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Marine Biological Laboratory Library

Woods Hole, Mass.

Presented by

ESTATE OF HERBERT W. RAND
January 9, 1964

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A TEXT-BOOK OF ZOOLOGY

VOL. I

MACMILLAN AND CO., LIMITED

LONDON . I'OMBAY . CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD

TORONTO

A TEXT-BOOK
OF ZOOLOGY

BY

T. JEFFERY PARKER, D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF OTAGO, DUNEDIN, N.2.

AND

WILLIAM A. HASWELL, M.A., D.Sc., F.R.S.

PROFESSOR OF BIOLOGY IN THE UNIVERSITY OF SYDNEY, N.S.XV.

Lit / j

*
iss.

IN TWO VOLUMES
VOL, I

WITH ILLUSTRATIONS

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1910

RICHARD CLAY AND SONS, LIMITED,

BREAD STREET HILL, E.G., AND

BUNGAY, SUFFOLK.

First Edition, 1898.
Second Edition, 1010.

PKEFACE TO THE EIRST EDITION

IN spite of its bulk, the present work is strictly adapted to the
needs of the beginner. The mode of treatment of the subject is
such that no previous knowledge of Zoology is assumed, and
students of the first and second years should have no more
difficulty in following the accounts of the various groups than is
incidental to the first study of a complex and unfamiliar subject.

There can be little doubt that the study of Zoology is most
profitably as well as most pleasantly begun in the field and by the
sea-shore, in the Zoological Garden and the Aquarium. In a
very real sense it is true that the best zoologist is he who knows
the most animals, and there can certainly be no better foundation
for a strict and scientific study of the subject than a familiarity
with the general appearance and habits of the common members
of the principal animal classes. But Zoology as a branch of
academical study can hardly be pursued on the broad lines of
general natural history, and must be content to lose a little in
breadth of view at least in its earlier stages while insisting upon
accurate observation, comparison, and induction, within the limited
field of Laboratory and Museum work.

A not uncommon method of expounding the science of Zoology
is to begin the study of a given group by a definition, the very
terms of which it is impossible that the student should under-
stand ; then to take a general survey of the group, illustrated by
casual references to animals and to structures of which it is highly
unlikely he has ever heard ; and, finally, to descend to a survey of
the more important forms included in the group. It will probably
be generally agreed that, from the teacher's point of view, this
method begins at the wrong end, and is hardly more rational than

vi PREFACE TO THE FIRST EDITION

it would be to deliver a coarse on the general characteristics of
English Literature, suitably illustrated by " elegant extracts," to a
class of students who had never read a single English poet or
essayist.

There can be no question as to the vast improvement effected in
zoological teaching by the practice of preceding the study of a
given group as a whole by the accurate examination of a suitable
member of it. With the clear mental image of a particular animal,
in the totality of its organisation, the comparison of the parts and
organs of other animals of like build becomes a profitable study,
and the danger of the comparative method that the student may
learn a great deal of the systems of organs in a group without
getting a clear conception of a single animal belonging to it is
much diminished.

The method of " types " has, however, its own dangers. Students
are, in their way, great generalisers, and, unless carefully looked
after, are quite sure to take the type for the class, and to consider
all Arthropods but crayfishes and cockroaches, and all Molluscs but
mussels and snails, as non-typical. For this reason a course of
Zoology which confines itself entirely or largely to " types," or, as
we prefer to call them, 1 examples, is certain to be a singularly
narrow and barren affair, and to leave the student with the
vaguest and most erroneous ideas of the animal kingdom as a
whole. This is especially the case when the number of examples
is small, each of the Phyla being represented by only one or two
forms.

In our opinion every group which cannot readily and intel-
ligibly be described in terms of some other group should be
represented, in an elementary course of Zoology, by an example.
\Vu have, therefore, in the majority of cases, described, in some
detail, an example of every important class, and, in cases where
the diversity of organisation is very great as in Crustacea and
Fishes two or more examples are taken. The student is thus
furnished with a brief account of at least one member usually
readily accessible of all the principal groups of animals.

By the time the example has been studied, a definition of the
class and of its orders will convey some idea to the mind, and will

1 Following a suggestion for which we are indebted to Dr. Alexander Hill,
Master of Downing College, Cambridge.

PREFACE TO THE FIRST EDITION vii

serve to show which of the characters already met with are of
distinctive importance, and which special to the example itself
In order to bring out this point more clearly, to furnish a connec-
tion between the account of the example and that of the class as a
whole, and to give some idea of the meaning of specific, generic,
and family characters, we have introduced, after the classification,
a paragraph giving the systematic position of the example, some-
times in more, sometimes in less detail.

Following the table of classification with its brief definitions
comes the general account of the group. This is usually treated
according to the comparative method, the leading modifications of
the various parts and organs being described seriatim. In a few
cases this plan has been abandoned and the class described order
by order, but this is done only when the deviations from the type
are so considerable as to lead us to think the comparative method
unsuitable for beginners. On the other hand, when all the classes
of the phylum present a very uniform type of structure, the
phylum is studied comparatively as a whole. The description of
each group usually ends with some account of its ethology and
distribution, and with a discussion of its affinities and of the
mutual relationships of its various subdivisions.

We have done our best to make the space devoted to each group
proportional to its complexity and range of variation, and to
subdue the natural tendency to devote most attention to the more
recently investigated classes, or to those in which we ourselves
happen to be especially interested. A few lesser groups have been
put into small type, partly to economise space, partly because they
seem to us to be of minor importance to the beginner.

Following out the plan of deferring the discussion of general
questions until the facts with which they are connected have been
brought forward, we have placed the sections on Distribution, on
the Philosophy of Zoology, and on the History of Zoology at the
end of the book. We have, however, placed a general account
of the structure and physiology of animals immediately after the
Introduction, and one on the Craniate Vertebrata before the
description of the classes of that division, but it will be obvious
that these deviations from the strictly inductive method were
inevitable in order to avoid much needless repetition.

After a good deal of consideration we have decided to omit all

viii PREFACE TO THE FIRST EDITION

references to the literature of the subject in the body of the work.
Anything like consistent historical treatment would be out of place
in an elementary book ; and the introduction of casual references
to particular discoveries, while they might interest the more
advanced reader by giving a kind of personal colouring to the
subject, could hardly fail, from their necessarily limited character, to
be misleading to the beginner, and to increase rather than diminish
his difficulties. We have, therefore, postponed all reference to the
history of the science to the concluding Section, in which the main
lines of progress are set forth, and have given, as an Appendix, a
guide to the modern literature of Zoology. The latter is intended
merely to indicate the next step to be taken by the student who
wishes to acquire something more than a mere text-book
knowledge. 1

The various Sections have been written by the authors in fairly
equal proportions, but the work of each has been carefully read
and criticised by the other, and no disputed point has been allowed
to stand without thorough discussion. We are therefore jointly
and severally responsible for the whole work.

A very large proportion of the figures have been specially drawn
and engraved for the book. Those in which no source is named
are from our own drawings, with the exception of Figs. 571, 572,
1017, 1018, 1019, 1022, 1059, 1063, and 1071, for which we are
indebted to Mrs. W. A. Haswell. Figs. 1002 bis, 1005 fo's,are from
photographs kindly taken for us by Mr. A. Hamilton. 2 Many blocks
have been borrowed from well-known works, to the authors and
publishers of which we beg to return our sincere acknowledg-
ments. All the new figures have been drawn by Mr. M. P. Parker.

1 In this connection we cannot resist the pleasure of quoting two passages,
exactly expressing our own views, from the preface to Dr. Waller's Human
Physiology, which came under our notice after the above paragraph was in type '.
"I have given a Bibliography after some hesitation, feeling that references to
original papers are of no use to junior students, and must be too imperfect to
be satisfactory to more advanced students. . . . Attention has been paid to
recent work, but I have felt that the gradually-formed deposit of accepted know-
ledge must be of greater intrinsic value than the latest ' discovery ' or the
newest theory. An early mental diet in which these items are predominant is
an unwholesome diet ; their function in elementary instruction is that of condi-
ments, valuable only in conjunction with a foundation of solid food."

2 The figures referred to are numbered 608, 609, 1080, 1081, 1082, 1085,
1128, 1132, 1140, 1063, and 1067 in the new edition.

PREFACE TO THE FIRST EDITION ix

We have received generous assistance from Professors Arthur
Dendy, G. B. Howes, Baldwin Spencer, and J. T. Wilson, and from
Mr. J. P. Hill and Dr. Arthur Willey. Professor W. N. Parker
has very kindly read the whole of the proof-sheets and favoured us
with many valuable suggestions, besides acting as referee in
numerous minor difficulties which would otherwise have cost a
delay of many weeks.

It is a mere truism to say that a text-book can never really
reflect the existing state of the science of which it treats,
but must necessarily be to some extent out of date at the time
of publication. In the present instance, the revises of the earlier
pages, giving the last opportunity for any but minor alterations,
were corrected in the latter part of 1895, and the sheets passed
for press in the middle of 1896. We are, therefore, fully alive to
the fact that much of our work already needs a thorough revision ,
and can console ourselves only by reflecting that "to travel hope-
fully is a better thing than to arrive, and the true success is to
labour."

We may mention, in conclusion, that, whatever may be the merits
or demerits of the book, it enjoys the distinction of being unique
in one respect. The two authors have been separated from one
another, during the greater part of their collaboration, by a
distance of 1200 miles, and the manuscript, proofs, and drawings
have had to traverse half the circumference of the globe in their
journeys between the authors on the one hand, and the publishers,
printers, artist, and engravers on the other. It will, therefore, be
readily believed that all persons concerned have had every oppor-
tunity, during the progress of the work, of exercising the supreme
virtue of patience.

PREFACE TO THE SECOND EDITION

A NEW edition of this Text-Book has been called for on some-
what short notice, and, had it not been for the assistance generously
rendered by Professor W. Newton Parker, who has helped me
greatly in the revision of the proofs, and has made a large number
of useful suggestions, it would have been impossible for me to have
completed the work within the time prescribed. Fortunately, also,
materials for the most important of the alterations and additions
had been already, to a certain extent, prepared.

The original plan of the work has not been in any way altered,
and, though all parts have been subjected to careful revision, there
is a good deal, especially in the descriptions of many of the
examples, which has not been materially changed. On the other
hand, some parts have been to a great extent re-written, and a
good many illustrations have been added, a fair proportion of which
are new to text-books of this description.

I have the pleasure of acknowledging assistance on special points
received from Professor J. P. Hill, Mr. S. J. Johnston, B.A., B.Sc.,
Mr. E. J. Goddard, B.A, D.Sc, and Mr. H. L. Kesteven, B.Sc, all
of the University of Sydney. A good many of the new illustra-
tions were re-drawn by W. Birmingham, Laboratory Assistant,
Department of Biology.

W. A. HAS WELL.

[ Lie * j

MASS.

CONTENTS

PAGE

PREFACE .... . v

CONTENTS OF SECTIONS IN VOL. I xiii

LIST OF ILLUSTRATIONS IN VOL. I. ... .... xix

TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM . . . xxxv
INTRODUCTION 1

SECTION I

THE GENERAL STRUCTURE AND PHYSIOLOGY OF ANIMALS .... 10

1. Amoeba 10

2. The Animal Cell . . .... 14

3. The Ovum : Maturation, Impregnation, and Segmentation : the

Germinal Layers 19

4. Tissues . 23

5. Organs 31

6. The Reproduction of Animals 40

7. Symmetry .... 41

8. The Primary Subdivisions or Phyla of the Animal Kingdom . 43

SECTION II

PHYLUM PROTOZOA ... 45

Class I. Rhizopoda . .46

1. Example of the Class Amceba proteus 46

2. Classification and General Organisation 47

Systematic Position of the Example ... 48

Appendix to the Rhizopoda 64

Class II. Mycetozoa . ... 66

1. Example of the Class Didymium difforme <>6

2. General Remarks . 67

Class III. Mastigophora . 67

1. Example of the Class Englena viridis . . '>7

2. Classification and General Organisation . .... 69
Systematic Position of the Example .... .70

Class IV. Sporozoa . 80

1. Example of the Class Mouoci/stis ay His 80

2. Classification and General Organisation .... .81
Systematic Position of the Example . ... 82

8

ft

xiv CONTENTS

PAGE

PHYLUM PROTOZOA continued.
Class V. Infusoria

1. Example of the Class Paramceciv.m caudatuin ....

2. Classification and General Organisation ... . 91
Systematic Position of the Example

Further Remarks on the Protozoa . . . . . 101

SECTION III

PHYLUM AND CLASS PORIFERA [PARAZOA] .

1. Example of the Class Sycon yelatinosum . . 105

2. Distinctive Characters and Classification ... . Ill
Systematic Position of the Example .

3. General Organisation ... 114

SECTION IV

PHYLUM COZLENTERATA . 128

Class I. Hydrozoa

1. Example of the Class Obelia ... . . . 128

2. General Structure and Classification 140

Systematic Position of the Example .... . 142

Additional Remarks ... 167

Class II. Scyphozoa . . 168

1. Example of the Class Aurelia aurita 168

2. General Structure and Classification 176

Systematic Position of the Example 177

Additional Remarks 184

Class III. Actinozoa , . 185

1. Example of the Class Tealia crassicornis 185

2. Distinctive Characters and Classification 193

Systematic Position of the Example . . . 196

3. General Organisation . 196

Class IV. Ctenophora 211

1. Example of the Class Hormiphora plumosa 211

2. Distinctive Characters and Classification 220

Systematic Position of the Example ... . 221

3. General Organisation . 222

Appendix to Ctenophora Ctenoplana and Codoplana . . . 225

The Relationships of the Coelenterata 226

Appendix to the Coelenterata The Mesozoa 230

SECTION V

PHYLUM PLATYHELMINTHES . 235

1. Examples of the Phylum 236

i. Planaria or Dendrocoeliim 236

ii. Fasciola hepatica . 240

iii. Tania soliuni 245

CONTENTS xv

PAGE

PHYLUM PLATYHELMINTHES continued.

2. Distinctive Characters and Classification 251

Systematic Position of the Examples 253

.">. General Organisation 254

4. Distribution, Mode of Occurrence, and Mutual Relationships . 283

Appendix to Platyhelminthes Class Nemertinea . . . 288

Distinctive Characters and Classification 295

SECTION VI

PHYLUM NEMATHELMINTHES 297

Class I. Nematoda ... 297

1. Example of the Class Ascaris lumbricoides 297

2. Distinctive Characters and Classification ..... 303
Systematic Position of the Example 304

3. General Organisation 305

Class II. Acanthocephala . . 312

Class III. Chatognatha .... 316

Appendix to Nemathelminthes 319

Family Chcetosomidce 319

,, Echinoderidce 319

,, Desmoscolecidce 320

Affinities and Mutual Relationships of the Nemathelminthes . 320

SECTION VII

PHYLUM TKOCHELMINTHES 322

Class I. Rotifera . . . 323

1. Example of the Class Brachionus rubens 323

2. Distinctive Characters and Classification 327

Systematic Position of the Example 329

3. General Organisation 330

Class II. Gastrotricha , 335

Appendix to Trochelminthes Dinophttea and Histriobdellea . 336

SECTION VIII

PHYLUM MOLLUSCOIDA 340

Class I. Polyzoa . . ... .340

1. Example of the Class Bugida avicularia 341

2. Distinctive Characters and Classification . . . 347
Systematic Position of the Example 348

3. General Organisation 348

Class II. Phoronida . . 355

Class III. Brachiopoda 360

1. Example of the Class Magellania lenticultirix . . . 360

2. Distinctive Characters and Classification . . . 366
Systematic Position of the Example 367

3. General Organisation 367

Mutual Relationships of the Classes of the Molluscoida . . 372

xvi CONTENTS

SECTION IX

PAGE

Phylum Eehinodermata 375

1. Example of the Asteroidea Asterias rubens or Anthenea flavescens. 375

2. Example of the Echinoidea Strongylocentrotyg or .Echinus . . 393

3. Example of the Holothuroidea Cucumaria or Colochinix . . 401

4. The Crinoidea Antedon rosacea 405

5. Distinctive Characters and Classification .410

Systematic Position of the Examples 41 4

6. General Organisation 415

SECTION X

PHYLUM ANNULATA 439

Class I. Chsetopoda 439

1. Examples of the Class 440

i. Nereis dumerilii . . . 440

ii. Lumbricus 454

2. Distinctive Characters and Classification 464

Systematic Position of the Examples 466

3. General Organisation 467

Appendix to the Chsetopoda Class MyzostOHlida .... 489

Class II. Gephyrea 491

1. Example of the Class Sipuncvlus nudus 492

2. Distinctive Characters and Classification 495

3. General Organisation 496

Class III. Archi-annelida . 503

Class IV. Hirudinea 506

1. Example of the Class Hirudo medicinalis and H. aiistr<di* . 506

2. Distinctive Characters and Classification 515

Systematic Position of the Example 517

3. General Organisation 517

4. General Remarks on the Annulata 523

SECTION XI

PHYLUM ARTHROPOD A 526

Class I. Crustacea 526

1. Examples of the Class 526

i. Apus or Lepidurus 526

ii. Astacus fluviatilis 539

2. Distinctive Characters and Classification 561

Systematic Position of the Examples 569

3. General Organisation 570

Affinities and Mutual Relationships 602

Appendix to Crustacea Class Trilobita 604

Class II. Onychophora 607

Class III. Myriapoda 614

1. Distinctive Characters and Classification 614

2. General Organisation 615

CONTENTS xvii

PAGE

PHYLUM ARTHROPODA continued.

Class IV. Insecta . 619

1. Example of the Class Periplaneta orientalis or P. americana . 619

2. Distinctive Characters and Classification 631

Systematic Position of the Example . . 636

3. General Organisation . . ... . 636

Class V. Arachnida . . . 653

1. Example of the Class Enscorpio or Bit thus 653

2. Distinctive Characters and Classification . ... 660

3. General Organisation 662

Appendix to the Arachnida the Pijcnoyonida, Linguatulida,

and Tardigrade 673

Relations of the Air-breathing Arthropoda 676

SECTION XII

PHYLUM MOLLUSCA . 680

Class I. Pelecypoda . 680

1. Example of the Class Anodonta and Unio 680

2. Distinctive Characters and Classification 694

Systematic Position of the Examples 696

3. General Organisation . . . ... 696

Class II. Amphineura . 712

1. Distinctive Characters and Classification 712

2. General Organisation . .... 713
Class III. Gastropoda ... . 721

1. Example of the Class Triton nodiferus .... . 721

2. Distinctive Characters and Classification . ... 732
Systematic Position of the Example .... . 734

3. General Organisation . . . 735
Appendix to the Gastropoda . .... 756

A. Class IV. Scaphoda . 756

B. Rhodope . . 758
Class V. Cephalopoda . . . 759

1. Examples of the Class 759

i. Sepia, .... 759

ii. Nautilus pompilius . 776

2. Distinctive Characters and Classification 789

Systematic Position of the Examples .... . 790

3. General Organisation . 790

General Remarks on the Mollusca 804

VOL. I

LIST OF ILLUSTRATIONS

VOL. I.

FIG. PAGE

1 . Amoeba proteus 10

2. Amoeba polypodia, fission 13

3. Alveolar theory of protoplasm 15

4. Reticular theory of protoplasm 1G

5. Diagrams illustrating karyokinesis 17

6. Ovum of Sea-urchin 19

7. Maturation and fertilisation of ovum ... ... 20

8. Segmentation of ovum 22

9. Gastrulation 22

10. Gastrula .23

11. Various forms of epithelium 24

12. Diagram to illustrate structure of glands 25

13. Gelatinous connective tissue 26

14. Reticular connective tissue 26

15. Fatty tissue ... ... .... 27

16. Hyaline cartilage 27

17. Fibro cartilage ...... 27

18. Bone ...... .28

19. Unstriped muscle 29

20. Striped muscle 29

21. Nerve-cells 30 1

22. Nerve-fibres ... 30'

23. Various forms of spermatozoa .30'

24. Viscera of Frog .... 33

25. Bones of human arm with biceps muscle 37

26. Nervous system of Frog 38

27. Hydra . . . 41

28. Diagram of axes of body 42

29. Radial symmetry 42

30. Amoeba, various species . . . 47

31. Protamoeba primitiva . 49

32. Quadrula, Hyalosphenia, Arcella, and Difflugia .... 49

33. Microgromia socialis 50

b 2

xx LIST OF ILLUSTRATIONS

FIG. PAGE

34. Platoum stercoreum .51

35. Various forms of Foraminifera 52

36. Shells of Foraminifera . 53

37. Hastigerina murrayi .... .... 54

38. Dimorphism and alternation of generations in Polystomella . . 55

39. Actinophrys sol ... 57

40. Actinosphserium eichhornii 57

41. Various forms of Heliozoa 58

42. Actinophrys sol, conjugation 59

43. Lithocireus annularis 60

44. Thalassoplancta brevispicula 61

45. Aulactinium actinastrum ......... 62

46. Actinomma asteracanthion ... 62

47. Collozoum inerme . . ...... 63

48. Chlamydomyxa labyrinth uloides .... ... 64

49. Labyrinthula 65

50. Didymium difforme 66

51. Euglena viridis . 68

52. Various forms of Flagellata 71

ij

53. Trypanosome . . . .72

54. H^ematococcus pluvialis 73

55. Pandorina morum 74

56. Volvox globator .... 75

57. Heteromita rostra ta . . 76

58. Various forms of Choanoflagellata 77

59. Various forms of Dinoflagellata .79

60. Noctiluca miliaris 79

61. Monocystis . . . 80

62. Gregariiia . 82

63. ,, development 83

64. Eimeria and Coccidium ... 84

65. Coccidium, life-history .... ... .85

66. Malaria parasite 86

67. Myxidium and Myxobolus 87

68. Sarcocystis miescheri . 88

69. Paramoecium caudatum 89

70. ,, ,, conjugation . .... 90

71. Various forms of Ciliata 94

72. .95

73. Vorticella ...... 96

74. Zoothamnium arbuscula ....... .97

75. Opanna ranarum . .... . . 98

76. Various forms of Tentaculifera . 100

77. Diagram showing the mutual relationships of the Protozoa . 103

78. Sycon gelatinosum . . 106

79. ,, ,, magnified ... .... 106

80. ,, ,, transverse section .... . 107

81. ,, ,, vertical section 108

LIST OF ILLUSTRATIONS xxi

PIG.

82. Sycon gelatinosum, pore-membrane . . . 109

83. ,, ,, apopyle . 109

84. External form of various Sponges . . .115

85. Ascetta primordialis ...... . 116

86. Diagrams of canal-system of various Sponges . .117

87. Vertical Section of Spongilla .... . . 118

88. Cells of ectoderm of Sponge . . . . . . 119

89. Development of tri-radiate spicule ... . 120

90. Skeleton of various Sponges . . 121

91. Various forms of Sponge spicules . 122

92. Pheronema Carpenter! . . ... 123

93. Larva of Clathrina blanca . ... ... 124

94. Development of Sycoii raphanus . 125

95. Obelia . . .130

96. ,, Vertical section of polype .... ... 132

97. Nematocysts of Hydra . . . 133

98. Tentacle of Eucopella . 134

99. Obelia, medusa ........ . . 135

100. Diagram of medusa ... . 136

101. Derivation of medusa from polype . 137

102. Projections of polype and medusa. . . 138

103. Development of zoophyte . . . 140

104. Bougainvillea ramosa . . . . 144

105. Various forms of Leptolinre . . 145

106. Ceratella . . 146

107. Hydra . . .147

108. Protohydra leuckartii ... . . 148

109. Various forms of leptoline Medusas ... . 150

110. Diagram illustrating formation of sporosac by degeneration of

medusa ............. 151

111. Early development of Eucope .... . 152
1J2. Two Trachymedusje . . . 154

113. Two Narcomedusse ..... . 154

114. ^Eginura, tentaculocyst ... . 155

115. Larva of ^Eginopsis ... . . 156

116. Millepora alcicornis, skeleton . . .157

117. Millepora, diagram of structure . .158

118. Stylaster sanguineus, skeleton . . 159

119. Halistemma tergestinum .... . 160

120. Diagram of a Siphonophore . . . . . . 162

121. Development of Halistemma ...... . . 163

122. Physalia .... .164

123. Diphyes campanulata ..... . . 165

124. Porpita pacifica ......... . 166

125. Graptolites ..... . 167

126. Aurelia aurita, dorsal and ventral views . . . . 169

127. ,, ,, side view and vertical section ..... 171

128. ,, ,, portion of umbrella with tentaculocyst . .172

xxii LIST OF ILLUSTRATIONS

TIG. , PAGE

129. Aurelia aurita, development 174

130. Tessera princeps 178

131. Lucernaria 178

132. Pericolpa quadrigata 179

133. Nausithoe 180

134. Charybdsea marsupialis . 181

135. Pilema pulmo . 183

136. Pelagia noctiluca, development 184

137. Tealia crassicornis, dissection and transverse section . . .186

138. Diagrammatic sections of Sea-anemone 188

139. Tealia crassicornis, section of tentacle 190

140. Nematocysts of Sagartia 190

141. Section of mesenteric filament of Sagartia .... . 191

142. Transverse sections of embryos of Actinia 193

143. Zoanthus sociatus 197

144. Hartea elegans 197

145. Corallium rubrum 198

146. Astrrea pallida 198

147. Pennatula sulcata 199

148. -Tubipora musica 199

149. Edwardsia claparedii 200

150. Antipathes ternatensis 201

151. Parantipathes and Schizopathes 202

152. Minyas 202

153. Alcyonimn palmatum 203

154. Gorgonia verrucosa 204

155. Structure of simple coral 206

156. Dendrophyllia and Madrepora ... .... 207

157. Adamsia palliata 209

158. llormiphora plumosa 211

159. ,, ,, dissection and transverse section . . . 212

160. ,, ,, diagrammatic sections 214

161. ,, ,, section of branch of tentacle .... 215

162. ,, ,, sense-organ 216

163. Ovum of Lampetia 217

164. Segmentation of oosperm in Ctenophora 218

165. Development of Ctenophora 218

166. Development of Callianira 218

167. ,, ,, (later stages) 219

168. Three Cydippida . ... 222

169. Deiopea kaloknenota 223

170. Cestus veneris ... 223

171. Beroe forskalii . . 224

172. Ctenoplana kowalevskii 225

173. Sections of embryos of Actinia and Beroe 228

174. Diagram illustrating the mutual relationships of the Coalenterata . 229

175. Dicyema paradoxum with infusoriform embryos .... 230

176. ,, ,, ,, vermiform ,, . . . . 230

LIST OF ILLUSTRATIONS xxiii

FIG. PAGE

177. Dicyema paradoxum, male 231

178. Rhopalura giardii, male . 232

179. ,, ,, female 232

180. Salinella, longitudinal section 233

181. ,, transverse ,, . . 234

182. Planaria, digestive and excretory systems 237

183. ,, nervous system 237

184. ,, reproductive system 239

185. Transverse section of a Planarian 240

186. Distomum hepaticum 240

187. ,, ,, section of integument 241

188. ,, ,, internal organisation 242

189. ,, ,, terminal part of reproductive apparatus . 243

190. ,, ,, development 244

191. Tsenia solium . 246

192. ,, ,, head 247

193. ,, ,, transverse section 247

194. ,, ,, proglottis . ' 248

195. ,, ,, ripe proglottis 250

196. ,, ,, development 251

197. Various Planarians 255

198. Gunda segmentata 256

199. Digenetic Trematodes ... 257

200. Gyrodactylus and Polystomum 258

201. Temnocephala 259

202. Actinodactylella .... .260

203. Tetrarhynchus . 261

204. Trenia echinococcus 261

205. Ligula ... . 262

206. Caryophyllrcus . 263

207. Gyrocotyle . . .... .263

208. Archigetes 263

209. Section of body- wall of a Triclad . .... .264

210. Parenchyma of Flat-worm .... .... 265

211. Diagram of Rhabdocoele 266

212. ,, ,, Polyclad ... 266

213. ,, ,, Triclad . . 267

214. Flame-cell ... 269

215. Reproductive organs of Mesostomum ehrenbergii .... 272

216. Development of a Polyclad 274

217. Miiller's larva ..'... 275

218. Embryos of Denclroccelum 276

219. Embryo of Temnocephala . 278

220. ' 279

221. A Cysticercoid 280

222. ,, with head evaginated 281

223. Cyst of Trenia echinococcus 282

224. Scolices ,, ,, ... 282

xxiv LIST OF ILLUSTRATIONS

FIG. PAGE

225. Scolex of Trenia echinococcus . ' . 282

226. Process of buckling in Microstomum 283

227 Diagram of the relationships of the Platyhelminthes and Nemer-

tinea . .287

228. Diagram of Nemertine . 289

229. Proboscis of Nemertine . . . 290

230. Tetrastemrna . .291

231. Anterior portion of Nemertine . . . 292

232. Proboscis of Hoplonemertean, retracted . . . 292

233. ,, ,, everted . 292

234. Transverse section of Nemertine . . ... 293

235. Vascular and excretory systems of Nemertine ... . 293

236. Pilidium . .294

237. Ascaris lumbricoides . 298

238. ,, ,, transverse section ... . . 299

239. ,, ,, muscle fibres . 300

240. ,, ,, dissection of female .... . 301

241 . Nervous system of Nematoda ... . 302

242. Ascaris lumbricoides, dissection of male organs . 303

243. Body- wall of platymyariaii Nematode . . 305

244. Ankylostoma duodenale ........ . 306

245. Transverse section of Gordius . ... . 306

246. Oxyuris . . 307

247. Gordius, anatomy ... . . 308

248. Development of Ascaris nigrovenosa . . . 309

249. Trichinella spiralis . 311

250. Two species of Echinorhynchus (Gigantorhynehus) . . 313

251. Echinorhynchus gigas, dissection of male . . . 314

252. ,, ,, female . 314

253. ,, ,, ,, nephridia . . 315

254. ,, ,, female organs . . . 315

255. Sagitta hexaptera .... . . 316

256. ,, bipunctata, transverse sections . 317

257. ,, ,, head . 317

258. ,, hexaptera, eye . 318

259. Development of Sagitta . ... 318

260. Chtetosoma . . 319

261. Echinoderes . 320

262. Desmoscolex . 320

263. A trochophore . . 322

264. Brachionus rubens, female . . 324

265. ,, ,, pharynx 325

2(56. ,, ,, male and female, with attached eggs . 326

267. Diagram of a Rotifer . . 327

268. Pataseison asplanchnus . 329

269. Typical forms of Rotifera . . 331

270. ... .332

271. ,, ,, mastax 333

> i

LIST OF ILLUSTRATIONS xxv

FIG. PAGE

272. Chsetonotus maximus 336

273. ,, ,, anatomy .... ... 336

274. Dinophilus fcsBniatus . 337

275. Stratiodrilus tasmanicus . 338

276. Bugula avicularia .... - . . . 342

277. Development of Bugula . 345

278. ,, ....-. .... 346

279. Larva of Bugula . 346

280. Plumatella . . . .349

281. Cristatella . . . 350

282. Lophopus . 351

283. Pedicellina . . .355

284. Phoronis australis 356

285. ,, ,, free end .... .... 357

28(1 ,, ,, internal organisation . .... 357

287- ,, ,, section 358

288. ,, ,, development . . . . 359

289. Magellan ia flavescens, shell .... .... 361

290. ,, lenticularis, anatomy 363

291. ,, flavescens, lophophore . 364

292. ,, muscular system .... .... 364

293. Terebratula, nervous system, &c. . .... 365

294. Typical Brachiopods . . . .368

295. ,, ,, anatomy .... ... 369

296. Development of Cistella . . .370

297. Larva of Cistella . 370

298. Development of Cistella . .371

299. Lophophore of embryo Brachiopod . . . . 372

300. Diagrams of phylactoloematous Polyzoon and Phoronis . . . 373

301. Starfish, oral aspect 376

302. ,, vertical section of arm . 378

303. ,, ambulacral system 379

304. Starfish, portion of vertical section of arm 380

305. ,, diagrammatic sections 381

306. Asterias rubens, digestive system 382

307. Astropecten, section of stone-canal 383

308. Anthenea flavescens, dissection from dorsal aspect .... 384

309. Asterias rubens, structure 385

310. Anthenea flavescens, lateral dissection 386

311. ,, ,, aboral surface 387

312. oral surface 387

313. Asterina gibbosa, development

314. . 390

315. ,, ,, larva .... 390

316. . . . . . 391

317. ,, exigua, young after metamorphosis 391

318. Asterina gibbosa, development . 392

319. Apical system of young Starfish 393

xxvi LIST OF ILLUSTRATIONS

FIG. PAGE

320. Echinus esculentus, peristome 394

321. Strongylocentrotus 395

322. Corona of Sea-urchin 396

323. Apical disc of Sea-urchin 397

324. Echinus, lantern of Aristotle 397

325. Sea-urchin, anatomy, lateral view 398

326. Echinoid, transverse section of ambulacral zone .... 399

327. Sea-urchin, anatomy, oral view 400

328. Cucumaria planci 401

329. Anatomy of a Holothurian 403

330. Antedon . .405

331. Aboral view of Antedon 406

332. Antedon disc 406

333. ,, transverse section of pinnule 407

334. ,, sagittal section 408

335. Anthenea, ventral view 419

336. Ophioglypha lacertosa 420

337. Astrophyton arborescens 421

338. Diagram of spine of Sea-urchin 422

339. Pedicellaria of Arbacia punctulata 422

340. Hemipneustes radiatus 423

341. Clypeaster sub-depressus 423

342. Metacrinus interruptus 424

343. Development of Echinoderms 431

344. ,, ,, Antedon 432

345. Stalked larva of Antedon . 433

346. Diagram to illustrate the relationships of the classes of Echino-

dermata 437

347. Nereis dumerilii 440

348A. ,, ,, parapodium 441

348e. ,, ,, setse 441

349. Nereis diversicolor, proboscis 443

350. Nereis dumerilii, anatomy 444

351. ,, ,, transverse section 445

352. ,, ,, nervous system 446

353. ,, ,, eye 447

354. ,, ,, nephridium 448

355. ,, ,, development 451

356. ,, ,, ,, 453

357. Lumbricus herculeus 454

358. ,, setee 455

359. ,, transverse section 456

360. ,, sagittal section 457

361. ,, nervous system 459

362. ,, nephridium 460

363. ,, reproductive organs 462

364. ,, development . . . 463

365. Polynoe setosissima . 467

LIST OF ILLUSTRATIONS xxvii

FIG. PAGE

366. Vermilia ccespitosa 468

367. Chietopterus 469

368. Setee of various Polychreta 470

369. Section of setigerous sac of an Oligochtete 470

370. Polynoe extenuata, anterior end 471

371. Polychreta, various, heads 472

372. Tubifex .473

373. Terebella 474

374. Aphrodita, enteric canal 475

375. Saccocirrus, transverse section 477

376. Phyllodoce, nephridium 479

377. Nephridia and coelomoducts 480

378. Diagram illustrating development of gonad of Polychreta . . 482

379. Spirorbis lams . 484

380. Eupomatus, development of trochophore 485

381. Autolytus cornutus, budding 487

382. Syllis ramosa 487

383. Serpulre with their tubes 488

384. Myzostomum 490

385. ,, anatomy 491

386. Sipunculus nudus, anterior extremity 492

387. ,, ,, tentacular fold 493

388. ,, ,, anatomy 494

389. ,, ,, nervous system 494

390. Bonellia viridis, female 497

391. Echiurus 497

392. Priapulus 498

393. Bonellia, anatomy ... 499

394. Echiurus, ciliated funnel 499

395. ,, anatomy 500

396. ,, nervous system 500

397. Bonellia, male 501

398. Echiurus, trochophore 501

399. Polygordius neapolitanus 503

400. Protodrilus 504

401. Polygordius neapolitanus, transverse section 504

402. ,, ,, trochophore 505

403. ,, ,, ,, later stage .... 505

404. Hirudo medicinalis . . 507

405. ,, ,, transverse section 508

406. ,, jaw 509

407. ,, australis, dissection from dorsal aspect . . . 510

408. ,, australis, ,, ,, left side . . . . .511

409. ,, medicinalis, nephridium 512

410. ,, diagram of blood-channels 513

411. ,, section of eye 514

412. ,, cocoon 515

413. Three Rhynchobdellicla . . 517

xxviii LIST OF ILLUSTRATIONS

FIG.

414. Proboscis of Clepsine . .518

415. Nephridium of Herpobdella ... 519

416. Pontobdella, nephridial system 520

417. Clepsine, development . .... 521

418. Diagram of origin of metamerism . 524

419. Diagram illustrating the relationships of the Annulata and

Trochelminthes . . .... 525

420. A pus cancriformis, dorsal aspect 527

421. Lepidurus kirkii, side view . 528

422. Apus glacialis, ventral aspect 529

423. ,, appendages . . 530

424. Lepidurus kirkii, sagittal section . 532

425. Apus, transverse section 534

426. ,, shell-gland ... .535

427. ,, cancriformis, nervous system 536

428. ,, structure of paired eye 537

429. ,, development 538

430A. Astacus fiuviatilis, male ... ... . 540

430 B. ,, ,, transverse section of abdomen .... 540

431. ,, ,, appendages . . . 543

432. ,, ,, articulations and muscles of leg .... 545

433. Section of skin and exoskeleton of Lobster 546

434. Articulations and muscles of abdomen of Crayfish .... 547

435. Astacus fluviatilis, dissection from right side . . . 548

436. ,, gills . . . . 550

437. ,, kidney ... .552

438. ,, ,, transverse section of thorax 553

439. ,, ,, diagram of circulation 554

440. ,, ,, nervous system 555

44 1 . , , , , reproductive organs .... . 557

442. ,, ,. formation of the blastoderm 557

443. ,, ,, early embryo 558

444. ,, ,, nauplius 559

445. ,, ,, section of embryo 560

446. ... ... 560

447. ,, ,, advanced embryo 501

448. Three Branchiopoda 571

44 J. ,, Cladocera . 572

450. Cypris .... . .... 573.

451. Cyclops and Calocalanus 574

452. Various forms of parasitic Eucopepoda 576

453. Argulus foliaceus . . 577

454. Lepas anatifera . .... . . 578

455. Balanus . .... 579

456. Sacculina carcini . 580

457. Nebalia geoffroyi 581

458. Paranaspides 582

459. Mysis oculata . . 582

460. Diastylis 583

LIST OF ILLUSTRATIONS xxix

FIG. PAGE

461. Gammarus ... . 584

4(52. Asellus .... . 585

403. Amphipoda .... . 586

464. Isopoda . . . 587

465. Shrimp and Prawn . . . . . 588

466. Scyllarus arctus ... . . . . 589

467. Pagurus bernhardus ... ... . 589

468. Cancer pagurus . . . 590

469. Typical Brachyura . . 591

470. Squilla . . . 592

471. Orchestia cavimana, anatomy 594

472. Euphausia pellucida .... . . . 595

473. Nervous system of Crab . 596

474. Cypris-stage of Lepas 598

475. Larvre of Crabs . . . 600

476. Diagram illustrating the mutual relationships of the orders of

Crustacea . (504

477. Dalmanites and Phacops . . 605

478. Triarthrus beckii . . 606

479. Peripatus capensis 607

480. ,, ,, ventral view of head 607

481. ,, anatomy 608

482. ,, trachea! pit .......... 609

483. ,, nephridium .... 610

484. ,, riovse zealandite, development ... . 611

485. ,, capensis .... 613

486. Scolopendrella immaculata ... . 615

487. Scolopendra . 616

488. Lithobius forficatus 616

489. Pauropus huxleyi ... 617

490. Strongylostoma, development 618

491. Periplaneta orientalis .... 620

492. ,, mouth-parts .... ... . 621

493. ,, americana, lateral view of head ... . 621

494. ,, muscular system .... . . 624

495. ,, anatomy . . . . . . . . . 625

496. ,, salivary glands . . 625

497. Trachea of caterpillar . . . 626

498. Periplaneta, tracheal system ... 627

499. ,, nervous system 627

500. ,, male reproductive organs .... . 628

501. ,, female reproductive organs ... . 628

502. Segmentation of ovum of Insect . 029

503. Ventral plate of embryo Cockroach . . . . . 030

504. Embryo Cockroach . . . 630

505. Lepisma . 632

506. Podura . 632

507. Locusta . . ... 633

508. Ephemera ... . . 033

xxx LIST OF ILLUSTRATIONS

FIG. PAGE

509. Aphis rosre . . . 633

510. Cicada ... ... . . 634

511. Culex and larva ..... 634

512. Gastrophilus equi 634

513. Pieris ... . . .635

514. Crioceris . 635

515. Section of integument of Insect 636

516. Mouth-parts of Honey-bee 637

517. ,, ,, Diptera 638

518. ,, ,, Lepidoptera 639

519. Digestive organs of Beetle 641

520. Nervous, trachea!, and digestive systems of the Honey-bee . . 642

521. Trachea] gills of Epheinerid . ... .643

522. Heart of Cockchafer ... .643

523. Nervous system of Diptera 644

524. Ocellus of Dytiscus larva 645

525. Chordotonal organ of Isopteryx 645

526. Sexual apparatus- of Honey-bee 646

527. Segmentation of ovum of Insect 648

528. Germinal layers and amnion of Insect 649

529. Development of Hydrophilus 650

530. ,, 650

531. Apis mellifica, queen, worker, and drone 652

532. Formica rufa .652

533. Euscorpio . 654

534. Ventral surface of cephalothorax and pre-abdomen of Scorpion . 654

535. Endosternite of Scorpion 655

536. Scorpion, anatomy, lateral view . . 657

537. ,, ,, dorsal ,, . . . .658

538. ,, development 659

5.'!9. Embryo of Scorpion 659

540. Chelifer bravaisii 662

541. Phrynus .... . 663

542. Galeodes dastuguei 663

543. Epeira diadema 664

544. ,, ,, chelicerre and pedipalpi of female .... 664

545. ,, ,, ,, ,, male 664

546. Sarcoptes scabiaei 665

547. Trombidium fuliginosum 665

548. Limulus ... . . 66(5

549. ,, ventral view 667

550. Eurypterus fischeri ... 668

551. Anatomy of dipneumonous Spider ...... 669

552. Limulus, sagittal section . 670

553. Lung-book of spider .... 670

554. Tracheal system of Spider 670

555. Gill-books of Limulus ... 671

556. Lateral eye of Euscorpio . . . : . . 671

557. Central eye of Euscorpio .... ... 672

LIST OF ILLUSTRATIONS xxxi

FIG. PAGE

558. Nymphon hispidum ........... 674

559. Pentastomum trenioidcs ........ . . 674

560. Macrobiotus hufelandi ......... . 675

561. Diagram to illustrate affinities of Arthropoda ..... 678

562. Anodonta cygnea ....... ... 681

563. ,, ,, interior of valve and animal removed from shell . 682

564. ,, ,, section of shell and mantle ..... 683

565. ,. cygnea, animal after removal of mantle-lobe . . . 685

566. ,, ,, dissection from left side ...... 686

567. ,, ,, structure of gills ....... 687

568. ,, ,, transverse sections . ..... 688

569. ,, diagram of circulation ... .... 690

570. ,, statocyst ...... .... 691

571. ,, early embryo .......... 692

572. ,, later embryos .......... 692

573. ,, advanced embryo ......... 693

574. ,, metamorphosis ......... 694

575. Anatomy of Pecten .... .... . 61(7

576. Valves of Mya, Modiola, and Vulsella . . . 698

577. Cardium edule . .......... 698

578. Venus gnidia ..... ... . 699

579. Scrobicularia piperata ...... .... 699

580. Solecurtus strigillatus ......... 700

581. Diagram of concrescence of mantle-lobes ...... 700

582. Requienia and Hippurites .... ..... 701

583. Teredo navalis ............ 701

584. Aspergillum ............ 702

585. Mytilus edulis . . .... 702

586. Nucula delphinodonta .......... 703

587. Gills of Pelecypoda ... 704

588. Gill-filaments of Mytilus .... . 705

589. Dissection of Poromya .... ..... 705

590. Donax, enteric canal ...... . 706

591. Nervous system and auditory organs of Nucula ..... 707

592. Eye of Pecten .... . . .708

593. Development of Ostrea ..... ..... 709

594. Veliger of Ostrea . .... . 709

595. Embryos of Cyclas ........... 710

596. Diagram illustrating the mutual relationships of the Pelecypoda . 712

597. Chsetoderma iiitidulum ..... ..... 713

598. Neomenia carinata . .......... 714

599. Chiton, spinosus, dorsal view ........ 714

600. ,, ventral view . . . . ..... .714

601. ,, valves of shell .......... 715

602. Chsefcoderma iiitidulum, longitudinal section ..... 716

603. Chiton, longitudinal section ......... 717

604. Nervous system of Amphineura ........ 717

605. Neomenia carinata, reproductive organs .... .718

606. Chiton, nephridial and genital systems ...... 719

xxxii LIST OF ILLUSTRATIONS

FIG. PACK

607. Chiton, development ... . . . . 720

608. Triton nodiferus, shell . . . 722

609. Triton ,, shell, median section 723

610. ,, ,, operculum . 724

611. ,, ,, lateral view of body 724

612. ,, ,, diagram of introvert 725

613. ,, ,, dissection from dorsal side 727

614. ,, ,, buccal mass 728

615. ,, ,, vertical section of buccal cavity . . V . 728

616. ,, ,, nervous system from dorsal side .... 730

617. ,, ,, ,, ,, and related parts, lateral view . 731

618. ,, ,, section ofeye 732

619. Diagrams of displacement of mantle-cavity, &c. .... 736

620. Solarium perspectivum 737

621. Terebra oculata ..... 738

622. Cypraea moneta 739

623. Doris tuberculata 739

624. Carinaria mediterranea 739

625. Limax . ..... 739

626. Sigaretus Irevigatus 740

627. Aplysia . . . 740

628. Shell-bearing Pteropoda 741

629. Atlanta peronii ... 741

630. Pterotrachea scutata 742

631. Helix nemoralis . ... 742

632. Pleurophyllidia lineata . 743

633. Patella vulgata ... . 743

634. Pulmonary cavity and related parts in Limax . . 743

635. Nervous system of Patella . . . 745

636. Nervous system of Aplysia ... . . 746

637. ,, ,, ,, Limmeus . . 746

638. Eyes of Gastropoda ... . . 747

639. Osphradium of Murex . . . 747

640. Reproductive oigans of Helix ... . 748

641. Ovotestis of Gastropoda . 749

642. Forms of egg-cases in Gastropoda 749

643. Segmentation and formation of germinal layers in Gastropoda . 751

644. Early development of Patella .752

645. Trochophore of Patella ... . 753

646. Later trochophore of Patella ... . 754

647. Veliger of Vermetus 755

648. Diagram illustrating the relationships of the Gastropoda . . 756

649. Dentalium, section of shell ... 756

650. ,, anatomy . 757

651. ,, larvae .... . 757

652. Rhodope . . 758

653. Sepia, cultrata . . . 760

654. Sepia ,, shell . . 762

655. ,, chromatophore . . . 762

LIST OF ILLUSTRATIONS xxxiii

1 '' 1(; - PACK

656. Sepia, cultrata, cranial cartilage .... . 763

657. ,, ,, nuchal cartilage 763

658. ,, ,, mantle-cavity 764

659. ,, officinalis, jaws . 765

660. ,, section of buccal mass . 766

661. ,, officinalis, enteric canal , 766

662. ,, cultrata, dissection of male from posterior aspect . . . 767

663. ,, ,, lateral dissection of male .... . 768

664. ,, officinalis, longitudinal section of ink-sac . . . 769

665. ,, cultrata, vascular system 770

666. ,, ,, cephalic ganglia 770

667. ,, ,, pedal and pleuro-visceral ganglia .... 770

668. ,, section of eye . 771

669. ,, cultrata, statolith 772

670. ,, officinalis, renal organs . 773

671. ,, ,, diagrammatic sagittal section of female . . . 774

672. ,, male reproductive organs 775

673. ,, sperms and spermatophore 775

674. Nautilus pornpilius, section of shell .... . 776

675. ,, ,, female in shell 778

676. Nautilus macromphalus, entire animal 779

677. Nautilus pompilius, lobe of foot 780

678. ,, ,, spadix 781

679. ,, ,, cephalic cartilage 781

680. ,, ,, mantle-cavity of male 782

681. ,, ,, dissection of male from left side . . . 784

682. ,, ,, arteries 785

683. ,, ,, renal sacs, ctenidia, &c 786

684. , , , , male reproductive organs 788

685. ,, ,, female ,, ,, 788

686. ,, macromphalus, egg . 789

687. Octopus vulgaris 791

688. Loligo vulgaris 792

689. Argonauta argo 793

690. Octopus lentus, male 793

691. Amphitretus pelagicus . . 794

692. Shell of Spirula .794

693. Spirula peronii 795

694. Ammonite . 795

695. Shell of Belemnite .... .796

696. ,, Argonauta argo 796

697. Segmentation of Loligo .... 798

698. Blastoderm of Sepia .... . . 799

699. ,, ,, sections 799

700. Development of Loligo 800

701. . - 801

702. . . 801

703. . 802

704. Diagram to illustrate the relationships of the Cephalopoda . . 804

VOL. I C

CLASSIFICATION OF THE ANIMAL KINGDOM

IN THIS BOOK.

KINGDOM ANIMALIA.

PHYLUM I. PROTOZOA.

,,

,,

Class I. RHIZOPODA.
Order 1. LOBOSA.

2. FORAMIXIFERA.

3. HELIOZOA.

,, 4. RADIOLARIA.
Class II. MYCETOZOA.
Class III. MASTIGOPHORA.
Order 1. FLAGELLATA.

2. CHOAXOFLAGELLATA.

3. DlXOFLAGELLATA.

,,

Order 4. CYSTOFLAGELLATA.
Class IV. SPOROZOA.

Order 1. GREGARIXIDA.
,, 2. COCCIDIIDKA.
,, -3. H.^MOSPORIDIA.
,, 4. MYXOSPORIDEA.
5. SARCOCYSTIDEA.
INFUSORIA.

Class V.

Order 1. CILIATA.

2. TEXTACULIFERA.

PHYLUM II. PORIFERA.

Class PORIFERA.
Sub-class I. Calcarea.
Order 1. HOMOC<ELA.

Order 2. HETEROCIELA.
Sub-class II. Hexactinellida.
,, III. Demospongia.

PHYLUM III. CCELENTERATA.

Class I. HYDROZOA.

Order 1. LEPTOLIXJE.

Sub-order a. AntJiomeduscc

Order 2. TRACHYLIN.E.
Class II. SCYPHOZOA.

Order 1. STAUROMEDUS.*;.
,, 2. COROXATA.
,, 3. CTBOMEDUS^.
,, 4. DJSCOMEDUS.*:.
Sub-order a.
,, b.

Class III. ACTINOZOA.
VOL. I

Sub-class I. Zoantharia.
Order 1. ACTINIARIA.

,, 2. MADREPORARIA.

., 3. AXTIPATHARIA.
Sub-class II. Alcyonaria.

Sub-order <t. Tr<'hyme<ln$'

,_,

,,

Order 3. HYDROCORALLIXA.

4. SIPHOXOPHORA.

5. GRAPTOLITHIDA.
4. ALCYOXACEA.

o. (TORGOXACEA.
0. PENXATULACEA.

C *

xxvi CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM III. CCELENTERATA continued,

Class IV. CTENOPHORA. Appendix to Ctenophora Ctenoplann

Order 1. CYDIPPIDA. and Cceloplana.

,, 2. LOB ATA. Order 1. PLATYGTENEA.

,, 3. CESTIDA.

,, 4. REROIDA. Appendix (II) to Coelenterata Mesozoa.

PHYLUM IV. PLATYHELMINTHES.

lass I. TURBELLARIA. Order 3. ASPIDOCOTYLEA.

Order 1. POLYCLADIDA. ,, 4. TEMNOCEPHALEA.

2. TRICLADIDA. Class III. CESTODA.

,, 3. RHABDOCCELIDA. Order 1. MONOZOA.

,, 2. POLYZOA.
Class II. TREMATODA.

Order 1. MOXOGENETICA. Appendix to Platyhelminthes Class

2. DIGENETICA. NEMERTINEA.

PHYLUM V. NEMATHELMINTHES.

Class I. NEMATODA. Class III. CHJETOGNATHA.

Order 1. NEMATOIDEA.

,, 2. NEMATOMOBPHA. Appendix to Nemathelminthes Chit-

tosomidce, Echinoderidce, and Desmos-
ClassII. ACANTHOCEPHALA. colecid*.

PHYLUM VI. TROCHELMINTHES.

Class I. ROTIFARA. Order 5. TROCHOSPH^RIDA.

Order 1. RHIZOTA. ,, 6. SEISONIDA.
,, 2. BDELLOIDA.

3. PLOIMA. Class II. GASTROTRICHA.
Sub-order a. Illoricata,

,, b. Loricata. Appendix to Trochelininthes Dino-

Order 4. SCIRTOPODA. pliilta, and Histriobdellea.

PHYLUM VII. MOLLUSCOIDA.

Class I. POLYZOA. Order 2. PHYLACTOL^MATA.

Sub-class I. Ectoprocta. Sub-class II. Endoprocta.

Order 1. GYMXOLJEMATA. Class II. PHORONIDA.

Sub-order a. Cyclostomata, III. BRACHIOPODA.

,, b. Cheilostomata. Order 1. INARTICULATA.

,, c. Ctenobtomata. ,, 2. ARTICULATA.

PHYLUM VIII. ECHINODERMATA.

Class I. ASTEROIDEA. Order 3. CLADOPHIUK^:.

Order 1. PHANEROZONIA. ,, 4. ZYGOPHIURJS.

,, 2. CRYPTOZONIA. Class III. ECHINOIDEA.
Class II. OPHIUROIDEA. Order 1. REGULARIA.

Order 1. LYSOPHIUR.I:. ., 2. CLYPEASTRIDEA.

,, 2. STREPTOPHIUK^-:. ,, 3. SPATANGOIDEA.

CLASSIFICATION OF THE ANIMAL KINGDOM xxxvii

PHYLUM VIII. ECHINODERMATA continued.

Class IV. HOLOTHUROIDEA.

Order 1. ELASIPODA.
,, 2. PEDATA.
,, 3. APODA.
Class V. CRINOIDEA.
Sub-class I. Monocyclica.

Sub-class II. Dicyclica.
Class VI. CYSTOIDEA.
,, VII. BLASTOIDEA.
,, VIII. EDRIASTEROIDEA.
IX. CARPOIDEA.

PHYLUM IX. ANNULATA.

Class I. CHJETOPODA.

Sub-class I. Polychaeta.

Order 1. ARCH

,, 2. PHANEROCEPHALA.
,, 3. CRYPTOCEPHALA.
Sub-class II. Oligochaeta.
Order 1. MICRODRILI.

,, 2. JV1 EGADRILI.

Appendix to the Chsetopoda-
MYZOSTOMIDA.

Class II. GEPHYREA.
Order 1. IXERMIA.
,, 2. ARM ATA.

Class III. ARCHI ANNELIDA.

,, IV. HIRUDINEA.
Order 1. RHYXCHOBDELLIDA.

,, 2. ARHYXCHOBDELLIDA.
-Class Sub-order 1. Gnathobdettida.

,, 2. Herpobdellida.

PHYLUM X. ARTHROPOD A.

Class I. CRUSTACEA.

Sub-class I. Branchiopoda.
Order 1. ANOSTRACA.
,, 2. NOTOSTRACA.
, , 3. CONCHOSTRACA.
,, 4. CLADOGERA.
Sub-class II. Ostracoda.
,, III. Copepoda.
Order 1. EUCOPEPODA.
,, 2. BRANCHIURA.
Sub-class IV. Cirripedia.
Order 1. EUCIRRIPEDIA.
,, 2. RHIZOCEPHALA.
Sub-class V. Malacostraca.
Order 1. MYSIDACEA.
,, 2. CUM ACE A.
,, 3. TAXAIDACEA.
,. 4. ISOPODA.
,, 5. AMPHIPODA.
Sub- order 1. Macrura.
,, 2. A nomura.
,, 3. Brachyurcbi
Appendix to Crustacea Class
LOBITA.

Class II. ONYCHOPHORA.
,, III. MYRIAPODA.
Sub-class I. Progoneata.

Order 1. PAUROPOUA.
,, 2. DIPLOPODA.
,, 3. SYMPHYLA.
Sub-class II. Opisthogoneata.

Order 1. CHILOPODA.
Class IV. INSECTA.
Order 1. APTERA.

2. ORTHOPTERA.

3. NEUROPTERA.

4. HEMIPTERA.
DIPTERA.
LEPIDOPTERA.
COLEOPTERA.
HYMEXOPTERA.

Class V. ARACHNIDA.

Order 1. SCORPIOXIDA.

PSEUDOSCORPIOX IDA.
PEDIPALPIDA.
SOLPUGIDA.
PHALAXGIDA.
6. ARAXEIDA.
,, 7. ACARIDA.
TRI- ,, 8. XIPHOSURA.

EURYPTERIDA.

to the Arachnida The

55

55

55

} 5

55

5 J

55

55

55

55

5.
6.

8.

2.

3.
4.
5.

8.
9.
Appendix

55

PYCXOGONIDA, LIXGUATULIDA, and TAR-

DIGRADA.

ZOOLOGY

INTRODUCTION

of Natural History which deals with

ERRATA.

PAGE.

*7, description of Fig. 5, for " atrosphere " read " astrophere."
52, description of Fig. 35, for " Rotalla" read " Rotalia."
71, description of Fig. 52, for " Astasiopis " read " Astasiopsis. "
74, line 9, for " divison " read " division."

Ill, line I, for " out" read "outer."

208, line 10, for " siphnozoids " read " siphonozooids. "

272, line 2, for " prostrate" read " prostate."

402, line 43, for " periphwmal " read " perihaemal."

450, line 7, for "Fig. 846" read "Fig. 347."

in me position ami ciicuctcuuis ui uncn m uuj.-u.cn vj.^^^. ^..^ w^^^
can fail to see that these animals, in spite of differences of size,
colour, markings, &c., are all, in the broad sense of the word,
' Cats." This is expressed in the language of systematic Zoology
by saying that they are so many species of a single genus.

According to the system of binomial nomenclature introduced by
Linngeus, each kind of animal receives two names one the generic

ZOOLOGY

INTRODUCTION

Zoology, the branch of Natural History which deals with
animals, is one of the two subdivisions of the great science Biology,
which takes cognisance of all organisms, or things having life, as
distinguished from such lifeless natural objects as rocks and
minerals. The second of the two subdivisions of Biology is
Botany, which deals with plants.

The subject-matter of Zoology, then, is furnished by the animals
which inhabit the land-surface, the air, and the salt and fresh
waters of the globe : the aim of the science is to find out all that
can be known of these animals, their structure, their habits, their
mutual relationships, their origin.

The first step in the study of Zoology is the recognition of the
obvious fact that the innumerable individual animals known to
us may be grouped into what are called species, the members of
which resemble one another so closely that to know one is to know
all. The following example may serve to give the reader a fairly
accurate notion of what Zoologists understand by species, and of
the method of naming species which has been in use since the time
of the great Swedish naturalist Linnoeus.

The Domestic Cat, the European Wild Cat, the Ocelot, the Leopard,
the Tiger, and the Lion are animals which agree with one another in
the general features of their organisation in the number and form
of their bones and teeth, in the possession of retractile claws, and
in the position and characters of their internal organs. No one
can fail to see that these animals, in spite of differences of size,
colour, markings, &c., are all, in the broad sense of the word,
' Cats." This is expressed in the language of systematic Zoology
by saying that they are so many species of a single genus.

According to the system of binomial nomenclature introduced by
Linnaeus, each kind of animal receives two names one the generic

E B

2 ZOOLOGY

name, common to all species of the genus ; the other the specific
name, peculiar to the species in question. Both generic and specific
names are Latin in form, and are commonly Latin or Greek in
origin, although frequently modern names of persons or places, with
Latinised terminations, are employed. In giving the name of an
animal, the generic name is always placed first, and is written
with a capital letter, the specific name following it, and being
written, as a rule, with a small letter. For instance, to take the
examples already referred to, the Domestic Cat is called Fclis
domestica, the European Wild Cat F. catus, the Leopard F. pardus,
the Tiger F. tigris, the Lion F. ho. Thus the systematic name of an
animal is something more than a mere appellation, since it indicates
the affinity of the species with other members of the same genus :
to name an animal is, in fact, to classify it.

It is a matter of common observation that no two individuals of
a species are ever exactly alike : two tabby Cats, for instance,
however they may resemble one another in the general characters
of their colour and markings, invariably present differences in
detail by which they can be readily distinguished. Individual
variations of this kind are of universal occurrence. Moreover, it
often happens that the members of a species are divisible into
groups distinguishable by fairly constant characters : among
Domestic Cats, for instance, we find white, black, tabby, gray, and
tortoiseshell Cats, besides the large long-haired Persian breed, and
the tailless Manx Cat. All these are distinguished as varieties of
the single species Felis domestica.

It is often difficult to decide whether two kinds of animals should
be considered as distinct species or as varieties of a single species, and
no universal rule can be given for determining this point. Among
the higher animals mutual fertility is a fair practical test, the
varieties of a species usually breeding freely with one another and
producing fertile offspring, while distinct species either do not
breed together or produce infertile hybrids or mules. Compare,
for instance, the fertile mongrels produced by the union of the
various breeds of Domestic Dog with the infertile mule produced by
the union of the Horse and Ass. But this rule is not without
exception, and in the case of wild animals is, more often than not,
impossible of application : failing it, the only criterion of a " good
species " is usually the presence of constant differences from allied
species. Suppose, for instance, that a naturalist receives for
description a number of skins of wild Cats, and finds, after an
accurate examination, that in some specimens the tail is two-thirds
the length of the body and the skin of a uniform reddish tint with
a few markings on the head, while in the rest the tail is nearly half
as long as the body, and the skin tawny with black stripes. If
there are no intermediate gradations between these two sets of
individuals, they will be placed without hesitation in distinct

INTRODUCTION 3

species : if, on the other hand, there is a complete series of grada-
tions between them, they will be considered to form a single
variable species.

As, therefore, animals have to be distinguished from one another
largely by structural characters, it is evident that the foundations
of a scientific Zoology must be laid in Morphology, the branch of
science which deals with form and structure. Morphology may be
said to begin with an accurate examination of the external
characters ; the divisions of the body, the number and position of
the limbs, the characters of the skin, the position and relations of
the mouth, eyes, ears, and other important structures. Next the
internal structure has to be studied, the precise form, position,
&c., of the various organs, such as brain, heart, and stomach, being
made out : this branch of morphology is distinguished as Anatomy.
And, lastly, the various parts must be examined by the aid of the
microscope, and their minute structure, or Histology, accurately
determined. It is only when we have a fairly comprehensive
knowledge of these three aspects of a given animal its external
characters, its rough anatomy, and its histology- -that we can with
some degree of safety assign it to its proper position among its
fellows.

An accurate knowledge of the structure of an animal in its
adult condition is not, however, all-sufficient. Nothing has been
made more abundantly clear by the researches of the last half-
century than that the results of anatomy and histology must be
checked, and if necessary corrected, by Embryology i.e. by the
study of the changes undergone by animals in their develop-
ment from the egg to the adult condition, A striking instance is
afforded by the common Barnacles which grow in great numbers on
ships' bottoms, piers, &c. The older zoologists, such as Linnaeus,
grouped these creatures, along with Snails, Mussels, and the like,
in the group Mollusca, and even the great anatomical skill of
Cuvier failed to show their true position, which was made out only
when Vaughan Thompson, about sixty years ago, proved, from
a study of the newly hatched young, that their proper place
is among the Crustacea, in company with Crabs, Shrimps, and
Water-fleas.

Given a sound knowledge of the anatomy, histology, and em-
bryology of animals, their Classification may be attempted that
is, we may proceed to arrange them in groups and sub-groups,
each capable of accurate definition.

The general method of classification employed by zoologists is
that introduced by Linnaeus, and may be illustrated by reference
to the group of Cats which we have already used in the explanation
of the terms genus, species, and variety.

We have seen that the various kinds of true Cat Domestic Cat,
Lion, Tiger, &c. together constitute the genus Felis. Now there

B 2

4 ZOOLOGY

is one member of the cat-tribe, the Cheetah, or Hunting Leopard,
which differs from all its allies in having imperfectly retractile
claws and certain peculiarities in its teeth. It is therefore placed
in a distinct genus, Cyncelurus, to mark the fact that the differences
separating it from any species of Felis are of a more fundamental
character than those separating the species of Felis from one
another.

The nearest allies of the Cats are the Hyaenas, but the presence
of additional teeth and of non-retractile claws to mention only
two points makes the interval between Hyaenas and the two
genera of Cats far greater than that between Felis and Cynselurus.
The varying degree of difference is expressed in classification by
placing the Hyaenas in a separate family, the Hycenidce, while
Felis and Cynaelurus are placed together in the family Fdidce.
Similarly, the Civets and Mongooses form the family Vivcrridcc ;
the Dogs, Wolves, Jackals, Foxes, &c., the family Ganidoc ; Bears,
the family Ursidce ; and so on.

All the foregoing animals have sharp teeth adapted to a flesh
diet, and their toes are armed with claws. They therefore differ
fundamentally from such animals as Sheep, Deer, Pigs, and Horses,
which have flat teeth adapted for grinding vegetable food, and
hoofed feet. The differences here are obviously far greater than
those between any two of the families mentioned above, and are
emphasised by placing the flesh-eaters in the order Carnivora,
the hoofed animals in the order Ungulata. In the same way
gnawing animals, such as Rats, Mice, and Beavers, form the order
Bodentia ; pouched animals, such as Kangaroos and Opossums, the
order Marsupialia ; and so on,

Carnivora, Ungulata, Rodentia, Marsupialia, &c., although
differing from one another in many important respects, agree in
the possession of a hairy skin and in the fact that they all suckle
their young, They thus differ from Birds, which have a covering
of feathers and hatch their young from eggs. The differences here
are considerably more important than those between the orders
of quadrupeds referred to, and are expressed by placing the latter
in the class Mammalia, while Birds constitute the class Avcs. In
the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c.,
form the class Heptilia; the slimy-skinned, scaleless Frogs, Toads, and
Salamanders the class Amphibia ; and the finned, water-breathing
Fishes the class Pisces.

Mammals, Birds, Reptiles, Amphibians, and Fishes all agree with
one another in the possession of red blood and an internal skeleton-
an important part of which is an axial rod or vertebral column-
and in never having more than two pairs of limbs. They thus
differ in some of the mosfc fundamental features of their organisation
from such animals as Crabs, Insects, Scorpions, and Centipedes,
which have colourless blood, a jointed external skeleton, and

INTRODUCTION 5

numerous limbs. These differences far greater than those be-
tween classes are expressed by placing the backboned animals
in the phylum or sub-kingdom Chordata, the many-legged,
armoured forms in the phylum Arthropoda. Similarly, soft-bodied
animals with shells, such as Oysters and Snails, form the phylum
Mollusca, Polypes and Jelly-fishes the phylum Coelcntcmta. And
finally the various phyla recognised by zoologists together con-
stitute the kingdom Animalia.

Thus the animal kingdom is divided into phyla, the phyla into
classes, the classes into orders, the orders into families, the families
into genera, and the genera into species, while the species themselves
are assemblages of individual animals agreeing with one another
in certain constant characters. It will be seen that the individual
is the only term in the series which has a real existence : all the
others are mere groups formed, more or less arbitrarily, by man.

To return to the animal originally selected as an example, it will
be seen that the zoological position of the Domestic Cat is expressed
as follows :-

Kingdom ANIMALIA.
Phylum CHORD ATA.
Class MAMMALIA.

Order CARNIVORA.
Family Fc lidcv.
Genus Felis.

Species F. domestica.

The object of systematic zoologists has always been to find a
natural as opposed to an artificial classification of animals.
Good instances of artificial classification are the grouping of Bats
with Birds on the ground that they both possess wings, and of
Whales with Fishes on the ground that they both possess fins and
live in the water. An equally good example of a natural classi-
fication is the grouping of both Bats and Whales under the head of
Mammalia because of their agreement, in all essential points of
anatomy, histology, and embryology, with the hairy quadrupeds
which form the bulk of that class.

With the older zoologists the difficulty was to find some general
principle to guide them in their (arrangement of animals some
true criterion of classification. It was believed by all but a few
advanced thinkers that the individuals of each species of animal
were descended from a common ancestor, but that the original

o

progenitor of each species was totally unconnected with that of
every other, having, as Buffon puts it, " participated in the grace
of a distinct act of creation." To take an instance all Wolves
were allowed to be descended from a pair of ancestral Wolves, and
all Jackals from a pair of ancestral Jackals, but the original pair in
each case was supposed to have come into being by a supernatural

6 ZOOLOGY

process of which no explanation could or ought to be offered.
Nevertheless it was obvious that a Jackal was far more like a
Wolf than either of them was like a Tiger, and that in a natural
system of classification this fact should be expressed by placing the
Wolf and Jackal in one family, the Tiger in another.

All through the animal kingdom the same thing occurs : no
matter what group we take, we find the species composing it
resemble one another in varying degrees, or, as it is sometimes ex-
pressed, have varying degrees of relationship to one another. On
the view that each species was separately created the word relation-
ship was used in a purely metaphorical sense, as there could of
course be no real relationship between two groups of animals
having a totally independent origin. But it was assumed that
creation had taken place according to a certain scheme in the
Divine Mind, and that the various species had their places in this
scheme like the bits of glass in a mosaic. The problem of classifica-
tion was thus to discover the place of each species in the pattern of
the unknown design.

The point of view underwent a complete change when, after the
publication of Darwin's Origin of Species in 1859, the Doctrine
of Descent or of Organic Evolution came to be generally
accepted by biologists. A species is now looked upon, not as an
independent creation, but as having been derived by a natural
process of descent from some pre-existing species, just as the
various breeds of Domestic Fowl are descended from the little
Jungle-fowl of India. On this view the resemblances between
species referred to above are actually matters of relationship, and
species are truly allied to one another in varying degrees since
they are descended from a common ancestor. Thus a natural
classification becomes a genealogical tree, and the problem of
classification is the tracing of its branches.

This, however, is a matter of extreme difficulty. Representing
by a tree the whole of the animals which have ever lived on the
earth, those existing at the present day would be figured by the
topmost twigs, the trunk and main branches representing extinct
forms. Thus the task of arranging animals according to their
relationships would be an almost hopeless one but for two
circumstances : one, that remains of many extinct forms have been
preserved ; the other, that the series of changes undergone by an
animal in its development from the egg often forms an epitome of
the changes by which, in the course of ages, it has been evolved
from an ancestral type. Evidence furnished by the last-named
circumstance is, of course, furnished by embryology : the study of
extinct animals constitutes a special branch of morphology to
which the name Palaeontology is applied.

The solid crust of the earth is composed of various kinds of
rocks divisible into two groups : (1) Igneous rocks, such as granite

INTRODUCTION 7

and basalt, the structure of which is due to the action of the
internal heat of the globe, and which originate below the surface
and are not arranged in layers or strata ; (2) Aqueous or sedimentary
recks, which arise by the disintegration, at the surface of the earth,
of pre-existing rocks, the fragments or debris being carried off by
streams and rivers and deposited at the bottom of lakes or seas.
Being formed in this way by the deposition of successive layers or
strata, the sedimentary rocks have a stratified structure, the lowest
being in every case older than the more superficial layers. The
researches of geologists have shown that there is a general order of
succession of stratified rocks : that they may be divided into three
great groups, each representing an era of time of immense but
unknown duration, and that each group may be subdivided into
more or fewer systems of rocks, each representing a lesser period of
time. The following table shows the thirteen rock-systems usually
recognised, arranged under the three great groups in chronological
order, the oldest being at the bottom of the list.

13. Quaternary and Recent.

m n . m A- 12. Pliocene.

. Oamozoic or lertiary. . < -,-, ,.-.

11, Miocene.

[ 10. Eocene.

9: Cretaceous.
II. Mesozoic or Secondary . 8. Jurassic.

Hr m

7. Inassic.

6. Permian.

5. Carboniferous.

I. Palaeozoic or Primary

4. Devonian.
3. Silurian.

2. Cambrian.
1. Laurentian.

Imbedded in these rocks are found the remains of various extinct
animals in the form of what are called fossils, In the more recent
rocks the resemblance of these to the hard parts of existing
animals is perfectly clear : we find shells hardly differing from
those we pick up on the beach, bones easily recognisable as those
of Mammals, Birds, or Fishes, and so on. But in the older rocks the
fossils are in many cases so different in character from the animals
existing at the present day as to be referable to no existing order.
We find Birds with teeth, great aquatic Reptiles as large as Whales,
Fishes, Molluscs, Crustacea, &c., all of an entirely different type from
any now existing. We thus find that the former were in many
cases utterly unlike the present animal inhabitants of the globe,
and we arrive at the notion of a succession of life in time t and are
even able, in exceptionally favourable circumstances, to trace back
existing forms to their extinct ancestors.

By combining the results of comparative morphology, embryology,

8 ZOOLOGY

and paleontology we get a department of Zoology called Phylo-
geny, the object of which is to trace the pedigrees of the various
groups. There are, however, very few cases in which this can be
done with any approach to exactness : most " phytogenies ' are
purely hypothetical, and merely represent the views at which a
particular zoologist has arrived after a more or less exhaustive
study of the group under discussion.

Animals may also be studied from the point of view
of Distribution. One aspect of this study is inseparable from
Palaeontology, since it is obviously necessary to mention in con-
nection with a fossil the particular system or systems of rocks in
which it occurs : thus we distinguish geological distribution or
distribution in time.

The distribution of recent forms may be studied under two
aspects, their horizontal or geographical distribution, and their
vertical or bathymetrical distribution. To mention the latter
first, we find that some species exist only on plains, others hence
called alpine forms on the higher mountains ; that some marine
shells, fishes, &c., always keep near the shore (littoral species), others
live at great depths (ctbyssal species), while others (pelagic
species) swim on the surface of the ocean. Among aquatic
animals, moreover, whether marine or fresh-water, three principal
modes of life are to be distinguished. There are animals, such
as Jelly-fishes, which float on or near the surface of the water,
and are carried about passively by currents: such forms are
included under the term Plankton. Most Fishes, Whales, and
Cuttle-fishes, on the other hand, are strong swimmers, and are able
to traverse the water at will in any direction ; they together consti-
tute the Nekton. Finally, such animals as Crabs, Oysters, Sponges,
Zoophytes, &c., remain permanently fixed to or creep over the
surface of the bottom, and are grouped together as the Benthos.

Under the head of geographical distribution we have such facts
as the absence of all Land-mammals, except Bats, in New Zealand
and the Polynesian Islands, the presence of pouched Mammals,
such as Kangaroos and Opossums, only in some parts of America
and in Australia and the adjacent islands, the entire absence of
Finches in Australasia, and so on. We find, in fact, that the
fauna i.e. the total animal inhabitants of a country is to a
large extent independent of climate, and that the fauna? of
adjacent countries often differ widely. In fact, it is convenient
in studying the geographical distribution of animals largely to
ignore the ordinary division into continents, and to divide the
land-surface of the globe into what are called zoo- geographical
regions. The characteristics of these regions will be discussed in
a future section ; at present it is only necessary, for convenience of
reference, to give their names and boundaries.

INTRODUCTION 9

1. The Hokirctic Region includes the whole of Europe, Asia as
for south as the Himalayas, Africa north of the Sahara, together
with the corresponding portion of Arabia, and North America as
for south as Mexico. For convenience of reference it is often
customary to divide this region into two : its Eurasian portion is
then called the Palcearctic, its American portion the Nearctic
region.

2. The Ethiopian Region includes Africa south of the Sahara,
Southern Arabia, and Madagascar with the adjacent islands.

3. The Oriental Region includes India, Ceylon, South China,
the Malayan Peninsula, and what are known as the Indo-Malayan
islands, i.e. those islands of the Malayan Archipelago which lie to
the west of a line called Wallace's line passing to the east of
the Philippines, between Borneo and Celebes and between Bali
and Lombok.

4. The Australian Region includes Australia, Tasmania, and the
Austro-Malayan islands, i.e. the islands of the Malayan Archipelago
lying to the east of Wallace's line.

5. The New Zealand Region includes New Zealand and the
adjacent islands, such as the Chatham, Auckland, and Campbell
groups.

6. The numerous groups of islands lying between Australia
and Southern Asia to the west, and America to the east, are
conveniently grouped together as the Polynesian Region.

7. The Neotropical Region includes the whole of South and
Central America and part of Mexico.

There are still two departments of zoological science to be
mentioned. As it is impossible to have a right understanding of
a machine without knowing something of the purpose it is in-
tended to serve, so the morphological study of an animal is im-
perfect without some knowledge of its Physiology, i.e. of the
functions performed by its various parts, and the way in which
they work together for the welfare of the whole. It is hardly
possible to give more than occasional references to physiological
matters in a text-book of Zoology, but in order to pave the way
for such references a brief account of the general principles of
Physiology will be given in the next section.

Not only may we study the action of a given animal's organs,
but also the actions of the animal as a whole, its habits, its
relations to other animals whether as friends, as enemies, or as
prey, to the vegetable kingdom, and to its physical surroundings,
such as temperature, humidity, &c. In a word, the whole question
of the relation of the organism to its environment gives us a final
and most important branch of Natural History which has been
called Ethology or Bionomics.

SECTION I.

THE GENERAL STRUCTURE AND PHYSIOLOGY

OF ANIMALS

"psd

1. AMCEBA.

IF we examine under the microscope a drop of water containing
some of the slimy deposit which collects at the bottom of pools of
rain-water and in similar situations, we occasionally find it to
abound in microscopic life ; and among the minute moving creatures
in such a drop we frequently find examples of a remarkable or-
ganism the Ama-la or Proteus Animalcule (Fig. 1). This is

a little particle of irregular
shape, which we should be
likely, on a cursory examina-
tion, to put down as motion-
less ; it appears somewhat like
an irregular particle of some
colourless glass-like substance
with a more granular central
portion. If, however, we make
an exact drawing of the out-
line of the Amoeba, and, after
an interval, compare the draw-
ing with the original, we find
that the drawing appears no
longer to represent what we
see ; a change has taken place
in the shape of the Amoeba ;
and careful observation shows that this change is constantly going
on : the Amoeba is constantly varying in shape. This change is
effected by the pushing out of projections or processes, called
pseudopods (>srf.), which undergo various alterations of size
and shape, and may become withdrawn, other similar processes
being developed in their place. At the same time careful

FIG. 1. Amoeba proteus, a living specimen.
c. rac. contractile vacuole ; nu. nucleus ;
ps<t. pseudopods. (From Parker's Biology,
after Gruber.)

SECT, i STRUCTURE AND PHYSIOLOGY OF ANIMALS 11

watching shows that the Amoeba is also, with extreme slowness,
changing its position. This it effects by a kind of streaming
motion. A projection forms itself on one side, and the entire
substance of the Amoeba gradually streams into it ; a fresh
projection appears towards the same side, the streaming move-
ment is repeated, and, by a constant succession of such move-
ments, an extremely gradual locomotion, which it often takes very
close watching to detect, is brought about. In these movements,
it is to be noticed, the Amoeba is influenced to some extent by
contact with other minute objects; when the processes come in
contact with small grains of sand or other similar particles their
movements are modified in such a way that the Amoeba, in its
slow progress onwards, passes on one side of them, so that it
might be said to feel its way among the solid particles in the drop
of sediment.

Judging from the nature of these movements, we are obliged to
infer that the substance of which this remarkable object is com-
posed must be soft and semi-fluid, yet not miscible with the water,
and, therefore, preserving a sharp contoui\ These and other
characteristics to be mentioned subsequently enable us to conclude
that we have to do with the substance of complex chemical com-
position termed protoplasm, which constitutes the vital material of
all living organisms whether animals or plants. In Amoeba the
protoplasm is in many cases clearly distinguishable into two parts,
an outer homogeneous, glassy-looking layer completely enclosing
a more granular internal mass.

Examination of the Amoeba with a fairly high power of the
microscope reveals the presence in its interior of two objects which
with a low power we should be likely to overlook. One of these
is a small rounded body with well-defined contour, which
preserves its form during all the changes which the Amoeba as a
whole undergoes. This is termed the nucleus (Fig. 1, nu.)\ it is
enclosed in an extremely delicate membrane, and consists of a
protoplasmic material differing from that which forms the main
bulk of the Amoeba in containing a substance which refracts the
light more strongly and which has a stronger affinity for certain
colouring matters. The other minute object to be distinguished
in the interior appears as a clear rounded space (c. vac.) in the
protoplasm. When this is watched it will be observed to increase
gradually in size till it reaches a maximum of, let us say, a fifth of
the total diameter of the Amoeba, when, by a contraction of its walls,
it suddenly disappears, to reappear presently and gradually grow
again to its maximum size. This pulsating clear space is the
contractile vacuole. Other clear spaces which do not pulsate are
the non- contractile vacuoles.

By watching the Amoeba carefully for some time we may be
enabled to observe that the movements of the protoplasm of the
body not only effect locomotion, but are connected also with the

12 ZOOLOGY SECT.

reception of certain foreign particles of organic nature i.e. either
entire minute animals or plants, or minute fragments of larger
forms into the interior of the protoplasm. A process of the
protoplasm is pressed against such a particle, which becomes sunk
in the soft substance, and passes gradually into the interior. Here
it becomes enclosed in one of the non -contractile vacuoles, and by
degrees partially or wholly disappears ; the part, if any, which
remains subsequently passes outwards from the protoplasm into
the surrounding water. The matter which disappears evidently
mixes with the" protoplasm and adds to its bulk. All, in fact, of
the matter of the foreign body that is capable of doing so, becomes
digested and assimilated by the protoplasm. The fluid in the
vacuole enclosing the food-particle (for such is the true nature of the
foreign body) probably contains some ingredient of the nature of a
ferment, which is able to act on certain substances and render
them more soluble or capable of being more readily taken up by
the protoplasm. This we infer mainly from what we know of the
digestion and absorption of food in the higher animals ; but the
fact, which has been established by experiment, that the Amoeba
is able readily to digest certain classes of organic substances, while
others, when taken into the interior of the protoplasm, remain
unaltered, seems to indicate that some special property, similar to
those possessed by the digestive ferments of the higher animals,
is present in the w r atery fluid surrounding the food-particle.

The movements of the Amoeba, slow and gradual though they
are, must involve a certain expenditure of energy or working power ;
this can only be derived from the energy of chemical affinity
which the protoplasm possesses in virtue of its complex chemical
composition. The protoplasm loses some of this energy by its
conversion into energy of movement. This loss implies the break-
ing up of the complex chemical ingredients of which protoplasm
is composed into simpler ones; the protoplasm falls a grade in
the scale of chemical compounds, and by its fall generates the
force by means of which the Amoeba moves. The energy of
chemical affinity which the protoplasm possesses is thus analogous
to the potential energy which the weight of a clock has when it is
wound up. As the weight, by virtue of its position, is able as it
falls to deal out working power so as to cause the movement of the
machinery of the clock, so the protoplasm is able, by the degra-
dation or decomposition of its complex compounds, to deal out
working power enabling the Amoeba to move. In the case of the
clock-weight there comes a time when all the potential energy is
expended ; the weight reaches its lowest limit, and unless it is
wound up again the clock stops. The like holds good of the
Amoeba ; the protoplasm is continually being used up decomposed
into compounds of a lower order and, in course of time, the whole
potential energy would become exhausted, were it not that a new

STRUCTURE AND PHYSIOLOGY OF ANIMALS

13

supply is being constantly received. This new supply of energy is
derived from the substance of the food-particles ; and this at the
same time maintains the bulk of the Amoeba, which, if food par-
ticles are absent from the water, gradually diminishes.

Accompanying the degradation, or destructive metabolism as it
is termed, of the protoplasm, and intimately connected with it, is
the passage inwards of oxygen from the air dissolved in the water,
and the passage outwards of carbonic acid gas. Oxygen is a
necessary agent in the process of destructive metabolism, and

FIG. 2. Amoeba polypodiain successive phases of division- The light spot is the contractile-
vacuole ; the dark the nucleus. (From Lang's Text-Book, after F. E. Schulze.)

carbonic acid is a constant waste-product of such action. This
interchange of oxygen and carbonic acid is the essence of the pro-
cess of respiration observable in all living things. In addition to
the carbonic acid given off in this process, other waste-products are
formed and have to be got rid of. In all probability the contractile
vacuole already referred to has to do with this process the process
of excretion since uric acid, which in higher animals is the typical
form assumed by such waste-products, is said to have been detected
in the interior of the contractile vacuole in the case of certain near
relatives of Amoeba.

When food is abundant the Amoeba increases in bulk more

14 ZOOLOGY SECT.

food being ingested than is required for simply maintaining the
size unaltered and soon a remarkable change takes place. The
processes become withdrawn, and a fissure appears dividing the
Amoeba into two parts (Fig. 2). This fissure grows inwards, and the
two parts become more and more completely separated from one
another, till eventually the separation becomes complete, and we
have two distinct Amoebae resulting from the division of the one.
While the protoplasm has been undergoing this division into
halves the nucleus has also divided, and each of the two new
Amoebae possesses a nucleus similar to the original one, and
developed from it by division. It is mainly by this simple process
of division into two, or binary fission as it is called, that rcpro-
duction or multiplication takes place in the Amoeba.

In spite of the great simplicity of its structure, the Amoeba
thus carries on a number of different functions. The practically
structureless particle of protoplasm is able to act on matter
absorbed as food in such a way as to alter the chemical composition
of the latter and to assimilate it ; it is able to carry on movements
of locomotion, as well as movements those involved in the
taking in of food particles which may be looked upon as move-
-raents of prehension ; it exhibits a certain degree of sensitiveness
or irritability, as shown by the modifications of its movements
which result from contact with foreign bodies ; it is able to
respire ; it carries on processes of excretion ; and, finally, it is
capable of reproducing its kind. It is these functions that charac-
terise living beings as distinguished from non-living matter.
What is specially characteristic of the living organism in general
when compared with a non-living object is the capacity of the
former to respond by changes in itself to influences operating on
it from without. In the case of such an extremely simple
organism as Amoeba, these changes are also, necessarily, extremely
simple ; but they are of a quite definite character. In addition
to the effects produced on its actions by mechanical obstacles and
the presence of food-particles, it can be shown by experiment that
Amoeba responds by definite changes in itself to such external
influences as changes in the amount of oxygen supplied, in the
quantities of various salts present, in the temperature, and in the
electric conditions of the water in which it lives. The power of
locomotion, the capacity for assimilating organic substances, and
the absence of two special compounds chlorophyll and cellulose-
are specially characteristic of the animal as distinguished from
the plant.

2. THE ANIMAL CELL.

In all but the lowest animals the various functions just enume-
rated are carried on by means of a more or less complex machinery

STRUCTURE AND PHYSIOLOGY IN ANIMALS

15

of organs muscles, alimentary or enteric canal, glands, heart and
blood-vessels, gills or lungs, nervous system, organs of excretion, and
organs of reproduction. But in all animals, however complex, the
same substance, protoplasm, which in Amoeba constitutes the
bulk of the body, is the essential and active part. Wherever in
the body active functions are being discharged and active changes
are going on, there we find protoplasm present ; where there is
no protoplasm there is no vital activity. In the earliest stages of
their existence all animals are formed entirely of protoplasm.
Every animal consists at first of a single minute particle of proto-
plasm, not widely different from an Amoeba. Soon this particle
divides into a number of parts which, instead of separating
completely from one another, like the parts of a divided Amoeba,
remain associated together, forming a clump of minute particles
of protoplasm. Such minute protoplasmic particles are termed
cells ; every animal consists, at first, of a single cell, and afterwards,
in all higher animals, this single cell becomes converted by division
and subdivision into a little cluster or clump of cells.

It is time that we should inquire more particularly into the
meaning of these two terms cell and protoplasm evidently so
important in the study of both plants and animals. Protoplasm,
we have already seen, is a semi-fluid, gelatinous, clear or finely
granular substance of complex chemical composition. It is known
not to be a definite compound, but to be a somewhat varying
mixture of chemical compounds, the most essential of which are
bodies of the class of proteids highly complex substances, into the
composition of which the elements carbon, hydrogen, oxygen,
nitrogen, and sulphur all

T 1

enter. Living protoplasm
always contains a large
amount of water. It is
soluble in weak acids or
weak alkalies ; and is
capable of being coagu-
lated - - rendered firmer
and more opaque - - by
the action of heat and
of strong alcohol. Its re-
action is slightly alkaline.
As regards its minute
structure, it is generally
acknowledged that there
are two kinds of sub-
stance in the protoplasm, in some cases more, in others less, dis-
tinctly marked off from one another. One of these kinds of material
is apparently of less fluid consistency than the other. According
to one view (alveolar theory) the two kinds are intimately com-

FIG. 3. Diagram to illustrate the alveolar theory of
protoplasm. (After Dahlgren and Kepner.)

16

ZOOLOGY

SECT

FIG. 4. Diagram to illustrate the reticular theory of
protoplasm. (After Dahlgreu and Kepner.)

bined in the form of an emulsion or froth, the one forming the
minute vesicles or bubbles in the froth, the other the ground
substance in which the bubbles are embedded (Fig. 3). Accord-
ing to another view (reticular theory), one of these substances,

the less fluid, appears to
be arranged in the form
of a network of threads,
composed of numerous
minute rounded granules
enclosing the second,
more fluid substance in
its meshes (Fig. 4).

To a particle of pro-
toplasm, typically con-
taining a nucleus in its
interior, constituting the
entire body of such a
simple organism as
Amoeba, and forming one
of the constituent ele-
ments of which a higher plant or animal is made up, the term cell
is applied. The word \vas first employed in reference to the micro-
scopic structure of plants, in connection with which it is much more
appropriate than in connection with the microscopic structure of
animals ; for a plant-cell has, nearly always, a definite, firm, enclos-
ing envelope or cell-wall (Fig. 5, I, c.w) a structure which is only
exceptionally present in the case of animals. In the interior of
the cell-protoplasm, or cytoplasm, is a body termed the nucleus,
similar to the nucleus of Amoeba, and usually of rounded shape, with
the appearance of being enclosed in a thin nuclear membrane
(A, nu.ni), perforated by numerous minute apertures. In the
nucleus is a single coiled thread, or a network of threads, or one
or more rounded clumps, of a substance chromatin (chr.) which
differs from ordinary protoplasm in having a stronger affinity for
most staining agents. A rounded body termed the nuclcolus
(nu), which usually occurs in the interior of the nucleus, is
formed either of a solid mass of chromatin, or of a substance
differing somewhat from chromatin in its properties, and less
strongly affected by staining agents. When the nucleus divides
during the process of division of the cell, its contents, more
particularly the chromatin, in many cases go through a remarkable
series of changes, to which the term haryoltinesis or mitosis is
applied.

At the time when this mitotic division is about to be
initiated, either one or two minute bodies (Fig. 5, A, c) are to be
distinguished situated close together in the cytoplasm in the
immediate neighbourhood of the nucleus. When only one of

STRUCTURE AND PHYSIOLOGY OF ANIMALS

17

these bodies is present at the outset it subsequently becomes
divided into two. These are the centrosomes minute masses of a
specially modified protoplasmic substance, capable of being
rendered conspicuous by certain staining agents, -" surrounded
by a light zone. The centrosomes, at first close together,
gradually separate from one another, a spindle-shaped bundle
of very fine fibres of achromatic : material the nuclear spindle

FIG. 5. Diagrams illustrating karyokinesis. A, the resting cell ; B, C, D, successive phases in
the formation and arrangement of the chromatiii loops and of the nuclear spindle ; E, F ,G,
separation of the two sets of daughter-chromosomes and their passage towards the poles of
the spindle ; H, I, division of the cell-body and formation of the two new nuclei ; c. centro-
some ; chr. chromatin ; cpl. cell-plate ; me', nucleoli ; nu. in. nuclear membrane ; s. atrosphere ;
sp. spindle. (From Parker's Biology, after Flemming, Rabl, &c.)

-extending between them (Fig. 5, C). At the same time,
or at an earlier stage, each centrosome has become the centre
of a system of fine achromatin fibres (apparently made up,
like the fibres of the spindle, of rows of granules) which are
arranged round it in a radiating manner, forming a structure

1 The term achromatin is usually applied to all the matter of the nucleus
that has not the special characteristics of chromatin ; but it applies to cytoplasmic
structures i.e. structures belonging to the body of the cell as well.

VUL. I C

18 ZOOLOGY SECT.

termed the attraction-sphere or astrospkere (Fig. 5. A, s). Meantime
important changes have been in progress in the nucleus. The
chromatin first becomes arranged in a close tangle (spireme), and
then becomes divided up into a number of parts the chromatin
segments or chromosomes which frequently have the form of loop-
like threads (Fig. 5, B, C, chr), but often assume other forms. The
number of chromosomes varies, but is constant throughout the
cells of the same species of animal. The nuclear membrane
disappears. Each of the chromatin segments splits lengthwise
into two parts the daughter-segments of the chromatin or daughter-
chromosomes (Fig. 5, B D), and with these the filaments of the
spindle become connected.

At this point the segments of the chromatin form a single
group the equatorial plate extending across the axis of the
spindle. The latter has shifted its position, so that its fibres now
run across the original site of the nucleus. Each daughter-segment
of the chromatin now separates from its fellow, so that two groups
are formed, each containing a similar number of chromosomes.
The two groups then move apart from one another, each approach-
ing the corresponding end or pole of the spindle with its
centrosome (Fig. 5, E G). How this movement is effected
is not definitely known ; it has been supposed that it is due to the
contraction of spindle-fibres attached to the centrosomes ; but
since there is no appearance of the fibres shortening or thicken-
ing, it is unlikely that this can be the true explanation.

When the groups have approached the extremity of the spindle,
the segments of each unite, and eventually the entire chromatin of
each of the two groups assumes the arrangement which the
chromatin of the original nucleus exhibited before division began.
A new nuclear membrane becomes formed around each chromatin
group, and the whole assumes the character of a complete nucleus
the daughter -nucleus (Fig. 5, H, I). It is of importance to
note that, though in this mitotic division of the nucleus of
the animal cell the centrosomes are so conspicuous that it
would appear as if they had an important share in controlling
the process, yet mitosis takes place during cell-division of the
higher plants on the same general lines as in animals though
centrosomes have rarely, if ever, been observed in plants higher
than the Mosses.

A furrow which appears on the surface of the cell-protoplasm (Fig.
5,H,I), surrounding it in the form of a ring in a plane at right angles
to the long axis of the spindle, deepens gradually so as to give rise
to a cleft, eventually completely separating the substance of the
cell into two halves. Each of these halves encloses one of the
daughter-nuclei, and has assumed the character of a complete
daughter -cell. During this process there is sometimes distinguish-
able along the line corresponding to the division line between the

STRUCTURE AND PHYSIOLOGY OF ANIMALS

19

two cells a narrow septum ; this is known as the cellulate (I., c.pl.~).
But a cell-plate is not of general occurrence in the division of the
animal cell.

In some instances the division of the nucleus is direct
or amitotic, the nucleus simply becoming separated into two
equal parts, without disappearance of the nuclear membrane
and without any complicated re-arrangement of the chromatin.

3. THE OVUM : MATURATION, IMPREGNATION, AND SEGMENTATION :

THE GERMINAL LAYERS.

Amoeba is simply an independent animal cell ; or, to express
the same meaning in another way, is a unicellular animal, and as
such it is a member of the phylum of the Protozoa or unicellular
animals. All the rest of the animal kingdom, forming the
division Metazoa, are multicdlular in the fully developed condition ;
but each of these multicellular
animals or Metazoa originates from
a single cell the ovum. The
ovum is a typical cell (Fig. 6),
usually spherical in shape, with
one or more enclosing membranes,
with cell-protoplasm enclosing a
nucleus (germinal vesicle) in which
are contained one or more rounded
masses of chromatin (germinal spot
or spots). The ovum may contain
in addition to the protoplasm a
quantity of non-protoplasmic nu-
trient material or yolk.

Before the process of impregna-
tion or fertilisation which gives
the impulse to development, the
ovum undergoes a change which is
termed maturation (Fig. 7, A). This consists, in essence, of
the throwing out of portions of the nucleus.' The latter
approaches the surface and divides, mitotically, into two parts
one coming to project on the surface and finally the projection being
completely separated off from the ovum as a rounded particle-
the first polar body (pol.). A second division of the nucleus
results in the throwing off of a second polar body ; and, after this
has been formed, the portion which remains in the ovum resumes
its central position and forms what is termed the female pro-
nudeus (B, $ pron.). The essential ultimate result of maturation
is the reduction of the number of chromosomes in the ovum by
one-half.

In the process of impregnation a very minute body, the male

c 2

FIG. 6. Ovum of a Sea-Urchin, showing
the radially striated cell-membrane,
the protoplasm, containing yolk-
granules, the large nucleus (germinal
vesicle), with its network of chro-
matiii and a large nucleolus (ger-
minal spot). (From Balfour's Em-
bryology, after Hertwig.)

20

ZOOLOGY

SECT.

cell, sperm-cell, or sperm, penetrates into the interior of the female
cell or ovum, and the nucleus which it contains the male pro-
nucleus (C, $ pron.) coalesces with the female pronucleus to form a
single nucleus called the segmentation nucleus (E, seg. nucl.). The

pol

mem.

menv

seg.nucL

FIG. 7. Diagram illustrating the maturation and fertilisation of the ovum. A, formation of first
polar body ; B, beginning of fertilisation, sperms approaching the micropyle ; C, formation
of the male pronucleus ; D, approximation of the male and female pronuclei ; E, formation
of segmentation-nucleus ; $> cent, female centrosome ; <*, cent, male centrosome ; mem. egg-
membrane ; microp. micropyle; pol. polar bodies ; 9 pron. female pronucleus ; $ pt'on. male
pronucleus ; seg. nucl. segmentation nucleus.

principal part in the process of fertilisation is thus played by the
two nuclei. The female centrosome disappears: a male centrosome
enters with the sperm.

Apparently in this process of fertilisation some attraction is

STRUCTURE AND PHYSIOLOGY OF ANIMALS 21

operative between the male and female cells. In many instances
a prominence (the receptive prominence) is pushed out by the
ovum at the point where the sperm enters. The female
pronucleus, leaving its former central position, approaches the
male cell as it enters. In most cases a single sperm alone enters
the ovum in impregnation. According to the older observers,
as soon as a sperm enters the ovum, a membrane is formed
around the latter hindering the penetration of additional sperms.
But it has now been shown that such a membrane occurs
only in certain cases, and is quite exceptional. That,
as a general rule, only one sperm penetrates into the ovum
appears to be due to the circumstance that, as a result of the
entry of the one sperm, the peculiar attraction above referred
to becomes in some way destroyed or diminished. But, though
the entry of one sperm only is usual, cases of the entry
of several polyspermy, as it is termed are by no means
rare, and would appear to be quite normal in some groups of
animals.

In some animals the ovum develops parthenogenetically i.e.
without any process of fertilisation by means of a male cell.
This is a normal phenomenon in certain families of insects,
for example. In a considerable number of marine invertebrate
animals it has been shown that -though gamogenesis, i.e. develop-
ment as the result of fertilisation of ovum by male cell, is
the normal process, yet parthenogenesis can be produced by
various artificial means. By adding various salts to the water
in which the ova are contained, by changes of temperature,
or by subjection to the action of carbonic acid gas, the ova,
in the absence of sperms, may be caused to give rise to normal
embryos. Such experiments on artificial parthenogenesis, as it
is termed, show that the entry of a male cell into the ovum
is not necessary for the development of the embryo even in
cases in which gamogenesis is normal ; but that other exciting
influences may bring about the same result.

Though, as stated above, the female pronucleus, under normal
circumstances, plays so important a role in the development, it
has been shown that it can be dispensed with. When unfertilised
ova of a sea-urchin are broken up, and fragments devoid of
nuclei are placed in water along with sperms, the fragments may
be fertilised ; and, the nucleus of the sperm taking the place
of the segmentation-nucleus, normal young, differing from those
produced in the usual manner only in their smaller size, may
be developed. This phenomenon is known as merogony.

The result of fertilisation is the formation of the impregnated
ovum, or oosperm as it is called. The oosperm, it is to be noted,
before development begins, consists in general of the primary
ovum minus the portions of the substance of its nucleus removed

22

ZOOLOGY

SECT.

in the polar bodies and also minm its centrosome, and plus
the sperm with its nucleus and centrosome.

On impregnation follows shortly the process of division already
brieflj' referred to, which is known as segmentation (Fig. 8).
This either affects the entire substance (holoNastic or complete

FIG. S. Various stages in the segmentation of the ovum. (From Gegeubaur's Comparative

Anatomy.)

segmentation) or only a part (meroblastic or incomplete seg-
mentation) of the oosperm. In the former case the ovum usually
contains little or no food-yolk, consisting exclusively, or nearly
so, of protoplasmic matter. The first stage in the process of
segmentation is the mitotic division of the segmentation-nucleus,
accompanied by the division into two parts of the substance
of the protoplasm the result being the formation of two cells,
each with its nucleus (Fig. 8). Each of these two cells then divides
-four cells being thus formed ; the four divide to form eight ;
the eight divide to form sixteen, and so on ; until, by the process
of division and subdivision, the oosperm becomes segmented into
a large number of comparatively small cells which are termed the
blastomeres. This mass of cells is spherical in shape, and the

ardv

ABC

FIG. 9. Gastrulation.
a'.'di. archenteron ; II. blastopore ; ecto. ectoderm ; endo, endoderin.

rounded blastomeres of which it is composed project on its sur-
face so as to give it somewhat the appearance of the fruit of
the mulberry, whence it is termed the mulberry body or morula
stage. The blastomeres next become arranged regularly in a
singl/3 layer the embryo (Fig. 9, A) assuming the form of a hollow

STRUCTURE AND PHYSIOLOC4Y OF ANIMALS

23

sphere, the Uasfosphere or llastula, with a wall composed of a
single layer of cells enclosing a cavity the segmentation cavity
or Uastoccele.

One side of the hollow blastula next becomes pushed inwards or
invaginated (Fig. 9, B, C\ as one might push in one side of a hollow
india-rubber ball, the result of this process of invagination, or
gastrulation as it is termed, being the formation of a cup the
gastrula (Fig. 10) with a double wall. The
cavity of the cup-shaped gastrula is the
archenteron or primitive digestive cavity ;
the opening is termed the blastopore, the
outer layer of the wall of the cup is the
ectoderm (or cpiUast), the inner the cndoderm
(or liypoUast). The ectoderm and endoderm
are the primary germinal layers of the em-
bryo ; from one or both of them are developed
the cells of a third layer the mesodcrm
(mcsoblast) which is subsequently formed
between them.

This mode of formation of the primary
germinal layers in holoblastic oosperms by a
process of gastrulation prevails in a number
of different sections of the animal kingdom.
In many animals, however, it becomes modi-
fied or disguised in various ways ; and in many meroblastic
oosperms it is doubtful if there occurs anything of the nature of
true gastrulation.

The cells of the three germinal layers give rise to the various
organs of the body of the fully -formed animal each layer having
a special part to play in the history of the development. As the
various parts of the embryo become gradually moulded from the
cells of the germinal layers, it becomes evident on comparison
that their internal structure the form and arrangement of their
constituent cells is undergoing gradual modifications, the nature
of which is different in the case of different parts. A differentia-
tion of the cells is going on in the developing organs, resulting in
the formation of a variety of different kinds of tissues.

FIG. 10. Gastrula in longi-
tudinal section ; a,
blastopore ; b, arch-
enteron ; c, endoderm;
d, ectoderm. (From
Gegenbaur's Compara-
tive Anatomy.)

4. TISSUES.

The cells of the tissues of the animal body differ greatly in
form in different cases. Some are rounded, others cubical, others
polygonal ; some are shaped like a pyramid, others like a cone,
others like a column or cylinder ; others are flattened and tabular
or scale-like. Cells situated on free surfaces are in many cases
beset at their free ends with delicate, hair-like structures or cilia
which vibrate to and fro incessantly during the life of the cell

24

ZOOLOGY

SECT.

(Fig. 11, a); sometimes there is on each cell a single, relatively long,
whip-like cilium, which is then termed a flagellum (/, g\ Cells

provided with cilia are
termed ciliated, such as
bear flagella flagellate
cells.

Some tissues are com-
posed entirely of cells.
Others, though originat-
ing from cells or by the
agency of cells, consist in
greater or less measure of
non-protoplasmic matter
formed between the cells.
Tissues composed en-
tirely of cells take the
form, for the most part,
of membranes covering
various surfaces, external
and internal. Such mem-
branes are known under
the general name of
epithelia (Fig. 11); they
may consist of a single
layer of cells (afi) or
may be many-layered
(i) ; the former are
termed non-stratified, the
latter stratified, epithelia.
The cells of an epithe-
lium may be flattened
(c, e\ their edges being
cemented together so as
to form a continuous
membrane ; or they may
be cubical or cylindrical
or prismatic (a, 1}) ; in
the case of a stratified
epithelium the cells may
be of different forms in
different strata (i). The
epidermis, which covers the outer surface of the body of an animal,
is an example of an epithelium ; sometimes it is stratified, some-
times unstratified ; its cells sometimes possess cilia, sometimes are
devoid of them. Lining the internal cavities of the body are
layers of cells, or epithelia, sometimes in a single layer, sometimes
in several layers, sometimes ciliated, sometimes non-ciliated.

FIG. 11 Various forms of epithelium, a, ciliated epi-
thelium ; b, columnar ; d, surface view of the sauie ;
c, tesselated ; e, the same from the surface ; /, flagel-
late epithelium with collars ; g, flagellate epithelium
without collars ; h, epithelium of intestine with
pseudopods ; i, stratified epithelium ; k, deric epi-
thelium of a marine planarian with pigment cells,
rod-cells, and sub-epithelial glands. (From Lang's
Comparative Anatomy.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

Glands (Fig. 12) are formed for. the most part by the modifica-
tion of certain cells of epithelia. In many cases a single cell of the
epithelium forms a gland, which is then termed a unicellular gland
(Fig. 12, A). The secretion (or substance which it is the function of
the gland to form or collect) gathers in such a case in the interior
of the cell, and reaches the surface of the epithelium through a
narrow prolongation of the cell which serves as the duct of the
gland (). In other cases the gland is multiccllular formed of a
number of cells of the epithelium lining a depression or infolding,
simple or complex in form, of the
latter (D-G). In the central
cavity of such a gland the secre-
tion collects to reach the general
surface or cavity lined by the
epithelium through the passage
or duct.

A series of tissues in which the
cells are, in most instances, sub-
ordinate, as regards bulk, to sub-
stances formed between them, is
the group known as the con-
nective tissues, including gela-
tinous connective tissue, retiform
connective tissue, fibrous connective
tissue, cartilage, and bone. In the
majority of forms of connective
tissue the cells lie embedded in
an intermediate substance called
the matrix or ground-substance
of the connective tissue.

In the case of gelatinous con-
nective tissue (Fig. 13) the ground-
substance (g) is of a gelatinous
character, sometimes supported
by systems of fibres (<?/), and the
cells are usually stellate or star-
shaped with radiating processes. Retiform or reticulate connective
tissue (Fig 14) consists of stellate or branching cells with pro-
cesses which are prolonged into fibres the fibres from neigh-
bouring cells joining so as to form a network. In this form of
connective tissue there is no true ground-substance the inter-
spaces between the cells being filled with other tissue elements.

Fibrous connective tissue, which is a very common form, has a
ground-substance containing gelatin, consisting mainly of numerous
fibres, usually arranged in bundles. Thicker yellow elastic fibres
may be present among the others, and may be so numerous as to
give the entire tissue an elastic character. Associated with fibrous

FIG. 12. Diagram to illustrate the structure
of glands. A, unicellular glands in an
epithelium ; B, unicellular glands lying
below epithelium and communicating
with the surface by narrow processes
(ducts) ; C, group of gland-cells ; D,
group of gland-cells lining a depression ;
E and F, simple multicellular gland ;
G, branched multicellular gland. (From
Lang.)

26

ZOOLOGY

SECT.

tissue, and produced by modification of its cells, is adipose or fatty
tissue (Fig. 15), which consists of masses of large cells in which the
protoplasm has more or less completely become replaced by fat,

b,

J

FIG. 13. Gelatinous connective tissue of a Jelly-fish ; e, epithelium ; g, gelatinous matrix
62, branching cells ; ej\ elastic fibres. (From Lang's Comparative Anatomy.)

the cells being bound together into groups and masses or lobules
by means of fibrous connective tissue.

In the case of cartilage, the matrix is of a firm but elastic

\rni7

"Fio. 14. Reticular connective tissue. (From Lang.)

character, sometimes quite homogeneous in appearance (hyaline
cartilage, Fig. 16), sometimes permeated by systems of fibres (fibro-
cartilaf/e, Fig. 17), which may be of an elastic nature (yellow elastic

STRUCTURE AND PHYSIOLOGY OF ANIMALS

27

cartilage}. The cells are usually rounded, and as a rule several
occur together in spaces scattered through the matrix ; sometimes
condensation of the matrix round each of the spaces in which the
cells are contained forms a cell-capsule. The outer surface is

Fia. 15. Fatty tissue ; F, fat-cells ; B, connective-tissue fibrils. (From Lang, after Eanvier.)

covered over by a fibrous membrane \hQpcrichondrium. Carti-
lage is frequently hardened by the deposition in the matrix of salts
of lime and is then known as calcified cartilage.

In lone or osseous connective tissue (Fig. 18) the matrix is exceed-
ingly dense and hard owing to its being strongly impregnated with
carbonate and phosphate of lime. It consists typically of numer-
ous thin lamellae, which are arranged partly parallel with the sur-
face, partly concentrically around certain canals the Haversian
canals (c) in which blood-vessels lie. The cells, or lone-corpuscles, lie

FIG. 10. Hyaline cartilage.

Fia. 1 7. Fibro-cart ilage.

in minute spaces the lacunce between the lamellae, and a system
of exceedingly fine channels the canaliculi extend from lacuna
to lacuna, containing fine protoplasmic processes by means of which
neighbouring cells are placed in communication with one another
The outer surface of the bone is covered by a vascular fibrous

28

ZOOLOGY

SECT.

membrane the periosteum which takes an active part in its
growth and nutrition.

The connective tissues are all more or less passive in the
functions which they perform, serving mainly for support and for
binding together the various organs. Muscular tissue, on the

other hand, has an active part to
play this being the tissue by
means of which, in general, all
the movements of the body of
an animal are brought about.
Muscular tissue varies greatly in
minute structure in different
groups of animals, and even in
different parts of the same ani-
mal. It consists of microscopic
fibres aggregated together into
large bundles or layers. These
fibres are composed of a sub-
stance the muscle-substan ce
which when living has the special
property of contractility, contract-
ing or becoming shorter and
thicker on the application of a
stimulus. There are two princi-
pal varieties of muscular tissue
to be distinguished, termed re-
spectively non-striated and striated
muscle. Each fibre of non-striated
muscle (Fig. 19) is usually a
single, greatly elongated cell,
sometimes brai::hed, with a single
nucleus ; it may contain a core
of unaltered protoplasm, or all
except the nucleus may be altered
into muscle-substance ; cross-
striation is absent. A fibre of
striated muscular tissue (Fig. 20)
is formed by the close union
of several cells which are repre-
sented by their nuclei (n). Some-
times there is a core of proto-
plasm ; but more usually the entire fibre is composed of muscle-
substance, with perhaps a remnant of protoplasm in the neigh-
bourhood of each nucleus. The substance of the fibre is crossed
by numerous transverse bands and striae, the precise significance
of which is a matter of controversy. The fibre is usually en-
closed in a delicate sheath the sarcolemma. Striated muscular

pplPSaJS r*lPw7 : T&

Fid. 18. Transverse section of compact
bone, a, lamellse concentric with the
outer surface ; b, lamellae concentric
with the surface of the marrow cavity ;
c, section of Haversian canals ; c', sec-
tion of a Haversian canal just dividing
into two ; rf, interstitial lamellae. (From
Huxley's Lessons in Physiology.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

29

tissue is specially characteristic of parts in which rapid movement
is necessary.

The principal elements of nervous tissue are nerve-cells and
nerve-fibres.

Nerve-cells (Fig. 21) vary greatly in form ; they are relatively

-71

FIG. 19. Non-striated muscle-cell ; /, substance of fibre ; n, nucleus ; p, unaltered protoplasm in
the neighbourhood of the nucleus. (From Huxley's Lessons in Physiology.)

large cells with large nuclei and one or several processes, one of
which is always continuous with a nerve fibre.

The nerve-fibres (Fig. 22), which are to be looked upon as greatly
produced processes of nerve-cells, are arranged for the most part
in strands which are termed nerves. The fibres themselves vary
greatly in structure in different classes of animals. In the higher
animals the most characteristic form of nerve-fibre is that which is
termed the medullated nerve-fibre. In this there is a central
cylinder the axis-cylinder or neuraxis (A, ax) which is the

A

B

-rz

IB*/

' 6

*l /

Fio. 20. Striated muscle. A, part of a muscular fibre of a Frog; B, portion of striated muscle
teased out to show separation into fibrillfe. (From Huxley's Lessons in Physiology.)
b, d, g, transverse bands and striae ; n, nuclei.

essential part of the fibre and is made up of numerous extremely
fine primitive fibrillcc ; this is surrounded by a layer of a white
glistening material the white substance of Schivann or medullary
sheath (med), enclosed in turn in a very delicate membrane the
neurilemma (ncur^).

The blood, the lymph, and other similar fluids in the body of an
animal may be looked upon as liquid tissues, having certain cells

30

ZOOLOGY

SECT.

-the corpuscles disseminated through a liquid plasma, which
takes the place of the ground-substance of the connective tissues.

ax
\

med

, ,

/

tr

/ B

'

i.

e

FIG. 21. Nerve-cells. A, multipolur ;
B, bipolar.

FIG. 22. Nerve-fibres. A, mcdullated ;
B, iion-medulated ; ax, neuraxis ;
med, medullary sheath; ncur,
neurilemnia.

In a large proportion of cases such corpuscles are similar to
Amoebae in their form and movements (amoeboid corpuscles, leuco-
cytes). In the blood of Vertebrates leucocytes occur along with
coloured corpuscles of definite shape containing the red-colouring
matter (Ju&rnoglMn) of the blood. The leucocytes are able, like
Amoebae, to ingest solid particles, and under certain conditions a

number of them may unite to-
gether to form a single mass of
protoplasm with many nuclei,
termed a plasmodium.

The characteristic cells of the
reproductive tissues are the ova
and the spermatozoa or sperms. The
ova (Fig. 6), when fully formed, are
relatively large, usually spherical
cells, sometimes composed entirely
of protoplasm, but usually with an
addition of nutrient food-yolk. Each

OVUm > aS Already mentioned, 611-

closes a large nucleus (germinal
vesicle) and in the interior of that
one or more nucleoli or germinal

S p fa ^l ie S p erms (Fig. 23) ai'6

extremely -minute bodies, nearly

always motile, usually slender and whip-like, tapering towards
one extremity, and commonly with a rounded head at the other.

FIQ. 23. Various forms of spermatozoa,
a, of a Mammal ; b, of a Turbellariaii
worm ; c, and d, and e, of Nematudo
worms ; /, of a Crustacean ; g, of a
Salamander ; h, the commonest form
with oval head and lon# flagellum.
(From Lang's Comparative Anatomy.)

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 31

The sperms are developed by a succession of cell-divisions from
certain cells the primitive male cells similar in character to
immature ova.

5. ORGANS.

The chief systems of organs of an animal are the integumen-
tary, the skeletal, the muscular, the alimentary or digestive, the
vascular, the respiratory, the nervous, the excretory, and the repro-
ductive.

The skin or integument consists in the majority of animals
of a cellular membrane the epidermis to which reference has
already been made, with, superficial to it, in many animals, a non-
cellular layer the cuticle, and below it usually a fibrous layer which
is known as the dermis. The epidermis may consist of a single
layer or may be stratified ; it is frequently ciliated, and some of
its cells frequently assume the form of unicellular glands. Modi-
fication of its superficial layers of cells gives rise frequently to the
formation of hard structures contributing to the development of
an cxoskeleton (vide infra).

The cuticle, when present, varies greatly in thickness and con-
sistency. Sometimes it is very thin and delicate ; in many
animals it becomes greatly thickened and hardened so as to form
a strong protecting crust, sometimes of a material termed chitin,
somewhat akin to horn in consistency, sometimes solidified by the
deposition of calcareous salts. The cuticle is to be looked upon as
a secretion from the cells of the epidermis; but the term is
frequently applied in the case of the higher animals in which a
cuticle in the strict sense of the term is absent either to a super-
ficial part of the epidermis, in which the cells have become altered
and horny, or to the whole of that layer.

The layer or layers of the integument situated beneath the
epiderm consist of fibrous connective tissue and muscular fibres,
constituting, as mentioned above, the derm or dermis.

The term skeleton or skeletal system is applied to a system
of hard parts, external or internal, which serves for the protection
and support of softer organs and often for the attachment of muscles.
This system of hard parts may be external, enclosing the soft
parts, or it may lie deep within the latter, covered by integument
and muscles : in the former case it is termed an cxoskelcton or
external skeleton ; in the latter an endoskeleton or internal skeleton.
In many groups of animals both systems are developed. An
exoskcleton is formed by the thickening and hardening of a part
or the whole of one of the layers of the integument enumerated
above ; or more than one of these layers may take part in its
formation. In many invertebrate animals, such as Insects.
Crustaceans, and Molluscs, it is a greatly thickened and hardened

32 ZOOLOGY SECT.

cuticle which forms the exoskeleton. The horny scales of Reptiles,
the feathers of Birds, and the fur of Mammals are examples of
an exoskeleton derived from the epidermis, while the bony
shell of Turtles and the bony scales of Fishes are examples of a
dermal exoskeleton.

When an endoskeleton is present, it usually consists either of
cartilage or bone or of both ; but sometimes it is composed of
numerous minute bodies (spicules) of carbonate of lime or of a
siliceous material.

A skeleton, whether internal or external, is usually composed
of a number of pieces which are movably articulated together,
and which thus constitute a system of jointed Severs on which the
muscles act.

The alimentary or digestive system consists of a cavity or
system of cavities into which the food is received, in which it is
digested, and through the wall of which the nutrient matters are
absorbed ; together with certain glands.

In the lowest groups in which a distinct alimentary or enteric cavity
is present it is not distinct from the general cavity of the body ;
but in all higher forms there is an enteric canal which is sus-
pended within the cavity of the body, and the lumen of which is
completely shut off from the latter. It may have simply the form of
a sac or bag with a single opening which serves both as mouth and
anus ; in other cases the sac becomes branched and may take the
form of a system of branching canals. In most animals, however,
the alimentary canal has the form of a longer or shorter tube
beginning at the mouth and ending at the anal opening (Fig. 24).
In most cases there are organs in the neighbourhood of the mouth
serving for the seizure of food ; these may be simply tentacles or
soft finger-like appendages, or they may have the form of jaws, by
means of which the food is not only seized, but torn to pieces or
pounded up to small fragments in the process of mastication. The
alimentary canal itself is usually divided into a number of regions
which differ both in structure and in function.

In general there may be said to be three regions in the ali-
mentary canal the ingestive, the digestive and absorbent, and the
egestive or efferent. The ingestive region is the part following
behind the mouth, by which the food reaches the digestive and
absorbent region. But, besides serving as a passage, it may also
act as a region in which the food undergoes certain processes,
chiefly mechanical, which prepare it for digestion. This ingestive
region may comprise a mouth-cavity or l>uccal cavity, a pharynx,
an cesopkagus or gullet, with sometimes a muscular gizzard which
may be provided with a system of teeth for the further breaking
up of the food, and sometimes a crop or food -pouch.

The digestive and absorbent region is the part in which the
chemical processes of digestion go on, and from which takes place

STRUCTURE AND PHYSIOLOGY OF ANIMALS

33

the absorption of the digested food-substances. Into this part are
poured the secretions of the various digestive glands, which act on
the different ingredients of the food so as to render them more
soluble. Through the lining membrane of this part the digested
nutrient matter passes, to enter the blood-system. This region
may present a number of subdivisions; nearly always there are
at least two a wide sac, the stomachy and a narrow tube, the
intestine.

The egestive or efferent region of the alimentary canal is the
posterior part of the intestine, in which digestion and absorption
do not go on, or only go on to a limited extent, and which serves

FIG. 24. General view of the viscera of a male FrogTj from the right side, a, stomach ; I, urinary
bladder ; c, small intestine ; ct, cloacal aperture ; d. large intestine ; e, liver ; /, bile-duct ;
(j, gall-bladder ; //, spleen ; i, lung ; Ic, larynx ; f, fat-body ; in, testis ; n, ureter ; o, kidney ;
p, pancreas ; s, cerebral hemisphere ; sp, spinal cord ; t, tongue ; u, auricle ; ur, urostyle ;
r, ventricle ; vs, vesicula seminalis ; w, optic lobe ; x, cerebellum ; y, Eustachian recess ;
z, nasal sac. (From Marshall.)

mainly for the passage to the anal opening of the faeces or
unabsorbed effete matters of the food.

The whole of the interior of the alimentary canal is lined
by a layer of cells the alimentary or enteric epithelium. The
form and arrangement of the cells of this epithelium vary greatly
in different groups of animals. Usually, they are vertically
elongated, prismatic or columnar, or pyramidal in shape ;
frequently they are ciliated. In some lower forms, the cells lining
the alimentary cavity have the power, like Amoeba, of thrusting
forth processes of their protoplasm (Fig. 11, Ji), and of taking minute
particles of food into their interior to become digested and absorbed
(intracellidar digestion). Sometimes they are all more or less
active in secreting a fluid destined to act on the food and render
it more soluble ; sometimes this function is confined to certain of
the cells, which have a special form ; very often the secreting cells

VOL. I

D

34 ZOOLOGY

line special little pouch-like, simple or branched glands, opening
by a passage or dud into the main cavity of the alimentary
canal. Besides these glands formed from specially modified cells
of the enteric epithelium there are nearly always present certain
large special glands, separate from the alimentary canal itself, but
opening into it by means of ducts. Of these the most generally-
occurring are the glands termed salivary glands, liver, and pancreas.
The salivary glands have the function of secreting a fluid called
the saliva, which, in many cases at least, has a special action on
starchy matters, converting them into sugar. The ducts of these
glands open always, not into the digestive, but into some part of
the ingestive region of the alimentary system.

The most important function of the liver properly so called
is one distinct from the process of digestion ; its secretion the
l)ile has, however, at least a mechanical effect on this process,
and assists the secretion of the pancreas in its effects upon fat.
In lower forms the organ to which the term liver is commonly
applied appears in many cases to combine the functions of a true
liver with that of a pancreas, and is thus more appropriately
termed hepato-pancreas or liver -pancreas.

The pancreas secretes a fluid the pancreatic juice which has
a very important effect in digestion. It renders substances of the
nature of albumins soluble by converting them into modifications
termed peptones ; it converts starch into the soluble substance
sugar ; it acts on fatty matters in such a way as to convert them
into emulsions which are capable of being taken up and absorbed,
and it effects the splitting up of part of the fat into fatty acids
and glycerine.

When the food has been acted on by the various digestive
secretions, the soluble part of it is fitted to be taken up. and
absorbed through the wall of the alimentary canal into the blood
(in animals in which a blood-system exists), or into the fluid
which takes its place. In the higher animals a part of the
soluble matter of the food passes directly into the blood contained
in the blood-vessels ; while another part is taken up by a set of
special vessels, the lacteals which are a part of the lymphatic
system, and reaches the blood indirectly.

In some of the lower groups of animals there is no system of
blood-vessels, and the nutrient matter of the food, absorbed
through the alimentary canal, merely passes from cell to cell
throughout the body, or is received into a space or series of spaces
containing fluid intervening between the alimentary canal and the
wall of the body. But in the majority of animals there is a system
of branching tubes containing a special fluid the blood, and it is
into this that the nutrient matter absorbed from the food sooner
or later finds its way. The blood has for one of its principal
functions the conveyance .of the nutrient matters from the

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 35

alimentary canal throughout the body, so that the various organs
may select from it the material which they require for the carrying
on of their functions. To carry out this office the blood is con-
tained in a complicated system of branching tubes or Mcod-vessels .

The essence of the process of respiration, as we have already seen,
is an interchange of oxygen and carbonic acid which takes place
between the tissues of an organism and the surrounding medium,
whether air or water. During the vital changes which go on in
the bodies of all animals, as in Amoeba, oxygen is constantly
being used up and carbonic acid being formed. The necessary
supply of oxygen has to be got from the air, or, in the case of
aquatic animals, from the air dissolved in the surrounding water.
At the same time the carbonic acid has to be got rid of. In the
lowest animals as for instance Amoeba, and. many of higher
organisation the oxygen passes inwards and the carbonic acid
outwards through the general surface of the body. But in the
great majority of animals there is a special set of organs the
organs of respiration having this particular function. In some
animals these organs of respiration are processes, simple or
branched, lined by a very delicate membrane, and richly supplied
with blood-vessels. Such processes are called gills or branchice ;
they are specially adapted for the absorption of oxygen dissolved
in water.

In other animals the oxygen is obtained directly from the air ;
and in such air-breathing forms the organ of respiration is very
often a sac, either simple or compound, termed a lung. The
interior of this sac is lined with an epithelium of extreme delicacy,
immediately outside of which is a network of microscopic blood-
vessels or capillaries with thin walls ; and the oxygen readily passes
from the air in the cavity of the lung through its lining and
the thin wall of the blood-vessel into the blood. In other air-
breathing forms the organs of respiration are trachea:, which are
ramifying tubes, by means of which the air is conveyed to all parts
of the body. In such forms, of which the Insects are examples, the
air is conveyed, by means of these tubes, from openings on the
surface of the body to all parts, and respiration goes on in all the
organs.

In order that the air or water in contact with the surface of the
lungs or gills may be renewed, there are usually special mechanical
arrangements. In many gill-bearing animals the gills are attached
to the legs, and are thus moved about when the animal moves its
limbs. In others certain of the limbs are constantly moving in
such a way as to cause a current of water to flow over the gills.
In air-breathing forms there is usually a pumping apparatus, by
means of which the air is alternately drawn into and expelled
from the lungs.

In a great number of animals there is in the blood a substance

D2

36 ZOOLOGY SECT.

called haemoglobin, which has a strong affinity for oxygen ; and the
oxygen from the air, when it enters the blood, enters into a state
of loose chemical combination with it. In this state, or simply
dissolved in the fluid plasma of the blood, the oxygen is conveyed
throughout the body.

Thus the blood, besides receiving the solid and liquid food from
the alimentary canal and carrying it throughout the body for
distribution, receives also the oxygen or gaseous food, and supplies
it to the parts requiring it. In all parts of the body in which
vital action is taking place chemical changes are constantly going
on. These chemical changes in the tissues, having for their result
the production of heat, motion, secretion, and nerve-action, are
for the most part of the nature of oxidations, and involve a constant
consumption of oxygen; while a product which becomes formed
as a result of this action is carbonic acid gas.

To carry out all the functions which it has to perform as a
distributor of nourishment and oxygen and a remover of carbonic
acid, the blood has to be moved about through the vessels to
circulate throughout the various organs. In the lowest forms in
which a definite blood-system is to be recognised, this movement
is effected in great measure by the general movements of the
body of the animal. In others certain of the vessels contract and
drive the blood through the system ; such contractions are of a
peristaltic character, the contractions being of the nature of con-
strictions running in a definite direction along the course of the
vessel, with an effect similar to that produced by drawing the
hand along a compressible india-rubber tube.

In all higher forms the movement of the blood is effected by
means of a special organ the heart. The heart is a muscular
organ which by its contractions forces the blood through the
system of vessels. In its simplest form it usually consists of two
chambers, both with muscular walls, the one, called the auricle,
receiving the blood and driving it into the other, which is called
the ventricle. The latter, in turn, when it contracts, drives the blood
through the vessels to the various parts of the body the return
of the blood backwards to the auricle from the ventricle being
prevented by the presence of certain valves, which act like folding
doors opening from the auricle towards the ventricle, but closing
when pressure is exerted in the opposite direction. In the higher
animals the heart becomes a more complex organ than this, with a
larger number of chambers and a more elaborate system of valves.
Carbonic acid, as already mentioned, is a waste-product con-
stantly being produced in the tissues and being carried off by the
blood to pass out by the gills or lungs. Besides the carbonic
acid, there are constantly being formed waste-substances of another
class viz., substances containing nitrogen, of which uric acid and
urea are the principal ultimate forms. These are separated from

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 37

the blood and thrown out of the body by a distinct set of organs
called renal organs, or organs of urinary excretion. The

form of these organs varies greatly in the different groups;
in many cases they are more or less intimately connected with
the genital system.

In place of the simple contractions and extensions of the proto-
plasm which constitute the only movements of Amoeba, the higher
animals are capable of complex and definite movements. These
are brought about by the agency of a set of organs termed the
muscles. A muscle is a band or sheet of muscular fibres
endowed in the living state with the property of contractility, by
virtue of which, when stimulated in certain ways, it contracts in
the direction of its length, becoming shortened, and, at the same
time, thickened (Fig. 25). The extremities of the muscle are

FIG. 25 . Bones of the human arm and fore-arm with the biceps muscle, showing the shortening
and thickening of the muscle during extraction and the consequent change in the relative
position of the bones viz., flexion of the fore-arm on the upper arm. (From Huxley's Physiology.)

frequently composed, not of contractile muscular fibres, but of a
form of strong fibrous connective tissue the tendon of the muscle.
The ends of the muscle are usually firmly attached to two different
parts of the jointed framework or skeleton, external or internal;
and, when the muscle contracts and becomes shortened, these two
parts are drawn nearer to one another.

In all but the most lowly-organised animals there is a system
of organs the nervous system by means of which a communi-
cation is effected between the various parts of the body, enabling
them to work in harmony, and by means of which also a communi-
cation is established between the organism and the external
world. The two essential elements of the nervous system the
nerve-cells and nerve-fibres have a regular arrangement which
varies in the different animal types both as regards structural
details and the relations borne to the other systems of organs ;
but there are to be recognised two chief parts or sets of parts-
the central and the peripheral.

38

ZOOLOGY

SECT.

vo

c

The central parts of the nervous system consist (Fig. 26) of
certain aggregations of nerve-matter known as nerve-ganglia,
containing a large number of nerve -eel Is ; a relatively large mass

of this matter may be

/

collected together to
form a "brain. To or
from these central parts
pass all the systems of
nerve-fibres, constituting
the peripheral part of the
system ; the former have
the office both of re-
ceiving impressions con-
veyed by the nerve-fibres
from the surface, from
the organs of special
sense, and from the in-
ternal organs, and of
sending off messages
through similar channels
to the various parts of
the body to muscles, to
glands, to alimentary
canal, and to vascular
system. When a move-
ment is to be effected
a message passes from
the nerve-centre along a
nerve-fibre to a muscle
and causes it to contract ;
when an organ requires
the amount of blood sup-
plied to it to be in-
creased or diminished a
message is conveyed
along a nerve-fibre and
causes the dilatation or
contraction of the blood-
vessels of the part ; and
a similar initiatory or
controlling influence is
exerted over the activities
of all the organs.

In certain groups of animals all the impressions from the
external world are received through the integument of the general
surface, and this is the case in all animals with the general
impressions of touch and of heat and cold. The sensitiveness of

sc.

FIG. 26. Nervous system of the Frog.
Howes's At lax.)

(From

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 39

the integument to such general impressions may be increased by
the presence in it of a variety of tactile papillae or corpuscles
having nerve-fibres terminating in them. In most animals, how-
ever, there are certain organs, the organs of special sense,
adapted to receiving impressions of special kinds eyes for the
reception of the impressions produced by light, ears for the recep-
tion of those produced by the waves of sound, olfactory organs or
organs of smell, and gustatory organs or organs of taste. The most
rudimentary form of eye is little more than a dot of pigment
which absorbs some of the rays of bright light these producing
a nerve-disturbance in certain neighbouring nerve-cells. To this
may be added clear, highly-refracting bodies which intensify the
effect. In the higher types of eye there are the same character-
istic parts the clear, highly-refracting substance, the pigment, and
the nerve-cells ; but each has undergone a development resulting
in the construction of an organ adapted to the reception of light-
impressions of a very definite character. The highly-refracting body
assumes the form of a lens for the focussing of the light-rays ; the
nerve-cells are arranged within a regular layer, the retina, from
which nerve-fibres pass to the central part of the nervous system ;
the pigment is so arranged as to absorb the light-rays and prevent
their passage beyond the retina, and in certain cases also lines a
diaphragm, the iris, with a central aperture through which the
rays of light are admitted to the central parts of the eye. In
some animals (Insects, Crustacea) the eye consists of a very large
number of independent elements, each with its refracting apparatus,
its nervous element, and its absorbing pigment.

The ear in its simplest form is a membranous sac or otocyst with
internally projecting stiff cilia, and containing a liquid in which
there lie a number of particles of carbonate of lime. The sound-
waves evidently set in vibration the liquid and its contained cal-
careous particles, and by means of these vibrations acting on the
cilia, an impression of a definite character is produced in the cells
of a neighbouring nerve-ganglion. In higher forms the apparatus
for receiving the vibrations becomes extremely complex, and there
is elaborated a nervous mechanism by which sounds of different
pitch and intensity produce impressions of a distinct character.
The organ of hearing usually possesses the additional function
of an organ ministering to the sense of rotation, and thus has an
important part to play in the maintenance of the equilibrium of
the body.

The essential elements of the reproductive organs the ova
and spermatozoa have already been briefly alluded to (p. 30).
The ova are developed in an organ termed the ovary, and the
sperms in an organ called the spermary or testis. Sometimes
ovaries and testes are developed in the same individual, when the
arrangement is termed monoecious or hermaphrodite ; sometimes

40 ZOOLOGY SECT.

the ovaries occur in one set of individuals the females and the
testes in another set the males, when the term unisexual or
dicecious is employed. Very frequently the male differs from the
female in other respects besides the nature of the reproductive
elements in size, colour, and the like ; when such differences are
strongly marked the animal is said to be sexually dimorphic. The
ova and sperms are usually conveyed to the exterior by canals
or ducts the ovarian ducts or oviducts, and the testicular ducts,
spcrmiducts, or vasa defer enlia. In some instances the ova are
impregnated after being discharged from the oviducts, and the
development of the young takes place externally; in other cases
the impregnation takes place in the oviduct, and the young
become fully developed in the interior of a special enlargement
of the oviduct termed the uterus. In the former case the animal
is said to be oviparous, in the latter viviparous ; but there are
numerous intermediate gradations between these two extremes.

6. THE REPRODUCTION OF ANIMALS.

In a limited number of groups of animals reproduction takes
place by means of cells corresponding to ova developed in organs
similar to ovaries, but without impregnation by means of sperms.
This phenomenon is known as parthenogenesis (cf. p. 21).

Besides the sexual process of reproduction by means of ova and
spermatozoa, there are in many classes of animals various asexual
modes of multiplication. One of these the process of simple
fission has been already not iced in connection with the reproduction
of Amoeba. The formation of spores is an asexual mode of multi-
plication which occurs only in the Protozoa, and will be described
in the account of that group. Multiplication by budding takes
place in a number of different classes of animals. In this form of
reproduction a process or bud (Fig. 27, ltd) is given off from some
part of the parent animal ; this bud sooner or later assumes the
form of the complete animal, and may become detached from
the parent either before or after its development has been
completed or may remain in permanent vital connection with the
parent form.

When the buds, after becoming fully developed, remain in vital
continuity with the parent, a sort of compound animal, consisting
of a greater or smaller number of connected units, is the result.
Such a compound organism is termed a colony, and the component
units are termed zooiil*. In some cases such a colony is produced
by a process which is more correctly termed incomplete fission
than budding.

Alternation of generations ; heterogamy ; paedogenesis.-
In the life-history of a considerable number of animals, a stage in
which reproduction takes place by a process of budding or fission

STRUCTURE AND PHYSIOLOGY OF ANIMALS

41

alternates with a stage in which there occurs a true sexual mode
of reproduction. Such a phenomenon is termed alternation of
generations or metagenesis. The term heterogamy is applied to
cases in which two different sexual generations usually a true
sexual and a parthenogenetic alternate with one another.
Pcedogenesis, or the development of young by a sexual process from

FIG. 27. Fresh-water polype (Hydra), two specimens, the one expanded, the other contracted,
showing multiplication by budding. W. 1 6t/.- W.3 buds in various stages of growth. (From
Parker's Biology.)

individuals that have not attained the adult condition, is a
phenomenon which is to be observed in some groups of animals.

7. SYMMETRY.

The general disposition or symmetry of the parts in an animal
presents two main modifications the radial and the bilateral.
The gastrula (p. 23) is the simplest and most generalised form among
multicellular animals or Metazoa ; but no adult animal retains
this simple shape. In the gastrula we may imagine a central
primary axis (Fig. 28, All) passing through the middle of the blas-
topore and of the archenteric cavity, and a series of secondary axes
(ab, cd,) running at right angles to this to the outer surface. In a
symmetrical gastrula the secondary axes would be all equal. Many

42

ZOOLOGY

SECT,

animals are in the adult condition similar in their symmetry to the
gastrula, except that there are special developments along a series
of regularly arranged radiating secondary axes ; these radial
developments may be in the form of tentacles or radially arranged
processes (Fig. 29), or may assume the character of a radial arrange-
ment of internal parts. Such an animal is said to be radially
symmetrical. The body of a radially symmetrical animal is capable

FIG. 28. Diagram of the axes of the body.
AB, primary axis ; ab, cd, secondary
axes. The lower figure is a transverse
section of the upper one showing its two
secondary axes. (From Gegenbaur.)

Fio. 20. Haciial symmetry. Letters as
in Fig. L'S. The processes at A are
the tentacles ; the lower figure repre-
sents the upper or oral surface. (From
Gegenbaur.)

of being divided into a series of equal radial parts or antimeres,
each of which is symmetrically disposed with regard to one of the
secondary or radial axes.

In animals which are not permanently fixed, locomotion usually
takes place in the direction of the primary axis of the body, and
one side, habitually directed downwards, becomes modified differ-
ently from the other which is habitually directed upwards : lower
or ventral surface becomes distinguishable from an upper or dorsal.
Thus the radial symmetry is now disturbed ; the secondary axes
have become unequal ; the dorso-ventral or vertical secondary axes

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 43

are, to a greater or less extent, different from the transverse or
horizontal secondary axes, and the body of an animal having such
a disposition of the parts is divisible into two equal lateral halves
or hemisomes by a median vertical plane passing through the
primary axis. This is the bilateral symmetry observable in all but
a few types of animals.

Sometimes the bilaterally symmetrical animal is unsegmented ;
sometimes it is divided into a series of segments or metamcrcs.
A distinct head may be present or absent. The head end or
anterior end is that which, save in exceptional cases, is directed
forwards in locomotion. It is towards this end that the organs of
special sense are situated, as well as the opening of the mouth and
the organs for the prehension and mastication of food. A head is
developed when the anterior part bearing these structures is
marked off externally from the rest. In segmented animals the
head consists of a number of segments amalgamated together, and
it contains the brain or the principal central ganglia of the nervous
system.

8. THE PRIMARY SUBDIVISIONS on PHYLA OF THE ANIMAL

KINGDOM.

The various systems of organ) digestive, circulatory, nervous,
excretory, etc. present under one form or another in all the higher
groups of animals, are variously arranged and occupy various
relative positions in different cases, producing a number of widely
different plans of animal structure. According as their structure
conforms to one or another of these great plans, animals are referred
to one or another of the corresponding great divisions or phyla of
the animal kingdom. That animals do present widely differing
plans of structure is a matter of common knowledge. We have
only to compare the true Fish, such as Cod, Haddock, etc., in a fish-
monger's shop with the Lobsters and the Oysters, to recognise the
general nature of such a distinction. The first-named are charac-
terised by the possession of a backbone and skull, with a brain and
spinal cord, and of two pairs of limbs (the paired fins) ; they belong
to the great vertebrate or backboned group the division Vertc-
Irata, of the. phylum Chordata. The Lobsters, on the other hand, in
which these special vertebrate structures are absent, possess a body
which is enclosed in a hard jointed case, and a number of pairs of
limbs also enclosed in hard jointed cases and adapted to different
purposes in different parts of the body some being feelers, others
jaws, others legs : their general type of structure is that which
characterises the phylum Arthropoda. The Oysters, again, with
their hard calcareous shell secreted by a pair of special folds
of the skin constituting what is termed the mantle, and with a
special arrangement of the nervous system and other organs which

44 ZOOLOGY SECT, i

need not be described here, are referable to the phylum Mollusca.
Other familiar animals are readily to be recognised as belonging to
one or other of these great phyla. A Prawn, a Crab, a Blue-bottle
Fly, a Spider, are all on the same general plan as the Lobster : they
are jointed animals with jointed limbs, and have the internal
organs occupying similar positions with relation to one another :
they are all members of the phylum Arthropoda. Again, a Mussel,
a Snail, and a Squid are all to be set side by side with the Oyster
as conforming to the same general type of structure : they are all
members of the phylum Mollusca. A Dog, a Lizard, and a Fowl,
again, are obviously nearer the Fish : they all have a skull and
backbone, brain and spinal cord, and two pairs of limbs, and are
members of the great group Chordata.

Altogether twelve phyla are to be recognised, viz. :-

I. Protozoa VII. Molluscoida

II. Porifera VIII. Echinodermata

III. Ccehnterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Nemathelminthes XL Mollusca

VI. Trochelminthes XII. Chordata

But these do not comprise all known animals. There are a
number of smaller groups which are only very doubtfully to be
associated with one or other of the phyla ; and it is in some cases
chiefly to avoid multiplication of the latter that such groups are
not treated as independent. Such forms, until their places are
more definitely fixed, are best dealt with as appendices to the
phyla to which they appear most nearly related.

SECTION II
PHYLUM PROTOZOA

IN the preceding section we learnt the essential structure of an
animal cell, and it was pointed out that in the lowest organisms
the entire individual consists of a single cell. All such unicellular
animals are placed in the lowest primary subdivision of the animal
kingdom the phylum Protozoa.

We have also learnt that cells vary considerably in character.
They may be amoeboid or capable of protruding temporary processes
of protoplasm called pseudopods ; flagellate, or produced into one
or more always a small number of threads having an intermit-
tent lashing movement ; ciliated, or produced into numerous
rhythmically moving threads of protoplasm ; or encysted, the proto-
plasm being enclosed in a cell- wall. Moreover, under certain
circumstances, amoeboid cells may fuse with one another to form
a plasmodium.

These well-marked phases in the life of the cell allow us to
divide the Protozoa into subdivisions called Classes. The same
organism may be amoeboid, flagellate, encysted, and plasmodial
at various stages of its existence, but nevertheless we find
certain forms in which the dominant phase in the life-history is
amoeboid, others which are characteristically flagellate or ciliated,
others again in which the tendency to form plasmodia is a
distinctive feature. In this way five well-marked groups of
unicellular organisms may be distinguished.

Class 1. RHIZOPODA. Protozoa in which the amoeboid form is
predominant, the animal always forming pseudopods. Flagella
are often present in the young, and occasionally in the adult.
Encystation frequently occurs.

Class 2. MYCETOZO A.- -Terrestrial Protozoa in which the plas-
modial phase is specially characteristic, as also is the formation
of large and often complex cysts.

Class 3. MASTIGOPHOUA. Protozoa in which the flagellate form

46 ZOOLOGY SECT.

is predominant, although the amoeboid and encysted conditions
frequently occur.

Class 4. SPOROZOA. Parasitic Protozoa without special loco-
motive parts in the adult. Encystation is almost universal, and
the young may be flagellate or amoeboid.

Class 5. INFUSORIA. Protozoa which are always ciliated, either
throughout life or in the young condition.

CLASS I. RHIZOPODA.

1. EXAMPLE OF THE CLASS Amoeba proteus.

Amoaba has been fully described in the preceding chapter ; it
will therefore be unnecessary to do more than recapitulate the
most essential features in its organisation.

It is an irregular mass of protoplasm (Fig. 30, E) about J mm.
in diameter, produced into irregular processes or pseudopods (psd)
of variable size and form and capable of being protruded and
retracted, often with considerable rapidity. The protoplasm is
divisible into a granular internal substance or endosarc and a clear
outer layer or ectosarc ; the difference between the two is hardly a
structural one, but depends simply on the accumulation of granules
in the central portion. The granules are, for the most part, various
products of metabolism proteinaceous or fatty.

Imbedded in the endosarc is a large nucleus (nu), of spherical form,
consisting of a clear achromatic substance, enclosed in a membrane,
and containing minute granules of chromatin. The contractile
vacuole (c. vac.),a, very characteristic structure of the Protozoa, lies
in the outer layer of the endosarc, and exhibits rhythmical move-
ments, contracting and expanding at more or less regular intervals.

Amoeba feeds by ingesting minute organisms (Fig. 30, c,/. vac.)
or fragments of organisms i.e., by enveloping them in its substance,
retaining them until the proteids they contain are dissolved and
assimilated, and then crawling away and leaving the undigested
remnants behind.

Amoeba are sometimes found to undergo encystation ; the
pseudopods are withdrawn and the protoplasm surrounds itself
with a cell- wall or cyst (D, cy\ from which, after a period of rest,
it emerges and resumes active life. The cyst is formed of a
ckitinoid material i.e., a nitrogenous substance allied in composi-
tion to horn and to the chitin of which the armour of Insects,
Crayfishes, etc., is composed.

Reproduction takes place by simple or Unary fission ; direct or
amitotic division of the nucleus is followed by division into two of
the cell-body (i). Occasionally two Amoebae have been observed to

II

PHYLUM PROTOZOA

conjugate or undergo complete fusion, but nothing is known of the
result of this process or of its precise significance in this particular

case.

A

FIG. 30. Amoeba. A, A. quarta; B, the same killed and stained ; C, A. protcus ; D, encysted
specimen ; E, A. proteus ; F, nucleus of same, stained ; G, A. verrucosa ; H, nucleus of same,
stained ; I, A. proteus, undergoing binary fission ; a, point of union of enclosing pseudopods ;
c. vac. contractile vacaole ; cy. cyst ; /. vac. food-vacuole ; nu. nucleus (numerous in
A. quarta) ; psd. pseudopod. (From Parker's Biology, after Leidy, Gruber, and Howes.)

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Rhizopoda differ among themselves in the character of
their pseudopods, which may be short and blunt or long and

48 ZOOLOGY SECT.

delicate ; in the number of nuclei ; and in the presence or absence
of a hard shell within or around the protoplasm. The following
four orders may be distinguished :

ORDER 1. LOBOSA.
Rhizopoda with short, blunt pseudopods.

ORDER 2. FORAMIMFERA.

Shelled Rhizopoda with fine, branched, and anastomosing
pseudopods.

ORDER 3. HELIOZOA.
Rhizopoda with fine,; stiff, radiating pseudopods.

ORDER 4. RADIOLARIA.

Rhizopoda having a shell in the form of a perforated central
capsule, and usually, in addition, a siliceous skeleton : the pseudo-
pods are long and delicate.

Systematic Position of the Example.

Amoeba proteus is one of many species of the genus Amaibcb,
belonging to the family Amcebidce, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell, among the Amcebidse. The genus
Amoeba is distinguished by the presence of one or more nuclei,
and of a contractile vacuole. In A. proteus the pseudopods are
of considerable length and sometimes branched, and there is a
single nucleus, having its chromatin in the form of scattered
granules.

ORDER 1. LOBOSA.

General Structure.- -The members of this group all agree
with Amoeba in essential respects, their most characteristic feature
being the short, blunt pseudopods. The chief variations in struc-
ture upon which the genera and species are founded have to do
with the number and character of the nuclei, the form of the
pseudopods, and the presence or absence of a shell.

In Amoeba itself there may be one (Fig. 30, E) or several (B)
nuclei, the chromatin of the nucleus may be arranged in various
ways (F, H), and the pseudopods may be prolongations of con-

II

PHYLUM PROTOZOA

49

siderable relative size (c), or mere wave-like elevations of the
surface (G). Sometimes specimens are found in which neither
nucleus nor vacuole is present ; these are placed in the genus

FIG. 31. Protamceba primitiva. Showing changes of form and three stages in binary fission.

(After Haeckel, from Parker's Biology.)

Protamosba (Fig. 31). Very probably, however, future investigation
will show this and other non-nucleate forms to possess a potential
nucleus in the form of minute scattered granules of chromatin.

The largest of the naked or shell-less Lobosa is Pelomyxa, which
may be as much as 8 mm. in diameter ; it is multi-nucleate and is

,

further distinguished by the presence of numerous non-contractile
vacuoles in the endosarc.

D

FIG. 32. A, Quadrula symmetric a; 15, Hyalosphenia lata; 0, Arcella vulgaris :

L>, Difflugia pyriformis. (From Lang's Comparative Anatomy.)

Skeleton. --We may understand the relation of the shelled to
the shell-less Lobosa by supposing an Amoeba to draw in the
pseudopods from the greater part of its body, and to secrete, from
that part only, a cell-wall ; such a cell-wall or capsule would differ

VOL. i

E

50

ZOOLOGY

feECT.

from a cyst in having an aperture at one end to allow of the
protrusion of pseudopods from a small naked area. This is exactly
what we find in Arcclla and its allies (Fig. 32, A-c), in which the
shell is chitinoid. A different kind of shell is found in Difflugia
(D), which secretes a gelatinous coating to which minute sand-
grains and other foreign particles become attached.

ORDER 2. FORAMINIFERA

General Structure.- -The members of this order differ from
the Lobosa in the fact that their pseudopods are long and delicate
and unite to form networks ; moreover, with few exceptions, they
agree with Arcella and its allies in possessing a shell. In the
majority of cases this shell is formed of calcium carbonate.

One of the simplest members of the group is Microgromia (Fig.
33). It consists of a protoplasmic body (B), with a single nucleus

FIG. 33. Microgromia socialis. A, entire colony ; B, single zooid ; C, zooid which
has undergone biliary fission, with one of the daughter-cells creeping out of the shell ;
D, flagellula ; c. vac. contractile .vacuole ; nu. nucleus; sh. shell. (From Biitschli's Protozoa,
after Hertwig and Lesser.)

(nu.) and contractile vacuole (c. vac.), enclosed in a chitinoid cell-
wall or shell (sh.) with an aperture at one end through which the
protoplasm protrudes and is produced into delicate radiating
pseudopods. The animal multiplies by binary fission, and the
individuals or zooids thus produced remain united in larger or
smaller clusters, or cell-colonies (A). Sometimes the cell-body of a.
zooid divides and one of the daughter-cells creeps out of the cell-
wall (C), and, after moving about for a time like an Amoeba, draws
in its pseudopods, assumes an oval form, and sends out two
flagella by means of which it is propelled through the water (D).
We shall find other instances in which the young of a Rhizopod is

ii PHYLUM PROTOZOA 51

a, flagellula, i.e. a cell provided with one or more flagella, which,
if its history were not known, would be included among the
Mastigophora.

Platoum (Fig. 34, A) is a form resembling Microgromia, but
illustrating a very interesting type of colony. The protoplasm
flows out of the mouth of the shell in the form of a long plate (B)

O- vac

B

FIG. 34. Platoum stercoreum. A, single zooid ; B, formation of colony ; c. vac. contractile
vacuole ; /. food particles ; nu. nucleus ; 5/1. shell. (From Biitschli's Protozoa, after
Cienkowsky.)

which sends off rounded side branches, and each of these, acquiring
a cell-wall, becomes a zooid of the simple cell-colony.

Gromia (Fig. 35, 1) leads us to the more typical Foraminifera.
The protoplasm of this form protrudes from the mouth (a) of the
chitinoid shell (sA.) and flows around it so that the shell becomes
an internal structure. The pseudopods are very long and delicate
and unite to form a complicated network, exhibiting a streaming
movement of granules and serving, as usual, to capture prey.

Skeleton. Squammulina (Fig. 3 5, c?) differs from Gromia mainly
in having the shell formed of calcium carbonate and possessing the
character of a hollow, stony sphere, with an aperture at one end.
It appears that all the calcareous Foraminifera begin life in this
simple form ; but in the majority of cases the adult structure
attains a considerable degree of complexity. The protoplasm of
the original globular chamber overflows, as it were, through the
aperture ; but, instead of forming an elongated plate from which
side buds are given off, as in Platoum, the extended mass rounds
itself off, and secretes a calcareous shell in organic connection with
the original shell, and communicating with it by the original
aperture. In this way a two-chambered shell is produced, and a
repetition of the process gives us the many-chambered shell found
in most genera. New chambers may be added in a straight line
(Fig. 36, 3\ or alternately on opposite sides of the original
chamber (o), or with each new chamber enclosing its predecessor
(4\ or in a flat spiral, each new chamber being larger than its
predecessor (7, 8), or in a spire in which the newer chambers

E 2

52

ZOOLOGY

SECT.

overlap the older (9, 10), or in an irregular spiral of globular
chambers (6), or in an extremely compact spiral in which the new
chambers completely enclose their predecessors (11\ In all cases

\

\

\

I ,'', f f '

I If//// /

'

^MiL^^-<-v~

i\- ' j HI H \ \\\\

i i i V it) . \

3.Squammulina

4.M i I i o I a

FIG. 35. -Various forms of Foraminifera. In ft, Miliola, a, shows the living animal;
6. the same killed and stained ; a. aperture of shell ; /. food particles ; nu. nucleus ; &h. shell.
(From Biitschli's Protozoa and Claus's Zoology.)

adjacent chambers communicate with one another either by a
single large hole or by numerous small ones : the protoplasm
is thus perfectly continuous throughout the organism. With the

II

PHYLUM PROTOZOA

53

increase in the number of chambers there is a multiplication of
the nucleus (Fig. 35, 4, b, nu).

Not only does the shell increase in size by the formation of new

I.Saccommina

2.Lagena

7. Discorbina

5< Spiroloculma

3.Nodosaria

4.Frondicularia G.CIobigerina

s.sk.

9.Planorbulina

ll.Nummtrlires

FIG. 30. Shells of Foraminifera. In 3, It, and 5, a shows the surface view, and b a section ;
8a is a diagram of a coiled cell without supplemental skeleton ; Sb of a similar form
with supplemental skeleton (s. sk.); and 10 of a form with overlapping whorls ; in lla half
the shell is shown in horizontal section ; b is a vertical section ; a. aperture of shell ; 1 15,
successive chambers, 1 being always the oldest or initial chamber. (After Carpenter, Brady,
and Butschli.)

chambers : individual chambers become larger. In this process
Jayers of calcareous matter are added to the shell from without by
the agency of a thin layer of protoplasm that extends over the

54

ZOOLOGY

SECT.

surface, a corresponding thickness being, probably, removed by
solution from the inner side at the same time.

The shell presents two leading types of structure apart from
the form and arrangement of the chambers : either it is of a
porcelain-like texture and provided with a single terminal aperture,
(Fig. 35, 4), r the texture is glassy and the whole shell perforated
with very minute apertures, through which, as well as through the
terminal aperture, pseudopods are protruded (Fig. 35, 2).

In many cases additional complexity is attained by the develop-
ment of what is called the supplemental skeleton (Fig. 36, 8b, s. sk.).
This consists of a deposit of calcium carbonate outside the original
shell ; it is traversed by a complex system of canals containing pro-
toplasm, and is sometimes produced into large spines. Foraminifera

sfo

FIG. 37. Has tiger ina murrayi. filsm. vacuolated protoplasm surrounding shell : psd.
pseudopods ; sh. shell ; xj>. spines. (After Brady.)

in which this secondary skeleton occurs are sometimes of consider-
able size 2-3 cm. in diameter and of extraordinary complexity.

Many Foraminifera resemble Difflugia in having a skeleton
formed of sand-grains, sponge-spicules, and other foreign bodies
cemented together by a secretion from the protoplasm (Fig. 36, 1).
Some of these are formed on the imperforate type, having the
protoplasm protruded from a single terminal aperture ; others on
the perforate type, small pseudopods being protruded between the
particles forming the shell.

In many cases the pseudopods are the only portions of proto-
plasm outside the shell, whereas in Gromia, as we saw, the shell
is invested with a layer of protoplasm, and is thus in strictness
an internal structure. In one of the calcareous forms with

II

PHYLUM PROTOZOA

55

perforated spiral shell, called Hastigerina (Fig. 37), a very remark-
able modification of this condition of things obtains. The shell

F

i &4r^ 8 **^ ' * /

. . *J& A""" i^^3^ "' *" Q

~^*>i O*"* ' '' SL/

*:-fS

^\:^'- f

r? t

-.!,.'-* ;. ; ^

t-L.

FIG. 38. Dimorphism and alternation of generations in Polystomella crispa. The arrows
indicate the direction of the life-cycle. A, young megaspheric individual; B, full-grown
megaspheric individual, decalcified ; C, megaspheric individual in the act of spore-formation,
the protoplasm leaving the shell in the form of flagellulas ; D, flagellula more highly
magnified ; E, microspheric individual developed from a flagellula ; F, microspheric individual
in the act of producing amoeboid embryos. (From Lang, after Schaudinn.)

.) is surrounded with a mass of protoplasm (plsm.) many times
its own diameter, and so full of vacuoles as to present a bubbly or

56 ZOOLOGY SECT.

frothy appearance. The shell itself, moreover, in this and allied
forms is provided with numerous delicate, hollow, calcareous spines
(sp. ), which are only to be seen in perfect, freshly-caught specimens.

Many Foraminifera exhibit the phenomenon of dimorphism :
the individuals of a single species occur under two distinct forms
(megaspheric andmicrospheric) differing from one another in the size
of the central chamber, the shape and mode of growth of the suc-
ceeding chambers, and the number and size of the nuclei (Fig. 38).

The reproduction of Foraminifera is mainly by spore-formation,
with or without conjugation. The protoplasm has been observed
in some to divide into minute masses which may be amoeboid or
may be of the nature of flagellulae each provided with a
flagellum. In some cases the flagellula3 have been observed to
conjugate in pairs. The young may develop shells while still
within the shell of the parent or only after becoming free. In the
dimorphic Foraminifera there is evidence of the occurrence of an
alternation of generations (p. 41) the megaspheric form alternat-
ing with the microspheric, and the latter being developed as a
result of a process of conjugation, the former without it (alterna-
tion of sexual and asexual generations).

Distribution. Gromia, Microgromia, and a few other forms
are found in fresh-water : one species has been found in damp
earth, but the great majority of the Foraminifera are marine,
some being pelagic, i.e. occurring at or near the surface of the
ocean, others abyssal, i.e. living at great depths. In the Atlantic,
large areas of the sea-bottom are covered with a gray mud called
Globigerina-ooze from the vast number of Globigerinae contained
in it.

From the palasontological point of view, the Foraminifera are a
very important group. Remains of their shells occur in various
formations from the Silurian period to the present day, certain
rocks, such as the White -Chalk (Cretaceous period) and the
Nummulitic limestone (Eocene), being largely made up of them.

ORDER 3. HELIOZOA.

General Structure. The Heliozoa are at once distinguished
from the preceding groups by the character of their pseudopods,
which have the form of stiff filaments radiating outwards from
the more or less globular cell-body, presenting very little move-
ment beyond the characteristic streaming of granules, and not
uniting to form networks.

One of the simplest forms is the common " Sun-animalcule,"
Actinophrys sol (Fig. 39). The body is nearly spherical, and
contains a large nucleus and numerous vacuoles, some of which,
near the surface, are contractile. Each of the stiff radiating
pseudopods has a firm axis, apparently composed of protoplasm,

II

PHYLUM PROTOZOA

57

which is traceable through the general protoplasm as far as the

nucleus. Living organisms are de-

voured in much the same way as in

Amoeba: each is ingested along with

a droplet of water, and is thus seen,

during digestion, to lie in a de-

finite cavity of the protoplasm,

called a food-vacuole. If the or-

ganism be small, processes of the

protoplasm are developed, and sur-

round and engulf it. If it be larger,

several pseudopods are applied to

it, their axial fibres becoming ab-

11 j ,1 r i

sorbed, and their substance envelops

if Anplncinrr if in a var-nnlp Tho
It, enclosing It in a VaCUO

animal can fix itself by means of

its pseudopods, the ends of which become viscid, and it is able

to crawl slowly by their means. Sometimes it floats freely in the

FIG. 3<k Actinophrys sol. a. axial

filaments of pseudopods ; . nucleus ;

P- pseudopod. (From Lang's Com-
ive. Anatomy, after Grenadier.)

rrucl

nu

FIG. 40. Actinosphserium eichhornii. A, the entire organism ; B, a small portion
highly magnified ; chr. chromatophore ; cort. cortex ; c. vac. contractile vacuole ; med. medulla ;
n u. nuclei. (From Blitschli's Protozoa, after Hertwig and LesseT.)

water, and it possesses the power of rising or sinking by some
unknown means.

Actinosphcerium (Fig. 40, A), another fresh-water form, is more

58

ZOOLOGY

SECT.

complex. The protoplasm is distinctly divided into a central mass,
the medulla or endosarc (B, med.), in which the vacuoles are small,

2. Nude aria

3.ClaHiru.lma

FIG. 41. Various forms of Helicrzoa. 3a, the entire animal; 8b, the flagellula ; c. rac.
contractile vacuole ; g. gelatinous investment ; nu. nucleus psd. pseudopods ; si: siliceous
skeleton ; sp. spicules. (From Biitschli's Protozoa, after Schulze and Greeff.)

and an outer layer, the cortex or ectosarc (cort.), in which they are
very large. There are numerous nuclei (nu.) and chromatophores
(chr.), the latter coloured green by chlorophyll, the characteristic
pigment of green plants.

II

PHYLUM PROTOZOA

59

Many genera form colonies. Numerous zooids may be united
by bridges of protoplasm into an open network, or the connecting
bridges may be shorter and the zooids more numerous, giving the
colony a more compact appearance.

Transitional stages occur between the naked genera already re-
ferred to and forms with a distinct skeleton. Sometimes the body
simply surrounds itself with a temporary gelatinous investment
(Fig. 41, #,#.), in other cases it is surrounded by a capsule of loosely
woven fibres through which the pseudopods pass, thus reminding
us of the state of things characteristic of perforate Foraminifera.

Fio. 42. Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A. two indi-
viduals in the first phase of conjugation ; B, beginning of the encystation ; C, maturation ;
D, completion of maturation ; E, coalescence of nuclei ; F, completion of the first spindle of
the zygote resulting from the conjugation. 1, axial filaments of the pseudopods ; 2, nucleus ;
3, spindles concerned in maturation ; A, 5, outer and inner layers of <jyst ; 6, polar bodies ;
7, nucleus formed by the union of the two nuclei; S, first (mitotic) division. (From Lang,
after Schaudinn.)

One genus has a shell formed of agglutinated sand-grains ; in
another (Fig. 41, 1) the skeleton consists of loosely matted needles
of silica. Lastly, in the graceful Clathrulina (3) the body is
enclosed in a perforated sphere of silica, quite like the skeleton of
many of the Radiolaria (p. 61).

Reproduction ordinarily takes place by binary fission ; a
peculiar form of budding has been observed, and spore-formation
also occurs, with or without encystation. ActinosphaBrium, for
instance, encloses itself in a gelatinous cyst and undergoes
multiple fission, forming numerous spores each enclosed in a
siliceous cell-wall. These resting spores remain quiescent

CO

ZOOLOGY

SECT.

throughout the winter, and in spring the protoplasm emerges
from each and assumes the form of the ordinary active Actino-
sphaBrium. In Clathrulina spore-formation takes place in the
active condition, and the spores (Fig. 4*1, 3 b) are flagellulse, each
being an ovoid body provided with two flagella.

Conjugation has been observed in some instances between two
or more individuals, which may separate again without any nuclear
changes taking place; or the conjugation may be followed by
a sexual process, comprising the coalescence of the protoplasm of
fhe two individuals and the coalescence of the nuclei (Fig. 42)
after each has given off a part of its substance (6), as in the
maturation of an ovum in multicellular animals (p. 19).

ORDER 4. RADIOLARIA.

The Radiolaria are a large and well-defined group of Rhizopods,
noticeable, in most instances, by the presence of a siliceous skeleton
of great beauty and complexity. They are all marine.

General Structure. --The most important characteristic of
the group is the presence of a perforated membranous sac, called

the central capsule (Fig. 43,
cent. caps.\ which lies em-
bedded in the protoplasm,
dividing it into intra-capsular
(int. caps, pr.) and extra-
capsular (ext. caps.pr.) regions.
In the intra-capsular proto-
plasm is a large and complex
nucleus (nu.\ or sometimes
many nuclei: from the ex-
tra-capsular protoplasm the
pseudopods (psd.) are given
off in the form of delicate radi-
ating threads, which in some
cases remain free, in others,
e.g. Lithocircus, anastomose
freely, i.e. unite to form net-
works. In one large section the Acantharia the pseudopodia
contain firm axial rods similar to those in the pseudopods of the
Heliozoa. There is no contractile vacuole, but in many forms the
extra-capsular protoplasm contains numerous large non-contractile
vacuoles, which give it the frothy or bubbly appearance noticed
previously in Hastigerina. The vacuolated portion of the proto-
plasm has a gelatinous consistency, and is distinguished as the
calymma.

The central capsule may be looked upon as a chitinoid
internal skeleton, reminding us of the shell of Gromia and of

SKel.

FIG. 43. Lithocircus annularis. cent. caps.
central capsule ; ext. caps. pr. extra-capsular
protoplasm ; iiit. caps. pr. intra-capsular pro-
toplasm ; mi. nucleus ; psd. pseudopods ; skel.
skeleton ; z. cells of Zoochlorella (After
Biitschli, from Parker's Biology.)

II

PHYLUM PROTOZOA

61

the perforated calcareous shell of Hastigerina with its investment
of vacuolated protoplasm. It is found in its simplest form in
Thalassoplancta (Fig. 44), in which it is spherical and uniformly
perforated with minute holes. In other forms, such as Lithocircus
(Fig. 43), it is more or less conical in form, and the apertures are
restricted to the flat base of the cone. Lastly, in the most complex
forms (Fig. 45), the membrane of the capsule is double, and there
are three apertures a principal one having a central position and
provided with a lid or operculum (op), and
two subsidiary ones on the opposite side.
In relation with the principal or lidded
aperture there is found in the extra-
capsular protoplasm a heap of pigmented
matter called the phceodium (ph.), prob-
ably partly of the nature of excreta. The
central capsule encloses, in addition to
the nucleus or nuclei, oil -drops, vacuoles,
proteid crystals, and pigment.

In some genera the central capsule is
the only skeletal structure present, but in
most cases there is in addition a skeleton
mainly external formed, as a rule, of
silica, but in one subdivision of the class
of a substance called acanthin, composed
of strontium sulphate, so transparent that
it can only be distinguished from silica
by chemical tests. The siliceous skeleton
may consist of loosely woven spines
(Fig. 44), but usually (and the acanthin
skeleton always) has the form of a firm
frame-work of globular, conical, stellate,
or discoid shape, frequently produced into
simple or branched spines. In the forms
with an acanthin skeleton the spines fre-
quently have inserted into them a number
of contractile filaments arising from the
gelatinous extra-capsular layer. A very
beautiful form of skeleton is exhibited by

Adinomma (Fig. 46), in which there are three concentric per-
forated spheres (A, sk. 1, sk. 2, sk. 3) connected by radiating spicules.
The outer of these spheres occurs in the extra-capsular protoplasm
(B, ex. caps, pr.), the middle one in the intra-capsular protoplasm,
and the inner one in the nucleus (nu).

Colonial forms are comparatively rare in this order, but occur
in some genera by the central capsule undergoing repeated
divisions while the extra-capsular mass remains undivided. In
this way is produced in Coltozoum for instance (Fig. 47, A, B, C)

FIG. 44. Thalassoplancta
brevispicula, part of a
section, km. central cap-
sule ; ip. intra-capsular
protoplasm ; n. nucleus,
containing nl. numerous
nucleoli ; ot. oil drops ; ca.
calymma ; rp. protoplasm
surrounding calymma ; s.
spicules. (From Lang's
Compamti ce A natomy, after
Haeckel).

62

ZOOLOGY

SECT.

FIG. 45. Aulactinium actinastrum. c. calymiua ; km. central capsule ; n. nucleus ; op
operculum ; ph. phaeodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

FIG. 46. Actinomma asteracanthion. A, the shell with portions of the two outer
spheres broken away; B, section showing the relations of the skeleton to the animal;
<< nt. rn/>s. central capsule ; ex. <<!/>.*. ///. extra-capsular protoplasm ; nu. nucleus ; sk. 1, outer,
sk. 2, middle, sk. 3, inner sphere of skeleton. (From Biitschli's Protozoa, after Haeckel and
Hertwig.)

II

PHYLUM PROTOZOA

63

a firm gelatinous mass, the calymma or vacuolated extra-
capsular protoplasm (D, vac.) common to the entire colony,
having embedded in it numerous central capsules (c. caps.) each
indicating a zooid of the colony. Collozoum may attain a length
of 3 or 4 cm.

Reproduction by binary fission has been observed in some
cases, and is probably universal. The nucleus divides first, then
the central capsule, and finally the extra-capsular protoplasm.

Spore-formation has been observed in Collozoum and some other
genera : the intra-capsular protoplasm divides into small masses,
each of which becomes a flagellula (Fig. 47, E, F) provided with a
single flagellum. In some instances all the spores produced are

,.: ?; WS^JHfcVaPN fJ w ?

FIG. 47. Collozoum inermei A C, three forms of the entire colony, iiat. size ; D, a small
colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with
vacuoles(rac.) ; E, spores containing crystals (c.) ; F, mega- and inicrospore. (From Butschli's
Protozoa, after Hertwig and Brandt.)

alike (E), and each encloses a small crystal (c.): in other cases (F)
in the same species the spores are dimorphic, some being small
(microspores), others large (megaspores). Their development has not
been traced.

Symbiosis. One most characteristic and remarkable feature
of the group has yet to be mentioned. In most species there occur
in the extra-capsular protoplasm (in the intra-capsular in some
cases) minute yellow cells (Fig. 43, 2.) which multiply by fission
independently of the Radiolarian. It has been proved that these
are unicellular organisms, sometimes regarded as plants (Class
Algae), sometimes as animals (Class Mastigophora of the Protozoa),
and named Zoochlorellce. This intimate association of two organisms
is called symbiosis : it is probably a mutually beneficial partner-
ship, the Radiolarian supplying the Zoochlorellse with carbon
dioxide and nitrogenous waste matters, while the Zoochlorellae

64

ZOOLOGY

SECT.

give off oxygen and produce starch and other food- stuffs, some
of which must make their way by diffusion into the protoplasm
of the Radiolarian.

APPENDIX TO THE RHIZOPODA.

CHLAMYDOMYXA AND LABYBINTHULA.

Chlamydomyxa (Fig. 48), of which two species have been described, has been
found living on Bog-mosses (Sphagnum) in Ireland and in Germany and

' <' i ;/ ' , '
;.', i . ;////.,

!-}''::'/.

l-ff\/',U~-"-

FIG. 48. Chlamydomyxa labyrinthuloides. A, active phase ; c.w. cell-wall ; /. frag-
ment of Alga ingested as food ; s/>. spindles in course of pseudopods ; B, resting-stage
numerous individuals in the cells of a fragment of Sphaijiium. ; a, specimen completely enclosed
in cell ; b and c, specimens which have emerged through the ruptured cell-wall ; C, specimen
multiplying by budding; I), by binary fission ; E, by internal tission. 1] may represent a
stage in spore-formation. (A, after Areher, B .E after (Jeddes.)

Switzerland. It may occur either in the active or in the resting condition. In
the latter (B, a, b, c) it consists of a mass of protoplasm with a number of
nuclei surrounded by a laminated wall of cellulose (p. 14). In the protoplasm are

II

PHYLUM PROTOZOA

65

numerous non-nucleated protoplasmic bodies or chromatophores, containing
chlorophyll and a green or brown colouring matter in varying proportions.
There are also a number of minute rounded bodies of a bluish tint probably com-
posed of reserve food-materials. In the young condition (a) the resting cells are
globular and microscopic, lying enclosed within the cells of the Sphagnum, but
as they grow in this confined space they become elongated and irregular, and
tinally burst through the wall of the moss-cell, forming masses (b, c) quite visible
to the naked eye. These may bud (C) or undergo binary fission (I)) ; or the
protoplasm, retreating from the cell- wall, may divide into numerous small
uninucleated amoeboid masses, each of which subsequently surrounds itself with
a new cell-wall (E).

During the whole of the resting stage there is nothing to distinguish Chlamy-
domyxa from a plant, and it would certainly be placed among the lower Algas
if the active phase of its existence were unknown.

In the active stage (A) the protoplasm protrudes from the ruptured cell-wall
in the form of stiff pseudopods produced into a complex network of extremely
delicate filaments, which are much branched and perhaps anastomose, and may
imite to form larger masses of protoplasm at a considerable distance from the
original cell. At the same time the bluish spheres (-sp. ) found in the resting
stage take on a spindle shape and travel slowly along the filaments.

In one of the two known species the protoplasm entirely leaves the cyst wall
and becomes free in the water.

The filaments are used to capture living organisms (/. ) which are digested by
the protoplasm surrounding them, the products of nutrition being conveyed
along the network to all parts of the organism. Thus in the active condition the
nutrition of Chlamydomyxa is holozoic, i.e. strictly like that of an animal, the
food consisting of living protoplasm. In the resting stage, on the other hand,
nutrition is purely holophytic, i.e. like that of an ordinary green plant, the food

B

rtu.

Fio. -Hi. "Labyrinthula vitellina. A, specimen crawling on a fragment of Alga (a.) ; c. ceils
travelling in the filaments. B, part of specimen in resting condition with heap of cells (c.) ;
C, a single cell from an actively moving specimen with connecting threads ; nn. nucleus.
(From Biitschli's Protozoa, after Gienkowsky.)

consisting of the carbon dioxide and various mineral salts dissolved in the water.
Chlamydomyxa multiplies in the resting condition by the formation of spores
each containing two nuclei. These give rise to flagellulse, the further history of
which has not been traced.

Labyrinthula (Fig. 49) in the resting stage (B) consists of a heap of small

VOL. I F

66

ZOOLOGY

SECT.

nucleated cells (c. ) connected by a homogeneous substance. In the active condi-
tion (A) it is produced into long delicate stiff filaments of pseudopodial character,
along which the cells (c.) travel, in the same manner as the spindles of Chlamy-
clomyxa. Labyrinthula has, therefore, the character not of a single cell, but of
a cell-colony, formed of numerous cells connected together. Chlamydomyxa, on
the other hand, has the character of a single multinucleate cell. There is thus
no close connection between these two aberrant forms : but both may, perhaps,
best be regarded as Rhizopoda with nearer relationships to the Foraminifera
(Gromia in particular) than to any of the other orders.

cvac

FIG. -00 Didymium differ me. A, two sporangia (spg. 1 and 2) on a fragment of leaf(/.).

B, section of sporangium, with ruptured outer layer (a.); and threads of capillitiuni (cp.).

C, a flagellula with contractile vacuole (c. i-ac.) and nucleus (.). D, the same after loss
of flagellum ; l>, an ingested Bacillus. E, an amcebula, F, conjugation of amoebulae to form
a small plasmodium, G, a larger plasmodium accompanied by numerous anicebulae ; sp.
ingested spores, (*\iter Lister.)

CLASS II. MYCETOZOA.
1. EXAMPLE OF TKE CLkSSDidymium difforme.

Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 50, G),
often several centimetres across, which crawls, like a gigantic Amoeba, over
the surface of decaying leaves. It shows the characteristic streaming move-

ii PHYLUM PROTOZOA 67

merits of protoplasm, and feeds by ingesting various organic bodies, notably the
Bacilli which always occur in great numbers in decaying substances. Numerous
nuclei are present.

After leading an active existence for a longer or shorter time, the protoplasm
aggregates into a solid lump, surrounds itself with a cyst, and undergoes multiple
fission, dividing into an immense number of minute spores. The cyst (Fig. 50,
A, spy. 1, spg. 2) is therefore not a mere resting capsule, like that of Amoeba,
but a sporangium or spore-case. Its wall consists of two layers, an inner of a
dark purple colour and membranous texture, formed of cellulose, and an outer of
a pure white hue, formed of calcium carbonate. Thus the whole sporangium,
which may attain a diameter of 3 or 4 mm., resembles a minute egg. From the
inner surface of the wall of the sporangium spring a number of branched
filaments of cellulose, which extend into the cavity among the spores and together
constitute the capillitium (B, cp. ).

The spores consist of nucleated masses of protoplasm surrounded by a thick
cellulose wall of a dark reddish -brown colour. After a period of rest the proto-
plasm emerges in the form of an amoeboid mass which soon becomes a flagellula
(C), provided with a single flagellum, a nucleus (nu.), and a contractile vacuole
(c. vac.). The flagellulse move freely and ingest Bacilli (D, b. ), and multiply by
fission : then, after a time, they become irregular in outline, draw in the
flagellum, and become amoeboid (E). The amcebulas thus formed congregate in
considerable numbers and fuse with one another (F), the final result being the
production of the great amoeboid mass (G) with which we started. There is no
fusion of the nuclei of the amcebulne. Thus Didymium in its active condition is a
plasmodium, i.e. a body formed by the concresence of amoebulse.

2. GENERAL REMARKS ON THE MYCETOZOA.

Speaking generally, the Mycetozoa differ from all other Protozoa in their
terrestrial habit. They are neither aquatic, like most members of the phylum,
nor parasitic, like many other forms, but live habitually a sub-aerial life on
decaying organic matter. They are also remarkable for their close resemblance
in the structure of the sporangia and spores to certain Fungi, a group of parasitic
or saprophytic plants in which they are often included, most works on Botany
having a section on the Myxomycetes or " Slime-fungi," as these organisms are
then called. They are placed among animals on account of the structure and
physiology of the flagellate, amoeboid, and plasmodial phases, which exhibit
automatic movements and ingest solid food. The Mycetozoa are sometimes
included among the Rhizopoda, a course which their very peculiar reproductive
processes appears to render inadvisable.

An interesting organism, called Protomyxa, probably belongs to this group. In
its plasmodial phase it consists of orange-coloured masses of protoplasm, about
1 mm. in diameter, which crawl over sea-shells by means of their long, branched
pseudopods, and ingest living prey. No nuclei are known. The protoplasm
becomes encysted and breaks up into naked spores, which escape from the cyst
as flagellulte, but soon become amoeboid and fuse to form the plasmodium.

CLASS III. MASTIGOPHORA.

1. EXAMPLE OF THE CLASS Euglcna viridis.

Euglena (Fig. 51) is a flagellate organism commonly found in
the water of ponds and puddles, to which it imparts a green colour.
The' body (E, H) is spindle-shaped, and has at the blunt anterior
end a depression, the gullet (F, a j s.), from the inner surface of which

F 2

68

ZOOLOGY

SECT.

springs a single long flagellum (fl.). According to recent observa-
tions the flagellum is not a simple thread, but is beset with delicate
cilium-like processes. The organism is propelled through the
water by the lashing movements of the flagellum, which is always
directed forwards ; it can also perform slow worm-like movements
of contraction and expansion (A--D), but anything like the free
pseudopodial movements which characterise the Rhizopoda is
precluded by the presence of a very thin membrane or cuticle which
invests the body. Oblique and longitudinal lines ; in the outer
layer of the protoplasm may be due to the presence of contractile
fibrils. There is a nucleus (nu.) near the centre of the body, and
at the anterior end a contractile vacuole (H, c. me.), leading into

H ft

f.vac

FIG. 51. Eugleua viridis. A I), four views illustrating euglenoid movements; E and H,
enlarged views ; F, anterior end further enlarged ; G, resting form after binary fission ; c. vac.
contractile vacuole in H, reservoir in E and F ; cy. cyst ; ,rf. flagellum ; m. mouth ; nu. nucleus ;
ces, gullet ; p. paramylum bodies ; pg. pigment spot ; r. (in H), reservoir. (From Parker's
Biology, after Kent and Klebs.)

a large non-contractile space or reservoir (r.) which discharges into
the gullet.

The greater part of the body is coloured green by the charac-
teristic vegetable pigment, chlorophyll, and contains rod-shaped
grains of paramylum (H, p.), a carbohydrate allied to starch. In
contact with the reservoir is a bright red speck, the stigma (pg.),
formed of a pigment allied to chlorophyll and called hcematochrome.
It seems probable that the stigma is a light-perceiving organ or
rudimentary eye.

Euglena is nourished like a typical green plant : it decomposes
the carbon dioxide dissolved in the water, assimilating the carbon
and evolving the oxygen. Nitrogen and other elements it absorbs
in the form of mineral salts in solution in the water. But it has

ii PHYLUM PROTOZOA 69

also been shown that the movements of the flagellum create a
whirlpool by which minute fragments are propelled down the
gullet and into the soft internal protoplasm. There seems to be
no doubt that in this way minute organisms are taken in as food.
Euglena thus combines the characteristically animal (holozoic) with
the characteristically vegetable (holophytic) mode of nutrition.
But, in all probability, the Euglena is in large measure saprophytic,
the products of the decay of organic matter dissolved in the water
being absorbed through the general surface.

Sometimes the active movements cease, the animal comes to
rest and surrounds itself with a cyst or cell-wall of cellulose (G),
from which, after a quiescent period, it emerges to resume active
life. It is during the resting condition that reproduction takes
place by the division of the body in a median plane parallel to
the long axis (G). Under certain circumstances multiple fission
takes place, and flagellulse are produced, which, sometimes, after
passing through an amoeboid stage, develop into the adult form.

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Mastigophora form a very extensive group, the genera and
species of which show a wonderful diversity in structure and habit.
The only character common to them all is the presence of one or
more flagella. Some approach plants so closely as to be claimed
by many botanists ; others are hardly to be distinguished from
Rhizopods ; while the members of one order present an interesting
likeness to certain peculiar cells found in Sponges.

The class is divisible into four orders as follows :-

ORDER I.--FLAGELLATA.

Mastigophora having one or more flagella at the anterior end
of the body.

ORDER 2. CHOANOFLAGELLATA.

Mastigophora having a single flagellum surrounded at its base
by a contractile protoplasmic collar.

ORDER 3. DINOFLAGELLATA.

Mastigophora having two flagella, one anterior, the other
encircling the body like a girdle.

ORDER 4. CYSTOFLAGELLATA.

Mastigophora having two flagella, one of which is modified
into a long tentacle, while the other is small and contained within
the gullet.

70 ZOOLOGY SECT.

Systematic Position of the Example.

Euglena viridis is one of several species of the genus Euglena,
and belongs to the family Englenidce, sub-order .Euglenoidea, and
order Flagcllata.

The presence of an anterior flagellum and the absence of a
collar, transverse flagellum, or tentacle, indicate its position among
the Flagellata. It is placed among the Euglenoidea in virtue of
possessing a single flagellum and a small gullet into which
the reservoir opens. The genus Euglena is distinguished
by its centrally placed nucleus, green chromatophore, red stigma,
and euglenoid movements. E. viridis is separated from other
species of the genus by its spindle-shaped body with blunt ante-
rior and pointed posterior end, and by the flagellum being some-
what longer than the body.

ORDER 1. FLAGELLATA.

The cell-body is usually ovoid or flask-shaped (Fig. 52, 6, 7, 9,
&c.), but may be almost ^globular (1), or greatly elongated (3).
Anterior and posterior ends are always distinguishable, the flagella
being directed forwards in swimming, and, as a rule, dorsal and
ventral surfaces can be distinguished by the presence of a mouth
or by an additional flagellum on the ventral side. They are,
therefore, usually bilaterally symmetrical, or divisible into equal and
similar right and left halves by a vertical antero-posterior plane.

Some of the lower forms have no distinct cuticle, and are able,
under certain circumstances, to assume an amoeboid form (2).
The curious genus Mastigamccba (4) nas a permanently amoeboid
form, but possesses, in addition to pseudopods, a single, long
flagellum. It obviously connects the Mastigophora with the
Rhizopoda, and indeed there seems no reason why it should be
placed in the present group rather than with the Lobosa. Simi-
larly, Dimorpka (5) connects the Flagellata with the Heliozoa : in
its flagellate phase (a) it is ovoid and provided with two flagella,
but it may send out long stiff radiating pseudopods, while retaining
the flagella, or may draw in the latter and assume a purely helizoan
phase of existence provided with pseudopods only (&).

Nuclei of the ordinary character are universally present. In
addition there is present in the cytoplasm near the base of the
flagellum a much more minute, deeply-staining body, which is termed
the Uepliaroblast (Fig. 53). This has sometimes been taken for a
micronucleus such as is general in the Infusoria, but it is not of
nuclear origin, and does not take an active part in any reproductive
processes.

The number of flagella is subject to great variation. There
mav be one (Fig. 52, 1-S\ two \9, 10), three (6), or four (7).
Sometimes the flagella show a differentiation in function ; in

II

PHYLUM PROTOZOA

71

Hetcromita, e.g. (Fig. 57) the anterior flagellum (fl. 1) only is
used in progression, the second or ventral flagellum (fl. 2) is trailed

4.MasMg-

amoeba

e.Dallingeria

8.0ikomonas

H.DInobryon 12,Syncrybfa 13. An

H.Rhijjidodendron

FIG. 52. Various forms of Flagellata. In 2, flagellate () and amoeboid (b) phases are
shown ; in 5, flagellate () and helio^oan (/;) phases ; in 8 are shown two stages in the in-
gestion of a food-particle (/)! ^''- chromatophores ; c. rac. contractile vacuole ;/*. food par-
ticle g. gullet; nm. nucleus ; 1. lorica ; p. protoplasm ; per, peristome ; v.i. vacuole of ingestion.
(Mostly from Btitschli's Protozoa, after various authors.)

behind when the animal is swimming freely, or is used to anchor
it to various solid bodies. In some (Trypanosomes, Fig. 53) the

72

ZOOLOGY

SECT.

flagellum (or one of them, if two are present) is attacked through-
out its length, or in the greater part of its length, to the edge
of a wavy protoplasmic flange, or undulating membrane, running
along the body.

There are also important variations in structure correlated with
varied modes of nutrition. Many of the lower forms, such as
Heteromita, live in decomposing animal infusions : they have
neither mouth nor gullet and take no solid food, but live by
absorbing the nutrient matters in the solution ; their nutrition is,
in fact, saprophytic, like that of many fungi. A few live as para-
sites in various cavities of the body of the higher animals. The
HcBmoflagdlata, an extensive group, live as parasites in the
plasma of the blood of various vertebrates. Most of these appear
to be harmless, but some are the causes of serious diseases in Man

FIG. 53. Try pano somes of Fishes, c. blepharoblast ; /. flagellum ; /a. and fp. (in A) anterior
and posterior flagella ; m. undulating membrane ; n. nucleus. (After Laveran and Mesnil.)

and other higher animals. One Euglena-like form lives as an
intra-cellular parasite within the cells of one of the lower worms.

Hcematococcus (Fig. 54), Pandorina (Fig. 55), Volvox (Fig. 56),
and their allies present us with a totally different state of things.
The mouthless body is surrounded by a cellulose cell-wall (c.t0.),
and contains chromatophores (chr.) coloured either green by chloro-
phyll or red by hsematochrome. Nutrition is purely holophytic,
i.e. takes place by the absorption of a watery solution of mineral
salts and by the decomposition of carbon dioxide. It is, there-
fore, not surprising that these chlorophyll -containing Flagellata
are often included among the Algas or lower green plants.

Other genera live in a purely animal fashion by the ingestion of
solid proteinaceous food, usually in the form of minute living
organisms : in these cases there is always some contrivance for
capturing and swallowing the prey. In Oikomonas (Fig. 52, 8),
we have one of the simplest arrangements : near the base of the
flagellum is a slight projection containing a vacuole (v.i.) ; the
movements of the flagellum drive small particles (/.) against this
region, where the protoplasm is very thin and readily allows the
particles to penetrate into the vacuole, where they are digested.

II

PHYLUM PROTOZOA

73

In Euglena, as we have seen, there is a short, narrow gullet, and
in some genera (9, g} this tube becomes a large and well-marked
structure.

Skeleton. While a large proportion of genera are naked or
covered only by a thin cuticle, a few fabricate for themselves a
delicate chitinoid shell or lorica (10, /.), usually vase-shaped and
widely-open at one end so as to allow of the protrusion of the
contained animalcule. In the chlorophyll-containing forms there
is a closed cell-wall of cellulose (Fig. 54, c.w.). One group of

Fro. 54. HsematoCOCCUS pluvialis. A, motile stage ; B, resting stage ; C, D, two modes
of fission ; E, Hcematococcns lacustris, motile stage ; F, diagram of movements of flagellum ;
chr. chromatophores ; c. vac. contractile vacuole ; c.v. cell-wall ; mi. nucleus ; uu'. nucleolus ;
pyr. pyrenoids. (From Parker's Biology.)

marine Flagellates have siliceous skeletons similar to those of the
K/adiolaria, with which they were originally classed.

In many genera colonies of various forms are produced by
repeated budding. Some of these are singularly like a zoophyte
(see Sect. IV.) in general form (Fig. 52, 11), being branched colonies
composed of a number of connected monads, each enclosed in a
little glassy lorica ; or green (chlorophyll-containing) zooids are
enclosed in a common gelatinous sphere, through which their
flagella protrude (12) ; or tufts of zooids, reminding us of the
flower-heads of Acacia, are borne on a branched stem (13). In
Volvox (Fig. 56) the zooids of the colony are arranged in the form
of a hollow sphere, and in Pandorina (Fig. 55) in that of a solid
sphere enclosed in a delicate shell of cellulose. Lastly, in RJiipido-

74

ZOOLOGY

SECT.

dendron (Fig. 52, 14} a beautiful branched fan-shaped colony is
produced, the branches consisting of closely adpressed gelatin-
ous tubes each the dwelling of a single zooid.

Binary fission is the ordinary mode of asexual multiplication,
and may take place either in the active or in the resting condition.
Haematococcus (Fig. 54) and Euglena (Fig. 51), for instance,
divide while in the encysted condition ; Heteromita (Fig. 57)

FIG. r>5. Pandorina morum. A, entire colony; B, asexual reproduction, each zooid.
dividing into a daughter-colony ; C, liberation of gametes ; D F, three stages in conjugation
of gametes; G, zygote ; H- -K, development of zygote into a new colony. (From Parker's
mjti, after Goebel.)

and other saprophytic forms while actively swimming : in the
latter case the divison includes the almost infinitely fine flagellum.
In correspondence with their compound nature, the colonial
genera exhibit certain peculiarities in asexual multiplication. In
JDiTiobryon (Fig. 52, 11) a zooid divides within its cup, in which
one of the two products of division remains ; the other crawls out
of the lorica, fixes itself upon its edge, and then secretes a new
lorica for itself. In Pandorina (Fig. 55) each of the sixteen zooids
of the colony divides into sixteen (B), thus forming that number of
daughter-colonies within the original cell-wall, by the rupture of

II

PHYLUM PROTOZOA

75

which they are finally liberated. In Voh-o.r (Fig. 56), certain zooids,
called parthenogonidia (A, a), have specially assigned to them
the function of asexual reproduction: they divide by a process
resembling the segmentation of the egg in the higher animals
(D^D 5 ), and form daughter-colonies which become detached and
swim freely in the interior of the mother-colony.

A very interesting series of stages in sexual reproduction is
found in this group. In Heteromita two individuals come together

a

H

FIG. 5t>. Volvox globator. A, entire colony, enclosing several daughter-colonies;
B, the same during sexual maturity ; C, four zooids in optical section ; Di D5, develop-
ment of parthenogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and
sperms; H, oosperm ; a, parthenogonidia ; .r/. nagellmu ; or. ovum ; or)/, ovaries; jig. pigment
spot ; spit, spermaries. (From Parker's Biology, after Colin and Kirchner.)

(Fig. 57, E 1 ) and undergo complete fusion (E 2 - -E 4 ) : the result of
this conjugation of the two gametes or conjugating cells is a thin-
walled sac, the zygotc (E 5 ), the protoplasm of which divides by
multiple fission into very minute spores. These, when first
liberated by the rupture of the zygote (E), are mere granules,
but soon the ventral or trailing flagellum is developed, and after-
wards the anterior flagellum (F^-F 4 ). In Pandorina (Fig. 55)
the cells of the colony escape from the common gelatinous envelope
(C) and conjugate in pairs (D, E), forming a zygote (F, G), which,
after a period of rest (H), divides and forms a new colony (K).

76

ZOOLOGY

SECT.

In some cases the conjugating cells are of two sizes, union always
taking place between a large cell or megagamctc and a small cell

E

FIG. 57. Heteromita rostrata. A, the positions assumed in the springing movements
of the anchored form ; B, longitudinal fission of anchored form ; C, transverse fission of
the same ; D, fission of free-swimming form ; E, conjugation of free-swimming with anchored
form ; E=>, zygote ; E 6 , emission of spores from zygote ; F, development of spores ; jt.l, ante-
rior ; rt.2, ventral flagellum. (From Parker's Biolor/y, after Dallinger.)

or microgametc. In Volvox (Fig. 56) this dimorphism reaches its
extreme, producing a condition of things closely resembling what

II

PHYLUM PROTOZOA

77

we find in the higher animals. Certain of the zooids enlarge and
form megagametes (B, ovy.), others divide repeatedly and give rise
to groups of microgametes (B, spy. E, F), each in the form of an
elongated yellow body with a red pigment-spot and two flagella.
These are liberated, swim freely, and conjugate with the stationary
megagamete (G), producing a zygote (H), which, after a period of
rest, divides and reproduces the colony. It is obvious that the
megagamete corresponds with the ovum of the higher animals,
the microgamete with the sperm, and the zygote with the oosperm
or impregnated egg.

It should be noticed that in the more complex cases of repro-
duction just described we meet with a phenomenon not seen in
cases of binary fission, viz., development, the young organism being
far simpler in structure than the adult, and reaching its final form
by a gradual increase in complexity.

IMonosiga. 2.Salpinaoeca.

S.Polyoeca. 4.Proferospongia.

FIG. 58. Various forms of Choanofiagellata. 2b illustrates longitudinal fission ; 2c, the pro-
duction of flagellulaj ; c. collar ; c. vac. contractile vacuole ; ft. flagellum ; I. lorica ; ?'..
nucleus ; s. stalk. (After Saville Kent.)

ORDER 2. CHOANOFLAGELLATA.

General Structure. The members of this group are distin-
guished by the presence of a vase-like prolongation of the proto-
plasm, sometimes double, called the c0//a?'(Fig.58,/,c.), surrounding
the base of the single flagellum (fl.). The collar is contractile, and,
although its precise functions are nob yet certainly known, there is

78 ZOOLOGY SECT.

evidence to show that its movements cause vortices in the water which
draw in small bodies towards the outside of the collar to which they
adhere. By degrees such bodies are drawn towards the base, and
each is received into a vacuole which moves back into the interior
of the protoplasm, another vacuole taking its place. The animalcule
may draw in both collar and flagellum and assume an amoeboid form.

The nucleus (nu.) is spherical, and there are one or two con-
tractile vacuoles (c. vac.), but no trace of mouth or gullet. Some
forms are naked (1), others (2) enclosed in a chitinoid shell or
lorica of cup-like form. A stalk (s.) is usually present in the
loricate and sometimes also in the naked forms.

The genera mentioned in the preceding paragraph are all simple,
but in other cases colonies are produced by repeated fission. In
Polywca, (3) the colony has a tree-like form, which may reach
a high degree of complexity by repeated branching. A totally
different mode of aggregation is found in Proterospongia (4), in
which the zooids are enclosed in a common gelatinous matrix of
irregular form.

Reproduction.- -The " collared monads," as these organisms
are often called, multiply by longitudinal fission (2b). In some
cases multiple fission of encysted individuals has been observed
(2c), small simple flagellulse being produced which gradually
develop into the perfect form.

The order is especially interesting from the fact that, with the
exception of Sponges, it is the only group in the animal kingdom
in which the collar occurs.

ORDER 3. DINOFLAGELLATA.

The leading features of this group are the arrangement of the two flagella
which they always possess, and- the usual presence of a remarkable and often
very beautiful and complex shell.

The body (Fig. 59, 1) is usually bilaterally asymmetrical, i.e. it may be
divided into right and left halves, which are not precisely similar. On the
ventral surface is a longitudinal groove (I. yr. ), extending along the anterior half
only, and meeting a transverse groove (t. gr.), which is continued round the body
like a girdle. From the longitudinal groove springs a large flagellum (fl. 1),
which is directed forwards and serves as the chief organ of propulsion ; a second
flagellum (fl. 2) lies in the transverse groove, where its wave-like movements
formerly caused it to be mistaken for a ring of small cilia.

The body is covered with a shell (2} formed of cellulose, and often of very
complex form, being produced into long and ornamental process, and marked
with stripes, dots, &c. Besides a nucleus and a contractile vacuole, the proto-
plasm contains chromatophores (1, chr.) coloured with chlorophyll or an allied
pigment of a yellow colour, called diatom in. Nutrition is holophytic or holozoic.

The foregoing description applies to all the commoner genera. Prorocentrum
(3) is remarkable for the absence of the transverse groove, while Polykrikos (4)
has no fewer than eight transverse grooves and no shell. The latter genus
also has stinging-capsules or n< matocyxts (a, b) in the protoplasm, resembling
those of Zoophytes (see Sect. IV.), and has numerous nuclei of two sizes,
distinguished as mwjaiuidei (nu.), and micronuclti (nu'.).

II

PHYLUM PROTOZOA

79

Reproduction is, as usual, by binary fission, the process taking place some-
times in a free-swimming individual, sometimes in one which has lost its flagella
and come to rest.

vac

Glenodinium

S.CeraHum 3.Prorocentrum

4.Polykrikos

FIG. 59.; Various forms of Dinoflaerellata. 2 shows the shell only ; ka is an undischarged,
and 6 a discharged stinging-capsule; chr. chromatophores ; fl. 1, longitudinal flagellum ;
fi. 3, transverse flagellum; 1. ;//. longitudinal groove; ntc. nematocyst ; nu. meganucleus ;
ntf. micronucleus ; py. pigment spot ; t. gr. transverse groove^ (From Btitschli's Protozoa.)

The Dinoflagellata are mostly marine. Some are phosphorescent. Certain
kinds occasionally occur in such abundance in bays and estuaries as to cause a
deep brownish or red discoloration of the sea- water.

ORDER 4. CYSTOFLAGELLATA.

This group includes only two genera, Noctiluca and Leptodiscus. A descrip-
tion of Noctiluca miliaris, the organism to which the diffused phosphorescence

of the sea is largely due, will serve
to give a fair notion of the leading
characteristics of the order.

Noctiluca (Fig. 60) is a nearly
globular organism, about ^ mm. in
diameter. It is covered with a
delicate cuticle, and the medullary
protoplasm is greatly vacuolated.
On one side is a groove from
which springs a very large and
stout flagellum or tentacle (bg. ), no-
ticeable for its transverse striation.
Near the base of this flagellum is
the mouth (m.), leading into a short
gullet in which is a second flagel-
lum (f. ), very small in proportion
to the first. On the side opposite
to the mouth is a strongly marked
superficial ridge. The light-giving
region is the cortical protoplasm.
Reproduction takes place by binary fission, the nucleus dividing indirectly.
Spore-formation also occurs, sometimes preceded by conjugation, sometimes not.

Fin. 00. Noctiluca miliaris. n. the adult
animal ; I, c. flagellulai ; l<r. tentacle ; /. flagel-
lum ; nt. mouth ; n. nucleus. (From Lang.)

80

ZOOLOGY

SECT.

The spores (b, c), formed by the breaking up of the protoplasm of the parent,
escape as flagellulaf.

CLASS IV. SPOROZOA.

1. EXAMPLE OF THE CLASS Monocystis agilis.

One of the most readily procured Sporozoa is the microscopic
worm-like Monocystis agilis (Fig. 61, A), which is commonly found
leading a parasitic life in the vesicular seminales of the common
Earthworm. It is flattened, greatly elongated, pointed at both
ends, and performs slow movements of expansion and contraction,
reminding us of those of Euglena. In this, the trophozoite or adult

FIG. (51. Monocystis. A, Trophozoites in different stages of contraction. B, encysted
gametocytes. C, division of gametocytes into gametes. D, conjugation of gametes to form
zygotcs. E, Cyst enclosing ripe spores formed from the zygotes. F, single spore, showing
the (8) sporozoites in its interior. G, group of developing sperm-cells of the earthworm,
enclosing a sporozoite in the centre. H, young trophozites still surrounded with the tails of
the degenerated sperms, nu, nuclei. (From Parker's Practical Zoology.)

condition, the protoplasmic body is covered with a firm cuticle,
and is distinctly divided into a denser superficial portion, the cortex,
and a central semi-fluid mass, the medulla. There is a large clear
nucleus (nu.) with a distinct iiucleolus and nuclear membrane, but
the other organs of the protozoan cell-body are absent : there is
no trace of contractile vacuole, of flagella or pseudopods, of mouth
or gullet. Nutrition is effected entirely by absorption.

Reproduction takes place by a peculiar and characteristic
process of spore-formation. Two individuals come together,
and become rounded off and enclosed in a common cyst (B). The
nucleus of each divides repeatedly, until a large number of nuclei
arc formed (C). Each of the nuclei becomes surrounded by a thin
layer of protoplasm. The minute cells thus formed, after

ii PHYLUM PROTOZOA 81

moving to and fro actively for a time, unite in pairs after the
substance of the two individuals has become coalescent (D). From
each of the cells or zygotcs that are formed by the union of two
of the original small cells or gametes, a spore is formed, so that the
cyst now comes to contain numerous small spores (E). These are
spindle-shaped bodies, each enclosed in a strong chitinoid case (F),
and thus differing in a marked manner from the naked spores
of the Rhizopoda and Mastigophora. The protoplasm and nucleus
of each spore then undergo fission, becoming divided into a number
of somewhat sickle-shaped bodies which are arranged within the
spore-coat somewhat like a bundle of sausages. By the rupture
of the spore-coat these falciform young or sporozoites are liberated,
and at once begin active movements, the thin end of the body
moving to and fro like a clumsy flagellum. The falciform young
appear, in fact, to be greatly modified flagellulse. They make their
way to the clumps of developing sperms, bore their way in, and
are thus found surrounded by sperm-cells in various stages of
development (G). After thus living an intracellular life for
a time, they escape (H) into the cavity of the vesicula and grow
into the adult form.

2. CLASSIFICATION AND GENERAL ORGANISATION.

The Sporozoa are exclusively parasitic, being the only group of
Protozoa of which this can be said. They have no organs of
locomotion and always multiply by spore-formation. The class is
divisible into the following five orders :

ORDER 1. GREGARINIDA.
Sporozoa in which the trophozoite is free and motile.

ORDER 2. COCCIDIIDEA.

Sporozoa in which the trophozoite is a minute intracellular
parasite.

ORDER 3. IL&MOSPORIDIA.

Sporozoa in which the trophozoite is amoeboid, and lives as
a parasite in the coloured blood-corpuscles of Vertebrates.

ORDER 4. MYXOSPORIDEA.

Sporozoa in which the trophozoite is amoaboid, but not intra-
cellular.

ORDER 5. SARCOCYSTIDEA.
Elongated Sporozoa, usually found in muscle.

VOL. I G

82

ZOOLOGY

SECT.

Systematic Position of the Example.

Monocystis agilis is a species of the genus Monocystis, belonging
to the family Monocystidce, of the order Gregarinida It is placed
in the Gregarinida on account of being free and motile in the tro-
phozoite state. The absence of partitions dividing the protoplasm
into segments indicates its position among the Monocystidse.
Monocystis is distinguished by its elongated form, by the absence
of any special apparatus in the cyst for the liberation and dispersal
of the spores, and by its spindle-shaped spores with thickened
ends, each producing 4 8 falciform young. The differences
between the species of Monocystis depend largely upon size.

ORDER 1. GREGARINIDA.

All the more typical members of the class belong to this
group. With the exception of Monocystis, already described, the
only genus to which it will be necessary to draw attention is
Gregarina (Figs. 62 and 63), the various species of which are
parasitic in the intestines of Crayfishes, Cockcoaches, Centipedes,

dcu,

spd

FIG. 6'2. -Gregarina. A, two specimens of G. blattannn partly embedded in enteric
epithelial cells of Cockroach ; B 1 , B-, two specimens of G. di(,m-ifiiu ; in B- the epimerite (.;>.)
is cast off ; C, cyst of <?. bln/tun'm, from which most of the spores have been discharged;
D, four stages in the development of G. {tirtant<:a. c>i. cyst ; den., deutomerite ; c/>. epimerite ;
ft. gelatinous investment of cyst; nu. nucleus ; pr. protomerite; -j>xd. 1, short pseudopod;
psd. ,?, long pseudopod ; sp. mass of spores ; spd. sporoducts. (From Biitschli's Protozoa.)

and other articulated animals. It differs from Monocystis in
having the medullary protoplasm of the adult divided into two
sections, an anterior, the protomerite (pr.), and a posterior, the
deutomerite (deu.\ in which the nucleus is situated. Anteriorly

II

PHYLUM PROTOZOA

83

to the protomerite there is sometimes found, especially in young
individuals, a third division, the cpimerite (cp.), which may be
provided with hooks (B 1 ), serving to attach the parasite to the
epithelium of the intestine of its host, by becoming embedded in
the substance of one of the cells. As maturity is reached the
epimerite is thrown off (B 2 ), and the parasite then lies freely in the
cavity of the intestine.

The cysts of Gregarina (C) are often very complex and
provided with delicate ducts (tyd.) in the thickness of the wall,

3

FIG. 03. Gregarina Development from the sporozoite. 1, cells of the digestive epithelium
of the host ; S, nuclei of the same ; 8, spore ; It, spore discharging sporozoites ('>) leaving
residual mass (6) ; 7, sporozoites in the act of entering epithelial cells ; S, the same as
intracellular parasites ; 9- 12, different stages in the growth of the young Gregarines into the
lumen of the intestine ; 13, epirnerite ; Ik. protomerite ; 15, deutomerite. (After Lang.)

through which the spores escape. In Gregarina gigantea of the
Lobster, the young (sporozoite) is liberated from the spore in the
form of a non-nucleated amcebula (D 1 ), with one long and one
short pseudopod (D 2 ) ; this divides by the long pseudopod (psd. 2)
becoming separated off, and each product of fission, developing a
nucleus, passes into the adult (trophozoite) form (D 3 , D 4 .) In
other species of Gregarina the sporozoites do not divide, but each
develops directly into the trophozoite (Fig. 63).

ORDER 2. COCCIDIIDEA.

Coccidium (Figs. 64, 65) and allied genera are parasites in the interior ot
cells, both in Vertebrates and Invertebrates. They live in the cells of various

G 2

84

ZOOLOGY

SECT.

organs, most frequently in those of the epithelium of the digestive canal. They
never inhabit blood-corpuscles. A few are intra-nuclear parasites. Two
distinct modes of irmltiplication occur by schizogony, a kind of multiple fission,
and by sporogony, a process of spore -formation preceded by conjugation between
male and female cells. The trophozoite, or adult phase, as we may term it, of
the parasite, grows to a certain size within the cell without destroying its
vitality the nucleus merely being pushed on one side. So far, in fact, from
impairing the nutrition of the cell, the presence of the parasite seems, in some
cases, for a time, rather to stimulate it At a certain stage of growth schizogony
(Fig. 65, b e) takes place. The nucleus divides to form a number of nuclei.
These migrate towards the surface, and each becomes surrounded by protoplasm,
w T ith the result that a number of small cells are formed. Each of these gives
rise to a club-shaped merozoite. The merozoites, when they become free, are
active bodies, which are able to penetrate into the interior of other epithelial
cells and develop into trophozoites like those from which they were derived.
This multiplication may take place on such an extensive scale that the

1 Ei

imena

S.CoccIdiutn

FIG. 04. Coccidiidea. A, adult Eimer'ta (E) in enteric epithelial cell (c?>.) of mouse ;
B, encysted form ; C, encysted form, the protoplasm contracting to form a spore ; D, formation
of falciform young(/.) in interior of spore (*/>.); E, spore with falciform young; F, adult
encysted form of CocciiHtim from liver of rabbit ; G, division into spores ; H, cyst containing
ripe spores (?/>.)> each with a single falciform young ; I, single spore with falciform young (/).
(From Biitschli's Protozoa, after Leuckart and Eimer."\

epithelium may be partially or completely destroyed. It is only, apparently,
when such extensive damage has been done, or is threatened, that multiplication
by sporogony takes place the invasion of a new host being by this process
rendered probable, and the continuance of the race being thus provided for in
the event of the death of the host in which the epithelium has become destroyed.
In this process certain of the merozoites, instead of developing into trophozoites,
grow more slowly ((/), and become converted into either micro- or megagame-
tocytes. Each of the former (k, j) gives rise by division to a number of narrow
biflagellate microgametes or sperms. Each of the megagametocytes (e, /), after a
process of the nature of maturation, forms a single rounded megagamete (ovum).
When this becomes fertilised by the penetration into it of a single microgamete, the
resulting body (zytjote or oosperni) divides to form a varying number of cells
each enclosed in a resistant cyst (/). These give rise to spores with a firm, chitinous
spore-membrane, each containing two or more falciform young or xporozoifes (I).
The cyst destroys the cell as it grows, and thus becomes free in the cavity by
which the epithelium is lined. The spores may thus pass out to the exterior,
and, if taken into the digestive canal of a new host, may liberate the now
active sporozoites, which may penetrate into epithelial cells (a) to become the
trophozoites with which the cycle began.

II

PHYLUM PROTOZOA

85

In some of the Coccidiidea this life cycle is modified in various ways, as, for
example, by the omission of schizogoiiy the trophozoites in such a case
developing directly into gametocytes.

Fio. 65. Life-History of Coccidium schubergi. a. penetration of epithelium cell of host by
sporozoite ; b-c, stages of multiple fission (schizogoiiy) ; d, gametocyte ; e, f, formation of
megagamete (ovum) ; g, fertilisation ; h, j, formation of microgarnetes (sperms) ; k, develop-
ment of fertilised ovum into four spores ; I, formation of two sporozoites (falciform young) in
each spore. (From Calkins, after Schaudinn.)

ORDER 3. H/EMOSPORIDEA.

These are Sporozoa which in the trophozoite condition live as parasites in
the interior of the coloured blood-corpuscles of all classes of Vertebrates, but
are occasionally found in other cells. In Man and in some other mammals and

86

ZOOLOGY

SECT.

in certain birds it has been found that their presence is the cause of various
feverish affections. The various forms of malaria in man have been proved to
be due to the presence in the blood-corpuscles of the patient of parasites
belonging to this order. The malaria-parasites, the history of which has been
carefully worked out, pass through a life-cycle comparable to that of Coccidium
described above. In the trophozoite stage (Fig. 66, A G) they live as amoeboid

D

r
f

Flo. 66. Life-History of Malaria Parasites. A-G, parasite of quartan fever, showing
development of trophozoite in a blood-corpuscle and the formation of merozoites ; //,
gametocyte of the same ; I-M, parasite of tertian fever to the formation of the merozoites ;
If, gametocyte ; 0-T, creseentio gametocytes of Laverania ; P-S, formation of micro-
gametes or sperms ; U- W, maturation of megagamete or ovum ; X, fertilisation ; Y, zygote.
a, zygote enlarging in stomach of mosquito ; b-e, passing into the body-cavity ; /, g, develop-
ment of the contents into a mass of sporozoites ; /t, sporozoites passing into the salivary
glands. (From Calkin's Protozoa, after Ross and Fielding Ould.)

intracellular parasites in the interior of the coloured corpuscles of their host.
Here they multiply by schizogony the products (merozoites) entering other
corpuscles. Some of the merozoites when they become established in the interior
of the corpuscles develop into rounded or crescentic bodies which become the
gametocytes (H, N, 0, T). In order that the life-cycle may be completed, it is
necessary that the parasite at this stage should be taken into the interior of a

II

PHYLUM PROTOZOA

87

second or intermediate host. In the case of the parasite of human malaria the
intermediate host is a mosquito of the genus Anopheles. On the mosquito
drawing up a drop of the blood of a malaria patient, all stages of the parasite
that occur in it are destroyed by the digestive juices of the insect with the
exception of the gametocytes ; these survive and form gametes in the
stomach of the mosquito. Each male gametocyte gives rise to a number of
slender filamentous microgametes (sperms, P, S) and each female gametocyte
forms a single megagamete (ovum). After maturation (U- -W) the megagamete
is fertilised (x) by one of the actively-moving microgamates, the result being the
formation of an active spindle-shaped ookinete. This perforates the stomach
wall and comes to rest in the subjacent tissues. It then becomese encysted
and increases greatly in size, bulging out into the body-cavity (b e). The
contents of the cyst eventually become divided up (f, g) into a large number of
long, narrow sporozoites. When the cyst becomes ruptured into the body-
cavity, these find their way to the salivary glands (h), and thence they may
readily be transferred to the blood-system of a human being when the mosquito
bites. Penetrating into the interior of coloured corpuscles they reach the
trophozoite condition.

The Hasmogregarines, which may most conveniently be referred to here, are
Sporozoa which live, like the malaria parasites, in the coloured blood-corpuscles
of all classes of Vertebrates ; but which in the mature or trophozoite condition
are not amoeboid, retaining the Gregarina-like form, and are therefore to be
regarded as belonging to the Gregarinida.

ORDER 4. MYXOSPORIDEA.

This group includes a small number of genera which are ainceboid
in the trophozoite phase, and which reproduce continuously by
spore-formation during that phase (Fig. 67, A). Many nuclei are present

FIG. 67. A, Myxidium lieberkiihnii, amoeboid phase; B, IWtyxobolus miilleri,

spore with discharged nernatocysts (ntc.); C, spores (psorosperms) of a Myxosporidian ;
ntc. nematocysts. (From Biitschli's Protozoa.)

in the amoeboid body, which may be of comparatively large size. The
spores (B) produced within the protoplasm of the trophozoite are provided
each with one or more bodies like the nematocysts of zoophytes and jelly-fish
[See Section IV]. Myxosporidea occur as parasites mainly of fishes
and amphibians, but very many occur in various groups of Invertebrates.
"Pebrine," the destructive silk- worm disease, is due to the presence of a
Sporozoan belonging to this order. A good example of the order is Myxidium,
found in the urinary bladder of the pike.

88 ZOOLOGY SECT.

ORDER 5. SARCOCYSTIDEA.

The best known form of this order is Sarcocystis (Fig. 68), which occurs in
the flesh of mammals, each parasite having the form of a long spindle embedded

FIG. CS. Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From
Butscmi's Protozoa, after Rainey.)

in a striped muscular fibre. They are often known as Rainey's or Mieschtr's
corpuscles. The protoplasm divides into spores from which falciform young are
liberated.

CLASS V. INFUSORIA.

1. EXAMPLE OF THE CLASS Paramcecinm caudatum.

Structure. Paramcecium, the "slipper-animalcule," is tolerably
common in stagnant ponds, organic infusions, &c. The body
(Fig. 69) is somewhat cylindrical, about J mm. in length, rounded
at the anterior and bluntly pointed at the posterior end. On the
ventral face is a large oblique depression, the luccal groom (hue. gr.\
leading into a short gullet (gul.\ which, as in Euglena, ends in the
soft internal protoplasm.

The body is covered with small cilia arranged in longitudinal
rows and continued down the gullet. The protoplasm is very
clearly differentiated into a comparatively dense cortex (cort.) and
a semi-fluid medulla (med.), and is covered externally by a thin
pellicle or cuticle, (cu.) which is continued down the gullet. The
cilia are continuous with the pellicle.

In the cortex are found two nuclei, the relations of which are
very characteristic. One, distinguished as the meganucleus (nu.),
is a large ovoid body staining evenly with aniline dyes, which,
when it divides, does so directly by a simple process of constriction.
The other, called the micronucleus (pa. nu.\ is a very small body
closely applied to the meganucleus; when it divides it goes
through the complex series of stages characteristic of mitosis
(p. 16).

The contractile vacuoles (c. vac.) are two in number, and are very
readily made out. Each is connected with a series of radiating
spindle-shaped cavities in the protoplasm which serve as feeders
to it. After the contraction of the vacuole these cavities are seen
gradually to fill, apparently receiving water from the surrounding

II

PHYLUM PROTOZOA

89

protoplasm : they then contract, discharging the water into the
vacuole, the latter rapidly enlarging while they disappear from

B

rac

C.TUC

me.

FIG. 69. Paramoecium caudatum. A, the living animal from the ventral aspect ; B, the
same in optical section : the arrow shows the covirse taken by food-particles ; C, a specimen
which has discharged its trichocysts ; D, diagram of binary fission ; buc. (jr. buccal groove ;
corf, cortex ; cu. cuticle ; c. vac. contractile vacuole ; /. vac. food vacuole j (iul. gullet ; med.
medulla; nu. meganucleus ; pa. mi. micronucleus ; trch. trichocysts. (From Parker's Biology. )

view ; finally the vacuole contracts and discharges its contents
externally.

The cortex contains minute radially arranged sacs called
trichocysts (trch.). When the animal is irritated, more or fewer of

90

ZOOLOGY

SECT.

these suddenly discharge a long delicate thread, which, in the
condition of rest, is very probably coiled up within the sac. In a
specimen killed with iodine or osmic acid the threads can fre-
quently be seen projecting in all directions from the surface (6').

Food, in the form of small living organisms, is taken in by
means of the current caused by the cilia of the buccal groove. The
food-particles, enclosed in a globule of water or " food-vacuole "
(/. vac.), circulate through the protoplasm, when the soluble parts
are gradually digested and assimilated. Starchy and fatty matters,
as well as proteids, are available as food, the digestive powers of
Paramcecium being thus considerably in advance of those of Amoeba.
Effete matters are egested at a definite anal spot posterior to the
mouth, where the cortex and cuticle are less resistent than else-
where. The whole feeding process can readily be observed in this
and other Infusoria by placing in the water some insoluble colour-
ing matter, such as carmine or indigo, in a fine state of division.

Reproduction. Multiplication takes place by transverse
fission (D), the division of the body being preceded by that of both
nuclei. As already mentioned, the meganucleus divides directly,
the micronucleus indirectly.

It has been proved, however, that multiplication by binary
fission cannot go on indefinitely ; but that after it has been repeated

Mg.nu

mi.nu

mJ.nu

FIG. 70. Paramoecium caudatum, stages in conjugation, gul. gullet ; mg. nu. meganucleus ;
mi. nu. micronucleus ; Mg. nu. reconstructed megauucleus ; Mi. nu. reconstructed micro-
nucleus. (From Parker's Biology, after Hertwig.)

a certain number of times it is interrupted by conjugation. In
this very remarkable and characteristic process two Paramcecia

ii PHYLUM PROTOZOA 91

become applied by their ventral faces (Fig. 70, A), but do not fuse.
The meganucleus (mg. nu.) of each breaks up into small masses,
which disappear, being apparently absorbed into the protoplasm.
At the same time the micronucleus (mi. nu.) of each divides,
each product of division immediately dividing again, so that each
gamete or conjugating body is provided with four micronuclei (B).
Two of these (mi. nu.' , mi. nu.") disappear; of the remaining two
one is distinguished as the stationary pronucleus, the other as the
active pronucleus. The active pronucleus of each Infusor now
passes into the body of the other and fuses with its stationary
pronucleus (D), each individual thus coming to possess a single
nuclear body derived in equal proportions from the two conjugat-
ing cells (E). The animalcules then separate from one another,
and the nucleus of each divides and gives rise to the permanent
mega- (G, Mg, nu.) and micronuclei (Mi. nu.).

2. CLASSIFICATION AND GENERAL ORGANISATION.

In the majority of the Infusoria the body is ciliated throughout
life, but in certain forms cilia are present only in the immature
condition, the adult being provided with peculiar organs of
prehension or tentacles. We thus get two orders, viz. :-

ORDER 1. CILIATA.
Infusoria provided with cilia throughout life.

ORDER 2.- -TENTACULIFERA.

Infusoria possessing cilia in the young condition, tentacles in
the adult.

Systematic position of the Example.

Paramoecium aurelia is one of several species of the genus
Paramcecium, belonging the family Parmcecidm, of the sub-order
Trichostomata, and order Ciliata. The presence of cilia in the
adult condition places it among the Ciliata : the presence of a
permanently open mouth into which food particles are swept by
the movement of the cilia, among the Trichostomata. The Para-
moecidse are free-swimming, asymmetrical, uniformly ciliated, with
a ventrally placed mouth. P. caudatum is about \--\ mm. in
length, its length about four times its breadth, rounded in front,
and bluntly pointed behind, and a single micronucleus is present.

ORDER 1. CILIATA.

This order presents a wider range of variations some of them
of a truly extraordinary character than any other group of
Protozoa.

92 ZOOLOGY SECT.

The form of the body is very varied : it may be ovoid (Fig.
71, 1), kidney-shaped (#), trumpet-shaped (#), vase or cup-shaped
(4, 9) ; produced into a long, flexible, neck-like process (5), or into
large paired lappets (6') ; flattened from above downwards, or
elongated and divided into segments reminding us of those of
a segmented worm (8}.

Most species are free-swimming, but some are attached to
weeds, stones, &c., by a stalk. This may be a purely cuticular
structure (9), or may contain a prolongation of the cortex in the
form of a delicate contractile axial fibre (Figs. 73 and 74, ax. /.),
which serves to retract the Infusor, its contraction causing the
stalk to coil up into a close spiral.

The arrangement of the cilia is also subject to great varia-
tion, and presents four chief types. In the holotrichous type, of
which Paramcecium is an example, the cilia are all small, equal-
sized or nearly so, and arranged in longitudinal rows (Fig. 69, Fig.
71, 1). The second or JieterotrwJwus type is seen in its simplest
form in Nyctotherus (Fig. 71, 2\ in which the left side of the
peristome is bordered by a row of specially large adored cilia, the
rest of the body being covered with small cilia. In Stentor (3)
the peristome is situated on the broad distal end of the trumpet-
shaped body, and the adoral band of cilia takes a spiral course.
This leads us to the peritrichous type of ciliation : in Vorticella
(Fig. 73) the vase-shaped body is, for the most part, quite bare of
cilia, but around the thickened edge of the peristome passes one
limb of a spiral band of large cilia united at their bases, the other
limb being continued round a raised lid-like structure, or disc, into
which the distal region is produced. This arrangement of cilia
reaches its greatest complexity in Episiylis plicatilis (Fig. 71, 9),
in which the ciliary spiral makes no fewer than four turns.

But it is in the hypotrichous type that the most extraordinary
modifications are found. The flattened body bears on its dorsal
surface mere vestiges of cilia in the form of very minute processes
of the cuticle, while on the ventral surface the cilia take the form
of large hooks, fans, bristles, and plates with fringed ends (Fig. 71,
7). The hooks and plates do not vibrate rhythmically like
ordinary cilia, but are moved as a whole at the will of the animal,
thus acting as legs. The hypotrichous Ciliata, in fact, in addition
to swimming freely in the water, creep over the surface of weeds,
&c., very much after the manner of Woodlice. One of the most
extraordinary forms in this group is Diophrys (7), the size and
arrangement of its polymorphic cilia giving it a very grotesque
appearance. In another genus (10) the distal end of the flask-
shaped body bears a circlet of large fringed cilia, giving the animal
the appearance of a Rotifer (vide Section VII.).

In addition to cilia, many genera possess delicate sheets of
protoplasm or undulating membranes in connection with the

n PHYLUM PROTOZOA 93

peristome. They contract so as to produce a wave-like movement
which aids in the ingestion of food. In some cases (Fig. 71, 11)
the undulating membrane (n, nib.) is a very large and obvious
structure.

Certain peculiar forms have yet to be mentioned. Multicilia (Fig.
71, 12) has an irregular body of varying form, and bears a small
number of very long flagellum-like cilia. Another genus in which
the cilia approach to flagella is LnpTiomonas (13), the ovoid body of
which bears a tuft of close-set cilia at its anterior end. Actino-
bolus (14) is remarkable for the possession, in addition to cilia, of
long retractile tentacles used for attachment. In Didinium ( 15)
the barrel-shaped body is encircled by two hoops of cilia.

As we have seen, the meganucleus in Paramcecium is ovoid : in
other genera it may be elongated and band-like (3, m-g. nu.), horse-
shoe-shaped (9), very long and constricted at intervals so as to
look like a string of beads (16), or much convoluted and branched
(17). In some genera the meganucleus undergoes repeated
divison, forming at last a very great number of small bodies only
discoverable by staining : this process of fragmentation of the nucleus
may proceed so far that the protoplasm of a stained specimen has
the appearance of being strewn with granules of chromatin. The
discovery of f this phenomenon has tended to throw doubt on the
reported total absence of a nucleus in some Rhizopods.

In nearly all species one or more micronuclei are present, the
number sometimes reaching nearly thirty. In Opalina (Fig. 75)
numerous nuclear bodies (nu.) are present, some of which on
account of their mitotic mode of division are to be regarded as
micronuclei, while the rest are meganuclei.

In Vorticella and other peritrichous genera there is a single
contractile vacuole (Fig. 73, c. vac), which, like that of Euglena,
opens through the intermediation of a reservoir into the vestibule.
In the remaining Ciliata there may be one, two, or many some-
times a hundred contractile vacuoles. They may be scattered
all over the cortex (Fig. 71, 18), or arranged in one or two rows
(8). The star-like arrangement of radiating canals, described in
Paramcecium, occurs in several genera : or there may be two long
canals, or the number of these channels in the protoplasm may
reach thirty (19, c). In some instances the protoplasm is hollowed
out by numerous non-contractile vacuoles (18, vac.) so as to
have a reticulate appearance, reminding us of the extra-capsular
protoplasm of Radiolaria.

Trichocysts, like those of Paramcecium, are found in many
holotrichous forms, but arc rarely present in the other subdivisions
of the order. In the peritrichous Epistylis umbellaria, however,
there are found numerous minute capsules (Fig. 71, 9, ntc.)
arranged in pairs, each containing a coiled thread. They are

?nth

mg.nu M,<

WS^$' ''-^ \ &

fey//. ..'- X

C-

vcw

A.-.*'. /-d V^i- ;.:),,-

*

2.Nycrorherus

S.Lacrymaria

10.TinMnnidium

3.E pisrylis

12.MulMcilia 13-Lophomonas

H.Cyclidium

H.Acfinobolus

. fore

,$sZSZ&~ law

'rmi.ntt \l\fvac

!8.Tracheliu$ mophryoglena
IdCondylos^ma r

17.0pa!inopsis

FIG. 71. Various forms of Ciliata. Pa shows part of a colony, 1> a single zooid, and
c a couple of nematocysts ; n. anus ; f. (in Ls) cuticle ; c. (in 1!) excretory canals ; c. rac.
contractile vacuole; ,/". vac. food vacuole; .'/. gullet; /////- nu, meganucleus ; -ini. nu. micro-
nucleus; intJt. mouth; nu. nucleus ; utc. nematocyst ; />. (in 15) a J'ftrama-civ.iii seized by
Didiiitiiuii', f. tentacle; i>. nth. undulating membrane; rac. non-contractile vacuole; rst.
vestibule. (From Biitschli's Protozoa, after various authors.)

II

PHYLUM PROTOZOA

95

obviously structures of the same character as trichocysts, and
their resemblance to the ncmatocysts so characteristic of Ccelenterata
(vide Section IV.) is singularly close.

Digestive Apparatus. --Man} parasitic forms (Fig. "71, 8, 17 ;
Fig. 75) have no mouth or gullet, and are nourished by absorption
of the digested food in the intestine of their host. The simplest
condition of the ingestive apparatus is found in Prorodon (Fig.
71, 1) and its allies, in which the mouth (mth.) is at one pole
of the ovoid body, and is closed except during the ingestion of
food, and the gullet (g.) is a short, straight tube. Such forms,
on account of the symmetrical disposition of their organs and the
want of differentiation of their cilia they are all holotrichous-
may be considered as the lowest or least specialised of the Ciliata.

*fi %

LDictyocysta

S.Thuricola
2. Pyxicola

5. Srichol-richa

FIG. 7:2. Various forms of Ciliata. In 1 the shell alone is shown ; m. contractile fibre ; op.
operculum. (From Butschli's Protozoa, after various authors.)

From them there is a fairly complete gradation to genera, like
Paramoecium, having the permanently open mouth on the left side
of the ventral surface, at the end of a well-marked buccal grove
or peristome. Vorticella (Fig. 73) and its allies are peculiar in
having the edge of the peristome (per.) thickened so as to form a
projecting rim, and in the development of an elevated disc (d.) from
the area thus enclosed : the mouth (mth.) lies between the peri-
stome and the disc, and between it and the gullet proper (gull.) is
interposed a section of the ingestive tube called the vestibule
into which the reservoir opens, and which contains the anal
spot. In Nyctotherus (Fig. 71, #) and some other genera there is,
instead of the temporary anal spot described in Paramcecium, a
distinct anal aperture (a.).

96

ZOOLOGY

SECT.

Most of the Ciliata are naked, having no shell or other form of
skeleton ; but in a few forms the body is provided with a shell or
lorica, formed of a chitinoid material, and reminding us of the

pen

FIG. 73. Vorticella. A, B, living specimens in different positions , C, optical section ; D 1 , D-,
diagrams illustrating coiling of stalk ; El, E-, two stages in binary fission; E3, free zooid ;
FI, F-, division into mega- and mierozooids ; G 1 , G 2 , conjugation ; H 1 , multiple fission of
encysted form ; H-, H : *, development of spores ; ax. f. axial fibre ; cort. cortex ; cu. cuticle ;
c. vac. contractile vacuole ; d. disc ; gull, gullet ; m. microzooid ; mth. mouth ; nil. mega-
nucleus ; per. peristome. (From Parker's Biology.)

similar structure found in so many of the Mastigophora. Some
(Fig. 7J, 4) have bell-like shells, variously ornamented, and in
others (Fig. 72, 1) the similarly shaped shell is perforated and
resembles the skeleton of some of the Radiolaria. A chitinoid
plate or operculum (Fig. 72, 2, op.) may be fixed to the edge of the
peristome, and, when the animal is retracted in its case accurately
closes the mouth of the latter, or a similar operculum (J) is

II

PHYLUM PROTOZOA

97

attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (w.), which acts as a retractor muscle.

Compound forms or colonies are common among the Peritricha,
rare in the other subdivisions. Many peritrichous forms occur as
branched, tree-like colonies, often of great complexity (Fig. 71, 9a;
Fig. 74). The stem of these may be a purely cuticular structure
and non-contractile (Fig. 71, 9, I), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 73, ax.f.}. In Ophridium
(Fig, 72, 4) the colony is an irregular mass, sometimes 3-4 cm. in
diameter, consisting of a gelatinous substance in which a delicate,
branching stem is embedded, each branch terminating in a zooid.
Some genera (Fig. 72, 5) secrete a hollow, brown, gelatinous tube,
branched dichotomously ; the end of each branch is the habitation
of one of the zooids.

Reproduction. Transverse fission is the universal method of
reproduction, the entire process taking from half an hour to two

F 2

--'"*

-

B

FIG. 74._ Zoothammum arbuscula. A, entire colony; B, the same, natural size ; C, the
same, retracted ; D, nutritive zooid ; E, reproductive zooid ; Fl, F 2 , development of reproduc-
tive zooid ; ax.f. axial fibre ; c. rac. contractile vacuole ; nu. nucleus ; n.z. nutritive zooid ;
r.z. reproductive zooid. (From Parker's Biology, after Saville Kent.)

hours in different species. In Vorticella (Fig. 73, E) and other
Peritricha the plane of division is parallel to the long axis of the
bell-shaped body, but as the distal surface probably corresponds
with the dorsal surface of such forms as Paramoecium, fission
is really transverse in this case also. In such simple Peritricha
as Vorticella division proceeds until two zooids are produced on
a single stalk ; one of the two then acquires a second circlet
of cilia near its proximal end, becomes detached (E 3 ), and, after
leading a free-swimming life for a time, settles down and develops
a stalk : in this way the dispersal of the non-locomotive species is
ensured. In many species of Zoothamnium (Fig. 74) the zooids
VOL. I H

98

ZOOLOGY

SECT.

are dimorphic : the ordinary bell-shaped forms (n.z.) divide in
the usual way, but as they remain attached, the process results only
in the increased complexity of the colony, not in the development
of a new one. The larger zooids (r. 2.) are globular and mouthless :
they become detached, swim off, and, after a short free existence,
settle down, develop a stalk (F), divide, and so form a new colony.
In Vorticella multiplication by ludding also occurs: a small
process is given off from one side (Fig. 73, F), develops a basal
circlet of cilia, and swims off as a microzooid, the parent individual

FIG. 75. Opalina ranarum. A, living specimen; B, stained specimen showing nuclei; C,
stages in nuclear division ; D F, stages in fission ; G, final product of fission ; H, encysted
form ; I, young form liberated from cyst ; K, the same after multiplication of the nucleus
has begun ; nu. nucleus. (From Parker's Biology, after Saville Kent and Zeller.)

or megazooid being left attached to the stalk. Obviously this
process is simply a modification of binary fission, the products of
division being of very different dimensions instead of equal-sized as
is the more usual case.

Spore-formation take place in Colpoda. The Infusor becomes
encysted, and divides into two, four, and finally eight masses, each
of which, becoming surrounded by a special investment, becomes
a spore. A somewhat similar process has been described in
Vorticella (Fig. 73, H} and others.

A peculiar kind of spore-formation, specially adapted to the
requirements of an internal parasite, takes place in Opalina

ii PHYLUM PROTOZOA 99

(Fig. 75), a parasite in the intestine of the Frog. Binary fission
takes place (D, E, F), and is repeated again and again so rapidly
that the daughter-cells are unable to grow to the adult size before
the next division. The final results of the process are small bodies
(G), each with only two or three nuclei instead of the large number
characteristic of the adult. These become encysted (H), and in
this passive condition are passed out of the Frog's intestine with
its faeces, frequently being deposited on water-weeds. All this
takes place during the Frog's breeding season : the tadpoles or Frog-
larvae feed upon the water-plants, and in doing so frequently take
in the spores or encysted Opalinoe along with their food. When
this occurs the cyst is dissolved by the digestive juices of the host,
and the protoplasm of the spore is set free as a rounded body
with a single nucleus (I), which rapidly grows into an adult
Opalina (K).

Conjugation, in the form of a temporary union accompanied by
interchange of micronuclei, has been described in Paramoecium
(p. 90), and takes place in many Ciliata. In others (e.g. Stylonychia
histrio) there is a complete union of the two gametes. In
Vorticella union is also permanent, and takes place, not between
two ordinary forms, but between one of the ordinary stalked
individuals, or megagametes, and a free-swimming, small form, or
microgamete, produced, as described above, by budding (G 1 , G' 2 ).
The essence of conjugation is the reception of nuclear material
derived from another individual : its effect appears to be a renewal
of vitality, usually manifesting itself in increased activity in
multiplication by fission.

ORDER 2. TENTACULIFERA.

Judged from the adult structure alone, the members of this
order would certainly be placed in a separate class of the Protozoa :
it is only in virtue of the facts of development that they are
united in a single class with the Ciliata.

The body may be globular (Fig. 76, la), ovoid (/b), or cup-
shaped (2a), but presents nothing like the variety of form met
with among the Ciliata. The distinguishing feature of the group
is furnished by the tentacles which are always present in greater
or less number, and which, in some cases at least, are the most
highly differentiated organs found in the whole group of Protozoa.
The characters of the tentacles vary strikingly in the different
genera.

In the common forms Acineta (2), and Podophrya (1\ the ten-
tacles spring either from the whole surface, or in groups from the
angles of the somewhat triangular body. Each tentacle is an elon-
gated cylindrical structure (7c), capable of protrusion and retrac-
tion, and having its distal end expanded into a sucker. It is, more-
over, practically tubular, the axial region consisting of a semi-fluid

H 2

100

ZOOLOGY

SECT.

protoplasm, while the outer portion is tolerably firm and resistant.
When partially retracted, a spiral ridge is sometimes observable

l.Podophrya

4.Dendrocometes c r- ,. 6. S f>haer obhrya

3.Rhynchero 5. Ephelota

! C VCU)

&<M/M.

, , \JBnfm\ H^W

wii-

!

T.Obhryodendron

c.wto

8.Ef>helora

9, Dendrosomo

FIG. 76. Various forms of Tentaculifera. la and b, two species of Fotlnphn/a ; c, a
tentacle much enlarged ; 2a, Acineta jolyi ; 2li, A. tuberosa ; in G the animal has captured
several small Ciliata ; 8a, a specimen multiplying by budding ; 6'fc, a free ciliated bud ; 9a, the
entire colony ; 9b, a portion of the stem ; He, a liberated bud ; a, organism captured as food ;
6.c. brood-cavity ; bd. bud ; c. vac contractile vacuole ; /. lorica ; mg. nu. meganucleus ;
mi. nu. micronucleus ; t. tentacle. (After Biitschli and Saville Kent.)

around the tentacle : this may indicate the presence of a band of
specially contractile protoplasm, resembling the axial fibre in the

II

PHYLUM PROTOZOA 101

stalk of Vorticdla. Infusors and other organisms are caught by
the tentacles (4, 0), the cuticle of the prey is pierced or dissolved
where the sucker touches it, and the semi-fluid protoplasm can
then be seen flowing down the tentacle into the body of the
captor, A single tentacle only may be present (3), or the tentacle
may be branched (4), the extremity of each branch being suc-
torial. In some forms there are no terminal suckers (5), and the
tentacles are waved about to catch the prey instead of standing
out stiffly as in Acineta. In other cases there are one or more
long, striated tentacles with tufted ends (7).

The nucleus may be ovoid (/a), horseshoe-shaped, or branched
(8, 9) : in many cases a micronucleus (1 a, mi. nu.) has been found
and it probably occurs in all. There are one or more contractile
vacuoles (c. vac.).

Some genera are naked (1) : others form a stalked shell or
lorica (a) like that met with in many of the Mastigophora.

The only colonial form is the wonderful Dendrosoma (9), in
which the entire colony attains a length of about 2 mm., and bears
an extraordinary resemblance to a zoophyte (vide Sect. IV.). It
consists of a creeping stem from which vertical branches spring,
and the various ramifications of these are terminated in Podo-
phrya-like zooids with suctorial tentacles. The nucleus is very
remarkable, extending as a branched axis throughout the colony
(b, nu.). Micron uclei of the ordinary character are present
as well.

Reproduction by Unary fission takes place in many species.
In Ephclota gemmipara (S) a peculiar process of budding occurs :
the distal end of the organism grows out into a number of
projections or buds, into which branches of the nucleus extend.
These become detached, acquire cilia on one surface, and swim
off (b). After a short active existence tentacles appear and the
cilia are lost. In this case budding is external, but in Acineta
tnberosa (2b) the buds become sunk in a depression, which is finally
converted into a closed brood-cavity (b.c.) : in this the buds take on
the form of ciliated embryos, which finally escape from the parent.
In Dendrosoma the common stem of the colony produces internal
buds (b, Id.).

Further Remarks on the Protozoa.

The majority of the Protozoa are aquatic, the phylum being
equally well represented in fresh and salt water. They occur
practically at all heights and depths, from 8,OOQ v to 10,000 feet
abovejsea-level, to a depth of from 2,000 to 3,000 fathoms. Some
forms, such as species of Amoeba and Groin ia, live in damp sand
and moss, and may therefore be almost considered as terrestrial
organisms. In accordance with their small size and the readiness
with which they are transported from place to place a large pro-

102 ZOOLOGY SECT.

portion of genera and even of species are universally distributed,
being found in all parts of the world where the microscopic fauna
has been investigated.

Numerous parasitic forms are known. Besides the entire class
of Sporozoa, species of Rhizopoda, Mastigophora, and of Infusoria
occur both as internal and external parasites. Species of Amoeba
are common in the intestines of the higher animals, and one
species has been found in connection with a cancerous disease in
Sheep. A ciliate Infusor, Icldhyophthirius, is found in the skin of
freshwater Fishes, where it gives rise to inflammation and death.

Many instances have been met with in our survey of the
Phylum of compound or colonial forms, the existence of which
seems at first sight to upset our definition of the Protozoa as
unicellular animals. But in all such cases the zooids or unicellular
individuals of the colony exhibit a quasi-independence, each, as a
rule, feeding, multiplying, and performing all other essential
animal functions independently of the rest, so that the only
division of labour is in such forms as Zoothamnium and Volvox,
in which certain zooids are incapable of feeding, and are set apart
for reproduction. In all animals above Protozoa, on the other
hand, the body is formed of an aggregate of cells, some of which
perform one function, some another, and none of which exhibit
the independent life of the zooid of a protozoan colony. It cannot,
however, be said that there is any absolute distinction between a
colony of unicellular zooids and a single multicellular individual :
Proterospongia and Volvox approach very near to the border-land
from the protozoan side, and a similar approach in the other
direction is made by certain animals known as Mesozoa, which will
be discussed hereafter (Sect. IV.). Moreover, the Mycetozoa, the
plasmodia of which are formed by the fusion of Amcebulae, the
nuclei of the latter remaining distinct and multiplying, are rather
non-cellular than unicellular. This point will also be referred to
at the conclusion of the section on Sponges (Sect. III.).

In each division of the Protozoa we have found comparatively
low or generalised forms side by side with comparatively high or
specialised genera. For instance, among the Rhizopoda, there
can be no hesitation in placing the Lobosa, and especially Prota-
moeba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 52. 2 and 8) and Heteromita are obviously the
lowest forms, Noctiluca and the Dinoflagellata the highest. But
whether the Rhizopoda, as a whole, are higher or lower than the
Flagellata, is a question by no means easy to answer. A flagellum
certainly seems to be a more specialised cell-organ than a
pseudopod, and some of the Mastigophora rise above the highest
of the Rhizopoda in the possession of a firm cortex and cuticle,

II

PHYLUM PROTOZOA

103

and the consequent assumption of a more definite form of body
than can possibly be produced by the flowing protoplasm of a
Foraminifer or a Radiolarian. On the other hand, the nucleus
of the Radiolaria is a far more complex structure than that of
the Mast rgophora:. and in Foraminifera, Radiolaria, and Heliozoa
the organism frequently begins life as a flagellula, a fact which,
on the hypothesis that the development of the individual recapitu-
lates that of the race, appears to indicate that these orders of
Rhizopoda are a more recently developed stock than at any rate
the lower Flagellata. These circumstances, and the fact that
Mastigamoeba might equally well be classed as a lobose Rhizopod
with a flagellum or as a Flagellate with pseudopods, seem to
indicate that the actual starting-point of the Protozoa was a form

Radiolaria
Foraminifera

Lobosa

Mycetozoa

Dinoflagellata

Cystoflagellata

Heliozoa Choano .
Flagellata

Tentaculifera

Ciliata

Flagellata

Sporozoa

FIG. 77. 'Diagram showing the mutual relationships of the chief groups of Protozoa.

capable of assuming either the amoeboid or the flagellate phase.
From such a starting-point the Lobosa, Foraminifera, Heliozoa,
Radiolaria, and Flagellata diverge in different directions, the first
four keeping mainly to the amoeboid form, but assuming the
flagellate form in the young condition, in the case of Foraminifera,
Heliozoa, and Radiolaria.

The Choanoflagellata, Dinoflagellata, and Cystoflagellata are
obviously special developments of the Flagellate type along
diverging lines.

As to the Ciliata, Midticilia and Lopliomcnas (Fig. 71,1% and 13)
appear to indicate the derivation of the order from the Flagellate
type, since their cilia are long and flageilum-like ; but the evidence
is not strong and no other is at hand. The derivation of the Tenta-
culifera from a ciliate type appears to be clear. The Tentaculifera
and the hypotrichous Ciliata are undoubtedly the highest develop-

104 ZOOLOGY sECi n

ment of the Protozoan series, since they show a degree of
differentiation attained nowhere else by a single cell.

The Mycetozoa appear to have been derived from the common
amoeboid-flagellate stock, since they are all predominantly amoe-
boid in the adult condition, flagellate when young. The Sporozoa
probably had a similar origin, but the characters of this class have
evidently been profoundly modified in accordance with their
parasitic mode of life.

The diagram on the previous page is an attempt to express
these relationships in a graphic form.

SECTION III
PHYLUM AND CLASS PORIFERA [PARAZOA]

The microscopic animals described in the preceding section
are, as already repeatedly pointed out, characterised by their
unicellular character, and in this respect stand in contrast to the
remainder of the animal kingdom. The animal kingdom is thus
capable of division into two great subdivisions, the Protozoa or uni-
cellular animals, and the Mctazoa or multicellular forms the latter
comprising all the groups that remain to be dealt with. In the
earliest stage of their existence all the multicellular animals or
Metazoa are, as already pointed out (p. 19), in a unicellular
condition, originating in a single cell, the fertilised ovum or
oosperm. By the process of segmentation or yolk-division the
unicellular oosperm becomes converted in all the Metazoa
into a mass of cells from which the body of the adult animal is
eventually built up. Of the Metazoa, the group which approxi-
mates most closely to the Protozoa is that now to be dealt vvith-
the Porifcra or Sponges. With all the other multicellular groups
the Sponges are so strongly in contrast that the Metazoa may
be regarded as falling into two main divisions the Porifera or
Parazoa, on the one hand, and all the rest of the Metazoa, grouped
together as Enterozoa, on the other.

1. EXAMPLE OF THE CLASS Si/con gdatinosum.

General External Appearance and Gross Structure.

Sycon gdatinosum^ one of the Calcareous Sponges, has the form of a
tuft, one to three inches long, of branching cylinders (Fig. 78),all con-
nected together at the base, where it is attached to the surface of a
rock or other solid body submerged in the sea. It is flexible, though
of tolerably firm consistency ; in colour it presents various shades
of gray or light brown. To the naked eye the surface appears
smooth, but when examined under the lens it is found to exhibit
a pattern of considerable regularity, formed by the presence of

1 This species is an inhabitant of southern seas. In all essential respects the
account of it given above will apply to S. ciliatum, a common European species
which differs chiefly in the absence of the pore-membranes,

J05

106

ZOOLOGY

SECT.

innumerable elevations of a polygonal shape, which cover the whole
surface and are separated off' from
one another by a system of depressed
lines. In these depressions between
the elevations are to be detected, under
the microscope, groups of minute
pores the ostia or inhalant pores.
At the free end of each of the cylin-
drical branches is a small but distinct
opening, surrounded by what appears
like a delicate fringe. When the
branches are bisected longitudinally
(Fig. 79), it is found that the terminal
openings (o) lead into narrow passages,
wide enough to admit a stout pin,
running through the axes of the
cylinders ; and the passages in the interior

FIG. 78. Sycon gelatinpsum.

Entire sponge, consisting of a
group of branching cylinders
(natural size).

FIG. 7tf. -Sycon gelatinosum.-A portion slightly

magnified; one cylinder (that to the right) bisected
longitudmally to show the central paragastric cavity
opening on the exterior by the osculum, and the
position of the incurrent and radial canals ; the
former indicated by the black bands, the latter,
dottediip. marks the position of three of the grot ips
of inhalant pores at the outer ends of the incurrent

canals ; o. oscuium.

of the various
branches join where the
branches join the pas-
sages thus forming a
communicating system.
On the wall of the
passages are numerous
tine apertures which re-
quire a strong lens for
their detection. The
larger apertures at the
ends of the branches
are the oscula of the
sponge, the passages the
paragastric cavities. If
a living Sycon is placed
in sea-water with which
has been mixed some
carmine powder, it will
be noticed that the
minute particles of the
carmine seem to be at-
tracted towards the sur-
face of the sponge, and
will often be seen to

P 8 " 88 intO its Substance

through the minute in-

, -,

halant pOrCS Or OStia

i J J

already mentioned as

nofnrrinfy in crrnnrm KP-
o 6 1<J1 4 X

tweeii the elevations on

Ill

PHYLUM AND CLASS PORIFERA

107

the outer surface. This would appear to be due to the passage of
a current of water into the interior of the sponge through these
minute openings dotted over the surface ; and the movement of
the floating particles shows that a current is at the same time
flowing out of each of the oscula. A constant circulation of
water would thus be seen to be carried on currents moved
by some invisible agency flowing through the walls of the sponge
to the central paragastric cavities, and passing out again by
the oscula.

If a portion of the S} 7 con is firmly squeezed, there will be
pressed out at first sea-water, and then, when greater pressure is

R

FIG. 80. Sycon gelatinosum. Section through the wall of a cylinder taken at right angles
to the long axes of the canals, highly magnified ; co, collencytes ; 1C, incurrent canals ;
ov. young ova ; R, radial canals ; sp. triradiate spicules.

exerted, a quantity of gelatinous-looking matter, which, on being
examined microscopically, proves to be partly composed of a
protoplasmic material consisting of innumerable, usually more or
less broken cells with their nuclei, and partly of a non-protoplasmic,
jelly-like substance. When this is all removed there remains
behind a toughish felt-like material, which maintains more or less
completely the original shape of the sponge. This is the skeleton
or supporting framework. A drop of acid causes it to dissolve
with effervescence, showing that it consists of carbonate of
lime. When some of it is teased out and examined under the
microscope, it proves to consist of innumerable, slender, mostly
three-rayed microscopic bodies (Figs. 80 and 81 , sp) of a clear
glassy appearance. These are the calcareous spicules which form

108

ZOOLOGY

FIG. 81. Sycon grelatinosum Transverse section
through the wall of a cylinder (parallel with the
course of the canals), showing one incurrent (1C),
and one radial (R) canal throughout their length ;
xp. triradiate spicules ; sp'. oxeote spicules of dermal
cortex (rfc.) ; sp". tetraradiate spicules of gastral
cortex ((]c.) ; ec. ectoderm ; en. layer of flattened cells
lining the paragastric cavity ; pm. pore-membrane ;
pp. prosopyles ; ap. apopylc ; di. diaphragm ; exc.
excurrent passage ; P.G. paragastric cavity ; em.
early embryo ; em', late embryo. The arrows in-
dicate the course of the water through the sponge.

SECT-

the skeleton of the
Sycon.

The arrangement of the
spicules, their relation to
the protoplasmic parts,
and the structure of the
latter, have to be studied
in thin sections of hard-
ened specimens (Figs. 80
and 81). An examination
of such sections leads to
the following results.

Microscopic struc-
ture. Covering the outer
surface of the sponge is
a single layer of cells the
dermal layer or ectoderm l
(Fig. 81, ec) through
which project regularly-
arranged groups of needle-
like and spear-like spicules
(sp'), forming the pattern
of polygonal elevations on
the outer surface. The
cells of the ectoderm are
in the form of thin scales,
which are closely cemented
together by their edges.
The paragastric cavities
are lined by a layer of
cells (en) which are, like
those of the ectoderm, thin
flattened scales. Running
radially through the thick
wall of the cylinders arc a
large number of regularly-
arranged straight passages.
Of these there are two sets,
those of the one set the
incurrent canals (Figs. 80

1 The terms ectoderm and
endoderm are here used as con-
venient terms for the outer and
inner layers of the Sponge,
though, as will appear later,
these layers differ completely in
their mode of formation from
the layers so named in the
higher phyla.

Ill

PHYLUM AND CLASS PORIFERA

109

and 81 1C) narrower, and lined by ectoderm similar to the ectoderm
of the surface ; those of the other set the radial or flagellate canals
(R) rather wider, octagonal in cross-section, and lined by endoderm
continuous with the lining of the paragastric cavity. The incurrent
canals end blindly at their inner extremities not reaching the
paragastric cavity ; externally each becomes somewhat dilated,
and the dilatations of neighbouring canals often communicate.
These dilated parts are closed externally by a thin membrane-
the pore-membrane (Fig. 81 pm, and Fig. 82), perforated by three
or four small openings (Fig. 82, p) the ostia already referred to.
The flagellate canals are blind at their outer ends, which lie at a
little distance below the surface opposite the polygonal projections
referred to above as forming a pattern on the outer surface ;
internally, each communicates with the paragastric cavity by a
short, wide passage the excurrent canal (Fig. 81 exc). Incurrent

'.

FIG. 82. Sycon gelatinosum. Sur-
face view of a pore-membrane highly
magnified ; p, ostium ; R. position of
the outer end of a radial canal.

FIG. 83. Sycon gelatine sum,

An apopyle surrounded by its dia-
phragm ; m. contractile cells.

and flagellate canals run side by side, separated by a thin layer of
sponge substance except at certain points, where there exist small
apertures of communication the prosopylcs (pp), uniting the
cavities of adjacent incurrent and flagellate canals. Each proso-
pyle is a perforation in a single cell termed a porocyte.

The ectoderm lining the incurrent canals is of the same char-
acter as that of the outer surface. The endoderm of the
flagellate canals, on the other hand, is totally different from that
which lines the paragastric cavity. It consists of cells of columnar
shape ranged closely together so as to form a continuous layer.
Each of these flagellate endoderm cells, or collared cells, or clwano-
cytes, as they are termed, is not unlike one of the Choanoflagellate
Protozoa (p. 77) ; it has a nucleus, one or more vacuoles, and, at
the inner end, a single, long, whip-like flagellum, surrounded at its
base by a delicate, transparent, collar-like upgrowth, similar to
that which has already been described as occurring in the
Choanoflagellata. If a portion of a living specimen of the sponge

110 ZOOLOGY SECT.

is teased out in sea-water, and the broken fragments examined
under a tolerably high power of the microscope, groups of these
collared cells will be detected here and there, and in many places
the movement of the flagella will be readily observed. The
flagellum is flexible but with a certain degree of stiffness,
especially towards the base, and its movements resemble those
which a very supple fishing-rod is made to undergo in the act of
casting a long line the movement being much swifter and
stronger in the one direction than in the other. The direction
of the stronger movement is seen, when some of the cells are
observed in their natural relations, to be from without inwards.
It is to these movements that the formation of the currents of
water passing along the canals is due. The collars of the cells in
specimens teased in this way become for the most part drawn back
into the protoplasm.

The short passage or excurrent canal, which leads inwards from
the flagellate canal to the paragastric cavity, differs from the
former in being lined by flattened cells similar to those of the
paragastric cavity ; it is partly separated from the flagellate canal
by a thin diaphragm (Fig. 81, di, and Fig. 88), perforated by a
large circular central aperture the apcrpyle (ap) which is capable
of being contracted or dilated : its opposite aperture of com-
munication with the paragastric cavity, which is very wide, is
termed the gastric ostium of the excurrent canal.

The effect of the movement of the flagella of the cells in the
flagellate canals is to produce currents of water running from
without inwards along the canals to the paragastric cavity. This
causes water to be drawn inwards through the prosopyles from
the incurrent canals, and, indirectly, from the exterior through the
perforated membranes at the outer ends of the latter.

Between the ectoderm of the outer surface and of the incurrent
canals, and the endoderm of the inner surface and of the flagellate
canals, are a number of spaces filled by an intermediate layer
the mesnglcea in which the spicules of the skeleton are
embedded. Each spicule is developed from cells termed sclcro-
blasts, which migrate inwards from the ectoderm. Each ray is
formed by the agency of a separate scleroblast, so that there are
three at least of the latter for each triradiate, and four for each
tetraradiate spicule. The spicules (Figs. 80 and 81, sp) are
regularly arranged, and connected together in such a way as to
protect arid support the soft parts of the sponge. Most are, as
already noticed, of triradiate form. Large numbers, however, are
of simple spear-like or club-like shape (sp') ; these, which are
termed the oxeote spicules, project on the outer surface beyond the
ectoderm, and are arranged in dense masses, one opposite the
outer end of each of the ciliated canals, this arrangement pro-
ducing the pattern already referred to as distinguishable on the

in PHYLUM AND CLASS PORIFERA 111

out surface. The thick outer layer in which the bases of these
oxeote spicules lie embedded, is termed the dermal cortex (do). A
thick stratum at the inner ends of the canals and immediately
surrounding the paragastric cavity is termed the (/astral cortex (gc).
It is supported by triradiate and also by tetraradiate spicules, one
ray of each of which (*p") frequently projects freely into the para-
gastric cavity, covered over by a thin layer of flattened endoderm
cells.

The mesoglcea itself, as distinguished from the spicules which
lie embedded in it, consists of a clear gelatinous substance
containing numerous nucleated cells of several different kinds.
Most of these are small cells of stellate shape, with radiating
processes the connective-tissue cells or collencytcs (Fig. 80, co) ;
others are fusiform ; a good many the amoeboid wandering cells
are Amoeba-like, and capable of moving about from one part of
the sponge to another.

Around the inhalant pores and the apopyles are elongated cells
(Figs. 82 and 83), sometimes prolonged into narrow fibres. These
are contractile effecting the closure of the apertures in question,
and are therefore to be looked upon as of the nature of muscular
fibres. In the case of the inhalant pores they are ectodermal ; in
that of the apopyles they are endodermal. A band of similar
fibres surrounds the osculum the oscular sphincter.

The sexual reproductive cells the ova (Figs. 80 and 81, ov) and
sperms are developed immediately below the flagellate endoderm
cells of the flagellate canals, arid in the same situation are to be
found developing embryos (cm, em'\ resembling in their various
stages those of Sycon raphanus, as described below.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

Sponges are plant-like, fixed, aquatic Metazoa, all, with the
exception of one family, inhabitants of the sea. The primary form
is that of a vase or cylinder, the sides of which are perforated by a
number of pores and in the interior of which is a single cavity;
but in the majority of Sponges a process of branching and folding
leads to the formation of a structure of a much more complex
character. The surface of the Sponge is covered by a single layer
of flattened cells the ectoderm l arid the internal cavities, or a
part of thorn, are lined by a second single layer the cndodcrm-
part or the whole of which consists of a single layer of choanocytcs,
i.e. columnar collared cells, each provided internally with a long
flagellum. Between these two layers is a quantity of tissue
usually of a gelatinous consistency the mcsogloba containing a
number of cells of various kinds. The wall of the Sponge is
pierced by a number of apertures. The skeleton or supporting

1 See footnote on p. 108.

112 ZOOLOGY SECT.

framework, developed in the mesogloea from cells derived from
the ectoderm, consists in some cases of fine, flexible fibres of a
material termed spongin ; in others of spongin-fibres supplemented
by microscopic siliceous spicules ; in others of siliceous spicules
alone ; in others of spicules of carbonate of lime. Reproduction
takes place both asexual ly by the formation of gcmmules, and
sexually by means of ova and sperms. The ovum develops into a
ciliated free-swimming larva, which afterwards becomes fixed and
develops into the plant-like adult Sponge.

The Sponges are sufficiently far removed in structure from the
rest of the Metazoa to justify us in looking upon them as con-
stituting one of the great divisions or phyla of the animal kingdom.
At the same time there is so much uniformity of structure within
the group that a division into classes is not demanded ; the phylum
Porifera contains a single class.

The class Porifera is classified as follows :

Sub-Class I. Calcarea.

Sponges with a skeleton of calcareous spicules, and with com-
paratively large collared cells.

ORDER 1. HOMOCCELA

Calcareous Sponges in which the internal lining membrane
consists throughout of flagellate collared cells.

ORDER 2. HETEROCCELA.

Calcareous Sponges in which the paragastric cavity is lined by
flattened cells, the collared cells being restricted to flagellate
canals or chambers.

Sub-Class II. Hexactinellida.

Sponges with six-rayed, tri-axon, siliceous spicules, and simple
canal system represented by unbranched or branched flagellate
chambers.

Sub-Class III. Demospongia.

Sponges either devoid of skeleton or with spongin fibres alone,
or a combination of spongin fibres and siliceous spicules, the
latter, when present, never six-rayed ; the canal system of the
Rhagon type (p. 118), usually complicated.

Systematic Position of the Example.

Sycon gelatinosum is one of many species of the genus Sycon.
Sycon is one of several genera of the family Syccttidw ; and the

in PHYLUM AND CLASS PORIFERA 113

family Sycettidce is one of several families of the order Heterocwla
of the class Calcarea. Among the families of the Heterocoela,
that of the Sycettidce is distinguished by the following features,
which characterise all its members :

" The flagellate chambers are elongated, arranged radially around
a central paragastric cavity, their distal ends projecting more or less
on the dermal surface, and not covered over by a continuous cortex.
The skeleton is radially symmetrical."

Of the genera into which the Sycettidce are divided, Sycon is
characterised as follows :-

" The flagellate chambers are not intercommunicating ; their
distal ends are provided each with a tuft of oxeote spicules."

The members of one of the other genera of the family Sycetta-
while possessing the general characteristics of the family, differ from
those of the genus Sycon in wanting the tufts of oxeote spicules ;
those of a third Sycantha have the flagellate chambers united
in groups ; the chambers of each group intercommunicating by
openings in their walls, and each group having a single common
opening into the gastric cavity. The members of this genus re-
semble Sycon, and differ from Sycetta, in the presence of tufts
of oxeote spicules at the distal ends of the flagellate chambers.

These distinctions between classes, orders, families, and genera
are of an entirely arbitrary character. No such divisions exist in
nature ; and they are merely established as a convenient way of
grouping the sponges and facilitating their classification. But a
classification of this kind, if carried out on sound principles, should
nevertheless have something corresponding to it in nature, inas-
much as the grouping of the various divisions and subdivisions
aims at expressing the relationships of their members to one
another. The members, for example, of the family Sycettidce are
all regarded, on account of the features which they possess in
common, as being more nearly related to one another than to the
members of the other families, and as 1 having been derived from a
common ancestor which also possessed those features the diver-
gences of structure which we observe in the different genera and
species being the result of a gradual process of change.

Within the limits of the genus Sycon, S. gelatinosum is distin-
guished from the rest as a group of individual Sponges all possess-
ing certain specific characters which it will be unnecessary to
detail here. But the individual Sponges referable to this species
frequently differ somewhat widely from one another : there are
numerous individual variations. If we compare a number of
specimens all possessing the species-characters of Sycon gelatino-
sum, we find that they differ in the number of branches, in the
shape of the cylinders some being relatively narrow, some re-
latively wide in the degree of development of the oscular crown

of spicules, in the ratio of the thickness of the wall to the width
VOL. I I

114 ZOOLOGY SECT.

of the contained paragastric cavities, and in many other more
minute points ; in fact, we find as a result of the comparison that no
two specimens are exactly alike. These differences are so great
that some very distinct races or varieties of S. gelatinosum have
been recognised, and some have received special names. Here
again, as in the case of the families and orders, the distinctions
are of an arbitrary character some writers on Sponges setting
down as several species what others regard merely as varieties of
one species. It is impossible, in fact, to draw a hard and fast line
of distinction between species and varieties. In the higher groups
of animals the attempt is made to establish a physiological dis-
tinction ; all the members of a species are regarded as being fertile
inter se, and capable of producing fertile offspring as a result of
their union ; but such a mode of distinguishing species is impos-
sible of application among lower forms such as the sponges. In
these lower groups, accordingly, a species can only be defined as
an assemblage of individuals which so closely resemble one an-
other that they might be supposed to be the offspring of a parent-
form similar to themselves in all the most essential features.
And, according to the view taken of the relative importance of
different points of colour, shape, and internal structure, the con-
ceptions of the species and their varieties and mutual relationships
formed by different observers must often differ widely from one
another.

3. GENERAL ORGANISATION.

General Form and Mode of Growth.- -The simplest Sponges
are vase-shaped or cylindrical in form, either branched or un-
branchedj and, if branched, with or without anastomosis or
coalescence between neighbouring branches. But the general
form of the less simple Sponges diverges widely from that of such
a branching cylinder as is presented by Sycon gelatinosum
(Fig. 78).

From the point to which the embryonic sponge becomes
attached it may spread out horizontally, following the irregulari-
ties of the surface on which it grows, and forming a more or less
closely adherent encrustation like that of an encrusting lichen
(Fig. 84, A). The surface of such an encrustation may be smooth ;
more commonly it is raised up into elevations rounded bosses,
cones, ridges or Iamella3 ; and the edges may be entire or lobed.
In other cases the sponge grows at first more actively in the
vertical than in the horizontal direction, and the result may be a
long, narrow structure, cylindrical or compressed, and more or less
branched (Fig. 84, />*). Sometimes vertical and horizontal growth
is almost equal, so that eventually there is formed a thick, solid
mass of a rounded or polyhedral shape (Fig. 84, C), with an even,
or lobed, or ridged surface. Very often, after active vertical growth

Ill

PHYLUM AND CLASS PORTFERA

115

has resulted in the formation of a comparatively narrow basal
part or stalk, the Sponge expands distally, growing out into lobes
or branches of a variety of different forms, and frequently anasto-
mosing. Sometimes, after the formation of the stalk with root-
like processes for attachment, the Sponge grows upwards in such
a way as to form a cup or tube with a terminal opening. Such a

A.Oscaria

C,E usbongia

B.Psammoclema

D. Poherion

Fin. 84. External form of various Sponges. A, Oscar ia. an encrusting form, with the
upper surface raised up into a number of rounded prominences ; J3, Psammoclema a
ramifying subcylindrical Sponge ; 0, Euspongria (toilet sponge), a massive form with
a broad base ; D, Poterion (Neptune's Cup), an example of a complex Sponge assuming
the form of a vase. (After Vosmaer.)

cup-shaped Sponge, exemplified in the gigantic Neptune's Cup
(Poterion, Fig. 84, D), is not to be confounded with the simple
vase or cup referred to above as the simplest type of Sponge,
being a much more complex structure with many oscula. Some-
times the Sponge grows from the narrow base of attachment into
a thin flat plate or lamella ; this may become divided up into a
number of parts or lobes, which may exhibit a divergent arrange-

i 2

116

ZOOLOGY

SECT.

ment like the ribs of an open fan. Often the lamella becomes
folded, and sometimes there is a coalescence between the folds,
resulting in the development of a honey-comb-like form of
sponge.

Sponges resemble plants, and differ from the higher groups of
animals, in the readiness with which, in many cases, their form

becomes modified during growth by
external conditions (environment).
Different individuals of the same
kind of Sponge, while still exhibiting
the same essential structure and the
same general mode of growth, may
present a variety of minor differences
of form, in accordance with differ-
ences in the form of the supporting
surface or in the action of waves and
currents.

Leading Modifications of
Structure. Sycon gelatinosum be-
longs to a type of Sponges interme-
diate between the very simplest forms
on the one hand, and the more com-
plex on the other. The simplest
type of Sponge-structure is that
of the so-called Ascctta or Olynthus
(Fig. 85). This is not a mature form
no adult Sponge retaining such
simplicity of structure. It is vase-
shaped, contracted at the base to
form a sort of stalk by the expanded
extremity of which it is attached ;
at the opposite or free end is the
circular osculum. So for there is a
considerable resemblance to Sycon
gelatinosum ; but the structure of its
wall in Ascetta is extremely simple.
Regularly arranged over the suri'ace
are a number of small rounded
apertures, the inhalant pores ; but,
sinje the wall of the Sponge is very thin, these apertures
lead directly into the central or paragastric cavity (Fig. 86 A),
the long passages or canals through which the communica-
tion is effected in Sycon being absent. The wall consists of the
same three layers as in Sycon, but the middle one, though it
contains a small number of spicules, is very thin. The ectoderm is
a thin layer of flat cells; the paragastric cavity is lined throughout
by choanocytes similar to those of the flagellate canals of Sycon.

FJO. 85. Olynthus stage of a simple
calcareous Sponge (Clalhrina). A
portion of the wall of the vase-like
sponge removed to show the para-
gastric cavity. (After Haeckel.)

Til

PHYLUM AND CLASS PORIFERA

117

A somewhat more complex type of structure than that of Ascetta
is exhibited by those
sponges in which the
wall becomes thick-
ened and perforated
by radially-arranged
canals, which open di-

rectly on the outer sur-
face by means of inhal-
ant pores or cstia, and
lead directly into the
paragastric cavity by
means of opapyles
the whole inner sur-
face as well as the
radial canals being
lined with flagellate
endoderm cells. In
forms which may be
regarded as represent-
ing the next stage
of development (Fig.
86, B : see also the
figures of Sycon gela-
tinosum), there are
formed by infolding
of the surface, in the
intervals between the
radial canals, canal-
like spaces, the incur-
nnt canals, lined by
ectoderm and com-
municating with the
exterior on the one
hand, either by a
wide opening or by
pores (ostia) perfor-
ating a pore-mem-
brane, and on the
other by means of
small openings, the
prosopyles (the equi-
valents of the inhalant
pores of the Olynthus),
with the radial canals.
Sponges similar to
Sycon gelatinosum,

/:v-: f *!/>#f$

'.: '. ::-',& /\V

&$&$&&

1 ''' '.-.;.! !':' .': ':.
\ '.::' r,^. .'.'' '.-:. -...
\:.-.;;'-.*-:?. \'v.>:?:.--.. :-::-.

FIG. 86. Diagram of the canal system of various sponges, the
ectoderm denoted by a continuous narrow line ; the flat-
tened endoderm by an interrupted line ; the flagellate
endoderm by short parallel strokes. A, cross-section
through a part of the wall of an Ascon ; B, cross-section
through a part of the wall of a Sycon ; C, cross-section
through a part of the wall of Leucillo con << xa ; V, vertical
section through Oscnrdln ; a, spaces of the incurrent canal
system ; b, spaces of the excurrent canal system ; os. oscu-
lum. (After Korschelt and Heider.)

118

ZOOLOGY

SECT.

but with flagellate canals arranged in groups, each group centred
round a main excurrent canal (Fig. 86, C) afford us the next
grade of advancing complexity. In these the incurrent canals may
form a branching system. In all the higher groups of Sponges
(Fig. 86, D and Fig. 87) the flagellate cells are confined to cer-
tain special enlargements of the canals the so-called *' ciliated
chambers " (C) and the rest of the canals are lined by flattened
cells.

Special names have been applied to the main types of canal-
system briefly sketched above. Forms in which the paragastric
cavity is lined by flagellate cells are said to belong to the Ascon
type, whether the paragastric cavity communicates directly or by
flagellate canals with the exterior. Forms in which there is a
paragastric cavity lined by flattened cells, and a system of radially

PG

CO

DP

In

FIG. 87. Vertical section of a fresh-water sponge (Spon^il la), showing the arrangement of the
canal-system. C. ciliated chambers ; DP. dermal pores ; Ex. excurrent canals ; GO. openings
of the excurrent canals ; PG, paragastric cavity ; SD. subdermal cavities ; 0. osculuni.
(Modified from Leuckart and Nitsche's diagrams.)

arranged flagellate chambers, are said to possess the Sycon type of
structure. Such Sponges as have small rounded flagellate cham-
bers (" ciliated chambers "), communicating in most cases by
narrow branching incurrent canals with the exterior (directly or
indirectly) on the one hand, and by similar excurrent canals with
the paragastric cavity on the other the flagellate cells being
confined to the flagellate chambers are said to possess the Rhagon
type of canal-system. In the lilt agon proper the arrangement 01
parts is very simple. The Sponge has a paragastric cavity opening
on the exterior by an osculum. Opening into this central cavity
by wide apopyles are a number of rounded chambers each com-
municating with the exterior by an inhalant pore (prosopyle).

The development of branches from the originally simple Sponge,
and the coalescence of neighbouring branches with one another,
greatly obscure the essential nature of the Sponge as a colony or
zooids similar to the branches of Sycon gelatinosum ; and this effect

Ill

PHYLUM AND CLASS PORIFERA

119

is increased by the development of a variety of infoldings of the
ectoderm which appear in the higher forms. The oscula dis-
tributed over the surface of the mass may indicate the component
zooids, but these are not always recognisable, being carried inwards
by the infoldings or closed up altogether.

A thicker or thinner specialised outer layer the dermal cortex
situated immediately below the superficial ectoderm, is present
in many Sponges. This is a layer of mesoglcea with special
skeletal elements, usually containing spaces and canals lined by
ectoderm (subdermal cavities, Fig. 87, S2)) which communicate
directly with the exterior, and, internally, usually with more
deeply situated spaces (subcortical cavities), from which the in-
current canals lead to the ciliated chambers. This dermal cortex
is present, though not highly developed, in Sycon gelatinosum
(Fig. 81, dc\ and the enlarged outer ends of the incurrent canals
lying in the dermal cortex and closed externally by the pore-
bearing membrane, may be regarded as representing dermal
cavities. In most higher sponges a special inner layer is
developed ; this is the gastral cortex, represented in a rudi-
mentary form in Sycon gelatinosum (Fig. 81, yc.) as the internal
layer with special spicules, in which the excurrent canals are
situated.

Histology. In the protoplasmic elements or cells of the
various groups of Sponges there is little variation, except
in minor points. The cells
of the ectoderm (Fig. 88)
are flattened, and very rarely
assume other forms ; in some
cases each flattened ecto-
dermal cell is provided with a
flagellum. Lining the paraga-
stric cavities and canals is a
layer of flattened cells similar
to those of the ectoderm, or of
flagellate collared cells. In
the gelatinous substance of the
mesoglcea are embedded connec-
tive-tissue cells, amoeboid wan-
dering cells, and, in certain

positions (around orifices), muscle-cells. Unicellular glands (see
p. 25) are present in some sponges, both calcareous and siliceous ;
also cells containing the pigment to which the bright colour
of many sponges is due, though in most cases the pigment is not
confined to special cells, but occurs scattered through the con-
nective-tissue cells and flagellate cells. Fresh-water Sponges are
green, owing to the presence of chlorophyll, the colouring matter
to which the prevailing green colour of plants is due.

FIG. 88. Cells of the ectoderm, very highly
magnified. (After Von Lendenfeld.)

120

ZOOLOGY

SECT.

The elements of the skeleton differ in character in the different
classes. In the Calcarea they consist of calcareous spicules, usually
tri-radiate in form. Each of these spicules is developed from special

cells the sclcroblasts (Fig.
89). In the remaining groups
of Sponges the skeleton
either consists of spongin
fibres alone (Fig 90, A),
or of siliceous spicules
alone, or of a combination of
spongin fibres with siliceous
spicules (B) : in some Demo-
spongia (the Myxospongia)
skeletal parts are altogether
absent. Spongin is a sub-
stance allied to silk in chemi-
cal composition : the fibres
are exceedingly fine threads,
consisting of a soft granular
core and an outer tube of
concentric layers of spongin.

FIG. 89. Development of a tri-radiate spicule of rni_. j.v rl~, "U V, A

Clathrina. sd, scleroblasts. (After Minchin.)

anastomose, or are woven and

felted together in such a way as to form a firm, elastic,
supporting structure. They are secreted by the activity of
certain cells in the mesogloea which are called the spongin-
blasts, derived from the ectoderm. In certain exceptional cases
the spongin assumes the form of spicules. The siliceous spicules
(Fig. 91) are much more varied in shape than the spicules
of the Calcarea, and in a single kind of Sponge there may be
a number of widely differing forms of spicules, each form having
its special place in the skeleton of the various parts of the Sponge-
body. In most forms siliceous spicules and spongin fibres
combine to form the supporting framework, the relative develop-
ment of these two elements varying greatly in different cases.
But in certain groups, including the common Washing-sponges
(Fig. 90 A), spicules are completely absent, and the entire
skeleton consists of spongin. In some forms which are devoid
of spicules, the place of these is taken by foreign bodies -
shells of Radiolaria, grains of sand, or spicules from other
sponges (Fig. 90, C). In others, again, such as the Venus's
Flower-Basket (Euplectella), the Glass-Rope Sponge (Hyaloncma),
and Pheronema (Fig. 92), the skeleton consists throughout of
siliceous spicules bound together by a siliceous cement.

Reproduction in the Sponges is effected either sexually or
asexual ly. The process by which, in all but the simplest forms of
Sponges, a colony of zooids is formed from the originally simple

Ill

PHYLUM AND CLASS PORIFERA

121

cylinder or vase, may be looked upon as an asexual mode of repro-
duction by budding. In some cases asexual multiplication also takes
place by the production of external buds ; in others of internal buds
in the shape of groups of cells called gemmules, which eventually
become detached and develop into new individuals. In the Fresh-

C.Spongelia

A.EusJDongia

B. Pachychalina

FIG. 90 Microscopic structure of the skeleton in various sponges. A, Euspongia, network
of spongin fibres; B, Pachychalina, spongin strengthened by siliceous spicules; C,
Spongelia, spongin strengthened by various foreign siliceous bodies, fragments of spicules
of other sponges, &c. (After Vosmaer.)

water Sponges (Spongillidce) multiplication takes place very actively
by means of such gemmules, each of which is a spherical group of
cells enclosed in an envelope composed of peculiarly shaped siliceous
spicules, termed ainpliidiscs (Fig. 91, right side). These gemmules
are formed in the substance of the Sponge towards the end of the

122

ZOOLOGY

SECT.

year ; they are set free by the decay of the part of the parent
sponge in which they are developed, and fall to the bottom. In
spring the contained mass of protoplasmic matter reaches the
exterior through an aperture in the wall of the gemmule, and
develops into the adult form.

All Sponges multiply by a sexual process by means of male
cells, or sperms, and female cells, or ova. These are developed
from certain of the amoeboid wandering cells of the inesoglcea,
which take up a special position, usually immediately below the
collared cells of the endoderm. Ova and sperms are developed in
the same Sponge, but rarely at the same time. The amoeboid cell
destined to form sperms divides into a number of small cells, giving
rise to a rounded mass of sperms. The latter, when mature, have
oval or pear-shaped heads and a long tapering appendage or tail.
Each amoeboid cell destined to form an ovum enlarges, and

FIG. 91. Various forms of sponge spicules. (From Lang's Text-Book.)

eventually assumes a spherical form. After a sperm has penetrated
into its interior and effected impregnation, the ovum usually
becomes enclosed in a brood-capsule formed for it by certain
neighbouring cells, and in this situation, still enclosed in the parent
Sponge, it undergoes the earlier stages of its development. The
boring Sponge, Cliond, is the only one, so far as known, in which
the early stages of development are passed through externally.

In all known cases there is a free-swimming ciliated larval
stage ; but the form assumed by the larva differs profoundly in
different Sponges. Of the simpler types of calcareous sponges
with a structure resembling that of the Olynthus, the development
has been followed out in the case of Clathrina blanca. In this
sponge segmentation is followed by the formation of an oval
blastula, the wall of which consists of a single layer of cells all
alike in character elongated, columnar, and flagellate. At one
pole of the blastula is seen a pair of cells which are of a different
character, being large, rounded, and granular. These are destined
to give rise to the archwocytcs, some of which form the repro-

PHYLUM AND CLASS PORIFERA

123

d active cells. Certain of the flagellate cells then withdraw their
flagella and pass into the internal cavity, becoming amoeboid. Soon

FIG. 92. Pheronema carpenter!, one of the Hexactinellida.
(From Wyville Thomson.)

a large number of these amoeboid cells come to fill up a great part of
the cavity of the larva, which now passes into a stage corresponding
to the planula larva of the Coelenterates (Sect. IV). This is the

124

ZOOLOGY

SECT.

larval form known as the par enchy mulct. The parenchymula
(Fig. 93) consists of three kinds of cells : (1) an external layer
of flagellate cells ; (2) an inner mass of amoeboid cells ; (3) the
two posterior granular cells. In this condition it becomes fixed,

and develops into the form of a flat plate
with an irregular outline. Most of the
amoeboid cells now migrate to the outer
surface, passing between the flagellate
cells and then becoming arranged outside
them to form the ectoderm. The flagel-
late cells now form an irregular mass
together with a number of non-flagellate
cells derived from the ectoderm, which
are destined to give rise to the porocytes.
A cavity appears in the mass, and becomes
surrounded by a layer of porocytes. The
cavity increases in size, and is soon seen
to be bounded not by the porocytes alone,
but in part also by flagellate cells. Sub-
sequently the flagellate cells come to
form the entire boundary of the cavity,
the porocytes passing outwards to become
perforated by apertures the inhalant
apertures in the wall of the sponge.
Among the flagellate cells and porocytes
there are also amoeboid cells derived from the two original granular
cells ; some of these give rise to the reproductive cells. The
scleroblasts are formed of certain ectoderm cells which migrate
inwards, and at an early stage arrange themselves in threes to give
rise to the tri-radiate spicules. The development of the sponge
becomes completed by the enlargement of the internal cavity
(paragastric cavity) which is now lined by flagellate cells, and by
the development of the osculum.

In Sycon the early stages (Fig. 94, a-c) differ somewhat from
those in Clatkrina llanca, and the embryo leaves the parent sponge
in the peculiar stage to which the name of amphiblastula is
applied. When the blastula is formed the greater part of its wall
consists of clear cells, with a number of granular cells the archaeo-
cytes at the posterior pole. The clear cells become elongated
and flagellate. The archseocytes pass into the internal (segmenta-
tion) cavity and become completely enclosed by the flagellate
cells (stage of so-called pscudogastrula}.

The cells at the posterior end then lose their flagclla and
become large rounded granular cells, so that after a time the
wall of the embryo comes to be composed in one half of the
flagellate cells that have remained unaltered, and in the other half of
the large granular cells. It is in this stage termed the amphi-

FIG. 93. Median longitudinal
section of the parenchymula
larva of Clathriiia blanca.
p.g.c., posterior granular cells
which give rise to the archpeo-
cytes. (From the Cambridge
Natural History, after Min-
chin.)

Ill

PHYLUM AND CLASS PORIFERA

125

blastnia (e) that the larval sponge becomes free. At a later
stage the flagellate cells become partly overgrown by the granular
cells, the latter eventually giving rise to the ectoderm of the
adult, while the former become the flagellate collared cells. The
larva becomes fixed by one side, and soon assumes a cylindrical

FIG. 94. Development of Sycon raphanus. , ovum ; b, c, ovum segmented?*, as seen from
above, c, lateral view ; if, blastula ; e, amphiblastula ; /, commencement of invagination ;
ff, . larva attached by its oral face ; h, i, young sponge A, lateral view ; /, as seen from above.
(From Sollas, after Schulze.)

form (Fig. 94, h, i}. An aperture which is developed at the free
end becomes the osculum, and small perforations in the sides of
the cylinder form the inhalant apertures. As the wall of the
cylinder increases in thickness by the growth of the mesogloea,the
radial canals are formed, the endoderm extending into them and
its cells becoming flagellate.

126 ZOOLOGY SECT.

The amphiblastula type of larva is characteristic of the Calcarea,
and is probably universal in that sub- class except in such primi-
tive forms as Glathrina.

In the SilicispongiaB, on the other hand, the typical larva is a solid
body with a superficial layer of ciliated cells, and an internal mass
of granular cells. From the former, apparently, the collared cells
of the flagellate chambers are formed : from the latter the external
ectoderm and the other elements of the body of the Sponge. The
granular cells break through the ciliated cells at one end and grow
over the latter as an investing layer. This is a remarkable reversal
of what, as will be seen subsequently, is to be observed in the
Ccelenterata and in fact in the rest of the Metazoa, but is readily
reconcilable with what takes place in Sycon and the more complex
Calcarea.

Distribution and Mode of Occurrence of Sponges, and
their Position in the Animal Series. Fossil remains of Sponges
have been found in various formations from those of the Cambrian
period onwards, the greatest abundance being found in the Chalk.
No extinct class or order has been detected, the fossil forms all
being members of existing groups. Some of the orders of existing
Sponges such as the Myxospongiae are incapable of being
preserved as fossils, and the fossil forms belong, as we should
expect, to the more highly silicified groups and to the more
complex groups of the Calcarea.

Fresh-water Sponges (Spongillidce) occur in rivers, canals, and
lakes in all the great divisions of the earth's surface. Marine
Sponges occur in all seas, and at all depths, from the shore
between tide-marks to the deepest abysses of the ocean. The
Calcarea and the true horny sponges (Ccratosci) are most abundant
in shallow water, and have not been found below 450 fathoms.
The Sponges found at the greatest depths are members of the
groups Hcxadindlida and Choristida.

Sponges do not appear to be edible by Fishes or even the higher
Crustaceans or Molluscs. Countless lower animal forms, however,
burrow in their substance, if not for food, at least for shelter, and
the interior of a Sponge is frequently found to be teeming with
small Crustaceans, Annelids, Molluscs, and other Invertebrates.
None of the Sponges are true parasites. The little Boring
Sponge, Cliona, burrows in the shells of Oysters and other bivalves,
but for protection and not for food. But a Sponge frequently lives
in that close association with another animal or plant to which the
term mcssmateism, or commensalism, is applied, associations which
benefit one or both. Thus some species of Sponge are never found
growing except on the backs or legs of certain Crabs. In these
cases the Sponge protects the Crab and conceals it from its enemies,
while the Sponge benefits by being carried from place to place and
thus obtaining freer oxygenation. Certain Cirri pede Crustaceans

in PHYLUM AND CLASS PORIFERA 127

(members of the order to which the Barnacles and Acorn-shells
belong) are invariably found embedded in certain species of Sponge.
Frequently a Sponge and a Zoophyte grow in intimate association,
so that they seem almost to form one structure. Thus the Glass-
rope Sponge (Hyalonema) is always found associated with aZoophyte
(Palythocu), and there are many other instances. Sponges often also
grow in very close association with certain low forms of plants
(Algae).

The position of the Porifera in the animal series is unquestion-
ably among the Metazoa. The view that they are compound
Protozoa is now no longer maintained since the significance of
the facts of their development has been fully recognised. A
Sponge is to be regarded as a colony of Protozoa only in the sense
in which the same may be said of one of the higher animals. It
consists of a complex of cells, some of which have a consider-
able degree of independence, and some of which have a close
resemblance to certain Protozoa ; but the same is true of one of
the higher animals, the difference being one of degree and not
of kind. Like the rest of the Metazoa, the Sponge develops from
the oosperm by a process of yolk-division.

But the Porifera are perhaps somewhat nearer the Protozoa
than are any of the other types of Metazoa ; and among the
Protozoa they appear to approach nearest to certain colonial
Flagellata. The genus Proterospongia (Fig. 58), already referred
to (p. 78), appears to be the member of the latter group which of
all known forms most closely resembles a sponge. Proterospongia
consists of a colony of collared Flagellates (Choanoflagellata)
embedded in a mass of gelatinous substance, in which there are
also amoeboid zooids similar to the amoeboid wandering cells of
Sponges.

But, while the Porifera are clearly Metazoa, and not Protozoa,
there is some room for difference of opinion as regards their
relationships to the Coslenterata, with which great phylum they
have been sometimes amalgamated. The reasons for and against
such an arrangement will be discussed in considering the
general relationships of the Coalenterata.

SECTION IV
PHYLUM CCELENTERATA

THE possession of an interval cavity lined by a special internal
layer of cells the endoderm in which the digestive and absorp-
tive functions are centred, distinguishes all the remaining groups
of Metazoa from the Parazoa or Sponges. The former are grouped
together under the comprehensive title of Enterozoa, or animals
with enteric cavity. The simplest Enterozoa have an internal
cavity in which there is no separation between the enteric
or digestive cavity and the coelome or body-cavity one con-
tinuous space representing both and opening on the exterior by
the aperture of the mouth. These constitute the phylum
Ccelenterata. They are all animals of a low type of organisation
with a conspicuous radial symmetry, disguising, in some cases,
a more obscure bilateral arrangement, which may be more
primitive.

The most familiar examples of Ccelenterata are the horny,
seaweed-like " Zoophytes/' or, as they are sometimes called,
" Corallines," to be picked up on every sea-beach Jelly-fishes,
Sea-anemones, and Corals. The phylum is divided into four classes
as follows :

Class 1. HYDROZOA, including the Fresh-water Polypes, Zoo-
phytes, many Jelly-fishes mostly of small size, a few Stony
Corals, and the peculiar Palaeozoic fossils known as Graptolites.

Class 2. SCYPHOZOA, including most of the large Jelly-fishes.

Class 3. ACTINOZOA, including the Sea-anemones, and the vast
majority of Stony Corals.

Class 4. CTENOPHORA, including certain peculiar Jelly-fishes
known as " Comb-jellies."

CLASS I. HYDROZOA.

1. EXAMPLE OF THE CLASS Obelia.

General Structure. Obelia is a common zoophyte occurring
in the form of a delicate, whitish or light brown, almost fur-like

128

SECT, iv PHYLUM CCELENTERATA 129

growth on the wooden piles of piers and wharfs. It consists of
branched filaments about the thickness of fine sewing-cotton : of
these, some are closely adherent to the timber, and serve for
attachment, while others are given off at right angles, and present
at intervals short lateral branches, each terminating in a bud-like
enlargement.

The structure is better seen under a low power of the
microscope. The organism (Fig. 95) is a colony, consisting of a
common stem or axis, on which are borne numerous zooids. The axis
consists of a horizontal portion (hydrorkiza) resembling a root or
creeping stem, arid of vertical axes, which give off short lateral
branches in an alternate manner, bearing the zooids at their ends.
At the proximal ends of the vertical axes the branching often
becomes more complex : the offshoots of the main stem, instead of
ending at once in a zooid, send off branches of the third order on
which the zooids are borne. In many cases, also, branches are found
to end in simple club-like dilatations (Bd. 1, 2) : these are imma-
ture zooids.

The large majority of the zooids have the form of little conical
structures (P. 1 P. 4), each enclosed in a glassy, cup-like invest-
ment or hydrotheca (h.th), and produced clistally into about two
dozen arms or tentacles (t) : these zooids are the polypes or hydranths.
Less numerous, and found chiefly towards the proximal region of
the colony, are long cylindrical bodies or blastostyles (bis), each
enclosed in a transparent case, the gonotheca (g.th), and bearing
numerous small lateral offshoots, varying greatly in form according
to their stage of development, and known as medusa-buds (m.bd). By
studying the development of these structures, and by a comparison
with other forms, it is known that both blastostyles and medusa-
buds are zooids, so that the colony is trimorphic, having zooids of
three kinds.

To make out the structure in greater detail, living specimens
should be observed under a high power. A polype is then seen
to consist of a somewhat cylindrical, hollow body, of a yellowish
colour, joined to the common stem by its proximal end, and pro-
duced at its distal end into a conical elevation, the mamifirium or
hypostome (mrib\ around the base of which are arranged the twenty-
four tentacles in a circle. Both body and manubrium are hollow,
containing a spacious cavity, the enteron (cnt), which communicates
with the outer world by the mouth (mth), an aperture placed at
the summit of the manubrium. The mouth is capable of great
dilatation and contraction, and accordingly the manubrium appears
now conical, now trumpet-shaped. Under favourable circum-
stances small organisms may be seen to be caught by the polypes
and carried towards the mouth to be swallowed.

The hydrotheca (h.th) has the form of a vase or wine-glass, and

is perfectly transparent and colourless. A short distance from its
VOL. i K

^

FIG. 95. Obelia sp. A, portion of a colony with certain parts shown in longitudinal section;
B, medusa ; C, the same with reversed umbrella ; D, the same, oral aspect ; Bd. 1, 2, buds ;
bis. blastostyle ; c<x. coenosarc ; ect. ectoderm ; end. endoderm ; ent. enteric cavity ; g.th.
gonotheca; h.th. hydrotheca ; I, lithocyst; m.bd. medusa-bud; mnb. manubrium ; msgl.
mesogloea ; mth. mouth ; p. perisarc ; P. 1, 2, 2, polypes ; rod. c. radial cauui ; t. tentacle ;
vl. velum.

SECT, iv PHYLUM CCELENTERATA 131

narrow or proximal end, it is produced inwards into a sort of
circular shelf (sh), perforated in the centre : upon this the base of
the polype rests, and through the aperture it is continuous with the
common stem. When irritated by a touch or by the addition of
alcohol or other poison the polype undergoes a very marked con-
traction : it suddenly withdraws itself more or less completely into
the theca, and the tentacles become greatly shortened and curved
over the manubrium (P. 2).

The various branches of the common stem show a very obvious
distinction into two layers : a transparent, tough, outer membrane,
of a yellowish colour and horny consistency, the perisarc (p), and
an inner, delicate, granular layer, the ccenosarc (cce), continuous
by a sort of neck or constriction with the body of each hydranth.
The ccenosarc is hollow, its tubular cavity being continuous with
the cavities of the polypes, and containing a fluid in which a
flickering movement may be observed, due, as we shall see, to the
action of cilia. At the base of each zooid or branch the perisarc
presents several annular constrictions, giving it a ringed appear-
ance : for the most part it is separated by an interval from the
ccenosarc, but processes of the latter extend outwards to it at
irregular intervals, and in the undeveloped zooids (Bd. 2) the two
layers are in close apposition.

In the blastostyle both mouth and tentacles are absent, the
zooid ending distally in a flattened disc : the hydrotheca of a
polype is represented by the gonotheca (ff.tli), which is a cylindrical
capsule enclosing the whole structure, but ultimately becoming-
ruptured at its distal end to allow of the escape of the medusa-
buds. These latter are, in the young condition, mere hollow off-
shoots of the blastostyle : when fully developed they have the
appearance of saucers attached by the middle of the convex
surface to the blastostyle, produced at the edge into sixteen very
short tentacles, and having a blunt process, the manubrium,
projecting from the centre of the concave surface. They are ulti-
mately set free through the aperture in the gonotheca as little
medusae or jelly-fish (B D), which will be described hereafter.

The microscopical structure of a polype (Fig 96) reminds us,
in its general features, of that of such a simple sponge as Ascetta,
but with many characteristic differences. The body is composed
of two layers of cells, the ectoderm (ecf) and the endoderm (end) :
between them is a very delicate transparent membrane, the
mesoglcea or supporting lamella (msgl), which, unlike the inter-
mediate layer of sponges, contains no cells and is practically
structureless. The same three layers occur in the manubrium,
the ectoderm and endoderm being continuous with one another at
the margin of the mouth. The tentacles are formed of an outer
layer of ectoderm, then a layer of mesoglcea, and finally a solid
core of large endoderm cells arranged in a single series. The

K 2

132

ZOOLOGY

SECT.

coenosarc, blastostyles, and medusa-buds all consist of the same
layers, which are thus continuous through the entire colony.

The perisarc or transparent outer layer of the stem shows no
cell -structure, but only a delicate lamination. It is, in fact, not a
cellular membrane or epithelium, like the ectoderm and endoderm,
but a cuticle, formed, layer by layer, as a secretion from the ectoderm
cells (see p. 31). It is composed of a substance of chitinoid or horn-
like consistency, and, like the lorica of many Protozoa, serves as a
protective external skeleton. When first formed it is of course in
contact with the ectoderm, but when the full thickness is attained

FIG. 96. Obelia sp. Vertical section of a polype, highly magnified ; cct. ectoderm ; end. endo-
derm ; ent. enteric cavity ; h.th. hydrotheca ; msgl. mesoglcea ; mth. mouth ; ntc. nematocysts ;
sh. shelf -like prolongation of hydrotheca ; t. tentacle.

the latter retreats from it, the connection being maintained only
at irregular intervals. In the same way the hydro- and gonothecse
are cuticular products of the polypes and blastostyles respectively :
in the young condition both occur in the form of a closely fitting
investment of the knob-like rudiment of the zooid (Fig. 95, Bd 1, 2).
The ectoderm has the general character of a columnar epithelium
(see p. 24), but exhibits considerable differentiation of its component
cells. It is mainly composed of large conical cells with their bases
outwards, and having between their narrow inner ends clumps of
small rounded interstitial cells, and occasional large branched nerve-

IV

PHYLUM CCELENTERATA

133

cells (Fig 98, nv.c). The tentacles and the manubrium contain,
in addition, a layer of unstriped muscle-fibres between the ectoderm
and the mesogloea : they are arranged longitudinally, and serve
for the rapid shortening of the tentacles (Fig. 98, mf). This
muscular layer is a derivative of the ectoderm, and may be looked
upon as a rudimentary mesoderm.

FIG. 97. Nematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb,
cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nematocyst ; nv.c. nerve-cell. (From Parker's
Biology, after Schneider.)

Embedded in the ectoderm are numerous clear ovoid bodies, the
stinging -capsules or ncmatocysts (Figs. 96 98 ntc\ organs closely
resembling those of Epistylis umbellaria (p. 93), and like them,
serving as weapons of offence. Each consists (Fig. 97, A) of a tough
ovoid capsule, full of fluid, and invaginated at one end in the form
of a hollow process continued into a long, coiled, hollow thread.
The whole apparatus is developed into an interstitial cell called a
cnidoblast (cnb), which, as it approaches maturity, migrates towards

134

ZOOLOGY

SECT.

nit

end

nv.c

the surface and becomes embedded in one of the large ectoderm
cells. At one point of its surface the cnidoblast is produced into
a delicate protoplasmic process, the cnidocil or trigger-hair (cnc) :
when this is touched for instance by some small organism
brought into contact with the waving tentacle the cnidoblast
undergoes a sudden contraction, and the pressure upon the stinging
capsule causes an instantaneous eversion of the thread (B), at the
base of which are minute barbs. The threads are poisonous, and

exert a numbing effect on the
animals upon which Obelia
preys.

The endoderm also has the
general character of a columnar
epithelium. In the body of the
polype the cells are very large
and have the power of sending
out pseudopods at their free
ends (Fig. 96), which apparently
seize and ingest minute portions
of the partly-digested food. As
in many Protozoa, the pseudo-
pods may be drawn in and long
fiagella protruded, the contrac-
tion of which causes a constant
movement of the food particles
in the enteron. Amongst these
large cells are narrow cells with
very granular protoplasm : they
are gland-cells, and secrete a
digestive juice. In the manu-
brium a layer of endodermal
muscle-fibres has been described
taking a transverse direction,
and so serving to antagonise
the longitudinal muscles and
contract the cavity. In the
tentacles (Figs. 96 and 98) the
endoderm (end) consists of a

single row of short cylindrical cells, nearly cubical in longitudinal
section: their protoplasm is greatly vacuolated and their cell-walls
so thick that they may be considered as forming a sort of internal
skeleton to the tentacles.

The structure of the medusae formed, as we have seen, by the

development of medusa-buds liberated from a ruptured gonotheca

-yet remains to be considered. The convex outer surface of the

bell or umbrella (Fig. 95, B D) by which the zooid was originally

attached to the blastostyle is distinguished as the ex-umbrella, the

f

Fio. 98. Tentacle of Eucopella. The
lower part of the figure snows the ex-
ternal surface, in the middle part the
ectoderm is removed and the muscular
and nervous layer exposed, in the upper
part these latter are removed so as to
show the core of endoderm cells ; ect.
ectoderm ; end. endoderm ; m /. muscle-
fibres ; ntc. nematocyst ; nu. nucleus ;
nv.c. nerve-cell. (After von Lendenfeld.)

IV

PHYLUM CCELENTERATA

135

concave inner surface as the sub-umbrella. From the centre of
the sub-umbrella proceeds the manubrium (jnnb}, at the free end
of which is the four-sided mouth (mth). Very commonly, as the
medusa swims the umbrella becomes turned inside out, the sub-
umbrella then forming the convex surface and the manubrium
springing from its apex (Fig. 95, C, and Fig. 99. A).

The mouth (Figs. 95, 96, 99, and 100, mth) leads into an enteric
cavity which occupies the whole interior of the manubrium, and
from its dilated base sends off four delicate tubes, the radial
canals (rad. c), which pass at equal distances from each other
through the substance of the umbrella to its margin, where they all
open into a circular canal (circ. c), running parallel with and close
to the margin. By means of this system of canals the food, taken

yen,

FIG. 99. Obelia sp. A, mature medusa swimming with everted umbrella ; B, one quarter
of the same, oral aspect ; circ.c. circular canal ; gon. goiiad ; /. lithocyst ; mnb. manubrium ;
mth. mouth ; rad. c. radial canal ; t. tentacle. (After Haeckel.)

in at the mouth and digested in the manubrium, is distributed to
the entire medusa.

The edge of the umbrella is produced into a very narrow fold or
shelf, the velum (Fig. 100, vl), and gives off the tentacles (t), which
are sixteen in number in the newly-born medusa (Fig. 95), very
numerous in the adult (Fig. 99). At the bases of eight of the
tentacles two in each quadrant are minute globular sacs (/),
each containing a calcareous particle or lithitc. These are the
marginal sense-organs or litkocysis : they were formerly considered
to be organs of hearing, and are hence frequently called olocysts :
in all probability their function is to guide the medusa by
enabling it to- judge of the direction in which it is swimming.
The marginal organs, in this case, may therefore be looked upon
as organs of the sense of direction.

The manubrium (Fig. 100, mnb) of the medusa consists of

136

ZOOLOGY

SECT.

precisely the same layers as that of the hydranth ectoderm,
mesoglcea, and endoderm. The ectoderm is continued on to the
sub-umbrella, and then round the margin of the bell on to the
ex-umbrella, so that both surfaces of the bell are covered with
ectoderm. The endoderm is continued from the base of the enteric
cavity into the radial canals and thence to the circular canal,
so that the whole canal-system is lined by endoderm. In the
portions of the bell between the radial canals there is found,
between the outer and inner layers of ectoderm, a thin sheet of
endoderm, the endoderm-lamella (end. lam), which stretches
between adjacent radial canals and between the circular canal
and the enteric cavity. In the bell, as in the manubrium, a

end torn

FIG. 100. -Dissection of a medusa with rather more than one-quarter of the umbrella and manu-
brium cut away (diagrammatic). The ectoderm is dotted, the endoderm striated, and the
mesoglcea black, circ. c. circular canal ; end. lam. endoderm lamella ; gon. gonad ; I. lithocyst ;
mnb. manubrium ; mth. mouth ; rail. c. radial canal ; vl. velum.

layer of mesoglcea everywhere intervenes between ectoderm and
endoderm.

The velum (vl) consists of a double layer of ectoderm and a
middle one of mesogloea : there is no extension of endoderm into
it. The tentacles, like those of the hydranth, are formed of a
core of endoderm covered by ectoderm, the cells of the latter
being abundantly supplied with stinging-capsules.

Comparison of Polype and Medusa. Striking as is the
difference between a polype and a medusa, they are strictly
homologous structures, and the more complex medusa is readily
derivable from the simpler polype- form. It is obvious, in the
first instance, that the apex of the umbrella corresponds with the
base of a hydranth (Fig. 101, A and D), being the part by which
the zooid is attached in each case to the parent stem : the mouth
with the manubrium are also obviously homologous structures in

IV

PHYLUM CCELENTERATA

137

the two cases. Suppose the tentacular region of a polype to be
pulled out, as it were, into a disc-like form (B), and afterwards to
be bent into the form of a saucer (C) with the concavity distal,

B

eel

FIG. 101. Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal,
and A', transverse section (along the line ab) of polype-form ; B, polype-form with extended ten-
tacular region ; C, vertical, and C', transverse section (along the line ab) of form with tentacular
region extended into the form of a bell ; D, vertical, and D', transverse section (along the line ah)
of medusa. The ectoderm is dotted, the endoderm striated, and the mesogloea black, dr. r.
circular canal; net. ectoderm; end endoderm; on/, lam. endoderm lamella; cnt. cm\ enteric
cavity; lujp. hypostome or manubrium ; mnb. manubrium ; m#r2. mesogloea ; mth. mouth;
nv. >ii?', nerve-rings ; t. tentacle ; c. velum. (From Parker's Bioloyy.)

i.e. towards the manubrium. The result of this would be a medusa-
like body (C, C') with a double wall to the entire bell, the narrow
space between the two layers containing a prolongation of the

138

ZOOLOGY

SECT.

enteron (ent. cct-v') and being lined with endoderm. From such a
form the actual condition of things found in the medusa would be
produced by the continuous cavity in the bell being for the most
part obliterated by the growing together of its walls so as to form

\

\

a,d- radiws

sub radius ~_,

per-radius

FIG 102 '-Projections of polype (A) and medusa (B), showing the various orders of radii;
yon. gonad ; mnb. manubrium.

the endoderm-lamella (D', cwl. lam], and remaining only along
four meridional areas the radial canals (rail, r), and a circular
area close to the edge of the bell the circular canal (dr. c).

While both polype and medusa are radially symmetrical, the increase in
complexity of the medusa is accompanied by a differentiation of the structures
lying along certain radii. If a polype is projected on a plane surface (Fig. 102, A),

iv PHYLUM CCELENTERATA 139

taken at right angles to its long axis, a large number of radii about twenty-
four can be drawn from the centre outwards, all passing through similar parts,
i.e. along the axis of a tentacle and through similar portions of the body and
manubrium. But in the medusa (B) the case is different. The presence of the
four radial canals allows us to distinguish four principal radii or per-radii. Half
way between any two per-radii a radius of the second order, or inter-radius, may
be taken ; half way between any per-radius and the inter-radius on either side a
radius of the third order, or ad-radius, and half way between any ad-radius and
the adjacent per- or inter-radius, a radius of the fourth order, or sub-radius. Thus
there are four per-radii, four inter-radii, eight ad-radii, and sixteen sub-radii.
In Obelia the radial canals, the angles of the mouth, and four of the tentacles are
per-radial, four more tentacles are inter-radial, and the remaining eight tentacles,
bearing the lithocysts, are ad-radial. The sub-radii are of no importance in this
particular form.

Reproduction. In the description of the fixed Obelia-colony
no mention was made of cells set apart for reproduction, like
the ova and sperms of a sponge. As a matter of fact, such sexual
cells are found only in their fully developed condition at least
in the medusae. Hanging at equal distances from the sub-umbrella,
in immediate relation with the radial canal and therefore per-
radial in position, are four ovoid bodies (Figs. 99 and 100, gon),
each consisting of an outer layer of ectoderm continuous with
that of the sub-umbrella, an inner layer of endoderm continuous
with that of the radial canal and enclosing a prolongation of the
latter, and of an intermediate mass of cells which have become
differentiated into ova or sperms. As each medusa bears organs
of one sex only (testes or ovaries, as the case may be), the individual
medusae, are dioecious. It will be noticed that the gonad has the
same general structure as an immature zooid an outpushing
of the body-wall consisting of ectoderm and endoderm, and
containing a prolongation of the enteric cavity.

Development.- -When the gonads are ripe, the sperms of the
male medusae are shed into the water and carried by currents to
the females, impregnating the ova, which thus become oosperms
or unicellular embryos. The oosperm undergoes complete seg-
mentation (Fig. 103, A F), and is converted into an ovoidal body
called a planula (G, H), consisting of an outer layer of ciliated
ectoderm cells and an inner mass of endoderm cells in which a
space appears, the rudiment of the enteron. The planula swims
freely for a time (H), then settles down on a piece of timber, sea-
weed, &c., fixes itself by one end (K), and becomes converted into
a Jiydrulct or simple polype (L, M), having a disc of attachment at
its proximal end, and at its distal end a manubrium and circlet of
tentacles. Soon the hydrula sends out lateral buds, and, by a
frequent repetition of this process, becomes converted into the
complex Obelia-colony with which we started.

This remarkable life-history furnishes the first example we have
yet met with among the Metazoa of alternation of generations, or

140

ZOOLOGY

8ECT.

metagenesis (see p. 41). The Obelia-colony is sexless, having no
gonads, and developing only by the asexual process of budding ;
but certain of its buds the medusae develop gonads, and from

o

FIG. 103 Stages in the development of two Zoophytes (A H, Laomedea, I M, Ewden-
drium) allied to Obelia ; A F, stages in segmentation ; G, the planula enclosed in the
maternal tissues ; H, the free-swimming planula ; I M, fixation of the planula and develop-
ment of the hydrula. (From Parker's Biology, after Allman.)

their impregnated eggs new Obelia-colonies arise. We thus have
an alternation of an asexual generation, or agamobium the Obelia-
colony, with a sexual generation, or gamobium the medusa.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Hydrozoa may be defined as multicellular animals in which
the cells are arranged in two layers, ectoderm and endoderm,
separated by a gelatinous, non-cellular mesoglcea, and enclosing
a continuous digestive cavity which communicates directly with
the exterior by a single aperture the mouth and is lined through-
out by endoderm. The ectoderm consists of epithelial cells, inter-
stitial cells, muscle-fibres, and nerve-cells. Certain of the inter-
stitial cells give rise to characteristic organs of offence the
stinging-capsules. The endoderm consists of flagellate or amoeboid
cells, gland-cells, and sometimes muscle-fibres. There are two
main forms of zooids, polypes or nutritive zooids, which are
usually sexless, and medusae or reproductive zooids. In corre-
spondence with its locomotive habits, the medusa attains a higher

iv PHYLUM CCELENTERATA 141

degree of organisation than the polype, having more perfect
muscular and nervous systems, distinct sense-organs, and a diges-
tive cavity differentiated into central and peripheral portions, the
latter taking the form of radial and circular canals. The repro-
ductive products are discharged externally, and are very commonly,
though not always, of ectodermal origin.

Many Hydrozoa agree with Obelia in exhibiting alternation of
generations, the asexual generation being represented by a fixed,
more or less branched hydroid colony, the sexual generation by a
free-swimming medusa. In other forms there are no free medusae,
but the hydroid colony produces fixed reproductive zooids. In
others, again, there is no hydroid stage, the organism existing only
in the medusa-form. Then, while in most instances the only
skeleton or supporting structure is the horny perisarc, there are
some forms in which the ccenosarc secretes a skeleton of calcium
carbonate, forming a massive stony structure or coral. Lastly,
there are colonial forms which, instead of remaining fixed, swim
or float freely on the surface of the ocean, and such pelagic species
are always found to exhibit a remarkable degree of polymorphism,
the zooids being of very various forms and performing diverse
functions.

Thus we have zoophyte colonies known to produce free medusae,
zoophyte colonies known not to produce free medusae, and medusa
known to have no zoophyte stage. Moreover, there are many
medusas of which the life-history is unknown, so that it is un-
certain whether or not a zoophyte stage is present. It is also
found that in some cases closely allied zoophytes produce very
diverse medusae, while similar medusae, in other cases, may spring
from very different zoophytes. For these reasons a sort of double
classification of the Hydrozoa has come about, some zoologists
approaching the group from the point of view of the zoophyte,
others from that of the medusa. On the whole the following
scheme seems best adapted for bringing before the beginner the
leading modifications of the class.

ORDER 1. LEPTOLIN.E.

Hydrozoa in which there is a fixed zoophyte stage, and in which
the sense-organs are exclusively ectodermal.

Sul-Ordcr a. Anthomedit.sce.

Leptolinse in which the polypes are not protected by hydrothecre or the
reproductive zooids by gonotheca? : the medusa? bear the gonads on the manu-
brium and have no lithocysts.

I

Sub-Order 1). Leptomedusce.

Leptolinse in which hydro- and gonothecce are present : the medusa? bear the
gonacls in connection with the radial canals and usually have lithocysts.

142 ZOOLOGY SECT.

ORDER 2. TRACHYLIN.E.

Hydrozoa in which no fixed zoophyte stage is known to occur,
all members of the group being locomotive medusae, some of which
have been proved to develop directly from the egg. The sense-
organs are formed partly of endoderm.

Sub-Order a. Trachymeduscv.

Trachylinse in which the tentacles spring from the margin of the umbrella,
and the gonads are developed in connection with the radial canals.

Sub-Order b. Narcomedusce.

Trachylinee in which the tentacles spring from the ex-umbrella, some dis-
tance from the margin, and the gonads are developed in connection with the
manubrium.

ORDER 3. HYDROCORALLINA.

Hydrozoa in w r hich a massive skeleton of calcium carbonate is
secreted from the ccenosarc, the dried colony being a coral.

ORDER 4. SIPHONOPHORA.

^

Pelagic Hydrozoa in which the colony usually exhibits extreme
polymorphism of its zooids.

ORDER 5. GRAPTOLITHIDA.

An extinct group of Hydrozoa, found only in rocks of Palaeozoic
age, in the form of the fossilised perisarc of the branched colonies.

Systematic Position of the Example.

Obelia, in virtue of the possession of gono- and hydrotheca9, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusse. It is placed in the family Oampanu-
lariidcv, distinguished by having cup-shaped theca? borne at the
ends of distinct branchlets : the genus Obelia is distinguished
from other genera of the same family by the fact that the
reproductive zooids are free-swimming medusae.

ORDER 1. LEPTOLIN.E.

The more typical members of this group agree in all essential
respects with Obelia, consisting of branched colonies bearing two
principal forms of zooids, which serve for nutritive and reproductive
purposes respectively.

General Structure.- -The form and size of the colonies are
subject to great variation : they may be little insignificant tufts
growing on shells, sea-weeds, &c., or may take the form of com-
plex trees three feet in height, and containing many thousand

IV

PHYLUM CCELENTERATA 143

zooids. The hydranths may be colourless and quite invisible to
the naked eye, or, as in some Tubulariae (Fig. 105, 5) may be bril-
liantly coloured, flower-like structures, nearly an inch in diameter.
The medusae may be only just visible to the naked eye, or, as in
jffiquorca, may attain a diameter of 38 mm., or about 15 inches:
they are often seen with great difficulty owing to the bubble-like
transparency of the umbrella ; but frequently the manubrium is
brightly coloured, or brilliant dots of colour the ocelli or eye-spots
may occur around the margin of the umbrella. They are also
frequently phosphorescent, the phosphorescence of the ocean being
often due to whole fleets of medusae liberated in thousands from
the hydroid colonies beneath the surface.

The two sub-orders of Leptolinae are distinguished by the
arrangement of the perisarc. In the Anthomedusae, of which
Bouyainvillea (Fig. 104) is a good example, the cuticle stops short at
the bases of the hydranths, and the reproductive zooids are not
enclosed in gonothecse. It is for this reason that, in classifications
founded on the zoophyte stage, the Anthomedusse are called Gymno-
Uastea or naked-budded zoophytes (see also Fig. 105, 1, 4, ) In
the Leptomedusse the cuticle is usually of a firmer consistency than
in the first sub-order, and furnishes hydrothecae for the hydranths
and gonothecaa for the reproductive zooids : they are hence often
classified as Cafyptoblastea or covered-budded hydroids. To this
group belong the commonest species of hydroids found on the sea-
shore, and often mistaken for seaweeds the " Sea-firs " or Sertu-
larians.

The medusae also exhibit characteristic differences in the two
sub-orders. In the Anthomedusse the umbrella is usually strongly
arched, and may even be conical or mitre-shaped (Figs. 104 ; 105, 7 ;
109, 1 and 2) : its walls are thick, owing to a great development of
the gelatinous mesogkeaof the ex-umbrella, that of the sub-umbrella
remaining thin ; and the velum is considerably wider than in Obelia.
But the most important characteristics are the facts that the
gonads(//cw) are developed on the manubrium and that lithocysts are
absent. Sense-organs are, however, present in the form of specks
of red or black pigment at the bases of the tentacles. These ocelli
(pc) consist of groups of ectoderm cells containing pigment, and it
has been proved experimentally that they are sensitive to light :
they are, in fact, the simplest form of eyes. In the Leptomedusae
the umbrella is usually less convex, thinner, and of softer consist-
ency than in the Anthomedusaa, the gonads are developed as buds
formed in connection with the radial canals and projecting from
the sub-umbrella, the velum is feebly developed, and sense-organs
take the form sometimes of ocelli, but usually of lithocysts.

In the majority of LeptolinaB the coenosarc, as in Obelia, con-
sists of a more or less branched structure attached to stones, timber,
seaweeds, shells,&c., by a definite root-like portion (hydrorhiza). The

144

ZOOLOGY

SECT. IV

curious genus Hydractinia (Fig. 105, 1) is remarkable for possessing
a massive coenosarc, consisting of a complex arrangement of
branches which have undergone fusion, so as to form a firm
brownish crust on the surfaces of dead gastropod shells inhabited
by Hermit-crabs. The constant association of Hydractinia with

FIG. 104. Bougainviilea ramosa. A, entire colony, natural size; B, portion of the same
magnified; C, immature medusa, dr. c. circular canal; ci(. cuticle or perisarc ; tnt. car.
enteric cavity; hyd. polype or hydranth ; ky/>. hypostome or maiiubriuni ; med. medusa; mnb.
niaimbrium ; rod. c. radial canal ; t. tentacle ; c. velum. (From Parker's Biology, after
Alhaan.)

Hermit-crabs is a case of commensalism : the hydroid feeds upon
minute fragments of the Hermit-crab's food, and is thus its com-
mensal or messmate ; and the Hermit-crab is protected from its
enemies by the presence of the inedible, stinging hydroid.
Hydractinia belongs to the Anthomedusas : the Leptomedusan

1. HydracHnia

2.Myriofhela

3. Corymorpha

ntf.cs- A ^.Syncoryne

JL

7. Sarsia

6. Clavarella

FIG. 105. Various forms of Leptolinae. In 1, a shows the entire colony, b a portion higlily
magnified ; in 7, a is a species producing medusa-buds from the manubrium, b from the bases
of the tentacles ; dz. dactylozooids ; m. and M. medusae ; rnnb. manubrium ; inth. mouth ;
oc. eye-spots ; rod. c. radial canals ; s. sporosacs ; s^. spines ; t, t l , <-, tentacles.

VOL. I L

146

ZOOLOGY

SECT.

Clat7irozoon, an Australian genus, resembles it in having branched
and intertwined coenosarcal tubes, the perisarc of which under-
goes fusion ; but the complex mass thus produced, instead of
forming an incrustation on a shell, is a large, abundantly branched,
tree-like structure, resembling some of the fan-corals or Gorgonacea
(vide infra). Ceratella (Fig. 100) has a similar fan-coral-like
appearance, with a branching axis composed of numerous inter-

v<

4 $ 9 jf&iJt Hrfsr^^Wa '^ )/

U/J$ILJK* /^^ % b

>N> >\S\ ^ <

i

\ s -^*-O lr-iv>v._^ i/i.i MPi (i

VV "- L ^=-'^v""C '-L. ^v

K ^ix a vT^f^ >

f*3rj? "v\ -y? -rs /y

&i ^3C &

">^Tf? -f^i

FIG. 106. Ceratella fusca. About nat. size. (From Hickson, after Baldwin Spencer.)

twining and anastomosing tubes ; but while Clathrozoon possesses
thecse, in Ceratella they are absent.

A great simplification of the colony is produced in Myriothela
(Fig. 105 #), in which the short coenosarc bears a single large
terminal hydranth, and gives off numerous slender branches which
bear the reproductive zooids (s). Even greater simplicity is found
in Corymorplia (3), in which the entire organism consists of a
single stalked polype, from the tentacular region of which the
medusae (m) arise.

IV

PHYLUM CCELENTERATA

147

But the simplest members of the whole class, with the exception
of one or two imperfectly known forms which will be referred to

cnc

_! TMS _

100 mm

SCALE FOR A

FIG. 107! Hydra. A, vertical section of entire animal ; B, portion of transverse section, highly
magnified ; C, two large ectoderm cells ; D, endoderm cell of H. viridis ; E, large uematocyst ;
F, small nematocyst ; G, sperm, a, ingested diatom ; bd. 1, bd. 2, buds ; chr. chromatophores ;
cnbl. cnidoblast ; cnc. cnidocil ; ect. ectoderm ; end. endoderm ; ent. cav. enteric cavity ; ent.
cav'. its prolongation into the tentacles ; fl. flagellum ; liyp. hypostome or manubrium ; int. c.
interstitial cells ; m. pr. muscle-processes ; mth. mouth : msgl. mesogloaa ; ntc. large, and ntc'.
small nematocysts ; nu. nucleus ; ov. ovum ; ovy. ovary ; pad. pseudopods ; *py. spermary ;
vac. vacuole. (From Parker's Elementary Biology, after Lankester and Howes.)

below, are the Fresh-water Polypes of the genus Hydra. The
entire organism (Figs. 27 and 107) consists of a simple cylindrical

L 2

148 ZOOLOGY SECT.

body with a conical hypostome and a circlet of six or eight
tentacles. It is ordinarily attached, by virtue of a sticky secretion
from the proximal end, to weeds, &c., but is capable of detaching
itself and moving from place to place after the manner of a loop-
ing caterpillar. The tentacles are hollow, and communicate freely
with the enteron. Both the body and the tentacles are highly
contractile, the contractions being effected by means of a layer of
fibres which run longitudinally. These fibres are processes the
muscle processes (C, m. pr.) of the large ectoderm cells. Similar
shorter muscle-processes of some of the endoderm cells run
circularly and antagonise the longitudinal fibres. Nematocysts are
abundant in the ectoderm. The endoderm cells are mostly
amoeboid and vacuolated. Each usually bears one or more flagella,
but these may be retracted. Glandular cells occur here and there.
Nerve-cells (multipolar) occur in both layers, but present no regular
arrangement. There is no perisarc. Buds (bd. 1, bd. 2) are
produced which develop into Hydrse, but these are always detached
sooner or later, so that a permanent colony is never formed. There

FIG. lOS.r-Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the

disc of attachment to the right.

are no special reproductive zooids, but simple ovaries (ovy) and
testes (spy) are developed, the former at the proximal, the latter
at the distal end of the body. Even simpler than Hydra are
Protoliydra (Fig. 108) and Microhydra, in which the tentacles are
absent.

Pelagohydra is also solitary, but is pelagic. The part .corres-
ponding to the base in Hydra here takes the form of a float, and
there are tentacles distributed over the surface of the float as well as
in the neighbourhood of the mouth ; medusae are developed from
processes on the float. Pelagohydra, however, is perhaps more
nearly related to the Siphouopfiora an order yet to be dealt with
-than to the LeptolinaB.

The polypes are usually cylindrical, as in Obelia, but in some
genera they are widened out into a vase-like form (Fig. 105, 5), in
others elongated into a spindle-shape (4). The tentacles may be
disposed in a single circlet, as in Obelia and Hydra, or there may
be an additional circlet round the hypostome (3, 5), or at the base of
the polype, or they may be scattered irregularly over the whole
surface (4\ In Myriothela (#) they are short, and so numerous
as to have the appearance of close-set papillae. In some forms

iv PHYLUM CCELENTERATA 149

they are knobbed at the ends, the knobs being loaded with stinging-
capsules (4).

In some species a- dimorphism of the hydranths obtains, some
of them being modified to form protective zooids. In Hydractinia
(7) these are simply mouthless hydranths with very short tentacles
abundantly supplied with nematocysts, capable of very active
movements, and called dactylozooids (dz). In Plumularia there are
small structures called "guard-polypes," resembling tentacles in
structure, with very numerous nematocysts, and each enclosed
in a theca. In Hydractinia the coenosarc is also produced into
spines (sp), which may be much modified zooids.

But the most remarkable modifications occur in the repro-
ductive zooids. In a large proportion of genera, both of
Anthomedusae and Leptomedusse, these take the form of locomotive
medusae, agreeing in general structure with the descriptions
already given. Each appears at first as a hollow bud-like process
of the blastostyle, or of an ordinary polype, or, more exceptionally,
of the coenosarc. This becomes constricted at the junction and
rounded off. The ectoderm at its free extremity becomes
thickened, and this thickening, as it grows, pushes the endoderm
before it, producing a sort of involution. In the interior of the
mass of ectoderm a cavity appears : this is destined to form the
sub-umbrellar cavity. The ectodermal partition that at first
separates the cavity from the exterior, becomes perforated and
most of it is absorbed, what remains round the edge going
to form the velum. The endoderm is reduced to a thin layer
except along four radial lines where it gives rise to the four
radial canals, the thin parts between going to form the endoderm
lamella.

In different families and genera the medusae exhibit almost end-
less variety in detail. As to size they vary from about 1 mm. in
diameter up to 400 mm. (16 inches). The number of tentacles
may be very great (Fig. 109, 2) or these organs may be reduced to
two (Fig. 109, 7), or even to one (Fig. 105, 3) ; in the last-named
cases it will be noticed that the medusa is no longer radially, but
bilaterally symmetrical, i.e. it can be divided into two equal and
similar halves by a single plane only viz., the plane passing
through the one or two tentacles. With the increase in the
number of the tentacles a corresponding increase in that of the
radial canals often takes place (Fig. 109, J).

Some medusae creep over submarine surfaces, walking on the
tips of their peculiarly modified tentacles (Fig. 105, 6) but the
majority propel themselves through the water in a series of jerks
by alternately contracting and expanding the umbrella, and so,
by rhythmically driving out the contained water, moving with
the apex foremost. In correspondence with these energetic move-
ments there is a great development of both muscular and

SECT. IV

PHYLUM CCELENTERATA

151

nervous systems. The velum and the sub-umbrella possess
abundance of muscle-fibres, presenting a transverse striation,
and round the margin of the umbrella is a double ring of nerve-
cells and fibres, one ring being above, the other below the at-
tachment of the velum (Fig. 101, D, nv, nv). The medusae thus
furnish the first instance we have met with of a central nervous
system, i.e. a concentration of nervous tissue over a limited area
serving to control the movements of the whole organism. It has
been proved experimentally that the medusae is paralysed by
removal of the nerve-ring. Over the whole sub-umbrella is a
loose network of nerve-cells and fibres connected with the nerve-
ring, and forming a peripheral nervous system.

In some medusae the circular canal communicates with the
exterior by minute pores placed at the summits of papillae, the

FIG. 110 Diagram illustrating the formation of a sporosac by the degradation of a medusa. A,
medusa enclosed in ectoderrnal envelope (es) ; B, intermediate condition with vestiges of
umbrella () and radial canals (ra) ; C, sporosac. ec. ectoderm ; en, endoderiu ; m, nianubrium ;
ov, ovary ; t, tentacle ; r, velum. (From Lang's Comparative Anatomy.)

endoderm cells of which contain brown granules. There seems to
be little doubt that these are organs of excretion, the cells with-
drawing nitrogenous waste-matters from the tissues and passing
them out through the pores. If we except the contractile
vacuoles of Protozoa, this is the first appearance of specialised
excretory organs in the ascending series of animals.

Besides producing gonads, some medusae multiply asexually by
budding, the buds being developed either from the manubrium
(Fig. 105, 7a), or from the margin of the umbrella (76) or the base
of the tentacles. The buds always have the medusa form.

In many Leptolinae the reproductive zooids undergo a degrada-
tion of structure, various stages of the process being found in
different species. Almost every gradation is found, from perfect
medusae to ovoid pouch-like bodies called sporosacs (Fig. 105, Ib,
5, s), each consisting of little more than a gonad, but showing an in-
^dication of its true nature in a prolongation of the digestive cavity

152

ZOOLOGY

SECT.

of the colony, representing the stomach of the manubrium (Fig.
110). We thus have a reproductive zooid reduced to what is
practically a reproductive organ. It is obvious that a continua-
tion of the same process might result in the production of
a simple gonad like that of Hydra : there is, however, no evidence
to show that the Fresh- water Polype ever produced medusae, and
the probabilities are that its ovaries and testes are simply gonads,
and not degenerate zooids. The case is interesting as showing
how a simple structure may be imitated by the degradation of a
complex one. It is quite possible, on the other hand, that the
reproductive organs of the Leptomedusae (Fig. 100) are sporosacs,
i.e. reproductive zooids, not mere gonads. In some rare cases the

FIG. 111. Early development of Eucope. A, blastuhvstage ; B, planula with solid endoderm ;
C, planula with enteric cavity ; at. enteric cavity ; ep. ectoderm ; liy. endoderm. (From
Balfour's Embryology, after Kowalevsky.)

sexual cells are not developed either in medusae or in sporosacs,
but are formed directly in the blasfcostyles.

In Obelia we found the medusae to be budded off from pecu-
liarly modified mouthless zooids the blastostyles. This arrange-
ment, however, is by no means universal : the reproductive zooids
-whether medusae or sporosacs may spring directly from the
coenosarc, as in Bougainvillea (Fig. 104), oT from the ordinary
hydranths (Fig. 105, 4 and 5). The primitive sex-cells, from which
ova or sperms are ultimately developed, are sometimes formed
from the endoderm or (more usually) ectoderm cells of the repro-
ductive zooid; but in many cases originate in the coenosarc, and
slowly migrate to their destination in the ectoderm of the gonad,
where they metamorphose in the usual way into the definitive re-
productive products, which when mature pass into the space below
the ectoderm of the gonad.

The development of the Leptolinas frequently, but not always,

IV PHYLUM CCELENTERATA 153

begins within the maternal tissues, i.e. while the oosperm or im-
pregnated egg- cell is still contained in the gonad of the medusae or
in the sporosac. The oosperrn divides into two cells, then into
four, eight, sixteen, &c. Fluid accumulates in the interior of the
embryo, resulting in the formation of a blastula or hollow globe
formed of a single layer of cells (Fig. Ill, A). The blastula
elongates, and the cells at one pole undergo division, the daughter-
cells passing into the cavity, which they gradually fill (B). At
this stage the embryo is called a planula : it consists of an outer
layer of cylindrical cells the ectoderm which acquire cilia, and an
inner mass of polyhedral cells the endoderm. In some cases the
planula arises by a different process : a solid morula is formed, the
superficial cells of which become radially elongated and form
ectoderm, the central mass of cells becoming endoderm. By
means of its cilia the planula swims freely, and before long a
cavity appears in the middle of the solid mass of endoderm, the
cells of which then arrange themselves in a single layer around
the cavity or enteron (C, al). The planula then comes to rest, fixes
itself at one end to some suitable support, and becomes con-
verted into a simple polype or hydrula by the attached end
broadening into a disc and the opposite extremity forming a
manubrium and tentacles. The hydrula soon begins to send off
lateral buds, and so produces the branched colony.

In Tubularia the oosperm develops, while still enclosed in
the sporosac, into a short hydrula. which, after leading a free
existence for a short time, fixes itself by its proximal end, buds, and
produces the colony. In Hydra development begins in the ovary,
and is complicated by the fact that the ectoderm of the morula
gives rise to a sort of protective shell : in this condition the
embryo is set free, arid, after a period of rest, develops into the
adult form.

ORDER 2. TRACHYLIN.E

General Structure. The members of this order are all
medusae : no zoophyte stage is certainly known in any of them, and
several species have been proved to develop directly from the egg.
They thus differ from the members of the preceding order in the
fact that no alternation of generations ordinarily occurs in their
life-history.

Most species are of small or moderate size, the largest not
exceeding 100 mm. (4 inches) in diameter. The gelatinous tissue
or mesogloea of the ex-umbrella is usually well developed, giving
the medusa a more solid appearance than the delicate jelly-fish of
the preceding order: this is well shown in Fig. 112, in which the
apical region of the umbrella has a comparatively immense thick-
ness. The tentacles are also stiff and strong, and are always solid
in the young condition, although they may be replaced in the
adult by hollow tentacles.

154

ZOOLOGY

SECT.

But the most characteristic anatomical feature of the group is
the structure of the sense-organs, which are club-shaped bodies
(Figs. 112 and 113, tc) consisting of an outer layer of ectoderm

I.Pel'aeue

2. G I o sso c o dor>

V.

FIG. 112.^Two Trachy medusae, dr. c. circular canal; J/OH. gonad ; mub. maiiubrium ; mtk.
mouth ; rail. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentacul'ocyst ; tg. tongue ;
vl. velum. (After Haeckel.)

rad.c

mth
l.Cunarcha

2.Polycol|3a

Fio. 113. Two XVarcoxnedusJe, -1 in vertical section, rton. gonad; mnb. rnanubrium ; mtlt.
mouth ; pr. peronium ; rad.r. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-root ;
v.l. velum. (After Haeckel )

enclosing a central axis of endoderm cells (Fig. 114): they have,
therefore, the structure of tentacles. They contain one or more
lithites, which are always derived from the endoderm. To

IV

PHYLUM CCELENTERATA

155

cct

-csut.

distinguish them from the lithocysts of Leptomedusse, and to mark
the fact that they are modified tentacles, they are called tcntaculo-
cysts. They may either project freely from the margin of the
umbrella, or may become enclosed in a pouch-like growth of
ectoderm and more or less sunk in the tissue of the umbrella.
Eyes occur in some, and are always of simple structure.

The two sub-orders of Trachylinae are characterised by the mode
of origin of the tentacles.

In Trachymedusse. as in the

/ *

preceding order, they arise
near the edge of the um-
brella (Fig. 112), but in the
Narcomedusoe they spring
about half-way between the
edge and the vertex (Fig.
113), and are continued, at
their proximal ends, into the
ielly of the ex -umbrella in
the form of " tentacle-roots "
(t.r).

As to the position of the
reproductive organs, there
is the same difference be-
tween the two sub-orders
of Trachylinoe as between
the two sub-orders of Lepto-

linse. In the Trachyrnedusge the gonads (Fig. 112, goti) are
developed in the course of the radial canals: in the Narcomedusse
(Fig. 113) they lie on the manubrium, sometimes extending into
the pouch-like offshoots of its cavity.

There is always a well-developed velum, which, as in Fig. 113, 1,
may hang down vertically instead of taking the usual horizontal
position. In the NarcomedusaB the manubrium is short ; in the
Trachymedusye it is always well developed, and is sometimes (Fig.
112, 2) prolonged into a long, highly contractile peduncle, having
its inner surface produced into a tongue-like process (tg) which
protrudes through the mouth. In some the gastric cavity is
situated in the manubrium, which in such a case is looked upon as
partly of the nature of a process cf the sub-umbrella (pseudo-
manubrium).

The simplest case of the development of TrachylinaB is seen in
dSginopsis, one of the Narcomedusse. The oosperm gives rise to
a ciliated planula, which forms first two (Fig 115), then four
tentacles, and a mouth, hypostome, and stomach. The larva of
^ginopsis is thus a hyrfrula, closely resembling the corresponding
stage of Tubularia. After a time the tentacular region grows out,
carrying the tentacles with it, and becomes the umbrella of the

FIG. 114. .ffieinura myosura, a tentaculo-
cyst highly magnified, ect. ectoderm ; end.
endoderm ; /. lithites ; ntc. nematocysts ;
nv.c. group of nerve-cells. (After Haeckel).

156 ZOOLOGY SECT.

medusa. Thus the actual formation of the medusa from the
hydrula of ^Eginopsis corresponds precisely with the theoretical
derivation given above (p. 136). It will be seen that in the present
case there is no metagenesis or alternation of generations, but that
development is accompanied by a metamorphosis that is, the egg
gives rise to a larval form differing in a striking manner from the
adult, into which it becomes converted by a gradual series 01
changes.

Metagenesis is, however, ribt quite unknown among the Trachy-
linse. In a parasitic Narcomedusa (Cunina parasitica) the planula

FIG. 115. Larva of JEginopsis. m. mouth; t. tentacle. (From Balfour, after Metschnikoff.)

fixes itself to the manubrium of one of the Trachymedusse which
serves as its host, and develops into a hydrula. But the latter, in-
stead of itself becoming metamorphosed into a medusa, retains the
polype form and produces other hydrulae by budding, these last
becoming converted into medusse in the usual way.

ORDER 3. HYDROCORALLINA.

The best-known genus of Hydroid Corals is Millepora, one species
of which is the beautiful Elk-horn Coral, J\l. alcicornis. The dried
colony (Fig. 116 A) consists of an irregular lobed or branched mass
of carbonate of lime, the whole surface beset with the numerous
minute pores to which the genus owes its name. The pores are
of two sizes : the larger are about 1 or 2 mm. apart, and are called
gastropores (B, g.p) ; the smaller are arranged more or less
irregularly round the gastropores, and are called dactyloporcs (d.p).
The whole surface of the coral between the pores has a pitted
appearance. Sections (C) show that the entire stony mass is
traversed by a complex system of branched canals, which com-
municate with the exterior through the pores. The wide vertical

IV

PHYLUM CCELENTERATA

157

canals in immediate connection with the gastropores are traversed
by horizontal partitions, the tabular (tl)\

In the living animal each pore is the place of origin of a zooid :
from the gastropores protrude polypes (Fig. 117, P) with hypostome
and four knobbed tentacles ; from the dactylopores long, filamentous,
mouthless dactylozooids or feelers (J).Z\ with irregularly disposed
tentacles : the function of these latter is probably protective and
tactile, like that of the guard-polypes of Plumularia and the
dactylozooids of Hydractinia. The bases of the zooids are con-
nected with a system of delicate tubes, which ramify through the

FIG. 116. Millepora alcicornis. A, part of skeleton, natural size ; B, portion of surface,
magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; tb. tabulu-.
(After Nicholson and Lydekker.)

canals of the coral and represent a much-branched ccenosarc,
recalling that of Hydractinia (p. 144).

The ccenosarcal tubes have the usual structure, consisting of
ectoderm and endoderm with an intervening mesogloea. From
the relative position of the parts it will be obvious that the cal-
careous skeleton is in contact throughout with the ectoderm of the
colony : it is, in fact, like the horny perisarc of the Leptolinse, a
cuticular product of the ectoderm.

The only other genus to which we shall refer is Stylaster (Fig.
118), which forms a remarkably elegant tree-like colony, abund-
antly branched in one plane, and of a deep pink colour. On the
branches are little cup-like projections with radiating processes
passing from the wall of the cup towards the centre, and thus

158

ZOOLOGY

SECT.

closely resembling the true cup-corals belonging to the Actinozoa
(vide infra). But in the case of Stylaster each " cup ' is
the locus, not of one, but of several zooicls a polype projecting
from its centre, and a dactylozooid from each of the compartments
of its peripheral portion. A calcareous projection, the style, the
presence of which is the origin of the generic name, rises up from
the tabula at the bottom of each pore.

The gonophores in most species of Millepora are developed in
certain of the pores in dilatations or ampulla ; in one species at

enrf eef

FIG. 117. Millepora. Diagrammatic view of a portion of the living animal, partly from the
surface, partly in vertical section. In the sectional part the ectoderm is dotted, the endoderm
striated, and the skeleton black, ect. ectoderm ; end. endoderm ; d.p. dactylopore ; D.Z.
dactylozooid ; g.p. gastropore ; mtk. mouth ; P. polype ; t. tentacle. (Altered from Moseley.)

the apices of the dactylozooids. They are medusae, but never
have the complete medusa-form, being devoid of velum, mouth,
radial canals and tentacles. Both male and female medusae
become free, but the period of free existence is very
short.

In Stylaster the medusoid character is much more completely
lost ; and the gonophores are more of the nature of sporosacs or

IV

PHYLUM CGKLENTERATA

150

degraded reproductive zooids lodged in special chambers (a) of
the coral.

The Hydrocorallina occur only in the tropical portions of the
Pacific and Indian Oceans, where they are found on the ; ' coral-

Fio. 11&. Stylaster sanguincus. A, portion of skeleton, natural size; B, small portion,
magnified ; a. ampullaj ; d.p. dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.)

reefs " partly or entirely surrounding many of the islands in
those seas. Fossil forms arc found as for back as the Triassic
epoch.

ORDER 4. SIPHOXOPHORA.

The diversity of form exhibited by the members of this order is
so great that anything like a general account of it would only be
confusing to the beginner, and the most satisfactory method of
presentation will be by the study of a few typical genera.

Hnlistemma (Fig. 119 A) occurs in the Mediterranean and other
seas, and consists of a long, slender, floating stem, to which a
number of structures, differing greatly in form, are attached. At one
-the uppermost end of the stem is an ovoid, bubble-like body con-
taining air i\iQ float or pneumatopfiore (pn). Next come a number
of closely set, transparent structures (net), having the general char-
acters of unsymmetrical medusa without manubria, each being a
deep, bell-like body, with a velum and radiating canals. During life
these swimming-bells or nectocalyccs contract rhythmically i.e. at
regular intervals drawing water into their cavities, and immedi-
ately pumping it out, thus serving to propel the entire organism

B

FIG. 111K Halistemma tergestinum. A, the entire colony; B, a single group of zooids.
< . cttfimsarc ; dz. dactylozooid ; /</</<. hydrophyllium or )>ract; wt. nectocalyx or swinniiing-
bell; ntc. battery of neinatocysts ; t>. idlj T po ; pn. pneumatophore or float; s, a', sporoeysts ;
t. tentacle. (After Claus.)

SECT, iv PHYLUM CCELENTERATA 161

through the water. Below the last nectocalyx the character of the
structures borne by the stem changes completely : they are of
several kinds, and are arranged in groups which follow one
another at regular intervals, and thus divide the stem into seg-
ments, like the nodes and internodes of a plant.

Springing from certain of the " nodes " are unmistakable polypes
(p}, differing however from those we have hitherto met with in
having no circlet of tentacles round the mouth, but a single long
branched tentacle (t) arising from the proximal end, and bearing
numerous groups or " batteries " of stinging-capsules (ntc). In
the remaining nodes the place of the polypes is taken by dactylo-
zooids or feelers (dz) mouthless polypes, each with an unbranched
tentacle springing from its base. Near the bases of the polypes
and dactylozooids spring groups of sporosacs (B, s, s'), some male,
others female ; and finally delicate, leaf-like, transparent bodies
the bracts or hydrophyllia (hph) spring from the " internodes " and
partly cover the sporosacs.

It is obvious that on the analogy of such a hydroid polype as
Obelia, Halistemma is to be looked upon as a polymorphic floating
colony, the stem representing a ccenosarc, and the various struc-
tures attached to it zooids the polypes nutritive zooids, the
feelers tactile zooids, the sporosacs reproductive zooids, the bracts
protective zooids, and the swimming-bells locomotory zooids. The
float may be looked upon as the dilated end of the stem, which
has become invaginated or turned-in so as to form a bladder
filled with air, its outer and inner surfaces being furnished by
ectoderm, and the middle portion of its wall by two layers of
endoderm, between which the enteric cavity originally extended
(Fig. 120, pn). The upper or float-bearing end is proximal-
i.e. answers to the attached end of an Obelia-stem : it is the
opposite or distal end which grows and forms new zooids by
budding.

In some Siphonophora the bracts contain indications of radial
canals, so that these structures, as well as the swimming-bells
and sporosacs, are formed on the medusa-type, while the hydranths
and feelers are constructed on the polype-type.

It will be noticed that the radial symmetry, so characteristic
of most of the Hydrozoa previously studied, gives way, in the
case of Halistemma, to a bilateral symmetry. The swimming-bells
are placed obliquely, and the mouth of the bell is not at right
angles to the long axis, so that only one plane can be taken
dividing these structures into two equal halves : the same applies
to the polype and feelers with their single basal tentacle. When
first formed the various zooids are all on one side of the stem, but
the latter becomes spirally twisted during growth, and so causes
them to arise irregularly.

VOL. I M

162

ZOOLOGY

SECT.

The egg of Halistemma gives rise to a ciliated planula re-
sembling that of the other Hydrozoa. At one pole the ectoderm
becomes invaginated to form the float (Fig. 121, ep], the opposite
extremity is gradually converted into the first polype (po), and

net

FIG. 120. Diagram of a Siphpnophore : the thick line represents endoderm ; the space ex-
ternal to it, ectoderm ; the internal space, the enteric cavity. c<x. ccenosnrc ; Jz. dact3 T lozooid ;
kph. hydrophyllium ; m<l. sporosae ; net, net', nectocalyces ; ntc. battery of nernatocysts ;
p. polype ; p/i. pneuinatuphore ; /. tentacle. (After Clans.)

a bud appears on one side which becomes the first tentacle (t).
By gradual elongation, and the formation of new zooids as lateral
buds, the adult form is produced ; the various zooids are all
formed between the first polype and the float, so that the two

IV

PHYLUM CCELENTERATA

163

become further and further apart, being always situated at the
distal and proximal ends of the colony respectively.

In an allied form (Agalma) the first structure to appear iiTthe embryo is not
the float, but the first bract, which grows considerably and envelops the growing
embryo in much the same way as the umbrella of a medusa envelops the manu-
brium. On this and other grounds some zoologists look upon the Siphonophore-
colony as a medusa the manubriuin of which has extended immensely and
produced lateral buds after the manner of some Anthomedusa? (Fig. 105, 7 a).

FIG. 121. Two stages in the development of Halistemma : the endoderm is shaded, the
ectoderm left white. <:p. pneumatocyst or air-chamber of pneumatophore ; hi/, endoderm
surrounding pneumatophore ; po. polype ; pp. pneumatophore; t. tentacle. (From Balfour,
after Metschnikoff.)

On this theory the entire ccenosarc is an extended manubrium, and the first or
primary bract is the umbrella. But frequently as in Halistemma a primary
bract is not formed, and when present there appears to be no reason against
regarding it as a lateral bud of the axis, of quite the same nature as the remaining
zooids.

In the well-known " Portuguese man-of-war " (Physalia) there
is a great increase in proportional size of the float and a corre-
sponding reduction of the rest of the coenosarc. The float (Fig.
122, pn) has the form of an elongated bladder, from 3 to 12 cm.
long, pointed at both ends, and produced along its upper edge
into a crest or sail (cr) : as a rule it is of a brilliant peacock-blue
colour, but orange-coloured specimens are sometimes met with.
At one end is a minute aperture communicating with the exterior.
There are no swimming-bells, but from the underside of the float
hang gastrozooids (p), dactylozooids, branching blastostyles
(gonodendra) with groups of medusoids looking like bunches
of grapes of a deep blue colour, and long retractile tentacles (t),

M 2

164

ZOOLOGY

SECT.

sometimes several feet in length and containing batteries of
stinging-capsules powerful enough to sting the hand as severely
as a nettle. The male reproductive zooid remains attached, as in

**-.

FIG. 122. Physalia : the living animal floating on the surface of the sea. cr. crest ; p. polype ;
pn. pneumatophore ; t. tentacle. (After Huxley.)

Halistemma, but the female apparently becomes detached as a
free medusa.

In Diphycs the float is absent. Two swimming-bells (Fig. 123-4, m)
of proportionally immense size are situated at the proximal end
of the coansarc, and are followed by widely-separated groups of
/ooids (/?), each group containing a polype (n) with its tentacles (*'),

IV

PHYLUM CCELENTERATA

165

a meduzoid (y), and a large enveloping bract (t). The stem often
breaks at the internodes, and the detached groups of zooids then
swim about like independent organisms.

Porpita is formed on a different type, and has a close general
resemblance to a medusa. It consists (Fig. 124) of a discoid

FIG. 123. Diphyes campanulata A, the entire colony; B, single group of zooids. ,
cosnosarc ; c, cavity of swiniming-bell ; c, groups of zooids ; (/, medusoid ; i, grappling line or
tentacle ; m, swimming-bell ; n, polype ; o, mouth of swimming-bell ; t, bract. (From
Parker's Biology, after Gegenbaur.)

body, enclosing a chambered chitinoid shell (s/i) containing air, and
obviously corresponding with the float of Physalia. The edge of
the disc is beset with long dactylozooids (tf) and from its lower
surface depend numerous closely set blastostyles provided with
mouths and bearing medusae, while in the centre is a single large

166

ZOOLOGY

SECT.

gastrozooid (%). The closely allied genus Velclla is of rhomboidal
form, and bears on its upper surface an oblique sail.

The reproductive zooids are liberated as free medusae. The
eggs give rise to young which have a close resemblance to flat
medusae with manubrium, marginal tentacles, and an air-chamber
or float developed in the ex-umbrella. Thus it is quite possible
that the Siphonophora of the Porpita-type may be medusae the
sub-umbrella of which has given rise to buds forming the feelers

FIG. 124. Porpita pacifica. A, from beneath ; B, vertical section, hij, large central gastro-
zooid ; hy'. blastostyles ; sh, chambered shell ; t, dactylozooids. (From Parker's Biology, after
Duperry and Koelliker.)

and blastostyles. But, as their early development is not known, it
is still quite legitimate to describe them in the same terms as the
other Siphonophora i.e. to consider them as hydroid colonies in
which the cosnosarc is represented by the discoid or rhomboid
body with its contained air-chamber.

ORDER 5. GRAPTOLITHIDA.

The " Graptolites " are fossil Hydrozoa found in the Upper Cambrian and
Silurian rocks. They are known only by their fossilised chitinoid skeleton, all
trace of the soft parts having, as in the majority of fossils, disappeared.

With one doubtful exception they are compound, consisting of an elongated
tube the perisarc of the common stem, having attached to it, either in a single

IV

PHYLUM CCELENTERATA

167

or a double row, numerous small projections, the hydrothecae (Fig. 125, h.lh).
The coenosarcal skeleton is strengthened by a slender axis, the virgula (v), the
proximal end of which is connected with a small dagger-shaped body, the
siculn (s\ supposed to be the skeleton of the primary
zooid by the budding of which the colony was pro-
duced. In connection with some species oval or
cup-like capsules have been found : these may be of
the nature of goiiothecre. But it must be added that
the evidence in favour of associating the Graptolites -^

with the Hydrozoa is by no means conclusive, and
reasons have been adduced for regarding them as
connected with groups much higher in the scale.

ADDITIONAL REMARKS ON THE HYDROZOA.

The vast majority of Hydrozoa are
marine, the only exceptions being Hydra,
found all over the world ; MicroJiydra, at
present known only in North America ;
Cordylophora, one of the Anthomedusse,
found in Europe, America, Australia, and
New Zealand ; Polypodium, also an Antho-
medusa, found in the Volga, where in
one stage of its existence it is parasitic on
the eggs of a Sturgeon; Limnocodiwn, a
doubtful Trachymedusa, hitherto found
only in a tank in the Botanical Gardens,
Regent's Park, where it was probably in-
troduced from the West Indies ; and Limnocnida, found in
Lake Tanganyika, Africa.

The oldest known Hydrozoa are the Graptolites, found first in
the Cambrian rocks ; Hydr actinia occurs in the Cretaceous epoch,
and Hydrocorallinse from the Cretaceous onwards.

Parasitism, although rare, is not unknown in the class. Poly-
podium, one of the Anthomedusse, is parasitic during part of its
existence, in the ovary of the Sturgeon ; and Cunina, one of the
Narcomedusse, is parasitic on a Trachymedusa.

In the section on the Protozoa we saw that while the majority
of species are independent cells, each performing alone all the
essential functions of an animal, others, such as Pandorina,
Volvox, and Proterospongia, consist of numerous unicellular
zooids associated to form a colony in which a certain division of
labour obtains, the function of reproduction, for instance, being
assigned to certain definite cells and not performed by all alike.
Thus the colonial Protozoa furnish an example of individuation,
numerous cells combining to form a colony in which the several
parts are dependent one upon another, and which may therefore
be said to constitute, from the physiological point of view, an
individual of a higher order than the cell.

FIG. 125. Graptolites.

A, Monograptus coionus;

B, DimorpJiograptus, both
magnified, hy. th. hydro-
theca ; s. sicula ; -c. vir-
gula. (After Nicholson
and Lydekker.)

168 ZOOLOGY SECT, iv

This is still more notably the case in the lower Metazoa, such
as Ascetta and Hydra, in which we have numerous cells combined
to form a permanent two-layered sac with a terminal aperture,
some of the cells having digestive, others tactile, others repro-
ductive functions. Thus while an Amceba or a Paramcecium is
an individual of the first order, Hydra and Ascetta are individuals
of the second order, each the equivalent of an indefinite number of
individuals of the first order.

In the Hydrozoa we see this process carried a step further.
Budding takes place and colonies are produced, the various zooids
of which each the equivalent of a Hydra instead of remaining
all alike, become differentiated both morphologically and physio-
logically, so as to differ immensely from one another both in form
and function. In Obelia, for instance, reproduction is made over
exclusively to the medusae, while in Halistemma we have zooids
specially set apart, not only for reproductive, but for tactile and
protective purposes. Thus in Halistemma and the other Siphono-
phora there is a very complete subordination of the individual
zooids to the purposes of the colony as a whole, the colony thus
assuming, from the physiological point of view, the characteristics
of a single individual, and its zooids the character of organs. In
this way we get an individual of the third order, consisting of an
aggregate of polymorphic zooids, just as the zooid or individual
of the second order is an aggregate of polymorphic cells or
individuals of the first order.

CLASS II. SCYPHOZOA.

1. EXAMPLE OF THE CLASS- -THE COMMON JELLY-FISH

(Aurelia aurita).

Aurelia is the commonest of the larger jelly-fishes, and is often
found cast up on the sea-shore, when it is readily recognisable by
its gelatinous, saucer-shaped umbrella, three or four inches in
diameter, and by having near the centre four red or purple horse-
shoe-shaped bodies the gonads lying embedded in the jelly.

External Characteristics. The general arrangement of the
parts of the body is very similar to what we are already familiar
with in the hydrozoan jelly-fishes. Most conspicuous is the
concavo-convex umbrella, the convex surface of which, or ex-
umbrella, is uppermost in the ordinary swimming position (Figs.
126 and 127, A). The outline is approximately circular, but is
broken by eight notches, in each of which lies a pair of delicate
processes, the marginal lappets (mg. Ip.) : between the pairs of
lappets the edge of the umbrella is fringed by numerous close-set
marginal tentacles (t).

S

'///;"*>,

/ ///fr<'^*-^

^m^^ f

"'a am

FIG. r2ti. Aurelia aurita. A, dorsal view, part of the ex-umbrella cut away to show part of
tht! stomach and one of the four gastric pouches ; B, ventral view two of the oral arms are
removed, a.r. '. adradial canal; g. f. gastric filaments; gon. gonads ; ft. j>. gastric pouch;
i.r. c. inter-radial canal ; m>i. I p. marginal lappet; ntih. mouth; or. a. oral arm; p.r. c. per-
radial canal ; s.y. p. sub-genital pit ; st. stomach ; t. tentacles.

170 ZOOLOGY

A narrow region of the umbrella adjoining the edge is very thin
and flexible: the structure thus constituted, with its marginal
notches and the fringe of marginal tentacles, is the velarium.
Unlike the true velum of the medusae of the Hydrozoa the
velarium contains endodermal canals.

In the centre of the lower or sub-umbrellar surface is a four-
sided aperture, the mouth (mth), borne at the end of an extremely
short and inconspicuous manubrium : surrounding it are four long
delicate processes, the oral arms (or. a*), lying one at each angle
of the mouth and uniting around it. Each arm consists of a
folded membrane, tapering to a point at its distal end, beset
along its edges with minute lobules, and abundantly provided
with stinging-capsules. The angles of the mouth and the arms
lie in the four per-radii, i.e. at the end of the two principal axes
of the radially symmetrical body : of the marginal notches with
their lappets, four are per-radial and four inter-radial.

At a short distance from each of the straight sides of the
mouth, and therefore inter-radial in position, is a nearly circular
aperture leading into a shallow pouch, the svJb-genital pit (s.g.p).
which lies immediately beneath one of the conspicuously coloured
gonads (gori). The sub-genital pits have no connection with the
reproductive system, and are probably respiratory in function.

Digestive Cavity and Canal-System. The mouth leads by
a short tube or gullet (gul), contained in the manubrium, into a
spacious stomach (st), which occupies the whole middle region of
the umbrella, and is produced into four wide inter-radial gastric
pouches (g.p), which extend about half way from the centre to
the circumference, and are separated from one another by thick
pillar-like portions of the umbrella-jelly. In the outer or peri-
pheral wall of each gastric pouch are three small apertures,
leading into as many radial canals, which pass to the edge of
the umbrella and there unite in a very narrow circular canal
(circ. c). The canal which opens by the middle of the three
holes, is of course inter-radial (i.r.c) : it divides immediately
into three, and each division branches again : the canals from the
other two holes are ad-radial (a.r.c), and pass to the circular canal
without branching. There is also an aperture in the re-entering
angle between each two gastric pouches : this leads into a per-
radial canal (r>.r.c), which, like the inter-radial, branches
extensively on its way to the edge of the umbrella.

The general arrangement of the cell-layers in Aurelia is the
same as in a hydroid medusa (Fig. 127, B). The main mass of
the umbrella is formed of gelatinous mesogloea, which, however,
is not structureless, but is traversed by branching fibres and
contains amoeboid cells derived from the endoderm. Both ex-
and sub-umbrellae are covered with ectoderm, and the stomach and
canal system are lined with endoderm, which is ciliated through-

IV

PHYLUM CCELENTERATA

171

out. Some observations seem to show that the short tube
described above as a gullet and a pair of the gastric pouches
are lined, not by endoderm, but by an in-turned portion of the
ectoderm, but this matter cannot be considered as definitely
settled.

It was mentioned above that in the free medusa the gonads
appear through the transparent umbrella as coloured horseshoe-

B

I. PC

FIG. 127. Aurelia aurita. A, side view, oiie-fomth of the umbrella cut away ; B, diagrammatic
vertical section, ectoderm dotted, endoderm strmted, mesogloea black, circ. c. circular canal ;
g. f. gastric filaments ; cion. gonad ; ft. p. gastric pouch ; gul. gullet ; h. hood ; i.r. c. inter-radial
canal ; tug. lp. marginal lappet ; mth. mouth ; or. a. oral arm ; s.g. p. sub-genital pit ; st.
stomach.

shaped patches. Their precise position is seen by cutting away a
portion of the ex-umbrella so as to expose one of the gastric
pouches from above (Fig. 126, A). It is then seen that the
gonad (gon) is a frill-like structure lying on the floor of the
pouch and bent in the form of a horse-shoe with its concavity
looking inwards, i.e. towards the mouth. Being developed from
the floor of the enteric cavity, the gonad is obviously an

172 ZOOLOGY SECT.

endodermal structure : when mature, its products ova or sperms
-are discharged into the stomach and pass out by the mouth.
Here, then, is an important difference from the Hydrozoa, in
which the generative products are usually located in the ectoderm,
and are always discharged directly on the exterior. The sexes
are lodged in distinct individuals.

Lying parallel with the inner or concave border of each gonad
is a row of delicate filaments (Fig. 126, 127, g.f), formed of endoderm
with a core of mesogloea and abundantly supplied with stinging-
capsules. These are the gastric filaments or phacellae : their
function is to kill or paralyse the prey taken alive into the
stomach. No such endodermal tentacles are known in the
Hydrozoa.

Muscular and Nervous Systems. The contractions of the
bell by which the animal is propelled through the water are

B

FIG. 128 Aurelia aurita. A, small portion of edge of umbrella, showing the relations of the
tentaculocyst ; B, vertical section of the same region (diagrammatic), ft, hood ; /, lithite ;
mrj. lp, marginal lappet ; oc, ocellus ; o/f. 1, o/f. 3, olfactory pits. (Altered from Lankester.)

effected by means of a muscular zone round the edge of the sub-
umbrella. The nervous system is formed on a different plan
from that of the hydroid medusae. Extending over the sub-
umbrellar surface between the superficial epithelial layer of
ectoderm and the muscular layer is a plexus of simple nerve-fibres.
This presents radial thickenings, most strongly developed
externally in the per-radii and iriter-radii, corresponding to the
position of the marginal notches and sense-organs. About the
base of each of the latter are special groups of nerve-cells. A
slight ring-like thickening of the plexus extends round the margin
in the neighbourhood of the marginal canal.

The sense organs (Fig. 128) are lodged in the marginal
notches in close relation with the nerve-patches : like the latter,
therefore, four of them are per-radial and four inter-radial. Each
consists of a peculiar form of sense-club or tentaculocyst, containing

iv PHYLUM CCELENTERATA 173

a prolongation of the circular canal, and thus representing a hollow
instead of a solid tentacle. At the extremity are calcareous con-
cretions or lithites (1) derived from the endoderm, and on the outer
side is an ectodermal pigment-spot or ocellus (cc). The
tentaculocysts are largely hidden by the marginal lappets (mg. Ip)
and by a hood-like process (A) connecting them ; and in connection
with each are two depressions, one on the ex-umbrella (plf. 1), the
other immediately internal to the sense-club (off. 2) : these
depressions are lined with sensory epithelium and are called
oljactory pits.

The development and life-history of Aurelia present several
striking and characteristic features. The impregnated egg-cell
or oosperm divides regularly and forms a morula, which, by accumu-
lation of fluid in its interior, becomes a blastula a closed sac with
walls formed of a single layer of cells. One end of this sac becomes
invaginated to form the gastrula. The blastopore or gastrula-
mouth does not completely close, the resulting two layered planula
(Fig. 129) differing in this respect, as well as in its mode of
formation, from the corresponding stage of a Hydrozoan.

The planula swims about by means of the cilia with which its
ectodermal cells are provided, and, after a brief free existence,
settles down, loses its cilia, and becomes attached by one pole.
At the opposite pole a mouth is formed, the process taking place
by a sinking-in or invagination of the surface so as to produce a
depression lined with ectoderm (B, s), the bottom of which
becomes perforated so as to communicate with the enteric cavity
(C, st) : the depression is the stomodceum, a structure of which
there is no trace in the Hydrozoa. On two opposite sides of the
mouth hollow processes grow out, forming the first two tentacles :
soon two others appear at right angles to these, the organism
thus being provided with four per-radial tentacles. Subsequently
four inter-radial and eight adradial tentacles appear. At the
same time the attached or proximal end is narrowed into a stalk-
like organ of attachment (E), and the endoderm of the enteric
cavity is produced into four longitudinal ridges, inter-radial in
position, and distinguished as the gastric ridges or tcenioles (D tn.).
The mouth (E, mth.') assumes a square outline, and its edges become
raised so as to form a short manubrium (mnb.} ; and, finally, the
ectoderm of the distal surface i.e. the region lying between the
mouth and the circlet of tentacles becomes invaginated in each
inter-radius so as to produce four narrow funnel-like depressions-
the septal funnels or infundibula (E and F, s. /.) sunk in the four
gastric ridges.

The outcome of all these changes is the metamorphosis of the
planula into a polype (E), not unlike a Hydra or the hydrula-stage
of the Leptolinse, but distinguished by a pronounced differentia-
tion of structure, indicated by the sixteen tentacles developed in

174

ZOOLOGY

SECT.

regular order, the stomodoeum, and the four gastric ridges with
their septal funnels. The Scyphozoon-polype is called a scyphula
or scyphistoma.

FIG. 129. Aurelia aurita, development. A. planula, erroneously represented as completely
closed ; B, C, formation of stomodteum ; D, transverse section of young scyphula ; E,
scyphula ; F, longitudinal section of same : the section passes through a per-radius 011 the
left of the dotted line, through an inter-radius on the right ; G, division of scyphula
into ephyrulae ; H, ephyrula from the side ; L, the same from beneath. In A D and
F the ectoderm is unshaded, the endoderm striated, and the mesoglcea dotted, a. lobes
of umbrella ; mnb. manubrium ; mth. mouth ; s.f. septal funnel ; st. stoniodseum ; t. tentacle ;
in. ttenioles. (From Korschelt and Holder's Embryology.)

The scyphula may grow to a height of half an inch, and some-
times multiplies by budding. After a time it undergoes a process

iv PHYLUM CCELENTERATA 175

of transverse fission (G), becoming divided by a series of constric-
tions which deepen until the polype assumes the appearance of a
pile of saucers, each with its edge produced into eight bifid lobes,
four per- and four inter-radial. Soon the process of constriction
is completed, the saucer-like bodies separate from one another,
and each, turning upside down, begins to swim about as a small
jelly-fish called an ephyrula (H, I). The umbrella of the ephyrula
is divided into eight long bifid a.rms (a) with deep (per-radial or
inter-radial) notches : it has of course carried away with it a
segment of the stomach with the gastric ridges of the scyphula :
during the process of constriction this becomes closed in on the
proximal or ex-umbrellar side, while on the sub-umbrellar side it
remains open, and its edges grow out to form a manubrium.
Round the margin there are the bases of eight per-radial and
inter-radial tentacles, each in the notch of one of the arms, and
eight ad-radial tentacles in the intervals between the lobes : the
latter disappear completely ; the former may persist as the
tentaculocysts. On each gastric ridge appears a single gastric
filament, soon to be followed by others, and in the notches at the
extremities of the eight arms tentaculocysts are now recognisable.
In the meantime the spacious enteric cavity is continued into the
eight arms in the form of wide radiating canals.

As the ephyrula grows the adradial regions at first deeply
notched grow more rapidly than the rest, the result being that
the notches become gradually filled up, and the umbrella, from an
eight-rayed star, becomes a nearly circular disc. Four oral arms are
developed and numerous marginal tentacles, and the ephyrula
gradually assumes the form of the adult Aurelia. It seems
probable that the sub-genital pits of the medusa are formed
from sections of the septal funnels of the scyphula.

Thus the life-history of Aurelia differs in several marked
respects from that of any of the Hydrozoa. There is, in a sense,
an alternation of generations as in Obelia, the gamobium being
represented by the adult Aurelia, the agamobium by the scyphula.
But instead of the medusa being developed either as a bud on a
branched colony, as in Leptolinas, or by direct metamorphosis of a
polype, as in Trachylinaa, it is formed by the metamorphosis of an
ephyrula developed as one of several transverse segments of a
polype ; so that the life -history might be described as a metamor-
phosis complicated by multiplication in the larval (scyphula)
condition, rather than a true alternation of generations.

It has been shown that, under exceptional circumstances, the
egg of Aurelia develops into scyphulse which do not undergo
transverse division, the entire scyphula becoming metamorphosed
into a single adult.

176 ZOOLOGY SECT.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Scyphozoa may be defined as medusoid Coelenterata, having
the same general structure and arrangement of the layers as the
medusoid Hydrozoa, but differing from them in the possession of
endodermal gastric tentacles ; in having gonads the sexual cells
of which are lodged in the endoderm and which discharge their
products into the digestive cavity ; in the absence of a true velum,
and in nearly all cases, in the presence of sense-organs in the form
of hollow sense-clubs or tentaculocysts. Whether a stomodseum
or ectodermal gullet occurs is uncertain. As in the Hydrozoa, the
medusa develops directly from the egg in some Scyphozoa, while
in others there is a sort of alternation of generations, a polype-
form (again obium) giving rise to the medusa-form (gamobium) by
a process of transverse fission. In the majority, however, nothing
is known of the life-history, the process of development having
been worked out only in a few cases.

As far as is known, the segmenting embryo gives rise to a gastrula
by invagination in all with the exception of Lucernaria. and its
allies : by the partial or complete closure of the blastopore a
planula is produced, at one end of which a second invagination
takes place, forming the stomodseum.

The Scyphozoa are divisible into four orders, as follows :

ORDER 1. STAUROMEDUS.E (LUCERNARIDA).

Scyphozoa having a conical or vase-shaped umbrella, sometimes
attached to external objects by an ex-umbrellar peduncle : no
tentaculocysts.

ORDER 2. CORONATA.

Scyphozoa having the umbrella divided by a horizontal coronary
groove : four to sixteen tentaculocysts.

ORDER 3. CUBOMEDUS.E.

Scyphozoa with a four-sided cup-shaped umbrella : four per-
radial tentaculocysts.

ORDER 4 DISCO-MEDUSA.

Scyphozoa with a flattened saucer- or disc-shaped umbrella:
not fewer than eight tentaculocysts four per- and four 'inter-
radial.

iv PHYLUM CCELENTERATA 177

Sub-Order a Semostomce.

Discomedusfc in which the square mouth is produced into four long oral
arms.

Sub- Order b Rhizostomcv.

Discomedusse having the mouth obliterated by the growth across it of the
oral arms : the stomach is continued into canals which open by funnel-shaped
apertures on the edges of the arms.

Systematic Position of the Example.

Aurelia aurita is one of several species of the genus Aurelia,
and is placed in the family Ulmaridce, the sub-order Semostontce,
and the order Discomedusce.

Its saucer-shaped umbrella and eight tentaculocysts place it at
once among the Discomedusse : the presence of a distinct mouth
surrounded by four oral arms places it in the first sub-order
or Semostomse. This group contains six families, characterised
mainly by differences in the canal system : the Ulmaridse are
distinguished by narrow branched radial canals opening into a
circular canal. Of the eight genera in this family, Aurelia stands
alone in having its tentacles attached on the dorsal or ex-umbrellar
side of the margin, and in the oral arms showing no trace of bi-
furcation. Eight species of Aurelia are recognised, A. aurita,
being distinguished by having the oral arms slightly shorter
than the radius of the umbrella, and by possessing a trichotomous
inter-radial canal and two unbranched adradial canals springing
from each gastric pouch.

ORDER 1. STAUROMEDUS.E (LUCERNARTDA).

Tessera (Fig. 130), formerly regarded as the simplest member of this group,
is now looked upon as probably not a mature form. It is described as a small
medusa about 4 mm. in diameter having the same general characters as the
scyphula-stage of Aurelia, except that the bell-shaped body is free-swimming.
The edge of the umbrella is surrounded by eight tentacles, four per-radial
(p.r.t.) and four inter-radial (i.r.t.), and movement is effected by a well-developed
system of circular and radial muscles.

Lucemaria (Fig. 131), a genus not uncommon on the British coasts, is in one
respect even more like a scyphula, since it is attached by a peduncle developed
from the centre of the ex-umbrella. The margin of the umbrella is prolonged
into eight short hollow adradial arms, bearing at their ends groups of short
adhesive tentacles (/.). As in the scyphula, each gastric ridge contains an
infundibulum, lined with ectoderm and opening on the sub-umbrella. The
gastric filaments (#./.) are very numerous a distinct advance on Tessera and
the gonads ((jon.) are band-like. There are no sense-organs in Lucernaria, but
in an allied genus, Halicystus, there are eight per-radial and inter-radial
marginal bodies (anchors) of the nature of reduced and modified tentacles, each
surrounded at its base by a cushion-like thickening containing many adhesive
cells. Internal to each anchor on the sub-umbrellar side is a pigment spot

VOL. I N

178

ZOOLOGY

SECT.

(rudimentary eye). Stenocyphus is an allied form which probably is able to
move by creeping (looping) movements like those of a leech. Capria has no

i.r

FIG. iso. Tessera princeps, A, external view; B, vertical section, r/./. gastric filament;
ijmi. gonad ; i.r. t. inter-radial tentacle; mub. manubriuni ; ruth, mouth; p.r. t. per-radial
tentacle ; st. stomach ; tn. tamiole. (After Haeckel.)

Fio. 131. Xiucernaria. A, oral aspect ; B, from the side, p. foot-gland ; 57. /. gastric filaments
gon. gonad ; mlh. month ; t. tentacles ; tn. tsenioles. (After Claus.)

tentacles. The Depaxtruhe have an almost entire margin fringed with
tentacles.

IV

PHYLUM CCELENTERATA

179

ORDER 2. CORONATA.

This group includes a number of rare and beautiful Medussft of curiously
complex structure, of which Pericolpa may be taken as an example. The
umbrella (Fig. 132) is usually conical, and is divided by a horizontal furrow
(coronary groove) into an apical region or cone, (en.) and a marginal region or

circ. 8

TfinJb

FIG. 132. Pericolpa quadriarata. A, external view; B, vertical section, circ. s. circular
sinus ; en. cone ; </. f. gastric filaments ; yon. gonads ; mg. lp. marginal lappets ; mnb. manu-
Lrium ; mtk. mouth ; pi I. I. pedal lobes ; st. stomach ; t. tentacles ; tc. tentaculocysts ; tn.
tjenioles. (After Haeckel.)

crown; the crown is again divided by a second, rather irregular horizontal
furrow into a series of pedal lobes (pel. I.), adjacent to the cone, and a series of
marginal lappets (my. lp.), forming the free edge of the bell. In some of the
Coronata, such as Pericolpa, the pedal lobes and marginal lappets correspond
(i.e. are in the same radii) ; in others (Periphylla, Ephyropsis) they alternate.

N 2

180

ZOOLOGY

SECT.

In Pericolpa four of the pedal lobes, inter-radial in position, bear
tentaculocysts (tc. ) ; four others, per-radially situated, give origin to long,
hollow tentacles ((,.). In the more complex genera there are eight additional
adradial tentacles.

The mouth (mth.) is very large, and leads by a wide manubrium (mnb.) into
a spacious stomach (st.}, which is continued quite to the apex of the cone. In
the wall of the stomach are four wide per-radial slits, leading into an immense
circular sinus (circ. .<?.). As in Lucernaria, there are four wide inter-radial im-
fundibula. The gastric filaments (g. y.) are very numerous, and the elongated
U-shaped gonads (/o.) are eight in number and adradial.

The coronary groove is characteristic of the group : but in other points -
such as the number of pedal and marginal lobes, tentaculocysts, and tentacles

f]

FIG. 133. Nausithoe. The entire animal from the oral aspect, ar. adradii ; g. gonads; g.f.
gastric filaments ; ir. inter-radii ; TO. circular muscle of sub-umbrella ; pr. per-radii ; rl. tenta-
culocysts ; sr. sub-radii ; t. tentacles. The black cross in the centre represents the mouth.
(From Lang's Comparative Anatomy.)

there is great variation. Pericolpa and its allies (Peromedusca} resemble the
Lucernarida and the members of the order Culo/nednw in the presence of
ttenioles and inter-radial septa : Ephyropsis and its allies (Cannostonur]
resemble the order Discophora in the absence of these structures. The scyphula
larva of Nausithoe (Fig. 133) lives as a parasite in the interior of a horny
sponge.

ORDER 3. CUBOMEMJS.E.

The Jelly-fishes forming this order are, as the name implies, of a more or less
cubical form, resembling a deep bell with somewhat flattened top and square
transverse section. They resemble the hydrozoan Medusa? more than any of the
other Scyphozoa. The best known species, Charybdcea marsupialix (Fig. 134), is
about 5 cm. in diameter and of very firm consistency.

IV

PHYLUM CCELENTERAT^

181

As in the lower Coronata, the margin of the umbrella bears four tentacles
(t.) and four tentaculocysts (tc.} but the position of these organs is reversed, the
tentaculocysts being per-radial, the tentacles inter-radial. The tentaculocysts
are set in deep marginal notches, and the tentacles spring from conspicuous
gelatinous lobes (I.}, which probably answer to the pedal lobes of the preceding
order. These pedal lobes sometimes bear a number of supplementary tentacles.

gon.

B

endMtm

circ.

FIG. 134. Charybdsea marsupialis . A, side view of the entire animal ; B, vertical section
passing on the left side through an inter-radius, on the right through a per-radius ;
C, transverse section, circ. c. circular canal; <in<(. lam. endoderm lamella; cn<L lam', its pro-
longation into the velarium ; g. f. gastric filaments ; gon. goiiad ; gon'. septum separating
gonads ; f. lappet ; m>ib. manubrium ; rod. p. radial pouch ; t. tentacle ; tc. tentaculocy.st ;
vl. velarium. (After Claus, somewhat altered.)

The margin of the umbrella is produced, in most cases but not in all, into a
horizontal shelf (vl.}, resembling the velum of the hydroid Medusa, but differing
from it in containing a series of branched vessels (end. lam'. ) continuous with the
canal-system and of course lined with endoderm. In the Hydrozoa, it will be
remembered, the velum is formed simply of a double layer of ectoderm with a

182 ZOOLOGY SECT.

supporting layer of mesoglcea. Such a false velum, like the produced thin edge
of the umbrella in Aurelia, is known as a velarium.

The mouth is situated at the end of a short manubrium (inn 6.) leading into a
wide stomach, from which go off four very broad shallow per-radial pouches
(rad. p.}, occupying the whole of the four flat sides of the umbrella, and
separated from one another by narrow inter-radial septa or partitions (mesenteries]
placed at the four corners. These pouches are equivalent to \vide radial canals,
and the partitions between them to a poorly developed endoderm lamella (end.
lam.}. At the margin of the umbrella the pouches communicate with one
another by apertures in the septa, so that a kind of circular canal is produced
(circ. c. ), which is divided into chambers by the mesenteries. Near the junction
of the gastric pouches with the stomach are the usual four groups of gastric
filaments (g. f. ).

The gonads (yon.) are four pairs of narrow plate-like organs, attached one
along each side of each inter-radial septum. The nervous system takes the form
of a sinuous nerve-ring round *the margin of the bell, bearing a distinct group of
nerve-cells at the base of each tentaculocyst and tentacle. The Cubomedusae are
the only Scyphozoa which, like the Hydrozoa, have a complete nerve-ring. The
tentaculocysts are very complex, each bearing a lithocyst and several eye-spots.

ORDER 4. DISCOMEDUS.E.

The preceding orders are all small ones, i.e., include a small number of genera
and species. The vast majority of Scyphozoa belong to the present order the
" Disc- jellies " or " Sea-blubbers " as ordinarily understood.

The umbrella is always comparatively flat, having the form of an inverted
saucer. The edge is produced primitively into eight pairs of marginal lappets,
but in some of the more highly differentiated forms the number both of lappets
and of tentaculocysts becomes greatly increased. Most of the Semostomae and
Rhizostomae are large, and one of the former group Cyanea arctica may attain
a diameter of 2 metres and upwards, while its marginal tentacles reach the
astonishing length of 40 metres or about 130 feet. But in spite of their size and
apparent solidity, the amount of solid matter in these great Jelly-fishes is extra-
ordinarily small ; some of them have been proved to contain more than 99 per
cent, of sea- water.

The marginal tentacles are hollow and often of great length in the Semostomaj
(Fig. 126), and altogether absent in the Rhizostomae (Fig. 135). In the
Semostomae there are four oral arms (Fig. 126, o r. a.), each resembling a leaf
folded along its midrib, and having more or less frilled edges : in the Rhizostomae
each of the original four arms (Fig. 135, or. a.) becomes divided longitudinally in
the course of development, the adult members of the group being characterised
by the presence of eight arms, often of great length, and variously lobed and
folded so as to present a more or less root-like appearance.

The arrangement of the enteric cavity and its offshoots presents an interest-
ing series of modifications. In no case are there any twnioles or inter-radial
septa (mesenteries). In the Semostomae (Fig. 126) the stomach-lobes give off
well-defined radial canals, which are frequently more or less branched, often
unite into complex networks, and sometimes open into a circular canal round the
margin of the umbrella.

In the Rhizostornae (Fig. 135, B) a similar network of canals is found in the
umbrella, but an extraordinary change has befallen the oral or ingestive portion
of the enteric system. Looking at the oral or lower surface of one of these Jelly-
fishes, such as Pilema, no mouth is to be seen, but a careful examination of the
oral arms shows the presence of large numbers hundreds, or even thousands in
some cases of small funnel-like apertures (B, C, s.mth.) with frilled margins.

IV

PHYLUM CCELENTERATA

183

Rhizostomes have been found with prey of considerable size, such as fishes, em-
braced by the arms and partly drawn into these apertures, which are therefore
called the suctorial mouths. They lead into canals in the thickness of the arms
(B, c. ), the lesser canals unite into larger, and then finally open into the stomach
(st.). We thus get a polystomatous or many-mouthed condition which is practi-
cally unique in the animal kingdom, the only parallel to it being furnished
by the Sponges, in which the inhalant pores are roughly comparable with the
suctorial mouths of a Rhizostome.

It has been found that this characteristic arrangement is brought about by
certain changes taking place during growth. The young Rhizostome has a single
mouth in the usual position, and more or less leaf-like arms, folded along the
midrib so as to enclose a deep groove, from which secondary grooves pass, like

St

s.mfft,

FIG, 13'). Pilema pulmo. A, side view of the entire animal ; B, vertical section, diagrammatic ;
C, one of the suctorial mouths, magnified, c. arm canal ; </. /. gastric filaments ; yon. gonads ;
or. a. oral arms ; rail. r. radial canal ; .?. mth. suctorial mouths ; st. stomach ; tl, t2, t3, tentacles
on oral arms. (After Cuvier, Glaus, and Huxley.)

the veins of a leaf, towards the edge of the arm. As development proceeds, these
grooves become converted into canals by the union of their edges, thus forming
a system of branching tubes opening proximally into the angles of the mouth and
distally by small apertures the suctorial mouths on the edges of the arms.
At the same time the proximal ends of the arms grow towards one another and
finally unite across the mouth, closing it completely, and forming a strong
horizontal brachial disc, which in the adult occupies the centre of the sub-
umbrellar surface.

The gastric filaments are usually very numerous. In the higher Rhizostom?e
a remarkable modification is produced in connection with the sub-genital pouches ;
the four pouches approach the centre and fuse with one another, forming a single
spacious chamber, the sub-genital portico, which lies immediately below the
floor of the stomach and above the brachial disc.

184

ZOOLOGY

SECT.

In many of the Discomedusa? development takes place in the same general
way as in Aurelia, i.e. the impregnated egg gives rise to a scyphula or asexual
polype stage, which, by transverse division, produces sexual medusae. In
Cassiopeia the scyphula arising from the fertilised ovum gives oft' buds which
become detached as free-swimming planulse, and these, coming to rest, develop
into scyphula?. But in other cases there is no alternation of generations, and
development is direct. For instance, in Pelagia (Fig. 136) one of the
Semostonue a blastula is formed which becomes invaginated at one end,

B

FIG. 136. Pelagia noctiluca : Three developmental stages, m. month ; r. marginal lappet ;
s. tentaculocyst. (From Korschelt and Heider, after Krohn.)

forming a gastrula. The blastopore or gastrula-mouth remains open, and a
considerable space is left between the invaginated endoderm and the ectoderm.
Next the mouth region becomes elevated, forming a manubrium, and around
this a circular depression appears the rudiment of the sub-umbrellar cavity
surrounded by a raised ridge, the umbrella margin, which soon becomes divided
into lobes, the marginal lappets. Up to this time the embryo is ciliated
externally, but soon the cilia disappear, and the little creatures assume somewhat
the form of an ephyrula, which gradually develops into the adult Pelagia.

ADDITIONAL REMARKS ON THE SCYPHOZOA.

The Scyphozoa are all marine, and the majority are pelagic, i.e.
swim freely on the surface of the ocean. A few inhabit the deep
sea, and have been dredged from as great a depth as 2,000 fathoms.
Nearly all are free-swimming in the adult state : some, however,
live on coral-reefs or mud-banks, and are found resting, in an
inverted position, on the ex-umbrella ; and a few, such as Lucern-
aria, are able to attach themselves at will by a definite ex-
umbrellar peduncle.

Many of the Scyphozoa are semi-transparent and glassy, but
often with brilliantly coloured gonads, tentacles, or radial canals.
In many cases the umbrella, oral arms, &c., are highly coloured,
and some species, e.g. Pelagia noctiluca, are phosphorescent. They
are all carnivorous, and although mostly living upon small

iv PHYLUM CCELENTERATA 185

organisms, are able, in the case of the larger species, to capture
and digest Crustaceans and Fishes of considerable size. In many
cases small fishes accompany the larger forms and take shelter
under the umbrella.

Considering the extremely perishable nature of these organisms,
and the fact that many of them contain not more than 1 per cent.
of solid matter, it is not to be expected that many of them should
have left traces of their existence in the fossil state. Nevertheless,
in the finely grained limestone of Solenhofen, in Bavaria, belong-
ing to the Upper Jurassic period, remarkably perfect impressions
of Jelly-fishes have been found, some of them readily recognisable
as Discomedusse.

CLASS III. ACTINOZOA.

1. EXAMPLE OF THE CLASS. A SEA- ANEMONE

(Tealia crassicornis).

Sea-anemones are amongst the most abundant and best known
of shore-animals. They are found attached to rocks, sea-weeds,
shells, &c., either in rock-pools or on rocks left high and dry by the
ebbing tide. Usually their flower-like form and brilliant colour
make them very conspicuous objects, but many kinds cover them-
selves more or less completely with sand and stones, and contract
so much when left uncovered by water, that they appear like soft
shapeless lumps stuck over with stones, and thus easily escape
observation. Any of the numerous species will serve as an
example of the group : the form specially selected is the " Dahlia
Wartlet" (Tealia crassicornis), one of the commonest British
species.

External characters.- -Tealia (Fig. 137, A) has the form of a
cylinder, the diameter of which slightly exceeds its height. It is
often as much as 3 inches (8 cm.) across, is of a green or red colour,
and habitually covers itself with bits of shell, small stones, &c. It
is attached to a rock or other support by a broad sole-like base,
sharply separated from an upright cylindrical wall or column, the
surface of which is beset with rows of adhesive warts or tubercles :
at its upper or distal end the column passes into a horizontal plate,
the disc or peristome. In the middle of the disc, and slightly
elevated above its surface, is an elongated slit-like aperture, the
mouth (mth.\ from which streaks of colour radiate outwards.
Springing from the disc and encircling the mouth are numerous
short conical tentacles (t.\ which appear at first sight to be
arranged irregularly, but are actually disposed in five circlets, of
which the innermost contains five, the next five, the third ten,

136

ZOOLOGY

SECT.

FIG. 137. Tealia crassicornis. A, dissected specimen ; B, transverse section, the half
above the line ab thnm^li tlie y""'- 1 ^ the lower half below the gullet.- <i. mes. directive
mesenteries; r/on. gnnads ; <n<i. gullet ; /. //;. longitudinal muscle; Ip. lappet; mcs. 1, primary,
me.?. 2, secondary, mcs. 3, tertiary mesenteries; mix. /. inesenteric filaments; milt, mouth ;
ost, 1, ost. 2, ostia ; p. m. parietal muscle ; syph. siphonoglyphe ; s. n>~ sphincter muscle ; t. m.
transverse muscle.

iv PHYLUM CCELENTERATA 187

the fourth twenty, and the fifth or outermost forty, making a total
of eighty.

Obviously the Sea-anemone is a polype, formed on the same
general lines as a Hydra or a scyphula, but differing from them in
having numerous tentacles arranged in multiples of five, and in
the absence of a hypostome, the mouth being nearly flush with the
surface of the disc. Its great size and bulk, and the comparative
firmness of its substance, are also striking points of difference
between Tealia and the polypes belonging to the classes Hydrozoa
and Scyphozoa.

Enteric System. Still more fundamental differences are found
when we come to consider the internal structure. The mouth does
not lead at once into a spacious undivided enteric cavity, but into
a short tube (gul.), having the form of a flattened cylinder, which
hangs downwards into the interior of the body, and terminates in
a free edge, produced at each end of the long diameter into a
descending lobe or lappet (lp.}. This tube is the gullet or stomodceum,
a structure we have already met with in the Scyphozoa, but which
here attains a far greater size and importance. Its inner surface is
marked with two longitudinal grooves (A arid B, sgph.\ placed one
at each end of the long diameter, and therefore corresponding with
the lappets : they are known as the gullet-grooves or siphonoglyphes.

The gullet does not simply hang freely in the enteric cavity,
but is connected with the body-wall by a number of radiating
partitions, the complete or primary mesenteries (mes. 1) : between
these are incomplete secondary mesenteries (mes. #), which extend
only part of the way from the body-wall to the gullet, and
tertiary mesenteries (mes. 3), which are hardly more than
ridges on the inner surface of the body-wall. Thus the entire
internal cavity of a Sea-anemone is divisible into three regions :
(1) the gullet or stomodceum, communicating with the exterior
by the mouth, and opening below into (2) a single main digestive
cavity, the stomach or mesenteron, which gives off (3) a number of
radially arranged cavities, the inter-mesenteric chambers or metentera.
It is obvious that we may compare the gullet and stomach with
the similarly named structures in the scyphu la-stage of Aurelia,
and the mesenteries with the gastric ridges ; indeed, there seems to
be little doubt that these structures are severally homologous. A
further correspondence is furnished by the presence of an aperture
or ostium (ost. 1) in each mesentery, placing the adjacent inter-
mesenteric chambers in direct communication with one another:
in Tealia a second ostium (ost. ) is present near the outer edge
of the mesentery. Moreover, the free edge of the mesentery
below the gullet is produced into a curious twisted cord, the
mcscntcric filament (mes. /.), answering to a gastric filament of
the Scyphozoa. In many Sea-Anemones the mesenteric filaments

188

ZOOLOGY

SECT.

are produced into slender threads the a ronl ia which may be
protruded through the mouth or through special apertures
(cinolidcs) of the body-wall (Fig. 138, A.)

The general arrangement of the cell-layers is the same as in
the two preceding classes. The body-wall (Fig 138) base, column,
and disc consists of a layer of ectoderm outside, one of endoderm
within, and between them an intermediate layer or mesoglcea,
which is extremely thick and tough. The gullet (f/nl.), which, like
that of the scyphula, is an in-turned portion of the body-wall, is
lined with ectoderm, and its outer surface i.e. that facing the
inter-mesenteric chambers is endodermal. The mesenteries (mes.)
consist of a supporting plate of mesogloea, covered on both sides by

B

mcs

nZ.

. c

FIG. 138. Diagrammatic vertical (A) and transverse (B) sections of a Sesu-anemone. The
ectoderm is dotted, the endoderm striated, the mesogloea' black, ac. acontium ; en. cinclis ;
gul. gullet ; -int. -nits. c. inter-mesenteric chamber ; mes. mesentery ; mcs. /. in-esenteric
filament ; ruth, mouth ; ost. ostiuin ; p. pore ; t. tentacle.

endoderm. The tentacles (t) are hollow out-pushings of the disc,
and contain the same layers.

Muscular System. Sea-anemones perform various charac-
teristic movements : the column may be extended or retracted, the
tentacles extended to a considerable length, or drawn back and
completely hidden by the upper end of the column being folded
over them like the mouth of a bag; the gullet, and even the
mesenteries, may be partially everted through the mouth ; and
lastly, the whole animal is able, very slowly, to change its position
by creeping movements of its base.

These movements are performed by means of a very well-
developed set of muscles. A mesentery examined from the surface

iv PHYLUM CCELENTERATA 189

is seen to be traversed by definite fibrous bands, the two most
obvious of which are the longitudinal or retractor muscle (Fig.
137, l.m.), running as a narrow band from base to disc, and the
parietal muscle (p.m.), passing obliquely across the lower and outer
angle of the mesentery. Both these muscles are very thick, and
cause a projection or bulging on one side of the mesentery,
specially obvious in a transverse section (B. l.m.) : a third set of
fibres, forming the transverse muscle (t.m), crosses the longitudinal
set at right angles, but is not specially prominent. The longi-
tudinal muscles shorten the mesentery, and draw the disc
downwards or towards the base, thus retracting the tentacles ; the
parietal muscles approximate the column to the base, and the
transverse fibres produce a narrowing of the mesentery and thus,
opposing the action of the longitudinal muscles, act as extensors of
the whole body. The withdrawal of disc and tentacles, during
complete retraction, has been compared to the closure of a bag by
tightening the string, and is performed in much the same way, the
string being represented by a very strong band of fibres, the
circular or sphincter muscle (s.m.), which encircles the body at the
junction of the column and disc.

The foregoing muscles can all be seen by the naked eye, or
under a low magnifying power. They are supplemented by fibres,
only to be made out by microscopic examination, occurring both in
the body-wall and in the tentacles. The latter organs, for instance,
are able to perform independent movements of extension and re-
traction by means of delicate transverse and longitudinal fibres.

It was mentioned above that the thickness of the longitudinal
and parietal muscles produces a bulging on one surface of the
mesenteries. A transverse section shows that the arrangement of
the mesenteries and of their muscles is very definite and charac-
teristic (Fig. 137, B). At each end of the gullet, opposite the
siphonoglyphe, are two mesenteries (d. mcs.), having their longi-
tudinal muscles turned away from one another : they are distin-
guished as the directive mesenteries, and, in the case of Tealia,
there are two couples of directive mesenteries, one at each end of
the long axis of the gullet. Of the remaining complete or
primary mesenteries there are four couples on each side (mcs. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary
mesenteries (mes. 2, mcs. S) are also arranged in couples, and in all
of them the longitudinal muscles of each couple face one another.

Symmetry .- -It will be noticed that Tealia, unlike the typical
hydrozoan and scyphozoan polypes, presents a distinct bilateral x//v/?-
mctry, underlying, as it were, its superficial radial symmetry. It
is divisible into equal and similar halves by two planes only, viz. a
vertical plane taken through the long diameter of the gullet, and a
transverse plane taken through its short diameter.

190

ZOOLOGY

SECT.

The general microscopic structure of a Sea-anemone is well
shown by a section through a tentacle (Fig. 139). Both ectoderm
(ect.) and endoderm (end.) consist mainly of very long columnar,
ciliated, epithelial cells, and the mesoglcea (m-sgl.) is not only ex-
tremely thick, but has the general characters of connective tissue,
being traversed by a network of delicate fibres with interspersed
cells. The middle la} T er has, in fact, ceased to be a mere gelatinous
supporting lamella or mesoglcea, and has assumed, to a far greater

ntc

FIG. 140. Three iiematocysts of
Sagartia. (After Hertwig.)

FIG. 139. Tealia crassicornis. Trans-
verse section of tentacle, ect. ectoderm ;
end. endoderm; l.m. longitudinal muscles ;
msgl. mesoglcea; nr.c. nerve -cells; nr.f.
nerve -fibres ; ntc. iiematocysts ; t. m.
transverse muscles. (After Hertwig.)

extent than in any of the lower groups, the characters of an inter-
mediate cell-layer or mesoderm.

Stinging-capsules occur in the ectoderm, and are also very
abundant in the mesenteric filaments. They (Fig. 140) resemble
in general characters the nematocysts of Hydrozoa, but are of
a more elongated form, and the thread is usually provided at
the base with very numerous slender barbs (B). Very fre-
quently the coiled thread is readily seen in the undischarged
capsule (A). Gland-cells (Fig. 141, <//.) are very abundant
in the ectodermal lining of the gullet and in the mesenteric
filaments : the latter are trilobed in section, and the gland-
cells are confined to the middle portion, the lateral divisions

IV

PHYLUM CCELENTERATA

191

being invested with ordinary ciliated cells (c.). In virtue of
possessing both stinging-capsules and gland-cells, the mesenteric
filaments perform a double function. The animal is very voracious,
and is able to capture and swallow small Fishes, Molluscs, Sea-
urchins, &c. The prey is partly paralysed, before ingestion, by the
nematocysts of the tentacles, but the process is completed, after
swallowing, by those of the mesenteric filaments. Then as the
captured animal lies in the stomach, the edges of the filaments
come into close contact with one another and practically surround

rite

FIG. 141. Transverse section of mesenteric filament of Sagartia. c. ciliated cells; gl. gland-
cells ; -iitc. nematocysts. (After Hertwig.)

it, pouring out, at the same time, a digestive juice secreted by their
gland-cells.

The muscles described above consist partly of spindle-shaped
nucleated fibres, and partly of muscle-processes, like those of
Hydra : the latter occur chiefly in the transverse muscular layer of
the tentacles and are endodermal, the longitudinal layer is formed
of distinct fibres of ectodermal origin : the great muscles of the
mesenteries are of course endodermal. Although always derived
either from the ectoderm or endoderm, many of the muscle-fibres
of Tealia undergo a remarkable change of position by becoming
sunk in the mesoglcea, and thus appearing to belong to that
layer (Fig. 139 /. m.). This fact is significant from the circum-

ZOOLOGY SECT.

stance that, as we shall see, the muscles of all animals above
Coelenterata are mesodermal structures.

The nervous system is very simple. It consists of a layer of
delicate fibres lying between the epithelial and muscular layers of
the ectoderm. Among the fibres are found nerve-cells (Fig. 139,
nc.c.\ often of large size, and occurring chiefly in the disc and
tentacles. Thus, as in the polype-forms previously described, the
nervous system is in a generalised condition, and shows no con-
centration into a definite central nervous system such as occurs
in Medusae.

Reproductive organs. Sea-anemones are dioecious, the sexes
being lodged in distinct individuals. The gonads ovaries or testes
-are developed in the substance of the mesenteries (Fig. 137,ffon.),
a short distance from the edge, and, when mature, often form very
noticeable structures. The reproductive products are obviously, as
in the Scyphozoa, lodged in the endoderm. The sperms, when
ripe, are discharged into the stomach and escape by the mouth :
they are then carried, partly by their own movements, partly by
ciliary action, down the gullet of a female, where they find their
way to the ovaries and impregnate the eggs.

The development of Sea-anemones resembles, in its main features,
that of Scyphozoa. The oosperm undergoes more or less regular
division, the details differing considerably in individual cases, and
becomes converted into a planula, an elongated ovoidal body with
an outer layer of ciliated ectoderm, and an inner layer of large
endoderm cells, surrounding a closed enteric cavity, usually filled
with a mass of yolk, which serves as a store of nutriment.

In this condition the embryo escapes from the parent, through
the mouth, swims about for a time, and then settles down, becom-
ing attached by its broader or anterior end. At the opposite or
narrow end a pit appears, the rudiment of the stomodseum ; this
deepens, and its lower or blind end becoming perforated, effects a
communication with the enteron.

The mesenteries are developed in regular order, but in a way which would
certainly not be suspected from their arrangement in the adult. First of all, a
single pair of mesenteries (Fig. 142; A, 1} grow from the body-wall to the gullet,
being situated one on each side of the vertical plane, at right angles to the long
diameter of the stomodaeum, and near one end of that tube. The enteron thus
becomes divided into two chambers, a larger or dorsal and a smaller or ventral,
and the embryo acquires a distinct bilateral symmetry. Next a pair of mesen-
teries (2) appear in the dorsal chamber, dividing it into a median and two lateral
compartments ; then a third pair (3) in the ventral chamber, producing a similar
division ; then a fourth pair (4) in the middle compartment of the dorsal
chamber ; then a fifth pair (B, 5) in the lateral compartments of the dorsal
chamber ; and a sixth (6) in the lateral compartments of the ventral chamber.
Soon the longitudinal muscles are developed, and the fate of these primitive
pairs of mesenteries can be seen. The third and fourth pairs become the two
directive couples of the adult ; another couple of primary mesenteries is consti-
tuted, on each side of the vertical plane, by one of the mesenteries of the first

IV

PHYLUM CCELENTERATA

193

and one of the sixth pair ; a third couple is similarly formed by a mesentery of
the second and one of the fifth pair. Thus it is only in the case of the directive
mesenteries that an adult couple coincides with an embryonic pair : in other
instances the two mesenteries of a couple are of different orders, belonging
to distinct embryonic pairs. The mesenteric filaments of the first cycle of

Std,

A

B

FIG. 142. Transverse sections of early (A) and later (B) stages of an embryo Sea-anemone
(Actinia.) The mesenteries are numbered in the order of their development ; std. stomo-
dseum. (After Korschelt and Heider.)

mesenteries are partly ectodermal, partly endodermal in origin, those of the
remainder entirely endodermal.

The tentacles are developed in a somewhat similar order to that of the
development of the mesenteries. The first to make its appearance is connected
with the larger or dorsal enteric chamber mentioned above : for some time it
remains much longer than any of its successors, and thus accentuates in a marked
degree the bilateral 'symmetry of the embryo.

It will be noticed that the development of the Sea-anemone is
accompanied by a well-marked metamorphosis, but that there is no
alternation of generations. In this respect its life-history offers a
marked contrast with that of Obelia.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Actinozoa are Coelenterata which exist only in the polype-
form, no medusa-stage being known in any member of the class.
The actinozoan differs from the hydrozoan polype mainly in
possessing a stomod^aum : it differs from the hydrozoan and many
scyphozoan polypes in the possession of mesenteries or vertical
radiating partitions, which extend inwards from the body- wall
and some of which join the stomodum. The free margins of the
mesenteries bear coiled mesenteric filaments, which appear to
answer to the gastric filaments of Scyphozoa, but may be partly
ectodermal in origin. The mesenteries are developed in pairs,

VOL. I O

194 ZOOLOGY SECT.

symmetrically on each side of a vertical plane : their final radial
arrangement is secondary.

The body-wall consists of ectoderm and endoderm separated by
a stout mesogloea containing fibres and cells. The stomodseum
consists of the same layers reversed i.e. its lining membrane is
ectodermal. The mesenteries are formed of a double layer of
endoderm with a supporting plate of mesogloea. Nematocysts,
frequently of a more complex form than those of Hydrozoa and
Scyphozoa, are present in the tentacles, body-wall, stomodseum,
and mesenteric filaments. The muscular system is well developed,
and contains both ectodermal and endodermal fibres and endo-
dermal muscle-processes. The nervous system consists of irregu-
larly disposed cells and fibres ; there is no concentration of these
elements to form a central nervous system.

/

The gonads are developed in the mesenteries, the sex-cells are
lodged in the endoderm, and the ripe sexual products are dis-
charged into the enteron. The impregnated egg develops into a
planula, which, after a short free existence, settles down and
undergoes metamorphosis into the adult form. Except in one
doubtful instance there is no alternation of generations.

In some Actinozoa the animal remains simple throughout life,
but in most members of the class an extensive process of budding
takes place, the result being the formation of colonies of very various
form and often of great size. Some kinds, again, resemble Tealia
in having no hard parts or skeletal structures of any kind : but the
majority possess a skeleton, formed either of carbonate of lime or
of a horn-like or chitinoid material, and developed, in most cases
though not in all, from the ectoderm.

The Actinozoa are classified as follows :

Sub-Class I. Zoantharia.

Actinozoa in which the tentacles and mesenteries are usually
very numerous and are frequently arranged in multiples of five or
six. The tentacles are usually simple, unbranched, hollow cones.
There are commonly two siphonoglyphes and two pairs of directive
mesenteries : the remaining mesenteries are usually arranged
in couples with the longitudinal muscles of each couple facing one
another.

ORDER 1. ACTIXIARIA.

Zoantharia which usually remain simple, but in a few instances
form small colonies. The tentacles and mesenteries are numerous,
and there is no skeleton. This order includes the Sea-anemones.

ORDER 2. MADREPORARIA.

Zoantharia which resemble the Actiniaria in the general
structure of the soft parts, but which usually form colonies, and

IV PHYLUM CCELENTERATA 195

always possess an ectodermal calcareous skeleton. This order
includes the vast majority of Stony Corals (Figs. 146 and 156).

ORDER 3. ANTIPATHARIA.

Compound, tree-like Zoantharia in which the tentacles and
mesenteries are comparatively few (6 24) in number. A skeleton
is present in the form of a branched chitinoid axis, developed from
the ectoderm, which extends throughout the colony. This order
includes the Black Corals" (Fig. 150).

Sub-Class II.- Alcyonaria.

Actinozoa in which the tentacles and mesenteries are always

u

eight in number. The tentacles are pinnate, -i.e. produced into
symmetrical brahchlets. There is never more than one siphono-
glyphe, which is ventral in position, i.e. faces the proximal end of
the colony. The mesenteries are not arranged in couples, and
their longitudinal muscles are all directed ventrally, i.e. towards the
same side as the siphonoglyphe.

ORDER 4. ALCYONACEA.

Alcyonaria in which the skeleton usually consists of calcareous
spicules or small irregular bodies occurring in the mesogloea, but
probably originating from wandering ectoderm cells. The common
" Dead men's lingers " (Alcyonium, Fig. 153) has a skeleton of this
type. In some cases the spicules become aggregated so as to pro-
duce a coherent skeleton, which may form a branched axis to the
whole colony, as in the precious Red Coral (Corallium, Fig. 145),
or a series of connected tubes for the individual polypes, as in
the Organ-pipe Coral (Tubipora, Fig. 148). In the " Blue Coral '
(Heliopora) the skeleton is a massive structure resembling that of
the Madreporaria. Most genera are compound ; a few, such as
Hartea which, however, is probably a larval form (Fig. 144)
are simple.

ORDER 5. GORGOXACEA.

Compound tree-like Alcyonaria, with a calcareous or horny
skeleton of ectodermal origin forming a branched axis throughout
the colony. Spicules are present in the mesogloea. There is no
siphonoglyphe. The beautiful *' Sea-fans " belong to this group
(Fig. J 54).

ORDER 6. PENNATULACEA.

Alcyonaria in which the colony is usually elongated, and has
one end embedded in the mud at the sea-bottom, while the
opposite or distal end bears the polypes, usually on lateral

196 ZOOLOGY SECT.

branches. The stem is supported by a calcareous or horny
skeleton. The polypes are dimorphic. The " Sea-pens" (Pennatula)
are the commonest members of this group (Fig. 147).

Systematic Position of the Example.

Tealia crassicornis is one of several species of the genus
Tealia: it belongs to the family Tcalidce, which, with several
other families, make up the tribe Hexactinicc, of the order
Actiniaria, of the sub-class Zoantharia.

The presence of numerous tentacles, arranged in multiples of
five, places it at once among the Zoantharia. The fact that it is
simple and devoid of a skeleton causes it to be assigned to the
Actiniaria. This order is divided into tribes characterised by
differences in the arrangement of the mesenteries, especially by
the presence of one or two couples of directive mesenteries,
and by the direction in which the longitudinal muscles face.
In the Hexactinise the mesenteries are all arranged in couples
with the longitudinal muscles of each couple facing one another,
except in the case of the two directive couples. The mesenteries
are in multiples of five, and the stomodseutn has two siphono-
glyphes and two lappets.

The family Tealidas is characterised by the possession of
numerous mesenteries, of tentacles of moderate length which are
completely covered by the closed-in disc during retraction, and
by the presence of a large endodermal sphincter muscle. The
genus Tealia is distinguished from other members of the same
family by being broader than high, by having numerous retractile,
equal-sized tentacles, and by the presence of longitudinal series
of warts on the column. The species crassicornis is distinguished
from other species of the genus by the warts being of approxi-
mately equal size.

3 GENERAL ORGANISATION.

The chief variations in the external form of the Actinozoa are
due to the diverse modes of budding : as we shall see, the structure
of the individual polypes or zooids is remarkably uniform at
least as regards all the essentials of their organisation.

Nearly all the Actiniaria or Sea-anemones are simple, and, in the
few instances where colonies are formed, these are usually small,
and contain a very limited number of zooids. In Zoanthus
(Fig. 143), for instance, the original polype sends out a horizontal
branch or stolon (st.}, from which new polypes arise. Besides the
Sea-anemones the only simple forms are certain Madreporarian
corals, such as Flabellum (Fig. 155, A, B), and three genera of
Alcyonacea, of which Hartca (Fig. 144) may be taken as an
example.

IV

PHYLUM CCELENTERATA

197

The simplest mode of budding is that just described in Zoan-
thus, in which new zooids are developed from a narrow band-like

B

FJC. 143 Zoanthus SOCiatus. A, entire colony ; st. stolon. B, transverse section, sgph.
siphonoglyphes ; d. <1. dorsal, and r. (I. ventral directive mesenteries. (After McMurrich and
Kurschelt and H eider.)

or tubular stolon (Fig. 143, st). A more usual method resembles that
with which we are already familiar in Hydrozoa, new buds being

formed as lateral outgrowths,
and a tree-like colony arising
with numerous zooids spring-
ing from a common stem or
coenosarc. Corallium and Gor-
gonia (Figs. 145 and 154) are
good examples of this type of
growth. In other cases the
buds grow more or less paral-
lel with one another, producing
massive colonies either of close-
set zooids or of zooids separ-
ated by a solid coenosarc. As
examples of this type we may
take Palythoa, the most com-
plex of the Actiniaria, and
many of the common Madre-
poraria, such as Astroea (Fig.
146). In the Sea-pens (Penna-
tulacea) the proximal end of
the elongated colony (Fig.
147) is sunk in the mud, and
FIG. 144. -Hartea eiegans. <ii. gullet ; the distal end bears zooids

><*. mesentery ; ;>. spicules ; t. tentacles. orrincrmcr PifViPv rlirprtlv frnm
(After Perceval Wright.) Springing tiy

198

ZOOLOGY

SECT.

the coenosarc or, as in Pcnnatula itself, from flattened lateral
branches. The stem itself is the equivalent of a polype.

A very peculiar mode of budding occurs in the Organ-pipe
Coral (Tubipom). The base of the original polype (Fig. 148)

grows out into a flattened expansion
from which new polypes arise, diverg-
ing slightly from one another as
they grow, and separated by toler-
ably wide intervals. The distal ends
of the polypes then grow out into
horizontal expansions or pliiforms
(pi.}, formed at first of ectoderm and
mesogloea only, but finally receiving
prolongations of the endoderm. The
platforms extend, come in contact
with one another, and fuse. In this
way platfoims of considerable extent
are formed (A, pi.), uniting the
polypes with one another. From
the upper surfaces of the platforms,
between the older polypes, new
buds arise, and in this way the colony tends to assume the form
of an inverted pyramid, the number of zooids, and consequently
the diameter of the colony, increasing pari iwssu with the vertical

Fio. 145. Corallium rubrum, por-
tion of a branch. (From Glaus,
after Lacaze-Duthiers.)

Kir;. 146, Astrsea pallida, the living colony. (After Dana.)

growth of the latter. The skeleton of this remarkable coral will
be referred to hereafter.

Although the general structure of the individual polypes

of the Actinozoa is, as mentioned above, very uniform, the varia-
tions in detail are numerous and interesting, especially among
the Actiniaria. One of the most important points to consider

IV

PHYLUM CCELENTERATA

199

FIG- 147. Peimatula sulcata. A, entire colony; B, portion of the same magnified.
/. lateral branch ; p. polype ; s. siphonozooid. (After Koelliker.)

t.-m>

FIG. 14S. Tubipora musica. A, skeleton of entire colony ; B, transverse sections of polype ;
C. single polype with tube and commencement of platform ; I), growth of new polypes from
platform. /. /. longitudinal muscles ; pi. p'*. polypes ; pi. platform ; MI ph. siphonoglyphe ; sp.
spicules ; std. stomocUeum. (After Cuvier, Quoy and Gaimard, and Hickson.)

200

ZOOLOGY

SECT.

B

is the arrangement of the mesenteries. In Edwardsia (Fig. 149),
a genus which burrows in sand instead of attaching itself to
rocks, &c., there are only eight mesenteries (B) the usual two
couples of directives, and two others on each side of the vertical
plane, having their longitudinal muscles directed ventrally, and
therefore not arranged in couples. The adult Edwardsia thus
corresponds with a temporary stage in the development of one of
the more typical sea-anemones, viz., the stage with eight mesen-
teries shown in Fig. 142, A. ; it is probably to be looked upon as
the most primitive or generalised member of the order. In

Zoanthus (Fig. 143, B) the dorsal
directives (d.d.) do not reach the
gullet, and each lateral couple con-
sists of one perfect and one small
and imperfect mesentery. In Ceri-
anthus, another burrowing form,
there is a couple of very small
ventral directives, and the remain-
ing mesenteries are very numerous,
not arranged in couples, and all
directed ventrally at their outer
ends, so as to have a very obviously
bilateral arrangement : in this genus,
as growth proceeds, new mesen-
teries are added on the dorsal side,
and not, as is usual, between already
formed couples. On the other hand,
the newly discovered Gyractis ex-
hibits a perfectly radial arrange-
ment : the mesenteries are all

arranged in couples with the longitudinal muscles facing one
another. Lastly, in all the more typical Sea-anemones (forming
the tribe Hexactinioe) there are either six, eight or ten pairs of
perfect mesenteries, which, as well as the secondary and tertiary
cycles, are all arranged in couples, the longitudinal muscles of
all but the one or two directive couples facing one another.

In the Madreporaria the mesenteries are arranged, so far as is
known, in the way just described for the HexactiniaB. In the
Antipatharia there are six primary, and sometimes either four or
six secondary mesenteries. In the whole of the Alcyonaria the
mesenteries are eight in number : they are not arranged in
couples, and their longitudinal muscles all face the same
way, viz., towards the ventral aspect (Fig. 148, B). In this
whole sub-class, therefore, the resemblance to Edwardsia is very
close, the main difference being that the longitudinal muscles
of the ventral directives lace inwards in the Alcyonaria,
outwards in Edwardsia.

FIG. 149 Edwardsia claparedii.

A, the entire animal ; t. tube. B.
transverse section. (After Andres,
and Korschelt and Heider.)

iv PHYLUM CCELENTERATA 201

The tentacles in Zoantharia are usually very numerous, and in
nearly all cases have the form of simple glove-finger-like out-
pushings of the disc. In Edwardsia, however, they may be
reduced to sixteen, and in some genera of Sea-anemones they are
branched. In the Antipatharia (Fig. 150) they vary in number
from six to twenty-four. When more than six are present, six
of them are larger than the others.

FIG. 150. Antipathes ternatensis, portion of a branch, showing three zooids and the horny
axis beset with spines. (From the Cambridge Natural History, after Schultze.)

In the Alcyonaria, on the other hand, the tentacles, like the
mesenteries, are eight in number and are always pinnate, i.e.
slightly flattened and with a row of small branchlets along
ach edge (Fig. 144). Many Actiniaria have the tentacles
perforated at the tip (Fig. 138, A, j>j>.) ; and in some species
these organs undergo degeneration, being reduced to apertures
on the disc, which represent the terminal pores of the vanished
tentacles and are called stomidia.

Many Sea-anemones possess curious organs of offence called
acontia (Fig. 138, A, and Fig. 157, ac.). These are long
delicate threads springing from the edges of the mesen-
teries : they are loaded with nematocysts, and can be protruded
through minute apertures in the column, called " port-holes " or
cinclides (en.).

Enteric System.- The gullet in the Actiniaria presents some
remarkable modifications. It is usually a compressed tube with two
siphonoglyphes, but in Zoanthus and some other genera the ventral
gullet-groove alone is present (Fig. 143, B), and in Gyractis both
grooves are absent, and the tube itself is cylindrical with a circular
mouth. The ordinary compressed form of gullet often assumes, in
the position of rest, an oo-shaped transverse section, owing to
its walls coming together in the middle and leaving the two ends
wide open. In most of the Antipatharia the zooid is drawn out
in the direction of the long axis of the branch (Fig. 151), and in
some it becomes constricted into three parts (B) which may have
the appearance of separate zooids, the central part containing the
gullet with the mouth, while the lateral parts each contains a gonad;
each of these apparent zooids bears two of the six tentacles ; the
median one has all six mesenteries attached internally to the gullet;
in each lateral part there is only the outer portion of one of the

202

ZOOLOGY

SECT.

transverse mesenteries. In such a form as Schizopathes (Fig. 151, B)
there is thus recognisable an arrangement of the parts which might

FIG. 151. Antipatharia. A, oral face of zooid of Parantipathes. B, oral face of zooid of

Schiznpathe*. (After Delage et Herouard.)

be interpreted as a dimorphism of the zooids, one set the parts
containing the mouth and gullet being regarded as yastrozooids,
and the others containing the gonads as gonozouids.

Fixed and Free Forms. A large proportion of Actinozoa are
permanently fixed, such, for instance, as most of the Stony Corals,
the Sea fans, Black Corals, &c. Most Sea-anemones are tempo-
rarily attached by the base, but are able slowly to change their
position : some forms, such as Edwardsia (Fig. 149) and Ocrianthus,
usually live partly buried in sand enclosed in a tube formed of
discharged stinging-capsules, the oral end with its crown of
tentacles alone being exposed : others, such as Peachia, live an
actually free life, habitually lying on the sea-bottom with the
longitudinal axis horizontal like that of a worm : a few, such as
Mini/as (Fig. 152), have the aboral end dilated into a sac containing
air and serving as a float ; by it? means
these animals can swim at the surface of
the sea, and are thus, alone among the
Actinozoa, pelagic.

Dimorphism. --With the exception of
one genus of Stony Corals, the Zoantharia
are all homornorphic, i.e. there is no dif-
ferentiation of the zooids of a colony. But
in the Alcyonaria dimorphism is common :
the ordinary zooids or polypes are ac-
companied by smaller individuals, called

siphonozooids (Fig. 147, s.), having no tentacles, longitudinal
muscles, or gonads.

None of the Actiniaria have a true skeleton : in some, how-
ever, there is a thick cuticle, and several kinds enclose themselves
in a more or less complete tube (Fig. 149). which may be largely
formed of discharged nematocysts. The simplest form of skeleton
is found in the solitary Alcyonarian genus Hartea (Fig. 144), already

FIG. 152. Minyas. f. float.
(After Andres.)

IV

PHYLUM CCELENTERATA

203

referred to, in which minute irregular deposits of calcium carbonate,
called spicules (sp.), are deposited in the mesogloea. A similar
spicular skeleton occurs in the " Dead-men's finger ' ( Alcyonium,
Fig. 153), where spicules of varying form are found distributed
throughout the mesogloea of the coenosarc. In Tubipora (Fig. 148),
the " Organ-pipe Coral," the mesogloeal spicules become closely
fitted together, and form a continuous tube for each polype, the
tubes being united by horizontal calcareous platforms (pi.) formed
by deposits of spicules in the expansions of the same name already
referred to. The skeleton of Tubipora is, therefore, an internal

FIG. 153. Alcyonium palmatum, A, entire colony ; B, spicules (After Cuvier.)

skeleton, and in the living state is covered by ectoderm. In the
Red Coral of commerce (Corallium, Fig. 145) the originally separate
spicules are embedded in a cement-like deposit of carbonate of
lime, the result being the production of an extremely hard and
dense branched rod, which extends as an axis through the coenosarc.
In the Blue Coral (Heliopora\ on the other hand, the stony
calcareous skeleton is not made up of fused spicules, but is solid
from the first.

Another type of skeleton is found in the Antipatharia (Fig. 150)
and in the Gorgonacea (Fig. 154). It also consists of an axial rod,
extending all through the colony and branching with it, but is

204

ZOOLOGY

SECT.

formed of a flexible horn-like material. Moreover it is not meso-
gloeal, but ectodermal in origin : in close contact with it is an
epithelium, from the cells of which it is produced as a cuticular
secretion, and this epithelium is formed as an invagination of the
base of the colony. In addition to its axis, Gorgonia contains
numerous spicules in the mesogloea of the coenosarc. In some

FIG. 154. Gorgonia verrucosa A, entire colony; B, portion of the same magnified, c.

coenosarc ; /. polype. (After Koch and Cuvier.)

of the Gorgonacea the axial skeleton is partly horny, partly
calcareous.

In the Sea-pen (Pennatula, Fig. 147) and its allies the stem of
the colony is supported by a horny axis which is unbranched, not
extending into the lateral branches. In this case the axis is
contained in a closed cavity lined by an epithelium, the origin of

iv PHYLUM CGELENTERATA 205

which is still uncertain. Spicules occur in the mesoglcea, some of
them microscopic, others readily visible to the naked eye.

In the Madreporaria we have a skeleton of an entirely different
type, consisting, in fact, of a more or less cup-like calcareous
structure, secreted from the ectoderm of the base and column of
the polype. When formed by a solitary polype, such a " cup-
coral " is known as a corallite : in the majority of species a large
number sometimes many thousands of corallites combine to-
form a corallum, the skeleton of an entire coral-colony.

The structure of a corallite is conveniently illustrated by that
of the solitary genus Flabellum (Fig. 155, A, B). It has the form
of a short conical cup, much compressed so as to be oval in section.
Its wall or tlicca (th.) is formed of dense stony calcium carbonate,
white and smooth inside, rough and of a brownish colour outside,
except towards the margin, where it is white. Its proximal or
aboral end is produced into a short stalk or peduncle, by which the
Coral is attached in the young state, becoming free when adult :
in many other simple Corals there is no stalk, but attachment to-
the support is effected by means of a flattened proximal surface
or basal plate (C, I. pi.). From the inner surface of the theca a
number of radiating partitions, the septa (scp.), proceed inwards or
towards the axis of the cup, and, like the mesenteries of a polype,
are of several orders, those extending furthest towards the
centre being called primary septa, the others secondary, tertiary,
and so on. Towards the bottom of the cup the primary septa
meet in the middle to form an irregular central mass, the columella
(col.). In some Corals the columella is an independent pillar-like
structure arising from the basal plate (D, col.).

In many Corals there is a distinct calcareous layer investing the
proximal portion of the theca, and called the epitheca (C, c.th.}. Some
species have the inner portions of the septa detached so as to form
a circlet of narrow upright columns, the pali. In others there are
horizontal partitions or dissepiments passing from septum to septum,
and in others, again, complete partitions or tabula 1 , like those of
Millepora (p. 157), extending across the whole corallite. In the
Mushroom-coral (Fungia), the corallite is discoid, the theca is con-
fined to the lower surface, and small calcareous rods, the synapticula'^
connect the septa with one another.

In the living condition the polype fills the whole interior of the
corallite and projects beyond its edge to a greater or less degree
according to its state of expansion (C). The proximal part of the
body- wall is thus in contact with the theca, which has the relation
of a cuticle, and is, in fact, a product of the ectoderm. The free
portion of the body-wall is frequently, in the extended state, folded
down over the edge of the theca so as to cover its distal portion.
The septa alternate with the mesenteries, each lying in the space
between the two mesenteries of one couple, and each being in-

206

ZOOLOGY

SECT.

vested by an in-turned portion of the body-wall (E, F). Thus the
septa, which appear at first sight to be internal structures, are
really external : they lie altogether outside the enteric cavity, and
are in contact throughout with ectoderm.

The ectodermal nature of the entire corallite is further proved by
its development. The first part to appear is a ring-shaped

GCfi.ff

Fio. 155. A, B, two views of Flabellum curvatum. C, semi-diagrammatic view of a simple
coral ; D, portion of a corallite ; E, F, diagram of a simple coral in longitudinal and transverse
section ; ectoderm dotted, endoderm striated, skeleton black, b. pi. basal plate ; col. colurn-
ella ; e. th. epitheca ; yul. gullet ; mes, mes. 1, mes. 2, mesenteries ; mes. f. mesenteric filaments ;
sep. septa ; t. tentacle ; th. theca. (A and B after Moseley ; C and D after Gilbert Bourne.)

deposit of carbonate of lime between the base of the polype and the
body to which it adheres : sections show this ring to be formed by
the ectoderm cells of the base. The ring is soon converted into a
disc, the basal plate, from the upper surfaces of which a number of
ridges arise, arrayed in a star-like fashion : these are the rudiments
of the septa Here, again, sections show that each septum corre-

IV

PHYLUM CCELENTERATA

207

spends with a radial in-pushing of the base, and is formed as a
secretion of the invaginated ectoderm. As the septa grow they
unite with one another at their outer ends, and thus form the theca.
In some cases, however, the theca appears to he an independent
structure.

The almost infinite variety in form of the compound corals is
due, in the main, to the various methods of budding, a subject
which has already been referred to in treating of the actinozoan
colony as a whole. According to the mode of budding, massive
Corals are produced in which the corallites are in close contact
with one another, as in Astraea (Fig. 146) ; or tree-like forms, such

FIG. 156. Dendrophyllia nigrescans, B, Madrepora aspera. co. corallites;

cs. coenosarc ; p. polypes. (After Dana )

as Dendrophyllia (Fig. 156, A), in which a common-calcareous stem,
the ccenenckyma,i$ formed by calcification of the ccenosarc (cs.), and
gives origin to the individual corallites. It is by this last-named
method, the ccenosarc attaining great dimensions and the indivi-
dual corallites being small and very numerous, that the most
complex of all Corals, the Madrepores (Madrepom, Fig. 156, B)
are produced.

The microscopic structure of corals presents two main varieties.
In what are called the aporose or poreless corals, such as Flabellum,
Astra^a, &c., the various parts of the corallite are solid and stony,
while in the perforate forms, such as Madrepora, all parts both of

208 ZOOLOGY SECT.

the corallites and of the connecting coenenchyma, have the charac-
ters of a mesh-work, consisting of delicate strands of carbonate of
lime united with one another in such a way as to leave interstices,
which in the living state are traversed by a network of interlacing
tubes, representing the coenosarc, and placing the polypes of the
colony in communication.

The Blue Coral (/fdiupora),one of the Alcyonacea, has a massive
coral lum the same general appearance as a Madreporarian. The
lobed surface bears apertures of two sizes, the larger being for the
exit of the ordinary polypes, the smaller for the siphnozoids.
Tabulae are present, and septum-like ridges, which, however, have
no definite relations to the mesenteries and are inconstant in
number.

Colour. The Actinozoa are remarkable for the variety and

t/

brilliancy of their colour during life. Every one must have noticed
the vivid and varied tints of Sea-anemones ; but most dwellers in
temperate regions get into the habit of thinking of Corals as white,
and have no conception of their marvellously varied and gorgeous
colouring during life. The Madrepores, for instance, may be pink,
yellow, green, brown, or purple : Tubipora has green polypes, con-
trasting strongly with its crimson skeleton ; and the effect of the
bright red axis of Corallium is greatly heightened by its pure white
polypes. In Heliopora the whole coral is bright blue ; the tropical
Alcyonidse are remarkable for their elaborate patterns and gor-
geous coloration ; and Pennatula, in addition to its vivid colours,
is phosphorescent.

In most cases the significance of these colours is quite unknown.
In some species, however, " yellow-cells " or symbiotic Algae have
been found in the endoderm, where they probably serve the same
purpose as the similar structures which we have already studied
in Radiolaria (p. 63).

Many Actinozoa, like many sponges (p. 126), furnish examples of
commensalism, a term used for a mutually beneficial association

ij

of two organisms of a less intimate nature than occurs in symbiosis.
An interesting example is furnished by the Sea-anemone Adam sin
palliata (Fig. 157). This species is always found on a univalve shell
-such as that of a Whelk inhabited by a Hermit-crab. The
Sea-anemone is carried from place to place by the Hermit-crab, and
in this way secures a more varied and abundant food-supply than
would fall to its lot if it remained in one place. On the other
hand, the Hermit-crab is protected from the attack of predaceous
Fishes by retreating into its shell and leaving exposed the Sea-
anernone, which, owing to its toughness, and to the pain erased
by its poisonous stinging-capsules, is usually avoided as an article
of food.

Other Sea-anemones such as the gigantic Discosoma of the
great Barrier-Reef are found associated with Small Fishes or

IV

PHYLUM CCELENTKRATA

209

Crustacea, which have their abode in the enteric cavity. In this
case the Fish secures shelter in a place where it is very unlikely to
be disturbed, and the two animals are strictly commensals or " mess-
mates " since they share a common table. A somewhat similar
instance is furnished by the Blue Coral (Heliopora), already referred
to more than once. The corallum contains, not only the apertures
for the polypes and siphonozooids, but also tubular cavities of

Fin. 157. Adamsia palliata, four individuals attached to a Gasteropod shell inhabited by
a Hermit-crab, ric. ac 1 . acontia ; s/t. shell of Gasteropod. (After Andres.)

an intermediate size, in each of which is found a small chsetopod
Worm, belonging to the genus Leucodorc. As the polypes are
frequently found retracted at a time when the Worms are protruded
from their holes in search of food, it is not surprising that the
latter should have been credited with the fabrication of the coral.
Trapezia, a genus of Crabs, always lives in interstices of a par-
ticular species of Madrepore.

The distribution of the Actiniaria is world-wide, and in
many cases the same genera are found in widely separated parts

VOL. i P

210 ZOOLOGY SECT.

of the world. They are, however, larger, and of more varied form
and colour in tropical regions, for instance on coral-reefs. The
largest reef-anemone, Discosoma, found also in the Mediterranean,
attains a diameter of 2 feet. Most members of the order are
littoral, living either between tide-marks or at slight depths, but a
few are pelagic, and several species have been dredged from depths
of from 10 to 2,900 fathoms.

The Madreporaria, taken as a whole, have also a wide distribu-
tion ; but the number of forms in temperate regions is small, and
the majority including the whole of what are called reef- building
Corals are confined to the tropical parts of the Atlantic, Indian,
and Pacific Oceans, flourishing only where the lowest winter tem-
perature does not sink below 68 F. (20 C.). Thus their northern-
most limits are the Bermudas in the Atlantic, and Southern Japan
in the Pacific ; their southernmost limits, Rio and St. Helena in
the Atlantic, Queensland and Easter Island in the Pacific : in other
words, they extend to about 30 on each side of the equator.
Moreover, they have a curiously limited bathymetrical distribu-
tion, flourishing only from high-water mark down to a depth of
about 20 fathoms, but not lower.

Many of the Pacific Islands are formed entirely of coral rock,
others are fringed with reefs of the same, and the whole east coast
of Northern Queensland is bounded, for a distance of 1,250 miles,
by the Great Barrier Reef, a line of coral rock more or less parallel
to and at a distance of from 10 to 90 miles from the land.
Such reefs consist of gigantic masses of coral rock fringed by living
coral, the latter growing upon a basis of dead coral, the interstices
of which have been filled up with debris of various kinds, so as to
convert the whole into a dense limestone.

The Antipatharia, and many of the Alcyonaria, such as the Gor-
gonacea and Pennatulacea, have also a world-wide distribution,
and, even in temperate regions, Black Corals and Sea-fans may
attain a great size : the members of both these groups, as well as
the Sea-pens, are found at moderate depths. The Red Coral is
found only in the Mediterranean, at a depth of 10 to 30 fathoms.
Tubipora and Heliopora have the same distribution as the reef-
building Corals.

From the palaeontological point of view, corals are of great im-
portance : they are known in the fossil condition from the Silurian
epoch upwards, and in many formations occur in vast quantities,
forming what are called coral limestones. The majority of fossil
forms are referable to existing families, but in the Palaeozoic era
the dominant group was the 2ingosa,ihe affinities of which are still
very obscure. In these the corallites are usually bilaterally sym-
metrical, the septa are arranged in multiples of four, and the cup
presents on one side a pit, the fossnla, where the septa are greatly
reduced.

IV

PHYLUM CCELENTERATA

211

CLASS IV. CTENOPHORA.

1. EXAMPLE OF THE CLASS Hormiphora plumosa.

External Characters. Hormiphora is a pear-shaped organism
about 5-20 mm. in diameter, and of glassy transparency (Figs. 158
and 159). The species //. plumosa is found in the Mediterranean ;
allied forms belonging either to the same genus (often called
Cydippe) or to the closely allied genus Plcurolrachia are common
pelagic forms all over the world.

From opposite sides of the broad end depend two long tentacles
.), provided with numerous little tag-like processes, and springing

FIG. 158. Hormiphora plumosa. A, from the side, B, from the aboral pole. mth. mouth ;
s. pi. swimming plates ; t. and b. tentacles. (After Chun.)

each from a deep cavity or sheath, into which it can be completely
retracted (Fig. 159, t.sh.). At the narrow end where the stalk
of a pear would be inserted is a slit-like aperture, the mouth
(mth.) : this end is therefore oral. At the opposite or aboral pole
is a slight depression, in which lies a prominent sense-organ (s.o.),
to be described hereafter.

But the most striking and characteristic feature in the external
structure of Hormiphora is the presence of eight equidistant meri-
dional bands (s.pl.), starting from near the aboral pole, and extend-
ing about two-thirds of the distance towards the oral pole. Each
band is constituted by a row of transversely arranged comb-like
structures, consisting of narrow plates frayed at their outer ends.
During life the frayed ends are in constant movement, lashing to
and fro, and so propelling the animal through the water. The combs

p 2

S.O

Fig. 159. Hormiphcra plumosa. A, dissected specimen having rather more than one
quarter of the body cut away. 13, transverse section ; diagrammatic, adr. c. adradial
canal ; inf. infundibulum ; inf. c. infundibular canal ; int. c. inter-radial canal ; rurd. i:
meridional canal ; iiilh. mouth ; orii. ovary ; per. c. pcr-radial canal ; s. o. sense-organ ; .s. pi.
swimming-plate ; tpy. spermary ; xtd. stomodamm ; std. c. stomodseal canal ; .s^</. / stunn>d;ral
ridges; t. tentacle ; t. li. }>ase <>f tentacle; t. r. tentacular canal ; /. .<(//. tentacular sheath.

SECT, iv PHYLUM CCELENTERATA 213

are, in fact, rows of immense cilia, fused at their proximal ends :
their presence and mode of occurrence arranged in meridional
comb-ribs or swimming-plates are strictly characteristic of the
class, and indeed give it its name.

It will be seen at once that apart from all considerations of
internal structure Hormiphora presents a similar combination of:
radial with bilateral symmetry as in some Hydrozoa, such as
Ctenaria (Fig. 109, 1), and as in the majority of Actinozoa. The
swimming-plates are radially arranged, and mark the eight adradii,
but the slit-like mouth and the two tentacles indicate a very
marked and characteristic bilateral symmetry. A plane passing
through the longitudinal axis of the body, parallel with the long
axis of the mouth, is called, as in Actinozoa (see p. 189), the vertical
plane : it includes two per-radii, which are respectively dorsal and
ventral. A plane at right angles to this, passing through both
tentacles, and including right and left per-radii, is called the
transverse plane.

Enteric System.- -The mouth leads into a flattened tube (Fig.
159, std.), often called the stomach, but more correctly the gullet or
stomodaium. It reaches about two-thirds of the way towards the
aboral pole, audits walls are produced internally into ridges (std.r.),
which increase the area for the absorption of digested food.
Living prey is seized by the tentacles, ingested by the aid of the
mobile edges of the mouth, and digested in the stomodseum, which
is thus physiologically, though not morphologically, a stomach.
The products of digestion make their way into the various parts
of the canal-system, presently to be described, and indigestible
matters are passed out at the mouth.

Towards its upper or aboral end the stomodseum gradually
narrows and opens into a cavity called the infundibulum (inf.),
which probably answers to the stomach of an Actinozoon or a
medusa, and is flattened in a direction at right angles to the
stomodaBum i.e. in the transverse plane. From the infundibulum
three tubes are given off: one, the infundibular canal (inf. c.), passes
directly upwards, and immediately beneath the aboral pole divides
into four short branches, two of which open on the exterior by
minute apertures, the excretory pores (Fig. 160, A, ex. p.). The two
other canals given off from the infundibulum are the pcr-radial
canals (per. c.) : they pass directly outwards, in the transverse plane,
and each divides into two inter-radial canals (int. c.), which in their
turn divide each into two adradial canals (adr. c.). These succes-
sive bifurcations of the canal-system all take place in a horizontal
plane (Fig. 160, B), and each of the ultimate branches or adradial
canals opens into a meridional canal (mrd. c.), which extends up-
wards and downwards beneath the corresponding swimming-plate.
Furthermore, each per-radial canal gives off a stomodceal canal
(std. c.), which passes downwards, parallel to and in close contact

fierc ex* so

adxc

S.pl

std.c

std

rritk

t.c

inf

FIG. 1(50. Ilorniiplicra plume sa, diagrammatic longitudinal (A) and transverse (B)
sections. The ectoderm is dotted, the endoderm striated, the mesogloea black, and the
muscular axis of the tentacles gray. Lettering as in Fig. 15'J, except ex. p. excretory pore.

SECT. IV

PHYLUM CCELENTERATA

215

with the stomodseum, and a tentacular canal (t. c.) which ex-
tends outwards and downwards into the base of the correspond-
ing tentacle. Each tentacle presents a thickened base (t. &.),
closely attached to the wall of the sheath, and giving off a long
flexible filament, beset with processes of two kinds one simple
and colourless, the other leaf-like, beset with branchlets, and of a
yellow colour.

Cell-layers.- -The body is covered externally by a delicate
ectodermal epithelium (Fig. 160), the cells from which the combs
arise being particularly large. The epithelium of the stomoda3um
is found by development to be ectodermal, that of the infundibulum
and its canals endodermal : both are ciliated. The interval between
the external ectoderm and the canal-system is filled by a soft jelly-
like mesogloea. The tentacle-sheath is an invagination of the ecto-

ad,.c

Vfesi

FIG. Kil. Hormiphora plumosa. A, transverse section of one of the branches of a
tentacle ; B, two adhesive cells (ad. c.) and a sensory ceil (s. c.) highly magnified, cu. cuticle
nu. nucleus. (After Hertwig and Chun.)

derm, and the tentacle itself is covered by a layer of ectoderm,
within which is a core or axis formed by a strong bundle of longi-
tudinal muscular fibres, which, as we shall see, are of mesodermal
origin, and which serve to retract the tentacle into its sheath.

Delicate muscle-fibres lie beneath the external epithelium and
beneath the epithelium of the canal-system, and also traverse
the mesogloea in various directions. The feeble development
of the muscular system is, of course, correlated with the fact
that the swimming-plates are the main organs of progression,
the Ctenophora differing from all other Ccelenterata in retaining
cilia as locomotory organs throughout life.

A further striking difference between our present type and the
Coelenterata previously studied is the absence, in Hormiphora, of
stinging-capsules. The place of these structures is taken by the
peculiar adhesive-cells with which the branches of the tentacles

216 ZOOLOGY SECT.

are covered. An adhesive-cell (Fig. 161, ad. c.) has a convex surface,
produced into small papilla, which readily adheres to any object
with which it comes in contact and is with difficulty separated.
In the interior of the cell is a spirally coiled filament, the
delicate inner end of which can be traced to the muscular axis of
the tentacular branch. These spiral threads act as springs, and
tend to prevent the adhesive-cells being torn away by the
struggles of the captured prey.

Both the central nervous system and the principal sense-
organ are represented by a peculiar apparatus situated, as already
mentioned, at the aboral pole. In this region is a shallow depres-
sion (Fig. 162, c. p.) lined by ciliated epithelium and produced in
the transverse plane into two narrow ciliated areas, the polar
plates (p. pi-}. From the depression arise four equidistant groups
of very large S-shaped cilia (sp), united to form as many springs (*p.),
which support a mass of calcareous particles (/.), like the lithites of

Fie. lt)2. Hormiphora plumosa, Sense-organ: b. bell; c. p. ciliated plate ; c. gr. ciliated
groove ; ex. p. excretory pore ; 1. lithites ; p. pi. polar plate ; sp. spring. (Modified from Chun.)

Hydrozoa and Scyphozoa. From each spring a ciliated groove (c. gr).
proceeds outwards, bifurcates, and passes to the two swimming-
plates of the corresponding quadrant. The lithitic mass, with
its springs, is enclosed in a transparent case or bell (b.), formed of
coalesced cilia. It appears that the whole apparatus acts as a
kind of steering-gear, or apparatus for the maintenance of equili-
brium. Any inclination of the long axis must cause the calcareous
mass to bear more heavily upon one or other of the springs : the
stimulus appears to be transmitted by the corresponding ciliated
groove to a swimming-plate, and results in a vigorous movement
of the combs. Thus the sensory pit acts as a central nervous
system, and the ciliated grooves as nerves. A sub-epithelial
plexus of nerve-fibres with nerve-cells extends all over the surface
of the body.

Reproductive Organs. --The animal is hermaphrodite, the
organs of both sexes being found in the same individual. The
yonads are developed in the meridional canals (Fig. 159, B), each of
which has an ovary (pvy.) extending along the whole length of one
side, a spermary (spy.) along the whole length of the opposite side.

IV

PHYLUM CCELENTERATA

217

The oigans are so arranged that in adjacent canals those of the
same sex face one another. It will be seen that the reproductive
products have, as in Scyphozoa and Actinozoa, the position of
endoderm-cells : whether they are developed, in the first instance,
from that layer is uncertain. When ripe, the ova and sperms are
discharged into the canals, make their way to the infundibulum,
thence to the stomodseum, and finally escape by the mouth. Im-
pregnation takes place in the water.

Development.- -The process of development has been traced
in several genera closely allied to Hormiphora, so that there is
every reason to believe that, in all essential particulars, the
following description will apply to that genus.

The egg (Fig. 163) consists of an outer layer of protoplasm (plstn.)
containing the nucleus (nu.), and of an internal mass of a frothy
or vacuolated nature (yk) : the hism

vacuoles contain a homo- -yL.

i x. i T- / "\ v.m-

geneous substance which

serves as a store of nutri-
ment to the growing embryo,
and apparently corresponds
with the yolk which we shall
find to occur in a large pro-
portion of animal eggs. En-
closing the egg is a thin
vitelline membrane (v.m.), sepa-
rated from the protoplasm by FlG 1G3 ._ 0vum of Lam p etia MM . nucleus;

a Considerable Space, filled With P ?s) - protoplasm ; r. m. vitullino membrane ;

. ,, yk. yolk. (After Chun.)

a clear jelly.

After impregnation the oosperm segments, but the details of
the process are very different from those we are familiar with in
the other Ccelenterata. The protoplasmic layer accumulates on
the side which will become dorsal, and the oosperm divides along
a vertical plane, forming two cells each with a sort of protoplasmic
cap (Fig. 164, A, plsm.). A second division takes place at right
angles to the first, producing a four-celled stage (B), and each of
the four cells divides again into daughter-cells of unequal size, the
result being an eight-celled embryo, each cell with a protoplasmic
cap at its dorsal end (C, D). Next a horizontal division takes
place, dividing off the protoplasmic caps as distinct cells, and so
producing a sixteen-celled stage (E, F) in which we can dis-
tinguish eight large, ventral, yolk-containing cells or megameres
(ing.), and eight small, dorsal, protoplasmic cells or micro-
meres (mi.).

The micromeres increase rapidly in number by division, and arc
further added to by new, small cells being budded off from the
megameres (Fig. 164, G, H, and Fig. 165, A). The result of this
increase is that the micromeres gradually overspread the megameres

218

ZOOLOGY

SECT.

(Fig. 165, C), the final result being the production of an embryo
consisting of a central mass of large yolk-containing cells (ma.),

FIG. 1(34. Segmentation of the oospsrm -.in Ctenophora. mg. meganieres ; mi. micromeres ;

plsm. protoplasm ; yk. yolk. (Modified from Korsehelt and Heider.)

partly surrounded by an epithelium-like layer, incomplete below,
of small cells (mi.). This stage corresponds with the gastrula of
preceding types, the micromeres forming the ectoderm, the mega-

FIG. 165. Three stages in the development of Ctenophora. ma. meganieres ; mi. micromeres.

(From Lang's Comparative Anatomy.)

meres the endoderm, and the ventral edge of the ectodermal
investment representing the blastopore. There is, however, no
archenteron or gastrula-cavity, and the stage has been produced,

B C

tne

me

Kic. hi*;. Three stages in the duvulopment of Callianira. <l. infundibiihim ; cc. ectoderm ;
en. endoderm ; me. mesoderm ; st. stomodajum. (From Lang's Comparative Anatomy.)

not by a process of invagination or tucking-in, but by one of epiboly

or overgrowth.

The endoderm-cells increase in number, and become much
elongated and arranged obliquely, their long axes radiating,

IV

PHYLUM CCELENTERATA

219

upwards and outwards, from the long axis of the entire embryo
(Fig. 166, A). Their lower (ventral) ends then become divided off,
forming a number of small cells, which constitute the rudiment
of a true middle cell-layer or mesoderm (A, me.). A kind of in-
vagination of the megameres with their mesoderm cells then takes
place, resulting in the formation of a cavity the infundibulum
(B, d.) bounded below by the megameres, now placed horizontally,
and above by the mesoderm. The mesoderm gradually retreats to
the dorsal surface (C), finally spreading out between the dorsal
ectoderm and the infundibulum. At the same time the ectoderm
cells bounding the aperture of the infundibulum grow into it so
as to line its ventral portion : in this way the ytomodseum (st.) is
produced. The remainder of the cavity widens out and becomes
the definite infundibulum (d.), and before
long sends off four adradial pouches, the
rudiments of the canal -system. At the
same time a gelatinous layer (Fig. 167, //.),
the mesoglcea, makes its appearance be-
tween the ectoderm and endoderm.

The later processes of development
may be described very briefly. The
canal-system gradually assumes its adult
complexity and the swimming - plates
appear. A thickening of the ectoderm
on each side of the body gives rise to
the epithelium of the tentacle and of its
pouch. The muscle-fibres forming the
axis of the tentacle (B, me.) are derived
from the mesoderm, which also gives rise
to the contractile fibres of the meso-
glcea (MC.J). The lithites are formed in
the ectoderm -cells of the apical pole, but
gradually make their way on to the free cn -
surface of the cells, and become supported
on four groups of fused cilia. Four outer
groups of cilia unite with one another to
form the bell (sJc.).

The most noteworthy points in this n
somewhat complex process of develop-
mcnt are the following :-

1. The distinction between a purely
protoplasmic part of the egg and a yolk-
containing portion. In the Hydrozoa
and Actinozoa the yolk-material is small

in amount and evenly distributed, the egg being described as
alecithal or yolkless. In the present instance the yolk is at first
accumulated in the centre of the egg, which is thus centrolecithal

li>7. Two later stages in the
development of Callianira.
d. infundibulum ; en. endoderm ;
a. mesogkea ; me. mesoderm ;
.s7,-. sense-organ ; .s/. stomodamm ;
L tentacle. (From Lang's Coin-
jxirative Anatomy.)

220 ZOOLOGY SECT.

or mid-yolked, but soon the protoplasm accumulates at one end
and the yolk at the opposite end of the developing embryo, pro-
ducing a tclokcitlial or end-yolked condition.

2. The fact that segmentation is unequal, there being a distinc-
tion into large cells or megameres, containing yolk, and purely
protoplasmic small cells or micromeres.

3. The formation of a peculiar type of gastrula by epiboly or
overgrowth, the ectoderm cells (micromeres) growing over and
partly enclosing the endoderm cells (megameres).

4. "The presence, for the first time in the ascending animal
series, of a true middle embryonic layer or mesoderm. In the
other Coelenterata, as well as in the Sponges, two embryonic layers
only are formed, and the intermediate layer of the adult is formed
by the comparatively late separation of muscle-cells and connec-
tive-tissue fibres either from ectoderm or endoderm. In the
present case a definite layer of mesoderm cells becomes separated
from the endoderm during the gastrula stage.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Ctenophora are pelagic Coelenterata in which the formation
of colonies is entirely unknown. No indication of a polype-stage,
so characteristic of the remaining Coelenterata, can be detected
either in the adult or in the embryonic condition. Ciliary move-
ment, instead of being a merely embryonic form of locomotion as
in the preceding classes, is retained throughout life, the cilia being
fused to form comb-like structures, which are arranged in eight
meridional rows or swimming-plates. Tentacles, when present,
are usually two in number, situated in opposite (right and left)
per-radii, and retractile into pouches. The enteron communicates
with the exterior by a large stomodseum which functions as the
chief digestive cavity. From the enteron is given off" a system of
canals, the ultimate branches of which are adradial and have a
meridional position, lying beneath the swimming-plates ; a single
axial canal is continued to the aboral pole, where it commonly
opens by two excretory pores. There are no gastric filaments.
The central nervous system is represented by a ciliated area on
the aboral pole, and is connected with a single sensory organ,
having the character of a peculiarly modified lithocyst. The
gonads of both sexes are lodged in the same individual, the ovaries
and testes being formed on opposite sides of the meridional canals.
The oosperm undergoes unequal segmentation, the gastrula is
formed by epiboly or overgrowth, and a definite mesoderm is
established during the gastrula stage. There is no alternation of

iv PHYLUM CCELENTERATA 221

generations ; but in some cases development is accompanied by a
well-marked metamorphosis.

The Ctenophora are divisible into four orders as follows :-

ORDER 1. CYDIPPIDA.

Ctenophora having two tentacles, retractile into sheaths, and
unbranched meridional and stomodreal vessels. The body is
either circular in section or is slightly compressed in the trans-
verse plane (Figs. 158 and 168).

ORDER 2. LOBATA.

Ctenophora having numerous non-retractile lateral tentacles
contained in a groove : the bases of the two principal tentacles are
also present, but have no sheaths. The stomodaeal and meri-
dional vessels unite with one another. The body is compressed in
the transverse plane, and is produced into two large oral lobes or
lappets and into four pointed processes or auricles (Fig. 169).

ORDER 3. CESTIDA.

Ctenophora having a band-like form, owing to the extreme
compression of the body in the vertical plane. The bases of the
two principal tentacles are present, enclosed in sheaths, and there
are also numerous lateral tentacles contained in a groove. Union
or anastomosis of the meridional and stomodoeal vessels takes
place (Fig. 170).

ORDER 4. BEROIDA.

Ctenophora having no tentacles. The mouth is very wide, and
the gullet occupies the greater part of the interior of the body.
The meridional vessels are produced into a complex system of
anastomosing branches (Fig. 171).

Systematic Position of the Example.

Hormiphora plumosa is a species of the genus Hormiphora, be-
longing to the family Pkurobrachiidcv and to the order Cydippida.

The presence of two tentacles, retractile into sheaths, and of
unbranched meridional canals places it in the order Cydippida.
In this order there are three families, amongst which the Pleuro-
Irachiidce are distinguished by the absence of any compression of
the body, the transverse section being circular. The genus
Hormiphora is distinguished by having a rounded body somewhat
produced at the oral pole, and by the aperture of the tentacle-

222

ZOOLOGY

SECT.

sheath being on a higher level than the funnel. In the species
plumosa the stomodreal ridges are of a brown colour, and the leaf-
like branchlets of the tentacles yellow.

3. GENERAL ORGANISATION.

Compared with the two former classes of Ccelenterates, the Hydrozoa and
Actinozoa, the organisation of the Ctenophora is remarkably uniform. This is
due to the fact that all the species are pelagic, none are colonial, and none form
skeletons. Nevertheless a very great diversity of form is produced in virtue of
differences in proportion and modifications of the tentacular and canal systems.

The Cydippida agree in all essential respects with Hormiphora, the most
important deviation from the type-form being the compression of the bocty in the
transverse plane in some genera, e.g. Euchlora (Fig. 168, 2], the result being an

mlti
l.Callianira

2.Euchlora

S.LampeMa

FIG. 168. Three Cydippida. ab. p. aboral process ; mth. mouth. (After Chun.)

oval instead of a circular transverse section, with the tentacles at the end of the
long axis. The aboral pole may be produced into wing-like appendages, as in
CaUianira (1), and in Lampetia (3) the mouth is so dilatable as to form, when
expanded, a sole-like plate by which the animal retains itself on the surface of the
water or creeps over submarine objects. In EnchJora rubra minute nematocysts
have been found, and there is reason to believe that it was by the modification

IV

PHYLUM CCELENTERATA

223

of these characteristic ccelenterate organs of offence that the adhesive cells of
Ctenophora were evolved.

The Lobata, for instance Deiopea, are distinguished, as their name implies,
by the presence of a pair of large lappets (Fig. 169 A, fy>.), into which the oral

mrd.c

Fir;. 109. Deiopea kaloknenota. A, adult ; B, young, aur. auricle ; lp. lappet ; I. t. lateral
tentacles ; nird. t. meridional canal ; mtli. mouth. (After Chun.)

surface is produced at either end of the vertical plane. Four of the swimming
plates are shorter than the others, and at their bases arise elongated processes
called auricle* (aur.), which bear swimming-plates. The meridional canals (mdr.c)
unite with one another, and, with the cesophageal canals, are continued into the
lappets, where they become curiously coiled. The principal tentacles are
usually absent in the adult, but are represented by their basal portion^, which
are small, situated at the oral end, and devoid of sheaths. From each tentacle-
base grooves are continued along the oral surface to the auricles, and from the
grooves depend numerous small lateral tentacles (l.t.}. In the young condition
the Lobata resemble such compressed Cydippida as Euchlora, having a pair of
long principal tentacles, no lappets, and unbranched vessels (B).

The Cextida are represented by the remarkable " Venus's Girdle" (Cestux
renerix), a band-shaped Ctenophore (Fig. 170) which sometimes attains a length

B

FIG. 170.- Cestus veneris. A, adult ; B, young. /. t. latci-al tentacles ; mth. mouth ; . p/J,

s. pi.'- swimming-plates ; t. tentacle. (After Chun.)

of 1 J metre, or nearly five feet. The body is greatly elongated horizontally in the
vertical, and compressed in the transverse plane, so as to have the form of a
ribbon, which progresses by undulations of the whole body as well as by the
action of its swimming-plates. Four of the swimming -plates (x.pl. 1 ) are very

224

ZOOLOGY

SECT.

small ; the other four (s.pl. 2 ) are continued all along the aboral edge of the body.

The bases of the two principal tentacles (V.) are large and are enclosed in sheaths,

and, as in Lobata, numerous small lateral tentacles (l.t.) spring from grooves

which, in the present case, are continued the whole length of the oral edge.

The young of Cestus (B) resembles a compressed Cydippid svhich undergoes

gradual elongation in the median plane.

Bei'o< ; , the principal genus of the Beroida, has the form of a cylinder (Fig. 171),

one end of which is rounded and bears the sense-organ, the other truncated

and occupied entirely by the immense mouth (m(h.}.
The greater part of the body is taken up by the
huge gullet; the infundibulum (inf.'), per-radial
and infundibular canals, &c. , all being crowded
into a small space at the aboral pole. The
meridional canals send off branches which unite
with one another, forming a complex network of
tubes, and at their -oral ends the four meridional
canals of each (right and left) side and the corre-
sponding stomodseal canal unite into a horizontal
tube, which runs parallel with the margin of the
mouth. There is no trace of tentacles either in
the adult or in the embryonic condition.

The Ctenophora .are usually perfectly
transparent, and quite colourless, save for
delicate tints of red, brown, or yellow
in the tentacles and stomodaeal ridges.
Cestus has, however, a delicate violet hue,
and when irritated shows a beautiful
blue or bluish-green fluorescence. Beroe
if \\ is coloured rose-pink.

Ctenophora are found in all seas from
the Arctic regions to the tropics. As is
to be expected from their perishable
?nlh na ^ ure > there is no trace of the group in
the fossil state.

A very remarkable fact has been made
c'hmi ) swimmin s- plates - (Aftcr out with regard to Bolina hydatina, one of

the Lobata, a Ctenophore which attains a

diameter of 25-40 mm. While still in the larval or cydippid con-
dition and not more than 0'5-2 mm. in diameter, it becomes
sexually mature, the gonads producing ripe ova and sperms ; and
the eggs are impregnated and develop in the usual manner. Soon
the gonads degenerate, the larva metamorphoses into the adult
form, and a second period of sexual maturity supervenes. This
precocious ripening of sex-cells, occurs as we shall see in other
animal groups, and is called pcedogenesis.

IV

PHYLUM CCELENTERATA

225

APPENDIX TO CTENOPHORA

CTENOPLANA AND COSLOPLANA.

Before leaving the Ctenophora mention must be made of two remarkable
organisms which have been supposed to connect the present class with the
Turbellaria Polycladida, or Planarians, a group of worms to be described in
the following section.

Ctenoplana (Fig. 172) is a small marine animal, nearly circular in outline,
flattened dorso-ventrally, and about 6 mm. in diameter. It has hitherto been

'^v^cs'-''^-'--' v : *\*\ ^/^' " ---.'."..;>' ".-

c "i i-} "-, .---. -'4V ~\-,/*//-- .~.-^.C--

%mm ;^r

-r.r

'

'" , - A A

- ' - ; \ ,-, - * * < *

\s\s-

JL/~

Fio. 172. Ctenoplana kowalevskii. A, from above ; B, from the side. r?. clefts ; r. r.
radiating ridges ; .. o. sense-organ. (After Korotnetf.)

found only twice once in the Indian Ocean and once in New Britain. Instead
of swimming freely, like a Ctenophoran, it creeps on its ventral surface, like
a worm. In the centre of the dorsal surface is a vesicle (.s-.o.) containing a mass
of lithites surrounded by eight radiating ridges (r.r.}, alternating with which
are as many clefts (cl.}, each containing a protrusible row of stiff processes,
resembling the swimming-plates of Ctenophora. The mouth is in the centre
of the ventral surface, and leads into a stomach, from which are given off
numerous anastomosing canals, as well as a vertical canal which passes upwards
and ends beneath the sense-organ. In diverticula of this system are found the
testes, which have independent ducts opening 011 the exterior. There are two
solid tentacles contained in sacs, and a nerve-centre lies beneath the sense-organ
(.s.o.). Beneath the ectoderm is a basement-membrane, which acts as an organ
of support, and the muscular system is complex. Near each tentacle is an
aperture leading into a branched canal which is probably excretory, like the
nephridial tubes of Flat-Worms. (See Section V.)

Cwloplana is found in the Red Sea. It is also flattened dorso-ventrally, but
is oval instead of circular in outline, its dimensions being about 6 by 4 mm. It
resembles Ctenoplana in its ventral mouth, dorsal sense-organ, paired retractile
tentacles, and complex system of anastomosing canals from the stomach. There
are, however, no swimming-plates, and the ectoderm is ciliated.

Nothing is known of the development of either genus.

Q

226 ZOOLOGY SECT.

Ctenoydana and C&loplana are perhaps best looked upon as forming an
additional, somewhat aberrant, order of the Ctenophora, viz.-

ORDER 5. PLATYCTENEA.

Flattened Ctenophora of creeping habit, with a pair of retractile lateral
tentacles. The costs? (swimming-plates), when present, are retractile. There are
no meridional canals, but a system of anastomosing peripheral vessels.

THE RELATIONSHIPS OF THE CCELENTERATA.

There can be little doubt that the lowest coelenterate form
known to us is the simple hydrozoan polype, represented by
Hydra and by the hydrula stage of many Hydrozoa. Somewhat
more complex, in virtue of its stomodaeum (if a true stomodseum
be indeed represented) and its gastric ridges and filaments, is
the scyphozoan polype, represented by the scyphula of Aurelia.
Still more complex is the actinozoan polype, or actinula, as
it may be called, with its large stomodseum, mesenteries and
mesenteric filaments, and elaborate muscular system. Speaking
generally, one may say that these three polype-forms represent as
many grades of organisation along a single line of descent.

The medusa-form in the Hydrozoa is, as we have seen, readily
derived from the hydrula by the widening out of the tentacular
region into an umbrella. We may thus conceive of the Trachy-
linse, or hydroid medusae with no fixed zoophyte stage, as being
derived from a pelagic hydrula.

The Leptolinas may be considered to have arisen in consequence
of the adoption of asexual multiplication, by budding, during the
larval or hydrula stage. Instead of the hydrula giving rise
directly to a medusa, we may suppose it to have formed a temporary
colony by budding, after the manner of the Hydra, the individual
zooids being ultimately set free as medusae. The next stage
would be the establisment of a division of labour, in virtue
of which a certain proportion only of the zooids became medusa3,
the rest retaining the polype-form, remaining permanently
attached, and serving for the nourishment of the asexual colony.

The Hydrocorallina appear to be a special development of the
leptoline stock, the nearest affinities of the order being with such
forms as Hydractinia.

The Siphonophora may be conceived as having originated from
a hydrula specially modified for pelagic life by the conversion of
the basic disc into a float something after the fashion of Minyas
(Fig. 152). In such a form extensive budding, accompanied by
division of labour, would give rise to the complex siphonophoran
colony.

The lowest Scyphozoa are the Lucernarida, some of which,

iv PHYLUM CCELENTERATA 227

however, show evidence of degeneration, so that it is quite possible
to conceive them as having been derived from more highly
organised forms, instead of springing directly from simple polypes
of the scyphula type. The Semostomse, Cubomedusse, and
Rhizostomae clearly represent three grades of increasing com-
plexity along the same general line of descent, the Coronata
diverging somewhat. It is to be noted, however, that such a
supposed line does not lead towards the simpler Actinozoa, but
towards a type which diverges from the latter as well as from the
Lucernarida, Cubomedusae and Peromedusae in the absence of
septa or mesenteries in the adult condition.

The close similarity of Edwardsia and the Alcyonaria in the
number and arrangement of the mesenteries seems to indicate the
derivation of both Zoantharia and Alcyonaria from a common
ancestor in the form of a simple actinozoan polype or actinula,
Edwardsia clearly leads us to the Hexactinise or typical Sea-
anemones, and the Madreporaria are undoubtedly to be looked
upon as skeleton-forming Hexactinia?.

The relationships of the Ctenophora to the other Ccelenterata are
very doubtful. Ctenaria, one of the Anthomedusaj (Fig. 109, 1\
presents some remarkable resemblances to a Cydippid, such as
Hormiphora. It has two tentacles, situated in opposite per-radii,
and each having at its base a deep pouch in the umbrella resem-
bling the sheath of Hormiphora. There are eight radial canals
formed by the bifurcation of four inter-radial offshoots of the
stomach, and corresponding with them are eight bands of nema-
tocysts diverging from the apex of the ex-umbrella. If these
striking resemblances indicate true homologies, we must compare
the whole sub-umbrellar cavity of Ctenaria with the stomodseum
of Hormiphora, the margin of the bell of Ctenaria with the mouth
of Hormiphora, and the mouth of Ctenaria with the aperture
between the stomodseum and the infundibulum of Hormiphora.
But, as we have seen, the gullet of Ctenophora is a true stomo-
doeum developed as an in-pushing of the oral ectoderm, and has
therefore a totally different origin from the sub-umbrella of a
medusa. Moreover, the tentacles of Ctenaria have no muscular
base contained in the sheath, but spring from the margin of
the umbrella as in other Hydrozoa : its gonads are developed in
the manubrium, not in the radial canals, and there is no trace of
an aboral sense-organ.

Of Hydroctena, which has also been supposed to afford us a
connecting link between the Hydrozoa and the Ctenophora, almost
the same may be said. Hydroctena is bell-like, and provided with
a velum. At its apex is an ampulla bearing two lithites supported
on spring-like processes of the epithelium. From the apex of the
gastric cavity a canal is given off which extends to the sense-
organ, where it terminates blindly, and from the sides a pair of

Q2

228

ZOOLOGY

SECT.

short canals, each of which terminates blindly at the base of the
corresponding tentacular sheath. Only two tentacles are present,
with sheaths at their bases : these are situated, not on the margin
of the bell, as in a medusae, but between it and the apex. There
are no traces of swimming plates, and, so far as the evidence at
present forthcoming goes, there is not sufficient evidence to
establish Ctenophoran affinities.

On the other hand, the resemblance between transverse sections
of an embryo Ctenophore (Fig. 173, B) and of an embryo Actinian

A

end

end

FIG. 173. Transverse section of embryos of Actinia (A) and Beroe (B), crt. ectoderm ;
end. ciidoderm ; inf. infundibulum. (After Chun.)

(A) is very striking, and the presence of a well- developed stomo-
dseum, and of gonads developed in connection with the endoderm
and discharging their products through the mouth, may be taken
as further evidences of affinity between the Ctenophora and the
Actinozoa.

The special characteristics of the Ctenophora are, however, so
numerous and so striking, and their development so utterly unlike
that of any of the other Ccelenterata, that in our present state of
knowledge it is impossible to determine their affinity with the
other classes with any degree of certainty.

As to the orders of Ctenophora, it seems tolerably clear that
both Lobata and Cestida are derived from cydippid forms, since
they both pass through, in the course of development, a stage
closely resembling the lower Cydippida. The Beroi'da are more
highly organised in certain respects, e.g. in the details of their
histology, than the other Ctenophora, and it seems quite possible
that they may be derived from tentaculate forms. Whether the
Platyctenea are primitive or specially modified, remains doubtful,
especially in the absence of data regarding their development ;
but the latter appears the more probable conclusion.

These relationships are expressed in the diagram on the
opposite page.

By many authors the Sponges have been looked upon as so
closely related to the Ccelenterata that they may be regarded
as members of the same great phylum. The points of resemblance
are readily to be recognised : the simple structure, with the large cen-
tral cavity into which a wide opening the mouth or the osculum,

IV

PHYLUM CCELENTERATA

229

as the case may be leads ; the absence of a well-developed meso-
derm, the fixed mode of life, and associated with it, the tendency
to form compound structures or colonies by a process of budding.
In addition, the occurrence of larval stages which have at least
a superficial correspondence in the two phyla would appear to
constitute an important connecting link. Bat a closer examina-
tion of the subject shows that some of these apparent points of
resemblance are superficial only, and establishes a number of
differences between Sponges and Coelenterates too important to
allow us to suppose that a close relationship exists. One of these
differences stands out beyond the others as the most radical. The
osculum of a sponge is found, when we trace the development of

Hexactinia^ Madreporaria
Cestida

Lobata / Rhizostomeue Semostomcfe

Cydippida. / / Beroida

V / / \/ Edwardsia^

Platyctenea y / Cororiatcfc V -^ Alcvonaria

-^ \ / ^**^^ \

Cubomeduscfe

ACTINULA

v

Lucernarida

Hydrocorallinoe
Leptolinofc

SCYPHULA

Siponophora
Trachylincfc

HYDRULA

Fni. 174. Diagram illustrating the mutual relationships of the Coelenterata.

the larva, to correspond in no sense with the mouth of the
Ccelenterate. This alone, apart from important differences in
the adult structure, such as the presence in the wall of the Sponge
of the system of inhalant apertures, the presence of the peculiar
collared endoderm cells, and the absence of stinging capsules,
would suffice to remove the Sponges from the Coelenterata, and
place them in a phylum apart. But not only is the grouping
of Sponges and Coelenterates in one phylum thus rendered
impossible by important differences in their structure and develop-
ment ; a comparison of the mode of formation of the embryonic
layers in the two groups shows such radical dissimilarity that it is
scarcely possible to find sufficient evidence for regarding them as
having been derived from the same metazoan ancestors, and there
is much to be said in favour of the view that they have originated
separately from the Protozoa.

230

ZOOLOGY

SECT.

APPENDIX (II.) TO THE CCELENTERATA.

THE MESOZOA.

Under the designation MESOZOA have been comprised certain lowly organised
animal forms, formerly supposed to afford us something of the nature of a
connecting link between the Protozoa and the Metazoa, but now more generally
looked upon as degenerate members of the latter subdivision. It has been
proposed to term them the 'Moruloidea, from the resemblance which they bear
to the morula stage in embryonic development.

They are all multicellular, with an ectoderm composed of a single layer of
cells ciliated in whole or in part, and an eiidoderm either composed of a single
elongated cell or of several cells : a mesogkea is not represented. The Mesozoa
comprise at least three families, the Dicy&midcR, the Heterocyemidce, and the
0) fhoneciidw, all the members of which are internal parasites.

The Dicyemidai are parasities in the kidneys of various Cuttle-fishes and
Octopods (Cephalopoda}. Dicyema (Fig. 175), the length of which is between

FIG. 175. Dicyema paradoxum,

with infusoriform embryos (ma