LIBRARY

A TEXT- BOOK
OF ZOOLOGY

BY THE LATE

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

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

AND

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

EMERITUS PROFESSOR OF BIOLOGY INT THE UNIVERSITY OF SYDNEY, N.S.W.

IN TWO VOLUMES
VOL. I

WITH ILLUSTRATIONS

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

1921

COPYRIGHT

First Edition, 1898.
Second Edition, 1910.
TVttW Edition, 192 r.

P3 3

Siol- U

PEEFACE TO THE FIRST 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
it would be to deliver a course on the general characteristics of

vi PREFACE TO THE FIRST EDITION

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 intelligibly
be described in terms of some other group should be represented,
in an elementary course of Zoology, by an example. We 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
serve to show which of the characters already met with are of

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

PREFACE TO THE FIRST EDITION

distinctive importance, and which special to the example itself.
In order to bring out this point more clearly, to furnish a connection
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
references to the literature of the subject in the body of the work.

viii PREFACE TO THE FIRST EDITION

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 bis, 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 acknowledgments.
All the new figures have been drawn by Mr. M. P. Parker.

We have received generous assistance from Professors Arthur

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 knowledge 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 condiments, valuable only in conjunction
with a foundation of solid food."

2 The figures referred to are numbered 618, 619, 1091, 1092, 1093, 1096,
1140, 1144, 1152, 1074, and 1078 in the 3rd edition.

PREFACE TO THE FIRST EDITION ix

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
hopefully 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. HASWELL.

PREFACE TO THE THIRD EDITION

I HAVE carefully revised all parts of the work, and, in addition
to introducing a number of minor alterations and some new
illustrations, have re-written certain portions, notably in Sections
VI., VIII. and X. Professor W. N. Parker has again rendered
invaluable help in the revision of the proofs and has made a number
of useful suggestions.

W. A. HASWELL.
SYDNEY,
March 2M, 1921.

CONTENTS

PAGE

PREFACES 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 15

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

Germinal Layers 20

4. Tissues 25

5. Organs 31

6. The Reproduction of Animals . - 41

7. Symmetry 42

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 — Amoeba proteus 46

2. Classification and General Organisation 47

Systematic Position of the Example 48

Appendix to the Rhizopoda 65

Class II. Mycetozoa 68

1. Example of the Class — Didymium difforme 68

2. General Remarks 69

Class III. Mastigophora 69

1. Example of the Class — Euglena viridis 69

2. Classification and General Organisation 71

Systematic Position of the Example 71

Class IV. Sporozoa 82

1. Example of the Class — Monocystis agilis 82

2. Classification and General Organisation ..... .83

Systematic Position of the Example. 84

xiii

xiv CONTENTS

1

PHYLUM PROTOZOA — continued. PAOE

Class V. Infusoria . 90

1. Example of the Class — Paramwcium caudatum .... 90

2. Classification and General Organisation 93

Systematic Position of the Example 93

Further Remarks on the Protozoa 103

SECTION III

PHYLUM AND CLASS PORIFERA [PARAZOA] 106

1. Example of the Class — Sycon gelatinosum 106

2. Distinctive Characters and Classification 112

Systematic Position of the Example 114

3. General Organisation 115

SECTION IV

PHYLUM CCELENTERATA 129

Class I. Hydrozoa 129

1. Example of the Class — Obelia 129

2. General Structure and Classification 141

Systematic Position of the Example 143

Additional Remarks 166

Class II. Scyphozoa 167

1. Example of the Class — Aurelia aurita 167

2. General Structure and Classification 175

Systematic Position of the Example 176

Additional Remarks 183

Class III. Actinozoa 183

1. Example of the Class — Tealia crassicornis . . . * .183

2. Distinctive Characters and Classification 191

Systematic Position of the Example 194

3. General Organisation 194

Class IV. Ctenophora 208

1. Example of the Class — Hormiphora plumosa 208

2. Distinctive Characters and Classification 217

Systematic Position of the Example 218

3. General Organisation 219

The Relationships of the Ccelenterata 223

Appendix to the Ccelenterata — The Mesozoa 227

SECTION V

PHYLUM PLATYHELMINTHES 232

1. Examples of the Phylum . . 233

i. Planaria or Dendroccelum 233

ii. Fasciola hepatica . 236

iii. Tcenia solium 243

CONTENTS xv

PHYLUM PLATYHELMINTHES — continued. PAGE

2. Distinctive Characters and Classification 248

Systematic Position of the Examples • f# 249

3. General Organisation 250

4. Distribution, Mode of Occurrence, and Mutual Relationships . . 279
Appendix to Platyhelminthes — Class Nemertinea .... 284
Distinctive Characters and Classification 290

SECTION VI

PHYLUM NEMATHELMINTHES 292

Class I. Nematoda 292

1. Example of the Class — Ascaris lumbricoides . . . . . 292

2. Distinctive Characters and Classification 298

Systematic Position of the Example 299

3. General Organisation . . . 299

Class II. Acanthocephala 307

Class III. Chaetognatha 310

Appendix to Nemathelminthes 313

Family Chcetosomatidce 313

„ Echinoderidce 314

,, Desmoscolecidce 314

Affinities and Mutual Relationships of the Nemathelminthes . 315

SECTION VII

PHYLUM TROCHELMINTHES 316

Class I. Rotifera 317

1. Example of the Class — Brachionus rubens 317

2. Distinctive Characters and Classification 321

Systematic Position of the Example 323

3. General Organisation 323

Class II. Gastrotricha 328

Appendix to Trochelminthes — Dinophilea and Histriobdellea . 330

SECTION VIII

PHYLUM MOLLUSCOIDA 333

Class I. Polyzoa ' . 333

1. Example of the Class — Bugula avicularia 334

2. Distinctive Characters and Classification . . . . . 340
Systematic Position of the Example 341

3. General Organisation 341

Class II. Phoronida 348

Class III. Brachiopoda 353

1. Example of the Class — Magellania lenticularis .... 353

2. Distinctive Characters and Classification 359

Systematic Position of the Example 359

3. General Organisation 360

Mutual Relationships of the Classes of the Molluscoida . . 365

xv i CONTENTS

SECTION IX

PAGE

PHYLUM ECHINODERMATA «, 368

1. Example of the Asteroidea — Asterias rubens or Anthenea flavescens 368

2. Example of the Echinoidea — Strongylocentrotus or Echinus . . 386

3. Example of the Holothuroidea — Cucumaria or Colochirus . . 393

4. The Crinoidea — Antedon rosacea 396

5. Distinctive Characters and Classification 401

Systematic Position of the Examples 405

6. General Organisation 406

SECTION X

PHYLUM ANNULATA 429

Class I. Chaetopoda 429

1. Examples of the Class 430

i. Nereis dumerilii 430

ii. Lumbricus 443

2. Distinctive Characters and Classification 454

Systematic Position of the Examples 455

3. General Organisation 457

Appendix I. to the Chaetopoda — Class Myzostomida . . . .477

Appendix II. to the Chaetopoda — Class Echiurida . . . • . 479

Class II. Sipunculoidea 484

1. Example of the Class — Sipunculus nudus 484

2. Distinctive Characters 488

3. General Organisation 488

Class III. Archi-annelida 491

Class IV. Hirudinea 494

1. Example of the Class — Hirudo medicinalis and H. australis . 494

2. Distinctive Characters and Classification 503

Systematic Position of the Example. 504

3. General Organisation 505

General Remarks on the Annulata 510

SECTION XI

PHYLUM ARTHROPODA 514

Class I. Crustacea 514

1. Examples of the Class 514

i. Apus or Lepidurus ......... 514

ii. Astacus fluviatilis . . . . . . . . .527

2. Distinctive Characters and Classification 548

Systematic Position of the Examples 555

3. General Organisation 556

Affinities and Mutual Relationships 587

Appendix to Crustacea — Class Trilobita 589

Class II. Onychophora . 591

CONTENTS xvii

PHYLUM ARTHROPODA — continued. PAQE

Class III. Myriapoda 598

1. Distinctive Characters and Classification 598

2. General Organisation 600

Class IV. Insecta 602

1. Example of the Class — Pcriplaneta orientalis or P. americana . 603

2. Distinctive Characters and Classification 615

Systematic Position of the Example . . . . . .621

3. General Organisation 621

Class V. Arachnida 637

1. Example of the Class — Euscorpio or Buthus . . . . . 638

2. Distinctive Characters and Classification 644

3. General Organisation 646

Appendix to the Arachnida — the Pycnogonida, Linguatulida,

and Tardigrada 657

Relationships of the Air-breathing Arthropoda .... 659

SECTION XII

PHYLUM MOLLUSCA 663

Class I. Pelecypoda 663

1. Example of the Class — Anodonta and Unio 663

2. Distinctive Characters and Classification 676

Systematic Position of the Examples 678

3. General Organisation 679

Class II. Ampbineura 694

1. Distinctive Characters and Classification 694

2. General Organisation 695

Class III. Gastropoda 702

1. Example of the Class — Triton rubicundus 702

2. Distinctive Characters and Classification 713

Systematic Position of the Example 715

3. General Organisation 715

Appendix to the Gastropoda 736

Class IV. Scaphopoda 736

Class V. Cephalopoda 738

1. Examples of the Class ' 738

i. Sepia 738

ii. Nautilus pompilius ......... 754

2. Distinctive Characters and Classification 766

Systematic Position of the Examples 767

3. General Organisation 768

General Remarks on the Mollusca . 780

INDEX . .... 783

ZOOLOGY VOL. I.

LIST OF ILLUSTRATIONS

VOL I.

no. PA.QE

1. Amoeba proteus 10

2. Amoeba polypodia, fission 14

3. Alveolar theory of protoplasm 16

4. Reticular theory of protoplasm 17

5. Diagrams illustrating karyokinesis . 18

6. Ovum of Sea-urchin 21

7. Maturation and fertilisation of ovum 22

8. Segmentation of ovum .......... 23

9. Gastrulation 24

10. Gastrula 24

11. Various forms of epithelium 25

12. Diagram to illustrate structure of glands 26

lo. Gelatinous connective tissue 27

14. Reticular connective tissue . 27

15. Fatty tissue 28

16. Hyaline cartilage 28

1 7. Fibro cartilage 28

18. Bone 29

19. Non -striated muscle 30

20. Striated muscle 30

21. Nerve-cells 31

22. Nerve-fibres 31

23. Various forms of spermatozoa 31

24. Viscera of Frog — 34

25. Bones of human arm with biceps muscle 38

26. Nervous system of Frog 7" 39

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. Trichospserium sieboldii 50

34. Microgromia socialis 51

35. Chlamydophrys stercorea 52

36. Various forms of Foraminifera . . 53

xix b 2

xx LIST OF ILLUSTRATIONS

FIG. PAGE

37. Shells of Foraminifera 54

38. Hastigerina murrayi 55

39. Dimorphism and alternation of generations in Polystomella . . 57

40. Actinophrys sol 58

41. Actinosphaerium eichhornii 58

42. Various forms of Heliozoa 60

43. Actinophrys sol, conjugation 61

44. Lithocircus annularis 62

45. Thalassoplancta brevispicula 63

46. Aulactinium actinastrum 64

47. Actinomma asteracanthion .64

48. Collozoum inernie ........... 65

49. Chlamydomyxa labyrinthuloides 66

50. Labyrinthula 67

61. Didymium difforme 68

62. Euglena viridis 70

63. Various forms of Flagellata 73

64. Trypanosome 74

65. Hsematococcus pluvialis 75

66. Pandorina morum 76

67. Copromonas subtilis 77

58. Volyox globator 78

59. Various forms of Choanoflagellata 79

60. Various forms of Din oflagellata 81

61. Noctiluca miliaris 81

62. Monocystis 82

63. Gregarina 84

64. „ development 85

65. Eimeria and Coccidium 86

66. Coccidium, life-history 87

67. Malaria parasites 88

68. Myxidium and Myxobolus 89

69. Sarcocystis miescheri .......... 90

70. Paramcecium caudatum . 91

71. „ „ conjugation 92

72. Various forms of Ciliata 96

73. „ „ 97

74. Vorticella 98

75. Zoothamnium arbuscula 99

76. Opalina ranarum 100

77. Various forms of Tentaculifera ........ 102

78. Diagram showing the mutual relationships of the Protozoa . .105

79. Sycon gelatinosum 107

80. „ ,, magnified 107

81. ,, ,, transverse section 108

82. ,, ,, vertical section 109

83. Sycon gelatinosum, pore-membrane 110

84. „ „ apopyle 110

LIST OF ILLUSTRATIONS xxi

FIG. PAGE

85. External form of various Sponges . .116

86. Ascetta primordialis 117

87. Diagram of canal-system of various Sponges 118

88. Vertical Section of Spongilla 119

89. Cells of ectoderm of Sponge .120

90. Development of tri -radiate spicule 120

91. Skeleton of various Sponges 121

92. Various forms of Sponge spicules 123

93. Pheronema carpenteri 124

94. Larva of Clathrina blanca 125

95. Development of Sycon raphanus 126

96. Obelia 131

97. ,, Vertical section of polype 133

98. Nematocysts of Hydra 134

99. Tentacle of Eucopella 135

100. Obelia medusa 136

101. Diagram of medusa 137

102. Derivation of medusa from polype 138

103. Projections of polype and medusa 139

104. Development of zoophyte 140

105. Bougainvillea ramosa . . . 144

106. Various forms of Leptolinse 145

107. Ceratella 146

108. Hydra 147

109. Protohydra leuckartii 148

110. Various forms of leptoline Medusae 150

111. Diagram illustrating formation of sporosac by degradation of

medusa 152

112. Early development of Eucope 153

113. »Two Trachymedusse . ... 154

114. Two Narcomedusse 155

115. ^Eginura, tentaculocyst 155

116. Larva of ^Eginopsis 156

117. Millepora alcicornis, skeleton . . 157

118. Millepora, diagram of structure 158

119. Stylaster sanguineus, skeleton 159

120. Halistemma tergestinum 160

121. Diagram of a Siphonophore 161

122. Development of Halistemma 162

123. Physalia 163

124. Diphyes campanulata 164

125. Porpita pacifica 165

126. Graptolites 166

127. Aurelia aurita, dorsal and ventral views . . . . . .168

128. „ „ side view and vertical section 170

129. ,, ,, portion of umbrella with tentaculocyst . . . 171

130. Aurelia aurita, development 173

131. Tessera princeps , 177

xxii LIST OF ILLUSTRATIONS

FIG. PAGB

132. Lucernaria 177

133. Pericolpa quadrigata 178

134. Nausithoe 179

135. Charybdsea marsupialis 180

136. Pilema pulmo 181

137. Pelagia noctiluca, development 182

138. Tealia crassicornis, dissection and transverse section . . .184

139. Diagrammatic sections of Sea-anemone 186

140. Tealia crassicornis, section of tentacle 188

141. Nematocysts of Sagartia 188

142. Section of mesenteric filament of Sagartia 189

143. Transverse sections of embryos of Actinia 191

144. Zoanthus sociatus 195

145. Hartea elegans 195

146. Corallium rubrum 196

147. Astrsea pallida 196

148. Pennatula sulcata • . 197

149. Tubipora musica . 197

150. Edwardsia claparedii 198

151. Antipathes ternatensis 198

152. Parantipathes and Schizopathes 199

153. Minyas 200

154. Alcyonium palmatum 200

155. Gorgonia verrucosa 201

156. Structure of simple coral 203

157. Dendrophyllia and Madrepora 204

158. Adamsia palliata 206

159. Hormiphora plumosa 208

160. ,, ,, dissection and transverse section . . . 209

161. „ „ diagrammatic sections 211

162. „ „ section of branch of tentacle . . . .212

163. ,, „ sense-organ 213

164. Ovum of Lampetia 214

165. Segmentation of oosperm in Ctenophora 215

166. Development of Ctenophora 215

167. Development of Callianira^ 215

168. „ ,, (later stages) 216

169. Three Cydippida 219

170. Deiopea kaloknenota 220

171. Cestus veneris . ' 220

172. Beroe f orskalii '. . . . . . .221

173. Ctenoplana kowalevskii 221

174. Tjalfiella 222

175. Hydroctena salenskii 225

176. Sections of embryos of Actinia and Beroe 226

177. Diagram illustrating the mutual relationships of the Ccelenterata . 227

178. Dicyema paradoxum with infusoriform embryos .... 228

179. , vermiform 228

LIST OF ILLUSTRATIONS xxiii

FIG. PAGE

180. Dicyema paradoxum, male 229

181. Rhopalura giardii, male 230

182. ,, ,, female 230

183. Salinella, longitudinal section 231

184. ,, transverse ,, 231

185. Planaria, digestive and excretory systems 234

186. ,, nervous system 234

187. „ reproductive system 236

188. Transverse section of a Planarian 237

189. Fasciola hepatica 237

190. ,, ,, section of integument 238

191. ,, ,, internal organisation 239

192. „ ,, terminal part of reproductive apparatus . . 240

193. „ ,, development 241

194. Tseniasolium 242

195. „ „ head 243

196. ,, „ transverse section 244

197. „ „ proglottis 245

198. ,, ,, ripe proglottis 246

199. „ „ development 247

200. Various Planarians . .251

201. Gunda segmentata 252

202. Digenetic Trematodes 253

203. Gyrodactylus and Polystomum 254

204. Temnocephala 255

205. Actinodactylella 256

206. Tetrarhynchus 257

207. Taenia echinococcus 257

208. Ligula 257

209. Caryophyllseus 258

210. Gyrocotyle 258

211. Archigetes ' 258

212. Section of body -wall of a Triclad 259

213. Parenchyma of Flat-worm 259-

214. Diagram of Rhabdoccele 261

215. „ „ Polyclad 261

216. „ „ Triclad 262

217. Flame-cell 264

218. Reproductive organs of Mesostomum ehrenbergii . . . .267

219. Developing egg of Planocera 269

220. Embryo of Planocera 270

221. Miiller's larva of Yungia 271

222. Embryos of Dendrocoelum 272

223. Embryo of Temnocephala 273

224. „ „ 274

225. A Cysticercoid 276

226. „ with head evaginated 276

227. Cyst of Tsenia echinococcus . . . 277

xxiv LIST OF ILLUSTRATIONS

FIG. PAGE

228. Scolices .277

229. Scolex of Tsenia echinococcus 277

230. Process of budding in Microstomum 278

231. Bilharzia haematobia 281

232. Diagram of the relationships of the Platyhelminthes and Nemertinea 283

233. Diagram of Nemertine 284

234. Proboscis of Nemertine 285

235. Tetrastemma ... 286

236. Anterior portion of Nemertine 287

237. Proboscis of Metanemertean, retracted 287

238. „ „ „ everted 287

239. Transverse section of Nemertean 288

240. Diagram of anterior end of Nemertean 289

241. Pilidium 290

242. Ascaris lumbricoides 293

243. ,, ,, transverse section 294

244. „ „ muscle fibres 295

245. „ „ dissection of female 296

246. Nervous system of Nematoda 297

247. Ascaris lumbricoides, dissection of male organs 297

248. Body-wall of platymyarian Nematode 300

249. Dochmius duodenalis 300

250. Transverse section of Gordius 301

251. Oxyuris . 302

252. Gordius, anatomy 303

253. Trichinella spiralis 305

254. Two species of Echinorhynchus (Gigantorhynchus) .... 307

255. Echinorhynchus gigas, dissection of male 308

256. „ „ „ female 308

257. „ „ „ nephridia 309

258. „ „ female organs 309

259. Egg of Echinorhynchus acus 310

260. Sagitta hexaptera 311

261. „ bipunctata, transverse sections 312

262. „ „ head 312

263. „ hexaptera, eye 312

264. Development of Sagitta .313

265. Chsetosoma . 313

266. Echinoderes 314

267. Desmoscolex 314

268. Trochophore 316

269. Brae hi onus rubens, female 318

270. „ „ pharynx ......... 319

271. ,, ,, male and female, with attached eggs . . 319

272. Diagram of a Rotifer 320

273. Paraseison asplanchnus 322

274. Typical forms of Rotifera 324

275. . . . 325

LIST OF ILLUSTRATIONS xxv

FIG. PAGE

276. Typical forms of mastax 326

277. Chaetonotus maximua 329

278. „ „ anatomy 329

279. Dinophilus tgpniatus 330

280. Stratiodrilus tasmanicus 331

281. Bugula avicularia 335

282. Development of Bugula 337

283. „ ., 338

284. Larva of Bugula 339

285. Plumatella 342

286. Cristatella 343

287. Lophopus 344

288. Pedicellina . 347

289. Phoronis australis 348

290. „ „ free end 349

291. ,, ,, internal organisation 349

292. „ „ section 350

293. Actinotrocha larva of Phoronis 351

294. „ „ „ 352

295. Magellania flavescens, shell . . 354

296. „ lenticularis, anatomy 356

297. ,, flavescens, lophophore 357

298. ,, muscular system 357

299. Terebratula, nervous system, &c 358

300. Typical Brachiopods 360

301. ,, ' „ anatomy 361

3C2. Development of Cistella 363

303. Larva of Cistella 363

304. Development of Cistella 364

305. Lophophore of embryo Brachiopod 365

306. Diagrams of phylactolsematous Polyzoan and Phoronis . . . 366

307. Starfish, oral aspect 369

308. ,, vertical section of arm .371

309. ,, portion of vertical section of arm 373

310. ,, diagrammatic sections . . 374

311. Asterias rubens, digestive system 375

312. Astropecten, section of stone-canal 375

313. Anthenea flavescens, dissection from do rsaljispect .... 376

314. Asterias rubens, structure 377

315. Anthenea flavescens, lateral dissection 378

316. „ „ aboral surface . 379

317. „ „ oral surface 379

318. Asterina gibbosa, development • . . .381

319. „ „ „ 382

320. „ „ larva 383

321. „ „ „ 384

322. „ exigua, young after metamorphosis 384

323. Asterma gibbosa, development . . ...... . . . 385

xxvi LIST OF ILLUSTRATIONS

FIG. PAGE

324. Apical system of young Starfish 385

325. Echinus esculentus, peristome 386

326. Strongylocentrotus 387

327. Corona of Sea-urchin 388

328. Apical disc of Sea-urchin 389

329. Echinus, lantern of Aristotle 390

330. Sea-urchin, anatomy, lateral view 390

331. Echinoid, transverse section of ambulacra! zone . . . .391

332. Sea-urchin, anatomy, oral view 392

333. Cucumaria planci 393

334. Anatomy of a Holothurian 395

335. Antedon 397

336. Aboral view of Antedon 397

337. Antedon disc 398

338. ,, transverse section of pinnule 399

339. ,, sagittal section . . . ' 400

340. Anthenea, ventral view 410

341. Ophioglypha lacertosa 411

342. Astrophyton arborescens 412

343. Diagram of spine of Sea-urchin . .413

344. Pedicellaria of Arbacia punctulata 413

345. Hemipneustes radiatus 414

346. Clypeaster sub-depressus 414

347. Metacrinus interruptus . . .415

348. Development of Echinoderms 422

349. ,, „ Antedon 423

350. Stalked larva of Antedon ^. 424

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

dermata 428

352. Nereis dumerilii 430

353. ,, ,, parapodium 431

354. „ „ setae 431

355. Nereis diversicolor, proboscis 433

356. Nereis dumerilii, anatomy 434

357. „ „ transverse section 435

358. ,, ,, nervous system 436

359. „ eye 437

360. ,, ,, nephridium 438

361. „ ,, development 440

362. „ „ „ 442

363. Lumbricus 444

364. „ setse 445

365. „ transverse section 446

366. ,, sagittal section 447

367. „ nervous system 448

368. „ nephridium 450

369. ,, reproductive organs 451

370. „ development 453

LIST OP ILLUSTRATIONS xxvii

FIO. PAGE

371. Polynoe setosissima 457

372. Galeolaria coespitosa 457

373. Chaetopterus 458

374. Setae of various Polychaeta 458

375. Section of setigerous sac of an Oligochsete 458

376. Polynoe extenuata, anterior end 459

377. Polychaeta, various, heads 460

378. Tu ifex 461

379. Terebella 462

380. Aphrodite, enteric canal 464

381. Saccocirrus, transverse section 466

382. Phyllodoce, nephridium 468

383. Nephridia and coelomoducts 469

384. Diagram illustrating development of gonad of Polychseta . . 470

385. Pionosyllis elegans 472

386. Spirorbis Isevis 473

387. Eupomatus, development of trochophore 474

388. Autolytus cornutus, budding . . . . . . . 475

389. Syllis ramosa 476

390. Serpulse with their tube* 476

391. Myzostoma 478

392. „ anatomy 479

393. Echiurus 480

394. „ anatomy . 481

395. ,, nervous system 481

396. Bonellia viridis, female 482

397. Bonellia, anatomy 482

398. Echiurus, ciliated funnel 482

399. Bonellia, male 483

400. Echiurus, trochophore 483

401. Sipunculus nudus, anterior extremity 485

402. „ „ tentacular fold .485

403. ,, „ anatomy 486

404. ,, ,, nervous system 486

405. Priapulus 490

406. Polygordius neapolitanus . . 491

407. Protodrilus 492

408. Polygordius neapolitanus, transverse section 492

409. ,, ,, trochophore 493

410. „ „ „ later stage . . . .493

411. Hirudo medicinalis 495

412. ,, „ transverse section 496

413. „ ,, jaw 497

414. ,, australis, dissection from dorsal aspect 498

415. „ australis „ „ leftside 499

416. ,, medicinalis, nephridium 500

417. ,, diagram of blood -channels 501

418. „ section of eye 502

xxviii LIST OF ILLUSTRATIONS

PIG. PAGE

419. Hirudo, cocoon 503

420. Three Rhynchobdellida . . 506

421. Proboscis of Clepsine 506

422. Nephridium of Herpobdella 507

423. Pontobdella, nephridial system 507

424. Clepsine, development 508

425. Diagram of origin of metamerism 511

426. Diagram illustrating the relationships of the Annulata and

Trochelminthes 513

427. Apus cancriformis, dorsal aspect 515

428. Lepidurus kirkii, side view 516

429. Apus glacialis, ventral aspect 517

430. ,, appendages 518

431. Lepidurus kirkii, sagittal section 520

432. Apus, transverse section 521

433. „ shell-gland 522

434. ,, cancriformis, nervous system 523

435. ,, structure of paired eye 524

436. „ development 525

437. Astacus fluviatilis, male 527

438. ,, ,, transverse section of abdomen .... 527

439. ,, ,, appendages 529

440. „ „ articulations and muscles of leg . . . . 532

441. Section of skin and exoskeleton of Lobster 533

442. Articulations and muscles of abdomen of Crayfish .... 534

443. Astacus fluviatilis, dissection from right side 535

444. „ „ gills 536

445. „ „ kidney . . 538

446. ,, ,, transverse section of thorax 539

447. ,, ,, diagram of circulation 540

448. ,, ,, nervous system 541

449. ,, ,, reproductive organs 543

450. ,, ,, formation of the blastoderm .... 544

451. ,, ,, ventral view of embryo 544

452. ,, ,, nauplius . . . 545

453. ,, „ sections of embryos 546

454. ,, ,, development of appendages 547

455. Three Branchiopoda 557

456. „ Cladocera 558

457. Cypris 559

458. Cyclops and Calocalanus . . . . 560

459. Various forms of parasitic Eucopepoda 561

460. Argulus f oliaceus 562

461. Lepas anatifera 564

462. Balanus 564

463. Sacculina carcini 565

464. Nebalia geoffroyi . . 566

465. Paranaspides 567

LIST OF ILLUSTRATIONS xxix

FIG. PAGE

466. Mysis oculata 567

467. Diastylis 568

468. Gammarus 569

469. Asellus 570

470. Amphipoda 571

471. Isopoda . 572

472. Shrimp and Prawn 573

473. Scyllarus arctus 574

474. Pagurus bernhardus 574

475. Cancer pagurus 575

476. Typical Brachyura 576

477. Squilla 577

478. Orchestia cavimana, anatomy 579

479. Euphausia pellucida 580

480. Nervous system of Crab 581

481. Cypris-stage of Lepas 583

482. Larvae of Crabs 584

483. Diagram illustrating the mutual relationships of the orders of

Crustacea 588

484. Dalmanites and Phacops 589

485. Triarthrus becki 590

486. Peripatus capensis 591

487. „ ,, ventral view of head 592

488. ,, anatomy 593

489. ,. tracheal pit 593

490. ,. nephridium 594

491. ,, novae zealandise, development 595

492. ,, capensis 596

493. Scolopendrella immaculata .- 598

494. Scolopendra 599

495. Lithobius forficatus 599

496. Pauropus huxleyi 600

497. Strongylostoma, development 602

498. Periplaneta orientalis 604

499. ,, mouth-parts 605

500. ,, americana, lateral view of head 605

501. ,, muscular system 607

502. „ anatomy 608

503. ,, salivary glands 609

504. Trachea of caterpillar 610

505. Periplaneta, tracheal system 610

506. ,, nervous system . . . 611

507. ,, male reproductive organs . . ... . .611

508. ,, female reproductive organs . . . . . .611

509. Segmentation of ovum of Insect 612

510. Ventral plate of embryo Cockroach 614

511. Embryo Cockroach 614

512. Lepisma 616

xxx LIST OF ILLUSTRATIONS

FIG. PAGE

513. Podura 616

514. Locusta 617

515. An Embiid 617

516. Psocus f asciatus 617

517. Mallophaga 617

518. Ephemera .618

519. Aphis rosse 618

520. Cicada 619

521. Culex and larva . . . . ' 619

522. Gastrophilus equi 619

523. Pieris 620

524. Crioceris 620

525. Panorpa communis 621

526. Section of integument of Insect 622

527. Mouth-parts of Honey-bee 622

528. „ „ Diptera 623

529. „ „ Lepidoptera 624

530. Digestive organs of Beetle 626

531. Nervous, tracheal, and digestive systems of the Honey-bee . . 626

532. Tracheal gills of Ephemerid 628

533. Heart of Cockchafer 628

534. Nervous system of Diptera 629

535. Ocellus of Dytiscus larva 630

536. Sexual apparatus of Honey-bee 631

537. Segmentation of ovum of Insect 633

538. Germinal layers and amnion of Insect 633

539. Development of Hydrophilus . 634

540. „ „ 635

541. Apis mellifica, queen, worker, and drone 636

542. Formica rufa 636

543. Euscorpio 639

544. Ventral surface of cephalothorax and pre-abdomen of Scorpion . 639

545. Endosternite of Scorpion . . . 640

646. Scorpion, anatomy, lateral view 641

547. „ „ dorsal „ 642

548. „ development .643

549. Embryo of Scorpion 644

550. Chelifer bravaisii 646

551. Phrynus 647

552. Galeodes dastuguei 647

553. Epeira diadema 648

554. ,, ,, chelicerae and pedipalpi of female .... 648

555. ,, ,, pedipalp of male 648

556. Sarcoptes scabisei 649

657. Trombidium fuliginosum 649

658. Limulus 650

559. ,, ventral view 651

560. Eurypterus fischeri 652

LIST OF ILLUSTRATIONS xxxi

FIG. PAGE

561. Anatomy of dipneumonous Spider 653

562. Limulus, sagittal section 654

563. Book-lung of spider 654

564. Tracheal system of Spider 655

565. Book-giU of Limulus 655

566. Lateral eye of Euscorpio 655

,567. Central eye of Euscorpio 656

568. Nymphon hispidum 657

569. Pentastomum taenioides 658

570. Macrobiotus hufelandi 658

571. Diagram to illustrate affinities of Arthropoda 661

572. Anodonta cygnea 664

573. ,. ,. interior of valve and animal removed from shell. 665

574. ,, ,. section of shell and mantle 666

575. .. ,, animal after removal of mantle-lobe . . . 667

576. .. ,. dissection from left side 668

577. ,. ., structure of gills 669

578. ,, ,, transverse sections 670

579. .. diagram of circulation 672

580. .. statocyst . .673

581. .. early embryo 674

582. M later embryos 675

583. ., advanced embryo 675

.584. ,, metamorphosis 676

585. Anatomy of Pecten 679

586. Valves of Mya, Modiola, and Vulsella 680

587. Cardium edule 680

588. Venus gnidia 681

589. Scrobicularia piperata 681

590. Solecurtus strigillatus 682

591. Diagram of concrescence of mantle -lobes 682

592. Requienia and Hippurites 683

593. Teredo navalis . . . .683

594. Aspergillum 684

595. Mytilus edulis 684

596. Nucula delphinodonta 684

597. Gills of Pelecypoda 685

598. Gill-filaments of Mytilus 686

599. Dissection of Poromya 686

600. Donax, enteric canal 687

601. Nervous system and auditory organs of Nucula .... 688

602. Eye of Pecten 689

603. Development of Ostrea . . . 690

604. Veliger of Ostrea . 691

605. Embryos of Cyclas 691

606. Diagram illustrating the mutual relationships of the Pelecypoda. 693

607. Chsetoderma nitidulum 695

608. Neomenia carinata . 695

xxxii LIST OF ILLUSTRATIONS

FIG. PAGE

609. Chiton spinosus, dorsal view 696

610. „ ventral view .696

611. „ valves of shell 696

612. Chaetoderma nitidulum, longitudinal section 697

613. Chiton, longitudinal section 697

614. Nervous system of Amphineura 698

615. Neomenia carinata, reproductive organs 699

616. Chiton, nephridial and genital systems 700

617. Chiton, development 701

618. Triton rubicundus, shell 703

619. „ „ shell, median section 703

620. „ „ operculum 704

621. „ „ lateral view of body 705

622. „ „ diagram of introvert 705

623. ,, ,, dissection from dorsal side 707

624. „ „ buccal mass . . 708

625. ,, ,, vertical section of buccal cavity .... 708

626. ,, „ nervous system from dorsal side . . .710

627. „ ,, ,, ,, and related parts, lateral view . 711

628. „ ,j section of eye 712

629. Diagrams of displacement of mantle-cavity, &c. . . . .716

630. Solarium perspectivum 718

631. Terebra oculata 718

632. Cyprsea moneta . . . 719.

633. Doris tuberculata . 719

634. Carinaria mediterranea . 719

635. Limax 719

636. Sigaretus Isevigatus 720

637. Aplysia 720

638. Shell -bearing Pteropoda 721

639. Atlanta peronii 721

640. Pterotrachea scutata 721

641. Helix nemoralis 722

642. Pleurophyllidia lineata 723

643. Patella vulgata 723

644. Pulmonary cavity and related parts in Limax 723

645. Nervous system of Patella 725

646. „ „ „ Aplysia 726

647. „ „ „ Limnseus 726

648. Eyes of Gastropoda 727

649. Osphradium of Murex 727

650. Reproductive organs of Helix 728

651. Ovotestis of Gastropoda 729

652. Forms of egg-cases in Gastropoda 730

653. Segmentation and formation of germinal layers in Gastropoda . 731

654. Early development of Patella 732

655. Trochophore of Patella 733

656. Later trochophore of Patella 733

LIST OF ILLUSTRATIONS xxxiii

KKJ. PAGE

657. Veliger of Vermetus 734

658. Diagram illustrating the relationships of the Gastropoda . . 736

659. Dentalium, section of shell 736

660. „ anatomy 737

661. „ larvse 737

662. Sepia cultrata 739

663. „ „ shell 740

664. „ chromatophore 741

665. „ cultrata, cranial cartilage 742

666. „ „ nuchal cartilage 742

667. „ „ mantle-cavity . . . . 743

668. „ officinalis, jaws 744

669. ,, section of buccal mass 744

670. ,, officinalis, enteric canal 745

671. ,, cultrata, dissection of male from posterior aspect . . . 746

672. „ „ lateral dissection of male 747

673. ,, officinalis, longitudinal section of ink-sac 747

674. ,, cultrata vascular system . . 748

675. „ „ cephalic ganglia 748

676. ,, „ pedal and pleuro- visceral ganglia .... 748

677. „ section of eye 749

678. „ cultrata, statolith 750

679. ,, officinalis, renal organs 751

680. „ ,, diagrammatic sagittal section of female . . . 752

681. ,, male reproductive organs 753

682. ,, sperms and spermatophore . 753

683. Nautilus pompilius, section of shell 754

684. „ „ female in shell 756

685. Nautilus macromphalus, entire animal 757

686. Nautilus pompilius, lobe of foot 758

687. „ „ spadix 759

688. „ „ cephalic cartilage 759

689. ,, „ mantle-cavity of male 760

690. ,, „ dissection of male from left side . . . 762

691. „ „ arteries 763

692. ,, ,, renal sacs, ctenidia, &c 764

693. ,, ,, male reproductive organs 765

694. „ „ female „ „ 766

695. „ macromphalus, egg 766

696. Octopus vulgaris 768

697. Loligo vulgaris 769

698. Argonauta argo 769

699. Octopus lentus, male 770

700. Amphitretus pelagicus '. . .770

701. Shell of Spirula 771

702. Spirula peronii 771

703. Ammonite 772

704. Shell of Belemnite . 772

xxxiv LIST OF ILLUSTRATIONS

FIG. r*«

705. Shell of Argonauta argo . . 7«o

706. Segmentation of Loligo 774

707. Blastoderm of Sepia 775

708. „ „ sections 77

709. Development of Loligo 776

710. „ „ 77r

711. „ „ 77",

712. „ ...... 778

713. Diagram to illustrate the relationships of the Cephalopoda . . 780

CLASSIFICATION OF THE ANIMAL
KINGDOM IN THIS BOOK

KINGDOM ANIMALIA.

PHYLUM I. PROTOZOA.

Class I. RHIZOPODA.

Order 1. LOBOSA.
„ 2. FILOSA.
,, 3. FORAMINIFERA.
„ 4. HEUOZOA.
,, 5. RADIOLARIA.
Class II. MYCETOZOA.
Class III. MASTIGOPHORA.
Order 1. FLAGELLATA.

„ 2. CHOANOFLAGELLATA.

3. DlNOFLAGELLATA.

Order 4. CYSTOFLAGELLATA
Class IV. SPOROZOA.

Order 1. GBEGARINIDA.
„ 2. COCCIDIIDEA.
„ 3. H^EMOSPORIDIA.
,, 4. MYXOSPORIDEA.

„ 5. SARCOCYSriDEA

Class V. INFUSORIA.

Order 1. CILIATA.

„ 2. TENTACULIFERA.

PHYLUM II.
Class PORIFERA.
Sub-class I. Calcarea.
Order 1. HOMOCCELA.

,, 2. HETEROCCELA.
Sub-class II. Hexactinellida.

PORIFERA (PARAZOA).

Sub-class III. Demospongia.
Order 1. TETRACTINELLIDA.
,, 2. MONAXONIDA.
„ 3. CERATOSA.
4. MYXOSPONGIA

PHYLUM III.

Class I. HYDROZOA.

Order 1. LEPTOLIN^E.

Sub-order a. Anthomedusce.
,, b. Leptomedusce.
Order 2. TRACHYLINJE.

Sub-order a. Trachymedusce.

„ b. NarcomeduscK.
Order 3. HYDROCORALLINA.
,, 4. SIPHONOPHORA.
„ 5. GRAPTOLITHIDA.
Class II. SCYPHOZOA.

Order 1. STAUROMEDUSJE.
„ 2. CORONATA.

„ 3. CUBOMEDUS^E.
,, 4. DlSCOMEDUS^E.

Sub-order a. Semostomce.
,. 6. RhizostomoB.

CCELENTERATA.
Class III. ACTINOZOA.
Sub-class I. Zoantharia.
Order 1. ACTINIARIA.
,, 2. MADREPORARIA.
,, 3. ANTIPATHARIA.
Sub-class II. Alcyonaria.
Order 4. ALCYONACEA.

5. GORGONACEA.

,, 6. PENNATULAOEA.

Class IV. CTENOPHORA.

Order 1. CYDIPPIDA

„ 2. LOBATA.

,, 3. CESTIDA

,, 4. BEROIDA.

,, 5. PLATYCTENEA.

Appendix to Coelenterata — Mesozoa

XXXV

xxxvi CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM IV. PLATYHELMINTHES.

Class I. TURBELLARIA. Order 3. ASPIDOCOTYLEA.

Order 1. POLYCLADIDA. ,, 4. TEMNOCEPHALEA.

„ 2. TRICLADIDA. Class III. CESTODA.

„ 3. RHABDOCOELIDA. Order 1. MONOZOA.

„ 2. POLYZOA (MEROZOA).
aass II. TREMATODA.

Order 1. MONOGENETICA. Appendix to Platyhelminthes — Class

„ 2. DIGENETICA. NEMERTINEA.

PHYLUM V. NEMATHELMINTHE S.

Class I. NEMATODA. Class III. CHJETOGNATHA.

Order 1. NEMATOIDEA.

„ 2. NEMATOMORPHA. Appendix to Nemathelminthes —

Chcetosomatidoe, Echinoderidas, and
Class II. ACANTHOCEPHALA. Desmoscolecidce.

PHYLUM VI. TBOCHELMINTHES.

Class I. ROTIFERA. Order 4. SCIRTOPODA.

Order 1. RHIZOTA. ,, 5. TROCHOSPH^RIDA.

„ 2. BDELLOIDA. „ 6. SEISONIDA.

„ 3. PLOIMA. Class II. GASTROTRICHA.

Sub-order a. Ittoricata. Appendix to Trochelminthes — Dino-

„ b. Loricata. philea and Histriobdellea.

PHYLUM VII. MOLLUSCOIDA.

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

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

Order 1. GYMNOL^EMATA . Class II. PHORONIDA.

Sub-order a. Cydostomata. „ III. BRACHIOPODA.

„ b. Cheilostomata. Order 1. INARTICULATA.

,, c. Ctenostomata. „ 2. ARTICULATA.

PHYLUM VIII. ECHINODERMATA.
SUB-PHYLUM I. ELEUTHEROZOA.

Class I. ASTEROIDEA. Order 4. ZYGOPHIUR^.

Order 1. SPINULOSA. a^ m ECHINOIDEA>

" ' VELATA- Order 1. REGULARIA.

„ 3. PAXILLOSA. ^ 2 CLYPEASTRIDEA.

„ 4. VALVATA. ^ 3 SPATANGOIDEA
„ 5. FORCIPULATA.

Class II. OPHIUROIDEA. Class IV. HOLOTHUROIDEA.

Order 1. LYSOPHIURJE. Order 1. ELASIPODA.

„ 2. STREPTOPHIUR^E, „ 2. PEDATA.

„ 3. CLADOPHIUR^E, 3. APODA.

CLASSIFICATION OF THE ANIMAL KINGDOM xxxvii

PHYLUM VIII. ECHINODERMATA — continued.
SUB-PHYLUM II. PELMATOZOA.

Class I. CRINOIDEA.

Sub-class I. Monocyclica.

,, II. Dicyclica.
Class II. CYSTOIDEA.

Class III. BLASTOIDEA.
„ IV. EDRIASTEROIDEA.
V. CARPOIDEA.

PHYLUM IX. ANNULATA.

Class I. CH^TOPODA.

Sub-class I. Polychaeta.

Order 1. ABCHI-CH^JTOPODA.

2. PHANEBOCEPHALA.
,, 3. CRYPTOCEPHALA.
Sub -class II. Oligocheeta.
Order 1. MICBODBILI.
,, 2. MEGADBILI.
Appendix I. to the Chsetopoda — Class
MYZOSTOMIDA.

Appendix II. to the Chsetopodt
Class ECHIURIDA.
Class II. SIPUNCULOIDEA.
„ III. ARCHI-ANNELIDA.
„ IV. HIRUDINEA.

Order 1. RHYNCHOBDELLIDA.
„ 2. ABHYNCHOBDELLIDA.
Sub -order 1. Gnathobdellida.
2. Herpobdellida.

PHYLUM X. ARTHROPODA.

Class I. CRUSTACEA.

Sub-class I. Branchiopoda.
Order 1. ANOSTBACA.

„ 2. NOTOSTBACA.

,, 3. CONCHOSTBACA.
„ 4. CLADOCEBA.
Sub-class JI. Ostracoda.
,, III. Copepoda.
Order 1. EUCOPEPODA.
,, 2. BBANCHIUBA.
Sub -class IV. Cirripedia.
Order 1. EUCIBRIPEDIA.
,, 2. RHIZOCEPHALA.
Sub -class V. Malacostraca.
Series I. Leptostraca.
„ II. Eumalacostraca.
Division I. Syncarida.
Order ANASPIDACEA.
Division 2. Peracarida.
Order 1. MYSIDACEA.

„ 2. CUMACEA.

„ 3. TANAIDACEA.

,, 4. ISOPODA.

„ 5. AMPHIPODA.
Division 3. Eucarida.

Order 1. EUPHAUSIACEA.
„ 2. DECAPODA.
Sub -order 1. Macrura.

Sub-order 2. Anomura.

„ 3. Brachyura.
Division 4. Hoplocarida.

Order STOMATOPODA.
Appendix to Crustacea — -Class TRI-
LOBITA.

Class II. ONYCHOPHORA.

„ III. MYRIAPODA.
Sub -class I. Progoneata.
Order 1. PAUBOPODA.
„ 2. DIPLOPODA ( CHILD -

GNATHA).
„ 3. SYMPHYLA.
Sub -class II. Opisthogoneata.
Order 1. CHILOPODA.

Class IV. INSECTA.
Sub -class I. Apterygota.
Order 1. THYSANUBA.

„ 2. COLLEMBOLA.

Sub -class II. Pterygota.
Order 3. OBTHOPTEBOIDEA.
Sub-order 1. Orthoptera*
,, 2. Isoptera.
„ 3. Embiidce.
„ 4. Psocidce.
„ 5. Mallophaga.

xxxviii CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM X. ARTHROPOD A — continued.

Class IV. INSECT A — cont.
Sub-class II. Pterygota — cont.
Order 4. NEUBOPTEBA.
,, 5. THYSANOPTEBA.
,, 6. HEMIPTEBA.
,, 7. DIPTEBA.
„ 8. LEPIDOPTEBA.
„ 9. COLEOPTEBA.
„ 10. MECOPTERA.
, 11. HYMENOPTEBA.

Class V. ARACHNIDA.

Order 1. SCOBPIONIDA.

„ 2. PSEUDOSCORPIONIDA.

,, 3. PEDIPALPIDA.

,, 4. SOLPUGIDA.

,,, 5. PHALANGIDA.

,, 6. ABANEIDA.

„ 7. ACABIDA.

,, 8. XlPHOSUBA.
,, 9. EUBYPTEBIDA.

Appendix to the Arachnida — The
PYCNOGONIDA, LINGUATULIDA, and
TABDIGBADA.

PHYLUM XI. MOLLUSC A.

Class I. PELECYPODA.

Order 1. PBOTOBBANCHIA.

„ 2. FlLIBBANCHIA.

„ 3. PSEUD O-LAMELLI -

BBANCHIA.
' ,, 4. EULAMELLIBBANCHIA.

Sub -order a. Integripalliata.

„ b. Sinupalliata.
Order 5. SEPTIBBANCHIA.
Class II. AMPHINEURA.
Order 1. PLACOPHOBA.

,, 2. APLACOPHOBA.
Class III. GASTROPODA.
Sub-class I. Streptoneura.
Order 1. ASPIDOBBANCHTA.
Sub-order 1. Docoglossa.

Sub-order 2. Rhipidoglovsa.
Order 2. PECTINIBBANCHIA.
Sub -order 1. Platypoda.

„ 2. Heteropoda.

Sub-class II. Euthyneura.
Order 1. OPISTHOBBANCHIA.
Sub-order 1. Tectibranchia.
„ 2. Nudibranchia.
Order 2. PULMONATA.
Appendix to the Gastropoda — Class
IV. SCAPHOPODA.
Class V. CEPHALOPODA.
Sub-class 1. Dibranchiata.
Order 1. DECAPOD A.
„ 2. OCTOPODA.
Sub-class II. Tetrabranchiata.

PHYLUM XII.

SUB-PHYLUM I. ADELOCHORDA.
Class ADELOCHORDA.

SUB-PHYLUM II. UROCHORDA.

Class UROCHORDA.

Order 1. LAHVACEA.
„ 2. THALIACEA.
Sub-order a. Gyclomyaria.
„ b. Hemimyaria.
Order 3. ASCIDIAOEA.

Sub -order a. Ascidice simplices.
„ 6. Ascidice compositce.

CHORDATA.

Order 4. LUCID A.

SUB -PHYLUM III. EUCHORDA.
Section I. ACRANIA

(CEPHALOCHORDA) .

Section II. CRANIATA

(VERTEBRATA).

Class I. CYCLOSTOMATA.

Order 1. PETBOMYZONTES.
2. MYXINOIDEI.

CLASSIFICATION OF THE ANIMAL KINGDOM xxxix

PHYLUM XII. CHORDATA— continued.

Class I. PISCES.

Sub-class I. Elasmobranchii.
Order 1. CLADOSELACHII.
,, 2. PLEURACANTHEI.
„ 3. ACANTHODEI.
,, 4. SELACHII.
Sub -order a. Protoselachii.

,, b. Ettselachii.
Section a. Squdlida.
ft. Rajida.

Sub -class II. Holocephali.

III. Teleostomi.
Order 1. CROSS OPTBBYOII.
,, 2. CHONDROSTEI.
„ 3. HOLOSTEI.
„ 4. TEI/EOSTEI.
Sub-order a. Physostomi.

,, b. Anacanihini.

„ c. Acanthopteri.

„ d. Pharyngognathi.

,. e. Plectognathi.

,, /. Lophobranchii.

Sub -class IV. Dipnoi.
Order 1. MONOPNEUMONA.

,, 2. DIPNEUMONA.
Appendix to Pisces — The Ostraco
dermi.

Order 1. HETEBOSTRACI.

„ 2. OSTEOSTRACI.

,, 3. ANTIARCHA.

Class III. AMPHIBIA.
Order 1. URODELA.
„ 2. ANURA.
,, 3. GYMNOPHIONA.
„ 4. STEGOCEPHALA.

Order 6. SAUROPTERYGIA.

,, 7. ICHTHYOPTERYGIA.
8. DlNOSAURIA.

„ 9. PTEROSAURIA.

Class V. AVES.
Sub -class I. Archseornithes.
Sub-class II. Neornithes.
Division A. Ratitse.
Order 1. MEOISTANES.
,, 2. APTERYGES.

„ 3. DlNORNITHES.

,, 4. RHE^:.
,, 5. STRUTHIONES.
,, 6. ^EPYORNITHES.
„ 7. GASTORNITHES.
Division B. Carlnatse.
Order 1. STEREORNITHES.
„ 2. ODONTOLC^:.
,, 3. ICHTHYORNITHES.
,, 4. PYGOPODES.
,, 5. IMPENNES.

„ 6. TURBINARES.

„ 7. STEGANOPODES.
,, 8. HBRODIONES.
„ 9. ANSERES.

„ 10. ACCIPITRES.

,, 11. CRYPTURI.

„ 12. GALLING.

„ 13. GRAIXJE.

„ 14. GAVJJC.

„ 15. LlMICOL^E.

„ 16. PTEROCLBTES.
„ 17. COLUMB^E.

„ 18. PSITTACI.

„ 19. STRIGES.
„ 20. PICARI^E.
,, 21. PASSERES.

Class IV. REPTILIA.
Order 1. SQUAMATA.
Sub-order a. Lacertilia.
„ 6. Ophidia.
,, c. Pyihonomorpha.
Order 2. RHYNCHOOEPHALIA.
„ 3. CHELONIA.
„ 4. THEROMORPHA.
5. CROCODILIA.

Class VI. MAMMALIA.

Sub-class I. Prototheria.

II. Theria.
Section A. Metatheria (MARSUPI

ALIA).

Order 1. POLYPROTODONTIA.
„ 2. DIPROTODONTIA.

xl

CLASSIFICATION OF THE ANIMAL KINGDOM

PHYLUM XII. CHORDATA — continued.

Class VI. MAMMALIA— cont.
Sub-class II. Theria — cont.
Section B. Eutheria.
Order 1. EDENTATA.
„ 2. CETACEA.
Sub-order a. Mystacoceti.

„ b. Odontoceti.
Order 3. SIRENIA.
„ 4. UNGULATA.
Section 1 . Ungulata vera.
Sub-order a. Perissodactyla.

„ b. Artiodactyla.
Section 2. Subungulata.
Sub -order a. Hyracoidea.

Sub -order b. Proboscidea.

Order 5. CARNIVORA.

Sub-order a. Carnivora vera.
„ b. Pinnipedia.

Order 6. RODENTIA.
„ 7. INSECTIVOBA.

8. CHIROPTEBA.
Sub-order a. Megachiroptera.
,, b. Microchiroptera.

Order 9. PRIMATES.
Sub-order a. Prosimii.

„ b. Anthropoidea.

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 Linnaeus.

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
VOL. I 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 Felis
domestica, the European Wild Cat F. catus, the Leopard F. pardus,
the Tiger F. tigris, the Lion F. leo. 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 Linna3us,
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 seventy 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 Cynaelurus.
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 Felidce.
Similarly, the Civets and Mongooses form the family Viverridce ;
the Dogs, Wolves, Jackals, Foxes, &c., the family Canidce ; 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
Rodentia ; pouched animals, such as Kangaroos and Opossums, the
order Marsupialia ; and so on.

Carnivora, Ungulata, Eodentia, 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 Aves. In
the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c.,
form the class Reptilia ; 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 most 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 Coelenterata. 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 — CHORDATA.
Class — MAMMALIA .

Order — CARNIVORA.
Family — Felidce.
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
progenitor of each species was totally unconnected with that of
every other, having, as Buff on 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. Kepresenting
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 sometimes appears to afford
an indication 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
rocks, 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.

f 13. Quaternary and Eecent.

12. Pliocene.
III. Camozoic or Tertiary ..jn Miocene

[lO. Eocene.
( 9. Cretaceous.

II. Mesozoic or Secondary . | 8. Jurassic.

I 7. Triassic.

I. Palaeozoic or Primary

6. Permian.
5. Carboniferous.
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
rocHk 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 Keptiles 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, 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 palaeontology 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 " phylogenies " 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 (abyssal 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 faunae 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 <0

1. The Holarctic Region includes the whole of Europe, Asia as
far south as the Himalayas, Africa north of the Sahara, together
with the corresponding portion of Arabia, and North America as
far 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
intended 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

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 Amoeba 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 (psd.), 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
watching shows that the Amoeba is also, with extreme slowness,

10

FIG. 1.— Amoeba proteus, a living specimen.
c. vac. contractile vacuole ; nu. nucleus ;
psd. pseudopods. (From Parker's Biology,
after Gruber.)

SECT. I STRUCTURE AND PHYSIOLOGY OF ANIMALS 11

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. Experiments show that the direction of its move-
ments is capable of being influenced from without by other agencies
— by light and heat, for example. So that we are led to conclude
that Amoeba has a fairly wide range of irritability or sensitiveness to
external influences or environment.

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 contour. The more or less fluid
character is evidenced not only by the nature of the movements,
but also by the facts that the clear spaces in the interior to be
presently noticed assume a rounded shape, and that if the Amoeba
be broken up, as it may be by pressure on the cover-glass, the frag-
ments become rounded off into droplets. 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 (exoplasm) completely
enclosing a more granular internal mass (endoplasm).

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 contains 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, or several spaces which subsequently coalesce into one.

12 ZOOLOGY SECT.

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
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 watery 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

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 13

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
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 or
catabolism as it is termed, of the protoplasm, and intimately con-
nected with it, is the passage inwards of oxygen from the air dissolved
in the water, and the passage outwards of carbonic acid gas (carbon
dioxide). Oxygen is a necessary agent in the process of destructive
metabolism, and carbonic acid is a constant waste-product of such
action. This interchange of oxygen and carbonic acid is the essence
of the process of respiration observable in all living things.
When Amoebae are placed in water from which all atmospheric
oxygen has been removed, all movement soon becomes arrested, the
pseudopods become withdrawn, the body becoming rounded off
and enclosed in a thin covering or cyst, and if the deprivation is made
to last long enough death and disintegration follow. Similar
results follow the presence of excess of carbon dioxide in the water.
In such an environment the Amoeba is unable to get rid of the carbon
dioxide which it is itself producing, and becomes poisoned by the
accumulation.

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 proba-
bility 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
food being ingested than is required for simply maintaining the
size unaltered — and soon a remarkable change takes place. The
processes are 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 is complete, and we
have two distinct Amoebae resulting from the division of the one.
While the protoplasm has been undergoing this division into

14

ZOOLOGY

SECT.

halves the nucleus has also divided, and each of the two new
Amcebse 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 repro-
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

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

of locomotion, as well as movements — those involved in the
taking in of food-particles — which may be looked upon as move-
ments 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

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 15

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, 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 distin-
guished from the plant.

In connection with these vital processes of Amceba the nucleus
plays an important role. If an Amoeba be cut in two, the part
containing the nucleus continues to comport itself like a normal
complete animal ; it continues to take in and digest food, and
eventually recovers its original size. The non-nucleated part, on
the other hand, though it may continue to live for a good many
days, is unable to absorb nourishment ; its movements lack
co-ordination and do not respond to stimuli in the nprmal way.
Eventually death takes place in about ten to thirty days as a result,
apparently, of the exhaustion of the store of potential energy — the
processes of destructive metabolism having continued without the
compensating processes of nutrition and assimilation.

It has already been stated that an Amoeba may be killed by being
subjected to excess of carbon dioxide. If this action takes place
gradually the Amoeba is able to withdraw its pseud opods and
assume the defensive, motionless, encysted state, in which it is able
better to resist unfavourable conditions. But there are many
poisons which cause more rapid, even instantaneous, disintegration.
Others cause immediate death without much alteration and by their
use it is possible to fix the Amoeba in such a way as to retain its
structure not greatly altered and preserve it permanently.

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
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

16 ZOOLOGY SECT.

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 thus 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 jjiai^oi a single cell, and^itexwards,
in alljEiglieiLanimals, this_single- cell Jje^ome&jLQnverted by division
and subdivision into a fittlejdusterjor 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. Proto-
plasm, we have already seen, is a semi-fluid, clear or finely
granular substance of complex chemical_cflmposition. It is known
* not to Be a definite

compound, but to be a
somewhat varying mix-

,/ ^Sm tiire, of cJiemicaL--e©m-

pourids, the most essential
of which are bodies of
the class of proteids —
highly complex sub-
stances, into the composi-
tion of which the elements
carbon, hydrogen, oxygen,
nitrogen, and sulphur all
enter. Living protoplasm

COntainS a

FIG. 3.-Diagram to illustrate the alveolar theory of p

protoplasm. (After Dahlgreu and Kepner.) amount of Water.

capable in the living condi-

tion of passing from the viscid or semi-fluid condition into a
gelatinous one in the whole or in parts of a cell. It is soluble in
weak acids and weak alkalies ; and is capable of being coagulated—
rendered firmer and more opaque — by the action of heat and of
strong alcohol. Its reaction is slightly alkaline. As regards its minute
structure, it is generally acknowledged that there are two kinds of
substance 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-
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

STRUCTURE AND PHYSIOLOGY OF ANIMALS

17

the second, more fluid substance in its meshes (Fig. 4). Apparently
both of these types of structure occur in the protoplasm in different
cases, sometimes even in intimate association.

To a particle of protoplasm, typically containing a nucleus in its
interior, constituting the entire body of such a simple organism as
Amoeba, and forming one of the constituent elements of which a
higher plant or animal is made up, the term cell is applied. The
word was first employed in reference to the microscopic 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, enclosing envelope
or cell-wall (Fig. 5, 7, 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.m), perforated by
numerous minute aper-
tures. The nucleus con-
tains a very complex
protoplasmic material
termed chromatin, which
differs from the cyto-
plasm in its strong affinity
for certain dyes or
staining agents and also
in containing phosphorus.
Chromatin is shown, by
its universal occurrence
and its persistence throughout the changes which the cell under-
goes, to be the most essential constituent of the nucleus.

In addition to the chromatin the nucleus contains linin, nuclear
sap, and plastin. The linin is of the same character as the reticular
or alveolar cytoplasm, and the nuclear sap corresponds to the more
watery portions of the latter. The plastin is related to chromatin
with regard to affinity to staining agents, but differs from it definitely
in that and other respects. The arrangement of these constituents
of the nucleus differs greatly in different nuclei and in the same
nucleus in different phases. The chromatin, in the resting con-
dition of the nucleus, i.e. when cell-division is not in progress, may
be distributed as granules throughout, with, or more rarely without,
a definite relationship with the reticulum or alveoli. In a typical
resting nucleus the chromatin granules, or chromioles, are strung
along the threads of the linin reticulum or in the interspaces between
the alveoli so that the chromatin in a stained nucleus stands out as
a conspicuous network. But in the nuclei of many unicellular

VOL. i c

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

18

ZOOLOGY

SECT.

animals the chromatin is all concentrated in a relatively large
central mass with or without other smaller aggregations, or in several,
sometimes many, granules of about equal size. The plastin is
usually collected together in a rounded mass — the nucleolus or
plasmosome — which may contain some chromatin.

When the nucleus divides during the process of division of the
cell, its contents, more particularly the chromatin, in most cases,

D

\i

FIG. 5. — Diagrams illustrating karyokinesis. .-1, the resting cell, B, C, D, successive phases in
the formation and arrangement of the chromatin 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; c. pi. cell-plate; nu'. nucleoli; nu. m. nuclear membrane ; s. astro -
sphere ; sp. spindle. (From Parker's Biology, after Flemming, Rabl, &c.)

go through a remarkable series of changes, to which the term
karyokinesis 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 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

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 19

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 1 material — the nuclear spindle
—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
termed the attraction-sphere or astrosphere (Fig. 5, A, s). Meantime
important changes have been in progress in the nucleus. The
linin network with its attached chromatin granules or chromioles
first becomes arranged in a close tangle (spireme), and then becomes
divided up in a very definite and orderly manner into a number of
par^s — 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 chro-
matin 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, //, /). 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

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.

c 2

20 ZOOLOGY SECT.

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
two cells a narrow septum composed of thickenings of the fibres of
the spindle ; this is known as the cell-plate (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.

In many of the Protozoa the nucleus in certain conditions gives
off parts of its substance, which pass into its cytoplasm as indepen-
dent chromatin bodies, the chromidia, which may give rise to new
nuclei. The chromatin masses in chromidia are distinguished as
the chromidiosomes.

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 multicellular 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 nucleoli (germinal spot or spots).
The ovum usually contains in addition to the protoplasm a
quantity of non-protoplasmic nutrient material or yolk.

Before the process of impregnation 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

STRUCTURE AND PHYSIOLOGY OF ANIMALS

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-
nucleus (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
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.).1 The
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
operative between the male and
female cells. In many instances a
prominence (the receptive promin-
ence) 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 t 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

1 In this coalescence the chromosomes of the male and female pronuclei do
not become fvised, but remain quite separate and retain their distinctness
during the nuclear divisions that follow.

FIG. G. — Ovum of a Sea-Urchin, show-
ing the radially striated cell-mem-
brane, the protoplasm, containing
yolk-granules, the large nucleus (ger-
minal vesicle), with its network of
chromatin and a large nucleolua
(germinal spot). (From Balfour's
Embryology, after Hertwig.)

22

ZOOLOGY

SECT

is a normal phenomenon in certain families of insects, for examplr
In a considerable number of marine invertebrate animals it has been
shown that though gamogenesis, i.e. development as the result of
fertilisation of ovum by male cell, is the normal process, yet parthe-

pol

B

went

rrwnv

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 ; (',
formation of the male pronucleus ; D, approximation of the male and female pronuclei ;
E, formation of segmentation-nucleus ; ? cent, female centrosome ; <$ cent, male centro-
some ; mem. egg-membrane ; microp. micropyle ; pol. polar bodies ; 9 pron. female
pronucleus ; £ pron. male pronucleus ; seg. nucl. segmentation nucleus.

nogenesis 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

STRUCTURE AND PHYSIOLOGY OF ANIMALS 23

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
•'f 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

Fia. 8. — Various stnges in the segmentation of the ovum. (From Gegenbaur's
Comparative Anatomy.)

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

On impregnation follows shortly the process of division already
briefly referred to, which is known as segmentation (Fig. 8).
This either affects the entire substance (holoblastic or complete
segmentation) or only a part (meroblastic or incomplete seg-
mentation) of the oosperm. In the former case the ovum usually
contains comparatively little or no food-yolk, consisting mainly
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
rounded blastomeres of which it is composed project on its sur-

24

ZOOLOGY

SEPT.

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
single layer — the embryo (Fig. 9, A) assuming the form of a hollow
sphere, the blastosphere or blastula, with a wall composed of a

arch,

ABC

FIG. 9. — Gastrulation.

arch, archenteron ; bl. blastopore ; ecto. ectoderm ; endo. endoderm.

single layer of cells enclosing a cavity — the segmentation cavity

or blastoccele.
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 epiblast),
the inner the endoderm (or hypoblast). The
ectoderm and endoderm are the primary
germinal layers of the embryo ; from one or
both of them are developed the cells of a
third layer — the mesoderm (mesoblast) —
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 modified or disguised in

various ways ; and in certain cases 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

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

STRUCTURE AND PHYSIOLOGY OF ANIMALS

25

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.

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 j
others are flattened and
tabular or scale - like.
Some are Amoeba-like or
amoeboid, resembling
Amoeba in their capacity
for developing pseudo-
pods. 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 (Fig. 11, a) ;
sometimes there is on each
cell a single relatively FIG n._Various forms of epithellum „_ ciliated epi.

long, Wnip-llke ClllUm, thelium ; b, columnar ; rf, surface view of the same ;

c, tesselated ; e, the same fyom the surface ; /, flagel-
late epithelium with collars ; g, flagellate epithelium
without collars; h, epithelium of intestine with
pseudopods ; t, strattMl epithelium ; fr, deric epi-

<->' A J

which is then termed a
flagellum (/, g). Cells

as , i, sjjatiHect epithelium : A", deric epi-

prOVlded With Cilia are thelium of a marine pjanarian with pigment-cells,
j -7". . 7 T rod-cells, and sub-epithelial glands. (From Lang's

termed Ciliated., SUCh as Comparative Anatomy.}

bear flagella flagellate cells.

Some tissues are composed entirely of cells. Usually the com-
ponent cells of a tissue are distinct ; but there are many examples

26

ZOOLOGY

SECT.

of tissues in which the cells have coalesced into an aggregation in
which cell boundaries have disappeared and the nuclei alone
indicate the originally separate elements : such a structure is termed
a syncytium. Other tissues, again, though originating from cells
or by the agency of cells, consist in greater or less measure of
non-protoplasmic matter formed between the cells. Tissues com-
posed entirely of cells take the form, for the most part, of membranes
covering various surfaces, external and internal. Such membranes
are known under the general name of epithelia (Fig. 11) ; they
may consist of a single layer of cells (a-k) 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 mem-
brane ; or they may be cubical
or cylindrical or prismatic (a, b) ;
in the case of a stratified epithe-
lium 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 some-
times possess cilia, sometimes are
devoid of them. Lining the
internal cavities of the body are
layers of cells, or epithelia, some-
times in a single layer, sometimes
in several layers, sometimes
ciliated, sometimes non-ciliated.

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 and 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 (B). In other cases the gland is multicellular— ioimed 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 secretion ^collects to reach the general
surface or cavity lined by the epithelium through the passage or duct.

FIG. 12. — Diagram to illustrate the struc-
ture of glands. A, unicellular glands in an
epithelium ; B, unicellular glands lying
below epithelium and communicating
with the surface by narrow processes
(ducts) ; (7, group of gland-cells ; D,
group of gland-cells lining a depression ;
E and F, simple multicellular gland ;
O, branched multicellular gland. (From
Lang.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

27

A series of tissues in which the cells are, in most instances, sub-
ordinate, as regards bulk, to substances formed between them, is
the group known as the

connective tissues, in- T^F"F7 r*-f- .•••—*,

eluding gelatinous connec-
tive tissue, retiform con- J\
nective tissue, fibrous ^
connective tissue, cartilage,
and bone. In the majority
of forms of connective

tissue the cells lie em- *f ^

bedded in an intermediate
substance called the matrix
or ground-substance of the
connective tissue.

In the case of gelatinous ? \

connective tissue (Fig. 13)
the ground-substance (g)
is of a gelatinous char-
acter, sometimes supported
by systems of fibres (ef),
and the cells are usually
stellate or starshaped with radiating processes. Retiform or
reticulate connective tissue (Fig. 14) consists of stellate or branch-
ing cells with processes which are prolonged into fibres — the

v-

tf

FIG. 13. — Gelatinous . connective' tissue of a Jelly-
fish, bi, epithelial cell passing (into the jelly; 62.
branched cells in the jelly ; e, epithelium ; g, gela-
tinous matrix ; ef, elastic fibres. (From Lang's
Comparative Anatomy.)

fibres from neighbouring
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-sub-
stance 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 tissue, and produced by modi-
fication of its cells, is adipose m fatty tissue (Fig. 15), which consists

FIG.

14. — Regular connective
Lang.)

tissue. (From

28

ZOOLOGY

SECT.

F

of masses of large cells in which the protoplasm has more or less
completely become replaced by fat, 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
character, sometimes quite homogeneous in appearance (hyaline

cartilage, Fig. 16), sometimes
permeated by systems of
fibres (fibro-cartilage, Fig.
17), which may be of an
elastic nature (yellow elastic
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 con-
tained forms a cell-capsule.
The outer surface is
covered over by a fibrous membrane — the perichondrium. Carti-
lage is frequently hardened by the deposition in the matrix of salts
of lime, and is then known as calcified cartilage.

In bone 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

FIG. 15. — Fatty tissue. F, fat-cells ; B, connective
tissue fibrils. (From Lang, after Ranvier.)

Fia. 16.— Hyaline cartilage.

FIG. 17.— Fibro-cartilage.

canals (c) — which contain blood-vessels. The cells, or bone-corpuscles,
lie in minute spaces — the lacuna — 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

STRUCTURE AND PHYSIOLOGY OF ANIMALS

29

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 pf the body of an
animal are brought about. Mus-
cular tissue varies greatly in
minute structure in different
groups of animals, and even in
different parts of the same animal.
It consists of microscopic fibres
aggregated together into large
bundles or layers. These fibres
are composed of a substance — the
muscle-substance — which when
living has the special property of
contractility, contracting or be-
coming shorter and thicker on the
application of a stimulus. There
are two principal varieties of
muscular tissue to be distin-
guished, termed respectively non-
striated and striated muscle. Each
fibre of non-striated muscle
(Fig. 19) is usually a single,

freatly elongated cell, sometimes
ranched, with a single nucleus ;
it may contain a core of un-
altered 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 represented
by their nuclei (n). Sometimes
there is a core of protoplasm ; but

more usually the entire fibre is composed of muscle-substance,
with perhaps a remnant of protoplasm in the neighbourhood
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 enclosed
in a delicate sheath — the sarcolemma. Striated muscular tissue

FIG. 38. — Transverse section of compact
bone, a, lamellae concentric with the
outer surface ; b, lamellae concentric
with the surface of the marrow cavity ;
c , sections of Hayersian canals ; c', sec-
tion of a Haversian canal just dividing
into two ; d, interstitial lamellae. (From
Huxley's Lessons in Physiology.)

30

ZOOLOGY

SECT.

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

\\

pIGj 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

essential part of the
fibre and is made up
of numerous extremely
fine primitive fibrillce ;
this is surrounded by
a layer of a white
glistening material — the
white substance of
Schivann or medullary
sheath (med), "ericlosed
in turn in a very deli-
cate membrane — the
neurolemma (neur).

The blood, the lymph,
and other similar fluids
in the body of an
animal may be looked
upon as liquid tissues,
having certain cells
— the corpuscles — disseminated through a liquid plasma, which
takes the place of the ground-substance of the connective tissues.
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 (haemoglobin) of the blood. The leucocytes are able, like
Amoebae, to ingest solid particles, and under certain conditions a

3

yae ^i

41

fjf!-
r/|v

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

STRUCTURE AND PHYSIOLOGY OF ANIMALS

31

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

„_, rned

dx i near

FIG. 21. — Nerve-cells. A, multipolar ;
B, bipolar.

FIG. 22.— Nerve-fibres. A, medullated
B, non-medullated. ax, neuraxis
med, medullary sheath ; neur,
neurolemma.

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, encloses a
large nucleus (germinal vesicle) and
in the interior of that one or
more nucleoli or germinal spots. The
sperms (Fig. 23) are extremely
minute bodies, nearly always motile,
usually slender and whip-like,
tapering towards one extremity, and
commonly with a rounded head at
the other. The sperms are de-
veloped by a succession of cell-
divisions from certain cells — the
primitive male cells — similar in
character to immature ova. During
the course of this development
(spermatogenesis) there is, as in the
maturation of the ovum (p. 20), a reduction of the number of
chromosomes in each nucleus by one half.

5. ORGANS.

The chief systems of organs of an animal are the integumen-
tary, the skeletal, the muscular, the alimentary or digestive, the

FIG. 23. — Various forms of spermatozoa.
a, of a Mammal ; b, of a Turbellarian
worm ; c, d, and e, of Nematode
worms ; /, of a Crustacean ; g, of a
Salamander ; h, the commonest form
with oval head and long flagellum.
(From Lang's Comparative Anatomy.)

32 ZOOLOGY

SECT.

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 exoskeleton (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 exoskeleton or
external skeleton ; in the latter an endoskeleton or internal skeleton.
In many groups of animals both systems are developed. An
exoskeleton 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 forma-
tion. In many invertebrate animals, such as Insects, Crustaceans,
and Molluscs, it is a greatly thickened and hardened 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.

i STRUCTURE AND PHYSIOLOGY OF ANIMALS 33

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 levers 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 buccal cavity, a pharynx,
an oesophagus 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
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 stomach, and a narrow tube, the
intestine.

The egestive or efferent region of the alimentary canal is the
VOL i D

34 ZOOLOGY

SECT.

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
mainly for the passage to the anal opening of the faces 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

Fio. 24. — General view of the viscera of a male Frog, from the right *ide. a, stomach ; b,
urinary bladder ; c, small intestine ; cl, cloacal aperture ; d, large intestine ; e, liver ;
/, bile-duct ; g, gall-bladder ; h, spleen ; t, lung ; k, larynx ; I, fat-body ; m, testis ;
n, ureter ; o, kidney ; p, pancreas ; s, cerebral hemisphere ; sp, spinal cord ; t, tongue ;
u, auricle ; ur, urostyle ; i\ ventricle ; vs, vesicula seminalis ; w, optic lobe ; ,r, cerebellum ;
y, Eustachian recess ; 2, nasal sac. (From Marshall.)

forth processes of their protoplasm (Fig. 11, h), and of taking minute
particles of food into their interior to become digested and absorbed
(intracellular 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
line special little pouch-like, simple or branched glands, opening
by a passage or duct 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, j
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

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 35

glands open always, not into the digestive, but into some part of
the ingestive region of the alimentary system.

The most important function of thQ^liver — properly so called —
is one distinct from the process of digestion ; its secretion — the
bile — 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 lactealst 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
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 blood-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 meo!ium,
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

D 2

36 ZOOLOGY SECT.

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 trachece, 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
called hcemoglobin, 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

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 37

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
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 Amosba, 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

ZOOLOGY

SECT.

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
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.

The central parts of the nervous system consist (Fig. 2G) of
certain aggregations of nerve-matter known as nerve-ganglia,
containing a large number of nerve-cells ; 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, con-
stituting the peripheral part of the system ; the central parts have
the office both of receiving impressions conveyed by the nerve-fibres
from the surface, from the organs of special sense, and from the
internal 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 movement 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 supplied to it to be increased or diminished a
message is conveyed along a nerve-fibre and causes the dilatation

FIG. 25. — Bones of the human arm 'and fore-arm with the
biceps muscle, showing the shortening and thickening of
the muscle during contraction 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.)

STRUCTURE AND PHYSIOLOGY OF ANIMALS

39

ol. n,

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 the integu-
ment to such general im-
pressions may be increased
by the presence in it of a
variety of tactile papillse
or corpuscles having nerve-
fibres terminating in them.
In most animals, however,
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, ol-
factory 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 pro-
ducing a nerve-disturbance
in certain neighbouring
nerve-cells. To this may
be added clear, highly-
refracting bodies which in-
tensify the effect. In the
higher types of eye there
are the same characteristic

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-

\

FIG. 26. — Nervous system of the Frog.
Howes 's Atlas.)

(From

40 ZOOLOGY SECT.

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. 31).
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
the ovaries occur in one set of individuals — the females— and the
testes in another set — the males, when the term unisexual or
dioecious 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,
spermiducts, or vasa deferentia. 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.

STRUCTURE AND PHYSIOLOGY OF ANIMALS

41

6. THE REPEODUCTION 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 binary
fission— has been already noticed in
connection with the reproduction of
Amoeba. The formation of spores
by multiple fission is an asexual mode
of multiplication 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, bd) 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 develop-
ment has been completed, or may
remain in permanent vital con-
nection with the parent form.

When the buds, after becoming
fully developed, remain in vital

•f. ., *., ' , FIG. 27.— Fresh- water polype (Hydra);

Continuity With the parent, a Sort two specimens, the one expanded, the

of compound animal, consisting &!tfS££*gfo?GP&*

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 zooids. In some cases such a colony is produced

by a process which is more correctly termed incomplete fission

than budding.

Alternation of Generations ; Heterogamy ; Psedogenesis. —
In the life-history of a considerable number of animals, a stage in
which reproduction takes place by a process of budding or fission
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.

.

in various stages of growth. (From
Parker's Biology.)

42

ZOOLOGY

SECT.

Pcedogenesis, or the development of young by a sexual process from
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. 24) is the simplest and most generalised form among
multicellular animals or Metazoa : but no adult animal retains

d L \--c

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

FIG. 29. — Radial symmetry. Letters as
in Fig. 28. The processes at A are
the tentacles ; the lower figure repre-
sents the upper or oral surface. (From
Gegenbaur.)

this simple shape. In the gastrula we may imagine a central
primary axis (Fig. 28, AB) 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
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-

I STRUCTURE AND PHYSIOLOGY OF ANIMALS 43

ment of internal parts. Such an animal is said to be radially
symmetrical. The body of a radially symmetrical animal is capable
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 : a 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
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 metameres.
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 OR PHYLA OF THE ANIMAL

KINGDOM.

The various systems of organs — 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

44 ZOOLOGY SECT, i

spinal cord, and of two pairs of limbs (the paired fins) ; they belong
to the great vertebrate or backboned group — the division Verte-
brata of the phylum Chordala. 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 ArtJiropoda. 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
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. Ccelenterata IX. Annulata

IV. Platyhelminthes X. Arthropoda
V. Nemathelminthes XI. 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. MYCETOZOA. — Terrestrial Protozoa in which the plas-
modial phase is specially characteristic, as also is the formation
of large and often complex cysts.
Class 3. MASTIGOPHORA. — Protozoa in which the flagellate form

45

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 L-RHIZOPODA.

1. EXAMPLE OF THE CLASS — Amoeba proteus.

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

Amoeba 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, the endosarc or
endoplasm, and a clear outer layer, the ectosarc or exoplasm ; 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,f. 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.

Amoebae are sometimes found to undergo ency station ; 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
chitinoid 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 in Amoeba proteus by simple or binary
fission ; direct or amitotic division of the nucleus is followed by
division into two of the cell-body (I). Occasionally two Amoebae

It

PHYLUM PROTOZOA

47

have been observed to copulate or undergo complete fusion, but
nothing is known of the result of this process or of its precise signi-
ficance in this particular case.

»__V; nil

iceba, A, A quarto, ; B, the same killed .and stained ; C, A. proteus ; D, encysted
E A proteus : F, nucleus of same, stained ; G, A. verrucosa ; H, nucleus of
', I, A. proleus, undergoing binary fission ; a, point of union of enclosing

dS : c. vac. contract, IR vnr-nnlp • MI /-Trof • f .,„,„ t^A ,r«^,,«i« .

Fia. 30.— Amoeba

specimen ; _

same, stained , *., ^j.. ±,,1/1*0,0, uuucigumg
pseudopods ; c. vac. contractile vacuole ;
(numerous in A. quarta) : psd pseudopod
and Howes.)

tituj jiooiuu , i*, JJVLIUI ui union 01 enclosing
V. cyst ; /. vac. food-vacuole ; nu. nucleus
From Parker's Biology, after Leidy, Gruber,

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
five orders may be distinguished : —

OEDER 1. — LOBOSA.

Rhizopoda with short, blunt pseudopods. There is a clear dis-
tinction between exoplasm and endoplasm.

ORDER 2. — FILOSA.

Rhizopoda with fine branched pseudopods which do not fuse
except near the bases : no exoplasm.

ORDER 3. — FORAMINIFERA.

Shelled Rhizopoda with fine, branched, and anastomosing
pseudopods.

ORDER 4. — HELIOZOA.
Rhizopoda with fine, stiff, radiating pseudopods.

ORDER 5. — RADIOLARLA.

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 Amoeba,
belonging to the family Amoebidce, of the order Lobosa. The blunt
pseudopods not uniting to form networks place it among the
Lobosa : the absence of a shell, among the Amcebidax 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

PHYLUM PROTOZOA

49

ways (F, H], and the pseudopods may be prolongations of con-
siderable relative size (C), or mere wave-like elevations of the
surface (G). Sometimes specimens are found in which neither

I

m

FIG. 31. — Protamoeba primitiva. Showing changes of form and three stages in binary
fission. (After Haeckel, from Parker's Biology.)

nucleus nor vacuole is present ; these are placed in the genus
Protamoeba (Fig. 31). This and other non-nucleate forms possess a
potential nucleus in the form of minute scattered granules of chro-
matin (chromidia).

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.

Skeleton. — We may
understand the relation
of the shelled to the
shell-less Lobosa by
upposing 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
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 Arcella 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 gela-
tinous coating to which
minute sand-grains and other foreign particles become attached.

The prevailing mode of multiplication is by means of binary
fission. But multiple fission also occurs. In such a case the animal
passes into the encysted condition, and nucleus and protoplasm

VOL. I E

FIG. 32. — A, Quadrula symmetric a ;
sphenia lata ; C, Arcella vulgaris ; D,

pyriformis.

alo-

_ ugia

(From Lang's Comparative Anatomy.)

50

ZOOLOGY

SECT.

undergo a process of division resulting in the formation of a number
of small bodies or spores which, when mature, are set free by the
rupture of the wall of the cyst. Each of these may develop directly
into the adult form. But in some cases it has been found that on
becoming free from the cyst they coalesce in pairs either with one
another or with similar bodies from other cysts. To such a

FIG. 33.— Trichospaerium sieboldii. 1, Adult of "A" form; 2, its multiplication by
fission and gemmation ; 3, division into amoeboid spores ; 4, development of one of these
into " B " form (5) ; 6, its multiplication by fission and gemmation ; 7, its division into
numerous minute bodies ; 8, discharge of these as biflagellate gametes ; 9, 10, copulation,
more highly magnified ; 11, zygote ; 12, transition towards " A " form. (After
Schaudinn.)

coalescence, which extends to the two nuclei as well as the cyto-
plasm, the term copulation is applied — the two copulating bodies
being gametes and the body resulting from the union a zygote. This
is the simplest form of sexual reproduction. A slight complication
is introduced when, as in Amoeba blattce, there are two sets of gametes
to be distinguished — a larger and a smaller — and copulation

II

PHYLUM PROTOZOA

51

takes place between one of the former and one of the latter. In
Paramosba, which is amoeboid in the adult condition and multiplies
by binary fission, when encystation takes place the contents of the
cyst become divided up to form flagellulcB, each provided with two
flagella. These are not known to copulate, but multiply by binary
fission and pass directly into the adult amoeboid form, the flagella
being lost. In Trichosphcerium (Fig. 33) there is an alternation of
generations— one generation (7, 2) reproducing by fission and
gemmation and then breaking up into amoeboid spores (3) which do
not copulate but develop (4) directly into the second generation (-5),
and this after reproducing by fission and gemmation (6), becomes,
resolved into bi-flagellate gametes (7, 8), whiclftopulate in pairs with
other similar gametes (.9, 10,11), the zygotes developing into the first

FIG. 34.— Microgromia socialis. A, entire colony ; B, single zooid ; C, zooid which
has undergone binary fission, with one of the daughter-cells creeping out of the shell ;
Z>, flagellula ; c. vac. contractile vacuole ; nu. nucleus ; sh. shell. (From Biitschli's
Protozoa, after Hertwig and Lesser.)

generation (1;J). In some cases copulation takes place, not between
specially produced gametes, but between adults ; -.this is usually
followed either by more active multiplication or by passing into a
motionless resting condition. A modification of copulation termed
autogamy occurs in Amoeba coli (as in a number of Protozoa of other
classes). In autogamy the Protozoan divides into two : each of the
two nuclei throws off a part of its substance and then unites with
the other.

Most of the Lobosa are free ; some occurring in fresh water,
others in the soil, others in the sea. But a number are parasites
in the bodies of higher animals. About six species of Amoeba are
known to occur as parasites in man, of which number, one — Amoeba
(Entamcebd) Jiistolytica — has been proved to be pathogenic, causing
dysentery and liver abscess.

K2

52 ZOOLOGY SECT.

ORDER 2. — FILOSA.

The order Filosa comprises a small number of Rhizopods having
affinities with some of the Lobosa on the one hand and with the
Foraminifera on the other. The pseudopodia are very fine and
thread-like, and become branched towards the ends : unlike those
of the Foraminifera, they do not coalesce with one another except
at or near the proximal ends and do not form networks. There is
no clear exoplasmic layer. The best known of the Filosa —
Euglypha — has a flask-shaped test composed of close-fitting hexa-
gonal siliceous plate*

o- vac

B

FIG. 35. — Chlamydophrys stercorea. A, single zooid ; -B, formation of colony ; c. vac.
contractile vacuole ; /. food particles ; nu. nucleus ; sh. shell. (From Biitschli's Protozoa,
after Cienkowsky.)

ORDER 3. — 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.
34). It consists of a protoplasmic body (B), with a single nucleus
(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).

PHYLUM PROTOZOA

53

We shall find other instances in which the young of a Ehizopod is
a flagellula, i.e. a cell provided with one or more flagella, which,

a.Squammulina 4.M i I i o I a

FIG. 36. — Various forms of Foraminifera. In 4, Miliola, a> shows the living animal ;
b, the same killed and stained : a. aperture of shell ; /. food particles ; nu. nucleus ; **.
shell. (From BQtschli's Protozoa and China's Zoology.)

if its history were not known, would be included among the
Mastigophora.

Ghlamydoplirys (Fig. 35, A) is a form resembling Microgromia,
but is said to form simple colonies by budding. In the simple

54

ZOOLOGY

SECT*

condition it has a chitinous shell with a single narrow opening :
it multiplies by binary fission, and by multiple fission produces
gametes which copulate.

2.Lagena

JqP

4.Frondicularia aClobigerina

s.sk

7. Discorbina

Q.PIanorbulina

ll.Nummulites

FIG. 37.— Shells of Foraminifera. In 3, 4. and 5, a shows the surface view, and b a section;
8a is a diagram of a coiled cell without supplemental skeleton ; 8b of a similar form
with supplemental skeleton (s. sic.) ; and 10 of a form with overlapping whorls ; in lla half
the shell is shown in horizontal section ; & is a vertical section ; a. aperture of shell ; 1 — 15
successive chambers, 1 being always the oldest or initial chamber. (After Carpenter, Brady ,
and Biitschli.)

Gromia (Fig. 36, 1) leads us to the more typical Foraminifera.
The protoplasm of this form protrudes from the mouth (a) of the
chitinoid shell (sh.) and flows around it so that the shell becomes
an internal structure. The pseudopods are very long and delicate

PHYLUM PROTOZOA

55

and unite to form a complicated network, exhibiting a streaming
movement of granules and serving, as usual, to capture prey.

Skeleton. — Squammulina (Fig. 36, 3) 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 ; and the extruded 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 repeti-

FIG. 38. — Hastigerina murrayi. phm. vacuolated protoplasm surrounding shell ; psd.
pseudopods ; sh. shell ; sp. spines. (After Brady.)

tion of the process gives us the many-chambered shell found
in most genera. New chambers may be added in a straight line
(Fig. 37, 3), or alternately on opposite sides of the original
chamber (5), 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
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 (IT). In all cases
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

56 ZOOLOGY SECT.

increase in the number of chambers there is a multiplication of
the nucleus (Fig. 36, 4, b, nu).

The shell presents two leading types of structure apart from
the form and arrangement of the chambers : either it is of a dense
porcelain-like texture and provided with a single terminal aperture
(imperforale, Fig. 36, 4), or the texture is more open and the whole
shell is perforated with very minute apertures, through which, as
well as through the terminal aperture, pseudopods are protruded
(perforate, Fig. 36; 2).

In many cases additional complexity is attained by the develop-
ment of an elaborate canal system in the more complicated perforate
forms, and, in certain cases, of what is called the supplemental skeleton
(Fig. 37, 8b, s. sJc.). This consists of a deposit of calcium carbonate
outside the original shell ; it is traversed by a complex system of
canals containing protoplasm and is sometimes produced into large
spines. Foraminifera in which this secondary skeleton occurs are
sometimes of considerable 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. 37, 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. A similar layer invests the surface in the
calcareous forms with perforate shells and gives off pseudopodia in
groups. In one of the calcareous forms with perforated spiral
shell, called Hastigerina (Fig. 38), a very remarkable modification of
this condition of things obtains. The shell (sh.) is surrounded with
a mass of protoplasm (plsm.) many times its own diameter, and so
full of vacuoles as to present a bubbly or 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 and microspheric) 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. 39).

The reproduction of Foraminifera is mainly by spore-formation,
with or without copulation. The protoplasm has been observed
in some to divide into minute masses which may be amoeboid or
may be of the nature of flagellulse — each provided with a

II

PHYLUM PROTOZOA

57

flagellum or a pair of flagella. In some cases the flagellulse have been
observed to copulate in pairs. The young may develop shells while

J-'K;. :J,l). — 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 flagellulae ; D, flagellula more
highly magnified ; E, microspheric individual developed from a flagellula ; F, microspheric
individual in the act of producing amoeboid embryos. 1, nucleiis ; 2, chromidia. (From
Lang, after Schaudinn.)

still within the shell of the parent or only after becoming free. In
the dimorphic Foraminifera there is evidence of the occurrence of an

58

ZOOLOGY

SEPT.

alternation of generations (p

FIG. 40. — Actinophrys sol. a. axial
filaments of pseudopods ; n. nucleus ;
p. pseudopod. (From Lang's Com-
parative Anatomy, after Grenacher.)

In the Atlantic, large areas of
gray mud called Globigerina
Globigerinae contained in it.

41) — the megaspheric form alter-
nating with the microspheric, and
the latter being developed as a
result of a process of copulation,
the former without it (alternation
of sexual and asexual generations).

Distribution. — Gromia, Micro-
gromia, and a few other forms are
found in fresh water : one species
has been found in damp earth, but
the great majority of the Fora-
minifera are marine, some being
pelagic, i.e. occurring at or near
the surface of the ocean, others
abyssal, i.e. living at great depths.

the sea-bottom are covered with a
-ooze from the vast number of

FIG. 41. — Actinpspliaerium eichhornii. A, the entire organism ; B, a small portion
highly magnified ; chr. chromatophore ; cort. cortex ; c. rac. contractile vacuole ; med.
medulla ; nu. nuclei. (From Butschli's Protozoa, after Hertwig and Lesser.)

From the palaeontological point of view, the Foraminifera are a
very important group. Eemains of their shells occur in various
formations from the Silurian period to the present day, certain

n PHYLUM PROTOZOA 59

rocks, such as the White Chalk (Cretaceous period) and the
Nummulitic limestone (Eocene), being largely made up of them.

ORDER 4. — 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,"
ActinopJirys sol (Fig. 40). The body is nearly spherical, and
contains a large nucleus and numerous vacuoles, one of which,
near the surface, is contractile. Each of the stiff radiating
pseudopods has a firm axis, apparently composed of protoplasm,
which is traceable through the general protoplasm as far as the
nucleus. Living organisms are devoured 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 definite cavity of the proto-
plasm, called a food-vacuole. If the organism be small, processes of
the protoplasm are developed, and surround and engulf it. If it
be larger, several pseudopods are applied to it, their axial fibres
becoming absorbed, and their substance envelops it, enclosing it in
a vacuole. The 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 water, and it possesses the
power of rising or sinking by some unknown means.

Actinosphcerium (Fig. 41, A), another fresh-water form, is more
complex. The protoplasm is distinctly divided into a central mass,
the medulla or endosarc (B, med.), in which the vacuoles are small,
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. The axial filaments of the pseudopods
are in relation each with one of the nuclei.

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.

There may be two nuclei of different kinds — one centrosome-like
body in the centre with the filaments of the pseudopods radiating
out from it, and the other a more superficially situated nucleus of
ordinary character : the former is derived from the latter.

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

60

ZOOLOGY

SECT.

(Fig. 42, 2, #.), 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.

c.vac

S.Nuclearia

3.Cla>hmlina

FlQ. 42. — Various forms of Heliozoa. 3a, the entire animal ; 3b, the flagellula ; c. rar.
contractile vacuole ; g. gelatinous investment; nu. nucleus; psd. pseudopods; ,vA\
siliceous skeleton ; sp. spicules. (From Biitschli's Protozoa, after Schulze and Greeff.)

One genus has a shell formed of agglutinated sand-grains ; in
another (Fig. 42, 1) the skeleton consists of loosely matted needles
of silica. Lastly, in the graceful Claihrulina (3) the body is

II

PHYLUM PROTOZOA

61

enclosed in a perforated sphere of silica, quite like the skeleton of
many of the Radiolaria (p. 62).

Reproduction ordinarily takes place by binary fission : a
peculiar form of budding has been observed, and spore-formation
also occurs, with or without encystation. Actinosphaerium, 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
throughout the winter, and in spring the protoplasm emerges
from each and assumes the form of the ordinary active Actino-
sphserium. In Clathrulina spore-formation takes place in the

A\

1-4-3

-\ 6

\7

Fia. 43. — Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A, two indi-
viduals in the first phase (^conjugation ; B, beginning of the encystation ; C, maturation;
Z>, 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 ; 4, 5, outer and inner layers of cyst ;
6, polar bodies ; 7, nucleus formed by the union of the two nuclei ; 8, first (mitotic)
division. (From Lang, after Schaudinn.)

active condition, and the spores (Fig. 42, 3 b] are flagellulse, each
being an ovoid body provided with two flagella.

Conjugation1 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
process of copulation, comprising the coalescence of the protoplasm
of the two individuals and the coalescence of the nuclei (Fig. 43)
after each has given off a part of its substance (6), as in the
maturation of an ovum in multicellular animals (p. 20). Autogamy
(p. 51) also occurs in both Actinophrys and Actinosphserium.

1 The term conjugation is here restricted to an association involving close
approximation without complete coalescence (copulation).

62

ZOOLOGY

SECT.

ORDER 5. — RADIOLARIA.

The Kadiolaria are a large and well-defined group of Khizopods,
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. 44, cent, caps.), which lies embedded in the
protoplasm, dividing it into intra-capsular (int. caps, pr.) and extra-
capsular (ext. caps, pr.) regions. In the intra-capsular protoplasm
is a large and complex nucleus (nu.), or sometimes many nuclei :
from the extra-capsular protoplasm the pseudopods (psd.) are given
off in the form of delicate radiating threads, which in some cases
remain free, in others, e.g. Lithe-circus, anastomose freely, i.e. unite
to form networks. In one large section — the Acantharia — the
pseudopodia, or some of them, 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-con-
tractile vacuoles, which give
it the frothy or bubbly ap-
pearance noticed previously
in Hastigerina. The vacuo-
lated portion of the proto-
plasm has a gelatinous
consistency, and is distin-
guished as the calymma. In
the Acanthometridae a
gelatinous sheath formed of
an extension of the calymma
invests the spicules of the
skeleton.

The central capsule may
be looked upon as a chiti-
noid internal skeleton, reminding us of the shell of Gromia and of
the perforated calcareous shell of Hastigerina with its investment
of vacuolated protoplasm. It is found in its simplest form in
Thalassoplancta (Fig. 45), in which it is spherical and uniformly
perforated with minute holes. In other forms, such as Lithocircus
(Fig. 44), 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. 46), 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 phcBodium (ph.), probably partly of the nature of

Inl. caps.fr
cent caps

caps.pr.

FIG. 44. — Lithocircus annularis. cent. caps.
central capsule ; ext. caps. pr. extra-capsular
protoplasm ; int. caps. pr. intra-capsular pro-
toplasm ; nu. nucleus ; psd. pseudopods ;
skel. skeleton ; z. cells of Zooxanthella. (After
Biitschli, from Parker's Biology.)

II

PHYLUM PROTOZOA

63

Ctf .__:

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 (the Acantharia) of a substance called acanlhin, composed of
strontium sulphate, so transparent that it can only b.e distinguished
from silica by chemical tests. The siliceous skeleton may consist
of loosely woven spines (Fig. 45), 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 frequently have inserted into them
a number of contractile filaments arising
from the gelatinous extra-capsular layer.
A very beautiful form of skeleton is exhi-
bited by Actinomma (Fig. 47), in which
there are three concentric perforated spheres
(A, sk. 1, sk. 2, sk. 3) connected by radia-
ting 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 pro-
duced— in Collozoum for instance (Fig. 48,
A, B, C) — 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
case^, and may be 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. 48, E, F) provided with
either one or two flagella. In some instances all the spores produced
are alike (E), and each encloses a small crystal (c.) : in other cases
(F) — in the same species — the spores are dimorphic, some being

FIG. 45. — Thalassoplancta
brevispicula, part of a
section, km. central cap-
sule ; ip. intra-capsular
protoplasm; n. nucleus,
containing nl. numerous
nucleoli ; of. oil drops ; ca .
calymma ; rp. protoplasm
surrounding calymma ; s.
spicules. (From Lang's
Comparative Anatomy , after
Haeckel.)

64

ZOOLOGY

SECT.

71-

FIG. 46. — Aulactinium actinastrum. c. calymma ; km. central capsule; n. nucleus
op. operculum ; ph. phaeodium. (From Lang's Comparative Anatomy, after Haeckel.)

cent, caps

rut,

FIG. 47. — Actinomma asteracanthion. A, the shell with portions of the two outer
spheres broken away ; S, section showing the relations of the skeleton to the animal ;
tent. caps, central capsule ; ex. caps. pr. extra-capsular protoplasm ; mi. nucleus ; sk. 1,
outer, sk. 2, middle, sk. 3, inner sphere of skeleton. (From Biitschli's Protozoa, after
Haeckel and Hertwig.)

PHYLUM PROTOZOA

65

small (microspores), others large (megaspores). Their development
has not been traced ; but in all probability the micro- and mega-
spores are gametes and copulate in pairs.

Symbiosis. — In most Kadiolaria there occur in the extra -
capsular protoplasm minute yellow cells (Fig. 44, z.), each enclosed
in a cell-wall, which multiply by fission independently of the
Radiolarian. It has been proved that these are unicellular organ-
isms, sometimes regarded as plants (Class Algse), sometimes as
animals (Class Mastigophora of the Protozoa), and named Zoo-
xanihellcB. This intimate association of two organisms is called
symbiosis : it is probably a mutually beneficial partnership, the
Radiolarian supplying the Zooxanthellse with carbon dioxide and
nitrogenous waste matters, while the Zooxanthellae 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.

} 48. — Collozoum inerme. A — C, three forms of the entire colony, nat. size ; D, a small
colony showing the numerous central capsules (c. caps.) and extra-capsular protoplasm with
vacuoles (vac.) ; E, spores containing crystals (c.) ; F, mega- and micro-spore. (From
Butschli's Protozoa, after Hertwig and Brandt.)

Thouglrbhe occurrence of symbiotic Algae is highly characteristic of the
Radiolaria, a similar association has been observed in various other Rhizopods,
notably in many Foraminifera. In the Radiolarian order Acantharia,
already referred to (p. 63), bodies long regarded as Zooxanthellse occur
mainly in the protoplasm of the central capsule ; but these have been shown
to be not symbiotes, but parts of the Radiolarian and devoid of cell-wall.

APPENDIX TO THE RHIZOPODA.

CHLAMYDOMYXA AND LABYRINTHULA.

Chlamydomyxa (Fig. 49), of which two species have been described, has been
found living on Bog-mosses (Sphagnum) in Ireland and in Germany and
Switzerland. It may occur either in the active or in the resting condition. In
VOL. I F

66

ZOOLOGY

SECT.

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 — the substance character-
istic of the cell-wall of the typical plant-cell (p. 17). In the protoplasm
are numerous non-nucleated protoplasmic bodies or chromatophores, con-
taining chlorophyll and a yellowish-brown colouring matter in varying
proportions. There are also a number of minute rounded bodies of a bluish
tint probably composed of reserve food-materials. In the young condition

a

FIG. 49.— Chlamydomyxa labyrinthuloides. A, active phase ; c.to. cell-wall ; /. frag-
ment of Alga ingested as food ; sp. spindles in course of pseudopods ; B, resting-stage —
numerous individuals in the cells of a fragment of Sphagnum ; a, specimen completely
enclosed in cell ; b and c, specimens which have emerged through the ruptured cell-wall -,
G, specimen multiplying by budding ; D, by binary fission ; E, by internal fission. E may
represent a stage in spore-formation. (A after Archer, B — E after Geddes.)

(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 finally burst through the wall of the moss-cell,
forming masses (6, c) quite visible to the naked eye. These may bud (C) or
undergo binary fission (D) ; 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).

II

PHYLUM PROTOZOA

67

During the whole of the resting stage there is nothing to distinguish
Chlamydomyxa from a plant, and it would certainly be placed among the
lower Algae 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 unite 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 con-
veyed 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

B

FIG. 50.— Labyrinthula vitellina. A, specimen crawling on a fragment of Alga (a.) ;
c. cells travelling in the filaments. £, part of specimen in resting condition with neap of
cells (c.) ; C, a single cell from an actively moving specimen with connecting threads ; nu.
nucleus. (From Butschli's Protozoa, after Cienkowsky.)

plant, the food 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
flagellulae, the further history of which has not been definitely traced, but
there is some evidence that copulation takes place between them.

Labyrinthula (Fig. 50), which lives parasitically on certain marine and
fresh- water Algse, in the resting stage (B) consists of a heap of small nucleated
cells (c.) connected by a homogeneous substance. In the active condition
(.4) 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-
domyxa. 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 (Groinia in particular) than to any of the other orders.

F 2

ZOOLOGY

SECT.

CLASS IL-MYCETOZOA.

1. EXAMPLE OF THE CLASS — Didymium difforme.

Didymium occurs as a whitish or yellow sheet of protoplasm (Fig. 51, G)
often several centimetres across, which crawls, like a gigantic Amoeba, over
the surface of decaying leaves. It shows the characteristic streaming move-

FIG. 51. — Didymium difforme. A. two sporangia (spg. 1 and 2) on a fragment of leaf (I.).

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

C, a flagellula with contractile vacuole (c. vac.) and nucleus (nu .). D, the same after loss
of flagellum ; ft, an ingested Bacillus. E, an amcebula. F, conjugation of amcebulse to
form a small plasmodium. O, a larger plasmodium accompanied by numerous amcebula) ;
sp. ingested spores. (After Lister.)

ments 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. Certain of these become differentiated as
sexual nuclei which throw off a portion of their substance and coalesce in pairs.
After leading an active existence for a longer or shorter time, the proto-
plasm 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. 51, A, spg. 1, spg. 2) is therefore not a mere resting capsule, like that

ii PHYLUM PROTOZOA t>9

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
protoplasm emerges in the form of an amoeboid mass which soon becomes a
flagellula (G), provided with a single flagellum, a nucleus (nu.), and a contractile
vacuole (c. vac.). The flagellulse move freely and ingest Bacilli (Z>, 6.), and
multiply by fission: then, after a time, they become irregular in outline,
draw in the flagellum, and become amoeboid (E). The amoebulse 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 amcebulae. Thus Didymium
in its active condition is a plasmodium, i.e. a body formed by the concrescence
of amcebulae.

2. GENERAL KEMABKS ON THE MYCETOZOA.

Generally considered, 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 resem-
blance 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 flagellulse, but soon become amoeboid and fuse to form the
plasmodium.

CLASS III.— MASTIGOPHORA

1. EXAMPLE OF THE CLASS — Euglena viridis.

Euglena (Fig. 52) is a flagellate organism commonly found in
the water of ponds and puddles, to which it imparts a green colour.
The body (A) is about (H mm. in length, is spindle-shaped, and has
at the blunt anterior end a depression, the gullet (A, B, gul.), from
the inner surface of which springs by two roots a single long flagellum
(fl.). 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 (C-F), but anything like the free pseudopodial move-
ments which characterise the Rhizopoda is precluded by the presence
of a very thin membrane or cuticle which invests the body. Oblique

70

ZOOLOGY

SECT.

and longitudinal lines in the outer layer of the protoplasm are due
to the presence ot elastic fibrils. There is a nucleus (nc, nu.) near the
centre of the body, and at the anterior end a contractile vacuole
(Ay c.v.), or more than one, leading into 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, chloro-
phyll, and contains
rod-shaped grains of
paramylum (A, p.),
a carbohydrate
allied to starch. In
contact with the
reservoir is a bright
red speck, the
stigma (st.), formed
of a pigment allied
to chlorophyll and
called hcemato-
chrome. It seems
probable that the
stigma is a light-
perceiving organ or
rudimentary eye.

Euglena is nour-
ished like a typical
green - plant : it
decomposes the
carbonlf d i'o x i d e
dissolved in the
fQ water, assimilating

the carbon and
evolving the oxy-
gen. Nitrogen and
other elements it
absorbs in the form
of mineral salts in
solution in the
water. But it has
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,

nu

FiG. 52. — Euglena viridis. A, view of entire organism,
showing details of structure (x about 1000); B, anterior
end, to show origin of flagellum, etc. ( x about 3000) ;
C — F, four views of the living organism, showing the
changes of form produced by the characteristic euglenoid
movements ; G, resting form after binary fission, showing
cyst or cell-wall, nuclei, and reservoirs of the daughter-
cells ; ch chromatophores ; c.v. contractile vacuole ; cy. cyst
or cell-wall; fl,. flagellum ;fl'. thickening on flagellum ; fl".
bifurcated base of flagellum; gul. gullet; nc., nu.. nucleus ;
ncl. "nucleolus"; p. paramylum bodies; r. reservoir ; st.
eye-spot or stigma. (From Parker's Practical Zoology: —
A, from Doflein ; B, from Dofleln. after Wager ; C—G, after
Saville Kent.)

n PHYLUM PROTOZOA 71

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 becomes quies-
cent, numbers of them coming together and secreting a gelatinous
scum, in which they lie embedded, on the surface of the water.
Each animal surrounds itself with a cyst or cell-wall of cellulose (6r),
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 GENEKAL 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 1. — FLAGELLATA.

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

ORDER 2. — CHOANOPLAGELLATA.

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.

Systematic Position of the Example.

Eugelena viridis is one of several species of the genus Euglena ,
and belongs to the family Euglenidce, sub-order Euglenoidea, and
order Flagellata.

The presence of an anterior flagellum and the absence of a
collar, transverse flagellum, or tentacle, indicate its position among

72 ZOOLOGY SECT.

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 chromatophores, red stigma,
and euglenoid movements. E. viridis is separated from other
species of the genus by its spindle-shaped body with blunt anterior
and pointed posterior end, and by the flagellum being somewhat
longer than the body.

ORDER 1. — FLAGELLATA.

In the Flagellata the cell-body is usually ovoid or flask-shaped
(Fig. 53, 6, 7, 9, &c.), but may be almost globular (1), or greatly
elongated (3). Anterior and posterior ends are always distinguish-
able, 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 rightfand 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 Mastigamceba (4) has 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, Dimorpha (<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 (6).

Nuclei of the ordinary character are universally present. In
addition there may be present in the cytoplasm at the base of the
flagellum or of each flagellum a very much more minute body which
is termed the blepharoplast (Fig. 54, 6), and in close relation to
the latter is a secondary nucleus or Jcinetonucleus (Jc.n).

The number of flagella is subject to great variation. There
may be one (Fig. 53, 1-3), two (9, 10), three (6), or four (7).
Sometimes the flagella show a differentiation in function ; in
many cases in which two flagella are present one only is used in
progression, ^the second or ventral flagellum is trailed behind when
the animal is swimming freely, or is used to anchor it to various
solid bodies. In some (Trypanosomes, Fig. 54) the flagellum (or
one of them, if two are present) is attached throughout 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

II

PHYLUM PROTOZOA

73

varied modes of nutrition. Many of the lower forms, such as
Heteromita, live in decomposing animal infusions : they have

3.Asrosiopsi
2.0ikomonas(?)

II.Dinobryon 12. Sy ncry {jfr, o 13. A^ho|>hysa H.Rhi|>iclodendron

FIG. 53. — Various forms of Tlagellata. 'In 2, flagellato (a) and amoeboid (ft) phases are
shown ; in 5, flagellate (a) and heliozoan (b) phases ; in 8 are shown two stages in the
ingestion of a food-particle (/.) ; chr. chromatophores ; e. vac. contractile vacuole ; /. food
particle : g. gullet ; nu. nucleus ; 1. lorica ; p. protoplasm ; per. peristome ; v.i. vacuole
of ingestion. (Mostly from Biitschli's Protozoa, after various authors.)

neither mouth nor gullet and take no solid food, but live by
absorbing the nutrient matters in the solution ; their nutrition is,

ZOOLOGY

SECT.

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
Hcemqflagellata (Binuckata), an extensive group, live for the most
part as parasites in the plasma of the blood of various vertebrates.
The best known of the Hsemoflagellata are the Trypanosomes (Fig.
54). These are long, narrow, flattened Mastigophora, usually pointed
at one or both ends, and often spirally twisted. They are provided

either with one flagellum or with two : when
two are present, one is free throughout and
directed forwards, the other runs backwards
and is attached along the edge of a wavy
protoplasmic flange or undulating mem-
brane (u.) ; when one is present it may be
either the free or the attached one. Each
flagellum has at its base a minute centro-
some-like granule, the blepfaroplast (6),
and this is connected by a slender thread
with a small body of nuclear character,
the kinetonuckus (k.n.), which is con-
nected by a thread with the primary
nucleus (n.) situated towards the middle
of the body. Sometimes the blepharo-
plast is actually situated within the
kinetonucleus and the flagellum appears
to arise directly from the latter. Most of
these appear to be harmless, but some are
the causes of serious diseases in Man
(" sleeping sickness ") and other higher
animals (tsetse-fly disease in cattle).

It- -H

--n

Mention may be made here of the Spiro-
chcetes, since a connection between them and
the Trypanosomes has been supposed to exist.
The true Spirochsetes (genus Spirochceta), as
distinguished from the smaller Trypanosomes
on the one hand, and the Spirilla (belonging to
the Bacteria) on the other — with both of which
they have been confounded — are non-flagellate,
wavy, flexible filaments with an undulating
membrane, which multiply by transverse binary
fission, and also, apparently, by the breaking up
of the filament into a number of extremely small rounded bodies capable of
developing into the adult form. Their affinities appear to be rather with the
Cyanophycece (Oscillatoria) among the lower plants than with the Protozoa.

HcBmatococcus (Fig. 55), Pandorina (Fig. 56), Volvox (Fig. 58),
and their allies differ somewhat widely from the other Flagellata,
and are sometimes regarded as constituting a distinct order

f-- .n.

FIG. 54. — A Trypanosome
with one flagellum. b, ble-
pharoplast ; k.n. kineto-
nucleus ; n. primary nu-
cleus; u. undulating mem-
brane. (After Minchin,
slightly altered.)

PHYLUM PROTOZOA

75

(Pliytoflagellata). The mouthless body is surrounded by a cellulose
cell-wall (c.w.), and contains chromatophores (chr.) coloured either
green by chlorophyll 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,
therefore, not surprising that these chlorophyll-containing Flagellata
are often included among the Algae 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. 53, 8)
we have one of the simplest arrangements : near the base of the
flagellum is a slight
projection contain-
ing a vacuole (v.i.) ;
the movements of
the flagellum drive
small particles (/.)
against this region,
where the proto-
plasm is very thin
and readily allows
the particles to
penetrate into the
vacuole, where they
are digested. 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 pro-
portion of genera
are naked or covered
only by a thin cuticle, a few fabricate for themselves a delicate
chitinoid shell, or lorica (10, I.), usually vase-shaped and widely
open at one end so as to allow of the protrusion of the con-
tained animalcule. In most of the chlorophyll-containing forms
there is a closed cell- wall of cellulose (Fig. 55, c.w.). One group of
marine Flagellates (Silicoflagellata) have siliceous skeletons similar
to those of the Radiolaria, 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

c.vat

FIG. 55. — Haematococcus pluvialis. A, motile stage ;
B, resting stage ; C, D, two modes of fission ; E, Hcemato-
coccus lacustris, motile stage ; F, diagram of movements of
flagellum ; chr. chromatophores ; c. vac. contractile
vacuole ; c.w. cell-wall ; nu. nucleus ; nu'. nucleolus ;
pyr. pyrenoids. (From Parker's Biology.)

76

ZOOLOGY

SECT.

(see Sect. IV.) in general form (Fig. 53, 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. 58) the zooids of the colony are arranged in the form
of a hollow sphere, and in Pandorina (Fig. 56) in that of a solid
sphere enclosed in a delicate shell of cellulose. Lastly, in Rhipido-
dendron (Fig. 53, 14) a beautiful branched fan-shaped colony is

FIG. 56.— -Pandorina morum. A, entire colony ; B, asexual reproduction, each zooid
dividing into a daughter-colony ; C, liberation of gametes ; D — F. three stages in copula-
tion of gametes ; O, zygote ; H— K, development of zygote into a new colony. (From
Parker's Biology, after Goebel.)

produced, the branches consisting of closely adpressed gelatinous
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. 55) and Euglena (Fig. 52), for instance,
divide while in the encysted condition ; Heteromita and other
saprophytic forms while actively swimming : in the latter case the
division includes the almost infinitely fine flagellum.

In correspondence with their compound nature, the colonial

PHYLUM PROTOZOA

77

genera exhibit certain peculiarities in asexual multiplication. In
Dinobryon (Fig. 53, 11) a zooid divides within its cup, in which
one of the two products of division remains ; the other crawls out

FIG. 57. — Copromonas subtilis. a. adult ; b, c, d, stages in binary fission ; e,f>.g, h, i, j,
stages in copulation with reduction (/, g) of the nuclei of the copulants, followed by a
return to a either directly (i) or through an encysted condition (&). (After Dobell.)

of the lorica, fixes itself upon its edge, and then secretes a new
lorica for itself. In Pandorina (Fig. 56) each of the sixteen zooids
of the colony divides into sixteen (B), thus forming that number of

78

ZOOLOGY

SECT.

daughter-colonies within the original cell- wall, by the rupture of
which they are finally liberated. In Volvox (Fig. 58), certain zooids,
called partJienogonidia (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
(Dl-D5), 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 a number of cases copulation has been
found to occur between ordinary individuals without any special
differentiation of gametes. The union of the nuclei (karyogamy)

H

Fid. 58.— Volvox glob a tor.

. A, entire colony, enclosing several daughter-colonies;

, the same during sexual maturity ; C, four zooids in optical section : D* — D5, develop-
ment of parthenogonidium ; E, ripe spermary ; F, sperm ; G, ovary containing ovum and
sperms ; H, oosperm ; a, parthenogonidia ; fl. flagellum ; ov. ovum ; ovy. ovaries • pa
pigment spot ; spy. spermaries. (From Parker's Biology, after Cohn and Kirchner.)

is always preceded in such cases by a reduction of their substance,
a process recalling the maturation of the ovum in higher animals
(p. 20). Copromonas (Fig. 57), which occurs in the faeces of frogs,
affords an example of this kind of copulation. Multiplication takes
place by binary longitudinal fission (a-d). Copulation also takes
place with reduction and karyogamy (e-f). This is not known to
recede any special form of multiplication, but the zygote or its
escendants by binary fission may pass into an encysted condition
(k) in which it is able to survive desiccation.

p
d

PHYLUM PROTOZOA

79

In Pandorina (Fig. 56) the cells of the colony escape from the
common gelatinous envelope (C) and copulate in pairs (D, E),
forming a zygote (F, G), which, after a period of rest (#), divides
and forms a new colony (K). In some cases the copulating cells
are of two sizes, union always taking place between a large cell or
megagamete and a small cell or microgamete. In Volvox (Fig. 58)
this dimorphism reaches its extreme, producing a condition of things
closely resembling what 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 fuse

n

IMonosiga.

2.Salpingoe ca.

S.Polyoeca. 4.Proferospongia.

FIG. 59. — Various forms of Choanoflagellata . 2b illustrates longitudinal fission ; 2c, the pro-
duction of flagellulse ; c. collar ; c. vac. contractile vacuole ; fl. flagellum ; 1. lorica ; nu.
nucleus ; s. stalk. (After Saville Kent.)

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.

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 collar (Fig. 59, 1, c.), surrounding

80 ZOOLOGY SECT.

the base of the single flagellum (Jl.). The collar is contractile, and,
although its precise functions are not yet certainly known, there is
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
PolyoBca (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 nagella
which they always possess, and the usual presence of a remarkable and often
very beautiful and complex shell.

The body (Fig. 60, 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 groom (I. gr.), 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.l), which is directed forwards and serves as the chief organ of propulsion ;
a second flagellum (Jl. 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, sometimes silicified,
and often of very complex form, being produced into long and ornamental
processes, and marked with stripes, dots, &c. Besides a nucleus, a contractile
vacuole and often an eyespot, the protoplasm contains chromatophores
(1, chr.) coloured with a yellowish or brownish pigment. Nutrition is holo-
phytic or holozoic.

The foregoing description applies to all the commoner genera. Proro-
centrum (3) is remarkable for the absence of the transverse groove, while
Polykrikos (4) has four to eight transverse grooves and no shell. The latter

II

PHYLUM PROTOZOA

81

genus also has stinging -capsules or nematocysts (a, b) in the protoplasm,
resembling those of Zoophytes (see Section IV.), and has numerous nuclei of
two sizes, distinguished as meganuclci (nu.) and micronuclei (nu'.). In the
Adinidce the characteristic groaves are absent.

1. Cleno dinium

2.CeraHur> 3-Prorocenrrum

4.Polykrikos

FIG. 60.— Various forms of Dinoflagellata. 2 shows the shell only ; 4a is an undischarged,
and b a discharged stinging-capsule ; chr. chromatophores ; fl. 1, longitudinal flagellum ;
fl. 2, transverse flagellum ; 1. gr. longitudinal groove ; ntc. nematocyst ; nu. meganucleus ;
nu'. micronucleus ; pg. pigment spot ; t. gr. transverse groove. (From Butschli's Protozoa.)

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.

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

ORDER 4. — CYSTOFLAGELLATA.

This group includes only two genera, Noctiluca and Leptodiscus. A
description of Noctiluca miliaris, the organism to which the diffused phos-
phorescence of the sea is largely
due, will serve to give a fair
notion of the leading character-
istics of the order.

Noctiluca (Fig. 61) 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.), noticeable for
its transverse striation. Near
the base of this flagellum is the
mouth (m.), leading into a short
gullet in which is a second
flagellum (/.), 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.

VOL. i a

Fia. 61.— Noctimca miliaris. a. the adult
animal ; b, c. flagellulse ; bg. tentacle ; /. flagel-
lum ; m. mouth ; n. nucleus. (From Lang.)

82

ZOOLOGY

SECT.

Reproduction takes place by binary fission, the nucleus dividing indirectly.
Spore-formation also occurs, sometimes preceded by conjugation, sometimes
not. The spores (ft, c), formed by the breaking up of the protoplasm of the
parent, escape as flagellulse.

CLASS IV.— SPOROZOA,

1. EXAMPLE or THE CLASS — Monocystis agilis.

One of the most readily procured Sporozoa is the microscopic
worm-like Monocystis agilis (Fig. 62, A), which is commonly found
leading a parasitic life in the vesiculse seminales of the common

H

-Flo. 62. — Monocystis. A, Trophozoites in different stages of contraction. B, encysted
gametocytes. (7, division of gametocytes into gametes. D, conjugation of gametes to form
zygotes. E, Cyst enclosing ripe spores formed from the zygotes. F, single spore, showing
the (8) sporozoites in its interior. O, group of developing sperm-cells of the earthworm,
enclosing a sporozoite in the centre. H, young trophozoites still surrounded with the tails
of the degenerated sperms, nu, nuclei. (From Parker's Practical Zoology.)

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
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. The innermost layer
of the cortex consists of contractile elements or myonemes which act
like the muscular fibres of higher animals. There is a large clear
nucleus (nu.) with a distinct nucleolus and nuclear membrane, but
the other organs of the protozoan cell-body are absent : there is

ii PHYLUM PROTOZOA 83

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), but
remain separate. The nucleus of each, after undergoing reduction-
changes, divides repeatedly, until a large number of nuclei are
formed (C). Each of the nuclei becomes surrounded by a thin
layer of protoplasm. The minute cells thus formed, after
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 zygotes 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, the falciform young or sporozoites,
which are arranged within the spore-coat somewhat like a bundle
of sausages. In all probability the spores pass through the digestive
system of a bird, pass out in its faeces, and only undergo further
development if taken into the intestine of an earthworm, when the
spore-coat becomes' dissolved or ruptured and the sporozoites are
set free. From the intestine they are able to migrate freely, and
pass to the ciliated funnels of the male reproductive system, entering
the cells of the ciliated funnels, in which they are said to live for a time
as intracellular parasites, and, after a time escape into the cavity of
the vesicula and become lodged in the centre of one of the spherical
bodies known as sperm-morulce, each made up of a protoplasmic core
with an investment of developing sperms (G). Ultimately they
become free as trophozoites surrounded for a time by degenerating
sperms (H).

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.

o2

84

ZOOLOGY

SECT.

ORDER 3. — H^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 amoeboid, but not intra-
cellular.

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

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

B

FIQ. 63.— Oregarina. A, two specimens of G. blattarum partly embedded in enteric
epithelial cells of Cockroach ; J51, J52, two specimens of G. dujardini ; in Bz the epimerite
(ep.) is cast off ; C, cyst of G. blattarum, from which most of the spores have been discharged;
Z), four stages in the development of G. gigantea. cy.cyst; deu. deutomerite ; ep. epimerite ;
g. gelatinous investment of cyst ; nu. nucleus ; pr. protomerite ; psd. 1, short pseudopod ;
psd. 2, long pseudopod ; sp. mass of spores ; spd. sporoducts. (From Biitschli's Protozoa.)

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.

Monocystis is one of the simpler representatives of the Gregarinida
as regards both structure and life-history. The structure of the
adult or trophozoite is complicated in Gregarina (Figs. 63 and 64)

II

PHYLUM PROTOZOA

85

and allied genera by the division of the body into two parts,
protomerite in front and deutomerite behind, by a sort of transverse
septum formed by an ingrowth of the layer of myonemes — the
nucleus being usually situated in the deutomerite. Sometimes the
protomerite is produced in front into a process ending in a rounded
enlargement, the epimerite, which may be provided with radiating
spine-like projections (Fig. 64). When it escapes from the spore
the sporozoite in nearly all cases (Fig. 64, 7) enters one of the cells
of the epithelium of the alimentary canal of some member of the
higher animal phyla, and lives and grows there for a time as an
intracellular parasite (8). As it grows it comes to project into the

3

-—n

FIG. 64. — Gregarina. Development from the sporozoite. 1, cells of the digestive epithelium
of the host ; 2, nuclei of the same ; 3, spore ; 4, spore discharging sporozoites (5) leaving
residual mass (6) ; 7, sporozoites in the act of entering epithelial cells ; 8, the same as
intracellular parasites ; 9-12, different stages in the growth of the young Gregarines into the
lumen of the intestine ; 13, epimerite ; 14, protomerite ; 15, deutomerite. (After Lang.)

cavity of the canal (9, 10, 11, 12), and may eventually become entirely
free therein or in the body-cavity, or remain attached to the cell by
the epimerite. The formation of spores in Gregarina and its allies,
as in Monocystis, is preceded by the close apposition to one another
of two trophozoites and their enclosure in a common cyst. Such
conjugating trophozoites in Gregarina, as also in Monocystis, are
not of the nature of gametes : they are cells in which gametes are
produced and are appropriately named gametocytes. The cells
that result from the division of the nuclei and protoplasm of the
two associated gametocytes are the true gametes : they copulate.
i.e. completely coalesce, in pairs, each pair developing into a spore.
The two gametes the copulation of which leads to the formation of
a spore may be entirely alike ; but in some at least of the Gregarinida

86

ZOOLOGY

SEOT.

it is possible to distinguish between two sets of gametes which may
be looked upon as male and female, copulation taking place between
members of the two sets.

The cysts of Gregarina (Fig. 63, C) are often very complex and
provided with delicate ducts (spd.) in the thickness of the wall,
through which the spores escape. In Gregarina (Porospora)
gigantea of the Lobster, the young (sporozoite) is liberated from the
spore in the form of a non-nucleated amcebula (Dl), with one long
and one short pseudopod (D2) ; this divides by the long pseudopod
(psd. 2) becoming separated off, and each product of fission, develop-
ing a nucleus, passes into the adult (trophozoite) form (IP, D4).
Such a multiplication by fission (schizogony) is repeated in such
cases through several generations till eventually a generation is
formed in which reproduction takes place by spore-formation ac-
companied by a sexual process. In other cases the sporozoites do
not divide, but each develops directly into the trophozoite (Fig. 64).

ORDER 2. — COCCIDIIDEA.

Coccidium (Figs. 65, 66) and allied genera are parasites in the interior of
cells, both in Vertebrates and Invertebrates. They live in the cells of various

organs, most fre-
quently in those
of the epithelium
of the digestive
canal. They
never inhabit
blood-corpuscles.
A few are intra-
nuclear parasites.
Two distinct
modes of multi-
plication occur —
by schizogony, a
kind of multiple
fission, and by
sporogony, a pro-
cess of spore -
formation pre-
ceded by copu-
lation between
male and female
cells. The tropho-
zoite, 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. 66, b-c) takes place. The nucleus divides to form a
number of nuclei. These migrate towards the surface, and each becomes
surrounded by protoplasm, with 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

1 Eimeria

2. Coccidium

Fro. 65. — Coceidiidea. A, adult Eimeria (E) in enteric epithelial
cell (ep.) of mouse ; B, encysted form ; C, encysted form, the
protoplasm contracting to form a spore ; D, formation of falci-
form young ( /.) in interior of spore (sp.) ; B, spore with falciform
young ; F , adult encysted form of Coccidium from liver of rabbit ;
G, division into spores ; H, cyst containing ripe spores (sp.), each
with a single falciform young ; I, single spore with falciform
young (/.). (From Bfltschli's Protozoa, after Leuckart and
Eimer.)

IT

PHYLUM PROTOZOA

87

which they were derived. This multiplication may take place on such an
extensive scale that the 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

Fio. 66. — Life-history of Coccidium schubergi. a. penetration of epithelium cell of host
by sporozoite ; b-c, stages of multiple fission (schizogony) ; d, gametocyte ; e,f, formation of
megagamete (ovum) ; g, fertilisation ; h, j, formation of microgametes (sperms) ; It,
development of fertilised ovum into four spores ; I, formation of two sporozoites (falciform
young) to each spore. (From Calkins, after Schaudinn.)

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 (d), and become

88

ZOOLOGY

SECT.

converted into either micro- or mega-gametocytes. Each of the former (h, j)
gives rise by division to a number of narrow biflagellate inicrogametes 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
(zygote or oosperm) divides to form a varying number of cells each enclosed in

FIG. 67. — Life-history of Malaria Parasites. A-G, parasite of quartan fever, showing
development of trophozoite in a blood-corpuscle and the formation of merozoites ; H,
gametocyte of the same ; I-M, parasite of tertian fever to the formation of the merozoites ;
N, gametocyte ; O-T, crescentic 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 ;
/, gr, development of the contents into a mass of sporozoites ; h, sporozoites passing into
the salivary glands. (From Calkin's Protozoa, after Ross and Fielding Quid.)

a resistant cyst (fc). These give rise to spores with a firm, chitinous spore-
membrane, each containing two or more falciform young or sporozoites (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

89

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

ORDER 3. — H^MOSPORIDEA.

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
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. 67, A-0) they live as
amoeboid 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 estab-
lished 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 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, 8) and each
female gametocyte forms a single megagamete (ovum). After maturation
(U—W) the megagamete is fertilised (x) by one of the actively-moving micro-
gametes, the result being the formation of an active spindle-shaped ookinete
or vermicule. This perforates the stomach-wall and comes to rest in the
subjacent tissues. It then becomes encysted and increases greatly in size,
bulging out into the body-cavity (b—e). The contents of the cyst eventually
divide up (/, 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. Pene-
trating into the interior
of coloured corpuscles
they reach the tropho-
zoite condition.

The Haemogregarines,
which may most con-
veniently be referred to
here, are Sporozoa which
live in the coloured
blood-corpuscles of all FTO 68 _4> myxidium lieberkiihnii, amoeboid phase ;
classes of Vertebrates, B, Myxobolus miilleri, spore with discharged nemato-

ntc

B

p
b

.

, and Reptiles ;

ut which, unlike the malaria parasites, in the mature or trophozoite con-
dition are not amoeboid, retaining the Gregarina-like form. Transference from
one host to another is effested by means of various blood-sucking Invertebrates.

90 ZOOLOGY SECT.

ORDER 4. — MYXOSPORIDEA.

This group includes a small number of genera which are amoeboid in the
trophozoite phase, and which reproduce continuously by spore-formation
during that phase (Fig. 68, A). Many nuclei are present 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.

ORDER 5. — SARCOCYSTIDEA.

The best-known form of this order is Sarcocystis (Fig. 69), which occurs in
the flesh of mammals, each parasite having the form of a long spindle embedded

Fia. 69. — Sarcocystis miescheri, adult form (s) in striped muscle of pig. (From
Biitschli's Protozoa, after Rainey.)

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

CLASS V.— INFUSORIA.

1. EXAMPLE OF THE CLASS — Paramoecium caudatum.

Structure. — Paramoecium, the " slipper-animalcule," is tolerably
common in stagnant ponds, organic infusions, &c. The body
(Fig. 70) is about ] mm. in length, somewhat cylindrical, but
flattened, with distinct upper and lower or dorsal and ventral
surfaces, and anterior and posterior ends, the latter rather more
pointed than the former. On the ventral face is a large oblique
depression, the buccal groove (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. Each
of the cilia is connected at its base with a very small basal granule
(rendered visible only by special staining of fixed specimens) situated
below the pellicle.

In the cortex are found two nuclei, the relations of which are
very characteristic. One, distinguished as the meganucleus (nu.),

IT

PHYLUM PROTOZOA

91

Tntc.e/7:

v.vac.

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 mega-
nucleus ; when
it divide's it
goes through
the complex
series of stages
characteristic of
mitosis (p. 18).
The contrac-
tile vacuoles (c.

-"> "&• ~ l- .^— S* 'V/TV+'I

vac.) are two in
number, and
are very readily
made out.
Each is con-
nected with a
series of radia-
ting spindle-
shaped cavities
in the proto-
plasm which
serve as feeders
to it. After
the contraction
of the vacuole
these cavities
are seen gradu-
ally to fill,
apparently re-
ceiving water
from the sur-
rounding proto-
plasm : they
then contract,
discharging the
water into the
vacuole, the
latter rapidly
enlarging while
they disappear
from view;

finally the vacuole contracts and discharges its contents externally.
The cortex contains minute radially arranged sacs called

nu.

FIG. 70. — Paramoecium caudatum. A, the living animal from
the ventral aspect ; B, the same in optical section : the arrow
shows the course taken by food-particles ; C, a specimen which has
discharged its trichocysts ; D, diagram of binary fission ; buc. gr,
buccaJ groove ; cort. cortex ; CM. cuticle ; c. vac. contractile
vacuole : /. vac. food vacuole ; gul. gullet ; med. medulla • nu.
meganucleus ; pa. nu. micronucleus ; trek, trichocysts. (From
Parker's Biology.)

92

ZOOLOGY

SECT.

trichocysts (trch.). When the animal is irritated, more or fewer of
these suddenly discharge a long delicate thread. In a specimen
killed with iodine or osmic acid the threads can frequently be
seen projecting in all directions from the surface (C).

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

Mg.rm

Fia. 71. — Faramoecium caudatuxn stages in conjugation. guL gullet ; mg. nu. mega-
nucleus ; mi. nu. micromicleus ; Mg. nu. reconstructed meganucleus ; Mi. nu. recon-
structed micronucleus. (From Parker's Biology, after Hertwig.)

mouth, where the cortex and cuticle are less resistant 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 tak§s place by transverse
binary fission (D), the division of the body being preceded by that
of both nuclei. As already mentioned, the meganucleus divides
directly, the micronucleus indirectly. Fission is never associated
with ency station as it is in Euglena.

Under certain conditions multiplication by fission is interrupted
by conjugation. In this very remarkable and characteristic process
two Paramoecia become applied by their ventral Surfaces (Fig. 71, A),

IT

PHYLUM PROTOZOA 93

but do not fuse. The meganucleus (mg. nu.) of each breaks up into
small masses, which eventually 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 conjugating body is provided with four
micronuclei (B). Three of these (mi. nu'., mi. nu" .) disappear ; the
fourth divides again into two, of which 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 conjugating cells (E). The animalcules then separate
from one another, and the nucleus of each divides and gives rise to
the permanent mega- and micro-nuclei (G, Mg. nu., Mi. nu.), the
original nuclear condition becoming completely established only
after the two animals have separated and have twice undergone
binary fission.

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 caudatum is one of several species of the genus
Paramoecium, belonging to the family Paramoecidce, 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 J — i 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.

94 ZOOLOGY SECT.

The form of the body is very varied : it may be ovoid (Fig.
72, 7), kidney-shaped (2), trumpet-shaped (3), 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. 74 and 75, 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 Paramoecium is an example, the cilia are all small, equal-
sized or nearly so, and arranged in longitudinal rows (Fig. 70, Fig.
72, 1). The second or heterotrichous type is seen in its simplest
form in Nyctotherus (Fig. 72, 2), in which the left side of the
peristome is bordered by a row of specially large adoral 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. 74) 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 Epistylis plicatilis (Fig. 72, 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,
while on the ventral surface the cilia take the form of large
hooks, fans, bristles, and plates with fringed ends (Fig. 72,
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

ii PHYLUM PROTOZOA 95

peristome. They contract so as to produce a wave-like movement
which aids in the ingestion of food. In some cases (Fig. 72, 11)
the undulating membrane (u. mb.) is a very large and obvious
structure.

Certain peculiar forms have yet to be mentioned. Multicilia (Fig.
72, 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 LopJiomonas (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, mg. 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 division,
forming at last a very great number of small bodies (chromidia) 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 this phenomenon has tended to throw doubt on the
reported total absence of a nucleus in some Ehizopods.

In nearly all species one or more micronuclei are present, the
number sometimes reaching nearly thirty. In Opalina (Fig. 76)
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. 74, 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. 72, 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 Badiolaria.

Trichocysts, like those of Paramoecium, are found in many
holotrichous forms, but are rarely present in the other subdivisions
of the order. In the peritrichous Epistylis umbellaria, however,
there are found numerous minute capsules (Fig. 72, 9, ntc.)
arranged in pairs, each containing a coiled thread. They are
obviously structures of the same character as trichocysts, and

l.Prorodon 2.Nlycrorherus

l2.Mulhcilia 13.Lophomonas

18-Trachelius IQ.Ophryoglena

" 16.Condvlosl-oma
IS.Didinium IZOpalinopsis

Fia. 72. — Various forms of Ciliata. 9a shows part of a colony, b a single zooid, and
c a couple of nematocysts ; a. anus ; c. (in 18) cuticle ; c. (in 19) excretory canals ; c, we.
contractile vacuole ; /. vac. food vacuole ; g. gullet ; mg. nu. meganucleus ; mi. nu. micro-

nimlaiia • *»/% mrm+h • nti niinlAiia • «//. npmnfrv^.'ct. • -n /in 1 r;\ a T>tirnvnn>ri.U'm Seized UV

SEOT. II

PHYLUM PROTOZOA

97

their resemblance to the nematocysts so characteristic of Coelenterata
(vide Section IV.) is singularly close.

Digestive Apparatus. — Many parasitic forms (Fig. 72, 8, 17 ;
Fig. 76) 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.
72, 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.
From them there is a fairly complete gradation to genera, like
Paramcecium, having the permanently open mouth on the left side

l.Dictyocysl-a

3.Thuricola ^.Ophrydium 5. S M chotricha
2.Pyxicola

Fra. 73. — Various forms of Ciliata. In 1 the shell alone is shown ; m. contractile fibre ; op.
operculum. (From Biitschli's Protozoa, after various authors.)

of the ventral surface, at the end of a well-marked buccal groove
or peristome. Vorticella (Fig. 74) 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. 72, 2) and some other genera there is,
instead of the temporary anal spot described in Paramcecium, a
distinct anal aperture (a.).

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
VOL. I H

ZOOLOGY

SECT.

similar structure found in so many of the Mastigophora. Some
(Fig. 72, 4) have bell-like shells, variously ornamented, and in
others (Fig. 73, 1) the similarly shaped shell is perforated and
resembles the skeleton of some of the Kadiolaria. A chitinoid
plate or operculum (Fig. 73, 2, op.) may be fixed to the edge of the
peristome, and, when the animal is retracted in its case, accurately

[JC

pen

FIG. 74. — Vorticella. A, B, living specimens in different positions ; (7, optical section ; D1,
Dz, diagrams illustrating coiling of stalk ; E1-, E%, two stages in binary fission • E'-', free
zooid ; FI, F2, division into mega- and microzooids ; G1, <?2, conjugation ; fl1, multiple
fission of encysted form ; fl2, Ha, development of spores ; ax. f. axial fibre ; corf, cortex ;
cu. cuticle ; c. vac. contractile vacuole : d. disc ; gull, gullet ; m. microzooid ; mth. mouth ;
nu. meganucleus ; per. peristome. (From Parker's Biology.)

closes the mouth of the latter, or a similar operculum (3) is
attached to the interior of the tube, and is closed by a contractile
thread of protoplasm (m.), 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. 72, 9a ;
Fig. 75). The stem of these may be a purely cuticular structure

PHYLUM PROTOZOA

99

and non-contractile (Fig. 72, 9, b), or may contain an axial
fibre or muscle, like that of Vorticella (Fig. 74, ax.f.). In Ophrydium
(Fig. 73, 4) the colony is an irregular mass, sometimes 4r-5 in. in
diameter, consisting of a gelatinous substance in which a delicate,
branching stem is embedded, each branch terminating in a zooid.
Some genera (Fig. 73, 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
hours in different species. In Vorticella (Fig. 74, E) and other
Peritricha the plane of division is parallel to the long axis of the
bell-shaped body. In such simple Peritricha as Vorticella division
proceeds until two zooids are produced on a single stalk ; one of
the two then

acquires a /-r\ A

second cir-
clet of cilia
near its prox-
imal end, be-
comes d e-
tached (1£3),
and, after
leading a
free - swim-
ming life for
a time, settles
down and
develops a
stalk : in this
way the dis-
persal of the
non- locomo-
tive species
is ensured.

In many species of Zoothamnium (Fig. 75) the zooids 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 com-
plexity of the colony, not in the development of a new one. The
larger zooids (r.z.) 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.

Spore-formation takes 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, gives rise
to a spore. A somewhat similar process has been described in
Vorticella (Fig. 74, H) and others.

H 2

. 75.— Zoothamnium arbuscula. A, entire colony ; B, the same,
natural size ; (7, the same, retracted ; D, nutritive zooid ; E^ repro-
ductive zooid ; F1, F2, development of reproductive zooid ; ax. f.
axial fibre ; c. vac. contractile vacuole ; nu. nucleus ; n.z. nutritive
zooid ; r.z. reproductive zooid. (From Parker's Biology, after Saville
Kent.)

100

ZOOLOGY

SECT.

71 ll

A peculiar kind of spore-formation, specially adapted to the
requirements of an internal parasite, takes place in Opalina
(Fig. 76), 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
(6r), each with only three to six 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 tad-
poles or Frog-
larvae feed upon
the water-
plants, and in
doing so fre-
quently take in
the spores or
encysted Opa-
linae along with
their food.
When this oc-
curs the cyst is
dissolved by
the digestive
juices of the
host, and the
protoplasm of
the spore is set
free and be-
comes divided
up into club-
shaped bodies,
each with a
single nucleus
(I). These are
gametes, which
copulate in pairs, the zygotes growing into adult Opalinse (K).

Conjugation, in the form of a temporary union accompanied by
interchange of micronuclei, has been described in Paramcecium
(p. 92), and takes place in many Ciliata. In others (e.g. Stylonychia
histrio) there is a complete union (copulation) 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 a number together from one of the fixed

FIG. TB.y-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 ; //, encysted form ; 7,
young form liberated from cyst ; K, the same after multiplication
of the nucleus has begun ; nu. nucleus. (From Parker's Biologu,
after Saville Kent and Zeller.)

ii PHYLUM PROTOZOA : •/ . 10;

individuals by a process of multiple fission. Aftfer
changes in the nuclei of the two conjugating individuals, during
which the microgamete contributes a micronucleus towards the
formation of the new nuclear apparatus of the megagamete, the
former shrivels and dies (Gl, 6r2). 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.

Though most Ciliata are free, many are parasites, mainly in the
alimentary canal of various Metazoa. Pathogenic forms are almost
unknown.

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. 77, la), ovoid (76), 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
elongated cylindrical structure (Ic), capable of protrusion and
retraction, and having its distal end sucker-like. It is, moreover,
practically tubular, the axial region consisting of a semi-fluid
protoplasm, while the outer portion is tolerably firm and resistant.
When partially retracted, a spiral ridge is sometimes observable
around the tentacle : this may indicate the presence of a band of
specially contractile protoplasm, resembling the axial fibre in the
stalk of Vorlicella. Infusors and other organisms are caught by
the tentacles (4, 6), 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 (la), horseshoe-shaped, or branched
(8, 9) : in many cases a micronucleus (la, mi. nu.) has been found,

102

ZOOLOGY

SECT.

and.it probably occurs in all. There are one or more contractile
Vacuoies (c. vac.).

Some genera are naked (1) : others form a stalked shell or
lorica (2a) like that met with in many of the Mastigophora.

7.0j>hryodendron

S.Epheloi-a

9. Dendrosoma

FiQ. 77.— Various forms of Tentaculifera. la and 6, two species of Podophrya ; c, a
tentacle much enlarged ; 2a, Acinetajolyi ; 2b, A. tuberosa ; in G the animal has captured
several small Ciliata ; 8a, a specimen multiplying by budding ; 56, a free ciliated bud ; 9a,
the entire colony ; 9b, a portion of the stem ; 9c, a liberated bud ; a, organism captured as
food ; b.c. brood-cavity ; bd. bud ; c. vac. contractile vacuole ; 1. lorica ; mg. nu. mega-
nucleus ; mi. nu. micronucleus ; t. tentacle. (After Butschli and Saville Kent.)

ii PHYLUM PROTOZOA 103

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 Section 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 meganucleus is very
remarkable, extending as a branched axis throughout the colony
(6, nu.). Micronuclei of the ordinary character are present as well.

Reproduction by binary fission takes place in many species.
In Ephelota gemmipara (8) 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 (6). After a short active existence tentacles appear and the
cilia are lost. In this case budding is external, but in Acineta
tuberosa (%, 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 (,9,6, bd.).

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 8,000 to 10,000 feet
above sea-level to a depth of from 2,000 to 3,000 fathoms. Some
forms, such as species of Amoeba and Gromia, 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-
portion of genera and even of species is 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 Ehizopoda, Mastigophora, and Infusoria
occur both as internal and external parasites.

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

104 ZOOLOfJY SECT.

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 (Section IV.). Moreover, the Mycetozoa, the
plasmodia of which- are formed by the fusion of Amcebulse, 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 (Section 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 Prot-
amoeba, at the bottom of the list, and the Radiolaria at the top.
Similarly, among the Mastigophora, such simple Flagellata as
Oikomonas (Fig. 53, 2 and 8) 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, and the consequent assump-
tion 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 Mastigophora ; 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 recapitulates 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 Mastigamoaba 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 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.

PHYLUM PROTOZOA

105

As to the Ciliata, Multicilia and Lophomonas (Fig. 72, 12 and 13)
appear to indicate the derivation of the order from the Flagellate
type, since their cilia are long and flagellum-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-
ment of the Protozoan series, since they show a degree of
differentiation attained nowhere else by a single cell.

Tentaculifera

Dinoflagellata

CystofJagellata

Ciliata

Radiolaria
Foraminifera

Lobosa
Mycetozoa

-Sporozoa
FIG. 78. — Diagram showing the mutual relationships of the chief groups of Protozoa.

The Mycetozoa appear to have been derived from the common
amoBboid-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 above is an attempt to express these relationships
in a graphic form.

Heliozoa 0.

/ Choano-

Flagellata
Flagellata

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 Metazoa 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. 20), 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 with —
the Porifera 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 — Sycon gelatinosum.

General External Appearance and Gross Structure.—

Sycon gelatinosum,'1 one of the Calcareous Sponges, has the form of a
tuft, one to three inches long, of branching cylinders (Fig. 79), all
connected 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..

106

SECT. Ill

PHYLUM AND CLASS PORIFERA

107

i/

FIG. 79. — Sycon gelatinosum.

Entire sponge, consisting of
a group of branching cylin-
ders (natural size).

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. 80), 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 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

fine 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

F». 80.-sycon selatino.«m. A portion s.ight,y

magnified; one cylinder (that to the right) bisected
longitudinally 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,
dotted; ip. marks the position of three of the groups
of inhalant pores at the outer ends of the incurrent

canals • o osculum

!T 3 l

through the minute in-

-nnrPQ nr ncrh'a
pOre& Or OStia

rnpntionpfl a«4

Occurring in groups be-

, -P , & , . *

tween tne elevations on

108

ZOOLOGY

SEOT.

fi

ft

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 Sycon is firmly squeezed, there will be
pressed out at first sea-water, and then, when greater pressure is
exerted, a quantity of gelatinous-looking matter, which, on being

examined micro-
scopically,proves
to be partly com-
posed of a proto-
plasmic material
consisting of in-
numerable usu-
ally more or less
broken cells with
their nuclei, and
partly of a non-
proto p 1 a s m i c,
jelly - like sub-
stance. When
this is all re-
moved there re-
mains behind a
toughish felt-like
material, which
maintains more
or less complete-
ly the original

n «

FIG. 81. — 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 ; or.
young ova ; R, radial canals ; sp. triradiate spicules.

shape of the sponge. This is the skeleton or supporting frame-
work. 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. 81 and 82, sp) of a clear glassy appearance. These
are the calcareous spicules which form the skeleton of the Sycon.

The arrangement of the spicules, their relation to the proto-
plasmic parts, and the structure of the latter, have to be studied
in thin sections of hardened specimens (Figs. 81 and 82). An
examination of such sections leads to the following results.

Microscopic Structure. — Covering the outer surface of
the sponge is a single layer of cells — the dermal layer or

Ill

PHYLUM AND CLASS PORIFERA

100

ectoderm1 (Fig. 82, ec)—
through which project
regularly-arranged groups
of needle-like and spear-
like spicules (spr), forming
the pattern of polygonal
elevations on the outer
surface. The cells of
the ectoderm (pinacocytes)
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 ecto-
derm, thin flattened scales.
Running radially through
the thick wall of the cylin-
ders are a large number of
regularly-arranged straight
passages. Of these there
are two sets, those of the
one set — the incurrent
canals (Figs. 81 and 82, 1C)
—narrower, and lined by
ectoderm similar to the
ectoderm of the surface ;
those of the other set — the
radial ox 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 blind-
ly at their inner extremi-
ties — not reaching the
paragastric cavity ; ex-
ternally each becomes
somewhat dilated, and

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.

Fia.82. — Sycon gelatinosum. Transverse section
through the wall of a cylinder (parallel with the
course of the canals), showing one incurrent (/C),
and one radial (R) canal throughout their length ;
sp. triradiate spicules ; sp'. oxeote spicules of
dermal cortex (dc.) ; sp". tetraradiate spicules of
gastral cortex (go.) ; ec. ectoderm ; en. layer of
flattened cells lining the paragastric cavity ; pm.
pore -membrane ; pp. prosopyles ; ap. apopyle ;
di. diaphragm ; exc. excurrent passage ; P.O. para-
gastric cavity ; em. early embryo ; em', late em-
bryo. The arrows indicate the course of the water
through the sponge.

110 ZOOLOGY . SECT.

the dilatations of neighbouring canals often communicate.
These dilated parts are closed externally by a thin membrane —
the pore-membrane (Fig. 82, pm, and Fig. 83), perforated by three
or four small openings (Fig. 83, 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. 82, exc.) Incurrent
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 prosopyles (pp) — uniting the
cavities of adjacent incurrent and flagellate canals. Each prosopyle
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

FIG. 83.— Sycon gelatinosum. Sur- FIG. 84.— Sycon gelatinosum.

face view of a pore-membrane highly An apopyle surrounded by its dia-

magnified ; p. ostium ; R. position of phragm ; m. contractile cells,
the outer end of a radial canal.

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 choano-
cytes, as they are termed, is not unlike one of the Choanoflagellate
Protozoa (p. 79) ; 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
is teased out in sea-water, and the broken fragments are 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,

in PHYLUM AND CLASS PORIFERA 111

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. 82, di, and Fig. 84), perforated by a
large circular central aperture — the apopyle (op)— 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 mesoglcea — in which the spicules of the skeleton are
embedded. Each spicule is developed from cells termed sdero-
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 spicute. The spicules (Figs. 81 and 82, sp) are
regularly arranged, and connected together in such a way as to
protect and 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 (spf) ; 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 flagellate canals, this arrangement pro-
ducing the pattern already referred to as distinguishable on the
outer surface. The thick outer layer in which the bases of these
oxeote spicules lie embedded is termed the dermal cortex (dc). A
thick stratum at the inner ends of the canals and immediately
surrounding the paragastric cavity is termed the gastral cortex (gc).
It is supported by triradiate and also by tetraradiate spicules, one ray

112 ZOOLOGY SECT.

of each of which (sp") frequently projects freely into the paragastric
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 collencytes (Fig. 81, 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. 83 and 84), 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. 81 and 82, ov) and
sperms — are developed immediately below the flagellate endoderm
cells of the flagellate canals, and in the same situation are to be
found developing embryos (em, 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 ectoderm1 — and the internal cavities, or a
part of them, are lined by a second single layer — the endoderm—
part or the whole of which consists of a single layer of choanocytes,
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 mesoglcea — containing a
number of cells of various kinds. The wall of the Sponge is
pierced by a number of apertures. The skeleton or supporting
framework, developed in the mesoglcea 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. Keproduction
takes place both asexually by the formation of gemmules, and

1 See footnote on p. 109.

in PHYLUM AND CLASS PORIFERA 113

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. 119), usually complicated.

ORDER 1. — TETRACTINELLIDA.
Demospongia with tetraxon spicules.

ORDER 2. — MONAXONIDA.
Demospongia with monaxon spicules.

ORDER 3. — CERATOSA.

Demospongia with skeleton of spongin fibres without siliceous
spicules.

ORDER 4. — MYXOSPONGIA.

Demospongia devoid of skeleton.

VOL. I I

114 ZOOLOGY SECT

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 Sycettidce ; and the
family Sycettidce is one of several families of the order Heteroccela
of the class Calcarea. Among the families of the Heteroccela,
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
resemble 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 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-

in PHYLUM AND CLASS PORIFERA 115

sum, we find that they differ in the number of branches, in the
shape of the cylinders, — some being relatively narrow, some
relatively wide — in the degree of development of the oscular crown
of spicules, in the ratio of the thickness of the wall to the width
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 f amilies 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 another
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-
branched ; 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. 79).

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. 85, .4). The surface of such an encrustation may be smooth ;
more commonly it is raised up into elevations — rounded bosses,
cones, ridges or lamellae ; 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

I 2

116

.ZOOLOGY

SECT.

branched (Fig. 85, B). 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. 85, C), with an even,
or lobed, or ridged surface. Very often, after active vertical growth
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-

B.Psammoclema

D. Poherion

FIG. 85.— External form of various Sponges. A, Oscaria, an encrusting form, with the
upper surface raised up into a number of rounded prominences ; B, Psammoclema, a
ramifying subcylindrical Sponge ; (7, Euspongia (toilet sponge), a massive form with
a broad base ; D, Foterion (Neptune's Cup), an example of a complex Sponge assuming
the form of a vase. (After Vosmaer.)

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
cup-shaped Sponge, exemplified in the gigantic Neptune's Cup
(Poterion, Fig. 85, 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-

ITT

PHYLUM AND CLASS PORTFERA

117

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-
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 differ-
ences of form, in accordance with
differences in the form of the support-
ing surface or in the action of waves
and. currents.

Leading Modifications of Struc-
ture.— Sycon gelatinosum belongs to a
type of Sponges intermediate between
the very simplest forms on the one
hand and the more complex on the
other. The simplest type of Sponge-
structure is that of the so-called Ascetta
or Olynthus (Fig. 86). This is not a
mature form— no adult Sponge retain-
ing 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 far there is a consider-
able resemblance to Sycon gelatino-
sum ; but the structure of its wall in
Ascetta is extremely simple. Regularly
arranged over the surface are a number of small rounded apertures,
the inhalant pores ; but, since the wall of the Sponge is very thin,
thesje apertures lead directly into the central or paragastric cavity
(Fig. 87, A), the long passages or canals through which the com-
munication 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.

FIG. 86.— Olynthus stage of a simple
calcareous Sponge (Clathrina). A
portion of the wall of the vase-like
sponge removed to show the para-
gastric cavity. (After Haeckel.)

118

ZOOLOGY

SECT.

A somewhat more complex type of structure than that of Ascetta

is exhibited by those
sponges in which the
wall becomes thick-

becomes
ened and perforated
b y radially - arranged
canals, which open
directly on the outer
surface by means of in-
halant pores or ostia,
and lead directly into
the paragastric cavity
by means of apopyles
— 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. 87,
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 in-
current canals, lined
by ectoderm and com-
municating with the
exterior on the one
hand, either by a
wide opening or by
pores (ostia) perforat-
ing a pore-membrane,
and on the other by
means of small open-
ings, the prosopyles
(the equivalents 01 the
inhalant pores of the
Olynthus), with the
radial canals. Sponges
similar to Sycon gela-
tinosum, but with

FIG. 87. — Diagram of the canal system of various sponges,
the ectoderm denoted by a continuous narrow line ; the
flattened 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 Leucllla convexa ; D, vertical
section through Oscar ella ; a, spaces of the in current canal
system ; b, spaces of the excurrent canal system ; os.
osculum. (After Korschelt and Heider.)

m

PHYLUM AND CLASS PORIFERA

119

flagellate canals arranged in groups, each group centred round a
main excurrent canal (Fig. 87, 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. 87, D, and Fig. 88) the flagellate cells are confined to certain
special enlargements of the canals — the so-called " ciliated cham-
bers " (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
arranged flagellate chambers, are said to possess the Sycon type of
structure. Such Sponges as have small rounded flagellate cham-

FIG. 88. — Vertical section of a fresh-water sponge (Spongilla), 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. osculum. (Modified from Leuckart and Nitsche's diagrams.)

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 Rhagon proper the arrangement of
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).

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 mesogloea with special
skeletal elements, usually containing spaces and canals lined by
ectoderm — (subdermal cavities. Fig. 88, SD) — which communicate

120

ZOOLOGY

SECT.

directly with the exterior, and, internally, usually with more
deeply situated spaces (subcortical cavities), from which the incur-
rent canals lead to the ciliated chambers. This dermal cortex
present, though not highly developed, in Sycon gelatinosum

is

(Fig. 82, 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

FIG. 89. — Cells of the ectoderm (pinaco-
cytes) very higlily magnified. (After
Von Lendenfeld.)

and very rarely assume other forms

regarded as representing dermal
cavities. In most higher sponges
a special inner layer is developed ;
this is the gastral cortex, repre-
sented in a rudimentary form in
Sycon gelatinosum (Fig. 82, c/c) 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 (pinacocytes) of the
ectoderm (Fig. 89) are flattened,

in some cases each flattened
flagellum. Lining the para-

ectodermal cell is provided with a
gastric 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 con-
nective-tissue cells, amoeboid
wandering cells, and, in cer-
tain positions (around orifices),
muscle-cells. Unicellular glands
(see p. 26) are present in
some sponges, both calcareous
and siliceous ; also cells con-
taining the pigment to which
the bright colour of many
sponges is due, though in most
cases the pigment is not con-
fined to special cells, but occurs
scattered through the connec
tive-tissue cells and flagellate
cells. Fresh-water Sponges are often green, owing to the presence
of chlorophyll, the colouring matter to which the prevailing green
colour of plants is due.
The elements of the skeleton differ in character in the different

IG. 90. — Development of a tri radiate spicule of
Clathrina. scl, scleroblasts. (After Minchin.)

Ill

PHYLUM AND CLASS PORIFERA

121

classes. In the Calcarea they consist of calcareous spicules, usually
tri-radiate in form. Each of these spicules is developed from special
cells — the scleroblasts (Fig. 90). In the remaining groups of Sponges
the skeleton either consists of spongin fibres alone (Fig. 91, A),
or of siliceous spicules alone, or of a combination of spongin fibres
with siliceous spicules (B) : in some Demospongia (the Myxospongia)

A.Eus^ongia

B.Pachychalina

FIG. 91. — 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, <fec. (After Vosmaer.)

skeletal parts are altogether absent. Spongin is a substance allied
to silk in chemical composition and contains a large amount of
iodine : the fibres are exceedingly fine threads, consisting of a soft
granular core and an outer tube of concentric layers of spongin.
These threads branch and anastomose, or are woven and felted

122 ZOOLOGY . SECT.

together in such a way as to form a firm, elastic, supporting struc-
ture. They are secreted by the activity of certain cells in the
mesogloea which are called the sponginblasts, derived from the
ectoderm. In certain exceptional cases the spongin assumes the
form of spicules. The siliceous spicules (Fig. 92) 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 development of these two elements varying greatly in
different cases. But in certain groups, including the common
washing-sponges (Fig. 91, 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. 91, C). In others, again, such as the Venus's
Flower-Basket (Euplectella), the Glass-Rope Sponge (Hyalonema),
and Pheronema (Fig. 93), the skeleton consists throughout of
siliceous spicules bound together by a siliceous cement.

Reproduction in the Sponges is effected either sexually or
asexually. In some cases asexual multiplication 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-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 often with peculiarly-shaped siliceous
spicules, termed amphidiscs (Fig. 92, right side). These gemmules
are formed in the substance of the Sponge towards the end of the
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.1 These are developed
from certain of the amoeboid wandering cells of the mesogloea,
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
an oval or pear-shaped head and a long tapering appendage or tail.
Each amoeboid cell destined to form an ovum enlarges, and

1 Sexually-produced larvae have not hitherto been found in the Hexactin-
ellida or Tetractinellida.

in

PHYLUM AND CLASS PORIFERA

123

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, Cliona, 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 kind— 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 archceo-
cytes, some of which form
the reproductive cells.
Certain of the flagellate
cells then withdraw their
flagella and pass into the
internal cavity, becoming
amoeboid. Soon a large
number of these amoe-
boid 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 (Section IV). This is the
larval form known as the parenchymula. The parenchymula
(Fig. 94) 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 flagellate 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. Subsequently the flagellate cells come to form the entire
boundary of the cavity, the porocytes passing outwards to become

FIG. 92. — Various forms of sponge spicules.
(From Lang's Text-Book.)

124

ZOOLOGY

SECT.

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 o f these
give rise to the re-
productive cells.
The scleroblasts
are formed of
certain ectoderm
cells which
migrate inwards,
and at an early
stage arrange
themselves i n
threes to give
rise to the tri-
radiate spicules.
The development
of the sponge be-
comes completed
by the enlarge-
ment of the in-
ternal cavity
(paragastric
cavity) which is
now lined b y
flagellate cells,
and by the de-
velopment of the
osculum.

In Sycon the
early stages
(Fig. 95, a-e)
differ somewhat
from those in
Clathrina blanca,
and the embryo
leaves the parent
sponge in the
peculiar stage to
which the name
of amphiblasiula
is applied. When
the blastula i s

formed the greater part of its wall consists of clear cells, with a
number of granular cells — the archaeocytes — at the posterior pole.

Ill

PHYLUM AND CLASS PORIFERA

125

The clear cells become elongated and flagellate. The archaeocytes
pass into the internal (segmentation) cavity and become completely
enclosed by the flagellate cells (stage of so-called pseudogastrula).

The cells at the posterior end then lose their flagella 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 un-
altered, and in the other half of the larger
granular cells. It is in this stage — termed
the amphiblastula (Fig. 95, e) — that the
larval sponge becomes free. At a later
stage the flagellate cells are partly over-
grown 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
fixes itself by one side, and soon
assumes a cylindrical form (Ti, 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, its cells remaining flagellate to
form the choanocytes of the radial canals.

The amphiblastula type of larva is characteristic of the Calcarea,
and is probably universal in that sub-class except in such primitive
forms as GlatJmna.

In the Silicispongiae, on the other hand, the typical larva is a solid
body with a superficial layer of flagellate 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 flagellate 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
Coelenterata — 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

FIG. 94.— Median longitudina
section of the parenchymula
larva of Clathrina blanca.
p.g.c., posterior granular cells
which give rise to the archaeo-
cytes. (From the Cambridge
Natural History, after Min-
chin.)

126

ZOOLOGY

SECT.

being members of existing groups.1 Some of the orders of existing
Sponges — such as the Myxospongise — are incapable of being
preserved as fossils, and the fossil forms belong, as we should
expect, to the more highly silicifiedjgroups and to the more complex
groups ofjthe Calcarea.

FIG. 95. — Development of Sycon raphanus. a, ovum ; b, c, ovum segmented — b, as seen
from above, c, lateral view ; d, blastula ; e, amphiblastula ; /, commencement of invagi-
nation ; g, larva attached by its oral face ; h, i, young sponge — h, lateral view ; i, as
seen from above. (From Sollas, after Schulze.)

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

1 The Archceocyathince, reef-forming "organisms of Cambrian age, may
have been nearly related to the Calcareous Sponges ; but this remains
doubtful.

in PHYLUM AND CLASS PORIFERA 127

between tide-marks to the deepest abysses of the ocean. The
Calcarea and, the true horny sponges (Ceratosa) 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 Hexactinellida 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 messmateism, 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 Cirripede Crustaceans
(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 a Zoophyte
(Palythoa), and there are many other instances. Sponges often also
grow in very close association with certain low forms of plants
(Algee).

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. 59), already referred
to (p. 80), 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)

128 ZOOLOGY SECT, m

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 Ccelenterata, 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 Ccelenterata.

SECTION IV
PHYLUM COELENTERATA

THE possession of an internal cavity lined by a special layer
of cells, — the endoderm, — in which the digestive and absorptive
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
Ccelentemta. 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 Palseozoic 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
VOL. i. 129 K

130 ZOOLOGY SECT, iv

growth on seaweeds, the wooden piles of piers, etc. It consists of
branched filaments about the thickness of fine sewing-cotton : of
these, some are closely adherent to the weed or 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. 96) is a colony, consisting of a
common stem or axis, on which are borne numerous zooids. The axis
consists of a horizontal portion (hydrorhiza) resembling a root or
creeping stem, and 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 immature
. 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 distally into about two
dozen arms or tentacles (t) : these zooids are the ^d^es^JM^^n^hs.
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 medusajwds (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 trimorphio, 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 manubrium or
hypostome (mnb), 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 enter on (enl), 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

FIG. 96. — Obelia sp. yl, 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 ; cce. coenosarc ; ect. ectoderm ; end. endoderm ; ent. enteric cavity ; g.th.
. gonotheca ; h.th. hydrotheca ; /. lithocyst ; m.bd. medusa-bud ; mnb. manubrium ; msgl.

ttr> mrmfh

vnrl /> rarliul

132 ZOOLOGY SECT.

narrow or proximal end, it is produced inwards into a sort of
circular shelf (sti), 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
1 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 (Ed. 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 (g.tti), 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
offshoots 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
medusce or jelly-fish (E — D), which will be described hereafter.

The microscopical structure of a polype (Fig. 97) 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 (ect) 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 mesogloea, and finally a solid
core of large endoderm cells arranged in a single series. The

IV

PHYLUM C(ELENTERATA

133

cgenosarc, 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 andendoderm,
but a cuticle, formed, layer by layer, as a secretion from the ectoderm
cells (see p. 32). 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
the latter retreats
from it, the con-
nection being
maintained only
at irregular inter-
vals. In the same
way the hydro-
and gono-thecae
are cuticular pro-
ducts of the
polypes and blas-
tostyles respec-
tively : i n t h e
young condition
both occur in the
form of a closely
fitting invest-
ment of the
knob - like rudi-
ment of the zooid
(Fig. $6,Bd.l, 2).

The ectoderm
has the general
character of a
columnar epithe-
lium (see p. 26),

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-
cells (Fig. 99, 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. 99, mf). This
muscular layer is a derivative of the ectoderm, and may be looked
upon as a rudimentary mesoderm.

Embedded in the ectoderm are numerous clear ovoid bodies, the

//.ft

FIGi 97.— Obelia sp. Vertical section of a polype, highly
magnified ; ect. ectoderm ; enrf.Sendoderm ; ent. enteric cavity ;
h.th. hydrotheca ; msgl. mesogloea ; mth. mouth ; ntc. nemato-
cysts ; sh. shelf-like prolongation of hydrotheca ; t. tentacle.

134

ZOOLOGY

SECT.

stinging-capsules or nematocysts (Figs. 97 — 99, ntc), organs closely
resembling those of Epistyhs umbellaria (p. 95), and, like them,
serving as weapons of offence. Each consists (Fig. 98, 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

FIG. 98. — Nematocysts of Hydra. A, undischarged ; .^.discharged ; C, nerve-supply ; cnb.
cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nematocyst ; nv.c. nerve-cell. (From
Parker's Biology, after Schneider.)

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 (J5), at the

IV

PHYLUM CCELENTERATA

135

-nu

nv.c

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. 97), 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 flagella 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. 97 and 99) the
endoderm (end) consists of a
single row of short cylindrical
cells, nearly cubical in longitu-
dinal section : their protoplasm
is greatly vacuolated and their
cell-walls are 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 gono-
theca — yet remains to be con-
sidered. The convex outer sur-
face of the bell or umbrella (Fig. 96, B — D) by which the zooid was
originally attached to the blastostyle is distinguished as the ex-
umbrella, the concave inner surface as the sub-umbrella. From the
centre of the sub-umbrella proceeds the manubrium (mnb), at the free
end of whi^h 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. 96, C, and Fig. 100, A).

The mouth (Figs. 96, 97, 100, and 101, mth) leads into an enteric
cavity which occupies the whole interior of the manubrium, and

ntc

FIG. 99.— Tentacle of Eucopella. The
lower part of the figure shows the ex-
ternal surface, hi 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.f.
muscle-fibres ; ntc. nematocyst ; nu.
nucleus ; nv.c. nerve-cell. (After von
Lendenfeld.)

136

ZOOLOGY

SEPT.

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
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. 101, vl), and gives off the tentacles (t), which
are sixteen in number in the newly-born medusa (Fig. 96), very
numerous in the adult (Fig. 100). At the bases of eight of the
tentacles — two in each quadrant — are minute globular sacs (l)>
each containing a calcareous particle or lithite. These are the
marginal sense-organs or lithocysts : they were formerly considered

FIG. 100. — Obelia sp. A, mature medusa swimming with everted umbrella ; B, one-quarter
of the same, oral aspect ; circ.c. circular canal ; gon. gonad ; I. lithocyst ; mnb. mami-
briutn; mth. mouth; rad. c. radial canal; t, tentacle. (After Haeckel.)

to be organs of hearing, and are hence frequently called otocysts :
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. 101, mnb) of the medusa consists of
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,

IV

PHYLUM CCELENTERATA

137

end font

ft.C

'/re

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 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
homol o g o u s
s t r u c t u res,
and the more
complex me-
dusa is readily
derivable from
the simpler
polype - form.
It is obvious,
in the first
instance, that
the apex of
the umbrella
c o r r e sponds
with the base
of a hydranth
(Fig. 102, 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
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,
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
enteron (ent. cav') 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
the endoderm-lamella (Z)', end. lam), and remaining only along
four meridional areas — the radial canals (rod. c), and a circular
area close to the edge of the bell — the circular canal (cir. c).

Fio. 101.— Dissection of a medusa with rather more than one-quarter
of the umbrella and manubrium cut away (diagrammatic). The
ectoderm is dotted, the endoderm striated, and the mesoglcea black ;
circ. c. circular canal ; end. lam. endoderm lamella ; gon. gonad ; /.
lithocyst; mnb. manubrium ; mth. mouth ; rad. c. radial canal; vl.
velum.

138

ZOOLOGY

SECT.

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.
103, A), taken at right angles to its long axis, a large number of radii — about

eel

FIG. 102. — Diagram illustrating the derivation of the medusa from the polype. A, longi-
tudinal, and A', transverse'section (along the^linejafr) of polype-form ; B, polpye-f orm with
extended tentacular regi9n ; 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', trans-
verse section (along the line ab) of medusa. The ectoderm is dotted, the endoderm striated,
and the mesogloea black, dr. c. circular canal ; ect. ectoderm ; end. endoderm ; end. lam.
endoderm lamella ; ent. cav. enteric- cavity ; hyp. hypostome or manubrium ; mnb.
manubnum ; msgl. mesogloaa ; mth. mouth : nv.t nv', nerve-rings : t. tentacle ; v. velum.
(From Parker's Biology.*)

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

TV

PHYLUM C(ELENTERATA

139

or per-radii. Halfway between any two per-radii a radius of the second order,
or inter -radius, may be taken ; halfway between any per-radius and the inter-
radius on either side a radius of the third order, or ad-radius, and halfway
between any ad-radius and the adjacent per- or inter-radius, a radius of the
fourth order, or su^-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 moutJ), 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.

FIG. 103.— Projections of polype U) and medusa (B), showing the various orders of radii
gon. gonad ; mnb. manubrium.

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

140

ZOOLOGY

SECT.

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. 100 and 101, 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

Fio. 104.— Stages in the development of two Zoophytes (A— H, Laomcdea, I—M, Euden-
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
development of the hydrula. (From Parker's Biology, after Allman.)

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. 104, 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-

IV PHYLUM CCELENTERATA 141

weed, &c., fixes itself by one end (K), and becomes converted into
a hydrula 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
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
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 i
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,
interstitial cells, muscle-fibres, and nerve-cells. Certain of the
interstitial 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- 1
spondence with its locomotive habits, the medusa attains a higher
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 medusas,
but the hydroid colony produces fixed reproductive zooids. In!
others, again, there is no hydroid stage, the organism existing only j
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

142 ZOOLOGY SECT.

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 divei
functions.

Thus we have zoophyte colonies known to produce free medusae,
zoophyte colonies known not to produce free medusae, and medusae
known to have no zoophyte stage. Moreover, there are many
^medusae of which the life-history is unknown, so that it is
uncertain 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.

T

ORDER 1. — LEPTOLIM;.

Hydrozoa in which there is a fixed zoophyte stage, and in which
the sense-organs are exclusively ectodermal.

Sub-Order a. — Anihomedusce.

Leptolinse in which the polypes are not protected by hydrothecae or the

I i reproductive zooids by gonothecae : the medusae bear the gonads on the

\manubrium and have no_ lithocysts.

Sub-Order b. — Leptomedusce.

- Leptolinae in which hydro- and gono-thecse are present : the medusa; bear
the gonads in connection with the radial canals and usually have lithocysts.

"

ORDER 2. — TRAOHYLIN^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. — Trachymedusce.

Trachyliiise 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.

Trachylinae in which the tentacles spring from the ex- umbrella, some
distance from the margin, and the gonads a*re developed in connection with
the manubrium.

PHYLUM CCELENTERATA 143

ORDER 3. — HYDROCORALLINA.

Hydrozoa in which a massive skeleton of calcium carbonate is
jreted from the coenosarc, 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 hydro-thecae, and
of gonads formed in connection with the radial canals, belongs to
the sub-order Leptomedusse. It is placed in the family Campanu-
lariidce, distinguished by having cup-shaped thecae 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
zooids. The hydranths may be colourless and quite invisible to
the naked eye, or, as in some Tubulariae (Fig. 106, <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
dZquorea, may attain a diameter of 380 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

144

ZOOLOGY

SECT. IV

Bougainvillea (Fig. 105) is a good example, the cuticle stops short at
the bases of the hydranths, and the reproductive zooids are not
enclosed in gonothecae. It is for this reason that, in classifications
founded on the zoophyte stage, the Anthomedusae are called Gymno-
blastea or naked-budded zoophytes (see also Fig. 106, J, 4, 5). In
the Leptomedusae the cuticle is usually of a firmer consistency than

in the first sub-
order, and fur-
nishes hydro-
thecas for the
hydranths and
gonothecae for
the reproductive
zooids : they are
hence often
classified as
Calyptoblastea or
covered - budded
hydroids. T o
this group belong
the commonest
species of hy-
droids found on
the sea - shore,
and often mis-
taken for sea-
weeds — the
" Sea-firs " or
Sertularians.

The medusae
also exhibit
c h a r a c teristic
differences in the
two sub - orders.
In the Anthome-
dusae the um-

*:)re^a

FiO. 105— Bougainvillea ramosa. A, entire colony, natural 1S

size ; B, portion of the same magnified ; C, immature medusa, stronffrV arched.
dr. c. circular canal ; CM. cuticle or perisarc ; ent. cav. enteric r

cavity ; 'Jiyd. polype or hydranth ; hyp. hypostome or manu- and may even D6
brium ; med. medusa ; mnb. manubrium ; rad. c. radial canal

t. tentacle ; v. velum. (From Parker's

after Allman.)

conical or mitre-
shaped (Figs. 105 ;
106, 7 ; 110, 1 and 2) : its walls are thick, owing to a great develop-
ment of the gelatinous mesoglcea of 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 (gori) are developed on the manubrium and that lithocysts
are absent. Sense-organs are, however, present in the form of specks

nmb 3. Corymorpha

6. Clavafella

FIG. 106.— Various forms of Leptolinae. In 1, a shows the entire colony, & a portion highly
magnified ; in 7, a. is a species producing medusa-buds from the manubrmm, 6 from the
bases of the tentacles; <fe. dactylozooids; m. and M. medusae ; mnb. manubrium ; mth.
mouth ; oc. eye-spots; rad. c. radial canals; s. sporosacs ; sp. spines ; t, fi, t*, tentacles .
VOL. I. L

146

ZOOLOGY

SECT.

of red or black pigment at the bases of the tentacles. These ocelli
(oc) 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 Leptomedusse
the umbrella is usually less convex, thinner, and of softer consis-
tency than in the Anthomedusse, 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.

It

FIG. 107.— Ceratella fuse a. About nat. size. (From Hickson, after Baldwin Spencer.)

In the majority of Leptolinse the ccenosarc, 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 (%<&wfea). The
curious genus Hydractinia (Fig. 106, 1) is remarkable~~T6f possessing
a massive ccenosarc, 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

IV

PHYLUM CCELENTERATA

147

Hermit-crabs is a case of commensalism : the hydroid feeds upon
minute fragments of the Hermit-crab's food, and is thus its com-

FIG. 108. — 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
nematocyst ; 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 ; hyp.
hypostome or manubrium ; int. c. interstitial cells : m. pr. muscle-processes ; mth. mouth ;
msgl. mesogloea ; ntc. large, and ntc'. small nematocysts ; nu. nucleus ; ov. ovum ; ovy.
ovary ; psd '. pseudopods ; spy. spermary ; vac. vacuole. (From Parker's Elementary
Biology, after Lankester and Howes.) «

mensal or messmate ; and the Hermit-crab is protected from its
enemies by the presence of the inedible, stinging hydroid.

L 2

148 ZOOLOGY SECT.

Hydractinia belongs to the Anthomedusae : the Leptomedusan
Claihrozoon, an Australian genus, resembles it in having branched
jand intertwined coenosarcal tubes, the perisarc of which under-
Igoes 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 p. 201). Ceratella (Fig. 107) has a similar fan-coral-like
appearance, with a branching axis composed of numerous inter-
twining and anastomosing tubes ; but while Clathrozoon possesses
thecae, in Ceratella they are absent.

A great simplification of the colony is produced in Myriothela
(Fig. 106, 2), in which the short ccenosarc bears a single large
terminal hydranth, and gives off numerous slender branches which
bear the reproductive zooids (s). Even greater simplicity is found
in Corymorpha (3), in which the entire organism consists of a
single-stalked polype, from the tentacular region of which the
medusae (m) arise.

But the simplest members of the whole class, with the exception
of one or two imperfectly known forms which will be referred to
below, are the Fresh-water Polypes of the genus Hydra. The
entire organism (Figs. 27 and 108) consists of a simple cylindrical

body with a
conical hypo-
stome and a
circlet of six

FIG. 109.— Protohydra leuckartii. (From .Chun, after Greeff.) • i , . ^

The mouth is to the left, the disc of attachment to the right. eignt

tacles. 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 looping
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 Hydras, but these are always detached
sooner or later, so that a permanent colony is never formed. There
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

iv PHYLUM COELENTERATA 149

Protohydra (Fig. 109) and Microhydm, in which the tentacles
are absent.

Pelagohydra is also solitary, but is pelagic. The part corre-
sponding 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 Siphonophora — an order yet to be dealt with
—than to the Leptolinae.

The polypes are usually cylindrical, as in Obelia, but in some
genera they are widened out into a vase-like form (Fig. 106, <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 j
surface (4). In Myriothela (2) they are short, and so numerous-
as to have the appearance of close-set papillae. In some forms
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
(1) 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
Anthomedusse and Leptomedusae, 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

{

-if

SECT, iv PHYLUM CCELENTERATA 151

diameter up to 400 mm. (16 inches). The number of marginal
tentacles may be very great (Fig. 110, 2), or these organs may be
reduced to two (Fig. 110, 1), or even to one (Fig. 106, 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. 110, 3). In addition to the
marginal tentacles longer or shorter oral tentacles may be present in
a whorl surrounding the mouth (Fig. 110, t').

Some medusae creep over submarine surfaces, walking on the
tips of their peculiarly modified tentacles (Fig. 106, 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- 1
ments there is a great development of both muscular and
nervous systems. The velum and the sub-umbrella possess
abundance of muscle-fibres, presenting a transverse striation,1
and round the margin of the umbrella is a double ring of nerve-
cells and fibres, one ring being above, the other below the attach-
ment of the velum (Fig. 102, D, nv, nvf). 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 medusa is paralysed byv
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 th&
exterior by minute pores placed at the summits of papillae, the
endoderm cells of which contain brown granules. There seems to
be little doubt that these are organsof^ excretion, the cells with-
drawing nitrogenous waste-matters~Irom 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. 106, 7a), or from the margin of the umbrella (76) or the base
of the tentacles : in one case they are formed on blastostyles
developed on the gonads. 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. 106, 76, 5, s),

152

ZOOLOGY

SECT.

each consisting of little more than a gonad, but showing an indica-
tion of its true nature in a prolongation of the digestive cavity
of the colony, representing the stomach of the manubrium (Fig.
111). 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. 101) are sporosacs,
i.e. reproductive zooids, not mere gonads. In some rare cases the
sexual cells are not developed either in medusae or in sporosacs,

but are formed
directly in the
blastostyles.

In Obelia we
found the me-
dusae to be bud-
ded off from
peculiarly modi-
fied mouthless
zooids — the
blastostyles.
This arrange-
ment, however,
is by no means

FIG. 111. — Diagram illustrating the formation of a sporosac by the
degradation of a medusa. A, medusa enclosed in ectodermal
envelope (es) ; B, intermediate condition with vestiges of
umbrella (u) and radial canals (ra) ; C, sporosac. ec. ectoderm;

en. endoderm ; m. manubrium ; 'ov. ovary ; t. tentacle ; v. velum. ,-m:,™

(From Lang's Comparative Anatomy.) Universal . tne

reproduct ive

zooids — whether medusae or sporosacs — may spring directly from
the coenosarc, as in Bougainvillea (Fig. 105), or from the ordinary
)iydranths (Fig. 106, 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 ccenosarc, and
slowly migrate to their destination in the ectoderm of the gonad,
where they metamorphose in the usual way into the definitive
reproductive products, which when mature pass into the space
below the ectoderm of the gonad.

The development of the Leptolinae frequently, but not always,
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 oosperm 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

IV

PHYLUM CCELENTERATA

153

formed of a single layer of cells (Fig. 112, A). The blastula
elongates, and the cells at one pole undergo division, the daughter-
cells passing into the cavity, which they gradually fill (B). Att
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 giving rise to 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-

FlG. 112. — Early development of Eucope. A, blastula-stage ; B, planula with solid endo-
derm ; C, planula with enteric cavity ; al, enteric cavity ; ep. ectoderm ; hy. endoderm.
(From Balfour's Embryology, after Kowalevsky.)

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, and, after a period of rest, develops into the
adult form.

154

ZOOLOGY

SECT.

ORDER 2. — TRACE YLINJE.

General Structure. — The members of this order are all
medusae : no zoophyte stage is certainly known in any of them
except Cunina parasitica, 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 100mm. (4 inches) in diameter. The gelatinous tissue
or mesoglcea of the ex-umbrella is usually well developed, giving
the medusa a more solid appearance than the delicate jelly-fish of

gen

1. Petas us

S.CIossocodon

FIG. 113. — Two Trachymedusae. dr. c. circular canal ; gon. gonad ; mnb. mnnubrium
mih. mouth ; rad. c. radial canal ; re. c. recurrent canal ; t. tentacle ; tc. tentaculocyst
tg. tongue ; vl. velum. (After Haeckel.)

the preceding order : this is well shown in Fig. 113, 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.

But the most characteristic anatomical feature of the group is
the structure of the sense-organs, which are club-shaped bodies
(Figs. 113 and 114, tc) consisting of an outer layer of ectoderm
enclosing a central axis of endoderm cells (Fig. 115) : they have,
therefore, the structure of tentacles. They contain one or more
lithites, which are always derived from the endoderm. To
distinguish them from the lithocysts of Leptomedusse, and to mark

IV

PHYLUM CCELENTERATA

155

the fact that they are modified tentacles, they are called tentaculo-
cysts. They may either project freely from the margin of the
umbrella, or may become enclosed in a pouch-like growth of

rad.c

mth
l.Cunarcha

2.Polycolt>a

FIG. 114. — Two Narcomedusae, 2 in vertical section, gon. gonad; mnb, manubrium ; mth.
mouth ; pr. peronium ; rad. c. radial canal ; t. tentacle ; tc. tentaculocyst ; t.r. tentacle-
root ; ?7. velum. (After Haeckel.)

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 Trachymedusae, as in the preceding
6rder, they arise near the edge of the umbrella (Fig. 113), but in the
Narcomedusae they spring about halfway between the edge and
the vertex (Fig. 114), and are
continued, at their proximal
ends, into the jelly of the ex-
umbrella in the form of " ten-
tacle-roots " (t.r).

As to the position of the re-
productive organs, there is
the same difference between the
two sub-orders of Trachylinse as
between the two sub-orders of
Leptolinse. In the Trachy-
medusaa the gonads (Fig. 113,
gon) are developed in the course
of the radial canals : in the
Narcomedusse (Fig. 114) 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. 114, 7,
may hang down vertically instead of taking the usual horizontal

end.

FIG. 115. — JEginura myosura, a tentaculo-
cyst highly magnified, ect. ectoderm ; end.
encloderm ; /. lithites ; ntc. nematocysts ;
n>\c. group of nerve-cells. (After Haeckel.)

156

ZOOLOGY

SECT.

position. In the Narcomedusse the manubrium is short ; in the
Trachymedusae it is always well developed, and is sometimes (Fig.
113, 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 of the sub-umbrella (pseudo-
manubrium).

The simplest case of the development of Trachylinae is seen in
dfiginopsis, one of the Narcomedusae. The oosperm gives rise to
a ciliated planula, which forms first two (Fig. 116), then four
tentacles, and a mouth, hypostome, and stomach. The larva of
^Eginopsis is thus a hydrula, 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
medusa. Thus the actual formation of the medusa from the

hydrula of
^Eginopsis
c o r r e sponds
precisely with
the theoretical
derivation
given above
(p. 137). It
will be seen
that in the
present case
there is no
metagenesis or
alternation of

generations, but that development is accompanied by a metamor-
phosis— 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 of changes.

Metagenesis is, however, not quite unknown among the Trachy-
linae. In a parasitic Narcomedusa (Cunina parasitica) the planula
fixes itself to the manubrium of one of the Trachymedusae which
serves as its host, and develops into a hydrula. But the latter,
instead of itself becoming metamorphosed into a medusa, retains the
polype form and produces other hydrulae by budding, these last
becoming converted into, medusae in the usual way.

ORDER 3. — HYDROCORALLINA.

The best-known genus of Hydroid Corals is Mittepora, one species
of which is the beautiful Elk-horn Coral, M. alcicornis. ' The dried
colony (Fig. 117, A) consists of an irregular lobed or branched mass

FIG. 116. — Larva of JEginopsis. m. mouth'; t. tentacle.
(From Balfour, after Metschnikoff.)

iv PHYLUM CCELENTERATA 157

of carbonate of lime (corallum), 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 dactylopores
(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
canals in immediate connection with the pores are traversed by ,
horizontal partitions, the tabula (tb).

. Y" .; « »

A,*%.'::- ff*~-9 * : ^

V : ^*-^

.;m '-vyf <*>*-*•• "& «B

* * * :>

, . '-•''

w

^ '•>

FIG. 1 17.— Millepora alcicornis. A, part of skeleton, natural size ; B, portion of surface,
magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; W.
tabulae. (After Nicholson and Lydekker.)

In the living animal each pore is the place of origin of a zooid :
from the gastropores protrude polypes (Fig. 118, G) with hypostome
and four knobbed tentacles ; from the dactylopores long, filamentous,
mouthless dactylozooids or feelers (D), 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
canals of the coral and represent a much-branched coenosarc,
recalling that of Hydractinia (p. 146).

The ccenosarcal tubes have the usual structure, consisting of
ectoderm and endoderm with an intervening mesoglcea. ^rom
the relative position of the parts it will be obvious that the calcare-
ous skeleton is in contact throughout with the ectoderm of the

158

ZOOLOGY

SECT.

colony : it is, in fact, like the horny perisarc of the Leptolianae, a
cuticular product of the ectoderm.

The only other genus to which we shall, refer is Stylaster (Fig.
119), which forms a remarkably elegant tree-like colony, abun-
dantly branched in one plane, and of a deep pink colour. On the

IV

CCELENTERATA

159

branches are little cup-like projections with radiating processes
passing from the wall of the cup towards the centre, and thus
closely resembling the true cup-corals belonging to the Actinozoa
(vide p. 202). But in the * case of Stylaster each " cup " is
the locus, not of one, but of several zooids — a polype projecting
fronxjts 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 cup.

The gonophores in most species of Millepora are developed in
certain of the pores in dilatations or ampulla ; in one species at
the apices of the
d a c t yl o zooids.
They are me-
dusae, but never
have the com-
plete medusa-
form, being de-
void of velum,
mouth, radial
canals and ten-
t a c 1 e s. Both
male and female
medusae become
free, but the
period of free
existence is very
short.

In Stylaster the
medusofd charac-
ter is much more
completely lost,

and the gonophores are more of the nature of sporosacs or de-'
graded 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-
reefs partly or entirely surrounding many of the islands in
those seas. Fossil forms are found as far back as the Triassic
epoch.

ORDER 4. — SIPHONOPHORA.

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.

Halistemma (Fig. 120, A) occurs in the Mediterranean and other

FIQ. 119.— Stylaster sanguineus. A, portion of skeleton,
natural size ; B, small portion, magnified ; a. ampullae ; d.p.
dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.)

B

FIG. 120.— Halistemma tergestinum. A, the entire colony ; B, a single group of zooiib
coe. coenosarc ; dz. dactylozooid ; hph. hydrophyllium or bract ; net. nectocalyx or swim
ming-bell ; ntc. battery of nematocysts ; p. polype ; pn. pneumatophore or float ; s., s'.
sporocysts ; t. tentacle. (After Claus.)

SECT. IV

PHYLUM CCELENTERATA

161

fin,

net

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 — the float or pneumatophore (pn). Next come a number
of closely set, transparent structures (net), having the general char-
acters of unsymme-
trical medusae with-
out manubria, each
being a deep, bell-
like body, with a
velum and radiating
canals. During life
these swimming-
bells or neclocalyces
contract rhythmic-
ally— i.e. at regular
intervals — drawing
water into their
cavities, and imme-
diately pumping it
out, thus serving to
propel the entire
organism 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 segments,
like the nodes and
internodes of a
plant.

Springing from
certain of the
" nodes " are un-
mistakable 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

VOL. I M

FIG. 121. — Diagram of a Siphonophore : the thick line repre-
sents endoderm ; the space external to it, ectoderm ; the
internal space, the enteric cavity, coe. coenosarc ; dz. dactylo-
zooid ; hph. hydrophyllium ; md. sporosac ; net., net'.
nectocalyces ; ntc. battery of nematocysts ; p. polype ; pn.
pneumatophore ; t. tentacle. (After Claus.)

162

ZOOLOGY

SECT.

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 (J9, s, sr), 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 pro-
tective zooids, and the
swimming-bells locomo-
tory 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

, . , , . .

Wnicn the enteric Cavity

originally extended (Fig.

FIG. 122. — Two stages in the development of Hall-
stemma : the endoderm is shaded, the ecto-

.

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

PHYLUM CCELEKTERATA

163

the latter becomes spirally twisted during growth, and so causes
them to arise irregularly.

The egg of HaHstemma gives rise to a ciliated planula resem-
bling that of the other Hydrozoa. At one pole the ectoderm
becomes invaginated to form the float (Fig. 122, ep), the opposite
extremity is gradually
converted into the first
polype (po), and 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
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 in the embryo is
not the float, but the first
bract, which grows con-
siderably and envelops the
growing embryo in much
the same way as the um-
brella of a medusa envelops
the manubrium. On this
and other grounds some
zoologists look upon the
Siphonophore-colony as a
medusa the manubrium of
which has extended im-
mensely and produced
lateral buds after the
manner of some Antho-
medusse (Fig. 106, 7 a). On
this theory the entire crenosarc 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-

M 2

. 123. — Physalia : the living animal floating on the
surface of the sea. cr. crest ; p. polype ; pn. pneu-
matophore ; t. tentacle. (After Huxley.)

164

ZOOLOGY

SECT.

spending reduction of the rest of the ccenosarc. The float (Fig.
123, 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), 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 Halisternma, but the
female apparently becomes
detached as a free medusa.
In Diphyes the float is
absent. Two swimming-
bells (Fig. 124, A, m) of
proportionally immense
size are situated at the
proximal end of the cceno-
sarc, and are followed by
widely-separated groups*of
zooids (5), each group con-
taining a polype (n) with
its tentacles (^), a meduzoid
(g), and a large enveloping
bract (t). The stem often
breaks at the internodes,
zooids then swim about like

Fio. 124. — Diphyes campanulata. A, the entire
colony ; B, single group of zooids. a. crenosarc ;
c. cavity of swimming-bell ; e, groups of zooids ;
g, medusoid ; t, grappling-line or tentacle ; m,
swimming-bell ; n, polype ; o, mouth of swim-
ming-bell ; t, bract. (From Parker's Biology,
after Gegenbaur.)

and the detached groups of
independent organisms.

Porpita is formed on a different type, and has a close general
resemblance to a medusa. It consists (Fig. 125) of a discoid
body, enclosing a chambered chitinoid shell (sh) containing air, and

IV

PHYLUM COELENTERATA

165

obviously corresponding with the float of Physalia : each of the
chambers communicates with the exterior by a couple of pores.
The edge of the disc is beset with long dactylozooids (t), and from its
lower surface depend numerous closely set blastostyles provided
with mouths and bearing medusae, while in the centre is a single
large gastrozooid (%). The mouth of the gastrozooid leads into
a wide gastric cavity. Between this and the pneumatophore is a
thick cellular mass through which ramify numerous canals belonging
to two systems. One of these systems is endodermal : it com-
municates with the endodermal cavities throughout the colony.

Try

Fia. 125. — Porpita pacifica, A, from beneath ; B, vertical section, hy, large central
gastrozooid : hy', blastostyles ; sh, chambered shell ; t, dactylozooids. (From Parker's
Biology, after Duperry and Koelliker.)

The other (trachea! system) is ectodermal and is lined internally
throughout by a chitinous layer ; it opens by numerous apertures
into the air-chambers of the float. The closely allied genus Velella
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 flab
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
and blastostyles: But, as their early development is not known, it

166

ZOOLOGY

SECT.

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
or a double row, numerous small projections, the hydrothecse (Fig. 126, h.lh).
The coenosarcal skeleton is strengthened by a slender
axis, the virgula (t>), the proximal end of which is
connected with a small dagger-shaped body, the
sicula (s), supposed to be the skeleton of the
primary zooid by the budding of which the colony
was produced. In connection with some species
oval or cup-like capsules have been found : these
maj be of the nature of gonothecse. 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 ; Microhydra, at present
known only in North America ; Cordylophora,
one of the Anthomedusae, found in Europe,
America, Australia, and New Zealand ; Poly-
E,Dimorphdgraptus,both podium, also an Anthomedusa, found in the

magnified, hy. th. hydro- \T -, -, . , ., . . ...

theca ; s. sicuia ; v. vir- Volga, where in one stage of its existence it is

IndLydtkker.)Nic son parasitic on the eggs of the Sturgeon ; Limno-

codium, a doubtful Trachymedusa, hitherto

found only in a tank in the Botanical Gardens, Regent's Park,

where it was probably introduced from the West Indies ; and

Limnocnida, found in Lakes Tanganyika and Victoria Nyanza and

in the river Niger.

The oldest known Hydrozoa are the Graptolites, found first in
the Cambrian rocks ; Hydractinia occurs in the Cretaceous epoch,
and Hydrocorallinae 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

FIG. 126.— Graptolites.

A, Monograplus colonus

iv PHYLUM CGELENTERATA 167

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.

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 Amoeba or a Paramoscium 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 OP 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.

mth

t.TTC

FIG. 127. — Aurelia aurita. A, dorsal view, part of the ex-umbrella cut away to show part
of the stomach and one of the four gastric pouches ; S, ventral view — two of the oral arms
are removed, a.r.c. ad-radial canal ; g. /. gastric filaments ; gon. gonads ; g. p. gastric
pouch ; i.r.c. inter-radial canal ; mg. Ip. marginal lappet ; mth. mouth ; or. a. oral arm ;
p.r. c. per-radial canal ; s.g. p. sub-genital pit : st. stomach ; t. tentacles.

SECT, iv PHYLUM CCELENTERATA 169

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.
127 and 128, 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).

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 sub-genital pit (s.g.p),
which lies immediately beneath one of the conspicuously coloured
gonads (gon). 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 halfway 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

170

ZOOLOGY

SECT.

^ angle between each two gastric pouches : this leads into a per-
radial canal (p.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. 128, B). The main mass of
the umbrella is formed of gelatinous mesogloea, which, however,
is not structureless, but is traversed by branching fibres and

Fia. 128. — Aurelia' .aurita. A, side view, one-fourth of the umbrella cut away ; B, dia-
grammatic vertical section, ectoderm dotted, endoderm striated, mesogloea black, circ. c.
circular canal ; g. f. gastric filaments ; gon. gonad ; g. p. gastric pouch ; gul. gullet ;
h. hood ; i.r. c. inter-radial canal ; mg. Ip. marginal lappet ; mth. mouth ; or. a. oral
arm ; s.g. p. sub-genital pit ; st. stomach.

contains amoeboid cells derived from the endoderm. Both ex- and
sub-umbrellse are covered with ectoderm, and the stomach and
canal system are lined with endoderm, which is ciliated through-
out. Some observations seem to ,show that the short tube
described above as a gullet and a part 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.

iv PHYLUM CGELENTERATA 171

It was mentioned above that- in the free medusa the gonads
appear through the transparent umbrella as coloured horse-shoe-
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. 127, 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
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.

A

FIG. 129. — Aurelia aurita. A, small portion of jedge of umbrella, showing the relations of
the tentaculocyst ; B, vertical section of the same region (diagrammatic), h, hood ;
1. lithite ; my. lp. marginal lappet ; oc. ocellus ; olf. 1, olf. 2, olfactory pits. (Altered
from Lankester.)

Lying parallel with the inner or concave border of each gonad
is a row of delicate filaments (Figs. 127, 128, ^./),formed of endodermj
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 the1
Hydrozoa.

Muscular and Nervous Systems. — The contractions of the
bell by which the animal is propelled through the water are
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 inter-radii, corresponding to the

172 ZOOLOGY

SECT.

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. 129) 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
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 (I) derived from the endoderm, and on the outer
side is an ectodermal pigment-spot or ocellus (oc). The
tentaculocysts are largely hidden by the marginal lappets (mg. Ip)
and by a hood-like process (h) connecting them ; and in connection
with each are two depressions, one on the ex-umbrella (olf. 7), the
other immediately internal to the sense-club (olf. 2) : these
depressions are fined with sensory epithelium and are called
olfactory 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. 130) differing in this respect, as well as in its mode of
formation, from the corresponding stage of a Hydrozoon.

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 ; if, as represented in Figs.
B and (7, the process takes place by a sinking-in or invagination of
the surface so as to produce a depression lined with ectoderm (B, st),
the bottom of which becomes perforated so as to communicate with
the enteric cavity (C, st), the depression is the stomodceum, a struc-
ture of which there is no trace in the Hydrozoa : there is some doubt,
however, of the occurrence of any such ectodermal involution in the
case of Aurelia. On two opposite sides of the mouth hollow pro-
cesses 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 ad-radial tentacle's 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 oitcenioles (D, tn.). The mouth (E, mth.) assumes
a square outline, and its edges become raised so as to form a short

IV

PHYLUM CCELENTERATA

173

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

FIG. 130. — Aurelia aurita, development. A, planula, erroneously represented as com-
pletely closed ; B, C> formation of stomodseum ; D, transverse section of young scyphula ;
E, scyphula ; F, longitudinal section of same : the section passes through a per-radius

tentacle ; In. taenioles. (From Korschelt and' Heider's Embryology.)

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

174 ZOOLOGY SECT.

of the Leptolinse, but distinguished by a pronounced differentiation
of structure, indicated by the sixteen tentacles developed in regular
order, the stomodseum, and the four gastric ridges with their septal
funnels. The Scyphozoon-polype is called a scyphula or scyphistoma.

The scyphula may grow to a height of half an inch, and some-
times multiplies by budding. After a time it undergoes a process
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 arms (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 ad-radial 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 Leptolinse, or by direct metamorphosis of a
polype, as in Trachylinse, 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.

iv PHYLUM CGELENTERATA 175

It has been shown that, under exceptional circumstances, the
egg of Aurelia develops into scyphulae which do not undergo
transverse division, the entire scyphula becoming metamorphosed
into a single adult.

2. GENERAL STRUCTURE AND CLASSIFICATION.

The Scyphozoa may be defined as medusoid Ccelenterata, 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 (agamobium) 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 stomodaeum.

The Scyphozoa are divisible into four orders, as follows : —

Scyphozoa having a conical or vase-shaped umbrella, sometimes
attached to external objects by an ex-umbrellar peduncle : no

ORDER 1. — STAUROMEDUSJS (LUCERNARIDA).

Da h
o e:

tentaculocysts.

ORDER 2. — CORONATA.

Scyphozoa having the umbrella divided by a horizontal coronary
groove : four to sixteen tentaculocysts.

ORDER 3. — CUBOMEDUS^;.

Scyphozoa with a four-sided cup-shaped umbrella : four per-
radial tentaculocysts.

' ORDER 4. — DISCOMEDUS^E.

Scyphozoa with a flattened saucer- or disc-shaped umbrella :
not fewer than eight tentaculocysts — four per- and four inter-
radial.

176 ZOOLOGY SECT.

Sub-Order a — Semostomce.

Discomedusse in which the square mouth is produced into four long oral
arms.

Sub-Order b — Rhizostomce.

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 Semostomce,
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 Semostomae. This group contains six families, characterised
mainly by differences in the canal system : the Ulmaridae 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
bifurcation. 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 ad-radial canals springing
from each gastric pouch.

ORDER 1. — STAUROMEDUS^E (LUCERNARIDA).

Tessera (Fig. 131 ), 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.

Lucernaria (Fig. 132), 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 ad-radial arms, bearing at their ends groups of short
adhesive tentacles (L). As in the scyphula, each gastric ridge contains an
inf undibulum, lined with ectoderm and opening on the sub-umbrella. The
gastric filaments (g.f.) are very numerous — a distinct advance on Tessera — and
the gonads (gon.) 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
(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
tentacles. The Depastridce have an almost entire margin fringed with
tentacles.

IV

PHYLUM CCELENTERATA
ORDER 2. — CORONATA,

177

This group includes a number of rare and beautiful Medusae of curiously
complex structure, of which Pericolpa may be taken as an example. The
umbrella (Fig. 133) is usually conical, and is divided by a horizontal furrow

9-f-

i.r.

frr.t

FIQ. 131. — Tessera princeps.**^, external view ; S, vertical section, ^./..jgastricifilament ;
gon. gonad ; i.r. t. inter-radial tentacle ; mnb. manubrium ; lmth.[mouth ; p.r. t. per-radial
tentacle ; st. stomach ; tn. tteniole. (After Haeckel.)

Fia. 132. — Lucernaria. A, oral aspect ; B, from the side. g. foot-gland ; g.f. gastric
filaments ; gon. gonad ; mth. mouth ; t. tentacles ; In. taenioles. (After Claus.)

VOL. T N

178

ZOOLOGY

SECT.

(coronary groove] into an apical region or cone (en. ) and a marginal region or
crown , the crown is again divided by a second, rather irregular horizontal
furrow into a series of pedal lobes (pd. L), adjacent to the cone, and a series of
marginal lappets (mg. 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.
In Pericolpa four of the pedal lobes, inter-radial in position, bear
tentaculocysts (tc.) ; four others, per-radially situated, give origin to long,

. 133. — Fericolpa quadrigata. ,4, external view ; B, vertical section, circ. s. circular
sinus ; en. cone ; g.f. gastric filaments : gon. gonads ; mg. Ip. marginal lappets ; mnb.
nmuubrium ; mth. mouth ; pd. L pedal lobes ; st. stomach ; t. tentacles ; tc. tentaculo-
cysts ; tn. tsenioles. (After Haeckel.)

hollow tentacles (L). In the more complex genera there are eight additional
ad-radial 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. s.). As in Lucernaria, there are four wide inter-radial in-

IV

PHYLUM CCELENTERATA

179

fundibula. The gastric filaments (g. /.) are very numerous, and the elongated
U-shaped gonads (gon.) are eight in number and ad-radial.

The coronary

pr

sr

groove
teristic

is charac-
of the
group : but in
other points — such
as the number of
pedal and marginal
lobes, tentaculo-
cysts, and tentacles
—there is great
variation. Peri-
colpa and its allies
(Peromedusce) re-
semble the Lucerna-
rida and the mem-
bers of the order
Cubomedusce in the
presence of tsenioles
and inter - radial
septa : EpJiyropsis
and its allies (Can-
nostomce) resemble
the order Disco -
phora in the ab-
sence of these
structures. The
scyphula larva of
Nau$ithoe(Fig. 134)
lives as a parasite
in the interior of a
horny sponge.

FIG. 134. — Nausithoe. The entire animal from the oral aspect.
ar. adradii ; g. gonads ; g.f. gastric filaments ; ir. inter-radii ;
m. circular muscle of sub-umbrella ; pr. per-radii ; rl. tentacu-
locysts ; sr. sub-radii ; t. tentacles. The black Across in the
centre represents the mouth. (From Lang's Comparative
Anatomy.)

ORDER 3. — CUBOMEDUSCE.

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 Medusae more than any of
the other Scyphozoa. The best known species, Charybdcea marsupialis
(Fig. 135), is about 5 cm. in diameter and of very firm consistency.

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.

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 Medusae, 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 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 (mrih.} leading into
a wide stomach, from which go off four very broad shallow per-radial pouches
(rad. f).}, occupying the whole of the four flat sides of the umbrella, and

N 2

180

ZOOLOGY

SECT.

separated from one another by narrow inter-radial septa or partitions (mesen-
teries) placed at the four corners. These pouches are equivalent to wide 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

Fia. 135. — Charybdaea 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 ;
G, transverse section, circ. c. circular canal ; end. lam. endoderm lamella ; end. lam', its
prolongation into the velarium ; g.f. gastric filaments ; gon. gonad ; gon'. septum separa-
ting gonads ; 1. lappet ; mnb. man'ubrium ; rad. p. radial pouch ; t. tentacle ; tc. tenta-
culocyst ; vl. velarium. (After Claus, somewhat altered.)

junction of the gastric pouches with the stomach are the usual four groups of
gastric filaments (g. f. ).

The gonads (gon.) 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

IV

PHYLUM CCELENTERATA

181

Cubomedusse are the only Scyphozoa which, like the Hydrozoa, have a com-
plete nerve-ring. The tentaculocysts are very complex, each bearing a litho-
cyst 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 primarily 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 Semostomse and

FIG. 136. — Filexua pulmo. A, side view of the entire animal ; B, vertical section, diagram-
matic ; C, one of the suctorial mouths, magnified, c. arm canal ; g.f. gastric filaments ;
gon. gonads ; or. a. oral arms ; rod. c. radial canal ; s. mth. suctorial mouths ; st. stomach ;
tl, 12, t3, tentacles on oral arms. (After Cuvier, Claus, and Huxley.)

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 extraordinarily 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 Semo-
stomse (Fig. 127), and altogether absent in the Rhizostomse (Fig. 136). In the
Semostomee there are four oral arms (Fig. 127, or. a.), each resembling a leaf
folded along its midrib, and having more or less frilled edges : in the Rhizo-
stomse each of the original four arms (Fig. 136, or. a.) becomes divided longi-
tudinally 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.

182 ZOOLOGY SECT.

The arrangement of the enteric cavity and its offshoots presents an interest-
ing series of modifications. In no case are there any taenioles or inter-radial
septa (mesenteries). In the Semostomae (Fig. 127) 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 Rhizostomae (Fig. 136, 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 (jB, G, s.mth.) with
frilled margins. Rhizostomes have been found with prey of considerable size,
such as fishes, embraced by the arms and partly drawn into these apertures,
which are therefore called the suctorial moutTis. 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 practically 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

A J^%, B ,^***\ C /r. • " %, the usual position,

and more or less
leaf - like arms,
folded along the
midrib so as to
enclose a deep
groove, from

afe- _^ s Which s60011^

"^r^O^^r^ ^i-^^Krr &< grooves pass, like

the veins of a leaf,
towards the edge

Fio. 137.— Pelagia noctiluca : Three developmentral stages, m.

mouth ; r. marginal lappet ; s. tentaculocyst. (Fom Korschelt development pro-

and Heider, after Krohn.) ceeds, 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 Rhizo-
stomae 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 imme-
diately below the floor of the stomach and above the brachial disc.

In many of the Discomedusse 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 off buds which
become detached as free-swimming planulae, and these, coming to rest, develop
into scyphulse. But in other cases there is no alternation of generations, and
development is direct. For instance, in Pelagia (Fig. 137) — one of the
Semostomae — a blastula is formed which becomes invaginated at one end

iv PHYLUM CCELENTERATA 183

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 lappet?. Up to this time the embryo is ciliated
externally, but soon the cilia disappear, and the little creatures assume some-
what 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
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

184

ZOOLOGY

SECT,

B

mes. /

mes. 3

FIG. 138. — Tealia crassicornis. A, dissected specimen ; B, transverse section, the half
above the line ab through the gullet, the lower half below the gullet, d. mes. directive
mesenteries ; gon. gonads ; gul. gullet ; l.m. longitudinal muscle ; Ip. lappet ; mes. 1,
primary, mes. 2, secondary, mes. 3, tertiary mesenteries ; mes. f. mesenteric filaments ;
mth. mouth ; ost. 1, ost. 2, ostia ; p.m. parietal muscle ; sgph. siphonoglyphe ; s.m.
sphincter muscle ; t. m. transverse muscle.

iv PHYLUM CCELENTERATA 185

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. 138, 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,
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 and 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. 2), which extend
only part of the way from the body-wall to the gullet, and

186

ZOOLOGY

SECT.

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 scyphula-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

B

FIG. 139.— Diagrammatic vertical U) and transverse (B) sections of a Sea-anemone. The
ectoderm is dotted, the endoderm striated, the mesoglrea black, ac. acontium ; en.
cinclis ; gul. gullet ; int. mes. c. inter-mesenteric chamber ; mes. mesentery ; mes. f.
mesenteric filament ; mth. mouth ; ost. ostium ; p. pore ; t. tentacle.

\

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. 2} 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
mesenteric filament (mes. /.), answering to a gastric ^ filament of
the Scyphozoa. In many Sea-anemones the mesenteric filaments
are produced into slender threads- — the acontia — which may be
protruded through the mouth or through special apertures
(cinclides] of the body-wall (Fig. 139, A).

The general arrangement of the cell-layers is the same as in
the two preceding classes. The body-wall (Fig. 139)— base, column,
and disc — consists of a layer of ectoderm outside, one of endoderm

IV PHYLUM CGELENTERATA 187

within, and between them an intermediate layer or mesogloea,
which is extremely thick and tough. The gullet (gul.), 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 mesoglcea, covered on both sides by
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
is seen to be traversed by definite fibrous bands, the two most
obvious of which are the longitudinal or retractor muscle (Fig.
138, 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 l>y 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
retraction 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

188

ZOOLOGY

SECT.

the mesenteries and of their muscles is very definite and charac-
teristic (Fig. 138, B). At each end of the gullet, opposite the
siphonoglyphe, are two mesenteries (d. mes.), 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 (mes. 1),
differing from the directive couples in having the longitudinal
muscles turned towards one another. The secondary and tertiary

ntc

end

FIQ. 140. — Tealia crassicornis. Trans-
verse section of tentacle, ect. ectoderm ;
end. endoderm ; l.m. longitudinal muscles ;
msffl. mesogloea ; nv. c. nerve-cells ; nv. f.
nerve-fibres ; ntc. nematocysts ; t. m.
transverse muscles. (After Hertwig.)

FIG. 141 . — Three nematocysts of
Sagartia. (After Hertwig.)

mesenteries (mes. 2, mes. 3) 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 sym-
metry, underlying, as it were, its superficial radial symmetry. It
is divisible into equal and similar halves by two planes only, viz., a
vertical or sagittal plane taken through the long diameter of the gullet,
and a transverse plane taken through its short diameter.

The general microscopic structure of a Sea-anemone is well
shown by a section through a tentacle (Fig. 140). Both ectoderm
(ect.) and endoderm (end.) consist mainly of very long columnar,

IV

PHYLUM CCELENTERATA

189

ntc

ciliated epithelial cells, and the mesogloea (msgl.) is not only
extremely thick, but has the general characters of connective tissue,
being traversed by a network of delicate fibres with interspersed
cells. The middle layer has, in fact, ceased to be a mere gelatinous
supporting lamella or mesoglcea, and has assumed, to a far greater
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. 141) 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 undis-
charged capsule (A).
Gland - ceUs (Fig.
142, gl.) are very
abundant in the
ectodermal lining
of the gullet and in
the mesenteric fila-
ments : the latter
are trilobed in sec-
tion, and the gland-
cells are confined to
the middle portion,
the lateral divisions
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
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

FIG. 142. — Transverse section of rnesenteric filament of
Sagartia. c. ciliated cells ; gl. gland-cells ; ntc. nematocysts.
(After Hertwig.)

190 ZOOLOGY . SECT-

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 mesogloea, and thus appearing to belong to that
layer (Fig. 140 l.m.). This fact is significant from the circum-
stance that, as we shall see, the muscles of all animals above
Ccelenterata 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. 140,
nv.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. 138, gon.),
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 gufiet 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. 1 43, 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 stomqdseum, 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-

IV

PHYLUM OELENTERATA

191

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 constituted, on each side of the vertical plane, by one of the mesenteries of
the first 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

std

B

FIG. 143. — 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-
daeum. (After Korschelt and Heider.)

cycle of 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 Coslenterata 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 stomodaeum : 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

192 ZOOLOGY

SECT.

and some of which join the stomodaeum. 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,
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. — ACTINIARIA.

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.

iv PHYLUM OELENTERATA 11)3

ORDER 2. — MADREPORARIA.

Zoantharia which resemble the Actiniaria in the general
structure of the soft parts, but which usually form colonies, and
always possess an ectodermal calcareous skeleton. This order
includes the vast majority of Stony Corals (Figs. 147 and 157).

ORDER 3. — ANTIPATHARIA.

Compound tree-like Zoantharia in which the tentacles and
mesenteries are comparatively few (6 — 24) in number. £ 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. 151).

Sub Class II — Alcyonaria.

Actinozoa in which the tentacles and mesenteries are always^
eight in number. The tentacles are pinnate, i.e. produced into]
symmetrical branchlets. 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 fingers " (Alcyonium, Fig. 154) 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 Eed Coral (Corattium, Fig. 146),
or a series of connected tubes for the individual polypes, as in
the Organ-pipe Coral (Tubipora, Fig. 149). 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. 145) —
are simple.

ORDER 5. — GORGONACEA.

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 mesoglcea. There is no
siphonoglyphe. The beautiful " Sea-fans " belong to this group
(Fig. 155).

VOL I. O

194 ZOOLOGY SECT.

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
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. 148).

Systematic Position of the Example.

Tealia ^prassicornis is one of several species of the genus
Tealia : it belongs to the family Tealidce, which, with several
other families, make up the tribe Hexactinice, 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 stomodseum has two siphono-
glyphes and two lappets.

The family Tealidae 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 endodemial 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 approximately
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 Zoanihus
(Fig. 144), for instance, the original polype sends out a horizontal
branch or stolon (st.)} from which new polypes arise. Besides the

IV

PHYLUM CCELENTERATA

195

Sea-anemones the only simple forms are certain Madreporarian
corals, such as Flabellum (Fig. 156, A, B), and three genera 'of
Alcyonacea, of which
Hartea (Fig. 145) may be
taken as an example.

The simplest mode of
budding is that just de-
scribed in Zoanthus, in
which new zooids are
developed from a narrow
band-like or tubular stolon
(Fig. 144, st). A more usual
method resembles that
with which we are already
familiar in Hydrozoa, new
buds being formed as lateral
out-growths, and a tree-like

Colony arising with numer- JTIG. 144— Zoanthus sociatus. A, entire colony ;

nn« vnnirlci «r»rinrrinrr frrkm a st- stolon. B, transverse section, sgph. siphono-

5 springing irom a glyphes ; d. d. dorsal, and v. d. ventral directive

Common Stem Or COSnOSarC. mesenteries. (After McMurrich and Korschelt and

Corallium and Gorgonia

(Figs. 146 and 155) are good examples of this type of growth. In
other cases the buds grow more or less parallel with one another,

producing massive colonies
either of close-set zooids or of
zooids separated by a solid
coenosarc. As examples of this
type we may take Palythoa, the
most complex of the Actiniaria,
and many of the common
Madreporaria, such as Astpcea
(Fig. 147). In the Sea-pens
(Pennatulacea) the proximal end
of the elongated colony (Fig.
148) is sunk in the mud, and
the distal end bears zooids
springing either directly from
the cosnosarc or, as in Penna-
tula itself, from flattened lateral
branches. The stem itself is
the equivalent of a polype.

A very peculiar mode of bud-
ding occurs in the Organ-pipe
\££ 'jj$T3P Coral (Tubipora). The base of

^Kk£^ the original polype (Fig. 149)

FIG. 145.— Hartea elegans. gul. gullet;; grOWS Out into a flattened CX-
("Ser'pIrS W^*3 ' '' ^^^ Pansion fl°m which neW P^P68

o 2

196

ZOOLOGY

SECT.

arise, diverging slightly from one another as they grow, and
separated by tolerably wide intervals. The distal ends of the
polypes then grow out into horizontal expansions or platforms
(pi), formed at first of ectoderm and mesogloea only, but

finally receiving prolongations of
the endoderm. The platforms ex-
tend, come in contact with one
another, and fuse. In this way
platforms 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 passu with
the vertical 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

FIG. 146. — Corallium rubrum, por-
tion of a branch. (From Glaus,
after Lacaze-Dnthiers.)

FIG. 147.— Astraea pallida, the living colony. (After Dana.)

the Actiniaria. One of the most important points to consider
is the arrangement of the mesenteries. In Edivardsia (Fig. 150),
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

IV

PHYLUM CCELENTERATA

197

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. 143, A ; it is probably to be looked upon as

the most primitive or generalised member
of the order. In Zoanthus (Fig. 144, B)
the dorsal directives (d.d.) do not reach
the gullet, and each lateral couple consists
of one perfect and one small and imperfect
mesentery. In Cerianthus, another bur-
rowing form, there are a couple of very
small ventral directives, and the remaining
mesenteries are very numerous, not
arranged in couples, and all directed
ventrally at their outer ends, so as to

Fia. 148.— Pennatula sulcata. A, entire colony; B, portion of the same magnified.

/. lateral branch; p. polype ; «. sjphonozooid. (After Koelliker.)

L.tn*

FIG. 149.-Tubipora musica. A, skeleton of entire colony ; B, transverse 'sections of polype
C, single polype with tube and commencement of platform ; D, growth of new polypes
fern platform I m. longitudinal muscles ; yi. y». polypes ; pJ. platform ; sgph. _siphono-
glyphe • sp spicules ; std : stomodfeum. (After Cuvier, Quoy and Gairaard, and Hickson.)

198

ZOOLOGY

SECT.

transverse section. (After Andres,

have a very obviously bilateral arrangement : in this genus, as
growth proceeds, new mesenteries are added on the dorsal side,
and not, as is usual, between already formed couples. On the

other hand, the genus Gyractis ex-
hibits a perfectly radial arrange-
ment : the mesenteries are all
arranged in couples with the longi-
tudinal muscles facing one another.
Lastly, in all the more typical
Sea-anemones (forming the tribe
Hexactinice) 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 mesen-
teries are arranged, so far as is
known, in the way just described

for the Hexactimae. In the Antl-

patharia 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. 149, 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 face inwards in the Alcyonaria, outwards
in Edwardsia.

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 six-
teen, and in some
genera of Sea-
anemones they are
branched. In the
Antipatharia (Fig.
151) they vary in
number from six
to twenty-four. When more than six are present, six of them
are larger than the others.

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
each edge (Fig. 145). Many Actiniaria have the tentacles

FIG. 151. — Antipathes ternatensis, portion of a branch,
showing three zooids and the horny axis beset with spines.
(From the Cambridge Natural History, after Schultze.)

iv PHYLUM CCELENTERATA 199

perforated at the tip (Fig. 139, A, p.) ; 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. 139, A, and Fig. 158, 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. 144, 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

FIG. 152.— Antipatharia. A, oral face of zooid of Parantipathes. B, oral face of
zooid of Schizopathes. (After Delage et Herouard.)

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. 152), 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 each of the lateral parts 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
transverse mesenteries. In such a form as Schizopatkes (Fig. 152, B)
there is thus recognisable an arrangement of the parts which might
be interpreted as a dimorphism of the zooids, one set — the parts
containing the mouth and gullet— being regarded as gastrozooids,
and the others containing the gonads as gonozooids.

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

200

ZOOLOGY

SECT.

FIG. 153. — Minyas.
/. float. (After
Andres.)

position : some forms, such as Edwardsia (Fig. 150) and Cerianthus,
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, habitu-
ally lying on the sea-bottom with the longitudinal
axis horizontal like that of a worm : a few, such
as Minyas (Fig. 153), have the aboral end dilated
into a sac containing air and serving as a float ;
by its 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 homomorphic, i.e. there is no differentiation
of the zooids of a colony. But in the Alcyonaria dimorphism is
common : the ordinary zooids or polypes are accompanied by

smaller individuals, called
siphonozooids (Fig. 148,
s.), having no tentacles,
longitudinal muscles, or
gonads.

None of the Actiniaria
have a true skeleton :
in some, however, there
is a thick cuticle, and
several kinds enclose
themselves in a more or
less complete tube (Fig.

150), which may be
largely formed of dis-
charged nematocysts.
The simplest form of
skeleton is found in the
solitary Alcyonarian
genus Hartea (Fig. 145),
already referred to, in
which minute irregular
deposits of calcium car-
bonate, called spicules
(sp.), are deposited in
the mesoglcea. A similar
spicular skeleton occurs
in the " Dead-men's finger " (Alcyonium, Fig. 154), where
spicules of varying form are found distributed throughout the
mesogloea of the ccenosarc. In Tubipom (Fig. 149), the " Organ-
pipe Coral," the mesoglceal spicules become closely fitted
together, and form a continuous tube for each polype, the

FIG. 154. — Alcyonium p alma turn, A, entire
colony; B, spicules. (After Cuvier.)

IV

PHYLUM CCELENTERATA

201

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
skeleton, and in the living state is covered by ectoderm. In the
Red Coral of commerce (Corallium, Fig. 146) 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

FIG. 155.— Gorgonia verrucosa. A, entire colony ; B, portion of the same magnified.
(After Koch and Cuvier.)

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. 151)
and in the Gorgonacea (Fig. 155). It also consists of an axial rod,
extending all through the colony and branching with it, but is

202 ZOOLOGY SFCT.

formed of a flexible horn-like material. Moreover it is not meso-
glceal, 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 mesoglcea of the coenosarc. In some
of the Gorgonacea the axial skeleton is partly horny, partly
calcareous.

In the Sea-pen (Pennatula, Fig. 148) and its allies the stem of
the colony is supported by a horny or calcareous 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 which is still uncertain. Spicules occur in the mesogloea,
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-
coral1 J 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. 156, A, B). It has the form
of a short conical cup, much compressed so as to be oval in section.
Its wall or theca (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, b. pi.}. From the inner surface of the theca a
number of radiating partitions, the septa (sep.), 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 (Z>, col.).

In many Corals there is a distinct calcareous layer investing the
proximal portion of the theca, and called the epitheca (C, e.ih.).
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 tabulce, like
those of Millepora (p. 158), extending across the whole corallite. \ In

TV

PHYLUM CCELENTEBATA

203

tlie Mushroom-coral (Fungia) the corallite is discoid, the theca is
confined 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

scf>-

FIG. 156. — 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 trans-
verse section ; ectoderm dotted, endoderm striated, skeleton black, b. pL basal plate ; col.
columella ; e. th. epitheca ; gul. gullet ; mes, mes. 1, mes. 2, mesenteries ; mes. /. mesentenc
filaments ; sep. septa ; t. tentacle ', th. theca. (A and B after Moseley ; C and D after
Gilbert Bourne.)

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 invested

204

ZOOLOGY

SECT.

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
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

FIG. 157. — Dendrophyllia nigrescans, li, XKTadrepora aspera.

co. corallites ; cs. coenosarc ; p. polypes. (After Dana.)

of the septa. Here, again, sections show that each septum corre-
sponds 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 be 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. 147) ; or tree-like forms, such

iv PHYLUM CCELENTERATA 205

as Dendrophyllia (Fig. 157, A), in which a common calcareous stem,
the coemnchyma, is formed by calcification of the coenosarc (cs.), and
gives origin to the individual corallites. It is by this last-named
method, the coenosarc attaining great dimensions and the indivi-
dual corallites being small and very numerous, that the most
complex of all Corals, the Madrepores (Madrepora, Fig. 157, B)
are produced.

The microscopic structure of corals presents two main varieties.
In what are called the aporose or poreless corals, such as Flabellum,
Astraea, &c., the various parts of the corallite are solid and stony,
while in the perforate forms, such as Madrepora, all parts both of
the corallites and of the connecting ccenenchyma 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 ccenosarc, and placing the polypes of the
colony in communication.

The Blue Coral (Heliopora), one of the Alcyonacea, has a massive
corallum of 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 siphonozooids.
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
brilliancy of their colour during lif e. Everyone 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
Alcyonidae 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 Kadiolaria (p. 65).

Many Actinozoa, like many sponges (p. 127), furnish examples of
commensalism, a term used for a mutually beneficial association
of two organisms of a less intimate nature than occurs in symbiosis.
An interesting example is furnished by various Sea-anemones
(Fig. 158) which live on univalve shells inhabited by Hermit-crabs.
The Sea-anemone is carried from place to place by the Hermit-crab,

206

ZOOLOGY

SECT.

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-
anemone, which, owing to its toughness, and to the pain caused
by its poisonous stinging-capsules, is usually avoided as an article
of food. Species of Peachia, which in the adult state burrow in
sand, in a larval condition live as parasites or commensals in the
radial canals of Scyphomedusse.

FIG. 158.— Adamsia palliata, four individuals attached to a Gastropod shell inhabited by
a Hermit-crab, ac. ac1. acontia ; sh. shell of Gastropod. (After Andres.)

Other Sea-anemones — such as the gigantic Discosoma of the
Great Barrier Reef — are found associated with small Fishes or
Crustacea, which have their abode in the enteric cavity. In this
case the Fish secures shelter in a place wjiere 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

IV PHYLUM CCELENTERATA 207

to more than once. The corallum contains not only the apertures
for the polypes and siphonozooids, but also tubular cavities of
an intermediate size, in each of which is found a small chsetopod
Worm, belonging to the genus Leucodore. 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 particular
species of Madrepore.

The distribution of the Actiniaria is world-wide, and in
many cases the same genera are found in widely separated parts
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, Bio 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. More-
over, they have a curiously limited bathymetrical distribution,
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
Gorgonacea 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, and at the
Cape Verde Islands : other species of Corallium occur on the coast

208

ZOOLOGY

SECT. IV

of Japan, at Mauritius and Madeira. Tubipora and Heliopora have
the same distribution as the reef -building Corals.

From the palseontological 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 Rugosa, the 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 fossula, where the septa are greatly
reduced.

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.

FIG. 159. — Hormiphora plumosa. A, from the side, B, from the aboral pole, mth,
mouth ; s. pi swimming plates ; t. and b. tentacles. (After Chun.)

159 and 160). The species //. plumosa is found in the Mediter-
ranean ; allied forms belonging either to the same genus (often
called Cydippe) or to the closely allied genus Pleurobrachia are
common pelagic forms all over the world.

From opposite sides of the broad end depend two long tentacles
(t.), provided with numerous little tag-like processes, and springing
each from a deep cavity or sheath, into which it can be completely
retracted (Fig. 160, t.sh.}. At the narrow end — where the stalk
of a pear would be inserted— is a slit-like aperture, the mouth

5.0

160. — Hormiphora plumosa. A, dissected specimen haying rather more than one quarter qf
the body cut away. B, section at right angles to long axis (horizontal) ; diagrammatic, adr. c.
adradial canal ; inf. infundibulum ; inf. c. infundibular canal : int. c. inter-radial canal ; mrd. c.
meridional canal ; mth. mouth ; ovy. ovary : par. c. per-radial canal ; s. o. sense-organ ; s. pi. swimming-
plate ; spy. spermary ; std. stomodaeum ; std. c. stomodseal canal ; std. r. stomodseal ridges ; t. tentacle ;
\t. b. base of tentacle ; (. c. tentacular canal ; t. sh. tentacular sheath.

VOL. I P

210 ZOOLOGY SECT, iv

(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
are, in fact, rows of immense cilia, fused at their proximal ends :
their presence and mode of occurrence — arranged in meridional
comb-ribs, costce, 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. 110, 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. An imaginary line passing
from the middle of the mouth to the sense-organ is the primary axis.
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. 188), the vertical or sagittal 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 or lateral plane. It is along these two
planes alone that the body is capable of division into approximately
equal halves.

Enteric System. — The mouth leads into a flattened tube (Fig.
160, std.), often called the stomach, but more correctly the gullet or
stomodceum. It reaches about two-thirds of the way towards the
aboral pole, and its 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
stomodaeum — i.e. in the transverse plane. From the infundibulum

212

ZOOLOGY

SECT.

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. 161, A, ex. p.). The two
other canals given off from the infundibulum are the per-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. 161, 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
with the stomodseum, and a tentacular canal (t. c.), which extends

ad.c

B

162. — Hormiphora plumosa. A, transverse section of one of the branches of a
tentacle ; B, two adhesive cells (ad. c.) and a sensory cell (s. c.) highly magnified. CM.
cuticle ; nu. nucleus. (After Hertwig and Chun.)

outwards and downwards into the base of the corresponding 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. 161), the cells from which tire combs
arise being particularly large. The epithelium of the stomodseum
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-
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-

iv PHYLUM CCELENTERATA 213

tudinal muscular fibres, which, as we shall see (p. 216), are of meso-
dermal 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, in a
sense, by the peculiar adhesive-cells with which the branches of
the tentacles are covered. An adhesive-cell (Fig. 162, ad. c.) has
a convex surface, produced into small papillae, 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

+rvrr» a\xra\r K^r fhp FIG. 163. — Hormiphora plumosa, Sense-organ, ft. bell:
VdV UJ c. p. ciliated plate ; c. gr. ciliated groove ; ex. p. excretory

Struggles of the Pore ; I. lithites ; p. pi. polar plate ; sp. spring. (Modified

from Chun.)

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. 163, 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
(sp.), which support a mass of calcareous particles (/.), like the
lithites of 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 (&.), 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

214

ZOOLOGY

SECT.

v.rrt

TLLL

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, and nerve-elements are also traceable in the mesoglcea.

Reproductive Organs. — The animal is hermaphrodite, the
organs of both sexes being found in the same individual. The
gonads are developed in the meridional canals (Fig. 160, B), each of
which has an ovary (ovy.) extending along the whole length of one
side, a spermary (spy.) along the whole length of the opposite side.
The organs 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 stomodaeum, and finally escape by the mouth. Im-
pregnation takes place in the
water.

Development. — The pro-
cess 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. 164) consists
of an outer layer of protoplasm
(plsm.) containing the nucleus
(nu.), and of an internal mass
of a frothy or vacuolated nature (yk.) : the vacuoles contain a homo-
geneous substance which serves as a store of nutriment to the growing
embryo, and apparently corresponds with the yolk which we shall
find to occur in a large proportion of animal eggs. Enclosing the
egg is a thin vitelline membrane (v. m.), separated from the proto-
plasm by a considerable space, filled with 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. 165, 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 ((7, D). Next a horizontal division takes
place, dividing off the protoplasmic caps as distinct cells/ and so

Fia. 164. — Ovum of Lampetia. nu. nucleus
plum, protoplasm ; v. m. vitelline membrane
yk. yolk. (After Chun.)

TV

-V

PHYLUM CCELENTERATA

215

producing a sixteen-celled stage (E, F) in which we can distinguish
eight large, ventral, yolk-containing cells or megameres (mg.),
and eight small, dorsal, protoplasmic cells or micromeres (mi.).

FIG. 165. — Segmentation of the oosperm in Ctenophora. mg. megameres ; mi. micromeres
plsm. protoplasm ; yk. yolk. (Modified from Korschelt and Heider.)

The micromeres increase rapidly in number by division, and are
further added to by new small cells being budded off from the
megameres (Fig. 165, G, H, and Fig. 166, A). The result of this

A 3 C

FIG. 166. — Three stages in the development of Ctenophora. ma. megameres ; mi. micromeres.
(From Lang's Comparative Anatomy.)

increase is that the micromeres gradually overspread the megameres
(Fig. 166, C), the final result being the production of an embryo
consisting of a central mass of large yolk-containing cells (ma.),

R B

FIG 167. — Three stages in the development of Callianira. d. infundibulum ; ec. ectoderm ;
en. endoderm ; me. small cells divided off from endoderm ; st. stomodaeum. (From
Lang's Comparative Anatomy.)

partly surrounded by an epithelium -like layer, incomplete below,
of small cells (mi.). This stage corresponds with the gastrula of

216

ZOOLOGY

SECT.

preceding types, the micromeres forming the ectoderm, the mega-
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,
not by a process of invagination or tucking-in, but by one of epiboly
or overgrowth. A gap which is left between the ectoderm cells
at the upper (aboral) pole— the pseudoblastopore — (Fig. 167, A)
soon closes up.

The endoderm-cells increase in number, and become much
elongated and arranged obliquely, their long axes radiating,

upwards and outwards, from the long
axis of the entire embryo (Fig. 167, A).
Their lower (ventral) ends then become
divided off, forming a number of small
cells (A, me.). A kind of invagination
and rotation of the megameres 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
small cells divided off from them. The
latter mainly give rise to the tentacular
canals. At the same time the ectoderm
cells bounding the aperture of the infun-
dibulum grow into it so as to line its
ventral portion : in this way the stomo-
dseum (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 the mesoderm (Fig. 168, me.),
with its gelatinous mesogloaa (#.), makes
its appearance between the ectoderm and
•me, ^0Pr ^T endoderm, its cells being derived from

F\teveio'imienT1oftercaiiieanirae ec^O(^erm ceu<s which migrate inwards,
d. infundibulum; en. endo- chiefly in the neighbourhood of the

derm ; g. mesoglcea ; me.
mesoderm cells, derived from

**»; tfc The later processes of development may
(From Lang's Comparative An- be described very briefly. The canal-
atom?/.) J /.-

system gradually assumes its adult com-
plexity and the swimming-plates appear. A thickening of the ecto-
derm 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, w^hich also gives rise
to the contractile fibres of the mesogloea (mer). The lithites are
formed in the ectoderm cells of the apical pole, but gradually make

iv PHYLUM CGELENTERATA 217

their way on to the free 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 (sk.).

The most noteworthy points in this somewhat complex process
of development 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 microlecithal. In the present instance
the yolk is at first accumulated in the centre of the egg, which is
thus centrolecithal or mid-yolked, but soon the protoplasm accumu-
lates at one end and the yolk at the opposite end of the developing
embryo, producing a telolecithal 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).

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 me-
ridional rows or swimming-plates (costse). 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
generations ; but in some cases development is accompanied by a
well-marked metamorphosis.

218 ZOOLOGY SECT.

The Ctenophora are divisible into five orders as follows : —

ORDER 1. — CYDIPPIDA.

Ctenophora having two tentacles, retractile into sheaths, and
unbranched meridional and stomodseal vessels. The body is either
circular in section or is slightly compressed in the lateral plane
(Figs. 159 and 169).

ORDER 2. — LOB ATA.

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 stomodseal and meridional
vessels unite with one another. The body is compressed in the
lateral plane, and is produced into two large oral lobes or lappets
and into four pointed processes or auricles (Fig. 170).

ORDER 3. — CESTIDA.

Ctenophora having a band-like form, owing to the extreme
compression of the body in the sagittal 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 stomodseal vessels takes
place (Fig. 171).

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. 172).

ORDER 5. — PLATYCTENEA.

Flattened Ctenophora of creeping or sessile habit, with a pair of
retractile lateral tentacles. The costae (swimming-plates), when
present, are short and deeply sunk. There are no meridional
canals; but there is a system of branching peripheral vessels
(Figs. 173 and 174).

Systematic Position of the Example.

Hormiphora plumosa is a species of the genus Hormiphora, be-
longing to the family Pleurobrachiidw 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-
brachiidce are distinguished by the absence of any compression of
the body, the transverse section being circular. The genus Hormi-

TV

PHYLUM CCELENTERATA

219

phora is distinguished by having a rounded body somewhat produced
at the oral pole, and by the aperture of the tentacle-sheath being
on a higher level than the funnel. In the species plumosa the
stomodaeal 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 Crolenterates, the Hydrozoa and
Actinozoa, the organisation of the Ctenophora, if we leave out of account the
Platyctenea, is remarkably uniform. This is due to the fact that nearly all

l.Callianira

2.Euchlora

3. L arrive Ha
FIG. 169.— Three Cydippida. ab. p. aboral process ; mth. mouth. (After Chun.)

the species are pelagic, none are colonial, and none form skeletons. Never-
theless a very great diversity of form is produced in virtue of differences in
proportiors and of 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 body in
the lateral plane in some genera, e.g. Euchlora (Fig. 169, 2), the result being an
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
Cattianira (/), 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.

220

ZOOLOGY

SECT.

The Lobata, for instance Deiopea, are distinguished, as their name implies,
by the presence of a pair of large lappets (Fig. 1 70, A, lp.), into which the oral
surface is produced at either end of the sagittal plane. Four of the swimming
plates are shorter than the others, and at their bases arise elongated processes

FIG. 170. — Deiopea kaloknenota. A, adult ; B, young, aur. auricle ; lp. lappet ; I. t.
lateral tentacles ; mrd. c. meridional canal ; mth. mouth. (After Chun.)

called auricles (aur.}, which bear swimming-plates. The meridional canals
communicate with the stomodseal canals, and from the connecting vessels
curiously coiled vessels (mrd. c.) are given off into the lappets. The principal
tentacles are usually absent in the adult, but are represented by their basal
portions, 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 (It.).
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 Cestida are represented by the remarkable " Venus's Girdle ': (Cestus
veneris), a band-shaped Ctenophore (Fig. 171) which sometimes attains a length
of 1^ metre, or nearly live feet. The body is greatly elongated horizontally in
the sagittal, and compressed in the lateral plane, so as to have the form of a
ribbon, which progresses by undulations of the whole body as well as by the

FIG. 171. — Cestus veneris. A, adult ; B, young. 1. t. lateral tentacles ; mth. mouth ;
s. pi*-., s. pi2, swimming-plates ; t. tentacle. (After Chun.)

action of its swimming-plates. Four of the swimming-plates (s.pl1.) are very
small; the other four (s.pl*.) are continued all along the aboral edge of the
body. The bases of the two principal tentacles (t.) are large and are enclosed
in sheaths, and, as in Lobata, numerous small lateral tentacles (It.) spring
from grooves which, in the present case, are continued the whole length of the

IV

PHYLUM CCELENTERATA

221

oral edge. The young of Cestu& (B) resembles a compressed Cydippid which
undergoes gradual elongation in the median plane.

Beroe, the principal genus of the Beroida, has the form of a cylinder (Fig.
172), one end of which is rounded and bears the sense-organ, the other truncated
and occupied entirely by the immense mouth
(mth.). The greater part of the body is taken
up by the huge stomodseum ; 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 com-
plex network of tubes, and at their oral ends
the four meridional canals of each (right and left)
side and the corresponding 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 Platyctenea are represented by four
genera — Ctenoplana, Ccdoplana, Tjalfidla and
Gastrodes.

Ctenoplana (Fig. 173) is a small marine
animal, nearly circular in outline, flattened
dorso- vent rally, and about 6 mm. in diameter.
It has hitherto been found only twice — once off
the west coast of Sumatra and once among the
islands to the east of Papua. While able to swim
freely, it is able also to creep on its ciliated oral
or ventral surface. When it is swimming the
animal draws downwards the edges of the disc
so that it becomes somewhat helmet-shaped
when viewed laterally. The organs of locomo-
tion are eight small, deeply sunk swimming
plates. In the centre of the aboral or dorsal
surface is a polar body or sense-organ with a
statolith, surrounded by a ring of small ciliated
tentacles which are disposed bilaterally with
reference to the transverse plane, but without
polar-plates (ciliated areas). Nerve-centres in the
form of several ganglia are situated in close relation to the polar body. The
mouth is in the centre of the ventral (oral) surface. There are two pinnate

ctr.

mih

FIG. 172.— Beroe forskalii.
inf. infundibulum ; mth.
mouth ; s.pl. swimming-
plates. (After Chun.)

FIG. 173. — Ctenoplana kowalevskii, dorsal aspect, t, t. tentacles ; tsh. tentacle sheaths ; ctr.
subtransverse costse; ess. sub-sagittal costse ; st. "stomach" (? stomodseum), 1, 2, 3, 4,
the four principal lobes of the infundibulum ; pf. sensory tentacles representing the
polar plates ; pg. pigment spots. (After Wllley.)

222

ZOOLOCJY

SECT.

retractile tentacles'. The canal-system is devoid of meridional canals, but
comprises a set of branching and anastomosing peripheral canals. The male
gonads only have been found : they are situated in diverticula of the canal
system and have been described as having independent ducts opening on the
exterior.

Coeloplana has been found in the Red Sea and on the coast of Japan. It
is also flattened dorso-ventrally, and further resembles Ctenoplana in its
ventral mouth, dorsal polar sense-organ with ganglia, paired retractile ten-
tacles, and anastomosing canals. There are, however, no swimming-plates,
and progression is effected only by creeping.

Nothing is known of the development of either of these two genera.

Tjalfidla (Fig. 174) was found in dredgings from a depth of 500 metres
off West Greenland. Instead of being free and pelagic like the Ctenophora
in general, or able to creep about like Co&loplana and Ctenoplana, Tjaljiella

e. *^JL.

FIG. 174. — Tjalfiella. Adult specimen with many embryos, side view ; oral (attached)
surface upwards, br. c. branching canals ; e., g., i. embryos ; I. tentacle ; t. b. tentacle-
base. (After Mortensen.)

is sessile in the adult condition, living attached to the stalk or polypes of an
Umbellula (Pennatulacea, p. 194). The body is elongated in the transverse
plane : shortened in the direction of the primary axis. The attachment is
effected by the oral surface by the agency of the inner surfaces of a pair of
chimney-like hollow processes, each with a wide distal opening and a ciliated
cavity. From the distal opening of each of these processes the corresponding
retractile tentacle (t.) is capable of being protruded. The slit-like mouth,
elongated, as in other Ctenophora, in the sagittal plane, opens out of a super-
ficial sub-oral cavity with folded walls with which the cavities of the chimney-
like processes are in free communication at their bases — the latter acting as
secondary mouths. There are no meridional or stomodseal canals, and in the
adult no swimming-plates : the polar (apical) sense-organ is reduced. There
are four pairs of reproductive organs, each comprising an ovary and testis
arranged opposite one another as in Hormiphora and the Ctenophora in general,

iv PHYLUM OCELENTERATA 223

but, in the absence of meridional canals, situated in rounded outgrowths of
the per-radial or transverse canal. The inter-radial canals give off lateral
branching, but not anastomosing, canals.

Tjalfiella differs from all the rest of the Ctenophora in being viviparous.
The young, escaping by rupture of the body-wall of the parent from the
brood-pouches (apparently in the canal-system) in which they are developed,
are provided each with eight swimming-plates and pursue a free existence for
a time, subsequently becoming attached and losing the swimming-plates.

Gastrodes, a parasite of Salpa (Vol. II.), is probably related to Ctenoplana
and Cceloplana.

[Bathyctena, also a deep-sea form, with some resemblances to Tjalfiella,
is more nearly related to the normal Cydippida.]

The Platyctenea are of interest owing to the possibility of their representing
an intermediate stage between Ccelenterates (through ordinary Ctenophores)
and Platyhelminthes (see Section V.).

The Ctenophora are usually perfectly transparent, and quite
colourless, save for delicate tints of red, brown, or yellow in the
tentacles and stomodseal ridges. Cestus has, however, a delicate
violet hue, and when irritated shows a beautiful blue or bluish-
green fluorescence. Beroe 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 nature, there is
no trace of the group in the fossil state.

A very remarkable fact has been made out with regard to Bolina
liydatina, one of the Lobata, a Ctenophore which attains a diameter
of 25-40 mm. While still in the larval or cydippid condition 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 im-
pregnated and develop in the usual manner. Soon the gonads de-
generate, 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 (p. 42).

THE RELATIONSHIPS OF THE CCELENTERATA.

There can be little doubt that the lowest ccelenterate 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 stomodseum (if a true stomodseum be indeed repre-
sented) 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 Trachylinse,

224 ZOOLOGY SECT.

or hydroid medusae with no fixed zoophyte stage, as being derived
from a pelagic hydrula.

The Leptolina3 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 establishment of a division of labour, in virtue of which a
certain proportion only of the zooids became medusae, 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. 153). 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,
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 Semostomae, 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 Hexactiniae.

The relationships of the Ctenophora to the other Ccelenterata are
very doubtful. Ctenaria, one of the Anthomedusae (Fig. 110, 7),
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

rv

PHYLUM CCELENTERATA

225

stomach, and corresponding with them are eight bands of nemato-
cysts 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-
dseum 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.

In the case
of Hydroctena,
found in Malay an
seas, the Cteno-
phoran resem-
blances, if only
superficial, are
much more strik-
ing. Hydroctena
(Fig. 175) is bell- F.G
like, and provided
with a velum (F.).

G. 175. — Hydroctena salenskii. ab. aboral sense-organ ;
M. manubrium ; t. tentacle ; V. velum. (From the Cambridge
Natural History, after Dawydoff.)

At its apex is an ampulla (ab.) 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 ends blindly, and from the sides a pair of 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 forth-
coming goes, there is not sufficient evidence to establish Cteno-
phoran affinities.

On the other hand, the resemblance between transverse sections
of an embryo Ctenophore (Fig. 176, B) and of an embryo Actinian
(A) is very striking, and the presence of a well-developed stomo-
daaum, and of gonads developed in connection with the endoderm
and discharging their products through the mouth, may be taken

VOL T.

226

ZOOLOGY

SECT.

end

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 is 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 Beroida 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
ten t ac ul a te
forms. Whether
the Platyctenea
are primitive or
specially modi-
fied r emai ns
doubtful ; but
the latter, in
view of the
larval develop-
ment of Tjalfiella, 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
central cavity into which a wide opening — the mouth or the
osculum, as the case may be — leads ; the absence of a well-developed
mesoderm, the fixed mode of life, and associated with it the ten-
dency to form compound structures 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 con-
stitute an important connecting link. But a closer examination
of the subject shows that some of these apparent points of re-
semblance are superficial only, and establishes a number of differences
between Sponges and Ccelenterates 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 the larva,
to correspond in no sense with the mouth of the Ccelenterate.
This alone, apart from important differences in the adult structure,

FIG. 176. — Transverse section of embryos of Actinia (A) and
Beroe (B). ect. ectoderm ; end. endoderm ; inf. infundibulum.
(After Chun.)

rv

PHYLUM CCELENTERATA

227

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 Coalenterata, and place them in a phylum
apart. But not only is the grouping of Sponges and Ccelenterates
in one phylum thus rendered impossible by important differences
in their structure and development ; a comparison of the mode
of formation of the embryonic layers in the two groups shows such

Hexactinia Madreporaria

Rhizostomerte Semostomtb
Beroida

Edwardsia

Coronatcfe

Cufciomedusce

ACTINULA

Lucernarida

Hydrocorallincie
Leptolincfe

SCYPHULA

HYDRULA
FIG. 177. — Diagram illustrating the mutual relationships of the Coolenterata.

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.

APPENDIX TO THE CCELENTEKATA.

THE MESOZOA.

Under the designation MESOZOA have been comprised certain lowly organ-
ised 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 " endoderm " either composed of a
single elongated cell or of several cells : a mesoglcea is not represented. The
Mesozoa comprise at least three families, the Dicyemidce, the Heterocyemidce,
and the Orthonectidce, all the members of which are internal parasites.

The Dicyemidce are parasites in the kidneys of various Cuttle-fishes and
Octopods (Cephalopoda). Dicyema (Fig. 178), the length of which is between
0-75 and 6 or 7 millimetres, consists of a head-part or calotte, and an elongated

Q 2

228

ZOOLOGY

SECT.

body. The form of the calotte varies a good deal, according to age ; in young
specimens it is isotropic (i.e., symmetrical around the long axis) ; in the adult
condition ventral and dorsal sides are distinguishable. It consists of a swollen
disc of four cells and a ring of four or five pole cells. The cells of the head all
bear cilia, which are shorter and thicker than those of the body-cells.

The body consists of a single large axial cell, and of a single layer of outer
cells which completely invest the axial cell. The outer cells which follow
immediately on the head are distinguishable from the rest by their granular
contents, and by their being dilated internally in such a way that the apex of
the axial cell is constricted.

The axial cell is either almost completely cylindrical or spindle-shaped, and
is covered in its entire extent by the outer cells. It presents a differentiated

A't;

VJ

a. 178. — Dicyema paradoxum,

with infusoriform embryos (males).
(From Bronn's Thierreich, after
Kolliker.)

FIG. 179. — Dicyema paradoxum,
with vermiform embryos. (From
Bronn's Thierreich, after Kolliker.)

cortical layer, beneath which the finely granular gelatinous contents are at
first homogeneous, but afterwards become vacuolated. In the middle of the
cell is a large oval or ellipsoidal nucleus.

The life-history is a true alternation of generations. The primitive nucleus
of the axial cell divides mitotically to form a smaller asexual germ-nucleus and
a larger nucleus — the definitive nucleus (somatic nucleus) of the axial cell.
Further germ-nuclei result from subsequent divisions. The germ-cells undergo
a process similar to segmentation. Of the cells thus formed one gives rise to
the axial cell of the embryo : the others, increasing in numbers and becoming

v PHYLUM CCELENTERATA 22J

smaller, gradually grow over the axial cell until they at length completely
enclose it. The embryo increases greatly in length, and escapes from the
interior of the parent, which it completely resembles, by perforating the body-
wall. This phase of the parent animal (Fig. 179) is the phase to which the term
nematogene is applied : the asexually developed young are the so-called vermi-
form embryo*. The latter swim about for a time in the fluid of the kidney of
the host ; afterwards they attach themselves by means of a head to certain
appendages — the venous appendages — of the walls of the cavity. A number of
generations of these asexually developed forms succeed one another until,
when the venous appendages of the kidney have become thickly infested with
the parasites, a change takes place, and a sexual process of multiplication
is initiated. In the interior of the nematogene a change is observable, and
female sexual individuals are formed instead of vermiform embryos. Unlike
the latter the former do not leave the body of the parent ; they also differ
in the non-development of an enclosing layer, the cells representing the
latter being converted into ova. In their development, as in that of
the vermiform embryos, the nucleus of the axial cell of the female divides
into two. One of these becomes the permanent somatic nucleus of the axial
cell : the other gives rise to the nucleus of the primitive ovum, and surrounds
itself with protoplasm. This divides to give rise to a number of ova, which
become more numerous till they come to fill the axial cell. Then the first
generation of ova is discharged into the protoplasm of the parent axial
cell : this first generation of ova is derived from the cells representing the
outer layer. Later, further generations of ova, which are the descendants of
the primitive ova, make their escape. These all eventually
wander away into the protoplasm of the axial cell of the
parent, increase in size, undergo a process of maturation,
and become fertilised. Fertilisation is effected by means of
typical tailed sperms developed in a second set of sexual
individuals, the males (Fig. 180), which were formerly
known as the infusorijorm embryos. In its mature form
the male is approximately pear-shaped, the narrower end
being posterior. Several axial cells are present : these
form the testes, in which the sperms are developed. They Dicyeina para-
are surrounded by the outer cells, which at the posterior doxum. (From
end take the form of a flat ciliated epithelium. The com- Ster^olS?^'
plete development of the sperms only takes place when
the young male leaves the host in which it was formed and seeks a new one ;
thus it is only by the sperms of a male from another host that the ova can be
fertilised. The males are developed from the fertilised ova and subsequently
escape, their development being similar to that of the vermiform embryos :
the phase of the parent form to which they are developed is that known as the
rJiombogene. After a number of generations of males have been formed in
this way, the rhombogene undergoes modification, and the last generation of
fertilised ova gives rise, not to males, but to vermiform embryos — i.e., to an
asexual generation — and with these the cycle beeins anew.

The Heterocyemidce, which are also parasites of the Cephalopoda, resemble
the Dicyemidse in most respects, but the head is wanting.

The family Orthonectidce comprises only two genera — Rhopalura and
Stcecharlhrum — which live as parasites in a Polyclad (Leptoplana), a Nemertine
(Linen*), an Annelid, and a Brittle -Star (Amphiura). In the stage that
represents the asexual form of the Dicyemidae the Orthonectid assumes the
character of a plasmodium, or mass of finely granular protoplasm containing
many nuclei, and is capable of active amoeboid movements. In the interior
of the plasmodia the sexual forms are developed. A nucleus of the plasmodium
surrounds itself with protoplasm and gives rise to a germ-cell, which by a
process of segmentation develops into the sexual stage. In some cases only
males are developed in one plasmodium and females in another : in others

230

ZOOLOGY

SECT.

both sexes are formed together in the same plasmodium. In some forms the
sexes are united. The sexual animals, especially the females (Fig. 182),
bear a considerable resemblance to the Dicyemidse, but instead of the axial cell
there are a number of cells, the ova or sperm-cells. The outer cells are
arranged in segments or rings. In front is usually a region composed of a few
rings in which the outer cells bear cilia which are directed forwards : then
comes a shorter region devoid of cilia, and behind that is the longest region,
having cilia directed backwards. In shape the body is usually spindle-like —
the males (Fig. 181) differing somewhat from the females. In about the
middle of the internal space of the male is the compact oval testis containing
small tailed sperms. Beneath the outer layer in the male, but not in the female,
is a layer of fibres sometimes regarded as muscular. The plasmodia multiply

FIG. 181.— Rhopalura giardii, male.
(From Brona's Thierreich, after Julin.)

Fio. 182.— Rhopalura giardi, female.
(From Bronn's Thierreich, after Julin.)

by fragmentation. The development of the embryos either goes on in the
intact plasmodium, or the latter breaks up and the embryos are to be found at
various stages free in the host.

In the development of a male from the germ-cell the first segmentation is
unequal. The further segmentation results in the formation of a solid morula.
The outer cells become differentiated into two distinct groups, the one giving
rise to the external layer of the anterior region, the other to that of the
posterior region of the body. The inner cells multiply and give rise to the
numerous small spermatocytes of the testis. The formation of the layer of
hbres only takes place later.

IV

PHYLUM CGELENTERATA

231

In the case of the female the segmentation appears to be equal from the
firsfc, and results in the formation of a blastula-like stage, which becomes
converted into a solid morula-like body by the passing inwards of a number of
cells. As in the male, the central cells multiply to form the sexual cells, and
the outer cells form the external layer with its segments. In all probability,
though this has not been actually proved, the mature sexual animals become
free from the plasmodia, and the females, after fertilisation, find their way to
another host where they are transformed into plasmodia, the germ-cells of
which are the fertilised ova.

FIQ. 183.— Saline 11 a, longitudinal section. (After Frenzel.)

To be mentioned also in connection with the Dicyemidae and Orthonectidse,
as perhaps allied with them, are the remarkable parasites Am&bophrya and
Lohmanella — the former living in certain Radiolarians (Protozoa), the latter in
the body-cavity of a F'ritillaria (Urochorda). These both resemble the groups
described above, and differ from the other Metazoa, in the presence of only a
single body-layer. This remarkable simplicity of body-structure occurs also
in Salinella, though too little is known with
regard to this animal to provide adequate data
for determining its affinities with certainty.

Salinella (Figs. 183 and 184), which has
only been found in artificial saline solutions,
is a minute animal in the form of a somewhat
depressed cylinder, open at both ends, and
with a wall composed of a single layer of
cells. The anterior end is somewhat pointed ;
around the anterior opening or mouth, which
is ventrally directed, is a circlet of from fifteen
to twenty long whip-like cilia. The posterior

aperture (anus), which is usually closed, is surrounded by a few stiff setae.
The ventral surface is flattened, and is covered with fine vibratile cilia, while
on the dorsal surface and the sides are regularly arranged rows of straight
" setae " (non-motile cilia). The internal cavity (enteron) is found to contain
sand, plant-fragments, and Bacteria ; its surface is beset with long cilia.
Multiplication is said to take place by transverse fission ; and a process of
conjugation followed by encystation has also been observed.

Trichoplax and Treptoplax, which have been supposed to be Mesozoa,
appear to be merely special modifications of developmental phases of Hydrozoa.

FIQ. 184. — Salinella, transverse
section. (After Frenzel.)

SECTION V

PHYLUM PLATYHELMINTHES

A NUMBER of classes of Metazoa, some a little, others very
decidedly, higher in organisation than the Ccelenterata, were
formerly regarded as constituting one great sub-kingdom or phylum
— the Vermes or Worms. The groups ordinarily referred to the
Vermes differ, however, very widely from one another : points of
agreement, except such as are merely negative, are, in fact, fre-
quently hardly recognisable : and rather than group together under
one common designation such a heterogeneous assemblage of forms,
it is usually considered to be more expedient to avoid the term
Vermes altogether, and to endeavour to divide the " Worms " into
phyla the members of which shall have points of positive resem-
blance to one another. The five phyla PlatyJielminihes, Nemathel-
minthes, Trochelminthes, Molluscoida, and Annulaia, with their
appendices, all consist of forms which are or have been comprised
in the Vermes. " They differ from the Ccelenterata in the presence
of three well-developed body-layers — of which the middle one, or
mesoderm, is of relatively predominant importance ; and for the
most part, in the much higher stage of complexity attained by the
various systems of organs. The first four phyla present no meta-
meric segmentation (p. 43) : in the Annulata, metamerism is more
or less strongly pronounced.

The Platyhelminthes or Flat- Worms are a group of soft-bodied,
bilateral, usually flattened animals, which are devoid of true
metameric segmentation. With a sufficient degree of uni-
formity of structure to render the phylum a fairly compact and
well-defined one, there is yet a considerable range in complexity,
from the simplest forms — certain of which have been supposed to be
nearly connected with the Ctenophora among the Coelenterata — to
the highest, which have all the various systems of organs very
much more highly developed. The body is built up from
three embryonic layers — ectoderm, mesoderm, and endoderm — as in
all higher groups of animals. An excretory vascular system of
a peculiar kind — the water-vascular or protonephridial system — is

232

SECT, v PHYLUM PLATYHELMINTHES 233

S

present in nearly all members of the phylum. A body-cavity (see
following Sections) is not present, the spaces between the various
organs and the wall of the body being filled up with a peculiar
form of connective-tissue termed the 'parenchyma. The egg is in
most instances composite, the egg-shell enclosing not only the
oosperm or impregnated ovum, but a quantity of nutrient material
or food-yolk, derived, in most instances, from a special set of glands,
the yolk or vitelline glands.

The main features which distinguish the Platyhelminthes from
the Ccelenterata are the pronounced bilateral symmetry with
the many secondary features which it involves, the presence
of a middle embryonic layer or mesoderm, and the non-occurrence
of fixed colonies formed by budding.

1. EXAMPLES OF THE PHYLUM.

i. A Fresh- water Triclad (Planaria or Dendrocaelum).1

General Features. — Species of fresh- water Planarians of the
genera Planaria and Dendroccelum are common in the mud at
the bottom of ponds of fresh water in all quarters of the globe.
They are small, thin, flattened worms a few millimetres in length,
broader at one end, the anterior, than at the other, the posterior,
which is more or less pointed. The animal (Figs. 185-187) is
very readily recognised to be bilaterally symmetrical, with an upper
or dorsal and a lower or ventral surface, right and left borders,
and anterior and posterior ends. The colour varies in different
species and in different individuals, but is usually gray, red,
brown, or black. Movements of locomotion in the direction of the
long axis of the body are recognisable in the living animal. Some-
times this is a steady gliding movement, which is brought about
by the action of a coating of vibratile cilia on the surface ; some-
times the worm moves along somewhat after the fashion of a
Leech, the ventral surface of the anterior end of the body being
of a sticky adhesive character, and performing the part of the
anterior sucker of the Leech.

Close to the anterior extremity on the dorsal surface are two
rounded black spots, the eyes (Fig. 186). On the ventral surface,
a considerable distance behind the middle of the body, is the
opening of the mouth (Fig. 185, mo.), and further back still, near
the posterior pointed end, is a smaller median opening, the genital
aperture (Fig. 187).

Dige^tive_Systej3a. — The mouth (Figs. 185, 186, mo.) leads through
a short mouth-cavity into a cylindrical thick-walled chamber, the
pharynx (ph.), which is highly mobile, and is capable of being

1 The account is sufficiently general to apply to species of either of these
genera.

234

ZOOLOGY

SECT,

thrust out as a proboscis through the mouth, beyond which it
may then be extended to a relatively considerable distance.
When retracted it lies within an enclosing muscular sheath. The

FIG. 185.— Planaria. Digestive and ex-
cretory systems, ex. openings of excre-
tory system ; int. intestine ; mo. mouth ;
o.ph. opening of pharynx. (After Jijima
and Hatschek.)

FIQ. 186. — Flanaria. Nervous system.
br, brain ; eye, eye ; I. ne. longitu-
dinal nerve ; ph. pharynx. (After
Jijima and Hatschek.)

cavity of the pharynx opens in front into the intestine (int.), which
almost immediately divides into three narrow main branches, one
running forward in the middle line, the other two running back-

v PHYLUM PLATYHELMINTHES 235

wards. Each of these three main branches gives off numerous
smaller branches, which in turn become branched, so that the
whole intestine forms a ramifying system, extending throughout
the greater part of the body ; all the branches terminate blindly,
an anal aperture being absent.

A system of vessels — the water-vessels or vessels of the excre-
tory system (ex.) — sends ramifications through all parts of the body.
There are two main, considerably coiled, pairs of longitudinal
trunks, right and left, which open externally on the dorsal surface
by means of several pairs of minute pores ; in front they are
connected together by a transverse vessel. ' The vessels of each pair
often join and separate again.- Each main trunk gives origin to a
number of branches, which in turn give off a system of extremely
fine capillary vessels, many of which terminate in flame-cells (Fig
217, p. 264). A flame-cell is a nucleated cell having in its proto-
plasm a small space into which one of the capillaries leads ; in this
space lies a bundle of vibratile cilia, or a single thick cilium, which
performs regular undulating movements, giving it somewhat the
appearance of a flickering candle-flame. Cilia are said also to
occur in the course of some of the capillaries, though this is
doubtful. This system of vessels is usually regarded as excretory ;
but it may also have a respiratory function.

A well-developed nervous system (Fig. 186) is present. At
the anterior end is a central knot of nerve-matter, the brain (br),
from which proceed backwards two longitudinal nerve-cords (I. ne.).
The brain consists partly of transverse fibres connecting together
the two longitudinal nerve-cords, partly of groups of nerve-
cells situated at the ends, or in the course of, the nerve-fibres.
The nerve-cords give off both internally and externally numerous
transverse branches, which divide into finer twigs ; the internal
branches of the two cords frequently anastomose, thus forming
commissures or connecting nerve-strands between the two. A
number of nerves extend forwards to the anterior margin, which
is highly sensitive.

Reproductive System. — The reproductive organs (Fig. 187)
are hermaphrodite or monoecious in their arrangement, both male
and female organs occurring in the same individual. The genital
aperture leads into a small chamber, the genital atrium or cloacat
which is common to both the male and the female reproductive
systems.

The male part of the apparatus consists of testes, vasa deferentia,
and penis. The testes (tes.) are numerous rounded bodies,
situated near the right and left borders. Two ducts, the right
and left vasa deferentia\(v.d.), run backwards from the neighbourhood
of the testes and unite in the middle line posteriorly. The median
duct formed by the union of the two vasa deferentia traverses a
protrusible muscular organ, the penis (p), to open into the genital

236

ZOOLOGY

SECT.

vil

cloaca. At the base of the penis, where the vasa deferentia meet,
the median canal is slightly enlarged to form a rounded dilatation,

the vesicula seminalis. Into the
median canal open the narrow ducts
of a number of unicellular glands,
the prostate glands (pr.).

The female part of the reproduc-
tive apparatus consists of ovaries
(germaria), oviducts, vitelline glands,
uterus, and muscular sac. The ger-
maria (ov.) are two in number —
small rounded bodies situated near
the anterior end, each connected
with an elongated duct, the oviduct.
The two oviducts (od.) unite pos-
teriorly to form a short median
common oviduct opening into the
genital atrinm. With this cavity are
connected also the uterus (ut.), a
median rounded chamber, and a
thick- walled muscular body, the
muscular sac (m.). Numerous branch-
ing tubes — the vitelline glands (vit.)
— open into the oviducts.

Reproduction is entirely sexual.
The oosperm is enclosed within a
protecting case or shell, which con-
tains also a quantity of food-yolk
derived from the vitelline glands.
When the larva has reached a cer-
tain stage it develops a temporary
larval mouth and gullet (see Fig.
222), and swallows the food-yolk,
by the aid of which it grows
rapidly. The larval mouth dis-
appears, and a new one — the per-
manent mouth — is developed in its
place. When the embryo leaves the
shell, it has assumed the character-
istic shape of the parent.

ii. The Liver-Fluke (Fasciola
hepatica).

General Features. — The Liver-
Fluke of the Sheep, which is to
be found in the interior of the larger bile-ducts of the infested
animal, is a soft-bodied worm of flattened leaf-like shape

vit

FIG. 187. — Flanaria. Reproductive
system, m. muscular sac ; ov.
germarium ; p. penis ; ph. pharynx ;
pr. prostate ; tes. testes ; ut. uterus ;
v . d. vas deferens : vit. vitelline
glands. (After Jijima and Hat-
schek.)

PHYLUM PLATYHELMINTHES1

237

od

FIG. 188. — Transverse section of aJPlanarian. circ. mus. circular muscular fibres ; ccec. in
testinal caeca ; dors. vent. mus. dorso-ventral muscular fibres ; eel. ectoderm ; ext . long. mus.
external layer of longitudinal muscular fibres ; int. central lumen of the intestine ; int.
long. mus. internal layer of longitudinal muscular fibres ; ne. nerve-cords ; o. d. oviducts ;
par. parenchyma ; test, testes ; v. def. vas deferens ; vit. vitelline glands. (From
Hatschek's Lehrbuch.)

(Fig. 189), with a triangular process, the head-lobe, projecting

from the broader end. The symmetry of the parts is distinctly

bilateral, as in the Planarian. Externally

the body is quite equilateral, the right and

left portions exactly balancing one another,

but, as will appear subsequently, this complete

symmetry does not extend to all the internal

organs.

The surface is devoid of vibratile cilia, but
is covered with innumerable minute spinules
or papillce, which are prolongations of the
homogeneous external layer or cuticle investing
the whole animal. At the extreme anterior
end of the triangular head-lobe is the small ria 189._ r a s c i o i a
opening of the mouth (mo.) surrounded by a
muscular oral sucker. A short distance back mouth; repr.
on the ventral surface, just behind the head- Sor^sucS. ; sckr' pos
lobe, is a second much larger posterior sucker
(sckr.). Between the two suckers, but rather nearer the pbsterior
one, is a median aperture, thegenitctl opening (repr.), through which a
curved muscular process, the cirrus or penis, may be protruded. In
the middle of the posterior end of the body is a minute opening,
the excretory pore (excr.).

Body-wall.— The body-wall (Fig. 190) is found on section to
comprise three layers : — (1) a homogeneous cuticle (cut.) of which
the spinules (sp.) are special developments ; (2) a layer of circularly
disposed muscular fibres (circ. mus.) : (3) a layer of longitudinal
muscular fibres (long. mus.). A cellular epidermis is wanting.
Beneath the muscles are numerous unicellular glands (gl.), the
ducts of which, in the form of processes of the cells, open on the

23R ZOOLOC4Y SECT.

outer surface. Internally, the interspaces between the organs
are filled by a peculiar form , of connective-tissue, the paren-
chyma.

Digestive System. — The • mouth (Fig. 191) leads to a small
rounded bulb-like body, the pharynx (ph.), with thick muscular
walls and a small cavity. From this a short passage, the oesophagus,
opens into the intestine. The latter (int.) is frequently a very

conspicuous structure, ow-
ing to its being filled^with
the dark biliary matter
mixed with blood on which
the Fluke feeds. It divides
almost immediately into
two main limbs, right and
left, and from each of these
are given off, both inter-
nally and externally, a
number of blind branches
or cceca, those on the inner

FIG. 190.— Fasciola hepatica. Section of the integu- side being short and Simple,

ment. circ. mus. layer of circular muscular fibres; , 1-1 ,1 ,1

cut. cuticle ; gl. unicellular glands ; long. mm. layer Willie tJlOSC On tne Ollter

of longitudinal muscular fibres ; sp. spinules. (After oirl« or/i Irmmvr anr\

Braun.) blue tiie

branched. The two limbs

of the intestine with their branches thus form, as in the Planarian,
a complicated system, the ramifications of which extend throughout
the whole of the body. There is no aperture of communication
between the intestine and the exterior, the only external opening
of the alimentary system being through the mouth.

A branching system of vessels — the water-vessels or vessels of
the excretory system— raim'fy throughout the body. A longi-
tudinal main trunk opens behind by means of the excretory pore
already mentioned as occurring at the posterior end. In front it
gives off four large trunks, each of which branches repeatedly, the
branches giving off smaller vessels, and these again still smaller
twigs, until we reach a system of extremely fine microscopic vessels
or capillaries. Each of these ends internally in a slight enlarge-
ment situated in the interior of a large cell, an excretory cell or
flame-cell, similar to a flame-cell of the Planarian.

The Liver-Fluke has a well differentiated nervous system,
which shares in the prevailing bilateral arrangement of the parts.
The central part of this system consists of a ring of nerve-matter
which surrounds the oesophagus and presents two lateral thicken-
ings, or ganglia, containing nerve-cells, and a single ganglion in the
middle line below. From this are given off a number of nerves,
of which the chief are a pair of lateral cords running back to the
posterior end and giving off numerous branches. There are no
organs of special sense.

v PHYLUM PLATYHELMINTHES 239

The reproductive organs (Fig. 191) are constructed on the
hermaphrodite plan, i.e. both male and female organs occur in
the same individual. The male part of the apparatus consists of
testes, vasa deferentia, and cirrus. The testes (te.) are two greatly
ramified tubes, which occupy the middle part of the body, one
situated behind the other. From each testis there runs forwards

^^ ^ ~f

111 ^$1 ;M> 0 i__.. v.s

od-4

FIG. 191. — Fasciola hepatica. Internal organisation. General view of the anterior portion
of the body, showing the various systems of organs as seen from the ventral aspect, ej.
ejaculatory duct ; /. female reproductive aperture ; int. anterior portion of the intestine
(the rest is not shown) ; od. commencement of oviduct ; ov. ovary (germarium) ; p. cirrus ;
ph. pharynx ; sh. shell-gland ; te. testes ; ut. uterus ; vdi. left vas deferens ; vd2. right vas
deferens ; vit. lobes of vitelline glands ; vs. vesicula seminalis. (After Sommer.)

a duct, the vas deferens, the two vasa deferentia (v.d.) opening
anteriorly into an elongated sac, the vesicula seminalis (v.s.), from
which a narrow tube— the ejaculatory duct (Fig. 192, ej.)— leads
to the male aperture at the extremity of the cirrus. The female
part of the reproductive apparatus consists of a single ovary
(germarium), an oviduct, a uterus, an ootype, vitelline glands, vitelline
ducts, and " shell-glands." The germarium (Fig. 191, ov.) is a

240

ZOOLOGY

SECT.

V

v.d

FIG. i92.-Fascioia hepatica. Ter-

minal part[of the reproductive appa-
ratus. <y. ejaculatory duct;/, female
aperture ; g. unicellular glands ; od.

branched tube situated on the right-hand side in front of the
testes ; the branches open into a common narrow tube, the
oviduct (od.). The vitelline glands (vit.) consist of very numerous,
minute, rounded follicles, which occupy a considerable zone in

the lateral regions of the body. On
each side are two large ducts,
anterior and posterior, uniting to
form a single main lateral duct,
right or left ; and these run nearly
transversely inwards to open into a
small sac, the yolk reservoir. From
this a single median vitelline duct
runs forwards for a short distance to
join the oviduct. Around the junc-
tion are grouped a mass of unicellular
shell-glands or accessory female glands
(sh. gl.), each of which is produced
into a narrow process or duct opening
into the end of the oviduct in the
region of the latter to which the term

ootuve is applied. The UterUS (ut.)

. y^ . ,

IS a Wide Convoluted tube, formed

by ?» imion of tke oviduct and
median vitelline duct ; in front it
opens close to the base of the cirrus.
When the cirrus is withdrawn, a small cavity, the genital atrium or
cloaca, is formed common to the external apertures of both male
and female ducts. A canal, termed the canal of Laurer, leads
from the junction of the oviduct and median vitelline duct to
open externally on the dorsal surface.

Development. — Each ovum on impregnation is surrounded
by a mass of vitelline matter or yolk derived from the yolk-
glands. It then becomes enclosed, while passing through the
ootype, in a chitinous shell, the substance of which is usually said
to be derived from the shell-glands.1 The completed egg remain?
for a little time in the uterus ; eventually it is discharged, and,
passing down the bile-ducts of the Sheep into the intestine,
reaches the exterior with the faeces. Active development only
begins at this stage, and, three to six weeks later, a portion of
the egg-shell at one end becomes separated off as a sort of
lid or operculum, and gives exit to the contained embryo. This,
the ciliated embryo or miracidium (Fig. 193, A), is a somewhat
conical body covered all over with vibratile cilia, and with two
spots of pigment, the eye-spots (eye), near the broader or anterior

1 The agency of the shell-glands in producing the material of the egg-shell
is not universally acknowledged. This point will be referred to in the section
dealing with the general organisation.

sucker ; v. d. vasa deferentia ; v. g.
vesicula seminalis. (After Sommer.)

PHYLUM PLATYHELMINTHES

241

end, which is provided with a triangular head-lobe (pap.). There
is an imperfectly developed intestine, and a pair of flame-cells,
each with a fine canal opening on the surface. The rest of
the interior is rilled with a mass of germ-cells. The ciliated
larva swims about in water, or moves over damp herbage
for a time, and perishes unless it happens to reach a water-snail,
(species of Lymnceus, or of Bulinus or Planorbis), as a parasite of
which it is alone able to enter upon the next phase in its life-history.
When it meets with the Snail, the embryo bores into it by means of

^

FIG. 193. — A — D. Development of Fasciola hepatica. A, ciliated
larva ; B, sporocyst, containing redise in various stages of develop-
ment ; C, redia, containing a daughter redia, and cercarise ; D, fully
developed cercaria. b. op. birth opening ; ent. enteron of redia ; eye,
eye-spots ; gast. gastrula stage of redia ; germ, early stages in the
formation of cercariae ; int. intestine of cercaria ; mor. morula stage
in the development of cercariae ; oes. oesophagus ; or. su. oral sucker ;
pap. head-papilla of ciliated embryo ; ph. pharynx ; proc. processes
of redia ; vent. su. ventral sucker. (After Thomas.)

the head-lobe, coming to rest in the pulmonary sac or some other
organ of the mollusc. Established in the interior of the Snail, it
loses its ectoderm and grows rapidly into the form of an elongated
sac, the sporocyst (Fig. 193, B), with an internal cavity containing
germ-cells and lined by a layer of cells, with remnants of the eye-
spots, and with flame-cells. The sporocyst may divide into two
similar bodies by a process of transverse fission, but this is excep-
tional. Eventually cells are budded off from the layer that lines the
internal cavity of the sporocyst or from the germ-cells, and these
VOL. i. R

242

ZOOLOGY

SECT.

undergo a process of segmentation similar to the holoblastic
segmentation of the impregnated ovum, resulting in the
formation of a morula, which becomes converted into a stage
resembling a gastrula. The gastrula elongates and gives rise

to a body called a redia
(0), which begins to move
about, and, eventually
forcing its way out of
the interior of the sporo-
cyst, finds its way to
some other part of the
Snail, usually the liver.
When fully formed, the
redia is a cylindrical
body with a pair of short
processes (proc.) near the
posterior end, and with a
circular ridge near the
anterior end. It possesses
a mouth leading to a
pharynx and simple sac-
like intestine, and there
is a system of excretory
vessels. In the interior
of the redia cells are
budded off and develop
into gastrulae, exactly as
in the case of the sporo-
cyst ; these gastrulse
either develop into a fresh
generation of rediae if the
season should be winter,
or, if it should be sum-
mer, give rise to bodies
termed cercarice. The
latter (D) are provided
with long tails : they
have anterior and pos-
terior suckers, and a
mouth and pharynx,

Fid 194.— Taenia solium. Entire specimen, followed by a bifid intes-

reduced ; cap. head. (After Leuckart.) ^^ An Opening) t^e

birth-opening (C, b. op.), is formed in the wall of the redia
near the circular ridge, and through this the cercarise escape ;
they move actively by means of their tails, and force their
way out of the body of the Snail. They then, losing the tail,
become encysted, attached to blades of grass or leaves of other

PHYLUM PLATYHELMINTHES

243

herbage. The transference of the larval Fluke in this stage to its
final host, the Sheep, is effected if the latter swallow the grass on
which the cercaria has become encysted. The young Fluke then
escapes from the cyst and forces its way .up the bile-ducts to the
liver, in which it rapidly grows, and, developing reproductive
organs, attains the adult condition.

iii. The Common Tape- Worm of Man (Tcenia solium).

General Features. — Tcenia solium occurs as a parasite in the
intestine of man. It has the form of a narrow ribbon (Fig. 194),
which may attain a length of several yards, attached at one end to
the wall of the intestine, the remainder hanging freely in the
interior. Towards the attached end the ribbon becomes very much
narrower than it is towards the opposite end ; and at this narrower
extremity is a small, rounded, terminal knob, which is known as the
head or scolex l ; the rest of the animal is termed the body or
strobila ; the narrow part immediately behind the head is some-
times called the neck. The attachment of the Tape- worm to the
wall of the intestine is slight and temporary ; it is effected by
certain organs of adhesion, the hooks and suckers on the head.

The head (Fig. 195) may be roughly described as pear-shaped,
but becomes four-sided at the broader end. In the middle of rhis
broader, anterior end is a rounded prominence,
the rostellum, round the base of which there is
a double row of usually about twenty-eight
curved and pointed chitinous hooks. The
rostellum is capable of being protruded and
retracted to a slight extent, and the position
of the hooks varies accordingly : when the
rostellum is fully retracted the points of the
hooks are directed forwards, and may even
meet in the centre ; as the rostellum is pro-
truded the hooks become rotated until their
apices come to be directed backwards. Four
cup-shaped suckers project slightly from the
surface behind the circlet of hooks.

The body or strobila has a jointed appear- FIG. 195.— Head of Taema
ance, owing to its .being made up of a string *Aft5™ eucffi n)med'
ofsegments, or proglottides — about 85(T alto-
gether (cf. p. 510). These are narrower and shorter in front, gradually
increasing in size towards the posterior free extremity. The neck,
or part immediately following the head, is devoid of any trace
of segmentation. The two surfaces of the proglottides are not to
be distinguished by any differences visible to the unassisted eye ;

1 Though very probable, it is not certain that this end of the Tape-Worm
actually corresponds to the anterior end in the Liver-Fluke, as will be
explained later.

B 2

244

ZOOLOGY

SECT

but that side towards which the female reproductive organs are
more nearly approximated is regarded as the ventral, the opposite
as the dorsal surface. On one border, alternately on the right
and left of each proglottis, is a little prominence, the genital
papilla, on which is the opening of a chamber, the genital cloaca,
into which both the male and female reproductive ducts open.

An examination of entire living and of preserved and stained
Tape- Worms under the microscope shows (1) that an alimentary
system is not present ; (2) that nervous and excretory systems are
represented ; ' (3) that there is a complete set of reproductive
organs, constructed on the same general plan as those of the
Liver-Fluke, present in each of the proglottides.

The nervous system consists of two not very well-defined
ganglia — united by a broad transverse commissure— in the head;
of slender nerves passing from these to the suckers, and of two

ut.

FIG. 196. — Transverse section of Tsenia soliuxn. c.m. circular layer of muscle ; ex. longitu-
dinal excretory vessel ; ne. longitudinal nerve : o.d. oviduct ; ov. ovary ; te. testes ;
trf. uterus. (After Shipley.)

longitudinal nerves which run backwards through all the proglot-
tides to the posterior end of the body. The ganglia seem to
correspond to the ganglia on the nerve-ring of the Liver-Fluke.

The excretory organs consist of a richly branched system
of excretory vessels. There are four main longitudinal trunks
(Fig. 197, can. excret.}, two near each lateral margin in the more
anterior part of the strobila ; in the more posterior region one of
these becomes lost on each side. The two pairs of longitudinal
vessels are connected together in the head by a ring-like vessel
and in each proglottis near its posterior margin by a straight,
transverse, connecting branch. Posteriorly the longitudinal trunks
open into a pulsatile caudal vesicle, communicating with the
exterior in the last proglottis. When the latter becomes thrown
off, the vesicle is lost with it, and, subsequently, the longitudinal
vessels^have their separate openings on the exterior. These

PHYLUM PLATYHELMINTHES

245

car, . 6Xcrct

main trunks of the excretory system give origin to a number of
branches, and these in turn give off numerous fine canalicules, or
capillaries, terminating in flame-cells, similar to those of the
Fluke.

The reproductive organs (Fig. 197), repeated in each fully
formed proglottis, are in essential respects very similar to those of
the Liver-Fluke. In the most anterior proglottides they are not
developed ; it is only at about the 200th proglottis that they first
appear : at first the male parts of the system are alone differen-
tiated ; then in the succeeding proglottides, till we approach near
the posterior extremity of the body, the female organs are like-
wise developed. In the most posterior segments modifications
and reductions of some of the parts take place, owing to the great
increase in size of the uterus. The male portion of the apparatus
consists of the testes with their efferent ducts, the vas deferens
(vas. def.), and the
cirrus, with its sac.
The testes consist of
numerous rounded
lobes situated
nearer the dorsal
than the ventral
surface, and ex-
tending throughout
the greater part of
the length and
breadth of the pro-
glottis. With each
lobe is connected a
fine efferent duct ;
the ducts of neigh-
bouring lobes unite
together to form somewhat larger ducts ; and the larger ducts,
receiving numerous tributaries, eventually open into the inner
extremity of the vas deferens, or main duct of the testis. The
vas deferens is a convoluted tube which extends outwards towards
the lateral margin (right or left as the case may be) of the
proglottis.

The terminal part of the vas deferens, which is somewhat
narrower than the rest, traverses a narrow protrusible process, the
cirrus, and opens at its extremity by the male genital aperture in
the genital atrium or cloaca. The cirrus is enclosed at the base
by a muscular sac, the cirrus-sac.

The ovary (germarium) (ov.) differs from that of the Liver-
Fluke in being a paired organ, consisting of two approximately
equal, right and4eft, halves. It is situated towards the posterior
border of the proglottis. Like that of the Liver-Fluke, it consists

FIG. 197. — A proglottis of Taenia solium with mature repro-
ductive apparatus, can. excret. longitudinal excretory canals
with transverse connecting vessels ; gl. vit. vitelline glands ;
nerv. 1. longitudinal nerves ; ov, ov. ovaries (germaria) ; por.
gen. genital pore ; schld. shell-glands ; uter. uterus ; raff.
vagina ; tas def. vas deferens. The numerous small round
bodies fire the lobes of the testes. (After Leuckart.)

246 ZOOLOGY SECT.

of a number of branching tubes, in the interior of which the ova
are developed. From opposite sides these tubes converge towards
the median line, where they open into the oviduct. A yolk-gland
(gl. vit.), of less relative extent than in the Liver-Fluke, consists
of a number of minute lobules ; a duct, the yolk-duct, which runs
forward from it, opens into the oviduct. The numerous lobules of
a rounded " shell-gland " (schld.) surround the yolk-duct where it
passes forward to join the oviduct ; and the many shell-gland ducts
open into the oviduct near its junction with the yolk-duct : this
part of the oviduct is the ootype — the part in which the egg becomes
completed. In front this passes into the uterus. The female
genital pore, situated in the genital atrium, leads into a narrow
passage which runs inwards and backwards towards the middle
line of the proglottis, where it ends in a dilatation usually filled
with sperms — the receptaculum seminis. From this a narrow duct
— the fertilising duct or spermatic duct — runs to join the oviduct.
The uterus, in the segments in which it first
makes its appearance, is a simple cylindrical
diverticulum of the oviduct ; it retains its
simple form as far back as about the -600th pro-
glottis, where it begins to branch, the ramifica-
tions increasing in extent and volume in the
posterior segments. It has no opening on the
exterior.

Masses of sperms (probably from the same
proglottis) pass in the act of copulation along
the vagina to the receptaculum seminis ; through
the fertilising duct they pass to the oviduct to
^er^se ^ne ova- &&• in the case of the Liver-
Fluke, the oosperm proper becomes surrounded
by a quantity of food-yolk developed in the yolk-glands,
and is then enclosed in a firm chitinous shell formed for
it by the secretion of the shell-gland. It then passes into
the uterus. The first completed eggs are found in the uterus
in some proglottis between the 400th and the 500th. From this
point backwards they rapidly accumulate, until the cavity of the
uterus, which now becomes branched, is filled and distended with
them. Eventually in the most posterior, so-called " ripe," pro-
glottides (Fig. 198), the uterus, with its contained accumulation of
eggs, becomes so large as to fill the greater part of the interior of
the proglottis, the remainder of the reproductive apparatus mean-
while having become absorbed.

Development. — When the ripe proglottides are detached they
pass to the exterior with the faeces of the host. For a time they
exhibit movements of contraction. The embryos contained within
the eggs have meantime assumed the form of rounded bodies, each
armed with six chitinoid hooks — the six-hooked or hexacanth embryo

PHYLUM PLATYHELMINTHES

247

or proscolex (Fig. 199, A) enclosed within two membranes. If the
proglottides, or the eggs which have escaped from them, should
now be taken into the alimentary canal of the Pig, which forms the
ordinary second host of the parasite, the hooked embryos, becoming
freed from their coverings, bore their way with the aid of their
hooks through the wall of the alimentary canal, and reach the
voluntary muscles. Here they increase greatly in size, and develop
into rounded cysts with a large cavity filled with watery fluid
(B). On the wall of the bladder, at one side, is formed a hollow

FIG. 109.— Development of Tapeworm. A, hexacauth embryo ; B, early stage of bladder
worm of Tcenia saginata ; C — E, stages in the formation of the scolex of the same ; C, the
invagination before the hooks and suckers have become developed ; A after the appearance
of the hooks and suckers ; E, partly evaginated ; F, fully evaginated scolex of T. solium
with caudal vesicle ; G, scolex of T. serrata with remains of the vesicle ; H, young tape-
worm of T. serrata. (After Leuckart.)

ingrowth, or invagination (C) ; and on the inner surface of this
are developed the hooks and suckers characteristic of the head or
scolex of the adult (D). When these are fully formed the hollow
ingrowth becomes everted (E), the suckers and hooks thus coming
to be situated on the outer surface (F), The whole embryo has
now the form of a bladder or vesicle, with which is connected
at one point a process having all the characters of the head and
neck of the mature Taenia solium ; this is the bladder-worm stage,
or cysticercus. If a portion of Pig's muscle containing cysticerci
which have not been killed by cooking is taken into the stomach

248 ZOOLOGY . SECT.

of Man, the bladder is thrown off, the scolex attaches itself to the
wall of the intestine by its hooks and suckers, and develops the
series of proglottides of the adult Tape- Worm.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Platyhelminthes are bilaterally symmetrical, usually dorso-
ventrally compressed animals, devoid t)f hard supporting skeleton —
either external or internal^and also of metameric segmentation ;
with three embryonic layers — ectoderm, mesoderm, and endoderm
— entering into the formation of the body. A body-cavity is not
present. There is a system of excretory vessels, communicating in
the majority of cases with the exterior, and furnished with ciliary
flames. There is no blood- vascular system. An enteric cavity
may be absent, may be rudimentary, or may be highly developed ;
it is never provided with an anal aperture. The completed egg
contains, in addition to the oosperm, a quantity of yolk-matter,
usually in the form of definite yolk-cells, and usually produced by
a special set of yolk-glands. Development is sometimes direct,
sometimes accompanied by a metamorphosis.

CLASS I.- TURBELLARIA.

Mostly non-parasitic Platyhelminthes with a ciliated cellular
epidermis ; with a digestive cavity.

ORDER 1. — POLYCLADDIA.

Flattened leaf-shaped Turbellaria, without separate yolk-glands ;
testes and ovaries numerous ; male and female, genital apertures
usually separate ; intestine complexly branched.

ORDER 2. — TRICLADIDA.

Turbellaria with elongate depressed body ; with numerous yolk-
glands, two ovaries, numerous testes ; a single genital aperture ;
intestine consisting of a median anterior division and two lateral
posterior limbs which are provided with side branches.

ORDER 3. — RHABDOCCELIDA, incl. ACCELA.

Comparatively small Turbellaria, with the body usually elongate
and c}dindrical or compressed : with simple, or nearly simple,
sac-like intestine ; with or without yolk-glands ; with one or two
ovaries and two or many testes.

CLASS II.— TREMATODA.

Ecto- or endo-parasitic Platyhelminthes devoid of cilia,1 or of a
cellular epidermis 2 ; with a well-developed digestive apparatus.

1 Except in certain species of Temnoccphala.

2 Except in the Tetnnocephalea and Actinodactylella.

v PHYLUM PLATYHELM1NTHES •->!<»

ORDER 1. — MONOGENETICA (HETEROCOTYLEA).
Mostly ectoparasitic Trematodes ; with, direct development.

ORDER 2. — DIGENETICA (MALACOCOTYLEA).
Endoparasitic Trematodes with complicated life-history.

ORDER 3. — ASPIDOCOTYLEA.

Endoparasitic Trematodes with direct development ; adhesive
apparatus in the form of a large sucker, which is divided by
septa into compartments, and occupies nearly the entire ventral
surface.

ORDER 4. — TEMNOCEPHALEA.

Trematodes with direct development, which live on the outer
surface or in the respiratory cavities of various animals— e.g., Crusta-
ceans ; most non-parasitic as regards their nutrition, with organs
of adhesion in the form of a simple posterior sucker and a system
of anterior or marginal tentacle-like appendages.

CLASS III.— CESTODA.

Endoparasitic Platyhelminthes without cilia and without diges-
tive cavity, the animal consisting in most cases of a rounded
head bearing organs of adhesion in the form of suckers and hooks,
and an elongated compressed body consisting of a string of similar
proglottides, each containing a complete set of hermaphrodite
reproductive organs.

ORDER 1. — MONOZOA.
The body not divided into proglottides.

ORDER 2. — POLYZOA (MEROZOA).
The body consisting of head or scolex, and string of proglottides.

Systematic Position of the Examples.

Planaria and Dendroccelum are genera of the family Planaridce
or fresh-water Planarians, which is one of the two families of the
order Tricladida, differing from the other family, the Geoplanidce
or Land Planarians, mainly in having the body less elongated and
more dorso-ventrally compressed.

The genus Fasciola, to which the Liver-Fluke belongs, is a
member of the family Distomidce of the Digenetic Trematodes.
The Distomidce are characterised by the following features : — They
have a cylindrical or more or less flattened body, always provided
with two suckers — the anterior terminal or nearly so, the posterior

250 ZOOLOGY . SECT.

ventral and either terminal, or in a varying position on the ventral
surface. A pharynx may be present or absent. The intestine is
always forked, the limbs simple or branched. The genital pore is
ventral, either median or lateral, sometimes at the posterior end.
There are two testes, sometimes fused into one, sometimes broken
up into more or less numerous follicles, but always provided with
only two vasa deferentia. There is a single germarium, not
uncommonly lobed or divided up into a number of separate parts.
A receptaculum seminis, or a Laurer's canal, or both, are present.
The vitelline glands are, in most instances, paired, more or less
richly branched, extending towards the lateral borders of the body.

The genus Fasciola is a member of the sub-family Fasciolince of
the Distomidce, .and this is distinguished from the other sub-families
by the following characteristics. The Fasciolince are broad, leaf-
like Distomidce, with the integument spinose or scaly. They have
a well-developed pharynx. The intestinal limbs are simple or
branched. The genital aperture is median, and situated in front
of the posterior sucker. The testes are situated one behind the
other, directly or obliquely : they are either simple, divided into
lobes, or branched. The ovary is immediately in front of the
testes, the uterus in front of the ovary. A Laurer's canal is
present. The receptaculum seminis is absent or small. Among
the many genera into which this sub-family is now divided the
genus Fasciola presents the following distinctive features : — The
anterior end is distinctly differentiated into a head-lobe ; the
intestinal limbs have long branched diverticula on the outer side,
short on the inner ; the gonads are all richly branched ; there is
no receptaculum seminis.

Tcenia solium is one of the many species of the genus Tcenia,
of the family Tceniadce, which is distinguished from the other
families of Cestodes by the possession of four suckers, with or
without a circlet of hooks, and by the development of well-defined
proglottides which become separated off when mature.

3. GENERAL ORGANISATION.

General External Features.-— As the name of the phylum
denotes, the body in the Platyhelminthes is, in the great majority
of cases, much compressed in the dorso-ventral direction ; very
thin, so that when very short it may be described as leaf-
like, or, when more elongated, as ribbon-like ; or thickish in
the middle and becoming thinner towards the margin. Some,
however, have the body comparatively thick, usually with a certain
amount of dorso-ventral compression ; a few are approximately
cylindrical or fusiform. The symmetry is always bilateral (p. 43),
the radial arrangement of parts so prevalent in the Coelenterata
and primarily, as we have seen, associated with a fixed or stalked

PHYLUM PLATYHELMINTHES

251

condition, never being observable. A Flat- Worm has dorsal and
ventral surfaces, right and left sides or borders, and anterior
and posterior ends. The anterior end is that which is directed
forwards in ordinary locomotion : it usually has some of the
features which distinguish a head-end ; but a distinct head is
rarely developed, and the mouth, when present, is usually placed
some distance back on the ventral surface.

In the Turbellaria (Fig. 200) the leaf -form is the prevailing one,
a shape resembling that described for Planaria being very common.
In many, however, the body is greatly elongated, and it may
assume the shape of a thin ribbon
with puckered edges, as in some
marine forms ; or may be thickened
and band-like, as in the Land
Planarians ; or it may approach
the shape of a cylinder, as in some
Rhabdocceles. A head-region is
not usually distinct ; but there
is always something to mark off
the anterior from the posterior end
— a difference in shape, the pre-
sence of eyes, and, sometimes, of
a pair of short tentacles ; in some
a slight constriction separates off
an anterior lobe, on which the eyes
are borne, from the rest of the
body. In others the anterior end
is retractile, and may be everted
as a, proboscis. The mouth is
never at the extreme anterior end,
but always ventrally placed, some-
times behind the middle. In some
Polycladida there is a small ventral FIG. 200.— various Planarians. A,

emf»L-<vr rvivVKaKhr wi+Ti a nnrvnla voluta ; B, Vortex; C, Monotus ; D,

SUCKer, prODaDly Wltn a COpUia- Thysanozoon; E, Rhynchodemus ; F,

torv function ; and in SOme Bipalium ; O, Polycelis. All natural size.

-T^T i n 1 1 ,1 ,1 • (After Von Graff.)

Rhabdocoeles both the anterior

and posterior ends, though not provided with suckers, are adhesive,
so that the animal can loop along like a Hydra or a Caterpillar.
There is never any external appearance of segmentation, though in
at least one exceptional instance (Gunda segmentata, Fig. 201) the
internal parts may be so disposed as to approximate to the meta-
meric arrangement (pseudo-metamerism). In such a case a number
of transverse muscular septa are present, imperfectly dividing the
body internally into a series of segments ; and various internal
organs— intestinal caeca, gonads, transverse commissures of the
nervous system — are arranged in pairs following this division. A few
Turbellaria multiply by budding, and these form long chains, having

Con-

252

ZOOLOGY

SECT.

eye

Icnq ne.

int

ovd-

something in common with the string of proglottides of a Cestode,

but differing radically,
as will be shown later,
in the mode of develop-
ment. Colour is very
general in the Turbel-
larian, though some are
transparent and colour-
less. The most vivid
coloration characterises
some of the marine
Polyclads, the Ehabdo-
cceles being compara-
tively obscure. The
surface is covered with
a coating of fine vibratile
cilia, the vibration of
which subserves respira-
tion as well as (in the
smaller forms) locomo-
tion. Among the ordi-
nary cilia are frequently
disposed longer whip-
like cilia or flagella,
likewise motile ; and
sometimes non-motile
(sensory) cilia may
occur here and there.

The Trematodes (Figs.
189, 202, 203, 204),
nearly related to the
Turbellarians in internal
organisation, resemble
them also in external
form, with certain modi-
fications connected with
a parasitic mode of life.
As in the latter class,
the leaf-shape prevails ;
an elongated form also
occurs, though more
rarely. The body is
usually thicker and more
_ solid than in most Tur-

Fio. 201.— Gunda segment at a. General view of the bellaria. The anterior

organisation, br. bruin; eye, eye; yen. ap. genital

aperture ; int. intestine with its ctvca ; Jong. ne. longi- end IS

tudinal nerve-cord ; or. ovary ; ovd. oviduct ; pe. penis ; rrrtT» +>

ph. pharynx ; te. testes ; ut. uterus. (After Lang.) lrom t

r»rtQfoM«r ^r
pOSteriOT by

PHYLUM PLATYHELMINTHES

253

B

its shape, by the arrangement of the suckers, and, in many of those
Trematodes that are external parasites, by the presence of eyes.
Suckers, present in the Turbellaria only in some of the Polycladida
and a few Tricladida, are universal in their occurrence. They are
always ventrally placed, their chief function being to fix the parasite
to the surface of its host in such a way as to facilitate the taking
in by the mouth of animal juices and epithelial debris ; their
number and arrangement vary considerably. There are nearly
always present an anterior set of suckers (or a single anterior
sucker surrounding the mouth) and a posterior set, or a single
large posterior sucker. The arrangement already described as
characterising the Liver-Fluke is that which is typical in the
digenetic forms — a single anterior and a single posterior sucker ; but
in some of the Digenetica the
posterior sucker is wanting.
Adhesive papillae on the dorsal
or ventral surface may supple-
ment the adhesive action of the
suckers (Fig. 202, B). In the
Monogenetica the suckers are
often more numerous ; in the
family Gyrodactylidce (Fig. 203,
A) there is no anterior sucker,
but at the posterior end one or
two discs armed with hooks ; in
the Polystomece (Fig. 203, B)
there is also a posterior disc on
which are six suckers with several
hooks ; in the Temnocephalea
(Fig. 204) there is no anterior
sucker, but the anterior end FIG.
develops a row (two only in
Scutariella) of adhesive ten-
tacles, while in Actinodactylella
(Fig. 205) a series of marginal tentacles is present in addition to
both anterior and posterior suckers. In the Aspidocotylea there is
only a single sucker ; but it extends over nearly the whole of the
ventral surface, and is complicated in structure owing to its cavity
being divided into a number of compartments by a system of
partitions.

Save in two exceptional cases (Temnocephala) vibratile cilia are
not known to occur on the surface in the adult condition ; in some
there are groups of non-motile cilia, situated on little conical
elevations — the tqctile cones. Pigment is rare in the endoparasitic
Digenetica, save in a few that five in the interior of transparent
animals ; though many appear coloured variously by the internal
organs shining through the translucent body-wall, or are stained

202. — Digenetic Trematodes. A,
Amphistomum ; B, Homalogaster. ff.p.
genital aperture ; m. mouth ; s. posterior
sucker ; te. testes ; vit. vitelline glands.
(After M. Braun.)

254

ZOOLOGY

SECT.

by some fluid derived from their host. Pigment occurs in some
of the ectoparasitic forms.

FIG. 203.— JYlonogenetic Trematodes. A, Gyrodactylus. cil. disc bearing hooks and pro-
cesses at the posterior end ; ent. intestine ; gl. unicellular glands whose ducts open on the
surface about the anterior end ; hi. caudal disc of the first embryo ; fa. caudal disc of the
second embryo ; mo. mouth ; oosp. oosperm ; ov. ovary ; p. penis ; ph. pharynx ; te. testes.
B, Polystomum. en. intestine ; g. p. genital pore ; mo. mouth ; ph. pharynx ; ov. ovary ; te.
testes ; u. uterus ; y., v. d. vas deferens ; vit. vitelline glands ; vit. d. vitelline ducts ; x
marks the position of the genito-intestinal canal connecting the oviduct with the intestine.
(From M. Braun.)

The relationship of the Cestoda to the Trematoda is, as will be
subsequently shown, fairly close ; but though there are connecting

PHYLUM PLATYHELMIKTHES

255

forms between the two classes, the shape of the average Cestode
is very different from that of such an average Trematode as the
Liver-Fluke. The body of an ordinary Cestode is of great length —
sometimes extending even to a good many feet — and relatively
narrow, being compressed into the form of a ribbon. One end.
which it will be
convenient to
designate ante-
rior (though it
may not, per-
haps, correspond
to the anterior
end in a Trema-
tode or a Turbel-
larian), is, in
most cases, at-
tached to the
host by means
of suckers and
hooks placed on
a rounded lobe,
the head or
scolex, connected
with the body
by a narrow part
or neck. The
head is usually
rather radially
than bilaterally
symmetrical,
with four suckers
and a 'circlet of
hooks. The
hooks, when pre-
sent, are borne
on a longer or
shorter retractile

process,

the

TOS-

EIG. 204. — Temnocephala minor, general view of the
organisation, c. cirrus ; e. s. ejaculatory sac ; g. c. genital
atrium ; t. intestine ; o. germarium : oo. ootype ; ph. pharynx ;
pr. prostate glands ; r. d. strands of ducts of integumentary
glands running forwards to the tentacles ; r. g. groups of inte-
gumentary (rhabdite-forming) glands ; r.v. receptaculum ; s.
sucker ; t. testes ; te. tentacles ; t. s. terminal sacs of excretory
system ; v. s. vesicula seminalis.

tellum, the long

axis of which is

in line with the

long axis of the

body. In Bothriocephalus and allied forms a pair of longitudinal

grooves take the place of suckers, and there are no hooks. In many

Cestodes parasitic in Fishes the head bears four prominent, thin,

folded flaps — the bothridia, which are exceedingly mobile, and are

used more as creeping organs than as organs of fixation. In relation

256

ZOOLOGY

SECT.

~x

to each of these bothridia, which, by coalescence, may appear to be
reduced to two, may be a small sucker of the ordinary kind. In
Tetrarhynchus (Fig. 206) there are four very long and narrow
rostella, or " proboscides," covered with hooklets, and capable of
being retracted into sheaths.

The Cestoda are devoid of mouth, and in most of them the

genital apertures are marginally placed, so that, externally, there

A^ ^^ is — except in

^^•^ ^/ the case of a

few in which the
genital apertures
are not marginal
— nothing to dis-
tinguish the dor-
sal surface from
the ventral. The
body, or strobila,
which is nar-
rower in front
than it is
further back,
is made up
throughout its
length of a series
of segments, or
proglottides,
which become
larger and more
distinctly
marked off from
one another as
we pass back-
wards. Tcenia
e chin o coccus
(Fig. 207) is
exceptional in
possessing only
three or four

FIG. 205. — Actinodactylella. b. c. bursa copulatrix ; br. brain ; -rvT

c. penis ; t. intestine ; ov. ovary ; ph. pharynx ; r. v. receptacu- p^

lum ; s. sucker ; t., t. testes ; ut. uterus ; v. vitelline glands ; a f p
v. s. vesicula seminalis.

and its allies —

Fig. 208), though the body has the normal elongated ribbon-like
form, the segments are not distinct, and in Caryophyllceus (Fig. 209),
Amphilina, Gyrocotyle (Amphiptyches — Fig. 210), and Archigetes
(Fig. 211) (which is perhaps merely a young stage of Caryo-
phyllceus), segmentation is entirely absent, the whole body in
these genera consisting of a single proglottis. The surface in
the Cestodes is devoid of cilia, and there is no pigment.

PHYLUM PLATYHELMINTHES

257

Integument and Muscular Layers. — In the Platyhelminthes
in general there are integumentary layers and underlying layers of
muscle, which are more
highly differentiated than
in the Coelenterates. But
considerable differences exist
in this respect between the
members of the three classes.
In the Turbellaria (Fig. 212)
there is, as already noticed
in the account given of the
Planarian, a distinct epider-
mis (ep.) in the form of a
layer of cells, most of which
are ciliated. A delicate
cuticle is usually, though not
always, distinguishable, in-
vesting the epidermis ex-
ternally. In one family the
cuticle is developed, along
the margin of the body, into
a series of chitinous bristles.
Among the ordinary epider-
mal cells there are in the
Polycladida numerous cells

containing short rod-like

KnrliPQ fViP rJinhfJiftxi (r~h \ • FiO. 206. — Tetrarhynchus." FlO. 207. — Taenia

Y V ' n. nervous system ; r. pro- echinococcus.

in the Other Orders OI the boscides; r*. sheaths, with (After Cobbold.)

m i_ n • J/L T- T-j-j. their muscles (rb.). (From

Turbellana these rhabdite- Leuckart, after Pintnftr.)
forming cells are sunk

deeply within the parenchyma, and, in the Rhabdocoela, have
very long ducts, formed of processes of the cells, by means of
which the rods, together with a viscid matter, reach the exterior

at certain points of the surface

/^^^ /^Sfr\ chiefly around the anterior ex-

tremity. The function of these
rhabdites is not in all cases cer-
tain ; they have been supposed
to add to the sensitiveness of
the parts in which they are
situated after the fashion of
hairs or nails, or to have a
skeletal function. In the Rhab-
FIO. 208.-Liguia. (After Leuckart.) docoela and Tricladida they

undoubtedly aid in adhesion, and

probably have the function of assisting in the entanglement
and capture of food. In certain of the Turbellaria stinging
VOL. i. s

ZOOLOGY

SECT.

capsules occur similar to those of the Ccelenterata, but these
seem to be derived from Hydra, Cordylophora, and other Coelen-
terates, which form their food. Adhesive cells with processes also
frequently occur in the epidermis. Beneath the epidermis is a
basement membrane (b. m.), which in the Polycladida is of a thick
resistant character, and contains stellate cells.

In a small number of the Trematoda three layers are distin-
guishable in the integument — a homogeneous, or nearly homo-

FIG. 209. — Caryophyllaeus.
d. g. vitelline duct ; d. st. vi-
telline glands ; e. excretory
pore ; k. mobile organ : od.
oviduct ; or. germarium ; p.
cirrus ; r. s. receptaculum se-
minis ; t. lobes of testes ; v. d.
vas deferens ; v. s. vesicula
seminalis ; w.g.o. female aper-
ture. (After Leuckart.)

FIG. 210. — Gyrocotyle (Amphiptyches).

eo. excretory opening ; mo. male open-
ing ; n. longitudinal nerve ; n'. anterior
nerve-ring ; n. r. posterior nerve-ring ; o.
opening of uterus ; o. ovary ; o'. recepta-
culum ovorum ; p. base of cirrus ; r.s. re-
ceptaculum seminis ; r. s. o. opening of
vagina ; *. sucker ; t. testes ; ut. uterus ;
r. 8. vesicula seminalis ; yk. vitelline
glands. (After Spencer.)

FIG. 211.—
Archigetes.

(After Leuckart.

geneous, outer cuticle ; a cellular, or at least nucleated, epidermis,
and a basement membrane ; but the cellular epidermal layer is
absent as such in the adult condition in the majority of the Trema-
todes, and there is only a homogeneous, non-nucleated outer layer,
which may be the modified epidermis, or may be the cuticle, with
or without a basement membrane. Rhabdite-forming and other

PHYLUM PLATYHELMTNTHES

259

unicellular glands derived from the epidermis are frequently
present beneath the integument.

In the Cestodes, as in the rfl

majority of the Trematodes, no
definite epidermis is present.
The external layer, sometimes
divided into two or more
strata, is of a homogeneous bm
non-cellular character, and is
usually termed cuticle. Beneath
this is a thin layer of paren-
chyma, the basal membrane.
Beneath this again is a layer
of fusiform cells, narrow pro-
longations of which pass to the
cuticle, into the inner part of
which they penetrate and
spread out into a thin layer.
These cells are by some authors

d.vrn.

rh.c

FIG. 212.— Section of the body-wall of a Triclad.

regarded as the Cells that b. m. basement membrane; c. m. circular
, ,1 ,• 1 -I layer of Jmuscle-fibres ; d. v. m. dorse-ventral

Secrete the CUtlCle ; but they layer Of muscle-fibres ; e.l. m external longi-

may be concerned in the

tudinal layer of muscle-fibres ; ep. epidermis ;
i. 1. m. internal longitudinal layer of muscle-

of nutrient matter, fibres ; p. parenchyma ; rh. rhabdites ; rh. c.
t , , rhabdite-forming cells. (After Jijima.)

and some of them are un-
doubtedly of the nature of nerve-cells and have nerve-fibres
connected with them.

The muscular layers of the body-wall vary somewhat in their
arrangement in the different groups of Platyhelminthes. Most

commonly there aie
an external layer of
circularly arranged
and an internal layer
of longitudinally ar-
ranged fibres ; fre-
quently layers of
fibres running in a
diagonal direction
are present also.

Characteristic of
the Flat-worms is a
peculiar form of con-
nective - tissue, the
parenchyma (Fig.
213) — mention of
which has already

FIG. 213.— Parenchyma of Distomum. a, b. intercellular been made m the
spaces; bm. basement membrane ; c. nuclei ; d. nuclei ; /qoe«T,iT,fir,ric ~f +>.0
ep. epidermis. (After Braun.) descriptions Ol tfce

s 2

d

260 ZOOLOGY SECT.

examples — presenting many varieties, filling up the interstices
between the organs and leaving only, in some instances,
very small spaces — sometimes regarded as representing the
body-cavity, or ccelome, which we shall meet with in other
groups of worms. Sometimes the parenchyma appears to con-
sist of distinct large cells with greatly vacuolated protoplasm,
with interspaces here and there in which groups of rounded cells
are enclosed. Sometimes the constituent cells run together, and
the parenchyma then appears as a nucleated, finely fibrillated,
vacuolated mass in which the boundaries of the cells are not
recognisable. Pigment occurs in the parenchyma in some Rhab-
docoele Turbellarians and a few Monogenetic Trematodes. In
some Turbellaria — species of Convoluta and Vortex — the paren-
chyma contains numerous cells enclosing chlorophyll or xantho-
phyll corpuscles ; these are symbiotic unicellular Algae, similar
in their mode of occurrence to the yellow cells which have been
referred to as found in the Radiolaria. Running through the
body, for the most part in a dorso-ventral direction, are numerous
slender muscular fibres, the fibres of the parenchyma muscle ;
many of these become inserted externally into the basement
membrane.

Great differences exist between the various groups of Platy-
helminthes as regards the development of the alimentary
system, differences which are, broadly, to be correlated with
differences in the mode of nutrition. Some of the Flat-worms
— the Turbellaria and some of the Monogenetic Trematodes —
procure their food, in the shape of small living animal or vege-
table organisms, or floating organic debris, by their own active
efforts. Others — the Digenetic Trematodes and the Cestodes—
having reached a favourable situation in the interior of their host,
remain relatively or completely passive. An alimentary canal is
completely absent in the last-nameS group, nutrition being effected
by the absorption of digested matter from the interior of the
animal in which the Cestode lives. In all the rest of the Platy-
helminthes there is an alimentary canal, which never opens on the
exterior by an anal aperture. All the Turbellaria (except some
Accela) and Trematoda have an alimentary apparatus consisting
of two well-defined parts — a muscular pharynx and an intestine.
The pharynx is usually a rounded muscular bulb, but is sometimes
(some Turbellaria) of a cylindrical shape ; it is usually capable of
eversion and retraction. Actinodactylella (Fig. 205) is exceptional
in having, in addition to a large muscular pharynx, an extensile
proboscis with a pin-shaped style, which becomes retracted within
the opening of the mouth. Unicellular glands open into the pharynx
in most cases.

The mouth is always ventral, but varies greatly in its position
on the ventral surface, being sometimes central, sometimes situated

PHYLUM PLATYHELMINTHES

261

behind, sometimes in front of, the middle of the length of the body.
In the most lowly organised group of Turbellaria (the Accela) the
intestine is represented merely by a vacuolated, nucleated mass of
protoplasm without, or with only an irregular, lumen. In the others

en

vs

. 214. — General plan of the
ucture of a Rhabdocoele
bellarian. be. bursa

pulatrix ; cn. brain ; e.
ye; g. germarium ; i. intest-
ae ; In. longitudinal nerve ;
i. mouth : ph. pharynx ;
.8. receptaculum ; s. unicel-
Jlar glands ; t. testis ; u.
terus ; v. vitellarium : vs.
esicula seminalis ; S ejacu-
itory duct; rf 9 common
enital aperture. (After Von
Jraff.)

cn

ov

FIG. 215. — General plan of the structure of a Folyclad. cn. brain ;
i., st. intestine ; In. longitudinal nerve cord ; m. mouth ; od.
oviduct; ov. ovary; ph. pharynx; phl. sheath of pharynx; t.
testes ; u. uterus ; vd. vas def erens ; vs. vesicula seminalis ; 6
male aperture; 9 female aperture. (After Von Graff.)

it is sometimes a simple sac (Rhabdoccele Turbellaria— Fig. 214, a
few Trematoda). with or without short lateral diverticula. In
the majority of the Trematodes it consists of a pair of simple canals ;
but in some, as in the Liver-Fluke, there is a pair of canals which

262

ZOOLOGY

SECT.

Zn

give off numerous branches. In the Polycladida (Fig. 215) there
is a central cavity from which numerous branching canals are
given off. In the Tricladida (Fig. 216) one median canal passes
forwards from the pharynx, and a pair of canals backwards from
it, all three giving off branches which again branch. In some
Polycladida there are minute pores, by means of which certain

of the canals are placed in commu
nication with the exterior. A num
ber of unicellular glands, which pro-
bably produce a digestive secretion,
open in many Trematodes and
Khabdocoeles at the junction of
pharynx and intestine.

A bilateral nervous system is
developed in all the Platyhelmin-
thes. Its elements are nerve-fibres
and nerve-cells. The nerve-cells,
which are usually bipolar, more
rarely uni- or multi-polar, lie in
the course of these fibres, with
which the substance of the cells is
in continuity. The degree of de-
velopment of a central part of the
nervous system, or brain, varies in
the different groups ; it is best
developed in some Polycladida and
some Monogenetic Trematodes. It
consists of numerous nerve-fibres
which here converge from the
various parts of the body and
pass across from one side to the
other, together with a central mass
of fine fibrils, and a number of
nerve-cells. It is situated in the
FIG. 2i6.~7Generai plan of the structure anterior portion of the body, almost
germa^iumft' median &i\mb of YhV in- m variably in front of the mouth,
testme; i2.' right limb; ?3. left limb; Where the peripheral part of the

In. longitudinal nerve-cord ; m. mouth ; * . ,

od. oviduct ; ph. pharynx ; t. testes ; nervOUS System IS DCSt developed,
a; v. vitellaria; vd. vas ag it ig jn ^ polyCla(Jida, the

Tricladida, and some Trematodes,
there are three pairs of longitudinal
nerve-cords running backwards from the brain throughout the
body, connected together by frequent transverse connecting
nerves, or commissures. To these there are sometimes super-
added fine net-works or plexuses of nerves, situated superficially
under the dorsal integument, or on both dorsal and ventral
surfaces. Sometimes nerves run forwards from the brain as well

v PHYLUM PLATYHELMINTHES 263

as backwards. In the Rhabdocoeles and some of the Trematodes
the whole system is simpler, and the number of longitudinal
cords fewer. In the Cestodes there are two principal longitudinal
trunks which run throughout the length of the body, and are
connected together in the head by commissures, variously thickened
to form ganglia representing the brain of other Platyhelminthes.

In addition to the tactile cones of some Trematodes and the
sensory cilia of the Turbellaria, already referred to, the sensory
organs of the Platyhelminthes are the eyes and the statocysts.
Eyes occur in the Turbellaria and some Monogenetic Trematodes,
but are wanting in the Digenetic Trematodes and in the Cestodes.
In some of the Polycladida they are extremely numerous, collected
into groups over the brain, and frequently arranged also round the
margin of the body. In the Rhabdocoeles and Monogenetic
Trematodes they are much less numerous — usually two to four.
In some cases each eye simply consists of a pigment spot ; to this
may be added a refractive body. When most highly developed
the eye is still of very simple structure, consisting of a cup formed
of one or more pigment-cells enclosing refractive bodies (rods), and
having nerve-cells in close relation to it with processes (nerve-
fibres) passing to the brain. The statocysts are sacs containing
stotoliths of carbonate of lime. The function of these bodies, which
occur only in a small number of the Turbellaria, is unknown ;
there is no sufficient evidence that they are organs of hearing ;
it is more likely that they are concerned with the maintenance
of equilibrium. Ciliated pits which appear to be sensory are
developed in some Rhabdocoeles in the head region.

The only vascular system present in the Platyhelminthes is
the system of water-vessels (protonephridia) which are commonly re-
garded as performing an excretory function. The arrangement of
these, the mode of ending internally of the finest branches, and the
way in which the system communicates with the exterior, vary greatly
in the different groups One or more main longitudinal trunks give
off branches which subdivide to form a system of* minute inter-
lacing branches or capillaries. In little spaces at the ends of the
capillaries are a number of highly characteristic structures — the
ciliary flames. Each ciliary flame consists of a bundle of vibratile
cilia ; typically each is situated in the interior of a cell — the
flame-cell (Fig. 217) — terminating one of the capillary branches.
But there are some cases in which there are several flames in each
flame-cell. The finer branches, and in some cases the larger trunks
also, are intracellular, and are to be looked upon as perforations
in linear rows of elongated cells. In the Cestoda, at least the
larger trunks are intercellular, being lined by an epithelium of
small cells. This system of water-vessels opens on the exterior
in a variel^Tof different ways : sometimes it opens by a number
of minute pores ; sometimes, as in the Liver-Fluke, there is a

264 ZOOLOGY SECT.

single posterior aperture ; frequently there are two. In the Tri-
cladida there are two longitudinal canals which open on the exterior
through special branches by a series of pores. In the Rhabdoccelida
there are either two longitudinal main vessels
or a single median one ; the communication
with the exterior in the former case may be
by a pair of ventral apertures, or indirectly
through the pharynx ; or there may be a
common short passage in which the two
trunks unite, opening by a posterior median
aperture. When a single main trunk is
present it opens at the posterior end of the
body. In the Trematodes there are usually
two principal longitudinal trunks, which
F T^SuSST P"* either unite behind to open at the posterior
k. termination of capillary ; en(J of the body, or (Monogenetica) remain

n. nucleus ; v. vacuoles ;

wf. ciliary flame. (After separate and open independently on the
dorsal surface, each having, where it opens,

a contractile excretory sac. In Temnocephala each dorsally opening
excretory sac has ramifying through its wall — which consists
mainly of a single large cell — a system of capillary vessels containing
ciliary flames.

In the Cestodes there are usually four longitudinal trunks, which
open through a contractile excretory sac at the posterior end of
the body. In many cases it has been shown that the main trunks
communicate with the exterior at intervals by means of fine canals.
The excretory sac is thrown off when the last proglottis becomes
separated off and does not in most cases become renewed, though
in at least one species of Tape- worm (Tcenia cucumerina) a new
vesicle is developed -again and again at the end of the body as a
fresh segment is thrown off. The main trunks are connected
together by a ring-vessel in the head and in some cases by a
transverse branch in each proglottis, and where the latter originate
from the main trunks are valves formed by folds of the wall of the
vessel. In the posterior region only two of the longitudinal
trunks (one on each side) may be retained.

The sexes are united in all the Platyhelminthes with a few
exceptions, and the reproductive organs are sometimes
somewhat complicated — presenting a remarkable advance on those
of the Ccelenterata. The male part of the apparatus consists of
testes, with their ducts, the vasa deferentia, often with a contractile
terminal enlargement or vesicula seminalis, a cirrus l or a penis,
and often prostate or granule glands. The female part com-
prises ovary or ovaries, receptaculum, oviduct, uterus, an ootype,

1 The term cirrus is here restricted to cases in which the terminal part of
the male duct, often provided with spines and other chitinous structures, is
involuted within a sheath when at rest.

v PHYLUM PLATYHELMINTHES 265

often a bursa copulatrix, " shell-glands" vitelline or yolk-glands,
and cement-glands. In most, though not in all, there is a single
or paired germarium (ovary), in which the ova are formed, and a
set oi'vitellaria or vitelline glands, producing material which sur-
rounds each of the mature impregnated ova before it becomes
enclosed in its shell. In some, on the other hand, ova and vitelline
matter are formed in the same organ — the germ-vitellarium. The
shell-glands were so named because they were usually supposed to
secrete the chitinoid substance of the egg-shells ; but the share
which they take in this process, if they take any, is uncertain, and
they are better called accessory female glands. The cement-glands
secrete a viscid material for causing the eggs to adhere together,
enclosing them in a cocoon or fastening them to some foreign body.
The oviduct is the passage by which the ova reach the exterior
from the ovary ; but an enlarged part of this passage, into which
ducts of the accessory glands open, is distinguishable as the ootype,
while a terminal part, leading to the female aperture, may be modi-
fied as a vagina. In some cases (Heterocotylean Trematodes) there is
a vagina or a pair of vaginae in the shape of a passage, or a pair of
passages, distinct from the oviduct and opening independently on
the exterior. A uterus in the form of an enlarged part of the
oviduct or of an outgrowth from the latter or from the atrium is
very usually developed for the reception of the completed eggs.
A special sac or bursa copulatrix, lined with spines, acts as the
female copulatory organ. A sac, the receptaculum, opening into
the oviduct or into the atrium (Figs. 204, 205, r.v.), may serve as a
reservoir for the semen received in copulation or for the vitelline
matter or yolk, or for surplus reproductive material. Male and
female ducts sometimes have separate and independent openings ;
but very usually there is a common chamber or genital atrium
into which both lead, opening on the exterior by a single
aperture.

In the Poly clad Turbellaria (Fig. 215) the testes are numerous,
and there are a corresponding number of fine tubes which
combine to form the two vasa deferentia, leading to the male
aperture with its penis. The latter is sometimes double or multiple.
The ovaries consist of numerous small rounded masses of cells, and
there are no separate yolk-glands. Numerous narrow oviducts
lead from the ovaries, and unite to form larger ducts ; these, in
turn, open into two elongated uteri, in which numerous large eggs
containing abundant yolk collect. The ducts of the uteri open
into a median egg-duct or vagina, with which the ducts of the
accessory glands communicate, and in which the eggs receive
their chitinoid investment, derived in this case from the yolk-
material of the substance of the ova. The vagina leads to the
female aperture, a part of it being, in some cases, surrounded
by a muscular sheath.

266 ZOOLOGY SECT.

In many cases the egg-duct gives off posteriorly a narrow duct
which usually terminates behind in a vesicle known as the accessory
sac or receptaculum : this may be double. In a few Polyclads
this duct opens on the exterior on the ventral surface some
distance behind the main female aperture, in one instance on the
dorsal surface. A genito-intestinal canal connecting this duct with
one of the intestinal caeca has been found in one Poly clad. In
most cases male and female apertures are distinct from one
another, the former being situated in front of the latter. But
sometimes, though rarely, both lead into a common chamber or
atrium with a single opening on the exterior.

In the Tricladida (Fig. 216) there are also numerous testes, but
the fine tubes connecting them with the two vasa deferentia are
absent. There are two germaria, situated far forwards, and
numerous yolk-glands. Two oviducts, into which the yolk is dis-
charged from the yolk-glands by a series of lateral apertures, lead
from the ovaries to unite in a median ootype or vagina, receiving
the ducts of glands which may secrete the substance of the cocoon.
The condition is thus intermediate between that observable in
most of the Rhabdocceles and that which characterises the Poly-
clads. Though germaria and vitellaria are separate, they have a
common duct, and might be regarded as distinct lobes of one
germo-vitellarium. A uterus is present, formed as an outgrowth
of the vagina or of the atrium, or as an independent sac or pair of
sacs opening independently on the exterior. There may be a
receptaculum seminis, and in some there is a duct of communica-
tion between this and the intestine (genito-intestinal canal). A
common genital atrium with a single external aperture receives
the ducts of both sexes.

In the Rhabdocceles (Figs. 214 and 218) there are usually only
two compact testes and two vasa deferentia leading to the unpaired
male aperture at the extremity of the cirrus. The prostate or
granule glands — a set of unicellular glands, which secrete round,
bright granules destined to mix with the sperms — are specially
well developed in the Rhabdocceles, and are present in some other
Turbellaria and in certain Trematodes. Ovaries (germ-vitellaria)
alone occur in some, separate germaria and vitellaria in others ;
there are either two germaria or one only. A receptaculum may
be present as a swelling or diverticulum of the main female
duct, or of the atrium. The terminal part of this duct may form a
muscular vagina, or there may be a muscular bursa copulatrix
developed from the wall of the atrium. A uterus is present in
most cases as an outgrowth from the wall of the atrium. Male
and female ducts have a common chamber or genital atrium with a
single external opening.

In the Acwla there are in nearly all cases separate male and
female apertures. The two testes are divided into numerous small

PHYLUM PLATYHELMINTHES

267

at

lobes. There are no vitellaria in most cases— the two ovaries
producing large ova containing abundant food-yolk and the sub-
stance of the egg-shell being formed from droplets in the latter.
Oviducts are absent in most cases. Into the main female genital
passage or antrum femininum opens a peculiar single or double sac
or bursa, usually provided with chitinous structures. In one case
at least (Heterochcerus) there is a
passage leading from the antrum
femininum to an aperture on the
dorsal surface.

Most of the Trematodes have
two testes (reduced to one in Aspi-
docotylea and Heterocotylea), usually
compact, sometimes branched ; in
a few instances there are four.
The vasa deferentia unite into a
median duct, which is dilated at
the base of the cirrus to form a
vesicula seminalis^ There is a
single oval or branched ger-
marium, and two sets of vitel-
line glands. A canal termed
Laurer's canal in some Malaco-
cotyleans, such as some species
of Distomum, leads from the ex-
terior to the oviduct or vitelline
duct. This may be replaced by a
receptaculum seminis, or both
structures may co-exist. The

Jistal part of the oviduct is en-
irged to act as a uterus. In the
leterocotylea there is a vagina,
which is sometimes paired, opening
on the surface independently of the
uterus : internally it communicates withjthe oviduct through the
main vitelline duct. In some of the Heterocotylea there is a genito-
intestinal canal occupying a corresponding position to Laurer's
canal, but opening into the intestine. In the Aspidocotylea this is
replaced by a stalked yolk-receptacle. There is nearly always a
genital atrium commop to the ducts of both sexes.

In the Temnocephalea (Figs. 204, 205) there^aie a genital atrium
and a single genital aperture. There are two pairs of compact
testes ; the right and left vasa deferentia unite in a vesicula semi-
nalis, and granule or prostate glands are well developed. The
cirrus has a chitinous tube and a variety of ^veraiJ)le spines. There
is a single compact germarium ; the oviduct has connected with it
a large receptaculum, and dilates posteriorly to form an ootype into

Tit

FIG. 218. — Reproductive organs of MCSD-
stomum ehrenbergii. dg. duct of
vitelline glands ; do. vitelline glands ; go.
common reproductive aperture ; ov.
ovary ; p. cirrus ; rs. receptaculum se-
minis ; s. pharynx ; t., t. testes ; ut. uterus;
vd. vas deferens. (From Claus, after von
Graff and Schneider.)

268 ZOOLOGY SECT.

which the " shell-glands " open. Actinodactylella alone has a bursa
copulatrix (Fig. 205, b. c.).

In the ordinary Cestodes each segment or proglottis contains a
set of reproductive organs similar to those of a Trematode. There
may be a single genital aperture leading into a genital atrium, into
which both male and female ducts open ; or the male and female
apertures may be distinct. The testis is divided into numerous
minute lobes, from which proceed a number of fine canals joining
together to form the vas deferens, at the extremity of which is the
chitinous cirrus. There are two germaria, and either a single
vitelline gland, or two. The oviduct has its origin in a sort of
isthmus connecting the two germaria. It receives a narrow
fertilising duct from the receptaculum seminis, and then the
vitelline ducts, and becomes surrounded by a rounded mass of
accessory glands to form the ootype, which is not definitely enlarged.
Further forward it gives off the uterus. The latter is at first a
simple cylindrical outgrowth from the oviduct, but it usually
becomes large and may be extensively ramified. It has no external
opening in most instances, so that the eggs only escape from it
by the breaking down of the proglottis or by dehiscence. But
in some (e.g., Dibothrioceplialus) it has an independent external
opening. The female aperture leads into a narrow canal — the
vagina — which ends in a receptaculum seminis from which the
narrow fertilising duct conveys the sperms to the oviduct. In a
small number of Cestodes the sexes are distinct.

The development of some of the Platyhelminthes (Rhabdoccela,
Monogenetic Trematodes) is direct — i.e., not complicated by the
occurrence of a metamorphosis ; in the Digenetic Trematodes,
the Cestodes, and some of the Planarians a metamorphosis
occurs.

The eggs of the Polyclads, each of which consists merely of the
fertilised ovum (oosperm) usually enclosed in an egg-shell, are,
in most instances, laid in large numbers embedded in a plate or
capsule of slimy secretion. The ovum divides first into two parts,
then into four. These are slightly unequal in size, one of them
being somewhat larger than the others, and by the position of this
and its relations to the other three the chief axes and planes of the
embryo are already determinable at this early stage. In the
notation adopted in following out the cell-lineage or order of
development of cell from cell in the embryo, this largest cell is
known as D : the other three are A, B, C — the lettering following
the direction of movement of the hands of a clock when looked at
from above. As shown by subsequent changes, D is posterior, B
anterior, A and C are lateral. From these four cells (megameres)
are given off above in succession four sets or quartettes, each com-
posed of four cells. The members of the first quartette are desig-
nated la, 16, Ic, and Id, the small letter of each indicating derivation

PHYLUM PLATYHELMINTHES

269

from the megaroere with the corresponding capital ; those of the
second quartette are 2a, 26, 2c, and 2d ; and so on with the others.
From the way in which the cells of the quartettes are given off,
the type of segmentation here exemplified, which prevails also in
the Annulata (Section X) and most of the Mollusca (Section XII),
is known as the spiral type. The cells of the first three quartettes
are all composed of cells (micromeres) smaller than those from
which they are derived. The formation of the third quartette
results in a stage (Fig. 219) in which the embryo consists
of thirty-two cells in all, since divisions take place in the
cells of the first and second quartettes. In later stages the
micromeres increase greatly by further division and extend as
a cap, the ectoderm, composed of a single layer of small cells
over all the upper part (animal) pole of the embryo ; then
spread further

/st quartette

..1st quartette

2nd

quartette

tarictte

as a thin layer
over the entire
surface till
only a slit-
like blastopore
is left on the
ventral side :
finally the
blastopore
closes and the
ectoderm
forms a com-
plete invest-
ment.

Meanwhile
the other em-
bryonic layers
have been es-
tablished. It

is one of the cells of the fourth quartette that is alone or mainly
responsible for the origin of the endoderm and a great part of the
mesoderm. The four original megameres, A, B, C, and D, now
reduced to quite small cells (Fig. 220, mac), after the separating off
of the fourth quartette take no further part in development, and
are ultimately absorbed. Of the four cells of the fourth quartette,
which are comparatively large, three become broken up into masses
of yolk material to be subsequently absorbed as food ; but they may
(Discoccelis) first give off cells which contribute to the formation
of the endoderm. The fourth (designated 4:d) gives rise to the
whole of the endoderm — or (in Discoccdis) the whole with the
exception of the portion derived from the other three cells of the
fourth quartette— and a considerable part of the mesoderm. ±d

2nd .
quartette 3D

FIG. 219. — Developing egg of Planocera
seen from the lower (vegetative) pole.

</'

thirty-two cell stage,
As the four original

megameres, A — D, have now each given off three cells in succession,
their designations are now 34— 3D. (From MacBride, after Surface.)

270

ZOOLOGY

SECT.

•mes

divides into two cells, an outer (4<f ) and an inner (4d2) ; the former
divides to give rise to the greater part of the endoderm (end). The
latter, dividing into two cells, each of which contributes a small cell
(end) to the pharyngeal part of the endoderm, gives rise to two strings
of cells (mes) extending obliquely forwards and spreading out to
form eventually a continuous layer constituting the greater part of
the mesoderm. The mesoderm in the neighbourhood of the stomo-

daeum is independently
developed from descen-
dants of the second
quartette.

The process by which
the germinal layers
have become formed is,
as in the Ctenophora
(p. 215), a process of
epibolic gastrulation.
The brain is developed
from a pair of thicken-
ings of the ectoderm
(Fig. 220, g) : these
unite into a common
mass from which the
longitudinal nerves are
formed as backward
outgrowths. The mouth
is developed as an in-
growth from the ecto-
derm in the position of
the former blastopore,

mac

FIG. 220. — Embryo of Flanocera. Diagrammatic
frontal section, passing through animal (upper) and
vegetative (lower) poles, mes. mesodermal bands
(products, with end'., of 4rf2) ; end. endoderm
(products of 4d2); end', small endoderm cells formed
from mesoderm rudiment ; mac. -vestigial rnegameres :
mes. ect. cells of ectodmnal origin derived from the
second quartette which migrate inwards to form
muscles of the stomodseum ; g. cells forming rudi-
ment of brain. (From MacBride, after Surface.)

and, as the embryo

becomes flattened, passes from the posterior end to what is destined
to be the ventral surface of the worm, while the muscular tissues of
the wall of the pharynx are formed from surrounding mesodermal
elements. The intestine is at first simple in form ; the caeca are
developed as a result of the formation of vertical mesodermal
septa which, growing inwards, constrict the enteric wall and the
enclosed mass of nutrient material. The embryo, which has
assumed an ellipsoidal shape, becomes flattened in the dorso-ventral
direction, and, having absorbed the greater part of the nutrient
matter, escapes by rupture of the egg-shell.

In many cases the embryo develops into a characteristic larval
form, such as that known as Muller's larva (Fig. 221). It assumes
an oval shape, with a series of eight elongated processes, fringed
with long cilia connected together into a continuous ciliated band.
There are eye-spots at the anterior end and a mouth in the middle
of the ventral surface. The form of the body alters after a time,

v PHYLUM PLATYHELMINTHES 271

becoming gradually longer and flatter, and the arms are
gradually reduced in length, till, eventually, they become com-
pletely absorbed.

The development of the Triclads is very different from that of
the Polyclads. Each egg-capsule or cocoon encloses a number of
oosperms and a quantity of yolk-cells. After segmentation a
blastoderm is formed composed of a rounded mass of cells
surrounded by the yolk-material, the cells of which become more
or less fused. Around the periphery of the blastoderm a layer
of cells forms a thin membrane — the ectoderm. About the
middle appears a rounded group of cells which passes to the
periphery and becomes connected with the ectoderm ; a cavity
is formed in its interior and it becomes converted into the
embryonal pharynx. Mean-
while a group of four cells
enclosing a cavity is modified
to form the foundation of the
endoderm, and increasing in
number gives rise to the em-
bryonal intestine, into which
the embryonal pharynx soon
opens, the latter opening on the
exterior by a mouth aperture.

The embryonal pharynx (Fig.

222, ph}) has the function of FIG. 221.— Mliller's larva of Yungia.
«;wallnwiTi(y thp vnllr maftpr with Lateral view. 2-6, ciliated processes.

swaiiowmg tne yoiK-matter witn (From MacBride, after Lang.)
which the embryonal intestine

becomes greatly distended. At a subsequent stage the embryonal
pharynx and intestine are aborted, and the former comes to be
represented merely by a mass of cells. In this a cavity arises
—the cavity of the permanent pharynx. The permanent intestine
becomes formed and the cavity of the pharynx opens into its lumen.
Subsequently the permanent mouth makes its appearance. The
brain is formed in the thickness of the blastoderm, and thus
appears to be of mesodermal origin, not of ectodermal, as in the
Polyclads.

In some Rhabdocceles (certain species of Mesostoma) two distinct
kinds of eggs are formed — summer and winter eggs. The oosperms
are deposited singly, and each, together with a mass of yolk-cells, is
enclosed in a chitinous, usually stalked, shell. Segmentation
takes place very much as in the Polyclads. No embryonal
pharynx is formed, the permanent pharynx performing the function
of swallowing the yolk-material : it appears to be of endodermal
derivation. The intestine arises from a group of cells — the
primitive endoderm — among which a cavity appears. But in
some Rhabdocoeles no definite intestinal epithelium is developed,
and the syncytial mass which represents it is only to be dis-

272

ZOOLOGY

SECT.

tinguished from the surrounding mesodermal syncytium by its
enclosing the remains of the yolk. No metamorphosis is known
to occur.

An account has already been given (p. 240, Fig. 193) of the
development and metamorphosis of the Liver-Fluke (Fasciola
hepatica) which may be looked upon as typical of the Digenetic
Trematodes in general. There is thus to be recognised in the
Digenetic Trematodes an alternation of generations comparable to
that which has been described as so general in the Ccelenterata.
In the Trematoda, however, it is to be observed, it is an alterna-
tion of a sexual, not with an asexual, but with a parthenogenetic
generation (the sporocyst), the ova of which develop into a second

FJG. 222.— Sections through embryos of Dendrocoelum lacteum (somewhat diagrammatic).
dzt yolk-cells ; ec, ectoderm ; en, endoderm ; ph,, embryonal pharynx ; phat permanent
pharynx ; wz, wandering cells. (From Korschelt and Heider, after Hallez.)

parthenogenetic generation (the redise) ; and these finally produce
larvae (the cercarise) capable of developing into the sexually
mature form. The term heterogeny is applied to a life-history
of this kind, in which several distinct generations succeed one
another in a regular series.

In some of the Distomidse the eggs, instead of becoming free as
in the case of the Liver-Fluke, are taken directly into the digestive
canal of the intermediate host, and there hatched out. The
sporocyst stage may take the form of a branching tube in the
interior of which cercarise are developed — the redia stage being
omitted. Sometimes the sporocyst becomes directly developed
into a redia instead of giving rise to a generation of the latter by

PHYLUM PLATYHELMINTHES

273

en

such a process of internal development as that described in the
case of. the Liver-Fluke. The cercariae in most Digenetic Trema-
todes only develop further if they succeed in establishing themselves
in a second intermediate host instead of merely becoming encysted
on the surface of herbage, as in the case of the Liver-Fluke. The
cercariae of different Trematodes differ greatly, particularly with
regard to the nature of the tail. In some forms the cercaria is
tailless : such cercariae do not become free, but are taken directly
— with the intermediate host in which they have been developed —
into the digestive canal of the final host.

Among the Heterocotylea, Gyrodactylus (Fig. 203, A) is vivi-
parous, and the remarkable phenomenon is observed that the
embryo (Aj), while
still within the body
of the parent worm,
develops another
embryo (hz) in its
interior, and this
again develops a
third. The rest of
the Heterocotylea
deposit eggs each of
which, within a
chitinous shell, con-
tains an oosperm and
a number of yolk-
cells. Usually there
is a stalk, and often
an operculum. In
general the develop-
ment appears to be
direct ; but Polysto-
mum passes through
a larval stage with
five rows of cilia, and in Diplozoon paradoxum, a parasite on the
gills of certain fresh-water fishes, in which there is also a ciliated
larval stage, the young animals do not become sexually mature
until two of them have permanently united with one another.

Temnocephala produces relatively large eggs, stalked or sessile,
with a thick chitinous shell, enclosing a single oosperm and a mass
of yolk-cells, which later become fused into a continuous mass.
Segmentation results in the formation of an irregular, massive
blastoderm composed of cells of several sizes. In this collects
a rounded group of larger cells, in the middle of which a space
appears (Figs. 223, 224). The space (endoccele) increases greatly
in size, the boundary cells becoming spread out to form a thin
layer, and approaches the periphery of the egg. A part of the

VOL. I. T

FIG. 223. — Section through the blastoderm of Temno-
cephala, showing early stage of endocoele (en).

274

ZOOLOGY

SECT.

en--

endocoele becomes subsequently rounded off to form the internal
lining membrane of the pharynx, the cavity of which remains cut
off from the exterior by a partition, which is the outer part of the
wall of the endoccele, until the young worm is ready to leave the
egg. A short prolongation backwards from this cavity ends blindly

in a mass composed
mainly of yolk, still con-
taining degenerate nuclei
and cells which have wan-
dered into it from the
blastoderm. Only at a
late stage, when the yolk
has become taken up, is
an arrangement of cells
recognisable in the form of
an intestinal epithelium
(endoderm) enclosing a
lumen (intestinal lumen).
An ectoderm likewise does
not appear as an embry-
onic layer, the epidermis
only appearing late, and
extending over the surface
evidently by modification
in situ of cells that have
become separated from the
main body of the blasto-
derm. The rudiments of
the brain are formed in
the blastoderm near the
wall of the endocoele, and
thus have no connection
with an ectoderm. The
excretory sacs and main
vessels are formed from a
small number of large
cells connected with the wall of the endocoele : subsequently
these rudiments shift their position to the dorsal surface on
which the sacs form their permanent apertures piercing the
epidermis. There is no metamorphosis of any kind — aU the organs,
including even the male part of the reproductive apparatus, being
well advanced towards full development before the young animal
leaves the egg.

The egg of a Cestode is similar in essential respects to that of a
Trematode : there is a tough, chitinoid membrane or egg-shell,
which encloses not only the ovum but a number of yolk-cells. The
result of segmentation is the formation of a superficial layer of cells

Fia. 224. — Longitudinal section through the entire
egg of Temnocephala with the shell removed,
showing blastoderm with developing endocoele (en).

v PHYLUM PLATYHELMINTHES 275

(ectoderm) and a central mass, all enclosed in a membrane composed
of a single layer of cells thrown off when the embryo escapes from
the egg. The ectodermal cells become ciliated, so tar as is known,
only in Bothriocephalus ; in the others they are thrown off or
ultimately absorbed without developing cilia. The central mass of
cells alone forms the embryo. The embryo, while still consisting
of a small number of cells, develops a series of six chitinous hooks.
These early changes all take place in the majority of Cestodes while
the egg is still in the uterus of one of the most posterior of the
proglottides of the parent worm. When the proglottis in question
becomes separated off, and has passed out from the body of the
final host, the eggs are discharged.

In order that development may proceed further, the embryo
must in most cases reach the interior of a second or intermediate
host. This is a passive migration, since the embryo of the Cestode is
still confined within the egg-shell, and the transference has to
take place in the water or food. The digestive fluids of this inter-
mediate host dissolve the egg-shell and set free the contained
six-hooked or hexacanth embryo, which bores its way by means of its
hooks to some part of the body in which it is destined to pass
through the next phase in its life-history, and there becomes
encysted.

The phase which follows presents two main varieties. In
cases in which the intermediate host is an invertebrate animal
the hooked embryo develops into a form to which the name of
cysticercoid is given ; when, on the other hand, the intermediate
host is a vertebrate, the form assumed is nearly always that
termed cysticercus, or 'bladder-worm. The cysticercoid form
(Figs. 225 and 226) is to be regarded as the more primitive and
less modified. Cysticercoids of various tape-worms occur in a
great variety of different invertebrates — e.g., Insects of all kinds,
Water-fleas, Centipedes, Earthworms. The hooked embryo loses
its hooks and develops into the cysticercoid in some part of the
invertebrate intermediate host. The cysticercoid consists of three
parts — a tape-worm head or scolex with the hooks and suckers of the
mature worm, a so-called body, and a caudal vesicle. Sometimes
there is a tail recalling to some extent the tail of a cercaria.
Sometimes the caudal vesicle is absent : when present, either from
the first, or as a result of later changes, it encloses the head as
well as the body after the manner of a cyst. While undergoing
these changes the cysticercoid is usually enclosed in an Adventitious
cyst formed for it by the tissues of its host, but it^often lies free
in the body-cavity. The transference to the final host is effected
by the intermediate host, or the part of it containing the cysti-
cercoid, being taken into the alimentary canal of the final host.
Sometimes, if the intermediate host is a relatively small animal,
such as a water-flea, this mayf afe place " accidentally " ; in other

T 2

276

ZOOLOGY

SECT.

cases the invertebrate intermediate host actually forms the food
of the final host. Thus a cysticercoid having as an intermediate
host an Earthworm is taken with the latter into the alimentary
canal of a Sea-Gull — its final host. In this way the cysticercoid
is set free in the alimentary canal of the final host, the head
becomes pushed out from the enclosing caudal vesicle and body
(probably owing to the stimulus of the higher temperature), so
that the suckers and hooks come into play and attach the young
tape-worm to the wall of the alimentary canal.

The cysticercus or bladder-worm differs from the cysticercoid
mainly in its much greater size and in the development of a

ros

etc

FIG. 225. — A Cysticercoid (Po!>jcei-ciis)
with the head and rostellum enclosed by
the caudal vesicle, a. aperture through
which evagination takes place ; 'fctf. body ;
c. cavity of cyst ; caud. caudal vesicle ; er.
aperture of excretory system ; ros. rostel-
lum ; s. sucker. (After Haswell and Hill.)

FIG. 226.— A 'Cysticercoid with the
rostellum evaginated. ros. rostellum ;
s., s. suckers ; f caud. caudal vesicle.
(After Haswell and Hill.)

relatively large caudal vesicle or caudal bladder. When the hooked
embryo has reached that part of the vertebrate host in which it
is destined to develop into the cysticercus it undergoes a remark-
able change ; it becomes greatly enlarged, and a cavity, filled with
fluid or with a very loose form of connective-tissue, appears in
its interior, so that it assumes the appearance of a relatively
large bladder. On one side of this bladder appears a small
invagination with a cavity opening freely on the exterior. On the
bottom of this is formed an elevation projecting into its interior ;
this is the rudiment of the rostellum on which the hooks are
borne ; at its base, on the inner surface of the side walls of the

PHYLUM PLATYHELMINTHES

277

invagination, appear the suckers. When inverted this invagination
corresponds closely with the head and body of the cysticercoid ;
the bladder corresponds to the caudal vesicle. Thus the chief
difference between a cysticercus and a cysticercoid is that in the

FIG. 227. — Cyst of Tacnia echinococcus with the devekmlng'daughter-cysts and scolices.

(After leuckart.)

former the caudal vesicle is relatively very large and that the order
of development of the parts is somewhat modified.

A very small number both of cysticercoids and cysticerci
multiply by proliferation — by the formation of more than one

tape-worm head from one
embryo. In the few instances
in which this occurs among

the cysticercoids the hooked
embryo gives rise, not directly
to a cysticercoid, but to a

228 — Scolioes of T. echinoccccus.
(After Cobbold.)

FIG. 229. — Separate scolex of
T. echinococcus. (After
Cobbold .)

mass of cells from which are given off a number of buds, each
developing into a cysticercoid with, the three parts already
described. One such form occurs in certain Earthworms, another
in a Myriapod (Glomeris limbatns).

Tcenia ccenums of the Dog has a bladder-worm stage in the
Sheep and Rabbit which gives rise to several tape- worm heads,
and the same holds good of Tcenia serialis from the Fox, But

278

ZOOLOGY

SECT.

the best known instance of multiple production of scolices in a
cysticercus is Tcenia echinococcus — well known as cause of the
disease termed Tiydatids, common in Man and in various domestic
animals. In this case the hooked embryo develops into a large
mother-cyst, from the interior of which daughter-cysts are budded off
(Fig. 22'?). Eventually from the walls of these daughter-cysts
there are formed numerous tape-worm heads,
or scolices (Figs. 228 and 229), which, when
fully formed, assume the appearance of
cysticercoids without the caudal vesicle.
These are readily detached, and should the
organ in which the cyst has been developed
be devoured by a Dog — which is the final
host of the parasite — some of these scolices
become attached to the wall of the intestine
and develop into the adult Tcenia echinococcus
— which are very small as compared with
the size of the cyst and as compared with
other tape-worms. The eggs, passing out
with the faeces of the Dog, may be taken
into the digestive canal of Man or of one of
the domestic animals, and the minute em-
bryos escaping, reach some organ, such as the
liver or lung, in which they are capable of de-
veloping into a comparatively enormous cyst.
Asexual reproduction also occurs in <
une Platyhelminthes. In some Rhabdoccele
Turbellaria (Microstomum) a process of bud-
ding (Fig. 230) results in the formation of
strings of sexual individuals which may
eventually separate ; the new bud is always
formed from the posterior end of the last
FIG. 23p.— Process of budding individual of the string.

~Tey 'e-scpot;' ^ The sporocyst stage in the Trematodes may,
vonmGraff") ® dfeBdy mentioned, multiply by budding or
fission. The formation of new proglottides
in the Tape-worm may be looked upon either simply as growth
accompanied by segmentation, or as asexual multiplication, accord-
ing as we regard the proglottides as segments of a simple animal
or as zooids of a colony. There is this essential difference between
the formation of proglottides and the asexual multiplication by
budding in Microstomum, that in the former case the proglottides,
when they have been formed by segmentation of the undivided
part behind the head, do not in turn give rise by budding to new
proglottides. Spontaneous transverse fission has been observed
in certain Tricladida, and is often followed by the regeneration of
the lost portion.

v PHYLUM PLATYHELMINTHES 270

6. DISTRIBUTION, MODE or OCCURRENCE, AND MUTUAL
RELATIONSHIPS.

Of all the great groups of the animal kingdom above the
Protozoa the Platyhelminthes are the widest in their distribution.
Members of the phylum occur on land, in fresh water down to the
bottom of some of the deepest lakes, on the sea-shore, in the deep
sea, and on the surface of the ocean ; and parasitic Flat-worms live,
in one phase or another, in animals of nearly every class of the
Metazoa.

As regards their mode of life, they present almost every possible
gradation between free-living forms which procure their food — con-
sisting of minute animals and plants — by their own exertions, and
forms that are only capable of living in a special part of the
interior of a certain other animal, and are quite incapable of pro-
curing food for themselves, living by the passive absorption of the
juices of their host or of its digested food. The Turbellaria are
for the most part free-living, and their food consists of small
Crustacea or the larvae of larger forms, Insect larvae, Water-mites,
Rotifers, small Worms, and the like ; or sometimes of Diatoms and
minute Algae of various kinds. Some, however, live a life of true
parasitism. Such are certain Rhabdocceles which are parasitic in
the alimentary canal of various Holothurians and Gephyreans (vide
Sections IX and X). In these there is correlated with the inactive
mode of life a tendency to degradation of structure, a degradatior
which is characteristic of parasites in general : the pharynx is
reduced in size as compared with that of non-parasitic allied
forms, not being required for the capture and swallowing of living
prey ; and the eyes, useless to an animal living in complete dark-
ness, are absent. Some of the Turbellaria, though not parasitic
in the strict sense, live in a state of commensalism with another,
larger, animal : that is to say, are more or less constantly associated
with it, living on its surface or in one of its cavities that open
freely on the exterior, and often sharing its food. An example of
this mode of life is the Triclad Edelloura, which lives on the surface
of the King-Crab (Limulus).

While a free existence is the rule in the Turbellaria, true
parasitism is the rule in the Trematodes, and is universal in the
Cestodes. The majority of the Monogenetic Trematodes are
external parasites, living on a part of the outer surface of a
larger animal, and feeding on mucus and other secretions of the
integument. Many are parasites on the gills of Fishes. A few,
however, inhabit the interior of various organs, and are true
internal parasites : one, for example (Polystomum), lives in the
urinary bladder of the Frog ; another (Aspidogaster) lives in the
pericardial cavity of a Fresh-water Mussel. At least one family of
Trematodes (the Temnocephalea) are not parasites at all in the

280 ZOOLOGY SECT.

strict sense of the term, living on the surface of the " host "
animal, depositing their eggs there, and being carried about
by it, but subsisting on minute living animals captured in the
water.

^ — The Digenetic Trematodes are all internal parasites, and in the
adult condition inhabit, in nearly all cases, the alimentary canal,
liver, or lungs of some vertebrate animal, swallowing the
digested food or various secretions of their host. But, as mentioned
before in the account given of their development, they are internal
parasites, not only in the adult condition, but throughout the
greater part of their life. After a short period of freedom as
ciliated larvae, they again enter into a state of parasitism as sporo-
cysts or rediae in a second host ; and, after a second free interval as
cercariae, may enter the body of a third host to become encysted.
The second host is, very generally, a Mollusc, and the cercaria
may become encysted in the same animal.

The Cestodes are, of all the Platyhelminthes, those that are most
modified in accordance with the condition of internal parasitism
in which they remain throughout life. The adult Cestode is
almost always an inhabitant of the alimentary canal of a verte-
brate. The intermediate host is frequently also a vertebrate —
commonly of a kind which is liable to become the prey of the
final host. In the case of Tcenia crassicollis of the intestine of
the domestic Cat, for example, the cysticercus stage occurs in the
livers of Rats and Mice ; the cysticercus of Tcenia serrata of the
Dog is found in Hares and Rabbits. But in many cases the inter-
mediate host is an invertebrate. In either case the passage from
one host to another is a passive translation, not an active
migration as in the Trematodes.

A few human parasites belong to the Trematoda, but none
that are of very common occurrence among Europeans. Fasciola
hepatica has occasionally been found in the human liver ; Dis-
tomum rathousii is a common intestinal parasite in China ;
Opisthorchis sinensis occurs in the liver of Man in China and Japan ;
Dicroc&lium lanceatum and various other species of the genus
occasionally occur in the human subject. Schistosomum hcema-
tobium and S. japonicum (Fig. 231), otherwise known as Bilharzia
hcematohia, and B. japonica, which differ from most other Trematodes
in being unisexual, are found in pairs in the sub-mucous tissue of
the human urinary bladder or rectum in various parts of Africa
and South America, in Arabia, the Philippines, and Japan. Eggs
with contained larvae are voided with the urine, and if they reach
water, the ciliated larvae or miracidia penetrate into the interior
of a fresh- water snail and give rise to sporocysts from which cercariae
are developed. The latter become free, and, if the opportunity
presents itself, may enter the human body through the skin or the
mucous membrane of the mouth, and, entering the venous system,

PHYLUM PLATYHELMINTHES

281

find their way usually to the veins of the bladder or the large intes-
tine and thence to the sub-mucous tissue, where they become
sexually mature.

The commonest human Cestode parasites among Europeans are
Tcenia solium and T. saginata (otherwise called T. mediocanellata).
The cysticercus stage of T. solium (Cysticercus celluloses) occurs,
as already stated, chiefly in the muscles of the Pig, that of
T. saginata in the muscles of the Ox ; and the relative prevalence
in different countries of these two Tape-Worms varies with the
habits of the people with regard to flesh-eating : where more
swine's flesh is eaten in an imperfectly
cooked state Tcenia solium is the more
prevalent ; where more beef, T. saginata.
Bothriocephalus latus, a very large tape-
worm without hooks, and with a pair of
longitudinal sucking-grooves on the head
instead of ordinary suckers, is a common
human parasite in Eastern countries. Its
cysticercus, which is elongated and solid,
occurs in the Pike and certain other fresh-
water Fishes.

Of all the Cestode parasites of man, how-
ever, the most formidable is one which
occurs in the human body, not in the
sexually mature or strobila condition, but
in that of the cysticercus. This is Tcenia
echinococcus , the presence of which produces
what is termed hydatid disease (p. 278).
The adult Tcenia echinococcus is a very
small tape- worm with only three or four FIG. 231.— stuiarzia bsema-

,1 - , f tobia. The female (?) Iyin<?

proglottides, occurring in the intestine or m a \entrai groove of the
the domestic Dog. The eggs passing out
with a liberated proglottis in the faeces
may reach the alimentary canal of Man
uninjured in drinking water, on the surface of salad vegetables,
and the like ; and, the egg-shells becoming dissolved, the contained
hooked embryos bore their way to the liver or the lungs or some
other organ. Arrived at its final destination, the embryo develops
into a cyst, which may become of enormous size. In the interior
of the primary or mother-cyst are developed a number of secondary
or daughter-cysts, and from the walls of these, both internally and
externally, are formed very numerous scolices in the way already
described (p. 278). Hydatid cysts are very common in some
domestic animals (Oxen, Sheep), as well as in Man. Various other
Cestodes occur in the bladder- worm stage occasionally in Man-
e.g., the Cysticercus cellulosce of Tcenia solium.
The most primitive of the Platyhelminthes are, without doubt,

canal ; ms. oral sucker of
male ; v.s. ventral suckers.
(After Leuckart.)

282 ZOOLOGY SECT.

some of the simplest Turbellaria, and it is among these that we
must look for the nearest existing relatives to the Coelenterata.
In none, however, is the relationship very close. Cceloplana
and Ctenoplana (p. 221) are probably rather to be looked upon
as Ctenophores specially modified in accordance with a creeping
mode of progression than as intermediate forms between Cteno-
phores and Turbellaria. The relationship with the Coelenterata is
shown, perhaps, most strikingly when we take into account the
development of the Turbellaria, in the earlier stages of which there
is to be recognised a marked tendency towards a radial symmetry.
In their development the Turbellaria, that is to say the Polyclads,
show some special points of resemblance to the Ctenophora ; the
ectoderm cells are formed and spread over the rest in a similar
way, and the bands of cilia have a disposition and mode of move-
ment that strongly bring to mind the ciliary swimming plates of
the Ctenophora. But though there is much to be said in favour
of the view that the Turbellaria and the Ctenophora were derived
from a common, not very distant, stock, the latter are too
specially modified to be looked upon as the direct ancestors of
the former.

The connection between the Turbellaria and the Monogenetic
Trematodes is very close — so much so that it is difficult to give
any characters of universal occurrence distinguishing all the
members of the two classes. The Trematodes are, in fact, to be
looked upon as Turbellaria some of whose external characteristics
— and, in the case of the Digenetica, whose life-history — have been
specially modified in accordance with a parasitic mode of life. It
is not unlikely that the Trematodes may be a polyphyletic group —
i.e., that different families may have become developed from
different families of Turbellaria altogether independently, some
of them appearing to be nearer the Rhabdocceles, others nearer
the Polyclads, and others, again, nearer the Triclads, in the majority
of their characters.

The remarkable life-history of the Digenetic Trematodes may,
as already pointed out, be looked upon as a special form of alter-
nation of generations — the alternation of a sexual with a pwdo-
genetic and parthenogenetic generation (heterogeny). The sporocyst
and redia are to be regarded as intercalated stages — as cercariae
which exhibit psedogenesis. The cercaria is the characteristic
larval stage of the Trematodes, and corresponds to the cysticercus
or cysticercoid of the Cestode. The most important difference
between these is in the presence in the former of an enteric cavity,
and its absence in the latter. There seems to be something to be
said in favour of the view that the enteric cavity of the cercaria is
represented by 'the frontal sucker of some scolices, and by the
zostellum of the majority.

Between the adult Cestodes and the Trematodes an intimate

PHYLUM PLATYHELMINTHES

283

relationship is traceable. Oaryophyllceus (Fig. 209) is a Cestode
which, but for the absence of an enteric cavity and the want
of organs of adhesion at the posterior end, is not far distant
from the Trematodes ; and the same might be said of Gyrocotyle
(Fig. 210), Amphilina, and Archigetes (Fig. 21 1).1 The most
important difference between a Cestode and a Trematode, in
addition to the absence of an enteric cavity in the former and
its presence in the latter, is the occurrence in the Cestodes of
strobilation. Ligula in a certain sense forms a connecting link in
this respect between the Trematode and the ordinary Cestode, the
body being elongated and the reproductive organs repeated as in

Monogenetica

Nemertinea

Polycladida

\Temnoc phalea
Tricladida

Rhabdocoelida

Digenetica Polyzoa
Monozoa

Lower Coelenterata
Fio. 232.— Diagram of the relationships of the Platyhelmlnthes (together with the Nemertinea).

the normal Tape- Worm, but there being no corresponding division
of the body into a string of definitely separated proglottides.

Of importance in connection with the subject of the relationship
of Trematodes and Cestodes is the question whether the scolex of
the latter is at the end corresponding to the anterior end of the
former, or whether it is the free end of the strobilia that is in
reality anterior. In favour of the latter conclusion is the fact
that the hooks of the hexacanth larva, developed at its anterior
end, are found in the cysticercoid to lie in the tail region, i.e., the
region most remote from that which develops the scolex, and thus
at the end which should represent the free extremity of the
strobila. On the other hand, the specialisation of the nervous
system to form quite definite and comparatively elaborate nerve-

1 It is possible, however, that in the last two forms we have, to do with
larval Cestodes which have failed to reach the mature stage, and have under-
gone a precocious development of the sexual apparatus.

284

ZOOLOGY

SECT.

centres (brain) in the scolex of some Cestodes (e.g., Moniezia]
tells in favour of the view that the scolex is anterior and
corresponds to a head.Jjjf

APPENDIX TO PLATYHELMINTHES.

ong.ne

CLASS NEMERTINEA.

General Features. — The Nemerteans are non-parasitic, un-
segmented worms, most of which are marine, only a few forms li ving

on land or in fresh water. They
are commonly looked upon as
nearly related to the Turbellaria
and were formerly included in that
class ; but in some respects they
are higher in organisation than
the Turbellaria, and they exhibit
certain special features distinguish-
ing them from the rest of the
lower Worms.

The body (Fig. 233) is nearly
always narrow and elongated,
cylindrical or depressed, unseg-
mented and devoid of appendages.
In length it varies from a few
millimetres to as much as twenty-
seven metres. In some cases there
is a short narrower posterior
region or " tail " ; a head is rarely
marked off from the body proper.
The entire surface is covered with
vibratile cilia, and frequently the
integument is vividly coloured.
Gland-cells of the epidermis secrete
a mucous matter, which may serve
as a sheath or tube for the animal.
The mouth (m.) is at or near the
anterior extremity on the ventral
aspect. Near it in front (rarely
united with it) there is an opening

FIG. 233. — Diagram Of the organs of a
Nemertine, from below, a. anus ; br.
brain ; div. caeca ; long. ne. longitudinal . , , , . ,

nerve-cords ; m. mouth ; n. nephridia ; through Which Can be protruded a

r -r™™- i~^«. muscular

or. ovaries ;
Hubrecht.^

pr. proboscis.

very long muscular organ, the
proboscis (pr.), the possession of

which is one of the most characteristic features of this class of
Worms. The proboscis is hollow ; when extended to its utmost, a
part still remains which is not capable of being everted. This
hollow tube (Fig. 234) is open in front, wfrere its edges are continuous

v PHYLUM PLATYHELMTNTHES 285

with the body-wall, and closed behind. Its wall in the eversible part
consists of an epithelium (internal when at rest) continuous with
the epidermis and similar to the latter, a basement membrane,
and either two or three layers of muscle, circular and longitudinal,
with an external thin epithelium of Hat cells. The circular muscular
fibres are not continued back on the non-eversible part, but the
longitudinal fibres pass backwards to form the retractor muscle, by
means of which the proboscis is attached to the sheath in which
it is enclosed, and by means of which also it is retracted. The
internal epithelium of the proboscis develops variously formed
and arranged papillae, and in most cases its cells form rods of a"
similar character to that of the rods or rhabdites of Turbellaria.
In the part between the eversible and non-eversible regions — a part

FK;. £84.-— Diagrammatic representation of proboscis : (A) in the retracted condition, (B) in the
everted condition, g. p. glandular portion of the proboscis ; m. muscle attaching the
proboscis to its sheath ; m. p. muscular portion of the proboscis ; p. p. in A, proboscis pore ;
p. p. in B represents the position of the proboscis pore in the retracted condition of the
proboscis ; p. s. proboscis sheath. (After Sheldon.)

which may itself become elongated and complicated in structure —
is developed in many Nemerteans (Metanemertini) a median
calcareous stylet (Figs. 237, 238) with groups ef smaller accessory
stylets at the sides. In the everted proboscis these are borne
at the free anterior extremity, and are thus capable of being
used as weapons. In Drepanophorus there are a number of small
stylets supported on a narrow curved plate, together with accessory
stylets. In the rest of the Nemerteans stylets are not developed.
It is by contraction of the muscular walls of the sheath, the
cavity of which (rhynchoccele) contains a corpusculated fluid, that
the proboscis becomes everted. The abundant nerve-supply of
the proboscis points to its being used partly as a tactile organ.

The outermost layer of the body-wall is an epidermis of
columnar cells, many of which are ciliated, while others are

286

ZOOLOGY

SECT.

cilgr
cer.g

tat-, ne

prob

neph

unicellular glands, some of
which are arranged in groups ;
these secrete the mucus with
which the surface is usually
covered, and which may form
a gelatinous tube. Beneath
the epidermis is a basement
membrane, very thin in most
cases, followed by the muscular
layers. In some Nemerteans
(whence called Dimyaria) there
are only two layers of muscular

dors.ves fibres, an outer circular and
an inner longitudinal ; in the
rest (Trimyaria) a third (longi-
tudinal) layer is superadded.
Another circular layer of mus-

op. neph cular fibres closely encompasses
the digestive canal. The inter-
space enclosed by the outer
muscular layers does not com-
prise any cavity corresponding
to a true coelome or body-
cavity, except, perhaps, the
cavities of the gonads, the
interspaces between the organs
being filled with parenchyma
(Fig. 239).

The digestive canal con-
sists of a tube which extends
throughout the length of the
body from the mouth— situated
near the anterior extremity on
the ventral side — to the anus
at the posterior extremity.1
The mouth is usually placed
some distance behind the pro-
boscis pore, but may be shifted
forwards so as to lie close to
the latter, or to be incorporated
with it. The first part of the

FIQ. 235.— Tetrastemma. General view of the digestive Canal is Usually a

J8H? m-raSf; Sti. && ^Svse0royf simple tabe-o»op%iM (stomo-
y^S^SS^S&^SSfi^t ^H"** may be more

nephridium; op. neph. nephridial aperture; Complicated, and divided into
prob1. eversible part of proboscis ; prob*. non-

eversible part of proboscis ; prob. ap. aperture * When a tail is present the

for the protrusion of the proboscis ; retr. mus. intpqtina mav or rnav not hf»
retractor muscle of the proboscis • at stylet Des™ne may. or may tot, I

(From Hatschek's Lehrbuch.) continued through it.

retr. mus

PHYLUM PLATYHELMINTHES

287

FIG. 236. — Anterior portion of
the body of a Nemertine.
br. brain-lobes ; n. lateral
nerves ; p. o. external open-
ing through which the pro-
boscis is everted ; p. s. pro-
boscis-sheath ; pr. proboscis.
(Esophagus and mouth shown
by dotted lines. (After
Hubrecht.)

various regions, sometimes with paired diverticula. Posteriorly it

opens into the intestine. The intestine, constituting by far the

greater part of the length of the canal, may be a simple uncon-

stricted tube, or may be only slightly con-
stricted at intervals by the paired gonads.

In most cases the constrictions correspond-
ing to the gonads are very deep, so that

the intestine comes to be provided with

two rows of lateral diverticula or caeca,

which may be branched. The caeca are

separated from one another by incomplete

transverse septa of dorso-ventral muscular

fibres — the arrangement of the caeca and

septa with the alternately arranged gonads

bringing about an appearance of imperfect

metamerism such as is observable in some

of the Platyhelminthes (Gunda, species of

Temnocephald).
The Nemerteans possess a system of

vessels usually regarded as representing a

blood-vascular system (Figs. 235 and

240), with well-defined walls consisting of

a layer of epithelium surrounded by a thin

layer of muscular fibres arranged circularly.

There are three principal longitudinal trunks — a median dorsal

(dors. ves. d.b., d.v.) and two lateral (lat. ves. l.v., l.b). The blood is,

in most cases, colourless, and contains rounded or elliptical, usually

colourless, corpuscles.

The excretory system has a considerable resemblance to that

of the Platyhelminthes. It
consists of a pair of longi-
tudinal vessels (Kg. 235, neph.)
which give off branches, by
one or several of which each
communicates with the ex-
terior. In one species of
Eupolia there are also ducts
opening into the alimentary
canal. The fine terminal
branches of the system are
provided with ciliary flames,
each situated in the midst of

FIGS. 237 and 238. — Proboscis of a Bleta- a arniTn nf rplls nnt in flip
nemertean, with stylet reserve-sacs arid ? §r?UP ,O1 euf> ^°

muscular bulb. Fig. 237 retracted, Fig. 238 interior of a single name-cell as

in most cases in the Flat- worms.

There are no special organs of respiration in any of the group.
But there is evidence that this function is carried out, in part at

288

ZOOLOGY

SECT.

least, by the taking in and giving out of water through the mouth
by the oesophagus.

The nervous system is in some respects more highly developed
than in the Turbellaria. The brain (Figs. 233 and 236 br., and Figs.
235, cer. g., and 240, n.g.d., n.g.v.) is composed of two pairs of
ganglia, dorsal and ventral, the ganglia of each pair being connected
together by commissures, the dorsal situated above, the ventral
below, the anterior part of the proboscis and proboscis sheath, and
both being above the mouth and oesophagus. From the brain
pass backwards a pair of thick longitudinal nerve-cords which
run .throughout the length of the body. Usually these are

p.

^-'- s.l

d.b

FIG. 239.— Diagrammatic transverse section of a KTemertean (Heteronemertini) through
the middle region of the body. b.m. basement membrane ; c.m. circular muscle layer ;
d.b. dorsal blood-vessel ; ep. epidermis ; ff- gonads ; int. intestine ; l.b. lateral blood
vessel ; l.m. longitudinal mu,sclevlayer ; n.c. lateral nerve cord ; n.L nerve plexus ; p.
proboscis ; p. s. proboscis aJj^th ; s.t. subcutaneous layer. (After Sheldon.)

lateral in position^ sometimes approximated dorsally, sometimes
ventrally. The lateral nerve-cords generally meet posteriorly in a
commissure usually situated above, but in one genus below, the
anus. A third median dorsal nerve of smaller size than the
lateral cords extends backwards from the dorsal commissure of the
brain. Associated with the nerve-cords in the Protonemertini and
the Heteronemertini is a nerve-plexus (Fig. 239, n.L) extending all
over the body. In the Metanemertiniy instead of a nerve-plexus,
there is a series of slender transverse connectives running across at
short intervals between the lateral nerve-cords, and from each cord
are given off numerous branches arranged with some regularity.

The position of the brain and lateral nerve-cords and the
nerve-plexus, or the system of commissures and nerve-branches,
varies in the different groups. In the Protonemertini . they
occupy the most primitive position, being quite superficially

PHYLUM PLATYHELMINTHES

289

-\ — v.s

situated at the bases of the epidermal cells. In the rest they are
deeper : in the Metanemertini they lie in the parenchyma within
the muscular layers. The median cord is always, except in the
Heteronemertines, superficially placed.

A remarkable apparatus connected with the nervous system
is that composed of a pair of peculiar structures known as the
cerebral organs. When most highly developed these consist
of a pair of ciliated tubes, opening externally in the region of the
brain and terminating internally in close relation to the dorsal
ganglion of the brain or a special ganglion distinct from the latter.
The external aperture may be
situated in a groove or furrow
(Fig. 235, til. gr.), vertical or
horizontal, of varying extent.
This apparatus may have a
respiratory function, more
especially for the oxygenation
of the substance of the brain,
but perhaps it has also a
sensory function. It has some
resemblance to the ciliated
pits developed in certain
Turbellaria.

Eyes are present in the
majority of Nemerteans, and
in the more highly organised
species occur in considerable
numbers. Sometimes they
are of extremely simple struc-
ture ; in other cases they are
more highly developed, having
a spherical refractive body
with a cellular " vitreous
body," and a "retina" con-
sisting of a layer of rods
enclosed in a sheath of dark
pigment, each rod having a
separate nerve-branch connected with it. Statocysts containing
statoliths have been found in only a few of the Nemerteans.

Reproductive System.— Most species are dioecious. The
ovaries (Fig. 233, ov.) and testes are situated in the intervals between
the intestinal caeca. The ovary or testis is a sac the cells lining
which give rise to ova or spermatozoa ; when these are mature each
sac opens by means of a narrow duct leading to the dorsal, rarely
to the ventral, surface, 'on which it opens by a pore. In all
probability the cavities of these hollow gonads are all that
represent the ccelome of higher forms.

VOL. i. u

l.n-

FIG. 240. — Diagram of anterior end of a
Nemertean (Metanemertini). a. n. anterior
nerves ; d. c. dorsal commissure ; d. n. dorsal
median nerve ; d. v. dorsal vessel ; /. v. lateral
vessel ; n. c. lateral nerve-cord ; n. d. g. dorsal
ganglion ; n. g. v. ventral ganglion ; p. p. pro-
boscis pore ; p. a. proboscis sheath ; v. c. ventral
commissure ; v. s. vascular ring or collar.
(From Sheldon, modified from Mclntosh.)

290

ZOOLOGY

SECT.

Development. — Some of the Nemerteans go through a meta-
morphosis ; in the others the development is direct. The

characteristic larval
form is the pili-
dium (Fig. 241).
This is a helmet-
shaped body with
side lobes like ear-
lappets, and a
bunch of cilia re-
presenting a spike.
In the metamor-
phosis two pairs of
ectodermal invagi-
nations, growing
inwards around the
intestine, fuse to -
gether and form
the integument and
body- wall of the
future worm, which
subsequently frees
itself from its in-
vestment and deve-
lops into the adult
form. In others
there is a ciliated
creeping larva
called the " larva
of Desor" in the
interior of which
the larval worm is

FIG. 241.— 1, Pilidium with advanced Nemertine worm ; /q^,™!™^

B, ripe embryo of Nemertes from interior of pilidium. developed

an. amnion, or investment of the embryo ; i. intestine ;

Ip. lateral pit ; n. nervous system ; ce. gullet ; pr. pro-
boscis ; st. stomach. (From Balfour, after Butschli.)

:n |"Up PA.QP r>f thp
pilidium.

DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nemertinea are ciliated, unsegmented worms with elongated
body, without distinct ccelome. There is an eversible proboscis
enclosed in a sheath and capable of being protruded to a great
length through an aperture situated usually in front of and
above the mouth. The intestine usually has distinct lateral
diverticula, and there is a posteriorly situated anus. There is a
blood- vascular system, and also a system of excretory vessels with
ciliary flames.

v PHYLUM PLATYHELMINTHES 291

ORDER 1. PROTONEMERTINI.

Dimyarian Nemertines with the lateral nerve-cords situated
outside the muscular layers. The mouth is situated behind the
brain. The proboscis is devoid of stylet.

ORDER 2. MESONEMERTINI.

Dimyarian Nemertines having the lateral nerve-cords withdrawn
within the musculature of the body-wall. The mouth is situated
behind the brain. There are no stylets.

ORDER 3. METANEMERTINI.

Dimyarian Nemertines, in which the lateral nerve-cords lie
inside the muscular layers in the parenchyma. The mouth is
situated in front ot the brain. The proboscis is provided with
stylets. A ventral (stomodaeal) caecum is present.

ORDER 4. HETERONEMERTINI.

Trimyarian Nemertines, in which the lateral nerve-cords are in the
muscle-layers, between the outer longitudinal and the circular layers.
The mouth is situated behind the brain. The proboscis hasno stylet.

The Nemerteans are almost exclusively marine ; and the greater
number live between tide-marks or at moderate depths ; a few
have been obtained from considerable depths. The comparatively
small number of terrestrial and fresh-water forms are all
Metanemertini. The Nemerteans progress for the most part by
slow crawling movements, leaving a track of slime behind them.
Some burrow freely in mud or sand, the proboscis being made use
of to help in the process. Some are able to swim by means of
undulating movements of the body. Nearly all are carnivorous,
and either capture living prey in the shape of small invertebrates
of various kinds, or feed on dead fragments. The chief function
of the proboscis is the capture of living prey, around which it
becomes coiled and then draws the prey towards the mouth. One
Nemertean lives in the interior of a Crustacean, and is probably a
true parasite. Others live, apparently as commensals or mess-
mates, in the pharynx or atrial cavity of Ascidians, or within the
mantle cavity of bivalve Mollusca.

A striking feature of the Nemerteans is the readiness with
which, on being irritated by handling or by the action of some
chemical agent, they break up transversely into fragments. This
takes place most freely when the body is highly charged with
sexual products, but is by no means confined to that condition.
The process probably takes place spontaneously under certain
circumstances. The broken-ofi fragments may remain alive for a
considerable time, and under suitable conditions regeneration of
the lost parts is readily effected, so that it is possible to look upon
the entire process as a form of asexual reproduction.

u 2

SECTION VI.

PHYLUM NEMATHELMINTHES.

THE members of the preceding phylum are characterised, as a
whole, by a marked dorso-ventral flattening. In the Worms
included in the present group the body is elongated and cylindrical,
whence their general name of Round- or Thread-worms. The
phylum includes the following classes : —

Class 1. NEMATODA. — The Round- worms in the strict sense of
the term. The best known forms are internal parasites, but many
genera and species are extremely abundant in fresh- and salt-water
as well as in the soil.

Class 2. ACANTHOCEPHALA. — The " Hook-headed Worms," a
group of formidable internal parasites.

Class 3. CH^ETOGNATHA. — The " Arrow-worms," a small group of
pelagic organisms.

The affinities of the Acanthocephala and Chsetognatha with the
Nematoda are somewhat doubtful, and the association of the three
classes is largely a matter of convenience.

CLASS I.— NEMATODA.

1. EXAMPLE OF THE CLASS — THE COMMON ROUND-WORM OF
MAN. (Ascaris lumbricoides.)

Ascaris lumbricoides is a common parasite in the human intes-
tine : a closely allied if not identical form (A. suillce) occurs in the
Pig, and another (A. megalocephakt) in the Horse. The following
description will apply to any of these. The female A. lumbricoides
is about 20-40 cm. (8-16 inches) long, and about 6-8 mm. (] inch)
in diameter ; the male is considerably smaller.

External Characters. — When fresh the animal is of a light
yellowish-brown colour : it is marked with four longitudinal
streaks, two of which, very narrow and pure white in the
living Worm, are respectively dorsal and ventral in position, and

292

SECT. VI

PHYLUM NEMATHELMINTHES

293

are called the dorsal and ventral lines (Fig. 242, d.l., v.l.) : the othei
two are lateral in position, thicker than the former, and brown in
colour, and are distinguished as the lateral lines. The mouth is
anterior and terminal in position, and is bounded by three lobes,
or lips, one median and dorsal (d. lp.}, the other two ventro-lateral
(v. lp.}. A very minute aperture on the ventral side, and about
2 mm. from the anterior end, is the excretory pore (ex. p.). At about
the same distance from the pointed and down-turned posterior end
is a transverse aperture with thickened lips, the anus (an.), which
in the male serves also as a reproductive aperture and gives exit
to a pair of needle-like chitinoid bodies, the penial setce (pn. s.).
In the female the reproductive aperture or gonopore is separate
from the anus, and is situated on the ventral surface about one-
third of the length of the body from the anterior end (Fig. 245,
gnp.). The sexes are also distinguished externally by the form of
the short tail, or post-anal portion of the body, which in the male

FIG. 242. — Ascaris lumbricoides. A, anterior end from above ; B, the same from below ;
C, posterior end of female, D, of male, side view. an. anus ; d. lp. dorsal lip ; d. I. dorsal
line ; ex. p. excretory pore ; p. papillae ; pn. s. penial setae ; v. 1. ventral line ; v. lp.
ventral lip. (After Leuckart.)

is sharply curved downwards (Fig. 242, D), while in the female (C)
its ventral contour is nearly straight.

Body-wall. — The outer surface of the body is furnished by a
delicate, transparent, elastic membrane, of a firm material of
albuminoid composition, the cuticle (Fig. 243, cu.}. It is divisible
into several layers containing a system of fine vertical channels (A.
megalocephala), and is wrinkled transversely, so as to give the animal
a segmented appearance. Beneath the cuticle is a protoplasmic
layer (der. epthm.) containing scattered nuclei and longitudinal fibres,
and representing a syncytial ectoderm — i.e., an ectoderm in which the
cell-boundaries are not differentiated, and whose cellular nature is
recognisable only by the nuclei. The cuticle is, as usual, a secretion
of the ectoderm.

Beneath the ectoderm is a single layer of muscular fibres (m.),
arranged longitudinally, and bounding the body-cavity. The
structure of the muscles is very peculiar : each (Fig. 244, A) has

294

ZOOLOGY

SECT.

the form of a spindle, striated longitudinally, and produced on its
inner face (i.e., towards the body-cavity) into a large and almost
bladder-like mass of protoplasm (p.) containing a nucleus (nu.).
Apparently the whole of this structure is derived from a single cell,
part of which has become differentiated into contractile substance
(c), the rest remaining protoplasmic. In transverse section the
contractile portion (B, c.) has the form of a plate bent upon itself
so as to be, as it were, wrapped round the protoplasmic portion
(p). The protoplasmic processes project to a greater or less extent
into the body-cavity, sometimes practically obliterating it, and are
produced into delicate filaments (/.) which take a transverse

ink

lal.1

ezc.is

ovy

FIG. 243. — Ascaris lumbricoides, transverse section. CM. cuticle ; rf. /. dorsal line ; der.
epthm. deric epithelium or epidermis ; ex. v. excretory vessel ; int. intestine : lat . I lateral
line ; w. muscular layer ; ovy. ovary ; ut. uterus ; v. v. ventral line. (After Vogt and
Yung.>

direction, and are mostly inserted into the dorsal and ventral
lines.

The muscular layer is not continuous, but is divided into four
longitudinal bands or quadrants, two dorso-lateral and two ventro-
lateral, owing to the fact that at the dorsal, ventral, and lateral
lines the ectoderm undergoes a great thickening and projects
inwards, between the muscles, in the form of four longitudinal
ridges (Fig. 243, d.l.9 v.v., lat. I.). The ridges are composed of fibres
continuous with the fibres of the ectoderm. It is this arrangement
that gives rise to the lines seen externally. The ridges forming
the lateral lines are much more prominent than the other two.

Digestive Organs. — The mouth leads into the anterior division
of the enteric canal, the pharynx, stomodceum or oesophagus (Fig. 245,
ph.) : its walls are very muscular, its cavity is three-rayed in cross-

VI

PHYLUM NEMATHELMINTHES

295

section, and it is lined by a cuticle secreted from the epithelial layer
and continuous, at the mouth, with that of the body-wall.
Posteriorly the pharynx opens into the intestine (int.), a thin-
walled tube, flattened from above downwards, and formed -of a
layer of epithelial cells bounded both internally and externally
by a delicate cuticle : it has no muscular layer (Fig. 243, int.).
Posteriorly the intestine narrows considerably to form the short
rectum, which has a few muscular fibres in its walls and opens
externally by the anus (Fig. 245, an.). The food, consisting of the
semi-fluid contents of the intestine of the host, is sucked in by
movements of the pharynx, and is then absorbed into the system
through the walls of
the intestine. The food
being already digested
by the host, there is no
need of digestive gland
cells, such as occur in
animals which prepare
their own food for
absorption.

It will be noticed
that in the above de-
scription the pharynx
is also called stomo-
dseum. This must not
be taken to indicate
that the two terms are
synonymous, but that,
in the present instance,
the epithelial lining of

FIG. 244.— Ascaris lumbricoides. A, a single muscle
fibre ; B, several fibres in transverse section with
portion of ectoderm (below), c, contractile substance ;
/. fibrous processes ; nu. nucleus ; p. protoplasmic
portion. (After Leuckart.)

the pharynx is derived
from the ectoderm,
being formed as an in-
turned portion of the outer layer of the body- wall. The epithelium
of the intestine, on the other hand, is endodermal, this portion of
the canal being derived from the archenteron of the embryo.

Between the enteric canal and the body-wall is a distinct space,
the body-cavity, containing a clear fluid and more or less
encroached upon by the protoplasmic processes of the muscle-cells.
The cavity is bounded externally by these processes, internally by
the outer cuticle of the intestine : there is no trace of epithelial lining
such as occurs in most of the higher animals. The body-cavity of
the Nematode, in fact, does not exactly correspond to the coelome
to be met with in most higher phyla. It is not to be derived,
directly or indirectly, from the archenteron of the embryo, and
it does not lie, like a true coelome, between layers of the
mesoderm.

ZOOLOGY

SECT.

f Is

III

The excretory system con-
sists oi two longitudinal canals
(ex. v.), one in each lateral line.
Anteriorly these pass to the ventral
surface, unite with one another, and
open by the minute excretory pore
(ex. p.) already noticed. The
system is not ciliated, contains no
flame-cells, and has no internal
openings. Each canal is an ex-
cavation in a single enormously
elongated cell with a single nucleus.

The nervous system consists
of a ring (nv. r.) surrounding the
pharynx and giving off six nerves
forwards and six backwards (Fig.
246). Of the latter two are of
considerable size, and run in the
dorsal and ventral lines respectively
(dln.,vln.). They are connected with
one another by transverse com-
missures (c.), and the ventral nerve
swells into a ganglion just in
front of the anus. The pharyngeal
nerve-ring contains nerve-cells, and
its ventral portion (un.) is thickened
and ganglion-like. The only sense-
organs are little elevations, the
sensory papillce (Fig. 242, p.), on
the lips.

The reproductive organs are
formed on a peculiar and very
characteristic pattern. The testis
(Fig. 247, ts.) is a long, coiled
thread, about the thickness of fine
sewing-cotton, and occupying a
considerable portion of the body-
cavity. At its posterior end it is
continuous with .the vas deferens,
the two passing insensibly into one
another so that the junction is
not visible externally. The vas
deferens, in its turn, becomes con-
tinuous with a wide canal, the
vesicula seminalis\(vs. sem.) which
opens by a short, narrow muscular
tube, the ductus ejaculatorius, into

VI

PHYLUM NEMATHELMINTHES

the rectum. Behind the rectum, and
opening into its dorsal wall, are paired
muscular sacs (s.), containing the penial
setce (pn. s.) already noticed. The anterior
end of the testis consists of a solid mass of
sex-cells ; passing backwards there is
found a cord or rachis occupying the axis
of the tube and having the sperm-cells
attached to it ; still further back the
sperms become gradually differentiated,
and are finally set free in the vas deferens.
The sperms are peculiar rounded cells
(Fig. 23, p. 31, c. d. e.) ; when transferred
into the body of the female they exhibit
amoeboid movements, but as long as they
remain in the male ducts they are non-
motile : they have no trace at any stage
of the characteristic tail of the typical
sperm. In this connection it may be
mentioned that the tissues of Ascaris are
remarkable for the total absence of cilia.

The organs of the female (Fig. 245) re-
semble those of the male, but are double
instead of single. There are two coiled,
thread-like ovaries (ovy.}, each passing in-
sensibly into a uterus (ut.). In the ovary,
as in the testis, the eggs are developed
in connection with an axial cord or rachis.
The two uteri unite in a short muscular
vagina (vag.) which opens, as already seen,
on the ventral surface of the body (gnp.)
at about one-third of the entire length
from the head.

Development. — The eggs are pro-
duced in immense numbers — at the rate,
it has been reckoned, of about 15,000 a
day. They are fertilised in the upper
part of the uterus, each becoming enclosed

FIG. 246.— Diagram of ner-
vous system of Nematoda.
c. commissures ; din. dorsal
nerve ; fo?w. posterior lateral
nerve ; on. upper and un.
under portion of nerve -
ring ; s.g. lateral swellings ;
vln. ventral nerve. (From
Lang, after Butschli.)

pn.$

FIG. 247.— Ascaris lumbricoides, posterior extremity of male, an.

dissected, an. anus; CM. cuticle ; der. epthm. epidermis ; m.
muscular layer ; pn.s. penial seta ; s. sac containing penial seta ;
ts. testis ; vs. sem. vesicula seminalis.

298 ZOOLOGY

SECT.

in a chitinoid egg-shell, and are passed out of the body of the host
with its faeces. The results of experiments render it probable that
infection is direct, without intermediate host, the embryo-containing
eggs being taken, in water, or in soil accidentally swallowed, into
the intestine of a new human host, in which the embryos, escaping
from the eggs, become mature.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Nematoda are Nemathelminthes having a cylindrical body
of great length in proportion to its diameter, and pointed at both
ends. The body-wall consists of a tough external cuticle, an
ectoderm in the form of a syncytium or protoplasmic layer con-
taining nuclei and rarely exhibiting cell-structure, and a single
layer of longitudinal muscular fibres which are interrupted along
one or more (dorsal, ventral, and lateral) lines. The body-wall
encloses a body-cavity containing a clear fluid, and more or less
encroached upon by processes of the muscle-cells or other meso-
dermal tissues. The enteric canal is straight, and consists of
pharynx, intestine, and rectum : the pharynx is a stomodseum.
The mouth is anterior and terminal, the anus ventral and situated
a short distance from the posterior end. Excretory canals, running
in the lateral lines, are usually present. The nervous system con-
sists of a pharyngeal ring containing nerve-cells and giving off
nerves forwards and backwards : of those passing backwards there
is either a single ventral-cord, or there are two chief cords, respec-
tively dorsal and ventral, of considerable size and extending to the
posterior end of the body. The Nematoda are in nearly all cases
dioecious : eggs are produced in immense numbers, and are impreg-
nated within the body of the female. The sperms are non-motile,
or perform amoeboid movements only after entering the female
organs. Cilia are wholly absent.

A large proportion of Nematoda are free-living, spending their
whole life in fresh- or salt-water, damp earth, decaying matter,
etc. ; the remainder are parasitic during the whole or a part of
life.

The class is divided into two orders.

ORDER 1. — NEMATOIDEA.

Nematoda in which the sub-cuticle is a nucleated protoplasmic
layer without cell-outlines. Two chief nerve-cords are given off
backwards from the pharyngeal ring and lie in the dorsal and
ventral lines. There are two excretory canals lying in the lateral
lines and opening anteriorly and ventrally. The gonads are
continuous with their ducts, and consist of long, more or less con-
voluted cords. This order includes nearly the whole of the free-
living Nematodes as well as the large majority of parasitic forms.

vi PHYLUM NEMATHELMINTHES 299

ORDER .2. — NEMATOMORPHA.

Nematoda in which the sub-cuticle consists of a layer of distinct
cells and the pharyngeal nerve-ring sends off a single large ventral
nerve-cord composed of three parallel strands well supplied with
nerve-cells. There are no excretory canals. The reproductive
products are discharged into the body-cavity. This order includes
a small number of greatly elongated, thread-like worms (species of
the genus Gordius), which are parasitic in the asexual, free-living
in the sexual stage ; and is usually regarded as comprising also the
genus Nectonema, which has only been found swimming in the sea.

Systematic Position of the Example.

Ascaris lumbricoides is one of many species of the genus Ascaris,
and belongs to the family Ascaridce of the order Nematoidea.

The absence of an epithelial lining to the body-cavity, and the
presence of elongated gonads continuous with their ducts, indicate
its position as one of the Nematoidea. Among the numerous
families constituting this order, the Ascaridae are distinguished by
the possession of three lips furnished with papillae, and by the
body of the male being curved ventrally and being provided with
penial setae. Ascaris is distinguished from the other genera of the
family by the absence of a bulb-like enlargement at the posterior
end of the pharynx, by the posterior extremity of the body having
the form of a short blunt cone, and by the presence of two penial
setae in the male.

3. GENERAL ORGANISATION.

External Characters. — The Nematoda vary much in size :
the little Anguillula, one of the commonest of aquatic animals, does
not exceed 1 mm. in length, while the dreaded parasite known as
the Guinea-worm (Filaria medinensis) is sometimes as much as
2 metres (6 feet) long. The length is always great in proportion
to the diameter, and the body is always bluntly pointed at the
anterior end and either pointed or forked posteriorly. One of the
most striking cases of disproportion between length and breadth is
exhibited by the free, sexual form of Gordius, one of the Nemato-
morpha ; it is found in earth or water and resembles a tangle of
brown string, the length being frequently as much as 15 or 16 cm.
while the diameter does not exceed 0*5 mm.

Body-wall. — The body is always covered by a cuticle secreted
by the deric epithelium or external ectoderm (sub-cuticle). The
cuticle is usually smooth, sometimes beset with spines or hooks,
sometimes ringed. In certain species it is raised "up to form a pair
of lateral fin-like folds running along the sides of the body for some
distance. Nectonema differs from all the rest in having two rows

300

ZOOLOGY

SECT.

of fine bristles running along either side of the elongated body.
A regular ecdysis or moulting of the cuticle at intervals takes place
in many Nematodes. The sub-cuticle in most cases takes the
form of a protoplasmic layer with scattered nuclei ; but in the
Nematomorpha (Fig. 250, ep.) it consists of a true epithelium — a
single layer of cells the outlines of which are quite distinct.

Beneath the ectoderm is a mus-
cular layer, which in many genera
has the same structure as in
Ascaris, i.e., consists of a single
layer of longitudinal fibres, inter-
rupted at the dorsal, ventral, and
lateral lines, each fibre being
spindle-shaped and produced into
a protoplasmic process which pro-
j ects into the body-cavity. But in
many forms (e.g., Strongylus) the
muscle-cells are flat rhomboidal

Fia. 248.— The body-wall of a platymyarian plates (Fig. 248), and each quad-
Nematode, spread out. lat. 1. lateral lines. * , -i , ,-,

(After Leuckart.) rant contains only two rows, tne

total number in a transverse

section being therefore eight. In Gordius the muscles are inter-
rupted along the ventral line only (Fig. 250), and in Nectonema
only along the dorsal and ventral lines, the lateral lines being
absent. In Gordius, but not in Nectonema, the protoplasmic
portions of the muscle-cells are directed outwards, not inwards.

Enteric Canal. — The mouth is frequently armed with spines
(Fig. 249, C), by means of which the worms draw blood from the

FIG. 249. — Dochznius duodenalis. A, male and female in coitu. B, anterior end,
showing — cv. gl. cervical glands ; ph. pharynx. C, mouth with spines ; D, posterior end
of male, with bursa. (After Leuckart.)

intestinal blood-vessels of their host. Many free-living forms have
a sharp stylet for piercing the tissues of the plants on which they
feed, and a suctorial apparatus for absorbing the juices. The
posterior end of the oesophagus is often dilated to form a globular

VI

PHYLUM NEMATHELMINTHES

301

chamber with muscular walls, the gizzard (Fig. 251, gz.). The only
specially interesting variation in the structure of the intestine
is that occurring in Trichinella (Fig. 253), one of the Nematodes
parasitic in Man, in which this part of the enteric canal consists of
a single row of perforated cells (zh): the lumen is therefore not
wter-cellular but wZra-cellular, like the gullet of an Infusor. In the
sexual stage of Gordius and in Nectonema the enteric canal under-
goes more or less complete degeneration, and in several other genera
there is no anus. The alimentary canal in some rare cases has
hollow appendages in the form of cesophageal glands or intestinal

FIG. 250. — Transverse section of Gordius (female) in the posterior region, cut. cuticle ; d. c.
dorsal canal ; div. diverticulum of lateral canal ; ep. deric epithelium ; int. intestine ;
I. c. lateral canal ; m. 1. muscle layer ; m. v. c. median ventral canal ; n. c. nerve-cord ;
par. parenchyma ; sp. th. spermatheca. (After Rauther.)

caeca. In Dochmius a pair of pear-shaped bodies of unknown
function, the cervical glands (Fig. 249, B, cv. gl.), lie one on each side
of the pharynx and probably open externally near the mouth.

In the Nematoidea the body-cavity is always a single continu-
ous chamber crossed in various directions by delicate fibres and
without epithelial lining. In Gordius the body is solid in the
larva, filled with polygonal cells (parenchyma), but fissures appear,
and, increasing in extent, eventually give rise to extensive canal-
like cavities — a median ventral and two lateral, with, in the female,
a narrow median dorsal, separated from one another for the most

302

ZOOLOGY

SECT.

•be

part by comparatively thin partitions of parenchyma sometimes
termed mesenteries. The median ventral canal (Fig. 250, m.v.c.)
of the body-cavity encloses the alimentary canal: the lateral
canals (I.e.) are associated with the development of the reproductive
apparatus. In the case of the lateral alone, and only in the female, is
there an epithelial lining. In Nectonema the
body-cavity is reduced in the female to a
narrow space between the wall of the ovary
and the muscular layer : in the male it is
considerably wider.

In the Nematoidea when definite excre-
tory organs exist, they take the form of
narrow longitudinal canals similar to those
described as occurring in Ascaris — each
canal being an excavation in a single enor-
mously elongated cell, and the system
having a median ventral opening. Some-
times only one of the canals is developed.
Flame-cells are never present. In the Nema-
tomorpha such a system does not occur.

In the Nematoidea the nervous system
has the structure already described in
Ascaris; it is, however, apparently absent
in some free-living forms. But in Gordius
,df it is much more highly developed : the
pharyngeal ring (brain) is of great thickness
and is continued into a single ventral cord
composed of three strands containing
nerve-cells. In Nectonema there is a similar
ring and three-stranded nerve-cord, and in
addition a large anal ganglion at the pos-
terior end. Eye-spots have been described
in the sexual form of Gordius.

The reproductive organs in all the
Nematoidea resemble those of Ascaris, the
only important variation depending upon
the fact that in the smaller forms the entire
genital tube (gonad plus gonoduct) is short
izzarT, and not coiled (Fig. 251, ts. and v. df.). A
Sis • ^ew forms are hermaphrodite, but, instead of
^(From having a double set of reproductive organs,
as in Platyhelminthes, organs of the ordinary
female nematode-type are present, and the gonads produce first
sperms and afterwards ova. Such animals are said to be pro-
tandrous (male products ripen first), and self -impregnation is as
effectually prevented as if the organs of the two sexes were distinct.
In Gordius and Nectonema (Nematomorpha) the sexes are

VI

PHYLUM NEMATHELMINTHES

303

distinct. In the female Gordius (Fig. 252, C) the lateral canals of
the body-cavity (ut.) are modified to act as ovaries and oviducts.
They give off a series of regularly arranged lateral diverticula from
the cells lining which the ova are derived (Fig. 252, A, and Fig.
250, div.). Behind, each of these lateral canals becomes a narrow
tube or oviduct. These open into a median chamber with glandular
walls, the uterus (vag.), and into this there also opens the duct of a
spermotheca or receptaculum seminis (spth.) for the reception and
storage of the sperms received in copulation. The uterus is con-
tinued backwards by an atrial cavity opening along with the intestine
in a cloacal aperture (Fig. 252, gnp.) at the posterior end of the
body. In the male (Fig. 252, B) the lateral canals are unsegmented

int

v.nv.cd va&

FIG. 252. — Gordius. A, horizontal section of female, showing ovaries (ovy) attached to mesen •
tery (mes.) ; b. w. body-wall. B, posterior extremity of male, sagittal section. 6. c.
coagulated mass of sperms (?) ; cl. cloaca ; int. intestine ; t. tail ; v. nv. cd. ventral nerve-
cord ; vs. sem. vesicula seminalis or sperm-sac. C, posterior extremity of female, sagittal
section, gnp. cloacal aperture ; spth. spermatheca ; ut. lateral canal ; vag. uterus,
followed posteriorly by atrium ; v. nv. cd. ventral nerve-cord. (After Vejdowsky.)

tubes — testes or sperm-sacs (vs. sem.) — from the cells bordering a
portion of which sperms are developed. From each of these at its
posterior end a narrow vas deferens passes backwards to open into
the cloaca (cl). The latter, with a ventrally-placed opening, has
a muscular wall and is probably capable of being everted to serve
as a penis. In Nectonema the entire genital apparatus is simpler
and the ovary is not segmented.

The development of Nematodes begins in some cases only
after the eggs have been laid : such eggs are almost always provided
with thick shells. In other cases the earlier stages, and sometimes
even considerably more advanced stages, are passed through in
the uterus of the mother, and a few (Triohinella spiralis, species of

304 ZOOLOGY SECT.

Ascaris, species of Filaria and others) are viviparous. Segmentation
is complete and usually unequal. In Ascaris the ovum divides first
into two cells, a dorsal and a ventral. The former becomes sub-
divided into two — anterior and posterior — and of these the latter
undergoes further division into two — a right and a left. The ventral
cell of the two-celled stage gives rise to a median row of four cells
of which the second becomes differentiated as the primordial
endoderm cell, which gives rise to the entire endodermal layer of
the mesenteron. The rest of the cells of this stage go to form the
ectoderm, the mesoderm and the sexual cells. A segmentation-
cavity is sometimes present, sometimes absent. The descendants
of the original dorsal cell undergo repeated division to form a layer
of numerous small cells (primitive ectoderm) that grows over the
rest, bringing about a kind of epibolic gastrulation. Of the ventral
row of four cells the most anterior divides into two pairs of cells
destined to give rise to the mesoderm bands as well as the ectoderm
of the stomodaeum ; the second, as already stated, is the primordial
endoderm cell ; the third divides transversely into two primordial
sexual cells, and the last divides into four caudal cells. These
last undergo rapid multiplication and give rise to the secondary
ectoderm, which pushes forwards the primary ectoderm as it spreads
over the surface and eventually covers the entire embryo as the
integumentary layer, the primitive ectoderm, now sunk beneath
the surface, probably going to form the nervous system, and some
cells given off from it contributing to the formation of the mesoderm.
The two mesoderm bands divide each into two — dorsal and ventral
— and produce the muscular layer of the body-wall. In certain
viviparous forms the elongation of the embryo into the vermiform
shape is preceded in the uterus of the mother by a flat disc-like
stage, and the gastrulation is completed by the curving downwards
and inwards of the thin edges of the disc until they come in contact
and fuse in the middle line ventrally.

Many of the Nematoda have a curious and complex life-
history : a few examples will be selected for description.

Rhabdonema nigrovenosum lives, in the sexual condition, in the
lungs of Frogs and Toads : it is remarkable among members of
the class in being hermaphrodite. The eggs are laid and the
embryos pass from the lungs into the enteric canal of the host, are
expelled with its faeces, and develop in water into a sexual
Nematode, called the Rhabditis-ioim, in which the sexes are
separate : in this the fertilised eggs develop in the body of the
female, and, when fully formed, make their way through the wall
of the uterus and proceed to devour the whole of the maternal
tissues, leaving nothing but the cuticle. Being set free, they live
in mud until they succeed in gaining access to a frog's mouth,
when they pass into the lung, develop hermaphrodite reproductive
organs, and so re-commence the cycle. It will be seen that we

VI

PHYLUM NEMATHELMINTHES

305

have here a peculiar form of alternation of generations, distinguished
not by the alternation of a sexual with an asexual form (meta-
genesis) as in Hydrozoa, but by the alternation of a hermaphrodite
with a dioecious form —
a variety of heterogamy
(page 41).

One of the most
terrible parasites of file
Man is Trichinella lfH-</e
spiralis (Fig. 253), a
minute worm, the male
(C) a little over 1 mm.
(iV m«) m length, the
female (B) about 3 mm.
(|- in.). In the adult or
sexual condition it lives
in the intestine of Man,
the Pig, and other
Mammals.

The adult females,
which are viviparous,
leave the cavity of the
intestine and bore into
its wall, usually reach-
ing the interior of one
of the lacteal vessels of
the lymphatic system.
Here they deposit their
young (B, e) to the
number of as many as a
thousand or more at a
time. These are carried
passively in the stream
of lymph, perhaps ulti-
mately in the blood-
stream, and thus distri-
buted throughout the
body. Eventually they
travel into the system
of voluntary muscles,
such as those of the
limbs, back, tongue,
etc. Each worm then
penetrates the sarco-
lemma of a muscle-
fibre and coils itself

FIG. 253. — Trichinella spiralis. A, encysted form,
in muscle of host ; B, female ; <7, male. bh. connective-
tissue envelope ; cy. cyst ; de. ejaculatory duct ;
e. embryos ; /. fat-globules ; h. testis ; m. f. muscle-
fibre ; oe. pharynx ; ov. ovary ; wo. gonopore ; zk,
perforated cells of intestine. (From Lang's Com-
parative Anatomy, after Glaus.)

spindle-shaped cyst
VOL. i.

up

in the

(cy.) is formed

muscle substance (A) ; a
round it, and the muscle
x

306 ZOOLOGY SECT.

undergoes more or less degeneration. This process gives rise to
various morbid symptoms in the host, but after some months the
cysts become calcined, and the danger to the infected individual is
over. The flesh of a " trichinised " human subject has been
estimated to contain 100,000,000 encysted worms, and that of an
infected pig 85,000 to the ounce. In order that further develop-
ment of the encysted and sexless Trichinae should take place, it is
necessary for the infected flesh of the host to be eaten by another
animal in which the worm is capable of living, e.g., that of Man
by a Pig or Rat, or that of a Pig by Man. When this is done the
cysts are dissolved by the digestive juices, the worms escape,
develop reproductive organs, and copulate, the young migrating
into the muscles and producing the disease as before. The
result of eating an ounce of " trichinised " or " measly " pork,
improperly cooked, might be the liberation in the humaD
intestine of perhaps 80,000 worms ; and, if half of these were
females, each producing 1,000 embryos, some 40,000,000 worms
would shortly begin to migrate into the muscles, and produce the
various symptoms of " trichiniasis."

It will be noted that in this case the parasite is able to exist in
various hosts, and that both sexual and asexual stages are passed
through in the same host, dispersal of the species taking place by
the flesh of an infected animal being eaten by another, either of
the same or of a different species.

The female Guinea-worm (Filaria medinensis) attains a length
of 30-200 cm. (1-6 ft.), and lives in the subcutaneous connective-
tissue of Man. The eggs develop in the uterus, and the new-born
young pass out of the body of the host through abscesses caused
by the presence of the parasite. If, as must often be the case,
they escape into water, they make their way into the body of a
Water-flea (Cyclops), which is the intermediate host, and in this
condition probably reach their human host once more in his
unfiltered drinking-water. Filaria bancrofti and other species are,
in the larval condition, parasites in the blood of man. The adult
females of F. bancrofti live normally in the lymphatic vessels.
They are viviparous, and the young when they escape reach the
blood and are thus distributed. Normally they are to be found in
the peripheral vessels only at night-time, when the superficial
vessels are more dilated and thus permit of their passage. They
are transmitted from one human host to another by the agency
of mosquitoes, which act as intermediate hosts. F. bancrofti is
very widely distributed in tropical countries, and is the cause of a
disease called filariasis, with a variety of symptoms — such as
anaemia, lymphatic tumours, elephantiasis.

VI

PHYLUM NEMATHELMINTHES

307

CLASS IL— ACANTHOCEPHALA.

This class contains a number of genera of parasitic worms, of which
Echinorhynchus is the chief. The present section will be devoted to this genus,
a not uncommon parasite in the intestine of Mammals, Birds, Reptiles,
Amphibians, and Fishes. The largest species, E. (Gigantorhynchus] gigas, is
found in the Pig (Fig. 254, A), and has once been recorded in the human
subject : it may attain, in the female, a length of 50 cm., or more than half a
yard. Most species are small, not exceeding 1 cm. in length.

External Characters. — The body is cylindrical, and ends in front in a
protrusible portion, the proboscis (A, p., B, pr.), which is cylindrical and is
covered with many rows of recurved chitinoid hooks. The worm lies with the
proboscis sunk in the wall of the intestine of its host, which is sometimes
riddled with holes formed in this way. In some species there is a distinct
neck (B, n.) between the proboscis and the trunk, and there may be a globular
dilatation at the anterior end of the neck. At the hinder end of the body is
a single aperture, the gonopore or reproductive aperture (gnp.) : connected

•jbr

B

FIG. 254. — A, Echinorhynchus (Gigantorhynchus)
gigas, female, from the Pig (nat. size); B, E.
lesiniformis, male, from the edible Frog (magni-
fied), b. bursa ; c. gl. cement glands ; gnp. gonopore ;
Im. lemnisci ; n. neck ; p. or pr. proboscis ; r. m.
retractor muscle of proboscis ; s. Ig. suspensory liga-
ment ; ts. testis ; v. vessel.

with this, in the male, is a protrusible, bell-like structure, the bursa (6.), which
acts as a copulatory organ, like the somewhat similar organ in certain Nema-
toda. There is no trace of mouth, anus, or excretory pore.

The body-wail is covered with a stout cuticle, beneath which is a striated
protoplasmic layer, probably representing the ectoderm. Then comes a layer
of transverse, and then one of longitudinal muscles. The body-wall thus
constituted encloses a spacious body-cavity or coelome containing a clear
fluid.

s 2

308

ZOOLOGY

SECT.

.far

In correspondence with the absence of mouth and anus there is no trace of
enteric canal, the Acanthocephala resembling in this respect the Cestoda, the
only other class of Metazoa which is entirely anen-
terous. Food is thus, as in tapeworms, taken
entirely by absorption by the general surface of the
body.

The proximal end of the proboscis is contained
in a muscular sheath sunk in the anterior end of
the trunk, and is provided with four retractor
muscles (Fig. 254, r.m.). The muscles of the sheath
are circular and act as protractors. At the sides of
the base of the proboscis two club-shaped organs,
the lemnisci (lm.), hang down into the body-cavity.
Their function is quite unknown, but they have

been compared with ^ •*•#

the cervical glands of
certain Nematodes (p.
301).

In the body-wall
run two longitudinal
vessels (v.) containing
a granular fluid, and
connected with a net-
work of fine canals in
the proboscis, bursa,
etc. The function of
these vessels is not
known with certainty :
they may have to do
with the absorption
and circulation of
nourishment.

The central ner-
vous system (Fig.
255, nv.) is represented
by a single large gang-
lion placed at the base
of the proboscis, and
sending off nerves in
various directions. In
the male there are
also two ganglia sup-
plying the reproduc-
tive organs. Organs
of sense are wholly
absent.

A pair of remark-
able excretory organs

or nephridia (Fig. 257) Fia. 255.— Echinorhynchus
have been found to gigas. Dissection of male.

i« -EWii™ *>• bursa ; c. gl. cement glands ; FIG. 256.— Echinorhynchus

• n-oT lm- lemnisci; nv. nerve- gigas. Dissection of female

rhynchus gigas. These ganglion ; pr. proboscis ; s. Ig. (semi-diagrammatic), b. bell ;

consist of a Dair of suspensory ligament ; ts.testis; lm, lemnisci; pr. proboscis;

.., , *\ . v. df. vas deferens. (After s. ovy. swimming ovaries ; ut.

rammed protoplasmic Leuckart.) uterus ; vg. vagina,

masses situated in the

body-cavity at the posterior end, near the genital aperture. In the interior
is a system of branching canals, the terminal branches of which, each contained
in one of the terminal lobes of the tree-like nephridium, are provided with

VI

PHYLUM NEMATHELMINTHES

309

ciliary flames ; at the end of each lobe are a number of fine perforations
placing the contained canal in communication with the body-cavity. The

stalk of each nephridium con-
tains a single main canal ; these
unite to form a wide median
dorsal channel which opens
behind in the female into the
unpaired portion of the oviduct
and in the male into the ejacu-
latory duct.

The greater part of the body-
cavity is occupied by the repro-
ductive organs. The sexes are
separate, and the female is
larger than the male. In both
sexes the gonads and their ducts
are connected with a great sus-
pensory ligament (s.lg.), which
extends backwards from the end
of the proboscis-sheath.

In the male there are two
ovoidal testes (Fig. 255, is.) con-

FIG. -57. — A, longitudinal section through the
terminal twigs of the nephridia of Echino-
rhynchus gigas ; highly magnified, a,
nucleus. £, a terminal twig more highly mag-
nified. 6, the porous membrane. (From Shipley,
after Kaiser.)

nected with the suspensory ligament. From each a
furnished with several vesiculce seminales or sacs for
the storage of spermatic fluid, passes backwards and
unites with its fellow to form an ejaculatory duct, with
which are connected about half a dozen cement glands
(c. gl.}. The ejaculatory duct opens into the bursa or
bell-like copulatory organ (6), and has at its opening a
small papilla acting as a penis.

In the female the ovary is connected with the
suspensory ligament (Figs. 256 and 258, s.lg.}. When
ripe, groups of ova — known as the " swimming
ovaries " (s. ovy.) — become detached and swim freely
in the body-cavity, where they are impregnated. The
ducts are very peculiar. Connected with the end of
the suspensory ligament is a muscular uterine bell (6.),
widely open anteriorly (Fig. 258, 2.) into the ccelome,
and having towards its posterior end a small aperture
or a pair of small apertures (?/), also communicating
with the ccelome. The bell is connected with a narrow
double passage leading to a uterus (ut.), which itself
opens by the genital aperture at the posterior end of
the body. The uterine bell performs rhythmical
swallowing movements, and as the eggs — containing
partly developed embryos — float in the ccelome they
are swallowed by the bell. The immature eggs, which
are globular, are passed back into the ccelome through
the posterior aperture (y] of the bell ; but the mature
eggs, which are spindle-shaped and covered with a
chitinous investment, make their way from the bell to
the uterus through the narrow passages, and so to the
vagina.

The early stages of development take place in the
ccelome. Segmentation is regular, and a peculiar form
of gastrula is produced, having neither archenteron nor
blastoccele — in other words the ectoderm and endo-
derm are in close contact with one another, and no

vas defer ens (v. df.),

-ut

FIG. 258. — Female organs
of Echinorhynchus.
b. uterine bell ; s. Ig.
suspensory ligamen t :
ut. uterus ; vg. vagina ;
x. y. apertures of bell ;
z. apertures leading
from bell to uterus,
(After Hertwig.)

310

ZOOLOGY

SECT.

central cavity is enclosed by the latter. The ectoderm layer, which is devoid
of cell-limits, secretes a cuticular membrane investing the embryo ; then a
second membrane is formed within the first, and a third within the second ;
the embryo thus comes to be enclosed in a triple
case (Fig. 259), which differs from an egg-shell in
being formed by the developing ectoderm. At
what will become the anterior end chitinoid hooks
appear.

At about this period the embryo is born, and
reaching the intestine of tjie host, is extiuded with
its faeces. Its further development depends upon
its being swallowed by an intermediate host, which,
in the case of E. gigas of the Pig is a maggot, the
larva of a Beetle, Cetonia aurata. The Echino-
rhynchi of fresh-water Fish have for their inter-
mediate host certain small fresh-water Crustacea
belonging to the genera Oammarus and Asellus.

Having reached the intestine of the intermediate
host, the chitinoid embryonic membranes are
dissolved by its digestive juices, and the embryo
either fixes itself to the wall of the intestine or
makes its way into the ccelome ; in either case it
soon begins to undergo further development. The
endoderm, hitherto a solid mass of cells, undergoes
a process of splitting, becoming divided into an
outer layer in contact with the ectoderm and a
solid central axis. The latter gives rise to the
reproductive organs and the suspensory ligament,
the outer layer to an epithelium, from which the
body-muscles arise ; the cavity formed by the
splitting of the endoderm is the ccelome. Part of
the proboscis and its sheath are also of endodermal
origin. The ectoderm gives rise to the protoplasmic
layer of the body-wall, to the whole system of
vessels, and to the lemnisci. The larval cuticle is
thrown off and a new one formed. The larva reaches
adult proportions and attains sexual maturity only
if the intermediate host is eaten by the permanent
host.

CLASS III.-CILZETOGNATHA.

The present group, like that just discussed, is a
very small one, containing only three genera
(Sagitta, Spadella and Krohnia) of curious arrow-
shaped worms, all but one species of which are
pelagic.

External Characters. — The body (Fig. 260) is
elongated and nearly cylindrical, and is divided into
food, trunk and tail, the head being marked off by

highly magnified, a. endo- its somewhat rounded form, while the junction of
cSt?nJ,uS&'hoeotderI(F;rom *™*& and tail is indicated by the ventrally placed
Shipley, after Hamann.) anus (a). The tail bears a horizontal expansion, or
« caudal fin (s. fl.), and there are also horizontal

lateral fins (fl.) — a single pair in Spadella, two pairs in Sagitta*

Body-wall.— There is no cuticle, but the outer layer of the body-wall
is formed by an epidermis or deric epithelium (Fig. 261, d. epthm), which,
instead of being syncytial as in the two preceding classes, is formed of several
layers of epithelial cells. Next comes a delicate basement membrane, and then

FIG. 259.— Egg of Echino-
rhynchus acus, en-
closed in a triple case ;

VI

PHYLUM NEMATHELMINTHES

311

a layer of muscles (w.), the fibres of which are striated and disposed longi-
tudinally in four bands — two dorso-lateral and two ventro-lateral — an
arrangement which recalls that of the corresponding layer in Nematoda.

Enteric Canal.— The mouth (Fig. 260, m.) „,-,,.

is a longitudinal slit -like aperture on the ventral
surface of the head ; on either side of it are
several sickle-shaped chitinoid hooks (Fig. 262,
gh.) which are moved by muscles in a horizontal
plane and serve as jaws. The anterior region
of the head also bears spines, and is strength- we-

ened by chitinoid plates and partly covered by
a hood-like fold of the integument.

The mouth leads by a muscular pharynx or
stomodaeum into a straight intestine (d.), which
extends through the trunk and opens by the
anus (a) at the junction of trunk and tail.

Coeiome. — At the junction of the head with
the trunk, and of the trunk with the tail, are
transverse partitions or septa, dividing the
ccelome into compartments. The trunk region
of that cavity is further subdivided by two
longitudinal partitions, the dorsal and ventral
mesenteries, which respectively connect the dorsal
and ventral surfaces of the intestine with the
body-wall : the tail-region of the coelome is
similarly divided into right and left chambers
by a longitudinal vertical partition (Fig. 261, A
and B).

There is no trace of vascular system or of ovtt-
excretory canals. The nervous system, on
the other hand, is much better developed than
in either of the preceding classes, in accordance
with a free life and active movements. On the
dorsal side of the pharynx is a comparatively
large 6mm (Fig. 262, g), which sends off on
each side a long nerve-cord, the cesophageal
connective (sc.). The two connectives sweep
round the enteric canal and unite on the ventral

surface, not far from the middle of the trunk, "^ilf^lli2— /to

in an elongated ventral ganglion (Fig. 260, bg.),
from which numerous nerves are given off. The
brain sends nerves to the eyes (Fig. 262, an.)
and to the olfactory organs (ro.), and is also
connected with two pairs of ganglia in the head,

which lie deeply sunk in the mesoderm : all the

rest of the nervous system retains its primitive

connection with the ectoderm.

Sensory Organs.— On the surface of the FIG.

body are numerous little papillae carrying stiff

bristle-like processes, and probably serving as

organs of touch. There are two eyes (Fig. 263),

situated one on each side of the dorsal surface

of the head: each is globular and contains

three biconvex lenses (L), separated by pigment

(p.) and surrounded by rod-like sensory cells

(rz.). Behind the head is a ring-like structure,

of the nature of an annular ridge of peculiarly modified and in part ciliated

cells (Fig. 262, ro.) : to this an olfactory function has been assigned.

260.— Sagitta hexap-
tera, from the ventral aspect
a. anus ; bg. ventral ganglion
d. intestine ; fl. lateral fins
ho. testis ; m. mouth ; ov.ovary
ovd. oviduct; sc. cesophageal
connective 4 sb. vesicula semin-
alis ; s. fl. tail fin ; sh, tail-
cavity; si. spermiduct. (From
Lang's Comparative Anatomy,
after Hertwig.)

312

ZOOLOGY

SECT.

Reproduction. — -The Chsetognatha are monoecious. The ovaries (Fig.
260, ov., Fig. 261, ovy.) are elongated organs situated one on each side of the
trunk-region of the coelome, and opening by a narrow oviduct just in front
of the posterior septum. The testes (Fig. 260, ho., Fig. 261, ts.) are similarly
situated in the tail-region of the ccelome, and have the form of narrow ridges

d.ef}tkrrt

Fio. 261. — Sagitta bipunctata. Transverse sections, A, of trunk ; B, of tail. coel. coelome ;
coel. epthm. layer of nuclei of the muscle-cells formerly regarded as a coelomic epithelium ;
rf. epthm. deric epithelium, diagrammatically shown as a single layer ; /. tin ; int. intestine ;
m. muscles ; ovy. ovary ; ts. testis. (After Hertwig.)

from which immature seminal cells are given off and develop into sperms in
the ccelome. The spermiducts or vasa deferentia are delicate tubes (si.)
opening at one end into the coelome by a ciliated funnel-like extremity, and
at the other end dilating into a reservoir or vesicula seminalis (sb.), which
opens externally in the posterior region of the tail.

FIG. 262V— Head of Sagitta bipunctata,

from above, an. optic nerve ; au. eye ; g.
brain ; gh. hooks ; rn. olfactory nerve ; ro.
olfactory organ ; sc. cesophageal connective.
(From Lang's Comparative Anatomy, after
Hertwig.)

FIG. 263.— Section of eye of Sagitta hexap-
tera. ep. epiderm ; I. lens ; p. pigment :
rz. visual cells ; st. rods. (From Lang's
Comparative Anatomy, after O. Hertwig.)

Development. — Internal impregnation takes place, and the oospenn,
segmenting completely and regularly, forms a typical gastrula by invagination
(Fig. 264, A). Two endoderm cells (g.) at the anterior end of the archenteron,
i.e. the end opposite to the blastopore, soon increase greatly, in size, and are
the rudiments of the gonads. This precocious differentiation of the sex-cells
is a point of considerable importance, as will be seen hereafter. Before long
these cells migrate into the archenteron and divide, forming a group of four

VI

PHYLUM NEMATHELMINTHES

313

cells (B, g.), two of which subsequently become the ovaries and two the testes.
At the same time two folds of endoderm grow into the archenteron from its
anterior end, partly dividing the cavity into three parts — a middle division
or mesenteron (d) — the rudiment of the intestine, and two lateral divisions—
the metentera, or ccelomic sacs (c. s.) — which give rise to the right and left
compartments of the coelome of the trunk. From the latter are given off in
front a pair of small head-cavities. Owing to the rapid elongation of the
embryo in the stages following, all the cavities become for a time obliterated :

FlG. 264. Tlirr<! stages in the development of Sagitta. bl. blastopore ; r.v. ruMomic sacs ; d.
mesenteron ; g. sexual cells ; pm. parietal layer of mesoderm ; st. stomodseum ; vm.
visceral layer of mesoderm. (From Lang's Comparatfre Anatomy, after O. Hertwig.)

subsequently the cavities of the enteric canal and ccelomic sacs reappear ;
the tail-region of the body-cavity is formed from the posterior, undivided
portion of the archenteron. The blastopore (bl.) now closes and an invagina-
tion of ectoderm — the stomodseum (st.) — takes place at the anterior end, and
finally communicates with the mesenteron.

From this it will be seen that the ectoderm of the gastrula gives rise to the
deric epithelium of the adult and to the epithelium of the pharynx, which is
therefore a stomodseum ; from the same layer the nervous system arises at a
later stage. The epithelium of the intestine arises
from the mesial (inwardly-turned) layers of the
two endodermal folds. The muscular layer of the
body-wall arises from the rest of the endoderm,
i.e., that portion of it which remains in immediate
contact with the ectoderm. Thus in Sagitta the
mesoderm is entirely derived from the endoderm
of the gastrula.

d

APPENDIX TO NEMATHELMINTHES.

1. Family Chcetosomatidce.

This family includes several genera of small
marine worms, Chcetosoma (Fig. 265), Tristi-
cochcpia, Rhabdoqaster, and others, which are
sometimes included among the Nematoda.

The body is elongated, its anterior region being
sometimes dilated to form a head. Either the
whole body or the dorsal surface only is beset
with fine setse, and there may be a double row of
movable chitinoid hooks round the head, remind-
ing us of the " jaws " of Sagitta. The ventral
surface bears posteriorly curious locomotor rods FIG. 265. — Mature female of
(/.), either hooked or with knobbed ends: by ^T^opS^!-'
these the animals crawl. The mouth is anterior testine ; c, anus ; d, d,
and terminal, the anus posterior and ventral, and °vaies ' e- generative pore ;
there is a muscular pharynx. The sexes are

314

ZOOLOGY

SECT.

separate. The male has a single testis : the vas deferens opens along with
the anus : there are two penial setae. The female has paired ovaries and a
single vagina opening near the middle of the body on the ventral side.

2. Family Echinoderidce.

Echinoderes is a minute marine worm of cylindrical form with a flattened
ventral surface. The body is segmented, or divided into rings, eleven or
twelve in number, all strongly cuticularised, and most of them bearing spines
(Fig. 266). The mouth is placed at the anterior, the anus at the posterior
end of the body : the former opens into a sac, which can be everted so as to
form a proboscis or introvert, and is armed with spines. The enteric canal
consists of a muscular pharynx and a straight intestine. A pair of sacs
opening by ciliated ducts on the tenth segment
appear to be excretory organs. The sexes are
separate : the gonads are paired sacs opening at
the posterior end of the body.

3. Family Desmoscolecidce.

Desmoscolex is also a minute marine worm,
having a globular head and a variable number of
segments (Fig. 267). The head bears four movable
chitinoid rods or setae, and a pair of similar struc -
tures occurs on many of the other segments. The
terminal mouth leads by a muscular pharynx into
a straight intestine : the anus is dorsal in position.

Fia 266 — Echinoderes, x about 210. b. spine; FIG. 267.— Female Desmoscolex,

c 's caudal spine : ph. pharynx ; s. and s'. spines ventral view, x 260. a. ovary.

o'n'the proboscis ; s.g. salivary glands ; st. stomach. (From Shipley, after Panceri.)
(After Hartog.)

The animal is dioecious ; the gonads have the form of simple sacs, the testis
opening along with the anus, the ovary on the ventral surface anterior to
the anus. The male has a pair of penial setae.

Trichoderma has some superficial resemblance to Desmoscolex, but is nearer
the ordinary Nematodes.

vt PHYLUM NEMATHELMINTHES 315

AFFINITIES AND MUTUAL KELATIONSHIPS OF THE
NEMATHELMINTHES.

The affinities of all the classes of Nemathelminthes are very
obscure, and the propriety of grouping them into a single phylum
is extremely doubtful. They all agree in being elongated, cylin-
drical worms with a body-cavity, which is sometimes of the nature
of a true coelome ; there is a certain resemblance between Nematoda
and Chsetognatha in the muscular system ; and the lemnisci of
Acanthocephala have been compared with the cervical glands of
Nematoda. Beyond these points there is little to unite the three
classes ; and, on the other hand, the proboscis of Acanthocephala
recalls the rostellum of Cestoda.

Very various views have been put forward as to the affinities of
the Chsetognatha. But, in the absence of adequate evidence of
any near relationship with higher phyla, they may be regarded as
having their nearest known relatives, even if very remote, in the
Nemathelminthes. In connection with this question, the Chaeto-
somidae, briefly described in the Appendix (p. 313), seem to
require consideration. Other possible relationships suggested by
the mode of development of the coelome from hollow diverticula of
the archenteron and by other features will be referred to in later
sections.

The three families placed as an Appendix to the phylum present
some undoubted resemblance to the Nematoidea : this is especially
the case in the reproductive organs of the Chaetosomatidae, and still
more in those of Desmoscolex. But the segmentation of the body
in both Desmoscolecidae and Echinoderidae and the presence of
setae show a certain resemblance to higher worms or Annulata,
which will be more fully appreciated when that phylum has been
studied.

SECTION VII

PHYLUM TROCHELMINTHES

THE typical larval form of a number of the groups which have
yet to be studied is a form which is known as the trochosphere
or, more usually, trochophore. It is necessary that a clear idea
should be formed at this stage of this important larva, reference
to which will very frequently be made in the sections that follow.
The general shape of a typical trochophore is oval or pear-like
(Fig. 268) with a broader and a narrower end and distinct bilateral

symmetry. Encircling
the body about the
middle, or rather
nearer the broad than
the narrow end, is a
double circlet of strong
cilia, the pre-oral circlet

li—wz

- wkr

*d.LM

v.LM
oe.LM

WAr

Neph
Mstr'

ED

(Wkr.) or prototroch,
situated on a corre-
sponding ring -like
thickening of the ecto-
derm ; behind the
mouth i.s often a
second circlet of cilia,
the post-oral circlet

FIG. 268.— Trochophore. A. anus; d.LM. dorsal (wkr.), and a ciliated

muscles ; ED. rectum ; J. stomach ; J1 intestine ; crrnnvp nr r i li a f p f\

Mstr. mesoderm-band ; n. nerves ; Neph. nephridia ; &iu

O. mouth ; Oe. oesophagus ; ceLM. cesophageal longi- streak USUally runs

tudinal muscle ; SP. apical plate ; v.LM. ventral muscle; -i i j e

r.LN. lateral nerve; Wkr, wkr., pre- and post-oral backwards trom it

bands of cilia; WS. apical cilia ; u-z. adoral cilia. (From olrmrr f>io TVMrlrlla r.f flio
Hertwig's Zoology, after Hatsehek.)

ventral surface. The

mouth, situated just behind the pre-oral circlet, leads into an
alimentary canal, which at first runs nearly transversely, and then
bends round so as to extend back towards the narrow end, near
which it opens on the exterior by an anal aperture. About the
middle of the broader (anterior) end of the trochophore is a thicken-
ing, the apical plate (SP.), projecting from which are usually a

SECT, vn PHYLUM TROCHELMINTHES 317

number of sensory cilia (WS.) ; and in many trochophores eye-spots
and a pair of short tentacles occur in close relation with the apical
plate, which is the nerve centre of the larva. A pair of tubes —
the excretory organs or nephridia (Neph.) — may be present.

In the higher groups in which this form of larva occurs, the
adult condition is attained by modifications and new developments
of so radical a nature that the transition from larva to adult is of
the nature of a metamorphosis. Sometimes the narrow part of the
larva elongates and becomes divided into a series of sections fore-
shadowing the metameres of the adult animal ; in other cases, in
which no metamerism occurs, radical changes of other kinds lead
to the adult form. But in all these higher groups, whatever the
nature of the changes involved, there is a metamorphosis, and the
adult animal is totally unlike the larva. In a small number of
forms now to be dealt with, however, there is no such radical
change, and the adult may be looked upon as a somewhat modified
trochophore. The groups thus associated together may not be
genetically related : they may have become independently
developed from trochophore-like ancestors, but the possession of
the general characters which have been referred to above renders
it convenient to group them together and regard them as con-
stituting a small but well-marked phylum. The groups referred
to are the Rotifera or Wheel-animalcules, together with the
Gastrotricha. Associated with these, though scarcely to be included
in the same phylum, are the Dinophilea and Histriobdellea.

CLASS L-ROTIFERA.

The Rotifers or " Wheel-animalcules " are microscopic creatures,
very abundant in pools, gutters, etc., and formerly classed with the
Infusoria, to which several of them bear a superficial resemblance.
But in spite of their minute size they are multicellular animals,
having an enteric canal, a spacious body-cavity, nephridial tubes,
gonads, a nervous system, and sense organs, and have therefore no
real relationship with the Protozoa.

1. EXAMPLE OF THE CLASS — Brachionus rubens.

External Characters. — Brachionus (Fig. 269) is one of the
commonest members of the class, being frequently found in abun-
dance hi ponds, ditches, etc. The female is about ^ mm. (TV in.) in
length, and is divisible into two distinct parts — a broad anterior
region, the trunk, and a slender movable tail (t.). The trunk is
enclosed in a glassy cuirass or lorica (lr.), formed by a thickening
of the cuticle and produced into several spines : the tail is wrinkled
superficially and ends in two slender processes, together forming
a kind of forceps. One surface of the trunk is flattened and,
owing to the position of the mouth, is considered as ventral ; the

318

ZOOLOGY

SECT.

opposite or dorsal surface is convex both from before backwards,
and from side to side.

The anterior portion of the body projects from the lorica in the
form of a transverse disc (tr. d) with a prominent edge fringed with
cilia : this is the trochal disc, and is one of the most characteristic
organs of the class. By the action of the cilia the animal is
propelled through the water, and, as in Vorticella, their successive
flexion gives an appearance of rotation to the disc or " wheel-
organ " whence the name of the class is derived. Within the circlet

FIG. 269. — Brachionus rubens, female. A, from the dorsal aspect ; B, from the right side,
a. anus ; br. brain ; d. f. dorsal feeler ; c. gl. cement-gland ; cl. cloaca ; c. /. ciliary lobes ;
c. v. contractile vesicle ; e. eye-spot ; int. intestine ; Ir. lorica ; 4. f. lateral feeler ; m. :
muscular bands ; nph. nephridial tubes ; ov. germarium : ph. pharynx ; st. stomach
t. tail ; tr. d. trochal disc ; vt. vitellarium. (After Hudson and Gosse.)

of cilia arise three prominences (c.l.) covered with cilia of large
size. The trochal disc is not perfectly symmetrical, but has at one
part of its circumference a depression in which the mouth lies :
this marks the ventral surface. The anus (a.) is dorsal in position,
and is placed at the junction of the tail with the trunk.

The body-wall consists of an epidermal layer, without cell-
limits, covered by a chitinoid cuticle : it is by a thickening of the
latter in the region of the trunk that the lorica is produced.

vn

PHYLUM TROCHELMINTHES

319

FIG. 270.— Pharynx of Brachionus
rubens. /. fulcrum ; m. manubrium ;
u. uncus ; r. ramus. (After Hudson and
Gosse.)

There is no continuous muscular layer, but several bands of
unstriped muscle (m.) pass from the lorica to the trochal disc
in front and to the tail behind, and act as retractors of those organs.
Digestive Organs. — The mouth (Fig. 272, mth.) lies, as
already mentioned, in the ventral region of the trochal disc, anterior
to the ciliary circlet but posterior to the three ciliated lobes ; it leads

by a short buccal cavity into a
pharynx (ph.) of peculiar struc-
ture known as the mastax, and
constituting one of the most
characteristic organs of the class.
The mastax is a muscular
chamber (Fig. 270) of rounded
form, and contains, as a thicken-
ing of its cuticular lining, an
elaborate apparatus for tri-
turating the food. In the
middle line is a forked struc-
ture, the incus, consisting of a
small base or fulcrum (j.) and of two branches or rami (r.). On
either side of the incus is a hammer-like structure, the malleus,
consisting of a handle or manubrium (m.) and of a toothed head or
uncus (u.). By means of the muscular walls of the chamber the
heads of the mallei are worked backwards and forwards upon the
forked uncus, and thus reduce the organ-
isms taken as food to a fine state of
division.

The pharynx leads by a short gullet into a
spacious stomach (Fig. 272, sZ.) having a wall
composed of very
large epithelial
cells, ciliated inter-
nally : with it are
connected paired
digestive glands.
The stomach opens
into a rounded in-
testine (int.), also
ciliated internally,
which communi-
cates, by means of
a short cloaca (cl.)
with the ex-
terior. The stomach
and intestine are formed from the archenteron of the embryo and
are therefore lined by endoderm : the rest of the enteric epithelium
is ectodermal, the pharynx being derived from the stomodseum,

cy,

FIG. 271. — Brachionus rubens. A, male ; B, female, with
attached eggs. c. gl. cement-glands ; cv. contractile vesicle ;
nph. nephridial-tube ; ov. ovum in body ; ov*. ova attached to
base of tail ; p. penis ; t. tail ; is. testis. (After Hudson and
Gosse.)

320 ZOOLOGY SECT.

the cloaca from the proctodaeum. Between the body-wall and the
enteric canal is a spacious body-cavity containing a fluid which
serves the purpose of blood and contains minute granules.

The excretory system consists of paired nephridial tubes
(Figs. 269 and 272, nph.) resembling those of the Platyhelminthes.
Their general direction is longitudinal, but they are a good deal
coiled and give off little tag-like processes ending in flame-cells.
Upon the end of each tag, projecting into the body-cavity, is a long
flagellum. The lumen of the tubes is intra-cellular : it is uncertain
whether or not the cavities of the flame-cells communicate with the
body-cavity by apertures in their walls. Posteriorly the nephridial
tubes open into a bladder or contractile vesicle (c. v.)} the contents
of which are discharged, by periodical contractions, into the
cloaca.

FIG. 272.— Diagram of a Rotifer, a. anus ; br. brain ; ci. pre-oral, and c2. post-oral circlet of
cilia ; c. gl. cement gland ; cl. cloaca ; CM. cuticle ; c. v. contractile vesicle ; d. ep. deric
epithelium ; d.f. dorsal feeler ; e. eye ; fl. c. flame-cells ; int. intestine ; m. muscles ; mth.
mouth; nph. nephridial tube ; ov. ovum ; ovd. oviduct : ovu. germarium : vh pharynx:
st. stomach ; vt. vitellarium.

Nervous System and Sense Organs. — There is a single
ganglion or brain (Figs. 269 and 272, br.), of proportionally large
size, situated at the anterior end of the body, above (dorsal to) the
mouth and pharynx. On the dorsal surface of the brain, where it
comes into contact with the body- wall, is a small red eye-spot (e.).
The only other organs which can be considered as sensory are
three structures known as tactile rods or feelers ; one of these (d.f.)
is a small cylindrical process tipped with stiff hair-like bodies,
which projects from the dorsal surface just behind the trochal disc :
the other two (I. /.) are paired, situated on the dorsal surface of
the lorica and not prominent.

The tail contains a pair of cement glands (c. gl.), by the secretion
of which the animal is able temporarily to attach itself.

Reproduction and Development. — The sexes are lodged
in distinct individuals, which present a striking degree of sexual

vn PHYLUM TROCHELMINTHES 321

dimorphism. The preceding description applies to the female, which
is the form most commonly met with. In addition to the organs
already mentioned, it has a germarium (ov., ovy.), connected with a
large vitellarium (vt.) and opening by an oviduct into the cloaca.

The male (Fig. 271, A) is a very minute creature, not more than
one-fourth the size of the female, and is strangely degenerate in
structure. The enteric canal is absent, the trochal disc simple in
structure, the nervous system and nephridial tubes greatly reduced,
and the greater part of the body occupied by a large testis (ts.)
which opens by a duct at the extremity of a protrusible, dorsally
placed penis (p.).

After extrusion the eggs are attached to the base of the tail
of the female (B, ov'.), where they undergo development : they are
of two sizes, the larger giving rise to females, the smaller to males.
Probably both kinds develop parthenogenetically, but in the
autumn thick-shelled winter eggs are produced which appear to
require fertilisation. These remain quiescent during the winter,
and in the spring develop into females.

2. — DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Rotifera are Trochelminthes of microscopic size. The
anterior end is modified into a retractile trochal disc, with variously
arranged cilia ; the posterior end usually forms a mobile and
often telescopically jointed tail. The mouth is anterior and more
or less ventral in position, the pharynx contains a chitinous
masticatory apparatus, and the anus is placed dorsally at the
junction of the trunk with the tail. There is a spacious body-cavity
devoid of epithelial lining. The excretory organs are a pair of
nephridial tubes provided with flame-cells. The central nervous
system consists of a single dorsal ganglion, with, in a few cases, a
smaller ventral or sub-ossophageal ganglion. The sexes are separate,
and the males are, in nearly all cases, smaller than the females
and degenerate in structure.

The class is divided into six orders as follows : —

ORDER 1. — RBIZOTA.

Rotifera which are fixed in the adult state by the truncated end
of the non-retractile tail.
Including Floscularia, Stephanoceros, Melicerta, etc.

ORDER 2. — BDELLOIDA.

Eotifera which both swim freely by means of the cilia of the
disc and creep after the manner of a Leech. The tail is telescopic
and forked distally.

Including Rotifer, Philodina, etc.

VOL. I. Y

322

ZOOLOGY

SECT.

ORDER 3. — PLOIMA.

Kotifera in which locomotion is performed by the ciliated disc
only. The tail is usually forked and more or less retractile.

Sub-order a. — Illoricata.

Ploi'ma in which the trunk is not covered by a lorica.
Including Hydatina, Polyarthra, Asplanchna, etc.

Sub-order b. — Loricata.

Ploiima in which a lorica is present.
Including Brachionus, Euchlanis, etc.

ce

ORDER 4. — SCIRTOPODA.

Kotifera provided with setose appendages moved by striped
muscles : skipping movements are performed by the aid of these,
as well as swimming movements by the trochal disc. The tail is

either absent or is repre-
sented by a pair of ciliated
processes.

Including Pedalion and
Hexarthra.
br

ORDER 5.—
TROCHOSPH^ERIDA.

Globular Kotifera having
the trochal disc represented
by an equatorial circlet of
cilia ; tail absent.

Including Trochosphcera
only.

ORDER 6. — SEISONIDA.

Marine parasitic Kotifers
(Fig. 273), with the trochal
disc reduced, the body long,
narrow, and ringed, with a
long slender neck-region,
and an elongated foot pro-

FIQ. 273.— Faraseison asplanchnus, female, . n , .

x 230. a. genital aperture ; br. ganglion ; /. Vlded at its extremity With

cv

. . . .

foot-glands ; m. mouth ; ma. mastax ; oe. oeso-
phagus ; ov. ovary ; st. stomach. (After Plate.)

-run-frtraf or\
Pei

VIT PHYLUM TROCHELMINTHES 323

Systematic Position of the Example.

Brachionus rubens is one of the several species of the genus
Brachionus : it belongs to the family Brachionidce, and to the
sub-order Loricata of the order Plo'ima.

It is placed in the order Ploima in virtue of its active swimming
habits and the absence of looping or skipping movements. The
presence of a distinct lorica places it in the sub-order Loricata.
The family Brachionidse is distinguished by having a box-like
lorica open at both ends, and a long, flexible, retractile tail with
wrinkled surface and forceps-like termination. In the genus
Brachionus the lorica is not marked with ridges, and the tail is
very long and perfectly retractile. In B. rubens the anterior edge
of the lorica is produced dorsally into six spines and is sinuous
ventrally.

3. GENERAL ORGANISATION.

External Characters. — The majority of the Rotifera are
free-swimming, being propelled rapidly through the water by the
action of the trochal disc. But in the Bdelloida (Fig. 274, 5), in
addition to this mode of progression, the animal performs looping
movements like those of a leech : the tail in this order is freely
jointed, the various segments fitting into one another like the tubes
of a telescope, and the body is fixed alternately by it and by the
anterior end, the trochal disc being kept retracted while the animal
moves in this way. Many of the Ploima also have a telescopic tail,
but in some, e.g., Asplanchna (Fig. 274, 6), this organ is absent. In
Pedalion (Fig. 275, 1) curious skipping movements are performed
by the aid of six hollow limbs or appendages, one dorsal, one
ventral, and two on each side. These curious organs are ter-
minated by feathered setae, and closely resemble the limbs of
some of the lower Crustacea : each is moved by two opposing
muscles which extend into its cavity (1, B, m). Three pairs of
similar appendages are present in the other genus of Scirtopoda,
Hexarthra (Fig. 275, 2), the resemblance of which -to the naupUus
larva of Crustacea is very striking ; and four genera of unarmoured
Ploima, e.g. Polyarthra (Fig. 274, 8), possess simple or fringed setae
moved by muscles attached to their bases.

In the Rhizota the adult is permanently fixed (Fig. 274, 1-4).
The end of the tail is devoid of the characteristic fork, and is
attached to plants or other supports. Moreover the animal is
surrounded by a tube into which it can retract itself completely,
protruding the anterior end with the trochal disc when undis-
turbed. In most instances, as for example in Floscularia (1) and
Stephanoceros (2), the tube is formed of a delicate, transparent,
gelatinous secretion of the epidermis ; but in Melicerta (3) it is
built up of rounded pellets, which the animal moulds in a cup-like

Y2

l.Floscularia 2. SrejDhan oceros 3.Melicerra

4.Melicerra S.Philodina 6.Asplanchna

10. N orhoica

ZHyuatiha

FIG 274 —Typical forms of Botifera :— 9 and lOjshow the lorica only. a. anus ; cl.tf*. ciliary
circlets'; int. intestine ; m. muscle : ph. pharynx ; st. stomach. (After Hudson and Gosse.)

SECT, vn

PHYLUM TROCHELMINTHES

325

depression on the dorsal surface and places in position one by one.
The pellets are usually formed of foreign particles, but in some
species are made of the animal's own faeces.

The ciliation of the trochal disc is subject to considerable
variation. In its simplest form the disc is surrounded by a single

II

S.Hexarhhra

3-Troch o sjjhaera

FIG. 275.— Typical forms of Rotifera. In 1, A shows the outer form, B the muscular system .
a. anus ; br. brain ; c1 c2. ciliary circlets ; cl. cloaca ; d. gf. digestive gland ; d. I. dorsal
limb ; e. eye-spot ; I. I., I. I', lateral limbs ; m, muscles ; mth, mouth ; nph. nephridial
tube ; ov. ovary ; ph. pharynx ; s. sense-organ ; i\ L ventral limb. (After Hudson and
Gosse (1 and 2) and Korschelt and Heider (3).)

circlet of cilia, within which lies the mouth. A modification of
this type may be produced by the prolongation of the ciliary
crown into long arm-like processes fringed with cilia, as in
Stephanoceros (2), or, as in Floscularia (1), into blunt elevations

326

ZOOLOGY

SECT.

bearing long stiff cilia like the pseudopodia of the Heliozoa. The
single circlet may be folded upon itself, or a second type may be
produced by the addition of a second circlet within and parallel to
the first. The mouth in this case is always placed between the two
circlets on the ventral side (Fig. 272), so that the inner or anterior
circlet is pre-oral and corresponds with the chief ciliary band
of a trochophore larva, while the outer or posterior circlet corre-
sponds with the post-oral band found in many worm-larvse. In the
curious globular Trochosphcera (Fig. 275, 3) there is a single
equatorial circlet, which is pre-oral, and a few post-oral cilia : here
the correspondence with the typical worm-larva is singularly
close. Lastly, both the pre- and post-oral circlets may be pro-
duced into more or less complex lobes, as in Melicerta (Fig. 274, 4),
or may be interrupted as in Brachionus, in which the pre-oral
circlet is represented by three distinct lobes, or as in Pedalion,
in which both circlets are divided into right and left moieties.
In one genus the trochal disc is absent.

•m,

FIG. 276. — Typical forms of mastax. A, forcipate type ; B, incudate type ; C, raraate type ;
/. fulcrum ; m, manubrium ; r. ramus ; u. uncus. (After Hudson and Gosse.)

Digestive Organs. — The typical form of mastax or pharyngeal
mill is that described in Brachionus (Fig. 270). There is an unpaired
incus consisting of a short stem or fulcrum (/.) and of two broad
branches or rami (r.), and a pair of mallei, each consisting of a
stout handle or manubrium (m.) and a broad, toothed head or
uncus (u.). In some forms all the parts of the apparatus become
very slender, the incus assuming the form of forceps (Fig. 276, A).
Or the mallei may be absent and the two rami movable upon
one another so as to convert the incus into a pair of forceps (B)
used to seize prey, the mastax being in this case protrusible.
Lastly, the fulcrum and manubrium may be absent, and the unci
and rami very strong and massive (C). Glands, supposed to be
salivary, open into the mastax or oesophagus.

The stomach is always large, and usually has a pair of digestive
glands opening into it : it may pass insensibly into the intestine,

vn PHYLUM TROCHELMINTHES 327

or the latter may be a distinct chamber of more or less globular
form. In the Khizota the intestine turns forwards so as to allow of
the anus being brought over the edge of the tube in defaecation
(Fig.^274, 4, a). In Asplanchna (6) the stomach ends blindly, the
intestine, cloaca, and anus being absent.

The excretory system is very uniform in structure. It con-
sists of a pair .of more or less coiled nephridial tubes, placed
longitudinally and giving off lateral branchlets which end in
flame-cells. The outer surface of each flame-cell usually bears
one or sometimes two flagella, which lie free in the body-cavity.
Frequently, but not always, the two tubes open posteriorly into
a contractile vesicle or bladder which discharges into the cloaca.

Nervous System and Sense-Organs. — The nervous system
usually consists of a single ganglion (Fig. 272, br.) towards the
dorsal aspect of the anterior part of the body, and representing the
brain or supra-cesophageal ganglion of the higher Worms : it sends
nerves to the muscles, trochal disc, and tactile organs. In some
cases a smaller ventral or infra-cesophageal ganglion is present
as well, connected with the first by a pair of slender ossophageal
connectives. Connected with the dorsal ganglion are a pair
of lateral longitudinal nerves which run backwards to the tail,
giving off branches in their course. One or more eyes (e.) are
usually present in close relation with the brain, and are sometimes
mere spots of pigment, but may be provided with a refractive
body or lens. The only other organs of sense are the tactile rods
(d.f., l.f.), of which there is usually one on the dorsal surface near
the anterior end of the body, and frequently two others, one on
each side of the trunk. They are more or less rod-like structures,
tipped with delicate sensory hairs and receiving nerves from the
brain.

Reproduction and Development. — In most cases the female
reproductive organs have the same general character as in Brachi-
onus, i.e. the gonad is unpaired (Fig. 269), consists of germarium and
vitellarium, and is provided with an oviduct (Fig. 272). But in
some of the Bdelloida, such as Philodina, there are two ovaries, not
divisible into germ-gland and yolk-gland, and the oviduct is absent.
The males are smaller than the females and degenerate in structure,
the enteric canal being atrophied (Fig. 271, A). There is a large
testis (t.) with a duct opening at the end of a protrusible penis (p.),
which is dorsal in all but Asplanchna, in which it, as well as the
cloacal opening of the female, appears to be ventral. Apparently
hypodermic impregnation sometimes takes place, i.e. the body-wall
of the female may be perforated at any place for the entrance
of the sperms.

Three kinds of eggs are produced : large and small summer eggs,
which always develop parthenogenetically, the larger giving rise to
females, the smaller to males ; and thick-shelled winter eggs, which

328 ZOOLOGY SECT.

probably require impregnation, and remain in an inert condition all
through the winter, finally developing in the spring. Most Rotifers
are oviparous, but some (Philodina, etc.) bring forth living young,
which are born by breaking through the body-wall or through the
cloaca, thus causing the death of the parent.

Segmentation is total and irregular, the oosperm dividing into
megameres and micromeres. An epibolic gastrula is formed, the
blastopore closes, and invaginations of ectoderm give rise to the
stomodaeum and proctodaeum. The tail is formed as a prolongation
of the postero-ventral region of the embryo, and contains at first
an extension of the endoderm. No metamorphosis is known to
take place in any member of the class.

Ethology. — A few Rotifers live in the sea, but the majority
are fresh-water forms, occurring in lakes, streams, ponds, and even
in puddles the water of which is rendered foul and opaque by mud
and sewage. Frequently the water in which they live is dried up,
and the thick-shelled winter eggs may then be widely dispersed
by wind. It is even stated that the adult animals may survive
prolonged desiccation and resume active life when again placed in
water. They are able to survive prolonged exposure to tempera-
tures far below the freezing point of water. Many forms cling
to the bodies of higher animals in order to obtain a share of their
food, thus leading a kind of commensal existence. Others go a
step further and become true external parasites, like Drilophaga
on a fresh- water Oligochaete (vide Section X), or Seison on the
little Crustacean Nebalia. Others, again, are internal parasites,
such as Albertia in the coelome of Earthworms and the intestine of
fresh- water Oligochaetes (Nais), and Notommata werneckii in the
cells of the fresh-water Alga Vaucheria.

Affinities. — The affinities of the Rotifera are very obscure.
Their general resemblance to the free-swimming larvae of Annelids
(phylum Annulata) is extremely close, and, in particular, the
curious Trochosphcera is, to all intents and purposes, a sexually
mature trochophore with a mastax. The excretory organs recall
those of the Platyhelminthes, and also resemble the provisional
nephridia or head-kidneys of Annulate larvae. Lastly, the hollow
muscular appendages of Pedalion and Hexarthra give those genera
a certain resemblance — which is probably, however, merely adaptive
—to the nauplius or free-swimming larva of Crustacea.

Class II. — GASTROTRICHA.

The GastrotricJia (Figs. 277 and 278) are a small group of minute fresh-
water animals, which are apparently allied, though certainly not very closely,
to the Rotifera, and are on that account placed in the present phylum. The
body is spindle-shaped with flattened ventral surface. The ventral surface
bears two longitudinal bands of cilia ; the dorsal is non-ciliated, but in some
forms bears a number of longitudinal rows of slender, pointed, cuticular
processes. The aboral end is narrow and usually bifurcated.

vn

PHYLUM TROCHELMINTHES

329

On the head are four tufts of flagella, which are partly sensory, partly
vibratile. The mouth, situated at the anterior end, leads by a narrow tube
into the thick-walled oesophagus. At the beginning of the latter are a number
of small chitinous denticles, and in front of them a circlet of setae. The
oesophagus leads to a wide elongated stomach followed by a short intestine
which terminates in an anal aperture at the posterior extremity. The

FIG. 277.— Chaetonotus maximus.
Highly magnified. (After Zelinka.)

yld.

FIG. 278. — Chaetonotus maximus (or-
ganisation), brn. brain ; gld. adhesive
gland ; mes. mesenteron ; mo. month ;
oes. resophagus ; ov. ovum ; ovar. ovary ;
retr. retractor muscles ; vent. mus. ventral
muscle. (After Zelinka.)

nephridia are a pair of unbranched coiled tubes each opening on the ventral
surface and terminating internally in a flame-cell. The nervous system con-
sists of a large dorsally and anteriorly situated cerebral ganglion or brain
giving off a pair of ventro-lateral longitudinal nerves. The sexes are united,
and there is no metamorphosis.

330

ZOOLOGY

SECT.

APPENDIX TO THE TROCHELMINTHES.

The Dinophilea and HistriobdeUea.

These are two isolated groups of minute animals which may most con-
veniently be dealt with in association with the Trochelminthes, since they
bear certain striking resemblances, now to one, now to another, member of
that phylum ; but they differ from all of them in the assumption of a simple
kind of metamerism (p. 43), by virtue of which they have claims to association
with the Annulata — a phylum to be treated of later. The Dinophilea are
free-living animals, mostly marine, one species living in brackish water.

The HistriobdeUea are
parasitic or commensal,
and live on the Euro-
pean lobster and the
Australian fresh-water
crayfishes.

Dinophilus (Fig. 279)
is a minute worm-like
marine animal with a
head or prostomium, a
body composed of from
five to eight segments
separated from one an-
other by constrictions,
and a short ventral tail.
The prostomium bears
two eye-spots and some
sensory hairs : it is
either covered uniformly
with cilia, or bears two
or three annular ciliated
bands apparently repre-
senting the prototroch
of the trochophore.
The body is in some of
the species uniformly
ciliated ; in others the
cilia are disposed in
rings corresponding to
the segments, except on
the ventral surface,
where the ciliation is

f

FIG. 279. — Dinophilus taeniatus. The left figure repre-
sents the dorsal surface of a young individual, x 76 ; the
mouth and alimentary tract are seen by transparency.
The right figure shows the anatomy of the male, x 38.
a. anus : b. rectum ; c. body-cavity ; d. vas deferens ;
m. pharynx ; n'. the first nephridium ; ce. entrance to
the oesophagus ; p., in left fig., prostomium ; p., in
right fig., penis ; st. stomach ; s. x. vesiculae seminalis.
(Fi

(From Sheldon, after Hanner.)

always uniform. The
mouth, which is situated
on the ventral aspect
of the prostomium, leads
into an alimentary canal consisting of oesophagus, stomach, and intestine,
all of which are ciliated ; the anus (an) is placed dorsally over the tail. A
protrusible muscular proboscis lies, when retracted, in a recess opening close
to the mouth. There is an imperfectly developed ccelome (hsemoccele ?)
which is crossed by strands of connective-tissue, and a ventral blood-vessel.
A nervous system is present, and consists of a large dorsal ganglion in the
prostomium, giving off two anterior and two posterior nerves or ventral
cords (sometimes segmented into a series of ganglia connected in each segment
by commissures), all situated in the epidermis.

The excretory system consists of a series of metamerically arranged pairs
of tubes (n/). The inner ends of these do not open into the body-cavity, but

VII

PHYLUM TROCHELMENTHES

331

are provided with peculiarly modified flagellate cells known as solenocytes, so
that these paired excretory tubes resemble closely the nephridia of some of
the Polychaeta (phylum Annulata ; see Section X). The sexes are separate.
In the male there is a conical ventral penis ; the last pair of nephridia act as
vesiculse seminales. In the ovary two sets of ova are developed, the larger
destined to give rise to females, and the smaller destined to form males.
They pass into the body-cavity and reach the exterior by an aperture on the
ventral surface in front of the anus.
A process of unequal segmentation
is followed by the formation of an
epibolic gastrula. What is known
of the development is in favour of
the view that Dinophilus is to be
looked upon as a trochophore-like
form that has made some progress
in the evolution of metamerism.

The Histriobdellea comprise only
the two nearly-allied genera Histrio-
bdella and Stratiodrilus (Fig. 280)—
the former found in the gill-cavities
and on the eggs of the European
lobster and the Norway lobster, the
latter in the gill-cavities of Australian
and Tasmanian fresh- water cray-
fishes. The animal is narrow, almost
cylindrical, with a well-marked head,
a body of six segments, and a nar
rower tail-region in which segmen-
tation is not clearly marked. The
head bears five tentacles (I1., tz., ts.)
tipped with non- motile sensory cilia,
and a pair of appendages or limbs
(La.) (retractile in Stratiodrilus), with
basal glands the ducts of which open
at their extremities. The head has
the mouth at or near its anterior
extremity on the ventral aspect.
The body bears, in Stratiodrilus ,
three pairs of two -jointed non-re-
tractile appendages or cirri (c1.,*?*.^8.)
tipped with non-motile cilia, and in
the male a pair of retractile append-
ages or claspers (cL). At the end of
the tail is a pair of large freely
movable appendages or legs (l.p.),
which are the chief organs of loco
motion : at the end of each of these
open the ducts of a mass of unicel-
lular glands. The anus is situated
posteriorly between the bases of the
legs. Opening from the mouth-
cavity on its ventral aspect is a muscular sac in which are enclosed,
when retracted, a system of chitinous jaws reducible to the same general
type as the mastax of the Rotifera, but with the relative position of
malleus and incus inverted. There is a highly developed nervous system
consisting of a large brain (br. c.) situated dorsally in the prostomium, «a pair
of cesophageal connectives, and a ventral nerve cord (n.c.) with a series of
ganglia which have a distinctly metameric arrangement. The excretory

FIG. 280.— Stratiodrilus tasmanicus,

male. ac. accessory gland of male ap-
paratus ; br. c. brain ; c1. c2. c3. cirri ; cl.
claspers (appendages peculiar to the male) ;
ex. excretory tubes ; gr. gld. granule-gland ;
I. a. anterior limb ; I. yl. gland at base of
anterior limb ; /. gld. gland at base of pos-
terior limb ; 1. p. posterior limb ; n. c.
nerve-cord ; p. penis ; t1. t2. t3. tentacles ;
ves. vesicula seminalis.

332 ZOOLOGY SECT, vii

system takes the form of ciliated tubes (ex.). closed internally, and showing a
tendency to metamerism : these in Stratiodrilus extend into the head. The
sexes are distinct : the male has a protrusible penis, directed ventrally.
There is no metamorphosis.

There seems to be some reason for believing that Dinophilus and the
Histriobdellea may help to bridge over the interval between the Trochelminthes
and the higher, segmented, worms or Annulata. In this connection the
Echinoderidse, which were noticed in an appendix to the last Section (p. 313),
have also to be kept in view.

SECTION VIII

PHYLUM MOLLUSCOIDA1

THE phylum Molluscoida comprises three classes — the Polyzoa
(including, provisionally, the Endoprocta), the Brachiopoda, and
the Phoronida. The members of these three classes are tolerably
widely divergent, so that it is somewhat difficult to frame a general
account of the entire phylum ; but the following are the most
important common features : —

There is, except in the Endoprocta, a body-cavity (ccelome), lined •
in most cases with a ccelomic epithelium, within which the ali-
mentary canal is suspended by means of mesenteries or by means
of funicular strands taking their place. The dorsal region of the
body is abbreviated, being represented only by a short space
between the mouth and anus, which are closely approximated.
There is a lophophore or tentacle-bearing ridge, usually of a horse-
shoe shape, containing a special compartment of the coelome, and
overhanging the mouth on its anal side there is in most cases a
sensitive process — the epistome — also containing a special com-
partment of the body-cavity. The central part of the nervous
system consists of a single ganglion (supra-oesophageal), or of two *
ganglia (supra-cesophageal and infra-cesophageal), or of a nerve-
ring. The nephridia when present are in nearly all cases a single/v
pair of ciliated tubes, which act also as gonoducts.

CLASS I.— POLYZOA.

The Polyzoa form colonies known as " Sea-mats," or " Coral-
lines," which in many cases bear a close general resemblance to
Hydroid Zoophytes, and only on a more minute inspection are
found to differ totally from the latter and to exhibit a very much
higher type of structure.

1 This and all the remaining phyla of the animal kingdom are characterised
by the possession of a true ccelome, i.e. of a cavity interposed between the
wall of the body and that of the enteron, and developed either directly by
outgrowth from the archenteron, or formed from clefts that appear in solid
masses o"f mesoderm cells. The only group hitherto dealt with in which a
definite ccelome is present is the Chsetognatha. In some of the groups which
are here comprised in the ccelomate phyla, however, as will be seen, the
ccelome is reduced, or entirely absent, or not typically developed.

333

334 ZOOLOGY SECT.

1. EXAMPLE OF THE CLASS — Bugula avicularia.

Bugula avicularia, the common Bird's-Head Coralline (Fig. 281),
occurs in brown or purple bushy tufts, two or three inches long, on
rocks, piles of jetties, and similar situations on the sea-shore in all
parts of the world. On a naked-eye examination it presents a
considerable resemblance to a Hydroid Zoophyte, and might readily
be taken for a member of that group. It consists of dichotomously
branching narrow stems, which are rooted by a number of slender
root-filaments. Each stem is found, when examined with a lens, to
be made up of a number of elements, the zocecia of the colony,
which are closely united together and arranged in four longitudinal
rows. The zooacia are approximately cylindrical in shape, but
broader distally than proximally, four or five times as long as broad,
and have, near the distal end, a wide crescentic aperture — the
" mouth " of the zooecium — on either side of which is a short blunt
spine. A rounded structure — the ocecium — in many parts of the
colony lies in front of each zooecium (Fig. 281, ooec.). On each
zocecium, except a few at the extremities of the branches, is a
remarkable appendage, the avicularium (awe.), having very much
the appearance of a bird's head supported on a very short stalk :
if the Bugula is examined under the microscope in the living
condition, the avicularia will be found to be in almost constant
movement, turning from side to side ; and a movable part, com-
parable to the lower jaw of the bird's head, will often be seen to
be moved in such a way that the mouth of the avicularium is
opened very widely and then becomes closed up with a quick
" snap." All the parts hitherto mentioned can be shown, by using
appropriate tests, to be composed of some material akin to chitin
in composition. The chitinous wall of the zooecia is the hardened
and thickened cuticle of the zooids, having beneath it the soft body-
wall.1 The anterior region of the body of the zooid forms an
introvert, i.e. is capable of being involuted like the finger of a
glove within the more posterior part : the cuticle covering this,
continuous behind with the thick ectocyst, is quite thin and flexible.
When the introvert is everted it is seen to bear at its anterior end a
circlet of usually fourteen long, slender filiform tentacles (tent.) on a
circular ridge or lop7tophQte_ surrounding the mouth of the zooid. The
tentacles are densely ciliated except along their outer surfaces : the
cilia vibrate actively in such a way as to drive currents of water,
and with them food-particles, towards the mouth (mo.) : they are
also capable of being bent in various directions. In the interior of
each is a narrow prolongation of the ccelome. In all probability,
besides bringing minute particles of food to the mouth of the zooid
by the action of their cilia, the tentacles are prehensile as well as

1 The terms ectocyst and endocyst are commonly applied respectively to the
hardened cuticle of the zooid and its soft body-wall.

THI

PHYLUM MOLLUSCOIDA

335

tactile, and also act as organs of respiration. When retracted they
become enclosed by the walls of the introvert as by a sheath —

tent

au

FIG. 281. — Bugula avicularia. Two zooids, magnified, an. anus ; avic. avicularia ; emb.
embryo enclosed in the ocecium ; funic. funiculus ; gast. muscular bands passing from the
stomach to the body-wall ; int. intestine ; mo. mouth ; ooec. : ooaciuin oes. oesophagus ; ov.
ovary ; ph. pharynx ; ret. parieto-vaginal muscles ; sp. spermatidia ; stom. stomach ;
tent, tentacles. The ganglion, which is not indicated, lies just below the middle of the
stroke from mo.

the tentacle-sheath. A pair of bands of muscular fibres — the parieto-
vaginal muscles (ret.) — passing to the introvert from the body- wall,
serve to retract the introvert and tentacles.

336 ZOOLOGY SECT.

The body-wall consists, in addition to the cuticle, of an epi-
dermis composed of a single layer of large flattened cells, two
muscular layers, the outer circular and the inner longitudinal, and
a layer of an irregular cellular tissue, or parenchyma.

The ccelome is extensive ; it is lined externally by the parietal
layer of parenchyma forming the innermost layer of the body-wall,
and internally by a visceral layer of the same tissue, ensheathing
the alimentary canal. Across the cavity between the parietal and
visceral layers of the parenchyma pass numerous strands of spindle-
s/shaped cells. A large double strand (funic.) passes from the
proximal or aboral end of the alimentary canal to the aboral
wall of the zocecium ; this is tike funictdus. A transverse partition
cuts off (though not completely) a small anterior compartment of
the coelome from the rest. The former surrounds the bases of
the tentacles, the narrow internal cavities of which are in com-
V/munication with it : this is known as the circular canal. The
coelomic fluid contains a number of colourless corpuscles or
leucocytes.

Alimentary Canal. — The mouth (mo.) leads into a wide
V chamber — the pharynx (ph.) — just behind the bases of the tentacles ;
from this a somewhat narrower short tube, separated by a constric-
tion from the pharynx, leads to the stomach (stom.) from which it is
«/ also separated by a constriction. The stomach gives off a long
conical prolongation or ccecum passing towards the aboral end of
i/ the •zocecium, to which it is attached by the funiculus. The
intestine (int) comes off from the oral aspect of the stomach, not
far from the oesophagus, with which it lies nearly parallel : it
terminates in a rounded anal aperture (an.) capable of being dis-
^ tended to a considerable size, situated not far from the mouth,
but outside the lophophore. The entire alimentary canal is lined
by an epithelium, which is ciliated throughout except in a portion
of the stomach : the cells of the epithelium, which are arranged
in a single layer, vary in length in different regions, being longest
in the pharynx, which is comparatively thick-walled. A pair of
slender muscles (gast.) passing from the body- wall to the stomach
1 act as retractors of the alimentary canal when the introvert is

drawn back.
J There are no blood-vessels.

The nervous system consists of a small round ganglion
situated between the mouth and the anus, giving off nerves to the
various parts ; organs of special sense are absent. Definite ex-
cretory organs do not occur in Bugula, the function of excretion
(i.e. the collection of the nitrogenous waste-matters) being appar-
ently carried on by the leucocytes and the cells of the funicular
tissue.

Reproductive Organs. — Ovary and testis are found to occur
together in the same zooid. They are both formed from specially

vm

PHYLUM MOLLUSCOIDA

337

modified cells of the parenchyma, either of the funiculus or of the
body-wall. The testis, developed from the cells of the funicular
tissue, gives origin to spherical masses of cells — the spermatidia
(sp) — which develop into sperms with very long motile tails. These
become free from one another and move about in the body-cavity
or in its prolongations into the tentacles. There is no spermiduct,
and it is doubtful if the sperms pass to the exterior. The ovary
(ov) is a small rounded body formed from the parietal layer of the
parenchyma about the middle of the zocecium ; it consists of only
a small number of cells of which only one at a time becomes a
A p

ect

Fra. 282. — Early stages in the development of Bugula. cent, central
corona ; ect. ectoderm ; end. endoderm ; seg. segmentation-cavity.

mature ovum, certain smaller cells forming an enclosing follicle.
The mature ovum is perhaps fertilised in the ccelome ; iBpass
into the interior of a rounded outgrowth of the zooeciiSfc— the
ooecium (OOBC.) — lined with parenchyma, and forming a sort of m
pouch in which it undergoes development. \ "

Development. — Segmentation (Fig. 282) is complete am
nearly regular. A blastula is formed having the shape of a
bi-convex lens. In the interior of the blastoccele or cavity
of the blastula, four cells (end.) — the primitive endoderm
cells — become distinguishable ; these increase in number by>
division, and form a mass of free cells which almost completely

VOL. T. z

338 ZOOLOGY SECT.

fill the blastocoele ; this mass apparently represents both
endoderm and mesoderm. Small cavities which appear in it

~ subsequently unite together to form the primitive coelome.
A very broad ring-shaped thickening — the corona (6r, cor.) —
is formed round the equator of the embryo and becomes
provided with cilia. A circular pallial groove arises on the
oral side of the corona. A sac-like, afterwards beaker-shaped
invagination of the ectoderm on what is destined to become
the oral side of the ciliated ridge, forms a larval structure, termed

s the sucker (Fig. 283, suck), which afterwards serves to fix the larva.
A second depression of the ectoderm in the region of the corona
on the oral side forms the ectodermal groove. At the aboral pole is
developed, also from the ectoderm, a second larval structure — the

\ calotte or retractile disc (disc), on which motionless sensory cilia

disc

cor

cor

SUClv

FIG. 283. — A, Larva of Bugula plumosa ; B, Sagittal-section of larva'of Bugula (diagram-
matic), cent, central tissue ; cor. corona ; disc, retractile disc ; e. ectodermal groove ; p.
pyriform organ ; pall, pallial groove ; suck, sucker. (From Korschelt and Heider, after
Barrois.)

appear. In close relation to the ectodermal groove is formed a mass
of cells, the pyriform organ (p.) which seems to have a sensory
function.

An alimentary canal is absent in the larva of Bugula when it
escapes from the ocecium. After an interval of free existence as a
ciliated larva, certain changes appear which lead to a very
i complete metamorphosis. The sucker becomes everted by a
strong contraction of the body, and fixes the larva to some foreign
object. The aboral side of the larva becomes greatly extended, so
that almost the entire integument of the primary zooid is devel-
oped from this part (i.e. from the region occupied by the retractile
disc and pallial groove). Accompanying the extension of the
aboral surface are the obliteration of the pallial groove and the
bending down of the corona towards the oral side. Thus the stage

vm PHYLUM MOLLUSCOIDA 339

of the larva termed the umbrella-shaped stage is reached. The
sucker is everted, and by means of it the larva becomes attached.
The edge of the " umbrella " becomes bent downwards, and
fused with the b^oad plate into which the sucker has ex-
panded, thus enclosing a circular cavity, the so-called vestibule
(Fig. 284, v.). The walls of this, consisting of the coronal cells
and a portion of the original sucker, become broken up and the
cavity is merged in the general cavity in the interior of the
larva. All the larval structures have now disappeared with the
exception of the basal plate of the sucker and the retractile disc.
The former gives rise to the basal part of the wall of the primary
zooecitun. From the latter, which becomes invaginated, or from
a sac which is developed to replace it, are developed both
the ectodermal and endodermal structures of the primary zooid.
Occupying the interior of the larva at this stage in addition to
this sac, there is only a mass of undifferentiated tissue derived
from the original central tissue together with that derived
from the disintegrated corona, pyriform organ, and part of the
sucker. The outer wall forms the wall of the primary zocecium, the
surface of which becomes covered with a chitinous cuticle or
ectocyst. Most of the internal mass goes to form a brown body,
which now becomes developed, but a part of it seems to form the
mesoderm of the zooid. A diverticulum
of the sac constitutes the first rudiment
of stomach and intestine ; a second
diverticulum forms the rudiment of the
oesophagus ; these become applied to
one another and fuse to form the con-
tinuous alimentary canal. The ganglion
arises as an invagination of the ecto-
derm in the space between mouth and
anus. The upper part of the cavity
of the primitive sac, after the rudi-
ment of the alimentary canal has been FIG. 284.— Longitudinal section of
separated off, forms a space termed the f^^fr.^SSSt
atrium ; the walls of this become con- of the zooid in the form of a

n '. , , .. sac; s. basal plate of everted

verted into the tentacle sneatn, while sucker ; v. vestibule. (From
on its base appear the rudiments of the figg? and Heider' after
tentacles and lophophore. During the

development of the organs of the adult zooid the brown body
becomes closely applied to the stomach and gradually absorbed.

The primary zooid thus formed gives rise asexually by a process
of repeated budding to the branching structure which has been
described. In many of the zocecia of a fully-developed colony no
zooid is found to be present, but, instead, there is a dark brown
body similar to that which occurs in the primary zocecium. This
is a zooid that has undergone degeneration — the lophophore

22

340 ZOOLOGY SECT.

tentacles, and alimentary canal having been absorbed. Such
degenerated zooids are capable of regeneration, the organs becoming
re-developed and the brown body re-absorbed.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Polyzoa are Molluscoida which, with one exception, form
colonies of zooids connected together by a common organic sub-
stance. There is a lophophore bearing a series of slender, ciliated,
post-oral tentacles. The anterior part of the body forms, in the
majority, a short introvert, within which the lophophore and the
tentacles are capable of being withdrawn. In some the pro-
stomium is represented by a small lobe — the epistome. The
alimentary canal is U-shaped, and the anus is anterior, within, or
just outside, the tentacular circlet. In most the nervous system
is represented only by a small ganglion between the mouth and
the anus. A cuticle, sometimes gelatinous, sometimes horny,
sometimes calcified, forms a firm exoskeletal layer for the support
of the colony. Nephridia (corresponding to the head-nephridia of
the trochophore) occur only in the Endoprocta. There is no
vascular system. The sexes are usually united. The majority of
Polyzoa occur in the sea ; a limited number are inhabitants of
fresh water.

Sub-Class I.— Ectoprocta.

Colonial Polyzoa with the anus outside the lophophore, with a
well-developed introvert and a spacious coelome.

ORDER 1. — GYMNOLJEMATA.

Almost exclusively marine Ectoprocta, with a circular lopho-
phore, and without an epistome.

Sub-order a. — Gydostomata.

Gymnolsemata with tubular calcareous zorecia having circular
apertures devoid of closing apparatus.
Including Crisia, Idmonea, &c.

Sub-order b. — Cheilostomata.

Gymnolaemata with calcareous or chitinous zooecia usually pro-
vided with opercula.

Including Bugula, Flustra (" Sea-mat "), Membranipora, Cellepora,
Selenaria.

Sub-order c. — Ctenostomata.

Gymnolaemata with chitinous or gelatinous zocecia provided with
a series of tooth-like processes closing the aperture when the
tentacles are retracted.

Including Alcyonidium, Serialaria, Paludicella.

vui PHYLUM MOLLUSCOIDA 341

\
ORDER 2. — PHYLACTOL^EMATA.

Fresh-water, Ectoprocta with horse-shoe-shaped lophophore and
with an epistome.

Including Cristatella, Plumatella, Fredericella.

Sub-Class II.— Endoprocta.

Colonial or solitary Polyzoa multiplying by the formation of
buds, which in Loxosoma soon become separated off, while in
Pedicellina they remain connected together by a creeping stolon.
The anus, as well as the mouth, is internal to the lophophore.
The introvert is slightly or not at all developed. A pair of ciliated
nephridial tubes are present.

Systematic Position of the Example.

Bugula avicularia is an example of the sub-order Cheilostomata
of the Gymnolaemata. It is a member of the family Bicellariidse,
which is characterised by the erect plant-like colony, with narrow
compressed branches, and attached by root-like fibres ; by the
avicularia, when present, being stalked and bird's-head shaped ; and
by the wide oblique apertures of the zooecia all facing in the same
direction. Bugula differs from the other genera of the family in
the arrangement of the zooecia in double or multiple rows, in their
close union, and in the avicularia, when present, being on the side
on which the mouth is situated. The various species differ in the
exact shape of the zooecia and of the avicularia.

3. GENERAL ORGANISATION.
Sub-Class I.— Ectoprocta.

The Ectoprocta and the Endoprocta differ so considerably from
one another that it is advantageous to deal with them separately.
The Ectoprocta are all colonial — the colonies being capable, in
most cases, like the colonies of hydroid zoophytes, of increasing in
size to an apparently indefinite extent by continuous budding.
The thickened cuticle which forms the support of the colony is
sometimes gelatinous, sometimes chitinous, sometimes chitinous
with sand-grains affixed, sometimes calcareous. The form of the
colony varies in different families and genera in accordance with
differences in the shape of the constituent zooecia, and differences
in their mode of budding and consequent arrangement. The
zooecia are sometimes tubular, sometimes ovoid, sometimes poly-
hedral. In some cases the buds are so developed that the colony
assumes the form of a thin, flat expansion, which may be encrusting,
and consist of a single layer of zooecia in close contact with one
another or connected together by tubular processes ; or. may be
erect, and with the zooecia either in one or two layers : sometimes

ZOOLOGY

SECT.

the lamellar colony thus formed may be fenestrated or divided into
lobes ; sometimes it is twisted into a spiral. In other cases the
colony, instead of being lamellar, has the form of an erect, shrub -
like structure, consisting of numerous cylindrical, many-sided, or
strap-shaped branches arising from a common root. Sometimes
there is a creeping cylindrical stolon, simple or branched, having
the zooids arranged along it in a single or double row. The colony
is free only in Cristatella (Fig. 286) — in which it performs creeping
movements, in some other (American) forms of Phylactolsemata
(in the younger stages of the colony), in one family of the
Cheilostomata — the Selenariidce (in which it moves along with
the aid of certain peculiar appendages — the vibracula — to be
described subsequently), and in one or two other cases.

stato

FIG. 285. — Plumatella, Portion of a colony, magnified, funic. funiculus ; gang, ganglion ;
int. intestine ; mo. mouth : OR. oesophagus ; repr. gonad ; retr. retractor muscle ; st. stomach ;
stato. statobJasts. (After Allman.)

The zooecia open on the exterior by means of circular, semi-
circular, or crescentic apertures, which in the Phylactolaemata and
the Cyclostomata among the Gymnolsemata are devoid of any special
closing apparatus ; while in the Cheilostomata there is a movable
lid or operculum closed by a pair of occlusor muscles when the
introvert is retracted ; and in the Ctenostomata there is a series of
lobes or teeth which close in together over the opening. The
cavities of the neighbouring zocecia are in some forms completely
cut off from one another by a continuation of the chitinous or
calcareous exoskeleton ; in others there is free communication ; in
others, again, there is communication through a number of minute
perforations.

The oral (anterior) part of the body of each zooid is, as already

vni

PHYLUM MOLLUSCOIDA

343

described in the case of Bugula, covered only with, a thin and
flexible cuticle, and forms an introvert capable of being retracted
into the interior of the zooecium. At the free end of the introvert
is the mouth surrounded by a lophophore bearing tentacles. The
tentacles are always simple, filiform, and hollow, each containing a
narrow diverticulum of the circular canal or anterior compartment
of the ccelome. They are beset with vibratile cilia by means
of which currents are created subserving alimentation and
respiration. They are also highly sensitive, and are capable
of being bent about in various directions by the contraction

FIG. 286. — Cristatella mucedo. Entire colony. (After Allman.)

of muscular fibres in their walls, so that they can be used
for prehension. In the Phylactolsemata (Fig. 285) the lopho-
phore is horse-shoe-shaped, in the Gymnolaemata (Fig. 281)
circular : in the former, but not in the latter, there is a ciliated
lobe, the epistome (Fig. 287, ep.) — which may have a sensory func-
tion— overhanging the mouth on the anal side. The retraction of
the introvert is effected by a pair of bands of muscular fibres, the
parieto-vaginal muscles, passing to it from the body-wall, and
by a pair of retractor muscles passing from the latter to the ali-
mentary canal.

v/^

344

ZOOLOGY

SECT.

Structure of Body-wall. — Beneath the cuticle is an epi-
dermis, consisting of a single layer of flattened polygonal cells,
firmly united together by their edges. Beneath this there
is usually, but not always, a layer of muscle, which is

arranged in two strata — an
external composed of circular,
and an internal of longitu-
dinal fibres. There is an ex-
r' tensive coelome lined in some
forms (PhylactolaBmata) by a
definite coelomic epithelium,
in part ciliated ; while in
others there is no such de-
finite, epithelium, but its place
is taken by thin parietal and
visceral layers of an irregular
cellular tissue — the paren-
chyma. Crossing the coelome
are strands, in some instances
very numerous, of spindle-
shaped cells. In some cases

FIG. 287. — Anterior portion of the body of , •, j

Lophopus, from the right side. an. anus ; tWO mesenteriC DandS SUS-
ep. epistome; ga. ganglion ; o. mouth ; pr. in- «ATirl f>,A oliTYVAnf or\r PQTIQ!

testine ; gt. oesophagus ; t. tentacles, cut off pena tH6 alimentary canal—

near the base. (From Lang's Comparative an anterior attached near the
Anatomy. After Allman.)

mouth and a posterior passing

from the caecum to the aboral end of the zooecium ; in most
cases the latter, to which the special name otfuniculus is given, is
alone present.

The alimentary canal has in all species the parts that have
been already described in the case of Bugula. In some of the
Cheilostomata it is stated that the cells of the oesophagus bear
numerous striated muscle-fibre processes. In some Ctenosto-
mata there is in addition a thick-walled chamber — the gizzard
— with chitinous teeth, between the oesophagus and stomach.

The nervous system consists of a single, sometimes bilobed,
ganglion (Fig. 285, gang., and Fig. 287, ga.) placed between the
mouth and the anal aperture, and of nerves passing from it
to the various parts. There are never any organs of special
sense, unless the epistome of the Phylactolaemata be of that
nature.

Nephridia are not known with certainty to exist in any of
the Ectoprocta. In some there is a pore through which water
enters the body-cavity, or a ciliated intertentacular tube opening
at the base of the tentacles. Excretion appears to be performed
by certain cells of the funicular tissue and of the parenchyma
or coelomic epithelium. These become loaded with' the products
of excretion, and are set free as leucocytes in the coelome, whence

vra PHYLUM MOLLUSCOIDA 345

they may pass out through the intertentacular tube^or may
accumulate in the cells of the brown body.

In many Ectoprocta the colony bears a series of remarkable
appendages — the avicularia — which are of the nature of modified
zooids. In typical cases the avicularium has the bird's-head-like
form that has been already described in the case of Bugula ;
sometimes it is completely sessile. A second set of movable
appendages found in some forms are the vibracula ; these are
long tapering whip-like appendages which execute to-and-fro
movements. Intermediate forms between avicularia and vibracula
show that the latter are extreme modifications of the former. The
avicularia are frequently found to have seized in their jaws minute
Worms or Crustaceans, and it is probable that their function, as
well as that of the vibracula, is defensive ; in the case of the
SelenariidcB, which form unattached colonies, it is said that the
movements of the vibracula subserve locomotion.

The impregnated ova in many cases undergo the early stages of
their development in certain dilatations of the colony (Fig. 281,
ocec.), and in many of the Gymnolsemata (Cheilostomata) these
ovicells or ocecia, as they are termed, take on a very definite shape.

Reproduction and Development. — In general the Ectoprocta
are hermaphrodite. Both ovary and testis are derived
from the layer lining the coelome (parenchyma or coelomic
epithelium as the case may be), or from the funicular tissue. The
testis may be single or double. The spermatidia, as in Bugula, or
the mature sperms, become free in the coelome. The ovary is very
generally situated towards the oral end or about the middle, the
testis towards the base. The mature ova escape into the coelome,
and in some forms there become impregnated apparently by the
spermatozoa of the same individual. The development of the
larva may take place in the ccelome or in a special diverticulum of
it ; in the Cheilostomata the fertilised ova pass into the ovicells ;
in some cases, both among the Phylactolsemata and the
Gymnolsemata, they are received into a sheath formed by the
tentacles of an imperfectly-developed zooid formed in a zoocecium
in which the original zooid had undergone degeneration.

In those cases in which the early stages of development are
passed through in the body-cavity of the parent, the ciliated
embryos may either escape through the zocecial aperture after
zooid has undergone degeneration, or through a special opening
formed for them in the wall of the zocecium. In some the fertilised
ova pass out through the intertentacular tube, which plays the part
of an oviduct. In Crisia and other Cyclostomata each of the ripe
ocecia is found to contain a large number of embryos, developed from
one ovum. The ovum in this genus segments to form a mass of
cells from which finger-like processes arise, the enci of each of these
becoming constricted off to form an embryo.

346 ZOOLOGY SECT.

Segmentation is total and approximately equal. The form of
the free-swimming larva varies considerably, but in most there is a
circular band with very long cilia, the corona, which may represent
the tentacular crown of the adult ; this divides the surface into
two regions — oral and aboral. The larva may or may not be
provided with a digestive canal. The aboral portion of the body
presents a ciliated retractile disc or calotte, which subsequently
becomes a closed sac destined to give origin to the primary zooid
of the colony ; on the oral side is the sucker by which the larva
afterwards becomes fixed. By a metamorphosis similar to that
which has been described in the case of Bugula (p. 338), a primary
zooecium with a primary zooid is developed from the previously
free ciliated larva. In Membranipora, which belongs to the Cheilo-
stomata, and in several genera of Ctenostomata, the larva (called
Cyphonautes when it was supposed to be an adult animal) is laterally
u compressed, with a bivalve shell. In the Cyclostomata the larva
is barrel-shaped, with the mouth at one end, and at the other a
prominence corresponding to the retractile disc. In the
Phylactolsemata the larva is in the form of a ciliated hollow cyst
from which the colony is formed by gemmation. A special form
of asexual multiplication by means of bodies termed statoblasts
(Fig. 285, stato) is observable in the Phylactolaemata. The
statoblasts are internal buds formed from the funiculus and
enclosed in a chitinous shell ; they are set free eventually by the
death and decay of the parent colony, and each gives rise to a small
zooid which fixes itself and develops into a colony.

Ethology and Distribution. — None of the Ectoprocta are
parasites in the strict sense of the term, but very many of them
live in intimate association with other organisms, often growing
over and through them so as to form with them one complex
structure. Certain genera are able by some means to excavate
minute burrows in the shells of bivalves.

The majority of Ectoprocta are marine ; but all the Phylacto-
laemata, together with Paludicella of the Ctenostomata, are in-
habitants of fresh water. The fresh-water forms inhabit both
running and stagnant waters ; they occur at all elevations and
are represented in all the great regions of the earth's surface.
The marine forms are most abundant at moderate depths ;
but representatives of the group have been dredged from as
great a depth as over 3,000 fathoms. In certain localities the
larger kinds grow in great luxuriance, so as to form miniature
forests.

Geologically the Ectoprocta are a very ancient group, being
represented in the Cambrian and later Palaeozoic formations by
forms which appear to have belonged mainly, if not exclusively,
to the Cyclostomata. In the later formations of the Mesozoic period
the Cheilostomata are also abundantly represented, and in the

VIII

PHYLUM MOLLUSCOIDA

347

Tertiary the latter sub-order greatly outnumbers the Cyclostomata.
The Tertiary Polyzoa flourished in certain localities in such
luxuriance that their remains form calcareous deposits of very
great extent.

Sab-Class II.— Endoprocta.

While the sub-class of the Ectoprocta comprises a large number
of genera, that of the Endoprocta includes only Pedicellina (Fig. 288),
Loxosoma, Umatella, Myosoma, Gonopodaria and Ascopodaria, with
one or two other less completely known forms. They are all
marine except Urnatella — an American fresh-water genus. The
feature indicated by the name of the sub-class — viz. the position
of the anus within the circlet of the tentacles, is an important
point of difference from
the rest of the class ; but
there are others of as great
or greater importance.

In none of the Endo-
procta is there a distinct
introvert. The body is
cup-shaped, with a rim
which is capable of being
inverted over a cavity
— the vestibule — within
which the tentacles can
be withdrawn, and which
contains both mouth and
anus. An epistome over-
hangs the mouth. The
ccelome is almost or quite
obliterated, the space
between the alimentary
canal and the wall of the
body being filled, more or less completely, with a gelatinous hyaline
matrix. A pair of nephridia are present. In Loxosoma they lie
one on either side of the oesophagus and open separately on the
exterior ; they are ciliated intra-cellular tubes, each of which
probably begins in a flame cell. In Urnatella the two nephridial
tubes unite to open into the cloaca — a diverticulum of the
vestibule. The ganglion (Fig. 288, gang.), situated between mouth
and anus as in the Ectoprocta, is bilobed in Loxosoma. Testes and
ovaries occur in the same individual in some, but appear to mature
at different times : they are provided with special ducts ; in others
the sexes are separate.

Pedicellina and Urnatella are colonial, Loxosoma solitary. In
Pedicellina (Fig. 288) there is a creeping stolon with which a
number of zooids are connected ; a diaphragm separates the body

FIG. 288. — Pedicellina. Showing successive stages
^numbered 1 to 6) hi the development of zooids by
budding, an. anus ; gang, ganglion ; mo. mouth ;
tent, tentacles (retracted). (After Hatschek.)

348 ZOOLOGY

S EOT.

of each zooid from the stalk. Gonopodaria ramosa has a branching
stalk. Urnatella has a disc of attachment with one to six, jointed,
branching stems. In Loxosoma, which is found attached to
various Annulata, two parts are distinguishable — the calyx or body
and the stalk. In the base of the latter is the so-called foot-gland,
consisting of a small number of granular cells arranged around a
central space opening on the exterior. Buds are formed, but
become detached before reaching maturity. Segmentation of the
ovum is complete, and a gastrula is formed by invagination.

Certain differences in the larval history have sometimes been
regarded as separating very widely the Endoprocta from the
Ectoprocta. The former, like the latter, have a free-swimming
ciliated larva, provided with a corona and a ciliated disc. This
develops directly into the primary zooid after becoming attached
by means of the oral surface. The ectoproct larva also, as stated
previously (p. 346), becomes attached by the oral
surface ; but any rudiments of a zooid — such
as an alimentary canal — which may have been
developed, become absorbed, and the primary
zooid is developed at the free or aboral end of
the larva, with its oral surface directed up-
wards, away from the base of attachment.
The difference, however, is not so important as
it may at first appear, for the parts of the larval
Endoproct do not remain in the reversed position
in which they are situated when attachment first
takes place, with the vestibule, mouth, and anus
directed downwards. Very soon a rotation is
observed to take place, by virtue of which the
vestibule and developing tentacles, with the
mouth and anus, become carried to their
permanent position on the free surface of the
animal.

CLASS II.-PHORONIDA.

The position of Phoronis and its close ally,
Phoronopsis, worm-like marine animals, is a
matter on which widely divergent views are held.
On account of certain strong resemblances to the
Polyzoa, and, more particularly, to the Phylac-
tola3mata they are most commonly looked upon
as related to that class and to the Brachiopoda,
and the Phoronida may thus conveniently be
dealt with as a class of the Molluscoida.

Phoronis (Fig. 289) lives in associations con-
FIG. 289.— Phoronis sisting of a number of individuals. These are
austraiis, natural usuauy iooked upon as having all been deve-

vni

PHYLUM MOLLUSCOIDA

349

-.-

FIG. 290. — Phoronis aus trails, free end,
magnified, an. anus ; ep. epistome ; mo.
mouth; nephr. nephridial aperture; neph.
nephridium. (After Ben ham.)

tent

loped from ova. But multiplication by fission has been proved

to occur actively in one species, and probably increase in the numbers

of the association is effected in this manner throughout the group.

Each worm is enclosed in a

membranous or leathery tube,

within which it is capable of

being completely retracted.

The body, which varies in

length in the different species

from 1-5 to 127 mm., is cylin-
drical, elongated, and unseg-

mented. At one end there is

a crown of numerous slender,

ciliated tentacles borne on a

horse-shoe-shaped lophophore,

the lateral cornua of which

are spirally coiled in the larger

species ; these are supported by a mesodermal skeleton and are

non-retractile.

Both mouth and anus (Fig. 290, wo., an.) are situated at this

tentacular extremity of the body;,
separated from one another by
only a short space. This short
space between mouth and anus
represents, as in the Polyzoa, the
greatly abbreviated dorsal surface ;y,
but it will be convenient to term'
this end of the animal the anterior,
and the opposite the posterior end :
the side of the elongated body
towards which the mouth is
approximated may be distinguished
as the oral, the opposite as the
anal. A broad lobe — the epistome
(ep.) — overhangs the mouth and
lies between it and the anus. Near
the anus open two ciliated nephri- ,
dial tubes (neph.) of mesodermal

FIG. 29i.— Phoronis austraiis, internal origin, which open internally each

organisation, af. bl. afferent blood vessel ; , , . , ,-, J , •

an. anus ; ef. bl. efferent blood vessel ; ep. by two apertures into the posterior

epistome; mes. mesentery : mo. mouth ; nT,Qrn-Uor ^f f^0 /Wlnrnp

nephr. p. nephridiopore ; nephr. d. duct Chamber OI tne CCBlOme.

of nephridium; nephrost nephrostome The CGelome, which is lined With

(internal opening of nephridium) ;«3s. 0280- i • -IT • T-IT

phagus ; red. rectum ; rect. mes. rectal a COelomiC epithelium, IS divided

(cufshoS: llf^BenhLmf '* 3 into two parts of very unequal ex-
tent by a partition or mesentery

which runs across just behind the tentacles. The anterior part is
in communication with cavities in the tentacles and the epistome.

oes
efbl

350

ZOOLOGY

SECT.

The posterior, and by far the most extensive part of the coelome,
occupies the whole of the length of the body behind the trans-
verse partition. It is subdivided into two by a median longi-
tudinal mesentery (Fig. 292, m.5 m.), which extends from the oral
to the anal surface and supports both limbs of the alimentary
canal ; and each of these is further subdivided by a longitudinal
mesentery extending from the body-wall to the oesophagus (ce.)
in the one compartment (usually termed the right), and to the
rectum (r.) in the other (left). The alimentary canal is bent on
itself to form a loop, as in the Polyzoa : it is distinguishable into

ef.u

m

FIG. 292. — Fhoronis, transverse section towards the anterior end. af. v. afferent blood-vessel ;
c. m. circular layer of muscular fibres ; ef. v. efferent blood-vessel ; ep. epidermis ; c. m. cir-
cular layer of muscle ; m., m. mesenteries ; ne. f. funnel-like opening of nephridium ;
OR. oesophagus ; r. rectum. (After Benham.)

oesophageal, gastric and intestinal regions. There is a closed system
of blood-vessels with contractile walls containing red bipod-corpuscles.
The nervous system lies immediately below the cells of the epidermis.
Nerve-elements are generally cfotributed over the surface, but are
specially concentrated in the form of a ring surrounding the
body just behind the mouth, but not enclosing the anus,
thickened into a ganglion between mouth and anus, and giving off
nerves to the tentacles. There are no organs of special sense.
Phoronis is hermaphrodite. Ova and sperms are developed in

vin

PHYLUM MOLLUSCOIDA

351

the coelome towards the posterior end from cells on the wall of one
of the large blood-vessels. When mature these pass out through !
the nephridia to the spaces enclosed by the tentacles, where the
ova are impregnated ( — according to another account, fertilisation
takes place in the ccelome — ), and they go through the early stages
of development fixed to the tentacles. The segmentation is com-
plete and slightly unequal : when four blastomeres are formed two
larger, darker endoderm and two smaller, clearer ectoderm cells are
to be distinguished. A blastula is formed with clearer ectoderm
cells on one side ; invagination takes place ; and, as the embryo
elongates, the blastopore is drawn out into a slit which eventually
becomes closed up behind, the anterior portion alone remaining!
open to form the mouth. The
anus is developed later as an
invagination in the position of
the posterior part of the
former blastopore. The meso-
derm arises from cells budded
off from the endoderm. The
prosocoele and mesoccele arise
by the formation of fissures;
the metaccele by a process of
folding off from the archen-
teron. A large pre-oral lobe is
formed, and the anus becomes
surrounded by a circlet of cilia
(Fig. 293, t. tr.). The part of
the body on which the anus is
situated becomes elevated into
a conspicuous process. Behind
the mouth there is a circlet of
cilia, and from this region
grow out a circlet of processes
—the rudiments of the larval
tentacles (Fig. 294, B). The
larva has now reached the stage
to which the term actinotrocha is applied. It has a large hood-like lobe
overhanging the mouth and a circlet of ciliated larval tentacles ;
the anus is situated on a prominent process.

There is a pair of larval excretory organs corresponding to those
of the trochophore larva ; these have no ccelomic apertures, but
are provided with solenocytes (see Section X, Annulata). They
apparently do not become converted into the nephridia of the
adult. A thickening of the ectoderm of the pre-oral lobe, sometimes
bearing eyespots, appears to represent the apical plate of the
trochophore. At the point where the oesophagus opens into it,
the gastric region of the alimentary canal gives off forwards in one

FIG. 293. — Actinotrocha larva ut Phoronis,

lateral view. ap. apical plate ; 1 . v. blood-
vessel ; m. mouth ; m. tr. circlet of cilia and
tentacles ; t. tr. circlet of cilia round anus.
(From MacBride, after Metschnikoflf )

352

ZOOLOGY

SECT.

species a pair of hollow diverticula, the cells of which contain
vacuoles like those of the neighbouring parts of the stomach itself
(Pig. 294, G, gl).

The ectoderm of the process on which the anus is situated
subsequently becomes involuted to form a deep pit (Fig. 294, A),
and rudiments of the adult tentacles (B, a.ten) are formed as a ring

aten

FIG. 294. — Actinotroclia larva of
Phoronis, three stages in the meta-
morphosis, seen from the side. A,
stage in which the rudiment of the
involution has appeared. B, stage
in which the involution is partly
everted. C, stage in which the
metamorphosis is almost complete.
a. ten. rudiments of adult tentacles;
gl. diyerticulum of gastric region ;
int. intestine ; inv. ectodermul in-
volution ; /. ten. larval tentacles ;
pr. I. prae-oral lobe ; st. stomach ;
t. tr. circlet of cilia round anus.
(From MacBride, after Metschnikoff.)

of processes at the bases of the larval tentacles. The metamor-
phosis from this point is completed with great rapidity. The larva
sinks to the bottom ; the pit at the side of the anal elevation
becomes everted (B, inv.) and the alimentary canal of the larva is
drawn into it (0), the projection thus formed, which grows out at
right angles with the long axis of the larva, becoming the body of

vni PHYLUM MOLLUSCOIDA 353

the future animal ; the larval tentacles and pre-oral lobe are thrown
off, and the lophophore is developed.

The Phoronida have a wide geographical distribution, occurring
in all the chief regions. Some of the species live about low- water
mark : others at moderate depths up to about thirty fathoms.
Some of the associations take the form o£ encrustations of the
matted tubes. In other cases the tubes lie in excavations in stone
or in the shells of Molluscs. In one species the tubes occupy
channels in the substance of the tube of a Sea-anemone.

CLASS III.-BRACHIOPODA.

The Brachiopoda are the fabricators of the well-known " Lamp-
shells " found in most parts of the world. They occur in the sea
at various depths, and were formerly classed under the Mollusca,
their characteristic bivalved shell being compared with that of
oysters, mussels, &c.

1. EXAMPLE OF THE CLASS — Magellania (Waldheimia) lenticularis
or M. flavescens.

Magellania lenticularis is found in great numbers, at moderate
depths, off the coast of New Zealand. An allied species, M. flavescens,
is equally common in the Australian seas, and several other species
are known in various parts of the world.

The body is entirely covered by a shell (Fig. 295) of oval form
and pink colour, composed of two pieces or valves, one of which, dis-
tinguished as the ventral valve (v. v), projects beyond the other
or dorsal valve (d. v), in the form of a short conical beak (b) perfor-
ated at the end by an aperture, the foramen (b), through which
passes a dark brown stalk or peduncle (Fig. 296, B, pd) of horny
consistency. In the natural state the peduncle is attached to a
rock or other support, and the animal lies with the ventral valve
uppermost and with the valves gaping slightly. The pointed or
peduncular end of the shell is considered to be posterior in position,
the opposite end or gape anterior.

It will be convenient to consider the shell first. Both valves are
deeply concavo-convex, of a pinkish or brown colour outside, white
within. The ventral valve (Fig. 295), as already stated, is produced
posteriorly into a beak (b), terminating in a foramen (/) for the
peduncle. The distal margin of the foramen is left incomplete by
the shell proper, but is closed by a small double plate, the deltidium
(d). Immediately anterior to the beak is the curved hinge-line
along which the valve articulates with its fellow, and just anterior to
the hinge-line the inner surface of the shell is produced into a pair
of massive, irregular hinge-teeth (t). On the inner surface of the
valve, towards its posterior end, are certain shallow depressions
marking the attachments of muscles (ad.m, d. m).

VOL. i. A A

354

ZOOLOGY

SECT.

The dorsal valve (D) has no beak, but its posterior edge forms
a hinge-line which is produced in the middle into a strong cardinal
process (c. p) with a curiously folded surface : when the two valves
are in position this process fits between the hinge-teeth of the
ventral valve, the hinge-teeth in their turn being received into
depressions (s) placed on.qacji side of the cardinal process. The
inner surface of thdon&f valve is produced into a median ridge or

af.tn'

FIG. 295.— Magellania flavescens. A, the entire shell from the dorsal aspect, and B, from
the left side ; C, interior of ventral valve, and D, of dorsal valve, ad. in. adductor impres-
sions ; b. beak ; c. p. cardinal process ; d. deltidium ; d. m. divaricator impressions ; d. v.
dorsal valve ; /. foramen ; p. m. protractor impressions ; s. tooth-socket ; s. I. shelly loop ;
sp. septum ; t. hinge-tooth ; v. aj. m. adjuster impressions ; v. v. ventral valve. (After
Davidson.)

septum (sp), continuous posteriorly with the cardinal process, and
attached on either side of the base of the latter are the two ends
of a delicate calcareous ribbon, the shelly loop (s. I), which projects
freely into the cavity enclosed between the two valves, and has the
form of a simple loop bent upon itself. The inside of the dorsal
valve also has muscular impressions.

Externally both valves present a series of concentric markings
parallel with the edge or gape : these are lines of growth, the

Vrn PHYLUM MOLLtJSCOlDA 355

shell beingjbuilt up by new layers being deposited within those
previously formed, and projecting beyond them so as to form a
'series of outcrops.

Microscopically the shell consists of prismatic rods or spicules
of carbonate of lime, placed obliquely to the surface and separated
from one another by a thin layer of membrane. It is also traversed,
perpendicularly to the surface, by delicate tubules which begin on
the inner surface in microscopic apertures and extend to within a
short distance of the outer surface.

The actual body of the animal (Fig. 296, B) lies at the posterior
end of the shell, occupying not more than a third of the space
enclosed between the two valves : it is consequently more or less
wedge-shaped in form, and presents dorsal and ventral surfaces in
contact with the two valves, and an anterior surface looking
towards the gape. The dorsal is of greater extent than the
ventral surface, so that the anterior surface is placed obliquely.

The dorsal and ventral regions are continued each into a flat
reduplication of the body- wall, closely applied to the correspond-
ing valve and containing a prolongation of the coelome. The two
flaps thus formed are the dorsal (d. m) and ventral (v. m) mantle-
lobes. They are fringed with minute setae (s) lodged in muscular
sacs, like those of Chaetopods (vide Sect. X.), and give off from their
outer surfaces hollow processes which extend into the tubules of
the shell mentioned above.

The large wedge-shaped space or mantle-cavity, bounded by the
mantle-lobes above and below, and behind by the anterior surface
of the body, is occupied by a huge and complex lophophore (Figs.
296 and 297, Iph), which springs from the anterior surface of the
body, and, like that of the fresh- water Polyzoa and of Phoronis,
has the general form of a horse-shoe. It is, however, peculiarly
modified : the two limbs of the horse-shoe curve towards one
another so as to adapt themselves to the mantle-cavity ; and the
middle of the concave edge, which is dorsal in position, is pro-
duced into a spirally coiled offshoot (lphr) which lies between the
two arms and is coiled towards the dorsal side. The lophophore
is hollow, containing a spacious cavity or sinus : its two main arms
also receive prolongations of the coelome into which the digestive
glands project : it is fringed throughout its whole extent with
long ciliated tentacles which form the outer boundary of a ciliated
food-groove, bounded on the inner side by a wavy ridge or lip
(Ip, lp'). By the action of the cilia microscopic particles are swept
along the food-groove to the mouth.

Digestive Organs. — The mouth (mth) is a narrow crescentic
aperture situated in the middle of the lophophore, towards its
convex or ventral edge, and is bounded dorsally by the lip. It
leads into a V-shaped enteric canal which consists of a gullet
passing upwards from the mouth, an expanded stomach (st), and a

A A 2

356

ZOOLOGY

SECT.

straight intestine (int.) which extends from the stomach downwards
and backwards towards the ventral surface and ends blindly,

.£1

FIG 996 — A, body of Magellania lenticularis, removed from shell ; B, sagittal section of
the entire animal. Both semi-diagrammatic, the lophophore being represented as of smaller
proportional size than in the actual animal (cf. Fig. 297). d.gl. digestive gland : d. m. dorsal
mantle-lobe ; d. t>. dorsal valve of shell ; gon\ gon*. gonads ; M. heart ; int. intestine ;
Ip, Ipi. lip ; Iph. lophophore ; Iph*. its coiled process ; mth. month ; nph. in B, neplmdium,
in A, nephridial aperture ; pd. peduncle ; pi. si. pallial sinuses ; s. setae ; st. stomach ; u. m.
ventral lobe of mantle ; v. v. ventral valve of shell.

there being no anus. On each side of the stomach, and opening
into it by a duct, is a large, branched digestive gland (d. gl). The
whole canal is lined with ciliated epithelium.

VIII

PHYLUM MOLLUSCOIDA

357

•mth.

The body- wall consists externally of an epidermis formed of a
single layer of cells, then of a layer of connective tissue, of a
cartilaginous consistency in many parts, and finally of a ciliated
ccelomic epithelium lining the
body-cavity. On the outer
surfaces of the mantle-lobes,
where they are in contact
with the shell, the epidermis
is replaced by a thin mem-
brane showing no cell-
structure.

The muscular system
(Fig. 298) is well developed.
Two large adductor muscles
(ad. m) arise on each side
from the dorsal valve, and
passing downwards, unite
with one another so as to
have a single insertion on the
ventral valve : their action is
to approximate the valves

and so to close the shell. A large and a small pair of divaricators
(d.m, d.m') arise from the ventral valves, and are inserted
into the cardinal process, which they depress : as this process
is situated posteriorly to the hinge-line, its depression raises

. 297. — Magellania^flayescens, the ventra
valve removed, c. p. cardinal process ; Iph. arm
of lophophore ; Iph1. its coiled process, with
the tentacles removed on the right side ; mth.
mouth. (After Davidson.)

.sh,

FIG. 298. — Muscular system of Magellania. ad. m. adductors ; b. beak ; d. aj. m. dorsal
adjusters ; d. m., d. m'. divaricators ; d. v. dorsal valve ; int. intestine ; mth. mouth ; pd.
peduncle ; pd. sh. sheath of peduncle ; p. m. protractor ; s. 1. shelly loop ; v. aj. m. ventral
adjusters ; v. v. ventral valve. (After Hancock.)

the rest of the dorsal valve and so opens the shell. Two pairs of
muscles arising, one from the ventral, the other from the dorsal
valve, and inserted into the peduncle, are called adjusters (aj. m) :

358

ZOOLOGY

SECT.

the peduncle being fixed, they serve to alter or adjust the position
of the animal as a whole by turning it in various directions.

The ccelome is a spacious cavity more or less encroached upon
by the muscles and other organs, and traversed by sheets and
bands of membrane which connect the enteric canal with the
body-wall, and thus act as mesenteries. The coelome is continued
into each of the mantle-lobes in the form of four canals or pallial
sinuses (Fig. 296, pi. si), the two outer of which are extensively
branched.

Blood-system. — Attached to the posterior region of the
stomach is a small, almost globular sac (h), which has been proved
to be contractile and is to be considered as a heart. Vessels have

been traced from
it to various parts
of the body, but
the relations of the
whole circulatory
system and the
course of the
circulation are
very imperfectly
known.

The excretory
organs consist of
a pair of very
large nephridia
(nph) lying one on
each side of the
intestine. Each
is funnel-shaped,
having a wide
inner opening or
nephrostome, with
plaited walls,
opening into the
coelome, and a
narrow, curved,
outer portion which opens into the mantle-cavity not far from the
mouth. As in many cases which have already come under our
notice, the nephridia act also as gonoducts.

The nervous system (Fig. 299) is a ring round the gullet pre-
senting supra- (g) and infra- (usg) oesophageal swellings or ganglia,
of which the infra-cesophageal is the larger. Nerves are given off
to the mantle, lophophore, &c. No special sense-organs are known.
Reproductive Organs. — The sexes are separate. There are two
pairs of gonads (Fig. 296, gori), one dorsal and one ventral, in the
form of irregular organs sending off branches into the pallial sinuses.

FIG. 299. — Anterior body-wall of Terebratula, to show nervous
system, &c. dm. dorsal mesentery ; g. brain ; gf. genital
folds ; n. nephridium ; nt. nephrostome ; ces. gullet ; ov.
ovary ; sc. oasophageal connective ; usg. infra-cesophageal
ganglion : vm. ventral mesentery ; dmn, Tin, ian, san. nerves.
(From Lang's Comparative Anatomy, after van Bemmelen.)

vin PHYLUM MOLLUSCOIDA 359

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Brachiopoda are Molluscoida in which the body is enclosed
in a shell formed of two parts or valves which are respectively
dorsal and ventral in position. The body occupies only a small
portion of the space enclosed by the shell, and is usually attached
to foreign objects by a posteriorly placed stalk or peduncle : it
gives off dorsal and ventral reduplications, the mantle-lobes, which
line the valves of the shell and enclose a large mantle-cavity.
From the anterior surface of the body is given off a lophophore
which surrounds the mouth, and is beset with ciliated tentacles.
There is a ridge-like pre-oral lip which is continued on to the
lophophore. The enteric canal is usually V-shaped, and is
divisible into gullet, stomach, and intestine : there is a pair of
digestive glands. The ccelome is spacious, and is continued into
the mantle-lobes. A heart is usually present, attached to the
stomach. The excretory organs are one or two pairs of nephridia
which act also as gonoducts. The nervous system is a ganglionated
circum-ossophageal ring : sense-organs are usually absent in the
adult. The sexes are separate or united. Development is accom-
panied by a metamorphosis.

The class is divided into two orders : —

ORDER 1. — INARTICULATA.

Brachiopoda in which the shell is mainly composed of chitinoid
material with a varying proportion of calcined substance : the
valves are not united by a hinge, and there is no shelly loop for the
support of the lophophore. An anus is present.

Including Lingula, Crania, Discina, &c.

ORDER 2. — ARTICULATA.

Brachiopoda in which the shell is formed of oblique prisms or
spicules of calcium carbonate : the two valves unite by a definite
hinge, and there is usually a shelly loop, for the support of the
lophophore, developed in connection with the dorsal valve. The
intestine ends blindly.

Including Magellania, Terebratula, Rhynchonella, Cistella
(Argiope), &c.

Systematic position of the Example.

The genus Magellania, of which there are several species,
belongs to the family Terebratulidse, and to the order Articulata.

The dissimilar valves of the shell articulated by teeth and
sockets, and the absence of an anus, place it among the Articulata.
The Terebratulidse are distinguished by an oval or rounded shell,
the structure of which is punctate, the dots corresponding with

360

ZOOLOGY

SECT.

blind tubes receiving processes of the mantle ; the beak of the
ventral valve is prominent, and has a foramen partly bounded by a
deltidium of one or two pieces : there is a shelly loop springing
from the hinge-line of the dorsal valve. The genus Magellania is
characterised by having the shelly loop fully half as long as the
shell itself, and by the presence of a median septum on the inner
face of the dorsal valve.

The specific differences between M. lenticularis and M. flavescens
are largely matters of detail, depending upon the precise form of
the shell and loop. More obvious differences are seen in the shell,
jvhich is pink, evenly-rounded, and short-beaked in M. lenticularis,
while in M. flavescens it is horn-coloured, almost pentagonal, and
has a prominent beak.

3. GENERAL ORGANISATION.

The shell presents two distinct types : in the Articulata, the
order to which Magellania belongs, the dorsal and ventral valves

FIG. 300.— Typical Brachiopoda. A, Lingula ; B, Crania ; C, Discina ; D, Terebratula
E, Cistella ; F, Spirifera ; 0, Kraussina. (After Bronn.)

are dissimilar, the dorsal valve having a cardinal process and usually
a shelly loop, the ventral a spout-like beak for the peduncle ; while
in the Inarticulata, of which Lingula is a good example (Fig. 300, A),
the two valves are nearly alike, and there is no shelly loop and no
beak. These differences are accompanied by differences in micro-
scopic structure ; in the Articulata the shell is dense and stony,
and is formed of obliquely placed calcareous prisms, while in

VIII

PHYLUM MOLLUSCOIDA

361

the Inarticulata it has no prismatic structure, but usually consists
of a chitinoid material more or less strengthened by calcareous
spicules, or of alternate chitinoid and calcareous layers. A system
of tubules, such as that described above in the case of the example,
occurs in many cases. Among the Articulata the loop may be
absent ; when present, it varies greatly in form and size, being
sometimes very small and simple (Fig. 300, C, D), sometimes bent
upon itself, as in Magellania, sometimes attached to the septum or
to the interior of the dorsal valve (E), sometimes, as in the extinct
Spirifera, represented by a complex double spiral (F), sometimes
reduced to short, paired rods springing from the septum (G). The
shell is secreted
partly by the
general surface
of the body,
partly by the
folds of the
mantle.

In some cases
spicules occur
scattered
through the
soft parts.

The majority
of both orders
are attached
by a longer or
shorter ped-
uncle which
passes between
the proximal
ends of the
valves in Lin-
gula (Fig. 300,
A), through a

perforation in the ventral valve in Discina (C), and through a
foramen in the spout-like posterior end of the ventral valve in the
Articulata. Crania (B) has the ventral valve fixed directly to
foreign objects, the peduncle being absent.

The lophophore is found in its simplest form in Cistella
(Fig. 301, A), in which it is a horse-shoe-shaped disc with very
short arms, attached to the dorsal mantle-lobe, of which it is a
prolongation, and surrounded with flexible tentacles which project
between the valves. From this the lophophore of Magellania,
which may be considered as typical for the Articulata, is easily
derived by an increase in size, and by the prolongation of the middle
region of the concave edge into a coiled offshoot. In the Inarti-

FIG. 301. — Dissections of A, Cistella ; B, Rhynchonella ; and C,
Iiingula. a. anus ; Iph. lophophore ; mth. mouth. (After
Schulgin arid Hancock.)

362 ZOOLOGY

SECT.

culata (C), and in Rhynchonella (B) among the Articulata, each
arm of the horse-shoe is coiled into a conical spiral, which in some
cases can be protruded between the valves. In all cases a section
of the arm shows the same general character, with a groove, one
of the lips of which is a simple ridge, while the other is the row of
tentacles.

The most noteworthy point about the muscular system is the
fact that the shell is both opened and closed by muscular action.
The dorsal valve may be taken to represent a lever of which the
hinge-line is the fulcrum, the cardinal process the short arm, and
tke main portion of the valve the long arm. The mode of action
of. the three principal sets of muscles — adductors, divaricators and
adjusters — has been described in the account of the example (p. 357).
In Lingula there is a very complex muscular system by means of
which the valves can be rubbed upon one another, or moved laterally
as well as opened and shut.

In the Articulata the enteric canal is V-shaped, as in Magel-
lania, the intestine being straight or nearly so, and ending blindly.
In the Inarticulata, on the other hand, the intestine is usually
coiled, and always ends in an anus (Fig. 301, 0, a), which generally
opens into the mantle-cavity, but in one genus (Crania) into a
pouch or sinus at the posterior end of the body between the
valves.

A heart is usually present, but the function of blood is per-
formed mainly by the coelomic fluid, which is propelled by the
cilia lining that cavity, and circulates both in the ccelome itself and
in the pallial sinuses, each sinus presenting — in Lingula at least —
both an outgoing and an ingoing current.

A single pair of nephridia, resembling those of Magellania,
occurs in all known genera except Rhynchonella, in which there are
two pairs, one dorsal and one ventral. Besides discharging an
excretory function they act as gonoducts.

The nervous system always takes the form of a circum-oeso-
phageal ring with ganglionic enlargements, the largest of which
is ventral or sub-cesophageal in position. Otocysts have been
described in Lingula, rudimentary eyes in Megerlia, and patches
of sensory epithelium in Cistella : with these exceptions sensory
organs are unknown.

There are usually four gonads, two dorsal and two ventral,
sending prolongations into the pallial sinuses. Some genera are
dioecious, others hermaphrodite, the epithelium of the gonads
producing, in the latter case, both ova and sperms.

The embryology and larval history of the Brachiopoda
have been worked out only in four genera belonging to the Articulata
and one (Lingula) belonging to the Inarticulata. The following
are the chief general characteristics of the development in the
former. In three of the four genera referred to the earlier stages

VIII

PHYLUM MOLLUSCOTDA

363

0Sftag cisieua

(Argiope). b. provisional setae;
W. blastopore ; me. mesen-

teron ; pv. cceiomic pouches.

are passed through in brood-pouches ; in the fourth the eggs are

only attached temporarily to the setae of the parent and become

free in a comparatively immature condition. Segmentation is

regular and complete, and results in the

formation of a ciliated blastula, the wall

of which is composed of cells of like

character throughout. This is converted into

a gastrula by invagination (Fig. 302, A) :

the blastopore narrows to a slit and gradu-

ally completely closes. The stomodaeum

arises as an invagination of the surface

ectoderm apparently in the position of the

anterior end of the former blastopore. An

ectodermal thickening in front forms the

apical plate from which the supra

cesophageal ganglion is derived, and a

second thickening behind the mouth on the

ventral surface subsequently gives origin to

the infra-cesophageal ganglion. Meanwhile

the CCelome Originates in a pair of SaCS
/T,. \ • i

(Fig. 302, pv), or a single sac subsequently
dividing into two— the cceiomic pouches-
growing out from the archenteron, and
eventually becoming completely closed off from it to give rise to
the ccelome, which is thus of the enteroccele-type as regards its
derivation. A groove, the mantle-groove, running transversely
divides the embryo into two regions, a broad
anterior and a narrower posterior. Folds
which are formed in front of the mantle-groove
are the beginnings of the dorsal and ventral
mantle-lobes. A groove which appears in
front of these separates off a head segment from
the rest, and the embryo is now superficially
divided into three segments or regions — head
region, body- or mantle- region and foot- or
peduncle- region (Fig. 303). The mantle-lobes
t£e increase in size and grow backwards over the
Sf mand two peduncular region : in them are developed four
bundles of setae (From groups of provisional setCB which project back-

the Cambridge Natural c ,r /T7V * 0/AO\ • /-v , n • i • i .1

History, after Kowai- wards (Fig. 303) i in Cistella, in which there are
no setae in the adult, these are thrown off subse-
quently : in the genera in which setae are present in the adult they
are also thrown off, but are replaced later by the permanent setae.
The head-region has in the meantime become broadened out and
in Cistella soon assumes the form of an umbrella-shaped disc
bordered with cilia and usually bearing eye-spots (Fig. 304, A),
and sometimes an apical tuft of long cilia. In this condition the

364

ZOOLOGY

SECT.

larva swims freely like a trochophore. After a time it comes to
rest and fixes itself by the peduncular segment. The mantle-folds
become reflexed so as to point forwards instead of backwards, thus
leaving the peduncular region exposed and covering the head-region
(Fig. 304, B). The umbrella-like head-region decreases in size,
and perhaps forms the lip, which is at first confined to the part
immediately dorsal to the mouth. The lophophore appears at

first on the inner surface of the dorsal
mantle-lobe, but gradually extends and
surrounds the mouth ; in its earlier stages
it is circular, but afterwards assumes the
horse-shoe form by sending out paired
extensions. In genera with a complex
lophophore, like Magellania, this organ
has at first a simple horse-shoe form
(Fig. 305, Iph). A shell is secreted by
the mantle-lobes, and the peduncular
region becomes the peduncle of the
adult.

The development of Lingula (Inarti-
culata) differs somewhat widely from
that of the Articulata. There is no
division by definite constrictions into
three regions. The head-region does not
expand to the same extent : it early
develops elevations which are the
rudiments of the first-formed tentacles.
.~~md The ridge from which the mantle-lobes
are developed grows forwards from the

/ nts outset. The ccelome is not formed by

outgrowth from the archenteron, but
appears as a pair of cavities in a pair of
masses of mesoderm-cells which are at

FIG. 304. — Two later stages in the n v j T* >• r ,1

development of cisteiia. A, nrst solid proliferations from the wall

free-swimming; B, after fixation. O « o^r • n

hs. peduncular region ; m.mantle ; Ol tjie . latter •

ms. body-region ; md. mesenteron ; coelome is not formed as an enterocoele

ivk. ciliated ring ; vs. head -region. , -,. •,

(From Lane's Comparative Ana- DUt as a SCniZOCCSle.

Distribution.— The Brachiopoda are

all marine. They are widely distributed geographically, and live
at various depths — from between tide-marks to 2,900 fathoms.
At the present day the class includes only about 20 genera and
100 species, but in past times the case was very different.
Brachiopods appear first in the lower Cambrian rocks, where the
existing genera Lingula and Discina are found. No more striking
examples can be adduced of persistent types — organisms which
have existed almost unchanged for the vast period during which
the whole of the fossiliferous rocks have been in process of for-

VTTT

PHYLUM MOLLUSCOIDA

365

rrilh

mation. Altogether 106 genera are known from the Palaeozoic rocks,
34 from the Mesozoic, and 21 in the Cainozoic and Recent periods.
Obviously the group is tending, though slowly, towards extinction.

Researches on fossil and recent forms
have shown the Brachiopoda to illus-
trate, in a remarkable manner, the
recapitulation theory already referred
to : the theory, that is, that ontogeny or
individual development is a more or less
modified recapitulation of phylogeny or
ancestral development. It has been
shown that there is a striking and
almost complete parallelism between the
stages in the development of the shelly
loop in such highly organised forms as
Magellania, and the entire series of
articulated Brachiopods from those with
the simplest to those with the most
complex loop.

MUTUAL RELATIONSHIPS OF THE
CLASSES OF THE MOLLUSCOIDA.

In adult structure Phoronis exhibits
marked resemblances to the Ectoprocta,
more especially to the Phylactolaemata
—resemblances which will be rendered clear by a comparison of
the diagrams A and B in Fig. 306. In both, the ventral side of the
body is greatly produced and elongated, and, by the approximation
of the mouth and anus, the dorsal surface is reduced to a very short
space between those two apertures. The form of the lophophore,
the presence of an epistome having similar relationships in the two
groups, and the fact that the ccelome is similarly developed in
both, point in the same direction. Some points which are supposed
to indicate relationships with the Annulata (Sipunculoidea) and
with the Chordata are referred to at a later stage.

The resemblances between the Brachiopoda and the other two
classes of the phylum are somewhat disguised by the development
of the shell, but are very obvious — more particularly when we
take into account certain features of the development. One of the
most striking points of resemblance between the three classes
is the presence of the lophophore with its tentacles ; in the earlier
stages of its development in the Brachiopod, as we have seen, this
structure (Fig. 305) has the horse-shoe shape which it retains in
the adult Phoronida and Phylactolaemata, and a lobe — the arm-
fold or lip (Ip) — comparable to the epistome, is present overhanging
the mouth. The end of the body of the Brachiopod with which
the peduncle is connected must correspond to the aboral extremity

FIG. 305. — Lophophore of embryo
of Terebratulina. d. gl. di-
gestive gland ; int. intestine ;
Ip. lip ; Iph. lophophore • mth.
mouth. (From Korschelt and
Heider, after Morse.)

366

ZOOLOGY

sfedi1.

in the Polyzoa, since this represents the part by which the larval
Polyzoan becomes fixed, the everted " sucker " of the latter being
evidently homologous with the foot-segment of the larval Brachio-
pod. The end of the body of the Brachiopod from which the
peduncle proceeds is thus the ventral portion. From the position
of the epistome and lophophore, it follows that the dorsal valve
of the Brachiopod, being on the same side of the mouth as the

B

fcnl

-lent

Fia. 306. — A, Diagrammatic median section of a phylactoloematous Polyzoan. an. anus ;
ep. epistome ; ep. cav. epistome-cavity ; funic. funiculus ; gang, ganglion ; int. intestine ;
mo. mouth ; neph. nephridium ; ces. oesophagus ; st. stomach ; tent, tentacles. B, diagram-
matic median section of Phoronis. nies. mesentery ; nr. nerve-ring. Other letters as
in A. (From Kor^chelt and Hcider, after Cori.)

epistome, lies on the side of the body corresponding with the anal
side of the Polyzoan, though the intestine is bent round in the
opposite direction and directed towards the ventral valve. The
supra-oesophageal ganglion of the Brachiopod represents the single
ganglion of the Polyzoa, though it is subordinate in importance to
the infra-oesophageal ganglion — not represented in the latter group.
Other important points of resemblance between the Brachiopoda
and the Phoronida are seen in the character of the nephridia and

vni PHYLUM MOLLUSCOIDA 36?

the presence in both of larval forms which may very well be looked
upon as modified trochophores.

The setae of Brachiopods, sunk in muscular sacs, are marks of
annulate affinities, since such organs are found elsewhere only
among Chaetopoda and Gephyrea (Sect. X.). The form of the
larva tells in the same direction, the eye-bearing head region
or prostomium and the provisional setae being very striking charac-
ters. But the segmentation of the Brachiopod is quite different
from that of the annulate larva, in which new segments are always
added behind those previously formed, and in which metamerism
always affects the mesoderm.

SECTION IX

PHYLUM ECHINODERMATA

THE phylum Echinodermata comprises the Starfishes (Asteroidea),
Sea-urchins (Echinoidea), Brittle-stars (Ophiuroidea), Feather-stars
(Crinoidea), and Sea-cucumbers (Holothuroidea). All exhibit a
radial arrangement of parts, which is recognisable as well in the
globular Sea-urchins and elongated Sea-cucumbers as in the star-
shaped Starfishes, Brittle-stars and Feather-stars. Another uni-
versal feature is the presence of a calcareous exoskeleton, sometimes
in the form of definitely shaped plates, which may fit together by
their edges so as to form a continuous shell ; sometimes merely in
the form of scattered particles or spicules. In very many the
surface is beset with tubercles or spines, from which feature the
name of the phylum is derived. The various systems of organs
attain a comparatively high degree of complexity. An extensive
coelome is present, developed in the embryo from hollow outgrowths
from the archenteron. The Echinoderms are rarely capable of
rapid locomotion, and are sometimes permanently fixed by means
of a stalk ; they never give rise to colonies by budding. With
hardly a single exception, all the members of this phylum are
inhabitants of the sea.

1. EXAMPLE OF THE ASTEROIDEA.

A Starfish (Asterias rubens or Anthenea flavescens).

General External Features of Asterias rubens. — The

body of the Starfish is enclosed in a tough, hard integument,
containing numerous plates, or ossicles as they are termed, of
calcareous material. This exoskeleton is not completely rigid in
the fresh condition, but presents a certain limited degree of flexi-
bility. The body (Fig. 307) is star-shaped, consisting of a central
part, the central disc, and five symmetrically arranged processes,
the arms or rays, which, broad at the base, taper slightly towards
their outer extremities. There are two surfaces — one, the aboral
or abactinal, directed upwards in the natural position of the living

368

SECT. IX

PHYLUM ECHINODERMATA

369

animal ; the other, the oral or actinal, directed downwards. The
aboral surface is convex, the oral flat ; the colour of the former is
much darker than that of the latter.

In the centre of the oral surface (Fig. 307) is a five-rayed
aperture, the actinostome, and running out from this in a radiating
manner are five narrow grooves, the ambulacral grooves, each extend-
ing along the middle of the oral surface of one of the arms to its
extremity. Bordering each of the ambulacral grooves there are
either two or three rows of movable calcareous spines, the
ambulacral spines. At
the central ends of the
grooves the ambulacral
spines of contiguous
sides of adjacent grooves
form five groups, the
mouth papillce, one at
each angle of the mouth.
External' to the am-
bulacral spines are three
rows of stout spines
which are not movable ;
and a third series runs
along the border separ-
ating the oral from the
aboral surface.

On the convex. aboral
surface there are a
number of short stout

Spines arranged in irre- FIG. SO?.— Starfish (Asterias rubens). General view of

gular rows parallel with
the long axes of the
rays. These are supported on irregularly-shaped ossicles buried in
the integument. In the soft interspaces between the ossicles are
a number of minute pores, the dermal pores, scarcely visible with-
out the aid of a lens. Through each of these pores projects
a very small, soft, filiform process, one of the dermal branchice
or papulce (Fig. 310, Resp. COB), which is capable of being entirely
retracted.

Very nearly, though not quite, in the centre of the aboral sur-
face is an aperture, the anus (Fig. 315), wide enough to admit
of the passage of a moderately stout pin. On the same surface,
midway between the bases of two of the rays, is a flat, nearly
circular plate, the surface of which is marked by a number of
radiating, narrow, straight, or slightly wavy grooves ; this is the
madreporite (mad.). The presence of this structure interferes to some
extent with the radial symmetry of the Starfish, two of the anti-
meres (p. 42), viz. those between which the madreporite is placed,

VOL. I. B Ti

the oral or actinal surface, showing the tube-feet.
(From Leuckart and Nitsche's Diagrams.)

370 ZOOLOGY SECT-

being different from the rest. There thus arises a bilateral sym-
metry, there being one vertical plane, and only one — that passing
through the middle of the madreporite and through the middle of
the opposite arm — along which it is possible to divide the Starfish
into two equal — right and left — portions.1 The two rays between
w^ich the madreporite lies are termed the bivium, the three
remaining the trivium.

Attached to the spines of the oral surface, in the intervals between
them, and in the intervals between the spines of the dorsal surface,
are a number of very small, almost microscopic bodies, which are
termed the pedicellarice (Fig. 310, Fed). Each of these is supported
on a longer or shorter flexible stalk, and consists of three calcareous
pieces — a basilar piece at the extremity of the stalk, and two jaws,
which are movably articulated with the basilar piece, and are
capable of being moved by certain sets of muscular fibres so as to
open and close on one another like the jaws of a bird. In some of
the pedicellariaB the jaws, when closed, meet throughout their
entire length, while in the case of others, mostly arranged in circles
round the spines on the aboral surface, one jaw crosses the other
at the end like the mandibles of a Crossbill. Such pedicellarise,
with three calcareous pieces, are termed forcipulate.

In a well-preserved specimen there will be seen in each of the
ambulacral grooves two double rows of soft tubular bodies ending
in sucker-like extremities ; these are the tube-feet or podia (Fig. 307).
In a living specimen they are found to act as the locomotive organs
of the animal. They are capable of being greatly extended, and
when the Starfish is moving along, it will be observed to do so by
the tube-feet being extended outwards and forwards (i.e. in the
direction in which the animal is moving), their extremities becom-
ing fixed by the suckers, and then the whole tube-foot contracting
so as to draw the body forwards ; the hold of the sucker
then becomes relaxed, the tube-foot is stretched forwards again,
and so on. The action of all the tube-feet, extending and con-
tracting in this way, results in the steady progress of the Starfish
over the surface. With the aid of the tube-feet the Starfish is
also able to right itself if it is turned over on its back.

At the extremity of each of the ambulacral grooves is to be
distinguished a small bright red speck, the eye (Fig. 310, A, oc),
over which is a median process, the tentacle (t), similar to the tube-
feet, but smaller and without the terminal sucker. The tentacles
have been ascertained by experiment to be olfactory organs, the
Starfish being guided .to its food much more by this means than
by the sense of sight.

Transverse Section of an Arm. — If one of the arms be cut
across transversely (Fig. 308 and Fig. 310, B) and the cut surface

1 The slightly eccentric position of the anal aperture introduces a corre-
spondingly slight inequality between the right and left portions.

PHYLUM ECHINODERMATA

371

examined, the aboral part of the thick, hard wall of the arm will
present the appearance of an arch (with its convexity upwards),
and the oral part the form of an inverted V, the ends of the
limbs of which are connected with the oral ends of the aboral
arch by a very short, flat, horizontal portion. Enclosed by these
parts is a space, a part of the ccelome or body-cavity, and below,
between the two limbs of the V, is the ambulacral groove. The
aboral arch is supported by a number of irregular ossicles and is
perforated by the numerous small dermal pores, through which the
dermal branchiae project.
The V-shaped oral part
of the body- wall — i.e. the
wails of the ambulacral
groove — is supported by
two rows of elongated
ossicles, the ambidacral
ossicles (Fig. 310, Amb.os),
which meet together at
the apex or summit of
the groove like the
rafters supporting the
roof of a house, but
with a movable articu-'
lation allowing of separa-
tion or approximation of
the two rows so as to
open or close the groove.
At the end of the ray
the ambulacral ossicles
end in a median ter-
minal ossicle. At the
edges of the groove a row of ossicles support the ambulacra
spines and prominent tubercles. Between the ambulacral ossicles
of each row are a series of oval openings, the ambulacral pores, one
between each pair of contiguous ossicles, and so arranged that they
form two rows on each side, one row higher than the other, the
pores of the higher row alternating with those of the lower. In
the ventral groove lie the contracted tube-feet (T. F.) : each tube-
foot is found to correspond to one of the ambulacral pores, so
that the former, like the latter, are arranged in a double alter-
nating row on each side of the groove. When the tube-foot is
drawn upon, it is seen to be continuous with one of a series of
little bladder-like bodies, which lie on the other side of the ambu-
lacral ossicles, i.e. in the cavity of the arm. These — the ampullce
(Figs. 308 and 310, amp.) — are arranged like the tube-feet, in a
double row on each side, a higher row and a lower, there being one
opposite each ambulacral pore. When one of them is squeezed,

Fio. 308. —Starfish. Vertical section through an arm.
amp. ampullae ; ep. epidermis ; rad. amb. radial vessel
of the ambulacral system ; rad. bl. v. points to the
septum dividing the perihspmal vessel into two parts
rad. ne. radial nerve of the epidermal system ; sp.
spaces in mesoderm of body-wall ; t. f. tube-feet.
(From Leuckart, after Hamann.)

372 ZOOLOGY SECT.

the corresponding tube-foot is distended and protruded, the cavities
of the tube-foot and ampulla being in communication by means of
a narrow canal running through the ambulacral pore and provided
with a valve. It is in this way that the foot is protruded in the
livimg animal : the corresponding ampulla being contracted by the
contraction of the muscular fibres in its walls, the contained fluid
is injected into the tube-foot and causes its protrusion, the return
of the water backwards through the canal being prevented by the
closing of the valve.

Vascular and Nervous Systems. — Running along the
ambulacral groove, immediately below where the ambulacral
ossicles of opposite sides articulate, is a fine tube, the radial ambu-
lacral vessel (Fig. 308, rad. amb), which appears in the transverse
section as a small rounded aperture. From this short side-branches
pass out on either side to open into the bases of the tube-feet.
Below the radial ambulacral vessel is a median thickening of the
integument covering the ambulacral groove : this marks the
position of the radial nerve (Fig. 308, rad. ne) of the epidermal
nervous system, and is traceable as a narrow thickened band running
throughout the length of the groove, and terminating in the eye
at its extremity, while internally it becomes continuous with one
of the angles of a pentagonal thickening of a similar character, the
nerve-pentagon, which surrounds the mouth. In thin sections
(Fig. 309) the ventral median thickening, or radial nerve (rad.
nerv.), as well as the nerve-pentagon, are seen to be thickenings of
the epidermis, consisting of numerous vertically-placed, fibre-like
cells, with their nuclei at their outer (lower) ends, intermixed with
longitudinal nerve-fibres and with nerve-cells. Above this, on each
side of the epidermal nerve-thickening constituting the radial
nerve, is a band of cells (d. nerv.) also of a nervous character. These
more deeply placed nerve-bands are the radial parts of the deep
nervous system : like the epidermal, the deep nervous system has a
central part in the form of a pentagon, which in this case is double,
surrounding the mouth. A third set of nerve elements (the aboral
or coelomic nervous system) extends along the roof of the arm super-
ficial to the muscles.

The two radial nerve-bands of the deep nervous system are
thickenings of the lining membrane of a space overlying the
radial nerve and underlying the radial ambulacral system. This
space (rad. bl. v.)} extending, like the other parts that have been
mentioned, throughout the length of the arm, forms part of a
system of channels, the perihcemal system, which have been regarded
as constituting a blood-vascular system. This radial perihcsmal
vessel or sinus, as it is termed, is divided longitudinally by a vertical
septum (sept.) into two lateral halves. Internally it com-
municates with an oral ring-vessel surrounding the mouth
and likewise divided into two by a septum. The inner division

IX

PHYLUM ECHINODERMATA

373

of the ring-vessel is connected with the axial sinus referred
to on p. 378.

In the septum dividing the radial perihaamal sinus into two runs
a strand of a kind of gelatinous connective tissue containing many
leucocytes and perforated by irregular channels or lacuna : this
is the radial strand of the lacunar or haemal system. Like the
radial vessels of the perihaBmal system, the radial strands of the
lacunar system are connected internally with an oral ring.

Structure of the Disc.
—When the aboral wall of
the central disc is dissected
away, the remainder of the
organs come into view (see
Fig. 313). The rows of am-
bulacral ossicles appear in
this view as ridges, the am-
bulacral ridges, one running
along the middle of the oral
surface of each arm to its
extremity, and extending in-
wards to the corresponding
angle of the mouth. At the
sides of each of these ridges
appear the rows of ampullse.
Within the pentagonal actino-
stome is a space, the peri-
stome, covered with a soft integument, and in the centre of
this is a circular opening, the true mouth, the size of which is
capable of being greatly increased or diminished.

Body-wall and Coelome. — The entire outer surface is covered
with a layer of ciliated epithelium, the epidermis or deric epi-
thelium (Fig. 310, Der Epithm), which is continued over the
various appendages and processes — the tubercles and spines, the
pedicellarise, the dermal branchiae, and the tube-feet. Beneath
it is a network of nerve-fibrils with occasional nerve-cells. The
mesoderm (Derm) of the wall of the body beneath this consists of
two layers, between which are a number of spaces : the ossicles (os)
are all, except the ambulacral ossicles and the inter-radial par-
titions, developed in the outer of these two layers. Each ossicle
consists of a close network of calcareous rods. Between contiguous
ossicles extend bands of muscular fibres.

The interior of the eoelome (Ccel.) or body-cavity is lined by a
ciliated epithelium, the ccelomic epithelium (Gael. Epithm.), which
not only covers the inner surface of the body-wall as the parietal
layer, but also forms an investment for the contained organs —
the various parts of the alimentary eanal and its appendages,
the gonads, the madreporic canal, ampullae, etc. In addition

FlQ. 309. — Starfish. Lower part of a vertical
section through the arm, to show the structure
of the radial nerve and the position of the
deep nervous system and radial perihsemal
vessels, d. nerv. strand of deep nervous system ;
rad. bl. v. radial perihgemal vessel ; rad. nerv.
radial nerve ; sept, septum of radial peri-
hsemal vessel ; sept', radial lacunar strand of
the haemal system <here represented as solid).
(After Cuenot.)

374

ZOOLOGY

SECT.

to this visceral layer of the peritoneum, the wall of the ali-
mentary canal and its cseca consists of a muscular layer and an
internal lining, the enteric epithelium or endoderm (Ent. Epthm).
The coelome is filled with a fluid, the ccelomic fluid, consisting
mainly of sea-water, but containing a number of amoeboid cor-
puscles (amosbocytes). These collect particles of waste-matters,
and when loaded with them pass to the exterior by traversing
the walls of the papulse. The latter consist of a muscular
layer, an external epidermal layer, and an internal peritoneal layer,

fee/

-EpiAm

FlQ. 310.— Diagrammatic sections of a Starfish. A, vertical section passing on the right
through a radius, on the left through an inter-radius. The olf-side of the ambulacra! groove
with the tube-feet (T. F.) and ampullae (Amp.) are shown in perspective. B, transverse
section through an arm. The ectoderm is coarsely dotted, the nervous system finely dotted,
the ectoderm radially striated, the mesoderm evenly shaded, the ossicles of the skeleton
black, and the ccelomic epithelium represented by a beaded line. Amb. os. ambulacral
ossicles ; Amp. ampullae ; An. anus ; C. Amb. V. circular ambulacral vessel ; C. B. V.
septum of ring perihsemal vessel ; Cd. cm. cardiac cseca ; Gael, crelome ; Ccel. Epithm.
coelomic epithelium ; Der. Epithm. deric epithelium ; Derm, mesoderm ; Ent. Epthm.
enteric epithelium ; Int. cce. intestinal cseca. Mdpr. madreporite ; Mes. mesentery ; Alth.
mouth ; Nv. R. nerve-ring ; oc. eye ; os. ossicles of body-wall ; Ovd. oviduct ; Fed.
pedicellarise ; p.h. perihsemal spaces ; Pyl. ccec. pyloric caeca ; Had. amb. v. radial ambulacral
vessel ; Rad. B. V. points to septum in the radial perihaemal vessel ; Rad. Nv. radial nerve ;
Resp. cce. dermal branchiae ; St. stomach ; St. c. stone-canal ; t. tentacle ; T. F. tube-feet.
(From Parker's Biology.)

the internal cavities being in free communication with the
ccelome.

Digestive System. — Thp. mouth is found to open through a
short passage, the oesophagus, into a wide sac, the cardiac division
of the stomach (Fig. 310, St, Figs. 313, 315, card. st). This is a
five-lobed sac, each of the lobes of which is opposite one of the five
arms. The walls of the sac are greatly folded, and the whole is
capable of being everted through the opening of the mouth, by
contraction of the muscular fibres of the body-wall, wrapped over

IX

PHYLUM ECHINODERMATA

375

some object desired as food, and then retracted into the interior,
the retraction being effected by means of special retractor muscles
(Fig. 313, retr) which arise from the sides of the ambulacral ridges.
This cardiac division of the stomach communicates aborally with
a much smaller chamber, the pyloric division of the stomach, and
this in turn opens into a
very short conical intest-
ine, which leads directly
upwards to open at the
anal aperture. The pyloric
division of the stomach is
pentagonal, each angle
being drawn out to form
a pair of large appen-
dages, the pyloric cceca
(Figs. 310, 3ll, 313, 315,
pyl. ccec). Each pair
o f pyloric caeca com-
mences as a cylindrical
canal or duct, the lumen
of which is continuous
with the cavity of the
pyloric chamber. This
soon bifurcates to form
two hollow stems, ex-
tending to near the ex-
tremity of the cavity of
the arm, and giving off laterally two series of short branches, each
having connected with it a number of small bladder-like pouches.
The walls of the pyloric caeca are glandular : they secrete a digestive
fluid, and are therefore to be looked upon as digestive glands. It is

found by experimenting with this
digestive fluid that it has an action
on food-matters similar to that
exerted by the secretion of the
pancreas in the Vertebrata, con-
verting starch into sugar, proteids
into peptones, and bringing about
the emulsification of fats. While
the pouches of the cardiac division
of the stomach are attached to

FIG. 312.— Transverse section through the , .,

madreporif canal of a Starfish (Astro- the Oral Wall OI the DOQV, the
pecten). (FiomlGegenbaur.afterTeuscher.) pyjorjc cseca are connected with

the aboral wall. From the short intestine are given off inter-
radially two hollow appendages, the intestinal cceca (Figs. 311 and
313, int. ccec), each with several short branches of irregular shape,
Ambulacral System. — Running downwards from the madre-

jbyl. coec,

FIQ. 311. — Asterias rubens. Digestive system, an.
anus ; card. st. cardiac division of the stomach ; int.
ccec. intestinal caeca ; madr. madreporite ; pyl. ccec.
pyloric coeca ; pyl. st. pyloric division of the stomach.
(From Leuekart.)

376

ZOOLOGY

SECT

porite to near the border of the mouth is an S-shaped cylinder,
Ufie madreporic or stone-canal (Fig. 315, mad. can). The walls of
this canal are supported by a series of calcareous rings, and project-
ing into it is a ridge which bifurcates to form two spirally rolled
lamellae occupying a considerable part of the lumen of the canal.
In some Starfishes, such as Astropecten (Fig. 312), the internal

FIG. 313. — Anthenea flavescens. Upper view of a dissection of the internal organs. The
aboral wall of the body, with the exception of a small portion round the anus and the madre-
porite, has been completely removed. One of the five intestinal cseca has been removed with
the exception of its proximal part. All the ovaries have been removed except one pair, and
four of the pairs of pyloric caeca have been cut away close to their bases. 1 — 5, the five rays
with their ambulacral ridges ; amp. ampullae ; an. anus ; int. ccec. intestinal cseca ; i. p. cut
ends of the inter-radial partitions ; mad. madreporite with the madreporic canal ; ov. ovaries ;
pol. yes. Polian vesicles ; pyl. ccec. pyloric caeca ; retr. retractor muscles inserted into the
cardiac division of the stomach.

structure is more complicated owing to the branching of the lamellae.
The interior of the madreporic canal communicates abov } with the
exterior through the grooves of the madreporite. At the bottom of
each of the grooves are a number of fine pores leading into vertical
canals of corresponding fineness, all lined, like the grooves and the
madreporic canal itself, with long cilia : most of these ope a below
into a dilatation of the madreporic canal, some into the ax;al sinus
referred to below. Below, the madreporic canal opens into a

IX

PHYLUM ECHINODERMATA

377

wide, five-sided, ring-like canal, the ring-vessel of the ambulacral
system. From this are given off the five radial ambulacral vessels,
passing to the extremities of the arms. From the pentagonal canal
are given off also in most Starfishes, but not in Asterias, a series
of five pairs of appendages, the Polian vesicles (Fig. 313, pol. ves) —
pear-shaped, thin-walled bladders with long narrow necks — which
are placed inter-radially. At the sides of the neck of each Polian

FIG. 314. — A, view of the under part of a specimen of Asterias rubens, which has been
horizontally divided into two nearly equal portions. B, enlarged view of the axial sinus,
stone-canal and genital stolon cut across, amb. oss. ambulacral ossicle ; amp. ampullae of
the tube-feet ; ax. s. axial sinus ; gon. gonad ; g. stol. genital stolon or axial organ ; marg.
marginal ossicle ; nerv. circ. nerve-ring ; oe. cut end of oasophagus ; pst. peristome ; ret.
retractor muscle of the stomach ; sept, inter -radial septum ; stone, c. stone-canal ; T.
Tiedemann's vesicle ; w. v. r. \vater-vascular ring-canal. (After MacBride.)

vesicle (except in the inter-radius containing the madreporic canal,
where there is one on one side only) project inwards a pair of
little rounded glandular bodies, the racemose or Tiedemann's vesicles
(Fig. 314, T), the cavity in the interior of each of which, opening
into the ring-vessel, is divided into a number of chambers.

The various parts of the ambulacral system of vessels have a
muscular wall and an internal lining epithelium in addition to the

378

ZOOLOGY

SECT.

coverings which they may derive, according to their situation,
either from the external epidermis or the internal coelomic epi-
thelium. The muscular layer is most strongly developed on the
tube-feet, where it consists of two strata, and is also well developed
on the ampullae and Polian vesicles.

The stone-canal is enfolded in the wall of a wider canal, the
axial sinus (Fig. 314, ax. s), which forms a part of the perihaemal
system already referred to. The axial sinus runs nearly vertically.
At its oral end it opens into the internal division of the oral ring
sinus ; aborally it approaches close to, if it does not actually open
into, an aboral ring sinus : it also communicates aborally with the
stone-canal, and opens on the exterior through certain of the pores
in the madreporite.

card.st ^f^^,^ ' " i

ip 7?i<zj[ . can

EiG."31 5. — Anthenea flavescens. Latera view of a dissection in which one of the rays and
a portion of a second have been removed, and in which the alimentary canal has been laid
open. amp. ampulla) ; an. anus ; card. st. cardiac pouch of the stomach ; int. coec. intestinal
caecum ; ip. inter-radial partition ; mad. madreporite ; mad. can. madreporic canal ; ov.
ovary ; pyl. ccec. pyloric caeca ; r. cut ends of the ring-vessel of the ambulacral system ;
ring v. position of the ring-vessel ; retr. retractor muscle of cardiac pouch of stomach ;
s. cavity of the stomach.

Accompanying the madreporic canal and also enfolded in the wall
of the axial sinus there is an organ — the axial organ (Fig. 314,
g. stol) — the relationships and function of which have given rise
to a considerable amount of difference of opinion. It is a fusiform
body, the interior of which assumes an appearance of com-
plexity largely due to both its inner surface (i.e., that turned
towards the axial sinus) and its outer (that facing the ccelome)
being folded in a complicated manner. The axial organ contains
strands of lacunar tissue, i.e. of the same tissue that composes the
so-called haemal system, and is intimately related with the
latter. Its essential morphological character, however, appears to
be that of a genital stolon. At its aboral end it is continuous with
a genital rachis, which, in the form of a ring, runs in the aboral
perihaemal sinus, and gives off branches to the gonads. There is

IX

PHYLUM ECHINODERMATA

379

evidence that the sexual cells originate in the aboral end of the

axial organ, and travel through the genital rachis and its branches

to the gonads, which are to

be looked upon as the

greatly expanded extremi-
ties of the latter. Strands

of the lacunar tissue accom-
pany the genital rachis and

its branches to the gonads.
Reproductive System.

—The Starfish is unisexual,

each individual possessing

either ovaries (Figs. 313,

314, and 315, ov) or testes,

which appear very similar

until they are examined

microscopically. They con-
sist of masses of rounded

follicles, like bunches of

minute grapes — a pair in

each inter-radial interval.

Ova and sperms are alike

developed from cells of

the same character as those

which become the amoebocytes of the ccelomic and other -cavities

of the body. The ducts, by
means of which the ova or
sperms reach the exterior,
open on the aboral surface
through a number of per-
forations on a pair of sieve-
like plates, situated inter-
radially close to the bases of
the arms.

Anthenea flavescens (Figs.
313, 315, 316, 317), a
common Australian Starfish,
which may be taken as an
example instead of Asterias
rubens, differs from the latter
in the following main points.

The animal consists of a
relatively/ large central disc
and five' relatively short arms,

J.VJ. t»J- I . AAAAlU^AA^Ct; ViCW UA Ul t*l OUHUUC, I'll*" * IT T

showing ambulacra^ grooves ^ with ^small which taper rapidly towards

their extremities. On the

Fiu. 3 Hi. —Anthenea, viewjof aboral surface,
showing granular ossicles, madreporite and
anus. (After Sladen.)

FIG. 317. — Anthenea, view of oral surface,

380 ZOOLOGY SECT

oral surface the comparatively broad, flat surfaces between the
ambulacral grooves are roughish, owing to the plate-like ossicles
being beset with a number of minute rounded tubercles or granules,
which, in the immediate neighbourhood of the ambulacral
grooves, assume the character of short, blunt spines. Here and
there among the tubercles, usually one in the middle of each
ossicle, are pedicettarice, which differ widely from those of Asterias.
Each pedicellaria in Anthenea is a small, narrow, oblong, calcareous
body, consisting of two parallel narrow valves or jaws : these,
instead of being supported on a flexible stalk, are articulated with
the edges of a slit-like depression on the surface of the flat ossicle,
and are thus on a level with the general surface. The term valvate
is applied to pedicellarise of this description. In a living Anthenea
many of the pedicellarise will be found to have their valves widely
open ; when they are touched the valves close together, gradually
opening again after a little time. The ambulacral spines bounding
the ambulacral grooves are flattened and blunt, and arranged
in fan-like fasciculi. Round the border separating the aboral and
oral surfaces the plates are arranged in two somewhat irregular
rows.

The aboral surface is strongly convex, but not uniformly so, there
being a more or less distinct depression in the form of a shallow
open groove, the inter-radial depression, opposite each of the intervals
between the arms. The surface is dotted over with numerous small
rounded tubercles, arranged in somewhat irregular radiating lines.
These aboral tubercles, though fewer than those on the oral surface,
are for the most part more prominent, so that they assume the
character of short spines. The ossicles on which they are borne
are star-shaped with six rays, a spine being borne in the centre of
each ossicle, and one on each of the rays. Between the ossicles the
surface is covered with a soft, slimy skin, perforated by a large
number of minute dermal pores, each of which is enclosed by a
minute irregular ring of calcareous matter ; each pore serves for
the lodgment of one of the dermal branchiae. Numerous pedi-
cellarise, similar to those on the ventral surface, but smaller, are
borne on the ossicles, usually taking the place normally occupied by
the central spine. The tube-feet are arranged in a single row on each
side of each ambulacral groove ; but the ampullce are in two rows,
an upper and a lower, and each tube-foot has two ampullse connected
with it, one of the upper row and one of the lower row.

Anthenea has vertical calcareous inter-radial partitions not
developed in Asterias. There are five bifid intestinal cceca, which
are narrow tubes, slightly enlarged and lobed at the extremities.

Development of a Starfish (Asterina gibbosa or A.
exigua.1 — In these Starfishes the reproductive apertures are

1 The development of these has been described in preference to that of the
examples, as it is more completely known.

IX

PHYLUM ECHINODERMATA

381

placed on the ventral surface. When the ova have been dis-
charged and impregnated, they adhere by means of a viscid
investment to the surface (rock or stone) on which they are laid,
and go through all the stages of their development in this position,
never passing through a free pelagic stage. The eggs are about
half a millimetre in diameter, and of a spherical shape. Each con-
sists of a perfectly opaque central mass of yellow or orange yolk,
and of a glassy layer enclosing this. After fertilisation the process
of segmentation begins by the division of the ovum into two blasto-
meres almost equal in size, but one, which may be termed cell
I., slightly smaller than the other (cell II.). Both I. and II. soon
afterwards divide, I. somewhat earlier than II. The resulting

FIG. 318. — Early stages in the development of a Starfish (Asterina gibbosa). A, eight-celled
stage ; B, stage of about thirty-two cells seen in section ; C, gastrula stage ; D, section of
early gastrula ; E, section of later gastrula. arch, archenteron ; blastoc. blastoccele ; blp.
blastopore ; ect. ectoderm ; end. endoderm. (Modified after Ludwig.)

four cells again divide, leading to the formation of an eight-celled
stage (Fig. 318, A), in which the four cells derived from I. form
an incomplete ring not closed below, and the four derived from
II. form an incomplete ring open above.

The eight cells then divide by meridional fissures into
sixteen, and a further division results in the formation of thirty-
two. The thirty-two cells become arranged in such a way as to
enclose a central cavity which had been present in the four-celled
stage : this stage (B) is the blastula ; the cavity is the segmenta-
tion-cavity or blastoccele. The number of cells in the wall of this
cavity increases by further divisions, and the whole surface becomes
covered with vibratile cilia. A process of imagination then
follows, one side of the blastula being pushed inwards to form
a double-walled cup or gastrula (C) opening on the exterior by

382

ZOOLOGY

SECT

an opening, the blastopore, which, at first very wide, gradually
becomes narrowed. At the same time the shape of the larva
alters, so as to be somewhaj elongated, the blastopore, lying at first
midway between the two poles, afterwards gradually drawing
nearer to what becomes the posterior end.

Of the two layers of the gastrula (D and E), the outer is the
ectoderm, the inner the endoderm ; between them is a space, at first
filled with gelatinous matter, in which cells soon appear, giving
rise subsequently to an intermediate mass of tissue, the
mesenchyme.

The cavity in the gastrula is early distinguishable into two
parts (Fig. 319, B) — that part into which the blastopore leads
(arch), and a wider terminal part (ent) ; the former becomes the
stomach and^intestine of the larva, the blastopore giving rise to the

larv.org

B

c/r/' ----- -

FIG. 319. — Later stages in the development pf the larva of Asterina gibbosa. A, newly
hatched larva, ventral surface, with the beginning of the larval organ at the anterior end and
with the larval mouth. B, dorsal half of an embryo of the same age as A. C, somewhat
older larva with larger larval organ, the ectoderm of the left side removed to expose the
alimentary canal and the walls of the body-cavity, arch, archenteron ; bl. p. blastopore ;
ect. ectoderm ; ent. enteroccele ; larv. mo. larval mouth ; larv. org. pre-oral lobe ; stom.
stomodseum. (From Ziegler's models.)

larval anus ; the latter is termed the enterocode (ccelome). The wall
of the enterocoele becomes thinner, and it gives off two lateral
swellings, the right and left enteroccelic pouches (C, ent), which
are closely applied to the sides of the larval alimentary canal : the
left pouch is soon seen to be larger than the right. The entero-
ccele is subsequently completely closed off from the enteric
canal. It now consists of three parts, an anterior undivided part,
and the two pouches, right and left. Of the latter the left grows
more rapidly than the right : both extend posteriorly in the space
between the enteric canal and the body- wall to coalesce posteriorly
in such a way as to give rise to the ccelome of the adult. The
anterior undivided part (anterior coelome) forms the ccelome of a
conspicuous larval structure, the pre-oral lobe, and it eventually
becomes cut off from the right and left pouches, giving off on

ht PHYLUM ECHINODERMATA 383

the left a five-lobed outgrowth, the liydroccele, which forms the
foundation of the entire ambulacral system of the adult : a right
hydrocoele is only represented by a small vesicle which in normal
embryos undergoes no further development. Before the hydro-
coele is developed and before the right and left ccslomic
pouches have become cut off, two apertures make their appearance
on the surface of the larva : one, on the ventral side, is the open-
ing of the stomodceum or larval mouth ; the other, on the dorsal
side, is the dorsal pore. The mouth subsequently opens into the
larval stomach, and for a time the enteric canal of the larva opens
on the exterior both by mouth and anus : soon, however, the
larval anus becomes closed up. The dorsal pore is developed as an
outgrowth of the anterior part of the enterocoele, a little to the left
of the middle line, meeting a thickening of the ectoderm about the
middle of the dorsal surface, where an aperture is formed.

larv.org

FIG. 320. — Larva of Asterina gibbasn. A, diagrammatic lateral view ; the alimentary
canal dotted, the ambulacral system striated, the ectoderm shaded. B, Larva seen from
the left as an opaque object, the body-wall of the left side removed ; hydroccele separated
<>if from left enteric sac and partly surrounding oasophagus. all. alimentary canal ; amb.
ambulacral system or hydrocoele ; dors. p. dorsal pore ; ent. enteric sacs and ccelome ;
larv. mo. larval mouth ; larv. org. pre-oral lobe ; ces. oasophagus of adult ; r, r. lobes of
hydroccele ; sept, septum between the enterocoelic sacs. ( A, after Ludwig ; B, from
Ziegler's models.)

The pre-oral lobe appears at an early stage as a dilatation at the
anterior end of the larva. This takes a dorso-ventral direction,
and assumes the character of an elongated, almost cylindrical,
hollow appendage at the anterior end of the larva, consisting of a
shorter ventral, and a longer dorsal, part. On the anterior surface
of the pre-oral lobe a flattened area appears surrounded by a raised
rim, which is beset with specially large cilia : this is the larval
organ. In the middle of the larval organ appears an elevation,
the rudiment of a sucker by means of which the larva becomes
attached when the metamorphosis is about to begin. At this
stage the larva (Fig. 321) is able to creep by contractions of the
pre-oral lobe, and also by the action of the cilia, more especially the
cilia of the larval organ.

The hydroccele, at first a five-lobed outgrowth of the entero-

384

ZOOLOGY

SECT.

tar is. or g

^>

i

Fio. 321. — Larva of Asterina, view
of the left side, showing the five-
lobed prominence (amb.) formed by
the developing ambulacral system
on what is destined to become the
ventral surface of the body of the
Starfish ; larv. org., pre-oral lote
with larval organ.

tent

coele, grows into the form of a horse-shoe with five lobes, each of
which represents one of thejradial parts of the ambulacral system,
the horse-shoe itself representing the ring-vessel. The rudiment
of the madreporic canal arises as a groove on the posterior wall of

the anterior coelome. This develops
into a canal leading from the hydro -
ccele to the anterior coelome, and
eventually entering into connection
with the dorsal pore, forms a tube,
the madreporic canal, leading from the
ring canal to the madreporite, of which
the dorsal pore represents the first-
formed aperture.

As the hydrocoele develops, its form
influences the external shape of the
larva ; on the left-hand side there
grows out a five-lobed elevation (Fig.
321, amb), each of the lobes corre-
sponding to one of the five lobes of the
hydrocoele. Each of the; latter then becomes divided, first into
three rounded processes (Fig. 320, B, amb), and then into five,
and these project freely
on the surface ; the
middle one is the rudi-
ment of the tentacle,
the lateral processes are
the first two pairs of
tube-feet. At the same
time five elevations of
the opposite wall be-
come evident, and give
rise to the beginnings
of the dorsal regions of
the arms (Fig. 323).

The transition from
the larval stage to the
condition of the five-
rayed Starfish (Fig. 322)
is effected by the abor-
tion of the pre-oral
lobe — (which, on the
larva becoming fixed
by means of the sucker,
degenerates into a temporary stalk and eventually becomes
completely absorbed)— by the further development of the arms and
tube-feet, and by cert.ain changes which take place in the internal
organs. Of these, one of the most important is the formation of

FIG 322.— Asterina exigua. Young Starfish shortly
after the metamorphosis has been completed, viewed
from the oral side. circ. amb. circular ambulacral
vessel ; dors. p. dorsal pore and madreporic canal ; rad.
amb, radial ambulacral vessel ; st. stomach ; tent .
tentacle ; t. f. tube-feet.

IX

PHYLUM ECHINODERMATA

283

a new mouth and oesophagus (Fig. 320, B, oss), the larval mouth and
oesophagus becoming abolished during the metamorphosis. The
new mouth is formed in the centre of the hydroccele (ring- vessel).
From the stomach, diverticula grow out radially into the developing

FIG. 323. — Views of the larva of Asterina gibbosa in the course of metamorphosis. A, larva
of eight days, from the right ; B, left, and C, right view of the larva of nine days ; 1-5,
lobes of hydroccele ; I-V, rudiments of arms. (From MacBride, after Ludwig.)

arms to give rise to the cseca ; and later the permanent anal
opening is formed on the dorsal surface.

When the first ossicles are definitely formed they present the
following arrangement (Fig. 324). In the middle of the abactinal
surface is a single
central plate (dors).
Around this are
five basals (bas)
one of which be-
comes merged into
the madreporite.
External to these,
five radials (rad)
appear somewhat
later. At the end
of each develop-
ing arm is a single
terminal or ocular
plate (term), which
is carried outwards
as the ambulacral
and adambulacral W— £^r/w<

Ossicles of the arm FIQ- 324.— Diagram showing the_relations of the chief plates of the

are developed,

supporting the

corresponding eye and tentacle. A ring of secondary radials or

infra-basals (sec. rad) is developed between the radials and the

VOL. I. CO

apical system in the young Starfish, an. anus ; has. basals ;
dors, central ; madr. madreporite ; rad. radials ; sec. rad.
secondary radials (infra-basals) ; term, terminal.

386 ZOOLOGY SECT.

central. In the adult, by Jhe intercalary development of numerous
additional ossicles, these primary plates of the apical system, as
it is termed, lose their original arrangement, and become no longer
recognisable.

2. EXAMPLE OF THE ECHINOIDEA.
A Sea-urchin. — (Strongylocentrotus or Echinus.)

General External Features. -- The Sea-urchin (Figs. 326
and 327) is globular in shape, but somewhat compressed in one

FIG. 325. — Echinus esculentus, peristome. 1, tube-feet of the lower ends of the radii ;
2, branchia ; 3, teeth ; 4, oral tube-foot (tentacle) ; 5, peristomial membrane (From
MacBride, after Kukenthal.)

direction, so that two poles are distinctly recognisable. At one of
these the degree of flattening is greater than at the other ; this is the
oral pole, the opposite pole being termed the anal or aboral. At
the oral pole is a rounded aperture, the mouth, through which may
be seen projecting five hard white points, the extremities of the
teeth, surrounding the mouth is a thin, soft membrane known as
the peristome or peristomial membrane (Fig. 325). At the anal pole
is a much smaller aperture, the anus, the space immediately
surrounding which is termed the periproct (Fig. 327).

The entire surface, with the exception of the peristome and
periproct, is bristling with spines — cylindrical, pointed, solid appen-

PHYLUM ECHINODERMATA

387

dages, the surface of which is longitudinally fluted. These are
movably articulated with the body so that they may be turned
about in all directions. When one of them is removed (see Fig.
343, p. 413), it is found that the joint is of the character of a ball
and socket, a concavity on the base of the spine fitting over a
hemispherical elevation on the surface of the Sea-urchin, and the
spine being retained in place and caused to move by means of a
capsule of muscular fibres enclosing the joint. Around the bases
of the large spines are a number of very small spinules. Here and

:'.2t5. — Strongylocentrotus, entire animal with the tube-feet extended. (From
Brehm's Tierleben.)

t-li< re among the spines are to be observed minute pedicellarice
Fig. 344, p. 413), which are comparable to the stalked
pedicellariae of Asterias ; but each has three jaws instead
of two, and a relatively long stalk, which is supported by a
slender calcareous rod. Here and there are to be found also small
rounded bodies termed the sphceridia, which are perhaps, like the
pedicellariae, to be looked upon as modified spines : they contain
glion-cells and are apparently organs of special sense, having
possibly the function of detecting changes in the composition of
the water.

o o 2

388 ZOOLOGY SECT.

Projecting from the surface among the spines all the way from
the peristome to the periproct will be observed five double rows of
tube-feet or podia (Fig. 326), which in a living specimen will be found
to be capable of great extension. These are similar to the tube-
feet of the Starfish, and have similar functions : the sucker-like
extremity of each is supported by a plate of calcareous matter.
Each double row of tube-feet occupies a meridional zone of the
surface, termed the ambulacral area, corresponding to the ambulacral
groove of the Starfish : the intermediate zones are termed the
inter -ambulacral areas. At the oral end of each ambulacral area

---4

-- 5

7-

FIG 327 — Corona of Echinus esculentus, from the aboral surface, showing the arrange-
ment of the plates of the corona. 1, the anus ; 2, periproct, with irregular plates ; 3, the
madreporite ; 4, one of the other genital plates ; 5, an ocular plate ; 6, an inter-ambulacral
plate ; 7, an ambulacral plate ; 8, pores for the protrusion of the tube-feet ; 9, tubercles.
(After MacBride.)

on the peristome (Fig. 325, 4) is a pair of appendages similar to tube-
feet, but shorter, and termed oral, tentacles. Ten shrub-like append-
ages, the dermal branchice (2), are situated in the peripheral part of
the peristome, ja pair opposite each inter-ambulacral area.

When the spines are removed, the body is found to be enclosed
in a rigid globular shell, or corona (Fig. 327) as it is termed,
formed of a system of plate-like ossicles, the edges of which fit
accurately and firmly together, and the surfaces of which are
ornamented with the rounded elevations or tubercles for the articu-
lation of the spines. These plates are arranged in ten zones, each

ix PHYLUM ECHINODERMATA 389

consisting of two rows, running in a meridional direction from the
edge of the peristome to the neighbourhood of the periproct. Of
the zones of plates there are two sets, each consisting of five, the
members of which alternate with one another. In the case of one
of these sets of zones — the ambulacral zones or ambulacral areas
already referred to — each of the plates is perforated towards
its outer end by two minute pores, the ambulacral pores, for the
protrusion of the tube-feet. In the other five zones, the inter-
ambulacral zones or areas, the plates are not perforated. At its
a^al_^ndjeach arear .ambulacral or inter-ambulacialT,_ends-_iiL_a.
single ttpic^lrAsite, so that the periproct is surrounded by a ring of
ten plates, ^^a^ical system--0i~ plates (Fig. 328). Of these, the
five that are situated at the ends of the ambulacral areas are
termed the \ ocular pieties r(oc), owing to the fact that each of them
bears a structure once supposed to be a rudimentary eye, but now
known to be a tentacle ; while the five opposite the inter-ainbulacral
areas are termed the genital plates
(gen), each of them being perforated
by an opening which is the aperture
of one of the five genital ducts — the
ducts of the ovaries or testes as the
case may be. One of these genital
plates (madr) has a swollen and
spongy appearance, which distin-
guishes it from the others : this is
the madreporite, through which, as in

the Case of the Structure of the Same FIG 328.— Apical system of plates-and

aboral extremities of zones of the shell

name in the Starfishes, the madreponc of a sea-urchin, amb. ambulacral
canal communicates with the exterior. SS^JSUS^^^ilt
The two ambulacral areas between JggjJt "(AtASaSti) perip>''
which the madreporite lies constitute
the bivium, the remaining three the trivium.

On the inner surface of the shell, close to the edge of the peri-
stome, there project inwards five processes, the auricles (Fig. 330,
aur), one opposite each ambulacral area. Within the ring of auricles
lies a complex structure termed Aristotle's lantern (Fig. 329).
This consists of the five teeth (e), the apices of which are to be
seen projecting through the mouth, together with a system of
ossicles. The teeth are long, curved, and pointed : proximally each
is supported by and partly embedded in a pyramidal ossicle, the
alveolus (a), consisting of two halves united by a longitudinal suture.
Firmly united to the base of the alveolus is a stout bar, the
epiphysis (6). Adjacent epiphyses are in close contact with one
another, and running inwards from their points of union are five
radially-directed, stout bars, the rotulce (c), the inner ends of which
unite to bound a circular aperture through which the oesophagus
passes. With the inner end of each rotula is movably articulated

390

^OOLOGY

SECT.

a more slender bar, the radius (d), which runs outwards, parallel
with, and closely applied to, the rotula, to end in a free, bifurcated
extremity. Aristotle's lantern as a whole is in the shape of a five-
sided pyra-
mid, at the
apex of which

Project the
ve teeth;
the pyramid is
hollow, con-
taining a pas-
sage which is
the beginning
of the oeso-
phagus. The
base has the

FlO."329.— Lantern of Aristotle of Echinus. A , fwo of the five chief appearance of
component parts apposexl and Viewed laterally. B, lateral, and ,TT1,™1 4-1* n

C internal view of a single part. a. alveolus, and a', suture with its <* Wilt,ei, LUC

fellow; b. epiphysis ; b'. suture with alveolus ; c. rotula ; d. radius ; fvrp nf wTiirli

e. tooth. (From Huxley's Invertebrates, after Miiller.) V1

is represented

by the five epiphyses, the spokes by the five rotulse with the five
radii in close contact with them, and the hub by the rounded
central aperture. Passing between the various ossicles of the
Fan tern, and from oc af, ^r^s

them to the auricles,
are systems of muscles
by means of the con-
tractions of some of
which the lantern as a
whole can be pro-
truded or retracted,
while the action of
others is to cause the
movements of the
alveoli by which the
teeth are brought to
bear on the food.

Nervous System.
—Passing outwards
through each auricle,
and running along the
inner surface of the
corona opposite the
middle of each ambu-
lacral area, is a radial nerve (Fig. 330, rad. ne). Within the
ring of auricles the five radial nerves are connected with a
nerve-ring (nerv. r) surrounding the mouth. At its distal end

rad.ne
rada,

FIG. 330.— Lateral view of the internal organs of a Sea-
urchin as seen on the removal of a half of the shell, ab. r
ves. haemal strand, aboral ring ; amb. r. ambulacral ring-
canal ; amp. ampullae ; an. anus ; aur. auricle ; cod.
ccelome ; int. intestine ; int. res. intestinal haemal strands;
mad. madreporite ; mad. can. madreporic canal ; mo.
mouth ; mus. muscles passing from the auricles to
Aristotle's lantern ; nerv. r. nerve-ring ; oc. ocular plate ;
or. r. ves. haemal strand, oral ring ; plex. axial organ ; pol.
ves. Polian vesicle ; rad. amb. radial ambulacral vessel ;
rad. ne. radial nerve ; siph. siphon ; sp. radial extension
of the ccelome surrounding the nerve ; t. f. tube-feet.

IX

PHYLUM ECHINODERMATA

391

each radial nerve is connected with the so-called eye (oc), borne
by the corresponding ocular plate. These parts correspond to
the epidermal nervous system of the Starfish, which, owing to
the ambulacral grooves having become closed in to form narrow
canals — the epineural canals '(Fig. 331, ep.), covered over by the
plates of the corona — is here more deeply situated ; the deep and
coelomic systems are only feebly developed.

Ambulacral System. — Internal to each radial nerve, and pur-
suing a corresponding course, runs a radial ambulacral vessel (Figs.
330 and 331). From this are given off on each side a series of short
branches to the tube-feet, with each of which is connected one of
a series of compressed sacs, the ampullce (amp), by two canals, one
passing through each of the two pores. At their oral extremities

amp

ois

FIG. 331 . — Diagrammatic transverse section of the ambulacral zone of an Echinoid. amb. oss.
ambulacral ossicle ; amp. ampulla of a tube-foot ; ep. epineural canal ; muse, muscles
attaching spine to its tubercle ; nefo. nervous ring in base of spine ; n. r. radial nerve-cord ;
oss. ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perih. radial perihsemal canal ;
pod. tube-foot ; wv. r. radial ambulacral vessel. (After MacBride.)

the five radial ambulacral vessels unite with a ring-vessel
surrounding the oesophagus. Appended to the ring-vessel are
five Polian vesicles (pol. ves.) in the form of small mammillated
bodies. A madreporic canal (mad. can.), corresponding to that of
the Starfish, but with soft membranous- walls devoid of ossicles,
runs from the madreporite at the side of the periproct to the
ring-canal. '

The enteric canal (Fig. 332, ali) is devoid of the radial caeca
which it presents in the Starfish : it is a wide, soft-walled tube,
which winds round the interior of the corona in its passage fi
the mouth to the anus, held in place by a band of threads, tl
mesentery, passing out from it to the inner surface of the shell.
It gives off a short diverticulum, the siphon (siph), which opens

392

ZOOLOGY

SECT.

amp

into it at both ends ; this, together with the intestine itself, probably
acts as an organ for the respiration of the coelomic fluid.

The coelome contains a fluid in which, as in the Starfish,
there are numerous corpuscles. Of these there are two kinds
— amoeboid corpuscles (amcebocytes) with long pseudopodia, and
vibratile corpuscles, which closely resemble sperms, having a rounded
head and a slender vibratile tail : the latter aid in bringing about
a constant circulation of the coelomic fluid.

The part of the coelome containing Aristotle's lantern is com-
pletely cut off from the rest by the arrangement of the membrane
enclosing the lantern, and the function of the branchiae on the
peristome is evidently the oxygenation of the coelomic fluid enclosed
in this compartment, which is known as the lantern-ccelome.

The perihsemal and haemal or lacunar systems, as well as

the axial organ, will
be referred to in the
account of the general
structure of the
phylum.

The reproductive
organs consist of five
masses of minute
rounded follicles (Fig.
332, ov) situated in
the anal portion of
the shell, and each
communicating with
the exterior by its
duct, which perforates
the corresponding
genital plate. The
sexes are distinct ; as
in the Starfish, there is
little difference to be
observed between the ovaries of the female and the testes of the
male until we come to examine their microscopic structure. The
genital rachides which in the Starfish connect the gonads
with the genital stolon (p. 378) are aborted in the adult
Sea-urchin.

The early stages in the development of the Sea-urchin are
very similar to the corresponding stages in the development
of the Starfish described on page 381. The bilateral larva
of the Sea-urchin, which is termed a pluteus, is provided with
a number of elongated arms or processes supported by delicate
calcareous rods. A metamorphosis, in which the bilateral larva
becomes converted into the radial adult, takes place as in^ the
Starfish.

FIG. 332. — Alimentary canal and other organs of Sea-
urchin as seen when the oral half of the corona has
been removed, ab. r. ves. aboral ring of the haemal
system ; ali. alimentary canal ; amp. ampullae ; int. ves.
intestinal blood-vessels ; lant. lantern of Aristotle ; ces.
oesophagus ; or. r. ves. oral ring-vessel of the haemal
system ; ov. ovary ; reel, rectum ; siph. siphon ; z.
teeth. (From Leuckart, partly after Cuvier.)

PHYLUM ECHINODERMATA

393

3. EXAMPLE OF THE HOLOTHUROIDEA.
A Sea-cucumber. — Cucumaria or Colochirus.

General External Features. — The body (Fig. 333) is elon-
gated, in shape not unlike a miniature cucumber, somewhat
irregularly five-sided, with an opening at each end. One end is
somewhat thicker than the other, and the opening at this thicker
(oral or anterior) end is the mouth, that at the opposite (aboral or
posterior) end is the anus. The body is five-sided, and along each
side there extends a double row
of podia or tube-feet. In Colo-
chirus there is a very distinct
ventral surface, into which three
of the five sides enter, distin-
guished by the absence of the
rows of tubercles that occur on
the dorsal portion of the surface,
and by the presence of three
distinct bands of tube-feet. This
ventral part of the body with
its three ambulacral areas is
the equivalent of the trivium
of the Starfish, the rest repre-
senting the bivium. On the
dorsal surface, instead of typical
tube-feet, there are papillae
devoid of sucking extremities,
and similar appendages take the
place of tube-feet at the ends
of the three ventral bands. In
Cucumaria the ventral surface
is less distinctly defined, but its

position is to J)e determined by FlG. 333.-Cucumaria planci. Entire anima 1
reference to the tentacles (vide seen from the ventral surface. (From

\« Hertwig's Lehrbuch, after Ludwig.)

p. 394) ; there are no papillae.

The ventral surface* is, it is to be noticed, parallel with the axis
joining mouth and anus, and the body, when compared with
that of the Starfish or Sea-urchin, is greatly drawn out in the
direction of the line joining mouth and anus.

There are no definite calcareous plates ; but the integument is
tolerably hard, owing to the presence in its substance of innumer-
able microscopic calcareous spicules, very variable in shape in
different species of Cucumaria, and in Colochirus having the form
of sieve-like or lattice-like plates, some of which are to be found
ever; in the walls of the tube-feet. The tube-feet are, like those
of the Starfish, used in locomotion, progression being effected by

394 ZOOLOGY SECT.

creeping with the ventral surface applied to the ground. In a
Sea-cucumber living undisturbed under natural conditions there
will be found protruding through the mouth a circlet of ten
tentacles, which are to be looked upon as greatly developed and
specially modified podia. These are tree-like in shape — a central
stem giving off a number of short branches, which may in
turn be branched — and they are highly sensitive and contractile.
Two of these tentacles will be seen to correspond to each of
the ambulacral areas. The pair situated opposite the middle
ambulacral area of the ventral surface are very much smaller than
the others, and will be observed to perform the special function of
pushing the food-particles into the mouth. All the tentacles are
drawn completely back within the mouth when the animal is
disturbed.

Structure of Body-wall. — When the wall of the body is
divided, it is found to consist, in addition to the hardened integu-
mentary layer, of two layers of muscle in addition to a thin layer
of cells, the peritoneum or ccelomic epithelium, lining the ccelome.
The outer layer of muscle is a complete, continuous layer of
muscular fibres which have a circular arrangement, i.e. are
arranged in a ring-like manner around the long axis of the body ;
while the inner layer is not continuous, consisting, in fact, merely
of five flattened bands which run longitudinally from the oral to
the anal extremities, each underlying one of the ambulacral areas.
In close contact with each of these bands, on its inner surface,
runs a radial ambulacral vessel (Fig. 334, rad. amb) together with a
radial nerve.

Ambulacral System. — Just behind the bases of the tentacles,
and surrounding the beginning of the oesophagus, is a circular
ambulacral vessel (ring, ves) which gives off the five radial vessels ;
these first run forwards and give off branches to the tentacles,
and then backwards, passing along the ambulacral areas and
giving off branches to the tube-feet, each of which is provided
with its ampulla. From the ring-vessel also arises a large pear-
shaped Polian vesicle (pol. ves), and a short sinuous canal, the
madreporic canal (mad. can), which ends in a perforated extremity—-
not situated, like the madreporite of the Starfish or Sea-urchin,
on the outer surface of the body, but in the interior of the coelome.

A nerve-ring surrounds the mouth and gives off the five radial
nerves.

Both perihaemal and ha3mal systems are well developed.
The latter comprises a ring-like strand (ri. bl. ves) situated close
to the nerve-ring and sending off five radial strands, as well as
dorsal and ventral strands (int. ves) accompanying the enteric
canal, and a plexus surrounding the left respiratory tree (p. 396).

The coalome contains a fluid in which float numerous amcebo-
cytes, similar to those of the Starfish, and also a number of

PHYLUM ECHINODERMATA

395

flattened nucleated corpuscles containing a red colouring matter
— hemoglobin — almost identical with that which gives the red
colour to the blood of the higher animals.

gen. op

— «=^ — gen du,

mad. can
int ves

Long frucs -

to ng rrut£

FIG. 334. — Internal organs of a Holothurian as «een when the body-wall is divided along
the middle of the, dorsal surface. 6. w. body-wall ; circ. WM«. circular layer of muscle ;
cl. cloaca ; cl. op. bloacal opening ; cuv. org. (Juvieran organs ; gen. op. genital aperture ;
gen. du. genital duct ; gen. gl. gonad ; int. intestine ; inter, oss. inter-ambulacral ossicles ;
int. ves. intestinal haemal strands ; long. mus. longitudinal band of muscle ; mad. can.
madreporic canal ; mes. mesentery ; pol. ves. Polian vesicle ; rad. amb. radial ambulacral
vessel ; rad. oss. ambulacral ossicles : ri. bl. ves. ring strand of haemal system ; resp.
respiratory trees ; ring-ves. ring-vessel of the ambulacral system ; stom. stomach. (After
Leuckart.)

The enteric canal is, as already mentioned, surrounded at its
oral extremity by the circlet of tentacles, and within these, when
they are fully exserted, is a narrow peristome with the mouth in

396 ZOOLOGY

SECT.

the centre. When the tentacles are retracted the peristome be-
comes inverted, so that peristome and tentacles are enclosed
within a chamber, the buccal chamber, into which the mouth leads.
Surrounding the oesophagus, which lies immediately behind the
buccal chamber, is a circlet of ten circum-oesophageal ossicles, five
ambulacral (rad. oss) in position, and five inter-ambulacral (inter,
oss). Through each of the former pass the corresponding radial
ambulacral vessel, haemal strand, and nerve: The alimentary canal
itself is a simple cylindrical tube, only indistinctly marked out
into oesophagus, stomach (stom), and intestine. It forms several
coils within the ccelome, to the wall of which it is attached by a
thin membranous dorsal mesentery, and terminates behind in a
comparatively wide chamber, the cloaca (cl).

Opening into the cloaca is a pair of remarkable organs of
doubtful function, the so-called respiratory trees (resp). Each of
these, beginning behind in a single tubular stem, becomes elabo-
rately branched in front, some of the branches reaching nearly to
the anterior end of the body-cavity. Each of the terminal branches
ends in a small enlargement or ampulla. Besides having to do,
most probably, with the respiration of the ccelomic fluid and with
the excretion of waste-matters, these organs have a hydrostatic
function ; it is through them also that, when the tentacles are with-
drawn, the overplus of fluid which would impede their retraction is
got rid of, and by their means, in like manner, that the quantity is
again increased when the tentacles are protruded again. In all
probability it is through the permeable walls of these organs that
additional supplies of sea-water are received into the coelome, and
thus reach the ambulacral system through the perforated end of
the madreporic canal.

Reproductive Organs. — The Soa-cucumber, like the Starfish
and Sea-urchin, has the sexes separate. Ovaries and testes (gen. gl)
are very like one another, and consist of bunches of tubular follicles,
which communicate with the exterior by means of a duct opening on
the dorsal surface some little distance behind the oral end (gen. ap.).

The early stages of development are very similar to those of
the Starfish (p. 381). The bilateral larva, however, assumes a shape
somewhat different from that of the Asteroidea, and is termed
the auricularia (Fig. 348) : it has a number of short processes
developed in the course of the ciliated bands. The larval mouth
and oesophagus, instead of being abolished as in the case of the
Starfish, persist to the adult condition.

4. THE CRINOIDEA.
A Feather-star. — Antedon rosacea.

General External Features. — In the Feather-star (Fig.
335), as in the Starfish, there are to be recognised a central disc and a

IX

PHYLUM ECHINODERMATA

397

series of five radiating arms. In the natural position of the animal
the side of the disc which corresponds to the oral or actinal
surface of the Starfish is directed upwards, and the aboral or

FIG. 335. — Antedon. Side view of entire animal (From Leuokart and Nitsehe's Diagrrnif,)

abactinal surface downwards. The five arms are bifurcated at
their bases ; they are feather-like and highly flexible, acting as
the locomotive organs of the animal, their alternate flexions and
extensions resulting
in a slow movement
through the water.
On the aboral side of
the disc are whorls
of slender, curved,
cylindrical appen-
dages, the cirri (Fig.
336), by means of
which the Feather-
star is enabled to
anchor itself tem-
porarily to a rock
or a sea-weed.

On the oral side
of the disc the body-
wall is soft and
flexible, containing
only scattered irre-
gular SpiculeS of FIG. 336. — Aboral view of Antedon. c. centro-dorsal ossicle ;
dr. cirrus : Rl, R2, R*, the three radial plates of one
matter ; column ; syz. syzygy or articulation. (After MacBride.)

"S?

calcareous

398

ZOOLOGY

SECT.

and in the centre of this surface is an opening, the mouth (Fig.
337, mo). From the mouth five very narrow ciliated grooves, the
ambulacral or food-grooves, radiate outwards towards the bases
of the arms, near which they bifurcate, so that ten grooves are
formed, one passing along the oral surface of each of the ten
arm-branches to its extremity. The anal opening (an) is likewise
on the oral surface, being situated on a papilliform elevation in
the interspace between two of the radiating canals.

The aboral side of the disc is occupied by a large, flat, pentagonal
ossicle, the centro-dorsal ossicle (Fig. 336, c ; and Fig. 339, CD),
bearing on its outer surface a number of little cup-like depressions,
with which the bases of the cirri are connected. The cirri (cirr)
consist each of a row of slender ossicles, covered, like all the rest
of the animal, with epidermis, and connected together by means

of muscular fibres.
Concealed from v'ew
by the centro-dorsal
ossicle is a thin plate
termed the " rosette "
(ros), formed by the
coalescence of the
basals of the larva.
At the sides are five
first radial ossicles
(R1), also partly con-
gealed ^py the centro-
dorsal i)ssicle : with
eaojr of these articu-
lates a second radial
(R2), which is visible
beyond the centro-
dorsal. With each of
the second radials arti-
culate two third radials
(R3), each forming the base of the corresponding arm-branch.

The ossicles of the arms — brachials (Br.1, Br.2) — are arranged
in a single row in each arm. They are somewhat elongated in
the direction of the long axis of the arm, strongly convex on their
aboral surfaces, longitudinally grooved on the oral surface, and
connected together by the investing epidermis and by bundles of
muscular fibres, by the contractions of which the movements of
the arms are brought about. Fringing the sides of each arm are
two rows of side-branches, or pinnules, each supported by its row
of connected ossicles, and each grooved along its oral surface.

The coelome contains numerous strands of connective-tissue
which serve to suspend the various organs.

Extending through the arms and pinnules between the supporting
ossicles and the ambulacral grooves are three canals, ciliated iifr

FIG. 337.— Antedon, oral (upper) surface of the central
disc. an. anus ; mo. mouth. (From Vogt and Jung.)

IX

PHYLUM ECHINODERMATA

399

parts, which are prolongations of the coelome (Fig. 338, cosl. can).
Two of these — the subtentacular canals — form a pair separated from
one another by a median septum underlying the ambulacral groove.
The other — the cceliac canal — runs between these and the sup-
porting ossicles (oss). The sub-tentacular canals and the cceliac
canal communicate with one another at the extremity of each
pinnule.

The enteric canal begins with a wide, funnel-shaped O3so-
phagus leading to a spacious stomach which gives off a number of
short, blunt diverticula and a pair of longer, narrower, " hepatic "
caeca, which are slightly branched at the ends. Distally the stomach
becomes contracted and opens
into a wide intestine, which winds
round the coelome, becoming
narrower where it passes upwards
to open on the exterior, the
terminal pa^t, or rectum, project-
ing as a tubular papilla on the
surface. In the living animal the
rectal tube is observed to undergo
frequent movements of contrac-
tion and dilatation, by means of
which water is drawn into and
expelled from the intestine ; so
that here, as in the Sea-urchin,
there would appear to be a. pro-
cess of intestinal respiration.

The ambulacral system con-
sists of a ring-vessel surrounding
the mouth, and a series of radial
vessels (Fig. 338, rad. amb.) which
run in the ambulacral grooves,
giving off branches to the pin-
nules. Connected with the radial
vessels and their branches are a
series of minute tubular ciliated appendages, the podia or so-called
tentacles (Fig. 339, tent.), which are homologous with the tube-feet
of the Starfishes and Sea-urchins, but are devoid of terminal suckers.
These are not organs of locomotion : they bear numerous sensory
papillae, and are therefore to be looked upon as tactile organs, but
they probably also have a respiratory function. Connected with
the ring-vessel are a number of ciliated tubular diverticula, the
water-tubes, which are suspended within the ccelome, and open
freely into it at their extremities. A large number of vessels with
minute ciliated openings — the water-pores (wat. p) — lead through
the affinal wall of the disc : these and the ciliated tubes are to be
considered as together representing the madreporic canal and its
openings in th^ Starfish and Sea-urchm.

flcial (ambulacral) nervous system ; ax. ne.
axial nerve ; cod. can. sub-tentacular and
coeiiac canals ; mus. muscles ; neur. ves.
radial sinus of the perihaemal system ; rad.

amb radial ambu£cral vessel g^g ofl

branches to the tentacles. Between the
paired sub-tentacular and unpaired coehac
canals is the genital rachis. The small round

"* *K the

400

ZOOLOGY

SECT.

The nervous system consists of two perfectly distinct parts —
supe/ficial and axial or aboral. A superficial radial nerve-ring
(ect. ne) surrounds the mouth, and from it are given off a series of
nerves — thickenings of the epidermis of the ambulacral grooves
and their offsets — which extend throughout the length of the
arms and pinnules. In the axis of the supporting ossicles of the
arm is an axial nerve (ax. co), which gives off branches (Fig. 338,
ax. ne) running through the axes of the ossicles of the pinnules.
The axial nerves are connected internally, not with the circum-
oral nerve-ring, but with a central body situated below the rosette,

in the interior
of the centro-
dorsal ossicle.
This, the cen-
tral capsule
(Fig. 339, cent,
caps), forms
the invest-
ment of a
body termed
the five-cham-
bered organ
(chamb. org)
divided into
five parts by
radial septa,
and continu-
ous with the

through the disc, passing through one radius and one inter-radius. r i A - f

amb. ambulacral vessels ; ax. co. axial nerve-cord passing through aDOrai end OI

the ossicles of the arm ; Brl Br.z brachial ossicles ; CD centre dorsal 4- "Up cr P "n i t a 1

ossicle ; cent. caps, central capsule ; chamb. org. chambered organ ; ° o c

Pro-
from

the five angles

of the central capsule combine to form a pentagonal ring from
which pass outwards the axial nerves of the arms: This system
controls the movements of the arms. Aborally the central capsule
gives off nerves to the cirri.

A system corresponding to the perihaemal system of the
Starfish is p esent, though reduced, and there is a highly-developed
and complicated lacunar or haemal system.

Numerous bodies termed the sacculi, the character of which
has given rise to much discussion, occur regularly arranged along
the ambulacral grooves and also in other parts. These are small
spherical bodies which become vividly coloured when treated with
staining agents. They are essentially collections of amoeboid cells
which may represent reserve materials, stored up for the nutrition
of the animal, or may contain excretory matters.

c7uimb.org

cenl.ca.ps

cirr

FIG. 339. — Antedon. Diagrammatic view of a median vertical section

cirr. cirri ; ect ne. ambulacral (epidermal) nerve-ring and radial stolon
nerve ; gen. st. genital stolon ; int. intestine ; mo. mouth ; R.i R*
R.z radials ; ros. rosette; tent, tentacles ; wat. p. water-pores. (After CCSSCS
Milnes Marshall.)

ix PHYLUM ECHINODERMATA . 401

The reproductive organs — ovaries or testes, as the case may
be — are lodged in the dilated bases of the pinnules, which become
considerably enlarged as the ova or sperms mature, those next to
the bases of the arms alone remaining sterile. When mature, the
sexual elements escape by means of short ducts. Each gonad
is one of the terminal parts of a system of cords of cells enclosed in
narrow tubes extending from a central part or genital stolon
(gen. st) — connected dorsally with the chambered organ — outwards
through the arms ; the terminal portions, lying in the pinnules,
are dilated to form the reproductive organs, and the cells
of their epithelium become developed into ova or sperms, while
the rest constitute a non-fertile connecting rachis. This system
is enclosed throughout by a plexus of haemal lacunae.

Like the rest of the Echinoderms, the Feather-star undergoes a
metamorphosis (Figs. 349 and 350). It passes through a free-
swimming ciliated larval stage, which is followed by a fixed
stalked stage known as the " pentacrinoid " larva on account of the
resemblance which it bears to the adult Pentacrinus, one of the
permanently fixed members of the same class. This fixed penta-
crinoid larva passes into the adult free-swimming Feather-star by
the development of the dorsal cirri, the elongation of the arms,
and the absorption of the stalk.

5. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Echinodermata are radially symmetrical animals, the radial
arrangement of whose parts imperfectly conceals a more obscure
bilateral symmetry. The surface is covered with an exoskeleton
of calcareous plates or ossicles, which usually support a system of
movable or immovable calcareous spines. There is a large body-
cavity or ccslome, and well-developed alimentary, nervous, and
vascular systems. A characteristic system of vessels, the ambu-
lacral system, is connected with the locomotion of the animal, as
well as with other functions : the organs of locomotion are in most
cases elastic and -contractile tubular bodies, the tube-feet, which are
appendages of the ambulacral system. Nearly all the systems of
organs of the animal partake to a greater or less extent of the
general radial form of the body. Reproduction is entirely sexual.
In the course of its development from the egg the Echinoderm
passes through a peculiar larval stage, in which the symmetry of
parts is bilateral, instead of radial as in the adult animal. All the
Echinodermata are marine.

The Echinodermata are classified as follows :—

SUB-PHYLUM I.-ELEUTHEROZOA.

Echinodermata devoid of a stalk, and always freely locomotive in
the adult condition : with a system of radial ambulacra in the form
VOL. i. B D

402 ZOOLOGY SECT.

of grooves or areas radiating out from the mouth, and containing
a double series of tubular appendages of the ambulacral system,
the tube-feet, usually employed in locomotion, and in the majority
of cases provided with terminal suckers : the anus usually aboral ;
the mouth on the surface that is habitually directed downwards, or
at the end habitually directed forwards in locomotion.

CLASS I.-ASTEROIDEA.

Free Echinoderms with star-shaped or pentagonal body, in
which a central disc and usually five arms are more or less readily
distinguishable, the arms being hollow, and each containing a
prolongation of the coelome and of its contained organs. There
are distinct oral and aboral surfaces, on the former of which the
anus and the madreporite are situated, and on the latter the
mouth and five narrow ambulacral grooves lodging the tube-feet.
The larva has the form either of a bipinnaria or of a brachiolaria.
This class includes the Starfishes.

ORDER 1. — SPINULOSA,

Asteroidea with only two rows of tube-feet in the ambulacral
grooves ; the dorsal surface covered with small close-set plates
bearing spines.

ORDER 2. — VELATA.

Asteroidea having the dorsal surface covered with a single mem-
brane or velum supported by sheaves of spines, or with a number of
such structures.

ORDER 3. — PAXILLOSA.

Asteroidea in which the dorsal surface is covered with paxillae.
The pedicellarise are never forcipulate.

ORDER 4. — VALVATA.

Asteroidea with granular surface without prominent spines ;
valvate pedicellariae.

Vt

ORDER 5. — FORCIPULATA.
Asteroidea with forcipulate pedicellariee.

CLASS II.-OPHIUROIDEA.

Star-shaped free Echinoderms, with a central disc and usually
five arms, which are more sharply marked of! from the disc than in
the Asteroidea and which contain no spacious prolongations of the
coalome. There are distinct oral and aboral surfaces. The anus
is absent ; the mouth, as well as the madreporite, on the oral

IX

PHYLUM ECHINODERMATA 403

surface. Except in one fossil order there are no ambulacral
grooves. The larva is a pluteus. This class includes the Sand-
stars and Brittle-stars (Figs. 341 and 342).

ORDER 1. — LYSOPHIUR.E.

Extinct Ophiuroids with ambulacral grooves.

Silurian and Devonian.

i

ORDER 2. — STREPTOPHIUR^E.

Ophiuroids in which the ambulacral ossicles articulate with one
another by simple ball-and-socket joints.

ORDER 3. — CLADOPHIUR^E.

Ophiuroids in which the ambulacral ossicles articulate with one
another by means of hour-glass-shaped surfaces. The arms may
be branched.

ORDER 4. — ZYGOPHIUR^:.

Ophiuroids in which the movement of the ambulacral ossicles on
one another is restricted by the presence of lateral processes and
pits.

CLASS III.-ECHINOIDEA.

Free Echinoderms with globular, heart-shaped, or disc-shaped
body enclosed in a shell or corona of close-fitting, firmly united
calcareous plates. The mouth is nearly always polar ; the anus
usually at the opposite (aboral) pole ; the madreporite is close to
the latter. There are no ambulacral grooves ; but the surface is
divided into alternating ambulacral and inter-ambulacral zones or
areas, which usually run from pole to pole. The larva is a pluteus.
This class includes the Sea-urchins, with the Heart-urchins and
Cake-urchins.

ORDER 1. — REGULARIA.

Echinoidea with globular corona containing, in most cases,
twenty meridional rows of plates. Mouth and anus polar. A
lantern of Aristotle is present. This order includes the Sea-
urchins.

ORDER 2. — CLYPEASTRIDEA.

Echinoidea with more or less flattened corona, with the mouth
central, the anus excentric. A lantern of Aristotle is present.
This order includes the Cake-urchins (Fig. 346).

ORDER 3. — SPATANGOIDEA.

Heart-shaped Echinoidea^ with the mouth and anus excentric. .
No lantern of Aristotle. This order includes the Heart-urchins
(Fig. 345).

D D 2

404 ZOOLOGY SECT.

CLASS IV.-HOLOTHUROIDEA,

Free Echinoderms with elongated, cylindrical or five-sided body,
having the mouth and anus at opposite extremities. The body-
wall is usually only supported by scattered ossicles or spicules.
There is no external opening to the madreporic canal (except in
some Elasipoda). The surface usually exhibits five ambulacral
areas : but these may be absent. There is a circlet of large oral
tentacles. The larva is an auricular ia. This class includes the
Sea-cucumbers and " Beche-de-mer."

ORDER 1. — ELASIPODA.

Holothuroidea with well-marked bilateral symmetry, with tube-
feet on the ventral surface (which is flattened) and papillae on the
dorsal. Confined to the deep sea.

ORDER 2. — PEDATA.

Holothuroidea with tube-feet either in longitudinal rows or
scattered irregularly over the surface.

ORDER 3. — APODA.

Holothuroidea devoid of tube-feet and of radial ambulacral
vessels.

SUB-PHYLUM 11,-PELMATOZOA.

Echinodermata which are usually fixed at the base, and usually
supported on a stalk composed of a row or rows of ossicles
(Fig. 347) : the mouth on the free surface, near or in the centre, and
having extending out from it on the oral surface a radially arranged
system of narrow, ciliated ambulacral grooves, having the function
of food-grooves, which may run between the plates of the theca,
on the surface of the theca, or along the oral surfaces of a system
of radial processes or arms given off from it. The tube-feet of
other Echinoderms, when represented, take the form of small,
tubular, strongly ciliated appendages (tentacles) without suckers :
the anus usually on the oral surface.

CLASS I.-CBINOIDEA.

Mostly fixed, stalked Pelmatozoa in which there is a theca
comprising five regularly arranged radial and five basal plates,
giving off five, usually branched, jointed processes or arms ; with
food-grooves radiating out from the mouth along the oral surfaces
of the arms, and extending along their branches : the central parts
of the ambulacral, nervous, and reproductive systems, and of the
coelome, lodged in the theca, send extensions through the arms.

This class comprises, , together with many extinct forms, the
only living Pelmatozoa.

ix PHYLUM ECHINODERMATA 405

SUB-CLASS I. — MONOCYCLICA.

Crinoidea in which the base of the theca comprises basals
only.

SUB-CLASS II. — DICYCLICA.

Crinoidea in which the base comprises basals and infra-basals.

CLASS II.- CYSTOIDEA.

Fixed, stalked, or sessile Pelmatozoa, with the plates of the theca sometimes
irregular, sometimes arranged in a regular radial system, with food- grooves
extending for a longer or shorter distance over the surface of the theca, some-
times on special plates lying above those of the latter, their terminal parts
extending on to a varying number of unbranched arms or " fingers " ; the
theca perforated completely or partially by numerous pores which are supposed
to have lodged respiratory processes.
Lower Silurian to Carboniferous.

CLASS III.-BLASTOIDEA.

Fixed Pelmatozoa with well-developed stalk, and theca with a regular
system of plates ; with five, rarely four, food-grooves radiating out from the
central mouth, and each borne on a special " lancet plate," the inter-radial
intervals between which are occupied by a corresponding number of oral or
" deltoid " plates. The grooves are bordered by a series of side plates bearing
small branches or " fingers " to which side branches of the grooves extend. In
the intervals between the grooves on the aboral sides of the deltoids are a whorl
of plates perforated by the apertures of groups of internally situated respiratory
folds (hydrospires). The anus is eccentrically situated on the oral surface.

Upper Silurian to Carboniferous.

CLASS IV. -EDRIASTEROIDEA.

Fixed (or sometimes free ?) Pelmatozoa, usually sessile, rarely with a short
stalk ; with sac-like, cushion- shaped or disc-shaped theca made up of numerous
plates devoid of any regular arrangement and without any appendages ; with
central mouth and five straight or curved radiating food-grooves bordered by
covering plates : anus and madreporite on oral side.

Cambrian to Carboniferous.

CLASS V.- CARPOIDEA.

Pelmatozoa with a well-developed stalk, with the body laterally compressed,
with only two food-grooves running out from the mouth. Theca composed of
numerous small irregular plates with larger lateral plates forming a framework
along the margins.

Cambrian and Silurian.

Systematic Position of the Examples.

Asterias rubens is a species of the genus Asterias, which, with
several others, constitutes the family Asteriidce of the order
Forcipulata. The family AsteriidcB is characterised among the
families of the Forcipulata by the following distinctive features : —
The ossicles of the aboral surface are small, unequal, reticulate
plates, bearing isolated or grouped spinelets (paxillw). The margin
of the actinostome is denned by the ambulacra! plates. The

406 ZOOLOGY SECT.

pedicellariae are of two forms, forceps-like and scissors-like. The
tube-feet are in four rows. Asterias differs from the other genera
of the family in having well-developed reticulate dorsal ossicles
bearing definite spines.

The Sea-urchins of which a short description has been given
are the genera Strongylocentrotus and Echinus, but the description
is sufficiently general to apply to any member of the family
Echinidce, to which these genera, with a number of others, belong.
The family Echinidce is one of about five families of the sub-order
Ectobranchiata, the members of which all differ from the other
sub-order — Entobranchiata — of the Regularia, or regular Sea-
urchins, in the possession of dermal branchiae, and in having the
auricles in the form of complete arches.

The Sea-cucumber (Cucumaria or Colochirus) is a member of
the Stichopoda — one of the families of the sub-order Dendrochirotce
of the Pedata, or foot-bearing Holothurians. The Dendrochirotce
differ from the Aspidochirotce — the other sub-order — mainly in
having arborescent instead of shield-shaped tentacles, and the
Stichopoda differ from the rest of the Dendrochirotce in having the
tube-feet arranged in five regular zones. The genus Cucumaria
is distinguished from the rest by having ten tentacles with the two
ventral smaller than the others. Colochirus is closely allied to
Cucumaria, the principal distinction being the presence in the
former of papillae taking the place of tube-feet in certain
situations, as already noted.

The Feather-star (Antedon rosacea) is a member of the family
Comatulidce, which is distinguished from the four other living
families comprised in the class Crinoidea of the Pelmatozoa by
the absence of a stalk in the adult condition.

6. GENEKAL ORGANISATION.

General Form and Symmetry. — Like the Ccelenterata, the
Echinodermata are radially symmetrical, the body being capable
of division into a series of sub-equal aniimeres along a series of
radiating planes at right angles to the principal axis. In the
majority of existing forms (Asteroidea, Ophiuroidea, and Crinoidea)
the radial symmetry is expressed in the external form of the body,
which is produced into a number of radially disposed parts, the arms
or rays, arranged around a smaller or larger central disc. But in the
Echinoidea the body is sub-spherical, and in the Holothuroidea
sub-cylindrical, the radiate arrangement being in these classes
indicated externally only by the distribution of the podia, and
internally by that of certain of the systems of organs.

Although, however, the general external form and the arrange-
ment of some of the internal organs in the Echinodermata indicates
a radial symmetry, it is invariably found that this radial arrange-

rx PHYLUM ECHINODERMATA 407

ment serves to hide a bilateral -symmetry which may be more
primitive. This is best marked in the larva, which has
pronounced bilateral instead of radial symmetry, but is quite
recognisable in the adult. In all Echinoderms there is, passing
through the primary axis, a plane — the median plane — along
which, and along which alone, the body is capable of being divided
into two equal — or, to speak more correctly, approximately equal —
right and left halves. The existence of such a single median
plane is, as already explained (p. 43), indicative of the bilateral
form of symmetry.

The body is most usually five-rayed (Ophiuroidea, most Aste-
roidea, Crinoidea), cylindrical (most Holothuroidea), or globular
(most Echinoidea), the surface in the two last cases being marked
by five bands or zones of tube-feet, which divide it into five
ambulacml and five inter-ambulacral areas. In the Ophiuroidea
and Asteroidea two of the rays — constituting the bivium — have
between them the madreporite, marking the position of the
madreporic canal of the ambulacral system ; the remaining three
rays form the trivium. The median plane passes through the
madreporite, and thus midway between the two rays of the
bivium, and bisects longitudinally the middle ray of the trivium.
A corresponding disposition of the parts is traceable also, as will
be subsequently shown, in the cylindrical and globular Echinoderms.

In all the Echinodermata aboral or abactinal and oral or actinal
surfaces are more or less distinctly recognisable. In the
Asteroidea, Ophiuroidea, and Echinoidea, the actinal surface is
that in the middle of which the mouth is situated, and which is,
in the natural position of the animal, directed downwards or
towards the surface to which it is clinging. The opposite abactinal
surface is, in the majority of the Asteroidea and Echinoidea,
marked by the presence of the anal aperture : in the Ophiuroidea
and some Asteroidea the anus is absent ; in some Echinoidea it is
situated on the border between the two surfaces, or even on the
oral surface. In the Crinoidea the oral surface, which is habitually
directed upwards in the natural position of the animal, bears both
mouth and anus, the former central, the latter eccentric and inter-
radial. In the fixed Crinoids the abactinal or aboral surface has
attached to its centre the distal end of the stalk ; in the free forms it
has connected with it whorls of slender curved appendages, the
dorsal cirri, by means of which temporary attachment is effected.
In the Holothurians, owing to the elongation of the body in the
direction of the line joining mouth and anus, oral and aboral surfaces
corresponding to those of the other classes are not distinguishable ;
but in many, as for example in Colochirus, there is a marked
difference between one surface— the dorsal, which is habitually
directed upwards, and another — the ventral, which is habitually
directed downwards,

408 ZOOLOGY SECT.

In considering the general external form in the various classes
of Echinoderms, we have to take into account the arrangement of
the tube-feet — the organs of locomotion— as these have important
relations to the other parts and to the whole plan of organisation
of the animal. These organs, as previously explained, are tubular
appendages with highly elastic and contractile muscular walls,
capable of being stretched out so as to extend a long way from the
surface of the body. In the majority of cases the tube-foot has at
its extremity a sucking-disc, by means of which it can be attached ;
in a few, however, this sucking-disc is absent.

The epidermis is ciliated in all but Holothuroidea. In the
subjacent dermal layers there are always present, except in Pelago-
thuria and Rhabdomolgus (Holothuroidea), calcareous bodies
or ossicles, varying very greatly in form and arrangement in the
different groups. Movable or immovable calcareous spines or
tubercles projecting on the surface are very general. Peculiarly
modified spines, termed pedicellarice, are commonly, though not
universally, present in certain parts in the Echinoidea and
Asteroidea. A pedicellaria consists in essence of two or three
calcareous jaw-like pieces or valves, movably articulated together,
and capable of being separated or approximated by the contraction
of bundles of muscular fibres ; sometimes there is a long stalk ;
sometimes (as in the case of Anthenea, p. 380) a stalk is absent ;
during life the jaws or valves keep opening and closing. That such
specialised structures have some important function to perform
there can be no doubt, but there is some uncertainty as to what
their special purpose is. According to some observers, the pedi-
cellariae of the Sea-urchin have been seen passing from one to
another the particles of faecal matter discharged from the anus,
and their function would thus appear to be a cleansing one. On
the other hand, it is stated that when a Sea-urchin is attacked
the spines may be bent aside from the assailed portion of the
surface so as to allow of the pedicellariae being brought to bear as
defensive weapons on the assailant, and from these and other
observations that have been recorded, both on Asteroids and on
Echinoids, it is concluded that the main function of these appen-
dages is to act as defensive organs. Pedicellariae are absent in the
Ophiuroids, but in the Euryalida there are peculiar hook-like
organs of adhesion, most abundant on the oral surface and towards
the extremities of the arms. The sphceridia, which have already
been referred to as occurring in the Sea-urchin, are only doubtfully
to be regarded as modified spines ; they are confined to the
Echinoidea. Also confined to that class are the clavulce — slender
spines covered with strong cilia, which occur in bands (fascioles) on
the surface of the Spatangoids. Larger spines, resembling the
clavulse in being covered with strong ciSa, occur also on the aboral
surface in the Clypeastroids and some Asteroids. The currents

rx PHYLUM ECHINODERMATA 409

produced by the action of their cilia serve to keep constantly
renewed the water in the neighbourhood of the anus and of the
respiratory podia or the papulae.

There are two principal systems of plates to be recognised,
an oral and an apical ; the former corresponding with the oral or
actinal, and the latter with the aboral or abactinal surface. The
former vary considerably in the different classes : the constant
elements are five orals, which may or may not be recognisable in
the adult. The apical system consists (1) of a central plate ; (2) of
five basals which are inter-radial in position ; (3) of five radials
which are radial in position. In the Asteroidea (Fig. 324) the
radials are late in making their appearance ; before they are
developed five terminal plates have become distinct, one at the end
of each rudimentary arm ; these are carried outwards by the
extension of the arm, and each supports the corresponding tentacle.
As a rule these plates of the apical system are only distinct in the
young condition. In the Ophiuroidea the arrangement resembles
that observable in the Asteroidea. In the Echinoidea (Fig. 328)
the basals (genitals) are perforated by the ducts of the repro-
ductive organs ; the radials (oculars) are perforated for the tentacles :
the central (anal) rarely persists as a single plate in the adult,
usually becoming broken up into a number of irregular plates. In
the stalked Crinoidea the term central has been applied to a plate
which is transformed into the disc of attachment at the base of
the stalk, but the correspondence between this and the similarly
named plate in the other classes is very doubtful ; the ossicles of
the stalk intervene between it and the basals. In the free forms
the uppermost segment of the larval stalk, uniting with the central
and the infra-basals, is transformed into a centro-dorsal plate, and
the basals nearly always unite into a rosette-plate, which is concealed
from view by the centro-dorsal and the radials. The apical system
of plates is apparently not represented in the Holothuroidea.

Modifications of Form in the Five Classes. — The general
shape in the Asteroidea is, as already pointed out, that of a
star. There is a central part, or central disc, from which proceeds
a system of radially disposed arms or rays. The central disc and
the rays are usually compressed in the vertical direction, as in
Anthenea and Asterina, but in some Starfishes the rays are
approximately cylindrical ; they nearly always taper distally. In
the majority of Starfishes, as in the examples described, the arms
are five in number, except in malformed individuals ; but in
some they are six, in others seven, eight, or more. The proportions
borne by the arms to the central disc are subject to considerable
variation. In some, as in Asterias, the arms are long, and the
central disc appears as little more than their point of union ; in
others, again, owing to coalescence of the arms, the whole Starfish
has the form of a five-sided disc, in which the arms are represented

410

ZOOLOGY

SECT.

only by the five angles ; while between these two extremes there
are numerous intermediate gradations. The Brisingidce differ
from all the rest of the class in having the arms almost as sharply
separated off from the central disc as in the Ophiuroids.

The abactinal or aboral and the actinal or oral surfaces are always
distinctly marked off from one another. In the middle of the
latter (Fig. 340) is the mouth, running out from which are five or
more narrow ambulacral grooves, one of which is continued along
the oral surface of each arm to its extremity. Near to, but not
quite in, the middle point of the aboral surface is the anal aperture,
absent in a few instances ; and on the same surface, nearer the

margin, between the two
rays of the bivium in the
five-rayed Starfishes, is the
madreporite, a finely grooved
calcareous plate perforated
by a number of minute aper-
tures. In some fossil Star-
fishes it is situated on the
oral surface. Sometimes in-
stead of one madreporite
there are several.

The wall of the body in
the Starfishes contains a
number of calcareous ossicles,
movably articulated together
and connected by bands of
muscle, so that, though the
body is firm, and in the dried
condition often quite rigid, the
arms are capable during life of
slow movements of flexion
and extension, enabling the
animal to creep through comparatively small fissures and crannies.
A special system of ossicles — the ambulacral ossicles — are arranged
in a double row along each ambulacral groove, the ossicles of the
two rows articulating movably with one another at the apex of the
groove. At the end of the arm the two rows of ambulacral ossicles
end in a terminal ossicle which supports the unpaired tentacle.
Spines are invariably present, but are sometimes confined to the
margins of the ambulacral grooves, in which position they are
movably articulated with the underlying ossicles. Tubercles take
the place of spines over most of the surface in many forms. In
Astropecten and other Paxillosa the ossicles of the aboral surface
take the special form to which the term paxillce is applied. Each
paxilla is a plate which is produced into a short rod, divided at its
extremity into a number of radiating processes.

FIQ. 340.— Anthenea. View of oral surface.
(After Sladen.)

IX

PHYLUM ECHINODERMATA

411

The tube-feet are arranged in a double row along each of the
ambulacral grooves, each connected through an aperture between
the ambulacral ossicles with an ampulla, or, exceptionally, with two
ampullae, situated in the coelome. Each double row of tube-feet
terminates at the extremity of the arm in an unpaired appendage,
the tentacle, which is tactile and olfactory, and not locomotive in
function. The tube-feet are provided (except in Astropecten) with
terminal suckers.

In the Ophiuroidea (Fig. 341) the central disc is much more
sharply marked off from the arms than in the Asteroidea. The
arms, which are usually five in number, rarely six, are comparatively
slender and cylin- ^

drical, tapering to-
wards the free ex-
tremities ; in one
group, the Euryalida
(Fig. 342), they are
branched. The
mouth is in the
middle of the oral
surface of the disc,
as in the Asteroidea,
but there are no
ambulacral grooves,
and there is no anal
aperture. Five pairs
of slits on the oral
surface (Fig. 341, C)
lead into the genital
bursse, which receive
the sperms and ova
from the gonads, and

which appear also to
act as Organs OI
respiration and

perhaps also of excretion. The surface is covered with thin
plate-like ossicles, usually beset along their edges with longer or
shorter spines ; sometimes irregular calcareous granules take the
place of plates. Hook-like organs of adhesion are present only in
the Euryalida. Each of the arms is supported by a row of internally
situated ambulacral ossicles. Tube-feet are present and are pro-
truded at the sides of the arms between the lateral plate-like
ossicles ; but they have no sucking-discs and no ampullae, and
locomotion is effected in the majority of the Ophiuroids by active
flexions and extensions of the arms. In one genus there is a pair
of fin-like appendages, supported by slender spines, on each joint
of the arms. The madreporite is situated inter-radially on the

. 341. — Ophioglypha lacertosa. A, outline, of the
natural size. B, central disc, aboral surface. C, the disc,
oral surface showing the mouth and genital fissures. (From
Nicholson and Lydekker's Paleontology.)

412

ZOOLOGY

SECT

oral, and not on the aboral surface as in the Asteroidea. In the
Euryalida there are five madreporites and five madreporic canals.

In the Echinoidea the body is either globular, or heart-shaped,
or flattened and disc-like. The exoskeleton is in the form of a
rigidly articulated system of calcareous plates, fitting closely
together by sutures, so as to form a continuous shell or corona.
Only Asthenosoma and allies, deep-sea forms, differ from all the rest
in having a corona possessing a certain degree of flexibility and
performing movements which are brought about by the contrac-

FIG. 342.— Astrophyton arborescens, aboral surface. (After Ludwig.)

tions of five longitudinal bands of muscle running along the
ambulacral areas on the inner surface.

In the globular forms, or regular Sea-urchins, the mouth is
situated at the oral pole of the globe, the anus at the aboral, and
the plates of the corona are in twenty regular meridional rows,
arranged in ten zones, five ambulacral and five inter-ambulacral,
as described in the account of Echinus, with peristome, periproct,
ocular and genital plates, and madreporite. Spines (Fig. 343),
pedicellaricB (Fig. 344), and sphceridia are present, as already
described (p. 387), the last-named appendages, however, being

IX

PHYLUM ECHINODERMATA

413

Fia. 343. — Diagram of
spine of Sea-urchin, show-
ing mode of articulation.
b. ligament ; m. muscle.
(From Leuckart.)

absent in one group. The spines are usually defensive organs
simply, but in some Sea-urchins they act also as the locomotive
organs, the animal moving by their agency along the sea-bottom.

The podia or tube-feet, which are arranged in a double row in
each ambulacral zone, are extremely extensible, and terminate in
sucking -membranes strengthened by a cal-
careous rosette. An unpaired tentacle, corre-
sponding to that of the Asteroidea, is sup-
ported on each of the ocular plates at the ends
of the ambulacral zones. Two podia in each
double row, situated on the peristome, are
likewise of the nature of tentacles (oral or
buccal tentacles), and are sometimes devoid of
sucking-membranes. Corresponding to the
dermal branchice of the Asteroidea are, in the
majority, five pairs of branched, hollow
appendages surrounding the peristome.

Surrounding the mouth are five teeth,
supported by an elaborate system of ossicles
(Aristotle's lantern, see p. 389), and a ring
of processes, the auricles, from the interior
of the corona, surrounds this and gives
attachment to some of the muscles by
which the ossicles are moved.

In the heart-shaped forms or Heart-urchins (Fig. 345) the
corona is heart-shaped, the mouth is usually more or less eccen-
trically placed on the oral surface, and the peristome is usually
transversely elongated ; the anus is on or near the border between
the two surfaces. The ambulacral areas do not run continuously,
but stop short at the margin (petaloid ambulacra) ; one of them, the
anterior, is usually unlike the others and
frequently devoid of pores. The genital and
ocular plates are in the middle of the aboral
surface, where the ambulacra converge, and
are thus widely separated from the anus ;
there are usually only four genital plates, and
the genital apertures may be reduced to two.
Slender spines beset the entire surface and are
the sole or chief organs of locomotion.
Modified (ciliated) spines, the clavulce, arranged
in narrow bands or fascioles, are variously
distributed round the anus and elsewhere ;
but are sometimes entirely absent. A few pedicellarise are present
in the neighbourhood of the mouth, and sphseridia also occur. The
" lantern of Aristotle," with its teeth, is not represented.

In the Clypeastridea or Cake-urchins the whole corona (Fig. 346)
is usually greatly compressed so as to assume the form of a disc,

Fio. 344. — Pedicellaria of
Arbacia punctulata.

(From Leuckart.)

414

ZOOLOGY

SECT.

sometimes notched at the edges or pierced by fenestrae. The mouth
is in the middle of the flat or concave oral surface, the anus eccen-

, trically situated near the
margin. The ambulacra are
•••• petaloid. The plates of the
-.:.- apical system are situated
about the centre of the
v \ * aboral surface, sometimes
surrounding a centro-dorsal
plate ; sometimes more or less
fused together. The spines are
exceedingly fine and hair-like,
those of the dorsal surface ciliated.
Sphseridia and pedicellariaB are
usually present, but clavulae are
absent. The dorsal podia are
flattened and respiratory. A n
" Aristotle's lantern " with teeth is
present, as in the globular forms,
but often much simplified.

In the Holothuroidea the body
is more or less elongated in the
direction of the axis joining mouth
with anus, which are placed at
opposite (anterior or oral, and
posterior aboral or anal) extremities
of the body. The shape is some-
times completely cylindrical, some-
times five-sided ; in many there
is more or less dorso- ventral com-
pression, and the dorsal and ventral
surfaces may differ greatly from one another. A flattened sole-
like ventral surface bearing the three rows of tube-feet of the
trivium is, as already stated, often distinguishable : it is most
distinctly developed in Psolus and allied
genera. In some Holothuroids the surface
is enclosed in an armour of close-fitting
plates ; but in the vast majority the body-
wall is comparatively soft, being strength-
ened merely by a great number of minute
ossicles of a variety of shapes. In Synapta
(Apoda) numerous minute anchor-like
spicules, each connected with a latticed
plate, project from the surface, and cause
the animal to adhere to soft bodies with
which it comes in contact. In the pelagic
Pelagothuria and in Rhabdomolgus, as Hertwig's Lehrbuch.)

FIG. 345.— Hemipneustes radiatus.
A, aboral, and B, oral surface. C, apical
plates. (From Bronn's Tierreich.) •

FIG. 346.— Clypeaster sub-
depressus, view of aboral
surface showing the peta-
loid ambulacra. (From

IX

PHYLUM ECHINODERMATA

415

already mentioned,
there are no hard parts
of any kind. Around
the mouth is a whorl of
tentacles — pinnate,
shield-shaped, or arbor-
escent. The tube-feet
are sometimes entirely
absent. When present
they are usually uni-
form in character
throughout, and may be
arranged in five regular
longitudinal rows, o r
scattered over the entire
surface. Sometimes, as
has already been stated
in the account of Colo-
chirus, the tube-feet of
the dorsal and even
some of those of the
ventral surface may
assume the form of
papillae. In the Elasi-
poda the tube-feet of
the dorsal surface are
remarkably modified,
taking the form of
greatly elongated pro-
cesses.

In the Crinoidea
the general shape is
that which has been
described in the case of
the Feather-star — star-
like, with a central disc
and a series of radiat-
ing arms, which usually
branch dichotomously.
In the stalked forms
(Fig. 347) a stalk, con-
sisting of a row of elon-
gated ossicles connected
together by bundles
of ligamentous fibres,
attaches the animal to
the sea-bottom. Along

FIQ. 347. — Metacrinus interruptus.
(After P. H. Carpenter.)

416 ZOOLOGY SECT.

some of the joints of the stalk are usually arranged a number of
slender, many- jointed appendages — the cirri. At its base the
stalk usually breaks up into a number of root-like processes ; distally
it becomes continuous with the central disc. The ossicles forming
the skeleton of the central disc are the basals and the radials : with
the latter articulate externally the brachials, a single row of which
gives support to each of the arms and its branches, while similar rows
of smaller ossicles support the pinnules — the lateral appendages
which fringe the arms in a double row. In the free forms the
stalk is absent in the adult condition, though present in the larva,
and from its terminal ossicle and other neighbouring plates is
formed by coalescence a plate — the centro-dorsal ossicle of the
disc. To the centro-dorsal ossicle are attached whorls of many-
jointed, slender, curved cirri.

The mouth in all the Crinoidea, with one exception (Actinometra),
is situated in the centre of the oral (upper) surface, and the anus
in all, with the same exception, is eccentric and inter-radial.
Running outwards from the mouth are a series of very narrow
ambulacral grooves, one of which extends along the oral surface
of each arm, giving off branches to the arm-branches and to the
pinnules. Bordering the ambulacral grooves and their branches
are a pair of rows of short tubular tentacles, which correspond
morphologically with the tube-feet of the other classes, but are
devoid of the terminal suckers, and are not locomotor, but probably
sensory and respiratory in function.

The ccelome in the Echinoderms is a wide cavity of entero-
ccelic origin lined by a ciliated ccelomic epithelium and containing
a corpusculated fluid. Prolongations of it pass out into the rays,
and, in the Ophiuroidea and Asteroidea, between the layers of the
body- wall. In the Crinoidea it contains numerous strands of
connective tissue. Special organs providing for respiration and
excretion through the medium of this fluid are the dermal branchiae
or papulce, the Stewart's organs, and the respiratory trees. The first
of these, which are confined to the Asteroidea and Echinoidea,
have been described in the accounts of the Starfish and Sea-urchin.
In most Asteroidea they occur only on the dorsal surface, but in
some forms they are present on the ventral surface as well. In
some of the Echinoids the place of dermal branchiae in providing
for the respiration of the compartment of the ccelome enclosing
Aristotle's lantern (lantern-ccelome) is taken by Stewart's organs,
simple or arborescent bodies which project inwards from the peri-
stome. The respiratory trees are referred to below in connection
with the enteric canal.

Some reference has already been made, in describing the general
form of the body, to the ambulacral system of vessels. A
ring-like circum-oral vessel (ring-vessel) in nearly all cases sends off
a series of radial branches, one passing along each of the rays or

ix PHYLUM ECHINODERMATA 417

ambulacral areas and giving off branches to the ampullae of the
tube-feet or to the tentacles. In most of the Holothuroidea
branches pass forwards to the circlet of shield-shaped or branched
oral tentacles, and in some cases there are vesicles or ampulla at
their bases. In the Apoda, in which tube-feet are wanting, radial
vessels are also absent, and the vessels to the tentacles come off
directly from the ring-vessel. In all the classes, except Crinoidea,
one or more bladder-like appendages — the Polian vesicles — are in
most cases connected with the ring-vessel. The racemose vesicles,
or Tiedemann's vesicles (p. 377), are characteristic of the Asteroidea.
In all, except the Crinoidea and the majority of the Holothuroidea,
there is a communication between the ring- vessel and the surround-
ing water through the madreporic canal. In the Asteroidea, and in
Cidaris among the Echinoidea, the wall of this tube is strengthened
by numerous calcareous ossicles. In the Asteroidea, Ophiuroidea,
and Echinoidea the communication with the exterior is through
the madreporite. The fine pores perforating the madreporite
and placing the madreporic canal in communication with the
exterior, and the madreporic canal itself, are lined with strong cilia
which move so as to drive a strong current inwards— the effect
being to keep all parts of the ambulacral system in a condition of
turgidity. In the few Holothuroids in which such a communication
exists (Elasipoda) there is usually a simple opening, but sometimes
a number of pores crowded together. In the remainder of the
Holothuroidea the distal end of the madreporic canal, or canals,
lies free in the interior of the body-cavity, with which it is placed
in communication by a number of perforations. In the Crinoidea
there is no madreporic canal ; but the ring- vessel is placed in
communication with the ccelome by means of a system of ciliated
water-tubes, while the ccelome communicates with the exterior
through a number of minute water-pores, which perforate the
oral body-wall. The fluid contained in the ambulacral system
is similar to that in the coelome, and contains similar corpuscles.
In one Ophiuroid, however, the ambulacral system contains
corpuscles coloured red with haemoglobin. Tiedemann's vesicles
appear to have the function of manufacturing the corpuscles.

It cannot be definitely stated that a blood-vascular system
exists in the Echinoderms. But two systems have been regarded
as playing the part of blood-vessels — the perihcemal system and
the hcemal system. Neither of these systems comprises vessels
with contractile walls, and there is no definite circulation of the
contained fluid. The perihsemal or, as it is sometimes termed,
pseudohaemal system, is present in all the classes of the phylum.
When typically developed (Asteroidea, Ophiuroidea) it consists of
a ring-like circum-oral vessel or sinus and five radial vessels given
off from it, together with an axial sinus and aboral ring-vessels.
These " vessels " are channels with a definite epithelial lining,

VOL. I. E E

418 ZOOLOGY SECT

and are of the nature of specialised parts of the coelome, from which
they are developed. In Asteroidea and Ophiuroidea the radial
and ring-vessels, which he between the corresponding parts of the
ambulacral and epidermal nervous systems, are divided into two
parts by a longitudinal septum, vertical in the radial, oblique in
the ring-vessel. The axial sinus is nearly vertical in direction and
partly encloses the axial organ in the way already described (p. 378).
At its oral end it opens into the inner division of the circum-oral
vessel : at its aboral end it opens into, or becomes closely applied
to, the aboral vessel, which is in the form of a ring giving off radial
branches towards the gonads : it may also communicate aborally
with several of the pore-canals of the madreporite, and opens into
the madreporic canal itself. In the Echinoidea the arrangement
of the parts is modified in certain important respects. An oral
ring-sinus is absent unless it be represented by the lantern-ccelome.
The radial vessels of the system do not open orally into the lantern-
ccelome : aborally they also terminate blindly, not opening into the
aboral ring-sinus. The axial sinus is largely encroached upon by the
axial organ : it terminates blindly at the oral end ; aborally it com-
municates with the madreporic canal and is not connected with the
aboral sinus. In the Holothuroidea there are five radial sinuses
extending through the ambulacral areas between the superficial
radial nerve and the radial ambulacral vessel, ending blindly
aborally and opening orally into an oral ring-sinus. There is no
axial sinus. In the Crinoidea the perihaemal system is greatly
reduced, though representatives of the radial sinuses are present in
the same situation as in the other classes.

The general disposition of the lacunar or so-called haemal
system in the Asteroidea has been described in the account given
of the structure of the Starfish (p. 373). Save for certain minor
alterations which are involved in the change in the position of the
madreporite, the system is arranged in the Ophiuroidea on the
same plan as in the Asteroidea. In the Echinoidea there is an
oral ring giving off five radial strands which in the greater part of
their course occupy the typical position between the superficial
radial nerve and the radial ambulacral vessel ; aborally they
terminate blindly. A gastro-intestinal system given off from the
oral ring is highly developed, and there are an axial plexus in the
axial organ and an aboral ring, with strands passing to the gonads,
as in the Asteroidea. In the Holothuroidea there is an oral rirlg
with radial strands, and a well-developed gastro-intestinal system.
In the Crinoidea this system of lacunae is highly developed and
complicated in arrangement.

Whatever be its functions, this system is not a system of blood-
vessels. It is made up of strands of a kind of gelatinous connec-
tive tissue, with many leucocytes, permeated in a very irregular
way by minute lacunae without definite walls. The great develop-

ix PHYLUM ECHINODERMATA 419

ment of the gastric and intestinal branches of this system in some
(Echinoids, Holothuroids) lends support to the view that its main
functions may be connected with the absorption and distribution
of nourishment.

The axial organ (genital stolon) of the Echinodermata is
closely connected both with the perihsemal and haemal systems.
Its general structure and relations in the Asteroidea have already
been described (p. 378). In the Ophiuroidea there is a close
correspondence with the Asteroidea, the chief differences being
such as are involved in the change in the position of the madreporite
from the aboral to the oral surface, and the resulting change in the
direction of the madreporic canal and associated axial sinus and
axial organ. In the Echinoidea the essentials are the same ; but
the axial organ has grown round the axial sinus so as to enclose it
completely.

The enteric canal varies in the five classes more than any
of the other systems of organs. It is a simple tube in the Holo-
thurians and Echinoids, passing spirally through the body from the
mouth at the oral pole to the anus at the opposite pole. In most
of the latter group a complex masticatory apparatus with five
teeth — the so-called " lantern of Aristotle " — is situated at its
anterior extremity ; the corresponding region in the Holothurians
is surrounded by a circlet of ossicles, which protect the nervous
and vascular rings and into which the longitudinal muscles of the
body-wall are inserted.

In the Echinoidea there is a tubular caecum, the siphon, con-
nected with the intestine. In the Holothurians the so-called
" respiratory trees " (absent in the Elasipoda and the Apoda) are
branched appendages — usually two in number, sometimes single
— of the cloaca or posterior wider portion of the intestine, and
the " Cuvierian organs " are simple filiform glandular tubes, also
connected with the cloaca.

The functions of the siphon and of the respiratory trees have
already been referred to in the accounts of Echinus and Cucumaria.
The Cuvierian organs, which occur only in a limited number of
Holothurians, correspond to undivided basal branches of the respira-
tory trees : they are defensive organs, the animal when attacked
throwing out numbers of these filaments, the secretion of which is
very viscid and assumes the character of slender threads which
may have the effect of entangling and hampering the assailant ;
but they may also have an excretory function.

In the Crinoidea the alimentary canal is simply a coiled tube
with both mouth and anal opening on the same (actinal) surface
of the body. In the Ophiuroids the central mouth leads into a
simple sac giving off short diverticula, and there is no anal
aperture. In the Asteroidea the alimentary canal is more complex
than in the other classes. The stomach is divided, as already

E E 2

420 ZOOLOGY SECT.

described in the account of the examples, into two portions, the
cardiac and the pyloric, the former giving ofi five large rounded
radial diverticula — the cardiac pouches or cardiac caeca, and the
latter five pairs of very long branched diverticula — the pyloric
or hepatic caeca. The intestine is short and conical, and opens, in
all but a few, by an anal aperture. In some Asteroidea (as in
Anthenea, Figs. 313 and 315) the intestine has connected with it
a system of five elongated bifurcated inter-radial intestinal caeca ;
in others (as in Asterias, Fig. 311) these are represented only by
two or three lobed diverticula. In one member of the class there
are also ten caeca connected with the oesophagus.

In the nervous system of the Echinodermata three distinct
parts, the relative development of which differs in the different
classes, are to be recognised. These are the epidermal or super-
ficial, the deep, and the coelomic or aboral. The epidermal system is
well developed in all the classes : its principal parts are a circum-
oral nerve-ring and radial branches, but a plexus of nerve-fibres
with occasional nerve-cells extends from it through the epidermis.
In the Ophiuroids the radial nerves and the ring nerve are similar
in their arrangement to what is to be observed in the Asteroids,
but are more deeply placed, being covered over by the investing
calcareous plates. The deep-lying nervous system is absent in
the Crinoidea, very feebly developed in the Echinoidea, but well
developed in the Asteroidea, Ophiuroidea, and Holothuroidea.
Its general arrangement has already been described in the account
of the Starfish. The aboral system is best developed in the Crinoidea
and is absent altogether in the Holothuroidea.

The sexes are distinct in all the Echinoderms, with one or two
exceptions ; but there is very rarely any trace of sexual dimorphism.
Asterina gibbosa, the Starfish the development of which has been
described (p. 381), is one of the exceptional hermaphrodite forms ;
the young animals of this species are male, producing sperms, but
at a later stage they become female and produce only ova. In the
family Synaptidae of the Apoda there are also numerous examples
of hermaphroditism, the animal at first producing ova, later only
sperms. In Amphiura squamata, an Ophiuroid, both ovaries and
testes are present at once. The gonads, ovaries or testes as the
case may^be, are branching bodies, inter-radial in position, and
usually in pairs. In the Asteroidea in general there are five pairs,
the ducts from which open usually on a special plate on the aboral
surface, but in one or two species on the oral surface. In Pentaceros
and some other genera the gonads are more numerous. In the
Echinoidea there are five ovaries or testes, the five ducts of which
open on the genital plates of the apical system. In the Ophiuroidea
there are ten pairs of gonads or groups of gonads, a pair in the walls
of each of five pair,s of genital bursce, which open on the exterior by
slits on the oral surface close to the mouth. In the Holothuroidea

ix PHYLUM ECHINODERMATA 421

there is only a single branched gonad, sometimes imperfectly
divided into two, with a duct opening on the dorsal surface not
far from the mouth. In the Crinoidea the ovaries and testes occupy
a remarkable position, being situated in the dilated bases of the
pinnules ; but, as in the other classes, they are connected by means
of a genital rachis running through the arm with a centrally situated
genital stolon (axial organ}.

Development and Metamorphosis. — A few of the members
of each class of Echinoderms are viviparous, in the sense that the
development of the young takes place in some sheltering cavity,
or brood-pouch, on the surface of the body of the parent. But in
most, development takes place externally, and the larvae are free-
swimming. The ovum in all undergoes regular and nearly equal
segmentation, resulting in the formation of a ciliated blastula,
which becomes invaginated so as to form a typical gastrula,
like that of some Ccelenterata. The invaginated cells form the
lining membrane (the endoderm layer) of an internal cavity — the
primitive alimentary cavity or archenteron ; the enclosing cells
form the ectoderm ; between the endoderm and ectoderm, and
derived from the former, appear the cells of the mesoderm or middle
layer. From the archenteron is given off a hollow outgrowth, the
enterocoele, from which are derived the body-cavity with its enclosing
peritoneal membrane, and the vessels of the ambulacral system with
their various appendages. In the Crinoidea the vesicle destined
to form the ambulacral system is developed independently of the
coelomic vesicles destined to form the body-cavity. A canal
opening on the exterior by a dorsally situated opening, the dorsal
pore (sometimes double), is formed by invagination from the surface
ectoderm, and comes into relation with a canal arising as an out-
growth from the rudimentary ambulacral system to form the
foundation of the madreporic canal of the adult. In the Crinoidea
five dorsal pores and five canals are developed, but the two sets of
structures do not enter into direct communication (see p. 399).

The part of the enterocoele (hydroccele) destined to give rise to
the ambulacral system, at first rounded, becomes compressed, and
subsequently divided round the border into five lobes. Each of
these lobes grows outwards to become developed subsequently into
one of the five radial ambulacral vessels of the Echinoderm ; the
central part of the hydroccele gives rise to the ring-vessel surrounding
the oesophagus.

The cilia, which at first (in the gastrula stage) covered the surface
of the larva uniformly, become restricted to a peri-oral band
(Fig. 348, 1, 2 por) surrounding a concave area on which the mouth
opens. A smaller adoral band (1, 7, aor) in the interior of the
mouth has the function of attracting nutrient particles. The
peri-oral band undergoes characteristic changes in the different
an<l I lie form of the larva at the same time becomes

422

ZOOLOGY

SECT

modified by the formation, except in the Crinoidea, of variously
arranged processes along its course. The resulting larva, echino-
pcedium or dipleurula, always exhibits marked bilateral symmetry.
It has a pre-oral lobe on which an apical plate comparable to that
of the trochophore (p. 316) may be developed.

FlQ. 348. — Diagram,-} of the development of the larva) of Echinoderms., 1, Primitive form
of Echinoderm larva ; 2 and:}, Development of an auricularia (Holothuroidea) ; 4, f>, and «,
Development of a hiphinaria ( Asteroidea) ; 7, 8, and 9, Development of a plutewt (Echi-
noidea and Ophinroidea). aor. in 1 and 7, adoral band of cilia, in 4, 5 and «. pro-oral loop ;
uli. alimentary ranal ; an. anus ; b, b. processes or arms ; mo. mouth ; por. in J, 2, 7 and i>,
peri-oral ciliated band and processes, in 3, 4, 5 and (>, itust-oral loop. (From Leuckart and
Nitsche's Diagrams.)

PHYLUM ECHINODEEMATA

423

In the Asteroidea the larva is either a bipinnaria (Fig. 348,
4 to 6) or a brachiolaria. The former has a series of bilaterally
arranged processes or arms ; the latter has, in addition, three
processes not developed in the course of the ciliated band and
used for fixation. The larva of Asterina, the development of which
has been described and illustrated on pp. 380-385, is a greatly
modified bipinnaria with the pre-oral lobe large and eventually
serving as a stalk, and the band of cilia confined to the edge of the
larval organ and devoid of the bilateral processes of the normal
bipinnaria. In at least one form the bipinnaria, developed in a
brood-pouch, adheres to the parent by means of the pre-oral lobe
which takes the form of a short stalk. In general the bipinnaria is
free-swimming and has a large pre-oral
lobe, the part of the ciliated band borne
on which becomes separated off as a
pre-oral loop (aor) from the rest or post-
oral loop (por). In the course of both
of these loops are the variously-
arranged paired processes. In both the
Ophiuroidea and the Echinoidea (Fig.
348, 7 to 9) the larva has the form
which is known as the pluteus. The
pluteus has a number of slender arms
directed forwards and supported by a
skeleton of delicate calcareous . rods :
the pre-oral lobe is reduced and the
ciliated band is undivided. The larva
of the Holothuroidea, the auricularia
(2 and 3), has a number of short pro- FIG 349._Free_9wimming Iarva 0

Cesses developed in the COUrse of the Antedon, from the left side.

ciliated bands ; subsequently, in the
pupa stage, the ciliated bands become
broken up into a series of ciliated
hoops encircling the body. Of the
Crinoidea the development of Antedon alone is known.
Blastula and gastrula stages occur as in the Starfish, but the
history of the archenteron and its diverticula is widely differ-
ent, though the outcome is the same — viz., the differentiation
of a primitive enteric canal, an anterior coelome, from which a
hydroccele becomes separated off, and a pair of ccelomic sacs.
The larva (Fig. 349) becomes barrel-shaped, and the pre-oral lobe,
which is not very conspicuous, develops an ectodermal thickening
with a tuft of sensory cilia. The vibratile cilia on the surface are
arranged in five transverse bands (7-F). Between the second and
third of these is a wide shallow depression, the vestibule or
stomodaeum (1), which does not communicate with the mouth.
After remaining in the free condition for a short time, the larva'

I-V, ciliated bands ; ba\ to ba5,
the five basals ; or\ to or5, orals ;
1, vestibule ; 2, intestinal vesicle ;
3, right enterocoele ; 4, calcareous
joints of the stalk ; 5, pedal plate.
(From Lang, after Seeliger.)

424

ZOOLOGY

SECT.

(Fig. 350) fixes itself by means of the pre-oral lobe, which elongates
into a stalk (11), the cilia meanwhile being lost, and the apical
plate absorbed. The vestibule becomes closed, and a solid rudiment
of the adult oesophagus arises in close apposition with it. Round
.the oesophagus the hydrocoele grows in the form
of a ring. The vestibule (5) with the oesophagus
and hydroccele are rotated so as to come to lie
at the free extremity. The radial canals first
appear as five tentacles which at first project into
the cavity of the vestibule, and subsequently—
when the latter opens out, as it soon does — on
the exterior. The oesophagus (8), meanwhile,
has become completed, and the mouth pierces the
bottom of the now open vestibular cavity. The
arms appear as five processes which soon bifur-
cate : the five radial canals become applied to
them and undergo a corresponding division.
The first plates are formed while the larva
is still in the free condition ; in the fixed con-
dition they undergo further development, and
extend into the arms as they grow. After
about six months this pentacrinoid larva be-
comes free by the absorption of the stalk and
develops into the adult Antedon.

In the transition from the bilateral
larva — pluteus, bipinnaria, brachio-
laria, or auricularia — to the radial
adult there is a marked metamorphosis.
As the adult form is developed on one
side of the larva, with its principal
axis at right angles to that of the
latter, the larval arms or processes
become absorbed. In the Holothu-
roidea and Ophiuroidea all the organs

FiG.sso.-staiked larva of Antedon, of, the larva are carried on into the
from the right side ; calcareous adult \ in the Asteroidea and Echinoidea

plates not represented. 1, right ,-1 -i ,-i

co3iomic sac ; 2, enteric cavity ; 3, tne larval moutn and. ossopnagus are

abolished and a new permanent mouth
and o^ophagus formed as a fresh in-
axial organ ; ii, fibrous strands in vagination from the surface. In the
(From Lang' after very limited number of Echinoderms
which are viviparous there is no such
marked metamorphosis ; but even in these the larva is at first
distinctly bilateral in its symmetry.

Ethology, etc. — The Echinodermata are without exception l
inhabitants of the sea. In the adult condition the majority creep
1 One species of Synapta is said to inhabit brackish water.

ix PHYLUM ECHINODERMATA 425

on the sea-shore or on the sea-bottom, the stalked Crinoids being
exceptional in their permanently attached condition ; but the
larvae of the great majority are pelagic — i.e. live swimming in the
upper strata of the ocean.

Echinoderms inhabit all depths of the sea, ranging from the
shore between low and high water limits to the greatest depths.
Members of all the classes are found at all depths ; but the stalked
Crinoids and the Elasipoda among the Holothuroidea are virtually
confined to the deepest waters of the ocean, only one genus of the
former and one species of the latter occurring in comparatively
shallow water. Echinoderms are found in the seas of all parts of
the globe. Like the majority of marine invertebrate groups, the
phylum is more abundantly represented, as regards the number of
genera and species as well as of individuals, in the warmer regions ;
the Crinoidea, the Holothuroidea and the Echinoidea are all much
more abundant in tropical and warm temperate seas than in colder
latitudes.

Echinoderms are of gregarious habits, large numbers of the
same species frequently being found closely associated together in
a comparatively narrow area. The movement of locomotion in
the Starfishes is, as previously described (p. 370), a slow creeping
one, through the agency of the tube-feet : the same holds good of
the Echinoidea and those of the Holothuroidea that possess tube-feet
(Pedata). The footless Holothurians (Apoda, such as Synapta)
creep along with the help of the tentacles. Most of the Ophiuroids
move by lateral flexions, sometimes sluggish, sometimes remark-
ably rapid, of the arms. The Comatulae, on the other hand, swim
along by the flexion and extension of the pinnate arms propelling
them through the water. Many Asteroids, Ophiuroids, and
Echinoids bury themselves in sand or mud ; others creep into
narrow fissures in rock or coral. Movements of manducation
are performed by the tentacles in the Holothurians : in the Star-
fishes the mouth papillae are separated from one another and the
cardiac part of the stomach everted in order to enfold the prey,
often of relatively large size. In those Echinoidea that possess a
lantern of Aristotle there are very powerful and efficient move-
ments of mastication. On the whole, as might be expected from
the comparatively highly developed muscular and nervous systems,
the co-ordination of movement is very much more complete in the
Echinodermata than in the groups already dealt with.

A remarkable characteristic of the Echinoderms is the faculty
of self-mutilation which many of them possess, together with the
capacity for replacing parts lost in this way or by accidental
injury. This is most marked in many Ophiuroids, some Asteroids,
and some Holothurians, and does not occur at all among the
Echinoids. Many Brittle-stars and some Starfishes, when removed
from Mif \\.-if PI, or when molested in any way, break off portions of

426 ZOOLOGY

SECT.

their arms piece by piece until, it may be, the whole of them are
thrown off to the very bases, leaving the central disc entirely bereft
of arms. A central disc thus partly or completely deprived of its
arms is capable in many cases of developing a new set ; and a
separated arm is capable in some instances of developing a new
disc and a completed series of arms. In some Starfishes (Ophiuroids
and Asteroids) a process of separation of the arms and their develop-
ment into complete individuals frequently occurs altogether inde-
pendently of injury, and seems to be a regular mode of reproduction
in these exceptional cases. Many Crinoids, also, readily part with
their arms when touched and are able to renew them again ; and
some, at least, are capable of renewing the visceral sac of the central
disc when it has become accidentally removed.

In the case of many Holothurians it is the internal organs, or
rather portions of them, that are capable of being thrown off and
replaced — the oesophagus, or the cloaca with the Cuvierian organs,
or the entire alimentary canal, being ejected from the body by
strong contractions of the muscular fibres of the body-wall, and
in some instances, at least, afterwards becoming completely
renewed.

Four out of the nine classes of the phylum Echinodermata —
the Cystoidea, Blastoidea, Edriasteroidea, and Carpoidea — are
represented only by fossil forms ; and these are found only in
rocks of the older (Palaeozoic) formations, no representatives
having survived to more recent times. Of the five classes that
have living members, one, the Crinoidea, was very much more
abundantly represented in the older geological periods than it
is at the present day, the remains of stalked Crinoids forming
great beds of limestone of Silurian to Carboniferous age : the
free Comatulse only appeared at a much later period. The other
classes, or at least the Echinoidea, Asteroidea, and Ophiuroidea,
were represented at a very early period by forms not very widely
different from those now living ; but the earliest Echinoids were
peculiar in having the number of rows of plates variable, and in the
plates overlapping one another. The Holothuroidea, owing to
their comparatively soft integument, were less fitted to leave any
remains in the form of fossils, and it is not till we come to the Meso-
zoic Period that undoubted traces of their existence are found.

Affinities. — The presence of radial symmetry was formerly
regarded as involving a near relationship with the Ccelenterata,
which were grouped with the Echinodermata under the comprehen-
sive class-designation of Radiata (see section on the History of
Zoology). But, leaving out of account the presence of a bilateral
symmetry underlying and partly concealed by the radial, we are led
by a study of the anatomy of the various systems of organs to the
conclusion that the Echinoderms are in no way closely or directly
related to the Ccelenterates. One very great and very important

ix PHYLUM ECHINODERMATA 427

difference between the two phyla consists in the presence in the
Echinodermata of an extensive ccelome or body-cavity lined by
mesodermal epithelium between the alimentary canal and the
body-wall. In addition to this the Echinoderms are characterised
by the possession of highly elaborated systems of organs — alimen-
tary, vascular, and nervous — such as occur in none of the Ccelen-
terates, all of which exhibit extreme simplicity in their internal
structure. A further point of difference, not perhaps of so much
importance, is the absence in the Echinoderms of any tendency to
form colonies of zooids by asexual multiplication by means of buds :
all Echinoderms are simple, i.e. non-colonial, animals, and each of
them is developed, save in certain very exceptional cases, as a
result of a sexual process from an impregnated ovum. In spite,
then, of the radial symmetry, we are forced to the conclusion that
the Echinodermata are not more nearly related to the Ccelenterata
than to some of the groups of Worms. They are, in fact, a singularly
isolated group, and we look in vain among the known members,
living and fossil, of other phyla for any really close allies. The
intermediate forms — whatever they may have been like — between
the Echinoderms and other groups have become extinct, and have
left no remains in the form of fossils, or such remains have not yet
been discovered. So difficult has it been found to connect the
Echinoderms with other animal types that it has even been proposed
to regard an Echinoderm as a radially arranged colony of zooids
connected together centrally, each ray being a zooid equivalent
to an entire simple worm-like animal. But the history of the
development is entirely at variance with such a view.

Whatever may have been the group of animals from which the
Echinodermata were developed, there is every probability that it
was a group with bilateral and not radial symmetry. The radial
symmetry is evidently, as has already been pointed out, of a
secondary character ; it is only assumed at a comparatively late
period of development, and even in the adult condition it does
not completely disguise an underlying bilateral arrangement of
the parts. Accordingly, within the phylum itself, it is reasonable
to regard those classes as the more ancient which have the radial
symmetry less completely developed. Again, the free condition
which characterises all existing Echinoderms, with the exception
of a few Crinoids, is probably less primitive than the attached,
since in other phyla the radial symmetry is co-ordinated with,
and seems to be developed on account of, a fixed, usually stalked
condition. Probably, then, stalked Echinoderms were the pro-
genitors of the existing free forms, and these were preceded by
primitive free forms with pronounced bilateral symmetry. It
appears to be most probable that this ancestral form possessed the
most essential features of the diplewrula larva (p. 422) ; i.e., that
it was a bilaterally symmetrical form with a pre-oral lobe, simple

428

ZOOLOGY

SECT. IX

alimentary canal with mouth on ventral surface and anus at
posterior end ; that it had a ccelome, originally developed from the
archenteron of the gastrula ; and that it had a band of strong
cilia running around the concave ventral surface. Such a
dipleurula-like form became converted, it is supposed, into a fixed
form, such as that represented by some of the extinct class of the
Cystoidea. The fixation must be supposed to have become
effected through the medium of the pre-oral lobe, and further
changes must have involved the shifting of the mouth to about
the middle of the free surface. From this primitive Cystoid, thus
regarded as the most primitive of all known Echinoderms, the
remaining classes, both fixed and free, might have been derived by
some such order of succession as that indicated in Fig. 351.

!!olothuroidea

Echinoidea

Asteroidea

Primitive Cystoid

Dipleurula
Flo. 351.— Diagram to illustrate the relationships of the classes of the Echinodermata.

According to another view, however, the most primitive of
existing Echinoderms are Synapta and its allies (Holothuroidea
apod a). The other Holothuroids are supposed, according to this
conception of the relationships of the various classes, to have been
derived from a Synapta-like ancestor. From the primitive stock of
the Holothuroids is supposed to have been derived a form which gave
origin to all the stalked classes. From this ancestral stalked Echmo-
derm, again, the remainder of the free classes — the Echinoidea,
Asteroidea, and Ophiuroidea — are regarded as having been descended.

Possible relationships between the Echinodermata and the
Chordata will be referred to in th* discussion of the affinities of
the latter phylum.

SECTION X
PHYLUM ANNULATA

THE phylum Annulata comprises six classes of Worms — the
Chcetopoda or Earthworms and marine Annelids, with the Myzo-
stomida and Echiurida, the Archi-Annelida, the Sipunculoidea, and
the Hirudinea or Leeches. These in general have the elongated
body divided externally into a number of rings, which represent a
division of the internal parts into a series of segments or metameres.
There is usually an extensive coelome, and there is in most a system
of blood-vessels. The nervous system consists in most cases of a
cerebral ganglion, cesophageal connectives, and a double ventral
nerve-cord, which is segmented into a series of ganglia. The organs
of excretion are in the form of metamerically arranged pairs of
tubes, the nephridia or segmented organs, closed internally or leading
from the coelome to the exterior ; and united with these, or distinct
from them, are a series of paired ducts, the ccehmoducts, for the
passage outwards of the reproductive elements.

CLASS I.-CHJETOPODA.

The Chaetopoda, comprising the Earthworms, Fresh- Water
Worms, and Marine Annelids, are Worms the body of which,
unlike that of a Flat- worm or a Round- worm, is made up of a series
of more or less completely similar segments or metameres, each
containing a chamber or compartment of the body-cavity and a
section of the alimentary canal and other organs. At the sides
of each are typically a pair of muscular processes, the parapodia,
which do duty as limbs, bearing bundles of setce (chcetce) or bristles,
and usually also certain tactile appendages, the cirri. There
is an extensive ccelome, incompletely divided into a series of
chambers corresponding to the segments by a series of muscular
partitions which act also as mesenteries, being attached internally
to the alimentary canal. The latter extends throughout the
length of the body ; the intestine is usually constricted, the con-
strictions -being either segmental, i.e. opposite the middle of the
segments, or inter-segmental, i.e. opposite the intervals between
the segments. There is a well-developed blood- vascular system

420

430

ZOOLOGY

SECT.

B

in the majority of the Chaetopoda, and organs of respiration in
the shape of gills or branchiae are usually developed. The
excretory organs are in the form of segmentally arranged pairs of
tubes, the nephridia. The nervous system consists of a bilateral
principal ganglion or brain situated in the prostomium, and a
double chain of ganglia extending throughout the body. The
sexes are in some distinct, in others united. When a definite
larval form occurs it is a trochophore.

1. EXAMPLES OF THE CLASS.
- a. Nereis dumerilii.1

General External Features. — Various species of Nereis
occur abundantly between tide-marks on the sea-shore, under stones,

and among sea-weed, in all parts of
the world. The worm varies con-
siderably in colour even in the same
species, the differences being partly
due to differences in the stage of
development of the sexual elements.
In N. dumerilii the prevailing colour
is some shade of violet, with a blush
of red in the more vascular parts
due to the bright red colour of the
blood. In shape (Fig. 352) the
body, which may be about 7 or 8
centimetres in length, is long and
narrow, approximately cylindrical,
somewhat narrower towards the
posterior end. A very distinct head,
bearing eyes and tentacles, is recog-
nisable at the anterior end ; the rest
is divided by a series of ring-like
narrow grooves into a correspond-
ing series of segments or metameres,
which are about eighty in number
altogether ; and each of these bears
laterally a pair of movable mus-
cular processes called the parapodia,
provided with bundles of bristles or
setoB (chcBtw). The head (Fig. 355)
consists of two parts, the pros-
tomium (prcest) and the peristomium (perist.). The former bears
on its dorsal surface four large rounded eyes, in front a pair of

1 Though Nereis dumerilii is here named as the example, and the majority
of the figures refer specially to that species, the description given would apply
almost equally well to a considerable number of species of the genus.

FIG. 352.— Nereis dumerilii,

natural size. A, Nerc-ix phase;
B, Heteroncreis phase. (After
Claparfede.)

PHYLUM ANNULATA

431

noto

rwuro

vent.cirr

FIG. 353. — Nereis dumerilii. A single para-
podium, magnified : ac. aciculum ; dors. cirr.
dorsal cirrus : neuro. neuropodium ; noto. noto-
podium ; vent. cirr. ventral cirrus. (After
Claparede.)

short cylindrical tentacles (tent), and further back a pair of

somewhat longer stout appendages or palpi (palp). The peri-

stomium, which has some

resemblance to the seg-
ments of the body, though

wanting the parapodia, bears

laterally four pairs of long,

slender, cylindrical tentacles

(perist. tent) : on its ventral

aspect is a transversely

elongated aperture, the

mouth. The segments of the

body differ little in external

characters from one another

throughout the length of the

worm. Each bears laterally a pair of parapodia, which in the

living animal ar<> usually in active movement, aiding in creeping,
or acting as a series of oars for propelling it
through the water. When one of the parapodia
(Fig. 353) is examined more attentively it is
found to be biramous, or to consist of two dis-
tinct divisions — a dorsal, which is termed the
notopodium (noto), and a ventral, which is called
the neuropodium (neuro). Each of these is
further subdivided into several lobes, and each
bears a bundle of setae. Each of the bundles
of setae is lodged in a sac formed by invagina-
tion of the epidermis — the setigerous sac — and is
capable of being protruded or retracted and
turned in various directions by strands of
muscular fibres in the interior of the para-
podium. In each bundle there is, in addition
to the ordinary setae, a stouter, straight dark-
coloured seta (ac.), the pointed apex of which
projects only a short distance on the surface ;
this is termed the aciculum. The ordinary setae
(Fig. 354) are exceedingly fine, but stifiish,
chitinous rods, of which two principal kinds are
recognisable : both have a terminal blade arti-
culating with the main shaft of the seta by a
distinct joint ; but in the one variety the shaft

Flmer\iiT7Isetoihighiy of the seta is finer than in the other, and the

Salp8ar1edtie)e.d' (^ieT terminal blade long, slender, and nearly

straight, whereas in the other variety it is

short and slightly hooked. On the dorsal side of the parapodium

is a short cylindrical, tentacle-like appendage, the dorsal cirrus

(Fig. 353, dors, cirr), and a similar, somewhat shorter appendage,

432 ZOOLOGY SECT.

the ventral cirrus (vent, cirr), is situated on its ventral side. The
last segment of the body, the anal segment, bears posteriorly a small
rounded aperture, the anus ; this segment is devoid of parapodia,
but bears a pair of appendages, the anal cirri, similar in character
to the cirri of the ordinary segments, but considerably longer.

On the ventral surface, near the bases of the parapodia, there is
in each segment a pair of very fine apertures, the openings of the
nephridia.

The enteric canal is a straight tube running throughout the
length of the body from the mouth to the anus. Between the
outer surface of this tube and the inner surface of the wall of the
body is a considerable space — the ccelome, body-cavity, or peri-
visceral cavity — filled with a fluid, the ccelomic fluid, containing
amoeboid corpuscles. The walls of the coelome (Fig. 357) are
lined with a thin membrane, the peritoneum or c&lomic epithelium,
of which the outer layer — that lining the body-wall — is the
parietal layer (par. peri), that covering the outer surface of the
alimentary canal the splanchnic or visceral layer (vise. peri). The
space is divided by a series of transverse partitions or septa passing
inwards from the body-wall to the wall of the alimentary canal
opposite the grooves between the segments, and thus dividing the
coelome into a series of chambers, each of which corresponds to one
of the segments. These partitions are not complete, spaces being
left around the alimentary canal and elsewhere through which
neighbouring chambers communicate.

The mouth leads into a wide cavity, the buccal cavity, con-,
tinued back into a pharynx (Fig. 356, ph). These two chambers
extend through the peristomium and the first to the fourth seg-
ments of the body. They are lined with a tolerably thick cuticle,
continuous with a similar layer lining the outer surface of the body,
and in the buccal cavity are a number of very small dark brown
chitinous denticles, which are very regularly arranged. The
posterior part of the pharynx (dentary region) has very thick walls
composed of bundles of muscular fibres, which are concerned
in the movements of a pair of laterally placed chitinous jaws.
Each jaw is elongated in the direction of the long axis of the body,
rounded at the posterior end or base where it is embedded in
muscle, pointed at the apex, which is strongly incurved ; the inner
edge is divided into a number of strong serrations or teeth : the
whole jaw might be compared to a pruning-hook with its cutting
edge deeply serrated (Fig. 355, B).

Behind the pharynx the alimentary canal narrows considerably
to form a tube, the oesophagus (ces), which runs through about five
segments to open into the intestine.

Running backwards and inwards from the wall of the peristomium
to the wall of the buccal cavity and pharynx are a number of
bands or sheets of muscle, the protractor muscles, by the contraction

PHYLUM ANNULATA

433

of which, and the pressure of the coelomic fluid, this anterior part
of the alimentary canal can be everted so as to form a proboscis
(Fig. 355), and thus the jaws are thrust forth and rendered
capable of being brought to bear on some small living animal or
fragment of animal matter, to be seized and swallowed as food.
The eversion is arrested at a certain point by means of a muscular
diaphragm passing from the wall of the buccal cavity to that of
the first body-segment. The proboscis is withdrawn again by a
retractor sheet of muscle, which passes inwards and forwards to be
inserted into the wall of the alimentary canal at the junction of
the pharynx and oesophagus.

Into the oesophagus open a pair of large unbranched glandular
pouches, or cceca (Fig. 356, gl), which probably are of the nature of
digestive glands. The intestine (int) is a straight tube of nearly
uniform character throughout, regularly constricted in each segment
—the constrictions becoming much deeper towards the posterior

P-,

-~jr

.3

FIG. 355. — Nereis diversicolor, x 4. Head with buccal region everted. A, dorsal view ;
B, ventral view, a, prostomium ; B, everted buccal region ; c, c', peristonaial tentacles,
1, 2, 3, 4 ; d, denticles ; e, eyes ; E, lower lip ; P. palp in A, entrance to pharynx in B ;
,/, jaw ; T, prostomial tentacle ; /, peristomium ; 17, parapodium of first body-segment.
(From the Cambridge Natural History.)

end of the body. The part of the intestine which lies in the last
segment is termed the rectum.

The wall of the alimentary canal (Fig. 357) consists (1) of the
visceral layer of the coelomic epithelium (vise, peri) ; (2) of a layer of
longitudinal muscular fibres (long, mus) ; (3) of a layer of circular
muscular fibres (circ. mus) ; and (4) of the enteric epithelium
(ent. ep), consisting of close-set, long, narrow cells. To these
layers is superadded in the buccal cavity and the pharynx an
internal chitinous cuticle, continuous with that of the general
outer surface.

Developmentally the buccal cavity and the pharynx constitute
the stomodceum, the rectum the proctodceum, the rest of the alimen-
tary canal the mesenteron.

The wall of the body consists of a cuticle, an epidermis or
deric epithelium, muscular layers, and the parietal layer of the
coelomic epithelium (par. peri). The cuticle (cut) is a thin chitinous
layer which exhibits an iridescent lustre due to the presence of two

VOL. i. F P

434

ZOOLOGY

SECT.

•^8

intersecting systems of fine lines ; it is perforated by numerous
minute openings, the openings of the epidermal glands. The epi-
dermis (ep) is very thin, ex-
cept on the ventral surface,
where it becomes consider-
ably thickened. It consists
of a layer of cells con-
taining numerous twisted
unicellular glands, which
are most abundant on the
ventral surface, particu-
larly near the bases of the
parapodia ; on the dorsal
surface the epidermis
contains plexuses of fine
blood-vessels. The mus-
cular layers are two in
number — an external, in
which the fibres run cir-
cularly (circ. mus), and an
internal, in which they
run longitudinally. The
latter is not a continuous
layer, but consists of four
bundles of fibres, two
dorso-lateral (dors. long,
mus) and two ventro-
lateral (vent. long. mus).

Nereis has a well-de-
veloped system of vessels
filled with blood of a
bright red colour. A
main dorsal vessel (Figs.
356 and 357, dors, vess)
runs from one end of the
body to the other above
the alimentary canal, and
is visible in places through
the body-wall in the living
animal. It, as well as the
majority of the vessels,

rte.cc

FIG. 356. — Nereis dumerilii. Semi-diagrammatic
view of the anterior portion of the body with the
dorsal body-wall removed, so as to show the ali-
mentary canal, the septa, the blood-vessels, and the
nephridia ; a portion of the intestine removed so as
to show the ventral blood-vessel and nerve-cord
which lie below, dors. vess. dorsal vessel ; gl. ceso -

phageal glands ; int. beginning of intestine ; ne. co. " -,-, ^'A A r rr n P « nnntraH- irvne
nerve-cordl; neph. nephridia ; as. oesophagus ; palp, \ U n Q

palp ; para, parapodia ; peritt. peristome; perist.tmt. * which are of a peristaltic
peristomial tentacles ; ph. pharynx with its jaws ; - ,
prcest. prostomium ; tent, prostomia1 tentacles ; vent. . Character — Waves OI COn-

* traction passing along the

wall of the vessel so as to cause the movement of the contained
blood. These peristaltic contractions are more powerful in the

PHYLUM ANNULATA

435

case of the dorsal vessel than in that of any of the others, and
11111 with great regularity from behind forwards, so as to drive a
current of blood in that direction. The contractions are brought
about partly by a series of muscular fibres which are arranged in
rings round the wall of the vessel at short intervals ; but the wall
of the vessel is itself contractile.

Along the middle of the ventral surface below the alimentary
canal runs another large longitudinal vessel, the ventral vessel (vent,
vess), in which the current of blood takes a direction from before
backwards. Connecting the dorsal and ventral vessels, there are in

ci/v. tnus
& & /cfons.vesa coU

W

dors. tariff rnu&

fros fieri
eht nuitt

7lOt Set

FIG. 357. — Nereis dumerilii. Semi -diagrammatic transverse section of the middle region of
the body. circ. mus. (external), circular layer of muscle of body-wall ; circ. mus. (internal)
circular layer of muscle of wall of enteric canal ; cod. cffilome ; cut. cuticle ; dors. long. mus.
dorsal longitudinal muscles of body-wall ; dors. vess. dorsal vessel ; ent. ep. enteric epithe-
lium ; ep. epidermis ; long. mus. longitudinal muscle of wall of enteric canal ; ne. co. nerve-
cord ; neph. nephridium ; neur. set. neuropodial setae and aciculum with their muscles ;
not. set. notopodial setae and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri.
parietal layer of ccelomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent,
vess. ventral vessel ; vise. peri, visceral layer of ccelomic epithelium. (The entire extent of
the ccelomic epithelium is not represented.)

each segment two pairs of loop-like transverse vessels which give
off branches to the parapodia, the alimentary canal, and neighbour-
ing parts. Some of these branches communicate with plexuses
of fine vessels in the interior of the lobes of the parapodia and in
the integument of the dorsal surface, and with dilatations or sinuses
situated in the bases of the parapodia. A delicate longitudinal
neural vessel accompanies the nerve-cord.

Nereis is devoid of any branchicB ; but there can be little doubt
that the lobes of the parapods with their rich blood-supply, and the

F F 2

436

ZOOLOGY

SECT.

areas of integument occupied by plexuses of blood-vessels, subserve
the function of respiration.

There is a well-developed nervous system (Fig. 358), which is
bilateral and metameric in its arrangement, like the other systems
of organs. Situated in the prostomium is a large bilobed mass
of nerve-matter containing numerous nerve-cells, the cerebral
ganglion or brain (c). This gives off tentacular nerves to the tentacles
and palpi, and two pairs of short thick optic nerves to the eyes.

Behind, two thick
nerve-strands, the
cesophageal connec-
tives (d), curve
round the mouth
in the peristomiurn
to meet on the
ventral aspect be-
hind the mouth
and below the
pharynx. The
oesophageal con-
nectives with the
cerebral ganglion
thus form a ring
around the ante-
rior part of the
enteric canal.
From them are
given off nerves to
the two anterior
pairs o f peristo-
mial tentacles.
Running back-
wards from the
point of union of
the cesophageal
connectives along
the entire length
of the body of the
worm, on the ventral aspect, is a thick cord of nerve-matter, the
ventral nerve-cord (h) . In each segment this cord presents a little dila-
tation from which nerves are given off to the various parts of the
segment ; and each of these enlargements is really double, consist-
ing of a pair of closely-united ganglia. The intermediate parts of the
cord, between successive pairs of ganglia, are also double, consisting
of a pair of longitudinal connectives enclosed in a common sheath.
Given off behind from the cerebral ganglion is a system of fine
nerves with occasional small ganglia, the stoniatogastric or visceral

FIG. 358. — Nereis. — Anterior portion of nervous system, com-
prising the brain (c), the oasophageal connectives (d), and the
anterior part of the ventral nerve-cord(A). (After Quatrefages.)

PHYLUM ANNULATA

437

re

system, distributed to the anterior part of the alimentary canal.
The first ganglion of the ventral cord, which is situated in the
third segment, represents at least two double ganglia which have
coalesced, as is shown by the fact that it gives off nerves to the
two posterior pairs of peristomial tentacles and to the first pair of
parapodia.

The tentacles and palpi, as well as the cirri, are probably organs
of the sense of touch. The only other sense-organs are the four
eyes and the two nuchal organs, all situated on the prostomium.
The eye (Fig. 359) consists of a darkly pigmented cup, the retina
(re.), with a small rounded aperture, the pupil, and enclosing a
mass of gelatinous matter, the lens (L). The wall of the cup is
composed of numerous long and narrow cells lying parallel with
one another in a radial direction. The outer end of each cell
narrows into a
nerve-fibre form- , //'

, * .-i - • — «*-xv*<^-^:-i_^- i \r i

ing part of the
optic nerve ; near
this end is a
nucleus ; the main
body of the cell is
dens ely pig-
mented ; the inner
part pro j ects
towards the lens
as a clear hyaline
rod(r). The cuticle
of the general
surface passes
over the eye, and

a rrmt,iTinAtirm nf FIG. 359. — Nereis. — SectionSthrough one of the eyes. co. cornea
cu. cuticle; J. lens; r. layer of rods ; re. retina. (After Andrews.)

the epidermis,

with its cells somewhat flattened, constitutes the cornea (co).
The nuchal organs consist of a pair of pits lined by a special
ciliated epithelium with gland-cells, situated in close contact with
the posterior part of the brain near the posterior part of the
prostomium on the dorsal side. They are regarded as olfactory
in function.

The organs which are supposed to perform the function of
excretion are a series of metamerically arranged pairs of tubes,
the segmental organs or nephridia (Figs. 356 and 357, neph, Fig.
360) occurring in all segments of the body with the exception
of several at the anterior and posterior ends. The nephridium
consists of two parts — a body and a narrow anterior prolonga-
tion. The body is of an irregular oval shape directed nearly trans-
versely, but slanting somewhat ; the outer end, situated in the
base of the parapodium near its middle, is much the narrower ;

438

ZOOLOGY

SECT.

the inner end is continuous with a narrow prolongation about
equal in length to the body, which runs forwards and inwards
to become attached to the mesentery. The external opening
or nephridiopore (ext. op) is a fine circular pore capable of being
widened or contracted, situated on the ventral surface not far
from the base of the ventral cirrus. It leads into a canal, ciliated
except in its most external part, which runs through the anterior
prolongation to its extremity, where it bends sharply back again and
runs to the body, through which it pursues an extremely tortuous
course to the outer end, and then bends back again and runs in

the anterior prolongation to the
extremity of the latter, where it
opens into the coelome through a
ciliated bell or funnel (fun), the
nephrostome, projecting through
the septum into the cavity of
the segment next in front of that
in which the body of the organ
lies. The edge of the nephrostome
is produced into a number of
narrow ciliated processes not
represented in the figure.
Throughout its course the canal
is excavated in a mass of nu-
cleated material of a granular
character not distinguishable into
cells.

On the dorsal side of each seg-
ment, in close relation to the
longitudinal muscular bundle, is
a specially developed ciliated
tract of the coelomic epithelium
of the nature of a short funnel
without external aperture, the

FIG.360.— Nereis dumerilii. One of the dorsal ciliated Organ. It IS pOS-

sible that at the time of sexual
maturity an aperture is formed
through the body-wall opposite
this funnel, and that thus a genital duct of a temporary character
becomes formed ; but no such opening has ever been observed.

Nereis is unisexual. The sexual elements, ova or sperms,
are formed from temporary masses of cells, ovaries or testes, which
are developed towards the breeding season by a proliferation of
the cells of the membrane (coelomic epithelium) lining the coelome
and the structures it contains. In Nereis dumerilii there is in the
male only a single pair of these proliferating masses of cells (testes),
situated in one of the segments between the nineteenth and the

nephridia. ext. op. external opening or
nephridiopore ; fun. internal funnel or
nephrostome (opening into the coelome ;
mes. transverse mesentery or septum.

x PHYLUM ANNULATA 439

twenty-fifth. But in other species of Nereis they are much more
numerous. These, during the season of their active development,
give off groups of cells which become disseminated throughout the
coelomic fluid. The original cells (mother-cells) undergo division
into smaller cells, each of which develops into a sperm with a
minute rod-shaped head and a long vibratile flagellum or tail. In
the female the ovaries (Fig. 357, ov), formed by a similar process
of proliferation, take the form of rounded masses of cells, meta-
merically arranged, surrounding the principal vessels throughout
the length of the body. The young ova become detached from
the ovaries, and attain their full development while floating about
in the ccelomic fluid. Both ovaries and testes dwindle after they
have given off the sexual cells, and at the non-breeding season of
the year are not to be detected.

Ova and sperms, when fully ripe, are discharged, reaching the
exterior probably through apertures temporarily formed by
rupture of the body-wall (cf. above), and impregnation takes place
by contact between the two sets of elements while floating freely
in the sea-water.

Nereis dumerilii is an extremely variable species. If we compare
a number of specimens, we find numerous individual differ-
ences between them. The most striking of these are differences
of colour and of the number of segments in the body ; but a careful
examination reveals many other points in which individuals differ.
Thus the precise form of the lobes of the parapodia, together with
the number of setae in the two bundles, varies ; so also do the relative
length of the tentacles, the number of teeth on the jaws, and the
number and arrangement of the denticles in the buccal cavity.
Not only are such individual differences common, but the species
occurs in two distinct forms or phases, which differ from one another
so widely that they have been referred to distinct genera. One
of these is the Nereis phase, which is that described in the preceding
paragraphs. A Nereis dumerilii may become sexually mature in
this form, or may first undergo a series of changes by which it
becomes converted into the second or Heteronereis phase (Fig. 352,5).
The principal changes which take place during this metamorphosis
are a great increase in the size of the eyes, and a marked modification
of the parapodia in the posterior portion of the body, the lobes
becoming larger and more leaf-like, and the setae of the Nereis
being superseded by others which are considerably longer, more
numerous, and somewhat oar-shaped. The Heteronereis, instead
of creeping about on the bottom, swims about actively through the
water by wriggling movements of the body combined with active
paddling movements of the parapodia with their long setae. After
a time the Heteronereis, like the Nereis, becomes sexually mature,
developing ova and sperms, the latter of which differ remarkably
in shape from those of the Nereis phase.

440 ZOOLOGY . SECT.

Development. — The egg of Nereis when first discharged is
enclosed in a transparent thick gelatinous envelope, within which
are two membranes — an outer, very thin and delicate, and an inner
(zona radiata), thicker and very distinctly striated in a radial
direction. The protoplasm of the ovum contains a number of
oil-drops and yolk-spherules. When fertilisation takes place
the yolk-spherules move away from what is destined to become
the upper pole of the egg, leaving a polar area composed of granular
protoplasm. The zona radiata disappears, and the contents of
the ovum undergo for a time amoeboid changes of form. Then
the spherical form is reassumed, two small bodies — the polar
bodies — are thrown off at the upper pole, and the process of

micro

macro

micro

macro

Fia. 361.— Nereis. Early stages in the development. A, lateral view of eight-celled stage ;
.B, the same from above ; C, stage of the formation of the first somatoblast ; D, stage at which
both somatoblasts are present ; macro, megameres ; micro, micromeres ; som. 1, som. 2, first
and second somatoblasts. (After Westinghausen.)

segmentation or cleavage begins (Fig. 361). This is of the spiral
type, and up to a fairly advanced stage corresponds very closely
with the segmentation of the Polyclad oosperm as described on
page 268. The oosperm divides first into two parts, then into four.
One of these (A , B) is larger than the rest. From these four cells — the
megameres — there are separated off in succession three sets or
quartettes of micromeres, making twelve in all. One of these,
belonging to the second set, somewhat larger than the others and
differing from them in its subsequent history, is termed the first
somatoblast (0, D, som. 1) ; a second somatoblast (som. 2) is soon
given off in the fourth quartette from the same megamere that
gave origin to the first.

The germinal layers are now "all established. The micromeres

x PHYLUM ANNULATA 441

constitute the ectoderm, destined to give rise to the epidermis and
all its derivatives, to the cerebral ganglion and nerve-cord, to the
oesophagus and rectum. The megameres eventually give origin
to the cells of the endoderm, forming the internal epithelium of
the alimentary canal. The second somatoblast gives rise
to the entire mesoderm of the Annelid and contributes a few small
cells to the endoderm of the intestine. As the micromeres multiply
by division, they form at first a cap of small cells over the upper
pole of the embryo ; eventually the cap extends so as completely
to cover the four megameres and the descendants of the somato-
blasts except at one point, the blastopore, at the lower pole, where
the investment remains for a time incomplete. When the
blastopore closes, the process of epibolic gastrulation is completed.
A thickening of the layer of ectoderm cells, the apical plate, in the
middle of what is destined to form the head-end of the embryo, is
the rudiment of the cerebral ganglion : in close relation to it are
formed a pair of pigment-spots, the larval eyes. From the middle
of the head-end projects a tuft of cilia (Fig. 362, A, ap. til.).
Encircling the body of the larva behind this is a thickened ridge,
the prototroch (prot), the cells of which develop strong cilia. Just
behind the prototroch the cells of the ectoderm become pushed
inwards in the middle of what will eventually become the ventral
surface, so as to line a sort of depression or pouch ; this is the
stomodo&um (si) or rudiment of the mouth and oesophagus. The
anus (an) does not appear until later ; the position which it will
subsequent!}^ occupy is indicated at this stage by a pigmented
area (pig. ar) marking the point at which the blastopore becomes
closed. The first and second somatoblasts divide to form a mass of
small cells which extend on the ventral surface behind the prototroch
and mouth, constituting what is termed the ventral plate ; of this
plate the more superficial cells are descendants of the first somato-
blast ; and those situated more deeply are derived from the
second somatoblast. A superficial thickening of the ectoderm
along the middle of the ventral plate is the rudiment of the ventral
nerve-cord (neur. pi) ; the deeper cells divide and extend to form a
pair of mesoderm-bands or muscle-plates, from which the muscles
of the body- wall are developed ; the muscular layers of the -wall of
the alimentary canal are derived from certain of the same set of
cells which migrate inwards from the lower end.

A pair of micromeres separated from the rest at an early stage
are destined to form the larval excretory organs, the head-kidneys or
larval nephridia : at first situated at the upper end, they sink
below the surface and migrate downwards till they come to lie below
the prototroch ; each then elongates, and a number of vacuoles
which have become formed in the interior coalesce in such a way
as to form a long, narrow canal. The embryo has now reached
the completed trochophore stage.

The endoderm cells become arranged so as to bound a canal-

np.pl

fr.brtd

prut

prut |Q|

l.rnus -V
itetsacs •*•

nenrpl

-prut

K±LjL-JMod

7/ara

—tent

FIG. 362. — Nereis. Later stages in the development. A, stage at which the prototroch and the
apical tuft of cilia first become distinct. B, somewhat later stage, hi which the stomodaeal
invagination is being formed, and the rudiments of the mesoderm bands are distinct ; C, late
trochophore stage, in which there are rudiments of the setigerous sacs ; D, somewhat later
stage, in which the parapodia have begun to become prominent and the provisional setae
project freely ; B, larva with three segments, an. anus ; ap. til. apical cilia ; ap. pi. apical
plate ; eye, eye ; fr. bod. frontal bodies ; int. intestine ; 1. mus. longitudinal muscle ; mes.
mesoderm ; mo. mouth ; neur. pi. neural plate ; para, parapodia ; pig. ar. pigmented area ;
prot. prototroch ; tens. h. sensory hairs ; set. sacs, setigerous sacs ; sorn. second somatoblast
and group of cells formed from it ; st. stomodaeum ; tent, peristomial tentacles. (After
E. B Wilson.)

442

SECT, x PHYLUM ANNULATA 4-13

like space, the beginning of the lumen of the middle part of the
alimentary canal (oesophagus and intestine, int), the cells subse-
quently giving rise to the enteric epithelium. This canal becomes
continuous in front with the stomodaeum, and behind with a
second smaller ectodermal invagination, the proctodceum, which
arises in the position of the former pigment-area. The part of
the larva behind the prototroch now elongates, and two pairs of
invaginations, the setigerous sacs (set. sacs), appear at its sides : in
the interior of these, to which a third pair is soon added, are
developed setae which grow out to a great relative length as the
larval or provisional setcs. Constrictions soon appear marking off
the first three segments, and at the same time the mesoderm bands
undergo a corresponding division into three pairs of mesoderm
segments. The mesoderm segments of each pair grow inwards
towards one another and surround the alimentary canal : in the
interior of each appears a cavity which is the beginning of a
segment or chamber of the coelome. As the two mesoderm
segments become closely applied to one another and unite around
the alimentary canal, their two cavities also come into close
relation, and eventually are separated from one another only by
thin vertical septa, forming dorsal and ventral mesenteries which
subsequently disappear. Successive mesoderm segments also
come into close relationship with one another, their cavities
eventually only remaining separated by thin transverse partitions,
which form the intersegmental septa.

The region in front of the prototroch becomes modified to form the
prostomium of the adult. The part immediately behind forms
the peristomium, which bears setae, and is to be looked upon as
the specially modified first segment. The body increases in length,
and additional segments with their setigerous sacs become dis-
tinguishable (E), until, on the development of the tentacles, the
outgrowth of the parapodia (para) with their cirri and the
permanent setae (which replace those first formed), the formation
of the full number of segments, and the completion of the internal
organs, the adult condition of the worm is attained.

6. THE EARTHWORM (Lumbricus).

General External Features.— The Earthworm (Fig. 363)
has a long narrow body, which may be described as approximately
cylindrical, but slightly depressed towards the posterior end.
Dorsal and ventral surfaces are readily recognisable, the latter
being much paler in colour than the former, and exhibiting a
slight flattening ; the anterior end is distinguishable in the living
animal as that which is directed forwards in the ordinary creeping
movements of the worm. The surface, as in the case of Nereis,
is very distinctly marked out into segments or metameres by a

444

ZOOLOGY

SECT.

series of ring-like constrictions ; the segments, which are very
numerous, amounting to about 150, are somewhat longer towards
the anterior end than they are further back.

At the extreme anterior end is a rounded lobe, the prostQWflwn,
immediately behind and below which is the opening of tnemouth.
Next to the prostomium is the most anterior segment, the peri-
stomium, which bounds the mouth behind. The eyes and tentacles
present in Nereis are not represented. On the most posterior

B

Fi(J, 363. — Lumbricus. A, entire specimen, lateral view ; B, ventral view of anterior portion
of the body, magnified. 1, prostomium ; 15, 33, fifteenth and thirty-third segments.
Each of the black dots represents a pair of setse. (After Vogt and Jung.)

segment, the anal segment, is a small median opening, the anal
aperture. A limited region of the body in front of the middle,
comprising five or six segments, has a swollen appearance ; this is
termed the clitellum. There are no parapodia like those of Nereis,
but running along the lower surface of the worm are to be recognised
with the aid of a lens four double rows of short bristles or setse
(Fig. 364), a pair of each row occurring in each segment, which thus
possesses eight altogether. The extremities of all these setae are
directed backwards, and they act as fulcra for the forward move-

X PHYLUM ANNULATA 445

ments of the worm on the surface of the ground or in the interior
of its burrow. The setse in the clitellum, and those in "the neigh-
bourhood of the genital apertures, are much slenderer than the rest.
Along the middle line of the dorsal surface, from about the eleventh
segment backwards, is a row of small apertures, one at the line of
division between each contiguous pair of segments : these, which
are termed the dorsal pores, perforate the body-wall and open
internally into the coelome. Through these coelomic fluid is capable
of being discharged, covering the surface with a thin layer which
may protect the worm from desiccation or from contact with
irritating substances. On the ventral surface are two rpws of
minute apertures — a pair on each segment — the excretory apertures
or nephrwliopores. On the ventral surface of the fifteenth segment
(Fig. 363, 7J) is a pair of slit like apertures with somewhat tumid
lips, the male reproductive apertures ; and on the segment imme-
diately in front — the fourteenth — are two smaller
rounded apertures, the female reproductive aper-
tures. In the intervals between the ninth and
tenth and tenth and eleventh segments are two
pairs of small pores, the openings of the recep-
tacula seminis.

The body-wall (Fig. 365) consists of a
cuticle, an epidermis or deric epithelium, a
dermis, muscular layers with associated con-
nective tissue, and, fining the inner surface, a
thin cellular membrane, the peritoneum or
coelomic epithelium. The cuticle (cut.) is similar
to that of Nereis, and has a similar iridescent
lustre ; it is perforated by numerous minute
apertures. The epidermis consists, except on
the clitellum, of a single layer of cells elongated
in the vertical direction : many of these cells have the character of
unicellular glands ; many others are sensory cells, and are con-
nected by fine nerve-fibres with the nerve-cord. On the clitellum
the epidermis is thickened, and blood-vessels extend between
the cells. Below the epidermis is a layer of connective tissue,
the dermis. The muscular fibres which make up the greater part-
of the thickness of the body- wall are arranged -in two principal
sets — a layer of circularly arranged fibres (circ. mus) situated
externally, immediately below the dermis, and a layer of longi-
tudinally arranged fibres (long, mus) situated internally. The
circular layer is interrupted at all the intervals between the segments ;
the longitudinal layer is interrupted along a series of longitudinal
lines, so as to be divided into seven bundles.

The setae (Fig. 364) are lodged in sacs, the setigerous sacs (see
Fig. 375), lined by a continuation of the epidermis. In the region
of the body in which the reproductive organs are lodged some of

446

ZOOLOGY

SECT.

these sacs are enlarged and glandular, and receive the special name
of capsulogenous glands.

The enteric canal (Fig. 366) is, as in Nereis, a tube which
runs through the entire length of the body from the mouth at the
anterior to the anus at the posterior end. As in the case of Nereis,
it lies in a cavity, the coelome, lined by a thin cellular membrane,
the peritoneum or ccelomic epithelium, and rilled with a fluid, the
coelomic fluid, containing colourless corpuscles. The ccelome is
divided into a series of chambers corresponding to the segments

dnrs \f

typh

cut

epid

neph

neph rost

71. CO

set

vent, v

FIG. 365. — Lumbricus, transverse section of the middle region of the body. circ. mus. layer of
circular muscular fibres ; ccel. cffilome ; cut. cuticle ; dors. v. dorsal vessel ; epid. epidermis ;
ext. neph. nephridiopore ; hep. layer of chloragen cells ; long. mus. longitudinal muscle ;
neph. nephridium ; nephrost. nephrostome ; n. co. nerve-cord ; set. setse ; sub. n. vess. sub-
neural vessel ; typh. typhlosole ; vent. v. ventral vessel. (After Marshall and Hurst.)

by a series of delicate transverse partitions, the septa or mesenteries,
consisting of folds of the peritoneal membrane enclosing muscular
fibres.

The mouth leads into a small buccal cavity. This is followed by
a much larger, thick-walled, rounded chamber, the pharynx (ph.).
From the wall of the pharynx there run outwards to the body-wall
a number of radially .arranged bundles of muscular fibres which,
when they contract, draw the pharynx backwards, and at the same
time dilate it. Behind the pharynx follows a comparatively
narrow tube, the oesophagus (ces), which extends through about seven

PHYLUM ANNULATA

447

segments. At the sides of the oesophagus, in each of the segments
ten, eleven, and twelve, is a pair of rounded projections. The
first pair — the cesophageal pouches— are hollow, and their cavities
are in communication
with the lumen of the
oesophagus (ces. gl). The
other two pair s — t h e
cakiferous glands — are
thickenings of the wall of
the oesophagus, the fluid
in the interior of which
is milky, owing to its
containing numerous par-
ticles o f ^ carbonate o f
lime ; the numerous
small cavities which they
contain are in communi-
cation with the cesopha-
geal pouches. Poste-
riorly the oesophagus is
continuous with a
rounded thin-walled
chamber, the crop (cr),
and this is followed by
a very thick - walled
chamber, also of rounded
form, the gizzard (giz).
From this the intestine
(ini) extends throughout
the rest of the length of
the body to the anal
aperture. It is wide, with
thick but soft walls, con-
stricted opposite the
septa, i.e. in the intervals
between the segments.
Running along the
middle of its dorsal sur-
face is a longitudinal
fold, the typhlosole (Fig.
365, typh), projecting
downwards into the
lumen. On the wall of the
intestine outside the muscular layers and surrounding the intestinal
blood-vessels are a number of granular, yellow cells — the cJdoragen
cells (hep) : these are specially abundant in the typhlosole. The ter-
minal part, situated in the last segment, is termed the rectum.

448

ZOOLOGY

SECT.

The whole alimentary canal is lined internally by a cuticle, which
is thicker in the gizzard than elsewhere, and by a single layer
of ciliated columnar epithelial cells, the enteric epithelium. Some
of these cells, more granular than the others, grouped in certain
regions — more particularly along the typhlosole — are of the nature
of unicellular digestive glands, secreting a digestive fluid. Others
seem to be specially concerned in the absorption of the digested
food. External to this is a layer of connective tissue, between
which and the external covering of yellow cells are muscular fibres,
of which there are two layers, an external longitudinal and an
internal circular. These layers are greatly thickened in the walls
of the pharynx and of the gizzard.

The Earthworm, like Nereis, has a well-developed vascular
system consisting of blood-vessels with well-defined walls. The

blood is bright red, the colour
being due to the same colouring
matter, viz. hcemoglobin, as in
the case of the blood of the
higher animals, occurring, how-
ever, not in corpuscles, but in
the liquid part or plasma ;
corpuscles are present, but they
,,, are colourless. The main trunks
are the dorsal, the ventral,
the sub-neural, the two lateral
neural, and a series of trans-
verse branches. The dorsal vessel
(Fig. 365, dors, v) runs along
the middle of the dorsal surface
between the body-wall and the
intestine ; it is readily visible
FIG. 36?.— Lumbricus. Anterior portion of shining through the former in the

% living worm. The ventral vessel
PWSt' prostomium' (vent.v) lies below the alimentary
canal, the sub-neural below this
again under the nerve-cord ; the lateral neural lie on either side of
the nerve-cord. The transverse branches correspond in number to the
segments ; they run round from the dorsal vessel to the ventral,
giving off branches in their course. Five of them, viz. those in
the seventh to the eleventh segments inclusively, are dilated and
pulsate rhythmically ; these have the function of driving the
blood through the system of vessels, and are hence frequently
termed the " hearts." The walls of the principal vessels are
contractile, and assist in bringing about the movement of the
blood, which is propelled in such a way as to run forwards in the
dorsal vessel and backwards in the ventral, its direction of move-

x PHYLUM ANNUL ATA 449

ment being regulated by a number of valves in the " hearts," the
dorsal vessel, and the chief vessels connected with it.

The nervous system (Fig. 367) consists of a dorsal bilobed
brain or cerebral ganglion and a double ventral nerve-cord together
with a pair of cesophageal connectives, by which the former is
connected with the anterior end of the latter. The brain, which is
of small size, is situated in the third segment, above the beginning
of the alimentary canal ; it is divided by a median constriction
into two lateral parts of pyriform shape with their broad ends in
contact. The connectives pass from this round the sides of the
alimentary canal to unite in the middle below with the anterior
end of the ventral nerve-cord. In this way a complete nerve-
ring or nerve-collar surrounds the anterior part of the enteric
canal in the third: segment. From thls~the ventral nerve-cord
extends backwards to the posterior end of the body, and in each
segment it presents a slight enlargement or ganglion, as it is
usually termed, most conspicuous in the more posterior segments.
The whole cord is double, consisting of two intimately united
right and left parts. From the brain, nerves are given off to
the prostomium ; and from the ventral cord three pairs of nerves
arise in each segment. From the ossophageal connectives a
series of stomatogastric nerves pass to the pharynx and neighbouring
parts of the alimentary canal.

The Earthworm is devoid of organs of sight or hearing. It
exhibits sensitiveness to bright light, the sensitive elements being
large cells of the epidermis devoid of pigment. The sense of
hearing appears to be absent ; but a faculty analogous to taste
or smell, enabling the animal to distinguish between different
kinds of food, is well developed. The goblet-shaped bodies, groups of
narrow epidermal cells, most abundant on the prostomjum and
peristpmiu-m, have probably to do with this faculty.

The organs' of excretion — the segmental organs or nephridia
(Fig. 368) — are similar to those of Nereis, but somewhat more
complicated. They are slender tubes which occur in pairs in all
the segments of the body except the first three and the last.
Externally each nephridium opens by one of the small nephridio-
pores which have already been mentioned as occurring on the
ventral surface ; internally it ends in a funnel-shaped ciliated
extremity with a crescentic slit-like aperture, the nephrostome
(nst), opening into the cavity of the segment in front of that in
which the external aperture occurs. The tube is thrown into
three loops attached to the posterior surface of the corresponding
septum by a fold of membrane. Two parts are clearly recognis-
able— an inner narrow and an outer wide part : in the former the
narrow central lumen is a perforation through the axis of a string
of cells, and is thus intracellular : it is lined in parts with cilia

VOL. I. G G

450

ZOOLOGY

SECT.

arranged in two rows ; in the latter (the terminal vesicle) the
passage is lined by cells, and is thus intercellular, and there is a
thick muscular investment. The nephridia are abundantly supplied
with blood by means of nephridial branches of the ventral vessel.

coe

ext

JL

FIG 368. — NephridiumofLumbricus (diagrammatic). — a. ampulla between ciliated and non-
ciliated parts of the intracellular canal ; oil. ciliated part of the intracellular canal ; coe.
investment derived from the ccelomic epithelium ; gzL nephridiopore ; Ic. non-ciliated part
of the intracellular canal ; mes. septum ; nst. nephrostome ; t.v. intercellular canal of the
terminal vesicle. /. — ///. the three principal loops. (From Meisenheimer, after Maziarski.)

Reproductive Organs. — The Earthworm is hermaphrodite.
There are two pairs of very small flattened testes (Figs. 366, 369, tet

PHYLUM ANNULATA

te'), partly divided into a number of digitate lobes, situated in the
tenth and eleventh segments. A pair of comparatively large sacs,
the anterior vesiculce seminales (ant. ves. sem), lie partly in the
cavity of the ninth segment, but extend into the tenth, where
they coalesce in the middle to form a large median sac of some-
what irregular form, the anterior sperm-reservoir (ant. sp. res).
The anterior pair of testes project into this, and the cells destined
to form the sperms, developed in the former, pass by dehiscence
into the large median cavity. On either side is a large ciliated
funnel, or rosette (fun), leading outwards from the interior of the

a,nt. ves. sem ctni. sp res

inl \ int n.co int I

v.eff"

ov.cL

FIG. 369. — Lumbricus. Reproductive organs, ant. sp. res. anterior sperm reservoir ;
ant. ves. sem. anterior left vesicula serainalis ; fun. funnel-like openings of vasa efferentia ;
int. intermuscular partitions ; mid. ves. sem. middle vesicula seminalis ; n. co. nerve-cord ; ov.
ovaries ; ov. d. oviducts ; post. sp. res. posterior sperm-reservoir ; post. ves. sem. posterior
vesicula seminalis ; rec. receptacula seminis ; te, anterior, and te', posterior testes ; v. eff.
anterior, and v. eff'. posterior vas efferens ; v. def. vasa deferentia. (After Vogt and Jung.)

reservoir. A second pair of vesiculse seminales (mid. ves. sem),
situated in the eleventh segment, also open into the anterior
sperm-reservoir. A third pair (post. ves. sem), situated in the
twelfth segment, unite in front to form the posterior sperm-reservoir
(post. sp. res), which lies in the middle of the cavity of the
eleventh segment. The posterior pair of testes have the same
relation to this as the anterior pair have to the anterior reservoir ;
and a posterior pair of ciliated funnels (fun) lead outwards from
its cavity. Each ciliated funnel passes into a narrow, somewhat
convoluted duct, the vas efferens, and the two vasa efferentia of each

G G 2

452 ZOOLOGY SECT.

side unite to form a vas deferens or spermiduct (v. def ), right or
left as the case may be, which passes almost straight backwards to
open by the corresponding male aperture on the fifteenth segment.

The female reproductive organs consist of a pair of ovaries, a
pair of oviducts with a pair of receptacula ovorum, and two pairs of
receptacula seminis. The ovaries (ov) are minute pear-shaped bodies,
which are situated in the thirteenth segment, attached to the
septum between the twelfth and thirteenth. The oviducts (ov. d)
are a pair of short tubes, each with a comparatively wide funnel-
shaped opening into the cavity of the thirteenth segment, and
extending backwards and outwards in the fourteenth segment to
open at the female aperture on the ventral surface of the latter.
The receptacula ovorum are a pair of reniform sacs which open into
the funnel-shaped ends of the oviducts. The receptacula seminis
or spermotheccB (rec) are two pairs of rounded sacs which open on
the exterior in the intervals between the ninth and tenth and tenth
and eleventh segments.

Though hermaphrodite, the Earthworm is not self-impregnating,
but two individuals provide for mutual fertilisation by an act of
copulation. The copulating individuals apply themselves together
by their ventral surfaces, the heads pointing in opposite directions,
and become attached in this position by the setae of the genital
region and by a viscid secretion from the clitellum and of the
capsulogenous glands (p. 446), situated in the neighbourhood
of the reproductive organs. The sperms from the male apertures
of each pass along temporarily formed grooves to the receptacula
seminis of the other.

When the ova are mature they are discharged from the ovary
into the cavity of the thirteenth segment, whence they pass out
to the exterior through the oviducts, to be enclosed in the cocoon
(vide infra), after having being detained for a time in the
receptacula ovorum.

Development. — The oosperms or fertilised ova of the Earth-
worm are enclosed, together with a quantity of an albuminous fluid
derived from the capsulogenous glands, in a cocoon, the wall of
which is formed of a viscid secretion from the glands of the
clitellum, hardened and toughened by exposure to the air. The
cocoon is deposited in the earth and the embryos develop into
complete, though minute, worms before they make their escape.
The segmentation is somewhat unequal. A flattened blastula
(Fig. 370, A) is formed, with a large but flattened segmentation-
cavity. This becomes invaginated to form a cylindrical gastrula
(B) ; the blastopore narrows and subsequently gives rise to the
mouth of the adult. A pair of large mesoderm cells are early marked
off from the other cells of the gastrula ; these undergo division
to form a pair of mesoderm bands (C, mes) composed of several
rows of small cells which grow forwards towards the mouth.

PHYLUM ANNULATA

453

By swallowing movements the embryo at this stage, having burst
through the enclosing viteUine membrane, takes in the albuminous
fluid in the interior of the cocoon, and increases rapidly in size.
As the embryo elongates, the mesoderm bands become divided
into segments, and the subsequent history of these is essentially
similar to what has been already described in the case of Nereis.
The ectoderm is thickened on each side along the line of the meso-

ect

tries

a. si op

Tries

0

FIG. 370. — Early stages in the development of Lumbricus , A, lateral view of flattened
blastula ; B, ventral view of gastrula with slit-like blastopore ; C, lateral view of later stage.
blastoc. blastocoele ; blastop. blastopore ; ect. ectoderm ; end. endoderm ; m. primary meso-
derm cell ; mes. mesoderm bands ; ner. cell from which the primitive nerve-cord (ne. co.)

takes origin ; nph. cells taking part in the formation of the nephridia ; st. stomodseum.
(After Wilson.)

derm bands, and the mass of ectoderm cells so formed becomes
arranged in a number of rows each originating behind in a larger
rounded cell or teloblast. The innermost of these rows (Fig. 370,
C, ner, ne. co) give rise to the ventral nerve-cord. The next two
rows (nph) are said by some observers to give rise to the nephridia
all but the funnels ; but according to others the nephridia, or at
least all their inner glandular portions, are of mesodermal derivation,

454 ZOOLOGY SECT.

The brain and cesophageal connectives are formed in continuity
with the rudiments of the ventral nerve-cord.

On the whole the development resembles that of Nereis, the
chief differences being such as may be traced to the non-occurrence
in the Earthworm of any free-swimming trochophore stage, with
the absence of such larval structures as the large pre-oral lobe,
the apical plate, the prototroch, and the larval nephridia or head-
kidneys.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Chaetopoda are Annulata with the body made up of distinct
metameres, which are usually numerous and similar throughout.
The metameres are provided with chitinous setae developed in sacs
(setigerous sacs) of the epidermis, and usually elevated on muscular
appendages, the parapodia. There is a large ccelome divided
internally into chambers by transverse septa, and not in com-
munication with the blood-vascular system, which is nearly
always highly developed. The ventral nerve-cord consists of a
chain of ganglia. The reproductive cells are formed by a prolifera-
tion of certain parts of the peritoneum or membrane lining the
coelome, and usually reach the exterior through ccelomoducts
or through modified or unmodified nephridia.

Sub-Class I.-POLYCH^ETA.

Chaetopoda with the sexes distinct, and the ovaries and testes of
simple character and metamerically repeated. Highly developed
parapodia are present, in most instances, bearing numerous long
setae. There is usually a definite head with eyes and tentacles,
and often cirri and branchiae on the segments of the body. A
clitellum is never developed. A metamorphosis takes place : the
larva is a trochophore. Nearly all the Polychaeta are marine.

ORDER 1. — ARCHI-CMTTOPODA.

Aberrant or primitive Polychaeta1 in which the nervous system
is not separated from the epidermis, and the ventral cord is not
segmented into ganglia. Only one genus (Saccocirrus).

ORDER 2. — PHANEROCEPHALA.

Polychaeta with protrusible pharynx usually armed with chitinous
jaws. There is a well- developed head. The segments are com-
pletely or nearly similar throughout the length of the body, and

1 The Archi-Chcetopoda are usually classed with the Polychceta, but their
alliancss are perhaps quite as close with the Oligochceta. In some respects
Saccocirrus resembles Polygordius and Protodrilus (Archi- Annelida, q.v.),
but is distinguished from them by the possession of setae.

x PHYLUM ANNULATA 455

the parapodia are usually equally developed throughout and
provided with cirri. The branchiae, when present, are not confined
to the anterior end.

ORDER 3. — CRYPTOCEPHALA.

Polychaeta devoid of protrusible pharynx and of jaws or teeth.
The head is frequently very small, and sometimes is devoid of
eyes or of tentacles, the prostomium sometimes much reduced and
covered over by the peristomium. The body is distinguishable, by
differences in the form of the segments, parapodia, and setae,
into two or even three regions. The parapodia are little prominent
in the posterior parts, and usually without cirri. The branchiae,
when present, are usually confined to the anterior end, and are
sometimes represented by modified cephalic palpi.

Sub-ciass XL— OLiGtocHJETA.

Chaetopoda with the sexes united, the reproductive system com-
plicated, the ovaries and testes compact and never more than two
pairs of each. No definite paropodia are developed and no cirri,
and only a small number of simple setae on each segment. The
head is not distinct. A clitellum is usually present. There is no
metamorphosis. Mostly terrestrial or fresh- water forms.

ORDER 1. — MICRODRILI.

Small Oligochaeta with relatively few segments, often multiply-
ing asexually. The male genital pores are on, or in front of, the
seventh segment. The vasa deferentia are short, opening on the
segment immediately behind that in which the internal apertures
are situated. The anterior part of the body is often distinguished
from the rest by a difference in the form and arrangement of the
setae. The clitellum, which is composed of only one layer of cells,
is situated comparatively far forward. Eye-spots are frequently
present.

ORDER 2. — MEGADRILI.

Mostly large Oligochaeta with many segments, never multiply
ing asexually. The male genital pores are behind the seventh
segment. The vasa deferentia are elongated, passing through two
or more segments. The anterior part of the body is never special-
ised as regards its setae. The clitellum, which consists of two
layers of cells, never begins in front of the twelfth segment.
Eye-spots are not developed.

Systematic Position of the Examples.

Nereis dumerilii is one of many species of Nereis differing from
one another in certain minor details of their structure — such as
the relative length of the palpi and tentacles, the size and form of

456 ZOOLOGY SECT-

the eyes, the shape of the parapodia, the form of the setae, and the
like. The genus Nereis differs from the other genera of the family
Nereidce, to which it belongs, in having the parapodia biramous
and the cirri simple, and in the presence of a series of denticles
in the buccal cavity in addition to the pair of jaws. The family
Nereidae differs from all the other families of the sub-order Nereidi-
formia of the Phanerocephala in the union of the following
characters : — The body is always elongated and made up of a
considerable number of segments. The prostomium is well developed,
and bears a pair of tentacles, a pair of palpi, and four eyes. The
peristomium is devoid of parapodia, and has four pairs of tentacles.
The parapodia are either uniramous or biramous ; both dorsal and
ventral cirri are present ; the setae are compound (articulated).
There is a pair of anal cirri. In the pharynx there is always a
pair of horny jaws, and usually a number of denticles in the buccal
cavity.

The members of the sub-order Nereidiformia are all character-
ised by the possession of well-developed tentacles and palpi, and
usually peristomial cirri. There are highly developed parapodia
with acicula, jointed setae, and dorsal and ventral cirri. The buccal
region of the enteric canal is eversible as a proboscis, and there are
usually horny jaws.

There are several species of the genus Lumbricus, differing from
one another in the general form of the body, the number of the
segments, the shape of the prostomial lobe, and other minor
points. All of them agree in the presence of the following features,
which characterise the genus and distinguish it from the many
other genera of the family Lumbricidce : —

The prostomium is dovetailed completely into the peristomium.
The setae are always in couples. There are longer and straighter
setae on the clitellum. The male apertures are always on the
fifteenth segment. There are three pairs of vesiculae seminales,
in the ninth, eleventh, and twelfth segments, connected across the
middle line in the tenth and eleventh by sacs enclosing the ciliated
funnels. There are two pairs of receptacula seminis always situated
in the ninth and tenth segments.

The family Lumbricidae is distinguished from the other families
of the sub-order Megadrili, which comprises all the Earthworms,
by the combination of the following features : —

The clitellum usually begins behind the twentieth segment and
occupies from six to nine segments ; it is incomplete ventrally.
Dorsal pores are present. The setae on the clitellum differ from
the others. The male apertures are not situated further back than
the fifteenth segment. There are three or four pairs of vesiculae
seminales, in the ninth to the twelfth segments. The testes and
ciliated funnels are usually in the tenth and eleventh segments ;
the female apertures on the fourteenth.

PHYLUM ANNULATA

457

3. GENERAL ORGANISATION.

The general form of the body in the

Chsetopoda is cylindrical, but in many, e.g.,
some members of the families Polynoulce
(Fig. 371) and Amphinomidce, there is a very
considerable degree of dorso-ventral com-
pression. In most the body is very long in
comparison with its breadth ; but this is not
a universal rule, the length being in some
cases not more than five or six times the
breadth. The surface is marked out by a
number of more or less distinct annular
constrictions or impressed lines into a corre-
FIQ. 37i.-poiynbe seto- spending series of segments or metameres,
sissima. Dorsal view which are usually very numerous, often some

of entire animal, with , . r J/L i_ •

the pharynx protruded, hundreds in number, though in some cases
there are not more than from twenty to

thirty. These segments are usually very similar throughout the
length of the body ; but in the Cryptocephala (Figs. 372, 373,
379) there
may be two
or even more
regions dis-
tinguishable
from one
another by
the form of
the segments
and of their
appendages.
In the OHgo-
chaeta there is
a thickened
zone, the cli-
tellum, com-
prising some-
times only
one segment,
sometimes a
number. Each
segment, with
certain excep-
tions to be
noted pre-
sently, bears FIG. 372.— A SerpuUd (Galeolaria coespitosa). Lateral view of animal
removed from its tube. abd. abdomen ; br. branchiae ; op. oper-

eitner a pair culum ; th. thorax,

abet

458

ZOOLOGY

SECT.

of parapodia or merely a greater or smaller number of setae. Para-
podia are lateral hollow processes of the body- wall bearing a number
of bristles or setae. Frequently the parapodium is divided horizon-

FIG. 373.— Chaetopterus. Natural
size of a young specimen. A, an-
terior region of the body ; B, middle
region ; C, hinder region, o, peri-
stomial cirri ; d, " sucker " ; e, the
great " wings " ; /, the first of the
three " fans " ; m, mouth. (From
Benham, after Panceri.)

FIG. 374. — Setae of various Polychaeta. (From
Claparede.)

tally into two distinct lobes or branches — a dorsal which is termed
the notopodium, and a ventral which is termed the neuropodium.

Even when this is not the
case there may be two
bundles of setae representing
the two parts. The setae
are nearly always chitinous ;
in Euphrosyne they are cal-
cined. They are always
solid, except in Euphrosyne,
entire, or divided into a
number of joints. In shape
(Fig. 374) they vary greatly
in different groups ; often
several very distinct forms

FIG. 375.— Section of the setigerous sac of an Oligo- °^ Se^8e are Present in

cha3te. &!, setigerous sac; 62, supplementary follicle different parts of each
with seta ; e, deric epithelium (epidermis) ; Im. -, . r . -.

longitudinal muscles of body-wall ; m, m. muscles parapodium of a Single
of the setigerous sac ; r.m, circular muscular layer ^Ji« ~f

of body-wall. (From Hatschek, after Vejdovsky.) Worm, Or in parapodia Ot

PHYLUM ANNULATA

459

different regions of the body. Some are exceedingly delicate and
hair-like, others needle-shaped, others compressed and sabre-like,
others bayonet-like. Very often there is a long, straight, narrow
part or handle with which is articulated a terminal blade, or
bayonet, or hook. Sometimes the setae are quite short, projecting
little beyond the parapodia, and are hook-like or comb-like.
Usually each bundle contains, in addition to the ordinary setae,
a stouter, straight, simple seta, which scarcely projects on the
surface ; this is termed the aciculum. Each seta, or each bundle of
setae, is lodged in a sac, the setigerous sac (Fig. 375), formed by an

ristlenb

FIG. .'576.— Polynoe extenuata. Dorsal view of anterior extremity, dors. cirr. dorsal cirri;
el. elytra ; perist. tent, peristomial tentacles ; prcest. prostomium. (After Claparede.)

invagination of the integument, and lined by cells continuous with
the epidermis. Each seta is derived from one of these cells, and is
to be looked upon as a specially developed part of the cuticle of
the general outer surface. The setigerous sacs are usually provided
with protractor and retractor muscles, by the action of which the
setae may be thrust out or retracted.

In addition to the setae the parapodium bears very commonly
certain soft appendages of a sensory character, the cirri (Fig. 353,
dors, cirr., vent. cirr.). There are usually both dorsal and ventral
cirri, the latter nearly always much smaller than the former. The

460

ZOOLOGY

SECT.

cirri are usually filamentous, sometimes jointed ; sometimes they
are laterally compressed and leaf -like. In Polynoe (Figs. 371 and
376) and its allies certain of the parapodia bear, instead of dorsal
cirri, flattened scales, the elytra (el.), richly supplied with nerves :
these are sometimes looked upon as modified dorsal cirri, but in
some members of the group cirri and elytra occur together on the
same segment.

In Sternaspis a ventral shield formed by a thickening of the

FlO. 377.— Heads of various Polychaeta (diagrammatic). A, Polynoid ; B, Syllid ; C, Nephthys ;
D, Eunice ; E, Phyttodoce ; J?t Trophonia. a, prostomium ; c, cirri of body-segments ; ci,
peristomial cirri (tentacles) ; c2, cirrus of first body-segment ; c3, cirrus of second body-
segment ; el', point of attachment of elytron ; p, palp ; *, nuchal organ ; t, tentacle ; I, peri-
stomium : //, ///, IV, segments. (From the Cambridge Natural History.)

cuticle in the posterior region of the body bears a number of setae
roundjits edge.

In the Oligochaeta (Fig. 378) the parapodia are absent as pro-
cesses of the body-wall, and are merely represented by a small
number of short setae each lodged in its sac ; cirri are not developed.
In certain Oligochaeta setae are absent.

The first segment or prostomium, together with the second or
peristomium, forms in many Polychaeta a very distinct head ; the
prostomium in such a case bears eyes and tentacles and contains
the cerebral ganglion ; on the peristomium is the opening of the

PHYLUM ANNULATA

461

mouth, and from it also arise the
peristomial tentacles. A ventral
pair of prostomial tentacles, some-
what thicker than the rest, are
sometimes to be distinguished,
and are termed the palpi. Neither
prostomium nor peristomium
bears parapodia, though an aci-
culum is sometimes developed in
the latter ; the prostomium, in
fact, is not quite correctly termed
a segment, being different from
the true segments both in struc-
ture and in mode of development.
In the Oligochseta there is no
definite head, tentacles are en-
tirely absent, and in the terres-
trial forms the prostomium does
not lodge the cerebral ganglion.
In Sternaspis spinosa the prosto-
mium is elongated and bifurcated
like the proboscis of the Echiurida
(vide infra).

The last segment is termed the
anal segment, owing to its bearing
the anal opening ; it usually also
differs from the preceding seg-
ments in wanting the parapodia
and in having a pair of special
cirri, the anal cirri.

Branchiae are borne on the
dorsal surfaces of more or fewer
of the segments in many of the
Polychaeta. Sometimes they occur
on all, or nearly all, the seg-
ments ; sometimes they are con-
fined to the middle region of the
body ; sometimes they are present
only at the anterior end, as in
the majority of the Polychaeta
living habitually in tubes (Figs.
372 and 379). In the Terebellidce
(Fig. 379) the branchiae are situ-
ated on the dorsal surfaces of
some of the anterior segments.
In the SerpulidcB (Fig. 372) they
form two incomplete lateral cir-
clets of elongated appendages

462

ZOOLOGY

SECT.

situated at the anterior end of the body, apparently repre-
senting modified palpi, and sometimes supported by a carti-
laginous skeleton ; one of them is enlarged to form a stopper or
operculum (op.), often armed with calcareous plates and spines, for
the closure of the mouth of the tube in which the annelid lives. In
shape the branchiae are sometimes filiform, sometimes compressed
and leaf-like, sometimes branched in a tree-like manner, some-

FIG. 379. — Terebella. (After Quatrefages.)

times pinnate. In Serpula (Figs. 372 and 390) and its allies each
branchia consists of an elongated stem on which are borne two
rows of short filaments. The surface of the branchiae is usually
ciliated. They are richly supplied with blood-vessels when a
blood- vascular system is developed ; in Glycera, in which there
are no blood-vessels, each branchia contains a diverticulum of the
coelome.

In the Oligochseta branchiae are rarely present ; but in certain

x PHYLUM ANNULATA 463

of the Microdrili there are metamerically arranged simple or
branched branchiae, sometimes retractile, on the segments of the
posterior region.

The body-wall consists of a cuticle, an epidermis, muscular
layers, and a layer of peritoneum. The cuticle, composed of a
chitinoid material, usually presents two systems of fine lines
intersecting one another at right angles : it is perforated in many
places by the ducts of the unicellular glands of the epidermis.
The epidermis consists of a single row of cells, in some cases with
smaller cells of replacement intercalated between their inner ends.
In shape the cells vary greatly in different families and often in
different parts of the body of the same worm, being sometimes
flattened, sometimes cubical or polyhedral, but more usually more
or less vertically elongated. Cilia occur on the surface in certain
parts in many Chsetopoda. Among the ordinary cells of the epi-
dermis there are usually numerous unicellular glands often containing
rod-like bodies. In the tubicolous forms these unicellular glands
are active in secreting the material for the construction of the tube.
In addition, the epidermis frequently contains sensory cells, which
are in many cases contained in certain special elevations or sensory
papillae.

The muscular part of the body-wall consists of two layers, in the
outer of which the fibres are disposed circularly, while in the inner
their arrangement is longitudinal. The circular layer is continuous,
or, more usually, is interrupted opposite the intervals between
the segments. The longitudinal layer is disposed in four bands in
the Polychaeta, two dorso-lateral and two ventro-lateral. In the
Oligochaeta it is divided by the setigerous sacs which pass through it.

The peritoneal or ccelomic epithelium consists of a single layer of
cells. These are usually non-ciliated, but are ciliated in the
Aphroditea, Glycera, and some others, the movement of the cilia
bringing about an active circulation of the coelomic or perivisceral
fluid in the coelome.

The body-cavity or coelome, a wide space intervening between
the wall of the body on the one hand and that of the enteric canal
on the other, is divided in many Chsetopoda by a series of transverse"
septa into a series of chambers corresponding to the segments.
The septa are not complete partitions, there being always apertures
of greater or less extent by which the cavities of neighbouring
segments communicate. The septa consist of double folds of the
peritoneum enclosing muscular fibres.

The enteric canal is an elongated and nearly always straight
tube, running through the entire length of the body from mouth
to anus. A number of different parts are usually distinguishable ;
but their disposition varies to a very great extent in the different
groups. The buccal cavity, into which the mouth leads, is followed
by a muscular pharynx ; these are both formed in the embryo by

464

ZOOLOGY

SECT.

invagination of the ectoderm, and therefore correspond to a
stomodceum. The muscular pharynx is absent in some of the Crypto-
cephala : when present it is frequently protrusible to a greater or
less extent (see Figs. 355, 371) ; around its extremity, when it is
fully protruded, are to be seen a circlet of papillae in some forms ;
and in many, one or more horny teeth, situated in its interior,
are brought into play. A gizzard with thick walls may follow upon
this protrusible pharynx, and is sometimes preceded by an
oesophagus, which may be dilated behind into a crop. The intestine
is nearly always more or less deeply constricted inter-segmentally,
and in the Aphroditea, or " Sea-mice " (Fig. 380), there are in each
of the segments (with the exception of one or two of the most anterior
and one or two of the most posterior) a pair
of cceca which are to a greater or less extent
branched at their extremities. In the Hesio-
nida a pair of caeca which open into the
anterior part of the intestine frequently con-
tain gas, and probably have a hydrostatic
function. In some of the terrestrial Oligo-
chaeta (Earthworms) a fold of the intestinal
wall, the typhlosole, projects into its lumen.
The intestine is straight in most, but is
somewhat coiled in the Chlorcemidce, Sternaspis,
and others. The wall of the alimentary canal
consists (1) of the visceral layer of peritoneum ;

(2) of longitudinally arranged muscular fibres ;

(3) of circularly arranged muscular fibres ;

(4) of enteric epithelium. The peritoneum on
the surface of the intestine has in many Chaeto-
poda its cells enlarged and granular to form
the so-called chloragen cells, which probably

South! have an excretory function. The enteric epi-
6, pharynx ; c, branching thelium is very generally ciliated ; it contains

cseca of intestine ; d, anus. , J ,& . J m,

(From Gegenbaur's Com- numerous gland-cells. The stomodaeum and
the proctodaeum are lined internaUy by a
cuticular layer, which is continuous with the cuticle of the general
surface. The anus is usually terminal in position, sometimes
directed towards the dorsal aspect. There is, in most instances, a
longitudinal mesentery running to the alimentary canal from the
dorsal body-wall ; sometimes a ventral mesentery is also present
bearing a corresponding relation to the ventral surface.

Some Chaetopoda are entirely devoid of blood-vessels. In one
family in which this is the case (the Glyceridce among the Phanero-
cephala), the perivisceral fluid, which assumes some of the functions
of the blood, contains numerous red corpuscles, the red colour of
which is due to the presence of haemoglobin (see p. 36). In the
majority of the Chaetopoda there is a highly developed vascular

\ PHYLUM ANNULATA 465

system. Sometimes the blood is colourless : very commonly it
is bright red in colour, owing to the presence of haemoglobin,
which is not confined to the corpuscles, but is dissolved in the
plasma. In Serpula and its allies the blood is bright green, owing
to the presence of a green colouring matter, which has an affinity
for oxygen similar to that possessed by hemoglobin.

The chief blood-vessels are usually dorsal and ventral longi-
tudinal trunks. These are connected together by metamerically
arranged transverse branches. In some of the Cryptocephala the
dorsal vessel is not present in the greater part of the length of the
body, its place being taken by a peri-intestinal sinus or a plexus of
vessels lying in the wall of the alimentary canal. This gives off in
front a short thick-walled dorsal vessel or " heart." The movement
of the blood is effected in most instances by peristaltic contractions
of the dorsal vessel, or of a peri-intestinal sinus or plexus, or of the
" heart " given off by the latter anteriorly, which have the effect of
driving the blood from behind forwards. In some instances, as in
the Earthworms and some Cryptocephala, specially dilated lateral
vessels are contractile, and by their pulsations bring about the
circulation of the blood through the system of vessels. Plexuses
of fine capillary vessels in the integument of various parts frequently
aid in respiration, and are particularly well developed in certain
forms in which definite organs of respiration are absent.

The nervous system consists of a cerebral ganglion or brain
and a double ventral chain of ganglia. The cerebral ganglion is
distinctly bilobed, and may be looked upon as composed of two
intimately united ganglia. It is almost invariably' situated in the
prostomium, though placed a little further back in the Earth-
worms ; it gives off branches to the eyes and tentacles. From it
there run backwards and downwards the paired cesophageal con-
nectives, which embrace the anterior part of the alimentary canal
between them, and below join the anterior end of the ventral chain
of ganglia. The latter always exhibits indications of being made
up of two lateral halves in the double character of the connecting
commissures and frequently of the ganglia themselves. One of
these double ganglia occurs in each segment, and from it a number
of nerves pass out to the various parts of the segment. In certain
Cryptocephala (Serpula and others) the two halves of the chain are
separated from one another by a wide space, across which trans-
verse commissures pass between the ganglia. Connected with the
cerebral ganglia, or with the cesophageal connectives, or with both,
there is a system of delicate stomatogastric nerves passing to the
walls of the anterior part of the alimentary canal. In the majority
of the Chaetopoda the cerebral ganglion and the ventral chain are
separated from the epidermis ; in some, howeverj the ventral chain
is in contact with the epidermis, and in certain primitive or aberrant
forms, the Archi-Chsetopoda (Fig. 381) and Sternaspis, the cerebral

VOL. I. H H

466

ZOOLOGY

SECT.

ganglion is in close union with the epidermis ; in these also the
ventral cord is not segmented into ganglia. Running longitudinally
through the ventral cord in many forms are certain giant fibres of
very large size ; though these may have rather a skeletal than a
nervous function, they are simply greatly enlarged and modified
nerve-fibres. Nerve-cells may be confined to the ganglia, or may
be distributed over the entire surface of the ventral cord. Giant
nerve-cells occur in some forms in certain regions. Small ganglia
are found frequently in various peripheral parts, more especially
at the bases of cirri or of sensory papillae.

The organs of special sense are eyes, tentacles and cirri,
nuchal organs, and otocysts. Eyes, absent in the Oligochaeta with

dors, vess

Fia. 381. — Saccocirrus, transverse section, to show the position of the nerve-cords, dors. vets.
dorsal vessel ; int. intestine ; ne. co. nerve-cord ; set. setae. (After Fraipont.)

a few exceptions and in some of the tube-forming Polychasta as well
as in a few free forms of that sub-class, are very general in their
occurrence. Their structure is, as a rule, very simple, but in some
forms reaches quite a high grade of development. Usually they are
confined to the prostomium, but Polyophthalmus, in addition to the
prostomial eyes, has pairs of eye-like organs — probably light-
producing (phosphorescent) organs — on many of the segments
of the body. Leptochone has a pair on each segment, and in Fabricia
there is a pair on the anal segment ; while in many species of
Sabella and all the species of Dasychone there are eyes on the branchial
filaments.
Most usually the eye is (as in Nereis, p. 437, Fig. 359) a

x PHYLUM ANNULATA 467

spherical capsule with a wall composed of a single layer of cells,
which are elongated on the inner side, i.e. the side turned towards
the brain, while on the outer side they are usually flattened. The
outer thin part of the wall of the capsule, or cornea, is some-
times united with the epidermis ; when the two layers remain
distinct, the outer one is the outer cornea, the inner the inner
cornea. In many cases a thickening of the surface cuticle over the
cornea forms a cuticular lens. The cells of the inner portion of
the wall of the capsule form the elements of the retina ; they are
long narrow cells, sometimes composed of three distinct segments
— (1) a clear rod, directed towards the central cavity ; (2) a middle
segment which is densely pigmented ; and (3) a segment contain-
ing the nucleus of the cell and directed towards the brain or the
optic ganglion, with which it is connected by a nerve-fibre.
Frequently the second and third segments are not to be separately
recognised, the whole of that part of the cell which contains the
nucleus being densely pigmented. A refractive mass fills the
interior of the capsule, and is sometimes distinguishable into a
firmer outer part, the lens, and a more fluid inner part, the vitreous
body. This refractive mass is often continuous with the cuticle
externally, and internally may be in continuity with the rods. In
some cases the structure of the eye is very much simpler. The
eyes on the branchial filaments of many tube-forming Polychaeta
consist each of a group of retinal cells having its own lens-like
body and quite independent of the others ; the eye is thus a
compound one.

Nuchal organs or ciliated pits (Fig. 377, B, s) are very general in
the Polychaeta. They consist of a pair of special ciliated areas or
pits on the head, eversible in certain cases.

Otocysts (statocysts) are only exceptionally present. They consist
of capsules of ciliated cells, in the fluid contained in which there
are one or several calcareous otoliths (statoliths).

Tactile cells of the epidermis, with or without a projecting tactile
hair or stiff cilium, are very common, especially on the prostomium
in the Oligochseta and on the tentacles and cirri in the Polychseta.
Groups of these are often aggregated together in papilla or goblet-
bodies, with special nerve-supply and often with a ganglion or a
single nerve-cell at the base.

The organs of excretion of the Chsetopoda are a series of
segmentally arranged tubes, the nephridia, of which a pair as a
rule occur in each of the segments of the body with the exception
usually of a few at the anterior and a few at the posterior end.
In its simplest form the nephridium is a curved tube, primarily
ectodermal in origin, ciliated internally, opening on the exterior by
a laterally-placed pore at the one extremity, and at the other ending
in a ciliated funnel or nephrostome, which opens into the ccelome
either of the same segment as that on which the external aperture

H H 2

468

ZOOLOGY

SECT.

* •-, solen ccyles

is situated (most Polychseta) or of the segment in front (all or
most Oligochaeta, some Polychaeta). The nephridia thus in such
cases effect a communication between the ccelome and the exterior,
and serve to carry off waste-products which have passed into the
coelomic fluid ; but in many instances the cells lining the tube are
active in separating out such waste-matters, and are loaded with
granules and concretions.

In many Polychseta, however, there is no ciliated ccelomic
aperture, the tube ending blindly internally, such a blindly ending
nephridium (Fig. 382) being frequently branched. On the inner
extremities in such cases, or on other parts of the tube, are situated
a number of peculiarly modified cells, the solenocytes, sometimes
separate, sometimes united together in groups. Each of these is
a rounded celMying in the ccelome, and connected with the
nephridium by^a^long, slender, tubular process : through the

lumen of the process extends
a single, extremely long, vibra-
tile flagellum, which may be
prolonged for some distance
in the interior of the nephri-
dium itself. The resemblance
between these solenocytes and
the flame-cells of Platyhel-
minthes will at once be
recognised.

In the Polychseta another
set of segmentally repeated
structures are frequently inti-
mately connected with the
nephridia. These are a series
of pairs of ciliated funnels,
the ccelomoducts, opening
widely into the ccelome, and,
in a typical case, communicating with the exterior. In Nereis
they are represented by the dorsal ciliated organ, and are not
known to open externally. When provided with external aper-
tures, as is usually the case, the ccelomoducts act as the
efferent ducts for the sexual elements. In many of the Poly-
chaeta they do not remain independent, but coalesce partially
or completely with the nephridia, and the functions of excretory
organs and reproductive ducts become combined in the one set of
" segmental organs " (Fig. 383). In some families of Polychseta
(Serpula and allies) there is a single pair of large nephridia in the
anterior region of the body, with smaller pairs in the posterior
segments, the former alone appearing to have an excretory function,
while the latter act exclusively as genital ducts. In Sternaspis
only a single pair of nephridia are present, which, though they have

nephridial—
canal

FIG. 382. — Inner branched end of nepliridium of
Fhyllodoce paretti, showing the nephridial
canal and the solenocytes. (After Goodrich.)

PHYLUM ANNULATA

469

small ciliated funnels, are not known to communicate with the
exterior.

In the Oligochseta the hephridia are usually simple, elongated
and coiled tubes, a pair or sometimes more than one pair in each
segment ; but, in some, these are replaced or supplemented in
certain of the segments, or, in all, by a branching system of tubes
with or without ciliated funnels.
Sometimes the ordinary neph-
ridia are not developed in the
segments lodging the reproduc-
tive organs, their place being
there taken by three pairs of
tubes of the nature of localised
ccelomoducts which become
modified to give rise to the
reproductive ducts ; but ordi-
nary nephridia may be present
in these segments as well. In
some Oligochaeta the nephridia
of the most anterior segments
open into the mouth or pharynx,
and have apparently taken on
the function of digestive glands
(peptonephridid), and aU the
nephridia of the posterior region
of the body in one species (Allo-
lobophora antipce), instead of
opening on the exterior, com-
municate with a pair of longi-
tudinal canals which posteriorly
open into a median vesicle
communicating with the rectum.

The permanent nephridia of
the adult Chsetopod are pre-
ceded in the larva by pro-
visional or embryonic nephridia
of a temporary character. These
have been found to occur in the
head (prostomium) of many
larval Oligochseta and Poly-
chseta. They are ciliated in-
tracellular tubes, sometimes

Fia. 383. — Diagram to illustrate the various
combinations of closed and open nephridia
and coclomoducts in the Polychseta.
la, Hypothetical stage with closed nephridia
and separate coelomoducts ; b, condition in
which the ccelomoducts have become united
with the nephridia : this occurs in Phyllo-
docidce and Goniadidce ; c, condition in which
the cffilomoduct becomes reduced to a ciliated
organ (N ephthyidce) ; Ila, combination of
nephridia with nephrostomes and separate
ccelomoducts (Dasybranchus} ; b, condition in
which " segmental organs " are formed by the
union of nephridia with nephrostomes and
coelomoducts (the most usual condition) ;
c, condition in which there are nephridia with
nephrostomes, and the coelomoducts are
reduced to ciliated organs (Nereis, etc.). The
nephridia are outlined with a thick line : the
coelomoducts striated. (After Goodrich.)

branched, which do not open
into the cavity of the prostomium. Sometimes solenocytes
occur at the inner ends of the branches or of the undivided tube.
Embryonic nephridia have also been shown to occur in the body
in certain forms.

470

ZOOLOGY

SECT.

Phosphorescence, the production of light rendering the
animal brilliantly luminous in the dark, occurs in a few cases
(various Polynoids, Chcetopterus, &c.). In some of these (Poly-
ophihalmus) the light is produced by eye-like organs in most of the
segments.

In the arrangement of the reproductive organs in the
Chaetopoda there is an essential difference between the two sub-
classes, the Oligochseta being hermaphrodite, and the Polychaeta,
with only a very few exceptions, unisexual. In the latter the
gonads, ovaries or testes as the case may be, are masses of cells
which are developed as the result of a proliferation of the ccelomic

perik

r&r.ffl

-vent.veas

JberiC

c

peril; — —

vent, vess

refir.yL

vent.

vess

ven£. vess

FlO. 384. — Diagram to illustrate the development of a gonad from the peritoneal (cffilomic)
epithelium in one of the Polychseta. 'peril, peritoneal membrane ; repr. gl. gonad (repro-
ductive organ) ; vent. vess. ventral vessel. (After E. Meyer.)

epithelium in certain positions (Fig. 384). Usually these organs,
which are only conspicuous about the breeding season, occur
in the great majority of the segments of the body ; sometimes they
are confined to a certain region. The exact place which they
occupy in the interior of the segment varies in different cases :
sometimes they surround one of the principal blood-vessels,
sometimes they are situated laterally, in the bases of the para-
podia. The sperms frequently undergo the final stages of their
development after they have become detached from the testes,
while floating in the coelomic fluid, and the same sometimes holds
good of the ova. Both sperms and ova appear to reach the
exterior, in[the majority of cases, through the " segmental organs,"

x PHYLUM ANNULATA 471

which may become modified and enlarged at the breeding season,
though in some forms it is stated that the reproductive cells escape
through temporary or permanent openings in the body-wall.
Impregnation takes place externally in nearly all.

In the Oligochaeta the reproductive organs are confined to a
certain limited region of the body. There are either, as in the
Earthworms, two pairs of testes, or a single pair, as in the aquatic
forms. The testes are small, and frequently become reduced to
mere vestiges in the adult animal, having mainly become broken
up into sperm-mother-cells, which in some way reach the vesiculae
seminales to undergo development into mature sperms. The
vesiculce seminales are comparatively large sacs, which vary in
number and arrangement in the different genera. One or two
median sperm-reservoirs, formed by the coalescence of pairs of
vesiculae, may be present. In the same segments as the testes, and
opening into the sperm-reservoirs when the latter are developed, are
either two or four ciliated funnels, according to the number of the
testes, leading into efferent ducts. All the four ducts, when four
are present, may remain distinct, or the two ducts of each side
may open into a common atrium, or they may unite to form a
common elongated vas deferens, opening at the male genital
aperture. In connection with the terminal part of each vas
deferens in many Oligochaets is a gland known as the prostate or
spermiducal gland. Near the aperture of the vas deferens in many
Earthworms are special setae, the penial setce.

There are never more than two ovaries, which, like the testes,
are of very small size. The ova may become mature in the ovary,
or groups of cells may be detached from the latter and one cell
in each group ripen into an ovum. A receptaculum ovorum
occasionally receives the ova after they leave the ovary. There
are two oviducts, which open by funnel-shaped apertures into the
coelome. Receptacula seminis are present.

Development. — The Oligochseta deposit the eggs in cocoons,
either buried in the earth or attached to water-plants. The
cocoon contains, in addition to a number of fertilised ova, a quan-
tity of an albuminous fluid which serves as nourishment to the
developing embryos. Segmentation is always unequal. In the
forms in which food-yolk is scanty there is a process of embolic
invagination (Lumbricus rubellus) ; in the others (Tubifex, &c.) the
process is of the epibolic type. In the former case a blastula and
an invaginate gastrula are formed in the way already described
in the case of the Earthworm. In Lumbricus trapezoides the
gastrula divides into two, each half subsequently giving rise to
an embryo. The micromeres spread over the megameres very
much as in the Polychseta. A pair of mesoderm cells early appears,
and by their division forms the mesoderm bands. No free larval
stage similar to the trochophore occurs in any of the Oligochseta,

472

ZOOLOGY

SECT.

but the stage intervening between the completion of the gastrula
and the commencement of the segmentation of the mesoderm
bands corresponds to the trochophore in essential respects ; and in
some forms there is recognisable a feebly developed circlet of cilia
comparable to the prototroch, and in some a pair of head-nephridia.
Impregnation and the development of the embryo take place

externally in all the Chaetopoda,
with a very few exceptions in which
development occurs in the ccelome
or in the interior of a dilated seg-
mental organ. In the Polychseta, in
the great majority of cases, fertilisa-
tion takes place by the sperms
coming in contact with the ova
when both have become discharged,
and the development of the embryos
enwt goes on while they are floating freely
in the sea. There are a few cases
in which the impregnated ova are
received into a sort of brood-pouch
and there pass through at least the
earlier stages of their development.
Such a brood-pouch is formed in
certain Phanerocephala by the
raising up of the integument on the
ventral surface. In some species
of Polynoe and allied genera, the
fertilised ova and the resulting
embryos adhere in masses to the
dorsal surface under the shelter of
the elytra ; in some other Polychseta
(certain Syllidea) they are stuck by
means of a viscid secretion to the
dorsal or the ventral surface (Fig.
385), or to the cirri. In certain
Cryptocephala (Fig. 386) they de-
velop in a cavity in the operculum ;
in others, in the interior of the
tube, between the body of the worm
and the inner surface of the latter,
or on its outer surface. In some,
again, though the ova do not remain in any way attached to the
parent worm, they may be deposited in clumps or packets enclosed
in gelatinous matter. Usually they have no other covering but the
egg-membrane.

The segmentation of the ovum in the Polychseta is unequal.
In the great majority the inequality between the megameres and

FIG. 385. — Fionosyllis elegans.

Dorsal view of female with advanced
embryos attached to the ventral sur-
face, d. c. dorsal cirri ; emb. embryos.
(From Potts, after Pierantoni.)

PHYLUM ANNULATA

473

micromeres is very marked. In some Serpulids, however, the differ-
ence is very slight, and the two sets of cells are at first scarcely
distinguishable. In such cases the cells arrange themselves in
such a way as to form the wall of a hollow sphere, the blastula,

OV

FIG. 386.— Spirorbis laevis, a hermaphrodite tubicolous Polychaet. Lateral view of entire
animal, ant. neph. anterior nephridium ; br. branchiae ; ces. oesophagus ; op, operculum with
developing embryos in its interior ; ov. ova ; sp. sperms ; st. stomach. (After Claparede.)

with an internal closed cavity, the segmentation-cavity. The
megameres, which may or may not have been distinct from the
first, lie on one side of the blastula ; and soon this side becomes
invaginated (Fig. 387, A), the result being the formation of an

474

ZOOLOGY

SECT

cmbolic gastrula. In the great majority of forms, however, an
epibolic gastrula is formed after the manner already described
in the case of Nereis ; but forms of the process of gastrulation
intermediate between these two extremes have been observed.
The blastopore of the gastrula, however formed, does not usually
give rise directly either to the mouth or to the anus. It becomes
elongated into a slit which becomes closed up, and the anus and
proctodaeum are formed by a fresh invagination in the original
position of its posterior end, while another invagination of the
ectoderm further forwards gives rise to the mouth and stomodseum.
The embryo then passes into the trochophore stage.

The arrangement of the cilia on the surface of the trochophore
varies in different Polychseta. Sometimes, though rarely, the

JZ*

FIG. 387.— A, B, C, three stages in the development of the trochophore of Eupomatus, from
the side. an. anus \fh. blastocoele ; m. polar cells of the mesoclerm ; md. mid-gut ; n. larval
head-nephridium ; ot. otplith ; sp. neural plate ; st. stomodceum ; wk, pre-oral ciliated ring;
tfti, post-oral ciliated ring. (From Lang's Comparative Anatomy, after Hatschek.)

pre-oral circlet is absent and the surface is covered uniformly with
cilia in addition to an apical tuft : such larvse are said to be atrochal.
Typically there are two circlets close together, the one pre-oral,
immediately in front of the mouth, and the other post-oral, immedi-
ately behind it. Sometimes, in addition to the pre-oral circlet,
there is a peri-anal circlet round the anal end (telotrochal larvse).
In some cases the pre-oral circlet is absent and the post-oral is
situated about the middle of the body (mesotrochal), or there may
be several between the mouth and the anal end (polytrochal).

The post-oral portion of the larva elongates, and traces of
segmentation become visible ; sometimes a series of constrictions
are developed before there is any trace of parapodia, sometimes
rudiments of the latter with their setae are developed first. The

PHYLUM ANNULATA

475

bud.

number of segments, at first very small, becomes added to from
behind as the body gradually elongates. The establishment of
external segmentation is accompanied by the division of the meso-
derm bands into a series of segments, the history of which has been
sketched in describing the development of Nereis. The ectoderm of
the ventral plate develops a median thickening which gives rise to
the ventral nerve-cord. Anteriorly this becomes connected by a
pair of thickenings at the sides of the mouth — the rudiments of the
cesophageal connectives — with the developing cerebral ganglion.

The completion of metamorphosis is brought about by the
increase in length of the body and concomitant increase in the
number of segments, by the full development of the various
systems of internal organs, and by the forma-
tion of the tentacles and other appendages.
The parapodia, when first formed, very usually
bear relatively long provisional setce, which are
subsequently thrown off to make way for those
of the adult.

Asexual reproduction by simple fission
followed by regeneration of the lost segments,
or by proliferation followed by fission, occurs
in certain groups of Chaetopoda both among
the Oligochaeta and the Polychseta. Simple
fission occurs in Salmacina, one of the Ser-
pulids : a constriction becomes formed at a
certain point towards the posterior end, rudi-
ments of a new set of cephalic branchiae bud
out on one side at this point, and this pos-
terior part becomes a distinct zooid, which is
eventually separated off and develops the full
number of segments characteristic of the adult.
This is not in any way a case of alternation of
generations, as both parent and offspring are
similar and sexual (hermaphrodite). In Nais
and Chcetogaster (Oligochaeta) there is multipli-
cation by proliferation of the segments at the
posterior end ; then the appearance of a constriction separating off
five or six of the most posterior segments followed by a fresh proli-
feration in front of the constriction ; and then a second constriction
appears five or six segments further forwards — the result being the
development of a chain of zooids which remain for a time connected
together. The sexual cells become fully developed only after the
zooids have separated from one another.

In some of the Syllidse there is a distinct alternation of generations.
The asexual worm developed from the ovum gives rise by a process
of posterior proliferation and constriction (Fig. 388) to sexual
zooids, a number of which may remain for a time connected together

FIG. 388. — Budding in one
of the Syllidae (Auto-
lytus cornutus) ; parent
stock with a male zooid
(bud) nearly ready to
become detached.
(After Agassiz.)

476

ZOOLOGY

SECT.

in a string before undergoing separation. These sexual zooids
become developed into mature males or females, which may be
remarkably unlike the parent form in the shape of the parapodia,

the character of the
setae, and other
points ; and in
some instances the
two sexes not only
differ from the
asexual parent form
but also from one
another, so that the
three forms, before
their relationship
was known, were
set down as repre-
senting three dis-

FIG. 38i».— Portion of Syllis ramosa. (From the Cambridge + :nn4. „«„«,

Natural History, after Mclntosh.)

Syllis ramosa

(Fig. 389), which occurs in the interior of certain deep-sea sponges,
is exceptional among the Chaetopoda in giving rise by lateral
branching to a colony from which
sexual zooids afterwards become
separated off.

Modes of Life, etc. — Very few
Chsetopoda are true parasites ; but a
considerable number are to be set
down as commensals, habitually asso-
ciating with another animal for the
sake of food and shelter. The Earth-
worms burrow in soil containing
decaying vegetable matter, passing
the mould through their intestine and
subsequently throwing it out in the
shape of castings on the surface.
They also feed on decaying leaves, and
sometimes on animal substances.
Some of the fresh-water Oligochaeta
(Tubificidce) manufacture tubes of
mud held together by a tenacious
secretion from the epidermal unicel-
lular glands. Some Of the Phanero- FIG. 39 J.—Serpulae with their tubes.

cephala form temporary tubes of a

gelatinous character, or more permanent parchment-like tubes some-
times strengthened by means of agglutinated sand-grains. But the
majority of the Phanerocephala, which for the most part prey on
other small animals, are not confined to tubes, but move about

x PHYLUM ANNULATA 477

freely. Some burrow in sand ; others even in harder substances,
such as the shells of Mollusca, or in limestone, shale, or sandstone.
Many Cryptocephala secrete tubes the substance of which is derived
from the epidermal glands. These tubes are sometimes membranous
or parchment-like, sometimes membranous but hardened by the
deposition of grains of sand or particles of broken shells or bits of
sea- weed ; sometimes (Fig. 390) they are of a hard, shelly, calcareous
character, sometimes composed entirely of foreign particles
cemented together ; very frequently they are permanently fixed to
foreign objects. Some, such as species of Polydora and Stylarioides,
near relatives of which construct tubes, excavate galleries in rock
or coral or in the shells of various Mollusca.

A few Polychseta, such as the Alciopidce and Tomopteris, as well
as, in a certain phase, the Nereidce and Syllidce, are pelagic ; but
the majority live on the sea-bottom. They occur in the greatest
abundance near the shore ; but are also found at all depths in the
ocean, the tube-dwelling forms being more abundant than the free
forms in the deeper zones.

Owing to the soft character of most of their parts, there are
comparatively few actual remains of Chaetopoda in the older
geological formations, though there are many burrows and tracks
which have been ascribed to members of that class. Tubes of
tubicolous Polychaeta have, however, been found in formations
dating from the Cambrian period onwards. Some tubes, not
distinguishable from those of the existing genus Spirorbis, are
found as far back as the Silurian ; and others, apparently closely
related to the living Serpula, as far back as the Carboniferous.
In addition there are a number of tubes of extinct forms ascribed
to the tubicolous Polychaeta. The horny jaws of various Polychseta
have been detected in strata from the Cambrian period onwards ;
and many tracks and burrows occurring in rocks of all ages are
ascribed, some with more, some with less certainty, to this group
of worms. No fossil remains of Oligochseta are known.

APPENDIX I. TO THE CELETOPODA.

CLASS MYZOSTOMIDA.

The Myzostomida are a group of worms which appear to have
their nearest relatives in the Chsetopoda, though possessing certain
special features of their own. They are all external, or, in one case,
internal, parasites of various Crinoids — both of the stalked and the
free varieties, or internal parasites of certain Starfishes. They are
disc-shaped animals (Fig. 391) (elongated in Stelechopus) devoid of
any trace of external segmentation. There are patches of cilia
here and there on both dorsal and ventral surfaces. At the sides
there are five pairs of parapodia (p), each with a chitinous hook and

478

ZOOLOGY

SECT.

a supporting rod ; in the intervals between these there are in
Myzostoma four pairs of small " suckers " ; and round the margin
are a series of ten or more pairs of cirri provided terminally with
motionless sensory cilia, and with a ventral groove lined by adhesive
cells. The mouth, usually situated at the anterior extremity, leads
into a muscular pharynx (Fig. 392, ph.) capable of being protruded
as a proboscis ; from this a narrow oesophagus leads to the stomach,
which gives off a number of branched lateral diverticula (da.). A
short cloaca (Ho.) leading from the stomach opens on the exterior,

in most cases at the
posterior end of the
body, sometimes on the
dorsal surface. There
is no distinct ccelome,
the space between the
alimentary canal and
the body-wall being
filled by connective-
tissue (parenchyma),
leaving only the cavi-
ties in which the sexual
elements are lodged.
Bundles of dorso-
ventral muscular fibres
form imperfect trans-
verse septa, as in some
Platyhelminthes.

There is no blood-
vascular system, and
specialised organs of
respiration are likewise
wanting. There is a
single pair of nephridia
with funnel-shaped in-
ternal apertures and
with external openings
either into the cloaca
or on the surface. The
nervous system comprises a large stellate ganglion situated ventrally,
probably representing a number of fused ganglia, and giving off a
number of nerves ; and two nerve-rings, one round the
oesophagus, the other round the pharynx, the two rings being
connected together by a series of longitudinal nerves. The
cesophageal ring presents a very obscure dorsal thickening, which
is the only representative of a cerebral ganglion.

Most of the Myzostomida are hermaphrodite. There is a pair of
ovaries formed by the proliferation of the layer of (coelomic)
epithelium covering the stomach ; and in the sexually mature

FIG. 391.— Myzostoma. I— X, cirri; m. mouth
p. parapodia ; s. suckers. (After von Graff.)

x PHYLUM ANNTJLATA 479

animal branching (coelomic) spaces in the parenchyma, between
the cseca, are found to be filled with ova (o). A posterior continua-
tion of these spaces (u) opens either into the cloaca or independently
of it. There are two elongated and usually branched testes (h),
each of which has a vas deferens leading to a vesicula seminalis (sb)
which opens near the lateral margin. In the hermaphrodite
forms the testes are matured before the ovaries, and may have
ceased to be functionaLbefore the ova become ripe.

FIG. 392. — Myzostoma. Diagrammatic view of the internal organs, c, cirri ; da, branches of
the stomach ; ed, hind-gut ; h, testes ; klo, aperture of cloaca : m, stomach ; mo, male genital
aperture ; o, ovaries ; p. parapodia, with hooks and supporting rod ; ph, pharynx ; php,
pharyngeal tentacles ; pht, pharyngeal pouch ; sb, vesicula seminalis ; m, suckers; M, uterus;
wo, female genital aperture. (From Lang's Comparative Anatomy, after von Graff.)

The development of the Myzostomida closely resembles that of
the Polychaeta. A trochophore larva is first formed, and this
becomes metamorphosed into a larva with provisional setae bearing
a close resemblance to that of Nereis (p. 443).

APPENDIX II. TO THE CILETOPODA.

CLASS ECHIURIDA.

The Echiurida are a small group of marine worms including the
genera Echiurus, Thalassema, Bonellia, Pseudobonellia, Hamingia,

480

ZOOLOGY

SECT.

ant set

Saccosoma, and Epithetosoma. They are unsegmented in the adult
condition and have no parapodia, but, except in the case of Sacco-
soma, have a pair of ventral setae in one or both sexes. The general
shape is cylindrical or ovoid, with the mouth anterior and the anus
posterior. Overhanging the mouth is a median appendage usually
termed the proboscis (absent in Saccosoma). This is sometimes of
great length, sometimes comparatively short, and is highly sensitive
and mobile, but is not retractile. In some cases it contains a
cavity communicating with the ccelome. There is a wide ccelome
undivided by mesenteries, but crossed by numerous muscular
strands which support the alimentary canal. The alimentary
canal is a greatly coiled tube with a short
thick-walled pharynx in front and an ovoid
rectum behind. With the intestine communi-
cates at both ends an elongated tube — the
siphon, comparable to that of the Echinoids —
and a ciliated groove runs throughout its length.
There is a closed blood-vascular system with a
peri-intestinal sinus, a dorsal vessel, and a
ventral vessel. The most important part of the
nervous system (Fig. 395) takes the form of an
unsegmented ventral nerve-cord giving off pairs
of nerves. In front this bifurcates, and the two
cords resulting from the division running for-
wards past the mouth, extend throughout the
length of the proboscis, at the anterior
extremity of which they unite to form a great
loop which shows no trace of ganglia. There
are no organs of special sense. Into the rectum
open a pair of caeca — the anal vesicles or pos-
terior nepkridia, which are probably the chief
excretory organs. These have appended to
them a number of ciliated funnels which open
into them from the coelome. Further forward
there is a pair of nephridia, one of which may
be aborted, or there may be two or three pairs.
The sexes are distinct and there may be extreme
sexual dimorphism. The larva is a trochophore.

Echiurus (Fig. 393) is approximately cylindrical, with a moderately
long proboscis which is excavated longitudinally on the ventral
surface by a ciliated channel at the posterior end of which the
mouth is situated. The surface is covered with small papillae
arranged in rings. On the ventral surface not far from the anterior
end is a pair of short simple pointed setae (ant. set), each lodged in
a setigerous sac and capable of being moved freely by means of
appropriate muscles. Surrounding the posterior (anal) end of the
body are a number of setae (post, set) arranged in one or two rings.

^rNfiw

t>ost.set

Fia. 393. — Echiurus,
entire animal, ant. set.
anterior setfe ; post,
set. posterior sete ;
prob. proboscis. (After
Greef.)

PHYLUM ANNULATA

481

The anal vesicles or posterior nephridia (Fig. 394, post, neph) are
narrow and elongated, and their funnels are sessile. There is a
pair of anterior nephridia (ant. neph) with nephrostomes and
nephridiopores situated ventrally behind the setae. The sexes
are alike and the ovary or testis is a ventrally situated median mass
of cells in the posterior half of the coelome. The anterior nephridia
act as genital ducts.

Thalassema is allied to Echiurus, but has a shorter proboscis and
no peri-anal setae. There are from one to three pairs of anterior
nephridia.

Bondlia (Fig. 396) is ovoid in shape and green in colour, and is
covered with large papillae which are not arranged in rings. The

t.neph

-post.neph

Fio. 394.— Echiurus, internal organisation.
an. anus ; ant. neph. anterior nephridia ; int.
intestine ; int. vess. intestinal vessel ; ces. oeso-
phagus ; post. neph. posterior nephridia ; set.
setigerous sacs ; vent. vess. ventral vessel.
(After Greef.)

FIG. 395. — Echiurus, general outline
of the animal, with the nervous system
(diagrammatic), ne. co. nerve-cord ;
ne. ri. nerve-ring. (After Greef.)

proboscis is very long and very extensible, and bifurcated terminally.
There are two ventral setae, sometimes more. There is only one
anterior nephridium (Fig. 397, ant. neph) which is greatly enlarged
and acts as a uterus. The anal vesicles (post, neph) are ovoid, and
the funnels terminate the branches of narrow branching tubes
given off from the main vesicles. It is the female which has these
characters : the male (Fig. 399) is minute, Turbellarian-like,
ciliated, with in most species a pair of setae, no proboscis, and a
reduced alimentary canal without mouth or anus. In the young

VOL. I. II

482

ZOOLOGY

SECT.

condition it enters the pharynx of the female and when sexually
mature establishes itself permanently in the nephridium.

Pseudobonellia is nearly related to Bonellia, but has two anterior

neph.fun

ant.neph

QV

post.neph.

FIG. 397.— Bonellia, general view of the internal organs.
an. anus ; ant. neph. anterior nephridium:; int. intestine ;
FIG. 396.— Bonellia viridis, entire neph. fun. nephrostome ; ces. oesophagus ; ov. ovary ;
animal (female) with the proboscis ph. pharynx ; post. neph. posterior nephridium ; prob.
moderately extended. (After Greef.) proboscis; rent. ress. ventral vessel. (After Greef.)

nephridia (uteri). The degenerate male, which Kas no setae, lies in
a median pit opening on the ventral surface between the two uteri.
Hamingia has a general resemblance as regards the female to
Thalassema, but there are no setae. There are two anterior
nephridia." The male, which has two setae, is
degenerate and parasitic in the female, as in
Bonellia.

Acanthohamingia has eight small setae.
Epithetosoma has an extremely long filiform
proboscis which, unlike that of the other
genera, is hollow, containing a prolongation
of the coelome. There are no setae. There is
a single nephridium and no anal vesicles.

In Saccosoma both proboscis and setae
appear to be absent. The male is not known.

ril ml , r -n i •

Development. - - The larva of Echmrus
(Fig. 400) has a well - developed pre-oral

FIG. 398. — One of the cili-

ated funnels of the poste-

PHYLUM ANNULATA

483

lobe with pre-oral and post-oral circlets of cilia, and in
other respects closely resembles the trochophore embryo of
a Chaetopod. The posterior part of the body elongates, and the
mesoderm-bands, developed as in the Chaetopoda, become divided
into as many as fifteen segments. A circlet of setae is developed at
the anal end, and subsequently the two ventral setae are formed
in the same manner as in the Chaetopoda. The pre-oral lobe
becomes narrowed to form the cylindrical proboscis of the adult ;

and the rudimentary segmentation
gradually disappears as development
advances.

In Bonellia there is unequal seg-
mentation, as in most Chaetopoda,
resulting in the formation of four
large megameres and eight small

coel

repr.ap

ves.sem

slow

Fro. 399. — Male of Bonellia. ali.
alimentary-canal ; coel. groups of
coeloraic cells destined to give
rise to sperms ; repr. ap. repro-
ductive aperture ; ves. gem. vesi-
cula seminalis. (After Greef.)

out

FIG. 400.— Trochophore of Echiurus,
an. anus ; ap. pi. apical plate ; int. in-
testine ; mo. mouth ; ne. co. rudiment
of nerve-cord ; ces. oasophagus ; oes.
conn, oesophageal connective ; stom.
stomach. (After Hatschek.)

micromeres : the latter multiply rapidly, and grow over the mega-
meres so as eventually to enclose the latter in a complete layer of
ectoderm, save at one point, where there is a gap, the blastopore.
Here the ectoderm bends inwards to give rise to a continuous
mesoderm layer superficial to the megameres. The blastopore
soon closes up. The megameres divide to form the cells of the
endoderm, among which a lumen only appears comparatively late ;
mouth and oesophagus are developed as an outgrowth, at first
solid, from the endoderm. The anus becomes formed still later by
invagination at the hinder end of the body ; and a pair of epidermal

i I 2

484 ZOOLOGY SECT!

vesicles which appear at its sides, developed as outgrowths from the
terminal part of the intestine, form the rudiments of the posterior
nephridia. A rudimentary pre-oral lobe becomes established.
The mesoderm remains unsegmented, but splits into somatic and
splanchnic layers going to form the muscular system, blood-vessels,
and other mesodermal organs. Before the alimentary canal is
formed the larva, which had previously been spherical with two
bands of cilia and a pair of eye-spots, becomes elongated and
dorso-ventrally compressed, and covered uniformly with cilia,
so as to present the general appearance of a Planarian. It becomes
converted into the adult female by a metamorphosis, including the
elongation of the pre-oral lobe to form the proboscis and the
development of the pair of setae of the adult. The male never goes
through this metamorphosis, but remains in the Planarian stage :
it at first adheres to the proboscis of a female, then enters the
oesophagus, and afterwards, when sexually mature, passes into the
cavity of the nephridium.

CLASS II.-SIPUNCULOIDEA,

The Sipunculoidea are marine Annulata devoid of any trace of
segmentation in the adult condition, without parapodia, and
without setae ; with an invaginable anterior body-region or introvert,
at the extremity of which is the mouth surrounded by tentacles.
The anus is anterior and dorsal. There is an extensive ccelome
filled with a corpusculated fluid, and not divided by septa. The
ventral nerve-cord is not made up of a series of ganglia. There is,
as a general rule, only a single pair of nephridia. The sexes are
separate ; the ovaries and testes simple masses of cells ; the
nephridia act as reproductive ducts. The larva is a modified
trochophore.

1. EXAMPLE OF THE CLASS — Sipunculus nudus.

General External Features. — Sipunculus occurs on sand at
moderate depths off the coast in most countries outside of the
tropics. It is an elongated worm of a cylindrical shape, somewhat
narrower towards one — the anterior — end. There is no trace of
division into segments. The anterior portion of the body, to the
extent of about a sixth of the total length, is capable of being
involuted within the part behind. The surface of this anterior
part, which is termed the introvert (Fig. 401), differs in appearance
from that of the rest of the body in being covered more or less
closely with chitinous papillae. The papillae of the posterior
portion of the introvert are shaped like the bowl of a spoon, with
the concavity turned towards the body-wall and the tip directed
backwards ; they are so closely arranged as to overlap one another
like the shingles of the roof of a house : further back they become

PHYLUM ANNULATA

485

post /bap

FIG. 401. — Anterior extremity of Sipun-
culus nudus. ant. pap. anterior
papillary region ; post. pap. posterior
papillary region ; tent, tentacular fold.
(After Ward.)

longer and narrower, mammilliform, and more scattered. When

the introvert is fully evaginated, there appears at its extremity

a horse-shoe-shaped fold of the integument, the tentacular fold

(tent.), which is lobed and plaited

(Fig. 402) so as to assume somewhat

the appearance of a circlet of

tentacles. For a little space imme-
diately behind the tentacular fold

the surface of the introvert is free

from papillae. The posterior portion

of the body is devoid of papillae,

but is marked out by a series of

narrow impressed lines into a

number of elongated four -sided

areas.
Body- wall. — The surface is

covered by a chitinoid cuticle having

an iridescent lustre similar to that

presented by the cuticle of Nereis

and Lumbricus, and due to the

same cause — viz., the presence 'of

two systems of intercrossing lines.

The papillae on the introvert are

local thickenings of this cuticular layer. Beneath the cuticle is an

epidermis consisting of a single layer of cells, usually sac-like, but

capable of being altered as a result of contraction or compression

into a spindle-like shape. Below the epidermis is a layer of

connective-tissue, the dermis, in
which, as well as to some extent in
the epidermis itself, are a number
of dermal bodies. Of these there
are three kinds — bicellular glands,
contained in papillae ; multicellular
glands, scattered through the in-
tegument and not contained in
papillae ; and sense-papillce, small,
rounded thickenings of the epider-
mis in the anterior region of the
introvert, with their summits
covered with cilia. There are
also numerous pigment-cells. A
number of canals branch through
the dermis, beneath which are three
layers of muscle — (1) an outer circular layer, continuous in the
introvert, but divided into annular bands in the rest of the body ;

(2) an oblique layer, well developed only between the origins of the
two retractor muscles of the introvert ; (3) a longitudinal layer

FIG. 402.— Tentacular fold of Sipun-
culus nudus. cer. org. cerebral
organ. (After Ward.)

486

ZOOLOGY

SECT.

which is separated by spaces into a series of parallel bands.

Between the bundles of the longitudinal
fenl layer of muscle runs a system of canals

which communicate with the body-
cavity by transverse branches.

There is a spacious ccelome, but it
is traversed in all directions by fila-
ments and strands of connective-tissue,
with which are mixed very fine muscular
fibres ; these mostly run from the wall
of the body to the alimentary canal.
Floating in the coelomic fluid are (1)
colourless amoeboid corpuscles ; (2)
rounded biconcave corpuscles, faintly
dors. retr-1iM^i'lfiM]L-rcc£ coloured by a substance, hccmerythrin,

m.n.co

dorsretr-

ocs

n.

ret

FIG. 403. — Dissection of the internal or-
gans of Sipunculus nudus. dors. retr.
dorsal retractor muscles of the intro-
vert ; int. intestine ; m. n. co. muscles
accompanying the nerve-cord ; n. ro.
nerve-cord : neph. nephridium ; ces.
oesophagus ; rect. rectum ; tent, tenta-
cular fold. (After Vogt and Jung.)

FIG. 404. — Anterior part of the nervous system
of Sipunculus nudus. can. o. ceb. cere-
bral organ ; corns, a?, oesophageal connective;
n. mu. ret. nerves to retractor muscles ; n. xpl,
splanchnic nerves ; n. ta. 1-4, nerves to ten-
tacular fold ; /, //, nerves from ventral cord ;
24, main mass of brain. (After Ward.)

x PHYLUM ANNULATA 487

which perhaps acts, like haemoglobin in many animals, as a carrier
of oxygen ; (3) reproductive elements ; (4) peculiar unicellular
ciliated bodies, the urns, which are developed by proliferation from
cells on the wall of the dorsal blood-vessel. These are comparable
in structure and function with the ciliated funnels of the Hirudinea

(?.».)•

The blood- vascular system is very feebly developed. It
consists of dorsal and ventral contractile vessels, the former known
as the " heart," communicating in front with a circular sinus
at the base of the tentacular fold.

The alimentary canal (Fig. 403) is a cylindrical tube of
uniform character throughout. It is twice the length^^the body,
running back from the mouth towards the posterip^nend, and then
bending sharply round to run forwards to the aims, the two limbs
being twisted spirally round one another. Running along the inner
surface of the entire length of the alimentary canal, with the excep-
tion of the terminal part or rectum, is a narrow groove. Connected
with the rectum is a narrow ccecum of variable length, which opens
into the beginning of the rectum. Two tuft-like groups of rectal
glands occur close to the anal opening.

The nervous system (Fig. 404) differs considerably from that
of the rest of the Annulata. There is a relatively small bilobed
cerebral ganglion situated on the dorsal aspect just behind the
tentacular circlet, to which it gives off on each side several pairs
of nerves. Arising from it anteriorly and dorsally are a number of
digitate processes lying in the ccelome. The cesophageal connectives
(corns. 03) which it gives off behind are greatly elongated ; from each
arise muscular nerves (n. mu. ret), and also a visceral nerve (n. spl)
passing to the alimentary canal. The two commissures unite behind
to form a ventral cord, which extends throughout the rest of the
length of the body. The ventral cord presents no appearance of
ganglia : it sends off laterally a large number of pairs of nerves
(/., //.) ; on section it appears distinctly double. Two delicate
muscular bands (Fig. 403, m. n. co.), which take origin anteriorly
from the body-wall, become attached to the nerve-cord, and follow
it throughout its length, giving off small branch-bands to accom-
pany the lateral nerves. A canal with ciliated, folded and pig-
mented walls, which opens in the middle line of the dorsal surface
just behind the tentacular fold (Fig. 402, cer. org.), extends back-
wards to the anterior ventral surface of the cerebral ganglion, where
it ends blindly. It is possible that this, the cerebral organ, may be
a sensory organ of some kind. Eyes are wanting. The digitate
processes of the cerebral ganglion, which bear a number of ciliated
cups along their edges, may be sensory in character.

Sipunculus has only a single pair of nephridia. These
(Fig. 403, neph) are situated tolerably far forwards, the external
openings being about 2 cm, in front of the anus. They are long,

488 ZOOLOGY SECT.

nearly straight tubes, of a brown or yellowish colour, and very
mobile in the living condition. Near the external opening, which
is situated at the anterior end, is the internal opening into the
coelome. The sexes are separate. There are no definite gonads
except at a certain season of the year, when cellular elevations
developed in the connective - tissue covering the ventral
retractor muscles of the introvert represent ovaries or testes
as the case may be. These give origin to cells which become
detached and develop into the fully-formed sexual elements
while floating about in the coelomic fluid. The nephridia act
as gonoducts.

2. DISTINCTIVE CHARACTERS.

The Sipunculoidea are Annulata with the body devoid of any
appearance of segmentation in the adult condition. There is a large
coelome, which is not divided into chambers by mesenteries or
septa. A blood- vascular system is sometimes present, sometimes
absent. The ventral nerve-cord is not composed of a chain of
ganglia. There is usually only one pair of nephridia. The sexes
are separate, the gonads simple, and the nephridia act as
gonoducts.

The larva is in most cases a trochophore.

3. GENERAL ORGANISATION.

The Sipunculoidea are a class of worms whose position among the
Annulata is determined more from a consideration of their develop-
ment than of their structure in the adult condition, though the
latter suggests a tolerably close affinity with the Chaetopoda. The
body of a Gephyrean is unsegmented, usually more or less com-
pletely cylindrical, broadest behind and narrowing towards the
anterior end. The surface is covered with a chitinous cuticle
developed often into papillae, or tubercles, or hooks. The anterior
part of the body is capable of being invaginated within the part
behind ; at the extreme anterior end of this invaginable part or
introvert, when it is evaginated, is the mouth, surrounded by a
circlet of sometimes pinnate, sometimes simple, tentacles, or by a
lobed and plaited tentacular fold. The prostomium is quite
rudimentary. The anus lies far forwards on the dorsal surface.

Body-wall. — Beneath the cuticle is an epidermis, which is
composed of a single layer of cells. Among the cells are unicellular
(rarely multicellular) glands, and sensory cells. The muscular
wall of the body consists of external circular and internal longitu-
dinal layers, sometimes with oblique and internal circular layers
superadded. There is an extensive undivided coelome, lined, as
in the case of the Chsetopoda, with a coelomic epithelium, which is
sometimes ciliated. Apertures (stomata) lead from the coelome
into variously arranged systems of canals in the wall of the body.

x PHYLUM ANNULATA 489

The alimentary canal is not definitely divided into regions,
but is a thin-walled tube, the oral part known as the pharynx, a
slightly narrower part succeeding as the oesophagus, the greater
part as the intestine, and the terminal part as the rectum. A ciliated
groove runs throughout the length of the canal. The intestine is
bent on itself, and spirally twisted as it runs forwards to the anal
opening, which, as already noted, is situated far forwards on the
dorsal surface : at the junction of intestine and rectum is a single
simple caecum or a pair of caeca ; and a number of small branching
glandular appendages are attached to the rectum close to the anal
opening.

There are no specialised organs of respiration in the
Sipunculoidea. A blood-vascular system is sometimes present,
sometimes absent. When present, as it is in most, it usually com-
prises a contractile dorsal vessel closely applied to the intestine,
and a peri-pharyngeal ring or plexus.

The nervous system (Fig. 404) consists of a nerve-ring,
sometimes greatly elongated, surrounding the anterior part of
the alimentary canal, with a dorsal and anterior thickening
representing a cerebral ganglion ; and of a nerve-cord, devoid of
ganglia, running backwards from this along the middle of the
ventral surface, and giving off pairs of branches at regular intervals.
Eyes of a very simple character, consisting of mere spots of pigment,
are present in some.

In the majority of cases there is a pair of nephridia or brown
tubes which open externally on the ventral surface, and internally
communicate with the ccelome by means of ciliated apertures, the
form and position of which vary in different cases. They act as
efferent ducts for the reproductive elements (gonoducts) ; but
their function as excretory organs has not been definitely established.
In many* cases one of these is absent.

The sexes are usually distinct, and the reproductive organs
are of very simple character, consisting merely of ridges or clumps
of cells (gonads), sometimes enclosed in a membrane, developed at
various points on the body-wall or on the wall of one of the main
blood-vessels. The cells of these ovaries or testes may develop
in situ into perfect ova or sperms ; more usually they become
detached, and undergo the later stages of their development while
floating in the ccelomic fluid.

The early stages of the development closely resemble those of
the embryo of one of the Polychseta, and a stage corresponding to
the trochophore of that class is developed, but with the mouth
situated further forward in front of the ring of cilia and with the
anus in front of the posterior extremity on the dorsal side. At no
stage in the development has any trace been observed of the tem-
porary segmentation which forms so marked a feature in the
development of Echiurus. But in at least one member of the

490

ZOOLOGY

SECT.

group the mesoderm undergoes segmentation, and the coelome is
formed by the coalescence of cavities which appear in these
segments.

Distribution, Affinities, etc.— The Sipunculoidea are all
marine. They are only capable of very slow creeping locomotion,
and live for the most part either in natural rock-fissures, or in
interstices between masses of fixed animals such as mussels or
ascidians, or in discarded shells of Mollusca or tubes of Polychseta,
or in burrows which they excavate for themselves either in sand or
mud, coral or rock. Their distribution is general ; and they occur
at considerable depths as well as in shallow water.

The resemblances between the Sipunculoidea and the Echiurida
have been and are regarded by many authors as sufficiently impor-
tant to justify the inclusion of both in one
class to which the name Gephyrea was given.
But apart from the embryological history,
the introvert of the Sipunculoidea with its
crown of tentacles, the forward dorsal position
of the anus, and the absence of true setae
are all points in which the members of that
class diverge from the Echiurida and the
Chsetopoda. At the same time it is in the
Echiurida probably that the Sipunculoidea
find their nearest known relatives.

The Priapuloidea, often classed with the Sipun-
culoidea, may be mentioned here. Priapulus (Fig.
405), species of which have been found widely
distributed at moderate depths in extra -tropical
seas, is a cylindrical worm, unsegmented but with
numerous superficial annulations in the posterior
half. At the anterior end is the mouth and at the
posterior the anus. There is a short introvert, but
no oral tentacles or tentacular fold. At the posterior
end is a hollow single or double process beset with
hollow papillae (resp.) : the lumen of this is in com-
munication with the ccelome, and the whole appa-
ratus is doubtless a respiratory organ or branchia.
The alimentary canal, which is not coiled, consists
of a muscular pharynx, a thin-walled intestine,
and a rounded rectum. There is no blood- vascular
system. The nervous system is in close connection
with the epidermis : it consists of a simple ventral
nerve-cord and a circum-cesophageal ring without

FIG. 405.-PriaPulus, entire brain-enlargement. The sexes are distinct The

animal, resp. posterior papillae, ovaries or testes are a pair of masses of follicles

(After Ehlers.) each having its main duct which opens on the

exterior close to the anus. The genital ducts with

their branches seem to act as excretory organs. The development is

unknown.

•e&b

PHYLUM ANNULATA

491

CLASS III.— ARCHI-ANNELIDA.

More primitive in some respects than the other Annulata are the Archi-
Annelida, comprising only the family Polyyordiidcp. They are marine worms
with narrow, elongated, cylindrical body. The prostomium (Fig. 406, Pr. st)
is small, the peristomium (Per. st) large. The segments (Mtmr) are only
faintly marked off externally for the most part, though the internal division
of the ccelome by means of septa is complete. Parapodia and setse are
absent, but the prostomium bears a pair of tentacles (t). Several pairs of
simple nephridia are present. The position of the nervous system (Fig. 408)
is more primitive than in the Annulata in general ; it is continuous with

fr.st

FIG. 406.— Polygordius neapolitanus. A, the living animal, dorsal aspect, about five times
natural size ; B, anterior end, lateral view ; C, ventral view of the same ; D, portion of the
body showing the metameres ; E, ventral view of the posterior extremity ; An. anus ;
An. seg. anal segment ; c. p. ciliated pit ; yr. grooves between metameres ; Mth. mouth ;
Mtmr. metameres ; p. papillae ; Per. st. peristomium ; Pr. st. prostomium ; s. papillae on
tentacles (t). (From Parker's Biology, after Fraipont.)

the epidermis, and not separated from it by mesodermal elements as in most
of the others. A pair of ciliated grooves (c. p.) are probably to be looked
upon as organs of special sense.

The family Polygordiidce includes two genera — Polygordius and Protodrilus.
There are a pair of prostomial tentacles, long in Protodrilus, short in Polygor-
ilius, and a pair of ciliated pits. The segmentation is only very indistinctly
marked externally in Protodrilus by circlets of cilia ; in Polygordius it is
indistinct in front, but better marked behind. In Polygordius lacteus a

492

ZOOLOGY

SECT.

series of tooth-like processes occur round the anus, and in front a circlet of
adhesive papillae. In Protodrilus there is a ventral ciliated groove. There
is a vascular system with dorsal and ventral longitudinal vessels. In each
segment is a pair of simple nephridia. In Protodrilus there are two ventral
nerve-cords, connected together by transverse commissures : in Polygordius
the cord (Fig. 408, V. Nv. Cd) is single ; in neither genus is there any trace of
ganglia. The sexes are united in most individuals of Protodrilus, ovaries
occurring in all the first seven segments and testes in some of those immediately

following. In Polygordius the sexes are
separate; the ovaries or testes (Fig. 408,
Spy) are developed in the posterior seg-
ments. There are no special reproductive
ducts.

FIG. 407.— Protodrilus, en-
tire animal, int. intestine ;
mus. oe. muscular append-
age of oesophagus ; oss. ceso-
phagus. (After Hatschek.)

Cd

FIG. 408.— Polygordius neapolitanus, transverse
section of a male specimen. Cod. Epthm. parietal layer
of ccelomic epithelium ; God. Epthm.' visceral or
splanchnic layer of the same ; Cu. cuticle ; Der.
Epthm. deric epithelium ; D. V. dorsal vessel ; Ent.
Epthm. enteric epithelium ; M. PI. muscle-plates ;
0. M. oblique muscles ; Spyt immature gonads :
V. Nv. Cd. ventral nerve cord continuous with deric
epithelium ; V. V. ventral vessel. (From Parker's
Biology," after Fraipont.)

The larva of Polygordius is a typical trochophore (Fig. 409), and its
metamorphosis into the adult worm (Fig. 410) takes place as in the Polychseta
in all essential respects.

Ctenodrilus and its ally Zeppdinia resemble Polygordius in the ectodermal
position of the nervous system and in the presence of ciliated pits ; but they
have a number of setae on each segment, and only a single pair of nephridia,
which are situated in the head. There is only a single (longitudinal) layer of
muscles in the body-wall. Ctenodrilus multiplies asexually by fission, and
is hermaphrodite and viviparous. Also sometimes supposed to be related
to the Archi-annelids is Nerilla, a minute marine worm with parapodia and
setae and with hollow cirri. [Compare Dinophilea and Histriobdettea,
Section VII.]

PHYLUM ANNULATA

493

B

An ci

FIG. 409. — Trochophore of Polygordius neapolitanus. A, lateral view of entire larva ;
B, diagrammatic vertical section ; C, transverse section through the plane ab in B ; An.
anus ; An. ci. anal cilia ; Bl. blastocoele ; Br. apical plate ; Ent. enteron ; M sd. mesoderm ;
Msd. bd. mesodermal bands ; Nph. head-kidney ; Oc. eye-spot ; Pr. or. ci. cilia of prototroch ;
Prc. dm. proctodseura ; Pt. or. ci. post-oral cilia ; St. dm. stomodseum ; F. Nv. Cd. ventral
nerve-cord. (From Parker's Biology, partly after Fraipont.)

FiO. 410.— Later stage in the development of Polygordius neapolitanus, in which the
posterior part of the trochophore has become elongated and segmented ; A, entire larva ;
B, vertical section ; C, transverse section along the plane ab in B ; Z)i — I)'--, three stages in
the development of the'somatic mesoderm; C<xl. coelome ; Ccel.'Epthm. coelomic epithelium;
Der. Epthm deric epithelium ; M. pi. muscle-plate ; Msd. (som), somatic mesoderm ;
M sd. (spl), splanchnic mesoderm ; o. eye ; t. tentacle. Other letters as in preceding
figure. (From Parker's Biology, partly after Fraipont.)

494 ZOOLOGY

SECT. X

CLASS IV.- HIRUDINEA.

1. EXAMPLE OF THE CLASS — The Medicinal Leech (Hirudo medici-
nalis and H. (Limnobdella) austmlis).

The medicinal Leech is found in ponds, swamps, and slowly
flowing streams in many parts of the world. H. medicinalis is the
common British species : H. australis is an allied Australian
form.

External Characters. — The Leech is a vermiform animal,
some 6-10 cm. (2-3 inches) in length, but is capable of contracting
and elongating itself so as to produce great alterations in form and
proportions. It moves by " looping " movements, and is also a
good swimmer. The body (Fig. 411) is depressed or flattened
dorso-ventrally, the dorsal surface convex, the ventral flattened.
The anterior end presents a ventrally directed cup-like hollow, the
anterior sucker (a. s.), in the middle of which is a small aperture,
the mouth (mth.). The hinder end bears a disc-like posterior sucker
(p. s.), also directed downwards, and at its junction with the trunk,
on the dorsal surface, is the very small median anus (a.). The
animal is brightly coloured, the dorsal surface in H. medicinalis
being longitudinally banded with alternate stripes of greenish-
grey and rusty red, the ventral surface greenish-yellow, spotted
with black : in H. australis the whole under-surface is rust-
coloured.

The whole body is encircled by close-set transverse grooves,
dividing it into annuli. These, like the annuli of some Earth-
worms, are more numerous than the true segments or metameres,
the study of the internal organs showing that, except at the two
extremities, each segment contains five annuli. There are also
external characters by which the actual segmentation is plainly
indicated. The rust-coloured streaks on the back of H. medicinalis
are spotted with black, and at every fifth annulus the spots are
larger than on the intervening rings : the annuli thus marked are
the second of their respective segments. Moreover, the same
rings bear on the ventral surface minute paired apertures, the
nephridiopores or excretory apertures (np. 1, np. 17) : of these there
are altogether seventeen pairs, marking the first ring of the seventh
and the second ring of the eighth to the twenty-third segments.

In front of the first and behind the last pair of nephridiopores
one important external mark of segmentation fails, but a further
indication is furnished by the presence on the middle ring of each
undoubted metamere of a number of delicate transparent elevations,
the segmental papillce (s.p.)} which have probably a sensory func-
tion. These structures are found along the whole length of the
body, and as they mark the middle (third) ring of all those segments,

mth

Q.p <'

/

(

\

/

A

„

XVI

/ . . . . . .

J \

I

I Hj

• • -

XVII

. . . ...

• • • • . ...

XVIII

yiv

«

• « . . ...

XX

... ... ;

-1

• • . . ...

XXI '

... ... s

1

j

)

J

• • • . . * . .

XXII

... ... ^

i

\

(

)

1

*. )

\

XX 111

7 ... ... )

B

XXIV

XXV

Fid. 411. — Hirudo medicinalis. A, dorsal ; B, ventral aspect ; a. anus ; a. s. anterior sucker;
e. 1. first, and e. 5. fifth pair of eyes; g. p. <S . male gpnopore ; g. p. 9. female gonopore;
mth. mouth ; np. 2. first, and np. 17. seventeenth pair of nephridiopores ; p. s. posterior
sucker ; *. p. segmental papilUe ; I. — XXVI. segments.

49G

ZOOLOGY

SECT.

the extent of which can be checked by the nephridiopores, it is
legitimate to assume their segmental value in the anterior and
posterior regions, where the controlling excretory apertures are
absent. By the clue thus furnished it is found that there are six
segments in front of that bearing the first pair of nephridiopores,
and three behind that bearing the last pair, making a total of
twenty-six metameres : of these the first seven and the last three
have less than the normal number of rings.

The anterior sucker bears on its dorsal surface five pairs of small
black spots, the eyes (e. 1, e. 5), the arrangement of which shows
them to be^special modifications of sensory papillae, since they

Lm,

dejb

b.t

njbk

-u.d

FIG. 412. — Hirudo medicinalis ; transverse section. b. t. botryoidal tissue ; c. m. circular
muscles ; cr. crop ; cr'. diverticula of crop ; cu. cuticle ; d. ep. epidermis ; d. s. dorsal sinus ;
d. v. m. dorso-ventral muscles ; I. m. longitudinal muscles ; 1. v. lateral vessel ; n. c. nerve-
cord ; nph. 1-4, nephridium ; n. s. nephrostomial sinus ; nst. ciliated funnel ; ts. testis; v. d.
vas deferens ; vs. vesicle of nephridium ; v. s. ventral sinus. (After Marshall and Hurst.)

occupy in the first five segments the precise position occupied in
the sixth and following segments by segmental papillae.

The perfectly definite and comparatively small number of
metameres in the leech offers a striking point of contrast with
what we have met with in the Chsetopoda, and is to be looked
upon as a mark of higher differentiation.

Body- wall. — The body is covered externally by a thin cuticle
(Fig. 412, cu.), which is constantly being cast off in patches and
renewed. Beneath it is an epidermis (d. ep.) consisting of hammer-
shaped cells, separated at their inner ends by spaces in which
blood-capillaries run. The blood is thus brought into close relation

x PHYLUM ANNULATA 497

with the surrounding water, and the skin becomes a highly efficient
respiratory organ. The space between the epidermis and the
enteric canal is filled by a peculiar form of connective-tissue,
consisting of a gelatinous matrix with interspersed cells and fibres,
many of the former large and branched. More immediately
surrounding the enteric canal is the peculiar and charac-
teristic botryoidal tissue (b. t.) consisting of branched canals, the
walls of which are formed of large cells loaded with black pigment.
This system of canals is in communication on the one hand with
the blood-vascular system and on the other with the greatly
reduced ccelome.

Numerous unicellular glands are produced from the epidermis :
the gland-cells themselves lie in the connective-tissue, and are con-
tinued into long ducts which open on the surface. Special glands
in the ninth, tenth, and eleventh segments secrete the substance
from which the cocoon is formed (vide infra, p. 503) : the segments
in question therefore constitute the clitellum.

The muscular system is well developed, and consists of an
outer layer of circular (c. m.) and an inner of longitudinal (I. m.)
fibres. There are also dorso-ventral fibres
(d. v. m.) passing vertically between the
pouches of the crop (vide infra), and radial
fibres extending from the wall of the enteric
canal to the integument : these take the
place of the septa of Chsetopods.

The alimentary organs are greatly
modified in accordance with the blood-
sucking habits of the animal. Surrounding
the mouth are three jaws, one median and
dorsal (Fig. 415, d.j.), the other two ventro-
lateral (v. l.j.). Each (Fig. 413) has the
form of a compressed muscular cushion,
with a sharp, evenly curved, free edge
covered with chitin, which is produced into FIG. 413.— One of the jaws of
numerous serrations or teeth : by means of its ^ifSJdL°euc?arti)Cinali8'
muscles each jaw can be moved backwards

and forwards through a certain arc, and the three, acting together,
produce the characteristic triradiate bite in the skin of the animal
upon which the Leech preys.

The mouth leads into a muscular pharynx (Figs. 414 and 415,
ph.) situated in the fourth to the eighth segments. Eadiating
muscles pass from its walls to the integument, and by their con-
traction dilate its cavity and suck in blood from the wounds made
by the jaws. Around the pharynx are numerous unicellular
salivary glands, which open close to the mouth : their secretion
has the effect of preventing the coagulation of the blood taken as
food.

VOL. 1. K K

498

ZOOLOGY

ph

SECT.

The pharynx com-
municates by a very
small aperture with
the second and largest
division of the enteric
canal, the huge crop
(cr.), a thin - walled
tube extending from
the eighth to the
eighteenth segment,
and produced into
eleven pairs of lateral
pouches (cr. 7, cr. 11),
the first ten of which
are directed outwards
and correspond each to
a segment, while the
eleventh (cr. 11) passes
directly backwards as
far as the twenty-
fourth segment. The
lumen is divided be-
tween the pairs of
pouches by transverse
septa, perforated by
central apertures, into
eleven chambers. The
crop is capable of great
dilatation, and its form
varies greatly accord-
ing to whether it is
empty or gorged with
blood. Posteriorly the
crop communicates by
a minute aperture with
the stomach (st.), a
tubular chamber with
a dilated anterior end,
and having its wall
produced internally
into a spiral fold : this
is the digestive portion
of the canal ; the blood
is passed into it from
the crop with extreme
slowness, and under-
goes an immediate
change, its colour turn-

PHYLUM ANNULATA

499

ing from red to green. The
digestion of a whole cropful of
blood takes many months. The
stomach is continued into a
narrow intestine (int.) : this
passes into a somewhat dilated
rectum (ret.) which turns
slightly upwards and opens by
the anus (an.) in the last
annulus.

The excretory system
consists of seventeen pairs of
nephridia (nph. 1-17), situated
in segments 7-23. A typical
nephridium (Fig. 416) has the
general form of a loop passing
upwards from the ventral
body-wall, produced into an
offshoot which extends in-
wards (mesially) to the corre-
sponding testis, and connected
posteriorly with a small bladder
or vesicle (vs.). The principal
loop is divisible into two chief
parts, the main lobe (m. I.) and
the apical lobe (a. I.), connected
with one another by a short
recurrent lobe (/. 1.) : the off-
shoot to the testis is known as
the testis-lobe (t. I.) ; it is abbre-
viated in H. australis.

All these parts are formed of
a close-set mass of gland-cells,
traversed by a complex system
of minute intra-cellular pas-
sages or ductules, which finally
unite into a comparatively wide
inter-cellular tube or duct : this
winds through the main and
apical lobes, and finally enters
the vesicle, which opens pos-
teriorly in the second annulus
of the segment. The free end
of the testis-lobe is swollen
into a lobed mass (Fig. 412, nst)
which lies in a sinus in connec-
tion with the testis. This lobed
body is a modified ciliated

500

ZOOLOGY

SECT.

funnel : it has a great number of small ciliated openings into the
sinus in which it lies. In most of the nephridia (all except the
first six pairs) modified ciliated funnels are present attached to
the inner ends of the nephridia, but these do not open into the
canals of the latter.

There is a complex blood-system containing, like that of
the Earthworm, red blood, the plasma coloured with haemoglobin
and containing sparsely distributed colourless corpuscles. But a
striking difference from the preceding annulate types is found in
the fact that the blood-containing spaces are of two kinds — blood-
vessels proper, having muscular walls ; and blood-sinuses, the walls
of which are devoid of muscle.

FIG. 416.— Nephriclium of Hirudo medicinalis. a. I. apical lobe; d. vesicle-duct;
m. 1. middle lobe ; np. nephridiopore ; nst. ciliated funnel ; r. I. recurrent lobe ;
t. I. testis-lobe ; vs. vesicle. The communication here represented as existing between
the ciliated funnel and the nephridial canals does not occur. (After Bourne.)

The two principal blood-vessels are lateral in position (Figs.
414 and 417, I. v.), running fore and aft at the level of the middle
of the nephridia and uniting with one another at the anterior and
posterior ends of the body. They send off branches both dorsally
and ventrally, some of which anastomose with one another. The
ultimate branches break up into capillaries in the integument,
nephridia, &c.

The two principal sinuses are respectively dorsal (Figs. 412 and
417, d. s.) and ventral (v. s.), the former lying just above the
enteric canal in the middle dorsal line, the latter occupying a
similar position on the ventral side, and enclosing the ventral
nerva-cord. The two sinuses are in connection with one another
posteriorly, and are also in communication, by means of their

PHYLUM ANNULATA

501

branches, with the capillaries of the skin. There is thus an
indirect connection, by means of capillaries, between the blood-
vessels and the sinuses, but no direct communication exists. The
sinuses in which the ciliated funnels are lodged open into the
ventral sinus. As we shall see more particularly in the general
account of the class, the vessels and sinuses represent a greatly
reduced coelome.

The nervous system is of the usual annulate type. There is
a small brain (Figs. 414 and 415, br.) situated above the anterior
end of the pharynx immediately behind the median dorsal jaw.
It is connected by a very short pair of oesophageal connectives
with the ventral nerve-cord, which consists of twenty-three well-
marked rounded ganglia (gn. 1-23), situated in the third or middle
ring of each segment, united by delicate double connectives

its

FIG. 417. — Diagram of principal blood-channels of Leech, d. s. dorsal sinus ; I. v. lateral vessel ;
v. s. ventral sinus containing nerve-cord.

and a slender median strand. The ganglion-cells are regularly
arranged in groups or packets. The first or sub-oesophageal
ganglion is larger than the others, and is shown by development
to be made up of five united pairs of embryonic ganglia : the last
ganglion is also of unusual size, and results from the fusion of six
pairs of ganglia distinct in the embryo. The whole ventral
nerve-cord is contained in the ventral sinus. Nerves are given off
from the ganglia, but not, as in the Earthworm, from the
connectives, in which, also, nerve-cells are wholly absent.

The principal sense-organs are the eyes, of which there are five
pairs (Figs. 411 and 418) situated round the margin of the anterior
sucker, on the dorsal side, one pair in each of the first five segments.
They occupy positions taken in the succeeding segments by seg-
mental sense-organs, with which they are obviously homologous.

502

ZOOLOGY

The structure of the eyes is peculiar : they are cylindrical in form
(Fig. 418), the long axis of the cylinder being at right angles to the
surface of the body. The outer layer is formed of black pigmented
tissue (pi.), surrounding a layer of large, clear, refractive cells (p.),

which occupy the greater part of the
organ. A nerve (n.) enters at one side,
and is continued up the axis of the
cylinder by a row of sensory cells.

The margin of the anterior sucker
also bears a large number of goblet-
shaped organs, which are very probably
organs of taste. The minute structure
both of these and of the segmental
sense-organs is very similar to that of
the eyes. The function of the seg-
mental sense-organs is unknown.

Reproductive Organs. — The Leech
is monoecious. There are nine or ten
pairs of testes (Figs. 414 and 415, ts.),
in the form of small spherical sacs,
situated in segments 12 to 20 or 21.
Each gives off from its outer surface a
narrow efferent duct, which opens into
a common vas deferens (v. d.). In the
tenth segment the vas deferens increases

FIG. 418. — Section of eyejf Leech. . . , ,V

c, cuticle ; dr. gland-cells ; ep. epi- in Width and forms a Complex COll,

the vesicula semmalis (v. sem.), from
Lang'S Comparative Anat' which is continued anteriorly a some-
what dilated muscular tube, the ductus
ejaculatorius (d. ej.). From each ejaculatory duct a narrow tube
passes to the base of the penis (p.), a curved eversible muscular
organ which opens on the ventral surface of the fourth annulus of
the eleventh segment, in the middle line. The base of the penis is
surrounded by a number of unicellular glands, which constitute
the prostate, and secrete a substance by which the sperms are
aggregated into masses called spermatophores.

The ovaries are coiled filamentous bodies, each enclosed in a
small globular ovarian sac (ov. s.), situated in the eleventh segment.
From each ovarian sac a short oviduct passes inwards and back-
wards, and unites with its fellow into a median duct, the walls of
which are supplied with albumen-secreting gland-cells. The
common oviduct opens into a curved muscular tube, the vagina (va.),
which opens in the middle line on the ventral surface of the fourth
annulus of the twelfth segment, i.e. one segment behind the male
aperture.

It will be noticed that the ovaries of the Leech form a single
pair, while the testes are multiple and segmental : also that, while

x PHYLUM ANNULATA 503

the gonads and efferent ducts of both sexes are paired, the penis
and the vagina are median and unpaired. In the latter respect
the contrast between the Leech and the Annulata previously
discussed is very striking. Further important peculiarities are the
enclosure of the ovary in a sac from which a duct leads directly to
the exterior, and the fact that the testes are hollow sacs discharging
the sperms into a cavity from which they pass directly to the
efferent ducts. In Chsetopods, it will be remembered, the gonads
lie freely in the ccelome, and their products — ova or sperms — are dis-
charged from their external surfaces and carried off either by
coelomoducts or by " segmental organs." It seems tolerably
certain that in the Leech the cavities both of the ovarian sacs
and of the testes represent shut-off portions of an almost obsolete
ccelome, and that their ducts are coelomoducts.

Development. — When breeding, two Leeches copulate, and one
impregnates the other by passing spermatophores through its
penis into the vagina. Simultaneous mutual impregnation
also been described. The clitellar seg-
ments (ninth ^to eleventh) secrete a
cocoon (Fig. 419), into which spermato-
phores, ova, and a quantity of albumen,
secreted by the gland-cells in the wall
of the median oviduct, are passed.
The animal then withdraws its head

, , FIG. 419.— Thecocoon'of Hirudo.
from the COCOOn, the two ends Of A, entire ; B, in section. (After

which close up by their own elasticity,

producing a closed capsule in which embryonic development takes
place. Segmentation is unequal, and results in the formation of a
globular embryo, which, after hatching, swims about in the cocoon,
actively devouring its albuminous contents, and finally escaping in
a form closely resembling the adult.

2. DISTINCTIVE CHARACTEES AND CLASSIFICATION.

The Hirudinea are Annulata in which the body consists of a
limited and definite number of segments, and is marked externally
by secondary rings or annuli, a variable number of which go to
a segment. The anterior end of the body is suctorial, and several
of the hindmost segments are fused to form a powerful sucking-
disc, which is directed downwards and backwards. The mouth
lies in the anterior sucker, the anus is usually dorsal and imme-
diately in front of the posterior sucker. The coelome is always
more or less obliterated by connective-tissue, and is represented
by sinuses of varying dimensions which contain blood. True
blood-vessels, with muscular walls, are also present. The nervous
system consists of a brain united by short ossophageal connectives
to a ganglionated ventral nerve-cord. The excretory organs are

uai imj

I

504 ZOOLOGY . SECT.

segmentally arranged nephridia. The sexes are united, the
testes numerous and usually segmentally arranged, the ovaries
a single pair. The testes have the form of sacs, and discharge
their products internally : the ovaries either have a similar struc-
ture or are band-like and enclosed in ovarian sacs, into which the
ova are set free. The penis and the vagina are unpaired, and open
by median apertures, the male anterior to the female, on the
ventral surface of the body. Development is usually direct, i.e.
unaccompanied by a metamorphosis. Leeches are either free-
living, or are permanently or intermittently parasitic : they
inhabit either the land, fresh-water, or the sea.

The class is divided into the following two orders : —

ORDER 1. — RHYNCHOBDELLIDA.

Hirudinea in which the anterior part of the body can be pro-
truded and retracted so as to form a proboscis or introvert.

This order includes Clepsine (Glossiphonia), parasitic on Snails,
Frogs, &c. ; Piscicola, on fresh-water Fishes ; Pontobdella and
Branchellion, on marine Fishes (Fig. 420).

ORDER 2. — ARHYNCHOBDELLIDA.
Hirudinea in which there is no proboscis.

Sub-order 1. — Gnaihobdellida.

Arhynchobdellida in which the mouth is provided with two,
or more usually three, toothed jaws.

This sub-order includes Hirudo, the common Leech, parasitic
on Vertebrata ; Aulostoma, the Horse-leech, free-living and
carnivorous ; Hcemadipsa, the Land-leech.

Sub-order 2. — Herpobdellida.

Arhynchobdellida in which the mouth is not armed with true
jaws.

This group includes Herpobdella (Nephelis) Trocheta, Orobdella,
etc. — all fresh- water or terrestrial forms.

Systematic Position of the Example.

Hirudo belongs to the family Hirudinidce, of the sub-order
Gnathobdellida.

The presence of jaws places it in the sub-order Gnathobdellida :
the possession of ten eyes, and the presence of five rings to all the
segments except a few at the anterior and posterior ends, dis-
tinguish it as a member of the family Hirudinida? : the genus
Hirudo is distinguished by the constant presence of 26 segments
and of 102 annuli.

x PHYLUM ANNULATA 505

3. GENERAL ORGANISATION.

In the essential features of their organisation the Leeches are a
very uniform group : there are, however, a few interesting modifi-
cations of structure which must be referred to.

Form and Size.— Most kinds do not exceed a few centimetres
in length, but the American species Macrobdella valdivania is said
to attain a length of 76 cm. (2J feet). The number of annuli to
a segment varies from two to fourteen and is constant in each
species, but the general form of the body is remarkably uniform,
the external differences between various species depending largely
on colour and on the development of papillae, which in some cases
are large and prominent. Setae are absent in all except one genus,
Acanthobdella, which has two pairs on each side of the first five
segments.

The proboscis (Fig. 421), the possession of which is distinctive
of the Rhynchobdellida, is simply the retractile anterior end of the
body, which, by the action of special muscles, can be drawn back
into a temporary sheath. The organ is thus an introvert, like that
of Sipunculoidea.

The chief differences in the structure of the enteric canal
depend upon the varying number, or, in some cases, the total
absence, of lateral pouches to the crop : for instance, the horse-
leech has only a single pair, corresponding to the eleventh pair in
Hirudo, while Nephelis has none at all. In the Rhynchobdellida
there is a distinct slender gullet (Fig. 421, gul.) leading from the
pharynx to the crop (cr.), and thrown into a coil when the proboscis
is retracted. Among the Gnathobdellida the median jaw is absent
in some land-leeches, and in other species all three jaws are rudi-
mentary or absent.

Blood-system and Coelome. — In the medicinal leech, as
we have seen, there are lateral vessels with contractile muscular
walls and dorsal and ventral sinuses with non-contractile walls.
All of these seem to be formed, in this and the rest of the Gnatho-
bdellida, from ccslomic spaces, and none of them, therefore, correspond
developmentally to the true blood-vessels of the Chaetopoda, which
are formed independently of the ccelome. In the rest, however,
the dorsal and ventral vessels are developed in the manner of true
blood-vessels, while the lateral vessels or sinuses are always
ccelomic in origin.

In Pontobdella, one of the Rhynchobdellida, there are dors'al
and ventral as well as lateral vessels, and lateral as well as dorsal
and ventral sinuses, and in each case the vessel is enclosed in the
corresponding sinus. The ventral sinus also contains the nerve-
cord and the ovaries, and offshoots of it surround the testes and the
nephrostomes. In the Rhynchobdellids in general the ccelomic
spaces remain fairly extensive, and are lined by a ccelomic epithelium.

506

ZOOLOGY

SECT.

S.CIefDsine

Another interesting condition occurs in Nephelis, in
which the middle region of the body contains a series
of paired, metamerically arranged spaces, surrounded
by botryoidal tissue and containing the nephrostomes.
Development shows that these cavities are derived

from true ccelomic
spaces in the embryo,
formed, as in Chaeto-
poda, by a splitting
of the mesoderm in
^ r each segment. Acan-
thobdella, already re-
ferred to as excep-
tional in the posses-
sion of setae, is also
the only member of
the class which has
a well-developed and
spacious ccelome,
divided by mesen-

3.BrcmcTh7llion teries into »!>umber
of segments.

1. Ron hob del la In most instances

FIG. 420.— Three Rhynchobdellida. br. gills ; pr. everted the skin, with its
)oscis. (1, after Bourne ; 2 and 3, after Cuvier.) abundant Supply of

capillaries, constitutes the only respiratory organ, but in
BrancMlion (Fig. 420, 3), a Rhynchobdellid para-
sitic on the Electric Rays (Torpedo and Hypnos),
and on one of the Australasian Skates (Raja
nasutd), differentiated respiratory organs or gills
(br.) are present in the form of delicate lateral
outgrowths of the
segments.

In most members
of the class the neph-
ridia are formed on
the same general type
as those of Hirudo,
but differ in the struc-
ture of the ciliated
funnels, which may
be more or less modi-
fied, as in Hirudo.
The funnels, where
they occur, never open

into the nephridial FIG. 421.— Proboscis of Clepsine. A, retracted ; B, eveite d ;
panalc TTank {urinal cr- cr°P > ffut' gullet; mth. mouth; pr, introveit ; s. gl.
Canals. J^acJl tunnel salivary glands. (After Bourne.)

fir

-mth

PHYLUM ANNULATA

507

leads by a narrow ciliated duct into a receptacle, in which leuco-
cytes laden with waste-matters are received from the coelomic
spaces and sinuses, subsequently
to undergo degeneration and
absorption (Fig. 422). By its
outer side this receptacle is in
close relation to the inner end
of the nephridium, and the
waste-matters from the disin-
tegrated leucocytes are no doubt
received into the nephridial
canals and thus passed out to
the exterior. In Hirudo and
Herpobdella (Nephelis) the re-
ceptacles appear to be the
organs in which new blood-
corpuscles are manufactured.
The ciliated funnels of the Hiru-
dinea correspond more closely
with the ccelomoducts or ciliated
organs of the Polychaeta than
with the nephrostomes ; they
are to be compared also with the
" urns " of the Sipunculoidea.
In the Rhynchobdellid Ponto-
bdella a very interesting modifi-
cation of the nephridial system
occurs. Instead of distinct neph-
ridia, there is found on the
ventral surface of the body a
very complex network (Fig. 423,
nph.), which sends off on each side of each segment a short branch

terminating in a ciliated funnel, and
a similar branch which opens ex-
ternally (np.). A similar modifica-
tion occurs in Branchellion.

The nervous system always
closely resembles that of Hirudo, as
also do the sense-organs. The
number of eyes is subject to con-
siderable variation : they may be
developed on the posterior sucker, or
may be absent altogether.

Reproductive Organs. — The

^i^^^^^fr^^ testes U8U*Uy have the. segmental
of nerve-cord ; np'. nephridiopore ; arrangement found in Hirudo, their
fiftffiraSfi&tf c'"' number varying from five to twelve

fel o( Herpobdella

(Clepsine). cr. crown-cells of funnel ; ex.
terminal cell of the nephridium; Ic. leuco-
cytes ; st. duct leading to receptacle \w. wall

°f (From Mei8enheimer> after

508

ZOOLOGY

SKC./T.

pairs. But in Herpobdella (Nephelis) they are very numerous,, and
are not arranged segmentally. In the Rhynchobdellida the
muscular penis is absent, its place being taken by an eversible
sac. The form of the ovary with its . containing sac in Hirudo
is exceptional. As a rule, there is an elongated hollow ovary,
producing ova from its epithelial lining, and thus agreeing very
closely in structure with the testis.

In Clepsine, a fresh- water Rhynchobdellid, copulation in the
ordinary sense of the word has never been observed, but one
individual has been seen to deposit one or more spermatophores on
any part of the body of another — often on the back. The spermato-
phore, which is nearly 3J mm. long, apparently exerts a solvent
action on the skin, since, after a short interval, the spermatic
substance streams through the skin into the coelomic spaces,
probably making its way at last to the ovaries. This extraordinary

-mlh

FIG. 424. — Six stages in the development of Clepsine. g. b. germinal bands ; mg. megameres ;
mi. micromeres ; mth. mouth. (After Whitman.)

process of hypodermic impregnation probably takes place in other
genera, but has been most closely followed in Clepsine.

It is in Clepsine that the early stages of development are
best known. Segmentation is unequal, the embryo consisting, in
the eight-celled stage (Fig. 424, A), of four large ventrally placed
megameres (mg.) and four dorsal micromeres (mi.). One of the
megameres, posterior in position, divides into two cells (B) : the
so-called neuronephroblast and mesoblast, the latter of which at
once divides into two. As shown by their subsequent history, the
neuronephroblast and the mesoblast correspond respectively to the
first and second somatoblasts of Nereis. The former divides and
subdivides to form two symmetrical groups of four cells each,
situated at the posterior pole. The number of micromeres
increases, at first apparently by division of the megameres. The
latter subsequently give off a number of small endoderm cells.

x PHYLUM ANNULATA 509

The embryo now consists of the three large megameres with a
number of endoderm cells, a cap of small micromeres forming an
ectodermal layer which is extending over the surface, with, at the
posterior pole, two symmetrical groups of neuronephroblast cells
(four in each), arid, somewhat deeper, the two mesoblast cells.
From each of the ten cells last mentioned new cells are given off
in front in such a way as to form ten rows of cells, five on each
side, four being derived from neuronephroblasts and one from
the mesoblast cell. These two sets of rows of cells constitute the so-
called germinal bands (g. &.). From their subsequent fate it is clear
that they correspond to the mesoderm bands of Nereis plus the
neural plate. They grow forwards, the ectoderm extending with
them, over the endoderm and megameres. At first they diverge
widely, but their anterior ends subsequently meet towards the
anterior end of the embryo. Later the intermediate parts of the
bands, originally widely separated from one another owing to their
divergence during growth, approach one another and meet along
the middle line of the ventral surface. The germinal bands give
rise to the nerve-cord, the mesodermal segments, and the nephridia.
The layer of micromeres not only gives rise to the whole ectoderm
but also forms the head — the germinal bands not extending into
that region. The embryonic enteric cavity (mesenteron) becomes
formed by arrangement of the endoderm cells round the three
megameres, which break up to form nutrient material or yolk
destined to become absorbed in nourishing the embryo. The
pharynx is formed by an invagination of the ectoderm which
joins the mesenteron. At this stage the embryo leaves the egg,
and soon escapes from the cocoon to pass through its later stages
attached to the ventral surface of the parent.

In the Gnathobdellida the young are hatched at an early stage
of development, and their megameres contain but little yolk :
they are nourished up to the time of leaving the cocoon on the
albumen with which the latter is filled. One member of this
order, Herpobdella (Nephelis), is remarkable for undergoing a
metamorphosis : the anterior end of the embryo is ciliated, and it
possesses a provisional pharynx and several pairs of provisional
nephridia. Paired masses of cells, the head-germs, are developed
in the head, and from these and the germinal bands the whole
body of the adult is produced, the greater part of the larval body
being cast off. This process closely resembles the development
of the pilidium larva of certain Nemerteans (p. 290).

Habits, Distribution, &c. — The majority of the Hirudinea
are inhabitants of fresh-water, and live, like the Medicinal Leech,
by sucking the blood of higher animals — Vertebrates or Molluscs.
It is doubtless in correlation with this intermittent parasitism — the
chance of finding a vertebrate host being an infrequent one — that
the crop has attained such vast dimensions, holding, in the case of

510 ZOOLOGY SECT.

the medicinal leech, as much blood as takes it a year to digest.
The allied species. Hirudo sanguisuga has been found in the nasal
passages of man, producing serious results, and being, to all
intents and purposes, an internal parasite. The same is the case
with the horse-leech, Hcemopsis vorax, taken in, when young, by
horses and cattle while drinking. It attaches itself to the pharynx
and may even descend the trachea. Others are permanent ecto-
parasites : for instance, Branchellion occurs on the outer surface
of the Skate, Electric Ray, and other Fishes, entire families of this
leech, including individuals of all sizes, being sometimes found
crowded together on a small area of skin, which is distinctly
marked by their powerful posterior suckers. Other fish-parasites
are Pontobdella, on Rays, and Piscicola, on fresh-water Fish.
Aulostoma, to which, as well as to Haemopsis, the name Horse-
leech is applied, is carnivorous, feeding on snails and other
Molluscs ; so also are Glossiphonia (Glepsine), Herpobdella (Nephelis),
and the gigantic Macrobdella. The last-named genus and some
others are of subterranean- habits, living in moist earth. The
Land-leeches (Hcemadipsa) live in the forests of many parts of
the world, and in spite of their small size, which does not exceed
30 mm. in length and 5 mm. in diameter, are much dreaded for
the persistent attacks they make on men and cattle.

Many genera are very widely distributed : for instance, the
Land-leeches (Hcemadipsa) occur in India, Ceylon, the East Indies,
Japan, Australia, and South America, a distribution which seema
to indicate that the group is one of great antiquity.

GENERAL REMARKS ON THE ANNULATA.

A special feature of the Annulata, as distinguished from the
phlya previously dealt with, is metamefic segmentation. In some
of the Platyhelminth.es, as we have seen, and in Gordius to a less
extent, there obtains a condition to which the term pseudo-
metamerism is applied. In such cases there is a serial repetition of
certain of the organs — gonads, diverticula of the intestine, nerve-
commissures, &c. — in such a way as to produce a jointed appearance,
though the body is not divided into definite segments. An appear-
ance resembling segmentation is produced also in certain Rhabdo-
coeles that multiply by budding, chains of zooids remaining connected
together for a time. In the strobila of the Cestodes we recognise a
condition which might be described as combining pseudo-meta-
merism with the formation of a chain of zooids. The condition of
true metamerism, as we observe it in the Annulata, is capable of
being deduced from a condition of pseudo-metamerism as it occurs
in Gunda (p. 251), the pseudo-metameres becoming converted into
true metameres by the development of inter-segmental constric-
tions and the completion of internal partitions. If we suppose that

PHYLUM ANNULATA

511

during this process serially repeated outgrowths of the enteron
became separated off to form series of coelomic sacs enclosing the
gonads, a condition would be reached not far removed from that
which characterises the Annulata. On the other hand, the meta-
meric condition is deducible from the condition of a linear colony
of zooids proliferating at the posterior end, the zooids, though
becoming each complete in itself, not, under ordinary circumstances,
becoming detached. The establishment of a closer connection
between the corresponding organs of the zooids in such a colony,
with the special differentiation of the anterior end, would result in
a condition closely resembling the metamerism of the Annulata. It
is conceivable that a condition of pseudo-metamerism was followed
by that of a linear series, not of zooids, but of comparatively
independent parts capable of readily reproducing the animal when
detached, and that a secondary closer connection established

Fia. 425. — Diagram to illustrate possible relations of the unsegmented to the metamerically
segmented worm. A, unsegmented worm with differentiated head end ; B, pseudo-meta-
merism ; C, linear series of zooids in which the first zooid differs in character from the others
and in which the formation of new zooids takes place at the posterior end ; D, metamerically
segmented worm.

between the organs of all the series of parts resulted in the meta-
meric condition.

It has to be borne in mind, however, in estimating the breadth
of the gap between the Annulata and the Platyhelminth that the
former differs from the latter not only in the presence of meta-
meric segmentation, but also in the possession of a totally distinct
type of nervous system. In this respect, in fact, Gordius (Nemat-
helminthes) is apparently nearer the Annulata than are any of the
Flat- worms.

Metamerism is not universal in the phylum. In some (Archi-
Annelida) it may be said to be incipient or rudimentary ; in others
(Echiurida and Sipunculoidea) vanishing or vestigial. The Archi-
Annelida are in this, as in some other respects, the most primitive
of the Annulata, and through them it seems possible to connect the
higher members of the phylum with such lower forms as Dinophilus

512 ZOOLOGY

SECT.

(p. 330) and the Histriobdellea (p. 331). The general occurrence of
the trochophore larva may be taken as pointing to descent from an
unsegmented ancestor having resemblances to the trochophore, and
a form like Dinophilus would afford us an intermediate link between
such a hypothetical ancestor and Polygordius or Protodrilus.

The position of the Sipunculoidea in the Annulata is, as already
noticed, a matter of doubt ; if we dissociate them from the Echiurida
there is little to connect them positively with the other members
of the phylum. But, on the whole perhaps, the evidence in favour
of regarding them as allied to the Echiurida, and through them with
the ChaBtopoda, is sufficiently strong. The segmentation has
become more completely aborted in the Sipunculoidea, and appa-
rently, in further adaptation to a sedentary life in fissures and
burrows, the anus has been displaced forwards as in several groups
of animals that are permanently fixed.

Affinities between the Phoronida (p. 348) and the Sipunculoidea
have often been supposed to exist, and by some zoologists it has been
proposed to unite the two groups in one class. It seems probable,
however, that the very manifest resemblances which undoubtedly
exist do not indicate a near relationship, but are the result of con-
verging modifications of originally widely different stocks. The
most striking of these points of resemblance are two — (1) the
approximation of the anus towards the oral aperture, and (2) the
presence of the tentacular circlet. But a study of the development
shows that these common features arise in totally different ways
in the two cases. The forward position of the anus in the Sipun-
culida is brought about by a gradual displacement resulting from
the growth of the aboral region of the body ; and the invagination
and evagination by which the corresponding result is attained in
Phoronis do not occur. Again, while in Phoronis the tentacles of
the adult may be looked upon as formed by the development of
processes along the line occupied by the post-oral circlet of cilia, in
the Sipunculida the tentacular lobes have nothing to do with the
post-oral circlet, but are formed by the growth of a series of lobes
from the margin of the mouth itself. The larva of the Sipunculida
again is, as already pointed out, very nearly related to the larva of
the Chaetopoda, and is a typical trochophore ; while the Actino-
trocha larva of Phoronis diverges somewhat widely from that type.

In adult structure, particularly in the absence of parapodia and
setae and the reduction of the ccelome, the Hirudinea differ
markedly from the Chaetopoda ; but a study of their earlier
developmental stages shows unmistakably their close connection
with the latter group, more particularly with the Oligochaeta ; and
the existence of an undoubted Leech (Acanthobdella) with setae
and with a well-developed ccelome traversed by mesenteries helps
still further to bridge over the gap between the two classes.

PHYLUM AXNULATA

513

The following diagram will serve to illustrate this view of the
relationships of the various groups referred to :—

Poly c ha efa

Myzosfomida

Sipunculoidea

Echiurida

Hirudinea

Archi -Annelida

Hlstriobdellea

Dinojahiiea Roh'fera

-GasfroCricha

Trochophore
FIG. 426. — Diagram illustrating the relationships of the Annulata and the Trochelminthea.

It should be added, however, that it is not likely that the
trochophore actually represents the ancestral form, since, to some
extent at least, its special features, such as the special arrangement
of the cilia, may be adaptations to a pelagic mode of life.

VOL. I.

L L

SECTION XI

PHYLUM ARTHROPODA

IN this large and important group of animals we meet with a
characteristic feature of the Chaetopoda, viz. metameric segmenta-^
tion, and also with more or less perfect bilateral symmetry 7~ftie"
mouth and anus are at opposite ends of the elongated body, and
the central nervous system consists of a dorsal brain and a double
ventral chain of ganglia. There is, however, an important advance
on the segmented Worms in the circumstance that each typical
segment bears a pair of appendages, distinguished from the simple
foot-stumps or parapodia of the Polychaeta in being divisible into
distinct limb-segments or podomeres, separated from one another
by movable joints and acted upon by special muscles. Arthropods
are also characterised by the almost universal absence of cilia, by
their muscles being nearly always of the striped kind, and by the
body-cavity largely corresponding not to a true coelome, but to
a hcemoccele, in free communication with the circulatory system and
developed from the latter.

The following are the most important subdivisions of the
phylum : —

Class 1. CRUSTACEA, including Crayfishes, Crabs, Shrimps,
Wood-lice, Barnacles, Water-fleas, &c.

Class 2. ONYCHOPHORA, including only the curious caterpillar-
like Peripatus, and a small number of closely related genera.

Class 3. MYRIAPODA, including the Centipedes and Millipedes.

Class 4. INSECTA, including the true or six-legged Insects, such
as Cockroaches, Locusts, Flies, Beetles, Butterflies, and Bees.

Class 5. ARACHNID A, including Spiders, Scorpions, Mites, &c.

CLASS I.-CRUSTACEA.

1. EXAMPLES OF THE CLASS.
a. Apus or Lepidurus.

Apus and Lepidurus are two closely allied Crustaceans found
in the fresh-waters of most parts of the world, but curiously local
in distribution and by no means common. They are so much

514

SECT. XI

PHYLUM ARTHROPODA

515

alike that, save in minor details, the same description will apply
to any species of either genus.

External Characters.— The animal (Fig. 427) is from 20 to
30 mm. in length, and has the anterior two-thirds of the dorsal
surface covered by a thin chitinous shell or carapace, beyond the
posterior edge of which the hinder part of the body (abd.) projects
as a nearly cylindrical structure distinctly divided into segments.
The last or anal segment
bears a pair of long
processes, the caudal
styles (a. /.), between
which, in Lepidurus, is
a flat scale-like post-
anal plate (Fig. 428).
On the dorsal surface of
the carapace, near its
anterior border, are the
paired eyes (E), closely
approximated in front,
diverging posteriorly.
Immediately in front
of them is a small
black median eye (e.),
and between their di-
verging posterior ends
a semi-transparent oval
area, the dorsal organ
(d. o.). Passing trans-
versely across the cara-
pace, a short distance
behind the dorsal organ,
is a shallow furrow, the
cervical fold, immedi-
ately posterior to which
a pair of coiled tubes
(sh. gl.) are seen, one on
each side of the carapace : these are the shell-glands or excretory
organs.

The carapace is attached only as far back as the cervical fold :
behind that level it is free, and, when lifted up or cut away
(Fig. 428), shows the greater part of the body of the animal, divided
into segments like the posterior portion. From the cervical groove
backwards about twenty-eight or thirty segments can be counted :
the region in front of the cervical groove shows no sign of segmenta-
tion, and is distinguished as the head. The segments have the
form of chitinous rings, often produced into small spines : each ring
slightly overlaps its successor, and is connected with it by a narrow

L L 2

Fio. 427. — Apus cancriformis, dorsal aspect, abd
abdomen ; a. /. caudal styles ; d. o. dorsal organ ; E •
paired eye ; e. median eye ; sh. gl. shell-gland ;
th. f. 1, endites of first thoracic foot. (From Bronn's
TMerreieh.)

516

ZOOLOGY

SECT.

area, the articular membrane, the chitinisation of which is less pro-
nounced than that of the rings themselves. By this arrangement

the segments
are freely mov-
able upon one
another in all
directions, the
articular mem-
branes acting
as joints. The
last or anal seg-
ment is pierced
by the terminal
anus (Fig. 431,
an.).

The ventral
surface of the
head is formed
by a flattened
sub-frontal plate
(Fig.429,5./.^.),
continuous
marginally with
the carapace.
The posterior
edge of this
plate is convex
backwards, and
is produced in
the middle line
into a shield-
shaped process,
the labrum or
upper lip (Ibr.),
which over-
hangs the
mouth. From
the sub-frontal
plate also arise,
on each side,
two delicate
processes, the
innermost
called the anten-
nule (ant. 7), the
outermost the
antenna (ant. 2) : these are the first two pairs of appendages. The

XI

PHYLUM ARTHROPODA

517

third pair consists of two strong toothed bodies of a deep brown
colour, placed one on each side of the mouth, and called the man-
dibles (md.). The remaining appendages form two rows of delicate
leaf -like processes, attached to the segmented portions of the body,
and overlapping one another from before backwards : their number
varies from forty to nearly seventy (th.f., abd.f.).

Appendages. — The antennule (Fig. 430, 7) consists of a bent
rod bearing delicate chitinous bristles or setae at its tip, and pre-
senting, at the bend, a joint, due to the presence of an articular
membrane. The appendage is thus
made up of two podomeres or limb-
segments, movably articulated to-
gether. x Its function is probably
tactile.

The antenna (2) is absent in
some species both of Apus and
Lepidurus : in A. cancriformis it
is a very delicate hook-shaped
unjoin ted structure, probably
functionless. As we shall see from
the study of development, it is a
vestigial organ.

The mandible (3) is also an un-
jointed appendage. It has the
form of a deeply concavo-convex
plate, strongly chitinised, and pro-
duced along its inner edge into
strong teeth. The mandibles lie
one on each side of the mouth, and
are so articulated that, by means
of muscles, their toothed edges can
be brought together in the middle
line, so as to rend the food.

The fourth and fifth appendages
are very small, and are probably
functionless or nearly so : they
follow one another just behind the
mandible, and are called the first and second maxillce. The first
maxilla (4) consists of two curved chitinous plates, the second of a
basal portion produced into two branches (5). Between the first
maxilla and the mandibles are a pair of delicate unjointed pro-
cesses, the paragnatha (Fig. 428, pgn.) : they form together a sort
of lower lip, and are not usually reckoned as appendages.

The foregoing appendages all spring from the unsegmented
anterior portion of the body or head. As we shall see,- however,
the succeeding limbs arise each pair from its own segment, so
that the presence of five pairs of appendages on the head may

42!). — Apus glacialis, ventral
aspect, abd. f. abdominal feet ; anil.
antennule ; ant2. antenna ; Ibr. labrum ;
md. mandible ; mx. first maxilla ; ov.
aperture of oviduct ; s.f. pi. sub-frontal
plate ; sh. gl. shell-gland ; th.f. thoracic
feet ; th.f. 1, first thoracic foot. (After
Bernard.)

518

ZOOLOGY

SECT.

be taken provisionally as an indication that this region of the
body is composed of five fused segments.

The sixth appendage (6) springs from the ventro-lateral region
of the first clearly marked segment, and is the first of the long
row of appendages plainly visible in a ventral view. It consists
of an axis formed of four podomeres (1-4), and bearing a number
of offshoots : six of these, called endites (en. 1 — en. 6), arise from
its inner or mesial border ; two, called exites (br.,fl.), from its outer
or lateral border. The proximal endite (en. 1) is small, and bears
strong spines ; in connection with its fellow of the opposite side
it is used to seize food-particles and pass them on to the mouth : it
is therefore conveniently distinguished as the gnathobase. The

5.2ndMaxilla

} -br

r**pr7
10. l!fAbdominal Fool-

6. 7^ Thoracic Poor

FIG. 430.— Typical appendages of Apus. 1— ^.podomeres of axis ; br. tract; en- I—en. 6 :
endites; fl. flabellum ; ov. ova. (After Lankester.)

distal endite is rudimentary (en. 6) : the remaining four (en. 2-5}
are long, jointed filaments. The proximal exite is nearly tri-
angular, and is called the flabellum (fl.)', the distal exite is oval, and
is known as the bract (br.) ; both probably serve a respiratory
function.

The seventh appendage (7) has only two podomeres in the axis,
and the endites are comparatively short and flat. The next eight,
i.e. those borne on the third to the tenth free segments, closely
resemble one another : each (S) has an unjointed axis and short
leaf-like endites, the whole appendage having a distinctly foliaceous
character. The sixteenth appendage — that of the eleventh free
segment — resembles its predecessors in the male, but in the

xt PHYLUM ARTHROPODA 519

female (9) is peculiarly modified. The distal portion of the axis
forms a hemispherical cup, over which the flabellum (fl.) fits like
a lid : in this way a capsule or brood-pouch is produced, which
serves for the reception of the eggs, and the appendage is dis-
tinguished as the oostegopod or brood-foot. The brood-feet and
the adjacent genital apertures allow of a very convenient division
of the body : all that region from the first free or postcephalic
segment to that bearing the oostegopods, both inclusive, is called
the thorax, and its appendages the thoracic feet : it consists of
eleven metameres. The remaining segments, from the twelfth
to the last inclusive, constitute the abdomen, and their appendages
are called the abdominal feet.

The abdominal resemble the thoracic feet in general characters,
having the same foliaceous form (10), with unjointed axis, small
leaf-like endites, and large flabellum and bract. They gradually
diminish in size from before backwards ; and, from the third abdo-
minal segment onwards, two or more pairs of appendages spring
from each segment, so that while the total number of abdominal
segments, in A. cancriformis, is twenty-two, and the five hinder-
most of these are without appendages, there are altogether fifty-
two pairs of abdominal feet. It seems probable that segments
bearing more than one pair of appendages represent two or more
fused — or, perhaps one should rather say, imperfectly differentiated
— metameres.

Body-wall. — The whole body is, as already mentioned, covered
by a layer of chitin of varying thickness, which constitutes an
exoskeleton or external supporting structure. Immediately under-
lying it is the deric epithelium or epidermis, from which the chitin
is secreted layer by layer. Thus the exoskeleton of Apus is a con-
tinuous cuticular structure, exhibiting segmentation in virtue of the
fact that, while comparatively thick and strong in places where
no movement is required, it is thin and flexible in the intervening
spaces, and thus allows of the movement of the harder parts upon
one another.

The setae, which occur on many parts of the body, and in parti-
cular fringe the appendages, are hollow offshoots of the chitinous
cuticle, containing a protoplasmic core continuous with the epi-
dermis. They thus differ fundamentally from the setae of
Chsetopods, which are solid rods sunk in muscular sacs.

The muscular system is well developed (Fig. 431). Under-
lying the epidermis is a layer of connective-tissue, and beneath
this is found, in the posterior or limbless part of the abdomen, a
layer of longitudinal muscles encircling the body and attached by
connective-tissue to each segment. In this way the muscular layer
is itself segmented, being divided by the connective-tissue insertions
into muscle-segments or myomeres. The action of these muscles is
to approximate adjacent segments : according as the fibres on the

520

ZOOLOGY

SECT.

dorsal, ventral, or lateral regions contract, the abdomen will be
raised, lowered, or turned sideways. In the limb-bearing portion

^._ of the abdomen and in the
thorax there is no longer a
continuous muscular tube,
but paired dorsal (dm.) and
ventral bands, which pass
respectively above and below
the origins of the limbs : the
dorsal bands arise in front
from the head-region, the
ventral from a strong fibrous
plate, the cephalic apodeme
(c. ap.), lying just behind the
gullet.

Each appendage is moved
as a whole by muscles pass-
ing into it from the trunk :
its various parts are acted
upon by delicate muscular
slips running to the various
podomeres of the axis and
to the endites, thus render-
ing them separately movable.
The only example we have
yet met with of appendages
moved by definite muscular
bands is that of the curious
Rotifer Pedalion (p. 323).
The muscles are all striped,
a character which applies to
the Arthropoda generally,
with the exception of the
Onychophora.

Digestive Organs.—
The mouth (Fig. 431, mth.)
is situated on the ventral
surface of the head, and is
bounded in front by the
labrum (Ibr.), on each side
by the mandibles, and be-
hind by the paragnatha.
The food appears to be
pushed forwards towards
the mouth by the toothed
bases of the thoracic feet, and is broken up by the mandibles, which
work laterally. The maxillaB are probably functionless, or nearly so.

PHYLUM ARTHROPODA

The mouth leads into a narrow gullet (gul), which passes upwards
and forwards into the head and enters a wide stomach (st.), from
which a straight intestine (int.) is continued back to the terminal
anus (an.) From each side of the stomach is given off a wide
tube (d.gl.) which branches extensively, its ramifications finally
ending in delicate caeca. The larger branches of these digestive
glands contain food in process of digestion : their ultimate caeca
secrete a digestive juice : the walls of the stomach itself are non-
glandular. The walls of the enteric canal consist of an inner layer
of epithelium and an outer layer of connective-tissue and muscle.
In the gullet and in the posterior end of the intestine the epithelium
secretes a thin cuticle, which thus comes to form the actual lining
of the cavity. It is shown by development that the portion of the
canal devoid of a chi-
tinous lining is formed
from the archenteron
of the embryo : the
gullet is developed
from the stomodaeum,
the posterior end of
the intestine from the
proctodaeum.

The body-cavity is
divided into several
parts by membranous
partitions (Fig. 432) :
there is a large median
cavity in which the
enteric canal (i) lies,
called the intestinal

•m

FIG. 432. — Transverse section of Apus
do. dorso-ventral muscles ; e. eggs
h. heart : i. intestine ;

cm. muscles to feet ;
dm. dorsal muscles ;
in. partition between

g. ovary ; n. heart ; i. mte(

intestinal and lateral sinus ; vm. ventral muscles. (From

Bernard.)

sinus : on each side of

this are lateral sinuses

containing the muscles ; and in the dorsal region is a median

cavity, the pericardial sinus. All these spaces are devoid of an

epithelial lining, and contain blood : as will become evident

later, they do not correspond with the coelome of the higher

worms.

The central organ of the circulatory system is the heart (Fig.
431, ht, and Fig. 432, h), a narrow tube contained in the pericardial
sinus. It is pierced laterally by several pairs of apertures or ostia
provided with valves opening inwards, and is continued in front
into a narrow tube, the cephalic artery (c. art.), which extends into
the head and gives off near its origin a pair of arteries to the shell-
glands (Fig. 429). When the heart contracts, the blood is driven
through these arteries to the head and carapace : it then travels
backwards in the intestinal sinus, passes to the limbs, and is
returned to the pericardial sinus, finally re-entering the heart,

522

ZOOLOGY

SECT.

during its diastole, through the ostia. The plasma of the blood
is coloured red by haemoglobin, and contains amoeboid corpuscles.

As already mentioned, the function of respiration is discharged
by the flabella and bracts of the feet, which are abundantly sup-
plied with blood and the movements of which ensure a constant
renewal of the water in their neighbourhood. The renal organ
or shell-gland (Fig. 433) consists of a coiled urinary tube (uc.)
lying between the two layers of the carapace and lined by gland-
cells. At one end the tube is connected with an end-sac (ts.),
also lined with glandular epithelium ; at the other it dilates into
a small bladder (b.) which opens on the second maxilla (m.).

The nervous system (Fig. 434) is constructed on the annulate
type. There is a squarish brain (br.) situated in the dorsal region

of the head, beneath
the eyes. From it a
pair of ossophageal con-
nectives pass back-
wards and downwards
to join the ventral
nerve-cord, which con-
sists of a double chain
of ganglia (gn. 1-4)
united by longitudinal
connectives and trans-
verse commissures so
as'to have a ladder-
like appearance. The
first pair of ganglia lies
immediately behind
the mouth, and sends
off visceral nerves
which join to form a

FIG. 433.— Shell-gland of Apus, diagrammatic, or. ring round the gullet,
cephalic artery ; b. bladder ; h. heart ; m. second maxilla ; «wr»11<vn in frrmt infr» &
t.i I. end-sac; uc. urinary tube. (From Bernard.)

small visceral ganglion

(v. gn.). Passing backwards the nerve-chain diminishes in size, and
comes to an end at about the level of the last pair of abdominal
feet (Fig. 431).

The origin of the nerves given off from the central nervous
system presents many points of interest. From the fourth ganglion
of the ventral cord backwards each pair of appendages has its own
pair of ganglia, the metameric correspondence between the limbs
and the nervous system being complete. The mandibles and the
first maxillae also receive nerves, each from their own pair of ganglia,
their serial homology with the more typical appendages being
thus confirmed. But the second maxillae receive their nerves
(mx. 2) from the connectives between the third and fourth ganglia :

XI

PHYLUM ARTHROPODA

52:*

the ganglion belonging to their segment may be assumed to have
atrophied. The antenna is supplied by a nerve (ant. 2) which
springs from the oesophageal connective, but which can be traced
backwards to the first ganglion of the ventral chain : this fact
may be taken as an indication that the antennae are serially homo-
logous with the jaws and feet— that they are, in fact, metameric
or post-oral appendages which have shifted forwards, one on
each side of the mouth, thus becoming pre-oral. The nerve of
the antennule (ant. 1) also springs from the cesophageal connec-
tive, but is traceable forwards to
the brain, where it is connected
with a special group of nerve-cells.
This has been explained by supposing
that the antennule is a post-oral
appendage the ganglion of which has
moved forwards along the ceso-
phageal connective and fused with
the brain — a process which actually
takes place with the ganglia of the
antennas in the higher Crustacea.
But it is also possible to consider
the antennules as pre-oral appen-
dages, belonging, like the prostomial
tentacles of Chsetopods, to the
prostomial region, and therefore
receiving their nerves from the
brain or prostomial ganglion. The
median and paired eyes are also
supplied by nerves from the brain.

Organs of Sense. — The setae
which occur on so many parts of the
body, and especially as fringes to the
limbs, are to be considered as organs
of touch : the only other organs of
special sense are the eyes. The

\ paired eyes are, as we have seen,

' situated on the dorsal surface of the
head, just over the brain : they are
covered by transparent cuticle form-
ing the cornea, beneath which is a narrow space or water-sac, com-
municating with the exterior by a pore, and therefore filled with
water. The eye itself is made up of a large number of radially
arranged elements called ommatidia (Fig. 435), each of which
consists of an outer and an inner portion. The outer portion is a
group of clear glassy cells (cc.) enclosing a transparent homogene-
ous vitreous body (cr.) : the whole of this portion of the eye serves to
efract the rays of light — it is the dioptric apparatus, like our own

th.f.l

Fi«. 434. — Nervous system of Apus
cancriformis. ant.l nerve to an-
tennule ; ant.2 to antenna ; br. brain :
gn. 1—4, first four ganglia of ventral
nerve-cord ; md. mandibular nerve ;
mx. 1, nerve of first maxilla ; mx. 2, of
second maxilla ; as. con., cesophageal
connective ; op., optic nerve ; ih. /. 1,
nerve of first thoracic foot ; v gn.
visceral ganglion. (After Lankester
and Pelseneer.)

524

ZOOLOGY

SECT.

lens and vitreous humour. The inner portion is a group of
sensory cells, constituting a retinula (re.), and enclosing a re-
fractive rod, the rhabdome (rh.) : the retinula is the actual per-
cipient part of the ommatidium, its cells being comparable to our
own rods and cones. The retinula3 of adjacent ommatidia are
separated from one another by cells full of black pigment (p.), so
that each ommatidium is in a state of optical isolation from its
fellows, and the whole eye is what is called a compound eye. The
optic nerve springing from the brain dilates into an optic ganglion,
from which fibres pass to the retinulse.

The median eye is an ovoid body, and consists of four groups
of large sensory cells enclosing a mass of pigmented tissue : it

FIG. 435. — Diagram of two ommatidia from tlie paired eyes of Apus. c.c vitreous cells ; cr. vit-
reous body ; cl. connective-tissue fibre ; hy. epiderm cells ; p. pigment cells ; r, inner parts
of ommatidia ; re. retinuloe ; rh. rhabdome. (From Bernard. )

is in immediate contact with the brain, and receives a narrow
canal from the water-sac beneath the cuticle of the paired eyes.

Reproductive Organs. — The large majority of individuals
both of Apus and Lepidurus are females ; males are of compara-
tively rare occurrence. The ovary (Fig. 431, ovy.) is a branched
tube occupying a considerable portion of the body-cavity in sexually
mature individuals. The walls of the tube are lined with epithelium,
and give rise to ova, which pass into the lumen of the tube and
thence to a duct (ovd.) opening on the eleventh or last thoracic

PHYLUM ARTHROPODA

525

segment. As in Leeches (p. 503), there is reason for thinking
that the cavity of the ovarian tube represents a shut-off portion of
the coalome, and the oviduct a nephridium. One species has
been shown to be hermaphrodite : in others males are occasionally
found, but reproduction appears to be, as a rule, parthenogenetic.

Development. — The eggs are centrolecithal, i.e., have an accumu-
lation of yolk in the centre surrounded by a superficial layer of
protoplasm.

The embryo is hatched in the form shown in Fig. 436, A. The

FIG. 436. — Three stages in the development of Apus. /*. frontal sensory organ ; L, digestive
gland ; s. carapace ; 1 — 4, cephalic appendages ; I — XIII, body-segments and appendages.
(From Lang's Comparative Anatowi/.)

body is oval, and is divisible into three regions — a large anterior or
head-region ; an intermediate trunk-region, the hinder part of which
already shows signs of segmentation (I-V) ; and a posterior bilobed
anal region. The head-region bears a single median eye, and a
pair of small un jointed appendages (7), each with two large setae
at its extremity : these become the antennules of the adult. The
trunk-region bears two pairs of appendages, the first of which (2)
is very large and fringed with setae, but is chiefly remarkable for
being biramous or two-branched — being formed of a proximal

526 ZOOLOGY SECT.

portion or stem, the protopodite ; a small inner branch, the endopodite ;
and a large outer branch, the exopodite. This second appendage
becomes the antenna of the adult, and may be called the antennary
foot : it is the chief organ of locomotion of the larva. The second-
trunk appendage is the mandibular foot (3), so called because it
becomes converted into the mandible of the adult : it is also
biramous. The only internal structure to be noted is the straight
enteric canal with its dilated anterior end or stomach : the mouth
opens between the bases of the antennary and mandibular feet,
and is bounded in front by a large labrum : the anus is at the
extremity of the anal region. This very peculiar and characteristic
larval form is called a nauplius.1

The nauplius swims freely, chiefly by vigorous strokes of the
great antennary feet, and after a time undergoes a series of moults
or ecdyses, the cuticle being cast off and the animal emerging
in the form shown in Fig. 436, B. The trunk-region has elongated,
new segments having been added, as in Cheetopods, between
those previously present and the anal region. The antennules
have become shifted backwards, and rudiments of a fourth pair
of appendages, the first maxillae (4), have appeared. The carapace
has grown out from the dorsal region of the head, and a peculiar
paired sense-organ (fs.) has appeared on the head.

After two more ecdyses the larva has assumed the form shown
in Fig. 436, C. Several new segments have been added, and the
anterior of these all bear leaf-like thoracic feet. The antennary
feet are still very large, and the bases of the mandibular feet have
become enlarged and toothed so as to form biting jaws. The
carapace (s) has increased greatly, and the caudal styles have
attained a considerable size. Further moults occur, new segments
are added with their appendages, the antennules and antennae
degenerate — the latter sometimes disappearing altogether, the
mandibles become reduced to the enlarged basal segment, and the
larva passes by almost insensible gradations into the adult form.

It will be seen .that the development of Apus proves clearly
that the antennae and mandibles are ordinary trunk-appendages,
homologous with the thoracic and abdominal feet : a comparison
of the antennary and thoracic feet of the larva supports the view
that the endopodite of the former corresponds with the fifth endite
of the latter, and the exopodite with the sixth endite. The
antennules are from the first unbranched or uniramous, and are
originally situated quite at the anterior region of the body : they
do not, therefore, show a complete correspondence with the remain-
ing appendages, and, as was inferred from their nerve supply,
may perhaps be considered as prostomial and not metameric
appendages.

1 More strictly metanauplius : the typical nauplius exhibits no segmenta-
tion of the trunk -region.

XI

PHYLUM ARTHROPODA

527

6. The Fresh-water Crayfish (Astacus fluviatilis).

Astacus fluviatilis is common in streams and rivers in England
and the continent of Europe ; allied species occur in Asia and

FM

Fid. 437. — Astacus fluviatilus, side view of male, a1, antennule ; a2, antenna ; ab, abdomen;
cth. eephalothorax ; kd, gill-cover ; r. rostrum ; 8, third maxillipede ; 9, first leg ; 10 — 13,
remaining legs ; 19, uropod ; XIV, first abdominal segment- XIX, sixth abdominal segment.
(From Lang's Comparative Anatomy.)

North America ; and fresh-water Crayfishes belonging to other
genera, but agreeing with Astacus in all essential features, are
found in America, Australia, and New Zealand.

External Char-
acters.— The body
o f the C r a y fi s h
(Fig. 437) is divided
into two regions —
a n anterior, the
cephalothorax (cth.),
which is unjointed,
and is covered by
a carapace resem-
bling that of Apus,
but of smaller pro-
portional size ; and
a posterior, the ab-
domen (ab), which
is divided into dis-
tinct segments,

movable Upon One FIG. 438.— Transverse section of abdomen of Crayfish. DA,

annfVior in a irar+inal dorsal abdominal artery ; EM, dorsal muscles of the abdomen;

n a Vei ACai Ep> gpace between tne p]eur0n and the appendage ; FM,

Diane The Cet)ha- ventral muscles of the abdomen ; M, muscles of the appendage ;

N, endopodite ; NG, nerve-ganglion ; P, protopodite ; PL,

lOtnoraX IS again pleuron ; PR, hind-gut ; S, sternum ; T, tergum ; V, ventral
j;_ • j^j :«j.rt abdominal artery; X, exopodite. (From Parker's Practical

divided into tWO zoology, after Marshall and Hurst.)

528 ZOOLOGY

regions — an anterior, the head ; and a posterior, the thorax—
by a transverse depression, the cervical groove. The divisions of
the body are thus the same as in Apus, but the abdomen alone
is movably segmented, owing to the fact that the carapace, in-
stead of being a purely cephalic structure continued backwards as
a loose fold over the thorax, is developed from the dorsal and
lateral regions of both head and thorax, and is free only at the
sides of the thorax, where it forms a flap or gill-cover (hi) on each
side, separated from the actual body-wall by a narrow space in
which the gills are contained (Fig. 444). The carapace is made of
chitin, strongly impregnated with carbonate of lime so as to be
hard and but slightly elastic.

The abdomen is made up of six segments and a tail-piece or
telson : the six segments (XIV-XIX) have a ring-like form,
presenting a broad dorsal region or tergum, a narrow ventral region
or sternum, and downwardly directed lateral processes, the pleura
—the last quite unrepresented in Apus. The telson is flattened
horizontally, and divided by a transverse groove into anterior and
posterior portions. All the segments and the telson are calcified,
and are united to one another by chitinous articular membranes :
the first segment is similarly joined to the thorax. Thus the exo-
skeleton of Astacus resembles that of Apus in being a continuous
cuticular structure, but differs from it in being discontinuously
calcified, so as to have the character of a hard jointed armour.

It has been stated that the abdominal segments are movable
upon one another in a vertical plane — i.e. the whole abdomen can be
extended or straightened, and flexed* or bent under the cephalo-
thorax : the segments are incapable of movement from side to
side. This is due to the fact that, while adjacent segments are
connected dorsally and ventrally by flexible articular membranes,
they present at each side a hinge (Fig. 442, h), placed at the
junction of the tergum and pleuron, and formed by a little peg-
like process of one segment fitting into a depression or socket in
the other. A line drawn between the right and left hinges con-
stitutes the axis of articulation., and the only possible movement
is in a plane at right angles to this axis.

Owing to the presence of the carapace, the thoracic region is
immovable, and shows no distinction into segments either on its
dorsal (tergal) or lateral (pleural) aspect. But on the ventral
surface the sterna of the thoracic segments are clearly marked
off by transverse grooves, and the hindmost of them is slightly
movable. Altogether eight thoracic segments can be counted.

The ventral and lateral regions of the thoracic exoskeleton are
produced into the interior of the body in the form of a segmental
series of calcified plates, so arranged as to form a row of lateral
chambers in which the muscles of the limbs lie, and a median
tunnel-like passage or sternal canal, containing the thoracic portion

XI

PHYLUM ARTHROPODA

529

of the nervous system. The entire endophragmal system, as it is
called, constitutes a kind of internal skeleton : its anterior end is
formed by a plate, the cephalic apodeme, having the same anatomical
relations as the similarly named structure in Apus.

The head exhibits no segmentation : its sternal region is
formed largely by a shield-shaped plate, the epistoma, nearly vertical
in position. The ventral surface of the head is, in fact, bent so as
to face forwards instead of downwards. The epistoma is bounded

ll.Uro|><

FIG. 439. — Typical appendages of Astacus. en. 1 — 5, podomeres of tmdopodite ; ep.
epipodite ; ex. exopodite \fl. flagella ; g. gill ; pr. 1, pr. 2, podomeres of protopodite ; 1 — 3
podomeres of axis of antennule. (After Huxley.)

laterally by the free edge of the carapace instead of passing
insensibly into it like the sub-frontal area of Apus, with which,
however, it agrees in having the labrum attached to the middle
of its posterior border. The cephalic region of the carapace is
produced in front into a large median spine, the rostrum (Fig. 437, r) :
immediately below it is a plate from which spring two movably

VOL. I. MM

530 ZOOLOGY SECT.

articulated cylindrical bodies, the eye-stalks, bearing the eyes
at their ends.

The appendages are seen at a glance to differ from those of
Apus in their vastly greater degree of differentiation : obvious at
a glance are the long feelers (Fig. 437, a. 1, a. 2) attached to the
head, the five pairs of legs (9-13) springing from the thorax, and
the little fin-like bodies arising from the sterna of the abdomen.
It will be convenient to begin with the last-named region.

The third, fourth, and fifth, segments of the abdomen bear
each a pair of small appendages, the swimming feet or pleopods
(Fig. 439, 10), the resemblance of which to the biramous limbs
of the larval Apus is obvious. There is an axis or protopodite
consisting of a very short proximal (pr. 1) and a long distal (pr. 2)
podomere, and bearing at its free end two jointed plates, fringed
with setse, the endopodite (en) and exopodite (ex). These appendages
act as fins, moving backwards and forwards with a regular swing,
and probably aiding in the animal's forward movements.
^ In the female a similar appendage is borne on the second seg-
ment, while that of the first is more or less vestigial. In the
male the first and second pleopods (9) are modified into incom-
plete tubes which act as copulatory organs, serving to transfer
the spermatophores to the body of the female. The sixth pair of
abdominal limbs (11) are alike in the two sexes : they are very
large, both endopodite and exopodite having the form of broad flat
plates : in the natural position of the parts they lie one on each
side of the telson, forming with it a large five-lobed tail-fin capable
of being spread out after the manner of a fan ; they are therefore
conveniently called uropods or tail-feet. The telson itself bears
no appendages.

The thoracic appendages are very different. The four posterior
segments bear long, slender, jointed legs (#), upon which the animal
walks : in front of these is a pair of very large legs terminating
in huge claws or chelce, and hence called chelipeds (Fig. 437, 9).
The three anterior segments bear much smaller appendages
more or less leg-like in form, but having their bases toothed to
serve as jaws : they are distinguished as maxillipeds or foot- jaws
(Fig. 439, 6, 7).

The structure of these appendages is best understood by a con-
sideration of the third maxilliped (7). The main portion of the
limb is formed of seven podomeres arranged in a single series,
strongly calcified, and — with the exception of the second and third,
which are fused — movably articulated with one another. The second
podomere, counting from the proximal end, bears a many-jointed
feeler-like organ (ex), and from the first springs a thin folded
plate (ep) having a plume-like gill (g) attached to it. Obviously
such an appendage is biramous, but with one of its branches
greatly in excess of the other : the first two segments of the axis

xi PHYLUM ARTHROPODA 531

(pr. 1, pr. 2) form the protopodite, its remaining five segments
(en. 1-5) the endopodite, and the feeler, which is directed out-
wards, or away from the median plane, the exopodite (ex). The
folded plate (ep) is called the epipodite : in the natural position
of the parts it is directed upwards, and lies in the gill-cavity
between the proper wall of the thorax and the gill-cover (Fig. 446).
Its position is thus very similar to that of the flabellum of Apus,
while the gill attached to it is comparable to the bract.

The five legs (8) differ from the third maxilliped in their greater
size, and in having no exopodite : in the fifth or last the epipodite
also is absent. The first three of them have undergone a curious
modification, by which their ends are converted into pincers or
chelcB : the fourth segment (en. 4) of the endopodite (sixth of the
entire limb) is produced distally so as to form a claw-like projec-
tion (en. 41), against which the terminal segment (en. 5) bites. The
first leg is much stouter than any of the others, and its chela is
of immense size and forms an important weapon of offence and
defence. The second maxilliped resembles the third, but is con-
siderably smaller : the first (6) has its endopodite greatly reduced,
the two segments of its protopodite large and leaf-like, and no gill
is connected with the epipodite.

As in Apus, the head bears a pair of mandibles and two pairs of
maxillae in relation with the mouth, and in front of that aperture
a pair of antennules and one of antennae. The hindmost appen-
dage of the head is the second maxilla (6), a markedly foliaceous
appendage : its protopodite (pr. 1, pr. 2) is cut up into lobes com-
parable with the four proximal endites in the thoracic feet of
Apus : its endopodite (en) corresponds with the fifth endite, while
the sixth endite is represented by the exopodite (ex), modified into
a boomerang-shaped plate, which, as we shall see, is an important
accessory organ of respiration. The first maxilla (4) is a very small
organ, having neither exopodite nor epipodite. The mandible (3)
is a large strongly calcified body, toothed along its inner edge,
and bearing on its anterior border a little three- jointed feeler-like
body, the palp, the two distal segments (en. 1, en. 2) of which
represent the endopodite, its proximal segment (pr. 2) together
with the mandible proper (pr. 1), the protopodite.

The antenna (2) is of great size, being nearly as long as the whole
body. It consists of an axis of five podomeres, the fifth or last
of which bears a long, flexible, many-jointed structure, oifiagellum
(fi), while from the second segment springs a scale-like body or
squame (ex). It is fairly obvious that the two proximal segments
represent the protopodite, the remaining three, with the flagellum,
the endopodite, and the squame the exopodite.

The antennule (1) has an axis of three podomeres (1-3) ending
in two many- jointed flagella (fl 1, and 2), which are sometimes
considered as endopodite and exopodite. But in all the other limbs,

M M 2

532

ZOOLOGY

SECT.

cert m.

as we have seen, the exopodite springs from the second segment
of the axis, and the probabilities are that there is no exact corre-
spondence" between the parts of the antennule and those of the

remaining appendages.

The eye-stalks, already noticed,
arise just above the antemmles and
are formed each of a small proximal
and a large distal segment. They
are sometimes counted as append-
ages serially homologous with the
antennae, legs, &c. But, as we have
seen in the case of Apus, the append-
ages of Crustacea are always formed
in regular order from before back-
wards ; the eye-stalks, on the other
hand, always appear later, both in
individual development and in the
Crustacean series, than the normal
anterior appendages. They are
therefore more properly to be looked
upon as articulated processes of the
prostomium, developed in connection
with the need for an increased range
of vision. Assuming this to be the
case, it will be seen that the body of
the Crayfish consists of a pro-
stomium, nineteen metameres, and
a telson. The prostomium bears
eye-stalks : the first five metameres
are fused with the prostomium to
form the head, and bear the an-
tennules, antennae, mandibles, first
maxillae, and second maxillae : the
next eight metameres (6th-13th)
constitute the thorax, and bear the
three pairs of maxillipeds and the
five pairs of legs : the remaining six
metameres (14th-19th), together
with the telson, constitute the abdo-
men, and bear five pairs of pleopods
and one of uropods.

The articulation of the vari-
ous podomeres of the appendages
is on the same plan as that of the abdominal segments (p. 528).
The podomeres are, it must be remembered, rigid tubes : they are
connected with one another by flexible articular membranes
(Fig. 440, art. m.), but at two points the adjacent ends of the

FIG. 440.— Portion of a leg of Asta-
cus, with the exoskeleton partly re-
moved, showing articulations and
muscles, art. m. articular mem-
brane ; en. 2 — .5, podomeres of endo-
podite ; ext. extensor muscles ; I.
flexors ; h. hinge.

XI

PHYLUM ARTHROPODA

533

tubes come into contact with one another and are articulated by
peg-and-socket joints (h.), the two joints being at opposite ends of
a diameter which forms the axis of articulation. The two podo-
meres can, therefore, be moved upon one another in a plane at
right angles to the axis of articulation and in no other direction,
the joints being pure hinge-joints. As a rule, the range of move-
ment is from the perpendicular to a tolerably extensive flexion
on one side — the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of universal
movement, owing to the fact that the axes of articulation vary
in direction in successive joints : the first joint of a limb bending,
for instance, up and down, the next backwards and forwards,
the next obliquely, and so on. In some cases, e.g. in the pleopods,
peg-and-socket joints are absent, the articulation being formed
merely by an annular articular mem-
brane, and movement being therefore
possible in any plane.

Body-wall. — The exoskeleton is pro-
duced into spines of varying form and
size, and many parts of it bear tufts or
fringes of setae, which also exhibit a
wide variation in size and form. It is
composed of a thick laminated chitinous
membrane (Fig. 441, cu.), more or less
impregnated with lime-salts, and is shed
periodically — once a year during adult
life. Beneath it is the epidermis (ep.

C.I

composed of a single layer of cells from FIG. 441.— vertical section of skin

and exoskeleton of Lobster.
c.t. connective-tissue ; cu. cuticle ;
ep. epidermis ; s. seta. (After

which the chitin is secreted, and under-
lain by a layer of connective -tissue (c.t.)
to which the muscles are attached.

The muscular system, like the exoskeleton, shows a great
advance in complexity over that of Apus. In the abdomen (Fig. 442)
the muscles are of great size, and are divisible into a smaller dorsal
and a larger ventral set. The dorsal muscles (d. m.) are paired
longitudinal bands, divided into myomeres, and inserted by con-
nective-tissue into the anterior border of each segment : anteriorly
they are traceable into the thorax, where they arise from the side-
walls of that region. When these muscles contract, they draw the
anterior edge of each tergum under the posterior edge of its pre-
decessor, and thus extend or straighten the abdomen.

The ventral muscles are extraordinarily complex. Omitting details,
there is on each side a wavy longitudinal band of muscle (c. m.),
nearly circular in section, which sends off a slip (ex.) to be inserted
into each segment above the hinge (h.) : the contraction of this
muscle must obviously tend to approximate the terga, and so aid
the dorsal muscles in extending the abdomen. Around this central

53 i

ZOOLOGY

SECT.

arb.m,

dm.

muscle is wrapped, in each segment, a band of muscle (env. m.) in
the form of a loop, the outer limb of which turns forwards and is
inserted into a sternum, while the inner limb turns backwards and
is inserted into another and more posterior sternum. The con-
traction of this enveloping muscle produces an approximation of
the sterna, and thus flexes the abdomen, the central muscle always
keeping the middle of the loop in place. The ventral muscles
are, like the dorsal, traceable into the thorax, where they arise

from the endophrag-
mal system (p. 529) :
their various parts
are connected by a
complex system of
fibres extending be-
tween the central and
enveloping muscles,
and connecting both
with their fellows of
the opposite side.
The flexor muscles are
immensely powerful,
and produce, when
acting together, a
sudden and violent
bending of the abdo-
men upon the cepha-
lothorax, causing the
Crayfish to dart back-
wards with great
rapidity.

It will be seen that
the body-muscles of
the Crayfish cannot
be said to form a
layer of the body-
wall, as in Chseto-
pods, the abdomen
of Apus, &c., but con-
stitute an immense fleshy mass, filling up the greater part of the
body-cavity, and leaving a very small space around the enteric canal.
In the limbs (Fig. 440) each podomere is acted upon by two
muscles situated in the next proximal podomere. These muscles
are inserted, by chitinous and often calcified tendons, into the
proximal edge of the segment to be moved, the smaller on the
extensor (ext.), the larger on the flexor (fl.) side, in each case half-
way between the two hinges, so that a line joining the two muscular
insertions is at right angles to the axis of articulation.

-d.m,

c.r

X

FIG. 442. — Four segments of abdomen of Crayfish in
sagittal section with muscles (diagrammatic'). A,
extension ; B, flexion ; art. m,, art. m'. articular mem-
branes ; c. m. central muscles ; d. m. dorsal muscle ;
ex. extensor slip of central muscle ; env. m. enveloping
muscle ; fl., fli, flexor slips ; ft. hinge ; st. sternum ; tg.
tergum.

XI.

PHYLUM ARTHROPODA

535

The digestive organs are con-
structed on the same general plan as
those of Apus, but present many strik-
ing differences (Fig. 443). The mouth
lies in the middle ventral line of the
head, and is bounded in front by the
labrum, at the sides by the mandibles,
and behind by a pair of delicate lobes,
the paragnatha. It leads by a short
wide gullet (oe) into a capacious
gizzard (sometimes termed stomach),
which occupies a great part of the in-
terior of the head, and is divided into
a large anterior division (cs), and a
small posterior division (ps) : the latter
passes into a narrow and very short
portion of the intestine, the mid-gut
(md), from which the rest of the in-
testine (hind-gut, hd) extends to the
anus (an), situated on the ventral sur-
face of the telson.

The outer layer of the enteric canal
consists of connective-tissue containing
striped muscular fibres : within this is
a single layer of columnar epithelial
cells. In the gullet and gizzard, and in
the hind-gut, the epithelium secretes a
layer of chitin, which thus constitutes
the innermost lining of those cavities.
It is proved by development that the
mid-gut, which has no chitinous lining,
is the only part of the enteric canal
developed from the mesenteron : the
gullet and gizzard arise from the stomo-
dseum, the hind-gut from the procto-
daeum. Thus a very small portion of
the enteric epithelium is endodermal.

In the anterior division of the gizzard
the chitinous lining is thickened and
calcified in certain parts, so as to form
a complex articulated framework, the

gastric

on which are borne a

median and two lateral teeth, strongly
calcified and projecting into the cavity
of the gizzard. Two pairs of strong
muscles arise from the carapace, and
are inserted into the gizzard : when

I 10. 443.— Astacus fluviatilis,
dissection from the right side.
aa. antennary artery ; ab. abdo-
men ; an. anus ; b. d, aperture of
Uuct of right digestive gland ;
If. 4, cheliped ; bm, ventral nerve-
cord ; cs. anterior division of
gizzard ; cth. cephalothorax ;
em, dorsal muscles ; fm, ventral
muscles ; ff. brain ; h. heart ;
fid, posterior part of intestine ;
Ir, left digestive gland ; md,
mid-gut ; o. ostium of heart ; oa.
right lateral (sternal) artery ; oaa,
dorsal abdominal artery ; oe,
gullet ; pi. 1 — 5, pleopods ; pi. 6,
uropod ; ps. posterior division of
gizzard ; sa. sternal artery ; t.
testis and telson ; uaa, ventral ab-
dominal artery ; vd. vas deferens ;
vdo, male genital aperture. (From
Lang, after Huxley.)

536

ZOOLOGY

SECT.

they contract they move the mill in such a way that the three
teeth meet in the middle and complete the comminution of the
food begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles in
the walls of the gizzard. The posterior division of the gizzard

Fio. 444.— Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed and
the gills undisturbed ; in M tue podo branchiae are removed and the outer arthrobranchiae
turned down. a\, antennule ; 02, antenna ; ab\, first ; abz, second abdominal segment ;
arb. 7 — 12, inner arthrobranchise ; arb'. 7 — 12 , outer arthrobranchise ; ep. 5, scaphognathite;
plb. 11 — 13, pleuro branchiae ; pdb. 7 — 13, podobranchs ; pi. 1, first pleopod ; 6 — 13, thoracic
appendages. (From Lang's Comparative Anatomy, after Huxley.)

forms a strainer : its walls are thickened and produced into
numerous setae, which extend quite across the narrow lumen and
prevent the passage of any but finely divided particles into the
intestine. Thus the gizzard has no digestive function, but is
merely a masticating and straining apparatus. On each side of
the anterior division is found at certain seasons of the year a
plano-convex mass of calcareous matter, the gastrolith.

XI

PHYLUM ARTHROPODA 537

The digestion of the food and to some extent the absorption of
the digested products are performed by a pair of large glands (lr.),
lying one on each side of the gizzard and anterior end of the
intestine. They are formed of finger-like sacs or cceca, which
discharge into wide ducts opening into the mid-gut, and are lined
with glandular epithelium derived from the endoderm of the embryo.
The glands are often called livers, but as the yellow fluid they
secrete digests proteids as well as fat, the name hepato-pancreas
is often applied to them, or they may be called simply digestive
glands. The Crayfish is carnivorous, its food consisting largely
of decaying animal matter. Microscopic glands occur in the
wall of the gullet.

The digestive organs and other viscera are surrounded by a
body -cavity, which is in free communication with the blood-vessels
and itself contains blood. As will be pointed out more particularly
hereafter, this cavity is to be looked upon as an immense blood-
sinus, and not as a true coelome.

There are well-developed respiratory organs, in the form of
gills , contained in a narrow branchial chamber, bounded internally
by the proper wall of the thorax (Fig. 446, ep), externally by the
gill-cover or pleural region of the carapace (led). Each gill con-
sists of a stem giving off numerous branchial filaments, so that
the whole organ is plume-like. The filaments are hollow, and
communicate with two parallel canals in the stem — an external,
the afferent branchial vein, and an internal, the efferent branchial
vein. The gill is to be considered as an out-pushing of the
body-wall, and contains the same layers — a thin layer of chitin
externally, then a single layer of epithelial cells, and beneath this
connective-tissue, hollowed out for the blood channels and con-
taining gland-cells, which will be referred to presently (p. 539).

According to their point of origin, the gills are divisible into
three sets — first, podobranchice or foot-gills, springing from the
epipodites of the thoracic appendages, from which they are only
partially separable ; secondly, arthrobranchice or joint-gills, spring-
ing from the articular membranes connecting the thoracic
appendages with the trunk ; and thirdly, pleurobranchice, or
wall-gills, springing from the lateral walls of the thorax, above the
attachment of the appendages. It is inferred from the study of
other Crayfishes, that a typical thoracic segment bears four gills,
one podobranch, two arthrobranchs, and one pleurobranch. But
in Astacus one or more of the gills in every segment are absent or
vestigial, and the table, or " branchial formula," on page 538 shows
the actual number and arrangement of these organs, ep standing for
epipodite, and r for the vestige of a gill.

By adding up the columns vertically we get the number of gills
in each segment ; ' by adding them horizontally, the number of each
kind of gill ; and by adding together the results obtained by both

VOL. i. M M*

538

ZOOLOGY

SECT.

methods, the total number of gills, viz., eighteen complete gills
with two vestiges and seven epipodites.

THORACIC
SEGMENTS.

I.

II.

III. IV.

V.

VI.

VII.

VIII.

TOTAI

Podobranchiae . .

Q + ep

l + ep

l + ep l+ep

1+cp

l+epl

l+ep

0

6+7ej

Arthrobranchise

0

i

2 2

2

2

2

0

11

Pleurobranchise

0

0

0

0

0

r

1

Ml

TOTAL

Q + cp

2 + ep

3 -HP

'

3 + ep

3 + r+rjo 3-fr — CJD

1

18 + 2,-+

The excretory organs differ both in position and in form

from those of
Apus. There are
no shell-g lands,
but at the base
of each antenna is
an organ of a
greenish colour,
the antennary or
green gland, by
which the function
of renal excretion
is performed. The
gland (Fig. 445) is
cushion - shaped,
and consists o f
three parts — (1) a
central saccule (s.)
of a yellowish
colour, occupying
the mid - dorsal
region, and con-
sisting of a sac
divided into
numerous c o m -
partments by par-
titions, and com-
municating with
(2) the outer or
cortical portion

/„ ~> \ nf Q rrrppn
. . \C' P')> "

s. saccule ; u\ p. white por- COIOUT Consisting

FIG. 445. — Diagram of kidney of Astacus fluviatilis. I, mi-
ravelled; II, the parts in their natural relations. W. bladder ;

c. p. cortical portion ; d. duct;
tion. (After Marchal.)

XI

PHYLUM* ARTHROPODA

530

of a glandular network formed of anastomosing canals, and com-
municating in its turn with (3) a white portion (iv. p.), formed of a
single tube partly converted into a sponge-work by ingrowths
of its walls. The whole organ is lined by glandular epithelium,
and the white portion discharges into a thin-walled sac or urinary
bladder (bl.) which opens by a duct (d.) on the proximal segment of
the antenna. The glands already referred to as occurring in the
gills are also supposed to have an excretory function.

The circulatory organs are in a high state of development.
The heart (Figs. 443, 446, h.) is situated in the dorsal region of the
thorax, and is a roughly
polygonal muscular organ
pierced by three pairs of
apertures or ostia (o.), guarded
by valves which open inwards.
It is enclosed in a spacious
pericardial sinus (Fig. 446,
pc.), which contains blood.
From the heart spring a
number of narrow tubes,
called arteries, which serve to
convey the blood to various
parts of the body. At the
origin of each artery from the
heart are valves which allow
of the flow of blood in one
direction only, viz., from the
heart to the artery. From
the anterior end of the heart
arise five vessels — the median
ophthalmic artery (Fig. 443,

OCl;), Which passes forwards to FIG. 446.— Transverse section of thorax of Cray-
fish., diagrammatic, abm. ventral abdominal
muscles ; bf. leg ; bm. ventral nerve cord ; d.
intestine ; dbm. dorsal muscles of abdomen ;
ep. wall of thorax ; h. heart ; k. gills ; Ted. gill-
cover ; 1. digestive gland ; ov. ovary ; pct
pericardial sinus ; sa.sn., sternal artery ; vs.

ventral sinus. The arrows show the direction of
the blood-current. (From Lang's Comparative
Anatomy.)

the eyes ; paired antennary
arteries (aa.), going to the
antennules,* antennae, green
glands, <fec., and sending off
branches to the gizzard ;
and paired hepatic arteries,
going to the digestive glands. The posterior end of the heart
gives off two impaired arteries practically united at their
origin, the dorsal abdominal artery (oaa.), which passes back-
wards above the intestine, sending branches to it and to the
dorsal muscles ; and the large sternal artery (sa.), which extends
directly downwards, indifferently to right or left of the intestine,
passing between the connectives uniting the third and fourth
thoracic ganglia, and then turns forwards and runs in the sternal
canal, immediately beneath the nerve-cord, sending off branches

540 ZOOLOGY SECT.

to the legs, jaws, &c. At the point where the sternal artery turns
forwards it gives off the median ventral abdominal artery (uaa.),
which passes backwards beneath the nerve-cord, and supplies the
ventral muscles, pleopods, &c.

All these arteries branch extensively in the various organs they
supply, becoming divided into smaller and smaller offshoots, which
finally end in microscopic vessels called capillaries. These latter
end by open mouths which communicate with the blood-sinuses
(Fig. 447, s.), spacious cavities lying among the muscles and viscera,

ht

pcda

St. 8

FIG. 447. — Diagram of the circulation in the Crayfish; heart and arteries scarlet; veins and
sinuses containing non-aerated blood, blue ; those containing aerated blood, pink. a. artery ;
af.br.v. afferent branchial vein ; br.c.v. branchio-cardiac vein ; ef.br.v. efferent branchial
vein ; ht. heart ; pcd.s. pericardial sinus ; s. sinus ; st.s. sternal sinus ; t>i. ostium with
valves ; v2. arterial valves. The arrows show the direction of the current.

and all communicating, mediately or immediately, with the sternal
sinus (st.s.)} a great median canal running longitudinally along the
thorax and abdomen, and containing the ventral nerve-cord and
the sternal and ventral abdominal arteries. In the thorax the
sternal sinus sends an offshoot to each gill in the form of a
well-defined vessel, which passes up the outer side of the gill and
is called the afferent branchial vein (af.br.v. ; see also Fig. 446).
Spaces in the gill-filaments place the afferent in communication
with the efferent branchial vein (ef.br.v.), which occupies the inner
side of the gill-stem. The eighteen efferent branchial veins open
into six branchiocardiac veins (br.c.v.), which pass dorsally in close
contact with the lateral wall of the thorax and open into the peri-
cardial sinus (pcd.s.).

The whole of this system of cavities is full of blood, and the
heart is rhythmically contractile. When it contracts, the blood
contained in it is prevented from entering the pericardial sinus by
the closure of the valves of the ostia, and therefore takes the only
other course open to it, viz., into the arteries. When the heart
relaxes, the blood in the arteries is prevented from regurgitating
by the valves at their origins, and the pressure of blood in the
pericardial sinus forces open the valves of the ostia and so fills

XI

PHYLUM ARTHROPODA

641

the heart. Thus in virtue of the successive contractions of the
heart and of the disposition of the valves, the blood is kept con-
stantly moving in one direction — viz., from the heart by the
arteries to the various organs of the body, where it receives carbonic
acid and other waste matters ; thence by sinuses into the
great sternal sinus ; from the sternal sinus by afferent branchial
veins to the gills, where it exchanges car- 2

bonic acid for oxygen ; from the gills by
efferent branchial veins to the branchiocar-
diac veins, thence into the pericardial sinus,
and so to the heart once more.

It will be seen that the circulatory system
of the Crayfish consists of three sections —
(1) the heart or organ of propulsion ; (2) a
system of out-going channels, the arteries,
which carry the blood from the heart to the
body generally ; and (3) a system of return-
ing channels, some of them, the sinuses.
mere irregular cavities ; others, the veins,
with definite walls, which return it from
the various organs back to the heart. The
respiratory organs, it should be observed,
are interposed in the returning current, so
that blood is taken both to and from the gills
by veins.

/Comparing the blood-vessels of Astacus
with those of a Chaetopod, it would seem
that^he ophthalmic artery, heart, and dorsal
abdominal artery together answer to a dorsal
vessel, part of which has become enlarged
and muscular and discharges the whole
function of propelling the blood. The hori-
zontal portion of the sternal artery, together
with the ventral abdominal, represent a
ventral vessel ; while the vertical portion of
the sternal artery is a commissure, developed
sometimes on the right, sometimes on the
left side, its fellow being suppressed. </

The blood when first drawn is colourless,
but after exposure to the air takes on a
bluish-grey tint. This is owing to the
presence of a colouring matter called hcemo-
cyanin, which becomes blue when combined with oxygen ; it is a
respiratory pigment, and serves, like haemoglobin, as a carrier of
oxygen from the external medium to the tissues. The hsemo-
cyanin is contained in the plasma of the blood : the corpuscles are
all colourless leucocytes,

FIG. 448. — Nervous system of
Astacus fluviatilis.
bg. sub-cesophageal gang-
lion ; eg. commissural
ganglion ; g. brain ; s, vis-
ceral nerve ; sc, cesopha-
geal connective ; y, post-
cesophageal commissure ;
IV— VIII, thoracic gang-
lia; 1-6, abdominal gang-
lia. (From Lang's Com-
parative Anatomy, after
Vogt and Yung.)

542 ZOOLOGY SECT.

The nervous system (Fig. 448) consists, like that of Apus,
of a brain (g) and a ventral nerve-cord, united by ossophageal
connectives (sc). But the right and left halves of the ventral
cord have undergone partial fusion, so that the ganglia, and in
the abdomen the connectives also, appear single instead of double.
Moreover, the brain supplies not only the eyes and antennules,
but the antennae as well, ,and it is found by development that
the two pairs of ganglia belonging to the antennulary and antennary
segments have fused with the brain proper. Hence we have to
distinguish between a primary brain or archi-cerebrum, the ganglion
of the prostomium, and a secondary brain or syn-cerebrum formed
by the union of one or more pairs of ganglia of the ventral cord
with the archi-cerebrum. A further case of concrescence of ganglia
is seen in the ventral nerve-cord, where the ganglia of the last
three cephalic and first three thoracic segments have united to
form a large compound sub-oesophageal ganglion (bg). All the
remaining segments have their own ganglia, with the exception of
the telson, which is supplied from the ganglion of the preceding
segment. There is a visceral system of nerves (s) supplying the
gizzard, originating in part from the brain and in part from the
cesophageal connectives.

Sensory Organs. — The eyes have the same essential structure
as the compound eye of Apus. The chitinous cuticle covering the
distal end of the eye-stalk is transparent, is divided by delicate
lines into square areas or facets, and constitutes the cornea.
Beneath each facet of the cornea is an ommatidium, optically
separated from its neighbours by black pigment, and consisting
of an outer segment or vitreous body, and an inner segment or
retinula, formed of sensory cells enclosing a rhabdome.

The antennules contain two sensory organs, to which have been
assigned the functions of smell and hearing respectively. The olfactory
organ is constituted by a number of extremely delicate olfactory
setce, borne on the external flagellum and supplied by branches of
the antennulary nerve. The so-called auditory organ is a sac
formed by invagination of the dorsal surface of the proximal
segment, and is in free communication with the surrounding water
by a small aperture. The chitinous lining of the sac is produced
into delicate feathered auditory setce, supplied by branches of the
antennulary nerve ; and in the water which fills the sac are minute
sand-grains, which take the place of otoliths, but, instead of being
formed by the animal itself, are taken in after each ecdysis, when
the lining of the sac is shed. Probably the main function of these
organs (statocysts) is connected with the equilibration of the body
rather than with the sense of hearing. Many of the setae on the
body generally have a definite nerve-supply, and are probably
tactile organs.

Reproduction. — The Crayfish is dioecious, and presents a very
obvious sexual dimorphism. The abdomen of the female is much

xi PHYLUM ARTHROPODA 543

broader than that of the male : the first and second pleopods of
the male are modified into tubular or rather spout-like copulatory
organs (Fig. 439, 9) ; and the reproductive aperture is situated in
the male on the proximal podomere of the fifth leg, in the female
on that of the third.

The testis (Fig. 449, B, t, u) lies in the thorax, just beneath the
floor of the pericardial sinus, and consists of paired anterior lobes
(t) and an unpaired posterior lobe (u). From each side goes off a
convoluted vets defer ens (vd), which opens on the proximal segment
of the last leg. The sperms are curious amoeboid bodies produced
into a number of stiff processes or pseudopodia (Fig. 23, /) : they
are aggregated into vermicelli-like spermatophores by a secretion
of the vas deferens. .

The ovary (A, ov, u) is also a three-lobed body, and is similarly
situated to the testis : from each side proceeds a thin- walled

FIG. 449. — Reproductive organs of Astacus fluviatilis. A, female; B, male. od. oviduct;
oe, its external opening ; ov. ovary ; t. testis ; M, unpaired posterior portion of gonad ; vd. vas
deferens. (From Lang's Comparative Anatomy, after Huxley.)

nridiwt (od), which passes downwards, without convolutions, to
open on the proximal segment of the third or antepenultimate
leg. The eggs are of considerable size and are centrolecithal.

As in Apus, both ovary and testis are hollow organs, discharging
their products internally. The ova, when laid, are fastened to the
setae on the pleopods of the female by the sticky secretion of glands
occurring both on those appendages and on the segments them-
selves : they are fertilised immediately after laying, the male
depositing spermatophores on the ventral surface of the female's
body just before oviposition.

544

ZOOLOGY

SECT.

Development. — The process of segmentation of the oosperm pre-
sents certain striking peculiarities. The nucleus (Fig. 450, A, nu)
divides repeatedly, but no corresponding division of the protoplasm
takes'place, with the result that the morula-stage, instead of being

FIG. 450.— Three stages in the formation of the blastoderm of Astacus fluviatilis. nu.
nuclei ; yp. yolk-pyramids. (From Korschelt and Heider, after Morin and Reichenbach.)

a heap of cells, is multinucleate but non-cellular. Soon the nuclei
thus formed retreat from the centre of the embryo, and arrange
themselves in a single layer close to the surface (B) : around each
of these protoplasm accumulates, the central part of the embryo
consisting entirely of yolk-material. We thus get a superficial
segmentation, characterised by a central mass of yolk and a

superficial layer
of cells collec-
tively known as
the blastoderm(C ) .
Subsequently the
yolk itself under-
goes a process
of segmentation,
becoming divided
into radiating
yolk pyramids
(y.p.}, each with
its base in con-
tact with one of
the cells of the
blastoderm and
its apex pointing
to the centre of
the egg : before
long, however,
these pyramids
fuse into an un-
divided mass of

FIQ. 451. — Ventral view of an embryo of Astacus fluviatilis,
gastrula stage, in order to show the ventral plate, c.l.
cephalic lobe ; inv. invaginated area of blastoderm (endoderm
disc) ; th. abd. thoracico-abdominal rudiments. (From
MacBride, after Reichenbach.)

yolk.

The first indications of the future Crayfish take the form of
thickenings on what will become the ventral surface. There are
at first five of these thickenings — two anterior, the head-lobes

XI

PHYLUM ARTHROPODA

545

'A—lhabd

(Fig. 451, c. L), on which the eyes subsequently appear ; two
somewhat further back, the thoracico-abdominal rudiments (th. abd) ;
and one, posterior and unpaired, the endoderm-disc (inv). On the
latter an invagination of the blastoderm takes place, giving rise to a
small sac, the archenteron, which communicates with the exterior
by an aperture, the blastopore. By this process the embryo passes
into the gastmla-stage, which, however, differs from the corre-
sponding stage in the types previously studied in the immense
quantity of food-yolk filling up the space (blastocoale) between
ectoderm and endoderm. Very soon the embryo becomes triplo-
blastic, or three-layered, by the budding of! of cells from the
endoderm in the
neighbourhood of
the blastopore :
these accumulate
between the ecto-
derm and endo-
derm, and consti-
tute the meso-
derm.

Before long the
blastopore closes,
converting the
archenteron into
a shut sac (Fig.
453, A ): the thora-
cic o - abdominal
rudiments unite
with one another,
forming a well-
marked OVal ele-

vatinn ^TTiw 4.^9
,iuii V-^B- *«^»

th. abd) and three

'f ,
pairs OI elevations

appear between it

and the head-lobes. These are the rudiments of the first three
pairs of appendages, the antennules (at1.), antennae (at.2), and
mandibles (mn.) : by their appearance the embryo passes into the
nauplius-stage, which in this case is passed through in the egg,
instead of being active and free-swimming as in Apus.

Between the bases of the antennules and antenna a pit appears,
which soon deepens and widens : this is the stomodceum (Fig. 453,
stdm.), and its aperture the mouth. A similar but narrower and
more cylindrical pit appears on the thoracico-abdominal rudiment :
this is the proctod&um (pcdm.), and its aperture the anus. For a
considerable time both stomodseum and proctodaeum remain in
the condition of blind sacs, but after a time they open into the

car

FIG. 452.—" Nauplius stage " in the development of Astacus
fluviatilis viewed from the ventral side. an. anus; at1.
rudiment of antennule; at*, rudiment of antenna; car. ridge
marking the first trace of the carapace ; c.l. cephalic lobe ; lab.
labrum ; m. mouth ; mn. rudiment of mandible ; pr.c. proto-
cerebrum ; th.abd. thoracico-abdominal rudiment. (From
MacBride, after Reichenbach.)

ZOOLOGY

SECT.

archenteron, a complete enteric canal being thus constituted. In
the meantime the endoderm cells lining the archenteron grow
outwards in a radial direction, ingesting the yolk as they do so,
until they take the form of long columns, in contact by their outer
ends with the ectoderm (Fig. 453, B).

ht~

Jbcdrtv

br

stdrrv
ntttv

ctbd,

ccn

Fro. 453. — Sections of embryos of Astacus. A, Nauplius-stage (compare Fig. 452) ; S, after
development of thoracic appendages (c/. Fig. 454). abd. abdomen ; an. anus ; br. brain ;
ect. ectoderm ; end. endoderm ; ent. enteron ; ht. heart ; mes. mesoderm ; we*1, splanchnic
layer of mesoderm ; mth. mouth ; pcdm. proctodaeum ; stdm. stornodaeum ; th. abd.
thoracico-abdominal rudiment ; v. nv. cd. ventral nerve-cord. (From Korschelt and
Heider, after Reichenbach.)

The thoracico-abdominal rudiment soon begins to increase
rapidly in length, but, being enclosed in the egg-membranes, it
grows not backwards but forwards, being in fact folded upon the
anterior part of the body in much the same way as the abdomen
of the adult during extreme flexion.

XI

PHYLUM ARTHROPODA

547

In the meantime
the post-mandibular
appendages are formed
in regular order from
before backwards : the
eye-stalks appear (Fig.
454, oc), as well as the
labrum (lab), and a
fold on each side of
the thorax, which is
the rudiment of the
carapace (car), and this
gradually extends dor-
sally until it meets
with its fellow of the
opposite side and
covers in the cephalo-
thorax. The embryo
now consists of a nearly
globular c e p h a 1 o -
thorax with a small
abdomen and a nearly
complete set of appen-
dages, all tucked in
under the cephalo-
thorax and closely
packed together within
the egg-membranes.
In this condition the
embryo is hatched,
and for some time
clings to the pleopods
of the mother or to
the empty egg-shells
by means of the pecu-
liarly hooked chelae of
its first pair of legs,
and is attached also
by a bundle of threads
secreted along the
posterior border of the
larval telson.

The development of
the principal internal
organs must be re-
ferred to very briefly.
From the ectoderm

oc

-pre
atlg

lab*

FU. 454— Astacus fluviatilis. Two views of eggs
showing stages in the development of the appendages.
A, stage in which the rudiments of maxillae have
appeared and in which the abdomen has become
forked. B, stage in which the rudiments of thoracic
appendages are appearing and in which the abdomen
is segmented, ab. abdomen ; at1, antennule ; at2, an-
tenna ; atlg. antennulary ganglion ; car. fold which be-
comes edge of the carapace ; caud. f. forked extremity
of abdomen ; e.g. cerebral groove which gives rise to
the optic ganglion ; lab. labrum ; mn. mandible ; mx*.
first maxilla ; mx*. second maxilla ; mxpi. mxp*. mxp".
maxillipedes ; oc. eye-stalk ; op. g. optic ganglion ; pr. c.
protocerebrum ; ret. retinulse ; th. rudiments of thoracic
appendages. (From MacBride, after Reichenbach.)

548 ZOOLOGY SECT.

arise not only the epidermis of the adult, but the epithelium of the
gullet and gizzard and of the hind-gut, the epithelium of the gills,
the nervous system, the vitreous cells and retinulae of the eyes, and
the epithelium of the auditory sac. From the endoderm arises the
epithelium of the mid-gut and of the digestive glands, the latter
being formed as tubular branching outgrowths of the archenteron.
The connective-tissues, the muscles, the vascular system, the
gonads, and perhaps the kidneys, are all of mesodermal origin.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Crustacea are Arthropods in which the five anterior seg-
ments are fused with the prostomium to form the head, while the
rest are usually divisible into two regions, the thorax and the
abdomen. More or fewer of the thoracic segments may be fused
with the head to form a cephalothorax. The head may bear a
median eye, which frequently disappears in the adult, and a pair
of compound eyes, both belonging to the prostomial region : the
latter frequently become elevated on jointed eye-stalks. The
appendages of the head are (1) the antennules, which are usually
considered as belonging to the first metamere ; (2) the antennae,
which are certainly post-oral or metameric appendages shifted
forwards to a pre-oral position ; (3) the mandibles or crushing jaws ;
(4) the first maxillae ; and (5) the second maxillae. The thoracic and
abdominal appendages are variously modified as jaws, legs, fins, or
accessory reproductive organs. With the exception of the anten-
nules, the appendages are typically biramous, consisting of a stem
or protopodite bearing two branches, the endopodite and exopodite.

The body is covered externally by a chitinous cuticle, which
becomes thickened and sometimes calcified in regions where no
movement is required, forming a series of hard parts or sclerites,
separated by flexible chitin : the whole chitinous cuticle thus
constitutes an exoskeleton. Typically there is one sclerite to each
metamere behind the head, and to each podomere in the appen-
dages, but concrescence of sclerites frequently takes place. The
exoskeleton is produced into setae, which are hollow processes of
the cuticle containing prolongations of the underlying epidermis.

Respiration takes place either by the general surface of the
body or by gills, which are hollow offshoots of the thoracic wall or
of the thoracic or abdominal limbs. The stomodaeum and proc-
todaeum form a considerable portion of the enteric canal, and are
lined with chitin : the mesenteron gives rise to digestive glands.
The body-cavity is divided into compartments, most of which
contain blood and are portions of the vascular system : the true
coelome may be represented by compartments of the body-cavity
not containing blood and by the cavities of the reproductive organs.
There is a vascular system which nearly always includes a con-

xr PHYLUM ARTHROPODA 549

tractile heart, formed as a muscular dilatation of a dorsal vessel,
and communicating by valvular ostia with an enclosing pericardial
sinus. The blood is taken from the heart to the various organs
by arteries, and is returned to the pericardial sinus by sinuses
and veins : the respiratory organs are interposed in the returning
current. The renal organs are peculiarly modified nephridia,
which may take the form either of shell-glands opening on the
second maxillae, or of antennary (green) glands opening on the
antennae.

The nervous system consists of a brain united by cesophageal
connectives with a ventral nerve-cord, formed of a double chain
of ganglia joined together by commissures and connectives. The
first three pairs of embryonic ganglia commonly unite to form
the brain, which is therefore a syn-cerebrum. The sexes are
separate or united : sexual dimorphism is common : partheno-
genesis frequently occurs. The sperms are either amoeboid with
radiating stiff processes or pseudopodia, or flagellate : the eggs
are usually centrolecithal, but may be telolecithal, or almost
alecithal. The muscles are striped, and there are no cilia.

Segmentation of the oosperm is usually superficial, but may
be complete or discoid. The embryo very usually has a distinct
nauplius-stage, which may be a free-swimming larva or may
be passed through before hatching, and is characterised by the
presence of three pairs of appendages which become the antennules,
antennae, and mandibles of the adult.

The Crustacea are classified as follows : —

Sub-class I.— Branchiopoda.

Crustacea with a varying number of body-segments, provided
with appendages of a uniform character, usually foliaceous, rarely
leg-like, the posterior region (abdomen) devoid of appendages and
provided with a pair of many- jointed or un jointed caudal styles.
A cephalic carapace is sometimes absent : when present it may be
either shield-like or bivalve. Paired eyes are usually present.
The antennules and the maxillae are reduced or absent : the
mandibles devoid of, or with a vestigial, palp. The larva is a
nauplius or metanauplius.

ORDER 1. — ANOSTRACA.

Branchiopoda in which a carapace is .not developed. The eyes
are stalked : the antennae are prehensile in the male, reduced
in the female. The appendages of the body-segments number
11 or 19 pairs. The caudal styles are not jointed.

This order includes Branchipus and Artemia.

VOL. I N N

550 ZOOLOGY SECT

ORDER 2. — NOTOSTRACA.

Branchiopoda in which there is a large dorsal shield-shaped
carapace. The eyes are sessile. The antennae are reduced. There
are 40 to 63 pairs of trunk appendages. The caudal styles are
many- jointed.

Including Apus and Lepidurus.

ORDER 3. — CONCHOSTRACA.

Branchiopoda with a carapace divided into two lateral portions
or valves like the shell of a bivalve mollusc, and enclosing the
entire animal. The antennae are biramous and are used as
swimming appendages. The eyes are sessile, coalescent. The
appendages of the body-segments number 10 to 27 pairs. The
caudal styles are in the form of unjointed, curved claws.

In this order are included Estheria, Limnetis (Fig. 455), and one
or two other, genera.

ORDER 4. — CLADOCERA.

Branchiopoda of small size with a bivalved carapace which
encloses the trunk but not the head. The eyes are sessile and
united together. The antennae are biramous and used as
swimming appendages. Only 4 to 6 trunk appendages ; caudal
styles unjointed, claw-like.

To this ord^-r belong Daphnia, Polyphemus, Leptodora (Fig.
456), etc.

Sub-Class II.— Ostracoda.

Crustacea with unsegmented or indistinctly segmented body,
bearing not mpre than four pairs of appendages on the trunk, the
limbless posterior part provided with a pair of caudal styles.
There is a well-eieveloped bivalved carapace. Paired eyes may be
present or absent. Both antennules and antennae are used in
swimming ; the latter are generally biramous. The mandibles
have a palp. The young escapes from the egg as a nauplius.

In this sub-class are comprised Cypris, Cythere, etc. (Fig. 457).

Sub-class III.— Copepoda.

Crustacea with elongated, distinctly segmented body, bearing
usually five pairs of limbs, the last four having the character of
biramous swimming appendages, sometimes with a sixth pair
which may be vestigial : the posterior region (abdomen) without
appendages, provided with a pair of caudal styles. The cephalic
dorsal shield is not extended backwards, but usually coalesces with
the exoskeleton of the first (and sometimes also the second) body-
segment. Paired eyes are absent except in the Branchiura. Both

r PHYLUM ARTHROPODA 551

antennules and antennae are usually well developed, and the latter
are sometimes biramous : they may both be used as swimming
organs or for prehension. The mandibles may be provided with a
palp. The young is a nauplius. In the parasitic forms more or
fewer of these general characteristics may become lost in the
adult.

ORDER 1. — EUCOPEPODA.

Free or parasitic Copepoda without paired compound eyes.
The appendages of the body-segments are devoid of a flagellum.
The genital apertures are situated on the seventh body-segment.
In this group are included (a) free-swimming forms, such as
Cyclops (Water-flea) (Fig. 458), and (6) parasitic forms or Fish-
lice — e.g., Ergasilus, Chondracanthus, Lerncea (Fig. 459).

ORDER 2. — BRANCHIURA.

Parasitic Copepoda with compound eyes and a suctorial mouth.
Some of the appendages of the body-segments are usually provided
with peculiar appendages — the flagella. The genital apertures are
situated on the fifth body-segment. This order includes the
Carp-lice, Argulus (Fig. 460), and two other genera.

Sub-class IV.— Cirripedia.

Imperfectly segmented Crustacea, which re always fixed in
the adult condition, and may be parasitic. The^j are usually six
pairs of biramous cirriform appendages of the body region. The
limbless posterior region (abdomen) is rudimentary, and is usually
provided with a pair of caudal styles. The carapace forms a pair
of folds, the mantle, completely enclosing the anir^al, and usually
supported by a system of calcareous plates giving rise to a hard
shell. Paired eyes are absent in the adult. The antennules of
the larva give rise to organs of attachment and become vestigial
in the adult : the antennae usually disappear. The mandibles have
no palp. The sexes are united in the great rnajority. The
young animal is hatched in the nauplius form and passes later
through a stage — the cypris stage — in which it is provided with
a bivalved shell.

ORDER 1. — EUCIRRIPEDIA.

Cirripedia some of which are parasitic, while the rest are non-
parasitic but are permanently fixed in the adult condition.
There are usually six pairs of biramous trunk appendages.

In this order are included (a) fixed forms such as Lepas
(Barnacle) (Fig. 461) and Balanus (Acorn-shell) (Fig. 462) and
(6) parasites — e.g., Petmrca, Alcippe, Proteolepas.

N N 2

552 ZOOLOGY SECT.

ORDER 2. — KHIZOCEPHALA.

Parasitic Cirripedia in which the body has undergone extreme
degeneration, and has lost all trace of appendages and of ali-
mentary canal in the adult condition.

Including Sacculina (Fig. 463) and Peltogaster.

Sub-class V,— Malacostraea,

Crustacea in which the body is always distinctly segmented and
is made up in all cases except the Leptostraca of an anterior region
(thorax) of eight segments, and a posterior (abdomen) of six, with a
terminal tail-piece or telson — the total number of segments,
leaving the prostomium and the telson out of account, being always
nineteen. The appendages of the thorax and abdomen are sharply
marked off from one another. The abdomen is devoid of caudal
styles. The exoskeleton of the head united with that of more or
fewer of the thoracic segments to form a cephalothoracic carapace.
Paired eyes are usually present and may be sessile or stalked.
The antennules are biramous in most cases. The mandibles are
provided with a palp. There is usually a metamorphosis, but a
nauplius-stage rarely occurs.

Series I. — Leptostraca (Phyllocarida).

Malacostraea in which the abdomen contains seven segments
and a telson — the last segment devoid of appendages, the telson
bearing a pair of caudal styles. There is a large bivalved carapace
with an adductor muscle, enclosing the greater part of the body.
The thoracic appendages are foliaceous, the abdominal biramous.

Includes only one order, the Nebaliacea, with Nebalia (Fig. 464)
and three allied genera.

Series II. — Eumalacostraca.

Malacostraea with six segments and a telson in the abdomen,
the latter never provided with caudal styles. Carapace never
bivalve. Thoracic appendages nearly always leg-like, but seldom
all uniform : their protopodite always made up of two podomeres
except in the Stomatopoda.

Division 1. — Syncarida.

Eumalacostraca devoid of carapace, with the first thoracic
segment united with the head or marked off from it by a groove.
Heart elongated, tubular.

ORDER ANASPIDACEA.

Syncarida in which the thoracic appendages are provided (except
the last or the last two) with exopodites, and (except the last)
with a double series of lamellar epipodites (gills). The abdominal

xi PHYLUM ARTHROPODA 553

appendages, except the first two in the male and the last in both
sexes, have the endopodite reduced or absent. The last pair of
abdominal appendages (uropods) expanded, and forming with
the telson a fan-like tail-fin. This, the only order of the Syncarida,
comprises the genera Anaspides, Koonunga, and Paranaspides
(Fig. 465). A related form is Bathynella, which, however, has only
one pair of abdominal appendages and no tail-fin.

Division 2. — Peracarida.

Eumalacostraca in which the carapace, when present, leaves at
least four of the thoracic segments free. Heart elongated, tubular.

ORDER 1. — MYSIDACEA.

Peracarida in which, though the carapace extends over the
greater part of the thorax, it does not coalesce dorsally with more
than the first three segments. The eyes, when present, are sup-
ported on movable stalks ; the antennules are biramous, and
the antennae have a scale-like exopodite or squame. The first
pair of thoracic appendages are specialised as maxillipedes. The
thoracic appendages (except sometimes the first and second pairs)
are biramous. The uropods with the telson form a broad fan-like
tail-fin.

This order includes Mysis (Fig. 466), Lophogaster, and other
genera.

ORDER 2. — CUMACEA.

Peracarida in which the carapace coalesces with the first three
or four segments of the thorax, is produced on each side to enclose
a branchial cavity, and in front is drawn out into a rostrum. The
eyes usually coalesce into one, which is not borne on a movable
stalk. The antennules are sometimes biramous ; the antennae
have no exopodites. Some of the thoracic appendages are
biramous. The telson may coalesce with the last segment of
the abdomen. The uropods are styliform, and there is no fan-
like tail-fin.

Includes Cuma (Bodotria), Diastylis, etc. (Fig. 467).

ORDER 3. — TANAIDACEA.

Peracarida in which the carapace coalesces with the first two
thoracic segments and is produced on each side to enclose a branchial
cavity. The eyes, when present, are usually supported on short
stalks which are not movable. The antennules are sometimes
biramous : the antennae may possess small exopodites. The first
pair of thoracic limbs are modified as maxillipedes. The second
and third thoracic limbs sometimes have vestigial exopodites.
The uropods are usually narrow.

This order includes Apseudes, Tanais, Leptochelia, etc.

554 ZOOLOGY SECT

ORDER 4. — ISOPODA.

Peracarida in which the dorsal exoskeleton of the head is. not
produced into a carapace, but in which the first and sometimes
also the second segment of the thorax coalesce with the head.
The eyes are sessile or borne on short processes which are not
movable. The antennules are nearly always uniramous : the
antennae sometimes bear a minute exopodite. The thoracic limbs
have no exopodites : the first pair are modified as maxillipedes ;
the rest are usually alike in character. The abdominal appendages
are usually biramous ; the rami function as branchiae. The body
is nearly always dorso-ventrally compressed. This is a large order
including many families, e.g. — Asellus, Phreatoicus, Anihura
Sphaeroma, Idotea, Oniscus, Bopyrus (Figs. 469, 471).

ORDER 5. — AMPHIPODA.

Peracarida with the characters of the preceding order, except
that (1) the body is nearly always laterally compressed ; (2) the
second and third pairs of thoracic appendages are nearly always
modified as prehensile organs (gnathopods) ; (3) there are vesicular
or lamellar branchiae attached to the bases of more or fewer of the
thoracic limbs ; (4) the abdominal appendages are distinguishable
into two sets, the three anterior pairs with many-jointed rami, the
three posterior (including the uropods) with unjointed styliform
rami.

Includes Orchestia, Gammarus (Fig. 468), Hyperia, Gaprella,
Cyamus (Fig. 470), and many other genera.

Division. 3. — Eucarida.

Malacostraca in which the carapace coalesces with all the thoracic
segments, forming a cephalothorax. The eyes are borne on movable
stalks. The heart is short, sac-like, and situated in the thorax.

ORDER 1. — EUPHAUSIACEA.

Eucarida in which none of the thoracic limbs take the form of
maxillipedes ; with a single series of branchiae (podobranchs)
attached to the bases of the thoracic limbs. The larva is a nauplius.

This is a comparatively small order of pelagic Malacostraca,
including EupTiausia (Fig. 479), TJiysanopoda, NyctipJianes, and a
few other genera.

ORDER 2. — DEC APOD A.

Eucarida in which the first three pairs of thoracic appendages
are modified as maxillipedes, with the branchiae usually in several
series — podobranchs, arthrobranchs, and pleurobranchs.

XT PHYLUM ARTHROPODA 555

Sub-order 1. — Macrura.

Decapoda with well-developed, elongated abdomen, which is
usually held in the extended position, and terminates in an expanded
fan-like tail-fin composed of the telson and the uropods. The
eyes are not enclosed in orbits. The antennules and antennae
are both large ; the former are not sunk in pits, and the antennae
usually have a scale-like exopodite (squame).

Included among the Macrura are (a) swimming forms — Penceus
and Palcemon (Prawns), Crangon (Shrimps), Lucifer, etc. ; and
(6) creeping forms — Homarus (Lobster), Astggm, Astacopsis, Para-
nephrops, Cambarus (Fresh- water Crayfishes), Palinurus (Rock-
lobsters), Scyllarus, etc. (Figs. 472, 473).

Sub-order 2. — Anomura.

Decapoda with the abdomen more or less reduced, usually held
in a flexed position, and not provided with such a well-developed
tail-fin as in the Macrura.

In most respects the Anomura are intermediate between the
Macrura and the Brachyura. Examples are the Hermit-crabs —
Pagurus (Fig. 474) and other genera, the Cocoa-nut crab — Birgus,
Galathea, Hippa, Porcellana, etc.

Sub-order 3. — Brachyura.

Decapoda in which the abdomen is greatly reduced, shorter
than the cephalothorax, and permanently flexed beneath it. The
antennules and the eyes are both capable of being retracted into
cavities. There is a metamorphosis comprising zocea and megalopa
stages.

Including the true Crabs such as Cancer, Maia, Grapsus, etc.
(Figs. 475, 476).

Division 4. — Hoplocarida.

Malacostraca in which the carapace does not coalesce with
at least the last four thoracic segments, so that the cephalothorax
is relatively short. In front of the head proper are two movable
segments, one bearing the stalked eyes, the other the antennules.
The branchiae are borne on the abdominal appendages. The heart
is elongated. There is a metamorphosis, but a nauplius stage
is not known to occur.

ORDER STOMATOPODA.

This, the only order of Hoplocarida, includes Squilla (Fig. 477),
Gonodactylus, and other genera.

Sytematic Position of the Examples.

The genera Apus and Lepidurus belong to the family Apodidce
of the order Notostraca of the sub-class Branchiopoda,

556 ZOOLOGY SECT.

The foliaceous character of the swimming-feet is alone sufficient
to assign them to the Branchiopoda, and the large number of seg-
ments (considerably more than ten) and swimming-feet, together
with the presence of the large shield -like carapace, decides their
position among the Notostraca.

They are placed in the family Apodidse in virtue of the elongated
body with 40-60 pairs of swimming-feet, diminishing in size from
before backwards and showing considerable differentiation ; and of
the elongated heart reaching to the twelfth post-cephalic segment.

Apus is distinguished by the absence of a post-anal plate, and
by elongated flagella (endites) to the first pair of thoracic feet ; in
Lepidurus the post-anal plate is present, and the flagella of the
first thoracic feet are short.

Astacus fluviatilis is one of several species of the genus Astacus,
belonging to the family Potamobiidce, tribe Astacoidea, sub-order
Macrura, order Decapoda, and sub-class Malacostraca.

The possession of a thorax made up of eight segments and an
abdomen of six places it among the Malacostraca : the presence of
a cephalothorax formed by coalescence of head and thorax, together
with that of movable eye-stalks, determines its position in the
division Eucarida : the modification of the first three pairs of
thoracic appendages as maxillipedes, and the arrangement of the
branchiae in three sets, place it in the Decapoda.

The possession of a squame to the antenna, and of legs having
all seven podomeres distinct — the first three pairs chelate, and the
first pair greatly enlarged — determine its position in the tribe
Astacoidea, which includes all the fresh-water Crayfishes and
the true Lobsters. The family Potamobiidse is distinguished by
having the podobranchise partly united to the epipodites, and by
possessing appendages on the first abdominal segment of the male
and usually on that of the female.

3. GENERAL ORGANISATION.

There is no class in the animal kingdom which presents so wide
a range of organisation as the Crustacea, or in which the deviations
in structure from the " type-form " are so striking and so interesting
. from their obvious adaptation to the mode of life.

The most interesting modifications are those connected with
the external characters and the structure of the append-
ages. As we have seen, the body consists of a prostomium, a
variable number of metameres, and an anal segment. The first
five metameres fuse with the prostomium to form a head, which,
as well as the anal segment, is homologous throughout the class.
On the other hand, there is no strict homology between the various
post-cephalic metameres in different forms until we come to the
Malacostraca, in which their number is constant.

PHYLUM ARTHROPODA

557

There is considerable diversity of form among the Branchiopoda.
Apus has already been described. Branchipus (Fig. 455, 1) and
Artemia (the Brine-shrimp) (Anostraca) are small shrimp-like
forms, the former living in fresh-water lakes, the latter in brine-
pools ; they have no carapace, and the eyes are raised on un jointed
stalks. In Limnetis (2), on the other hand, and in Estheria (3)
the carapace takes the form of a shell, formed of two parts
or valves, united in Estheria by a hinge, and resembling the
shell of a cockle or other bivalved mollusc. The limbs have the
same general structure as those of Apus, but the antennae are
often of considerable size, and are sometimes modified into
prehensile organs.

ant.1

ard.2

ant.2

3. Estheria

FIG. 455. — Three Branchiopoda. In 3, a is the shell ; b the animal with one valve of the
shell removed, ant1, antennule ; ant2, antenna; ht. heart; m. adductor muscle ; md.
mandible ; ov. ovary ; a. unpaired process from head ; p. copulatory appendages ; sh.gl.
shell-gland ; t. testis. (After Gerstaecker.)

In the Cladocera, of which the common fresh-water Daphnia
(Fig. 456, 1) is a good example, there is a great reduction in size
(1-2 mm.), and a corresponding shortening of the body by a
reduction in the number of metameres. Segmentation is very
imperfect, and the whole body, but not the head, is covered
by a large, folded carapace. The abdomen, is turned downwards
and is in constant movement, sweeping out any foreign particles
which may have made their way among the feet. Between the
abdomen of the female and the posterior part of the carapace
is a large brood-pouch (br.p.), in which the eggs are stored. The

558

ZOOLOGY

SECT.

paired eyes (E) have fused into a single organ, which exhibits
a constant trembling movement. The antennules (ant. 1)
are small, the antenna? (ant. 2) very large, biramous, and con-
stitute the chief organs of locomotion. The mandibles are large,
the second maxillae absent in the adult, and there are usually
five pairs of leaf-like swimming-feet (/) on the thorax. The
abdomen is devoid of appendages. Many of the Cladocera have
an extraordinarily grotesque form (2, 3), owing to the peculiar
shape of the head, the immense antenna?, and the great hump-like
brood-pouch.

CLTlt.Z

1 . D a b h n i a

FIG. 456. — Three Cladocera. ant. 1, antennule ; ant. 2, antenna ; br. brain ; br.p. brood-
pouch ; E. eye ; d.gl. digestive gland ; /. swimming feet ; ht. heart ; md. mandible ;
sh.gl. shell-gland. (1 after Claus, 2 and 3 after Gerstaecker.)

The Qstracoda are usually not more than 1-2 mm. in length,
and are found both in fresh- and sea- water. One of the commonest
genera isjtypzis, which occurs in immense numbers in stagnant
pools. Cythere is a common marine form.

The body (Fig. 457) is unsegmented, and is completely enclosed
in a carapace (A), the right and left halves of which are articu-
lated together along the dorsal edge so as to form a bivalved shell
(C)t which may be variously ornamented or sculptured. The
valves are opened by the elasticity of a ligament, which passes
from one to another at the hinge, and are closed by a large adductor
muscle (m.), which extends transversely from valve to valve, its

XI

PHYLUM ARTHROPODA

559

insertions giving rise to markings on the shell (A, m.), often of
systematic value.

At the anterior end is a median eye (e), and in some forms
compound eyes are present as well. There are only seven pairs
of appendages. The antennules (ant. 1) and antennae (ant. 2) are
large, and the latter usually biramous. The mandible (md.) has
a large leg-like palp and a flabellum-like offshoot. The first
maxilla (mx.l) also bears a large plate resembling a flabellum of
Apus. The last cephalic appendage (second maxilla, mx.2) is
jaw-like in some forms (Cypris), leg-like in others (Cy there). The
only thoracic appendages are two or four pairs of slender legs
(f.l, f.2). The abdomen (abd.) is devoid of appendages, and is
terminated by a pair of small caudal styles.

FIG. 457. — A, external view of Cypris ; li, the same with the appendages exposed by the
removal of the left valve of the shell ; C, transverse section ; D, a single sperm, abd.
abdomen; ant.l, antennule ; ant.2, antenna; d.gl. digestive gland; e. median eye;
/.7, f.2, thoracic feet ; int. intestine ; m. adductor muscle ; md. mandible ; mx.l, mx.2,
maxillae ; ov. ovary ; sh. shell ; t. testis. (After Gerstaecker and Zenker.)

The diversity of form among the Copepoda is so great that it
will be advisable to consider s^paraTelyThe free-swimming
Eitcopepoda, the parasitic Eucopepoda, and the Branchiura.

The free-swimming Eucopepoda are well represented by the
common water-flea (Cyclops), found everywhere in fresh and
brackish water, and easily recognisable, in spite of its minute
size, by its elongated form, its rapid, jerky movements, and by the
egg-sacs of the female.

Cyclops (Fig. 458, 1) has been compared in form to a split pear,
the broad end being anterior, and the convex surface dorsal.
The first thoracic segment is fused with the head, and the
cephalothorax (c. th.) thus formed is covered with a carapace

560

ZOOLOGY

SECT,

produced in front into a short spine or rostrum (r), near the base
of which, on the dorsal surface, is the median eye (e). There are
five free thoracic segments : the last (ih. 6) bears the genital
aperture, and is fused in the female with the first abdominal

inl.f

\

2-Calocalanus

FIG. 458. — Free-swimming Eucopepoda. 1«, female Cyclops, from the right side ; b, dorsal
view ; c, antenna of male ; d. swimming-foot, abd.l, first abdominal segment ; an. anus ;
ant.l, antennule ; ant.2, antenna ; c. ih. cephalothorax ; e. median 6ye ; en. endopodite ;

e.s. egg-sac ; ex. exopodite ; ov. ovary ; pr.l, pr.2, protopodite ; r. rostrum; s./. swimming-
feet ; th.2, th.6, thoracic segments. (After Huxley, Gerstaecker, Hartog, and Gies

Giesbrecht.)

segment (abd. 1). There are four abdominal segments : the last
bears the dorsal anus (an), and a pair of caudal styles produced
into plumed setae.

The antennules (ant. 1) are very large, and are the principal
organs of locomotion. In the male they are modified (c) — by a

XI

PHYLUM ARTHROPODA

561

peculiar form of joint and long setae — as clasping organs, used
for holding the female during copulation. The antennae (ant. 2)
are comparatively short and uniramous. Mandibles and maxillae
are present, and the first four thoracic segments bear biramous

a.nt.2,

s*

7. Lernaea

5. Lesheira

6.T racheliasfes

FIG. 459. — Various forms of parasitic Eucopepoda. 4a, female ; 4b, male, ant.l, antennule;

.s. egg-sac ; f.l, f.2, thoracic feet ; h, head ; M . male ;
(After Gerstaecker, Clans, Cuvier, and G. M. Thompson.)

ant.2, antenna ; e. median eye ; e.s. egg-sac ; f.l, f.2, thoracic ff
mx.2. second maxillae; n, "neck." '*«•.«•««*•*•«<*» r>io,ic nin^

swimming-feet (la, s.f.), those of the right and left sides being
connected by transverse plates or couplers. The fifth thoracic
segment bears a pair of vestigial limbs : the abdominal segments
are limbless.

562

ZOOLOGY

SECT.

si

Some of the pelagic marine Eucopepoda (Fig. 458, 2) are re-
markable for their brilliant colours and for the extraordinary
development of their setae, especially those of the caudal styles.

The parasitic Eucopepoda, or Fish-lice, present a very interesting
series of modifications, illustrating the degeneration of structure
which so often accompanies parasitism. Ergasilus (Fig. 459, 1)

"*'- Yj-U / is found on the Sills

of the Bass (Morone

labrax) ; it is readily
recognisable as a
Copepod, but the ap-
pendages are greatly
reduced, the antennae
modified into hooks
for holding on to the
host, and the eyes
absent. Anthosoma
(2), found in the
mouth of the Por-
beagle Shark (Lamna
cornubica), has recog-
nisable appendages,
but the form of the
body is much modi-
fied by the develop-
ment of curious
overlapping lobes.
Nicothoe (3), found
on the gills of the
Lobster, has antennae
and mouth - parts
modified for suction :
the abdomen is nor-

FlG. 460.— Argulus foliaceus, young male. 01, antennule ; H1&1, but the thorax

og, antenna; ab. abdomen ; &i— 64, thoracic feet: d. ;c Tvrv^nnorl i^f/^ Imrro
digestive glands connected with intestine ; kfl, anterior or Jr L

suctorial feet ; kf2, posterior or leg-like portion of second lobes which ffive it a
maxillae ; pa. paired eye ; r. rostrum ; sd. shell-gland ; st. . ' n& ,,

stylet ; ts. testis ; ua, median eye. (From Lang's Compara- CUTlOUSlv deiormed
live Anatomy.) T m

appearance. mChon-

dracanthus (4), the various species of which are parasites on the gills
of Bony Fishes, there is, at the first glance, nothing to suggest that the
animal is a Crustacean, except the characteristic copepod egg-sacs :
the body is depressed, unsegmented, and produced into crinkled
lobes, and it requires careful examination to discover that antennules,
hooked antennae (ant. 2)— used for attachment— mandibles, maxillae,
and two pairs of legs (f.l, f.2) are present. The male (6) is of
higher organisation than the female, but of minute size — about TV
the length of its mate — and is permanently attached to her body,

xi PHYLUM ARTHROPODA 563

close to the genital aperture (a, M). In Lerncea (7) and its allies
the body is vermiform with a curiously lobed anterior end : the
maxillae are adapted for piercing the skin of the host and sucking
its juices, and there are minute vestiges of feet. In Lesteira (5)
the degradation is even more marked : the female reaches a large
size — 70 mm. in length, excluding the egg-sacs — and is found
with the swollen head between the skin and flesh of a fish
(Genypterus blacodes), and the rest of the body hanging freely
into the water. Lastly, in Tracheliastes (6) the second maxillae
(mx.2) are greatly enlarged, and form a characteristic organ of
attachment.

Argulus (Fig. 460) is the most familiar example of the Branchiura, or
Carp-lice. It is an external parasite on fresh- water Fishes (Carp, Stickleback,
&c.), not permanently attached like the degenerate forms just described, but
crawling freely over the surface of the host. The body consists of an oval
flattened cephalothorax, and a small bilobed abdomen (db. ). The mandibles
and maxillae are piercing organs enclosed in a sucking-tube or proboscis (r.),
in front of which is a median tube ending in a spine (st.). The second maxillae
are divided into two portions, the anterior of which (kf. 1} are modified into
sucking-discs by which the parasite clings to the surface of its host, and there
are four pairs of swimming-feet (b? — 64). Alone among the Copepoda the
Branchiura have no egg-sacs, and they are exceptional also in the possession
of compound eyes (pa-).

The most familiar examples of the Eucirripedia are the Barnacles
found on ships' bottoms, piles, &c., and the Acorn-shells or Sessile
Barnacles which occur in immense numbers on rocks between
tide-marks in all parts of the world.

The common Barnacle (Lepas anatifera) is attached by a long
stalk or peduncle (Fig. 461, A, p), covered with a wrinkled skin,
and bearing at its distal end the body proper enclosed in a sort of
bivalved carapace, formed by a fold of the skin, and strengthened
by five calcareous plates. Of these one is median and dorsal, and
is called the carina (c) ; two are lateral and proximal, the scuta (s) ;
and two lateral and distal, the terga (t). During life the carapace
is partly open, and from the ventrally placed aperture delicate
setose filaments are protruded and keep up a constant grasping
movement : these are the endo- and exopodites of the biramous
thoracic feet, of which there are six pairs. Removal of the carapace
shows the feet to arise from a vermiform unsegmented body (B),
attached on the ventral aspect to the stalk and carapace by its
anterior end, while its posterior end is free and terminates in a
long filament, the penis (p), immediately dorsal to which is the
anus. The mouth is ventral and anterior, and is provided with
a pair of mandibles and two pairs of maxillae. There are no
antennae ; at first sight the antennules appear to be absent, but
a careful examination shows the presence of a pair of minute
structures (of) on the proximal or attached surface of the stalk,
and embedded in the cement by which the animal is fixed to its

564

ZOOLOGY

SECT.

support ; these are the antemmles, and their position relatively to
the mandibles shows that the stalk is formed by an elongation of
the anterior region of the head.

Fio. 461. — Lepas anatifera. A, the entire animal ; B, anatomy, ai, antennule ; c. carina •
cd. cement-gland ; 1. digestive gland ; m. adductor muscle ; oil. oviduct ; ov. ovary ; p. (in B)
penis and (in 4) peduncle ; s. scutum ; t. tergum and testis ; vd. vas deferensr (From
Lang's Comparative Anatomy, after Darwin and Claus.)

The Sessile Barnacles or Acorn-shells (Balanus) have no stalk
(Fig. 462), the head-region being short and broad. The scuta (s)

ad

sc

QD WQ

Fia. 462. — Balanus. A, external view ; B, anatomy. a\. antennules ; ad. adductor muscle ;
m. muscles of scuta and terga ; o. edge of parapet ; ov. ovary ; ovi. oviduct ; s. scutum ;
apet ; t. tergum ; wo. female aperture. (From Lang's Comparative Anatomy, after

XI

PHYLUM ARTHROPODA

565

and terga (t) support a valvular carapace, through the opening of
which the feet are protruded, and the whole animal is surrounded
by a sort of parapet (sk) formed of six calcareous pieces. One
of these, dorsal in position, is the carina, the others appear to be
represented by small calcifications developed on the peduncle of
certain stalked forms such as Pollicipes.

Many of the Eucirripedia are parasitic. Some of these (Petmrca,
&c.), parasitic in Actinozoa, resemble the attached forms in
essential respects ; others (Alcippe), parasitic in the shells of
Molluscs and Cirripedes, have abdominal but no thoracic feet.
Proteolepas, also parasitic on other Cirripedes, has a maggot-like,
segmented, limbless body, and a suctorial mouth.

FIG. 463. — Sacculina carcini, on abdomen of crab. br. branchial region of crab ; /,
hepatic region ; d, intestinal region ; ks, body of parasite ; p. peduncle; mb, basilar
membrane, giving off root-like processes whicli are seen extending through the body of
the host. (From Lang's Comparative Anatomy, after Delage.)

The Rhizocephala are represented by Sacculina (Fig. 463), parasitic
on Crabs, and Peltogaster on Hermit-Crabs. Both genera have the
appearance of an immense tumour (ks) on the abdomen of the host,
showing no sign of segmentation, no appendages, no mouth or
anus. From the attached end go off a number of delicate root-
like filaments, which extend through the body of the host and
absorb nutriment. Obviously degeneration is here as complete as
it can well be, and nothing but the developmental history of the
parasite (p. 583) would justify its inclusion among the Crustacea.

VOL. i o o

566

ZOOLOGY

SECT

The most striking general character in the external features of
the Malacostraca is the limitation in the number j^egments. The
headHas~tFe same composition as in the Entomostraca, but the

thorax is invariably formed
of eight segments, and, except
in the Phyllocarida, the
abdomen of six segments and
a telson. The limbs are strik-
ingly modified for the perform-
ance of various functions.

The Phyllocarida are inter-
esting from the fact that
they are annectent or linking
forms between the Branchio-
poda and the Copepoda on
the one hand, and the higher
Crustacea, particularly the
Schizopoda and Decapoda,
on the other. The order
contains only three genera,
the commonest of which,
Nebalia (Fig. 464), is a little
shrimp-like marine Crusta-
cean about 6-8 mm. in
length. The body is divisible
into head, thorax, and abdo-
men, all having the normal
malacostracan number of -
segments except the abdo-
men, which is formed of
eight segments, the last bear-'
ing caudal styles — structures
not found elsewhere in the sub-
class. There is a bivalved
cephalic carapace (s), closed
by an adductor muscle (sm)
and extending backwards to
the fourth abdominal seg-
ment : it is terminated in front
by a movable rostrum (r).
The eyes (a) are large,
raised on

movably articulated stalks.^
The antennules (aT) and antenna (a2) are large, the mandibles
(md.) have palps (mt), and the exopodite of the second maxilla4
(mxt) has the form of a slender filament which acts as a " cleaning-

FIG. 464. — Nebalia geoffroyi, male. a. eye;
a\, antennule ; a%, antenna ; c, head ; brf. thoracic
feet ; d, intestine ; h. heart ; km, gizzard ; md.
mandible ; mt, mandibular palp ; mxt. exo-
podite of second maxilla ; PI — p±, pleopods ;
r. rostrum ; s, carapace ; sm, adductor muscle ;
t. testis; I — VIII, thoracic segments. (From compound, and
Lang's Comparative Anatomy, after Glaus.) rn -, . . i

XI

PHYLUM ARTHROPODA

567

foot" to keep the cavity of the carapace free from foreign
bodies. There are eight thoracic appendages (brf), all of them leaf-
like, and recalling those of Apus. The first four abdominal appen-
dages (pl — j»4) are large biramous swimming-feet, like those of
Copepods ; the fifth and sixth (p5, p{.) are small and uniramous.

FIG. 465. — Paranaspides lacustris, . x4. a1, antennules ; a2, antennae; Ab.l, first
abdominal segment ; ep, epipodites or gills on the thoracic legs ; md, mandible ; Pl.l, first
abdominal appendage ; T, telson ; Th.8, eighth free thoracic segment ; U, uropod. (After
Geoffrey Smith.)

•

The Syncarida (Anaspidacea) (Fig. 465) are small, shrimp-like,
fresh-water Crustaceans, which, though resembling the rest of the
Malacostraca (Eurnalacostraca) in the presence of only six segments
in the abdomen and the absence of caudal styles, differ from them in

FlG. 466. — My sis oculata.

end. endopodite ; ex. exopodite ; ot. otocyst ; p, brood-pouch.
(After Gerstaecker.)

the possession of a combination of features which connect them more
closely with certain fossil forms of Carboniferous age. Thus there
is no carapace, the thoracic appendages are provided with slender
respiratory exopodites, and bear a double series of epipodites or

o o 2

568

ZOOLOGY

SECT.

branchiae ; there are stalked eyes and a fan-like tail-fin formed of
the telson and the expanded uropods.

The Mysidacea (Fig. 466) are
small, transparent, shrimp - like
forms, mostly from 2 — 6 mm. in
length. They agree with the Cray-
fish in the general form of the body,
in the union of the head and thorax,
in the presence of a carapace — which
leaves some of the posterior thoracic
segments free — and in the number
both of segments and appendages,
but present several interesting
characters indicating a lower grade
of organisation. One of the most
notable of these is the absence of
differentiation in the thoracic appen-
dages, which, though they have a
leg-like and not a leaf-like form, are
all alike, none of them being modi-
fied into maxillipedes, except to a
very slight degree in some forms.
Moreover, the legs all possess exopo-
dites (ex), thus retaining the primi-
tive biramous or " split-footed "
form which is lost in the Decapoda.
The first five pleopods are large in
the male, small in the female : the
sixth is a uropod, i.e., assists the
telson in the formation of the charac-
teristic malacostracan tail-fin : there
is no trace of the entomostracan,
caudal styles.

The Cumacea are also a very
small group : Diastylis (Fig. 467) is
a good example. They are little
shrimp-like animals, differing from
all the Malacostraca previously con-
sidered in having poorly developed
sessile eyes, sometimes fused to-
gether, and in some genera altogether
absent. The carapace (cth) is so
small as to leave the five posterior
segments (th IV — VIII) uncovered.
The first two pairs of thoracic limbs
are maxillipedes, the last six, legs :
of these two or three pairs have
exopodites (ex).

\

FIG. 467.— Diastylis stygia. 01, an-

tennule ; 0% , antenna ; 06.1 — ab.-j , ab-
dominal segments ; cth. cephalothorax ;
en, endopodite ; ex, exopodite ; p.i, p.$ ,
pleopods ; IV-VII, th VIII, free thoracic
segments. (From Lang's Comparative
Anatomy, after Sars.)

xi PHYLUM ARTHROPODA 56, i

The Tanaidacea, the Isopoda, and the Amphipoda are often
grouped together under the heading of Arthrostraca. These orders,
particularly the two last, comprise a great number of genera and
species, many of them strangely modified in correspondence with'
special habits of life. The best known examples of the Amphipoda
are the little Fresh-water Shrimp (Gammarus, Fig. 468) and the
Sandhoppers (Talitrus, Orchestia) so common on the sea-shore. Of
the Isopoda very convenient examples are Asellm (Fig. 469),
common in fresh- water, and the well-known Wood-lice or Slaters
(Oniscus, Fig. 471, 1), found under almost any piece of wood,
stone, &c., which has lain undisturbed on the ground for a
few weeks.

The body is usually compressed or flattened from side to side in

FIG. 468. — Gammarus neglectus. abd.l — abd.6, abdominal segments ; ant.l, antennule ;
ant. 2, antenna ; cth. cephalothorax ; E. eye ; j. f. 1, first jumping-foot ; 1. 1 — I. 7, legs ;
mxp. maxillipede ; os. oostegite ; ov. ova ; s.f.l, first swimming-foot ; th.2 — th.8, free thoracic
segments. (After Gerstaecker.)

Amphipods (Fig. 468), depressed or flattened from above down-
wards in Isopods (Fig. 469). The normal malacostracan number
of segments is present, but the first thoracic segment is always
united with the head, so that the apparent head is really an incom-
plete or partial cephalothorax (c.th). In the Tanaidacea (Tanais,
&c.) the second segment of the thorax also unites with the head,
and such forms — sometimes included under a distinct sub-order,
Anisopoda — form a transition to the other Malacostraca, and
especially the Cumacea. In the Amphipoda and Isopoda the pos-
terior seven thoracic segments (th.2 — th.8) are free, and those of the
short abdomen are usually free in Amphipods (Fig. 468, abd. 1-6),
often more or less fused in Isopods (Fig. 469, abd). In some

570

ZOOLOGY

SECT.

Isopoda the thoracic segments are produced laterally into large
and prominent pleura.

The eyes (E) are compound and usually sessile : they are, how-
ever, stalked in some of the less specialised members of the order,
a circumstance which lends support to the view that the sessile
eyes have, in this particular group, arisen by the atrophy of eye-
stalks. The antennae (ant. 2) as well as the antennules (ant.l) are
uniramous, or the former bear a minute exopodite. The first pair
of thoracic appendages (mxp) are modified to form maxillipedes,

FIG. 469.— Asellus aquations. A, dorsal; B, ventral view, abd, abdomen; ant.l, antennule ;
ant.2, antenna ; bp. brood-pouch ; c.th, cephalothorax : E, eye ; l.l — 1.7, legs ; pl.l — pl.6,
pleopods ; th.2 — th.8, free thoracic segments. (After Gerstaecker.)

which are sometimes united together in the middle line so as to
form a sort of lower lip. The remaining seven thoracic appendages
take the form of legs (1.1-1.7) which are usually arranged in two
groups, four of them directed forwards and three backwards, or
vice versa. The legs end either in simple claws or in large sub-
chelse (p. 578) : vestigial expedites are present in some Tanaidacea.
In the female, certain of the legs bear flat plates, the oostegites
(Fig. 468, os), probably modified epipodites, which enclose a brood-
pouch for the reception of the eggs. In Amphipods the gills are
also borne on the legs.

PHYLUM ARTHROPODA

571

The abdominal appendages are very different in the two orders.
In Amphipoda the first three are biramous swimming-feet (Fig.
468, s.f.), the last three peculiar stiff processes used for jumping
(j.f). In Isopods more or fewer of the pleopods have broad plate-
like endo- and exopodites (Fig. 469, pl.3), the former thin and
vascular and acting as gills : the sixth pair (pi. 6) are either leg-
like or aid in the formation of a tail-fin.

Interesting modifications occur in both sub-orders. Among the
Amphipoda, Phronima (Fig. 470, 1) is a marine form of glassy

3. C a JDP e Ma

2. Cy a mu s
FIG. 470. — Amphipoda. 3, a, male ; b, female. (After Gerstaecker, and Bate and Westwood.)

transparency, the female of which inhabits a transparent barrel-
like structure — the test of a pelagic Tunicate — in which she brings
up her young. Caprella (3) is a singular creature in which the
abdomen is quite vestigial, and the rest of the body, as well as
the appendages, extremely slender. It creeps about on colonies
of Hydrozoa and Polyzoa, to the branches of which its own form
and colour are so closely assimilated as to render it difficult of
detection. The allied Cyamus (Whale-louse) (2) is parasitic on the
skin of whales : it also has a vestigial abdomen, but the body —
exceptionally among Amphipods — is broad and depressed, and
the legs are curiously swollen.

Among the Isopoda, one of the most interesting forms is the
common Wood-louse (Fig. 471, 1), which is almost unique among
Crustacea for its perfect adaptation to terrestrial life. The allied

572

ZOOLOGY

SECT.

" Pill-bugs " (Armadillidium, 2) have the habit of rolling them-
selves up into a ball when disturbed. Cymoihoa and its allies are
large species (6-8 cm. in length) parasitic in the mouths of Fishes,
where they hold on to the mucous membrane with their short,
clawed legs : their mouth-parts are often modified for sucking. In
the Bopyrini, found in the gill-cavities of various Crustacea, para-
sitism is accompanied by great degeneration and asymmetry,
as well as by a notable degree of sexual dimorphism, the males
(3 b, m) being very small and permanently attached to the bodies of
the females. Lastly, in Cryptoniscus, parasitic on Crabs, the adult
female (4 6) has no trace of crustacean organisation, and it is only
by the study of development that its true systematic position can be
guessed.

In the division Eucarida, the EupJiausiacea (Fig. 479) are pelagic
forms in which none of the thoracic appendages are modified so
as to take the form of maxillipedes, and in which there is only

a single series of
branchiae (podo-
branchs).

Amongst the
Decapoda are in-
cluded nearly all
the largest and
most familiar
Crustacea — the
Prawns and
Shrimps, Lob-
sters, Crayfishes,
and Crabs. The
>r end with cephalothorax is

(After Cuvier, alwayg c Q m.

pletely covered

by the carapace. The three anterior pairs of thoracic appendages
are modified into maxillipedes, which retain the original biramous
character, but the five posterior pairs are enlarged, and form legs,
which are always — except as an individual variation — devoid of
exopodites in the adult.

In the Shrimps and Prawns (Fig. 472) the body is compressed,
and the exoskeleton is not calcified. The abdomen is very large
in proportion to the cephalothorax, and has a peculiar bend close
to its junction with the thorax. The legs are very slender, ar^e
used for swimming, not walking, and sometimes one pair, sometimes
another, is enlarged to form the chelipeds. The rostrum is large
— sometimes longer than the rest of the carapace — and the eye-
stalks, antennae, and legs may attain extraordinary dimensions.

The Lobsters and fresh-water Crayfishes agree with Astacus in
all essential details, but the sea-Crayfishes (Pcdinurus) present some

2. Armadillidium.

3. Gyge.

4. Cryptoniscus.

FIG. 471.— Isopoda. 3, a, entire animal ; 6, _
attached male (m) ; 4, a, larva ; b, adult female.
Claus, and Gerstaecker.)

xr

PHYLUM ARTHROPODA

573

striking modifications. There are no chelae, the legs all ending in
simple claws : the antennae are of immense size, and their proximal
segments are fused with one another and with the carapace, quite

2.Pala.emon.
Fia. 472.— Shrimp (dorsal view) and Prawn (side view). (After Cuvier.)

crowding out the epistoma : the rostrum is reduced, or even
vestigial, and the pleopods are very broad and fin-like. In Scyllarus
(Fig. 473) and its allies the body is broad and depressed, the bases
of the legs widely separated from one another by the broad

574

ZOOLOGY

SECT. ,

FIG. 473. — Scyllarus arctus.
ant.l, antennule ; ant.2, antenna ;
E, eye. (After Cuvier.)

sterna, the antennae (ant. 2) short
and plate-like, and the eye-stalks (E)
enclosed in socket-like grooves of
the carapace. Most of these charac-
ters show an approximation to what
is found in the Crabs.

Of the Anomura, the Hermit-
Crabs (Pagurus, &c., Fig. 474) are
very strangely modified in relation
with their peculiar mode of life.
They are always found inhabiting
the empty shells of Gastropods
(Whelks, Periwinkles, &c.), the
abdomen, which has become spirally
twisted, completely enclosed within
the shell and only the cephalothorax
protruding. In correspondence with
this mode of protection, the abdo-
men is soft, having only vestiges
of terga (t) on the dorsal side, and

its appendages are more or less atrophied except the sixth

pair (up), which take the form of a pair of hooks, and are

used to hold on to

the columella of

the shell. The fifth

pair of legs (1.5)

are much reduced,

and in some species

one of the chelipeds

is greatly enlarged

and its chela (ch)

acts as an oper-

culum, completely

closing the mouth

of the shell when

the animal is re-
tracted, or both che-
lipeds are enlarged

to perform this

function. As the

Hermit-Crab grows

it takes up its abode

in larger and larger

shells, sometimes

killing and remov-
ing piecemeal the FIG. 474.— Pagurus bernhardus. ch. chela or

flrst riSnt le§ '« '•*> L5> fourth and fifth legs ;
t> abdominal terga ; up. uropods. (After Bell.)

XI

PHYLUM ARTHROPODA

575

Other Anomura, such as the Cocoa-nut Crab (Birgus), Hippa, &c.,
approach the Brachyura in the short, more or less permanently
flexed abdomen, but are clearly separated from them by the
structure of the head and its appendages.

In the Brachyura, or true Crabs, we reach the highest degree of
specialisation known among the Crustacea. The cephalothorax

Kni. 475. — Cancer pagurus. A, dorsal, B, ventral aspect, ant.l, antennule ; ant.2>
antenna ; abd.l, abti.3, abd.7, abdominal segments ; E eye-stalk ; l.l, 1.5, legs ; mxp.3, third
maxillipedes. (After Bell.)

(Fig. 475) is always of great proportional breadth, and is frequently
much broader than long. The abdomen, on the other hand, is
greatly reduced, its sternal region is uncalcified, and it lies per-
manently flexed in a groove on the very broad thoracic sterna, so

576

ZOOLOGY

SECT.

as to be often quite hidden in a view from above. In correspondence
with this the pleopods are much reduced, the male retaining only two
pairs as copulatory organs, the female four pairs for the attachment

FIG. 476. — Typical Brachyura. (After Bell and de Haan.)

of the eggs. The uropods are absent, so that there is no tail-fin.
The eye-stalks (E) are contained in orbits or sockets of the carapace,
which are so prolonged that the eyes appear to arise behind the
antennules and antennae. Both pairs of feelers are small, and the

XT

PHYLUM ARTHROPODA

577

bases of the antennules are contained in sockets or fossettes. The
third maxillipedes (mxp.) are broad, flat, and valve-like, not leg-
like as in the Macrara. The first legs (LI) form chelipeds often
of great size : the remaining legs generally end in simple claws,
but in the Swimming-crabs (Fig. 476, 1) the distal segment in the
fifth pair is flattened and forms a fin. The range of variation in
form, proportions, colour, markings, &c., among Crabs is very
great (Fig. 476).

Unlike the Decapoda, the Stomatopoda form a very small order,
comprising a few genera varying from the size of a Shrimp to that
of a Lobster. Squilla (Fig. 477) is the best known genus.

The abdomen (a: — a7) is very large in proportion to the
cephalothorax, and the carapace (cth), which is thin and uncalcified,
leaves the last three thoracic segments (VI — VIII) uncovered.
The rostrum is movably articulated, and covers the anterior head-

FIQ. 477. — Squilla. ai, antennule ; 02, antenna ; ai — a/, abdominal segments ; br, gills ; cth,
cephalpthorax ; p, copulatory organ ; p\ — p5, pleopods ; p6> uropods ; VI — VIII, free
thoracic segments ; 1 — 8, thoracic appendages. (Prom Lang's Comparative Anatomy.)

region, which is divided into two distinct segments, the first bearing
the large stalked eyes, the second the antennules. This arrangement
appears to support the view that the antennulary region is a
metamere distinct from the prostomium ; but the division in
question is absent in the larva, and does not appear till the proper
segmentation of the body is established : probably it has a physio-
logical meaning, and is connected with the necessity of extreme
mobility of the eyes and olfactory organs in an animal which lives
in a burrow with only the anterior end of the head exposed.

The antennule (at) has three flagella ; the antenna (a2) a single
flagellum and a very large exopodite. The first five pairs of
thoracic limbs (7 — 5) are turned forwards towards the mouth, and
act as maxillipedes ; the second of these — corresponding with the
second maxillipede of Astacus — is very large (2), and its distal
segment is turned back and articulated to the penultimate segment
like the blade of a pocket-knife to the handle. In this way a very

578 ZOOLOGY SECT

efficient weapon called a sub-chela is produced, both the segments
of which are produced into strong spines. The remaining three
thoracic appendages (6 — 8) are slender legs provided with exopo-
dites : the last of them has a styliform copulatory organ (p)
developed from its proximal segment. The pleopods are large and
biramous : the first five (pl — jp.) have gill-filaments (br) attached
to their plate-like exopodites : the sixth (p^) form large uropods
or lateral tail-lobes, as in Astacus.

With regard to the texture of the exo skeleton, there is
every graduation from the delicate polished cuticle of most
Branchiopoda, Ostracoda, Copepoda, &c., through the calcified but
still flexible cuticle of Astacus, to the thick, tuberculated, stony
armour of many Crabs (Fig. 476, 3), or the shelly pieces of Cirri-
pedes. The exoskeleton is secreted from a single-layered ectoderm,
and undergoes periodical moults or ecdyses. There is no trans-
verse layer of muscle, and the longitudinal layer is broken up
into paired dorsal and ventral bands. As a rule, each limb-
segment is acted upon by two muscles : the joints are nearly
always hinge- joints.

The body-cavity consists of several chambers separated from
one another by partitions. In Palcemonetes, one of the Prawns,
there is a median dorsal chamber enclosing the ophthalmic artery,
and not containing blood : it is probably a portion of the coelome
in the strict sense of the word. The cavities of the gonads are
also coelomic, and the ducts by which they communicate with the
exterior are probably modified ccelomoducts. In addition to these
cavities there is a large central space, in which the enteric canal,
digestive glands, gonads, &c., lie ; paired lateral spaces containing
portions of the shell-gland ; spaces in the limbs ; and the peri-
car dial sinus, in which the heart lies. All these cavities contain
blood, and constitute a kind of secondary body-cavity, formed by
the enlargement of blood-vessels, which have largely replaced the
true coelome. Such a secondary or blood-containing body-cavity
is called a hcemocoele.

The enteric canal consists of a vertical gullet, an expanded
" stomach," and a nearly straight horizontal intestine. In some of
the Cladocera the intestine is coiled, but this is quite exceptional.
In the lower Crustacea, part or the whole of the " stomach " is
formed from the mesenteron, but in Malacostraca both gullet and
" stomach " (gizzard) are developed from the stomodseum. A
" gastric mill " is present in Malacostraca, and a rudiment of
such an apparatus occurs in Ostracoda. The digestive glands
are usually branched caeca formed as offshoots of the mesenteron :
in the Isopoda and Amphipoda (Fig. 478, I) they are unbranched
caeca extending into the abdomen : in Stomatopoda they consist
of ten metamerically arranged organs opening into the intestine.
In Amphipods there is an unpaired intestinal caecum (ud) or a
pair of caeca which may have an excretory function (hd). So-called

XI

PHYLUM ARTHROPODA

579

salivary glands, opening on the labrum, have been found in several
genera.

In most of the Branchiopoda, Ostracoda, Copepoda, and
Cirripedia, respira-
tion takes place by
the general surface
of the body,and the
only respiratory
organs are speci-
ally modified parts
of the appendages.
In the stalked
Barnacles, how-
ever, there are
delicate processes
attached to the
feet, which are
supposed to be
rudimentary gills.
Amongst the Mala-
costraca also, the
Phyllocarida,
many Mysidacea,
and the Cumacea
have no special-
ised respiratory
organs, but the
Euphausiacea
possess tufted
p o d o b r a n c h i ae
(Fig. 479) quite
uncovered by the
carapace. In the
Decapoda the gills
may be either
plume-like, as in
Astacus and its

allies, Or the Spft FIG. 478.— Orchestia cavimana, male.

cylindrical gill-
filaments may be
replaced by flat
plates, as in Crabs
and many Prawns.
It is in this order
only that we find the three types of gill described in Astacus,
and the examination of numerous forms leads to the conclusion
that the typical or theoretical branchial formula for the group is
as shown in the table on page 580.

^*ct

a, eye

antennule ;

) antenna ; aoa, anterior aorta ; aop, posterior aorta ; bm, ventral
nerve-cord ; br, gills ; C+T, cephalothorax ; de, vas deferens ; ed,
rectum ; ehd, entrance of excretory caecum into intestine ; g,
brain ; h, heart ; hd, excretory caecum ; kf, maxillipede ; I,
digestive glands ; od, anterior part of gonad in which small ova
are often found in young males ; oe, gullet ; p\ — $7, abdominal
segments ; sm. "stomach " ; ud, intestinal caecum ; vs. vesicula
seminalis ; t, testis ; // — VIII, free thoracic segments. (From
Lang's Comparative Anatomy, after Nebesky.)

580

ZOOLOGY

SECT.

Actually, however, this formula never occurs, as there is always
more or less reduction in the number of gills. Palinurus has the
highest number known, viz., twenty-one, and in the Common Crab
the total number is only nine.

THORACIC

SEGMENTS.

L

I.J. III.

IV.

V.

VI.

VII.

VIII.

TOTAL.

Podobranchiae

l + ep

l + ep

l + ep

l + ep

l + ep

l+ep

l + ep

l+ep

, 8 + 8ep

Arthrobranchiae

2

2

2

2

2

2

2

2

16

Pleurobr anchi 83

1

I

1

1

1

I

1

I

8

TOTAL . .

4 + ep

±+ep\4 + ep

± + ep

± + ep

4 + ep\± + ep

4 + ep

32 + 8ry,

Many Crabs live on land, and the gills are enabled to discharge
their function in virtue of the moisture retained in the nearly
closed gill-chamber. In the Cocoa-nut Crab (Birgus) the upper
part of the gill-chamber is separated from the rest and forms an
almost closed cavity into which vascular tufts project : it thus
functions as a true lung. Probably the inner surface of the gill-
cover or branchiostegite performs a respiratory function in the
Crayfishes.

In Amphipoda, also, the gills (Fig. 478, br) are outgrowths of
the thoracic limbs : in Isopods they are the modified endopodites
of the second to the fifth pleopods ; in some of the terrestrial

FIG. 479.— Anterior portion of Euphausia pellucida. a\, antennule ; 02, antenna;

first abdominal segment ; au, eye ; br.i — br.8> podobranchiae ; cth. cephalothorax ; en.i, en.z,
endopodites of first two thoracic limbs ; ex.i — ex.6, exopodites of first six thoracic limbs ;
h. heart ; /, digestive gland; m, "stomach " ; ov. ovary ; ovd. oviduct ; I — VIII, proto-
podites of thoracic limbs. (From Lang's Comparative Anatomy.)

XI

PHYLUM ARTHROPODA

581

forms, in adaptation to aerial respiration, a system of air-tubes is
developed in the exopodites ; in Stomatopoda, gill-filaments (Fig.
477, br) spring from the exopodites of the first to the fifth pleopods.
Moreover many Crustacea perform rhythmical contractions of the
intestine, taking in and expelling water : such anal respiration
is common among the lower groups, and is especially noticeable in
Cyclops.

The heart is absent in many Copepods (including Cyclops), in
some Ostracoda (including Cypris), and in Cirripedia : it is an
elongated tube with several pairs of ostia in Euphyllopoda, Lepto-
straca, Stomatopoda, Anaspidacea, Tanaidacea, Isopoda, and
Amphipoda (Fig. 478, h) ; in Cladocera and Decapoda it is shortened
to an ovoid sac with one or more pairs of ostia.

Excretory Organs. — In many larval Crustacea two pairs of
modified mesonephridia are present — the antennary glands opening
on the bases of the antennae, and the
maxillary or shell-glands opening on the
bases of the second maxillae. But as deve-
lopment proceeds one pair nearly always
atrophies, the maxillary gland alone being
usually retained in the Branchiopoda,
Ostracoda, Copepoda and Cirripedia, the
antennary gland in the Malacostraca. In
the Stomatopoda, however, there is no
antennary gland, and the function of renal
excretion may be discharged by a pair of
glandular tubes opening into the rectum ;
and in Amphipoda, though antennary
glands are present, an excretory function
is also assigned to a caecum or a pair of
caeca opening into the posterior end of the
mesenteron. In some of the Cirripedia the
maxillary gland is described as opening into
one of the compartments of the body-
cavity like a typical nephridium.

The nervous system is always formed
on the ordinary arthropod type, as described
in Apus and Astacus, and the chief varia-
tions it presents are connected with the greater or less amount of
concrescence of ganglia. In the sessile Barnacles and in the Crabs
(Fig. 480) this process reaches its limit, the whole ventral nerve-cord
being represented by a single immense thoracic ganglion (bg).

The sense-organs are mostly of the same character as those of
the two examples. The median or nauplius-eye always occurs in
the larva, and can frequently be shown to exist in the adult of
even the higher groups (Decapoda). The Cirripedia and many
parasitic Copepods are eyeless in the adult, as also are certain
VOL. i P P

bg. thoracic ganglion ; eg.
commissural ganglion ; g.
brain ; m. gizzard ; sc.
oesophageal connective ; sg.
visceral nerves ; y. post-ceso-
phageal connective. (From
Lang's Comparative Anat-
omy, after Milne-Edwards.)

582 ZOOLOGY SECT.

subterranean Malacostraca. Olfactory setae occur, as a rule, on the
antennules, and the auditory organs (or statocysts) of Decapoda
are open sacs in the basal segment of the same appendages, but
in Mysidacea they occur as closed cysts (Fig. 466, ot) in the endo-
podites of the uropods.

Reproduction. — In most Crustacea the sexes are separate, but
hermaphroditism occurs in some Branchiopods, in nearly all Cirri-
pedes, and in certain parasitic Isopods (Cymoihoa). In the latter
case the animals are protandrous, male organs being developed first,
and female organs at a later stage. In many Cirripedia minute
complemental males are found attached, like parasites, to the body
of the ordinary or hermaphrodite individual, the male organs of
which appear to be inadequate for the full discharge of the ferti-
lising function. Sexual dimorphism is almost universal, and
reaches its maximum in the parasitic Copepods and Isopods already
referred to.

The gonads are always a single pair of hollow organs discharging
their products into a central cavity or lumen, whence they pass
directly into the gonoducts and so to the exterior. The gonads may
be simple or branched, and frequently there is more or less con-
crescence between those of the right and left sides, as in Astacus
and Cyclops. The sperms vary greatly in form, and are usually
motionless : in Cirripedia, however, they are motile, and in Ostra-
coda they perform movements after reaching the female ducts.
In some Ostracoda they are about three times as long as the animal
itself (Fig. 457, D). In many Branchiopoda and Ostracoda reproduc-
tion is parthenogenetic. In Daphnia, for instance, the animal
reproduces throughout the summer by parthenogenetic summer eggs
which develop rapidly in the brood-pouch (Fig. 456, 1, br. p.).
In the autumn winter eggs are produced, which are fertilised by the
males : they pass into the brood-pouch, a portion of which becomes
specially modified and forms the ephippium or saddle. At the next
moult the ephippium is detached and forms a sort of bivalved
capsule in which the eggs remain in an inactive state during the
winter, developing in the following spring.

Development. — In some Crustacea (Lucifer, Euphausia, and
others) segmentation is complete, and a hollow blastula is formed :
in others division of the nucleus into a number of daughter nuclei
is followed by their migration towards the surface, where, each
becoming surrounded by protoplasm, they form a layer of cells
(blastoderm) enclosing a central mass of yolk (centrolecithal egg
with superficial blastoderm) : in others, again, the egg is telolecithal,
and the protoplasm, accumulated at one pole, divides so as to form
a disc of cells which afterwards spreads over the whole yolk.

Development is always accompanied by more or less metamor-
phosis. In most Branchiopoda the young is hatched in the form of
a nauplius (Fig. 436, A), and further changes are of the same char-

xi PHYLUM ARTHROPODA 583

acter as in Apus. In Cladocera development is direct, the nauplius-
stage being passed through in the egg, and the young hatched in
a form closely resembling the adult. In one of the Cladocera,
however, Leptodora (Fig. 456, 3), while development of the summer
eggs is direct, the winter eggs give rise to free nauplii. In the
Ostracoda the nauplius is peculiar in having a bivalved shell and
all three pairs of appendages uniramous. In all the Copepoda
there is a free nauplius, which, in the parasitic forms, leads a free
existence for a time, and then attaches itself to its particular host
and undergoes retrograde metamorphosis.

In the Cirripedia, also, there is a free nauplius, the body of which
is often produced into long spines. After several moults, the
nauplius passes into a form called the Cypris-stage (Fig. 481),
characterised by the presence of a bivalved shell, like that of an
Ostracod : the antennules (at.1) have become modified into organs

oc

FIG. 481.— Cypris-stage of Lepas fascicularis. add. adductor muscle of carapace ; aU,
antennules ; caud.f. caudal styles ; ex. excretory organ (shell-gland) ; fix. disc for fixation ;
fix. gl. fixing gland ; gn. jaws ; int. intestine ; lab. labrum ; m. mouth ; oc.1, simple eye ;
oc.2, compound eye ; th. thoracic legs. (From MacBride, after Willemoes-Suhm.)

of adhesion by the development of the penultimate segment into a
disc, the antennae have disappeared, and six pairs of swimming-feet
like those of a Copepod have made their appearance : there are
paired compound eyes, and the shell is closed by an adductor
muscle. After leading a free existence for a time, the Cypris-
larva attaches itself by its antennules, aided by the secretion
of cement-glands, and becomes a pupa : the carina, terga, and
scuta appear beneath the shell, and within the skin of the mouth-
parts and legs of the pupa appear the corresponding appendages
of the adult. In Lepas the anterior region of the head grows out
into a peduncle. The pupal integument is then thrown off, the
paired eyes disappear, and the adult form is assumed.

In Sacculina a still more extraordinary metamorphosis takes
place. The young is hatched as a nauplius, but without alimentary
canal, and passes into a Cypris-stage. In this condition, after a

p p 2

584

ZOOLOGY

SECT.

brief free existence, it attaches itself to the body of a young Crab,
near the base of a seta, by means of its antennae. The thorax with
its appendages is thrown off, and the rest of the body is converted
into a rounded mass of cells. The antennae perforate the cuticle
of the host, and, through the communication thus formed, the
mass of cells passes into the interior of the Crab, and is carried by the
movement of the blood until it comes to rest in the thorax. The
Sacculina now sends out root-like processes, grows immensely,
and, pressing upon the body-wall of the Crab, causes atrophy
of the tissues : this allows the now greatly-swollen parasite to
project on the exterior as the tumour-like adult described above
(p. 565).

FIG. 482.— Larvse of Crabs. A. Zocea-stage of Maia ; S, Megalopa-stage of Fortunus.
h. heart ; a^- - ae, abdominal segments ; 1, antennule ; 2, antenna ; I— VIII, thoracic appen-
dages. (From Lang's Comparative Anatomy, after Claus.)

The embryo of Euphausia leaves the egg as a typical free-swim-
ming nauplius ; this passes into what is called the protozocea-stage,
distinguished by the possession of an elongated, unsegmented
abdomen without appendages. After successive moults, the rest
of the appendages appear, and the adult form is assumed. In
Mysis (Fig. 466) the nauplius is maggot-like, and undergoes develop-
ment in the brood-pouch, emerging in a condition closely resembling
the adult.

The development of the Decapoda presents a very interesting
series of modifications. In two genera of prawns (Penceus and
Lucifer) the embryo leaves the egg as a nauplius, and passes by
successive moults through a protozosea stage, a zocea-stage, with
segmented but limbless abdomen, and a mysis or schizopod-stage

xi PHYLUM ARTHROPODA 585

in which it resembles an adult Mysis, having exopodites to all
the thoracic limbs.

In the Crabs the nauplius stage is passed through in the egg,
and the young is hatched in the form of a peculiarly modified
zoaea (Fig. 482, A), with an immense cephalothorax produced into
spines, large stalked eyes, and a slender abdomen. This passes
by successive moults into the megalopa-stage (B), which resembles
an adult Macruran, having an extended abdomen with well-
developed pleopods. The megalopa passes by successive moults
into the adult form.

In the Lobster (Homarus) both nauplius and zoaea-stages are
passed through in the egg, and the embryo is hatched in the mysis-
stage with exopodites to all the thoracic limbs. In the Rock-
lobster (Palinurus), and its allies, the newly hatched young is a
strangely modified mysis-form called a Glass-Crab or Phyllo-
soma : it has broad, depressed cephalic and thoracic shields of
glassy transparency : the abdomen is very small and the legs
extremely long and biramous. Lastly, in the Fresh-water Cray-
fish the young resemble the adult in all but proportions and certain
unimportant details of structure. Thus in the series of Decapoda
we get a gradual abbreviation in development, stages which are
free larval forms in the lower types being hurried through before
hatching in the higher.

The larvae of Stomatopoda are grotesque little creatures with a
very large spiny carapace. In Amphipoda there is no free larval
form, but in Isopoda the young leave the egg in the form of a
curious maggot-like modification of the nauplius, which remains
in the brood-pouch until it has attained the adult form.

Ethology. — The Crustacea are remarkable for their very perfect
adaptation to the most various conditions of life : they occur in
fresh-water, in the sea, in brine-pools, in subterranean caves, and
on land : of the marine forms some are littoral, some pelagic, some
abyssal, descending to over 3,000 fathoms. One species of Copepod,
Pontellina mediterranea, may almost be considered as aerial : it
is described as taking long flying leaps out of the water, after the
manner of a Flying-fish. Some, like Lobsters, Crayfishes, &c., are
solitary ; others, like Shrimps, are gregarious, occurring in immense
shoals. Most of them either prey on living animals or devour
carrion, but, as we have seen, the Barnacles are fixed, and feed on
minute particles after the fashion of many of the lower animals,
and the members of more than one order are parasites remark-
able for their deviation from the typical structure of the class
and their adaptation to their peculiar mode of life. In size they
present almost every gradation from miscroscopic Water-fleas
to Crabs two feet across the carapace, or four feet from tip to
tip of the legs.

586 ZOOLOGY SECT.

As to geographical distribution, all the chief groups are cosmo-
politan, and it is only among the families, genera, and species that
matters of interest from this point of view are met with. Fossil
remains are known from very ancient periods. The oldest forms
are usually referred to the Phyllocarida, and occur from the Cam-
brian to the Trias. The shells of Ostracoda are also known from
the Cambrian upwards, and those of Cirripedia from the Silurian.
Peracarida are known from Palaeozoic times, but are rare as fossils :
the earliest Macruran is a shrimp-like form from the Devonian,
while the highly differentiated Brachyura are not known with
absolute certainty until the Cretaceous period.

It was in the Crustacea that the recapitulation theory so often
alluded to was first worked out in detail. Embryology shows
that all Crustacea may be traced back in individual development
to the nauplius, upon which follows some kind of zoaea-stage, many
of the lower forms progressing no further. But in Malacostraca
the zoaea is followed by the mysis stage, which is permanent in
Schizopods, transient in Decapods. It was certainly a tempting
hypothesis that this series of forms represented as many ancestral
stages in the evolution of the class. But we have to remember
that all such free larvae are subject to the action of the struggle
for existence, and have no doubt been modified in accordance with
their own special needs and without exclusive reference to their
ancestors or to the adult species into which they finally change.

Many Crustacea present instances of protective and aggressive
characters, i.e., modifications in form, colour, &c., which serve to
conceal them from their enemies or from their prey. Probably
the most striking example is that of certain crabs (Paramithrax),
which deliberately plant Sea-weeds, Sponges, Alcyonarians, Zoo-
phytes, &c., all over the carapace, and are thus perfectly concealed
except when in motion. Another Crab, a species of Dromia, carries
a relatively immense Ascidian or Sea-squirt on its back, and in
another member of the same family the hinder legs are used to
hold umbrella-wise over the back a single valve of a bivalve shell.

Several instances of commensalism occur in the class. The
association of Hermit-crabs with sea-anemones, has already been
referred to (p. 205) : another interesting example is the occurrence
of the little Pea-crab (Pinnotheres) in the mantle-cavity of Mussels.
Other Decapods are found in the intestines of Sea-urchins and
Holothurians, and one genus of Crab lives in a cavity in a Coral,
the aperture being only just sufficient to allow of a due supply
of food and water.

It is in Crustacea that we find the first indication of characters
the purpose of which appears to be their attractiveness to the
opposite sex. The immensely enlarged and highly coloured chelae
of some male crabs (Gelasimus, Fig. 476, 2) are said to be used for
attracting the female as well as for fighting. The sound-producing

xi PHYLUM ARTHROPODA 587

organs of some Decapoda have probably also a sexual significance.
The Rock-lobster (Palinurus vulgaris) has a soft chitinous pad on
the antenna, which it rubs against a projecting keel on the sternal
region of the head, producing a peculiar creaking sound ; and
AlpTieus, another Macruran, makes noises by clapping together the
fixed and movable ringers of its large chelae. The fact that these
sounds can be produced at the will of the animals seems to show
that the latter undoubtedly possess a sense of hearing, and that
the auditory sac is not merely an organ of the sense of direction.

Affinities and Mutual Relationships. — That the Crustacea
belong to the same general type of organisation as the articulated
worms is clear enough. The advance in structure is shown in
the reduction in number and in the differentiation of the segments,
and in the concrescence of those at the anterior end to form a
head ; in the hardening of the cuticle into sclerites so as to form
a jointed armour ; in the jointing and mobility of the limbs ; and
in the differentiation of the dorsal vessel into a heart by which
the propulsion of the blood is alone performed. The resemblance
of the foliaceous limbs of Phyllopods to the parapodia of the
higher worms is so striking that one can hardly believe it to be
without significance. On the other hand, the absence of transverse
muscles and of cilia, and the replacement of the coelome by blood-
spaces, are fundamental points of difference from any known
Chaetopod.

As to the mutual relations of the various orders, the Branchio-
poda, with their very generalised structure and parapod-like
limbs, may be taken as the base of the series. The Ostracoda,
Copepoda, and Cirripedia are best conceived as derivatives, along
separate lines, of an ancestral form common to them and the
Branchiopoda. By a differentiation of the post-cephalic limbs,
and a reduction in the number of segments, the branchiopod
type easily passes into that of the Phyllocarida, which, though
they nearly conform to the malacostracan type of segmentation,
have still marked traces of relationship with lower groups in the
presence of caudal styles and in their bivalved carapace and
foliaceous thoracic appendages. Next to these in ascending order
would come the Cumacea with their cephalic carapace coalescent
with the first three or four thoracic segments and bounding branchial
cavities at the sides of the thorax, but with — as more primitive
features — a biramous character in some of the thoracic appendages
and the absence of the fan-like tail-fin. Then a little higher, the
Arthrostraca (Tanaidacea, Isopoda and Amphipoda) and the
Anaspidacea may be supposed to have branched off from the main
trunk at about the same level, and may be regarded, on account
of a 'number of resemblances, as having had a common origin from
it. Probably the Anaspidacea are to be looked upon as more

588

ZOOLOGY

SECT.

primitive than the other two groups in view of the less advanced
coalescence of the first thoracic segment with the head, the absence
of specialised maxillipedes, and the biramous character of the
thoracic limbs ; but, on the other hand, they show a higher develop-
ment in the possession of the fan-like tail-fin and the stalked
movable eyes such as characterise the Decapoda.

A stage nearer the latter group are the Mysidacea, with their
single pair of maxillipedes, their stalked eyes, their rudimentary

Brachyura

Macrura

ArJ-hros^aca

\ Mysidacea
Anaspidacea

Trilobi>a

Ancmur*a

Euphausiacea

Sfomaf-opoda

Phyllocarida

Copepoda

Cirripedfa

Annulaha
FIG. 483. — Diagram illustrating the mutual relationships of the orders of Crustacea.

podobranchise and their fan-like tail-fin ; but these still show some
primitive features, more especially in their incomplete t cephalo-
thorax and their biramous thoracic appendages. But without
doubt it is in the Euphausiacea that we find the nearest connections
with the Decapoda. This is shown, in spite of the absence of
maxillipedes, in their completed cephalothorax, their series of
podobranchiae, and sac-like heart, in addition to their stalked eyes
and fan-like tail-fin.

XI

PHYLUM ARTHROPODA

589

From the Euphausiacea the Macrura are derivable by the
differentiation of three pairs of foot- jaws and the disappearance of
the exopodites of the legs. In the series of the Macrura we find,
on passing from the Prawns through such forms as Astacus,
Palinurus, and Scyllarus, a gradual shortening of the abdomen,
accompanied by a broadening and flattening of the whole body.
In Birgus, Hippa, &c., this process goes a step further, and the
abdomen becomes permanently flexed under the cephalothorax,
thus leading to the high degree of specialisation found in the Crabs.

These relationships are expressed in the diagram on page 588.

APPENDIX TO CRUSTACEA.

Class TRILOBITA.

The Trilobita are extinct Arthropods peculiar to, and characteristic of, the
Palaeozoic rocks : they are specially abundant from the upper Cambrian to
the Carboniferous. They are often found in a wonderfully good state of
preservation, owing to the hard exoskeleton covering the dorsal surface : the

ffl

Fia. 484.— Dalmanites socialis, dorsal aspect ; B, the same rolled up ; C, under-side of
head of Fhacops fecundus. c.sh. cephalic shield ; e. eye ;f.c. fixed cheek ;/.«. frontal
suture ; gl. glabella ; Ibv. labrum ; m.c. movable cheek ; p. pygidium ; pi. pleura ; s.f.p.
sub-frontal plate ; th. thorax. (After Gerstaecker.)

greater part of the ventral region and the appendages were, however, very
delicate, and are preserved only in exceptionally favourable cases.

The body is depressed, more or less oval in outline, and divided into three
regions, the head (Fig. 484, c.sh), the thorax (th), and the abdomen (p), all of
which usually present an elevated median Bridge and depressed lateral portions,

590

ZOOLOGY

SECT.

whence the trilobation generally characteristic of the group. The head is
covered by a carapace or cephalic shield (c.sh), the elevated median region of
which, known as the glabella (gl), usually presents three or four transverse
grooves, probably indicating the presence of four or five segments. The lateral

regions of the carapace are
divided by an oblique line of
separation, the frontal or
facial suture (/.<§.), into an
inner or mesial portion, the
fixed cheek (f-c), continuous
with the glabella, and an
outer free portion, the mov-
able cheek (m.c) ; the latter
bears the large paired com-
pound eye (e). In some cases
there is an indication of a
dorsal organ, like that of
Apus, on the last cephalic
segment. Ventrally the cara-
pace is continued, as in
Apus, into a sub -frontal
plate (C, s.f.p), to the pos-
terior edge of which is
attached a large labrum or
hypostome (Ibv). In many
Trilobites the hypostome
bears a pair of small com-
pound eyes. The posterior
angles of the carapace are
often produced into spines.

The thorax (th) is com-
posed of a variable number
(2-29) of movably articu-
lated segments, which are
commonly trilobed, consist-
ing of a median region or
axis, and of lateral pleura
(pi) often produced back-
wards and downwards into
spines. The abdomen is
covered by a caudal shield or
pygidium (p), formed of a
variable number of fused
segments. Owing to the
mobility of the thorax, the
Trilobites were able in many
cases to roll themselves up

like Wood-lice (£). Each of
the segments, with the sole
exception of the last or
anal, bore a pair of appen-

JFiQ. 485.— Triarthrus becki, x 2£. A, ventral
surface with appendages; Ep, metastome; Hy, hypo-
stome. B, second thoracic appendage, en. endo-
podite ; ex. exopodite x 12. (From the Cambridge
Natural History, after Beecher.)

uages

The appendages are known only in a few cases. Quite recently a single
pair of antennae (Fig. 485) has been shown to exist in one species, probably
attached to the sub-frontal plate. There are no true jaws. Four pairs of
biramous leg-like cephalic appendages have been demonstrated, and the
thorax bears slender biramous legs with endo- and exopodites, and bearing
spiral gills. Similar limbs are present on the abdomen.

XI

PHYLUM ARTHROPODA 591

The larvae of several species of Trilobites have been found in the fossil
state. In some of these stages the body consists only of carapace and
pygidium in the youngest, and the thoracic segments are subsequently
intercalated in regular order. In other species the earliest stage has the
form of a rounded plate, the posterior portion of which elongates and segments
to form the thorax and abdomen. Nothing is known of the larval appendages,
and none of the stages hitherto discovered can be considered as nauplii.

The precise systematic position of the Trilobites is uncertain, but their
nearest affinities seem to be, on the whole, with such Branchiopoda as Apus ;
the relationship is, however, by no means a close one.

CLASS II.-ONYCHOPHORA.

The class Onychophora comprises only the aberrant arthropod
Peripatus, with several sub-genera (or closely allied genera), which
differs very widely in certain important features of its organisation
from all the rest of the Arthropoda, and in some respects enables
us to bridge over the interval between the latter and some of the
lower phyla, more particularly the Annulata.

General external features. — Peripatus (Fig. 486) is a cater-
pillar-like animal of approximately cylindrical form, and not divided

FIG. 486. — Peripatus (Perlpatopsis) capensis, lateral view. (From Balfour.

into segments ; it has a fairly well-marked head, and a series (14 —
42, according to the species) of pairs of short stumpy appendages.
The integument is thrown into a number of fine transverse wrinkles,
and is beset with numerous conical papillae, each capped with a
little chitinous spine. The head (Fig. 487) bears a pair of antennae,
a pair of eyes, a pair of jaws, and a pair of short processes known
as the oral papillce. The antennae are made up of a number of
short rings bearing minute spines. The eyes are constructed
somewhat after the model of the chaetopod eye as described on
p. 437. On the surface of the oral papillae are situated the apertures
of a pair of glands — the slime-glands. Each jaw is composed of
two curved, falciform, pointed, chitinous plates, the inner toothed
on its posterior concave edge ; they lie at the sides of the mouth
enclosed by a circular lip. The jaws, as well as the oral papillae,
are developed as modified limbs.

The legs are not jointed, but rows of papillae give them a ringed
appearance ; each consists of a conical proximal part and a small
distal part oifoot, the latter terminating in a pair of horny claws.

The ventral surface is reddish in colour, the dorsal darker :
the latter presents an elaborate pattern — which varies greatly in
different individuals — produced by minute mottlings of various

592

ZOOLOGY

SECT.

colours and tints, green, red, and brown, and the arrangement of
these in stripes and bands.

Body-wall and body-cavity. — The wall of the body consists
of a cuticle, a layer of deric epithelium with an underlying layer
of fine fibres, a layer of circular muscular fibres, and one of
longitudinal muscular fibres divided into a series of bundles. A
layer of epithelium lines the wall of the body-cavity and invests
the contained organs. Incomplete muscular partitions divide the
cavity into a median and two lateral compartments, in addition to
the pericardium, or space in which the heart is lodged ; the lateral
compartments send prolongations into the legs. The body-cavity

is not a coelome,
but, as shown by
its development, a
haemoccele — an ex-
tension of the blood-
vascular system —
as in the Crustacea.
The enteric
canal (Fig. 488)
begins with a small
buccal cavity, formed
secondarily by the
union of a ring of
papillae and folds
surrounding the
true mouth into a
circular lip : it en-
closes the bases of
the jaws and bears
on its roof a slight
prominence, the

Fia. 487.— Ventral view of head of Peripatus (Peripatopsis) ton9ue> ^ltn. a rOW
capensis, with antenna?, jaws, oral papillae, and first pair OI small Spines Or
of legs. (After Balfour.) teeth. This is

followed by a thick- walled pharynx (phar.) leading to a narrow
oesophagus. The part which follows, the mesenteron or stomach-
intestine, a wide somewhat thin- walled tube, extends nearly to the
posterior end of the body. The narrower rectum leads to an anal
aperture situated on the last segment of the body. A diverticulum
leading backwards from the buccal cavity, where it passes into the
pharynx, receives the secretion of two long, narrow, tubular salivary
glands (sal. gld.).

Circulatory system. — The heart is an elongated tube running
through nearly the entire length of the body. It presents a
number of pairs of valvular ostia arranged segmentally — i.e., one
opposite each pair of legs. It is enclosed in a pericardial sinus

XI

PHYLUM ARTHROPODA

593

imperfectly cut off from the general body-cavity by a longitudinal

partition.

The organs of respiration (Fig. 489) are delicate, unbranched

or rarely branched, tracheal tubes, lined with a thin chitinous layer

exhibiting fine transverse striations. Groups of these open in

little depressions of the integu-
ment, the tracheal pits (tr.p.), the
external openings of which are
known as the stigmata (tr.o.).
The stigmata are usually distri-
buted irregularly over the surface,
with a tendency to arrangement
in rows in P. capensis. By
means of these tubes air is con-
veyed to all parts of the body.

A series of pairs of glands, the
coxal or crural glands (Fig. 488,
cox. gld.), lie in the lateral com-
partments of the body-cavity, and
their ducts open on the lower
surfaces of the legs just outside
the nephridial apertures. They
are absent in the female except

-tr.o.

Cin

FIG. 488.— Dorsal view of the internal organs FiQ. 489.— Section through a tracheal pit

of Peripatua. an. anus ; ant. antennae ; and diverging bundles of tracheal tubes

brn. brain ; cox. gld. coxal gland of the seven- of Peripatus. tr. tracheae; tr. c. cells

teenth leg ; S gen. male genital aperture ; in walls of tracheae ; tr. o. tracheal stigma ;

ne. co. nerve-cord ; neph. nephridia ; or. tr. p. tracheal pit. (From Camb. Nat. Hist.,

pap. oral papillae ; phar. pharynx ; sal. gld. £ after Balf our.)
salivary gland ; si. gld. slime gland ; stom.
stomach. (Combined from Balf our.)

in P. capensis, and their number and arrangement differ in the
males of the various species. Also opening on the ventral surfaces
of the legs are a series of thin-walled vesicles — the coxal organs :
these occur in both sexes and are capable of eversion and retraction.
A pair of large glands — the slime-glands (si. gld.) — opening at

594

ZOOLOGY

SECT.

the extremities of the oral papillae, may be modified coxal glands :
the secretion of these is discharged in the form of a number of
fine viscid threads when the animal is irritated, and appears to
serve a defensive purpose.

The nervous system consists of a brain (brn.) situated in the
head, and of two longitudinal nerve cords (ne. co.) which run parallel
with one another throughout the body to the posterior end, where
they join together behind the anal aperture. A number of very
fine transverse commissures, more numerous than the segments
(i.e., than the pairs of limbs), connect the two cords together to
form a ladder-like nervous system comparable to that of some
of the Flat Worms. The cords are very slightly swollen opposite
each pair of limbs : nerve-cells cover them uniformly throughout
their entire length. The brain gives off nerves to the antennae.

The nerves to the jaws are
given off just where the
brain passes into the longi-
tudinal nerve cords.

The excretory organs are
nephridia (Fig. 490) of the
type of those of the Annu-
lata, situated in pairs in the
lateral compartments of the
body-cavity, and opening on
the lower surfaces of the legs
bases. Each nepliri-

J

.c. i, s.c. 2, s.c. 3, s.c. 4, successive regions of closed internal vesicle, repre-

coiled portion ; s.o.t., third portion of nephridium r *.«

broken off at p.f from the internal vesicle, which Senting a Section OI * tne

BaK^P' ^* *»<*"*'*«• ***-•«*" co3lome, into which by a

funnel-shaped aperture leads

a looped tube (s.c.). and a dilated terminal vesicle (s.), situated close to
the external opening. The salivary glands are, as shown by the study
of their development, specially modified nephridia, as apparently
also are a pair of glands — the anal glands — opening close to the anus.

Reproductive organs. — Peripatus has the sexes distinct. In
the female there are two tubular ovaries, a pair of oviducts, and
two uteri, the latter in the form of long curved tubes which unite
behind in a median vagina opening on the exterior on the ventral
surface, between the legs of the last pair or behind them. In the
oviparous forms the opening is situated at the end of a long cylin-
drical process — the ovipositor. In some species, connected with
each uterus where it leaves the ovary, are two diverticula — the
receptaculum seminis and receptaculum ovorum. In certain species
one or other of these may be absent.

In the male there are two tubular testes, each with a narrow vas
efferens opening by a funnel-like aperture into a vesicula seminalis ;

XI

PHYLUM ARTHROPODA

595

this is followed by a long, narrow, coiled vas deferens. The two
vasa deferentia unite together to form a median tube — the ductus
ejaculatorius — opening on the exterior, in the same position as the
vagina of the female. The wall of the ^ proximal part of the
ejaculatory duct is glandular, and secretes a substance forming
complicated cases which enclose masses, of sperms to form
spermatophores.

Development. — The differences between the species of Peripatus
as regards the segmentation of the egg and the formation of the
germinal layers as described by various observers are very con-
siderable. Nearly all the species are viviparous, but in some the
egg, before the completion of embryonic development, is enclosed
in a well-formed shell, and in certain species the eggs pass out to
the exterior before the emergence of the embryo. In some species

FIG. 491. — Two early stages in the development of Peripatus novae -zealandiae. A, trans-
verse section of an ovum in which the yolk is nearly covered by the blastoderm (bl.) ; B,
transverse section of an ovum in which the blastopore (blp.) is formed. (After Sheldon.)

the egg encloses a considerable amount of food-yolk, in others the
quantity of food-yolk is small, and nutriment is obtained from the
parent.

In P. novce-zealandice there is a superficial segmentation. The
first segmentation-nucleus is itself superficial, and segmentation
results in the development of a number of nuclei, each with its
island of protoplasm, which arrange themselves on what is destined
to become the dorsal side (Fig. 491, A), opposite the site of the
future blastopore, while some pass inwards to the central part of
the ovum. The peripheral nuclei multiply rapidly and grow round
the yolk so as completely to enclose it except on a small space
(blastopore) in the middle of the ventral side (B). There a thicken-
ing takes place, and an involution of the lips of the blastopore
results in a sort of invagination, the floor of the invagination-cavity
being formed of yolk with scattered nuclei.

596

ZOOLOGY

SECT.

In another species — Peripatopsis capensis — the segmentation is
total. A peripheral ectodermal layer becomes formed, enclosing a
central mass of cells — the endoderm — except at one point where a
small area, the blastopore, is uncovered.

In accordance with the smaller size of the ova and the relationship
of the embryo with the wall of the uterus, the South American

FIG. 492. — Three somewhat later stages in the development of Peripatus capensis, showing
the mode of closure of the blastopore and the appearance of the primitive segments. A,
stage in which the blastopore (bl.) has the form of an elongated slit ; B, stage in which the
blastopore is closing up in its middle part ; C, stage in which the blastopore has become
closed up except the anterior part which has gone to form the mouth (mo.), and the
posterior part which has formed the anus (an.) ; the whole embryo has now become strongly
curved towards the dorsal side. (After Balfour.)

species show a totally different mode of development. The eggs,
which are almost entirely devoid of yolk, undergo a total and
tolerably equal process of segmentation. Even at this stage the
embryo, which increases considerably in size, appears to receive
nutrient lymph from the uterine wall. Later an intimate connection

xi PHYLUM ARTHROPODA 597

is established between the embryo and a modified area of the
uterine epithelium, the placenta thus formed evidently providing,
like the placenta of Mammals, for the nourishment of the embryo.

In P. capensis (Fig. 492) proliferation of cells gives rise to an
oval thickening behind the elongated blastopore. The mesoderm
takes its origin at this point and extends forwards in the form of
two germinal bands, one on the right of the blastopore and the
other on the left. These bands undergo a division into rudiments
of segments — the division beginning in front. The lips of the
blastopore meanwhile become approximated, and fuse throughout
the greater part of their length, leaving only an anterior and a
posterior opening ; these go to form the mouth and the anus
respectively. The division into segments soon becomes well
marked. The cavities in the mesoderm of the segments give rise
only to the internal vesicles of the nephridia and the generative
ducts, which thus alone represent the ccelome. The nephridia
themselves are developed as ingrowths of the ectoderm. At
the anterior end the head lobes become distinguishable. The body
elongates, and the head and trunk become differentiated. The
limbs now arise as ventro-lateral outgrowths which are developed
from before backwards. The jaws originate in the same manner
as the limbs, as external projections, and only later become enclosed
by folds that give rise to the buccal cavity of the adult.

Distribution. — The various species of Peripatus are all terres-
trial, and are found in damp localities, under bark, or dead timber,
or stones. Some twenty-nine species occur in the Neotropical
region ; one in South America ; eight in Africa ; four in Malaya ;
one in New Britain, and eight in Australasia.

Relationships. — Peripatus is the most primitive of existing
Arthropods, and presents some striking points of resemblance to
the Chaetopoda. The development is in the main arthropodan,
especially as regards the mode of segmentation (at least in the
forms with much food-yolk, which are probably the more
primitive), the mode of closure of the blastopore, and of the
development of the germinal bands. Arthropodan also are the
relatively large size of the brain and the presence of tracheae, the
character of the heart with its pairs of ostia, together with the
clawed appendages, and the jaws in the form of modified limbs.
The nephridia, on the other hand, and their modification in certain
segments to form the gonoducts, which are ciliated internally,
are annulate in character ; and in all probability the slime glands
and coxal glands correspond to the setigerous sacs of the Chsetopoda.
The nervous system is peculiar, and is most nearly paralleled among
the Platyhelminthes and the Mollusca. Also peculiar, and serving
to distinguish Peripatus from the rest of the Arthropoda, are the
large number of stigmata and their irregular arrangement, the
presence of only a single pair of jaws, and the nature of the cuticle.

VOL. I Q Q

598

ZOOLOGY

SECT.

CLASS III.-MYBIAPODA.1

The class Myriapoda, including the Centipedes and the Millipedes,
consists of tracheate Arthropoda, which present many features
of resemblance to the Insects. There is a distinct head, bearing
many- jointed antennae, a pair of eyes, and two or three pairs of
jaws ; the body is not distinguishable into regions, but consists
of a number of similar segments, each bearing either one pair of
legs or two pairs. A system of air-tubes or tracheae, similar to
those of Peripatus and the Insects, opens by a series of stigmata,
usually in considerable numbers, on the sides or lower surfaces of
the segments.

A. — DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Myriapoda are tracheate Arthropoda in which there is a
head, bearing antennae and jaws, and a trunk made up of a number
of similar segments, provided with leg-like appendages. Groups
of ocelli are present on the head.

Sub-Class I.-PROGONEATA.

Myriapoda in which the genital apertures are situated far
forwards towards the anterior end of the
body.

ORDER 1. — PAUROPODA.

Progoneata with ten trunk-segments
and nine pairs of legs, one pair to each
segment except the first. Antennae with
several flagella. Tracheae not known.
The order includes only the two genera
Pauropus (Fig. 496) and Eurypauropus.

ORDER 2. — DIPLOPODA (CHILOGNATHA).

Progoneata with a body composed of
a considerable number of apparent seg-
ments, each of which, with the exception
of the first three, bears two pairs of legs
and represents two true segments united.
FIG. 493. — scoiopendreiia There are no maxillipedes.

immaculata. (From Leuck
art, after Latzel.)

This order includes the Millipedes.

1 As will appear subsequently, the class Myriapoda, as formerly understood,
comprises two groups which are separated from one another by such important
differences that they might very well be looked upon as constituting two
distinct and independent classes. The old class Myriapoda is retained here as
a matter of convenience, and the two constituent groups are ranked as sub-

XI

PHYLUM ARTHROPODA
ORDER 3. — SYMPHYLA.

599

Progoneata in which there are not more than twelve leg-bearing
segments, and in which there is only a single pair of branching
tracheae, the two external apertures of which are situated in the
head. Not more than three pairs of jaws. Feet with two claws.

a.nt

-Urn

slom.

inf

FIG. 494. — Scolopendra. (From
Cuvier's Animal Kingdom.)

FIG. 495. — Ziithobius forficatus seen from the
ventral side. ant. antennae ; brn. brain : cox. ap.
coxae of appendages ; ft. 15, fifteenth pair of legs ;
int. intestine ; maL Malpighian tubes ; mxp.
maxillipedes ; ne. co. nerve cord ; oes. oesophagus ;
stom. stomach. (From Leuckart.)

This order includes only the two genera Scolopendrella (Fig. 493)
and Scutigerella.

Sub-Class II.— OPISTHOGONEATA,

Myriapoda in which the genital apertures are situated at the
posterior extremity of the body.

Q Q 2

600 ZOOLOGY SECT.

ORDER 1.- — CHILOPODA (SYNGNATHA).

Opisthogoneata with numerous (15 — 173) 'trunk-segments, each
bearing a single pair of legs. Numerous tracheae opening in pairs
of stigmata on the sides of a number of the segments. Four pairs
of jaws, including a pair of poison-jaws. Feet with a single claw.

This order includes the Centipedes (Fig. 494) and Scutigera.

GENERAL ORGANISATION.

External Features. — The head in the Myriapoda is as well
marked off as in an Insect ; it appears to be composed of about
four fused segments. The antennce consist sometimes of many,
sometimes of comparatively few segments ; in Pauropus they are
branched. A pair of eyes, situated on the dorsal surface of the
head, consist of aggregations of ocelli except in
Scutigera, in which there are compound eyes,
differing, however, in their structure from those
of Insects. There are a movable labrum, a pair of
mandibles, and two pairs of maxillce. The
mandibles have no palps ; one or both pairs of
*Jj&-~£^Sae maxillae usually possess palps ; the second pair
~i ' of maxillae are in some groups more or less

united together. In the Chilopoda the first pair
of legs of the trunk are specially modified to
act as poison- jaws (maxillipedes), by means of
which the Centipede inflicts its poisonous bite.

The number of segments in the body varies
from 10 to 173. In the Millipedes the dorsal
walls of the segments are very strongly arched ;
in the Centipedes the segments are all dorso-
ventrally compressed, with distinct tergal and
Leuckart, after Lat- sternal shields separated laterally by intervals of
comparatively soft skin on which the stigmata
open. In the Chilopoda each segment bears a pair of jointed
legs ; of these the most anterior pair is extended forwards,
as already stated, to form a pair of poison-jaws (maxillipedes), at
the extremity of the pointed terminal joint of which opens the
duct of a poison-gland. In the Diplopoda each segment behind
the fourth or fifth bears two pairs of legs, the four or five most
anterior having only one pair each. In most of the Diplopoda
the appendages of the seventh segment are modified in the male
to form copulatory organs.

The integument and body- wall do not differ widely from
those of Insects (see p. 621). The exoskeleton is a thickened
chitinous cuticle which is calcified in Diplopoda. Odoriferous
glands are present in most Diplopoda on some of the body-
segments, and open on the dorsal surface. Scolopendrella possesses
spinning glands.

xi PHYLUM ARTHROPODA 601

The alimentary canal is straight, and is much simpler in
character than that of the Insecta. There is a pair of salivary .
glands ; and one or two pairs of Malpighian tubes, having a renal
function, open into the beginning of the hind-gut.

The heart is a greatly elongated tube, divided into a number
of chambers.

The respiratory system resembles that of Insects, which will
be fully dealt with later, consisting of air-tubes or trachea. There
is one pair of stigmata in each true segment in the Diplopoda (two
pairs in each apparent segment). Each stigma leads into an air-
chamber from which a large number of tracheae are given off. In
some Diplopoda the tracheae are branched, in others unbranched :
the tracheae of one group do not anastomose with those of other
groups. In the Chilopoda the number of stigmata is in most cases
less than the number of segments, and the tracheae anastomose,
often forming longitudinal trunks which may extend throughout
the body. In Scutigera the stigmata are unpaired and dorsal,
and each leads into a large air-chamber which gives off on either
side a large number of radially arranged short air-tubes — the whole
forming a sort of lung.

In the Symphyla there are only two stigmata, and these are
situated on the head.

The nervous system is, in accordance with the form of the
body, much less concentrated than in the Insecta (see p. 628).
There is a brain, a pair of cesophageal connectives, and a ventral
nerve-cord consisting of a series of double nerve-ganglia, one in each
segment, with double connectives between them. The double
character of the ventral cord is much more distinctly marked in
the Chilopoda than in the Diplopoda, the ganglia are more distinct,
and the first three are intimately united together into an infra-
cesophageal mass. A sympathetic or visceral nervous system is
present, at least in the Diplopoda.

The sexes are always separate. There is usually an unpaired
gonad with paired ducts. In the Chilopoda the single genital
aperture is situated at the posterior end of the body : in the
Diplopoda and Pauropoda the two apertures are placed far forwards
towards the anterior end.

The ovum, as in most Arthropods, contains a large quantity of
food-yolk. The centrally-placed segmentation-nucleus divides so
as to give rise to a number of nuclei, this division being accom-
panied by a partial division of the yolk into a number of masses.
The nuclei then, for the most part, migrate to the surface, some
being left behind in the yolk. Those that reach the surface, sur-
rounded each by its little clump of protoplasm, become arranged
into a continuous superficial layer of cells — the blastoderm. On
the surface of this appears a thickening — the ventral plate — and
along the thickening is formed a groove which may perhaps repre-

602

ZOOLOGY

SECT.

sent the blastopore, though the endoderm is formed by direct
modification of the cells in the interior of the yolk. Stomodseum
and proctodseum are developed as invaginations of the surface
layer. The thickening of the blastoderm gives rise to a pair of
germinal bands in which rudiments of the segments soon become
recognisable. Larval membranes do not occur.

In some of the Diplopoda there is a metamorphosis, such as
will shortly be described in the embryo Insect, and the larva
(Fig. 497, B) has a singular superficial resemblance to an Insect,
owing to the presence at first of only three pairs of appendages on
the anterior trunk region.

Fossil remains of Myriapoda have been found in strata as far
back as the Devonian. The more ancient fossil forms are not

FIG. 497.— Two stages in the development of Strongylostoma, one of the Diplopoda.
A, early stage in the formation of the larva, which already exhibits distinct segments;
at. antennae. B, larva immediately after hatching. (From Balfour, after Metschnikoff.)

capable of being grouped in the same orders as the living repre-
sentatives of the class, and are looked upon as constituting at
least two orders, the members of which are all extinct. While the
Progoneata, and, more especially, the Symphyla, show marked
resemblances to the Insecta — more particularly to some of the
members of the order Aptera, the Opisthogoneata have features
connecting them through the Onychophora with the Annulata.

CLASS IV.-INSECTA.

The class of Insects (comprising the Cockroaches, Grasshoppers,
Dragon-flies, House-flies, Butterflies, Beetles and Bees, with their
many allies), though it is a very extensive one — including as it does
a larger number of species than any of the other classes of the
Arthropoda — is yet characterised by a remarkable degree of uni-
formity, no such extremes of modification occurring as are observable
within the class Crustacea.

XT PHYLUM ARTHROPODA 603

Characteristic of all the members of the class is the presence of
three clearly-defined regions — the head, thorax, and abdomen.
There are present on the head, antenna?, mandibles, and two pairs
of maxillae, the jaws being variously modified in the different
orders. All Insects have three pairs of thoracic legs, and most
have either one or two pairs of wings likewise borne on the thorax ;
the abdomen is not provided with paired appendages.

The organs of respiration are tracheae similar to those of the
Myriapoda.

The various systems of internal organs attain a very high grade
of structure in all the higher groups of Insects. In most the
development is complicated by the occurrence of a strongly-
marked metamorphosis. Insects are terrestrial or aerial, only a
few groups living on the surface of, or immersed in, fresh or salt
water ; but many are aquatic throughout their larval condition.

Many groups of Insects are remarkable for the high grade of
their intelligence as compared with the members of other classes
of the animal kingdom. This manifests itself mainly in a number
of instincts, often of a remarkable character, having to do with
the protection and rearing of the young ; and in some cases leading
to the formation of communities consisting of individuals of various
different kinds (workers, soldiers, sexual individuals) for mutual
support and protection.

1. EXAMPLE OF THE CLASS— THE COCKROACH (Periplaneta
orientalis or P. americana).

The Cockroach, familiarly known by the misleading title of
" Black Beetle," is a common pest of kitchens, bakeries, and store-
rooms. It is nocturnal in its habits, rarely coming out of its
lurking-places in the daytime, and is almost omnivorous in its
diet. It is a good example of the Insecta, not only on account of its
large size, which renders it convenient for dissection, but also
because of its generalised structure, which makes it a fairly central
member of the class, devoid of any extreme modifications.

Three regions are very distinctly recognisable in the body of
the Cockroach (Fig. 498). In front is the head, elongated vertir
cally, bearing the very long, slender feelers or antennce and the
large eyes, and contracted behind to form a narrow neck. In the
middle is the thorax, consisting of three segments, bearing the
three pairs of legs and the two pairs of wings. Behind is the
abdomen, consisting of ten segments covered over above by the
wings in the male. The entire surface is invested by a chitinous
cuticle, which is especially thickened on the head, on certain parts
of the thorax, and on the anterior pair of wings.

The head consists of four parts — the epicranium behind, com-
prising the region between and behind the eyes ; the dypeus,

604

ZOOLOGY

SECT.

or portion extending vertically downwards ; and two lateral parts,
the gence, in front. The eyes are a pair of reniform black patches
on the sides of the head ; each is seen when examined with a lens
to be divided into a number of minute hexagonal areas or facets,
like those in the eye of the Crayfish. Borne in sockets just below
the eyes are the long, slender, highly mobile feelers or antennce,
each made up of a large number of small segments, the first three
being larger than the others. Internal to the base of each antenna
is a rounded white space — thefenestra — the nature of which is not

IIG. 498.— Feriplaneta orientalis, male. A, dorsal view. £, ventral view x 2i. ab.l,

abj ab.9, ab.10, first, second, ninth, and tenth segments of abdomen ; ant. antennse ;~e. cerci;

t fi^PfV8 ; c?:,coxa of third IeS ; E- eye ; «*• elytra ; ep. epicranium ;/. fenestra ;fe. femur

hird leg; M. head; lg.l,lg.2,lg.3, legs; l.p. labial palp; Ir. labrum-; mn. mandible; m.p.

maxillary palp ; p.p. style on ninth abdominal segment, internally to which a podical plate

is seen ; th.l (th. in B), th.2, th.3, segments of thorax; ti. tibia ; tr. trochanter; ts. tarsus;

w. posterior wing.

known, but which may be an abortive representative of the simple
eyes or ocelli found in most Insects.

Movably articulated with the lower or ventral end of the clypeus
is a broad plate, the labrum or upper lip (Fig. 499, Ibr.) overhanging
the aperture of the mouth. Below the genae and articulating with
the sides both of the epicranium and of the clypeus are a pair of
stout mandibles (Fig. 499, md., and 500, man.) which work hori-
zontally like those of the Crayfish ; their inner edges are divided
into a number of teeth. Behind the mandibles are a more flexible

XI

PHYLUM ARTHROPODA

605

viz;

pair of jaws — the first pair of maxillce (mx.v max.1). Each maxilla

exhibits a structure comparable to the fundamental type of the

appendages of the Cray-
fish : — a basal part or

protopodite, consisting

of two segments (podo-

meres), supporting an

internal ramus or endo-

podite, and an external

ramus or exopodite. If m.<?

The former consists of \^^) mi

two parts : an inner, \iiSm4&5r

pointed, hard blade —

the lacinia (mi.), and

an outer, softer, more

elongated — the galea

(me.). The exopodite

forms a palp, the maxil-
lary palp (pm.), consist-
ing of five podomeres.

Behind these are the

second maxillae, which

are reducible to the

same type, but which

have their two basal

segments (those of the

protopodites) united to-
gether in the middle

line to form two median

sclerites, known respectively as mentum (m.) and submentum (sm.),

so that the two appendages form a sort of lower lip called the

labium. The endopodites taken to-
gether constitute what is termed
the ligula ; each is divided into two
parts like the endopodite of the
first maxillae. The exopodites form
-cerv three-jointed palps, the labial palps
(pi.). A soft pear-shaped median
process — the tongue, lingua or hypo-
pharynx, is attached to the upper
side of the mentum : just behind it
is a median slit supported by two
chitinous pieces — the entrance to

FIG. 500. — Periplaneta americana. |/U0 T^ar-Trm?-
Lateral view of the head and its ap- tlie pudiyux.

pendages. cerv. one of the cervical The neck, or narrow region be-

sclentes ; ey. eye ; gen. gena ; man. , ' n j-vxgivij.

mandible ; max.*, first pair of max- tween the neaa proper and the

illse ; max.2, second pair of maxilla; J.-L j £ jS

thorax, is covered for the most part

FIG. 499. — Mouth parts of the Cockroach. Ibr. labrum;
m. mentum ; md. mandible ; mxi, anterior pair of maxillao ;
me. and mi. outer and inner divisions of the first and
second pair of maxillae ; mx.2, second maxillse ; pi. labial
palp ; pm. maxillary palp ; st. stipes ; sm. submentum.
(From Lang's Comparative Anatomy.)

606 ZOOLOGY SECT.

by a thin flexible cuticle, but supporting it are eight thickened
and hardened patches — the cervical sclerites (cerv.).

Each of the three segments of the thorax — known respectively
as prothorax, mesothorax, and metathorax — is covered over dorsally
by a chitinous plate — the tergum, and ventrally by another — the
sternum. The tergum and sternum of each segment are distinct
from one another, not united into a continuous sclerite as in the
Crayfish. The tergum of the prothorax is larger than that of the
other two segments, and overlaps the neck above. Attached to
the anterior border of the tergum of the mesothorax in the male
are the anterior wings or elytra — a pair of thick opaque plates,
which, in their ordinary position, extend backwards over the
abdomen to some little distance beyond its extremity. Articulating
with the tergum of the metathorax are the posterior wings — a pair
of extremely delicate membranous expansions, which, when at rest,
are folded up longitudinally, like a fan, under the elytra. In the
female of P. orientalis the wings are only represented by small
vestiges. Attached to the sternum of each segment of the thorax
is a pair of legs. Each leg consists of a stout flattened proximal
podomere or coxa ; a small second, or trochanter ; a third, the/emw,
similar to the coxa but narrower ; a fourth slender and spinose, the
tibia ; and finally the tarsus or foot, composed of six very short
segments provided ventrally with patches of setae to give adhesive
power ; the last segment (pulvillus) is armed in addition with a
pair of claws (Fig. 498).

Of the segments of the abdomen the most posterior are over-
lapped by those just in front. Each is enclosed in a dorsal tergum
and a ventral sternum, both of which are thinnish and flexible —
the terga and sterna of successive segments overlapping one another
from before backwards. The eighth and ninth terga are hidden
from view by being overlapped by the seventh. The tenth is
produced backwards into a thin flexible plate, the posterior border
of which presents a deep notch ; below this is the opening of the
anus, at the sides of which are a pair of small hard plates — the
podical plates ; at the sides of the tergum are a pair of many-
jointed palp-like bodies — the cerci. The sternum of the first
abdominal segment is rudimentary. In the male that of the ninth
bears a pair of short styles. In the female the sternum of the
seventh is very much more prominent than in the male. The
genital aperture is situated on the ventral aspect of the posterior
extremity of the abdomen beneath the anal opening.

When compared with the Crayfish, as regards the external
anatomy, the Cockroach is found to differ (1) in the arrangement
of the segments into regions ; (2) in the form and position of the
appendages. The head and thorax together correspond to the
cephalothorax of the Crayfish, but comprise fewer segments ; the
abdomen contains a larger number of segments. The single pair

XI

PHYLUM ARTHROPODA

fi07

of antennae probably correspond to the antennules of the Crayfish
—the antennae of the latter not being represented. On this view
the homologies of the anterior appendages in the two animals may
be expressed in the following table : —

CRAYFISH. COCKROACH.

Antennules.
Antennae.
Mandibles.
First maxillae.
Second maxillae.
First maxillipedes.
Second maxillipedes.
Third maxillipedes.

Antennae.

Absent.

Mandibles.

First maxillae.

Second maxillae (labium)

First legs.

Second legs.

Third legs.

add.cox

abd.ccac
cxi.Jem

Kepresentatives of the five pairs of thoracic legs of the Crayfish
would thus appear to be absent in the Cockroach, and evanescent
rudiments, no traces of which remain in the adult, alone represent
in the latter the well-developed abdominal appendages of the
former.

In the living Cockroach respiratory movements are to be
observed, in which the abdomen becomes alternately expanded
and contracted ; these move-
ments bring about the alternate
inhalation and exhalation of air
through certain apertures — the
stigmata — at the sides of the
body. Two of these are situated
on each side of the thorax, one
between the prothorax and meso-
thorax, and the other between
the mesothorax and the meta-
thorax. Eight occur on either
side in the abdomen between the
terga and sterna of the seg-
ments. Just internal to each
spiracle the main air-tube or
trachea into which it leads pre-
sjjnts an elastic ring or spiral,
acting as a valve for closing the
passage.

The principal Sets of mUSCleS pia. 501.— Ventral portion of the muscular
f ^ trnnlr nf fhp fWlrrnarh arp system of the Cockroach, add.cox. ad-
ductor of coxa ; abd. cox. abductor of coxa ;

(1) the longitudinal Sternal mUSCleS ext fern, extensor of femur ; 1st terff stern.
}'. ~~-,7 \ i • -I p first tergo-sternal ; long, stern, longitudinal
(Fig. 501, long. Stem.), Which form sternal ; obi. stern, oblique sternal. (After

a transversely segmented sheet, Mia11 and Denny->

extending between adjoining sterna of the thorax and abdomen ;

(2) oblique sternal muscles (obi. stern.}, confined to the abdomen ;

long, stern

M. stern
terg.stern,

608 ZOOLOGY SECT.

and (3) longitudinal tergal muscles, best developed in the abdomen.
The various segments of the limbs are capable of being flexed or
extended on one another, as in the Crayfish, by the contractions of
special muscles. The wings are little used, the female Cockroach
being incapable of flight, and the male not a strong flier : accordingly
the wing muscles are not very strongly developed.

Between the body-wall and the alimentary canal is a cavity
taking the place of the coelome, but in reality forming a specially
developed part of the blood- vascular system (hmmocoele). This is
bounded externally by an irregular wall, formed of a mass of poly-
gonal cells constituting the fat-body.

Digestive system. — The mouth opens into a buccal cavity or
pharynx, which receives the ducts of the salivary glands (Fig. 502,

^£«

•mx.flp

FIG. 502. — Semi-diagrammatic view of the internal organs of female Cockroach, dissected
from the left side. The heart is not represented, abd.1, abd.5, first and fifth abdominal seg-
ments ; abd. gang.*, sixth abdominal ganglion ; an. anus ; ant. antennary nerve ; brn. brain ;
cer. cercus ; ccec. caeca ; coll. colleterial glands ; cr. crop ; gizz. gizzard ; gon. gonapophyses ;
inf. gang, sub-oesophageal ganglion ; int. intestine ; Ib. pip. labial palp ; I. ov. left ovary ;
malp. Malpighian tubes ; mx. pip. maxillary palp. ; od, points to the external opening of the
median oviduct (vagina) ; ces. oesophagus ; opt. optic nerve ; rec. rectum ; r.ov. right ovary ;
sal. glA. salivary glands ; sal. rec. salivary receptacle (left) ; sal. du. salivary ducts, indicating
the point at which the median duct of the salivary glands unites with the median duct of
the salivary receptacles ; spir. stigmata ; st. 7, sternum of the seventh segment ; te. 10,
tergum of the tenth segment ; th1, th2, th*, first, second, and third segments of the thorax ;
thor. gang1, first thoracic ganglion.

sal. gld.). Each gland is divided into two lobes, made up of
numerous ramifications. In close relation to each gland is an
elongated thin- walled sac — the salivary receptacle (sal. rec.). The
duct given off from the salivary receptacle joins that of the opposite
side and the median duct thus formed is joined by a single duct
(sal. du.), formed by the union of the two ducts of the salivary

xi PHYLUM ARTHROPODA 609

glands ; the common duct thus formed opens into the buccal
cavity (Fig. 503).

From the buccal cavity there proceeds backwards a narrow
oesophagus (ces.), which leads to an elongated dilatable sac — the
crop (cr.). On this there follows the proventriculus or gizzard (gizz.)
— a pear-shaped chamber with the broad end directed forwards,
its chitinous internal lining raised up into a number of teeth. A
narrow passage leads from this to the stomach or mesenteron — a
wide tube with glandular walls ; from its anterior end are given
off eight tubular hepatic cceca (hep. CCB.) — blind tubes somewhat
narrower than the stomach. The point of junction of the stomach
with the intestine is marked by the presence of very numerous
thread-like yellow appendages — the Malpighian tubes (malp.) —
which are the renal organs of the animal. The intestine (int.)
terminates in a dilated portion — the rectum (ret.) — the walls of
which are longitudinally folded. Of the entire alimentary
canal only a small part — the stomach — with the appended
hepatic caeca, is of the nature
of a mesenteron, the region in
front being a stomodaeum,
and that behind a procto-
daeum.

The heart, with the aorta
into which it is continued in
front, is the only part of the
blood-system with definite
walls. It is an elongated

tube, closed behind, Open in FIG. i303.— Right salivary gland-and salivary recep-
P . tacle of Cockroach. (After Miall and Denny.)

front, running along the

middle line of the abdomen and thorax immediately beneath the
terga, enclosed in a pericardial sinus which, as in the Crayfish, is
part of the haemoccele. Internally the tube is divided by valves
opening forwards into a number of chambers ; its walls are perforated
by a series of pairs of valvular apertures or ostia. Running from
the wall of the heart to the terga are a series of segmentally-arranged
fan-shaped bundles of muscles — the alary muscles (Fig. 533, m.).
T^eSe fibres are partly inserted into the wall of the heart, the
-iumen of which they dilate, partly into a membrane underlying the
pericardial sinus.

Respiration takes place through the instrumentality of a
system of air-tubes or trachece (Figs. 504 and 505), opening on the
surface at the stigmata, to which reference has already been made.
These tracheae form a richly ramifying system extending to all
parts of the body. They possess a chitinous internal lining,
supported by means of a spirally-wound, fibre-like thickening. By
means of this system of air-tubes air is conveyed throughout the
body to all parts, and there is thus ensured the rapid and

610

ZOOLOGY

SECT.

complete oxygenation which the functional activity of the Insect
requires.

The nervous system consists of a brain (Fig. 502, brn., and
506, br.), a sub-cesophageal pair of ganglia (infr. gang.), three thoracic
(Fig. 506, thor. 1, #, and 3) and six abdominal pairs of ganglia (the
members of each pair being united), a system of connectives uniting
the ganglia together, and a series of nerves given off to the various
parts of the body. The brain consists of a bilobed mass of nerve-
matter situated in the head, and divisible into two parts, anterior
and posterior. From the anterior part is given off on each side the

FIG. 504. — Portion of a trachea of a
Caterpillar. B, C, D, branches ; a,
cellular layer ; b, nuclei. (From Gegen-
baur.)

Fia. 505.— Cockroach. View of
the arrangement of the principal
trunks of the tracheal system.
(After Miall and Denny.)

optic nerve passing to the eye to become expanded into an optic
ganglion, and from the posterior part the nerves to the antennae.
It is supported by a chitinous framework — the tentorium. From
the brain there run backwards a pair of oesophageal connectives
(conn.), passing, one on each side of the oesophagus, downwards
and backwards to the sub-cesophageal ganglia. The latter, which
are situated between the submentum and oesophagus, give off a
pair of connectives, passing backwards to the first thoracic ganglia.
From the sub-cesophageal ganglia are given off the nerves to the
labrum, the mandibles, and both pairs of maxillae. The three
airs of thoracic and six of abdominal ganglia are connected together

XI

PHYLUM ARTHROPODA

611

lficr.2

Ihcr 3

into a chain by a series of double connectives ; the last pair of

abdominal ganglia, situated in the sixth segment of the abdomen

(abdO), are larger than the others, and

supply the segments behind. A visceral

nervous system, ramifying on the

anterior part of the alimentary canal,

is connected with the two cesophageal

connectives by two nerves, which join

above the oesophagus to form a median

frontal ganglion.

The organs of special sense are
the eyes, the antennae, and the palpi.
The eyes are compound — each being
made up of a large number of simple
elements similar to those that go to
make up the eye of Apus (see p. 523).
The antennae and palpi, together with
the anal cerci, act as organs of touch.
In addition, certain setae on the
antennae appear to have an olfactory
function.

Reproductive organs. — In the
male the testes (Fig. 507, test.) are a
pair of small bodies which lie in the
fourth and fifth segments of the
abdomen immediately below the terga.
From these a pair of delicate tubes, the
vasa deferentia, lead to the vesiculce
seminales, two tufts of whitish caeca,
which together constitute what is
known as the " mushroom-shaped gland " ; these open into the
anterior end of the ejaculatory duct (duct, ej.), an unpaired tube

506. — Cockroach. General
view of the nervous system. aM6,
sixth abdominal ganglion ; ant. an-
tennary nerve ; br. brain ; conn.
cesophageal connective ; inf. sub-
cesophageal ganglion ; opt . optic
nerve ; thor.l, thor.2, thor.3, first,
second, and third -thoracic ganglia.
(After Miall and Denny.)

FIG. 507. — Cockroach. Male
reproductive organs, lateral
view. duct. ej. ductus ejacula-
torius with mushroom-shaped
gland ; stern. 7, sternum of
seventh segment of abdomen ;
terg. 7, tergum of the same
segment ; test, testis. (After
Miall and Denny.)

ill.gld

FIG. 508. — Cockroach. Female
reproductive organs, coll. gld. col-
leterial glands ; od. oviducts ; ov.
ovaries. (After Miall and Denny.)

612

ZOOLOGY

SECT.

pen

seg

blast

with muscular walls opening on the exterior immediately below
the anus. Around the genital aperture are a series of chitinous
processes, the gonapopTiyses, which subserve copulation.

In the female there are two groups of ovarian tubes or ovarioles,
each group or ovary (Fig. 508, ov.) consisting of eight. The
ovarioles of each group are united together anteriorly, where they

are connected by a ligament to
the dorsal body-wall. Pos-
teriorly each group is connected
with a lateral oviduct (od.).
Each ovarian tube has a beaded
appearance, owing to its con-
taining a row of ova, which
increase in size posteriorly. The
two oviducts unite to open by
a median aperture on the
sternal surface of the eighth
segment of the abdomen. A
pair of unsymmetrical sacs
opening together in the middle
of the sternum of the ninth
segment constitute the sper-
motheca or receptaculum seminis.
A pair of ramifying glandular
tubules, the colleterial glands
(coll. aid.), open behind the
spermotheca. A series of chitin-
ous gonapophyses, which aid in
carrying the eggs, are situated
between the female genital
aperture and the anus.

Development. - - The eggs
are enclosed about sixteen
together in chitinous capsules,
the substance of which is
secreted by the colleterial
glands. They are laterally
compressed, concave on one
side (the future ventral side),
convex on the other (the future
dorsal side).

The mature egg in the lower part of the ovary is enclosed in a
follicle composed of a single layer of cells, within which is the thin
chitinoid egg-shell or chorion perforated by a number of micropylar
apertures. After the processes of maturation and fertilisation, the
segmentation-nucleus undergoes division, the result being the
formation of a number of irregular amoeboid cells, which are

Wast

yk.c

FIG. 509.— A — D, successive stages in the seg-
mentation of the ovum of an Insect ; blast.
blastoderm ; peri, peripheral protoplasm ; seg.
segmentation cells ; ylc. yolk ; yTc. c. yolk-cells.
(From Korschelt and Heider, after Bloch-
mann.)

xi PHYLUM ARTHROPODA 613

distributed through a considerable portion of the yolk. These
(Fig. 509) all migrate to the surface, where they multiply rapidly
and form a layer, the blastoderm, which becomes thickened along
the ventral surface by the cells being elongated in a vertical
direction. From the blastoderm a number of cells pass inwards
into the substance of the yolk, where their function is to convert
the yolk-material into various soluble substances for the nourish-
ment of the blastoderm. The ventral thickening of the latter is
the ventral plate : its cells proliferate, and the plate comes to be
several cells thick : in front it becomes broader — an indication of
the position of the future head-lobes. At the opposite end there is
a specially thickened area of the ventral plate with a slight depres-
sion on its surface ; the depression perhaps represents the blastopore,
since it is from this point forwards that the formation of the meso-
derm proceeds. The latter is formed as a longitudinal band which
bifurcates in front in the position of the head-lobes.

The mode of origin of the endoderm in the Cockroach is not
known with certainty. It appears beneath the mesoderm, in two
separate portions, as a thin layer of cells — one portion, the anterior,
coming into relation with the beginnings of the stomodaeum, which
arises as an invagination from the surface in the region of the head-
lobes, and the other, the posterior, uniting with the proctodaeum,
a similar ectodermal invagination at the posterior end of the ventral
plate. These two rudiments of the endoderm grow towards one
another, and eventually meet to form a continuous layer destined
to form the wall of the mesenteron. The ventral plate early becomes
divided by a number of narrow transverse lines which indicate the
boundaries of the future segments.

Kudiments of appendages (Figs. 510, 511) appear on the head
and thorax, and a series also appears on the abdomen ; all of the
latter, however, subsequently disappear with the exception of the
last j^air, which give rise to the cerci. The segment on which the
rudiments of the antennae appear is at first post-oral in position,
but subsequently becomes fused with a pre-oral segment (pros-
tomium), so that the antennae acquire their permanent pre-oral
position only secondarily. The prostomial segment, the antennary
segment, a segment devoid of appendages, the segment bearing
the rudimentary mandibles, and those bearing the two pairs of
maxillae — six segments in all — unite to form the head of the adult.

Then follows the appearance of the larval membranes. On
either side arises a fold of the blastoderm ; and the two folds
grow inwards and eventually unite over the body of the embryo,
forming a complete two-layered covering for it. The outer layer
is termed serosa, the inner amnion.1

1 This term is derived from one used in the Vertebrata, in which there is
an analogous membrane, occupying, however, a dorsal instead of a ventral
position as regards the body of the embryo.

VOL. I R R

614

ZOOLOGY

SECT.

Each of the two mesoderm bands undergoes transverse division
into a series of segments (somites), which become hollow and are
then closely applied to one another, eventually coalescing, so
that the cavities of all of them unite to form the coelome, the
outer walls becoming applied to the ectoderm to form a somatopleure,
or lamina consisting of somatic layer of mesoderm and of ectoderm ;
the inner being applied to the endoderm to form a splanchnopleure,
or lamina consisting of splanchnic layer of mesoderm and endoderm.
When the two rows of somites, right and left, become approximated

FIG. 510. — Ventral plate of embryo Cock-
roach (Blatta germanicaj, isolated
from the yolk. as. amnion and serosa ; at.
antennary lobe : col. brain ; cpl. caudal
plate ; Ib. labrum ; md. mandible ; ma;1,
mxz, first and second maxillae ; pi, rft vs,
legs. (After Wheeler.)

FIG. 511. — Embryo Cockroach just after
the rupture of the amnion and serosa,
lateral view of entire egg. Letters as in
preceding figure. In addition, at. fatty
body ; ast. caudal styles • b. cephalic end of
yolk ; oc. eye. (After Wheeler.)

dorsally, special cells — the cardioblasts — separated off from them
combine to form the wall of the tubular heart. Long before this
is formed, however, the sides of the embryo are observed to undergo
regular pulsations. The original coelome — the space between the
somatopleure and splanchnopleure — has become converted into,
or replaced by, a blood-sinus (haemocoele), divided up into a number
of smaller lacunae containing plasma by films of connective tissue.
The ventral plate gradually grows upwards at the sides, and

xi PHYLUM ARTHROPODA 615

eventually its borders meet and unite along the dorsal middle line,
the entire yolk thus becoming enclosed by it.

The ventral nerve-chain is developed from a groove of the
ectoderm, bounded by thickenings which become detached from
the surface-ectoderm and form the chain of ganglia. The brain
is developed from a pair of ectodermal thickenings. That part
which is developed in the prostomial region — the archicerebrum —
becomes united with that developed in the following two segments
to form the completed brain or syncerebrum.

It can hardly be said that the Cockroach undergoes a metamor-
phosis, the young Insect when it escapes from the egg differing
from the adult only in its smaller size and in the absence of wings,
which grow out subsequently from the terga of the meso- and
metathorax. Between its hatching and its complete development
the young Cockroach undergoes no fewer than seven " moults "
or ecdyses, in which all the chitinous parts become thrown off and
renewed.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Insecta are air-breathing Arthropoda in which the body
consists of three well-marked regions — head, thorax, and abdomen ;
the head devoid of external segmentation, nearly always bearing
compound eyes, with a pair of antennae situated on the prostomium,
mandibles, and two pairs of maxillae ; the thorax of three segments
each bearing a pair of legs, and the second and third usually bearing
wings ; the abdomen composed of a varying number of segments
(7 — 11), which are devoid of appendages in the adult condition.
A liver is absent, but salivary glands are always present. There
is an elongated tubular heart, divided into eight chambers, situated
in the abdomen ; the vessels themselves are not highly developed.
The Insecta are, almost without exception, air-breathers, and the
organs of respiration take the form of branching tubes, the tracheae,
by means of which air is conveyed to all parts of the body. The
nervous system and sense-organs reach a high level of com-
plexity. The excretory organs are a number of blind tubes, the
Malpighian tubes, appended to the intestine. The sexes are
separate ; development is sometimes direct, more usually compli-
cated by metamorphosis.

Sub-Class I.— APTERYGOTA.

Insecta which are completely devoid of wings and pass through
no metamorphosis.

ORDER 1. — THYSANURA.

Wingless Insecta with an abdomen of ten segments, more or
fewer of which bear small appendages, and with two or three slender,

R R 2

(516 ZOOLOGY SECT.

many-jointed cerci on the anal segment. Compound eyes and ocelli
are sometimes present, sometimes absent. This order includes
the " Silver-fish " (Lepisma, Fig. 512), and other genera — Japyx,
Machilis, Campodea.

ORDER 2. — COLLEMBOLA.

* Wingless Insecta with an abdomen of six or fewer segments,
without paired appendages, but, in most cases, with a peculiar

springing apparatus on the
last or penultimate segment.
Compound eyes never present.
This order includes the Spring-
tails (Podura, Fig. 513) and
others.

FIG. 512.— Lepisma. (After Guerin and FIG. 513.— Podura. (After

Percheron.) Guerin and Percheron.)

Sub-Class II.— PTERYGOTA.

Insects the great majority of which have wings in one or both
sexes in the adult condition. A complete or incomplete meta-
morphosis.

ORDER 3. — ORTHOPTEROIDEA.

Insects with incomplete metamorphosis, with biting jaws, and
with cerci of varying character at the extremity of the abdomen.

Sub-order 1. — Orthoptera.

Orthopteroidea in which there are two pairs of wings of which
in most cases the anterior pair are harder and more opaque, the
posterior pair more delicate and capable of being folded up like
a fan beneath the anterior pair. The metamorphosis is incomplete.

In this sub-order are included a large number of families capable
of being arranged in three main groups, viz. — (1) the Saltatoria
or Leapers (Grasshoppers, Locusts, Fig. 514, and Crickets) ; (2) the

XI

PHYLUM ARTHROPODA

617

Gressoria or Walkers (Stick-and-Leaf Insects, Praying Insects);
and (3) the Gursoria or Runners (Earwigs, Cockroaches).

Fia. 5U. — Lpcusta. (From Cuvier'd
Animal Kingdom.)

FIG. 515. — AuEmbiid (Oligotoma michaeli) inag-
nifled. (From the Cambridge Natural History,
after McLachlan.)

Sub-order 2. — Isoptera.

Orthopteroidea which form organised communities composed of
soldiers, workers, and male and female sexual individuals. Mem-
branous anterior and posterior wings are developed in the sexual
individuals, but are soon thrown off.

This sub-order comprises only the Termites or " White Ants."

Sub-order 3. — Embiidce.

Minute Orthopteroidea with rather long and narrow bodies
(Fig. 515) ; winged or wingless — the wings when present mem-
branous, not caducous but persistent. Communities are not
formed.

Sub-order 4. — Psocidce.

Minute Orthopteroidea with comparatively short thick bodies,
slender antennae, and usually with
delicate membranous wings, the an-
terior pair the larger.

This sub-order (Fig. 516) includes
Book-lice and Death-watches.

FIG. 516.— A winged Psocid (Psocus
fasciatus), magnified. (From the Cam-
bridge Natural History, after McLachlan.)

FIG. 517.— One of the Mallophaga in-
habiting the common fowl. (From the
Cambridge Natural History, after Piaget.)

618

ZOOLOGY

SECT.

Sub-order 5. — Mallophaga.

Small, flat, wingless Orthopteroidea with very slight metamor-
phosis, with biting mouth-parts.

This sub -order comprises only the Biting Lice or Bird Lice
(Fig. 517), which are external parasites of birds and mammals.

ORDER 4.— NEUROPTERA.

Pterygota with two pairs of mem-
branous wings. The larvae live in
water.

This sub -order includes the Per-
lidae, the Dragon-flies (Odonata), the
May-flies (Fig. 518), the Ant-lions
and the Caddis-flies.

ORDER 5. — THYSANOPTERA.

FIG. 518. — Ephemera (May-fly) and
larva. (After Gu&rin and Percheron.)

slight metamor-
phosis, with sucking mouth-parts,
and usually with four narrow
fringed wings.

This order comprises the insects,
usually of very small size, known
as Thrips.

ORDER 6. — HEMIPTERA (RHYNCHOTA).

Insects in which wings are usually present, sometimes similar,
sometimes dissimilar, and in which there is a jointed suctorial

v-

FlG. 519.— Aphis rosae and larva. (From Cuvier's Animal Kingdom.

rostrum formed from the labium, enclosing the jaws in the form of
piercing organs. The prothorax is free from the other segments
of the thorax. The metamorphosis is incomplete.

XI

PHYLUM ARTHROPODA

619

This order includes Bugs, Water-bugs, Scale-insects, Plant-lice
(Fig. 519), and Cicadas (Fig. 520). Lice (Anoplura), wingless
parasites of mammals, are usually looked upon as degenerated
members of the Hemiptera.

Fig. 520. — Cicada. (After Guerin and Percheron.)

ORDER 7. — DIPTERA.

Insects provided (except in the Fleas) with a single pair of
transparent membranous wings,
representing the anterior pair of
other orders. The mouth-parts are

\

FIG. 521. — Culex (mosquito) and larva.
(After Guerin and Percheron.)

FIG. 522.— Bot-fly of the horse (Gastro-
philus equi). a, mature insect ; b,
egg attached to a hair ; c, d, and e, stages
in the larval development. (After Brehm.)

adapted for piercing and sucking. The prothorax is fused with the

other segments of the thorax. There is a complete metamorphosis.

This order includes Gnats and Mosquitoes (Fig. 521), House-flies

620

ZOOLOGY

SECT.

and Blow-flies, Bot-flies (Fig. 522), Crane-flies, and " Daddy-long-
legs," to which may be added the greatly modified, wingless, para-
sitic group, the Fleas.

FIG. 523.— Butterfly (Pieris), with caterpillar and chrysalis stages. (After Guerin and

Percheron.)

OEDEE 8. — LEPIDOPTEEA.

Insects with both pairs of wings well developed and covered
with scales (modified hairs). The maxillae are modified to form

an elongated sucking tube, which can be
rolled up spirally ; the other parts of the
mouth are rudimentary, with the excep-
tion of the labial palpi. The pro thorax
is fused with the mesothorax. The
metamorphosis is complete.

This order includes Butterflies (Fig. 523)
and Moths.

OEDEE 9. — COLEOPTEEA.

Insects in which the anterior pair of
wings take the form of hard horny wing-
cases, or elytra, which, when at rest, are
folded up along the back and cover over
the folded-up membranous posterior
wings. The prothorax is movable on the
other segments. The jaws are fully
developed, and adapted for biting and chewing. The metamor-
phosis is complete.
This order includes the true Beetles (Fig. 524).

Fio. 524.— Beetle (Crioceris,

with larva. (After Guerin
and Percheron.)

XI

PHYLUM ARTHROPODA

621

ORDER 10. — MECOPTERA.

Insects with four elongated membranous wings (sometimes
rudimentary or absent). The mouth-parts are adapted for biting
and are borne at the end of a deflexed rostrum. The larvae are
caterpillar-like with numerous
pro-legs and are carnivorous.

This order includes the
Scorpion-flies (Fig. 525).
-

ORDER 11. — HYMENOPTERA.

Insects in which both pairs of
wings are present and mem-
branous. The mouth-parts are
adapted both for biting and
licking. The prothorax is united
with the other segments of the
thorax. There is a complete
metamorphosis.

Included in this order are
Bees (Fig. 541) and Wasps,
Ants (Fig. 542), Gall-flies, and FlG 525_A Scorpion Fly (Panorpa com.

Ichneumons. munis), male. (After Sharp.)

Systematic Position of the Example.

The Cockroach is a member of the order Orthoptera, of which
there are three divisions, the Cursoria, to which the Cockroaches
belong ; the Gressoria, comprising the MantidoB and Phasmidce, or
Stick-and-Leaf insects and their allies ; and the Saltatoria, in-
cluding the Grasshoppers, Locusts, and Crickets. The division
Cursoria comprises, in addition to the Earwigs, the single family of
the Cockroaches (Blattidce), characterised by the deflexed head, the
flat oval body, the large prothoracic tergum, the long antennae,
the three pairs of legs similar, with large coxae entirely covering
the sternal surface of the thorax, the five- jointed tarsi, and the
presence of anal cerci. Periplaneta belongs to a section of the
family distinguished from the rest by the femora being spiny
underneath, and by the valvular character of the last sternum in
the female.

3. GENERAL ORGANISATION.

The exoskeleton of the Insecta (Fig. 526) consists of a chitinous
cuticle (cut.), which varies in hardness and thickness in different
Insects and in different parts of the body of the same Insect, but is
very rarely calcified. Frequently it presents hexagonal markings ;
sometimes it is perforated by numerous pores ; sometimes it is

622

ZOOLOGY

SECT.

FIG. 526. — Section through
the integument of an
Insect, base, basement
membrane ; cut. layers
of the cuticle; epi.
epidermis ; set. seta.
(After Miall and Denny.)

covered with thin scales ; in many cases it is developed into tactile
hairs or setce, which may be scattered over the body, or may be
located only on certain of the appendages —
the antennae, the maxillary and labial palpi,
and the tarsi of the legs. In some, glands
are present in the integument — odoriferous,
honey-secreting, or wax-forming glands ;
poison glands are present in connection with
an abdominal sting in certain Insects ;
spinning glands, forming a silky material, are
confined to the larvae.

The head presents no trace of segmenta
tion, but the history of its development in-
dicates that it may be looked upon as
composed of a prostomium and about five
segments, intimately united together. It varies a good deal
in shape, but always presents the regions that have already been
described in the case of the Cockroach. Of these the epicranium
is the most extensive ;
the clypeus, situated in
front of it, supports the
labrum ; the gence are
situated laterally, and a
median piece, the gula,
occupies the middle of the
ventral surface. Some-
times the head is sunk
within the anterior part ^tfL *
of the thorax ; some-
times it is free from the
latter ; and there may
be, as in the Cockroach, a
short narrow region or
neck, covered with soft
skin, supported only by
isolated cervical sclerites,
on the ventral aspect.

The three segments of
the thorax — pro-, meso-,
and metathorax^&iQ usu-
ally firmly united to-
gether ; but in some
Insects the prothorax is

mnvfl'hlp iinon fhp nfliAr FlG- 527.— A, mouth-parts of the Honey-bee (Apis

mellifica) ; £, the two pairs of maxillae, au. eye ;

Segments I it IS USUallv a- antenna ; c. cardo ; ep. epipharynx ; Ibr. labrum ;

TT , f ., ., J li. ligula ; m. mentum ; mm, mxit first pair of maxillae ;

tne Smallest OI the three md. mandible ; pi. labial palpi ; pm. palp of the first

cofrrYionfc TTI ^o^ 4-V.^ Pair of maxillae ; prg. paraglossa ; sm. submentum;

segments. In eacn tne ^m. stipes of the flrst maxiiia3, (From Lang.)

XI

PHYLUM ARTHROPODA

623

exoskeleton consists of dorsal or tergal and ventral or sternal
elements, sometimes separate from one another laterally, sometimes
united together in such a way as to form complete rings round the
segments. Laterally projecting processes or pleura are sometimes
developed.

The abdomen contains from seven to eleven segments, enclosed
in tergal and sternal shields. In some Insects the first abdominal
segment is united with the thorax so as to appear to belong to the
latter region.

The appendages of the head are four pairs, as in the Cock-
roach ; but a considerable variation is observable in the different

\JLX

ft*

liu. 528. — Mouth- parts of the Diptera. A, of Tabanus ; B, of Culex. Lettering as in
preceding figure : hp. hypopharynx ; oc. ocellus. (From Lang.)

orders, especially as regards the jaws. In certain of the Aptera
an additional pair — the so-called maxillulce — occur between the
mandibles and first maxillae. In a few eyes are absent. Most
have large compound or faceted eyes, and many have simple eyes
or ocelli as well ; in a few groups the latter are alone present.
The antennae vary in shape in different groups and sometimes
even in the sexes of the same species. They may be tapering,
moniliform, club-shaped, pectinate, or plume-like. In addition to
functioning as tactile appendages they bear the olfactory setae, and
there seems reason to believe that they act also as organs con-
cerned in the maintenance of the equilibrium of the body. The

624

ZOOLOGY

SECT.

mandibles are always one-jointed, and differ from those of the
Crustacea in never being provided with a palp. An arrangement
of the mouth-parts adapted for biting or chewing has already been
described in the case of the Cockroach : this type is characteristic of
the order Orthoptera, to which the Cockroach belongs, and a very
similar type characterises the Coleoptera. In the Hymenoptera
(Fig. 527) the mouth-parts are adapted both for biting and for
licking; the mandibles (md.) and maxillae (mx^) are sharp and
lancet-like ; the middle part of the labium is produced into a long
median tongue (ligula, li.) at the sides of which are a pair of

accessory tongues or para-
glossoB (prg.). In the Hemi-
ptera there is a proboscis
formed from the labium
and enclosing the stylet-like
mandibles and maxillae . In
the Diptera (Fig. 528) the
mandibles (md.), usually
not developed in the males,
are biting or piercing
organs, while the basal parts
of the labium form a pro-
boscis (inx.2) enclosing a
spine or seta (hp.) — which
is a process from the hypo-
pharynx — and sometimes
stylet-like maxillae (mx.^.
In the Lepidoptera
(Fig. 529) the mandibles are
aborted in the adult, and the
maxillae are developed into,
elongated half-tubes, which
when applied together form

FIG. 529.— Mouth-parts of the Lepidoptera. B, the Q r>nrn™WP fn"h» (0* '\ pa-ne»Klo

second maxilla). Lettering as in preceding figures : a Complete tUDe (ST.) CapaDle

pi. labial palp ; pm. palp of the anterior maxillae ; of being coiled UP in a Spiral
sr. sucking tube. (Prom Lang.) -. .fi -i -,

manner under the head, the

extremity provided with hooks or spines for rupturing the nectaries
of flowers.

Appendages of the thorax. — Each of the segments of the
thorax bears a pair of five-jointed legs ; the terminal section or
tarsus being made up of a number of short segments, usually five,
and ending in a pair of claws, often with an adhesive pad or sucking
disc between them, or in a single claw. In accordance with varia-
tions in the uses to which they are put, considerable differences are
observable in the form of the legs in different groups of Insects.
In most they are adapted for walking, and are long and slender ;
in some they are expanded to enable them to act as swimming

xi PHYLUM ARTHROPOD A 625

paddles ; in some the first pair are prehensile, and develop a sub-
chelate extremity ; in others, again, the legs, or the first pair of
them, are stout and adapted for burrowing. In addition to the
legs the meso- and metathorax may each bear a pair of wings.
The wings are thin transparent expansions of the integument of the
body, supported by a system of branching ribs or nervures consisting
of chitinous material with branches of the tracheae, nerves, and
tubular diverticula of the body-cavity. In most Lepidoptera the
wings are opaque, owing to their being covered with numerous
overlapping microscopic scales, to which the various colours of the
wing are due. In some insects — e.g., Beetles and Orthoptera — the
posterior wings alone are delicate and membranous, the anterior
pair being converted into hard or tough cases — the elytra — which
when folded up cover over and protect the delicate posterior wings.
In some Beetles the elytra are permanently united together along
the back of the Insect. In some Insects (Bugs) the anterior wings
are chitinous at the bases only. In the Diptera the anterior
wings alone are developed, the posterior being represented by
vestiges — the halteres or balancers. In the Strepsiptera, or Bee-
parasites, an aberrant group of Neuroptera, on the other hand,
it is the anterior pair that are vestigial. In some Insects
(Spring-tails, Lice, Fleas) wings are entirely absent in all stages.
In others again they are present in one sex — usually the male —
and absent or vestigial in the other. In the Aptera there is no
vestige whatever of wings at any stage, and this, taken in connection
with the simplicity of the structure in other respects, seems to
indicate that in these Insects we have to do with the descendants
of a primitive group in which wings had not yet become developed.

The segments of the abdomen are mostly devoid of paired
appendages in the adult condition (except in the Thysanura),
though vestiges of them may be present in the young at an early
stage. Each segment is enclosed in dorsal tergal and ventral
sternal plates, which usually remain separate laterally, but may be
united. At the extremity of the abdomen there are frequently
appendages which are perhaps of the nature of limbs, having the
function of stings, ovipositors, and genital processes.

Haemocoele. — The cavity intervening in an Insect between the
body-wall and the various internal organs does not correspond,
as already explained (p. 578), to the ccelome of other groups, but is
found, when we study its mode of development, to be a hcemocoele
— an extended part of the blood-vascular system. The ccelome is
apparently represented only by the lumen of the reproductive organs.

A fat-body is always present, either in the larval condition or
throughout life. It consists of a mass of polygonal cells bounding
the hsemocoele externally. When young the cells are nucleated
and possess a protoplasmic body. At a later stage a fluid loaded
with minute granules takes the place of the protoplasm, and

626

ZOOLOGY

SECT.

crystals containing uric acid are formed. These crystals afterwards
become absorbed ; their appearance and subsequent absorption
would seem to point to the probability that the fat-body is con-
cerned in separating out nitrogenous waste matters, which subse-
quently reach the exterior through the Malpighian tubes. Its
chief function is to serve as a reserve-store of nutrient material.

Digestive system. — Some Insects do not feed in the adult
condition, and when this is the case the mouth may be absent,

FIG. 530. — Digestive apparatus of
a Beetle (Carabus auratus).

ad, anal glands : ab, their muscu-
lar appendages ; cd, stomach ;
ed, hind gut ; in, crop ; k, head
with mouth-parts ; ce. oasopha-
gus ; py. proventriculus ; vm.
Malpighian tubes. (From Lang,
after Dufour.)

FIG. 531. — Nervous, tracheal, and digestive systems of
the Honey-bee, a. antenna ; era, compound eye : b\,
b<2, bz, the three pairs of legs ; cm, stomach ; ed, hind-
gut ; hm, honey stomach (crop) ; rd, rectal glands ; st,
stigmata ; tb, vesicle of tracheal system ; vm, Mal-
pighian vessels. (From Lang's Comparative Anatomy.)

as, for example, is the case in the Day-flies (Ephemeridce). When
a mouth is developed, as it is in the vast majority of Insects, it
is situated on the lower aspect of the head, bounded in front by
the labrum, and behind by the labium. It leads into the buccal
cavity or pharynx, into which open the ducts of a pair of salivary
glands, each of which often has associated with it a thin- walled
sac or salivary receptacle. Also in the neighbourhood of the mouth
in such larval Insects as spin a cocoon, the ducts of a pair of

11 PHYLUM ARTHROPOD A 627

spinning glands open. A projection of the roof of the mouth-
cavity (epipharynx) is present in some Insects ; in others it is
replaced by a projection from the floor, the hypopharynx or lingua.

The alimentary canal is nearly always considerably longer than
the body ; it is longer in vegetable-feeding than in carnivorous
forms. The pharynx leads into a long, narrow passage — the
cesophagus (Figs. 530 and 531 w.) — which dilates behind into a
crop (in) for the storage of food. The place of this in sucking Insects
is taken by a stalked sac, usually termed the sucking stomach. The
essential processes of digestion are carried on in an elongated
chamber with glandular walls — the stomach (cd) — which may be
divided into several parts. Sometimes between the crop and
stomach is intercalated a muscular-walled chamber, frequently
containing chitinous teeth, the proventriculus or gizzard (pv).
Appended to the stomach at its anterior end are, in many Insects,
a varying number of tubular blind pouches, the hepatic cceca. At
its junction with the small intestine there open a number (from 2
to over 100) of narrow tubular appendages, the Malpighian tubes
(vm), which are the organs of renal excretion. In the cases in which
the development of the alimentary canal has been traced, it has
been found that the Malpighian tubes mark the point where the
mesenteron passes into the proctodaeum, and it is assumed that
this holds good generally. The lumen of the tubes is sometimes
filled up with cells. In some insects, the Malpighian tubes open
into a paired or unpaired sac — the urinary bladder. The intestine
is usually elongated, and its posterior portion (ed.) is dilated to
form a wide rectum (r.), which opens on the exterior by an anal
aperture situated on the ventral side of the last segment of the
abdomen. Anal glands (ad.), producing an odoriferous secretion,
often open into the rectum.

The tracheal system (Fig. 531) communicates with the ex-
terior through a number of apertures — the stigmata (st) — which
vary in the details of their arrangement in the different orders.
They are always protected against the entry of foreign particles by
some means — either by being surrounded by special bundles of hairs,
or by being provided with a special sieve-like membrane. In
most cases they are capable of being closed by muscular action.
In some Insects, mainly those adapted for active flight, such as the
Hymenoptera, the tracheal system is dilated in certain parts of the
body to form comparatively large air-sacs or air-reservoirs (tb). In
the aquatic larvae of some Insects there is a series of soft external,
simple or divided, processes — the tracheal gills (Fig. 532) — attached
to the abdominal segments and richly supplied with trachea, which
have no communication with the exterior ; in others rectal gills
are developed — soft lamellaa on the inner surface of the rectum.

The blood-vascular system is, in comparison with the other
systems of organs, not very highly developed, the need of an

628

ZOOLOGY

elaborate system of vessels being greatly diminished by the way in
which all the tissues and organs are supplied with oxygen through
the system of tracheae. The blood is colourless or faintly yellowish
or greenish, and contains colourless corpuscles. A contractile
dorsal vessel or heart (Fig. 533) extends through the abdomen — and
sometimes the thorax — immediately below the terga. Its cavity is
divided internally into a series of chambers by a system of valves.
In its walls are a series of slits or ostia, by which a communication
is effected between the internal cavity and a surrounding pericardial
sinus. Alary muscles, fan-shaped bundles of fibres, arise from the
terga and are in part inserted into the heart, causing or assisting
in causing its dilatation and the opening of the ostia. In front the
heart gives origin to a main vessel, or aorta (a).

FIG. 532. — Thorax and anterior abdominal segments
of a larval Ephemerid with tracheal gills.
HF, hind-wings ; tki, tkz, tk3, tracheal gills ; tl,
longitudinal tracheal trunks ; VF, fore-wings.
(From Lang's Comparative Anatomy.)

FIG. 533. — Heart of Cock-
chafer (Melolontha). a,
aorta ; m, m, alary muscles.
(From Gegenbaur.)

The nervous system (Figs. 531 and 534) is on the same general
plan as in the Crustacea. There is a double supra-cesophageal
ganglion or brain, a sub-cesophageal ganglion, also double, and a series
of thoracic and abdominal pairs of ganglia, which are closely united
together in the middle line. The brain is relatively large in the
higher Insects, and is divided into several lobes. It gives off nerves
to the antennae, the ocelli and the labrum, and on each side arises
a large lobe — the optic ganglion — on which the compound eye
rests. A pair of cesophageal connectives pass backwards on either
side of the mouth from the brain to the sub-cesophageal ganglion.
These connectives are usually very short, and, as a consequence, the
brain and sub-oesophageal ganglia are closely approximated. From
the latter there originate nerves to the appendages of the mouth — the
mandibles and the two pairs of maxillae. There are sometimes three

XI

PHYLUM ARTHROPODA

620

pairs of thoracic and as many as eight of abdominal ganglia in
the adult insect ; but in many cases there is a greater or less degree
of concentration of the ventral ganglionic chain (Fig. 534), and in
some of the Diptera this reaches such an extreme that all the ventral
ganglia, with the exception of the sub-oesophageal, are united
into one elongated mass. The Insects, like the higher Crustacea,
possess a visceral or sympathetic nervous system, connected with the
oesophageal connectives, and passing backwards on the oesophagus
and crop.

The most highly developed organs of special sense are the
large compound eyes. The surface of the compound eye is marked

FIG. 534. — Nervous systems of four species of Diptera to illustrate various degrees of concen-
tration. A, non-concentrated nervous systems of Chironomus plumosus with three
thoracic and six abdominal ganglia ; B, nervous system of Empis stercorea with two
thoracic and five abdominal ganglia ; C, nervous system of Tabanus bovinus, with one
thoracic ganglion and with the abdominal ganglia closely approximated ; D, nervous
system of Sarcophaga carnaria, with all the ganglia of the ventral chain united
together with the exception of the sub-cesophageal. (From Lang's Comparative Anatomy.)

out, as in the case of the Crayfish, into a great number of minute
hexagonal facets, each of which indicates one of the elements
(ommatidia) of the eye. Of these there may be as many as 28,000
(Dragon-fly). When the eye is examined in section, each omma-
tidium is found to consist of a cornea-lens — the outer surface of
which forms the facet — a crystalline cone, and a rhabdome. The
crystalline cone is not always developed, its place being taken
in the eyes of some Insects by four crystal cells. The rhabdome is
an elongated rod. Beneath the rhabdomes is afenestrated membrane,
beneath which, again, is a dense plexus of nerve-fibres. Nerve-
fibres pass through the fenestrated membrane and terminate in a
delicate sheath which encloses each rhabdome, the sheath, together

630 ZOOLOGY SECT.

with the nerves that end in it, constituting the retinula. Pigment
surrounds the crystalline cones and retinulae.

The ocelli, or simple eyes (Fig. 535), consist of a biconvex
transparent thickening of the cuticle — the lens — and beneath it
of a group of specially modified epidermal cells. Some of these,
situated beneath the lens, form a transparent mass, the vitreous
body, another set of elongated cells being arranged to form the
retina.

The antennae and palpi are the organs of touch, and these appen-
dages seem to be also the seat of the olfactory sense. A number of
minute processes sometimes sunk in pits, and each having a special
nerve-plate connected with it, are regarded as being specially con-
cerned with this sense ; and similar processes or pits on the maxillae
and the epipharynx are perhaps connected with the sense of taste.
The results of experiments on the action of the antennae seem to

lead to the conclusion that
one of their main functions
is to act as organs for
regulating the equilibrium
of the body.

Peculiar nerve-endings,
supposed to be auditory,
occur in various parts of
the body. These are dis-
tinguished as chordotonal
or tympanic, according to

FIG. 535.— Section through the ocellus of a young the nature of that part of
Dytiscus larva, ct. cuticle : gk, cells of the , i i • i

vitreous body ; hy, epidermis ; I, cuticular lens ; no, the apparatus W Jl 1 C n

2FterGSnea^her:)retiIialcells;sf'rods' (From Lang' functions by vibrating in

response to the sound-
vibrations. In the chordotonal organs this is a tense thin chord ;
in the tympanic a tense membrane or tympanum. The name of
Johnston's organs is given to auditory organs of a tympanic type
which occur in the second segment of the antennae in the majority
of insects.

In certain Insects — the Fireflies and Glow-worms, belonging to the
order Coleoptera — occur luminous organs for the production of
light.

Sounds are emitted by many Insects, and are produced by a
variety of different means. Often the sound is the result of the
rubbing together of opposed rough surfaces of the integument.
The chirp of the Grasshopper, for example, is produced by the
rubbing of the femur of the last pair of legs over a series of ridges
on the anterior wing, and that of the Locust by the rubbing against
one another of the roughened basal parts of the first pair of wings.
In other cases the sound results from the rapid vibratory move-
ment of the wings ; this is the case with the buzzing of many

XI

PHYLUM ARTHROPODA

631

Diptera and Hymenoptera. Again, the humming sounds charac-
teristic of many of the last-named order are produced partly by
the vibrations of the wings in flight, partly by the vibration of
leaf-like appendages in the tracheae set in motion by strong ex-
piratory currents of air. The loud shrill note of the Cicada is
produced by the rapidly recurring contractions of the fibres of a
muscle inserted into a stiff chitinous membrane, the result being
a series of crackling sounds, which follow one another so rapidly
as to give rise to a continuous note.

Reproductive organs. — The sexes are always separate in
Insects, as in Arthropoda in general ; and the males and females
are very commonly distinguishable from one another by various
modifications of
form and of color-
ation. There are
two ovaries, each
of which consists
of a greater or

smaller number of \|p^^ ^f''y
narrow tubes or T/% */A/Vd

ovarioles ; in each oci % 'j/t

of these the ova ^w/ <*<

are arranged in a
single row — the
early stages i n
their formation
being situated at
the anterior end,
the more mature

OVa towards the FIG. 536.—^4, female and B, male sexual apparatus of the Honey-
•n n e f a " ^ee ' ' accessory glands ; de, common ejaculatory duct ;

ffd, poison-glands ; gb, poison-vesicle ; ks, bulb of the stinging
F a P h apparatus ; md, rectum, twisted back and cut off ; nva, acces-

sory sac of the vagina (bursa copulatrix) ; od, oviduct ; ov,

OI OVanan ovary ; p, penis ; rs, receptaculum seminis ; sd, colleterial

gland ; t, testes ; va, vagina ; vd, sperm-ducts. (From Lang's
tubes Opens into a Comparative Anatomy.)

lateral oviduct,

and the two -lateral oviducts, right and left (Fig. 536, A, od.), in most
cases unite behind to form a median oviduct or vagina (va.), which
opens towards the posterior end of the abdomen. Connected with
this median oviduct, or opening close to it, are receptacula seminis
(rs.) and colleterial or cement-glands (sd.). Sometimes there is a
copulatory sac, or bursa copulatrix (nva.). In the male the paired
testes (B, t.) vary greatly in form : sometimes each is a long, narrow
tube ; sometimes several such tubes combine to form the testis ; or
it may be of more compact rounded form and entire or lobed.
Each testis has a slender duct or vas defer ens (B, vd), the two
vasa deferentia uniting to form a median ejaculatory duct. A
vesicula seminalis is .appended to each vas deferens or to the

« a 9

632 ZOOLOGY SECT.

ejaculatory duct. Accessory glands, opening into the vas deferens
or the ejaculatory duct, secrete cementing material for uniting the
sperms into masses, the spermatophores. In most instances the
eggs are laid shortly after their fertilisation, only a comparatively
few forms, such as the Aphides or Plant-lice, many Diptera, and
some Coleoptera, being viviparous. Some Insects, such as the
Aphides and the Bees and Wasps, as well as some Lepidoptera
and Neuroptera, present us with the unusual phenomenon of
parthenogenesis ; i.e., ova are formed, as in ordinary female insects,
in organs corresponding to the ovaries of the latter, and are developed
without fertilisation. In the case of the Aphides, an autumn
generation of completely-developed males and females is followed
by a spring generation consisting entirely of females ; these are
both parthenogenetic and viviparous. In the Bees, the workers
(imperfectly developed females) occasionally produce ova which,
without fertilisation, develop into drones (males). In a few species
of one or two groups, including the Scale-Insects (Coccidce) and
Gall-Insects (Cynipidcz), males are never developed, so that repro-
duction is exclusively parthenogenetic. Pcedogenesis accompanies
parthenogenesis in certain Diptera ; i.e., the larvce or pupce produce
ova and embryos without impregnation.

The eggs when laid are protected from injury by a number of
methods ; they may be firmly fixed to the substratum, buried in
the earth, or laid in the interior of certain plants or even of animals.
The deposition of the eggs, by means of ovipositors, in the leaves
or other parts of plants gives rise to swellings — the so-called galls,
in the interior of which the young Insects live. In the case of
many Insects the eggs are enclosed in a cocoon ; in others they are
surrounded by gelatinous or waxy material. The eggs are, for the
most part, of relatively considerable size. In form they vary, but
the long oval prevails in most instances. The ripe egg is enclosed
in two egg-membranes — an inner, the vitelline membrane, produced
by the egg itself, and an outer, the chorion, formed from the follicle,
cells. The chorion, which usually exhibits a more or less elaborate
pattern, has one or more apertures or micropyles for the entry of
the sperm. The contents are distinguishable into two layers —
a superficial, consisting of protoplasm, and a central, of nutrient
yolk.

Development. — The segmentation is usually of a type already
referred to (p. 581) as very common among the Crustacea, viz.,
superficial segmentation. The actual segmentation (Fig. 537)
has chiefly been observed in the case of certain Insects with very
little yolk ; but there can be very little doubt that in ordinary
forms with abundant yolk the process is in essence the same. The
segmentation-nucleus, originally situated near the middle of the
ovum, divides into a number of nuclei, and most or all of these
migrate towards the surface, and arrange themselves in the form

XI

PHYLUM ARTHROPODA

633

of a sphere almost parallel with the latter ; eventually they
reach the peripheral protoplasm

at the surface, which then be-
comes divided into cell-areas
corresponding with the nuclei.
The layer of cells thus formed
constitutes the blastoderm. This

vent.pl

blast

ser

amn

ond ect

amn.f

blast

blast

ect amn ser

"blast

yk. v

Fiu. 537. — A — 1), successive stages in the
segmentation of the ovum of an Insect.
blast, blastoderm ; •peri, peripheral proto-
plasm ; sey. segmentation-cells ; yk. yolk ;
ykc. yolk-cells. (From Korschelt and
Heider, after Blochmann.)

blast

FIG. 538. — A — C, transverse sections through
the developing ovum of an Insect at suc-
cessive stages to show the mode of develop-
ment of the germinal layers and of the
amnion. amn. amnion ; amn. f. fold of the
amnion ; amn. cav. cavity of the amnion ;
blast, blastoderm covering the yolk ; ect.
ectoderm ; end. endoderm ; gast. invagination
of ventral plate ; ser. serosa ; vent. pi. ventral
plate ; yk. yolk. (After Korschelt and
Heider.)

thickens along one side to form the ventral plate, as already described
in the case of the Cockroach (p. 613), and the changes which this

634

ZOOLOGY

SECT.

structure undergoes, together with the mode of formation of the
appendages, are similar in most members of the class, except that
in most Insects the formation of the mesoderm and endoderm is
associated with a more or less distinct invagination (Fig. 538). The
same holds good of the formation of the amnion and the further de-
velopment of the mesoderm and endoderm. In some cases there is
developed between the serosa and the true amnion a space filled
with yolk, and the ventral plate appears sunk within the yolk.
The nervous system is developed from the ectoderm in the manner
indicated in the account of the Cockroach (p. 615). The tracheal
system is derived from a series of pairs of segmentally arranged
ectodermal involutions (Fig. 540, st).

* Metamorphosis. — In some instances the young Insect, when
it escapes from the egg-membranes, has exactly the form of the

-of

an

BW

FIG. 539.— A— E, ventral view of five stages in the development of Hydrophilus. a and b,
points at which the blastopore first closes ; a/, edge of the amnion fold ; a/', caudal fold ;
a/", paired head-fold ; an. antenna ; es, terminal segment ; g, pit-like invagination to form
the rudiment of the amnion cavity; k, procephalic lobes ; r, groove-like medio-ventral in-
vagination ; s, germinal bands covered by the amnion. (From Lang, after Heider.)

parent, except that, as a rule, the wings have not yet grown. But
in most cases there is a metamorphosis. In some this is com-
paratively slight and gradual, the adult Insect differing from the
larva only in comparatively unimportant points, and the segments
and appendages of the latter becoming directly converted into
those of the former. Such a metamorphosis, in which there is no
quiescent stage, is said to be incomplete. The term complete .is
applied to the metamorphosis of the majority of Insects, in which
the larva differs so completely from the imago, or perfect Insect,
in external form, the nature of the appendages, and the internal
organisation, that there is need of a quiescent or pupa stage, during
which the whole animal, or a considerable part of it, undergoes
an entire transformation. The metamorphosis is complete in the
Diptera, Lepidoptera, Coleoptera, and Hymenoptera, absent or
incomplete in the other orders. In many Diptera the body of the

XI

PHYLUM ARTHROPODA

635

larva or " maggot " is completely worm-like, without any appen-
dages, and without any distinct head. In other cases (Lepidoptera,
&c.) there is a distinct head ; the three thoracic segments have
three pairs of jointed legs, and the abdominal segments short
unjointed pro-legs (Fig. 523). In most instances the larvae differ
widely from the adults in their food and mode of life ; very generally
the jaws are adapted for biting, even when the mouth of the adult is
suctorial. After a longer or shorter period passed in this larval
condition, in which it is usually active and very voracious, the
young Insect passes into a quiescent or pupa stage, during which
it remains passive, en-
closed in a tough integu-
ment, while a more or less
complete reconstruction of
the organs goes on, result-
ing in the development of
all the parts of the perfect
Insect. The development
of the new parts takes
place from certain patches
of cells, the imagined discs,
present in the larva.

In the Diptera the larva
or maggot is sometimes
completely devoid of jaws.
In some Diptera, how-
ever, the jaws are well
developed, and there is a
distinct head. After fre-
q u e n t moultings the
maggot passes either into

a quiescent Or pupa Stage FlQ. 540.— J. an_d B, later stages of thT embryo of

enclosed in a hard skin, or
into the stage of an active
aquatic pupa, which swims
about actively in water
and may possess tracheal
gills.

In the Lepidoptera the larvae (" caterpillars ") are worm-like,
but with well-developed jaws, three pairs of jointed thoracic legs,
and a number of unjointed stumpy abdominal legs (pro-legs).
Lepidopterous larvae are often brilliantly coloured, are very active,
and feed with voracity, chiefly on leaves and other succulent parts
of plants. Eventually they spin a cocoon of a silky substance,
enclosed within which, and covered with a tough skin, they pass
through a quiescent or pupa condition — the condition of the
chrysalis (Fig. 523). From the interior of this the imago subse-

Hydyophilus with the rudiments of the ex-
tremities ; in B the abdominal appendages are
visible, a. anus ; an. antenna ; g, rudiment of the
ventral nerve-chain ; m. mouth ; md. mandible ;
mxi, first maxilla ; mxz, second maxilla ; PI, P2,
P4, thoracic legs ; p±, p5, p7, p$, rudiments of the
appendages of the first, second, fourth, and sixth
abdominal appendages ; st. stigmata ; vk, prosto-
mium. (From Lang, after Heider.)

636 ZOOLOGY SECT.

quently emerges with all the parts of the adult Insect fully
formed.

In mode of life there is a very considerable difference between
different orders and families of Insects. Some are parasites in the
strict sense throughout life. This is the case, for instance, in the
Strepsiptera (Bee-parasites), the females of which live permanently
lodged between the joints of the abdomen of their hosts. The
Lice and Bird-lice are external parasites throughout life ; Bugs and
Fleas, though not adhering to their hosts, are parasites as regards
their diet. Many Insects are parasites in the larval condition,

FIG. 541. — Honey-bee (Apis mellifica). a, queen (perfect female) ; b, worker (imperfect
female), and c, drone (male). (After Brehm.)

though free in the adult state. This holds good, for example, of
the larvae of the Ichneumons, which develop in the interior of the
bodies of other insect-larvae ; also of the larvae of the Bot-flies
(Fig. 522), which inhabit the alimentary canal of mammalian hosts
(Horses, Oxen, Sheep, Rhinoceroses, Tapirs). The blood-sucking
Insects act in certain cases as the carriers or intermediate hosts of
the protozoan or bacterial parasites that are the causes of various
diseases in man. Thus, as was stated in the account of the malaria-

Fio. 542. — Red Ant (Formica rufa) ; male, worker, and female. (After Brehm.)

parasite (Section II, p. 89), mosquitoes are the means of conveying
that disease from one person to another.

In accordance with the high grade of the structure of their
various systems of organs, Insects exhibit a correspondingly high
degree of functional activity. The quantity of food consumed
and assimilated is great in comparison with the bulk of the body,
and the energy expended in muscular contractions is of very con-
siderable amount. It is estimated that while the muscular force
exerted by a Horse bears a ratio of about O7 to its own weight
(reckoned as 1) the muscular force of an Insect bears a ratio to
its weight of from about 14 to about 23. Insects are also dis-
tinguished among the Invertebrata by the keenness of their
senses. The sense of sight is, as we should expect from the

xi PHYLUM ARTHROPODA 637

elaborate character of the optic organs, the most highly developed,
many Insects having been shown by experiment to have a keen
sense of colour ; but a sense of smell, the seat of which is in the
antennse and palpi, can be shown to exist in a high degree, and
the parts about the mouth bear nerve-endings concerned in a well-
developed sense of taste. A sense of hearing does not appear to
be universally present, but is well marked in such forms as produce
sounds. At the same time Insects are remarkable for the instincts,
often leading to results of an elaborate character, which guide
them in the pursuit of food and the protection and rearing of
their young. Among the insects which are the most highly
endowed in this respect are some — the Ants, Bees, Wasps, and
Termites — which live together in organised associations or com-
munities, the various individuals composing which are distinguish-
able into sexual individuals, neuter workers, and soldiers (Figs. 541
and 542), each specially organised for the part which it has to play
in the economy of the community.

Distribution in time. — The earliest known fossil remains of
Insects have been found in rocks of Silurian age. A good many
fossil Insects have been found in the Devonian ; but they only
become abundant in the Carboniferous. All the Palaeozoic Insects
belong to a group which has been regarded as a distinct order,
and has been named the Palceodictyoptera. The members of this
group are characterised rather by the absence of the special
characteristics of any of the existing orders than by any positive
features of their own ; but different families of the order approxi-
mate to a certain extent towards the groups of living Insects.
Amongst them, for example, are forms representing the Cock-
roaches and the Phasmidaa among the Orthoptera ; others repre-
senting the modern Day-flies among the Neuroptera ; others the
Coleoptera.

Of the existing orders, the Neuroptera, Orthoptera,. and
Coleoptera are first found in the Trias ; the Hemiptera, Diptera,
Hymenoptera, and Lepidoptera in the Jurassic.

CLASS V.-ARACHNIDA.

The class Amchnida, comprising the Scorpions and Spiders, the
Mites and Ticks, the King-crabs, and a number of other families,
is a much less homogeneous group than the Insecta, approaching
the Crustacea in the variety which it presents in the arrangement
of the segments and their appendages. In most members of the
class, however, there is an anterior region of the body — the cephalo-
thorax — representing both head and thorax, and a posterior part,
or abdomen, which is typically composed of a number of distinct
segments ; in some cases cephalo thorax and abdomen are amalga-
mated. There are no antennse in the adult Arachnid, though

638 ZOOLOGY SECT.

rudiments of them have been found in the larvae of some species.
The first pair of appendages of the cephalothorax (probably repre-
senting the antennae of the Crayfish) are the chelicerce ; the second
are the pedipalpi, the representatives of the Crayfish's and Cock-
roach's mandibles. Behind these are four pairs of legs. The
organs of respiration are sometimes tracheae, similar to those of
the Insects, sometimes book-lungs — sacs containing numerous
book-leaf-like plates : sometimes leaf-like external appendages
or gills.

1. EXAMPLE OF THE CLASS. — THE SCORPION (Euscorpio or

Buthus).

Scorpions are inhabitants of warm countries — the largest kinds
being found in tropical Africa and America. They are nocturnal
animals, remaining in holes and crevices during the day, and
issuing forth at night to hunt for their prey, which consists of
Spiders and Insects. These they seize with their pincer-claws
and sting to death with their caudal spine, afterwards sucking
their juices.

There are a number of different species of Scorpions, divided
into several genera, which differ from one another in comparatively
unimportant points, so that the following general description will
apply almost equally well to any of them.

External features. — A Scorpion (Fig. 543) has a long narrow
body, in superficial appearance not unlike that of a Crayfish.
There is a small cephalothoracic shield or carapace, covering over
dorsally a short anterior region — cephalothorax or prosoma. This
is followed by a long posterior region or abdomen, the terminal
part of which in the living animal is habitually carried over the
back (Fig. 546), constituting the " tail," at the end of which the
sting is placed. The carapace bears a pair of large eyes about its
middle, and several pairs of smaller eyes on the antero-lateral
margin. The anterior, broader part of the abdomen, which is
termed the pre-abdomen or mesosoma, consists of seven segments,1
each of which is enclosed in firm, chitinous, dorsal and ventral
plates, or terga and sterna. The tergum and sternum of each
segment are separated from one another laterally by intervals
of soft skin, except in the seventh, where they are united laterally
for a longer or shorter distance. The posterior, narrower part of
the abdomen, known as the post-abdomen or metasoma, consists of
five segments, each enclosed in a complete investing ring of hard
chitinous matter. Articulating with the last segment of the
post-abdomen is a terminal appendage, the caudal spine or sting,
swollen at the base and acutely pointed at the apex, where open

1 Originally there are eight, but the original first loses its distinctness in
embryonic life.

XI

PHYLUM ARTHROPODA

639

the ducts of two poison-glands. The anal opening is situated on
the ventral surface of the last segment of the post-abdomen,
immediately in front of the sting.

The aperture of the mouth, which is very small, is at the anterior
end of the cephalothorax on its ventral aspect ; a lobe which over-
hangs it in front is the labrum. On each side of the mouth is a
three-jointed appendage — the ohelicera (Fig. 544, chel.) — which is
terminated by a chela. Behind these are the very large pincer-
claws or pedipalpi (ped.), each composed of six podomeres and
terminating in a powerful chela. The basal joint of each pedipalp

chel

FIG. 543. — Euscorpio. (From
Cuvier's Animal Kingdom.)

FIG. 544. — Scorpion. Ventral surface of the
cephalothorax and pre-abdomen. chel. cheli-
cerse ; op. operculum ; pect. pectines ; ped.
pedipalpi ; stig. stigmata. (From Leuckart
and Nitsche's Diagrams.)

has a process which bites against the corresponding process of the
other pedipalp, these processes thus performing the function of
jaws. Following upon the pedipalpi are four pairs of walking
legs, each composed of seven podomeres, the last of which is
provided with curved and pointed horny claws. The basal segments
of the first two pairs of walking legs are modified so as to perform
to some extent the function of jaws.

All the six pairs of appendages hitherto described — the cheli-
cerse, the pedipalpi, and the four pairs of walking legs — belong to
the cephalothorax. The first segment of the pre-abdomen (Fig.
544) has a narrow sternum, on which there is a soft rounded

640

ZOOLOGY

SECT.

median lobe divided by a cleft ; this is termed the
operculum (op.) ; at its base is the opening of the genital duct. To
the sternum of the second segment of the pre-abdomen are attached
a pair of remarkable appendages of a comb-like shape — the pectines
(pect.) — each consisting of a stem, along the posterior margin of
which is a row of narrow processes, somewhat like the teeth of a
comb ; the function of these appendages is doubtful, but is probably
sensory. The remainder of the segments of the pre-abdomen, and
all those of the post-abdomen, are devoid of appendages. The
sterna of the third, fourth, fifth, and sixth
segments of the pre-abdomen, which are very
broad, bear each a .pair of oblique slits — the
stigmata (stig.) — leading into the pulmonary
sacs.

In the interior of the cephalothorax, over the
nervous system, is a cartilaginous plate — the
endosternite (Fig. 545) — which serves .to give
of scorpion. (liter attachment to muscles, and is comparable to

Lankester.) ,, , ,. , ' r . , -<vrv\

the cephalic apodeme of Apus (p. 520).

All the appendages of the Scorpion are post-oral in position, and
the most anterior — the chelicerse — are probably best regarded as
corresponding to the antennse of the Crayfish, the equivalent of
the Crayfish's antennules and of the antennse of the Cockroach
not being present. The pedipalpi would then be the homologues
of the mandibles of the Insect and the Crustacean.

FIG. 545. — Endosternite

CRAYFISH.

Antennules.
Antennae.
Mandibles.
First maxillae.
Second maxillae .
First maxillipedes.
Second maxillipedes.
Third maxillipedes.

COCKROACH.

Antennse.
Absent.
Mandibles.
First maxillse.
Second maxillae.
First legs.
Second legs.
Third legs.

SCORPION.

Absent.
Chelicerse.
Pedipalpi.
First legs.
Second legs.
Third legs.
Fourth legs.

Digestive system. — The narrow mouth leads into a large
chamber with elastic walls, the pharynx ; this is capable of being
greatly dilated by the action of a number of radiating bundles of
muscular fibres which run outwards from it to the walls of the
cephalothorax, the result of this being to cause suction through
the mouth, by which means the juices of the Scorpion's prey are
drawn in. A second dilatation, to which a narrow oesophagus
leads, receives the ducts of a pair of salivary glands (Fig. 547,
sal. gld.). Upon this follows the mesenteron (mesent.), which is
an elongated, wide, straight tube, with glandular walls, corre-
sponding to the stomach of the Insect. Opening into the

XI

PHYLUM ARTHROPODA

641

mesenteron are five pairs of narrow tubes (Figs. 546 and 547,
hep. du.) leading into the substance of a large glandular body,
usually termed the liver (hep.), though its hepatic functions are
doubtful. Into the long narrow intestine, in the last segment of

the pre-abdomen, open one or two pairs of delicate tubes — the

Malpighian tubes (mal.) — which act as the organs of renal excretion.

Circulatory organs. — An elongated tubular heart (Fig. 546,

hrt.) lies in the pre-abdomen enclosed in a pericardial sinus ; it is

642

ZOOLOGY

SECT.

divided internally by folds into a series of eight chambers ; into
each of these chambers the blood passes by a pair of valvular
apertures or ostia. The heart ends both in front and behind in
main arteries, the anterior and posterior aortce (ant. art., post, art.) ;
and a series of pairs of lateral arteries are given off from the various
chambers. The anterior aorta soon bifurcates to form a pair of
vessels which embrace between them the oesophagus, and meet
below in a median ventral trunk which runs backwards above the
nerve-cord. The blood carried to the various parts of the body
by the arteries is gathered up into a large ventral sinus from which
it passes to the book-lungs. From these it is carried by a series of

veins to the pericardial sinus to
enter the heart through the
ostia.

The organs of respiration
in the Scorpions are in the form
of pulmonary sacs or book-lungs
(pul.), the stigmata or external
openings of which have already
been referred to. Each pul-
monary sac is a compressed
chamber lined with a thin

chel

stomo-

saigld

mesent

mat

-jbroct

fw]b die

cuticle. The lining membrane
is raised up into numerous
delicate laminae lying parallel
with one another like the leaves
of a book. Into the numer-
ous narrow spaces between the
laminae the air penetrates, and
oxygenates the blood which
enters the interior of the laminae
from the ventral sinus.

A pair of coxal glands, situated
near the base of the fifth pair
of appendages, are, in the
embryo Scorpion, represented
in the second to the sixth segments by tubes which grow out from
the ccelomic sacs ; in the adult Scorpion only the one pair persists,
in the form of a closed gland, and its function is quite uncertain.

The nervous system is constructed on a plan which bears a
considerable resemblance to that of the Crayfish and that of the
Cockroach. There is a bilobed cerebral ganglion or brain (Fig. 546,
brn.) from which nerves are given off to the eyes ; a nerve-collar
formed of a pair of cesophageal connectives unites ventrally in a
sub-cesophageal ganglion, forming the anterior part of a ventral nerve-
cord (ne. co.). The connectives and sub-oesophageal ganglion give
rise to the nerves of the first six pairs of appendages and of the

FIG. 547. — Dorsal view of the internal organs
of Scorpion, chel. chelicerse ; hep. liver ;
hep. du. hepatic ducts ; mal. Malpighian
tubes ; mesent. mesenteron ; proct. intestine ;
sal. gJd. salivary glands : stomo. stomodseum.
(From Leuckart, after Elan chard.)

XI

PHYLUM ARTHROPODA

643

operculum, the pectines, and the two following segments. The
first ganglion behind the suboesophageal ganglion appears in the
eleventh segment (reckoning the cephalothorax as made up of six) ;
behind which a ganglion occurs regularly in each segment as far back
as the fourth of the post-abdomen.

The organs of special sense are the eyes and pectines. The
lateral eyes (Fig. 566) are similar in character to the simple eyes or
ocelli of Insects. The two larger central eyes (Fig. 567) differ from
them in having the retinal cells arranged in groups as in the com-
pound eye, but resemble them in the presence of a single cuticular
lens.

Reproductive organs. — In the male the testes consist of two
pairs of longitudinal tubules united by cross branches. These
are connected with a
median vas deferens, the
terminal portion o f
which, provided with
accessory glands, is modi-
fied to form a double
penis ; its external open-
ing is just behind the
operculum, as already
noticed. There is an un-
paired ovary, which is
made up of three longi-
tudinal tubules with
transverse connecting
branches ; the oviducts
open on the operculum.

Scorpions are v i v i -
parous. The eggs, which
are spherical or oval, and
in most species, contain
a large amount of food-
yolk, lie in a follicle
formed of a diverticulum
occurs in the follicle

FIG. 548.— Three surface views of the ventral
plate of a developing Scorpion. A,
before the appearance of segments ; -B,after
five segments have become formed ; C, after
the appendages have begun to be formed.
(From Balfour, after Metschnikoff.)

or

of the oviduct. Fertilisation either
after the egg has escaped into the
oviduct. The further development takes place in the oviducts ;
and, when born, the young Scorpion differs from the parent very
little save in size.

Development. — The segmentation is of the type to which the
term discoidal is applied. On one pole are formed a number of cells
in the form of a one-layered disc or cap, which gradually spreads
over the yolk. On this appears a thickening — the ventral plate
(Fig. 548) corresponding to that of the Insect. A longitudinal
groove which appears on the surface of this may be regarded as
representing an elongated blastopore (Fig. 548, A). The cells of the

644

ZOOLOGY

SECT.

blastoderm of the ventral plate become divisible into three layers —
ectoderm, endoderm, and mesoderm. The mesoderm undergoes
division into a series of masses which are hollowed out to form the
primitive segments (B) and their cavities. Embryonic membranes —
serosa and amnion — are formed as in the Insects. When about ten
segments have become distinguishable, the rudiments of appendages
(Fig. 548, 0, and Fig. 549) appear in the form of hollow processes
of the segments on either side of the middle line. Behind the
rudiments of the thoracic limbs appear a series of seven pairs of

abdominal appendages (ab l — 7) ;
the first early disappear ; the
place of the second is after-
wards taken by the operculum ;
the third develops into the pectines.
The four posterior pairs become
aborted ; the book-lungs appear
as pits at their bases.

2. DISTINCTIVE CHARACTERS AND
CLASSIFICATION.

The Arachnida are air-breathing
2 Arthropoda in which the body is
usually distinguishable into two
regions — cephalothorax and abdo-
men. The cephalothorax bears
sessile, usually simple eyes, two
pairs of jointed appendages — the
chelicerse and pedipalpi — and four
pairs of legs. There are no an-
tennae. The organs of respiration,

U. i>*£7. JillllUlJ'U UL OWJ.JJ1UU V.AU.BVU1-- -, -,-, . -,

pio carpathicus), with rudimentary when present, are usually either
d^fTch^SrSSdlmSS tracheae or book-lungs, but in the

SSSTii, safiTa. •& « xip110^™ take the form of book-

legs ; ped. pedipaip ; stom, opening of gills. Heart and vascular system

stomodaeum ; v.g, ganglia of ventral n . . i -i

nerve cord. (From MacBride, after are usually present ; tne heart is

tubular, like that of the Insects.

The sexes are nearly always separate, and there is usually no meta-
morphosis.

The class is divided into the following orders : —

ORDER 1.— SCORPIONIDA.

Arachnida in which the body consists of a continuous cephalo-
thorax and an abdomen, the latter consisting of an anterior broader
pre-abdomen of seven segments, and a posterior, narrower post-
abdomen of five segments, with a caudal spine in the form of a
sting. There are small chelate chelicerse and large chelate pedipalpi.

FIG. 549. — Embryo of Scorpion (Euscor-

±1 PHYLUM ARTHROPODA 645

A pair of comb-like pectines occur on the second segment of the
pre-abdomen. The organs of respiration are four pairs of book-
lungs in the third, fourth, fifth and sixth segments of the pre-
abdomen.
This order includes the Scorpions.

ORDER 2. — PSEUDOSCORPIONIDA.

Arachnida in which there is a continuous cephalothorax, some-
times marked dorsally with two transverse grooves, and a broad
abdomen, not divided into pre- and post-abdomen, and not pro-
vided with a sting. The chelicerae are very small, the pedipalpi
similar to those of the Scorpions. The organs of respiration are
a system of tracheae. A pair of spinning glands are present.

This order includes the Book-scorpions (Fig. 550).

ORDER 3. — PEDIPALPIDA.

Arachnida in which the body consists of unsegmented cephalo-
thorax and flattened abdomen of eleven to twelve segments. The
chelicerse are simple, the pedipalpi simple or chelate, and the
first pair of legs terminate in a many-jointed flagellum. The
organs of respiration are two pairs of book-lungs on the second
and third segments of the abdomen.

This order includes the Scorpion-spiders (Fig. 551).

ORDER 4. — SOLPUGIDA.

Arachnida with three regions — head, thorax (of three segments),
and abdomen (of ten segments). The chelicerae are chelate ; the
pedipalpi elongated and leg-like. The organs of respiration are
tracheae.

This order includes Galeodes (Fig. 552).

ORDER 5. — PHALANGIDA.

Arachnida with an unsegmented cephalothorax, and an abdomen
of six segments. The chelicerse are chelate, the pedipalpi leg-like.
The organs of respiration are tracheae. No spinning glands are
developed.

This order includes the Harvestmen.

ORDER 6. — ARANEIDA.

Arachnida in which the body is composed of an undivided
cephalothorax and an unsegmented abdomen, which is usually soft
and rounded, and attached to the cephalothorax by a narrow neck.
The chelicerae are sub-chelate, with poison-glands ; the pedipalpi
simple. The organs of respiration are book-lungs alone, or book-
lungs combined with tracheae.

This order comprises all the true Spiders (Fig. 553).

VOL. i T T

646 ZOOLOGY SECT.

ORDER 7. — ACARIDA.

Arachnida in which the body exhibits no division into regions.
The mouth-parts are adapted either for biting or piercing and
sucking. The organs of respiration, when present, are in the
form of tracheae.

This order includes the Mites and Ticks (Figs. 556 and 557).

ORDER 8. — XIPHOSURA.

Arachnida in which the body consists of a cephalothorax covered
over by a broad carapace, and an abdomen of seven firmly united
segments, with a long narrow tail-piece or telson. The cephalothorax
bears a pair of short chelate appendages and five pairs of legs. The
abdomen bears in front a pair of united plate-like appendages,
forming the operculum, followed by five pairs of flat appendages
overlapped by the operculum. The organs of respiration are lamelli-
fofm gills attached to the abdominal appendages.

This order includes the King-crabs (Limulus, Figs. 558 and
559).

ORDER 9. — EURYPTERIDA.

Arachnida with a relatively small cephalothorax, followed by
twelve free segments and a terminal, elongated, narrow telson.
There are a pair of pre-oral leg-like or chelate appendages and
four more leg-like appendages on the cephalothorax, the last ex-
panded to form swimming paddles. A broad operculum is situated
immediately behind the cephalothorax. There are pairs of lamellate
appendages on certain of the anterior free segments. The exo-

skeleton is characteristically sculptured.
This order includes only a number of

extinct (Palaeozoic) forms of large size

(Fig. 560).

3. GENERAL ORGANISATION.

The external form in the Scorpionida
has already been sufficiently described.
Most nearly related to that order in this
respect are the Pseudoscorpionida or
Book-scorpions and their allies. In these
(Fig. 550) there is an unsegmented
cephalothorax, or the carapace is crossed
'ii by two transverse grooves which may
Lan8 s m(*icate segmental divisions. There is a
broad abdomen consisting of eleven or
twelve segments ; the post-abdomen is not represented, nor the
caudal sting. The chelicerae are small ; the pedipalpi are large, and
resemble those of the Scorpions in their chelate form. Spinning
glands are present.

XI

PHYLUM ARTHROPODA

FIG. 551.— Phrynus. (From Cuvier's Animal Kingdom.)

648

ZOOLOGY

SECT.

FIG. 553. — Spider (Epeira diadema).

The Pedipalpi, or Scorpion-spiders (Fig. 551), are intermediate in
some of their external features between the Scorpions and the
Spiders. The abdomen is broad and marked out into a series

of eleven or twelve distinct seg-
ments ; in one of the genera of the
order there is a short post-abdomen
formed of the last three segments,
with an elongated, many- jointed
anal filament. The chelicerae end in
simple claws ; they are probably
provided with poison-glands ; the
pedipalps are very long, either claw-
like or chelate ; the first pair of legs
are very long and slender, their
terminal part made up like an-
tennae of numerous short joints.
There are eight eyes on the cara-
pace, two larger central, and six smaller marginal.

The Solpugida (Fig. 552) have, at least superficially, the appearance
of being intermediate between the Insecta and the other groups
of Arachnida.
The cephalo- A

thoracic region
is divided by a
constriction
into two parts,
head and
thorax, the
latter made up
of three Beg-
in e n t s . The

chelicerae are chelate ; the pedipalpi resemble the legs, and are used
in locomotion. The first pair of legs are attached to the head.
The abdomen is distinctly segmented, and there is no caudal

appendage. A pair of poison-glands
open at the bases of the chelicerae. There
are two simple eyes on the head.

In the true Spiders (Fig. 553) the
abdomen is rounded, unsegmented, and
separated off from the cephalothorax by
a constriction. The chelicerae (Fig. 554, A)
are sub -chelate, and the duct of a large

riEpeira~diadema°f if*m P°ison-glan(l opens at the extremity.
iv. v. podomeres : 66, sac ; spk. The pedipalpi (Fig. 554, B) are elon-

spiral tube. (After Leuckart.) g§^ sii-j^ted,8 and end in simple

extremities ; in the male (Fig. 555) the terminal joint is modi-
fied to serve for the reception and transference of the sperms.

FIG. 554. — A, Chelicerse, and B, pedipalpi of female of Epeira
diadema. (After Leuckart.)

bb

XI

PHYLUM ARTHROPODA

649

At the extremity of the abdomen is the spinning apparatus or
arachnidium (Fig. 561, arach.). This consists of four or six eleva-
tions, the spinnerets, usually
jointed, probably derived from
embryonic rudiments of abdo-
minal appendages. On the sur-
faces of these open the numerous
fine ducts of the spinning glands
(sp. glds.)} secreting the material
of which the spider's web is com-
posed. The fine threads of viscid
secretion issuing from the ducts
harden on exposure to the air,
and are worked up into the web
by means of the posterior legs.
There are six or eight eyes on
the carapace.

In the spider-like Phalangida,
or " Harvestmen," the cephalo-
thorax is not constricted off FIQ-
from the abdomen. The chelicerae
are chelate, the pedipalpi short and leg-like, the legs long and

slender.

In the Acarida,
or Mites and Ticks
(Figs. 556 and 557),
the distinction into
regions is no longer
recognisable. The
form of the mouth-
parts varies some-
what in the different
families. Some-
times the basal
portions of the
pedipalpi form a
sucking proboscis
enclosing the stylet-
like chelicerae,
modified to form
piercing organs ;
sometimes these
appendages are
claw-like or chelate.
The legs vary
somewhat in shape

FIG. 557.— Water mite (Trombidium fuliginosum), female. ; Jift^

chel. chelicerae ; ped. pedipalpi. (After Leuckart.) * n t H 6 Clllierent

650

ZOOLOGY

SECT.

groups, according as they are used for prehension, for creeping,
for running, or for swimming ; they end usually in two claws,
between which there may be discs or stalked suckers.

In the Xiphosura or King-crabs (Fig. 558), the body consists of
two well-marked regions — cephalothorax and abdomen. The former
is covered over by a wide, dorsally convex, sub-crescentic shield or
carapace, bearing two large compound eyes, and two smaller simple
eyes. The segments of the abdomen (seven in number) are united

"together, being covered dor-
sally by a continuous abdo-
minal carapace. At the
posterior end is attached a
very long, narrow, caudal
spine or telson. The anterior
appendages (Fig. 559) re-
semble those of the Scorpion.
In front of the mouth is a
pair of short, three- jointed,
chelate appendages, the cheli-
cerce (1), at the sides of a
labrum (rostrum) or upper lip.
Behind these follow a series
of five pairs of legs, the bases
of all of which, with the
exception of the last, are ,
covered with spines, and have
the action of jaws, while the
extremities are for the most
part chelate. The first pair
of appendages of the abdomen
are flat plates, which are
united together in the middle
line and together form the
broad operculum (operc.) over-
lapping all the posterior
appendages ; on its posterior
face are the two genital
apertures. The posterior
appendages, of which there
are five pairs, ^are thin flat plates to which the gills are attached ;
each of them is divided by a suture into a small inner ramus or
endopodite, and a larger external ramus or exopodite. Between
the sixth pair of appendages is a pair of processes, the chilaria.
In the Eurypterida (Fig. 560) there is a small cephalothorax
bearing a pair of large eyes and a pair of ocelli, and an elongated
segmented region containing twelve segments, followed by a
narrow pointed telson. There are usually five pairs of limbs

FIG. 558. — Limulus. Dorsal aspect.
(After Leuckart.)

PHYLUM ARTHROPODA

651

ceph

cjberc

surrounding the mouth and, with the exception of the first, toothed
at the bases in order to perform the functions of jaws ;. the last
pair are stouter than the others and are expanded so as, apparently,
to assume the character of swimming paddles. Certain of the
more anterior of the free segments bear paired lamelliform appen-
dages which probably carried the branchiae, as in the Xiphosura.
The exoskeleton is in many cases elaborately sculptured.

A cartilaginous in- ^^ ^^^^

ternal endosternite
of the same nature as
that which has been
described as occurring
in the Scorpions is
found in Limulus and
in certain Spiders, but
not in the other groups.

Coxal glands,
similar to those that
have been described
in the Scorpion, occur
also in most Spiders,
in the Solpugida and ' '^

Phalangida, in some
Acarida, and in the
Xiphosura. In the
Solpugida and Phalan-
gida they occur in the
bases of the last pair
of legs ; in the Ara-
neida and Xiphosura,
as in the Scorpion,
they are found on the
bases of the fifth pair
of appendages.

Alimentary sys-
tem.— The mouth of
the Spiders leads into
a pharynx (Fig. 561,
ph) followed by a
narrow oesophagus
(ces.) expanded behind into a special sucking stomach (suck. St.). The
mesenteron (mesent.) gives off in the cephalothorax a pair of large
diverticula from each of which arise five narrow branches (CCBC.),
the last four of which enter the bases of the legs ; in the abdomen
it is surrounded by a mass of cells commonly termed " liver " (hep.),
the ducts of which open into it. The rectum or proctodceum gives
off dorsally a large cloacal sac (rect. CCBC.) into which open a pair of

FIG. 559. — Ventral view of Limulus. 1 — 6,iappendagesof
cephalothorax ; abd. abdomen ; ceph. cephalothorax ;
open, operculum, behind which are seen the series of
abdominal appendages : tels. caudal spine or telson.
(After Leuckart.)

652

ZOOLOGY

SECT.

narrow tubes, the so-called " Malpighian tubes," which are not the
homologues of the tubes so named in the Insects, being, with the
cloacal sac, of endodermal and not ectodermal derivation.

In the Pseudoscorpionida the mesenteron, which is bent into a
loop, gives off three diverticula ; the proctodseum has also a
diverticulum. In the Solpugida the mesenteron also gives off
diverticula ; the occurrence of Malpighian tubes is doubtful. In
the Acarida there are always diverticula, the number and arrange-
ment of which vary,
connected with the
mesenteron. There
are usually two long
Malpighian tubes
which may be fused
mesially.

In the Xiphosura,
the mouth (Fig. 562,
mo.), which is situated
some distance behind
the anterior extremity
of the body, leads into
a suctorial pharynx,
followed by a stomach,
which opens into the
elongated mesenteron ;
the proctodseum, a
short tube with folded
walls, opens on the
exterior at the pos-
t e r i o r extremity of
the abdomen. Into
the mesenteron, as in
the Scorpion, open the
ducts of a large gland,
usually termed the
" liver " (I liv.}.

A heart is absent
in many of the Mites.
In the other Arach-

nida a heart is present and has the same general form as in the
Scorpions, though always more concentrated.

In the various orders the organs of respiration differ a good
deal in their character. In the Pseudoscorpionida they take the
form of branching trachece similar to those of Insects. In the
Pedipalpi there are two pulmonary sacs or book-lungs similar to
those of the Scorpions. In the Solpugida there is a system of
tracheae. In :the Spiders there are either four pulmonary sacs

FIG. 560. — Eurypterus fischeri (Silurian).
(From Nicholson and Lydekker.)

XI

PHYLUM ARTHROPODA

653

(Fig. 563), or two pulmonary sacs and a system of tracheae (Fig. 564).
Tracheae are present in the Phalangida and also in the majority
of the Acarida.
In the Xipho-
sura the organs
of respiration
are external
appendages or
gills (book-gills),
in the shape of
delicate laminae
attached to the
abdominal ap-
pendages (Fig.
565).

The nervous

•*
system is, in

most instances,
more concen-
trated than in
the Scorpions.
There may be
one or two
separate abdo-
minal ganglia
behind the mass
formed by the
united cephalo-
thoracic and
anterior abdo-
minal (Pseudo-
scorpionida,
Pedipalpida,
some Araneida,
Solpugida, Pha-
langida). In
most of the
Araneida and
in the Acarida
all the abdo-
minal are united
with all the
cephalothoracic
ganglia to form

a single mass perforated by the oesophagus, the part lying behind,
which is much the larger, representing the ventral nerve-cord.
Sense-organs. — Eyes are present in all except in some of the

654

ZOOLOGY

SECT.

Acarida. Their number and arrangement have been given with
the external characters of the groups. They are all (Fig. 566) of
the type of the ocelli or simple eyes of Insects, except the central
eyes of the Scorpions (Fig. 567) and the compound eyes of Limulus.
The former are intermediate in character between ocelli and faceted

FIG. 562. — Diagrammatic view of a median longitudinal section of Limulus. abd. app. ab-
dominal appendages ; an. anus ; brn. brain ; chil. chilaria ; hep. du. opening of one of the
hepatic ducts ; M. heart ; int. intestine ; 1. liv. " liver " ; mo. mouth ; ne. co. nerve-cord ;
oes. 03sophagus ; operc. operculum ; tels. telson ; ven. sinus, venous sinus ; 1 — 5, legs. (From
Leuckart, partly after Packard.)

eyes, possessing the single cuticular lens (lens) of the ocellus, and
resembling the faceted eye in having the retinal cells arranged in
groups corresponding to ommatidia. Each retinula, composed of
five cells, contains a thick axial rod or rhabdome (rhabd.).

In Limulus the compound eye has a continuous chitinous cornea-
lens of the nature of. a thickening of the cuticle. This, though
non-faceted, differs from the corresponding
part in the compound eye of the Scorpion in
being produced internally into a number of
conical papillae, each of which lies over one
of the ommatidia and may be looked upon as
its lens.

A considerable variety is observable in the
exact arrangement of the parts of the repro-
ductive apparatus in different groups of
the Arachnida. In general, testes or ovaries
are either paired or (more rarely) unpaired
tubes, with paired vasa deferentia or oviducts,
which unite in a median duct opening on the

FIG. 563.— Book-lung of
Spider (Zilla callo
phylla).

. axis; b,

(Fronfiiertwig.)stl exterior by an unpaired genital opening. Vivi-
parity is exceptional. In the Spiders the
ovaries (Fig. 561, ov.) are two wide tubes, on the surface of which
follicles project prominently ; sometimes they unite into a single
circular ovary. There are two short oviducts even when the ovary
is single ; these unite in a median vagina, which opens on the
exterior by a median genital aperture at the base of the abdomen.

XI

PHYLUM ARTHROPOD A

655

One, two, or three receptacula seminis (rec. sem.) are present, and
either open into the vagina or independently on the surface. In
the male there are two elongated tubular testes with two narrow
and often greatly coiled efferent ducts, which unite in a short

FIG. 564. — Main branches
of the tracheal system
of a Spider. .<?«.
stigma. (From Hertwig,
after Bertkau.)

FIG. 565. — One of the book-gills of Limulus, with
the appendage to which it is attached. (After
Lankester.)

median vas deferens, the aperture of which is on the base of the
abdomen between the stigmata of the first pair. The pedipalpi
of the male (Fig. 555) are modified to act as intromittent organs :
the terminal segment is swollen, and contains a twisted tube (sph.)
into which the sperms from the reproductive aperture are received
in order to be transferred ^ens

in the act of copulation ^r\—

to the reproductive aper-
ture of the female. The
eggs of spiders are laid
in nests or cocoons, and
are usually guarded by
the mother, sometimes
carried about by her.

In their mode of life
the Arachnida present
almost as great a diver-
sity as the Insecta.
Some Acarida are para-
sites throughout life.
Most of the other groups
of Arachnida are pre-
daceous — preying for the
most part on Insects or other Arachnids. To capture the Insects
which constitute their food, the majority of Spiders construct a web
formed of the threads secreted by the arachnidium. The primary

nervf

FIG. 566. — Section of the lateral eye of Euscorpius
italicus. int. intermediate cells ; lens, cuticular
lens ; nerv. c. terminal nerve-cells ; nerv . f. nerve-
fibres of optic nerve ; rhabd. rhabdomes. (After
Lankester and Bourne.)

656

ZOOLOGY

SECT-

function of the threads formed from the secretion of the spinning
organ is to constitute the material for the manufacture of a cocoon
enclosing the eggs, and in some Araneids this is the sole purpose to
which they are devoted. In others there is added a nest for the
protection of the eggs and of the parent itself ; this in many cases
becomes a permanent lurking-place which the Spider inhabits at
all seasons, and from which it darts out to capture its prey ; in the
Trap-door Spider the nest has a closely fitting hinged lid. In very
many Spiders the secretion is used mainly to form the web by
means of which the prey is snared, and frequently also a nest in
which the Spider lies in wait. A subsidiary function of the threads
is to aid in locomotion, the Spider being enabled by means of

them to let itself down
lens safely from considerable

/ heights, and even to

float in the air.

Some of the Mites, as
already mentioned, are
parasitic ; others feed
on various kinds of
fresh or decaying
animal or vegetable
substances. Most free
Acarida are terrestrial ;
some are aquatic.

The Xiphosura are
marine, living at a
depth of a few fathoms

FIG. 567.— Section of the central eye of Eusc orpins, in Warm Seas, burrowing

Letters as in preceding figure, pigm. cells containing •„ QoT,,q . fl^i'r f™r1 ™n

pigment ; vtr. vitreous body (a specialised part of the m bcmu 5 L

ectoderm). (After Lankester and Bourne.) sjsts of Various kinds

of marine Annelids.

Geological History. — The most ancient of the living groups of
the Arachnida are the Scorpions, which are represented in Silurian
rocks by various fossil forms not differing very widely from those
existing at the present day. The earliest known fossil Spiders
have been found in deposits of Carboniferous age ; and remains of
Pedipalpida occur in the same formation. In Tertiary deposits
there have been found representatives of all the principal groups
of living Arachnida.

The earliest fossil-remains of Xiphosura that have been found
occur in strata of the Triassic period. Other fossil species occur
in later formations. These are all nearly related to the living
species of Limulus. The Eurypterida, as already noted, are
entirely Palaeozoic, ranging from the Lower Silurian rocks to the
Devonian.

xi PHYLUM ARTHROPODA 657

APPENDIX TO THE ARACHNIDA.

THE PYCNOGONIDA, LINGUATULIDA, AND TARDIGBADA.

These three groups, though not in any way related to one another, and of
doubtful relationships to the Arachnida, are, as a matter of convenience,
mentioned together here.

THE PYCNOGONIDA.

These are marine Spider-like Arthropods (Fig. 568) in which the body
consists of a cephalothorax composed of an anterior proboscis (s), three head-
segments, and one thoracic segment, followed by three free thoracic segments
and a rudimentary abdomen (ab.). The cephalothorax bears usually four
simple eyes and four pairs of appendages, the first of which may be chelate.
To these succeed a pair of ovigerous legs (3), and the first pair of thoracic
legs (4). The free thoracic segments bear lateral processes for the articulation

FIG. 568. — Nymphon hispidum. 1 — 7, appendages ; ab. abdomen ; 8. proboscis.
(From Lang, after Hoek.)

of the remaining three pairs of legs. A number of Antarctic species have
five pairs of legs. The rudimentary abdomen (ab.) is devoid of appendages.

Diverticula from the mesenteron penetrate for a considerable distance into
the limbs, Malpighian vessels are^absent. There is a tubular heart with two
or three pairs of ostia. Organs of respiration are absent. The nervous
system consists of brain, sub-cesophageal ganglia and three other ganglia in
the cephalothorax, and one or two small pairs in the abdomen. The testes in
the male are partly, and the ovaries in the female either partly or completely,
contained in the bases of the thoracic appendages on which they open. In the
male 4-7 cement-glands are situated in the fourth joints of certain of the
appendages ; their secretion cements the eggs together into masses which are
carried on the ovigerous legs of the male, and in one species on those of the
female also.

A metamorphosis occurs in most cases. The larva usually has three pairs
of 'appendages, so that it bears a superficial resemblance to a nauplius ; but
the appendages are simple, and in other respects the larva has no essential
likeness to the nauplius form. Additional segments with their appendages
are formed behind the original three until the form of the adult is completed.

658

ZOOLOGY

SECT

Different kinds of Pycnogonids occur at various depths from between tidal
limits to considerable depths in the ocean. The larvae of the species of one
genus are internal parasites in certain hydroid Zoophytes.

THE LlNGTJATULlDA OR PENTASTOMIDA.

The Linguatulida (Fig. 569) are parasitic animals, which, when super-
ficially examined, present little appearance of affinity with the Arthropoda.
The body is completely worm-like, not divided into regions, and presenting
only a superficial annulation, which in no way corresponds with division of
the body into segments. The sole representatives of limbs are four hooks
(hie.) at the sides of the mouth. The muscular fibres are striated. The
alimentary canal is simple and straight, and Malpighian tubes are absent.

bwcc
sbyl

FIG. 569.— Pentastomum taenioides,

young female/an. anus ; gang, ganglion ;
hk. hooks ; mo. mouth ; ees. oasophagus ;
ov. ovary ; ovd. oviduct ; rec. sew. re-
ceptaculum seminis ; sex. ap. sexual
aperture ; stom. stomach : ut. uterus.
(After Leuckart.)

FIG. 570.— Macrobiotus hufelandi.

I — IV, appendages ; frwcc.buccal cavity ;
gld. accessory gland ; mal. Malpighian
tube ; ov. ovary ; reef, rectum ; sali.
salivary glands ; stom. stomach ; styL
teeth. (From Hertwig's Lehrbuch,
after Greef and Plate.)

Heart and organs of respiration are wanting. The nervous system is greatly
reduced. A narrow, nerve-collar surrounds the oesophagus, presenting no
brain- enlargement, and connected behind with a single ventral nerve-mass.
Organs of special sense are absent.

Some species of Pentastomum are in the adult condition parasites in the
lungs of snakes. One species (Pentastomum tcenioides) inhabits certain
cavities — the frontal sinuses and maxillary antra connected with the nasal
chambers — in the Dog and Wolf. Its embryos, escaping and falling on grass
and other herbage, which form the food of Hares and Rabbits, are taken up
by these animals, and perforating the wall of the alimentary canal, by means
of a boring apparatus composed of several chitinous pieces, lodge themselves
in the liver, where they become encysted and undergo a metamorphosis.

xi PHYLUM ARTHROPODA 659

Afterwards they leave the cysts and move about. If the young Pentastomum
should be received into the mouth of a Dog (still contained probably in most
cases in the tissues of the Hare or Rabbit) it may find its way to the frontal
sinuses or maxillary antra, there to undergo its final transformation into the
adult form. The larva possesses two pairs of short legs.

THE TARDIGRADA.

The Tardigrada ("Bear-animalcules") are soft-skinned animals (Fig. 570)
of minute size, not exceeding a millimetre in length. The body is unsegmented
and not distinguishable into regions, except that in some a slight constriction
separates off an anterior part or head from the rest. The mouth is provided
with a sucking proboscis. There are four pairs of short unjointed legs
(I. — IV.), the last of which is terminal, and each is provided with two or
four claws. The mouth is surrounded by papillae ; the buccal cavity contains
a pair of horny, sometimes partly calcified, teeth (styl.}. The ducts of a pair
of salivary (?) glands (soli) open into the cavity of the mouth ; there is a
muscular pharynx (ph.], a narrow oesophagus, and a wide mesenteron (stom.) ;
the anus is sub-terminal, situated in front of the last pair of limbs. A pair of
tubes (mal.) which open into the terminal part of the intestine are perhaps
representatives of Malpighian tubes. The muscles are all non-striated.
There are no organs of respiration, and heart and blood-vessels are likewise
absent. There are a brain and a ventral nerve-cord with four ganglia. Two
eye-spots situated at the anterior end are the only representatives of organs
of special sense. The gonads in both sexes are saccular, and open into the
terminal part of the intestine. Segmentation is complete and regular. The
young animal at one stage has only two pairs of rudimentary legs, but develops
the full number before being hatched. The larva possesses a head and four
distinct segments.

Some of the Tardigrada live among damp moss ; others in fresh or in salt
water.

RELATIONSHIPS OF THE AIR-BREATHING ARTHROPODA. 1

Notwithstanding the existence of some striking superficial
resemblances between the Arachnida and the Insecta, the evidence
afforded by anatomy and embryology points to the conclusion that
there is no direct genetic relationship between the two groups.
The occurrence in both of a peculiar form of respiratory organs,
the tracheae, seems at first sight to indicate such a relationship ;
but the evidence of an independent origin is so strong that it
must be supposed that the tracheae have been independently
developed in the two classes. The most important points of
difference are — the separation of head and thorax in the Insecta,
the mode of development of the eyes, the presence in the Arachnida
of an extensive " liver," and (perhaps) the endodermal origin of
the Malpighian tubes in the latter class.

Resemblances between Limulus and the Scorpions are readily
apparent. In both there is a cephalothorax bearing six pairs of
appendages, together with two median and several lateral eyes.
The appendages in both are all originally post-oral, the first

1 The Xiphosura, and also the Pentastomida, though not air-breathing, are
discussed here.

660 200LOGY SECT.

pair becoming pre-oral in course of growth, and the ganglia
belonging to it coalescing with the brain. The upper lip between
the bases of these appendages is similarly developed in both.
The pair of processes situated behind the sixth pair of appendages,
which in Limulus form the chilaria, are represented in the Scorpions
by a small pentagonal plate in front of the operculum. The
abdomen of Limulus corresponds to the pre- and post-abdomen of
the Scorpion ; it contains only eight segments, inclusive of the
telson ; but there is evidence, from a comparison with certain
fossil forms, that the telson represents several united metameres.
A certain amount of correspondence is also traceable in the appen-
dages of the abdomen. In both the first pair form the operculum ;
in the Scorpion the second pair form the pectines, while -the rest
disappear ; in Limulus all persist as the lamelliform appendages
to which the book-gills are attached. In structure there is con-
siderable similarity between the book-gills of Limulus and the
book-lungs of the Scorpion, but how far they are equivalent to
one another remains doubtful in view of the difference in their
position, the book-gills being attached to the dorsal surface of the
abdominal appendages and the book-lungs sunk within the
segments.

The presence in both of the large " liver," the presence of a circum-
cesophageal artery, of a cartilaginous endosternite, and of a pair
of coxal glands on the basal joints of the fifth pair of appendages,
are some of the points of correspondence in the internal anatomy.

While Limulus is thus closely related to the Scorpions on the
one hand, it exhibits, on the other, indications of affinities with the
Trilobites, a group of extinct Arthropods probably finding their
nearest existing allies in the Branchiopod Crustacea (p. 556). This
resemblance to the Trilobites is most marked in the stage — the
trilobite-stage — in which the young King-crab escapes from the
egg. Certain fossil representatives of the Xiphosura come still
nearer to the Trilobites than the adult Limulus, and thus increase
the probability that there is a genetic connection between the two
groups.

It seems probable that the air-breathing Arachnida were
derived through Limulus-like ancestors from primitive Crustacea,
and that the tracheae were developed without genetic relationship
with those of the other air-breathing groups— perhaps as modifica-
tions of the pulmonary sacs, the latter having been originally
derived from gills like those of Limulus. That air-tubes can be
developed in air-breathing members of what are, fundamentally,
aquatic groups is shown by the case of certain terrestrial Isopoda
among the Crustacea (p. 580).

There is a very evident close relationship between the Myriapoda,
i.e., the Progoneata, and the Insecta. The Insects are more highly
specialised, and have their structure modified in adaptation to a

PHYLUM A&TH&OPODA

661

special mode of locomotion, but the resemblances in many respects
are very strong. One of the most striking points of difference
is the indefiniteness in the number of the segments in the Myriapoda,
and their constant and definite arrangement in the Insecta. The
well-defined thorax of the Insects is wanting in the Myriapods in
general, but certain of the segments following the head differ from
the rest in various respects, and might be looked upon as con-
stituting a thoracic region. The presence in both groups of a
sharply marked-off head bearing antennae and jaws is an important
point of resemblance ; so is the absence in both of the voluminous
" liver " of the Crustacea and Arachnida. The gap between the
two classes is narrowed by two converging groups — the Symphyla

Air- breaking
Arach nids

Insecta

Mynofjoda

Onycho

Pycnogonida
Crustacea

Xi[jliosura
Euryfjfenda

Primih've Arrhrofsods

Annelida

FIG. 571.

among the Myriapoda on the one hand, and the wingless — and in
other respects primitive — Apterygota among the Insecta on the
other.

While the Insecta thus appear to be nearly related to the Pro-
goneata, there are indications of relationship between the Opistho-
goneata and the Onychophora, and, through these, to the Chsetopoda.
The elongated, homonomously segmented body, the well-defined
head with its antennae, the occurrence of similar appendages on
all the body-segments, all point in this direction. Accordingly,
instead of placing the branchiate Arthropoda in one group and
all the air-breathing forms in another, and deriving the latter from
the former, we should probably express more correctly the affinities

VOL. i u u

662 ZOOLOGY SECT, xi

of the various groups of Arthropods by some such scheme as tL it
expressed in the diagram (Fig. 571).

Here an intermediate link between Annelida and the existn 3
Arthropoda is supposed to have been constituted by hypothetic *•
primitive forms from which Peripatus, the Insecta, and tl *
Myriapoda are supposed to have been evolved in the one direction,
and the Crustacea, Eurypterida, Xiphosura, and air-breathing
Arachnida in the other.

On account mainly of general resemblances to the Spiders, the
Pycnogonida have frequently been grouped with the Arachnida,
and attempts have been made to homologise their appendages
with those of the Spiders and Scorpions. There is at least one pair
more in the Pycnogonida ; and either the last pair would have to
be set down as corresponding to the vestigial first abdominal pair
of the ordinary Arachnida, or the ovigerous legs would have to be
reckoned, not as independent appendages, but as parts of the
second pair, a view for which there is some ground. A close
relationship with the Arachnida, however, cannot be traced, and
their affinities are perhaps best expressed, as in the diagram, by
connecting them with the Arachnid branch of the Arthropod tree
at a point below that at which the air-breathing forms had become
developed from forms allied to the Xiphosura.

The position of the Pentastomida is a matter of uncertainty.
In the absence of organs of respiration and excretion, the only
feature in the adult which distinctly points to arthropod affinities
is the striated character of the muscular tissue. The presence of
two pairs of legs in the larva, however, is sufficient to confirm the
view that they are aberrant and probably degenerate Arthropods,
while leaving it uncertain in what class they find their nearest
allies. The Tardigrada are still more aberrant in some respects.
They differ from Arthropods in general in the absence of external
segmentation in the adult state, in the simple unjointed character
of the appendages, in the absence of striation in the muscular
fibres, and in the absence of organs of respiration and circulation.
It is impossible to place them in any of the great classes, and they
are perhaps best looked upon as a special offset of the Arthropod
tree given off near the base.

i*

SECTION XII

PHYLUM MOLLUSCA

THE Mollusca, like the Arthropoda, form one of the chief divisions
of the animal kingdom, both as regards diversity of organisation
and number of genera and species. They are sharply distinguished
from Arthropods by the absence of segmentation, and by having,
as a rule, an exoskeleton in the form of a shell, usually external,
sometimes internal. An enumeration of the Classes of the Phylum
will serve to give some notion of its extent.

Class 1. — PELECYPODA, including the bivalved shell-fish, such
as Mussels, Cockles, Oysters, &c.

Class 2. — AMPHINEURA, including the Chitons and their allies.

Class 3. — GASTROPODA, including the univalved shell-fish, such
^as Periwinkles, Whelks, Snails, Slugs, &c.

Class 4. — SCAPHOPODA, including the Tooth-shells.

Class 5. — CEPHALOPODA, including the Cuttle-fishes, Squids,
Octopods, and Nautili.

CLASS I.— PELECYPODA.

1. EXAMPLE or THE CLASS — THE FRESH- WATER MUSSELS
(Anodonta and Unio).

Fresh-water Mussels are found in rivers and lakes in most parts
of the world. Anodonta cygnea, the Swan-mussel, is the commonest
species in England ; but the Pearl-mussel, Unio margartiifer, is
found in mountain streams, and other species of the same genus
are universally distributed.

The Mussel (Fig.^572) is enclosed ir* a brown shell formed of
two separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand, with the
valves slightly gaping, and in the narrow cleft thus formed a delicate,
semi-transparent substance (m.) is seen — the edge of the mantle
or pallium. The mantle really consists of separate halves or
lobes corresponding with the valves of the shell, but in the position
of rest the two lobes are so closely approximated as to appear
simply like a membrane uniting the valves. At one end, however,
the mantle projects between the valves in the form of two short

<*3 u u 2

664

ZOOLOGY

SECT.

tubes, one (ea\ sph.) smooth-walled, the other (in. sph.) beset with
delicate processes or fimbrice. By diffusing particles of carmine
or indigo in the water it can be seen that a current is always passing
in at the fimbriated tube, hence called the inhalant siphon, and out
at the smooth or exhalant siphon. Frequently a semi-transparent,
tongue-like body (ft.) is protruded between the valves at the opposite
side from the hinge and at the end furthest from the siphons :
this is the foot ; by its means the animal is able slowly to plough its
way through the sand or mud. When the Mussel is irritated the
foot and siphons are withdrawn and the valves tightly closed.
In a dead animal, on the other hand, the shell always gapes, and it
can then be seen that each valve is lined by the corresponding lobe
of the mantle, and that the exhalant siphon is formed by the union
of the lobes above and below it and is thus an actual tube ; but

tip a

FIG. 572.— Anodonta cygnea. The entire animal. A, from the left side ; 7?, from the
posterior end. d. p. a. dorsal pallial aperture ; ex. tph. exhalant siphon ; ft. foot ; in. sph.
inhalant siphon ; Iff. ligament ; m. mantle ; urn. umbo. (After Howes.)

that the boundary of the inhalant siphon facing the gape of the shell
is simply formed by the approximation of the mantle-lobes, so that
this tube is a temporary one.

The hinge of the shell is dorsal, the gape ventral, the end bearing
the siphons posterior, the end from which the foot is protrude*7
anterior : hence the valves and mantle-lobes are respective!}
right and left.

In a dead and gaping Mussel the general disposition of the parts
of the animal is readily seen. The main part of the body lies be-
tween the dorsal ends of the valves : it is produced in the middle
ventral line into the keel-like foot, and on each side, between
the foot and the corresponding mantle-lobe, are two delicate
striated plates, the gills (Figs. 576-578). Thus the whole animal
has been compared to a book, the back being represented by tkj
hinge, the covers by the valves, the fly-leaves by the mantle -lobe
the first two and the last two pages by the gills, and the remaind
of the leaves by the foot.

The shell. — When the body of the mussel is removed from

XII

PHYLUM MOLLUSCA

h.L

the shell the two valves are seen to be united, along a straight
hinge-line (Fig. 573, A, h.L), by a tough, elastic substance, the
hinge-ligament (Figs. 572 and 578, Ig.) passing transversely from
valve to valve. It is by the elasticity of this ligament that the shell
is opened : it is closed, as we shall see, by muscular action : hence
the mere relaxation of the muscles opens the shell. In Anodonta
the only junction between the two valves is afforded by the liga-
ment, but in
Unio each is pro-
duced into strong
projections and
ridges, the hinge-
teeth, separated

by grooves or 1 ' Ufct..X^*w

sockets, and so
arranged that
the teeth of one
valve fit into
the sockets of
the other.

The valves are
marked e x t e r-
nally by a series
of c'oncentric
lines "(Fig. 572)
parallel with the
free edge or gape,
and starting
from a swollen
knob or eleva-
tion, the umbo
(um.), situated
towards the an-
terior edge of
the hinge-line.
These lines are
lines of growth.
The shell is

thickest at the umbo, which represents the part first formed
in the young animal, and new layers are deposited under this
original portion, as secretions from the mantle. As the animal
grows each layer projects beyond its predecessor, and in this way
successive outcrops are produced giving rise to the markings in
question. In the region of the umbo the shell is usually more or
less eroded by the action of the carbonic acid in the water.

The inner surface of the shell also presents characteristic markings
(Fig. 573, A). Parallel with the gape and at a short distance from

FIG. 573. — Anodonta cygnea. A, interior of right valve ; B, the
animal removed from the shell, a. ad. anterior adductor or its
impression ; a. r. anterior retractor or its impression ; d. gl.
digestive gland, seen through mantle ; ex. sph. exhalant siphon ;
ft. foot ; gl. gills, seen through mantle ; h. 1. hinge-line ; in. sph.
inhalant siphon ; M. kidney, seen through mantle ; k. o. Keber's
organ, seen through mantle ; m. mantle ; p. ad. posterior
adductor or its impression ; pc. pericardium, seen through
mantle ; pi. 1. pallial line ; pi. m. pallial muscles ; p. r. posterior
retractor or its impression ; pro. protractor or its impression.

666

ZOOLOGY

SECT.

it is a delicate streak (pi. I.) caused by the insertion into the shell
of muscular fibres from the edge of the mantle : the streak is hence
called the pallial line. Beneath the anterior end of the hinge the
pallial line ends in an oval mark, the anterior adductor impression
(a. ad.), on to which is inserted one of the muscles which close the
shell. A similar but larger posterior adductor impression (p. ad.)
lies beneath the posterior end of the hinge. Two smaller markings
in close relation with the anterior adductor impression mark the
insertion of the anterior retractor (a. r.), and of the protractor (prc.)
of the foot : one connected with the posterior adductor impression
is that of the posterior retractor (p. r.). From all these im-
pressions faint converging lines can be traced to the umbo : they
mark the gradual shifting of the muscles during the growth of the

animal.

The shell consists of three
layers. Outside is a brown
horn-like layer, the peri-
ostracum (Fig. 574, prc.),
composed of conehiolin, a
substance allied in composi-
tion to chitin. Beneath this
is a prismatic layer (prs.)
formed of minute prisms of
calcium carbonate s e p a -
rated by thin layers of con-
chiolin ; and, lastly, forming
the. internal part of the
shell is the nacre (n.), or
" mother-of-pearl," formed
of alternate layers of car-
bonate of lime and con-
chiolin arranged parallel to
the surface. The perio-
stracum and the prismatic layer are secreted from the edge of the
mantle only, the pearly layer from the whole of its outer surface.
The hinge-ligament is continuous with the periostracum, and is to
be looked upon simply as a median uncalcified portion of the shell,
which is therefore, in strictness, a single continuous structure.

By the removal of the shell the body of the animal (Fig. 573, B)
is seen to be elongated from before .backwards, narrow from side
to side, produced on each side into a mantle-lobe (m.) and con-
tinued ventrally into a keel-like visceral mass (Fig. 575, v.m.), which
passes below and in front into the foot (ft.). Thus each valve of
the shell is in contact with the dorso-lateral region of the body
of its own side together with the corresponding mantle-lobe, and
it is from the epithelium (Fig. 574, ep.l) covering these parts that
the shell is formed as a cuticular secretion. The whole space

FIG. 574. — Vertical section of shell aiid mantle of
Anodonta. c.t. connective-tissue layer of
mantle ; ep.l, its outer epithelium ; ep. 2, its
inner epithelium ; n. nacreous layer of shell ;
prc. periostracum ; prs. prismatic layer. (After
Glaus.)

XII

PHYLUM MOLLUSCA

667

between the two mantle-lobes, containing the gills, visceral mass,
and foot, is called the mantle-cavity.

A single layer of epithelial cells covers the whole external sur-
face, i.e. the body proper, both surfaces of the mantle, the gills,
and foot ; that of the gills and of the inner surface of the mantle
(Fig. 574, ep. .?) is ciliated. Beneath the epidermis come connective
and muscular tissue, which occupy nearly the whole of the interior
of the body not taken up by the viscera, the ccelome being, as we
shall see, much reduced. The muscles are all unstriped, and are
arranged in distinct bands or sheets, many of them very large and
conspicuous. The largest are the anterior and posterior adductors
(Figs. 573 and 575, a. ad., p. ad.), great cylindrical muscles, pass-
ing transversely across the, body andjnserted at either end on to

I. m, rob

FIG. 575. — Anodonta cygnea. the animal with most of the left mantle-lobe removed,
a. anus ; a. ad. anterior adductor ; a. r. anterior retractor ; au. auricle ; d. p. a. dorsal
pallial aperture ; ex. sph. exhalant siphonr;/«. foot ; in. sph. inhalant siphon ; kd. kidney ;
I. ext. gl. left external gill-lamina ; I. ext. pip. left external labial palp : I. int. gl. left internal
gill-lamina ; /. int. pip. left internal labial palp ; 1. m. cut edge of left mantle-lobe ; mth.
mouth ; p. ad. posterior adductor ; pc. pericardium ; p. r. posterior retractor ; pro. pro-
tractor ; ret. rectum ; r. m. right mantle-lobe ; v. ventricle ; v. m. visceral mass.

the valves of the shell, which are approximated by their con-
traction. Two muscles of much smaller size pass from the foot
to the shell, which they serve to draw back ; they are the anterior
(a. r.) and posterior (p. r.) retractors. A third muscle (prc.),
inserted on to the shell, close to the anterior adductor, has
its fibres spread fan-wise over the visceral mass, which it serves to
compress, thus forcing out the foot and acting as a protractor of
that organ. The substance of the foot itself consists of a complex
mass of fibres, the intrinsic muscles of the foot, many of which also
act as protractors. Lastly, all along the border of the mantle is a
row of delicate pallial muscles (Fig. 573, pi. m.), which, by their
insertion into the shell, give rise to the pallial line already seen.

The ccelome is reduced to a single ovoidal chamber, the peri-
cardium (Fig. 575, pc.), lying in the dorsal region of the body and

668

ZOOLOGY

SECT.

containing the heart and part of the intestine ; it is lined by coelomic
epithelium. In the remainder of the body the space between the
ectoderm and the viscera is filled by the muscles and connective-
tissue.

Digestive organs. — The mouth (Fig. 575, mth.) lies in the
middle line, just below the anterior adductor. On each side of
it are two triangular flaps, the internal and external labial palps
(1. int. pip., I. ext. pip.) ; the external palps unite with one another
in front of the mouth, forming an upper lip ; the internal palps are
similarly united behind the mouth, forming a lower lip ; both are
ciliated externally. The mouth leads by a short gullet (Fig. 576,
gul.) into a large stomach (st.), which receives the ducts (d.d.) of a

II

I P*o ^

rcl

d.p*

in.sph

int I

gon

FIG. 576. — Anodonta cygnea. Dissection from the left side. a. anus ; a. ad. anterior
adductor ; a. ao. anterior aorta ; a. v. ap. auricula-ventricular aperture ; bl. urinary bladder ;
e. pi. gn. cerebro-pleural ganglion ; d.d. duct of digestive gland ; d. gl. digestive gland ;
d. p. a. dorsal pallia! aperture ; ex. sph. exhalant siphon : ft. foot ; g. ap. genital aperture ;
gon. gonad ; gul. gullet ; i. 1. j. inter-lamellar junction : in. sph. inhalant siphon ; int. intes-
tine ; kd. kidney ; ra. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ;
pc. pericardium ; pd. gn. pedal ganglion ; r. ap. renal aperture ; r. au. right auricle ;
ret. rectum ; r. p a. reno-pericardial aperture; st. stomach ; ty. typhlosole ; v. ventricle ;
v. gn. visceral ganglion ; w. t. water tubes ; x. aperture between right and left bladders.

pair of irregular, dark-brown digestive glands (d.gl.). The intestine
(int.) is given off from the posterior end of the stomach, descends
into the visceral mass, where it is coiled upon itself, then ascends
parallel to its first portion, turns sharply backwards, and proceeds,
as the rectum (ret.), through the pericardium — where it traverses the
ventricle of the heart — and above the posterior adductor, finally
discharging by the anus (a.) into the exhalant siphon, or cloaca.
The wall of the rectum is produced into a longitudinal ridge, or
typhlosole (ty.), like that of the Earthworm, and two similar ridges
begin in the stomach and are continued into the first portion of
the intestine. The stomach contains, under certain conditions,
a gelatinous rod, the crystalline style.

XII

PHYLUM MOLLUSCA

669

On each side is a single gill or ctenidium composed of two
plates or lamina, an inner and an outer. We have thus right outer
and inner gill-laminae, and left outer and inner gill-laminae (Fig.
575, 1. ext. gl.y 1. int. gl.). Seen from the surface, each lamina presents
a delicate double striation, being marked by faint lines running
parallel with, and by more pronounced lines running at right
angles to, the long axis of the organ. Moreover, each lamina is

-*^38>" * c

*sa£^y* ~-

i^

FIG. 577. — Anodonta cygnea. A, transverse section of outer, and B, of inner gill-lamina ;
C, diagram of gill-structure ; Z), transverse section of gill filament, b. c. blood-corpuscle ;
6. w. blood-vessels ; ch. chitin ; /. branchial filaments ; ep epithelium ; i. f. j. inter-
filamentar junction ; i. 1. inner lamella ; i. 1. j. inter-lamellar junction ; o. I. outer lamella ;
os. external ostium ; os' internal ostium ; r, chitinuus rods ; 10. t. water-tubes. (A, B, and D
after Peck.)

double, being formed of two similar plates, the inner and outer
lamella, united with one another along the anterior, ventral, and
posterior edges of the lamina, but free dorsally. The lamina has
thus the form of a long and extremely narrow bag open above
(Figs. 576, 577, and 578) : its cavity is subdivided by vertical bars of
tissue, the inter-lamellar junctions (i. l.j.), which extend between the
two lamellae, and divide the intervening space into distinct compart-
VOL. T u u*

670

ZOOLOGY

SECT.

ments or water-tubes (w.t.), closed ventrally, but freely open along
the dorsal edge of the gill. The vertical striation of the laminae is
due to the fact that each lamella is made up of a number of close -
set gill-filaments (/.) : the longitudinal striation to the circumstance
that these filaments are connected by horizontal bars, the inter-

Ult

FIG. 578.— Anodonta cygnea. Three transverse sections, A, B, C, au. auricle ; bl. urinary
bladder ; ext. gl. external gill-lamina ; ft. foot ; U.i. inter-lamellar junction ; int. intestine ;
int. gl. internal gill-lamina ; kd. kidney ; k.o. Keber's organ ; Ig. ligament ; m. mantle ; p.ad.
posterior adductor ; pc. pericardium ; ret. rectum ; s.br.c. supra-branchial chamber ; sh.
shell ; ty. typhlosole ; v. ventricle ; re. vena cava ; v. gn. visceral ganglion. (After Howes,
slightly altered.)

filamentar junctions (i.f.j.). At the thin free or ventral edge of the
lamina the filaments of the two lamellae are continuous with one
another, so that each lamina has actually a single set of V-shaped
filaments, the outer limbs of which go to form the outer lamella,
their inner limbs the inner lamella. Between the filaments, and

xii PHYLUM MOLLUSCA 671

bounded above and below by the inter-filamentar junctions, are
minute apertures, or ostia (os.), which lead from the mantle-cavity
through a more or less irregular series of cavities into the interior
of the water-tubes. The filaments themselves are supported by
chitinous rods (r.), and are covered with ciliated epithelium, the
large cilia (Fig. 577, D) of which produce a current running from
the exterior through the ostia into the water-tubes, and finally
escaping by the wide dorsal apertures of the latter. The whole
organ is traversed by blood-vessels (b. v.).

The mode of attachment of the gills presents certain features of
importance. The outer lamella of the outer lamina is attached
along its whole length to the mantle (Fig. 578) : the inner lamella
of the outer and the outer lamella of the inner lamina are attached
together to the sides of the visceral mass a little below the origin
of the mantle : the inner lamella of the inner lamina is also attached
to the visceral mass in front (A), but is free further back (B). The
gills are longer than the visceral mass, and project behind it, below
the posterior adductor (C), as far as the posterior edge of the mantle :
in this region the inner lamellae of the right and left inner laminae
are united with one another, and the dorsal edges of all four laminae
constitute a horizontal partition between the pallial cavity below
'and the exhalant chamber or cloaca above. Owing to this arrange-
ment it will be seen that the water-tubes all open dorsally into a
supra-branchial chamber (s. br. c.) continuous posteriorly with the
cloaca and thus opening on the exterior by the exhalant siphon.

The physiological importance of the gills will now be obvious.
By the action of their cilia a current is produced which sets in
through the inhalant siphon into the pallial cavity, through the
ostia into the water-tubes, into the supra-branchial chamber, and
out at the exhalant siphon. The in-going current carries with it
not only oxygen for the aeration of the blood, but also Diatoms,
Infusoria and other microscopic organisms, which are swept into
the mouth by the cilia covering the labial palps. The out-going
current carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in producing
the food-current is of more importance than their respiratory
function, which they share with the mantle.

The excretory organs are the kidneys or urocceles (portions
of the true ccslome), situated one on each side of the body just
below the pericardium. Each consists of two parts, a brown
spongy glandular portion or kidney (Fig. 576, kd.), and a thin-
walled non-glandular part or urinary bladder (bl.), which com-
municates with its fellow anteriorly by a large oval aperture (x).
The two parts lie parallel to one another, the bladder being placed
dorsally and immediately below the floor of the pericardium :v
they communicate with one another posteriorly, while in front each
glandular part opens into the pericardium (r. p. ap.), and the bladder

072

ZOOLOGY

SECT.

on to the exterior by a minute aperture (r. ap.), situated between
the inner lamina of the gill and the visceral mass. Thus the whole
organ, often called, after its discoverer, the organ of Bojanus, is
simply a tube bent upon itself, opening at one end into the coelome,
and at the other on the external surface of the body : it has thus
the normal relations of a nephridium, but is of coelomic not ecto-
dermal derivation. The epithelium of the bladder is ciliated, and
produces an outward current.

An excretory function is also discharged by a large glandular
mass of reddish-brown colour, called the pericardial gland or Keber's
organ (Fig. 578, B, k.o.). It lies in the anterior region of the body
just in front of the pericardium, into which it discharges.

CLfl.

FIG. 579. — Diagram of the circulatory system of Anodonta. Vessels containing aerated
blood red, non-aerated blue, af.br.v. afferent branchial veins ; ao. aorta : art. 1, artery to
mantle ; art. 2, artery to body generally ; au. auricle ; ef.br.v. efferent branchial veins ;
nph.v. nephridial veins ; pc. pericardium ; v. ventricle ; v. c. vena cava. The arrows^ show
the direction of the current.

The circulatory system is well developed. The heart lies in
the pericardium and consists of a single ventricle (Figs. 576, 578,
and 579, v.) and of right and left auricles (au.). The ventricle is
a muscular chamber which has the peculiarity of surrounding the
rectum (Figs. 576 and 578, B) : the auricles are thin- walled chambers
communicating with the ventricle by valvular apertures opening
towards the latter. From each end of the ventricle an artery is
given off, the anterior aorta (Fig. 576, a. ao.) passing above, the
posterior aorta (p. ao.) below the rectum. From the aortse the
blood passes into arteries (Fig. 579, art. I art.%) which ramify all over
the body, finally forming an extensive network of vessels, many

XII

PHYLUM MOLLUSCA

673

ot

of which are devoid of proper walls and have therefore the nature
of sinuses. The returning blood passes into a large longitudinal
vein, the vena cava (v. c.), placed between the nephridia, whence
it is taken to the kidneys themselves (nph. v.), thence by afferent
branchial veins (af. br. v.) to the gills, and is finally returned by
efferent branchial veins (ef. br. v.) to the auricles. The mantle has
a very extensive blood supply, and, as mentioned above, probably
acts as the chief respiratory organ : its blood (art.l) is returned
directly to the auricles without passing through either the kidneys
or the gills. The blood is colourless and contains leucocytes.

The nervous system is formed on a type quite different from
anything we have yet met with. On each side of the gullet is a
small cerebro-pleural ganglion (Fig. 576, c. pi. gn.) united with its
fellow of the opposite side by a nerve-cord, the cerebral commissure,
passing above the gullet. Each
cerebro-pleural ganglion also
gives off a cord, the cerebro-
pedal connective, which passes
downwards and backwards to a
pedal ganglion (pd. gn.) situated
at the junction of the visceral
mass with the foot : the two
pedal ganglia are so closely
united as to form a single bilobed
mass. From each cerebro-
pleural ganglion there further
proceeds a long cerebro-visceral
connective which passes directly
backwards, through the kidney,
and ends in a visceral ganglion
(v. gn.) placed on the ventral side
of the posterior adductor muscle. The visceral, like the pedal
ganglia, are fused together. The cerebro-pleural ganglia supply
the labial palps and the anterior part of the mantle ; the pedal
the foot and its muscles ; the visceral the enteric canal, heart, gills,
and posterior portion of the mantle.

It will be seen that the cerebral commissures and cerebro-pedal
connectives, together with the cerebro-pleural and pedal ganglia,
form a nerve-ring which surrounds the gullet : the cerebro-pleural
ganglia may be looked upon as a supra-oesophageal nerve mass
corresponding in part with the brain of Annelids and Arthropods,
and the pedal ganglia as an infra-cesophageal mass representing the
ventral nerve-cord.

Sensory organs' are poorly developed, as might be expected in
an animal of such sedentary habits. In connection with each
visceral ganglion is a patch of sensory epithelium forming the
so-called olfactory organ or, better, ospkradium, the function of

FIG. 580.— Statocyst of Anodonta. a, b,c, c',
cenular layers surrounding the statocyst ;
ot- statolith. (From the Cambridge Natural
History.)

674

ZOOLOGY

SECT.

mes

which is apparently to test the purity of the water entering by the
respiratory current. Close to each pedal ganglion a minute statocyst
(" otocyst ") (Fig. 580) is sometimes found, the nerve of which is
said to spring from the cerebro-pedal connective, being probably
derived from the cerebral ganglion. Sensory cells — probably
tactile — also occur round the edge of the mantle, and especially
on the fimbriae of the inhalant siphon.

Reproductive organs. — The sexes are separate. The gonads
(Fig. 576, gon.) are large, paired, racemose glands, occupying a
considerable portion of the visceral mass amongst the coils of
the intestine : the testis is white, the ovary reddish. The gonad
of each side has a short duct which opens (g. ap.) on the surface
of the visceral mass just in front of the renal aperture.
In ^the^breeding season the _ eggs, extruded from the genital

rk aperture, pass into the

mes supra-branchial chamber

and so to the cloaca.
There, in all probability,
they are impregnated by
sperms introduced with
the respiratory current.
The oosperms are then
passed into the cavities
of the outer gill-laminae,
which they distend enor-
mously. Thus the outer
gill-laminae act as brood-
pouches, and in them
the embryo develops

TIG. 581.— Early embryo of Anodonta. eh, vitelline into the peculiar larval

membrane ; ent. archenteron ; m. micropyle ; mes. meso- r -i

derm ; rk, polar cells ; s&, shell-gland ; sz, lateral cells; lOrm presently to DC

w, cilia. (From Korschelt and Heider's Embryology.) described.

Development. — Segmentation of the oosperm is complete, but
unequal. A gastrula is formed by the invagination of the mega-
meres into the micromeres, but the archenteron (Fig. 581, ent.)
thus formed is quite small and insignificant, and has no physiological
importance until a late period of larval life. Certain of the cells of the
gastrula are budded off into the blastocosle, where they accumulate
and form the mesoderm (mes.). At about the same time a deep
invagination (sd.2) is formed, which might easily be mistaken for
the archenteron, but is really a very characteristic molluscan organ,
the shell-gland : it marks the dorsal surface of the embryo. The
posterior end is distinguished by a tuft of long cilia.

The shell-gland becomes converted into a plate of long, cylin-
drical cells (Fig. 582, sd.), from which an impaired shell (s.) is
secreted. This is replaced before long by a bivalved shell of
triangular form, its ventral angles produced into incurved hooks

XII

PHYLUM MOLLUSCA

beset with spines (Fig. 583, sJi). At the same time the body of
the larva, which has hitherto been an undivided mass projecting
between the two valves of the shell, becomes cleft from below
upwards, and thus divided into a single dorsally-placed body

B.

mes

FIG. 582. — Two later stages in the development of Anodonta. ent. archenteron ; mes. meso-
derm ; «. shell ; sd. shell-gland ; so. sense-organs ; u\ cilia. (From Korschelt and Heider's
Embryology.)

proper, and paired — right and left — mantle-lobes. Upon the latter
peculiar brush-like sense-organs make their appearance, and on
the ventral surface of the body is formed a glandular pouch, which
secretes a long thread, the provisional byssus (/). The mesoderm

R.

B.

So.

_-sh.

so.

cT,"K> "— ' ^~^<

^sm.

IV. "'"

FIG. 583. — A, advanced embryo of Anodonta. B, free glochidium. /, provisional byssus
s. shell ; sh. hooks : sm. adductor muscle ; so. sense-organs ; w. cilia. (From Korschelt and
Heider's Embryology.)

cells give rise to a single immense adductor muscle (sm), the fibres
of which extend from valve to valve.

The larva is now called a glochidium : it remains in the brood-
pouch, nourished by a secretion from the walls of the latter, and
entangled with its fellows by means of the byssus. At this stage
the outer gill-lamina appears as if stuffed full of closely
aggregated sand-grains. Before long the larvae are ejected

676

ZOOLOGY

SECT.

B.

IV.

f.

through the exhalant siphon, and if they happen to come in
contact with a passing Stickleback or other fresh-water fish,
fix themselves on some part of its body by means of the hooked
valves. The glochidia of Unio usually attach themselves to the
gills, those of Anodonta to the skin or the fins. In this position

they become encysted by
an overgrowth of the
skin or mucous mem-
brane of the hosty and
are nourished by its
juices absorbed through
processes of the mantle.
They thus lead a truly
ectoparasitic existence
for about ten weeks.

While in this condi-
tion a metamorphosis
takes place. The pro-
visional byssus and
sense-organs disappear
(Fig. 584), and immedi-
ately posterior to the
former an invagination,
the stomodceum (m), is
formed, and soon com-
municates with the arch-
enteron. The posterior
end of this cavity is in
close contact with the
ectoderm, so that the
anus is formed by a
simple process of rupture,
and without the develop-
__^____ ment of a proctodseum.

FIG. 584.— Three stages in the metamorphosis of Ano- The foot (fu) arises as a

donta. d. enteric canal ; /. provisional byssus ; fu. rn(*r\\an vpnfral plpvatirvn

foot ; g. lateral pits ; *. rudiments of gills ; m. mouth ; median

sh. shell ; sm. adductor muscle ; so. sense-organs ; w. behind the mouth, and On
cilia. (From Korschelt and Holder's Embryology.) , . ^ . . ,

each side 01 it two

papillae (Jc) appear, the rudiments of the gills. The larva is
now fitted for free existence ; it drops from its host, and gradually
assumes the adult form and mode of life.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Pelecypoda are bilaterally symmetrical, compressed Molluscs,
in which the mantle consists of paired right and left lobes, secreting
a bivalved calcareous shell. There is no distinct head. The
ventral region of the body is differentiated into a muscular foot,

fu.

Xtl PHYLUM MOLLUSCA 6?7

which is usually ploughshare- or tongue-shaped : in some cases
there is a byssus-gland posterior to the foot, which secretes a mass of
horny fibres, the byssus, by which the animal may be permanently
attached. There are two gills or ctenidia, one on each side : the
chief function of the gills is the production of a respiratory and food-
carrying current of water. The body is covered by a one-layered
epidermis, which is ciliated on the gills and on the inner surface of
the mantle. The muscular system is well-developed, the largest
muscles being either one or 'two adductors, which close the shell,
and several bands connected with the foot and byssus ; the
muscles are usually unstriped. The ccelome is reduced to a
dorsally-placed pericardium. The mouth is bounded by two pairs
of flat, triangular tentacles or labial palps, the cilia of which
serve to carry food-particles to the mouth : the enteric canal is
coiled, and is formed mainly from the mesenteron : there are large
paired digestive glands : the rectum passes through the pericardium,
usually perforates the ventricle, and ends above the posterior
adductor. The heart is contained within the pericardium, and
consists of a median ventricle and of right and left auricles : the
blood, which is usually colourless, is taken from the ventricle to
the body by one or two aortse, and is returned partly directly,
partly by way of the renal organs and gills, to the auricles. The
renal organs are a single pair of coelomic kidneys, which
usually open at one end into the pericardium, at the other on the
exterior. The nervous system consists typically of four pairs of
ganglia called respectively cerebral, pleural, pedal, and visceral :
the cerebral and pleural of each side are usually fused into a single
cerebro-pleural ganglion. The chief sense-organs are statocysts
and osphradia or water-testing organs. The sexes are separate or
united : there are no accessory organs of reproduction. Develop-
ment is accompanied by a metamorphosis, which usually includes
a trochophore stage.

The classification of the Pelecypoda is as follows : —

OEDEB 1. — PROTOBRANCHIA.

Pelecypoda in which the gills take the form of a single pair of
plume-like organs or ctenidia, each with two rows of flattened
gill-filaments. The foot is not compressed, but has a flattened
ventral surface or sole upon which the animal creeps. There are
two adductor muscles.

This group includes only four genera — Nucula (Fig. 596), Yoldia,
Leda, and Solenomya.

ORDER 2. — FILIBRANCHIA.

Pelecypoda in which there is a pair of plate-like gills formed
of distinct V-shaped filaments : interfilamentar junctions are either

678 ZOOLOGY SHOT.

absent or formed by groups of interlocking cilia : interlamellar
junctions are either absent or non- vascular. As a rule there are
two adductor muscles, but the anterior may be greatly reduced or
absent.

Including the Noah's ark shell (Area), Sea-mussel (Mytilus,
Fig. 595), Anomia, Trigonia, &c.

ORDER 3. — PSEUDO-LAMELLIBRANCHIA.

Pelecypoda in which the gills are plaited so as to present
vertical folds : the interfilamentar junctions may be ciliary or
vascular : the interlamellar junctions vascular or non-vascular.
There is a single large (posterior) adductor muscle. The shell is
frequently inequivalve.

Including the Scallop (Pecten, Fig. 585), Oyster (Ostrea), Pearl
Oyster (Meleagrina), Lima, Pinna, &c.

ORDER 4. — EULAMELLIBRANCHIA.

Pelecypoda in which the gill-filaments are united by vascular
interfilamentar and interlamellar junctions, firm, basket-like gills
being the result : the gills may be smooth or plaited. There are
two equal-sized adductor muscles.

Sub-order a. — Integripalliata.

Eulamellibranchia in which the siphons are small or absent
and the pallial line on the shell is entire.
Including the Fresh- water mussels (Anodonta and Unio).

Sub-order b. — Sinupalliata.

Eulamellibranchia in which the siphons are of considerable
size, and the pallial line is inflected to form a sinus.

Including the Cockle (Cardium), My a, Pholas, Teredo (Ship-
worm), Aspergillum, &c. (Figs. 587-590).

ORDER 5. — SEPTIBRANCHIA.

Pelecypoda in which the gills are reduced to a horizontal
muscular partition. There are two adductor muscles.
Including Poromya, Cuspidaria, &c.

Systematic Position of the Examples.

Anodonta and Unio are two genera belonging to the family
Unionidce, sub-order Integripalliata, order Eulamellibranchia.

Their complex basket-like gills are alone sufficient to place
them among the Eulamellibranchia. The incomplete ventral
siphon and the correlated entire pallial line (see p. 666) indicate
their position among the Integripalliata. The regular shell, with

XII

PHYLUM MOLLUSCA

679

thick brown periostracum and large external ligament, the elon-
gated branchial or inhalant aperture, the long, compressed foot,
and the absence of a byssus, place them among the Unionidae.
Anodonta is distinguished from Unio by the absence of hinge-
teeth.

3. GENERAL ORGANISATION.

The most important variations in structure in the present class
are connected with modifications of the gills, the foot, the muscular
system, and the siphons. With the structure of the muscles and
of the siphons are correlated important variations in the shell
which are of great systematic value, especially in cases where, as
with fossils, the shell is the only part available for examination.

XIV

-VIII

,

FIG. 585. — Anatomy of Pecton. I, palpi ; II, foot ; III, aperture of gonad into kidney ; IV
external renal aperture ; V, male, and VI, female portion of gonad ; VII, pallial eye ;
VIII, visceral ganglion ; VIII', gill ; IX, anus ; X, striated portion of adductor ; XI,
smooth portion ; XII, retractor of foot ; XIII, heart ; XIV, liver ; XV, stomach. (From
Pelseneer's Mollusques.)

In all the Protobranchia, some of the Filibranchia, such as
Area, and all the Eulamellibranchia and Septibranchia, there
are two almost equal-sized adductor muscles, as in Anodonta.
In many Filibranchs, such as the common sea-mussel (Mytilus
edulis), the anterior adductor becomes greatly reduced and the
posterior correspondingly enlarged ; and in another species of the
same genus (M. latus) the anterior adductor has completely
atrophied, the function of closing the shell being performed by the

ZOOLOGY

sisc*.

great posterior adductor alone. In Anomia and in the Pseudo-
lamellibranchs there is a single immense adductor (Fig. 585, X, XI)
placed nearly in the middle of the greatly shortened body, and
known to represent the posterior adductor — both from the fact that
the rectum passes over it, and from the circumstance that, in the
embryo Oyster, two adductors are present, the anterior of which
atrophies, while the posterior enlarges to form the single muscle
of the adult.

These peculiarities in the muscular system bear their mark
upon the shell, in which impressions corresponding to the

FIG. 586.— Left valves of A, Mya B, Modiola; C, Vulsella. The uppef dotted line
passes through the hinge-lines, the lower connects the anterior and posterior adductor
muscles. (From the Cambridge Natural History.)

adductors are clearly marked on the inner surface (Fig. 586). The
whole class is, in fact, frequently classified on this basis, species
with equal-sized adductors (Protobranchs, some Filibranchs, and
all Eulamellibranchs and Septibranchs) being called Isomyaria (A),
those with a large posterior and a reduced anterior adductor (most
Filibranchs) Heteromyaria (B), and those with large centrally
placed posterior and no anterior adductor (Pseudolamellibranchs
and Anomia among Filibranchs) Monomyaria (C).

In many forms, such as Nucula (Fig. 596), Ostrea, &c., the right
and left mantle-lobes are quite free from each other, so that there

are no siphons.
In Anodonta and
Unio, as we
have seen, the two
lobes unite so as
to enclose a
dorsal or exhalant
siphon, a ventral
or inhalant siphon
being forme d

FIG. 587 — Cardium edule. A, exhalant siphon ; B, inhalant

siphon ; F, foot. (From the Cambridge Natural History.) sition of the lobes

ventrally. In such cases the pallial muscles in their neighbourhood
act as retractors of the short and imperfect tubes thus formed. In
other species a second concrescence of the mantle-lobes takes place
so as to convert the inhalant siphon into an actual circumscribed

XII

PHYLUM MOLLUSCA

681

aperture or short tube. In the Sinupalliata the two siphons are

prolonged into distinct muscular tubes (Fig. 587, A, B) which, in

the position of extension, project beyond the posterior margin of the

shell and may even be considerably longer than the body. Under

these circumstances the posterior pallia! muscles become enlarged

to form retractors of the siphons, and the portion of the pallial

line from which they arise is,

as it were, pushed forwards

so as to form a bay or pallial

sinus (Fig. 588, p.s). Thus

the shells of species with well-

developed siphons are sinupalliate,

or have an indented pallial line,

while those with small or no

siphons are integripalliate, or have

an entire pallial line. The larger

the siphons the stronger are their

muscles and the deeper is the

pallial sinus : when very large

they cannot be completely re-

tracted, and the posterior border

of the shell then gapes perman-

ently. Ihe Siphons may be

separate (Fig. 589) or united
(Fig. 590). They are specially adapted for species of burrowing
habits, which are able to remain buried in the mud or sand, only
the ends of the siphons being exposed for the supply of aerated
water and food, and even these can be instantly withdrawn in the
event of danger.

In addition to their union posteriorly to form the siphons, the
mantle-lobes may concresce to a greater or less extent along their

FIG. 588. — Venus gnidia, inner surface of
left valve, al, anterior lateral tooth ; am,
anterior adductor impression ; c, cardinal
teeth ; I. ligament ; lu. lunule ; p, pallial

Cambridge

sinus ; u. umbo. (From

FIG. 589.— Scrobicularia piperata, in its natural position, partly buried in sand. Am,
exhalant siphon ; B, inhalant siphon. (From the Cambridge Natural History.)

ventral border (Fig. 591), forming a more or less tubular invest-
ment for the body, and leaving an anterior pedal aperture for the
protrusion of the foot. Their anterior portions may also be united V
to form a sort of hood.

To return to the shell, the muscular impressions and the pallial
line on which have already been referred to. As a general rule
the right and left valves are alike, or nearly so, the shell being

VOL. I XX

682

ZOOLOGY

SECT.

therefore equivalve. Each valve is inequilateral, being divided into
unequal portions by a line drawn from the umbo to the gape. It
will be remembered that in the Brachiopoda, the only other class
of bivalved animals, the precise opposite is the case, the shell being
equilateral and inequivalved. Some Pseudo-
lamellibranchs are, however, nearly equilateral
and markedly inequivalved, such as the scallop
(Pecten), and the inequivalve character is still
more marked in the oyster, in which the right
valve is deeply concavo-convex and permanently
attached to a rock, while the [left is flat and
forms a sort of lid. This condition of things
reaches its maximum in the extinct Hippurites
(Fig. 592, B), in which the left valve has the
form of a long tube closed at one end by
the flat lid-like right valve. In the extinct
Requienia (A) the left valve is spirally coiled,
so that it resembles a snail-shell, and its aper-
ture is closed by the flat lid-like right valve :
in Diceras, also extinct, both valves are coiled.
The hinge-teeth (Fig. 588) vary greatly in
form and size or may be absent altogether : the
hinge-ligament is usually band-like, but in
Pecten takes the form of a cylindrical cord.
The variations in form, ornamentation, colour,
&c., among the many thousand known species
of shell are too numerous to mention ; but
reference must be made to peculiar modifica-
exhaiant siphon, the tions found in certain burrowing forms. In

two united at SS. ~, 7 . , , . ,& ,

(From the Cambridge Pholas, a siphonate genus which burrows in

stone, the shell is weak and brittle, and

additional calcareous pieces are developed between the two valves.

In Teredo (Fig. 593), the so-called Ship-worm, which causes great

FIG. 590. — Solecurtus
strigillatus. s. a/,
inhalant siphon, s. ef,

FIG. 591. — Diagram illustrating the various degrees of union of the mantle-lobes, b.o, byssal
aperture ; /. foot ; s. a, exhalant siphon ; s. b, inhalant siphon ; 1, first point of union
between siphons ; 2, second, between inhalant siphon and foot ; 3, third, between byssal
aperture and foot. (From the Cambridge Natural History.)

XII

PHYLUM MOLLUSCA

683

destruction by boring into piles, ships' timbers, &c., the valves (V.)
remain very small and weak but movable, and the general surface
of the mantle secretes a continuous shelly tube which lines the
burrow. In Aspergillum (Fig. 594), which lives buried in sand,
there is a similar but wider calcareous
tube, with which the valves are com-
pletely fused, and the anterior end of
the tube which appears above the sur-
face of the sand is closed by a plate
perforated with numerous holes like
the rose of a watering-pot. The larval
shell is sometimes, though not always,
distinguishable at the apex of each
valve in the Pelecypoda in general.
In Niwula, Area, &c., the foot

FIG. 592. — A, Requienia ammonea ; B, Hip-
purites cornu-vaccinum. a, right
valve ; /, point of fixation. (From the Cam-
bridge Natural History.)

(Fig. 596, /.) presents what may be con-
sidered as its most primitive form,
having a flat ventral surface or sole
upon which the animal creeps. Far
more common is the ploughshare-like
form we are already familiar with in
Anodonta and Unio, adapted for slowly
making its way through sand or mud.
In a few forms, e.g. Trigonia and Cardium
(Fig. 587), it is bent upon itself and is
capable of being suddenly straightened so
as to act as a leaping organ : in Mytilus it is cylindrical (Fig. 595, F) :
in the Oyster it is absent. In addition to the anterior and posterior
retractors and a pair of protractors, the foot is sometimes provided
with a levator muscle, particularly well developed in Nucula and
its allies.

x x 2

FIG. 593.— Teredo navalis, in
a piece of timber. P, pallets
(small calcareous plates support-
ing the siphons) ; SS. siphons ;
T, tube ; V, valve of shell.
(From the Cambridge Natural
History.)

684

ZOOLOGY

SECT.

FIG. 594. — Aspcrgillum.

(After Sowerby.)

FIG. 595.— Mytilus e dulls, attached by byssus (By.)
to a piece of wood. F, foot; S, exhalant siphon.
(From the Cambridge Natural History.)

OLCt

If,

pap

FIG. 596. — Adult specimen of KTucula delphinodonta, represented as seen from the right
side. Reconstructed to show internal organs. Fully grown specimens are 4 mm. long.
aa. anterior adductor muscle ; bg. byssal gland ; eg. cerebral ganglion ; /. foot ; g. gill ;
h. heart ; int. intestine ; lp. labial palp ; ess. oesophagus ; ot. statocyst ; pa. posterior ad-
ductor muscle ; pap. palp-appendage ; pg. pedal ganglion ; sio. stomach ; vg. visceral gan-
glion. (After Drew.)

XII

PHYLUM MOLLUSCA

685

Immediately posterior to the foot a byssus-gland is frequently
found : it secretes a silky substance in the form of threads which
serve to anchor the animal permanently or temporarily. It is by
means of the byssus that the Sea^-mussel (Mytilus) is attached to
the rocks (Fig. 595, By) : in Pinna the threads are fine enough to
be woven in a fabric. In Lima the threads of the byssus are spun
into a kind of nest in which the animals lie protected, and in species

FIG. 597. — Half transverse sections of various Pelecypoda to show the chief kinds of gill.
A, Nucula ; E, Amusium ; C, Area ; D, Mytilus ; E, Anodonta ; F, Poromya.

a. aperture in branchial septum ; b. v. bk^d-vessel ; ft. foot ; t. /. inner row of filaments ;
i. g. inner lamina ; i. I. inner lamella ; i. I. j. interlamellar junctions ; m. mantle ; o. f. outer
row of filaments ; o. g. outer lamina ; o. I. outer lamella ; sep. branchial septum. (Modified
from Korschelt and Heider, and Lang.)

of Modiola similar modifications of the byssus occur. In such
forms as Mytilus the muscles which ordinarily serve to retract the
foot are inserted mainly into the byssus : the latter being fixed,
they serve to rotate the animal in various directions, or, in other
words, act as adjusters and also as retractors of the byssus. It
must be borne in mind that the definite byssus just described is not
homologous with the provisional byssus of Anodonta (p. 675)
which lies in front of the mouth.

686

ZOOLOGY

SECT.

The gills or ctenidia are two in number, right and left. Each
consists of a horizontal axis bearing two rows of filaments, outer
and inner, which are outgrowths from it. In the Protobranchia
(e.g. Nucula) the filaments are short, compressed, and free from one
another (Figs. 596, g, and 597, A). In
Atnusium (B) the gill-filaments are much
elongated and thread-like instead of trian-
gular.

In the common Ark-shell (Area, G) a
great change is seen. The gill-filaments are
delicate and somewhat flattened threads,
each bent upon itself into the form of an
elongated U, and therefore consisting of a
proximal or fixed limb and a distal or free
limb. The flexure takes place in such a
FIG. 598.— Four giii-fiiaments way that the free limb is external in the

of BXytilus. c.j. ciliary , r*-\ • , i • j_r

junction ;/. filaments. (From outer row of filaments, internal in the inner
Natural His- TQW Adjacent filaments are loosely united
by groups of large interlocking cilia (see
Fig. 598), placed at regular intervals, and in this way all the outer
and all the inner limbs of the filaments are respectively joined to-
gether so as to convert each longitudinal row of U-shaped filaments
into a double plate, fairly coherent unless the ciliary junctions
are forcibly vm

separated. In ix vn VI

this way the
single c t e n i -
dium of Nucula
has given place
to two plate-
lik e simple
laminae, each
formed of an
outer and an
inner lamella :
the inner
lamella of t h e
outer and the
outer lamella of
the inner
laminae are
united along

their dorsal edges, the line of junction representing the axis of the
ctenidium : the outer lamella of the outer and the inner lamella
of the inner laminae are free dorsally.

In Mytilus (Fig. 597, D) the gill is strengthened by the develop-
ment of delicate non- vascular bars or interlamellar junctions

FIG. 599. — Dissection of Poromya. I, anterior palp; II, foot;

III, lamella on branchial septum ; IV, valve of branchial aperture ;

IV, anal siphon ; V, posterior adductor ; VI, posterior retractor of
foot ; VII, heart ; VIII, ovary ; IX, branchial septum ; X, anterior
adductor. (From Pelseneer.)

XII

PHYLUM MOLLUSCA

687

between the two limbs of each filament. In Lucina these
junctions are large and provided with blood-vessels ; and vascular
bars of tissue, the interfilamentar junctions, replace the ciliary
junctions of the lower forms. Thus by a regular series of grada-
tions the ctenidium is replaced by the complex double gill we are
already familiar with in Anodonta. In all the higher forms the
outer lamella of the outer lamina unites with the mantle and
the inner lamella of the inner lamina with the visceral mass, while,
posterior to the latter, the inner lamellae of the right and left
inner laminae unite with one another. The blood-vessels, which
are confined to the filaments in the simpler types, occur also in the
interfilamentar and interlamellar junctions in the more complex
forms of gills. In the Septibranchia the gills are degenerate,

FIQ. 600. — Sagittal section of part of enteric canal of Donax. I, lower lip ; II, intestine ; III,
pyloric csecum ; IV, crystalline style ; V, cuticle ; VI, stomach ; VII, gullet ; VIII, upper
lip ; IX, mouth. (From Pelseneer.)

being represented by a horizontal muscular partition or septum
(Fig. 597, F, and Fig. 599, IX), which divides the inhalant and
exhalant chambers from one another. Kespiration in this case is
performed entirely by the internal face of the mantle.

Digestive Organs. — The mouth is anterior ; in forms with two
adductor muscles it is always placed immediately behind the
anterior adductor. It is usually bounded by two pairs of labial palps
which sometimes attain a relatively immense size (Fig. 596) ; there
is never any trace of jaws or other masticatory apparatus. The
convolutions of the intestine are sometimes very complex. The
crystalline style either lies freely in the stomach and anterior part
of the intestine, or is contained in a caecal pouch of the stomach
(Fig. 600), which may be prolonged into one of the lobes of the

G88

ZOOLOGY

SECT.

XIV

XIII

mantle. The anterior end of the style, which projects into the
stomach, appears to be slowly dissolved by the digestive juice,
forming a sort of cement to enclose the hard particles of the food
and prevent any harmful effect on the mucous membrane. It is
possible also that the dissolved substance of the style may play
the part of a digestive secretion, since it appears to contain a
substance of the nature of a digestive
ferment capable of acting upon starchy
matters.

The excretory organs or kidneys
occur in their simplest form in the Proto-
branchia, in which they have the form of
cylindrical curved tubes, opening at one
end into the pericardium and at the other
on to the exterior ; the whole organ is
lined with glandular epithelium, and) has
no communication with its fellow of the
opposite side. In the higher forms the
organ becomes differentiated into a secreting
portion or kidney, which is very spongy in
texture, and opens into the pericardium,
and a non-secretory portion or bladder,
which opens externally. Frequently there
is a communication between the right and
left nephridia, and in some genera, such as
the Oyster, the organs become extensively
branched. Also taking a share in the pro-
cess of excretion are the pericardial glands,
or Keber's organs, glandular developments
of the wall of the pericardium.

Circulatory Organs. — The heart is
cuia.""i, pieuTaigwagUon; usually perforated by the rectum, but lies

II, pleuropedal connective; lj ^,r , •, • -vr i /tn* trv/» 7\

in, common connective from altogether above it in JNucula (Fig. 596, h)
and some other genera ; the ordinary
arrangement seems to have been brought

posterior paiiiai nerve; viii, about by the heart becoming folded over

ospnradial ganglion; IX, . •• ..*'.. -, •i-iii •. TJ_I

visceral connective; x, state- the intestine and united below it. In the
xn ' to 'external Baperture • Oyster and some other forms the heart is
S& SRftSSMSl M™ the rectum. In Area the ventricle

nerve ; XV, nerve to palps ; is divided into two by a Constriction.
XVI, cerebral ganglion. T. r, r j J/L

(From Peiseneer.) -c ores are orten lound. on tne surface 01

the foot, and it has been asserted that

through them the external water mixes with the blood ; this, how-
ever, is certainly not the case : the blood-system is everywhere
closed. The blood is red in some forms (e.g., Area) owing to
haemoglobin in the corpuscles ; in some cases it is of a bluish
tint owing to the presence of hsemocyanin.

FIG. 601. — Nervous system and
"auditory" organs of Nu-

XII

PHYLUM MOLLUSCA

The nervous system is found in its most primitive condition
in Nucula (Fig. 601). Instead of the single cerebro-pleural ganglion
of Anodonta there are, on each side, distinct cerebral (XVI) and
pleural (I) ganglia, each united by a connective with the pedal.

The most characteristic sense-organs are the statocysts

(" otocysts ") and the osphradia. The statocyst — " auditory " or
directive organ — is always placed in the foot, in close relation to

FIG. 602. — Vertical section of eye of Pcctcn. 1, cornea ; 2, lens ; 3, external epithelium ;
4, blood-sinus • 5, retina ; 6, pigmentary layer ; 7, optic nerve. (From Korschelt and
Heider.)

the pedal ganglion, sometimes embedded in the latter. The
statocysts are developed as involutions of the ectoderm and retain
their connection with the exterior in Nucula (Fig. 596, ot) and some
others. In most cases they become closed sacs. The cavity is
usually ciliated, but the cilia may be wanting. Each statocyst
may contain a number of minute statocones or, more usually, a
single, larger statoliih. The nerves supplying the statocysts are
given off not from the pedal ganglia, but from the cerebro-pedal

690 ZOOLOGY SECT.

connectives, and their fibres are derived from the cerebro-pleural
ganglia.

The osphradia — " olfactory " or water-testing organs — are patches
of sensory epithelium with an accessory or osphradial ganglion
situated in immediate relation with the visceral ganglia (Fig. 601,
VIII), but connected by nerve-fibres with the cerebro-pleural
ganglia. Patches of sensory epithelium, very similar to the
osphradia, and called the abdominal sense-organs, occur one on each
side of the anus in Area and other forms devoid of siphons, and a
similar organ has been described beside the retractor muscles of
the siphons in several Sinupalliata.

In a few instances eyes are present, but never in what we are
accustomed to consider as the normal position for such organs,

B * C

r

U rk. sd ' "

rn.es.

E.

6*.

•me!?.

Fia. 603. — Five stages in the development of Ostrca. a. anus ; bl. blastopore ; m. mouth ; ma.
stomach; mes. mesoderm ; rk. polar bodies ; s. shell ; sd. shell -gland ; sm. anterior adductor ;
w. pre-oral circlet of cilia. (From Korschelt and Heider.)

at the anterior or head-end of the body. They occur, in fact, in
the only situation where they can be of any use, namely, along the
edge of the mantle. The best-known form in which they occur is
the common Scallop (Pecteri), which has a single row (Fig. 585, VII)
all round the mantle-border. Each has a cornea (Fig. 602, I), a
cellular (not cuticular) lens (2), a retina (5) — formed of cells, the
inner ends of which are modified into visual rods, and an optic
nerve (7), one branch of which spreads over the front of the retina
and sends branches backwards to the visual rods. In this
peculiarity, as well as in the cellular lens, the eye of Pecten is
singularly like that of Vertebrates. The pallial eyes of Pelecypoda
are probably to be looked upon as modified tentacles. The only
cephalic eyes that occur in this class are a pair of small but well-

XII

PHYLUM MOLLUSCA

691

developed organs which occur in the bases of the most anterior
filament of the inner lamina of the gill in Mytilus and some other
genera.

Reproduction and Development. — Most Pelecypoda are
dioecious, but several hermaphrodite forms are known. Some of
these, such as some Oysters,
are protandrous, the gonad
producing first sperms and

afterwards ova : in others
part of the gonad serves as
an ovary, part as a testis,
the two opening into a
common duct : in others
again there is a distinct
ovary and testis on each
side opening by separate
ducts. There are never any
accessory organs of repro-
duction, such as sperma-
theca, penis, &c. Fertili-
sation frequently takes

place in the Water after the FIG- 604.— Veliger larva of Ostrea. a. anus ; dm

, . j 0 dorsal longitudinal muscle ; 1. " liver " ; m. mouth

eggS are laid. Segmentation ma. stomach ; s._shell ; sm. adductor muscle ; ss.

is total and follows the
spiral type. One of the
four megameres is very much larger than the other three. The
gastrula is formed either by invagination or by epiboly. A shell-
gland (Fig. 603, sd.) is formed as an invagination of the dorsal
surface, a stomodseum (m) as an invagination of the ventral surface,
and the larva of most forms, unlike that of Anodonta or Unio,

H

hinge of shell; Vel. velum; vm. ventral longitu-
dint! muscie. (From Korschelt and Heider.)

FK; OU5. — Two embryos of Cyclas. a. anus ; by. byssus-gland ;/. foot ; g. gonad ; k, gill ;
m. mouth ;m + l. stomach and " liver " ; mr. edge of mantle ; n. kidney ; p. pericardium ;
*', unpaired shell ; s", rudiment of paired shell ; sd. shell -gland ; vd. gullet ; vel. velar area.
(From Korschelt and Heider.)

692 ZOOLOGY SECT.

passes into a stage in which it closely resembles the trochophore
of Chaetopods (Fig. 603), having a prototroch and an apical tuft
in the middle of the prostomium. There is also an ectodermal
thickening on the prostomium which becomes the cerebral ganglion,
and a similar ventral thickening which gives rise to the pedal
ganglion and corresponds with the rudiment of the ventral nerve-
cord in Polychaeta. The pelecypod trochophore is, however, dis-
tinguished from the corresponding stage in Worms by the presence
of the shell-gland, which soon secretes an unpaired shell. The
prostomial region grows out into a thickened retractile rim bearing
the pre-oral circlet of cilia, and called the velum (Fig. 604, Vel.) :
the larva at this stage is distinguished as a veliger — a very charac-
teristic mulluscan phase of development. The shell soon becomes
bivalved and extends ventrally on each side, paired processes of the
dorsal region of the body accompanying it and forming the mantle-
lobes. A projection grows out from the ventral surface, between
mouth and anus, and forms the foot (Fig. 605, /), and on the sides
of the body the gill-filaments (k) arise as a row of fine processes,
at first simple but afterwards becoming bent upon themselves so as
to assume a V-shape. Eyes are often present in the larva at the
base of the velum.

General Remarks.— Although none of the Pelecypoda are
microscopic, they present a considerable range in size, from the
minute Nitcula, about 4 mm. long, to the Giant Clam (Tridacna
gigas) of the Indian and Pacific islands, which is sometimes 60 cm.
(two feet) in length and 500 pounds in weight.

Many pelecypod shells are white or dull brown in colour, but
in several genera brilliant tints are the rule, the various species of
Scallop (Pecten) being specially remarkable in this respect. The
inner surface of the shell often exhibits beautiful iridescent tints,
noticeably in the so-called Pearl-oyster (Meleagrina) and the
Australian Trigonia. As far as is known, the colours are all what
are called "non-significant," i.e., are of no physiological or ethological
importance. In this connection the formation of pearls by some
species must be mentioned : they are deposits of nacre formed
usually round encysted parasitic worms, either between the mantle
and shell or in the soft parts. They are produced, amongst other
species, by the " Pearl-oyster " (Meleagrina margaritifera) and
by the Pearl-mussel (Unio margaritifera). Some species, such
as the common boring Pholas. are phosphorescent.

Most Pelecypoda are sluggish in habit, progressing only by slow
contractions of the foot, and some are permanently fixed during
adult life by the byssus, or are only able to change their position
after throwing off the byssus, which becomes replaced by a new
one. The Scallops, however, swim freely by clapping the valves
together. The Cockles (Cardium), Trigonia, &c., jump by sudden
movements of the foot, and the Razor-fish (Solen) jerks itself

xn PHYLUM MOLLUSCA 693

forward by suddenly withdrawing its foot and thus ejecting water
through the siphons. The only parasitic genus is Entovalva,
found in the gullet of a Holothurian.

Pelecypoda are abundant both in fresh water and the sea ; the
marine forms are mainly littoral. None are pelagic or terrestrial.
They are very abundant in the fossil condition, occurring in all
formations from the Upper Cambrian upwards, and, owing to
their gregarious habits, frequently forming extensive deposits or
shell-beds. The oldest forms are all iso- or hetero-myarian ; the
monomyarian types (Pseudolamellibranchia) appear first in the
Carboniferous, and the Siphoniata not until the Triassic period.
The modern genus Area dates from the Upper Cambrian, and thus
furnishes as striking an example of a " persistent type " as some
of the Brachiopods.

There seems to be little doubt that the Protobranchia, and
especially Nucula, exhibit the most primitive type of pelecypod

SINUPALLIATE
EULAMELLIBRANCHIA

INTEGRIPALLIATE
EULAMELLIBRANCHIA

PSEUDO-LAMELLIBRANCHIA

HETEROMYARIAN
FILIBRANCHIA

ISOMYARIAN
FILIBRANCHIA

PROTOBRANCHIA

FIG. 606. — Diagram illustrating the mutual relationships of thelPelecypoda.

organisation, as indicated by the plume-like gills with separate
filaments, the simple kidneys, and the distinct cerebral and pleura!
ganglia ; absence of concrescence is always a mark of low or
generalised organisation. The Filibranchia with imperfectly united
gill-filaments come next, and are divisible into two groups — iso-
myarian with equal-sized adductors, and heteromyarian with more
or less atrophied anterior and proportionally enlarged posterior
adductor ; the latter group is to be looked upon as the more special-
ised, and leads to the Pseudolamellibranchia (monomyarian type)
in which the anterior adductor disappears completely in the adult,
while the posterior is immensely enlarged and assumes a central
position. Similarly, the isomyarian Filibranchia lead to the
Eulamellibranchia, which are equal-muscled, but have the gill-
filaments united into a complete basket-work. In the Eulamelli-

694 ZOOLOGY SECT.

branchia, lastly, there is a gradual series of stages from compara-
tively generalised forms with free][mantle-lobes up to the highly
specialised species with large siphons. That the Pseudolamelli-
branchia and the siphoniate Eulamellibranchia are to be looked
upon as the highest members of the class is indicated not only by
morphological evidence, but by their comparatively late appearance
in time.

CLASS II.— AMPHINEURA.

The Amphineura are a class of marine Mollusca formerly grouped
with the Gastropoda, but now recognised as sufficiently far removed
from the latter to require separation as a distinct class. The
commonest, as well as the most highly organised, of the Amphineura
are the Chitons, a group of remarkably sluggish Limpet-like Molluscs
with a shell composed of eight pieces. The other^members of the
class are lowly organised, comprising the simplest forms, all devoid
of a shell, of the entire phylum.

^/ 1. DISTINCTIVE CHAKACTERS AND CLASSIFICATION.

The Amphineura may be defined as bilaterally symmetrical,
more or less elongated Mollusca, with terminal mouth and anus,
either devoid of a shell, or possessing one which consists of eight
median valves. The mantle contains numerous spicules of carbon-
ate of lime, and is not divided into paired lateral lobes. The
ctenidia are either absent, or there is a single pair, or they occur
as a circlet round the anus, or as two lateral rows situated between
the edge of the mantle and the side of the foot. A radula (vide infra)
is sometimes present, sometimes absent. The nervous system
consists of two pairs of nerve-cords, pedal and pallial, connected
in front with a nerve-ring.

The class is divisible into two orders :

ORDER 1. — PLACOPHORA.

Amphineura with a broad foot, and with a shell which consists
of eight transverse valves. There is a row of ctenidia on either side.
This order includes the Chitons.

ORDER 2. — APLACOPHORA [SOLENOGASTRES].

Amphineura with an elongated body covered completely by the
mantle, without shell, but with calcareous spicules. There is no
foot, but generally a ventral longitudinal groove along which
usually runs* a low ciliated ridge. In some there is a posterior
cavity (cloaca or mantle-cavity) containing a pair or a circlet
of ctenidia.

This order includes Neomenia, Proneomenia, Chcetoderma, and
a number of other genera.

XII

PHYLUM MOLLUSCA

695

607. — Chaetoderxna
nitidulum. a. anus ;
m. mouth. (From the
Cambridge Natural His-
tory.)

2. GENERAL ORGANISATION.

External Features. — The Aplacophora are distinguished by
their worm-like body, sometimes elongated and narrow and capable
of being coiled into a spiral, sometimes comparatively short and
thick. In most instances there is little differ-
ence in external appearance between the an-
terior and posterior ends. In some species of
Chcetoderma (Fig. 607) alone is there in sexually
mature specimens a "head," separated off
from the body by a constriction, as well as a
posterior cloacal region which is similarly
marked off. A shell is completely absent.
The mantle covering the surface possesses a
cuticle, in the substance or on the surface of
which are spicules of calcified material. In
many cases the dorsal surface is beset with
uni- or multi-cellular epidermal papillae.
Along the middle of the ventral surface runs,
in most instances, a groove, in some cases
merely represented by a narrow strip from
which the cuticle and spicules are absent.
The ventral groove, when present, usually
contains a slight longitudinal ridge, and this groove with its
contained ridge is all that in these simple forms represents the foot,
an organ so highly developed in other Molluscs. In Chsetoderma it is
entirely absent. With the ventral groove is connected in front
an anterior ciliated groove, while behind it is in direct communica-
tion with the cavity of the cloaca.

In Proneomenia ctenidia are absent. In the remaining genera
there is either a pair or a circlet of gills in the
form of simple or complex folds situated in the
cloaca — a cavity at the posterior end of the body
into which the anus opens (Fig. 612).

In Chiton (Figs! 609 and 610) the body is
dorso-ventrally compressed, convex above, and
presents below a broad flat foot (narrow in
Chitonellus) which acts not only as an organ for
effecting creeping movements, but also as a sucker

Cambridge Natural P P,. * , & . , , J ,,

History.) for enabling the animal when at rest to adhere

firmly, like a Limpet, to the surface of a rock.
The head region is not distinctly separated off, and is not provided
with eyes or tentacles.. The most remarkable external feature of
Chiton is the presence on the dorsal surface of a calcareous shell
(Figs. 609 and 611) made up of no fewer than eight transversely
elongated pieces or valves, arranged in a longitudinal row, articu-
lating together and partly overlapping one another. They are

FIG. 608. — Neomenia
carinata. a. anus ;
gr. ventral groove ;
m. mouth. (From the

696

ZOOLOGY

SECT.

sometimes partly, sometimes completely, covered over .by the
mantle. Each, valve consists of two very distinct layers, a more

Fia. 609.— Chiton spinosus, dorsal view.
(From the Cambridge Natural History.)

FIG. 610. — Chiton, ventral view. an. anus ;
cten. ctenidia ; ft. foot ; mant. mantle edge ;
mo. mouth ; pip. palp. (After Pelseneer.)

superficial and a deeper, the latter formed of compact calcareous
substance, the former perforated by numerous vertical canals for
the lodgment of the sense-organs to be pre-
sently referred to. External to the valves
the dorsal integument (mantle) of Chiton
and its allies is usually beset with a number of
horny or calcified tubercles and spicules.
The mantle develops only very slight lateral
flaps, and under cover of these, in mere
grooves which represent the mantle-cavities
of other Molluscs, are a series of small
ctenidia (Figs. 610 and 616, cten.) to the
number of from fourteen to eighty. The
mouth and anus are both median, situated
at the anterior and posterior extremities
respectively.

Alimentary System. — In the Aplaco-
phora the mouth is usually a longitudinal,
rarely (Chcetoderma) a transverse, slit, situated
ventrally near the anterior extremity. There
is a buccal cavity, with a radula1 in some
cases (Fig. 612, rad), and in others a single
tooth supporting smaller den-

F,<,6ii.-c*iton, valves of
shell. (From the Cambridge tides : sometimes teeth, are entirely absent.

There are both salivary and buccal glands.

1 For a description of the structure of this characteristic organ see the
account of Triton (p. 708).

I XII

PHYLUM MOLLUSCA

Very characteristic of the group as compared with other Molluscs
is the presence of a straight intestine devoid of coils, and
having connected with it a single dorsal caecum and usually a
double row of lateral caeca, itn the Placophora the buccal cavity

Ibrn

—cten.

FIG. 612. — Chaetoderma nitidulum , longitudinal section, an. anus; brn. brain ; ccec. gland-
ular cseca of mesenteron ; cten. ctenidium ; dia. diaphragm separating off the posterior
portion of the body ; gon, gonad ; mo. mouth ; peri, pericardium and heart ; rod. radula ;
reel, rectum. (After Simroth.)

always contains a well-developed odontophore and radula. The
intestine is elongated and coiled. There are salivary glands and
a large paired liver (Fig. 613, liv.)./

Body-cavity. — In the Aplacopnora the interstices between the

organs and the body-
wall are filled with a
form of connective-
tissue with muscular
fibres ; a vertical
diaphragm (Fig. 612,

••••m ^3p±!li\ dia.} separates the

n^€p/r~~^~~~^^LL i posterior part of the

body, containing the

FIG. 613. — Diagrammatic longitudinal section of Chiton, -rkorinarrlinrn (<n0<r<i\

specially intended to show the relations of the parts of P ( L \Pen)>

the ccelome, which, except the genital parts, are bordered from the rest. In the

with a thick line. an. anus ; ent. enteric cavity ; ft. foot ; ..-.,

gon. gonad ; hd. head-lobes ; M. heart ; liv. liver ; mo. PlaCOphora

mouth; neph. kidney; peri, pericardial cavity. (From *frjc ^ ClOT~"
Simroth, after Haller.)

peri

VOL. T

an exten-
Y Y

698

ZOOLOGY

SECT.

sive cavity, lined with a ccelomic epithelium, and divided into three
completely separated parts — the pericardium, the genital cavity,
and the general body-cavity.

Vascular System. — The vascular system of the Aplacophora
is very rudimentary. There is a heart enclosed in a pericardium
(Figs. 612 and 615, peri) and composed, when best developed, of
an auricle and a ventricle.

In Chiton there is a well-developed heart (Fig. 613, Jit.) consisting
of a median ventricle and two lateral auricles. The pericardial
cavity in which it lies is a space of considerable extent in the
posterior region of the body, below the two last valves of the shell.
The Nervous System consists in the^ Aplacophoja (Fig. 614,
A,B,C) of four longitudinal nerve-cords — twu^tetial and two pleural.
These are connected together by an cesophageal ring, thickened
dorsally into a single or double cerebral ganglion ; and in front of
this is a second, more slender stomatogastric nerve-ring with small

ganglia. The
pedal cords
(v.v) may pre-
sent in front a
pair of gang-
lionic thicken-
ings connected
by a commis-
s u r e, and
further back
-pe there may be a
D series of e n -

FIG. 614. — Nervous system of Amphineura. A, Proneomenia ; largements
B, Neomcnia ; C, Chcetoderma ; D, Chiton, c, cerebral ganglia ; -, -,

I, I, pleural cords ; pc. posterior commissure ; s, stomatogastric United. Dy COm-

missures. The
pleural cords

(I, I) are connected behind, above the rectum, by a commissure
(p, c) which usually bears a median enlargement. Sometimes a
union takes place posteriorly between the cords of the two pairs.
There are no eyes, or statocysts, or tentacles. The dorsal epidermal
papillae are perhaps sensory. Some have a sensory frontal lobe and
a sensory pit or elevation in the middle line of the dorsal surface
near the posterior end.

In the Placpphora (Fig. 614, D) there is an oesophageal nerve-
ring consisting of a thicker dorsal cerebral portion not differentiated
into ganglia, and a thinner ventral buccal commissure. The cerebral
part sends off nerves to the labial palps, the lips, and the buccal
apparatus. Two pairs of longitudinal nerve-cords, pedal and
pleural, are given off posteriorly : the former, from which arise
nerves to the foot, are joined by numerous commissures passing
beneath the enteric canal ; the latter, which send off nerves chiefly

X

commissure or ring, with ganglia ; v, v, pedal cords. (From the
Cambridge Natural History, after Hubrecht.)

xn

PHYLUM MOLLUSCA

(599

on/

•pert

to the mantle and the ctenidia, are united together by a supra-
rectal commissure at the posterior end of the body. Near its
origin each pleural cord gives off a slender visceral commissure,
which unites with its fellow of the opposite side : two small ganglia
lie in this visceral commissure near the middle line. The large
cords contain nerve-cells throughout their length.

The conspicuous organs of special sense present on the head of
Gastropods (vide infra), are absent in the Placophora, as in the
Aplacophora. A pair of processes situated in front, at the sides of
the mouth, have the character of labial palps. In the buccal
cavity there are cup-shaped
gustatory organs supplied with
nerves from the cerebral com-
missure, and in front of the
odontophore is a thickening of
the epithelium — the subradular
organ — containing nerve-end-
ings. Remarkable sensory
organs, the micrcesthetes and the
megalcesthetes, lie in the canals
already mentioned as occurring
in the superficial layer of the
shell-valves. The megalsesthetes
may take the form of eyes, with
cornea, lens, pigment-layer, iris,
and retina ; in some cases the
lens is absent.

Reproductive and Renal
Organs. — In the Placophora
the sexes are distinct : in the
Aplacophora, with the exception
of Chsetoderma, they are united.
In the Aplacophora (Fig. 615),

• f rn ±.

Wltn the exception OI UiiaetO-

derma, the gonads are paired.

The sexual products pass into

the pericardial cavity and thence

are carried to the exterior by a pair of ducts (coslomoducts)

opening into the cloaca. Renal organs are unknown in the

Aplacophora.

In the Placophora (Fig. 616) there are two symmetrical kidneys,
each opening internally into the pericardium by*"a ciliated funnel-
like opening (n. peri, ap), and externally (neph. ap) between two
of the posterior ctenidia some little distance in front of the anus.
Each consists of a looped main tube, into which open numerous
minute tubules which ramify among the viscera. The testis and
ovary (gon) are similar in appearance, differing only in colour

Y Y 2

FIG. 615. — Neomenia carinata, reproduc-
tive organs, cop. copulatory organs ; gon.
gonads enclosed in extensions of the peri-
cardial cavity ; gonod. gonoducts ; peri, peri-
cardium ; reel, receptaculum seminis. (From
Simroth, after Wiren.)

700

ZOOLOGY

SECT.

when the products are mature. Each is an unpaired sac marked
by a series of slight lateral .constrictions. There are two gonoducts,
each opening immediately in front of the corresponding nephridial
duct.

Little is known of the development of the Aplacophora. The
eggs undergo complete segmentation, and give rise to a gastrula
by invagination. This develops into a form of trochophore with
a ciliated ring, the prototroch. The larva is provided for a time

with a row of seven
calcareous plates on
the dorsal surface.

The eggs of Chiton
are fertilised in the
mantle-cavity, where
in one species they
are retained until the
embryos are fully de-
veloped. In other
cases they are laid
singly or in strings.
The segmentation is
complete, and corre-
sponds very closely
with that of the Poly-
chaeta (p. 440). The
four megameres give
off three quartettes
of micromeres, which
give rise to the ecto-

-(pen.GLp
.peri.cip
ne ph. a. p

FIG. 616. — Chiton, nephridial and genital systems, an-
anus : cten. ctenidia ; gen. ap. genital aperture ; gon. gonad ;

\ ap.
peri-

derm. The mega-
meres give origin to
the endoderm, and
the mesoderm origin-
ates in a cell given
off by the posterior
megamere Mi-
crome res and
megameres at first
arrange themselves in such a way as to form a somewhat
flattened blastula, one side of which (vegetal pole) is composed
of a comparatively small number of large endoderm cells. Then
follows the invagination of the cells of the vegetal side and the
resulting formation of a gastrula : this soon becomes elongated
in the direction of the future long axis.

Two rings of cells surrounding the embryo develop cilia (Fig.
617, cil.), and owing to the double circlet thus formed an anterior
and a posterior region are distinguishable in the larva. The

XII

PHYLUM MOLLUSCA

701

blastopore becomes shifted from its original posterior position
forwards on the ventral surface until it comes to be situated just
behind the circlet of cilia ; it undergoes elongation, and an invagina-
tion of ectoderm round its anterior end forms the mouth (mo.) and
stomodseum. A ventral diverticulum of this forms the rudiment
of the radular sac (rd.). By greater relative growth of the post-
oral part the embryo assumes the form of a pear ; and in this
trochophore stage, with a pre-oral circlet and a bunch of cilia in
the middle of the apical area, it becomes free in the case of certain
of the species, while in others it remains enclosed in the egg up
to a later stage of development. As yet there is no anus, that
aperture, with the proctodseum, being formed later by invagination.
An apical plate is not present in the early larva ; but the rudiments
of the cerebral ganglia (C, cer. </.),. which subsequently appear at
the apical pole, probably represent it. Primitive nephridia, such

B

mesant

FIG. 617. — Chiton, development. A, general view of larva ; B, section of early, and C, of
' later trochophore. cole, calcifications (rudiments of shell) ; cer, g. cerebral ganglion ; cti.
ciliary ring; cil. t. ciliary tuft at apical pole ; eye, eye ; ft. gl. foot-gland ; me*, mesoderm ;
mesent. mesenteron ; mo. mouth ; rd. radular sac ; sp. spines ; vise. g. visceral ganglion.
(From Korschelt and Heider, after Kowalewsky.)

as occur in Annulate and many Molluscan trochophores, are not
present.

The post-oral region now becomes greatly elongated ; the meso-
derm increases greatly in extent, and forms two weU-defined streaks,
which are afterwards divided into parietal and visceral layers
with a coelomic space between them. The post-oral part of the
embryo now presents an appearance resembling rudimentary seg-
mentation. This is due to the development of a series of rudiments
of the eight pieces of the shell (A, cede.), each of which is
formed independently after the fashion of the entire shell of other
Mollusca.

Ethology, Distribution, <&c. — All the Amphineura are
marine. The Placophora occur at all depths, though most abundant

702 ZOOLOGY SECT.

on the shore between tidal limits. The Aplacophora, on the
other hand, are rare in very shallow water, and absent altogether
from the littoral zone : some have been found at considerable
depths (down to 1,250 fathoms). Some of them burrow in mud,
others live in association with various colonial Coelenterates.
The Placophora are all vegetable feeders, their food consisting of
minute algae and diatoms. When at rest, they adhere firmly
to the surface of a rock or a block of coral by means of the
sucker-like foot. When forcibly detached the animal curls itself
up into a ball, and will only after a considerable time slowly extend
itself again : all its movements are extremely sluggish.

The Aplacophora have no hard parts that would be recognis-
able in the fossil condition ; but numerous fossil Placophora are
known from the Silurian onwards. The valves of the Silurian
genera differ from those of recent forms in the absence of the
articulations.

CLASS III,— GASTROPODA.

The Gastropoda, including the Snails and Slugs, Limpets,
Whelks, Periwinkles, Sea-hares, and the like, are Mollusca in
which there is, as a rule, a shell composed of a single piece,
and in which the mantle is not divided into two lateral folds as
in the Pelecypoda. The body is inequilateral, owing to
one-sided development. There is a well-developed ventral foot,
usually with a broad, flat surface on which the animal creeps.
A head-region bearing eyes and tentacles is distinguishable in
front of the foot. The alimentary canal is characterised by the
presence in the buccal region of a peculiar organ, the odontophore,
present also in some of the Amphineura, bearing rows of minute
chitinous teeth. Plume-like ctenidia are usually present. A
metamorphosis occurs in the development, during which the young
Gastropod passes successively through trochophore and veliger
stages. The majority of the families of Gastropoda are marine,
a few of these being pelagic ; but some inhabit fresh water, and
others are terrestrial.

1. EXAMPLE OF THE CLASS. — THE TRITON (Triton rubicundus).^

Triton is a marine Gastropod living in shallow water, usually
close inshore. The species to which the following description
specially applies has a very wide range, from the English Channel
to the South Pacific, and occurs as a fossil as far back as the Miocene.
In most respects the English Whelk (Buccinum undatum) will be
found to conform to the description.

The shell (Fig. 618) is a very hard and dense calcareous structure,
presenting no trace of division into valves such as compose the

1 Or Charonia rubicunda.

XII

PHYLUM MOLLUSCA

703

shell of the fresh-water Mussel, and lacking also its bilateral
symmetry. It is in the form of an elongated hollow cone closely
wound round a central axis. The apex of the cone is the organic
apex of the shell, corresponding to the umbo of the fresh-water
Mussel, and is the point from which the growth of the shell has
proceeded : the base is represented by the wide oblique opening
—the mouth or peristome of the shell. Starting from the apex along
the internal cavity of the spirally -wound cone, in order to reach

^ ^^

FIG. 018. — Shell of Triton rubicundus.

Natural size.

FIG. 619. — Longitudinal median section of the
shell of Triton rubicundus.

the mouth in an adult shell, we pass completely round the central
axis five times — i.e. the spiral consists of five turns. In following
the turns, the direction taken is to the right, that is to say, the
spiral of the shell is a right-handed or dextral one. The axis
(Fig. 619) is in the shape of a twisted shelly rod — the columella —
containing a narrow lumen ; it is formed by the close union of
the axial portions of the wall of the spiral. The windings of the
spiral are marked on the outer surface of the shell by a narrow

704 ZOOLOGY SECT.

impressed spiral line or suture, parallel with which are numerous
fine ridges and depressions — the lines of growth ; the increase in
size of the shell takes place in the direction of these lines, not at
right angles to them as in the shell of the fresh-water Mussel,
and the lines that more strictly correspond to the lines of growth
of the latter are excessively fine striae which run transversely to
the stronger lines. At certain points, usually three in a full-grown
shell, the spiral is interrupted by a transversely directed edge
which appears to overlap the succeeding portion ; this edge marks
the position which the mouth of the shell has occupied during
regularly recurring periods of arrest of growth, probably annual.

The mouth of the shell is bordered on the side turned away
from the columella by a prominent rim or outer lip of the peristome ;
this is produced at the extremity farthest from the apex of the
shell into a spout-like process — the siphoned process. The promi-
nent edge of the peristome is in relation to the dorsal surface
of the body of the animal ; the opposite side has no prominent
edge, but is rounded off to form a smooth inner lip ; a couple of
ridges on this inner lip towards the apical end aid the animal in
drawing itself out after it has become re-
tracted into the interior of the shell. The
outer lip is in relation to the dorsal surface
of the body of the animal, the inner lip in
relation to the ventral surface ; the siphonal
FIG. 620.— opercuium process is for the lodgment of a spout-like
cundSs*.011 process of the edge of the mantle — the

siphon.

When removed from the water, or disturbed in any other way,
the animal becomes completely withdrawn into the interior of
the shell, when the latter is observed to become closed by a plate
— the opercuium (Fig. 620) — which fits accurately across the passage
some distance internal to the peristome. The opercuium is an
oval plate of chitinoid material hardened by calcareous deposits ;
like the shell itself, it exhibits lines of growth marking what has
been its edge at successive stages in the development of the shell.

The minute structure of the shell is in the main similar to that
of the fresh-water Mussel (p. 666). Its outer surface is covered
with a thin layer of uncalcified chitinoid material, the periostracum,
beneath which is a thick prismatic layer, and, lining the inner
surface, a layer of nacre.

External Features of Soft Parts. — The Triton is able to
extend itself to a considerable degree beyond the mouth of the
shell ; but a portion of the body always remains concealed in the
interior, even when the animal has extended itself to its utmost,
the body being, like that of the fresh-water Mussel (and of nearly
all the Mollusca), organically connected with the shell. In Triton
the connection is by means of a strong muscle — the columellar

XII

PHYLUM MOLLUSCA

705

muscle (Fig. 621, col. m.) — which extends from the concave right
side of the animal to the columella, into which it is inserted ; it
is by means of this muscle that the anterior portion of the body,
capable of being thrust out through the mouth of the^shell, may
again be withdrawn.

If the Triton be examined in the extended condition (Fig. 621)
it will be found to present a distinct head, which bears dorsally

liv

tent

FIG. 621. — Lateral view of the body of a female specimen of Triton rubicundus, removed
from the shell, moderately extended, col. m. columellar muscle ; cott. collar of mantle ;
eye, eye ; liv. liver ; mant. cav. mantle-cavity ; meso. mesopodium ; op. operculum ;
ow. ovary ; prop, propodium ; *iph, siphon ; stom. stomach ; tent, tentacles.

a pair of appendages — the tentacles (tent) — of a sub-cylindrical
shape, slightly compressed towards their bases, and narrowing
somewhat towards their free extremities ; these are capable of
being extended and contracted, but not of being completely
retracted. Bach bears on its outer side, some little distance from
the base, a prominent eye. At the anterior end of the head on
its ventral aspect is the opening of the mouth. When the
animal is feeding, an elongated cylin-
drical introvert (Fig. 622), comparable
to that of Sipunculus (p. 484), is ex-
tended forwards, bearing the mouth at
its anterior end ; at other times the

introvert is Completely involuted within FIQ. 622.— Diagram of the introvert

J ^s m*»«4i,M.M :,-v i.-x*-, ,-,: + ,, ^1 :^ « i ^^^

the head and anterior portion of the
body. In the male, on the right-hand
side of the body some little distance
behind the head, is a long, narrow fleshy
process, broader at the base than at
the free end, and deeply grooved longi-
tudinally ; this is the penis. Running
back from its base is a narrow groove with prominent lips — the
sperm-groove, continuous with that of the penis ; in the female these
parts are not represented.

Foot. — On the side of the body (ventral) which the animal
applies to the surface of the ground when it extends from the

in-l.ro

of Triton, in longitudinal sec-
tion, as it appears when almost
completely extended. The black
lines represent the wall of the
alimentary canal ; the cross-
hatched part the wall of the
introvert ; the dotted line marks
the position of the opening
through which the introvert
passes, intro. introvert ; mo.
mouth ; ces. lumen of oesophagus.

706 ZOOLOGY

SECT, xn

shell is a flat surface elongated in the antero-posterior direction.
The wall of the body in this region is composed of a dense mass
of muscular fibres : this is the principal part of the foot (pro-
podium and mesopodium combined) ; the posterior portion (meta-
podium) is a thick process projecting behind this and bearing
the operculum on its surface. The foot is highly contractile, and
it is by means of contractions passing over it in a succession of
undulations that the animal creeps along, dragging after it the
rest of the body enclosed in the shell. In the middle line of its
flat surface, nearer the anterior than the posterior end, is a slit-
like aperture leading into a cavity lined with unicellular glands—
the pedal gland.

When the remainder of the body has been removed from the
shell, it is found to be twisted up into a coil — the visceral spiral,
corresponding to the spire of the shell within which it was lodged.
This is unsymmetrical, the axis of the spiral being directed not
straight backwards, but backwards, upwards, and to the right.
The external asymmetry of the body is not strongly marked in
the part which is capable of being protruded from the shell, but
is still recognisable ; and an examination of the internal organs
shows a marked excess of development on the left-hand side,
i.e. the side which corresponds with the longer outer side of the
spiral of the shell. The surface of the part of the animal which
is capable of being pushed out from the shell is covered with a
thick integument, which is darkly pigmented except on the lower
surface of the foot. Over the visceral spiral the mantle forms
a thin, delicate, colourless layer. Anteriorly the mantle becomes
thickened and pigmented, and at the posterior limit of the pro-
trusible part gives rise to a thickened ridge, the collar (Fig. 621,
coll.), forming a semicircle over the dorsal and lateral regions.
In the middle the collar is not in close contact with the body,
but leaves a large cleft leading into a very wide space extend-
ing backwards for a considerable distance. This space, which
is formed by an infolding of the mantle, is termed the mantle- or
pallial-cavity. In it are to be found the ctenidium, the osphradium,
and the anal, excretory, and reproductive apertures. The wall
of the cavity is much folded and plaited, and contains a quantity
of glandular tissue, the plaits being most numerous on the right-
hand side in front of the anus.

The ctenidium or gill (Fig. 623, cten.) is closely applied to the
wall of the mantle-cavity to the left of the middle. It consists of
a main stem, with which are connected a row of delicate flexible
laminae set at right angles to it : these are broadest in the middle,
becoming smaller towards the ends.

The osphradium (osph.) lies close to the ctenidium on its outer
side, in the natural position of the parts (reversed in the figure
owing to the reflection of the wall of the mantle-cavity). It

FIG. 623. — Triton rubicundus. Dissection of the internal organs of a female, viewed from
the dorsal side. The roof of the mantle-cavity has been divided by a longitudinal incision
and the flaps laid out, that on the left bearing the ctenidium and osphradium, and that
on the right the rectum and terminal part of the oviduct. The muscular dorsal wall of
the body and the introvert have been divided so as to bring into view the anterior part of
the alimentary cunal and a portion of the nervous system. The buccal cavity has been
tilted up and opened so as to show the odontophore, and the esophagus has been cut
through near the anterior end. A portion of the ventral wall of the crop has been re-
moved so as to bring the internal folds into view, and the interior of the nephridium with
the contained portion of the intestine has been exposed. Note. The complete course
of the intestine through the nephridium is not shown : the stomach is not seen, being
hidden by the nephridium, and the ovary is not represented, an.^inus ; ant. aort. anterior
aorta ; aur. auricle ; buc. buccal cavity ; cer. buc. con. cerebro-buccal connective ; cer. g.
cerebral ganglia ; crop, crop ; cten. ctenidium ; int. intestine : jaw, jaw ; /. buc. g. left buccal
ganglion ; /. sal. ff/.ileft salivary gland ; neph. kidney ; neph. ap. renal aperture ; od. odonto-
phore ; ces. oesophagus ; <KS'. anterior end of same, cut and turned aside ; osph. osphradium ;
ovid. oviduct : ovid', terminal thick-walled portion of oviduct ; pleur. g. pleural ganglion ;
post. aort. posterior aorta : post. ces. posterior oasophagus ; rad. s. radula sac ; rect. rectum ;
r. sal. gl. right salivary gland ; sal. du. salivary duct ; siph. siphon ; supra, g. supra-ceso-
phageal visceral ganglion ; tent, tentacle ; vent, ventricle.

708

ZOOLOGY

SECT.

od.ortt

FIG. 624.— Triton rubicundus. Interior of
the buccal mass, from above, magnified. I. jaw,
left jaw ; odont. lateral surface of odontophore ;
rad. radula ; r. jaw, right jaw.

presents a central axis, connected at right angles with which are two
rows of close-set delicate lamellae. Like the ctenidium, it is closely
applied to the wall of the mantle-cavity throughout its length.
The laminae of the osphradium are supplied with nerves which

ramify over them, and its
function seems to be to ascer-
tain the condition of the
water that enters the mantle-
cavity.

Digestive System. — The
mouth, situated at the an-
terior end of the introvert,
leads into a large chamber
with muscular walls, the
buc&tt '-"mty {Ibuc.}. At the
sides'^' rthe entrance to this
cavity the ravesting-euSgk is
thickened to form two dis-
tinct horny plates — the
(Figs. 623, 624, and
jaw). The jaws are flexible,
and on examination under the microscope are found to be com-
posed of numerous rows of minute bodies, the denticles ; tfcs
anterior edge is minutely denticulated. From the floor of the
cavity rises an elevation, the odontophore (Fig. 623, od.t Fig. 624,
odont.}, which is somewhat elongated in the direction of the long
axis of the body and compressed laterally . ^Over the summit of
the odontophore
runs longitudin-
ally " a narrow
strap - like body,
the radula or
lingual ribbon
(F^igs. 624 and
625, rod.), beset
with numerous
minute horny
teeth arranged in
transverse r<
Posteriorly t h fs
toothed ribbon
passes into a nar-
row curved pouch
— the radular sac (Fig. 623, rad. s, Fig. 625, rad. sac.) — extending
backwards from the posterior and lower aspect of the buccal
cavity. Anteriorly it does not extend beyond the odontophore-
prominence. The latter contains cartilages (Fig. 625, cart.)

JCL11S

rcLcl-sac

FIG. 625. — Triton rubicundus. Diagrammatic longitudina i
vertical section of buccal cavity, bod. cav. body-cavity ; cart.
cartilage of odontophore ; jaw, right jaw ; ces. (Esophagus ; rad.
radula ; rad. sac. radula sac.

xn PHYLUM MOLLUSCA 709

serving for the support of the whole apparatus, and is capable
of being extruded, with the radula which it bears, through the
opening of the mouth, by the contraction of sets of protractor
muscular fibres. Muscles inserted into the odontophore also
effect the movements of the radula by means of which it
produces a rasping effect on the food, and probably the radula
itself is capable of a certain degree of to-and-fro movement on the
membrane (sub-radular membrane) which lies beneath it. The
entire buccal cavity is capable of being drawn forwards towards
the mouth opening, or backwards into the introvert, by the con-
traction of strands of muscular fibres passing from its wall to the
wall^of the body.

• From the buccal cavity runs backwards a long narrow tube
with^-sftcutria^edr-walls — the oesophagus (Figs. 623 and 625, oes.).
^Posteriorly this ope1' ^.to a large^e^oidr-eac — the crop (Fig. 623,
crop). The outer &a .ace of the crop appears marked with
numerous close-set fine lines, transverse or oblique in direction ;
and, when the cavity of the organ is opened, it is found that these
correspond to numerous delicate folds whicJL extend far inwards,
and almost completely block up the lumen^ On either side of the
crop is a large gland — the salivary glanctyFig. 623, 1. sal. gl., Fig. 627,
r. sal. gl.) — partly composed of a compact glandular substance,
partly of spongy tissue in which the secretion collects. The two
salivary glands are unlike in size and shape, that on the left-hand
side being much longer than that on the right. ^Each has a narrow
duct (sal. du.) which runs forwards and inwards to the dorsal aspect
of the oesophagus, where the two come into close apposition, becom-
ing embedded in the wall of the oesophagus, along which they run
forwards to open into the buccal cavity.

From the crop leads backwards and to the left a narrow cylin-
drical tube — the posterior oesophagus. On this follows a stomach
(Fig. 621, stom.) which is in the form of a U-shaped tube partly
embedded in the substance of the digestive gland or " liver," the
hepatic ducts from which open into it. The tubular stomach is
followed by a somewhat narrower tube — the intestine (Fig. 623, int.).
This enters the cavity of the nephridium, round the interior of which
it bends ; and, leaving it at its right-hand side, runs forwards in a
straight course -^s-fefee-ree^m-(^ec^), embedded in the glandular
wall of the mantle-cavity, ^o near the anterior end, where it
terminates in a short, freely projecting, spout-like portion, with
the anus (an.) at its extremity.

The digestive gland or " liver " forms a mass of reddish-brownN
glandular follicules which compose the greater part of the bulk of J
the visceral coil.

Vascular System. — Close to the base of the ctenidium, behind
it and a little to the right, is the heart, lodged, like that of the
fresh-water Mussel, in a cavity, the pericardium, lined by a trans-

710

ZOOLOGY

SECT.

FIG. 626.— Triton rubicundus. Nervous system, from the dorsal side.
cer. buc. con. cerebro-buccal connective ; cer. g. cerebral ganglion ; col.

mantle-nerve ; opt. n. optic nerve ; ped. con. cerebro-pedal and pleuro-
pedal connectives ; ped. g. pedal ganglia ; pi. g pJeural ganglion ; r.
abd. g. right abdominal ganglion ; r. br. n. right branchial nerve; r. buc.
gang, right buccal ganglion ; r. mant. n. right mantle-nerve ; supra, g.
supra-intestinal visceral ganglion ; tent. n. nerve to tentacle • vise n
visceral nerve-branches.

parent, mem-
brane— the peri-
car dial mem-
brane. The heart
consists of two
chambers, an
auricle (Fig. 623,
aur.) and a ven-
tricle. The
auricle, which is
the smaller of the
two, is situated
somewhat in
front of the
ventricle, close
to the ctenidium,
from the main
central vessel of
which it receives
the blood. The
ventricle (vent.)
is of somewhat
pyramidal shape,
but with the
edges rounded
off. Its wall is
extremely thick
and muscular.
Passing out from
the ventricle
towards the right
is a thick artery,
which soon
divides into two,
one running for-
wards, the other
backwards — the
anterior (ant.
aort.) and pos-
terior (post, aort.)
a o r t CB. The
former is a very
large trunk which
runs forward s
below the pos-
terior oesopha-
gus, crop, and

xii PHYLUM MOLLUSCA 711

anterior oesophagus, giving ofE branches to the region of the head
as it goes. The posterior aorta, narrower than the anterior, passes
into the visceral spiral, where it breaks up into branches for the
supply of the various parts. The blood-system consists in large
measure of sinuses, as in the fresh-water Mussel, and the general
course of the circulation is similar to what has already been
described in that Mollusc (p. 672).

Excretory System. — There is only one kidney (neph.),
a large organ situated dorsally, behind the pericardium. It is
a sac with thick, glandular, and highly vascular walls, the inner
surface of which is thrown into numerous complex folds. In front
it communicates directly by a large aperture (neph. ap.) with the
mantle-cavity, and by a narrower passage with the pericardium.

The nervous system (Figs. 626 and 627) is more highly
elaborated than in the fresh-water Mussel. Two pairs of nerve-

cer.buc.con*

FIG. 627. — Triton rubicundus. Lateral view of nerve-ganglia and related parts. Letters
as in Fig. 626 ; in addition— ant. aort. anterior aorta ; cr. crop ; ces. oesophagus ; sal. du.
salivary duct ; r. sal. ffl. right salivary gland.

ganglia — the cerebral (cer. g.) and the pleural (pi. g.) — lie close
together over the posterior part of the oesophagus, just where it
passes into the crop. The right and left cerebral ganglia are
fused together in the middle line, though separated by a con-
striction, and the ganglia of the two pairs are placed very close
together, though quite distinct. From each cerebral ganglion
there passes forwards a stout cerebro-buccal connective (cer. buc. con.)
to a buccal ganglion (r. buc. g.) situated on the posterior surface of
the buccal chamber. Also given off anteriorly from the cerebral
ganglia are optic nerves (opt. n.) to the eye and tentacular nerves
(tent, n.) to the tentacles. From each cerebral ganglion passes
downwards and forwards a stout cerebro-pedal connective, and from
each pleural ganglion a pleuro-pedal connective, to a large pair of
closely-united pedal ganglia (Figs. 626 and 627, ped. g.) embedded
in the upper layers of the muscles of the foot, to which they give
off numerous nerves. The right pleural ganglion gives off behind a

712

ZOOLOGY

SECT.

supra-intestinal visceral connective, which bends across to the left,
over the oesophagus, and, some distance to the left of the alimentary
canal, expands into a triangular supra-intestinal visceral ganglion
(supra, g.), situated below the superficial layer of muscular fibres.
The left pleural ganglion gives off an infra-intestinal visceral con-
nective, which passes obliquely backwards and to the right, below
the alimentary canal, to a ganglion situated a little to the right
of the middle line — the infra-intestinal visceral ganglion (infra, g.).
The supra-intestinal ganglion gives off a nerve which runs towards
the osphradium and ctenidium, which it supplies with branch
nerves, and unites with a stout mantle-nerve (I. mant. n.), which is
given off from the left pleural ganglion. The right pleural also
gives off a stout connecting nerve to the infra-intestinal ganglion.
From the supra- and infra-intestinal ganglia the left and right

visceral connectives are continued
backwards and unite behind in the
neighbourhood of the stomach ;
each ends in a triangular abdo-
minal ganglion (1. abd. g. ; abd. r.
g.), and these are joined by a
transverse commissure, from which
a number of visceral nerves (vise,
n.) are given off. A remarkable
torsion of the nerve connectives is
here to be observed, the two
visceral connectives becoming
twisted into the form of the
figure 8.

The organs of special sense
of Triton, in addition to the ten-
tacles and the osphradium, which
have been already referred to, are
the eyes and the statocysts. The eye (Fig. 628) is a rounded
invagination of the epidermis with an inner wall or retina (ret.}
composed of pigmented and sensory cells. The latter (retinophores)
are elongated cells narrowed at their central free ends, and pro-
duced at the opposite extremity to become continuous with nerve-
fibres of the optic nerve. The former (retinulce) have their free
extremities much enlarged, and surround the slender ends of the
retinophores. A layer of short rods (rds.) lies within the retina
proper. The outer wall is thin, and, with the overlying epidermis,
forms a transparent cornea. In the interior of the eye is a clear
rounded lens (I.) of dense cuticular matter secreted by the cells
of the retina ; this is surrounded by a less dense vitreous body.

The sexes are distinct. There is a single gonad — ovary or testis
as the case may be — lodged in the visceral spiral. The sperm-
duct is a white tube, thickish and much convoluted where it leaves

FIG. 628.— Triton. Section of the eye.
co. cornea ; ep. epidermis ; /. lens ; opt.n.
optic nerve ; rds. layer of rods (the line
is not continued far enough inwards) ;
ret. retina.

xii PHYLUM MOLLUSCA 713

the testis. narrower and straight distally ; it opens in front in the
mantle-cavity into the proximal end of tfre sperm-groove, which,
as already mentioned, runs forwards along the right side and
becomes continuous with the groove traversing the penis. The
oviduct (Fig. 623, ovid.) is proximally a very delicate tube with
colourless, transparent walls. This runs forwards to the right
side of the mantle-cavity, where it assumes the character of a stout
tube (ovid'.) with thickened glandular walls, which passes forwards
close to and parallel with the rectum, and opens on the exterior
near the anus.

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Gastropoda are unsymmetrical Mollusca, with a mantle
which is not divided into two lateral portions, and usually a shell,
which does not consist of two lateral valves, but of a single,
unsymmetrical, usually spirally coiled valve, enclosing a visceral
mass of corresponding form. There are, typically, two plume-like
ctenidia enclosed in a mantle-cavity, but there may be only one ;
and in air-breathing forms ctenidia are not developed, respiration
taking place through the wall of the mantle-cavity itself. A
distinct head bearing eyes and tentacles is present in the majority.
The foot is situated behind the head, and usually has an extensive
flattened ventral surface. The buccal cavity contains an odonto-
phore. The kidney is usually single. The nervous system contains
distinct cerebral and pleural, besides pedal, visceral, abdominal,
and buccal ganglia. The sexes are sometimes separate, sometimes
united. The larva passes through trochophore and veliger stages.

Sub-Class I.- Streptoneura.

Gastropoda in which the visceral connectives are in most cases
twisted into a figure of 8, and in which the sexes are distinct.

ORDER 1. — ASPIDOBRANCHIA.

Streptoneura with the nervous system but little concentrated :
the pedal ganglia are produced into long cords with the anterior
ends of which the pleural ganglia are fused ; the cerebral ganglia
wide apart ; the osphradium little developed. There is nearly
always a single ctenidium or a pair, plume-like and free distally.
The auricles and the kidneys are usually paired.

Sub-Order 1. — Docoglossa.

Aspidobranchia in which the pleural ganglia are not connected
with the opposite visceral connective. The eye is in the form of
an open pit, without lens. There are two osphradia, a single jaw,
and no operculum. The visceral mass is conical.

This section includes the Limpets (Patellidce).

VOL. i z z

714 ZOOLOGY SECT.

Sub-Order 2. — Rhipidoglossa.

Aspidobranchia in which each pleural ganglion is connected
with the opposite visceral connective. The eye is a closed sac
and contains a lens. There are nearly always a single osphradium,
a pair of jaws, and two auricles.

This sub-order includes the Ear-shells (Haliotidce), Trochus,
Turbo, and others.

ORDER 2.— PECTINIBRANCHIA.

Streptoneura with a somewhat concentrated nervous system.
There is a single osphradium which is often pectinate. The
primarily right ctenidium is alone developed. The heart has a
single auricle. The ctenidium consists of a stem with a single row
of lamellae, attached throughout its length to the wall of the
mantle-cavity.

Sub-Order 1. — Platypoda.

Pectinibranchia with the foot flattened ventrally, at least in
front. Jaws are nearly always present.

This sub-order includes the Cowries, the Vermetes, the Tritons,
the Whelks, the Cones, and a number of other groups.

Sub-Order 2. — Heteropoda.

Pelagic Pectinibranchia with the foot laterally compressed and
bearing, at least in the male, a ventral sucker. The visceral sac
and mantle form only a small part of the mass of the body. Jaws
are absent.

Sub-Class II.— Euthyneura.

Gastropoda in which the visceral connectives are not twisted _
into a figure of 8, and in whicn the sexes are united.

ORDER 1. — OPISTHOBRANCHIA.

Marine Euthyneura with aquatic respiration, the auricle of the
heart usually posterior to the ventricle. The mantle-cavity, when
present, opens by a wide aperture.

Sub-Order 1. — Tectibranchia.

Opisthobranchs provided in nearly all cases with a mantle and
a shell, nearly always with a true ctenidium, and an osphradium.

This section comprises the Aplysiidce, or Sea-hares, and several
other families, including certain pelagic Gastropoda, some shell-
bearing, some shell-less, formerly regarded as constituting a distinct
class — the Pteropoda.

xn PHYLUM MOLLUSCA 715

Sub-Order 2. — Nudibranchia.

Opisthobranchs which are devoid of a shell in the adult condition,
and have no true ctenidia or osphradia, respiration being carried
on by means of secondary branchiae usually arranged in a circlet
around the anus, or in rows on the dorsal surface, or laterally under
the edge of the mantle.

This sub-order includes Doris, Eolis, Tethys, and other shell-less
forms.

OEDEE 2. — PULMONATA.

Euthyneura devoid of ctenidia, respiration being carried on
through the walls of the mantle-cavity, which has a narrow con-
tractile aperture.

This sub-order includes the Land-Snails and Slugs.

Systematic Position of the Example.

Triton rubicundus is one of several species of the genus Triton,
which is the only member of the family Tritonidce belonging to
the sub-order Platypoda. The family Tritonidae differs from the
other families of the sub-order in the possession of a proboscis, of
a well-developed, but not greatly elongated, siphon, and of a short
foot,

3. GENEEAL OEGANISATION.

External Features, Symmetry, &c. — FewT Gastropods make
an approach towards even superficial symmetry, and in cases in
which there is a near approximation towards such a state of
things, it seems clear, from the results of the study of develop-
ment and of a comparison with allied forms, that the symmetry
presented is not primitive, but has been secondarily acquired —
such symmetrical forms having been derived from unsymmetrical
ancestors.

The departure from symmetry is most marked in the majority
of the Streptoneura. It may be said to be due to the develop-
ment of a protective shell composed of one piece and extensive
enough to be capable of enclosing all the soft parts ; and to
the extension of the foot on the ventral side as an elongated
muscular creeping organ. The development of the shell rendered
necessary an arrangement of the parts whereby the mantle-cavity
with the anus, the ctenidia, and the excretory apertures should
come to be situated in the neighbourhood of the opening of the
shell, i.e., towards the head-end of the animal. The mantle-
cavity and associated parts (pallial complex, as the whole is termed)
had, therefore, to be shifted forward from its primitive posterior
position, and this was probably effected by arrest or retardation of
growth on one side and active extension on the other.. In the
majority of cases it is the right side the growth of which becomes

VOL. I 7 7*

716

ZOOLOGY

SECT.

retarded, and, in consequence, it is on the right side that the
pallial complex comes to travel forwards. The effect is as if, the
bead retaining its symmetry, the parts between it and the anus
had undergone a process of rotation or torsion through about 180°
around a vertical axis passing in a dorso- ventral direction — the

C

r. vise. com

FIG. 629. — Diagrammatic representation of the displacement of the mantle-cavity and asso-
ciated parts in the Gastropoda. Enteric canal blue, blood-system red. A represents
a nearly symmetrical arrangement ; in B, C, D are represented successive stages of dis-
placement of the mantle-cavity to the right and forwards ; in E the anus and (primarily)
right ctenidium have passed the middle line. an. anus : aort. aorta ; cer. g. cerebral
ganglion ; 1. cten. left ctenidium ; 1. vise. com. left visceral connective ; rnant. mantle ;
mo. mouth ; neph. ap. renal apertures ; ped. g. pedal ganglion ; pi. g. pleural ganglion ;
r. cten. right ctenidium ; r. vise. com. right visceral connective ; vise. com. visceral con-
nectives. (After Korschelt and Heider.)

direction of torsion being opposite that of the movement of the
hands of a watch (Fig. 629).

With regard to the spiral form assumed by the shell in all highly
developed Gastropods, it can only be pointed out here that, given
the necessity for complete protection, compactness, and a provision
for continuous growth, the spirally-coiled cone is the form of shell

xn PHYLUM MOLLUSCA 717

best adapted to all the conditions. A straight cone, however
directed, would be a great impediment to active progression, and
the coiling in a compact spiral would seem to be the line of
development best adapted to secure concentration and strength.

The rotation around a dorso-ventral axis is not the only form of
torsion leading to the markedly unsymmetrical disposition of parts
observable in most Gastropoda. There is also a process of torsion
around a horizontal axis which, the head and foot being regarded
as fixed, results in the visceral mass enclosed in the spiral shell
coming to occupy a more or less dorsal position with the apex
directed backwards and to the right.

The shifting of the pallial complex in many Streptoneura
proceeds so far that the complex completely or partially passes
across the middle line, and the anus comes to be situated to the
left of the mouth. The displacement of the pallial complex
involves two important series of changes in the internal organs, in
addition to the suppression of certain structures to be referred to
presently. In the first place there is necessarily involved a
throwing of the enteric canal into a loop (Fig. 629), and in the
second place the long pleuro-visceral connectives (vise. com. )
become twisted in such a way as to assume the form of the figure 8
—the right connective becoming supra-intestinal and the left
infra-intestinal.

Universally accompanying the process of forward displacement
of the pallial complex, except in Haliotis and Fissurella and allied
Ehipidoglossa, occurs the reduction of its paired parts. Thus in
all the more highly developed Streptoneura the primitively right
(topographically left) ctenidium alone persists, and one of the two
kidneys is alone fully developed and functional.

In the Euthyneura there are distinguishable various stages in a
process of detorsion by which the torsion tends to be reversed and
the pallial complex carried back towards the posterior end along
the right side. The pleuro-visceral connectives lose their twisted
arrangement in nearly all such cases ; but there is the same
reduction of the paired parts of the pallial complex as in the
Streptoneura.

The shell in the adult limpets (Patella and allied genera) is in
the form of a short cone. In most of the Gastropoda it is in the
shape of a spiral with the turns usually in close contact with one
another, the inner walls of the turns coalescing to form an axial,
hollow or solid column — the columella. The portion of the shell
projecting inwards between the turns of the spiral sometimes
becomes absorbed. In certain cases, on the other hand, the cavity
of the apical portion of the spiral may be cut off from the cavity of
the rest of the shell by the formation of a transverse partition, the
animal then becoming restricted to the basal portion ; or several
such partitions may be formed. By far the greater number of

718

ZOOLOGY

SECT.

(From the

Natural History.)

such spiral shells are dextral, i.e. if we begin at the apex of the

spiral to reach the opening of the shell
we have to pass from left to right, with
the columella always on our right-hand
side : in a few cases, however, the spiral
is sinistral, taking the opposite direction
from that of the ordinary dextral shell.
The form of the shell varies with
the degree of obliquity with which the
whorls are set on the axis. When
the obliquity is very slight (Fig. 630) the
spiral is nearly flat ; when the obli-
quity is great, an elongated tapering
shell such as that represented in Fig. 631
"Cambridge [s the result. Sometimes the later whorls
completely cover over the earlier ones,
so that the spiral form of the shell is concealed. Sometimes only
the apical portion of the shell is spiral, the re-
mainder being a straight or sinuous cylinder.
The spiral form of the shell and the parts enclosed
in it, as well as the direction of the spiral,
whether dextral or sinistral, are, it may be here
pointed out, very fundamental features of the
organisation of the Gastropod, and are fore-
shadowed at an early stage in the segmentation
of the ovum. The mouth of the shell has usually
a prominent margin or peristome, which is some-
times entire and continuous, sometimes broken
by a deep notch or a spout-like process or canal,
formed in connection with the development of a
spout-like prolongation of the mantle, the siphon,
which lies in it. The mouth of the shell in many
Gastropoda is capable of being closed by means of
an operculum borne on the foot. In some terrestrial
forms in which an operculum is absent, the opening
may be closed up during winter by a layer of
hardened mucous matter to which the name of
epiphragm is applied. The margin of the mantle
in some cases bears a series of tentacles. Lateral
folds of the mantle are in some of the Gastropoda
(Fig. 632) reflected over the shell and may com-
pletely cover it. In some cases these folds unite
by their edge, so that the shell comes to be
enclosed in a complete sac of the mantle ; such
enclosed shells are always imperfectly developed
and incapable of covering the body. Thus in Fw 631 _shell of
Aplysia and some other Opisthobranchs the shell Terebra'ocuiata.

xn

PHYLUM MOLLUSCA

719

is greatly reduced, thin and horn-like, and concealed within the
mantle, while in the
nudibranch members of
the same sub-order it
is entirely absent (Fig.
633). The shell is also
completely absent in
some of the pelagic
forms (Heteropoda and
Pteropoda) ; in others,
though present and ex-
ternal, it is too small to
enclose the animal (Fig. 634). In the slugs, among the Pulmonata,
the shell is vestigial and in most cases is concealed by the mantle

(Fig. 635).
The foot varies in the extent of its

development in the different families

FIG. 632.— Cypraea moneta (Cowrie). Showing the
mantle, provided with marginal tentacles, partly en-
veloping the shell. Br. siphon ; MM. mantle ; F. foot ;
T. tentacles at the edge of the mantle. (From Cooke,
after Quoy and Gaimard.)

FIG. 633.— Doris (Archidoris)
tuberculata. a. anus ; br.
branchiae ; m, penis ; rh, rh,
tentacles. (From the Cam-
bridge Natural History.)

FIG. 634. — Carinaria mediterranea. a. anus ;
br. branchia ; /. foot ; i. intestine ; ra. mouth ; p.
penis ; s. sucker ; sh. shell ; t. tentacles. (From the
Cambridge Natural History.)

po

of the class. It usually presents an elongated flat ventral surface
on which the animal creeps by wave-like contractions of the
muscular tissue. An exceptional case is that of Caecum, in which

the creeping movement is
entirely due to the action of
cilia covering the ventral
surface. In the typical
Gastropods the foot is usually
distinguishable into three
parts, a middle part or meso-
podium, which is the most
important, with a smaller
anterior propodium and pos-
terior metapodium. In many
burrowing forms (Fig. 636)

FIG. 635.— A Slug (Umax). PO, pulmonary ,1 rnYvnorlinm is

aperture (From the Cambridge Natural History.) ^ne prOpO( im IS

720

ZOOLOGY

SECT.

developed and sharply marked off to act as a burrowing organ. In
a few cases a pair of tentacles — the pedal tentacles — are situated at
the anterior end of the foot ; still rarer is a pair of similar appen-
dages at the posterior end. The whole foot becomes reduced in the
few Gastropods that remain fixed. The metapodium very usually

FIG. 636.— Sigaretus laevigatus, exemplifying great development of propodium (pr.) and
metapodium (met.), in a burrowing Gastropod. The shell has been removed. /. meso-
podium ; 1. " liver " ; s. ap. aperture of siphon ; t. t. tentacles. (From the Cambridge
Natural History, after Quoy and Gaimard.)

in the Streptoneura bears a disc or stopper — the operculum already
referred to — usually horn-like, rarely completely calcified, more
commonly horn-like with a thin calcareous investment — by means of
which the aperture of the shell is closed when the animal is retracted.
In some forms, such as the Sea-hares
(Aplysia, Fig. 637), the foot develops a pair
of lateral lobes — the parapodia — which act as
fins ; and in the Pteropods (Fig. 638), which
are specially modified for a pelagic existence,
these constitute the largest part of the foot.
In the Heteropoda (Figs. 639, 640), which are
also pelagic, the foot is also modified to act
as a swimming organ. In one family of ;this
sub-order (Fig. 639) all three parts of the
foot are well developed, the mesopodium bears
a sucker, and the metapodium an operculum ;
in the rest the mesopodium is alone well de-
veloped and forms a laterally-compressed,
vertically-elongated fin. The term epipodium
is applied to a ridge or fold, which, when
FIG. 637,-Apiysia, dorsal best developed, runs around the entire side
view. v, parapodia. of the foot, and may be beset with papillse

(After Keferstein.) i Ti *

or tentacle-like processes.

A pedal gland is present in the majority : it is a simple or branched
invagination of the integument, lined by mucus-secreting cells.
Very commonly, as in Triton, it opens on the exterior in the middle
line of the ventral surface of the foot.

The Gastropoda have a \Yell-marked head, separated from the
body by a constriction or neck. The mouth, situated at the anterior

xn

PHYLUM MOLLUSCA

721

Fra. 638. — Shell-bearing Pteropoda. /. /. flns ; 1. liver ; o. ovary ; sh. shell. (From Cooke,

after_Souleyet.)

FIQ. 639.— Atlanta peronii. a, cerebral ganglia; b, pedal ganglia; e. eye ; g, ctenidia;
h. heart ; Jc, kidney • i. " liver " ; m. mouth ; o. ovary p. operculum ; t. testis.

ctcn

Fia. 640.— Pterotrachea scutata. ali. alimentary canal; cten. gills; eye, eye ; fl. float";
wo. mouth ; prob. proboscis ; repr. gonad ; sh. shield covering a portion of the dorsal
surface ; su. sucker.

722 ZOOLOGY

end of the Lead on its ventral aspect, is in many instances provided
with a protrusible proboscis or introvert, sometimes of considerable
length. In the dorsal surface of -the head are a pair of tentacles
which vary a good deal in shape, but are usually cylindrical or club-
shaped. In most cases the eyes are situated on tubercles at the
bases of the tentacles, or elevated towards the middle ; but in the
snails and slugs (Pulmonata, Fig. 641) the eyes are elevated on the
extremities of a second, longer, pair of tentacles (oc. tent) placed
behind the first.

The mantle is usually developed into a fold — the mantle-flap —
originally posterior, but subsequently becoming shifted round, in
the course of the displacement already referred to, to the right-
hand side. This covers over a cavity — the mantle-cavity — situated
anteriorly, in which are situated the anal and renal apertures and
the ctenidia. The edges of the mantle-flap may become united
together in such a way as to form a chamber opening on the exterior

oc.lenc

FIG. 641. — Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye-
bearing tentacles ; pulm. opening of pulmonary sac ; tent, anterior tentacles. (After
Pelseneer.)

by a comparatively narrow opening. In many of the Prosobranchia
the edges of this aperture are drawn out- into a spout-like prolonga-
tion open ventrally — the siphon — which lies in the corresponding
prolongation of the peristome of the shell and serves as a channel
for the ingress and egress of water. In some Gastropods, however,
there is no definite mantle-cavity, the anus, nephridial apertures,
and ctenidia merely lying under cover of a comparatively slightly
developed lateral mantle-flap. Usually there is on the inner surface
of the mantle a glandular area — the pallial mucus-gland.

Respiratory Organs. — There are typically two ctenidia, one
on the right side and the other on the left, contained in the mantle-
cavity ; but in the great majority of the Streptoneura and branchiate
Euthyneura the primitively right (actually left) ctenidium alone is
retained. In those Gastropoda that possess two ctenidia, and in
many forms with only one, the axis of the ctenidium bears two
rows of compressed filaments, and is attached only towards its
base. But in the majority of those with one ctenidium there

'II

PHYLUM MOLLUSCA

723

is, as in Triton, only a single row of filaments retained, and the
organ is attached throughout its length.

ve

Tn.e

FIG. 642.— Pleurophyllidia lineata,

from the ventral surface, a. anus ;
br. secondary branchiae : m. mouth ;
s. o. sexual opening. (From the Cam-
bridge Natural History.)

FIG. 643.— Patella vulgata, seen from the
ventral side. /. foot ; g. /. circlet of gill-
lamellae ; m. e. edge of the mantle ; raw. attach-
ment-muscle • si. slits in the attachment-
muscle : sh. shell ; v. efferent branchial vessel ;
»'. aorta ; ve. smaller- vessels. (From the Cam-
bridge Natural History.)

pul.v

In the Nudibranchs true ctenidia are absent, but their place
as breathing organs is taken by a number of secondary branchice,

sometimes simple,
sometimes branched
or pinnate processes,
which are distributed
over the dorsal sur-
face, as in Eolis ; or,
as in Doris (Fig. 633),
form a circlet sur-
rounding the anus ; or,
as in Pleurophyllidia
(Fig. 642), a row on
each side beneath the
mantle-flap.

In the limpets
and its allies,
the true

ctenidia are r e p r e -
sented only by a pair of vestiges, and respiration is carried
on by a number of secondary branchiae (g. (.) in the form of
lamella situated between the short lateral fold of the mantle

vent

ciorl

eph

FIG. H44. — Pulmonary cavity and related parts in a slug
(Xiimax). aort. aorta ; aur. auricle ; neph. kidney ;
peric. pericardium, laid open ; pul. ap. pulmonary aperture ;
pu7. v. pulmonary vein with its ramifications ; rect. rectum;
ur. ureter ; rent, ventricle. (After Pelseneer.)

724 ZOOLOGY

SECT.

and the foot. In the Pulmonata, and in some members of other
groups, ctenidia are absent, and the mantle-cavity, completely
enclosed except for a small rounded opening, has the function of a
pulmonary sac or lung (Fig. 644), its roof being richly supplied
with blood-vessels : in the aquatic forms its function is apparently
as much hydrostatic as respiratory. In one family of Pulmonata,
the pulmonary chamber gives off a number of branching air-tubes
or trachea. In some of the Pulmonata there is a return to a
completely aquatic mode of respiration accompanied by the develop-
ment of secondary gills — vascular processes of the wall of the
mantle-cavity.

Near the base of each ctenidium is an elevation — the
osphradium — corresponding to the body of that name in other
Mollusca and having a similar function.

Digestive Organs. — In many Streptoneura there is a long
introvert, capable of being everted and retracted, at the
extremity of which the mouth is placed. A single curved horny
jaw lies on the roof of the buccal cavity in the Pulmonata ; in
most Streptoneura (asjn Triton) the place of this is taken by
two lateral pieces.

A characteristic feature of the alimentary canal of the Gastro-
poda, which, however, they share with some Amphineura and with
the Cephalopoda, is the possession of an odontophore and radula,
a typical example of which has been described in that of Triton.
In the different groups differences are observable in the odonto-
phore as regards the proportions of the parts, and the size, form,
and arrangement of the teeth. The structure and relations of the
alimentary canal are similar to those already described in Triton,
and salivary glands and " liver " (hepato-pancreas) are always
present. The former may be tubular, but are usually botryoidal :
the latter varies in relative extent and in the arrangement of its
lobes in different forms.

In some Opisthobranchia the stomach contains a series of teeth
which are sometimes sharp and chitinous, sometimes plate-like and
calcined. Frequently a special development of the cuticular lining
of the stomach forms a hard rod — the crystalline style, lodged in
a caecum and comparable to the body of the same name in the
Pelecypoda (p. 687). A pyloric caecum is frequently appended to
the stomach. The intestine is long and thrown into folds in the
vegetable-feeding forms, short and straight in the carnivorous.
In some cases, e.g. Haliotis, it traverses the ventricle, in others
the pericardium ; in others it passes through the nephridium. In
Eolis (Nudibranchia) the stomach gives off a number of glandular
cseca which penetrate into the interior of the secondary branchiae
or cerata on the dorsal surface ; these^caeca take'the place of the
" liver " of other Gastropoda. In some of the Pectinibranchia
there is a peculiar adrectal gland, situated at the side of the rectum

xn

PHYLUM MOLLUSCA

725

and secreting a colourless fluid, which in Murex and Purpura turns
purple on exposure to the air, and was anciently used as a dye —
the " Tyrian purple."

The heart is, as in other Molluscs, enclosed in a special cavity —
the pericardium — a specialised part of the ccelome, communicating
with the cavity of the kidneys. It consists usually, as in Triton,
of two chambers — auricle and ventricle ; but in some, e.g. Haliotis,
there are two auricles and a ventricle. In the Opisthobranchia,
as already mentioned, it lies in front of the ctenidia ; in the Strep-
toneura at the side or behind. Given off from the apex of the
ventricle is a large vessel which soon bifurcates to form anterior
and posterior aortae. These are the main trunks of the arterial
system, which is more highly developed than in the Pelecypoda ;
the finest branches terminate
in sinuses, as in the latter
class.

The nervous system
varies considerably in the
different groups in regard to
the arrangement of the
ganglia and their commis-
sures and connectives.

In the majority the
arrangement is nearly that
which has been described as
occurring in Triton. There
is a pair of cerebral ganglia
usually closely united, but
in Patella (Fig. 645) widely FIG. 645.
separated, situated over the
gullet, and giving off behind
a pair of visceral nerve-cords,
in the course of which there is placed laterally a pair of
pleural ganglia, and which are united together behind - in a
median abdominal ganglion (or a paired ganglion, as in Triton).
In the course of these visceral cords there is also a pair of visceral
ganglia. A pair of pedal ganglia, united together by a transverse
commissure and joined to the cerebral and pleural ganglia by
connectives, give off behind one or two pairs of pedal nerves, as
already mentioned. A pair of buccal ganglia are connected by
slender nerves with the cerebral. At the base of each osphradium
is usually a small osphradial ganglion connected by a slender nerve
with the visceral. In most Streptoneura (Fig. 629), in accordance
with the torsion of the visceral mass, the visceral cords are twisted,
as already described in the case of Triton, into a figure of 8 : the
right visceral ganglion becomes displaced to the left and comes
to lie above the alimentary canal (supra-intestinal), while the

Nervous system of Patella, cer. g.
cerebral ganglia ; mant. n. mantle-nerves ;
osphr. g. osphradial ganglia ; ped. g. pedal
ganglia and pedal nerve-cords ; pi. g. pleural
ganglion. (After Spengel.)

726

ZOOLOGY

SECT.

left becomes displaced to the right and comes to lie below the
alimentary canal (infra-intestinal).

In Patella (Fig. 645) the pedal ganglia
(ped. g.) give origin to a pair of elongated
pedal nerve-cords. In Haliotis and Fissurella
there is a similar pair of pedal cords which are
connected together by transverse commissures,
and, in the latter genus, join one another
posteriorly.

In the Euthyneura (Fig. 646), except in Actceon
and Chilina, the visceral cords are not caught
up in the twist of the visceral mass, and do
not cross one another. They unite behind in
a ganglion (abd. g.) which represents both the
abdominal and the supra-intestinal visceral.

In the Snails and other Pulmonata (Fig. 647)
the ganglia of the nervous system are more
closely aggregated together. A pair of cerebral
ganglia overlie the oesophagus, and below it is
a mass of ganglia in which are to be made out
a pair of pedal ganglia and at least two pairs
of ganglia representing the pleural and visceral.
A pair of small buccal ganglia are connected with
the cerebral by means of slender connectives.

The organs of special sense are the eyes,
the statocysts, and the osphradia. In nearly
all cases there are two cephalic eyes (Fig. 648),

the position of which has already been referred to in the account

given of the external characters. In structure they are simplest

in Patella (A), where each con-
sists of a pit-like depression,

lined by pigmented cells con-
nected with nerve-fibres. In the

majority they have the structure

described in the case of Triton.

In certain species of Oncidium,

a littoral Pulmonate, there are

numerous eyes of a simple type

scattered over the dorsal surface.

In this case the optic nerve

pierces the retina and the cells

of the latter have their free ends

directed away from the centre of

the eye, as in Pecten (see p. 690)

and in the Vertebrata, instead of

FIG. 646. — Nervous sys-
tem of Aplysia (Opis-
abd.

ganglion ; cer. g. cere-
bral ganglion ; osphr.
g. osphradial ganglion ;

(After Spengel.)

-, . . -i •»«• -ii

towards it, as in Other MolluSCa.

Tile internal cavity of the eye

FIG. 647 —Nervous system of Limnaeus (Pui-

monata). abd. g. abdominal ganglion ; cer. g.
cerebral ganglion ; osphr. g. osphradial gang-

*1 *' °' pleural

xn

PHYLUM MOLLUSCA

727

in Oncidium is occupied by a refractive body composed of a
few large transparent cells.

The statocysts are usually placed in close relation to the pedal
ganglia, but are always innervated from the cerebral. An olfactory

FIG. 648.— Eyes of Gastropoda. A, Patella; B. Trochus; C, Turbo; D. Itturex.
ep. epidermis ; /. lens ; op. n. optic nerve ; r. retina ; v. h. vitreous humour. (From the
Cambridge Natural History, after Helger.)

organ is present in the shape of groups of cells on the tentacles,
in which the fibres of an olfactory nerve terminate.

The osphradia are prominences, usually of simple form, situated

close to the base of the
ctenidium. In many of the
branchiate Streptoneura (Fig.
649), as already mentioned
in the case of Triton (see
p. 706, Fig. 623), the primi-
tively right osphradium,
which is alone developed,

FIG. 649— Transverse section of osphradium of j,^^,^^. ft.~ fnrrn nf p npp
BXurex. br. n. branch nerve passing to lamina ; assumes tne L a pet

*A?terasSyi °Sphr' n' main osphradial nerve> tinate body with a central

ridge, on either side of which

is a row of close-set lateral laminae, and is commonly termed the
parabranchia from its resemblance in appearance to a gill. In

728

ZOOLOGY

SECT.

some cases it is of even more complicated shape than in Triton,
owing to the branching of the lateral ridges.

The excretory organs or kidneys of the Gastropoda are
dorsally placed glandular tubes or chambers, which communicate
internally with the pericardium, and open on the exterior, either
directly or through a duct — the ureter. Both right and left kidneys
may be present, though unequal in size, the one situated to the
right of the anus being larger than that situated to the left ; or

the former may alone

&

*tb

^cilbgrl

rec.sem

be developed (Euthy-
neura). In a very
limited number of Gas-
tropoda the gonad
opens into the kidney.
The sexes are sepa-
rate in nearly all the
Streptoneura, united
in the Euthyneura.
Special gonoducts are
present, except in one
or two forms in which
the nephridia perform
that function. In the
unisexual forms the
reproductive appara-
tus is of a compara-
tively simple character,
consisting merely of a
racemose reproductive
organ, ovary or testis
as the case may be,
situated dorsally in the
visceral spiral, with

FIG. 650. — Reproductive organs of Helix, alb. gl. albumen-

gland ; ds. dart-sac ; flag, flagellum of the penis ; herm. the ffOnoduct opening
gl. hermaphrodite gland or ovotestis ; herm. d. duct of
ovotestis ; muc. gl. mucous gland ; muc. gl. ap. apertures of
mucous glands into vestibule; ovid. oviducal part of the ,,;«.>, fV,Qr>rl
common duct; ovid. ap. aperture of oviduct into vesti- rignt-nana

£

iar |IOrwardS On tne

bule; pen. penis; rec.~ sem. receptaculum seminis:; rec. m the male, a penis.
sem. ap. aperture of receptaculum seminis ; sp. d. sperm . .. . , f .

duct ; sp. d'. spermiducal part of common duct. (After Which IS gTOOVCd longl-

tudinaUy and non-
retractile. In the hermaphrodite forms, such as the Pulmonata
(Fig. 650), on the other hand, a considerable degree of complexity
is observable. There are an ovotestis or "^hermaphrodite gland "
(herm. gl., Fig. 651, A) — some of the follicles of which produce
ova while others produce sperms, a convoluted " hermaphrodite
duct " (herm. 'd.), an albumen-gland, in which the albumen
of the relatively large eggs is formed, and sometimes a separate
oviduct and sperm-duct leading to a common genital opening ;

PHYLUM MOLLUSCA

729

sometimes there is a single duct undivided throughout ; some-
times there is incomplete division. A receptaculum seminis
(rec. sem.) is connected with the oviduct, and also a number
of narrow accessory oviducal glands (muc. gl.) ; frequently a gland
termed prostate is connected with the sperm-duct, and there are
an eversible sac — the sac of the dart (ds.) — containing a crystalline
stylet, and a penis (pen.), which is perforated by a canal and is
capable of being retracted by a special muscle. In the Pulmonata
the first part of the duct (" hermaphrodite duct " proper) is simple,
and serves for the passage both of ova and sperms : the middle
part is incompletely divided internally into two passages, one
serving as oviduct, the other as sperm-duct. In the distal part
oviduct and sperm-duct are completely separate. Where the
sperm-duct enters the penis, there is given off a long, slender,
tapering diverticulum, the flagellum (flag-), in which the sperms
are made up into elon-
gated masses or sper-
matophores.

Development.—
The limpets (Patella)
are exceptional in lay-
ing the eggs one by
one and unfertilised
— impregnation tak-
ing place in the water
after they have been
discharged. In almost
all the Gastropoda
fertilisation is internal,
and the eggs are laid in
great masses, embedded in jelly, each egg haying its own hyaline
envelope. Very often the mass of spawn consisting of the jelly-like
substance, with the eggs embedded in it, attains a relatively con-
siderable size. In form it varies greatly : frequently it is in the shape
of long strings which are cylindrical or band-like : sometimes several
such strings are twisted together into a cord. Sometimes the
spawn is fixed to sea-weed or other objects ; sometimes it is un-
attached, and may float about freely. In the Streptoneura (Fig.
652), instead of being embedded in a jelly-like mass, the eggs are
enclosed in a firm parchment-like capsule, in which is contained,
in addition to the eggs, a quantity of an albuminous fluid, serving
to nourish the developing embryos. The shape of the capsule
varies greatly in the different genera : sometimes it is stalked,
sometimes sessile ; in some cases there is a lid or operculum, the
opening of which permits the embryos to escape. Very commonly
large numbers of these capsules are aggregated together, and usually
they are attached to a rock or a sea- weed or similar object. In

FIG. 651. — Follicles of tire ovotestis of the Gastropoda. A,
of Helix hortensis (Pulmonata): B, of the Eolidae.
a. a, ova ; b, masses of sperms ; c. common efferent duct.
(From Gegenbaur.)

730

ZOOLOGY

SECT.

many cases only a limited number — sometimes only one— of the
embryos contained in the capsule become developed, the rest
serving as nutriment for the survivors.

In the land Pulmonata each ovum may be embedded in gelatinous
matter enclosed in a firmer envelope, and a number of them are
arranged in a string ; sometimes a large number are embedded
in a rounded gelatinous mass. Usually, as in some species of
Helix and other genera, the outer layers of the albumen-like
substance enclosing the egg become toughened and impregnated
with salts of lime, so as to assume the character of a calcareous
shell ; a number of such eggs, which are of relatively considerable
size, are laid in holes exca-
vated in the earth.

In a few marine and
fresh-water Gastropoda the
ova undergo their develop-
ment in the body of the
parent, enclosed in an en-
largement of the oviduct
which serves as a uterus.

The egg contains a con-
siderable quantity of food-
yolk, which may be evenly
distributed, or a clear pro-
toplasmic and an opaque
yolk-laden segment may be
distinguishable. There is
a fairly close agreement
throughout the class in the
nature of the segmentation
(Fig. 653), which is very
similar in the early stages
to that of Nereis and the

PWtrmnrla in frpnprnl (r> FlG- 652.— Forms of egg-cases in Gastrgpoda. A and

erai (p. ^ pyrula or Busycon; B, Conus; C,

440) and to that Of a Poly- Voluta musica; E Ampullaria. (From

i -i / r»/»r>\ T 11 the Cambridge Natural History.)

clad (p. 268). In all cases

it is total, sometimes equal at first, but soon afterwards becoming un-
equal. The first four blastomeres, A, B, C, D (cf. Fig. 219) are
usually equal or nearly so. From these a succession of four quar-
tettes of smaller cells (micromeres) become divided off, the larger
cells being the megameres. The former then increase by division
and form a cap of small cells (ectoderm) on the surface of the
megameres. Of the cells of the fourth quartette, three become
endoderm cells, contributing to the endodermal lining of the
mesenteron. The fourth becomes the parent cell of the mesoderm.
The megameres themselves eventually become converted into
endoderm cells with the exception of D, which, before becoming

XII

PHYLUM MOLLUSCA

731

an endoderm cell, gives off externally its daughter-cell of the
fourth quartette destined, as in the Polychseta (p. 441), to
give rise to the mesoderm. A segmentation-cavity is developed
between the micromeres and the megameres, and the result is
the formation of the blastula, one side of which (vegetal pole) is
greatly thickened owing to its consisting of the large megameres,
the opposite side (animal pole) being made up of micromeres.
This may become a gastrula by epiboly or over-growth of the

TTtlC

7n.es

FIQ. 653. — Diagram of the segmentation and formation of the germinal layers of the Gastro-
poda. A and B, lateral view ; C — F, viewed from the animal (upper) pole ; H, from the
vegetal (lower) pole ; G, in optical section ; eat. ectoderm ; end . endoderm ; mic. micromeres ;
meg. megameres ; mes. mesoderm ; poL polar bodies. (After Korschelt and Heider.)

ectoderm over the megameres ; or, if the segmentation-cavity is
of considerable size (Paludina), an invagination takes place.

The two larval stages, the trochophore and the veliger, are
characteristic of the development of the Gastropoda. The former
is most typically developed in Patella ; in other Gastropods it
undergoes more or less modification. In Patella (Fig. 654) there
is a ciliated blastula (A) which has on one side the large megameres.
The latter become enclosed by the micromeres, and the foundation
of the mesoderm is laid in the manner already described. The

732

ZOOLOGY

SECT.

blastopore is situated at the vegetal pole, destined to become
the hinder end of the larva, but it soon changes its position and
extends forwards on the ventral side, and a ciliated ring — the
prototroch or future velum — is formed. Subsequently the position
of the blastopore becomes still further shifted and its form U-shaped
and then slit-like. It undergoes elongation (Fig. 655, A) and

B

shgl

FIG. 654. — Earlier stages in the development of Patella. A, blastula ; B, beginning of endo-
dermal inyagination ; C, completion of gastrula ; D, frontal section of somewhat later stage.
ap. pi. apical plate ; bl. blastopore ; endm. endo-mesoderm cell ; end. endoderm ; mes. meso-
derm ; mesent. mesenteron ; prot. prototroch ; sh. gl. shell-gland. (From Korschelt and
Heider, after Patten.)

eventually becomes partly closed up, the closure taking place
from behind forwards ; in most cases in the position of the anterior
part a sinking-in of the ectoderm forms the rudiment of the
stomodseum. The originally solid mass of endoderm develops a
lumen, and its cells become arranged to form the enteric epithelium.
From the posterior end, where the mesoderm cells are situated,
proceed two very regularly formed mesoderm-bands (Fig. 655, B).

Xtt

PHYLUM MOLLUSCA

733

On the dorsal surface the shell-gland has already appeared as a
pit lined by elongated ectoderm cells ; on the surface of this appears

FIG. 655. — A and B, Trochophores of Patella at different stages. In A are to be seen the
circular -blastopore and the two foot-elevations ; in B the blastopore is drawn out, at the
sides of it are the two mesoderm bands. (From Korschelt and Heider, after Patten.)

the embryonic shell. The rudiment of the foot (Fig. 655, A)
arises at a remarkably early stage as two protuberances lying on
the ventral side of the
posterior end of the
larva at the sides of
the blastopore ; these
coalesce to form the
median foot.

The larva (Fig. 656)
has now assumed the
trochophore form. The
pre-oral part is large
and convex, with an
apical plate on which is
borne a bunch of long
cilia, and near it two
small ciliated elevations,
each consisting of a single
cell. The pre-oral part
of the larva then be-
comes much flattened,
and the apical plate (ap.
pi) increases in size and
importance. At the pos-
terior end is a bunch of

VOL. I

ctn.c

FIG. 656. — Later trochophore of larva of Patella in
longitudinal section, an. c. anal cells with cilia ; ap. pi.
apical plate ; /. foot ; mes. mesoderm cells ; mesent.
mesenteron ; mo. mouth ; rad. rudiment of radu la-sac ;
sh. shell. (From Korschelt and Heider, after Patten.)

3 A

734

ZOOLOGY

SECT.

cilia which are borne on two special large cells, the anal cells (an. c).
The embryonic shell becomes saucer-shaped. A slight ridge in the
neighbourhood of the shell represents the border of the mantle.
The mid-gut (mesent) has become considerably widened : a
diverticulum from it is recognisable, and this afterwards opens
on the exterior to form the anus. A diverticulum of the fore-
gut (rad) at the same time forms the rudiment of the radular
sac. The statolith-sacs appear as depressions of the ectoderm at
the sides of the mouth : these grow inwards and become sac-like,
subsequently lying at the sides of the foot, which has meantime
attained a considerable size.

The trochophore-stage, which is so well marked in the case
of Patella, occurs in other Gastropods, though, as a rule, presenting

lent

tent

sh

FIG. 657. — Veliger stage of Vermetus. cer. g. cerebral ganglia ; eye. eye ; /. foot ; mo. month ;
ot. statocyst ; sh. shell ; tent, tentacle ; vel. velum. (After Lacaze-Duthiers.)

modifications perhaps traceable to the enclosure of the embryo in
an egg-shell and to the presence of much food-yolk. The history
of the blastopore is not the same in all cases ; in Paludina it becomes
converted into the anus ; in some the mouth is developed from
its anterior portion ; in others the stomodaeal invagination arises
after its complete closure, or may, with the mantle-cavity, only
become developed after the symmetry has been disturbed by torsion.
In most of the Gastropoda the pre-oral circlet or velum (Fig. 657,
vel.) undergoes a development not observable in the Pelecypod
embryo, and becomes greatly extended as a bilobed flap, the strong
cilia with which it is bordered rendering it a very efficient organ
of locomotion for the larva. With the full development of the
velum the larva passes into the veliger stage (Fig. 657). In this

xn PHYLUM MOLLUSCA 735

stage the shell (sh.) increases in size, loses its simple form, and
begins to develop a spiral. A cleft-like depression in the border
of the mantle on the right-hand side forms the rudiment of the
mantle-cavity in which, later, the gills are developed. The anus
when it first appears may be symmetrically placed, but later
becomes shifted to the right side and forwards as well as dorsally.
A pair of larval nephridia are developed, having a remarkable
resemblance to the larval nephridia of the trochophore of Poly-
chseta, but of mesodermal origin. Each consists of a longer or
shorter tube, usually intra-cellular, opening on the exterior at one
end, and at the other terminating in one or several solenocytes.
The foot (/.) may attain a considerable development during the
veliger stage, and on its posterior and dorsal part appears the
operculum. Two little processes on the velar area develop into
the tentacles (tent.), and the eyes (eye) appear at their bases. As
the foot and other organs advance in development the velum
decreases in size and gradually aborts, but in some cases a
portion of it persists as the subtentacular lobes or labial tentacles
in the neighbourhood of the mouth.

In the Pulmonata the velum is not well developed, except in
Oncidium, though the trochophore stage is well marked.

The young Gastropod is at first bilaterally symmetrical ; the
prevailing asymmetry is the result of unequal growth of the two
sides of the body. In the majority of cases it is the left side that
grows more actively than the right, a result of which is that the
posterior parts — the anus and the region surrounding it — are dis-
placed forwards towards the right, the space between the anus and
the mouth on that side undergoing little or no increase in length.
In the Opisthobranchia and the Pulmonata the anus with the
mantle-cavity and its contents become displaced forwards towards
the anterior end ; in most of the Streptoneura the anus, &c., in
their displacement forward pass beyond the middle line, one of the
most striking effects of which is the crossing of the pleuro- visceral
connectives already referred to.

Ethology and Distribution. — Only a few aberrant families of
Gastropoda are parasites. Most are aquatic, all the most primitive
forms being' inhabitants of the sea. Of the marine families the
majority move by creeping over the sea-bottom, some burrowing
in mud or sand, some in solid rock ; some are able to float in
a reversed position, adhering to frothy mucus secreted by the
glands of the foot ; certain exceptional forms such as Vermetus
are fixed in the adult condition by the substance of the shell.
A few families — the Heteropoda and the Pteropoda — are specially
modified for a pelagic mode of existence, and swim through the
water by flapping movements of the lobes of the foot, which act
as fins. Gastropods are found in the ocean at considerable
depths — up to nearly 3,000 fathoms. Many forms, however, are

3 A 2

736

ZOOLOGY

SECT.

inhabitants of fresh water, while many Pulmonata are terrestrial,
and occur even towards the summits of the highest mountains.

Fossil Gastropoda are known from almost the earliest fossil-
bearing rocks, and all the major divisions of the class are repre-
sented in formations of Palaeozoic age.

The mutual relationships of the various groups of Gastropoda
are shown in the following diagram (Fig. 658) : —

Plafypoda Heherojaoda

Rhi|3idoglossa

Docoglossa

Pulmona^

TecHbranchia

Nudibranchia

Scajahopoda

FlQ. 658. — Diagram to illustrate the relationships of the Gastropoda.

APPENDIX TO THE GASTEOPODA.

CLASS IV.— SCAPHOPODA.

The Scaphopoda or Elephant's tusk-shells are aberrant marine Molluscs
comprising only three genera — Dentalium, Siphonodentalium, and Pulsellum.
The body is elongated so as to be almost worm-like, with complete bilateral
symmetry. The mantle-folds are almost completely united to form a
cylindrical tube enclosed by the shell (Fig. 659), which is in the form of a
delicate, curved tube, open at both ends and wider at the anterior or oral
end than at the other. The foot (Fig. 660, /) is narrow, trilobed at the

extremity or provided with a ter-
minal disc, capable of being pro-
truded through the oral opening
of the shell, and used for burrow-
ing in sand. The mouth is
situated on a short oral pro-
boscis, and is sometimes sur-
section'of rounded by lobed processes or
pinnate palpi. Further back are
a pair of tentaculiferous lobes,

each bearing a large number of filiform tentacles, which are probably respira-
tory in function. The mouth leads into a buccal cavity containing an odonto-
phore. Connected with the mesenteron is a large bilobed digestive gland (I.).
The anus is situated ventrally behind the base of the foot. The vascular
system is extremely simple, consisting of sinuses without definite walls, and
there is no distinct heart, though in the neighbourhood of the rectum there

FIG. 659.— Dentalium, longitudinal
shell. (After Keferstein.)

XII

PHYLUM MOLLUSCA

737

is a specially contractile part of the principal sinus. Two tubes open near
the anus, the right one acting as a gonoduct, the left (k) entirely renal in
function. The sexes are distinct. There is an elongated unpaired gonad (g. ),
divided by lateral incisions into a number of lobes, occupying all the posterior
and dorsal parts of the body. Anteriorly it narrows to form the gonoduct.

The nervous system consists of paired cerebral,
pleural, pedal, and visceral ganglia ; the cerebral
ganglia are situated close together. There are no
eyes or statocysts.

In the gastrula stage the embryo, which is provided
with cilia, becomes free. The ciliated cells are
arranged in a characteristic manner in three rows
which, at first situated close together about the
middle of the body, become shifted at a later stage
near the apical pole, and amalgamated into a broad
band representing the pre-oral circlet of other
molluscan larvae ; at the same time a bunch of cilia
previously developed at the apical pole becomes
more conspicuous and a considerable part of the
general surface covered with more delicate cilia.
The blastopore, at first terminal, is shifted forwards
on the ventral surface until it comes to be imme-
diately behind the ciliated circlet. At its anterior
end an invagination gives rise to the mouth and
stomodseum.

The larva (Fig. 661) has now attained the stage of
a trochophore, in which, however, both apical plate
and primitive nephridia are wanting. A shell-gland
is developed, and soon the rudiment of the shell
appears. The post-oral region, at first inconsiderable
in size, soon undergoes an increase, until it forms
eventually by far the longest part of the body, while
the pre-oral region almost completely aborts. When
the post- oral region has attained a certain size, there
are developed on it two lateral folds, the rudiments
of the mantle (B), which grow inwards towards the
middle ventral line, and later on unite by their free
margins. The pre-oral circlet or velum changes its form — at first it is
conical, later it becomes plate-like, and is then gradually reduced, the

FIG. 660. — Dentalium,
anatomy, a. anterior
aperture of mantle ; /.
foot ; g. gonad ; k.
kidney ; /. digestive
gland. (From the
Cambridge Natural
History, after Lacaze-
Duthiers.)

FIG. 661. — Veliger of Dentalium. A, longitudinal section of a larva 14 hours old ; B, larva
of 37 hours ; C, longitudinal section of larva of 34 hours, m. mouth ; v, v, velum. (From
Cookc, after Kowalewsky.)

738 ZOOLOGY SECT.

larva sinking to the bottom ; and though still occasionally swimming with
the aid of the velum, coming to use the foot as a creeping organ. The shell
now increases in size step by step with the growth of the mantle, and bends
round the body of the larva until its edges meet and coalesce in the ventral
median line. Later it assumes the elongated conical form, curved towards
the dorsal side, characteristic of the adult. The foot at the same time
elongates and takes on the characteristic three-lobed shape.

Class V.- CEPHALOPODA.

The Cephalopoda, including the Cuttle-fishes, Squids, Octopods,
and Nautili, are marine Mollusca of a high grade of organisation.
There is a very definitely-formed head, bearing a pair of highly-
developed eyes, and surrounded by the anterior portion of the foot,
modified into arms or tentacles. The body is bilaterally symmetrical.
The posterior part of the foot is modified to form a funnel leading
out from the large mantle-cavity. A shell is sometimes present,
sometimes absent. When present it is usually internal, but
sometimes external, and in the Nautili is capable of containing
the body of the animal.

1. EXAMPLES OF THE CLASS.
i. THE CUTTLE-FISH (Sepia1).

Cuttle-fishes are marine Molluscs, which live usually at a depth
of a few fathoms, but often come into shallower water, and are
frequently caught in the trawl or the seine. The animal arrests
attention when compared with Unio or Triton by the strength,
and more particularly the rapidity, of its movements ; by the
possession of a pair of eyes resembling in size and complexity
those of a Fish ; and by various other features, all pointing to
a higher grade of organisation than is attained by the members
of the classes of Mollusca dealt with in the preceding pages.

External Features.— The Cuttle-fish (Fig. 662) has a distinct
head, bearing ten long arms, and a pair of large, highly-developed
eyes. The head is connected with the body by a constricted
region or neck. The trunk is elongated and shield-shaped, the
base of the shield being towards the head. The long axes of
head and trunk are in line with one another. Not only the head,
but also the trunk, is completely equilateral, in which respect
there is a marked contrast to Triton ; and this symmetry extends
to most of the systems of internal organs. The free extremity of
the head bears the mouth, and is accordingly termed the oral
extremity, the opposite extremity, the apex of the shield-shaped
body, is the aboral end. The surfaces of the shield are anterior

1 Most of the figures have reference to a common Australian species —
S. cultrata — but the differences between the various species of the genus are
slight and unimportant, and the description given will apply fairly well to any
oth r species.

XII

PHYLUM MOLLUSCA

739

or antero-dorsal and posterior or postero-ventral, its borders right
and left. The anterior surface is to be distinguished by its darker
colour, and by the firmness of the body-wall, due to the presence
in this position of a hard internal shell.

The aperture of the mouth is surrounded by the bases of the
ten arms. These are in pairs, situated to the right and left of the
median plane. All of them, with
the exception of the fourth pair
(the most anteriorly situated pair
being reckoned as the first), are
stout at the base and taper towards
the extremity. When extended
they are about two-thirds of the
length of the body. The outer
surface of each (i.e. that turned
away from the mouth) is strongly
convex, the inner flat, and beset
throughout its length with a
number of suckers, which are
arranged in four longitudinal rows.
Each sucker is in the form of a
shallow cup, supported on a short,
thick stalk ; the lip of the cup
is membranous, and immediately
within it is a narrow, horny
rim. Into the floor and walls of
the cup are inserted numerous
muscular fibres. When the sucker
is being brought into use it is
firmly applied to the surface of
the object ; by the contraction
of the muscular fibres the cavity
of the cup is then enlarged, and
a partial vacuum is formed, the
result being firm adhesion, owing
to the pressure of the surrounding
water. The fourth pair of arms,
usually known as the tentacles, are
comparatively long and narrow,
and provided with suckers only
towards their free ends, which are somewhat thickened and club-
like. In the male the fifth arm on the left side presents a slight
modification, some of the suckers being absent. This is an in-
dication of a change termed hectocotylisation, which, as will be
pointed out in the general account of the class, assumes in some
cases a very remarkable character. As the nerves which supply
them are derived from the pedal ganglia, there is no doubt that

FIG. 662.— Sepia cultrata. Entire
animal viewed from the antero-dorsal
aspect.

740

ZOOLOGY

SECT.

the arms of Sepia represent a portion of the foot of other Molluscs ;
but there is some doubt as to whether they correspond to the fore-
foot or to the epipodia of the Gastropoda. The head-region,
comprising as it does the arms (which are the chief part of the
foot) and the head proper, is termed the cephalopodium.

The trunk is covered over by the thick integument of the mantle,
which terminates toward the oral end in a ridge round the neck.
Anteriorly this ridge projects as a prominent rounded lobe under
cover of which the head can be partially retracted. Posteriorly it
forms the oral lip of the opening of a large cavity bounded by
the mantle — the mantle-cavity — which extends along the entire
posterior face of the body almost to the apex. The wide cleft
between the oral edge of the mantle and the posterior surface of
the body is not the only aperture leading into the mantle-cavity.
On the oral side of this cleft is a large tube — the
funnel (Fig. 667, inf.)— opening on the exterior
behind the neck, and internally communicating
by a wide aperture with the mantle-cavity. The
cleft is capable of being almost completely closed
by the apposition of a pair of oval projections
(mant. cart.) of the inner surface of the posterior
mantle-wall near its oral border, and a pair of
concave depressions (inf. cart.) on the opposite
(posterior) face of the funnel. The funnel is thus,
under ordinary circumstances, the main outlet of
the mantle-cavity. As such it not only carries to
the exterior the effete water of respiration, the
faecal matters from the intestine, and the products
of the excretory and reproductive organs, but also
takes an important part in locomotion, the most
important movements of the Cuttle-fish — by
which it darts rapidly through the water in the
direction of the aboral pointed end of the body
—being effected by rhythmical contractions of
the muscular walls of the mantle-cavity causing jets of water
to be forced in the oral direction through the funnel. The
free passage of water inwards through the funnel is prevented
by the presence in its interior of a flap-like valve opening outwards.
The water required for respiration and in locomotion is thus drawn
in, not through the funnel, but through the partially-closed slit-
like pallial aperture previously referred to. The funnel seems,
from the source of the nerves which supply it, to be, like the arms,
a specially modified part of the foot.

Fringing each lateral margin of the body is a thin muscular
fold — the fin — which is used as a swimming organ.

The anterior wall of the body exhibits, as already mentioned, a
hard and resistant character owing to the presence of the internal

FIG. 663.— Shell
Sepia cultrata,
posterior view. Re-
duced.

xn

PHYLUM MOLLUSCA

741

shell (Fig. 663). This is completely enclosed in a sac of the
mantle. Like the body itself, it is bilaterally symmetrical. In
shape it may be described as leaf-like, with a rounded and
comparatively broad oral end, and a narrower aboral end, provided
with a sharp, anteriorly-projecting spine. The posterior surface is
convex ; the anterior convex towards its oral end, but deeply
concave aborally, and bounded laterally by thin prominent wing-
like ridges which converge to meet at the aboral extremity. The
main mass of the shell consists of numerous, closely-arranged, thin
laminae of calcareous composition, between which are interspaces
containing gas. On the surface is a thin layer of chitinoid material,
and slightly thicker strips of similar composition run along the
margins.

The living Cuttle-fish will be observed to undergo frequent
changes of colour, and blushes of different hues are to be observed
passing over the [surface.
These are due to the presence
of numerous contractile pig-
ment - containing cells or
chromatophores (Fig. 664)
situated in the deeper layers
of the integument over the
entire surface. The chromato-
phores are flattened sacs with
elastic walls, the contracting
tendency of which is capable
of being counteracted by the
action of bundles of muscular
fibres radiating outwards from
the edge of the sac into the sur-
rounding tissues. When these
radiating fibres are in action the
edge of the. chromatophore is
drawn outwards in different directions, and as a result the flattened
sac becomes more expanded and thinner, the pigment being spread
out into a thinner layer. When the fibres are relaxed the elasticity
of the wall comes into play, and the chromatophore contracts, the
contained pigment resuming its former arrangement. A peculiar
iridescence which, in addition to the play of colours, is recognisable
in the integument of Sepia is due to the presence of a number of
cells, the iridocysts.

When the mantle-cavity is laid open (Fig. 667) there is seen
on each side of it one of the two plume-shaped ctenidia (cten.).
In the middle line of the posterior surface, close to the internal open-
ing of the funnel, is the anal aperture (an.) situated at the oral
extremity of a longitudinal tube — the rectum. On either side of
the rectum is a much narrower projecting tube with a terminal

771

Fia. 664.— Chromatophore of Sepia, magnified.
nuc. nuclei in wall of sac ; pigm. pigment ;
rod. mus. radiating strands of muscle. (After
Vogt and Jung.)

742 ZOOLOGY SECT.

opening — the excretory aperture (neph.). On the left-hand side is
the opening of the sperm-duct or oviduct (ovid.) as the case may be.
In addition to the shell, which is an important protective
structure, and gives support to the muscles of the fins, Sepia also
has a remarkably well developed internal skeleton composed of
cartilage. An important part of this — the cranial cartilage (Fig.

665) — protects the principal nerve-
centres, encloses the statocysts, and
gives support to the eyes. Other
cartilages support the bases of the
I arms. A thin shield-shaped plate —
the nuchal cartilage (Fig. 666) — lies
on the posterior surface of the neck.
The pair of elevations on the pos-
terior wall of the funnel and the
corresponding depressions on the
anterior surface of the body are

-T . ej

J». 665,-Sepia cnltrata, crania, *°™ eaC,h On * ^m plate of cartl-

cartilage seen from the posterior aspect, lage, and. Other thin Cartilages
with the cavities of the statocysts ex- _,. ,1 n i r ,1 n

posed, eye, position of eye indicated by Support tne bases OI the tins,
dotted line; ot. statocyst ; pall. n. Aliman + a-nxr CUrcta-m TVio

pallia! nerve ; vitc. n. visceral nerves. Alimentary fcystem.—

mouth is surrounded by a thin,

lobed peristomial membrane, within which is a circular lip (Figs.
669, 672, perist.) beset with numerous papillae. Lodged within the
circular lip is a pair of powerful horny jaws (Fig. 668, Fig. 669,
jawl, jaw 2 ; Fig. 670,^'. ; Fig. 672, jaw). These have somewhat
the appearance of the beak of a parrot, the posterior jaw being
larger and more strongly bent than the other, which it partly
encloses. The mouth leads into a thick-walled buccal cavity, which
contains an odontophore bearing numerous minute horny teeth.
The oesophagus (Figs. 669 and 670, ce ; Fig. 672/ces),
following on the buccal cavity, is a narrow straight
tube, which runs between the halves of the
" liver " towards the aboral end of the body. It
opens into a rounded thick- walled stomach (st.),
and, close to the pyloric aperture leading from the
latter into the intestine, opens a wide ccecum (c.).
The alimentary canal at this point bends sharply
round upon itself, and the intestine runs nearly FIG. eee.— sepia
parallel with the oesophagus to open into the StSSeT'
mantle-cavity as already described.

A pair of glands (Fig. 670, s.g. ; Fig. 672, sal.), which are
commonly termed salivary, though their functional correspondence
with salivary glands has not been proved, are situated in the
head behind the cranial cartilage. The ducts of these two glands
run inwards and unite to form a median duct, which opens into
the buccal cavity. The name of " liver " (Fig. 670, I, I. ; Fig. 671,

xn

PHYLUM MOLLUSCA

743

liv.) or digestive gland is given to a large brown glandular mass which
extends from the neighbourhood of the salivary glands nearly to
the aboral end of the body. It consists of two partly united right
and left portions, each of which has a duct opening into the
cavity of the alimentary canal opposite the point where stomach,
caecum, and intestine meet. Surrounding the ducts and opening

ma.TLl.ca.i-l

FIG. 667. — Sepia cultrata, female seen from the postero-ventral aspect, the wall of the
mantle-cavity divided along the middle line and the two flaps thus formed spread out so as
to expose the contents, ac. nid. accessory nidamental glands ; an. anal aperture with its
lateral appendages ; /. membranous fold attaching the ctenidium to the wall of the mantle-
cavity ; inf. external opening of funnel ; inf. cart, infundibular cartilage ; ink. d. ink-duct ;
ink. s. ink-sac ; lig. ligamentous band which extends from the anterior wall of the mantle -
cavity to the ovary, cut across ; liv. " liver " ; I. cten. left ctenidium ; I. neph. left renal

\jCk\Lvy IAJ UI1C UTCU.J, l^UU C»\yj.vn30 , «-«<l>. Al V ^i , _

aperture ; I. nid. left nidamental gland ; I. st. g. left stellate ganglion ; rnant. cart, mantle-
cartilage ; mo. rnouth ; mus. neck-muscles ; ov. ovary ; ovid. oviduct ; reel, rectum.

into them are masses of minute vesicles (Fig. 670, b, d.) ; the
secretion of these has the property of converting starchy matters
into sugar ; they sometimes, though without sufficient reason,
receive the name of pancreas.

744

ZOOLOGY

SECT.

Immediately below the thin integument of the anterior wall of
the mantle-cavity lies a characteristic organ — the ink-sac (Fig.
667, ink. s. ; Fig. 670, i.). This is a pear-shaped body, a portion of
the interior of which is glandular and secretes a black substance —
the ink or sepia — which collects in the main cavity of the sac and
is discharged by a cylindrical duct opening into the rectum close to
the anal aperture. When the Cuttle-fish is startled it discharges
the ink, which, mixing with the water in the mantle-cavity, is

ejected through the funnel as a black
cloud, under cover of which the
animal may escape from a threatened
attack.

buc

FIG. 668.— Sepia officinalis, jaws.
A, in situ ; £, removed and slightly
enlarged. (From the Cambridge
Natural History.)

FIG. 669. — Sepia, median section through the buccal
mass. g. buc. buccal ganglia ; g. stom. stomato-
gastric ganglia ; gust, supposed gustatory organ ;
jaw 1, posterior jaw ; jaw 2, anterior jaw ; ce. oeso-
phagus ; Tpcrist. circular lip ; rad. radula. (After
Keferstein.)

Vascular System.— The heart (Figs. 671, 672, and 674) of the
Cuttle-fish consists of a ventricle and two auricles. The ventricle
(vent.), which is divided into two lobes by a constriction, is somewhat
obliquely placed, but the rest of the vascular system is almost
completely equilateral. At its oral end the ventricle* gives off
a large vessel — the oral aorta (aort.) ; aborally it gives origin
to a much smaller aboral aorta (aort'), which bends over the
ink-sac and supplies the aboral portions of the body. The arteries
which lead off from the aortse communicate by their ultimate
branches with a system of capillaries, and these with a system

XII

PHYLUM MOLLUSCA

745

of veins. A large median vein, the vena cava (v. cav.), runs from
the head to the neighbourhood of the rectum, in front of which
it bifurcates to form the left and right afferent branchial veins
(/. off. br. v.} r. aff. br. v.), each running through the cavity of
the corresponding renal organ to the base of the gill, where it is
joined by veins from the aboral region. At the base of the gill
the afferent branchial vein becomes dilated to form a contractile
sac — the branchial heart (r. br. Tit.) — appended to which is a rounded
body of a glandular character — the appendage of the branchial
heart, representing the pericardial glands
of the Pelecypoda. The afferent branchial
vein runs through the axis of the branchia,
giving off branches as it goes. The
blood is carried back to the ventricle
on either side by a dilated contractile
vessel, the auricle or efferent branchial
vein (1. aur.t r. aur.).

The ccelome (Fig. 680) is a pouch of
considerable size, divided by a constric-
tion into oral and aboral parts. The
former is the pericardium, or cavity in
which the heart is lodged ; it gives off
a pair of diverticula, right and left, each
lodging the corresponding branchial
heart, and communicates by a pair of
apertures with the cavities of the kidneys
or renal sacs. The aboral part of the
ccelome forms the capsule (gonoccele)
which encloses the ovary or testis.

The paired, plume-shaped ctenidium
lies parallel with the long axis of the
body. It is attached throughout the
greater part of its length to the wall of
the mantle-cavity by a thin muscular
fold, and consists of numerous pairs of
delicate lamellae, the surface of which is
increased by the presence of a complex
system of foldings. Internally the
lamellae are not completely in contact, an axial canal being left
through which the water penetrates freely to all parts of the gill.
The blood carried to the gill by the afferent branchial vessel
passes in a System of minute branches through the lamellae, and is
gathered up again into vessels which open into the main efferent
vessel leading to the auricle.

Nervous System. — Though parts homologous with those
of Triton are recognisable in the nervous system of Sepia, their pro-
portions and arrangement indicate a higher grade of organisation.

FIQ. 670. — Sepia officinalis,

enteric canal, a. anus ; b. d. duct
of one of the portions of the diges-
tive gland ; b. m. buccal mass ; c.
caecum ; i. ink-sac ; t. d. ink-duct ;
j. jaws ; 1. I. digestive gland ; a?,
oesophagus ; p. pancreatic appen-
dages ; r. rectum ; s. g. salivary
glands ; st. stomach. (From the
Cambridge Natural History.)

746

ZOOLOGY

SECT.

The cerebral, pedal, and pkuro-visceral ganglia (Fig. 675.), all of
relatively large size, are closely aggregated together around the
oesophagus, supported and protected by the cranial cartilage. The
cerebral ganglia (cer. g.) are fused together into a rounded mass,
lodged in a hollow of the cranial cartilage, and covered over

n t.c

FIG. 671. — Sepia cultrata, male specimen seen from the postero-veutral aspect, the
mantle-cavity opened as in Fig. 667, the posterior body-wall partly dissected off, so as to
expose the organs in the visceral sac, the ink-sac and duct removed, aort'. aboral aorta ;
app, appendage of left branchial heart ; ccec. caecum ; inf. cart, funnel cartilages ; liv.
digestive gland ; I. abd. v. left abdominal vein ; I. aff. br. left afferent branchial vessel ; I. aur.
left auricle ; /. br. ht. left branchial heart ; I. cten. left ctenidium ; I. neph. left kidney ; I. st. g.
left stellate ganglion ; mant. cart, mantle-cartilage ; mo. mouth surrounded by peristomial
membrane ; pen. penis ; prost. prostate ; r. abd. v. right abdominal vein ; r. cten. right
ctenidium ; reef, rectum ; r. ven. app. appendages of right afferent branchial vessel ; te.
testis ; te. v. vein to testis ; va. valve of funnel ; vent, ventricle.

anteriorly by a strong fibrous membrane. Laterally are given off
a pair of short thick processes — the optic nerves or optic stalks
(opt. st.) — which expand almost immediately into large masses —
the optic ganglia (opt. g.) — in immediate contact with the eyes.
At the sides and posteriorly a pair of very thick commissural

PHYLUM MOLLUSCA

747

bands of nerve-matter pass round the oesophagus to unite with the
pedal and pleuro-visceral ganglia, which lie behind. The pedal

ganglia (Fig. 676) are, like
the cerebral, united into a
single mass ; orally this is
prolonged and expanded into
a broad mass from which the
ten brachial nerves (br. n.) are
given off to the arms. The

pf~

brccx.r

Lclen.
la.

Lb

Fia. 672. — Sepia cultrata, lateral dissection of male.
The left-hand half of the head has been removed by an
approximately median longitudinal section, the buccal
mass, however, being left intact ; the funnel and the
anterior and posterior walls of the mantle-cavity are
likewise bisected longitudinally. The left ctenidium with
the left renal sac and left branchial heart have been
removed from their natural position and displaced back-
wards so as to expose the other organs. The digestive
gland with its ducts and the pancreatic appendages have
been removed, but the position of the former is indicated
by a dotted line . app. appendage of left branchial heart ;
aort. aorta ; aort'. aboral aorta ; buc. buccal mass ; br. cart.
section of cartilage supporting the arms ; cer. g. cerebral
ganglia ; giz. caecum ; ink. s. ink-sac ; inf. funnel ; jaw,
jaw ; 1. aur. left auricle'; I. br. ht. left branchial heart ;
/. cten. left ctenidium ; liv. position of digestive gland ;
1. neph. left renal sac ; n. cart, nuchal cartilage ; oes.
oesophagus ; ot. cavity of statocyst laid open ; ped. g.
section of pedal ganglion ; perist. points to circular lip
with peristomial membrane surrounding it ; post. v. abdo-
minal vein ; r. aur. right auricle ; r. cten. right ctenidium ;
reel, rectum ; sal. salivary gland ; sh. shell ; st. stomach ;
te. testis ; va. valve of funnel ; v. cav. vena cava ; vent.
ventricle.

pleuro-visceral ganglia, also united into one,
contact with the pedal behind the oesophagus.

FIG. 673— Sepia officinalis,

longitudinal section of ink-
sac, a. anus ; d. ink-duct ;
i. g. ink-gland ; i. r. cavity
of ink-sac ; o. orifice of ink-
gland ; r. rectum ; sp. sphinc-
ter muscles. (From the Cam-
bridge Natural History, after
Girod.)

are in immediate

748

ZOOLOGY

SECT.

Besides the optic nerves the cerebral ganglia also give off a pair
of slender nerves which join a smaller pair of closely united buccal

a or I

FIG. 674. — Sepia cultrata, lieart and main blood-vessels from the posterior aspect, ant. ao,
aort. aorta ; aort'; aboral aorta ; app. appendage of right branchial heart ; eff. br. v. right
efferent branchial vessel ; ink. a. artery to ink-sac ; ink. v. vein from ink-sac ; L aff. br. v.
left afferent branchial vessel ; 1. aur. left auricle ; ov. v. deep ovarian vein ; ov. v'. superficial
ovarian vein ; pall. v. pallial vein ; r. abd. v. right abdominal vein ; r. aff. br. v. right afferent
branchial vein ; r. cten. right ctenidium ; r. br. ht. right branchial heart ; v. cav. vena cava ;
ven. app. venous appendages ; vent, ventricle.

br. n.

FIG. 675. — Sepia cultrata, 'cephalic
ganglia, from the anterior aspect, ao.
aorta ; buc. buccal ganglion ; cer. buc. con.
cerebro-buccal connective ; cer. g. cere-
bral ganglion ; opt. g. optic ganglion (re-
moved on the left side) ; opt. st. optic
stalk ; pall. n. pallial nerve ; pi. g. pleural
ganglion ; vise. n. visceral nerves.

FIG. 676. — Sepia cultrata, anterior view
of pedal and pleuro-visceral ganglia after
removal of the cerebral and optic, br. n.
brachial nerves ; conn, connectives be-
tween the cerebral and the pedal and
pleuro-visceral ganglia (cut across) ; inf. n.
nerve to funnel ; pall. n. pallial nerves ;
vise. n. visceral nerves.

ganglia (Fig. 675, buc.), situated close to the buccal mass on the
anterior aspect of the oesophagus. The buccal ganglia again
(which are sometimes looked upon as separated portions of the

XII

PHYLUM MOLLU8CA

740

cerebral) are connected by slender connectives with a pair of
stomatogastric ganglia (Fig. 669, g. stom.), also closely united, situated
on the posterior aspect of the oesophagus. Besides the ten brachial
nerves, each of which, expanding at the base of the arm into a
brachial ganglion, runs along the axis of the arm to its extremity,
the pedal ganglia also give off nerves to the funnel, and also a
pair to the statocysts ; but the latter are found, when their fibres
are traced to their origin, to be derived from the cerebral ganglia.
The pleuro-visceral ganglia give off two visceral nerves (Fig. 676,
vise, n.) supplying the various internal organs, one pair of branches,
the branchials, having each a branchial ganglion at the base of the

scl.carl

corn,

set- c Ctrl

orb.ccL.rt

FKJ. 677. — Sepia, section of eye. cil. proc. ciliary processes ; corn, false cornea ; ir. iris ; Jens,
lens ; opt. g. optic ganglion ; orb. cart, orbital cartilage ; rds. rods ; ret. retina ; scl. cart.
sclerotic cartilage. (From Vogt and Jung, after Hensen.)

ctenidium, and running along its axis to its extremity. Two other
ganglia of considerable size — the visceral and the gastric — occur in
the course of this system. The pleuro-visceral ganglia also give
off two very stout pallial nerves (pall, n.) which run through the
neck to the inner surface of the mantle-cavity, where each expands
into a large, flat, pallial or stellate ganglion (Fig. 667, I. st. g.}, which
is visible in front of the ctenidium when the mantle-cavity is opened.
From the outer edge of this arise a number of nerves supplying the
various parts of the mantle.

The organs of special sense of the Cuttle-fish are much more
highly developed than those of Triton. The eyes (Fig. 677) are

VOL. i 3 it

?50 200LOGY

supported by curved plates of cartilage forming a sort of orbit,
connected with the cranial cartilage. The significance of the
various parts of the eye will not be fully understood till the struc-
ture of that of the Vertebrata has been studied. A transparent
portion of the integument covering the exposed face of the eye is
termed the false cornea (corn). The eyeball has a firm wall, or
sclerotic, strengthened by plates of cartilage (scl. cart). Externally,
i.e., on the side turned towards the surface of the head, this presents
a large opening — the pupil. The part of the sclerotic which imme-
diately bounds the pupil is termed the iris (ir.) ; it contains muscular
fibres by whose action the size of the pupil can, to a limited extent,
be increased or diminished. Just internal to the iris and projecting
slightly through the pupil is the lens — a dense glassy-looking
body of a spherical shape. The lens consists of two plano-
convex lenses in close apposition ; it is supported by an annular
process — the ciliary process (cil. proc.) — projecting inwards from the
sclerotic. Between the two parts of the lens lies a thin layer of
cells — the cornea. The lens with the ciliary process divides the
cavity of the eye into two portions, a smaller outer — the cavity of
the aqueous humour, containing water, and a larger inner, con-
taining a gelatinous substance — the vitreous humour. Over the wall
of this inner chamber extends the retina (ret), the sensitive part of
the eye, in which the optic nerve-fibres derived from the optic
ganglion terminate. The retina is of somewhat
complicated structure, mainly composed of a layer
of close-set parallel rods (rds), which immediately
(I bound the cavity of the eye, with externally a

/j layer of retinal cells which are in communication

sr •-, with the rods internally and with the optic nerve-

\ fibres externally.
f^ \ In immediate contact with the eye, in addition

FIG 678 - Sepia tO the °Ptic g^g^On, is a large soft bod7 of

cuitrata. stato- unknown function, the so-called optic gland or
nth, highly magni- white ^ Bundles of muscuiar fibres bring

about limited movements of the eyeball in various
directions. An integumentary fold of the character of an eyelid is
capable of being drawn to some extent over the false cornea.

The statocyst (" otocyst ") (Fig. 665), though not of such compli-
cated structure as the eye, is very much more highly developed
than that of the Pelecypoda or Gastropoda. The two statocysts are
embedded in the cartilage of the posterior portion of the cranium
close to the pleuro-visceral ganglion. The cavities of the two
organs, which are about 3 mm. in diameter, are separated by a
median cartilaginous septum. The inner surface presents a
number of rounded and pear-shaped elevations, and is lined with a
flattened epithelium raised up on the posterior surface into a
ridge or crista acustica and a macula acustica composed of large

xn

PHYLUM MOLLUSCA

751

cylindrical cells provided at their free extremities with short cilia,
and produced at their bases into processes continuous with nerve-
fibres derived from the statocyst-nerve. Enclosed in the cavity
of the statocyst and attached to the macula is a large statolith
(Fig. 678) of dense composition and complicated form. The
function of the statocysts as organs of hearing is quite unproved ;
it has been shown by experiment that their removal leads to a loss
of the power of co-ordinating the movements in such a way as to
maintain the equilibrium.

v.cav

jnecL.s

s.f

FIG. 679. — Sepia officinalis, excretory organs. «W. w. abdominal vein ; ap. 1, funnel-like
opening from the pericardium ; ap. 2, aperture of communication between the left and the
median renal sac ; br. ht. branchial heart ; ink. s. t>. ink-sac vein ; med. s. median sac ;
pall. t>. pallial vein ; ur. renal aperture (ureter) ; v. caw. vena cava ; ven. app. venous
appendages of the afferent branchial veins. (From Vogt and Jung, after Grobben.)

Supposed to be olfactory in function is a pair of ciliated pits,
which open by slits on the surface behind each eye ; among the
ciliated cells lining the pit are numerous narrow sensory cells con-
nected at their bases with the fibres of a nerve derived from a
small ganglion situated close to the optic ganglion. A small eleva-
tion (Fig. 669, gust), covered with papillae, on the floor of the buccal
cavity just in front of the odontophore, is perhaps an organ of taste.

The excretory organs or kidneys of Sepia (Figs. 679 and
680) are a pair of thin-walled sacs, which open into the mantle-

3 B 2

752

ZOOLOGY

SECT.

liv

CLp

cavity by the conspicuous excretory apertures already described.
On either side is an aperture (ap. 1) placing the cavity of the sac in
communication with the pericardium, and the right and left sacs
communicate with one another orally and aborally. From their
posterior junction is given off a median diverticulum (Fig. 680,
med. s), into which the pancreatic follicles (pane.) project.
Through each excretory sac runs the corresponding afferent branchial

vein, formed by the
bifurcation of the vena
cava, and surrounding
it are masses of gland-
ular tissue (Fig. 679,
ven. app), by whose
agency the process of
renal excretion (the
products of which, in
the shape of a nitro-
genous excretory sub-
stance called guanin,
are to be detected in
fenl the internal cavity) is
br.hl carried on.

Reproductive
system. — In the male
the testis (Fig. 681, te.)
forms a compact mass
of minute tubules
situated in the aboral
region of the body
and enclosed in a
capsule. The single
spermiduct (v. def.) is
a greatly convoluted

F.o.eso.-Septa officinal, diagram of a median tube which leads from

vertical section of a female specimen, to show the trie Cavity OI the Cap-
relations of the cavities, ap. aperture between the i , i ,-» i rl

secondary body-cavity (pericardium) and the lateral SU16 towards tne leit ,
nephridial sac: br. ht. branchial heart ; cod. coelome;

Ictl.s
coeL

ink.s

it opens into £

nephridial sac ; liv. liver ; med. s. median nephridial sac ; gated vesicula

ov. ovary ; ov. ap. aperture leading from oviduct to &7 . . N

secondary body-cavity ; pane, pancreatic appendages ; dllS (VeS.), tO WillCil IS

sh. shell ; st. stomach: ur. ureter; vent, ventricle. l^^rl <, rAn-nArAnv

(From vogt and Jung, after Grobben.) appended a glandular

body, the prostate

(pr.). In the interior of the vesicula seminalis the sperms are
rolled up by the action of a system of grooves and ridges into
long narrow bundles of about 2 cm. in length, each of which
becomes enclosed by a chitinoid capsule of a narrow cylindrical
shape, forming a spermatophore (Fig. 682, B) ; at one end of
the spermatophore is a complicated apparatus of the nature

XII

PHYLUM MOLLUSCA

753

of a spring for causing the rupture of the wall and the discharge
of the sperms. The vesicula seminalis expands into a wide sac —
the spermatophoral sac or Needham's sac (Fig. 681, sp. s.) — in the
interior of which the spermatophores are stored. This opens into
the mantle-cavity by the aperture already described at the extremity
of the penis to the left of the middle line.

In the female the ovary (Fig. 667, ov.) occupies a position
corresponding to that of the testis in the male, and is enclosed in

spr*

Fia. 681. — Sepia, reproductive organs of male. pn. penis ;
pr. prostate ; sp. s. sperm-sac ; te. testis ; v. def. vas
deferens ; res. vesicula seminalis. (After Keferstein.)

FIG. 682. — Sepia. A, sperms,
highly magnified ; B, sperma-
tophore. sp. mass of sperms ;
spr. spring apparatus by
which the wall of the sperma-
tophore is ruptured. (From
Vogt and Jung.)

a similar capsule, with the cavity of which
the lumen of the oviduct is continuous.
An axial swelling bears numerous follicles, each containing a
single ovum supported on a long slender stalk ; the different ovaare
in various stages of development. At the breeding season the
ovary becomes a compact mass of ova, which assume a polygonal
shape owing to mutual pressure. The oviduct (ovid.) is a wide
tube, opening, as already described, into the mantle-cavity to the
left of the rectum. Occupying a conspicuous position on the anterior
wall of the mantle-cavity of the female is a pair of large flattened
glands of somewhat oval outline, the nidamental glands (nid),
situated to the right and left of the ink-duct. In the long axis of
each is a median canal, on either side of which is a range of closely-

754

ZOOLOGY

SECT

set delicate lamellae ; the median canal opens into the mantle-
cavity by a slit bounded by a number of plaits situated at the
narrower oral end. The nidamental glands secrete the viscid
material by means of which the eggs when deposited adhere together
in masses. A glandular mass of unknown function, known as the
accessory nidamental glands (ac. nid.), lies at the sides and around
the oral ends of the nidamental glands proper.

ii. THE PEARLY NAUTILUS (Nautilus pompilius).

The three living species of Nautilus, of which N. pompilius is
the best known, are inhabitants of moderately shallow water about
the shores and coral-reefs of the South Pacific, usually swimming

FIG. 683. — Section of the shell of Nautilus pompilius, showing the septa (s, s), the ?eptal
necks (s. n., s. «.), the siphuncle, si. (represented by dotted lines), and the large body-
chamber (ch). (From the Cambridge Natural History.)

near the bottom, and probably rarely, if ever, coming voluntarily
to the surface. The body is enclosed in a calcareous, spirally-
coiled shell (Fig. 683), into which the entire animal can be with-
drawn for protection. The cavity of the shell is divided by a
system of septa into a series of chambers, the last and largest of
which, opening widely on the exterior, alone lodges the body of the
animal. Between the animal and its shell there is a direct organic
connection through the intermediation of a narrow, tubular, vascular
prolongation of the visceral region, which perforates the entire
series of the septa to the apex of the spiral. This tube, which is
termed the siphuncle (si.), has its wall supported by scattered

xii PHYLUM MOLLUSCA 755

spicules of carbonate of lime ; but, in addition, as it passes through
each septum, there is produced over it for some distance a shelly
tube — the septal neck (s. n.) — continuous with the substance of the
septum. The apical or initial chamber presents a small scar, the
cicatrix, which may indicate the original presence of the larval
shell, or protoconch, which has fallen off in the course of develop-
ment.

When the shell of the Nautilus is compared with that of Triton
some points of resemblance, together with important points of
difference, will be at once recognised. In both the growth of the
shell has taken place in such a way as to produce a gradual and
regular increase in the width of the internal cavity, from the apex
to the mouth, the result being a form of shell which, if it were
straightened out, would be a long cone. In both the growth has
not taken place in a straight line, but in a spiral, and a spiral of
so close a character that successive turns are in immediate contact
and their walls fused together. But in Nautilus all the turns of
the spiral are in the same plane ; the spiral, in other words, is a
flat one, as has already been found to be the case in certain of the
Gastropoda (p. 718), whereas in Triton the spiral is an elongated
helix : in other words, the spiral of Nautilus is that of a watch-
spring, that of Triton that of a corkscrew. The possession by
Nautilus of the series of septa marking the position which the
animal has occupied at successive stages in its growth is another
striking difference. Moreover the relations of the soft parts of
the shell are radically different in the two cases. In Triton the
body is attached to the shell by the columellar muscle ; in
Nautilus the main organic connection is by means of the siphuncle ;
for, though it is chiefly through the pressure exerted by two great
lateral masses of muscle (Fig. 684, mus.) that the Nautilus retains
its hold of the shell, the muscular fibres are not attached to the
latter in the same intimate way as those of the columellar muscle
of Triton, but are inserted into a horny cuticular membrane inter-
vening between the muscle and the shell. Again, while the curva-
ture of the body of Triton with the enclosing shell is towards the
ventral side (endogastric), in Nautilus it is towards the dorsal side
(exogastric).

When the animal is removed from the shell it is found to possess
two regions, a distinct and relatively large, obtusely conical head
bearing eyes and a system of tentacles, and a rounded sac-like
trunk. Both head (or cephalopodium) and trunk are very slightly
compressed, the direction of the compression being, as in Sepia,
from the antero-dorsal towards the postero- ventral side, and
they are almost completely bilaterally symmetrical, only a very
slight disturbance of the symmetry being discernible. The mouth,
situated at the free extremity, is provided with a pair of relatively
enormous, partly calcified jaws (Fig. 684), Surrounding the mouth

ZOOLOGY

s K< -r.

is a series of bilaterally arranged lobes which represent the fore-
foot or the epipodia of other Molluscs. These are beset with
numerous slender, three-sided tentacles, each provided with an
elongated tubular sheath, in the interior of which the greater part
of the tentacle in the retracted condition lies enclosed, only a
small portion protruding. Minute ring-like markings on the
tentacle are due to the presence of a number of annular constric-
tions, which give the tentacle a transversely ridged character.

Jaws in/

ten*

,.}i,id^

7na.nl -

FIG. 684. — Nautilus pompilius, diagrammatic lateral view of a female specimen, enclosed
in its shell, cart, cartilage ; cten. ctenidia ; hd. hood ; inf. funnel ; jaivs, jaws ; mant. mantle ;
mant'. dorsal mantle fold overlapping the coil of the shell ; mus. position of lateral mass of
muscle : nid. nidamental glands ; sept, first septum ; siph. siphuncle. (After Keferstein.)

There are no suckers : but the ridged surfaces enable the tentacle
to adhere firmly to rough objects. The tentacles are arranged in
two series, an outer and an inner. The outer, which are borne on
an annular muscular ridge of the foot, are nineteen on each
side in both sexes. Anteriorly this muscular ridge is thickened to
form a massive lobe — the hood (Figs. 684, 685, hd.) — in which
there is a concavity for the reception of the coil of the shell. The
hood bears two tentacles, and has the appearance of being com-
posed of the immensely developed sheaths of these, completely

PHYLUM MOLLUSCA

757

fused together in the middle line : on each side the enlarged
sheaths of a second pair of tentacles are closely applied to, though
not completely coalescent with, the hood, being separated from the
latter by a narrow groove. The hood, with these two enlarged
sheaths, is covered with a thickened tuberculated skin, and acts

sh-

w.f -

FIG. 685. — -Nautilus macromphalus, adhering to the substratum in a vertical position by
means of its tentacles, e. eye ; h. hood ; n. m. nuchal membrane detached from coil of
shell ; o. t. ophthalmic tentacles ; sh. shell ; w. f. wing of funnel. (After Willey.)

after the manner of an operculum for protecting the tentacles
and other soft parts about the head. Altogether there are forty-
two tentacles of the outer series, including four ophthalmic tentacles,
one situated on the oral and another on the aboral side of each
eye. The latter (ophthalmic) differ from the rest in being highly

758

ZOOLOGY

SECT.

sensitive, ciliated, and with the ridges on the inner side produced
into lamellae. The tentacles of the inner series differ strikingly in
number and arrangement in the two sexes. In the female there
are two inner lateral lobes, right and left, quite symmetrically
developed, each bearing twelve tentacles, and an inner posterior
lobe (Fig. 686) divided by a deep median notch into two,
each half bearing twelve to fourteen tentacles. On the middle of
the oral surface-of the latter, close to the median notch, is an oval
patch raised up into numerous closely set ridges (organ of Owen).
In the male the inner posterior lobe with its ridged organ is only
represented by a median posterior body consisting of two oval

LfO.1

FIG. 686. — Tuner posteripr lobe of foot of female of Nautilus pompilins, with neighbouring
parts of cephalopodium. ow. organ of Owen ; t. one of the tentacles of the outer ring ;
vol. organ of Valenciennes. (After Willey.)

elevations, each divided into a number of folds (organ of van der
Hoeven). The internal lateral lobes are greatly modified, four of
the tentacles on either the right side or the left, usually the latter,
being modified to form a structure termed the spadix (Fig. 687),
which is supposed to represent the hectocotylised arm of the male
Sepia. It has the form of a large compressed cone formed by the
union of the enlarged sheaths of three of the tentacles. The
corresponding tentacles themselves are in the adult male enor-
mously thickened, and the outer surface of the most posterior (3)
is covered with regularly arranged rows of minute pits. A fourth
tentacle, much smaller than the others, is closely applied to the
outer surface of the organ. In the internal lateral lobe, right or

XII

PHYLUM MOLLUSCA

759

FIG. 687. — Nautilus pompilius ,

spadix of full-grown male, seen
from the outer side. 1, 2, 3, 4,
modified tentacles ; 1, withdrawn
into its sheath, its position and
shape indicated by the dotted
line ; 3, the flattened tentacle
with the rows of minute cavities ;
x, patch of modified integument.
Two-thirds of the natural size.
(After Haswell.)

left as the case may be, opposite that bearing the spadix, the

latter is represented by a group of four tentacles forming what is

termed the anti-spadix.
A further difference between the male

and the female with regard to the foot

is the presence in the latter, but not in

the former, on the inner surface of the

outer ring, close to the inner posterior

lobe on either side, of an area thickly

beset with delicate membranous ridges

(organ of Valenciennes, Fig. 686, vol.).
On the posterior side of the head is a

funnel corresponding with that of Sepia,

but extending further forwards ; this,

however, does not form a completely

closed tube, the edges of its right and

left moieties being simply in apposition

posteriorly without being united to-
gether. Near the oral end is a large,

somewhat triangular valve arranged like

that of Sepia.

*~* There is an internal skeleton of carti-
lage (Fig. 688), as in Sepia, but its

relationships with the nerve-ganglia

are much less intimate in the case of Nautilus than in that of
Sepia.

Mantle and Mantle - cavity. — The
mantle is produced around the head into a
free flap, longer and looser than the mantle-
flap of Sepia. Dorsally this splits into two
layers, reflected over the convexity of the
shell, which fits into a hollow behind the
hood. Ventrally and posteriorly the mantle
encloses a large mantle-cavity (Fig. 689),
corresponding to that of Sepia. In this are
lodged two pairs of ctenidia (cten.), having
the same general structure as the single
pair present in Sepia. Between the bases
of the ctenidia of each side is a small knob-

pilius, cartilaginous in- ... , . . , T . V1

temai skeleton. (After like elevation, the oral osphradium (ant. os.),1

and behind the bases of the more aborally

situated pair are two compressed, bilobed projections, more or

1 As in Sepia, it is convenient to use the term oral for parts towards the
mouth end, and aboral for those situated towards the opposite extremity, the
same terms being also used to indicate relative position of different parts. The
relative position of the parts is, however, for the sake of simplicity given here
as they lie when the mantle -cavity is opened by turning back its thin postero-
ventral wall.

760

ZOOLOGY

SECT.

less completely united in the middle so as to form a transverse
ridge ; these are the aboral osphradia (p. os.). In the middle line
of the mantle-cavity is the anus (an.), a large aperture with minutely
lobed margin, situated on a slight elevation, but by no means so
prominent as in Sepia. On each side are two apertures, the oral

r. ant.cs

I. neph.

an.

p. OS

pl.neph
L.visc.ap

FIG. 689. — Nautilus pompilius, interior of mantle-cavity of a male specimen with the
postero-ventral wall reflected, a. I, neph. oral left renal aperture ; an. anus ; cten. ctenidia ;
en. eye ; /. funnel ; 1. $ ap. left reproductive aperture indicated by a bristle passed through
it ; I. vise. ap. left viscero-pericardial aperture ; n. s. Needham's sac ; pen. penis ; pt. neph.
aboral left renal aperture ; p. os. aboral osphradia. ; r. ant. os. right oral osphradium ; v. n.
visceral nerves. (After Willey.)

and aboral renal apertures (Fig. 689, a. I. neph., pt. neph.), corre-
sponding to the single pair of Sepia, but not elevated on papillae.
Close to each posterior renal aperture is an opening — the viscero-
pericardial (1. vise, ap., r. vise, ap.) — leading into the pericardial
section of the cceloine ; these are not represented in Sepia. In
both sexes there are two reproductive ducts, right and left ; but

xii PHYLUM MOLLUSCA 761

in both the right alone appears to be functional, and the left is much
the smaller. The opening of the right sperm-duct of the male is
situated on a cylindrical prominence — the penis (pen.) — placed
close to the middle line. In the female the nidamental glands are,
as in Sepia, conspicuous objects when the mantle-cavity is exposed ;
but they are mainly situated on its posterior instead of its anterior
wall.

Enteric Canal. — The mouth is surrounded by a circular lip
beset with numerous papilla. There is a pair of jaws (Fig. 690,
jaw) of similar shape to those of Sepia, but much more powerful,
and calcined towards the tips. The buccal mass is a large rounded
body with thick muscular walls. On the floor of the contained
cavity is a large and prominent odontophore (odont.), with long
and pointed, curved teeth. In front of the odontophore is a large
bilobed soft prominence, the tongue (tong.). Behind the odonto-
phore, between it and the opening of the oesophagus, are one large
median and two lateral tongue-like prominences beset with papillae ;
on the inner surface of the latter are the apertures of a pair of salivary
glands.

The oesophagus (CBS.) becomes dilated aborally into a very
spacious crop (cr.) for the storage of the food, which consists of
small prawn-like Crustaceans and small Fishes broken up by the
jaws and radula. This opens into a rounded stomach (stom.) having
very much the appearance of the gizzard-like caecum of Sepia.
The intestine (int.), shortly after it leaves the stomach, develops
a rounded ca3cum (ccec.) with complexly folded walls, into which
the ducts of the digestive gland or " liver " open. The intestine
does not pass straight to the anus as in Sepia, but first bends
round in a short coil. The ink-sac and duct of Sepia are not
represented." There is a very large digestive gland divided into
four main portions or lobes, each of which is made up of a number
of lobules. The ducts (" bile-ducts," b. du.), opening as above
mentioned into the caecum, have a series of small diverticula which
may represent the pancreatic appendages of Sepia.

The ccelome consists of the pericardium and the gonocoele — the
cavity in which the gonad is enclosed : these communicate with
one another by three apertures. The pericardium contains the
ventricle, the four auricles, and parts of the renal glandular appen-
dages. It communicates with the exterior by the viscero-pericardial
apertures.

Heart and Vascular System. — The blood-system consists of
the heart, the arteries and veins, and certain large spaces constituting
the haemoccele. The latter consists of three chief parts— the
peristomial, peri-cesophageal, and peri-hepatic haemocceles, the first
surrounding the buccal mass, the second the oesophagus, and the
third the liver.

The ventricle (Figs. 690 and 692, vent.) is a bilobed, transversely

l.tsnl.irit

--^

\ buc.pap

LefS.br:

FIG. 690.— Nautilus pompilius, dissection of the internal organs of a male, from the left
side. The funnel and the hood have been divided by a longitudinal median section. A
portion of the wall of the buccal cavity has been removed to show the odontophore and
the tongue, ace. gl. vesicula seminalis ; an. anus ; aort. oral aorta ; aort', posterior pallial
artery ; b. du. bile-ducts ; buc. n buccal nerves ; buc. pap. papillae of peristomial membrane ;
cer. g. cerebral ganglion ; cose, caecum ; cr. crop ; hd. good ; inf. funnel ; inf. n. infundibular
nerve ; int. 1, partof intestine between stomach and caecum ; int. 2, partof intestine following
caecum ; jaw, larger (posterior) jaw ; I. eff. br. v. left efferent branchial vessels ; /. tent.
int. left internal tentacular lobe ; need. s. JSTeedham's sac : odont. odontophore ; ce'. style
passed from buccal cavity into the opening of the oesophagus ; ces. oesophagus ; olf. n.
olfactory nerve ; opt. n. optic nerve ; oto. statocyst ; pott. n. pallial nerves ; ped. g. pedal
ganglion ; pi. g. pleural ganglion ; r. eff. t». right efferent branchial vessel ; retr. retractor
muscle of the buccal mass ; r. liv. right lobe of " liver " ; stom. stomach; test, testis ; tong.
tongue-shaped elevation of the floor of the mouth ; va, valve of funnel ; ven. c. vena cava ;
vent, ventricle.

SECT.

PHYLUM MOLLUSCA

763

— reel

placed, muscular sac, very similar to that of Sepia. On either side
there open into it two auricles or efferent branchial vessels, one
from each of the four ctenidia. The ventricle gives off a large
main aorta (aort.), which passes to the head after giving off arteries
to the stomach, the crop, the digestive gland, and the mantle,
From the aboral surface of the ventricle arises a smaller artery, the
lesser aorta, which immediately bifurcates. One of its branches —
the posterior pallial artery (Fig. 691, post. pall, a.) — passes to the
area of the mantle applied to the septum, bifurcates to supply this
area, and gives off a branch to the siphuncle. The other — anterior
pallial (ant. pall, a.)
— after giving off arte-
ries to the intestine
and rectum, and to
the branchiae and
osphradia, passes to
the muscular edge of
the mantle, bifurcating
anteriorly. Three geni-
tal arteries (gen. a. l,f
?, J), supplying the
various parts of the
reproductive appara-
tus, are likewise given
off directly from the
ventricle.

A large vena cava
(Figs. 690 and 692,
ven. c.) occupies a
position corresponding
closely with that which
it occupies in Sepia.
It presents the re-
markable peculiarity FIG. 691.— Nautilus pompilius (male), origin of pallial

•L anA fronitol QT-foi-ioa nnt <n«7 ti jmt.firir>r nallia] art.ttrv !

of being in free com-
munication by numer-
ous (valvular) aper-
tures with the general
cavity of the haemocosle. At its aboral end it presents a dilatation
from which four afferent branchial veins (Fig. 692, a. I. aff, p. I. aff,
p. r. aff, r. ant. aff.) — two right and two left — proceed to the
corresponding ctenidia, at the bases of which veins from the abof al
region join them. There are no branchial hearts.

The renal organs (Fig. 692) are, like the ctenidia and the
afferent and efferent vessels, four in number, instead of two as in
Sepia. Each renal sac (I. neph. s., r. neph. s., I. post. neph. s., r. post,
neph. s.) opens into the mantle-cavity, as already stated, by an orifice

and genital arteries, ant. pal. a. anterior pallial artery ;
eff. br. v. efferent branchial veins ; gen. a. 1, artery to
vesicula seminalis (v. sem.) ; gen. a. 2, testicular artery
and its branches ; gen. a. 3, artery to pyriform sac ; n. 8.
spermatophore-sac ; post. pall. a. posterior pallial artery ;
P>jr. pyriform sac ; reel, rectum ; test, testis. (After Willey.)

764

ZOOLOGY

SECT.

which is not drawn out into a tube. There is no communication
between the cavities of the different sacs, and thus no median
chamber as in Sepia, and there is also no communication with the
pericardium. The cavities are found to contain phosphate of
lime. Into each projects, from the corresponding afferent
branchial vein, a compact rounded group of venous appendages
(ren. app.), consisting of two symmetrical portions. Internal to
these, each afferent vein has connected with it a second group of
glandular appendages, which are cylindrical or club-like in form ;
they project, not into the nephridial sac, but into the peri-
cardial compartment of the coelome. They have been compared
with the appendages of the branchial heart of Sepia, but differ in
their relations to the renal appendages.

FIG. 692. — Nautilus pompilius, renal sacs, with ctenidia and other related parts, as seen
from the posterior aspect ; the boundaries of the four renal sacs represented by dotted
lines, a. I. aff. left oral afferent vessel ; cten. right ctenidia ; I. neph. ap. left oral renal
aperture ; I. neph. s. left renal sac ; I. post. neph. ap. left aboral renal aperture ; /. post,
neph. s. left aboral renal sac ; l.v.ap, left viscero-pericardial aperture ; p. 1. aff. left aboral
afferent vessel ; p. r. aff. right aboral afferent vessel ; r. ant. aff. right oral afferent vessel ;
r. ant. aur. right oral auricle ; ren. app. renal appendages ; r. neph. ap. right renal aperture ;
r. neph. s. right renal sac ; r. post. aur. right aboral auricle ; r. post. neph. s. right aboral renal
sac; r. v. ap. right viscero-pericardial aperture ; ven. c. vena cava; vent, ventricle ; vise,
per. s. viscero-pericardial sac.

Nervous System. — Nautilus differs strikingly from Sepia, and
somewhat resembles Chiton (p. 698, Fig. 614) in the form assumed
by the central parts of the nervous system v(Fig. 690, cer. g.),
distinct ganglia being absent. A very thick nerve-collar,
the posterior portion of which is double, surrounds the oesophagus.
The anterior part of the collar (Fig. 690, cer. g.) represents the
cerebral ganglia, the oral portion of the posterior part the pedal
(ped. g.), the aboral portion the pleuro-visceral (pi. g.) ; while the
lateral parts, not distinctly marked off from the rest, represent the
cerebro-pedal and cerebro-pleural connectives. From the cerebral
ganglia pass nerves to the buccal mass, to the olfactory organs
(olf. n.) and the statocysts, and a pair of very thick optic nerves

XII

PHYLUM MOLLUSCA

765

supply the eyes (opt. n.). The pedal ganglion gives off numerous
nerves to the tentacles and the funnel, and from the pleuro-visceral
arise pallial and visceral nerves.

Sense Organs. — The statocysts (otocysts) are a pair of sacs
embedded in recesses close to the cerebral ganglia, but not enclosed
in the cartilage of the endoskeleton ; each contains a number of
microscopic statocones. An olfactory function is ascribed to a
process (the rhinophore) with a ciliated pit at its base, situated on
the aboral side of the eye. The ophthalmic tentacles (Fig. 685, o.t.)
are supposed to act as accessory olfactory organs. The osphradia
contain ganglion-cells, are beset with sensory cilia, and are
undoubtedly organs of special sense.

The eyes, situated at the sides of the head, are very large but
extremely simple in structure, presenting a marked contrast to
those of Sepia, and scarcely comparable to those of any

other animal with the excep-
tion perhaps of Patella (p. 726).
Each is of the shape of a
saucer, attached to the head
f by its convex side by means of
a short thick stalk, the mouth
being closed in by a slightly
convex disc, with a circular
aperture at about its centre."
A slightly raised rim runs
round close to the margin on
the posterior half, and a narrow
groove extends inwards from

post aort \ this. to the central aperture.

In the interior of the cup is
neither lens, vitreous humour,
FIG. 693.- Nautilus pompiiius, male repro- nor iris. The sea-water, pas-

ductive organs, ace. vesicula seminalis ; eff. ,, , ,1 «i— «1

vess. efferent branchial vessels ; /. gen. op. Sing in tnrOUgn tne Central

left genital opening; post. aort. posterior aT»ATfnrp rlirppfhr Kafhp« fhp

aorta ; pyr. pyriform appendage ; r. gen. ^pei 1C, (. ^biy UdUll

op. right genital [opening ; sp. s spermato- retina, which is spread over

phore-sac ; test, testis ; vent, ventricle.

the interior in a thick layer.

Reproductive Organs. —The gonad (testis, Fig. 693, test, or
ovary, Fig. 694, ov.), like that of Sepia, is single and median, enclosed
in a special sac towards the aboral end of the body. The duct
is paired in both sexes, but in both the right alone appears to
be functional. In the male a large glandular vesicula seminalis,
in which the spermatophores are formed (ace.), is connected with
the right duct, and this appears to be represented on the left-hand
side by a vestige — the so-called pyriform sac (pyr.), situated close
to the ventricle. The distal part of the right duct dilates to form
a receptacle, the spermatophoral sac or Needham's sac (sp. s.), and
opens, nearly in the middle line at the end of a prominence — the

VOL. I 3 C

766

ZOOLOGY

SECT.

penis (Fig. 689, pen.). In the female the right oviduct has a
glandular dilatation, which is supposed to be an albumen gland.
The ova are of large size, greatly exceeding those of Sepia in
dimensions, containing a large proportion of food-yolk. Nidamental
glands are present, but are mainly situated, as already pointed
out, on the posterior instead of the anterior wall of the mantle-
cavity. Each egg becomes enclosed in an elaborate capsule (Fig.
695), probably moulded by the agency of the organ of Owen, on
the inner posterior lobe of the foot of the female (Fig. 686). The
development is not known.

FlO. 694. — Nautilus pompilius, female reproductive FIG. 695. — Egg of Nautilus

organs, alb. albumen-gland ; I. gen. op. left genital macromphalus, enclosed

opening ; ov. ovary ; pyr. pyriform appendage ; r. gen. in its capsule. (After Willey.)
op. right genital opening ; vent, ventricle. (After
Lankester and Bourne.)

2. DISTINCTIVE CHARACTERS AND CLASSIFICATION.

The Cephalopoda are bilaterally symmetrical Mollusca, which
have the main part of the foot displaced forwards to the neigh-
bourhood of the mouth and divided into a series of arms bearing
suckers, or of lobes bearing tentacles,, while the remainder of the
foot forms a funnel for the egress of water from the mantle-cavity.
The visceral mass is symmetrical and not coiled. The mantle
encloses posteriorly and ventrally a large mantle-cavity, in which
are situated the ctenidia and the renal, reproductive, and anal
apertures. The shell may be absent or rudimentary ; when present
and well developed, it may be internal or external, undivided,
or divided internally by septa into a series of chambers. There is
an internal cartilaginous skeleton, supporting and protecting the
nerve-centres and giving attachment to muscles. The mouth is
provided with a pair of horny jaws, and an odontophore is present.

xtt PHYLUM MOLLUSCA 767

In the majority there is an ink-gland with a duct opening into
the rectum. The ctenidia and kidneys are either two or four in
number. The nervous system is highly developed ; and the prin-
cipal nerve-ganglia are aggregated together around the oesophagus.
The sexes are separate ; the segmentation of the ovum is meroblastic,
and there is no metamorphosis.

Sub-Class I.— Dibranchiata.

Cephalopoda in which the main part of the foot assumes the
character of a circlet of either eight or ten arms, bearing suckers,
and surrounding the mouth. The funnel forms a complete tube.
The shell is usually internal ; when external its cavity is not divided
by septa. There are two ctenidia, two kidneys, and two branchio-
cardiac vessels or auricles. An ink-gland and duct are present.

ORDER 1. — DECAPODA.

Dibranchiata possessing ten arms, with stalked suckers provided
with horny rims, and with a well-developed internal shell.

This order includes the Cuttle-fishes, Squids, Spirula, and others,
as well as the extinct Belemnites.

ORDER 2. — OCTOPODA.

•

Dibranchiata provided with eight arms, the suckers on which are
sessile and devoid of horny rims : with or without slight vestiges
of an internal shell. An external shell, secreted by a speciaUy-
modified pair of arms, is present in the female Argonaut only.

This order includes the Octopods and the Argonauts.

Sub-Class II.— Tetrabranchiata.

Cephalopoda in which the main part of the foot has the
character of lobes bearing numerous tentacles. The funnel does
not form a complete tube. There is an external, spiral, chambered
shell. There are four ctenidia, four kidneys, and four auricles.
An ink-gland is absent.

This sub-class includes only one living genus, Nautilus, but the
Ammonites and other extinct forms are usually referred to it.

Systematic Position of the Examples.

The genus Sepia is a member of the family Sepiidm of the order
Decapoda, which is distinguished from the seven other families of
the order by the combination of the following features : — The body is
compressed and comparatively broad ; the fins are narrow and
elongated ; the internal shell consists almost entirely of calcareous
material.

Nautilus is the sole living representative of the sub-class Tetra-
branchiata.

3 o 2

768

ZOOLOGY
3. GENEEAL ORGANISATION.

SECT.

The uniformity of structure among the Dibranchiate Cephalopoda
is very great, and, as already stated, Nautilus is the only living
member of the Tetrabranchiata, so that comparatively little has
to be said to supplement the descriptions of the two examples.

External Features. — The general external shape differs very
little in the different members of the Dibranchiata : the body in
some is more elongated than in others ; the degree of compression
likewise varies. Fins may be absent, and the animal may progress
entirely by creeping with the aid of the long arms, or by swimming
by the movements of the arms, or under the propulsion of a current
of water forcibly ejected through the funnel by the contraction
of the muscular mantle (Fig. 696). When fins are present they

FIG. 696. — Octopus vulgaris. A, at rest; B, in motion. /. funnel, the arrow sh<nving
the direction of the propelling current through the water. (From Cooke, after Merculiano.)

may take the form of a continuous lateral flap as in Sepia, .but, more
usually, are of the nature of flattened lobes situated towards the
aboral extremity of the body (Fig. 697) ; in Ctenopteryx they have
the character of fringes of filaments. The arms vary in length
and proportions and in the form and arrangement of the suckers.
Eight arms are present in the Octopoda and ten in the Decapoda.
In the former group the Argonauts (Fig. 698) have, in the female,
one pair of arms (wa.) flattened and expanded at the extremities
for the secretion and support of the shell (sh.). In the Decapoda
one pair of arms, the fourth, is always specially modified, as in
Sepia, to act as prehensile appendages or tentacles usually capable

XII

PHYLUM MOLLUSCA

769

of being partly or entirely

retracted within certain

sacs situated at their

bases. In nearly all one

of the arms in the male

is specially modified (or

hectocotylised) to act as

an intromittent organ.

This modification is only

very slight in Sepia and

confined to the base, and

is most marked in certain

of the Octopoda (Fig.

699), including the Argo-
nauts. In the latter,

before the breeding

season, the third arm in

the male is found to be

represented by a rounded

sac, which subsequently

bursts and sets free the

elongated hectocotylised

arm. Spermatophores

are taken by the arm

from the genital opening,

and in the act of copula-
tion the entire arm is

detached and left in the mantle-cavity of the female. In other

cases the arm is not
detached. The
suckers are some-
times stalked,
sometimes sessile,
sometimes armed
with hooks, some-
times replaced by
hooks. In many
ca'ses the arms are
united by a web-like
fold, the interbrachial
membrane (Fig. 700)
which may reach
nearly to their ex-

FIG. 698.— Argonauta argo, female, showing the relations tremities.

of the animal to the shell in the living state, the arrow Jj} the T 6 t T a -

showing the direction of movement. /. funnel ; m. mouth, , , . ,

with jaws projecting ; sh. shell, with arms as seen through brancJliata tne Series
it; wa, webbed arm clasping the shell. (From Coolie, r „ r .ji^j

alter Lacaze-Duthiers.) Ol groups Ot slender,

FIG. 697. — ZiOligo vulgar is.

view ; B, horny internal
Keferstein.)

A, entire animal, dorsal
shell or pen. (From

770

ZOOLOGY

SECT.

ringed, sheathed tentacles, situated on lobes of the foot surrounding
the mouth, take the place of the arms, and suckers are not
present. In the males the spadix probably represents, functionally
at least, the hectocotylised arm of the Dibranchiata.

FIG. 699. — Octopus lentus, male specimen, showing the structure of the hectocotylised arm
(h. a.). (From Cooke, after Verrill.)

In all the Dibranchiata the funnel is a complete tube. In the
Nautilus, on the other hand, as we have seen, the folds which form
the funnel have their edges merely in apposition, and not united.
A valve, such as has been described in Sepia, occurs in most
Decapoda and in Nautilus, but is absent in the Octopoda.

Chromatophores, similar to those of
Sepia, are universal in the Di-
branchiata but absent in Nautilus.

Shell.— The shell of Nautilus is
the most complete and yet in a
certain sense the most primitive.
As already stated, it is an external
shell of a spiral - character, divided
internally by septa into a series of
chambers. The last of the chambers
is occupied by the body of the
animal ; the rest are filled with gas.
Perforating the middle of all the
septa in succession is a spiral tube
—the siphuncle — continuous with

E,a. -oo.-Ampwtret«s pelagic™, the <*ntro-dorsal region of the

course
the

Nautilus shifts forward at intervals
into a newly-formed chamber, and a new septum is formed
closing the latter off from the cavity last occupied. It is only
after the last septum has been formed that the animal attains
sexual maturity.

0. 700.-Araphitretus pelagicus, .

an Octopod with the arms united by a Visceral prominence. In the C(

gU&JHV&A' &!!«£> of its growth the body of

XII

PHYLUM MOLLUSCA

771

Of existing Dibranchiata, Spirula alone has a shell (Fig. 701)
comparable to that of Nautilus. The shell of Spirula is of spiral
form, the turns
of the spiral,
however, not
being in con-
tact. Intern-
ally it is divided
into chambers
by a series of
septa, and these

are perforated ^*% B '^JP N^9BEBVF' c

by a siphuncle. A

But the initial
chamber (proto-
conch), instead
of being, like
the initial
chamber in

Nautilus, simi- FIG. 701. — Shell of Spirula. A, outside view; B, showing last
chamber and position of siphuncle ; C, in section, showing the

lar to the Others septa and the course of the siphuncle ; D, shell broken to show th«

fVirmo-Ti em oil or convexity of the inner side of the septa ; E, portion of a septal

tnougn smaller, neck (After Cooke>)

is dilated into

a spherical shape, constricted off from the succeeding chamber,

and has passing through it a tube — the prosiphon — not continuous
with the siphuncle. Again, as will be
seen by comparing Figs. 684 and 702,
the relation of the soft parts to the shell
is the reverse of what obtains in Nautilus,
the shell of Spirula curving backwards
(endogastric curvature), that of Nautilus
forwards (exogastric curvature). Moreover
the shell of Spirula is an internal structure,
being almost completely covered by the
mantle.

The shell of the extinct Ammonites (Fig.
703), which are usually referred to the
Tetrabranchiata, resembles that of the
Nautilus in many respects, being a cham-
bered spiral shell with a large terminal
chamber, and with a siphuncle. The chief
external difference is in the form of the
sutures, or lines of union of the edges of
the septa with the side wall of the shell ;
sucker ; /. funnel ; s^ ~s~2. these are more or less complexly lobed,

projecting portions ot the . in- , • • -*f , •/ -r>

shell, the internal part of instead of being entire as in Nautilus. But

».it*Au ;^ s«,i:^«4-rtj v.., J«A*« j . ,

in one important respect the shell at aj*

772

ZOOLOGY

SECT.

FIG: — 7CRT— An^-Ammonite

(Ceratites nfiaosus). s. lobed
sutures.

Ammonite differs from that of Nautilus and approaches that of
the dibranchiate Spirula. At the apex of the spiral is an initial
chamber or protoconch, which is dilated and separated from the
first of the ordinary chambers by a constriction, and has passing
into it a prosiphon not continuous with the siphuncle. The

Ammonite was also characterised by the
possession of a paired or unpaired
calcified structure, called the aptychus,
not represented in any existing form.
The aptychus was most probably endo-
skeletal. Young Ammonites, each with
its aptychus, have been found within
the shell of the parent, in which they
must have remained protected during
their development.

In the ordinary decapod Dibranchiata
the shell may consist of three parts —
a horny pen or pro-ostracum, a calcareous guard, and a part termed
the phragmacone. The last, which alone represents the shell of
Spirula, has the form of a cone divided internally by a series of
septa perforated by a siphuncle. These parts are most completely
developed in the extinct genus Belemnites, in which the shell
(Fig. 704) consists of a straight, conical,
chambered phragmacone (phr.), with a
siphuncle, enclosed in a calcareous sheath,
the guard, produced into a horny or cal-
careous plate, the pro-ostracum (pen.). In
Sepia the spine-like projecting point repre-
sents the guard, and the main substance of
the shell is to be looked upon as the pro-
ostracum and phragmacone. In the Squids
(e.g., Loligo) the shell (Fig. 697, B) is long,
narrow, and completely horny ; it corre-
sponds to the pro-ostracum, the phrag-
macone being entirely absent.

In Octopus the shell is represented only
by a pair of vestiges with which muscles
are connected. In Argonauta there is no
shell in the male, but the female has an
external shell (Fig. 705) of a remarkable
character. This is a delicate spiral structure
the internal cavity of which is not divided
into chambers. It is .not secreted by the mantle like the shells of
other Mollusca, but by the surfaces of a pair of the arms ending
in expanded disc-like extremities, which become applied to its
outer surface (Fig. 698) ; its chief function is to carry the eggs.
An internal cartilaginous skeleton is present not only in

pen

phr

i m i ostracum ; phr. phrag-
moruiir. (From Nicholson
untl 1/ydokkrr's I'lihrmit-
ology.)

xn

PHYLUM MOLLUSCA

773

Sepia and Nautilus, as already described, but in all the Cephalo-
poda. Such an internal skeleton occurs in other groups — some
Chsetopoda (p. 462), Crustacea, and Arachnida (p. 651) — but
attains a much more elaborate character in the present group than
in any other Invertebrates.

The plume-shaped gills, lodged in the mantle-cavity, are two
in number in all the Dibranchiata, as in Sepia. In the Tetra-
branchiata there are four gills, similar in general character to
those of the Dibranchiata.

The ccelome in the Dibranchiata has the extent already
indicated in the case of Sepia, except that in the Octopoda
the oral part does not exist. In Nautilus it encloses, besides the
heart and gonad, a part of the glandular appendages of the afferent
branchial vessels. In the Dibranchiata the pericardial portion
communicates with the renal sacs ; in Nautilus this communication
is absent, but the coelome opens on the exterior by two symmetrical
viscer o-peri-
cardial orifices
placed at the side
of the openings
of the aboral
kidneys.

Alimentary
Organs. — Jaws
similar to those
of Sepia are pre-
sent in all the
members of the
class ; in Nauti-
lus, instead of
being completely
horny, they are
partly calcified,
salivary glands,
general charm-in

FIG. 705. — Shell' of Argonauta argo.

Buccal mass, oesophagus, stomach, intestine,
and digestive gland are all of the same
throughout all the members of the class.
In some of tin*. Dihni.nrhia.ta, such as Octopus, tlio.ro arc two
pairs of salivary glands. In Nautilus the salivary glands are
absent, so far as known, the (esophagus is dilated to form a sort
of crop, and the stomach is gizzard-like. In that genus also the
ink-gland, general in the Dibranchiata, is absent, and there is a
caeca] append age to the intestine ; the digestive gland is four-lobed,
each lobe having its duct. The so-called pancreas, described in
Sepia, is similarly developed in all the Dibranchiata, and is present
also, though only feebly developed, in the Tetrabranchiata. .

Heart and vascular system are well developed in the
Cephalopoda, and their structure and arrangement closely corre-
spond with what has been described in Sepia, except that in

774

ZOOLOGY

SECT.

Tetrabranchiata there are, as already stated, in accordance with
the double number of gills, four auricles instead of two, and
branchial hearts are absent.

Nervous system and Sense-organs. — The ganglia of the
central nervous system are in all closely aggregated together round
the oesophagus, as already stated to be the case in Sepia ; and the
general disposition is the same as that described. In Octopus the
ganglia are much less sharply marked off. In Nautilus, as already
mentioned, there is less concentration, and distinct ganglia are not
recognisable. All the Dibranchiata possess highly developed eyes
similar to those of Sepia, elevated on stalks in certain cases ; but
in Nautilus the eyes are of a much simpler character, each con-
sisting of a sac opening on the exterior by a small rounded aperture,
lined internally by a two-layered retina similar to that of Sepia,
but without lens, vitreous humour, or cornea. In the embryo
of the Dibranchiata, the eye passes through a stage in which it is in
the condition of an open cup similar to the adult eye of Nautilus.
Osphradia are present, as already mentioned, only in the Tetra-
branchiata ; but in both the Dibranchiata and the Tetrabranchiata

certain sensory processes or de-
pressions conjectured to possess
an olfactory function are de-
veloped on the head. Statocysts
are universally present.

All the Dibranchiata have two
kidneys or renal sacs similar
pOSl in character to those of Sepia, and
communicating with one another ;
in Octopus they are completely
united. In the Tetrabranchiata
there are four kidneys, each open-
ing on the exterior.

The sexes are distinct in all
the Cephalopoda, and in addition
. TOG.— segmenting ovum of x,oiigo. to the hectocotvlised arm there

(From Korschelt and Haider, after Watas*.) external

dors

vent

frequently

differences between male and female. In all the Dibranchiata
the arrangement of the gonads and gonoducts is, as regards general
features, similar to what we find in Sepia. In Octopus, however,
there are two oviducts instead of one, and in one other member
of the Octopoda (Eledone moschata) the same holds good of the
spermiducts.

Development. — The development of the Dibranchiata alone is
known. The eggs are very large, containing a relatively large
amount of food-yolk. They are usually laid in masses or strings
embedded in a soft, gelatinous, or a tougher, more leathery
substance, usually attached to some foreign body ; in some cases

XII

PHYLUM MOLLUSCA

775

each egg, enclosed in a gelatinous sheath, has a longer or shorter
stalk. A chorion or delicate transparent egg-membrane, in which

• ^M^SslMl^^

^^v •n»IS*'«v-- -HAwr

^^&^ •3S*$L-

=£2.$& ^&~\

\-bl

FIG 707 —Sepia, blastoderm at a late stage of segmentation, bl. blastoderm ; yk. yolk. (From
Korschelt and Heider, after Vialleton.)

there is an aperture — the micropyle — immediately invests the
egg itself. In shape the egg is oval or spherical. The greater part
of the compara-
tively small
quantity of pro-
toplasm lies as
a disc-like ele-

yh.&p

vation on the
surface of the
yolk on the side
of the egg at
which the mi-
cropyle is situ-
ated. Contin-
uous with this
germinal disc is
a thin layer of
peripheral pro-

toplasm invest-
inff the entire

ovum.

B

€P

7Q8 _Sections through the edge of the blastoderm of Sepia at
three successive stages, bl. blastoderm ; yk. yolk : yk. ep. yolk-
epithelium. (From Korschelt and Heider, after Vialleton.)

Segmentation (Figs. 706 and 707) is incomplete, being confined

776

ZOOLOGY

SECT.

to the germinal disc. At an early stage in the process of division
the blastoderm exhibits a distinct bilateral symmetry. This
meroblastic segmentation results in the formation of a nearly
circular blastoderm, the outer cells of which tend to separate off.
At first the blastoderm consists of only a single layer of cells —
the ectoderm, which gradually extends. At a later stage a second
layer (Fig. 708, B, C) appears below the margin of the blastoderm,

sh.al

FlU. 709. — Early stages iu the development of Loligo. A, stage at which tin: rudiments of
the eyes and of the shell-gland are first distinguishable ; B, later embryo from the oral
side ; C and D, from the anal side. ant. /. /. anterior funnel fold ; ar. rudiments of arms ;
den. ctenidia ; eye, eye ; mo. mouth : mnnt. rudiment of mantle ; oto. statocyst ; post. /. /.
posterior funnel fold ; «s7/. <//. shell-gland ; yk. s. yolk-sac. (After Korschelt and Iteirlrr.)

and extends inwards until it comes to underlie the greater part
of the embryonic part of the blastoderm : separating this from the
yolk is a thin layer of uncertain derivation — the yolk-epithelium
(Fig. 708, yk. ep.). There is some doubt as to the nature of the
second layer ; it certainly gives rise to the mesodermal structures,
and by some observers it is also said to form the epithelium of the

Xtl

PHYLUM MOLUJSCA

mesenteron. From whatever source it may be derived, the latter
becomes distinguishable as a cell-plate which becomes converted

ctn

Fio. 710. — Two later stages in the development of Loligo. A, from the funnel side; S,
obliquely from above. Letters as in preceding figures ; ne. cart, nuchal cartilage. (After
Korschelt and Heider.)

B

man-l

yk.s

j,-Ki_ 7 1 1. — Two stages in the development of Loligo, later than those represented in Fig. 710.
From the anal or funnel side. Letters as in preceding figure ; in addition, fin, fins ; fun.
funnel. (After Korschelt and Heider.)

into a vesicle at first opening below against the yolk epithelium,
there never being any direct communication with the yolk. An

778

ZOOLOGY

SECT.

extensive stomodseum eventually opens into this ; a proctodaeum
is merely represented by the ectodermal pit forming the anus.

About the middle of the blastoderm appears a thickening of
a cap-like shape, the edges of which become raised above the
general level of the blastoderm ; this is the rudiment of the
mantle. On the surface of this is developed a depression which
subsequently forms a closed sac — the shell-gland (Fig. 709, sh.gl.).
Below the mantle — i.e. nearer the vegetal pole — appear two eleva-
tions, each with a pit-like depression, which are the rudiments of
the eyes ; and still nearer the vegetal pole a series of paired eleva-
tions, the rudiments of the arms.

After the complete enclosure of the yolk by the blastoderm, the
mouth (mo.) is developed as an oval depression between the rudi-

— clen

FIG. 712. — Two late stages in the development of Loligo, seen from the funnel side.
as in preceding figures. (After Korschelt and Heider.)

Letters

ments of the eyes. Immediately in front of the edge of the mantle
appear two short ridges, the beginnings of the gills (cten.), and
a pair of folds — the posterior funnel-folds (post. f. /.) — which are
formed between these and the eyes, are the first rudiments of the
funnel ; the greater part of which, however, is formed from a second
pair of folds — the anterior funnel-folds (ant.f.f.) — developed further

xti . PHYLUM MOLUJSCA 779

forwards. Behind the anterior funnel-folds appear two pit-like
depressions, which subsequently develop into the statocysts.

The elevations on which the eyes (eye) are situated become
more and more prominent. The eyes themselves are formed
from a part only of these elevations ; each is a pit which sub-
sequently becomes closed to form a vesicle — the optic vesicle :
later an ingrowth of the ectoderm over this gives rise to the
lens.

The embryo covers only a part of the egg, and as it develops, it
withdraws itself more towards the animal pole, at which the
germinal disc was originally situated — a constriction, which soon
becomes very deep, separating it off from the rest of the egg ; the
latter, consisting of the greater part of the yolk enclosed in a thin
layer of blastoderm, forms a rounded appendage of the embryo —
the yolk-sac (yk. s.). The yolk-sac undergoes contractions, which
are due to the action of contractile cells in the thin mesoderm
lining it, and by this means the yolk is forced into the interior of
the body of the embryo.

The anus appears as an aperture situated on a little papilla —
the anal papilla. A row of cilia, which are developed in the
neighbourhood of the mouth in some forms, perhaps represent the
velum or pre-oral circlet of other molluscan embryos. The mantle
now increases in extent, and its margins become more prominent.
The anterior funnel-folds grow out and unite in the middle
line ; and these, with the posterior folds, go to form the
completed funnel together with the " neck-muscles." For a time
the edges of the two folds which form the funnel remain free ;
eventually they coalesce into a complete tube.

The edges of the mantle grow out into prominent folds to form
the mantle-cavity, into which the gills are drawn. Lateral out-
growths have already given rise to the rudiments of the fins. The
arms grow out into more and more prominent processes on which
the suckers are developed, the fourth pair — the prehensile arms
(ar. 2) — soon becoming distinguishable from the rest by their
greater length.

As the embryo increases in size, the yolk is gradually absorbed,
and the yolk-sac decreases in bulk, until, when the embryo leaves
the egg, it has almost completely disappeared.

Distribution. — The Cephalopoda are all marine, and range from
tidal limits to a considerable depth. A large number (Loligo, &c.)
are pelagic and move together in great shoals. Sepia lives chiefly
between stones and in rock-fissures in the littoral zone, and often
burrows in sand. Octopus constructs a den or shelter of stones to
which it always returns after excursions in search of food. Cephalo-
pods are, nearly without exception, carnivorous. In length they
range from an inch or two to as much as fifty feet — the gigantic
members of the group, such as Architeuthis, being by a long way

780 ZOOLOGY

the largest of invertebrate animals. Like the other classes of
Mollusca they are most abundant in tropical and warm-temperate
seas.

If the Ammonites are to be included among the Tetrabranchiata,
that sub-class was most abundantly represented during the
Mesozoic period. The nautiloid Tetrabranchiata were most
abundant in the Palaeozoic epoch, during which there lived a
great variety of forms of this group, the shell being straight
(Orthooeras), or curved (Phragmoceras), or in a flat spiral with
the turns not in contact, or in a helix, or a flat close spiral
(Nautilus and others). The earliest representatives of the Nautiloids
are found in rocks of Cambrian age ; they are comparatively
scarce in the Mesozoic epoch and in the Tertiary, and are repre-
sented at the present day only by the genus Nautilus itself. The
Ammonites are mainly Mesozoic, the representatives found in
the earlier rocks (from the Upper Silurian onwards) being few in
number and simpler in structure than the more typical later
forms. The oldest fossil representatives of undoubted Dibranchiata
belong to the extinct order of the Belemnites, which flourished
in the Mesozoic period from the Trias to the Cretaceous, and
survived in scanty number into the Tertiary. Unlike the Tetra-
branchiata, the Dibranchiata would appear to have reached their
maximum at the present cfity.

The mutual relationship of the various groups of Cephalopoda
are indicated, as nearly as the information at our disposal will
allow, in the following diagram (Fig. 713).

Decapoda

Belemnit-es
NatfMloids

Ammonifes

FIG. 713. — Diagram to illustrate the relationships of the groups of Cephalopoda.

GENERAL KEMARKS ON THE MOLLUSCA.

.

The Mollusca, like the Arthropoda, form an extremely well-
defined phylum, none of the adult members of which approaches
the lower groups of animals in any marked degree. There are,
however, clear indications of affinity with " Worms," especially
in the frequent occurrence of a trochophore stage in development,
and in the occurrence, in Amphineura and some of the lower Gastro-

xii PHYLUM MOLLUSCA 781

poda, of a ladder-like nervous system resembling that of some
Turbellaria and of the most worm-like of Arthropods — Peri patus.
The head-kidneys or primitive nephridia of the molmscan and
annelid trochophore are very similar, though developmentally
unlike, and are probably homologous with certain types of nephridial
tubes found in " Worms " from Platyhelminthes to Chsetopoda.

If the occurrence of the trochophore be taken as a guide towards
the ancestry of the Mollusca, it need not necessarily be regarded
as leading back to the Annulata. In fact the absence of segmental
repetition of parts in all, with the exception of Nautilus, would
seem to indicate the derivation of the phylum from a group in
which metamerism had not arisen. It will be readily recognised
that the gap between the typical trochophore and certain forms of
Turbellarian larvae (Miiller's larva) is not a very wide one, and
might be covered by adaptation of the larval Flat-worm to a freer
pelagic life. If we were to suppose that the most primitive Mollusca
were derived from Turbellarian-like ancestors, the conversion of a
larva of the type of Miiller's larva into a larval form like the mol-
luscan trochophore would also have to be postulated. This might
involve a common platyhelminth origin for Annulata and Mollusca,
with subsequent extreme divergence — a divergence in which the
respective trochophores would take part, though in a limited degree.
The chief changes which the adult animal would have to undergo
in order to assume the character of a primitive Mollusc on this
supposition would be — (1) the development of some kind of
protective layer of hard material, perhaps composed at first of
spicules in a thickened integument, on the dorsal surface — the
rudiment of the shell ; (2) the greater development of the muscular
layers of the body-wall on the ventral ^de to give rise to a more
efficient and specialised creeping organ than was possessed by the
Turbellarian ancestor ; (3) the development of specialised respira-
tory organs in the form of ctenidia — a change rendered necessary
by the great reduction in the available respiratory area brought
about by the development of the shell ; (4) the formation of an
anus and proctodaeum ; and (5) the development of a coelome.

With regard to the relationships of the various classes of
Mollusca, the following points are some of the most important to
be borne in mind.

The simplest, but not necessarily the most primitive, members
of the phylum are undoubtedly the Protobranchia among Pelecy-
poda, and the Aplacophora among Amphineura. The latter take
the lowest rank in virtue of the absence of both foot and shell, but
the possession by some of a radula indicates a comparatively high
degree of specialisation. On the other hand, while there is no
indication of an odontophore, even in a rudimentary condition, in
the Pelecypoda, the foot and shell are well developed even in Nucula
and its allies. There is no actual evidence to show that the foot

VOL. I 3D

782 ZOOLOGY SECT, xn

and shell have been lost by degeneration in the Aplacophora or the
odontophore in Pelecypoda ; and it would appear, therefore, that
the two groups are to be derived independently from some primitive
form.

The facts that the pelecypod shell, at its first appearance, is
univalve, and that the foot of the Protobranchia is of the creeping
type and their ctenidia plume-like, suggest the derivation of the
class from a form resembling a simple type of Gastropod with no
odontophore and with undisturbed bilateral symmetry. The
Amphineura are also bilaterally symmetrical, with paired ctenidia,
kidneys, and auricles ; and the fact that these organs are also paired
in the lower Gastropoda seems to point to a common ancestor for
Pelecypoda, Amphineura, and Gastropoda, which was bilaterally
symmetrical, had a creeping foot, a simple shell, paired auricles,
kidneys, and gills, and no odontophore.

While the leading feature in the evolution of the Pelecypoda
has been the splitting of the mantle into two halves and the
resulting bivalve shell, the most noticeable fact in that of Gastro-
poda, apart from the appearance of the odontophore, has been the
torsion of the visceral mass, producing a characteristic asymmetry.
In the Cephalopoda, on the other hand, the primitive bilateral
symmetry is retained, and the most characteristic special feature
of the group is the extraordinary modification of the foot into arms
or tentacles, and funnel. The class is raised far above the remain-
ing Mollusca by its wonderfully high organisation, especially of
the nervous system and the eye, and there is nothing to indicate
close relationship with any of the lower classes beyond the general
conformity to the molluscan plan of organisation and the presence
of an odontophore. The Cephalopods form, in fact, a singularly
isolated group. Palaeontology has not hitherto given any indica-
tion of their origin, and embryology is equally silent ; the absence
of a free larva, and the profound modification in development
produced by the enormous mass of food-yolk, sharply separating
them from all other members of the phylum.

INDEX

3 D 2

INDEX

All numbers refer to pages : words in italics are names of families, genera and
species : words in thick type are names of higher divisions : words in small
capitals are names of examples. Numbers in thick type are numbers of pages
on which there are figures : an asterisk after a number indicates a definition
of the term or of the group.

ABACTINAT,, 368*, 410

Abdomen, of Apus, 519 : Aatacu*, 528 :
Periplaneta, 603, 606

Aboral, 368*, 410

Absorption, 33

Aby^.pal species, 8*

Acantharia, 62, 63, 65

Acanthin, 63*

Acanthobdella, 505, 506 .

Acanthocephala, 293* : External char-
acters, '307, 308: Body-wall, 307 :
Body-cavity, 307, 398 : Proboscis,
308 : Vessels, 308 : Nervous system,
308 : Excretory organs, 308, 309 :
Reproductive organs, 309 : Develop-
ment, 309, 310

Acanihohamingia, 482

Acanthometridce, 62

Acarida, 646, 649, 651, 652, 653, 655, 656

Accessory female glands, 240*, 265*

Achro matin, 19*

Aciculum, 431*

Acineta, 101, 102, 103

Accela, 248, 261, 266, 267

Acontia, 186*, 199, 206

Acorn-shells, 551

Actceon, 726

Actinal, 368*, 410

Actinia, 191, 223

Actiniaria, 192*, 194, 195

Actinobolus, 95, 98

Actinodactylella, 253, 256, 260, 268

Actinometra, 416

Actinomma asterac-tnthion, 63, 64

Actinophrys sol, 58. 59, 61

Actinosphcerium, 58, 59, 61

Actinostome, 369*

Aclinotrocha, 351, 352, 512

Actinozoa, 129 : Example, 183 : Dis-
tinctive characters and classifica-
tion, 191 : Systematic position of
example, 194 : General organisa-
tion, 1 9 1 : Budding, 195 : Structure
of polypes, 196 : Enteric system
199 : Fixed and free forms, 199
Dimorphism, 200 : Skeleton, 200
Colour, 205 : Commensalism, 205
Distribution, 207

Actimda, 223*, 227

Adamsia palliata, 205, 206

Adductor impressions, 685, 666, 679,
680, 681

Adductor muscles, 357*, 666, 675, 676,
677, 679, 680

Adhesive cells, of Hormiphora, 213*
Turbellaria, 258

Adinidce, 81

Adipose tissue, 27*, 28

Adjusters, 357*

Adradius, 139

Adrectal gland, 724

JEginopsis, 156

JSginura, 155

JEquorea, 143

Affinities — See Relationships

Aijalma, 163

Agamobium, 141*, 175

Aggressive characters, in Crustacea,
586

Air-sacs, of Insscts, 627, 628

Albertia, 328

Alciopidfp., 477

Alcippe, 551, 565

Alcyonacea, 193*, 195, 197, 200

Alcyonaria, 193*, 194, 195, 197, 199,
200, 224

785

786

INDEX

Alcyonidce, 205

Alcyonidium, 340

Alcyonium, 193, 200

Alecithal egg, 217*

Algce, 65

Alimentary canal — See Digestive
system

Alimentary system — See Digestive
system

Allolobophora antipce, 469

Alpheu*, 587

Alpine forms, 8*

Alternation of generations — See Meta-
genesis

Alveolus, of Sea-urchin, 389*

Ambulacral area, 389

Ambulacral grooves, 369*

Ambulacra] ossicles, 371, 373, 374, 410

Ambulacral pores, 371, 389

Ambulacral ridges, 373

Ambulacral spines, 369, 380

Ambulacral system, of Astoria*, 375 :
Echinus, 389, 391: Sea-cucumber,
394, 395 : Antedon, 399 : Echino-
dermata, 407

Amitotic division, 20*

Ammonites, 767, 772, 780

Amnion, of Periplaneta, 613* :
Scorpion, 644

AMCEBA, 10, 11, 12, 13, 14, 15, 46 :
Pseudopods, 46, 47 : Endosarc, 46 :
Ectosarc, 46 : Contractile vacuole,
46, 47 : Encystation, 46 : Fission,
46 : Systematic position, 48

Amoeba coli, 51

Amoeba histolytica, 51

Amcebidce, 48

Amo3bocytes, 374, 392

Amoeboid cells, 25

Amce^ophyra, 231

Amoabula, of Didymium, 68, 69 : of
Gregarina, 84, 86

Amphiblastula, 124*, 125, 126

Amphidiscs, 122*, 123

Amphilina, 256, 283

Amphineura, 663, 694* : Distinctive
characters and classification, 694 :
General organisation, 695 : External
features, 695, 696 : Ctenidia, 695 :
Alimentary system, 696, 697 : Body-
cavity, 697 : Vascular system, 698 :
Nervous system, 698 : Reproductive
and renal organs, 899, 700 : Develop-
ment, 700, 701 . Ethology, distribu-
tion, &c., 701, 702

Amphinomidce, 457

Amphipoda, 554*. 569, 570, 571, 572,
573, 578, 580, 581, 587 "

Amphiptyches, 256, 258

Amphistomum, 253

Amphitretus pelagicus, 770

Amphiura, 229

Amphiura squamala, 420

Ampullae, 159*, 371*, 391, 394

Ampidlaria, 730

Amusium, 685, 686

Anal filament, 648

Anal glands, of Peripatus, 594

Anal respiration, 581

Anal spot, of Paramcscium, 92

Anal vesicles. 480*

Anaspidacea, 552*, 567, 581, 587

Anaspides, 553

Anatomy, 3*

Anchors, 176

Anr/uillula, 299

Anisopoda, 569*

Annulata, 430* : General remarks on,
510 : Relationships, 512, 513

Annuli of Leech, 494

ANODONTA, 663 : Shell, 664, 665, 666 :
Body, 666, 667 : Muscles, 667 :
Ccelome, 667, 668 : Digestive organs,
668 : Gills, 669 : Excretory organs,
671, 672 : Circulatory system, 672,
673 : Nervous system, 673 : Sensory
organs, 673, 674 : Reproductive
organs, 674 : Development, 674,
675, 676 : Systematic position, 678,
679

Anodonta cyqnea, 663, 664, 665, 667,
668, 669, 670

Anomia, 678, 680

Anomura, 555*, 574, 575

Anopheles, 89

Anoplophyra, 96

Anoplura? 619

Anostraca, 549*, 557

ANTEDON ROSACEA, 396, 397 :. General
external features, 396, 397 : Ossicles,

398 : Ccelome, 398 : Enteric canal,

399 : Ambulacral system, 399 :
Nervous system, 400 : Perihaemal
and haemal system, 400 : Sacculi,

400 : Reproductive organs, 401 :
Metamorphosis, 401, 423, 424 : Sys-
tematic position, 406 : Develop-
ment, 423, 424

Antenna, of Astacus, 531 — See also Ap-
pendages

Antennary gland, of Astacus, 538, 539

Antennary glands, 538, 581

Antennule, of Astacus, 531 — See also
Appendages

ANTHENEA FLAVESCEXS, 376, 378, 379,
' 380

Anthomedusse, 142*, 144, 145, 149

Anthowma, 561, 562

Anthura, 554

Antimeres, 43*, 406

Antipatharia, 193*, 198, 199, 201,207

Antipathes, 198

Antispadix, 759

Ant-lions, 618

Ants, 621, 636, 637

Aorta— See Vascular system

INDEX

787

Aphides, 632

Aphis rosce, 618

Aphrodite, 464

Aphroditea, 463, 464

Apical plate, of Trochosphere, 316 : of
Articulata, 363*

Apical system of plates, 385, 389

Apia mellifica, 622, 631, 636

Aplacophora, 694*, 695, 696, 697, 698,
699, 700, 702, 781

Aplysia, 718, 719, 720, 726

Aplysiidce, 714

Apoda (Holothuroidea), 404*, 417, 419

Apodidce, 555, 556

Apopyle, 109, 110, 111*

Appendages, of Rotifera, 323 : Apus,
517, 518 : Aatacus, 529, 530 : Crus-
tacea, 556 : Peripatus, 591 : Peri-
planeta, 605, 606, 607 : Insecta, 623 :
Scorpion, 639, 640

Apseudes, 553

Apterygota, 615*, 661

Aptychus, 772*

APUS, 514 : External characters, 515,
516 : Appendages, 517, 518 : Body-
wall, 519: Muscular system, 519:
Digestive organs, 520, 521 : Body-
cavity, 521 : Circulatory system,
521 : Respiration, 522 : Renal
organ, 522 : Nervous system, 522,
523 : Organs of sense, 523, 524 : Re-
productive organs, 524 : Develop-
ment, 525 : Systematic position, 555

Apus, 518, 521, 522, 524, 525

Apus cancrijormis, 515, 523

Apus glaciali*, 517

Aquatic pupa, 635

Arachnida, 515, 637*, 659, 660, 661,
662 : Example, 638 : Distinctive
characters and classification, 644 •
General organisation, 646 : External
form, 646-651 : Endosternite, 651 :
Coxal glands, 651 : Alimentary svs-
tcm, 651 : Heart, 652 : Organs "of
respiration, 652 : Nervous system,
653 : Sense organs, 653 : Repro-
ductive apparatus, 654 : Mode of
life, 655 : Geological history, 656 :
Appendix, 657

Arachnidium, 649

Araneida, 645*, 648, 651, 653, 656

Arbacia punctulata, pedicellaria, 413

Area, 678, 679, 683, 685, 6S6, 688, 690,
693

Arcella, 49

Archceocyathince, 126

Archseocytes. 123

Archenteron, 24*

Archi-Annelida, 428, 491*, 492, 493,
511, 513

Archi-cerebruni, 542 : of Feriplamta,
615

Archi-Chaetopoda, 454*, 465

Archidoris, 719

Archigetes, 256, 258, 283

Architeuthis, 779

Argiope, 359

Argonauta argo, 769, 772

Argonauts, 767, 768, 769, 772

Argulus, 551, 562, 563

Arhynchobdellida, 504*

Aristotle's lantern, 389*

Ark-shell, 686

Armadillidium, 572

Arrow worms, 292

Artemia, 549, 557

Arthrobranchise, 538

Arthropoda, 514* : Affinities of air-
breathing, 661, 662

Arthrostraca, 569*, 587

Articulata, 359*, 360, 361, 362 : Shell,
360, 361

Asc.aridcc, 299*

ASCARIS LUMBRTCOIDES, 292 : External
characters, 292 : Body-wall, 293 :
Digestive organs, 294, 295 : Body-
cavity, 295 : Excretory system, 296 ;
Nervous system, 296, 297 : Repro-
ductive organs, 296, 297 : Develop-
ment, 297 : Systematic position, 299

Ascaris megalocephala, 292

Ascaris suilla, 293

Ascetta, 117

Ascon, 118, 119*

A scopodaria, 347

Asellus, 554, 569, 570

Asexual reproduction, 41 : in Amoeba,
46 : Heliozoa, 61 : Radiolaria, 63 :
Euglena, 71 : Flagellata, 76 :
Choanoflagellata, 80 : Dinoflagellata,
81 : Cystoflagellata, 82 : Sporozoa,
83 : Gregarinida, 86 : Coccidiidea,
86 : Hsemosporidea, 89 : Myxo-
sporidea, 90 : Paramxcium, 92 :
Ciliata, 99 : Tentaculifera, 103 :
Sponges, 122 : Actinozoa, 195 :
Platyhelminthes, 278 : Bugula, 339 :
Chaetopoda, 475

Aspergillum, 6"/8, 683, 684

Aspidobranchia, 713*

Aspidochirotse, 406

Aspidocotylea, 249*, 253, 267

Aspidogaster, 279

Asplanchna, 322, 323, 324

Astacopsis, 555

Astacus, 555

ASTACUS FLUVTATILJS, 527, 535: Ex-
ternal characters. 527 : Abdomen,
528, 534 : Thoracic region, 528 :
Head, 529 : Appendages, 529, 530 :
Articulations, 532 : Body-wall, 533 :
Muscular system, 533, 534 : Diges-
tive organs, 535 : Respiratory
organs, 536, 53h/ : Excretory organs,
538, 539 : Circulatory organs, 539,
540, 541 : Nervous system, 541, 542 :

788

INDEX

Sensory organs, 542 : Reproduction,
542, 543 : Development, 544, 545,
546, 547 : Systematic position, 556

Astasiopsis, 73

ASTERIAS RUBENS. 368 : General ex-
ternal features, 368, 369, 370 : Trans-
verse section of an arm, 370. 371 :
Vascular and nervous systems, 372 :
Structure of the disc, 373 : Body-
wall and coelome, 373 : Digestive
system, 374, 375 : Ambulacral sys-
tem, 375, 376, 377 : Reproductive
system, 378, 379 : Systematic posi-
tion, 405

Asteriidcp, 405*

Asterina, development, 380, 381, 382,
383, 384, 385, 386

Asterina gibbosa, 381, 382, 383, 385, 420,
423

Asteroidea, Example, 368 : Develop-
ment, 380 : Distinctive characters
and classification, 402 : Apical sys-
tem, 409 : Modifications of form,
409, 410 : Ccelome, 416 : Ambula-
cral system, 416, 417 : Blood-
vascular system, 417, 418 : Haemal
system, 418 : Axial organ, 419 :
Enteric canal, 419, 420 : Nervous
system, 420 : Reproductive organs,
420, 421 : Development, 421 :
Ethology, 424, 425

Asthenosoma, 412

Astracoidea, 556*

Astrcea, 195, 196, 204

Astropecten, 410, 411

Astrophyton, 412

Astrosphere, 17, 18*

Atlanta peronii, 721

Atrium, 245, 339

Atrochal, 474*

Attraction -sphere, 19*

Auditory organs, 39, 40, 630

Aulactinium actinastrum, 64

Aulottoma, 504, 510

AURELIA AURITA, 167 : External char-
acteristics, 168, 169 : Digestive
cavity and canal system, 169, 170 :
Cell-layers, 170: Gonads, 171:
Gastric filaments, 171 : Muscular
and nervous systems, 171, 172 :
Sense organs, 171, 172 : Develop-
ment and life-history, 172, 173, 174,
175 : Systematic position, 176

Auricle, of heart, 37*

Auricles, of Sea-urchin, 389 : of Cteno-
phora, 220*

Auricularia, 396, 404, 422, 423

Australian region, 9*

Autogamy, 51, 61

Autolytus cornutus, 475

Avicularium, 334, 335, 345*

Axes, 42*, 43

Axial fibre, 94, 98, 99

Axial nerve, 400

Axial organ, 378, 392, 419, 421

Axial sinus, 373, 378

Axis -cylinder, 30*, 31

B

>ALANUS, 551, 563, 564, 565, 581
Barnacles, 3, 514, 551, 563, 564, 579,

581

Barrier reef, 207

Basal plate, of coral, 202*, 203, 204
Bathyctena, 223
Bathynella, 553
Batteries, 161*
Bdelloida, 321*, 323, 327
Bdelloura, 279
Bear-animalcules, 658, 659
Bee -parasites, 625, 636
Bees, 514, 602, 603, 621, 626, 631, 632,

636, 637

Beetles, 514, 602, 603, 620, 625
Belemnites, 767, 772, 780
Benthos, 8*
Berenice, 150
Beroe, 221, 223, 226
Beroida, 218*, 221
Bicellariidce, 341*
Bicellular glands, 485*
Bilateral symmetry, 42, 43*
Bile, 35*

Bilharzia hcematobia, 280, 281
Bilharzia japonica, 280
Binomial nomenclature, 1*
Binucleata, 74
Biology, 1*
Bionomics, 9*
Bipalium, 251
Bipinnaria, 402, 423
Bird-lice, 636
Bird's-Head Coralline, 334
Birgus, 555, 575, 580, 589
Birth-opening, 242*
Bivium, 370, 407 : of Sea-cucumber, 393
Black coral, 193*, 198, 199, 207
Blastoccele, 24*
Blastoidea, 405*, 426, 428
Blastomeres, 23*
Blastopore, 24*
Blastosphere, 24*
Blastostyle, of Obelia, 130*, 131 :

Leptolinge, 151 : Porpita, 165
Blastula, 24*
Blatta — See Periplaneta
Blattidce, 621, 637
Blepharoblast, 72*, 74
Blood, 30*, 35*
Blood-corpuscles, 30
Blood -vascular system — See Vascular

system

Blood-vessels, 35
Blow-flies, 620

INDEX

789

Blue coral, 193

Bodotria, 553

Body-cavity — See Coelome
Body-wall, of Sea-anemone, 185 :
Hormiphora, 212 : Liver-fluke, 237 :
Platyhelminthes, 257 : Nemertinea,
285 : Ascaris. 293 : Nematoda, 299 :
Chsetognatha, 310 : Brachionus
ruhens, 318 : Bugula, 336 : Ecto-
procta, 344 : Magellania, 355 : As-
terias, 373 : Sea-cucumber, 394 :
Nereis, 433 : Earthworm, 445 :
Chsetopoda, 463 : Sipunculus, 485 :
Sipunculoidea, 488 : Hirudo, 496 :
Apus, 519 : Astacus, 533 : Crustacea,
578 : Peripatus, 592 : Myriapoda,
600 : Insecta, 621

Bojanus, organ of, 672

Bolina hydatina, 223

Bone, 27, 28*, 29

Bone -corpuscles, 28*

Bonellia, 479, 481, 482, 483

Book-gills, 653

Book-lungs, 642, 652

Book-scorpions, 646

Bopyrini, 572

Bopyrus, 554

Botany, 1*

Bot-fly, 620, 636

Bothridia, 255*

Bothriocephalus, 255, 275, 281

Bothriocephalus latus, 281

Botryoidal tissue, of Leech, 497

Bougainvillea, 144, 152

Brachial disc, of Discomedusse, 182 .

Brachial ossicles, 398

Brachiolaria, 402, 423

Brachionidce, 323*

Brachionus, 317, 318, 319, 320, 321,
322, 323

BRACHIONUS RUBENS : External char-
acters, 317, 318: Body-wall, 318:
Digestive organs, 319 : Body -cavity,

320 : Excretory system, 320 : Ner-
vous system and sense organs, 320 :
Reproduction and development, 320,

321 : Systematic position, 323
Brachiopoda, 332, 353* : Example,

353 : Distinctive characters and
classification, 359 : Systematic posi-
tion of example, 359 : General or-
tanisation, 360 : Shell, 360, 361 :
picules, 361 : Peduncle, 361 : Lo-
phophore, 361 : Muscular system,
362 : Enteric canal, 362 : Heart,
362 : Nephridia, 362 : Nervous sys-
tem, 362 : Gonads, 362 : Develop-
ment, 362, 363, 364, 365 : Distri-
bution, 364

Brachyura, 555*, 575, 576, 577, 586

Bract, 160, 162*, 518

Brain, 38*

Branch f.llion, 504, 506, 510

Branchiae, 36* : of Asterias, 369 : Sea-
urchin, 388: Polychfeta, 461, -462 :
Oligochaeta, 462 : Branchellion, 50/TT
Astacus, 537 : Crustacea, 578 : Aiio-
donta, 668, 669, 670, 671 : Pele-
cypoda, 686 : Triton, 706 : Gastro-
poda, 723

Branchial formula, of Astacus, &c., 538

BranchioDoda, 549*, 555, 556, 557, 578,
579, 581, 582, 587, 660

Branchipus, 549, 557

Branchiura, 551*, 559, 562, 563

Brine-shrimp, 557

Brisingidce, 410

Brood-cavity, 103

Brood-pouch, 363, 519

Brown body, 339

Brown tubes, 489*

Buccal cavity, 33

Buccal tentacles, 413

Buccinum undatum, 702

Budding, 41* — Sea Asexual reproduc-
tion : in Turbellaria, 251

Buff on, 5

Bugs, 619, 625, 636

Bugula, 340

BUGULA AVICULARIA, 334, 335 : Body-
wall, 336 : Coelome, 336 : ^Aiimen-
tary canal, 336 : Nervous system,
336 : Excretory organs, 336 : Re-
productive organs, 336, 337 : De-
velopment, 337, 338, 339 : Sys-
tematic position, 341

Bugula plumosa, 338

Bulinus, 241

Bursa copulatrix, 265, 266, 267, 268, 631

Busycon, 730

BUTHUS, 638 : External features, 638,
639, 640 : Digestive system, 640,
641 : Circulatory organs, 641, 642 :
Organs of respiration, 642 : Nervous
system, 641, 642, 643 : Organs of
special sense, 643 : Reproductive
organs, 643 : Development, 643, 644

Butterflies, 514, 602, 620

Byssus, 677, 684, 685 : provisional,
675, 685

Byssus-gland, 677, 685

G

.'ADDIS-FLIES, 618

Cake-urchins, 403*, 408, 413, 414
Calcarea, 113*, 121, 122, 123, 125
Calcareous spicules, of Sponges, 108,

109, 120, 122, 123
Calciferous glands, 447
Callianira, 215, 216, 219
Callitiara, 150
Galocalanus, 560
Calotte, 227*, 338
Calymma, of Radiolaria, 62

790

INDEX

Calyptoblastea, 144*

Gambarus, 555

Cambrian, 7

Campanulariidce, 143*

Campodea, 616

Canaliculi (bone), 28*

Canals, Haversian, 28*, 29 : incurrent,
radial or flagellate, excurrent of
Sponge, 108, 109*, 110, 118: of
Medusa, 136, 137

Canal system of Sponges, 118, 119

Cancer, 555, 575

Cannostomae, 179

Capillaries, 36*

Capillary vessels, 235
4 Capillitium, of Mycetozoa, 68, 69

Caprella, 554, 571

Capria, 176

Capsulogenous glands, 446*

Carabus auratus, 626

Carapace, Apus, 515 : Astacus, 527 :
of Scorpion, 638

Carboniferous, 7

Cardinal process, 354*

Cardioblasts, 614*

Cardium, 678, 680, 683, 692

Carina, of Cirripedia, 563

Carinaria mediterranea, 719

Carp -lice, 551

Carpoidea, 405*, 426

Cartilage, 27, 28* : Hyaline, 28 :
Fibrous, 28 : Yellow elastic, 28 :
Calcified, 28

Caryophyllceus, 256, 258, 283

Cassiopeia, 182

Caudal cells, 304

Caudal spine, 650

Caudal styles, of Apus, 515

Caudal vesicle, 244*, 275

Cell, 16, 17*, 18 : Forms, 25 : Ciliated,
25, 45 : Flagellate, 25, 45 : Amoe-
boid, 45 : Encysted, 45

Cell, animal, 15, 16*

Cell-colony, 52*, 67

Cell-division, 18, 19

Cell-lineage, 268*

Cell-plate, 18, 20*

Cell -wall, 17*

Cellepora, 340

Cellulose, 15, 66, 69, 71, 75, 76, 80

Cement glands, 265, 320

Centipedes, 514, 598, 600

Central capsule, of Radiolaria, 62 : of
Antedon, 400

Central nervous system of Medusae, 151

Centro -dorsal ossicle, of Antedon, 397,
398

Centrolecithal egg, 217*, 525

Centrosome, 18*

Cephalic apodeme, of Aput, 520 : Anta-
eus, 529

Cephalopoda, 663, 738* : Examples,
738, 754 : Distinctive characters and

classification, 766 : Systematic posi-
tion of the examples, 767 : General
organisation, 768 : External fea-
tures, 768 : Shell, 770 : Internal
skeleton, 772, 773 : Gills, 773 :
Coelome, 773 : Alimentary organs,
773 : Heart and vascular system,
773, 774 : Nervous system and
sense organs, 774 : Osphradia, 774 :
Statocysts, 774 : Kidneys, 774 :
Sexes, 774 : Development, 774, 775,
776, 777, 778, 779: Distribution,
&c., 779 : Relationships, 780

Cephalopodium. 740

Cephalo thorax, of Astacus, 527

Cerata, 724

Ceratella, 148

Ceratella fusca, 148, 148

Ceratites nodosus, 772

Ceratium, 81

Ceratosa, 113*, 127

Cercaria, 241, 242*

Cerci, 606

Cerebral organs, 289*, 487

Cerianthus, 197, 200

Cervical fold, of Apus, 515

Cervical glands, 301

Cervical groove, of Astacus, 528

Cervical sclerites, 606

Cestida, 218*, 220

Cestoda, 249*, 254, 255, 256, 257, 258 ,
259, 260, 263, 264, 268, 274, 275, 276,
277, 279, 280, 281, 282, 283, 284 :
Example, 243

Cestus veneris, 220

Cetonia aurata, 310

Chajta?, 430— See Seta

Chcetoderma, 695, 696, 697, 699

Chcetogaster, 475

Chsetognatha, 292*, 310* : External
characters, 310 : Body-wall, 310 :
Enteric canal, 311 : Ccelome, 311 :
Nervous system, 311 : Sensory or-
gans, 311, 312 : Reproduction, 312 :
Development, 312, 313

Clcetonotus, 329

Chsetopoda, 429* : Examples, 430, 443 :
Distinctive characters and classi-
fication, 454.: Systematic position of
examples, 455 : General organisa-
tion, 457 : General form, 457 : Para-
podia and seta?, 458, 459, 460, 461 :
Branchiae, 461 : Body-wall, 463 :
Coelome, 463 : Enteric canal, 46H :
Blood-vessels, 464 : Nervous sys-
tem, 465 : Organs of special sense,
466 : Organs of excretion, 467, 468,
469 : Phosphorescence, 470 : Re-
productive organs, 470 : Develop-
ment, 471, 472, 473, 474, 475 :
Asexual reproduction, 475, 476 :
Mode of life, &c., 476, 477 : Appen-
dices, 477, 479

INDEX

791

Chcetopterus, 458, 470

Chcetosoma, 313

Chcetosomidce, 313*

Chalk, 59

Charybdcea marsupialis, 179, 180

Cheilostomata, 340*, 341, 342, 344,
345, 346

Chelse, 530*

Chelicerse, 638*, 646, 648, 649, 650

Chelifer bravaisii, 646

Chelipeds, 530*

Chilaria, 650

Chilina, 726

Chilognatha, 598*

Chilopoia, 600*, 601

Chironomus, 629

Chitin, 32, 46*

Chiton, 663, 694, 695, 696, 697, 700, 701

Chitonellvs, 695

Chlamydomyxa, 65, 66

Chlamydophrys stercorea, 52, 53, 54

Chlorcemidce, 464

Chloragen cells, 447

Chlorophyll, 14, 59, 66, 70, 75, 120

Choanocytes, 108, 109, 110*, 112

Choanoflasrellata, 71* : General struc-
ture, 79 : Collar, 79, 80 : Colonies,
79, 80 : Reproduction, 80

Chcetopterus, 458

Chondracanthus, 551, 561, 562

Chordotonal nerve -endings, 630

Chorion, of Cephalopoda, 775 : of In-
sects, 612

Choristida, 127

Chromatin, 17*, 18, 20

Chromatophores, Actinosphoerium, 58,
59 : Chlamydomyxa, 66 : Flagellata,
73, 74 : Dinoflagellata, 80, 81 :
S3Pia, 741 : Cephalopoda, 770

Chromidia, 20*, 49

Chromidiosonies, 20*

Chromioles, 17*, 19

Chromosome, 18, 19*

Chrysalis, 635

Cicada, 619, 631

Cicatrix, 755

Cidaris, 417

Cilia, 25*

Ciliary flames, 263

Ciliary process, 750

Ciliata, 93* : Form of body, 94, 95, 96,

97, 98, 99, 100 : Stalk, 94, 98, 99 :
Arrangement of cilia, 94, 96, 98 :
Undulating membranes, 94, 96 :
Meganucleus, 95, 96 : Micronuclei,
95, 96 : Contractile vacuole, 95, 96,

98, 99 : Non-contractile vacuoles,
95, 96 : Trichocysts, 95, 96 : Diges-
tive apparatus, 96, 97 : Skeleton,
lorica, 96, 97, 98 : Operculum, 97,
98 : Colonies, 96, 97, 98, 99 : Re-
production, 99, 100 : Conjugation,
100, 101

Ciliated chambers, 119

Ciliated pits, 467*

Cinclides, 186, 199

Circulation — See Vascular system

Circulatory system — See Vascular sys-
tem

Cirri, 397*, 429

Cirripedia, 551*, 578, 579, 581, 582,

• 583, 586, 587

Cirrus, 237

Cirrus sac, 245

Cistella, 359, 360, 361, 362, 363, 364

Cladocera, 550*, 557, 558, 581, 583

Cladophiurae, 403*

Class, 4*

Classification, 3*, 5 : of Rhizopoda,
46 : Mastigophora, 69 : Sporozoa,
82 : Infusoria, 90 : Porifera,
112 : Hydrozoa, 141 : Scyphozoa,
175 : Actinozoa, 191 : Ctenophora,
217 : Platyhelminthes, 248 : Ne-
mertinea, 290 : Nematoda, 298 :
Rotifera, 32 1 : Polyzoa, 340 :
Brachiopoda, 359 : Echinodermata,
401 : Chsetopoda, 454 : Sipun-
culoidea, 48'8 : Hirudinea, 503 :
Crustacea, 548 : Insecta, 615 :
Arachnida, 615 : Pelecypoda, 676 :
Amphineura, 694 : Gastropoda, 713 ;
Cephalopoda, 766

Clathrina, 117, 120, 123, 125

Clathrina blanca, 123, 124, 125

Clathrozoon, 148

Clathrulina, 60, 61

Clavatella, 145

Clavulse, 408*

Cleaning foot, 566, 567

Clepsine, 504, 506, 507, 508, 509, 510

Cliona, 123, 127

Clitellum, 444, 457, 497

Cloaca, 245

Clypeaster subdepressus, 414

Clypeastridea, 403*, 408, 413

Clypeus, 603, 604*, 622

Cnidoblast, 134*

Cnidocil, 134*

Coccidce, 632

Cocci iiiiea, 83* : Characteristic fea-
tures, 86, 87, 88, 89

Coccidium, 86, 87

Cockchafer, 628

Cockles, 663, 678, 680

Cockroach — See Periplaneta

Cockroaches, 514, 602, 617, 621, 624,
633, 634, 637, 640, 642

Cocoa-nut crab, 555

Cocoon, 452, 471

Codonella, 96

Ccslenterata, Classes, 129 : Examples,
129, 167, 183, 208 : Relationships,
223-227 : Appendix, 227 : Rela-
tionships to Sponges, 226

Coeliac canal, 399

792

INDEX

Ccelome and body cavity, of Ascaris,
295 : Nematoda, 301 : Acantho-
cephala, 307: Chsetognatha, 311
Buguli, 336 : Endoprocta, 347
Phoronis, 349 : Maqetlania, 358
Asterias, 373 : Sea-urchin, 392
Sea-cucumber, 394 : Antedon, 398
Echinodermata, 416 : Nereis, 432
Chsetopoda, 463 : Sipuncidus, 486
Gephyrea, 488 : Apus, 521 : Crus
tacea, 578 : Peripatus, 592 : Insecta
625 : Anodonta, 667 : Amphineura
697 : Sepia, 745 : Cephalopoda, 773
Coelomoducts, 429, 469
Cfploplana, 221, 222, 282
Coenenchyma, 205*
Ccenosarc, 132*

Coleoptera, 620*, 624, 630, 632, 637

Collar of choanoflagellata. 79, 80

Collared cells, 108, 110*, 112

Collared monads, 79, 80

CoUembola, 616*

Collencytes, 108, 112*

Colleterial glands, 612

Collozoum, 63, 65

COLOCHIRUS, 393 : General external
features, 393 : Structure of body-
wall, 394 : Ambulacra! system, 394 :
Nerve-ring, 394 : Perihaemal and
haemal systems, ?94 : CcBlome, 394 :
Enteric canal, 395 : Reproductive
organs, 396 : Development, 396 :
Systematic position, '406

Colony, 41* : of Foraminifera, 53 :
Heliozoa, 59 : Radiolaria, 63 :
Flagellata, 75, 76 : Choanoflagellata,
80 : Ciliata, 98 : Tentaculifera, 103 :
Obelia, 130: Leptolime, 143 : Actino-
zoa, 195 : Polyzoa, 333 : Bugula, 334 :
Ectoprocta, 341 : Endoprocta, 348

Colpoda, 99

Columella, of Coral, 202* : of Triton,
703

Column, 185*

Comatidce, 425, 426

Comatulidce, 406

Comb -jellies, 129

Comb-ribs, 210*

Combs, of Hormiphora, 208*, 209, 210

Commensalism, in Sponges, 127 : Hy-
dractinia, 147 : Actinozoa, 205 :
Platyhelminthes, 279 : Chaetopoda,
476 : Crustacea, 586

Complemental males, 582

Conchiolin, 666*

Conchostraca, 550*

Condylostoma, 96

Cone, of Coronata, 178

Cones, 714

Conjugation, 61* : of Amceba, 47 :
Foraminifera, 56 : Heliozoa, 61 :
Flagellata, 79 : Cystoflagellata, 82 :
Paramoecium, 92 : Ciliata, 100

Connective tissue, 27* : Gelatinous,
27: Fibrous, 27: Fatty, 27, 28:
Retiform, 27

Connective tissue cells of Sponges, 112

Contractile vacuole, 10, 12*, 13, 14, 46,
47, 88, 70, 79, 80, 91

Contractility of muscles, 29, 38*

Conus, 730

Convoluta, 251- 260

Copepoda, 550*, 559, 578, 579, 581, 582,
583

Copromonas subtili.*, 77, 78

Copulation, 50*, 78

Coral, Aporose, 205* : Black, 193, 207 :
Blue, 1 93, 205 : Fossil, 208 : Organ
pipe, 193, 195 : Perforate, 205* :
Red, 193, 201, 207 : Reef -building,
207 : Stony, 129, 193, 199

Coral limestones, 207, 208

Coral reefs, 159, 207

Corallines, 129, 333

Corallite, 202*

Corallium, 193, 195, 196, 201, 205, 207

Corallum, 204*

Cordylophora, 166, 258

Cornea, 750* : false, 749, 750*

Corona, of Polyzoa, 338 : Sea-urchin,
388

Coronary groove, 175, 177, 178*

Coronata, 175*, 177, 178, 179

Corpuscles, 30 : amosboid, 30 : Mie-
scher's or Rainey's, 90

Cortex, of Actinosphcerium, 58, 59 :
Monocystis, 82 : Paramoecium, 90,
91 : Sponges, 111, 119, 120

Corymorpha, 145, 148

Couplers, 561

Covered-budded Hydroids, 144

Cowries, 714

Coxa, 606

Coxal glands, Peripatus, 593 : Scor-
pion, 642 : Arachnida, 651

Coxal organs, 593

Crabs, 515., 555, 572, 575, 578, 580, 581,
585, 589

Crane-flies, 620

Crangon. 573

Crania, 359, 360

Crayfish, 527 — See Astacus

Crayfishes, 515, 555, 572, 580, 585

Cretaceous, 7

Crickets, 621

Crinoidea, 368, 404* : Example, 396 :
Distinctive characters and classifica-
tion, 404 : Apical system, 409 : Modi-
fications of form, 415 : Coelome, 416 :
Ambulacral system, 417 : Blood-
vascular system, 418 : Hsemal sys-
tem, 418 : Enteric canal, 419 : Ner-
vous system, 420 : Reproductive
organ?,' 421 : Development, 421 :
Ethology, 425, 426
Crioceris, 620

INDEX

793

Crisia, 340

Crista acustica, 750*

Cnstatella, 341, 343

Crop, 33

Crown, of Coronata, 178

Crural glands, 593

Crustacea, 514, 662 : Example a, 514 :
Example 6, 527 : Distinctive char-
acters and classification, 548 : Sys-
tematic position of the examples,
J>55 i General organisation, 556 : Ex-
ternal characters and structure of
appendages, 556 : Texture of the
exoskeleton, 578 : Body-cavity, 578 :
Enteric canal, 578 : Respiratory
organs, 579 : Heart, 581 : Excretory
organs, 581 ; Nervous system, 581 :
Sense organs, 581 : Reproduction,
582 : Development, 582 : Ethology,
585 : Affinities and mutual relation-
ships, 587 : Appendix, 589

Cryptocephala, 455*, 457, 464, 465,
472, 477

Cryptomnnas, 73

Cryptoniscus, 572

Crystalline style, 668 : of Gastropoda,
724

Ctenaria, 150, 224, 225

Ctenidium, 669, 670, 671, 686 : of
Triton, 706 : of Gastropoda, 722, 723

Ctenodrilus, 492

Ctenophora, 129, 208 : Example, 208 :
Distinctive characters and classifica-
tion, 217 : Systematic position of the
example, 218 : General organisation^
219, 220, 221, 222, 223 : Relation-
ships, 223

Ctenoplana, 221, 222, 282

Ctenopteryx, 768

Ctenostomata, 340*, 342, 344, 346

Cubomedusse, 175*, 179, 180, 181

Cucumaria — See Colochirus

Cucumaria planci, 393

Culex, 619, 623

Cuma, 553

Cumacea, 553*, 568, 579, 587

Cunarcha, 155

Cunina, 166

Cunina parasitica, 156

Cup-coral, 202

Cursoria, 617, 621

Cuspidaria, 678

Cuticle, 32*

Cuttle-fish, 663, 738, 767

Cuvier, 3

Cuvierian organs, 419

Cyamus, 554, 571

Cyanea arctica, 181

Cyanophycece, 74

Cyclas, 691

Cyclidium, 96

Cyclops, 551, 559. 560, 581, 582

Cyclostomata, 340*, 342, 345, 346, 347

Cydippe, 208

Cydippida, 218*, 219

Cymothoa, 572, 582

Cynipidm, 632

Cyphonautes, 346

Cyprsea, 719

Cypris, 550, 558, 559, 581

Cypris stage, of Cirripedes, 551, 583

Cyst, 13*, 71 : Amoeba, 46*, 47 : Didy-

mium, 68 : Euglena, 70, 71 : Mono-

cystia. 82, 83
Cysticercoid, 275
Cystic rcuft, 247, 275, 276
Cysticercus celluloses, 281
Cystoflagellata, 71* : Characteristic

features, 81

Cystoidea, 405*, 426, 428
Cythere, 550, 558, 559
Cytoplasm, 17*

'ACTYLOPORES, of Millepora, 157* :

Stylaster, 159

Dactylozooids, 149*, 164 : of Mille-
pora, 158, 159 : Stylaster, 159 :

Halistemma, 160, 162
Daddy long-legs, 620
Dahlia wartlet, 184, 185
Dallingeria, 73
Dalmanites sociaUs, 589
Daphnia, 550, 558
Darwin, 6
Dasychone, 466
Daughter-cell, 20*
Daughter-chromosomes, 19*
Daughter-cysts, 278
Daughter-nucleus, 19*
Daughter-segments, 19*
Day-flies, 626, 637
Dead men's fingers, 193, 200
Decapoda, 554*, 556, 572, 579, 581, 582,

584, 585, 586, 587, 588
Decapoda (Cephalopoda), 767*, 768,

781
Degeneration, in Copepoda, 562, 563 :

in Cirripedia, 565 : in Tsopoda, 572
Deiopea, 220
Deltidium, 353*
Deltoid plates, 405
Demosponsia, 113*
Dendrochirotse, 406*
Dendroc<xlum — See Planaria
Dendrocometes, 102
Dendrophyllia, 204, 205
Dendrosoma, 102, 103
Dentalium, 736, 737
Dentary region, 432
Denticles, 432
Depastridce, 176
Derm (dermis), 32*

794

INDEX

Dermal branchiae, 369, 388, 416 : of
Echinoidea, 388

Dermal cortex, 109, 111*, 119, 120

Dermal pores, 369

Dermis, 32*

DesmoscolecidcB, 314, 315

Desmoscolex, 314, 315

Destructive metabolism, 13*

Deutomerite of Gregarina, 84, 85*

Development of Sponges, 122, 123 :
Sycon, 124, 125, 126 : Obelia, 140 :
Leptolince, 152 : Trachylince, 156 :
Aurelia, 172, 173, 174 : Sea-ane-
mone, 190 : Hormiphora, 214 :
Dicyemidce, 228, 229 : Ehopalura,
229', 230 : Planaria, 236 : Liver-
Fluke, 240, 241 : Tcenia, 246, 247 :
Platyheiminthes, 268-278 : Nemer-
tinea, 290: Ascaris, 297 : Nematoda,
303, 304 : Chjetognatha, 312, 313 :
Bract ionus rubens, 321 : Rot if era,
328 : Bugula, 337 : Ectoprocta, 345 :
Endoprocta, 348 : Pkoroni*, 351,
352 : Brachiopoda, 362, 363, 364 :
Asterina, 380-386 : Sea-urchin, 392 :
Sea-cucumber, 396 : Echinodermata,
421 : Antedon, 423, 424 : Nereis,
440, 441, 442, 44:t : Lumbricus, 452,
453, 454 : Chrctopoda, 471 : Echi-
urida, 482, 483, 484 : Hirudo, 503 :
Hirudinea, 508, 509 : Apus, 525,
526 . Astacus, 544, 545, 546, 547 :
Crustacea, 582, 583 : Peripatus, 595,
596, 597 : Myriapoda, 601 , 602 :
Periplaneta, 612, 613 : Scorpion, 643,
644: Anodonta, 674, 675, 676:
Pelecypoda, 691, 692 : Amphineura,
700, 701: Gastropoda, 729-735:
Scaphopoda, 737, 738 : Cephalopoda,
774-779

Devonian, 7

Diastylis, 553, 568

Dibothriocephalus, 268

Dibranchiata, 767*, 770, 771, 772, 773,
774, 780

Diceras, 682

Dichyocysta, 97

Dicroccelium lanceolatum, 280

Dicyclica, 405*

Dicyema, 227, 228, 229

Dicyemidce, 227, 228, 229

Didinium, 95, 96

DlDYMIUM DIFFORME, 68, 69

Differentiation, 25*

Difflugia, 49

Digenetica, 249*, 253, 260, 263, 266,
267, 268, 272, 273, 280, 282

Digestion, intracellular, 34

Digestive glands, 33

Digestive svstem. 33* : of Paramcecium,
92 : Ciliata, 97 : Aurelia, 169, 170 :
Sea-anemone, 185 : Actinozoa, 199 :
Hormiphora, 210, 212 : Planaria,

233, 234 : Liver-Fluke, 238 : Platy-
helminthes, 260, 261, 262 : Nemer-
tinea, 286, 287 : Ascaris, 294, 295,
296 : Nematoda, 300, 301 : Choeto-

fnatha, 311 : Brachionus rubens,
19, 320 : Rotifera, 326, 327 : Bugula,
336 : Ectoprocta, 344 : Endoprocta,
347 : Phoronis, 349 : Magellania,
355, 356 : Asterias, 374, 375 : Sea-
urchin, 391, 392 : Sea-cucumber,
395, 396 : Antedon, 399 : Echino-
dermata, 419, 420 : Nereis, 432:
Lumbricus, 446, 447, 448 : Chseto-
poda, 463, 464 : Sipunculus, 486,
487 : Sipunculoidea, 489 : Hirudo,
497, 498 : Hirudinea, 505 : Apus,
520, 521 : Astacus, 535, 536, 537 :
Crustacea, 578 : Peripatus, 592, 593 :
Myriapoda, 601 : Periplaneta, 608,
609 : Insecta, 626, 627 : Scorpion,
640, 641, 642 : Arachnida, 651, 652 :
Anodonta, 668 : Pelecypoda, 687,
688 : Amphineura, 696, 697 : Triton,
707, 708, 709 : Gastropoda, 724 :
Scaphopoda, 736 : Sepia, 742, 743,
745: Nautilus, 761, 762: Cephalo-
poda, 773

Dimorpha, 72, 73

Dimorphism, in Foraminifera, 56 : in
Radiolaria, 65 : in Flagellata, 79 :
in Ciliata, 99

Dimorphism, sexual, 40*

Dimorphograptus, 166

Dimyaria, 286
• Dinobryon, 73, 77

Dinoflagellata, 71* : Characteristic
features, 80, 81

Dinoplilea, 317, 330

Dinophilus, 330, 331, 511, 512

Dioecious, 40*, 140*

Diophrys, 94, 96

Diphyes, 164

Dipleuruia, 422*

Diplomita, 73

Diplopoda, 598*r 600, 601, 602

Diplozoon, 273

Diptera, 619*, 623, 624, 625, 629, 631,
632, 634, 635, 637

Directive mesenteries, of Sea-anemone,
188*

Disc, 185*

Disc -jellies, 181

Discina, 359, 360, 361

Discoccelis, 269

Discoidal segmentation, 643

Discomedusse, 175*, 181, 182, 183

Discorbina, 54

Discosoma, 206, 207

Dissepiments, 202*

Distinctive characters — See Classifica-
tion
• Distomidce, 249*, 250

Distomum, 259

INDEX

795

DISTOMUM HEPATICUM, 236 — See
Fasciola hepatica

Distomum rathousii, 280

Distribution, 8 : of Sponges, 125 :
Hydroco rollinse, 159 : Scyphozoa,
183 : Actinozoa, 207 . Ctenophora,
223 : Platyhelminthes, 279 : Ecto-
procta, 346 : Bracaiopoda, 364 :
Sipunculoidea, 490 : Hirudinea, 510 :
Onychophora, 597 : Pelecypoda,
692 : Amphineura, 701 : Gastropoda,
735 : Cephalopoda, 779

Distribution, geographical, 8

Distribution, geological, 8* : of Fora-
minifera, 58 : Sponges, 125, 126 :
Corals, 208 : Brachiopoda, 364, 365 :
Chsetopoda, 477 : Insecta, 637 :
Arachnida, 656 : Cephalopoda, 780

Distribution, vertical, 8*

Divaricate r?, 357

Dochmius duodenalis, 300, 301

Docoglossa, 714*, 736

Doctrine of descent, 6

Donax, 687

Doris, 715, 719, 723

Doris (Archidoris) tuberculata, 719

Dorsal cirri, of Antedon, 397

Dorsal organ, of Apus, 515*

Dorsal pores, of Earthworm, 445

Dorsal surface, 43*

Dragon flies, 618, 629

Drepanophorus, 285

Drilophaqa, 328

Dromia, 586

Ductus ejaculatorius, 296*

Dytiscus, 630

E.

E

IAR, 40*

Ear-shells, 714

Earthworm — See Lumbricus

Earthworms, 429

Earwigs, 621

Ecdyses, of Apus, 526

Echiniidce, 406*

Echinoderes, 314

Echinoderidce, 314

Echinodermata, 368* : Examples, 368,
386, 393, 396 : Distinctive charac-
ters and classification, 401 : Sys-
tematic position of examples, 405 :
General organisation, 406 : General
form and symmetry, 406 : Systems
of plates, 409 : Modifications of
form, 409 : Coelome, 416 : Ambula-
cral system, 416 : Blood-vascular
system, 417 : Haemal system, 418 :
Axial organ, 419 : Enteric canal,

419 : Nervous system, 420 : Sexes,

420 : Development and metamor-
phosis, 421 : Echinopsedium, 422 :

Ethology, &c., 424 : Self -mutilation
and regeneration, 425 : Affinities, 426

Echinoidea, 368, 403* : Example, 386 :
Distinctive characters and classifi-
cation, 403 : Apical system, 409 :
Modifications of form, 409 : Dermal
branchiae, 416 : Stewart's organs,
416 : Ambulacra! system, 416 :
Blood-vascular system, 417 : Haemal
system, 418 : Axial organ, 419 :
Enteric canal, 419 : Nervous system,
420 : Reproductive organs, 420 :
Development, 421 : Ethology, 424

Echinopcedium, 422*

Echinorhynchus, 307, 308, 309, 310

ECHINUS, 386 : General external fea-
tures, 386, 387, 388 : Corona, 388 :
Aristotle's lantern, 389, 390 : Ner-
vous system, 390 : Ambulacral sys-
tem, 390, 391 : Enteric canal, 391,
392 : Coelome, 392 : Blood-vascular
system, 392 : Reproductive organs,
392 : Development, 392 : Syste-
matic position, 406

Echiurida, 429, 461, 479*, 480, 481, 482,
483, 484, 511, 512

Echiurus, 479, 480, 481, 482, 483

Ectobranchiata 406*

Ectocyst, 334*

Ectoderm, 24* — See Body-wall

Ectoprocta, 340*, 341 : Structure of
body-wall, 344 : Alimentary canal,
344 : Nervous system, 344 : Ne-
phridia, 344 : Avicularia, 345 :
Vibracula, 345 : Reproduction and
development, 345 : Ethology and
distribution, 346

Ectosarc, 46*

Edriasteroidea, 405*, 426

Edwardsia, 198, 200, 224, 227

Eggs — See Development

Eimeria, 86

Ejaculatory duct, 239

Elasipoda, 404*, 419, 425

Eledone moschata, 774

Elephant's tusk shells, 736

Eleutherozoa, 401*

Elk-horn coral, 156, 157

Elytra, of Polychseta, 460 : Cock-
roach, 606 : Coleoptera, 620, 625

Embiidse, 617*

Embryology, 3* — See Development

Empis, 629

Emulsions, 35*

Encystation, 45*, 61

Endites, 518

Endocyst, 334*

Endoderm, 24*

Endoderm-disc, of Aslacus, 545

Endoderm lamella, 137*

Endogastric, 755*

Endophragmal system, of Astacus, 529

Endoplasm, 11*, 46*

796

INDEX

Endopodite, 526*

Endoprocta, 332, 341*, 347 : Vestibule,
347 : Nephridia, 347 : Cloaca, 347 :
Ganglion, 347 : Testes and ovaries,
347 : Foot -gland, 348 : Develop-
ment, 348

Endosarc, 46*

Endoskeleton, 32*

Endosternite, 640*, 651

End-sac, 522

Entamcetia histolytica, 51

Enteric canal — See Digestive system

Enteroco3le, 382

Enterozoa, 106

Entobranchiata, 406*

Entovalva, 693

Environment, 9*

Eocene, 7

Eolis. 715, 723, 724

Epeira diadema, 648

Ephelota, 102, 103

Ephemera, 618

Ephemeridce, 626, 637

Ephippium, 582

Ephyropsis, 178, 179

Ephyrula, of Aurelia, 174*

Epiblast, 24*

Epibolic gastrulation, 270

Epiboly, 216*

Epicranium, 603*, 622

Epidermis, 24*, 32 — See Body-wall

Epimerite of Gregarinida, 84, 85*

Epineural canals, 391*

Epipharynx, 627

Epiphragm, 718

Epiphysis, of Sea-urchin, 389

Epipodite, 531*

Epipodium, 720*

Epistoma, of Astacus, 529

Epistome, 333, 344, 349

Epislylis plicatilis, 94, 96

Epistylis umbellaria, 94, 95

Epitheca, 202*

Epithelia, 25, 26* : "N on -stratified, 25,
26* : Stratified, 25, 26*

Epithelium, enteric, 34*

Epithetosoma, 480, 482

Equatorial plate, 19*

Equivalve, 682*

Ergasilus, 551, 561, 562

Estheria, 550, 557

Ethiopian, 9*

Ethology, 9* : of Platyhelminthes, 279 :
Rotifera, 328 : Ectoprocta, 346 :
Echinodermata, 424 : Cha&topoda,
476 : Hirudinea, 509 : Crustacea,
585 : Insecta, 636 : Arachnida, 655 :
Pelecypoda, 692 : Amphineura, 701 :
Gastropoda, 735

Eucarida, 554*, 572

Euchlanis, 322

Euchlora, 219

Eucirripedia, 651*, 663, 565

Eucope, development, 153

Eucopella, 135

Eucopepoda, 551*, 559, 560, 561

Eudendrium, 140

EUGLENA VIRIDIS, 69, 70, 92 : Syste-

matic position, 71
Euglenidce, 71
Eugleno.dea, 71
Eutflypha, 52
Eulamellibranchiata, 678*, 679, 680,

693, 694

Eumalacostraca, 552*, 567
Euphan&a, 554, 580, 582, 584
Euphausiacea, 554*, 572, 579, 580, 584,

588

Euphrosyne, 458
Euphyllopoda, 581
Euplectella, 122
Eupolia, 287
Eupomatus, 474
Euryalida, 408, 411, 412
Eurypauropus, 598
Eurypterida, 646*, 650, 651, 652, 656,

662 - u.

Eurypterus ftacheri, 652
EUSCORPTO — See BUTHTJS
Euscorpius carpal! icus, 644
Euscorpius itaiicns, 655, 656
Euspongia, 116i, 121
Euthyneura, 714*, 717, 722, 726, 728
Evolution, 6
Examples, of Rhizopoda, 46 : Myce-

tozoa, 68 : Mastigophora, 69 : Sporo-

zoa, 82 : Infusoria, 90 : Porifera,
Hydrozoa, 129 : Scyphozoa,
Actinozoa, 183 : Ctenophora,
Platyhelminthes, 233, 236,
Nematoda, 292 : Rotifera,
Polyzoa, 334 : Branchiopoda,
Asteroidea, 368 : Echinoidea,
Holothuroidea, 393 : Crino-
396 : Chsetopoda, 430, 443 :

106
167
208
243
317
353
386
idea

Sipunculoidea, 484 : Crustacea, 514,
527 Insecta, 603 : Arachnida, 638 :
Pelecypoda, 663 : Gastropoda, 702 :
Cephalopoda, 738, 754

Excretion, 13*, 37

Excretory aperture, 742*

Excretory pores, 212, 293

Excretory system, of Leptolinse, 151 :
Planaria, 235 : Liver-Flu ke, 238 :
Tcenia, 244 : Platyhelminthes, 263 :
Nemertinea, 287 : Ascaris, 296 :
Nematoidea, 302 : Echinorhynchus,
308 : Brachionus rubens, 320 : Roti-
fera, 327 : Bugula, 336 : Ectoprocta,
344 : Endoprocta, 347 : Phoronis,
349 : Magellania, 358 : Brachiopoda,
362 : Nereis, 437, 438 : Lumbricus,
449, 450 : Chsetopoda, 467, 468, 469 :
Sipunculus, 487 : Sipunculoidea,
489 : Hirudo, 498, 499, 500 : Hiru-
dinea, 506, 507 : Apus, 522 : Astacus,

INDEX

797

538, 539 : Crustacea, 581 : Peri-
patus, 594 : Periplaneta, 609 : In-
secta, 627 : Scorpion, 641 : Arach-
nida, 651 : Anodonta, 671 : Pele-
cypocfca, 688 : Amphiiieura. 699.
700: Triton, 711 : Gastropoda, 728 :
Scaphopoda, 737 : Sepia, 751, 752 :
Nautilus, 763, 764 : Cephalopoda,
774

Excurrent canals, of Sponges, 109, 110*

Exhalant siphon, 664

Exites, 518

Exogastric, 755*

Exoplasm, 11*, 46*

Exopodite, 526*

Exoskeleton, 32*— See Body-wall

External features, of S>/con, 106, 107 :
of Porifera, 1 1 5, 116, Il7, 118: Obelia,
131 : Aurelia, 168, 169 : Sea-
anemone, 184, 185 : Aetinozoa, 194 :
Hormiphora, 208, 209, 210 : Planaria
and Dendroccelum, 233 : Liver-Fluke,
230, 237 : Tcenia solium, 242, 243 :
Platyhelminthes, 250 : Nemertinea,
284 : Ascaris lumbricoides, 292 :
Nematoda, 299 : Echinorhynchus,
307 : Chsetognatha, 310 : Brachionus
rubens, 317, 318: Rotifera, 323:
Buqula aricularia, 334 : Ectoprocta,
341, 342, 343 : Endoprocta, 347 :
Phoronis, 348, 349 : Magellania, 353,
354 : Brachiopoda, 359 : Asterias
rubens, 368, 369, 370 : Anthenea
tiwescens, 379, 380 : Sea-urchin, 386,
387,' 388 : Sea-cucumber, 393 : Ante-
don rosacea, 396, 397, 398 : Echino-
dermata, 406 : Ophiuroidea, 406,
407 : Holothuroidea, 407 : Aste-
roidea, 409, 410 : Echinoidea, 412 :
Crinoidea, 415 : Nereis, 430, 431 :
Lumbricus, 443, 444 : Chsetopoda,
457, 458 : S'ipunctdus nudus, 484,
485 : Sipunculoidea, 488 : Priapu-
loidea, 490 : Archi-aimelida, 491 :
Hirudo, 494, 495, 496 : Hirudinea,
505 : Apus, 515 : Astacus, 527, 528 :
Crustacea, 556 : Peripatus, 591. 592 :
Myriapoda, 600 : Periplaneta, 603,
604 : Insecta, 621 : Scorpion, 638,
639 : Arachnida, 646 : Anodonta,
663, 664 : Pelecypoda, 679 : Am-
phineura, 695, 696 : Triton, 704 :
Gastropoda, 715 : Scaphopoda, 736 :
Sepia, 738, 739 : Nautilus, 755 :
Cephalopoda, 768

Ex-umbrella, 135*

Eves, 39* : of Euglena, 70 ; Medusce,
'155: P lanaria, 233, 234: Platy-
helminthes, 263 : Nemertinea, 289 :
Nematoda, 302 : Chsetognatha, 311 :
Brachionus, 320 : Rotifera, 327 :
Dinophilus, 330 : Brachiopoda, 362 :
Asterias, 370 : Nereis, 437 : Chseto-

VOL. I

poda, 466, 467 : Hirudo, 502 : Apus,
523, 524 ; Astacus, 542 : Crustacea,
581,582: Periplaneta. 611 : Tnsecta,
629, 630 : Arachnida, 653, 655, 656 :
Pelecypoda, 689. 690 : Chiton, 699 :
Triton, 712 : Gastropoda, 726, 727 :
Sepia, 749, 750 : Nautilus, 765

JL1 ABRICIA, 46(>

Facets, 542*

Facial suture, 590*

Fasces, 34*

Falciform young — See Sporozoites

Family, 4*

FASCIOLA HEPATICA, 236 : General fea-
tures, 237, 239 : Body -wall, 237, 238 :
Digestive system, 238 : Water-
vessels, 238 : Nervous system, 238 :
Reproductive organs, 239, 240 :
Development, 240, 241 : Syste-
matic position, 250

Fascioles, 408*, 413*

Fasciolince, 250

Fat, 27*, 28

Fat body, of Periplaneta, 608

Fauna, 8*

Feather-star — See Antedon rosacea

Feeding, method of, Amceha, 12, 46 :
Actinophrys, 59 : Chlamydomyxa, 67 :
E^glena, 70, 71 : Flagellata, 73, 75 :
Choanoflagellata, 80 : Monocystis,
83 : Tent aculif era, 101

Femur, of Cockroach, 606

Fenestrffi, of Periplaneta, 604*

Ferment, 12*

Fertilisation — See Impregnation

Fever, parasite of, Quartan, 88, 89 :
Tertian, 88, 89

Fibres, nerve, 30*, 31

Fibro- cartilage, 28*

Filaria bancrofti, 30^, 306

Filaria medinensis, 299, 306

Filariasis, 306

Filibranchia, 677*, 678, 679, 693. 694

Filosa, 48*, 52

Fimbriae, of Fresh-water Mussels, 664

Fire-flies, 630

Fission, 14, 41*, 46, 49, 50, 51, 52, 61,
66, 71, 75, 76, 80, 81, 83, 92, 103

Fissurella, 717, 726

Five-chambered organ, 400*

Fixed cheek of Trilobites, 589, 590

Flabellum, 202, 203, 205

Flagella of Copepods, 551*

Flagellata, 71* : Cell body, 72, 73 :
Flagella, 70, 73 : Modes of nutrition,
75 f Skeleton, 75 : Colonies, 75, 76 :
Asexual multiplication, 76 : Sexual
reproduction, 78

3 E

798

INDEX

Flagellate canals, of Sponges, 108. 109*,
110, 119

Flagellate cells, 110*, 119

Flagellulge, 51*, 56, 60, 61, 63, 67, 69,
71, 80, 81, 82

Flagellum, 25*, 69, 70, 71 : of Astacus,
531

Flame-cells, 235, 263, 264

Flat-worms — See Platyhelminthes

Fleas, 620, 625, 636

Flies, 514, 602

Float, 160, 161*

Floscularia, 321, 323, 324, 325

Flustra, 340

Folliculina, 96

Food vacuole, of Actinophrys, 59 :
Paramvcium, 92*

Food-yolk, 31

Foot, of Anodonta, 664 : Pelecypoda,
683 : Amphineura, 695 : Triton, 705,
706 : Gastropods, 719 : Sepia, 739,
740 : Nautilus, 756 : Cephalopoda,
768

FoQt -gland, 348

Foramiaifera, 48* : General structure,
51, 52, 53, 54 : Skeleton, 53, 54, 55,
56 : Protoplasm, 55, 56 : Dimor-
phism, 56*, 57 : Reproduction, 56 :
Distribution, 58

Forcipulata, 402*

Forcipulate pedicellaria?, 370*

Formica rufa, 636

Fossettes, 577*

Fossils, 7*

Fossula, 208

Fredericella, 341

Fresh-water Crayfish, 527

Fresh -water Mussel — See Anodonta

Fresh-water Sponges, 126, 127

Fresh -water Worms, 429

Fritillaria, 231

Frondicularia, 54

Frontal suture, 590*

Fulcrum (of Rotifera), 319

Furigia, 203

Funiculus, 336*, 344

Funnel, 770

Funnel folds, 770

G.

a

ALATHEA, 555
Galea, 605
GaUodes, 645, 647
Galeolaria coespitosa, 457
Gall-flies, 621, 632
Gall -insects, 632
Galls, 632*
Gametes, 50*, 78 : of Flagellata, 76,

79* : Sporozoa, 82, 83
Gametocytes, 82, 85*, 88, 89
Gammarus, 554, 569

Gamobium, 141*, 175

Gamogenesis, 22*

Ganglia, 38, 238

Gastral cortex, 109, HI*, 120

Gastric filaments, of Aurelia, 171*

Gastric mill, 578 : of Astacus, 535

Gastric ostium, 111*

Gastric ridges, of Aurelia, 172* :
pouches, 169*

Gastrodes, 221, 223

Gastrolith, 536*

Gastrophilus equi, 619, 620

Gastropoda, 663, 702* : Example, 702 :
Distinctive characters and classifica-
tion, 713 : Systematic position of ex-
ample, 715: General organisation,
715 : External features, symmetry,
&c., 715: Shell, 717: Foot, 719:
Head, 720 : Mantle, 722 : Respira-
tory organs, 722 : Osphradium, 724 :
Digestive organs, 724 : Heart, 725 :
Nervous system, 725 : Organs of
special sense, 726: Excretory organs,
728 : Reproduction, 728 : Develop-
ment, 729, 730, 731, 732, 733, 734,
735 : Ethology and distribution, 735 :
Relationships', 736 : Appendix, 736

Gastropores, of Millepora, 157*

Gastrotricha, 317. 328*, 329

Gastrozooicls, 164, 199

Gastrula, 24*

Gastrulation, 24*

Gelasimus, 576, 586

Gemmules, 112, 122*

Gense, 604*

Generic, 1*

Genital cloaca, 245* : rachis, 378, 421 :
stolon, 378, 421 : plates, 389 : bursse,
420 : operculum, 640*

Genital system — See Reproductive sys-
tem

Genus, 1*

Geological distribution — See Distribu-
tion, geological

Geoplanidcr, 249

Gephyrcea, 490

Germarium, 236*, 265, 267

Germinal bands, of Clepsine, 509 : of
Peripatus, 597

Germinal disc, 775

Germinal lay^r=i, 24*

Germinal spot, 20*, 21, 31

Germinal vesicle, 20*. 21

Germ-vitellarium, 265

Giant Clam, 692

Giant fibres, 466

Giant nerve -cells, 466

Giqantorhynchus, 307, 308, 309, 310

Gill-cover, of Astacus, 528

Gills,' 36*— See Respiration

Gizzard, 33

Glabella. 590*

Glands, 26*

INDEX

799

Glands, Multicellular, 26* : Unicellular,

26* : Ducts, 26, 34 : Salivary, 34*
Glandular portion, 671*
Glass-crab, 585
Glass -rope sponge, 122, 127
Olenodinium , 81
Globigerina, 54, 58
Glochidium, 675
Glomeris, 277
Glossiphonia, 504, 510
Glossocodon, 154
Glow-worms, 630
Glycera, 462, 463
Glyceridce, 464
Gnathobase, 518
Gnathobdellida, 504*, 505, 509
Gnats, 619

Goblet -shaped bodies, 449
Goblet-shaped organs, of Leech, 502
Gonads — See Reproductive system
Gonapophyses, 612*
Gonoccele, 761*
Gonodactylus, 555
Gonodendra, 164*
Gonoducts, 489*
Gonopodaria, 347
Gonopore, 293

Gonotheca, of Obelia, 130, 131
Gonozooids, 199*

Gordius, 299, 300, 301, 302, 303, 511
Gorgonacea, 193*, 201, 202
Gorgonia, 195, 201
Granule glands, 264, 266*
Granules, 380
Grapsus, 555
Graptolithida, 143*, 166
Grasshoppers, 602, 616, 621, 630
Green gland, of Astacus, 538
Gregarina, 84, 85, 86
Gregarina blattarum, 84
Gregarina duyardini, 84
Gregarina gigantea, 84, 86
Gregarinida, 83* : Characteristic fea-
tures, 84, 85
Gressoria, 617, 621
Gromia, 53, 54, 56, 58
Guard, 772*
Guard-polypes, 149*
Guinea-worm, 299, 306
Gula, 622*

Gullet, 33 — See Digestive organs
Gunda segmentata, 251, 252, 287
Gustatory organ, 39*
Gyge, 572

Gymnoblastea, 144*
Gymnolaemata, 340*, 341, 343, 345
Gy ractis, 198, 199
Gyrocotyle, 256, 258, 283
Gyrodactylidce, 253
Gyrodactylus, 254, 273

VOL. I

XlABiTS — See Ethology

Hcemadipsa, 504, 510

Haemal system, 373

Haematochrome, 70, 75

Hcematococcus, 74, 75

Haemerythrin, 486*

Haemocoele, of Crustacea, 514, 578 :
Peripatus, 608 : Insecta, 625

Haemocyanin, 541

Haemoflagellata, 74

Haemoglobin, 30, 36

Hcemopsis vorax, 510

Haemosporidea, 84* : Characteristic fea-
tures, 88, 89

Halicystu*, 176

Haliotidce, 714

Hatiotis, 717, 724, 725, 726

Halistemma, 159, 160, 161, 162, 163, 167

Halistemma tergestinum, 160> 162

Halteres, 625

Hamingia, 479, 482

Hartea, 193, 195, 200

Harvest -men, 645, 649

Hastigerina, 55, 56

Haversian canals, 28*, 29

Head, 43*

Head-germs, 509*

Head -kidneys, 441

Head-lobes, of Astacus, 554 : of Fasci-
ola, 237*

Heart, 36*

Heart — See Vascular system

Heart-urchins, 403*, 408, 413, 414

Hectocotylisation, 739, 769*

Heliopora, 193, 201, 205, 206, 208

Heliozoa, 48* : General structure, 58,
59, 60, 61 : Colonies, 59 : Skeleton,
59, 60 : Reproduction, 61 : Conjuga-
tion, 61

Helix, 728, 729

Helix nemoralis, 722

Hemipnetistes radiatus, 414

Hemiptera, 618*, 619, 624, 637

Hemisomes, 43*

Hepatic caeca, 399

Hepato-pancreas, 35, 537

Hermaphrodite, 40*

Hermit-crabs, 555, 574 : and Hy-

dractinia, 146, 147
Herpobdella, 504, 507, 508, 509, 510
Herpobdellida, 504*
Hesionida, 464
Heterochxrus, 267
Heteroccela, 113*
Heterocotylea — See Monogenetica, 249,

267

Heterocyemidce, 227, 229
Heterogamy, 41*, 305
Heterogeny, 272, 282
Heteromita, 73
Heteromyaria, 680*

3 E*

800

INDEX

Heteronemertini, 288, 291*

Heteronereis, 430, 439

Heteropoda, 714*, 719, 720, 735, 736

Heterotrichous, 94*, 96

Hexacanth embryo, of Taania, 246 :
Cestodes, 275

Hexactinellida, 113*, 122, 124, 127

Hexactinice, 194, 198

Hexarthra, 322, 323, 325, 328

Hinge -ligament, 665

Hinge-line, 354, 665

Hinge -teeth, 354, 665

Hippa, 555, 575, 589

Hippurites, 682, 683

Hirudiaea, 429, 494 : Example, 494 :
Distinctive characters and classifica-
tion, 503 : General organisation, 505 :
Form and size, 505 : Setae, 505 :
Proboscis, 505 : Enteiic canal, 505 :
Blood-vessels, 505 : Respiratory or-
gans, 506 : Nephridia, 506, 507 :
Nervous system, 507 : Sense organs,
507 : Reproductive organs, 507 :
Development, 508, 509 : Habits,
dis ribution, &c., 509, 510, 512

Himdinidce, 504*

Hirudo, 503, 504, 507, 510

Hirudo australis, 494, 498, 499

HIRUDO MEDICINALIS, 494 : External
characters, 494, 495 : Body-wall,
496 : Muscular system, 496, 497 :
Alimentary organs, 497, 498 : Ex-
cretory system, 498, 499, 500 : Blood-
system, 500, 501 : Nervous system,
501 : Sense organs, 501, 502 : Re-
productive organs, 502, 503 : De-
velopment, 503 : Systematic posi-
tion, 504

HIRUDO QUINQUESTRIATA — See H.

MEDICINALIS

Hirudo sanguisuga, 510

Histology, 3*

Histriobdellea, 330, 331, 332, 512

Holarctic region, 9*

Holoblastic segmentation, 23*

Holophytic nutrition, 67*, 70, 75, 80

Holothuria — See Sea-cucumber

Holothurian — See Colochirus

Holothuroidea : Example, 393 : Dis-
tinctive characters and classifica-
tion, 404 : General form, 407 : Modi-
fications of form, 414 : Coelome, 416 :
Ambulacral system of vessels, 416 :
Blood-vascular system, 417 : Haemal
system, 418 : Axial organ, 419 :
Enteric canal, 419 : Respiratory
trees, 419 : Cuvierian organs, 419 :
Nervous system, 420 : Reproductive
organs, 420 : Development, 421 :
Ethology, 424

Holotrichous, 91, 94*, 97

Holozoic nutrition, 67*, 70, 80

Homalogaster, 253

Homarus, 555, 585

Homoccela, 113*

Hood, 756*

Hook-headed worms, 293

Hoplocarida, 555*

HORMIPHORA PLUMOSA, 208 : External
characters, 208, 209 : Enteric sys-
tem, 210, 211 : Cell-layers, 212, 213 :
Nervous system, 213 : Sense organs,
213 : Reproductive organs, 214 :
Development, 214, 215, 216, 217 :
Systematic position, 218

Host, of parasite, 86 : intermediate, 89

House-flies, 602, 619

Hyaline cartilage, 28*

Hyalonema, 122, 127

Hyalosphenia, 49

Hybrids, 2*

Hydatids, 278*

Hydatina, 322, 324

Hydra, 134, 147, 148, 153, 166, 167, 258

Hydractinia, 145, 146, 148, 149, 166

Hydranths, of Obelia, 130*, 149

Hydrocoele, 383

Hydrocorallina, 143*, 156, 157, 158,
159, 166

Hydroctena, 225

Hydroids, naked - budded, 144* :
covered-budded, 144*

Hydropkilus, 634, 635

Hydrophyllia, of Halistemma, 160, 162*

Hydrorhiza, 130*

Hydrospires, 405*

Hydrotheca, of Obelia, 130*, 131, 166

Hydrozoa, 129* : Example, 129: General
structure and classification, 141 :
Alternation of generations, 141:
Systematic position of example, 143 :
General remarks on, 166

Hydrula, 140, 141*, 153, 227

Hymenoptera, 621*, 624, 626, 627, 631,
634, 637

Hyperia, 554

Hypoblast, 24*

Hypodermic impregnation, 327, 508

Hypopharynx, 627

Hypostome, of Obelia, 130*

Hypotrichous, 94*, 96

_LCTINEUMONS, 62], 636

Idmonea, 340
Jdotea, 554
Illoricata, 322*
Imaginal discs, 635
Imago, 634
Impregnation, 21*
Impregnation, hvpodermic, 327, 508
Inarticulata, 359*, 360, 361, 362
Incurrent canals, of Sponges, 108, 109*,
110, 118

INDEX

801

Incus (of Rotifera), 319*
Individual of the first order, 167*
Individual of the second order, 167*
Individual of the third order, 167*
Individual differences, in Nereis du~

merilii, 439

Individual variations, 2*, 114, 439
Individuation, in Hydrozoa, 167
Inequilateral valves, 682*
Infundibula, of Attrelia, 173*
Infundibular canal, 212*
Infusoria, 46* : Example, 90 : Classi-
fication, 93 : Systematic position of
example, 93
Infusoriform embryos, of Dicyemidse,

228, 229*

Inhalant pores, 107, 118
Inhalant siphon, 664
Ink-sac, of Sepia, 744
Inseeta, 514, 602*, 660, 661, 662 :
Example, 603 : Distinctive charac-
ters and classification, 615 : Syste-
matic position of the example, 621 :
General organisation, 621 : Exo-
skeleton, 621, 622 : Head, 622 :
Thorax, 622 : Abdomen, 623 : Ap-
pendages of head, 622, 623, 624 :
Appendages of thorax, 624 : Abdo-
men, 625 : Hsemoco?le, 625 : Fat
body, 625 : Digestive system, 626,
627 : Malpighian tubes, 627 : Tra-
chea! system, 627 : Blood -vascular
system, 627 : Nervous system, 628,

629 : Organs of special sense, 629,

630 : Luminous organs, 630 : Pro-
duction of sounds, 630 : Reproduc-
tive organs, 631 : Eggs, 632 :
Development, 632, 633, 634, 635 :
Metamorphosis, 634 : Mode of life,
636 : Distribution in time, 637

Integripalliata, 678*
Integripalliate, 681*
Integument — See Body-wall
Inter-filamentar junctions, 670
Inter-lamellar junctions, 669
Inter-mesenteric chambers, 186*
Internodes, of Siphonophora, 161
Inter-radius, 139*
Interstitial cells, of Obelia, 133
Intertentacular tube, 344
Intestinal caeca, 375, 380*
Intestine, 489*
Introvert, of Polyzoa, 334 : Sipuncttlus,

484 : Sipunculoidea, 488 : Triton,

705 : Gastropoda, 724
Invagination, 24*
Iridocytes, 741*
Iris, 40, 750*
Irritabilitv, 11*
Isomyaria^ 680*
Isopoda, 554*, 569, 570, 571, 572, 578,

581, 582, 587, 660
Isoptera, 617*

€/ APYX, 616

Jaws, 33

Jelly-fish, Common (Aurelia aurita), 167

Jelly-fishes, 129

Johnston's organs, 630*

Jurassic, 7

K

K

.ARYOGAMY, 61, 78*

Karyokinesis, 18
Keber's organ, 672, 688
Kidneys — See Excretory system
Kinetonucleus, 72*, 74
King-Crabs, 637, 646, 650, 651
Kingdom, 5*
Koonunqa, 553
Kraussina, 360
Krolmia, 310

_LjABiAL PALPS, 605 : of Pelecypoda,

668

Labium, 605
Labrum, 604 : of Apus, 516 : of Astacus,

529 : of Periplaneta, 604 : of Scorpion,

639

Labyrinthula, 67
Lacinia, 605
Lacrymaria, 96
Lacteals, 35
Lacunae (bone), 28*
Lacunar system, 373
Laijena, 54

Lamellae (of bone), 28, 29
Lampetia, 214, 219
Lamp -shells, 353
Lancet plate, 405
Land -snails, 715
Lantern ccelome, 392
Laomedea, 140

Lappets, of Sea-anemone, 185
Larva, of Desor, 290
Larval membranes, of Periplaneta, 613*
Larval organ, 383
Lateral plane, 210*
Latreillia, 576
Laurentian, '<
Lanrer's canal, 240
Laverania, 88
Leaf -insects, 617, 621
Leda, 677

Leech — See Hirudo
Leeches — See Qirudinea
Lemnisci, 308*
Lemnobdella — See Hirudo
Lens, 40 — See Eye
Lepas, 551, 563, 564

3 E* 2

802

INDEX

Lepas anatifera, 563, 564

Lepas fascicularis, 583

Lepidoptera, 620*, 624, 625, 632, 634,

635, 637

Lepidurus — See Apus
Lepidurus kirkii, 516, 520
Lepisma, 616
Leptochelia, 553
Leptochone, 466
Leptodiscus, 81
Leptodora, 550, 557, 583
Leptolinse, 142* : General structure,
143, 144, 145, 146 : Porisarc, 143 :
Medusae, 144, 145 : Ccenosarc, 146 :
Polypes, 149 : Reproductive zooids,
149 : Development, 152, 153
Leptomedusse, 142*, 144, 146, 149
Leptostraca, 552*, 566, 581
Lerncea, 551, 561, 563
Lesteira, 561, 563
Leucilla convexa, 118
Leucocytes, 30
Leucodore, 207
Lice, 619, 625, 636

Ligula, 256, 257, 283

Ligula, 605*

Lima, 678, 685

Limax, 719, 723

Limnceus, 726

Limnetis, 550, 557

Limnobdella — See Hirudo

Limnocnida, 166

Limnocodium, 166

Limpets, 702, 713, 723 — See Patella

Limulus, 650, 651, 654, 655, 656, 660

Linens, 229

Lingua, 627

Lingual ribbon, 708

Linguatulida, 657, 658, 659. 661

Lingula, 359, 360, 361, 362, 364

Linin, 17*

Linnaeus, 1, 3

Liquid tissues, 30

Lithite, 136*

Lithites, of Aurelia, 172

Lithobius forficatus, 599

Lithocircus annularis, 62

Lithocysts, 136*

Littoral forms, 8*

Liver, 35*

Liver-Fluke — See Fasciola hepatica

Liver-pancrea?, 35

Lobata, 218*, 220, 223

Lobosa, 48*, 49 : General structure, 48,
49 : Skeleton, 49

Lobsters, 555, 572, 585

Locomotion, of Amoeba, 12, 46 : Helio-
zoa, 59 : Euglena, 69 : Flagellata,
72 : Choanoflagellata,*80 : Sporozoa,
82 : Paramoecium, 92 : Tentaculi-
fera, 101

Loco motor rods, 313
Locusta, 617

Locusts, 514, 616, 617, 621, 630
Lohmanella, 231

Loligo, 772, 774, 776, 777, 778, 779
Loligo vulgaris, 769
Lophog aster, 553
Lophomonas, 95, 96, 105
Lophophore, 333, 334, 355, 361
Lophopus, 344

Lorica, of Ciliata, 97 : Flagellata, 73,
75 : Choanoflagellata, 79, 80 : Ten-
taculifera, 102 : Rotifera, 317
Loricata, 322*, 323
Loxosoma, 341, 347, 348
Lucernaria, 175, 176, 177
Lucernarida — See Stauro medusae, 175
Lucifer, 555, 584
Lucina, 687
Lumbricidce, 456*

LUMBRICUS. 443 : General external fea-
tures, 443, 444: Body-wall, 445,
446 : SeTse, 445 : Setigerous sacs,
445 : Enteric canal, 446, 447 : Vas-
cular system, 448 : Nervous system,
449 : Organs of excretion, 449, 450 :
Reproductive organs, 450, 451 :
Development, 452, 453 : Systematic
position, 456
Lumbricus rubellus, 471
Lumbricus trapezoides, 471
Luminous organs, 630
Lung, 36* : of Scorpion, 642 : of Pul-

monata, 724
Lymph, 30*
Lysophiuree, 403*

M

ACHILIS, 616

Macrobdella valdiviana, 505

Macrobiotus hufelandi, 658

Macrura, 555*, 586, 587, 588

Macula acustica, 750

Madrepora, 204, 205

Madreporaria, 193*, 195,196, 198,202,
204, 205, 207, 224

Madrepores, 204, 205, 207

Madreporic canal, 376, 391, 394

Madreporite, of Starfish, 369, 410 : of
Sea-urchin, 389

MAGELLANIA, Shell, 353, 354 : Body,
355, 356 : Mantle-lobes, 355 :
Mantle -cavity, 355 : Lophophore,
355, 356, 357 : Food-groove, 355 :
Digestive organs, 355, 356 : Body-
wall, 357 : Muscular system, 357 :
Coelome, 358 : Blood system, 358 :
Excretory organ?, 358 : Nervous
system, 358 : Reproductive organs,
3*58 : Position of example, 359

Magellania flavescens, 353, 354

Maggot, 635*

Mala, 555, 581, 584

INDEX

803

Malacocotylea, 249— See Digenetica
Malacostraca, 552*, 556, 566, 578, 579,

581, 582, 586
Malaria parasites, 88, 89
Malleus (of Rotifera), 319
Mallqphaga, 618*

Malpighian tubes, 601, 609 : of Scor-
pion, 641 : of Arachnida, 652 : of
Tardigrada, 659

Mandibles — See Appendages

Manducation, 425*

Mantidce., 621

Mantle, of Anodonta, 663, 666 : Pele
cypoda, 680 : Amphineura, 694
Triton, 706 : Gastropoda, 722
Scaphopoda, 737 : Sepia, 741
Nautilus, 759

Mantle -groove, 363

Mantle lobes, 355

Manubrium, of Medusae, 130, 135 : of
Obelia, 131 : of Brachionus, 319

Marginal lappets, of Aurelia, 168, 169*,
178

Margiial sense organs, 136*

Marginal tentacles, of Aurelia, 168,
169*

Marine Annelids, 429

Mastax of Rotifera, 319

Mastiyamceba, 72, 73

Mastigophora, 45* : Example, 69 :
Classification, 71 : General organisa-
tion, 71 : Systematic position of the
example, 71

Matrix of connective tissue, 27*

Maturation, 20*, 22

Maxilla — See Appendages

Maxillary palp — See Appendages

Maxillipeds — See Appendages

Maxillulae, 623*

May-flies, 618

Measly pork, 306

Mecoptera, 621*

Medulla, of Actinosphcerium, 58, 59 :
Monocyrtis, 82 : Paramcecium, 90, 91

Medullary sheath, of Nerve'-fibre, 30*.
31

Medusa-buds, of Obelia, 130

Medusae, of Obelia, 135, 136

Megadrili, 455*, 456

Megagametes, of Flagdlata, 79* : Spo-
rozoa, 87, 88, 89 : Ciliata, 100, 101

Megagametocyte, 87, 88

Megalaesthetes, 699

Megalopa stage, 585

Megameres, 700, 730 : of Ctenophora,
215, 216 : of Polyclad, 268, 269 : of
Chiton, 700 : of Gastropoda, 730

Meganucleus, of Dinoflagellata, 81 :
Paramoscium, 90, 91, 95 : of Ciliata,
95

Megaspheric forms, 56, 57

Megaspores, of Radiolaria, 65

Megazooid, of Vorticella, 98, 99

Meleagrina, 678, 692
Meleagrina margaritifera, 692
Meiicerta, 321, 323, 324, 325
Melolontha, 628
Membranes, 26

Membranipora, 340, 346

Mentum, 605

Meridional canal, 212

Meroblas;ic segmentation, 23*

Merogony, 23*

Merozoa, 249* — See Polyzoa

Merozoite, 86*, 87, 88

Mesenchyme, 382

Mesenteric filaments, of Sea-anemone,
186* : of Flabellum, 203

Mesenteries, of Sea-anemone, 180*,
182, 185, 186, 190, 191 : Arrange-
ment in Actinozoa, 196

Mesenteron, 186, 640*

Mesoblast, 24*

Mesoderm, 24*, 216

Mesoderm bands, of Ascaris, 304 : of
Periplaneta, 614

Mesoglrea, 111*, 112 : of Obelia, 132*

Mesonemertini, 291*

Mesopodium, 706, 719

Mesosoma, 638*

Mesostomum, 267

Mesothorax, 606

Mesotrochal, 474*

Mesozoa, 227

Messmateism — See Commensalism

Metabolism, 13*

Metacrinus interruptus, 415

Metagenesis, 41* : of Foraminifera, 57,
58 : Obelia, 141 : Leptolinse, 152 :
Trachylinae, 156 : Aurelia, 172 :
Liver-Fluke, 240 : Tcenia, 246 :
Platyhelminthes, 270 : Nematoda,
304

Metameres, 43*, 510

Metamerism, 510, 511

Metamorphosis, of Trachylinae, 156 :
Sea-anemone, 191 : Platyhelminthes,
268 : Nemertinea, 290 : Phoronis,
351, 352 : Asterina, 381 : Echino-
dermata, 421 : Antedon, 423 : Chse-
topoda, 475 : Apus, 525 : Crustacea,
582 : Insecta, 634

Metamorphosis, retrogressive — See De-
generation

Metanauplius, 526

Metanemertini, 291*

Metapodium, 706, 719

Metathorax, 606

Metazoa, 20*, 106

Metentera, 186*

Micraesthetes, 699

Microdrili, 455*, 463

Microgametes, of Flagellata, 79* : Spo-
rozoa, 86, 87, 88 : Ciliata, 100, 101

Microgametocyte, 88

Microgromia, 51, 52

804

INDEX

Microhydra, 149, 166

Microlethical egg, 217

Micromeres, of Ctenophora, 215, 216 :
of Polyclad, 269 : of Chiton, 700 : of
Gastropoda, 730

Micro nucleus, of Dinoflagellata, 81 : of
Ciliata, 95 : of Paramcecium, 91, 92

Micropyle, 22 : of Cephalopoda, 775

Microspheric forms, 56, 57

Microspores, of Radiolaria, 65

Microstomum, 278

Microzooid of Vorticella, 98, 99

Miescher's corpuscles, 90*

Migration of parasites, 280*

Miliola, 53

Millepora, 156, 158

Millepora alcicornis, 157, 158

Millipedes, 514, 598

Minyas, 200, 224

Miocene, 7

Miracidium, 240*

Mites, 514, 637, 646, 649, 652, 656

Mitosis, 18*, 19

Mitotic division — See Mitosis

Modiola, 680, 685

Mollusca, 663* : General remarks, 780

Mollusc oida 333* : mutual relation-
ships of the classes, 365

Monaxonida, 113*

Mongrels, 2*

Moniezia, 284

Monocyclica, 405*

MONOCYSTIS AGILIS, 82, 83 : Syste-
matic position, 84

Monoecious, 40*

Mono'enetica, 249*, 253, 254, 260,
262, 263, 264, 268, 273, 279, 282,
283

Monomyaria, 680*

Monnsiga, 79

Monotus, 251

Monozoa, 249*

Morphology, 3*

Morula, 24*

Moruloidea, 227*

Mosquitoes, 619, 636

Mother-cyst, 278

Mother-of-pearl, 666

Moths, 620

Mouth papillae, 369*

Movable cheek, 589, 590*

Mulberry body, 24*

Mailer's larva, 270

Multicellular, 20*

Multicellular gland, 26*

Multicilia, 95, 96

Multiple fission, of Euglena, 71

Murex, 725, 727

Muscle, striated, 29*, 30 : non-striated,
29*, 30

Muscle processes, Hydra, 148

Muscles, 37*, 38

Muscular fibres of Sponges, 112

Muscular system, of Aurelia, 171 : Sea-
anemone, 184, 187 : Magellania,
357 : Brachiopoda, 362 : Hirudo,
496, 497 : Apus, 519 : Astacus, 533,
534 : Crustacea, 578 : Periplaneta,
607 : Anodonta, 667 : Pelecypoda,
679

Muscular tissue, 29*, 30

Mushroom coral, 203

Mussels, 663

My a, 678

My a arenaria, 680

Mycetozoa, 45* : Example, 68, 69 :
Sporangium, 68, 69 : Capillitium, 68,
69 : Spores, 68, 69 : Flagellula, 68,
69 : Plasmodium, 68, 69 : General
remarks, 69 : Protomyxa, 69

Myo meres, of Apus, 519

Myonemes, 82*

Myosoma, 347

Myriapoda, 514, 598*, 660, 661, 662 :
Distinctive characters and classifica-
tion, 598 : General organisation, 600 :
External features, 600 : Integument
and Body-wall, 600 : Alimentary
canal, 601 : Heart, 601 : Respira-
tory system, 601 : Nervous system,
601 : Reproduction, 601 : Ovum,
601 : Fossil remains, 602

Myriothela, 144, 148, 149

Mysidacea, 553*, 568, 579, 582, 584,
588

Mysis, 553, 567, 584

Mytilus, 678, 679, 685, 686, 687, 691

Mytilus edulis, 679, 684

Mytilus latus, 679

Myxidium lieberkuhnii, 89, 90

Myxobolus mulleri, 89

Myxospongise, 113*, 121, 126

Myxosporidea, 90* : Characteristic
features, 89

Myzostorna, 478, 479

Myzostomida, 429, 477*, 478, 479

N.

N

ACRE, 666*

Nais, 475

Nareomedusae, 142*, 155, 156

Natural History, 1*

Nauplius, 525, 526

Nausithoe, 179

Nautiloids, 663, 767

Nautilus macromplial'is, 757, 766

NAUTILUS POMPILIUS, 754 : Shell, 754 :
External characters of soft parts,
755 : Mantle and mantle -cavity, 759 :
Enteric canal, 761 : Coplome, 761 :
Heart and circulation, 761 : Renal
organs, 763 : Nervous system, 764 :
Sense organs, 765 : Reproductive
organs, 765 : Systematic position, 767

INDEX

805

Nearctic region, 9*

Nebalia, 552, 566, 567

Nebeliacea, 552

Neck, of cockroach, 603

Nectocalyx, 161

Nectonema, 299, 300, 301, 302, 303

Needham's sac, 753*, 765

Nekton, 8*

Nemathelminth.es, 293* : Appendix,
313 : Affinities and relationships, 315

Nematocyst, 81*, 89, 90, 97, 133, 134*

Nematoda, 292* : Example, 292 : Ex-
ternal characters, 293 : Body-wall,
293, 294 : Digestive organs, 294,
295 : Body-cavity, 295 : Excretory
system, 296 : Nervous system, 296,
297 : Reproductive organs, 296, 297 :
Development, 297, 298, 303, 304 :
Distinctive characters and classifica-
tion, 298 : General organisation, 299 :
Life-history, 304

Nematogene, 228, 229*

Nematoidea, 299*, 301, 302

Nematomorpha, 299*, 300, 302

Nemertinea, 283, 284* : General fea-
tures, 284, 285, 286, 288, 289 : Body-
wall, 285, 286, 288 : Digestive canal,

286 : Blood-vascular system, 286,

287 : Excretory system, 286, 287 :
Respiration, 287, 288 : Nervous
system, 286, 288, 289 : Cerebral
organs, 286, 289 : Eyes, 289 : Stato-
cysts, 289 : Reproductive svstem,
289 : Development, 290 : Distinc-
tive characters and classification, 290

Neomenia, 694. 695, 698, 699

Neotropical region, 9*

Nephelis, 504

Nephridiopore, 438, 445, 494

Nephridium, 429, 437*, 438, 449, 450,
467, 468, 487, 499, 500, 672 : pro-
visional, 469

Nephrostome, 358, 438, 449, 467

NereidoB, 456*, 477

Nereidtformia, 456*

NEREIS, External features, 430, 431 :
Enteric canal, 432 : Coelome, 432 :
Body-wall, 433 : Vascular system,
434, 435 : Nervous system, 436 :
Sense organs, 437 : Excretory organs,
437, 438 : Reproductive organs, 438 :
Individual variation, 439 : Develop-
ment, 440, 441, 442, -443: Syste-
matic position, 455

Nerilla, 492

Nerve-cell, 30*, 31

Nerve-fibres, 30*, 31

Nerve ganglia, 38

Nerve pentagon, 372

Nervous system, 15, 30, 38*, 39 : Obelia,
133 : Leptolinae, 151 : Aurelia, 171 :
Tealia, 190 : Hormiphora, 213 :
Planaria, 234, 235 : Fasciola, 238 :

Tcenia, 244 : Platyhelminthes, 261,
262, 263 : Nemertinea, 288, 289 :
Ascaris, 296, 297 : Nematoidea, 302 :
Acanthocephala, 308 : Chaetognatha,
311, 312: Brachionus, 320: Roti-
fera, 320, 327 : Dinophilea, 330 :
Gastrotricha, 330 : Polyzoa, 344 :
Endoprocta, 347 : Phoronis, 350 :
Magellania, 358 : Brachiopoda, 362 :
Starfish, 372 : Sea-urchin, 390 :
Holothurian, 394 : Antedon, 400 :
Echinodermata, 420 : Nereis, 436 :
Lumbricus, 448, 449 : Chaetopoda,
465, 468 : Myzostomida, 478 : Echiu-
rida, 4 80, 481 : Sipunculus, 486, 487 :
Sipunculoidea, 486, 489 : Archi-
annelida, 491, 492: Hirudo, 501:
Hirudinea, 507 : Apus, 522, 523 :
Astacus, 541, 542 : Crustacea, 581 :
Peripatus, 594 : Myriapoda, 601,
610, 611 : Periplaneta, 610, 611 :
Insects, 628, 629 : Scorpio, 641, 642 :
Arachnida, 653, 654 : Mussels, 673 :
Pelecypoda, 688, 689 : Amphineura,
698, 699 : Triton, 710, 711 : Gastro-
poda, 725, 726 : Scaphopoda, 737 :
Sepia, 745, 746, 747, 748, 749 :
Nautilus, 762, 764 : Cephalopoda,
774

Nervous tissue, 30*

Nervures, 625*

Neuraxis, 30*, 31

Neurolemma, 30*, 31

Neuronephro blast, 508*

Neuropodium, 431*, 458

Neuroptera, 618*, 625, 632, 637

New Zealand region, 92

Nicothoe, 561, 562

Nidamental gland, 753*

Noah'-s Ark shell, 678

Noctiluca, 81

Nodes, of Siphonophora, 161

Nodosaria, 54

Nomenclature, binomial, 1*

Non-contractile vacuoles, 11*, 62, 95,
96

Noteus, 324

Notholca, 324

Notommata werneckii, 328

Notopodium, 431*, 458

Notostraca, 550*

Nuchal cartilage, 742

Nuchal organs, 437*

Nuclear membrane, 17*, 18, 82

Nuclear sap, 17*

Nuclear spindle, 18, 19*

Nuclearia, 60

Nucleolus, 18*, 20, 82

Nucleus, 11*, 15, 17, 20, 21

Nucula, 677, 680, 683, 684, 685, 686,
688. 689, 692, 693

Nudibranchia, 715*, 719, 723, 724, 736

Nummulites, 54

806

INDEX

Nyctiphanes, 554
Nyctotherus, 94, 96, 97
Nymphon hispidum, 657

0,

'BF.LIA, General structure, 129, 130,
131 : Microscopic structure, 132, 133 :
Medusae, 135, 136, 137 : Comparison
of polype with medusa, 137, 138, 139 :
Reproduction, 139 : Development,
140 : Systematic position, 143

Occlusor muscle, 342

Ocelli, 143, 172

Octopoda, 663, 767*, 768, 769, 770,773,
780

Octopus, 768, 770, 772, 773

Oct or chandra, 150

Ocular plates, 388, 389

Odonata, 6J8

Odontophore, 702, 707, 708, 709.

(Esophagus — See Digestive system

Oikomonas, 73, 75

Olfactory organs, 39*

Olfactory pits, 172*

Oligochaeta, 455*, 457, 460, 461, 462,
463, 464, 466, 467, 468, 469, 470,
471, 475, 476, 477

Olynthus, 117

Ommatidium, 523*, 524

Oncidium, 727, 735

Oniscus, 554, 569, 572

Onychophora, 515,591*, 661 — SeePm-
patus

Ocecium, 334*

Ookinete, 89*

Oosperm, 23*

Oostegites, 570*

Oostegopod, 519*

Ootype, 239, 240

Opalina, 95, 100

Opalinopsis, 96

Operculum, of Radiolaria, 62*, 64 :
Ciliata, 97, 98 : Gastropoda, 704*,
705, 720

Ophioglypka, 411

Ophiuroidea, 368, 402* : Distinctive
characters and classification, 402* :
General form and symmetry, 406,
407, 408 : System of plates, 409 :
Modifications of form, 410, 411,
412 : Ooplome, 416 : Ambulacra!
system, 417 : Blood-vascular sys-
tem, 417, 418 : Haemal system,
418 : Axial organ, 419 : Enteric
canal, 419 : Nervous system, 420 :
Reproductive organs, 420 : De-
velopment, 422, 423 : Ethology,
&c., 425, 426

Ophrydium, 97, 99

Ophryodendron, 102

Ophryoglena, 96

Opisthobranchia, 714*, 718, 724, 725,

735

Opisthogoneata, 599*, 602, 661
Opisthorchis sinensis, 280
Optic gland, 750
Optic nerve-fibres, 750
Optic vesicle, 779
Oral arms, 169*
Oral surface, 369*
Oral tentacles, 151, 413
Orchestia, 554, 569, 579
Order, 4*
Organ of Bojanus, 672* : of Owen,

758* : of van der Hoeven, 758* :

of Valenciennes, 759
Organic evolution — See Evolution
Organism, 1*
Organ-pipe Coral, 193
Organs, 31*
Oriental region, 9*
Orobdella, 504
Orthoceras, 780

Orthonectidce, 227, 229, 230, 231
Orthoptera, 616*, 621, 624, 625, 637
Orthopteroidea, 616*
Oscarella, 118
Oscaria, 116
Oscillatoria, 74
Oscular sphincter, 112*
Osculum, 107*

Osphradium, 673*, 674, 712, 774
Ossicles, 368*
Ostia, 107*, 110, 521
Ostium, 186*
Ostracoda, 550*, 558, 559, 578, 579,

581, 582, R86, 587
Ostrea, 678, 680, 683, 688, 690, 691
Otocyst, 40*, 467, 673, 674. 689
Ovariole, 612*, 631
Ovary, 40*
Ovicells, 345*
Oviduct, 40*, 240
Oviparous, 40*
Ovipositor, 594
Ovum, 20*, 21, 31
Owen, organ of, 758*
Oxeote spicules, 109, 111*
Oxygen, oxidation, 36, 37
Oxyuris, 302
Oysters, 663, 678, 680

JL ACHYCHALINA, 121
Paedogenesis, 42*, 223*, 632
Pagurvs, 555, 574
Palaearctic region, 9*
Palcemon, 555, 573
Palcemonetes, 578
Palaeodictyoptera, 637
Palaeontology, 6*

INDEX

807

Palinurus, 555, 572, 580, 585, 587

Pallial complex, 715

Pallial groove, 338

Pallial line, 666*

Pallial muscles, 667

Pallial sinus, 358

Pallium — -See Mantle

Palpi, 461

Paludicella, 340, 346

Paludina, 734

Pali, 202*

Palythoa, 127, 195

Pancreas, 35*

Pancreatic juice, 35*

Pandorina, 74, 75, 76, 77

Papillae, 415*

Paragastric cavity, 107*, 109

Paragnatha, 517*, 535

Paramithrax, 586

Par amoeba, 51

Paramcecidce, 93

PARAM.JECIUM, 90, 91, 92, 93 : Syste-
matic position, 93

Paramc&cium caudatum, 91, 92, 93

Paramylum, 70*

Paranaspides, 553, 567

Paranephrops, 555

Parapodium, 429, 430, 431, 458, 720

Paraseison asplanchnus, 322

Parasitism, of Amoeba, 51 : Protozoa,
74, 83, 84, 86, 89, 97 : Ciliata, 101 :
Hydrozoa, 166 : Mesozoa, 227 :
Rotifera, 328

Parazoa, 106

Parenchyma, 233*, 259, 260, 301, 344

Parenchyma muscle, 260

Parenchymula, 123*

Parthenogenesis, 21*, 41*, 282, 632

Parthenogonidia, 78*

Parthenope, 576

Patella, 717, 723, 725, 726, 727, 729,
731, 732, 733, 734

Patellidce, 713

Pauropoda, 598*, 601

Pauropus, 598, 600

Paxillge, 405

Paxillosa, 402, 410

Peachia, 200, 206

Pea-crab, 586

Pearl -mussel, 692

Pearl-oyster, 678

Pearls, 692

Pebrine, 90*

Pecten, 678, 679, 682, 689, 690, 692

Pectines, 640*

Pectinibrancbia, 714*, 724

Pedal gland, 706*, 720

Pedal lobes, 178

Pedal tentacles, 720

Pedalion, 322, 323, 325, 328

Pedata, 404*, 425

Pedicellarise, 370, 374, 380, 387, 408

Pedicellina, 341, 347

Pedipalpi, 638, 647, 648

Pedipalpida, 645*, 647, 648, 652, 653,
656

Peduncle, 202*

Pelagia, 182, 183

Pelagic species, 8*

Pelage-hydra, 149

Pelaqothuria, 414

Pelecypoda, 663 : Example, 663 : Dis-
tinctive characters and classification,
676* : General organisation, 679 :
Adductor muscles, 679 : Shell, 680 :
Siphons, 680 : Foot, 683 : Byssus
gland, 685 : Gills, 685, 686, 687 :
Digestive organs, 687 : Excretory
organs, 688 : Circiilatory organs,
688 : Nervous system, 688, 689 :
Sense organs, 689 : Reproduction
and development, 690, 691, 692 :
General remarks, 692 : Mutual rela-
tionship, 693

Pellicle, 90

Pelmatozoa, 404*, 425

Pelomyxa, 49

Peltogaster, 552, 565

Pen, 772*

Penceus, 555, 584

Penial setae, 293, 299

Pznnatula, 194, 195, 197, 202, 205

Pennatulacea, 194*, 195, 197, 202

Pentacerns, 420

Pentacrinoid larva, 401*, 424

Pentacrinus, 401

Pentastomida — See Linguatulida

Pentastomum tcenioides, 658

Peptonephridia, 469*

Peptones, 35

Peracarida, 553*, 586

Perforate corals, 205*

Pericardial sinus, 521*

Perichondrium, 28*

Pericolpa, 178, 179

Perihsemal system, 372*

Periosteum, 29*

Periostracum, 666*

PERIPATUS, 514, 591 : External fea-
tures, 591, 592 : Body-wall and
body-cavity, 592 : Enteric canal,
592, 593 : Circulatory system, 592 :
Organs of respiration, 593 : Coxal
and slime glands, 593, 594 : Nervous
system, 594 : Nephridia, 594 : Re-
productive organs, 594 : Develop-
ment, 595, 596, 597 : Distribution,
597 : Relationships, 597

Peripatus capensis, 591, 592, 593, 594,
596, 597

Peripatus novos-zealandios, 595

Periphylla, 178

PEREPLANETA AMERICANA, 603, 604 :
Head, 603, 604 : Neck, 605 : Thorax,
606 : Abdomen, 606 : Respiratory
movements, 607 : Muscles, 607 :

INDEX

Hsemocoele, 608 : Digestive system,
608, 609 : Renal organs, 609 : Heart,
609 : Respiration, 609, 610 : Ner-
vous system, 610, 611 : Organs of
special sense, 611 : Reproductive
organs, 611 : Development, (512, 613,
614, 615 : Systematic position, 621

Periplaneta orientals, 603, 604, 605

Periproct, 386*

Perisarc, 131, 132, 133

Peristaltic movements, 37*, 434

Peristome, 94, 97, 98, 185*, 373*, 386*

Peristomium, 430*, 444, 460

Peritoneum, 445*

teritrichous ciliation, 94*, 98

Peri visceral cavity, 432*

Periwinkles, 663

Perlidce, 618

Peromedusse, 179

Per-radius, 139*

Petaloid ambulacra, 413

Petasus, 154

Petrarca, 551, 565

Phacellse, 171*

Phacops fecundus, 589

Phseodium, 62, 64

Phalangida, 645*, 649, 651, 653

Phanerocephala, 454*, 456, 464, 472,
476

Pharynx — See Digestive system

Phasmidce, 621, 637

Pheronema carpenteri, 122, 124

Philodina, 321, 324, 327

Pholas, 678, 682, 692

Phoronida, 333, 348, 353

Phoronis, 348, 349, 350, 351, 352

Phragmacone, 772*

Phragmoceras, 780

Phreatoicus, 554

Phronima, 571

Phrynus, 647

Phylactolaemata, 341*, 342, 343, 344,
345, 346

Phyllocarida, 552*, 566, 567, 579, 581,
586, 587, 588

Phyllodoce paretti, 468

Phyllosoma, 585

Phylogeny, 8*

Phylum, 5*, 43*

Physalia, 163, 164

Physiology, 9*

Phytoflagellata, 75

Pieris, 620

Pigment, 70, 120

Pilema, 181

Pilidium, 290

Pill-bug, 572

Pinacocytes, 120*

Pinna, 678, 685

Pinnotheres. 586

Pinnules, 398*

Pionosyllis elegans, 472

Piscicola, 504, 510

Placenta, 597

Placophora, 694*, 697, 698, 699, 701,
702

PLANABIA, 233 : General features, 233,
234, 236 : Digestive system, 233,
234: Water vessels, 234, 235:
Nervous system, 234, 235 : Re-
productive system, 235, 236 : Syste-
matic position, 249

PlanaridcB, 249

Plankton, 8*

Planocera, 269, 271

Planorbis, 241

Planorbulina, 54

Plant lice, 619, 632

Planula, 140*, 153, 172, 173, 190

Plasma, of blood, 30

Plasmodium, 31*, 68, 69

Plasmosome, 18*

Plastin, 17*, 18

Platyctenea, 218*, 221, 222, 223

Platyhelminthes, 233, 248* : Ex-
amples, 233, 236, 243 : Systematic
position, 249 : General external
features, 250 : Integument and
muscular layers, 257, 259 : Paren-
chyma, 259 : Alimentary systems,

260, 261, 262 : Nervous system,

261, 262, 263 : Water-vascular
system, 263, 264 : Reproductive
organs, 264, 265, 266, 267 : De-
velopment, 268, 269. 270, 271, 272,
273, 274, 275, 276, 277, 278:
Asexual reproduction, 278 : Dis-
tribution, occurrence, and relation-
ships, 279, 283 : Appendix, 284

Platypoda. 714*, 715

Pleopod, 530*

Pleurobrachia, 208

Pleurobrachiidce, 218

Pleurobranchia, 536, 537*

Pleuron, 528*

PleurophyUidia, 723

Ploitna, 322*, 323

Plumatella, 341, 342

Plumularia, 149

Pluteus, 392, 403, 422, 423

Pneumatophore, 160, 161*

Podia, 370*, 413

Podical plates, 606*

Podobranchia, 536, 537*

Podomere, 514*

Podophrya, 101, 102

Podura, 616

Polar body, 21*, 22

Polar plates, 213*

Polian vesicles, 376, 377*, 391, 394, 417

Pollicipes, 565

Polyarthra, 322, 323, 331

Polycelis, 251

Polychseta, 454*, 460, 461, 463, 466, 467,

468, 469, 470, 471, 472, 474, 475,

477

INDEX

809

Polycladida, 248*, 251, 252, 253, 257,
258, 261, 262, 263, 265, 266, 268,
269, 270, 282, 283

Polycolpa, 155

Polydora, 477 .

Polygordiidce, 491

Polygordius lacteus, 491

Polygordius neapolitanus, 491, 492, 493

Polykrikos, 80, 81

Polymorphism, 142*

Polynesian region, 9*

Polynoe, 457, 459, 460

Polynoe extenuata, 459

Polynoe setossima, 457

Polynoidce, 457, 470

Polyceca, 79, 80

Polyophthalmus, 466, 470

Polype, 130*, 149

Polyphemus, 551, 558

Polyphyletic group, 282*

Poly podium, 166

Polyspermy, 21*

Polystomatous, 182*

Polystomece, 253, 254

Polystomella, 57

Polystomum, 254, 273, 279

Polytrochal larvse, 474*

Polyzoa, 333*: Example, 334: Dis-
tinctive characters and classifica-
tion, 340 — See Ectoprocta and Endo-
procta

Polyzoa (Cestoda), 249*

Pontellina msditerranea, 585

Pontobdella, 504, 505, 506, 507

Porcellana, 555

Pore-membrane, 109, 110*

Porifera, Example, 106 : Distinctive
characters and classification, 112 :
General form and mode of growth,
115 : Leading modifications of struc-
ture, 117 : Histology, 120 : Skeleton,
120 : Reproduction, 122 : Develop-
ment, 122 : Distribution, affinities,
&c., 125, 226

Porocyte, 110*, 123

Poromya, 678, 685, 686

Porospora giyantea, 86

Porpita, 164, 165

Port-hole — See Gin elides

Portuguese Man-of-War, 163

Portunus, 576

Post-abdomen of Scorpion, 638

Potamobiidce. 556*

Potcrion, 116

Prawns, 572, 573

Pre-abdomen of Scorpion, 638

Priapuloidce, 490

Priapulus, 490

Primary axis, 42*, 210*

Primitive ectoderm, 304*

Primordial eridoderm cell, 304*

Primordial sexual cells, 304*

Prismatic layer, 666

Proboscis, 234

Proglottides, 243*

Progoneata, 598*, 602, 660, 661

Pro -legs, 635

Proneomenia, 694, 698

Pronucleus, active and stationary, 93 :

male and female, 21*, 22, 23
Pro-ostracum, 772*
Propodium, 706, 719
Prorocentrum, 80, 81
Prorodon, 96, 97
Proscolex, 247*
Prosiphon, 771*

Prosobranchia — See Streptoneura
Prosoma, 638
Prosopyle, 109, 1 10*
Prostate, 264, 266*, 471
Prostomium, 430*. 444, 460
Protamceba, 49
Protandrous, 302*
Protective characters, 586
Proteids, 16*
Proteolepas, 551, 565
Proterospongia, 79, 80, 127*
Proteus Animalcule, 10
Prothorax, 606
Protobranchia, 677*, 679, 686, 688,

693, 781

Protoconch, 755*, 771
Protodrilus, 491, 492, 512
Protohydra, 148, 149
Protomerite, 84, 85*
Protomyxa, 69
Protonemertini, 291*
Protonephridial system, 232*, 263
Protoplasm, 11*, 15, 16*, 17
Protopodite, 526*, 530, 531
Prototroch, 316*, 441
Protozoa, 45
Protozoaea stage, 584*
Protractor muscle, 666
Proventriculus, 609*
Psammoclema, 116
Pseudoblastopore, 216*
Pseudobonellia, 479, 482
Pseudogastrula, 125*
Pseudo-lamellibranchia, 678*, 680, 682,

693, 694

Pseudo-manubrium, 156*
Pseudo -metamerism, 251*
Pseudopod, 10*, 45*, 46*, 47 : of

Amoeba, 10*, 46*, 47
Pseudo-scorpionida, 645*, 646, 652, 653
Psocidse, 617*
Psocus Jasciatus, 617
Psolus, 414
Psorosperms, 89
Pteropoda, 714*, 720, 721, 735
Pterotrachea, 721
Pterygota, 616*
Pulmonary sac, 724*
Pulmonata, 715*, 719, 722, 724, 726,

728, 729, 730, 735, 736

810

INDEX

Pulsellum, 736

Pupa, of Cirripedia, 583 : of Insects,

634

Purpura, 725

Pycnogonida, 657*, 658, 662
Pygidium, 590*
Pyloric caeca, 374, 375
Pyrenoids, 75
Pyriform organ, 338
Pyriform sac, 767
Pyrula, 730
Pyxicola, 97

Q

R

Q

UADRULA, 49

;ACHIS, 297*
Radial canals, 108, 109*, 110, 118
Radial symmetry, 42, 43*
Radiata, 426

Radii, orders of, 138, 139
Raiiolaria, 48* : General structure,

62 : Central capsule, 62 : Skeleton,

63 : Colonial forms, 63 : Reproduc-
tion, 63 : Symbiosis, 65

Radula, 708*

Radular sac, 708*

Rainey's corpuscles, 90*

Raphidiophrys 60

Razor-fish, 692

Receptacula ovorum, 452*

Receptacula seminis, 452*

Rectum, 489*

Red coral, 193, 201, 207

Redia, 242*

Regularia, 403*, 406

Relationship, 6*

Relationships, of Protozoa, 104 :
Sponges, 127, 226 : Ccelenterata,
226 : Platvhelminth.es, 282 : Nema-
thelminthe's, 315 : Rotifera, 328 :
Dinophilea, 330 : Molluscoida, 365 :
Echmodermata, 426, 490, 512 :
Sipunculoidea, 490, 512 : Annulata,
510, 587 : Crustacea, 587 Peripatus,
597, 662 : Air-breathing Arthropoda,
597, 659 : Pelecypoda, 693 : Gastro-
poda, 736 : Cephalopoda, 780 :
Mollusca, 780

Relationships, diagrams of : of Pro-
tozoa, 105 : Coelenterata, 227 :
Platyhelminthes, 283 : Echinoder-
mata, 428 : Annulata and Trochel-
minthes, 513 : Crustacea, 588 :
Arthropoda, 661 : Pelecypoda, 693 :
Gastropoda, 736 : Cephalopoda, 780

Renal organs — See Excretory system

Reproduction, Reproductive System,
15, 31. 32, 40, 41 : Amoeba, 14, 46,
47 : Foraminifera, 56, 57, 58 :

Heliozoa, 61 : Radiolaria, 63 : My-
cetozoa, 69 : Euglena, 71 : Flagel-
lata, 76, 77, 78, 79 : Choanoflagel-
lata, 80 : Dinoflagellata, 81 : Cysto-
flagellata, 82 : Monocystis, 82, 83,

85 : Gregarinida, 86 : Coccidiidea,

86 : Haemosporidea, 89 : Para-
mcccium, 92 : Ciliata, 99 : Tenta-
culifera, 103 : Porifera, 122 : Obelia,
139 : Leptolinae, 149 : Trachylinao,
155 : Hydrocorallina, 159 : Siphono-
phora, 163, 164, 165 : Aurelia, 171 :
Tealia, 190 : Actinozoa, 195 . Hor-
miphora, 214 : Mesozoa, 229, 230,
231 : Planaria, 235, 236 : Fasciola,
239, 240 : Tcenia, 245, 246 : Platy-
helminthes, 264, 265, 266, 267, 268,
278 : Nemertinea, 289 : Ascaris,
296, 297 : Nematoda, 302 : Acan-
thocephala, 308, 309 : Chaetognatha,
312 : Brachionus. 320, 321 : Roti-
fera, 327 : Dinophilea, 331 : Bugula,
336, 337 : Ectoprocta, 345 : Endo-
procta, 347 : Phoronis, 350, 351 :
Magellania, 358 : Brachiopoda, 362 :
Starfish, 378, 379 : Sea-urchin, 392 :
Holothurian, 396 : Antedon, 401 :
Echinodermata, 420 : Nereis, 438 :
Lumbricus, 450, 451, 452 : Chaeto-
poda, 470, 471 : Myzostomida, 478,
479 : Sipunculus, 488 : Sipuncu-
loidea, 489 : Archi-annelida, 492 :
Hirudo, 502 : Hirudinea, 507, 508 :
Apus, 524, 525 : Astacus, 542, 543 :
Crustacea, Peripatus, 594, 595 :
Myriapoda, 601 : Periplaneta, 611,
612 : Insects, 631 : Scorpio, 643 :
Arachnida, 654, 655 : Mussel, 674 :
Pelecypoda, 691 : Amphineura, 699,
700 : Triton, 712 : Gastropoda, 728,
729 : Scaphopoda, 737 : Sepia, 752,
753 : Nautilus, 765, 766 : Cephalo-
poda, 774

Requienia, 682, 683

Reservoir (Eughna), 70

Respiration, 13*, 35

Respiratory organs, 36* : Starfish, 369 :
Sea-urchin, 392 : Holothurian, 396 :
Cha?topoda, 461 : Apus, 518, 522 :
Astacus, 536, 537 : Crustacea, 579 :
Peripatus, 593 : Myriapoda, 691 :
Periplaneta, 609, 610 : Insects, 626,
627 : Scorpion, 642 : Araohnida,
653, 654, 655 : Mussel, 669 : Pele-
cypoda, 635, 686 : Triton, 706 : Gas-
tropoda, 722, 723 : Sepia, 745 :
Nautilus, 759 : Cephalopoda, 773

Respiratory trees, 396*, 419

Retina, 40*

Retinal cells, 750

Retinophore, 712*

Retinula, 524*, 542, 712

Retractor muscles, 666

INDEX

811

Rhabdites, 257*

Rhabditis-form, 304

Rhabdocoelida, 248*, 251, 252, 257, 260,
261, 262, 263, 264, 266, 267, 268, 271,
278, 279, 283

Rhabdog aster, 313

Rhabdome, 524*

Rhabdomolgus, 414

Rhabdonema nigrovenosum, 304, 305

Rhagon-type of Sponge, 119*

Rhinophore, 765*

Rhipidodendron, 73, 76

Rhipidoglossa, 714*, 736

Rhizocephala, 552*, 565

Rhizopoda, 45* : Example, 46 : Classi-
fication and general organisation, 47

Rhizostomse, 176*, 181, 182, 224

Rhizota, 321*, 327

Rhombogene, 229*

Rhopalura, 229, 230

Rhyncheta, 102

Rhynchobdellida, 504*, 505, 506,507,508

Rhynchodemus, 251

Rhynchonetta, 359, 361

Rhyncocoele, 285*

Rhyncota, 618*

Ring-vessel, 264*

Rock-lobster, 555

Rock systems, 7

Rocks, igneous and aqueous, 6*, 7*

Rods of retina, 750*

Rosette, 398*

Rosette (Earthworm), 451

Rosette plate, 409

Rostellum, 243*

Rostrum, 529*

Rotalia, 53

Rotation, sense of, 40

Rotifer, 320, 321

Rotifera, 317* : Distinctive charac-
ters and classification, 321 : Ex-
ternal characters, 323, 324, 325 :
Digestive organs, 326 : Excretory
system, 327 : Nervous system and
Sense organs, 327 : Reproduction
and Development, 327 : Ethology,
328 : Affinities, 328
Rotulse, 389*
Round -worms, 292 — See Nemathel-

minthes
Rugosa, 208*

S

ISABELLA, 466
jSaccammina, 54
Saccocirrus, 454, 466
Saccosoma, 480, 482
Sacculi, 400*

Sacculina, 552, 565, 583, 584
Sagartia, 188, 189
tiagitta, 310, 311, 312, 313
Sagittal plane, 210*

Sail, of Siphonophora, 164, 165

Salinella, 1231

Saliva, 34*

Salivary glands, 34*

Salivary receptacle, 608

Salmacina, 475

Salpingceca, 79

Saltatoria, 616, 621

Sand-hopper — See Orchestia, 554

Saprophytic nutrition, 71*, 74

Sarcocystidea, 84, 90

Sarcocystis, 90

Sarcolemma, 29*

Sarcophaga, 629

Sarcoptes scabicei, 649

Sarsia, 145

Scale -insects, 619, 632

Scallop, 678

Scaphopoda, 663, 736*, 737, 738

Schistosomum hcematobium, 280

Schistosomum japonicum, 280

Schizogony, 86*, 87, 88

iSchizopathes, 199

Schizopod-stage, 584, 585

Schizopoda, 586

Scirtopoda 322*, 323

Sclerite, 548*

Scleroblast, 111*, 120, 121, 124

Sclerotic, 750

Scolex, 243*, 275

Scolopendra, 599

Scolopendrella, 598, 600

SCORPION — See BUTHUS

Scorpionida, 644*, 656, 659, 660

Scorpion -flies, 621

Scorpion-spiders, 645

Scorpions, 514, 637, 656, 659, 660

Scrobicularia piperata, 681

Scuta, 563, 564

Scutariella, 253

Scutigera, 600, 601

Scutigerella, 599

Scyllarus, 573, 574

Scyllis ramosa, 476

Scyphistoma — See Scyphula

Scyphozoa, 129 : Example, 167 : Struc-
ture and classification, 175 : Syste-
matic position of example, 176 :
Additional remarks on, 183

Scyphula, 174*, 175, 227

Sea-anemones, 129, 183, 192, 194, 197,
198— See Tealia

Sea-blubbers, 181

SEA- CUCUMBER, External features, 393 :
Structure of body-wall, 394 : Am-
bulacral system, 394 : Nervous -
vascular systems, 394 : Ccelome, 394,
395 : Enteric canal, 395, 396 : Re-
productive organs, 396 : Develop-
ment, 396 : Systematic position, 406

Sea-cucumbers, 404 — See Holothu-
roidea

Sea-fans, 193, 201

812

INDEX

Sea-firs (Sertularians), 144

Sea-hares, 702

Sea-mats, 333 — See Polyzoa

Sea -mice, 464

Sea-mussel, 678

Sea-pens, 194

SEA-URCHIN, External features, 386,
387, 388 : Corona, 388 : Aristotle's
lantern, 389, 390 : Nervous system,
390, 39 J : Ambulacral system, 391 :
Enteric canal, 391, 392 : Ccelome,
392 : Blood-vascular system, 392 :
Reproductive organs, 392 : Develop-
ment, 392 : Systematic position, 406

Sea-urchins, 403* — See Echinoidea

Secondary axis, 42*, 43

Secondary ectoderm, 304

Secretion, 26*

Segment, 43*

Segmental organ — See Nephridium

Segmentation of oosperm, 23*

Segmentation - cavity — See Blast o c oele

Segmentation-nucleus, 21*, 22

Seison, 328

Seisonida, 322

Selenaria, 340

Selenariidce, 342, 345

Self-mutilation, 425

Semostomse, 176*, 181, 182, 224

Sense Organs, 39* : Obelia, 136 : Tra-
chylinse, 154, 155 : Aurelia, 171,
172 : Cubomedusae, 181 : Hormi-
phora, 213 : Ctenoplana, 221 : Tjal-
fiella, 222 : Platyhelminth.es, 263 :
Nemertinea, 289: Chsetognatha, 311,
312 : Brachionus, 320 : Rotifera,
327 : Starfish, 370 : Sea-urchin,
388 : Nereis, 437 : Lumbricus, 448 :
Sipunculus, 487 : Hirudo, 501, 502 :
Hirudinea, 507 : Apus, 523, 524 :
Astacus, 542 : Crustacea, 581, 582 :
Peripatus, 594 : Periplaneta, 611 :
Insects, 629, 630 : Scorpio, 643,
655 : Arachnida, 653, 654. 655 :
Mussel, 673, 674 : Pelecypoda, 689,
690 : Amphineura, 698, 699 : Triton,
712 : Gastropoda, 726, 727 : Sepia,
749, 750 : Nautilus, 765 : Cephalo-
poda, 774 — See also Eyes, Auditory
organs, Olfactory organs, Gustatory
organs, Tactile orgams, Osphradia

Serse papillae, 296, 485

SsriA, External features, 738, 739 :
Shell, 740, 741 : Chromatophores,

741 : Mantle-cavity, 741 : Internal
skeleton, 742 : Alimentary system,

742 : Ink-sac, 744, 769 : Vascular
system, 744 : Coelome, 745 : Ctenidia,
745, 746 : Nervous system, 745, 746,
747, 748 : Sensory organs, 749, 750,
751 : Excretory organs, 751, 752 :
Reproductive organs, 752, 753, 754 :
Systematic position, 767

Sepia cultrata, 738, 739, 740, 741, 742,
743, 748, 747, 748, 750

Sepiidce, 767*

Septal funnel, 173*

Septai neck, 755*

Septibranchia, 678*, 679, 680

Septum, 202*

Serialaria, 340

Serosa, 613*, 644

Serpula, 457, 462, 465, 468, 476, 477

Serpulidcr, 457, 461, 473, 475, 476

Sertularians, 144

Setae, 428, 430, 431, 445 : provisional,
363, 443

Setigerous sac. 431*, 442, 443, 445, 446

Sexual dimorphism, 40*

Sexual generation, 141

Shell, Magellania, 353, 354, 355 :
Brachiopoda, 360, 361 : Mussel,
664, 665 : Pelecypoda, 680 : Chiton,
695, 696 : Triton, 702, 703, 704 :
Gastropoda, 717, 718: Scaphopoda,
736 : Sepia, 740, 741 : Nautilus,
754, 755 : Cephalopoda, 770, 771

Shell -gland, 240, 515, 674, 776, 778 :
(Apus), 515*, 522* : (Crustacea), 581

Shelly loop, 354*

Ship-worm, 678

Shrimp, 514, 555, 572, 573

Sicula, 166*

Sigaretus, 720

Silicispongiae, 125*

Silicoflagellata, 75

Silver-fish, 616

Sinupalliata, 678*, 681, 690

Sinupalliate shells, 681*

Sinus, 541

Siphon, 391, 419, 704

Siphonal process, 704*

Siphoniata, 693

Siphonodentalium, 736

Siphon oglyphe, 185*

Siphonophora, 143*, 159, 160, 161, 162,
163, 164, 165, 166, 224

Siphonozooid, 200*

Siphons, inhalant and exhalant, 664*,
680

Siphtmcle, 754*

Sipunculoidea, 365, 484*, 511, 512:
Example of the class, 484 : Dis-
tinctive characters, 488 : General
organisation, 488 : Body-wall, 488 :
Alimentary canal, 489 : Blood-
vascular svstem, 489 : Nervous sys-
tem, 489 : Eyes, 489 : Nephridia,
489 : Reproductive organs, 489 :
Development, 489, 490 : Distribu-
tion, Affinities, &c., 490; 512

SIPUNCULUS NUDUS, General external
features, 484, 485 : Body-wall, 485 :
Ccelome, 486 : Blood-vascular sys-
tem, 487 : Alimentary canal, 486,
487 : Nervous system, 486, 487 :

INDEX

813

Nephridia and gonads, 487, 488 :
Systematic position, 512
Skeleton, 32* : Lobosa, 49 : Foramini-
fera, 55, 56 : Heliozoa, 59 : Radio-
laria, 63 : Flagellata, 75 : Ciliata,
97, 98 : Porifera, 120 : Actinozoa,
200 : Sepia, 742 : Cephalopoda,
772, 773— See also Shell and Body-
wall

Skin, 32*— See Body-wall

Slime glands, 591

Slugs, 663, 715

Snails, 663, 715'

Solarium, 718
Solecurtus strigillatus, 682

Solen, 692

Solenocytes, 331*, 351, 468
Solenogastres, 694*, 695, 696, 697,
698, 699, 700, 702, 781

Solenomya, 677

Solpugida, 645*, 647, 648, 651, 652, 653

Somatoblast, 440

Somatopleure, 614*

Spadella, 310

Spadix, 758*, 759

Spatangoidea, 403*, 408, 413

Species, 1*, 114*

Specific name, 2*

Sperm, 21, 31*

Spermary, 40*

Spermatidia, 337*

Spermatogenesis, 31*

Spermatozoon — See Sperm

Spermiduct, 40*

Sperm-mo rulae, 83*

Spermotheca?, 303*, 452*

Sperm-reservoirs, 471

Sphamdia, 387*, 408, 412

Sphseroma, 554

Sphcerophrya, 102

Spicules, 32*, 200, 361

Spiders, 514, 637, 645. 648, 651, 653,
656

Spinnerets, 649*

Spinning-glands, 646, 649

Spinules, 237*

Spinulosa, 402

Spiral segmentation, 269*

Spire me, 19*

Spirifera, 360, 361

Spirilla, 74

Spirochcetes, 74

Spiroloculina, 54

Spirorbis, 477

Spirorbis Ice.vis, 473

Spirilla, 767, 771, 772

Splanchnopleure, 614*

Spongelia, 121

Spongilla, 119

SpongiUidcB, 122, 126

Spongin, 112*, 120, 122

Spongin-blasts, 122*

Sporangium (Mycetozoa), 68, 69

Spore, 41, 50*, 69, 82, 83, 85, 87, 88,
89, 90

Spore formation, 41, 56, 61, 63, 67, 68,
69, 82, 83, 85, 86, 87, 89, 90, 99

Sporocyst, 241*

Sporogony, 86*, 87

Sporosac, 151*

Sporozoa, 46* : Example, 82 : Classi-
fication and general organisation, 83

Sporozoites, 82, 83*, 85, 86, 87, 88, 89

Springtails, 616, 625

Squame, 531*

Squammulina, 53, 55

Squids, 767

Squilla, 577

Stcecharthrum, 229

STARFISH, External characters, 368,
369, 370 : Transverse section of
arm, 370, 371 : Vascular and nervous
systems, 372 : Structure of disc,
373 : Body-wall and ccelome, 373 :
Digestive system, 374, 375 : Ambu-
lacral system, 375 : Reproductive
system, 378, 379 : Development,
380, 381, 382, 383, 384, 385, 386:
Systematic position, 405

Starfishes, 3H8— See Asteroidea

Statoblasts, 346*

Statocones, 689*, 765

Statocysts, 263*, 289, 467*, 542*, 674,
689, 765*, 774

Statolith, 263, 289, 467*, 689*

Stauromedusse, 175*, 176, 177, 224

Stelechopus, 477

Stenocyphus, 176

Stentor, 94, 96

Stephanoceros, 321, 323, 324

Sternal canal, 528*

Sternaspis, 460, 461, 464, 465, 468

Sternaspis spinosa, 461

Sternum, 528*

Stewart's organs, 416*

Stichopoda, 406

Stichotricha, 97

Stick-insects, 617, 621

Stigma (Euglena), 70

Stigmata, 593*, 607, 627, 640

Stinging capsule — See Nematocyst

Stolon, 194*

Stomach — See Digestive system

Stomatogastric system, 437

Stomatopoda, 555*, 577, 578, 581

Stomidia, 199*

Stomodaeal canal, 212*

Stomodseum, 172*, 210

Stone-canal, 376*, 377, 391

Stony-corals, 129, 193, 199

Stratiodrilus tasmanicus, 331, 332

Strepsiptera, 625, 636

Streptoneura, 713*, 715, 717, 720, 722,
724, 725, 727, 728, 729, 735

Streptophiurae, 403*

Strobila, 243*, 256

814

INDEX

Strongylocentrotus, 386, 387, 389, 406—

See Sea-urchin
Strongylofitoma, 602
Strongylus, 300

Stylarioides, 477

Stylaster, 158, 159
Style, 159*

Stylonychia, 100

Subcortical cavity of sponges, 120*

Sub-cuticle, 299*'

Subdermal cavity of sponges, 119*

Sub-genital pit (Aurelia), 169*

Sub-genital portico, 182*

Sub-kingdom — See Phylum

Sub-mentum, 605

Sub-radius, 139*

Sub-tentacular canal, 399

Sub-umbrella, 135*

Succession of Life in time, 7

Sucker (Sepia), 739*

Sucking-disc, 408

Suctorial mouths, 182*

Summer eggs, 582

Supplemental skeleton, 54, 56*

Supporting lamella — See Mesoglcea

Swimming-bell — See Nectocalyx

Swimming ovaries, 309

Swimming-plate, 210*

Sycantha, 114

Syce.tta, 114

Sycettidce, 114*

SYCON : External characters, 106,
107 : Microscopic structure, 108,
109, 110 : Systematic position, 114 :
Development, 124, 125, 126

Sycon gelatinosum, 117, 118

Sycon raphanus, 126

Sycon-type of sponge, 118, 119*

Syllida, 486, 487

Syllis ramosa, 476

Symbiosis, 65*

Symmetry, 42*, 43 : Polype and
Medusa, 138 : Tealia, 188, 190

Symphyla, 598, 599*, 600, 602, 661

Synapta, 414, 425, 428

Synapticula, 203*

Synaptidce, 420

Syncarida, 552*, 567

Syn-cerebrum, 542*, 615

Syncoryne, 145

Syncrypta, 73

Syncytial ectoderm, 293*

Syncytium, 26*, 293*

Syngnatha, 600*, 601, 602

T

~L

ABANUS, 623, 629
Tabulae, 15,*, 202
Tactile cones, 253*, 263
Tactile organs, 39

Tcenia coenurus, 277

Tcenia crassicollis, 280

Tcenia cucumerina, 264

Tceniadce, 250

Tcenia echinococcus, 256, 257, 277, 278,
281

Tcenia mediocanellata, 281

Tcenia saginata, 281

Tcenia serialis, 277

Tcenia serrata, 280

T^NIA SOLIUM, 243, 281 : General
features, 242, 243, 244 : Nervous
system, 244 : Excretory organs, 244,
245 : Reproductive organs, 245, 246 :
Development, 246, 247 : Systematic
position, 250

Tamiole, 172*

Talitrus, 569

Tanaidacea, 553*, 569, 570, 587

Tanais, 553, 569

Tape-worm — See Tsenia and Cestoda

Tardigrada, 657, 658, 659*, 661, 662

TEALIA : External characters, 184,
185 : Enteric system, 185, 186 :
Cell layers, 186, 187 : Muscular
system, 184, 187, 188 : Symmetry,

188 : Microscopic structure, 188,

189 : Nervous system, 190 : Repro-
ductive organs, 190 : Development,
190, 191 : Systematic position, 194

Tealidce, 194*

Tectibranchia, 714*, 736

Teloblast, 453*

Telolecithal egg, 217*

Telotrochal larvae, 474*

Telson, 528*

Temnocephala, 253, 255, 264, 273, 274

Temnocephalea, 249*, 253, 255, 256,
264, 267, 273, 274, 279, 283

Tendon, 38*

Tentacle roots, 155*

Tentacle sheath, 208*

Tentacles, 33*, 93

Tentacular canal, 212

Tentaculifera, 93* : Body and ten-
tacles, 101, 102 : Nucleus, contractile
vacuoles, shell, colonies, reproduc-
tion, 101, 102, 103

Tentaculocyst, 155*, 172

Tentorium, 610*

Terebella, 462

Terebellidce, 461, 462

Terebra, 718

Terebratula, 358, 359, 360

Terebratulidce, 359*

Terebratulina, 365

Teredo, 678, 682, 683

Terga, 563*

Tergum, 528*

Termites, 617

Tessera, 176, 177

Testis — See Spermary

Tethys, 715

I XI) EX

815

Tetrabranchiata, 767*, 769, 770, 771,

773, 774, 780
Tetractinellida, 113*, 122
Tetramita, 73
Tetrarhynchus, 256, 257
Tetrastemma, 286
Thalassema, 479, 481
Thalassoplancta, 63
Theca, 202*

Thomson, J. Vaughan, 3
Thorax, of Apus, 519 : Astocus, 528 :

Periplaneta, 606 : Solpugida, 648
Thread-worms, 292 — See Nemathel-

minthcs
Thuricola, 97
Thysanopoda, 554
Thysanoptera, 618*
Thysanozoon, 251
Thysanura, 615*, 625
Ticks, 637, 646 — See Acarida
Tieclemann's vesicles, 377*, 417
Tintinnidium, 96
Tissues, 25*

TjalfleUa, 221, 222, 223, 226
Tomopteris, 477

Tooth -shells, 663 — See Scaphopoda
Tracheae, 36*, 593, 724
Tracheal gills, 627, 628
Trachea! system (Porpita), 165
Tracheliastes, 561*
Trachelius, 96
Trackelomonas, 73
Trachylinee , 142* : General structure,

154 : Sense organs, 154 : Tentacles,

155 : Reproductive organs, 154,
155 : Development, 155

Trackymedusge, 142*, 154, 155, 156

Translation, 280*

Transverse plane, 210*

Trap-door Spider, 656

Trapezia, 207

Trematoda, 248*, 252, 253, 254, 255,
256, 258, 259, 260, 261, 262, 263, 264,
265, 266, 267, 268, 272, 273, 274, 278,
279, 280, 282, 283 : Example, 236

Treptoplax, 231

Triarthrus becki, 590

Trichina, 305, 306

Trichinetta, 301, 305, 306

Trichinella spiralis, 303

Trichiniasis, 306

Trichocyst, 91*, 92

Trichoplax, 231

Trichoitomata, 93

Tricladida, 248*, 249, 251, 253, 257,
259? 262, 264, 266, 271, 278, 279

Tricosphcerium sieboldii, 50, 51

Tridacna fjigas, 692

Trigger-hair — See Cnidocil
•nia, 678, 683, 692

Trilobita, 589, 590, 591, 660

Trimorphism, 130*
'i, 286

Tristicothceta, 313

TRITON RUBICUNDUS, 702 : Shell, 702,
703, 704 : External features of soft
parts, 704, 705 : Foot, 705, 706 :
Visceral spiral, 706 : Mantle, 706 :
Ctenidium, 706 : Osphradium, 706 :
Digestive system, 707, 708, 709 :
Vascular system, 709, 710, 711 :
Excretory system, 711 : Nervous
system, ^710, 711, 712 : Sensory
organs, 712 : Reproductive organs,
712, 713 : Systematic position, 715

Tritonidce, 715*, 725

Trivium, 370*, 407

Trochal disc, 318*

Trochelminthes, 316* : Appendix, 330

Trocheta, 504

Trochophore — See Trochosphere

Trochosphcera, 322, 325, 326, 328

Trochosphaerida, 322*

Trochosphere, 316, 430

Trochus, 714, 727

Trombidium fuliginosum, 649

Tropho/oite, 82

Trypanosomes, 72, 74

Tube-feet, 369, 370*, 387, 388, 393

Tubifex, 461, 471

Tubificidce, 476

Tubipora, 193, 195, 196, 197, 200, 201,
205, 208

Tubularia, 145, 153

Tubularice, 143, 145

Turbellaria, 248*, 251, 252, 253, 257,
259, 260, 261, 262. 263, 264, 265, 266,
278, 282 : Example. 233

Turbo, 714, 727

Tympanic nerve -endings, 630*

Tympanum, 630*

Typhlosole, 447*, 464, 66S

u

U LMARIDM. 176*
Umbo, 664, 665
Umbrella, 169*
Uncus, 319*

Undulating membrane, 74*, 94, 95, 96
Unicellular animals, 20*
Unicellular gland, 26*
Unio, 662, 678 — See Anodonta
Unio margaritifera, 662, 692
Unionid'jk, 678*
Unisexual, 40*
Urea, 37
Uric acid, 13, 37
Urinary bladder, 671*
Urinary organs, 37*
Urnatella, 347, 348
Urns, 487*
Urocoeles, 671*
Uropods, 530*
Uterine bell, 309
Uterus, 40*

816

INDEX

V

V ACUOLE, contractile, 10, 12*, 13, 14
46, 47, 47,68, 70, 79, 80, 91, 95, 102
non-contractile, 11*, 62, 95, 96

Valenciennes, organ of, 759*

Valvata, 402*

Valvate pedicellarise, 380*

Van der Hoeven, organ of, 758*

Variation, individual, 2*, 114, 439

Variety, 2*, 115*

Vascular system, 35 : Nemertinea
286, 287 : Acanthocephala, 308
Phoronis, 350 : Magellania, 358
Brachiopoda, 362 : Starfish, 372
Antedon, 400 : Echinodermata, 417
Holothurian, 418: Nereis, 434
Lumbricus, 448 : Sipunculus, 487
Sipunculoidea, 489: Archi annelida
492 : Hirudo, 500 : Hirudinea, 505
Apus, 521 : Astacus, 539 : Crus
tacea, 581 : Peripatus, 592 : Peri
planeta, 609 : Insects, 627., ' 628
Scorpio, 641, 642 : Arachnida, 652
Mussel, 672, 673 : Pelecypoda, 688
Amphineura, 698 : Triton, 709, 710
Gastropoda, 725 : Scaphopcda, 736
Sepia, 744: Nautilus, 761, 763
Cephalopoda, 773, 774

Vas deferens — See Spermiduct

Vegetal pole, 731

Velarium, 169*. 179

Velata, 402*

Velella, 165

Veliger, 691, 692*, 731, 734

Velum, 136*, 137, 169, 692, 732

Ventral plate, 601*

Ventral surface, 43*

Ventricle, 37*

Venus gnidia, 681

Venus's Flower-basket, 122

Venus's Girdle, 220

Vermes, 233

Vermetes, 714

Vermetus, 734, 735

Vermiform embryos, 229

Vertebral column, 4

Vertical plane, 210*

Vestibule, 97*

Vibracula, 342, 345*

Vibratile corpuscles, 392

Virgula, 166*

Visceral spiral, of Triton, 706

Vitellaria, 265

Vitelline or yolk glands, 233*, 265

Vitelline membrane, 214

Vitreous body, 523*

Viviparous animals, 40*
Vohita, 730
Volvox, 74, 78, 79

Vortex, 260

Vorticella, 94, 95, 96, 97, 98, 99

Vulxella, 680

W

ALDHhIMIA.— See Magellania
Wallace's line, 9*
Wandering cells, 112
Wasps, 621, 632, 637
Water-bugs, 619
Water-flea, 514, 551
Water-pores, 399*, 417
Water-sac, 523

Water-tubes, 399*, 417, 669, 670
Water-vascular system, 232*, 23:?
Whale -louse, 571*
Wheel -animalcules — See Rot if era
Wheel-organ, 318
Whelks, 663, 714
White body, 750

White substance of Schwann, 30*
Winter eggs, 321, 582
Wood-louse, 514, 554 — -See Oniscu

X

iphosura, 646*, 650, 651, 652,
656, 660, 661, 662

Y

ELLOW CELLS — See Zooranthel
Yellow elastic cartilage, 28*
Yoldia, 677
Yolk, 20*, 21
Yolk-epithelium, 776*
Yolk or vitelline glands, 233*, 265
Yolk reservoir, 240
Yolk-sac, 778, 779*
Yungia, 271

Z,

IEPPELIXIA, 492
Zilla caUophylla, 654
Zosea, 5S4*, 585
Zoantharia, 192*, 194, 198, 224
Zoanthus, 194, 195, 197, 199
Zocecium, 334*
Zoo -geographical regions, 8, 9
Zooid, 41*, 51, 76, 77, 78, 278
Zoology, 1*
Zoophyte, 129
Zoothamnium. 99
Zooxanthella, 62, 65
Zygophiurae, 403*
Zygote, 50*, 76, 78, 79*, 82. 83*

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