\
Ooy^ vc^"^^ •
THE MACMILLAN COMPANY
NEW YORK • BOSTON • CHICAGO • DALLAS
ATLANTA ■ SAN FRANCISCO
THE INVERTEBRATA
tA Manual for the Use of Students
by
L. A. BORRADAILE
Fellonjo of Sekvyn College, Cambridge
and
F. A. POTTS
Fellwo of Trinity Hall, Cambridge
with Chapters by
L. E. S. EASTHAM
Professor of Zoology in the Uni-versity of Sheffield
and
J. T. SAUNDERS
Felloe of Christ's College, Cambridge
SECOND EDITION
NEW YORK: THE MACMILLAN COMPANY
CAMBRIDGE, ENGLAND: AT THE UNIVERSITY PRESS
1935
/
Copyright, 1935, by
THE MACMILLAN COMPANY
All rights reserved — no part of this book may be
reproduced in any form without permission in writing
from the publisher, except by a reviewer who wishes
to quote brief passages in connection with a review
written for inclusion in magazine or newspaper.
PRINTED IN THE UNITED STATES OF AMERICA
PREFACE TO THE FIRST EDITION
This book is intended for the use of students who have completed
a year's study of the principles of zoology and of the anatomy and
physiology of a series of invertebrate types such as is provided by any
of several elementary textbooks in use in this country. The types
commonly included in these books — various Protozoa, Hydra , Ascaris^
and the Liver Fluke, Earthworm, Leech, Crayfish, Cockroach, Pond
Mussel, and Starfish — are not here described in detail. We have
endeavoured to provide the student with a classification of the
Invertebrata which proceeds as far as is usual in an honours course,
with a concise statement of the characteristic features of each of the
groups mentioned, and with a more detailed statement and discussion
of matters of importance or interest concerning them. The choice of
examples has been difficult, and we have not always been able to
include all those we should have wished, but a fairly full account of
certain representative genera has been given.
The writing of the book has been shared among us as follows:
Chapters I-IV, X, XI (except Onychophora), XII, XVIII and XIX
have been written by L. A. Borradaile, Chapters V (except Cteno-
phora), VII-IX, XIII, XV-XVII, and the Onychophora in Chapter
XI by F. A. Potts, Chapter VI and the Ctenophora in Chapter V by
J. T. Saunders, and Chapter XIV jointly by F. A. Potts and L. E. S.
Eastham, but each of us has read and criticized the work of the others.
We desire to express our grateful thanks for valuable advice and
criticism to Dr S. J. Hickson, Professor D. Keilin and Dr S. M.
Manton; for much care bestowed upon the illustrations to Messrs
A. P. Hayle, J. F. Henderson, and C. F. Pond; and for valuable
assistance in the preparation of the index and in other matters to
Mr B. Newman.
For permission to reproduce illustrations acknowledgment is due
to Professor G. H. F. Nuttall; Messrs Geo. Allen & Unwin, Ltd.
(Textbook of Zoology, Sedgwick) ; Messrs A. & C. Black, Ltd. (Treatise
on Zoology, Lankester); the Council of the Cambridge Philosophical
Society (Biological Reviews); Cambridge University Press (The De-
termination of Sex, Doncaster, Plant Biology, Godwin, Ciliary Move-
ment, Gray, Zoology, Shipley and MacBride, Primitive Animals,
Smith, Palceontology , Wood); Herrn Gustav Fischer, Jena (Ergebnisse
u. Fortschritte der Zoologie, Lehrbuch der Protozoenkunde) ; Herren
Walter de Gruyter & Co. (Handbuch der Zoologie) ; the Council of the
Linnean Society of London (Zoological Journal) ; Messrs Macmillan
& Co., Ltd. (Cambridge Natural History , Harmer and Shipley, Human
vi PREFACE
Protozoology^ Hegner and Taliaferro, Textbook of Comparative
Anatomy, Lang, Textbook of Zoology , Parker and Haswell); Messrs
Methuen & Co., Ltd. (Textbook of Entomology , Imms); Oxford Uni-
versity Press {The Animal and its Environment and Manual of Zoology,
Borradaile). Acknowledgment to the authors of the works from which
these illustrations are taken is made in the legends.
THE AUTHORS
CAMBRIDGE
February, 1932
NOTE TO THE SECOND EDITION
The book is now eighty pages longer. The additional matter is chiefly
in Chapters IV and XIV. In other chapters a number of smaller
additions and corrections have been made to the text and to figures.
Each chapter has been revised by its writer, but the revision has
been submitted to the other authors of the book.
Our grateful thanks for various assistance are due to Drs A. M.
Bidder, O. M. B. Bulman, S. M. Manton, and C. F. A. Pantin, and
to Messrs L. E. R. Picken, J. D. Robertson, and P. Ullyott. Dr A.M.
Bidder very kindly communicated to us certain facts, as yet unpub-
lished, which are stated in the second half of p. 595.
THE AUTHORS
CAMBRIDGE
February, 1935
TABLE OF CONTENTS!
CHAPTER I
Introduction: The Invertebrata page i
CHAPTER n
Subklngdom Protozoa
7
Phylum Protozoa]
Class Mastigophora (Flagellata)
45
Subclass Phytomastigina
46
Order Chrysomonadina
50
Order Cryptomonadina
50
Order Euglenoidina
52
Order Chloromonadina
54
Order Dinoflagellata
54
Order Volvocina
56
Subclass Zoomastigina
58
Order Rhizomastigina
60
Order Holomastigina
60
Order Protomonadina
63
Order Polymastigina
65
[Suborder Polymastigina s.str'.
[Suborder Diplomonadina
[Suborder Hypermastigina
Class Sarcodina (Rhizopoda)
68
Order Amoebina
68
Order Foraminifera
72
Suborder Monothalamia
72
Suborder Polythalamia
74
Order Radiolaria
76
[Suborder Peripylaea (Spumellaria)]
[Suborder Actipylaea (Acantharia)]
[Suborder Monopylaea (Nassellaria)]
[Suborder Tripylaea (Phaeodaria)]
Order Heliozoa
83
Order Mycetozoa
86
Class Sporozoa
87
Subclass Telosporidia ^
88
Order Coccidiomorpha
88
Suborder Coccidia
89
Suborder Haemosporidia
91
^ The names of groups which are mentioned in the text but not under a
separate heading are here placed in square brackets.
(^orn
vm TABLE OF CONTENTS
Order Gregarinidea
page 93
Suborder Schizogregarinaria
93
Suborder Eugregarinaria
95
Appendix : Piroplasmidea
98
Subclass Neosporidia
100
Order Cnidosporidia
100
[Suborder Myxosporidia]
[Suborder Microsporidia]
[Suborder Actinomyxidea]
Order Haplosporidia
102
Order Sarcosporidia
102
Class Ciliophora
102
Subclass Ciliata
102
Order Holotricha (Aspirigera)
106
Suborder Prociliata
106
Suborder Astomata
107
Suborder Gymnostomata
107
Suborder Vestibulata (Hymenostomata) 109
Order Heterotricha
109
Suborder Polytricha
109
Suborder Oligotricha
no
[Tribe Tintinnina]
[Tribe Entodiniomorpha]
Order Hypotricha
III
Order Peritricha
III
Order Chonotricha
114
Subclass Suctoria
114
CHAPTER III
Subkingdom Parazoa
117
[Phylum Porifera]
Class Calcarea
126
Class Hexactinellida
126
Class Demospongiae
126
[Order Tetractinellida]
[Order Monaxonida]
[Order Keratosa]
[Order Myxospongiae]
CHAPTER IV
Subkingdom Metazoa 128
TABLE OF CONTENTS IX
CHAPTER V
Phylum Coelenterata
page 146
Subphylum Cnidaria
152
Class Hydrozoa
153
Order Calyptoblastea
154
Order Gymnoblastea
154
Order Hydrida
154
Order Trachylina
164
Suborder Trachomedusae
164
Suborder Narcomedusae
164
Order Hydrocorallinae
165
Order Siphonophora
166
Order Graptolithina
169
Class Scyphomedusae (Scyphozoa)
172
[Order Stauromedusae]
[Order Discomedusae]
Class Actinozoa (Anthozoa)
180
Order Alcyonaria
180
Order Zoantharia
186
Subphylum Ctenophora
193
[Class Ctenophora]
[Order Tentaculata]
[Order Nuda]
CHAPTER VI
Acoelomate Triploblastica
197
Phylum Platyhelminthes
198
Class Turbellaria
213
Order Acoela
213
Order Rhabdocoelida
214
Order Tricladida
214
[Suborder Paludicola
[Suborder Maricola]
[Suborder Terricola]
Order Polycladida
215
Class Trematoda
216
[Order Temnocephalea]
Order Heterocotylea
218
Order Malacocotylea
220
Class Cestoda
223
Order Monozoa
225
TABLE OF CONTENTS
Order Merozoa P^^^e 225
[Suborder Tetraphyllidea]
[Suborder Diphyllidea]
[Suborder Tetrarhynchidea]
[Suborder Pseudophyllidea]
[Suborder Cyclophyllidea]
CHAPTER VII
Phylum Nemertea ^33
Phylum Rotifera J^
Phylum Gastrotricha ^^
CHAPTER VIII
Phylum Nematoda ^
Phylum Nematomorpha ^57
Phylum Acanthocephala 5
CHAPTER IX
[Coelomata] ^^^
Phylum Annelida ^^^
Class Chaetopoda
Order Polychaeta ^^^
Order Oligochaeta ^^^
Class Archiannelida ^94
Class Hirudinea ^^^
Class Echiuroidea 3
Class Sipunculoidea 3 4
CHAPTER X
Phylum Arthropoda
CHAPTER XI
305
Subphylum Onychophora 3^7
Subphylum Trilobita 3 3
CHAPTER XII
Subphylum Crustacea ^2
Class Branchiopoda 353
Order Anostraca 35
[Order Lipostraca]
TABLE OF CONTENTS xi
Order Notostraca p(^g^ 360
Order Diplostraca 362
Suborder Conchostraca 362
Suborder Cladocera 362
[Tribe Ctenopoda]
[Tribe Anomopoda]
[Tribe Onychopoda]
[Tribe Haplopoda]
Class Ostracoda 368
Class Copepoda 370
Class Branchiura 376
Class Cirripedia 376
Order Thoracica 377
Order Acrothoracica 382
Order Apoda 382
Order Rhizocephala 382
Order Ascothoracica 385
Class Malacostraca 386
Subclass Leptostraca 390
Subclass Hoplocarida (Stomatopoda) 391
Subclass Syncarida 392
Subclass Peracarida 393
Order Mysidacea 393
Order Cumacea 393
Order Tanaidacea 395
Order Isopoda 395
Order Amphipoda 400
Subclass Eucarida 403
Order Euphausiacea 403
Order Decapoda 404
[Suborder Penaeidea]
[Suborder Caridea]
[Suborder Palinura]
[Suborder Astacura]
[Suborder Anomura]
[Suborder Brachyura]
CHAPTER XIII
Subphylum Myriapoda 418
Class Chilopoda 418
Class Diplopoda 422
[Class Pauropoda]
[Class Symphyla]
xii TABLE OF CONTENTS
CHAPTER XIV
Subphylum Insecta (Hexapoda)
page 425
Class Apterygota
463
Order Thysanura
463
Order CoUembola
463
Order Protura
465
Class Pterygota
466
Subclass Exopterygota
466
Order Orthoptera
466
[Suborder Cursoria]
[Suborder Saltatoria]
Order Dermaptera
468
Order Isoptera
469
Order Plecoptera
471
Order Embioptera
471
Order Psocoptera
471
Order Odonata
472
[Suborder Zygoptera]
[Suborder Anisoptera]
Order Hemiptera or Rhynchota
474
[Suborder Heteroptera]
[Tribe Cryptocerata]
[Tribe Gymnocerata]
[Suborder Homoptera]
[Tribe Auchenorhyncha]
[Tribe Sternorhyncha]
Order Ephemeroptera
481
Order Mallophaga
483
Order Anoplura
483
Order Thysanoptera
485
Subclass Endopterygota (Holometabola)
485
Order Neuroptera
485
Order Mecoptera
486
Order Trichoptera
486
Order Lepidoptera
487
[Suborder Homoneura]
[Suborder Heteroneura]
Order Coleoptera
492
[Suborder Adephaga]
[Suborder Polyphaga]
Order Hymenoptera
495
[Suborder Symphyta]
[Suborder Apocrita]
TABLE OF CONTENTS Xlll
Order Diptera
page 504
[Suborder Orthorrhapha]
[Suborder Cyclorrhapha]
Order Aphaniptera
512
Order Strepsiptera
5H
CHAPTER XV
Subphylum Arachnida
515
Class Scorpionidea
520
Class Eurypterida .
524
Class Xiphosura
526
Class Araneida
530
Class Acarina
533
[Order Notostigmata]
[Order Cryptostigmata]
[Order Prostigmata]
[Order Stomatostigmata]
[Order Heterostigmata]
[Order Parastigmata]
[Order Mesostigmata]
[Order Metastigmata]
Class Phalangida 537
[Class Palpigradi]
[Class Pedipalpi]
[Class Pseudoscorpionidea]
[Class Solifugae]
Class Pantopoda (Pycnogonida) 538
Incertae sedis:
Class Tardigrada
539
Class Pentastomida
541
CHAPTER
XVI
Phylum Mollusca
543
Class Amphineura
547
Order Polyplacophora
547
Order Aplacophora
548
Class Gasteropoda
550
Order Streptoneura (Prosobranchiata)
563
Suborder Diotocardia (Aspidobranchiata) 563 (564)^
Tribe Rhipidoglossa 563
Tribe Docoglossa 563
^ References in brackets are to the pages on which examples are described.
Xll TABLE OF CONTENTS
CHAPTER XIV
Subphylum Insecta (Hexapoda)
page 425
Class Apterygota
463
Order Thysanura
463
Order CoUembola
463
Order Protura
465
Class Pterygota
466
Subclass Exopterygota
466
Order Orthoptera
466
[Suborder Cursoria]
[Suborder Saltatoria]
Order Dermaptera
468
Order Isoptera
469
Order Plecoptera
471
Order Embioptera
471
Order Psocoptera
471
Order Odonata
472
[Suborder Zygoptera]
[Suborder Anisoptera]
Order Hemiptera or Rhynchota
474
[Suborder Heteroptera]
[Tribe Cryptocerata]
[Tribe Gymnocerata]
[Suborder Homoptera]
[Tribe Auchenorhyncha]
[Tribe Sternorhyncha]
Order Ephemeroptera
481
Order Mallophaga
483
Order Anoplura
483
Order Thysanoptera
485
Subclass Endopterygota (Holometabola)
485
Order Neuroptera
485
Order Mecoptera
486
Order Trichoptera
486
Order Lepidoptera
487
[Suborder Homoneura]
[Suborder Heteroneura]
Order Coleoptera
492
[Suborder Adephaga]
[Suborder Polyphaga]
Order Hymenoptera
495
[Suborder Symphyta]
[Suborder Apocrita]
TABLE OF CONTENTS Xlll
Order Diptera page 504
[Suborder Orthorrhapha]
[Suborder Cyclorrhapha]
Order Aphaniptera 512
Order Strepsiptera 514
CHAPTER XV
Subphylum Arachnida 515
Class Scorpionidea 520
Class Eurypterida . 524
Class Xiphosura 526
Class Araneida 530
Class Acarina 533
[Order Notostigmata]
[Order Cryptostigmata]
[Order Prostigmata]
[Order Stomatostigmata]
[Order Heterostigmata]
[Order Parastigmata]
[Order Mesostigmata]
[Order Metastigmata]
Class Phalangida 537
[Class Palpigradi]
[Class Pedipalpi]
[Class Pseudoscorpionidea]
[Class Solifugae]
Class Pantopoda (Pycnogonida) 538
Incertae sedis:
Class Tardigrada
539
Class Pentastomida
541
CHAPTER
XVI
Phylum Mollusca
543
Class Amphineura
547
Order Polyplacophora
547
Order Aplacophora
548
Class Gasteropoda
550
Order Streptoneura (Prosobranchiata)
563
Suborder Diotocardia (Aspidobranchiata) 563 (564)^
Tribe Rhipidoglossa 563
Tribe Docoglossa 563
^ References in brackets are to the pages on which examples are described.
XIV TABLE OF CONTENTS
Suborder Monotocardia (Pectinibran-
chiata)
p(^ge 563 (565)
Tribe Rachiglossa
563
Tribe Taenioglossa
563
Subtribe Platypoda]
Subtribe Heteropoda]
Tribe Toxiglossa
563
Order Opisthobranchiata
567
Suborder Tectibranchiata
567
Suborder Nudibranchiata
567
Order Pulmonata
569
Suborder Basommatophora
570
Suborder Stylommatophora
570
Class Scaphopoda
572
Class Lamellibranchiata
573
Order Protobranchiata
579 (582)
Order Filibranchiata
579 (583)
Order Eulamellibranchiata
579 (585)
Order Septibranchiata
579
Class Cephalopoda (Siphonopoda)
587
Order Dibranchiata
588
Suborder Decapoda
588
Tribe Belemnoidea
588
Tribe Sepioidea
588
Tribe Oegopsida
588
Tribe Myopsida
589
Suborder Octopoda
589
Order Tetrabranchiata
602
Suborder Nautiloidea
602
Suborder Ammonoidea
602
CHAPTER XVII
Phylum Polyzoa
606
Class Endoprocta
612
Class Ectoprocta
613
Order Phylactolaemata
613
Order Gymnolaemata
613
Suborder Cyclostomata
613
Suborder Cheilostomata
613
Suborder Ctenostomata
613
Phylum Brachiopoda
613
Class Ecardines
618
Class Testicardines
618
Phylum Chaetognatha
618
Phylum Phoronidea
622
TABLE OF CONTENTS
XV
CHAPTER XVIII
Phylum Echinodermata P^g^ 623
[Subphylum Eleutherozoa]
Class Asteroidea 634
Class Ophiuroidea 638
Class Echinoidea 640
Order Endocyclica 647
Order Clypeastroida 647
Order Spatangoida 647
Class Holothuroidea 648
Order Aspidochirotae 652
Order Pelagothurida 652
Order Elasipoda 652
Order Dendrochirotae 652
Order Molpadida 652
Order Synaptida (Paractinopoda) 652
[Subphylum Pelmatozoa]
Class Crinoidea 654
Class Amphoridea 658
Class Carpoidea 658
Class Thecoidea (Edrioasteroidea) 658
Class Cystoidea 658
[Order Diploporida]
[Order Rhombifera]
Class Blastoidea 659
CHAPTER XIX
Phylum Chordata 660
Subphylum Hemichorda (Enteropneusta s.lat.) 662
[Class Enteropneusta (Balanoglossida)]
[Class Pterobranchia]
Subphylum Tunicata (Urochorda) 669
Class Larvacea 679
Class Ascidiacea 680
Class Thaliacea 681
Order Pyrosomatida (Luciae) 685
Order Salpida (Herhimyaria) 685
Order Doliolida (Cyclomyaria) 686
[Subphylum Cephalochorda]
[Subphylum Vertebrata]
CHAPTER I
INTRODUCTION
The Invertebrata have long since ceased to constitute one of the
primary divisions in the scientific classification of the Animal King-
dom. Their name is now no more than a convenience for designating
a group of phyla with which it is often necessary to deal as a whole.
The primary lines of real cleavage in the Animal Kingdom divide it,
not into Vertebrata and Invertebrata, but into three unequal sec-
tions, the Protozoa, Parazoa and Metazoa, which are ranked in the
following chapters as subkingdoms.
Between the Protozoa, which are without cellular differentiation
and contain a large group of photosynthetic members, and the Meta-
zoa, in which such differentiation is always strongly marked and
photosynthesis is absent, there is a gulf which is in fact far deeper
than that which sunders the Protozoa from the lower plants. The view,
indeed, has been put forward that these two components of the
Animal Kingdom are not, as is usually held, directly related to one
another, but arose, with the Plants, as entirely distinct branches of an
ancestral stock of living beings. The Parazoa or sponges — unique
among many-celled organisms in possessing collared flagellate cells —
are probably derived from the Protozoa by an origin distinct from
that by which the latter group gave rise (if they did so indeed) to the
Metazoa.
Within the Metazoa, the most significant difference is that which
exists between the Coelenterata or Diploblastica and the triploblastic
phyla which constitute the rest of the subkingdom. The Coelenterata,
which typically start life as a simple, two-layered, ciliate larva, the
planula, either retain throughout life the two-layered condition, or
depart from it only by the immigration, late in development, of cells
from the two primary layers (ectoderm and endoderm, p. 128) into
the space (blastocoele) between those layers. The triploblastic animals
always possess a true third layer (mesoderm) which is early developed
and forms important organs. They are the great majority of animals,
and compose a number of phyla.
The brigading of these phyla is a difficult task — one, indeed, which
it is at present impossible to effect completely. Two main stocks,
however, stand out fairly clearly. The Annelida, Arthropoda and
Mollusca — by the plan of their central nervous system, the mode and
position of origin of their mesoderm, the types of cleavage of the ovum
(p. 281) and of larva (the ^roc^oj'/)^er^) which the Annelida and Mollusca
2 THE INVERTEBRATA
share, and the presence of a cuticle and segmentation which the Anne-
lida and Arthropoda have in common — constitute one of these stocks.
The other comprises the Echinodermata and Chordata. Its members
have a central nervous system which is not on the annelid plan and is
peculiar in retaining its epithelium (p. 136); they exhibit a common
mode of origin of the mesoderm, primitively as hollow pouches, from
the gut wall ; they possess, or give indications of, three primary meso-
dermal segments ; the cleavage of their ova is entirely different from
that which is characteristic of the Annelida and MoUusca, and
between the larvae of the lowest chordates (the Enteropneusta) and
those of certain echinoderms there is a remarkable and detailed re-
semblance.
The remaining phyla, smaller and less important, are hard to
relate either to the foregoing groups or to one another. By the type of
cleavage of their ova and the possession of flame cells (p. 202), the
Platyhelminthes and Nemertea seem to be akin to the annelid stock.
Their lack of coelom is a difficulty in this respect. The structure of the
adults of the Rotifera and of the larva of the Polyzoa, which has the
character of a trochosphere, might link these groups to the same stock.
Some other small phyla (Brachiopoda, Chaetognatha) have possibly
distant relationship to the echinoderm-chordate grouping. Others,
notably the Nematoda, are more difficult to place.
In the great assemblage of triploblastic phyla, the backboned
animals, or Vertebrata properly so-called, stand as a branch of one
phylum, the Chordata. Yet their considerable numbers, the size,
high organization, and intelligent activity of their members, and the
fact that Man is one of them, give them an importance so great that
they have always been the subject of a distinct department of zoo-
logical study, and were at one time regarded as a primary branch of
the Animal Kingdom. That standing they have lost; but it is still
necessary for many purposes to treat them apart.
The term " Invertebrata" is retained to cover all the non-chordate
phyla and the chordates other than the Vertebrata. In that sense it
is used in this book. Only the Cephalochorda (Amphioxus), which,
though they are not vertebrates, have much in common with those
animals, are left aside as best studied with them.
The limits of the several phyla are, with one or two exceptions,
agreed among zoologists. As much cannot be said for the lower
grades of the classification. Different views upon phylogeny, and
considerations of convenience, lead to many divergences as to the
extent and rank of the various divisions in the systems preferred by
different authorities; and even when there is agreement as to the
limits of a group different names may be applied to it. In no two
works will quite the same arrangement be found. This fact should
ORGANISM AND ENVIRONMENT 3
be borne in mind by the student in using the table of classification
which will be found as the Table of Contents at the beginning of
this book.
In surveying the diverse organisms which constitute the Inverte-
brata, the student should bear in mind the following principles.
The most fundamental of the characteristics of living organisms is
the way in which, in the face of an environment which presents as
many dangers as opportunities, they hold their own by making ad-
justments within themselves. This statement applies equally to the
struggle for existence of the individual and to the slow racial adjust-
ments which we know as evolution.
The term environment^ is a collective name for all the external
things which affect any living being. Four principal factors constitute
the environment — the ground or "substratum" (if any) upon which
the organism stands, the "medium" (water or air) which bathes it,
the heat and light which it receives from the sun's rays or can lose to
its surroundings, and the other organisms in its neighbourhood. Of
these factors the substratum has in most cases relatively little im-
portance, and we may dismiss it now.
The medium, on the other hand, is of enormous importance. Meet-
ing all parts of the surface of objects that it contains, it exerts every-
where a pressure upon them, supports them, may transport them,
affects the movements they execute, and controls all exchange,
whether of matter or of energy, between them and the world about
them ; and from it animals obtain their supplies of free oxygen, often
of water, and sometimes of food. If it be liquid, according as the
concentration of substances dissolved in it be greater or less than that
within the organism water and solutes will tend to pass to or from the
body of any animal which is not covered by an impermeable cuticle.
This exchange is of the utmost importance, both as a danger by up-
setting equilibria within the body and as an advantage by facilitating
the excretion of substances which are harmful in the organism. It is
controlled by the surface layer of protoplasm, which either is, or
owing to surface tension behaves as, a delicate membrane that has the
power of actively regulating, to some extent, the passage of substances
through itself, and by the activity of the organs of excretion. If the
medium be gaseous, according to the amount of water vapour it
contains water will tend to evaporate to it from the surface of the body.
This is important owing to the necessity for the intake of water by the
^ The student may occasionally be puzzled by the phrase "internal en-
vironment". This bizarre contradiction in terms is sometimes applied to what
we shall presently call the "internal medium" (p. 132).
4 THE INVERTEBRATA
mouth to compensate for it, and also because the latent heat of the
evaporation lowers the temperature of the body. Whether the medium
be liquid or gaseous, it offers, according to the free gases it contains,
varying possibilities of interchange of oxygen and carbon dioxide
with organisms. This has naturally extremely important effects upon
respiration.
The loss or gain of heat tends, of course, to affect the temperature
within organisms, and with this the chemical processes of the latter
vary, being, as is usual in such processes, slowed as the protoplasm
becomes colder and quickened when it is warmed, and being brought
finally to a stop when its minute organization is destroyed either by
the coagulation of certain of its proteins by heat or by the freezing of
its water. Every organism is tuned to work within a range of tem-
perature peculiar to it. "Warm-blooded" animals keep their tem-
perature within proper limits by active chemical and physical means;
"cold-blooded" animals (to which all invertebrata belong) are in this
respect at the mercy of their surroundings except in so far as they
can circumvent them by their habits. Light, while it is essential for
photosynthetic organisms, has chemical effects of importance in many
others, and in all which possess sense organs capable of appreciating
it is a source of stimuli from the external world.
Relations between an animal and other organisms in its surround-
ings are almost always based in the long run on nutrition. Either such
organisms serve the animal for food, or they attack it to make it their
food, or they are competitors for a common food-supply, or in rarer
cases they assist it, or obtain its assistance, in the quest for food or in
defence against enemies which would use it or them for food. Only
between members of opposite sexes of the same species are there
relations of another kind, namely those which are concerned with
reproduction. The coming of organisms into relation with one another
usually involves the receipt of stimuli and more or less complicated
behaviour, with the use of organs of locomotion and prehension.
The action of the environment upon the organism will be seen to
be threefold: (i) it affects it mechanically, as by transporting it from
place to place, by the impact of adjacent objects, or by the attacks of
enemies ; (2) it affects the working of the living machine by the com-
pulsory introduction or abstraction of materials (water, salts, etc.) or
of energy ; (3) it directly stimulates it to activity, which may be an in-
evitable response, such as the movement of certain organisms towards
light (phototaxis) or be dependent upon conditions existing at the
moment in the organism ; or it may inhibit such activity. Besides such
action the environment may affect the organism negatively, by failure
in respect of food, oxygen, or some other necessity which the organism
is dependent upon obtaining from its surroundings. Where such
ORGANISM AND ENVIRONMENT 5
failures occur from time to time the organisms have usually means of
enduring them (reserve stores, resting stages, etc.).
In proportion as the organism is unable to resist these influences
of the environment it is liable upon occasion to be harmed by them.
The process of evolution has been the development of organisms in
such a way as to set them free from such influences in respect of their
proper environments. Its results may be classed under three heads,
(i) Some, such as the acquirement of a cuticle or of a habit of burrow-
ing or of hibernation, merely fend off or avoid the action of the en-
vironment : these involve the least increase in the complexity of the
organism. (2) Others, such as the formation of organs for the ex-
cretion of the excess of water, provide for remedial action: in these,
as a rule, more complicated machinery is formed. (3) Others, such
as the development of a nervous system or of organs of locomotion
or weapons of oifence, bring it about that the action which results
from the receipt of stimuli is turned to the best advantage by the
organism: these cause a considerable, often a very great, complication
of the living machine.
Thus a general outcome of evolution is the forming of more com-
plex, that is of " higher", organisms. But a relatively simple organism
may, in its proper environment, enjoy as much autonomy as in other
circumstances is possessed by one that is more highly organized. This
is notably true of many parasites.
Some of the results of evolution, as for instance the formation of a
nervous system or of a cuticle, are such as to increase the independence
of the organism in all circumstances. Others, however, such as the
substitution of pulmonary for branchial respiration, or of absorption
for ingestion of food, are of value only in particular environments or
modes of life, and even unfit the organism for other ways of living.
Thus two distinct phenomena underlie the diversity of the Animal
Kingdom — an increase in the autonomy of the individual, and the
specialization of animals for particular modes of life.
Every species, however good a fight it maintains, is threatened with
extinction owing to the continual loss of individuals, always by the
action of its environment and usually also by that "natural death"
which appears to await all organized protoplasm that is not periodic-
ally reorganized.^ In reproduction^ howev^er, the individual provides
by fission for the maintenance of its race. In the lower organisms the
protoplasm of the body retains ascertain plasticity, and in these there
is very often an asexual process of reproduction in which the new
construction that is necessary to organize at least one of the products
^ See p. 27. It is possible that in some of the least highly organized
metazoa natural death either is no more inevitable than in protozoa or is long
delayed.
6 THE INVERTEBRATA
of fission, and often goes on in both, is carried out with cells (or, in
protozoa, with organized protoplasm) which existed as such in the
parent. In more highly organized animals the only protoplasm which
retains the required plasticity is that of the germ cells, and conse-
quently such animals have only the sexual reproduction which these
cells perform. The germ cells (gametes), before they reconstitute the
adult body, normally undergo the process known as conjugation or
syngamy, which is not an essential part of the reproductive process
but a provision for heritable variation whereby the race becomes
adaptable to its surroundings. Conjugation can only be performed by
uninucleate individuals and therefore, while in protozoa it sometimes
takes place between adults (hologamy, p. 31), in metazoa it always
requires the production of uninucleate young (the ova and sperma-
tozoa). The lower metazoa reproduce both by means of these gametes
and also asexually . In the higher animals, as we have seen, reproduction
is solely by gametes, though conjugation may be suspended for one
or more generations by the development of unfertilized ova (partheno-
genesis).
CHAPTER II
THE SUBKINGDOM PROTOZOA
The Protozoa are sundered from the rest of the Animal Kingdom by
a perfectly sharp distinction. The distinction consists in this : that in
the body of a protozoon, whether there be one nucleus, or a few, or
many, no nucleus ever has charge solely of a specialized part of the
cytoplasm; whereas in other animals there are always many nuclei,
each in charge of a portion of cytoplasm which is specialized for a
particular function, such as contraction, or conduction, or secretion.
Stated thus, the definition of the Protozoa is quite unambiguous.
Unfortunately, ambiguity is usually imparted to this subject by the
introduction of a concept, that of the "cell", which has a different
extension for different authorities. If that concept, primarily of use
in other connections, is to be introduced here, we must frame our
definition in one of two ways, according to the meaning which we
attach to the word "cell". If we apply this term to every nucleus
together with its cytoplasm, we must define the Protozoa as " animals
which consist of one cell or of several cells which are all alike, save
sometimes the reproductive cells ". If, on the other hand, we give the
term "cell" its earlier extension, applying it only to the specialized
units of nucleus and cytoplasm which together compose the bodies
of the higher animals and plants, we shall define the Metazoa as
"cellular animals" and the Protozoa as "non-cellular". It will then
be convenient to employ the term "energid" for application to any
nucleus with its cytoplasm, whether they together constitute the body
of a protozoon or a cell of a metazoon.
In any case the facts remain the same, and they provide one of the
main sources of the interest which the study of the Protozoa offers,
namely the carrying out of the processes of life, and often of a complex
life, by an organization which, though it may visibly be of correspond-
ing complexity, is purely cytoplasmic. Considered in this light the
structure of, for instance, the more complicated ciliates and flagellates
is exceedingly instructive. In three other respects the Protozoa are
peculiarly interesting. In their bodies dead " formed " material, how-
ever plentiful it be as a covering or scaffold for the body, never as-
sumes the importance which it has as ground substance or skeleton in
the Metazoa, where the size of the body is such that the protoplasm
cannot maintain its organization without support against forces that
tend to deform it. Consequently, in observing the physiology and
behaviour of a protozoon, we are seeing in the actual protoplasm of an
8 THE INVERTEBRATA
intact organism processes which in an intact metazoon we observe as the
activities of a complex in which protoplasm is masked and conditioned
by other components of the body : in short, in the Protozoa we observe
the normal activities of protoplasm more directly than in the Metazoa.
Again, a life cycle comprising more than one generation, which is com-
paratively rare among metazoa, is universal among protozoa, and its
varieties are extraordinarily interesting. Finally, while every meta-
zoon is thoroughly an animal, the Protozoa present an unbroken series
from wholly plant-like organisms, through various intermediates, to
members whose nutrition and behaviour are those of animals — or
rather, as we shall see, there are several such series.
The Protozoa are all of small size. Most of them are minute, ranging
from a few thousandths of a millimetre to a little over one millimetre
in length: a few reach dimensions of several, or even of many centi-
metres, but these for the most part consist of a relatively thin layer of
protoplasm (certain mycetozoa). With the small size of protozoa is
probably to be connected, not only, as we have seen, the relative un-
importance to them of dead skeletons, but also their characteristic type
of organization. In larger organisms, the regions differentiated for
special purposes must usually be correspondingly larger, and therefore
require the services of nuclei of their own, the absence of which is the
hall-mark of a protozoon. The actual size varies very much in each
group. It is, on the average, least in the Mastigophora. The order of
magnitude of certain representative species may be gathered from the
approximate magnifications stated for figures below.
The bodies of the Protozoa vary greatly in shape. Whereas in each of
the metazoan phyla there is a fundamental type of body form to which
the members of the phylum conform in essentials, however aberrant
from it they be, the Protozoa have no such type. When the surface of
the protoplasm is virtually fluid and is not retained by a shell, it takes,
while it is at rest, a spherical form. When there is a firm surface layer,
the individual tends, if it be a flagellate, to have an egg or spindle shape,
if it be a ciliate to be bilateral with a spiral twist at one end, in the
Suctoria to be cup-shaped. Concerning the body form of the Sporozoa,
which are parasitic, no generalization can be made. Bodies of any of
these shapes may be anchored, and have then usually a stalk, which
may be of dead material as, for instance, in Acineta and Codosiga
(Figs. I, 49), or a part of the living protoplasm. In the latter case it
has generally a cuticular covering, as in Vorticella (Fig. 2), but may
be naked (various flagellates).
Stalked forms, and occasionally others, may be colonial; that is to
say, a number of zooids, each having a nucleus and the shape and
complete organization of an individual of related solitary species, are
united by protoplasmic connections to form a single living being. The
PROTOZOA
^vest.
^--myn
Fig. 2.
Fig. I.
Fig. I. Two species of Actneta, x loo. After Saville-Kent. A, A. grandts.
B, A. lemnarum.
Fig. 2. Vorticella: a generalized figure of an individual, magnified, with a
portion of the stalk further enlarged, c.vac. contractile vacuole; ci. outer
ciliary wreath; ci.' inner wreath, of the peristome; cu. cuticle] f.vac. food
vacuole; ?neg. meganucleus; mi. micronucleus ; myn. myonemes of bell; myn/
myoneme of stalk (containing elastic fibrils); pel. pellicle; ph. "pharynx"
(terminal portion of vestibule); ppm. protoplasm of stalk; pst. peristome;
res. reservoir; i/.?He. undulating membrane; vest, vestibule.
lO
THE INVERTEBRATA
zooids of a colony are usually all alike, but differentiation may exist
between them, in that certain of them are specialized for the produc-
tion of new colonies, which is not performed by the other zooids
(various volvocina. Figs. 3, 46 ; etc.). Colonies arise by the division of
a single primary zooid, whose fission is not carried to completion, so
that its products do not entirely separate. Their origin is therefore
usually said to be a form of asexual reproduction. It may, however,
also be looked upon from another point of view, as the repetition,
within a continuous mass of protoplasm, of the nucleus and the other
organs coincidentally. In this aspect, the colony is seen to have
features in common with other multinucleate conditions of protozoa,
such as (i) that oi Hexamitus (Fig. 4), etc., in which a unitary body has
SomAttc cctU
Fig. 3. Colonial volvocina. a, Eiidorina, a spherical motile colony of thirty-
two similar zooids all capable of division, b, Pleodorina illinoiensis , a spherical
motile colony consisting of thirty-two zooids, of which" four at one end of the
colony constitute a "soma", which dies when the other twenty-eight zooids
divide, c, Pleodorina californica. The "somatic cells" constitute about half
the colony. After West and Fritsch.
two similar sets of organs, one on each side of the body, or several sets,
with a nucleus assigned to each, (2) that oi Polykrikos (Fig. 40 B), etc.,
in which there are several nuclei, and several sets of the other organs
of the body, but the repetition (merism) of the nuclei and that of the
other organs do not correspond, and (3) that of Opalina (Fig. 5),
Actinosphaeriwn (Fig. 33), etc., in which there are numerous nuclei,
but only one set of the other organs of the body. Multinucleate
masses of protoplasm are known as syncytia. Syncytia which, like
those cited above, arise by the division of an original nucleus in the
mass of protoplasm are known as symplasts. An entirely different kind
of syncytium arises by the union of uninucleate individuals, whose
nuclei remain distinct in the resulting body. Such syncytia are known
PROTOZOA
II
as Plasmodia. They are found in the Mycetozoa (Fig. 73) and
occasionally elsewhere.
Pseudocolonies, consisting of distinct individuals united only by
stalks, tubes, etc., of dead material, are formed by various mastigo-
phora (Fig. 38) and vorticellids.
Fig. 5-
Fig. 4. Hexamitus i?itestinalis, x 3800. After Dobell. axs. axostyle; ba.gr.
basal granules of flagella; nu. nucleus.
Fig. 5. Opalina ranarian, x 150. From Bronn. ecp. ectoplasm; 7iu. nuclei.
The, protoplasm of living protozoa is often apparently homogeneous,
apart from inclusions such as granules of reserves and other manu-
factured substances, chromosomes, skeletal structures, and so forth.
Sometimes, however, there is visible in it an apparent meshwork of
12 THE INVERTEBRATA
a more viscid substance, with a more fluid constituent in its meshes.
Actually, the structure is then that of a foam, and the meshwork is an
optical section of the walls of bubbles or alveoli which contain the
liquid constituent. In the nucleus the more viscid constituent is the
linin meshwork, the liquid the karyolymph', in the cytoplasm the
meshwork is the spongioplasm, and its contents the enchylema} The
gelation to which this structure is due is produced by fixing reagents
in many cases in which it does not exist in life. There is, perhaps, no
fundamental distinction between the alveoli and the smaller of the
spaces known as vacuoles of which so much use is made in the physio-
logy of the Protozoa — for storage, as the site of chemical processes
such as digestion, for drainage, for hydrostatic functions, etc. The
largest vacuoles have often a definite wall of their own.
The surface of the protoplasm is protected in various ways.
{a) Sometimes, as in some amoebae, it is apparently quite fluid.
Then, however, there exists upon it an extremely thin membrane,
known as the plasmalemma, which has the power of regulating the
exchange of materials between the organism and the watery medium
in which it lives. Without this power the protoplasm would soon be
poisoned or dissolved, {b) In other cases, the surface layer is semi-
solidified (gelated) as a visible, firm, but living pellicle. This is often
** sculptured" in a pattern, as in Paramecium (Fig. 85 B, C). (c) Inter-
mediate conditions connect the pellicle with the cuticle^ a. close-fitting
dead membrane which may be nitrogenous, as in Monocystis, or of
carbohydrate, as in many plant-like flagellates. In typical dino-
flagellates (Fig. 40 A) it is composed of stout plates of cellulose.
(d) Again, there may be a shell from which protoplasm can issue
through an opening. Such a shell may be nitrogenous, as in Arcella
(Fig. 59), etc., of a nitrogenous basis with foreign bodies built into
it, as in Difflugia and Rhabdammina (Figs. 60, 6 A), of siliceous plates
as in Euglypha (Fig. 7), calcareous, as in most foraminifera (Fig. 6
B, C), or of cellulose, as in the spores and sclerotium of the My-
cetozoa. It is said that mineral shells always contain a groundwork
of organic material. They are often composed of several chambers,
and may be perforated by numerous small pores. Houses are loose-
fitting, wide-mouthed shells (Fig. 38 C). Cysts are temporary shells
without opening, {e) Finally, there may be an external lattice, which
is pseudochitinous in Clathrulina (Fig. 8) and siliceous in the
Silicoflagellata (Fig. 38 F), or a case of calcareous pieces (Coccolitho-
phoridae. Fig. 38 E). The siliceous lattice of many radiolarians is part
of an internal skeleton.
The term ectoplasm is applied to any conspicuously differentiated
outer layer of the protoplasm, and denotes very different conditions
^ The term hyaloplasm has been used in this, but also in other, senses.
PROTOZOA
13
in different organisms — in Amoeba, a stratum which, save at its
surface, is only unlike that below it in not containing granules; in
various planktonic protozoa (Figs. 32, 33) a highly vacuolated layer
Fig. 6. Shells of foraminifera. A, Rhabdarrmmia abyssorum, x 4-5. B, Nodo-
saria hispida, x 18. C, Globigerina bulloides, x 55. After various authors.
whose low specific gravity confers buoyancy; in the Ciliophora and
many flagellates and sporozoa a stout pellicle with an underlying layer,
the cortex, which is said to be stiffer than the internal protoplasm
{endoplasm) and may exhibit differentiations of various kinds.
H
THE INVERTEBRATA
Occasionally the protoplasm contains structures (trichocysts of
ciliates and mastigophora, Fig. 9, so-called ''nematocysts" in certain
dinoflagellates, Fig. 40, pole capsules of neosporidia, Fig. 82), from
which threads can be shot out upon the surface of the body. The
function of these threads is often doubtful, but it has been shown that
the trichocysts of Paramecium are fixing organs, others which lie
around the mouth of their possessor {Cyathomonas, Fig. 39 C; etc.)
seize prey, and the pole capsules serve to anchor spores to the lining
of the host's gut. The threads of " nematocysts " and pole capsules
Fig. 7. Euglypha alveola ta, x 60. From Hegner and Taliaferro, after Calkins.
are coiled up in vesicles before they are shot out ; those of trichocysts
are formed by the stiflening of an extruded secretion.
The motile organs of the Protozoa are of several kinds, each of which
is mainly found in one of the classes of the phylum. Pseudopodia are
temporary protrusions of protoplasm. They are of various types —
blunt lobopodia (Figs. 54, 59), fine filopodia (Fig. 7), branching and
anastomosing rhizopodia (Figs. 61, 65), and axopodia (Fig. 71) with
an internal supporting filament. They are used in various ways and
for various purposes. Their mode of formation is not fully understood,
but it is clear that, at least in many cases, they do not arise, as has been
PROTOZOA
15
alleged, by alterations in the surface tension of the protoplasm, and
it is probable that the movement {amoeboid movement) in the course
of which they are formed is not fundamentally different from the
movements of muscles, or cilia, or flagella. Granules may often be
seen to stream up and down the axopodia and rhizopodia. Flagella
are lashes, long and usually few in number, which by a rowing
(Fig. 10) or by an undulating motion (Fig. 11) draw or propel the
body or attract particles to it. In the rowing stroke the flagellum is
^,^r«rjn.
-^h'.
-nuc.
Fig. 8.
Fig. 9.
Fig. 8. Clathritlina. A, Ordinary individual, x 200. B, Binary fission within
the lattice. C, Escaped fiagellate individual. After Leidy.
Fig. 9. Part of a longitudinal section through a Paramecium showing the
trichocysts at the end of the body discharged, and in the endoplasm material
for the replacement of trichocysts. From Saunders, after Mitrophanov.
71UC. meganucleus; tr. undischarged trichocysts; tr.' discharged trichocysts;
tr.m. material for replacing trichocysts.
held rigid and slightly concave in the direction of the stroke; in
recovering its position it bends as it is drawn back, so that less
resistance is offered to the medium. When, as is usually the case, the
flagellum beats obliquely, or the undulations pass around as well as
along it, the body rotates as it advances, or if it be fixed a whirlpool
is set up. Down each flagellum runs an internal thread, the axial
filament^ which, on entering the body or at some distance within it,
joins a basal granule.^ The latter is in most cases connected to the
^ This structure is sometimes called the blepharoplast, but as that name has
also been applied to parabasal bodies its use is best avoided.
.'-10
^ -11
■^--12
13
Fig. lo. Simplest type of movement of flagellum of Monas during rapid
forward movement, a, 1-7. Successive stages in preparatory stroke. Note the
flexure begins at the base and spreads to the tip. b, 8-13. Successive stages
in the effective stroke. Note the rigidity of the flagellum. The arrow indicates
the direction of movement of the organism. After Krijgsman.
Fig. II.
m
Fig. 12.
Fig. II. Peranema. a, Slow forward movement with undulations restricted
to the tip of the flagellum. 6, Rapid forward movement with undulations
along the whole length of the flagellum. After Verworn.
Fig. 12. o, Flagellum of Tracheloynonos (Euglenoidina) showing axial filament
and sheath. 6, Transverse section of the flagellum. After Doflein.
PROTOZOA 17
nucleus by a thread or threads known as rhizoplasts (Fig. 47 A).
Sometimes it lies against the nucleus. Rhizoplasts may connect it
to other structures, notably in many parasitic flagellates to a body of
unknown function called the parabasal body. The " kinetonucleus "
of Trypanosoma (Fig. 47 E) is a body of this class, which possibly in-
cludes structures of more than one kind. Sometimes, as in Trypano-
soma^ a flagellum runs for some distance parallel with the surface of
the body and is connected to it by a film of protoplasm known as an
undulating membrane^ which must be distinguished from the structures
of the same name which are formed by the fusion of cilia. When there
are two flagella, it often happens that one is trailed behind the body
and the other directed forwards (Figs. 47 D, 53 C, 70 B). Flagella are
often used for anchoring, and sometimes appear to have a sensory
function. Cilia are smaller and more numerous lashes which by a
rowing action repeated by one after another of them in "metachronal
rhythm " (Fig. 13) cause movements of the animal or of the water near
it. Like flagella they have each an internal filament, a basal granule,
and a rhizoplast, which, however, does not connect with the nucleus.
Often cilia are united into compound organs, which may be conical
am, paddle-like membranellae (Fig. 14), or undulating membranes
(Fig. 90). Many protozoa which possess a definite body form are able
temporarily to alter it by contractions of the protoplasm stretching
the pellicle (metaboly), and in various cases this contractility is
localized in fibrils, known as myonemes^ situated in the ectoplasm.
Systems of fibres which ramify from a central mass known as the
"motorium" and have been thought to be of the nature of a nervous
system have been described in various ciliates ; in some of these cutting
the fibres is said to destroy the co-ordination between different sets
of ciliary organs. It is possible also that the rhizoplast system of
flagellates may have a conducting function. Setise organs are possessed
by various protozoa in the form of specialized flagella and cilia in
which the tactile sense is highly developed, and by many of the plant-
like flagellates as pigment spots (eye-spots), which may be provided
with a lens. A chemical sense seems to be indicated by the fact that
food is often recognized at a distance, and also probably in some of the
cases of discrimination in ingestion (p. 19).
Internal skeletal structures are found in many members of the
phylum. They may be part of the living protoplasm, as the axial fibres
of axopodia and the axostyles which lie in the midst of the body of
various mastigophora (Trichomofias, Fig. 50; etc.) and probably the
central capsules of the Radiolaria, or of dead inorganic matter, as the
skeletons of the Radiolaria (Fig. 69).
The Protozoa present every type of nutrition exhibited by organ-
isms, except that of the "prototrophic" bacteria, which perform
i8
THE INVERTEBRATA
chemosynthesis by the use of energy obtained from reactions between
inorganic substances. Holophytic nutrition/ however, is found, among
protozoa, only in certain of the Mastigophora (see below). Of the
holozoic members of the phylum, some feed by amoeboid action.
Fig. 13-
- - - vib.
_«L .^ha.lam.
■- tl.fi.
Fig. 14.
ha.fi.
Fig. 15-
Fig. 13. Diagram illustrating the optical appearance given by a profile view
of cilia beating in metachronal rhythm. After Verworn.
Fig. 14. Three membranellae from the adoral wreath of Stentor. After
Doflein. ha.fi. basal fibre of the rhizoplast system; ha.lam. basal lamellae;
tl.fi. terminal fibres; vih. vibratile elements; the band beneath each of these
represents the fused basal granules of the constituent cilia.
Fig. 15. Paramecium, showing the motorium lying near the vestibule, and the
fibrils which radiate outwards. After Rees.
^ The nutrition of an organism is said to be holophytic when it is eflfected,
as in typical plants, by the building up of complex organic substances from
simple inorganic ones by use of the energy of certain of the sun's rays (photo-
synthesis). The radiant energy is obtained by means of the green, yellow, or
brown structures known as " chromatophores " or "chromoplasts" (e.g. the
PROTOZOA 19
Usually this can be done at any point of the surface, as in the familiar
case of Amoeba^ but most of those flagellates which perform amoeboid
ingestion do so in a particular region only. Other protozoa swallow
through a permanent mouth. The true mouth is the spot at which
the food passes below the ectoplasm. It may be (a) a bare patch of
endoplasm, (b) the opening of an excavation (oesophagus) in the
endoplasm, (c) the bottom of a depression (vestibule) in the ecto-
plasm, (d) the junction of a vestibule and an oesophagus. Any
passage, whether oesophagus, or vestibule, or compounded of both,
through which food enters is called a gullet, though not all cavities to
which this name is applied are actually used in feeding. The opening
of a gullet is the cytostome, which when there is a vestibule is not the
true mouth. Gullets are found in many of the Mastigophora and most
of the Ciliata. In ciliates either of the kinds may be present (p . 1 04) . In
the Mastigophora the gullet is at least sometimes ectoplasmic, but its
morphology needs further investigation. A gullet may be supported
by skeletal rods (Figs. 39 E, 89 A), and is then often dilatable: a
vestibule may have ciliary apparatus, trichocysts, etc. for taking food
(Figs. 39 C, 90). The Suctoria (Fig. i) draw the protoplasm of their
prey into their bodies through tentacles. The details of ingestion into
the protoplasm differ considerably in diflFerent organisms. In some
amoeboid forms the cytoplasm comes into contact with the food at
once, either by flowing over it or by its adhering to the surface and
being drawn in ; others enclose the particles to be swallowed without
touching them, either by arching over them, as Amoeba proteus does,
or by excavating a vacuole for their reception. In some at least of the
organisms whose food is driven into a gullet, a vacuole forms for it,
apparently by the pressure of the water forced in, and on reaching a
certain size nips off. Often, but by no means always, discrimination
is exercised between particles which appear equally capable of being
swallowed. It is doubtful whether this discrimination is concerned
solely with such properties as the size and shape of the particles or
also with their chemical qualities. Solid food is digested in food
vacuoles, which usually contain visible fluid and in which the reaction
is often first acid and then alkaline. Live food dies during the acid
phase, and protein is digested during the alkaline phase. Protozoa have
chloroplasts of green plants). In this mode of nutrition the simple materials
of the food are absorbed through the surface of the body. In holozuic nutrition
complex organic substances are swallowed through temporary or permanent
openings as in the majority of animals. In saprophytic (or saprozoic) nutrition,
practised by certain organisms, including among others various parasites,
which are in contact with solutions of organic matter, relatively complex
carbon compounds are taken, but these are absorbed through the body sur-
face. The modes of nutrition classed under this head vary greatly in the
complexity of the substances they require.
20 THE INVERTEBRATA
not been shown to digest fat, but can usually dissolve starch, and some-
times cellulose. The latter faculty becomes of great importance when
they are symbionts in the alimentary canal of animals whose food
consists of plant tissues (pp. 68, m). In a few cases {Balaiitidium,
some Amoebae) contractions of the protoplasm divide large morsels
into fragments. Often, but not, for instance, in foraminifera, the
food vacuoles circulate in the cytoplasm ; sometimes they do this along
a regular track. Their circulation is often due to streaming of the
endoplasm, but sometimes (ciliates, etc.) it is brought about by
peristalsis of the cytoplasm. Defaecation of the indigestible remains
of food takes place at any part of the surface when there is no pellicle,
but in pelliculate forms at a fixed spot. Sometimes there is a per-
manent rectal passage lined by ectoplasm (Fig. 88 B, an). Saprophytic
forms range from some which can subsist on mixtures of substances
as simple as aminoacids and acetates (or even, as Polytoma can, upon
ammonium acetate alone), to parasites whose food probably differs
chemically but little from that of holozoic forms.
Reserve materials^ for use at times when nutriment is not being
taken or when some process, such as rapid multiplication, is making
heavy demands upon the resources of the organism, are stored by
most protozoa, and are often conspicuous, as granules, vacuoles,
crystals, etc., in the cytoplasm. The carbohydrates starch, para-
mylum (in the Euglenoidina), and leucosin (in the Chrysomonadina)
are formed by holophytic organisms and by some colourless forms
related to these (as by Polytoma, Fig. 24, Peranema, etc.). Glycogen
is stored by parasitic and other anaerobic forms, in which it is per-
haps split with evolution of energy, as in various anaerobic metazoa.
Protein reserves are common in holozoic species. Nucleic acid
("volutin") is widespread, probably as a reserve for the nucleus. Oil
reserves also occur in practically all groups — a rather remarkable fact,
in view of the apparent inability of the Protozoa to digest fats. In
phosphorescent forms (dinoflagellates, radiolarians) the oxidation of
fats is the source of the emission of light.
The nitrogenous excreta of the Protozoa appear to be most often
ammonia compounds, less often urea, and occasionally urates. Ex-
cretion doubtless frequently takes place from the general surface of
the body. Sometimes there are recognizable in the cytoplasm granules
or crystals of urates or phosphates, which may be expelled with the
faeces but appear in other cases to be redissolved. Their material is
then perhaps passed into the contractile vacuoles. The latter are spaces
filled with water which periodically undergo collapse with expulsion
of their contents to the exterior. In the simplest cases, as in the
familiar laboratory types Amoeba, Chlamydomonas and Actino-
sphaerium (Figs. 23, 33c.z;«c.), the contractile vacuoles are solitary,
PROTOZOA
21
spherical cavities, one or more in number according to the organism;
over these in pelliculate genera there is a soft patch in the pellicle
through which discharge takes place. Sometimes, as in Eiiglena and
Fig. 17.
Fig. 16.
Fig. 16. Paratnecium caudatiini from the ventral side, x 375. After Doflein.
CV, contractile vacuoles: in the anterior one the main vacuole has just dis-
appeared by discharging, and is about to be reconstituted from the accessory
vacuoles, which are at their maximum size ; in the posterior one the accessory
vacuoles having re-formed the main vacuole are themselves re-forming; f.v.
food vacuoles ; me. meganucleus ; 7ni. micronucleus ; v. vestibule.
Fig. 17. The cycle of action of the contractile vacuole and its canals in Para-
mecium. From Lloyd, after Putter.
Paramechifn, they are accompanied by accessory vacuoles by whose
contents they are reconstituted (Figs. 39 D, 16, 17) and which in some
ciliates (Fig. 89 B, C) extend as long canals through the cytoplasm.
22 THE INVERTEBRATA
Another complication sometimes exists in the presence of a "reser-
voir" through which the vacuole communicates with the exterior,
either directly, as in Peranema (Fig. 39 E), or by way of the gullet, as
in Euglena and Vorticella (Figs. 39 D, 2). At least some contractile
vacuoles appear to have a lining membrane, and it is probable that
they are not entirely abolished at systole. The fact that these organs
are commoner in freshwater protozoa than in marine or parasitic
species suggests that their primary function may be the discharge of
water, which must enter the body when the surrounding medium has
a lower osmotic pressure than the protoplasm. Possibly, however,
they serve also as organs of excretion.
Respiration no doubt takes place upon the whole surface of the
body. It has been supposed that the contractile vacuoles subserve
this function, but, while they no doubt remove carbon dioxide in
solution, it is difficult to see how their activity could cause the entry
of oxygen.
Many protozoa either regularly or occasionally pass a period of
their lives in a cyst. The cysts may be coats of jelly or stronger cover-
ings, usually organic, but sometimes, as in the Chrysomonadina,
chiefly composed of inorganic material. The function of the cyst is
nearly always to shield the organism, either from unfavourable cir-
cumstances or from stimuli which would interfere with some process,
such as reproduction or digestion, but in a few cases it facilitates
syngamy by keeping gametes together. Encystment is less common
among species which live in the relatively equable conditions of the
sea, than in freshwater and parasitic forms. Cysts which do not sub-
serve reproduction may be resistance cysts, against drought, alterations
of concentration, or the appearance of poisonous substances in the
surrounding medium, either in the habitat in which encystment takes
place or in those encountered in the course of distribution. They
may on the other hand be resting cysts, which enable the organism
to proceed undisturbed with digestion or photosynthesis or by quies-
cence to conserve its energy during starvation. Cysts which subserve
reproduction may be gamocysts, in which union of gametes takes place
(gregarines. Figs. 76-78), oocysts, containing a zygote, or sporocysts
containing several small individuals produced by fission. The oocyst
frequently becomes a sporocyst by fission of the zygote. Reproductive
cysts are often also resistance cysts.
The nuclei of the Protozoa (Fig. 18) vary greatly in structure.
They usually contain masses of some size composed of various
materials. Such masses, when they consist only of the substance
known as plastin (which takes acid stains), are known as nucleoli',
if they also contain chromatin (basic-staining) they are artiphi-
nucleoli. A single central mass is an endosome : it may be a temporary
PROTOZOA
23
aggregation, as, for instance, in Actinosphaertum, but more often is
permanent except, sometimes, at division. Such a permanent en-
dosome is usually a nucleolus or an amphinucleolus, but is said
sometimes to consist solely of chromatin or of achromatic matter.
A permanent endosome consisting of plastin or chromatin, or both, is
.nul
kar.^
^'kan
nu.
Fig. 18. Nuclei of Protozoa. A, Polystomella crispa. B, Amoeba proteus.
C, Actinosphaerimn eichhorni. D, Naegleria bistadialis. E, Polytoma tivella.
F, Ceratiiim fusus. G, Stentor coeriileus. All highly magnified, to various
degrees. After various authors, cen.'i possible centriole; kar. karyosome,
containing a centriole in D; nu.' nucleoli or amphinucleoli.
known as a karyosome. Two principal types of protozoan nuclei — the
dense and the vesicular — may be distinguished; there are, however,
intermediates between them, and they do not characterize each a
distinct branch of the phylum, but the dense appears to have arisen
more than once from the vesicular. In nuclei of the dense type the
achromatic part has a relatively firm consistency, and often exhibits,
24
THE INVERTEBRATA
at least in fixed specimens, a fine meshwork. The plastin is in masses
scattered through the nucleus, or occasionally in a single excentric
mass. The shape is often not spherical (Figs. 87-90). The meganuclei
of ciliophora and dinoflagellate nuclei belong to this type, which
otherwise is rare. In vesicular nuclei the achromatic part is more fluid
and its meshwork, if any, is coarse. The plastin may be in several
masses under the nuclear membrane, but usually is in a karyosome.
spd.
.-cen.
nn.mr.^
r^-, rr::^^'^=^^^Zll.
^ "-C^^
-^^ry^i^E—^^
• — =^^ \ ;'^^
':A!^^=^
:r=^^p"^^
• i ' :. : '
Fig. 19. Mitosis (paramitosis) of the sporogony oi Aggregata eberthi. After
Belar. A, Interphase between divisions. B, Early metaphase. C, Anaphase
beginning. D, Later anaphase. E, Early telophase. F, Later telophase.
cen. centriole; chr. chromosomes; 7iu.me. nuclear membrane; spd. spindle.
The modes of division of protozoan nuclei are also very various.
Many were formerly classed as amitoses but are now regarded as un-
usual types of mitosis. True amitoses are rare, and perhaps occur only
in the meganuclei of the Ciliophora. The mitoses are sometimes (Fig.
58) practically identical with those of the Metazoa, but are usually
more or less aberrant. The "division centre" by which mitosis is
initiated may be a centrosome consisting of centrosphere and
centriole, or may be either of the latter two entities alone. The
centrosphere often forms a plate or cap at each pole of the nucleus.
Most often the nuclear membrane remains intact throughout the
PROTOZOA
25
process. The division centre may be intranuclear or extranuclear ;
when it is an extranuclear centriole, it is often identical or associated
with the basal granule of a flagellum. Cases in which the chromosomes
are distinct and on the whole behave like those of metazoa are known
as eumitoses. Another set of mitoses, known as paramitoses (Fig. 19),
differ from those of the Metazoa in that the chromosomes do not
shorten in the metaphase, and are not symmetrically arranged on the
equator of the spindle (if such be visible); and their longitudinal
Fig. 20. Stages in the mitosis (cryptomitosis) of Haplosporidium limnodrili.
After Granata. A, Resting nucleus : the spindle here persists. B, Metaphase.
C, Anaphase. D, Telophase, clir. chromatin; kar. karyosome; spd. spindle.
halves, when they separate, hang together to the last at one end so
that they appear, though deceptively, to divide transversely. In a
third set, known as cryptomitoses (Fig. 20), there are no distinct
chromosomes but the chromatin merely concentrates as a mass upon
the equator of a spindle, whose fibres may not be visible, and divides
into two halves which travel to opposite poles. Intermediate cases
connect cryptomitoses with eumitoses. Paramitoses occur in cocci-
dians, dinoflagellates, and the spore formation of radiolarians, crypto-
mitoses for the most part in parasitic and coprozoic forms, as in
Haplosporidium and Naegleria.
26 THE INVERTEBRATA
In certain cases mitoses repeated several times without dissolution
of the nuclear membrane give rise to polyenergid nuclei which possess
numerous sets of chromosomes, the sets being finally liberated to
form each a daughter nucleus. The polyenergid condition is probably
always a provision for spore formation, and may (as in the coccidian
Aggregatd) occur only as a transient phase before sporulation, but in
other cases (radiolarians) it persists for a long time, the nucleus
dividing meanwhile as a whole by "giant mitoses'* in which all the
chromosomes take part. A remarkable process in Amoeba proteus is
possibly to be interpreted as the multiplication of units in a poly-
energid nucleus by cryptomitoses.
In the Ciliophora (other than the Opalinidae and the Chonotricha)
the nucleoplasm, which in other protozoa is contained in one nucleus
or in several which are all alike, is divided into two portions, a large
amitotic meganucleus which breaks up periodically in *'endomixis"
(see p. 35) and also at conjugation, and one or more small micronucleiy
by division of one of which the pronuclei of conjugation are provided
and the meganucleus replaced when the latter disintegrates. Indi-
viduals without micronuclei have been observed and kept through
several asexual generations. Thus the meganucleus is capable of
conducting by itself the normal vegetative existence of the individual,
though the absence of this nucleus at syngamy shows that it does not
establish the characters of the race. That function must be performed
by the micronucleus, but, since the latter does not exist without the
meganucleus, save for a brief period during conjugation, it presumably
does not regulate the life of the individual. The chromatin of the
meganucleus is known as trophochromatin, that of the micronucleus
as idiochromatin.
A similar distinction between trophochromatin and idiochromatin
is discernible in various other protozoa. In the Opalinidae (Fig. 21 C)
and Chonotricha there are two sets of chromosomes, an outer and an
inner, which divide successively at mitosis. The members of the outer
set (megachromosomes), larger and less regular than those of the inner,
are held to represent the meganucleus of other ciliophora : the material
of which they are composed is known to be cast out of the nuclei of
the Opalinidae before gamete formation. The members of the inner
set {microchromosomes) represent the micronucleus. In various cases
of gamete and spore formation by members of other classes, especially
of the Sporozoa, there is a destruction (Fig. 21 A, see legend), or
a casting out from the body (Fig. 21 B), of a portion of nuclear
substance which is probably trophochromatin. It has been suggested
also that the obscurity of cryptomitoses is due to a veil of tropho-
chromatin dividing amitotically around the idiochromosomes. It may
be that all protozoa contain chromatin in both these conditions; and
PROTOZOA
27
it is perhaps in this respect, as well as in restriction of function, that
the cells of metazoa differ from protozoa.
From the fact that, in many cases at least, the trophochromatin is
periodically destroyed and replaced, and from further facts which we
shall cite in discussing the significance of conjugation, it would appear
that trophochromatin, or some part of the protoplasm associated with
it, in the course of its regulative activity eventually becomes effete
and is replaced from the idiochromatin, which is not liable to that
fate. Perhaps the possession by protozoa of the facility for this re-
Fig. 21. Nuclear phenomena in protozoa. A, Extrusion of the germ nucleus
froin the somatic nucleus in Gregarifia cimeata before gamogony. The somatic
nucleus will break up and disappear. B, Extrusion of the substance of the
endosome from the gamont of Eimeria schubergi, the germinal part of the
nucleus remaining in the centre of the body. C, Mitosis in Opalina ranarinn,
megachromosomes in anaphase in outer part of nucleus, microchromosomes
in metaphase internal to them. A, after Milojevic; B, after Schaudinn;
C, after Tonniges.
placement, and the lack of such facility in the body cells of metazoa,
is the explanation of the fact that protozoa are not subject to the
"natural death" which eventually overtakes the body of a metazoon.
The loss of trophochromatin during the formation of gametes is
not to be confused with the reduction division of maturation. Reduc-
tion divisions, however, have been seen in members of all classes of
the Protozoa, and it maybe suspected that a process of this kind occurs
in all species in which there is syngamy. Such divisions are sometimes
{Actinophrys, etc.) strikingly similar to those of the Metazoa, but in
other cases {Paramecium, etc.) are peculiar. The reduction division
28
THE INVERTEBRATA
usually closely precedes syngamy, as in the Metazoa, but in the
Telosporidia and Volvocina it takes place in the first division of the
zygote, so that for the whole of the rest of its life history the organism
is haploid.
In many protozoa there are present in the cytoplasm, scattered or
massed into a group, numerous granules which, like chromatin, take
basic stains and are known collectively as the chromidium (Fig. 22 A).
They appear to arise from the nucleus, and have been said, but
probably incorrectly, in some cases to give rise to nuclei by con-
densation. They may appear upon occasion or be present through the
greater part of the life cycle. Their function is uncertain and probably
not always the same.
Th.Q. fission of the Protozoa takes place in several ways. Whether in
Fig. 22. Arcella. After Swarczewsky. A, Vegetative individual. B, In-
dividual in process of budding, chm. chromidium ; nu. nucleus ; op, opening
of shell ; sh. shell.
asexual reproduction or in the formation of gametes, it may be:
{a) equal binary fission, the familiar mode of division of Amoeba,
Paramecium, and a vast number of other cases; {b) budding, in which
one or more small products separate from a parent body, as in
Arcella (Fig. 22 B), the Suctoria (Fig. 96), etc.; {c) repeated fission,
in which equal divisions give rise to four or more young which do not
separate till the process is completed, as in Chlamydomonas (Fig. 23),
the microgamete formation of Volvox (Fig. 44), etc.; or (d) midtiple
fission, in which the nucleus divides several times without division
of the cytoplasm, which finally falls into as many parts as there are
nuclei, usually leaving behind some residual protoplasm, which may
contain nuclear matter. Multiple fission is seen in the spore formation
of numerous protozoa, as Amoeba, sporozoa (Fig. 74 C, D; Gg, G3;
K, L), etc. The fission of multinucleate protozoa, such as Actino-
sphaerium, Opalina (Fig. 86), etc., to form multinucleate offspring by
PROTOZOA
29
division of the cytoplasm without relation to that of the nuclei, is
known as plasmotomy. It is usually binary, but occasionally takes
place by budding or is multiple. The plane of simple binary or of
repeated fission is often transverse to the principal axis — if there be
one — of the body, but in most flagellates it is longitudinal. Repeated
longitudinal fission in which the daughter individuals remain in
position is called radial; such fission is common in the green flagellates
of the order Volvocina (e.g. some species of Chlamydomonas, Fig. 23
Fig. 23. Chlamydomonas. A, C angidosa, x 1000. B-D, the same, in fission
(radial). E-H, C. /ow^wi/g'wa in fission (pseudotrans verse). Highly magnified.
After Dill, with modifications, c.vac. contractile vacuole; cph. chromato-
phore; cu. cuticle; e. eye-spot; nu. nucleus ; /)_yr. pyrenoid.
A-D). Sometimes an individual in longitudinal fission shifts in its
cuticle during the process, till the plane of division becomes transverse.
Fission of this kind is said to be pseudotransverse : it is seen, for in-
stance, in some Chlamydomonas (Fig. 23 E-H). In Polytoma (Fig. 24)
the only vestige of longitudinal fission consists in a slight obliquity of
the first division of the nucleus.
Each type of fission takes place in some cases in a cyst and in others
without encystment.
BRARY
MASS.
30
THE INVERTEBRATA
The fate of flagella at fission varies. Sometimes, as in Chlamy-
domonas and Polytoma^ they are lost, early or late in the process. In
other cases they are retained. When this happens in an organism with
a single flagellum, that organ has been said sometimes to be split
longitudinally, but usually, if not always, a second flagellum grows
out from the basal granule, which divides. When several flagella are
present and persist, they are distributed between the ofi^spring, each
of which grows new flagella to complete its equipment. Probably,
a new flagellum always grows from a basal granule. Chromatophores
divide, and if numerous may do so independently of the fission of the
Fig. 24. Polytoma uvella, x about 1300. A, Ordinary individual, showing
nucleus, eye-spot, contractile vacuoles, flagella with basal granules, and
starch grains, the latter confined to the hinder part of the body. B-E, stages
of the first fission. In C and D the flagella are omitted. After Dangeard.
body. Contractile vacuoles and other organs rarely (Euglena) divide,
but are usually shared as the flagella. Complex organs, however, are
often destroyed (dedifferentiated) and remade by the individual that
receives them.
Conjugation^ or syngamy,^ the union of two nuclei, accompanied by
the fusion of such cytoplasm as each may possess, is, so far as our
^ The name conjugation is by some authorities restricted to the peculiar
process by which syngamy is accomplished in most ciliophora (p. 33).
^ The union of nuclei is karyogatny : in most cases of syngamy it is accom-
panied by plasmogamy or the fusion of cytoplasm, but in typical ciliophora
one of each pair of fusing nuclei has perhaps no cytoplasm ; and autogamy
(p. 33) is said sometimes to occur between nuclei whose cytoplasm has never
been divided. Plastogamy (p. 36) is plasmogamy without karyogamy.
PROTOZOA 31
knowledge goes at present, by no means universal in the Protozoa.
Especially among the Mastigophora, but also in other groups as in
the Amoebina, there are many cases in which it appears not to occur.
Probably, however, in the majority of species it either is known or
may reasonably be inferred to take place. The energids by which it
is performed, known here, as in all organisms, as gametes^ may be
either merogametes, formed by special acts of fission and smaller than
the ordinary energids of the species, or hologametes, not formed by
special fissions, and as large as, or larger than, the ordinary energids.
Syngamy between like gametes is known as isogamy, that between
unlike gametes as anisogamy. The simplest cases of the process are
those instances of isogamy in which two full-sized ordinary indi-
viduals unite. Such unions are known as hologamy and are rare,
though they occur in Copromonas (Fig. 39 F') and a few other species.
The fact that nearly all protozoa in which it is certainly known to
occur are coprozoic (p. 43) suggests that it is an adaptation to special
conditions — perhaps to brief duration of the active stage — and is
not, as might be assumed, the primitive form of syngamy. In all
other cases the gametes are special individuals, and one at least is
a merogamete. They may be isogamous or anisogamous, and in the
latter case one — \ht female gamete — is less active than the other, which
is the male gamete. Nearly always the female gamete (macrogamete)
is larger than the male {microgamete), and often it is a hologamete.
In the latter case the process is known as oogamy. As examples of
isogamy of merogametes we may cite the syngamy of Polystomella
(Fig. 66C-E) and of some Chlamydomonas (e.g. C. steini).
Anisogamy occurs independently in many genera, and has more
than once become oogamy. An interesting series of grades in this
respect is provided by the Volvocina. Chlamydomonas euchlora ex-
hibits the transition from isogamy to anisogamy. By undergoing
different numbers (2-6) of divisions, its individuals form merogametes
of several different sizes, but these pair indifferently, some unions
being isogamous, some anisogamous. C. brauni and other species
(Fig. 25) form merogametes of two sizes and are definitely ani-
sogamous. Volvox (Fig. 46) and related forms have an anisogamy
in which the female gamete is a hologamete (oogamy). A similar
series is shown by the Sporozoa. The syngamy of some species of
Monocystis, for instance, is isogamy of merogametes, that of others
is anisogamy of merogametes with various degrees of unlikeness
between the gametes, and that of the malaria parasite (Fig. 75) and
its relations is anisogamy between a hologamete and a merogamete.
Syngamy, whether isogamous or anisogamous, nearly always is exo-
gamous, that is, takes place between the offspring of different parents.
Since the male and female gametes are usually formed by distinct
32
THE INVERTEBRATA
parents, sex may be said to exist among protozoa, but it is rarely that
(as in the sporozoon Cyclospora, etc.) the sexes may be distinguished
Fig. 25. Anisogamous syngamy of Chla?nydomonos. a-h, C. media, after
Klebs. a, Vegetative individual. 6, Eight gametes produced inside the cuticle
of the parent, c, Single gamete, d, Gamete before syngamy showing proto-
plasm at one end of the cuticle, e, /, g, Syngamy between two gametes of
unequal size : the individuals slip out of the cellulose wall at the time of fusion,
the cilia withdraw and there is a complete fusion of the individuals, h, The
resultant thick-walled zygote, z, j, k, C. braiini, after Goroschankin : in this
species the gametes fuse whilst yet within their cellulose walls and there is
marked anisogamy, the small gamete slipping into the cuticle of the larger
gamete, j, Shows the nuclei, chloroplasts and pyrenoids of the two gametes
still separate, k. Shows the fused nuclei.
by other features. In many of the Telosporidia (e.g. Monocysfis,
Fig. 78) sexual congress may be held to occur, in that individuals,
male and female in cases of anisogamy, apply themselves together at
PROTOZOA 33
the time of gamete formation, and their gametes unite each with one
from the other parent. Hermaphroditism appears in the CiUophora^
(except the Opalinidae and Chonotricha). Here congress (Fig. 26)
takes place between two individuals (conjugants) in each of which the
meganucleus (see above) disintegrates, and the micronuclei divide to
form a number of nuclei — perhaps a reminiscence of the formation
of numerous merogametes. All but one of these nuclei disappear,
and the survivor divides to form a male pronucleus, which passes over
into the partner, and a female pronucleus which, in possession of the
cytoplasm of the parent, awaits the arrival of the male pronucleus of
the partner. Fusion now takes place between the male and female
pronuclei in each of the pair of conjugants, the latter separate, and
by the division of their zygote nuclei mega- and micronuclei arise.
Two hermaphrodites have formed each a male and a female gamete
and cross-fertilization has taken place .^ In the Vorticellidae (Fig. 26 B)
the individuals which enter into congress differ, one being of the
ordinary size and fixed, the other small and free-swimming. The
smaller arises from an ordinary individual, as a bud or by repeated
fission. After reciprocal fertilization of the type just described, the
smaller partner perishes, its endoplasm being sucked into the larger.
This curious simulation of sexual dimorphism by hermaphrodites
occurs in a less marked form in other ciliates.
A remarkable process known as autogamy, in which a nucleus
divides into two which after maturation immediately reunite, occurs
in Actinophrys and Actinosphaerium (see pp. 83-86), and possibly in
some other cases.
Parthenogenesis is known to occur in members of at least three of
the four classes of the phylum. The clearest case is presented by
Actinophrys, when gametes which have failed in attempt at cross-
fertilization develop parthenogenetically (p. 86): it is interesting that
one of these gametes is a functional male. Individuals of Polytoma
which are potential gametes will grow and divide, and the same is
true of the gametes of some species of Chlamydomonas and Haema-
tococcus when syngamy has been missed. The endomixis of ciliates
(p. 35) is a phenomenon of this kind.
Since it is comparatively easy to observe the conditions which pre-
cede and the results which follow syngamy in the Protozoa, many
experiments and observations have been made upon those creatures,
with a view to discovering the signifi^cance which the process has for
^ Actinophrys (p. 83) may be said to be hermaphrodite, and so perhaps are
many of the Radiolaria. But it is not certain that the " gametes " of this group
are not parasitic dinoflagellates. (See p. 80.)
^ Occasionally (Collinia, Dendrocofuetes) the conjugants also exchange
halves of their meganuclei. The latter, however, always disintegrate.
34
THE INVERTEBRATA
organisms in general. Most of these researches have been carried out
upon ciliata. They have led to two theories: (i) that syngamy effects
a periodical rejuvenescence of the organism ; (2) that it produces new
Fig. 26. Conjugation of ciliates. A, Paramecium. B, Vorticella. Both at the
moment of the exchange of male pronuclei, meg. fragments of disintegrating
meganuclei ; mi. micronuclei : in each the male and female pronuclei produced
by the division of a single nucleus still hang together by their spindle, but are
parting, the male to pass into the other conjugant, the female to remain behind ;
mi.' abortive division products of the original micronucleus.
Fig. 27. A diagram of the behaviour of the micronuclei during the conjuga-
tion of Paramecium caudatum. From Borradaile. The white circles represent
the portions which degenerate.
types of individual and therefore gives the species more chances of
survival in the struggle for existence.
(i) Cultures of protozoa in which conjugation is prevented are
liable after a time to fall into an unhealthy condition known as de-
PROTOZOA
35
pression, in which the nucleus^ is overgrown, the body stunted, divi-
sion retarded, and the various organs and functions increasingly de-
generate, until finally digestion ceases and the organisms die. From
this condition conjugation will recover a culture which is not too far
gone. It was held that depression was the senility of the organism —
ultimately of the same nature as that which in the Metazoa destroys
the parent body, while the gametes, after syngamy, continue the
existence of the species — and the conclusion was drawn that in both
cases the effect of the union of nuclei was rejuvenation. Now, how-
ever, it is known that depression is a disease, which by more natural
methods of culture can be avoided without conjugation. It is true that
in cultures of ciliates there has been observed a periodical waxing
<^
Cg5>>c2g><^
Fig. 28. A diagram of the nuclear changes in Paramecium aurelia during
endomixis. From Robertson, after Jennings. The white circles represent
degenerating nuclei. Fissions take place between D and E, and between H
and I.
and waning of vitality of which the low points in some cases coincide
with conjugation ; but in other cases there occurs at these points not
conjugation but a process known as endomixis, which closely resembles
the procedure in conjugation, but takes place in solitary individuals
and does not involve syngamy. In this process (Fig. 28) the mega-
nucleus is destroyed and replaced by one of the products of the
division of the surviving micronucleus, as in conjugation. It would
appear from these facts that the invigorating effects of conjugation
are due not to the true syngamy (union of nuclei) but to the accom-
panying replacement of the meganucleus, which probably has become
effete (see p. 27). If, as has been suggested (p. 26), those protozoa
^ In ciliophora the meganucleus.
36 THE INVERTEBRATA
which have no meganucleus have in their nuclei trophochromatin
which is destroyed at syngamy, this conclusion may be extended to
them also.
(2) Variety in a protozoan species is of three kinds : (a) that which
results from the production of different combinations of genes at
syngamy, and is permanent, forming races {pure lines) like those
which exist in higher organisms in the absence of cross-fertili-
zation; such pure lines have, for instance, been found in respect
of body-length in cultures of Paramecium, each line in the culture
breeding true so long as asexual reproduction continues; (b) that
which results from the spontaneous appearance of mutations ; this also
is permanent; it has been studied in Ceratium and other genera;
(c) that which results from modification of the individual by the direct
action of the environment; this, like mutation, produces differences
between individuals of a pure line, but it is not permanent, though it
may be inherited for several generations before it disappears. It
would seem that, apart from the occasional appearance of mutations,
the permanent varieties in a species are produced only by syngamy.
Here may be mentioned the union of individuals by fusion of
their cytoplasm, the nuclei remaining distinct, which is practised by
the Mycetozoa (Fig. 73 F) and in some other cases. This process,
which is not syngamy, is known as plastogamy, and its product as a
Plasmodium.
The life of a protozoon passes in the course of generations through
a cycle in which individuals of different kinds succeed one another.
The life cycles of various protozoa differ greatly, being related to the
vicissitudes of the environment of the species and to the need for
distribution as well as to the recurrence at intervals of conjugation,
but it is possible to formulate a type of which all of them may be
regarded as variants. After a period of "vegetative" existence and
increase by asexual reproduction, during which the individuals are
known as agamonts, there appears a generation known as gamonts
because they produce gametes, the latter unite in pairs, and the zygote
or sporont gives rise to a generation of sporozoites which, becoming
agamonts, repeat the asexual part of the cycle. The table on p. 37
shows this typical life history.
In comparing this table with the actual course of the cycle in any
species, the student should remember:
(i) that in each part of the cycle fission may take place in any of the
modes described above, and that the agamogony of a species may
proceed in more than one of these ways (as, e.g., that of Amoeba
proteus by binary or multiple fission) ;
(2) that in the cycle of most protozoa there is a point at which ad-
justment must be made to unfavourable conditions, either recurring
PROTOZOA 37
in the local habitat or met with in the course of the distribution of the
species (e.g. freshwater forms and parasites); and that at this time
(a) the ordinary agamogony is suspended, (b) the syngamy, if any,
usually takes place, (c) there is often a phase of protective encystment,
(d) there is often very rapid multiplication by multiple or repeated
fission, which may be the sporogony, the gamogony (eugregarines),
or an agamogony;
(3) that any part of the cycle may be omitted ; in such cases it is
most often the sporogony which is dropped, but many species appear
to omit gamogony, and in a few (e.g. Monocystis and the other eugre-
garines) there is no agamogony;
Agamont (Schizont, Meront)
Agamogony (Schizogony, Merogony)
Agametes (Schizozoites, Merozoites)
Growth of Agametes
Agamont of second generation
Agamogony repeated
Gamont
Gamogony
Gametes
Syngamy
Zygote (Sporont)
Sporogony
QQ 0 0 0 © © © Sporozoites
/
0
Growth of sporozoite ■
Agamont
Fig. 29. A table of the life history of a protozoon.
(4) that a reduction division may occur at either of two points in the
cycle — shortly before syngamy (most cases), or directly after the
formation of the zygote (the Telosporidia and Volvocina) — and that
correspondingly either the diploid or the haploid phase may extend
over the greater part of the life history.
The term spore is applied to various phases of the life history in a
way which is liable to cause confusion, {a) Strictly speaking, perhaps,
it should be applied only to the products of repeated or multiple
fission of a zygote (sporont). (b) Most often, however, it is used to
denote the products of any repeated or multiple fission, {c) In a few
38 THE INVERTEBRATA
cases (e.g. the " ciliospores " of the Suctoria) it is applied also to
products of budding. A cyst in which several spores are enclosed is
a sporocyst. Individual spores may be enclosed in spore cases^ when
they are chlamydospores (as those of the Mycetozoa, Fig. 73 A), or
naked, when they are gymnospores. The latter may be amoeboid
(amoebulae or pseudopodiospores, e.g. Amoeba, Polystomella, Fig. 66 C),
flagellate {flagellulae or flagellispores, e.g. Polystomella, Fig. 66 B,
Chlamydomonas), or ciliate {ciliospores, e.g. the Suctoria, Fig. 87 H).
Spores may be gametes (e.g. the Mycetozoa, Chlamydomonas), or
serve for the distribution of the species, when, if they are motile, they
are known as "swarm spores". The sporoblasts of many telosporidia
(e.g. Plasmodium, Fig. 75, 16-18) are spore-like bodies which are
not set free, but give rise under cover to another generation of spores.
The so-called spores of such sporozoa as Monocystis (Fig. 78 G, H)
are really minute sporocysts, enclosing several spores ("falciform
young").
The life history of the individual protozoon usually exhibits little
change save increase in size. Sporozoites and other spores, however,
may diff^er considerably from the adults into which they grow. This
difference reaches its height in the ciliospores of the Suctoria.
The behaviour of a living being is that part of its life which consists
in action upon the outer world. Like the rest of life it comprises
activity of various kinds — mechanical, chemical, etc. — which in some
cases, as in the direction of locomotion to or from the light or the
shooting out of trichocysts, is immediately due to external circum-
stances {stimuli), while in others, as in the beating of cilia which
continues even when the organism is encysted, it is not. Both these
sorts of activity are so ordered that in normal circumstances they
conduce to the welfare of the organism. The reactions of the Protozoa
to stimuli are at least superficially analogous to the reflexes of the
Metazoa. Study of them has chiefly been directed to those which
result in locomotion. Such reflexes are of two kinds, topotaxis and
phobotaxis. In topotaxis the organism orientates itself in relation to
the stimulus, and moves either towards or away from it. This is the
less common mode of reaction in protozoa, but it appears to be per-
formed by some in the neighbourhood of food, by gametes in their
union, and by certain green flagellates {Volvox, sometimes Euglena,
etc.) in approaching the light.
In phobotaxis , which has been studied in many protozoa of various
groups, the only circumstances which act as stimuli are those which
are "unfavourable", that is from which the organism withdraws;
and in doing this it is not repelled in a straight line, but turns away
at an angle which has no necessary relation to the direction of the
stimulus, and may again bring the individual into the unfavourable
PROTOZOA
39
Fig. 30. A diagram of the path by which a protozoon is directed by phobo-
taxis into the zone of optimum concentration of a substance diffusing from
a particle. From Fraenkel, after Kiihn. C, Centre of diffusion. The arrows
show the direction of movement of the protozoon, the circles define zones of
equal concentration, the circles of dots and dashes enclose the optimal zone.
J.-. .
•-^
>' .■.. ; .^
J ......
^
' * • • ,
• •.
• •'■.ife-'.' ■
'
<•■•■.•■•
"r
> ■ ■ r
^• •
r
1
2
3
•it V-
•v. /;•
1 \
N .••■•■ Y
"\.>-S 5>y»;\^v-...»'. -.ly
Fig. 31. Chemophobotaxis of Bodo sulcatus. From Fraenkel, after Fox.
1-5, Positions successively taken up by the members of a culture placed under
a coverslip. The position in which the individuals gather in each case is that
of the optimum concentration of oxygen, which alters as the supply of the
element is lessened by the action of the flagellates.
40 THE INVERTEBRATA
circumstances. The reaction is then repeated. Thus the organism is
shepherded by its reactions in the direction of the optimum con-
ditions. Fig. 30 shows the path of an individual in the neighbourhood
of a particle whence is diffusing some substance of which a certain
concentration is optimal for the species to which the individual
belongs. Any departure from this concentration turns the moving
individual, so that it is led to and kept in a zone in which the optimum
exists. Fig. 31 shows how members of a culture of the flagellate Bodo
sulcatus, when placed under a coverslip, find by this reaction the
optimum concentration of oxygen, which is at first in the middle of
the field and recedes as the organisms use up the supply of the element.
The number of ways in which a protozoon can respond to stimuli
is at most small, but the response to a stimulus by an individual in
many cases depends not only upon the nature of the stimulus but
also upon the condition of the individual at the moment (hunger,
fatigue, etc.).
The relation of protozoa to their environment is governed primarily
by the fact that, owing to their small size, any cuticle which is thick
enough to protect their protoplasm from loss of water or poisoning
by substances in the medium has the effect of immobilizing the
organism. Hence in the active phase they are only found in water or
in damp places on land, and are peculiarly susceptible to variations in
the composition of the medium. Purely holophytic protozoa are also
dependent upon the presence of sunlight. Save for these restrictions,
members of the phylum are found in every environment in which any
other species of organism can exist. In the sea they are plentiful alike
in the plankton and in the benthos, and occur at all depths. Their
planktonic members are liable to possess the same peculiarities which
appear in members of other phyla in the same conditions — spininess
(Figs. 6C, 67, 69), phosphorescence, buoyancy, etc. In attaining a
low specific gravity they often show an expedient of their own, namely
the presence in their protoplasm of vacuoles of water of lower saline
content than the medium in which they are suspended (radiolarians,
Glohigerina, heliozoa ; Figs. 32, 33, 69, 71). In fresh waters their species
have the same cosmopolitan distribution as other small freshwater
organisms. Most of them, however, are severely restricted, in all
the localities in which they are found, by the necessity for conditions
which only occur in some one type of environment, and often even
there only during certain seasons or (as in the case of the dung fauna)
for yet siiorter periods. In this matter protozoa are particularly subject
to the/)H of the medium, its dissolved organic contents, and its saline
contents. Thus Po/_y/om« flourishes in an acid medium, ^S^zVoj-Zowwrn re-
quires a slightly alkaline one, and Acanthocystis pronounced alkalinity.
Euglena viridis and Polytoma live in highly nitrogenous mi\is\ons,Acti-
PROTOZOA
41
B
Fig. 32. Radiolaria without skeleton. A, Thalassicolla pelagica, x 20. After
Haeckel. B, Collozoum inerme. C, A central capsule of the same, more highly
magnified. After Doflein. cal. calymma; cps. central capsule; nu. nucleus;
oil, oil globule; ^5. pseudopodium ; j^.c, "yellow cells" (Zooxanthellae).
42
THE INVERTEBRATA
nosphaerium and Paramecium caudatum in less highly organic infusions,
Volvox and Amoeba proteus in much purer waters, Haematococcus in
rain water. As a rule the marine and freshwater faunas are restricted by
conditions of salinity, but Polystomella ranges from the sea into brack-
ish waters. For many holophytic protozoa the amount of sunlight is
important. Others, as Euglena gracilis, bleach in the absence of light,
but can still flourish if the presence of organic matter in solution makes
c.vac.
f.vac.
Fig. 33. Actinosphaerium eichhorni, x 180. From Leidy. The endoplasm is
crowded with food vacuoles containing diatoms, and nuclei are represented
in the figure by the dark areas, c.vac. contractile vacuole ;/.z;<2c. food vacuole
which has just swallowed a rotifer; ps. pseudopodia.
saprophytic nutrition possible. Holozoic species must of course have
their proper food; in infusions they appear as this becomes plentiful,
first, after the bacteria, those whose diet is purely bacterial, such as
Monas and Colpoda, then those, such as Stylonichia, that feed upon the
first comers, and so on ; though some bacterial feeders, as Paramecium,
are rather late to appear. Temperature has also an influence upon
protozoan faunas. The powers, possessed by freshwater protozoa, of
distribution across inhospitable regions and of surviving unfavourable
PROTOZOA
43
conditions at home are no doubt due to the faciUty with which they
form resistance cysts (p. 22). In various cases all the unsuitable con-
ditions of the environment indicated above have been found to induce
encystment, and encysted protozoa have been discovered in dust from
the most remote desert regions.
The protozoa which live in dung (coprozoic species) and in decaying
bodies, and those of very foul waters, are branches of the aquatic
fauna: they include many flagellates, Umax amoebae (p. 69), and
ciliates, and the conditions in which they are in the active state may
exist only for a very short period. These faunas merge on the one hand
into that of intestinal parasites, and on the
other into that of damp earth. In the latter
there is a large population, some of whose
members {Euglena, Arcella, Paramecium^
etc.) are of common occurrence elsewhere.
It has important effects upon the fertility
of the soil, by devouring valuable bacteria.
Perhaps the only truly subaerial members
of the phylum are certain mycetozoa.
Parasitic members are included in nearly
all the principal divisions of the phylum,
but not in the Radiolaria or Volvocina.
The Sporozoa are exclusively parasitic.
The relations of parasitic protozoa to their
hosts are of all degrees of intimacy : they
may be merely epizoic (as Spirochona,
p. 114), ectoparasitic (as Oodinium, P- 55)j
inhabitants of internal cavities (as Opa-
lina,p. 1 06), tissue parasites (as Myxobolus,
p. 100), or intracellular (as Plasmodium,
p. 91). They show, according to their de-
gree of parasitism, the same peculiarities
as other parasites — reduction of organs of
locomotion, simplicity of form, means of
fixation, the liberation of numerous young (in the Sporozoa), etc.
Some, as Entamoeba histolytica, are harmful by destroying for their
own nutriment the tissues of the host : more by secreting poisonous
substances, as the malaria parasites do. Many are specific to a par-
ticular host or hosts. Not infrequently there are two successive hosts
belonging to difl^erent phyla: both of these may be invertebrates, as
with Aggregata, which passes from the crab to the octopus, but more
often one is a vertebrate and the other an invertebrate. In such cases
it is often possible to decide which was the original host, and this
proves sometimes to be the vertebrate and sometimes the inverte-
Fig. 34. Oodinium poucheti,
parasitic on Oikopleura. A,
An Oikopleura bearing the
parasites. B, A free spore of
the parasite, pst. parasite, on
the tail of the host. The trunk
of the Oikopleura is enclosed
in the newly-secreted and not
yet expanded "house".
44 THE INVERTEBRATA
brate. It is interesting that the two most dangerous protozoan para-
sites of Man, the sleeping-sickness and malaria parasites, differ in
this way (pp. 63, 91).
Symbiosis^ of various kinds is practised by both holophytic and holo-
zoic protozoa. Instances of this are described below, on pp. 47, 68,
m, 193.
The division of the phylum into the four classes, Sarcodina, Mastigo-
phora, Ciliophora, and Sporozoa, characterized by the presence or
absence in the predominant phase of the life history of the several
types of motile organs, will be familiar to the student. Two attempts
have been made to brigade these classes into subphyla. One con-
trasts the Sarcodina under the name of Gymnomyxa with the other
classes, or Corticata^ on the ground that the latter possess a firm ecto-
plasm. The other contrasts the Ciliophora with the rest of the classes
(Plasmodroma), which lack cilia and a meganucleus. Neither of these
systems is satisfactory, for each is probably grounded, not upon a
fundamental cleavage of the phylum, but upon the specialization of
one branch of it.
The ancestral group of the Protozoa is probably the Mastigophora.
This is fairly evident as concerns the Sporozoa — a class highly
adapted to parasitism, and often possessing a flagellated phase — and
the Ciliophora, also a greatly specialized group, which possesses in
the cilia organs easy to derive from flagella. The Sarcodina, on the
other hand, were formerly held to be ancestral to all protozoa, on
account of the supposedly primitive condition of their protoplasm.
But neither the structure nor the behaviour of amoeboid organisms is
really simple; their holozoic nutrition is a less easy process and is
much less likely to be primitive than photosynthesis, which is common
in the Mastigophora ; the sporadic occurrence of amoeboid forms in
various groups of the Mastigophora probably indicates that the latter
have more than once given rise to organisms resembling the Sarcodina;
and, finally, the Sarcodina very commonly have flagellate young,
but the Mastigophora do not have amoeboid young. The Mastigo-
phora, indeed, are probably not only the basal group of the Protozoa
but also' not far removed from the ancestors of all organisms, for they
alone present (and often can alternate) the modes of nutrition both
of plants and of animals ; and their characteristic organ, the flagellum,
occurs in the zoospores of plants, in bacteria, and in the spermatozoa
of metazoa.
^ The term symbiosis has been used in various senses. It is here applied
to all cases of partnership between two organisms of which one lives within
the body of the other and both derive benefit from the association. It is
sometimes restricted to cases, such as those described on p. 47, in which the
infesting partner is photosynthetic.
PROTOZOA 45
The connection between the Protozoa and the Metazoa in the family
tree of the Animal Kingdom is an interesting but a very obscure
problem. Concerning it three theories are held. The first, supported
by the morphological resemblance of the uninucleate protozoon to a
cell in the body of a metazoon, and of Volvox to the blastosphere stage
in the development of such a body, holds that the metazoon is a
colony of protozoa, each differentiated as a whole for some function
in the body which they compose. The second, based on the fact that
the protozoon, which performs equally all the processes of life, is
thus physiologically equivalent not to one cell but to the whole body
of a metazoon, holds that the Metazoa arose from multinucleate
protozoa by the nuclei taking in charge each a local, differentiated
portion of the cytoplasm. The third, based on the fact that, save for
their mode of nutrition, the Metazoa have — in their cellular structure,
nuclear division, maturation of gametes, etc. — more in common with
multicellular plants than with the Protozoa, holds that the earliest
organism we can as yet envisage was multinuclear and photosynthetic,
and gave rise independently to the Metazoa and, by reduction of the
body, to flagellates, and so to the Protozoa, which on this view are
not truly members of the Animal Kingdom.
Class MASTIGOPHORA (FLAGELLATA)
Protozoa which in the principal phase possess one or more flagella;
may be amoeboid, but are usually pelliculate or cuticulate; are often
parasitic but rarely intracellular; have no meganucleus; and do not
form very large numbers of spores after syngamy.
The reproduction of the Mastigophora is in most cases by equal
longitudinal fission. The way in which in many of the solitary
Volvocina this becomes transverse has been described above (p. 29).
In the Dinof^agellata fission is oblique or transverse. The fission may
be simply binary or repeated. The number of fissions often varies in
the same species, and is usually greater in the formation of gametes
than in asexual reproduction. Binary fission in forms which have not
a stout cuticle usually occurs in the free-swimming stage, but may
take place in a cyst or jelly case, as, for instance, occasionally in
Eiiglena viridis. In forms with a stout cuticle, as in the Volvocina, the
protoplasm shrinks from the cuticle, which serves as a cyst. Repeated
fission usually occurs in a cyst. The^fate of the flagella at fission has
been dealt with on p. 30. The mitoses (see p. 25) in this group range
from beautiful eumitoses to the extremest cryptomitoses, the latter
generally in parasitic forms. Paramitosis occurs in the Dinoflagellata.
In many genera syngamy is not known to occur. Among those in
which it does, all degrees of difference between gametes are found,
46 THE INVERTEBRATA
and in particular among the Volvocina there are interesting cases
intermediate between hologamy and merogamy, and between isogamy
and anisogamy. Thus in Polytoma the age at which the products of
fission unite varies in a species, so that some are merogametes while
others, delaying, become hologametes ; in Pandorina (p. 58) isogamy
and anisogamy are facultative ; and various species of Chlamydomonas
(see p. 31) make up a series in which there is a transition from com-
plete isogamy to a pronounced anisogamy which rises to oogamy in
Volvox and other colonial forms.
The zygote is very commonly encysted.
The Mastigophora fall into a number of fairly well-defined orders.
It is convenient to group these by their nutrition into two subclasses
— the Phytomastigina, containing orders most of whose members are
holophytic (see p. 18), and the Zoomastigtna, which have no holo-
phytic members — but all the orders of the Phytomastigina contain
some colourless members, whose nutrition is purely saprophytic,
and all except the Volvocina include colourless holozoic forms.
Owing to this fact it is impossible to frame a definition which will
enable every member of each subclass to be recognized as such with-
out comparison with other species. Certain characteristics, however,
distinguish most members of the Zoomastigina from most of the
colourless Phytomastigina. These characteristics are stated below, in
the section which deals with the Zoomastigina.
Subclass PHYTOMASTIGINA
Mastigophora which possess chromatophores, and species without
chromatophores which closely resemble such forms.
There can be no doubt, for reasons which have been given above,
that this subclass contains the most primitive members of the phylum.
Its nutrition is extraordinarily interesting from that point of view.
Some of its species, notably among the Volvocina, are purely holo-
phytic. Others are normally also saprophytic, and some of these, like
Euglena, can upon occasion practise this mode of nutrition alone. Yet
others, like Polytoma, have become colourless, and are purely sapro-
phytic. Others again are both holophytic and, by amoeboid in-
gestion, holozoic. These lead insensibly to similar forms, members of
the Zoomastigina (Monas, etc.), which, being without chromato-
phores, have not the faculty of photosynthesis, but are purely animal
in their nutrition. Some of the coloured forms which possess a pit
that is called a gullet are said to take food with it, and thus to combine
holophytic and holozoic nutrition. In any case certain of their relatives
which have lost the chromatophores {Cyathomonas, Peranema, etc.)
take solid food through a similar gullet. Most of the holozoic forms
MASTIGOPHORA 47
are probably also saprophytic. Certain species {OchromonaSy etc.) are
known to make use of all three modes of nutrition. Thus all ways of
obtaining nutriment meet in this group.
The species which practise photosynthesis do so, like plants, by
means of chromatophores, of which they may possess one, two, or
many. The chromatophores are plate- or cup-shaped masses of proto-
plasm of a green, yellow, or brownish colour, owing to the presence
in various proportions of the pigments chlorophyll, xanthophyll,
carotin, etc. The chlorophyll absorbs the rays of sunlight whose
energy is used in photosynthesis. The green chromatophores are
known as chloroplasts, the yellow as xanthoplasts . Often there are to
be seen in or on the chloroplasts the protein bodies known as pyrenoids ,
which act as centres of starch formation. A red pigment, haemato-
chrome, is frequently present, diffused through the cytoplasm. In
bright light it spreads over the surface and is believed to shield the
chloroplasts from excess of certain rays. A small red spot of carotin,
sometimes darkened by another pigment, is generally present in
photosynthetic species, and probably acts as a rudimentary eye,
making the organism sensitive to light, which is of such importance
in its nutrition.
The holophytic forms are usually capable of passing into a resting
phase, in which the flagella are withdrawn, the body rounded off, a
cyst or jelly case secreted, and the organism closely resembles a plant
cell. Division may take place in that condition, establishing a pseudo-
colonial stage known as the palmella, and from this there may be
built up a branched body (Fig. 38 D, D^) which simulates those of the
lower algae. Plant-like forms of this kind occur in every order of the
group. It is indeed impossible to define the Phytomastigina from the
Algae, and the members of this subclass are regarded both by
botanists and by zoologists as coming within the scope of their
sciences.
Many of the coloured species are liable to produce colourless In-
dividuals. This happens in two ways: the chromatophores may be-
come bleached owing to the animal living in darkness ; or the rate of
division of the chromatophores may lag behind that of the body, so
that eventually there are produced offspring in which there are no
chromatophores (" apoplastid " individuals). These facts show how
the colourless species may have arisen.
Members of various orders of the Phytomastigina (cryptomonads,
a chrysomonad, a chlamydomonad, and perhaps dinofiagellates) are
known to live in the resting stage as symbionts in holozoic organisms
(other protozoa, sponges, coelenterates, worms, etc.). Nearly all are
yellow or brown (Zooxanthellae) ; most green symbionts {Zoochlorellae)
are algae belonging to the Protococcaceae. An exception to this is the
48 THE INVERTEBRATA
chlamydomonad of the genus Carteria which lives as a zoochlorella in
the tissues of the turbellarian worm Convoluta roscoffensis (Figs. 35,
36). The photosynthetic partner in these symbioses benefits by a supply
of carbon dioxide and the nitrogenous excreta of its host ; the latter
has waste matters removed, is supplied with oxygen, and sometimes
draws on the supply of carbohydrates manufactured by the guest,
though it is rarely, as Convoluta, unable to dispense with this nutri-
ment, and often, as the reef corals (p. 193), makes no use of it. If kept
in the dark it is apt to devour the guest. A photosynthetic organism
is specific to a particular host species. In some cases the two partners
are capable of living apart; in others, they are mutually dependent.
a.
epd.
■pyr.
Fig. 35- Fig. 36.
Fig. 35. A section through a portion of the superficial tissues of Convoluta
roscoffensis, showing symbionts belonging to a species of Carteria (Chlamy-
domonadidae, Volvocina). From Keeble. ci. cilia of epidermis; epd. epi-
dermis; gr.c. "green cells" (symbionts); nu. nucleus of symbiont; pyr.
pyrenoid.
Fig. 36. A free individual of the species of Carteria which is symbiotic in
the resting stage with Convoluta roscoffensis. From Keeble. chl. chloroplast ;
e. eye-spot; nu. nucleus ; ^jyr. pyrenoid.
The plant organism usually enters the host by being ingested but not
digested. It may be passed on from one generation to the next in
asexual reproduction or even, as with the green Hydra, in the ovum,
but is often lost in the gametes of its host, so that the zygote must be
reinfected. Protozoan hosts in symbiosis are usually members of the
Radiolaria (Figs. 32 A, 37, 69 A) or Foraminifera, but various ciliates,
Noctiluca, etc., also harbour holophytic symbionts. Zooxanthellae are
commonest in marine hosts, zoochlorellae in fresh water.
The amoeboid faculty possessed by some members of the group
may be limited to ingestion, but is often exhibited also in locomotion.
Certain forms with such locomotion lose their flagella for shorter or
longer periods : some may have done so altogether. When species with
PHYTOMASTIGINA
49
nu
amoeboid movement become colourless they are only to be separated
from the Sarcodina by certain features (of their nuclei, cysts, swarm
spores, etc.) which prove them to be related
to various mastigophora.
Of the orders of the Phytomasttgina, that
which contains the most highly organized
members is the large and protean group
Dinoflagellata, characterized by the posses-
sion of two flagella, one longitudinally di-
rected and the other transverse, usually in
a groove around the body but in a few cases
twisted about the base of the longitudinal
flagellurrL Three of the remaining orders
differ from the rest in the possession, in the
anterior part of the body, of a pit ("gullet ") Fig- 37- Lithocircus annu-
r 1 • u\u a 11 11 laris. After Lankester. c/)s.
or groove, from which the flagella usually ^^^^^^^ ^^^^^^^. ^^ /^.
arise. One of these, the Cryptomonadina, cleus; ^or. pore plate; j^.c.
has simple contractile vacuoles and its carbo- "yellow cells ".
hydrate reserves are of starch : it is held by
some authorities to be related to the ancestors of the dinoflagellates.
The second, the Euglenoidina, has a more complex contractile vacuole
system, and its reserves are of paramylum. The third is the little
group Chloromonadina, which differs from the Euglenoidina in having
oil reserves only and in the delicacy of its pellicle. The orders without
groove or gullet are the Volvocina^ the most plant-like of the Masti-
gophora, with green chromatophores (except in a few colourless
genera) and starch reserves; and the Chrysomonadina, by some re-
garded as the most primitive members of the class, which have
yellow or brown chromatophores and no starch reserves and are
often capable of becoming amoeboid.
Each of these groups exhibits most or all of the varieties of nutri-
tion and motility which have been mentioned above. Each of them
possesses {a) coloured, flagellate, solitary forms which constitute most
of its membership, {b) coloured species, whose individuals pass most
of their time in a non-flagellate condition, as a palmella, which is
sometimes of branched, plant-like form, {c) colourless saprophytic
forms, and {d), except in the Volvocina, colourless holozoic forms.
More than one order has purely amoeboid members, non-flagellate
throughout the greater part or all of-their existence. The support
which this versatility gives to the view that the Mastigophora, and in
particular the Phytomonadina, are near the base of the genealogical
tree of organisms has already been mentioned.
50 THE INVERTEBRATA
Order CHRYSOMONADINA
Yellow, brown, or colourless phytomastigina ; without starch reserves,
but usually with leucosin and oil ; without gullet or transverse groove ;
often amoeboid.
The genera briefly mentioned under this and the following orders
illustrate the range of variety within the group.
Chrysamoeba (Fig. 38 A, A^). One flagellum; two yellow chromato-
phores; no skeleton. Egg-shaped when swimming, but on the sub-
stratum becomes amoeboid and may lose flagellum. Ingests food by
pseudopodia. In fresh waters.
Ochromonas (Fig. 38 B). As Chrysamoeba^ but with two unequal
flagella; and usually one chromatophore.
Dinobryon (Fig. 38 C). Two unequal flagella; two yellow chro-
matophores. Secretes a flask-shaped house, which in some species
adheres to those of other individuals to form a pseudocolony. In fresh
waters.
Hydrurus (Fig. 38D-D2). One flagellum; one chromatophore.
Passes most of its life in the resting stage, which by division forms
a plant-like growth (see p. 47). In fresh waters.
Rhizochrysis. Flagella normally lacking; one chromatophore; body
naked and permanently amoeboid.
Leucochrysis . As Rhizochrysis, but colourless.
Silicoflagellata (or Silicoflagellidae) . One flagellum; numerous
yellow chromatophores ; a lattice-work case of hollow, siliceous bars.
Marine, planktonic, e.g. Distephanus (Fig. 38 F).
Coccolithophoridae . One or two equal flagella; two chromatophores
(sometimes green) ; a case composed of calcareous plates {coccoliths)
or rods (rhabdoliths) enclosing the body. Marine, planktonic, e.g.
Syracosphaera (Fig. 38 E).
Order CRYPTOMONADINA
Green, yellow, brown, or colourless phytomastigina; with starch (and
occasionally also oil) reserves ; with gullet or with longitudinal groove,
without transverse groove; very rarely amoeboid.
Many of the yellow members of this group live in the resting stage
as symbionts in other organisms.^
Cryptomonas (Fig. 39 A). Two flagella; two chromatophores,
usually green; a gullet. Marine and in fresh waters.
^ Owing to certain features of their nucleus and its mode of division these
symbionts have been held to be related to the Dinoflagellata. Their other
features, however, are those of the Cryptomonadina.
Fig. 38. Chrysomonadina. A, ChrysamoSa radians in the flagellate phase,
X 1250. Ai, The same in the amoeboid phase. B, Ochromonas sp., x iioo.
C, Dinobryon sertularia, x 750. D/' Flant" oi Hydrurus. Dj , Tip of a branch
of the same. D2 , Flagellate stage (" s warmer ") of Hy drums. E, Syracosphaera
pulchra, x 2000. F, Distephanus speculum, x 800. After various authors,
with modifications, cph. chromatophore ; cth. coccolith; leu. leucosin;
nu. nucleus.
52 THE INVERTEBRATA
Chrysidella (Fig. 39 B). Two flagella; two yellow chromatophores ;
a groove anteriorly. Symbiotic in foraminifera, radiolarians, etc.
Cyathomonas (Fig. 39 C). Two flagella; chromatophores absent.
Holozoic, seizing food by trichocysts in the gullet. In fresh waters.
Chilomonas. Two flagella ; chromatophores absent ; gullet very deep
and narrow. Saprophytic. In foul fresh waters.
Phaeococcus. Normally in the palmella phase. Marine and in fresh
waters.
Order EUGLENOIDINA
Phytomastigina which have numerous green chromatophores or are
colourless ; with reserves of paramylum and sometimes also oil ; with
gullet; with contractile vacuole opening by a "reservoir", usually
into the gullet; without transverse groove; with stout pellicle, usually
with metaboly ("euglenoid movement").
Euglena (Fig. 39 D, D'). A typical member of the group, with
chromatophores ; one flagellum, arising from the bottom of the gullet,
double at base, and connected by two rhizoplasts to a basal granule
behind the nucleus ; pyrenoids present only in a few species ; paramylum
reserves; and contractile vacuole fed by accessory vacuoles. The
nutrition is interesting. Most species, at least, can live and multiply,
with purely holophytic nutrition. All, however, flourish better if
traces of aminoacids be present. If the medium be rich in organic
substances, the use which is made of these varies with the species.
Most, including E. viridis, can take in organic combination nitrogen,
but not carbon ; a minority, including E. gracilis, can also obtain carbon
in that way. In the dark, if suitable compounds, especially peptones,
be present, the latter set of species bleach and live as saprophytes. It
has not been established that Euglena uses its gullet to take solid food.
Fresh waters, and infusions.
Peranema (Figs. 11, 39 E). Without chromatophores; gullet sup-
ported by rods and can open or close. Saprophytic and holozoic.
Paramylum reserves formed. In infusions.
Copromonas { = Scytomonas , Fig. 39 F, F'). Without chromato-
phores; body pear-shaped; no metaboly; gullet long and narrow.
Nutrition holozoic, chiefly by bacteria. Coprozoic in dung of frogs.
After some days of binary fission syngamy takes place between
ordinary individuals (hologamy), the nuclei first throwing out two
"polar bodies". Some zygotes encyst; others continue to divide.
Finally all encyst. The cysts are washed away and swallowed by a
frog or toad with its food. They pass uninjured through the gut and
hatch in the moist faeces, where alone the active stage exists.
Colacium. Normally in the palmella phase, forming branched,
plant-like growths.
Fig. 39. Cryptomonadina and Euglenoidina. A, Cryptomonas ovata, x 900.
B, Chrysidella schaudinni, in the resting stage. C, Cyathomonas truncata,
X 1000. U, Euglena viridis, x 400. D', A longitudinal section of the anterior
end of the same, more highly magnified. E, Pera?tema trichophorum, x 850.
F, Copromonas siibtilis, x about 1700. F', A pair of the same, beginning to
conjugate, less highly magnified. After various authors, with modifications.
c.vac. contractile vacuole ; c.vac' accessory contractile vacuole ; cph. chromato-
phore; e. eye-spot ; /.z^«c. food vacuole;^, flagellum; ^j/. gullet; nn. nucleus;
pmy. paramylum grains; res. reservoir; rod. stiflfening rods of gullet; stch.
starch grains; tri. trichocysts.
5^. THE INVERTEBRATA
Order CHLOROMONADINA
Phytomastigina which have numerous green chromatophores or are
colourless ; with reserves of oil ; gullet ; and complex contractile vacuole ;
without transverse groove ; possessing a delicate pellicle, or amoeboid.
Vacuolaria. Typical, bright green members of the group, which
pass much of the life history in the palmella stage. In fresh waters.
Order DINOFLAGELLATA
Phytomastigina which have numerous yellow, brown, or green
chromatophores or are colourless; with reserves of starch or oil or
both; with complex vacuole system; with two flagella, one directed
backwards and usually in a longitudinal groove (sulcus) and the other
transverse, usually in a more or less spiral groove (annulus) ; usually
with an armour of cellulose plates, but sometimes amoeboid.
The complex vacuoles of dinoflagellates are not, as was held, con-
tractile, but contain water driven into them through their external
pores by the action of the flagella. Their function is unknown.
Possibly they are hydrostatic, or alimentary, or both.
The plane of fission is oblique, but resembles the longitudinal
fission of other Mastigophora in passing between the two flagella.
Fission may be within or without a cyst: in either case it may be
simply binary or repeated ; within a cyst it is sometimes multiple. The
products of repeated binary fission of pelagic forms sometimes hang
together for a considerable time as a chain. The occurrence oisyngamy
is suspected but has not yet been proved beyond doubt.
The typical members of this order are free-living and highly
organized, but it includes forms which are greatly degenerate and
only recognizable as belonging to it while they are spores. The
members may be holophytic, saprophytic, or holozoic, feeding in
the latter case by pseudopodia either from a spot on the sulcus or at
any point. They are usually pelagic, sometimes parasitic, and for the
most part marine.
Ceratium (Fig. 40 A). Typical, armoured, holophytic species; with
three long spines. In freshwater forms the chromatophores are green ;
in marine species they are yellow or brown.
Dinophysinae. Pelagic genera, often of bizarre form, with the
annulus at one end of the body, and the shell in two lateral plates.
Polykrikos (Fig. 40 B). Soft-bodied species; colourless and holo-
zoic; with the flagella and other external features repeated several
times along the axis of the body, and the nucleus also repeated,
but not in correspondence with the other features (see p. 10). The
protoplasm contains peculiar nematocyst-like organs. Holozoic.
PHYTOMASTIGINA
55
Oodinium (Fig. 34). Thin-cuticled ; pear-shaped; colourless; living
as an ectoparasite on marine pelagic animals, and possessing the
typical dinoflagellate organization only in the spore stage.
Dinamoehidium. Colourless and holozoic; completely Amoeba-like
in the ordinary phase, but forming dinoflagellate swarm spores in a
fusiform cyst.
Noctiluca (Fig. 41). (Formerly placed in an independent order —
Cystoflagellata.) Large, peach-shaped forms; colourless and holo-
zoic; with highly vacuolated protoplasm; a stout pellicle; and, in
nil.
tr.fl.-
t^-ann.
ntc.--i
Fig. 40. Dinoflagellata. A, Ceratium macroceras, x about 300. B, Polykrikos
schwarzi, x 250. C, A discharged "nematocyst" of Po/j'/en'i^o^. After various
authors, with modifications, ann. annuli; cph. chromatophore ; cu. cuticle;
In.fl. longitudinal flagellum; ntc. "nematocyst" ; nu. nucleus; sul. sulcus;
sut. suture between plates of cuticle ; tr.fl. transverse flagellum.
the groove of the peach, an elongate mouth, a small flagellum, a
structure known as the tooth which is said to represent the transverse
flagellum, and a strong tentacle, homologous with a similar structure
in certain more normal dinoflagellates. Noctiluca is phosphorescent.
Like other dinoflagellates it reproduces by binary fission and by spore
formation after multiple fission. The spores are more dinoflagellate-
like than the adult. Marine, pelagic.
Dinothrix. Normally in the palmella phase, forming thread-like
growths. Marine.
56
THE INVERTEBRATA
Order VOLVOCINA
Phytomastigina which have usually a flask-shaped, green chromato-
phore, with one or more pyrenoids, but are sometimes colourless,
though never holozoic; form starch reserves, even when colourless;
have no gullet or transverse groove ; possess usually a cellulose cuticle
and often haematochrome; and regularly undergo syngamy.
Of all the Mastigophora, the members of this order most closely
resemble the typical plants.
Fig. 41.
Fig. 41. Noctiluca, x 65. A, Ordinaiy individual. B, Spore formation.
C, A spore. After various authors, with modifications, fl. flagellum; nu.
nucleus; ten. tentacle; tth. tooth.
Fig. 42. Haematococcus lacustris, x 475. After West. A-C, Individuals in
ordinary phase, showing strands of protoplasm from body to cuticle. D-F,
Successive stages in fission. G, H, Individuals in resting phase.
Chlamydomonas (Figs. 23, 25). Typical solitary members of the
order, with two flagella ; an eye-spot ; a close-fitting cellulose cuticle ;
and one pyrenoid. The various species exhibit isogamy, anisogamy,
and intermediate conditions (see p. 31). In fresh waters.
Polytoma (Fig. 24). A colourless Chlamydomonas', retaining the
eye-spot (usually) and the habit of starch formation; but with the
VOLVOCINA
57
cuticle composed of some substance which does not give the cellulose
reaction. Nutrition saprophytic by means of simple substances (fatty
acids, aminoacids, etc.). Syngamy is facultatively hologamy or
merogamy, isogamous or anisogamous, according to the age of the
gametes. In infusions of decaying animal substances.
Carter ia (Figs. 35, 36). Differs from Chlamydomonas in having
four flagella. It is probably a species of this genus that is symbiotic
in the turbellarian Convoluta roscojfensis .
Fig. 43. Pandorina. From Godwin, a. The adult colony of sixteen similar
flagellated zooids, x 200. h, A colony undergoing asexual reproduction,
X 450 — each zooid has divided to form a daughter colony which still remains
within the parent body. Some of the colonies have already produced flagella,
and will shortly break out of the wall which enclosed the parent, b-g, Stages
in sexual reproduction — b, Motile gametes, c, Stage immediately after fusion
of two gametes, d, Later stage showing flagella withdrawn, e, Later stage
showing resting zygote with thickened wall. /, Motile individual produced
by the zygote on germination, g. New colony produced by vegetative division
of the motile individual.
Haematococcus ( = Sphaerella, Fig. 42). Differs from Chlamy-
domonas in that there is a wide space, traversed by protoplasmic
threads, between body and cuticle; several pyrenoids. Much hae-
matochrome is often present. Isogamous. Common in collections
of rainwater.
Pandorina (Fig. 43). Spherical, fi:ee-swimming colonies of 16 or
32 green pear-shaped zooids, each with the organization of the solitary
members of the order, closely pressed together with the narrow end
inwards and the flagella outwards. An additional cellulose envelope
containing mucilage encloses the whole colony. The colonies are re-
produced in two ways: (i) asexually, by the repeated fission of each
58 THE INVERTEBRATA
zooid to form a group of 16 like the parent colony, the dissolution of
the colonial and zooid envelopes, and the setting free of 16 young
colonies ; (2) sexually, by the division of each zooid and the setting
free of its products as gametes which, except in size, resemble ordinary
zooids. Since the number of fissions in the formation of gametes
differs in different colonies, the gametes differ in size. They unite in-
differently, so that some of the unions are isogamous, though most are
anisogamous. The zygote, after a period of encystment, becomes a
free flagellate and divides to form a colony. In fresh waters.
Eudorina (Fig. 3 a). Colonies which differ from those of Pandorina
in that : {a) the zooids are spaced on the inside of the common en-
velope, though connected by strands of protoplasm; {b) the sexual
reproduction is strongly anisogamous, since in some colonies the
zooids do not divide but, becoming somewhat larger, act as macro-
gametes, while in others each zooid divides into a bundle of 16-64
slender individuals (microgametes), which are set free and fertilize the
individuals of a macrogamete (female) colony.
Pleodorina (Fig. 36, c). Rather larger colonies which differ from
those of Eudorina in that some of the zooids do not perform repro-
duction. These zooids, which are smaller than the rest, are termed
"somatic".
Volvox (Figs. 44-46). Large, subspherical colonies resembling in
general features those of Pleodorina but with smaller and more
numerous zooids, of which a much smaller proportion is reproductive.
Those zooids which perform asexual reproduction are known as
parthenogonidia : the plates of young zooids which arise by their radial
fission, curving into spheres to form the new colonies, bulge into the
hollow of the parent colony, where they remain for a time before they
are set free. The clusters (antheridia) of microgametes arise in the
same way. In some species the microgametes are considerably modi-
fied, being pale, very slender, and bearing their flagella in the middle
of their length. Male, female, and asexual reproductive zooids may
be found in any combination in a colony. Details of the structure of
the colonies are shown in Figs. 45, 46.
Subclass ZOOMASTIGINA
Mastigophora which do not possess chromatophores and are not
otherwise practically identical with coloured forms.
By one or more of the following peculiarities of the Zoomastigina
most members of the group are distinguished from most colourless
members of the Phytomastigina.
I. The Zoomastigina never have starch or other amyloid re-
serves.
MASTIGOPHORA
59
2. They often have more than two flagella. This is very rare in the
Phytomastigina.
Fig. 44, Volvox aureus. After Klein, a (x 180), A medium-sized colony
showing as round black dots the numerous "somatic cells" of which it is
made up; the protoplasmic connections between them, and the cell-walls,
can only be made visible by staining. The colony contains three types of re-
productive units : daughter colonies (d.c.) produced asexually by division of
a single zooid; ripe macrogametes or young zygotes (z); and young "an-
theridia" (an) whose contents are dividing up and will eventually form
microgametes. b, A colony of microgametes which has just escaped from the
antheridium. c, Mature antheridia as seen in surface view of a colony; in two
the microgametes are seen sideways, and in two endways.
3. With a single exception,^ it has not yet been established that
syngamy occurs in any of them.
4. Many of their parasitic members possess parabasal bodies.
^ Helkesimastix, a coprozoicmember of the Protomonadina, performs hologamy.
6o
THE INVERTEBRATA
Order RHIZOMASTIGINA
Zoomastigina with one or two flagella, and the whole surface of the
body permanently amoeboid.
Mastigamoeba (Fig. 47 A). One flagellum; numerous, finger-like
pseudopodia. In fresh waters.
^Mm
-(mo)— I
V.Cc.l.
V42.
Fig. 45. Diagrams to show the structure of the colony of two species of
Volvox. After Janet. V.a.i. Surface view of a small part of the colony of
V. aureus. V.a.z. Section through a similar region. V.g.i. and V.g.2. show
V. globator in the same way. The zooids are very different in shape in the two
species, but in both they have been separated by the formation of mucilage
{mu) by the cell-walls; the unaltered middle layer of the walls {m.l.) is still
visible. Protoplasmic strands (p.c), fine in the one species and thick in the
other, connect the zooids. Each zooid, with its curved chloroplast (ch) often
containing more than one pyrenoid, its eye-spot, and two flagella, has the
structure of a Haematococcus.
Order HOLOMASTIGINA
Zoomastigina with numerous flagella, and the whole surface of the
body capable of amoeboid action.
Multicilia. Spherical, with 40 or 50 flagella scattered evenly over
the whole surface, at any point on which food can be ingested by
amoeboid action. A marine species with one nucleus; freshwater
species multinucleate.
Fig. 46. Volvox. After Janet and Klein, a, V. aureus, a daughter colony of
small size seen through the layer of zooids of the parent colony; the opening
left in the young colony at its formation is shaded, b, V. aureus, a single
macrogamete among the ordinary somatic zooids; abundant protoplasmic
filaments connect it with surrounding zooids and it contains large nucleus (a)
and chloroplast (ch). c, V. aureus, a plate of mature microgametes just
liberated from an antheridium and now beginning to separate. Each contains
nucleus (n), eye-spot (y), flagella, a chloroplast (ch), and pyrenoid (p). d, V.
globator, diagrammatic section through the middle of an old colony showing
three large daughter colonies projecting into the interior of the parent colony
which is full of thin mucilage with a radiating structure, e, V. globator, similar
section to d, showing three antheridia {an) in different stages of maturity and
three large macrogametes {e). Both types of organ have been formed from a
single zooid of the parent sphere into the interior of which they now project.
In d and e, the flagella of the somatic zooids have been omitted.
62
THE INVERTEBRATA
f-vac.-/-
Fig. 47. Zoomastigina. A, Mastigamoeba aspera, x about 300. B, Oikomonas
termo, x 2000. C, Monas vulgaris, x 2000. D, Bodo saltans, x 2000. E, Try-
panosoma hrucei, x 2800. F, Crithidia sp., x 2300. After various authors,
with modifications, ba.gr. basal granules of flagella ; bri. bristle-like processes
borne by the surface of the protoplasm; c.vac. contractile vacuole ;/.z;ac. food
vacuole; fl. flagellum; fl.' trailing fiagellum; M. position of mouth-spot;
nu. nucleus; p.by. parabasal body; ^5. pseudopodium ; rh. rhizoplast; u.me.
undulating membrane.
ZOOMASTIGINA
63
Order PROTOMONADINA
Zoomastigina with one or two flagella; amoeboid movement, if
present, not active over the whole surface of the body ; and no extra-
nuclear division centre.
Monas (Fig. 47 C). Two unequal flagella. Ingestion at base of
flagella. Except for absence of chromatophores much resembles
Ochromonas among the Phytomastigina and is probably related to
that genus. In fresh waters and infusions.
Bodo (Fig. 47 D). Two rather unequal flagella, of which one trails
freely behind and is used for temporary anchoring. Ingestion at a
spot near the base of the flagella. In infusions and coprozoic.
—fl-
%
-u.me.
-nn.
B
~~p.by.
Fig. 48. A diagrammatic comparison of various Trypanosomidae. A, Her-
petomonas. B, Leishmania. C, Crithidia. D, Trypanosoma, ba.gr. basal
granule;^, flagellum; nu. nucleus; p.by. parabasal body; u.me. undulating
membrane.
Oikomonas (Fig. 47 B). One flagellum. Ingestion of food as in
Monas. This genus bears the same relation to certain uniflagellate
Chrysomonadina that Monas bears to Ochromonas. In fresh waters
and soil.
Trypanosomidae (Fig. 48). Parasites, with one flagellum; a slender,
usually pointed shape; a strong pellicle without ingestion spot; a
parabasal body; and no contractile vacuole. This family, which con-
tains many dangerous parasites of man and domestic animals, appears
to have originally infested invertebrates and to have obtained access
64 THE INVERTEBRATA
to vertebrates owing to the latter being subject to attack by the original
hosts. The original mode of infection was by faeces. The species of
each genus assume, in certain circumstances, the forms characteristic
of other genera. The following are the principal genera.
Herpetomonas { = Leptomonas) . Basal granule and parabasal body
at one end, near the origin of the flagellum. Parasitic in the gut,
principally of insects, but also of other invertebrates and of reptiles.
Leishmania. Oval bodies containing a nucleus, parabasal body,
basal granule and rhizoplast, but with no flagellum, infesting the
tissues of vertebrates, and transferred by flies of the genus Phleboto-
muSy in whose gut they assume the form of Herpetomonas. In Man they
cause kala-azar and Oriental sore.
Crithidia (Fig. 47 F). Flagellum starts from a basal granule near
the middle of the long, slender body, to which the flagellum is united
by an undulating membrane; parabasal body placed between the
basal granule and the nucleus. Parasitic in the gut of insects.
Trypanosoma (Fig, 47 E). As Crithidia^ but the basal granule of
the undulating membrane and the parabasal body are beyond the
nucleus, towards the non-flagellate end. Many species, all parasitic
in the blood and other fluids of vertebrates, and nearly all (not T.
equiperdum) distributed by a second, invertebrate, host, which is
usually an insect for terrestrial species and a leech for aquatic species.
In the invertebrate the trypanosome passes for a time into a condition
in which it resembles Crithidia^ and during which it is incapable of
reinfecting the vertebrate. Reinfection is in some species (e.g. T. lewisi
in the rat, transmitted by a flea) by the invertebrate or its faeces being
swallowed by the vertebrate ; this is probably the original mode of
obtaining entry to the vertebrate host. Other species (e.g. T.gambiense^
transmitted by a tsetse fly) are reintroduced to the vertebrate by the
bite of the invertebrate. T. equiperdum, parasitic in horses, in which
it is the cause of " dourine", is transmitted by coitus and has dispensed
with the invertebrate host.
Most, if not all, of the pathogenic species have a wild host with
which they are in equilibrium and in which they are non-pathogenic.
T. lewisi, non-pathogenic in the blood of the rat, has a period of
intracellular multiple fission in the stomach of the flea and then passes
into the rectum of the latter, where it changes from the crithidial to
the trypanosome form and becomes capable of reinfecting the verte-
brate, which it accomplishes in the manner mentioned above. T. cruzi,
the cause of Chagas' disease in Man in South America, is non-patho-
genic in the armadillo. It is transmitted by the bug Triatoma, in which
it probably has an intracellular stage, and becomes infective in the
faeces. In the vertebrate host, it passes most of its time, and reproduces,
as a Leishmania form, in the tissues. T. gambiense and T. rhodesiense^
ZOOMASTIGINA 65
causes of sleeping sickness in man when they have passed into the
cerebrospinal fluid, and T. brucet, the cause of African cattle sickness,
are non-pathogenic in antelopes. Their crithidial stage is passed in the
salivary glands of the tsetse (Glossina), reproduces by binary fission,
and is not intracellular. They are transmitted to the vertebrate host
by the bite of the fly.
Choanoflagellata (or Choanoflagellidae) . Uniflagellate, generally
fixed, forms; with a protoplasmic collar around the base of the flagel-
lum. Ingestion by attraction of particles by the flagellum to the outside
of the collar, adherence to this, and transference by streaming of
protoplasm to the base of the collar, where they are received by a
vacuole which is formed between the cuticle, if present, and the proto-
'■■f-vac.
Fig. 49. Choanoflagellata. A, Monosiga brevipcs, x 1200. B, Codosiga um-
bellata, x 310. Both after Saville-Kent. C, Ingestion in Codosiga. f.vac.
food vacuole. The dotted lines show the currents set up by the flagellum,
the small arrow the transport of the food particles on the collar.
plasm (Fig. 49 C): defaecation within the collar. There is usually a
stalk, generally not of living matter. This may branch, and thus unite
numerous zooids. Examples are Monosiga (Fig. 49 A), solitary, with
protoplasmic stalk; Codosiga (Fig. 49 B), branched, with cuticular
stalk.
Order POLYMASTIGINA
Zoomastigina with two to many, generally with more than three,
flagella, and an extranuclear division centre.
The genera here placed in one order are usually separated as Poly-
mastigina, Hypermastigina, and Diplomonadina. They are the most
highly organized members of the Mastigophora.
Trichomonas (Fig. 50). (One of the Polymastigina sensu stricto.)
Body roughly egg-shaped ; with four flagella, of which one is directed
66
THE INVERTEBRATA
backward and united to the body by an undulating membrane ; a cyto-
stome near the broad anterior end; and an axostyle which projects
from the posterior end. The united basal granules act as a division
centre, possibly in virtue of a centriole concealed among them. The
cytostome is used for ingestion. A staining body which follows the
base of the undulating membrane has been regarded as the parabasal
body, but a deeper-lying structure is now asserted to represent that
Fig. 50. Trichomonas niuris, semidiagrammatic. From Hegner and TaHa-
ferro, after Wenrich. axs. axostyle; ce. compound basal granule which acts
as a centriole; ch.gr. inner row of chromatic granules; ch.gr.' outer row of
chromatic granules; ch.rd. chromatic basal rod of undulating membrane;
ch.rg. chromatic ring at the emergence of the axostyle ; fl. anterior flagella ;
fl,.' posterior flagellum; kar. karyosome; M. mouth (cytostome); nu. nucleus;
p.hy. parabasal body; u.me. undulating membrane; u.me.' posterior flagellum
lying along the edge of the undulating membrane.
organ. In the cytoplasm, a number of " chromatic granules " are also
present. Several species, parasitic in various cavities of vertebrates,
including the mouth, intestine, and vagina of man.
Hexamitus { = Octomitus, Fig. 4). (Diplomonadina.) Body elon-
gate ; without gullet ; presenting strong bilateral symmetry ; and bear-
ing on each side four flagella, three anterior and one posterior, the
basal granules of the foremost being united; and an axostyle. Two
nuclei are present, one on each side of the body, near the anterior
POLYMASTIGINA 67
group of basal granules, with which they are connected. Intestinal
parasites of vertebrates.
Giardia { = Lambliay Fig. 51). (Diplomonadina.) Shaped like a half-
nu
Fig. 51.
Fig. 52.
Fig. 51. Giardia intestinalis, from the intestine of man. Semidiagrammatic.
axs. axostyle (axoneme) ; ba.gr. basal granules ; ce. centriole ; cone, ventral con-
cavity (" sucker "); yz. fibre around concdivity ; fl., fl.' , fl.'\ fl.'" anterolateral,
posterolateral, ventral, and caudal flagella; kar. karyosome; la.sd. lateral
shield, the thickest part of the body; nu. nucleus; p.by. parabasal body;
rh. rhizoplasts.
Fig. 52. A diagram of the structure of Trichonympha campanula, showing a
portion of each layer. From Hegner and Taliaferro, after Kofoid and Swezy.
alv.l. alveolar layer; ant.fl. anterior flagella; ba.gr. rows of basal granules;
ce. point at which the spindle arises in division; chr. chromatin granules in
nucleus; ecp. ectoplasm; enp. endoplasm; f.b. food bodies; lat.fl. lateral
flagella; long.my. longitudinal myonemes; nu. nucleus; obl.f. oblique fibres
(rhizoplasts) ; /)e/. pellicle ; /)05f.yZ. posterior flagella; ^wr/.rd'^. surface ridges of
pellicle; tr.niy. transverse myonemes.
pear, broad end forwards, with, on flat side, a concavity for adhesion.
Organization as Hexamitus but all flagella in middle or hinder region.
Parasitic in intestine of man and other mammals.
Trichonympha (Fig. 52). (Hypermastigina.) Body narrower in
68 THE INVERTEBRATA
front than behind; provided with very numerous flagella arranged in
three distinct sets; without gullet. At the front end is a papilla. The
ectoplasm, thin behind, is strong and complex in the fore part of the
body, where it is composed of the following layers: (i) a pellicle,
sculptured into longitudinal ridges, (2) a layer containing longi-
tudinal rows of the basal granules of the flagella, (3) a layer containing
a network of rhizoplasts ("oblique fibres"), (4) an alveolar layer,
(5) a layer of transverse myonemes, (6) a layer of longitudinal my-
onemes. In the conical front region on which the first set of flagella
stand, the rhizoplasts and basal granules are merged to form con-
verging strands with which the flagella are connected. At division this
conical apparatus acts as a division centre, dividing first and forming
the spindle between its halves as they separate. Possibly it does so in
virtue of a concealed centriole. Trichonympha is symbiotic with ter-
mites, in whose gut it lives (p. 435). The termite devours wood but is
unable itself to digest it. The digestion is performed by the protozoon,
which obtains in return food and lodging. Wood particles are con-
tained in the endoplasm of the hinder part of the body of Tricho-
nympha, into which they are ingested by the cupping-in of this region
under the action of the myonemes of the forepart.
Class SARCODINA (RHIZOPODA)
Protozoa which in the principal phase are amoeboid, without flagella;
are usually not parasitic; have no meganucleus; and, though they may
have a phase of sporulation, do not form large numbers of spores after
syngamy.
With the exception of the Amoebina and Foraminifera, which are
undoubtedly closely related, the orders of this class have much less
affinity with one another than have those of the Mastigophora. In
all of them flagellate young and gametes are common.
Order AMOEBINA
Sarcodina which have no shell, skeleton, or central capsule; whose
pseudopodia never form a reticulum and are usually lobose ; and whose
ectoplasm is never vacuolated.
Thus defined, the group excludes forms such as Arcella which
differ from its members practically only in the possession of a shell.
These forms, however, are also connected with the typical Forami-
nifera by intermediates (as Lieberkuhnia and Allogromia) . There is,
indeed, a continuous series from naked amoebae to such foraminifera
as Polystomella, and the drawing of a boundary line between the
groups of which they are typical is a matter of convenience.
PROTOZOA 69
Naegleria (Fig. 53). Small amoebae which live in various foul in-
fusions ; possess a contractile vacuole ; and in certain conditions pass
into a biflagellate phase. Naegleria is placed here rather than among
the Rhizomastigina because it is most often in the non-flagellate con-
dition, its flagellate phase, though fully grown, is not known to per-
form reproduction, and the general features of the amoeboid phase
are those of the amoebina of the Umax group, most of whose members
appear to have no flagellate phase. These organisms form one or two
_ fcon.vac.
Fig. 53. Naegleria bistadialis, X 800. Partly after Kiihn, in Doflein. A,
Amoeboid condition. B, Transition to flagellate condition. C, Flagellate
condition, coti.vac. contractile vacuole; rh. rhizoplast.
broad pseudopodia, are given to assuming a slug-like shape with one
pseudopodium at the foremost end, and have a very simple nucleus
with a large karyosome.
Vahlkampfia, also found in foul infusions, is a typical member of
the Umax group.
Amoeba (Fig. 54). Typical amoebae, with numerous pseudopodia;
contractile vacuole; and no flagellate phase. Various species, of which
the commonest three are shown in the figure. The true A. proteus is
the largest of the common Amoebae, has a lens-shaped nucleus and
yo
THE INVERTEBRATA
longitudinal ridges on the ectoplasm, forms spores endogenously in
the unencysted condition, and does not normally feed on diatoms,
which form a great part of the food of A. duhia.
Entamoeba (Figs. 55, 56). Parasitic amoebae; without contractile
vacuole. Reproduction during most of the life history is by binary
fission. Finally encystment takes place and in the cyst the nucleus
divides several times. The cysts pass out of the host and infect a new
^■f£-..5>;.*" Jia-tf'- '..'<?-!i' :
^.VV'%^•lL-■<a•;^•:v^r.•£•i
^ -St ..•,■■' - ■ - '> ■-'■'
1^
><^-
■\^^
-••'■•'''?;
._.;:®
#?;
Fig. 54. Amoebae. From Hegner and Taliaferro, after Schaeffer. A, A. pro-
teus. a^, Equatorial view of nucleus, a^, Polar view of nucleus, a^. Equatorial
view of nucleus in the folded condition often seen in this species, a*, Crystal
of the kind found distributed in the endoplasm of the species. B, A. discoides.
6^, 6^, Equatorial and polar views of nucleus. 6^, Crystal. C, A. dubia. c^,c^.
Equatorial and polar views of the nucleus, c^-c^", Crystals and concretions.
Dimensions in microns : A, 600 in length. B, 450 in length. C, 400 in length.
a^, 46 X 12. b^, 40 X 18. c\ 40 X 32. a*, maximum 4-5. b^, maximum 2-5.
^3_^io^ maxima 10 to 30.
individual, in which they are dissolved and set free their contents,
which divide into uninucleate young. The cysts must remain in a fluid
medium if they are to cause reinfection. Several species exist, occur-
ring in various vertebrates and invertebrates. E. coli is a harmless
commensal in the colon of man, feeding on bacteria, etc. E. histolytica
{ = E. dysenteriae), a parasite which often causes dysentery and oc-
casionally abscesses of the liver and other organs, differs from E. coli
in having a distinct ectoplasm, in the central position of the karyo-
ecp. enp.
B /• vac.
Fig. 55. Entamoeba, x about 2000. After Dobell and O'Connor. A, E. histo-
lytica. B, £". to/t. ^.c. ingested red blood corpuscles; ecp. ectoplasm; enp.
endoplasm ; f.vac. food vacuole ; kar. karyosome, eccentric in E. coli ; nu.
nucleus ; ps. pseudopodium.
or /
Fig. 56. Fig. 57.
Fig. 56. A diagram of the life cycle of Entamoeba histolytica, a-f, Encystment
and formation of amoebulae. I-III, Binary fission (in gut of host).
Fig. 57. Pelomyxa palustris. Partly after Doflein. f.p. undigested particles
swallowed with food ; ^(y. glycogen granules; nu. nuclei (stained).
72 THE INVERTEBRATA
«
some and in certain other features of the nucleus (Fig. 55), and in
forming only four, instead of eight, nuclei in the cyst. This species
breaks up by digestion cells of the intestinal epithelium and other
tissues, absorbs the soluble products, and ingests portions of the
destroyed cells and also red corpuscles.
Pelomyxa (Fig. 57). Large, multinucleate species, living in, and
feeding by ingesting, the mud of stagnant fresh waters rich in vegetable
debris. The cytoplasm contains glycogen granules (see p. 20).
Order FORAMINIFERA
Sarcodina w^hich have either a shell or reticulate pseudopodia or,
usually, both these features ; and in pelagic species a vacuolated outer
layer of protoplasm.
The shell may be of one or of several chambers, and is composed in
different cases of different materials, nitrogenous, calcareous, siliceous,
or of foreign particles.
The pseudopodia may be lobose, filose, reticulate without streaming
of particles along them, or reticulate with streaming. The latter type
alone is found in the Polythalamia.
The reproduction of the single-chambered forms (Monothalamia)
is both by binary and by multiple fission. In binary fission, Lie-
herkUhnia and Trichosphaerium divide the shell. In the rest, a portion
of the protoplasm emerges from the old shell and secretes a new one
(Fig. 58), the nucleus or nuclei divide, one of the products of each
passing into the protruded protoplasm while the other remains in the
old shell, and the two portions of protoplasm break apart. Multiple
fission usually produces amoebulae, sometimes flagellulae. The latter
are known or suspected to be gametes. In these forms there does not
usually appear to be a regular alternation of sexual and asexual repro-
duction. In the Polythalamia binary fission does not occur, and in
some of them, perhaps in all, there is a more or less regularly alternate
production of asexual amoebulae and flagellate gametes.
Suborder MONOTHALAMIA
Foraminifera, usually of freshwater habitat; with non-calcareous,
single-chambered shells; whose pseudopodia are rarely reticulate;
and whose protoplasm does not extend as a layer over the shells.
Arcella (Figs. 22, 59). Shell pseudochitinous, shaped like a tam-o'-
shanter cap, finely sculptured; pseudopodia lobose; two or several
nuclei and a chromidium present. Gas vacuoles in the protoplasm
are said to contain oxygen and to have a hydrostatic function. Re-
production by binary fission, or by budding to form amoebulae with
fine pseudopodia {Nucleariae). In fresh waters.
Fig. 58. Binary fission of Eiiglypha alveolata, x about 450. From Hegner
and Taliaferro, after Schewiakoff. A, B, C, D, Successive stages in the mitosis,
with formation and occupation of a new shell.
att.
ah.
Fig. 59. Arcelladiscoides, x 500. From Leidy. A, Seen from above. B, Seen
from the side, optical section, ott. thread attaching animal to inner surface
of shell; f.vac food vacuole; g.vac. gas vacuole; nu. nucleus; op. edge of
opening into shell ; ^^. pseudopodia; j/?. shell.
74
THE INVERTEBRATA
Diffiugia (Fig. 60). Shell of sand grains, etc., united by organic
secretion, pear- or vase-shaped; pseudopodia lobose; one or two
nuclei and a chromidium present. Gas vacuoles sometimes formed.
In fresh waters.
Euglypha (Figs. 7, 58). Shell resembling that of Difflugia but
formed of siliceous plates secreted by the animal; pseudopodia filose.
In fresh waters.
Trichosphaerium. Flat, encrusting forms, with a jelly coat; finger-
like pseudopodia protruding through separate openings in the coat;
and numerous nuclei. Reproduction alternately by escape of amoe-
bulae and of biflagellate isogametes ; but both generations can perform
plasmotomy. Marine.
sh.
Fig. 60. Fig. 61.
Fig. 60. Difflugia urceolata, x 100. After Leidy. sh. shell composed of
particles of sand containing body of the animal ; ps. pseudopodia.
Fig. 61. Lieberkuhnia wagneri. After Verworn.
Lieberkiihnia (Fig. 61). Shell thin, flexible, egg-shaped, with
mouth directed to one side; pseudopodia reticulate. Shell divided at
binary fission. Marine and in fresh waters.
Suborder POLYTHALAMIA
Foraminifera, nearly always of marine habitat; usually with a shell
of several chambers, which is most often calcareous, but sometimes
with one chamber or no shell ; whose pseudopodia are reticulate ; and
whose protoplasm extends as a layer over the shell.
The external layer of protoplasm can be withdrawn into the shell.
FORAMINIFERA
75
The shells of this group are typically many-chambered and cal-
careous, but a fair number are one-chambered, and most of these and
some of the many-chambered shells are composed of foreign particles
{arenaceous). Either kind may be imperforate or perforate by numerous
small pores, but most of the non-calcareous shells are imperforate.
The one-chambered shells are of various shapes. They usually grow
A B
Fig. 62. A, Section of a foraminifer in which each septum is formed of a
single lamella. B, One in which the septum is formed of two lamellae and
a supplemental layer is present. After Carpenter, a, passages between the
chambers; h, septum; c, anterior wall of last chamber; d, supplemental
skeleton.
A B
Fig. 63
Selsea.
form,
Dimorphism of Numniulites laevigatus, Bracklesham Beds (Eocene),
From Woods. A, Section of the entire shell of the megalospheric
X 9. B, Section of the central part of the microspheric form, x 9.
by extension at their openings. Shells with more than one chamber
grow by the addition of chambers. The protoplasm bulges from the
mouth of the shell and there secretes around itself a new chamber into
which opens the previous mouth. The chambers may be arranged in
a straight line, as in Nodosaria (Fig. 6B), or in a spiral, as in Poly-
stomella, etc. (Figs. 62, 63, 65), or occasionally irregularly; and the
shell may be strengthened by the deposition, upon their original
76 THE INVERTEBRATA
walls, of a supplemental layer (Fig. 62 B). The nuclei, where there
is more than one, bear no constant relation to the chambers.
In many species the shells are dimorphic, the two forms (Figs. 63,
66) being distinguished by the size and arrangement of the first formed
chamber, which is small in one (the microspheric form) and larger in
the other (megalospheric) . These forms correspond to the alternation
of generations in the life cycle (Fig. 66), the microspheric form, which
usually becomes multinucleate at an early stage, reproducing asexu-
ally by multiple fission, while the megalospheric form, which remains
uninucleate till it is about to reproduce, produces gametes.
Most foraminifera are creeping organisms, but the Globigerinidae
are planktonic and have, correspondingly, vacuolated ectoplasm and
long slender spines on the shell. The shells of such forms, falling to
the bottom, form an important constituent of many deep-sea oozes.
Allogromia (Fig. 64). Shell one-chambered, egg-shaped, pseudo-
chitinous. Marine and in fresh waters.
Rhabdammina (Fig. 6 A). Shell one-chambered, straight or forked,
tubular, composed of foreign particles. Marine.
Nodosaria (Fig. 6B). Shell perforate, calcareous, consisting of
several chambers arranged in a longitudinal row, the mouth of each
chamber opening into the next younger and larger. Marine.
Polystomella (Figs. 65, 66). Shell perforate, calcareous, consisting
of numerous chambers, arranged in a flat spiral, and complicated as
follows in the details of their architecture: each whorl is equitant,
i.e. overlaps the previous whorl at the sides and thus hides it; the
mouth is replaced by a row of large pores ; backward pockets {retral
processes) stand along the hinder edge of each chamber; the sup-
plemental layer contains a system of canals filled with protoplasm.
Marine. The life cycle of this genus, which shows the alternation of
generations described above, has been followed in detail (Fig. 66).
Nummulites (Fig. 63). As Polystomella but with more chambers.
Marine. Includes, besides recent forms, large fossil species in Eocene
limestones.
Globigerina (Fig. 6C). Shell perforate, calcareous, chambers fewer
and less compact than in Polystomella, arranged in a rising (helicoid)
spiral, and bearing long spines. External layer of protoplasm frothy,
with large vacuoles by which the specific gravity is reduced. Marine,
pelagic. Its shells are common in oceanic oozes and in chalk.
Order RADIOLARIA
Marine, planktonic Sarcodina, which have no shell but possess a
central capsule and usually a skeleton of spicules ; whose pseudopodia
are fine and radial and usually without conspicuous axial filament;
and the outer layer of whose protoplasm is highly vacuolated.
Fig. 64. Allogromia oviformis, x 230, but the pseudopodia less than one-
third their relative natural length. From M. S. Schultze. sh. shell; ppm. pro-
toplasm surrounding shell; ps. pseudopodia, fusing together in places and
surrounding food particles such as diatoms, which have adhered to the
pseudopodia owing to the stickiness of the latter, and are digested in situ,
without the formation of visible food vacuoles around them.
78
THE INVERTEBRATA
Fig. 65. Polystomella crispa, x 45. After M. S. Schultze. sh. shell ;/)/)m. a
mass of protoplasm formed by the fusion of pseudopodia ; ps. pseudopodia.
The retral processes are darkly shaded : the external protoplasm is not visible.
FORAMINIFERA
79
Fig. 66. Stages in the life cycle of Polystornella. Semidiagrammatic. A, Me-
galospheric form, decalcified and stained. B, Shell of the same surrounded by
escaping gametes. C, D, Conjugation. E, Zygote. F, Microspheric form,
decalcified and stained. G, Shell of the same surrounded by escaping amoe-
bulae. H, Young megalospheric individual with three chambers, gam.
gametes ; i.wh. inner whorl of spiral ; o.wh. outer whorl ; nu. nucleus, ret.pr. re-
tral processes; i, first chamber.
8o
THE INVERTEBRATA
The pseudopodia branch, and to some extent join: they are said to
contain an axial filament and they show streaming of granules. The
central capsule is a pseudochitinous structure, of varying shape accord-
ing to the species, which encloses the nucleus and some cytoplasm
containing oil globules. It is perforated by pores, which by their
arrangement characterize the suborders, being evenly distributed in
the Peripylaea {Spume llaria), gathered into groups in the Actipylaea
(Acantharta), concentrated into one *'pore plate" in the Monopylaea
(Nassellaria), and represented by three openings or "oscula" in the
Tripylaea {Phaeodaria). The spicules are usually siliceous, but in one
group (Acantharta) they are said to be of strontium sulphate. They are
rarely absent, occasionally loose, but usually united into a lattice-work
1/ V
Fig. 67. Fossil Radiolaria. From Woods. A, Lithocampe tschernyschevi,
Devonian. B, Trochodisciis longispmus, Carboniferous. C, Podocyrtis schom-
burgki, Barbados Earth (Tertiary). A and C, Nassellaria; B, Spumellaria.
(Figs. 67, 68), which is often very complicated, with projecting spines.
The latter may be radial but do not meet at a central point except in the
Acantharia. The outer layer of the body differs from that of the pelagic
Foraminifera in that the vacuoles are contained in a layer of jelly
(calymma) traversed by strands of protoplasm, which secrete it and
the vacuoles, and in that it cannot be withdrawn.
There is no contractile vacuole.
The Radiolaria reproduce by binary fission and by spore formation.
The spores found in them are sometimes alike (isospores) and some-
times of two kinds (anisospores) . The latter are held to be gametes, and
it is said that union between them has been observed. On account of
their resemblance to the Dinoflagellata it has been suggested that they
belong to parasitic members of that group. It is possible, on the other
RADIOLARIA
8l
hand, that the Radiolaria have an alternation of generations like that
of the Foraminifera.
Peculiarities of the mitoses in this group have been mentioned
above (pp. 25, 26).
Symbiotic flagellates, known as "yellow cells" {Zooxanthellae, see
pp. 47, 50), are present in large numbers in the cytoplasm of many
of the Radiolaria.
Thalassicolla {Fig. 32 A). (Suborder Spumellaria.) Skeleton absent
or represented by some loose siliceous spicules; one nucleus; yellow
cells in extracapsular protoplasm.
-Hn.
nu.
Fig. 68. A, Heliosphaera inermis, x 350. After Hertwig. B, The skeleton of
Actinomma. After Biitschli. sk. skeleton; cps. central capsule; 7iu. nucleus.
The yellow cells are shown, but not labelled, in A.
Collozoum (Fig. 32 B). As Thalassicolla, but with central capsules
united by their extracapsular protoplasm into a colony; and each
capsule contains several nuclei.
Heliosphaera (Fig. 68 A). As Thalassicolla, but the skeleton has
the form of a lattice-work on the surface of the body.
Actinomma (Fig. 68 B). As Heliosphaera, but the skeleton consists
of several lattice spheres, formed successively at the surface as the
animal grows, with radial struts joining them. Ultimately the inner-
most sphere may lie in the nucleus.
Acanthometra (Fig. 69 A). (Suborder Acantharia.) A skeleton of
radial spicules of strontium sulphate meeting centrally in the central
capsule; nuclei numerous; yellow cells intracapsular. Remarkable
structures known as "myophrisks", surrounding the spines of this
82
THE INVERTEBRATA
sp.
osc.~.
Fig. 69. Radiolaria. A, Acanthometra elastica, after Hertwig. B, Aulac-
tinium actinastrum, after Haeckel. cps. central capsule; mph. myophrisk;
nu. nucleus; osc. oscula of central capsule; phae. phaeodium; ps. pseudo-
podium; sp. spine; y.c. yellow cells.
SARCODINA 83
genus at their junction with the calymma, are contractile and are used
in the regulation of the diameter of the body.
Lithocircus (Fig. 37). (Suborder Nassellaria.) A siliceous skeleton
in the form of a ring, bearing spines. Yellow cells extracapsular.
Aulactinium (Fig. 69 B). (Suborder Phaeodaria.) A skeleton of
hollow, radial, compound, siliceous spicules, not meeting in the
centre; nuclei two; central capsule with three oscula, one of which is
surrounded by a mass of coloured granules (the phaeodium^ from
which the suborder is named). Like the rest of the Phaeodaria this is
a deep-sea form and does not possess yellow cells.
Order HELIOZOA
Sarcodina, generally of floating habit and freshwater habitat ; without
shell or central capsule; sometimes with siliceous skeleton; with
spherical bodies; typical axopodia; and usually a highly vacuolated
outer layer of protoplasm.
The locomotion of members of this group, in the ordinary phase,
is effected as rolling, due to contraction of successive pseudopodia
in contact with the ground so that the body is pulled over. The
pseudopodia usually show streaming of granules. When they bend,
which they do to enclose prey which has adhered to one of them, their
axial filaments are temporarily absorbed at the bend. Protoplasm
from the pseudopodia then surrounds the prey and streams with it
inward to the endoplasm, where a food vacuole is secreted around it.
Contractile vacuoles are present.
Asexual reproduction is usually by binary fission (or plasmotomy in
multinucleate forms), sometimes by budding. Sexual processes have
only been thoroughly investigated in Actinophrys and Actinosphaerium,
where they take the form of autogamy (see below).
Dimorpha (Fig. 70), one of the Helioflagellata, a small group of
organisms which is usually appended to the Heliozoa, bears somewhat
the same relation to that order that Naegleria bears to the Amoebina.
It has a biflagellate and a heliozoan phase, and can pass from one to
the other. In the latter it retains the flagella, whose filaments share
a common basal granule with those of the axopodia, and has no
vacuolated layer or protecting case. In fresh waters.
Actinophrys (Figs. 71, 72). Unprotected ; with one nucleus, against
which the central filaments of the axopodia end; no skeleton. Auto-
gamy (or more correctly paedogamy^) takes place as follows: the
pseudopodia are withdrawn and a jelly cyst formed. Binary fission
now takes place, so that two individuals lie side by side in the cyst.
^ Paedogamy is a kind of autogamy in which not only the nucleus but
also its cytoplasm divides and reunites.
Fig. 70. Dimorpha mutatis. Partly after Blochmann. A, In the flagellate
phase, alive. B, In the heliozoan phase, stained, with pseudopodia as if alive.
ba.gr. basal granule ; es^n. chromatic matter which will condense to form the
endosome', f. vac. food vacuole ;yZ. flagella; nu. nucleus ;^s. pseudopodia.
Fig. 71, Actinophrys sol, x about 800. From Bronn. ecp. ectoplasm;
enp. endoplasm; c.vac. contractile vacuole ; /.I'ac. food vacuole; nu. nucleus;
ps. pseudopodium.
Hl'LIOZOA 85
Each divides mitotically twice, throwing out as a polar body one
product of each division. The first of these two divisions is a reduction
division. The two individuals now fuse, one behaving as a male by
T^;^^W.
.-»*a«=*^
Fig. 72. A-F, Successive stages in the autogamy of Actinophrys sol. After
Belar. ecp. ectoplasm; enp. endoplasm; ./.ielly coat;^^. pseudopodium put
out by 3, the male gamete, towards ?, the female gamete.
sending out a pseudopodium towards the other, and a strong inner
cyst forms around the zygote. After a while the latter undergoes
binary fission and the two products escape from the cyst. Occasion-
ally two individuals enter a jelly cyst together and then either the two
86 THE INVERTEBRATA
gametes of each undergo cross-conjugation with those of the other,
or there is one cross-conjugation and the remaining gamete of each of
the two original individuals performs parthenogenesis. In fresh and
marine waters.
Actinosphaeriutn (Fig. 33). Unprotected; with many nuclei,
against which the central filaments of the axopodia do not end. In
preparation for autogamy the nuclei are reduced in number and the
cytoplasm divides into as many corpuscles as there are nuclei. Each
of these then undergoes a process similar to that which occurs in
Acttnophrys, forming a zygote which hatches as an independent
individual. In fresh waters.
Clathrulina (Fig. 8). Animal enclosed in a stalked, pseudochi-
tinous lattice sphere; one nucleus. At binary fission, one product
becomes a biflagellula and swims away. In fresh waters.
Order MYCETOZOA
Plasmodial Sarcodina, living usually in damp places on land ; which
have in the active phase no shell, skeleton, or central capsule, but in
the quiescent phase a cyst of cellulose; possess numerous, blunt
pseudopodia; and are usually distributed by air-borne, cellulose-
coated spores.
The life history of a typical mycetozoon is as follows. The adult
Plasmodium is a sheet of protoplasm containing many thousands of
nuclei and numerous contractile vacuoles. In it there are to be seen
veins along which streaming takes place, alternately towards and
from the periphery. It feeds in a holozoic manner, usually upon de-
caying vegetable matter, sometimes (Badhamia) on a living plant. In
drought it breaks up into numerous multinucleate cellulose cysts
which constitute the sclerottum. It prepares for reproduction by
condensing at certain points, at each of which it forms a cellulose spo-
rangium, often stalked. In the sporangium is a capillitium of cellulose
threads and entangled in the capillitium are uninucleate, cellulose-
coated spores, whose formation is preceded by a reduction division.
When the sporangium is ripe it bursts and the spores are dissemin-
ated by wind, etc. In damp surroundings they open and liberate each
an amoebula which becomes a flagellula. The flagellulae perform
syngamy and the zygote again becomes an amoebula. The amoebulae
tend to fuse and form small plasmodia. By multiplication of their
nuclei the adults arise.
Chondrioderma (Fig. 73). On bean stalks.
Badhamia. On fungi, especially Stereum.
Plasmodiophora. In turnips, causing " finger-and-toe " disease. No
sporangia. Distribution by flagellulae in soil.
PROTOZOA
Class SPOROZOA
87
Protozoa which in the principal phase have no external organs of
locomotion or are amoeboid ; are parasitic, and nearly always at some
stage intracellular; have no meganucleus; and form after syngamy
large numbers of spores, which may be sporozoites or undivided
zygotes.
The two subclasses, Telosporidia and Neosporidia, of this class
have little in common, and their association in classification is a
matter of convenience.
m-nu-
nu
Fig. 73. Various stages of Chondrioderma difforme. From Strasburger.
A, A spore hatching. B and C, Flagellulae. D, Young and E, Older amoe-
bulae. F, Amoebulae fusing to form plasmodium. All x 540. G, Portion of
Plasmodium, x 90. nu. nucleus.
Though upon analysis the type of life history characteristic of the
Telosporidia is found to differ profoundly from those of the Neo-
sporidia, all sporozoan life histories are complicated. Usually they
comprise all the phases indicated in the scheme on p. 37, though in
the Eugregarinaria (and perhaps in the Actinomyxidea) agamogony is
omitted. Each phase, moreover, is liable to be elaborated. The term
sporoblast is applied to certain stages in various life histories, but un-
fortunately the stages so named are not all comparable with one
another. In the Telosporidia it denotes either the zygote or the
88 THE INVERTEBRATA
products of the first of two successive multiple fissions whereby the
sporozoites and other spore-like stages often arise. In the Neosporidia
it denotes the syncytia (of different origins in different groups) from
which by differentiation of cells complex spores are formed.
Subclass TELOSPORIDIA
Sporozoa in which the adult of the vegetative stage has only one
nucleus; and comes to an end with spore formation; and the spore
cases, if present, are simple structures, which nearly always contain
several sporozoites.
The vegetative stage {trophozoite) has usually a definite shape, but
in some haemosporidia is amoeboid. Its fission (agamogony), if such
occur, is multiple, and is usually known as schizogony^ the term schizo-
zoites or merozoites being applied to the offspring. Its single nucleus
only divides to form those of the young into which this stage breaks up,
but owing to such division the body may be for a while multinucleate.
The trophozoite of one of the two orders (the Coccidiomorpha)
remains intracellular: in the other order (the Gregarinidea) it after
a time outgrows its cell host. Save in one suborder (Eugregarinaria),
it passes through the usual phase of agamogony before giving rise to
gamonts, but in the Eugregarinaria agamogony is omitted, and the
members of the single vegetative generation become gamonts, which
provide for the increase of the species by the formation of many
gametes in both sexes. The gamonts may be free or intracellular.
Free individuals are often able to adhere by a sticky secretion, form-
ing what is known as a syzygy. When gamonts so adhere (Figs. 76,
6; 77 B) they do so in pairs^ whose members are to be the parents of
gametes that will unite reciprocally. Syngamy is isogamous in a few
of the Gregarinidea, but is usually anisogamous, and in the Cocci-
diomorpha becomes an oogamy (p. 31). In some cases, perhaps in
all, the first division of the zygote is a reduction division, so that
nearly the whole of the cycle is haploid.
The little group Piroplasmidea, whose members in some respects
resemble the Telosporidia, are best placed as an appendix to this
subclass.
Order COCCIDIOMORPHA
Telosporidia in which the adult trophozoite remains intracellular;
and the female gamete is a hologamete.
Typically the members of this order are parasites of the gut, but
more than once they have come to infest the blood. One such invasion
gave rise to the suborder Haemosporidia. The rest of the group con-
stitute the Coccidia.
^ The term syzygy should perhaps be restricted to such pairs.
SPOROZOA 89
Suborder COCCIDIA
Coccidiomorpha, for the most part gut parasites ; of which the zygote
is non-locomotory ; the sporozoites are nearly always encased; and
the gamonts often form a syzygy.
Eimeria (Fig. 74) is parasitic in the intestinal epithelium of various
vertebrates and invertebrates. E. schubergi, from the intestine of the
centipede Lithobtus, may be described as a type of the suborder. The
spherical trophozoite (agamont) undergoes schizogony (agamogony)
by multiple fission within the epithelial cell which it inhabits. The
spindle-shaped schizozoites (agametes) being set free into the cavity
of the organ, each infects another cell in which it grows like its parent.
After some days of this there occur fissions in which the young on
invading a host cell grow into adults unlike their parents and of two
kinds — male and female gamonts. Each female gamont extrudes
stainable matter from its nucleus and thus becomes a female holo-
gamete. In the male gamont the nucleus divides several times, and
the daughter nuclei are set free with portions of the cytoplasm as
biflagellate male gametes, which are thus merogametes. The gametes
leave the host cell and unite while free in the gut cavity. The zygote
nucleus undergoes what is probably a reduction division and encysts.
Within its cyst (the oocyst) it divides by multiple fission into four
sporoblasts each of which forms a cyst of its own (a secondary sporo-
cyst) in which it divides into two sporozoites. Thus sporogony takes
place in two stages. In each of these there is some residual proto-
plasm. Meanwhile the oocyst has passed out of the host in the faeces.
Infection of a new host takes place by contamination of food by the
encysted spores, which hatch in the intestine.
Aggregata is remarkable among coccidians for having two hosts.
Its agamogony takes place in crabs and involves a generation of sporo-
blasts, but is not repeated. A cuttlefish, devouring a crab, ingests the
agametes, which in the new host proceed to become gamonts. After
gamogony with flagellate male gametes, fertilization, and sporogony,
the spores, containing four or more sporozoites, are passed with the
faeces of the mollusc and swallowed by a crab.
Adelea is parasitic in the epithelium of the gut of Lithobius. Its life
history resembles that of Eimeria, but the gamonts, which differ con-
siderably in size, the male being the smaller, become free and form
a syzygy in the gut, though without encystment. The male gametes
are consequently not under the necessity of reaching the female by
swimming, and are not flagellated.
Haemogregarina has become completely a blood parasite, and has
a life history closely resembling that of the Haemosporidia, with the
sexual process in an invertebrate host (see below). Since, however, it
90
THE INVERTEBRATA
undergoes syzygy, the organism would appear to belong to the Adelca
stock, whereas the Haemosporidia are probably related to Eimeria.
Fig. 74. A diagram of the Hfe cycle of Eimeria schubergi. A, Infection of a
cell of the intestinal epithelium of the host. B, Growth of the agamont.
C-E, Agamogony (schizogony). F, G, Gamogony. H, Conjugation (syn-
gamy). I-L, Division of the encysted zygote into sporoblasts. M, Division
of each sporoblast within its cyst into two sporozoites. The oocyst containing
the sporocysts is passed out of the host and swallowed by another. N, Escape
of the sporozoites in the intestine of the new host.
Schellackia and Lankesterella, which have no syzygy, are transitional
to the Haemosporidia, under which (on p. 91) their life histories are
described.
COCCIDIOMORPHA 9I
Suborder HAEMOSPORIDIA
Coccidiomorpha, always true blood parasites; which have naked
sporozoites ; a locomotory zygote {ookinete) ; and no syzygy.
The members of this suborder are intracellular blood parasites of
vertebrates and contain granules of pigment (melanin) derived from
the haemoglobin of the host — a feature which is lacking in the blood
parasites that belong to the Coccidia. They are transmitted from one
vertebrate host to the next by a blood-sucking invertebrate. Their
agamogony and the formation of their gamonts take place in blood
cells of the vertebrate host, but their gametes are formed, and ferti-
lization takes place, in the invertebrate. A series of intermediate cases
shows how this condition may have arisen.
(i) Schellackia (suborder Coccidia), parasitic in the gut of a
lizard, leaves the gut epithelium after schizogony and completes its
cycle in the subepithelial tissues. In order to reach a new host it has
therefore to rely on transference by a carrier instead of passing out
with the faeces. To accomplish this, the sporozoites enter blood vessels,
get into red corpuscles, and are sucked up by a mite. The blood-
sucker, however, does not inject the parasite into the new vertebrate
host, but is swallowed, so that the parasite infects the host through
the gut epithelium, in which its schizogony is still performed.
(2) Lankesterella (suborder Coccidia), parasitic in frogs, passes its
whole cycle in the epithelioid lining of blood vessels, the sporozoites
being transferred, as in Schellackia^ in red corpuscles, which are
sucked up by a leech. Infection is still through the gut of the verte-
brate, whose wall the sporozoites pierce on their way to the blood
vessels.
(3) Haemoproteus (Haemosporidia), parasitic in birds, has its
schizogony alone in the blood vessel walls, the sexual part of the cycle
being remitted to the invertebrate host. The parasite enters the red
corpuscles not as a sporozoite but earlier, as the young stage of the
gamont, which grows up in the corpuscle. At the same time a change
in the mode of infection has taken place, the blood-sucker injecting
the sporozoites into the blood vessels of the vertebrate host. Thus the
parasite has completely abandoned the gut wall and become a true
blood parasite.
(4) Plasmodium (Haemosporidia), the cause of malaria and ague in
man, is parasitic in the red blood-corpuscles of mammals and trans-
mitted by the mosquito Anopheles. Its schizonts (trophozoites), as
well as its gamonts, inhabit red corpuscles.
The trophozoites of Plasmodium (Fig. 75) are amoeboid. In the
young stage they are rounded and each contains a large vacuole which
gives it the appearance of a ring. They undergo schizogony in the
92
THE INVERTEBRATA
Fig. 75. A diagram of the life cycle of Plasniodimn vivax. After Borradaile.
1-7, Schizogony (Merogony), asexual reproduction which takes place in
man. 8—13, Gamogony and syngamy, which take place in the stomach of a
mosquito. 14-20, Sporogony by the zygote (sporont), which takes place in
the body cavity of the mosquito, i, Infection of a red corpuscle. 2, Signet-
ring stage. 3, Amoeboid stage. 4, Full-grown schizont preparing to divide.
5, Multinucleate stage. 6, Rosette stage, corpuscle breaking up. 7, Free
schizozoites. 8, Infection of red corpuscles by young gamonts. 9, Full-
grown gamonts free in the mosquito's stomach. 10, 11, Formation of gametes.
12, Conjugation. 13, Zygote in the ookinete condition. 14, Invasion by zy-
gote of endoderm cell of mosquito. 15, Encystment. 16, Sporoblasts formed
by division of zygote (sporont). 17, 18, Formation of sporozoites. 19, In-
vasion by latter of salivary gland. 20, Sporozoites injected into blood of a man.
TELOSPORIDIA 93
red corpuscles, which then break up, setting free the schizozoites
(merozoites) and also products of the metaboUsm of the parasite which
cause fever. After some generations, gamonts similar to those of
Eimeria appear, but remain quiescent unless sucked up by a mosquito,
in whose gut the female gamont becomes a spherical macrogamete, the
male gamont throws off whip-like microgametes, and syngamy takes
place. The zygote becomes elongate and active (an ookinete), and
bores its way through the wall of the mosquito's stomach, on the
outside of which it becomes encysted (oocyst). Here its nucleus
divides and it breaks up into sporoblasts which in turn produce
spindle-shaped sporozoites. The oocyst now bursts, setting the sporo-
zoites free in the blood of the insect. They make their way into the
salivary glands and are injected with the saliva into a mammalian
host, where they give rise to trophozoites which infest the red
corpuscles.
Three species of Plasmodium infest man — P. vivax which sets free
a generation of schizozoites in forty-eight hours, P. malariae which
does so in seventy-two hours, and P. falciparum whose schizogony
occurs at more irregular intervals. Since the attacks of fever take place
when the corpuscles break up and set free the toxins formed by the
parasites, the fever caused by P. vivax returns every third day and is
known as "tertian ague", and that caused by P. malariae ("quartan
ague") recurs every fourth day, while P. falciparum causes irregular
(quotidian) fevers which are more or less continuous. These latter
are the "pernicious malaria" of the tropics. The morphological
differences between the species are small, but P. vivax is distinguished
by the active movement of its pigment granules and the large number
(15-24) of its schizozoites, P. malariae by the sluggishness and often
quadrilateral form of its amoeboid stage, P. falciparum by the paucity
of its pigment and by its curved, sausage-shaped gamonts.
Order GREGARINIDEA
Telosporidia in which the adult trophozoite becomes extracellular;
and the female (as well as the male) gametes are merogametes.
Intestinal and coelomic parasites of invertebrates, especially of
arthropods and annelids.
Suborder SCHIZOGREGARINARIA
Gregarinidea which undergo schizogony.
Schizocystis (Fig. 76). Parasitic in the intestine of the larvae of
dipterous flies. The young trophozoite attaches by one end to the gut
epithelium of the host. Its nuclei multiply. When ripe it undergoes
multiple fission. The products (schizozoites) either repeat asexual
94
THE INVERTEBRATA
reproduction or become gamonts. These undergo syzygy, coencyst-
ment, and gamogony. The gametes unite, and the zygotes form small
oocysts ("spore cases") within the gamocyst. In its case each zygote
divides into a bundle of sporozoites. The spores are set free and
swallowed by new members of the host species, in whose intestine the
spore cases are digested and the process repeated.
Fig. 76. A diagram of the life cycle of Schizocystis. 1-4, Schizogony. 5,
Gamonts. 6, Syzygy. 7-9, Gamogony in a cyst (gamocyst). 10, 11, Syn-
gamy. 12, Freed spore case containing sporozoites resulting from sporogony.
Ophryocystis (Fig. 77). Parasitic in the Malpighian tubules of
beetles. The cushion-shaped trophozoites are attached to the host's
cells by branched processes. After several generations of schizogony,
they become free gamonts, enter into syzygies, encyst, and within the
gamocyst undergo two divisions, whereby each forms one definitive
gamete and a binucleate enveloping cell which perhaps represents
abortive gametes. Syngamy then takes place, and the zygote divides
GREGARINIDEA
95
to form within the enveloping cells a parcel of eight sporozoites in a
case. Thus each syzygy produces only one pair of gametes and results
in only a single spore.
Suborder EUGREGARINARIA
Gregarinidea which have no schizogony.
The adult trophozoite has a stout cuticle and the ectoplasm con-
tains myonemes, longitudinal or transverse, or both. Partitions of the
ectoplasm without myonemes may (Fig. 80 F) divide the body into
«?* >*«%'"«'*; 'r.Va •. fr V, -V >* i'l*.' •-"»\' *' %
str.
B
-gam.
n^-^yg-
spz.-j
Fig. 77. Stages in the life history of Ophryocystis mesnili. A, Agamont, on
the epithelium of a Malpighian tubule of the host. B, Syzygy. C, Formation
of a cyst (gamocyst) and multiplication of nuclei. D, Formation of gametes.
E, Zygote. F, Spore case with sporozoites, still enclosed in residual proto-
plasm of gamonts. gam. gamete; nu. nuclei of agamont; nu.' gamete nucleus;
nu." nuclei of enveloping (residual) protoplasm; spz. sporozoites; str. striated
border of epithelium of Malpighian tubule; zyg. zygote.
three segments — the epimerite or fixing organ, protomerite^ and deuto-
merite, which latter contains the nucleus. When ripe the trophozoites
become gamonts, joining in syzygies of two which together form a
gamocyst and give rise to gametes (iso- or anisogametes according to
species) by multiple fission in which residual protoplasm remains.
Syngamy takes place within the cyst between the gametes of one
parent and those of the other. The zygotes secrete small oocysts
{pseudonavicellae) of their own, and within these divide into several
sporozoites ("falciform young"). Passing out of the host these are
96 THE INVERTEBRATA
swallowed by another of the same species, within which their cysts
are digested and a new infection begins by the sporozoites invading
cells of the host. These they eventually outgrow, and lie in a cavity
of the host, either entirely free or attached by an epimerite.
Fig. 78. A diagram of the life cycle of Monocystis. A, Trophozoite adhering
to the seminal funnel of the host. B, Encysted syzygy. C, Formation of
gametes. D, Conjugation. E, Encystment of zygotes. F, Multiplication of
nuclei of the same. G, Formation of sporozoites (only four of the eight are
shown). H, Release of sporozoites in intestines of new host. I, Infestation of
sperm morula, ext. external coat of gamocyst; gam. gametes; int. internal
coat of gamocyst; res. residual protoplasm; spc. cells of sperm morula; spe.
tails of withered spermatozoa adhering to parasite; spz. sporozoites; zy
zygote.
In comparing this life cycle with that of Etmeria, given above, it
should be noted that in the gregarines, whose female gametes are
merogametes and numerous, the "spores" (small sporocysts each
EUGREGARINARIA
97
containing several sporozoites) are each the whole product of a zygote
(i.e. are oocysts), whereas in the coccidians, where the female gamete
Fig. 79. Monocystis. From Borradaile. A, M. magna, x 25. B, M. lumbrici,
X 85. The latter is covered with the tails of spermatozoa, the offspring of the
sperm mother-cell in which it was embedded.
Fig. 80. Gregarina longa, from larva of Tipula, the Daddy-long-legs. Highly
magnified. After Leger. A, B, C, D, E, Stages of the development of G. /ow^a
at first within and then pushing its way out of one of the cells of the intestine
of the Tipula larva. F, Mature form. c. cell of intestine of host; nu. its
nucleus ; pst. parasite.
is a hologamete, the zygote forms, by means of a generation of
sporoblasts, several such spores in its oocyst.
Monocystis (Fig. 79). Without divisions of the body. Parasitic in
98 THE INVERTEBRATA
seminal vesicles of earthworms. Several species, some isogamous,
others anisogamous. The spores escape either down the vasa defer-
entia of the host or by the latter being eaten by a bird, whose faeces
contain them intact. Swallowed by another worm, their cases are
digested and the sporozoites traverse the intestinal wall to reach the
vesiculae seminales, where they enter sperm mother-cells, in which
they pass their earlier stages.
Gregarina (Fig. 80). All three divisions of the body present.
Parasitic in the alimentary canals of cockroaches and other insects.
The gamocyst develops into a complicated structure with ducts for
the discharge of the pseudonavicellae.
Appendix to the Telosporidia
Order PIROPLASMIDEA
Protozoa, parasitic in red blood-corpuscles and transmitted by ticks ;
which have no external organs of locomotion; perform agamogony
by binary fission; conjugate as hologametes; and after syngamy
become motile zygotes which divide in a cyst into numerous, naked
sporozoites.
The members of this group are of doubtful affinity. In the general
course of the life-cycle they resemble the Telosporidia, but in the
possession by the trophozoite of part of a flagellar apparatus, and
in that the gametes are both hologametes, they diff^er from the
other members of that subclass. An interesting feature of their
life history is that they are transmitted in the ovum from one
generation of the invertebrate host to the next. They are at present
only known from mammals and ticks.
Piroplasma { = Babesia). Infests various mammals (cattle, dogs,
monkeys) and causes the red-water fever of cattle and other diseases.
The trophozoites, in red corpuscles, are pear-shaped and unpig-
mented, and have a rhizoplast and basal granule as if for a flagellum.
When taken into the alimentary canal of a tick they become gametes
and form zygotes, which are ookinetes (p. 91), bore through the gut
wall of the host, and reach its ovary. There they enter ova in which
they are transmitted to the next generation of the tick. They encyst
in the ova and divide into amoeboid sporoblasts (sporokinetes) which
are distributed as the cells of the host divide and by their own active
migration. Thus some reach the salivary glands. There they become
multinucleate and break up into sporozoites, which are injected with
the saliva into a new mammalian host.
SPOROZOA
99
Fig. 8i. A diagram of the life cycle of Piroplasma. After Dennis, with
modifications. 1-5, agamogony in red corpuscles ; 6, gamete ; 7, 8, conjugation
in gut of ticks; 9-12, passage of zygote through walls of gut and oviduct into
ovum and encystment there; 13, 14, formation of sporoblasts ; 15, migration
of sporoblast; 16, sporoblasts, now multinuclear, in cell of rudiment of
salivary gland of tick of second generation; 17 (less magnified), acinus of
salivary gland containing sporozoites formed by break-up of sporoblasts.
al. wall of gut; od. wall of oviduct; ov. ovum.
lOO THE INVERTEBRATA
Subclass NEOSPORIDIA
Sporozoa in which the adult of the vegetative stage is a syncytium ;
which usually forms spores continuously within itself; and the spore
cases are usually complex structures, which, except in the Actino-
myxidea, contain only one germ.
Order CNIDOSPORIDI A
Neosporidia whose spores possess pole capsules.
The formation of the spores in this group is a complex process of
which the details and the relation to the typical life cycle of the
Protozoa have not yet been completely elucidated. The following
scheme provisionally co-ordinates the facts that have been estab-
lished concerning it. In the syncytium (Fig. 82), which is the
agamont and which often multiplies by plasmotomy, there arise,
perhaps by the coming together of nuclei, bodies known 2iS pansporo-
blasts^ each composed of a couple of envelope cells with one or more
cells known as sporoblasts. The nucleus of each sporoblast divides and
the sporoblast gives rise to a complex, multicellular spore, composed
of a case of two or three pieces, each with an underlying nucleus, one
to five nematocyst-like pole capsules ^ each with a nucleus, and one or
more germs. In most cases the germ is single and at first has two
nuclei, which later fuse. Here we may regard the sporoblast as a
gamont and the products of its division as homologues of gametes, of
which some become the accessory cells of the spore and two (those
which the germ at first possesses) the definitive gametes. In one
group, however (the Actinomyxidea), there are several germs (often
as a syncytium), and syngamy takes place not between nuclei in a
germ but at an earlier stage, between pairs of cells in the pansporo-
blast, each zygote becoming a sporoblast. Here the sporoblast is a true
sporont, and the products of its division are homologues of sporozoites,
of which some become the accessory cells of the spore and the others
(the germs) are the definitive sporozoites. It is a remarkable, but
apparently an established, fact, that syngamy thus takes place at
different stages in the formation of essentially similar spores.
Infection of new hosts is by the mouth, and the function of the
pole capsules is, by discharging their threads, to anchor the spore to
the gut wall. A schizogony may precede pansporoblast formation.
Of the three suborders of the Cnidosporidia, the Myxosporidia have
two or four pole capsules in the spore, the Microsporidia one, and the
Actinomyxidea three. The latter group also differ from the other two
in respect of the germs, as mentioned above.
Myxobolus (Myxosporidia, Fig. 82). Large syncytia in the tissues
of various freshwater fishes. Some species are harmless, others
dangerous pests.
Fig. 82. A diagram of the life cycle of a typical member of the Myxosporidia.
The schizogony shown here (D-F) probably often does not occur. A, Escape
of germ. B, Migration within host. C, Infection of a cell of the latter. D-G,
Schizogony and reinfection. H, Multiplication of nuclei. I, Appearance of
first pansporoblast. J, Appearance of mare pansporoblasts and multiplica-
tion of syncytium by plasmotomy. K, L, Development of spores in a pan-
sporoblast. M, Fully formed spore, before conjugation. N, Ripe spore after
conjugation, env. envelope cell; nu.' nucleus of spore case; nu." nucleus of
pole capsule ; spb. undifferentiated sporoblast with nuclei which will become
those of the germ, spore case, and pole capsules; vac. vacuole containing
glycogen sometimes found; zy.nu. zygote nucleus.
102 THE INVERTEBRATA
Nosema (Microsporidia). The syncytium early breaks up, first into
binucleate forms and finally into single sporoblasts. In the intestinal
epithelium of insects. A serious pest of the silkworm, causing the
disease known as pebrine, and of the bee.
Sphaeractinomyxon (Actinomyxidea). The whole body is reduced
to a single pansporoblast, as in all members of the suborder. The
spores are without the spines found in related genera. In annelids.
Order HAPLOSPORIDIA
Neosporidia whose spores possess cases with a lid, but have no pole
capsules.
This order contains certain parasites which infest aquatic in-
vertebrates. They are perhaps derived from the Cnidosporidia by
loss of the pole capsules.
Haplosporidium^ parasitic chiefly in annelids, is the typical genus.
Order SARCOSPORIDIA
Neosporidia whose spores do not possess cases or pole capsules.
These organisms are tubular syncytia with a radially striped ecto-
plasm, parasitic in the muscle fibres of mammals, and reproducing by
simple, sickle-shaped spores.
Sarcocystis (Fig. 83). In various mammals, occasionally in man.
Class CILIOPHORA
Protozoa which, at least as young, possess cilia ; are never amoeboid ;
if parasitic are very rarely intracellular ; nearly always possess a mega-
nucleus; and do not, after syngamy, form large numbers of spores.
This class, though some of its parasitic members are of compara-
tively simple structure, contains the most highly organized Protozoa.
Facts concerning sundry of the organs and processes in its members
(the ciliary apparatus, p. 17 ; the contractile vacuole system, pp.21 , 22 ;
the nucleus, p. 26; conjugation, p. 33 ; etc.) have been stated above.
The life history, except for the remarkable process of conjugation
undergone by most of the class, is relatively uncomplicated. In
particular, though the nuclear peculiarities of the typical members
of the group render inevitable certain special features in the metagamic
divisions, there is no true sporogony.
Subclass CI LI AT A
Ciliophora which as adults possess cilia; and which do not possess
suctorial tentacles.
The morphology of this group is much aff"ected by the disposition
of the apparatus used in obtaining nutriment. The food may be ab-
PROTOZOA
103
sorbed through the surface: the shape of the body is then simple
(Figs. 5, 87 A). Nearly always, however, there is a mouth. In some of
the lower genera this is anterior and terminal, or nearly so (Fig. 89 A),
but usually it is removed to one side of the body (Fig. 87 E). This side
is then said to be "ventral", and that opposite to it is ** dorsal".
»'
III
;S^;".-J : *
>-l^'^
■,-■1
\mM}m
m
U— m.
^\:r:\::m A
■>-
B
Fig. 83. Sarcocystis lindemanm, from the vocal cords of man. After Baraban
and Saint-R^my. A, Longitudinal section of muscle fibres, showing the
parasite in situ in a fibre, x 300. B, Enlarged portion of outer region of para-
site, showing the striated wall, and some of the compartments which contain
the spores. C, A single spore, x 1600. m. muscle fibre; w. wall of parasite.
The mouth, either is merely a soft patch of exposed endoplasm or
possesses a gullet (p. 19). In a relatively few cases (including all those
in which the mouth is terminal and a few of those in which it is
104 THE INVERTEBRATA
ventral) the mouth is at the surface of the body : in such cases the
gullet, if there be one, is an oesophagus, excavated in the endoplasm
and capable of being opened and closed to seize the prey which is
of some size. Most often, however, there is a vestibule. This, to which
also the name '* gullet" is often applied, is a depression leading to the
mouth, incapable of being closed, lined by inturned ectoplasm, and
containing a ciliary apparatus, which usually includes one or more
undulating membranes. By this apparatus the minute objects which
constitute the food of all ciliates that have a vestibule are drawn in,
being meanwhile, in some cases at least, entangled by a mucous
secretion. At the bottom of the vestibule lies the true mouth; some-
times an oesophagus is present (Stentor) or is represented by a cleft in
the endoplasm (Paramecium). The inner part of the vestibule may be
free from cilia, and so simulate an oesophagus {Paramecium, Vorti-
cella).^ The vestibule is usually approached by 2i peristome. This is a
groove, of varying dimensions, which leads from the front end along
the ventral side to the opening (cytostome) of the gullet. It is not
straight, but runs in a longer or shorter spiral round the body, so
that the anterior end of the latter is spirally deformed (Figs. i6, 84 A).
The higher forms have along what is primarily the outer edge of the
peristome a food-gathering row of cirri or membranellae, the adoral
wreath (Fig. 90, ad.mae.). Typically, the spiral is open, as in Para-
mecium, but in some cases, as in Stentor (Figs. 84B, 89 C), it has con-
tracted, so that it lies coiled as a crown at the anterior end. In such
cases the animal is usually fixed temporarily or permanently by the
opposite end.
The members of one order (Hypotricha) are depressed dorso-
ventrally, and have a flat ventral side, along which the peristome runs
and which is usually provided with a complex apparatus of cirri
(Figs. 90, 91). The animal applies this side to the substratum, in
locomotion upon which certain of the cirri are used. The dorsal side
is naked save for a few " sensory " cilia. It is probably from such forms
that the familiar bell-animalcules and their relations (Peritricha) are
derived. In these, the shape of the body and the position of the peri-
stome at first suggest that the morphological peculiarities of the
group are due to an evolution similar to that by which such forms
as Stentor came into being— but the fact that the peristome, which in
all other ciliates that- possess it curves clockwise, is in the Peritricha
twisted in the opposite direction, makes this view impossible. The
origin of the Peritricha may be explained as follows (Fig. 84). In
^ It is possible that this is a true oesophagus. Other regions are sometimes
to be distinguished in the vestibule: in Paramecium, for instance, its outer
section has trichocysts and cilia but no membranes, its middle section
membranes only.
CILIATA
105
hypotrichous forms which had taken to fixing themselves to the sub-
stratum by that (ventral) side which they applied to it, the mouth,
being no longer of use in its ventral situation, moved to the left side.
The peristome accordingly came to run along the edge of the body,
around which it became continued on the dorsal surface. In dorsal
aspect its direction is of course reversed; and the adoral wreath has
come to be internal. The body, in correspondence with the changed
habit of life, has shortened, till its outline, seen from above, is circular,
and has deepened. Thus the oral-aboral axis of the Peritricha is not
anteroposterior as in Stentor, but dorsoventral.
per.--
\UM4UUA
Jj^r-vesL
vest
Fig. 84. A diagram of the disposition of the peristome in various ciHata.
A, Ventral view of a typical heterotrichous form. B, Similar view of Stentor.
C, Ventral (aboral) view of a peritrichous form without stalk, such as Tricho-
dina (Fig. 86 D). C, Dorsal (oral) view of the same. ad. adoral wreath of
membranellae ; vest, vestibule ; per. peristome.
The general surface of the body is in the lower and in some of the
higher genera uniformly covered with cilia, but most of the more
highly organized forms are naked save where there stand certain
special pieces of ciliary apparatus. The ectoplasm (Fig. 85) has a
definite and often complicated structure. There is always a tough
pellicle, which is frequently sculptured. Under it is often an alveo-
lar layer of minute, regular vacuoles. When there are myonemes,
these lie on the inner walls of larger canal vacuoles of this layer. Under
it again is usually a layer, the cortex, whose firm consistency prevents
the granules, vacuoles, etc., of the endoplasm from entering it, though
it may possess small granules of its own. The basal granules of the
cilia lie immediately below the alveolar layer; trichocysts are im-
bedded in the cortex. Either the cortex or both it and the alveolar
io6
THE INVERTEBRATA
layer may be absent. In Paramecium the cortex is covered by a thick
pellicle which possibly contains a minute alveolar layer.
rid. tri. fl!
B
Fig. 85. Details ofthe ectoplasm of ciliates. After Wetzel. A, Frontonia leucas
(Vestibulata). B, Paramecium. C, The same, surface view. alv. alveolar layer;
ba.gr. basal granules; cor. cortex; enp. endoplasm ; yZ. flagellum;^.' insertion
of flagellum ; pel. pellicle ; rid. ridges of pellicle ; tri. trichocyst.
Order HOLOTRICHA (ASPIRIGERA)
Ciliata which do not possess an adoral wreath ; and which nearly all
have uniform ciliation of the whole surface of the body.
This order is a collection of relatively simply organized ciliates,
some of which are primitive while others are degenerate through
parasitism.
Suborder PROCILIATA
Holotricha without mouth ; and without differentiation of meganuclei
from micronuclei.
Opalina (Figs. 5, 21 C, 86). With several, usually many, nuclei,
which are all alike. In each nucleus, however, there can be dis-
Fig. 86. Opalina ranarum. From Borradaile. A, Ordinary individual in
longitudinal fission. B, The same in transverse fission. C, Small encysted
individual (distributive phase). D, Gamete. E, Encysted zygote.
CILIATA 107
tinguished two kinds of chromosomes, which are held to represent
the chromatin of the mega- and micronuclei of other ciUates. The life
history differs from that of other members of the class in that syngamy
is of the normal type. The agamont, parasitic in the rectum of a frog
or toad, reproduces by binary plasmotomy. In the spring the plasmo-
tomy outruns the nuclear divisions so that there arise small individuals
with few nuclei. These encyst and pass out of the host. Swallowed
by a tadpole, they hatch, and give rise to uninucleate gametes, of two
sizes (anisogamous). After fusion of the gametes the zygote encysts
for a while, issues, and by nuclear division becomes the adult
agamont.
Suborder ASTOMATA
Holotricha without mouth; but with mega- and micronuclei.
Unlike Opalina, the members of this group are probably not
primitive but degenerate through parasitism.
Collinia. Parasitic in the blood-spaces of the gills of Gammarus and
other crustaceans.
Anoplophrya (Fig. 87 A). Reproduction by repeated budding at
one end of the elongate body, forming a chain. Parasitic in various
annelids.
Suborder GYMNOSTOMATA
Holotricha with a mouth, whose gullet, if any, is without ciliary
apparatus (i.e. an oesophagus); and with mega- and micronuclei.
Ichthyophthirius . Subspherical, with a mouth at one pole and short
gullet ; numerous contractile vacuoles near the surface of the body ;
and a meganucleus, but no micronuclei visible in the adult. Para-
sitic in various freshwater fishes, where it lies in blisters in the skin.
When it is full-grown, it falls out of the host, encysts, and forms by
repeated fission a number of small ciliospores, each of which has a
mega- and a micronucleus, the latter having appeared during the
process, perhaps from within the meganucleus. The spores infect new
hosts. A sexual process of the nature of autogamy has been described,
but is very doubtful.
Prorodon (Fig. 89 A). Ovoidal, with mouth at one pole, a deep
gullet which is supported by skeletal rods and is capable of opening
and closing; one mega- and one micronucleus. In fresh waters.
Loxodes. Compressed, with mouth as a mere slit in the pellicle on
the ventral edge of the body, overhung by the beak-like anterior end;
numerous mega- and micronuclei ; a row of vacuoles containing ex-
creta along the dorsal border, and a contractile vacuole at the hinder
end. In fresh waters.
axL.
c.vac/
F'
G
H
Fig. 87. Various Ciliophora. A, Anoplophrya prolifera, x 200, after Saville-
Kent. B, Entodinium caudatum, after Schuberg. C, Tintinnidium inqidlinwn,
after Faure-Fremiet. D, Trichodina pedicidus , x 450, after Biitschli. E, Col-
poda steini, x 1300, after Wenyon. F, Sphaerophrya sol, in the free stage,
X 170, after Biitschli. F', The same, dividing subequally to form a ciliated
bud. G, Dendrocometes paradoxus, x 250, after Biitschli. H, Free bud of
Tocophrya quadripartita , after Biitschli. ad. adoral wreath; c.vac. contractile
vacuole; ci. rows of body cilia; ci.' aboral ring of cilia; ci." girdle of cilia;
cu. cuticle of host; esm. endosome of meganucleus (an unusual feature);
f.vac. food vacuole; hk. hooks; meg. meganucleus; mi. micronucleus ; ten.
tentacles; vest, vestibule.
CILIATA
109
Suborder VESTIBULATA (HYMENOSTOMATA)
Holotricha with a mouth and a gullet (vestibule) which is permanently
open and usually possesses an undulating membrane ; and with mega-
and micronuclei.
Colpoda (Fig. 87 E). Kidney-shaped; with large vestibule on con-
cave side; but no undulating membrane; and no peristome. Fission,
binary or repeated, takes place in a cyst. Common in infusions, fresh-
water and marine.
Colpidiiim. As Colpoda\ but with undulating membrane. Common
in infusions, freshwater and marine.
Paramecium (Figs. 9, 15, 16, 17, 26, 85). Slipper- or pear-shaped
according to species; with undulating membranes^; and peristome.
Common in infusions, freshwater and marine.
mrp
Fig. 88. Ciliata from the rectum of the frog. From Borradaile. A, Balan-
tidium entozoon, x 65. B, Nyctotherus cordiformis, x 130. a«. anus ; cy. con-
tractile vacuole ; meg. meganucleus ; mi. micronucleus ; v. vestibule.
Order HETEROTRICHA
Ciliata which possess a gullet, permanently open and provided with
undulating membrane; an adoral wreath, curving clockwise; and
most often on the rest of the body a uniform covering of cilia ; and
whose body is not depressed.
Suborder POLYTRICHA
Heterotricha which retain the uniform ciliation of the general surface
of the body.
Balantidium (Fig. 88 A). Egg-shaped; the peristome a deep groove
at the anterior end. Parasitic in the rectum of frogs, the intestine of
man (where it is occasionally harmful), etc.
Nyctotherus (Fig. 88 B). Kidney-shaped; with permanent anus.
Parasitic in the rectum of frogs, the intestine of man, etc.
^ There are three of these, each composed of four rows of cilia. See p. 104,
no
THE INVERTEBRATA
Spirostomum (Fig. 89 B). Rod-shaped; with the peristome as a
long groove; meganucleus beaded; several micronuclei. In fresh
waters and marine.
Stentor (Fig. 89 C). Long and funnel-shaped ; attached by the base,
but often frees itself to swim ; meganucleus beaded ; several micro-
nuclei. The animal is very highly contractile. In fresh waters.
M.
,rods
mi.-Wti^'r-ri-;
-meg.
■vac!
c.vac.
c.vac.'
cvac?
Fig. 89. Ciliata. After various authors. A, Prorodon teres, x 500. B, Spiro-
stomum ambiguum, x 150. C, Stentor coeruleus, x 50, ad.zv. adoral wreath;
c.vac. contractile vacuole; c.vac' accessory vacuole; c.vac." accessory canal;
f.vac. food vacuole ; ecp. ectoplasm ; M. mouth ; meg. meganucleus ; mi. micro-
nucleus ; per. peristome ; rods in protoplasm around gullet.
Suborder OLIGOTRICHA
Heterotricha of shortened form; with the body cilia reduced to a few
rows or absent.
This suborder contains two tribes of very different habits, the
pelagic Tintinnina, and the Entodiniomorpha, forms of bizarre shape
parasitic in the alimentary canal of mammals, chiefly in the stomach
of ruminants. Both suborders have an anterior peristome with very
strong membranellae, and are naked on the rest of the body, save
sometimes for a few cilia or patches of cirri.
Tintinntdium (Fig. SyC). (Tintinnina.) Cup-shaped ; anchored by
an aboral process into a chitinoid case. In fresh waters and marine.
CILIATA III
Entodinium (Fig. 87 B). (Entodiniomorpha.) With three posterior
processes, of which the largest is said to serve as a rudder. In the
rumen and reticulum of sheep and oxen. Like others of the tribe,
these organisms are present in such numbers that they are believed
to be symbionts which play a part in the nutrition of the host, render-
ing the vegetable food more easily assimilable by feeding on it and
being in turn digested further on in the alimentary canal. Infection
of the host is probably by cysts on grass.
Order HYPOTRICHA
Ciliata with depressed body; a gullet, permanently open and pro-
vided with undulating membranes ; an adoral wreath, curving clock-
wise ; the dorsal cilia represented only by a few stiff hairs ; and on the
ventral side usually an elaborate system of cirri and other ciliary
organs.
The animals can swim but spend much of their time crawling over
solid objects by means of the cirri.
Stylonichia (Figs. 90, 91). A typical example. Common in in-
fusions.
Kerona. With a less highly developed ciliary system than Styloni-
chia. Ectoparasitic on Hydra,
Order PERITRICHA
Ciliata, for the most part permanently fixed by the aboral surface;
with a gullet, permanently open and provided with undulating mem-
brane; an adoral wreath, curving counter-clockwise; and on the rest
of the body no cilia, save those of an aboral ring in the free-swimming
species and stages.
The conjugation of members of this group has. been discussed on
p. 33, their morphology on pp. 104, 105. The anus and contractile
vacuole open into the deep vestibule, perhaps owing to an extension
of the depression of the ectoplasm which forms the latter. The mega-
nucleus is horseshoe-shaped.
Trichodina (Fig. 87 D). Dice-box shaped; with aboral ring of cilia
for swimming, enclosing a ring of hooks for temporary attachment.
Ectoparasitic on Hydra and other animals.
Vorticella (Figs. 2, 92). Shaped like a solid, inverted bell, with, in
place of the handle, a stalk which consists of a prolongation of the
body, and is clad in a cuticle and contractile by means of a myoneme.
Solitary. In fresh waters and marine.
Carchesium (Fig. 93). As Vorticella, but colonial. In fresh waters.
Epistylis. As Carchesium, but the stalk is purely cuticular and non-
contractile. In fresh waters and marine.
112
THE INVERTEBRATA
fr.cir.'^'-zZ
^u.me.
^nd.ci.
-jyrexi.
int.u.me.
f.vac-
ahxin-^t:
se.ct.^--
- (^ - -^j-^A n.cir.
pt.cir.
Fig. 90. Stylonichia mytilus, in ventral view, x 200. After various authors.
ab.cir. abdominal cirri; ad.mae. adoral membranellae ; An. position of anus
(on dorsal side) ; An.cir. anal cirri; c.vac. contractile vacuole; c.vac/ accessory
canal of the same; end.ci. endoral c\\i2i\ f .vac. food vacuole ;/r.aV. frontal cirri;
gu. gullet; int.u.me. internal undulating membrane; lip. projecting lower lip
of peristome; mar.cir. marginal cirri; meg. meganucleus; mi. micronucleus ;
pre.ci. preoral cilia; pt.cir. posterior cirri; se.ci. "sensory" cilia of dorsal
surface ; ii.me. preoral undulating membrane (another undulating membrane
is present but is omitted, to simplify the figure).
Fig. 91. Stylonichia mytilus , from the left side. After Biitschli.
CILIATA
"3
Fig. 92.
■?,
Fig. 93.
Fig. 92. A group of individuals of Vorticella in various phases of the life
history. From Borradaile. a, Ordinary individual, b, The same contracted,
c, Ordinary fission, d, A later stage of the same, e. Free-swimming individual
produced by ordinary fission. /, /', Two modes of fission to form microcon-
jugants (/, budding;/', repeated fission of one product of a binary fission).
gy Conjugation.
Fig. 93. Carchesium epistylidis, x 100. After Saville-Kent.
114
THE INVERTEBRATA
Order CHONOTRICHA
Ciliata, permanently sessile by the posterior end upon the bodies of
Crustacea; with the peristome represented by a spiral funnel at the
anterior end, coiled clockwise, ciliated inside, and leading to the
mouth; and the rest of the body naked.
A small but very interesting group which shares with the Prociliata
two characteristics not found elsewhere in the class, namely (i) that
their nuclei are of one kind only and at mitosis form two sets of
chromosomes (see p. 26), (2) that they form numerous gametes,
mi.-if--^
Fig. 94. Fig. 95.
Fig. 94. Spirochona gemmipara, x 520. ach. achromatic part of nucleus
(centrosphere) ; chr. chromatin ; mi. micronucleus (which divides within mega-
nucleus, where it appears when division is impending).
Fig. 95. A diagram of the formation of an internal bud by one of the Suctoria.
which unite in the same way as those of members of the other classes
of the phylum. In the Chonotricha the reproduction, both sexual and
asexual, is carried out by buds. The nucleus contains a large achro-
matic mass which acts as a division centre.
Spirochona (Fig. 94). Shaped like a slender vase. On the gills of
Gammarus, etc., in fresh and marine waters.
Subclass SUCTORIA
Ciliophora of which all but a few primitive forms lose their cilia in
the adult; and which possess one or more suctorial tentacles.
A few members of the group are free ; a few are endoparasitic ; most
CILIOPHORA
115
are attached, and these have usually a cuticular stalky which is often
expanded at the end to form a shallow cup in which the animal sits or
a deep one which encloses it.
The suctorial tentacles contain a tube, lined by ectoplasm, which
opens at the end, where there is often a knob. In some species there
are also solid, sticky tentacles, used to capture prey.
Reproduction by simple binary fission does not occur. In a few cases
fission is equal or almost so (Podophrya, Sphaerophrya, Fig. 87 F'), but
here one of the products differs from the parent in losing its tentacles
and acquiring cilia and thus resembles the buds of other species. This
happens whether the parent be a stalked or a floating form. Most
species multiply by typical budding. The buds may be external
(Fig. 96 B) or formed in brood pouches (Fig. 95) from which they
Fig. 96. Ephelotagemmipara. After Hertwig. A, Ordinary individual, x 150.
B, Budding individual, c.vac. contractile vacuoles ; nu. meganucleus (stained) ;
processes of this form the meganuclei of the buds, as in all the budding of
the Suctoria (cf. Figs, 87 F', 95).
escape when they are ripe. External budding is the more primitive,
internal the commoner process. In either, one bud or more than one
may be formed at a time. The buds (Fig. 87 H), whether external or
internal, are usually ciliated and at first without tentacles; the cilia
form a girdle round the body, with sometimes the vestige of an adoral
wreath. Certain species form also unciliate and often tentaculate oflF-
spring by external budding. Some species will, in unfavourable cir-
cumstances, resolve practically the whole body into one internal bud
which swims away, leaving the pellicle and stalk behind.
Conjugation is of the same nature as in the Ciliata. Two individuals
become united by pseudopodia-like processes of protoplasm, their
meganuclei break up, and their micronuclei form pronuclei which
unite reciprocally. Often, however, the conjugants do not break
apart, but one detaches itself from its stalk to unite permanently with
Il6 THE INVERTEBRATA
the other. It is not known what happens to the two zygote nuclei in
these cases.
The arrangement of the larval cilia in rings, the prevalence of a
sessile habit, the frequent inequality of conjugants, and sometimes
the absorption of one of these by its partner, suggest the derivation of
this subclass from a form which resembled the Peritricha.
Sphaerophrya (Fig. 87 F, F'). Spherical species; which are at first
free and provided with knobbed tentacles on all sides ; afterwards be-
come endoparasites in ciliates ; and are then without tentacles. Fission
equal or somewhat unequal ; in the parasitic stage it is repeated before
the young escape. Parasitic in Paramecium^ etc.
Ephelota (Fig. 96). Stalked; not seated in a cup; bearing tentacles
distally. Reproduction by external, usually multiple, budding.
Marine.
Acineta (Fig. i). Stalked; the stalk expanding to form a shallow
cup. Reproduction by internal budding. In fresh waters and marine.
Dendrocometes (Fig. 87 G). Body lens-shaped; without stalk; with
branched arms which end in several pointed tentacles. Reproduction
by formation of one internal bud. Sessile upon the gills of Gammarus.
CHAPTER III
spi.2
THE SUBKINGDOM PARAZOA (PORIFERA)
Multicellular organisms ; invariably sessile and aquatic ; with a single
cavity in the body, lined in part or almost wholly by collared flagellate
cells; with numerous pores in the body wall through which water
passes in, and one or more larger openings through which it passes
out; and generally with a skeleton, calcareous, siliceous, or horny.
The members of this phylum are the sponges.
The simplest sponge is a little creature, osc.
known as the Olynthus (Fig. 97), which
is found only as a fleeting stage in the
development of a few of those members
of the group which possess calcareous
skeletons ; but the bodies of all sponges
may be regarded as derived from it,
even though it may not appear as a stage
in their life history. It is a hollow vase,
perforated by many pores, and having at
the summit a single large opening, the
osculum. Through the pores water con-
stantly enters it, to pass out through the
osculum. Herein it and its kind difl^er
from all the Metazoa, using the principal
opening not for intaking — as a mouth — ■
but for casting out. The wall (Fig. 98) of
the vase consists of two layers, (a) a
gastral layer, composed of collared flagel-
late cells resembling the Choanoflagellata
(p. 65) and known as choanocytes, stand-
ing side by side but not touching, which
lines the internal cavity or paragaster
except for a short distance within the Fig. 97. The Olynthus of a
rim; and (b) a dermal layer, which makes simple calcareous sponge, with
1 ^ ' r 1 1 • 1 r part of the wall cut away to
up the greater part of the thickness of ^^p^^^ ^^e paragaster. osc. os-
the wall and is turned in a little way at culum ; po. pore ; spi. spicule,
the rim. This layer again consists of
two parts, (i) a covering layer of flattened cells, known as pinacocytes,
rather like those of a pavement epithelium, but with the power of
changing their shape; and (ii) the skeletogenous layer, between the
covering layer and the gastral layer. The skeletogenous layer consists
ii8
THE INVERTEBRATA
of scattered cells, with a jelly in which they are imbedded. The most
numerous of these cells are engaged in secreting spicules of calcium
carbonate by which the wall is supported. They wander from the
covering layer into the jelly, and then each divides into two, and the
resulting pair secrete in their protoplasm, which is continuous, a
needle-like spicule which presently outgrows them. Most often the
original spicule cells come together in threes before this process, so
that the three spicules which they secrete become the rays of a three-
rayed compound spicule. This lies in the wall with two rays towards
the osculum and one away from it. Sometimes a fourth cell joins the
others later, and forms a fourth ray which projects inwards towards
the paragaster. Often there are simple spicules which project from
the surface of the sponge. Other cells, known asporocytes, of a conical
shape, extend through the jelly, having their base in the covering layer
am
Fig. 98. Part of a longitudinal section of the wall of an Olynthus, including
a portion of the rim of the osculum. From Borradaile. a.m. amoeboid cell ;
ch. choanocyte; e.' flat covering cells (pinacocytes) of dermal layer; e.'' similar
cells lining the rim of the osculum ;;. jelly; por. pore; pc. young porocyte;
pc' fully developed porocyte; sp. spicule; sp.c. spicule cell.
while their apex reaches the paragaster between the choanocytes. Each
is pierced from base to apex by a tube, which is one of the pores. Be-
sides these cells of the dermal layer, there are in the jelly wandering
amoeboid cells which appear, in some cases at least, to belong neither
to the gastral nor to the dermal layer, but to be descended inde-
pendently from blastomeres of the embryo. Some of them become
ova; others, it is believed, give rise to male gametes; the rest are
occupied in transporting nutriment and excreta about the sponge.
There are no nerve or sense cells in this or any other sponge.
The current which flows through the body is set up by the working
of the flagella of the choanocytes. It carries with it various minute
organisms which serve the sponge for food, being swallowed, in some
way which is still in dispute, by the collar cells. These digest the food,
rejecting the indigestible parts into the space within the collar; and
passing on the digested food to amoebocytes, which visit them to
obtain it.
PORIFERA
119
No sponge remains at this simple stage throughout its life. At the
least the body branches and thus complicates its shape, and then often
new oscula appear at the ends of the branches (Fig. 99). A higher
grade is reached when, as in the calcareous sponge Sycon (Fig. 100),
the greater part of the vase is covered with blind, thimble-shaped out-
growths, regularly arranged, and touching in places, but leaving
inh.c.
Fig. 100.
Fig. 99. A branched calcareous sponge of the first (Ascon) type. From
Sedgwick, after Haeckel.
Fig. 100. A semidiagrammatic view of a simple Sycon, opened longitudinally,
with a portion of the wall enlarged. i7ih.c. inhalent canal; fl.c. flagellated
chamber.
between them channels, known as inhalant (or afferent) canals, whose
openings on the surface of the sponge are often narrowed and are
known as ostia. The thimble-shaped chambers are known as flagellated
chambers, and are lined by choanocytes, but these are now lacking
from the paragaster, where they are replaced by pinacocytes. Water
enters by the ostia, passes along the inhalant canals and through the
I20
THE INVERTEBRATA
pores, now known as prosopyleSy into the excurrent canals, leaves
these through the openings, known as apopyles^ by which they com-
municate with the paragaster, and flows outwards through the
osculum. A third grade is found in sponges such as the calcareous
sponge Leucandra (Fig. loi), where the wall of the paragaster is folded
a second time, so that the flagellated chambers, instead of opening
direct into the paragaster, communicate with it by exhalant (or efferent)
canals lined with pinacocytes.
The three grades of sponge structure (Fig. 102), in which suc-
cessively the choanocytes line the whole paragaster, are restricted to
flagellated chambers, or are still further removed by the presence of
exhalant canals, are known as the *' Ascon ", *' Sycon ", and *' Leucon "
Fig. 1 01. Diagram of a section of the wall of the sponge Leucandra aspersa,
showing the direction of the currents. After Bidder.
grades. In many of the sponges whose canal systems are of the third
grade, the flagellated chambers are no longer thimble-shaped, but
small and round. As the canal system has grown more intricate, com-
plication has taken place also in the skeletogenous layer. It has grown
thicker, forming outside the flagellated chambers a layer known as the
cortex, in which the inhalant canals ramify; and there appear in it
branched connective tissue cells which can change their shape.
The sponges which we have so far considered have skeletons com-
posed solely of calcareous spicules, and their choanocytes are re-
latively large. They constitute a comparatively small group, the class
Calcarea. The majority of the phylum are without calcareous spicules
and have relatively small choanocytes. They have usually siliceous
PORIFERA
121
spicules, of which there exist many different types (Fig. 103), cha-
racteristic of various groups of sponges, while minor differences
distinguish those of the species, which are often only separable by
this means. A horny substance, spongin, may occur as a cement
uniting spicules, as fibres in which spicules are imbedded, or as a
fibrous skeleton from which spicules are absent. The sponges in
which the skeleton is in the latter condition constitute the horny
f\
0
far
por
OHQ
fit
^^ exh c
Oit
lO^^
CP
ost
ost
r
I
ink c
ink c 3 4
Fig. 102. Diagrams of the canal systems of sponges. Partly after Minchin.
I, Ascon grade. 2, Sycon grade. 3, Leucon grade. 4, Leucon with small,
round flagellated chambers, exh.c. exhalant canal; inh.c. inhalant canal;
fl.c. flagellated chamber; osc. osculum; ost. ostium ;^ar. paragaster ; /)or. pore.
sponges (Keratosa), of which the bath sponge {Euspongia, Fig. 104)
is an example. Foreign bodies (sand grains, etc.) are often imbedded
in the spongin fibres. In a few cases (Myxospongiae) there is no
skeleton. The choanocytes of non-calcareous sponges are always
restricted to flagellated chambers. Almost without exception these
are arranged as in calcareous sponges of the Leucon type, and in
most cases the system is made still more intricate by ramifications of
the paragaster, the irregular appearance of numerous oscula, which
122
THE INVERTEBRATA
Fig. 103. Various types of sponge spicules. From Woods, a- e, are from Demo-
spongiae, /, from a hexactinellid, g and h, from extinct groups of sponges,
J, from Calcarea. a, With one axis (monaxon). b and c, With four axes
w-^u^^'u"' ^ '^ ^ "calthrops", c a "triaene" spicule), d and e, Irregular. /,
With three axes (triaxon ; four six-rayed spicules united as part of a continuous
skeleton by additional deposits), j, A three-rayed compound spicule formed
by the union of monaxons.
PORIFERA
123
put it into communication with the water at many points, and the
appearance of "subdermal cavities" and other complications in the
outer part of the body.
The non-calcareous sponges fall into two very distinct classes —
the Hexactinellida, in which there is always a siliceous skeleton of six-
rayed spicules (Fig. 103/), the jelly is absent, and the flagellated
chambers are thimble-shaped, as in the simpler Sycons; and the
Demospongtae, in which the skeleton, if present, does not contain six-
rayed spicules of silica, jelly is present, and the flagellated chambers
are almost invariably small and rounded (Fig. 106 C).
ff.c.
Fig. 104. A diagram of the structure of a bath sponge (Euspongia). From
Borradaile. exh.c. exhalant canal; inh.c. inhalant canal; fix. flagellated
chamber; osc. osculum; ost. ostia; sd.c. subdermal cavity; sk. one of the
principal pillars of the skeleton, containing imbedded sand grains; sk.' minor
fibres of the skeleton.
Sponges have free larvae, of several different kinds, but all covered,
wholly or in part, with flagellate cells, by which they swim. The re-
markable feature of the metamorphosfes by which these larvae become
the fixed adults is that the flagellated cells pass into the interior,
develop collars, and become the choanocytes (Fig. 106).
Asexual reproduction is found throughout the group. It takes
place by the outgrowth and separation of external buds, or by the
formation of internal buds or gemmules, enclosed in stout coats. In
124
THE INVERTEBRATA
some cases (Spongillidae) the gemmules are remarkable in that they
originate as clumps of the amoeboid cells of the parent. They will
stand freezing or drought, and carry the species through unfavourable
conditions. The power of regeneration and repair is possessed by
sponges in a high degree, and they can be propagated artificially by
cuttings.
Sponges are found in all parts and at all depths of the sea. Only
one family, the Spongillidae, occurs in fresh water, but its members
are plentiful and widespread.
ost. ih.ch. *'^'
i ih.ch.
ih.ch.
ost.
spi—
Fig. 105. Section of a portion of Grantia extusarticulata. Highly magnified.
From Dendy. ost. openings of the inhalant canals (ostia) ; ih.ch. inhalant
canal ; prp. openings of inhalant canals into flagellated chamber (prosopyles) ;
fl.c. flagellated or collar ceils (choanocytes) ; y?.c/z. flagellated chamber; spi.
spicules ; ap. exhalant opening (apopyle) of flagellated chamber.
The affinities, and therefore the systematic position, of the phylum
Porifera have been the subject of much dispute. In that their bodies
consist of many "cells", they might seem to be metazoa. But they
differ from all members of that group in several important respects.
In no metazoon are choanocytes found. In none is the principal
opening exhalant. In none is there during development an inversion
whereby a flagellated outer covering becomes internal. Lastly, and
perhaps most significantly, in a sponge the *' cells" are far less special-
ized and dependent upon one another than the cells of a metazoon.
Many of them can assume various forms, becoming amoeboid,
PORIFERA
125
collared, etc. Many are isolated in the jelly, and when they touch they
are often not continuous. No nervous system co-ordinates their
activities. Even the choanocytes, though the sum of their efforts
produces a current, do not keep time in their working. In short, the
Porifera are practically colonies of protozoa. Moreover, it would
Fig. 106. A, Larva (Amphiblastula) of Sycon raphanus. B, The same
with flagellated cells invaginating. After Schulze. ca. segmentation cavity;
der.c. dermal cells ; fl.c. flagellated cells. C, Section of flagellated chamber of
Spongilla lacustris. From Vosmaer. ap. apopyle ; nu. nucleus ; vac. vacuole.
seem that they took origin from choanoflagellate mastigophora. Now
opinion is, as we have seen, not unanimous that the Metazoa arose as
colonies of protozoa, and in any case it is unlikely that they sprang
from choanoflagellates. Thus the sponges, in spite of certain super-
ficial resemblances to the Metazoa, have no real similarity to, and
probably no genetic affinity with, that subkingdom. For this reason
126 THE INVERTEBRATA
it is best that, in a classification of animals, they should be given,
under the name of Parazoa, the same rank as the Protozoa and the
Metazoa.
Class CALCAREA
Sponges with skeletons consisting solely of calcareous spicules ; and
with large choanocytes.
Clathrina. A meshwork of Ascon tubes. The nuclei of the choano-
cytes are at the bases of the cells. British.
Leucosolenia. A clump of erect Ascon tubes, each of which may be
branched, connected at their bases. The nuclei of the choanocytes
are apical. British.
Sycon (Fig. loo). A simple vase with a canal system of the second
type, having the thimble-shaped outgrowths little adherent to one
another. The nuclei of the choanocytes are apical. British.
Grantia. Differs from Sycon in that the outgrowths which contain
the flagellated chambers adhere in many places and are covered by a
cortex (Fig. 105). British.
Leucandra. Canal system of the third type (Fig. loi). Nuclei of
choanocytes basal. British.
Class HEXACTINELLIDA
Sponges with a purely siliceous skeleton composed of six-rayed
spicules; with small choanocytes and thimble-shaped flagellated
chambers ; and without jelly, the soft parts of the body being united
solely by a meshwork of trabeculae furnished by branching cells of
the dermal layer.
A deep-sea group.
Euplectella^ Venus' flower basket, and Hyalonema, the glass-rope
sponge, have both been dredged in British waters. Both harbour
various commensal crustaceans. On the rooting-tuft of long, fine
spicules, which is the "glass-rope" of Hyalonema, grows an epizoic
anemone of the genus Epizoanthus.
Class DEMOSPONGIAE
Sponges whose skeleton, if present, does not contain six-rayed spicules
of silica, and may be purely siliceous, or composed of silica and
spongin, or of spongin alone ; whose flagellated chambers have small
choanocytes and are usually small and rounded; and which possess
jelly.
PORIFERA 127
Cliona (Monaxonida^). A cosmopolitan genus, which bores into
the shells of molluscs and into calcareous rocks.
Halichondriay the crumb-of-bread sponge (Monaxonida). A com-
mon British littoral form, usually of encrusting growth.
Spongilla (Monaxonida). A member of the family of freshwater
sponges mentioned on p. 124. Cosmopolitan.
Euspongia, the bath sponge (Keratosa). Medit., W. Indies, etc.
Hippospongia (Keratosa). A sponge of the same kind with a coarser
texture due to the inclusion of much foreign matter in its skeleton.
Oscarella (Myxospongiae). British, has no skeleton.
^ Orders of Demospongiae : Tetractinellida, with tetraxon spicules (Fig.
103); Monaxonida, with monaxons; Keratosa, with spongin skeleton; Myxo-
spongiae, without skeleton.
CHAPTER IV
THE SUBKINGDOM METAZOA
The fundamental difference in histology which distinguishes the
Metazoa from the Protozoa has already been described in Chapter ii.
Something must here be said concerning the main features of the
organization of the Metazoa.
The simplest type of bodily architecture in this subkingdom is that
with which the student is familiar in Hydra, where the body consists
of a sac with one opening, and with the wall composed of two
cellular layers and a layer of secreted jelly between them. The inner
layer is the endoderm. It consists of cells specialized for the processes
of digestion, and the cavity which it lines is for the reception of food.
The outer layer is the ectoderm: by its cells relations with the en-
vironment are regulated. Some of these cells form a protective and
retaining sheet ; among them stand others which are sensitive ; others
— nerve-cells — lying below the sheet, are branched so as to serve
for the transmission in various directions of the stimuli received by
the sense cells : together they form a nerve-net. At the base of both
ectoderm and endoderm there lie muscle fibres — which in Hydra are
elongate contractile processes of the retaining cells but in other
animals of this type are often whole cells that have left the surface.
Lastly, from certain undifferentiated cells at the base of the ectoderm
there are formed the generative cells.
When we compare this organization with that of a protozoon we
observe that the cellular structure of the metazoon, primarily, perhaps,
necessitated by its size (p. 8), has the following result : by isolating the
units specialized for the performance of particular functions it (a) re-
moves most of them from the direct action of the outer world, (6) makes
it possible that groups of them should constitute independent organs,
and {c) enables the relations of such organs, both with the environ-
ment and with one another to be regulated by intervening cells and
internal media. Already in the simple case we have examined these
facts are turned to advantage. Under the protection of the layer
which remains in contact with the outer world there are established
a special organ of digestion and a system for distributing stimuli
which are received by distinct units on the surfaces. Other ele-
ments (muscular, genital) are beginning to separate. In the following
pages we shall see this process of separation and differentiation carried
much further. Its result is that the activities of the organism are less
and less liable to interference from or suppression by the environ-
METAZOA 129
ment, either through the unregulated exchange of substances or by
unregulated stimuli. We shall see also, how the machinery which is
fashioned in this way varies in correspondence with the environment.
In the phylum to which Hydra belongs, the Coelenterata, the body
is always of the type just described, whatever form the sac or its layers
may assume, though the jelly may contain cells, sometimes plentiful,
of various kinds — muscle fibres, skeleton forming cells, and amoeboid
corpuscles — which have migrated into it from the ectoderm or endo-
derm. In all other metazoan phyla there is between ectoderm and
endoderm a third layer, the mesoderm, which usually is more bulky
than either of the other layers and forms the greater part of the body.
The phyla which possess this layer are known as Triploblastica —
three-layered animals — while the Coelenterata are Diploblastica. It
is true that the mesoderm is partly foreshadowed by the cells which
are present in the jelly of many coelenterates, but mesoderm is more
plentiful than the cells in the jelly generally are, it contains important
organs and usually definite systems of spaces (see p. 131), and its
rudiment appears very early in the development of the individual.
Every triploblastic animal, however, passes through a stage — the
gastrula — in which it consists only of ectoderm and endoderm. Save
in this essential feature, the gastrulae of diff'erent animals may be
extraordinarily unlike, and, especially when the animal is developed
from a very yolky tgg, they are sometimes very difficult to recognize
as such; but where the gastrula is well formed, as in the familiar de-
velopment of Amphioxus or in that of a starfish (Fig. 438), its two-
layered wall may always be found to contain a cavity, the archenteron,
which possesses a single opening, the blastopore. The ectoderm and
endoderm are separated by a space, which is often a mere crack, but
may be much wider, and contains a fluid or a slight jelly. This space
is known as the blastocoele, and when, as in the cases cited above, the
gastrula arises by the dimpling-in (invagination) of the wall of a one-
layered hollow vesicle or blastula, the blastocoele begins as the cavity
of the blastula.
The mesoderm, whose appearance converts the gastrula into a
triploblastic body, is not a single entity, but contains components
which originate in two diff'erent ways, namely:
{a) Cells which migrate from ectoderm or endoderm, or from
mesoderm of the other kind, into the blastocoele; this kind of
mesoderm (Fig. 438, mch.) is known as mesenchyme, and is comparable
to the cells which invade the jelly of coelenterates.
{b) Cells which constitute the wall of the cavity known as the
coelom. This kind of mesoderm is called mesothelium. In some cases,
as in Amphioxus, the starfish, Sagitta, and the Brachiopoda (Figs. 462,
438, 430, 427 A), it arises as pouches of the archenteron which separate
130 THE INVERTEBRATA
from the latter, their cavity becoming the coelom and their wall the
mesothelium. In other cases it arises as solid outgrowths or layers
shed off from the wall of the archenteron, and coelomic cavities after-
wards appear in it. This happens, for instance, in the tadpole. In yet
other cases a single pole cell or teloblast^ as in annelids (Fig. 196) and
molluscs, or a group of a few cells, as in arthropods, separate, on each
side of the embryo, from the rudiment of the endoderm, and multiply
so as to form a band of cells in which coelomic cavities appear. A
coelom which arises as a pouch from the archenteron is known as an
enter ocoele ; one which arises in a mass of mesothelium is a schizocoele.
In the lower triploblastic phyla (Platyhelminthes, Nemertea,
Nematoda, etc., p. 197) there is no mesothelium. Chaetognatha have
no mesenchyme. In most phyla, both kinds of mesoderm develop. ^
We must now consider the organs formed by each of the three
layers.
i. Endodermal organs. After giving rise to mesoderm, the archen-
teron becomes the rudiment of the alimentary canal. Except in
Platyhelminthes, its blastopore is in various ways replaced by two
openings,^ so that it has both mouth and anus. Its wall, the endoderm,
forms the lining of the alimentary canal, except in those regions,
known as fore gut or stomodaeum and hind gut or proctodaeum, which
are formed by a tucking-in of the ectoderm at the mouth and anus.
The endoderm also gives rise to the various diverticula of the mid gut,
such as the liver and other digestive glands, the lungs of vertebrata,etc.
A true stomach is an enlargement of the mid gut.
Digestion was perhaps originally entirely intracellular in the endo-
derm cells, and many of the lower animals still have intracellular
digestion, though this is usually preceded by an extracellular process
which by dissolving certain components of the food enables the re-
mainder to be reduced to particles small enough to be taken up by
the cells. In the annelids, arthropods (except certain ticks, p. 534),
cuttlefishes, and Chordata digestion is entirely extracellular. The
enzymes secreted vary with the food : in carnivorous animals such
as cephalopods and starfishes they are principally proteases, in feeders
on vegetable tissues they are largely carbohydrases, in omnivores such
as the crayfish and cockroach and holothurians they are adapted to
deal with all classes of food-stuffs. Considering the importance of
cellulose both as a potential food-stuff and as cell walls which enclose
more valuable foods, it is remarkable that cellulases should be rare
(PP-435, 559>587)-,
Both intracellular ingestion and absorption are not always confined
^ Mesenchyme is scanty in the lower Chordata.
^ The most primitive way is probably that of Peripatus (p. 319), in which
the middle of the blastopore closes and the ends become mouth and anus.
METAZOA 131
to the alimentary canal proper but may take place in digestive glands
or "livers", as for instance in those of the mussel, the snail, and the
crayfish, but not in those of cuttlefishes or vertebrates. It is said that
in various bivalve molluscs and in holothurians amoeboid corpuscles
pass through the endoderm, take up particles in the gut, digest them,
and, returning, distribute the products. The presence of a cuticle in
the ectodermal portions of an alimentary canal does not always prevent
absorption there (e.g. in the fore gut of some insects). Finally it should
be noted that some animals perform a part of their digestion externally
to the body J as the starfish by extruding its stomach (p. 636), and
various insects, mites, earthworms, etc. by pouring out saliva ; and that
in other cases bacterial or protozoan symbionts (pp. 68, iii) play a
part in the digestion of food — particularly of celluloses — in the gut.
The food of all animals contains amino acids, usually as protein,
for the manufacture of the proteins needed in the repair and growth
of protoplasm. Much amino acid, however, is deaminated^ the car-
bonaceous residue being oxidized, together with the carbohydrate
and fat which the food usually also contains, for the liberation of
energy, and the ammonia excreted in various forms by various organs
presently to be mentioned.
ii. Mesodermal organs. Since mesothelium gives rise to mesen-
chyme, it is often difficult to distinguish between the two and to
decide what part each plays in the formation of organs ; but broadly
speaking it can be said that the connective and endoskeletal, the
vascular, and some muscular tissues arise from mesenchyme, while
in coelomata the peritoneum and the organs derived from it — gonads
(ovaries and testes), mesodermal kidneys, etc. — and the principal
muscles arise from mesothelium.
Within the massive layer of mesoderm, cavities are necessary for
sundry purposes. Channels must be provided for the transport of
various materials — the products of the digestion of food, the gases of
respiration, water, the waste products of metabolism, which are
usually eliminated with the excess of water, and the substances known
as hormones which are secreted by certain organs as messengers to
regulate the activity of others. The germ cells, which are sheltered
in this layer, must be given access to the exterior. Often there must
also be spaces to give play to movements of the viscera. Such facilities
are provided by two systems of cavities, the primary and secondary
body cavities^ of which either or both jnay be present.
{a) The primary body cavity, sometimes known as the haemocoele,
is to be regarded, morphologically, as representing that part of the
blastocoele which is not obliterated by the mesenchyme cells or by a
solid matrix or fibres secreted by them. Its fluid contents, containing
free mesenchyme cells ("corpuscles"), are the blood and lymph, and
132 THE INVERTEBRATA
it has usually the form of a branching system of vessels (''vascular
system") through which the fluid is caused to circulate by the con-
traction of muscular fibres in the wall of some portion of it which is
known as a heart. In some cases, however, the haemocoele forms
large "perivisceral" sinuses around the internal organs. It never
contains germ cells or communicates with the exterior.
Since the haemocoele fluid is in intimate relation with the tissue,
its composition is a matter of very great importance to the animal.
It bears to the tissues much the same relation that the external
medium bears to the body as a whole and is on that account often
spoken of as an internal medium} If it be changed the working of the
organism is influenced. It is liable to be fouled by poisonous waste
products of metabolism and these must be removed from it and ex-
creted or so changed as to be harmless. It is liable to alteration by
diffusion between it and the external medium, and in proportion as
this can take place the animal will be at the mercy of its surroundings.
To maintain it in a constant condition in respect of the substances
which it might exchange with a particular external medium two
agencies are at work — the guardianship, active or passive, of the pro-
tective sheet of ectoderm and of any cuticle or other covering which the
latter may secrete, and the activity of the excretory organs, especially
in the excretion of water. The effectiveness of these agencies varies.
The independence of the body fluids from the external medium is
least in some marine animals, such as echinoderms and certain
molluscs: in these the fluids closely resemble sea water both in the
ions present and in the total osmotic pressure. In a series of others,
independence grows, and it is highest, in the sea, in teleostean fishes.
In fresh water animals the composition of the blood is kept entirely
different from that of the external medium. In land animals there is
of course no question of the exchange of solutes, and unless the loss
of water were reduced to a minimum life would be impossible. It is
an interesting fact that, though the resemblance of the body fluids
in fresh water and land animals to sea water is much less than that
of marine animals, something of it still remains, no doubt because
protoplasm came into being in sea water and still requires to be
bathed by a fluid which somewhat resembles the latter. The principal
differences are an increase in potassium and a decrease in magnesium
and SO4 ions and a lower total osmotic pressure.
The blood is the principal means of transport within the body.
A very important part of its freight is oxygen. Its capacity for
^ The fluids of the secondary body cavity (coelomic fluids) are also internal
media, but less intimate and therefore chemically less important than the
blood. In echinoderms, tiowever, they are probably more important than the
fluid of the vestigial haemocoele (lacunar system, p. 629).
METAZOA 133
this gas, however, would be quite insufficient for it to maintain the
metabolism of an active animal if the gas were carried in mere solution.
This deficiency is met, when necessary, by the presence in the blood
of respiratory pigments. These bodies are compounds of a protein
with a nitrogenous pigment which contains a metal. They are re-
lated to one another, to chlorophyll, and to the colourless substance
cytochrome which is very widely distributed in the protoplasm of
animals and plants, where it plays a part in the regulation of oxida-
tions. They form very labile addition compounds with oxygen, which
they can thus take up in the organs of respiration and carry to the
tissues, where they yield it up by dissociating under the lower oxygen
tension, undergoing at the same time a change in colour. The most
important of them are haemoglobin, which contains iron and is red,
chlorocruorin, also containing iron, which is green, and haemocyanin,
containing copper, which is blue when oxygenated. Haemoglobin is
present in Vertebrata, where it is carried in the "red corpuscles",
and sporadically in many invertebrates, as in the earthworm, where
it is in solution in the plasma. Chlorocruorin is found in solution in
the blood of various polychaete worms, haemocyanin in solution in
the blood of the higher Crustacea, the king-crab (Limulus), and various
molluscs. Both haemoglobin and haemocyanin are slightly different
compounds in different animals, and with these differences are as-
sociated differences in the pressure at which they take up or yield
oxygen. Broadly speaking, the blood pigments of animals which live
under conditions of low oxygen pressure take up the gas at a lower
pressure than those which live under high oxygen pressure. On the
other hand they do not maintain so high a pressure in the tissues.
Independently of such differences, the haemocyanins are less efficient
oxygen carriers than the haemoglobins. In tracheate arthropods,
where air is brought direct to the tissues by a system of tubes, there
are no blood pigments.
The blood of the higher invertebrates contains in solution a con-
siderable amount of protein, of which the respiratory pigment, if
present, is only a part. This protein is comparable with the organic
ground substance of a skeletal tissue. It is not a food for the tissues
but by maintaining the osmotic pressure of the blood it is of im-
portance in regulating the distribution of water between that fluid
and the tissues, and, since proteins combine with both acids and
alkalies, it helps to neutralize excess of either of these. In vertebrates
some of this protein provides the material for clotting, by which loss
of blood or injury is prevented; but invertebrates, when they form a
clot, do so from material furnished by corpuscles.
(b) The secondary body cavity or coelom is from the first completely
surrounded and separated from the blastocoele by the mesothelium,
134 THE INVERTEBRATA
which is derived, as we have seen, from the endoderm. This cavity
has various forms, but is rarely tubular and never possesses a heart.
Usually it constitutes one or more large perivisceral spaces around the
heart, alimentary canal, and other organs. It will be noted that the
perivisceral cavity which surrounds the internal organs of most triplo-
blastic animals, so that these organs are unaffected by the movements
of the body wall and are able freely to perform movements of their
own, may be either coelomic or haemocoelic, but is usually coelomic
(Fig. 107 a-c). In the Arthropoda, where the perivisceral function
of the coelom is entirely usurped by the haemocoele {d~g), the former
space is reduced to small cavities in the gonads and excretory organs.
In animals which possess a coelom, the gonads are derived from
its walls, and either the germ cells are shed into a coelomic perivisceral
cavity or the gonad itself contains a cavity which is a separated portion
of the coelom.
The coelom communicates with the exterior. The communication
is usually made through organs belonging to one or other of the types
known as "nephridia" and "coelomoducts", though it occasionally
takes place through openings of other kinds, such as the dorsal pores
of the earthworm and the abdominal pores of fishes.
Nephridia and coelomoducts are organs which meet the need for
the passage to the exterior of products of organs derived from or
imbedded in the mesoderm. Their characteristic features are as
follows :
(a) The nephridial system is primarily an organ which serves the
mesenchyme, though it may come to lie in the coelom, and in certain
annelids communicates with that space. It is for the most part intra-
cellular, and consists of tubes, often, at least, of ectodermal origin,
usually branched and bearing at the end of each branch a solenocyte
or flame cell (see p. 202). It may be continuous or divided into seg-
mental units, the nephridia. Water, probably containing excreta, is
shed by the protoplasm of the tubes, and passes out in the current
set up by the action of the flame cells or by cilia.
{b) Coelomoducts are mesodermal passages which open at one end
to the exterior and at the other usually into the coelom, though the
coelomic opening may lead only into a minute vesicle of the coelom,
or even be lost altogether. They may (i) be solely excretory, the
excreta being shed into them by gland cells in their walls, or borne
into them by a current of fluid from the coelom through the coelomic
opening of the organ, or derived from both these sources (see p. 141) ;
(2) combine excretion with the function of conducting the germ cells
to the exterior; (3) be simply gonoducts, which was perhaps their
original function.
Many annelida possess compound excretory organs formed by the
METAZOA
135
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136 THE INVERTEBRATA
union in various ways of nephridia with coelomoducts or other meso-
dermal elements (see p. 276). In such cases the nephridia acquire a
communication with the coelom, and excreta or germ cells may pass
from it through them. In other groups, as in some Crustacea, a
coelomoduct is supplemented or in great part replaced by an ecto-
dermal component, but there is no evidence that this component
represents a nephridium.
iii. Ectodermal organs. The ectoderm gives rise to the epidermis
(epithelium which covers the body), to certain glands, to the ne-
phridia, to the principal external organs of sense, and to the nervous
system (in nearly all cases ; there are nerve cells under the endoderm
of certain coelente rates, and a part of the nervous system of the
Echinodermata is remarkable in being formed from the peritoneum
and therefore mesodermal).
The epidermis with some underlying mesodermal connective tissue
known as the dermis constitutes the skin. In invertebrates it is
columnar or syncytial, in vertebrates it is stratified. In the lower
invertebrates its cells are usually ciliated, which was probably the
original condition. The cilia subserve locomotion, the taking of food,
or respiration (p. 139). When unciliated its protective function is
often increased by a cuticle. To it belong various glands, especially,
in naked epithelia, mucous glands whose secretion is protective, in
aquatic animals against parasites, in terrestrial against desiccation.
Others form cuticular structures, cement, poisons, etc.
The nervous system was no doubt primitively situated immediately
below epithelia, having arisen by specialization of epithelial cells
for the transmission of impulses due to stimuli received upon the
surface — for the most part, presumably, upon the ectoderm. In
many cases (the Coelenterata, Echinodermata, Hemichordata, some
annelids, etc.) it remains there, but usually it is in a deeper, more
protected situation. All triploblastica possess a central nervous system.
This arose as a condensation of the primitive nerve-net of branched
cells, portions of which may remain unchanged. The central nervous
system was formed in different positions in different animals. In
those which have a long axis it has the form of cords along that axis.
The cords may be paired or unpaired, lateral, ventral, or dorsal.
Anteriorly they pass into an enlargement, the "brain" or cerebral
ganglion, connected with the principal organs of distant sense. In
Chordata the central nervous system is hollow, its removal from the
surface being not, as usual, by separation from the epithelium, but
by the folding-in of the strip of epithelium which it adjoins and
which remains to line its cavity. A similar condition is seen in some
echinoderms. From the central nervous system nerves proceed to
various parts of the body.
METAZOA 137
The nerve-net is joined by processes from the bases of the sense
cells. Probably at an early stage in the evolution of the Metazoa
stimuli were transmitted only by such processes, running directly
from the sense cells {receptor cells) to end against the muscle or other
cells which are set in action through them {effector cells). This con-
dition, however, is now rare, occurring only in the tentacles of some
coelenterates : nearly always there are nerve-cells which have left the
surface layer, whose processes continue those of the sense cells and so
extend and complicate the system of communications. In the primitive
condition of the nervous system, as seen, for instance, in Hydra, the
nerve-cells have numerous similar branches, forming a network over
which messages pass in all directions from any point of stimulation.
That there is co-ordination in the action which results is due only to
the fact that the messages do not evoke responses equally in all the
effector cells.
The condensation of nerves and a central nervous system out of
this network is due to a change in the form and arrangement of the
elements of which it is composed. The change, which is already fore-
shadowed in certain parts of the bodies of coelenterates (p. 150),
consists in processes of the nerve cells elongating in particular
directions and thus forming paths of conduction which in the higher
triploblastica are isolated by the loss of the rest of the network. As this
system is perfected its elements become neurones — cells with one main
process, the axon or nerve fibre, along which the impulse passes from
the cell body. The axon ends by breaking up into a tuft of branches, the
terminal arborization, from which a stimulus is given either to another
nerve cell or to an effector cell. Thus instead of spreading in all
directions the impulse is conducted to a definite destination: the
interference of the environment in the affairs of the organism is
regulated. The cell body may be a sense cell in the epithelium, or it
may be internal. In the latter case it possesses other processes — the
dendrons — which by fine branches — the dendrites — receive stimuli
from the axons of other neurones. Two neurones at least are con-
cerned in the transmission of an impulse. In the simplest case the
axon of a sense cell (or of a cell whose dendriies receive stimuli from
a sense cell)^ transmits the impulse to a neurone whose axon conducts
it to the effector cell. This process is known as a refiex and the arrange-
ment of neurones as a reflex arc. The impulse is passed from the first
neurone to the second in an exchange station — the central nervous
system. The nerve fibres run to and from this station in bundles which
are the nerves. This arrangement is not only, as w^e have seen, more
^ In the Vertebrata the cell bodies of the afferent fibres from the receptors
are moved far inwards to lie in the dorsal root ganglia, receiving impulses by
a dendron nerve fibre which is longer than the axon.
138 THE INVERTEBRATA
precise but, since one efferent neurone can serve several afferent,
more economical of fibres than the nerve-net. Usually, moreover,
the system is complicated, and in the highest animals it is enormously
complicated, by the branching of axons, which increases the number
of efferent fibres an afferent fibre can affect, and by the intervention,
in the central nervous system, of intermediate neurones between
those which are directly afferent and efferent. By this the number
of afferent fibres which an efferent fibre can serve is increased. For
the efficient working of this system it is essential that impulses should
pass over it in one direction only, and thus should not leak from one
path to another and affect organs for which they were not destined.
That is provided for in the following way. Where the terminal
branches of an axon meet the dendrites of another neurone the two
are not continuous but interlace without joining, making what is
called a synapse. Their discontinuity makes an obstacle, over which
impulses can pass in one direction only, from axon to dendrites. The
mode of passage of impulses from the one to the other and again from
efferent neurone to effector cell is not at present known. It is perhaps
an electrical process, but it involves the production of a chemical
that probably affects the sensitivity of the recipient cell. In that case
the transmission of a nervous impulse includes a process which
recalls that other mode of communication, mentioned above, in
which the chemical messengers known as hormones are distributed
through the vascular system.
Stimuli received from the nervous system, like other stimuli, may
inhibit as well as cause activity. This is very important, because when
an action is to be performed activity which hinders it must be
abolished. Thus, for instance, a contracting muscle may by a reflex
inhibit contraction in an opposing muscle: the circular and longi-
tudinal muscles of the earthworm are an example of such a system
(p. 261). A similar end is obtained in a different way in the muscles
which open and close the claws of crabs and lobsters, where each
muscle fibre has two nerve fibres, one excitatory and the other in-
hibitory, and impulses from the central nervous system pass simul-
taneously to the excitatory fibres of one muscle and the inhibitory
fibres of the other. Further, one neurone may inhibit another, and
thus inhibition may be effected not only through but in the central
nervous system.
Concerning the way in which the central nervous system, which
in the lowest animals that possess it is merely a relay station where
impulses from the principal sense organs are multiplied and distri-
buted, later takes on inhibitory functions, and later still develops
"functional units" for co-ordination, and concerning the control of
the latter by the brain, something is said on pp. 198, 261, 448
METAZOA 139
iv. Certain organs are formed in different animals from different
layers. Organs of respiration may be covered or lined by ectoderm,
as are the gills of crustaceans and annelids, the external gills of the
tadpole, and the lungs of snails ; or by endoderm, as are the gills of
a fish or the lungs of vertebrates. The skin, when it is naked or
covered only by thin cuticle is always respiratory, and in many small
animals is the only organ of respiration. Cilia may keep water in
movement over it so as to renew the supply of oxygen, and when
there is a vascular system a rich blood-plexus may increase the
efficiency of the skin, as in earthworms. In larger animals there are
usually localized organs of respiration. In these the surface is in-
creased by folding or branching either outwards or inwards, the blood
supply is richer than elsewhere, and there is some means of con-
stantly renewing the medium. Organs of aquatic respiration are
usually projections, known as gills : the water around them is renewed,
either by muscular movements of the body, of limbs which bear the
gills or of structures in their neighbourhood, or by ciliary action.
Organs of aerial respiration may be those that were originally used
for aquatic respiration: this is especially the case when they are en-
closed in a chamber which protects them; sometimes, as in snails
and land crabs, such a chamber becomes itself converted into a re-
spiratory organ by the vascularizing of its lining. In other cases, as
in terrestrial vertebrates, there are developed for this function cavities
in the body into which air is drawn. In order to prevent damage to
the epithelium by desiccation, a layer of moisture is always main-
tained over air-breathing surfaces, either by exudation or by special
glands or by water-retaining hairs, etc. Consequently aerial respira-
tion is in the long run aquatic respiration, with the difference that the
supply of oxygen in the layer of water over the respiratory epithelium
is maintained not by renewal of the layer but by diffusion from ad-
jacent air. It is maintained that on that account aerial respiration is
less efficient than aquatic, and this argument is supported by the fact
that the respiratory area of air-breathing animals is greater than that
of related forms which have aquatic respiration.
What has been said in the foregoing paragraph does not apply to
the tracheate arthropods, whose respiration does not take place
through an epithelium with the intermediation of the blood, but
by the bringing of air directly to the tissues by a system of fine
ectodermal tubes — the tracheal system (p. 440).
The relative effect upon respiration of the pressures of carbon
dioxide and of oxygen differs according as the animal is living in water
or in air. In clean waters the pressure of carbon dioxide varies little,
because any excess of the gas is removed by the formation of carbon-
ates and bicarbonates, but the pressure of oxygen is easily lowered as
140 THE INVERTEBRATA
the amount in solution is used up. In air, on the other hand, free
carbon dioxide will accumulate, but there is a large supply of oxygen.
Consequently aquatic respiration will sooner be affected by changes
in the pressure of oxygen in the medium, aerial respiration by changes
in the pressure of carbon dioxide. In foul waters, however, free
carbon dioxide may be present in such quantities as to be an important
factor. In such waters, and in the habitat of many internal parasites,
free oxygen may be practically absent. Many animals which live in
such circumstances obtain energy by an anaerobic process, the
complex molecules of carbohydrates being decomposed to form
simpler ones without the intervention of free oxygen. To this end the
animals in question lay up in their tissues large quantities of the
starch-like substance glycogen of which carbohydrate stores in
animals are usually composed. Thirty per cent, of the dry weight of
an Ascaris, and nearly half that of a tapeworm, consist of this sub-
stance. The glycogen is converted into glucose (dextrose) and then
decomposed, according to the following equations:
(i) (C,n,,0,)n-^nU,0 = nC,U,,0,
(ii) C,U,,0, = 2C,Ufi,
(Glucose) (Lactic acid)
This process of course yields considerably less energy than would be
obtained by total oxidation. Apparently, at least in many animals,
it cannot go on indefinitely unless the lactic acid be removed. This
may happen either by the acid being discharged into the surrounding
fluid and swept away by movements of the latter (which would occur,
for instance, in the host's intestine), or by an access of oxygen with
which some of the lactic acid is oxidised so as to give energy for
building the rest back into glycogen. In the latter case the process
becomes ultimately aerobic. It is probable, indeed, that even in
aerobic animals the process by which energy is liberated is at first
anaerobic, but that this phase is quickly followed by one in which,
by the use of oxygen, a part of the product is destroyed and the rest
built back, so that the process as a whole appears aerobic. Thus
anaerobic animals differ from those that are aerobic only in the length
of time for which the anaerobic process goes on.
Organs of excretion are even more various, in kind and in origin,
than those of respiration. If the removal of carbon dioxide from the
body be disregarded, there are two processes to be considered here — •
the excretion of water and that of solids. In the lower aquatic animals,
whose surface is in various degrees permeable to water, the removal
of the latter from the body is, as we have seen, a matter of very great
importance: it was probably the original function of the nephridial
system and is an essential part of that of excretory coelomoducts. But
METAZOA 141
the removal of solids — both of solutes which have entered from with-
out and of the nitrogenous products of metabolism — is also essential,
and in most animals advantage is taken of the outgoing water to remove
the solids. It is to provide for this as well as to meet the loss due to
evaporation, that terrestrial animals must take in water by the mouth.
In coelenterates excretion probably takes place from the general
surface of the ectoderm, and perhaps also from the endoderm. In
triploblastic animals without a perivisceral cavity ectodermal in-
growths— the nephridia, mentioned above — permeate the mesen-
chyme and perform excretion. In the Nematoda there are lateral
ducts in the ectoderm, which probably subserve excretion. In
animals with a coelomic perivisceral cavity excreta are shed into the
cavity (or carried into it by such cells as the ''yellow cells" of the
earthworm) ; and removed to the exterior by nephridia (which may,
as in the earthworm, open to the coelom), by the mesodermal coelomo-
ducts, or in other ways, as in echinoderms, which shed excreta from
the coelomic fluid through the respiratory organs (gills, respiratory
trees). In echinoderms also solid excreta, perhaps not nitrogenous,
are removed by amoebocytes which pass to the exterior through the
gills. In the Vertebrata the excretory portions of the coelom are in the
adult separated, and imbedded as the Malpighian capsules in the mass
of coelomoducts which forms the kidney. During its passage along
the nephridial tube or coelomoduct the fluid containing excreta re-
ceives additional substances secreted by the walls of the tube ; and
in the terrestrial vertebrates, in which it originates as an exudation
filtered out under blood-pressure in the Malpighian capsules, water
and some of its solid contents are regained by absorption from it. In
the Arthropoda, where the perivisceral cavity is haemocoelic, the
excretory organs are still often coelomoducts (segmental organs of
Peripatus, antennal and maxillary glands of crustaceans, coxal glands
of arachnids) : attached to or imbedded in these are vestiges of the
coelom (end-sac). Instead of, or in addition to, these organs, tubular
diverticula of the ectodermal or endodermal parts of the alimentary
canal often perform excretion in this phylum (Malpighian tubes,
certain of the ''hepatic" coeca of crustaceans). In the insects the
"fatty body" contains a temporary or permanent deposit of excreta
removed from the circulation. In ascidians excreta are similarly laid
up as concretions by mesodermal cells. Various other organs which
are known or supposed to have an excretory function will be men-
tioned in later chapters. The nitrogenous excreta vary in chemical
constitution in diff^erent animals. Their variety appears to depend
partly on the fact that the products of the decomposition of protein,
ammonia compounds, are toxic and accordingly, unless they can be
speedily discharged from the body, are converted into such substances
142 THE INVERTEBRATA
as urea, guanin, and uric acid, which are relatively harmless. In aquatic
animals, where plenty of water is available to carry off the excreta
rapidly, the latter are principally ammonia compounds. In terrestrial
animals it is necessary to expend energy in converting them into
substances such as those mentioned above.
The only triploblastic animals which have a rigid skeleton of great
importance are the Echinodermata and Vertebrata, in which it is
internal and mesodermal, and the Arthropoda, in which it is a cuticle
secreted by the ectoderm and is therefore primarily external, though
ingrowths of it may form a kind of internal skeleton to which muscles
are attached.
The muscular system is in coelenterates derived from ectoderm or
endoderm, in triploblastica almost entirely from mesoderm, though
some muscles of Crustacea arise from ectoderm. In Coelomata fibres
from the mesenchyme form only minor muscular structures, such as
the walls of blood vessels; the great masses of muscle are mesothelial.
In the lower animals the fibres mostly lie parallel to the layers from
which they arose, forming a sheet in the gut wall and another, known
2iS ihtdermomuscular tube, undtrth.t^'kin. In these sheets there is always
a longitudinal and usually also a circular layer; sometimes diagonal
fibres are added. The movements which such layers bring about are
changes of size and shape of regions of the body or gut wall by the
contraction of one set of fibres with relaxation of the other, their
action being aided by the changes in turgor which are caused by the
compression of the fluids they enclose. When there is a skeleton
muscular action is different. The dermomuscular tube is now broken
up into muscles which pull upon pieces of the skeleton and so move
parts of the body. When the skeleton is internal and the body wall
remains flexible, more or less of the dermomuscular sheet remains.
It is lost when there is a stiff cuticle. The muscles of limbs are pro-
vided by outgrowth from the dermomuscular layer.
In the lower animals the muscular fibres are usually varieties of the
unstriped kind. In other cases, chiefly in higher animals (vertebrates,
AmphioxuSy arthropods, part of the adductor muscle in the scallop,
etc.) there appears a new type, the striped fibre, more swift and
powerful in action but more dependent upon the nervous system;
it has lost the power of automatic rhythmical contraction and of
retaining without nervous stimuli a certain degree of contraction,
known as "tone". Some striped fibres, however, retain one or other
of these powers ; thus the fibres of the heart of vertebrates contract
automatically and those of the adductor of the claw of crabs and
lobsters automatically maintain tone. Tone may also be maintained
in striped fibres by the nervous system. In some cases (adductors
of the scallop and of crustacean claws, spines of sea-urchins, etc.) a
METAZOA 143
muscle contains two sets of fibres, one of which by rapid contraction
brings an organ into a certain posture, in which it is held by tonic
contraction of the other (the *' catch" fibres).
Most of the energy expended by an animal is liberated in its con-
tractile tissues. It is obtained, normally from carbohydrates, by a
process which, as we have seen (p. 140), is at first anaerobic and then
aerobic (see also p. 662).
The rudiments of the gonads may be situated either in ectoderm,
endoderm, or mesoderm. In Coelomata they always arise in meso-
thelium. However, since they are often recognizable as early in
development as the layers, and the cells of which they are composed
may migrate from one layer to another, and they do not form tissues,
they are best regarded as an independent entity.
The body constituted by the elements described above has usually
a bilateral symmetry, though this is rarely exhibited completely by
all the systems. In the Coelenterata and Echinodermata, however,
there is a radial symmetry. It is interesting to find that a sessile life,
for which such symmetry seems particularly advantageous, is cha-
racteristic of the Coelenterata, and was probably adopted by the
ancestors of all the Echinodermata. The terms ventral and dorsal,
which belong by right respectively to those aspects of a bilateral
animal which are normally turned to and from the ground or sub-
stratum, are sometimes conveniently applied to a pair of structures
by which two sides may be distinguished in the body of an animal
whose symmetry is predominantly radial. They should, however,
never be applied to the oral and aboral aspects of such an animal.
Meristic repetition of organs of the body is common in Metazoa. It
may, as in parts of the body of annelids, affect practically all systems,
so that there is a complete segmentation of the body into similar
somites, or may be confined to certain organs. In the latter case it is
important to distinguish between {a) the repetition of single organs
in an unsegmented animal, as the ctenidia and shell plates are in-
dependently repeated in the mollusc Chiton, and {b) the condition,
presented for instance by the Vertebrata and by much of the body
of many arthropods, in which a formerly more complete segmentation
now affects only some of the systems to which it at one time extended.
The student should beware of thinking that the segmentation of all
animals which present the phenomenon is derived from that of a
common ancestor. The strobilation of the Cestoda in preparation
for the detachment of reproductive units is a very different matter
from the segmentation of the Annelida, and that again is far from
being, as is sometimes assumed, certainly the same thing as the seg-
mentation of the Vertebrata.
The anterior end of a bilateral animal is the site of the principal
144 THE INVERTEBRATA
sense organs, of the "brain", and usually also of the mouth, and is
often obviously differentiated as a head. In a segmented animal this
cephalization may extend to one or more of the anterior somites ; and
these usually become part of the head, losing their individuality in
the way mentioned in the preceding paragraph, and only betraying
their existence by the presence of certain of their organs (ganglia,
appendages, etc.).
In the process of development by which the body peculiar to the
species is reconstituted from the ovum, the early stages are of
necessity much unlike the adult; but because the general features
must arise before the more special ones, and because general features
are shared by animals, the young resembles other young animals
which have reached the same stage. Since the more special a feature
is the fewer are the animals which share it, as the young approaches
the adult form the circle of other animals whose young it resembles
narrows. Since the evolution of its species consisted in the appear-
ance of the same special features, its development (ontogeny) roughly
recapitulates its evolution (phylogeny), but its features at any moment
are not those of the adult of some ancestor but those of the corre-
sponding young stage of that ancestor, and it is only because that
stage was preparing the features of its own adult that there is re-
capitulation of the latter. Not all features of young animals, however,
are anticipatory of those of their adults. Some of them — the em-
bryonic membranes of the higher vertebrates and the ciliated bands
of echinoderm larvae for instance — are adaptations to the needs of
the young only and disappear in the adult. Such features are said to
be caenogenetic . In respect of them development in no sense re-
capitulates adult phylogeny. Now it is held that in some cases a
young animal, becoming sexually mature at an early stage (as in the
well-known instance of the axolotl which may breed as a tadpole),
has cut out permanently its later stages and started a new course of
evolution from a young stage of an ancestor. This is known as neoteny,
and in it caenogenetic features may be taken up into the new adult
form: it may, for instance, account for some of the peculiarities of the
Larvacea (p. 679), the Cladocera (p. 362), and Leucifer (p. 415). A
young animal which is developing within an egg shell or in the womb
of its mother is known as an embryo : one which is fending for itself
is a larva. A stage which is larval in one animal has often become
embryonic in another. Embryonic development is said to be " direct."
Actually it is no more so than that which is larval. Caenogenetic
features are found in both, those which are most conspicuous being
in larvae organs of locomotion and feeding, in embryos the presence
of yolk or means of obtaining from the mother the nutriment which
the embryo cannot acquire from the outer world.
METAZOA 145
The yolk, which varies in amount in all phyla, is responsible for
much of the difference between animals at corresponding stages,
especially the youngest. The student will recall that from this cause
the cleavage of the ovum which in Amphioxus is complete and equal,
is in the frog complete but unequal and in the chick incomplete.
The blastula has in Amphioxus a large cavity, in the frog a small
one, in the crayfish (Fig. 202) is full of yolk, and in the chick is a
disc upon the yolk. The mode in which the establishment of the
two-layer stage (gastrulation) takes place is also partly affected by
yolk, though evidently other factors are concerned. In the Planula
(Fig. 152) it is by immigration, in Amphioxus and the crayfish by
invagination, in the frog largely by overgrowth (epiboly), in the chick
by delamination.
All these modes of development are repeated sporadically in
various groups of Metazoa: thus the early stages of the mollusc
Paludina and the crustacean Leucifer are analogous to those of
Amphioxus^ those of the squid and the scorpion, members of the
same phyla, to those of the chick. On the other hand, certain features
of cleavage are constant through whole phyla and groups of phyla.
The cleavage of coelenterates and echinoderms is radial (Fig. 196, i),
that of chordates is bilateral, that of polyclads, nemerteans, annelids,
and molluscs is spiral (p. 281). Determinate cleavage, in which the
part of the body to be formed by each blastomere is fixed from the
first, as in the case described on p. 282, is common to the spirally
cleaving groups but occurs in a quite different manner in the tunicates.
Again, while the mesotheHum of annelids, arthropods, and molluscs
is laid down as a pair of ventral bands proliferated from behind,
that of other coelomata arises from the wall of the definitive enteron
(p. 129).
An important function of many larvae is the distribution of the
species. This is often effected by their being planktonic. Among the
larval types adapted to that existence is a series whose members have
delicate tissues and a large blastocoele, whereby their buoyancy is
increased, and strongly ciliated bands, often drawn out into processes,
whereby swimming and feeding take place. To this series belong
Miiller's larva (Fig. 155), the Pilidium (Fig. 169), the trochospheres
(Figs. 197, 374, 420), the Actinotrocha (Fig. 432), the Dipleurulae
(Fig. 439), and the Tornaria (Fig. 466). The student should beware
of supposing that these types are phylogenetically related. With one
or two exceptions, their resemblance is probably an instance of
convergent adaptation.
CHAPTER V
THE PHYLUM COELENTERATA
Metazoa, either sedentary or free-swimming, with primarily radial
structure ; the body wall composed of two layers of cells, the ectoderm
and endoderm, and between these a layer secreted by them which is
originally a structureless lamella (mesogloea) but usually contains cells
derived from the primary layers ; within the body wall a single cavity,
the enteron, corresponding to the archenteron of the gastrula, having
a single opening for ingestion and egestion, and often complicated by
the presence of partitions or by the formation of diverticula or canals ;
digestion partly intracellular; the nervous system a network of cells;
commonly with the power of budding, by which either free indi-
viduals or colonial zooids may be formed; and whose sexual repro-
duction typically produces an ovoidal, uniformly ciliated larva, known
as the planula^ which has at first a solid core of endoderm.
Thus defined, this phylum contains the whole of the diploblastic
animals, that is, those in which the space (blastocoele) between ecto-
derm and endoderm is either devoid of cells, or contains only cells
derived late in development by immigration from ectoderm or en-
doderm. Of such animals there are two very distinct stocks — the
Cnidaria, characterized by muscular movements, which possess
nematocysts (p. 148), and are reducible either to the polyp or to the
medusa type (p. 150); and the Ctenophora, which retain the ciliary
locomotion of the planula, are without nematocysts, and are not to be
assigned either to the polyp or to the medusa type.
In the Coelenterata the Metazoa are at the beginning of their
evolution and we have a primitive type with great potentialities,
though these animals have also already acquired specialized features.
The tissues consist of two single layers of cells, the ectoderm
and endoderm, which constitute a thin body wall surrounding the
central cavity (Fig. 108): the only increase in thickness and com-
plexity of the body wall that is possible is by development of a
gelatinous intermediate layer. Thus, while the typical polyps like
Hydra have a very thin layer of this kind, it has become thicker, very
much folded and penetrated by cells in the actinozoan polyps and
exceedingly thick in the larger jellyfish, forming not only a kind of
internal skeleton but even a reservoir of food
The principal type of cell found in the tissues, both ectoderm and
endoderm, of the primitive coelenterate is the musculo-epithelial cell
which is columnar in shape and only differs from similar epithelial
COELENTERATA
H7
cells in the higher Metazoa in the fact that it is produced into one or
two contractile fibres, which are imbedded in the mesogloea. Such
a tissue unit resembles a protozoon in the fact that different parts of the
cytoplasm carry on different functions although they are not separated
by any partition from each other nor provided with separate nuclei.
An endodermal cell of Hydra has an inner border which can be
produced into flagella, by
means of which the fluid ^,, 7/(. cn.f. cn.L ',-./•
of the body cavity is kept
in motion: or these may
be retracted and the cell
instead puts out pseudo-
podia to engulf particles
of food. In the interior
of the cell, beyond the
border, the food is con-
tained in vacuoles where
it is digested, and finally
the external border of the
cell is produced into
permanent cell organs,
the muscle fibres or tails
already mentioned, in
which the cytoplasm can
contract with much
greater force and rapidity
than in any other part
of the cell. Among the
endodermal cells, how-
ever, some are met with
Fig. 1 08. Diagrammatic longitudinal section
of the body wall of Hydra. From Manton.
c. crystal; cb. cnidoblast; en. cnidocil; d.f. de-
veloping flagellum; d.n. developing nemato-
cyst; e.f. ectodermal muscle fibre; e.i. inter-
stitial cell of ectoderm; e.m. ectodermal mus-
culo-epithelial cell; en.f. endodermal muscle
fibre ; en.i. interstitial cell of endoderm ; /. flagel-
lum ; f.v. food vacuole ; g. nerve cell ; g.c. gland
cell; i.f. food inclusion; m. mesogloea; n. ne-
matocyst; p. pseudopodium ; s.c. sense cell;
su. supporting cell.
of a more specialized type : gland cells which pour into the cavity a
digestive secretion (for the preparatory or extracellular digestion), and
sense cells, found also in the ectoderm, which are thread-like, with
a short projecting process. Both these kinds have no muscle tails.
A type of cell which is even more characteristic of the Coelentetata
(except the Ctenophora) than the musculo-epithelial cell is the thread
cell or cnidoblast. Though this would appear to have reached the
highest peak of specialization it must be pointed out that within the
limits of a single cell many functions are performed and a machinery
developed which would be formed from a number of different kinds
of cells in the higher Metazoa. A thread cell (Fig. 109) is formed from
an undifferentiated interstitial celF: part of the cytoplasm becomes
^ This is a type of cell which preserves an embryonic character and may
develop into germ cells and musculo-epithelial cells as well as cnidoblasts.
148 THE INVERTEBRATA
glandular and forms a large vacuole, filled with a poisonous fluid and
lined with a chitinous membrane, of complicated structure. The
whole of this secreted body is called the nematocyst. Another part of
the cytoplasm round the nematocyst develops muscular fibrillae, and
by their contraction the "explosion" of the nematocyst is caused, an
action so violent that the whole cell may be cast out of the animal as
a consequence. The external part of the thread cell develops a short
Fig. 109. Nematocysts of Hydra, large penetrating type. A, Undischarged.
B, Discharged. From H. attenuata. After P. Schulze. C, Discharged but re-
tained within its thread cell. After Will. bar. barbs; br.w. (stippled) wound
in chitin {ctn.) of prey possibly caused by mechanical action of smaller barbs,
the continuation {con.) being due to the solvent action of a fluid from within
the nematocyst; cnc. cnidocil; fil. filament; Id. lid; la. lasso; m. muscular
fibrils ; nu. nucleus of thread cell.
sensory process, the cnidocil, which bores through the cuticle of the
musculo-epithelial cell in which it lies and comes into contact with
the water. The stimulation of these cnidocils, for example if the animal
is touched by the appendage of a wandering crustacean, causes a dis-
turbance which is transmitted through the body of the cnidoblast to
the muscle fibres of the nematocyst to cause explosion, so that within
a single cell we have the receptor and effector organs which are
necessary for a very remarkable reflex action independent of the
nervous system. Lastly, the nematocyst may be attached to the base
COELENTERATA
149
of the thread cell by the lasso, an organ which helps to restrain the
force of the explosion. From the high degree of differentiation and
the independence of action these cells might almost be considered
as separate organisms within the coelenterate if their development
were not to be traced from the interstitial cells.
The nervous system of coelenterates is one of their most character-
istic organs, composed of cells of a special type which are only to be
Fig. no. Diagram of Hydra to show the nervous net. n.c. nerve cells.
demonstrated by difficult methods of staining. Over the surface of
the mesogloea on both sides among the muscle tails there is spread a
network of cells (Fig. no) with very small cell bodies and many fine
branches which appear to anastomose with each other and also con-
nect with the sense cells in the ectoderm and endoderm. Synaptic
junctions such as occur elsewhere in the Metazoa have, however,
been recently demonstrated in the Scyphomedusae and so possibly
150 THE INVERTEBRATA
occur in the rest of the coelenterates. A sense cell is shown in
Fig. 108 s.c. It has a rod-like process projecting from the surface and
at its other end it ends in slender branches which join with those of
the nerve cells. Such sense cells respond to touch and probably also
to light and chemical stimuli. If a polyp is touched with a wire the
disturbance is transmitted in all directions by the nerve net and
results in a general contraction of the muscular system, which may
last for long periods. In some cases coelenterate polyps are only
capable of expansion in the absence of light.
This "nerve net " is the most primitive type of nervous system. The
cells which compose it differ from the nerve cells of higher Metazoa
in their simple structure, and above all in the fact that they are ar-
ranged in a diffuse fashion, and not aggregated along particular lines.
This is at any rate true for the most primitive polyps : in the medusae
and the more differentiated polyps the nerve cells tend to con-
centrate in special parts but not in such a fashion as to form any kind
of a central nervous system.
Much of the interest of the coelenterates lies in the conflict between
the two modes of life, an easy sedentary existence and a wandering
or rather freely-drifting life which demands a larger measure of
activity and a greater elaboration of structure and physiological de-
velopment. The two types of individual which correspond to these
modes of life are the Polyp and the Medusa. There are large divisions
of the coelenterates in which only one type is present, while in the
others they may even be united in the same species and the same
colony of that species. A survey of the phylum is very largely con-
cerned with the variations of these types and the combination of them
in the life histories of the different coelenterates.
The polyp (Fig. 1 1 1 A) is an attached cylindrical organism with a
thin body wall consisting of two single layers of ectoderm and endo-
derm separated by a narrow structureless lamella. At the free end an
oral cone occurs and at its apex the mouth opening into the enteron.
The oral cone (in the Hydrozoa) is surrounded by a number of
tentacles, which are usually very extensible and armed with batteries
of nematocysts, by which the living animals, on which the coelenterate
feeds, are caught. Tentacles contain a prolongation of the endoderm
which may form a tubular diverticulum of the enteron or a solid core.
The medusa (Fig. 1 1 1 C) is a free-living organism differing from the
polyp in the great widening of the body, especially along the oral
surface, and the restriction of the enteron by the increase in thickness
of the structureless lamella on the aboral side of the endoderm, so
that while a central ^^^^nc cavity remains, the two endodermal surfaces
have come together peripherally to form a solid two-layered endoderm
lamella except along certain lines, where the canal system is developed
COELENTERATA
151
radiating from the gastric cavity. The oral cone becomes the manu-
brium ; the rim which bears the original tentacles of the polyp is now
separated widely from the mouth by differential growth and drawn
downwards in the formation of the bell. Very often a secondary set
of oral tentacles are developed on the manubrium. The radial sym-
metry of the polyp is more strongly emphasized in the medusa by the
can.c
—ten.
Fig. III. Diagram to illustrate the relation between polyp and medusa.
A, Polyp. B, An imaginary intermediate form. C, Medusa. Ectoderm black,
endoderm cross-hatched, mesogloea stippled, can.c. circular canal ; can.r. radial
canal; end.l. endoderm lamella; ent. enteron; M. mouth; mb. manubrium;
or.c. oral cone ; ten. tentacle ; vm. velum. The velum, present in many medusae,
is absent in Obelia.
radial development of the canal system. The muscular system of the
bell is greatly developed by the substitution of a type of cell in which
the muscular processes form a long striated fibre while the epithelial
part is greatly reduced; such a cell is capable of rapid rhythmical
contraction. The nervous system may be partially concentrated to
form a nerve ring and well-defined sense organs occur in connection
with this. In this phylum, the lowest of the Metazoa, the gametes are
152
THE INVERTEBRATA
of the type which is found throughout that great animal division ; the
maturation divisions make their typical appearance here. Eggs and
spermatozoa respectively are nearly always borne by different in-
dividuals or colonies. After fertilization the egg segments by equal
divisions until firstly, a single layer of cells (ectoderm) arranged to
enclose a central cavity constitutes the blastula. Then, by the migration
of cells into this cavity, it becomes filled up with tissue (endoderm)
Fig, 112. Development of a hydroid polyp. After Merejkowsky. A, Forma-
tion of endoderm in the blastula, by budding from the pole. B, Planula with
solid core of endoderm. C, Appearance of enteron ; endoderm cells beginning
to arrange themselves as a single layer.
while the ectoderm becomes ciliated. Such a larva with a solid core
of endoderm is a planula (Fig. 112). It is capable of wide distribution
by currents and may live for a considerable period before settling
down. A split appears in the endoderm, the first appearance of the
enteron, and the larva sinks to the bottom and attaches itself by
one end. At the other end a mouth and tentacles appear and the
creature becomes a polyp. There are a few exceptions to this in the
phylum in which the egg develops directly into a medusa.
SUBPHYLUM CN ID ARIA
Coelenterata referable to two types, the fixed /)o/_y/) and the free w<?^w5«;
locomotion usually by muscular action; possessing nematocysts.
They are divided into the following classes :
Hydrozoa. Cnidaria, nearly always colonial; typically with free
or sessile medusoid phase, arising as buds from the polyp-colony: no
vertical partitions in the enteron; medusae with a velum and nerve
ring; tentacles of polyp usually solid; ectodermal gonads; and an
external skeleton.
ScYPHOMEDUSAE. Cnidaria in which the polyp stage is incon-
spicuous and may be absent altogether: the polyp, where present,
CNIDARIA
153
gives rise to medusae by transverse fission (strobilation) ; with vertical
partitions (mesenteries) in the enteron of some forms and larval
enteron of others ; velum and nerve ring absent ; endodermal gonads ;
and skeleton absent.
AcTiNOZOA. Solitary or colonial cnidarian polyps without medu-
soid phase; vertical partitions (mesenteries) in the enteron; endo-
dermal gonads; with or without a skeleton.
Fig. 113. The planula of a hydromedusan, Clava squamata. A and B, Swim-
ming about in the sea. C, Coming to rest on a rock. D, Developing tentacles,
oral cone and stolon, or.c. oral cone ; stn. stolon. Magnified. From Allman.
Class HYDROZOA
The most typical life histories of the "hydroids" are those in which,
the phenomenon of '* alternation of generations" is presented. That
is, there is a regular alternation of phases, hydroid colonies giving
rise to free-swimming medusae and the fertilized eggs laid by the
medusae each giving rise to a new colony of polyps. In the first two
orders of the Hydrozoa, the Calyptoblastea and the Gymnoblastea,
alternation of generations is well shown in the typical genera. As will
be shown, there is a progressive suppression of the medusoid "genera-
tion " in other members of these orders. In the other orders there is,
however, complete suppression of the polyp phase in the Tracho-
medusae and Narcomedusae, and in the Siphonophora remarkable
colonies are found which appear to^^have originated by budding from
a medusa.
The following orders are contained in the class :
Calyptoblastea {Leptomedusae). Hydrozoa in which the coenosarc
is covered by a horny perisarc, produced over the nutritive polyps as
hydrothecae and over the reproductive individuals as gonothecae;
154 THE INVERTEBRATA
the medusae flattened, with gonads on the radial canals, and usually
statocysts.
Gymnoblastea (Anthomedusae). Hydrozoa in which the coenosarc
is covered by a horny perisarc which stops short at the base of the
polyps and reproductive individuals; the medusae bud-shaped, the
depth of the bell greater than the width, with gonads on the manu-
brium and eyes, but not statocysts.
Hydrida. Hydrozoa existing as solitary polyps without medusoid
stage; tentacles hollow; without perisarc, the polyps being capable of
locomotion; gastrula forms a resting stage encased in an egg shell.
Trachylina. Hydrozoa in which the medusoid is large and the
hydroid phase minute. The latter either forms medusa buds or being
represented by the planula larva metamorphoses into a medusa.
Statocysts with endodermal concretions : generative organs lying on
the radial canals or on the floor of the gastric cavity.
Hydrocorallinae. Hydrozoa existing as fixed colonies with an ex-
ternal calcareous skeleton into which the usually dimorphic polyps
can be retracted.
Siphonophora. Hydrozoa existing as free-swimming, polymorphic
colonies, without perisarc, derived by budding from an original
medusiform individual.
The Graptolithina (see p. 169) are probably another order of the
Hydrozoa and certainly belong to the class.
Orders CALYPTOBLASTEA, GYMNOBLASTEA,
HYDRIDA
We will take as examples of these orders Obelia, belonging to the
Calyptoblastea and Bougainvillea to the Gymnoblastea, both of which
produce free-swimming medusae, and then describe Tubularia with
its sessile gonophores. The series ends with Hydra (Hydrida).
In a colony of Obelia (Fig. 114) root-like hollow tubes (the hydro-
rhiza) run over the surface of attachment, such as a seaweed, and
from these spring free stems, which branch in a cymose fashion
giving ojff the polyp heads (hydranths) on alternate sides. At the
growing ends of the main branches are produced buds which develop
into hydranths, and towards the base of the branches in the axils of
the hydranths, polyps modified for reproduction, the blastostyleSy
occur. The whole system of tubes which connect up the individual
polyps is the coenosarc, and it must be understood that the enteron
or cavity of the colony is continuous and common to all its members.
The rhythmical contraction of the hydranths causes currents which
distribute the food obtained by some individuals to those parts of the
colony where feeding is not taking place. As in all Calyptoblastea the
CALYPTOBLASTEA
155
coenosarcis completely invested by the cuticular secretion, the perisarc^
composed of chitin and produced to form cups round the hydranths
(hydrothecae) and the blastostyles (gonothecae). The hydranth of Obelia
Fig. 114. Part of a branch of Obelia sp. To the left a portion is shown in
section. After Parker and Haswell. ect. ectoderm; end. endoderm; mth.
mouth; coe. coelenteron; esc. coenosarc; psc. perisarc; hth. hydrotheca;
bis. blastostyle ; med. medusa-bud ; gth. gonotheca.
is an expansion of the coenosarc, ending in a prominent oral cone, sur-
rounded by a single ring of rather numerous tentacles, which have a
solid core of endoderm cells. The blastostyle has a mouth but no
156 THE INVERTEBRATA
tentacles; the body wall proliferates to form distinct individuals, the
medusae. Those nearest the mouth of the gonotheca mature first, and
they are liberated as they mature.
The medusa of Obelia (Fig. 115), the
type of the Leptomedusae, is like a
shallow saucer, the middle of the
concave (subumbrellar) surface being
produced into a short manubrium.
The rim of the medusa bell is '^^l^ /^3^^ ? I^S-^en.
furnished with a large number of
short tentacles. Like all medusae
belonging to the Hydromedusae, it
has four radial canals, running from
the gastric cavity to the circular canal. Fig. 115. Free-swimming me-
On the course ofthe radial canals and, dusa of Ohelia. From Shipley
at the end of a short branch, patches ^^^ MacBride ca«.r. radial canal;
r . 1 1 11 ^ J R- gonad: M. mouth at end
of the subumbrellar ectoderm are ^^ j^anubrium; ot. otocyst; ten.
modified to form the gonads. The tentacles,
germ mother-cells originate in the
ectoderm of the manubrium, pass through the endoderm and along
the radial canals to the gonads and then migrate into the ectoderm
again. Only male or female germ cells are produced by each medusa.
At regular intervals in the circumference are eight sense organs, the
statocysts. They are tiny closed vesicles, lined with ectoderm and
filled with fluid in which minute calcareous grains occur. The epi-
thelial lining not only secretes these but is also sensory: the impact
of the grains on the cells produces a stimulus which is transmitted
through the nerves to the muscles, and if the position of the medusa
should be abnormal the muscles contract in such a way as to right the
bell of the animal.
Another characteristic of the hydrozoan medusa is the velum (which
is practically absent in Obelia), a narrow internal shelf running inside
the border of the subumbrellar cavity. This is largely composed of
ectodermal circular muscles, separated by a horizontal partition of
structureless lamella. At its base is a double nerve ring: the inner
half of this is concerned with the subumbrellar musculature (and, in
the Trachylina only, the outer with the sense organs).
The ripe ova are shed into the water by the rupture of the gonad,
and fertilization takes place here. Segmentation leads to the formation
first, of a hollow blastula, and from this, by the immigration of cells
at one pole, the elongated planula larva (Fig. 112) with a solid core of
endoderm is formed. It is ciliated and swims freely for a time, eventu-
ally settling down by its broader end, while the other end develops
a mouth and tentacles surrounding it. The endoderm delaminates to
GYMNOBLASTEA
157
form the enteron. From the base of this first formed polyp there is
an outgrowth along the surface of attachment which is the beginning
of the hydro rhiza. From this the rest of the colony is developed.
Fig. 116. Bougainvillea fructuosa, x about 12. From AUman. A, The fixed
hydroid form with numerous hydroid polyps and medusae in various stages
of development. B, The free-swimming sexual medusa which has broken
away from A.
In Bougainvillea (Fig. 116) the polyps belong to the gymnoblastic
type to be described for Tubularia. The creeping hydrorhiza gives
off branches, one of which is seen in the figure, and from these
158 THE INVERTEBRATA
numerous individuals are budded. Most of these are polyps (hy-
dranths), distinguished from those of the Calyptoblastea by the fact
that the perisarc stops short at the base of the polyp and does not
form a hydrotheca. The medusoid individuals take their origin directly
from the coenosarc each as a simple bud, within which is developed
a single medusa which eventually divests itself of a thin covering,
breaks from its stalk and swims away. Several may spring from the
same stem, but this may also bear normal polyps. There is here no
blastostyle, or polyp modified for budding off medusae, and this con-
dition, in which polyps and medusae belong to the same grade of
differentiation from the coenosarc, is possibly to be regarded as
primitive, that of Obelia as secondary. In Eudendrium an intermediate
stage occurs. Medusae are budded off from the stalk of a normal
polyp, and as soon as this budding commences the polyp loses its
tentacles, diminishes in length and may be said to become a blasto-
style.
Tiibularia (Fig. 117) occurs as a colony of large polyps with long
stalks springing from a hydrorhiza of insignificant extent. At the base
of the polyp the stalk forms a swelling ; there the perisarc stops. There
is an oral cone surrounded by a ring of tentacles and also a ring of
larger (aboral) tentacles at the broadest part of the polyp. Both kinds
of tentacles are solid, with an axis of vacuolated endoderm cells
placed end to end, which have a skeletal value. In Fig. 118 part of
the phenomenon of digestion is illustrated. A crustacean has been
swallowed and lies in the stomach {s). After preliminary digestion a
fluid mass of half-digested material is formed and, by alternate
contraction of A the stomach and B the spadix (manubrium) of the
gonophore together with the basal swelling of the polyp, the food is
forced into contact with all the absorptive epithelium of the polyp
and gonophore and also pipetted along the cavity of the stalk.
The reproductive individuals originate from hollow branched
structures springing from the polyp itself between the oral and aboral
tentacles. Each polyp has several of these branches, and from each
branch a number of reproductive individuals arise. The branch is
usually termed a blastostyle, although it is only part of an individual
and not a modified polyp as in Obelia. Each of the buds it produces,
however, has the structure of a medusa but remains attached to the
parent polyp as long as it lives. Like the free-swimming medusa of
Bougainvillea it conforms to the anthomedusan type, the depth of the
medusa bell exceeding the width and the gonads being situated on the
manubrium (spadix). This sessile medusa is called a gonophore. As
seen in Fig. 119 A, the radial and circular canals are formed as in
Obelia, and four very short tentacles occur opposite the radial canals
on the margin of the bell ; but the entrance to the subumbrellar cavity
GYMNOBLASTEA
159
is very much constricted compared to Obelia or a free-swimming
anthomedusa. Another modification is that the eggs, which are large
and yolky, when Hberated from the gonad are fertiHzed in the sub-
umbrellar cavity and develop there through the planula stage into
ten.or.
\---prs.
Fig. 117.
Fig. 118.
Fig. 117. Median vertical section through a polyp of T«6w/flna. 6/^f. blasto-
style ; can.c. circular canal ; end.dig. digestive endoderm ; end.vac. vacuolated
endoderm, forming supporting core of tentacles ; gnph. gonophores ; Mmouth ;
prs. end of perisarc; t. testis; ten.ab. aboral and ten.or. oral tentacles. Slightly
altered from Kukenthal.
Fig. 118. Tiibularia with food. A, with stomach s, contracted and the basal
swelling sw. and the spadix of the gonophore sp. expanded. B, The reverse.
St. cavity of stalk. Arrows denote direction of fluid movement. From Beutler.
an advanced larva called the actinula (Fig. 119B) which is really a
polyp of Tubularia with a short stem. At this stage it makes its way
out of the shelter of the gonophore and fixes by its aboral end. As a
rule, only one of these large eggs can be produced at one time and
i6o
THE INVERTEBRATA
a ripe gonophore generally contains two larvae of different ages, one a
planula and the other an actinula, which may be seen protruding from
the aperture of the bell.
In such gonophores the neuromuscular structures of the bell are
hardly developed at all, the mouth never opens and there are no
evident sense organs. In the medusae called Lizzia and Margellium,
common plankton forms whose polyp stages are not known, we see
the normal anthomedusan type. In both of these there are a number
of short tentacles, arranged in groups round the margin of the bell,
and four double tentacles at the end of the manubrium. Lizzia
can.c.
end. la
--ten.on
Fig. 119. Longitudinal sections through gonophores of TM^M/ana. A, Young
male. B, Female with larvae. In B the details of tissues are omitted ; in A
ectoderm is black, endoderm cross-hatched, end. lam. endoderm lamella;
act. actinula larva ; pla. planula larva with rudiments of aboral tentacles ;
mb. manubrium; ov. ovum; stk. stalk of polyp. Other letters as in Fig. 117.
Original.
possesses eight "eyes" (Fig. 120 1) which are little patches of ecto-
derm, in which some of the cells develop pigment while others elongate
and end in rods. The latter are concluded to be the light-perceiving
cells. There is also an outer enlargement of the cuticle which serves
to concentrate light on the organ and may be called a lens. Though
there is no direct evidence that these organs have a relation to light,
they have in a simple form all the structural elements of the eye of
higher animals. Margellium (Fig. 121) has no eyes but apparently
suffers no disability from their absence : probably the light-perceiving
cells are scattered over the general surface of the ectoderm. "Eyes"
are however a general character of the Anthomedusa as "Ears" (as
statocysts may be broadly termed) are of the Leptomedusa.
GYMNOBLASTEA
i6i
Among the hydroids with sessile medusoids or gonophores there
are many forms in which the medusoid structure is lost, and a bud-
like structure is found in which a transverse section shows simply
plg.c. /per.c
-c.end.
Fig. 1 20. I. A, Eye oiLizzia koellikeri seen from the side, magnified, B, The
same seen from in front. C, Isolated cells of the same. From O. and R. Hert-
wig. Is. lens; per.c. percipient cells; pig.c. pigment cells. II. Radial section
through the edge of the umbrella of Carmarina hastata showing sense organ
and velum, a.n. auditory nerve; c.end. continuation of endoderm along
aboral surface; cm. circular muscles of velum; l.n.r. lower nerve ring;
msg. mesogloea ; ntc. nematocysts ; ra.v. radial canal running into circular
canal, both lined by endoderm; tct. sense organ or tentaculocyst ; u.n.r.
upper nerve ring; vm. velum.
Fig. 121. Margellium, example of an anthomedusan. M. mouth; man. manu-
brium; t. testis; ten. or. oral and ten.m. marginal tentacles. Original.
successive layers of ectoderm with generative cells, structureless
lamella and endoderm round an enteron which does not open by a
mouth. In forms like this the migrations of germ cells, mentioned as
f%.
l62 THE INVERTEBRATA
occurring in Obelia, are very noticeable. Thus in Eudendrium
(Fig. 122 D) the germ cells are often to be distinguished making their
way along the coenosarc towards developing gonophores. If this
degeneration of medusae is followed to its conclusion, a stage is
arrived at in which there are no special reproductive buds at all, but
the generative cells occur in the body of the hydroid. This is the con-
dition in Hydra ^ where the multiplication of the interstitial cells at
different positions produces testes or ovaries. In the latter case each
ovary contains a single egg of a size unusual in the Hydrozoa, which
grows by the ingestion of its sister oocytes and the conversion of their
protoplasm into yolk spherules as in Tubularia. This phenomenon
appears to be a consequence of the habitat of the genus. As in so
many other freshwater animals, a free-swimming stage is omitted
from the early history and the period of larval development is passed
in the shelter of the egg shell ; when the gastrula stage has been arrived
at and the yolk is mostly absorbed, development is suspended during
a resting stage of three or four weeks. After this the young Hydra pokes
its oral end out of the shell and, after creeping about for a short time,
frees itself and develops a mouth and tentacles. Other characters which
differentiate /fyd/ra from the majority of hydroids are the solitary habit,
which it shares with some Gymnoblastea, the hollow tentacles and the
complete absence of a stiffperisarc, this enabling the animal to execute
its characteristic looping movements. It is often pointed out that the
presence of a distinct migratory phase, the medusa, would entail a
serious disadvantage on Hydra ; it is suggested that the medusae might
be swept out to sea and lost. Hydra usually lives in ponds and is there-
fore hardly subject to this danger, but at the same time the embryo
in its horny egg shell is admirably fitted for dispersal, for example in
mud on the feet of migratory birds. This modification of reproductive
habits in Hydra is paralleled in the freshwater sponges with their
gemmules, the freshwater polyzoa with their statoblasts and the
cladoceran Crustacea with their ephippial eggs. It must, however, be
mentioned that a remarkable group of freshwater medusae occur
which belong to the Trachylina, and a stage occurs in their life history
which has sometimes been compared with Hydra and named a
separate genus {Microhydra) of hydroid polyps. This is, however, an
interesting case of convergence.
The following genera of Calyptoblastea may be shortly mentioned :
Plumularia (Fig. 122 A) with a creeping hydrorhiza, giving off
plume-like branches, each of which bears a series of hydrothecae on
one side only; hydrothecae small, so that the polyps cannot be com-
pletely retracted within them; beside the nutritive polyps a second
smaller kind (nematophore), without mouth, but with long amoeboid
HYDROZOA
163
B, 5-'"'-^'>'-tX'',r« IhotCmSr^ cells into gono-
164 THE INVERTEBRATA
processes which engulf decaying polyps, epizoic organisms like
diatoms and protozoa and larvae of other epizoic forms.
Sertiilaria (Fig. 122 B) with a creeping hydrorhiza, more or less
branching stems which bear opposite hydrothecae; hydrothecae
large, so that the polyps can completely retract within them.
The following genera of Gymnoblastea may also be mentioned :
Cordylophora, living in fresh or brackish water (Norfolk Broads),
polyps with scattered filiform tentacles.
Pennaria (Fig. 122C) with two kinds of tentacles, oral capitate
and aboral filiform; nematocysts of very large size; medusae de-
generate but become free when gonads are mature.
Hydr actinia^ with spreading plate-like perisarc covered by naked
coenosarc, very often found coating a shell inhabited by a hermit
crab; with spiral dactylozooids and sessile gonophores.
Podocoryne, as Hydr actinia, but with free medusae.
The polyp forms of many medusae, both Antho- and Lepto-
medusae, are unknown.
Order TRACHYLINA
This group consists of forms in which the medusoid develops directly
from the egg and the polyp has either been reduced to a minute
fixed individual or is represented only by the planula larva which
metamorphoses into a medusa. The possession of sense tentacles with
endodermal concretions is an important character. There are two
suborders :
Trachomedusae. Trachylina with sense tentacles in pits or vesicles
and with gonads situated in the radial canals ; with marginal tentacles
on the edge of the umbrella. Examples: Geryonia, Limnocodium,
Carmarina (Fig. 120 II), Limnocnida.
Narcomedusae. Trachylina with sense tentacles not enclosed and
marginal tentacles inserted some distance aborally from the edge of
the umbrella ; with gonads on the oral wall of the stomach. Example :
Cunina.
The inclusion of the following freshwater forms in the order is
provisional :
Limnocnida is a remarkable freshwater form found in the Central
African lakes. Up till the present only male medusae have been found
in Lake Tanganyika and female in Victoria Nyanza. Asexual repro-
duction by budding takes place from the margin of the bell. Other
species occur in Rhodesia and the Indian rivers.
Craspedacuta (Limnocodium) w^s first known from the Victoria Regia
tank in the Royal Botanic Gardens at Kew, but has now been dis-
covered in various North American rivers and has even colonized
HYDROZOA 165
ponds and canals in England. It has a polyp -like stage, Mtcrohydra,
which has a certain likeness to Hydra.
Order HYDROCORALLINAE
The forms included in this group are mostly associated with reef
corals in tropical seas. The main part of the colony consists of a much
branched hydro rhiza with frequent anastomoses. Instead of secreting
a horny perisarc as the Calyptoblastea and the Gymnoblastea do, the
ectoderm lays down an exoskeleton consisting of calcareous grains,
which becomes bulky and solid. It may be either massive or encrust-
ing or branching. From pits in the surface of the colony arise the
AacL
tab.
Fig. 123. Diagrammatic section through Millepora showing a gastrozooid
with a dactylozooid (dact.) on each side of it and an ampulla (amp.) with a
medusa enclosed in it; can.i, the living canals, shown in black, and can.2, the
degenerating canals, shown as lines, constitute the hydrorhiza, and the
skeleton is represented by stippling ; med. a medusa just liberated ; tab. tabulae
in a gastropore. Slightly altered from Hickson.
polyps. These are of two types (Fig. 123). First there are the in-
dividuals of normal structure with a mouth surrounded by tentacles
{gastrozooids) : these nourish the colony. Then there are the dactylo-
zooids which are much longer and more slender. They have no mouth
but they possess scattered capitate tentacles and may form a ring
round a gastrozooid, in which case it is readily observed that their
function is to catch prey and hand it to the central gastrozooid for
digestion. Besides the polyps there are the medusae, which, as in
Bougainvillea, are budded directly off from the coenosarc: they are
lodged in pits of the skeleton called ampullae^ but their liberation has
been observed in Millepora. It is supposed, however, that their free-
living existence is very brief.
l66 THE INVERTEBRATA
Order SIPHONOPHORA
The Siphonophora are colonial animals which exhibit the maximum
development of polymorphism found in the Coelenterata or indeed
in any group of the Animal Kingdom. They are pelagic and each
colony originates from a planula which metamorphoses to form a
single medusiform individual (Fig. 124B nec.^ which later drops off
from the colony), from the exumbrellar side of which springs a
coenosarcal tube budding off all the other members of the colony
(Fig. 124 B gst. etc.). It usually happens that those which are de-
veloped first are needed to buoy up and propel the young colony.
Consequently the first individual is either medusiform or else forms
an apical float or pneumatophore, the epithelium of which secretes gas
(Fig. 124 A/)w., B nee}). There may also be formed from the ectoderm
of the first formed individual an oleoeyst containing a drop of oil. The
succeeding medusiform individuals resemble the bell of an antho-
medusa, with velum, musculature and canal system but lacking the
manubrium, and they are called neetoealyees: while the most primi-
tive siphonophores have only a single one there may be a series of
them. Following these the coenosarc in one type of colony (Fig. 124 A)
grows to a great length and buds off at intervals along its length
similar assemblages of individuals. Such an assemblage is known as
a eormidium, and may consist of (i) a shield-shaped hydrophy Ilium
which covers the rest of the cormidium, (2) a gastrozooid resembling
the manubrium of a medusa, with a mouth, and a tentacle usually
branched, (3) a mouthless individual, the daetylozooid^ with a tentacle
usually of great length and provided with strong longitudinal muscles,
and (4) a gonozooid (or individual bearing gonophores) which may or
may not have a mouth. The gonophores often resemble those found
in some of the Gymnoblastea like Tubularia. Such forms as those
described above are the genera Halistemma, Diphyes and Muggiaea.
In other cases the coenosarc is not a linear stolon but a massive
body from which are budded off innumerable cormidia, in which
gastrozooids, dactylozooids and gonozooids are all crowded together
to form a compact colony. In Physalia (Fig. 125 B), the ** Portuguese
man-of-war ", there is an enormous cap-shaped pneumatophore which
floats above the surface of the water. There are no neetoealyees, but
the colony is borne hither and thither by the wind and countless
numbers are cast up on the lee shores. The dactylozooids of Physalia
hang suspended from the colony and form a drift net ; when they are
touched by a fish the nematocysts discharge and the fish is captured.
The tentacles contract and the prey is drawn up until the gastrozooids
can reach it. The lips of these are spread out over the surface of the
fish until it is enclosed in a sort of bag in which it undergoes the first
SIPHONOPHORA
167
^>can.r.
cor.
—med.
Fig. 124. Development of the siphonophore colony. A, Diagram of the
possible combinations of individuals in a colony. The continuous gastro-
vascular system is shown in black, pn. pneumatophore ; nee. nectocalyx;
hyd. hydrophyllium ; gst. gastrozooid; dac. dactylozooid with its tentacle;
gnz. gonozooid ; cor. cormidium ; esc. coenosarc ; ten. branched tentacle,
sometimes springing from the base of the gastrozooid. B, Early stage of colony
of Muggiaea, showing two generations -of nectocalyces, nec.^, nec.^ can.r. the
radial canals of the first nectocalyx. Other lettering as in A. nec.^ is lost later
and nec.^ becomes the single permanent nectocalyx of the colony ;/)w. is really
an oleocyst and not a pneumatophore. C, Sarsia, an anthomedusan, for
comparison, showing budding of daughter medusae from the end of the
radial canals, mnb. manubrium; med. daughter medusae. A, altered from
Hertwig; B, after Chun; C, after Allman.
i68
THE INVERTEBRATA
Stage of its digestion. Physalia can catch and devour a full-grown
mackerel, and the poison of its nematocysts is so virulent as to en-
dac/-~-
Fig. 125. Examples of Siphonophora. A, Velella. Altered from Haeckel.
Vertical section, showing the cavity of the pneumatophore (stippled) and
produced into branching gas tubes, the tracheae (tra.), and a network of
endodermal tubes (black), which arise from the cavity of the gastrozooid and
gonozooids (black); ?ned. medusa buds. Other letters as in Fig. 124. B, Phy-
salia showing the "drift net" arrangement of the tentacles of the dactylo-
zooids.
danger human life. In Velella (Fig. 125 A) the disc-shaped colony
has a superficial resemblance to a single medusa. The pneumato-
HYDROZOA 169
phore consists of a chitinous disc containing a number of chambers
and raised into a vertical ridge which forms a sail. On the under
surface there is a single large gastrozooid in the centre, a larger
number of gonozooids surrounding it and a fringe of dactylozooids
at the margin. The gonozooids produce buds which actually escape
as free medusae. The coenosarc consists of a mass of tissue which is
traversed by endodermal tubes placing in communication the cavities
of the gastrozooid and the gonozooids, and ectodermal tubes (tracheae)
which are prolongations of the gas cavity of the pneumatophore. This
tropical form is often brought in large numbers to the shores of
Devon and Cornwall by the Gulf Stream.
The medusae and nectocalyces of the Siphonophora are very
similar to the Anthomedusae. Medusae like Sarsia (Fig. 124C) may
bud off other medusae either from the bell or the manubrium, but
the Siphonophora are probably not to be regarded simply as a colony
of medusae connected by coenosarc. A further change has gone on
in which organs have been displaced from their original position. The
manubrium has come to lie outside the primary medusa bell, forming
a gastrozooid (Fig. 124 B, gst.) at the beginning of the main coeno-
sarcal axis. No manubria corresponding with the medusa bells of the
nectocalyces are present. In the cormidia the hydrophyllium which
may be a modified bell, the gastrozooid and the tentacles may be
quite separate from one another while the complete medusoid form
is shown only by the fixed gonophores (Fig. 124 A and B).
In more specialized siphonophores owing to the shortening of the
main axis the displacement of parts is more extreme and the com-
ponent parts of the cormidia no longer recur in the typical groups,
all kinds of organs being crowded together. Lastly, with the great
development of the gas-secreting pneumatophore, the medusa bell
is suppressed.
While the above description gives an impression of the order re-
garded as colonial animals the siphonophores must be primarily
considered as coelenterates exhibiting growth variability to such an
extent that the identification of the component structures as organs
or individuals is difficult and of purely academic interest.
Order GRAPTOLITH INA
Extinct, probably planktonic, animals ; if related to the Hydrozoa, the
polyp generation is dominant, the medusoid generation unfossilized
or possibly represented by the prosicula; the individuals are budded
oflF from one another and remain in contact with the parent ; there is
no definite coenosarc; and the perisarc is produced round the polyps
as hydro thecae.
1 70
THE INVERTEBRATA
Graptolites are represented in the earliest fossiliferous rocks, the
Cambrian, Ordovician and Silurian. Though we know nothing of
their soft parts, the exoskeleton was horny or chitinous and so may be
well preserved. It resembles in general development that of the
colonies of the Calyptoblastea, in that it was produced round the
polyps to form definite hydrothecae. The graptolites, however, differ
from calyptoblast hydroids because new individuals have the appear-
ance of being budded off directly from older ones rather than from
m.
h.2
)ThA
A « C
^'^- -^- Fig. 1.7.
Fig. 126. A. Young colony of Monograptus, showing prosicula (/)), meta-
sicula (m) and developing hydrothecae i and 2, with growth-lines. After
Kraft. X15. B. More mature colony of Mowo^rop/M^. X2-5. C. Early part
of the colony of Climacograptus, showing sicula {s) partly enclosed by the
early hydrothecae. After Wim an. X15.
Fig. 127. A, Didymograptus \-fractus. Ordovician. Early part of the poly-
pary. After Elles. si. sicula ; cr.c. crossing-canal ; i, first hydrotheca ; 2, second
hydrotheca. X5. B, Tetragraptus similis. Lower Ordovician. Young form
with virgula and disc. After Ruedemann. x 4.
a common coenosarc. Each colony originates from a conical body
called a sicula^ the exoskeleton of the first formed individual, consist-
ing of the pro- and meta-sicula (Fig. 126 A). From the side of the
metasicula a bud is formed, which develops into the first hydrotheca,
and from this is produced the second, and so on. In this way a linear
series of polyps is produced which are arranged in a slender lamella
(stipe) y the hydrothecae being in contact and the cavity of the colony
being continuous. This is the simplest arrangement, and is seen in
Monograptus (Fig. 126 A and B) where the hydrothecae are all on one
GRAPTOLITHINA
171
side of the stipe. In Didymograptus (Fig. 127) the second hydrotheca
grows across the sicula to open on the opposite side, and the first and
second hydrothecae go on budding independently, so that we have a
colony with two stipes or branches. By another modification later,
hydrothecae bud off two individuals instead of one, and colonies like
Tetragraptus (Fig. 128) and Bryograptus are formed. In Diplograptus
and Climacograptus (Fig. 126 C) there is a biserial stipe, either formed
of two uniserial stipes growing back to back and thus separated by a
xi.
Fig. 128.
Fig. 129.
Fig. 128. Tetragraptus. Ordovician Rocks, a, central disc.
Fig. 129. Diplograptus foliaceus from the Utica Slate, New York, x f ,
After Ruedemann. For description see text.
median septum, or as a result of alternate budding throughout the
colony.
In the absence of a coenosarc the graptolites were not attached by
a creeping hydrorhiza, such as occurs in Calyptoblastea. There was,
however, a thread coming off from the end of the sicula which ended
in a disc, by which it is supposed that the graptolites were attached
to floating seaweed (Fig. 127 B). It is also possible that some grapto-
lites were independent planktonic organisms with a pneumatocyst or
other kind of float. Such a pneumatocyst appears to be shown in
Diplograptus (Fig. 129) as a square central body from which a number
of stipes radiate. There is also a circle of round bodies which are
possibly gonophores, as they contain siculae. In any case the grapto-
lites were true pelagic organisms and their floating habit gave them
172 THE INVERTEBRATA
a universal distribution in the Palaeozoic oceans. A series of life
zones may be traced in the rocks which were there laid down, each
characterized by a definite assemblage of graptolites, and these may be
traced throughout the world. By a careful consideration of these
graptolite successions the main line of evolution of the group has been
worked out. It is now concluded that actual genetic relationship is
best traced by the characters of the hydrothecae. The earlier forms
have very simple hydrothecae, but the shape becomes gradually more
complex. On the other hand the genera were usually founded on the
number of branches or stipes in the colony, such as Bryograptus with
many stipes in the Cambrian, Tetragraptus with four in the Lower
Ordovician, and Didymograptus with two in Lower and Middle
Ordovician. These genera succeed each other in geological age, and
so we may suppose that they constitute an evolutionary series. In
reality they constitute not one but several series. Thus there is the
same type of hydrotheca (which we will call A) in Bryograptus callavei^
Tetragraptus hicksi and Didymograptus affinis, while another type (B)
is common to B. retroflexus, T. denticulatus and D . fasciculatus . The
genera of graptolites as at present constituted are thus open to criti-
cism ; it would be more correct to classify all the species into hydro-
thecae of type A as one genus, and those into type B as another. In
the genus Monograptus, which is the last and most abundant of the
graptolites, though the form of the colony is simple, the hydrothecae
vary tremendously, and it is obvious that we have here grouped to-
gether the descendants of many different genera undergoing com-
paratively rapid evolutionary changes.
Certain forms, whose relationship is not clear, occur very com-
monly at certain horizons in the Cambrian and Ordovician and less
commonly in later rocks up to the Carboniferous and are grouped
together as *' dendroid" graptolites. It is possible that they are
closely related to the Calyptoblastea. They differ from the "true"
graptolites in showing polymorphism, the thecae being generally
interpreted as having enclosed feeding individuals (corresponding to
the thecae of the true graptolites), gonozooids (or perhaps protective
individuals) and budding individuals.
Class SCYPHOMEDUSAE (SCYPHOZOA)
This class contains the common jellyfishes of temp>erate and colder
seas, some of which are of extraordinary size, like Cyanea arctica^
the diameter of whose disc is a couple of yards.
The simplest type of Scyphomedusae is found in the division
known as the Stauromedusae, two members of which, Haliclystus and
Lucernaria (Fig. 130), are not uncommon on the British coasts.
SCYPHOZOA 173
adhering to the blades of Zoster a or Laminaria. It has a narrow stem
arising from its exumbrellar surface, by which it attaches itself
temporarily to seaweed. The edge of the bell is divided into eight
Fig. 130. Longitudinal sections through A, Lucernaria and B, a strobilizing
scyphistoma of Chrysaora. In A the section passes through an interradius, on
the left on the exact line of the mesentery so as to show the subumbral pit
and on the right to one side so as to show the face of the mesentery. In B only
the right side of the section passes through an interradius. C, Transverse
section through Lucernaria along the line x x in A. D, Ephyra larva of
Aurelia. can.c. circular canal ; g. gonad ; g.f. gastral filament ; e.s. exumbrellar
stalk ; 7nes. mesentery ; ni.L' longitudinal muscle of mesentery ; su.p. subumbral
pit; ten.s. tentacle becoming later a tentaculocyst ; I.R. interradius; P.R, per-
radius; A.R. adradius with first indication of canal. A and C, altered from
Bourne; B, after Heric; D, original.
lobes, on each of which are several short tentacles and the adhesive
organs which are called marginal anchors} There is no velum and
tentaculocysts are absent. The manubrium is well developed and the
^ Absent in Lucernaria.
, THE INVERTEBRATA
I opens into a spacious gastric cavity which is divided by four
|ns, the interradial mesenteries^ into four broad chambers which
to be perradial. The mesenteries are vertical walls projecting
frpnii tke body wall and composed of endoderm with an internal layer
ofimaiogloea. They have a free edge centrally, while on each side a
vdrtlcal series of gastric filaments project into the enteron, and a
parallel series of gonads stand nearer the body wall. The perradial
chambers do not quite extend to the edge of the bell : a circular canal
is'cut off from the rest of the enteron. Also in the interradial position
) and penetrating the whole length of the mesentery is an ectodermal
'invagination, the subumbral pit.
The Stauromedusae only exist as individuals of this structural
type, superficially more like a polyp than a medusa, but usually sup-
posed to be a medusa, and the egg develops into an individual exactly
resembling the parent.
The vast majority of the Scyphomedusae belong to the subdivision
Discomedusae, which includes our type Aurelia aurita (Fig. 131), the
commonest British jellyfish, but one whose distribution is world
wide.
It has a similar external appearance to that of Obelia^ save for the
difference in size, the margin of the bell being surrounded by very
numerous short tentacles. The manubrium is well developed and the
corners of the mouth are drawn out into four long frilled lips along
the inside of which are ciliated grooves leading into the gullet. The
gullet is very short and opens into the endodermal stomach. This is
produced into four interradial pouches in the lining of which the
genital organs develop as pink horseshoe-shaped bodies. Parallel to
the internal border of the gonads there is a line of gastric filaments
which project freely into the lumen of the pouch. The endodermal
cells of which they are composed contain batteries of thread cells
which kill any living prey taken into the stomach. The gastric pouches
of Aurelia occupy the position of the mesenteries of Lucernaria\, and
the subgenital pits occurring underneath the gonads and lined by ecto-
derm correspond to the subumbral pits of the simpler form. The
broad perradial pouches in Lucernaria have disappeared owing to the
great growths of the mesogloea and the restriction of the gastric
cavity to a central position. There is, however, an extensive canal
system running from the gastric cavity to the circular canal which
is all that represents the former extension of the gastric cavity. It
consists of eight branched and eight unbranched canals : four of the
branched canals are interradial and four perradial: the eight alter-
nating unbranched canals are called adradial.
In this elaborate "vascular" system there is a circulation of fluid
produced by the cilia of the lining epithelium working in definite
SCYPHOZOA
175
directions (Fig. 132). The water drawn in by the mouth passes first
into the gastric cavity and then the gastric pouches ; thence by the
adradial canals to the circular canal. It returns thence by the branched
interradial and perradial canals to exhalant grooves on the oral arms.
The whole circulation takes about twenty minutes, and it serves to
maintain a constant supply of food to all parts of the body. Food
Fig. 131. Aurelia aurita. Somewhat reduced. From Shipley and MacBride.
M. mouth; oa. oral arm; tn. tentacles on the edge of the umbrella; p.cn. one
of the branching perradial canals ; there are four of these, and four similar
interradial canals ; the perradial canals correspond to the primary stomach
pouches of the hydratuba, the interradial to the pouches of the medusa;
a.cn. one of the unbranched adradial canals; c.cn. the circular canal;
tct. marginal lappets hiding tentaculocysts ; g.fil. gastral filaments, just outside
these are the genital organs.
undergoes its preparatory digestion in the stomach : the half-digested
fragments are swept by the cilia on the round described above and
may be ingested by any of the endodermal cells of the canal system
and become available for local needs. The gastrovascular system thus
at once fulfils the functions of the digestive and circulatory systems
of higher animals.
176 THE INVERTEBRATA
The neuromuscular system is further developed than in even the
medusoid individuals of the Hydrozoa. The muscles are ectodermal,
and each cell is almost entirely converted into contractile protoplasm
vi^ith a cross-striated pattern forming an elongated fibre; physio-
logically they are capable of rapid rhythmic contraction and not of slow-
tonic contraction like the muscle of a sea anemone (p. 193). The fibres
p,cn.
I. en.
a,cn
p.en.
x.cn.
Fig. 132. Diagram showing the course of ciliary circulation (see arrows) in
the genital pits and other organs of an adult Aurelia. After Widmark.
A, interradius ; B, perradius ; gen. gonad ; gg.cn. gastrogenital canal ; gst.p.
gastric pouch; i.cn. interradial canal; o.o.a. opening on oral arm. Other
letters as in Fig. 131.
are arranged as a circular musculature over the peripheral part of the
subumbrella. The nerve net is also confined to the ectoderm and is
concentrated in the neighbourhood of the tentaculocysts. There is
no true velum, but a pseudovelum consisting of an internal flange
which is not occupied by muscles and a nerve ring as in the Hydrozoa.
The tentaculocysts are the characteristic sense organs of the
SCYPHOZOA 177
Scyphomedusae (but are present also in some Trachylina in the
Hydrozoa). They are minute tentacles which project at the end of the
interradial and perradial canals, which are continued into them. The
edge of the bell projects over them as a hood. In each apical endoderm
cell of the tentacle there is a crystal which according to some authors
is calcium oxalate. On one side of the tentacle is a pigment spot which
may be an ocellus, and near it are two pits lined with sensory epi-
thelium and said to be olfactory. In the neighbourhood of these
tentacles, then, all the senses appear to be localized. The tentaculo-
cyst (Fig. 133) is made up of two parts, a club-shaped projection
heavy at its distal end, and a pad of sensory epithelium immediately
beneath it. If the medusa is tilted from the normal horizontal position
the club of the highest tentaculocyst will press more firmly against its
sensory pad, and the club of the lowest tentaculocyst less firmly.
Fig. 133. Diagram of tentaculocyst of Aurelia: A, in horizontal position;
B, with medusa tilted, the tentaculocyst t being pressed down upon the
sensory epithelium se. ; h, hood.
Whatever tentaculocyst is highest produces greatest stimulation : this
alone controls the rate of beating of the bell, which has been shown to
be 50 % greater than normal when the animal is tilted through 90°.
Further the state of excitation of the highest tentaculocyst does not
allow complete relaxation of the musculature of the section of the
bell nearest to it between successive beats. This means that less water
is driven downwards at each beat from the uppermost half of the bell
than from the lower half, with the result that the bell automatically
rights itself. The Scyphomedusae are excellent subjects for experi-
ment, and if cut into ribbons will still live and their muscles function.
If the tentaculocysts are cut out one by one the rhythmic movements
of the bell continue until the last is removed when they suddenly
cease. After that, drastic stimulation, tactile or chemical, is necessary
to make the muscles contract.
The gonads are situated, as has been already stated, in the floor of
the stomach, and the ripe gametes are liberated into the genital pouch.
178 THE INVERTEBRATA
The eggs are fertilized as soon as they become free by spermatozoa
from another individual which are drawn^into the mouth along with
the food. They pass through the canals to the opening on the oral
arms (Fig. 132, o.o.a.) and undergo the first stage of their development
enclosed in pouches at the side of the oral grooves. Little opaque
patches along the side of the lips are to be seen with a lens, and when
dissected out they prove to be masses of planula larvae. The planula^
is eventually set free, but soon attaches itself to stone or weed and
develops into a small polyp, without perisarc, the hydratuba, which
Fig. 134. Strobilation of Aurelia aurita. From Sars. A, Hydratuba on
stolon which is creeping on a Laminaria. The stolon is forming new buds at
I and 2. B, Later stage or scyphistoma, x 4. The strobilation has begun.
C, Strobilation further advanced, x 6. D, Free-swimming ephyra stage,
showing first appearance of unbranched adradial canals, x 7-5, seen from
below. E, The same seen in profile, X7'5.
eventually grows sixteen long and slender tentacles. Internally this
stage has the same structure as Lucernaria with four interradial
mesenteries, which are invaded by vertical ectodermal pits, and form
perradial pouches between. At the base of the hydratuba a horizontal
stolon grows out, and off this fresh hydratubae may be budded
(Fig. 1 34 A). They may separate from the parent as in Hydra. During
the winter the whole hydratuba is segmented by transverse horizontal
furrows. This process is termed strobilation (Fig. 134 B). In each
of the disc-like segments so produced, marginal growth at once begins,
^ In Aurelia the formation of the planula sometimes takes place by in-
vagination of the blastula.
SCYPHOZOA 179
eight notched lobes being formed, four of which are interradial and
four perradial. In each notch there is a short tentacle and this becomes
a tentaculocyst. Each lobe is provided also with two short lateral
tentacles, but these disappear. A prolongation of the gastric cavity
into each lobe indicates the beginning of the branched perradial and
interradial canals, and at a little later stage the adradial canals also
appear (Fig. 134 D). The gastric filaments are also seen as four pairs
in the interradial mesenteries.
The Scyphistoma is the name given to the segmented body and each
of the segments is an Ephyra larva (Figs. 130 D, 134). They lie upon
each other like a pile of saucers, connected, however, by strands of
tissue in which run the muscles of the interradial mesenteries con-
tinuous throughout the pile of individuals. These muscles contract
violently at intervals until the communicating strands snap and one
by one the ephyrae swim away. The ephyra develops into the adult
by the filling up of the adradial notches in the margins as well as by
the growth of the bell as a whole. The mesogloea increases enorm-
ously in thickness, causing the two layers of the endoderm to come
together as a solid lamella except where the canals occur. The mesen-
teries lose their attachment and cease to exist as partitions with the
collapse of the enteron, but their position is marked by the gastric
filaments. The basal part of the scyphistoma remains and grows new
tentacles, and after a resting period as a hydratuba may strobilate
again.
The life history of the sessile form may thus be summarized. The
hydratuba feeds and buds in the summer, continues to feed and stores
food in the autumn but ceases to bud, strobilates in the winter, grows
new tentacles in the spring and feeds and buds again. In this the
Scyphomedusae show features in common with the life history of
the hydroid colonies and the freshwater Hydra.
The Rhizostomeae are a division of the Scyphomedusae in which
the four lips around the mouth are vastly developed and folded, and
the central mouth itself is narrowed and in a number of forms en-
tirely closed. It is replaced by thousands of small "sucking mouths "
which lie along the course of the closed-in grooves of the lips. These
lips now constitute organs of external digestion. Small copepods and
even fish are enclosed by the lips, digested and the fluid absorbed
through the "sucking mouths" which are too small to admit solid
particles of any size. The young medusa of Rhizostoma still has a
central mouth, but in the adult of that and other forms, e.g. Pilema
here figured (Fig. 135), it is entirely closed. Cassiopeia is a semi-
sedentary form, which lies with its exumbrellar surface upwards on
the mud of mangrove swamps. The bell pulsates gently and brings in a
constant stream of plankton organisms which are seized by the lips.
i8o
THE INVERTEBRATA
The mode of development described above is typical in the Scypho-
medusae. There are, however, certain exceptions. In the genus
Pelagia the medusa develops directly from the egg into an ephyra
larva, and in Cassiopeia the hydratuba only produces a single ephyra
at a time, a condition which is obviously primitive compared with
Aurelia, "polydisc" strobilation being a secondary adaptation for the
more effective spread of the species.
Fig- 135- Diagrammatic longitudinal section through Pilema. Enteron and
its branches shown in black, many "sucking mouths" along the lips, can.r.
radial canal ; sg.p. subgenital pit.
Class ACTINOZOA (ANTHOZOA)
Solitary or colonial coelenterates with polyp individuals only:
coelenteron divided by mesenteries: stomodaeum present: genital
cells derived from endoderm.
They are divided into the two orders Alcyonaria and Zoantharia.
Order ALCYONARIA
Actinozoa with eight mesenteries and eight pinnate tentacles ; stomo-
daeum with a single siphonoglyph (ciliated groove) ; skeleton internal,
consisting of spicules in the mesogloea, occasionally supplemented
by an external skeleton ; longitudinal muscles on the ventral faces of
the mesenteries.
As a type of the order we will describe Alcyonium digitatum, *' Dead
men's fingers", a colonial form which occurs below low-tide mark,
attached to stones, in various sizes and shapes, but usually in broad-
lobed masses. A small portion or lobe of a colony is shown in Fig. 136,
and it is seen that the polyps project in life from the general surface
of the colony. The ectoderm, mesogloea and endoderm of the polyps
ALCYONARIA
I«I
are of course continuous with the same layers in the coenosarc of the
colony, but while the ectoderm is only a thin skin composed of a
single layer of cells spread over the surface of the whole colony, the
mesogloea is expanded to form a bulky mass of jelly which is traversed
by the endodermal tubes of the polyps. These run parallel with each
other without joining for considerable distances, but they are con-
nected by other endodermal tubes which are much more slender, so
that, like a hydroid colony, the alcyonarian colony has a common
coelenteric system.
Fig. 136. Diagram of section through colony of Alcyonium showing ex-
tended polyps with pinnate tentacles and coenosarc. Original. The direction
of water-circulation is shown by arrows. The mesogloea is indicated by dots
and the spicules it contains by small crosses. D, dorsal and V, ventral sides
of polyp ; bd. endodermal bud which will give rise to a new polyp ; ect. ecto-
derm ; end. endoderm ; end.s. and sol. solenia, solid endoderm strands ; mes.d.
the two dorsal mesenteries; mes.' the other mesenteries; std. stomodaeum.
The polyps are delicate and withdraw on the slightest stimulus, the
oral disc with its crown of tentacles being pulled inside the enteron
by the contraction of longitudinal muscles running in the mesenteries
and attached to the oral disc. By a continuation of this contraction
the whole column of the polyp is introverted ("turned outside in",
as with the finger of a glove). This is the condition in which preserved
colonies oi Alcyonium are nearly always found, and tangential sections
l82 THE INVERTEBRATA
through the superficial layers of the colony are rather difficult to
interpret in consequence.
There is no oral cone in the actinozoan polyp, but the mouth is an
elongated slit and is situated in the middle of a circular flattened area,
the oral disc, which is surrounded by the tentacles. It does not open
directly into the enteron but into a tube lined with ectoderm, the
stomodaeum or gullet^ which communicates with that cavity. The
whole of the stomodaeum is ciliated, but at one end of it there is a
groove which is lined with specially strong cilia which draw water in
at the mouth. This is the siphonoglyph, and it is said to occupy a
ventral position, but the student must be warned that there is no
homology between surfaces so termed in the coelenterates and in the
higher Metazoa. ,
Internally the enteron is divided up by eight vertical folds of the
body wall, the mesenteries, which project so far into the cavity of the
enteron that their upper parts join with the stomodaeum. Below
the level of this organ they end in an enlarged free edge, the mesenteric
filament. The foundation of the mesentery is the mesogloea, which is
not much thicker here than in the body wall but is folded in the
muscular region of the mesentery. On both sides it is covered with
endodermal epithelium. While in the hydroid polyp there is little
differentiation into regions, in the actinozoan polyp the endodermal
cells specialized for various functions are arranged in strips of tissue
occupying definite positions on the mesenteries. This may be seen in
the sections of a polyp in Figs. 140 and 141. It must in the first place
be explained that the presence of the siphonoglyph and the elongation
of the stomodaeum are an indication that on the original radial sym-
metry of the polyp a bilateral symmetry has been imposed, and on
each side of the axis of the stomodaeum the mesenteries correspond
exactly in arrangement. Now the muscular endodermal cells are
concentrated on the ventral side of each mesentery and into a narrow
part of it to form a longitudinal retractor muscle. In the section below
the siphonoglyph the ipesenteric filament is seen, and this consists of
different elements in the different mesenteries. One pair of mesen-
teries, which are " dorsal " in position, are distinguished from the rest
in having a filament which is flattened in cross-section, and is covered
by very large ciliated cells (Fig. 140 F). They work in concert with the
cells of the siphonoglyph to produce a current of water which is
drawn in at the mouth and flows right along the ventral side of the
tubes through the system, bearing with it oxygen and food for the
tissues which are contained in the depths of the colony. The cilia of
the dorsal mesenteries are responsible for the return current which
makes its way out of the polyp by the dorsal side of the stomodaeum.
These two mesenteries are much longer than the rest, as may be seen
ALCYONARIA 183
in Fig. 136, mes.d., and their persistence throughout the endodermal
tubes is necessary for the maintenance of the exhalant current. In
contrast with this the remaining six mesenteries have rounded
filaments covered with an epitheUum consisting largely of gland
cells. Also the germ cells arise near the free border (Fig. 140 F).
Small organisms caught by the tentacles and introduced into the
enteron are embraced by these mesenteric filaments and held fast
while the fluid from the glands brings about a disintegration and
partial digestion of the tissues. Solid fragments of food resulting
from this are ingested by individual endodermal cells and the diges-
tion completed. Not only do the dorsal mesenteric filaments differ
from the others in function but they are ectodermal while all the rest
are endodermal.
The mesogloea of Alcyonium is invaded by cells from the ectoderm
which form in their cytoplasm aggregations of calcium carbonate
with a characteristic shape which are called spicules. As the spicules
develop the secretory cells migrate into the deeper parts of the colony.
They are present in such numbers as to give a certain quality of
solidity to the colony, and on its death the spicules it contains remain
behind as a not inconsiderable mass. The part which alcyonarians
consequently play in the formation of coral reefs, though secondary,
is not unimportant. The mesogloea, as has been mentioned above, is
traversed by hollow strands of endoderm (solenia) which communicate
between the polyp tubes and also by solid endodermal strands which
may play some part in the secretion of the jelly of the mesogloea.
From the solenia, where they approach the surface, small buds are
formed which develop into new polyps.
The gonads are developed at the breeding season, from groups of
endodermal cells near the filaments, but they only occur on the six
ventral mesenteries. The eggs are comparatively large and pass very
slowly up the enteron and out of the stomodaeum, being fertilized
outside the polyp and developing into a planula larva. After a free-
swimming period this fixes and becomes a single individual which by
budding gives rise to a colony.
Variation in the Alcyonaria occurs mostly in the method of forma-
tion of the colonies and the skeleton. The simplest form is found in
Cornularia and Clavularia. From the original polyp a creeping stolon
with a single endodermal tube is given oflF, and this gives rise at in-
tervals to polyp buds, which may in turn produce fresh stolons. The
coenosarc of the colony thus forms a network like a hydroid colony.
In Alcyonium, as already described, the elongated polyps are crowded
together in bundles and fused along nearly the whole of their length,
the ectoderm and mesogloea of adjacent polyps being continuous, and
the endodermal tubes in frequent communication. The mesogloea
184 THE INVERTEBRATA
thickens enormously. In the red coral Corallium rubrum (Fig. 137)
there is an upright branched colony with a rigid axis composed ojf
spicules compacted together which is the precious coral of commerce.
This is clothed by the delicate tissue of the coenosarc from which the
short polyps arise and which contains a network of endodermal tubes,
some of which run along the parallel grooves which are sometimes to
be seen on the surface of a piece of precious coral. The mesogloea
contains spicule-forming cells derived from the ectoderm, and these
travel inwards and add their secretion to the central skeleton. This
form occurs at considerable depths in the Mediterranean and the seas
of Japan. Dimorphism, as described below for Pennatula, also occurs
here.
Fig. 137. Section transverse to the axis of Corallium. After Hickson.
A, autozooid; Ax, skeletal axis; S, siphonozooid without tentacles. The
ectoderm is indicated by the outer line, the mesogloea by stippling and the
endodermal network (solenia) by the irregular spaces in the mesogloea.
The gorgonians (suborder Gorgonacea) also have upright branching
colonies. The supporting axis has, however, an origin, different to
the last, being horny and not calcareous and secreted by the ectoderm
on what is really the outer surface of the animal. As secretion is con-
fined to an invagination of the basal epithelium which burrows into
the whole length of the colony, it appears to be an internal skeleton.
The gorgonians are a remarkable feature in shallow tropical seas,
forming groves and thickets which challenge comparison with the
plant forms of the land (Fig. 138).
In Pennatula and its relations (suborder Pennatulacea) a single
axial polyp grows to a relatively enormous length, sometimes as much
ALCYONARIA 185
as three or four metres , and contains a long horny axis which is possibly
endodermal. The secondary polyps are budded off from endodermal
tubes which ramify in the much thickened mesogloea of the body wall
of the primary polyp, and belong to two types of individuals, the
normal autozooids which feed the colony and the stphonozooids, with
reduced mesenteries and enlarged siphonoglyph, whose only function
is to maintain the circulation of water in the canals of the colony. The
autozooids in Pennatula are arranged in rows side by side to form
Fig. 138. Gorgonians (two species on the left) and hydrocorallines (on the
right) growing on a coral reef in Florida. From an underwater photograph
by Professor W. H. Longley.
equal and regular lateral branches on each side of the axis giving the
colony its feather-like form, and the siphonozooids are mainly found
on the back of the axis. A colony has a limited but remarkable power
of movement and can burrow into sand or mud by its basal stalk.
In two genera, Tubipora (the organ-pipe coral) and Heliopora (the
blue coral), which are widely distributed on coral reefs, a continuous
calcareous skeleton is developed resembling that of reef corals. The
polyps of Tubipora are elongated and parallel and connected by stony
platforms which are traversed by the endodermal tubes. But while
i86
THE INVERTEBRATA
in Tuhipora there is an internal skeleton developed as in Cor allium,
by the fusion of spicules in the mesogloea, in Heliopora the skeleton
is secreted by a layer of ectodermal cells and not composed of spicules.
In Heliopora (Fig. 139) there are on the surface of the colony larger
pits (thecae) occupied by the polyps and smaller pits which lodge
tubular processes of the network of solenia : the same skeletal cha-
racters also occur in the fossil Heliolites which closely resembles it
and was a dominant type in Palaeozoic coral reefs. Tubipora too has
a Palaeozoic representative in Syringopora}
Fig. 139. Diagrammatic section through the edge of a colony of Heliopora.
After Kukenthal. The skeleton is shown as deep black, the ectoderm and
endoderm as lines and the mesogloea by stippling, pol. polyp ; sol. network
of solenia parallel to the surface of the colony ; tub. vertical tubules arising
from this network ; th. theca.
Order ZOANTHARIA
Actinozoa with mesenteries varying greatly in number, typically
arranged in pairs, the longitudinal muscles of which face each other,
except in the case of two opposite pairs, the directives , in which the
muscles are on opposite sides; tentacles usually simple, six or some
multiple of six in number; mesenteric filaments trefoil-shaped in
section; stomodaeum with two ciliated grooves; typically a calcareous
exoskeleton, but this may be entirely absent.
The coelenterate animals which are included in this group fall into
two apparently different categories, the sea anemones, which are
usually single individuals and never possess any kind of skeleton, and
the madreporarian corals, which are usually colonial animals and
always have an ectodermal exoskeleton. The polyps, however, may
all be referred to the same type of structure, and the presence or
absence of a skeleton or of the colonial habit are matters of secondary
importance compared with this.
^ The relationship between these recent alcyonarians and the Palaeozoic
corals is denied by some authors.
ZOANTHARIA 187
In its main structural lines the zoantharian polyp resembles the
alcyonarian type. The stomodaeum is elongated in the same plane but
possesses two siphonoglyphs instead of one. There are tentacles which
are hollow, unbranched, and often very numerous. The mesenteries
are like those of Alcyonium, but their arrangement and the structure
of the mesenteric filament is very different. Numbers and grouping
of mesenteries vary greatly within the limits of the Zoantharia itself.
The simplest form, and that most like Alcyonium (Fig. 140 A), is found
in the small burrowing sea anemone, Edwardsia (Fig. 140 C). Here
there are eight mesenteries with bilateral symmetry, as in Alcyonium.
In six of these the longitudinal muscles are on the same side, facing
ventrally, while the remaining pair have the muscles facing outwards
and dorsally, so that the arrangement is different from that in the
Alcyonaria.
In the typical sea anemone, such as Actinia, and in coral polyps,
the mesenteries are arranged in cycles (or generations). There are six
couples of primary mesenteries in the first cycle, and these are the
largest and alone reach as far as the stomodaeum. In four of these
pairs the muscles face each other ; in the other two pairs, the directives,
they face away from each other. The secondary mesenteries, which
are much smaller, are situated in the spaces between two adjacent
pairs (exocoeles), never between two members of a pair (entocoeles).
Finally, there may be tertiary and even quaternary mesenteries, al-
ways in exocoelic spaces of the generation preceding, making third
and fourth cycles. This *' hexactinian" type, in which the mesenteries
are present in multiples of twelve, is derived from that in Edwardsia,
as may be seen in the development of some of the Zoantharia, for
example another small burrowing anemone, Halcampa. In this there
is first of all an Edwardsia stage (Fig. 140 C) with eight mesenteries.
From this the hexactinian type is derived quite simply by the sub-
sequent growth of four additional mesenteries with muscles on their
dorsal faces. These belong to the first cycle and join up with the
stomodaeum, and they arise in such positions as to complete, with
pre-existing mesenteries, four pairs with muscles facing each other.
These four mesenteries in Halcampa never develop a mesenteric
filament, but the complete adult arrangement, as seen for instance in
Actinia mesembryanthemum, the commonest of our British anemones,
is given in Fig. 140 E. In such a form as Peachia, often used in labora-
tories on account of its simplicity, there are slight deviations from the
type. There is no second siphonoglyph (sulculus) and the second cycle
of mesenteries is incomplete, none of them having a mesenteric fila-
ment, while the pairs in two exocoeles are completely absent (Fig.
140 B).
The mesenteric filament of the Zoantharia (Fig. 141 B, C) is tre-
i88
THE INVERTEBRATA
mes.dir.
^ -m. I.
mes!
mes.din
--mes.dir.
mes.fi I.
-ov.
Fig^i40. Diagrammatic transverse section of corals and sea anemones
AAlcyomum. BPeacha. CEd^oardsia. T^, Aulactinia. E, Typical a^fan
m the region of the stomodaeum. F. Alcyonium below the stomodaeum
mes. primary mesentery; mes.dir. directive mesenteries; mes.^ mes^ mes *'
filamen't?^' '^TV ^."^^^^f ^^"^^ mesenteries; mes.fil. ciliated mesenteric
filaments m./. longitudinal muscle; ov. ovaries; sip. siphonoglyph ; sip.'
sulculus. The direction of the top of the page is dorsal ^
ZOANTHARIA
189
foil-shaped in section, and while the functions of digestion and
water-circulation are in the Alcyonaria performed by different
filaments, here they are performed by different parts of the same fila-
ment. Thus, near the stomodaeum, the central part of the filament of
a sea anemone or coral is crowded with digestive gland cells and also
with nematocysts, while the wings are covered with strongly ciliated
epithelium which maintains a current. In the lower part of the me-
sentery the filament is exclusively digestive in function : the cells of
Fig. 141. A, Vertical section through a sea anemone showing primary
(right) and secondary (left) mesenteries (dotted) from the endocoelic side.
ac. acontia ; ap. aperture in mesentery ; g. gonads ; x. ciliated region, and y. di-
gestiv'e region, of mes.f. mesenteric filament ; M. mouth ; or.s. oral sphincter ;
retr. longitudinal retractor muscle ; ten. tentacle. B, Transverse section
through the ciliated region of mesenteric filament at x. C, Similar section
through the digestive region at y. c.ph. phagocytic cells filled with carmine
and fish fragments; gl.c. gland cells; nem. nematocysts; z. zooxanthellae.
After Stephenson.
the wings are phagocytic, as is shown by feeding with carmine. From
the central part of the filament free threads called acontia are produced
in some anemones, which are loaded with nematocysts and may be
shot out of the mouth or of special-pores in the body wall when the
polyp is stimulated.
In the corals the skeleton is secreted by the ectoderm, but only by
that part of it which forms the basal disc, A flat plate of calcium car-
bonate is laid down first of all by the whole of the disc, but almost at
once the epithelium is thrown into radial folds and into a circular fold
190
THE INVERTEBRATA
which encloses them, and in these are formed vertical walls which
rise from the plate ; the circular wall is called the theca and the radial
wall septa (Fig. 142 A). The latter are formed in spaces between the
Fig. 142. Skeleton formation in the Zoantharia. A, Oral view of a young
coral polyp with the beginning of the skeleton seen through the transparent
tissues. B, Vertical section through a later stage. C, Development of a colony
showing budding from the extrathecal zone. D, Division of a polyp, pol.
polyp before division; pol.' polyp after division and subsequent growth:
skeleton of pol. shown in black (as in earlier diagrams) and that of pol.' by
stippling ; th. theca ; sep. first formed part of septum ; ex.th. extrathecal portion
of polyp or colony. E, Lophohelia. Skeleton of colony, soft parts indicated by
dotting, pol. polyp ; pol.' polyp about to divide ; th. theca with septa indicated ;
cch. coenenchyme. F, Astroides. After van Koch. Tangential section of young
form fixed on cork {ck.). ect. ectoderm ; end. endoderm ; cal. granular secretion
of calcium carbonate forming the basal disc ; mes. mesentery ; sep. septum.
mesenteries. The continued secretion by such a form as the English
solitary coral Caryophyllia produces a cup of limestone, of which the
tapering basal portion is solid but which has a shallow apical de-
ZOANTHARIA ^ 191
pression, which is traversed by the radiating vertical septa and con-
tains in the centre a more or less regular vertical rod, the columella.
The depression always tends to become filled up by the secretory
activity of the general surface of the basal disc, but the building up
of the theca and septa keeps pace with this. It is difficult at first to
realize that this is an exoskeleton and that in a massive structure like
a brain coral the actual living tissue is a mere film on the surface of a
great hemispherical mass of calcium carbonate which it has secreted.
It is not surprising to learn that such colonies with a diameter of a
yard or more have a life span of a hundred years or so.
With regard to the actual mechanism of lime secretion the view
most generally held is that illustrated by Fig. 142 F, which shows a
coral larva which has fixed upon a piece of cork. The skeleton as
shown in a section is, when first laid down, a series of spheroidal
masses of calcium carbonate, which thus appear to be a secretion of
the ectoderm cells, issuing from the cells as a solution and immedi-
ately crystallizing out as irregular masses. Another suggestion is that
ammonium carbonate excreted by the coral meets the calcium salts
of the sea water and carbonate of lime is precipitated round the ecto-
derm; and still another, that calcium carbonate is stored up in the
ectoderm cells and when the cells are full they drop out of the
epithelium and are added to the skeleton.
Coral colonies exist in the most diverse shapes and forms (Fig. 143),
from the slender tree-like colonies of many Madrepora to the massive
rounded forms like Pontes. Each colony is formed from a single
planula which settles down and forms a polyp. From this first in-
dividual the hundreds of thousands of polyps in a large colony are
formed by division or gemmation. An example of division is given
in Fig. 142D. In such a case when the polyp has reached a certain
size the oral disc becomes elongated in the direction of the long axis
of the mouth, tentacles and mesenteries increase in number, and
finally a transverse constriction divides first the mouth, then the disc
and lastly the whole polyp. The division of the polyp is followed by
that of the theca. In the Meandrine corals (brain corals) the polyp
elongates enormously and the mouth divides but not the theca, and
so we get the curious thecae running more or less parallel to each
other which recall the convolutions of the human brain. In Lopho-
helia (Fig. 142 E) division is equal, but while one of the polyps re-
sulting from it continues to grow the other marks time; the axis of
growth changes sides at each division and the result is a colony
showing cymose branching.
In Fig. 142 B it is shown that part of the coral polyp overlaps the
theca. It is this extrathecal zone which gives rise to young polyps
when a colony is formed by gemmation (Fig. 142 C). The bud and the
192
THE INVERTEBRATA
parent remain connected by their extrathecal portions, and this con-
stitutes the coenosarc of the colony. The gaps between the thecae of
the colony are filled up by calcareous material secreted by the coeno-
sarc and called coenenchyme ,
P.
Mo.
Fa
Fu.
M.
Fig. 143. Photograph of a pool on a coral reef (Great Barrier Reef), showing
various types of zoantharian corals. Fa. Favia, with circular thecae ; Fu. the
free coral Fungia, a single polyp ; M. Meandrina, the brain coral ; Mo. Monti-
pora, a branched coral; P. Pontes. (Photograph by Dr S. M. Manton.)
The polyps of the Zoantharia attain a higher physiological grade
than those found elsewhere in the coelenterates. The sea anemones,
like Hydra, in the absence of any external skeleton, are capable of
locomotion, especially in the case of burrowing forms. The muscles
of the body are arranged in such a way as to bring about many different
kinds of movements. Thus, while the longitudinal muscles of the
COELENTERATA I93
mesenteries cause a longitudinal retraction of the polyp, the transverse
muscles of the mesenteries in the neighbourhood of the stomodaeum
open the mouth when they contract, and the longitudinal muscles of
the tentacles when these are touched by particles of food contract so
that the tentacle bends towards the mouth and helps to push the food
inside it. The muscular system is for the most part under the control
of the nerve net. Although there is no central nervous system the
amount of contraction produced is proportional to the strength of
the stimulus. If a sea anemone is violently stimulated, e.g. touched by
a glass rod in any part the stimulus is transmitted to every muscle
and the whole animal shrinks to a shapeless lump. The process of
feeding is extremely complex and involves the action of the muscles,
the cilia and the glands. In a sea anemone like Metridium^ which lives
on the minute animals of the plankton, when these approach the oral
disc they are stunned by the nematocysts, snared by the mucus of
the glands of the tentacles, transported by cilia to the tips of the
tentacles, and pushed by the tentacles towards the mouth, which
gapes to receive them. Most remarkable of all, the cilia of the lips,
which normally maintain the outwardly flowing respiratory current,
reverse their beat to sweep the food into the enteron. While there
is this remarkable co-ordination of activities in feeding the nerve net
preserves the individuality in action of the parts so that the severed
tentacle of a sea anemone is able to execute movements just as if it
was still in place on the appropriate stimulation. In another common
anemone, Tealia, there are no cilia on the tentacles and oral disc, and
feeding takes place entirely by the muscular movement of the tentacles .
Sea anemones and corals are often nocturnal, remaining contracted
by day, expanding and feeding at night. In such corals as Lobo-
phy Ilium the tentacles are capable of enormous extension. In the
forms which feed by day like Fungia the tentacles are shorter and the
food is collected more by the action of cilia on the tentacles and oral
disc and less by the seizing of organisms by the arms and withdrawal
to the mouth. A remarkable biological feature is the frequent
presence of commensal algae (compare Hydra viridis) in the tissues
(Fig. 141 B, z.). This is especially the case in reef corals, in which
the most recent investigations show that the algae are of no nutritive
value while the oxygen they liberate in the tissues has no relation to
the needs of the coral. On the other hand the fact that they remove
excreta from the coral tissues is of importance.
SuBPHYLUM CTENOPHORA
Free and solitary Coelenterata ; whose active locomotion takes place
by ciliary action ; which are not reducible either to the polyp or to the
medusoid type ; and are without nematocysts, but possess '* lasso cells ".
194
THE INVERTEBRATA
The Ctenophora, apart from certain aberrant forms, are globular,
pelagic, transparent animals living in the surface waters of the sea.
They are usually classed with the Coelenterata, but they differ from
other members of that phylum in several important respects, notably
in the entire absence of nematocysts.
Two British forms are easily procurable, Pleurohrachia pileus and
Hormiphoraplumosa. Pleurohrachia pileus is about the size of a small
Fig, 144. Hormiphora pliimosa. After Chun. Side view. M. mouth leading
via stomodaeum into infundibulum ; ab.p. aboral pole with sense organ ;
ab.fu. aboral funnel of infundibulum ; pa.can. paragastric canal running to-
wards oral pole; 8, one of the eight meridional comb plates; ca. one of the
eight canals running towards 8; tn.po. a tentacular pouch; tn. a tentacle;
gel. gelatinous material.
hazel nut, while Hormiphora plumosa (Fig. 144) is rather smaller.
They are transparent and ovoid. At one pole is the mouth; the only
other openings into the alimentary canal are two small pores near the
sense organ. At the other pole is the sense organ marked as a small
CTENOPHORA I95
Spot lying in a slight depression. The surface of the body is beset by
eight meridional rows of comb plates formed of strong cilia borne
upon modified ectodermal cells. The general surface of the body is
not ciliated.
On opposite sides of the body are two tentacles set in pouches. The
tentacles have muscular bases and are capable of being protruded
from the pouches or withdrawn again. They are usually about half as
long again as the body when fully extended. The tentacles are armed
with cells of a special type called "lasso cells" or coUoblasts, which
take the place of nematocysts. Each colloblast consists of a sticky
head having at its base a spiral thread wound round a stiff central
filament. The tentacles are used for catching the prey which is
entangled by the sticky heads of the colloblasts.
The mouth leads through a stomodaeum lined with ectoderm into
a space, the infundibulum, lined with endoderm. From the in-
fundibulum four canals radiate outwards ; each of these divides into
two and then runs under the comb plates as the subcostal canals. Two
more canals lead out from the infundibulum and run directly without
branching to the base of the tentacles. There are also two paragastric
canals running alongside the stomodaeum.
At the opposite pole to the mouth, the aboral pole, is the elaborate
sense organ formed of small round calcareous bodies united into a
morula. This morula is supported on four pillars of fused cilia and is
covered by a roof also formed of fused cilia. Ciliated furrows lead
out from the sense organ to the comb plates and are believed to assist
in carrying stimuli to the comb plates from the sense organ.
The comb plates are the locomotor organs. When at rest the tip of
a plate is directed towards the oral pole. In movement a rapid beat
of the plate is directed aborally and the cilia then return slowly to
rest. The ctenophore therefore moves slowly through the water with
the oral end in front. Each plate of the comb beats in succession, the
first plate to beat being the one at the aboral end and the remainder
following in succession. This type of beating, which is common in
ciliary movement, is termed "metachronal" (see p. 17). It gives the
appearance of waves travelling down the comb from the aboral to the
oral pole. Ordinarily all the eight rows of plates beat in unison, but
interference with the aboral sense organ destroys this unison.
The main substance of the ctenophore, which fills the space be-
tween the ectoderm and the endoderm, is a gelatinous material in
which are found strands of muscle. Immediately beneath the ecto-
derm lies a subcuticular layer of muscle and nerve fibres which, in
appearance, closely resembles the arrangement found in the Turbel-
laria. It is important to note that the whole musculature of the
Ctenophora is derived from the mesenchyme. There are no musculo-
epithelial cells.
196 THE INVERTEBRATA
Ctenophores are hermaphrodite ; the male and female gonads occur
close to each other in the subcostal canals. Self-fertilization probably
occurs. It is a remarkable fact that, if the first two segments of the
dividing egg of a ctenophore be separated a half larva will develop
from each segment. In the egg, therefore, the organ forming sub-
stances must be localized. If these half larvae be kept until generative
organs develop, the missing half is then regenerated. In contrast to
this behaviour in the Ctenophora, the separated blastomeres of the
cnidarian egg as far as the sixteen-celled stage will develop each into
a complete animal.
The Ctenophora are divided into two orders: (i) Tentaculata^
possessing tentacles, to which the majority of forms belong ; (ii) Nuda,
without tentacles, to which belongs only the genus Beroe.
Most of the Tentaculata have the ovoid shape, similar to that seen
in Pleurobrachia, but some are flattened in a peculiar manner. Cestus
Veneris, Venus' Girdle, is flattened laterally and the body is drawn
out into a narrow band, two inches wide and nearly a yard long. It is
found in the surface waters of the Mediterranean.
The Platyctenea, a group of Tentaculata to which belong the forms
Coeloplana and Ctenoplana, are flattened dorsoventrally. The flatten-
ing is produced by the expansion outwards of the stomodaeum so that
the whole of the ventral surface corresponds to the stomodaeum of
the normal types. Ctenoplana lives in the surface waters of the sea
and retains traces of the swimming plates, but Coeloplana crawls over
the rocks and seaweed, and resembles a turbellarian. It has lost the
swimming plates and developed pigment, but it still retains the sense
organ and the two tentacles. The gut system is irregularly branched
and the muscular system is highly developed for crawling purposes.
One member of the group, Gastrodes, is a parasite in the body of
Salpa. Its chief interest, however, is in the larva, which is a planula,
found nowhere else among the Ctenophora, and thus provides the
strongest piece of evidence for the close relationship of the Cteno-
phora with the Coelenterata.
CHAPTER VI
THE ACOELOMATA: PLATYHELMINTHES
Under this title are grouped the phyla Platyhelminthes, Nemertea,
Rotifera, Nematoda, Gastrotricha, Acanthocephala and Nemato-
morpha (the three last of which are very small groups). The animals
contained in these are unsegmented forms with mesenchyme (p. 129)
and the space between the gut and the body wall (when it exists)
is a primary body cavity filled with fluid (e.g. Rotifera). The turgor
of the body cavity fluid when present has a determining role in the
preservation of the form of the body (e.g. Nematoda, and Rotifera).
Generally speaking this space with its contained fluid plays the part
of a circulatory system, but in the Nemertea the body cavity is re-
duced to a series of canals which constitute the first vascular system
in the animal kingdom. This primary body cavity has no definite
epithelial boundaries and so can be easily distinguished from a true
coelom. It tends to be invaded by mesenchyme cells; in the Platy-
helminthes these completely fill it, forming a characteristic tissue
(parenchyma), and in the Nematoda the cavity appears to be also
completely occupied by a very few enormous vacuolated cells: the
vacuoles simulate a body cavity.
The excretory organ is of nephridial type (or it may be derived
from this as in Nematoda). It is a canal, closed at the internal end,
intracellular or intercellular, with some hydromotor arrangement
which maintains a flow of fluid to the exterior. In the simplest cases
there is a continuous ciliation of the inner wall of the canal (some
Turbellaria). Usually, however, the ciliation has disappeared over
most of the canal but is strengthened and diflFerentiated in others;
the characteristic units of the system, the flame cells, being now found.
Flame cells may be situated in the course of the canal in some forms
but usually constitute the terminal organ (Fig. 149). This system
though usually spoken of as "excretory" is primarily concerned with
the regulation of fluid content and is often absent in marine forms
(e.g. Turbellaria Acoela, p. 213). A nerve net is usually present and
from this are differentiated an anterior "brain" and some longi-
tudinal nerves. The reproductive system is that in which differences
between and within the groups principally occur: these differences
are to be regarded as adaptations to the varying conditions of life.
198 THE INVERTEBRATA
PHYLUM PLATYHELMINTHES
Free-living, bilaterally symmetrical, triploblastic Metazoa; usually
flattened dorsoventrally ; without anus, coelom or haemocoele; with
a flame-cell system; and with complicated, usually hermaphrodite,
organs of reproduction.
The name Platyhelminthes is given to a division of that hetero-
geneous collection of animals which in Linnaeus' time were called
Vermes. The Vermes included everything that looked like a worm,
but appearances have since been found to be deceptive and the
collection has been broken up into separate phyla, one of which is the
Platyhelminthes or flatworms. Of all the worm-like animals the flat-
worms are undoubtedly the most primitive, for they alone show
relationships to the Coelenterata.
The phylum Platyhelminthes falls naturally into three classes:
(i) Turbellaria, (ii) Trematoda, (iii) Cestoda.
Of these the Turbellaria are with few exceptions free-living, while
the Trematoda and Cestoda are all, without exception, parasites. It
is in the Turbellaria that we see most clearly the typical organization
of a platyhelminth, for in the Trematoda and Cestoda the parasitic
habit has induced a considerable departure from the structure of the
free-living ancestor. In shape the Platyhelminthes are flattened, they
are not segmented and do not possess a coelom. The ectoderm is
ciliated in the Turbellaria, but the ciliation is lost in the two parasitic
groups and there are further modifications. The gut, which is present
only in the Turbellaria and Trematoda, has but one opening which
serves both as mouth and anus, and in this respect reminds us of the
Coelenterata. Between the ectoderm and the endoderm which con-
stitutes the lining of the gut there exist a large number of star-shaped
cells with large intercellular spaces forming a mass o{ parenchymatous
tissue. The nervous system consists essentially of a network as in the
Coelenterata, with the important diflference that there is an aggre-
gation of nerve cells at the anterior end which, in the free-living
forms almost always takes the form of a pair of cerebral ganglia, and
that certain of the strands of the network stretching backwards from
these cerebral ganglia are often more distinct than others and merit
the name of nerve cords (Fig. 145). There is, therefore, the beginning
of a definite central nervous system. There are no ganglia other than
the cerebral, but in the general nervous network nerve cells and
nerve fibres are mixed together.
By operating on the animals in different ways it is possible to show
what functions the different parts of the nervous system have. If the
cerebral ganglion of a Polyclad is removed, the body of the animal
PLATYHELMINTHES
199
remains permanently quiescent after the operation. This state of quies-
cence is not however due to a loss of co-ordination in the motor
system. Stimulation of the anterior end can evoke all the normal
ce.ga.-/--
M.-l--,
?—
Fig. 145. The nervous system of Acoela, to show the nerve strands of the
network. After Steinmann. ce.ga. cerebral ganglion (brain); M. mouth;
(S and ?, male and female openings respectively.
forms of locomotion, and this shows that the nerve net and not the
cerebral ganglion is responsible for the correlation of the different
parts of the musculature. The primitive central nervous system which
200
THE INVERTEBRATA
here takes the form of a cerebral gangHon is best regarded as a
development in connection with the special sense organs, from which
it receives stimuli. The cerebral ganglion functions as a relay system
in which the stimuli received from the special sense organs are re-
inforced, often extended in time, and then passed on to the nerve
net. When this sensory relay has been destroyed by removing the
cerebral ganglia, the nerve net is no longer excited to bring the
muscular system into action, although this may still be done by
artificial stimuli.
Sense organs occur in adults only in the free-living Turbellaria,
where they may take the form of eyes, otocysts, tentacles and ciliated
CB <^ \ <Si) (S) €D ~--ect.
-pt-c.
-nu.
Fig. 146. Eye of Planaria luguhris. From Hesse Doflein. ect. ectoderm;
nu. nucleus of pigment cell; pi.c. cup-shaped pigment cell forming retina;
sp.c. special light-sensory nerve cells with fibrillae {ff.) extending to retina.
Arrow indicates line of vision.
pits in the ectoderm. They may also occur in the free stages in the life
history of the Trematoda and Cestoda. The eyes occur on the dorsal
surface where they are visible as dark spots. The retina is formed of
cup-shaped cells, which are heavily pigmented. The interior of the
cup is filled with special nerve cells, varying in number from two
to thirty, the fibrillae of which touch the retina, and the fibres at the
other end are joined together to form an optic nerve leading to the
brain. There is no lens, but the ectoderm over the eye is not pig-
mented and so permits light to pass through it (Fig. 146). It should
be noted that in this simple eye, as in the extremely complicated organ
found in the vertebrates, the light has to pass through the sensory
cells of the nervous system before it reaches the retina, for they are in
PLATYHELMINTHES
201
front of, not behind, the retina. This type of eye is easily seen and
studied in the common freshwater planarians. In Planaria lugubris,
the eye has only two sight cells, while in Planaria lac tea there are
thirty.
Special sensory cells which act as receptors for the appreciation of
changes in the composition of the surrounding medium (chemo-sensory
receptors) or to changes in the flow of water past the surface of the
body (rheotactic receptors) are situated just below the ectoderm.
Their endings project through the ectoderm and form the actual
receptor organ. The taste receptors are spread uniformly over the
Ventral
Fig. 147. Diagrammatic transverse section through the anterior end of a
planarian at the level of the cerebral ganglion. After v. Gelei. a and b rheo-
tactic sensory cells and their endings; c chemo-serisory cells and e their
endings (where the endings of sensory cells of this type occur the cilia are
absent) ; d taste cells and their endings; / the two cerebral ganglia in section.
surface of the body in the Rhabdocoelida, but tend to be more
numerous near the mouth. The endings of the taste receptors project
among the cilia and are of the same length as these. The rheotactic
receptors are confined to certain areas; their endings project among
the cilia and are slightly longer than these. Special chemo-sensory
receptors with short nerve endings that project only just above the
surface of the ectoderm occur in definite areas or grooves on the head.
Here the cilia and rhabdites are absent. These areas are known as
auricular organs. These sensory organs may also be sunk into pits,
which as they are provided with long cilia for driving the water into
them, are known as ciliated pits (see Fig. 150).
202 THE INVERTEBRATA
The tentacles are projections of the body wall near the anterior end.
They are found in the Turbellaria only, but are not present in all
these. When present they are quite distinct and have very long cilia
which, by their motion, set up currents which pass the water over
special sensory areas and so lead us to suppose that their use is for
water-testing, or searching for food. Occasionally these tentacles may
be sunk into pits.
A statocyst occurs in primitive forms of the Turbellaria. It is
situated above the brain and suggests a connection with the Coe-
lenterata where such sense organs are common, but as we know
nothing of its nervous supply it is difficult to make a proper com-
parison.
An excretory system exists in nearly all Platyhelminthes. In the
Acoela, however, it is absent. The excretory system usually consists
of main canals, running down either side of the body (Fig. 148).
The position of the openings of these main canals to the exterior varies.
The main canals are fed by smaller branches which are ciliated, while
the main canals are not. These smaller branches again branch many
times and finally end in an organ known as aflame cell (Fig. 149). The
large canals are often quite easily visible in living specimens, but the
flame cell is exceedingly small and can only be seen in transparent
forms as in the cercaria larvae of the Trematoda. The flame cell itself
consists of a cell with branched processes extending amongst the
parenchyma cells. Attached to the cell are a number of cilia which
move together in the lumen of the canal with a flickering movement.
It is from this flickering motion that the cell derives its name. It is
generally believed that excretion of substances into the lumen of the
tube is performed by the cells forming the wall of the tube itself.
The flame cells represent concentrations of the originally complete
ciliary lining of the canal and their function is to maintain a hydro-
static pressure which will cause the excreted substances to move
down the lumen of the tube to the exterior (see also p. 197).
Movement in the Platyhelminthes is effected in two ways. The
animal may creep over a surface by the motion of the ectodermal cilia,
the surface being freely lubricated when necessary, as is the case
in land forms, by the discharge of slime from the ectodermal slime
glands. More rapid movement is effected by the general musculature
of the body which causes a series of undulations to pass backwards
along the flat body and urges it forward (Fig. 150). The musculature
of a platyhelminth consists of a covering of muscle lying just below
the ectoderm and composed of two layers, an outer circular and an
inner longitudinal layer, except in the Cestoda and in the pharynx
of the Turbellaria where the outer muscles are the longitudinal and
the inner the circular.
PLATYHELMINTHES
203
~V--l,exc.ca,
— - — ^* QIU.
-. '-= ct.
".exc.po.
Fig. 149.
Fig. 148.
Fig. 148. The excretory canal system in Dendrocoelum lacteum. After Wil-
helmi. exc.po. excretory pores opening to exterior; l.exc.ca. lateral longi-
tudinal main excretory canals.
Fig. 149. Terminal organ of an excretory canal, the flame cell. After
Wilhelmi. ca. excretory canal; ci. bundle of cilia forming the "flame";
nu. nucleus of flame cell ; p. cytoplasm of flame cell.
204 THE INVERTEBRATA
Passing through the parenchyma and running dorsoventrally are
strands of muscle which are attached at either end to the dorsal and
the ventral muscle layers. The muscles themselves consist of fibres
formed of a homogeneous transparent material that shows no trace
of any structure. These fibres are produced by a special cell, the
myoblast, which is often to be seen lying alongside the fibre it has
produced.
The outer covering of a platyhelminth differs according to the
group to which it belongs. In the Turbellaria the outer covering is
formed of ectodermal cells. These are usually large and flat, sometimes
with peculiar branched nuclei as in Mesostomum, or smaller and with
round nuclei as in the majority of forms. Externally the cells are
ciliated, the cilia being arranged in tracts over the surface of the body.
Inside the cells are seen a number of crystalline, rod-shaped bodies,
e.
gen.po.
Fig. 150. Planaria, x about 4. From Shipley and MacBride. e. eye;
ci.pit, ciliated pits at side of head; M. mouth at end of protruded pharynx;
ph. outline of the pharynx sheath into which the pharynx can be with-
drawn ; gen.po. generative pore.
known as rhabdites. Although much has been written about rhabdites
their function remains obscure. They are a secretion, more or less
firm, which dissolves and becomes liquid in contact with water. They
are formed in special cells, lying either between the ectoderm cells or
just beneath them in the parenchyma, and distributed thence ,to the
ectoderm cells. Rhabdites are usually absent from the ectoderm cells
in the neighbourhood of sense organs. It will be noticed that when
Turbellaria are placed for preservation in an irritant fluid such as
acetic acid the body becomes covered with an opaque white layer.
Whether this opaque layer is produced from the rhabdites or from
the slime glands which occur in certain regions of the body is not
certain.
Immediately below the ectoderm lies the basement membrane. This
is a thin transparent structureless layer, which probably assists in
preserving the general shape of the body and serves as an attachment
for the muscles which lie immediately beneath it.
PLATYHELMINTHES
205
The basement membrane is continuous over the body except where
it is penetrated by the openings of gland cells. It is absent beneath
the ectoderm overlying the sensory areas. In certain parts of the
ectoderm, notably in the pharynx of the Tricladida, the nuclei of
the ectoderm cells sink through the basal membrane and its under-
lying muscle layer and come to lie in the parenchyma attached to the
cells by long strands of protoplasm (Fig. 151). In the Trematoda
y/^ii>H n.net.
Fig. 151. Transverse section through the outer layer of pharynx of a triclad.
Altered from Steinmann. ba.mefnb. basal membrane ; circ.m. layer of circular
muscles; ect. ectoderm; gl. glands; long.m. layer of longitudinal muscles;
n.net. nervous network; nu. nuclei of ectoderm cells; ra.m. radial muscles.
and the Cestoda, the ectoderm cells have all sunk into the paren-
chyma, and the body is covered by a thick cuticle secreted by the
ectoderm cells.
The parenchyma (also called the^mesenchyme), which fills the interior
of the body, is of very different structure in different Platyhelminthes.
It is generally formed of cells with long irregular processes and much
intercellular space. Within these cells are small granules and particles,
which stain readily. Their appearance and number vary according to
the state of health of the animal, whether it is starved or fed, and they
2o6 THE INVERTEBRATA
are probably, therefore, products of secretory activity formed after
the assimilation of food and destined eventually to be converted into
rhabdites or the slime which flows from the slime glands. The
parenchyma is no mere padding tissue. It probably serves for the
transport of food materials, and certain cells in it provide for the
repair of lost parts of the body. These free cells of the parenchyma
retain their embryonic condition and do not become vacuolated or
branched. They are smaller than the branched cells of the paren-
chyma and scattered among them in normal circumstances, but when
an injury occurs they migrate to the cut surface, where they collect
in large numbers and proceed to regenerate the tissues lost by injury.
The digestive system of the platyhelminth differs entirely from that
of the higher animals in that it is a sac with one opening only, which
serves both for the entry of the food and the exit of the faeces, and
not a tube with a mouth and anus serving separately for the entry and
exit of food. In the simplest forms, in many of the Rhabdocoela, the
sac is a straight wide tube with no diverticula (Fig. 152), while in
others the gut is branched. In the Tricladida the gut has three main
branches. A muscular structure lined by an inturning of the ecto-
derm surrounding the mouth forms the pharynx. The pharynx itself
may lie in a pit of the ventral body wall, called the pharynx pouchy
from which it can be protruded or withdrawn. The epithelial lining
of the gut cavity consists of large cells without cilia, the cell walls
of which are often difficult to distinguish. A muscular wall to the
gut is present, but is so exiguous as to avoid identification in many
forms, and it appears therefore as if nothing separates the cells of
the gut from the parenchyma. It is possible for food substances to
pass not only from the lumen of the gut into the cells lining it, but
also from the parenchyma. Thus when Turbellaria are starved they
can consume certain organs lying in the parenchyma (ovaries, testes,
etc.) by passing these into the gut cells or into the lumen of the gut
for digestion.
The Turbellaria are carnivorous and will eat small living Crustacea
or worms which are caught by the protrusion of the pharynx. A
sticky secretion, derived from the slime glands and perhaps the rhab-
dites, is immediately poured over the prey, which is thus wrapped up
in sHme. If the object is small enough it is ingested whole into the
gut. Here digestion proceeds. Fat is digested in the lumen of the gut,
but the digestion of other substances takes place in vacuoles in the
cells of the gut wall. Animals which have recently died are also eaten
by Turbellaria, and an effective trap can be made by placing a freshly
killed worm or a Gammarus or two in a jampot and lowering it to
the bottom of the stream or pond. The Turbellaria are able to "scent
out" the food, and all those within a wide area collect in the pot for
PLATYHELMINTHES
207
ves.sem.
-rec.sem.
Fig 152. Dalyelliaviridu, dors^Uie^. From Bresslau. ^^^ b"^"'^^^^;^;'-;^
MTha^^xfri-' -S«-iul'senfinis; <. test.s; ...... ves.cula
seminalis ; vit. vitellarium.
208
THE INVERTEBRATA
the feast. When the animal is too large to be ingested whole, the
pharynx is attached to the prey and worked backwards and forwards
with a pumping motion, while at the same time a disintegrating
digestive fluid is poured out from the walls of the pharynx. Particles
of food are thus pumped up into the gut cavity and digested in the
same way as the living prey. In the Trematoda, also, the cells lining
the gut have a certain limited power of amoeboid movement at their
exposed edges, and intracellular digestion is apparently the usual
method.
The Turbellaria are able to go without food for long periods, but
during starvation they grow smaller and smaller. Stoppenbrink
starved Planaria alpina, keeping them entirely without food, while as
a control he kept a similar collection supplied with food. His results
are given in the table below. The measurements are in millimetres.
Date
Fed
Starved
Largest
Smallest
Largest
Smallest
Lgth
Bdth
2
2-5
2-5
2-5
Lgth
Bdth
Lgth
Bdth
Lgth
Bdth
i6. iii. 03
15. vi. 03
15. ix. 03
15. xii. 03
13
17
17
17
10
12
13
14
I
2
2
13
10
7
3h
2
I
10
6
4
2i
I
t
T
3
This reduction in size is accompanied by the absorption and
digestion of the internal organs, which disappear in a regular order,
the animal using these as food in the manner already described. The
first things to go are the eggs which are ready for laying, then follow
the yolk glands and the remainder of the generative apparatus.
Finally the ovaries and the testes disappear, so that the animal is
reduced to sexual immaturity. Next the parenchyma, the gut and the
muscles of the body wall are reduced and consumed. The nervous
system alone holds out and is not reduced so that starved planarians
differ in shape from the normal forms in having a disproportionately
large head end, the bulk of which is the unreduced cerebral ganglion.
On feeding these starved forms will regenerate all the lost organs and
return to the normal size, like Alice when she ate the right half of the
mushroom.
It is in the generative organs that the Platyhelminthes show the
greatest complexity of organization (Figs. 165, 166). With rare excep-
tions the Platyhelminthes are hermaphrodite. The generative pore is
variably placed but it is usually to be found in the middle line of
the ventral surface not nearer to the anterior or posterior end than
PLATYHELMINTHES
209
v.d. —
ves.scm.-
f/en.at.^
Tpfnini of^enl r„ *''^^^"■"'' " "^ " p\^naH.n. After Steinmann. g.o.
oviduct °/ovar' 1 ^^""-"'-^ ',"? '""'°''' "■•"■°'- '""^<="'^'- organ; orf.
deferens , ws.jem. vesicula seminalis ; vit. vitellarium.
210 THE INVERTEBRATA
one-quarter or one-fifth the length of the body. This pore leads into a
space known as the genital atrium. Into the genital atrium open the
separate ducts leading from the male and female portions of the
generative system, together with other accessory organs. The homo-
logies of the various accessory portions of the generative organs in
the three different groups are difficult to ascertain. Names are often
used which were applied to organs before their homologies were
ascertained, and this increases the confusion.
In studying the generative systems in actual specimens elaborate
reconstruction from sections is often necessary, as the heavy pig-
mentation obscures them when the animal is viewed by transmitted
light. In transparent specimens careful staining will bring to light
most of the parts but it often requires considerable skill and practice
to identify these parts.
The organization of the platyhelminth generative system may be
reduced to a general plan as follows. The testes are round bodies, often
very numerous, having a lining of cells which give rise to the sperma-
tozoa. From the testes lead out ducts, the vasa ejferentia, which,
uniting, form the vas deferens. There are usually two vasa deferentia
collecting the sperm from the testes on either side of the body. The
ends of the vasa deferentia are often distended and act as vesiculae
seminales. The vasa deferentia unite and lead into a pear-shaped bag
with very muscular walls. This is the penis. At rest it opens into the
genital atrium, but during copulation it is extruded through the genital
pore to the exterior and pushed into the genital pore of another in-
dividual. The penis is usually seen very easily, being one of the most
conspicuous parts of the genital apparatus.
The female portion of the generative system consists of the ovary,
which produces the ova, and the vitellarium, which supplies the ova
with yolk and a shell. The shell substance is liquid and hardens later.
This division into ovarium and vitellarium (or "yolk gland" as it is
sometimes called) occurs throughout the Platyhelminthes, but it is
probably an elaboration of the more usual arrangement of forming
the yolk in the ovary, an arrangement which occurs in the primitive
Acoela and in the Polycladida. The ovaries discharge their ova into an
oviduct which is enlarged near the point of this discharge and thus
forms a receptaculum seminis. Here fertilization occurs. The oviduct
next receives the opening of the vitelline ducts. After the opening of
the vitelline ducts the duct continues as the ductus communis, and
leads into the genital atrium. At the junction of the oviducts and
vitelline ducts there is a thickening of the walls of the duct and
certain glands, the "shell" glands, pour a secretion on to the egg
which probably assists in hardening the shell. This thickening is
indistinct in the Turbellaria but is very marked in the Trematoda,
PLATYHELMINTHES 211
and the structure there receives the name of ootype, because it is the
place where the egg is shaped before being passed into the uterus for
storage. In the Trematoda the ductus communis is long and coiled
and serves for the storage of eggs. It is called the " uterus ", but it is
not of course homologous with the "uterus" of the Rhabdocoelida
which will be described shortly, nor with the ''uterus " of the Cestoda
which is again probably a different organ.
The genital atrium receives not only the openings of the male and
female organs but also certain accessory organs. In the Rhabdocoelida,
of which Mesostoma is an example, there open out from the genital
atrium on either side the paired uteri (Fig. 165, i), in which the eggs
are stored before laying. In Dalyellia (Fig. 152) the fertilized eggs
pass into the parenchyma. There is another opening which leads into
a short muscular receptacle, the bursa copulatrix. The bursa copulatrix
receives the penis of another individual during copulation. Sperm
is deposited here but remains only for a short time before being
expelled by muscular contractions and received into the oviduct
where it is collected near the ovary in the true receptaculum seminis.
In the Tricladida the uterus and the bursa copulatrix are replaced
by organs, the homologies of which are doubtful. These are the
unpaired stalked gland organ and the unpaired muscular gland organ.
The stalked gland organ is often called the "uterus" but it has not
been observed to contain eggs. It is regularly present, whereas the
muscular gland organ is often absent. It has recently been shown that
the stalked organ serves as a bursa copulatrix and receives tem-
porarily the penis and the sperm of another individual.
During copulation the ventral surfaces of two animals are applied
together so that the genital openings lie opposite to each other. The
penes are extruded through the genital opening of one copulant into
the genital opening of the other. There is a mutual exchange of sperm.
Since the ova are ripe at the same time as the sperm, and as, in many
forms, there is only one common genital opening to the exterior,
special precautions are necessary to prevent self-fertilization. To
ensure that cross-fertilization shall take place a great elaboration of
the structures surrounding the genital atrium has occurred, resulting
in that complication of the genitalia, which is so characteristic of the
Platyhelminthes .
In freshwater Tricladida copulation occurs fairly freely among
animals kept in glass jars, where they are easily observed. When the
penis is retracted its lumen is closed so that sperm cannot escape
into the genital atrium, whence it might find its way up the oviduct
(Fig. 154). When the penis is thrust out through the genital opening
during copulation it is dilated on extrusion, so that the lumen is
opened. This dilation also causes the penis to fill completely the
212
THE INVERTEBRATA
genital atrium and opening, so that the opening of the oviduct into
the genital atrium is blocked and no sperm can enter or ova escape.
At copulation the penis of one animal is squeezed past the penis of the
other into the genital atrium. It cannot enter the oviduct, since this
is blocked and so it is received into the stalked gland organ, where the
sperm is temporarily deposited. After copulation is finished, the penes
are withdrawn and the sperm is transferred from the stalked gland
organ to the oviduct. The arrangement of the organs round the genital
atrium in the Tricladida varies considerably. In Bdellocephala, for
example, the penis is reduced and, when extruded, does not fill the
genital atrium sufficiently to block the opening of the oviduct. In
vc's^em.
mini.0K
g.o.
Fig- 154- Longitudinal vertical section through region of the genital atrium
in Dendrocoelum lacteum. After Ullyott and Beauchamp. c.o. common
opening to exterior; g.o. opening of genital atrium (gen.at.); fl.p. flagellum
of penis; mus.or. muscular organ; od. oviduct; p. penis; sp. stalked gland
organ (bursa copulatrix) ; ves.sem. expanded portion of vas deferens forming
a vesicula seminalis.
this case a flap of skin has developed which is drawn over the opening
of the oviduct, when the penis is extruded.
After the sperm is transferred to the oviduct, it moves up to the
receptaculum seminis at the top, near to the point of discharge of
the ova. The ova are fertilized in the oviduct and then move down
towards the genital atrium, receiving on the way the products of the
vitellaria. On arrival in the genital atrium a cocoon is shaped and made
ready to be deposited. When laid it is usually attached to weeds,
sometimes by a stalk.
The parasitic Trematoda and Cestoda are unaffected by the sea-
sons and are perpetually producing eggs. But in the Turbellaria the
season of egg-laying varies. In some, for example Dendrocoelum lac-
teum, the generative system is in full working order all the year round,
PLATYHELMINTHES 213
in others, for example Planaria alpina, the eggs are only produced
during the winter months. Mesostomum produces two kinds of eggs
which are called ''summer" and "winter" eggs. The "winter" eggs
have a thick shell and are well supplied with yolk; they remain in the
uterus and escape only with the death of the parent. The "winter"
Qg^ can remain dormant for a long period. The "summer" egg is very
thin-shelled and has very little yolk. The development is very rapid
and the young embryos are seen moving in the uterus of the parent
seventy-two hours after the appearance of the eggs. They escape by
the genital pore and their formation does not involve the death of
the parent. The term "winter" and "summer" egg is not entirely
apposite, for "winter" eggs are often found in midsummer. The
"winter" egg is a method of carrying the species over unfavourable
conditions which may develop in winter or in summer. The ' ' summer "
egg is a means for rapid multiplication when conditions are favour-
able.
Asexual reproduction occurs commonly in the Turbellaria. In
Microstoma lineare the hinder end buds off new individuals which re-
main attached for some time so that chains of three or four individuals
in different stages of development are often seen. Planarians undergo
autotomy, cutting themselves in two by a ragged line which traverses
the middle of the body. Lost parts are easily regenerated in the
Tricladida and the group is a favourite one for experimental work on
regeneration.
Having thus provided the reader with a general account of the
organization of a platyhelminth it will now be possible for us to follow
the systematic arrangement of the phylum, to define the divisions and
to point out features of interest in various forms and life histories.
Class TURBELLARIA
The Turbellaria may be defined as Platyhelminthes which are nearly
all free living and not parasitic, which retain the enteron; which have
a cellular, ciliated outer covering to the body; which usually have
rhabdites; and which do not form proglottides. Suckers are very
rarely present.
The systematic arrangement of the Turbellaria is based primarily
on the structure of the gut. There are four orders : (i) Acoela, (ii) Rhab-
docoelida, (iii) Tricladida, (iv) Polycladida.
Order ACOELA
In these the gut is not hollow but consists of a syncytium formed by
the union of endodermal cells. There is no muscular pharynx.
Primitive features are the nerve net and the fact that the germarium
and vitellarium are not separated. Convoliita roscojfensis is the best
214 THE INVERTEBRATA
known member of this division. It lives between the tidemarks on
sandy shores. Imbedded in the parenchyma are algal cells which
live in a symbiosis (p. 47) with the Turbellarian. The photo-synthetic
products of these algal cells provide a source of nourishment for the
animal. Convoluta henseni, another member of this order, is a rare
platyhelminth that has adopted a planktonic habitat.
Order RHABDOCOELIDA
In these forms (Fig. 152) the gut is straight and the mouth is near the
anterior end. The gut may or may not have lateral pouches. In the
more primitive members of this order, oivjhich. Microstomum lineare is
a common example, found in fresh water, the germarium and the vitel-
larium are not separated. Another well-known member of this group is
Dalyellia viridis, common in freshwater ponds in Britain and remarkable
for the elaborate chitinous structure of the penis. Mesostoma ehrenbergi
and M. quadr angular e, the latter X-shaped in cross-section, both
occur in freshwater ponds. They are large and transparent and form
the best objects for studying the structure of the group. Plagiostomum
lemani is a form with side pouches to the gut.^ It occurs at the bottom
of deep lakes in temperate regions. Otoplana also has side pouches to
the gut but is chiefly remarkable for possessing an otocyst overlying
the brain.
The Rhabdocoelida occur in both fresh and salt water; marine
forms are, however, very small.
Order TRICLADIDA
In this group the gut is divided into three main divisions with numer-
ous lateral diverticula from each division. The mouth has shifted back-
wards to the middle of the body. There are three well-recognized
divisions of this order, separated according to habitat : the Paludicola
or freshwater forms, the Maricola or marine forms, and the Terricola
or land forms. The Paludicola are all fairly large forms in contrast
with the Maricola which are small, no more than 2-4 mm. long. To
the Paludicola belong the three commonest freshwater Turbellaria
in Britain: Dendrocoelum lacteum, a white form, Planaria lugubris^z.
black form, and Polycelis nigra, a rather smaller black form easily
recognized by the ring of eyes round the anterior edge of the body.
Perhaps the best known member of the Maricola is Procerodes lobata
( = Gunda segmentata) in which the side diverticula of the gut are
regularly arranged, with testes and excretory openings between them,
giving the appearance of a segmented animal. The Terricola often
reach a very large size — as long as 50 cm. They are often brightly
^ These forms, with side pouches to the gut, are sometimes placed in a
separate order called Alloiocoela.
TURBELLARIA
215
coloured with stripes down the dorsal surface. Bipalium kewense is
a cosmopolitan tropical form that often turns up in greenhouses. It
is often a foot long and is easily recognized by the axe-shaped head.
Rhynchodemus terrestris, a small form 6-8 mm. long, is a British
representative of this division. It is found in damp situations under
the bark of decaying trees and fallen timber.
Order POLYCLADIDA
These are entirely marine. The gut has many diverticula leading out
from a not very conspicuous main stem. The mouth has shifted to the
ciL'
— ciV
Fig. 155. Miiller's larva of a Polyclad, Cycloporus papillosus Lang. Ventral
view, cil.^ large cilium at anterior end; e. eyes; l.^ L^ l.^ projecting lobes,
the edges of which have cilia longer than those on the general body surface
(there are eight of these lobes, there being one similar to 1} and another pair
similar to /.^ on the dorsal surface); m. mouth; cil} large cilium at posterior
end. (Altered from Kiikenthal.)
posterior end. The germarium and the vitellarium are combined into
one organ but there are separate male and female openings. The ovum
is entolecithal, i.e. it has the yolk inside it as in the Acoela. In all other
Platyhelminthes the ovum is ectolecithal, i.e. it has no yolk inside it
but is surrounded inside the egg shell with yolk cells, which break
down when development begins. The early embryological stages in
the development of the Polycladida resemble, as might be expected,
those of the Acoela, but there are however four macromeres instead
of two as in the Acoela. A further point of difference is that in the
2l6 THE INVERTEBRATA
Polycladida the entry of the ovum by the sperm takes place after the
extrusion of the polar bodies, whereas in other Turbellaria this
follows the entry of the sperm. These facts have inclined modern
authorities to the belief that the Polycladida are more nearly related
to the primitive Acoela than to the Rhabdocoelida and Tricladida.
A further point of interest in this group is that development is not
direct. It leads to the production of a larva, known as "Miiller's
larva" (see Fig. 155), which is characterized by projecting processes
and a band of ciha. As we have seen (p. 145), projecting processes
(arms) and bands of cilia are characteristic of the larvae of many
forms belonging to several phyla ; but their presence is probably an
adaptive feature and it is unwise to base phylogenetic speculations
on them. "Miiller's larva" is a planktonic, and therefore a dis-
tributive stage, in the life history. At metamorphosis, when the
animal adopts the crawling progression of the adult, the larva loses
the projecting arms and the bands of cilia, while at the same time it
loses its rotundity, becoming flattened and elongated.
Some members of this group attain a considerable size, six inches or
more in length. A small sucker is found in some forms behind the
genital pore. Thysanozoon, a member of this order, has the dorsal
surface covered with papillae into which run coeca from the intestine.
In Yimgia there are similar papillae also containing diverticula of the
gut, some of which open to the exterior.
Class TREMATODA
The Trematoda may be defined as Platyhelminthes which are para-
sitic (or, in Temnocephalea, epizoic) ; which retain the enteron ; which
in the adult have outside the ectoderm a thick cuticle; which have
suckers; usually, but not always, a sucker on the ventral surface in
addition to one surrounding the mouth; the ventral sucker is sub-
divided in some forms and may also be stiffened with a ringlike
chitinous skeleton.
The Trematoda are linked to the Turbellaria by the little group of
animals which constitutes the order Temnocephalea containing the
genus Temnocephala and one or two others. These animals have a very
discontinuous distribution and live attached to the surface of fresh-
water animals,chiefly Crustacea. They do not feed on their host but use
it as a resting place from which they catch rotifers, Cyclops, and other
small water animals for food. The possession of five tentacles at the
anterior end makes the group easily recognizable (Fig. 156). The epi-
dermis is retained as a nucleated syncytium which secretes outside it a
thick cuticle. In the region of the tentacles rhabdites occur. The mouth
is anterior, the gut has the same shape as in the Rhabdocoela. There is
a large sucker at the posterior end with the common male and female
PLATYHELMINTHES
217
opening in front of it. The nervous system is of the primitive network
type, but the ovary and vitellarium are separate. Many authors place
the Temnocephalea with the Turbellaria, basing their claims to be
associated with this class rather than the Trematoda on the presence
of some scanty cilia, rhabdites, a basal membrane and the absence
of any chitinous thickening to the sucker and the absence of Laurer's
canal. They are symbionts rather than parasites, which further dis-
tinguishes from the Trematoda, but their thick cuticle and their
syncytial ectoderm are undoubtedly Trematodan in character.
l-v£ sue
CU.
circ.in.
I lovg.m.
( ect.c.
-par.c.
— vesx.
Fig. 156.
Fig. 157-
Fig. 156. Temnocephala minor, x 12. After Haswell. g.o. genital opening;
M. mouth; sue. sucker; ten. tentacles.
Fig. 157. Transverse section through body wall of a trematode. After
Benham. ba.memh. basement membrane; cire.m. circular muscle layer;
CU. cuticle; ect.c. ectoderm cell; long.m. longitudinal muscle layer; par.c.
parenchyma cell; sp. spine; ves.c. vesicular cell (present in many trematodes).
The rest of the Trematoda are all parasitic but they resemble in
general shape the Turbellaria. They have retained the mouth, which
is anteriorly placed, and the gut, which, however, is bifid, a shape not
found in the Turbellaria. As in the Turbellaria, the gut may have
lateral diverticula which branch freely. The Trematoda have, how-
ever, lost the external ciliation of the Turbellaria (Fig. 157). The
2l8 THE INVERTEBRATA
ectoderm is represented by cells sunk into the parenchyma in much
the same way as nuclei of the ectodermal cells in the pharynx of the
Tricladida. But the outer portion of the cell is lost in the Trematoda
and its place is taken by a thick cuticle, which is often armed with
spines. Suckers are always present for attachment to the host and
are of large size. The presence of these suckers and their shape makes
it possible to divide the Trematoda proper into two orders : (i) Hetero-
cotylea, (ii) Malacocotylea.
Order HETEROCOTYLEA
In the Heterocotylea there is a large posterior sucker stiffened with
chitinous supports. It is often subdivided, as in Octohothrium or
Polystomutn (Fig. 158). In the Malacocotylea the sucker is not always
posterior, it often moves forward on the ventral surface so that, as in
Fasciola, it comes to lie one-third of the body-length from the anterior
end. It is never provided with chitinous supports. All the Hetero-
cotylea are ectoparasites with the single exception of Po/y^^owwm which
occurs in the bladder of the common frog, of which from 3 to 10 per
cent, are infected by it. They are confined to one host only. The
Malacocotylea are all internal parasites and pass from one host to
another at certain stages in their life history. In the Heterocotylea the
excretory pores are paired and lie near the anterior end of the body,
whereas in the Malacocotylea the excretory system discharges to the
exterior through a single median pore placed at the posterior end of
the body. In the Heterocotylea there are separate openings for the
male and female portions of the generative system, while in the
Malacocotylea there is but one common opening. In the Heterocotylea
there is a pair of ducts leading from the ootype to the exterior indepen-
dently from the male and female ducts, usually called the vaginae.
The vaginae are inconspicuous as a rule, but in Polystomutn their
openings are very clearly marked by two prominences on either side
of the body about one-fifth of the body-length from the anterior end
(Fig. 158). Corresponding ducts do not occur in the Malacocotylea.
The nervous system of the Heterocotylea is more primitive than that
of the Malacocotylea, but in both groups it is stereotyped and does
not vary as it does in the Turbellaria. In both groups it consists of a
cerebral ganglion with six cords leading posteriorly. In the Hetero-
cotylea there are irregular commissures between the cords, while in
the Malacocotylea the commissures are few in number and regular.
Life history of the Heterocotylea. The usual habitat of this order is
on the gills of fishes where they often live isolated. Self-fertilization
must therefore be practised, but copulation has been observed in
Polystomutn and also in Diplozoon, where it is permanent. The
members of this order probably cause considerable inconvenience
TREMATODA
219
to their hosts, but the numbers infesting one host is seldom very
considerable and they have no economic importance as parasites. The
eggs when laid are normally attached to the body of the host, Poly-
stomum being exceptional in laying the eggs in the bladder whence they
vas def.
Fig. 158. Polystomum integerrimum, ventral view, showing the reproductive
system. After Zeller. g.i. genito-intestinal canal; g.o. common genital open-
ing; ho. booklet; M. mouth; oot. ootype; ov. ovary; p. penis; sue. sucker;
t. testis; ut. uterus with eggs inside it; vag. vagina; vag.po. vaginal pore;
vas def. vas deferens ; vit. vitellarium ; vit.d. vitelline duct.
pass out to the exterior into water. The egg hatches as a larva with eye-
spots and a large ventral posterior sucker. It swims by means of cilia
which are arranged in bands round the body. These larvae make their
way to some particular spot on the host after being free-swimming
for a time. As soon as they attach themselves the ciliary covering is
cast off and the generative organs develop. The larva of Polystomum
220 THE INVERTEBRATA
seeks out a tadpole, dying within twenty-four hours if one is not found.
If a tadpole is reached, the parasite fastens itself on to the gills, where
its ciliary covering is cast and it then creeps into the bladder to wait for
three years before becoming sexually mature. The larvae may, how-
ever, attach themselves to the external gills, where a copious supply
of nourishment induces such rapid growth that the animal becomes
sexually mature in five weeks and produces eggs. But it dies when
the tadpole metamorphoses, and thus it never reaches the bladder.
In Dtplozoon, which lives attached to the gills of the minnow, the
larvae attach themselves to the gills of the host, but they do not
develop generative organs until they meet another larva. If such a
meeting occurs the larvae fuse across the middle. After fusion the
generative organs develop and the animals grow in such a manner
that the vas deferens of one form is permanently connected to the
genital atrium of the other. They thus remain throughout their lives
in permanent copulation.
Another form which displays a variation of the usual type of
history is Gyrodactylus which occurs on the gills of freshwater fish.
In Gyrodactylus the ovary and the vitellarium are not separated, as
is the general rule in the Trematoda, but constitute one organ. A
single egg ripens at a time and, after fertilization, develops into an
embryo in the uterus. Before the first embryo leaves the mother a
second younger one appears inside it so that we thus have a con-
dition of three generations one inside the other, and the conditions
are such that the youngest embryo must develop without fertilization.
This feature of the development of one larva with another without
the agency of fertilization is common in the life histories of the
Malacocotylea but Gyrodactylus is the one member of the Hetero-
cotylea in which it occurs.
Order MALACOCOTYLEA
The life history of Fasciola (Fig. 159) may be taken as the type of life
history commonly found in the group. For details of this life history
the reader is referred to elementary textbooks.
In the Malacocotylea the adult is always, with rare exceptions,
parasitic in some vertebrate host, the sporocyst and redia stages are
always parasitic in a mollusc. Two hosts are always, and three may be
necessary for complete development. Divergence from the type of
life history recorded for Fasciola may come about by (i) a generation,
the redia stage, being omitted, (ii) the sporocyst forming by budding
a second generation of sporocysts within which the cercariae arise,
(iii) the cercaria requiring to encyst in a host and to await this host
being eaten by the final host before reaching sexual maturity as in the
case of Gasterostomum fitnbriatum, where the sporocyst develops in
ves.sm.^^
Fig. 159. Diagram of reproductive and nervous system of Fasciola hepatica,
X about 8. From Leuckart. M. mouth ; ph. pharynx ; n. nerve ring ; In.n. chief
longitudinal nerve; al. beginning of alimentary canal; p. opening of penis;
ves.sm. vesicula seminalis ; iit. uterus ; ov. ovary ; sh.gl. shell gland ; a.t. anterior
testis; pt.t. posterior testis; y.gl. yolk glands; vas de. vas deferens.
222 THE INVERTEBRATA
the liver of Anodoriy the cercaria encysts in the roof of the mouth of
the roach and only reaches sexual maturity when the roach is
swallowed by a perch.
In Distomum macrostomum^ which is parasitic in the gut of thrushes,
there is no free-living stage in the life history. The eggs, passed out
with the faeces of the bird, are eaten by a snail, inside which the sporo-
cyst develops. The sporocyst finds its way into one of the tentacles.
It there develops pigment, being brightly coloured in bands of green
and red, while its presence stops the snail from withdrawing this
tentacle. Presumably this brightly coloured object attracts the bird
which devours the snail and infects itself by setting free the cercariae
from the sporocyst.
-e.po.
Fig. i6o. Schistosoma: the male (cJ) is clasping the female (?) in the gynae-
cophoral groove (gr.). e.po. excretory pore; M. mouth; v.suc. ventral sucker.
After Fritsch.
Schistosoma ( = Bilharzia) is a parasite of man, living as an adult
in the abdominal veins (Fig. i6o). It is long and thin and well
adapted for this habitat. It is one of the rare examples of dioecious
trematodes. The male, however, does not lose touch with the female
once he has found her, but carries her permanently in a fold of the
ventral body wall. The eggs are laid in the blood vessels and, being
provided with a sharp spike, they lacerate the walls of the capillaries
and pass into the bladder. Immediately the urine is diluted the
miracidia hatch, but they wait for dilution before hatching. The
second host is a water snail. The cercariae swim freely in the
PLATYHELMINTHES 223
water, and in districts in China and Egypt where the disease is com-
mon they swarm. Bathing, washing or drinking the infected water
allows the cercaria to enter the final host. The cercariae penetrate the
skin with great rapidity and, entering the blood system, make their
way to the abdominal veins where they become mature. The disease
can be prevented by strict sanitary measures in regard to water, and
it can be cured by the administration of compounds of antimony to
infected patients. That the disease is a very old one in Egypt is shown
by the discovery of Schistosoma eggs in the kidneys of mummies of
the twentieth dynasty (i 250-1000 B.C.).
The hatching of miracidia from the egg of Schistosoma is dependent
on the dilution of the urine by fresh water and this serves to emphasize
the fact that the stages in the life history of all parasites are ultimately
connected with environmental conditions. The egg ofFasciola hepatica
does not hatch unless the pYi of the water in which it is deposited is
below 7-5, the optimum point apparently being about pYL 6-5. If
the eggs are kept in water more alkaline than pYi 7-5 the embryo
remains within the shell and eventually dies.
The identification of a cercaria with an adult is a task which requires
great patience, and many cercaria are known which have not been as
yet connected with an adult. Almost any mollusc, if dissected care-
fully under a hand lens, will provide specimens of rediae and cercariae,
although infected specimens may be more common in some localities
than in others. The tail of a cercaria is often an elaborate structure.
Some have rings and chitinous stiffenings, while the well-known
Bucephalus larva of Gasterostomum is a cercaria with a forked tail
(Fig. 161).
Class CESTODA
The Cestoda may be defined as endoparasitic Platyhelminthes in
which the enteron is absent and the ciliated ectoderm has, in the adult,
been replaced by a thick cuticle. In the parenchyma lime cells occur
(see Fig. 162). Proglottides are usually formed.
The Cestoda as a group have felt the influence of the parasitic habit
more than the Trematoda. They have dispensed altogether with a gut,
there is no mouth, and they absorb their food through the skin. As
they live always in the alimentary canal of vertebrates they are con-
veniently situated for this purpose and the amount of food available
to them probably counterbalances the difficulties attendant on dis-
pensing with the usual method of digesting and assimilatiiig food.
The ectoderm cells have sunk into the parenchyma after secreting a
cuticle as in the Trematoda, but this cuticle is thicker and divided into
layers. Immediately beneath the cuticle are the longitudinal muscles.
The circular muscles are incomplete at the edges. In transverse
224
THE INVERTEBRATA
yl'Ory.
Fig. i6i. Bucephalus larva (cercaria) of Gasterostomum fimbriatum. After
Benham./.^a//, forked tail ; gl.org. glandular organ ; M. mouth ; phar. pharynx.
ectc. c.rn. ^^^^^
u,t. cut, I m.fi.: /^^^ ^^
ca.
Fig. 162. Transverse section through a mature proglottis of Taenia, x about
12. From Shipley and MacBride. cut. cuticle; ect.c. ectoderm cells sunk into
the parenchyma; m.fi. longitudinal muscle fibres cut across; cm. layer of
circular muscles; lime c. lime cell; ov. ovary; t. testis with masses of germ
cells forming spermatozoa; In.exc.ca. longitudinal excretory canal; In.n.
longitudinal nerve cord; iit. uterus; od. oviduct.
CESTODA 225
sections the circular muscles appear to divide the parenchyma into
two regions, an outer cortical zone, where occur the cut ends of the
longitudinal muscle together with calcareous bodies, and an inner or
medullary zone, where the generative system lies (Fig. 162).
The Cestoda may be divided into two orders : (i) Cestoda Monozoa,
(ii) Cestoda Merozoa.
Order CESTODA MONOZOA
These are small forms which live in the gut of fishes, usually Elasmo-
branchs. They resemble a trematode in shape and in the fact that
they do not form proglottides, but they have no gut. They have at
one end a "frilled" organ which serves for attachment, and a small
sucker at the other end. An example of this order is Amphilina. It is
difficult from the structure to say which end is the anterior and which
the posterior, for the nervous system consists of two cords running
down either side of the body with a single similar commissure at
either end. But when the animal moves it has the ''frilled" organ in
front so that is spoken of as the anterior end.
Order CESTODA MEROZOA
These are distinguished from the Cestoda Monozoa by the fact that
they all have the power of budding and so reproducing asexually, re-
sembling in this respect the turbellarian Microstoma lineare. The
adult worm has a scolex which is provided with organs of fixation
such as hooks, suckers or folds (Fig. 163). The scolex is usually
buried in the intestinal mucosa of the host. Behind the scolex
comes the neck, the most slender portion of the body, which may or
may not be sharply marked off^ from the scolex. It is in the neck that
asexual reproduction occurs, fresh segments being continually cut off
and, as they grow larger, pushed by the formation of new segments
away from the scolex. The segment so formed is called a proglottis.
The proglottis is not truly comparable with the new individuals pro-
duced in Microstoma lineare. Through each proglottis run the ex-
cretory canals and the nervous strands which are common to all
(Fig. 162). The proglottis when first cut ofiF from the neck region is
devoid of generative organs, but these develop as it becomes more
mature. When the generative organs are mature, fertilization of the
ova occurs, the ovaries and the testes disappear, and the uterus alone
remains to store the eggs. When the proglottis reaches this stage it is
"ripe " and breaks off to pass out with the faeces (Fig. 164). Despite
its connection with the scolex, each proglottis must be regarded as an
individual for it contains a full set of generative organs both male and
female.
sue:
Fig. 163. Taenia solium. Slightly magnified. From Shipley and MacBride.
A, Entire worm, showing head and proglottides, sue/ sucker on head; g.po.
genital pores; prog, ripe proglottis. B, Head. rost. rostellum; ho. hooks; sue.
suckers; stroh. commencement of strobilization. C, Ripe proglottis broken
off from worm. od. remains of vas deferens and oviduct; ut. branched
uterus crowded with eggs.
CESTODA MEROZOA
227
The Structure of the scolex is of importance for it forms the basis
of the classification of the Cestoda Merozoa. In the tapeworms
occurring in the gut of fishes the scolex may have two or four suckers
and the neck may be sharply separated from the region where bud-
ding occurs. In these tapeworms the scolex is often armoured with
chitinous projections and hooks, and the number of the proglottides
is usually small. The tapeworms occurring in the mammals {Cyclo-
phy Hided) are, with one exception, characterized by a head which
bears four suckers at the sides, and, on a projection at the top, called
the rostellum, is a crown of hooks.
ln,exc.ca, tr.excca
vasde
Fig. 164. Diagram of a ripe proglottis of Taenia solium, x about 10. From
Cholodkowsky. In.exc.ca. longitudinal excretory canal; tr.exc.ca. transverse
excretory canal ; vas de. vas deferens ; vag. vagina ; ov. ovary ; y.gl. yolk gland ;
sh.gl. shell gland ; ut, uterus ; t. testes ; ln,n. longitudinal nerve.
As a general rule the more primitive cestodes are found in the
lower vertebrates, while the advanced types are found in the mam-
mals. The evolutionary stage of the parasite is therefore closely
related to that of its host. A notable exception to this rule is Diboth-
riocephalus latus, the Broad Tapeworm of man, which belongs to
a group of tapeworms occurring more commonly in the guts of
fishes. The scolex of Dibothriocephalus has two suckers on either side
of the head. These suckers are of the nature of flabby folds sharply
distinct from the well-defined cuplike suckers of the Cyclophyllidea.
The generative organs are of the same type as is found generally
throughout the Platyhelminthes. There is a single opening for both
male and female organs. From the ootype there leads out a duct which
228 THE INVERTEBRATA
is called the uterus and is used for the storage of eggs, but it is
doubtful whether it is homologous with the uterus of the
Trematoda.
The life history of a cestode is a complicated combination of sexual
and asexual reproduction. One, two or three hosts may be necessary.
The egg passes to the exterior with the faeces. It contains inside it
an embryo armed with six hooks called an "onchosphere". The egg
case takes diiferent shapes ; in Dibothriocephalus latuSy which is a more
primitive type of cestode, the covering of the embryo is ciliated.
In the Cyclophyllid tapeworms, which constitute the most advanced
group of the Cestoda the ciliary covering is lost. In Dipylidium
caninum, the adult of which occurs in the alimentary canal of the
cat or dog, it is replaced by an albuminous coat with a chitinous
Uning inside, while in most of the other forms only the chitinous
covering persists. The egg hatches as an onchosphere after being
swallowed by the first host. The onchosphere then penetrates the
wall of the alimentary canal using its hooks for this purpose and
lodges somewhere in the peritoneal cavity of the host. Here it
develops suckers and a scolex. In primitive forms such as Dibothrio-
cephalus, the larval cestode rests inside the first host, a Cyclops, at
a stage of its development known as the plerocercoid stage. This
stage is ovate in shape and the generative organs are undeveloped
and there are no signs of proglottides. The Cyclops is then eaten by
a freshwater fish, after which the larva, or plerocercus, bores through
the wall of the alimentary canal and rests in the body cavity where
it grows still further, reaching the metacestode stage. Proglottides
can be distinguished in the metacestode stage but the generative
organs are not fully mature. Growth now ceases but the metacestode
stage is often inconveniently large for the body cavity, causing it to
bulge. Sticklebacks thus infected with the metacestode of Schisto-
cephalus gasterostei are commonly found. The adult in this case
reaches maturity when eaten by a bird. Man acquires Dibothriocephalus
latus, a nearly related form, by eating pike infected with the meta-
cestode. In the Cyclophyliidea the resting stage in the first host is
the "bladder worm" (or cysticercus). The onchosphere on reaching
its resting place becomes hollowed out into a ball filled with fluid.
A depression then forms in the wall of the sphere and becomes an
inverted scolex. In Taenia serrata, the common tapeworm of the dog,
the bladder stage in the rabbit (to which the name Cysticercus pisi-
formis was given before the connection with the adult was discovered)
has but one head inverted into the cyst. In the bladder-worm stage
of Taenia coenurus, which is found in the brain of the sheep and
causes the disease known as "gid" or "staggers", many heads are
formed and invaginated into the cyst so that multiple infection may
CESTODA MEROZOA 229
occur when a sheep is devoured and torn to pieces by dogs or wolves.
In Taenia echinococcus ^ the adult of which lives in the alimentary
canal of the dog and is remarkable for having but three proglottides,
the cysticercus stage is found in domestic animals and also in man in
countries where men live in close association with dogs. The cyst
stage is very large and the bladder may contain a gallon or more of
fluid. Such a cyst, known as a " hydatid ", rapidly proves to be fatal.
It is particularly dangerous and difficult to eradicate because the
walls of the cyst have the power of budding off asexually daughter
cysts. A still further development of asexual budding in the cysti-
cercus stage occurs in Staphylocystis, where the onchosphere imbeds
itself in the liver and then develops a stalk or stolon which buds off
cysts which are detached and fall into the body cavity of the host.
Where the cysticercus is swallowed by the final host the head is
everted from the bladder, the bladder is digested and proglottides
forthwith make their appearance from the neck region of the scolex.
So far as is known the production of proglottides continues for the
duration of the life of the host.
The subdivision of the Cestoda Merozoa depends on the shape of
the scolex. There are five divisions, the last of which contains the
forms commonly found as adults in the alimentary canal of the
Mammalia and is the only group of economic importance.
(i) Tetraphyllidea. The four suckers are usually stalked out-
growths of the scolex. Parasitic in fish, amphibia and reptiles.
Onchosphere enters a copepod and develops into a larva known as a
plerocercoid, in which condition it remains until the copepod is
eaten, when it develops into the adult. Size moderate usually
20-30 cm. long but occasionally as small as i cm. or as large as.
I metre.
(ii) Diphyllidea. There are two suckers only and the scolex has a
long neck armed with spines. There is only one family and one genus,
Echinobothrium, which is found in the spiral intestine of Selachians.
The larva, which is of cysticercoid form, is found in the prawn
Hippolyte.
(iii) Tetrarhynchidea. These have four suckers each provided with
a long spiniferous retractile process. The adult is parasitic in the
alimentary canal of Elasmobranchs and especially Ganoids. The larva
which may be of either the procercoid or cysticercoid type occurs in
marine invertebrates of many kinds, fish and occasionally reptiles.
(iv) Pseudophyllidea. The scolex has two suckers which may
be absent in some forms, there is no clearly marked neck and
hooks are usually absent. Occasionally as in Triaenophorus , a
common parasite of freshwater fish, the external divisions between
the proglottides are indistinct and these are only indicated by the
230 THE INVERTEBRATA
regularly placed openings of the uterine birth pores. The majority
of these are parasitic as adults in freshwater fishes, but Dibothrio-
cephalus latus occurs in man and Bothriotaenia in birds. Archigetes
is parasitic as an adult in body of tubilex, an oligochaete worm living
in fresh water. The larva is a plerocercoid which in some forms,
Caryophyllaceus and Archigetes develops gonads paedogenetically so
that there is no adult with proglottides. These paedogenetic forms
closely resemble the Cestoda Monozoa in appearance.
(v) Cyclophyllidea. The scolex bears four cup-shaped suckers and
has a rostellum with a crown of hooks.
The Cyclophyllidea comprise the majority of the common tape-
worms. Those infesting the gut of mammals all have a scolex closely
resembling that of Taenia with four well-defined suckers and a circlet
of hooks. Those found in the gut of fish have a more elaborate
scolex. The number of proglottides varies considerably, the smallest
number (3) is found in Taenia echinococcus ^ while many forms have
hundreds of proglottides and are several yards in length. The pro-
glottides never drop off before they are mature, as they may do in
the other groups and develop generative organs later, consequently
the separated proglottides always contain fully developed oncho-
spheres. Two interesting forms may be mentioned. Dipylidium cani-
num is a tapeworm infesting the alimentary canal of dogs and cats.
Each proglottis has a double set of generative organs with two
separate generative openings, a feature which gives the animal its
name, but which may occur in other forms. The first host is the flea,
and puppies and kittens are early infected by catching and eating
these insects. The mature proglottis has a double set of male and
female generative organs with an opening on either side. Hymenolepis
nana is one of the smallest tapeworms. The adult has ten to twenty
proglottides and only measures half an inch in length. It occurs in
children in certain places, particularly Lisbon and New York, where
it is said to be increasing. It is remarkable among tapeworms for
being the only one known to go through all its life history in one
host. The embryos bore into the intestinal wall where they pass
through the cysticercus stage and emerge again into the alimentary
canal when adult.
The homologies of the various ducts of the genitalia of the Platy-
helminthes (Figs. 165, i66) present great difficulties. While one or two,
the oviduct and the vas deferens for example, are quite clearly homo-
logous throughout, the homologies of others, particularly the accessory
organs such as uterus, bursa copulatrix, vagina, are very doubtful.
The ** uterus" of the Trematoda is clearly the ductus communis of
the Turbellaria greatly elongated and used for egg storage, while the
vii. — --
Fig. 165.
gen.at.
— vas def.
Fig. 166.
Figs. 165 and 166. Diagram of the arrangement of genital organs and ducts
in the Platyhelminthes. i. Rhabdocoelida. 2. Tricladida. 3. Trematoda,
Heterocotylea. 4. Cestoda, Dibothriata. b.c. bursa copulatrix (stalked
organ); d.c. ductus communis; gen.at. genital atrium; Laur.c. Laurer's
canal; ov. ovary; p.s.o. pear-shaped (muscular) organ; pe. penis; rec.sem.
receptaculum seminis; sh.gl. shell glands; test, testis; ut. uterus; ut.op.
uterine opening to exterior; vag. vagina; vag.op. opening of vagina to ex-
terior; vas def. vas deferens; ves.sem. vesicula seminalis; vtt. vitellarium;
d* 6f $ op. common opening of genital atrium to exterior.
232 THE INVERTEBRATA
vagina of the Cestoda is the same, but the relation of the *' vagina" of
the Heterocotylea or the ** uterus" of the Cestoda remains at present
obscure.
If the vagina of the Cestoda is homologous with the uterus of
the Trematoda, the uterus of the Cestoda, which is a single duct,
may correspond with the vagina of the Trematoda, which is however
a paired structure. The homologies of the ducts in the Trematoda are
further complicated by the presence of Laurer's canal, a duct leading
out of the ductus communis and opening to the exterior in the
Malacocotylea but into the gut in the Heterocotylea. The bursa
copulatrix and the muscular pear-shaped organ, which open into the
genital atrium in the Turbellaria, are accessory reproductive organs
which are probably not represented in the parasitic forms. (See
Figs. 165 and 166.)
CHAPTER VII
THE NEMERTEA ROTIFERA AND
GASTROTRICHA
PHYLUM NEMERTEA
Elongated flattened unsegmented worms with a ciliated ectoderm and
an eversible proboscis lying in a sheath on the dorsal side of the
alimentary canal, with which it is not connected; no perivisceral
body cavity, the spaces between the organs being filled with paren-
chyma; aHmentary canal with mouth and anus; excretory system
with flame cells; a blood vascular system; gonads simple, repeated;
sexes separate; sometimes a larval form {Pilidium).
The Nemertea in their general organization resemble the Platy-
helminthes very strongly. In certain positive features they have
advanced, e.g. in the development of a proboscis independent of the
gut, in the presence of a vascular system, and a second opening, the
anus, into the alimentary canal, but in the simplicity of the gonads
and absence of hermaphroditism the Nemertea are less specialized than
the Platyhelminthes. There can be no doubt, however, that the two
phyla are very closely connected, although the presence of an anus
and a vascular system is an enormous advance.
The proboscis (Figs. 167, 168) is the most characteristic organ of
the nemerteans. It lies in a cavity (rhynchocoel), completely shut off
from the exterior, which has muscular walls (the proboscis sheath),
and is attached to the posterior end of the sheath by a retractor
muscle which is really the solid end of the piroboscis. The proboscis
may be compared with the finger of a glove with a string tied to
the inside of the tip ; when the proboscis is at rest the string, i.e. the
retractor muscle, keeps it turned inside out within the sheath ; when
the muscles of the proboscis sheath contract and press upon the fluid
in the rhynchocoel the proboscis is everted, but never completely,
because the retractor muscle keeps it from going beyond a certain
point. At this point, in the Metanemertini, is a diaphragm cutting
off the apical part of the proboscis cavity, and mounted on this is a
spike or stylet with reserve stylets in pouches at the side (Fig. 168 C).
This part of the cavity probably contains a poisonous fluid which is
ejected through a canal in the diaphragm into wounds caused by the
stylets. The proboscis in this class of nemerteans is thus a formidable
weapon. In other nemerteans, though the stylet is not developed, the
proboscis is prehensile and can be first coiled round its prey and then
nep.
al.c.
d.v.
■gen.op.
St. intjd.
'•?• Pf- P*- ect
mt.
Fig. 1 68.
an.
Fig. 167.
Fig. 167. Diagrammatic dorsal view of nemertine. From Kukenthal. al.c.
alimentary canal; an. anus; brn. cerebral ganglia; c.o. cerebral organ; c.v.
connecting and d.v. dorsal vessel; e. eye; gen. op. genital openings; l.v.
lateral vessel l.n. lateral nerve; M. mouth; nep excretory system and np.o.
one of its pores ; pb. proboscis in the rhynchocoel.
Fig. 168. A, Longitudinal vertical section of a metanemertine to show the
relation of the various cavities. After Benham. brn. cerebral ganglia; int.
intestine ; int.d. coecum ; pb. proboscis ; pb.' solid non-eversible part of the
former, attached to the proboscis sheath and acting as a retractor muscle ;
ps. proboscis sheath ; re. rhynchocoel ; rh. rhynchodaeum ; std. stomodaeum ;
St. stomach. B, Transverse section of a palaeonemertine, passing through a
pouch of the intestine on the right and an ovary on the left. After Coe.
ect. ectoderm; l.v. lateral blood vessel; l.n. lateral nerve; m.c. circular
muscles; m.l. longitudinal muscles; ov. ovary; p.al. pouch of intestine;
par. parenchyma. Other letters as above. C, Proboscis of a metanemertine
to show the diaphragm and the stylets sy. and sy.' After Bresslau.
NEMERTEA 235
retracted to bring it within reach of the mouth. Some forms use the
proboscis to aid in burrowing. The part of the proboscis in front of
the brain is called the rhynchodaeum.
The ectoderm is completely ciliated : there are gland cells amongst
the ciliated epithelium; within this are layers of, first, circular, and
then longitudinal, muscles. There is a nerve net which in the most
primitive nemerteans lies at the base of the ectoderm cells, in others
between the circular and longitudinal muscles, and in the most ad-
vanced forms within both layers of muscle. While the nervous system
is thus extremely primitive there are concentrations of the nerve net
to form lateral nerve cords and a pair of cerebral ganglia above the
mouth, each cerebral ganglion being divided into a dorsal and ventral
lobe and connected by commissures above and below the proboscis
sheath. The dorsal lobe is subdivided into an anterior and posterior
part: the posterior part is in close relation with an ectodermal pit, the
cerebral organ, which is situated in some forms in a lateral slit. As
yet, however, the control of the movements of the organism is not
dependent on the cerebral ganglia. There are occasionally eyes of
simple structure.
Inside the muscle layers the body is filled with parenchyma like
that of the Platyhelminthes (Fig. 168 B), but in it are one, two or
three longitudinal vessels, connected together by transverse vessels
with contractile walls, which constitute the vascular system. The blood
is generally colourless, but has corpuscles which sometimes contain
haemoglobin. The circulation is assisted by the movements of the
body. It can hardly be supposed that the blood system, situated so
deeply in the body, can be respiratory in function.
The alimentary canal is a straight tube, the mouth and anus being
nearly or quite terminal. The excretory system is formed by a pair
of canals situated laterally, each of which communicates with the
exterior by one or several pores and gives off many branches, ending
internally in flame cells like those of the Platyhelminthes. In some
cases the end organs come into contact with the blood vessels. The
generative organs are series of paired sacs alternating with the
pouches of the mid gut and these each develop at the time of maturity
a short duct to the exterior.
Most nemerteans develop directly, but in some a pelagic larva with
a remarkable form of metamorphosis is found. This larva is known as
the Pilidium (Fig. 169). A conical gastrula with a flattened base is
first formed by invagination and it passes into the Pilidium by the
following changes. A band of cilia round the base constitutes the
prototroch and forms the locomotory organ of the larva ; it is drawn
out into two lateral lappets. An apical sense organ is formed by a
thickening of the ectoderm. Two cells migrate into the blastocoele and
236 THE INVERTEBRATA
break up into a tissue called mesenchyme, which is partly converted
into larval musculature and partly remains undifferentiated until
needed as raw material for the adult organs. The gut is connected
with the exterior by an ectodermal oesophagus, ending in a large
mouth on the flattened base between the lappets. Thus a creature
appears which has many resemblances to the trochosphere larva to be
described later.
Inside this larva the young nemertean is produced (Fig. 169 A, B).
Five ectodermal plates (imaginal discs) sink below the surface
Fig. 169. Pilidium larva. A, Side view of late form enclosing young nemer-
tean. After Korschelt and Heider. B, Frontal view of earlier stage showing
the imaginal discs. The anterior unpaired invagination is continued to form
the proboscis. After Burger, al. alimentary canal; ap.o. apical organ;
amn. ectoderm of the amnion; ect. ectoderm of the adult; M. mouth; mesc.
mesenchyme of Pilidium ; ns. nervous system ;/)r. prototroch ; rh. rhynchocoel.
and each forms the floor of a sac. Eventually these sacs join
round the gut and a continuous cavity is formed separating the adult
inside from the larval skin (sometimes known as the amnion) which
is thus its protecting husk while it develops. The imaginal discs
join together and form the secondary or adult ectoderm. The Pilidium
continues to swim about with the little nemertean inside it, even when
the organs of the latter are developed and cilia cover its surface so
that the adult moves freely as if a parasite of the larva. At length it
bursts through the tissues of the amnion and the latter sink like a
discarded mantle.
NEMERTEA AND ROTI FERA 237
The nemerteans are classified as follows :
Palaeonemertini. Proboscis without stylets; cerebral ganglia and
lateral nerves in the ectoderm or between the two layers of muscles,
Carinella.
Metanemertini. Proboscis armed with stylets; lateral nerves
within all the muscle layers. Tetrastemma, Geonemertes^ Malacobdella.
Heteronemertini. Proboscis without stylets; a second layer of
longitudinal muscles outside the circular muscles ; lateral nerve cords
lie between the two. Linens, Cerebratulns.
PHYLUM ROTIFERA
Minute animals, unsegmented and non-coelomate, typically with a
ciliated trochal disc for locomotion and food collection, a complete ali-
mentary canal with anterior mouth and posterior anus, and a muscular
pharynx with jaws of a special type ; excretory system with flame cells
joining the hind gut to form a cloaca ; no blood system or respiratory
organ ; very simple nervous system ; sexes separate, two kinds of eggs,
one developing immediately without fertilization and the other, which
is fertilized, thick-shelled and developing only after a resting period.
This group contains a large number of forms of great interest to
the microscopist which are easily obtained from many kinds of fresh
water. They are, generally speaking, the smallest of all metazoa. They
vary little in structure and present a remarkable similarity to the
trochosphere larva. It must be admitted that the Rotifera are on a
lower stage of organization than the annelids and molluscs which
possess this larva and may even be related to a common ancestor of
these phyla. On the other hand, the Rotifera come near to the Platy-
helminthes, the Gastrotricha and Nematodes.
An elastic external cuticle covers most of the body. Under this is
a syncytial ectoderm ; a continuous layer of muscles forming a body
wall is absent (as in the Arthropoda), but isolated bands of muscle,
chiefly longitudinal, traverse the body (or perivisceral) cavity (Fig.
171).
What is the true nature of the body cavity is a question which has
never been properly answered. It is a wide space between ectoderm
and endoderm, traversed by muscles, and is neither a coelom nor a
haemocoele in the narrower sense, but probably only a derivation of
the segmentation cavity of the gastrula (the blastocoele), as in the
trochosphere larva. But they do possess a body cavity and not a solid
parenchyma, and so differ from the Platyhelminthes. Their excretory
system is, however, very similar to that of the latter phylum, and in
the union of the excretory duct with the gut the rotifers resemble
certain specialized trematodes.
238
THE INVERTEBRATA
B
Fig. 170. Hydatina senta. A, Female, ventral view. Original. B, Male,
side view. After Wesenberg Lund. al. rudiment of alimentary canal;
la.an. lateral antenna; bl. bladder; brn. brain; cng. cingulum; cl.a. cloacal
aperture ; d.an. dorsal antenna ;f.c. flame CG\\\g.gl. gastric gland ; ect. syncytial
ectoderm of trochal disc; M. mouth; mc. circular and ml. longitudinal
muscle cells ; ms. muscular mastax and trophi ; np. nephridium, intracellular
duct represented by double dotted line; ov. ovary; oe. oesophagus;/), penis
retracted; ped.gl. pedal gland; pa. papillae with large cilia; st. stomach;
t. testis; tro. trochus; vit. vitellarium.
ROTIFERA
239
Like the Nematoda they consist of a small number of cells and all
the tissues, except the cells of the velum, may lose their cell boundaries
and become syncytial. Not only is there a superficial resemblance
to heterotrichous ciliates in the Protozoa but the tendency to the
acellular condition carries this a step further.
Hydatina senta may be taken as a type of the group (Fig. 170).
jC.m.c-
ecdu.
ect.<
Fig. 171. A, Side view, diagrammatic, from Shipley and MacBride, and B,
tansverse section of a female rotifer. An. anus (cloacal aperture); c.m.c.
circular muscle cell; cu. cuticle; D. dorsal; e. eye; ecdu. excretory duct
(nephridium) ; ect. ectoderm; int. intestine; l.m. longitudinal muscle; m.
muscle; od. oviduct; tc. trochus; V. ventral. Other letters as in Fig. 170.
The female is pear-shaped, the posterior end being the stalk. The
anterior end is flattened and form^ the trochal disc. This is, in many
rotifers, bordered by a double ciliated ring, the velum, the outer part
of which (the cingulum) is the original velum and is composed of
strong cilia. The inner is called the trochus. Between the two rings,
which are thus preoral and postoral respectively, is a ciliated groove
in which is situated the mouth. The velum in life gives the impression
240 THE INVERTEBRATA
of revolving wheels, the reason for the scientific name of the group.
In Hydatina the cingulum forms a complete ring and the trochus is
reduced to a double transverse row of cilia; in the groove between
them is situated a number of papillae on which are stiff cilia. (In
Copeus and other creeping forms there are no trochus and cingulum
but cilia cover the trochal region and part of the ventral surface. This
is said to be a primitive arrangement.) The posterior end is called the
foot and it terminates in a pincer-shaped appendage, on which open
glands with a sticky secretion. By means of this apparatus the rotifer
can anchor itself in the intervals of its free-swimming life. The dorsal
surface of the rotifer is marked out by the position of the cloaca!
aperture just in front of the foot ; on this surface immediately behind
the velum is a sense organ, the dorsal antenna^ and below it the brain.
There are also two lateral antennae ; all three are prominences bearing
stiff sense hairs. Elsewhere the body is covered by a thin, smooth,
transparent cuticle secreted by the ectoderm.
The food, which consists of micro-organisms of various kinds, is
swept by means of the ciliary currents of the disc into the mouth and
then through the oesophagus into the muscular pharynx or mastax
which is provided with chitinous jaws, the trophic which are in con-
stant movement and, in Hydatina^ masticate the food as it passes
through. This first part of the alimentary canal is ectodermal and
constitutes the stomodaeum. Then follows the endodermal stomach,
lined with ciliated epithelium, in which digestion takes place. ^ Two
gastric glands open into it anteriorly. A narrow intestine leads into the
cloaca, into which the excretory system also opens. The latter consists of
two lateral ducts , coiled at intervals , consisting of perforated cells placed
end to end into which flame cells (vibratile tags) open frequently but
irregularly. Anteriorly the ducts communicate by a transverse vessel
just behind the disc and posteriorly they open into a pulsating vesicle
which expels its contents into the cloaca. It has been calculated that
in some species this bladder expels a bulk of fluid equal to that of the
animal about every ten minutes.
The single ovary is a bulky organ : it is divided into a small gerinar-
ium (the ovary proper) and a much larger vitellarium or yolk gland
which occupies much of the space between the stomach and the body
wall. The ovary is continued into a duct which opens into the cloaca.
The female is still the only individual known in many kinds of
rotifers. It was not until 1848 that a male rotifer of any kind was
described. In only a few species is the male equal in size and organi-
zation to the female. In all the rest there is a more or less pronounced
sexual dimorphism. In Hydatina (Fig. 170 B) the male has no
^ Digestion is usually extracellular, but in Ascopus and other rotifers it is
intracellular.
ROTIFERA 241
alimentary canal, but the ciliated disc, musculature and excretory-
system are well developed. Usually the male is not only smaller but
its ciliated disc and the alimentary canal are very much reduced and
the excretory system may be absent. The chief organ is the large
testis^ usually filled with ripe spermatozoa, which opens by a median
dorsal penis in many cases. Where the penis is absent the tapering
hinder end may be inserted in the cloaca of the female. Finally, it may
be mentioned that in one large family, the Philodinidae, which in-
cludes the genus Rotifer^ no male has ever been found.
Two kinds of reproduction occur in the rotifers as in the cladoceran
Crustacea, but in this case there are two kinds of females, one of which
always reproduces parthenogenetically, the eggs developing to form
females (female producers), while the other may reproduce bisexually.
In this second type (male producers) there are eggs, often smaller than
the female eggs, which develop quickly by parthenogenesis into males.
At various seasons after the appearance of these male eggs there are
produced by the same individual also other eggs, distinguished by a
thicker shell, and these have been fertilized by the spermatozoa of the
just hatched males injected through the skin. These *' resting" eggs
are fertilized "male eggs" and they only develop after a dormant
period into females.
The reproduction of a rotifer runs through a cycle in which at first
only parthenogenesis occurs but which is terminated by sexual re-
production. In rotifers which are typical members of freshwater
plankton, the cycles run to a time-table. There are "dicyclical"
rotifers like Asplanchna, which have two sexual periods, one in spring
and the other in autumn, while other forms like Pedalion are " mono--
cyclical" and have only a sexual period in the autumn passing the
winter as resting eggs. In rotifers like Hydatina, which inhabit
puddles and ponds, the sexual periods are very frequent and begin
soon after the resting eggs have hatched. The resting egg is a stage in
which the species can survive when the puddle dries up. Sexual re-
production can be brought on in cultures by alteration of the external
conditions.
Besides the environmental types which have been mentioned above
as free-swimming and inhabiting larger and smaller bodies of water,
the following rotifers may also be mentioned :
Stephanoceros and Floscularia (Fig. 172 bis) are sedentary forms
which secrete a protecting gelatinous tube into which they can with-
draw rapidly. Melicerta is another sedentary form which produces
a tube formed out of mud particles or its own faeces.
Callidina and other genera are terrestrial forms which can remain
for a great part of the year in a dried-up condition but come to life
immediately when moistened by rain. Such forms are found, for
242
THE INVERTEBRATA
l.v.m.
Fig. 172 bis.
Fig. 172.
Fig 172. Ventral view of Chaetonotus. From Kukenthal. an. anus; brn.
brain- Z.m. longitudinal muscle; l.v.m. lateroventral muscle; M. mouth;
mg. mid gut; nep. nephridium; od. oviduct; ov. ovary; ped. foot; ped.gL.
pedal gland ; ph. pharynx.
Fig 1726/5. A, Floscularia cornuta. Female within its gelatinous tube.
From Hudson. i3, F. campanulata Male, ves.sem. vesicula seminalis ;/>. penis.
GASTROTRICHA 243
instance, in roof gutters and amongst moss. The group to which these
forms belong is called the "bdelloid" or leech-like rotifers, because
they not only swim, but progress by a looping method like that of
Hydra or a leech.
PHYLUM GASTROTRICHA
Minute wormlike, unsegmented animals, with certain tracts of the
skin ciliated, the cuticle often forming bristles and scales; a non-
cellular hypodermis, forming adhesive papillae, longitudinal muscle
cells which do not form a continuous sheath; straight alimentary
canal consisting of a muscular pharynx like that of the nematodes
and a mid gut without diverticule ; a pair of nephridia in freshwater
representatives; a nervous system consisting of a cerebral ganglion
and two lateral cords ; hermaphrodite individuals in one division of
the phylum (Macrodasyoidea) and parthenogenetic females in the
other (Chaetonotoidea) ; the single female aperture opening near
the anus, and the male aperture when present variable in position.
Development direct and cleavage total.
These small animals (Fig. 172) are usually elongated and creep
or swim by means of their cilia or move in a leech-like manner
using their musculature. They feed on minute animals and plants
which are sucked in by the pharynx.
The Gastrotricha have features in common with the Rotifera, such
as the external ciliation, the bifid foot and the excretory system with
flame cells, but in the character of the gut they recall the Nematoda.
CHAPTER VIII
THE NEMATODA, NEMATOMORPHA
AND ACANTHOCEPHALA
PHYLUM NEMATODA
Unsegmented worms, with an elongated body pointed at both ends;
ectoderm represented by a thin sheet of non-cellular hypodermis, con-
centrated to form two lateral lines and to a less degree dorsal and
ventral midlines^ secreting an elastic cuticle, made of protein, not
chitin, usually moulted four times in the life of the individual; cilia
absent from both external and internal surfaces; a single layer of
muscle cells underneath the hypodermis, divided into four quadrants,
each muscle cell being elongated in the same direction as the body
and composed of a peripheral portion of contractile protoplasm and
a larger internal core of unmodified protoplasm which sends a process
to a nerve ; the space between the body wall and the gut sometimes
filled by a small number of highly vacuolated cells, the vacuoles
joining together and simulating a perivisceral cavity ; excretory system
consisting of two intracellular tubes running in the lateral lines;
nervous system made up of a number of nerve cells rather diff^usely
arranged but forming a circumpharyngeal ring and a number of
longitudinal cords of which the mid-dorsal and mid-ventral are the
most important; sense organs of the simplest type; sexes usually
separate, gonads tubular, continuous with ducts, the female organs
usually paired, uniting to open to the exterior by a ventral vulva, the
male organ single, opening into the hind gut, thus forming a cloaca,
in a diverticulum of which lie the copulatory spicules ; spermatozoa
rounded and amoeboid, fertilization internal; alimentary canal
straight and composed of two ectodermal parts, the suctorial fore gut
and the hind gut and an endodermal mid gut without glands or
muscles ; segmentation of egg complete and bilateral in type, develop-
ment direct, larvae only differing slightly from adult.
The nematodes appear to occupy an isolated position, but many of
their characters, though more specialized, resemble those of the
Platyhelminthes and Rotifera. They are certainly closely related to the
Acanthocephala, Gastrotricha, and the Nematomorpha. One of their
peculiar features is certainly secondary, namely the absence of cilia.
There are in some nematodes cilium-hke processes to the internal
border of the endoderm cells ; in one case active movement has been
reported. The excretory canals, when the absence of flame cells is
NEMATODA 245
taken into account, are seen to resemble those of the Platyhelminthes.
Nearly all the other characters may be called primitive. The sim-
plicity of organization, the absence of segmentation at all stages and
a vascular system, the diffuse nature of the nervous system and the
structure of the muscle cells are all signs of a lowly origin. But it is
still maintained by some that these features are not primitive but
degenerate and that the origin of the phylum is to be sought in the
arthropods, probably in the parasitic forms of that group (the
degenerate arachnids called linguatulids). If this view is taken it must
be supposed that the parasitic nematodes are the most primitive
members of the phylum and that some of their descendants became
less and less parasitic, until entirely free-living forms came into
existence. This would be an extraordinary reversal of evolution for
assuming which at present there are no grounds.
The view taken in this book is that the free-living nematodes are
ancestral to the parasitic forms and that there is no real connection
between the arthropods and the nematodes. Not only do the
nematodes present no indications of segments or appendages at any
point of the life history but also the cuticle is of an entirely different
chemical composition in the two phyla, and the loss of cilia most
likely a phylogenetically recent phenomenon in the nematodes as in
the parasitic platyhelminthes.
The anatomy of the nematodes is best known from the study of
Ascarts which is one of the largest members of the group and the only
one adapted for dissection in class. Full accounts of this form are
given elsewhere, but the following points must be emphasized. In
Ascarts (Fig. 173) there appears to be a wide space between the
muscle layer and the endoderm cells, with no epithelial boundary,
walls, but on closer examination it is seen to be occupied by a very
small number of greatly vacuolated cells, and what appears to be a
continuous cavity is really the confluent vacuoles of adjacent cells,
and so the term "intracellular" may be applied to it. This arrange-
ment has not been verified in many other nematodes but connective
tissue cells can usually be demonstrated in the space. They may be
phagocytic; the enormous branched cells of Ascarts (Fig. i75)>
lying on the lateral lines, take up in their tiny corpuscle-like divisions
such substances as carmine and indigo which are injected into the
body.
A striking feature of the histology of Ascarts is the presence of
greatly enlarged cells. Not only do the body cavity cells show this,
but in the excretory system the greater part of the canal is contained
in the body of one cell which divides into two limbs each running the
whole length of the body on opposite sides.
As a simple type of nematode the genus Rhabditis (Fig. 174) will
246
THE INVERTEBRATA
be described, as it is seen alive as a transparent object under the
microscope. Most species are free-living. They are obtained by
allowing small pieces of meat to decay in moist earth. The larvae
vs^hich exist in an "encysted" condition in the soil are attracted by
the products of decay, and in a few days become sexually mature.
m,co.
g.c.n.
oes.
lat.l.
cx.c.
cut.
m.n
^v.n.
Fig. 173. Diagrammatic transverse section through Ascaris in the region of
the oesophagus, showing the single large cell occupying the space between
the body wall and the gut. Original, ait. cuticle ; d.n., v.n. dorsal and ventral
nerves ; g.c.n. nucleus of giant cell, cytoplasm dotted, vacuoles {vac.) shown
as clear spaces; ex.c. excretory canal; hyp. hypodermis; lat.l. lateral line;
m.co. contractile part of muscle cells; in.t. tails of the muscle cells running
toward the nerves in the median lines ; oes. oesophagus with three gland
cells gl.c. and radiating muscles ni.r. which increase the lumen of the oeso-
phagus and cause suction. The number of muscle cells in each quadrant is
much greater than in the drawing.
Great numbers of adults and young can then be scraped off the surface
of the meat in the liquefied matter formed by bacterial decomposition.
It will be seen that the animal progresses by alternate contractions
of the muscles on each side of the animal, which bend the animal into
S-shaped curves and enable it to wriggle slowly through thick hquid
or on soil. The cuticle which covers the body is thin, tenacious but
elastic. It enables the animal to keep an almost constant round cross-
cav.
Fig. 174. Rhabditis. Altered from Maupas. A, Mature female. B, Mature
male. C, Ventral view of hind end of male, slightly turned to one side so that
the vas deferens is seen only to the right of the alimentary canal. D, Side
view of hind end of male to show the relations of the cloaca. E, Encysted
larva enclosed in the stretched skin {cut.) of the last moult, an. anus ; h.cav.
buccal cavity; c.b. copulatory bursa; c.sp. copulatory spicule; cl. cloaca;
ex.a. excretory aperture; gl.c. gland cells ;/.^. fore gut; ni.g. mid gut; h.g.
hind gut; n.c. nerve collar; ov. ovary; o. egg ready to be fertilized; o. eggs,
one just fertilized, the other in the two-cell stage; p.v. pharynx with its
valves ; rec.sem. receptaculum seminis ; t. testis ; ut. uterus ; va. vagina ; vd. vas
deferens.
248 THE INVERTEBRATA
section and length ; in the presence of such a cuticle and the absence
of circular muscles the peristaltic movements of a worm like
Lumbricus are impossible. A cross-section through Rhahditis shows
a similar structure to Ascaris, though the muscle cells are much less
numerous (only two to each quadrant) : each cell contains a number
of contractile fibrils arranged in a different way to those in the Ascaris
cell. The body cavity has not been investigated; that of Ascaris has
therefore been described above.
The alimentary canal consists first of all of an ectodermal fore gut
lined by cuticle in which the following parts can be distinguished:
(i) a mouth, surrounded by papillae, opening into a narrow buccal
cavity^ with parallel sides, (2) an oesophagus, with muscular walls and
a small number of unicellular glands, forming two swellings, the
oesophageal bulbs. The posterior of these (the so-called pharynx)
exhibits rhythmical pumping movements, caused by the contraction
of the radial muscles which enlarge the cavity of the bulb and open
the valve formed by the thickened cuticle. In this way the surrounding
fluid is drawn into the oesophagus : no solid particles much larger than
bacteria can be admitted through the narrow lumen. When the
muscles relax and the cavity disappears the fluid is driven on into the
midgut. This is composed of a single layer of cells, which internally are
naked but externally have a fine cuticle. These are entirely absorptive
in function, gland cells being absent. There are no muscles, but the
gut contents are circulated by the locomotory movements of the
animal. The hind gut which follows is lined with cuticle and opens at
the ventrally situated anus. Near the anus is a sphincter muscle, but
there are also dilator muscles running from the hind gut to the body
wall, and during the periodic contraction of these the gut contents are
evacuated. The alimentary canal of the nematodes as thus seen in
action represents a type simplified because the animal usually lives on
food which has been split up into easily assimilable substances — in this
case by bacterial action, in the case of Ascaris by the ferments of the
living host — and this is passed with great rapidity through the
alimentary canal by the pumping action of the oesophagus.
In addition there are easily seen in living Rhabditis the ventral
aperture of the excretory canal, not far behind the mouth, and when
the animal is compressed under the coverslip the coiled line of the
excretory canal ; the only part of the nervous system which can be so
seen is the ring round the oesophagus.
The genital organs are of the type seen is Ascaris but simpler. In the
female there are two tubular gonads bent once on themselves, dis-
charging by a single genital aperture, situated about half-way between
^ In some free-living nematodes which are carnivorous (e.g. Mononchus) the
buccal cavity is very wide and rotifers and other animals are taken into it.
NEMATODA
249
)ex.c.
—HI.
-oe.t.
'h.g.
Fig. 175-
Fig. 176.
Fig. 175. Dissection of an Ascarid to show position of the branched excretory
cells. /./. lateral lines ; ex.c. excretory cells. After Nassonow.
Fig. 176. Mermis. Showing the blindly ending oesophagus and the isolated
mid gut, the cells of which are full of fat globules, oe.t. end of oesophagus ;
f.b. mid gut; h.g. hind gut; sti. stylet. Original.
250 THE INVERTEBRATA
the head and the tail. The ovary is a short syncytial tube, the nuclei
becoming larger and larger and the centre of more definite and larger
aggregations of cytoplasm and yolk nearer the uterus. Finally, there
is a single ovum discharged at a time into the oviduct ; as soon as this
happens another ripens in its place. To reach the uterus the egg has
first to pass through a portion of the oviduct {receptaculum seminis)
filled with the amoeboid spermatozoa of the male. Fertilization takes
place, a shell is formed and at the same time maturation proceeds.
The two uteri join to form the median vagina. In this the fertilized
egg develops and the young larva is formed and may hatch within the
vagina. The stages of segmentation are seen nowhere with such ease
or clearness as in a small transparent nematode of this kind.
The male, on the other hand, has only a single gonad. The apical
testis is syncytial like the ovary. Nearing the vas deferens a zone may
be seen of free spermatocytes and in the vas deferens itself can be seen
large numbers of rounded spermatozoa. The genital duct opens into
the gut to form a cloaca. This contains a dorsal pocket in which is
secreted a chitinous apparatus consisting of two converging rods, the
copulatory spicules ^ with a grooved connecting piece to hold the points
together. The pocket has a special muscle which protrudes the spicules
from the anus (cloacal aperture). To each side of this aperture is a
lateral cuticular flange, supported by ribs, which meets its fellow at
the root of the drawn-out tail. This acts as a sucker (copulatory bursa) ^
by which the male retains its position on the body of the female until
the spicules are thrust through the female aperture and keep the
female and male apertures both apposed and open. Then by the con-
traction of the muscles of the cloaca the spermatozoa are expelled and
passed into the vagina of the female. Here they become amoeboid
and travel up the uteri so that they can meet the ova as the latter
are discharged.
Besides the normal condition in which males and females are pro-
duced in equal numbers, many species of Rhabditis occur in which
there is a remarkable disparity in numbers of the sexes. For a thou-
sand females there may be only ten or twenty males, and they are
lethargic in their sexual activities. The females, on the other hand,
have developed a curious kind of hermaphroditism. When the gonad
first becomes ripe a number of spermatozoa are produced. Afterwards
the gonad produces nothing but eggs which are fertilized by the in-
dividual's own spermatozoa, and after these are exhausted nothing
but sterile eggs are laid. Experiment has proved that in these animals
self-fertilization may occur for an immense number of generations
without any deterioration of the species.
In Rhabditis, as in the majority of nematodes, there are four moults.
After the second moult the animal may remain within the loosely
NEMATODA 251
fitting skin as a so-called "encysted" larva which possesses, however,
the power of movement. The protection of the cast skin and possibly
other factors enables this stage in the life history to resist desiccation
and to remain in a state of dormant metabolism until some odour of
decaying substances attracts the larvae and the opportunity of rapid
reproduction is given for a brief period.
This third larval period is characteristically the period of wandering
in many nematodes, and this is seen in a remarkable manner in the
classical life history of Ancylostoma (Fig. 177). These animals hve
attached in the adult stage to the mucous membrane of the human
small intestine, sometimes in such numbers as to present an aspect
comparable to the pile of a carpet. They feed on the intestinal tissues
and only accidentally rupture the blood vessels, causing anaemia in
the host. The females are fertilized in situ and eggs are laid, which
begin to segment before they pass out into the faeces. The rest of the
life history may be shown as follows :
(i) First larval form (rhabditoid) with a buccal cavity like Rhabditis.
This lives in the soil for three days before the first moult, which
produces the
(2) Second larval form which moults after two days, the skin re-
maining as a cyst round this strongyloid larva (3). In this stage the
animal becomes negatively geotropic and thigmotropic, ascending
through the soil and being specially attracted to the moist skin of
human beings. This they penetrate by way of the hair follicles, though
occasionally the larva enters the gut by the mouth. In the former
event, the minute larva is able to make its way through the skin to
lymph spaces and to blood vessels, eventually being swept into the
circulation by the vena cavae to the right auricle, thence to the right
ventricle and then to the lung. In the pulmonary capillaries this
career is ended and the larvae make their way into the alveolar
cavities of the lung. They then travel by the bronchi and the trachea
to the oesophagus and so to the intestine. Here the animal is freed
from the second skin, producing the larva without buccal capsule. The
third moult produces the last larval stage towards the fifth to seventh
day and this is termed the larva with provisional buccal capsule (4).
Finally, about the fifteenth day the fourth moult produces the worm
with the definitive buccal capsule (5), and in three to four weeks from
hatching the parasite has become sexually mature and is attached to
the epithelium of the intestine. -
This most important human parasite shows in its earliest stages the
structure and the free-living habit of the primitive form Rhabditis,
and it is noteworthy that there are many species of the latter genus
which have already become parasites.
It may, however, be supposed that a less specialized life history is
252
THE INVERTEBRATA
Fig. 177. Nematodes parasitic in man. A,B and C,Ancylostoma. After Looss.
A, Adult worm attached to epithelium of small intestine of the host, with some
of the tissue of the latter sucked into the buccal cavity of the worm. d.g. dorsal
gland; la. lancet; oe. oesophagus; v.t. ventral tooth; tis. host tissue, lacerated
by the lancets and partly digested ; vil. villi of small intestine. B, Larvae pene-
trating the skin of mammal, x. through the horizontal fissures of the epi-
dermis ; y. along the hair follicles ; z. larvae which have arrived in the lymph
vessels of the subdermis ; ep. epidermis. C, Copulatory bursa of adult male,
spread out to show the arrangement of the rays. D. dorsal; V. ventral;
sp. copulatory spicule. D, Filaria bancroftt, longitudinal vertical section
through a mosquito (Stegomyta) to show wandering of the larvae. After Bahr.
a, larvae just swallowed and now in the mid gut (mg.) ; some migrating through
the gut wall ; b, larvae developing in the thoracic muscles (th.m.) ; c, larvae
which have finished development (8-15 days) migrating in the haemocoele
of the head ; d, larvae in the blood space of the labium, which they leave by
rupturing the body wall when the mosquito bites; cr. crop; ph. pharynx;
pr. proboscis ; sg. salivary glands.
NEMATODA 253
that of the species of Oxyuris in which the tgg is swallowed by the
host and the remaining stages of development take place in the gut.
It is said that several successive generations of the parasite may occur
within the same host. On the other hand, the wandering habit of
nematodes is a fundamental character and even forms in the first stage
of parasitism (facultative) may penetrate host tissues.
The life histories of the principal nematode parasites of man and
domestic animals are summarized on pp. 254-5. They are arranged
in a definite order passing from the simplest type in Haemonchiis to
the most specialized life histories in Filaria.
Two other classes of nematode parasites merit particular attention.
They are, respectively, parasites of plants and insects.
Plant parasites. Nematodes are particularly fitted for a parasitic
life in plants by reason of their form and activity and their capacity
(at the end of the second larval stage) for resisting desiccation and
other unfavourable conditions. They are small enough, as larvae, to
obtain entrance through the stomata of leaves, and sometimes possess
dart-like projections of the buccal lining which enable them to
penetrate the cell walls of plants. They feed on cell sap and by
their interference with the life of the host plant cause the for-
mation of galls, wilting and withering of the leaves, and stunting of
the plant.
Tylenchus tritici passes through a single generation in the course of
the year, and infects wheat. The animal becomes adult when the grain
is ripening and a pair, inhabiting a single flower, produce several
hundred larvae. Instead of the grain a brown gall is produced, and
in this the larvae (after moulting twice) may survive for at least
twenty years. If the grain falls to the ground the larvae may remain
there over the winter or may escape into the soil. When the corn
begins to grow in the spring they enter the tissues of the plant and
make their way up the stem to the flower, where they speedily mature.
The great interest of this life history lies in the easy adaptation of the
parasitic life history to the annual cycle of the wheat plant and the
extreme capacity for survival in a dormant and desiccated condition
until the right plant host becomes available. Tylenchus devastatrix,
on the other hand, may pass through several generations in the year
and attacks indiscriminately clover, narcissi bulbs and onions, and
many other useful plants. Heterodera (Fig. 178 D) is a parasite of the
roots of tomatoes, cucumbers and beets, and is remarkable because
the female attaches herself in larval life to a rootlet from which she
sucks a continuous flow of sap. She is fertilized by wandering males
and grows enormously, becoming lemon-shaped. Inside the body
thousands of larvae are produced, which escape into the soil and live
there until the opportunity arises for infection of fresh roots.
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256 THE INVERTEBRATA
Insect parasites. Four of these may be mentioned, though other life
histories are also of great interest.
In Mermis (Fig. 176) a curious reversal of the typical nematode
life cycle occurs. The sexual forms are all free-living either in the soil
or fresh water. On summer days after showers the sexual forms of
Mermis nigrescens exhibit a curious tropism, leaving their haunts two
or three feet in the ground and crawling up the stems of plants, but
disappearing when the sun grows warm. The eggs are laid in the
ground and when the larvae hatch they pierce the skin of insect larvae
Fig. 178. Insect and plant parasites. A, Atractonema. Female showing the
beginning of the prolapsus of the uterus, which has proceeded in Spherularta,
B and C, until it is far larger than the rest of the worm. In C, the body is a
minute appendage (bd.) of the prolapsed uterus (ut.pr.) not much longer than
one of the greatly enlarged cells of the latter. D, Heterodera ; i and 2, larvae
attached to a rootlet with their heads imbedded in its tissues ; 3, the full-grown
female (on a smaller scale), removed from the plant. The alimentary canal is
shown in black to emphasize that its growth c auses the increase in size of the
parasite. 0. ovary; Mi. uterus. A-C, after Leuckart; D, after Strubell.
and wander into the body cavity where they nourish themselves by
absorbing fluid food through the cuticle. The mid gut has become a
solid body, having no connection with the mouth and anus, and in it
fat is stored up which serves as raw material for the production of
eggs. When the animals become sexually mature they escape into the
soil.
In Tylenchus dispar (a form which is thus placed in the same genus
as the well-known plant parasites) the adult female and innumerable
larvae are found in the body cavity of the bark beetle, Ips, during the
winter. Allantonema has similar relations to another bark beetle,
Hylobius. The female is enormously developed, the uterus and other
NEMATODA AND NEMATOMORPHA 257
female organs occupy the whole of the body, the gut having entirely
disappeared. In the spring the larvae (having undergone two moults)
bore through the walls of the end gut and undergo further develop-
ment in the **frass" (faeces of the beetle). The male develops pre-
cociously and fertilizes the female which, when it becomes mature,
is still of normal proportions. After fertilization the females (only)
infect the beetle larvae which by this time have appeared. Entrance
is obtained by means of a "dart" exactly like the similar organ in
the plant parasites. In the body cavity the female Allantonema grows
rapidly, and when metamorphosis occurs and the mature bark beetle
seeks another tree to form a new colony, it is full of larvae.
Spherularia (Fig. 178 B, C) is a parasite of the humble bee. In the
summer the moss and soil near the bee's nest is inhabited by the
sexually mature worms, and after fertilization has taken place the
female wanders into the body cavity of the insect, as in the preceding
life histories. Though the number of cells in the somatic tissues of the
bee is said not to increase in number there is an enormous growth in
size of the vagina which becomes prolapsed and forms eventually an
organ many times the size of the rest of the body, which remains
attached for some time but eventually disappears. The parasitized
humble bees, after passing the winter in their nests, tend to emerge
early. In the spring very often inactive bees may be caught which
prove, on dissection, to contain one or more of these enormous
sausage-shaped bodies, each of them full of eggs and larvae, which
escape through the gut wall and become free-living.
Atractonema (Fig. 178 A), a parasite of the Cecidomyidae (p. 509),
has a similar life history.
PHYLUM NEMATOMORPHA
As in Nematoda but lateral line and "excretory" canal absent,
nervous system consisting of a dorsal "brain" and a single ventral
cord, genital ducts in both sexes opening into the hind gut to form a
cloaca, development very characteristic — gastrulation by invagination
and a larva with peculiar boring organ which infects insects.
In addition it should be mentioned that the alimentary canal is
always more or less degenerate and the body cavity may either be
occupied by parenchymatous tissue or by reduction of this becomes
more or less empty of cells.
An example of this group is Gordius robustus with the following life
history. The adults are found in brooks from October till May when
they copulate. The sperm is not directly introduced into the cloaca,
but placed in masses on the body near it. The eggs are laid in the
water and the larvae soon hatch. By using the boring organ which
they possess they find their way into the body cavity of crickets which
258 THE INVERTEBRATA
live near the water. There they remain and grow until the autumn
when they leave the host and enter the water again as mature animals.
Other forms have similar life histories. Parachordodes first infects
chironomid larvae and then these are eaten by the second host, the
beetle Calathus in which they grow to maturity.
OV.S.
,0V.
-in.
cu.
n.c.
al.c.
Fig. 179. Transverse section through Parachordodes. al.c. alimentary canal;
cu. cuticle; m. muscular layer; n.c. nerve cord; ov. ovary; ov.s. ovarian
sinus; par. parenchyma; pa. dorsal sinus.
Fig. 180. Larvae of Gordius in the leg of an insect.
PHYLUM ACANTHOCEPHALA
As in Nematoda, but possessing an eversible proboscis provided with
hooks for attachment and glandular organs (the lemnisci)^ cuticle
delicate, hypodermis containing a peculiar lacunar system, a layer of
AC A NTH O CEP HAL A
259
circular muscles as well as longitudinal, nervous system consisting of
a brain and two lateral cords, excretory organs, which when they
occur are nephridia with modified flame cells ; body cavity without
parenchyma but traversed by a tubular organ, the ligament, containing
gonads, eggs developing inside the body until the provisional hooks of
cem.gl.
Fig. 181. Neoechtnorhynchus. cent. gl. cement gland; g.nu. giant nucleus;
/. lemnisci; pr. proboscis; pr.s. proboscis sheath; retr.pr. retractor muscle
of proboscis ; t. testis ; v.d. vas deferens,
the pharynx are formed, larvae developing further when laid in water
and eaten by a crustacean, becoming mature when eaten by a verte-
brate after which the animals attach themselves to the wall of the
intestine by their proboscis.
An example of these is Echinorhynchus proteus, the adult of which
lives in ducks and the larvae in Gammarus.
CHAPTER IX
THE PHYLUM ANNELIDA
Segmented worms in which the perivisceral cavity is coelomic ; with
a single preoral segment (prostomium) ; with a muscular body wall
in which externally the elongated muscle cells are arranged with their
longitudinal axes across the width of the worm (circular layer) while
internally their axes are parallel to the length of the worm (longitudinal
layer) ; with a central nervous system consisting of a pair of preoral
ganglia connected by commissures
with a pair of ventral cords which
usually expand in each segment to
form a pair of ganglia from which
run nerves to all parts of the seg-
ment; with nephridia and coelomo-
ducts; and the larva, if present, of
the trochosphere type.
While the above definition is the
only one that can be applied to all
the annelids, typical representatives
of the phylum can also be described
as possessing a definite cuticle and
bristles or r/?«e^ae composed of chitin,
arranged segmentally, imbedded in
and secreted by pits of the ectoderm
(Fig. 182). The cuticle is thin and
not composed of chitin, thus differing
from that of the Arthropoda. r-u . f r a •
^ , 111 Fig. 182. Chaeta 01 Lumbricus in
Four classes compose the phylum. ^^^^ ^^j^ ^^^^^^^ f^^^ Stephen-
Of these the largest and most typical son. cu. cuticle ; ect. ectoderm ; ch.
is that of the Chaetopoda, which are chaeta; cm. circular muscles ; /)r.m.
well segmented, have a spacious peri- protractor and n.m. retractor mus-
9 , 11 cles : per. peritoneum : joL. follicle
Visceral coelom and a ways possess ^^^y^,^ formative cdl of chaeta
chaetae. All these characters are (^^^^ nucleus),
primitive. The Archiannelida is a
small group characterized by small size, ciliation of skin, loss of external
segmentation and often of chaetae. Several members of the group,
however, like Saccocirriis, retain chaetae. It is almost certain that the
archiannelids are derived from the chaetopods by a process of
simplification. The Leeches or Hirudinea are adapted to a specialized
mode of life — ectoparasitism — and their whole organization is affected
ANNELIDA 261
by it. They retain the segmentation characteristic of the phylum in most
of their organs but the coelom is usually much restricted and broken up
into a system of small spaces and the chaetae are lost. In one primitive
form, Acanthobdella, there are chaetae and a spacious perivisceral
coelom in the anterior segments. In all leeches the anterior and
posterior suckers and a hermaphrodite reproductive system, closely
paralleled in a subdivision of the Chaetopoda, the Oligochaeta, show
the speciahzation of the group. The Echiuroidea and Sipunculoidea
are two groups of burrowing marine worms in which segmentation
has been almost entirely suppressed in the adult but is sometimes
shown in the larvae by mesoblastic somites or ganglion rudiments.
Chaetae are lost except in a few forms, but a large perivisceral coelom
is preserved.
Class CHAETOPODA
Well-segmented Annelida, with chaetae and a spacious perivisceral
coelom, usually divided by intersegmental septa.
In a typical chaetopod there is a distinct preoral region or pro-
stomium and a postoral body composed of many segments. Each
segment owes its distinctness to the development in the larva of a
pair of mesoblastic somites which join round the gut, the cavities
which develop in them becoming the perivisceral cavity of the adult
segment. At the same time the larval ectoderm (epiblast) develops
segmentally repeated organs : the ganglia^ swellings in the continuous
ventral nerve cords, the nephridia or excretory organs and the chaetae.
In the Polychaeta, one of two orders into which the Chaetopoda are
divided, the chaetae are borne in groups upon processes known as
parapodia, whose projection from the body wall is due to the develop-,
ment of special muscles for moving the chaetae. In the other order,
the Oligochaeta, there are no parapodia. The chief feature of the
nervous organization is that the musculature of all parts of the body is
co-ordinated by metamerically repeated intra- and intersegmental re-
flexes (Fig. 183). In each segment there is, for example, a correlation
of the circular and longitudinal muscles by the segmental nerves which
acts so that contraction of one brings about automatically relaxation of
the other. Then there are nervous connections between adjacent
segments which act so that excitation of a muscle layer in one segment
leads to excitation of the same layer in the other segment. By the
working together of the inter- and intra-segmental reflexes the normal
peristaltic movement of Lumbricus and other chaetopods is brought
about.
There is also a system of giant fibres, three in number, running
along the whole length of the ventral nerve cord. These are responsible
for the reactions which require immediate co-ordination of the whole
262
THE INVERTEBRATA
body in response to excitation of the higher centres, the supra- and
subpharyngeal ganglia. The rapid contraction of the whole of the
longitudinal musculature in response to a noxious stimulus is an
example of this kind of reaction. On p. 200 it was shown that the
primary function of the primitive central nervous system is that of a
sensory relay. In the annelids there is added the second great func-
tion, that of inhibition. A Nereis, which has had the suprapharyngeal
cm.
Fig. 183. A, Longitudinal and B, transverse sections of Lumbricus to show
the musculature and its innervation. In A, segment 2 shows neurones con-
stituting an intrasegmental reflex arc, segments 3 and 4 show those which
make up an intersegmental arc. B shows the distribution in the body wall
of two segmental nerves and their branches, al.c. alimentary canal; cm.
circular muscles ; g.f. giant fibre ; l.m. longitudinal muscles.
ganglia removed, moves about ceaselessly, showing that a function
of the ganglia in the normal animal is the inhibition of movement.
If the supra- and subpharyngeal ganglia are both removed then the
animal is permanently quiescent, a condition like that of a polyclad
turbellarian when the cerebral ganglia are removed.
The coelom is bounded by an epithelial layer, ihQ peritoneum^ which
gives rise to i\\& gonads ^ which in polychaets are usually developed in
most of the segments, to t)\& yellow cells ^ which play a part in the work
of nitrogenous excretion, and to the coelomoducts by which the eggs
ANNELIDA 263
and sperm pass from the coelom to the exterior. In most of the poly-
chaets the eggs are fertiHzed externally, forming a trochosphere larva,
the method of reproduction thus conforming to that of other marine
groups. In the terrestrial and freshwater oligochaets (as in leeches)
fertilization is internal and the young hatch in a form resembling the
parent. There is no doubt that the former mode of development is
more primitive.
The nephridia are essentially tubes developed from the ectoderm
which push their way inwards so that they project into the body
cavity. In some polychaets they end blindly — this is the primitive
condition. In the majority of chaetopods they have acquired an
opening (nephrostome) into the body cavity itself. In some cases there
is a partial fusion with a mesodermal element, the coelomoduct, so
that a compound tube consisting mainly of ectoderm but partly of
mesoderm exists {nephromixium). Nephromixia may take on the
functions of coelomoducts where these do not exist independently.
All types of tubes are termed here segmental organs.
The head and accompanying sense organs may be well developed,
for instance, in some of the pelagic Polychaeta where the eyes are re-
markably complex. In such cases the brain (prostomial ganglia) may
attain a structure almost as complicated as in the higher arthropods.
The head processes (tentacles, palps) vary greatly. While they may
be very complicated in the Polychaeta, they are frequently absent in
burrowing members of that group and invariably so in the Oligo-
chaeta.
The blood system also varies greatly. In small forms it is absent
altogether. Typically it consists of a dorsal vessel in which the blood
moves forward, and a ventral vessel in which it moves backward and
from which the skin is supplied with venous blood. The whole of the
dorsal vessel (Fig. 201) is usually contractile : there may also be vertical
segmental contractile vessels which are usually called ''hearts". In
some forms, for example Pomatoceros (Fig. 186 C), there are no
separate dorsal and ventral vessels but a sinus round the gut : the peri-
stalsis of the latter brings about the movements of the blood. While
the whole of the skin is sometimes richly supplied with blood vessels
and usually performs an important part in the aeration of the blood
there are often branched segmented processes which may rightly be
called gills (Arenicola (Fig. 189)): the alimentary canal is probably
a respiratory organ too. While haemoglobin is often present in the
blood, usually in solution, a related pigment, chlorocruorin, which is
green, occurs in many tubicolous polychaets. The variable state of the
mechanism of respiration is shown by the fact that one species of
a genus (the polychaet. Poly cirrus) may possess haemoglobin while
another has no respiratory pigment.
264 THE INVERTEBRATA
The Chaetopoda are, in this work, divided into the following
orders: (i) Polychaeta, (ii) Oligochaeta. To the latter, however, the
Hirudinea are very closely related.
Order POLYCHAETA
Marine Chaetopoda with numerous chaetae arising from special
prominences of the body wall called parapodia ; usually with a distinct
head which bears a number of appendages; nearly always dioecious,
with gonads extending throughout the body and external fertilization ;
with a free-swimming larva, the trochosphere.
The structure of the Polychaeta is very variable and dependent on
the habit of life, both externally (especially the head appendages and
parapodia) and internally (especially the segmental organs). The
variation in methods of reproduction is also very characteristic. For
these reasons an account will first be given of some of the very large
number of families into which the Polychaeta are divided, in which
a rough oecological grouping is adopted. A summary of the variation
in segmental organs and reproductive habits follows at the end.
'^Eunicidae. Eunice^ Leodice
(the Palolo worm).
Nereidae. Nereis.
Syllidae. Syllis, Myri-
anida.
Phyllodocidae. Eulalia,
Aster ope.
Polynoidae. Aphrodite,
. Lepidonotus, Panthalis.
Chaetopteridae. Chaeto-
pterus.
Terebellidae. Terebella,
Amphitrite.
Serpulidae. Pomatoceros,
Filigrana.
Sabellidae. Sabella, Spiro-
gr aphis.
Arenicolidae. Arenicola
without jaws.
Glyceridae. Glycera with
jaws.
The errant Polychaeta with unmodified
head and armed eversible pharynx
(proboscis); fitted for an active life
but often living in tubes ; very often
greatly modified in structure and
physiology at the sexual season.
The true tubicolous Polychaeta, much
modified for the collection of micro-
scopic food ; anterior part of gut not
eversible and jaws absent ; inhabiting
tubes which they rarely or never
leave.
The burrowing Polychaeta with re-
duced head ; with proboscis.
The errant Polychaeta
The external structure is known to the elementary student through
the type Nereis (Fig. 184). The prostomium bears two kinds of
POLYCHAETA 265
filiform, tactile appendages, the tentacles which are dorsal and the
palps which are ventral ; there are also one or two pairs of eyes upon
it. The anterior part of the gat (pharynx) is eversible and serves for
grasping food ; its lining may be chitinized in places to form the jaws
and paragnaths oi Nereis or teeth as in Syllis. These are not necessarily
^^^^•^•->^ . pr. ten.
ac^
r^ieur.
Fig. 184. Nereis. A, Dorsal view of head and ifirst trunk segments with
everted pharynx. B, Side view of same with pharynx retracted. C, Para-
podium of unmodified type. D, Parapodium of Heteronereis. E, Example of
unmodified compound chaeta. F, Oar-shaped compound chaeta of Hetero-
nereis. The peristomium is stippled, pr. prostomium; ten. tentacle;^, palp;
ten.c. peristomial cirri ; d.c. dorsal cirrus ; v.c. ventral cirrus ; not. notopodium ;
neur. neuropodium ; /./. foliaceous outgrowths of parapodia ; ac. aciculum ;
ch. chaetae ; ch. oar-shaped chaetae ; j. jaws ; pg. paragnaths.
the sign of a carnivorous habit but may be used for cutting up pieces
of seaweed or boring in sponges. ,
The ordinary trunk segment has a double parapodium consisting
of a dorsal notopodium and a ventral neuropodium ^ usually with rather
different types of chaetae. A dorsal cirrus and a ventral cirrus are
nearly always present ; they are filiform structures but may be modi-
fied to form pectinate gills [Eunice) or plate-like elytra (Polynoidae).
266
THE INVERTEBRATA
From the conical noto- and neuropodia spring a bundle of chaetae ;
the chaetal sacs project into the coelom and each bundle is supported
by an enlarged and wholly internal chaeta — the aciculum, which also
forms the point of origin of the parapodial muscles. The chaeta are
of two kinds, simple and compound.
The segment (or segments) just behind the mouth, forming the
peristomium, is, however, much modified. There are no notopodia or
neuropodia (except in occasional species, which retain chaeta-bearing
Fig. 185. Errant Polychaeta. Peristomial segments stippled to show extent of
cephalization. Anterior end. A, Syllis, single peristomial segment; pharynx
retracted in sheath, ap. aperture of pharynx sheath cavity ; M. mouth ; p. palp ;
ph. pharynx ; ph.sh. cavity of pharynx sheath ; pro. proventriculus ; t. tooth ;
ten. tentacle. B, Eulalia, three peristomial segments and five pairs of ten-
tacular cirri, pharynx protruded, covered with papillae. B , Parapodium with
leaf-like dorsal and ventral cirri, notopodium only represented by dorsal
cirrus neuropodium with compound chaetae. C, Asterope, head with five
tentacles and three pairs of tentacular cirri (ten.c.) ; conditions in the head
region largely governed by the presence of the enormous eyes. Pharynx
protruded.
processes as a primitive feature). But the cirri remain as the peri-
stomial cirri in pairs consisting of a dorsal and ventral member. In
Nereis there are two pairs of peristomial cirri on each side, indicating
the fusion of two segments to form the peristomium. In some families
(Syllidae) (Fig. 185 A) this is constituted by a single segment, but
usually two or more have been pressed forward towards the mouth
and modified. This is the first indication of the process of cephali-
zation carried much further in the arthropods and vertebrates.
The worms in this group used to be definitely classed as the
POLYCHAETA 267
Errantia or free-swimming forms, but a great number of them (e.g.
the Nereids) do live in tubes which, however, they can leave and
reconstruct anew. The most beautiful example of tube building in the
Polychaeta is furnished by Panthalis^ a polynoid. In this the chaetal
ch.m.x
,m.d.v.
nep.
n.c. C
Fig. 186. Transverse sections through different types of Polychaeta.
A, Aphrodite. After Fordham. B, Arenicola, middle region. After Ashworth.
C, Pomatoceros, thorax. Original, al. alimentary canal ; cm. coecum of mid gut ;
ch.m. matted notopodial chaetae ; cil. ciliated groove ; d.v. and v.v. dorsal and
ventral blood vessels; el. elytron; m.c, m.d.v., m.l. circular, dorsoventral and
longitudinal muscles; ohl.m. oblique muscles; nep. nephridium; neur. neuro-
podium ; not. notopodium ; n.c. nerve cord ; sin. sinus ; th.m. thoracic mem-
brane; v.cir. ventral cirrus.
pits of the notopodium produce not stiff bristles but plastic threads
which are woven by the comb-like ventral chaetae and the shuttle-Hke
action of the anterior parapodia into a continuous fabric which forms
the lining of the mud-covered tube. Aphrodite^ the sea mouse (Fig.
186 A), is a short, broad form which burrows in mud, and though it
268 THE INVERTEBRATA
does not form a separate tube it covers its back with a blanket made
from interwoven chaetal threads similarly formed from the noto-
podium. Between this blanket and the back is a space into which
water is drawn by a pumping action of the dorsal body wall, being
filtered through the matted chaetae. In this there are special plate-
like modifications of the dorsal cirri — the elytra — round which
circulates the water from which they possibly obtain dissolved
oxygen. In other polynoids (e.g. Lepidonotus^ which lives under
stones but does not burrow) the elytra can have no respiratory
function but are probably protective, spreading over the whole or
greater part of the back (sometimes bits of sand or shell are attached
to special papillae). Not all the dorsal cirri are modified to form
elytra: typical filiform cirri are placed on alternate segments.
Aphrodite has remarkable segmental coeca of the alimentary canal
in which takes place digestion of the fine food particles which pass a
sieve at the junction with the intestine.
The diagnostic features of Nereis and other genera mentioned in
the classification are given below.
Nereis (Fig. 184). Two tentacles, two palps; pharynx with two jaws
and twelve groups of paragnaths; noto- and neuropodium each
double; chaetae all compound; most species have a special sexual
form (Heteronereis).
Eunice. Five tentacles, two palps; pharyngeal armature well
developed; a single peristomial segment; gills in many segments;
chaetae simple and compound.
Eulalia (Fig. 185 B). Five tentacles, no palps; pharynx very long
with soft papillae only ; three peristomial segments ; dorsal and ventral
cirri leaf-like; chaetae all compound.
Asterope (Fig. 185 C). Similar to Eulalia but a pelagic polychaet
with transparent body and enormous eyes of complicated structure.
Syllis (Fig. 185 A). Three tentacles, two fused palps; pharynx
enclosed in a pharynx sheath with a single conical tooth and a mus-
cular proventriculus which functions as a pump; no notopodium.
Autolytus (Fig. 194 B). Like Syllis but pharynx long, with a circle
of teeth; no ventral cirrus. Myrianida has similar characters.
The true tubicolous Polychaeta
Here the prostomium has become much smaller and its appendages
enormously modified and increased. The peristomium may be pro-
duced into a collar which in some forms grows round the prostomium
and encloses a funnel-like cavity at the bottom of which lies the
mouth. The food consists of small animals or plants or organic debris
and it is collected by ciliary mechanisms. In the terebellids (Fig.
187 A), serpulids (Fig. 188) and sabeHids, the appendages of the head,
tenf.'x
Fig. 187. Tubicolous Polychaeta. Terebellid (Loimia). A, Side view of
young form taken from its tube. A', Side view of pelagic larva in its gelatinous
case. After D. P. Wilson. Chaetopterus pergamentaceus . Original. B, Side
view of worm in tube. Arrows show the direction of the water currents.
B', Dorsal view of anterior part to show the ciliated grooves. Original.
Arrows show the direction of the food currents, abd. abdomen ; col. peristomial
collar; cup, organ for forming foodballs; e. eye; fan., mu. fan. muscles for
moving fans; gl.sh. mucous glands; iVf. mouth; neur. neuropodia forming
suckers for attachment of worm to tube ; not. notopodia ; not. 4, notopodia
with enlarged chaetae in 4th chaetiferous segment; not. 10, food-collecting
notopodia; ot. otocyst; tent, tentacle; thor. thorax; up.l. upper lip, the lower
lip (l.l.) is a prominent structure to the right of the tentacles.
270 THE INVERTEBRATA
which probably correspond to tentacles, are very numerous. Each
tentacle has a ciliated groove running from the tip to the mouth and
along this minute particles may be seen to travel. In the terebellids
these tentacles are extensible and capable of independent movement
when separated from the body. In the serpulids and sabellids, they
are rather stiff branched structures, which can, however, curl up
when withdrawn into the tube ; they sometimes bear eyes and some-
times are wonderfully pigmented.
Besides the food-collecting tentacles there are gills in the tere-
bellids. These are branched processes, usually three pairs, situated
just behind the head, full of circulating blood. In the serpuHds and
sabellids, there are no special respiratory organs but the whole
surface of the body serves for the exchange of gases.
In the terebeUids the tubes are composed of a soft cementing
substance mixed with mud or a parchment-like material to which
adhere sand grains, sponge spicules, foraminifera or fish-bones. It
is usually porous (so that change of water can take place through it)
and the animal occasionally leaves its shelter; there are at least two
openings to the exterior. The tube of the chaetopterids is parchment-
like but in the serpulids there is a groundwork of mucin in which
carbonate of lime is laid down. In the latter family there is only one
opening from which the crown of tentacles emerges but never any
more of the body. The tentacles are violently withdrawn in obedience
to any such stimulus as touch or change of illumination.
In all the types except Chaetopterus the body is divided into two
regions, an anterior thorax and a posterior abdomen. The thorax is
composed of segments in which the notopodium is a conical structure
with capillary chaetae while the neuropodium is a vertical ridge in
which are imbedded short-toothed chaetae called uncini^ which only
just project from the body wall. It is suggested that the notopodium
assists movement up and down the tube while the neuropodia are
braced against the tube and maintain the worm in position. In the
abdomen the arrangement of the parapodia is different, and in the
serpulids and sabellids the uncini become dorsal and the simple
chaetae ventral (introversion).
In the serpulids (Fig. 188) the peristomium is similar to the other
thoracic segments but it is produced into a collar which folds back
over the ventral surface and sides and secretes successive hoop-shaped
rings which are added to the tube. Other features are the thoracic
membrane f^ lateral frill possibly respiratory, and the operculum, a much
enlarged and stopper-like branch of a tentacle which exactly closes
the mouth of the tube when the animal is retracted.
The renewal of water round the body is of the utmost importance
in respiration. It is brought about by undulatory movements of the
POLYCHAETA
271
abdomen and sometimes by sharp rhythmic contractions and ex-
pansions of the body which pump the body in and out of the tube.
The great development of the dorsal bands of longitudinal muscle
seen in a transverse section of a serpulid (Fig. 186 C) is characteristic
of the tubicolous worm. Another typical modification seen in the
serpulids and sabellids is the median ciliated groove, which starts from
the anus, runs along the ventral surface of the abdomen, turning on
to the dorsal surface when the thorax is reached. It serves to conduct
the faeces to the mouth of the tube.
Fig. 188, 'D[2igv2i.m oi Pomatocer OS triqueter \n Its tube. Original. The aperture
of the tube is represented in black : the top and base of the tube are shown
by vertical lines {tb.), the sides not represented so that the thorax can be seen
within. The collar {col.) is shown by stippling, folded back over the top and
sides of the tube ; and the thoracic membrane also by stippling. The collar is
transparent showing the prostomium and the lip of the tube beneath. The
fact that the tube is composed of successive rings is indicated in the neigh-
bourhood of the aperture (ann.). ap.tb. aperture of tube; neur. neuropodia;
not. notopodia; op. operculum ; />r. prostomium; fen. tentacle.
Chaetopterus (Fig. 187 B) is probably the most modified of all
tubicolous worms. It lives in a parchment-like tube which is U-
shaped with at least two apertures. There is a peristomial collar as in
other tubicolous worms, but the tentacles are a pair of rudimentary
processes. A very complicated mechanism exists for obtaining food,
which can be observed by taking a live Chaetopterus from its tube and
replacing it within a glass tube of the same calibre in an aquarium.
The worm fits very loosely in its tube and there is plenty of room for
a current of water to sweep through from end to end. Such a current
is maintained by the rhythmical oscillation of the fans (fused noto-
podia) of the middle region. Food particles contained in the current
are entangled in mucus secreted by the dorsal surface of the anterior
region, and ciliated currents, working in grooves in the enlarged
272 THE INVERTEBRATA
notopodia of the tenth chaetiferous segment, carry these strings of
mucus to the cup-shaped organ where they accumulate to form a ball
of food which is carried forward in a dorsal groove to the mouth.
The burrowing Polychaeta
Arenicola marina (Fig. 189) is the type of a burrowing polychaet
and it has a rounded cross-section like an earthworm. In its division
of the body into regions, the modification of the parapodia, and the
internal anatomy it resembles the tubicolous worms. The prostomium
is much reduced, however, without any appendages and there is an
eversible pharynx, covered with minute papillae, which is the organ
for locomotion through the sand as well as for feeding. In general
form it thus resembles an earthworm : the chief obvious difference is
the presence of gills and parapodia. It is divided into three regions :
the anterior, consisting of the peristomium, an achaetous segment,
and six segments which have a notopodium with capillary setae and
a neuropodial ridge with chaetae resembling uncini (crotchets); the
median, the segments of which have gills in addition; and the pos-
terior, in which parapodia and chaetae are entirely lost.
The body wall consists of the typical circular and longitudinal
muscle layers as in Lumbricus, and by their alternating contraction and
expansion the peristaltic movements which are characteristic of the
earthworm and other burrowing forms are carried out. In Nereis and
other surface-living forms progression takes place in two ways,
(i) By alternate flexing of the two sides swimming movements are
brought about. The longitudinal muscles, which are arranged in four
bundles, are much more important than the circular and are capable
of rapid contraction. (2) By successive movement of the parapodia
crawling movements occur (as in a centipede), the special parapodial
muscles coming into action. In tubicolous forms peristalsis occurs,
but the longitudinal muscles are even more important than in Nereis
for the violent movements of contraction which withdraw the animal
into its tube. They form a bulky dorsal mass and resemble the columella
muscle of the gasteropod in their action (Fig. 186 C).
Arenicola is the most convenient polychaet type for dissection
and therefore the following details of internal anatomy are given
(Fig. 190). In several prominent features it differs from Lumbricus and
also from Nereis or Eunice. The body cavity is spacious, it is not en-
croached upon by the longitudinal musculature, and the vertical septa
which primitively separate the body cavities of the segments have
nearly all disappeared. Only the three anterior septa and an indefinite
number of the most posterior are preserved. In the greater part of the
body the coelom is thus uninterrupted. In its general development
the alimentary canal resembles that of the earthworm. The muscular
POLYCHAETA
273
pharynx, however, is not well developed, the oesophagus is a thin-
walled tube with no such development as the gizzard of the earthworm
"'^'^^ T /?>'••
not.~^
in.r.{
Fig. 189. Arenicola marina. Side view. After Ashworth. ac/i. ist achaetous
segment; a.r. anterior region; m.r. median region; not. notopodium; neur.
neuropodium; pr. prostomium; p.r. posterior region; per. peristomium;
phar. pharynx.
and it bears only a single pair of coeca. The intestine is the longest
part of the gut, the seat of digestion and absorption, and it is invested
by a layer of yellow cells. The blood system, which also contains
-s.o.
—nephr.
yiot.m.
.not.m.
Fig. 190. General dissection of Arenicola marina. After Ashworth. aff.b.v.,
eff.b.v. afferent and efferent vessels of gills and body wall; an. auricle,
d.v., l.v. dorsal and lateral blood vessels; nephr. nephrostome; not.m. noto-
podial muscles; oe.p. oesophageal pouch; sep. septa; s.o. segmental organ;
s.i.v. subintestinal blood vessel ; ven. ventricle ; v.n.c. ventral nerve cord ; v.v.
ventral blood vessel. The direction of the flow of blood is indicated by arrows.
POLYCHAETA 275
haemoglobin in solution in the plasma, differs slightly from that of
Lumbricus : there is a single pair of large hearts, each divided into a
ventricle and auricle which connect the important lateral intestinal
vessels from which the branches supplying the gills are derived with
the ventral vessel.
The circulation for that region just behind the heart may be ex-
pressed as follows : lateral vessels->auricle^ventricle-> ventral vessel
-^afferent vessel to body wall and gill->efferent vessel to subintestinal
vessel->intestinal plexus->dorsal vessel or lateral vessel. The dorsal
vessel does not communicate directly with the heart.
The segmental organs are, like the gills, only found in the middle
region. They are prominent organs lying beneath the oblique muscles,
remarkable for the large size of the nephrostome, the dark secretory
bag-like portion, the cells of which contain insoluble excreta, and
the small gonad which lies just behind it. In Arenicola as in Lum-
bricus the gonads are restricted to a small number of segments, but
the reproductive cells are shed into the body cavity at maturity and
completely fill it.
In Glycera the prostomium is narrow and conical, the tentacles
being very small. It possesses a very large proboscis armed with four
sharp teeth. The parapodia are reduced in size, and bear compound
chaetae and in its internal structure too Glycera comes nearer to the
errant worms than does Arenicola.
The excretory and reproductive organs of the Polychaeta
Now that a survey of the chief types of the Polychaeta has been
made a brief description of the segmental organs found in the group
will be given. These are tubes, repeated in successive segments,
which serve to convey the excretory and generative products from the
coelom to the exterior. They are primarily divided into nephridia^
derived from ectoderm, and coelomoducts , formed from mesoderm.
The typical nephridium is a closed tube, whose bhnd end projecting
into the coelom is fringed with solenocytes, cellular organs which have
a very close resemblance to the flame cell of Platyhelminthes and
Rotifera. Such "closed" nephridia (protonephridia) are found in
the Phyllodocidae, Glyceridae and Alciopidae. But in the majority
of the Polychaeta and all Oligochaeta there is another type of
"open" tube, which usually serves for the escape of excreta, and this
possesses a small funnel or nephrostome. It may, however, take over
the function of the coelomoduct and carry sperm or eggs to the
exterior. In this case the nephrostome becomes wider and the tube
more glandular. The familiar example of the open tube is the
nephridium of Lumbricus, which is purely excretory. In this type
276
THE INVERTEBRATA
the tube consists of ectoderm : the funnel of the nephrostome in Lum-
bricus (and probably of other forms) is derived from a single ecto-
dermal cell. The coelomoduct is entirely formed from mesoderm and
usually has a wide coelomic funnel easily distinguished from the
typical nephrostome. The oviducts and the sperm ducts oi Lumbricus
are coelomoducts. In a family of the Polychaeta called the Capitel-
lidae there are coelomoducts in most segments of the body serving as
gonoducts (Fig. 191 I, D). In the majority of Polychaeta they have
co.d-
nep.
-nmx._
cor.
,nephr.
co.d.
Fig. 191. Segmental organs of Polychaeta. After Goodrich. I, Transverse
section (right half) of body segment showing combinations of nephridia and
coelomoducts. A, Hypothetical. B, Phyllodocidaeand Alciopidae. C, Neph-
thyidae and Glyceridae. D, Capitellidae. E, Capitellidae. F, Nereidae.
co.d. coelomoduct; cor. ciliary organ; nep. "closed" nephridium; nep.o.
"open" nephridium; nmx. nephromixium. II, Segmental organ of Vanadis
(Alciopidae). nephr. coelomic funnel; sol. solenocytes.
however disappeared altogether and their function is otherwise
performed.
There is in addition a type of organ called a nephromixium which is
formed by the union of a nephridium and coelomoduct. In the
Alciopidae the separate components of the nephromixium are clearly
seen (Fig. 191 II). Here the union is with the closed nephridium but
in the Capitellidae and many other polychaet families there are
open nephridia and these have often an intimate fusion with the
coelomoducts. Thus in one form of the Capitellidae shown in Fig.
POLYCHAETA
277
191 I, E the funnel of the coelomoduct has completely fused round
the opening of the nephridium.
In Nereis, Nephthys and Glycera the functional segmental organ is
an open nephridium, but a rudiment of the coelomoduct, which
does not open to the exterior, the so-called ciliary organ, occurs in
each segment. In the majority of polychaets, however, the coelomo-
duct has disappeared altogether.
co'.d.
nephr. nepi
nu.
^~"^} — nep.
Fig. 192.
Fig. 192. Segmental organ of G/om/)/zoma (Hirudinea). After Oka. Showing
mesodermal part with ciliated nephrostome and a single cell of the ectodermal
part, with intracellular duct. nu. nucleus.
Fig. 193. Development of Megascolides australis (Oligochaeta). At the
posterior end the nephridia are single ; traced anteriorly they break up into a
number of loops each of which becomes a separate micronephridium (nep.').
al. gut; sep. septa. Other letters as in Fig. 191 for both figures.
Then again there may be a great difference between the nephridia
in different parts of the same worm. In the serpulids, terebellids and
other families there are one to three pairs of long segmental organs
situated anteriorly. In most of the segments behind there are short
funnels in the body wall which are open nephridia but serve for the
escape of the eggs and sperm. There is thus a division of labour
between the segmental organs in tubicolous worms: the anterior are
specialized for excretion, the posterior are genital ducts.
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278 THE INVERTEBRATA
The closed nephridium appears to be the most primitive type of
segmental organ and a survival of the time when the coelom had not yet
developed. The open nephridium is far commoner in the Chaetopoda
with their extensive coelomic cavities. The origin of the coelomoducts
is doubtful. They may be thought to have arisen as genital ducts but
now the nephridia often serve for the escape of the gametes.
The gonads in the polychaets are usually patches of the peritoneal
epithelium, repeated in most of the segments, proliferating until
a great number of the germ cells have been detached into the
body cavity which they almost entirely fill and where they undergo
maturation (Fig. 199 A). When ripe they reach the exterior usually
through the segmental organs, but occasionally the body wall ruptures
and so opens a way of escape.
Like so many other marine animals the polychaets thus liberate
eggs and sperm freely into the sea, fertilization taking place externally.
This habit is associated in many forms with the phenomenon of
swarming in which a worm, usually crawling or burrowing on the
sea bottom, when sexually mature rises to the surface and swims
vigorously, eventually discharging its genital products and sinking to
the bottom as suddenly as it rose. In most nereids this occurs irregu-
larly through the summer months, but in at least two forms (Leodice
viridis, the ** Palolo " of the reefs of the Southern Pacific, and Leodice
fucata of the West Indian reefs) the phenomenon (Fig. 194 E) has
acquired the strictest periodicity. As the day of the last quarter of
the October-November moon dawns the Pacific Palolo breaks oflf the
posterior half of its body, already protruding from the mouth of its
burrow in the coral rock, and these fragments rise to the surface in
such quantities that the water writhes with worms and is later milky
with the eggs and sperm discharged. Immediately afterwards the
remaining anterior end begins to regenerate the missing portion, but
a whole year elapses before the gametes are again ripe — even two days
before spawning occurs fertilization cannot be brought about arti-
ficially. In the West Indian species the phenomenon is similar but
takes place in the third quarter of the June-July moon.
In the syllids the phenomena of swarming are vastly more varied.
The whole animal may produce germ cells and swarm. Usually how-
ever the gonads are confined to the posterior part of the body which is
detached as a free-swimming unit ; this often develops a head but never
jaws and pharynx. It can live for some time but not feed. In the
majority of forms a single bud is produced, but in Autolytus (Fig.
194 B) and Myrianida 2i proliferating region is established at the end of
the original body and from this a chain of sexual individuals is budded
off, the oldest being situated most posteriorly. The whole chain may
be found swimming at the surface, the original worm dragging after
Fig. 194. Diagrams of reproduction in the Polychaeta. A-D, Syllidae.
E, Eunicidae. A, Syllis with the posterior region forming a reproductive in-
dividual. B, Autolytus with a chain of reproductive individuals budded off
successively from pr.r. a proliferating region. C, Trypanosyllis gemmipara,
longitudinal section through the end of a budding stock showing two kinds
of reproductive individual, rs. a single individual which contains al. , the
continuation of the alimentary canal {al.) of the stock, and rs. successive rows
of individuals without alimentary canal formed from the proliferating cushion,
pr.r. D, Syllis ramosa, showing branching of the asexual stock and budding of
reproductive individuals, r.s., r.s. , from parapodia of the stock. E, Diagram
of the swarming of the Palolo worm, Leodice. a, mature female protruding
posterior end from its burrow; b, male — the sexual part of which has just
become detached ; c, sexual fragments swimming up to the surface and d,
discharging the eggs and sperm. In e they are emptied and sink to the bottom ;
/, parent worm regenerating the sexual region.
28o
THE INVERTEBRATA
it the chain of sexual individuals which one by one detach and lead
a short independent existence. In some species of Trypanosyllis
(Fig. 194 C) the zone of proliferation is in the form of a cushion of
tissue on the ventral surface of the last
two segments and this produces not a
linear series of buds but successive trans-
verse rows, amounting to more than a
hundred — the fully formed sexual indi-
vidual possesses a head but no vestige of
an alimentary canal. The extraordinary
branching form, Syllis ramosa (Fig.
194 D), shows remarkable capacity for
heteromorphic growth in the production
of sterile side branches from the stock
and reproductive buds.
In the syllid there is usually no noto-
podium during asexual life but during
the maturation of the gonads the para-
podium is reconstructed, a notopodium
being formed from which spring bundles
of long capillary swimming chaetae,
while a corresponding development of
new muscles takes place. Even greater
is the change in the parapodia of the
maturing nereids. The muscles of the
asexual period break down and the
fragments are digested by leucocytes Fig. 195. A Heteronereis. )^ho-
before the new muscles are formed. The tograph of specimen stained
parapodium of the sexual form, the in borax carmine and mounted
Heteronereis /is produced into memhr^n- in Canada balsam Notice the
- .,, J ^ . , r enormously developed eyes,
ous frills and contains a new type ot j^^^ peristomial cirri, anterior
oar-shaped chaeta (Fig. 184 D, F). The unmodified trunk region, pos-
eyes become immensely larger and the terior modified region with
animal itself very sensitive to light. The parapodia sloping backwards
TT • J • r ^ ui 4-u and darker appearance owmg
Heteronereis does in fact resemble those ^^ ^^^^^^^^ of gonads.
members of the Phyllodocidae and
Alciopidae which have become perma-
nently pelagic. The increase in the surface of the parapodia may
be useful in swimming and floating: it has without doubt some
connection with the increased gas exchange associated with an
active life.
It is easy to see in the swarming habit an adaptation for securing
fertilization of the greatest possible number of eggs. There are
remarkable cases in the syllids (Odontosyllis) where the meeting of
POLYCHAETA 281
the sexes is facilitated by the exchange of light signals, and in the
nereids the discharge of sperm may only be brought about by the
influence of a secretion from the swarming female. Discharge of
the gametes is nearly always followed by the death of the sexual
individual.
The fertilized egg gives rise to an unsegmented larva, the trocho-
sphere, which is described in the next section.
Development of the Polychaeta
The cleavage of the egg in the Polychaeta and the Archiannelida,
the polyclad Turbellaria, the Nemertea and the Mollusca follows
almost exactly the same plan. Division occurs rhythmically, affecting
the whole or greater part of the blastomeres at the same time. The
first two divisions are equal, producing four cells (Fig. 196, 2) lying
in the same plane, which are called A, B,CyD; each cell in its further
cleavage resembles the others and gives rise to one of the quadrants of
the embryo. D tends to be larger than the others and becomes the
dorsal surface of the embryo, while B is ventral, A and C lateral. The
next divisions (third, fourth and fifth) are unequal and at right angles
to the first two and result in three quartettes of micromeres being
divided oflf successively from the macromeres as A, B, C and D are
then termed. The region in which the micromeres lie is the upper or
animal pole of the embryo, while the macromeres form the vegetative
pole. The micromeres are not directly over the macromeres from
which they are formed but in one quartette they are all displaced to
the right, while in the next they will be displaced to the left of the
embryonic radius and the next to the right again. The cleavage is
therefore said to be of spiral type and successive cleavage planes are at
right angles. At a later period it is replaced by cleavage in which
there is no alternation of the kind described above, and the result is
that the embryo becomes bilaterally symmetrical.
The rest of the description is drawn from the Polychaeta but can
be applied with slight modifications to the other groups.
The cells of the first three quartettes give rise to the ectoderm of
the larva and of the adult. The sixth division, however, results in the
separation from the macromeres of a fourth quartette which is com-
posed of cells differing notably in size and density from those of the
first three. Of the fourth quartette d^ (Fig. 196, 4) alone produces
the mesoderm, while the other three, «*, Z>* and c*, reinforce the
macromeres to form the endoderm. The mesoderm is, however; only
in course of differentiation during larval life and a larval mesoderm
or mesenchyme is produced from which particularly the musculature of
the trochosphere is fashioned. The mesenchyme is derived from the
inward projections of cells of the second and third quartettes.
282 THE INVERTEBRATA
Gastrulation (Fig. 196, 7). The amount of yolk in the macromeres
determines the character of the cleavage within certain limits and the
type of gastrulation. In forms like Polygordius with very little yolk
the micromeres and macromeres are nearly the same size and gastru-
lation takes place by invagination; in Aremcola, Nereis and nearly all
Polychaeta and Mollusca the micromeres are much smaller than
the macromeres, and as they divide to form the ectoderm they grow
round the massive macromeres and an **epibolic" gastrula is formed.
The cells of the fourth (and fifth) quartettes approach each other
from the two sides. The mesoblast cell (d^) begins to withdraw from
the surface into the blastocoele, and the blastopore, that is the un-
covered surface of the macromeres, becomes much smaller and slit-like.
Eventually as gastrulation is completed the lips of the blastopore join
in the middle, the same cells meeting each other in every case, leaving
an anterior opening which becomes the mouth and a posterior, which
closes, but in the neighbourhood of which the anus of the trocho-
sphere arises later. The blastopore therefore represents the ventral
surface of the larva. At the same time the macromeres withdraw into
the interior to form a second cavity, the archenteron, bringing with
them the cells of the fourth and fifth quartettes («*, ^^, c^\ a^, c^, d^).
The somatohlast (d^) breaks up into a large number of cells to form
the ventral plate.
The change from gastrula to trochosphere (Fig, 197) follows quickly
and with little further cell division. The first quartette of micromeres
have by this time been diflFerentiated (Fig. 196, 5) into (i) the apical
rosette, consisting at first of four small cells and becoming the apical
organ of the trochosphere ; (2) the cells of the so-called annelid cross
which alternate with those of (i) and form the cerebral ganglia;
(3) the prototroch, forming four groups of cells which constitute the
preoral ciliated ring of the trochosphere; and (4) the intermediate
girdle cells, forming most of the general ectoderm of the part in front
of the prototroch, which is called the umbrella. The expansion of the
subumbrellar ectoderm, i.e. that behind the prototroch, is due to the
proliferation of a single cell in the second quartette of micromeres,
d^ (the somatoblast (Fig. 196, 6)). It forms a plate which spreads from
its originally dorsal position round the sides, the two wings uniting
behind the mouth to form the ventral plate, becoming the ventral
body wall. The descendants of this single cell thus make up nearly
the whole of the subumbrellar ectoderm. Its sisters a^, b^, c^ give
rise to the stomodaeum and are tucked in at the mouth at the close of
gastrulation. This marks the completion of the alimentary canal. The
young trochosphere now possesses a very thin outer epithelium,
thickened in the region of the apical disc and the equatorial ring of
cilia, the prototroch, and in the region of the ventral plate, which is
POLYCHAETA
283
Fig. 196. Partly after Dawydoff. 1-3, Diagrams of radial and spiral cleavage.
4-7, Development of Nereis, i, Eight-cell stage in a radial type, e.g. Echino-
derm. 2, Spiral cleavage, four-cell stage just before cleavage, leading to
eight-cell stage, sp. spindle. 3, Macromeres stippled. 4, Diagram of seg-
menting egg (Nereis), seen from the animal pole, showing the macromeres,
the first three quartettes of micromeres and the mesoblast cell (^*) in the
fourth quartette. 5, Later stage, also^rom animal pole, to show the rosette
cells (a^^-d^^), the annelid cross (indicated by stippling), the four groups of
prototroch cells (horizontal shading and cilia) and the intermediate girdle cells
(a^^-d^^). 6, Vertical section through the same stage as 4, along the line XY.
7, Vertical section through a later stage to illustrate gastrulation : cells derived
from d^ (cross-hatched) growing over the macromeres, the mesoblast cell
withdrawing into the interior.
284
THE INVERTEBRATA
the rudiment of a large part of the trunk of the aduh worm. It will
form ventral nerve cord, chaetal sacs and the ventral and lateral ecto-
derm of the trunk. The larval gut opens by a mouth in the equatorial
region and consists of an ectodermal oesophagus (stomodaeum) open-
ing into the endodermal stomach and an ectodermal hind gut opening
to the exterior by an anus. The cavity between the ectoderm and the
gut (blastocoele) is spacious and traversed by the pseudopodia-like
processes of the mesenchyme cells, larval muscles and nerves, and
also contains the two larval nephridia, each of which is composed of
two hollow cells placed end to end, one of which contains a "flame'*
of cilia. They are descended from the first quartette of micromeres
and sink in from the surface.
mesc
^-prt.
DCS.
an.v.^
Fig. 197. Trochosphere larva of £'M/)owaiw^. Side view. After Shearer, ap.o.
apical organ; e. eye; prt. preoral ciliated ring; hk. "head kidney", larval
nephridium ; otc. otocyst ; mes. mesodermal band ; an. anus ; an.v. anal vesicle ;
bl.c. blastocoele ; mesc. mesenchyme ; oes. oesophagus ; st. stomach.
The trochosphere drifts hither and thither in the sea, swimming
feebly by the action of the ciha of the prototroch and sometimes also
by secondary postoral rings of cilia (e.g. metatroch formed from cells
of the third quartette). During this pelagic existence the rudiments of
the adult worm continue their development — which is best traced in
Polygordius — the apical organ develops into the prostomium of the
adult with brain, tentacles and eyes, while the trunk rudiment formed
by the proliferation of the ventral plate and the mesoblast cell grows
backwards as an ever-lengthening cyHndrical process containing the
end gut. In the ectoderm of this is developed ventrally the rudiment
of the ventral nervous system, while to the sides of this and internally
are the mesodermal strips (derived from the single cell d^), which
POLYCHAETA 285
show at once metameric segmentation (Fig. 198), first as pairs of solid
blocks, then with cavities, to form the somites. Each of these box-like
mesodermal segments has then an inner wall which is applied to the
gut (splanchnic mesoderm) and an outer (somatic mesoderm) lying
under the ectoderm. The right and left rudiments meet in the middle
lane and are only separated by the dorsal and ventral mesenteries
which are formed by their apposed walls, while the anterior and
posterior borders of each segment are septa. At the same time the
adult nephridia develop from ectoderm rudiments and the blood
vessels differentiate in the septa and mesenteries.
A
B
Fig. 198. Development of PoZy^orJm^. After Woltereck. A, Trochosphere
with rudiment of prostomium and trunk. B, Metamorphosing larva with the
prostomium and trunk brought close together by the contraction of the
longitudinal muscles and the umbrella of the trochosphere shrivelled and
about to be discarded. Three segments only of the trunk are shown, brn.
brain ; e. eye ; m.l. longitudinal musculature ; m.l. part of the same which by
contraction brings the prostomium and trunk rudiments into contact; M.
mouth ; nep. protonephridium with solenocytes ; pre. prostomium ; prt. pro-
totroch ; mtr. metatroch ; oe.c. oesophageal commissure ; ten. tentacle.
The advanced larva (Fig. 198 A) thus consists of two rudiments of
the adult body, separated by the body of the larval trochosphere.
They are joined by a pair of longitudinal muscles and of nerves, and
in one species of Polygordius metamorphosis of the larva into the
adult is brought about by the shrivelling up of the larval tissues and
the drawing together and the union of the head and trunk assisted by
the contraction of these muscles (Fig. 198 B). The larval mouth re-
mains in the adult. After metamorphosis the animal sinks to the
bottom and begins its adult life.
286 THE INVERTEBRATA
Order OLIGOCHAETA
Chaetopoda, nearly all land and freshwater forms, with a compara-
tively small number of chaetae, not situated on parapodia, with pro-
stomium distinct but usually without appendages ; always hermaph-
rodite, the male and female gonads being few in number (one or
two pairs), situated in fixed segments of the anterior region, the male
always anterior to the female; with special genital ducts (coelomo-
ducts) opening by funnels into the coelom, spermathecae , and a
clitellum present at sexual maturity ; with reproduction by copulation
and cross-fertilization; eggs being laid in a cocoon, developing
directly without a larval stage.
In addition the pharynx is not eversible and pharyngeal teeth (such
as frequently occur in the Polychaeta) are absent, except in one small
family, the Branchiobdellidae, which have ectoparasitic habits similar
to the leeches and resemble them in some particulars of structure.
Though the chaetae are not borne on parapodia they are usually
divided into two bundles or groups on each side which roughly
correspond to the noto- and neuropodia. They may be classified into
hair chaetae which are long and fine (dorsal chaetae of Stylaria) and
shorter chaetae which are rod-like (Lumbricus) or needle-like. The
point of the needle is single- or double-pronged. There is not, how-
ever, the great variety found in the Polychaeta.
Certain main features of the reproductive system (Fig. 199) are the
salient characters of the group. Its members are, without exception,
hermaphrodite, and with a single possible exception cross-fertilization
only is possible. The restriction of the gonads to a few segments occurs
also in some sabellids among the Polychaeta and in some archiannelids.
The sexual cells are shed into the coelom either into the general
coelomic cavity as in the Polychaeta or into special parts of it divided
off from the rest (seminal vesicles of Lumbricus) where they mature.
Spermathecae are usually present to contain the spermatozoa received
from another worm in copulation. The clitellum is a special glandular
development of the epidermis whose principal function is the secretion
of the substance of the cocoon and the albuminoid material which
nourishes the embryo. It is a secondary sexual character which is only
present in the reproductive season in most Oligochaeta, but the earth-
worms [Lumbricus^ Allolobophora) used in zoological laboratories in
this country always possess it. Both the clitellum and the cocoon pro-
duced by it are found in the Hirudinea. It may also be mentioned that
many oligochaets have special copulatory chaetae, sometimes hooked
for grasping the other worm or with a sharp point for piercing it.
For the purposes of the elementary student it is probably best to
recognize that the Oligochaeta contain two well-marked oecological
OLIGOCHAETA 287
types, the ''earthworm", a larger burrowing terrestrial form, and the
aquatic oligochaet which is much smaller and simpler in structure.
It is probable that the former type is the more primitive ; the aquatic
oligochaet shows many characters which resemble those of the archi-
annelids and are most likely due to a process of simplification. The
reasons for the conclusion that the aquatic oligochaets are not the
oldest of these groups are given below.
The Earthworms
These are divided into a number of families of which the most
important are the Lumbricidae, containing Lumbricus and Allolobo-
phora^ and the Megascolecidae which is the largest of all.
The primitive forms in all families resemble Lumbricus in the
following characters. There are a large number of segments and each
one is furnished with eight chaetae arranged in pairs and all on the
ventral side of the worm. A series of dorsal pores is found along the
back in the intersegmental grooves. The alimentary canal is cha-
racterized by a large muscular pharynx by which the food is sucked
in, with many glands, the secretion of which is used in external
digestion. The oesophagus in one part of its length gives rise to one
or more pairs of diverticula, the cells of which secrete carbonate of
lime {oesophageal pouches and glands). At the end of the oesophagus
or the beginning of the intestine there is a thick-walled gizzard in
which the food is masticated with the aid of the soil particles. The
intestine has a dorsal ridge, the typhlosole^ to increase the absorptive
surface. The nervous, muscular and circulatory systems exist through-
out the earthworms with little variation from the condition in
Lumbricus.
The reproductive system (Fig. 199 C) consists essentially of two
pairs of testes in segments 10 and 11 and one pair of ovaries in
segment 13, followed by ducts which open by large funnels just be-
hind the gonads and discharge to the exterior in the next segment in
the case of the oviduct, and several segments behind in the case of the
sperm duct. The testes, at least, are enveloped by sperm sacs (vesiculae
seminales) which are outgrowths of the septa, and in the cavity of
these the sperm undergo development. In some earthworms there are
no sperm sacs and this condition, resembling that in the Polychaeta, is
probably the earliest in the group. There are two pairs of spermathecae
in the region in front of the testes. In the neighbourhood of the male
external aperture there are spermiducal [prostate) glands which do not
actually open into the sperm duct. A single pair of segmental organs
(open nephridia) is present in each segment.
The variations which occur in more specialized members of all
families are as follows. The chaetae may increase in number and come
A
8pth.}j^
Fig. 199. Reproductive organs of the Chaetopoda. A, Polychaeta (longi-
tudinal section of Serpula intestinalis to one side of the middle line). Original.
Oligochaeta. B, Naididae, diagrammatic. After Stephenson. C, Lumbricus
terrestris, diagrammatic. After Hesse, at. atrium ; cl. clitellum ; ect. ectoderm ;
m.c. circular and m.l. longitudinal muscles; o. ovary; od. oviduct; o.s. ovisac;
ov. ovum; s.f. funnels of vas deferens; sp.s. sperm sac; s.v. seminal vesicle;
spth. spermatheca ; t.s. testis sac ; t. testis ; v.d. vas deferens. D, dorsal and
V, ventral. The numbers are those of the segments, the vertical lines are septa.
OLIGOCHAETA 289
to be arranged in a complete ring round the body [perichaetine). The
dorsal pores may disappear. The oesophagus may lose its calciferous
glands and the gizzard may be absent or develop into several. The
reproductive organs vary in small but important particulars. There
are nearly always two pairs of testes in segments 10 and 11 and one
pair of ovaries in segment 13, but the testes may be reduced to a single
pair. There are usually two pairs of spermathecae but the number
varies and occasionally they are absent altogether. Th^ prostate glands
(of unknown function) are nearly always present in earthworms
except in the Lumbricidae.
The simplest method of copulation in earthworms is that found in
Eutyphoeiis, where the end of the sperm duct can be everted to form
a penis. This is inserted into the spermathecal apertures and the
spermatozoa thus pass directly from one worm to another. It is
obvious that the mechanism of copulation is far more complicated in
the Lumbricidae. Here the worms come into contact along their
ventral surfaces and each becomes enveloped in a mucous sheath. Close
adhesion is secured between the clitellum of one worm and the seg-
ments 9 and 10 of the other, partly by embracing movements of the
clitellum and partly by the chaetae of the same region being thrust
far into the body wall of the partner. The sperm passes out of the
male aperture and along the seminal groove to the clitellum ; how it
enters the spermathecae of the other worm has never been observed.
The cocoons are formed some time after copulation. The worm
forms a mucous tube as in copulation. The cocoon is then secreted
round the clitellum and finally the albuminous fluid which nourishes
the embryo is formed between the cocoon and the body wall and the
worm frees itself from the cocoon by a series of jerks. All three
products, mucus, cocoon substance and albumen, are secreted by the
clitellum and each probably by a distinct type of cell. The eggs are
sometimes extruded and passed backwards into the cocoon while it
is still in position on the clitellum but the spermathecae eject the
spermatozoa when the cocoon passes over them.
The embryo oi Lumbriciis is illustrated in Fig. 200. The prototroch
is absent but the gut and stomodaeum are developed early to absorb
the albumen in the cocoon. There are two mesoblast pole cells at the
hinder end which bud off the mesodermal strips: there are three
ectodermal pole cells on each side, the ventralmost a neuroblast
forming half the nerve cord and the two others nephroblasts giving rise
to longitudinal rows of cells which divide up to form the nephridia.
A primitive kind of nephridium in the Oligochaeta is that de-
scribed in Lumbricus, of which there is a pair for each segment, the
nephrostome projecting through the septum and opening into the
cavity of the segment in front. A great many modifications of this
290
THE INVERTEBRATA
arrangement exist especially in the Megascolecidae. Here, in addition
to the type already described which is distinguished as a mega-
nephridium, there are micronephridia of which enormous numbers may
exist in a single segment (2500 in Pheretima). These are small tubes
which may or may not open into the coelom by a nephrostome. They
may exist in the same segment as a pair of meganephridia. There is
^end.
Fig. 200. Embryo of Lumbricus foetidus. After E. B. Wilson. A, Lateral
view of an embryo in which the mesoblast is unsegmented. B, Ventral view
of the same embryo. C, Longitudinal section of a later embryo a little to one
side. D, Transverse section of ventral part of the same embryo along the
line XY in C. brn. brain; coe. coelomic cavity of mesoblastic somites; ect.
ectoderm; end. endoderm; ent. enteron; M.t. mesoblastic teloblast; npb.
nephroblasts ; nrb. neuroblasts ; nep. nephridia ; sep. septa ; std. stomodaeum.
good evidence for supposing that an originally single meganephridium
has been broken up into a multitude of micronephridia. In the
development of the earthworm Megascolides the segmental organs
first appear as cords of cells like meganephridia. These are thrown
into a series of loops and each loop is separated from the rest as a
micronephridium.
OLIGOCHAETA 29I
Other modifications are those in which the nephridia open into the
ahmentary canal instead of to the exterior. They may ho. peptonephridia^
opening into the interior part of the ahmentary canal ; whether they
have a digestive function is not known. On the other hand they may
unite to form a longitudinal duct (or ducts) which discharges into
the hind end of the intestine. Whether there is any physiological
meaning for the variations in the segmental organs of the earthworms
is entirely unknown.
There is a well-developed blood circulation. Blood flowing through
the parietal and dorso-intestinal vessels of each segment is collected
in the dorsal vessel. It is prevented from returning by an elaborate
system of valves (Fig. 201). Waves of peristaltic contraction beginning
at the hind end of the dorsal vessel and continued by the "hearts"
press it forwards and ventralwards into the ventral vessel which is the
main distributing channel.
The aquatic Oligochaets
As a type of these, Stylaria, belonging to the family Naididae, will
be shortly described (Fig. 202). This is a transparent worm rather
less than a centimetre long found crawling on water weed. The
prostomium bears minute eyes and is produced into a long filiform
process. In most of the segments there are two bundles of chaetae
on each side, the dorsal consisting of hair chaetae and needle chaetae,
while the ventral has only "crotchets" with a double point. The first
four segments have no dorsal bundles (incipient cephalization).
The alimentary canal is simpler in character than that of Lumhricus,
3. gizzard being absent. The intestine is ciliated and the action of the
cilia brings in from the anus a current of water which probably assists
respiration. The testes (Fig. 199 B) develop in segment 5 and the
ovaries in segment 6, while a pair of spermathecae is found in the
testis segment. The sexual cells develop in the seminal vesicle and
the ovisac which are unpaired backward pouchings of septa 5/6 and
6/7 respectively. The male ducts open by a funnel on septa 5/6 and
discharge into an atrium, which is lined by the cells of the prostate.
While sexual individuals are often met with and can be recognized
at once by the appearance of the opaque clitellum in segments 5-7,
individuals reproducing asexually are much commoner. Chains of
worms attached to one another may be found, and the existence of
one or more zones of fission, where new segments are being formed
and separation of two individuals will take place, is easily observed
under the microscope.
Stylaria is a delightful object of study. The operation of many of
the organs can be easily observed with a low power and the results
292
THE INVERTEBRATA
form a useful supplement to work with Lumbricus in understanding
oligochaet organization.
d.i.v.
Fig. 201,
Fig. 202.
Fig. 201. Dorsal vessel oi Lumbricus to show connections and valves. After
Johnston, d.v. dorsal vessel ; vessels leading to dorsal vessel ; d.i.v. from sub-
intestinal vessel, p.v. from subneural vessel (parietal vessel); sep. septum;
va. valves open with dilation and va. closed with contraction of the dorsal
vessel.
Fig. 202. Stylariaproboscidea. Original. Dorsal view. /ew. median prostomial
process ; cr. crop ; e. eye ; M. mouth ; ph. pharynx ; oe. oesophagus ; int. intestine
(stippled); sep. septum. The four anterior segments have hooked ventral
chaetae {ch.v.) only, the rest with long dorsal hair chaetae (ch.d.) as well.
From the above account it will be seen that Stylaria differs from
Lumbricus not only in its small size and transparency but also in the
number and appearance of the chaetae — which give it a certain
OLIGOCHAETA
293
resemblance to the Polychaeta. The reproductive organs, however,
are entirely different from those of the latter group and it is in this
system that the real contrast between polychaet and oligochaet lies.
The aquatic oligochaets when they are of small size often show re-
duction of the vascular system, ciliation of the under surface (in one
form, Aeolosoma), and a nervous system of embryonic type. These are
characters which may be primitive but, as in the archiannelids, so
here, they are probably the results of simplification; it is generally
agreed that the replacement of sexual by asexual reproduction is a
secondary feature, and the frequency with which it is found in the
aquatic Oligochaeta shows them to be, on the whole, specialized types.
Fig. 203. Blood circulation in Lumhriculus variegatus. After Haffner. A,
Head and anterior region showing dorsal and ventral vessels joined by a
network of vessels round the gut. B, Single segment of the middle region
with a much closer plexus. C, Posterior end with a continuous sinus round
the gut connected at intervals with the dorsal and ventral vessels. An. anus ;
bl. blind contractile sac of the dorsal vessel {d.v.) ; M. mouth ; pi. plexus ;
sin. sinus; v.v. ventral vessel.
Two common genera, Tubifex and Lumhriculus, are larger worms
which in their appearance have more resemblance to earthworms.
A brief description of them follows.
Tubifex. A small red worm with rather numerous chaetae in the
dorsal and ventral bundles belonging to various types; without
gizzard; testes and ovaries in segments 10 and 11 respectively.
It lives in the mud at the bottom of ponds and lakes with its head
buried and its tail waving in the water; the latter movements are
respiratory. They draw water from upper layers which contain more
294 THE INVERTEBRATA
oxygen : when the oxygen content of the water in general falls a greater
length of the worm is protruded and its movements become more
vigorous. A great deal of detritus passes through its alimentary canal
so that Tubifex plays the same sort of part in fresh water that the
earthworms play on land.
Lumbriculus resembles Tubifex superficially but has only eight
chaetae in a segment, placed as in Lumbricus; chaetae double pointed;
not often met with in sexual state but reproduces habitually by
breaking up into pieces each of which regenerates the missing
segments.
In this worm the primitive nature of the blood system is well seen
(Fig. 203). At the posterior end there is a continuous sinus round the
gut, in the middle region this becomes resolved into a dense plexus
of capillaries and at the anterior end there is the beginning of a seg-
mental arrangement.
Class ARCHIANNELIDA
Small marine annelids with simplified structure, parapodia and chaetae
being usually absent.
This group was founded to receive two genera, Polygordius and
Protodrilus, which were formerly considered to be primitive forms
from which the larger groups of annelids might be derived. From
time to time other genera have been included which show some, but
not all, of the characters which distinguish the original genera. The
series of diagnoses of the best known genera given below starts with
Polygordius and works back to forms which come very close to the
Chaetopoda. There can be little doubt that the Archiannelida are
derived from this latter group by the loss of some of its distinctive
features (e.g. parapodia and chaetae), and retention of juvenile
characters (ciliation and connectionof nervous system with epidermis).
These changes are also found within the limits of the Polychaeta, and
if it was not that other characters link up its members the group might
well be considered as a family of polychaets. Dinophilus comes late
in the series because, though evidently related, it does stand rather
apart. It has a superficial resemblance to a small turbellarian
enhanced by the great reduction of the coelom.
Polygordius (Fig. 198 B) with elongated cylindrical body, head
with two tentacles and ciliated pits; without parapodia or chaetae;
with segments of the coelom separated by septa with a pair of seg-
mental organs opening into each by nephrostomes; with longitudinal
muscles in four quadrants, the circular muscles being usually absent;
with a reducedvascular system and nerve cords lying in the epidermis ;
with a trochosphere larva, Fig. 198 A.
ANNELIDA
295
Protodrilus . As in Polygordius but with segmentation marked ex-
ternally by ciliated rings and with a longitudinal cihated groove in the
middle of the ventral surface ; with a ventral muscular pharyngeal sac ;
hermaphrodite.
,ten.
Fig. 204. Examples of the Archiannelida. A, Nertlla, dorsal view of female:
A', parapodium. B, Saccocirrus, side view of anterior end. C, Histriobdella,
dorsal view of male. D, Dinophilus, dorsal view of male. amp. ampulla of ten-
tacle {ten.)\ a.f. anterior foot; ch. bundle of chaetae; cl. clasper; cil.b. bands
of cilia ; e. eye ; gen.s. genital segment ;]. jaw ; /. eyes ; nep. nephridia ; o. ovary ;
od. oviduct ; p. penis ; ph. pharynx ; p.f. posterior foot ; p.p. palp ; rm. rectum ;
St. stomach; v.s. vesicula seminalis; f.. testis. A, B, after Goodrich; C, after
Shearer; D, after Harmer.
A single species, P. chaetifer^ has recently been discovered with four
short chaetae in each segment.
Saccocirrus (Fig. 204 B). As in ProtodriluSy but with chaetae
296 THE INVERTEBRATA
arranged in a single bundle on each side of each segment ; with separate
sexes, each with complicated genital apparatus, the females with
spermathecae and males with a pair of protrusible penes in each
segment behind the oesophagus.
Nerilla (Fig. 204 A). As in Protodrilus, but with two bundles of
chaetae separated by a single cirrus on each side of each segment;
three prostomial tentacles and a pair of palps ; with separate sexes and
a reduced number of genital segments (three in male, one in female),
three pairs of sperm ducts uniting at a common median genital
aperture, and two oviducts with separate genital apertures.
Dinophilus (Fig. 204 D) with very short flattened body consisting
of only five or six segments, a ciliated ventral surface and ciliated ring
in every segment; without septa, dorsal and ventral mesenteries, and
a vascular system; with greatly reduced coelom and longitudinal
muscles; five pairs of *' closed" nephridia; separate sexes, male with
median penis injecting spermatozoa into female through skin, female
with eggs of two sizes, the smaller giving rise to males and the larger
to females.
Histriobdella^ which may be mentioned here (Fig. 204 C), is a
parasite of the eggs of the lobster, having no chaetae but two pairs
of "feet" by which it executes acrobatic movements. It resembles
Dinophilus in its reduced coelom and musculature but has jaws, and
from the structure of these it has been claimed that Histriobdella is a
much modified polychaet belonging to the family Eunicidae.
The value of the Archiannelida to the elementary student of
zoology is that they illustrate an evolutionary process which may be
called simplification or reduction (but not degeneration), and which
is not unlike the changes which parasitic forms have undergone.
Class HIRUDINEA
Annelida with a somewhat shortened body and small, fixed number
of segments, broken up into annuli and without chaetae (except in
Acanthohdella) or parapodia ; at the anterior and posterior ends several
segments modified to form suckers; coelom very much encroached
upon by the growth of mesenchymatous tissue and usually reduced to
several longitudinal tubular spaces (sinuses) with transverse com-
munications. Hermaphrodite, with clitellum. Embryo develops in-
side cocoon.
In the typical leeches the constitution of the body is remarkably
constant. There is a prostomium and thirty-two body segments; an
anterior sucker (in the centre of which is the mouth) is formed from
the prostomium and the first two segments, and a posterior from the
last seven. Both suckers are directed ventrally. The subpharyngeal
ANNELIDA
297
"ganglion" (Fig. 205 B) is composed of four single ganglia fused
together and the posterior "ganglion" of seven. Between them lie
twenty-one free ganglia, and the number of segments is estimated by
summation of all the ganglia. The number of annuli to a segment
varies in different forms.
Fig. 205. Anterior part of nervous system in A, Lumhricus. After Borradaile.
B, Hirudo. After Leydig. The brain in both consists of a single dorsal pair
of gangHa belonging to the prostomium. In Lutnbricus the subpharyngeal
ganglion (sbp.) and lower part of the circumpharyngeal commissures give
off nerves to segments i (peri.) peristomium, 2, 3 and so belong to three
segments. In Hirudo the subpharyngeal mass consists of four (or five) pairs
of ganglia fused together, e. eyes ; M.c. mouth cavity ; 7. jaws ; pr. prostomium ;
ph. pharynx (with network of visceral nerves) ; so. sense organs. /
The alimentary canal is highly characteristic and consists of the
following parts, (i) A muscular pharynx with unicellular salivary
glands. In the Gnathobdellidae, which includes Hirudo, there are
three chitinous plates or jaws. In the Rhynchobdellidae (Fig. 206),
there is a protrusible ^ro^o^m surrounded by 2i proboscis sheath. (2) A
short oesophagus follows, leading into (3) the mid gut (crop) which is
298 THE INVERTEBRATA
often provided with lateral coeca, varying in number, and is used for
storing up the blood or other juices of the host. This is kept from
coagulating by the ferment (anticoagulin) contained in the salivary
secretion [Hirudo). In the mid gut a very slow digestion takes place,
the blood appearing almost unchanged even after several months.
(4) An intestine, which is also endodermal, and has, in Hirudo, a pair
of diverticula. (5) A very short ectodermal rectum discharging by the
anus, which is dorsal to the posterior sucker.
The body wall consists of a single layer of ectodermal cells between
which blood capillaries penetrate, a dermis with pigment cells and
blood vessels, and an outer circular and inner longitudinal layer of
muscles. The muscle fibres have a characteristic structure, consisting
rm
Fig. 206, Glossiphonia as example of the Rhynchobdellidae. Dorsal view.
an. anus ; cr. crop (black) ; int. intestine (stippled) ; oe. oesophagus ; pb. pro-
boscis; ps. proboscis sheath; rh. rhynchodaeum ; rm. rectum; sa.gl. salivary
glands.
of a cortex of striated contractile substance and a medulla of un-
modified protoplasm. Inside the musculature are masses of mesen-
chymatous tissue : in the Gnathobdellidae this is pigmented and forms
the botryoidal tissue, the cells of which are arranged end to end and
contain intracellular capillaries filled with a red fluid.
The mesenchyme almost completely occupies the space which is
the perivisceral cavity in the earthworm. There are, however, longi-
tudinal canals, constituting the sinus system, and these represent the
remnants of the coelomic spaces ; there are always dorsal and ventral
and often (e.g. Glossiphonia, Fig. 207 B) two lateral sinuses, and there
are numerous transverse canals in each segment. Into this reduced
coelom the nephrostomes open and the gonads are found in it. The
HIRUDINEA
299
blood system consists of two contractile lateral vessels (and in the
Rhynchobdellidae of dorsal and ventral vessels running inside the
corresponding coelomic spaces). These vessels all communicate with
m.c.
ect.
t. n.s.
v.s
n.c.
Fig. 207. Transverse sections of Hirudinea to show the progressive restriction
of the coelom. A, Acanthobdella, B, Glosstphonia, C, Hirudo. In A the coelom
{coe.) is continuous but encroached upon by growth of parenchyma (stippled).
In B it is broken up into a system of sinuses, d.s. dorsal; v.s. ventral; h.s.
hyperdermal sinus ; l.s. lateral and i.s. a network of intermediate sinuses. In
C the sinuses (outlined in black) are reduced in size, and there is no inter-
mediate network, n.s. the nephrostomial sinuses, branches of the ventral sinus,
contain the testes {t)\ botryoidal tissue {h.t.) is present; ch. chaetae; cm.
coecum; cr. crop; d.v. dorsal, l.v. lateral, v.v. ventral blood vessel; ect. ecto-
derm; gl. glands; ni.c. circular, m.l. longitudinal muscles; nep. nephridium;
n.c. nerve cord ; oe. oesophagus ; per. peritoneum ; s.o. sense organs.
one another. They also communicate with the sinuses of the coelom
and with the capillaries of the botryoidal tissue, as has been shown
by careful injection. This astonishing condition is unique, but a
parallel may be drawn with the vertebrate in which the lymphatic
300 THE INVERTEBRATA
system communicates both with the coelom and the blood system.
The pecuhar functions of the lymphatic system are not shared by
the botryoidal vessels which have no particular connection with the
The nervous system is of the usual annelidan type but characterized
by the fusion of ganglia anteriorly (Fig. 205) and posteriorly. There
are segmental sense organs in the form of papillae, and on the head
some of these are modified to form eyes and the so-called "cup-
shaped organs".
The nephridia consist of two tubes, one ending in a nephrostome,
the other with an external aperture ; their lumina do not communicate
(Fig. 192); the nephrostomes open into a branch of the ventral or
the lateral sinus. The testes^ of which there are often several pairs
(nine in Hirudo), and the single pair of ovaries are also present as closed
vesicles in the sinuses and are derived from the coelomic epithelium,
but in distinction from the rest of the annelids they are continuous
with their ducts. The separation of the genital part of the coelom from
the rest, begun in the Oligochaeta, here becomes complete. The
testes discharge into a common vas deferens on each side; the two
vasa unite anteriorly to form a median penis. Similarly the two
oviducts join and the eggs pass through a single albumen gland and
vagina to the exterior. The spermatozoa, united in bundles, are
deposited on the body of another leech and appear to make their way
through the skin to the ovaries where fertilization occurs. The eggs
are laid in cocoons, the case of which is formed by clitellar glands in
the same way as in Lumbricus.
The Hirudinea may be divided as follows :
AcANTHOBDELLiDAE, a family intermediate between the Oligochaeta
and the Hirudinea, containing the single genus Acanthobdella.
Rhynchobdellidae, marine and freshwater forms, with colourless
blood, protrusible proboscis and without jaws.
Gnathobdellidae, freshwater and terrestrial forms, with red
blood without a protrusible proboscis but usually with jaws.
Family Acanthobdellidae.
Acanthobdella (Fig. 207 A), a parasite of salmon, is a hnk with the
Oligochaeta. In it the specialized hirudinean characters are only partly
developed. There is no anterior sucker but a well-developed posterior
sucker formed from four segments. The total number of segments is
twenty-nine compared with thirty-two in the rest of the group. There
are dorsal and ventral pairs of chaetae in the first five body segments
and the coelomic body cavity is a continuous perivisceral space, in-
terrupted only by segmental septa as in the Oligochaeta. It is, however,
restricted by the growth of mesenchyme in the body wall and split
ANNELIDA 30I
up into a dorsal and ventral part in the clitellar region. The so-called
testes (really vesiculae seminales) are tubes running through several
segments, filled with developing spermatozoa and their epithelial wall
is continuous with that of the perivisceral coelom, another primitive
feature. The vasa deferentia, moreover, open into the testes by typical
sperm funnels.
It is interesting to find that in the Branchiobdellidae, a family of
the Oligochaeta, parasitic on crayfish, there is the same sort of leech-
like structure: a posterior sucker, annulated segments, absence of
chaetae and presence of jaws. But the condition of the coelom,
nephridia and generative organs is so like that of the Oligochaeta that
the family must remain in that group.
Family Rhynchobdellidae.
Pontohdella, parasitic on elasmobranch fishes.
Glossiphonia (Fig. 206), a freshwater leech feeding on molluscs like
Limnaea and Planorbis and on the larvae of Chironomus ; body ovate
and flattened ; hind gut with four pairs of lateral coeca ; eggs laid in
the spring, the young when hatched attaching themselves to the
ventral surface of the body of the mother.
Family Gnathobdellidae.
Hirudo, the medicinal leech, at one time a common British species
but now extinct ; jaws armed with sharp teeth.
Haemopis^ the horseleech, common in streams and ponds, which it
leaves to deposit its cocoons and in pursuit of prey ; jaws armed with
blunt teeth, which cannot pierce the human skin; a single pair of
coeca in the mid gut.
This leech is carnivorous, devouring earthworms, aquatic larvae of.
insects, tadpoles and small fish. The land leeches of the tropics, of
which Haemadipsa may serve as an example, live in forests and swamps
and, mounted on leaves and branches, wait until a suitable mam-
malian prey presents itself.
The following classes, the Echiuroidea and the Sipunculoidea,
were formerly classed together as the Gephyrea. There is, however,
good reason for separating them in spite of their general similarity,
which is possibly due to the fact that they are both composed of
burrowing animals and have lost their segmentation.
Class ECHIUROIDEA
Annelids which show few signs of segmentation, with a spacious
coelomic cavity, a well-developed prostomium, a terminal anus, a
single pair of ventral chaetae, sometimes several pairs of segmental
organs, and in Echiurus a trochosphere larva in the nervous system of
302
THE INVERTEBRATA
which there appear to be as many as fifteen pairs of ganglionic
swelHngs (Fig. 208).
Echiurus, with a spoon-shaped prostomium, two pairs of segmental
organs and a trochosphere larva.
Bonellia (Fig. 209 A, B) with a prostomial proboscis bifurcated at
m.
mes.
Fig. 208. Echiuriis. Ventral view of larva to show segmentation of posterior
end. After Baltzer. a. anus ; ch. chaeta-forming cell ; com. neural commissure ;
l.m. longitudinal muscle; m. mouth; rnes. mesoderm ; /)w. larval nephridium
with solenocytes ; vn. ventral nerve cord composed of many neuromeres.
the end, capable of enormous elongation and extremely mobile; a
single segmental organ (brown tube) ; the female is the typical in-
dividual and the males are reduced to small ciliated organisms, like
a turbellarian, which live in the segmental organ of the female.
It is now known that larvae of Bonellia carry the potentialities of
both sexes. If they develop independently they become females. If
ANNELIDA
303
m.retr.
Fig. 209. Bonellia viridis. A, Female. B, Male from nephridium of female.
After Spengler. C, Sipunculus. From Shipley and MacBride. a. frilled
membrane surrounding the mouth ; al. alimentary canal ; al. degenerate
alimentary canal of male; an. anus; an.v. anal vesicle; ht. brown tube
(nephridium); ch. position of chaetae; cil.gr. ciliated groove; an. coecum of
gut; d.v. dorsal blood vessel; e. cut ends of intestine; g. anal glands; m.retr.
retractor muscle of anterior end ; M. mouth; n.c. nerve cord; nephr. nephro-
stome; oe. oesophagus; o. ovary; op. o male reproductive aperture; pr.
greatly enlarged prostomium; sp. spermatozoa.
304 THE INVERTEBRATA
they should come into contact with the body of the adult female, she
exercises (probably through the action of some specific secretion) a
largely repressive effect on further development, but a male gonad
is formed.
Thalassema. The British representative, T. neptuni, has a single pair
of segmental organs. In two Japanese species, T. taenioides and T.
misakiensis, these have been greatly multiplied so that in the former
there are 200 pairs rather irregularly arranged. From a consideration
of these forms it appears that the multiplication of the segmental
organs is a secondary phenomenon.
Class SIPUNCULOIDEA
Annelids with a spacious uninterrupted coelomic cavity and few signs
of segmentation: without prostomium in adult; chaetae always ab-
sent, anterior part of body invaginable into posterior part ; anus dorsal
and anterior ; with a single pair of segmental organs (brown tubes) ;
in Phascolosoma a trochosphere with three pairs of mesoblastic somites
which soon disappear.
Sipunculus (Fig. 209 C) and Phascolosoma are British genera.
CHAPTER X
THE PHYLUM ARTHROPODA
Bilaterally symmetrical, segmented Metazoa; with, on some or all of
the somites, paired limbs, of which at least one pair function as jaws;
a chitinous cuticle, which usually is stout but at intervals upon the
trunk and limbs flexible so as to provide joints; a nervous system
upon the same plan as that of the Annelida ; the coelom in the adult
much reduced and replaced as a perivisceral space by enlargement of
the haemocoele ; without true nephridia, but with one or more pairs
of coelomoducts as gonoducts and often as excretory organs; and
(except in Peripatus) without cilia in any part of the body.
The Arthropoda have much in common with the Annelida, and
must be regarded as derived from the same stock as the Polychaeta
in that phylum. The key to most of their peculiar features is an in-
crease in the thickness of the cuticle. This brings with it the necessity
for joints ; and the stout, jointed limbs can now be adapted for various
purposes to which those of polychaetes were not convertible. Always
at least one pair of them become jaws ; with this is usually associated
the specialization for sensory functions of one or two pairs which have
come to stand in front of the mouth, and thus the process of cephaliza-
tion, begun in the polychaetes, proceeds further here. Other limbs
commonly become legs. In order to move the complex of hard pieces
constituted by the jointed cuticle, the continuous muscular layer of
the body wall of an annelid has become converted into a system of
separate muscles ; with this, and with the fact that turgescence of the
body wall is no longer a factor in locomotion, is perhaps connected
the replacement of the perivisceral coelom by a haemocoelic space.
The loss of the nephridia which in annelids lie in the coelom is prob-
ably due to the reduction of that cavity. An interesting feature of
difference between the Arthropoda and Annelida is the absence from
the former phylum of the chetae, imbedded in and secreted by pits
of the skin, which characterize the annelids; though bristles, formed
as hollow outgrowths of the cuticle, are common on arthropods. This
difference, too, may be connected with the difference in the stoutness
of the cuticle. Lastly, it is perhaps that thick covering, hindering the
loss of water by evaporation from the surface of the body and pro-
viding the skeleton which the lack of support from the medium
necessitates, which has enabled arthropods very successfully to invade
the dry land. Like those of all other phyla, their earliest known
members, the trilobites, were aquatic. Of their surviving groups.
SOMITES AND LIMBS
Somite
Onychophora
Arachnida
Scorpionida
Trilobita
I...*
Preantennae
Embryonic
?
2...
Jaws
Chelicerae
Antennae
3-.-
Oral Papillae
Pedipalpi
ist biram. limbs
4...
I St pair of legs
I St pair of legs
2nd
5..-
2nd „
2nd
3rd
6...
3rd
4th
7...
4th
5th
8...
Embryonicft
9...
CO
Genital operc. ? cJ
10...
Pectines
II...
0
I St Lung books
12...
*3
2nd
13...
C3
to
3rd
CO
1
14...
15...
'u
a
4th
No limbs
<4-l
u
16...
0
I St som. Metasoma
a
17. .
8...
19...
c4
■M
0
2nd
3rd
4th
at
G
J2
20...
CO
5th
J3
u
21...
22...
CO
0
C3
23...
24...
25...
26...
C
CO
1
Cfl
27...
28...
29...
Last pair of legs
Postseg-\
mental
Embryonic
Telson
Telson
region
* Eyes and frontal organs belong to a presegmental region which may have median
** If the superlinguae be maxillules (see p. 463), the limbs behind them stand on
t Terga fused in Scolopendra, free in Lithobius. ff Chilaria in Limulus.
§ This somite appears to have no limbs, because the limbs of the 8th and 9th somites
cJ indicates the position of the male opening, ? that of the female.
OF ARTHROPODA
Crustacea
Malacostraca
Insecta
Chilopoda
(Scolopendra)
Diplopoda
(Julidae)
Embryonic
Embryonic
Embryonic
?
Antennules
Antennae
Antennae
Antennae
Antennae
Embryonic
Embryonic
Embryonic
Mandibles
Mandibles
Mandibles
Mandibles
Maxillules
(ist) Maxillae**
ist Maxillae
Embryonic
Maxillae
Labium (2nd Maxillae)
2nd Maxillae
Maxillae
(ist) Maxillipeds
ist pair of legs
Maxillipeds ^ x
Collum
2nd Thoracic limb
2nd ,,
ist pair of legs j
ist pair of legs
3rcl
3rd
2nd ,,
2nd „ ?c?§
4th
ist Abd. som.
3rd „
3rd pair of legs
5th
2nd ,,
4th
4th „ 1
6th „ ?
3rd
5th
5th „ J
7th
4th
6th
bo
8th
5th „
7th
ist Abd. limb
6th
8th
cts
n
2nd „
7th
9th
3rd
8th „ V
loth ,,
4th
9th „ (styles) 0
nth
5th
loth ,, som.
i2th
■bM
6th
nth „ (cerci)
13th
°^
••
14th
15th
i6th
17th
i8th
19th
20th
-^ .id
^ 0
'o ^
0 ^
0
0
N
C
rrt
2ISt ,, X
^
••
Genital limbs ? c^
Limbless somite
Telson
Embryonic
Telson
Telson
mesoblast of its own, and may bear various ganglia which enter into the procerebrum.
somites 6, 7, etc.
X Lithohius has 15 pairs of legs.
have each moved forward one somite.
3o8 THE INVERTEBRATA
only one, the Crustacea, remain predominantly of that habit. No other
invertebrate phylum has so large a proportion of terrestrial members.
A more detailed survey of the organization which we have now out-
lined, necessitates a brief exposition of the principal groups into which
the phylum falls. One small section stands apart from the rest. The
Onychophora have a thin cuticle, without joints; a continuous mus-
cular body wall; eyes (p. 310) of annelid type; only one pair of jaws,
which moreover are constructed on a different principle from those
of other arthropods, biting with the tip and not with the base of the
limb ; and a long series of coelomoducts, of which the pair that are
the oviducts are ciliated. Only in this group, too, does the first somite
bear a pair of limbs : in all others that somite is an evanescent, em-
bryonic structure without external representation in the adult. In
all these respects the Onychophora show a lower degree of develop-
ment of the peculiar features of arthropods than the rest of the
phylum.
The remaining groups of the phylum fall into two sharply different
sections, the crustacean-insect-myriapod section and the arachnid
section. In the first of these sections, the first pair of limbs (those of
the second somite) are antennae, the succeeding pair, if present, are
also antennae, the third pair are mandibles, and behind these limbs
are one or more pairs of additional jaws (maxillae). In the crustaceans
and insects there is commonly a pair of compound eyes of a complex
type peculiar to these animals. The trilobites belong to this section,
but their appendages, behind the first pair are undifferentiated. In
the arachnid section none of the limbs have the form of antennae or
mandibles, the first pair (chelicerae) being usually chelate, the second
chelate, palp-like, or leg-like, and the third to sixth pairs leg-like,
though often some of the postcheliceral limbs possess biting processes
(gnathobases) on the first joint. The members of this section never
possess true compound eyes of the crustacean-insect type.
The Crustacea differ from the Insecta and Myriapoda in possessing
a second pair of antennae, and nearly always in being truly aquatic.
The Insecta differ from the Myriapoda in possessing only three pairs
of legs, and usually in the possession of wings.
The series of somites which, with small pre- and postsegmental
regions, constitutes the body of an arthropod is marked out, by
differences in width, fusions of somites, or features of the limbs, into
divisions known as tagmata. In the Onychophora, Crustacea, In-
secta, and Myriapoda, the foremost tagma is a short division, known
as the head, which carries the antennae and mouth parts, and the rest
of the body, known as the trunk, is often divided into two sections
called thorax and abdomen. In the Arachnida, the foremost tagma is
the prosoma ("cephalothorax"), and carries legs as well as the limbs
ARTHROPODA 309
used in feeding, while the divisions, if any, of the hinder part of the
body {opisthosoma or "abdomen") are known as the mesosoma and
metasoma. It is important that the student should recognize that each
of these divisions varies in size, and that consequently none of them
comprises in all arthropods the same somites, so that, for instance,
the thorax of an insect is a quite different entity from that of a cray-
fish. The most significant variation is that of the head, which, as the
organization of its possessor becomes higher, increases in size, taking
in behind somites whose appendages become jaws, while, by altera-
tion in the position of the mouth, it adds others, whose limbs become
antennae, to its preoral sensory complex. Thus, while the head of the
Onychophora comprises only the first three somites, and only the first
of these is preoral, in the Crustacea there are in the true head six
somites (including the embryonic first somite), of which three are
preoral, and thoracic somites, whose limbs (maxillipeds) function as
jaws, are often united with the head.
The paired limbs of arthropods present an enormous variety of
form, and attempts have been made to reduce them to a common type.
Some of the evidence suggests an archetype with a nine-segmented
axis bearing on the median side of the first segment a biting process
(gnathobase) and on a more distal segment an outer branch (exopo-
dite) ; but there are difficulties in the way of assuming this in all cases,
and the problem is still far from solution.
The arthropod cuticle has a thin, impermeable, non-chitinous
external layer (epicuticle) and a thick, elastic, permeable, lamellar
inner layer (endoctiticle),\sLrge\y composed of chitin ^, the outer lamellae
usually hardened, often by salts of lime. From time to time during
the growth of the animal, the hard outer layers of the cuticle are
separated by solution of the inner layers by an enzyme, ruptured,
and shed in a moult or ecdysis. A new cuticle which has formed
under it then expands to accommodate the body.
The nervous system of arthropods contains, in typical instances, on
two longitudinal ventral cords and in a dorsal brain, a pair of ganglia
for each somite, but where the somites are fused there is often a fusion
of their ganglia, and where they bear no limbs their ganglia may be
absent. The brain is a complex structure composed of the ganglia of
the somites which have become preoral (though in a few Crustacea
the antennal ganglion remains postoral), of paired ganglia for certain
primitively preoral presegmental &ense organs (eyes, frontal organs),
and sometimes also of a median anterior element {archicerebrum, in
the strict sense). The ganglia of the first somite are known as the
protocerebrum ; with the ganglia anterior to them they constitute the
procerebrum {archicerebrum of Lankester). The ganglia of the second
^ Chitin is an amino-polysaccharide which resists most solvent agents.
310 THE INVERTEBRATA
somite are the deutocerebrum or mesocerebrum ; those of the third
somite are the tritocerebrum or metacerebrum. The identity of some of
these gangha may be lost, even in development. Concerning the
functions of the central nervous system something is said on p. 448.
The eyes of the Onychophora are a pair of simple, closed vesicles,
each with its hinder wall thickened and pigmented and its cavity oc-
cupied by a lens secreted by the wall. The eyes of all other arthropods
(Fig. 211) consist of one or more units each of which is in essence a
cup, or a vertical bundle, of cells, over which the cuticle of the body
forms a lens. The cells which compose the bottom of each cup are
(except in the median eye of the Crustacea) arranged in a sheaf or
sheaves called retinulae; in the midst of each retinula is a vertical rod,
known as the rhabdom, secreted by the cells of the sheaf in vertical
sections which, when they are distinct, are known as rhabdomeres.
Each bundle-unit has one such retinula. Sometimes in the cups the
retinulae are surrounded by cells which bear on their free ends short
rods of the same nature as the rhabdomeres. The retinula cells contain
pigment and there is a ring of strongly pigmented cells around the
cup. The eye units occur {a) as single cups each with several retinulae
(ocelli of insects. Fig. 211 C"), {b) as groups of similar cups placed
contiguously (eyes of myriapods), {c) as eyes composed of a number
of small cups, each with a single retinula, united together (lateral eyes
of Limulus), {d) as true compound eyes (Fig. 212) composed of a
number of bundles of cells, each bundle {ommatidium) complex in
structure and containing two or more refractive bodies, but each
probably representing a narrowed and deepened cup. Compound
eyes of this type are found in crustaceans and insects. They vary much
in detail, but essentially the structure of an ommatidium is as follows
(Fig. 211 D). At its outer end is a transparent portion of the general
cuticle of the body, usually thickened to form for the ommatidium a
biconvex lens. Under this lie the epidermal cells which secrete it
(corneagen cells): the lens is one of the facets of the eye. Under the
corneagen cells comes a bundle of two to five vitrellae or crystal cells,
grouped around a refractive body, the crystalline cone, which they
have secreted. The vitrellae taper inwards and their apex is clasped
by a second bundle of cells, four to eight in number, which together
form the retinula. Like the vitrellae the retinular cells secrete in the
axis of the ommatidium a refractive body. This is the rhabdom, and
is made up of rhabdomeres, one for each of the cells. Each retinular
cell passes at its base into a nerve fibre which pierces the basement
membrane of the eye and enters the optic ganglia. Around each
ommatidium, separating it from its neighbours, there are usually
pigmented cells, known as iris cells. The eyes of arachnids, other than
the lateral eyes of Limulus, simulate the ocelli of insects, but are
Opt.hs
opt.n.^ ^
tr.com.
Fig. 2IO. A plan of the nervous system of Chirocephahis. ob.z, second ab-
dominal somite ; an.' antennulary nerve ; ati/' antennary nerve ; an."co?n. com-
missure for fibres which unite antennary ganglia ; brti. brain ; fr. nerve to
frontal organ; ga. ganglion of ventral cord; tn.e. nerves to median eye; md.
mandibular nerve; 7nx.' maxillulary nerve; 7nx." maxillary nerve; oe. oeso-
phagus; oe.com. circumoesophageal commissure; opt.l. optic lobes; opt.n.
optic nerve; th.\, first thoracic somite; th.iz, nerve of last thoracic somite;
tr.com. transverse commissure of ventral cords.
312
THE INVERTEBRATA
thought, from details of their structure, to have been formed by the
degeneration of compound eyes resembling the lateral eyes of
Limulus. The median eye of the Crustacea (Fig. 226) is composed of
Fig. 211. Diagrams of a series of eyes of arthropoda. A, Hypothetical start-
ing point of the series. B, Cells have sunk in to form a retinula. The units of
the lateral eyes of Limulus are substantially in this condition. C, C", Cells
from the sides have closed in over the retinula. C, Hypothetical stage in the
evolution of an ommatidium from a cup with a single retinula. C", Actual
condition of many ocelli of insects, etc. : the cup has several retinulae. D, An
ommatidium. b.me. basement membrane of retinular layer; c.c. central cell;
cgn. comeagen cells; en. crystalline cone; cu. cuticle; Is. lens; n. nerve fibre;
pig. pigmented cells which form a ring in the outer part of the ocellus; pig.'
outer iris cells; pig." inner iris cells; rd. "visual rods"; ret. retinular cells;
rh. rhabdona; vit. vitrellae; vit.hu. vitreous humour.
three cups, which may (some copepods) separate widely. The paired
eyes probably do not, as has been suggested, represent a pair of ap-
pendages. The foremost, or preantennal, somite, to which they would
in that case belong, possesses, in Peripatus and as a rudiment in
ARTHROPODA
313
embryonic stages of centipedes and certain insects, an appendage
which co-exists with the eye.
In most compound eyes, the pigment, both in retinular and in pig-
ment cells, flows to and fro, being in dim light retracted towards the
inner or outer ends of the cells so as to leave the sides of the om-
matidia exposed, and in bright light extending so as to separate the
ommatidia completely. In many diurnal insects it is permanently in
the latter position. Vision takes place in two ways according to the
situation of the pigment. When the latter is extended, in each omma-
B
n.ji.
f>opUga.
-w.
opt.n.
Fig. 212. The eye of Astaciis. A, The left eye. B, A portion of the cornea
removed, to show the facets. C, A longitudinal section of the eye. w. muscles
which move the eye; n.fi. nerve fibres; oynm. ommatidia; opt.ga. optic
ganglion; opt.n. optic nerve.
tidium there falls on the retinula a narrow pencil of almost parallel
rays. There is then mosaic vision, an apposition image, composed of as
many points of light as there are ommatidia, being formed on the
whole retinal layer. When the pigment is retracted, each ommatidium
throws a complete image of the greater part of the field of vision, and
the images together form a superposition image, falling in such a way
that their corresponding parts are superposed. Superposition images
are less sharp than apposition images, but are formed with less loss of
light. Compound eyes are especially adapted for perceiving the move-
ments of objects, owing to the way in which such movements affect
a series of ommatidia in succession.
314 THE INVERTEBRATA
The alimentary canal of the Arthropoda possesses at its mouth and
anus involutions of ectoderm, lined by cuticle, which are respec-
tively the stomodaeum or fore gut, and proctodaeum or hind gut.
These may be short, but in the higher Crustacea and Insecta form a
considerable part, and sometimes nearly the whole, of the canal. The
cuticular lining of fore and hind gut is shed at moulting. The lining
of the fore gut sometimes provides teeth for triturating or bristles for
straining the food. Digestion is extracellular, save in certain acarina.
The respiratio7i of aquatic arthropods, other than those which are
but little modified from terrestrial ancestors, is sometimes, if the
animal be small, efi'ected only through the general integument of the
body, but usually takes place by means oi gills (branchiae). These are
nearly always external processes, known as epipodites, which stand on
the bases of the limbs, and are often branched or folded. Among
terrestrial arthropods, some of the Arachnida possess luftg books,
which are generally held to have arisen by the enclosure of gill books,
such as those on the limbs of Limulus, each within a cavity of the
ventral side of the body. The remainder of the terrestrial Arthropoda
breathe by means of tracheae, which are tubular involutions of the
ectoderm and cuticle which convey air to the tissues. In some
arachnids tracheae are present as well as lung books. Usually tracheae
are branched, and strengthened by a spiral thickening of their
chitinous lining. The study of the phylogeny of the Arthropoda leads
to the conclusion that a tracheal system has arisen independently in
the Onychophora, the Arachnida, and the Insecta and Myriapoda.
Among the Crustacea, tufts of tubes which resemble tracheae are
found in the abdominal appendages of woodlice.
The vascular system is an '* open " one. That is, be the arteries long
or short, they end by discharging their blood not into capillaries in
the tissues from which veins conduct it to the heart, but into peri-
visceral cavities, known as sinuses, which bathe various organs. From
these sinuses the blood collects into a pericardial sinus ("pericar-
dium"), part of the haemocoelic system, which surrounds the heart.
The latter is a longitudinal dorsal vessel, perforated by ostia by which
it receives its blood from the pericardial sinus. Among the conse-
quences of the structure of the vascular system are a low blood pres-
sure and liability to severe bleeding from wounds. The latter danger
is met, especially in the Crustacea, by very rapid clotting of the blood.
Haemoglobin is present in the plasma of certain of the lower crustace-
ans and a few insects, haemocyanin in Limulus, scorpions, and some
spiders.
The coelom appears in the embryo as the cavities of a series of
mesoderm segments (" mesoblastic somites", Fig. 352). It never
assumes a perivisceral function, and in the adult is represented
ARTHROPODA
315
only by the cavities of the gonads and of certain excretory organs and
occasional vestiges elsewhere.
The excretory organs of arthropods are of very various kinds. True
nephridia appear never to be present. Coelomoducts are present in a
number of cases, though in the absence of perivisceral coelom they
end internally each in a small coelomic vesicle or "end sac". These
are found in the Onychophora in a long series of segmental pairs. In
Crustacea there is either a pair of coelomoducts on the third (an-
tennal) somite or a pair on the somite of the maxillae, or, rarely, both
mdm
ect.
Fig. 213. Early stages in the development of Astacus. After Morin and
Reichenbach. A-C Cleavage, D gastrulation. a. anterior end of embryo;
blp. blastopore; ect. ectoderm; end. endoderm; mdm. mesoderm; nu, nuclei;
pt. posterior end of embryo; v, yolk; yp, "yolk pyramids", due to the
transitory appearance of divisions of the yolk corresponding to the superficial
cells.
these pairs are present. In various crustaceans other glands, some
ectodermal, some mesodermal, appear to have an excretory function,
and sometimes replace both pairs of coelomoducts, which become
vestigial. In arachnids, coelomoducts open on one or two of the pairs
of legs. They are known as coxal glands, but are not homologous with
the glands to which that name is applied in certain crustaceans. Mal-
pighian tubules are tubular glands which open into the alimentary
canal near the junction of mid and hind gut in the Arachnida, Insecta,
and Myriapoda. In archnids they, are of endodermal origin, but in
insects and myriapods they are part of the ectodermal hind gut. It
is interesting that the subphyla differ in the nature of their nitro-
genous excreta. In the Crustacea these are principally ammonia
compounds and amines, in the Insecta they are urates, in the Arach-
nida guanin.
3l6 THE INVERTEBRATA
Nearly all the muscular tissue of arthropods is composed of striped
fibres, but in Peripatus only the fibres of the jaw muscles are striped,
and among the higher groups certain exceptions to the rule are known
(some visceral muscles, etc.).
The gonads are always, owing to the reduction of the coelom,
directly continuous with their ducts, which are probably coelomo-
ducts. These have no constant position of opening in the phylum. In
the Crustacea they nearly always open at the hinder end of the thorax.
In the Arachnida their opening is similarly near the middle of the
body. In the Onychophora, Insecta, and centipedes they open near
the hinder end, but in the remaining groups of the Myriapoda their
opening is not far behind the head.
Typical features of the embryonic development are shown in Figs.
213, 316, and 352. The ova are generally yolky, and their cleavage
is typically of the kind knownas"centrolecithal",in which (Fig. 213)
the products of division of the nucleus come to lie in a layer of
protoplasm upon the surface of a mass of yolk which thus occupies
the position of a blastocoele. The mode of gastrulation varies from
invagination (Fig. 213 D) to obscure processes of immigration and
delamination. The formation of the mesoblast as a pair of ventral
bands (Fig. 316), proliferated in primitive cases from behind, has
already been mentioned (p. 130). As in annelids (p. 285), the meso-
blast bands segment, and in most cases the segments (" mesoblastic
somites") develop coelomic cavities (Fig. 352). The haemocoele
arises by separation of the germ layers. The heart is formed by the
dorsal ends of the mesoblast segments approximating. The nerve
cords are proliferated from the ventral ectoderm (Fig. 352 A).
In spite of the yolky eggs, there is a great variety of larval stages,
though direct development is also frequent. The series of somites,
which in the adult is often obscured by the loss, obsolescence, or
fusion of some of its members, is usually more distinct in the embryo
or larva, where the presence of a somite which it is difficult or im-
possible to recognize at a later stage is frequently indicated by one or
more of three criteria: a pair of segments of mesoblast (mesoblastic
somites), a pair of segmental ganglia, and a pair of limbs or limb
rudiments.
CHAPTER XI
THE SUBPHYLA ONYCHOPHORA AND
TRILOBITA
The two groups of animals with which this chapter deals both present
in an apparently primitive condition features which are characteristic
of the phylum Arthropoda. One at least of them existed in the Palaeo-
zoic period. For these reasons, each of them has been regarded as
giving indications concerning the ancestry of the Arthropoda. Where-
as, however, the Trilobita are related rather closely to the Crustacea
and more distantly to the other subphyla, the Onychophora are, as
has been stated above, widely divergent from the rest of the
Arthropoda. Some authorities, indeed, prefer to treat this group
as an independent phylum. It must at least be regarded as repre-
senting a branch which parted at a very early date from the main
arthropod stock. The trilobites are indisputable arthropods, on the
line of descent which gave rise to the Crustacea and perhaps to
other subphyla.
SUBPHYLUM ONYCHOPHORA
Tracheate Arthropoda with a thin, soft cuticle and a body wall con-
sisting of layers of circular and longitudinal muscles ; head not marked
off from the body, consisting of three segments, one preoral, bearing
preantennae, and two postoral, bearing jaws and oral papillae respec-
tively, also with eyes which are simple vesicles ; the remaining segments
all alike, the number varying according to the species, each bearing a
pair of parapodia-like limbs which end in claws and contain a pair of
excretory tubules ; stigmata of the tracheal system scattered irregularly
over the body; cilia present in genital organs; development direct.
The animals which constitute this very important class are few in
number and uniform in structure, all being placed in the genus Peri-
patus divided into many subgenera (Fig. 214). They are distributed
discontinuously over the warmer parts of the world and occur in very
retired positions which are permanently damp as, for instance, be-
neath the bark of dead trees and under stones. They have a superficial
resemblance to other crawling animals which are found in the same
places, like myriapods, slugs and earthworms, and until their anatomy
was well known v/ere classed, by different investigators, with all three
of these. Certain of the characters of Peripatus such as the feebly de-
veloped sense organs, the simple structure of the jaws and feet and
the soft skin may be linked with the environment in which they lurk
3l8 THE INVERTEBRATA
away from light and enemies. Yet it can hardly be doubted that the
Onychophora are a division of the Arthropoda which has preserved
more primitive features of an ancestral race than any other living
Fig. 214. Peripatus capensis, x very slightly. From Sedgwick.
forms, terrestrial or aquatic. Such features are in all probability the
thin cuticle, the muscular body wall, the annelid-like eye, the small
number of head segments, the complete series of segmental excretory
organs, the presence of cilia and possibly also the parapodia-hke
limbs.
The thinness of the cuticle is responsible for the absence of external
segmentation (save for the repetition of the appendages). The head
(Fig. 215) bears three pairs of appendages which are none of them
Fig. 215. Peripatus capensis, S- Ventral view of anterior end. ant. pre-
antenna; o.p. oral papilla; 7. jaw; i, first trunk appendage. After Sedgwick.
very highly developed. While elsewhere in the arthropods the first
segment is present in the embryo but disappears in the adult, here it
persists and bears a pair of appendages which may be caW^dpreantennae
(to distinguish them from antennae). They are rather long and very
mobile, but not retractile like the tentacles of the slug. The next seg-
ment bears the jaws, which are not unlike enlarged claws of the trunk
appendages and so bite with the tip and not the side. They are more-
over tucked within the oral cavity. But they are borne on muscular
papillae arising in the embryo and must without doubt be regarded
as appendages.
ONYCHOPHORA
319
The trunk appendages are short and conical, hollow, bearing at
their distal ends spinose pads and a retractile terminal 'foot" with
two recurved claws.
The adult body cavity is haemocoelic but the embryonic coelom
is well developed. In the development of Peripatus just after the
gastrula stage the blastopore becomes elongated, the anterior part
']m V !\§jf,a
D
pr.s
Fig. 216. A, Transverse section through Peripatus. e.g. crural gland; e.m.
circular muscles ; d.l.m. dorsal longitudinal muscles ; g. genital organ ; h. heart ;
ha. haemocoele; int. intestine containing^, peritrophic membrane; n. excre-
tory tubule; n.c. nerve cord; ob.m. oblique muscles; pf. pericardial floor; s.g.
slime gland; s.l.g. salivary gland; v.l.ni. ventral longitudinal muscle. B, An
excretory organ of Peripatus. bl. bladder ; c.d. ciliated part of duct ; d. duct ;
e.s. coelomic sac ; ex. external aperture. C, Part of the ventral and lateral body
wall of P. capensis to show irregular distribution of tracheae {tr.). D, Ventral
view of embryo of P. capensis to show six pairs of mesoblastic somites.
pr.s. primitive streak; and blastopore closed in the middle to form mouth (M.)
and anus* {an.). A and B, after Manton; C, after Moseley ; D, after Balfour.
giving rise to the mouth, the posterior to the anus, while the median
part closes (Fig. 216). Behind the blastopore is a primitive streak
which forms the paired mesoblastic somites. The anterior pair move
in front of the mouth and help to provide the mesoderm of the
tentacular segment. None of the rest become preoral. In all seg-
ments the somites early acquire a cavity, the coelom, and later divide
into two. Of these the ventral part migrates into the appendage as
this is formed, and eventually becomes part of the segmental excretory
320 THE INVERTEBRATA
organ. The other part approaches its fellow in a mid-dorsal position
to form the heart lying between them (cf. Fig. 352 A) and while in the
anterior region it mostly disappears, those of the posterior segments
fuse longitudinally to form two tubes which become the gonads
(Fig. 217).
At the same time the gaps between the organs become filled with
blood. A dorsal part of the haemocoele so formed is marked off by a
partition as the pericardium. This contains the heart, a long tube with
a pair of ostia in nearly every segment. There are, however, no other
blood vessels, so that the condition of the circulatory system is by no
means so advanced as in the higher Crustacea and the more primitive
arachnids.
The possession of the perivisceral haemocoele almost diagnoses the
group as arthropods, but it was the discovery of the tracheae which led
to the inclusion of Pen]^«^w^ in that phylum. The stigmata are scattered
over the surface of the body most thickly on the sides and ventral sur-
face, several occurring in each segment. Each stigma leads into a pit,
penetrating the muscle of the body wall, from which arise bundles of
minute air-containing tubes which end in the various organs of the
body (Fig. 216 C). It can hardly be doubted that these tracheae are
definitely arthropodan in type : their most significant difference from
those of other forms is in their non-segmental character. Their
irregular distribution is only possible because they originate as pits in
soft skin; when once a cuticular exoskeleton has been established
tracheae can only be excavated in the joints between segments.
Probably then the Onychophora have never had a more definite
cuticle than they possess at present; if they had, tracheae have been
acquired since it was lost.
The alimentary canal consists of short ectodermal fore gut and hind
gut and a very long endodermal mid gut, lined by a peritrophic mem-
brane (p. 434) which is thrown off periodically. The fore gut consists
of a buccal cavity into which open the large salivary glands and a mus-
cular suctorial pharynx. The mid gut possesses no separate glands.
The excretory tubules (Fig. 216 B) are composed of a distal terminal
bladder, a coiled secretory canal and a ciliated duct which opens
into a much reduced coelomic vesicle. The bladder and probably the
whole of the canal are formed from ectoderm, the rest from meso-
derm. It can perhaps be said then that the tubule is a modified
coelomoduct which has attained its present condition by the tucking-
in of ectoderm at its external opening. The tubules form a complete
series, but some of them have been converted into uses other than
excretion. Thus the tubules corresponding to the oral papillae form
the salivary glands and are much larger and more complex than in
other segments. The anal glands and the gonoducts themselves have
ONYCHOPHORA
321
the same origin. Only the tubules corresponding to the jaws and the
first three trunk appendages disappear.
The sexes are separate in Peripatus and the gonads paired, but the
ducts unite to form a median passage opening just before the anus.
In the male the filiform spermatozoa are bound up in spermatophores
in the upper part of the vas deferens ; the lower part is muscular and
ejaculatory in function. Fertilization is internal. The ovaries are
embraced by a funnel, the receptaculum ovorum, which communi-
Fig. 217. Diagram of transverse sections through, embr^'^os of Peripatus
capensis to show the haemocoele and the coelom in the following stages: A,
before the haemocoele has appeared; B, when the somite has divided into
dorsal and ventral parts ; C, when these parts have separated and the heart is
formed ; D, at time of hatching. After Sedgwick, i , alimentary canal ; 2, coelom
(cavity of mesoblastic somite, dividing into 2 cavity of gonad and 2, coelomic
part of excretory tubule ; in C and D the tubule is shown divided into 2 ,
coelomic sac and 2 , canal); 3, haemocoele, 3 , heart.
cates with an oval receptaculum seminis. The eggs are fertilized then
at the proximal end of the oviduct : they vary in size according to the
species. In the larger, development takes place at the expense of the
yolk and the secretions of the uterine wall; but the embryos from
smaller eggs become attached to the uterine wall and a placenta is
formed. Cilia have been described in parts of the genital tract.
There are other derivatives of the ectoderm, the crural glands
(Fig. 216 A, e.g.), found on all the legs except the first and consisting
322
THE INVERTEBRATA
of a simple sac ; and a single pair of slime glands discharging on the
oral papillae, made up of a much branched secretory part and a large
reservoir. The slime can be shot out and entangle an enemy. It is
never used in obtaining food.
S op.
Ofl.
Fig. 218. Peripatus capensis, S, dissected to show the internal organs, x 2.
After Balfour, an. anus ; ant. preantenna ; brn. supraoesophageal gangHa ;
c.oes. circumoesophageal commissure; e.g. enlarged crural gland of last pair
of legs ; m.g. mid gut ; o.p. oral papilla ; ph. pharynx ; sal.gl. salivary gland ;
sl.gl. slime gland; v.n.c. ventral nerve cord; i, 2, 10, appendages of the trunk
segments; 4, excretory tubule of the fourth segment; <S op. male aperture.
The nervous system (Fig. 2 1 8) consists of a pair of supraoesophageal
ganglia from which the preantennal nerves are given off, a pair of cir-
cumoesophageal commissures and two ventral cords which are widely
ARTHROPODA 323
separated and connected by about ten transverse strands in each
segment. There are shght enlargements in each segment which can
be regarded as incipient gangha, but the whole nervous system is
primitive for an arthropod or even an annelid and can be best
compared to that of Polygordiiis in the annelids or Chiton in the
molluscs.
The Onychophora are not known as fossils, but all that has been
said indicates that they came off from the main arthropodan stock at
a very early stage when a typical haemocoele had been developed and
cephalization had commenced, but when the epithelium had not finally
specialized in the production of chitin and was still ciliated in places.
SuBPHYLUM TRILOBITA
Palaeozoic Arthropoda with the body moulded longitudinally into
three lobes; one pair of antennae; and, on all the postantennal so-
mites, appendages of a common type which has two rami and a
gnathobase.
The Trilobita were marine organisms and were very numerous in
the Cambrian and Silurian but became extinct by the Secondary
period. Their body was oval and depressed, and
consisted of a head and a segmented trunk, of
which the anterior somites were movable on
one another, but the hindermost, in varying
number, were nearly always united to form a
tagma known as the pygidium. The body could
usually be rolled up like that of a woodlouse.
Along its whole length longitudinal grooves pig. 219. 0/.m/.c«fa-
divided lateral pleural portions from a middle ractes, from the Lin-
region. In the head, this middle region is known . gula Flags. Natural
as the glabella and transverse furrows usually s^^^- Froi"" Woods,
mark out more or less distinctly five somites. The pleural portions
of the head are known as the cheeks, and each bears in most species
a sessile compound eye. On each cheek a longitudinal facial suture
divides an outer from an inner area, passing immediately internal to
the eye. The posterolateral angles of the cheeks are often produced
backward as spines. Under the head a large labrum or hypostoma
projects backward below the mouth, behind which is a small rneta-
stoma.
The antenna is uniramous and multiarticulate and is the only pre-
oral appendage. Since it is the foremost of five head appendages it
has the same position as the antennule of the Crustacea, with which
it is probably homologous. In that case it would seem likely that a
true first somite had already, as in modern Crustacea, become merged
324
THE INVERTEBRATA
in the anterior region of the head. Traces of a groove which exist in
some species may perhaps indicate its existence.
The remaining limbs are all of one type, though there is a gradual
progressive modification from one end of the series to the other. Each
has two rami. Of these, one, usually held to be the outer ("exopo-
dite"), bears a long fringe of bristles, while the other (*'endopodite")
is leg-like and divided into six joints. It is supposed by some authori-
ties that the bristle fringe on the so-called exopodite was on the inner
side of the limb, and was used for collecting food, like the fringes on
Fig. 220, Triarthrus becki, from the Utica Slate (Ordovician) near Rome,
New York. After Beecher. A, View of the ventral surface showing append-
ages, etc. h, hypostome (labrum) ; m, metastoma. x f . B, Diagrammatic
section through the second thoracic somite, a, " endopodite " ; b, " exopodite ".
C, Dorsal view of second thoracic leg. a, "endopodite"; b, "exopodite";
c, "protopodite" with gnathobase. Enlarged.
the trunk limbs of branchiopoda (p. 355), but this surmise is not
generally accepted. From the basal portion of the limb a process for
the manipulation of food, the gnathobase, projects towards the middle
line. The configuration of the basal portion (protopodite), and the
relation of the rami to it, are obscure. The telson is without limbs.
The Trilobita hatched as a larva, the Protaspis, which was sub-
circular, and consisted, like the Nauplius larva of the Crustacea,
principally of head. In its further development there appear, first the
pygidium, and then free somites between the pygidium and the head,
TRILOBITA
325
new somites being added in front of the telson while those at the front
end of the pygidium become free.
Fig. 221. Development of a trilobite. After Barrande. A-D, Sao hirsuta,
Cambrian, Bohemia. A, Earliest stage (Protaspis), x 12. B, Later stage, with
three somites behind the head, x 12. C, With more distinct glabella furrows
and four somites behind the head, x 12. D, With six somites behind the
head, x 10.
It is probable that the majority of the trilobites lived upon the sea-
bottom in shallow to moderately deep water, but others appear to have
been adapted to burrowing, pelagic, and deep-sea conditions.
CHAPTER XII
THE SUBPHYLUM CRUSTACEA
Arthropoda, for the most part of aquatic habit and mode of respira-
tion ; whose second and third somites bear antennae ; and their fourth
somite a pair of mandibles.
The Crustacea are essentially aquatic arthropods. That fact alone
makes it possible that in them the same appendages should combine
the functions of locomotion (by swimming), feeding (by gathering
particles from the water), respiration (by exposing a thinly covered
surface to the medium), and the reception of sensory stimuli. There
is perhaps no extant crustacean in which all four functions are thus
combined — unless we may regard the trunk limbs of the Branchiopoda
(see below) as sense organs in a minor degree — but not uncommonly
three, and perhaps usually two, are performed by the same limb. In
the lowest members of the subphylum — the "phyllopod" Branchio-
poda (such creatures as the fairy shrimp, Chirocephalus, shown in
Fig. 236) — a long series of somites of the trunk bear similar append-
ages which all function alike in swimming, respiration, and the gather-
ing of food. Evolution within the crustacean group appears to have
proceeded mainly by the specialization, for particular functions, of
particular appendages of an ancestor which possessed along the
whole length of the body a numerous series of limbs, of which all,
except probably the first pair (antennules), were as much alike and
capable of at least as many functions as those which the Branchio-
poda now possess upon the trunk. Such a condition existed in the
Trilobita, but in all modern Crustacea the appendages of the head
are already specialized for various uses, and in most members of
the group the specialization has gone farther. Moreover, it has taken
place in more than one way. Limbs which in one crustacean are adapted
to some particular function are in others specialized for quite different
services.
Two other factors, added to, or perhaps consequent upon, the
specialization of limbs, have taken part in bringing about the great
variety of organization which exists in the Crustacea. One is a short-
ening of the body. As the efficiency of the limbs increases by speciali-
zation, there occurs a lessening of their number, and finally the re-
duction or loss of the somites whose limbs have thus disappeared.
The reduction, which has occurred independently in every class, has
taken place in the hinder part of the body, though as a rule the ex-
treme hind end (telson) is relatively unaffected. The other factor is
CRUSTACEA 327
the development, from the hinder part of the head, of a skin fold —
the carapace — by which the important anterior region of the body is
overhung and protected, and the setting up in the surrounding water
of currents for purposes of respiration and feeding is facilitated. Not
all crustaceans possess the carapace: in some it has perhaps never
existed, others have discarded it. In those which have it, its extent
varies : in extreme cases it encloses the whole body.
The transformation of the external make-up of the body is of
course reflected in the internal organization, which shows corre-
sponding concentrations of function and differentiation of the contents
of somites.
On these general lines evolution has given rise to six Classes of
Crustacea. We must now briefly survey them, (i) In the Bran-
chiopoda feeding is performed by the limbs of the trunk. In the
"phyllopod" groups of this class, mentioned above, it is only on the
head that difi^erentiation among the appendages has proceeded to any
considerable extent. Of the head limbs each, as we have seen, is
specialized for some particular function, such as the service of the
senses or the manducation of food. On the trunk the limbs, which
are numerous, are still similar and all subserve at least the functions
of feeding and respiration. In the order Anostraca, to which Chiro-
cephalus belongs, there is no carapace, and the trunk limbs, whose
similarity is very strong, retain the function of swimming. In the
order Notostraca (Fig. 242), also phyllopodous, there is a carapace
but it is wide and shallow and does not enclose the trunk limbs, and
they are still used for swimming. A certain degree of diflFerentiation
exists between these limbs, the anterior pairs for instance being
capable of clasping objects. In both the foregoing orders limbs have
been dispensed with on some of the hinder somites. The remaining
phyllopod group, the Conchostraca (Fig. 243), are united with the
non-phyllopod group Cladocera as the order Diplostraca. In the
members of that order (except a few aberrant cladocera) the carapace
encloses the trunk limbs, which are not used for swimming, that
function being taken over by the antennae. The Conchostraca alone
among branchiopods retain limbs on all their trunk somites like the
trilobites, but as in the Notostraca there is a certain degree of diff'er-
entiation between the members of the series. In the Cladocera (Fig
244), the highest group of the Branchiopoda, a compact and very
efficient feeding apparatus is formed by some half-dozen pairs of
limbs, the trunk is correspondingly shortened, and even so some of the
hinder somites are limbless. In certain members of this group, such
as the water-flea Daphnia there is a high degree of differentiation be-
tween the trunk limbs (Fig. 245). (2) A similar habit of body is even
more strongly developed in the class Ostracoda (Fig. 248), which are
SOMITES AND LIMBS
Somite
Apus
Daphnia
Ov/)rw
Cyclops
I...
No limbs
No limbs '
No limbs A
No limbs A
2...
Antennules
Antennules
Antennules
Antennules
3.--
Antennae
Antennae
Antennae
Antennae
4...
Mandibles
Mandibles
Mandibles
Mandibles
5--.
Maxillules
-
Maxillules
Maxillules
Maxillules
6...
Maxillae
Maxillae (ves.)
Maxillae
Maxillae
7...
Thor. limbs i
Thor. limbs i
Thor. limbs i
Maxillipeds
8...
2
2;
2(??.
Thor. limbs 2 j
9...
3
3
3
10...
4
4
4
II...
5
\
5
12...
6
„ 5/
a/ " ^ ^""-^
13...
7
Thor. som. 7?
'^lThor.som.7(^)c??|
Abd. som. i /
14...
8
8
w
i5--.
9
9
2
16...
10
ui
3
17...
He??
en
0
18...
19...
Abd. som. i
2
^ CO
4)
1
20...
21...
34-. •
>>
>>
>>
3^
4
17]
0 0
12 s ^
in w 0
^ £
<U jO
11
OS
w <u 0
1 -^
2^s
S
0
0
35.-.
36...
37--.
„ i8^
19
20
.a
1°
38...
21
0
z
39---
22J
Telson
Telson 0
Telson
Telson
with rami
with rami
with rami
with rami
"Abdominal "somites are those between the last genital somite and the telson. S indicates
female, c, uniramous and vestigial, d, genital operculum of female represents seventh
OF CRUSTACEA
Lepas
No limbs
Antennules
Lost in adult
Mandibles
Maxillules
Maxillae
Thor. limbs i ?
2
3
4
5
6d^
Nebalia
o
-a
CO
o
Telson
with rami
No limbs ^
Antennules
Antennae
Mandibles
Maxillules
Maxillae
Thor. limbs i
2
3
4
5
6?
7
Abd. limbs i
3
4
5
6
Abd. som. 7
Gammarus
Telson
with rami
No limbs
Antennules
Antennae
Mandibles
Maxillules
Maxillae
Maxillipeds .
Legs I
II
III
IV
V?
VI
VII s
Abd. limbs i
2
3
4
5
6
Telson
Astacus
No limbs
Antennules
Antennae
Mandibles
Maxillules
Maxillae
Maxillipeds I
11
„ III
Legs I
„ 11
„ III?
„ IV
„ V6^
Abd. limbs i
2
3
4
5
6
Somite
Telson
the position of the male opening, ? that of the female, a, joined but distinct, b, fused in
thoracic limb : the somite which bears it is often reckoned as the first abdominal.
330 THE INVERTEBRATA
very short-bodied and completely enclosed in a bivalve shell formed
by the carapace. Whereas, however, in the Cladocera it is by trunk
limbs that food is gathered, in the Ostracoda that function is per-
formed by limbs of the head. The trunk limbs, which have lost the
functions of swimming and respiration as well as that of feeding,
serve relatively unimportant subsidiary purposes, and are reduced, at
most, to two pairs. Some members of the class carry shortening to
an extreme pitch by contriving to dispense with one or both of these
pairs. (3) The members of the class Copepoda (Fig. 249) also feed by
means of appendages on the head, though they use these differently
from the Ostracoda. In contrast to that group they have no carapace,
and they have retained a trunk of some ten somites, of which the first
half-dozen bear limbs which are specialized organs of swimming. The
hinder part of the trunk is without appendages, save a pair of styles
on the telson, often shows coalescence of somites, and may become
a mere stump. Some of those members of this class which are para-
sitic lose in the adult female the segmentation and most, or even all,
of the appendages. (4) In the small class of parasites known as
Branchiura (Fig. 254), which are sometimes placed in the Copepoda,
but differ from that group in possessing compound eyes and in other
important respects, there are carapace-like lobes at the sides of the
head, but these do not enclose the trunk, and the general build of
the body and the form and function of the thoracic limbs simulate
those of a copepod. The abdomen is much reduced. (5) The class
Cirripedia or barnacles, which as larvae attach themselves by their
antennules to some object upon which they henceforward lead a
sedentary life under the protection of a large, mantle-like carapace,
bear, upon the same trunk somites as do the Copepoda, limbs which,
like those of the latter group, are biramous. These appendages, how-
ever, are used , not for swimming, but for gathering food-particles from
the water ; while of the head appendages the antennae are absent and
the others are much reduced and not used in gathering food. The least
specialized members of this class are, in respect of segmentation and
appendages, on a par with the best-segmented of the Copepoda. Most
cirripedes, however, (the ordinary barnacles, Fig. 255) have lost the
whole of the hinder (abdominal) region of the trunk. Others are
deficient in the appendages of further somites, and the series ends
with the sac-like parasites of the order Rhizocephala (Fig. 260).
(6) The class Malacostraca (the highest crustaceans, including various
"shrimps", slaters, sandhoppers, crayfishes, etc.) obtain their food
with the limbs on the anterior region (thorax) of the trunk, and, in
primitive cases in which it is gathered as particles, strain it from the
water with the last pair of appendages of the head (the maxillae). The
thoracic limbs retain also the function of locomotion and normally
CRUSTACEA 331
are adapted for respiration by the presence upon them of gills, which
are usually protected by a carapace of moderate size. Thus this region
of the body of the Malacostraca is, in its own ways, as many-functioned
as the corresponding part of the trunk of Chirocephalus . The Malacos-
traca maintain in typical cases (Figs. 269, 282) the swimming function
of the limbs on the hinder portion (abdomen) of the trunk, and some
of the class have found other uses (ovigerous, copulatory, etc.) for
these appendages. Accordingly there is seldom any reduction in the
fixed number of fourteen (or fifteen) trunk somites which, arranged
always in a thorax of eight and an abdomen of six (or seven), cha-
racterizes the class. Nevertheless in all but one of the orders the
abdomen has lost a somite, in the crabs (Fig. 284) and some others
of the highest order {Decapoda) it is reduced, and in a few members
of the class it is a limbless and unsegmented stump.
The name Entoviostraca was formerly used in the classification of
the subphylum, to distinguish from the Malacostraca a division con-
taining all the other classes. Since, however, these differ from one
another as widely as each of them does from the Malacostraca, the
name is no longer used in classification but is only a convenient
designation for the lower crustacean classes as a whole.
When feeding is restricted to a few limbs it is often, though not
always, accomplished in some other way than by the original habit
of gathering food in small particles. Continuous and automatic
straining-out of such particles, which is practised (though in different
modes) by the most primitive members of all classes except the
Branchiura, is superseded in various members of different classes
by the intermittent seizure, by particular limbs, of particles of some
size, and this by the grasping of larger objects, which may lead to a
predatory habit. Finally, either of these modes of feeding may be
replaced in parasites by suction or absorption, through organs which
do not always represent appendages at all. (Parasites, however, are not
known among the Branchiopoda or Ostracoda.) Needless to say, each
change in the mode of obtaining nutriment has entrained numerous
alterations in organs other than those by which the food is actually
taken, as in the means of locomotion, sense organs, weapons of offence,
etc. On the other hand, adaptations to mere differences of habitat,
in the Crustacea, as in other arthropods, are, as a rule, strikingly small.
There is, for instance, remarkably little difference between a land
crustacean and its nearest marine^relatives. Pelagic genera, however,
are sometimes considerably modified.
We must now proceed to review in more detail the common organi-
zation of the Crustacea and the variation which it presents throughout
the group.
The cuticle of a crustacean is, save for the joints, usually stout
332 THE INVERTEBRATA
relative to the size of the animal, but is thinner and flexible in many
parasitic genera. It is often strengthened by calcification, and in
certain ostracods, barnacles, and crabs this gives it a stony hardness.
In each somite there mayor may not be distinguishable the dorsal plate
or tergite (tergum) and ventral sternite {sternum) usual in arthropods.
The tergite may project at each side as a pleuron.
There are embryological indications that the body should be re-
garded as containing, besides the somites, an anterior presegmental
region, to which the eyes belong, corresponding to the prostomium of
a worm, and a. postsegmental region or telson, on which the anus opens.
Each somite, except the first, which is purely embryonic, may bear a
pair of appendages, though it is rarely that the appendages of all the
somites are present at the same time. The somites never all remain
distinct in the adult. Always some of them are fused together and
with the presegmental region so as to form a head, and often there is
also fusion of them elsewhere.
Nearly always the somites are grouped into three tagmata, differ-
entiated by peculiarities of their shape or appendages, and known as
the head, thorax, and abdomen. These, however, are not morpho-
logically equivalent in different groups. The head always contains,
besides the region of the eyes and the embryonic first somite, the
somites of five pairs of appendages — two, the antennules and an-
tennae, preoral; and three, the mandibles, maxillules, and maxillae,
postoral. More somites are often included in the actual head, but as
the additional appendages (maxillipeds) then usually show features
of transition to those behind them, and as the fold of skin which forms
the carapace first arises from the maxillary somite, the true head is
held to consist only of the anterior portion of the body as far as that
somite inclusive. There is evidence of an earlier head, carrying only
the first three pairs of limbs which alone exist in the Nauplius larva,
and still indicated in some cases (as in Chirocephalus , Anaspides,
Fig. 269, and My sis, Fig. 265) by a groove which crosses the cheek
immediately behind the mandible. This mandibular groove is distinct
from the true cervical groove which often (as in Astacus, Fig. 283)
marks the boundary between head and thorax : the two grooves may
co-exist, as in Apus and in Nephrops. The Crustacea, indeed, ad-
mirably illustrate the way in which the process of *'cephalization"
tends, in arthropods as in vertebrates, to extend backwards and to
involve more and more segments. With it has gone a backward shift-
ing of the mouth, which in the Crustacea now stands behind the third
somite, with two pairs of appendages (antennules and antennae) in
front of it. The commissure which unites the ganglia of the antennae
still passes behind the mouth, and may usually be seen, as in Astacus
(Fig. 225), crossing from one of the circumoesophageal commissures
CRUSTACEA 333
to the other. The head of the Crustacea is unlike, and less specialized
than, those of other arthropods in that its limbs are not entirely re-
stricted to sensory and alimentary functions but often have also other
uses, such as swimming, the setting up of currents, or prehension.
The head, though it varies in extent, is of the same nature through-
out the group, being primarily, like the heads of other animals, the
seat of the principal organs of special sense and of rtianducation. On
the other hand, the two tagmata known as the thorax and abdomen,
which usually can be recognized in, and together compose, the post-
cephalic part of the body or trunk, vary much more in extent, and each
of them has in the several groups no constant feature save its position
relative to the other. The precise boundary between thorax and ab-
domen is sometimes difficult to fix. The names, as they are com-
monly used, are in this respect inconsistently applied, denoting in
some groups limb-bearing and limbless regions, in others the sections
of the trunk which lie before and behind the genital openings. For
the sake of consistency we shall adopt the convention that the somite
which bears the genital openings (or the hinder such somite when, as
sometimes happens, the male opening is on a somite behind that of
the oviduct) is always the last somite of the true thorax. In this sense,
in certain cases (copepods, cladocera), somites which are commonly
called abdominal are strictly to be reckoned as thoracic. In respect of
segmentation the trunk varies from the condition of a limbless stump
in certain ostracods to the possession of more than sixty somites in
some of the Branchiopoda.
A structure very commonly found in crustaceans is the shell or
carapace, a dorsal fold of skin arising from the hinder border of the
head and extending for a greater or less distance over the trunk. Its
size varies greatly. In the Ostracoda (Fig. 248) and most concho-
stracans (Fig. 243) it encloses the whole body, extending forwards
at the sides so as to shut in the head. In other cases (cirripedes, Fig.
255, most cladocera, Fig. 244), it only leaves part or the whole of the
head uncovered. In typical malacostraca it covers the thorax (Fig.
283), but in some it is a short jacket, leaving several thoracic somites
uncovered (Fig. 272), and in some (the Syncarida, Isopoda, and Am-
phipoda, Figs. 269, 274, 278) it has disappeared. In the Anostraca
(Fig. 236) and Copepoda (Fig. 249) it was perhaps never present. It
may be a broad, flat shield over the back, as in Apus (Fig. 242), but
is usually compressed, and in the Conchostraca and Ostracoda be-
comes truly bivalve, with a dorsal hinge. In the Cirripedia it is an
enveloping mantle, usually strengthened by shelly plates (Fig. 257).
In the Conchostraca, Ostracoda, Leptostraca, and Cirripedia it has
an adductor muscle, but the adductors of these groups vary in position
and are not homologous. The carapace may fuse with the dorsal side
334 THE INVERTEBRATA
of some or all of the thoracic somites (the Cladocera, most of the
Malacostraca) : such somites are not on that account alone to be
regarded as included in the head, though they may become so. The
chamber enclosed by the carapace is known in various cases by various
names as gill chamber, mantle cavity, etc., and performs important
functions in sheltering gills or embryos, directing currents of water
which subserve feeding or respiration, etc. In front, the carapace is
continuous with the dorsal plate which represents the terga of the
head, the cervical groove, if present, marking the boundary between
them. We shall apply the term dorsal shield to the structure composed
of the dorsal plate of the head with the carapace, if the latter be
present.^ The dorsal plate of the head may be prolonged in front as
a projection which is called the rostrum (Fig. 283, rs).
A glandular patch or patches on the dorsal surface of the head, near
its hinder limit, in many of the Branchiopoda, in Anaspides, and in the
young stages of various other crustaceans, is known as the dorsal
organ or neck gland. It is used by cladocera and conchostraca for
temporary fixation. In other cases its function is not known. Possibly
the organs to which this name is given are not all homologous. They
must not be confused with the "neck organ" of branchiopods (see
p. 342).
Of the appendages or limbs of the Crustacea, the first, or antennule,
is a structure sui generis, not comparable in detail with any of the
others. Typically it is uniramous, and though in many of the Mala-
costraca it has two rami, these are probably not homologous with the
rami, described below, of other appendages. The remaining limbs
may all be reduced to one or other of two types — the "biramous"
limb usually so-called, to which most of them more or less clearly
conform, and the phyllopodium, to which belong the trunk limbs of
the Branchiopoda and some other appendages, chiefly maxillules and
maxillae and notably the maxilla of the Decapoda. The name by
which the first of these types is generally known refers to the fact that
limbs which best represent it fork distally into two rami. Since, how-
ever, the phyllopodium possesses the same two rami, and bears them,
though not as a distal fork, yet in the same way as a great number of
limbs of the first type, it is well not to use a name which might imply
that there is a constant difference in respect of the rami between the
limbs of the two types. We shall therefore called the first type the
stenopodiiim, referring to its usually slender form (Gk. orevos,
narrow).
^ These terms have been used in various senses. In the usage here pro-
posed, when there is no carapace fold the dorsal shield is the dorsal plate
of the head together with the terga of the somites (if any) that are fused with
the head.
CRUSTACEA
335
In the stenopodium (Figs. 222 D-G, 223) the two rami — an inner
endopodite and an outer exopodite — are set upon a common stem, the
protopodite. In many cases the protopodite bears also, on its outer
Fig. 222. Limbs of Crustacea. Not drawn to scale. After various authors.
A, Maxilla of Mysis larva of Penaeus (Decapoda). B, Maxilla of Acanthosoma
larva of Sergestes arcticus (Decapoda). C, Second trunk limb of female
Cyclestheria hislopi (Conchostraca). D, Mandible of Calamis. E, Thoracic
limb of Nebalia. F, Mid-thoracic limb of an euphausid. G, Swimmeret of
the crayfish, bp. basipodite; br. branchia (epipodite); cp. coxopodite; en. en-
dopodite; ep. epipodite; ex. exopodite ;y?6. flabellum (exopodite) ; ^«. gnatho-
base; pr.cp. precoxa; 1-9, endites or segments of the limb.
side, one or more processes known as epipodites (Fig. 223, ep). In
limbs in which the type is most perfectly developed the two rami are
subequal and are borne distally upon the protopodite (Fig. 222 G),
336 THE INVERTEBRATA
but in most cases the endopodite is the larger, and forms with the
protopodite an axis, the corm^ on which the exopodite stands laterally
(Figs. 222 E, F, 223, 286). In a few instances the exopodite is the
larger.
The protopodite most often has two joints, a proximal coxopodite
and a distal basipodite. In certain cases, however (as in the antenna
of the Mysidacea and Asellus, the last three thoracic limbs of the
Stomatopoda, certain swimming limbs of the Branchiura (Fig. 254 B),
and less clearly in many other instances), a basal joint, the precoxa or
pleuropodite, precedes the coxopodite; moreover the basipodite may
be divided into two joints — the probasipodite, which then usually bears
the exopodite, and the metabasipodite or preischiopodite . This con-
dition is seen most clearly on the thorax of the malacostracan genera
Anaspides and Nebalia (Fig. 222 E), where the two components of the
basipodite are separate in some limbs and fused in others; it is less
obvious in other cases in which it occurs. Thus the full possible
number of joints in the protopodite is four. Some authorities, how-
ever, prefer to regard the preischium as part of the endopodite, in
which case the true protopodite has only three joints at the most.
Epipodites, when they are present, are borne upon the precoxa
(proepipodites) or coxopodite (fnetepipodites).
The endopodite is usually segmented. If the preischium be not
reckoned to it, its maximum number of joints is five. These are found
on the thoracic limbs in the subclass Eucarida of the Malacostraca
as, for instance, in the crayfish, where they are named, in order from
the base outwards, the ischiopodite , meropodite^ carpopodite^propodite^
and dactylopodite .
In the subclass Peracarida, however, the five joints to which the above
names are usually given are not homologous with those so designated in the
Eucarida. Here the true carpopodite and propodite have fused, but the pre-
ischiopodite, which in the Eucarida is probably fused with the ischiopodite,
remains distinct, so that the distal part of the corm has still five joints.
With the four possible joints of the protopodite these segments of
the endopodite make up a total of nine in the corm of the limb. Some-
times a subdivision of certain of the joints into many jointlets or
annuli occurs. This may be seen in some of the thoracic limbs of
mysidacea and of certain prawns (Pandalus, etc.) and in many an-
tennae. A slender, many-jointed, terminal portion of either ramus is
known as 3. flagellum . The exopodite is often unsegmented, but when
segmented usually possesses a flagellum. It does not possess a
standard number of joints. It is more often absent or reduced than is
the endopodite.
The phyllopodium (Figs. 222 A, C, 224, 238), is a broader and flatter
limb than the majority of stenopodia. Its cuticle is usually thin, and
CRUSTACEA
337
then the shape of the limb is maintained largely by the pressure of
blood within it. In these cases the flexibility is such that no joints are
needed. There is in this limb an axial portion or corm which bears on
the median side a row of lobes known as endites, and on the outer side
one or more lobes known as exiles. Of the latter the more distal,
standing usually opposite the third or fourth endite from the base and
often known as the flabellum, is the homologue of the exopodite of the
biramous limb. Exites proximal to this are epipodites. That next to
the flabellum is the branchia (metepipodite) ; any which may be pre-
yflX.
7__^
^ ii
Q-J^
^ W—ex
t&:
"^j^-^ew.
5—
2--4—
Fig. 223.
Fig. 224.
Fig. 223. The second thoracic limb of Anaspides. After Caiman, with an
addition, en. endopodite; ep. epipodite; ex. exopodite ; ^jc. flexure of limb;
I, position of precoxa or pleuropodite, represented, according to Hansen, by
an isolated area of chitinization of the thoracic wall; 2, coxa or coxopodite;
3, basis or basipodite (probasipodite) ; 4, preischium or metabasipodite;
5, ischium or ischiopodite ; 6, merus or meropodite; 7, carpus or carpopodite;
8, propus or propodite; 9, dactylus or dactylopodite (apical joint).
Fig. 224. Tenth thoracic limb of Apiis. br. branchia; flb. flabellum; gn.
gnathobase; ep. epipodite; ex. exopodite; 1-5, segments of the limb; z'-^'t
endites; 6, last endite or apical lobe.
sent proximal to the branchia are proepipodites (Fig. 238, pr. ep.). The
flabellum typically overhangs its attachment proximally as well as
distally. In the latter direction it may extend so far as to form with
the distal part of the corm a pair of equal rami (Fig. 222 C). The
homology of the endopodite then becomes apparent. This ramus
corresponds to that part of the corm of the phyllopodium which is
distal to the insertion of the flabellum or exopodite. Of the endites ^
that which stands at the base of the limb is usually diflFerent in form
from the rest and used in one way or another for manipulating the
338 THE INVERTEBRATA
food. It is known as the gnathobase. The most distal " endite " is the
termination of the corm, and is better called the "apical lobe". It
has often special functions. The number of the endites varies. In the
Branchiopoda it is commonly six, but in the Anostraca (Fig. 238)
there are indications of seven. It is greatest in the larval maxilla of
certain decapoda (Fig. 222 A), where it is nine, which, as we have
seen, is the maximum number of segments in the corm of the steno-
podium. The suggestion, made by this fact, that the segmentation of
the phyllopodium by endites corresponds with that which the corm
of the stenopodium owes to the presence of joints, is strengthened by
the fact that in some of the maxillae in question (Fig. 222 B), and in
that of Calanus (Fig. 251), which also has nine segments, the limb is
jointed and the joints fall between the endites or, where these are
lacking, precisely complete their number. A less regular jointing of
the same kind is present in some other phyllopodia (Apus, Fig. 224 ;
etc.). In both kinds of limb, also, the position of the exopodite bears
the same relation to the segmentation, being usually upon the third,
occasionally upon the fourth segment, while epipodites stand on the
first or second segment. Endites are rare on stenopodia, but a
gnathobase is always present in the mandible (Fig. 222 D) and some-
times in other limbs, and a few other such processes occur.
A limb of either type may differ from that type in the lack of any
of its parts. Notably the loss of the exopodite is liable to produce
from either a uniramous limb. Moreover, though the two types are
very distinct in cases in which they are perfectly developed, as in the
swimmerets of Astacus (Fig. 222 G) and the trunk limbs of Apus
(Fig. 224), there are many limbs which depart more or less from
either type in the direction of the other — as, for instance, from the
stenopodial type in the shape of the exopodite (Fig. 254 B), or, as
stated above, in the relation of the latter to the rest of the limb, or
from the phyllopodium in the proportions of the rami or the reduction
of the endites.
The comparison just made between the phyllopodium and the
stenopodium leaves untouched the question which of them is the
more primitive, that is, more resembles the limbs of the ancestral
crustacean. On this point there is an old and as yet unsettled con-
troversy. As proof of the primitiveness of the stenopodium it is
pointed out (i) that this limb is more widespread than the phyllo-
podium, (2) that it occurs in the Nauplius larva (p. 352), the early
phyllopod Lepidocaris (p. 360), and the trilobites, in all of which it
is likely to be primitive, (3) that it more nearly approaches the form
of the majority of parapodia of the Annelida, from which the Crus-
tacea are held to have taken origin. In demonstration of the ancestral
nature of the phyllopodium it is urged (i) that typical stenopodia
CRUSTACEA 339
with subequal rami borne distally upon a protopodite are compara-
tively rare and usually occur in highly specialized crustaceans (Cope-
poda, Cirripedia, Malacostraca), (2) that the biramous limbs of the
Nauplius and Lepidocaris are not primitive but adaptive, the relations
of the rami of the limbs of trilobites are problematical, and the ad-
mittedly primitive Branchiopoda possess phyllopodia, (3) that the
unjointed, turgid, lobed phyllopodium more nearly resembles the
parapodia of certain annelids in which the neuropodium is axial, than \
the stenopodium resembles the normal biramous parapodium.
Concerning the functions of particular members of the series of
limbs, and the corresponding modifications of their structure, little
can be said that would hold good throughout the subphylum. There
is an immense variety in these respects. The antennules and antennae
are primarily sensory, and perhaps usually possess something of that
function when they are also capable of swimming, prehension, attach-
ment, etc. In the Nauplius larva (Figs. 235, 258) the antennules are
uniramous and the antennae biramous, and they normally retain these
conditions in the adult. The mandibles always play, by means of their
strong gnathobase, some part in preparing the food, whether by
chewing or by piercing for suction, but the distal part of the limb
{palp) may aid in locomotion or set up feeding currents. They gener-
ally lose in the adult the biramous condition which they have in the
Nauplius. The maxillules and maxillae tend to be phyllopodia. The
maxillules have usually the function of passing food to the mouth but
may serve other ends. The maxillae have various functions in con-
nection with feeding and respiration. The limbs of the thorax perform
in various cases practically every function for which appendages are
used. If a crustacean walks, it is usually by means of these limbs.
Often in one or more of them the last joint can be opposed to the joint '
which precedes it, forming a chela (or a subchela), so that the ap-
pendage is adapted for grasping. Modification of the hinder thoracic
or anterior abdominal limbs in connection with reproduction is
common. Abdominal limbs are lacking save in certain of the Branchio-
poda and most of the Malacostraca. When they are present they are
commonly used for swimming, for setting up currents of water, or
for carrying eggs and young.
Three elements of minor importance complete the external make-
up of the Crustacea. In front of the mouth is a labrum or upper lip ;
behind the mandibles is a lower lip or metastoma, usually cleft into
a pair of lobes known as paragnatha ; and on the telson usually (but
in no adult malacostracan except the Leptostraca, Fig. 267) is a pair
of caudal rami forming the caudal /w;t«.
Appendages which are lost are regenerated at subsequent moults ;
and the highest members of the group possess an elaborate mechan-
340 THE INVERTEBRATA
ism for autotomy — the breaking-ofF of limbs which have been injured
or which have been seized by enemies.
An internal skeleton is usually present in the form of ingrowths of
the cuticle, known as apodemes, which serve for the insertion of
muscles. Sometimes (notably in the Decapoda, Figs. 233, apo.; 290,
enph.) they unite to form a framework, the endophragmal skeleton. In
the Notostraca, a mesodermal tendinous plate, the endosternite y lies
under the anterior part of the alimentary canal.
The nervous systems of Crustacea exhibit a very complete series of
stages from the ideal arthropod condition (see p. 309), to the ex-
tremest concentration. That of the Branchiopoda (Fig. 210) is in a
very primitive state, having the antennal ganglia behind the mouth
as the first pair of the ventral ladder, distinct ganglia for the following
somites, and widely separated ventral cords. In the lower members
of the Malacostraca {Nebalia, some mysids, etc.), the antennal ganglia
have joined the brain and the ventral cords are closer together, but
otherwise the primitive condition is retained. In other crustaceans
various degrees of concentration of the ventral ladder are found, be-
ginning with the establishment of a suboesophageal ganglion for the
somites of the mouth parts (Fig. 225, s.oes.), and ending in the forma-
tion, in the crabs (Fig. 290) and some other forms, of a single ventral
ganglionic mass. In the Rhizocephala one ganglion (Fig. 261, ga.)
supplies the whole body. The brain contains ganglia for the eyes
(optic lobes), for the first or preantennulary somite (protocerebrum) ,^
and for the antennules (deuto- or mesocerebrum). Except in the Bran-
chiopoda it also contains the antennal ganglia {trito- or metacere-
brum). A visceral ("sympathetic") system is present. In its main
features the functioning of the nervous system resembles that of
insects (p. 446).
Sense organs are well developed in the free members of the group.
Eyes are of two kinds, the compound eyes, of which a pair is usually
present except in the Copepoda and adult cirripedes, and the median
eye. Details of the structure of the compound eyes have been given
above (p. 310). They may be sessile or stalked, and the latter con-
dition has given rise to a theory that they represent a pair of append-
ages. Since, however, there are no somites corresponding to their
ganglia and since at their first appearance in the embryo they are
sessile, this view is not generally accepted (see also p. 312). The
median eye (Fig. 226) is the eye of the Nauplius larva, and it persists
in most adults, though it is generally vestigial in the Malacostraca. It
consists of three pigmented cups, one median and two lateral, each
^ As in other arthropods, the name procerebrum is given to the anterior
part of the brain, composed of the protocerebrum, the optic lobes, and some-
times other gangha which are not connected with paired hmbs.
CRUSTACEA
341
n. ch
st.ar.
v%8,n.' ■
Fig. 225. A semidiagrammatic view of central nervous system of Astacus.
From Borradaile. ab. i, ab. 6, the first and sixth abdominal ganglia; cer.
cerebral ganglion (brain); c.oes. circumoesophageal commissure; I.e. longi-
tudinal commissures of ventral cord; n.ab.l. nerves to abdominal limbs;
n.an.' nerve to antennule; n.an." nerve to antenna; n.ch. nerve to cheliped;
n.m. nerves to limbs adjoining the mouth; opt.n. optic nerve; s.oes. sub-
oesophageal ganglion; st.ar. sternal artery; th. i, th. 6, first and sixth thoracic
ganglia; vis.n. nerve to proventriculus ; vis.n.' nerve to hind gut.
342
THE INVERTEBRATA
of which is filled with retinal cells whose outer ends are continued as
nerve fibres. Thus the sense cells are inverted, as in the eyes of verte-
brata. Sometimes each cup has a lens. In some of the Copepoda the
lateral cups are removed from the median one and developed as a
pair of lateral eyes. Senses other than sight are subserved by various
modifications of the bristles which exist on the surface of the body
and contain nerve fibrils in their protoplasmic contents. Most of these
bristles are branched in various ways and have tactile functions, in-
cluding that of appreciating the resistance of the water to movements.
In the Decapoda and Syncarida on the basal joint of the antennule
(Fig. 227) and in the Mysidae on the endopodite of the sixth ab-
dominal appendage there is a pit whose wall bears such hairs while
the hollow usually contains sand grains (most decapods) or a cal-
careous body formed by the animal (Mysidae). These organs are
Fig. 226. A horizontal section through the median eye of Cypris. After
Claus. a, position of the median (ventral-anterior) cup, which is not in the
plane of section ; Is. lens ; n. nerve fibres ; pig. pigment layer ; rd. visual rod ;
ret. retinal cells.
statocysts for the sense of balance. Olfactory hairs or aestheiascs
(Fig. 227 B, E) with delicate cuticle stand on most antennules and on
many antennae. A pair of groups of cells, sometimes surmounted by
setae, standing on the front of the head and known as frontal organs,
are found in many crustaceans and are supposed to be sensory. They
are present as two papillae in the Naiiplius larva (Fig. 258, ten.). The
iiuchal sense organ or "neck organ" of many branchiopods is a group
of cells on the upper side of the head containing refractive bodies and
connected to the brain by a special nerve. Its function is unknown.
As is well known, most crustaceans are pigmented. The pigments
are of various colours — red, orange, yellow, violet, green, blue, brown,
black, etc., though not all are found in any one species. The majority
of them are lipochromcs, though the brown and black are melanins.
For the most part they are contained in branched cells (chromato-
CRUSTACEA
343
phores), but some of the blue, and perhaps of certain others, is
diffused in the tissues. The chromatophores may He in the epidermal
layer, in the dermis, or in the connective tissue of deeper organs. Their
aes
op.-
Fig. 227. The antennule of Astaais. A, The right antennule, seen from the
median side with the basal joint opened and the flagella cut short. B, Two
joints of the distal part of the outer flagellum, enlarged, after Huxley. C, The
basal joint of the left antennule, from above. D, Two hairs from the stato-
cyst. E, An aesthetasc. aes. aesthetasc ; en. inner flagellum ; ex. outer flagellum ;
grn. sand grains; w. nerve; n.f. nerve fibre; op. opening of statocyst, overhung
by a fringe of hairs ; stc. statocyst.
behaviour has been studied in various malacostracans. The pigment
is often caused to expand or contract, which it does by flowing into
and out of their processes. In this it is affected by light, responding
both to intensity of illumination and to the nature of the background,
344 THE INVERTEBRATA
but only rarely to colour (wave-length). In light of high intensity or
on a light-absorbing (e.g. dull black) background it expands; in light
of low intensity or on a light-dispersing (e.g. dull white) background
it contracts. Different pigments are affected to different degrees, and
thus both the degree and the pattern of the coloration of a sensitive
species (notably, for instance, of many prawns), changes with its
surroundings — usually, in nature, in such a way as to render the animal
inconspicuous. The response to intensity of illumination is due to
direct action of the light upon the chromatophores and will thus take
place even in blinded animals; the response to background depends
upon the eyes. The eyes, however, do not act through nerves to the
chromatophores, but by causing certain endocrine glands to pass
hormones into the blood.
The alimentary canal (Figs. 233, 236, 244, 278, 289) is with very
rare exceptions straight, save at its anterior end, where it ascends
from the ventral mouth. The/or^ gut and hind gut (stomodaeum and
proctodaeum), lined with cuticle inturned at the mouth and anus,
leave a varying length of mid gut (mesenteron) between them. The
intrinsic musculature, sometimes supplemented by extrinsic muscles
running to the body wall, is strongest in the fore gut, whose lining
sometimes develops teeth or hairs. In the Malacostraca (Fig. 228)
these elements become a more complex proventriculus ("stomach"),
with a ''gastric mill" and a filtering apparatus of bristles which
strains particles from the juices of the food, the mill and filter
being often in separate "cardiac" and "pyloric" chambers. The
mid gut usually bears near its anterior end one or more pairs of
diverticula ("hepatic coeca"), which serve for secretion and absorp-
tion and may branch to form a "liver". This gland, however,
unlike the liver of vertebrates, forms all the enzymes necessary
for the digestion of the food and absorbs from its lumen the products
of digestion. It stores the reserves in the form of glycogen and fat.
Occasionally there is an anterior median dorsal coecum. Coeca
are also sometimes found at the hinder end of the mid gut: these
are more often median. In a few cases the hind gut is absent and the
mesenteron ends blindly. In the Rhizocephala and the monstrillid
copepods (p. 374) the alimentary canal is absent throughout life,
for these animals absorb through the skin during the parasitic
period enough nutriment to last through an entire life history.
Digestion is extracellular. The fore gut is frequently the seat of
mechanical processes, and sometimes of chemical action by juices
secreted by the mid gut diverticula, but never of absorption. The
latter process as well as most of the chemical work is performed by
the mid gut, including the hepatic diverticula. In the hind gut the
faeces are passed to the anus, being in some entomostraca sheathed in
CRUSTACEA
345
a so-called *' peritrophic membrane " composed of a mucoid substance
secreted by certain cells of the epithelium.
The principal excretory organs of the Crustacea are two pairs of
glands, known as the antennal and maxillary glands, which open (Fig.
233, k.op.) at the bases of the appendages from which they take their
names. They are very rarely (Lophogastridae) both well developed at
md.g. I ij
vlv.^ lap.
Fig. 228. The left-hand side of the fore gut and mid gut of Astaciis, viewed
from within. Semidiagrammatic. bri. bristles which form part of the filtering
apparatus ; cd. cardiac chamber of the " stomach " ; cm. median dorsal coecum
of the mid gut; h.g. hind gut; l.d. opening of left liver duct; la. p. lateral,
finely-filtering pouch of the pyloric chamber leading to opening of liver;
la.tth. lateral teeth; m.' , m." , left anterior and posterior gastric muscles; mdg.
mid gut; me.tth. median tooth; oe. oesophagus; oss. i-oss. 6, cardiac, urocar-
diac, prepyloric, pyloric, pterocardiac, and zygocardiac ossicles — calcifications
of the cuticle of the stomach which constitute the mechanism of the gastric
mill: when the cardiac and pyloric ossicles are pulled in opposite directions
by the contraction of the muscles m.' and m." attached to them, the teeth are
brought together; press, chamber which presses the liquid out of the food
into the upper passage {u.p.) and the lateral pouches {la. p.) ; py. pyloric
chamber; 11. p. upper passage leading to mid gut; vlv.^ one of a pair of valves
which guide the faecal residue from the press into vlv."^, an enclosing valve
which conducts it to the hind gut; vlv.^ one of a pair of valves which guard
openings of liver ducts.
the same stage in the same species, but one may succeed the other as
a functional organ in the course of the life history: the antennal gland,
for instance, is the larval excretory organ of the Branchiopoda, but
the maxillary gland is that of the adult ; and the Decapoda, whose
adult kidney is the antennal gland, sometimes use as larvae the
maxillary gland instead. The maxillary gland is the more widespread
346 THE INVERTEBRATA
as an adult organ, the antennary gland being functional in the adult
only in certain of the Malacostraca. In the Ostracoda and Leptostraca
both are vestigial in the adult. Each of these glands (Figs. 229-231)
has an end sac and a duct leading from the end sac to the exterior. The
end sac is always mesodermal and doubtless represents a vestige of
the coelom. The duct is sometimes (in the Malacostraca probably
^sac.
Fig. 229. The maxillary gland of Estheria. After Cannon, sac. end sac.
Fig. 230. Diagrams of the antennal gland of the early metanauplius of
Estheria. After Cannon. A, The whole gland. B, The sphincter in section.
duct, intracellular ectodermal duct; ect. ectoderm; sac. end sac (coelomic);
sph. sphincter cells.
always) a multicellular, mesodermal structure, and sometimes intra-
cellular and of ectodermal origin. At the junction of end sac and duct
there is often a sphincter. The antennal gland of the Decapoda is
usually very complicated. That of the crayfish lacks extensions of the
bladder which lie among the viscera in many other genera, as in crabs.
All the parts of the organs are excretory, and the function of the
CRUSTACEA
347
sphincter of the end sac is perhaps to prevent the passage back into
that vesicle, which secretes ammoniacal compounds, of poisonous
products excreted in the duct.
These glands are probably the remaining members of a series of
segmental excretory organs. Their mesodermal portions are no doubt
Fig. 23 1 . Diagrams of the green gland of Astacus. From Parker and Haswell,
after Marchal. I, The organ unravelled. II, A section of the organ with the
parts in their natural positions, bl. bladder; c.p. cortical (green) portion of
the gland ; d. terminal duct; s. end sac; zo.p. medullary (white) portion of the
gland.
coelomoducts, homologous with those of the Annelida, their ecto-
dermal portions probably are not the homologues of nephridia but
represent ectodermal glands such as are common in the Crustacea.
Various other glands, mostly of doubtful morphological significance,
which occur in different crustaceans have been shown, or are sus-
348 THE INVERTEBRATA
pected, to have an excretory function. Thus, in Nebalia, eight pairs
of ectodermal glands at the bases of the thoracic limbs are excretory,
while in ostracods a pair of rather complex glands, also of ectodermal
origin, which lie between the folds of the shell in the antennal region,
may have a similar function. Excretion appears also sometimes to
be performed by coeca of the mid gut — as by some of those of the
barnacles and by the posterior pair of amphipods — or by cells of
the epithelium of the mid gut itself.
Respiration in many of the smaller crustaceans, notably in the Cope-
poda, takes place through the general surface of the body. In forms
with stouter cuticle or more bulky bodies this is supplemented or re-
placed by the use of special organs upon which the cuticle remains
thin. The most important of such organs are the lining of the cara-
pace, if that structure be present, and certain epipodites which are
known as gills and in many of the Malacostraca have their surface
increased by branching or folding (Figs, 222 F; 285; 287, i, 2). In
the Decapoda incorporation of the precoxa with the flank of the body
has brought it about that some of the gills (proepipodites, p. 337),
stand in that position and not upon the actual limbs (Fig. 232). Such
gills are known as "pleurobranchiae". In the Isopoda respiration is
effected by the broad rami of the abdominal limbs. Renewal of the
water upon the respiratory surfaces may be brought about by the
movements of the limbs upon which they are located, but often certain
appendages bear special lobes adapted to set up a current under the
carapace and thus to flush the chamber in which the gills and the
carapace lining are situated.
Some land crustaceans have no special adaptations for respiration
in air. In others the gill chamber is adapted, by the presence of
vascular tufts of the lining of the carapace, for use as a lung. The
woodlice, which are terrestrial members of the Isopoda, are remark-
able in approaching in their respiration the principle employed by
normally terrestrial arthropods, for the integument of their ab-
dominal limbs is invaginated to form branching tubes which resemble
tracheae.
The vascular system is seen in its most primitive condition in the
Branchiopoda Anostraca (Chirocephalus, Fig. 236). Here the heart (h.)
runs the whole length of the trunk, situated above the gut in a blood
sinus known as the pericardium, with which it communicates by a
pair of ostia in each somite except the last. In front it is continued
into the only artery, a short aorta, from which the blood flows direct
into the sinuses of the head and thence through those of the trunk to
the pericardium, eddies from a main ventral sinus supplying the
limbs. In all other Crustacea, except the Stomatopoda, the heart, if it
be present, is in some degree shortened, and in the Malacostraca (Fig.
CRUSTACEA
349
233) a system of arteries interposes between the heart and the sinuses,
leaving the former by several vessels, which conduct the blood to the
organs. In the Eucarida (Euphausiacea and Decapoda) the heart is
shortened to a compact shape and has three pairs of ostia; in most of
Fig. 232. Part of the left side of a late larva of the prawn Penaeus to show the
origin of the gills. Slightly magnified. After Claus. L^-L^, the first to fifth
legs; Mi-M^, the first to third maxiUipeds; la, 2a, 3a, ya, distal series of
rudiments, standing upon the coxopodites ; from these rudiments arise the
mastigobranchiae (see p. 408), and on the second maxilliped a podobranch also ;
16, 56, ic, 2C, 7c, members of two series of rudiments, standing where the
membrane of the joint between the coxopodite and the body will develop ;
from these respectively the anterior and posterior members of pairs of arthro-
branchiae arise ; i </, 5 J, 7 J, members of a fourth series of rudiments, standmg
on the basal parts (precoxae) of the limbs; from this series will arise the
pleurobranchiae, which, owing to the taking up of the precoxae into the body,
will stand on the side of the thorax.
the Cladocera it is a sac (Fig. 244, h.) with only one pair. In the Cirri-
pedia and many of the Copepoda and Ostracoda the heart is absent
and the blood is kept in movement only by the movements of the body
and alimentary canal. In the parasitic copepod Lernanthropus and
.^ ." .« ••> o i
>>
antenna
liking leg
tomach "
rd ; e. eye
phthalmi
k. kidne;
5 " w O O
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^ !? :: u .
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e; an
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erve
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^ *.
ride, an.'
ing; cA. ch
age; oe. oe
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ral thoraci
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shown bu
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CO rt r> ^2 "^ rrt
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; genit
minal
om. n(
; v.th.
§.2
hiple
male
abdo:
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^ <u
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n ; S op.
the last
al gangli
;terior a
4-* ^iZ^
c
nd side.
; tel. telso]
ion; ab.6,
ga. cerebr
rtery (pos
« g
•2.2
CO S^
a, rt
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ea C/3 ■'-' r>" c5
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t4H « a (u c
- = 8.S-g
<U "^ Ui w O
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IT ti
rom t'
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ifiedf
nt. int
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CO Clc
ed f]
f ant
mod
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dors
CO (U
■•-; o ., o •
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ppen<
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erv ;
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"S-C =^ 2j ^
'CUS fliivi
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bdomina
on ; cv.g.
sternal ;
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_2 .. rt i: .
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CO V '
CRUSTACEA
351
some related genera there is a remarkable system of closed blood
vessels without a heart.
The blood IS a pale fluid, which bears leucocytes except in ostracods
and most copepods. It contains in the Malacostraca the copper-
containing respiratory pigment haemocyanin (p. 133). In various
entomostraca, notably in Lernanthropus , just mentioned, haemo-
globin has been found.
r.ov.
Fig. 234. A, Male reproductive organs of Astacus fluviatilis. From Howes.
r.t. right anterior lobe of testis ; med.t. median posterior lobe of testis ; vas de.
vas deferens ; op. external opening of vas deferens ; leg, right fourth ambula-
tory leg on which the vas deferens opens. B, Female reproductive organs of
Astacus fluviatilis. From Howes, r.od. right oviduct :'the left oviduct is shown
partly opened ; r.ov. right lobe of ovary ; l.ov. left lobe of ovary with the upper
half removed to show the ovarian cavity, which is the remains of the coelom
and into which the ripe ova drop; op. external opening of oviduct; leg, right
second ambulatory leg on which the oviduct opens.
As is usual with animals that are free and active, the sexes are
separate in the great majority of the Crustacea, though the Cirripedia,
which are sessile, certain of the parasitic Isopoda, and a few excep-
tional species in other groups, are hermaphrodite. Parthenogenesis
takes place in many of the BranchioJDoda and Ostracoda, and in these
it is often only at more or less fixed intervals that sexual reproduction
occurs. The male is usually smaller than the female and in some
parasites is minute and attached to her body. He has often clasping-
organs for holding his partner, and these may be formed from almost
352 THE INVERTEBRATA
any of the appendages. He may also possess organs for the transfer-
ence of sperm: these may be modified appendages or protrusible
terminal portions of the vasa deferentia. The gonads of both sexes
(Fig. 234) are hollow organs from which ducts lead directly to the
exterior. Primarily there is one gonad on each side, but they often
unite more or less completely above the alimentary canal. The ducts
usually open near the middle of the body, though the male openings
of cirripedia and some cladocera are almost terminal and the female
opening of cirripedia is on the first thoracic somite. Save in the Cirri-
pedia, the Malacostraca, and some of the Cladocera, the ducts of the
two sexes open upon the same somite.
The spermatozoa are very varied in form and often of complex
structure ; usually, but not always, they are immobile. They are trans-
ferred to the female, often in packets (spermatophores) . The ova have
usually much yolk, and meroblastic, centrolecithal cleavage (Fig,
Fig. 235. A ventral view of the first Nauplius of Cyclops. After Dietrich.
an.' antennule; an." antenna ;^n. gnathobase; Ibr. labrum; md. mandible.
213 A-C), but sometimes are less yolky and undergo total cleavage.
Gastrulation may be by invagination (Fig. 213 D) or by immigration.
Occasionally the eggs are set free at laying, but in the great majority of
cases they are retained for a time by the mother, either in some kind
of brood pouch or adhering in some way to her body or appendages.
Development is not infrequently direct, but in most cases involves a
larval stage or stages.
Typically, the crustacean hatches as a Nauplius larva (Fig. 235), a
minute creature, egg-shaped with the broad end in front, unseg-
mented, but provided with three pairs of appendages — the antennules,
which are uniramous, and the antennae and mandibles, which are
biramous and should each bear a gnathobasic process or spine
directed towards the mouth, though those of the mandibles are often
not developed at first. The antennal ganglia are as yet postoral (see
p. 340). The median eye is the only organ of vision. A pair of frontal
organs (p. 342) are present as papillae or filaments. There is a large
CRUSTACEA 353
lab rum. Fore, mid and hind guts can be recognized in the alimentary-
canal. Antennal glands may be present. This larva is found in some
members of every class of the Crustacea, though among the Mala-
costraca only certain primitive genera possess it, and in the Ostracoda
it is modified by having already at hatching a precociously developed
bivalved carapace. In every class, however, it is also often passed over,
and becomes an embryonic stage within the egg membrane or in a
brood pouch, the animal hatching at a later stage, such as the Meta-
nauplius and Zoaea mentioned below, or even almost as an adult.
In the Branchiopoda and Ostracoda the Nauplius is transformed
gradually into the adult, adding somite after somite in order from
before backwards by budding in front of the telson, much as somites
are added to the trochosphere in the development of annelids, while
by degrees the other features of the adult develop. The early stages
of this process, which possess more somites than the Nauplius, but
have not yet the adult form, are known as Metanauplii. The carapace
is often foreshadowed quite early by a dorsal shield, which later grows
out behind and at the sides to assume the form which it has in the
adult, and the appendages, at first mere buds, gradually take on their
final shapes.
In most cases, however, the process just described is modified.
{a) It makes a sudden great advance at one moult. In the Cirripedia
the late Nauplius passes with a leap to the so-called Cypris larva,
which has many of the features of the adult : a similar leap takes the
copepod Metanauplius to the first " Cyclops'' stage (p. 373) and those
of Malacostraca to the Zoaea. {b) Certain structures may be pre-
cociously developed. In those of the Malacostraca which have
Nauplii, the Metanauplius is followed by stages, known as Zoaeae, in
which the abdomen is well developed, while the thorax, though it
already possesses in front a few pairs of biramous appendages, is
still rudimentary in its hinder part. In these larvae also the last pair
of abdominal limbs usually appears, or comes to functional develop-
ment, before the others. Zoaeae, however, most often are not
preceded by a free Nauplius but appear as the first free stage (Fig.
291 A), (c) Temporary retrogression of certain organs takes place
during the development of some of the Malacostraca: this affects some
of the thoracic limbs in certain Stomatopoda and the prawn Sergestes,
abdominal swimmerets and the antennule in the prawn Penaeus.
Class BRANCHIOPODA
Free Crustacea with compound eyes ; usually a carapace ; the mandi-
bular palp very rarely present and then as a minute vestige; and at
least four, usually more, pairs of trunk limbs, which are in most cases
broad, lobed, and fringed on the inner edge with bristles.
354 THE INVERTEBRATA
The Branchiopoda are, on the whole, the most primitive class of
the Crustacea. This is seen in the varying and usually large number
of their somites, the usually small amount of differentiation in the
series of limbs on the trunk, the vascular system of the lower members
of the group (p. 348), and the nervous system of all (p. 340). Their
mouth parts, on the other hand, are small and simple in structure, a
condition in which they are not primitive but exhibit reduction.
Nearly all of them are, like sundry other archaic animals, of freshwater
habitat, and their characteristic mode of feeding is the taking, by
means of setae upon their trunk limbs, of particles of detritus or
plankton from suspension in the water.
The primary divisions of the class have been mentioned on p. 327.
The most conspicuous differences between them are in the cara-
pace, the compound eyes, the antennae, the trunk limbs, and the
telson.
The carapace is very variously developed. In the Anostraca it is
not present. The Notostraca have it as a broad, shallow cover over
the back. In the normal Cladocera ("Calyptomera") it bends down
at the sides to enclose the trunk as a shell, which forms a brood pouch
over the back. In the two groups of aberrant Cladocera which (though
they are probably not closely related) are together known as "Gym-
nomera" this shell has shrunken to a dorsal brood pouch leaving the
trunk partly or wholly uncovered. In the Conchostraca it forms in
the same way as in the Cladocera a shell, but here the head is usually
enclosed as well as the trunk, and there is a distinct dorsal hinge of
thin cuticle separating two valves which can be closed by an adductor
muscle situated in the maxillulary somite. Usually the carapace leaves
the trunk free within it, but in the Cladocera it fuses with two — in
Leptodora (p. 368) with all — of the thoracic somites.
The antennae, which in the Nauplius are biramous and natatory,
retain this condition in the adult of those forms (Diplostraca) in
which the enclosing carapace has deprived the trunk limbs of the
swimming function, and also in the extinct Lepidocaris (Lipostraca).
In the recent Anostraca the antennae are stout but uniramous and not
natatory; in the male they are adapted to clasping the female. In the
Notostraca, which apply the head to the ground in feeding, they are
reduced to uniramous vestiges.
The trunk limbs (except in the aberrant Cladocera which constitute
the Gymnomera) are phyllopodia (p. 336) which bear on the median
side endites furnished with feathered bristles and on the outer side,
besides the exopodite or flabellum, a thin-walled branchia and often
also one or two proepipodites. With these appendages the Anostraca
and Notostraca swim, and all members of the class breathe and gather
food. Beating rhythmically forward and backward with a movement
BRANCHIOPODA 355
which each pair starts a little earlier than the pair in front of it, they
cause, bya pumping action which shall be described presently(p. 358),
a flow of water into the median gully whose sides are formed by the
two rows of limbs, thence outwards into the spaces between each
limb and its neighbours in front and behind, and then backwards.
This current brings with it the particles which serve for food, bathes
the branchiae, and causes, in the Anostraca and Notostraca, forward
movement of the body. As the water passes outwards, the food
particles are, by the bristles on the endites, strained off and retained
in the median gully. The apparatus varies in detail with the nature
of the food. In the Notostraca, which feed mainly by stirring up,
with the tips of their thoracic limbs, detritus on the bottom and then
filtering it, the bristles on the endites are not adapted to straining
out fine particles (which therefore escape with the outgoing current)
but detain coarser particles. This is perhaps the primitive mode of
feeding of the Branchiopoda, and may even be inherited from the
Trilobites. In the Anostraca and Diplostraca there is a special ap-
paratus for filtering off fine particles. This consists of a close set row
of long, finely feathered setae, placed on the edge of the endites and
so disposed as to cover the opening from the median gully to the space
between the limb and its neighbour behind. Members of these orders
which derive part or all of their food from detritus have various kinds
of apparatus, composed of bristles, for removing the coarse particles
and passing them backwards to be either swept away with the out-
going stream or broken up for food by the hinder members of the
series of limbs. Finally, the material gathered is passed forwards to
the mouth in a median "food groove" along the belly by a current
whose causation is a matter of dispute. The feeding apparatus whose
principles have just been described differs greatly in detail in different
branchiopods, and reaches its highest complication in the tribe of
cladocera known as Anomopoda, to which the common water-flea
Daphnia belongs. Examples of it are described more fully below.
The Gymnomera have slender, mobile, jointed trunk limbs with
which they manipulate the relatively large organisms which serve
them for food.
The telson is in the Anostraca subcylindrical, with the caudal rami
as elongate plates or styles ; in the Notostraca it has the rami long and
many-jointed, and is in Lepidurus produced backwards on the dorsal
side as a plate. In the typical Diplostraca it is flexed ventrally and
produced backwards laterally into a pair of strong, curved, toothed
claws, and can be brought forward ventrally to clear the gully between
the limbs. In the Gymnomera it has re-straightened.
The compound eyes are in the Anostraca stalked (in Lepidocaris they
appear to have been absent). In the remainder of the class they are
356 THE INVERTEBRATA
sessile and covered by an invagination of the outer cuticle, which forms
a shallow chamber over them.
Artemia salina (p. 359) and a few marine cladocera are the only
members of the class whose habitat is not in fresh water.
Throughout the group, thick-shelled eggs capable of resisting
drought or freezing are produced by sexual reproduction. Often
there is also parthenogenesis, the eggs of which are usually thinner
shelled than those that are sexually produced (see p. 367).
The name Phyllopoda, which is applied sometimes to the whole
class and sometimes to its members exclusive of the Cladocera, is
on account of this ambiguity best not employed in systematic
nomenclature.
Order ANOSTRACA
Branchiopoda without carapace ; with stalked eyes ; with antennae of
a fair size but not biramous ; with the trunk limbs numerous and all
alike; and with the caudal rami unjointed, and flat or subcylindrical.
We may take as an example of this group, Chirocephalus diaphanus
(Fig. 236), one of its two British representatives. This creature turns
up from time to time in temporary pools of water in various districts.
It is about half an inch in length, transparent, and almost colourless,
save for the reddened tips of most of the appendages and of the ab-
domen, the black eyes, and often a green mass of algae in the gut. It
is incessantly in motion, swimming on its back. Its delicate appear-
ance, and the iridescent gleaming of the bristles on its appendages as
they are moved have earned it the name of the fairy shrimp. The body
is long, subcylindrical, and enlarged anteriorly to form the head^ upon
which the mandibular groove (p. 332) is conspicuous. The head has
in front a median eye and a neck organ (p. 342), and bears at the sides :
{a) the large, stalked compound eyes; (b) the antennules, slender, un-
jointed, and ending in a tuft of sense-hairs ; (c) the stout antennae^ tri-
angular in the female but in the male (Fig. 237) elongate, two-jointed,
and carrying on the inside at the base a complicated, lobed "frontal
appendage " which comes into play when the limb is used for clasping
the female ; {d) the mandibles, whose bases are prominent at the sides
of the head, while the remaining part of each of them is directed to-
wards the mouth as a process with a blunt, roughened end. Below,
the head bears {a) the large labrum which is directed backwards under
the mouth ; {b) the paragnatha, a pair of small, hairy lobes behind the
mouth; [c) the maxillules, a pair ot small triangular plates fringed
by long bristles ; {d) the maxillae, which are microscopic vestiges, each
bearing three spines.
Behind the head come eleven thoracic somites which bear each a
pair of phyllopodia. Fig. 238 shows that these possess all the typical
BRANCHIOPODA
357
features of such limbs but are remarkable for the distal position of
the exopodite and for the very long basal endite, which may be
simply the gnathobase (p. 337) but probably represents also the
th.M
h. a I.
md.
Fig. 236. A female of Chirocephalus diaphanus. The animal is seen from the
right-hand side in the morphological position : normally it swims upside down.
ab. I, ab.y, first and seventh abdominal somites; al. alimentary canal; an/ an-
tennule; an." antenna; e. compound eye; e.' median eye; egg p. egg pouch;
h. heart; Ibr. labrum; Ir. liver; 7nd. base of mandible; nk.on. neck organ;
ov. ovary; rayn. ramus of caudal fork; tel. telson; th. 11, eleventh thoracic
limb; th. 12, twelfth thoracic somite.
nk.on.
>an:
Fig. 237. Fig. 238.
Fig, 237. A front view of the head of a male Chirocephalus. an.' antennule;
an." antenna; e. compound eye; e.' median eyQ\ fr.ap. frontal appendage;
nk.on. neck organ.
Fig. 238. A thoracic limb of Chirocephalus, mounted flat. br. branchia;
bri. bristles which strain out the food; ep. epipodite; ex. exopodite ; ^6. flabel-
\\xra\pr.ep. proepipodites ; 1-7, endites.
second endite. The fringe of long bristles on the median border
is, in life, directed backwards, roughly at right angles to the main
plane of the limb. The twelfth thoracic somite, upon which are
358 THE INVERTEBRATA
the genital openings, is fused ventrally with the first abdominal. In
the male, it bears a pair of ventrolateral processes in each of which is
the terminal portion of a vas deferens, with a protrusible penis which
probably represents an appendage. In the female there is here a
median, ventral, projecting egg pouch, which, like the penes, is held
to represent a pair of limbs. The abdomen consists of seven simple,
limbless somites and a telson which bears a pair of caudal rami as
narrow, pointed plates, fringed with bristles.
The alimentary canal begins with a short, vertical fore gut, or
oesophagus. This leads to a mid gut which continues as far as the
telson, where it is succeeded by the hind gut or rectum. The mid gut
is somewhat wider in the head, where it is known as the stomachy than
in the trunk, where it is called the intestine. From the stomach pro-
ceeds a pair of sacculated diverticula ("liver"). The /oo^ consists
partly of coarse detritus gathered by the trunk limbs from the bottom
of the pool, and partly of small organic particles, especially unicellular
algae, which are strained off from the water by the trunk limbs in the
following manner (Figs. 239, 240). The space which exists between
each limb and that behind it is enlarged at the forward stroke, which
finishes with the limbs vertical, and narrowed at the back stroke, which
ends with them roughly horizontal, lying against the body. During
the forward stroke the enlarging of this space exerts a suction. The
proepipodites, exopodite, and large distal endite are drawn back by
the suction and pressed back by the resistance of the water, till they
reach the limb behind and so convert the space just mentioned into
a chamber which is closed except on the median side, where it is
separated only by the backwardly directed bristle fringe from the
median gully between the limbs of the right and left sides. From this
gully, therefore, water is drawn into the chambers at the sides as they
enlarge, particles which it contains being strained off by the bristles
and remaining in the gully. The latter is of course replenished by the
entrance of water from the ventral side. During the back stroke, the
chambers, as they become smaller and the pressure of the water in
them rises, open owing to this pressure lifting the structures which
had closed them ; and the water they contain is driven out and back-
ward in two ventrolateral streams, the animal being driven forwards.
Thus the same movement of the limbs serves both for the gathering
of food and for swimming. The particles which are retained in the
median gully are drawn dorsalwards because the suction of the side
chambers is greatest where they enlarge most, at the bases of the
limbs, and so get into a median food groove of the ventral surface.
There they are carried forward to the mouth by a minor stream, which
is said to be caused by the escape forwards at the bases of the limbs
of some of the water contained in the lateral chambers at a certain
ANOSTRACA 359
phase of the movement. The food is agglutinated by a sticky secretion
produced by glands in the labrum, and pushed by the maxillules
between the mandibles, which pound it and pass it into the mouth.
The organs of excretion are a pair of maxillary glands (p. 345),
situated in the hinder part of the head and the first thoracic somite.
They are wholly of mesodermal origin. The nervous system (Fig. 210)
and the vascular system have been described above (pp. 340 and 348).
^ht gonads are a pair of tubes lying one on each side of the alimentary
canal in the abdomen, and are continuous in front each with a short
duct. The vasa deferentia lead to the penes, the oviducts to a median
Fig. 239. A diagrammatic view of a Chirocephalus swimming on its back.
The arrows show the direction of the currents set up by the action of the
thoracic limbs, the dotted line the course of the gathered particles in the food
groove.
Fig. 240. Thoracic limbs of Chirocephalus seen from the median side in two
phases of their action. A, The forward stroke : water is being drawn through
the fringe of bristles into the space between the limbs, which is enlarging.
B, The backward stroke: water is being driven backwards out of the space
between the limbs, which is contracting.
uterus in the egg pouch. The eggs are enclosed in stout shells and will
remain alive in dry mud for many months. The larva at hatching is a
late Nauplius in which, though there are no appendages behind the
mandibles, the trunk is already distinct from the head.
Artemia salina^ the other British species of anostracan, occurs in
various parts of Europe in salt lakes and marshes and in pans in
which brine is being concentrated. It can endure a very high con-
centration of salt, and some of its minor features change with the
degree of the concentration, so that it has been described under
different specific names. It differs from Chirocephalus in having only
six abdominal somites and in the form of the antennae of the male.
360 THE INVERTEBRATA
Lepidocaris (Suborder Lipostraca), a minute, blind, freshwater form
from the Middle Devonian, was closely related to the Anostraca which
survive (Euanostraca), but differed from them in the following, among
other respects. It had biramous antennae which recall those of the
Cladocera; a clasping organ on the maxillule of the male, instead of on
the antenna; and the trunk limbs without branchiae and differentiated
into two sets — the first three pairs adapted for gathering food, with
gnathobase and with the last endite directed inwards and the exo-
podite lateral, and the remaining pairs adapted for swimming, with
the last endite and the exopodite directed distally side by side at the
end of the limb.
Order NOTOSTRACA
Branchiopoda with a carapace in the form of a broad shield above the
trunk ; the compound eyes sessile and close together ; the antennules
and antennae much reduced ; the trunk limbs numerous, the first two
.fi nid.
Fig. 241. A ventral view of the head region of Lepidurus glacialis. From
Caiman, a.' antennule; a." antenna; gn. gnathobase; L. labrum (turned
forwards); /. paragnathum; md. mandible; mx.' maxillule; mx." maxilla.
pairs of them differing considerably from the rest ; and slender, multi-
articulate caudal rami.
This order contains only the genera Apus and Lepidurus^ which
differ in but minor features. Apus cancriformis (Fig. 242) is British,
but is now very rarely found in these islands. The head is broad and
depressed, fiat below and arched above, and forms with the carapace
a horseshoe-shaped structure, which bears the eyes above and the
small antennules and antennae beneath, at some distance from the
sharp front edge. There is a dorsal organ, which is not used for fixa-
tion, but no nuchal sense organ. From under the carapace the hinder
part of the trunk projects backwards, ending in two long, jointed
caudal rami. The genital opening is on the i ith of the trunk somites.
Each of these bears a pair of limbs until the 13th (second of the ab-
domen) is reached, after which there are two to five pairs to a somite
br. Jib. car. th.^^
c
Fig. 242. Apus caficr if omits. A, Dorsal view. car. carapace; d.on. dorsal
organ ; e. compound eye ; e.' median eye ; ram. ramus of caudal fork ; sh.gl. shell
gland (maxillary gland) seen through the carapace ; th.i, processes of the first
thoracic limb. B, Ventral view. ab. abdominal limbs; ab.' limbless somites
of the abdomen; an.' antennule; car. carapace; Ibr. labrum; ?nd. mandible;
mx.' maxillule; mx." maxilla ;^^n. paragnathum; ram. ramus of caudal fork;
th.i, first thoracic limb; th. 10, tenth thoracic limb; th.ii, egg pouch on
eleventh thoracic limb. C, Side view with the left half of the carapace cut
away. br. branchia;^6. flabellum. Other letters as above.
362 THE INVERTEBRATA
as far as the 28th somite. Five limbless somites separate this from the
telson. The first thoracic limb is a modified phyllopodium, with the
endites slender and many-jointed, very long in Apus though shorter
in Lepidurus (Fig. 241). The second thoracic limb is less modified in
the same direction, the endites being shorter and unjointed. The re-
maining trunk limbs (Fig. 224) are normal phyllopodia: they decrease
in size from before backwards, and those of the thorax have the endites
well chitinized and mobile. Feeding is most often upon detritus (see
p. 355), the flat underside of the head being applied to the bottom
during the process, but the animals also devour the dead or living
bodies of organisms, clasping them with their strong thoracic limbs
and rasping fragments from them with the endites. The Notostraca
swim well, but can also crawl with their thoracic limbs or clamber
with the anterior pairs.
The limbs of the genital somite are in the female modified for
carrying eggs, the flabellum fitting as a lid over a cup formed by the
distal part of the axis. Pvlales are rare, reproduction being normally
by parthenogenesis.
Order DIPLOSTRACA
Branchiopoda with a compressed carapace which usually encloses the
trunk and its limbs ; the compound eyes sessile and apposed or fused ;
the antennae large and biramous ; four to twenty-seven pairs of trunk
limbs, often considerably diflPerentiated ; and the telson usually ending
in a pair of curved claws.
Suborder CONCHOSTRACA
Diplostraca with 10-27 pairs of trunk limbs; the carapace provided
with adductor muscle in the maxillulary somite and with hinge, not
fused with thoracic somites, and usually enclosing the head; and
nearly always a Nauplius larva.
No member of this order is British.
The animals haunt the bottom and are mainly or exclusively
detritus feeders, dealing differently with fine and coarse particles
(P-355)-.
Estheria (Fig. 243) is a common European genus. A thoracic limb
of a related but exotic form is shown in Fig. 222 C.
Suborder CLADOCERA
Diplostraca with 4-6 pairs of trunk limbs ; the carapace without hinge
or adductor muscle, fused with two or more thoracic somites, and
not covering the head ; and without Nauplius larva (save in Leptodora).
The members of this suborder are the water fleas. They fall into
four tribes. Of these, the first, known as Ctenopoda, show affinities
BRANCHIOPODA 363
with the lower Branchiopoda in that their trunk limbs, of which there
are six pairs, are all alike and all strain food from the water, the
gnathobase projects, and the heart is elongate. The shell is well
developed and covers the trunk limbs. Sida, which may be taken
add
ram.
ThA
Fig. 243. Estheria ohliqua. From Caiman, after Sars. A, Shell of female,
from the left side. B, Male seen from the side after removal of left valve of
shell, add. adductor muscle; aw.' antennule; an." antenna; md. mandible;
ram. caudal ramus; Th. i, first thoracic limb.
among weeds in pools in various, parts of Britain, is one of the
Ctenopoda. Penilia, one of the few marine cladocera, is another.
The second tribe of the Cladocera, known as Anomopoda, contains
most of the genera of the suborder. Its members retain a well-
developed shell, but the trunk limbs, of which there are often only-
five, and sometimes only four, pairs, are highly differentiated for
364 THE INVERTEBRATA
various parts of the process of feeding, only some of them doing the
actual filtering off of the food particles. The gnathobases of the filter-
ing limbs do not project but are enlarged to bear most of the filter
fringe. The heart is a short sac in the first two trunk somites.
Daphnia and Simocephalus ^ common British forms, found swim-
ming in ponds and ditches, are examples of this tribe. Simocephalus
(Fig. 244) differs from Daphnia in possessing a cervical groove (p. 332),
and in lacking a median dorsal spine which in Daphnia stands on the
hinder edge of the carapace. The following description applies to both
genera. The head is bent downwards, so that the median eye and the
small antennules are ventral to the antennae. A large, sessile com-
pound eye, formed by the fusion of a pair, stands in front. Above it
is a nuchal sense organ. Of the rami of the antennae one has four
joints and the other three, and both bear long, feathered setae. The
mouth parts are much like those of Chirocephalus (p. 356). The seg-
mentation of the trunk is obscure. The first two somites are fused
with the head, as is shown by the position of their appendages. Behind
these are three fairly distinct limb-bearing somites (so that there are
in all five pairs of trunk limbs), and then three that are limbless and
hardly distinguishable and a telson, which is compressed and produced
on each side of the anus into a toothed plate, bearing terminally a spine
that may represent a furcal ramus. The third free somite is longer
than the others and bears its limbs in the hinder part, which suggests
that it is the fifth of the six pairs of Sida which is missing here. The
limbless region is commonly known as the "abdomen". Two strong
dorsal processes on it close the brood chamber behind.
The structure of the trunk limbs is shown in Fig. 245. Together
they form a food-gathering mechanism which is very efficient be-
cause, instead of all working in the same way as those of the Anostraca,
they are diflterentiated in adaptation to diff^erent parts of the task. The
third and fourth pairs form a pumping and straining apparatus (Fig.
246) which in principle is the same as those formed by the limbs of
Chirocephalus, but has for side walls the carapace, against which the
proepipodites play, and is closed behind by a barrier formed by the
fifth pair. The broad exopodites of the third and fourth pairs open
and close the ventral side of the apparatus as they flap to and fro
under the pressure of the water. The long, feathered bristles of the
first and of the distal part of the second pair guard the ventral opening
of the median gully and keep too large particles from being drawn
into it. The complex set of bristles upon the large endite or "gnatho-
base" (which corresponds both to the first and to the second endite
of the ideal series) in this limb play some part — exactly what is dis-
puted— in bringing the food to the mouth. Glands in the lab rum
produce a sticky secretion as in Chirocephalus.
CLADOCERA
365
- -mx:
vas de
Fig. 244. A, Side view of male Shnocephalus sima. Highly magnified. From
Shipley and MacBride. aii.' antennules"; an." antennae; t. testis; vas de. vas
deferens ; hep. hepatic diverticulum ; h. heart ; sh.gl. shell gland ; mes. mid gut ;
nk.on. neck organ. B, Side view of female Shnocephalus sima, magnified to
the same extent as A. From Cunnington. an.' antennules; an." antennae;
md. mandibles; mx.' maxillules; Th.i-Th.5, thoracic limbs; hep. hepatic
diverticulum; h. heart; ov. ovary; bdp. brood pouch; sh.gl. shell gland;
brn. brain; md.g. mid gut; 7ik.on. neck organ.
pnep.
D
Fig. 245. Thoracic limbs of Daphnia pulex. After Lilljeborg. A, First.
B, Second. C, Third. D, Fifth Hmb. ap. apical lobe of endopodite; br.
branchia; en. endopodite, indistinctly divided, on the 3rd and 4th limbs, into
three joints which are not shown; ex. exopodite ; /r. fringe of bristles which
strains out the food : normally this fringe stands vertical to the plane of the
limb (see Fig. 246, bri.), but it has been mounted flat for drawing; the part of
the limb upon which it stands probably corresponds to the gnathobase and
two succeeding endites; "gn." "gnathobase"; /)r.e/). proepipodite.
CLADOCERA 367
The alimentary canal resembles that of Chirocephalus (p. 358), but
the coeca are unb ranched. The food on being swallowed passes direct
to the middle part of the mesenteron, where it is digested, and then
forwards to the anterior region and the coeca, where the digested
products are absorbed and the indigestible residue sent backwards to
be formed into faecal pellets in the hinder part of the mid gut. The
maxillary gland lies in the carapace.
The gonads are simple, elongated sacs lying in the trunk and con-
tinuous with their ducts, which open in the male on the telson, in the
female dorsally behind the last limb. The eggs are yolky. They are of
two kinds, "summer" eggs which have relatively little yolk and de-
p.ch.
me.gy.
-th.3
Fig. 246. A diagram of a transverse section through the thorax of Daphnia.
After Storch. bri. bristles of the fringes which strain out the food ; bri.' bristles
of the second pair of thoracic limbs which guard the opening of the median
gully; car. carapace; d. dorsal surface of the thorax; fd.gr. food groove;
me.gy. median gully or filter chamber ; p.ch. chambers between the limbs : the
enlargement and contraction of these chambers by the movements of the
limbs set up a pumping action by which water is caused to flow through the
bristle fringes from the median gully; pr.ep. proepipodites, playing upon the
carapace and closing the pumping chambers at their outer sides; 1/1.2-4,
sections through the thoracic limbs, which being directed backwards are cut
transversely: each limb underlies that behind it.
velop rapidly by parthenogenesis in the brood pouch of the mother,
and "winter" eggs with much yolk which need fertilization and
develop slowly. The winter eggs are fertilized in the brood pouch, but
then the cuticle of the carapace, which has thickened, is thrown off
as a case — the ephippium — in which they are contained. They go
through the early stages of segmentation within a short time, but after
this a period of quiescence sets in, during which they may be. dried
or frozen without injury. Sexual reproduction takes place at certain
times only, normally twice a year. After the winter eggs develop in
spring, there are for some half-dozen generations no males, and re-
production proceeds by parthenogenesis. Then, about May, a genera-
368 THE INVERTEBRATA
tion appears in which males are present. In this sexual and asexual
reproduction go on side by side. The same thing occurs again in
autumn or at other times when, in unfavourable circumstances, such
as cold or starvation, males appear. It is interesting to note that,
since parthenogenesis is never suspended by all the females, there is
nothing to show that a sexual phase in the life cycle is necessary.
The normal cladocerans which compose the tribes Ctenopoda and
Anomopoda are often united under the name Calyptomera in contrast
to the remaining two tribes, which are known as Gymnomera. These
are aberrant forms whose food consists of planktonic organisms
relatively much larger than the particles upon which Daphnia feeds.
Their carapace has shrunk till it forms only the brood pouch and
leaves free the comparatively slender, prehensile trunk limbs with
which the food is handled, and their eyes are prominent and adapted
to sighting moving objects. They are often bizarre in form.
Polyphemus, a British freshwater genus, is an example* of the Tribe
Onychopoda. It has a long telson, but the head and "abdomen" are
not elongate and the carapace does not fuse with the hinder part of
the "thorax". The trunk limbs have gnathobases. In Evadne and
Podon, marine members of the tribe, the telson is not elongate.
Leptodora (Fig. 247), the only member of the Tribe Haplopoda, is
a pelagic inhabitant of certain fresh waters in Britain and elsewhere.
The body is long and slender owing to elongation of the head and of
the " abdomen", in which the segmentation is distinct. The fore part
of the trunk bears six pairs of slender, jointed, uniramous limbs,
without gnathobases. The carapace has fused with all the somites of
this region and projects behind it as a brood pouch. The winter tgg
gives rise to a Nauplius, the only instance of a larva in the Cladocera.
Class OSTRACODA
Free Crustacea, with or without compound eyes; with a bivalve
carapace and an adductor muscle; a mandibular palp, usually bi-
ramous; and not more than two recognizable pairs of trunk limbs,
these not being phyllopodia.
The small crustaceans which compose this class differ little in the
general form of the body but show very great variety in that of their
appendages. All their cephalic limbs are well developed and complex;
the trunk limbs are uniramous and one or both pairs may be lost.
The adductor is in the maxillulary somite. There is often a gastric
mill and usually a pair or more of hepatic coeca : the latter and the
gonads may (Cypris) extend into the shell valves. Both antennal and
maxillary glands are present, both have ectodermal ducts, and both
CRUSTACEA
369
are without opening in the adult. Other glands may be excretory.
The NaupliuSy if present, has a bivalve shell. There are among the
ostracods freshwater and marine, pelagic and bottom-living forms.
Parthenogenesis is common among them, and in some males have
never been found.
fh.1
-ram.
Fig. 247. A female of Leptodora kindti. After Lilljeborg. an.' antennule;
an." antenna; car. carapace; mdg. mid gut; ov. ovary; ram. ramus of caudal
fork; tel. telson; th.i, first thoracic limb; trk.g, ninth trunk somite.
Cypris (Fig. 248) is a common British freshwater genus. It swims
well, by means of its antennae, but i& not pelagic. It is omnivorous,
feeding on algae, small animals, detritus, etc., and taking its food in
various ways. Large objects are pushed into the shell by the antennae
or pulled in by the mandibles, finer particles drawn in by the action of
the epipodites of the maxillules (whose fan of setae is conspicuous in
the figure), gathered by long bristles on the palps of the mandibles,
370 THE INVERTEBRATA
and passed towards the mouth by the endites and endopodites of the
maxillules, assisted by the gnathobase of the maxillae. The first trunk
limb is used in crawling, and the second in cleaning. Cypris lacks the
compound eyes and the heart, which are found in some other members
of the class — for instance in the marine Cypridina, which is also
characterized by a large antennal exopodite, turned outwards in a
notch of the shell for rowing.
an.—
an:
jTh.Q
ram.
Fig. 248. Lateral view of Cypris. After Zenker, an/ antennules; an." an-
tennae; md. mandibles; mx.' ist maxillae; mx." 2nd maxillae; Th.i, Th.2,
thoracic limbs ; ram. ramus of caudal fork ; e. eye.
Class COPEPODA
Free or parasitic Crustacea, without compound eyes or carapace;
with biramous or uniramous palp, or with none, on the mandible;
and typically with six pairs of trunk limbs, of which the first is always
and the sixth often uniramous, the rest biramous, and none are
situated behind the genital aperture (i.e. on the abdomen).
The form of the body varies greatly in the members of this class,
from the pear-shaped or club-shaped free-swimming genera to the
distorted, unsegmented, and sometimes even limbless adults of some
of the parasites. In all cases in which the segmentation is complete
the number of somites is the same — sixteen, including a preantennu-
lary somite but not the telson — throughout the group, but the actual
tagmata, which do not conform to the limits of the head, thorax, and
abdomen, are not uniform in all members of the class.
We shall take as an example of the group the little freshwater
crustacean Cyclops (Fig. 249) which, though it is not one of the most
primitive members of the Copepoda, is well segmented and can be
obtained everywhere in ponds and ditches. The shape of this animal
is that of a slender pear with a stalk. The front part of the pear is
unsegmented; this is a compound head or "cephalothorax", com-
posed of the true head and the first two thoracic somites : beneath, in
front, it bears a blunt projection, the rostrum. The rest of the broad
CRUSTACEA
part of the body contains three somites, the third to fifth of the thorax.
The cephalothorax and these free thoracic somites are produced at
an'.
Fig. 249. Cyclops. A, Dorsal view of female. Partly after Hartog. An.
position of anus ; aw.' antennule; «/z." antenna; e.^. egg sacs; e. eye; g.som.
compound somite, consisting of the last thoracic (bearing the genital opening)
and the first abdominal; od. oviduct; ov. ovary; ram. ramus of caudal fork;
5-^^ spermatheca or pouch for receiving the spermatozoa of the male ; ut.
uterus: i.e. pouch of the oviduct into which the eggs pass before being shed.
B, Ventral view of male. ab. abdomen; an/ antennule; an." antenna; cop.
copula; e. eye; Ibr. labrum; ynd. mandible; mx.' maxillule; w^c." maxilla;
7nxp. maxilliped; pgn. paragnathum; rant, ramus of caudal fork; tel. telson;
th. 2, th. 6, thoracic limbs.
the sides into low pleural folds. The stalk begins with a short somite
which is united to, but distinguishable from, that which succeeds it.
The next somite bears the genital openings and is therefore, on the
372
THE INVERTEBRATA
convention we have adopted (p. 333), the last somite of the true
thorax, but is usually reckoned as the first of the abdomen ; in the
female it is fused with the somite which succeeds it. Two free ab-
dominal somites and a telson, which bears two styliform, setose
caudal rami, complete the body. The somites of the thorax bear
limbs, which will be described presently. The
limbs of the somite of the genital opening are
present in the female only, and in her are re-
duced to the condition of small valves over
the openings of the oviducts. The abdominal
somites are without limbs in either sex. It
will be seen that the actual tagmata of Cyclops
are not the head, thorax, and abdomen, how-
ever the limit between thorax and abdomen
be fixed, but are a cephalothorax of eight
somites (including the preantennulary), a
mid-body (sometimes, but unsuitably, named
the " metasome ") of three somites, and a hind
body or **urosome" of five somites and the
telson.
On the head, the median eye is well de-
veloped. The antennules are long, uniramous,
provided with sensory hairs, divided into
seventeen segments, and in the male bent as
hooks to hold the female. The antennae are
shorter, slender, uniramous, and four-jointed.
The mandibles (Fig. 250, md.) have a toothed
blade (gnathobase) projecting towards the
mouth and a papilla, bearing a tuft of bristles,
which represents the palp. The maxillules
have a large gnathobase and small endopodite
and exopodite. The maxillae are uniramous.
The maxillipeds (first pair of thoracic limbs) Fig 250. Mouth parts of
f ^ f , . 1 • 1 Cyclops, r rom bedgwick,
are also uniramous; they stand immediately ^f^^j. ciaus. en. endopo-
internal to the maxillae. The 2nd to ^th dite; ex. exopodite; md.
thoracic limbs, of which the 2nd stands on the mandible ;mx/maxillule;
head, are biramous, with broad, flat, spiny J^'^-" maxilla ;wj£:;).maxil-
rami (Fig. 249 B). The protopodites of each
pair are united by a transverse plate or "copula" so that they move
together in swimming. The thoracic appendages of the 6th pair are
small and uniramous.
The swimming of Cyclops is of two kinds — a slow propulsion by the
antennae and antennules, and a swifter progression brought about
by the use of the swimming limbs (2nd to 5th pairs) of the thorax. In
7nxp
COPEPODA
373
the more primitive, pelagic copepods (Calanus, etc.) which have
biramous antennae and biramous palps on the mandibles, the an-
tennules do not take part in swimming. Such copepods feed by an
automatic straining of particles from the water, though their ap-
paratus for this purpose (see below) is very different from that of the
Branchiopoda. Cyclops, on the other hand, in a manner of which the
details are not understood, seizes its food particles from time to time.
The alimentary canal is of much the same nature as that of Chiro-
cephalus but without mid gut diverticula. It possesses well-developed
extrinsic muscles, of which those that run from its anterior region to
the adjoining body wall produce rhythmical displacements of the
canal and so cause a movement of the blood, while the dilators of the
rectum draw in water which is believed to subserve respiration. Special
organs for circulation and respiration are wanting in Cyclops, though
other copepods have a saccular heart. Maxillary glands are present —
probably entirely mesodermal. The ventral cords of the nervous
system are concentrated into a single ganglionic mass. The gonads are
single median structures which lie above the gut in the first two
thoracic somites. The ducts are paired. In the female a large,
branched uterus adjoins the ovary on each side, communicating with
the lateral opening on the urosome by an oviduct which at its termina-
tion receives a duct from the spermatheca. The latter is median, in
the same segment as the oviducal openings, with a median entrance
of its own. The male transfers his spermatozoa to the female in a
spermatophore. The eggs when laid are cemented into a packet {(^gg
"sac") which hangs from the opening of the oviduct, and are thus
carried until they hatch. The possession of a pair of such packets
gives a characteristic appearance to the females of Cyclops, as to those
of many other copepods. In some genera, however, there is a single
median packet, and in a very few the eggs are laid into the water.
The larva hatches as a typical Nauplius (Fig. 235). This is suc-
ceeded by several Metanauplius stages, and then suddenly at a moult
takes on the first Cyclops stage, which has the general form of the
adult but lacks appendages behind the 3rd pair of swimming limbs
and also the somites of the urosome. In five successive Cyclops stages
the missing somites appear, the tale of limbs being meanwhile
completed.
Calanus, which is marine and pelagic in all parts of the world, often
occurring in enormous shoals which ^re an important item of food for
fishes and whales, is in several respects more primitive than Cyclops,
having the antennae and mandibular palps (Fig. 222 D) biramous,
well-developed and biramous limbs on the 6th thoracic somite, and
only one postcephalic somite in the cephalothorax. The 6th thoracic
somite is included in the mid-body, not in the urosome. The primi-
374
THE INVERTEBRATA
tive custom of feeding by the automatic straining of food particles
from the water is retained : the feeding current eddies from the swim-
ming current which the antennae, mandibles, and maxillae set up,
and is strained through a fringe of bristles on the maxillae (Fig. 251).
The parasitic habit has been adopted by members of very different
families of copepods, and to very various degrees even by members
of a single family. Every stage may be found between normal, free-
living forms and the most degenerate parasites. Parasitic forms often
have a suctorial proboscis, which is formed by the upper and lower
lips enclosing mandibles adapted to piercing. Such a proboscis is not
necessarily accompanied by a high degree of degeneration. The life
histories of parasites are often complicated, and may involve remark-
able changes of habit. Degenerate forms usually reach one of the
ex-
Fig. 251. The maxilla of Calanus. ex. small prominence which perhaps re-
presents the exopodite; i and 2, endites representing the first two segments;
9, terminal segment.
Cyclops Stages and may pass through them all before they begin to
degrade. Often the male is less degenerate than the female: he may
be free-swimming while she is sedentary, or may be much smaller
and cling to her body. It is only possible here to mention a few of the
numerous genera of these interesting parasites.
Notodelphys, commensal in the pharynx of ascidians, is clumsy
bodied, and has a large dorsal egg pouch on the 5th and 6th thoracic
somites, but can swim and is sometimes captured outside the host.
Monstrilla has a very remarkable life history. The adults of both
sexes are free-swimming, as are the newly-hatched Nauplii^ but the
intermediate stages are parasitic in various polychaets, where they
absorb nourishment by means of a pair of long, flexible processes
which represent the antennae. In this stage they lay up a food supply
for the entire life cycle, throughout which the animals are without
functional mouth parts or alimentary canal.
COPEPODA
375
Chondracanthus (Fig. 252), which infests the gills of various marine
fishes, has in the adult stage a large female, whose body is produced
into irregular, paired lobes and her appendages degenerate, though
the mouth has not a proboscis but is flanked by the three pairs of
minute, sickle-shaped jaws. The males are small, retain more of the
Fig. 252.
Fig. 252. Chondracanthus gibb OS us . After Claus, A, Female. B, Male, more
highly magnified, al. alimientary canal; an.' antennule; an." antenna; e. eye;
e.s. egg " sac " ; nixp. maxilliped ; t. testis ; th. 2 and 3, thoracic limbs ; vas d. vas
deferens; cJ, males attached to females.
Fig. 253. Stages in the life history of Lernaea. A, Metanauplius. B, First
Cyclops stage. C, "Pupa". D, Sexual stage: coition. E, Ripe female.
an.' antennule; an." antenna ; ^z a;, secretion of a gland by which fixation is
effected ; hd.pr. processes of the head of the female which are imbedded in the
tissues of the host; mxp. maxilliped; sip. siphon; th.z, second thoracic limb;
ram. ramus of caudal fork.
copepod organization than the female, and cling by hook-like
antennae to her body.
Caligus, ectoparasitic, mainly in the gill chambers of fishes, is
clumsily built and has a suctorial proboscis, but retains the power of
swimming. Its sexes do not differ greatly.
Lernaea (Fig. 253) hatches as a Nauplius and at the first Cyclops
376 THE INVERTEBRATA
Stage becomes parasitic on the gills of a flat fish, deriving nourish-
ment from its host by means of suctorial mouth parts. Here it passes
into a "pupal" stage in which the power of movement is lost and
retrogressive changes have taken place. Presently it regains the power
of swimming and leaves the host in an adult copepod stage. In this
stage impregnation takes place. The male develops no further, but
the female attaches herself to the gills of a fish of the cod family,
where by a great development of the genital somite she becomes con-
verted into a vermiform parasite, anchored into the host by processes
that grow out from her head, and retaining only the now relatively
minute appendages of the thorax.
In Herpyllobius, parasitic on annelids, the female is reduced to a
mere sac, drawing nourishment from the host by rootlets and bearing
minute males which are also sac-like.
Xenocoeloma, also parasitic on annelids, is represented in the host's
body only by the gonads, which are hermaphrodite, and some
muscles, enclosed in a cylindrical outgrowth of the host's epithelium
which forms a body wall for the vestiges of the parasite and contains
a gut-like prolongation of the host's coelom.
Class BRANCHIURA
Crustacea, temporarily parasitic on fishes; which possess compound
eyes ; a suctorial mouth ; carapace-like lateral expansions of the head
which are fused to the sides of the first thoracic somite ; an unseg-
mented, limbless, bilobed abdomen with a minute caudal furca; and
four pairs of thoracic limbs, which are biramous, with usually a
proximal extension of the exopodite.
The members of this group in many respects superficially resemble
the Copepoda, with which they are generally placed, but differ from
that class in certain important features, notably in the possession of
compound eyes, the lateral head-lobes, the opening of the genital
ducts between the fourth pair of thoracic limbs, and the phyllopod-
like proximal overhang of some of the thoracic exopodites (Fig, 254 B).
The carp-lice, as the Branchiura are called, are found both on fresh-
water and marine fishes. They are good swimmers. The females
deposit their eggs on stones and other objects. The larvae differ little
from the adult.
Argulus (Fig. 254), the principal genus, has a pair of suckers on
the maxillae and a poison spine in front of the proboscis. A.foliaceus
is common on freshwater fishes in Britain and the Continent.
Class CIRRIPEDIA
Fixed and for the most part hermaphrodite Crustacea ; without com-
pound eyes in the adult ; with a carapace (except in rare instances) as
CRUSTACEA
377
a mantle which encloses the trunk; with usually a mandibular palp,
which is never biramous; and typically with six pairs of biramous
thoracic limbs.
The great majority of the Cirripedia are extremely unlike the rest
of the subphyllum, and would not be recognized as crustaceans at all
by the layman. The familiar members of the class are the ordinary
barnacles (Thoracica). Besides these, however, it contains several
groups of related organisms, of which the parasitic barnacles (Rhizo-
cephala) are the best known. The Ascothoracica link the class to other
crustaceans.
,an .
sip.- — ^rrrrz^
Fig. 254. Arguliis. A, A ventral view of a female of A. americanus. From
Caiman, after Wilson. B, The second left swimming limb of A. foliaceus.
After Hansen. An. position of anus; an.' antennule; an." antenna; e. paired
eye; ex. exopodite; mx.' maxillule; mx." maxilla; ram. ramus of caudal furca;
in some species the rami stand immediately on each side of the anus;
sip. siphon, or suctorial proboscis; sp. poison spine.
Order THORACICA
Cirripedia with an alimentary canal ; six pairs of biramous thoracic
limbs; no abdominal somites; and permanent attachment by the
preoral region.
We shall take as an example of this group the common goose
barnacle, Lepas (Figs. 255, 257 A), found all the world over on float-
ing objects in the sea. It hangs by a stalk or peduncle which, as we
shall see, represents the foremost part of the head, greatly elongated
378
THE INVERTEBRATA
but Still bearing at its far end the vestiges of the antennules, imbedded
in a cement by which it is held fast. The glands which produce the
cement are contained in the peduncle, and open on the antennules.
tgm.
an!
Fig- 255. A view of Lepas anatifera, cut open longitudinally to show the
disposition of the organs. From Leuckart and Nitsche, partly after Claus.
stk. stalk; cna. carina; tgm. tergum; scu. scutum; an.' antennule; md. man-
dible with "palp" in front; mx.' ist maxilla; mx." 2nd maxilla; Th. the six
pairs of biramous thoracic limbs; Ihr. labrum; M. mouth; oe. oesophagus;
Ir. "liver" coeca; st. stomach; An. anus; ov. ovary; od. oviduct; t. testes;
ves.sem. vesicula seminalis; p. penis; cent, cement gland and duct; add. ad-
ductor scutorum muscle, which closes the carapace; mtl.ca. mantle cavity,
i.e. the space intervening between the carapace and the body.
The rest of the body is known as the capitulum, and is completely
enclosed in the carapace or mantle, a fleshy structure strengthened
by five calcified plates — a median dorsal carina, and on each side two
known as the scutum and tergum. The scuta are anterior to the terga,
that is, nearer to the peduncle. The mantle cavity opens by a long slit
LEPAS 379
on the ventral side. Within the mantle cavity lies the body, turned
over on its back with the appendages upwards (or downwards, as the
animal hangs) and connected with the peduncle and mantle only at the
extreme anterior end, where there is a preoral adductor muscle by which
the sides {valves) of the mantle can be drawn together and so the
opening closed. The antennae, which should be somewhere in this
region, are absent. The prominent mouth is overhung by a large
labrum. At its sides stand the mandibles, which have a flat, toothed
process towards the mouth and a large, uniramous, foliaceous palp^
and the maxillules, simple structures with a fringe of strong bristles
on the notched median edge. A pair of simple, hairy lobes, united by
a median fold, which shut in the mouth and its appendages from be-
hind, represent the maxillae. The six pairs of thoracic limbs or cirri
have each two long, many-jointed, hairy rami, curled towards the
mouth. They are successively longer from before backwards. A
couple of filamentous epipodites ("gills") stand on the protopodite
of the first pair. Behind the cirri stands a long median ventral penis ^
and behind this again is the anus, with a pair of vestigial caudal rami.
The animal feeds by thrusting out the cirri through the mantle
opening and withdrawing them with a grasping motion, whereby
particles are gathered from the water by the setae upon the limbs. If
it be molested the motion ceases and the valves are drawn to. The
alimentary canal has an oesophagus (stomodaeum) directed forwards
from the mouth to the long wide stomach which bears several coeca
around its commencement and tapers behind into an intestine. Com-
plicated maxillary glands open on the maxillae. There is no heart or
system of blood vessels. The nervous system has a suboesophageal
ganglion, and a separate ganglion for each pair of cirri behind the first.
Lepas is hermaphrodite. The ovaries lie in the peduncle and the
oviducts open on the bases of the first pair of thoracic limbs, much
further forwards than is usual in Crustacea. The testes are branched
tubes which lie at the sides of the alimentary canal and in the basal
parts of the cirri. Each vas deferens enlarges into a vesicula seminalis
whose duct joins that of its fellow in the penis. Impregnation takes
place by the penis depositing a mass of spermatozoa on either side
of the mantle cavity of a neighbouring individual, near the opening
of the oviduct. It is possible that isolated individuals may be self-
fertilized. The ova undergo their early development within the mantle
cavity of the mother attached in a flat mass, the ovarian lamella, by
a glutinous secretion manufactured by the terminal enlargement of
the oviduct, to a fold of the mantle which projects on each side from
near the junction with the body and is known as an ovigerous frenum.
The young are set free as Nauplii, characterized, as are those of
nearly all cirripedes, by a pair of lateral frontal horns , on each of which
380 THE INVERTEBRATA
opens a unicellular gland (see Fig. 258). These are processes of a
dorsal shield which in later stages acquires other spines. After several
moults the larva suddenly passes into the so-called Cypris stage. It is
now enclosed in a bivalve shell with an adductor muscle, and possesses
a pair of compound eyes. The antennules of this stage possess near
their ends a disc on which opens the cement gland. The antennae have
disappeared. There are six pairs of biramous thoracic limbs and a
small abdomen of four somites. The Cypris larva becomes fixed by
the discs on its antennules, and its body rotates within the shell, so
M ' I I
th. M. e! e. an'.
scu.
na.
B
Ih. e'. CC' al. y- an'.
Fig. 256. Diagrams of three stages in the metamorphosis of Lepas. From
Korschelt and Heider. A, The Cypris stage. B, The attached larva (pupa).
C, The young Lepas. ab. abdomen; al. aHmentary canal; an.' antennule;
car. cuticle of carapace of larva, not yet shed; cna. carina; e. compound eye;
e.' median eye; M. mouth; scu. scutum; tgm. tergum; th. thoracic limbs;
X. origin of carapace fold ; y. a ventral fold of the head.
that the ventral surface is directed backwards (Fig. 256 A, B). Now
the shell and body are rotated upwards on the antennae so that the
adult position is assumed (Fig. 256 C); meanwhile the shell plates
appear, the preoral region elongates to form the peduncle, and the
abdomen disappears.
Scalpellum (Fig. 257 C, D) attaches itself to fixed objects, usually
in deep waters. It diff^ers from Lepas in possessing a number of ad-
ditional plates on the capitulum, and scales of a similar nature on the
peduncle. It is more remarkable in possessing what are known as
complemental males . A few species of the genus are composed entirely
THORACICA 381
of hermaphrodites as Lepas is. In most, however, some individuals
are without female organs. These individuals are always smaller than
those which possess ovaries, and live within, or at the opening of, the
mantle cavity of the latter. In some species they almost perfectly
scu. ^'tgm.
--cna.
Fig. 257. Cirripedia Thoracica. A, Lepas anatifera. B, Balamus. C, Scal-
pellum vulgare. D, Male of the same, enlarged. A— C, after Darwin; D, after
G. Smith, cna. carina ; cnl. carinolateral ; e. vestige of eye ; la. lateral ; op. open-
ing of mantle cavity; rst. rostrum; rstl. rostrolateral ; scu. scutum; stk. pe-
duncle ; t. testis ; tgm. tergum ; S, dwarf males.
resemble these in organization, but usually they are more or less de-
generate, being sometimes even without an alimentary canal. As a
rule the more degenerate live within the mantle cavity of the partner,
the less degenerate on its mantle edge. In certain species, which have
382 THE INVERTEBRATA
very degenerate males, the large individuals are without testes, so that
the sexes are separate. The function of the complemental males is
probably the effecting of cross-fertilization, for the species which
possess them are of solitary habit. The phenomenon perhaps arose
from the settling of young hermaphrodite individuals on the stalk of
old ones, which is common in stalked barnacles.
Balanus (Fig. 257 B), the common acorn barnacle, differs from
Lepas in the lack of a stalk, and in having an outer wall of skeletal
plates homologous with some of the extra pieces on the capitulum of
Scalpellum .
Order ACROTHORACICA
Cirripedia of separate sexes ; with an alimentary canal ; fewer than six
pairs of thoracic limbs; and no abdominal somites; permanently
sessile on the preoral region, in which the antennules are absent and
the cement glands much reduced.
These are minute creatures whose females live in hollows which
they excavate in the shells of molluscs, while the males are degenerate
and have the same relation to the female as have those of the species
of Scalpellum in which the sexes are separate.
Alcippe, British, lives in the columella of whelks, etc.
Order APODA
Hermaphrodite Cirripedia; without mantle, thoracic limbs or anus;
whose body is divided by constrictions into rings.
Proteolepas, the only known member of the order, is a small,
maggot-like animal found by Darwin in the mantle cavity of the
stalked barnacle Alepas. The antennules, by which it is attached, and
the mouth parts, are those of a cirripede. Since the mouth is terminal,
at least some of the more anterior of the eleven rings cannot represent
somites.
Order RHIZOCEPHALA
Cirripedia which are parasitic, almOst exclusively on decapod Crus-
tacea ; have at no time an alimentary canal ; and in the adult neither
appendages nor segmentation; make attachment in the larva by an
antennule; and are in the adult fastened to the host by a stalk from
which roots proceed into the host's tissues.
Sacculina (Figs. 258-261), parasitic on crabs, is the best known
example of this group. Its life history is a very remarkable one. It
starts life as a Nauplius (Fig. 258 A), with the characteristic frontal
horns of cirripede Nauplii but without mouth or alimentary canal.
The Cypris larva (Fig. 258 B) clings to a seta of a crab by one of its
CIRRIPEDIA
383
antennules. The whole trunk, with its muscles and appendages, is
now thrown off and a new cuticle formed under the old one, with a
dart-like organ which is thrust through the antennule and the thin
cuticle at the base of the seta of the crab into the body of the latter.
Through the dart the remnant of the larva, a mass of undifferentiated
cells surrounded by a layer of ectoderm, passes into the host's body
cavity. Carried by the blood it becomes attached to the under side
of the intestine (Fig. 259). There rootlets begin to grow out from it
and eventually permeate the body of the crab to the extremities of the
Fig. 258. Larval stages of Sacculina. From G. Smith. A, Nauplius, B,
Cypris. A.i, antennule; A. 2, antenna; Ab. abdomen; E. undifferentiated
cells; F. frontal horn with gland cells; GL gland cells; Md. mandible; Ten.
frontal tentacles (frontal organs) ; Tn. tendon.
limbs. Meanwhile a knob also grows from the mass; forms within
itself a mantle cavity surrounding an internal "visceral mass" which
contains the rudiments of genital organs and a ganglion ; presses upon
the ventral integument of the abdomen of the host, whose cuticle
is thus hindered from forming at that spot ; and consequently at the
next moult of the crab comes to project freely under the abdomen,
where it may be found in the adult condition.
The phenomenon known as parasitic castration is exhibited by
crabs attacked by Sacculina. The moult at which the parasite becomes
external produces a change in the secondary sexual characters in the
384 THE INVERTEBRATA
new cuticle. The male crabs have a much broader abdomen, reduced
copulatory styles (these may disappear altogether), and abdominal
svvimmerets (which carry the eggs in the female, and are absent in
the normal male). There is, in short, a marked tendency to the female
type. In the female crabs there is also a change, but this is held to be
not towards the male but towards the juvenile type. The gonads dis-
appear, but cases have been observed in which the parasite has been
killed and months afterwards what was probably an originally male
crab has regenerated a hermaphrodite gonad. Parasitic castration is
d.i —
A B
Fig. 259. Stages in the development of Sacculina upon the mid gut of a crab.
From G. Smith. A, Early stage. B, Later stage, b. swelhng caused by the
body of the Sacculina; c.t. central tumour upon which the body arises; dd.y
d.s. inferior (posterior) and superior (anterior) diverticula of the gut of the
host; n. "nucleus" or rudiment of the body of the Sacculina; op. opening of
a cavity in the central tumour, the "perisomatic cavity", from which the
definitive body eventually protrudes (not the mantle opening); rt. roots;
X. final position of the parasite.
the most evident expression of a remarkable and at present ill-under-
stood interference by the parasite with the general metabolism of its
host.
Thompsonia (Fig. 262), parasitic on crabs, hermit crabs, etc., is an
extraordinary case of extreme reduction by parasitism, in which an
arthropod is degraded to the level of a fungus. The rootlets of the
parasite are widely diffused through the host. Their branches in the
CIRRIPEDIA 385
limbs give off sacs which become external at a moult of the host.
These sacs contain neither ganglion, generative ducts, nor testes, but
only a number of ova in a space of doubtful nature. When they are
ripe the ova have become (probably by parthenogenesis) Cypris
larvae, which are set free by the formation of an opening. There is
no parasitic castration of the host.
,CS^MU
Uh.
slk.
, Sac.
Fig. 260.
Fig. 260. A specimen of the shore crab (Carcinus) bearing a Sacculina. op.
mantle opening; Sac. Sacculina; stk. stalk.
Fig. 261. A vertical section of Sacculina at right angles to the plane of
greatest breadth. From Caiman, at. atrium of oviduct; ^a. ganglion;^/, col-
leteric gland opening into atrium; o. eggs in mantle cavity; op. opening of
mantle cavity; ov. ovary; rt. roots; stk. stalk; t. testis.
Order ASCOTHORACICA
Parasitic cirripedia, which have an alimentary canal from which
diverticula extend into the mantle ; six pairs of thoracic appendages ;
and a segmented or unsegmented abdomen ; and are not attached by
the preoral region.
These animals are parasitic and often imbedded in the tissues of
their hosts. They are an early branch^of the cirripede stock which has
retained the abdomen, in some cases well segmented and provided
with movable caudal rami, and has not the characteristic mode of
fixation by the antennules, or frontal horns in the NaupUus.
Laura (Fig. 264), imbedded in the tissues of the antipatharian
386 THE INVERTEBRATA
Gerardia, has the mantle in the form of a very spacious sac with a
narrow opening. Its abdomen has two somites and a telson,
Syiiagoga (Fig. 263), external parasite on Antipathes, has a bivalve
mantle, from which usually protrudes the long abdomen of four
somites and a telson. It is possible that this is an immature stage
of an animal which is more retrograde when it is adult.
Fig. 262.
Fig. 263.
Fig. 262. An abdominal limb of the prawn Synalpheus infested by Tho?np-
sonia, x 120. From Potts, bl. blind branch of root system which after further
development will become an external sac; en. endopodite of limb of host;
ex. exopodite of the same; mtl. mantle of sac; stk. stalk; vm. visceral mass,
occupied entirely by the ovary.
Fig. 263. Synagoga mira. After Norman, ab.i, first abdominal somite; ait.'
antennule ; car. mantle (carapace) ; M. mouth ; ram. ramus of caudal fork ;
tel. telson ; th. thoracic limbs.
Class MALACOSTRACA
Crustacea with compound eyes, which in typical members of the
group are stalked ; typically a carapace which covers the thorax ; the
mandibular palp, if present, uniramous ; a thorax of eight somites and
abdomen of six (rarely seven), all (except the 7th abdominal) bearing
appendages; and a complex proventriculus.
The Malacostraca contain a very large number of species, which
exhibit great diversity. Nevertheless they are capable of reference to
a common type in respect of more features than the members of any
CRUSTACEA 387
Other group, though the Copepoda approach them in this. The ideal
malacostracan has twenty somites, including the preantennulary and
excluding the telson. Of these, six belong to the head (p. 332), eight
constitute the thorax, and six the abdomen. This number is only
departed from in the Leptostraca, which have an additional somite
at the end of the abdomen. (In the embryos of Mysidacea such an
additional somite is present, but in the adult it has fused with that
which precedes it.) The female openings are always on the 6th thoracic
somite, and the male on the 8th. A carapace encloses the thorax at
sk.hst.
Fig. 264. Laura gerardiae. After Lacaze-Duthiers. A, The animal intact,
attached to the skeleton of its host, after removal of the soft tissues of the
latter. B, A view obtained by opening the mantle along the dorsal side.
a. anterior end; Ir. liver, branching in mantle; mtl. mantle; ov. ovary;
sk.hst. skeleton of the host.
the sides. The median eye is vestigial in the adult, and the compound
eyes stalked. The antennules are biramous, as they are in no crus-
tacean of any other group. The antennae have a scale-like exopodite
by extending which the animal keeps its body level in the water. The
mandibles have uniramous palps and the part which projects towards
the mouth is cleft into "incisor" and "molar" processes. The maxil-
lules have two endites (on the first and third joints) and the maxillae
four, grouped in twos. The thoracic limbs have a cylindrical, five-
jointed endopodite (p. 336), used when the animal has occasion to
walk or to grasp large particles of food, a natatory exopodite, and two
388 THE INVERTEBRATA
respiratory epipodites. The abdominal appendages are biramous;
those of the first five pairs (pleopods) slender and fringed and used in
swimming, those of the last pair (uropods) broad, turned backward,
and forming with the telson a tail-fan, used in rapid backward move-
ment. There are no caudal rami. (The Leptostraca are the only
members of the class which possess these rami in the adult.) Food is
chiefly collected as particles in a stream which is set up by the
action of the maxillae and which passes forwards through a filtering
fringe of bristles upon the median margins of those appendages.
This type is said to possess the caridoid fades . It is adapted prim-
arily to swimming and is best exhibited in the small, prawn-like,
pelagic forms, formerly classed together as Schizopoda but now dis-
tributed, as the orders Mysidacea (Fig. 265) and Euphausiacea, to the
Fig. 265. A female oi Mysis relicta. After Sars. bd.p, brood pouch;
md.gr. mandibular groove ; sta. statocyst.
two main subclasses of the Malacostraca (see below). Departures
from it are many and important, and most of its features have dis-
appeared more than once independently. Thus the carapace, the
inner ramus of the antennule, the scale of the antenna, the mandibular
palp, exopodites of thoracic limbs, etc., have been lost in various
branches of the malacostracan tree. Only the number of the somites
and the size of the tagmata are constant, save in the case of the Lepto-
straca already mentioned and in certain parasitic isopods. Departure
from the caridoid facies is associated with the abandonment of the
swimming habit for crawling or burrowing, and when that happens
the animal ceases to gather food by filtration and adopts other modes
of feeding, for which its limbs, and particularly the thoracic endo-
podites, become variously modified — as, for instance, by the de-
velopment of chelae.
MALACOSTRACA 389
An exceptionally large number of members of this class have direct
development. Of those which possess larvae only a few {Euphau-
siacea, a few of the Decapoda) hatch in the Nauplius stage. A special
characteristic of the larval development of the Malacostraca is the
occurrence of a zoaeal stage (p. 353), in which the carapace and tag-
mata are present, the abdomen is better developed than the hinder
part of the thorax, and the animal swims by biramous maxillipeds.
In crabs, hermit crabs, and some related families the Zoaea is
succeeded by a Metazoaea, which differs from it in having uni-
ramous rudiments of thoracic limbs behind the maxillipeds. In
other forms with larval development there is at this stage a prawn-
au'.
-l'ih.2
thA-
ab.G + t.
th.8
ab.i)+(i+t.
Fig. 266. Malacostracan larvae. A, Zoaea of Porcellana. B, Schizopod of
the lobster, C, Phyllosoma of Palinurus. D, Young Erichthus of a stomatopod.
ab. abdomen; an/ antennule; en. endopodite; ex. exopodite; t. telson; th.
thorax. The numerals indicate the somites or theirappendages.
like Schizopod larva ('' Mysis'' stage), with biramous limbs on all
the thoracic somites, which is not always preceded by a Zoaea.
The Malacostraca fall into two large groups and three smaller ones.
Of the latter, the Leptostraca retain, in the hinder end of the abdomen,
a primitive condition, which has been lost in the other groups. The
Stomatopoda (Hoplocarida) stand alone in possessing two free pseudo-
somites in the anterior part of the head, certain peculiarities of the
thoracic limbs, and peculiar gills oathe abdominal appendages. The
Syncarida unite certain features which are characteristic of other
groups. The large groups Peracarida and Eucarida contain most of the
members of the class. The former of these two divisions is character-
ized by possessing a brood pouch, formed by plates (oostegites) upon
the thoracic limbs, in which the young undergo a direct development.
390
THE INVERTEBRATA
and by the freedom of some or all of the thoracic somites from the
carapace. The Eucarida do not possess a brood pouch and usually
have larval stages, their heart is a short chamber in the thorax, and
their carapace fuses with the dorsal side of each thoracic somite.
Independently in each of these two groups the caridoid facies has
been lost to various degrees, so that the members of each can be
roughly arranged in a series which, starting with prawn-like "schizo-
pods", ends in the Peracarida with the woodlice and in the Eucarida
with the crabs.
Subclass LEPTOSTRACA
Malacostraca with a large carapace provided with an adductor muscle
and not fused with any of the thoracic somites; stalked eyes; the
Fig. 267. A female of Nebalia bipes. From Caiman, after Claus. a.' an-
tennule; a." antenna; ah} and ab.^ first and sixth abdominal limbs; add. ad-
ductor muscle of carapace; /. ramus of caudal furca; p. palp (endopodite)
of maxillule; r. rostrum; t. telson; i, 7, first and seventh abdominal somites.
thoracic limbs all alike, without oostegites, biramous, and usually
foliaceous; seven abdominal somites, of which the last bears no ap-
pendages; and caudal rami on the telson.
Nebalia (Fig. 267) is the commonest and typical genus of this
group. A^. bipes, the British species, may be found between tide-
marks, under stones, especially in spots which are foul with organic
remains. Nebalia has a rostrum, which is jointed to the head. The
antennae have no scale, while the antennules are unique in possessing
one. The carapace has an adductor in the region of the maxilla and
encloses the four anterior abdominal somites. The thorax is short.
MALACOSTRACA 391
Its limbs (Fig. 222 E) are flat. Their endopodite is narrow and
possesses five indistinct joints. Sometimes the long basipodite
is divided and its distal region added to the endopodite as a
preischium (p. 336). The exopodite is broad and there is a very
large epipodite, which serves as a gill. (The related Paranebalia,
however, has a slender exopodite with a flagellum, and a small epipo-
dite.) The first four pairs of abdominal limbs are large and biramous,
the fifth and sixth small and uniramous.
The alimentary canal possesses a proventriculus of relatively simple
type, several pairs of simple mid gut coeca, and an unpaired posterior
dorsal coecum. The heart is long, reaching from the head to the 4th
abdominal somite. The nervous system is of primitive type (p. 340).
The excretory organs have been alluded to on pp. 346, 348.
The 2imTsi2\ feeds by straining particles from the water by means of
an elaborate arrangement of setae of different kinds on the thoracic
limbs, the necessary currents being set up by a pumping action of the
same limbs. These work upon a principle similar to that employed by
the Branchiopoda, the exopodites and epipodites acting as valves for
pumping chambers between the limbs, but it is the backward stroke
that enlarges the chambers, and they are closed by the forward flap-
ping of their valves. Development is direct, the embryos being carried
between the thoracic limbs of the mother, held in by the long setae
on the limbs, but not glued to them like the eggs of the crayfish.
Subclass HOPLOCARIDA (STOMATOPODA)
Malacostraca with a shallow carapace which is fused with three
thoracic somites and leaves four uncovered ; two free pseudosomites
on the head ; stalked eyes ; the first five thoracic limbs subchelate and
Fig. 268. A male Squilla fna?itis. From Caiman, a.' antennule; a." antenna;
p. penis; sc. scale (exopodite) of antenna ; th.\ thr, th}, first, second, and last
thoracic limbs.
the last three biramous ; no oostegites ; a large abdomen whose first
five pairs of limbs bear gills on the exopodites, while the sixth forms
with the telson a tail fan; and a large, branched "liver".
392
THE INVERTEBRATA
The 2nd thoracic limb bears a large, raptorial subchela. The ali-
mefitary canal has a rather simple proventriculus and a large branched
"liver"; the latter and the gonads extend along the large abdomen.
In the nervous system eight pairs of ganglia are fused as the sub-
oesophageal ganglion. The heart is very long, reaching from the
head to the fifth abdominal somite. The excretory organs are maxillary
glands. The larvae are pelagic and of the same general type as the
Zoaea but with a peculiar facies of their own (Fig. 266 D).
The members of the subclass are all marine, and for the most part
live in burrows.
Squilla (Fig. 268) occurs in British waters.
Subclass SYNCARIDA
Malacostraca without carapace; with eyes stalked, sessile or absent;
most of the thoracic limbs provided with exopodites and none of them
chelate or subchelate ; no oostegites ; a tail fan ; and simple coeca on
the mid gut,
A small group of freshwater malacostracans with a combination of
features which forbids their inclusion in either of the other subclasses.
In typical genera, they possess most of the features of the caridoid
VIII
n cgr
Fig. 269. Anaspides tasmaniae, x 3. From Woodward, cgr. mandibular or
"anterior cervical" groove; ii, viii, second and eighth thoracic somites;
I, 6, first and sixth abdominal somites.
facies except the carapace ; and the relatively slight differentiation of
thorax from abdomen is a primitive character possessed by no other
member of the class.
Anaspides (Figs. 223, 269), from pools at 4000 feet in Tasmania, is
a normal member of the group.
Bathynella, from subterranean waters in Central Europe and Eng-
land, small, degenerate, and eyeless, has various limbs reduced or
absent and the first thoracic segment free.
MALACOSTRACA 393
Subclass PERACARIDA
Malacostraca whose carapace, if present, does not fuse with more
than four thoracic somites ; whose eyes may be stalked or sessile ; and
which possess oostegites; a more or less elongate heart; and a few
simple coeca on the mid gut.
A large subclass, containing several orders, which range from the
prawn-like Mysidacea, in which the caridoid facies (pp. 387, 388) is
practically intact, to the Isopoda and Amphipoda (slaters and sand-
hoppers) in which the carapace is lost and other features are greatly
modified. The important common characters which all these orders
possess are the presence of oostegites and the retention of the young,
which are directly developed, in a brood pouch formed by those
organs. Certain peculiarities, however, of the mandibles, which bear
behind the incisor process a movable structure known as the lacinia
viobilis, of the thoracic limbs (p. 336), etc., are also possessed in
common by the Peracarida.
Order MYSIDACEA
Peracarida with a carapace which covers most or all of the thoracic
somites; the eyes (when present), stalked; the scale of the antenna
well developed ; exopodites on most or all of the thoracic limbs, of
which one or two pairs are maxillipeds; and a well-formed tail fan.
Small, usually pelagic crustaceans, most of which are marine,
though a few occur as "relicts" or immigrants in fresh waters. They
are mostly carnivorous, but take vegetable matter in the course of
feeding. Small food particles are obtained in a current set up by the
maxillae (p . 388) and when there are no gills also by a whirling action of
the thoracic exopodites, and are strained off by the maxillae: large
food masses are seized by the endopodites of the thoracic limbs.
My sis (Figs. 265, 270), British, possesses a statocyst on the endo-
podite of each uropod, but has not the branched gills (thoracic
epipodites) which are found in some of the Mysidacea (Lophogas-
tridae). Its respiration takes place through the thin lining of the
carapace, under which a current is drawn from over the back by the
action of the epipodites of the maxillipeds (first pair of thoracic limbs).
Order CUMACEA
Peracarida with a carapace which covers only three or four thoracic
somites but is on each side inflated into a branchial chamber and pro-
duced in front of the head to lodge the expanded end of the exopodite
of the first thoracic limb; eyes (when present) sessile; no exopodite
on the antenna or endopodite on the maxilla ; three pairs of maxilli-
394
THE INVERTEBRATA
t-car.
bri.-
th.lfed.
Fig. 271.
Fig. 270.
Fig. 270. Maxilla of Mysis. bri. bristles used in straining out the food;
en. endopodite; ex. exopodite; 1-6, segments.
Fig. 271. Part of a transverse section through the hinder region of the head
of Hemimysis. After Cannon and Manton. hri. bristles of the fringes on the
maxillae by which food particles are strained out ; car. the edge of the cara-
pace; hd. head; mx.' base of maxillule; mx." section of maxilla; th. 1, section
of first thoracic limb; th. i, ed. section of endite of first thoracic limb.
The arrows show the direction of the currents. Note that the outgoing
water from the food current joins that of the respiratory current, which comes
down from under the carapace.
car.
an'.
Th.4
Th.6
Fig. 272, Female Diastylis stygia. After Sars. ab. abdomen; ab.6, appendage
of the sixth somite of the abdomen; an.' first antenna; car. carapace; gill, gill
borne on first maxilliped and seen through the carapace; Th. free part of
thorax; Th.4-Th.8, fourth to eighth thoracic limbs. The male has pleopods
and long antennae.
PERACARIDA 395
peds; a large epipodite, bearing a gill, on the ist thoracic limb and
natatory exopodites on some of the others; and slender uropods,
which do not form a tail fan.
Small, marine organisms which live in mud or sand and are highly
specialized, especially in their respiratory mechanism, for that
habitat. The first thoracic exopodites form a valved exhalant siphon
with the carapace lobes which lodge them.
Diastylis (Fig. 272) is a British genus.
Order TANAIDACEA
Peracarida with a very small carapace, covering only two thoracic
somites, with which it fuses; eyes (if present) on short, immovable
stalks; a small scale, or none, on the antenna; thoracic exopodites
absent or vestigial, a branchial epipodite on the maxilliped; and
slender uropods, which do not form a tail fan.
Small, marine crustaceans, usually inhabiting burrows or tubes,
which are in an intermediate condition between the Cumacea and
Isopoda in respect of the loss of the caridoid facies.
Apseudes (Fig. 273 A), and Tanais, which differs from it in having
short, uniramous antennules and liropods and no antennal scale, and
lives in a mass of fibres it secretes, are British genera.
Order ISOPODA
Peracarida without carapace ; with sessile eyes ; the body usually de-
pressed; the antennal exopodite absent or minute, the thoracic limbs
without exopodites, the first pair modified as maxillipeds, the re-
mainder usually alike ; the pleopods modified for respiration, and the
uropods usually not forming a tail fan. (Any of these features may
be absent in the adults of parasitic forms.)
The Isopoda are a large group and exhibit much variety. We will
study as an example Ligia, the shore slater (Fig. 274), found just
above tidemarks in Britain and most parts of the world. This creature
has a depressed, oval body, the cephalothorax, formed by fusion of
the ist thoracic somite with the true head, lying in a notch on the
anterior edge of the 2nd somite of the thorax. Two large, sessile com-
pound eyes take up the sides of the head. The abdomen continues the
outline of the thorax, and its 6th SQmite is fused with the telson. The
anteiiniiles , which are usually short in isopods, are here minute. The
antennae are of a good length, which is due to the elongation of the
two joints which precede the flagellum. The mandibles, unlike those
of most isopods, lack the palp, but otherwise they are complicated,
having between the incisor and molar processes a row of spines and
396
THE INVERTEBRATA
an:
ah.Q + t.
Fig. 273. Malacostraca. A, Apseudes ; B, Cyamus ; C, Phrojiima 9 ; D, Leucifer S.
ab. abdomen; an.' antennule; an." antenna; the antennae of Cyamus are
minute and those of Phronima $ reduced to a tubercle containing the green
gland; br. gill; e.' , e." the two sections of the eye oi Phronima; t. telson;
th. thorax. The numerals indicate somites or their appendages.
MALACOSTRACA
397
the movable structure known as the lacinia mobilis (Fig. 275 A, la. mo.)
which is characteristic of the Peracarida. The maxillules and maxillae
are less well developed than those of most isopods. The maxillipeds
are broad and close the mouth region from behind. The rest of the
thoracic limbs are uniramous and leg-like. Their coxopodites are fused
with the body, so that the brood pouch plates (oostegites) of the female,
which are epipodites of the legs, seem to arise from the sterna. The
first five pairs of abdominal limbs are broad, with plate-like, re-
spiratory endopodite and exopodite. The endopodite of the second
pair of the male is produced into a copulatory style. The uropods have
slender, styliform rami. The alimentary canal has an elaborate pro-
ventriculus, adapted, not to chew the food, but to press the juices
Fig. 274. Dorsal and ventral views of Ligia oceanica.
From the Cambridge Natural History.
out of it and to strain off solid particles from them ; and there are three
pairs of mid gut coeca. The heart lies in the hinder part of the thorax
and in the abdomen, where blood returns from the respiratory limbs
to the pericardium. The nervous system has a concentration of ganglia
in the abdomen as well as one for the mouth parts. The gonads are
paired, and the testes bear three follicles, characteristic of the Isopoda
(see Fig. 276 A). The young when set free from the brood pouch re-
semble the adult but lack the last pair of legs. Ligia is omnivorous,
but chiefly eats Fucus. It gnaws with its mandibles, feeding hurriedly
at low tide.
Armadillidium, the common woodlouse, is more completely ter-
restrial in its habits than Ligia. Its antennae and uropods are short
and thus permit the body to roll up into a ball in the familiar manner.
398
THE INVERTEBRATA
The air tubes on the abdominal limbs have been alluded to on
P-348.
Asellus (Fig. 276 A), the hog slater, is a common freshwater crus-
tacean. It differs from Ligia, among other ways, in having all the
abdominal somites fused, a flagellum on the antennule, a palp on the
mandible, and free coxopodites on the legs.
Idotea, common among weeds, etc., on the British coast, differs
from Ligia in having the last four abdominal somites fused with the
telson and the uropods turned inwards as valves to cover the pleopods.
cp.-
Fig. 275. Limbs of Li^g/a. A, Mandible. B, Maxillule. C, Maxilla. D, Max-
illiped. E, Third abdominal limb. cp. coxopodite; en. endopodite; ex. exo-
podite; mc. incisor process; la.mo. lacinia mobilis; niol. molar process;
pr. protopodite; spi. spine row. i & 2 first two joints, fused; i', 3', endites.
Many of the Isopoda are parasitic. Among these there is found
every grade from well-organized temporary parasites to some which
are as adults mere sacs of eggs. Aega (Fig. 276 B), a fish louse, has the
ordinary isopod form, though heavily built, and with piercing mouth
parts and some of the legs hooked. Its broad uropods form a tail fan.
Bopyrus (Fig. 277 A), in the gill chamber of prawns, with dwarf males,
is more degenerate but still recognizable as an isopod. Cryptoniscus
(Fig. 277 B), a " hyperparasite " on members of the Rhizocephala and
a protandrous hermaphrodite, is extremely degenerate. Many of these
parasites produce parasitic castration (see p. 383).
ISOPODA
399
Th.Q
Fig. 276. A, Asellus aquaticus. Male viewed from above. From Leuckart
and Nitsche. an.' antennule; an." antenna; ah. 6, the last pair of abdominal
limbs ; t. testes with their efferent canals : the nervous system is shown in
black ; Th. z-Th. 8, thoracic limbs. B, Aega psora. B', Maxillule of the same.
All after Sars.
400
THE INVERTEBRATA
mzp.
...thA.
.00.
op.bd.
Fig. 277. A, Bopyrus fougerouxi: a female in ventral view. From the Cam-
bridge Natural History, after Bonnier, mxp. maxiiliped; th.4, fourth thoracic
Hmb (third leg); 00. oostegite; (^, male attached to female. B, Cryptoniscus
paguri: ripe female stage in ventral view. After Fraisse. M. mouth; op.bd.
line along which brood pouch will open ; rsp. one of two openings through
which a respiratory current passes to and from brood pouch.
Order AMPHIPODA
Peracarida without carapace ; with sessile eyes ; the body usually com-
pressed; no antennal exopodite; the thoracic limbs without exo-
podites, the first pair modified as maxillipeds, the remainder of more
than one form, the second and third usually prehensile; the pleopods
when fully developed divided into two sets, the first three pairs with
multiarticulate rami, the last two resembling the uropods, which do
not form a tail fan.
We will take as an example of this order Gammarus (Figs. 278-280),
of which closely related species occur in Britain in fresh waters and
between tidemarks in the sea. The body of this animal is compressed
and elongated, with the ist thoracic somite fused to the head and no
sharp distinction between the thorax and abdomen, which are of
nearly equal length. At the sides of the head are pleural plates. The
pleura of the thorax are short; but large, hinged coxal plates on the
legs take their place. All the segments of the abdomen are free. The
telson is deeply cleft. The antennules have two fiagella; the uniramous
PERACARIDA
401
antennae are much like those oi Ligia. The mandibles have the same
parts as those of Ligia ^ with a palp. The maxillules, maxillae, and
maxillipeds are shown in Fig. 279. The maxillipeds are united by the
fusion of their coxopodites. The first two pairs of legs are subchelate,
the third and fourth pairs are turned forwards and help the subchelae
in feeding, the last three pairs are turned backwards and used when
the animal crawls on its side. The first three pairs of abdominal limbs
med.dxm
cpx,
^{' Th.
j^^^^:^^=^^>
\-T/u7
p.ca.
Fig. 278. Gammariis neglectus. Female bearing eggs seen in profile. From
Leuckart and Nitsche, after G. O. Sars. cpx. cephalothorax; Th. free thoracic
somites; ab. the six abdominal somites; an.' antennule; an." antenna;
md. mandible; mx.' maxillule; mx." maxilla; ?nxpd. maxilliped; Th.2~
Th.8, thoracic limbs; ab.i-ab.2, three anterior abdominal limbs for swim-
ming; ab.4-ab.6, three posterior abdominal limbs for jumping; h. heart with
six pairs of ostia; or. ovary; /lep. hepatic caecum; p.ca. posterior caeca of
the alimentary canal; med.d.cni. median dorsal caecum; al. alimentary canal;
n.sy. nervous system ; o. ova in egg pouch, formed from oostegites on the
coxae of the second, third and fourth thoracic limbs; tel. telson (cleft).
are used in swimming and to direct water towards the gills, the last
three pairs are used together to kick the ground in jumping. Simple
gills (epipodites) are found on the coxopodites of the legs, and ooste-
gites on those of the third to fifth pairs in the female (Fig. 280). The
alimentary canal has a single-chambered but complex proventriculus,
two pairs of *' hepatic" coeca, and a pair of coeca at the hinder end
of the mid gut which have been supposed to be excretory. The prin-
cipal organs of excretion are antenna! glands. The heart extends from
402
THE INVERTEBRATA
the 7th to the ist thoracic somite. The young are born with all their
legs. The females with young are carried by males. After they have
inc.,
la.vio.-
spi.- — ''"/4
mol.
A hr.
Fig. 279. Fig. 280.
Fig. 279. Mouth parts of Gammarus. A, Mandible. B, Maxillule. C, Maxilla.
D, Maxillipeds. en. endopodite; inc. incisor process; la.?fio. lacinia mobilis;
mol. molar process;/)//), palp; spi. spine row; 1-3, segments of limb.
Fig. 280. A diagram of a transverse section through the thorax of Ga?nmartis.
br. branchia; bp. basipodite; cp. coxopodite; cp.' coxal plate; g. gonad;
h. heart; hep. "hepatic" coeca; int. intestine; n. nerve cord; 00. oostegite.
th.3.
Fig. 281. Caprella gra?tdimana. From the Cambridge Natural History, after
P. Mayer, ab. abdomen; br. gills; th.2, th.H, thoracic somites.
parted with the young they moult and are immediately re-impreg-
nated. When the cuticle has set they are liberated.
Caprella (Fig. 281), slender-bodied and living upon seaweeds,
hydroids, etc., has two thoracic somites in the cephalothorax, no legs
MALACOSTRACA 403
on the 4th and 5th thoracic somites, all the remaining legs subchelate,
and the abdomen reduced to a minute stump.
Cyamus, the whale louse (Fig. 273 B), is a Caprella with a short,
wide body, adapted to its habit and habitat.
Phronhna (Fig. 273 C), marine and pelagic, often inhabiting pelagic
tunicates, jellyfish, etc., is transparent and has a large head with
immense eyes.
Subclass EUCARIDA
Malacostraca with a carapace which is fused with all the thoracic
somites; stalked eyes; no oostegites; a short heart situated in the
thorax; and a large, branched "liver".
The differences between the two orders which compose this sub-
class are not great. The small, prawn-like Euphausiacea are not far
from the lower genera of the true prawns, members of the Decapoda.
Order EUPHAUSIACEA
Eucarida in which the exopodite of the maxilla is small ; none of the
thoracic limbs are maxillipeds; there is a single series of gills, and
these stand upon the coxopodites of thoracic limbs; and there is no
statocyst.
Fig. 282. Nyctiphanes norzoegica. Slightly magnified. From Watase. The
black dots indicate the phosphorescent organs.
The Euphausiacea are marine and pelagic, and at times form an
important part of the food of whales. Like many pelagic animals
they possess (in nearly all species), phosphorescent organs, which in
this case are complex and situated on various parts of the body. They
are filter feeders. Most (perhaps not all) are hatched as Nauplii, and
subsequently pass through stages of the Zoaea type.
Nyctiphanes (Fig. 282) is a British example of the group.
404 THE INVERTEBRATA
Order DECAPODA
Eucarida in which the exopodite (scaphognathite) of the maxilla is
large; three pairs of thoracic limbs are more or less modified as
maxillipeds, and five are " legs " ; there is usually more than one series
of gills, of which some (podobranchiae) stand upon the coxopodites
of thoracic limbs, others {arthrobranchiae) upon the joint-membranes
at the bases of the limbs, and others (pleurobranchiae) upon the sides
of the thorax; and a statocyst is usually present in the proximal joint
of each antennule.
The Decapoda owe their name to the condition of the hinder five
pairs of thoracic limbs, which are adapted for locomotion, typically
by walking but sometimes by swimming. Often, however, as in the
crayfish, one of these pairs bears large chelae and is incapable of the
locomotory function: others may also be incapacitated for it, as, for
instance, the two small hinder pairs of the hermit crabs (Fig. 292) .
Only in some of the lower genera is there any vestige of the exopodite
upon these five pairs.
This order contains the most highly organized crustaceans. Among
its members there is great diversity in the habit of body and in the
form of the appendages, but two principal types can be observed. In
the first or macrurous type the caridoid facies is in the main retained,
the body is long and subcylindrical or somewhat compressed, the
abdomen is long and ends in a tail fan, the appendages are usually
slender, and any of the legs may be chelate. An example of this type,
the common crayfish, Astacus (Figs. 283 ; 212, 213, 222 G, 224, 226,
227, 231, 233, 234, 286), is described in most textbooks of elementary
zoology. The second or brachyurous type — which is not confined to
the Brachyura sensu stricto but occurs independently in various
members of certain groups, known collectively as the Anomura, that
are intermediate between the macrurous divisions of the order and
the Brachyura — has the cephalothorax greatly expanded laterally and
more or less depressed, while the abdomen is reduced and folded
underneath the cephalothorax. In it the appendages are as a rule
shorter and stouter than in macrurous forms, and only the first pair
of legs has a true chela.
The suborders Penaeidea (primitive prawns), Caridea (prawns and
shrimps), Astacura (crayfishes and lobsters), and Palimira (craw-
fishes and bear-crabs) are macrurous. They are for the most part
swimmers, though some of them, as the Astacura and Palinura, do
more walking than swimming. The suborders Anomura and Brachyura
are walkers, though some of the crabs have their own ways of swim-
ming by means of flattened legs. The Brachyura proper are dis-
tinguished from other brachyurous forms by the occurrence in nearly
DECAPODA
405
M '-'
4o6
THE INVERTEBRATA
BRACHYURA
407
all the latter of well-formed uropods, which the true crabs do not
possess, and by a fusion of the edge of the carapace with the epistome,
a sternal plate which lies in front of the mouth (see p. 410).
As an example of the Brachyura we shall describe Carcinus maenaSy
the common shore crab of Britain (Figs. 284, 285, 287-291). The
depression to which is due the difference in shape between the
cephalothorax of this typical crab and that of a crayfish or prawn has
brought it about that in a transverse section (Fig. 285) the carapace
cb.sp.
en^
£p<
Fig. 285.
Fig. 286.
Fig. 285. Diagram of a transverse section through a branchial chamber of
Carcinus maenas. From Borradaile. a.l.e. anterolateral edge; cp. coxopodite;
eb.sp. epibranchial space; ep.i, mastigobranch (distal part of epipodite) of
the first maxilliped; ep.^, mastigobranch of the third maxilliped; hy. hypo-
branchial space ; i.r. inner layer of branchiostegal fold ; o.r. outer layer of the
same; proc. process of flank of thorax, which meets branchiostegite and
separates two of the openings above the legs into the chamber.
Fig. 286. The left third maxilliped of Astacus. cp.set. coxopoditic setae;
en. endopodite; ep. epipodite; ex. exopodite.
has at the sides (where, as the branchiostegite, it covers the gills), not
an arched profile but runs out almost horizontally and is then bent
in, at an angle {a.l.e.) which is more acute in the anterior part of the
body than in the hinder part, to end against the flank above the coxo-
podites of the legs. At the angle, the branchiostegite, viewed from
above, describes the lateral part of the outline of the body. That out-
line begins between the eyes, where in the crayfish the rostrum stands,
with the/row^, a low, three-toothed lobe. On each side of this is the
orbity an excavation of the surface of the head for the reception of the
4o8 THE INVERTEBRATA
eye. From the orbit the notched anterolateral edge curves outwards
and backwards as a crest on the branchiostegite, forming with its
fellow and the front a semicircle. From each end of the semicircle
a slightly concave posterolateral edge carries the outline slanting
inwards to the short, transverse /)0^/mor edge of the carapace.
To return to the transverse section : the thin inner layer of the fold
which makes the branchiostegite is not so much drawn out as the
stout outer layer, so that a considerable space is left between them.
In the hinder region the two layers are not very widely separated,
and there are in this space only blood channels and connective tissue,
but anteriorly branches of the liver and gonad intrude there. The
edge of the branchiostegite fits close against the flank of the thorax
and the exopodites of the maxillipeds, leaving however the following
openings: (i) small slits, one above each leg, (2) a large opening in
front of the coxopodite of the chela, (3) a still larger opening in front
of the mouth. These openings lead to and from the gill chamber. In
th; flattening of the body, the lateral wall of the thorax has come to
face in great part upwards, so that the gills instead of being directed
vertically from their attachments, are directed more or less hori-
zontally inwards over the convex, mound-like inner wall of the gill
chamber. The gills are of the kind known as phyllobranchiae. That is,
the axis of each, instead of bearing filaments as in the gills of the cray-
fish {trichobranchiae), has on either side a row of plates, set close like
the leaves of a book. The podobranchiae stand out from the base of
an epipodite, which bears also a slender process known as a mastigo-
branchia. In the crayfish the gill lies along this and is fused with it
(Fig. 286). The first maxilliped has a mastigobranchia without a
podobranchia. The gill series of Carcinus is shown in the following
table :
Mxpd
I
Mxpd
II
Mxpd
III
Leg I
(Cheli-
ped)
Leg II
Leg
III
"rf
LegV
Total
Podobranchiae
Anterior
Arthrobranchiae
Posterior
Arthrobranchiae
Pleurobranchiae
Mastigobranchiae
(I)
I
I
(I)
I
I
I
(I)
I
I
2
I
I
I
2
3
2
2
(3)
Total
(I)
2 + (l)
3+(i)
I
—
9 + (3)
The mastigobranchiae lie in the gill chamber, that of the first
maxilliped in the epibranchial space above (external to) the gills and
CARCINUS
409
those of the second and third maxillipeds in the hypobranchial space
below the gills. Their function is the cleaning of the gills.
In front, the gill chamber narrows to an exhalant passage^ which
contains the scaphognathite and leads to the large anterior opening.
The scaphognathite, working to and fro, drives water out of this
opening and so draws in a current through the other apertures. The
opening in front of the chela can be closed by a flange on the coxopo-
dite of the third maxilliped, and so the current can be regulated. The
pb.mp^s
pb.mp.2
plb.i
ar.ch.
Fig. 287. A dorsal view of the organs in the left branchial chamber of Car-
cinus maenas. From Borradaile. ab. abdomen; ar.ch. arthrobranchs of the
cheliped; ar.mp.z, arthrobranch of the second maxilliped; ar.m^.3, arthro-
branchs of the third maxilliped; ep. i, base of the mastigobranch of the first
maxilliped; i.r. inner layer of the branchiostegal fold, reflected; pb.mp.2,
pb.mp.2, podobranchs of the first and second maxillipeds; pel. pericardial
lobe, a thin fold of the body wall, of undetermined function; plb.i, plb.2,
first and second pleurobranchs ; p.hy. posterior opening of the hypobranchial
chamber; scl. sclerite which keeps open the entrance to the exhalant passage;
scp. scaphognathite.
water which enters this opening is prevented from taking a short cut
to the exhalant passage by a large expansion of the base of the mastigo-
branch of the first maxilliped, which directs it under the gills. The
current from the openings over the legs also passes under the gills.
All the water then passes upwards through the gills into the epi-
branchial space above them and so to the exhalant passage. Thus the
gills are thoroughly bathed.
Owing to the width of the body the sterna are more easily dis-
tinguished than in the crayfish. Those of the maxillulary to second
410 THE INVERTEBRATA
maxillipedal somites are fused into a triangular mass. In front of the
mouth the plate known as the epistorne represents the mandibular and
antennal sterna. From this a ridge extends to the median rostral
tooth, separating two sockets in which stand the antennules. A down-
ward process from the front, abutting on the basal joint of the antenna,
separates each of these sockets from the orbit of its side. The two-
jointed eyestalk arises close to the median line and passes through a
gap between the frontal process and the antennal base to enlarge
within the orbit.
The abdomen is reduced to a flap, turned forwards and closely ap-
plied to the sterna of the thorax. Its ventral (upper) cuticle is thin.
It is broader in the female than in the male, in which its 3rd to 5th
somites are fused. Two small knobs on the 5th thoracic sternum,
fitting into sockets on the 6th somite of the abdomen, lock the two
together as by a press button.
The antcnjmles have short flagella and can fold back into the sockets
mentioned above. The antennae also have a short flagellum. They have
no exopodite (scale) and their coxopodite is represented by a small
operculum over the opening of the antennary ("green") gland. The
month parts are shown in Fig. 288 . In the mandibles, the biting edge
(incisor process) is toothless and the molar process reduced to a low
mound behind the biting edge. The palp is stout and the first two of
its three joints are united. The maxillules and maxillae have the usual
endites well developed. The scaphognathite of the maxilla is shaped
to fit the exhalant passage of the gill chamber. The maxillipeds have
epipodites produced into long, narrow mastigobranchs, fringed with
bristles which brush the gills. The flagella of their exopodites are
turned inwards and the endopodite of the first of them is expanded
at the end and helps to border the exhalant opening for the respiratory
current. The third pair are broad and enclose the mouth area from
below. The legs lack an exopodite and have the usual joints (p. 336)
in the stout endopodite, but the basipodite and ischiopodite are
united. The first leg has a strong chela: concerning the physiology of
its muscles something is said on pp. 138, 142. The others differ from
those of the crayfish chiefly in that none of them are chelate. The
animal, as is well known, walks sideways with them. Abdominal limbs
are present in the female only on the 2nd to 5th somites. On a short,
one-jointed protopodite they bear two long, equal, simple rami, covered
with setae to which, as in other decapods, the eggs are attached by a
covering secreted by dermal glands. In the male, the abdomen bears
limbs only on its first two somites, and they are uniramous and adapted
for transferring the sperm, the endopodite of the second working as
a piston in a tube formed by that of the first.
In feeding the food is seized by the chelae, which place it between
DECAPODA 415
Of the various examples of the order which are mentioned below,
all except Leucifei\ Birgus, and Gecarcinus occur in British waters.
The most aberrant member of the Decapoda is the minute, pelagic
Leucifer (Fig. 273 D), which has a very slender, macrurous body with
an extremely elongate head, long eyestalks, no limbs on the last two
thoracic somites, no chelae, and no gills. Like the normally built
prawn Penaeus and the rest of the group (Penaeidea) to which both
belong, Leucifer starts life as a Nauplius.
Leander, the common prawn, one of the Caridea, is macrurous like
the crayfish, but built for swimming rather than walking, with
phyllobranchiae, and with chelae only on the first two pairs of legs.
Crangon, the shrimp, is related to Leander but has a broader and
flatter body, a very small rostrum, and the first leg subchelate.
Nephrops, the Norway lobster, one of the Astacura, differs from
the crayfish in minor points, among others in having the podobranchs
free from the mastigobranchs.
Homarus, the lobster, differs from Nephrops in size, form of
chelae, etc.
Palinurus, the crawfish or spiny lobster, one of the Palinura, differs
from the crayfishes and lobsters in having a small spine in place of
the rostrum, no antennal scale (exopodite), and no chela on any leg.
It has a peculiar broad, thin schizopod larva, the Phyllosoma
(Fig. 266 C).
Eupagurus (Fig. 292), the hermit crab, one of the Anomura,
lives in the empty shells of gastropod molluscs. It has a large, soft
abdomen, containing the liver and gonads, twisted to fit into the
shell, and without appendages on the right side, save for the uropods,
of which both pairs are present, roughened, and serve to hold on the
shell. The first three pairs of legs are as in a crab, the last two small
and chelate.
Birgus, the robber crab, is a hermit crab which has grown too large
to use the shells of molluscs, and has accordingly re-developed ab-
dominal terga. It lives on land in the Indopacific region, and is
adapted to aerial respiration by the presence of vascular tufts on the
lining of the gill chambers. Its Zoaeae are marine.
Lithodes, the stone crab (Fig. 293), is by origin a hermit crab, but
has lost the habit of living in shells and so thoroughly taken on the
build of the true crabs that only some asymmetry of the abdomen and
a few other minor points of structure betray its ancestry, even the
uropods being absent.
Galathea, the plated lobster, another of the Anomura, is lobster-
like but has the abdomen bent under the thorax, and the last leg small
and slender and folded into the gill chamber.
Porcella?ia, the china crab, related to Galathea^ has a form of body
Fig. 292. Eiipagurus bernhardus, S. ab. 3, third abdominal limb; tel. telson;
th.S, last thoracic limb; sc. scale of antenna.
Fig. 293. Lithodes maia, ?, in ventral view. From the Cambridge Natural
History. ab.-T,, lateral plates of the third abdominal somite; 06.5, left lateral
plate of the fifth abdominal somite; mar. marginal plate; Te.t, telson and
sixth abdominal somite, fused ; th. 8, brush-like last thoracic limb.
DECAPODA 417
resembling that of the true crabs, but possesses uropods. Fig. 266 A
shows its remarkable zoaea.
Cancer, the edible crab, is a member of the Brachyura, nearly re-
lated to Carcinus but more heavily built, without the slight powers of
swimming possessed by the latter, and differing in other small points.
Gecarcinus, containing land crabs of the tropics, differs from Car-
cinus and Cancer in the shape of the third maxillipeds, which gape,
the sternal position of the male opening, and the highly vascular lining
of its swollen gill chambers. Its Zoaeae are marine.
Maia, the spiny spider crab, is narrow in front, with bifid rostrum
and feeble chelae, and a habit of decking itself with seaweed for
concealment.
CHAPTER XIII
THE SUBPHYLUM MYRIAPODA
Land-living tracheate arthropods, usually elongated, with numerous
leg-bearing segments; a distinct head with a single pair of antennae,
a palpless mandible and at least one pair of maxillae ; tracheal system
with segmentally repeated stigmata, tracheae usually anastomosing;
eyes, if present, clumps of ocelli; mid gut without special digestive
glands, hind gut with Malpighian tubules; young hatching at a stage
resembling the adult but possessing fewer than the adult complement
of segments.
It has long been recognized that the group Myriapoda as defined
above contains two chief divisions which are here treated as classes,
one of which, the Chilopoda, is more closely related to the Insecta than
the other, the Diplopoda. It is, however, convenient to retain the
group, though the similarity of the chief members is probably more
superficial than natural.
Classification. Chilopoda (Opisthogoneata), centipedes; Diplo-
poda, millipedes (with two smaller classes, the Symphyla and the
Pauropoda, these form the Progoneata, all distinguished by having
the genital opening near the anterior end of the body).
Class CHILOPODA
Carnivorous arthropods with the genital opening situated at the hind
end of the body (opisthogoneate) ; body segments all similar (at least
in the more primitive members of the division), body usually flattened
dorsoventrally ; ocelli present, head bears also antennae and three
pairs of jaws (mandibles and two pairs of maxillae) ; the ist body
segment bears a pair of poison claws; the rest, each a single pair of
ambulatory limbs, except the last two, which are legless ; blood system
consists of a dorsal heart and a ventral vessel connected by an anterior
pair of aortic arches ; tracheae typically branch and anastomose and
have a spiral lining; gonads dorsal to gut.
The type used for the study of this division is the centipede,
Lithobius (Fig. 294), which is found under bark and stones, and is
a much more active creature than the millipede, lulus, which is found
in the same situation. The chitinous exoskeleton is flexible and is
moulted frequently. The body is flattened dorsoventrally and the legs
in each pair are widely separated. The head consists of six segments all
represented by coelomic sacs in the embryo which disappear in the
MYRIAPODA
419
aca^
ac
Jv.n.c
... ue.se.
Fig. 294.
Fig. 295-
Fig. 294. Lithohius forficatus. Dorsal view of whole animal, i, antenna;
2, maxilliped; 4, pair of walking legs.
Fig 295. Lithohius forficatus, S- Dissected to show internal organs, ant
anfenna; amb. walking legs; ac.gl. accessory glands; a/, alimentary canal;
mt. Malpighian tubules; p.cl. poison claw; sal.gl. salivary gland, t. testis,
ve.se. vesicula seminalis ; v.n.c. ventral ner\^e cord.
Both from Shipley and MacBride.
420
THE INVERTEBRATA
adult, including a preoral and (between the antennae and the mandibles)
an intercalary. All segments except the first are originally postoral
but in development the mouth moves back and comes to lie between
the mandibles. The number of head segments is the same as in the
embryo insect and the crustacean, and a remarkable homology may be
observed between the chilopod and insect head appendages. Thus
hd.c-
/-.
/ /
ant.
Abr.
^md.
.^-
mx-
hd.c.
t.mxp.-
/..-./Y^^Qx^
mxp.
I
Fig. 296. Ltthobiusforficatiis. Original. Ventral view of head and two succeed-
ing segments in a specimen boiled in potash and mounted in Canada balsam.
On the right of the observer the maxilliped, and the sternum belonging to it, is
lightly stippled : on the left the maxilla is more coarsely stippled, ant. antenna ;
amb. I , base of the first ambulatory appendage ; hd.c. head capsule ; Ibr. labrum ;
md. mandible ; mx. maxilla ; ?nx.' first maxilla ; mxp. maxilliped (poison claw) ;
t.mxp. tergum of the maxilliped.
the antennae are jointed mobile appendages varying in length; the
mandibles are toothed plates without palps, the ist maxillae consist of
a basal portion bearing inner and outer lobes, while the 2nd maxillae
are usually fused together to form a sort of labium and possess a
palp-like jointed structure (Fig. 296). The difference between the
mouth parts of an orthopteran insect and a chilopod lies in the
reduction in size of the two pairs of maxillae in the latter, which
CHILOPODA
421
cox.
is possibly connected with the great development of the first pair
of trunk appendages as maxillipeds, which are four-jointed, the
distal joint being a sharp claw perforated by the opening of the poison
gland, while the proximal joint is enlarged and meets its fellow in the
middle line to form an additional lower lip.
The body segments in Lithobiiis number eighteen. Of these, the
ist carries the poison claws (maxillipeds), the 17th the genital opening
and usually a pair of modified appen-
dages, the gonopods, and the last
(telson), which is greatly reduced in
size and not seen in Fig. 297, the
anus, while the 2nd to i6th have
each a pair of seven-jointed walking
legs. Each segment has a broad
tergum and sternum and between
them a soft pleural region with a few
small chitinous sclerites and the stig-
mata. In Lithobius and the group of
chilopods to which it belongs, the
terga are alternately long and short
(Fig. 294). Only the segments which
have long sterna have stigmata, but
all have walking legs. In other centi-
pedes, e.g. Scolopendra (see Table, genital segment,
pp. 306-7), the terga are equal gonopods.
throughout.
The alimentary canal consists of a short fore gut into which open
two or three pairs of salivary glands, a very long mid gut without any
associated glands, and a short hind gut into which open a pair of
Malpighian tubules.
The vascular system is better developed than in insects. The
heart runs the whole length of the body and possesses in each segment
not only a pair of ostia but also lateral arteries. It ends anteriorly in
a cephalic artery and a pair of arteries which run round the gut and
join to form a supraneural vessel. The arteries branch and open into
haemocoelic spaces. There is a pericardium and below it a horizontal
membrane, perforated and provided with alary muscles as in insects.
In the respiratory system the tracheae branch and anastomose and
possess a spiral thickening, but in the remarkable form Scutigera the
stigmata are unpaired and dorsomedian in position and the tracheae
are unb ranched and simple in structure.
The reproductive organs (Fig. 295) consist of an unpaired ovary or
testis, with a duct which divides into two and passes round the end
gut to open by the median genital opening. There are two pairs of
--'g.pod.
Fig. 297. Lithobius forficatus, ?.
Hind end, ventral side. 16, last
segment bearing ambulatory legs ;
cox. coxopodite of last leg; 17, pre-
hearing g.pod.
422
THE INVERTEBRATA
accessory glands and in the male two vesiculae seminales. Spermato-
phores are formed but it is doubtful whether copulation occurs.
Lithobius lays its eggs singly and buries them in the earth. The young
are hatched with seven pairs of legs.
The nervous system comprises a cerebral ganglion supplying the
antennae and the eyes, a suboesophageal ganglion giving branches to
the other head appendages and the maxillipeds, and a ventral chain
with a pair of ganglia in each leg-bearing segment.
Class DIPLOPODA
Arthropods with the genital opening situated on the 3rd segment
behind the head (progoneate) ; trunk segments arranged in an anterior
region {thorax) of four single segments and a posterior region [abdomen)
of double segments, each with two pairs of legs; body usually cylin-
drical; skeleton strengthened by a calcareous deposit; ocelli present,
head bears also short club-shaped antennae, mandibles and a single
pair of maxillae; vascular system well developed as in Chilopoda;
tracheae arise in tufts from tracheal pouches, do not anastomose;
— 1
Fig. 298. lulus terrestris, sometimes called the "wire-worm", x about 3^,
From Koch, i, antennae; 2, eyes; 3, legs; 4, pores for the escape of the
excretion of the stink glands.
gonads ventral to gut ; young hatch usually in a stage with three pairs
of legs and development takes place gradually.
Though the head of the adult millipede appears to have fewer
segments than that of the Chilopoda a study of the embryo shows
that there are really the same number. An intercalary segment exists
between the antennal and mandibular segments and behind the
mandibles a pair of rudimentary appendages appear but soon vanish.
These are the first maxillae: the second maxillae (labium) persist in
the adult.
lulus is one of the commonest genera of millipedes. It is vege-
tarian. It has an elongated body, consisting of a large number of
segments (up to seventy), which can be rolled into a ball. The head
(Fig. 299 A) carries a pair of short antennae with seven joints. The
labrum is continuous with the front of the head and is a toothed
plate; the mandibles, which have no palp, bear a movable tooth and
DIPLOPODA
423
a ridged and toothed plate; behind them is an organ known as the
gnathochilarium (Fig. 299 C), which, in structure and position, recalls
Fig. 299. lulus terrestris. A, Side view of anterior end. an. antenna; ah.
abdomen ; col. collum (ist thoracic segment) ; e.e. clump of ocelli ; gn. gnatho-
chilarium ; g.op. genital opening (on basal joint of 2nd pair of legs) ; hd.
head ; Ibr. labrum ; 77id. mandible ; th. thorax. B, A segment detached from
the rest. st77i. sternum; tg77i. tergum. C, Gnathochilarium seen from inner
side. (The parts — basilar plate — which it is suggested may belong to the
segment of the collum are stippled.) c.b. central body; hyp. hypostoma; LI.
lamellae linguae; merit, mentum; p.m. promentum; p.p. palps; st. stipes.
D, Diagram of four rings seen in side view (above) when the animal is
stretched out straight ; (below) when- it is coiled in a spiral. The dorsal part
of the ring is clearly seen to be longer than the ventral. A, B and C, original ;
D, after Kukenthal.
the labium of insects and, like it, is formed by the junction of paired
appendages, the principal part of it by the appendages of the labial
424 THE INVERTEBRATA
segment. Also a postlabial segment contributes to it forming the
basilar plate. The tergite of this segment, however, forms what is ap-
parently the first segment after the head. This is known as the collum\
there are no stigmata and no separate appendages, though the first pair
of legs appears to belong to it they are those of the second segment.
The next three have a single pair of ambulatory legs apiece, a pair of
ganglia and a pair of stigmata, and in the embryo a pair of coelomic
sacs. These four segments maybe said to constitute the thorax, though,
as related above, the first takes part in the formation of a head
structure. The genital openings are situated in the basal joint of the
second pair of legs, which appear to arise from the second segment,
but really belong to the third.
Behind this is the abdomen consisting of an indefinite number of
double segments (up to a hundred in lulus). The exoskeleton of a
body segment consists of a tergum and two sterna. In the double
segment of lulus (Fig. 299 B) the sclerites of two segments are fused
together to make a continuous ring. The sterna carry two pairs of
stigmata and legs. In the embryo there are two pairs of coelomic
sacs ; there are two ostia in the heart and two pairs of ganglia. In
lulus the sterna are much shorter than the terga and also much
narrower so that the legs come oif very close together ; also the terga
are narrower in front so that they can be telescoped into the terga in
front. The diagram here given (Fig. 299 D) shows that this relation
occurs when the diplopod body is straightened out ; when the animal
rolls up the adjacent rings are completely disengaged.
The stigmata are elongated slits, which can be closed by a valve,
and they communicate with a tracheal pocket from which spring two
thick bunches of unbranched tracheae. These are of two sorts; one
long and slender, the other shorter and thicker with a spiral lining.
In other millipedes {Glomeris) the tracheae may become much longer
and branch but they never anastomose.
The circulatory system is in a stage of development higher than
that of the insects. The alimentary canal bears a pair of long
Malpighian tubules arising from the hind gut.
The legs consist of the same elements as in the insect, but the
tarsus is divided into three joints, the last of which carries a claw. In
the male the first leg is modified for copulation and in the 7th segment
there is an auxiliary copulatory apparatus, consisting of processes
used for transferring sperm into the vagina of the female. These
processes may occur together with legs and so are not homologous
with them. There are no similar organs in the female. The generative
glands are unpaired with ducts opening on the 3rd body segment. The
eggs are yolked and are laid after copulation in a nest made of hard
earth. The mother keeps watch over them before hatching.
CHAPTER XIV
THE SUBPHYLUM INSECTA (HEXAPODA)
Tracheate Arthropoda in which the body is divided into three distinct
regions, the head, thorax and abdomen. The head consists of six
segments and there is a single pair of antennae ; the thorax consists
of three segments with three pairs of legs and usually two pairs of
wings; the abdomen has typically eleven segments and does not
possess ambulatory appendages; genital apertures situated near the
anus (Fig. 300).
th.
an
Fig. 300. Lateral view of a grasshopper to show external segmentation
typical of insects. From Metcalf and Flint, ab. abdomen ; an. antenna ; ex.
coxa; e. eye;fe. femur; hd. head; ///. thorax; ti. tibia; to. trochanter; ts. tarsus.
The head is enclosed by an exoskeleton which consists of several
plates or sclerites, both paired and unpaired, united together, having
no clear relation to the segmentation of the head. The segments are
indicated by the paired appendages, the ganglia of the nervous system
(neuromeres) and the coelomic sacs which can be demonstrated in
sections of the embryo but which disappear later. Thus the six seg-
ments are indicated by the evidence which follows :
Segment
Preantennal
Antennal
Intercalary
Mandibular
Maxillary
Labial
Neuromere
Protocerebrum
Deutocerebrum
Tritocerebrum
Mandibular ganglion
Maxillary ganglion
Labial ganglion
Appendage
Antenna
Embryonic
Mandible
Maxilla
Labium
426 THE INVERTEBRATA
In the embryos of most generalized insects only, are coelomic sacs
present in all head segments.
In addition to compound eyes there are simple eyes or ocelli, of
two kinds. Lateral ocelli are usually the sole type of eye in larval
insects and represent the larval counterparts of the compound eyes
which function in the adult. Dorsal ocelli on the vertex of the head
of adult insects are structures distinct from the lateral and co-exist
with compound eyes. The ocellus consists of a single cornea, a trans-
parent area of cuticle which usually forms a lens-like body, the cells
which secrete it, and the visual cells arranged in groups, the retinulae,
having in the centre the optic rod or rhabdome. The compound eyes
(as described more fully in the section on the Arthropoda) possess a
cornea which is divided into a number of facets; corresponding to
each facet is a group of visual cells, the ommatidium. The current
theory of mosaic vision states that each ommatidium, isolated from
its neighbours by a coat of pigment, conveys to the retinula at its base
only such rays of light as travel parallel to the axis of the ommatidium.
The total impression is that of a mosaic composed of as many separate
pictures as there are ommatidia, every picture different from its
neighbours, but all combining to form a single "coherent" picture.
The compound eye has probably the advantage that it can detect
movements of the smallest amplitude. It gives, however, only a vague
idea of the details of objects, for there is no focussing apparatus and
only objects very close to the eye can be perceived clearly. In some
insects the eye is divided into two parts : a dorsal with coarse facets
which probably only serves to detect variation in illumination, and a
ventral with finer facets which gives fairly definite images of objects.
Possibly in some insects the first function in night vision the second by
day. It must also be mentioned that experiments show that many
insects can distinguish colours. The development of flower colour and
pattern is generally supposed to have taken place simultaneously with
that of the aesthetic senses of insects.
The antennae are a pair of appendages consisting usually of many
joints. They are sometimes filiform but may show complicated varia-
tions in structure. In all cases they carry sense hairs, particularly
those which serve an olfactory function ; it is well known that in some
insects the removal of the antennae or coating them with paraffin wax
destroys the olfactory sense, but this is not always the case.
The mouth is bordered dorsally by the labrum, a median plate or
sclerite which is underlain by the membranous roof of the mouth —
the epipharynx.
The first two pairs of mouth parts, mandibles and first maxillae,
lie at the sides of the mouth, while the second maxillae, invariably
fused together, bound the mouth posteriorly, and are known as the
INSECTA
427
labium. Such a fusion characterizes the maxillipeds of certain Crus-
tacea. The primitive and generaHzed condition is, undoubtedly, that
which is found in insects which feed on soHd food, e.g. the Cockroach
or MachiHs. The mandible which is rarely jointed, represents the
toothed basal segment of an, originally, jointed limb, and corre-
sponds in form and function, to that of Crustacea, but never possesses
a palp (Fig. 301). Each first maxilla is articulated to the head by a
Fig. 301. Mouth parts of Machilis (Petrobius) maritimiis. After Imms.
I, Mandible. 2, Maxilla. 3, Hypopharynx (li.) and superlinguae {sL).
4, Labium, c. cardo; g. galea; gl. glossa; /. lacinia; Ip. labium; m. post
mentum ; 7nx.p. maxillary palp ; pf. palpifer ; pg. paraglossa ; p?n. prementum ;
pgr. palpiger; s. stipes.
basal segment, the cardo. The succeeding segment, the stipes, carries
an outer palp bearing sclerite, and distally, bears two lobes, inner
spiny lacinia, and outer hood-like galea. In the labium, the basal
plates corresponding to cardo and stipes of the two sides, are fused
to form the suhmentum and post mentum. The more distal prementum
bears a palp at either side and a number of lobes, typically four, between
428 THE INVERTEBRATA
them, known collectively as the ligula. Where there are four as in the
Cockroach, the two median glossae are defined from the outer para-
glossae. Each of these lobe? is further divided into two in Machilis
(Fig. 301). Arising from the floor of the mouth is a small sclerite, the
hypopharynx, which bears the salivary aperture. A pair of sclerites,
superlinguae, are normally fused to the sides of the hypopharynx.
Fig. 302 indicates the similarity between the insectan and crus-
tacean mouth parts. Such an attempt at a comparison is only possible
with the more generalized mouth appendages of the Insecta.
With the evolution of different feeding habits, the structure of the
mouth parts, just described, has been departed from in a variety of
ways. Comparative and embryological study, however, clearly reveals
a uniformity of plan throughout, and the student must realize that the
modifications to be met with in bugs, butterflies, bees and flies are
Fig. 302. To show the resemblance between the insectan maxilla and labium
and the biramous limb of the Crustacea. From Imms, after Hansen. A, Bi-
ramous crustacean appendage. B, Insectan maxilla. C, Maxillipeds of a
Gammarid crustacean. D, Insectan labium, end., end.i endites; en. endo-
podite; ep. epipodite; ex. exopodite; sni. submentum.
all referable to the basal plan as exemplified in the mouth parts of
Blatta or Machilis.
The thorax is separated from the head by a flexible neck region
usually containing cervical sclerites, which, however, have not any
segmental value. It consists of three segments — the prothorax, which
carries a pair of legs but no wings, the mesothorax and the metathorax,
which each bear a pair of legs and, typically, wings. The legs are made
up of five main segments, the coxa and trochanter (both of which are
small), the femur and tibia (which form the greater part of the limb),
and the tarsus (which is usually further subdivided into a number of
joints, and ends in a pair of claws with a bifid cushion between them
called the pulvillus). Of the many adaptations exhibited by the legs of
insects the jumping type found in grasshoppers, the digging type in the
mole-cricket Grylloialpa, the swimming type in the water beetles like
INSECTA 429
Dytiscus, the prehensile type in the fore legs of the praying insect
Mantis may be mentioned, in addition to the ordinary running type
as seen in a cockroach. Modifications for the production or reception
of sound as in the Orthoptera and for the collection of food (the combs
and pollen baskets of bees) are also familiar.
The wines of an insect are thin folds of the skin flattened in a
horizontal plane, arising from the region between the tergum and
pleuron. A section of a wing bud shows two layers of hypodermis,
the cells of which are greatly elongated (Fig. 320). Into the blood
space between the layers grow tracheae, and when in a later stage the
two layers of hypodermis come together and the basement mem-
branes meet and fuse, spaces are left round the tracheae which form
the future longitudinal wing veins. These spaces contain blood and
sometimes a nerve fibre during development. The cuticle round the
veins is much thicker than in the general wing membrane, so that the
veins are actually a strengthening framework for the wing. The
number and arrangement of the veins is highly characteristic of the
difi^erent groups. Though the majority of insects possess wings there
are important orders which are wingless. Some such as those to
which the fleas and lice belong are secondarily so, because of their
parasitic habit. Others, however, constituting the large division
Apterygota, are primitively wingless, and these, both on morpho-
logical and palaeontological evidence, must be regarded as the most
ancient types known.
Among many orders of insects, there has developed a tendency for
the two pairs of wings to act as one. This is accomplished by various
devices which couple the fore and hind wings together, on each side.
In the scorpion flies, e.g. Panorpa, bristles project back from the
posterior or jugal lobe of the fore wing to overlie the anterior border
of the hind wing. Corresponding bristles to these constituting the
frenulum, project forwards from the anterior border of the hind wing,
and overlie the posterior border of the fore wing. In most Lepidop-
tera, frenular bristles of the hind wing are held in position by a group
of curved setae forming a retinaculum on the fore wing. The two
wings of a side, in Hymenoptera are coupled by a row of hooks —
the hamuli — on the anterior border of the hind wing, engaging in
a fold of the posterior border of the fore wing.
In other orders, we find one pair of wings diverted to uses other
than flight, the latter operation, then, being dependent on one pair
of wings. The fore wings, for instance, of Orthoptera and of Dermap-
tera, are protective to the more delicate folding flight wings, behind
them. The elytra, or fore wings of beetles, are similarly protective,
and are held passively extended while the second pair of wings propel
the animal through the air. In the males of Strepsiptera, the anterior,
430 THE INVERTEBRATA
and in the male Coccid bugs and all Diptera, the posterior wings
are minute structures, flight being performed by the remaining
pair, which are normally developed. Thus, either by linking two
pairs of wings together, or by dispensing with one pair, flight is
commonly brought about by one functional unit on each side of the
body. The variations in form, consistency, and size of the wings are
briefly dealt with under the different orders.
Simple up-and-down movements of the wings are sufficient to
account for the elementary phenomena of insect flight. In moving
through the air the anterior margin remains rigid but the rest of the
membrane yields to the air pressure; so that when the wing moves
downward it is bent upwards (cambered) ; as the wing moves upward
the membranous part is bent downwards, therefore, by becoming
deflected the wing encounters a certain amount of pressure from be-
hind which is sufficient to propel it. The faster the wings vibrate the
more they are cambered, the greater the lateral pressure and the
faster the flight. Smaller insects have as a rule a greater rate of wing
beat. Thus a butterfly may make only 9 strokes a second while a bee
makes 190 and a housefly 330. The wing muscles of insects thus con-
tract immensely faster than those of any other animals. It is inter-
esting to note that the intracellular respiratory pigment, cytochrome,
occurs in high concentration in them.
To bring about wing movement direct muscles attached to the wing
base and others called indirect inserted on the body wall are employed.
The extent to which direct and indirect muscles are present varies.
In the Odonata a direct musculature is strongly developed, the
muscles being attached to the intucked wing base. In the specialized
orders Lepidoptera, Diptera and Hymenoptera, indirect muscle action
is responsible for most of the movement and those muscles attached
directly to the wing base serve for folding the wing to a position of
rest as well as for flight purposes.
Fig. 303 represents diagrammatically the condition in the winged
aphides. The thorax is a box whose roof is capable of being arched
and flattened by longitudinal and dorsoventral muscles respectively.
Since the wing base has two points of attachment, (i) to the pleural
plate, and (ii) to the edge of the tergum, the wing operates as a lever
of the second order. The arching of the tergum raises the wing base
and depresses the wing, while a flattening of the tergum depresses the
wing base and raises the wing.
The abdomen consists of a series of segments less dift'erentiated
than those of the head and thorax. The number is eleven, as seen to
be present in the embryo insect (with the addition of a transient
telson) and in primitive groups (Thysanura and Odonata). In other
groups, the nth segment is represented by the podical plates which
INSECTA
431
bear the cerci anales (as for instance in the cockroach). In specialized
insects the apparent number of abdominal segments may be greatly
reduced .
In insect embryos rudiments of appendages are borne on each of
the abdominal segments, but these rudiments disappear in the adult
except in the Apterygota. Only those which become the cerci anales
in the nth segment are frequently retained. In the 8th and 9th
t.irM-
V
.IV. a
l.rti.
P
.w.a
Fig. 303. To illustrate the mechanism of wing movement in an Aphid.
Wing depression: A, left side view of mesothorax; B, transverse section.
Wing elevation: C, left side view of mesothorax; D, transverse section.
dv.tn. dorsoventral muscles; l.m. longitudinal muscles; p.zv.a. pleural wing
attachment; t.w.a. tergal wing attachment. Effective muscles shown by-
dotted lines in A and C. After Weber.
segments in the female and the 9th segment in the male there are
paired structures known as gonapophyses which perform various
reproductive functions (oviposition in the female, copulation in the
male). It is highly probable that these are modified appendages.
The alimentary canal (Fig. 304) varies greatly in length; in many
larvae it is no longer than the animal itself, but in certain types of
insects like the Homoptera, which feed on plant juices, it is much
coiled and may be several times the length of its possessor. It consists
432
THE INVERTEBRATA
of an ectodermal stomodaeum or fore gut, an endodermal mid gut
and an ectodermal proctodaeum or hind gut. The fore gut consists of
(a) the buccal cavity succeeded by (b) the pharynx, which may be
muscular and form a pumping organ (Fig. 328 A), (c) the oesophagus,
which has a posterior dilatation, the crop. This functions as a food
reservoir and may have a diverticulum enormously developed in
sucking insects to store the liquid food. Lastly there is (d) the pro-
mi.
Fig. 304. General view of internal organs of Apis ?nellifica as seen from above ;
musculature and tracheal system not shown. From Carpenter, an. antenna;
bn. brain; co. colon; cr. crop; e. eye; ga. ganglion; ?ng. mid gut; 7nt. Mal-
pighian tubule; oe. oesophagus ; r?n. rectum; sa.gl. salivary glands (three types
are shown); pv. proventriculus ; il. ileum.
ventriculus or gizzard, most typically developed in insects which eat
hard food as in the Orthoptera. The chitinous lining of the fore gut
is here greatly thickened and the sphincter muscles in this region
control the passage of food between fore gut and mid gut. Into the
buccal cavity discharge the salivary glands (Fig. 304), which may as
in the cockroach have a very similar function to those of the mammal,
in producing enzymes for the digestion of carbohydrates. In other
INSECTA
433
insects, however, they are specialized in ways which are mentioned
later. Such glands are usually associated with the labium; in some
insects, however, mandibular and maxillary glands are found.
The mid gut (Fig. 305) is lined by a layer of cells frequently all
similar, which perform almost the whole task of digestion and ab-
Fig- 305. A, Longitudinal section of wall of oesophagus of a termite.
B, Longitudinal section of mid gut of termite in secretory phase. C, Trans-
verse section of mid gut of Blatta in resting phase. After Imms. bm. base-
ment membrane; c, chitinous intima; cm. circular muscle; cr. crypt; ep.
cellular layer ; e. enteric epithelium ; h. striated hem ; l.m. longitudinal muscles ;
nc. group of regenerative cells ; pm. peritoneal membrane.
sorption of all classes of foodstuffs. While secreting, the cells break
down and their contents are discharged into the gut cavity. In the
absorptive phase the border of the cells has a striated appearance. The
same cell may be capable of both absorption and secretion, but the
epithelium as a whole often passes through rapid cycles which necessi-
tate the constant supply of fresh cells. These are found (Fig. 305) in
434 THE INVERTEBRATA
the troughs of folds or bottoms of pits into which the mid gut epithelium
is thrown. In many insects the surface is increased by the formation
of long diverticula, the pyloric caeca, the cells of which are not in any
way diiferent from the rest of the epithelium. These vary greatly in
number. Though the mid gut epithelium has not an internal chitinous
lining there is a curious chitinous tube, free in its cavity, the peritrophic
membrane, which is, however, secreted by special cells of the pro-
ventricular region (which may be ectodermal). Its function and place
in digestion are not understood.
In certain cases, however, the mid gut is differentiated into func-
tional regions. The first part of the mid gut of the tsetse fly, for
instance, is concerned with water absorption which reduces the meal
of blood to a viscid mass. Digestion of the food takes place in a region
behind this, and in the lowest region of the mid gut absorption is
effected. These functional regions are histologically distinct. In the
cockroach in which no such histo-physiological distinction exists
between the several parts of the mid gut, it appears that much di-
gestion takes place in the crop to which place the enzymes from other
parts may have to pass to meet the food before its further passage
backwards. The so-called gizzard has been shown in this case to act
not only as a triturating organ, but as a complicated sphincter
guarding against the passage of any but the finest particles from the
crop to the mid gut. After digestion has proceeded in the crop as
the result of salivary and other secretory activity, the food passes
through the gizzard, there to be triturated ; and so on to the mid gut to
meet the enzymes produced by the walls of this part of the gut.
Resorption of the digested food takes place in the mid gut as well as
in the hind gut. The hind gut begins where the Malpighian tubules
enter the alimentary canal and is usually divided into a small intestine
or ileum, a large intestine or colon, in both of which the chitinous lining
is sometimes folded and produced into spines, and a short globular
rectum. In most insects rectal glands in the form of thickened patches of
epithelium occur .These have been shown to absorb water from the faeces
and therefore play a most important part in water conservation.
Though the digestive enzymes of insects in the main belong to the
same classes as those of mammals there are many significant differ-
ences. An omnivorous insect like the cockroach produces all the
classes of enzymes except that represented by pepsin which is peculiar
to vertebrates. Then also the enzymes of insects appear to work in a
rather more acid medium than do the enzymes of mammals. Finally
the specialization in feeding habits in insects is responsible for the
absence of enzymes which are not wanted and either the acquisition
of enzymes not generally found in the Animal Kingdom or the
formation of a symbiotic partnership.
INSECTA 435
Thus when we compare the cockroach with such forms as the tsetse
fly (Glossina) and the blow-fly Calliphora^ we find these latter deficient
in certain enzyme classes, the former in carbohydrases, the latter in
tryptases and peptidase. The evolution of the habit of feeding on
blood (which consists so largely of proteins) involves the loss of the
enzymes which digest carbohydrates and fats. Similarly the blow-fly
which exists on a diet in which carbohydrates are predominant has
to a certain extent lost its proteolytic and lipolytic enzymes.
This principle has an even wider application. In the leaf-mining
caterpillars of the Lepidoptera, certain species are restricted to the
upper and others to the lower parenchymatous layer of the leaf. If
an egg of one species is accidentally deposited in the wrong layer of
the leaf, death of the larva ensues owing to its inability to digest the
proteins of that layer. Thus each species, it is said, has enzymes which
are specialized in the narrowest degree for digestion not only for the
proteins of a single plant but for those of a particular part of that plant
(all others being unsuitable). Sucking forms, like Aphis, explore
diff^erent regions of the plant tissue and it may perhaps be inferred that
they have a wider range of enzymes than the leaf-miners.
Most interesting of all is the relation of phytophagous insects to
cellulose, which is incapable of digestion by any vertebrate. Only a
few wood-boring beetle larvae [Cerambycidae) have been shown to
possess an enzyme which digests cellulose. The great majority of
insects do not possess a cellulase and as all plant cell contents are
contained within cellulose envelopes, it is clear that digestion can
only follow when either protoplasm is released by mechanical injury
of the cell-wall or the enzymes are able to penetrate the cell-wall and
act upon the contained protoplasm. In lepidopterous caterpillars,
which digest vegetable protoplasm with much greater success than
do mammals, the latter explanation has been shown to be true.
The insects which live on wood (excluding the Cei^ambycidae) can
be divided into two classes: (i) those, like bark beetles, which feed
on fungi, growing in their tunnels, and (2) those which harbour sym-
biotic organisms in special parts of their alimentary canal. In the
latter class maybe mentioned the wood-boring larvae of certain crane-
flies and of death-watch beetles (e.g. Xestobium). In these cases the
supposed symbiotic organism is the yeast, Saccharomyces. How it
assists in the assimilation of wood is not known. On the other hand,
those of the termites which eat wood in normal life always contain
the flagellates belonging to Trichonympha and other genera (p. 68)
living free in the intestine. The absolute dependence of certain
termites on the flagellate is shown by the fact that when the flagellate
fauna is removed (which can be done without harming the termite
by heating to 40°) the termites will starve although they continue to
43^ THE INVERTEBRATA
eat their usual diet. Since the trichonymphids are known to digest
wood inside their own bodies, it is probably only indirectly that the
termites benefit from the wood, the flagellates being their immediate
source of food. Termites will live on a diet of cellulose (e.g. cotton
wool) but not when the last traces of nitrogenous material have been
removed.
The majority of so-called saprophagous insects are really phyto-
phagous, in that they feed on yeasts, and micro-organisms effecting
the decomposition of the decaying matter. The house-fly is probably
such a case. Blow-fly larvae feeding on decaying meat do, however,
employ proteolytic enzymes, and to this extent are truly saprophagous,
as is also the dung beetle Geotrupes. The flesh-fly Luctlia, though
saprophagous in this way, still requires the microflora of the decaying
food to complete a diet suitable for full development, these organisms
supplying the vitamines necessary for growth.
The great range of environments occupied by insects as a whole is
largely an expression of their diverse feeding habits, and few materials
have escaped their attentions. In addition to the foods mentioned
above, may be noted keratin, which undergoes fermentative digestion
in the larval gut of the clothes-moth Tmea biselliella. Silk can be
utilized as the sole diet of the museum beetle Anthrenus museoruniy the
amino-acids in this case supplanting both fats and carbohydrates.
The saliva of various insects shows great variety according to their
habits; thus the larva of the tiger-beetle (Ctcindela), the flesh-eating
larvae of flies, e.g. Sarcophaga, and the aquatic larva of Corethra, pour
their saliva, which contains a proteolytic enzyme, on their food and
suck up the products of digestion {external digestion). Bees, with their
reliance on pollen and honey as food, have four different kinds of
salivary glands. These probably serve different purposes such as to
invert sugars, to ensure preservation of food by adding formic acid,
and to predigest pollen in the manufacture of " bee bread" on which
the young are fed. The proportion of carbohydrate to fat and protein
in the food after the early stages of feeding, determines whether a
larval bee shall become a queen (fertile female) or a worker (sterile
female). The former is fed throughout on a richer protein diet pre-
pared from pharyngeal glands while the latter has its diet changed to
pollen and nectar containing a higher carbohydrate content. In
wood-boring larvae the secretion of a mandibular gland softens the
wood and thus assists mastication, while, in caterpiUars, silk pro-
duction is the main function of labial glands.
The principal excretory organs are the Malpighian tubules, opening
into the anterior end of the hind gut, and therefore are just as much
ectodermal structures as the nephridia of annelids. The proof of their
function is the presence of crystals, which can be identified micro-
INSECTA 437
chemically as uric acid, inside the cells and in the lumen of the tubule.
A mass, mainly of uric acid, is found in the hind gut of pupating in-
sects, having been deposited there by the tubules. But in addition
nitrogenous end products are found in the nephrocytes (cells found
commonly associated with the fat body and the pericardium), the
fat body and the hypodermis in quantities which increase with age,
and in the hollow wing scales of certain butterflies, e.g. Pieridae, so
that it appears that the mechanism of the Malpighian tubules for
ridding the body of the insect of nitrogenous excreta is by no means
efficient.
Of non-nitrogenous excretory products may be mentioned the
carbonates of calcium, potassium and magnesium. Calcium car-
bonate may be excreted in the integument, but in many cases it is
eliminated by the Malpighian tubules, either gradually, e.g. Droso-
phila, or expelled en masse by way of the blood and the hypodermis
during pupation, e.g. Ascidia, the celery fly. In this latter example
the recrystallization of the compound on the inner wall of the
puparium (see p. 509) may serve to strengthen the weakness of the
latter.
To the majority of insects the matter of water conservation is of
considerable importance. It is interesting, therefore, to find that in
a blood-sucking bug (Rhodnius) the proximal part of the Malpighian
system is concerned with the withdrawal of water from the lumen of
the tubules, and its return to the body cavity. In this insect, the
distal parts of the tubes secrete into their lumen potassium and
sodium urate, water and base returning to the blood through the
walls of the proximal parts of the tubes (Fig. 306). A circulation of
water and base exists, therefore, within the system, similar to that
obtaining in the vertebrate kidney.
The circulatory system. There is, first, a heart, primitively con-
sisting of thirteen chambers, each corresponding to a segment, with
a pair of ostia guarded by valves precluding outflow, at the base of
each chamber. The blood is driven forward in these by muscular
action of the heart wall, and passes into an anterior aorta which opens
into the general body cavity in the head region. The haemocoelic body
cavity is very spacious and the blood bathes all the organs. There is a
dorsal horizontal diaphragm perforated by many holes, which separ-
ates off the pericardium in which the heart lies, and attached to this
are paired alary muscles, the outer ends of which are inserted in the
terga (Fig. 307). By their contraction the passage of blood from the
body cavity into the pericardium and heart is facilitated. Though the
circulatory system is usually simple, accessory vessels are known,
which direct blood backwards along the nerve cord and upwards
towards the pericardium in the metathorax (in the moth Protoparce)
438
THE INVERTEBRATA
Fig. 306. Diagram of a longitudinal section through the gut of Rhodnius
prolixus to show the entrance of Malpighian tubules. Amp. ampulla in
wall of hind gut, the long cells of which project into the rectum and with-
draw water from the excretory matter ; Mg. mid gut ; Mt.D. distal part of
Malpighian tubule at which place water and excretory matter enter the tube;
Mt.P. proximal part of Malpighian tubule from which water is returned to
the body cavity, leaving the excretory material in a granular state ; Rg. rectal
gland in wall of rectum. Modified after Wigglesworth.
Fig- 307- Transverse section through dorsal part of the abdomen of Apis
mellifica to show attachment of heart to the body wall and to the diaphragm
by the alary muscles (al.m.). After Snodgrass. (The insertion of the alary
muscles in the tergum is not shown.) dg. diaphragm ;/.6. fat body; h. heart;
mg. mid gut; mt. Malpighian tubule; tra. trachea.
INSECTA 439
Further, in certain insects accessory hearts are present which assist
in the circulation through special regions (in the thorax of the beetle
Dytiscus and in the bases of the legs of Aphids where they propel blood
through the wings and legs of these forms respectively). The much
reduced system is on the whole greatly in contrast with the complex
arrangements of the decapod Crustacea and of such Arachnids as
Limulus and the Scorpions where the respiratory pigment haemo-
cyanin renders the blood of the greatest importance in respiration.
The part played by blood in respiration introduces a topic which can
only adequately be considered with the tracheal system next to be
described. In anticipation of that account it may suffice to note the
following points. The walls of the tracheae are freely permeable to
gases and there must therefore occur an exchange of gases between
the blood and the air in the air in the tracheae. In some insects the
walls of air sacs within the tracheal system become intucked so as
to form "inverted tracheae" through which blood circulates, thus
giving rise to an organ which may act as a veritable lung, e.g. Sphinx
and Crabro. Though these facts suggest a special oxygen-carrying
function for the blood, it appears that its oxygen capacity is no greater
than can be accounted for by physical solution. Haemocyanin does
not occur and to this fact must be put down the rather vestigial nature
of the blood system in insects. Haemoglobin occurs in a few, e.g. the
larva of the midge Chironomus, the male apparatus of the water bug
Macrocorixa and in certain tracheal cells of the horse-fly Gastro-
philus. This pigment may be derived from intracellular cytochrome
and its occurrence be of the nature of a chemical accident of little
functional significance. On the other hand it may serve, as it appears
to do in Chironomus, as a means of enabling the animal to utilize
oxygen when this occurs only at low tensions in the surrounding
medium. The occurrence of chlorophyll invariably owes its origin to
the food plant. Of the several kinds of blood cells which exist, per-
haps those which play an important part in the histolysis of larval
tissues during the pupation of holometabolous insects, e.g. the blow-
fly Calliphora, are of most interest.
Associated with the blood are the following cellular tissues, the
fat body, the nephrocytes, the oenocytes, the corpora allata, and in
various beetles, the photogenic organs. The fat body consists of
closely adherent cells, in the vacuoles of which products of
digestion are stored up. Fats, albuminoids and glycogen occur in this
way. In addition are found urates showing that this organ serves for
excretion. Oenocytes occur as bunches of large cells close to the
spiracles in the abdomen and develop as hypodermal invaginations
in these places. The corpora allata arise similarly in the mandibular
segment and subsequently come to lie above the oesophagus behind
440
THE INVERTEBRATA
the brain. These small compact glands are probably hormonic in
function though the precise nature of them (as of oenocytes) awaits
elucidation. 1 Photogenic organs, found in the glow-worm larva and
the female beetle Lampyris, possess a rich supply of tracheae and
produce light by the oxidation of luciferin by the enzyme luciferase.
In the insects the tracheal system characteristic of terrestrial Arthro-
poda attains its most complete development. The ectodermal tubes
of the system form a network of which every part is in communication
a.s.
Fig. 308. Tracheal system of the locust, Dissosteira Carolina. Modified from
Vinal. A, Side view. B, Dorsal view, the lower half to show air sacs, the
upper half to show tracheal supply to the alimentary canal, a.c. alimentary
canal; a.s. air sacs; l.t. longitudinal trunk; sp. spiracles.
with every other part. Typically it communicates with the exterior
by two pairs of openings called stigmata or spiracles on the thorax and
eight pairs on the abdomen (Fig. 308). The main branches leading
from the stigmata not only divide into finer capillaries leading to the
adjacent organs but communicate by means of lateral trunks with each
other. The capillaries or tracheoles never end blindly in the blood but
always in the cells of the body, whether muscular or glandular or
connective tissue, so that normally the oxygen is conveyed directly
^ It has recently been shown in Rhodnius that they act as ductless glands
controlling metamorphosis.
INSECTA
441
to the latter without the intervention of the blood. These end tubes,
as may be seen in Fig. 309, are of the smallest calibre and their lumen
is intracellular. The chitinous lining, which in the main tracheae is
strengthened, forming the spiral threads which prevent collapse of
the tubes, in the tracheoles is thinned down so much that gaseous
diffusion can take place easily between the cell fluid and the lumen
of the tube.
The system is further elaborated to secure regular circulation of
air in the main passages. Thus the stigmata are oval slits which can
be closed and opened in various ways — usually by valves operated by
Fig- 309. Tracheal end cell and tracheoles from silk gland of caterpillar,
Phalcra bucephala. From Imms, after Holmgren, c. tracheoles ; e. end cell ;
t. trachea.
special muscles. Respiratory movements can easily be observed in
such insects as wasps and grasshoppers. They are effected by the
alternate contraction of the abdomen in its vertical axis by tergo-
sternal muscles and recovery to the original form usually by the
elasticity of the abdominal sclerites. Abdominal contraction with
open spiracles results in expiration, but if the spiracles are closed the
air already in the system will be forced into the finer capillaries where
the oxygen pressure is thus increased.
In some Orthoptera it has been found that certain stigmata are
normally inspiratory and others expiratory. Thus, in various grass-
442 THE INVERTEBRATA
hoppers (Fig. 308), the first four pairs are open at inspiration and
closed in the expiratory phase, while the last six pairs are open in the
expiratory phase and closed at inspiration. It follows that an air
circulation through the main trunks is set up, aiding considerably in
the diffusion of gas through the whole system. Air sacs (as mentioned
above) in the form of thin-walled diverticula of the main tracheae
occur in many insects (Fig. 308), particularly those, such as bees,
migratory locusts and house-flies, with the power to fly for prolonged
periods. These also assist considerably in the circulation of air
through the tracheal system owing to the ease with which they can
be compressed.
Thus to assist respiration in typical insects a neuro-muscular
mechanism has been evolved which ensures some control of the
ventilation of the tracheal system. Spiracular closing mechanisms
and compressible air sacs are important in this process. But though
a circulation of air certainly does take place in some, there are forms,
such as lepidopterous larvae, which exhibit no respiratory movements
and so, it may be inferred, possess no means of ventilating the air
tubes. Forces of diffusion have been shown to be adequate to supply
oxygen to the tissues of such examples as have no ventilating mechan-
ism. These same forces will also explain the transfer of oxygen from
the wider to the narrower air-containing tracheae. With regard to the
ultimate problem concerning the way in which the air reaches the
cell, modern theory on insect respiration assumes that the blind ends
of the tracheolar tubes are bounded by a membrane which is im-
permeable to lactic acid and such metabolites. Each tracheole con-
tains a variable amount of fluid, the height of the column of which is
determined in a state of equilibrium by hydrostatic pressure and
capillarity on the one hand and by forces of osmotic pressure in the
tissue fluids and of atmospheric pressure on the other. If now the
osmotic pressure of the tissue fluids, for any reason, increases, water
will then be absorbed from the tracheole tubes and the column of air
will be made to extend more deeply into the tissue. It has been shown
that muscular activity of insects is associated with such withdrawal
of water from the tracheoles. The evidence points to the conclusion
that the change from glycogen to lactic acid which accompanies
muscle contraction would provide the necessary osmotic changes to
withdraw water from the tube and so bring the column of air to the
tissues when and where their need is greatest (Fig. 310). From the
air column, thus brought deeply into the tissues, the oxygen must
diffuse into the surrounding tissue fluids.
The control of respiratory movements by nerve centres is of in-
terest. Though each nerve ganglion of the ventral chain serves as a
centre for the respiratory movements of its own segment, there are
INSECTA
443
certain regions of the nervous system which exercise a controlHng
influence over the respiratory activity of the insect as a whole. One
example will serve to illustrate this point. The nymph of the dragon-
fly Libellula pumps water for respiratory purposes into its rectum.
In the natural state it responds to changes in oxygen content of water
Fig. 310. Diagrams to illustrate the theory of tracheal respiration. A, Trache-
ole ending in resting muscle; B, In active muscle, i, trachea; 2, tracheole
cell; 3, parts of tracheoles containing air; 4, parts of tracheoles containing
liquid; 5, muscle. After Wigglesworth.
quite readily, by increasing the rate of its respiratory movements,
when there is oxygen lack ; reducing such movements in water satur-
ated with oxygen. When however the prothoracic ganglion is de-
stroyed, respiratory movements continue evenly, without reference
to the oxygen tension of the water.
444
THE INVERTEBRATA
There are thus primary respiratory centres, each responsible for
movement in its own segment, and specially localised secondary
centres, which can influence those movements in accordance with
the demands for oxygen. The site of the secondary centre varies in
different animals, but never appears to lie in the head. Just as
secondary centres respond to oxygen lack, so have they been shown
to respond to the influence of carbon dioxide.
Though the above remarks would apply to the majority of insects,
there are many stages of reduction in the group, culminating in the
wingless Collembola, many of which have no tracheae at all, gaseous
exchange taking place through the skin.
Aquatic insects fall into two physiological groups. The first
is distinguished by direct breathing, at least one pair of functional
Fig. 311.
Pupa of Anopheles maculipennis. After Nuttall and Shipley.
/. respiratory funnel.
Spiracles being retained. In the water beetle Dytiscus the abdominal
spiracles communicate with a supply of air under the elytra which
is renewed when the beetle comes to the surface : in the larva of the
mosquito the spiracles are open to the air while the animal is sus-
pended from the surface film (Fig. 312).
The second group includes the early stages of the Odonata,
Plecoptera, Ephemeroptera and Trichoptera. These have no func-
tional spiracles but breathe by means of tracheal gills — expansions of
the body wall through whose thin walls respiratory exchange between
the animal and the water is effected according to the laws of diflf"usion
(Fig. 333). They are usually external but in certain dragonfly nymphs
{Aeschna and Libellula) the rectal wall is raised into such gills and
respiration is effected by pumping water in and out through the anus.
INSECTA
445
Fig. 312. Larva of Anopheles maculipennis After Nuttall and Shipley-
r feeding brush; c. antenna; d. maxilla; .. thorax;/, spiracles, g. palmate
hairs for suspending from surface film; /. anal gills.
446 THE INVERTEBRATA
Certain larvae show an even more complete adaptation to life in water
in that though they possess a tracheal system this is entirely closed
from the exterior and in their early stages it is filled with fluid. Such
forms respire of necessity by a process of simple diffusion through the
general integument, e.g. Chironomus and Simulium.
Reproduction. The sexes of insects are separate, leery a purchasiy
a remarkable exception, being the only known self-fertilizing her-
maphrodite in the class. The usual method of reproduction is by de-
position of yolky eggs following copulation. The egg, except in many
parasitic Hymenoptera, is richly supplied with yolk and invested with
a vitelline membrane and further protected by a hard shell or chorion.
The chorion exhibits different degrees of external sculpture and it is
perforated at some point or points to allow of sperm penetration.
The spermatozoa, which are of the filiform type, may be transmitted
to the female in the form of a spermatophore. Though insects are
on the whole prolific creatures capable of producing large numbers
of eggs, a few cases are met with where females only lay a few eggs
in the course of their life. Thus, in the viviparous tsetse flies, a single
egg is passed to the uterus about every nine or ten days. The larva is
there nourished by special "milk" glands till it is fully fed when it
is passed out for immediate pupation. Viviparity and reduced tgg
production are here obviously associated with one another. In a large
number of cases reproduction is effected without the intervention of
the male. This phenomenon of parthenogenesis is best seen in the
aphides or plant lice where several generations resulting in the pro-
duction of parthenogenetic females are passed through. The racial
advantage accruing from this greatly increased reproductive capacity
is obvious.
Parthenogenesis is in certain cases, e.g. among the family Cecido-
myidae of the order Diptera, found to occur in larval forms. In
Miastor, a form living in decaying wood and bark, reproduction in
this manner (paedogenesis) occurs for the greater part of the year. These
larvae contain prematurely-developed ovaries from which as many as
thirty larvae may grow. In summer, larvae occur which are morpho-
logically different from the paedogenetic forms. These summer larvae
pupate and the small midge-like flies which emerge lay four or
five large eggs; from these a further series of paedogenetic larvae
arises.
Among a few of the parasitic Hymenoptera, e.g. some Chalcididae,
the phenomenon oi polyembryony has been observed. This consists in
the development of more than one embryo from a single Qgg. In
Copidosoma gelechiae, which parasitizes a caterpillar living on the
goldenrod Solidago, a hundred or more embryos may result from the
deposition of a single egg.
INSECTA
447
Organs of reproduction (Fig. 313). In the male the testes are usually
small paired organs lying more or less freely in the body cavity. The
extent to which they are divided into follicles, and the form of follicle,
vary in different orders. Thus, in the Diptera, each testis is unifoUi-
cular, while in the Orthoptera a multifollicular condition prevails.
Each follicle is divided into Sigermarium or formative zone, a zone of
growth and maturation, and a zone in which spermatids are trans-
formed into spermatozoa. In multifollicular testes the connection
between each follicle and the main duct is known as the vas efferens
Fig. 313. Diagram of reproductive organs of A, a male, and B, a female honey
bee. C, Longitudinal section of an ovariole of Dytisciis marginalis. A and B
after Comstock. ac.gl. accessory gland; be. bursa copulatrix; cgl. colleterial
gland; ed. ejaculatory duct;/, follicle cells; ge. germarium; ov. ovary; od. ovi-
duct ; o. ovum ; sc. spermatheca ; t. testis (multifollicular) ; vd. vas deferens ;
V. vagina; ve.se. seminal vesicles.
and each testis leads to the median ejaculatory duct by a vas deferens
which is swollen at some point to form a seminal vesicle. The ejacu-
latory duct opens between the 9th and loth abdominal sterna in
association with the external genital plates (gonapophyses) of copu-
latory significance. Accessory glands of various kinds and little
understood function are usually found associated with the genital
ducts.
The female organs (Fig. 313) consist of ovaries, oviducts, sperma-
thecae, colleterial glands and a bursa copulatrix.
448 THE INVERTEBRATA
Each ovary consists of a number of ovarioles, corresponding to the
testicular follicles of the male. Reduction of the ovary to a single
ovariole occurs in such insects as Glossina^ the tsetse fly, where the
minimal number of eggs is produced.
Each ovariole (Fig. 3 13) is tubular and contains zones corresponding
to those met with in the follicle of the testis. In addition to the de-
veloping ova, nutritive cells are found in association with the latter.
Such cells are concerned with the transference of yolk to the growing
ova and they or other cells may entirely encircle the ova, round which
they secrete the chorion or outer egg shell.
The ovarioles forming an ovary are connected together anteriorly
in the body cavity by their peritoneal coverings, known at this point
as terminal filaments, and these are attached either to the body wall
or the pericardial diaphragm, thereby maintaining the ovary in
position.
The oviducts leading from the ovaries unite in the middle line to
form a common duct which widens to form the vagina immediately
before reaching the exterior on or between the 8th, 9th and loth
abdominal sterna.
Colleterial glands providing fluid for the formation of an ootheca
(a case surrounding the eggs), or a sticky secretion for fastening eggs
to surfaces, usually open into the vagina. The pouch for the reception
of spermatozoa is the spermatheca. It is an ectodermal invagination,
lined by chitin and provided with a muscular coat. The spermatheca
opens into the vagina or into the bursa copulatrix, this being an in-
vagination of the body wall around the genital aperture adapted for
receiving the intromittent organ of the male.
The fiervous system of insects (Fig. 314) consists of a dorsal brain
and a ventral double chain of ganglia connected by longitudinal and
transverse commissures. The anterior three pairs of ganglia of the
ventral chain are always fused to form the suboesophageal ganglion^
the nerves from which supply the mouth parts. The suboesophageal
ganglion is united by paraoesophageal connectives to the brain.
The brain consists of three pairs of closely fused ganglia which
supply the eyes, antennae and labrum respectively (see p. 425). In
addition to this is the sympathetic system (Fig. 314 B, C) which
supplies the muscles of the alimentary canal and of the spiracles.
In the insects, and indeed the arthropods in general, there has been
a great advance over the stage of nervous organization in the annelids.
The complex nature of the appendages and the necessity of co-ordi-
nating groups of these for locomotion, feeding and so on, has led to
the association of special parts of the nervous system with these
functions. We will call each such part a "functional unit". Each
functional unit is to some extent self-regulating and is not dependent
INSECTA
449
for its autonomous action on the higher centres. For example a de-
capitated wasp can still walk and if a limb be removed from one side,
compensating movements of the remaining five legs enable the animal
to walk in a straight line. But the working together of the functional
units concerned, into different reactions, is controlled by the brain,
ocn.
Fig. 314. Nervous system of a grasshopper. After Uvarov. A, Ventral
chain. B, Brain and associated nerves. C, Optical section through head.
Mtrn. antennary nerve and ganglion; dc. deutocerebrum ; ocn. ocellar nerve;
op.ga. optic ganglion; pc. protocerebrum ; sug. suboesophageal ganglion;
syg. sympathetic ganglia ; tc. tritocerebrum.
and the inhibitory character of that control is shown when the ganglia
are removed. A "decerebrate" bee will try to fly, walk, feed and
polish its abdomen all at the same ^ime. This is because no inhibi-
tion is being exercised on the functional units, which themselves
remain intact in spite of the removal of the higher centres.
Sense organs. There can be no doubt that insects perceive stimuli
similar to those causing sensations in ourselves. They are sensitive
45° THE INVERTEBRATA
to the waves of light and sound, to changes of temperature, to chemi-
cal stimuli by contact or at a distance, e.g. as in the sensations of taste
and smell, and to tactile impressions. The sensory equipment is
complicated, and the solution of the functional problem which many
of its parts present is not made easier by the fact that though the
principle of the reaction may be the same as in ourselves, insects often
react to stimuli of an amplitude which is beyond our receptive
capacity. For instance, they do react to pitches of sound which
the human organ cannot detect, and though they do not appreciate
the full spectrum in colour vision, they can perceive ultra-violet rays.
No matter what the sense organ may be, the fundamental element
is the sensilla. In the case of a simple sensory hair {trichoid sensilla)
the following elements are present: a trichogenous cell which gives
rise to the seta ; a hair membrane cell which produces the fine mem-
brane at which the seta is articulated to the body wall, and a bipolar
nerve cell which lies within the trichogenous cell (Fig. 315). Such
sensillae are generally tactile, though in certain cases olfactory, gusta-
tory and heat-perceiving functions have been shown to rest in them.
Olfactory sensillae commonly occur on the antennae. These are
generally placoid (with plate-like cuticle covering the sense cell) or
coeloconic (where the covering plate is thin and sunk in a depression
below the surface) (Fig. 315). But though the antennae are usually
olfactory in function, this sense is also located elsewhere, since re-
moval of the antennae does not entirely inhibit olfactory sensation.
The power of insects to diffuse scents from special glands is well
known. These serve for defence, or to attract the sexes to each other,
and their prevalence, and wide distribution throughout the class,
postulate the existence of an olfactory sense. In moths the faculty
possessed by males of discovering the exact position of unpaired
females is of so astonishing a character that many observers have dis-
believed the olfactory explanation, and resorted to theories of etheric
wave-transmission. The production of a volatile chemical is clear,
however, in those cases where male moths have assembled at an
empty box in which a female had been recently housed. It is com-
paratively simple to demonstrate the existence of a taste sense in
insects. Preferences for sugar to other substances in solution can
readily be shown in a feeding butterfly. To find however, for example
as in Pyrameis, the red-admiral butterfly, that the taste organs lie in
the feet, is perhaps sufficient reason for using the term chemo-tactile ^
for a sense which has no exact parallel in our own experience. Taste
organs occur also in the mouth, and on the palps of the mouth-parts.
Many insects, such as grasshoppers and cicadas, are provided with
sound receptors known as tympanal organs, with which are incor-
porated chordotonal sensillae. Each of the latter consists of a sense
INSECTA
451
cell, to one end of which is attached a nerve fibre. To the other end
is connected a rod or scolopale which ends in an apical thickening or is
free to vibrate in the fluid protoplasm of an enveloping cell. The
whole structure is attached to the hypodermis by covering cells at
one end and by a ligament at the other (Fig. 315 D).
Fig. 315. Insect sensillae. A, Trichoid sensilla. B, Placoid sensilla. C, Coelo-
conic sensilla. Tc. trichogenous cell; He. hair-membrane cell; Nc. nerve
cell. D, Chordotonal sensilla. Cc. cap cell to scolopale ; Ec. enveloping cell ;
Ek. end knob of scolopale; H. hypodermis; L. ligament of attachment;
Nc. nerve cell; S. scolopale and V. its fluid filled vacuole. A, modified
from Eltringham after Snodgrass. B, modified from Imms after Hess. C and
D from Imms.
Scolopale sensillae of this type may or may not be associated with
a tympanum or ear drum. When they are, as in cicadas and grass-
hoppers, there is clear evidence of response to sound waves set up by
sound-producing organs possessed by themselves.
452 THE INVERTEBRATA
In the numerous cases in which no tympanum capable of respond-
ing to sound waves exists, a precise function is not clearly indicated.
According to some, they may act as rhythmometers, i.e. co-ordinators
of the rhythmical movements of the insect's body. A more probable
function is that of perceiving vibratory stimuli from without.
The organs of vision have been dealt with in Chapter x, and it is
perhaps enough to mention that the ommatidia of which the com-
pound eye is built up, are specialized sensillae of hypodermal origin,
essentially similar to those already mentioned.
Embryology . Though Arthropod eggs vary in the amount of yolk
contained within them they are for the most part yolky and are
ceiitrolecithal in type (p. 316). To this feature must be ascribed those
distorting influences which make Arthropod development so different
from that of other invertebrates.
Among insects it is only in the primitive Apterygota and in many
parasitic Hymenoptera that are found small, comparatively yolkless
eggs which undergo total cleavage. But though these may represent
the primitive condition, they cannot be taken as typical of modern
insects.
The typical 3^olky egg is provided with a vitelline membrane and
a stout chorionic shell which is commonly sculptured. After fertiliza-
tion, incomplete cleavage sets in, a process involving only the suc-
cessive mitoses of nuclei. In this early stage, therefore, the egg is a
syncytium of very yolky cytoplasm in which lie the cleavage nuclei.
These wander to the peripheral cytoplasm, there to form an outer
cellular layer or blastoderm (Fig. 316 A and B). In this latter, occurs
a thickening, thus separating embryonic from extra-embryonic
blastoderm and in its relation to the yolk the embryo now resembles
an inverted chick embryo, but, as might be expected, its method of
differentiation is highly different.
Gastrulation proceeds as follows. From the middle line of this
embryo certain cells pass inwards towards the yolk by invagination,
by proliferation or by their overgrowth by cells of the germ band
lateral to them. This enclosed cell mass is mesoderm (together with
endoderm in certain cases). The plate left outside constitutes the
ectoderm (Fig. 316 C and D). In such cases where endoderm is not
included in the enclosed mass as above, this layer rises from growth
centres, anterior and posterior, at the places where the stomodaeum
and proctodaeum will appear or already have differentiated. The result
in any of these cases is a three-layered embryo relegated to the ventral
side of the egg, i.e. beneath the yolk. It consists of a layer of outer
ectoderm, within which is the mesoderm from which segmental
somites develop. Against the yolk lies the endoderm destined to form
the mid gut. The mesoblastic somites give rise on their upper borders
A'
Ect.
C
Ect
EM.
D'
End. H f
Fig. 316. To illustrate the main features of insect embryology. A, B,C, Sagittal
sections. A', B', C, Transverse sections through corresponding stages.
D, Sagittal section of embryo with germ layers present. C", Transverse
section through C-stage embryo in mouth region. D', Transverse section
through D-stage embryo. E, Transverse section through older embryo with
haemocoele between endoderm and ectoderm (in which nerve cord is
developing). The mesoblastic somite on each side has given rise to the
gutter-like heart rudiment, to the fat-body and to the muscles of body wall
and gut. Embryonic membranes, the amnion A and the serosa 5 cover the
embryonic rudiment. Bl. blastoderm; Ect. ectoderm; End. endoderm;
Exe. extra-embryonic blastoderm; F. fat-body; G.B. germ band; H. haemo-
coel; ///. heart rudiment; M. mesoderm; Mc. muscles; Pr. proctodaeum;
St. stomodaeum ; Y. yolk. After Eastham.
454 THE INVERTEBRATA
to the heart rudiments, and on their outer and inner borders to the
muscles of body wall and gut respectively. The lower border of each
somite breaks down to form fat-body. In so doing the coelomic
cavity disappears, and, minute as it always was, becomes continuous
with those spaces arising by separation of the germ layers from each
other, viz. the haemocoele. This latter as in all Arthropods consti-
tutes the main body cavity (Fig. 316 E).
Metamorphosis. Insects, like all other arthropods, attain their
maximum size by undergoing a succession of moults or ecdyses. The
number of moults which an insect passes through is fairly constant
for the species, and the form assumed by the animal between any
two ecdyses is termed an instar. The animal's existence is thereby
made up of a succession of instars, the final one being the adult. In
the simplest and most generalized insects the several instars are very
similar to one another and only differ from their appropriate adults
in the absence of wings and the incomplete development of the re-
productive system. Where the adult is primitively wingless, as in
silver fish and springtails (Fig. 323), the change from young to adult
is so slight as to be ignored, and metamorphosis, involving only a
development of the reproductive system, is conveniently regarded
as being absent. The insect orders falling in this category are grouped
under the heading Ametabola.
In winged insects, however, the winged adult is in sharp contrast
to the wingless young stage. Such forms are said to undergo a meta-
morphosis (Fig. 341). The degree of metamorphosis varies consider-
ably, irrespective of wings, in winged insects according as the young
stages resemble their adults or not. A growth stage of a cockroach,
for instance, possesses the general appearance of the adult. On the
other hand the young stage of a housefly is a grub and has no re-
semblance to the final instar with its wings, elaborate body form and
mouth parts (Fig. 349).
Metabolous insects, those passing through a distinct metamor-
phosis, are therefore further divided into two subclasses, (i) the
Heterometabola, e.g. the cockroach, and (ii) the Holometabola, e.g.
the fly. A classification of insects based on degree of metamorphosis
is therefore possible and such a basis for classification is used in all
modern systems.
The orders composing the Heterometabola are the Orthoptera,
Dermaptera, Hemiptera, Isoptera, Embioptera, Psocoptera, Ano-
plura, Thysanoptera, Plecoptera, Ephemeroptera, Odonata, Mallo-
phaga, the last three orders being sometimes classed as Hemimetabola
owing to the young stages being aquatic and distinguished from the
adults by the possession of features adapting them to life in water.
The young stages of all the Heterometabola, however, strongly re-
INSECTA
455
semble their adults in body form, type of mouth parts, and the
possession of compound eyes, and are known as nymphs (Fig. 317).
Fig. 317. Metamorphosis of a capsid bug {Plesiocaris vagicollis). After
Petherbridge and Hussain. 1-4, nymphal instars; 5, imago.
The orders composing the Holometabola are the Neuroptera,
Mecoptera, Trichoptera, Lepidoptera, Coleoptera, Strepsiptera,
456 THE INVERTEBRATA
Hymenoptera, Diptera, and Aphaniptera. The young stages of these
are known as larvae and differ from their aduUs in body form, mouth
parts, and the absence of compound eyes. So great is the difference
between the larva and the aduh that an instar known as the pupa
has been specialized to bridge the gulf (Fig. 341). This stage, one of
apparent rest, is actually one of great physiological and developmental
activity, and it is here that many larval tissues, e.g. the muscles and
the alimentary canal, are broken down by phagocytic or other action
and the new adult tissue is built up from many growth centres,
generally known as imaginal discs. A less obvious prepupal instar is
also present, enabling the change from larva to pupa to be effected.
It may reasonably be assumed that metamorphosis of the Holo-
metabola has arisen through larval and adult specialization going on
concurrently but in opposite directions, and it is not surprising to
find among the orders composing this group, as for instance in many
Coleoptera, larvae which are rather nymph-like in that they are well
chitinized and possess well-developed legs, and mouth parts re-
sembling those of the adults (Fig. 318 A).
The forms of larvae vary considerably and indicate to a great extent
the degree of metamorphosis passed through. A campodeiform larva
(Fig. 3 18 A) is one strongly resembling certain members of the
ametabolous Thysanura and possesses well-developed legs, biting
mouth parts, antennae and cerci, e.g. many Coleoptera. An eruci-
form larva (Fig. 318B) is fleshy and thin-skinned, its legs are often
in the form of supporting struts rather than organs of active loco-
motion, and there are no cerci. Further, prolegs are often found on
the abdomen, e.g. caterpillars of Lepidoptera and sawflies (Fig. 344).
A grub (Fig. 318 C) is an apodous larva which in other respects re-
sembles the cruciform type, e.g. certain Diptera, Coleoptera and
Hymenoptera.
Pupal modifications are also found; thus the exarate type, cha-
racteristic of the Hymenoptera, Mecoptera, Neuroptera, is that in
which the cases, in which the adult appendages lie, are free of any
attachment to the body (Fig. 341). In obtect pupae (Fig. 338), wing
and leg cases are fused to the body wall, e.g. most Lepidoptera and
Diptera. In the most specialized Diptera the last larval skin is re-
tained as a barrel-shaped puparium over the pupa within. Such pro-
tected pupae are called coarctate (Fig. 349).
In the Heterometabola the development of adult form is a gradual
process and the appendages, including mouth parts, antennae and
legs, grow directly into those of the adult. Wings in such forms
develop gradually as external dorsolateral extensions of the meso- and
metathoracic body wall (Fig. 317). All the Heterometabola have
such a wing development and therefore the alternative name Ex-
opterygota is often given to the group.
INSECTA
457
Larvae of the Holometabola on the other hand possess, for the
most part, mouth parts having a form and mode of working different
from that of their aduks, their legs are reduced in size and complexity
or even absent, and they show no sign of external wing growth. It
is in the pupal stage that adult appendages appear for the first time
on the surface.
Though the many forms of larvae may be regarded as adaptive
modifications of a primitive type (for example the eruciform larva as
Fig. 318. Types of coleopterous larvae. A, Campodeiform larva of Ptero-
stichus, Caraiiidae (original). B, Eruciform larva of Melolontha, Scarabaeidae
(original). C, Legless larva oi Phyllobius urticae, Curculionidae. After Rymer
Roberts.
an adaptation to a sedentary life among abundant food) their origin
may be explained by reference to embryology. In the development
of insects a germ band lies ventrM to the yolk and this undergoes
development from before backwards progressively, into segments
which bear limbs. At an early stage (Fig. 3 19 A), the cephalo-thoracic
segments and appendages may be present while the abdomen is as
yet unsegmented. A little later (Fig. 319 B), the abdomen becomes
segmented and later still (Fig. 319 C), on these segments embryonic
458 THE INVERTEBRATA
legs occur. Finally these abdominal embryonic legs disappear and
the insect may then hatch with thoracic legs only (Fig. 319 E). These
Fig. 319. To illustrate the principles of Berlese's theory. A, B and C are
3-> 3*- and 4-day embryos oi Hylatoma berberidis in the Protopod oligomero,
Protopod polymero and Polypod stages of development. D is the Oligopod
stage of Melolontha. Corresponding with these are the early larval forms,
E, oi Platygaster (protopod), F and G of Figites anthomyiarum (protopod poly-
mero and polypod respectively) and H, of Sitaris (campodeiform or oligopod).
A, B, C and D, after Graber; E, after Kulagin; F and G, modified after
James; H, after Korschelt and Heider. A, antenna; A i, first abdominal leg;
Md. mandible; Mz, second maxilla; Mi +2, first and second maxillae ; T3,
metathoracic leg.
Stages are known as the Protopod oligomero (imperfectly segmented
abdomen), Protopod polymero (segmented abdomen), Polypod (with
INSECTA 459
abdominal legs), and Oligopod (thoracic legs only) stages respectively.
It is noteworthy that among larvae there are forms resembling these
different stages. Thus the first stage larva of Platygaster is in a Proto-
pod condition (Fig. 3 19 E). The first stage larva of Phaenoserphus is in
a Polypod state. The first two stages in the larval life history of the
Cynipid Figites resemble Protopod and Polypod embryonic stages
respectively (Fig. 319 F, G). The Campodeiform larvae of Carabid
beetles, of certain Trichoptera, and of the Neuroptera, resemble the
final Oligopod stage of embryonic development (Fig. 319 H). Because
of facts of this nature, it has been suggested by Berlese in a theory
which now carries his name, that the moment of eclosion from the
^gg> perhaps decided by the amount of yolk, is one of significance in
determining the form of the larva. Thus in the Holometabola, the
stage in which hatching occurs corresponds with one or other of the
embryonic phases alluded to. Some insects hatch as veritable em-
bryos, i.e. as protopod, others as polypod, or oligopod larvae. A
fourth larval form, the apodous grub of Diptera, of many Hymenop-
tera and of some Coleoptera may be derived by degeneration from
the preceding oligopod stage.
The theory further maintains that in Heterometabolous insects, the
above stages, with the exception of the Apodous, are always passed
through in the egg, and emergence from the egg in these insects
occurs as a nymph which has thus reached in embryonic life a higher
stage of differentiation than any larva.
The natural corollary of this theory is that certain if not all of the
nymphal stages of the Heterometabola correspond to the prepupal
and pupal instars of the Holometabola.
The development of adult appendages in the larva is only one of
the many aspects of metamorphosis. The wings which suddenly
appear in the pupa of the butterfly grow gradually through each of
the five larval instars, but instead of growing externally as in the
Heterometabola (Exopterygota) they arise as outgrowths from the
bottom of intuckings of the body wall. In other words an accom-
modating fold of the body wall forming a sac, opening at the surface
by a minute pore, hides the growing wing bud within it and this is
the main difference between endopterygote and exopterygote develop-
ment.
At pupation the sac carrying the wing disc or bud at its base be-
comes straightened out by contraction of its walls and the wing bud
is thereby brought to view. Similar limb buds are to be found for the
adult legs and mouth parts, which always grow in association with the
corresponding larval organs. Such buds are known collectively as
imaginal discs and their existence characterizes all endopterygote
insects (Fig. 320).
460
THE INVERTEBRATA
Fossil record. Though the insects form an undoubted natural group
— all its members being referable to some generalized form, possess-
ing among other things mouth parts similar to those of the cockroach,
ivr.-
Fig. 320. The internal development of a wing in the larva of the butterfly
Pieris rapae as seen in transverse sections. A, Instar i. B, Instar 2. C, In-
star 3. D and E, Instar 5. ch. chitin; hy. hypodermis; m./. middle lamella;
p.m. peripodal membrane ; tch. tracheoles within the veins ; tel. tracheole cells ;
tra. trachea; v. vein; wr. wing rudiment.
efficient for chewing solid food, an ii-segmented abdomen, a 3-seg-
mented thorax and a 6-segmented head, and two pairs of membranous
wings carrying parallel longitudinal veins with a reticulum of cross
veins between them — the orders are clearly defined. They are not
INSECTA 461
easily linked together by intermediate forms and the story of evolution
within the subphylum consists rather of disjointed sentences than
a continuous theme. The two divisions already mentioned, however,
the Exopterygota and Endopterygota, are natural groups which we
may for convenience call the "generalized" and the "specialized"
respectively. The former have for the most part biting mouth parts
(the Hemiptera forming an important exception), while the latter
have their mouth parts modified in many remarkable ways enabling
them to tap sources of food forbidden to the others, such as the in-
ternal fluids of plants and animals and the deeply hidden nectar of
modern flowering plants. Moreover, the life cycle in these two divi-
sions is very difl^erent, the exopterygote (hemimetabolous) insects
having a gradual metamorphosis with external wing growth and the
endopterygote (holometabolous) forms having a complex meta-
morphosis with internal wing growth and a pupal stage intercalated
in the life history to bridge the gulf between dissimilar larvae and
adults.
From a morphological study alone one is driven to the conclusion
that the insects with biting mouth parts and simple metamorphosis
are the most primitive — i.e. more nearly resembling the ancestral
forms than the Endopterygota. It is of great interest therefore to
find that the palaeontological record, though discontinuous, supports
the conclusions drawn from comparative anatomical investigations.
The first records of insects are to be found in rocks of the Devonian
period. Here they consist of remains which, though fragmentary,
suggest that wingless insects similar to our present-day Apterygota
abounded then. If they were as soft-bodied as those we know to-day
the poverty of the record can well be understood and it is fairly certain
that thysanuroid insects similar to the silver fish Lepisma existed,
throughout the Devonian age.
There is abundant evidence, however, that winged insects existed
in the Carboniferous period. There were insects with prominent
meso- and metathoracic wings, with lateral wing-like expansions on
the prothorax, and shorter pleural processes on the abdomen. The
order Palaeodictyoptera in which such forms have been placed has
given rise to much speculation as to the origin of wings, one idea being
that wings are hyper-developments, on the appropriate segments, of
lateral processes which occurred on all segments behind the head.
In rocks of the same period have been found forms so similar to
our modern cockroaches that it is xlifficult not to place them in the
same family, mouth parts and wing venation being almost identical
in the ancient and modern types. Since such forms have existed from
the Carboniferous till to-day the student making his first essay into
the intricacies of entomology by dissecting the cockroach should keep
462 THE INVERTEBRATA
in mind that he is dealing with a very ancient type — a real aristocrat
among insect species !
In both the Ephemeroptera and Odonata we find many generalized
characters — in the mouth parts and the reticulate wing venation —
and these orders had their origin in the Permian, when forms assigned
to the two orders Protephemeroptera and Protodonata abounded.
Even as early as this, these orders had taken to a nymphal aquatic
existence. In the Permian rocks we find primitive dragonflies, stone-
flies and Hemiptera of which the Heteroptera with their character-
istic half-horny anterior wings appear to be the more recent develop-
ment.
Up to this stage none of the important endopterygote orders had
made their appearance.
The mandibulate Mecoptera form an order which is more general-
ized in structure among the Endopterygota, and Permian Mecoptera
from Kansas and New South Wales have been discovered which have
wing features that link up five of the important higher orders, the
Diptera, Trichoptera, Lepidoptera, Neuroptera and Mecoptera.
The highly specialized Hymenoptera make their first definite ap-
pearance in the sawfly form in the Jurassic, but remains from the
Permian have been described as Protohymenoptera. These had two
pairs of wings of equal size without coupling apparatus and a venation
of a generalized hymenopteran type.
Hymenoptera of the specialized kinds — the bees, wasps, ants — are
found first in the Tertiary period. In the same way we find nemato-
ceran Diptera (craneflies, etc.), in the Upper Lias, but not till the
Tertiary age do we find forms more nearly resembling our highly
organized blowflies, etc. Little can be said here of the Lepidoptera
except that they occur in the Tertiary period.
The Coleoptera are far older geologically than the Diptera, Lepi-
doptera and Hymenoptera. Already there were water beetles, weevils
and the leaf-eating chrysomelids in the Triassic, and recognizable
beetle remains, though scarce, have been extracted from the Upper
Permian. This is not without interest, since the Coleoptera as we
know them to-day possess, particularly in their mouth parts, a number
of features which place them in the generalized category.
Now if we consider the order of events hinted at in the above brief
account, it will be seen that though the ancestors of the Hymenoptera,
Diptera and Lepidoptera may have existed in the Permian, the latter
age with the Carboniferous was essentially one of insects with in-
complete metamorphosis and with no feeding mechanism for dealing
with flowering plants. It has been suggested that the change from
the perpetual warmth and humidity of the Carboniferous to the
transitional epoch of the Pernio- Carboniferous with its glacial con-
INSECTA 463
editions may have accounted for the onset of metamorphosis, the pupal
stage being evolved for the purpose of surviving cold periods while
in a quiescent state.
The most interesting fact, however, is that the main evolution of
our specialized bees, flies and butterflies coincided in point of time
with the evolution of the flowering plants to which by their manner
of feeding they are now on the whole so inseparably bound.
Class APTERYGOTA
Primitively wingless insects carrying on the abdomen a varying
number of paired appendages other than the external genitalia and
cerci. Metamorphosis slight or absent.
Order THYSANURA (Bristle-tails)
Biting mouth parts (Fig. 301); antennae many-jointed; compound
eyes present; abdomen of eleven segments, some or all of which bear
styliform appendages which probably represent the coxites of limbs
no longer present; anal cerci usually jointed, rarely (e.g. Japyx) in
the form of forceps.
Lepisma saccharina (Fig. 321), the common "silver fish" which
inhabits dwellings of man, and Machilis (Petrobius) maritimuSy found
above high-tide mark along the sea shore and estuaries, are common
examples. In Machilis (Figs. 322, 301) interesting features are pre-
sented by the well-developed superlinguae and the jointed mandibles
both of which are primitive characters. The superlinguae in Machilis
are paired structures attached to the hypopharynx and possess inner
and outer lobes and a palp-like process. This superficial resemblance
to maxillae gave considerable weight to the view that an additional
head segment was involved. Embryological evidence in support of
this conclusion is of a doubtful nature, and the most acceptable view
to take is that the superlinguae are processes attached to the hypo-
pharynx and perhaps homologous with the paragnaths of Crustacea.
Order COLLEMBOLA (Springtails)
Small wingless insects with biting mouth parts deeply withdrawn
into the head ; compound eyes absent ; 6-segmented abdomen which
often carries three pairs of highly modified appendages serving the
purposes of adhesion and jumping; a tracheal system is commonly
absent and there are no Malpighian tubules; metamorphosis absent.
Four-jointed antennae, ocelli and postantennal sensory organs are
characteristic features of the head.
464 THE INVERTEBRATA
There are no tarsi on the legs, claws being borne by the tibiae. The
I St abdominal segment carries a ventral tube which is moistened by
a glandular secretion from behind the labium poured down a ventral
Fig. 321. Lepisma saccharina. Fig.
From Imms, after Lubbock.
322. Machilis (Petrobius) maritimus.
From Imms, after Lubbock.
groove running along the middle of the thorax. This ventral tube, re-
garded as adhesive, is formed by the fusion of the embryonic append-
ages of this segment. On the ventral side of the 3rd segment, the
INSECTA 465
nearly complete fusion of a pair of appendages has resulted in the
formation of the hamula, which engages the furcula prior to leaping.
The latter is a forked structure representing a pair of limbs of the 4th
segment (Fig. 323). By contraction of the extensor muscles of the
furcula the latter is pulled down out of contact with the hamula and
the animal is propelled forwards into the air.
The absence of tracheae is a secondary feature due to the small size
of the animals rendering surface respiration sufficient for their mode
of life.
Collembola have a wide distribution. They are found along the
sea shore between tidemarks and submerged by each tide, e.g.
Anurida maritima. Common aquatic forms are denizens of fresh
waters, e.g. Podura aquatica. They have been reported to be so
1^^:
TTlO
Fig. 323. A, Axelsonia (Collembola). B, Hamula of Tomoceros showing
c. basal piece, and r. its rami. From Imms, after Carpenter and Folsom.
p. ocular pigmented area; v. ventral tube; h. hamula; m., d. and mc. caudal
furcula.
abundant in Arctic zones as almost to cover the snow, and in Europe
sometimes to be present in such large numbers that the progress of
railway trains is impeded owing to their having prevented the wheels
from gripping the rails.
Order PROTURA
Minute insects without wings, eyes or antennae ; with piercing mouth
parts deeply inserted in the head capsule ; with abdomen of twelve
segments, the first three of which bear papillae.
This is a small group of doubtful affinities. Its members are found
in decaying organic matter. The fact that on hatching the abdomen
is 9-segmented and that subsequent moults bring about the full
number of segments is regarded by some authorities as sufficient
ground for their inclusion in a class distinct from the Insecta. An
example is Acerentomum doderoi of Europe.
466 THE INVERTEBRATA
Class PTERYGOTA
Subclass EXOPTERYGOTA
Order ORTHOPTERA
Insects with generalized biting mouth parts; ligula 4-lobed, con-
sisting of inner paired glossae and outer paraglossae ; fore wings rather
narrow and somewhat hardened {tegmina) ; hind wings membranous ;
abdomen usually with jointed cerci of short or moderate length;
ovipositor generally present.
This order comprises terrestrial insects of large size which have
great powers of running and jumping. There are many flightless
species in all the families (cf. the female of Blatta orientalis).
The main structural features are exemplified by Periplaneta, the
cockroach. Its generalized character is shown by the character of
the mouth parts, the nervous system (six abdominal ganglia), the
circulatory system (heart with thirteen chambers, three in the thorax
and ten in the abdomen), and the obvious ten segments of the
abdomen.
The order is divided into two sub -orders, the Cursoria in which the
legs are of approximately equal size and the Saltatoria in which the
last pair of legs are modified for jumping (Fig. 324). The former
consists of the Blattidae (cockroaches) which are swift-running,
omnivorous forms, usually tropical in their distribution, the Mantidae
(praying insects), which are carnivorous, with modified raptorial fore
legs, and the Phasmidae (stick and leaf insects), some of which are
immensely elongated and attenuated to resemble sticks or twigs, while
others have laminar expansions of the skin that give the animal a re-
semblance to leaves, which is closer in the female than in the male.
The female phasmid at any rate is almost motionless, and the habit of
feigning death is commonly developed in the family. All these cha-
racters help to protect the female from observation in the plants
which it frequents and of which it eats voraciously.
In the Saltatoria there are the Acridiidae (locusts and short-horned
grasshoppers), the Locustidae (long-horned grasshoppers), and the
Gryllidae (crickets). The latter include a form remarkably adapted
for a burrowing life, namely Gryllotalpa. Nearly all these insects are
vegetarian, and in the Acridiidae, while the species commonly live
a solitary existence and are harmless, under certain conditions a form
with a gregarious and migratory instinct develops in countless num-
bers which invade cultivated districts causing incalculable harm.
Thus in the case of Locusta migratoria, when environmental con-
ditions favour an increase in numbers, there is an inevitable trend
towards the production of swarming migrants, i.e. the gregarious
INSECTA 4^7
phase. The subsequent decline in numbers leads to the production
of solitary non-migrants, i.e. the solitary phase. The two phases
differ morphologically, biologically and in distribution so distmctly
as to have been regarded as distinct species. Between them are
transient individuals which form a series with no fixed characters,
Fig. 324. Pachytylus migratorius. A grasshopper. Natural size.
From Shipley and MacBride.
merging imperceptibly into the gregarious phase at one end and into
the solitary phase at the other. .
A very characteristic feature of the Saltatoria is the possession ot
stridulating organs. In one type, exhibited by the cricket Gryllus, a
file on one of the anterior wings is rubbed over a scraper on the other.
468 THE INVERTEBRATA
In another type, e.g. Locusta, a row of pegs on the hind limb is rubbed
against a thickened area of the fore wing. Where there are organs for
producing sound, there are also organs for perceiving it. These are
tympana^ chitinous ear drums, which can be set in vibration and then
affect special auditory sense organs. The auditory organs may be
found on the front tibiae or on the ist abdominal segment. The
posterior wings of the Saltatoria possess many parallel longitudinal
veins with a network developed between these by numerous cross
veins. They fold in a fan-like manner, a line of folding, the anal suture,
separating a prominent posterior "anal" area of the wing from the
main part of the wing in front. Besides the fully winged forms, like
locusts, there are found in the several families all stages of wing re-
duction to mere scales as in certain stick insects, or to their complete
absence as in Grylloblatta.
Fig. 325. Forficula auricularia. Male. From Imms, after Chopard.
Order DERMAPTERA
Insects with biting mouth parts ; ligula two-lobed ; fore wings modi-
fied to form short leathery tegmina; cerci unjointed, always modified
into forceps; metamorphosis slight.
The common earwig, Forficula auricularia (Fig. 325) is the best
example of this small but definite order. It comprises a number of
small, usually nocturnal insects, omnivorous in diet. The female
deposits the eggs in the soil, remains with them until they hatch, and
even protects them afterwards. The hind wings have a characteristic
venation and fold up along transverse as well as longitudinal furrows.
INSECTA 469
thus contrasting with the Orthoptera. When unfolded, the wing
presents the appearance of a half wheel, the " spokes " radiating back-
wards from the anterior border, which is greatly strengthened. The
large posterior membranous portion corresponds to the anal wing
area of Orthoptera, that part corresponding to the anterior area of the
latter order having been greatly strengthened by the coalescence of
a number of longitudinal veins. The forceps are organs of defence
and offence. In Labidura they are used for seizing the small animals
on which this form lives.
Order ISOPTERA (Termites or White ants)
Social and polymorphic insects with biting mouth parts ; four-lobed
ligula; wings very similar, elongate and membranous, capable of
being broken off along a line at the base ; cerci short ; metamorphosis
slight.
The animals of this order abound everywhere in the tropics. Like
the true ants they have types of individuals (castes), specialized for
the purpose of reproduction, labour and defence (Fig. 326). The
termite community usually contains a dealated royal pair^ the king
and queen, who are the founders of the colony, and also supple-
mentary reproductory individuals of two kinds: {a) winged, which
normally serve for the formation of new colonies, and {b) wingless,
which become capable of reproduction if occasion demands. There is
usually a vast number of sterile wingless individuals belonging
to two castes, the workers and soldiers. The termite nests may be
merely series of burrows in trees, dry timber or in the ground, or they
may be huge mounds made of earth cemented together with the saliva
of the termites. Those living in the ground excavate the soil of the
tropics, turning it over and enriching it just as earthworms do in
temperate regions.
Their food consists chiefly of wood and other vegetable matter and
many species are extremely harmful, e.g. Neotermes, which damages
structural timbers, and Calotermes militarise which bores into and
does much harm to tea plants in Ceylon.
The winged sexual forms in several colonies usually swarm at the
same time so enabling intercrossing between members of different
colonies to take place, and of the countless numbers a few individuals
escape the attacks of birds and ojther animals and alight and cast
their wings.
A single pair forms a new colony first of all by making a small
burrow, the nuptial chamber. The first formed young are mostly
workers, and having themselves been tended to maturity by their
parents take over the nursing of the young. The queen becomes
470
THE INVERTEBRATA
enormous and helpless and is fed by the workers ; she lays eggs at an
incredible rate, up to a million eggs a year, it is said.
It is now known that digestion and growth of wood-eating termites
can only go on when there is a protozoan fauna of trichonymphids
(p. 67) and other flagellates in the hind gut. The fragments of wood
Fig. 326. Hamitermes silvestri Hill. Tropical Australia. After Tillyard.
A, Neoteinic queen. B, Winged male. C, Worker. D, Soldier. E, Nymph.
are ingested by the Protozoa and converted into sugars, being largely
stored up in the form of glycogen. The flagellates seem to form the
main food source to the termites, the wood having been already
digested within them.
Termites may forage by night for plant food and the genus Termes
also cultivates in its nest fundus gardens . The fungus which grows on
INSECTA 471
a bed of chewed vegetable matter serves as the food for the royal pair
and the nymphs.
The workers and soldiers differ from the sexual individuals, not
only in their sterility, but also in having more powerful mandibles.
In the soldiers the head can produce a protective secretion and the
mandibles are greatly specialized for defence (Fig. 326). Both these
castes consist of males and females, though secondary sexual cha-
racters are not very marked.
If, as is stated, slight caste differences are already apparent in the
newly hatched young, caste-formation cannot be a matter of nutrition.
Order PLECOPTERA (Stoneflies)
This is a small order of mandibulate insects with a heterometabolous
metamorphosis. Though in possession of two pairs of well-developed
wings, they are weak fliers and do not move far from their aquatic
breeding grounds. Prominent, elongate antennae and cerci are cha-
racteristic features, as also are the three-jointed tarsi. According to
some authorities the wing venation represents a primitive type. Much
variation in venation is, however, found in the order.
The nymphs are always aquatic, for the most part inhabiting swift-
flowing streams with stony beds. They possess the antennal and cereal
features of the adult and breathe by means of gill tufts in various
positions. In some cases gill vestiges are found on adults though these
are not aquatic. Like most aquatic insects, they have a wide distri-
bution, the most generalized families being found in southern, the
most specialized in northern, regions. Perla maxima is a common
species found in European streams.
Order EMBIOPTERA
Small insects with elongated and flattened bodies ; two pairs of similar
wings with reduced venation; females apterous; cerci two-jointed,
generally asymmetrical in male; metamorphosis absent in female,
slight in male.
These insects are widely distributed in the warmer parts of the
world. Many are gregarious, living in tunnels formed of silk produced
by tarsal glands, e.g. Embia major from the Himalayas.
Order PSOCOPTERA (Booklice)
Small insects, either winged or wingless; with biting mouth parts;
thoracic segments distinct ; wings with reduced venation from which
cross veins are largely absent; metamorphosis slight.
These insects are to be found on bark and leaves of trees and feed
on lichens and dry vegetable matter. The eggs are laid on the bark
472 THE INVERTEBRATA
or leaves and covered by a protecting sheath of silk by the female,
e.g. Peripsocus phaepterus.
Atropus pulsatoria, the booklouse, is found in damp dark rooms
and feeds on the paste of book bindings, wallpaper, etc.
Order O DON AT A (Dragonflies)
Predaceous insects with biting mouth parts; two similar pairs of
wings with characteristic reticulate venation; prominent eyes and
small antennae ; elongated abdomen with accessory male genitalia on
the 2nd and 3rd sterna; metamorphosis heterometabolous ; nymphs
aquatic, possessing a modified labium known as the mask.
The members of this order are large insects, and in the Carboni-
ferous period genera existed which had a wing expanse of two feet.
They are strong and rapid fliers, catching their food in the form of
small insects, on the wing. The forwardly directed legs play an im-
portant part in catching the prey and holding it w^hile it is masticated
by the mouth parts.
The thorax has a peculiar obliquity of form, the pleural sclerites
being directed downwards and forwards at each side with the result
that the leg bases are carried forwards towards the mouth and the
wing bases backwards.
The wings (Fig. 327) have a complex venation of a reticular nature,
characteristic features being a stigma or chitinous thickening of the
wing membrane near the apex, a nodus or prominent cross vein at
right angles to the first two longitudinal veins, and a complex of
veins near the wing base known as the triangle, Fig. 327. There is no
coupling apparatus. All the mouth appendages are strongly toothed,
maxillae and labium assisting the mandibles more efficiently in
mastication than in most insects with biting mouth parts.
Though the male pore is on segment 9 of the abdomen, the copu-
latory apparatus is found in the sternal region of segments 2 and 3.
Before copulation, spermatozoa are transferred to this apparatus.
The male then grasps the female in the region of the prothorax by
means of his posterior abdominal claspers. While in flight in this
tandem position the female turns her abdomen down and forwards
and receives sperm from the accessory copulatory apparatus of the
male. Dragonfly eggs are laid in water or on water weeds. The
nymphs breathe by means of tracheal gills and are of two kinds:
(i) those with external gills in the positions of cerci anales and caudal
fWaments—Zygoptera, (ii) those with gills on the walls of the rectum
— Anisoptera. In the latter case water is pumped in and out through
the anus, and this action may be made use of in locomotion — the
sudden expulsion of water causing a rapid forward movement on the
Fig. 327. The emergence of the dragonfly Aeschna cyanea. After Latter.
474 THE INVERTEBRATA
part of the nymph. The nymphs are, however, on the whole slow-
moving creatures, lurking well camouflaged among water weeds
while in wait for their prey. The main diff^erence between the mouth
parts of the nymph and imago concerns the labium. In the adult this
has normal proportions, but in the nymph the mentum and sub-
mentum are elongated and capable of being shot out rapidly from the
folded resting position, so impaling the prey, e.g. a tadpole, on the
labial hooks.
Order HEMIPTERA or RHYNCHOTA (Bugs)
Mouth parts for piercing and sucking ; palps absent ; labium forming
an incomplete jointed tube which receives dorsally two pairs of
slender stylets (maxillae and mandibles) ; wings usually two pairs, the
anterior harder than the posterior; metamorphosis gradual.
The existence of this large order of insects has largely been de-
pendent on the store of easily obtainable food which exists in the sap
of flowering plants and the mouth parts form an efficient mechanism
for obtaining this. There are, however, families like the Reduviidae
and Cimicidae (bed bugs) and the various water bugs (e.g. Nepa^
water scorpion, and Notonecta, back-swimmer) which feed on animal
juices. On either count they are of immense economic importance,
not only for the damage which the loss of sap and blood causes to the
host organism, but also because they open the way for bacterial in-
fection and carry the agent of such diseases as "mosaic disease"
among cultivated plants and trypanosomiasis among mammals.
The antennae are usually short. The labium projects from the head
as a rostrum which is jointed, and dorsally grooved to carry the
stylets (Fig. 328). At its base the groove does not exist but the lab rum
roofs over an enclosed space. The stylets are modified mandibles and
maxillae which are withdrawn at their base into divergent pockets in
the head, but converge and interlock as they pass into the space
between the labrum and labium and into the groove of the latter, in
which they fit tightly; where the inner pair of stylets (the maxillae)
meet together there are left two narrow channels, of which the dorsal
serves for the inward passage of the food juices and the ventral for the
outward flow of the saliva (Fig. 328). At rest the rostrum is bent
beneath the body, and when the insect feeds it is extended forward
and the stylets projected to penetrate the host tissues (Fig. 328). In
some plant-feeding species the stylets are immensely long and very
slender and it is difficult to explain the mechanism by which they
are forced into the tissues as far as the vascular bundles, but the
mechanical insertion of the stylets is greatly assisted by a solvent
action of the saliva which appears to loosen the plant cells from one
another and to allow the stylets to pass between. In Aphis rumicis
INSECTA
475
the phloem cells of the plant are eventually pierced and their contents
sucked out. The pumping action is performed by the muscles of the
pharynx.
This order comprises a large number of families which in the
following scheme of classification are arranged in two suborders, the
Heteroptera and the Homoptera. The Heteroptera have wings which
are horny distally, but membranous apically (Fig. 331). The pro-
Fig. 328. Mouth parts of the Hemiptera. A, Sagittal section through head
of Graphosoma italiciim. After Weber. B and C, Diagrams of mouth parts
and adjacent region of the head. C is a transverse section across B at the
point X X. After Imms. Ibyn. labium; Ibr. labrum; nid. mandible;
mx. maxilla; ph. pharynx; ph.p. muscles of pharyngeal pump; sty. stylets.
boscis is terminal and free. In the Homoptera the fore wings have a
homogeneous texture and are ^often membranous. The head is
ventrally flexed so as to bring the base of the proboscis into contact
with the anterior coxae (Fig. 329).
There are two tribes of insects within the Heteroptera, {a) those
which are aquatic and whose antennae are obscure, the Cryptocerata^
and {b) mostly terrestrial forms with conspicuous antennae, the Gym-
476 THE INVERTEBRATA
nocerata. The former are noteworthy for their numerous adaptations
to aquatic life. They commonly lay their eggs in the tissues of sub-
merged plants. Many, e.g. the water boatman, Corixa, and back-
swimmer, Notonecta, have powerful legs fringed with hairs which, by
the simultaneous movement as members of pairs, propel the animals
through the water as oars do a boat. They breathe air at the surface
film, making use either of a terminal abdominal tube (Nepa) or of
unwettable hairs between which air is trapped to enable the animal
to breathe during its periods of complete immersion (Notonecta).
Among the Gymnocerata may be mentioned the bed bug, Cimex,
an ectoparasitic insect, with vestigial wings, flattened body and
prominent claws. It inhabits human dwellings, and its retiring habits
coupled with its power to fast for long periods make it a difficult
creature to eradicate when once it is established. The shield bugs
{Pentatomidae) are phytophagous. The mesothoracic tergum is greatly
enlarged to extend at least as far over the abdomen as the junction
between the horny and membranous parts of the wing when these are
at rest. The red bugs (Pyrrhocoridae) are also phytophagous. Certain
species, e.g. of Dysdercus, are known as "stainers" from their habit
of feeding on cotton-bolls into which they inject a micro-organism
responsible for the appearance of a red stain on the fibre. The
Capsidae are almost exclusively phytophagous, some of their members
being very serious pests of our English orchard trees and shrubs.
Plesiocoris, until recent times restricted to such trees as willow, now
attacks black currant bushes, apple trees, etc. An exception to this
phytophagous habit is found in Cyrrtohinus mundulus which sucks the
eggs of the sugar cane hopper, Saccharicida, so eflPectively controlling
this pest in Hawaii. In the family Reduviidae are many forms which
transmit trypanosomiasis, in the tropics, e.g. Rhodius prolixus.
The extent to which the head flexure has brought the point of
emergence of the rostrum into the thoraco-sternal region forms the
basis for the separation of the Homoptera into two tribes. The least
modified from the heteropterous condition in this respect are the
Auchenorhyncha (Fig. 329 B). These are all active animals and though
the rostrum is close to the thorax it clearly arises from the head. Here
belong the cicadas, frog hoppers, tree hoppers, and leaf hoppers.
Cicada septendecim is an example with a life cycle which may last
as long as seventeen years. Eggs are deposited in holes in the twigs
of trees. From here the newly hatched nymphs fall to the ground, into
which they burrow to feed on the tree roots. A stage resembling the
pupa of holometabolous insects is passed through before final
emergence.
The second tribe is known as the Steniorhyncha. In these forms the
rostrum appears to arise from between the fore limbs. The antennae
INSECTA
477
are well developed and do not possess a terminal spine (arista) — a
feature characteristic of the first series. To this group belong the
Fig. 329. Lateral views of proboscides of Rhynchota to illustrate the difference
between the Heteropterous condition (A), and the Homopterous condition (B).
A, Deraecoris fasciolns, modified after Knight. B, Zammara tympimnm
(Cicadidae). m. niesothorax; iv. wing.
scale insects, Coccidae. Females of these are wingless, often scale-
like, and devoid of legs. The winged males have atrophied mouth
47^ THE INVERTEBRATA
parts and the second pair of wings are reduced to short clawed
structures. Well-known examples are Pseudococcus the mealy-bug,
Tachardia lacca the lac-insect of commerce and Aspidiotus perniciosus
the San Jose scale-insect of citrus trees.
Fig- 330- Types of Rhynchota. A, Macrotrista angularis (Homoptera, Cica-
didae). B, Aphis rumicis (apterous viviparous female). C, Winged viviparous
female of same. B and C, after Davidson.
The plantlice (Aphididae), Fig. 330, notable for their wide dis-
tribution and for their prolific reproduction, have transparent wings
and a two-jointed tarsus, that of the Coccidae being one-jointed.
Wax-secreting cornicles are borne dorsally in the abdomen.
In the last family the reproductive phenomena are of immense
scientific importance. A comparatively simple life cycle is that of
Aphis rumicis. The winter is passed on the spindle tree Euonymus as
eggs laid in the autumn after the fertilization of females. In spring
INSECTA
479
these eggs hatch, giving wingless parthenogenetic females which
produce young viviparously. A variable number of these partheno-
genetic generations is passed through in the summer and then winged
parthenogenetic females occur which migrate to another host (the
bean or other plants), and there reproduce, giving rise to generations
of parthenogenetic females which eventually produce winged females
which migrate back again to the primary host, the spindle tree Euony-
Fig. 331. External anatomy of Leptocoris trivittatus with wings spread on
one side. After Essig. an. antenna; he. hemielytron.
mus. This generation gives rise to oviparous females which copulate
with winged males, migrants from the secondary host plant.
In other forms, such as Phylloxera vastatrix, the notorious pest of
vineyards, the life history is immensely complicated and involves
migrations between root and stem of the host plant. The reproductive
capacity of these insects is most remarkable and is fortunately offset
by the number of enemies which they possess.
480 THE INVERTEBRATA
The following summary will assist in the understanding of the life
cycle of Aphis rumicis :
Fertilized eggs laid in autumn
I
Viviparous parthenogenetic females I Euonymus
Winged migrant parthenogenetic females
^ . . .
Wingless parthenogenetic viviparous females
I !- Vicia {aha
Winged viviparous females (autumn) J
I
Winged males x Wingless oviparous females]
\ \ Euonymus
Eggs laid in autumn J
The cyclical reproductive phenomena in aphides as just described
raise important problems relating to the intrinsic differences between
sexual and parthenogenetic individuals, and to the environmental
conditions governing the occurrence of these phases in any life
cycle.
Fertilized eggs produce only strictly parthenogenetic females. These
multiply by diploid parthenogenesis, i.e. the eggs retain the full
complement of chromosomes and are not capable of fertilization.
Eventually come individuals capable of bearing sexual forms, sexu-
parae. The sexual forms arising from these produce germ cells
undergoing normal reduction and which are therefore haploid.
It follows then that fertilisation will restore diploid partheno-
genesis. Sexual differences are indicated in the chromosomes; the
female of Aphis saliceti possessing six, of which two are sex chromo-
somes; the male only five, one only being a sex chromosome. Sexual
reproduction leads however only to the production of parthenogenetic
females and not to males and females in equal numbers, as might
be expected. This appears to be due to the fact that in the maturation
of sperms, those with only two chromosomes die. Fertilization there-
fore is always between sperms and ova each with three chromosomes,
of which in each case two are normal chromosomes {autosomes) and
one is a sex chromosome X. The capacity of females with six
chromosomes to produce male offspring with only five is due to the
fact that in the maturation of male-producing parthenogenetic eggs,
reduction in the number of chromosomes only affects the sex {X)
chromosomes, one remaining in the tgg^ the other going to the polar
body. In this way a parthenogenetic female with six chromosomes,
i.e. 4 + XJ\r, gives rise to males with only five, i.e. 4 + X.
INSECTA 481
A complete analysis of the environmental conditions governing the
onset of sexual phases after a period of parthenogenetic reproduction
is yet to be made. Food, temperature and light seem to be important,
and of these a reduction of the last mentioned factor seems to be
associated with the production of sexual winged individuals.
Though the order contains insects for the most part harmful to
man and his property, a few are useful in that they yield the dyestuffs
Kermes (females of Kermes ilicis) and Cochineal {Dactylopius coccus) ^
and the resin stick-lac {Tachardia lacca). The usually harmful plant-
sucking habit is being put to good use in Queensland where the
coccid bug, Dactylopius tomentosus, is employed against the prickly
pear cactus with considerable success.
Order EPHEMEROPTERA (Mayflies)
Vestigial mouth parts reduced from the biting type; wings mem-
branous with a reticulate vena-
tion ; the hinder pair small ; caudal
filament and cerci very long (Fig.
332). The nymphs are aquatic and
an active winged stage known as
the subimago occurs before the
last moult yields the adult.
The eggs are laid in water,
either scattered over the surface
or attached to stones, etc., by the
female, which enters the water
for the purpose.
The nymphs at first possess no
gills but subsequent instars bear
on the abdomen movable tracheal
gills (Fig. 333), which may be
branched or lamellate, exposed or
protected in a branchial chamber.
The body form^varies with the
habits. Thus inhabitants of fast-
flowing streams have flattened
bodies with legs provided with
strong clinging claws, e.g. Ecdyo-
nurus. Those which live in clear
still water have a stream-lined
form for rapid movement, e.g.
Chloeofi, while burrowing types
have fossorial legs, e.g. Ephemera,
and are often provided with protective gill opercula, e.g. Caenis. The
Fig. 332. Ephemera vidgata.
From Imms.
482 THE INVERTEBRATA
mouth parts are of the biting type, and the two-jointed mandibles and
well-developed superlinguae are features of importance. The nymphs
are essentially herbivorous. Nymphal life is usually of long duration :
Fig- 333 Nymphal instars of Heptagenia. After Imms. A, Third instar.
B, Seventh instar. C, Eighth instar. a, a\ b and c, gills belonging to these
instars respectively; w. wing rudiment.
as many as twenty-three instars may occur. In order to emerge, the
fully fed nymph creeps out of the water on to a plant stem. A moult
gives rise to the winged subimago stage. This flies away and after a
period which varies, according to the species, from a few minutes to
about twenty-four hours, a final moult yields the adult which enjoys,
INSECTA 483
as the name of the order implies, a similarly short life. In the adult
the mouth parts are vestigial, no feeding is done, and the alimentary
canal, full of air, serves no longer for digestion.
Economically these insects are of importance in so far as they con-
stitute a proportion of the food of freshwater fishes, the adults being
caught by fish during their nuptial dance, and the nymphs being de-
voured by bottom-feeding fish.
Order MALLOPHAGA (Biting lice)
These insects are ectoparasites of birds (less frequently of mammals).
Their reduced eyes, flattened form and tarsal claws are features corre-
lated with this mode of life. Unlike the Anoplura they have no
piercing mechanism and devour with biting mouth parts small
particles of feathers, hair, or other cuticular matter.
The common hen louse, Menopon pallidum (Fig. 334), may be taken
as an example. The head is semicircular in form and articulates with
a prothorax which is freely movable on the rest of the body, a tagma
formed by the fusion of the meso- and metathorax with the abdomen.
The mouth is placed ventrally on the head and surrounded by biting
mandibles and less prominent ist and 2nd maxillae.
Eggs are laid separately on feathers or hairs and the life cycle is
completed in about a month — the young instars resembling the adult
in form and habit.
The various families of biting lice are strictly confined to particular
groups of birds, indicating that evolution of the parasites has pro-
ceeded concurrently with that of their bird hosts.
Order ANOPLURA (Sucking lice)
Ectoparasites of mammals, with mouth parts adapted for piercing the
skin and sucking the blood of their hosts. The eyes are ill-developed
or absent. The single-jointed tarsus carries a large curved claw ad-
mirably adapted for clinging to the host. The thoracic segments are
fused, and a flattened abdomen of nine segments possesses large
pleural areas allowing the body to swell on feeding.
The minute mouth parts are accommodated at their bases in a
stylet sac which is a diverticulum ventral to the pharynx. There are
two stylets of which the dorsal is a paired structure, the halves of which
maintain contact with each other Idistally to form a half-tube which
is completed by the ventral stylet. This also consists of two elements.
Between the dorsal and ventral stylets lies the salivary duct which
appears to be a modification of the hypopharynx. The stylet complex
can be sufficiently everted so as to make contact with the skin. Into
484 THE INVERTEBRATA
the wound is poured the salivary fluid, and the mouth funnel is
thrust in to enable the blood to be sucked up by the pharyngeal pump.
Embryological evidence tells us that the ist maxillae unite to form the
dorsal stylet, the ventral being formed by the labium. A pair of
mandibles also develops but these remain in a rudimentary and un-
chitinized condition.
Pediculus humanus, the body louse (Fig. 335), is associated with the
spread of many diseases, such as typhus and relapsing fever. The
disease known as trench fever, prevalent in all war areas during the
Great War, has also been shown to be transmitted by this insect.
Eggs are laid attached to hairs of the body or clothing, and the three
instars passed through before attainment of the mature state closely
resemble the adult.
msth
mtfli.
Fig. 334-
Fig. 335.
Fig. 334. Hen louse, Menopon pallidum. Dorsal view, showing biting
mandibles by transparency, an. antenna; md. mandible; mxp. maxillary
pd\ip',pth. prothorax; msth. mesothorax; mtth. metathorax.
Fig. 335. Body louse, Pediculus humanus. After Imms.
The louse has been found to lay about ten eggs daily, depositing
in all about three hundred. Temperature plays a big part in con-
trolling the development of these animals. Under average conditions,
the life cycle is completed in about three or four weeks.
INSECTA
485
Order THYSANOPTERA (Thrips)
Minute insects with asymmetrical piercing mouth parts; prothorax
large and free ; tarsus two- or three-jointed with terminal protrusible
vesicle; two pairs of similar wings, provided with a fringe of pro-
minent long hairs, veins few or absent; metamorphosis slight,
including an incipient pupal instar.
These insects are for the most part plant feeders, a few being
carnivorous. They are regarded as serious pests in that they rob the
plant of sap. They also often cause malformations and in some cases
inhibit the development of fruit. Parthenogenesis is of frequent
occurrence. In the case of the pea thrips, Kakothrips robustus, the
eggs are inserted in the stamen sheath of the flower and the nymphs
emerging feed on the young fruit, inhibiting its growth. Later they
feed on the soft tissues of pea pods, causing scar-like markings. The
nymphs leave the plant and bury themselves deeply in the ground,
where they remain till the following
spring, when they pupate. Common
thrips of importance are Taeniothrips
inconsequens of pears and Anapho-
thrips striatus of grasses and cereals.
Subclass ENDOPTERYGOTA
Order NEUROPTERA
(Alder flies, lacewings, antlions)
Rather soft-bodied insects with biting
mouth parts; two similar pairs of
membranous wings held in a roof-
like manner over the body when at
rest. The wings have a primitive type
of venation, a distinguishing feature
being the ladder-like arrangement of
veins along the anterior border. The
abdomen is without cerci. The larvae
are invariably carnivorous — campo-
deiform, with biting or suctorial
mouth parts. Aquatic larvae usually
possess abdominal gills.
The alder fly, Sialis, may be taken
as an example with an aquatic larva.
In June and July the adults fly rather
sluggishly in the neighbourhood of
water. They lay eggs in clusters on
grass blades and leaves overhanging water, and the larvae on hatching
Fig. 336. Larva of Sialis lutaria.
From Inims, after Lestage.
486 THE INVERTEBRATA
fall into the water. In this larva (Fig. 336), more than in any other, the
paired segmented tracheal gills on the abdomen show a great resem-
blance to paired limbs. Pupation takes place in the moist earth near the
water's edge. The larva of Sialis differs from those of the majority of
Neuroptera in that its mouth parts are of the biting type, whereas in
antlion larvae and the larvae of lacewings, etc., the mouth parts are
adapted for piercing the skin and sucking the juices of animal prey.
For this purpose, the points of the mandibles and maxillae are used
for piercing, and the mandibles, being grooved, form with the closely
fitting maxilla a tube up which the fluid is drawn. The carnivorous
habit of neuropterous larvae plays an important part in insect pest
control, for example, larvae of lace wing flies feed largely on aphides.
Order MECOPTERA (Scorpion flies)
A small order of insects distinguished by their vertically directed and
elongated head capsule carrying the biting mouth parts at its end;
two pairs of similar wings with a simple venation in which a number
of cross veins divide the whole area into a number of nearly equal
rhomboidal cells.
The male genitalia are prominent and the terminal segments of the
abdomen carry them in a dorsally curved position in the manner of
the scorpion's tail. The cruciform larvae are caterpillar-like and may
possess prolegs on all segments of the abdomen. This feature, together
with the presence of a large number of ocelli on the head (there may
be twenty or more on each side), readily distinguishes these larvae
from those of the Lepidoptera.
Panorpa communis, the common English scorpion fly, lays eggs in
crevices in the soil and the larvae hatching from these feed on decay-
ing organic matter. Pupation occurs in an earthen cell and the life
cycle is an annual one. Much information is still wanting on the life
histories of the members of this order.
Order TRICHOPTERA (Caddis flies)
Medium-sized insects with bodies and wings well clothed with hairs ;
mandibles vestigial or absent; maxillary and labial palps well de-
veloped; two pairs of membranous wings, with few cross veins and
held in a roof-like manner when at rest.
The cruciform larvae are aquatic and usually live in cases formed
of such material as particles of wood, sand, small shells, etc. A pair
of hooked prolegs on the last abdominal segment which assists in
adhering to the case is a characteristic feature.
The eggs are laid in or near water and the larvae quickly cover
INSECTA 487
themselves with some foreign substance (Fig. 337), building a form
of tube from the wide end of which the head projects. Respiration
is effected by tracheal gills generally found on the abdomen, water
currents being passed through the tubular case by the undulatory
movements of the body. The larvae may be herbivorous or carnivor-
ous. Pupation usually takes place within the case after the openings
to the case have been closed by silk. The pupa is provided with large
mandibles by means of which it releases itself before the emergence
of the adult. The free pupa swims to the water's edge by means of its
mesothoracic legs and shortly afterwards the adult emerges. Common
caddis flies are Phryganea, Limnophilus and Rhyacophila.
Fig. 337. A, B, C, D, Cases of Trichoptera. A, Hydroptila maclachlani.
B, Odontocerum. C, Phryganea. D, Hydropsyche, pupal case. E, Halesus.
guttatipennis. After Imms.
Order LEPIDOPTERA (Butterflies and moths)
Mouth parts of the imago usually represented only by a sucking pro-
boscis formed by the maxillae; two pairs of membranous wings,
clothed with flattened scales, as also is the body; metamorphosis
complete; larvae cruciform with masticating mouth parts, with three
pairs of legs on the thorax and often five pairs of prolegs on the
abdomen; pupae obtect, either enclosed in a cocoon or an earthen
case, or free.
The imagines live on the nectar of flowers, and to absorb this a
highly specialized proboscis has been formed from the greatly elon-
gated galeae of the maxillae, each being grooved along its inner face
and locked to its neighbour (Fig. 338). The laciniae are atrophied
and the maxillary palp is usually much reduced. The mandibles are
488 THE INVERTEBRATA
nearly always absent and the labium is represented by a transverse
plate and a pair of three-jointed palps.
Each half of the proboscis is a tube in itself into which passes blood
from the head, and also a trachea and a nerve. Across the cavity of
this tube there pass a number of diagonal muscles, the contraction of
which causes the whole organ to roll up into its characteristic position
beneath the head and thorax (Fig. 339). How the proboscis is ex-
tended is not fully understood; in all probability, blood pressure
plays an important part.
The length of the proboscis in many cases corresponds to the depth
of the corolla of the flower which the species frequents, and in the
Sphingidae (hawkmoths) is greater than that of the body. Sometimes
Fig* 338. A, Tryphaetia pronuba, with venation and frenulum (Jr.); ^ con-
dition on right. Original. B, Obtect pupa of Platyhedra gossypiella. After
Metcalf and Flint.
the organ is reduced or absent and the animal does not then feed
in the adult state at all.
The beginnings of the proboscis can be traced in primitive forms.
In the Micropterygidae there are biting mandibles and maxillae of
the type usually found in insects which masticate their food: in
Micropteryx there is no proboscis, the animal feeding on pollen; in
Eriocrania the mandibles are non-dentate, the laciniae are lost and
the galeae form a short proboscis.
The characteristic feature of the wings is the clothing of scales
(Fig. 338). These latter are formed by enlarged hypodermal cells,
and their main function appears to be the presentation of colour due
either to striation of the surface causing interference colours, or in
lesser degree to the pigment they contain (like the uric acid of the
Pieridae). There also occur "scent scales" which may have a sexual
INSECTA 489
significance. Several methods of wing coupling have been developed
independently in the order. In addition to the type already referred
to on p. 429 and consisting oi frenulum and retinaculum^ there is the
further method met with in the ghost moths in which a jugal lobe
from the fore wing engages the anterior border of the hind wing. In
other forms there is neither frenulum nor jugum and the wings are
Fig- 339- Head and proboscis of a moth. A, Front view. B, Side view. After
Metcalf and Flint. C, Transverse section of proboscis. After Eltringham.
dp. clypeus; drn. diagonal muscles; e. eye; ep. epipharynx; gal. galea;
Ibr. labrum; l.h. locking hooks; Ip. labial palp; md. mandible; 7nxp. ma.xillary
palp; n. nerve; tra. trachea.
coupled by a considerable overlap of the two wings of a side, e.g. the
butterflies {Papilionina).
In the females of certain Lepidoptera the wings are totally lost and
the animals are confined to the food plant on which they spend their
larval life. The male is attracted to the female, under these circum-
stances, by scent.
Lepidopterous larvae (Fig. 344 A-C) have three thoracic and ten
abdominal segments with nine pairs of spiracles situated on the pro-
49^ THE INVERTEBRATA
thorax and first eight abdominal segments. The mandibles are typi-
cally strong and dentate; the maxillae are stumpy and consist of a
cardo, stipes and single maxillary lobe with a two- or three-jointed
palp : the labium has a large mentum, a prementum bearing a median
spinneret and small two-jointed palps.
The thorax bears three pairs of legs, and the abdomen five pairs of
prolegs on segments 3-6 and 10. Such prolegs are different from the
typical insect limbs, being conical and retractile with hooks on the
apex (Fig. 344 C). In many families there are less than five pairs of
prolegs, and in Micropteryx there are eight pairs.
These larvae feed almost exclusively on flowering plants (excep-
tions being the Lycaenid caterpillars which are carnivorous, feeding
on aphides or entering ants' nests and devouring the larvae). Their
digestive enzymes are modified for dealing with plant tissues.
The pupa, which is disclosed after the last larval moult, is usually
protected by a cocoon previously prepared by the larva. In the case
of Tortrix moths the cocoon is largely composed of leaves drawn to-
gether by silk strands. In others, e.g. the silkworm moth, Bombyx
mori, it is composed of silk and from it the silk of commerce is pre-
pared. Agglutinated wood particles form a hard cocoon in the puss
moth, Dicranura. In Pieris, the pupa is naked and attached to the
substratum by the hooked caudal extremity, the cremaster^ and by a
delicate girdle of silk about its middle. In the most primitive forms
(e.g. Micropterygidae) the pupae are free, their segments are free to
move and the appendages are not fused to the body. Obtect pupae,
in which only few segments are movable and the appendages are
fused to the sides of the body, are most common, e.g. Platyhedra
(Fig. 338 B). Free or incompletely free pupae often emerge from the
cocoon before the emergence of the adult.
Lepidoptera are almost invariably harmful in the larval stage, few
plants being free from their attacks, and some of the world's most
serious insect pests, such as the cotton bollworm, Platyhedra gossy-
piella^ and the gypsy moth, Porthetria dispar, are included in this
order.
The order is divided into two suborders. In suborder I, Homo-
neura, the fore and hind wings have venations which are almost
identical. To this primitive feature may be added that of the included
family Micropterygidae whose mouth parts are mandibulate and the
structure of whose maxillae and labium are easily comparable with
those of the cockroach.
The ghost moths or swifts (Heptalidae) are also included in this
suborder. These nocturnal insects have vestigial mouth parts and
short antennae. Their jugate type of wing coupling has already been
described. In certain species^t.g. Hepialus humuli, the female searches
INSECTA 491
for the male prior to mating. The larvae live in the ground and are
white and hairless.
The second suborder, Heteroneura, is more specialized in that the
venation of the hind wing has undergone reduction and so presents
a venational form very different from that of the Homoneura. Here
are included the vast majority of moths and all butterflies. Since the
families are distinguished largely on venational characters no attempt
will be made to deal with them in a classificatory scheme.
Among the numerous families of this suborder may be mentioned
the Tineid moths — small species still retaining maxillary palpi and
possessing narrow fringed wings, with a frenular bristle on the hind
wing for coupling purposes. Tinea hiselliella is one of the clothes
moths whose larvae can live on the keratin of woollen goods.
The goat moths (Cossidae) are large moths without maxillary palps
and with a frenular coupling apparatus. These are nocturnal, and lay
their eggs on trees. Their larvae tunnel in timber, e.g. Cossus.
Ephestia the flour moth and Plodia the meal moth are most im-
portant as pests of stored products, while Chilo is a form whose larva
bores into the shoots of the sugar cane in India. Galleria the wax moth
inhabits beehives in most parts of the world, having become arti-
ficially distributed. These belong to the family Pyralidae.
Hawk moths (Sphingidae) are large stoutly-built moths whose fore
wings are much larger than the hind ones. A further feature is the
obliquity of the outer margin of the wings. The proboscis is long and
the antennae, which are thick, end in a hooked tip. The larvae have
ten prolegs and usually bear an upturned spine or process on the back
of the last segment.
Of slender build are the geometer moths {Geometridae). They are
weak in flight and a coupling mechanism is not always present on the
wings. Some species, e.g. Cheimatobia the winter moth, are wingless
as females. The family gets its name from the fact that in most of
the larvae, prolegs are borne by segments six and ten of the abdomen
only. Such larvae, in consequence, walk by looping the body, bringing
the hind segments near to the thoracic and so appear to be measuring
distances along the surface walked upon.
The owl moths or Noctuidae are the most dominant family of the
order. They usually fly at night and to this fact is related their sombre
colouring which assimilates the insects to their surroundings when
resting during the day. The larvae, are almost hairless, and in such
forms as pupate in the ground the pupa is naked. Tryphaena pronuba
(Fig. 338) is a common species whose larvae devour roots. The
larvae of nearly related species known as cut worms and army worms
rank among the worst insect pests of North America.
In the above mentioned forms, collectively known as moths, the
492 THE INVERTEBRATA
antennae taper to a point and the frenular coupling apparatus is
common. The remainder forming the superfamily Papilionina may
be grouped for convenience as butterflies, whose antennae are clubbed
and on whose wings there does not occur a frenulum.
Here are found the Whites, e.g. Pieris, the larvae of many of which
are restricted to a cruciferous diet, and the Blues and Coppers in
which the metallic colouring on the wings and the larvae tapering
towards both extremities are distinguishing features. There are also
the Swallow-tails, e.g. Papilio, in which the hind wings are commonly
extended into tail-like prolongations. Finally may be mentioned the
skippers, so-called because of their erratic darting flight quite distinct
from the sustained flights of other forms.
Order COLEOPTERA (Beetles)
Biting mouth parts ; fore wdngs modified to form horny elytra which
meet along the mid-dorsal line; hind wings membranous — folded
beneath the elytra — often reduced or absent; prothorax large and
mobile; mesothorax much reduced; metamorphosis complete, larvae
(see p. 457) campodeiform or cruciform or, more rarely, apodous.
In the larvae the head is well developed (Fig. 318) and the mouth
parts are of the biting type, resembling those of the adults. The most
primitive larvae are those of the campodeiform type (found for instance
among the Cicindelidae (tiger beetles), Carahidae (ground beetles) and
the Staphylinidae (rove beetles)). They are very active in movement
and often predaceous, with well-developed antennae and mouth
parts, and chitinized exoskeleton. In the eruciform type (Fig. 318 B),
found among plant-eating forms like the lamellicorn beetles, the legs
are shorter, and the animal much less active in its search for food,
the body bulkier and cylindrical. Finally there is the apodous type
which is found in the Curculionidae (the weevils), in which not only
are the thoracic legs lost but the antennae and mouth parts are reduced
(Fig. 318 C). The apodous and eruciform larvae usually live inside
the soft tissues of plants or beneath the soil attached to roots.
The relation which these larval forms bear to one another is indicated
by the larval stages passed through in the life history of the oil beetle,
Meloe, the larvae of which are parasitic on solitary bees of the genus
Andrena. The first instar is known as the triiingulin. This is an active
campodeiform larva which attaches itself to its host after searching
actively for it. The second instar which is enclosed with an abundance
of honey in the cell of the bee is intermediate in form between the
campodeiform and the eruciform types, legs being present, but very
small. The third stage is a legless maggot. From this series it may be
inferred that the form of larva in Coleoptera is related to the ease or
difficulty with which food is obtained.
INSECTA
493
In such a large order of insects it is to be expected that all manner
of habit and food will be found. Beetles occur in large numbers in
water, soil, and plant tissues. Circumscribed environments like dung,
rotting vegetation, wood and fungi are never without prominent
coleopteran associations. A large number, such as many coccinellids
(lady birds), carabids, e.g. Carabus violaceus, and staphylinids, e.g.
Ocypus olens, are carnivorous and to this extent useful insects. On the
other hand, among the phytophagous forms are to be found some of the
most serious agricultural pests, the boll weevil, Anthonomus grandis,
causing so much damage to the cotton crop in America that it has
Fig. 340. External anatomy of Calosoma semilaeve, with left elytron and wing
extended. After Essig. aw. antenna; e/. elytron; />. palp; 5^. spiracles.
been seriously proposed to cease growing cotton for a period of time
in order to eradicate this pest. A large number cause considerable
damage to timber, probably the most notable being Xestobiiim riifo-
villosum, the death-watch beetle, destructive to structural timber.
The order falls into tw^o suborders, the Adephaga and the Polyphaga.
The Adephaga are distinguished by filiform antennae, a five-
jointed tarsus and a larva of the campodeiform type, with a tarsus
bearing two claws.
To this group belong those families including the large water
beetle Dytiscus, the ground beetles Carabus and Calosoma (Fig. 340),
the tiger beetle Cicindela, and the aquatic whirligig beetles Gyrinus.
494
THE INVERTEBRATA
The second suborder, the Polyphaga, includes a large number of
families grouped into several superfamilies the members of which
show much variation. There is a tendency towards reduction in the
number of tarsal joints from five to three, and though some forms
possess filiform antennae, clavate (clubbed), geniculate (elbowed),
Fig. 341. The hornet, Vespa crabro. A, Larva. B, Pupa. C, Adult S'
Fig. 342. Three types of Coleoptera. A, Ocypus olens (Staphilinidae). B,
Scarahaeus Thomsoni (Scarabaeidae). C, Corymbites cupreus (Elateridae).
and lamellate (segments extended to form a "book" of closely
arranged leaves or lamellae) antennae occur, as in the Coccinellidae,
Curculionidae and Scarabeeidae respectively. Larvae vary from the
INSECTA 495
campodeiform to the legless grub, but where a tarsus is present it
invariably carries only one claw.
The Staphylinidae range from carnivorous to phytophagous forms,
and, as adults, are characterized by the short elytra which leave the
abdomen exposed. The larvae are campodeiform, closely resembling
those of ground beetles, e.g. Ocypus (Fig. 342 A). Meloidae or oil
beetles also have short elytra but these being wider at the base than
is the prothorax are readily distinguished from members of the
staphilinid group. The interesting changes undergone by their larvae
during metamorphosis have already been mentioned.
The Chrysomelidae or leaf beetles are exclusively phytophagous.
Their bodies are rounded and smooth, and are often high-coloured
with a metallic lustre. Antennae of these beetles are filiform and
relatively short (e.g. Phyllotreta^ the flea beetle).
Weevils belonging to the family Curculionidae are easily dis-
tinguished by their greatly extended head, forming a rostrum at the
end of which mouth parts are borne. Anthonomus grandis the cotton
boll weevil of America, and Ceuthorrhynchus the turnip gall weevil, are
typical examples. The larvae are apodous.
The chafer beetles {Scarabceidae), Fig. 342 B, have lamellate
antennae. Their legs are often fossorial and bear four-jointed tarsi.
Characteristic of these is the fat bodied cruciform larva, almost
incapable of movement, and which feeds on roots, e.g. Melolontha
(Fig. 318). Aphodius is a dung beetle whose larva develops in the
faecal matter of farm animals.
The family Coccinellidae (ladybirds) is of extreme importance, its
members being carnivorous in young and adult stages, aphids and
scale insects figuring very largely in their diet. The beetle is smooth
and rounded, with head concealed beneath the prothorax. The four-
jointed tarsus appears to possess only three joints, owing to the
small concealed third joint, e.g. Coccinella of Europe. Novius
cardinalis is a classical example of a predatory insect being used in the
biological control of the scale-insect, Icerya purchasi, of citrus trees.
Order HYMENOPTERA (Bees, wasps, ants, sawflies, etc.)
Mouth parts adapted primarily for biting and often secondarily for
sucking as well; two pairs of membranous wings coupled together
by booklets fitting into a groove, hind wings smaller; ist segment of
the abdomen fused to the thorax, and a constriction behind this
segment commonly found; an ovipositor always present, modified
for piercing, sawing, or stinging; metamorphosis holometabolous ;
larvae generally legless, more rarely cruciform, with thoracic and
abdominal legs; pupae exarate, protected generally by a cocoon.
49^ THE INVERTEBRATA
This order is remarkable for the great specialization of structure
exhibited by its members ; for the varying degrees to which social life
has developed, and for the highly evolved condition which parasitism
has reached.
Specialization of structure is evidenced in the mouth parts of the
an.
Fig- 343 • Head and extended mouth parts of the honey bee, Apis mellifica.
After Cheshire, an. antenna; gal. galea; gs. glossa; Ibr. labrum; Ip. labial
palp; md. mandible; mxp. maxillary palp; oc. ocellus; pg. paraglossa.
Hymenoptera. The biting mouth parts of the phytophagous and
carnivorous sawflies closely resemble those of the cockroach. In the
wasps, e.g. Vespa, which are predaceous, the mouth parts are adapted
for licking as well as for biting. The maxillary laciniae are reduced
but the galeae are enlarged into broad setose membranous lobes which
absorb juices. A correspondingly large bilobed glossa occurs on the
labium.
INSECTA
497
The next important line of evolution is that concerned with the
development of a mechanism for obtaining juices from deeply placed
nectaries of flowers. For this purpose, e.g. in Apis^ the honey bee
(Fig. 343), a complicated tubular proboscis is formed. The glossae
of the labium have become fused and elongated, the paraglossae
remaining small. The labial palps enclose the fused glossae (median
^%{{+^C
Fig. 344. Caterpillar of Lepidoptera, A, B, C, and of Hymenoptera, D, E.
A, Larva of Tryphaena pronuba. B, Its head capsule. C, An abdominal leg.
D, Larva of apple sawfly, Hoplocampa testndinea. E, Head capsule of latter.
an. antenna; dp. clypeus; /r. frons; lb. labium; Ibr. labrum; ind. mandible;
mx. maxilla; oc. ocellus; v. vertex.
lobes of the labium), they being concave on their inner surfaces.
Outside these the large hood-like galeae of the maxillae form an
additional enclosing jacket.
The glossa is grooved along its dorsal surface and fluid passes up
this by capillarity, assisted by movements of the proboscis. It is
finally pumped up by pharyngeal action, the labial palps and maxil-
498 THE INVERTEBRATA
lary glossae undoubtedly playing an important part in maintaining
a complete tube. The mandibles are now no longer biting organs but
tools used for manipulating material such as pollen and wax. Such
a feeding mechanism is the climax in an evolutionary process which
has involved in succession the fusion of the glossa lobes, as in the
sawtlies, the lengthening of the basal joints of the labium and maxilla
as in ColleteSy and finally the elongation of the glossa, e.g. Apis and
Bombus.
The highly complex social life found in the bees, ants and wasps,
in which caste development is a feature of prime importance, is fore-
shadowed in the interesting behaviour of solitary wasps and bees.
The supply of food to the larva by progressive feeding , instead of mass
provisioning, appears to enable the parent to become acquainted with
its offspring, and this establishment of family life may be regarded
as the forerunner of the complex social state of higher forms .^ A
second important feature in the development of social life has been
the phenomenon of trophallaxis . Among wasps, for instance, the
worker taking food to a grub receives in turn a drop of saliva from
the grub. This is eagerly looked for by the workers, and it is suggested
that it is the mutual exchange of food between young and adult which
engenders in the adult an interest in the welfare of the colony. A
third important feature in social development has been the exploita-
tion of a particular form of food material which can be obtained in
large quantities, e.g. pollen and honey.
The phenomenon of parasitism (Fig. 345) is highly developed in
the Hymenoptera; Ichneumons, Chalcids and Proctotrypids being
almost entirely parasitic. Almost all orders of insects are affected by
the activities of these very important insects, Qgg, larval, pupa, and
adult stages being parasitized.
From the foregoing it will be seen that some of the most important
insects are included in this order. The sawflies are important as
agricultural pests. Flower- visiting bees are of great value in the
pollination of flowers. Carnivorous wasps do good by devouring
other insect pests such as aphides, while to a large extent the parasitic
Hymenoptera are useful in checking the depredations of phyto-
phagous insects.
Two main types of larvae are found in this order, the legged larva
of the sawflies (Fig. 344 D) and the legless form of bees, wasps and
ants (Fig. 341 A). The sawfly larva has a superficial resemblance to
the lepidopterous caterpillar, but is distinguished by its single pair of
^ In English species of the wasp Odynerus the egg is laid in a cell and
sufficient caterpillars stored to serve as food for the whole of the larval life
(mass provisioning). Certain African species of this genus supply their growing
larvae from day to day with fresh caterpillars (progressive feeding).
INSECTA
499
Fig. 345. A and B, Exarate pupae of Phaenoserphus viator. C, Pupae of
same projecting from empty skin of host, the ground beetle larva, Pterostichns.
After Eastham. s. spiracle; t. invagination to form tentorium.
500 THE INVERTEBRATA
ocelli and the absence of crotchets or spines on the abdominal legs.
The prolegs of the abdomen occur on different segments in the two
forms under consideration as reference to Fig. 344 clearly shows.
The order, falls naturally into two suborders, the Symphyta and the
Apocrita.
Suborder I, the Symphyta, includes those species with the most
generalised form, both as adults and as larvae. None of them show
the highly specialised habits and instincts which characterize most
of the remaining suborder, and with few exceptions they are phyto-
phagous. The first abdominal segment is not perfectly fused to the
metathorax nor is the fusion accompanied by the constricted waist so
characteristic of the remaining Hymenoptera (Fig. 346 D). The ovi-
positor is used in oviposition as a saw or drill for piercing plant tissues.
The trochanter is two-jointed. Larvae are cruciform (Fig. 344 D) and
in addition to thoracic legs, certain of the abdominal segments often
carry prolegs devoid of distal spines or crotchets.
To this group belong the wood-wasps, the ovipositors of which are
used as a drill for perforating growing timber, in which the eggs are
laid. The six-legged, strong-headed larva bores through the wood (in
the case of Sirex gigas, this stage lasts as long as two years), pupation
occurring near the surface of the affected timber, from which the
adult bites its way out. The sawflies (Fig. 346 D), with saw-like ovi-
positors, are most important as agricultural pests, and are distinguished
from the wood-wasps by their softer bodies, their smaller size, and
by the presence of two apical spurs on the anterior tibiae, e.g.
Nematus ribesii the gooseberry sawfly.
The second suborder, the Apocrita, includes all the remaining
Hymenoptera. The second abdominal segment is invariably con-
stricted to form a narrow waist or petiole, the first segment being
firmly amalgamated with the thorax (Fig. 346). Larvae are apodous
when full grown.
Ichneumon flies (Fig. 346 A) are distinguished by their slender
curved antennae, and by the stigma on the wing. The ovipositor is
long and issues far forwards beneath the abdomen. The larvae of
Lepidoptera and of sawflies are their commonest hosts. Rhyssa
parasitizes the larvae of Sirex.
Cynipid flies have similarly slender antennae, but by the absence
of the stigma on the wing, and by their reduced venation are easily
distinguished from the foregoing. Many of these are plant gall-
formers, e.g. Neuroterus responsible for oak galls, and Rhodites for
the pin-cushion galls of roses. Others, e.g. Eucoila, are parasitic on
fly larvae.
Chalcid wasps (Fig. 346 B) also have a venation of the wing which is
so reduced as to present no closed cells. The antennae are geniculate
INSECTA
501
or elbowed. Though most of these small wasps are parasites, e.g. of
lepidopterous and dipterous larvae, and of homopterous nymphs, a
Fig, 346. Types of Hymenoptera. A, Cryptus ohscuriis (Ichneumonoidea) ;
B, Bruchophagus funebris (Chalcididae), after Howard. C, Polistes aurifer
(Vespidae), after Essig. D, Pamphilus sp. (Tenthredinidae), original.
E, Monomorium minimum (Formicidae), after Essig.
few feed as larvae on plant tissues such as Harmolita which produces
galls on grasses.
5^2 THE INVERTEBRATA
In ichneumons, chalcids and cynipids the ovipositor issues far for-
wards beneath the abdomen, and these insects differ in this feature from
the Proctotrypidae in which the ovipositor is terminal. Dipterous larvae
are often parasitized by these insects, as are also the eggs of Orthop-
tera and Hemiptera. Many hyper-parasites, i.e. parasites of other
parasites, occur in this family. Phaenoserphus is parasitic on carabid
beetle larvae (Fig. 345), and Inostemma is an egg-parasite of dipterous
gall midges.
Whereas parasitism is a character, largely though not wholly,
common to the foregoing families, the ants, wasps and bees next to
be considered show a tendency, in varying degrees, towards the
development of the social habit.
The ants (Formicotdea) are social, polymorphic insects in which
two segments are involved in the formation of the abdominal petiole.
Further, this petiole is always characterized by the possession of one
or two nodes (Fig. 346 E). The females are endowed with a well-
developed sting, the modified ovipositor. Polymorphism reaches its
highest degree of complexity in this group, as many as twenty-nine
morphologically different castes having been recognized. Some of
these are pathological phases due to infection by parasites, e.g.
Nematode worms or other Hymenoptera. In such colonies as produce
winged forms of both sexes, mating takes place during a nuptial flight
in which several colonies in one neighbourhood indulge at the same
time. This ensures intercrossing between individuals from different
colonies. The females then cast off their wings and start colonies in
the ground, each one for itself. The workers are sterile females, whose
power to lay eggs in certain circumstances may return. For instance,
when a colony loses a queen several workers may, under the influence
of suitable diet, take her place. In addition to the environmental
complexity which a social existence involves, the lives of ants are
further complicated by association with other organisms. Some, e.g.
certain myrmecine ants, have adopted an agricultural habit, living on
fungi which they specially cultivate. Others gather seeds from which
they destroy the radicle to prevent germination, special chambers or
granaries in the nest being constructed for their storage. The pastoral
habit characterizes others, a symbiotic relation being set up with such
insects, e.g. Aphides, as exude fluids which are palatable to the ants.
In addition to associations of this kind there are numerous others of
an indifferent or little understood nature, but which may range from
the symbiotic to the parasitic. Finally may be mentioned the slave-
makers; Formica sanguinea, for instance, captures from the colonies
of F. fusca, pupae which on emergence serve as slaves in the colony
which has adopted them.
The wasps of the super-family Vespoidea are both social and
INSECTA 503
solitary in habit. In these, the abdominal petiole is smooth (Fig. 346 C)
and, in species with a worker caste, this is always winged. The pro-
thoracic tergum extends back towards the wing base. Among solitary
species may be mentioned Odynerus which stores with caterpillars
its nest in which its larvae are developing. Pompilid wasps are ex-
clusively predatory on spiders. Certain forms have adopted the
"Cuckoo" habit, laying their eggs in the nests prepared and pro-
visioned by other species. Thus the ruby wasp Chrysis usurps the
nest of Odynerus. Mutilla behaves similarly towards many solitary
bees and wasps and has been bred out from the puparia of the tsetse
fly. Social wasps, e.g. Vespa, live in nests commonly constructed of
paper obtained in the form of wood pulp by these insect architects.
The larvae living in closely arranged cells on horizontal combs are
fed on insect food gathered by the workers. In early summer, our
common social wasps are useful in the control of such insects as plant
lice, etc. Later in the season, however, their liking for sweet fruits
may make them a nuisance both in the garden and in the home. In
autumn the colony perishes, fertilized females being the only sur-
vivors. Vespa germanica and Vespa vulgaris are common English
wasps. Vespa crabro is the Hornet (Fig. 341).
Closely resembling these are those wasps belonging to the super-
family Sphecoidea, the distinctive feature of which is the possession
of a prothoracic tergum which does not extend back as far as the wing
bases. These are all solitary predaceous forms, which sting their prey
and so paralyse it before placing it in the larval cells which have been
previously prepared, e.g. Sphex. A tendency towards the social habit
is exhibited by Bembex which leaves its larval cells open and so can
provision its young from day to day on small flies.
The super-family Apoidea includes the social and solitary bees.
Distinctive of bees are the dilated hind tarsi and the plumose hairs
of the head and body to which pollen adheres. Inner metatarsal
spines of the posterior legs comb the body hairs free of pollen, this
being then transferred to the outer upturned spines (pollen basket)
of the hind tibia of the opposite side. These legs are further adapted
by possession of special spines for the manipulation of wax plates
when being removed from the abdomen. The median glossa is also
characteristic and in certain solitary forms, e.g. Anthophora and all
the social bees, e.g. Apis and Bombus, is greatly elongated along with
the parts other than the mandibles^ for gathering nectar from deep-
seated nectaries of flowers. Larvae are fed exclusively on pollen,
nectar and salivary fluids. Megachile, the leaf cutter, is a solitary bee
which makes cells of neatly cut leaf fragments. Each cell containing
an egg is stored with honey and pollen. Such cells are commonly
made in the walls of houses, the mortar being removed for this purpose.
504 THE INVERTEBRATA
Andrena constructs burrows in the ground and, though solitary, is
usually found in groups of individuals occupying a common terrain
which may include a "village " of several hundred nests. Nomada has
adopted the ** cuckoo" habit.
Bombus enjoys a social existence similar to that of Vespa in that
only impregnated females survive the winter.
The colony of the Honey bee Apis mellifica has more permanence,
only the males dying off in the autumn to leave the rest of the colony
to hibernate. The nest is of wax, an exudation from abdominal glands
of the worker (sterile female), and a material known as propolis of
vegetable origin serves to fasten parts of the nest together and to
render the whole weatherproof.
The workers are graded according to age into nurses^ who see to the
welfare of the larvae by incorporating salivary juices with their food,
ventilators who by wing-fanning set up currents in the nest or hive
to reduce the temperature and to evaporate the honey, scavengers or
cleaners^ 2ind foragers who collect pollen and nectar. The changes from
nursery work to house work and to field work are necessitated by
changes in glandular capacity as age increases. Though the density
of the population of the colony determines to some extent when a
queen with a number of workers will depart from the hive as a
swarm, it appears that this event is also dependent on the relative
proportions of the above age-groups among the working caste .^
Order DIPTERA (Flies)
Insects with a single pair of functional wings, the hind pair repre-
sented by stumps (halteres) (Fig. 347); mouth parts suctorial and
sometimes piercing or biting, usually elongated to form a proboscis ;
prothorax and metathorax small and fused with the large mesothorax ;
metamorphosis complete, larvae cruciform and always apodous, the
head frequently being reduced and retracted; pupa either free or
enclosed in the hardened larval skin (puparium).
This is a very large and highly specialized order of insects. The
imagines are mostly diurnal species, feeding on the nectar of flowers,
but a number are predaceous, living on other insects (e.g. the robber
flies), while some, e.g.Tachinids, are parasites. A further development
which takes place in several families is the acquisition of blood-
^ In Bees, Wasps and Ants, haploid parthenogenesis results in the pro-
duction of males. A fertilized (diploid) female has control over the fertiliza-
tion of eggs which she lays. If an egg is fertilized by sperm from the
spermatheca a female (diploid) offspring develops. If not, a male offspring
(haploid) develops. Whether the young female produced in the former case
becomes a worker (sterile) or a queen (capable of fertilization) depends on
nutrition. Contrast this with diploid parthenogenesis in Aphids (p. 480).
INSECTA
505
Fig- 347- Anopheles maculipennis, ?. After Nuttall and Shipley.
506 THE INVERTEBRATA
sucking habits. The representatives of this oecological class are of
great importance because they harbour and transmit pathogenic
organisms, causing such diseases as malaria, sleeping sickness,
elephantiasis, yellow fever and some cattle fevers.
The several kinds of mouth parts which have been developed in
the Diptera have departed widely from the primitive biting type.
There is always a proboscis formed principally by the elongated
labium, ending in a pair of lobes, the labella. This labium serves as
a support and guide to the remaining mouth parts which are enclosed
within it (Fig. 348).
The most complete system is to be found in the gadflies, e.g.
Tahanus and Chrysops. Within the groove of the labium are to be found
a pair of mandibles and a pair of maxillae, sword-like piercing
organs by means of which the wound through the skin of mammals
is made. Into the wound so formed is inserted a tube composed of
the eptpharynx, an elongated chitinization of the roof of the mouth to
which the lab rum is fused, and the hypopharynx, a corresponding
elongation of the mouth floor. The blood passes into this tube, being
drawn up by the pharyngeal pump within the head. The hypopharynx
carries a duct down which the salivary fluid is passed. Besides this,
the proboscis of a gadfly can be used for taking up fluids exposed at
surfaces. Such exposed fluid is drawn into small channels, the pseudo-
tracheae, which converge to a central point on the underside of the
labellar lobes. There it meets the distal end of the epi-hypopharyngeal
tube, up which it passes.
The mouth parts of the female mosquito (Fig. 348 A) in principle
diff"er from those described above only in the absence of a pseudo-
tracheal membrane on the labellar lobes and the more slender and
elongated labium. Mandibles are absent in the males, maxillae being
represented only by palps in this sex. The housefly Musca (Fig. 348 D,
E, F) has lost all piercing mechanism, mandibles being absent, maxillae
only being represented by the palps, and the mouth parts consist of a
folding labium with highly developed pseudotracheal membrane on
the labellar lobes and prominent epi-hypopharyngeal tube. Musca
feeds largely on fluid matter but in the presence of soluble solid food,
e.g. sugar, solution is effected by regurgitating alimentary fluid on it.
By means of small chitinous teeth situated round the point to which
the pseudotracheae converge, surfaces of solids can be scraped so
enabling enzymes in the regurgitated fluids to act rapidly.
The tsetse fly, Glossina (Fig. 348 B), also possesses no mandibles
and only the palps of the maxillae. It does, however, feed on mam-
malian blood after piercing the skin. In this form the whole labium
is rigidly chitinized; the labellar lobes, from which all traces of
pseudotracheae have disappeared, are small and provided with
*■ mx.
Fig. 348. Types of mouth parts of the Diptera. A, Culex pipiens, ?. B,
Glossina suhmorsitans. C, Transverse section through proboscis of Culex.
D, Transverse section through proboscis of a muscid fly. E, Proboscis of a
muscid fly, extended and with left labellar lobe removed. F, Proboscis of a
muscid fly, half folded, an. antenna ; e. eye ;/.c. food channel ; hyp. hypopharynx ;
Ibm. labium; Ibl. labellum; Ibr.ep. labrum-epipharynx ; md. mandible; mx.
maxilla; mxp. maxillary palp; ph. pharynx; ph.p. pharyngeal pump; pstra.
pseudotracheae ; sd. salivary duct. A-D, after Patton and Cragg; E, F,
original.
508 THE INVERTEBRATA
chitinous teeth which make the wound. Thus a second kind of blood
sucking mechanism has been evolved from a form like Musca, which
only possessed the faculty of sucking fluid from surfaces.
The larvae of Diptera are among the most specialized in the Insect
Kingdom. Legs have been entirely lost, and the head and spiracular
system have undergone varying degrees of reduction. Thus the most
generalized larvae are at the same time eucephalous, i.e. with complete
head capsule, and peripneustic ^ i.e. with lateral spiracles on the ab-
n
■\
f
t""
■'"1
■"■ f
V--
B
Pig. 2^f). Early stages of the Diptera. A, Larva of Musca domestica. Ace-
phalous amphipneustic type. B, Empty puparium of Musca domestica.
C, Pupa of Musca domestica removed from puparium. D, Larva of Bibio sp.
Eucephalous peripneustic type. A, B, and C after Hewitt; D, original.
sp. spiracle.
domen, e.g. Bihio (Fig. 349 D). In the most specialized forms, on
the other hand, we find the acephalous larva whose head capsule is
entirely wanting. Such acephalous larvae may be either amphi-
pneustic^ with only prothoracic and posterior abdominal spiracles, or
metapneustic, where only two spiracles are retained at the posterior
end of the body. The first instar larva of Musca is metapneustic,
subsequent instars being amphipneustic (Fig. 349 A).
The eucephalous larva develops into an exarate pupa from which
the adult emerges by a longitudinal slit on the thorax. The pupa
INSECTA 509
resulting from the acephalous larva, on the other hand, is coarctate,
the last larval skin being retained as a protective puparium, tracheal
connections maintaining contact between the pupa within and the
larval skin outside it. Final emergence of the fly in this case clearly
involves two processes, {a) the liberation of the fly from its pupal skin,
and (b) its further liberation from the puparium. The latter splits
transversely (Fig. 349 B), the top being thrust away by an eversible
head sac, xhtptilinum^ which such flies possess. The features of meta-
morphosis just described are characteristic of many flies and by de-
fining one of the suborders constitute an important basis of modern
classifications (Fig. 349 C).
The suborder Orthorrhapha includes all those flies which are liber-
ated by means of a longitudinal split in the mid-dorsal line of the
pupal case. Such flies possess no ptilinum. Many of these, the Ne-
matocera, have slender antennae and usually pendulous maxillary
palpi. Their larvae are eucephalous with horizontally biting mandibles
and their pupae are free. To this series belong the Crane-flies (Fig.
3 50 A) , the larvae of which often damage cereal crops by devouring their
roots. The Culicidae (Fig. 347) are the gnats and mosquitoes, the
piercing proboscis of which has already been described. They are
further distinguished by their wings which are fringed with scales.
Both larvae and pupae are aquatic, the former being metapneustic,
the latter propneustic (with anterior spiracles only). With the blood-
sucking habit of these flies has evolved an association with certain
organisms which when transmitted to man cause disease. Anopheles
is concerned with the transmission of malaria. Stegomyia transmits
the causative organism of Yellow Fever while Culexfatigans, a widely
distributed tropical form, is a carrier of the thread-worm Filaria
bancrofti^ the cause of elephantiasis.
Nearly related to these are the Chironomidae (midges), the mouth
parts of many of which are not adapted for piercing and sucking. A
few of these, however, do suck blood, e.g. the midges of the genus
Forcipomyia, whose larvae breed, some in water, others behind the
bark of trees.
The Cecidomyidae (Fig. 350 C) are the gall-midges distinguished by
their beaded antennae adorned with whorls of setae. The larvae of a
few of these are parasitic. Some are predaceous, but others, forming
a large majority, are phytophagous, forming galls in plant tissues, e.g.
of grasses. Contarinia pyrivora is the pear midge, the larvae pf which
develop in the flowers of the pear so as to abort fruit production.
Miastor lives behind tree bark in the larval state and as mentioned
above is noteworthy for the phenomenon of paedogenetic partheno-
genesis.
Another family of blood-sucking flies, known as the Simuliidaef
510
THE INVERTEBRATA
consists of small flies with a hump-backed appearance and with broad
wings. The spindle-shaped larvae live in running water and are cha-
racterized by the possession of prothoracic prolegs and an anal pad
provided with setae by means of which they cling to rocks etc. in the
Fig. 350. Types of Diptera. Tipula ochracea (Tipulidae). B, Chrysops
caecutiens (Tabanidae). C, Contarinia nasturtii (Cecidomyidae). D, Hypo-
derma bovis (Cyclorrhapha, Oestridae). C, from Smith after Taylor ; D, from
Smith after Theobald.
rapidly flowing water of their environment. Still included in the
suborder Orthorrhapha are the flies with short antennae, the Brachy-
cera. Though included in this scheme with the Orthorrhapha their
venational characters indicate a close relation with the Cyclorrhapha.
In general, the basal joints of the antennae are larger than the terminal
INSECTA 511
ones, these being reduced in number as compared with the nemato-
ceran condition. The maxillary palpi are porrect (not pendulous).
Their larvae are hemi-cephalous with vertically biting mandibles and
the pupae are free and spiny. From this vast assemblage of flies we
may mention the Tabanidae or gad-flies (Fig. 350 B). These flies, to the
mouth parts of which reference has already been made, are of stout
build and possess large eyes occupying a large part of the head surface.
Though a few transmit disease organisms (Chrysops dimidiata, the
vector of the worm. Filaria loa is responsible for Calabar swelling in the
natives of West Africa), the majority are harmful chiefly through the
annoyance which their bites occasion. Tabanid eggs are usually laid
on the leaves of plants overhanging water and their carnivorous
larvae are either aquatic or ground dwellers. The robber flies (Asilidae)
are large bristly flies with a backwardly directed proboscis. They feed
on all kinds of insects which they paralyse with their salivary fluid, and
their legs, being strong and provided with powerful claws, are well
adapted for grasping the prey.
The Empidae, flies of more slender build, exhibit similar habits.
Their larvae are terrestrial as are also those of the preceding family.
Suborder Cyclorrhapha. These flies emerge from a pupa which is
enclosed in the last larval skin or puparium and the commonly trans-
verse or circular split in the latter, for release of the adult, gives the
name to this suborder. It is therefore really a larval feature which
establishes the position of these flies in the classification.
The antennae are three-jointed, the last of which is greatly en-
larged, carrying a dorsal spine or arista. The maxillary palpi are one-
jointed and porrect. A crescentic suture on the head lies above and
encloses the bases of the antennae. This, known as the frontal suture,
is a narrow slit along the margins of which the wall of the head is
invaginated to form the ptilinal sac, the eversion of which enables the
adult to emerge from the puparium. The extent to which the frontal
suture is developed and the ptilinum persists, varies. The Syrphidae,
for instance, have usually no persistent ptilinum and the frontal suture
is not well-developed. All larvae have a vestigial head, and are either
amphi- or metapneustic.
The Syrphidae (hover-flies) form an important family of brightly
coloured flies, whose most obvious mark of distinction is the posses-
sion of a false longitudinal vein lying about the middle of the wing.
Their larvae are amphipneustic, leathery grubs, some of which
devour Aphidae (Syrphus), others hving in decaying material being
saprophagous {Eristalis), others again being phytophagous (Merodon,
the bulb-fly).
The remainder may be considered under the heading of muscid
flies. The frontal suture is prominent and the ptilinum persists. Many
512 THE INVERTEBRATA
families are included here, to some of which belong such serious
agricultural pests as the frit-fly of oats^ Oscinus frit^ and Chlorops
taeniopus the gout-fly of barley. In such cases, the larvae bore into
the growing shoot, or into the stem. Larger and better known are
the saprophagous house-fly Musca and the blow-fly Calliphora. The
larva of Hypoderma lineatum is parasitic in the bodies of cattle
causing "warbles" on the backs of affected animals, while Gastro-
philus equi, the bot-fly, is parasitic as a larva in the alimentary tract of
horses.
The Tachinidae are important as parasites, chiefly of larval
Lepidoptera. Thus Ptychomyia remota is responsible for the very
effective control of the Levuana moth, Levuana iridescens, of Fiji.
Blood-sucking muscids are important, e.g. Glossma, as the vector
of trypanosomiasis causing sleeping sickness of man and cattle disease
in Africa. The tsetse flies are pupiparous, larvae being nourished by
special glands opening into the genital tract. The larvae are deposited
as soon as fully grown and pupate immediately.
A number of members of this order present a greatly modified
structure resulting from an ectoparasitic habit. They are known as
the Pupipara, being similar in their viviparity to Glossina. The follow-
ing examples may be quoted. Hypobosca is a winged leathery fly with
body dorso-ventrally compressed, and is an ectoparasite of cattle.
Melophagus is a wingless species, similarly associated with sheep,
familiarly known as the sheep tick. Nycteribia is a wingless form
parasitic on bats.
Order APHANIPTERA (Fleas)
Wingless insects, ectoparasitic on warm-blooded animals; laterally
compressed with short antennae reposing in grooves; piercing and
sucking mouth parts, maxillary and labial palps present; coxae large;
tarsus five-jointed; larva legless; pupa exarate, enclosed in a cocoon.
These insects are perfectly adapted to an ectoparasitic existence
by their laterally compressed bodies, prominent tarsal claws, well-
developed legs suitable for running between the hairs of their host
and for jumping, and by their mouth parts (Fig. 351). They only
exhibit slight relationship to one other order, viz. Diptera, by their
metamorphic features and to a less degree by their mouth parts.
The mouth parts consist of a pair of long serrated mandibles, a
pair of short triangular maxillae with palps, and a reduced labium
carrying palps. There is a short hypopharynx and a larger labrum-
epipharynx reminiscent of the Diptera. The labial palps, held together,
serve to support the other parts, a function which is performed by the
labium in the Diptera. In piercing, the mandibles are most important
and the blood is drawn up a channel formed by the two mandibles
INSECTA
513
and the labrum-epipharynx. The thoracic segments are free and there
are never any signs of wings. Though the eggs are laid on the host
they soon fall off and are subsequently found in little-disturbed parts
of the haunts of the host. Thus in houses they come to lie in dusty
carpets and unswept corners of rooms. In a few days the larvae hatch
and feed on organic debris. The legless and eyeless larva possesses
a well-developed head and a body of thirteen segments. At the end
Fig. 351. The life history of the flea, Ctenocephalus cams. From Imms, after
Howard, a, egg; b, larva in cocoon; c, pupa; d, imago; e, larva of flea,
Ceratophyllus fascia tus ; f, antenna of imago.
of the third larval instar a cocoon is spun and the creature turns to
an exarate pupa from which the adult emerges, the whole life cycle
occupying about a month in the case of Pulex irritans.
Pulex irritans is the common flea of European dwellings, but by
far the most important economically is the oriental rat flea, Xeno-
psylla cheopis, which transmits Bacillus pestis, the bacillus of plague
from the rat to man. It appears that this bacillus lies in the gut of
514 THE INVERTEBRATA
the flea and the faeces deposited on the skin of the host are rubbed
into the wound by the scratching which follows the irritation from
the bite.
Ceratophyllus fasciatus, the European rat flea, also transmits the
plague organism as also can Pulex irritanSy but since the latter does
not live successfully on rats, it is never likely to prove a source of
trouble*
Order STREPSIPTERA
Small parasitic insects, allied to the Coleoptera, with winged, free-
living males and larviform females, which never leave the interior of
their host.
Sty lops causes great modification of its host, the bee (Andrena).
CHAPTER XV
THE SUBPHYLUM ARACHNIDA
Arthropods with fully chitinized exoskeleton ; the anterior part of the
body (prosoma), never divided into head and thorax, consisting of six
adult segments, the first (preoral) with prehensile appendages
(chelicerae) usually three-jointed, the second (postoral) with append-
ages either sensory or prehensile (pedipalps) and the remaining four
ambulatory ; the posterior part (opisthosoma) consisting of thirteen
segments and a telson in the most primitive forms but tending to
become shortened, the first (pregenital) segment differing from the
rest, the second bearing the genital opening; respiratory mechanisms
of various types usually developed in the anterior part of the opistho-
soma ; coxal glands of coelomic origin in the 2nd to 5th prosomatic
segments ; larval forms absent except in Limulus.
As has been pointed out in the introduction to the Arthropoda, the
Arachnida are distinctly marked off from the rest of the phylum by
the character of their appendages and especially by their chelicerae
which furnish so strong a contrast to the sensory antennae, elsewhere
found in the phylum. Moreover, nowhere else (except perhaps in
trilobites) are true jaws absent, the prolongation of the basal joint of
the anterior limbs toward the mouth (gnathobases) serving the arach-
nids for mastication. In the divisions of the group is found the greatest
diversity in form, for though by no means active creatures, arachnids
have become adapted to many kinds of environment.
Besides the segments enumerated in the preamble, there is in the
embryo of most arachnids a precheliceral segment (Fig. 352 B, C).
The variation in the segments of the prosoma is confined to minor
details, the chelicera preserving much the same characters throughout
the group, only losing a joint in the Araneae, and being either chelate
or subchelate ; the pedipalp, however, varies according to its function,
being chelate in the scorpion and the Pedipalpi, which seize their prey
by means of it, modified for purposes of fertilization in the spiders,
and merely an ambulatory appendage in Limulus. In most forms
the tergites of the segments are fused together, but in the Pedipalpi
and the Solifugae the last two prosomatic segments are entirely free.
It is in the opisthosoma and its segments that the greatest amount
of variation can be seen. The pregenital segment (Fig. 352 B, C) is
always developed in the embryo, but tends to disappear in the adult.
Thus in the Palpigradi, Pedipalpi and Pseudoscorpionidea it forms
a distinct segment; in Limulus it is represented by a pair of rudi-
Op^'iO
r ~ -iCOC.
pro- 1
o-'>p--Jf-
^eox.gl-
pet.--S-
-(/OM.
co-
rnet.
Fig. 352. The development of the Arachnida. A, Transverse section of a spider
embryo (Theridium), after Morin, showing the coelomic sacs (coe.) and the
formation of the heart (h.). n.r. nerve rudiment ; y. yolk with contained cells.
B, Sagittal section of a spider embryo after Wallstabe. Coelomic sacs of
pre. precheliceral segment, pro. i, pro. 6, first and last limb-bearing segments
of the prosoma, prg. pregenital segment, op. 2, second, and op. 10, tenth seg-
ment of the opisthosoma. C, Diagram of the scorpion embryo, altered from
Dawydoff. Coelomic sacs of pre. precheliceral segment; 1-6 limb-bearing
segments of the prosoma; prg. pregenital segment; gop. segment of genital
operculum ; pet. segment of pectines ; Igp. segments of first three lung books ;
eo.d. coelomoducts which never reach the exterior; cox.gl. coxal glands; gon.
gonoducts; g. gonad. D, Embryo of the scorpion Buthus earpathieus, after
Brauer. Stage showing che. the chelicerae; pp. pedipalps; the four other
appendages of the prosoma; prg. the pregenital segment and appendages;
8, appendage forming genital operculum, succeeded by those which form
the pectines and the lung books; 13, last of these; 7net. metasoma.
ARACHNIDA 517
mentary appendages, the chilaria\ it is entirely missing in the adult
scorpions. In addition to this segment there is a maximum of twelve
segments and a terminal appendage, the telson, which is attained only
by the embryo scorpions and the eurypterids; the Palpigradi and
Pseudoscorpionidea have one less. In all these cases, there is a
differentiation of the segments into two regions, the meso- and meta-
soma. In Limuliis there are six segments only, but in the related
extinct genus, Hemiaspis, there are three more. The Solifugae show
ten. In the spiders, mites and phalangids, the body is much
shortened; the phalangids have the anterior segments united to the
prosoma. Lastly, the telson may be a sting in the scorpions, a jointed
sensory flagellum in the Palpigradi, a fin in some eurypterids or a
digging stick in others and in Limulus.
A typical feature is the suctorial alimentary canal. The mouth is
usually narrow and situated just behind the chelicerae; only in
Limulus has it moved backwards, become enlarged and surrounded
by the basal joints (gnathobases) of all the prosomatic appendages ; in
the scorpions the appendages of the 2nd-4th segments form gnatho-
bases; the Palpigradi and Solifugae have no gnathobases. In all
arachnids, except Limulus^ the food is fluid and is drawn through a
narrow oesophagus into a sucking stomach and thence into a straight
mid gut, which is by far the longest part of the gut, and receives the
openings of the digestive coeca ; often, as in scorpions, there are several
of these, segmentally repeated, very much branched and forming a
compact " liver "-hke organ. There may be important salivary glands
entering the fore gut as in the scorpions. Posteriorly the mid gut,
except in Limulus, gives off Malpighian tubules. The hind gut is short.
The respiratory organs of the Arachnida are distributed as follows,
(i) *'Gill books" in the aquatic form, Limulus, and probably in the
extinct eurypterids. (2) "Lung books" in the terrestrial scorpions
and Pedipalpi. (3) A combination of lung books and tracheae in the
spiders. (4) Tracheae alone in the Solifugae, Pseudoscorpionidea,
Phalangida and Acarina. (5) Lastly, in the Palpigradi, smaller acarines
and other forms, there are no special respiratory organs and exchange
of gases takes place through the skin.
As the Arachnida apparently form a natural group, efforts have
been made to derive these various methods of respiration one from
the other. The gill books (Fig. 353) are stated to be the most primitive
respiratory organs. They are piles of leaflets, in which blood circulates,
attached in each segment to the posterior face of freely oscillating
plates, which are possibly appendages, resembling the abdominal
appendages of the Isopoda which are also respiratory in function.
There is a special muscular mechanism for opening and shutting the
leaflets in the water and thus facilitating gaseous exchange. In the
5l8 THE INVERTEBRATA
lung books of the scorpion there are also parallel leaflets, which are
sunk into pits with a confined opening (pneumostome). The air
circulates between these leaflets, but there is no evidence that air is
actively pumped in and out of the lung. Gaseous exchange then
appears to be entirely due to diflFusion. In spiders, however, a
complicated system of muscles has been described which bring about
expiration by compressing the lung. Inspiration follows by the
elasticity of the chitin lining.
Fig- 353- Longitudinal section through the opisthosoma oi Limulus, showing
four of the five gill books. From Shipley and MacBride. i, operculum;
2, second gill book; 3, muscle which moves the gills up and down; 4, blood
vessels; 5, muscle which raises the operculum.
Fig- 354- Diagram of respiratory organs of the Arachnida, After Kingsley.
A, Two segments with appendages (gill books), bearing leaflets on their pos-
terior face as in Limulus. B, Appendages partly (right) and wholly (left)
withdrawn into pits of the ectoderm so that the flat appendage forms the
floor of the pit and the leaflets are internal, a. anterior.
It is generally supposed that the lung books of scorpions are derived
from gill books by the withdrawal of the leaflets into special pouches,
the lungs (Fig. 354). The appendages or plates disappear or form the
floor of the lung and the leaflets appear as folds of the lining. Lung
books, according to this view, are organs which, originally intended
for aquatic use, have been slightly adapted for terrestrial life, but while
the scorpions in their long history have shown no capacity for further
ARACHNIDA
5^9
development, the rest of the Arachnida have developed the typical
arthropod tracheal system. The spiders, at least, have passed through
a primitive lung-book stage from which they have not all emerged.
In fact they show all the stages of replacement of lung books by
tracheae, which actually arise as diverticula of the lung itself. Thus
we have the following stages in the spiders :
(i) Two pairs of lung books and no tracheae in the families
Atypidae, Liphistiidae and Aviculariidae.
Fig- 355- Respiratory organs of spiders. After MacLeod. A, Horizontal
section through the opisthosoma of Argyroneta. i, stigma opening into a
cavity from which arise bundles of 2, terminal and 3, lateral tracheae;
4, lung book with leaflets in section. B, Longitudinal section through lung
book of a spider, i, pneumostome or stigma; 2, free edge of leaflet; 3, air
space between leaflets ; 4, blood space within leaflet.
(2) An anterior pair of lung books and a posterior pair of stigmata,
opening into tracheae, in the majority of families.
(20) An anterior pair of lung books, the posterior pair of stigmata
and tracheae having entirely disappeared, in the family Pholcidae.
(3) Two pairs of stigmata, both opening into tracheae, in the family
Caponiidae.
These form a complete series. The adherents of the theory that
lung books have given rise to tracheae claim that, on the whole, those
spiders which have two pairs of lung books are the most primitive
520 THE INVERTEBRATA
in Other respects. It may be pointed out, however, that there is also
a connection between the degree of development of tracheae in a
family and the activity of its members. In inert forms, there may be
reduction or even total loss of the tracheal system.
In all the forms in which lung books or gill books are present, there
are processes in the embryo which can be identified as rudiments of
appendages, on the anterior abdominal segments (Fig. 352 D). On
the posterior border of these processes, leaflets develop at the same
time as an invagination forms the lung cavity above them, so that the
limb itself forms part of the floor of the cavity. On the whole then,
embryology may be said to show the origin of lung books from gill
books, and the comparative anatomy of spiders indicates that lung
books have been replaced by tracheal systems. But there lie outside
this series arachnid groups, like the Acarina, with tracheal systems
of a different kind, which can only be derived with difficulty from the
respiratory system of the other forms and may have had a separate
origin.
In the arachnids, the mesoblast is formed as two lateral bands
which segment into somites, just as does the same tissue in the anne-
lids. The somites correspond with the external segmentation and in
each one of them appears a coelomic cavity. This is best seen in the
scorpions (Fig. 352 C) and the spiders (Fig. 352 B). They are formed
near the ventral surface and extend on the one hand into the appendage
and on the other towards the dorsal middle line, where the extensions
from the two sides meet and form the heart between them. They also
form diverticula varying in the different groups, which are the remains
of a complete series of metamerically segmented coelomoducts. In the
scorpions, the embryo (Fig. 352 C) shows five pairs of these, in seg-
ments 3, 4, 5, 6 and 8. In only one case, that of segment 5, do the
coelomoducts reach the external surface, and persist in the adult as a
pair of excretory organs, the coxal glands. In segment 8 they grow
towards the middle line and form the mesodermal part of the gono-
ducts. The other coelomoducts disappear and the coelomic sacs are
resolved into mesenchyme which fills up the spaces of the body and
forms the muscles, the blood and the fat body. In Limulus there are
also a pair of coxal glands, which in development arise from the
coelomic somites of no less than six segments, of which only segment
5 sends out a duct opening to the exterior.
Class SCORPIONIDEA
Arachnids with the prosoma covered by a dorsal carapace; the
opisthosoma divided into a mesosoma and metasoma distinct from
one another, containing twelve segments and a telson; chelicerae
SCORPIONIDEA
521
and pedipalps both chelate; four pairs of walking legs; the first
mesosomatic segment carries the genital operculum, the second the
pectines, and the next four each a pair of lung books; the metasoma
comprises segments reduced in size to form a flexible tail for wielding
the terminal sting (the telson) and bears no appendages. Viviparous.
Fig. 356. Scorpio swammerdami, x f. From Shipley and MacBride. A, Dorsal,
B, Ventral view, chc. chelicera ; pp. pedipalp ; e.l. lateral and e.m.^ median
eyes; g.op. genital operculum; pet. pectines; 3, 4, 5, 6, walking legs of the
prosoma; 9, 10, 11, 12, stigmata of right side; 13, last segment of mesosoma;
pi. soft tissue of pleura ; met. i , first segment of metasoma ; tel. telson.
The tergum of the prosoma bears a group of lateral eyes near the
anterior border and a pair of median eyes, but some scorpions are
blind. On the ventral surface there are inward projections from the
522 THE INVERTEBRATA
basal joints of the pedipalps and the first two pairs of walking legs,
which are masticatory in function (gnathobases). The walking legs
are six-jointed and end in double claws. Between the basal joints of
the last pair is a plate, the metasternite, which represents the fused
sterna corresponding to these limbs; the sterna of the other pro-
somatic segments are not represented. At the beginning of the meso-
soma there is in the embryo a pregenital segment with two limb
rudiments. This disappears without leaving a trace in the adult. The
two succeeding segments bear appendages: (i) the genital operculum^
a small plate covering the openings of the genital ducts, which is
formed by the union of two rudiments of appendages ; (2) the pectineSy
flap-like structures attached by a narrow base with a distal border of
chitinous spines like the teeth of a comb. They are tactile in function
and derived from embryonic limb rudiments. There are no other ex-
clusively sensory organs (except the eyes) on the body of the scorpion,
but there are sense hairs scattered over the surface and more numerous
on the pedipalps than elsewhere.
The lung books are found on segments 3-6 of the mesosoma. The
7th segment is without any external segmental organs. As has been
already mentioned, there are, in the embryo, seven pairs of meso-
somatic appendages (Fig. 352 D), those on the embryonic pregenital
segment and on the six succeeding segments. Of these the 4th-7th
never develop to more than papillae, but folds develop on their
posterior surface and the skin behind is tucked in to form the lung
sacs. When the sacs are complete, the folds become the leaves of the
lung book. In the internal space of these folds, the blood circulates
and is presumably aerated; it contains the respiratory pigment,
haemocyanin. The circulatory system of the scorpion is remarkably
complete (Fig. 357). The heart consists of seven chambers (in the 7th-
13th segments), into each of which a pair of ostia opens and from each
there leave a pair of lateral arteries. In addition, there is an anterior
and a posterior aorta, the former dividing into many branches in the
prosoma, and one of these passes backwards as a supraneural
artery. The arteries end in tiny vessels and many of these communicate
with the special ventral sinus, which supplies blood to the lung books.
Muscles run from the roof of this to the floor of the pericardium, and
when they contract the ventral sinus enlarges and draws venous blood
into it. When they relax, blood is forced into the lung books, whence
it is returned to the pericardium by segmental vessels.
A minute mouth opens into the pharynx which is suctorial, with
elastic walls which can be drawn apart by muscles. A short oeso-
phagus succeeds, and into this open the salivary glands. The endo-
dermal mid gut is long and narrow and receives throughout its course
several pairs of ducts which lead from the digestive glands. These
SCORPIONIDEA
523
524 THE INVERTEBRATA
together form a bulky mass, filling up the dorsal part of the meso-
somatic body cavity. The food passes into the cavity of these to be
digested. It consists mainly of insects, which are chewed by the
gnathobases and the juices sucked up by the action of the pharynx.
The beginning of the short hind gut is marked by the Malpighian
tubules.
The nervous system consists of a supraoesophageal ganglion which
supplies the eyes, a large suboesophageal complex which gives
branches to all the adult appendages, and two ventral cords which
bear ganglia in the last seven segments.
The sexes are separate and the gonads constitute a network. The
spermatozoa are filiform and fertilization is internal, being preceded
by a courtship, described in lively fashion by Fabre as danse a deux.
Scorpions are viviparous. Sometimes the eggs are rich in yolk and
the young develop entirely at its expense ; in Scorpio and other genera
the eggs are small and yolk is entirely absent. In this case the young
develop in lateral sacs of the uterus, attached to the mother by a kind
oi placenta. The young, when hatched, are sometimes carried on the
mother's back.
The earliest scorpions are found in the Silurian, and it is of con-
siderable interest that the first genus, Palaeophonus, was a marine
animal. It closely resembles the terrestrial scorpions, except in its
shorter and broader limbs without claws, and in the absence of
stigmata.
Class EURYPTERIDA
Extinct aquatic arachnids resembling the scorpions in the number
and arrangement of the segments of the adult; the division of the
abdomen into meso- and metasoma is not quite so marked ; chelicerae
short and three-jointed, chelate; the next four segments bear append-
ages which are often similar (but the pedipalps may be chelate); in
the last (6th) prosomatic segment the appendages are always larger
than the rest and are broad and paddle-shaped; first and second pairs
of mesosomatic appendages unite to form the genital operculum ; the
first five mesosomatic segments bear indications of leaf-like branchiae ;
metasoma ends in a structure (telson) of variable form ; mouth has
moved backwards and is surrounded by gnathobases of all the limbs.
The great interest of this group lies in its similarity to the scorpions.
There was, however, much more variety in external structure in these
aquatic arachnids and they sometimes attained a length of six feet.
Not only is there fundamental agreement in the segmentation and the
division into meso- and metasoma, but also in characters like the shape
and usually the size of the chelicerae, and the telson, which in primitive
eurypterids has a recurved sting-like form. Slimonia (Fig. 358 B)
ARACHNIDA
525
has a slightly modified telson. In one eurypterid {Glyptoscorpius)
structures have been described which correspond to the pectines in
position and structure. If this is substantiated, it constitutes a
remarkable resemblance in detail.
Fig. 358. Diagram of extinct Arachnida. A, Pterygotus osiliensis, dorsal
view. After Schmidt. B, Slimonia (restoration of ventral surface by M.
Laurie). C, Hemiaspis limuloides, dorsal surface. From Woods. All Silurian
forms. Segments and appendages numbered to correspond ; Arabic numerals
in Pterygotus and Roman in Slimonia. chc. chelicerae (segment i); 6. meta-
stoma; d. compound eyes; e. simple eyes;^.op. genital operculum; tel. telson.
A few special characters may be mentioned here. On the ventral
surface a structure called the metastoma is seen which possibly re-
presents the pregenital segment. Branchiae undoubtedly existed, but
their exact nature is not known. Possibly the sterna of the segments
526 THE INVERTEBRATA
which carried them were membranous and the branchiae were tucked
in under them. There are five pairs and the first of these corresponds
in position to the pectines of the scorpion (except possibly in Glypto-
scorpius). Thus, when the ancestors of the scorpions became terrestrial,
we may suppose that the first pair of respiratory appendages remained
external and took on a sensory function, while the rest helped to form
the lung books.
Minute forms with incompletely developed abdomen and enlarged
eyes have been found which are thought to be the pelagic larvae of
eurypterids. The adults were in all probability carnivorous forms,
which crept and swam and sometimes burrowed at the bottom of
shallow seas. In Pterygotus (Fig. 358 A) and Eurypterus there are
adaptive modifications of the telson for swimming and burrowing
respectively.
Class XIPHOSURA
Aquatic arachnids with a broad prosoma divided by a hinge from the
opisthosoma in which the first six segments are present and fused
together dorsally; they bear six pairs of biramous appendages, of
which the first form an operculum on which the genital apertures
open and the remaining five carry the gill books ; chelicerae of usual
arachnid type, pedipalps not distinguished from the four pairs of
ambulatory appendages which follow ; mouth far back surrounded by
gnathobases of all the postoral limbs ; caudal spine present possibly
representing the lost abdominal segments as well as the telson;
pregenital segment represented by rudimentary appendages, the
chilaria.
Limulus (Figs. 359, 360), which is the sole living representative of
the group, is evidently more affected by specialization than either the
scorpions or eurypterids, and it is on this account that the attempts
which have been made to indicate the king crab as an ancestral form
to higher groups have usually been regarded as ingenious but illusory.
It is essentially a shore-living, burrowing animal. Like a crab, its
carapace is compact, dorsoventrally flattened and expanded laterally,
so that the animal can shovel its way under sand and mud. Its legs
are tucked under the carapace and the hinder pair kick out the sedi-
ment behind. To protect the gill books from this rough treatment, the
operculum completely covers the appendages which bear them. But
Limulus has not lost its tail, and an observer, watching the creature in
an aquarium, will contrast it unfavourably for grace and efficiency with
a crab. Its swimming movements, principally brought about by the
flapping of the abdominal appendages, are slow and clumsy, and we
can hardly consider it except as a sedentary animal.
The chelicerae are small, chelate and three-jointed, as is usual in
XIPHOSURA 529
In all the others the appendages almost meet in the middle line, but
remain distinct. From the posterior surface of the exopodite arise
about two hundred branchial leaflets. The appendages are provided
with muscles by which the flapping movements are made which propel
the animal in a leisurely way through the water and circulate water
amongst the leaflets.
The mouth occupies a subcentral position under the carapace,
surrounded by the gnathobases. Worms and small molluscs from the
shore mud are seized by the chelae and, after mastication by the
gnathobases, stuffed into the mouth, which leads to the fore gut con-
sisting of an oesophagus and a chitin-lined "stomach"; the mid gut
is long and into it open two pairs of ducts from the digestive glands.
These glands are very well developed and fill up much of the space
inside the cephalothorax. There are no Malpighian tubules and no
salivary glands in Limulus.
The circulatory system is very complete and like that of the scorpion
in its main lines. A unique feature is the complete investment of the
ventral nervous system by an arterial vessel which corresponds to the
supraneural vessel of the scorpion.
The nervous system is of a very concentrated type. The supra-
oesophageal ganglia supply the eyes and are fused with the ganglia
of all the succeeding segments as far as the opercular segment to form
a ring round the oesophagus. From this a double ventral cord ex-
tends into the opisthosoma, swelling into ganglia in each of the "gill-
book" segments. Median and lateral eyes (p. 310) are present.
The coxal (brick red) glands arise from six segments in the embryo
and open on the fifth pair of legs.
The reproductive organs consist of a network of tubules com-
municating with the exterior by paired ducts opening on the genital
operculum. The eggs are laid far up on the shore at spring tides in holes
dug for them by the mother, and the male, which comes ashore clinging
to the carapace of the female, spreads the sperm over them, a method
of fertilization very similar to that of the frog. The eggs are heavily
yolked and the young hatch as a planktonic larva in a condition re-
sembling the adult but with an opisthosoma showing separate segments
and without the caudal spine. The larva, which swims by means of the
abdominal appendages, as in the adult, has been called the *'Trilo-
bite" stage, because of an extremely superficial likeness to that group.
While Limulus has existed since the Trias without any modification,
it is of considerable interest that in the Palaeozoic ver}'' similar animals
occur, in which there are three additional segments and a rather
shorter caudal spine, indicating that the latter organ has been formed
at the expense of the posterior opisthosomatic segments. These ani-
mals are Hemiaspis (Fig. 358 C) and Bunodes.
530 THE INVERTEBRATA
Class ARANEIDA
Arachnids with prosoma covered by a single tergal shield but head
marked off by groove ; opisthoSoma ("abdomen ") separated by waist,
soft, rarely having any trace of segmentation, two to four pairs of
spinnerets and several kinds of spinning glands ; chelicerae two-jointed,
subchelate; pedipalps modified in male for transmission of sperm.
In the embryo spider, the segmentation of the opisthosoma is in-
dicated by the presence of coelomic cavities of which there are ten
(Fig. 352 B) ; there are also five pairs of rudimentary appendages, the
first of these disappears, the next two assist in forming the lung books,
and the fourth and fifth become the spinnerets. When more than two
pairs of spinnerets are present the additional ones are split off from
pre-existing spinnerets. Embryology thus shows that the existing
forms with apparently unsegmented opisthosoma are descended from
ancestors with nearly the full number of segments typical of arachnids.
The chelicerae (Fig. 364) contain a poison gland in the basal joint.
Spiders have developed to an extreme the tendency, so common in the
arachnids, towards adopting a carnivorous diet. While most of the
spiders on account of their size can only obtain suitable supplies of
food from insect life, some are able to attack larger forms, even birds
in the case of Mygale. Besides the poison glands which cause the
immediate death of the prey, there are salivary glands in the under lip
which produce a proteolytic ferment. A fly which is caught by a
spider is pressed against the mouth by the gnathobases of the pedi-
palps, a drop exudes from time to time and in a couple of hours the
morsel of flesh has been externally digested and the resulting fluid
sucked into the spider's alimentary canal by the pulsations of the
"stomach", the chitinous exoskeleton of the prey remaining as an
empty husk. This method of feeding is a leading characteristic of the
group.
The diagram (Fig. 361) shows the main features of the anatomy
of the spider. The oesophagus, after dilating into the sucking stomach,
is succeeded by the mid gut which immediately sends out two main
lateral branches forward with coeca running into the limbs. It passes
back through the opisthosoma and gives place to the hind gut where
the Malpighian tubules are given off. The main feature is the digestive
gland which is a dorsal diverticulum of the mid gut, richly branched
and filling the opisthosoma on each side of the heart. In this the latter
stages of digestion take place. The hind gut is short and dilated into
a stercoral pocket where faeces accumulate. The heart is situated in a
distinct pericardium in the opisthosoma, has three pairs of ostia, and
gives off an anterior and a posterior aorta and three lateral arteries on
each side. In contrast to the scorpion and Limulus there are no definite
ARANEIDA
531
arterioles, but the blood is finally collected into sinuses which feed
the lung books when these are present.
The nervous system is more concentrated than in the scorpion,
consisting of a supraoesophageal ganglion supplying the eyes (Fig. 363 )
and a suboesophageal complex supplying the rest of the body. Two
non-ganglionated nerves pass backwards to the opisthosoma.
The diagram (Fig. 361) shows a lung book opening in the anterior
part of the opisthosoma and the details of the structure are exhibited
in Fig. 355. The "leaves" of the book are seen to be thin plates with
an internal space for the circulation of the blood. They are dotted
with short chitinous spines (not seen) and fused with the walls of the
lung. The cavity of the lung only communicates by a narrow opening
d.dig. Pf"^' h.
Fig. 361. Diagram of a spider, Epeira diademata, showing the arrangement
of the internal organs, x about 8. From Warburton. an. anus; ar. artery;
brn. brain ; chc. chelicera ; cm. caecum of mid gut in ambulatory limb ; d.dig.
ducts of digestive gland ; e. eye ; ga.sb. suboesophageal ganglion ; gl.ac, gl.ag.,
gl.am., gl.t. aciniform, aggregate, ampulliform and tubuliform glands ; h. heart
with three ostia ; Ing. lung book ; M. mouth ; m.d. dorsal muscle of sucking
stomach; 7ng. mid gut; m.t. Malpighian tubule; o. ov.ary ; p.gl. poison gland;
pcm. pericardium ; sp. spinneret ; s.p. stercoral pocket of hind gut ; s.st. sucking
stomach; v. vessel bringing blood from lung book to pericardium.
with the outside air. Respiratory movements for the renewal of the
pulmonary air have not been recorded by most observers and the
method of respiration cannot be very efficient. In this diagram (Fig.
361) the tracheae are not shown, but in Fig. 355 A a horizontal section
through the opisthosoma is shown in which the same ingrowth has
given rise to a lung book and a bundle of tracheae. The character of
the tracheae is well seen. They spriAg from a long pocket in parallel
series and do not branch as in the insects, but they have the typical
structure, strengthened by a spiral ridge of the chitinous lining. This
form [Argyroneta) shows a richly developed tracheal system, but in
other forms, particularly spiders with slow movements, the number
532
THE INVERTEBRATA
of tracheae is much reduced, even to a single pair from each stigma.
The variations in the development of the tracheae are recorded in the
opening section on the Arachnida (p. 519).
Fig. 363.
Fig. 362.
Fig. 362. Pedipalp of Tegenaria guyonii, the large house spider. From
Shipley and MacBride. i, coxa; 2, gnathobase, the so-called maxilla;
3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus; 8, palpal organ.
Fig. 363. Front view of head of Textrix denticulata. From Warburton.
I, head; 2, eyes; 3, basal joint of chelicerae; 4, claw of chelicerae.
1-..
Fig. 364. Diagrammatic view of expanded palpal organ. From Shipley and
MacBride. i, tarsus; 2, bulb; 3, vesicula seminalis, and 4, the opening of
its duct which is protected by 5, the conductor; 6, haematodocha which is dis-
tended with blood when the palpal oigan is expanded; 7, alveolus; 8, tarsus.
The spinning glands are shown in the diagram in the ventral part
of the abdomen. In a web spinner like Epeira, there are five types of
glands of diverse structure and function, all opening by minute pores
on the spinnerets. Thus the ampulliform glands supply the radial lines
ARACHNIDA 533
of the webs, and the spiral Hnes are made by the aggregate glands
which furnish the viscid fluid which covers them. The egg cocoon is
formed by the tubidiform glands and these glands are absent in the
males. The aciniform glands manufacture the cords which are wrapped
ound the prey caught in a web, and the pyriform glands make the
attachment discs by which a silk thread is anchored to the ground.
Such a spider as this is well adapted for its sedentary life in a web. It
has immensely long legs compared to the size of the body and on the
ground moves slowly and uncertainly. But its legs end in claws and
spines, by which it not only can cling with absolute safety to the elastic
threads of the web, but which it also uses to weave the threads of silk
as they come out of the spinnerets. Thus the web spinners represent
the greatest specialization of the group; there are, however, other
forms like the wolf spiders (Lycosidae) and the jumping spiders
(Salticidae), which are just as predaceous as the Epeiridae but by no
means so sedentary. They run swiftly after their prey or jump
suddenly on it. They may only possess two ampulliform glands which
secrete a "drag line" which they leave behind them as they move.
The web spinner relies almost entirely on its sense of touch and
the vibration of the lines of the web affecting the tactile hairs of the
limbs is the guide to the entangled prey. Eyes, though present, are
not efficient. But the hunting spiders find their victims by sight and
have a remarkable range of vision. This is not only used in the pursuit
of food but also in the elaborate courtships which are characteristic of
these two families, during which the male executes the most fantastic
dances.
The generative apertures are found between the aperture of the
anterior pair of lung books and the spinnerets. Fertilization is internal
and before the male is ready to fertilize the female the sperm must be
transferred to his modified pedipalps (Fig. 362). The terminal joint
of these is greatly enlarged and contains a complicated tubular vesicular
seminalis. A drop of seminal fluid is emitted either on to a small web
spun by the male or on some surface like a leaf and the palps are then
applied to the fluid and the seminal vesicle charged. After this court-
ship begins, and at the close the palps are inserted into the genital
opening of the female; the spermatozoa are stored in spermathecae.
The eggs are laid in a cocoon.
Class ACARINA (Mites and ticks)
Arachnids with a rounded body with no boundary between the
prosoma and the opisthosoma ; basal segments of the pedipalps united
behind the mouth ; no gnathobases to the four walking limbs.
These forms are usually minute except in the case of the parasitic
534 THE INVERTEBRATA
ticks. They are, variously, scavengers, ectoparasites on all sorts of
plants and "hangers on" of all sorts of animals, but in the last case
they become, by the modification of the chelicerae and pedipalps,
blood-sucking parasites.
In the most free-living of them, like the aquatic and predatory
Hydrachnidae, the chelicerae are clawed piercing weapons and the
pedipalps leg-like with sensory hairs. The chelate condition of the
chelicerae may be seen in the cheese mite, Tyroglyphus (Fig. 366),
which is a typical saprophyte living on cheese only when it has begun
to decay. The pedipalps are here no longer leg-like.
In a tick Hke Argas (Figs. 365 A, 367, 368), the pedipalps are sensory,
but the chelicerae and the median hypostome are elongated and con-
verted into serrated cutting tools ; a sucking channel is formed between
these. The mouth is usually minute and leads into a sucking pharynx
and then into an endodermal stomach which gives rise to caeca in the
ticks, where there are also salivary glands of large size opening into
Fig. 365. Dorsal surface of A, Argas (Argasidae) and B, Amhlyomma, $
(Ixodidae). From Nuttall and Warburton. The Argasidae are distinguished
by the leathery skin, diversified by discs which mark the insertion of muscles ;
the Ixodidae by the hard scutum, which covers the whole body of the male
and the anterior part of the body in the female (B).
the pharynx. The saliva is said to contain an anticoagulin, as in leeches,
and this renders easier the gradual digestion of the blood which is
taken into the stomach. A remarkable phenomenon without parallel
in the Arthropoda is the occurrence of intracellular digestion in some
acarines. The cells of the stomach put out pseudopodia and the blood
plasma is taken into vacuoles where it is digested.
The circulation is extremely degenerate. No heart has been ob-
served with certainty and the blood system is lacunar in mites, but
in the tick, Argas, there is a single-chambered pulsating vessel with
a pair of ostia and an aorta running forward to a periganglionic sinus.
The respiratory organs are tracheae, long and convoluted. These open
ACAR[NA
535
Fig. 366. Tyroglyphus siro, seen from the ventral side. A, Female. B, Male.
Magnified. From Leuckart and Nitsche. an. anus; chc. chelicerae; pp.
pedipalps; 3, 4, 5, 6, first, second, third and fourth walking legs; rep.ap.
reproductive opening, flanked by two suckers on each side ; su. suckers at side
of anus.
,chc.d.
hyp.
chc.s.
Fig- 367. Ventral view of capitulum (false head) of Argas persicus, S. From
Nuttall. b.cap. basis capituli; chc.d. digit, and chc.s. shaft of chelicera;
hyp. hypostome; pp. four-jointed "palp" (pedipalp); p.h., h.h. postpalpal
hair, posthypostomal hair.
536 THE INVERTEBRATA
by stigmata, the position of which varies in the main divisions of the
group. In the Notostigmata there are four pairs of dorsal stigmata in
the first four opisthosomatic segments; in the Cryptostigmata, four
pairs of stigmata at the bases of the four walking legs ; in the rest there
^xhc-sh.
Fig. 368. Argas persicus, S. Median longitudinal section showing the pro-
boscis, alimentary canal and reproductive systems. Altered from Nuttall.
The chelicerae are seen within the cheliceral sheath (chc.sh.). They are thrust
forward by the contraction of dorsoventral body muscles (m.r.ch.) and cut their
way into the host by the digits (chc.d.) which are moved by their flexor muscles
{m.f.d.). The barbed hooks of the hypostome {hyp.) are thrust into the wound
and keep the tick in place, an. anus ; b.cap. basis capituli ; brn. concentration
of nervous system ; cav.sh. cavity of cheliceral sheath ; coe. caecum of the
stomach; end. endosternite ; h. heart; M. mouth cavity; oe. oesophagus; ph.
pharynx with radiating muscles; r.s. rectal sac; st. stomach; sal.d., sal.gl.
salivary duct and gland ; ts. testis ; vd. vas deferens ; w.gl. white (accessory) gland.
is a single pair of stigmata in varying positions, in front of the chelicerae
(Prostigmata), between the chelicerae and pedipalps (Stomatostig-
mata), between the pedipalps and ist walking legs (Heterostigmata),
the 2nd and 3rd legs (Parastigmata), the 3rd and 4th (Mesostigmata),
ARACHNIDA 537
and behind the 4th legs (Metastigmata). If we regard the opistho-
somatic position of the breathing organs as primitive it is difficult to
see how these varying arrangements have come to pass in the
acarines.
The life history of the parasitic forms is of great interest, especially
that of the ticks or Metastigmata. These are divided into the Ixodidae
(Fig. 365 B) and Argasidae. The former live permanently on one host;
the life of Boophilus bovis, attached to the cow, is only interrupted
by the necessity of moulting and reproduction. Though compelled to
withdraw its mouth parts when the skin is being cast, the tick plunges
them into the host again at the same place, as soon as possible after the
completion of the process. In many other cases the ticks fall off before
every moult and have to seek a new host afterwards. The argasids,
however, in the full-grown state, make only short visits to the host to
suck blood, lasting for a few hours. In these last cases the young can
go without food for months and the full-grown tick for years. In the
course of several of these meals the six-legged larva develops into an
eight-legged nymph which becomes sexually mature only after further
development. Copulation may take place several times, spermato-
phores being inserted, but the sperm in these can only escape and
reach the ovary after the female again feeds. But in all cases when
fertilization of the eggs has once occurred, the female falls to the
ground and after laying her eggs dies.
Many kinds of ticks carry disease, e.g. in both the following cases
caused by Spirochaeta, Texas fever of cattle (Boophilus annulatus)
and the relapsing fever of man (Ornithodorus moubata). Also certain
small parasites of the blood corpuscles {Piroplasma), in severe diseases
of cattle, are carried largely by Rhipicephalus .
Class PHALANGIDA
Arachnids with prosoma covered by a single tergal shield and united
to the opisthosoma by its whole breadth; opisthosoma always seg-
mented ; chelicerae three-jointed and chelate ; pedipalps leg-like ; two
simple eyes.
These creatures, with their enormous elongated legs, are familiar
objects in the summer; the active predaceous forms are supposed to
live for a single season only, but some representatives are slow-moving
and live longer. They feed on insects and other arthropods and suck
their juices. The walking legs have the same number of joints as^
spiders, but the tarsus is multiarticulate. The opisthosoma contains at
least ten segments. The animal breathes by tracheae and there are
two stigmata on the first sternum of the opisthosoma, opening on each
side of the reproductive aperture from which emerges a long pro-
538 THE INVERTEBRATA
trusible process, which is an ovipositor in the female, a penis in the
male.
The Notostigmata mentioned above (p. 536) are forms transitional
between the acarines and the phalangids.
Class PANTOPODA (PYCNOGONIDA)
Arachnida, in which the opisthosoma has disappeared, with the
exception of the pregenital segment which bears legs on which the
genital pore opens.
Fig. 369. A phalangid, Oligolophus spinosus, adult <S, xz. i, chelicerae;
2, pedipalps; 3, 4, 5, 6, walking legs. From Shipley and MacBride.
These extraordinary animals, e.g. Nymphon (Fig. 370 A), are all
marine and semisedentary, crawling slowly over seaweed and seden-
tary animals. They consist of the following regions: (i) tht proboscis,
a prolongation of the prosoma with the mouth at the tip; (2) four
segments fused together bearing four eyes, the chelicerae, the pedi-
ARACHNIDA 539
palps, the ovigerous legs which are present in both sexes and the first
pair of walking legs; (3) three free segments bearing the remaining
pairs of walking legs. The body is usually very small while the legs
are enormously elongated. They have eight joints. The proboscis con-
tains a sucking pharynx preceded by a filter of chitinous hairs which
prevents any but fluid food from proceeding further. The small
stomach gives off" digestive coeca which extend into the legs and other
appendages. The common British form, Pycnogonum Itttorale, is found
firmly attached by the terminal claws of the legs to the sides of sea
anemones into which it inserts the proboscis and sucks the juices. There
is a dorsal heart with three pairs of ostia; respiration is cutaneous.
The nervous system consists of supraoesophageal ganglia and a
ventral chain with suboesophageal and three or four other ganglia.
The sexes are separate and the males carry the eggs on the oviger-
ous legs. The gonads, like the alimentary canal, are branched and open
on the 4th segment of the legs (the last pair of legs in Pycnogonum or
all four pairs in Phoxichilidium femoratum). In the latter species the
larvae are hatched as six-legged creatures, which form cysts in the
polyps of the gymnoblast hydroid, Coryne.
Four small classes, Pseudoscorpionidea, Pedipalpi, Solifugae and
Palpigradi, are undoubtedly arachnids, but can merely be mentioned
here.
The two small classes following have been associated with the
arachnids but no sufficient reason can be advanced for this. They
both exhibit simplicity of structure ; in the case of the Pentastomida
this is due to parasitism, but in the Tardigrada some of the traits of
primitive arthropods may be preserved. In some ways the Tardigrada
resemble Peripatus and their development is said to be of a very
primitive type. But the size and specialized habitat incline the author
to regard this as a case of "simplification" such as is met with in the
Archiannelida (p. 294).
Class TARDIGRADA
Minute arthropods with four pairs of stumpy legs ending in claws,
with oral stylets and a suctorial pharynx, without definite circulatory
or respiratory systems.
Representatives of this group, e.g. Macrobiotus (Fig. 370 B), are
found, for instance, in moss and in the sediment of rain gutters. They
are minute and often very transparent animals, with a thin and
flexible cuticle. The body is usually short and flattened; the tardi-
grades have been compared to the tortoises among the vertebrates,
from their slow and awkward gait. The mouth opens into a tube in
540
THE INVERTEBRATA
which work the two chitinous stylets; a suctorial pharynx, the wall
of which is composed of radiating muscular fibres, follows. Into the
pharynx opens a pair of salivary glands. The animals pierce the wall
of plant cells with the stylets and suck the sap by the action of the
pharynx. Then comes a narrow oesophagus leading into a capacious
stomach, and lastly the rectum, which is joined by two short tubes
which probably represent Malpighian tubules, and by the duct of
the gonad.
Fig. 370. A, A/jym/)/ion, an example of the Pantopoda. After Mobius. ^6. pro-
boscis with mouth; chc. chelicera; pp. pedipalp; ol. ovigerous leg; i, 2, 3,
first three ambulatory legs ; ops. opisthosoma bearing, 4, last pair of ambu-
latory legs. Fusion of first four segments indicated by stippling. B, Macro-
biotus, ?, dorsal view. Modified from Greeff. cl. cloaca; cor. corpuscles in
body cavity; gl. dorsal accessory gland; M. mouth; m.t. Malpighian tubule;
od. oviduct; oe. oesophagus; o. ovary; ph. suctorial pharynx; sa.gl. salivary
glands ; st. stomach ; sty. stylets.
The perivisceral cavity contains no connective tissue cells but is
crowded with numerous rounded corpuscles and traversed by bands
of longitudinal muscle. The nature of the cavity is not known but
the existing account of the embryology describes pairs of coelomic
pouches arising as outgrowths of the archenteron, as in the echino-
derms.
ARACHNIDA
541
The legs resemble the appendages of Peripatus and each is termin-
ated by two forked claws. The last pair are terminal and the anus opens
between them. The nervous system consists of suprapharyngeal, sub-
pharyngeal and four pairs of trunk ganglia, the latter corresponding
to the appendages.
Physiologically the Pantopoda are interesting in their capacity for
resisting desiccation. Like the rotifers and nematodes with which
they are associated in habitat they shrivel up with loss of water,
absorbing it again and returning to life at the next rain.
Class PENTASTOMIDA
Elongated vermiform parasites with a secondary annulation and two
pairs of claws at the sides of the mouth; without respiratory or
circulatory systems.
an. ->:
Fig. 371. A, Demodex folliculorum, ventral view. After Blanchard. A mite
living in the hair follicles of man and domestic animals. B, Linguatula
taenioides. After Leuckart. Ventral view, at the stage when it is eaten by
the second host. al.c. alimentary canal; an. anus; cl. claws; M. mouth.
The commonest example, Linguatula taenioides, lives in the nasal
passages of carnivorous mammals; the larvae, in which the claws of
542 THE INVERTEBRATA
the adult are borne on prominences which may be called limbs, live
in other mammals, chiefly herbivorous. The eggs are* passed out of
the host, the larvae climb on to plants and are eaten by hares or
rabbits ; they traverse the wall of the gut and encyst in other tissues,
often the liver. After a period of growth they wander once more
through the body ; they may at this stage be eaten by the second host
and after wandering through the body reach the nasal passages. The
larvae do resemble certain parasitic mites (Fig. 371 A) and for that
reason the group has been classed with the arachnids.
CHAPTER XVI
THE PHYLUM MOLLUSCA
Unsegmented coelomate animals with a head (usually well developed),
a ventral muscular/oo/ and a dorsal visceral hump ; with soft skin, that
part covering the visceral hump (the mantle) often secreting a shell
which is largely calcareous, and produced into a free flap or flaps to
enclose partially a mantle cavity into which open the anus and the
mesoblastic kidneys (usually a single pair) ; a pair of ctenidia (organs
composed of an axis with a row of leaf-like branches on each side,
contained in the mantle cavity, originally used for breathing) ; having
an alimentary canal usually with a buccal mass, radula and salivary
glands, and always a stomach into which opens a digestive gland or
-^na.
ped.g.
Fig. 372. Comparison between annelidan and molluscan organization. Side
views of A, post-trochosphere larva of Annelida with segmenting trunk;
B, veliger larva of Paludina (Mollusca) before torsion. After Naef. Ali-
mentary canal shown by stippling, an. anus ; brn. brain or suprapharyngeal
ganglion of annelid ; ce.g. cerebral ganglion of Mollusca ; F. foot ; M. mouth ;
ma. mantle; ped.g. pedal, pl.g. pleural, pa.g. parietal, sbg. subpharyngeal,
vis.g. visceral ganglia; vm. velum.
kepatopancreas ; with a blood system consisting of a hearty a median
ventricle and two lateral auricles, arterial system and venous system
often expanding into a more or less extensive haemocoele, with haemo-
cyanin as respiratory pigment ; a nervous system consisting of a cir-
cumoesophageal ring, often concentrated into cerebral and pleural
ganglia, pedal cords or ganglia and visceral loops; coelom, varying
in development, but always represented by the pericardium, the cavity
of the kidneys (which communicates with the pericardium), and the
cavity of the gonads; often with larvae of the trochosphere type.
While we do not know exactly what the ancestral molluscs looked
like, we can make a very shrewd guess at their structure. They
possessed the molluscan characters given in the definition above and
544
THE INVERTEBRATA
they resembled the diagrammatic creature shown in side view in
Fig. 349 A. They had a head with tentacles, a flat creeping foot, a
conical visceral hump covered by a mantle which possibly contained
numerous calcareous spicules and not a complete shell, and a posterior
mantle cavity into which opened the median terminal anus and the
common apertures of the kidneys and the gonads, and which also
dig.gl.
gxpe. pcd. ^„
M-c. 8h.p.
ma.c
ped.g. p.v.C. an. k.op. A
vis.g.
-pa.g.
ped-ij. B
pig. vis.g.
pcd.g.-l —
ped.g.
Fig- 373- Types of Mollusca. Side view. Partly after Naef. A, Ancestral
mollusc. B, Amphineura. C, Gasteropoda. D, Lamellibranchiata (Nucula,
a primitive type). The head-foot is stippled to contrast with the visceral
hump and mantle. The course of the alimentary canal is indicated by double
dotted lines. In A the mantle cavity has its original posterior position, in C
it has become anterior, while in B and D it has extended forward on both
sides of the body, becoming very spacious in D. a.a. anterior adductor
muscle; an. anus; au. auricle; ce.g. cerebral ganglion; ct. ctenidium; dig.gl.
digestive gland ; F. foot ;g.coe. genital coelom ; k. kidney ; k.op. kidney opening;
M. mouth; ma. mantle; ma.c. mantle cavity; op. operculum; p.a. posterior
adductor muscle; pa.g., ped.g., pl.g. parietal, pedal, pleural ganglia; plm.
palp-lamella ; p.pr. palp-proboscis ; pcd. pericardium ; p.v.c. pleurovisceral
(palliovisceral in B) commissure ; sh.p. shell plates ; st. stomach ; ven. ventricle ;
vis.g. visceral ganglia.
contained the ctenidia. In the alimentary canal the fore gut formed a
muscular body, the buccal mass, and a radula (p. 557) and the mid gut
an oesophagus, stomach and digestive glands and intestine. The heart
had a median ventricle and a pair of auricles. The perivisceral coelom
reduced by the development of an extensive haemocoele (p. 556) is
represented by the pericardium with which communicates in front
MOLLUSCA 545
the cavity of the gonads and at the sides the two coelomoducts
("kidneys"). In the nervous system there were as in annehds and
arthropods, a circumoesophageal commissure or brain which may or
may not have been ganglionated, ventral pedal cords, a visceral
commissure coming from the pleural part of the brain, and a pallial
commissure in the mantle edge. From this beginning diverged the
different groups which we know to-day. The chitons (Amphineura),
which have departed least from the ancestral structure, became elon-
gated but limpet-like forms (Fig. 373 B), their visceral hump being
protected by eight shell plates, their mantle cavity extended all round
the foot while instead of a single pair of ctenidia many such pairs arose.
The Gasteropoda remained as short creeping forms (Fig. 373 C) ; they
are characterized by the growth of the visceral hump dorsally, but
unequally so that it has coiled in a spiral (which is covered by a single
shell). This caused a readjustment of the visceral hump which has
revolved (usually to the right) on the rest of the body through 180°
(torsion) and the mantle cavity is now anterior. The Lamellibranchiata
(Fig. 373 D) are flattened from side to side, the whole body being
covered by two mantle lobes secreting two shell valves, united by a
median hinge. The ctenidia inside the greatly enlarged mantle cavity
have developed into huge organs of automatic food collection and so
the head, rendered unnecessary and withdrawn into the mantle cavity,
has become vestigial. Similarly the foot has lost its flat sole and has to
be extended out between the valves to move the animal.
In the Cephalopoda, though there is an unequal growth of the
visceral hump relative to the rest of the body, as in gasteropods, it is
coiled in a plane spiral, but there is no torsion, the mantle cavity re-
maining posterior. The primitive forms in the group (Fig. 402 A) have
an external shell which is divided into chambers, and those behind the
body chamber contain gas. This has had a great effect on the develop-
ment of the group, for by diminishing the specific gravity of the
animals it has enabled them to become more or less free-swimming.
They have tended, with the loss of the shell, to become more and
more efficient swimmers, and this is associated with the development
of their predatory habits. The anterior region shows a kind of trans-
formation new to the molluscs in its partial modification into circum-
oral prehensile tentacles for seizing food. Lastly, and in connection
with all these changes, the brain and sense organs have become
enormously developed and the cephalopods are seen to be one of the
most progressive groups of invertebrates.
Characteristically the ectodermal epithelium of the mantle secretes
a shell in the Mollusca and in most of them the method of secretion
is the same. The original shell is laid down by the mantle of the
veliger larva (Fig. 374 B), but all extension takes place by secretion at
546 THE INVERTEBRATA
its edge (Fig. 377). The outer shell layer, periostracum, formed of
horny conchiolin, is first produced in a groove and then the prismatic
ap.o.
?^-prt.
nres.
-^mes.
Fig. 374. Patella coerulea. A, Trochosphere larva, sagittal section. B, Early
veliger larva, sagittal section C, Veliger larva, frontal section to show meso-
derm bands. After Patten, ap.o. apical organ; end. endoderm; F. foot;
int. intestine; M. mouth; mes. mesoblast pole cell and derivatives; m. em-
bryonic muscle cells; prt. prototroch; sh. shell; st. stomach; t.t. telotroch;
D and V dorsal and ventral.
/ , vm.
an.-
vm.
an.
sh.
Fig. 375. Veliger larvae. A, Ostrea edulis, side view. After Yonge. Ciliary
currents shown by arrows. Suspended material is thrown by the action of
the large cilia of the velum on to the ciliated tract, ct., imbedded in mucus
and carried to the mouth, M., then through the oesophagus into the stomach,
St. The style, shown by stippling, projects from the style sac, s.s., in which it
rotates ; many particles are imbedded in this. After leaving the stomach the
material passes through the coiled intestine (dotted) and by the rectum, rm.,
out into the mantle cavity, ma.c. Other letters: an. anus; a.m. adductor
muscle; dig.gl. digestive gland; F. foot; sh. shell; vm. velum. B, Dreissensia,
ventral view. After Meisenheimer.
layer, largely consisting of calcite or arragonite, is secreted underneath
it by the cells of the thickened edge. The innermost nacreous layer
(also mostly CaCOg) is, however, formed by the cells of the whole of
MOLLUSCA 547
the mantle, and under such conditions as occur in the formation of
pearls this general epithelium is capable of secreting any of the three
shell layers.
In the Mollusca the development of the trochosphere takes place in
a fashion identical with that described for the annelid. In the diagram
given here for Patella, we see the completion of gastrulation and the
appearance of the ciliated rings of the trochosphere (Fig. 374 A); also
the single large cell which gives rise to the mesoderm. Then in
Fig. 374 B we see the early veliger with an internal organization
similar to the annelid, with apical organ, larval nephridia and proto-
troch. The figure shows, however, organs which are not present in
the annelid. On the dorsal side between the prototroch and the anus
the larval ectodermal epithelium forms the rudiment of the mantle
and even at this early age secretes the first shell. On the ventral side,
there is a prominence which is the foot (formed by the union of two
rudiments). The single mesoderm cell gives rise first of all to two
regular mesoderm bands ; and by the development of a cavity in each
of these, right and left coelomic sacs are formed ; then instead of seg-
menting as in the annelid, these largely break up into single cells,
some elongating and becoming muscle cells (Fig. 374 C). It is because
there is never any commencement of segmentation in the embryonic
mesoderm in molluscs that we have the strongest grounds for be-
lieving that molluscs never had segmented ancestors. The trocho-
sphere is followed by a second free-swimming stage, the veliger (Fig.
375), in which the prototroch develops into an organ, the velum, of
increased importance, which serves not only for locomotion but also
for feeding, the cilia creating a current which brings particles into
the mouth. In the veliger stage the foot increases in size and the
shell often becomes coiled in the Gasteropoda.
Class AMPHINEURA
Mollusca with an elongated, bilaterally symmetrical body, the mouth
and anus at opposite ends; with a head, without tentacles or eyes,
tucked under the mantle, which occupies the whole of the dorsal
surface, and contains various kinds of calcareous spicules imbedded
in cuticle, sometimes united to form continuous shells; a flattened
foot sometimes reduced ; a nervous system (Fig. 398 A) without definite
ganglia, the ganglion cells being evenly distributed along the length
of the nerve cords, and composed of a circumoesophageal commissure
and two pairs of longitudinal cords [pedal and palliovisceral), each
pair united by a posterior commissure dorsal to the rectum ; a radula;
usually a trochosphere larva.
PoLYPLACOPHORA. Shore-living amphineura with flat foot which
548 THE INVERTEBRATA
occupies the whole ventral face of the body ; mantle containing eight
transverse calcareous plates as well as spicules ; in the mantle groove
which runs entirely round the body there is a more or less complete
prst,^
prsm. I
nac.
I /ex.ma.
I
-in.ma'
prsm: -^prst:
Fig. 376.
Fig. 377.
Fig. 376. Ventral view of Chiton to show external and internal bilateral sym-
metry. Mantle cavity finely stippled, the divisions of the coelom, shown
above the foot, coarsely stippled, an. anus ; ct. ctenidia ; e.k.a. external kidney
aperture; i.k.a. internal kidney aperture; gen. gonad; g.a. genital aperture;
k. kidney ; M. mouth ; pcm. pericardium.
Fig. 377. Vertical section through the edge of the mantle of Mytilus. ex.ma.
external, in.ma. internal surface of mantle;^/, gland cells ; nac. nacreous layer;
prsm. prismatic layer of shell; prsm.' prisms arising at external border; prst.
periostracum ; prst.' periostracum arising in a fold of the mantle ; ov. ova in
the mantle tissue. After Field.
row of ctenidium-like gills on each side, Chiton (Figs. 373 B, 376),
Craspedochilus .
Aplacophora. Worm-like Amphineura in which the foot is absent
or represented by a median ridge in a ventral groove and the mantle
correspondingly enlarged. No shell plates but spicules only. Mantle
AMPHINEURA 549
cavity perhaps represented by a small cloacal chamber at the posterior
end, gills present (Chaetoderma) or absent (Neomenia).
Craspedochilus is a small mollusc found underneath stones between
tidemarks. It looks like an elongated limpet and has exactly the same
habits, browsing on small algae and returning after excursions to a
centrally situated home. In dorsal view there are seen the eight shell
plates which articulate with one another and allow the animal to roll
up like a woodlouse. Each plate is composed of two layers, the upper
or tegmentum and lower or articulamentum. Both are calcareous, but
the tegmentum is traversed by parallel canals containing ectodermal
tissue which end on the surface in remarkable sense organs ; some of
these have the structure of eyes (the aesthetes). Young individuals,
which possess a full equipment of aesthetes are negatively phototropic.
As, however, the valves become corroded and covered with encrusting
organisms they become indifferent to light. The part of the mantle
which surrounds the shells is called the girdle and this contains the
spicules which are characteristic of the Amphineura as a whole.
On the ventral surface is seen the head, which does not project from
under the shelter of the mantle. It bears no eyes and no tentacles, and
is separated from the foot by a narrow groove. The mantle groove is
shallow, running completely round the animal and containing a vary-
ing number of branchial organs, each of which resembles a ctetiidium.
There may be only six on each side crowded together at the posterior
end, or they may occupy the whole groove from the head to the anus.
It is probable that the forms with a small number of branchiae are the
most primitive, and from the fact that the branchiae are graded in
size it seems likely that one of them (the largest) is the original one
and the others are derived from it. At any rate the repetition of the
branchiae does not mean that the chitons were once metamerically
segmented animals. There is no trace of any segmentation of the
mesoblast in the larva and there is no correspondence between the
numbers of the shell plates and of the branchiae.
The mantle groove also contains the anus in the middle line
posteriorly, on each side, the renal apertures just in front of it, and
the genital apertures a little further forward. In this entire symmetry
of the various apertures the chitons differ from any living gasteropods.
The internal anatomy presents the features attributed above to the
ancestral molluscs. Another feature which is probably primitive is
the uniform distribution of nerve cells in the nerve cords and the
consequent absence of ganglionic enlargements. The cords are con-
nected by many commissures which form a nerve plexus (Fig. 398 A).
A point of great interest is the palaeontological antiquity of the
group, forms with eight shell valves occurring in the Ordovician.
The Aplacophora are simplified forms, often worm-like in appear-
550 THE INVERTEBRATA
ance. Besides the primitive character found in the chitons they have
others, one of which is the free communication of the parts of the
coelom. The gonads open into the pericardium and a pair of coelomo-
ducts (probably corresponding to the kidneys) convey the gametes
from the pericardium to the exterior. The radula varies greatly, all
stages from absence to a type with several transverse rows of teeth
being found. This condition may also be considered as primitive.
Class GASTEROPODA
Mollusca with a distinct head bearing tentacles and eyes, a flattened
foot, and a visceral hump which exhibits the phenomenon of torsion
in various degrees and is often coiled; always exhibiting bilateral
asymmetry to a certain extent; typically with a shell secreted in a
single piece; nervous system with cerebral, pleural, visceral and
usually pedal ganglia and a visceral loop ; a radula ; often a trocho-
sphere larva.
We can safely say that the Gasteropoda are descended from sym-
metrical unsegmented ancestors (p. 543), and that the most prominent
differences among their present-day representatives are due to the
varying degrees in which they exhibit the phenomena of torsion. The
ancestors of the Gasteropoda had not been affected by torsion. They
possessed a symmetrical body with a straight alimentary canal ending
in a posterior anus. On each side of this was a ctenidium, that is, a
breathing organ composed of an axis with a row of leaf-like branches
on each side. The ctenidia may have been free on the surface when
they first arose, but they were soon contained in the posterior mantle
cavity which developed with the visceral hump.
Many characters belonging to the primitive mollusc are still pre-
served in the gasteropods, the head with tentacles, the nervous system
with cerebral, pleural, and pedal ganglia, the radula, the ventricle
with two auricles and the two kidneys. Lastly, there is a flat creeping
foot and a visceral loop formed by a connective from each pleural
ganglion uniting with its fellow in the neighbourhood of the ctenidia.
In the alimentary canal of molluscs there is a tendency for digestion
and resorption to be confined to a dorsal diverticulum of the alimen-
tary canal which develops into the digestive gland (liver). The growth
of this causes the formation of a projection, the visceral hump, and a
looping of the alimentary canal. This projection grows until it falls
over, and this is the first step in the coiling of the visceral hump which
is such a characteristic feature of the gasteropods. Growth proceeds
until, in the snail, for instance, the visceral hump would, if uncoiled,
be longer than the whole of the body. Owing, however, to the fact
that one side of the hump grows faster during development than the
MOLLUSCA 551
Other, the whole organ is twisted into a compact spiral which can be
arranged so as not to interfere with the balance of the animal while
crawling.
In all gasteropods with coiled shells the mantle cavity is anterior,
the opening directed forward and the coiling of the visceral hump is
directed posteriorly. But in the development of these forms from the
larva (Fig. 378) the mantle cavity first makes its appearance behind
the visceral hump, and at a particular stage the visceral hump rotates
in a counter-clockwise direction through an angle of 180° on the rest
of the body (Fig. 378 D). This is what is known as torsion, and as
shown above it is entirely distinct from the coiling of the visceral
hump which precedes it, though it may have been necessitated by the
Fig. 378. To show torsion in Paludina vivipara. After Naef. Embryos seen
from the side (A, B, D) and behind (C). A, Almost symmetrical stage, with
mantle cavity behind but anus twisted a little to the right. B, Stage showing
90° of torsion, mantle cavity and anus to the right. C, Torsion at almost the
same stage as B. D, Stage showing i8o° of torsion and the adult condition.
an. anus; m.c. mantle cavity; op. operculum; vm. velum.
antecedent phenomenon. Only the narrow neck of tissue (and the
organs which pass through it), between the visceral hump and the
rest of the body, is actually twisted ; but the orientation of the mantle
cavity and its organs is changed (Fig. 379). Before torsion the
ctenidia and the anus point backwards, the auricles are behind the
ventricle. After torsion the ctenidia project forward, the auricles are
in front of the ventricle; the mantle cavity opens just behind the head.
The uncoiled visceral loop has been caught in the twisting and one
connective laid over the other, one passing over the intestine and the
other underneath, but both coming together near the anus and com-
pleting a figure of eight. The whole process takes only two or three
552
THE INVERTEBRATA
minutes in Acmaea so that it can hardly be brought about by differ-
ential growth. Muscular contractions must play their part.
The large majority of gasteropods belong to the order which ex-
hibits torsion in full development. It is called Prosobranchiata,
because of the anterior position of the gills, or Streptoneura, because
of the coiled visceral loop. The periwinkles, whelks and limpets of
our shores, the freshwater Paludina, and many others belong to it.
The order may, however, be divided into two groups, a primitive one
in which the two ctenidia and consequently the two auricles are pre-
served {Diotocardia represented by Patella, Fissurella and Haliotis)
(Fig. 380 A-C), and a more specialized one in which the right (primi-
tive left) gill, its auricle and even the right kidney have disappeared
M.
max.
Fig. 379. Diagram to illustrate torsion, when seen from above. A, Ancestral
gasteropod. B, 90° of torsion. C, Torsion completed (180°). After Naef.
an. anus; au. auricle; ce.g. cerebral ganglion; M. mouth; max. mantle cavity;
pa.g. parietal, ped.g. pedal, pl.g. pleural, vis.g. visceral ganglia ; v. ventricle.
{Monotocardia, represented by Littorina, the periwinkle, and Buc-
cinumy the whelk) (Fig. 380 D). Some of the Diotocardia, like Trochus,
are in an intermediate state in which, though the right gill has dis-
appeared, there is still a rudiment of the corresponding auricle. Be-
sides this fundamental difference, there are others. For example, in
the Monotocardia, special generative ducts are developed (cp. also the
penis of the male Buccinum), while in the Diotocardia, the generative
organs open to the exterior through the right kidney.
It is possible that the disappearance of the organs of one side is to
be regarded as the consequence of the processes concerned in torsion
and that in the Diotocardia the phenomenon cannot be regarded as
having reached its climax. On the other hand, there is a large division
MOLLUSCA
553
of gasteropods called the Opisthobranchiata which show that the
changes occurring in torsion are to a certain extent reversible. They
Fig. 380. Mantle cavities of streptoneurous gasteropods. A, Patella,
ctenidia absent. B, Fissurella, ctenidia equal ; kidneys unequal, like those of
Patella. C, Haliotis, right ctenidium smaller; ciliary currents shown by
arrows, exhalant shown emerging from the three most recently formed holes
in the shell. D, Biiccinum, male, with single set of pallial organs, an. anus ;
au. auricle; ct. ctenidium; e. eye; exh.c. exhalant current; gon. gonad; inh.c.
inhalant current ; k.l. left, and k.r. right, kidney ; M. mouth ; m.s. shell muscle ;
mu.gl. mucous glands ; op.s. opening from mantle cavity through shell ;
osp. osphradium ; op.g.k. opening from gonad into kidney ; p. penis ; pb. pro-
boscis; p.c.m. pericardium; rhyn. rhynchocoele ; siph. siphon; ten. tentacles;
vd. vas deferens ; ven. ventricle ; int. intestine ; Sop. male aperture ; rm. rectum.
have the ctenidium pointing backwards, the auricle behind the
ventricle and the visceral loop untwisted and symmetrical. There are
554 THE INVERTEBRATA
some forms (Bullomorpha (Fig. 387 C, D)) included in the Opistho-
branchiata which possess a complete coiled shell, but show only 90°
of torsion, so that the anus and the ctenidium point laterally instead
of anteriorly. The visceral loop also shows untwisting and the forms
in this division are thus supposed to show partial reversion of torsion
or detorsion. Forms like this pass into the typical opisthobranchs
with complete detorsion, in which the shell is reduced or lost, the
ctenidium directed posteriorly and the visceral loop is completely
untwisted {Aplysia (Fig. 388 A)). The Opisthobranchiata, it is plainly
seen, are derived from the Monotocardia amongst the Streptoneura,
since they have only a single ctenidium, a single auricle and a single
kidney. They have not attained to complete bilateral symmetry, be-
cause the mantle cavity is still on the right side where yet present
(tectibranchs), and the anus and genital aperture both open there.
The disappearance of the shell and the consequent uncoiling of the
visceral hump, if not the cause of detorsion, is a constant accompani-
ment of the phenomenon. When it is complete, the mantle cavity and
even the ctenidium may disappear and we arrive at the group known
as the Nudibranchiata. In forms like Eolis (Fig. 388 C) their descent
is shown by the fact that they possess a veliger larva with a coiled
visceral hump which undergoes torsion (which reverses later). The
adult shows evidence of streptoneurous ancestry in the presence of
the anus at the right-hand side. In Doris (Fig. 388 B) the anus and
renal aperture are median, but the genital aperture is still situated on
the right side.
The last division of the Gasteropoda is the Pulmonata, which is
usually united with the Opisthobranchiata to form the group Euthy-
neura. But "euthyneury " or symmetry of the nervous system (more
particularly the "visceral" part of it) is arrived at in different ways
in the two divisions. In the Opisthobranchiata, as shown above, it is
by detorsion. In the Pulmonata, however, the shell is retained and
the visceral hump coiled in typical members of the group (land snails).
But the visceral loop is shortened and untwisted at the same time
(Fig. 387 A, B), and finally it is incorporated with its ganglia into the
circumoesophageal nerve collar, so that the nervous system becomes
symmetrical. The most primitive members of the Pulmonata still show
a twisted visceral loop which is beginning to shorten. All the group
have lost the ctenidium but they retain the single auricle which shows
them to be derived from the Monotocardia. This was brought about
by a chain of circumstances involving migration from sea to shore.
The type of the Gasteropoda which is usually given for dissection
is Helix (either H. aspersa, the common English garden snail, or
H. pomatia, the edible snail). It possesses many features which are
common to the whole of the Gasteropoda, but as has been seen above,
GASTEROPODA 555
the order Pulmonata to which Helix belongs is the most speciaHzed
and probably the latest developed division. Helix is a terrestrial
animal breathing by a kind of lung, while the majority of gasteropods
are marine animals breathing by gills, and besides the complications
which this involves, the reproductive system is hermaphrodite with
the most elaborate provision of glands and ducts which serve to
produce eggs well stored with nourishment and are arranged so as
to assure cross-fertilization. In the account of Helix which follows
an attempt is made to distinguish clearly between the purely gastero-
pod features and the adaptive features which belong to the Pulmonata.
The body of a snail is composed of three regions, the head, foot and
visceral hump. The visceral hump is all that part which is covered by
the shell when the animal is expanded, while the head and the foot
make up the remainder outside the shell. There is no boundary be-
tween the latter two regions. The German zoologists refer to the
whole as the " Kopffuss " (the " head foot "), and this can be retracted
as a whole within the shell by the action of the columella muscle (Fig.
381). The foot is particularly characteristic of the Gasteropoda. It
possesses a flat ventral surface underlain by longitudinal muscle fibres.
If a snail is observed crawling up a pane of glass, a series of rippling
waves of contraction of very small amplitude are seen to pass regularly
over the surface of the foot. They are co-ordinated by the action of a
nervous network, such as occurs in the lower invertebrates (Fig. 1 10).
The gliding movement of a snail indeed resembles that of a turbel-
larian, and we actually find that in some marine gasteropods the
surface of the foot is clothed with cilia, which beat in unison, though
they are perhaps capable of inhibition by the central nervous system.
In most water snails, however, the foot moves by muscular contrac-
tion. To fit this kind of movement for passing over a hard dry surface,
there is in the snail a copious secretion of slime from a pedal (mucous)
gland which runs dorsal to the foot and opens just ventral to the
mouth. As soon as the slime emerges it is spread out as a smooth bed
of lubricating fluid along which the snail moves.
There are two pairs of tentacles on the head of the snail. The first
are shorter and are supposed to be the seat of the sense of smell ; the
second bear a pair of simple eyes (Fig. 409 B) at their tip. Both are
hollow and have attached to the inside of the tip a muscle whose con-
traction turns them outside in. The mouth is a transverse slit just
ventral to the first pair of tentacles. On the right side of the body not
far below and behind the second pair of tentacles is the reproductive
aperture. On removing the shell, the junction of the visceral hump
with the rest of the body is seen anteriorly as a thickened collar which
is the edge of the mantle and the seat of secretion of the principal
layers of the shell. It is fused to the head of the snail except for a
556 THE INVERTEBRATA
round hole on the right side which is the aperture of the mantle cavity
or pneumostome. In the marine gasteropods the mantle cavity has a
wide opening to the exterior, though a part of the mantle border
(siphon) is modified to form a special channel by which fresh water for
breathing may be drawn in by the action of the cilia clothing the gill.
But in the air-breathing pulmonates where the cavity is converted
into a lung, the injury of delicate respiratory tissues by evaporation
must be avoided, and a pumping mechanism for renewal of air
established. The restriction of the respiratory aperture is one of the
necessary modifications. If a section is drawn across the lung of a
snail it will be seen that the mantle forms the roof of the cavity and
is covered with ridges in which run pulmonary veins converging to-
wards the auricle. The floor of the cavity is arched and has a layer of
muscles, which contract rhythmically. When they contract, the arch
flattens and air is drawn in and at the limit of contraction a valve slides
across the pneumostome. When the muscles relax, the cavity de-
creases in size and exchange of gases with the blood in the roof vessels
is facilitated by the increase of pressure of the contained air. Then the
pneumostome opens and air is expelled ; the subsequent contraction
of the floor muscles brings in a fresh supply. This "breathing " is not
so regular or so frequent as in a vertebrate ; moreover, it may cease
altogether in the winter when the snail hibernates.
In dissection, a cut is made underneath the collar and another
under the rectum and the roof of the mantle cavity turned back
so as to show the pericardium enclosing the ventricle and single
auricle, and the kidney, which is a yellow organ consisting of a
number of folds covered by cells containing uric acid. The ureter is
a thin-walled tube which runs along the right border of the mantle
cavity parallel to the rectum and opens just behind the pneumo-
stome and above the anus. Here again is a diff'erence from the marine
gasteropods in which the anus and kidney aperture discharge inside
the mantle cavity, faeces and urine being swept away by the respira-
tory current. The pericardium and the kidney represent the coelom
in the snail and, as is usual in Mollusca, their common derivation is
shown by the connection of the cavities by the renopericardial canal.
The coelom, though thus represented, does not constitute the peri-
visceral cavity. On cutting the floor of the mantle cavity and con-
tinuing the cut forward towards the mouth a large body cavity is
revealed which contains the anterior part of the alimentary canal and
the greater part of the reproductive organs. This is a haemocoele
almost as well developed as that of arthropods. Its connection with
the rest of the blood system and the general course of the circulation
may be briefly described here as follows : the ventricle pumps arterial
blood through a single aorta which soon divides into an anterior aorta
GASTEROPODA
557
supplying the buccal cavity and a posterior which supplies the visceral
hump. The terminal branches of these arteries eventually communi-
cate with the general haemocoele (stippled in Fig. 381) and this
discharges into the cir cuius venosus leading to the lung and heart.
The alimentary canal (Fig. 382) commences with the buccal mass.
On the roof of the mouth is a small transverse bar, the jaw, and in
conjunction with this works the radula, which is a strip of horny base-
ment membrane on which are fastened many rows of minute recurved
ped.art
haem.
Fig. 381. Helix pomatia. Diagram of the circulation and haemocoelic spaces.
The pulmonary veins, ventricle and arteries are shown in black; the veins
and haemocoelic spaces are indicated by stippling. Only a few of the arteries
are shown and a small portion of the arterial capillary network in the posterior
part of the foot. The course of the columella muscle and its branches is
indicated. The direction of the blood flow is shown by arrows, aff.v. afferent
veins; ao. aorta; art.cap. arterial capillaries; au. auricle; buc. buccal mass;
col. columella, col.m. columella muscle; cr. crop; c.v. circulus venosus; haem.
haemocoelic spaces; k. kidney; n.col. nerve collar; ped.art. pedal artery;
pul.v. pulmonary veins ; ten. tentacles ; ven. ventricle.
teeth. It is formed in a ventral diverticulum of the buccal cavity called
the radula sac (Fig. 383) in which proliferating tissue is constantly
producing transverse rows of cells called odontoblasts, each of which
helps to form a tooth, and other cells which secrete the basement
membrane. The whole radula is pressed forward by the new growth
so that fresh surfaces are constantly coming into use as the old part
is worn away. The radula is supported by masses of tissue, resembling
cartilage, which also serves for the attachment of muscles, and the
whole forms the rounded organ which is the buccal mass.
558 THE INVERTEBRATA
The buccal cavity is succeeded by the oe^phagiis, which widens
out into the crop, which in Hfe contains a brown hquid secreted by the
'Miver". On the side of the crop are the branching white salivary
glands, which empty their secretion by two ducts running forward
into the buccal cavity. The secretion is partly mucus, partly digestive
fluid containing an enzyme acting on starch. The crop is succeeded
:ped.(i.
muc.gh
Fig. 382. Helix pomatia. A, Section of alveolus of the digestive gland.
ab.c. absorptive cells; cal.c. calcareous cells; cil.c. ciliated cells of liver tube;
f.c. ferment cells. After Meisenheimer. B, Diagrammatic side view of animal
dissected to show the alimentary canal and nervous system. Original. An.
anus; ao. aorta; au. auricle; buc. buccal mass; biic.g. buccal ganglion; ce.g.
cerebral ganglion ; cr. crop ; dig.gl. digestive gland ; d.d. openings of digestive
ducts (the ducts represented by black lines); F. foot; h.gl. hermaphrodite
gland; int. intestine; k. kidney; muc.gl. mucous gland; n.n. nerve net in
surface of foot; oc.ten. oculiferous tentacle; ot. otocyst; pa.g., ped.g., pl.g.
parietal, pedal and pleural ganglia; pal.n. pallial nerve; rad.s. radula sac;
sal.d. salivary duct ; sal.gl. salivary gland ; spt. spermatheca (duct broken off
short) ; st. stomach ; va. valves directing food into digestive ducts ; ven.
ventricle; vis.g., vis.n. visceral ganglia and nerve.
by the stomach; this is imbedded in the digestive gland (liver), which
occupies most of the visceral hump. The ** liver", though apparently
solid, is composed of a number of tubes and the end portion {alveolus)
of each tube is glandular; the rest is ciliated and serves to introduce
small fragments of food into the active alveolus. The alveoli contain
cells of three kinds, secretory, resorptive and lime-containing (Fig.
GASTEROPODA
559
382 A). The secretory cells produce the brown fluid found in the crop ;
this contains a ferment which dissolves the cellulose of plant cell walls
and liberates the protoplasmic contents, no portion of which is di-
gested in the crop or stomach. But these contents in the form of small
granules are actually introduced into the alveoli of the liver and there
taken up and digested by the resorptive cells which possess intra-
odp.'^-
Fig. 383. Vertical longitudinal section through head of iiTe/iA;. After Meisen-
heimer. cart.r. cartilaginous support of radula; ce.g. cerebral ganglion;
j. jaw ; M. mouth ; m.r. muscles of radula ; odp, odontophore (radula sac) ;
oe. oesophagus ; p.g. pedal ganglion ; rad. radula ; v.g. visceral ganglion.
odh.
Fig. 384. Vertical longitudinal section through the radula sac of Helix
pomatia. After Meisenheimer. odb. four rows of odontoblasts secreting a
tooth, to. ; a. the most anterior row of odontoblasts which, together with the
basal epithelium, i.ep., of the radula sac, secrete the basal membrane, b?n., to
which the teeth, to., are attached. As the odontoblasts complete the secretion
of a tooth they are succeeded by fresh cells from the epithelium of the radula
sac, s.ep., pressing fonvard in the direction of the arrow and themselves
reinforce the basal epithelium.
cellular proteolytic enzymes.^ A combination of extra- and intra-
cellular digestion is highly characteristic of Mollusca, but in the
possession of a cellulose-dissolving ferment Helix stands almost alone
^ In carnivorous gasteropods digestion follows a different course. The
glands of the alimentary canal secrete proteolytic enzymes and the digestion
of protein takes place in the stomach and not in the cells of the digestive gland
(see Murex, p. 566).
560 THE INVERTEBRATA
in the Animal Kingdom, and may be indeed said to be physiologically
adapted to a plant diet (cp. Teredo^ p. 587). The intestine runs from
the stomach, within the liver, and then as the rectum in the roof of
the mantle cavity.
The reproductive organs are extremely complicated (Fig. 385 A),
but a function has been assigned to each part of what appears to the
elementary student as an unmeaning tangle of tubes. Eggs and sperm
are produced in the same follicle of the ovotestis, a small white gland
o.d. gph. sptd.
Fig. 385. Helix pomatia. A, Reproductive organs. B, Section through the
copulatory organs of two mating snails at the moment of the transference of
the spermatophores. After Meisenheimer. The organs of the two individuals
are indicated by shading sloping in different directions, al.gl. albumen gland ;
d.s. dart sac; fl. flagellum; m.gl. mucous glands; o.d. oviduct; p. penis;
rec.sem. receptaculum seminis; ret. p. retractor muscle of the penis; sph.
spermatophore ; spt. spermatheca ; spt.d. spermathecal duct ; sp.d. sperm duct ;
o gl. hermaphrodite gland (ovotestis), and ^ d. duct.
in the apex of the visceral hump. But while ripe sperm is found
throughout a large part of the year, mature eggs only occur for a very
short space indeed. Both eggs and sperm pass from the ovotestis to
the albumen gland through the hermaphrodite duct, the terminal
portion of which is a pouch [receptaculum seminis) where sperm is
stored and fertilization is said to occur. After fertilization, the eggs
enveloped in albumen from the gland enter the rather voluminous
female duct, which runs almost straight to the exterior. They then
GASTEROPODA 561
receive a calcareous shell secreted by the epithelium of the duct. The
terminal portion of the duct is the thick-walled muscular vagina, into
which open the mucous glands , the dart sac and the spermathecal duct.
The sperm, on the other hand, passes down a male duct which is at
first only partly separate from the female duct, the cavity of both ducts
being in communication until the male duct leaves the company of
the female duct altogether, slips under a muscle, and joins the penis
at its junction with the slender flagellum. In this latter the sperma-
tozoa are compacted together and enclosed in its secretion to form
spermatophores . The penis is muscular and has a special retractor penis
muscle also attached to it. Both vagina and penis open into a common
genital atrium, with an opening to the exterior far forward on the
right side.
Cross-fertilization is the rule in nearly all species of Helix but cases
of self-fertilization have been known. Usually, however, there is re-
ciprocal fertilization, preceded by a remarkable preparatory event in
which two snails approach each other and evert the genital atrium so
that the male and female apertures appear externally. The dart sac
mentioned above contains a calcareous sculptured weapon, the dart,
which can be secreted anew very quickly by the epithelium of the sac.
This is propelled by the muscles of the sac out of the female aperture
when the other snail is almost in contact — in fact the two darts are
launched almost simultaneously, with such force that they pierce the
body wall, traverse the cavity and are found imbedded in various
internal organs. Some time after this drastic stimulation, the two
snails approach each other again and reciprocal fertilization takes
place, the penis of each individual being inserted in the vagina of the
other (Fig. 385 B). The following account of further events has been
given and shows, as in the earthworm, the remarkable complexity of
the arrangements which are made to prevent self-fertilization in such
common hermaphrodites. The foreign spermatophores find their way
up the spermathecal duct to the terminal spermatheca, where the
chitinous covering of the spermatophore is dissolved, and the sper-
matozoa set free. These now retrace their path to the junction with the
female duct and then move up that duct to the fertilization pouch. Ferti-
lization takes place in May or June but the eggs are not laid till July.
It is said that the foreign sperm remains in the pouch during this
time, and that immediately before ovulation the sperm produced by
the individual itself degenerates within the hermaphrodite duct so
that the eggs pass down the duct without any danger of being self-
fertilized and meet the foreign sperm at the end. After fertilization,
the egg cell passes down the oviduct where it is enveloped with such
quantities of albumen that the diameter of the albumen envelope is
20-30 times that of the egg cell itself. In the outer layer of albumen
562 THE INVERTEBRATA
a skin appears, and in this crystals of calcium salts are laid down
which aggregate to form a definite shell. The eggs are laid in July and
August in small holes in the earth and hatch after about twenty-five
days of development.
In the autumn the snail loses its appetite and hides, often in
company with large numbers of its fellows, under leaves, making a
small hole in the ground with its foot and shell in which it lies with
the aperture upwards. The head and foot are withdrawn into the shell
cent.
Wflr.v>,
lat.
aamaa
cent.
Fig. 386. Radula of various types. A, Docoglossate (Patella). Stout teeth
used for rasping encrusting layer of algae off rocks: radula of relatively
enormous length; the teeth are quickly worn away. B, Rhipidoglossate
{Haliotis). Lateral and central teeth as in Patella, used in browsing on algae
growing on stones. The marginals, of which only about half are shown, are
probably used as a sieve to prevent fragments of food of too great size entering
the oesophagus. C, Rachiglossate (Buccinum). Teeth of carnivorous type,
with sharp cusps. D, Toxiglossate (Conus). Specialization of carnivorous
type, in which only two teeth (laterals) remain in each row, are hollow, and
are used as poisoned daggers, carrying the secretion of the salivary glands.
cent, central, lat. lateral, mg. marginal teeth.
and the edges of the mantle approximate to form an almost complete
disc filling up the aperture, leaving only a small hole for breathing.
They secrete a membrane (epiphragma) mostly composed of Ca3(P04)2 .
Several such membranes may be found behind each other. In this
winter sleep the snail remains for about six months ; respiratory move-
ments are carried on slowly and the heart beats sink from about
10-13 to 4-6 per minute. The rate of heart beat is closely dependent
on the temperature, and at a temperature of 30° C. is from 50 to 60
beats per minute.
GASTEROPODA 563
Order STREPTONEURA (PROSOBRANCHIATA)
Gasteropoda which exhibit torsion, nearly always with a shell and an
operculum, with a visceral loop twisted in the form of a figure
of eight, the mantle cavity opening anteriorly, the ctenidia in
front of the heart, and separate sexes.
Classification
Suborder Diotocardia (Aspidobranchiata). Strep toneura always
with two auricles and sometimes two ctenidia, the ctenidia
with two rows of leaflets (aspidobranch), and the genital
products discharging to the exterior by means of the right
kidney.
These are divided into two main tribes according to the
characters of the radula :
Rhipidoglossa possessing a radula composed of rows of
numerous narrow teeth diverging like the ribs of a fan.
Haliotis^ Fissurella.
DocoGLOSSA possessing a radula with rows consisting each of a
few strong teeth, very long and used for browsing on the
algal covering of stones. Patella^ Acmaea.
Suborder Monotocardia (Pectinibranchiata). Streptoneura
with a single auricle and ctenidium, the ctenidium always with
one row of leaflets (pectinibranch), with a single osphradium
resembling an aspidobranch gill, the gonads with separate
ducts opening far forward in the mantle cavity and in the
male forming a penis.
These are divided into four tribes, each with a distinct type
of radula, of which three are mentioned below:
Rachiglossa: predatory animals; radula with not more than
three teeth in a row; always with a siphon. Buccinurrij
the whelk. Purpura feeds largely on barnacles. Nassa.
Taenioglossa : radula normally with seven teeth in each row.
Natica feeds on shell fish. Littorina, the periwinkle,
amphibious. S trombus progresses by leaping. Paludiha and
Ampullaria, fresh water. -
This tribe also includes a pelagic section, the Heteropoda
(Pterotrachea). The rest are called the Platypoda.
Toxiglossa: radula with two elongated teeth in each row; a
poison gland. Conus (Fig. 386 D).
564
THE INVERTEBRATA
Suborder DIOTOCARDIA
Haliotis, the ormer (Figs. 380 C, 390 A), is a greatly flattened gastero-
pod which lives between tidemarks, as far north as the Channel
Islands, browsing on seaweed and eating all kinds of dead organic
material. It can move with considerable speed (5-6 yards a minute),
Fig- 387. To illustrate origin of euthyneury in the Pulmonata, A, B, and the
Opisthobranchiata, C, D. After Naef. A, Chilina. The left parietal ganglion
(Lpa.g.) has moved forward owing to the shortening of its plural connective.
B, A pulmonate belonging to the Basommatophora. The corresponding con-
nective on the other side has shortened also, the visceral loop has become
untwisted and the nerve ganglia are concentrating. C, Actaeon, with short
spire and broad shell mouth, ctenidium and anus pointing to the right.
D, Bulla, showing slightly greater detorsion without spire, the shell mouth
opening to the right and anus pointing posteriorly: left parietal ganglion
drawn over right connective so that visceral loop is untwisted, an. anus;
au. auricle; ct. ctenidium; Lpa.g. left, r.pa.g. right parietal ganglion; ma.c.
mantle cavity; v. ventricle.
but adheres very firmly to stones. The mantle cavity is very spacious
and contains two ctenidia, the left being rather the larger, each with two
rows of filaments. The mantle has a slit which runs in the roof of the
mantle cavity, its position being shown by a row of holes in the shell
which serve for the escape of the exhalant current. The anus opens
GASTEROPODA 565
at the posterior end of the mantle cavity and the two kidneys on each
side of the anus. There is a well-marked visceral loop and the pedal
nerve centres have the form of long cords in which ganglion cells are
evenly distributed. The gonad has no ducts but the genital cells are
discharged into the right kidney. The radula has numerous marginal
teeth arranged in a fan-like manner (rhipidoglossate type).
Fissurella, the keyhole limpet (Fig. 380 B), is so-called because of
the hole which perforates the mantle and the apex of the shell. It
possesses two equal ctenidia. The visceral hump and shell are com-
pletely uncoiled, but in other respects it resembles Haliotis and
possesses the same type of radula.
Patella, the limpet (Fig. 380 A), represents a type of complete
adaptation to life on an exposed coast between tidemarks. Its conical
shell only shows coiling in its early stages and offers the minimum of
resistance to the waves. As in the above forms there is no operculum,
but the mollusc cannot be detached from rocks without using great
force, owing to the enormous power of the pallial muscles which press
the shell against the rock. The mantle cavity is restricted anteriorly
and the ctenidia have disappeared, though the osphradia connected
with them are present as minute yellow specks. But a secondary
mantle cavity extends all round between the foot and the mantle and
contains a series of folds which are known as pallial gills . In the re-
lated Acmaeidae there are various stages of the loss of the ctenidia
and their replacement by pallial gills. The enormously elongated
radula is composed of very strong teeth and there are a small number
of marginals (docoglossate type). This type of radula is suited for the
feeding habits of the limpet, which scrapes the crust of minute algae
off the surface of rocks. Limpets have a remarkable "homing " sense,
returning after excursions for food to the same spot, which may be
marked by a depression in the rock.
Suborder MONOTOCARDIA
Biiccinum, the whelk (Fig. 380 D), lives between low-water mark and
100 fathoms. It is active and carnivorous, feeding on living and dead
animals, which it grasps by means of its foot. It has a remarkable
and highly developed proboscis which can be retracted within a
proboscis sheath. The true mouth is situated at the end of the pro-
boscis. The radula (of the rachiglossate type) is used for rasping
away flesh, but it can even bore holes in the carapace of Crustacea.
There is only a single ctenidium with a single row of filaments.
This is the primitive right member of the pair, though situated on
the left of the mantle cavity. A very prominent organ is the bipectin-
ate osphradium, which is easily mistaken for a ctenidium. There is a
566 THE INVERTEBRATA
single kidney which is not used for the passage of the genital products.
The gonads have separate ducts and in the male there is a penis.
The eggs are laid in capsules which usually contain several hundred
and the capsules are attached to each other, forming the sponge-like
masses so often flung up by the tide.
Murex is nearly related to Buccinum and also carnivorous. It has
been recently shown that the salivary glands and the "liver" all
contain the same proteolytic enzymes. These have been separated
by adsorption and found to comprise a proteinase, a carboxy-poly-
peptidase, an aminopolypeptidase and a dipeptidase. These are just
such enzymes as occur in the vertebrates and the higher Crustacea,
but in contrast to vertebrates in Murex there is no division of labour
amongst the digestive organs.
Fig. 388. Pterotrachea. co.g. cerebral ganglion; cr. crop; ct. ctenidium;
e. eye; /. foot (fin); ped.g. pedal ganglion; su. sucker; t. teeth which prevent
large particles of food from passing before digestion ; vis.g. visceral ganglion ;
vis.h. visceral hump.
LittorinUy the periwinkle, is interesting because it exhibits tend-
encies toward a terrestrial habit which is reflected in its structure. In
certain species the filaments of the ctenidium are extended over the
roof of the mantle cavity to form a kind of vascular network not un-
like that in Helix and other pulmonates. Littorina rudis lives almost
at highwater mark and spends more of its life in air than in water.
The structure of this form is very similar to Buccinum but it has
no proboscis and is not carnivorous.
Paludina, on the other hand, is a freshwater form of common
occurrence in this country which still preserves the ctenidium and so
must be regarded as a direct immigrant from sea water into fresh
water. It possesses a kind of uterus in which embryos of relatively
enormous size are developed.
Pterotrachea (Heteropoda) (Fig. 388) is an inhabitant of the open
sea with many adaptations to pelagic life. It is laterally compressed ;
the tissues are transparent except for the digestive gland and peri-
GASTEROPODA 567
cardium compressed into a small visceral hump. The animal swims
ventral surface uppermost, using its foot as a fin. The sucker is a
rudiment of the crawling surface. It is predaceous, seizing worms
and other animals with its radula and swallowing them whole.
Order OPISTHOBRANCHI ATA
Hermaphrodite gasteropods which are descended from Streptoneura
which have undergone torsion but themselves show a reversal of
torsion (detorsion); with the mantle cavity, where present, tending to
occupy a posterior position again, the shell to become smaller, in-
ternal or entirely absent and the single ctenidium to disappear and
be replaced by accessory respiratory organs or by the whole external
surface becoming a respiratory organ.
The opisthobranchs are classified as follows:
Tectibranchiata. Opisthobranchiata which often have a shell
and nearly always a mantle cavity and ctenidium. Actaeon, Bullae
Aplysia, Cavolinia.
NuDiBRANCHiATA. Opisthobranchiata usually of slug-like habit
which have neither a shell, nor a mantle cavity, nor a ctenidium.
EoliSy Doris.
Aplysia (Fig. 389 A), the sea hare, is found crawling on seaweeds
which form its food. The younger forms occur in rather deeper water
and are red in colour, matching the red algae on which they occur,
while the larger individuals, between tidemarks, devour green sea-
weeds such as Ulva and are olive-green. The head possesses two pairs
of tentacles, the anterior being large and ear-like (hence the animal's
name), while those of the second pair are olfactory in function and
have each a simple eye at their base. From the sides of the foot in
the posterior region rise two upwardly directed flaps, the parapodia :
by using these the animal can swim. The mantle is reflected over the
shell so as to cover all except a small area and the mantle cavity lies to
the right of this with the ctenidium pointing backwards, while the
anus is at the posterior end. In the walls of the mantle cavity are
unicellular glands which secrete the purple pigment ejected by the
animal when it is molested. There is a single generative aperture and
a single duct for the sperm and ova but a seminal groove runs forward
from the aperture to the head and reciprocal fertilization is impossible.
The only internal characters which need be mentioned are the
nervous system, with its well-developed but perfectly symmetrical
visceral loop, and the alimentary canal which, in front of the stomach,
is dilated into a crop, lined with horny plates, in which the seaweed
is masticated before digestion.
^^-pten.
cer.--
Fig- 389. Opisthobranchiate molluscs, in dorsal view. A, Aplysia, with
parapodia {par.) turned to the side to show the mantle cavity; nervous system
and buccal mass indicated by dotted lines. B, Doris, with position of heart
indicated beneath the mantle. C, Eolis. After Alder and Hancock. C, One
of the cerata of Eolis, shown in section, c.c. ciliated canal communicating
with hcp.c. the hepatic caecum, a diverticulum of the intestine; c.s. cnidosac,
opening to the exterior and containing numerous nematocysts ingested in its
cells. D, Cavolinia with alimentary canal (a/.) seen through the transparent
tissues and the direction of the ciliated currents on the epipodia indicated
by arrows. After Yonge. Other letters: a/, alimentary canal; an. anus; an.
auricle ; ce.g. cerebral ganglion ; cer. cerata ; ct. ctenidium ; epi. epipodia ;
2 op. genital aperture; op.s. opening of shell sac; p. penis; pa.g. parietal,
pl.g. pleural, ped.g. pedal ganglia; par. parapodia; sem.gr. seminal groove;
ten. tentacles; vis.g. visceral ganglion; M. mouth; ven. ventricle.
GASTEROPODA 569
Cavolinia (Fig. 384 D) is an example of the Pteropoda (sea butter-
flies), a special group of the Opisthobranchiata which are modified for
pelagic life. They usually have a transparent uncoiled shell in the
form of a quiver or a vase, from the aperture of which projects the
foot in the form of two fins, the epipodia. By the slow flapping move-
ment of these the pteropods progress through the water. There are
ciliated tracts on the fins, and by the action of the cilia on these, small
organisms are sifted from the water and collected in the mouth, the
radula assisting in swallowing. Limacina is a pteropod with a coiled
shell.
Eolis (Fig. 384 C) is a nudibranch which possesses a series of dorsal
processes (the cerata)^ which contain diverticula of the digestive gland,
each of w'hich opens to the exterior at the tip of the process. The
animal feeds on hydroids or sea anemones, and while most of the food
is digested or passes out of the anus, the nematocysts are collected in
terminal sacs in the cerata and when the animal is irritated they are
ejected and everted. This is a unique example of the use in defence
by one animal of the oflPensive weapons of another. The cerata are
often brilliantly coloured and experiments with fish show that sea
slugs are avoided on account of their "warning" patterns.
Hermaea is another nudibranch with similar cerata, which have not,
however, openings to the exterior. The animal feeds on green algae
(Siphonales). The radula, in each row of which there is only a single
sharp tooth, forms a saw by which the cell wall of the alga is opened.
Then by dilatation of the buccal cavity the fluid protoplasm is sucked
out.
Doris (Fig. 384 B), the sea lemon, a short flattened nudibranch,
sluggish in movement, which feeds on incrusting organisms like
sponges. There is a tough mantle, which is usually pigmented and
often resembles the feeding ground, and is reinforced by calcareous
spicules. Anteriorly there is a single pair of short tentacles and
posteriorly a median anus surrounded by a tuft of accessory gills. In
front of the anus is the median kidney aperture. The nervous system
is centralized round the oesophagus, and the generative aperture
occurring on the right side is the only external organ which is
asymmetrical.
Order PULMONATA
Hermaphrodite gasteropods, most of which exhibit torsion and have
a shell (but no operculum), but which have a symmetrical nervous
system, the symmetry being due to the shortening of the visceral
connectives and the concentration of the ganglia in the circum-
oesophageal mass; with a mantle cavity which has become a lung,
without a ctenidium, but with a vascular roof and a small aperture
570 THE INVERTEBRATA
(pneumostome) ; with a single kidney; without a larva, development
being direct from an egg richly supplied with albumen.
The Pulmonata are thus classified :
Basommatophora. Pulmonata with eyes at the base of the posterior
tentacles. Limnaea, Planorbis.
Stylommatophora. Pulmonata with eyes at the tip of the posterior
tentacles. Helix, Arton, Testacella.
A few members of the Basommatophora are marine but these are
all shore forms and breathe air. The group, like the Opisthobranchiata,
must have been derived from the Streptoneura Monotocardia, as they
possess a single kidney. While they are usually united with the
Opisthobranchiata to form the Euthyneura, which includes all forms
in which the visceral loop is untwisted, there is no real justification
for the establishment of the group, for the " euthyneurous " condition
is one which has been arrived at in two different ways, by detorsion
in the Opisthobranchiata and by shortening of the visceral com-
missures in the Pulmonata. The important characters of the Pul-
monata are those associated with the assumption of the terrestrial
habit, namely the existence of the lung and the physiological cha-
racters correlated therewith. So strongly impressed are these that in
almost all the forms which have secondarily returned to water (to
fresh water as a rule), the lung continues to function as such and
never contains water. Limnaea^ for example, may be observed in an
aquarium to approach the surface of the water at frequent intervals,
expel a bubble of air from the lung and protrude the pneumostome
through the surface film for a fresh supply. There are, however, a
few species {Limnaea abyssalis) which live at great depths in lakes,
and here the mantle cavity is full of water.
The other general characters of a pulmonate have been given at
the beginning of the chapter in the description of Helix. They include
the concentrated nervous system (it will be seen in Fig. 390 B that
the visceral loop of Limnaea is not so much shortened as that of Helix ;
in other respects also it is a more primitive form), the complicated re-
productive system, with its adaptations for cross-fertilization, and the
digestive tract, specialized for the consumption of vegetable food.
Helix, as has been seen, is thoroughly adapted for this purpose, but
in the case of some of the slugs there is an exception to the general
rule in the development of the carnivorous habit. This culminates in
such a form as the predaceous Testacella, which pursues earthworms
underground and seizes them with the aid of the strong recurved
teeth of the radula which can be thrust out of the mouth, the everted
buccal cavity forming a huge proboscis. When the worm is swal-
ce.g.
vis.g-
ped.g. ce.g
os.g.
Fig. 390. Comparison of gasteropod nervous systems. From Shipley and
MacBride. A, Haliotis tuberculata. B, Limnaeaperegra. ce.^. cerebral ganglia ;
ct. ctenidium; ma.n. nerve to mantle; os.g. osphradial ganglia; pa.g., ped.g.,
pl.g. parietal, pedal and pleural ganglia ; ped.n. non-ganglionated pedal nerves
of Haliotis connected by commissures ; vis.g. visceral ganglia.
572 THE INVERTEBRATA
lowed it is digested in a large crop by the action of the juices of the
digestive gland.
The reduction of the shell is shown in the slugs, some of which,
like Testacella, have a small cap-like shell, which cannot possibly con-
tain the visceral hump, while others have an internal horny disc like
the shell of Aplysia and still others none at all. The mantle cavity of
slugs opens by a pneumostome but there are no respiratory move-
ments as in Helix. In other respects the organization of the slugs is
very similar to that of snails.
The details of reproduction and development are uniform through-
out the group, but in some snails like Bulimus, the amount of albumen
added as food for the developing embryo is so great that the egg is
the size of a bantam's tgg.
Class SCAPHOPODA
Bilaterally symmetrical Mollusca with a tubular shell open at both
ends, a reduced foot used for burrowing, a head with many pre-
hensile processes, a radula, separate cerebral and pleural ganglia;
ctenidia absent and circulatory system rudimentary; and a trocho-
sphere larva.
This is a small group of molluscs which in some ways stands be-
tween the Gasteropoda and the Lamellibranchiata. They are greatly
specialized for burrowing. Thus the shell is tubular and perforated
at the apex. The foot emerges from the wider opening, while the apex
remains above the surface of the sand when the animal is burrowing,
and serves alike for the entrance of water into and its exit from the
mantle cavity. The head is proboscis-like in form and has none of the
usual sense organs, but in Dentalium (Fig. 391), the one common
genus, there are extensible filaments, the captacula, with sucker-like
ends, which arise from the dorsal side of the head and serve partly
as sense organs and partly for seizing the food. The foot is conical
and can be protruded for use as a digging organ.
There is a well-developed radula, a mantle, which in the larva is
produced into two lobes (which fuse later), a nervous system with
separate cerebral and pleural ganglia and a symmetrical visceral loop.
The kidneys are paired; they do not have an opening into the peri-
visceral coelom. These characters, with the exception of the first and
last, bring the Scaphopoda near to the primitive lamellibranch. In
the two following morphological features the group is so specialized
that it stands apart from any other division of the Mollusca.
There are no ctenidia, respiration taking place by means of the
m?ntle. The circulatory system is remarkably simplified and there is
no distinct heart.
MOLLUSCA 573
The gonad discharges into the right kidney as in the Diotocardia
among Gasteropoda.
Fig. 391. Diagram of the structure of Z)ew^a/m?«. Ahered from Naef. Head
and foot stippled, an. anus; buc. buccal mass, with radula; ce.g. cerebral
ganglion; eta. captacula; dig.gl. digestive gland; F. foot; gon. gonad, com-
municating with the cavity of the left kidney {k.) ; M. mouth ; yyia. mantle ;
max. mantle cavity; pl.g. pleural ganglion; ped.g. pedal ganglion; sh. shell;
St. stomach; vis.g. visceral ganglion. The mollusc is represented buried in
sand, except for the perforated narrow end, through which both the inhalant
and exhalant currents (shown by arrows) flow.
Class LAMELLIBRANCHIATA
Mollusca in which typically the body is bilaterally symmetrical, much
compressed from side to side and completely enveloped by the mantle
which is divided into two equal lobes ; each lobe secretes a shell valve,
the two valves being joined dorsally by a ligament and hhige and closed
ventrally by the contraction of one or two transverse adductor muscles
574 THE INVERTEBRATA
the head is rudimentary, eyes, tentacles and radula being absent ; there
is a pair of labial palps with the mouth situated between them ; the
foot is ventral, without a crawling surface but usually wedge-shaped
and adapted for progression in mud or sand ; there are two ctenidia
in the mantle cavity, often greatly enlarged and with a complicated
structure; their cilia, together with those of the labial palps, form
a mechanism for the collection of small food particles ; the sexes are
nearly always separate, and there is a trochosphere and a veliger larva
in the marine forms.
The development of the ctenidia (Fig. 392) is the outstanding
morphological and physiological character of the lamellibranchs. The
arrangement of the shell valves, which allows the mantle cavity to
extend the whole length of the body, also makes possible a great ex-
tension of the ctenidia. The axis increases in length and the branches
on each side not only increase in length, h&covmng filaments ^ but also
turn up at the ends so that there is a descending and an ascending limb.
The limbs of adjacent filaments are connected together by ciliary
junctions (Mytilus), or by growth of tissue (Anodonta), so that thus all
the filaments are joined together to form gill plates, each gill plate
consisting of two lamellae formed from all the ascending and all the
descending limbs respectively. The lamellae are united by cords of
tissue which constitute the interlamellar concrescences. The extent to
which the gills are welded together to form continuous plates is the
distinction between the three main groups of the Lamellibranchiata,
the Protobranchiata [Nucula), the Filibranchiata [Mytilus) and the
Eulamellibranchiata (Anodonta). But even in the last-named group
there are left occasional holes through which water passes into the
interlamellar spaces then into the epibranchial space dorsal to the gills.
Belonging to the same physiological system are the labial palps, two
folds, one in front of the mouth and one behind, which are turned
backwards and prolonged on each side of the visceral mass so as to
form two pairs of richly ciliated triangular flaps, embracing the
anterior end of the ctenidia, and enclosing a groove which leads to
the mouth.
In the anterior part of the mantle cavity the axis of the gill is at-
tached to the side of the animal dorsal to the foot, which here forms
a vertical partition dividing the cavity into a right and left half. The
mantle cavity continues behind the foot, however, and here the up-
turned ends of the inner rows of filaments of both ctenidia are united
so that the mantle cavity is now divided by a horizontal partition into
an upper or epibranchial cavity and a lower main cavity. The former
opens at the dorsal siphon, the latter at the ventral siphon. A constant
current of water is maintained during activity, entering by the ventral
siphon, passing through the gill lamellae, and leaving by the dorsal.
a.v.
.1/. ^ — > -
,-»r.v.
//.ci7..^ -^^^ 'a^IN.
l.cil
il.j.--
Fig. 392. The ctenidia of the LameHibranchiata. A and B, Mytilus. C,
Anodonta. A, Vertical transverse section through ctenidium of one side.
av afferent vein; a.l ascending, d.l. descending limb of filament; cil.d.
ciliated discs ; e.v. efferent vein ; F. foot ; il.s. interlamellar space ; il.j. mter-
lamellar concrescence ; ma. mantle ; muc. mucus travelhng ventrally (m direc-
tion of arrows) and muc.' collected in the food groove; pl.c plicate canals;
r V renal veins. B, Horizontal section through two adjacent filaments, bl.c.
blood ceWs; ch. chitinous rods; cil.j. ciliary junction; f.cil., fi.cil., /.a/ frontal,
latero-frontal, lateral cilia. The arrows denote the direction of the food
current and the path of the food particles it contains. C, Horizontal section
through a gill. Lettering as above.
57^ THE INVERTEBRATA
From this the animal separates its food in the form of minute plants
and fragments of organic debris. The current can easily be demon-
strated by pipetting a suspension of carmine particles in the neigh-
bourhood of the siphons, and the details of the process worked out
by observing the motion of the coloured granules over the surfaces
of the mantle cavity when one of the shells and its mantle lobe have
been removed. In this way the direction of the ciliary currents of the
ctenidia which transport the food particles can be demonstrated
(Fig. 393). On entering the wide mantle cavity the velocity of the
inhalant current is checked, and the heavier particles sink down and
are taken up by the ciliary currents of the mantle which run towards
the posterior region in the neighbourhood of the siphons. The main
ingoing current with the smaller particles of carmine is drawn over
the surface of the ctenidium and impinges against the individual fila-
ments. Their structure and the distribution of the groups of cilia
which all perform different functions is shown in the diagram of a
transverse section through a ctenidium (Fig. 392 B). That the main
current of water is drawn into the mantle cavity at all is the result of
the activity of the lateral cilia. When the current which they have
drawn to the ctenidium impinges on its surface the large latero-frontal
cilia perform their task of deflecting the particles on to the face of the
filaments where they come under the influence of the frontal cilia,
which produce a constant stream down over the surface of the
ctenidium towards its ventral edge. During the passage the particles
in the stream become entangled in mucus, and on reaching the edge
the string-like masses of food and mucus are directed by other cilia
along the edge in the direction of the mouth, travelling partly in the
''food groove''. When the labial palps are reached the collected
material may, according to its nature, either be swept straight into
the mouth or come under the influence of cilia working along re-
jection paths which direct it away from the mouth and toward the
outgoing circulation on the mantle (Fig. 393 B).
This complicated but well co-ordinated ciliary mechanism is
nearly always working when the lamellibranch is covered with water,
and the amount of water which passes through the mantle cavity of
a single mussel is surprisingly large. But it must be remembered that
this current also serves the purpose of respiration, though the ex-
change of gases takes place through the medium of the mantle rather
than the ctenidia. At low tide the animal must close its shell and COg
accumulates within the mantle cavity. This chemical change depresses
ciliary activity and finally brings the cilia to rest, so that the store of
oxygen in the tissues is conserved. When the tide rises, however,
the cilia immediately resume activity.
Though the majority of the lamellibranchs have the power of
LAM ELL IB RANG HI AT A
577
movement it is thus seen that they feed in the manner of a sedentary
organism, and it is not surprising that there are many fixed and
burrowing forms among them.
inh.s.
Fig. 393. Diagrams to show ciliary currents of Mytilus. Adapted from Orton.
A, Food currents with left lobe of mantle removed to show the outer lamella
only of the left gill, and the two palps of the left side separated and not embrac-
ing the front end of the gill as they normally do in life. The vertical arrows re-
present the currents caused by the frontal cilia, those at the bottom of the gill
the main food current running to the mouth and that at the top of the gill the
exhalant current, x. represents a curtain which prevents the inhalant current
from directly impinging on the surface of the gill, an opportunity being thus
afforded for a preliminary rejection of particles, by. byssus threads ; ct. outer
lamella of left ctenidium ; exh.c. course of exhalant current shown by arrows
in the epibranchial chamber, the roof of which is indicated by dots ; F. foot ;
inh.s. left lip of inhalant siphon ; inh.c. inhalant current ; M. mouth ; pp. palps.
B, Rejection currents. Mytilus with foot and the gills removed so as to show
the interior of the right lobe of the mantle. The direction of the currents
caused by the cilia is shown by arrows. The palps of the left side and the
anterior end of the outer left gill remain and the rejection current marked by
three parallel arrows is shown. The colleQtor current runs along the groove
under the mantle edge to the pouch x. aa. anterior adductor muscle ; by.m.
muscles of the byssus ; pa. posterior adductor muscle. Other letters as above.
A short oesophagus leads directly into the stomach, which is a wide
sac receiving on each side the ducts of the digestive gland which is
578 THE INVERTEBRATA
similar to that of Helix ^ but contains only one kind of cell. This cell
takes up the finely divided food which reaches the gland and digests it
by intracellular ferments. The intestine runs into the foot and makes
one or more loops, eventually returning to near the hind end of the
stomach. It then passes through the pericardium where it is usually
surrounded by the ventricle, ending as the rectum. The peculiarity
of the digestive system is the presence of a diverticulum of the
intestine, the cells of which secrete a crystalline style (Fig. 394) ; some
cilia in the diverticulum rotate this and others move it forward so that
at its free end, projecting into the stomach against a structure called
the^fl5inc^/zi>/^, it is constantly worn away and the style material mixed
with the contents of the stomach. It is composed of protein to which
is adsorbed an amylolytic ferment and it may be broken down and
re-formed periodically. There is no doubt that this represents a special
Fig. 394. A, Section of part of the alimentary canal of Donax. cent, caecum
of the intestine containing est. crystalline style ; g.s. gastric shield ; int. in-
testine ; M. mouth ; oe. oesophagus ; st. stomach. B, Transverse section across
the caecum showing cil. ciliated epithelium and est. crystalline style composed
of concentric layers of material. After Barrois.
provision for the digestion of carbohydrates and it is also found in
somegasteropods. For the rest, digestion of proteins and absorption
take place in the digestive gland, the cells of which have a surprising
power of taking up solid particles. In the oyster, it may be mentioned,
there is an extraordinary abundance of leucocytes which wander here,
there and everywhere, through the body. It has been shown that
they enter the stomach and ingest diatoms and other food particles
there, speedily digesting them and wandering over the body afterwards,
so that they play a unique part in the transport of food.
The lamellibranchs are most conveniently classified by the structure
of their ctenidia. We have firstly three groups which can be arranged
in an evolutionary series, showing the ctenidia to become larger,
more complex and solid organs. Lastly there is an isolated group, the
Septibranchiata, in which the habit of life has completely changed
and the ctenidia have practically disappeared :
LAMELLIBRANCHIATA
579
Protobranchiata Nucula.
FiLiBRANCHiATA Myttlus, Pecteti.
EuLAMELLiBRANCHiATA Ostrea, Cyclas, Cardium, Mya, Anodonta.
Septibranchiata Poromya, Cuspidaria.
In Fig. 395 A, B, the difference is seen between the Protobranchiata
with their short and simple filaments and the next two groups in which
each filament is greatly elongated and upturned so that descending
and ascending limbs can be distinguished. The contrast between the
Filibranchiata and the Eulamellibranchiata is expressed by Fig. 392,
in which a transverse section through a "gill" is shown, showing the
component filaments separate in the first case, save for the ciliary
junctions, united in the second. Lastly, in Fig. 395 C, it is seen that
in the Septibranchiata, the ctenidia are replaced by a horizontal mus-
cular partition (which moves up and down like the piston of a pump)
Fig- 395- Vertical sections of Lamellibranchiata to show different stages of
development of the ctenidia. A, Protobranchiata. B, Filibranchiata and
Eulamellibranchiata. C, Septibranchiata. The arrows in C show the direction
of the flow of water through the " diaphragm ", when the latter moves down-
wards. After Sedgwick, from Lang.
with apertures connecting the ventral and dorsal divisions of the
mantle cavity.
The ciliation of the filaments is the same in all the first three divi-
sions. Even in the Protobranchiata, the ciliary apparatus for food-
collecting has been developed as in the rest of the group, and it has
been pointed out that there are ciliated discs, adjacent pairs of which
act as ciliary junctions and hold the filaments together to form
lamellae. There is, moreover, a subdivision of the mantle cavity into
inhalant (ventral) and exhalant (dorsal) chambers in spite of the small
size of the ctenidia.
The blood system of the lamellibranchs is best explained by refer-
ence to that of Myttlus, the common mussel (Fig. 396). Here the
heart, as in Anodonta, consists of a ventricle surrounding the rectum
and two auricles, each of which opens into the ventricle by a narrow
canal and is attached by a broad base to the wall of the pericardium
580 THE INVERTEBRATA
'over the insertion of the ctenidia into the mantle. A single vessel, the
anterior aorta (a posterior aorta is also present in Anodonta), leaves
the ventricle, dilates into an aortic bulb and then divides into many
arteries. Of these, the most important are the pallial arteries going to
the mantle and the arteries forming part of the visceral circulation
(the gastrointestinal, hepatic and terminal arteries, the last named
supplying the most anterior part of the body including the foot). The
arteries break up into a network of vessels in all the tissues and these
Viscera
p.n
Plicate
canals
Fig. 396. Diagram of the circulation in Mytilus to show the greater import-
ance of the part of the system in the mantle and pHcate canals. Of the blood re-
turned from the viscera a much smaller proportion is sent through the ctenidia.
Slightly altered from Field. An. auricle; bl. bladder of kidney opening into
the pericardium ; aff.c.v. afferent, eff.c.v. efferent ctenidial vein ; l.v. longi-
tudinal vein of kidney; p.ar. pallial artery; pc. pericardium; v. ventricle with
rectum, represented by a dotted line, passing through it.
join to form veins and sinuses which are largely situated on the inner
side of the mantle and the superficial parts of the body. The skin,
being bathed in water and devoid of any cuticular covering which
might hinder diffusion, is a general organ of respiration and the mantle
is the most important part of it. Most of the blood from the pallial
circulation is returned to the network of vessels in the kidney through
the ribbon-like organs, known as plicate canals, which extend along
the mantle just above the insertion of the ctenidium.
LAMELLIBRANCHIATA 581
The visceral vessels likewise return blood to the kidney network
so that practically the whole of the blood passes through the excretory
organ and is purified. A part of the blood from the kidney network
enters the ctcjiidial circulation, discharging into the longitudinal
afferent branchial vein, which gives off to each filament a vessel which
descends one side and ascends the other. The ascending vessels join
to form a longitudinal efferent vessel, which discharges into the longi-
Viscera
Ipdney
eff.c.v.-
.aff.c.v.
Ctenidia
Fig. 397. Circulation of Anodonta. A, Simplified diagram to show the course
of the blood, indicating the relative importance of the various branches.
Vessels returning arterial blood to the heart shown in black. B, Transverse
section of Anodonta to show part of the course of the circulation. In the foot,
F., the veins run into the vena cava cut in section, from which a small part
of the blood is returned direct to the auricle in the dorsal wall of the bladder,
bl., the rest through the kidney, k., longitudinal afferent vessels, Iv/, and
thence to the afferent system of vessels in the ctenidium, ajf.c.v. On the other
side the efferent system of vessels, eff.c.v., is shown returning blood to the
longitudinal vessels at the base of the ctenidia, v.'\ from which it passes to the
auricle, au., through an irregular system of blood spaces. The pallial circula-
tion is not shown here. sbr. epibranchial space; v.c. vena cava; ven. ventricle.
tudinal vein of the kidney. Into this longitudinal vein is collected the
blood from the kidney network in general and by this channel blood
is returned into the auricle. It will be seen that the branchial cir-
culation is not important in Mytilus ; in Anodonta (Fig. 397) it is more
developed.
In Anodonta (Fig. 397) where the foot is larger than in Mytihis
and movement more continuous the pedal artery is more impor-
582 THE INVERTEBRATA
tant than the visceral arteries. The veins from the foot and the vis-
cera join to form a pedal sinus and this opens into the vena cava. The
junction of these is marked by a sphincter muscle (Keber's valve).
This sphincter is closed when the foot is extended. The relaxation of
the muscles and the pumping of the blood into the sinuses of the foot
bring about the swelling of the foot. When the foot is retracted the
blood is largely contained in spaces in the mantle. The pallial cir-
culation is maintained during movement when the visceral circulation
is interrupted as described above.
While the Protobranchiata have a nervous system with four distinct
pairs of ganglia (Fig. 373 D) in the remainder of the class the number
is reduced to three by the iFusion of the cerebral and pleural ganglia
(Fig. 398 B).
The sexes are usually separate in the Lamellibranchiata, but some
species of Ostrea and Pecten are always hermaphrodite, while this
condition is frequent in Anodonta. In the Protobranchiata the gonad
discharges into the kidney, but in most forms there is a separate
generative aperture. While most marine forms and the freshwater
Dreissensia have trochosphere and veliger larvae, some lamellibranchs
incubate the embryos within the ctenidia, and in the family Unionidae,
which includes Anodonta, the larvae are much modified {Glochidium).
When they are ripe the mother liberates them if a fish swims near her,
and they attach themselves to the gills or fins and become encysted
there. After a parasitic life which varies greatly in length they escape
from the cyst as young mussels.
Order PROTOBRANCHIATA
The best-known representative is Nucula (Fig. 373 D). It has a shell
of very characteristic appearance with numerous teeth on the hinge
line and a foot which, when fully extended, has a flat ventral surface
which has been compared with that of the gasteropod. But instead of
creeping by means of it the animal uses it for burrowing ; it is folded
up (as is seen in the diagram), and thrust into the mud, then opened
out and used as a holdfast, and the contraction of the retractor muscles
draws the body below the surface. While the surface of the ctenidium
is so small that the organ is of little use for feeding, the labial palp is
enormous and divided into three parts. One of these is a kind of
proboscis which is thrust out of the shell and collects food by ciliary
currents. This is sorted and forwarded to the mouth by the other two
parts without the intervention of the ctenidium.
The nervous system has distinct cerebral and pleural ganglia and
the gonads have retained their original connection with the kidneys.
These and some less important characters show that Nucula and its
LAMELLIBRANCHIATA 583
relations are probably the most primitive of living lamellibranchs.
The specialization of the labial palps has had as its consequence the
partial suppression of the ctenidia, which remain in an undeveloped
condition. In this respect the Protobranchiata can hardly be held to
resemble the ancestral lamellibranch.
Order FILIBRANCHIATA
Mytilus (Fig. 393). While the majority of lamellibranchs are semi-
sedentary, the sea mussel has developed the sedentary tendency and
marks a half-way stage to the oyster which remains fixed through
Fig. 398. Nervous system of A, Chiton, B, a lamellibranch. Dorsal views.
The outline of the mantle edge is indicated by a dotted line, buc.c. buccal
commissure and ganglia; ce.c. cerebral commissure; ce.pl. g. cerebro-pleural
ganglion; pal.n. pallial nerve; ped.g., ped.n. pedal ganglion and nerve; p.v.c.
palliovisceral commissure; sb.rad.c. subradula commissure; vis.g. visceral
ganglion.
adult life. The mussel lives in association in beds between tidemarks
where the conditions are favourable. The very extensible foot is
tongue-like in shape with a groove on the ventral surface which is
continuous with the byssus pit posteriorly. In this a viscous secretion
is poured out which enters the groove and hardens gradually when it
comes into contact with sea water. The tip of the foot is pressed
against the surface to which the muSsel attaches itself, and in a cup-
like hollow which ends the groove the attachment plate is formed at
the end of the byssal thread. When one byssal thread has been formed
the foot changes its position and secretes another thread in another
place. The byssus thus consists of a mass of diverging threads arising
584 THE INVERTEBRATA
from the byssiis pit and by means of it the animal is firmly attached
to stones or other mussels. But mussels, particularly when young,
creep about both by using the cup at the tip of the foot as a sucker and
also by forming a path of threads along the surface of the substratum,
as can be easily seen in the laboratory. While the development of the
byssus is the most outstanding characteristic of the mussel, it may
also be mentioned that a pair of simple eyes are developed, anterior
ten.
Fig- 399- Pecten maximus, general anatomy, right valve and ctenidium re-
moved. After Dakin. add.u. unstriped and add.s. striped adductor muscle ;
an. anus; au. auricle; b.gr. byssal groove; ct.' descending and ascending
lamella of left ctenidium ; e. eye ; /. foot ; int. intestine ; l.p. labial palp ;
M. mouth ; o. ovary ; oe. oesophagus ; st. stomach ; t. testis ; ten. tentacles of
mantle; ven. ventricle; vm. velum.
to the inner ctenidial lamella ; these are an inheritance from the larval
mussel. The invasion of the mantle by the generative organs is another
peculiar point. In the breeding season the aeration of blood in the
mantle is reduced and the plicate canals (Fig. 396) become the chief
organ of respiration.
Pecten (Fig. 399). There are two common British species, P.
maximus and P. opercularis, which are commonly known under the
name of "scallops". The animal is found free and it moves not by
LAMELLIBRANCHIATA 585
the ordinary lamellibranch method but by swimming. The two valves
are unequal, the right being larger and more convex, and the animal
rests on this valve; in P. opercularis the valves are almost equal. In
swimming the valves open and close very rapidly, forcing out the
water between them. Usually the water is forced out dorsally on each
side of the hinge line and the animal moves with the free ventral
border forward ; but on sudden stimulation the current passes out
directly ventrally and the hinge line becomes anterior. There is a
single large adductor muscle: this is divided into two parts and the
larger of these serves for the rapid contractions which cause swimming
movements ; the fibres are transversely striated ; the smaller part has
fibres which are capable only of strong long-continued contraction
and keep the valves closed. (Cp. Chapter iv, p. 143.)
The foot is very much reduced, but it has nevertheless a distinct
function, that of freeing the palps and gills from sharp and disagree-
able foreign material; in the larva it is used actively in locomotion.
The ctenidia, while resembling the typical filibranch gill of Mytilus
in general, differ in the possession of two kinds of filaments and in the
vertical folding of the gills. The larger principal filaments lie at the
bottom of the troughs between successive folds and the descending
and ascending limbs of each principal filament are connected by a
sheet of tissue, the inter latnellar septum. In one species, Pecten
tenuicostatiis, there are organic connections between filaments instead
of ciliary junctions only, and the existence of this condition is a valid
criticism of the classification of the lamellibranchs by ctenidial
structure.
Pecten is hermaphrodite. The ovary has a very vivid pink colour
when the eggs are ripe. The testis lies behind it and is cream-coloured.
The remaining feature to be noted is the presence of a large series of
stalked eyes (Fig. 409 D), of a very complicated structure, at regular
intervals all round the mantle.
Order EULAMELLIBRANCHI ATA
Anodonta (Figs. 392 C, 397). Many of the characters of this fresh-
water genus are described above.
Ostrea (Fig. 400). In this form the adult is always fixed by the left
(the larger) valve. As in Pecten, there is only one adductor muscle
(the posterior) in the adult (but the spat possesses two equal muscles),
and this is divided into two parts, one with striated the other with
non-striated fibres. The foot has disappeared entirely ; the two auricles
are fused together. Of great interest are the reproductive habits : it
has been established that individuals of O. edulis function alternately
as males and females. Spawning tends to take place at full moon as
586 THE INVERTEBRATA
in some echinoderms. Another point of physiological importance is
the great part which leucocytes play in digestion ; the lumen of the
alimentary canal is invaded and diatoms and similar bodies ingested,
digested and transported by the leucocytes into the connective tissue.
A figure of the veliger larva of Ostrea is given (Fig. 375 A) to show
the ciliary currents by which food is obtained, the crystalline style,
which is revolved by the action of the cilia of the style sac, and the
foot, which is lost in the adult.
ex.c
ven.'
^int.
d.bx.'
Fig. 400. Ostrea edulis, general anatomy, right valve and mantle removed.
After Yonge. Lettering as in Fig. 374; in addition: in.c. inhalant and ex.c.
exhalant chamber; d.b.c. division between above chambers. Arrows indicate
direction of currents.
Teredo (Fig. 401) is the most specialized of the boring lamelli-
branchs. While most lamellibranchs burrow in mud, others tend to
work in consolidated sediments such as Pholas in chalk and sandstone,
and Saxicava in the hardest limestone. Teredo and Xylophaga bore in
wood. The latter makes shallow pits, but Teredo, working with ex-
traordinary speed, excavates long cylindrical tunnels (sometimes as
much as a foot in a month or two). The wood is reduced to sawdust
by the rotatory action of the two shell valves, in which the adductor
muscle fibres maintain a rhythmical contraction. The sawdust is
swallowed by the animal and is largely retained in a relatively
MOLLUSCA 587
enormous caecum of the stomach, but a great deal of the material
passes into the cavity of the digestive gland and is there ingested by
the epithelial cells. There is no doubt that Teredo has developed
enzymes which are almost unique in the Animal Kingdom, which
digest cellulose and hemicellulose. The structure of the animal
is remarkable for the extraordinarily long siphons and mantle
cavity; while the mantle often lays down a calcareous lining to
the tube and always a pair of calcareous valves, the pallets, which
close the mouth of the tube when the siphons are retracted. The foot
/ 1 v T V
^•^- di.(jh ct' int. cm
Fig. 401. Teredo, represented boring in wood. The sawdust formed by the
rotatory movement of the shell valves, sh., is shown entering the mouth, M.,
and the faecal pellets of undigested wood are shown as black masses in the
exhalant chamber, exh.c. Other letters : an. anus ; au. auricle ; ct. ctenidium ;
ct.' continuation of ctenidium as a ciliated ridge over the visceral mass; cm.
caecum of stomach filled with wood; c.s. position of crystalline style sac;
di.gl. digestive gland; F. foot; int. intestine; z«^.c. inhalant current; pcd.
pericardium ; pit. palette ; sh. left valve of shell ; ven. ventricle. Original.
is very much reduced. A constant current into and out of the mantle
cavity is maintained by ciliary action, and the ctenidia, though so
greatly modified and elongated, constitute a collector mechanism;
but it does not seem that diatoms obtained in this way form any part
of the normal food of the creature, which exists almost entirely on the
carbohydrates furnished by wood which also contains small quantities
of proteins.
Class CEPHALOPODA (SIPHONOPODA)
Bilaterally symmetrical Mollusca with a radula and a well-developed
head which is surrounded by a crown of mobile and prehensile ten-
tacles, sometimes held to be part of the foot, which certainly forms the
588 THE INVERTEBRATA
funnel or siphon, a muscular organ, originally bilobed, used for the
expulsion of water from the mantle cavity; one or two pairs of typical
ctenidia; coelom sometimes exceedingly well developed, the genital
part being continuous with the pericardium; typically a chambered
shell in the last chamber of which the animal lives, though in most
modern representatives it is reduced and internal or wholly absent;
nervous system greatly centralized and eyes of great size and often
complex type; eggs heavily yolked and development direct.
The Cephalopoda fall into two groups, in one of which (Tetra-
branchiata) there are two pairs of ctenidia and a w^ell-developed
external shell, w'hile the members of the other (Dibranchiata) have
one pair of ctenidia and either one internal shell or none at all. Of the
Tetrabranchiata Nautilus is the only living member; of the Dibran-
chiata, Sepia, a common form in the Mediterranean and elsewhere,
is a convenient type. The organization of the group will best be
understood from a description of these examples. As Sepia is the
more easily obtained we shall describe it first and in more detail,
though it is in many respects less primitive than Nautilus.
Order DIBRANCHIATA
Cephalopoda with a single pair of ctenidia and kidneys; shell in-
ternal, enveloped by the mantle and in various degrees of re-
duction; 8-10 tentacles; the two halves of the funnel only seen
in the embryo; chromatophores present; eyes of complex
structure.
Classification
Suborder Decapoda. Dibranchs with ten tentacles and wdth a
well-developed coelom. Internal shell consisting of phrag-
mocone, rostrum and proostracum or very much simplified.
(i) Tribe Belemnoidea. Fossils from Mesozoic rocks which
have given rise to the following tribes :
(2) Tribe Sepioidea. Decapoda with specially modified 4th
pair of tentacles which can be retracted into pits; eyes with
a cornea, internal shell sometimes with phragmocone bent
ventrally: fins not united posteriorly; shore and bottom
living forms. Spirula, Sepia, Sepiola.
(3) Tribe Oegopsida. Decapoda with anterior chamber of eye
open; tentacles usually all alike; suckers often modified to
form hooks ; shell only represented by a horny gladius ;
strong swimmers. Includes many abyssal forms wdth
phosphorescent organs; some gigantic forms, like Archi-
teuthis, 60 feet long.
CEPHALOPODA 589
(4) Tribe Myopsida. Decapoda with a cornea in the eye, a
simple gladius, specially elongated 4th pair of tentacles, not
retractile into pits; fins united posteriorly; shore forms.
Loligo (Fig. 411 D).
Suborder Octopoda. Dibranchs with eight tentacles and a reduced
coelom. Octopus, Argonauta, Opisthoteuthis .
Sepia officinalis^ is a shallow-water form, in which the shell has
become internal. The general disposition of the organs remains much
as it would be if the animal inhabited the last chamber of a shell like
that of Nautilus (cf. Fig. 402 A and B). The whole body is cylindrical.
At one end, which would have projected from the shell, is the head
with the mouth in the centre and the two relatively enormous eyes at
the sides. Round the mouth are the tentacles (arms) for seizing prey
which are often considered to be part of the foot. Four pairs of these
are short and stout and covered with suckers on their inner surface.
The fourth pair (counting from the dorsal surface) are long and can
be retracted into large pits at their base ; there are suckers only at their
free end. The left hand member of the fifth pair in the male is slightly
modified by suppression of the suckers. At one side, called pos-
terior, is the mantle cavity, and protruding from its opening is
the funnel, which is the remaining part of the foot. The visceral
hump is the conical apex of the animal. Instead then, of being
protrusible like that of a lamellibranch or used for gliding like
that of a gasteropod, the main part of the cephalopod foot is
greatly modified for respiratory purposes. In view of the fact that
there is no boundary between the head and the foot in molluscs, dis-
cussion as to whether the tentacles are part of the head or the foot is
difiicult and unimportant.
The shell has become internal and is a rather substantial plate
which acts as an endoskeleton. The absence of a figid envelope has
made it possible for the mantle to become very mobile and to develop
thick muscular layers, circular muscles running round the mantle
cavity and longitudinal running towards the apex of the hump. When
the latter contract and the former relax the mantle cavity enlarges and
draws in water which circulates round the ctenidia ; when the reverse
action takes place the first effect of the contraction of the circular
muscles is to draw the mantle lobe tight round the neck and then,
when the contraction reaches its height, the water is expelled through
the funnel. In rest these movements are gentle and rhythmic and
only effect the change of water necessary for respiration. At the same
time the animal is usually swimming slowly forward by the undula-
^ This description of the structure and habits of Sepia applies generally to
all the well-known Decapoda.
fn.^
rm.
brn. oe. dig-gl-
rad. V ^ \
sh.
St.
Fig. 402. Diagrammatic median sections through A, Nautilus and B, Sepia
for comparison of the organization of the Tetrabranchiata and Dibranchiata
respectively. Altered from Naef. brn. brain; cm. caecum; ct. ctenidia;
dig.gl. digestive gland ; fn. funnel ; g.sep. genital septum ; h. heart ; ho. hood ;
i.s. ink sac;y. jaws; k. kidney; Ip. lips; mt. mantle; mt.' dorsal extension in
Nautilus; o. ovary; oe. oesophagus; rad. radula; rm. rectum; sep. septa; sh.
shell; sip. siphuncle; st. stomach; t. testis. Sepia is shown in the expiratory
position with the mantle pressed against the funnel, and the valve of the latter
flat against its wall. In the inset C, the inspiratory phase is seen with the
mantle relaxed to allow the entry of water as shown by the arrow, and the
valve of the funnel opened so as to prevent the passage of water.
CEPHALOPODA 59I
tory movement of the lateral fins. But if Sepia is alarmed or excited
the muscles contract violently and the spasmodic ejection of water
through the funnel causes the animal to dart quickly backw^ards.
Equally by turning the funnel backward it can move quickly forward.
Not only is the mantle highly muscular but the dermis contains large
cells filled with pigment, the chromatophores ^ which can be dilated by
the contraction of radiating muscle fibres attached to the cell wall.
By alternate contraction and expansion of the chromatophores, waves
of colour are made to pass rapidly over the surface of the animal. The
colour change which is brought about in this way may be to a certain
extent a response to the character of the background but it is also
stated to be the expression of emotions.
Sepia swims with the longest axis horizontal, the upper flattened
surface is that under which the shell lies and the lower the mantle-
cavity surface. These surfaces are dorsal and ventral respectively and
the mouth and tentacles are anterior. All round the mantle in the
horizontal plane rises a horizontal fin by which the gentler swimming
movements are effected.
When the mantle cavity is opened as shown in Fig. 403, the funnel
is seen with its narrow external and wide internal openings, and at
the base of it two sockets which fit corresponding knobs on the
mantle. This locking arrangement ensures that the mantle fits tightly
on the neck and so that all water is expelled by the funnel. At the
anterior end of the visceral hump is seen the central anus at the end
of a long papilla, so placed as to discharge the faeces directly into the
cavity of the funnel, the shorter renal papillae immediately on each
side, and on the left side only the genital aperture, also at the end of a
long papilla. More posterior still are the large and typical ctenidia.
On the face of the visceral hump in mature animals the accessory
genital glands are seen through the skin ; the chief of these are the
shell-forming nidamental glands of the female which occupy a con-
siderable area. Between these and in front of them are the accessory
nidamental glands. Posterior to them is the ink sac, usually seen
through the integument from which a narrow duct runs ventral to
the rectum, opening into it a short distance behind the anus. The
first step in dissection is to strip off the skin and then dissect out the
gland and its duct as carefully as possible. It usually contains a large
amount of the ink, which is composed of granules of melanin
pigment formed by the oxidation of the aminoacid tyrosin by the
agency of an enzyme, tyrosinase. This substance is ejected into the
mantle cavity and through the funnel to form a "smoke cloud"
when the animal is attacked.
The next stage in dissection is the opening up of the kidneys by
cutting through the thin outside wall. It will at once be seen that the
592
THE INVERTEBRATA
Fig. 403. Ventral view of male of Sepia officinalis with mantle cavity opened
to expose its contents, an. anus; dep.m. depressor muscle of the funnel;
e. eye; /z. fin; g.pap. p^enital papilla; hec. hectocotylized arm;/, jaw; k.pap.
papilla bearing external aperture of kidney ; kn. cartilaginous knob on mantle
which fits into the socket, soc. ; vis.m. visceral mass. Other letters as in Fig.
402. From Shipley and MacBride.
CEPHALOPODA
593
cavity of the organ contains a large amount of spongy excretory tissue,
developed round the veins which run straight through the kidney.
Just inside the renal papilla is a small rosette which carries the reno-
pericardial aperture. This leads into the long narrow renopericardial
canal running in the outer wall of the kidney and opening posteriorly
into the pericardium^ a wide space lying behind the kidneys which is
dep.m. i^\ ^
an.
Fig. 404. Sepia officinalis. Dissection from the ventral side to show kidneys
and blood vessels. Arrows show the direction of flow of blood, abd.v. ab-
dominal vein; a.ao. anterior aorta; au. auricle; ajf.v. afferent branchial vein;
br.ht. branchial heart ; eff.v. efferent branchial vein ; k.d. opening into dorsal
sac of kidney (see arrow); k.t. excretory tissue surrounding the vena cava;
pal.v. pallial vein; p.ao. posterior aorta; r.p.a. opening into kidney cavity of
the renopericardial canal, r.p.c; st.g. stellate ganglion; ven. ventricle; v.cav.
vena cava. Other letters as in Figs. 402, 403. Original.
only separated by an incomplete partition from the still more spacious
genital coelom occupying the apex of the visceral hump (Fig. 405).
The median ventricle and the two lateral auricles are spindle-
shaped bodies arranged in a line at right angles to the longitudinal
axis of the body. Arterial blood is sent to the body from the ventricle
by an anterior aorta running dorsal to the oesophagus towards the
head and ^. posterior aorta\ venous blood returns to the heart fro
/
'<b/4^
'^
<p'
LIBRARY i^
594
THE INVERTEBRATA
head by a very important vessel, the vena cava^ which splits in the
kidney region into two branchial veins, which run to the so-called
branchial hearts, special muscular dilatations which pump blood
through the capillaries of the ctenidia. The blood which is oxygenated
there is sucked out of the ctenidium by the expansion of the auricle.
Blood is also returned directly to the branchial heart from the mantle
by the abdominal veins (and a smaller pair), and by the unpaired
genital and ink sac veins which run first into the right branchial vein.
pan.'
coec
Fig. 405. Vertical section of Sepia officinalis to show the relation of the divi-
sions of the coelom. After Grobben. coec. coecum; dig.gl. digestive gland
(" liver ") ;/m. funnel \g.coe. genital coelom ; h. heart ; i.s. ink sac ; k.pap. external
opening of kidney; k.t. excretory tissue; nid. nidamental gland; o. ova; pan.
"pancreatic" tissue surrounding the duct of the digestive gland; pcd. peri-
cardium; r.p.a. opening into the kidney of the renopericardial canal, r.p.c;
sh. shell; Z), dorsal; V, ventral.
In describing the alimentary system it must first be mentioned that
Sepia, as a type of the Decapoda, possesses ten tentacles of which the
fourth pair are longer than the others. These two tentacles have a
slender stem and a swollen terminal portion to which the suckers are
confined. Each tentacle can be rapidly extended and attached to the
living prey, and with equal rapidity retracted into a pit near the
mouth, thus bringing the food into the reach of the other tentacles,
which hold it while it is being devoured. Round the mouth are frilled
lips and just within it are the characteristic beaks, corresponding to
the jaws of the gasteropod, which bite upon each other. The buccal
CEPHALOPODA
595
mass is large and contains a well-developed radula and is traversed
by the narrow oesophagus. Just within the buccal mass is the first
pair of salivary glands and immediately in front of the digestive gland
is the second pair, which produce not a digestive juice but a poison.
In Octopus, which lives largely upon crabs, the prey is seized and
bitten by the beaks, a drop of the poisonous saliva entering at the
same time by the punctures in the carapace and causing almost im-
mediate death. This is true of Sepia also which lives upon prawns and
shrimps. The food is bitten into pieces by the jaws — sometimes of
considerable size — and passed down the oesophagus (which though
narrow is capable of considerable distension) to the muscular, non-
8.6 mc.(/
p.sal.gl. ce.g
oe.
digjjl ->y;-
ma.n
VI
is.g. ot. ! ped.g. \
fu'.n- bra.g. yw.
Fig. 406. Vertical section through head of Sepia officinalis showing buccal
mass (coarsely stippled) and brain (black) surrounded by the cartilaginous
skull (finely stippled), a.sal.gl. anterior salivary gland; bra.g. brachial
ganglion with brachial nerves coming off from it; ce.g. cerebral ganglion;
dig.gl. "liver"; /w./z. nerve to funnel {fu.) coming off from pedal ganglion;
y. beaks ; ma.n. mantle nerve ; oe. oesophagus ; ot. otocyst ; ped.g. pedal gang-
lion ;/).5a/.£/., p.sal.gl. posterior salivary duct and gland; rad. radula; s.buc.g.,
i.biic.g. superior and inferior buccal ganglia ; vis.g. visceral ganglion. Original.
glandular stomach. Here it is mixed with the secretion from the
digestive gland and the digested food passes to the spiral coecum.
This contains an elaborate ciliary mechanism which removes solid
particles from the coecum, leaving only liquid products of digestion
to be absorbed there. The digestive gland consists of a solid bilobed
gland ("liver") and a more diffuse and spongy part ("pancreas").
Both are enzyme-producing, but the "pancreas" (which in Sepia is
suspended in the kidney sac) is al^o partly excretory. The single
duct opens into the coecum, but a groove guides its secretion into
the stomach. The "liver" is the principal "storage organ" for food
reserves ; it seems probable that these only reach the 'gland from the
blood-stream, and that food is all absorbed in the alimentary canal,
596 THE INVERTEBRATA
and does not enter the liver. In this respect the cephalopods appear
to differ from the majority of invertebrates.
The nervous system of Sepia is of great interest from the large size
and intimate association of the ganglia round the oesophagus, which
form a genuine "brain" (Figs. 407, 408) in which special centres for
the co-ordination of vital activities and for the simple reflex actions
have alike been detected. In contrast to vertebrates there is a con-
centration of nerve cells in the brain, only a few outlying ganglia
being present. For the protection of this large nervous mass a "skull "
has been developed composed of a tissue very similar to cartilage,
which also forms the supports of the fins and tentacles. The nerve net
found in the foot of gasteropods is absent.
cer.
VIS.
Fig. 407. Lateral view of the brain of a cephalopod (Eledone?) to show the
localization of function. After Buddenbrock. al.c. alimentary canal; buc.
buccal ganglion ; cer. the different divisions of the cerebral ganglion ; brae.
brachial ganglion; ped. pedal ganglion; vis. visceral ganglion; the various
reflex centres A for biting, B for swallowing, C for swimming forward, D
for creeping and climbing, E for closing and F for relaxing the suckers, G
for in-breathing and H for out-breathing.
The brain consists of the following ganglia : dorsally the cerebral or
supraoesophageal, ventrally (i) tho^ pedal, divided into the brachial (the
motor centre for the tentacles) in front and the infundibular (supply-
ing the funnel) behind, and (2) the visceral supplying the mantle and
the visceral hump. The cerebral ganglia are much more differentiated
than any of the others. They can be divided into separate regions
which co-ordinate the movements of organs for the performance of
such complicated actions as feeding, swimming and creeping. In the
visceral ganglia there are also two sharply defined centres which
control the movements of the whole mantle in in-breathing and out-
breathing respectively as well as numerous small centres, the stimu-
lation of which causes contraction of small muscle patches in the
mantle, while in the brachial ganglia there are separate centres for
gripping by the suckers and for letting go.
CEPHALOPODA
597
From the cerebral ganglia there run forward a pair of nerves which
end at the border of the buccal mass in a pair oi superior iwrca/ ganglia ;
a circumoesophageal commissure links up these with the inferior
ten.n.
Fig. 408. Nervous system of Sepia. After Hillig. bra.g. brachial ganglion ;
br.g. branchial ganglion and nerve; ce.g. cerebral ganglion; gas.g. gastric
ganglion ; rna.n. mantle nerve ; olf.n. olfactory pit and nerve ; op.g. optic
ganglion ; s.buc.g. superior buccal ganglion ; st.g. stellate ganglion ; sym.n.
sympathetic nerve ; ten.n. brachial nerves ; vis.g., vis.n., visceral ganglion
and nerve.
buccal. From the visceral ganglia there is a pair of nerves running to
the very prominent stellate ganglia in the mantle ; there is also a vis-
ceral loop which sends off branches to the gills and a sympathetic loop
ending in the gastric ganglion between the stomach and the caecum.
598 THE INVERTEBRATA
The infundibular ganglion gives off a pair of nerves to the funnel and
the brachial ganglia a separate nerve, which carries a ganglion on
its course, to each arm.
In the dissection of the nervous system a general view of the
different parts of the brain is best obtained by making a longitudinal
vertical section with a sharp scalpel. Such a section is shown in Fig.
406. Afterwards the dissection of the nerves coming away from the
brain can be carried out.
Sepia possesses very large eyes (Fig. 409 C), similar in their structure
and development to those of a vertebrate. In the embryo, the eye
originates as an ectodermal pit, the lining of which forms the retina
and the contents of which become the vitreous humour. The pit
closes up and at the point of closure the interior part of the lens
is formed. Later appears a circular fold which forms the iris, limiting
the pupil of the eye and forming an outer eye chamber which is finally
enclosed by the growth of a cornea. The external half of the lens is
formed at the same time. A special ciliary muscle regulates the posi-
tion of the lens. When it is relaxed the eye is focussed on the distance :
when it contracts, increasing the pressure of the vitreous humour and
so pushing the lens forward, the eye is focussed on near objects.
The ovaries and the testes are simply parts of the wall of the coelom.
The ova are cells of large size ; they are nourished by other peritoneal
cells, the follicle cells, which surround the ova and pass on food from
the special blood supply. The surface of contact between these cells
and the egg is increased by folding. When ripe the ova escape into the
genital coelom and pass into the genital duct. This has a terminal
glandular enlargement and there are also the nidamental glands, un-
connected with the genital ducts, which have already been mentioned.
These secrete an elastic substance which forms the egg envelope.
The sperm pass similarly into the genital coelom and then by a
very small aperture into the sperm duct which is modified to form in
turn the seminal vesicle, the prostate gland and the terminal reservoir,
called Needham's sac. All these play their part in the formation of
the remarkable spermatophores, elastic tubes which by an elaborate
arrangement burst and liberate the spermatozoa after copulation. The
spermatophores are passed directly from the extended genital papilla
into the funnel and then on to one of the arms (the hectocotylus)
which is modified for the purpose of transferring the sperm to the
female. In Sepia, the modification shows itself only by the suppres-
sion of some rows of suckers at the base of the arm, but in other
forms it is profoundly modified. In Octopus, the end of the arm
is spoon-shaped and the arm is extended so as to enter the mantle
cavity of the female. In other octopods, a cyst, in which the sper-
matophores are stored, is formed at the end of the arm ; from it a long
CEPHALOPODA
599
filament is protruded. In Philonexis and Argonauta the modified arm
is charged with spermatozoa, inserted into the mantle cavity of the
female and then detached. This arm was described by early observers
as a parasitic worm and named Hectocotylus .
4d..
cor.
cor.
pig.cp.
op.n.
Fig. 409. Eyes of Mollusca. A, Nautilus. B, Helix. C, Sepia. D, Pecten
(inverted type), cil.m. ciliary muscle; cor. cornea; d.ret. distal and pr.ret.
proximal layers of the double retina of Pecten ; ir. iris ; /. lens ; Id. eyelids ;
op.g.y op.n. optic ganglion and nerve; pig. ep. pigmented epithelium ; ret. retina ;
tap. tapetum ; vit.h. vitreous humour. In Sepia the cartilage is shown in black.
Other Dibranchiata. The members of this group are classified in
two suborders, whose members respectively possess, like Sepia, ten
arms (Decapoda), or, like Octopus, only eight (Octopoda). In no
member of either division is there any known form in which the shell
is external ; in all cases the shell is more or less rudimentary or, in the
case of the Octopoda, entirely absent. There is a well-known and
6oo THE INVERTEBRATA
extremely numerous fossil group, the Belemnoidea (Fig. 410 B), in
which impressions of the entire creature show the internal shell, the
ink sac, and the ten arms beset with hooks. The shell consists of a
chambered phragmocone , protected by a thickened guard, and with an
anterior plate, the proostracum. It may well have been derived from
a nautiloid form like Orthoceras (Fig. 410 A), as may be seen in the
accompanying series of diagrams, in which the soft parts are of
course partly conjectural. In a rare living form, Spirula (Fig. 410 C),
''U:,:::;-___^ ^P' sip.
A^
phr. sip.
y
prst. ^^w
'"c
Fig. 410. Series of Cephalopoda to illustrate the evolution of the internal
shell. After Naef. A, Orthoceras, Palaeozoic. B, Belemnites, Mesozoic.
C, Spirulirostra, Tertiary. C, Spirula and D, Sepia, living. (D', enlargement
of posterior end of D.) I'he reflection of the mantle over the shell is indicated
by a dotted line. This is incomplete in Orthoceras, but the shell is completely
internal in the rest. gd. guard; phr. phragmocone ; prst. proostracum;
sep. septa; sip. siphuncle.
the chambered shell is reduced, but not quite so much as is the case
in the belemnites. It is coiled and there is no guard or proostracum.
Both are, however, present in the related fossil Spirulirostra (Fig.
410 C). Finally, in Sepia (Fig. 410 D) the guard is represented by the
minute rostrum and, according to one interpretation, one side of the
phragmocone has expanded to cover the surface of the proostracum,
the septa forming the oblique calcareous partitions of the cuttle bone,
while the other side forms a minute lip in which the septa are crowded
together (Fig. 410 D'). The siphuncle (p. 602) is a short wide funnel
in between the two sides.
CEPHALOPODA
6oi
pro-
In LoUgo there is only a horny pen^ which represents the
ostracam, while in the Octopoda there is no skeleton at all.
The Dibranchiata are specialized in two ways. The first is for a
pelagic life; their bodies become elongated, fins develop and they
become transparent. They may, exceptionally, develop such speed in
Fig. 411. External appearance of Dibranchiata. a. Octopus swimming back-
ward, b, Octopus asleep, c, Sepia swimming gently, d, Loligo in the act of
catching prey, e, Sepia becoming active.
the water that they take off from the surface and glide for considerable
distances through the air, in the mariner of the flying fish, aided by
their spreading fins (Todarodes Sagittarius). Loligo (Fig. 411 ^) is a
well-known example of the pelagic type and may be seen in aquaria
swimming in troops, keeping their distances and turning with military
precision.
6o2 THE INVERTEBRATA
The second mode of specialization is for a semisedentary life on
the bottom. In this the body is short and the arms, which are much
larger and more mobile than in the other type, are used for crawling.
Octopus (Fig. 41 1 a) hides itself among stones and seeks its prey only
at night. Sepia and Sepiola^ though capable of active movement,
spend long periods of rest half-covered with sand, assuming by means
of chromatophore expansion brown ripple-marking on their mantles.
The most sedentary form is the flattened Opisthoteuthis ^ which is
almost radially symmetrical and has a remarkable resemblance to a
starfish ; the arms are all joined together and form a suctorial disc by
which the animal applies itself to a rock.
Order TETRABRANCHIATA
Cephalopoda with well-developed calcareous shells. Living forms
with two pairs of ctenidia and kidneys; arms very numerous,
without suckers ; eye simple ; chromatophores absent ; funnel in
two halves.
Suborder Nautiloidea, with membranous protoconch, central
siphuncle and simple suture line, e.g. Nautilus^ Orthoceras.
Suborder Ammonoidea, with calcareous protoconch, marginal
siphuncle and usually complicated suture line, e.g. PhylloceraSy
Baculites,
A brief description of Nautilus, the only surviving cephalopod with
an external chambered shell, must be given here. The shell is coiled
in a plane spiral ; the earliest formed portion was membranous and is
represented by a small central space. In the ammonoids there is a
calcareous chamber, the protoconch, in this position. Succeeding this
are the numerous chambers, separated from each other by the curved
septa, each one marking a stage in the animal's growth. As the shell
is added to, the animal moves forward and from time to time shuts off
a space (the chamber) behind it by the secretion of a new septum. The
terminal living chamber is much larger than the rest and is occupied
by the body of the animal. All the others contain gas (which differs
from air in its smaller proportion of oxygen) ; by means of this the
heavy shell is buoyed up in the water and the animal can swim freely.
The septa are perforated in the middle and traversed by the siphuncle
which is a slender tubular prolongation of the visceral hump. It
contains blood vessels and probably secretes gas into the chambers to
maintain a constant pressure.
The relations of the different parts of the body in Nautilus are easily
compared with those in Sepia (Fig. 402). The shell coils forward over
the neck of the animal (exogastric) ; the mantle cavity is posterior as
in all cephalopods. In other words differential growth of the vis-
CEPHALOPODA 603
ceral hump is not here associated with torsion. The mantle is thin and
adheres to the shell; it cannot therefore be associated with the re-
spiratory and locomotory movements. The "head foot" is produced
into two circles of arms which are very numerous; they are re-
tractile and adhesive but have no suckers. The anterior part of the
region where it touches the shell is very much thickened to form the
hoody and when the animal is retracted into the living chamber the
hood acts as an operculum. The third region of the head foot is the
funnel^ here composed of two separate lobes.
The other principal points in which Nautilus differs from the rest
of the living cephalopods are as follows :
(i) There 2ir&four ctenidia and/owr kidneys, without renopericar-
dial apertures. The pericardium opens independently to the exterior
by a pair of pores. The fact that in the most primitive cephalopod
now existing there is a kind of segmentation of the body cavity and
mantle organs has been taken to support the origin of the cephalopods
from a metamerically segmented ancestor. This "segmentation"
may, however, be secondary. Certainly the absence of a renoperi-
cardial connection is not a primitive feature. There is nothing to
prove that the fossil chambered-shell cephalopods had four ctenidia
and four kidneys.
(2) There are very simple eyes (Fig. 409 A) consisting of an open
pit with no lens, the surface of the retina being bathed by sea water.
This appears to be a primitive feature, but Nautilus is nocturnal and
the eyes may have undergone reduction.
(3) There is no ink sac in Nautilus^ nor apparently in the other
forms grouped in the Tetrabranchiata.
Nautilus lives at moderate depths on some tropical coasts. It either
swims"^near the bottom or crawls over the rocks, pulling itself along
by its arms like Octopus (Fig. 411 b). The gentler swimming move-
ments are caused by the contraction of the muscles of the funnel only ;
the more violent movements are probably caused by the animal
suddenly withdrawing into the shell, thus expelling the water from
the mantle cavity. It is nocturnal and gregarious and a ground feeder.
The chief interest of Nautilus lies in the fact that it is the sole living
representative of a vast multitude of cephalopods with external
chambered shells which flourished between the earliest Cambrian
and the late Cretaceous period, a space of time embracing much the
longest part of the history of life on the earth. After being the
dominant type of marine invertebrate in the Mesozoic they suddenly
became extinct, and the Cephalopoda are now mainly represented by
the Dibranchiata with their internal shells.
The Tetrabranchiata are divided into two groups, the nautiloids
and the ammonoids. The first of these contains Nautilus and other
604 rUV. TNVERTERRATA
forms which agree with it in the position of the sipluincle ami (he
vshape of the septum. They re:icli their maxiiiunn clevelojiment in the
early I'alaeo/.oic, where tlie dominant forms have straiglit shells like
Orihoccras and Actinoceras, which were sometimes as much as 8 feet
long. It is diflicult to suppose that shelled animals of this size were
anythinjr other than sedentary organisms. There is a tendency for
the shell to become coiled in odier forms, exhibiting itself first in
Fijr. 412.
B
T^ijT- 413-
Fir. 412. Nauti/us nuicrowf^liolus ;ulhon"n)^ to the suhstratviin by moans of its
tontaclos in a vertical position. It usually lies horizontally. After Willey.
The shell shows alternate li^ht and dark hands which resemble "ripple-
marking", f//"- dorsal muscular attachments of the funnel ; c. eye ; hd. hood ;
mf. mantle; o. /(•//. ophthalmic tentacles.
I'^i^. 413. A, Phylhiccrds fictcrof^ltyUiim, from the Lias: a part of the shell has
been removeil to expose the sutures, x ,|. H, Suture line of Phyll<ncras lictcro-
l^hylhtni, from the Lias: the arrow indicates the position of the siphuncle
and points towards the aperture of the shell. I'rom Woodward. Natural size.
slightly curved forms like Cyrtoceras, then in loosely coiled forms like
(jyroCcras and finally in the closely coiled Ndutilus. There is also the
reverse tendency, ami in Lituilcs the young shell is closely coiled but
in adult life it straightens out completely.
The ammonoids appeared first of all in the middle of the Palaeozoic
but reached their zenith in the Mesozoic. From the beginning of the
'IVias onward new families, genera ;md sj^ecies are ceaselessly evolved.
These are differentiated by the sliajie and sculj^turc of the shell
CEPHALOPODA
605
whorls, but particularly by the patterns of the suture line^ that is, the
junction line of the septum and tlie outer shell (V\^. 413). These
patterns reach the greatest complexity. A great deal of interest at-
taches to the fact that in these characters the earlier formed chambers
of an ammonoid individual usually differ from those of the adult shell
(h'igs. 413, 414, 415). 'J'here may, in fact, be several changes in the
life of an individual and the succession of such changes has been re-
corded as evidence for tracing the descent of particular ammonoids.
The most striking manifestation of the phenomenon is afforded by
Of
ni
<^i
Es Li S
Li Lz I
a «
i '«• 4^5-
Fig. 414.
Fi;^. 414. Baculites chicoensis Chalk. After Perrin Smith.
Fig. 415. Suture lines of Baculites to show the variation in development at
different af^es. a, first, h, second and c, sixth suture lines of B. rhkoensis \
d, adult septum of B. capensis ; E. external lobe ; Es. external saddle ; /. internal
lobe ; //J , L^ , first and second lateral lobes ; .Sj , first lateral saddle ; .S'2 , internal
(dorsal) saddle, a-c after Perrin Smith, d 'dftcr Spath.
ammonoid stocks, particularly in the Cretaceous, in which the ap-
proach of extinction is heralded by " uncoiling" in various stages. In
Scaphttes the shell is coiled in youth but later straightens out and
finally hooks back. In Baculites (Fig. 414) only the very earliest
chambers forjn a coiled shell; nearly the whole of the shell is straight.
But the suture lines, though tending to become simplified, show the
type of the family from which the uncoiled form is derived, and it is
possible to show quite definitely that such genera as ** Scaphttes" and
''Baculites" are not natural but polyphyletic; both scaphoid and
baculoid forms occur in different lines of descent.
CHAPTER XVII
THE MINOR COELOMATE PHYLA
PHYLUM POLYZOAi
Coelomate unsegmented animals, always sedentary and nearly always
colonial; with a circumoral ring [lophophore) of ciliated tentacles, and
a U-shaped alimentary canal; usually with a ciliated free-swimming
larva; asexual reproduction by budding.
The ordinary individuals in a colony of polyzoa at first sight re-
semble hydroid polyps — in their general shape, size and circle of
tentacles. Closer inspection shows that they are triploblastic animals.
In the majority of the Ectoprocta each individual consists of two
distinct parts, the zooecium or body wall and the polypide^ consisting
of the alimentary canal, the tentacles and the tentacle sheath (which
contains the tentacles when contracted). The polypide can be entirely
retracted within the zooecium and, as will be seen below, has a much
shorter life than the latter.
The form chosen for illustration, Plumatella (Fig. 416), belongs
to an order (Phylactolaemata) of the Ectoprocta in which the lopho-
phore is not a simple circle, as is the case in the other order, the
Gymnolaemata, but is horse-shoe-shaped. A small ridge, the epistomey
overhangs the mouth in this group but not in the Gymnolaemata.
The mouth opens into the oesophagus which passes into a capacious
stomach with a caecum which is attached by a strand of mesoderm,
the funiculus , to the body wall. From the upper end of the stomach,
the intestine runs to the anus which is situated just outside the
lophophore. The food, consisting of small organisms like diatoms
and protozoa, is collected by the cilia of the lophophore and trans-
ported through the whole of the alimentary canal by cilia.
The body cavity (also in all Ectoprocta) is a true coelom containing
a colourless fluid, and the cells which line it give rise to the germ
cells. Polyzoa are hermaphrodite; the testes are formed on the
funiculus and the ovary on the body wall. When the germ cells are
ripe the so-called ifiter tentacular organ often appears ; this is a tube
which opens within the lophophore and serves for the escape of the
genital products. Part of the coelom is shut off from the rest by an
incomplete septum, as the ring canal which is prolonged into the
tentacles. The intertentacular organ opens internally into this.
The nervous system is represented by a single ganglion, situated
^ Often called Bryozoa.
POLYZOA 607
between the mouth and the anus, and many nerves chiefly supplying
the tentacles and gut. There are no special sense organs. No trace of
a vascular system exists.
A remarkable phenomenon very characteristic of the Polyzoa is the
formation of the brown body. Tentacles, gut, in fact the whole of the
m,retr.
staU — IL'
Fig. 416. View of right hali of Plumatellafungosa, slightly diagrammatic. After
Allman and Nitsche. an. anus ; bzv. body wall ; ep. epistome ; fn. funiculus ;
ga. ganglion ; int. intestine ; Iph. lophophore ; M. mouth ; m.retr. retractor
muscles of polypide; oe. oesophagus; ov. ovary; p.m. parietal muscles; sp.
spermatozoa; st. stomach; stat. statoblast; t. testis; t.' the same, more
mature ; tb. wall of tube ; ten. tentacles.
polypide, degenerates and forms a brown, compact mass. A new poly-
pide is regenerated from the zooecium and the brown body often
comes to lie in the new stomach and is evacuated through the anus.
This periodical renewal of the individual is a normal process in most
polyzoa. It was formerly explained as due to the accumulation of
6o8 THE INVERTEBRATA
excreta in the absence of specific excretory organs. It can, however,
be hardly doubted that animals so small and with so great an area of
epithelium in contact with the water are able to rid themselves of
excreta in a simpler fashion.
As triploblastic metazoa with a centralized nervous system the
Polyzoa possess a more efficient contractile mechanism than the
hydroids. The most prominent feature of this is the parietal system
of muscles which circle round the body wall. By their contraction
the internal pressure is raised and the polypide protruded. The re-
tractor muscle which runs from the lophophore to the opposite end of
the zooecium has an opposite action to the parietal system . The Polyzoa
are fascinating but exasperating objects under the microscope: they
emerge with infinite caution from the zooecium and withdraw with
incredible rapidity. With the lophophore a flexible part of the body
wall is also invaginated and this is called the tentacle sheath.
The colonies of polyzoa differ greatly from those of hydrozoa in
their habit and this is largely due to the absence of a connecting coeno-
sarc. They are often incrusting like Membranipora and Flustra (hence
the name of '* sea mats "), with all their zooecia packed closely together
in a single layer ; they may also be slender or massive ; in the latter
case they have a superficial resemblance to the actinozoan corals.
While the outer layer of the body wall is often horny or flexible it
frequently becomes incrusted with calcium carbonate and thus
rendered rigid.
In the incrusting Polyzoa, especially the Cheilostomata, the zooecia
are rigid boxes, in contact with one another along all four sides and
with the substratum at the bottom. These are usually strongly calcified
and only the top of the box, the frontal surface, is flexible (Fig. 417 A,
B). The parietal muscles, which in primitive polyzoa formed a con-
tinuous layer of circular muscles as in Chaetopoda, here form de-
tached groups running from the side walls through the coelom to the
frontal surface. When the muscles contract the latter is depressed and
the lophophore is protruded. The process of calcification may extend
to the frontal membrane and the mechanism of protrusion has then
to be altered. In one large group of the Cheilostomata, there is a
membranous diverticulum of the ectoderm under the calcareous
frontal surface. This is called the compensation sac (Fig. 417 C); to its
lower surface the parietal muscles are attached. When they contract
and the tentacles are extruded the sac fills with water, and when they
relax the sac empties.
Polymorphism is a feature of polyzoan as it is of hydrozoan colonies.
Perhaps the most remarkable modifications are to be seen in the in-
dividuals known as vibracula and avicularia of such forms as Bugula
(Figs. 418 A, 419). The vibracula are nothing more than long bristles
POLYZOA 609
which are capable of movement and often act in concert throughout a
part of the colony, sweeping backwards and forwards over the surface,
preventing larvae and noxious material from settling on the colony.
The avicularia resemble the head of a bird, possessing a movable
mandible which is homologous with the operculum of an unmodified
polyp, and this is provided with powerful muscles. The avicularia
suddenly snap their jaws and enclose as in a vice small roving animals
which touch them, particularly the larvae of incrusting animals. In
the most primitive cases, an avicularium is found in the same position
in the colony as an ordinary zooecium and may even possess a func-
ten.8.
cn.f.s. C;8. ten.8.
jopc.
Fig. 417. Protrusion of the polypide in two types of cheilostomatous Polyzoa.
Membranipora. After Harmer. A, With polypide retracted. B, With polypide
protruded. C, A form with a calcareous frontal wall. An. anus; ca. calcified
cuticle of zooecium ; c.s. compensation sac ; f.s. frontal surface ; ga. ganglion ;
int. intestine ; oe. oesophagus ; ope. operculum ; m.retr. retractor muscle of
polypide; p.m. parietal muscles; st. stomach; ten.s. tentacular sheath.
tional polypide. Further evolution led to displacement of the avicu-
laria so that they became appendages of other zooecia, situated near
the orifice. The two kinds of modified individuals thus perform tasks
which in the Hydrozoa are allotted to the dactylozooids and in the
Echinodermata to the pedicellariae.
Most of the Polyzoa are marine an~d are amongst the most familiar
objects of the beach. A complete division, the Phylactolaemata, are
freshwater. The marine forms possess a variety of free-swimming
larvae, which are of the trochosphere type. In the Phylactolaemata,
certain internal buds called statoblasts are formed from lens-shaped
6io
THE INVERTEBRATA
masses of cells on the funiculus and are enclosed by chitinous shells.
The polypides die down during the winter and in the spring the
statoblasts germinate and produce new colonies.
avc-
Fig. 418, Polymorphism of Polyzoa. After Harmer. A, Bugula. Portion of a
colony, avc. avicularium; ovc. ovicell; tentacles of ordinary individuals,
ten. protruded, ten/ retracted. Crista. B, Portion of colony with ovicell (ovc),
surface view. B', Section through ovicell to show emb.' primary embryo;
emb." secondary embryos ;/o/. follicular tissue.
Fig. 419. An avicularium of Bugula. Magnified. From Hincks. b. beak;
C. chamber representing the body cavity of the modified individual; dm.
muscle which opens, om. muscle which closes the mandible on the beak;
md. mandible, the operculum of the modified cell ; p. stalk.
The free-swimming larvae may be assigned to the " trochosphere "
type. In most cases they are much modified and only the larvae of
the Entoprocta and the Cyphonautes larva among the Ectoprocta
possess an alimentary canal and are able to feed. The Cyphonautes is,
POLYZOA
6ll
then, the typical form (Fig. 420). It possesses a bivalve shell, each
valve being triangular. The apical organ and ciliated ring (correspond-
ing to the prototroch) can be seen projecting from between the valves,
and in addition there are various characteristic organs, such as the
internal sac, by which attachment is effected, prior to metamorphosis,
and the pyriform organ of unknown function. On attachment the
alimentary canal degenerates and the first individual of the colony
is formed from a polypide bud consisting of an internal layer of
Fig. 420. Cyphonautes larva seen A, in side view, B, in oral view. al.c. ali-
mentary canal; An, anus; ap.o. apical organ; cil.r. ciliated ring; coe. coelom ;
i.s. internal sac ; M. mouth ; p.o. pyriform organ ; ve. vestibule.
ectoderm and an external of mesoderm. The ectoderm gives rise to
the tentacles and tentacle sheath, the ganglion and the alimentary
canal of the new polypide. A polypide bud which develops in exactly
the same way is formed in the course of regeneration after the forma-
tion of a brown body. In the Endoprocta the larva fixes by its oral
surface and undergoes metamorphosis into the adult in the course
of which the mouth rotates upwards (compare Cirripedes, Fig. 256).
The alimentary canal does not degenerate.
In the Cyclostomata it is probable that the fertilized egg never de-
6l2
THE INVERTEBRATA
velops into a single individual but always into a large number by what
is known as embryonic fission, such as occurs in the parasitic Hymen-
optera. A much modified zooecium, the so-called ovicell, serves as
a brood pouch and in that the primary embryo is formed and attached
to follicular tissue which supplies nourishment. Masses of cells are
nipped off to form the secondary embryos each of which becomes a
free-swimming larva.
Fig. 421. Section through an endoproctous polyzoan. Altered from Ehlers.
An. anus; at. atrium; ga. ganglion; g.gl. gonad; g.op. genital opening;
int. intestine ; M. mouth ; mr. sphincter muscle of circular flap of body wall ;
oe. oesophagus ; par. parenchyma ; st. stomach.
Classification
Class Endoprocta (Fig. 421). Simple and archaic polyzoa in which
the anus is situated inside the lophophore ; without coelom, the
space between gut and body wall being filled with paren-
chymatous tissue; with non-retractile tentacles which can be
covered by a circular flap of the body wall, provided with a
sphincter muscle; with a pair of protonephridia ending in flame
cells, and gonads with a duct of their own; with a trochosphere
larva. Pedicellina, Loxosoma.
BRACHIOPODA 613
Class EcTOPROCTA. Polyzoa with anus outside the lophophore; with
a coelomic body cavity and a lophophore retractile into a tentacle
sheath; without definite excretory organs.
Order Phylactolaemata. Freshwater Ectoprocta with a horseshoe-
shaped lophophore, an epistome and statoblasts. Plumatella^
Cristatella.
Order Gymnolaemata. Ectoprocta mostly marine, with a circular
lophophore, without an epistome, with various types of trocho-
sphere larva.
Suborder Cyclostomata with tubular zooecia, aperture without
operculum, embryonic fission characteristic. Crista.
Suborder Cheilostomata, with aperture of zooecium closed by an
operculum. Bugula, Flustra, Memhranipora.
Suborder Ctenostomata with aperture of zooecium closed by a
folded membrane when the lophophore is retracted.
Alcyonidium.
It is possible that the Endoprocta should be separated from the
Ectoprocta as a distinct phylum and associated with forms like the
rotifers.
PHYLUM BRACHIOPODA
Coelomate unsegmented animals with a bivalve shell which is always
attached, the valves being respectively dorsal and ventral in position;
a complex ciliated circumoral organ, the lophophore^ which maintains
a circulation of water in the mantle cavity and leads food currents to
the mouth.
The group contains only marine animals with a strong but super-
ficial resemblance to the lamellibranchs among the Mollusca. In the
Palaeozoic and Mesozoic it was more abundantly represented than
the Mollusca, but at the present day it contains but few genera and
species. Of the former Terebratula and Waldheimia (in which the
valves meet to form a hinge and which belong to the Testicardines)
are found in deep water off our own coasts. Examples of hingeless
forms (Ecardines) are Crania which occurs abundantly in shallow
water in the West of Ireland, and Lingula, which is not found in
Britain, but in the tropics is sometimes exceedingly abundant in
mud between tidemarks.
In such forms as Waldheimia and Terebratula (Figs. 422, 423), the
ventral shell valve is larger than the ^dorsal and has a posterior beak
or umbo perforated by a round aperture through which passes the
stalk for attachment to a stone or rock. Each valve is secreted by a
corresponding mantle flap, but in a way which diflFers from the corre-
sponding process in the Mollusca. The mantle epithelium is produced
6l4 THE INVERTEBRATA
into minute papillae which traverse the substance of the shell. The
cells, of which the papillae are composed, are often of a minutely
di.ql.
r
Fig. 422. Longitudinal section of Magellania {Waldheimia) slightly to the
left of the middle line. After J.J. Lister, bw. body wall ; di.gl. digestive gland ;
h. heart; int. intestine; Ip. dorsal lip of Iph. lophophore; M. mouth; tn.d.
muscles running from dorsal valve to ventral ; nphr. nephrostome ; ri. vertical
ridge on dorsal valve; st. stomach; stk. stalk; tn. tentacles of lophophore;
tn.t. terminal tentacles.
Fig. 423. Fig. 424.
Fig. 423. Terehratula semiglobosa, Upper Chalk. A, Dorsal; B, Lateral view,
fl, posterior ; b, anterior ; a-b, length ; c-d, breadth ; e-f, thickness ; g-h, hinge
line, X f . From Woods.
Fig. 424. Vertical section of shell of Magellania {Waldheimia) flavescens.
a, prismatic layer; b, periostracum ; c, outer calcareous layer; d, e, canals
traversing the calcareous layers, containing the mantle papillae in life.
After King.
branching type which resemble the bone corpuscles of vertebrates.
It must be supposed that the papillae are concerned with the secretion
and growth of the shell. Each valve (Fig. 424) is composed of an outer
BRACHIOPODA 615
layer of organic material (pertostracum)^ under which is a thin layer
of pure calcium carbonate and a thick inner prismatic layer composed
mainly of calcareous but partly of organic material. The shell valves
are opened and closed by a muscle system which is much more
complicated than that of the lamellibranchs.
The hinge line is posterior and the mantle cavity is thus anterior.
On opening the shells it is seen to be largely occupied by a compli-
cated organ known as the lophophore of which a description follows.
The mouth is placed in a transverse groove which is bounded, dorsally
by a continuous lip and ventrally by a row of tentacles. The groove
is enormously extended and its boundaries drawn out laterally into
two arms which are often coiled spirally in these and other members
of the phylum. The tentacles are long and may be protruded from
the shell opening. The cilia on the tentacles and on the mantle sur-
Fig. 425-
-jiL.ped.
^■;-
Fig. 425. Crania attached to a stone in the act of feeding with protruded
tentacles. AA, ingoing, B, outgoing currents. After Orton.
Fig. 426. Lingula in positions of life in mud (indicated by stippling), i, feed-
ing position with peduncle {ped.) extended; 2, position when peduncle is
contracted ; ch. chaetae fringing entrance to shell. Arrows indicate currents.
faces produce two ingoing currents of water at the sides opposite the
two arms of the lophophore; the outgoing current is central, between
the two arms (Fig. 425). This ciliary mechanism is similar to that of
the lamellibranch ctenidium. On each side the current of water is
6l6 THE INVERTEBRATA
directed between the tentacles of the lophophore, and the smaller and
lighter particles suspended in it are sieved away and pass into the
ciliated buccal groove and so towards the mouth. Heavier particles
drop on to the ventral mantle lobe and are removed by outgoing ciliary
currents and sudden clapping movements of the valves. When the
ingoing currents have passed between the spirals of the lophophore
they unite in the median dorsal part of the mantle cavity and become
the outgoing current. The lophophore of Testicardines is supported
by calcareous processes of the dorsal valve (the brachial skeleton)
which assumes diverse and diagnostic forms in the different genera.
The mouth leads into a ciliated alimentary canal. There is a stomach
into which opens the digestive gland composed of branching tubes in
the cavity of which most of the digestion takes place. In Waldheimia
the intestine ends blindly, but in Lingula and Crania there is an anus.
The coelom is spacious and divided into a right and left half by a
dorsoventral mesentery; transverse mesenteries also exist. It is pro-
longed into the lophophore and tentacles and into the mantle as the
pallial sinus. A pair of segmental organs, short tubes with large
nephrostomes, which also function as generative ducts, are situated
in the coelom; their external openings are at the sides of the mouth.
The generative organs are developments of the coelomic epithelium
and eggs and sperm alike dehisce into the body cavity. The sexes are
usually separate in the brachiopods.
The blood system is very little developed and consists only of a
longitudinal vessel in the dorsal mesentery, in one region of which a
contractile vesicle may be distinguished as the heart, and a number
of vessels running forward to the mouth and backward to the mantle
and generative organs ; all end blindly.
The nervous system mainly consists of a supraoesophageal and
a suboesophageal ganglion in front of and behind the mouth respec-
tively, connected by circumoesophageal connectives. A nerve runs
to each tentacle but no special sense organs are known.
Lingula (Figs. 426, 427 H) is a persistent form, which has lived since
the earliest period of which we have an organic record, the Cambrian,
in precisely the same stage of development, if we can judge from the
hard parts. It lives in mud or sand and has a very long contractile
stalk by which it roots itself and can withdraw from the surface. The
opening of the shell is usually situated near the surface and the mantle
secretes chaetae, like those of annelids, which project from the an-
terior border, and with the help of mucus and the mantle border form
inhalant siphons at the side and an exhalant siphon in the middle. The
shell valves are equal in size and horny in consistency, being composed
of alternating layers of chitin and calcium phosphate.
Crania (Fig. 425) is a form without a stalk. The ventral valve is fiat
BRACHIOPODA
617
arch^
arm /(|^ ^ P'.^- 'V^^^
mM^^.
^>
Fig. 427. Development of Brachiopoda. A, Section of larva at end of
gastrulation showing the two coelomic pouches originating from the archen-
teron. B, Larva divided into three regions. C, Differentiation of the preoral
region (oblique shading) and mantle lobes (stippling). D, Turning forward
of mantle lobes and shrinking of preoral region. E, Appearance of the arms
of the lophophore (one shown), preoral region now represented by lip. F, In-
ternal view of dorsal valve showing the first stage in development of lopho-
phore as a tentacular ring. G, Further development by extension of the dorsal
lip and the division of the ring into two arms. The ciliated groove is indicated
by stippling and the movement of food to the mouth by arrows. H, Larva
oi Lingula, corresponding to F. al.c. alimentary canal; An. anus; arch, arch-
enteron ; arm, one arm of the lophophore ; coe. coelomic pouch ; Ip. dorsal lip ;
M. mouth; w./. mantle lobe; stk. stalk; ten. tentacles; ch. chaetae; e. eyes.
Altered from Delage and Herouard, after various authors.
6l8 THE INVERTEBRATA
and fixed by its whole surface to a rock; the dorsal valve is conical.
The tentacles of the lophophore are protruded from the shell margin.
The Brachiopoda have free-swimming larvae which are usually
divided into three regions, an anterior like the preoral region of the
trochosphere, a median region in which the two lobes of the mantle
are early produced, and a posterior one, hidden by the mantle lobe,
which becomes the stalk (Fig. 427 B). The mantle lobes develop four
bundles of chaetae (Fig. 427 C), and then turn forward to envelop
the anterior region (Fig. 427 D). This now begins to develop the
lophophore (Fig. 427 E, F, G) and shell valves form on the mantle
lobes, while the posterior region grows into the stalk.
The coelom develops as a pair of pouches or a single pouch from
the archenteron (Fig. 427 A). Though the presence of mantle lobes,
the presence of chaetae and the resemblance of the larva to a trocho-
sphere relates the Brachiopoda to the annelid-mollusc stock, there
is no evidence of segmentation and they cannot come very close to the
Annelida ; but possibly are nearer to the MoUusca. On the other hand
the enterocoelic development of the body cavity suggests affinities
to the echinoderms and chordates.
Classification
EcARDiNES. Brachiopoda having shells with no hinge, no internal
skeleton, and alimentary canal with an anus. Lingula^ Crania.
Testicardines. Brachiopoda having shells with hinge and internal
skeleton, without anus. Terehratula^ Waldheimia.
PHYLUM CHAETOGNATHA
Coelomate animals with an elongated body divided into three regions,
head, trunk and tail, and with lateral and caudal fins ; head with a
pair of eyes and two groups of chitinous teeth and jaws; cerebral
ganglion and ventral ganglion (in the trunk) connected by circum-
oesophageal commissures ; body wall containing a layer of longitudinal
muscle cells of peculiar type arranged in four quadrants ; alimentary
canal straight ; no localized excretory or respiratory organs or vascular
system; hermaphrodite and cross-fertilizing; free-swimming larva.
The structure of an individual of this small and homogeneous
group is shown in Fig. 428. Very little need be added to the definition.
The muscles are of a primitive type, each elongated cell consisting
of a core of unmodified cytoplasm and an outer shell ring of con-
tractile substance ; they have thus some resemblance to those of the
nematodes. The chaetognaths are, however, capable of executing
very rapid movement by suddenly contracting these longitudinal
muscles and are able to pounce upon and capture their food, which
CHAETOGNATHA
619
\—al.c.
■ga.v.
consists of diatoms, copepods and larvae of various kinds including
fishes, in fact of most of their plank-
tonic neighbours. These are seized
by the hook-like jaws and swallowed
whole.
The coelom is well developed with
a distinct epithelial lining, and it is
divided into two halves by a complete
median and vertical mesentery, and
also by two transverse septa into three
chambers corresponding to the head,
the trunk and the tail. Of these the
head cavity is mainly occupied by the
jaw muscles, while in the trunk and
tail cavities are developed the ovaries
and the testes respectively. l^\iQ ovaries
(Fig. 429) are elongated solid organs
attached laterally to the body wall.
Traversing each ovary on its inner
side is a duct with a blind anterior
end [oviduct)', this encloses a second
duct [sperm pouch) also with a blind
anterior end and with indefinite walls, /
containing sperm derived from an- '
other animal. Both ducts open into fn.
a small bulblike seminal receptacle with \
an external aperture just in front of \
the second septum. The maturing
egg is fertilized by a spermatozoon
which passes into the ovary from the
second duct and the zygote then passes
through the wall of the oviduct and
then to the exterior.
There is a solid testis in each half of
the tail cavity and from these sperm
mother cells are constantly budded off
into the coelom, which is thus filled
with sperm in all stages of develop-
ment. The sperm passes into vasa jrig. 428. Sagittahexaptera.\en-
deferentia, which are long tubes with tralview, X2h- After O. Hertwig.
a small internal opening behind the ale. alimentary canal; An. anus;
testes and a terminal dilatation, the /": ^^^^^''•''- ^!^^^^^ ganglion;
vesicula seminalis , ^\i\ch opens to the ^ol^^\sp:'^'inLluAe^^^^^
exterior. deferens ; v.s. vesicula seminalis.
■ov.
od.
\--ts.
>—v. s.
620
THE INVERTEBRATA
The eggs are laid in the sea and develop rapidly, passing through
typical blastula and gastrula stages, after which the coelom is deve-
loped as a pair of anterolateral pouches of the archenteron (Fig. 430 A).
After gastrulation two cells become very prominent. These are the
mother cells of the generative organs. The primary coelomic cavity is
mes.
Fig, 429. Transverse section through middle of trunk of Sagitta bipunctata.
After Burfield. al.c. alimentary canal (intestine) ; gl.c. gland cells (the cells
which are not stippled are absorptive cells) ; lat.fn. lateral fin ; mes. mesentery ;
od. oviduct; ov. ovary (covered by endothelium); sp.d. sperm pouch.
std.
hd-coe.
-\-<il.c.
Fig. 430. Larvae of Sagitta showing formation of coelomic pouches from the
archenteron. After Burfield. In A the pouches still open into the archen-
teron. In B the pouches forming the head coelom have completely separated
oflF from the archenteron and the archenteric folds have grown back so as
partly to separate off the second pair of pouches, al.c. alimentary canal ; bis.
blastopore; ect. ectoderm; end. endoderm; g.c. genital cells; hd.coe. head
coelom ; M. mouth ; std. stomodaeum.
divided up first of all by the separation of the head cavity (Fig. 430 B)
and at a later stage by a second septum between trunk and tail, which
divides the genital cells, which now number four, into an anterior
pair, the mother cells of the ovaries, and a posterior pair, those of the
testes.
CHAETOGNATHA
621
Sagitta bipunctata is one of the most characteristic and cosmo-
pohtan members of the plankton and is a typical pelagic organism
ms.s.
\ ' I / I I
Fig. 431-
Fig. 432.
Fig. 431. Phoronis. Altered from Delage and Herouard. Sagittal section
to show half the lophophore, the alimentary canal, one of the nephridia
and most of the vascular system. The middle part of the section is omitted.
An. anus ; ajf.v. afferent and eff.v. efferent vessels ; ep. epistome ; ga. ganglion ;
int. intestine ; Iph. lophophore ; Iph.v. lophophoral vessel giving off a branch
to each tentacle; nep.a. nephridial aperture; nephr. nephrostome; 7i.r. nerve
ring; oik ovary; org. lophophoral organ (paired); sep. septum between two
divisions of the coelom ; st. stomach ; ts. testis ; ten.v. tentacular vessel.
Fig. 432. Actinotrocha and its metamorphosis into the adult. After various
authors. A, Actinotrocha larva with ciliated lobes (cil.l.) and rudimentary
visceral sac (vis.s.). B, \isceral sac evaginated. C, Growth of visceral sac and
decrease in size of preoral lobe (pr.L). D, Metamorphosis : mouth and anus
approximate, preoral lobe becomes epistome (ep.), ciliated lobes of larva
are seen in the mouth and new tentacles "are beginning to grow. An. anus ;
M. mouth. Alimentary' canal shown by stippling, visceral sac by oblique
shading.
with its glassy transparent body and its powers of vertical migration ;
off the coast of California it lives at a depth of 15-20 fathoms during
622 THE INVERTEBRATA
the day and the greater part of the night, but at sunrise and sunset it
rises to the surface, the Hght intensity and temperature there being
at an optimum for the species at those times.
The Chaetognatha are a very early offshoot of the coelomate stock
and cannot very well be compared to any other phylum. While it is
tempting to liken the tripartite division of the coelom in Chaeto-
gnatha with that in echinoderms and protochordates, it must be
realized that in Sagitta the two transverse septa arise at different
times and for different reasons. There is, however, a true tail here
which is elsewhere found only in the Chordata and the development
of the body cavity is enterocoelic.)
The fossil, Amtskwia, occurring in the Cambrian, has been assigned
to this group, but it appears to differ from the living forms in the
absence of a septum between trunk and tail and in the presence of
tentacles on the head.
PHYLUM PHORONIDEA
Coelomate unsegmented animals, sedentary, hermaphrodite and tubi-
colous, with a horseshoe-shaped lophophore, an epistome, a vascular
system with haemoglobin, and two excretory organs.
This is a very small group: the genus Phoronis (Fig. 431) includes
most of the species. They are all marine animals, usually of incon-
siderable size, and like all sedentary forms they have a free-swimming
larva; this is called an Actinotrocha and it can be referred to the
trochosphere type. It passes into the adult by a remarkable meta-
morphosis which is illustrated in Fig. 432.
Phoronis has a strong resemblance to a polyzoan like Plumatella but
it differs from such a form in the presence of a vascular system and
in other respects.
CHAPTER XVIII
THE PHYLUM ECHINODERMATA
Coelomate animals; bilaterally symmetrical as larvae, radially sym-
metrical as adults ; whose dermis contains calcareous ossicles ; whose
coelom in the larva consists of three segments, and in the adult forms
a perivisceral cavity and several intricate systems of spaces, one of
the latter being a water vascular system which pushes out the surface
of the body as a series of delicate tentacles, the podia or tube feet ;
whose vascular system is represented by strands of lacunar tissue;
whose principal nervous system remains in contact with the ectoderm
from which it arose (though it may be invaginated with the latter) ;
which have no nephridia; and whose gonads discharge direct to the
exterior by special ducts.
The group includes the animals familiarly known as starfishes
(Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea),
sea cucumbers or trepangs (Holothuroidea), and sea lilies (Crinoidea)
(Fig. 435)-
The great unlikeness between these animals and all other coelomata
is chiefly due to the radial symmetry which they assume at meta-
morphosis and which distorts all their systems of organs to its own
mould. The radii, which are nearly always five in number, diverge
from the mouth. The surface of the body upon which the mouth lies
is known as the oral or ambulacral, the opposite surface as the aboral
or abambulacral. The terms ''ventral" and "dorsal" should not be
applied to these surfaces, for they correspond not to the ventral and
dorsal but to the left and right sides of the larva. The anus, if present,
lies usually on the aboral side, but in the Crinoidea it lies on the oral
side. The alimentary canal runs a straight or devious course from
mouth to anus. The other systems consist each of a ring around the
axis which passes through the mouth and the middle of the aboral
side, and a tube or cord along each radius. The radii are constituted
by the presence of the radial members of the various systems. The
areas between the radii are known as interradii. Most of the systems
lie close under the ambulacral surface, and the tube feet project from
it, forming radial bands known as the ambulacra. In the Asteroidea
and Crinoidea the tube feet of each ambulacrum stand on either side
of an ambulacral groove at the bottom of which lies the highly nervous
strip of epithelium which is the radial "nerve cord". In the other
classes the ambulacral groove is roofed in, forming an epineural canal
over the nerve cord. In the Asteroidea, Ophiuroidea and Crinoidea
624 THE INVERTEBRATA
the body is prolonged as arms in the direction of the radii, and the
ambulacral and abambulacral surfaces are subequal in extent. On the
other hand, in the spherical or cushion-shaped Echinoidea and the
sausage-shaped Holothuroidea, the body is compact, and the am-
bulacral surface extends over most of it, leaving only in the Echinoidea
a small, and in the Holothuroidea a minute, aboral area opposite to
I'igr- 433- Asterias ruhe?ts. A, Part of an oral view: in one of the arms shown
the adambulacral spines have closed over the ambulacral groove ; in the others,
the radial nerve can be seen. B, An aboral view of the disc, showing the
madreporite. C, The tip of an adambulacral spine, showing pedicellariae.
the mouth (Fig. 434). Externally and internally the symmetry is
never quite perfect. At best the presence of the madreporite (see
below), or of the anus, or of a genital opening, differentiates one of the
interradii, and in some echinoids and holothurians a new and con-
spicuous bilateral symmetry has developed, and affects a number of
organs.
ECHINODERMATA 625
In life, the Crinoidea are fastened to the ground by a stalk which
arises from the middle of their aboral surface, and, though a few of
them break free when they are adult, the mouth is directed upwards
by them all. The other existing groups (Eleutherozoa) are free. In
the Asteroidea, Ophiuroidea, and Echinoidea the mouth is directed
downwards. The Holothuroidea apply one side of the long body to
the ground, so that the mouth is directed horizontally (Fig. 435).
The tube feet {podia), whose function was perhaps originally a
sensory or food-collecting one, are (or some of them are) in the
nrmrinnnnnni^^. ^^^^'I'tnnnniTOP^
or
abo
2 6r
abo
^or
abo -J
Fig. 434. Diagrams to show the relative extent of the oral and aboral sur-
faces, and to compare the form of body, in the several classes of the Echino-
dermata. All the diagrams are in the same morphological position. From
Borradaile. i, Asteroidea. 2, Ophiuroidea. 3, Echinoidea. 4, Holothuroidea.
5, Crinoidea. abo. aboral surface; or. oral surface.
Asteroidea, Echinoidea, and Holothuroidea adapted, by the presence
of suckers at their ends, to walking. Probably they always subserve
respiration, and in the "irregular" echinoids some of them are
modified for this function. They may also be modified for seizing
food. They are protruded and retracted by alterations of the pressure
of the fluid within them by the action of the water vascular system
(see below).
The epidermis is usually ciliated, but not in ophiuroids or, except
in the ambulacral groove, in crinoids. Usually, also, it contains gland
cells and sense cells, the latter with their bases prolonged into fibrils
which enter a plexus, formed by them and by branched nerve cells,
626
THE INVERTEBRATA
among the bases of the epithelial cells — a nerve- net. The characteristic
ossicles of the dermis may be scattered, so as merely to impart a
leathery consistency to the skin, or united by muscles as a skeleton,
or firmly apposed so as to constitute an armour. Some of them usually
project as spines, over which the epidermis may presently wear away.
Pedicellariae (Figs. 433 C, 449) are sets of two or three spines ar-
ranged to bite together as pincers. They are of various types, often
complicated, but only occur in asteroids and echinoids.
The alimentary canal differs greatly in the several groups. It is
axial in the Asteroidea and Ophiuroidea, coiled in the other classes.
It possesses various diverticula in different cases, but not large glands
,1!^'
T^^r:rti^^:
^^P^^
Fig- 435- Representatives of classes of the Echinodermata, in their natural
positions. A, A starfish (Asteroidea). B, The shell of a sea urchin (Echinoi-
dea). C, A holothuhan. D, A sea lily (Crinoidea).
like those which are common in other phyla. The anus is lacking in
the Ophiuroidea and a few asteroids, and when present is more or less
excentric except in the Holothuroidea.
The coelom of the adult is present as several distinct systems of
spaces, of which the following are the most important: (i) the large
perivisceral cavity in which lie all the principal viscera; (2) the peri-
haemal system, consisting of a radial vessel (in asteroids divided longi-
tudinally by a vertical septum in which lies the principal "blood"
strand) along each radius, and a ring vessel around the mouth, all
lying immediately above the main nerve cords; (3) the aboral sinus
system enclosing the genital rachis and gonads (see below); (4) the
water vascular system (Fig. 436), which lies above the perihaemal
system, and consists of a ring around the mouth, a tube, known as the
stone canal because its wall is frequently calcified, leading to an open-
r w.v.
ECHINODERMATA 627
ing known as the madreporite (see below), a radial vessel along each
radius, and lateral branches from the radial vessels to the tube feet,
which, when the latter are used for walking, possess swellings known
as ampullae ^ by whose contractions the
feet are extended; (5) the madreporic
vesicle^ an inconspicuous cavity of mor-
phological importance (see below);
(6) the axial sinus. This is a space which
varies greatly in its development. It is
conspicuous in the Asteroidea, small in
the Echinoidea and Ophiuroidea, very
small in the Holothuroidea, merged in
the perivisceral cavity in the Crinoidea.
It communicates with the exterior (or, as
will be seen, in most holothurians with
the coelom) by a pore or set of pores
situated in one of the interradii. This
opening constitutes the madreporite.
The stone canal opens into the axial
sinus just below the madreporite, and so
the latter serves as the opening of the
stone canal. In the Asteroidea and
Echinoidea the madreporite is a con-
spicuous structure on the aboral side, pierced by many pores. In the
Ophiuroidea it is on the oral side, and has one pore, or only a few
pores. In most of the Holothuroidea it becomes detached, in the
course of development, with its tiny axial sinus, from the body wall,
and hangs into the perivisceral cavity, with which, instead of with the
exterior, it now makes communication, by a number of pores. In this
group, by meristic repetition, there may be several or many stone
canals, each with an "internal madreporite". In the Crinoidea, the
stone canals, of which there are several, end each by a single opening
into the perivisceral cavity, and the latter communicates by a number
of pores with the exterior.
In the bilateral larva {Dipleurula) ^ the coelom (Fig. 437) is present
as three pairs of sacs, of which the first is preoral. The second pair is
connected by a passage with the first: the third is independent. In
outline, the relation between these sacs or segments of the larval
coelom and the coelomic spaces of the adult is as follows : the peri-
visceral cavity of the adult is formed by the fusion of the main portions
of the hinder pair ; the aboral sinus system becomes separated from
the perivisceral cavity; the perihaemal system arises as outgrowths
from the left hinder cavity (in some cases it receives a component also
from the left anterior cavity); the water Vascular system ("hydro-
Fig. 436. A diagram of the
water vascular system of a
starfish. FromBorradaile.am/).
ampulla ; mad. madreporite ;
r.w.v. radial water vessel ; st.c.
stone canal; ?./.z;. vessel of tube
foot; w.z;.r. water vascular ring.
628
THE INVERTEBRATA
code") is formed by the transformation of the left second cavity (the
right second cavity disappearing) ; the axial sinus is the persistent left
anterior cavity, its madreporite being derived from a "water pore"
which puts that cavity into communication with the exterior. The
opening of the stone canal into the axial sinus is the remains of the
connection between the left anterior cavity and the left second cavity,
which latter, as we have seen, becomes the water vascular system.
The madreporic vesicle is budded off from the right anterior cavity
(the rest of which disappears) ; in the larva this vesicle pulsates ; it
probably represents the pericardium of the Hemichorda, which
retains its contractile function in the adult (pp. 667, 668).
All echinoderms except the Holothuroidea possess a peculiar
structure known as the axial organ, composed of connective and
lacunar ("vascular") tissue, with cells derived from the genital rudi-
coe. 2/ coe. ir
por.^^.^
pro.
n.pU '
"^An.
coe. 1 1 M
coe. 3/
Fig. 437. A diagram of the arrangement of the coelom in the ideal Di-
pleurula. From Sedgwick. An. anus; coe. il. left anterior coelom; coe. 2/.
left middle coelom ; coe. 7.r. right middle coelom ; coe. 3/. left hinder coelom ;
coe. ir. right hinder coelom; M. mouth; n.pt. neural plate on apex of preoral
lobe ; por. water pores (only the left of these normally appears ; it becomes
the madreporite); pro. preoral lobe or prostomium.
ment, known as the genital stolon. The axial organ adjoins the axial
sinus where the latter is present; in the Crinoidea it lies in the axis
of the body. Its function is unknown ; it has been regarded as a heart
for the lacunar system on account of contractions which it is said to
perform, and as an organ of excretion because in echinoids it takes up
carmine injected into the body cavity. Of excretion in the echinoderms
little is known. It appears to be performed by the wandering out,
through the walls of the gills, of amoeboid cells laden with granules
of excreta, by the organs of respiration, and by the intestine, but no
constant and conspicuous organs subserve it alone. There are no
nephridia. The nitrogenous excreta consist largely of ammonia
compounds and contain practically no urates.
Respiration is performed through a variety of structures, some of
which expose the coelomic fluid to the external water, Vv^hile others
ECHINODERMATA 629
carry the water into the body and expose it to the fluid in the coelom.
To the first class belong the podia, and the "gills" of asteroids and
echinoids; to the second belong the "genital bursae" of ophiuroids
and the respiratory trees of holothurians.
The vascular system of other animals is represented in the Echino-
dermata by a system of strands of a peculiar lacunar tissue, containing
intercommunicating spaces which have no epithelioid lining. Ulti-
mately, this system is of the same nature as the blood vessels (haemo-
coele) of other animals, since both are systems of spaces derived from
the blastocoele and filled by a fluid matrix containing free cells ; but
in appearance, and probably in the mode of its functioning, it is very
different. A ring of lacunar tissue surrounds the mouth, lying in or
immediately above the perihaemal ring and giving off in each radius
a strand or "vessel" which similarly lies above the radial perihaemal
canal. Another portion of the system lies in the axial organ and con-
nects the oral ring with an aboral ring, which accompanies the genital
rachis (see below) and sends strands to the gonads. In the Echinoidea
and Holothuroidea two strong "dorsal" and "ventral" vessels from
the oral ring accompany the alimentary canal, running on opposite
sides of that organ and giving off a plexus of branches which ramify
on it, and in holothurians also in a perforated fold of the peritoneum.
A "vascular" plexus is also present on the alimentary canals of other
groups. Contractions are said to have been observed in parts of the
system, but it is very doubtful whether anything in the nature of a
regular circulation takes place in it, though it probably maintains
communication by diffusion between various parts of the body.
With rare exceptions, the sexes of echinoderms are separate. The
genital organs are remarkable for their simplicity. They possess neither
organs of copulation, nor accessory glands, nor receptacles for the
retention of ova, nor a reservoir for the storage of sperm in either sex,
and they discharge direct to the exterior and not, as is usual in coelo-
mate animals, through the coelom or through ducts proper to that
cavity. Nevertheless they arise in ontogeny from the coelomic wall.
The genital system consists, except in the Holothuroidea, of the
genital stolon, a collection of cells in the axial organ ; the genital rachis,
a ring connected with the stolon (aborally in the Asteroidea, Ophiu-
roidea, and Echinoidea, orally in the Crinoidea); the gonads proper,
which are sacs or tubes, often branched, borne upon long or short
branches of the rachis and varying in number in the different groups ;
and the short ducts, lacking in the Crinoidea. In the Holothuroidea
there is only one gonad, which lies in the "dorsal" interradius and
has a duct in the dorsal mesentery and a vestigial stolon lying upon
the duct, but no rachis.
The nervous system consists of networks of fibrils and nerve cells
630 THE INVERTEBRATA
underlying various epithelia, though in places denser and, by the
parallel arrangement of the fibrils, modified into "nerves". It
is remarkable not only for remaining in this primitive condition,
but for being partly derived from mesodermal epithelia. It is in
three parts: (i) the ectotieural system underlying the whole ectoderm
as a plexus (see p. 625) and thickened {a) along each ambulacrum
as a radial nerve, (b) around the mouth as a nerve ring, which con-
nects and has been found by experiment to co-ordinate the radial
nerves {a and b are, with a strip of epithelium, removed from the
surface of the body save in asteroids and crinoids), (c) as branch nerves
to such structures as tube feet, spines, etc.; (2) the deep oral system
underlying the mesodermal epithelium of the perihaemal vessels and
having a distribution similar to that of the ectoneural system but less
extensive than the latter and in particular defective in the Echinoidea ;
(3) the aboral or apical system, also mesodermal in origin, developed from
the peritoneum of the aboral body wall. This system is best developed
in the Crinoidea, where it is removed from the general peritoneum
and enclosed in the ossicles. Here it has the form of a nerve along
each arm and a complex central station in the "chambered organ"
(see below). In the Asteroidea it runs as a cord above the peritoneum
of each arm, the cords meeting in the middle. In the Ophiuroidea
and Echinoidea it is a ring in the aboral sinus. It is not found in the
Holothuroidea. The mesodermal nervous systems are principally
motor, innervating the muscles which move the internal skeleton:
through connecting fibres they receive stimuli from the ectoneural
system.
The Echinodermata are poorly provided with sense organs. There
is a general sensitiveness of the epithelium of the body, at least to
tactile stimuli, which is heightened in the podia and in the terminal
tentacle which stands at the end of each radial water vessel in the
Asteroidea, Ophiuroidea, and Echinoidea. The olfactory sense is
perhaps also located in the podia or in some of them, especially in
those that are situated around the mouth and in the Holothuroidea
are developed into tentacles. An eye-spot is situated at the base of
each terminal tentacle in the Asteroidea, and certain holothurians
possess statocysts in the skin.
All echinoderms are marine in habitat. Few of them are pelagic:
none are parasitic. Only the Crinoidea are fixed, and some of these
are only temporarily so.
The majority of members of the phylum have free, pelagic larvae ;
though some, as Asterina, pass a considerable time in the egg mem-
brane and have larvae which are not pelagic ; and a few, chiefiy polar
or deep-sea species, keep the young in brood pouches until they have
the adult form. The eggs of the species which possess pelagic larvae
ECHINODERMATA 631
are small ; the others larger and more yolky in proportion to the late-
ness of the stage at which they are set free. Fertilization takes place
in the sea or in brood pouches. Cleavage (radial, Fig. 196, i) is total
and forms a hollow, one-layered blastula (Fig. 438 A). This, by in-
vagination or unipolar ingrowth, forms a gastrula with a wide blasto-
coele into which typical mesenchyme cells wander from the wall of
coe.-
^-cil.bd.
-stoni.
\
/
"An.
Fig. 438. Stages in the development of Asterias vulgaris. After Field.
A, Section of blastula. B, Section of gastrula. C, Section of older gastrula.
D, Three days' larva from the right-hand side. An. anus; arch, archenteron;
blc. blastocoele ; hip. blastopore ; cil.bd. ciliary band ; coe. rudiment of coelom ;
ect. ectoderm; ertd. endoderm; ent. enteron; mch. mesenchyme; mth. meso-
thelium; stom. stomodaeum.
the archenteron. The blastopore becomes the anus, and the mouth is
formed by the breaking through of a stomodaeum. Meanwhile the
archenteron has budded off, at the anterior end, a vesicle which, by
processes that differ in detail in different cases, will give rise to the
three segments of the coelom described above (p. 627). The future
ventral side of the larva becomes concave. The larva is now known as
the Dipleurula. The cilia which uniformly covered the blastula be-
come sparse over most of the body but, except in the Crinoidea, grow
632 THE INVERTEBRATA
Stronger and more numerous in a longitudinal band around the ventral
concavity. This band is the organ of locomotion. Growing more
rapidly than the rest of the ectoderm, it becomes thrown into folds,
the larval arms (which have nothing to do with the arms of adult
echinoderms), whose length and arrangement differ so as to character-
ize a special type of larva in each class (Fig. 439). In the Auricularia
larva of the Holothuroidea the body is elongate and the band lengthens
fore and aft and outlines a strong preoral lobe. The Bipinnaria of the
Asteroidea resembles the Auricularia in general features, but in it the
Fig. 439. Diagrams of echinoderm larvae. The postoral part of the early
ciliated band is drawn heavily (except where remote), the preoral part cross-
hatched. A, Early stage with simple continuous band. B, Auricularia.
Q, Bipinnaria. U, Pluteus. E, Crinoid larva, y^n. anus; M. mouth; /)r.' pre-
oral band; pr." corresponding part of continuous band; pt. postoral band.
border of the preoral lobe separates completely from the rest of the
longitudinal band. In the Plutei of the Ophiuroidea and Echinoidea
the band remains continuous, but forms only a small preoral lobe, and
the postanal region of the body develops greatly, while the slender
arms are supported by calcareous rods. The Pluteus of the Ophiuroidea
(Ophiopluteus) has a different appearance from that of the sea urchins
ECHINODERMATA 633
(Echinopluteus), owing to the fact that the former of these larvae has
fewer arms than the latter and that in it the arms known as the " pos-
terolateral arms" are the largest and are directed forwards, whereas
these arms, if they are present in the Echinopluteus , are there small
and directed outwards or backwards. The larva of the Crinoidea has
no longitudinal band, but five rings of strong cilia around the body.
In the development of the Holothuroidea the Aiiricularia is succeeded
by a stage known as the pupa, in which the longitudinal band breaks
up and rearranges itself into a series of five transverse rings some-
what resembling those of the crinoid larva. The Bipinnaria of the
Asteroidea is succeeded by a Brachiolaria which differs from it in
possessing in the preoral region three processes by which the larva
can hold fast to objects.
The larvae become transformed into adults by a metamorphosis
which differs in the several classes. In all it involves an alteration of
the position of the mouth, which in groups other than the Crinoidea
is removed to the left side, and in the Crinoidea to the posterior end,
taking with it the coelomic cavities of the left side. The fate of the
several divisions of the larval coelom has been described above
(p. 627). In the Crinoidea and Asteroidea the larva becomes fixed by
the preoral lobe at the time of metamorphosis, 2. fixation disc develop-
ing for the purpose. In crinoids the fixation persists, at least until the
adult is completely formed. In starfishes it is only temporary.
The fixation of the sea lilies, and the fact that starfishes are fixed
when the bilateral symmetry of the larva changes to the radial sym-
metry of the adult, are interesting facts in view of the fixation which
is general in the other great group of radially symmetrical animals,
the Coelenterata. Radial symmetry is essentially the symmetry' of a
sessile animal, which is in the same relation with its surroundings on
all sides, whereas bilateral symmetry is that of a travelling animal,
which needs differentiation not only of the upper side from that which
faces the ground, but also of the fore from the hind end. It is likely
that at one time all echinoderms were fixed, and that those which are
now free retain the radial symmetry of their sessile ancestors. This
supposition is supported by the fact that the earliest known fossil
members of the phylum were fixed.
For the rest, the Dipleurula and its metamorphosis suggest that
the early sessile echinoderms were descended from a free, bilateral
ancestor ; and the close resemblance between the Auricularia and the
Tornaria larva of Balanoglossus, together with the history of the
coelom (see p. 660), and the nature of the nervous system, indicate an
affinity between that ancestor and the Enteropneusta.
634
THE INVERTEBRATA
Class ASTEROIDEA
Star-shaped or pentagonal Echinodermata ; whose arms contain caeca
of the alimentary canal, and are usually not sharply marked off from
the disc ; which have an aboral madreporite ; open ambulacral grooves ;
and usually both suckers on the tube feet, and pedicellariae.
The ossicles (Fig. 440) of the body wall of a starfish may, as in the
familiar Asterias, constitute a toughening meshwork, or may have
the form of more closely set plates, but are not united to form a con-
tinuous shell. Along the sides of the arms run two rows of strong
pieces, the supero- and infer o-marginal ossicles^ which are hidden in
Asterias but in many genera appear on the surface. The ossicles bear
spines^ which vary much in size and shape and arrangement, being
often longer than the stumpy structures on the back of Asterias.
Around and between the spines are usually pedicellariae of various
-sp ped'
,gill
>oss
-amb.oss.
Fig. 440. A diagram of a transverse section of the arm of a starfish. From
Borradaile. ab.m. muscle which straightens the arm; ad.oss. adambulacral
ossicle; ad.sp. adambulacral spine; amb.oss. ambulacral ossicle; amp. am-
pulla of tube foot; m.' muscle which opens the ambulacral groove; ped/ one
of the small pedicellariae with crossed jaw ossicles; ped." one of the large
pedicellariae whose jaw ossicles are not crossed; r.b.v. radial "blood vessel";
t.f. tube foot. Other letters as in Fig. 441.
kinds, the most perfect of which is theforcipulate, found in Asterias j
which has a basal ossicle: its jaws may be straight or crossed. Over
interspaces between the ossicles arise delicate, hollow outgrowths,
tht gills, into which the perivisceral cavity is prolonged. Above each
ASTEROIDEA
635
O ex K -M
636 THE INVERTEBRATA
ambulacral groove runs a double row of large, transversely placed,
ambidacral ossicles^ movable upon one another by muscles. Each has
a smaller adambulacral ossicle at its outer end. Adambulacral spines
stand on the adambulacral ossicles. In Asterias they are long, and
bear groups of large pedicellariae of the kind with uncrossed jaws.
They can be turned inwards to protect the ambulacral grooves.
The mouth leads through a short oesophagus (Fig. 441) into a large
sac-like stomach, with two retractor muscles in each arm. Above is
a five-sided pyloric sac, from each angle of which, separately or, as
in Asterias, by a short common duct, arises a pair of branched /))'/onc
caeca, which are slung, each by a double mesentery, from the roof
<^,-
^M^
Fig. 442. Echinaster sentus, in the act of devouring a mussel.
From Shipley and MacBride. mad. madreporic plate.
of an arm: the epithelium of these secretes the digestive ferments
and stores nutriment. From the pyloric sac a short, conical rectum,
bearing in Asterias two glandular rectal caeca, rises to the anus, which
is slightly excentric, in the interradius which is next, clockwise, after
that of the madreporite. Animals of any kind that can be seized serve
{or food, and usually the stomach can be extruded to envelop and
digest prey which are too large to be swallowed. Some species clasp
bivalves with their arms (Fig. 442) and pull them open with the tube
feet so that the everted stomach can be applied to the soft parts of
the mollusc.
In each interradius a stiff septum projects into the perivisceral
ASTEROIDEA
637
cavity between the arms. To the septum in the interradius of the
madreporite is attached a sac, the axial sinus, and into this, so as to
appear to lie in it, project the axial organ and the stone canal, whose
wall is calcified and infolded so as to increase its surface. Orally, the
stone canal joins the water vascular ring, which bears nine small
Fig. 443. Part of the aboral half of a starfish (Asterias ruhens) removed, with
the alimentary canal, from the rest of the body, and viewed from within. One
lobe of the stomach has been cut away, and another partly turned back. The
detached figure represents an enlarged view of the axial sinus and adjoining
structures. From Borradaile. abo.m. aboral muscle; ax.o. axial organ; ax.s.
axial sinus ; l.st. one of the lobes of the stomach ; oes. oesophagus ; py.c. pyloric
caecum ; py.d. pyloric duct ; py.s. pyloric sac ; r.an. rectal caecum ; sep. septum ;
st.c. stone canal.
TiedemanrCs bodies, of gland-like structure, and often, but not in
Asterias, several stalked sacs, the Polian vesicles. The radial water
vessel of each arm lies under the ambulacral ossicles, and between
them and the radial nerve is the perihaemal vessel, divided by a
septum in which runs the "blood vessel". The gonads are ten in
638 THE INVERTEBRATA
number, shaped like bunches of grapes and varying in size with the
season. They are attached to the body wall by their ducts, which open
one on each side at the base of each arm.
Asterias (Figs. 433, 436, 440, 441, 443). A typical member of the
class. Its principal features have been mentioned above. British.
Astropecten. Without anus; without suckers on the tube feet; and
with conspicuous marginal ossicles. Lives on a bottom of hard sand,
into which it burrows, and upon which its tube feet are adapted to
walk. British.
Asterina. With the arms short and wide, so that the body is
pentagonal ; and without pedicellariae. Has a shortened development,
with a larva which is not a Bipinnaria. British: between tidemarks.
Brisinga. With numerous, long, slender arms, sharply distinct
from the disc, which is small. A deep-sea genus.
Class OPHIUROIDEA
Star-shaped Echinodermata ; whose arms are sharply marked off from
the disc and do not contain caeca of the alimentary canal; with
madreporite on the oral side; ambulacral groove covered; tube feet
without suckers; and no pedicellariae.
The special features of the organization of a brittle star are con-
nected with the fact that the animal moves, not by means of its tube
feet, but by pushing and pulling upon surrounding objects with its
arms. In adaptation to this the arms are sharply distinct from and
freely movable upon the disc, on the underside of which they are
inserted. They are armoured by skeletal plates (Figs. 444, 445), in an
upper, two lateral, and an under series. The epidermis is vestigial;
there is a strong cuticle ; spines on the side plates give grip ; and the
under plates, covering in the ambulacral groove, which is thus con-
verted into an epineural canal, protect the nerve cord during the
movements of the arm. The ambulacral ossicles of each pair fuse to
form one of a series of vertebrae, which articulate by an arrangement
of knobs and sockets and can be moved upon one another in various
directions by four muscles. The large vertebrae reduce the peri-
visceral cavity in the arm to a canal, in which there is no room for
caeca of the alimentary canal. The nerve cord bears ganglia corre-
sponding to the muscles between the vertebrae and formed by increase
of the coelomic (deep oral) component of the cord. The perihaemal
vessel is shallow and not divided by a septum. The tube feet have no
suckers and no ampullae and are often provided with warts of sense
cells.
The alimentary canal (Fig. 446) is a mere bag, not protrusible
through the mouth, which is armed with an arrangement of spines
ECHINODERMATA 639
serving as teeth. The food of some species consists of animals cap-
tured by the arms : others shovel mud into the mouth with the ad-
jacent tube feet and digest the food it contains. There is no anus.
The madreporite, aboral in the young, becomes oral in the adult
Fig. 444. A, Ophiura. Oral surface of disc and part of the arms. B, Ophio-
glypha. Aboral surface. From Woods. 6. buccal plates; c/. upper ("dorsal")
plates of arms; g. genital slits; /. lateral plates of arms; r. radial plates;
V. under ("ventral") plates of arms.
ver.
In.m.
tiss. cqe. ^^^^^
:^ la.plL /V^
epin. ', Ta
ra.n.
.'peh:\ %J^
Fig. 445. A section through an arm of an ophiuroid. Diagrammatic, magni-
fied. From Shipley and MacBride. coe. coelom; d.plt. upper plate; epin.
epineural canal ; la. pit. lateral plate ; In.ni. longitudinal muscle ; ped.ga. pedal
ganglion ; ra.n. radial nerve cord ; ra.peh. radial perihaemal canal ; ra.wv.
radial water vascular canal ; sp. spine; tf. tube foot; tiss. soft tissue supporting
plates; ver. "vertebra"; v. pit. under plate.
because the disc, growing independently of the arms,- and faster
aborally, comes to overhang in the interradii. In coming over, the
madreporite brings with it the axial sinus, stone canal, axial organ
and madreporic vesicle, which are all orally placed. The gonads open,
not directly to the exterior, but into genital bursae, of which one opens
640 THE INVERTEBRATA
on each side of the base of each arm (Fig. 444 A, g). The ectoderm
lining the bursae retains its cilia and causes currents which subserve
respiration.
Ophiura, Ophiocoma, Ophiothrix, Amphiura. British genera, separ-
ated by relatively unimportant differences, which are chiefly evident
in the ossicles and spines. Amphiura is hermaphrodite and viviparous.
B.w.
^wv-sy.
ext.ir.iii.i
or.plt.
Fig. 446. A diagram of a section of an ophiuroid, passing through an inter-
radius and part of the opposite arm. B.w. body wall ; coe. coelom ; coe.' coelom
of the arm; coe". perioesophageal sinus; epin. epineural canal; ext.ir.m. ex-
ternal interradial muscle; g.rch. genital rachis lying in the aboral sinus;
in.ir.m. internal interradial muscle; M. mouth; w. nerve ring; n.' radial
nerve ; or.f. oral tube foot ; or.plt. oral plate ; Po. Polian vesicle ; ra.peh. radial
perihaemal canal; tth. teeth; tor. ossicle known as torus angularis; tr.m. trans-
verse muscle ; wv.sy. water vascular system : to the left the circumoral ring,
to the right the radial vessel; i, ist ambulacral ossicle, which is displaced
into an interradius and known as a "peristomial plate"; 2, 3, 4, 2nd to 4th
ambulacral ossicles which form "vertebrae"; 2', extension of first vertebra
towards an interradius; i, ii, iii, ist to 3rd under ("ventral") plates.
Class ECHINOIDEA
Globular, cushion-shaped, or discoidal Echinodermata, without
arms; with small abambulacral area, in which lies the madreporite;
ambulacral grooves covered ; tube feet ending in suckers ; numerous
long spines; and pedicellariae.
The characteristic form of body of the Echinoidea is such as would
result if the arms of a starfish were drawn up into the body by shrink-
age of the aboral surface.
We shall describe the anatomy of this group by an account of a
typical member of it — Echinus esculentus, a large species common in
Britain. This animal (cf. Fig. 447) has the shape of a sphere with one
side flattened, slightly polygonal in equatorial outline. In the middle
of the flattened side is the mouth. Under the delicate, ciliated epi-
dermis an armour, the shell or corona, composed of dermal plates
firmly sutured together, encloses most of the body, but at the two
poles there are leathery areas, the peristome around the mouth, and
the periproct in which the anus lies excentrically. The corona (Fig.
ECHINODERxMATA 641
448) is composed of twenty meridional rows of plates, two in each
radius (ambulacrum), and two in each interradius. The plates of the
ambulacra are distinguished by the presence on them of tfie pores
for the tube feet. These pores are in pairs, since each ampulla com-
municates with its tube feet by two canals. Thus water can circulate
in and out of the tube feet and respiration is facilitated. At the aboral
pole each radius ends in a single ocular plate ^ which bears the opening
of the terminal tentacle, and each interradius in 2i genital plate which
abuts upon the periproct and bears the opening of a gonoduct. One
of the genital plates bears also the madreporite. All the plates are
studded with bosses of various sizes, to which articulate the concave
amh.
:/2'^
-ijill
^crist.
Fig. 447. Echinus rniliaris from the oral side. amb. ambulacrum; gill, gill;
inter, interambulacrum ; perist. peristome.
bases of the large and small spines and the pedicellariae. The spines,
unlike those of starfishes and brittle stars, which are moved with the
ossicles under them, have muscles of their own. These are in two sets,
an outer one which causes movements, and an inner " catch" muscle
(p. 143) which holds the spine firmly in position. On level ground the
spines take part at times in locomotion, the animal using them like
stilts. The pedicellariae (Fig. 449), which have three jaws, are of
several kinds. Gemmiform pedicellariae have stiff stalks and globular
heads with a poison gland in each jaw. The tridactyle kind have a
flexible stalk and long jaws. The ophiocephalous kind are smaller and
have a flexible stalk and broad, toothed jaws. The trifoliate pedicellariae
642
THE INVERTEBRATA
—por.
-g.plt.
amb.-.
Fig. 448. A diagram of an aboral view of the dried shell of an Echinus. The
spines, pedicellariae and tube feet have been removed. From Shipley and
MacBride. amb. ambulacrum; An. anus; ho. bosses which bear the spines;
g.plt. genital plate with genital pore ; jun. line of junction of ambulacral and
interambulacral plates ; mad. madreporic plate ; oc.plt. ocular plate ; por. pores
through which tube feet protrude ; /)/)r. periproct (leathery skin around anus).
Fig. 449. Pedicellariae of Echinus miliaris. Enlarged, but not accurately
to scale. A, Trifoliate. B, Ophiocephalous. C, Thdactyle. D, Gemmiform.
ECHINOIDEA 643
are the smallest and have very flexible stalks and broad, blunt jaws. It
is said that the gemmiform kind are weapons of defence against large
foes, the tridactyle against small, the ophiocephalous seize small
animals for food, and the trifoliate destroy debris. The peristomial
edge of the corona is indented in each interradius by two notches,
where stand the ^/7/^— delicate, branched outgrowths of the body wall,
each containing a cavity which is continuous with the lantern coelom
e. . aos.
A7i. / ,inad.
mad.ves.
-amp:
Fig. 450. A diagram of a vertical section of Echinus, passing on the left
through a radius and on the right through an interradius. Certain structures
not immediately in the plane of section are shown. A, Whole section. B, The
region of the madreporite. amp. ampulla of tube foot; amp.' madreporic
ampulla; An. anus; aos. aboral sinus, with genital rachis; arc. arch ossicle
of jaw; aur. auricula; d.b.v. "dorsal blood vessel"; e. pigment spot in ocular
plate; epin. epineural canal; g.sto. genital stolon; gill, external gill; j. jaw (not
strictly in section) ; j.' lower part of the same ; l.coe. lantern coelom ; M. mouth ;
m. muscle (protractor) which pulls down the jaw and protrudes the tooth;
m.' muscle (retractor) which pulls back the jaw; mad. madreporite; mad.ves.
madreporic vesicle ; n. nerve ring ; oe. oesophagus ;^er.coe. perivisceral coelom ;
ra.n. radial nerve ; ra.wv. radial water vessel ; rm. rectum ; rot. rotula (the com-
pass which overlies this is omitted) ; sh. shell (corona) ; sip. siphon ; st. stomach ;
stc. stone canal; T.bd. Tiedemann's body ("Polian vesicle"); tf. tube foot;
tth. tooth; v.b.v. "ventral blood vessel"; wvr. water vascular ring.
(see below). Ten little plates on the peristome around the mouth
carry openings for the ten short, stout, sensory buccal tube feet, the
proximal pair of podia in each radius.
The mouth, which is surrounded by five strong, slightly projecting,
chisel-shaped, interradial teeth, leads into a relatively narrow oeso-
phagus, whose lower part is enclosed in a framework, known as
Aristotle's lantern (Figs. 450, 451), which supports the teeth. The
644
THE INVERTEBRATA
ijA.. marf.
rm,-
aur."
Fig. 451. Aristotle's lantern of Echinus esculentus. From Shipley and Mac-
Bride, mnp. ampullae of tube feet; arc. arch of jaw; aur. auricula; comp.
compass; g.sto. genital stolon; ia. interambulacral plates of shell; j. jaw;
m. protractor muscle of jaw; m.' retractor of jaw; m." elevator of compass;
m.'" depressors of compasses ; mad. madreporite ; oe. oesophagus ; rm. rectum ;
stc. stone canal; T.bd. Tiedemann's body ("Polian vesicle"); tth. tooth;
tth.' soft upper end of tooth; v.h.v. "ventral blood vessel"; w.v.r. water
vascular ring. The rotulae, which underlie the compasses, are not seen.
ECHINOIDEA
645
lantern consists of five composite y^w;^, each clasping a tooth, and five
radial pieces, known as rotulae, which unite the jaws aborally. The
teeth can be moved outwards and inwards by muscles running from
the jaws to radially placed arches, known as the auriculae, which arise
from the inside of the corona near the lantern. Under each auricula,
which perhaps represents a pair of ambulacral ossicles, runs a radial
nerve, with its epineural canal, and the radial perihaemal canal,
*' blood vessel ", and water vessel. Within the lantern is a space, known
rrrv.
aumpr-
Fig. 452. An oral view of a sea urchin, with part of the shell removed to
show the course of the alimentary canal. After Cuvier. amp. ampullae at
base of tube feet; arc. arch; b.r. "blood" ring; d.b.v. "dorsal blood vessel";
/.jaw; Ian. lantern of Aristotle (displaced); M. mouth surrounded by five
teeXh {tth.)\ oe. oesophagus, coiled intestine and rectum; ov. ovaries with
oviducts ; per. fold of peritoneum supporting genital rachis ; rm. rectum ;
sip, siphon; st. stomach; v.b.v. "ventral blood vessel".
as the lantern coelom, which is an enlarged perihaemal ring. Muscles,
running from the auriculae to slender ossicles, known as compasses,
which overlie the rotulae, can raise and depress the roof of the lantern,
and thus pump the fluid of its coeldm into and out of the gills. In
some urchins, but not in Echinus, pouches of the lantern coelom
project upwards into the perivisceral cavity. These are known as
Stewart's organs or internal gills and when they are present external
gills are often lacking.
The oesophagus enlarges into a flattened tube, the stomach (Fig. 452),
646 THE INVERTEBRATA
which runs horizontally round the body in a clockwise direction as
viewed from below, suspended from the shell in festoons, by strings
of tissue. At its beginning, there is a short caecum, and it is accom-
panied by a small, cylindrical tube, the siphon, which opens into it at
either end. From its distal end a tract similar to it, the intestine, re-
turns in the opposite direction and then ascends as the narrower
rectum to the anus. The food consists chiefly of seaweed.
The water vascular ring has five Tiedemann's bodies. It is situated
above the lantern, and the radial vessels run downwards and out-
wards from it between the jaws and under the auriculae and then
FiR- 453- A diagram of a section across a radius of Echinus. From Shipley
and MacBride. amh. ambulacral plate; amp. ampulla; ho. boss for articula-
tion of spine ; ect. ectoderm ; epin. epineural canal ; m. muscles which move
the spine; ra.n. radial nerve cord; ra.peh. radial perihaemal canal; ra.wv.
radial water canal ; sp. spine ; t.f. cavity of tube foot.
meridionally under the radial plates of the corona, to end each in the
pigmented tentacle of an ocular plate. Each is accompanied in its
course under the shell by the radial nerve cord, epineural canal, and
perihaemal canal (Fig. 453). It is said that a small radial "blood
vessel" (not shown) runs between the perihaemal canal and water
vessel. From the water vascular ring the stone canal, which is not
calcified, ascends vertically to the madreporite, accompanying the
axial organ, which surrounds the small axial sinus. Under the
ECHINOIDEA 647
madreporite, however, the sinus is free and enlarged and forms an
"ampulla" into which the stone canal opens.
The oral ring of the lacunar system lies below the water vascular
ring. The features of this system have been described on p. 629. The
gonads are five large masses hanging into the perivisceral cavity from
the region of the genital plates. The rachis is degenerate in the adult.
Echinus is an example of the regular sea urchins (order Endocyclica).
The class contains two other orders, Clypeastroida and Spatangoida,
known collectively as the irregular urchins (Exocyclica), in which the
anus, with its periproct, is displaced from the apical position which it
occupies in the regular forms, and lies in an interradius, known as the
posterior interradius, so that the body, which is considerably or very
much flattened, has a marked bilateral symmetry. The madreporite
remains in position and extends over the region vacated by the peri-
proct. In most of the irregular urchins (though not in certain primi-
tive forms known as Holectypoida or Protoclypeastroida) the aboral
parts of the ambulacra are expanded to an oval shape (petaloid) and
bear flattened, respiratory tube feet. These peculiarities are associated
with the habit, possessed by typical members of both orders, of living
partly or wholly buried in sand (see below).
Order ENDOCYCLICA
Echinoidea in which the mouth is central, the anus remains within
the apical area, and the ambulacra are not petaloid.
Echinus (Figs. 447-453). A typical example, described above.
Order CLYPEASTROIDA
Echinoidea in which the mouth is central and furnished with a
lantern, the anus outside the apical area, the dorsal parts of the am-
bulacra nearly always petaloid, and the body usually much flattened.
The members of this order live at or near the surface of the sand,
and walk by means of the tube feet, which are ver)/- numerous. They
extract food from the sand, which they shovel into the mouth by
means of the teeth.
Clypeaster. A typical member of the group, of large size, wide-
spread in tropical waters.
Echinocyamus. Small, oval, and not extremely flattened. E.pusillus
is a British species.
Order SPATANGOIDA
Echinoidea in which the anus and often also the mouth are excentric,
the lantern has disappeared, the dorsal parts of the ambulacra are
petaloid, and the body cushion-shaped or heart-shaped.
648 THE INVERTEBRATA
Typical members of this order live buried at some depth in the
sand and move, not by means of their tube feet, but by ploughing
their way with numerous, curved, flattened spines. In such forms
the body has a heart-shape, owing to the depth of the anterior am-
bulacrum, which differs from the rest and has special tube feet,
capable of great elongation and provided with fringed discs. These
gather sand rich in food, which is then pushed into the mouth by
stout buccal tube feet.
Spatangus and Echinocardium (Fig. 454) are typical members of
the order, found in British waters. Echinocardium comes into shallower
water than Spatangus, burrows deeper, and differs in respect of the
arrangement of the spines.
-esp.tf.
buctf.
Fig. 454. Echinocardium cordatum. A, From the aboral; B, From the oral
side, buc.tf. buccal tube feet, where the ambulacra converge upon the
mouth ; resp.tf. respiratory tube feet at the side of a petal : both kinds much
contracted.
Class HOLOTHUROIDEA
Sausage-shaped Echinodermata, without arms ; without recognizable
abambulacral area ; usually without external madreporite in the adult ;
with the ambulacral grooves covered ; some of the tube feet modified
into tentacles around the mouth, and some or all of the rest, if present,
provided with suckers ; a muscular body wall containing very small
ossicles; no spines; and no pedicellariae.
The typical form of body of the Holothuroidea is well seen in the
members of the widely distributed genus Holothuria (Fig. 457 B), to
which the familiar British "cotton spinner" belongs. It is such as
would result if in a regular echinoid the ossicles were reduced and
the body drawn out in the oro-anal axis, the madreporite with the
ECHINODERMATA 649
gonad which adjoins it being displaced along their interradius to a
position not far from the mouth, and the other gonads lost. As has
been explained on p. 627, the madreporite usually becomes internal.
It is so in Holothuria.
Owing to the presence in one interradius of the primary madre-
porite and the gonad, the body always possesses a rudimentary bi-
lateral symmetry. In many cases, as in Holothuria^ this symmetry
becomes conspicuous owing to the fact that the animal constantly
applies to the ground the three radii of the side opposite to the madre-
poric interradius, and this side becomes differentiated as "ventral"
from the "dorsal" side which contains the madreporic interradius
with the two radii which adjoin it. The differentiation consists in a
more or less marked flattening of the ventral side and the loss, or
Fig- 455- A diagram of a transverse section of a holothurian. d.b.v. dorsal
" blood vessel " ; d.i. dorsal interradius ;^. gonad ; int. 1,2, 3, the three stretches
of the intestine ; l.d.i. left dorsal interradius ; l.d.ra. left dorsal radius ; l.ra. left
radius; l.v.i. left ventral interradius; mes. mesentery; r.d.i. right dorsal inter-
radius; r.d.ra. right dorsal radius; r.ra. right radius; r.v.i. right ventral inter-
radius; v.b.v. ventral "blood vessel"; v.ra. ventral radius.
conversion into pointed sensory papillae, of the tube feet on the
dorsal side. The tube feet may be confined to the radii or may, as in
Holothuria^ spread over the interradii, obliterating externally the
radial arrangement, though internally the radial structures retain
their position.
The tentacles may be much branched {dendritic)^ provided with
lateral branches only {pinnate)^ or, as in Holothuria, furnished only
with a terminal circle of short branches, which themselves branch
{shield-shaped). Shield-shaped tentacles are retractile owing to the
presence of ampullae. Dendritic tentacles do not possess ampullae
but are withdrawn by means of retractor muscles, inserted into the
radial pieces of the calcareous ring around the oesophagus (see below),
which pull in the tentacular crown as a whole. Pinnate tentacles are
withdrawn by retractor muscles or by ampullae or by both.
650 THE INVERTEBRATA
In the body wall, under the dermis, which contains minute ossicles
of a form characteristic of the species, there are transverse muscles
between the radii and longitudinal muscles under the radii. The radii
contain the same structures as in the Echinoidea. Only one tube runs
from the radial water vessel to each tube foot.
The alimentary canal (Fig. 456) is slung to the body wall by a
mesentery. It runs (except in Synapta) an S-shaped course, looping
almost the whole length of the body — backwards in the mid-dorsal
interradius, forwards in the left dorsal interradius, and finally back-
wards in the right ventral interradius to the anus. It starts as an
oesophagus , enclosed in a calcareous ring of ten ossicles, five radial and
five interradial, which has been thought to represent the auriculae
and lantern of an echinoid. The oesophagus is followed by a short
muscular region known as the stomach, this is succeeded by a thin-
walled intestine which forms the greater part of the canal, and finally
there is a short, wide cloaca. Into the latter usually open two long,
branched respiratory trees, whose ramifications end in thin-walled
ampullae through which water, when pumped in by contractions of
the cloaca, passes into the body cavity, carrying oxygen to the coelomic
fluid, and so to the organs. In //o/o^^«n« and a number of other genera
the lower branches of the respiratory trees are converted into Cuvierian
organs, tubes covered with a sticky substance which in sea water
elongate and form a mass of sticky threads. When the animal is at-
tacked or otherwise irritated a violent contraction of the muscles of
the body wall sets up in the perivisceral cavity a pressure which
ruptures the cloaca and drives out the Cuvierian organs (and often
subsequently the rest of the alimentary canal). The enemy is en-
tangled by the sticky threads. Except in the Dendrochirotae, the food
is extracted from sand or mud which is shovelled into the mouth by
the tentacles. Dendrochirotae entangle small organisms on their sticky
tentacles and, putting the latter one by one into the mouth, contract
upon them and, by drawing them out, strip ofi" the catch.
The axial organ is represented only by a cord-like genital stolon
near the gonoduct. The aboral sinus and vascular ring, genital rachis,
and apical nervous system are absent. The lacunar system consists of
an oral ring, radial "vessels", "dorsal and ventral vessels" of the
alimentary canal, and a plexus on the latter. In the middle part of the
intestine of Holothuria and many other genera, the "dorsal vessel"
hangs from it on a perforated fold of the peritoneum, but remains
connected with it by a plexus known as the rete mirabile. In perfora-
tions between the strands of this plexus the branches of the left
respiratory tree are entangled. The condition of the water vascular
system is that described on p. 627; for that of the genital system see
p. 629.
HDLOTHIJHOI F)K A
-ten.
amp. 0^y^^^
651
■ In.m.
F\ii. 456. A view of Holothuria tuhulosa, somewhat diminished. The animal
is opened along the left dorsal interradius and the viscera are exposed. After
Ludwig. al. alimentary canal; amp. ampullae of tentacles; d. cloaca; ^. re-
productive organ ; In.m. radial longitudinal muscle partly cut away ; plx. dorsal
"blood plexus"; Po. Polian vesicle; ra.zvv. radial water vessel; v.b. ventral
"blood vessel"; re. respiratory trees; stc. stone canals; ten. tentacles; wvr.
water vascular ring.
652 THE INVERTEBRATA
The Holothuroidea are divided into six orders. Of these the
Aspidochirotae and Dendrochirotae contain between them the bulk
of the members of the class.
Order ASPIDOCHIROTAE
Holothuroidea with shield-shaped tentacles; no retractor muscles,
but tentacle ampullae ; podia on the trunk ; the madreporite internal ;
and respiratory trees.
Holothuria (Figs. 456, 457 B).
Order PELAGOTHURIDA
Holothuroidea of pelagic habit ; with shield-shaped tentacles ; no re-
tractor muscles, but large tentacle ampullae which push out the body-
wall ; no podia on the trunk ; the madreporite external ; and no respira-
tory trees.
Pelagothuria (Fig. 457 E). The only pelagic holothurian. The
animal swims by a webbed circle of projections caused by the en-
largement of the tentacle ampullae.
Order ELASIPODA
Deep-sea, benthic Holothuroidea with shield-shaped tentacles; no
retractor muscles or tentacle ampullae; podia on the trunk; the
madreporite external or internal; and no respiratory trees.
Deima (Fig. 457 F, F').
Order DENDROCHIROTAE
Holothuroidea with dendritic tentacles; retractor muscles but no
tentacle ampullae ; podia on the trunk ; the madreporite internal ; and
respiratory trees.
Cucumaria (Fig. 457 A). Body pentagonal, with two rows of tube
feet on each radius and usually no other podia except the tentacles.
British.
Order MOLPADIDA
Holothuroidea of burrowing habit; with slightly pinnate or un-
branched tentacles; tentacle ampullae, and sometimes also retractor
muscles; without podia on the trunk; with respiratory trees; and
with internal madreporite.
Trochostoma (Fig. 457 C).
Order SYNAPTIDA (PARACTINOPODA)
Holothuroidea of burrowing habit; with pinnate tentacles whose
ampullae are vestigial; with retractor muscles; without radial water
vessels, or podia on the trunk; or respiratory trees; and with internal
madreporite.
Synapta (Fig. 457 D). Ossicles anchor-shaped. British.
te
»wi
tf. i
ten.
Fig. 457. Examples of the orders of the Holothuroidea A, Cucumana (Den-
drochirotae). B,Holothurta (Aspidochirotae) in dorsal view. B 1 he same
fn ventral vew, with one tentacle enlarged. The bare strip does not represent
an Inter adTus C, Trochostoma (Molpadida). D, Synapta (Synaptida)
e" pZo^h^^ (Pel'agothurida). F. D.z... (Elasipoda) m do-l view^F The
same, in ventral view. ten. tentacle, enlarged; tf. some of the tube feet,
extended.
654
THE INVERTEBRATA
Class CRINOIDEA
Echinodermata with branched arms; the oral surface directed up-
wards; attachment during the whole or part of their life by a stalk
which springs from the aboral apex; suckerless tube feet; and open
ambulacral grooves; and without madreporite; or spines; or pedi-
cellariae.
The majority of the members of this class are extinct, and of those
that survive the typical, stalked forms (Fig. 461) live in deep water
Fig. 458. Antedon bifida in oral view. From Sedgwick, after Claus.
A. anus; M. mouth.
and are less familiar than the shallow water feather stars {Antedon
and Actinometra) which, when they are adult, break off from their
stalks and swim by waving their arms. It will therefore be convenient
to choose one of the latter to illustrate the anatomy of the group.
Antedon rosacea^ the common feather star, may be dredged in ten
fathoms of water off the coast of England. Its body is composed of a
small central region or calyx and five pairs of long, slender arms^ each
bearing a double row of alternate branchlets known as pinfiules. On
CRINOIDEA 655
the convex aboral side, the calyx bears in the middle a knob, formed
by the centrodorsal ossicle, which is the stump of the stalk and is
fringed with numerous slender, jointed cirri, each ending in a small
hooked claw and used for temporary attachment.
The flat top of the calyx is covered with a leathery tegmen, in the
middle of which is the mouth, while the anal papilla stands in one of
the interradii. Five amhulacral grooves start from the mouth, where
'd^ra-n.
cir.
cir.g.
Fig. 459. A transverse section through the disc and base of an arm of An-
tedon rosacea. After Ludwig. at. various sections of alimentar^^ canal ; ax.coe.
axial part of perivisceral coelom, free from trabeculae; Br^, Br-, first and
second brachial plates; CD. centrodorsal piece; ch.on. "chambered organ"
in centre of aboral nervous system ; cir. cirri ; cir.g. branches of chambered
organ and genital stolon in cirri; coe. coelomic canals of arm; d.ra.n. radial
nerve of aboral nervous system; epm. epithelium of ambulacral groove;
g.sto. genital stolon; M. mouth; m. muscles connecting radial and brachial
plates ; n. nervous layer of ambulacral groove ; or.zvv. circumoral water vas-
cular ring, giving off, on the left in the figure, a canal to one of the tube feet
around the mouth ; por. pore canals ; R^, R^, R^, first, second and third radial
plates ; ra.wv. radial water canal ; stc. stone canals ; trb. trabeculae traversing
the coelom.
they are separated by five low triangular flaps, the oral valves, and
radiate across the tegmen, each running to one of the pairs of arms,
to supply which it bifurcates. The groove on each arm gives a branch
to each pinnule. Along their whole course the grooves bear a row of
finger-shaped podia on each side, and they and the podia are ciliated.
Down this system a current set up by the cilia conveys particles
656 THE INVERTEBRATA
gathered from the water to the mouth for food. The podia can only
serve for respiration and to increase the ciliated surface : they are not
prehensile, and it is said that only those around the mouth are sensory.
Everywhere except in the grooves, the ectoderm is vestigial and
cuticulate like that of the Ophiuroidea. The dermis, which on the
oral side is merely leathery, contains on the aboral side a skeleton of
large ossicles. This consists of: (i) the centr odor sal \ (2) the small
rosette (formed by the fusion of five larval pieces known as basals),
which is internal and roofs a cavity, presently to be described, in the
centrodorsal ; (3) in each radius, three radials, of which the first is
usually not visible externally; (4) in each arm, a row of brachials;
(5) in each pinnule, a row of pinnularies', (6) in each cirrus, a row of
cirrhals, which are hollow. The ossicles of the appendages of the body
are movable upon one another by muscles.
The alimentary canal consists of a short, vertical oesophagus^ a wide
stomach, curved horizontally around the axis of the calyx and bearing
two long diverticula and some low pouches, and a short intestine^
which ascends to the anus.
The perivisceral coelom (Fig. 459) of the calyx is traversed by
numerous calcified strands (trabeculae) . In the arms there are present
(i) a pair of subtentacular canals, (2) aboral to these, a genital canal,
(3) aboral to this again, a coeliac canal, which is derived from the right
posterior coelom of the larva. All these canals lead from the peri-
visceral cavity. It is said that there is a tiny perihaemal vessel in each
arm but no oral perihaemal ring. There is no genital (" aboral") ring
sinus. In the hollow of the centrodorsal ossicle lies what is known as
the chambered organ (Fig. 459, ch.on.). This consists of five radial
compartments, derived from the larval right posterior coelom; its
wall is richly nervous and constitutes the centre of the aboral or
apical nervous system. From the centre issue five interradial nerves,
which branch and form a complicated plexus (Fig. 460) with a co-
ordinating circular commissure, and from this plexus radial nerves
supply the arms and pinnules. Nervous prolongations of the cham-
bered organ also pass down the cirri. The whole of this system is
enclosed in the ossicles of the adult, but it originates from the wall
of the adjacent coelom. It controls the movements of the animal. If
it be destroyed they cease ; but the ectoneural system (which has the
same arrangement as that of a starfish) can be cut away without
affecting the movements.
The axial organ lies in the axis of the body. Starting as a slender
strand in the centre of the chambered organ where the walls of the
chambers meet, and enlarging in the perivisceral cavity, it narrows
again orally, where it is continuous with a circular genital rachis.
From this again genital cords pass down the arms in the genital canals
Fig. 460. A diagram of the
apical nervous system of
Antedon. From Sedgwick,
after Ludwig. a. axial cords
of the arms ; Bi, first
brachial ossicle; CD, centro-
dorsal; Ri, Rz, R2, radiah.
B
,'perist.
Fig. 461. Pelmatozoa. A, Rhizocrinus, x about 2^. From Sars. B, Thecal
plates of a cystoid with diplopores. C, Plates with pore-rhombs. D, Edrio-
aster bigsbyi. After Bather, amb. ambulacra on which the covering plates
remain; An. anus; flp. flooring plates of ambulacra; ia. interambulacrum ;
?}iad. madreporite ; /)m5L peristome ; /)or. pores (for tube feet?).
658 THE INVERTEBRATA
and so reach the pinnules, where they enlarge into gonads. The genital
cells are dehisced by rupture when ripe. The lacunar system has an
oral ring and "vessels" from this to a plexus on the stomach and to
the lacunar tissue of the axial organ. It is doubtful whether radial
vessels are present.
The water vascular ring closely surrounds the mouth, and from it
numerous stone canals open without madreporites to the perivisceral
cavity, which in turn communicates by many isolated pores, lined by
cilia, with the exterior. This arrangement is due to the fusion of the
larval axial sinus with the perivisceral cavity and subsequent multi-
plication of stone canals and pores (see p. 627). There are no am-
pullae, but the diameter of the canals can be varied by muscular
strands which traverse them.
Two other recent crinoids may be mentioned here. Pentacrinus is
a deep-water form with a long, jointed stalk, bearing whorls of cirri
at intervals. The adult, like Antedon, breaks free and swims by waving
its arms, but trails its stalk behind it. Rhizocrinus (Fig. 461) has a
jointed stalk without cirri except at the distal end, where some
branching root-cirri are developed. By these the animal is perman-
ently rooted. It is found at great depths in the Atlantic.
EXTINCT CLASSES
Echinoderms belonging to several groups now extinct are numerous
as fossils in Palaeozoic rocks. They are all sessile by the aboral side,
a fact whose significance has been mentioned above (p. 633). Their
body wall contains an armour (theca) of plates, in which mouth, anus,
and madreporite can often be identified. These echinoderms are
usually classified with the Crinoidea as Pelmatozoa, in contrast to the
free members of the phylum, which constitute the Eleutherozoa.
The following are the groups referred to in the foregoing paragraph :
Amphoridea. The most primitive echinoderms. Body sac-like, its
skeleton showing no food grooves or other traces of the ambulacral
system. Aristocystis, Ordovician.
Carpoidea. Stalked, and with secondary bilateral symmetry owing
to compression upon a plane in which lie mouth, madreporite, and
anus. Two food grooves on the theca have been described. Sometimes
there are two arm-like spines at the ends of the oral edge. Trochocystis,
Cambrian; Placocystis, Silurian.
THEComEA (Edrioasteroidea). Cushion-shaped, without stalk or
arms. Five food grooves, provided with covering plates, radiate from
the mouth. Stromatocystis, Cambrian ; Edrioaster (Fig. 461), Ordovician.
Cystoidea. Body sac- or vase-shaped, with stalk, and with food
grooves, either on the theca (epithecal) or on special ossicles (exothecal).
The grooves may be carried partly or wholly by brachioles — arm-like
ECHINODERMATA 659
processes of the oral side of the body. The plates of the theca are
hexagonal, and bear pores, either in pairs (diplopores) or in diamond
patterns (pore-rhombs). The members of the group fall into two
divisions as follows :
DiPLOPORiDA, with diplopores and epithecal grooves. Eucystis,
Ordovician.
Rhombifera, with pore-rhombs and exothecal grooves. Echino-
sphaera, Ordovician; Lepadocrinus, Silurian.
Blastoidea. Highly organized forms, typically with ovoid, stalked
body. Certain plates of the theca (basals, radials, deltoids, lancets, etc.)
are uniform in arrangement throughout the group. The ambulacra
are bordered by rows of pinnule-like brachioles, and at the sides of the
ambulacral grooves run elongated internal pouches, the hydrospires,
which open to the exterior at the oral end by spiracles. Pentremites
Carboniferous.
CHAPTER XIX
THE PROTOCHORDATA
Most of the members of the phylum Chordata do not come within
the scope of this book. But, though a position in that phylum is ac-
corded by all authorities to the Tunicata and by many also to the
Hemichorda, it is often convenient to treat of both these groups with
the other invertebrate animals, and we shall take that course. It will
be well, however, first to indicate what are the features which the
groups in question share with the other chordate animals — the Verte-
brata proper, and the Cephalochorda {Amphioxus), which are usually
studied with the Vertebrata. The Chordata are bilaterally symmetrical,
coelomate metazoa which have in common certain fundamental
features stated in the following paragraphs.
(i) With the single exception of the minute, sessile Rhabdopleura^
every member of the group possesses, at least in its early stages,
lateral perforations from the pharynx to the exterior which are known
as gill clefts or, by the name which is applied to those of them which
in vertebrata do not bear gills, as visceral clefts. Moreover, the gill
clefts of the Cephalochorda, the Hemichorda (except Cephalodiscus),
and many tunicates, have the further resemblance that the perfora-
tions which originate them are subsequently divided by tongue bars,
so that each gives rise to two of the definitive clefts. It is probable
that the original function of the "gill" clefts was the filtering off" of
food from water taken in through the mouth. This function they still
retain in the lower members of the phylum {Balanoglossus, Am-
phioxus, tunicates, many fishes), though something of a respiratory
function is perhaps always superadded to it.
(2) In all the Chordata the central nervous system (a) arises from a
median dorsal strip of ectodermal epithelium from which it never
parts, (b) is, except in the trunk cord of Balanoglossus and the whole
central nervous system (ganglion) of Cephalodiscus and Rhabdopleura,
removed from the surface of the body, by invagination or by over-
growth of the epithelium at its sides, so as to form a tube, lined by its
persistent epithelium. These features of the nervous system of the
Chordata have analogies in that of the Echinodermata (pp. 624, 630).
(3) The common features of the coelom of the Chordata are more
obscure. The coelom of the Hemichorda and the Cephalochorda
arises, as in the Echinodermata (p. 631), by pouches of the arch-
enteron forming three segments, of which the anterior (the proboscis
cavity or head cavity) is at least in its beginning median and com-
PROTOCHORDATA
66 1
municates on the left side (and in the Hemichorda often on both sides)
by a pore with the exterior. In the Hemichorda the three segments
retain their entity throughout life : the first does not divide into lateral
halves, the second (collar cavities) acquires a pair of pores to the ex-
terior, the third (trunk cavities) does not undergo transverse division.
In the Cephalochorda, the first divides into two halves, of which the
left, by the opening out of its pore, becomes Hatschek's pit in the
ectodermal depression known as the wheel organ, the second forms
the first mesoderm segment (mesoblastic somite) and some cavities
around the mouth, the third subdivides to form all the mesoderm
segments except the first. In the Vertebrata the coelom forms as a
split in a mass of mesoderm, though there are indications that the
mesoderm rudiment should be regarded as a solid pouch arising in
coe. 3'.
Fig. 462. A diagrammatic longitudinal section of an embryo of Amphioxus.
From Shipley and MacBride. al. alimentary canal; coe. i. anterior coelom
or head cavity; coe. 2. middle coelom (collar cavity), which becomes first
mesoblast segment ("mesoblastic somite"); coe. 3. hinder coelom (trunk
cavity); coe. 3'. mesoblast segment dividing off from coe. 3; n. neural canal;
ne. neurenteric canal; neii. neuropore.
the same position as the hollow pouches of the Cephalochorda. The
head cavity is represented by the premandibular segment of the em-
bryo, a median structure with an opening to the exterior in the form
of a communication with the ectodermal invagination for the pituitary
body. Certain peculiarities of the mandibular segment indicate that
it is the homologue of the first mesoderm segment in the Cephalo-
chorda and so of the collar cavity. The remaining segments must re-
present those of the Cephalochorda and so the trunk cavity of the
Hemichorda. In the Tunicata the mesoderm arises as a solid mass
in the same position as the pouches of the Cephalochorda, but the
coelom, except for certain doubtful vestiges, is non-existent.
(4) Except in the Hemichorda, the notochord, a skeletal rod which
arises from the endoderm of the median dorsal line of the gut, runs
the whole, or a considerable part, of the length of the body. In the
662 THE INVERTEBRATA
Hemichorda the dorsal side of the gut at the anterior end, over the
mouth, forms a skeletal outgrowth into the proboscis. This out-
growth has received the same name as the notochord, on the theory
that it represents the anterior portion of that structure.
(5) With the exception of the Hemichorda, all the Chordata possess
the tail^ a postanal prolongation of the body in the direction of its
main axis, without viscera, but containing extensions of the other
principal organs — muscles, nerve cord, notochord, and, in the Verte-
brata, backbone. A true tail is found only in the Chordata, In them
it is a very important organ, used primarily in locomotion and main-
taining position, though it may become an organ of prehension or a
weapon.
An interesting biochemical confirmation of the morphological find-
ing that these subphyla, including the Hemichorda, are a unity, lies
in the fact that all of them possess a phosphagen^ which is a compound
of creatin, whereas the phosphagen of non-chordate animals is a
compound of arginin. Moreover these two phosphagens have been
found together, on the one hand, in the lantern muscles of a sea
urchin, a member of that invertebrate phylum which shows most
affinity with the Chordata (p. 2); and on the other hand in Balano-
glossus, the chordate which shows most affinity to the Echinodermata.
SuBPHYLUM HEMICHORDA^
Chordata without tail, atrium, or bony tissue; with notochord re-
stricted to the preoral region ; central nervous system partly or wholly
on the surface of the body ; and three primary segments of the coelom
retained in the adult in corresponding, externally visible regions of the
body, the foremost of which is preoral.
This small group contains the Enteropneusta, burrowing worms of
the genus Balanoglossus and related, slightly diflferent genera, and
the Pterobranchia, the remarkable little organisms Cephalodiscus and
Rhabdopleura, which live at considerable depth in the sea, in tubular
houses which they secrete for themselves by their proboscis.
The body oi Balanoglossus (Fig. 463) has a conical preoral lobe, the
proboscis, which behind, by a narrow stalk, joins the short, wide collar
region. This overhangs in front the stalk of the proboscis and behind
the beginning of the long trunk. Each of these regions contains one
of the three segments of the coelom, the proboscis segment undivided,
^ A phosphagen is a labile compound of an aminoacid with phosphoric acid.
It is intimately associated with muscular contraction, being broken down
during activity and reconstituted during rest. This is a more immediate source
of energy than glycolysis.
^ Some authors give the name Enteropneusta to the whole of this sub-
phylum.
PROTOCHORDATA 663
the collar and trunk segments each in two lateral halves. (The trunk
cavities send forward into the collar a pair of " perihaemal " pro-
longations at the sides of the dorsal blood vessel, mentioned below.)
A pair, left and right, or a single, \th, proboscis pore opens at the base
.Irk.
Fig. 463. A Dolichoglossiis kozvalevskii, x i. From Spengel.
col. collar; M. mouth; pro. proboscis; sit. gill slits; trk. trunk.
of the organ. The mouth opens on the ventral side, between the over-
lap of the collar and the proboscis stalk. A pair of collar pores open
backward from the collar cavities into the first gill pouch (see below).
Dorsolaterally on the first part of the trunk is on each side a row of
664 THE INVERTEBRATA
numerous small gill openings; these lead into deep pouches which
communicate with the pharynx each by a tall opening virtually
divided into two by a tongue bar, which hangs from the dorsal side
but does not quite join the ventral side as do the tongue bars in
Amphioxus. In the region of the gills, and a little way behind it, the
trunk is somewhat flattened above or has a pair of lateral ridges or
folds, known as tht genital pleurae because when they are present the
gonads lie in them. Behind this hranchiogenital region the trunk
becomes more cylindrical and tapers gradually, as the abdominal
region, to the anus, which is terminal.
The proboscis and collar are used in burrowing. They are distended
by the taking in of water by the action of cilia in the tubes leading to
the pores, and contracted by muscles in the body wall, the water being
thus driven out. The proboscis first enters the mud and the collar
follows and, by distending, gives a purchase. for drawing forward the
trunk.
The body is covered by a ciliated epithelium, with gland cells and
at its base a net of nerve fibrils to which processes of epithelial
cells contribute. This net is thickened along the dorsal and ventral
median lines of the trunk in the form of nerve cords, which are united
by a ring thickening immediately behind the collar. The dorsal cord
alone is continued into the collar, and here it is somewhat thicker and
is invaginated to form a tube, by which arrangement it is protected
during the movement of this prominent part of the body. On the
stalk of the proboscis the cord communicates with the general
net on that organ. There are no special sense organs. No dermis
interposes between the epithelium and the muscles of the body wall.
The alimentary canal (Fig. 464) is straight. From the buccal cavity
in the collar, the hollow notochord (see p. 661) projects forward into
Fig. 464. A diagram of a median longitudinal vertical section of a typical
member of the Enteropneusta.
With the exception of the branchial blood vessels, structures which are
not median are shown by interrupted lines.
aff.br., aff.i., aff.ph., off. pro. afferent vessels to gill clefts, intestine, pharynx,
and proboscis; ce.n.sy. central nervous system in collar; cm. coecum of noto-
chord; col. collar; col.ca. collar cavity (mesentery lacking here) ; d.b.v. dorsal
blood vessel; d.b.zv.v. vessel from dorsal body wall; d.rt. "dorsal root";
eff.br., eff.glo., eff.i., eff.pro. efferent vessels from gill clefts, glomerulus (cut
short), intestine, and proboscis; glo. glomerulus; h. heart; lat.ph.v. lateral
pharyngeal vessel; w. muscles of proboscis; M. mouth; mes.d.coL, mes.d.trk.,
mes.v.coL, mes.v.trk. mesenteries, dorsal and ventral, of collar and trunk;
nch. notochord; /)./). proboscis pore;/)cw. pericardium; pro. proboscis ; /)ro.
ca. proboscis cavity; sep. proboscis septum ; sk. notochord skeleton; sk' . pro-
cess of sk. at side of mouth; si. gill slit; trk. trunk; trk.ca. trunk cavity (at
point where mesentery is lacking) ; v.b.v. ventral blood vessel; v.b.zc.v. vessel
to ventral body wall.
ENTEROPNEUSTA
665
d.rt^
col.ca.
col.--
ce.n.sy.-.
mes.d.coL-
d.b.v..
eff.br., ^
trkxa.^^
mes.d.trk..- \
d.b.w.v
-<tff.hr.
v.b.w.v.
mes.vJrk.
Fig. 464. (Legend on previous page.)
666
THE INVERTEBRATA
the hinder part of the proboscis, strengthening the neck of that
structure and supporting a group of organs (heart, pericardium,
glomerulus), which form with it the proboscis complex: at its root is
a skeletal thickening of its basement membrane. Backwards, the
buccal cavity leads into the pharynx, from which the gill slits open.
coei
Fig. 465.
Fig. 466.
Fig. 465. A longitudinal horizontal section through G/owo6a/a«M^. Diagrani-
matic. From Shipley and MacBride. al. alimentary canal ; br.s. branchial sac
with external opening; col. collar; col.ca. collar cavity; d.b. dorsal blood
vessel;^, reproductive organs ; jg'/om. glomerulus; h. hezrt; pent, pericardium;
peh. perihaemal cavity ; por' . collar pore ; por. proboscis pore ; pro. proboscis ;
pro.ca. proboscis cavity; trk. trunk.
Fig. 466. A Tornaria larva. From Sedgwick, after Metschnikoff. a, From
the left-hand side, b, From the dorsal side. The larva is in the regressive
stage, and numerous secondary foldings of its ciliated rings have been lost.
An. anus; ap. apical sense plate; coe.' rudiment of anterior (proboscis)
coelom ; coe." , coe.'" rudiments of right middle (collar) and hinder (trunk)
coeloms; M. mouth ;^cm. pericardium.
In most species there is a ventral gutter, below the gill slits, leading
to the intestine, which lies in the abdominal region. Along this gutter
passes the mud which the animal swallows for food, excess of water
leaving by the gill slits, which thus act as a straining apparatus.
The blood vessels are for the most part mere crevices between the
HEMICHORDA 667
basement membranes of the ectoderm, endoderm, and mesoderm,
which otherwise are everywhere in contact, having nomesenchymatous
connective tissue between them. A dorsal vessel above the ahmentary
■pro.
Fig. 467. A zooid of Rhabdopleura normani removed from its tube and seen
from the right-hand side. From Lang. An. anus; arm. arms; M. position
of mouth; por.' position of collar pore; pro. proboscis; stk. stalk; trk. trunk.
canal widens over the notochord in the hinder part of the proboscis
into a space known as the heart. This is covered by a vesicle known
as the pericardium, whose lower wall, contracting, communicates
pulsations to the blood. From the heart the blood passes into a plexus
668 THE INVERTEBRATA
contained in an organ known as the glomerulus, which is formed by a
puckering of the hinder wall of the proboscis cavity around the end
of the notochord. It is thought that this organ acts as a kidnev, taking
waste matters from the blood and throwing them into the proboscis
cavity, whence they are expelled through the proboscis pores when
the organ contracts. From the glomerulus the blood is gathered into
two vessels which lead backwards one on each side to a ventral vessel
below the gut. From this vessel is supplied a plexus in the wall
of the alimentar}- canal, including the bars between the gill openings.
From this plexus blood passes to the dorsal vessel. The blood flows
forwards in the dorsal vessel and backwards in the ventral.
The sexes are separate. The gonads are mere sacs lying at the sides
in the anterior region of the trunk. When they are ripe, openings
break through from them to the exterior. Though they have no con-
nection with the coelom of the adult, they are developed from the
coelomic wall.
In most species the tgg is small, and development passes through
a pelagic lar\-al stage kno\\Ti as the Tornaria (Fig. 466), which closely
resembles the Auricularia lar\-a of holothurians (p. 632), but differs
in possessing a perianal band of cilia in addition to the longitudinal
band, and in the presence of a couple of eyespots in the patch of
epithelium which bears the apical tuft of cilia. The larv^a presently
sinks to the bottom and undergoes a gradual transformation into the
adult, retaining its original symmetry-. The pulsating vesicle, which
in echinoderms becomes the madreporic vesicle of the adult (pp. 627,
628), is in Balanoglossus the rudiment of the pericardium. In some
species the Q^g is larger and there is no Tornaria stage. In all, how-
ever, cleavage of the OMjm is complete and gastrulation is bv in-
vagination.
Cephalodiscus and Rhabdopleura (Fig. 467) are minute animals in
which, owing to a protrusion of the ventral surface, the body is vase-
shaped and the gut drawn down into a U , so that the anus opens
upwards. The collar bears in Rhabdopleura two and in Cephalodiscus
several, hollow, branched arms which by means of cilia collect the food
of the animal. On the forepart of the trunk are in Cephalodiscus the
single pair of gill clefts and the pair of gonadial openings, in
Rhabdopleura only a gonadial opening on the right side. On the belly
is a peduncle which bears buds. In Cephalodiscus these become free;
in Rhabdopleura they remain in continuity with the parent so that a
colony of zooids is formed. Both genera have all the characteristic
features of Balanoglossus, save that Rhabdopleura has no gill clefts or
glomerulus, and that in both the dorsal nerve patch of the collar is not
invaginated.
PROTOCHORDATA 669
SuBPHYLUM TUNIC AT A {UROCHORDA)
Chordata without coelom, segmentation, or bony tissue ; with a dorsal
atrium in the adult; notochord restricted to the tail, which is present
in the larval organization only; the central nervous system removed
from the surface of the body and in the adult degenerate ; and a test,
usually largely composed of a substance (tunicin) related to cellulose.
In the adult form, the members of this group are extraordinarily
unlike the rest of the phylum. They have lost all the characteristic
features of chordate animals except the gill clefts, and are rather
shapeless objects which lead a sluggish existence by means of an
organization of a low grade. Most of them are sessile, and there is no
doubt that this habit has established the peculiarities of the group.
We shall describe the organization and life of the Tunicata by
giving an account of a typical example, Ciona intestinalis of the British
coasts, one of the simple "ascidians". The adult of this animal
(Fig. 468) is a subcylindrical sac, which reaches a height of several
inches, sometimes nearly a foot, seated by the blind posterior end
upon some solid object on the bottom, and at the other bearing two
openings, a terminal mouth or branchial opening and an atrial opening,
seated on a tubular projection a little way below the mouth. This
projection, which marks the dorsal side of the animal, is known as
the atrial siphon. Beyond its origin the body narrows as the oral
siphon towards the mouth. The latter is surrounded by eight small
lobes with red pigment spots between them. The atrial opening has
six lobes. Both apertures can be narrowed and virtually closed. When
the animal is in water and has them open a current may be seen to
set in at the mouth and out at the atrial opening. By sudden con-
tractions of the body water may be forced out of both of them.
The body is covered by a tough, translucent test, remarkable for
being composed largely of tunicin, a substance closely related to
cellulose, and therefore not to be expected in an animal. The test is
a cuticular secretion of the ectoderm, but contains cells of mesodermal
origin which have wandered into it, and ramifying tubes in which
blood circulates, which enter it at a point near the base of the sac.
Below the test lies the true body wall or mantle, which contains
numerous longitudinal and transverse strands of muscle by which the
shape of the body can be altered, and sphincters around the openings,
where test and mantle are tucked in for a short distance. "
The alimentary canal (Fig. 469) begins as a tube, the stomodaeum
or buccal cavity, lined by the inturned test. This leads to the very
large pharynx, 3. circlet of tentacles standing at the junction. A short
prebranchial zone of the pharynx lies between the tentacle ring and
xht peripharyngeal band — a couple of ciliated ridges, which run round
670
THE INVERTEBRATA
at.
[fr—
- hrn.
m.-
A
Wm
-S'OP'
—An.
■inL
■St.
m
—slk.
Fig. 468. Ciona intestinalis. From Shipley and MacBride. The live animal
seen in its test ; some of the organs can be seen, as the test is semitransparent
S« anu ; at. Atrial onfice; hrn. bram; g. reproductive organs; S-oPj-n^f
Openings; int. intestine; M. mouth; m. muscles; pph peripharyngeal ring,
St. stomach; stk. stalk attached to a rock; ten. tentacular ring.
TUNICATA
671
Fig. 469. Ciona intestinalis dissected from the left side. The atrial cavity and
the pharynx have both been opened by longitudinal incisions, and part of
the intestine cut away. An. anus ; at. op. atrial opening; d.tub. dorsal tubercle;
est. endostyle; est.' caecum of the same; h. heart; int. intestine; lang. languet;
M. mouth; nitl. mantle; od. oviduct; oe. opening from pharynx to oeso-
phagus; ov. ovary; ph. pharynx; pph. peripharyngeal ridges and groove;
rm. rectum ; sn.gl. subneural gland ; st. stomach ; ten. tentacles ; vas de. vas
deferens.
672 THE INVERTEBRATA
the pharynx, with a groove between them. In the dorsal middle line
of the prebranchial zone stands the dorsal tubercle. This is the pro-
tuberant, horseshoe-shaped opening 0^2. ciliated funnel v^hich. receives
the duct of a subneural glatid that lies under the brain. The function
of the gland is unknown. The funnel is innervated from the brain and
is supposed to be sensory.
The rest of the pharynx constitutes the spacious branchial chamber.
The lateral walls of this chamber consist of a basket work, formed by
the subdivision of the original gill clefts. The openings of the basket
work (Fig. 470) are known as stigmata. They are longitudinally
elongate and stand in transverse (dorsoventral) rows. Between them
are transverse and longitudinal bars. The inner surface of this basket
Fig. 470. Portions of the pharyngeal wall of Ciona intestinalis. From Sedg-
wick, after Vogt and Yung. A, From within. B, From the outside, a, in-
ternal longitudinal bar ; 6, transverse bars of the first order ; c, transverse bars
of the second order; d, papillae; e, transverse bars of the third order;
/, /', stigmata.
work is crossed by internal longitudinal bars, slung from it and bearing
papillae which project into the branchial cavity. All the bars of this
apparatus are hollow and contain blood. The epithelium which covers
them is ciliated, the cilia being longer on the sides of the stigmata.
Ventrally the basket-work walls are separated by a narrow, longi-
tudinal, imperforate tract known as the endostyle. This is folded into
a groove (Fig. 471) lined by an epithelium which is glandular and
ciliated in alternate longitudinal strips, arranged in a manner similar
to those in the endostyle of Amphioxus. To right and left a ciliated
strip runs beside the groove. Posteriorly the groove passes into a
caecum and the lateral ciliated strips curve up, as the retropharyngeal
band, to the opening of the oesophagus, which is dorsal at the hinder
TUNICATA 673
end. Anteriorly, the same strips are continuous with the posterior
peripharyngeal ridge, on each side of a gap in the latter. Dorsally,
the lateral walls are separated by a hyperpharyngeal band, from which
there hangs down into the branchial cavity a row of processes, the
languets, which are curved to one side.
The stigmata lead, not directly to the exterior, but into a cavity,
the atrium, which opens externally at the atrial opening, is lined by
ectoderm, and is placed dorsally like a saddle upon the branchial
chamber, surrounding the latter completely except {a) in front, and
{b) in the median line ventrally, posteriorly, and for a short distance
from the front end dorsally. The atrium is crossed by vascular trabe-
cule from the branchial basket work to the mantle. Its median dorsal
Fig. 471. A diagram of a transverse section of Ciona, with an enlarged view
of the endostyle. The thick arrows show the course of the main current, the
thin arrows that of the food particles, at. atrium; ci., ci.' short and long cilia;
est. endostyle ; gl. gland cells ; lang. languet.
part is known as the cloaca, the parts at the sides of the branchial
chamber as the peribranchial cavities. One important difference be-
tween this atrium and that of Atnphioxus should be noted. The atrium
of the Cephalochorda is ventral: that of the Tunicata is dorsal.
By the apparatus just described the animal feeds. The working of
the cilia at the sides of the stigmata drives water through the latter,
from the pharynx to the atrium, whence it passes out by the atrial
opening as the current which has already been mentioned. This re-
sults in water being drawn in through the mouth to replenish the
pharynx. Mucus secreted by the gland cells of the endostyle is by the
cilia of that organ passed on to the inner face of the branchial basket
work, over which by further ciliary action it is passed to the dorsal
languets. These receive it with their curved ends, slung in which it is
674 THE INVERTEBRATA
worked backwards as a rope to the oesophagus. As it passes over the
pharyngeal wall particles brought in with the current through the
mouth are entangled in it, to be carried to the hinder part of the
alimentary canal and there digested if they be fit for food. A similar
function is performed by mucus which passes dorsalwards along the
peripharyngeal band.
The short oesophagus leads backwards to a fusiform stomach. From
this an intestine, whose ventral wall projects inwards as the typhlo-
sole does on the dorsal side of the intestine of the earthworm, loops
forwards to become the rectum, which runs a straight course half-way
along the atrium, lying near the dorsal side of the pharynx. The
epithelium of the digestive part of the alimentary canal is ciliated,
and a gland ramifies in the wall of the stomach, into which it opens
by a duct. The faeces are cast out by the outgoing current from the
atrium.
The stomach and intestine lie in a section of the body known as
the abdomen, which is behind (basal to) the region (called the thorax)
in which the pharynx and atrium are situated. The viscera just men-
tioned are enclosed in a perivisceral space, known as the epicardial
cavity, which is of a very peculiar kind, since it is formed by two thin-
walled outgrowths from the pharynx, one on each side of the retro-
pharyngeal band. Epicardial diverticula of the pharynx are found in
many tunicates (p. 677), but it is only in Ciona that they expand and
form a perivisceral cavity, applying their walls as a peritoneum to the
contained organs. In this cavity lies also the heart, a V-shaped tube
placed in the intestinal loop, near the hinder end of the endostyle.
The heart has no proper wall but is formed by the folding-in of one
side of the tubular pericardium, which on this side is muscular. The
other blood vessels also have no walls of their own, but are mere
vacuities between various structures. In these respects the Tunicata
resemble the Hemichorda (see p. 666). Each end of the heart is
continuous with a blood vessel. The vessel from one end runs forwards
under the endostyle and communicates with the blood spaces in the
branchial bars: these in turn join a vessel, in the hyperpharyngeal
band, which gives off branches to the digestive organs, gonads, and
body wall. To these same organs runs the vessel from the opposite
end of the heart. The course of the circulation is remarkable. The
heart for several beats drives the blood towards one end and then
reverses its action. Thus the blood passes, at one time, like that of a
fish, through the gills to the rest of the system, and at another in the
opposite direction. The plasma is colourless, but contains nucleated
corpuscles, some of which are of various colours owing, remarkably
enough, to the presence of compounds of vanadium.
The animal is a hermaphrodite. The ovary lies between the stomach
TUNICATA 675
and intestine as a compact mass ; the testis ramifies over the stomach
and intestine. The genital ducts run side by side along the rectum and
at some distance beyond its end open into the cloaca. The vas de-
ferens is the narrower, and has a patch of red excretory cells around
its enlarged end, where it opens by a rosette of small pores. Fertili-
zation takes place in the water, and the spermatozoa will not unite
with ova from the same individual.
The central nervous system is reduced to a single elongated solid
ganglion (brain) on the dorsal side between mouth and atrial opening.
From its ends nerves are given off. There are no organs of special
sense unless the pigment spots between the lobes of the mouth be
functional for the perception of light.
Not the least striking feature of the remarkable organization which
has just been described is the absence of any space that can with cer-
tainty be identified as coelom. Epicardium, pericardium, the cavities
of the gonads, and even those of the closed excretory vesicles that lie
around the intestine in many ascidians have been held to be of that
nature, but there is no uncontrovertible evidence on this point con-
cerning any of them. Nephridia are also absent. Excretion, so far as
is known, is performed only by the cells mentioned above, which
store urates as solid concretions.
So far, the student will have seen little ground for regarding Ciona
as a chordate animal. When, however, we turn to consider its life
history, no doubt remains upon this point. The eggs are small, though
they contain some yolk ; their cleavage is total and at first nearly equal.
The early stages of development much resemble those of Amphioxus,
but differ in that the cells which are to form the rudiments of various
organs are very early recognizable (determinate cleavage), and that
the mesoderm, which arises from the sides of the archenteron, does
so, not as pouches, but as clumps of cells. Eventually there is formed
a larva, about a quarter of an inch in length, which is known as the
Appendicularia larva, and often as the "ascidian tadpole". This
creature (Fig. 472 A) has a tail about four times as long as its trunk.
In the tail are a notochord, a hollow dorsal nerve cord, a muscle band
on each side, and a few mesenchyme cells. Dorsal and ventral median
flaps of the test serve as fins, the tail being a swim.ming organ. In the
trunk, notochord and muscle bands are lacking, and along with the
alimentary canal the brain and pericardium are found. The mouth lies
dorsally at some little distance from the front end. It leads through
a short oesophagus into a large phafynx, in which the endostyle is
already well developed. There is no branchial basket work, but on
each side a gill slit leads from the pharynx into an ectodermal pouch,
which in turn opens dorsolaterally. Later the two pouches become
united above the pharynx and thus the atrium comes into existence.
676 THE INVERTEBRATA
Meanwhile the gill clefts increase in number by the breaking through
from the pharynx of new clefts and the subdivision of existing clefts,
in the course of which they pass through a U -stage with tongue bars.
(The basket work is ultimately established by the formation across
each gill cleft of longitudinal bars which divide it into stigmata.) From
the pharynx leads the rest of the alimentary canal, which early shows
rudiments of oesophagus, stomach, and intestine, the latter curving
dorsally and eventually opening into the left half of the atrium.
d.n.c.
trTcga.
int. I fi.
A*
atop.
Fig. 472. Diagrams of the metamorphosis of an ascidian larva. A, The larva
at the time of fixation. B, Midway in the metamorphosis. C, The meta-
morphosis completed, ad.ga. adult ganglion; at. right rudiment of the
atrium; at.op. atrial opening; ce.ves. cerebral vesicle; ci.f. ciliary funnel ;
d.n.c. dorsal nerve cord; e. eye; epic, epicardium; est. endostyle; fix. fixation
papillae; ga. ganglion; g.s. gill slits; h. heart; int. intestine; M. mouth; nch.
notochord; st. stomach; stat. statolith; trk.ga. trunk ganglion.
The dorsal nerve tube of the tail is in the trunk enlarged to form
the brain. The hinder part of this is thick-walled and is known as the
trunk ganglion (it does not become the "ganglion" of the adult). The
anterior part is larger than the trunk ganglion and for the most part
thin-walled, and is known as the cerebral vesicle. Dorsally on the
right it is differentiated to form the eye, a cup whose cavity is directed
inwards, filled with pigment. On the floor a stalk projecting into the
vesicle carries a concretion, the statolith, probably a sense organ for
TUNICATA 677
balance. Presently the vesicle acquires an opening into the dorsal
side of the pharynx, near the mouth. The pericardium arises towards
the end of larval life as an outgrowth from the ventral side of the
pharynx. It does not form the heart until metamorphosis. The front
end of the body is a prominent chin, and bears three fixation papillae
of glandular cells. Except for the tips of these papillae, the animal is
entirely covered with test, which even closes the mouth, so that feed-
ing is impossible. After swimming for a short time, the larva fixes
itself to some solid object by the papillae, and proceeds to undergo a
metamorphosis (Fig. 472 B, C), by which it assumes the adult form.
The tail is devoured from within by phagocytes. By growth in the
region between the chin and the mouth, the latter and the atrial
opening are shifted back until they point upwards from the region of
fixation. Meanwhile, the central nervous system degenerates, save
for certain portions of the cerebral vesicle, which forms from its
hinder region the ganglion of the adult and from its ventral and an-
terior region the subneural gland and the ciliated funnel ; the pharynx
develops in the way described above ; the heart is formed ; the epicar-
dial diverticula grow out from the pharynx; and the gonads arise
from a mass of mesoderm.
Ciona is a solitary animal. Some other tunicates resemble it in this
respect, but a large number establish by budding colonies of zooids,
each zooid having the essential features of an individual of Ciona. In
a few cases {Perophora, Fig. 476), the zooids are free from one another
save at their bases, where they are united by the stolon from which
they were formed. In most genera, however, the zooids of a colony
are imbedded in a common test, with only the mouths and cloacal
openings at the surface (Figs. 473, 477). In such cases the original
connection between the zooids is lost, though their atrial openings
usually join in a common cloaca. In the pelagic genera Salpa and
Doliolum and their relatives buds are formed but, instead of remaining
together as a colony, eventually become free and lead a solitary
existence.
Budding is accomplished in various ways, (a) It most often takes
place from a ^/o/o«, which is a median ventral, tubular outgrowth of the
visceral (abdominal) region of the parent, usually containing an inner
tube that consists of the united distal portions of the two epicardial
diverticula of the pharynx, and some mesenchyme cells in a blood
space between this tube and the stolon wall. The epicardial tube will
form the alimentary canal of the buds. Often it also forms the atrium
and the nervous system. In the class Thaliacea, however (see below),
the stolon is more complex and contains special tubes or strands of
cells for the atrium and gonads and sometimes also for the nervous
system. (/>) In other cases (Perophora and Clavelina) the stolon con-
678 THE INVERTEBRATA
tains, not an epicardial tube, but a longitudinal septum of mesoderm ;
and the internal organs of the bud are formed by the complication of
a vesicle which arises by the hollowing out of a mass of mesoderm
that comes into being in a swelling at the end of the stolon, (c) In
Botryllus and its allies budding is effected in yet another way. These
c.cl.
ect.
bud. sin.
est.
Fig. 473. Fig. 474.
Fig. 473. A diagram of a zooid, with a portion of one of its neighbours,
imbedded in an ascidian colony. The shaded area is the common test. bud.
newly formed bud; c.cl. common cloaca; cL, cl.' cloacas of two zooids; epc.
epicardium; est. endostyle; h. heart; ov. ovary; ph. pharynx; rm. rectum;
St. stomach ; stn. stolon ; t. testis ; vas de. vas deferens.
Fig. 474. Diagrams of the budding of tunicates. A, transverse section of
the stolon of a zooid such as that shown in Fig. 473 ; B, similar section of
stolon of Pyrosoma or Salpa ; C, part of a transverse section of a zooid of
Botryllus. at. atrium ; atr. tube from which atrium is formed ; bl. blood space ;
bud. bud arising; ect. ectoderm; end. endoderm; epc. epicardium; est. en-
dostyle ; g. strand from which gonads are formed ; mdm. mesoderm strand ;
n. strand from which ganglia are formed.
genera, which, unlike Ciona but like most solitary ascidians, possess
no epicardium (epicardial diverticula), form their buds by paired
outgrowths that are of quite a diflPerent kind from the stolon, for they
arise from the atrial wall and each contains an inner vesicle which is
a prolongation of the epithelium that lines the atrium of the parent :
this vesicle forms the internal organs as well as the atrium of the bud.
TUNICATA 679
It should be noticed that in budding the origin of the organs takes place
without regard to the germ layers from which they arise in the develop-
ment of the ovum, for the endodermal inner tube of ordinary stolonial
budding often forms atrium and nervous system, which should be of
ectodermal origin, and the ectodermal (atrial) inner vesicle of the
"pallial" budding of Botryllus forms the alimentary canal, which
should be endodermal, while in Perophora and Clavelina all these
organs arise from a mass of mesoderm.
A zooid which arises by budding is known as a blastozooid {blasto-
zoite): one which arises from an ovum is an oozooid. The oozooid,
which in the Thaliacea differs considerably from the blastozooid,
has always lost the power of sexual reproduction. In the Salpida and
Doliolida the blastozooid has lost the power of budding, so that there
is a regular alternation of generations.
The Tunicata fall into three classes. Of these, one, the Larvacea^
only comprises a few little animals which spend the whole of their
lives in the larval condition, developing genital organs and repro-
ducing without metamorphosis. The other two classes both attain the
adult form, but whereas in one of them — the Ascidiacea — the animals
are sedentary and have both branchial and atrial openings directed
away from the substratum, the members of the other — the Thaliacea
— are pelagic and swim by driving water out of the atrial opening,
which is at the opposite end of the body from the mouth.
Class LARVACEA
Tunicata in which the sexually mature form retains the organization
of the larva.
The test is not composed of tunicin . It forms a remarkable ' ' house ' '
that does not adhere to the animal, which from time to time leaves it
and secretes a new one. The habitat is pelagic, and food is filtered
from the water by an apparatus which forms part of the house and
through which water is caused to flow by the movements of the tail.
The organization of the animal diflFers from that of the ascidian
larva described above in various points, of which the following are
the most important. Gonads are present in the hinder region of
the body: nearly always they are hermaphrodite and protandrous.
The tail is attached to the ventral side, near the hinder end of the
body. The two simple gill clefts open ventrolaterally directly to the
exterior. The intestine also opens directly to the exterior, ventrally or
on the right-hand side. The brain. is a compact fusiform ganglion,
and the existence of a cavity in it or in the nerve cord is doubtful.
There is no eye and a statocyst lies beside the brain on the left. In
certain of these respects the animal resembles the larva of Doliolum
(Fig. 483 A).
68o THE INVERTEBRATA
The Larvacea are now generally regarded as an instance of neoteny
(p. 144).
•e
Fig- 475- Oikopleura albicans in its house. Magnified. From Borradaile.
b. the body of the animal ; t. its tail. Movements of the tail cause water to
enter through two funnels {i) provided with gratings by which coarse particles
are strained out. The water is directed (curved arrow) through a filtering
apparatus (/) which removes food particles : these are sucked out of the filter
by the animal. When the pressure rises sufficiently, the water opens a spring
door {e) at the broad end and passes out (wavy arrow), driving the house in
the opposite direction. The animal can escape by pushing open a door {e')
at the base of the beak. It does not return, but secretes a new house, s.
streamers on the house.
Oikopleura (Figs. 43 A, 475). Common in British waters.
Class ASCIDIACEA
Tunicata in which the adult is sedentary and has no tail ; a degenerate
nervous system; an atrium which opens dorsally; a stolon (if any)
of simple structure; and several gill clefts, which are nearly always
divided into stigmata by external longitudinal bars.
The colonial members of this group are known as "compound
ascidians" and are sometimes classed together as Ascidiae compositae.
But they are not of one origin ; some of them have stolonial budding
and dorsal languets and are related to such solitary forms as Ciona;
the others, with pallial budding and a continuous dorsal lamina in
place of the languets, are related to solitary forms, such as Ascidia,
which have no epicardium and possess a dorsal lamina.
Ciona (Figs. 468-471). Described above.
Clavelina. Resembles Ciona in general features but has a stolon
and forms clusters of individuals by the breaking off of buds from
the ends of the stolon branches ; the stolon has a mesodermal septum ;
the zooids and their tests are free from one another.
Polyclinum. As Clavelina; but the stolon contains an epicardial
tube; and the zooids are imbedded in a common test with only the
branchial and atrial openings at the surface.
TUNICATA
68 1
Ascidia. Solitary; without epicardium ; with the viscera at the side
of the body, not in an abdomen; with dorsal lamina in place of
languets. British.
Perophora (Fig. 476). Colonial; the zooids are free from one
another but connected at their bases by a stolon with a mesodermal
septum; and have dorsal lamina and viscera at the side as in Ascidia.
Botryllus (Fig. 477). Colonial; the zooids imbedded as in Poly-
clinum\ but with pallial budding; and with dorsal lamina, and viscera
at the side.
ch
M.
Fig. 476. . Fig. 477.
Fig. 476. Part of a colony of Perophora. After Lister, with modifications.
al. alimentary canal; at. atrium; est. endostyle; M. mouth; ph. pharynx;
sep. septum of stolon; stn. stolon; swd. seaweed on which the colony is
growing; z. zooids; z.' young zooid.
Fig. 477. Two groups of individuals of Botryllus violaceus. Magnified. After
Milne Edwards, cl. opening of common cloaca of the group; M. mouth
opening.
Class THALIACEA
Pelagic Tunicata in which the adult has no tail ; a degenerate nervous
system; an atrium which opens posteriorly; a stolon of complex
structure; and gill clefts which are not divided by external longi-
tudinal bars.
Thus defined, the group includes the Pyrosomatida (Luciae),
which are transitional from the Ascidiacea, with which they are
usually placed, though by their essential peculiarities they belong
682
THE INVERTEBRATA
here. The three orders of the class differ considerably, though two of
them (Pyrosomatida and Salpida) are more nearly related to one
another than either is to the third (Doliolida).
In all, the muscular strands of the mantle are arranged as rings
which encircle the barrel- or lemon-shaped body. These are com-
plete, but feeble and present at the ends of the body only, in Pyrosoma,
strong but usually incomplete ventrally and convergent dorsally in
-f-m.
Fig. 478. Fig. 479.
Fig. 478. The asexual form (oozooid) of Salpa democratica-miicronata. From
Sedgwick, after Claus. at. atrial opening ; Br. " gill " (hyperpharyngeal band) ;
est. endostyle; M. mouth; Ma. test; Nu. "nucleus"; Stn. stolon; Wg. ciliated
pit.
Fig. 479. Pyrosojna. A, A colony. B, The same cut open longitudinally.
the Salpida (Fig. 478), strong, complete and regular in the Doliolida
(Fig. 483). Their contractions cause (in Pyrosoma, assist) the loco-
motion of the animal by driving water from the atrial opening — in
the Salpida and Doliolida direct to the exterior, in Pyrosoma (Fig.
479) into the lumen of a cylindrical colony and thence through the
THALIACEA
683
Fig. 480. A diagram of the budding of the first individuals of a colony of
Pyrosonia by the cyathozooid. A, B, C, Successive stages, at. atrial opening
of cyathozooid ; bd. bud ; stn. stolon.
Fig. 481. The end of a stolon of Salpa democratic a-miicronata, showing part
of chain of young sexual individuals (blastozooids) about to be set free. From
Sedgwick, after Claus. at. atrial opening of a zooid; Af. anus; Br. "gill";
C. heart; End. endostyle; M. mouth; N. ganglion; Nu. "nucleus"; Ov.
ovary; Wb. peripharyngeal band; Wg. ciliated funnel.
684
THE INVERTEBRATA
sn.'gl
oe.
Fig. 482. A semidiagrammatic view of left side of Salpa. From Herdman.
An. anus; at. atrial aperture; emb. embryo in ovisac; est. endostyle; gill,
"gill" (hyperpharyngeal band);^a. nerve ganglion; h. heart; Igt. the single
languet ; M. branchial aperture ; m. muscle bands ; oe. oesophagus ; pbr. peri-
branchial cavity; ph. pharynx;/)^/?, peripharyngeal band; sn.gl. subneural
gland ; st. stomach ; t. testes ; tst. test.
Fig. 483. Doliolum and its life history. From the Cambridge Natural History,
after Uljanin and Barrois. A, The larva, from the right-hand side. B, The
same view of the adult asexual individual (oozooid). C, Dorsal view of the
hinder region of an older oozooid, more highly magnified. D, Stolon, more
highly magnified. E, Probud in migration, more highly magnified. F, Gas-
terozooid. At. atrial opening; b. probuds; Br. branchial opening; cl. cloaca;
d.p. dorsal process (cadophore); est. endostyle; h. heart; Lb. lateral buds;
m.b. median buds; n.g. nerve ganglion; ot. otocyst; p.c. pericardial cavity;
stig. stigmata; stk. stalk; stn. stolon.
THALIACEA 685
single external opening of the latter. The gill clefts are in Pyrosoma
numerous (up to fifty), tall dorsoventrally, and crossed by internal,
though not by external, longitudinal bars. In the Salpida the first-
formed cleft persists and becomes in the adult a single gigantic open-
ing which occupies the entire side of the pharynx. The Doliolida
have a varying number (few in the oozooid, more numerous in the
blastozooid) of short openings.
In Pyrosoma and the Salpida the egg is retained long in the parent :
in Pyrosoma it is yolky and meroblastic ; in the Salpida the embryo is
nourished through a placenta. Development is direct, the tailed
larval stage being omitted; and the buds formed by the oozooid on
its stolon (which has a single epicardial tube) hang together for some
time as a chain. In Pyrosoma this chain (of four zooids) coils into a
circle around the body of the degenerate oozooid [cyathozooidy Fig.
480), and its members then bud in such a way as to form a cylindrical
colony, closed at one end, composed of blastozooids. This is the form
in which the animals pass their free existence, the oozooid never
leaving the body of its parent. In the Salpida oozooid and blasto-
zooids are alike well developed and free swimming, and the blasto-
zooids, of which there is a long chain, though they may coil into a
circle (Cyclosalpa), are incapable of budding and eventually break
away in groups (Fig. 481). In the DolioHda there is a tailed larva, and
the buds formed on the stolon (in which the epicardial tubes remain
separate) break free one by one, though they subsequently make
attachment to a dorsal process of the mother, by whom they are
carried for some time.
Order PYROSOMATIDA (LUCIAE)
Thaliacea which have no larval stage; whose oozooid is degenerate
and retained within the parent; whose stolon contains a single epi-
cardial tube; and whose blastozooids at first form a short chain, but
subsequently by budding constitute a cylindrical colony of ascidian-
like individuals.
Pyrosoma (Fig. 479). The only genus. The colonies vary in length
from an inch or two to several feet, and are phosphorescent, from
which fact the generic name is derived.
Order SALPIDA (HEMIMYARIA)
Thaliacea which have no larval stage ; whose oozooid is well formed
and free; whose pharynx has no lateral walls, owing to enlargement
of the primary pair of gill clefts ; and whose blastozooids are incapable
of budding, but adhere as a chain from which they eventually break
free in groups.
686 THE INVERTEBRATA
All that is left of the walls of the branchial chamber (Fig. 482) is
the endostyle and a dorsal (hyperpharyngeal) bar, known as the
"gill", which runs in a slanting direction along an immense internal
cavity formed by the confluence of the branchial and atrial chambers
through the absence of lateral branchial walls. The animal is as
transparent as glass, save for a small, coloured "nucleus" where the
stomach and intestine are situated.
Salpa (Figs. 478, 481). The chain of blastozooids is band-like.
Cyclosalpa, The chain of blastozooids forms rings.
Order DOLIOLIDA (CYCLOMYARIA)
Thaliacea which have a tailed larval stage; whose oozooid is well
formed and free; whose pharynx has several stigmata on each side;
and whose blastozooids break free one by one from the stolon as buds
which subsequently make attachment to a dorsal process (cadophore)
of the parent, by whom they are carried for some time.
The larva has the barrel-shaped body of the adult with a tail at-
tached ventrally at the hinder end: it lies free in its test. Dorsally
behind it has already the rudiment of the cadophore. In the adult
oozooid the cadophore elongates, as the buds, wandering round the
body from the ventral stolon, begin to settle down and develop. The
bodies which break off from the stolon are known as probuds. They
travel by the pseudopodial activity of certain of their ectoderm cells,
and on arriving upon the cadophore divide several times to form the
definitive buds. These fasten themselves in a lateral and a median
row on each side. Eventually they grow into individuals of three
kinds, which co-operate in a remarkable manner. Those of the lateral
row become gasterozooids which gather food for the community,
those of the median row phorozooids which act as nurses, and others
gonozooids carried by the phorozooids. The latter presently break free
with their charges, which are ultimately liberated to reproduce
sexually.
Doliolum (Fig. 483).
INDEX
Names of genera and species are printed in italics.
The figures in thick type refer to illustrations upon the pages indicated.
References to details of illustrations are only given in special cases.
When the first of several figures after a word is followed by a semicolon
it indicates the principal reference to the subject.
An asterisk (*) signifies that some of the series of figures which immedi-
ately follow it refer to particular genera only.
Reference to the summaries of the characteristics of the groups is only
made when information there given is not to be found elsewhere.
Where the passage referred to extends over more than one page, as a rule
only the first page is indicated.
Abambulacral surface, 623
Abdomen, of Arachnida, see Opis-
thosoma ; of Arthropoda, 308 ; of
Crustacea*, 308, 330, 331, 332,
333, 353, 358, 360, 364, 368, 370,
372, 380, 385, 386, 387, 389, 395,
400, 403, 404, 410, 415; of In-
secta*, 308, 430, 457, 458, 460,
483, 486; of lulus, 424; of Poly-
chaeta, 270
"Abdomen" of Tunicata, 674, 677
Abdominal limbs, of Crustacea*,
328-9, 331, 339, 360, 388, 391, 397,
401,410, 4i5;ofInsecta, 431,458,
459, 463, 465 ; of lulus, 424
Abdominal region of Balanoglossus,
664
Aboral aspect, side, or surface, 143 ;
of Echinodermata, 623
Aboral sinus system, 626, 627, 650
Acantharia, 80, 81
Acanthobdella,.20o; 261, 296, 299
Acanthobdellidae, 300
Acanthocephala, 258; 197, 244
Acanthocystis, 40
Acanthometra, 81 ; elastica, 82
Acanthosoma, 335
Acarina, 533; 517, 520
Accessory nidamental gland, 591
Acephalous larva, 508
Acerentomum doderoi, 465
Aciculum, 266
Acineta, 116; 8, 9; grandis, 9; lem-
narum, 9
Acmaea, 552, 563
Acmaeidae, 565
Acoela, 213, 215
Acoelomate Triploblastica, 197
Acontia, 189
Acrothoracica, 382
Actaeon, 564, 567
Actinia, iSj ; mesembryanthenium, 187
Actinoceras, 604
Actinometra, 654
Actinomma, 81
Actinomyxidea, 102; 100
Actinophrys, 83 ; 27, 33 n. ; sol, 84, 85
Actinosphaeriwn, 86; 10, 20, 23, 28,
33, 40, 42, 83 ; eichhorni, 23, 42
Actinotrocha larva, 622; 145, 621
Actinozoa, 180; 153
Actinula larva, 159
Actipylaea, see Acantharia
Adambulacral ossicles, 636
Adambulacral spines, 636
Adductor muscles, of Cirripedia,
379; of claws, 142; of Concho-
straca, 354; of Crustacea, 333; of
Lamellibranchiata*, 142, 573, 585;
of Leptostraca, 390
Adelea, 89
Adephaga, 493
Adoral wreath, 104
Aega, 398 ; psora, 399
Aeolosoma, 293
Aeschna, 444; cyanea, 473
Aesthetascs, see Olfactory hairs
Aesthetes, 549
Afferent branchial vein, 581
Afferent canals, see Inhalant canals
Agametes, 37
Agamogony, 37; 88
688
INDEX
Agamonts, 36
Aggregata, 89 ; 24, 43
Alary muscles, 437, 438
Alciopidae, 276, 276
Alcippe, 382
Alcyonaria, 180
Alcyonidium, 613
Alcyonium, 181; i8i, 188; digitatum,
180
Alepas, 382
Algae, Symbiotic, 47, 193
Alimentary canal, 130; of Antedon,
656; of Arachnida, 517; of Ara-
neida, 530; of Arenicola, 272; of
Argas, 534; of Arthropoda, 314; of
Asteroidea, 636; of Balanoglossus,
664; of Brachiopoda, 616; of Car-
cinuSy 412; of Chaetognatha, 618;
of Chirocephalus, 358; of Ciona,
669 ; of Crustacea, 344 ; of Cyclops,
373 ; of Daphnia, 367 ; of Echino-
dermata, 626 ; of Echinus, 643 ; of
Gammanis, 401 ; of Gastrotricha,
243; of Helix, 557; of Hirudinea,
297; of Holothuroidea, 650; of
Insecta, 43 1 ; of lulus, 424 ; of
Lamellibranchiata, 578 ; of Lepas,
379; of Ligia, 397; of Limulus,
529 ; oi Lithobius, 421 ; of Mollusca,
544, 550; of Nebalia, 391; of
Nematoda, 248 ;of Nematomorpha,
257; of Nemertea, 235; of Oligo-
chaeta, 287 ; of Ophiuroidea, 638 ;
of Pantopoda, 539; of Peripatus,
320; of Phoronis, 621 ; of Polyzoa,
606; of Pterobranchia, 668; of
Rotifera, 240, 241 ; of Salpida, 685 ;
of Scorpionidea, 522; of Sepia,
594; of Stomatopoda, 398; of
Stylaria, 291 ; of Tardigrada, 539;
of Testacella, 572; of the trocho-
sphere, 282. See also Digestive
system
Allantonetna, 256
Allogromia, 76 ; 68 ; oviformis, 'J'J
Alloiodoela, 214 n.
Allolobophora, 286, 287
Alternation of generations, see Life
cycle
Alveolar layer, 105
Alveoli, of digestive gland, 558; of
protoplasm, 12
Afttblyomma, 534
Ambulacra, 623
Ambulacral grooves, 623 ; 636, 655
Ambulacral ossicles, 634, 638, 645
Ambulacral surface, 623
Ametabola, 454. See also Apterygota
Aminoacids, 20, 52, 57, 131
Amiskzvia, 622
Amitoses of Protozoa, 24
Ammonoidea, 602, 604
Amnion, Insect, 453 ; Nemertean, 236
Amoeba, 69; 13, 19, 20, 28, 38; dis-
coides, 70; dubia, 70, 70; proteus,
69; 19, 23, 26, 36, 42, 70
Amoebina, 68
Amoeboid movement, 15
Amoebulae, 38
Amphiblastula larva, 125
Amphilina, 225
Amphineura, 547
Amphinucleoli, 22
Amphioxus, 129, 142, 145, 660, 661,
672, 673, 675, See also Cephalo-
chorda
Amphipneustic, 508
Amphipoda, 400
Amphitrite, 264
Amphiura, 640
Amphoridea, 658
Ampulla of stone canal, 647
Ampullae, of Hydrocorallinae, 165;
of tube feet, 627
Ampullaria, 563
Anaerobic animals, 140
Anal papilla, 655
Anal suture, 468
Anaphothrips striatus, 485
Anaspides, 392; 332, 334, 336, 337;
tasmaniae, 392
Ancestral group of Protozoa, 44
Ancylostoma, 254; 251, 252
Andrena, 492, 504, 514
Animal Kingdom, i ; 45
Animal pole, 281
Anisogamy, 31 ; 32, 46, 56-8 (passim),
88. See also Gametes
Anisoptera, 472
Anisospores, 80
Annelid cross, 282
Annelida, 260; i, 2
Annuli, of crustacean limbs, 336 ; of
leeches, 297
Annulus of dinoflagellates, 54
Anodonta, 574, 575, 579, 580, 581,
581, 582, 585
Anomopoda, 363; 355, 368
Anomura, 404
Anopheles, 509 ; maculipennis, 505 ;
444, 445
AnopLophrya, 10'] ; prolif era, 108
Anoplura, 483
Anostraca, 356; 327, 354, 355, 360
Antedon, 654, 657; bifida, 654;
rosacea, 654; 655
Antennae, 306-7, 308, 309; First, of
Crustacea, see Antennules; of
Crustacea (second pair)*, 327,
328-9, 332, 336, 339, 342, 354,
356, 359, 360, 362, 364, 369, 372,
373, 379, 380, 387, 390, 395, 397,
401, 410; of Insecta*, 426, 463,
471, 474, 475, 476, 490, 492, 494,
495, 500, 509, 510, 511, 512; of
Myriapoda*, 420, 422; of Trilo-
bita, 323 ; so called, of Rotifera,
240; of Peripatus, see Preantennae
Antennal glands, 345 ; 346, 401, 414
Antennules, 326; 328-9, 332, 334,
339, 342, 356, 360, 364, 372, 378,
380, 382, 383, 385, 387, 388, 390,
395, 400, 410
Anterior aorta, of Araneida, 530; of
Carcinus (ophthalmic artery), 412 ;
of Helix, 556; of Insecta, 437; of
Lamellibranchiata, 580; of Scorpi-
onidea, 522; of Sepia, 593
Anterior cervical groove, 392
Anterolateral edge, 408
Antheridia, 58, 59
Anthomedusae, 154; 158, 160, i6i,
164. See also Gymnobiastea
Anthotiomus gratidis, 493, 495
A?ithuphora, 503
Anthozoa, see Actinozoa
Atithrenus museorutn, 436
Antipathes, 386
Anurida maritima, 465
Anus, 130; of Amphineura, 549; of
Balanoglossiis, 664 ; of Brachiopoda,
6i6; of Echinodermata*, 623, 626,
63 1, 636, 639, 640, 647 ; of Echiuroi-
dea, 301; of Gasteropda, 551; of
Gastrotricha, 243 ; of Haliotis, 564,
565 ; of Helix, 556; of Lepas, 379;
of Mollusca, 544 ; of Nemertea, 233,
234; of Opisthobranchiata*, 544,
567, 569 ; of Peripatus, 3 1 9 ; of Poly-
chaeta, 284 ; of Pterobranchia, 668 ;
of Sepia, 591 ; of Sipunculoidea,
304. See also Alimentary canal
INDEX 689
Aorta, of Anostraca, 348; of Helix,
556. See also Anterior aorta,
Posterior aorta
Aperture, see Opening
Aphaniptera, 512
Aphididae, 478
Aphids, 439
Aphis, 435; rumicis, 478; 474, 478,
480 ; saliceti, 480
Aphodius, 495
Aphrodite, 267 ; 264, 267, 268
Apical lobe, 338
Apical nervous system, of Crinoidea,
656; of Echinodermata, 630; of
Holothuroidea, 650
Apical organ, 282, 284, 611
Apical rosette of trochosphere, 282
Apis, 497, 498, 503; ynellifica, 432,
438, 496, 504
Aplacophora, 548
Aplysia, 567; 554, 568, 572
Apocrita, 500
Apoda, 382
Apodemes, 340
Apodous larvae, see Larvae
Apoidea, 503
Apoplastid phytomastigina, 47
Apopyles, 120
Appendages, Paired, see Limbs
Appendicularia larva, 675
Apposition image, 313
Apseudes, 395, 396
Apterygota, 463 ; 429, 431, 452
Apus, 360; 328, 332, 333, 337, 338,
362; cancriformis, 360; 361
Aquatic oligochaets, 291
Arachnida, 515; 308
Araneida, 530
Arcella, 72; 12, 28, 28, 43, 68; discoi-
des, 73
Archenteron, 129; of polychaete em-
bryo, 282
Archiannelida, 294; 260
Archicerebrum, of Lankester, see
Procerebrum; setisu stricto, 309
Archigetes, 230
Architeuthis, 588
Arenaceous shells, 75
Arenicola, 272; 263, 264, 267, 275,
282; marina, 272; 273, 274
Argas, 534; S34', persicus, 535, 536
Argonauta, 589, 599
Argulus, 376; 377; americanus, ZlTt
foliaceus, 376; 377
690
INDEX
Argyroneta, 531; 519
Arion, 570
Arista, 477, 511
Aristocystis, 658
Aristotle's lantern, 643 ; of Echinus
esculentus, 644
Armadillidium, 397
Arms, of Brachiopoda, 615; of
Crinoidea, 654; of Dibranchiata,
589, 594, 599, 600, 602; of Echino-
dermata, 624, 632; of Echinoderm
larvae, 632, 633; of Pterobranchia,
668
Artemia salina, 356, 359
Arterial system, of Araneida, 531 ; of
Carcinus, 414; of Helix, 556; of
Lamellibranchiata, 579 ; of Litho-
bius, 421 ; of Malacostraca, 348; of
Scorpionidea, 522. See also Aorta,
Vascular system
Artery, Antennary, Dorsal abdominal
(posterior aorta), Gastric, Hepatic,
Ophthalmic (anterior aorta), ster-
nal. Ventral abdominal. Ventral
thoracic, of Crustacea Decapoda,
350, 412, 413; Cephalic, Supra-
neural, of Chilopoda, 421 ; Gastro-
intestinal, Hepatic, Pallial, Ter-
minal, of Lamellibranchiata, 580;
Lateral, Supraneural, of Arach-
nida*, 522, 529, 530; Pedal, of
Mollusca*,557, 581. See also Aorta
Arthrobranchiae, 404
Arthropoda, 3^5; i, 2, 317
Articulamentum, 549
Ascaris, 245 ; 246, 248 ; lumbricoides,
254
Ascidia, 681 ; 680
Ascidia (Insecta), 437
Ascidiacea, 680; 679
Ascidiae compositae, 680
Ascidian tadpole, 675 ; 676
Ascon grade, 120
Ascopus, 240 n.
Ascothoracica, 385 ; 377
Asellus, 398; 336; aquaticus, 399
Asexual reproduction, 5 ; of Aquatic
oligochaetes*, 293, 294; of Meta-
zoa, see Budding, Strobilation ; of
Porifera, 123 ; of ProtOzoa, see Aga-
mogony, Schizogony, Sporogony;
of Turbellaria, 213
Aspidiotus perniciosus, 478
Aspidobranchiata, see Diotocardia
Aspidochirotae, 652
Aspirigera, see Holotricha
Asplanchna, 241
Astacura, 404
Astacus, 404 ; 315, 329, 332, 338, 341,
343, 345, 347, 407 1 fluviatilis, 350,
351, 405
Asterias, 638; 634, 636, 637; rubens,
624, 637 ; vulgaris, 631
Asterina, 638 ; 630
Asteroidea, 634; 623
Asterope, 268 ; 264, 266
Astomata, 107
Astroides, 190
Astropecten, 638
Atractonema, 256, 257
Atrial opening, 669, 677, 679
Atrial siphon, 669
Atrium, Genital, see Genital atrium;
of Cephalochorda, 677 ; of Tuni-
cata*, 673, 675, 677, 678
Atropus pulsatoria, 472
Auchenorhyncha, 476
Aulactinia, 188
Aulactinium, 83 ; actinastrum, 82
Aurelia, 174-80, 176, 177; aurita,
174; 175; Strobilation of, 178, 178
Auricles, of Arenicola, 275 ; of
Gasteropoda, 552; of Lamelli-
branchiata, 579 ; of Mollusca, 544 ;
of Sepia, 593
Auriculae, 645
Auricular organs, 201
Auricularia larva, 632 ; 632, 668
Autogamy, 33 ; 83
Autolytus, 268; 278
Autosomes, 480
Autotomy, 340
Autozooids, 185
Avicularia, 608
Axelsonia, 465
Axial filament, 15
Axial organ, 628; of Crinoidea, 656;
of Holothuroidea, 650
Axial sinus, 627; of Echinoidea, 646
Axon, 137, 138
Axopodia, 14, 15
Axostyles, 17
Babesia, see Piroplasnia
Bacillus pestis, 513
Baculites, 605; 602, 605; cape?isis,
605 ; chicoensis, 605
Badhamia, 86
INDEX
691
Balanoglossus, 662; 633, 660
Balantidiimi, 109; 20; entosoon, 109
Balanus, 382 ; 381
Basal disc, 189
Basal granule, 15
Basal ossicles, 656
Basement membrane, 204
Basilar plate, 424
Basipodite, 336
Basommatophora, 570
Bathynella, 392
Bdellocephala, 212
Bdelloid rotifers, 243
Beaks of Cephalopoda, 594
Beds, Mussel, 583
Behaviour, 38; of Protozoa, 38
Belemnites, 600
Belemnoidea, 588, 600
Bembex, 503
Berlese's theory, 459; 458
Beroe, 196
Bibio, 508; 508
Bilateral cleavage, 145
Bilateral symmetry, 633; of Actino-
zoa, 182; of Ciliata, 8; of Echino-
derm larvae, 623, 627, 633; of
Echinoidea, 624, 647 ; of Holo-
thuroidea, 624, 649 ; of Metazoa, 143
Bilharzia, see Schistosoma
Binary fission of Protozoa*, 28 ; 36,
45, 54, 70, 72, 74, 80, 83, 85, 86,
98, 115
Bipaliimi kewense, 215
Bipinnaria larva, 632; 632
Biramous limb, see Stenopodium
Birgus, 415
Bladder worm, see Cysticercus
Blastocoele, 129; i, 131, 133, 237
Blastoidea, 659
Blastopore, 129, 130 and n.
Blastostyles, 154
Blastozoite, see Blastozooid
Blastozooid, 679 ; 685
Blastula, 129, 145 ; of Coelenterata,
152; of Echinodermata, 631; of
Obelia, 156
Blatta, 428, 433, 466 ; orientalis, 466
Blepharoplast, 15 n.
Blood, 131, 132, 133; of Arenicola,
273, 275; of Chaetopoda, 263; of
Ciona, 674; of Crustacea, 351; of
Insecta, 439; of Scorpion, 522
Blood vessels, see Arterial system.
Artery, Vascular system
"Blood vessels" of Echinodermata,
see Lacunar system
Bodo, 63 ; saltans, 62 ; sulcatus,
Chemophobotaxis of, 39, 40
Body, of Amphipoda, 400 ; of Areni-
cola, 272 ; of Balanoglossus, 662 ; of
Cephalodiscus and Rhabdopleura,
668; of Crustacea, 332; of Cteno-
phora, 195; of Echinodermata*,
623, 640, 648; of Isopoda, 395 ; of
Medusa, 150; of Metazoa, 128; of
polyp, 150; of Porifera, 117; of
Protozoa, 8; of Tubicoleus Poly-
chaeta, 270. See also Symmetry
Body cavity, of Nematoda, 245 ;
Perivisceral, see Perivisceral cavity ;
Primary, see Haemocoele; Secon-
dary, see Coelom
Body wall, of Hirudinea, 298 , of
Holothuroidea, 650; of Metazoa,
128, 129; of polyps, 150; of Roti-
fera, 237
Bofubus, 498, 503, 504
Bonibyx mori, 490
Bonellia, 302 ; viridis, 303
Boophilus annulatus, 537; bovis, 537
Bopyrus, 29^', /(JUgsrouxi, 400
Bothriotaenia, 230
Botryllus, 681; 678, 678, 679; vio-
laceus, 681
Botryoidal tissue, 298
Bougainvillea, 157; 158, i6s', fruc-
tuosa, 157
Brachial ossicles, 656
Brachial skeleton, 6i6
Brachiolaria larva, 633
Brachioles, 658
Brachiopoda, 613; 2; Development
of, 617
Brachyura, 404, 407
Brachyurous type, 404
Brain, 136; 138, 144; of Acantho-
cephala, 259; of Acoelomate Tri-
ploblastica, 197; of Arthropoda,
309; of Branchiopoda, 340; of
Ciona, 675 ; of Crustacea, 340 ; of
Insecta, 448, 449; of Nemato-
morpha, 257; of Polychaeta, 264;
"of Sepia, 596, 596. See also
Ganglion, Cerebral
Branchia, of Phyllopodium, 337, 354;
oi Salpa, see'' GiW
Branchiae, see Gills
Branchial chamber, 672
692
INDEX
Branchial hearts, veins, 594
Branchial opening, 669
Branchiobdellidae, 286, 301
Branchiogenital region, 664
Branchiopoda, 353; 326, 327, 331,
334, 340
Branchiura, 376; 330, 331
Breathing, see Respiratory move-
ments
B rising a, 638
Brown body, 607
Bruchophagus funebris, 501
Bryograptus, 171; 172; callavei, 172;
retroflexusy 172
Bryozoa, name for Polyzoa, 606 n.
Buccal capsule, 251
Buccal cavity, of Balanoglossus , 664;
of Helix, 557; of Insecta, 432; of
Nematoda, 248; of Tunicata, 669.
See also Alimentary canal
Buccal mass, 557; of Helix, 557; of
Sepia, 594-5
Buccal tube feet, 643
Buccinum, 565 ; 552, 553, 562, 563
Budding, of Alcyonaria, 183; of
Cysticerci, 229 ; of Hydratuba, 178 ;
of Hydrozoa*, 152, 154, 156, 158,
161, 164, 165, 166, 169; of Madre-
poraria, 191; oi Microstoma, 213;
of Polychaeta, 278 ; of Protozoa*,
28, 72, 83, 114, 115; of Ptero-
branchia, 668; of Stylaria, 291;
of Tunicata*, 677, 678, 680, 681,
683, 684, 685. See also Colonies
Bugula, 608, 610, 613
Bulimus, 572
Bulla, 564, 567
Bunodes, 529
Bursa copulatrix, of Insecta, 447,
448; of Turbellaria, 211, 230, 232
Buthus, Internal anatomy of, 523 ;
carpathicus. Embryo of, 516
Byssus pit, 583
Caddis flies, see Trichoptera
Cadophore, 686
Caeca, Mesenteric, see Mes. caeca
Caecum, of Echinoidea, 646; of
Polyzoa, 606 ; of Sepia, 595
Caenis, 481
Caenogenetic features, 144
Calabar swellings, 255
Calanus, 373; 335, 338, 374
Calathus, 258
Calcarea, 126; 120
Calcareous ring in Holothuroidea,
650
Caligus, 375
Callidina, 241
Calliphora, 435, 439, 512
Calosoma, 493 ; semilaeve, 493
Calotermes militaris, 469
Calymma, 80
Calyptoblastea, 153; 154, 162, 172.
See also Leptomedusae
Calyptomera, 354, 368
Calyx, 654
Campodeiform larva, 456, 459 ; of
Coleoptera, 492, 493, 495
Canal system, of Medusae, 150, 151,
156, 1 74 ; of Sponges, 119,1 20, I2I
Cancer, 417
Capillitium, 86
Capitellidae, 276
Capitulum, 378
Caprella, 402 ; grandimana, 402
Capsidae, 476
Captacula, 572
Carabus, 493 ; violaceus, 493
Carapace, 333; 327; of Branchio-
poda, 327, 354; of Calyptomera,
354; of Carcinus, 407; of Cirri-
pedia, 330, 333 ; of Cladocera, 327,
354, 362; of Conchostraca, 327,
333, 354; of Gymnomera, 354; of
Lepas, 378; of Leptodora, 368; of
Malacostraca, 334, 388; of Neba-
lia, 390; of Ostracoda, 330, 333;
of Peracarida, 393
Carchesium, iii; epistylidis, 113
Carcinus, 385, 408, 417 ; niasnas, 407 ;
406, 407, 409, 411, 412, 413, 414
Cardium, 579
Cardo, 427
Caridea, 404
Caridoid facies, 388
Carina, 378
Carinella, 237
Cartfiarina, i6l, 164
Carotin, 47
Carpoidea, 658
Carpopodite, 336
Carteria, 57; 48, 48
Caryophyllaceus , 230
Caryophyllia, 190
Cases of Trichoptera, 487
Cassiopeia, 179, 180
Catch fibres, 143
INDEX
693
Caudal furca (rami), 339, 355, 358,
360, 364, 372, 379, 388, 390
Cavolinia, 569; 567, 568
Cecidomyidae, 509
Cells, 7 ; assuming various functions,
128; Corneagen, 310; Flame, 202;
134, 197, 203, 235, 240, 243, 27s;
Interstitial, 147 and n. ; Iris, 310;
Lasso, 193, 195; Musculo-epithe-
lial, 146; of Porifera, 117, 118, 124;
Pole, 130, 289; Sensory, 128, 147,
149, 201, see also Sense organs;
Somatic of Volvocina, 10, 58;
Thread, 147; Yellow, 262. See
also Choanocytes, Myoblast, Oeno-
cytes, Pinacocytes, Porocytes, etc.
Cellular animals, 7; structure, 124, 128
Cellulases, 130, 435, 559, 587
Cellulose, 12, 56, 86, 130, 131
Central capsule, 80
Central nervous system, 136; 137,
138, 199; of Annelida, 262; of
Chordata, 660; of Tunica ta, 675.
See also Brain, Ganglion, Nervous
system
Centrodorsal ossicle, 655; 656
Cephalization, 144; of Crustacea,
332; of Polychaeta, 266
Cephalochorda, see Amphioxus
Cephalodiscus, 668 ; 660, 662
Cephalopoda, 587
Cephalothorax, of Arachnida, see Pro-
soma ; of Brachyura, 404 ; of Cope-
poda, 370, 371, 372; oiLigia, 395
Cerambycidae larvae, 435
Cerata, 569
Ceratium, 54; 36; macroceras, 55
Ceratophyllus fasciatusy 514; 513
Cerci anales, 431
Cerebral ganglia, see Brain; Gang-
lion, Cerebral
Cerebral organ, 235
Cerebral vesicle, 676, 677
Cerebratulus, 237
Cervical groove, 332
Cervical sclerites, 428
Cestoda, 223; 198; Merozoa, 225,
227; Monozoa, 225, 230
Cestus Veneris, 196
Ceuthorrhynchus, 495
Chaetae, 260, 260 ; 305 ; of Acantho-
bdella, 300 ; of Archiannelida*, 294,
295, 296; of Chaetopoda, 261 ; of
Echiuroidea, 301 ; of Oligochaeta*,
286-94 (passhn) ; of Polychaeta*,
265-72 (passim); of brachiopod
larva, 618
Chaetoderma, 549
Chaetognatha, 618; 2
Chaetonotus, 242
Chaetopoda, 261 ; 260
Chaetopterus, 264, 270, 271 ; perga-
mentaceus, 269
Chambered organ, 656; 630, 655
Chambers, of cephalopod shells, 602;
of Foraminifera shells, 75
Cheeks of Trilobita, 323
Cheilostomata, 613; 608
Cheimatobia, 491
Chela, 339
Chilaria, 517, 526
Chilina, 564
Chilo, 491
ChilomonaSy 52
Chilopoda, 418
Chirocephalus, 311, 326, 327, 331,
332, 348, 357, 359, 359, 364, 367,
373; diaphanus, 356, 357
Chironomidae, 509
Chironomus, 439; 301, 446
Chitin, 309 n. ; 260
Chiton, 548; 143, 548, 583
Chlamydomonas , 56; 20, 28, 29, 29,
30, 31, 32, 33, 38, 46, 57; angulosa,
29; brauni, 31, 32; euchlora, 31;
longistigma, 29; media, 32; steini,
31
Chlamydospores, 38
Chloeon, 481
Chlorocruorin, 133; 263
Chloromonadina, 54; 49
Chlorophyll, 47
Chloroplasts, 47; 19 n.
Chlorops taeniopus, 512
Choanocytes, 117; 118, 119, 120,
123, 124, 125, 126
Choanoflagellata (Choanoflagellidae),
65
Cho7idr acanthus, 375 ; gibbosus, 375
Chondrioderyna, 86 ; difforme, 87
Chonotricha, 114; 26, 33
Chordata, 660; 2
Chorion, of egg of Insecta, 446
Chromatophores, of Cephalopoda,
591; of Crustacea, 343, 344; of
Phytomastigina*, 47; 18, 30, 46,
50-6 (passim)
Chromidium, 28
694
INDEX
Chromoplasts, see Chromatophores
of Phytomastigina
Chrysamoeba, 50 ; radians, 51
Chrysaora, 173
Chrysidella, 52; schaudinni, 53
Chrysis, 503
Chrysomelidae, 495
Chrysomonadina, 50; 20, 63
Chrysops, 506; 255; caecutiens, 510;
dimidiata, 511
Cicada septendecim, 476
Cicindela, 436, 493
Cilia, 13; of Ciliata, 104, 105; of
Ciona pharynx, 672, 673 ; of Coe-
lenterata, 174, 182, 189, 193; of
mollusc gills, 556, 576; of Turbel-
laria, 201, 202, 204. See also Epi-
dermis
Ciliary junctions, 574
Ciliary organ, 277
Ciliata, 102; 109, IIO
Ciliated band, of Dipleurula, 631-2;
of Tornaria, 668
Ciliated funnel, see Dorsal tubercle
Ciliated pits, 201 ; 204
Ciliated ring, see Ciliated band,
Metatroch, Prototroch, Velum
Ciliophora, 102; 24, 26, 33, 44
Ciliospores, 38
Cimex, 476
Cimicidae, 474
Cingulum, 239
Ciona, 680. See also C. intestinalis;
intestinalis , 669 ; 670, 671, 672
Circulation of blood, 132; in Ano-
straca, 348; in Araneida, 530; in
Arenicola, 275 ; in Arthropoda,
314; in Balanoglossus, 667-8; in
Chaetopoda, 263 ; in Ciona, 674 ;
in Copepoda, 373 ; in Echino-
dermata, 629; in Entomostraca,
349; in Insecta, 437; in Malaco-
straca, 349 ; in Mollusca*, 556, 579 ;
in Nemertea, 235 ; in Ostracoda,
349 ; in Scorpionidea, 522. See also
Vascular system
Circulation of food, in Alcyonium,
183; in Aurelia, 174; in Daphnia,
364, 367; in Obelia, 155
Circulatory system, see Vascular
system
Circulus venosus, 557
Circumoesophageal connectives, see
Nervous system
Cirrhal ossicles, 656
Cirri, of Crinoidea, 655 ; of Protozoa,
17; of Thoracica, 379
Cirripedia, 376; 330, 333, 349, 351,
352, 353
Cladocera, 362; 144, 327, 354
Classification, vii, i, 2; of Brachio-
poda, 618; of Cephalopoda, 588;
of Demospongiae, 127 ; of Gastero-
poda, 563; of Hemiptera, 475-6;
of Lamellibranchiata, 579; of
Myriapoda, 418; of Opistho-
branchiata, 567 ; of Polychaeta, 264 ;
of Polyzoa, 612 ; of Protozoa, 44 ; of
Pulmonata, 570; of Radiolaria, 76
Clathrina, 126
Clathrulina, 86; 12, 15
Clava squamata, 153
Clavelina, 680; 677, 679
Clavularia, 183
Cleaners, see Scavengers
Cleavage of ovum, affected by yolk,
14s; Centrolecithal, 316, 352, 452;
Determinate, 145, 675; of Archi-
annelida, Polychaeta, Polyclada,
Mollusca, Nemertea, 281; of Ar-
thropoda, 316; of Balanoglossus,
668 ; of Ciona, 675 ; of Coelenterata,
151, 196; of Crustacea, 352; of
Ctenophora, 196; of Echino-
dermata, 283, 631; of Pyrosoma,
685; Radial, 283; Spiral, 281
Climacograptus, 170, 171
Cliona, 127
Clitellum, 286; 289, 291
Cloaca, of Holothuroidea, 650; of
Nematoda, 250; of Rotifera, 240;
of Tunicata, 673
Clypeaster, 647
Clypeastroida, 647
Cnidaria, 152; 146
Cnidoblast, 147
Cnidocil, 148
Cnidosporidia, 100
Coarctate pupae, 509 ; 456
Coccidae, 477
Coccidia, 89
Coccidiomorpha, 88
Coccinellidae, 495 ; 494
Coccinella, 495
Coccolithophoridae, 50
Coccoliths, 50
Cocoons, of earthworms, 289 ; of In-
secta*,490,495,5i3;of spiders, 533
INDEX
695
Codosiga, 65 ; 8 ; wnbellata, 65
Coelenterata, 146; i, 129, 196
Coeliac canal, 656
Coelom, 133 ; 2, 129, 130, 135, 141 ; of
Acanthohdella, 300; of Annelida,
260; of Archiannelid genera, 294,
296 ; of Arenicola, 272 ; of Arthro-
poda, 314; of Balayioglossus, 662;
of Brachiopoda, 616, 618; of
Cephalochorda, 660; of Chaeto-
gnatha, 619; of Chaetopoda, 260,
262; of Chordata, 660; of Cri-
noidea, 656 ; of Crustacea, 346 ;
of Dipleurula, 627 ; of Echino-
dermata, 626, 633 ; of Echiuroidea,
261 ; of Hemichorda, 660, 661 ; of
Hirudinea, 261, 298, 299, 300; of
Mollusca, 543; of Peripatus, 319;
of Polyzoa, 606 ; of Sepia, 593 ; of
Sipunculoidea,26i ;ofSnail,556;of
Tunicata, 661, 675 ; of Vertebrata,
661. See also Pericardium
Coelomoducts, 134; 140, 141; of
Arthropoda, 315; of Chaetopoda,
262 ; of Crustacea, 347 ; of Mol-
lusca, 545 ; of Polychaeta, 275, 276,
277
Coeloplana, 196
Coenosarc, of Calyptoblastea, 153 ; of
Polyzoa, 608
Colacium, 52
Coleoptera, 429 ; three types of, 494
Coleopterous larvae, 457
Collar, of Balanoglossus, 662 ; of Chae-
topterus, 270; of Choanoflagellata,
65 ; of Pterobranchia, 668
Collar cavities, 661 ; of Balano-
glossus, 663
Collar cells, see Choanocytes
Collar pores, 663 ; of Balanoglossus,
663
Collembola, 463 ; 444
Colleterial glands, 447, 448
Colletes, 498
Collinia, 107; 33 n.
Colloblasts, see Lasso cells
Collozoum, 81 ; inerme, 41
Collum, 424
Colon, see Large intestine
Colonies, of Alcyonaria, 180, 183; of
Carchesium, 1 1 1 ; of Hydroids*, 153,
154, 157, 158, 162, 163, 164, 165;
of Polyzoa, 606, 608; of Protozoa,
8; of Rhabdopleura, 668; of Si-
phonophora, 166, 167, 168, 169; of
Syllis rarnosa, 280; of Tunicata,
677, 680, 685 ; of Volvocina, 57, 58 ;
of Zoantharia, 186
Colpidium, 109
Colpoda, 109; 42; steini, 108
Columella muscle, 555
Compasses, 645
Compensation sac, 608
Complemental males, 380
Compound ascidians, 679
Compound eyes, see Eyes
Conchostraca, 362; 327, 333, 354
Conjugants, 33
Conjugation, 6,30. See also Syngamy
Contarinia nasturtii, 510 ; pyrivora,
509
Contractile vacuoles, 20; of Euglena,
52 ; of Heliozoa, 83 ; of Protozoa, 22
Conus, 562, 563
Convoluta roscoffensis , 213 ; 48,48, 57 ;
henseni, 214
Copepoda, 370; 330, 333
Copeus, 240
Copidosoma gelechiae, 446
Copromonas, 52; 31 ; subtilis, 53
Coprozoic Protozoa, 43; 31
Copulatory bursa, 250; spicules, 250
Corallium, 184, 186; rubrum, 184
Corals, of Alcyonaria, 185; of Hy-
drocorallinae, 165; of Madre-
poraria, i86, 189-92; of Polyzoa,
608
Cordylophora, 164
Corethra, 436
Corixa, 476
Corm, 336
Cormidium, 166
Cornularia, 183
Corona, 640
Corpuscles, 131. See also Blood
Cortex, of Ciliata, 105; of Porifera,
120; of Protozoa, 13
Corticata, 44
Corymbites cupreus, 494
Coryne, 539
Cossidae, 491
Cossus, 491
Cotton spinner, see Holothuria
Course of circulation, see Circulation
of blood
Coxa, 428
Coxal glands, of Arachnida*, 520,
529; of Arthropoda, 315
696
INDEX
Coxopodite, 336
Crabro, 439
Crangon, 415
Crania, 616; 613, 615, 618
Craspedacuta, 164
Craspedochilus, 549 ; 548
Cremaster, 490
Crinoidea, 654; 623
Crista, 610, 613
Cristatella, 613
Crithidia, 64; 62, 63, 64
Crop, of Gasteropoda, 558; of In-
secta, 432 ; of Opisthobranchiata,
567
Crotchets, 291
Crural glands, 321
Crustacea, 326; 308
Crustacean-insect-myriapod section,
308
Cryptocerata, 475
Cryptomitoses, 25
Cryptomonadina, 50 ; 49, 53
Cryptomonas, 50; ovata, 53
Cryptoniscus, 398 ; paguri, 400
Cryptus obscurus, 501
Crystal cells, see Vitrellae
Crystalline cone, 310
Crystalline style, 578
Ctenidia, 543 ; of Cephalopoda*, 588,
591, 603; of Chiton, 549; of
Gasteropoda*, 550, 551, 552, 553,
554, 564, 565, 567; of Lamelli-
branchiata*, 574, 575, 576, 577,
578, 579, 582, 585, 587; of Mol-
lusca, 543
Ctenidial circulation of Lamelli-
branchiata, 581
Ctenocephalus canis, 513
Ctenophora, 193; 146
Ctenoplana, 196
Ctenopoda, 362; 368
Ctenostomata, 613
Cucumaria, 652; 653
Culexfatigans, $og ; pipiens, 507
Culicidae, 509
Cumacea, 393
Cunina, 164
Cup-shaped organs, 300
Curculionidae, 494, 495
Cursoria, 466
Cuspidaria, 579
Cuticle, 2, 5, 131, 132, 136; of An-
nelida, 260; of Arthropoda, 309;
of Crustacea, 331; of Nematoda,
245; of Protozoa, 12; of Rotifera,
237; of Trematoda, 218
Cuvierian organs, 650
Cyamus, 403 ; 396
Cyanea arctica, 172
Cyathomonas, 52 ; 14, 46 ; truncata, 53
Cyathozooid, 685 ; 683
Cyclas, 579
Cyclestheria hislopi, 335
Cyclomyaria, see Doliolida
Cyclophyllidea, 230; 227
Cycloporus papillosus, 215
Cyclops, 370; 216, 228, 255, 328, 352,
353, 371, 372
''Cyclops'' larvae, 373; 353, 374,375
Cyclorrhapha, 511
Cyclosalpa, 686; 685
Cyclospora, 32
Cyclostomata, 613
Cyphonautes larva, 610; 6ll
Cypridina, 370
Cypris, 369; 328, 342, 368, 370
''Cypris" larva, 380; 353, 380, 382,
383, 385
Cyrrtohinus mundulus, 476
Cyrtoceras, 604
Cysticercus, 228; 229, 230; pisi-
formis, 228
Cystoflagellata, see Noctiluca
Cystoidea, 658
Cysts, 22; 12. See also Gamocysts,
Oocysts, Sporocysts
Cytostome, 19; 104
Dactylopius coccus, tomentosiis, 481
Dactylopodite, 336
Dactylozooids, 165; 166
Dalyellia viridis, 214; 207
Daphnia, 364; 327, 328, 355, 367;
pulex, 366
Dart, 257
Dart sac, 561
Dead men's fingers, see Alcyonium
digitatum
Deamination, 131
Death, 5
Decapoda, Cephalopoda, 588, 599;
Crustacea, 404; 331
Deep oral nervous system, 630
Deima, 652; 653
Demodex folliculorum, 541
Demospongiae, 126; 123
Dendrites, 137
Dendritic tentacles, 649
INDEX
697
Dendrochirotae, 652; 650
Dendrocoelum lacteum, 203, 212, 212,
214
Dendrocometes, 1 16 ; 33 n. ; paradoxus,
108
Dendroid graptolites, 172
Dendron, 137
Dense nuclei, 23
Dentalium, 572; 573
Depression, 35
Deraecoris fasciolus, 477
Dermal layer, 117
Dermaptera, 468 ; 429
Dermis, 136
Dermomuscular tube, 142
Determinate cleavage, 145, 675
Detorsion, 554
Deutocerebrum, 310, 340
Deutomerite, 95
Development, 144. See also Embry-
ology, Larvae, Life cycle
Diastylis, 395 ; stygia, 394
Dibothriocephalus, 227, 228 ; latus,
227, 228, 230
Dibranchiata, 588
Dicranura, 490
Dicyclical rotifers, 241
Didymograptus , 171 ; 170, 172; affinis,
172; fasciculatuSy 172; v-fractus,
170
Difflugia, 74; 12; urceolata, 74
Digestion, 130; by Acarina, 534; by
Alcyonium, 183 ; by Arenicola, 273 ;
by Arthropoda, 314; by Aurelia,
175; by Coelenterata, 147; by
Crustacea, 344; by Daphnia, 367;
by Helix, 558-60; by Insecta, 434-
6; by Lamellibranchiata, 577-8;
by Oligochaeta, 287; by Ostrea,
586; by Physalia, 167; by Proto-
zoa, 19; by Rotifera, 240; by Tere-
do, 587; by Turbellaria, 206; by
Zoantharia, 189. See also Alimen-
tar>' canal. Circulation of food,
External digestion
Digestive caeca, see Digestive gland
Digestive gland, 130, 131; of Arach-
nida*, 517, 522, 529, 530; of
Brachiopoda, 616; of Mollusca*,
543, 558, 569, 577, 595- See also
Liver, Mesenteric caeca
Digestive system, oi Alcyonium, 182;
of Aurelia, 174; of Ctenophora,
195, 196; of Medusae, 150; of
Platyhelminthes*, 206, 213, 214,
216, 217, 223 ; of Zoantharia, 187.
See also Alimentary canal, Enteron
Dimorpha, 83 ; mutatis, 84
Dimorphic shells, 76
Dinamoebidium, 55
Dinobryon, 50; sertularia, 51
Dinoflagellata, 54; 49, 50 n., 80
Dinophilus 296 ; 294, 295
Dinophysinae, 54
Dinothrix, 55
Diotocardia, 564; 552, 563
Diphyllidea, 229
Diphyes, 166
Dipleurula, 631 ; 145, 627, 628
Diploblastica, 129. See also Coelen-
terata
Diplograptus, ly i;foliaceus, 171
Diplomonadina, 65, 66, 67
Diplopoda, 422
Diplopores, 659
Diploporida, 659
Diplostraca, 362; 327, 354, 355
Diplozoon, 218; 220
Diptera, 504; 430, 508, 510
Dipylidiu?n caninum, 228, 230
Direct wing muscles, 430
Directives, 187
Discomedusae, 174
Dissosteira Carolina, 440
Distephanus, 50; speculum, 51
Distomum macrostomum, 222
Division of protozoan nuclei, 24
Docoglossa, 563
Dolichoglossus kozvalevskii, 663
Doliolida, 686; 679, 682, 685
Doliolum, 686 ; 677, 679, 684
Donax, 578
Doris, 569; 554, 567, 568
Dorsal and ventral, 143; aspects of
bilateral animals, 143 ; aspects of
Ciiiata, 103; aspects of Holothu-
roidea, 649; aspects of Sepia, 591 ;
" blood vessels " of Echinoidea and
Holothuroidea, 629, 643, 650;
mesenteries of Alcyonaria, 182;
structures in radial animals, 143
Dorsal antenna, 240
Dorsal blood vessel, of Arthropoda,
see Aorta ; of Balanoglosstis, 667 ;
of Chaetopoda, 263, 275; of
Rhynchobdellidae, 299
Dorsal "blood vessels" of Echino-
dermata, 629, 643, 650
698
INDEX
Dorsal cirrus, 265 ; lamina, 680 ; organ,
334 ; pores, 287 ; shield, 334 and n. ;
siphon, 574; tubercle, 672
"Dorsal" plates of Ophiuroidea, 639
Dorsolateral antennae, 240
Drag line, 533
Dreissensia, 546, 582
Drift net of Physalia, 166
Drosophila, 437
Ductus communis, 210, 211, 230
Dysderciis, 476
Dytisciis, 429, 439, 444, 493; fnar-
ginalis, 447
Earthworms, 287
Ecardines, 618
Ecdyonurus, 481
Ecdysis, see Moulting
Echinaster sentus, 636
Echinohothrium, 229
Echinocardimn, 648 ; cordatum, 648
Echinocyamus, 647 ; pusiUus, 647
Echinodermata, 623 ; 2, 136, 142, 143
Echinoidea, 640; 623
Echinopluteus, 633
Echinorhynchus proteus, 259
Echinosphaera, 659
Echinus, 647. See also E. escidentus;
esculentus, 640, 644; miliaris, 641,
642
Echiuroidea, 301 ; 261
Echiurus, 302; 301, 302
Ectoderm, 128; i, 129, 130, 132, 136,
139, 141, 142, 143, 146, 150, 152,
156, 176, 183, 201, 204, 235, 237,
244, 281, 282
Ectoneural system, 630 ; of Crinoidea,
656
Ectoplasm, 12 ; 19, 44, 68, 70, 76, 95,
102, 104, 105
Ectoprocta, 613; 611
Edrioaster, 658 ; bigsbyi, 657
Edrioasteroidea, see Thecoidea
Edivardsia, 187; 188
Effectors, 137, 138
Efferent canals, see Exhalant canals
Egg sac, 373
Eggs and Egg laying, of Arachnida*,
524, 529, 533; of Arthropoda, 316;
of Balanoglossus, 668 ; of Chaeto-
gnatha, 619, 620; of Ciona, 675 ; of
Cnidaria*, 152, 156, 159, 162, 174,
178, 183; of Crustacea*, 352, 356,
359, 362, 367, 373, 376, 379, 385,
410; of Ctenophora, 196; of Dino-
philus, 296 ; of Echinodermata, 630,
631 ; ofHirudinea,30o; of Insecta*,
452, 468, 471, 472, 476, 478, 481,
483. 484, 485, 486, 491, 500, 503,
504n., 511, 513; of /z//u^, 424 ; of
Myriapoda*, 422, 424; of Mol-
lusca*, 555, 561, 566, 572, 585,
598; of Nematoda*, 250, 254, 255,
256, 257, 259 ; of Oligochaeta, 289 ;
of Pantopoda, 539; of Peripatus,
321 ; of Platyhelminthes*, 210, 212,
215, 219, 220, 222, 228; of Poly-
chaeta, 275 ; of Rotifera, 241 ; of
Thaliacea, 685
Eimeria, 89 ; 96 ; schubergi, 27. 89,
90
Ejaculatory duct, 447
Elasipoda, 652
Eledone, 596
Eleutherozoa, 658; 625
Elytra, of Coleoptera*, 429, 492, 495 ;
of Polychaeta, 268
Embia major, 471
Embioptera, 471
Embryo, 144
Embryology (.?. str.), 145 ; of Arach-
nida, 516, 520; of Arthropoda,
316; of Asterias, 631 ; of Brachio-
poda, 617, 618; of Chaetognatha,
619, 620; of Chordata, 660; of
Ciona, 675 ; of Coelenterata*, 152,
156, 162, 178 n., 196; of Crustacea,
352; of Echinodermata, 631; of
Insecta, 452, 453, 457, 458, 459 ; of
Lumbricus, 289, 290; oi Peripatus,
321; of Polyzoa, 611; of Tardi-
grada, 450 ; of Thaliacea, 685 ; of
trochospheres, 281
Embryonic fission, see Polyembryony
Enchylema, 12
End gut, see Hind gut
End sac, 315, 346
Endites, 337, 338
Endocyclica, 647
Endoderm, 128; i, 129, 130, 136,
139, 141, 142, 143, 281 ; of Coelen-
terata*, 146, 147, 150, 152, 155,
158, 161, 174, 181, 182, 183, 195.
See also Enteron
Endoderm lamella, 150; 179
Endomixis, 35; 26
Endophragmal skeleton, 340
Endoplasm, 13
INDEX
699
Endopodite, of Crustacea, 335; 336,
337; of Trilobita, 324; of Xipho-
sura, 528. See also Limbs
Endoprocta, 612
Endopterygota, 485
Endopterygote wing-formation, 459
Endosome, 22
Endosternite, 340
Endostyle, 672, 673, 675
Entamoeba, 70; 71; coli, 70; dysen-
teriae, see E. histolytica; his-
tolytica, 70; 43, 71
Enterobius vermicularis , 254
Enteron, 146, 150, 151, 154, 157, 161.
See also Archenteron, Digestive
system
Enteropneusta, 662 and n.; 2, 665
Entocoeles, 187
Entodiniomorpha, no
Entodinium, in; caudatum, 108
Entomostraca, 331
Envelope cells, 100
Environment, 3; 129
Enzymes, Digestive, 130; 344, 434,
558, 559, 566
Eolis, 569; 554, 567, 568
Epeira, 532; diademata, 531
Epheolota, j 16 ; gemmipara, 115
Ephemera, 481 ; vulgata, 4S1
Ephemeroptera, 481 ; 444
Ephestia, 491
Ephippium, 367
Ephyra larva, 179; 173
Epibolic gastrulation, 145, 282
Epibranchial space, of Decapoda,
408 ; of Lamellibranchiata, 574
Epicardial cavity, 674; diverticula,
674, 677 ; tube, 674, 677, 678
Epicardium, see Epicardial cavity, etc.
Epicuticle, 309
Epidermis, 136; in the several phyla,
see names of phyla. See also
Ectoderm
Epimerite, 95
Epineural canal, 623 ; of Echinoidea,
646; of Ophiuroidea, 638
Epipharynx, 426. See also Mouth
parts of Insecta
Epiphragma, 562
Epipodites, 314, 335, 336, 337. ^See
also Gills of Crustacea, Metepipo-
dites, Oostegites, Proepipodites
Epistome, of Decapoda, 410; of
Polyzoa, 606
Episiylis, in
Epizoanthiis, 126
Equitant whorls, 76
Erichthus larva, 389
Eriocraniay 488
Eris talis, 511
Eruciform larvae, 456; 457; of
Coleoptera, 492, 495 ; of Sym-
phyta, 500
Estheria, 362; 346; obliqua, 363
Euanostraca, 360
Eucarida, 403 ; 389, 390
Eucephalous larva, 508 ; 509
Eucoila, 500
Eucystis, 659
Eudendrium, 158; 162, 163
Eudorina, 58; 10
Euglena, 52; 21, 22, 38, 43, 46;
gracilis, 42, 52 ; viridis, 40, 45, 5-, 53
Euglenoid movement, see Met?/boly
Euglenoidina, 52; 20, 49, 53
Eugiypha, 74; 12; alveolata, 14; 73
Eugregarinaria, 95 ; 88
Eulalia, 268 ; 264, 266
Eulamellibranchiata, 579; 574, 585
Eumitoses, 25
Eunice, 268; 264, 265, 272
Eupagurus, 415; berr.hardus, 416
Euphausiacea, 403; 388, 389
Euplectella, 126
Eupomatus, Trochosphere of, 284
Eur>'pterida, 524
Eurypterus, 526
Euspongia, 127; 121, 123
Euthyneury, 554, 570
Eutyphoeus, 289
Evadne, 368
Evolution, 3, 4, 5, 144
Exarate pupae, 456
Excreta, see Excretion
Excretion, 134; by Arthropoda, 315;
by Crustacea, 345-7; by Echino-
dermata, 628; by Metazoa, 134,
140; by MoDusca, 556; by Proto-
zoa, 20; by Tunicata, 675. Set
also Excretory organs
Excretory organs, 140; 3, 134;
of Arachnida, 315, 520; of Ar-
thropoda, 315; of Balanoglossus,
668; of Crustacea, 315, 345; see
also Antennal glands, Maxillary
glands; of Insecta, 315, 436; of
Metazoa, 140; of Myriapoda, 315;
of Nematoda, 245, 248 ; of Nemer-
700
INDEX
Excretory organs (cont.)
tea, 235; of Onychophora, 315,
320; of Platyhelminthes, 202; of
Polychaeta, 275; of Rotifera, 241.
See also Coelomoducts, Coxal
glands, Glomerulus, Kidneys,
Malpighian tubules, Nephridia
Exhalant canals of Porifera, 120
Exhalant passage of Carcinus, 409
Exites, 337
Exocoeles, 187
Exocyclica, 647
Exogamous syngamy, 31
Exopodite of Crustacea, 335, 336,
337; of Trilobita, 324; of Xipho-
sura, 528. See also Limbs
Exopterygota, 466; 456
Exopterygote development of wings,
459
External digestion, by Araneida, 530;
by Insecta, 436; by Oligochacta,
287; by Rhizostomeae, 179; by
Turbellaria, 208
External medium, see Medium
Extracellular digestion, 130; 147
Extrathecal zone, 191
Exumbrellar surface, 173, 179
Eyes, Compound, 310, 313; Crus-
tacean median, 340, 342 ; of Arach-
nida*, 310, 521, 529, 532, 533; of
Arthropoda, 310, 312; of Ascidian
tadpole, 676; of Chae*ognatha,
618 ; of Chaetopoda, 263, 265, 268,
280, 291 ; of Crustacea*, 310, 313,
340, 355, 356, 364, 372, 376, 380,
387, 395; of Hirudinea, 300; of
Hydrozoa, 160, 161; of Insecta,
310, 426, 452; of Mollusca*, 549,
555, 570, 584, 598, 599, 603; of
Myriapoda*, 417, 422; of Nemer-
tea, 235; of Onychophora, 310,
317; of Polychaeta, 263, 265; of
Trilobita, 323 ; of Turbellaria, 200,
200. See also Eye -spots
Eye-spots, of Asteroidea, 630; of
Protozoa, 17
Eyestalk, 410
Facial suture, 323
Falciform young, 38; 95
Fasciola, 220; 218; hepatica, 221, 223
Fat body, 437, 439
Favia, 192
Feeding, of Actinozoa*, 183, 193;
of Arachnoidea, 517, 522, 529,
530, 534, 539; of Asteroidea, 636;
of Balanoglossus, 666 ; of Brachio-
poda, 615, 616; of Branchiopoda*,
354, 356, 358, 359, 362, 364, 367;
of Carcinus, 410; of Cephalopoda,
594; of Chaetognatha, 619; of
Chaetopterus, 271 ; of Chordata,
660 ; of Ciliata, 104 ; of Ctona, 674 ;
of Copepoda, 373 ; of Crinoidea
(Antedon), 656; of Crustacea, 326-
7, 331; of Cyprts, 369; of Dioto-
cardia*, 565 ; of Echinoidea*, 646,
647, 648; of Errant Polychaeta*,
265 ; of Filter-feeding Malacos-
traca*, 388, 391, 393, 403; of
Gastrotricha, 243 ; of Holothu-
roidea*, 650 ; of Holozoic Mastigo-
phora*, 46, 49, 52, 54, 63, 64, 65,
68 ; of Hydatina, 240 ; of Hydro-
corallinae, 165; of Lamellibran-
chiata*, 574. 576, 577, 582, 587,
587; of Lepas, 379; of Monoto-
cardia*, 565; of Nematoda, 248;
of Nemertea, 233 ; of Oikopleura,
680 ; of Ophiuroidea, 639 ; of Opis-
thobranchiata*, 567, 56?^; of Phy-
salia, 166, 168; of Polyzoa, 607 ; of
Protozoa, 19; of Pulmonata*, 570;
of Sarcodtna*, 70, 77, 83, 86; of
Streptoneura, 564, 565, 566, 567;
of Suctoria, 115 ; of Temnocephala,
216; of Trilobita, 324; of Tubi-
colous Polychaeta, 268, 270; of
Turbellaria, 206; of Veliger, 586;
of Zoantharia, 193
Feet, of Histriobdella, 296 ; of Ony-
chophora, 319
Female gametes, 31 ; of Metazoa, see
Eggs; of Porifera, 118; of Proto-
zoa, 31, 33, 85, 89-96 (passim)
Femur, 428
Figites, 459 ; antomyiarum, 458
Filaria, 253 ; bancrofti, 255 ; 252, 509;
loa, 255, 511 ; medinensis, 255
Filibranchiata, 579; 574, 583
Filograna, 264
Filopodia, 14
Finger-and-toe disease, 86
Fission, of Metazoa, see Budding,
Strobilation ; of Protozoa, see
Fission of Protozoa
Fission of Protozoa*, 28 ; Binary, 28,
29, 36, 45, 52, 54, 70, 72, 74, 80, 83,
INDEX
701
Fission of Protozoa* (com.)
85, 86, 1 15 ; by budding, 28, 29, 72,
83, 115; Longitudinal. 29, 45, 54;
Multiple, 28, 36, 54, 64, 70, 72, 88,
89. 93. 95; Oblique, 45; Pseudo-
transverse, 29, 29, 45 ; Radial, 29, 29,
58; Repeated, 28, 29, 45, 54, 57;
Transverse, 29. See also Plasmctomy
Fissurella, 565; 552, 553, 563
Fixation disc, 633
Fixation papillae, 677
Flabellum, 337
Flagella, 15
Flagellata, see Mastigophora
Flagellated chambers, 119
Flagellispores, see Flagellulae
Flagellulae, 38
Flagellum, of Crustacean limbs, 336 ;
of Helix, 561
Flame cells, see Cells, Flame
Flatworms, see Platyhelminthes
Floscularia, 241 , 242
Flustra, O08, 613
Follicle cells, 598
Follicles, Gonadial, 447, 560
Food, 131. See also Feeding
Food grqove, of Branchiopoda, 355;
of Chirocephalus, 358 ; of Lamelli-
branchiata, 576
Foot, Molluscan, 544, 547 ; of Am-
phineura*, 547, 548; of Cephalo-
poda*, 589, 603 , of Gasteropoda*,
545, 550, 555, 567, 569; of Lamel-
libranchiata*, 547, 581, 582, 583,
585, 587; of Scaphopoda, 572
Foot of Hydatina, 240
Foragers (Bee workers), 504
Foraminifera, 72; 20, 68
Forceps, 468, 469
Forcipomyia, 509
Forcipulate, 634
Fore gut, see Stomodaeum
Forficula auricularia, 468 ; 468
Forynicafusca, 502; sanguinea, 502
Formicoidea, 502
Fossil Arachnida*, 524, 529 ; Brachio-
poda*, 613,616; Cephalopoda, 600,
603 ; Chaetognatha (Amiskwia),
622; Corals, 186; Echinodermata,
633, 658; Foraminifera, 76; Grap-
tolithina, 169; Insecta, 460; Lipo-
straca, 360; Polyplacophora, 549;
Radiolaria, 80; Trilobita, 325
Frenulum, 429, 489, 492
Frilled organ, 225
Front of Carcinus, 407
Frontal appendage, 356; cilia, 576;
horns, 379; organs, 342; surface,
608; suture, 511
Front onia leucas, 106
Functional nervous unit, 448
Fungia, 193; 192
Fungus gardens, 470
Funiculus, 606
Funnel, of Cephalopoda*, 589, 591,
603 ; of segmental organs, 275, 276,
277
Furca, caudal, see Caudal furca
Furcula, 465
Galathea, 415
Galea, 427
Galleria, 491
Gametes, 31 ; 6 ; of Ciliophora, 33 ; of
Cnidosporidia, 100; of Foramini-
fera*, 72, 74, 76, 79; of Heliozoa*,
3 1 , 85, 86 ; of Mastigophora, 45 ; of
Metazoa, see Eggs, Spermatozoa ;
of Monocystis, 31, 97 ; of Myceto-
zoa, 38, 86; of Opalina, 107; of
Protozoa, 31, 33, 36, 38 ; of Radio-
laria, 80; of Telosporidia*, 88-97;
of Volvocina*, 31, 32, 38, 45, 56-8
Gammarus, 400; 107, 114, 116, 259,
329, 402; neglectus, 401
Gamocysts, 22 ; 94
Gamogony, 37
Gamonts, 36; 37, 88-97, 100
Ganglia, of Ophiuroidea, 638; of
ventral cord, 261, 309, 373, 414,
422, 448, 524, 529, 539, 541
Ganglia, System of, see Nervous
system
Ganglion, Antennal, 309, 340; Bra-
chial, 596; Cerebral (Supraoeso-
phageal), 136, 198, 235, 322, 422,
524, 529, 539, 541, 550, 572, 582,
596; see also Brain; Gastric, 597;
Inferior buccal, 597 ; Infundibular,
596, 598; of Ciona, 67$, 677; of
Rhizocephala, 340, 383 ; Pedal, 550,
596; Pleural, 55°, 572, 582; Pro-
-stomial, see Cerebral ; Suboesopha-
geal, 296-7, 340, 379, 422, 434,
524, 529, 531, 539, 541; Superior
buccal, 597; Supraoesophageal,
see Cerebral; Trunk, of Appendi-
cularia larva, 675 ; Visceral, 596
702
INDEX
Gasteropoda, 550
Gasterostomum, 222; fimbriatum^ 220,
224
Gasterozooids, of Doliolida, 686;
684; of Siphonophora, 166, 169
Gastra) layer, 117
Gastric cavity, 150; filaments, 174;
glands, 240; mill, 344; shield, 578
Gastrodes, 196
Gastrophilus, 439; equi, $12,
Gastrotricha, 243; 197, 237, 244
Gastrovascular system, 175
Gastrula, 129; of Echinodermata,
631 ; of Polychaeta, 282
Gastrulation, 145; 129; of Arthro-
poda, 316; of Aurelia, 178; of
Crustacea, 353 ; of Echinodermata,
631; of Insecta, 542; of Nemato-
morpha, 257 ; of Nemertea, 235 ; of
Obelia, 1 5b ; of Polychaeta, 282 ; 283
Gecarcinus, 417; 415
Gemmules, 123
Generative organs, General mor-
phology of, 131, 134, 143; of Am-
phineura, 548; of Arachnida*, 524,
529, 533> 537; of Archiannelida*,
294, 296; of Arthiopoda, 316; of
Balanoglossus, 664, 668; of Brachi-
opoda, 6i6 ; of Chaetognatha, 619 ;
of Chaetopoda, 262, 288 ; of Chilo-
poda, 421 ; of Cnidaria*, 156, 174,
177, 183; of Crustacea*, 351, 352,
359, 362, 367, 373, 376, 379, 385,
397, 414, 415 ; of Ctenophora, 196 ;
of Diplopoda, 424; of Echino-
dermata, 629, 637-8, 639, 647,
658 ; of Hirudinea, 300, 301 ; of In-
secta, 447, 447 ; of Lamellibranchi-
ata*, 582, 584, 585 ; of Nematoda,
248; of Nemertea, 235; of Oligo-
chaeta*, 286, 287, 289, 291, 293;
of Onychophora, 321 ; of Opistho-
branchiata, 567, 569; of Platy-
helminthes*, 208, 215, 218, 225,
227, 230, 231 ; of Polychaeta, 262,
275, 278; of Pulmonata, 560, 570;
of Rhabditis, 248 ; of Rotifera, 240,
241 ; of Scaphopoda, 573 ; of Sepia,
598; of Streptoneura*, 552, 565,
566; of Tunicata, 674-5, 679
Generative pore, see Generative
organs
Genital aperture, opening, organs,
system, see Generative organss
Genital atrium, of Helix, 561; of
Platyhelminthes, 210; of Stylaria,
291
Genital bursae, 629, 639; canal, 656;
coelom, 593; cords, 656; ducts, see
Gonoducts; operculum, 522; plate,
641 ; pleurae, 664; stolon, 629. See
also Axial organ
Genital rachis, 629; of Crinoidea,
629, 656; of Echinoidea, 629, 647
Geometridae, 491
Geonemertes, 221
Geotrupes, 436
Gephyrea, 301
Gerardia, 386
Germarium, 447 ; 240
Germs (gametes), see Gametes; of
Cnidosporidia, 100
Geryonia, 164
Giardia, 67 ; intestinalis , 67
Gid, 228
"Gill" of. Salpa, 686
Gill books, 517; 314, 526, 527,
529
Gill chamber of Decapoda, 408
Gill clefts, of ascidian tadpole, 675 ;
of Balanoglossus, 660, 664, 666 ; of
Cephalodiscus, 660, 668 ; of Chor-
data, 660; of Thaliacea, 681, 685;
of Tunicata, 660, 672, 675, 676,
685
Gill filaments, lamellae, plates, 574
Gill pouches, slits, see Gill clefts
Gills, of Arthropoda, 314; of Aste-
roidea, 629, 634; of Crustacea*,
348, 379, 391, 393, 394, 40i, 403,
404, 408, 409 ; see also Epipodites ;
of Echinodermata, 629; of Echi-
noidea*, 629, 643, 645 ; ofLimulus,
314, 517, 526, 527, 529; of Mol-
lusca, see Ctenidia ; of Poly-
chaeta*, 263, 270, 275 ; of Salpida,
685 ; Tracheal, see Tracheal gills
Girdle of Polyplacophora, 549
Gizzard, of Earthworms, 287 ; of In-
secta, see Proventriculus
Glabella, 323
Glands, Aciniform, 533; Aggregate,
533 ; Ampulliform, 532 ; Antennal,
see Antennal glands; Maxillary,
see Maxillary glands; Mucous,
561; Oesophageal, 287; of ali-
mentary canal, see Alimentary
canal. Digestive gland. Liver;
INDEX
703
Glands {cont.)
Pedal, 535; Prostate, see Pro-
state glands; Pyriform, 533; Shell
of Platyhelminthes, 210; Spermi-
ducal, see Prostate glands; Spin-
ning, 532; Tubuliform, 533
Globigerina, 76 ; 40 ; buLloides, 13
Globigerinidae, 76
Glochidium, 582
Glomeris, 424
Glomerulus, 668
Glossae, 428
Glossina, 506; 435, 448, 512; sub-
morsitans, 507
Glossiphonia, 301 ; 277, 298, 299
G los sob alarms y 666
Glycera, 275 ; 264, 277
Glycogen, 140; 20, 72
Glyptoscor pills, 525 ; 526
Gnathobase, 309; in Arachnida*,
517, 522, 527, 532; in Crustacea*,
338, 357, 363, 364, 368; in Trilo-
bita, 324
Gnathobdellidae, 300; 297
Gnathochilariurn, 423 ; 423
Gonads, 131, 134, 143; of Arthro-
poda, 316; of Balanoglossus y 668;
of Chaetopoda*, 262, 278, 286;
of Coelenterata*, 156, 174, 183,
196; of Crustacea*, 351, 352, 359,
367, 373. 397, 414; of Echinoder-
mata*, 629, 637, 639, 647, 658;
of Nemertea, 233. See also
Generative organs
Gonapophyses, 431, 447
Gonoducts, 134, 275. See also
Generative organs
Gonophore, 158
Gonopods, 421
Gonothecae, 155
Gonozooids, of Doliolida, 686 ; of
Siphonophora, 166
Gordius robustus, 257
Gorgonacea, 184
Grantia, 126; extusarticulata, 124
Graphosoma italicum, 475
Graptolites, 172
Graptolithina, 169; 154
Gregarina, 98 ; cuneata, 27 ; longa, 97
Gregarinidea, 93 ; 88
Grub, 456; 459
Grylloblatta, 468
Gryllotalpa, 466 ; 428
Gryllus, 467
Guard, 600
Gullet, of Metazoa (Oesophagus),
see Alimentary canal; of Phyto-
mastigina, 46, 49; of Protozoa*,
19; 46, 49, 50, 52, 103-4, 107, 109
Gunda segmentata, see Procerodes
lobata
Gymnocerata, 476
Gymnoblastea, 154; 157, 162, 164.
See also Anthomedusae
Gymnolaemata, 613
Gymnomera, 368; 354, 355
Gymnornyxa, 44
Gymnospores, 38
Gymnostomata, 107
Gyrinus, 493
Gyroceras, 604
Gyrodactylus, 220
Haemadipsa, 301
Haematochrome, 47 ; 57
Haetnatococcus, 57; 33, 42; lacustris,
56
Haemocoele, 131, 134; of Arthro-
poda, 314; of Echinodermata, 132,
629 ; of Helix, 556 ; of Insccta, 437,
454; of Mollusca, 543; of Peti-
patus, 319, 320; of Rotifera, 237.
See also Vascular system
Haemocyanin, 133; 351, 439, 522,
543
Haemoglobin, 133; 235, 263, 314,
439
Haemogregarvia, 89
Haernonchus, 253, 254
Haemopis, 301
Haemoproteus , 91
Haemosporidia, 91 ; 88
Halcampa, 187
Halesus guttatipennis, 487
Halichondria, 127
Haliclystus, 172
Haliotis, 564; 552, 553, 562, 563;
tiiberculata, 571
Halistemma, 166
Hamitermes silvestri, 470
Hamula, 465
Hamuli, 429
Haplopoda, 368
Haplosporidia, 102
Haplosporidiimi, 102; 25; limnodrili,
Harmolita, 501
Hatschek's pit, 661
704
INDEX
Head, 144; of Arthropoda, 308, 309;
of Chaetopoda, 263 ; of Crustacea*,
308, 309, 332, 334, 356, 360, 362,
364, 368, 370, 372, 377, 387, 389,
395> 400; of Insecta*, 308, 425,
463; of Mollusca*, 544, 549, 550,
555, 559, 567, 572, 589; of Myria-
poda*, 308, 418, 420, 422; of
Onychophora, 308, 309, 318; of
Trilobita, 323, 324
Head cavity of Chordata, 660
Head foot, 555, 589, 603
Head kidney, 284
Heart, 132; of Arachnida*, 520, 522,
530, 534, 539; of Arthropoda, 314;
of Balanoglossus , 667 ; of Brachio-
poda, 616; of Ciona, 674, 677; of
Crustacea*, 348, 363, 364, 370,
373, 390, 391, 392, 397, 401; of
Insecta, 437; of Mollusca*, 544,
550, 551, 553, 554, 556, 572, 579,
593
Hearts, Branchial, 594; oi Arenicola,
275 ; of Chaetopoda, 263 ; of
Oligochaeta, 291
Heat, 4
Hectocotylus, 598
Heliolites, 186
Heliopora, 186, 186
Heliosphaera, 81 ; inermis, 81
Heliozoa, 83
Helix, 554; 559, 599; aspersa, 554;
pomatia, 554, 557, 558, 559, 560
Helkesimastix, 59 n.
Hemiaspis, 517; 529; limuloides, 525
Hemichorda, 662; 628, 660, 661
Hemimetabola, 454
Hemimyaria, see Salpida
Hemimysis, 394
Hemiptera, 474; 454, 475, 477, 478
Hepatic caeca, diverticula, see Mesen-
teric caeca
Hepatopancreas, see Digestive gland
Hepialidae, 490
Hepialus huniuli, 490
Heptagenia, Nymphal instars of, 482
Hermaea, 569
Hermaphrodite duct, 560
Hermaphroditism, of Chaetognatha,
619-20; of Crustacea*, 351, 376,
379, 381, 382, 398; of Ctenophora,
196 ; of Gastrotricha, 243 ; of Hiru-
dinea, 296 ; of Icerya, 446 ; of
Mollusca*, 555, 567, 582, 585; of
Nematoda, 250; of Oligochaeta,
286; of Platyhelminthes, 208; of
Polyzoa, 606 ; of Protodrilus, 295 ;
of Protozoa, 33 ; of Tunicata*, 674,
679, See also Mutual fertilization
Herpetomonas, 64
Herpyllobius, 376
Heterocotylea, 218
Heterodera, 253 ; 256
Heterometabola, see Exopterygota
Heteronemertini, 237
Heteronereis, 268 ; 265, 280, 280
Heteroneura, 491
Heteropoda, 566
Heteroptera, 475
Heterotricha, 109
Hexactinellida, 126; 123
Hexactinian, 187
Hexamitus, 66; 10; intestinalis, il
Hexapoda, see Insecta
Hind gut, see Proctodaeum
Hinge of Lamellibranchiata, 573
Hippospongia, 127
Hirudinea, 296; 260
Hirudo, 301 ; 297, 297, 298, 299, 300
Histriobdella, 296 ; 295
Holectypoida, 647
Hologametes, 31
Hologamy, 3 1 ; 6
Holomastigina, 60
Plolometabola, see Endopterygota
Holophytic nutrition (Photosyn-
thesis), 18-19 n.; I, 44, 45 5 of
Phytomastigina*, 18, 40, 46, 48,
49, 52, 54; of Protozoa, i, 18, 44
Holophytic Protozoa, see Holophytic
nutrition
Holothuria, 648, 652, 653; tubulosa,
651
Holothuroidea, 648; 623, 649, 653
Holotricha, 106
Holozoic nutrition, T8-i9n.; of
Phytomastigina*, 46, 49, 52, 54;
of Protozoa, 18, 42; of Zoomasti-
gina, 46. See also Digestion,
Feeding
Holozoic Protozoa, see Holozoic nu-
trition
Homarus, 415
Homoneura, 490
Homoptera, 475 ; 431, 476
Hood of Tetrabranchiata, 603
Hoplocampa testudinea, 497
Hoplocarida, see Stomatopoda
INDEX
705
Honniphora plumosa, 194, 194
Hormones, 131 ; 138, 344, 440
Houses, of Larvacea, 679 ; of Pro-
tozoa, 12; of Pterobranchia, 662
Hyalonema, 126
Hyaloplasm, 12 n.
Hydatid cyst, 229
Hydatina, 240, 241 ; senta, 238, 239
Hydra, 162; 48, iii, 128, 146, 147,
149 ; attenuata, 148
Hydractinia, 164
Hydranths, 154, 155, 158. See also
Polyps of Hydrozoa
Hydratuba, 178
Hydrida, 154. See also Hydra
Hydrocoele, see Water vascular
system
Hydrocoraliinae, 165; 154
Hydroids, see Hydrozoa
Hydrophyllium, 166
Hydropsyche, 487
Hydruptila maclachlani, 487
Hydrorhiza, 154
Hydrospires, 659
Hydrothecae, 155, 162, 164, 172
Hydrozoa, 153, 163
Hydrurus, 50; 51
Hylatoma berberidis, 458
Hylobius, 256
Hymenolepis nana, 230
Hymenoptera, 495 ; 430, 456, 501
Hymenostomata, see Vestibulata
Hyper mastigina, 65, 67
Hyperparasitism, by Cryptoniscus,
398; by Hymenoptera, 502
Hyperpharyngeal band, 673
Hypobosca, 512
Hypobranchial space, 409
Hypoderma lineatum, 512; bovis,
510
Hypopharynx, 428; 463, 483, 506
Hypostoma, see Labrum
Hypostome, 534
Hypotricha, 11 1; 104
Icerya purchasi, 446, 495
Ichthyophthirius, 107
Ideal malacostracan, 387
Ideal mollusc, 544
Idiochromatin, 26
Idotea, 398
Ileum, see Small intestine
Imaginal discs, 459
Imperforate Foraminifera, 75
Incisor process, 387
Indirect wing muscles, 430
Infero-marginal ossicles, 634
Inhalant canals, 119
Inhibition, 138
Ink sac, 591
Inostemma, 502
Insecta, 425 ; 308
Instar, 454
Intercalary segment, 418, 420, 422,
425
Interlamellar concrescences, 574;
septum, 585; spaces, 574
Internal environment, 3 n.
Internal gills, see Stewart's organs
Internal longitudinal bars, 672
Internal madreporite, 627
Internal medium, 132; 3 n. See also
Blood
Internal sac of Polyzoa, 6ii
Internal skeleton, of Alcyonaria*,
183-6 (passim) ; of Crustacea, 340;
of Echinodermata*, 626, 634, 638,
640, 643, 650, 656 ; of Metazoa, 1 30 ;
142; of Porifera, 118, 121, 123; of
Protozoa, 7, 12, 17; of Triplo-
blastica, 142
Interradial mesenteries, 174
Interradii of Echinodermata, 623
intertentacular organ, 606
Intestine, see Alimentary canal
Intracellular body cavity, 245
Intracellular digestion, 130; 147, 206,
208, 240, 534, 559, 578
Invertebrata, i ; 2
Ips, 256
Irregular echinoids, 647 ; 625
Ischiopodite, 336
Isogamy, 31 ; 46, 56, 74, 88. See also
Gametes
Isopoda, 395
Isoptera, 469
Isospores, 80
luliis, 422; 418; terrestrisy 422, 423
jfapyx, 463
Jaws, of Arthropoda, 308, 309; see
also Mouth parts ; of Chaetognatha.
"618 ; of Echinoidea, 645 ; of Helix,
557; of Onychophora, 308, 318;
of Sepia (beak), 594
Jelly, of Coelenterata, see Structure-
less lamella; of Porifera, 118, 123
Jugal lobe, 429
70 6
INDEX
Kakothrips robustus, 485
Karyogamy, 30 n. See also Syngamy
Karyolymph, 12
Karyosome, 23, 24
Keratosa, 127 and 11.; 121
Kermes ilicis, 481
Kerona, iii
Kidneys of molluscs, 556; 545, 550,
552, 554, 565, 566, 570, 572, 573,
580, 581, 582, 591, 603. See also
Renal openings
Kinetonucleus, 17. See also Para-
basal body
Labial hooks, 474
Labial palps, of Insecta, see Mouth
parts of Insecta; of Lamelli-
branchiata, 574; of Protobran-
chiata, 582
Lahidura, 469
Labium (Second maxillae of Insecta),
see Maxillae
Labrum, of Crustacea, 339; of In-
secta, 426; of Trilobita, 323. See
also Mouth parts
Lacinia, 427
Lacinia mobilis, 393 ; 397
Lacunar system, 629; of Antedon,
658; of Echinoidea, 629, 647; of
Holothuroidea, 629, 650
Lacunar tissue, 629
Lamblia, see Giardia
Lamellibranchiata, 573
Lampyris, 440
Languets, 673
Lankesterella, 91 ; 90
Lantern coelom, 645
Large intestine of Insecta, 434
Larvacea, 679: 144
Larvae, Actinotricha, 621, 622; Ac-
tinula, 159; Amp hib las tula, 125;
Appendicularia, 675 ; Argulid, 376 ;
Auricularia, 633, 668; Bipinnaria,
632; Brachiolaria, 633; Crinoid,
633; "Cyclops'', 373; 353, 374,
375; Cyphonautes, 610, 611 ; "Cy-
pris", 380; 353, 382, 385; Dipleu-
rula, 631; 627; Echinoderm, 630,
632; Echinopluteus , 633; Ephyra,
179; Erichthus, 389; Euphausid,
403 ; Glochidium, 582 ; Insect, 456 ;
see also names of orders ; Megalopa,
414; Metanaiiplius, 353, 373;
Meiazoaea, 389; Miiller's, 145,
215, 216','' My sis' \ see Schizopod ;
Nauplius, 352; see also Nauplius
larvae; Nematode, 251-9 (passim);
Ophiopluteus, 632 ; of Brachiopoda,
618; of Gordius, 257, 258; of Panto-
poda, 539; of Pentastomida, 541,
542; of Porifera, 123; Phyllosoma,
415; Pilidium, 235; Pla?iula, 152;
see also Planula larva ; Pluteus,
632; Protaspis, 324; Rhabditoid,
251 ; Schizopod, 389; Stomatopod,
392; Tornaria, 668; 633; "Trilo-
bite", of Ltmulus, 529; Trocho-
sphere, 282 ; see also Trochosphere
larva; Veliger, 546, 547, 582;
Zoaea, 353 ; see also Zoaea larvae
Larval arms of Echinodermata, 632
Larval nephridia, 284
Lasso cells, 193
Lasso of nematocyst, 148
Lateral cilia, 576
Lateral lines of Nematoda, 244
Latero-frontal cilia, 576
Laura, 385
Laura gerardiae, 387
Laurer's canal, 217, 232
Leander, 415
Legs, of Arachnida*, 308, 521, 527,
533, 537 ;of Arthropoda, 305 ; of In-
secta*, 308, 428, 464, 466, 481, 490,
492, 495, 503, 511, 512; of Mala-
costraca*, 397, 398, 401, 402, 403,
404, 410, 415 ; of Myriapoda*, 421,
424; of Onychophora, see Feet
Leishmania, 64
Lemnisci, 258
Leodice, 264; 279; fucata, 278;
viridis, 278
Lepadocrinus, 659
Lepas, 377; 329, 379, 380; anatifera,
378, 381
Lepidocaris, 360; 338, 339, 354, 355
Lepidonotus , 264, 268
Lepidoptera, 430, 435
Lepidurus, 360; 355, 362; glacialis,
360
Lepisma, 461 ; saccharina, 463 ; 464
Leptocoris trivittatus , 479
Leptodora, 368; 354, 362; kindti, 369
Leptomedusae, 153; 156, 160, 164.
See also Calyptoblastea
Leptomonas, see Herpetomonas
Leptostraca, 390; 333, 346, 388, 389
Lernaea, 375 ; 375
INDEX
Lernanthropus, 349, 351
Leucandra, 126;- 120; aspersa, 120
Leucifer, 415; 144, 145, 396
Leucochrysis, 50
Leucon grade, 120
Leucosolenia, 126
Levuana iridescens, 512
Libellula, 443, 444
Lieberkiihnia, 74; 68, 72; wagneri, 74
Life cycle, 8 ; of Actinomyxidea, 100 ;
of Aphididae, 478 ; of Cecido-
myidae, 446 ; of Cestoda, 228 ; of
Cladocera, 367; of Cnidosporidia,
100, loi, 102 ; of Coccidia*, 89, 90,
91; of Coccidiomorpha, 88; of
Coelenterata, 150; of Doliolida,
684 ; of Foraminifera, 72 ; of Gre-
garinidea*, 93, 94, 95, 96, 98; of
Haemosporidia*, 89, 90, 91 ; of
Hydrozoa*, 153,15 5-66 (passim) ; of
Malacocotylea*, 220, 223 ; of My-
cetozoa, 86 ; of Neosporidia, 87 ; of
Piroplasmidea, 98, 99; of Poly-
thalamia, 72, 76, 79 ; of Radio-
laria, 80; of Rotifera, 241; of
Scyphoniedusae*> 178, 178, 180;
of Sporozoa, 87 ; of Telosporidia,
88 ; of Trypanosomidae, 64 ; of
Tunicata, 677
Life history, of Alcyonaria, 183; of
Arthropoda, 3 1 6 ; of ascidians, 675 ;
of Brachiopoda, 6i6; of Chaeto-
gnatha, 620 ; of Copepoda, 373 ;
374. 375, 376; of Crustacea*, 352;
of Echinodermata, 630; of Hetero-
cotylea, 218; of Insecta, 454: see
also names of orders; of Lepto-
straca, 391; of Malacostraca, 389;
of MoUusca, 545 ; of Nematoda*,
251-7; of Nemertea, 235; of
Pelagia, 180; of Peracarida*, 393,
397, 402 ; of Polychaeta, 282 ; of
Polyzoa, 610; of Porifera, II7; of
Protozoa, 38; of Siphonophora,
166; of Trilobita, 324. See also
Embryology, Larvae, Life cycle
Ligament of Acanthocephala, 259
Light, 4, 38, 40, 47
Ligia, 395; 398; oceatiica, 397
Ligula, 428
Limacina, 569
Limax amoebae, 43 ; 69
Limbs, of Arachnida, 308, 575 ; of
Arthropoda, 305, 306-7, 309; of
707
Crustacea, 326, 327, 328-9. 33°,
334; of Onychophora, 317, 319; of
Trilobita, 324. See also Ab-
dominal limbs, Antennae, Anten-
nules, Legs, Mandibles, Maxillae,
Maxillules, Mouth parts, Pleopods,
Thoracic limbs, Trunk limbs,
Uropods
Limnaea, 301, 570; abyssalis, 570;
peregra, 571
Limnocnida, 164
Limnocodium, see Craspedacuta
Linmophilus , 487
Limulus, 526; 306 n., 310, 312, 515,
517, 518, 520; Polyphemus, 527,
528
Lineus, 237
Lingiiatula taenioides, 541, 541
Lingula, 616; 615, 617, 618
Linin, 12
Lipostraca, 360; 354
LithobiuSy 418; 89, 306 n., 307 n. ;
forficatus, 419, 420, 421
Lithocampe tschernyschevt, 80
Ltthocircus, 83 ; annularis, 49
Lithodes, 415 ; maia, 416
Littorina, 566; 552, 563; rudis, 566
Lituites, 604
Liver, 130, 131; of Arachnida, 517;
of Crustacea*, 344, 358, 391, 403,
414; of Helix, 558, 560; of Sepia,
595. See also Digestive gland,
Mesenteric caeca
Living chamber of Nautilus, 602
Lizzia, 160; koellikeri, 161
Lobophyllium, 193
Lobopodia, 14
Locomotion of Protozoa*, 15, 83
Locusta, 468 ; migratoria, 466
Loimia, 269
Loligo, 589, 601, 601
Longitudinal band, see Ciliated band
Longitudinal fission of Protozoa*,
29; 45, 54
Lophohelia, 191 ; 190
Lophophore, of Brachiopoda', 615 ; of
Polyzoa, 606
Loxodes, 107
Voxosotna, 612
Lucernaria, 172; 173, 178
Luciae, see Pyrosomatida
Lucifer, see Leucifer
Lucilia, 436
Lumbricidae, 287, 289
7o8
INDEX
Lumbriculus, 294 ; 293 ; variegatus,
293
LumbricuSy 287; 260, 261, 262, 275,
276, 286, 289, 292, 297 ; foetidus,
290; terrestrisy 288
Lung, 555, 556, 557, 570
Lung books, 314, 517, 518, 521, 522,
530, 531
Machilis (Petrobius) maritimus, 427,
427, 428, 463
Macrobioius, 539, 540
Macrocorixa, 439
Macrogametes, see Female gametes
Macromeres, 281, 282
Macrotrista angularis, 478
Macrurous type, 404
Madreporic vesicle, 628 ; 627, 639
Madreporite, 627; 624, 628, 639, 641,
646, 648, 649, 652, 658
Magellania flavescens, 614
Maia, 417
Malacobdella, 237
Malacocotylea, 220; 218
Malacostraca, 386; 330, 344, 346,
348, 351, 352
Malaria, 91
Malaria parasite, see Plasmodium
Male eggs of Rotifera, 241
Male gametes, 31; of Metazoa, see
Spermatozoa ; of Porifera, 118; of
Protozoa*, 31, 32, 33> 85, 89, 93, 96
Mallophaga, 483
Malpighian capsules, 141
Malpighian tubules, 315; 141; of
Arachnida*, 315, 517, 524, 530,
540; of Insecta, 315, 434, 436, 437,
438; of Myriapoda*, 315, 421, 424
Mandibles, 308; of Crustacea, 308,
332, 339, 352 ; of Insecta, 308, 426,
427 ; of Myriapoda*, 308, 420, 422.
See also Mouth parts
Mandibular groove, 332
Mandibular palps, 339, 369, 372,
373, 379, 387, 388, 395, 401, 410,
412
Mantis, 429
Mantle, of Brachiopoda, 613, 615,
6i6, 618 ; of Cirripedia*, 330, 333,
377, 378, 386; of Mollusca*, 544,
545, 547, 549, 555, 55^, 564, 5^5,
567, 569, 572, 580, 582, 584, 587,
589, 591, 603; of Tunicata, 669,
682
Mantle cavity, or groove, of Brachio-
poda, 615, 616; of Cirripedia*,
378, 379, 383 ; of Mollusca*, 544,
545, 549, 550, 551, 554, 556, 564,
565, 566, 567, 572, 574, 576, 579,
587, 589, 591, 602
Mantle flap or fold, see Mantle
Manubrium, 150-1, 156, 158, 160,
166, 169, 173, 174
Margellium, 160; 161
Marginal anchors, 173
Maricola, 214
Mass provisioning, 498 n.
Mastax, 240
Mastigamoeba, 60; aspera, 62
Mastigobranchiae, 408
Mastigophora, 45 ; 8, 44, 46
Maxillae (Both pairs of), 308 ; 306-7 ;
of Crustacea, 308 ; of Insecta, 426-
7; 307, 308, 427, 428; of Myria-
poda*, 307, 308, 420, 422. See
also Mouth parts
Maxillae, First, of Crustacea, see
Maxillules, Mouth parts
Maxillae, Second, of Crustacea, 332,
335, 339, ^^^ '^^•^o Mouth parts
Maxillary glands, 345, 346, 359, 367,
373, 379, 392
Maxillipeds, 307 ; of Crustacea*, 332 ;
328-9, 372, 393, 397, 40ir 403,
404, 410; of Lithobius, 421
Maxillules, 332, 339, 379, 410. See
also Mouth parts
Meandrina, 192
Mecoptera, 486
Medium, 3, 132. See also Internal
medium
Medusa, The, 150, 151, 152
Medusae, of Hydrczoa*, 153, 156,
158-62 (passim), 165, 166, 169; of
Scyphozoa*, 174, 179, 180
Megachile, 503
Megachromosomes, 26
Megalopa larva, 414, 414
Megalospheric form, 76
Meganephridium, 290
Meganucleus, 26; 24, 33, 35, 44, 107,
no, 115
Megascolecidae, 287, 290
Megascolides, 290; australis, ZTJ
Melanin(s), 591; 91, 342
Melicerta, 241
Meloe, 492
Meloidae, 495
Melolontha, 457, 458, 495
Melophagus, 512
Membranellae, 17, 104
Membranipora, 608, 609, 613
Menopon pallidum, 483 ; 484
Mentum, 427
Mermis, 256; 249; mgrescens, 256
Merodon, 511
Merogametes, 31, 89
Meropodite, 336
Merozoites, see Schizozoites
Mesenchyme, 129; 130 and n., 131,
134, 142, 197; of Acoelomata, 197;
of Ctenophora, 195; of Echino-
dermata, 631; of Hirudinea, 298;
of Nemertea, 236; of Platyhel-
minthes, 205; of trochosphere, 281
Mesenteric caeca, of Aphrodite, 268 ;
of Arachnida*, 315, 517, 534, 539;
of Crustacea*, 344, 358, 367, 379,
391, 392, 393, 397, 412; of
Echinodermata*, 626, 636, 646,
656; of Insecta, 434. See also
Digestive gland, Liver
Mesenteric filaments, 182, 187
Mesenteries, of Actinozoa, 182, 187,
188, 190; of Holothuroidea, 650;
of Polychaeta, 285 ; of Scyphozoa,
174, 178
Mesenteron (Mid gut), 130; of Crus-
tacea*, 344, 358, 367, 412; of
Hirudinea, 297 ; of Insecta, 433 ; of
Nematoda, 248
Mesoblast, see Mesoderm
Mesoblastic somites, see Mesoderm
segments
Mesocerebrum, see Deutocerebrum
Mesoderm, 129; i, 131, 134, 142,
143 ; in the trochosphere, 281, 285 ;
of Arachnida, 520; of Arthropoda,
316; of Chordata, 661, 675; of
Insecta, 452. See also Mesen-
chyme, Mesoderm segments, Me-
sothelium
Mesoderm segments (Mesoblastic
somites), 316; 135; of Annelids,
285; of Arachnida, 516, 520; of
Arthropoda, 314, 316; of Chaeto-
poda, 261, 285 ; of Chordata, 661 ;
of Onychophora, 319
Mesogloea, see Structureless lamella
Mesosoma, 309, 517, 522, 524
Mesostoma, 211; 204 ; ehrenbergi, 2x4 ;
quadr angular e, 214
INDEX 709
Mesothelium, 129; 131, I33, 135.
142, 143. See also Mesoderm
segments
Metabasipodite, see Preischiopodite
Metabola, see Pterygota
Metaboly, 17, 52
Metacerebrum, see Tritocerebrum
Metacestode stage, 228
Metachronal rhythm, 17; 195
Metameric segmentation, 285
Metamorphosis, see Life history
Aletanauplius larva, 353, 373
Metanemertini, 237; 233
Metapneustic, 508; 509, 511
Metasicula, 170
Metasoma, 309, 517, 521, 524
Metasome, 372
Metasternite, 522
Metastoma, of Crustacea, 339: of
Eurypterida, 525 ; of Trilobita, 323
Metatroch, 284
Metazoa, 128 ; i, 7, 8, 27, 35, 45, 124
Metazoaea larva, 389
Metepipodites, 336, 337. See also
Branchia
Metridium, 193
Miastor, 446, 509
Microchromosomes, 26
Microfilaria diurna, 255 ; nocturna,
255
Microgametes, see Male gametes
Microhydra, 165; 162
Micromeres, 281
Micronephridia, 290
Micronuclei, 26; 33, 34, 35, 107, no,
115
Micropterygidae, 488, 490
Micropteryx, 488, 490
Microspheric form, 76
Microsporidia, 102
Microstoma lineare, 214; 213
Mid gut, see Mesenteron
Mid-gut caeca, see Mesenteric caeca
Milk glands of tsetse fly, 446
Millepora, 165, 165
Mitoses of Protozoa, 24
Molar process, 387
Mollusca, 543 ; i, 2; Types of, 544
Molpadida, 652
Mo?7as, 63; 16, 42, 46; vulgaris, 62
Monaxonida, 127 and n.
Monocyclical rotifers, 241
Monocystis, 97; 12, 31, 37, 38, 96, 97;
lumbrici, 97; magna, 97
yio
INDEX
Monograpttis , 170; 170, 172
Monomorium mhiimiitn, 501
Mononchus, 248 n.
Monopylaea, 80
Monosiga, 65 ; brevipes, 65
Monothalamia, 72
Monotocardia, 563 ; 552, 565
Monstrilla, 374
Montipora, 192
Mosaic disease, 474
Mosaic vision, 313
Motile organs of Protozoa, 14
Moulting, 250, 257, 309, 373, 454 .
Mouth, Position and shape of, in
Arthropoda, 309 ; in ascidian tad-
pole, 675, 677 ; in Balanoglossus,
663; in Brachiopoda, 615; in
Chilopoda, 420; in Coelenterata*,
150, 151, 152, 160, 161, 165, 166,
174, 179, 182, 191, 194; in Echino-
dermata*, 623, 625, 631, 640, 647,
655; in Helix, 555; in Hirudinea,
296; in Hydatina, 239; in Insecta,
426; in Lepas, 379; in Peripatus,
319; in Platyhelminthes*, 198,
214, 215, 216, 217; in Protozoa*,
19, 103, 107; in Trilobita, 323; in
Triploblastica, 130, 144
Mouth, see also Alimentary canal
Mouth parts (limb-jaws and lips), of
Arthropoda, 305, 308; of Crus-
tacea*, 308, 339, 354, 356, 364,
369, 372, 374, 379, 387, 397, 398,
401,402, 410,411 ; of Insecta*, 308,
426, 427, 428, 463, 465, 466, 468,
469, 471, 472, 474, 475, 477, 481,
483, 485, 486, 487, 489, 490, 492,
496, 506, 507, 509, 511, 512; of
Myriapoda*, 308, 420, 422; of
Onychophora, 318
Mucous glands of Helix, 561
Muggiaea, 166, 167
Miiller's larva, 216; 145, 215
Midticilia, 60
Multiple fission of Protozoa*, 28, 36,
54, 64, 72, 76, 88, 89, 93, 95
Murex, 566; 559 n.
Musca, 506, 508, 512; domestica,
508
Muscle(s), Alary, 437, 438; Adduc-
tor, see Adductor muscles ; Colu-
mella, 555; Retractor, see Re-
tractor muscles. See also Muscula-
ture
Muscle fibres, 128, 142; of Arthro-
poda, 316; of Chaetognatha, 618;
of Coelenterata*, 147, 151, 176;
of Nematoda, 244; of Pecten, 585 ;
of Peripatus, 316; of Platyhel-
minthes, 204
Muscular gland organ, 211
Musculature, of Actinozoa*, 182,
187, 188; of Arthropoda, 305; of
ascidian tadpole, 675 ; of Aster-
oidea, 634; of Brachiopoda, 615;
of Cephalopoda, 589; of Chaeto-
gnatha, 618; of Chaetopoda, 261,
262, 266, 272 ; of Ciona, 669 , of
Crinoidea, 656; of Ctenophora,
195 ; of Echinoidea, 641, 644, 645 ;
of Gasteropoda, 555 ; of gill books
and lungs, 517, 518; of Hirudinea,
298; of Holothuroidea, 649, 650;
of Medusae, 151, 156, 176, 179; of
Metazoa, 142; of Nematoda, 244,
246; of Nemertea, 235; of Ony-
chophora, 308, 317; of Ophiu-
roidea, 638, 639; of Polyzoa, 608;
of Platyhelminthes, 202 ; of Roti-
fera, 237 ; of Thaliacea, 682 ; of
the trochosphere, 285 ; of wings,
430, 431
Mutations, 36
Mutilla, 503
Mutual fertilization, by Ciliophora,
33 ; by Helix, 561 ; by Oligochaeta,
289; by Platyhelminthes, 211
Mya, 579
Mycetozoa, 86; 8, 11
My gale, 530
Myoblast, 204
Myonemes, 17
Myophrisks, 81
Myopsida, 589
Myrianida, 268 ; 264, 278
Myriapoda, 418; 308
Mysidacea, 393 ; 387, 388
Mysis, 393; 332, 394; relicta, 388
'^ My sis" larva, see Schizopod larva
Mytilus, 583 ; 548, 574, 575, 577, 579,
580, 581
Myxobolus, 100; 43
Myxospongiae, 127 and n.
Myxosporidia, 100
Nacreous layer, 546
Naegleria, 69 ; 25 ; bistadialis, 69 ; 23
Narcomedusae, 164; 153
Nassa, 563
Nassellaria, see Monopylaea
Nattca, 563
Nauplius larvae, 352; 324, 332, 338,
339, 340, 342. 352, 354, 359, 368,
373, 374, 375, 379, 382, 383, 385,
389, 403, 415
Nautiloidea, 602, 603
Nautilus, 602; 588, 590, 599, 604;
macromphalus, 604
Nebalta, 390; 329, 335, 336, 340, 348;
bipes, 390, 390
Neck of Cestoda Merozoa, 225
Neck gland, see Dorsal organ ; organ
(Nuchal organ), 342; 334, 356, 364
Nectocalyces, 166
Needham's sac, 598
Nematocysts, 148; 150, 166, 189,569
Nematoda, 244; 2, 130, 141, 197
Nematomorpha, 257; 197, 244
Nematus rihesii, 500
Nemertea, 233; 130, 197
Neoechinorhynchus, 259
Neoinenia, 549
Neosporidia, 100; 87, 88
Neoteny, 144
Neotermes, 469
Nepa, 474, 476
Nephridia, Nephridial system, 134;
136, 140, 141, 197, 261, 275-8, 277,
300
Nephrocytes, 437; 439
Nephromixia, 263, 276
Nephrops, 415; 332
Nephrostome, 263; 275, 276, 289,
290
Nephthys, 277
Nereis, 268 ; 262, 264, 265, 272, 277,
282, 283
Nerilla, 296 ; 295
Nerve-cord, see Nervous system
Nerve fibre, 137 and n. ; 138
Nerve net, 128, 136, 137, 138, 150;
of acoelomate Triploblastica, 197;
of Balanoglossus, 664; of Coelen-
terata*, 149, 176, 195; of Echino-
dermata, 626, 629; of Platy-
helminthes, 198, 217; Origin of
centres and nerves in*, 136, 137,
150, 197, 198, 630, 664
Nerve rings, of Echinodermata, 630;
of Medusae, 151, 152, 153, 156, 176
Nerves, 137; 136. See also Nervous
system
INDEX 711
Nervous system, 136; 5, 128, 142; of
Acanthocephala, 259; of Anne-
lida*, 260; I, 261, 287, 289, 294,
300; of Arachnida*, 524, 529, 531,
539; of Arthropoda, 300; of
Brachiopoda, 6i6; of Chaeto-
gnatha, 618; of (Jhordata, 660; 2;
of Ciona, 675, 676, 677 ; of Coelen-
terata*, 149, 151, 156, 176, 195; of
Crustacea*, 340; 311, 373, 379,
383, 391, 392, 397, 413, 414; of
Echinodermata, 629, 638, 646, 650,
656; of Gastrotricha, 243; of
Hemichorda, 664, 668; of Insecta,
448 ; of Lithobius, 422 ; of Mol-
lusca*, 545, 549, 550, 554, 555,
570, 571, 572, 582, 596; of Nema-
toda, 245 ; of Nematomorpha, 257 ;
of Nemertea, 235 ; of Onycho-
phora, 322; of Platyhelminthes*,
198, 199, 217, 225; of Polyzoa,
606 ; of Rotifera, 237
Neurones, 137
Neuropodium, 265
Neuroptera, 485
Neuroterus, 500
Nidamental glands, 591
Noctiluca, 55 ; 48, 56
Noctuidae, 491
Nodosaria, 76; hispida, 13
Nodus of Odonata, 472
Notnada, 504
Non-cellular animals, 7
Notochord, 661 ; of ascidian tadpole,
675 ; of Hemichorda, 662, 664
Notodelphys, 374
Notonecta, 474, 476
Notopodium, 265
Notostraca, 360; 327, 354, 355
Novius cardinalis, 495
Nuchal sense organ, see Neck organ
Nucleariae, 72
Nuclei, in Metazoa and Protozoa, 7,
27; of pansporoblasts, 100; of
Protozoa, 22, 23 ; Plurality of, in
Protozoa, 10; Position of, in
choanocytes, 126
Nucleoli, 22
"Nucleus", of Rhizocephala, 384; of
Thaliacea, 686
Nucula, 582; 544, 574, 579
Nuda, 196
Nudibranchiata, 567; 554
Nummulites, 76 ; laevigatus, 75
712
INDEX
Nuptial chamber, 469
Nurses, 504
Nutrition, 17 ; of Mastigophora, 46 ; of
Phytomastigina, 46, 47, 49 ; of Pro-
tozoa, 17, 18 ;of symbionts, 48. See
also Holophytic nutrition, Holozoic
nutrition, Saj^ophytic nutrition
Nycteribia, 512
Nyctiphanes, 403 ; tjorwegica, 403
NyctotheruSy 109; cordiformis, 109
Nymphon, 538, 540
Nymphs, 455
Obelia, 154; 151, 155, 156, 158
Obhque fission, 45
Obtect pupae, 456, 490
Ochromonas, 50; 47, 51, 63
Octobothrium, 218
Octomitiis, see Hexainitus
Octopoda, 589; 599
Octopus, 589, 595, 598, 599, 601, 602
Ocular plate, 641
Ocypus, 495 ; olens, 493, 494
Odonata, 472 ; 430, 444
Odontoblasts, 557
Odontocerum, 487
Odontosyllis, 280
Odynerus, 498 n., 503
Oegopsida, 588
Oenocytes, 439, 440
Oesophageal bulbs, 248
Oesophageal pouches, 287
Oesophagus, see Alimentary canal
Oikomonas, 63 ; termo, 62
Oikopleura, 43, 680; albicans, 680
Olenus cataractes, 323
Olfactory hairs, 342
Oligochaeta, 286; 261, 263, 301
Oligolophus spinosus, 538
Oligopod stage, 458, 459
Oligotricha, no
Olynthus, 1 17; 1 17
Ommatidium, 310; 426
Onchosphere, 228, 229, 230
Onychophora, 317; 308. See also
Peripatus
Onychopoda, 368
Oocysts, 22; of Gregarinidea*, 94,
95, 97
Oodinium, 55 ; 43 ; pouched, 43
Oogamy, 31, 46
Ookinete, 91, 93, 98
Oostegites, 389; 393, 397, 401
Ootheca, 448
Ootype, 211
Oozooid, 679; 685
Opalina, 106; 10, 28, 43, 107; ran-
arum, II, 27, 106
Opalinidae, see Prociliata
Opening (Aperture), Atrial, see
Atrial opening; Excretory, see Ex-
cretory organs, Renal openings ;
Genital, see Generative organs; of
Mantle cavity, see Mantle. See
also Anus, Mouth, Oscula, Ostia,
Pneumostome, Pores
Operculum, 270
Ophiocoma, 640
Ophioglypha, 639
Ophiopluteus, 632
Ophiothrix, 640
Ophiura, 639, 640
Ophiuroidea, 638 ; 623, 625, 626, 627,
629, 630, 632
Ophryocystis, 94; mesnili, 95
Opisthobranchiata, 567; 553, 568
Opisthosoma, 309 ; 515, 528, 530, 537.
See also Mesosoma, Metasoma
Opisthoteuthis , 602; 589
Optic lobes of Crustacea, 340
Oral aspect (side, or surface), 143;
of Echinodermata, 623
Oral cone, 150; 155, 158; disc, 182;
siphon, 669; valves, 655
Organisms, 4
Ornithodorus moubata, 537
Orthoceras, 600 ; 600, 604
Orthoptera, 466; 429, 432, 454
Orthorrhapha, 509, 510
Oscar ella, 127
Oscinus frit, 512
Oscula, of Porifera, 117, 119, 120; of
Radiolaria, 80, 83
Osphradia, 565
Ossicles, of Echinodermata, 626 ;
Adambulacral, 636 ; Ambulacral,
636, 638, 645 ; Basal, 656 ; Brachial,
656; Centrodorsal, 655, 656; Cir-
rhal, 656; Infero-marginal, 634;
of Holothuroidea, 648, 650; Pin-
nulary, 656; Radial, 656; Rosette,
656; Supero-marginal, 634. See
also Auriculae, Skeletal plates
Ossicles, System of, in Asteroidea,
634; in Crinoidea, 656; in Echi-
noidea (plates), 641 ; in Holothu-
roidea (calcareous ring), 649, 650;
in Ophiuroidea, 638
INDEX
713
Ostia, of heart, 314, 349, 421. 437,
522, 530; of sponges, 119
Ostracoda, 368; 327, 331, 333, 34^,
349, 351, 353
Ostrea, 585; 579, 582; eduhs, 546,
585, 586
Otocyst, see Statocysts
Otoplana, 214
Ova, 6. See also Eggs
Ovarian lamella, 379
Ovarioles, 448
Ovary, see Generative organs
Ovicell, 612
Oviducts, see Generative organs
Ovigerous frenum, 379; legs, 539
Ovipositor, of Insecta*, 431, 466,
495, 502; of Phalangida, 538
Ovo testis of Helix, 560
Oxidation, 131; 140, 143
Oxyuris, see Enterobius vermicularis
Pachytylus migratorius , 467
Paedogamy, 83
Paedogenesis, 446
Paired limbs, see Limbs
Palaeonemertini, 237
Palaeophonus, 524
Palinura, 404
Palinurus, 415 ; 389
Pallial arteries and circulation of
Lamellibranchiata, 580
Pallial budding, 679 ; gills, 565 ; sinus,
616
Palliovisceral cords, 547
Palmella, 47
Palolo worm, see Leodice viridis
Palps, Labial, see Labial palps;
Mandibular, see Mandibular palps ;
of Acarina, see Pedipalps; of
Polychaeta*, 265, 268
Paludicola, 214
Paludina, 566; 145, 543, 552, 563;
vivipara, 551
Paniphilus, 501
Pancreatic tissue, 594
Pandalus, 336
Pandorina, 57; 46, 57
Panorpa, 429 ; communis, 486
Pansporoblasts, 100
Panthalis, 267 ; 264
Pantopoda, 538
Papilio, 492
Papilionina, 489, 492
Parabasal body, 17
Paracordodes, 258, 258
Paractinopoda, see Synaptida
Paragaster, 117
Paraglossae, 428
Paragnatha, 339. See also Mouth parts
Paramecium, 109; 12, 14, 15, 18, 21,
21, 27, 28, 34, 42, 43, 104 and n.,
106, 116; aurelia, 35; caudatum,
21. 34, 42
Paramitoses, 25
Paranebalia, 391
Paraoesophageal (Circumoesopha-
geal) connectives, see Nervous
system
Parapodia, of Aplysia, 567 ; of Poly-
chaeta*, 261, 264-75 (passim)
Parasites, 5. See also Parasitic habits
Parasitic castration, by Isopoda, 398;
by Rhizocephala, 383
Parasitic habits, of Acanthocephala,'
259 ; of Acarina*, 534, 537 ; of Ano-
plura, 484; of Aphaniptera, 512;
of Branchiobdellidae, 301 ; of Cili-
ata*, 106, 107, 109, no, in; of
Cirripedia, 382 ; of Copepoda, 374 ;
of Crustacea, 33 1 ; of Cyamus, 403 ;
of Dinoflagellata, 54; of Diptera,
512; of Entamoeba, 70; of Hemi-
ptera, 476 ; of Hirudinea, 300-1 ; of
Histriobdella, 296; of Hymeno-
ptera*, 498-502 (passim) ; of Iso-
poda, 398 ; of Mallophaga, 483 ; of
Nematoda*, 245-57 (passim); of
Nematomorpha, 257; of Penta-
stomida, 541 ; of Plasmodiophora,
86; of Platyhelminthes*, 198, 216;
of Polymastigina*, 66 ; of Proto-
zoa, 43 ; of Sporozoa*, 87 ; of
Trypanosomidae, 63
Parazoa, see Porifera
Parenchyma, Parenchymatous tissue,
205; 197, 198, 206, 225
Parthenogenesis, 6 ; of Crustacea*,
351, 362, 367, 369; of Gastro-
tricha, 243 ; of Insecta*, 446, 479,
480, 485, 504 n. ; of Protozoa, 33;
of Rotifera, 241
Parthenogonidia, 58
Patella, 565 ; 547, 552, 553, 562, 563 ;
coeridea, 546
Peachia, 187; 188
Pecten, 584; 142, 579, 582, 599;
maximus, 584, 584; opercularis,
584, 585; tenuicostatus, 585
7H
INDEX
Pectines, 522, 525
Pectinibranchiata, see Monotocardia
Pedal cords, 545, 547, 565; sinus,
582
Pedalion, 241
Pedicellariae, 626; Crossed, 634;
Gemmiform, 641, 642; Ophioce-
phalous, 641, 642; of Asteroidea,
634; of Echinoidea, 641; Tridac-
tyle, 641, 642 ; Trifoliate, 641, 642 ;
Uncrossed, of Asteroidea, 634
Pedicellina, 612
Pediculus humanus, 484 ; 484
Pedipalpi, 512
Pedipalps, 515, 522, 524, 530, 532,
533. 534, 539
Peduncle, see Stalk
Pelagia, 180
Pelagothuria, 652; 653
Pelagothurida, 652
Pellicle, 12; 49, 105, 106
Pelmatozoa, 658; 657
Pelomyxa, 72 ; palustris, 71
Pen of Loligo, 601
Penaeidea, 404
Penaeus, 335, 349, 353. 4i 5
Penilia, 363
Penis, see Generative organs
Pennaria, 164; 163
Pennatula, 184
Pennatulacea, 184
Pentacrinus, 658
Pentastoinida, 541 ; 539
Pentatoniidae, 476
Pentremites, 659
Peptonephridia, 291
Peracarida, 393 ; 336, 389
Pera?teina, 52; 12, 20, 22, 46; tri-
chophoruni, 53
Perforate F'oraminifera, 75
Peribranchial cavities, 673
Pericardial sinus, see Pericardium
Pericardium, of Arthropoda, 314; of
Cephalopoda, 593 ; of Ciona, 674,
677 ; of Crustacea, 348 ; of Entero-
pneusta, 667 ; of Insecta, 437 ; of
Mollusca, 543 ; of Snail. 556
Perichaetine, 289
Perihaemal coelom of Enteropneusta,
663 ; system of Echinodermata*,
626; 637, 638, 645, 656
Periostracum, of Brachiopoda, 615;
of Mollusca, 546
Peripatus, 317; 130 n., 141, 305, 312,
316, 319, 539; capensis, 318, 319,
321, 322
Peripharyngeal band, 669
Periplaneta, 466
Peripneustic larva, 508
Periproct, 640
Peripsocus phaepterus, 472
Peripylaea, 80
Perisarc, 155
Perisomatic cavity, 384
Peristome, of Ciliata, 104; of Echi-
noidea, 640
Peristomial cirri, 266
Peristomium, 266 ; 268, 272
Peritoneum, 131, 136, 262, 278
Peritricha, iii; 104
Peritrophic membrane, of Crustacea,
345; of Insecta, 434; of Onycho-
phoia, 320
Perivisceral cavity, 134, 141; of
Acanthobdella, 300 ; of Arthropoda,
314; of Chaetopoda, 261 ; of Ciona^
674; of Echinodermata, 626; of
Mollusca, 543; of Rotifera, 237.
See also Coelom, Haemocoele
Per la maxima, 471
Pernicious malaria, 93
Perophora, 681 ; 677, 679, 681
Perradial, 174
Petrobius maritimus, 464
Phaenoserphus , 459, 502 ; viator, 499
Phaeococcus, 52
Phaeodaria, see Tripylaea
Phaeodium, 83
Phalangida, 537
Phalera bucephala, 441
Pharj'nx, see Alimentary canal
Phascolosoma, 304
Pheretima, 290
Philodinidae, 241
Philonexis. 599
Phlebotomus, 64
Phobotaxis, 38
Pholas, 586
Phoronidea, 622
Phoronis, 622; 621
Phorozooids, 686
Phosphagen, 662 and n.
Phosphorescent Protozoa, 20
Photogenic organs of Insecta, 439
Photosynthesis, see Holophytic nu-
trition
Phoxicfiilidiuni femoratum ,539
Phragmocone, 600
INDEX
715
Phronima, 403 ; 396
Phryganea, 487, 487
Phylactolaemata, 613; 609
Phyllobins urticae, 457
Phyllobranchiae, 408
Phylloceras, 602 ; heterophyllum , 604
Phyllopoda, 356
Phyllopodium, 336; 334
Phyllosoma larva, 415
Phyllotreta, 495
Phylloxera vastatrix, 479
Physalia, 166; 168
Phytomastigina, 46; 59
Pieridae, 437
Pieris, 490, 492 ; rapae, 460
Pigments, Blood, 133 ; see also Chlo-
rocruorin, Haemocyanin, Haemo-
globin ; of Crustacea, 342 ; of
Haemosporidia, 91 ; of Lepidop-
tera, 488 ; of Phytomastigina, 47.
See also Chromatophores, Melanins
Pilema, 179; 180
Pilidiiim larva, 235; 145, 233, 236
Pinacocytes, 117
Pinnate tentacles, 180, 649, 652
Pinnulary ossicles, 656
Pinnules, 654
Piroplasma, 98, 99, 537
Piroplasmidea, 98 ; 88
Placenta, of Onychophora, 321; of
Salpida, 685 ; of Scorpionidea, 524
Placocystis, 658
Plagiostomum lemani, 214
Planaria, 204; alpina, 208, 213;
lactea, 201; lugiibris, 201, 214
Planorbis, 301, 570
Planula larva, 152; i, 145, 146, 156,
178, 183, 196
Plasmodia, 11, 86
Plasmodiophora, 86
Plasmodium, 36
Plasmodium, 91 ; 38, 43 ; falciparum,
93 ; matariae, 93 ; vivax, 92, 93
Plasmodroma, 44
Plasmogamy, 30 n.
Plasmotomy, 29, 83, 100, 107
Plastin, 22, 23, 24
Plastogamy, 36 ; 30 n.
Platygaster, 458, 459
Platyctenea, 196
Platyhedra, 490 ; gossypiella, 490 ; 488
Platyhelminthes, 198; 2, 130, 197,
215, 230
Plecoptera, 471 ; 444
Pleodorina, 58; calif ornica, 10 ; illi-
noiensis, 10
Pleopods, 388. See also Abdominal
limbs of Crustacea
Plesiocarls vagicollis, 455
Plesiocoris, 476
Pleurohrachia, ig6 ; pileus, 194
Pleurobranchiae, 348 ; 404
Pleuron, 332
Pleuropodite, see Precoxa
Plicate canals, 580
Plodia, 491
Plumatella, 606, 6 1 2, 622 ;fungosa, 607
Plumidaria, 162; 163
Pluteus larva, 632; 632
Pneumatophore, 166
Pneumostome, of Arachnida, 518; of
Pulmonata*, 556, 570, 572
Podia, see Tube feet
Podical plates, 430
Podobranchiae, 404
Podocorj^ne, 164
Podocyrtis schomhurgki, 80
Podophrya, 115
Podura aquatica, 465
Pole capsules, 14, 100
Polian vesicles, 637 ; 640, 651
Polistes aurifer, 501
Poly cells nigra, 214
Polychaeta, 264; 261, 263
Polycirrus, 263
Polycladida, 215
Polyclinufn, 680
Polydisc strobilation, 180
Polyembr^'ony, of Hyrnenoptera,
446 ; of Polyzoa, 612
Polyenergid nuclei, 26
Polygordius, 294; 282, 284, 285, 285
Polykrikos, 54; 10, 55; schzcarzi, 55
Polymastigina, 65
Polyp, The, 150; 151, 152, 153
Polyphaga, 493, 494
Polyphemus, 368
Polypide, 606
Polyplacophora, 547
Polypod stage, 458, 458
Polyps, of Anthozoa, 180-93 (Pas-
sim); of Hydrozoa*, 153, 154-66
" (passim); of Scyphozoa*, 174, 178,
180
Polystomella, 76; 31, 38, 42, 75, 79;
crisp a, 23, 78
Polystomum, 218; 219; integerrimum,
219
7i6
INDEX
Polythalamia, 74; 72
Polytoma, 56; 20, 29, 30, 33, 40, 46;
uvella, 23, 30
Polytricha, 109
Polyzoa, 606 ; 2
PoTtiatoceros, 263 ; 264, 267 ; triqueter,
271
Pontobdella, 301
Porcellana, 415 ; 389
Pore plate of Radiolaria, 80
Pore-rhombs, 659
Pores, Collar, ^ee Collar pores ; Dorsal,
see Dorsal pores; of Porifera, 117;
Proboscis, see Proboscis pore ;
Water, 658, see also Madreporite
Porifera, 117; i, 125
Porites, 191 ; 192
Porocytes, 118
Poromya, 579
Porthetria dispar, 490
Portuguese man-of-war, see Physalia
Posterior aorta, of Araneida, 530; of
Astacus, 350; of Carcinus, 412; of
Helix, 557; of Lamellibranchiata,
580; of Scorpionidea, 522; of
Sepia, 593
Posterior interradius of Echinoidea,
647
Posterolateral arms of Plutei, 633
Posterolateral edge of Carcinus, 408
Postsegmental region of Crustacea,
332
Preantennae, 318; 306
Preantennal somite, 306, 318, 425
Prebranchial zone, 669
Precheliceral somite (segment), 515;
516
Precoxa, 336
Pregenital somite (segment) of Arach-
nida.5i5;5i6
Preischiopodite (Preischium), 336
Prementum, 427
Preoral lobe of Echinodermata, C32
Preoral region, of Annelida, 260 ; see
also Prostomium; of Arthropoda,
308; of Crustacea, 331, 332; of
Echinodermata, see Preoral lobe ;
of Enteropneusta, see Proboscis.
See also Preseginental region,
Preoral somites
Preoral somites, of Arachnida, 515;
of Arthropoda, 309; of Chilopoda,
420; of Crustacea, 332; of Ony-
chophora, 319
Presegmental region of Crustacea,
332
Primary body cavity^ see Haemocoele
Primary embryo, 612
Prismatic layer, of Brachiopod shell,
615 ; of Molluscan shell, 546
Probasipodite, 336
Proboscis, of Acarina, 534, 537; of
Bonellia, 302 ; of Branchiura, 377 ;
of Buccinum, 565 ; of Chaetopoda,
264 ; of Enteropneusta, 662 ; of
Hemiptera, 475 ; of Lepidoptera,
487; of Nemertea, 233; of Panto-
poda, 538, 539; of Rhyncho-
bdellidae, 297 ; of Suctorial Cope-
poda, 374, 375, 376
Proboscis cavity, of Balanoglossus,
662, 668; of Chordata, 660
Proboscis complex of Balanoglossus,
666 ; pore of Balanoglossus, 663
Proboscis sheath, of Buccinuvi, 565 ;
of Nemertea, 233 ; of Rhyncho-
bdellidae, 297
Probuds, 687
Procerebrum, 309, 340 n.
Procerodes lobata, 214
Prociliata (Opalinidae), 106; 26, 33
Proctodaeum, 130; of Arthropoda,
314; of Crustacea, 344; of trocho-
sphere, 284
Proctotrypidae, 502
Proepipodites, 336; 337, 348, 354,
358, 364
Progressive feeding, 498 n.
Prolegs, 456, 490, 491, 500, SIC
Proliferating region, of Syllidae, 278 ;
of Tapeworms (neck), 225, 227
Proostracum, 600
Propodite, 336
Propolis, 504
Prorodon, 107; teres, IIO
Prosicula, 170
Prosobranchiata, see Streptoneura
Prosoma (" Cephalothorax" of Ar-
achnida), 308, 515, 521, 526, 530,
533, 537
Prosopyles, 120
Prostate glands, of Oligochaeta, 287,
289, 291 ; of Sepia, 598
Prostomium, of Annelida, 260; of
Chaetopoda*, 261, 264, 268, 272,
284, 291 ; of Echiuroidea*, 301,
302; of Hirudinea, 296; of Sipun-
culoidea, 304
INDEX
717
Protaspis larva, 324, 325
Proteolepas, 382
Protephemeroptera, 462
Protobranchiata, 582; 574, 579
Protocerebrum, 309, 340
Protochordata, 660
Protoclypeastroida, 647
Protococcaceae, 47
Protocdnch, 602
Protodonata, 462
Protodrilus, 295 ; 294 ; chaetifer, 295
Protohymenoptera, 462
Protomerite, 95
Protomonadina, 63
Protoparce, 437
Protoplasm of Protozoa, 1 1
Protopod stages (oligomero, poly-
mero), 458, 458
Protopodite, 324, 335, 336, 372, 379
Prototroch, of Cyphonautes, 611; of
Pilidium, 235 ; of trochosphere, 282
Protozoa, 7 ; i, 126
Protozoa and Metazoa, Connection
bet\Aeen, 45
Protura, 465
Proventriculus, of Carcinus, 412; of
Crustacea, 344 ; of Earthworms,
see Gizzard; of Insecta, 432; of
Ganimarus, 401 ; of Ligia, 397
Pseudococcus, 478
Pseudocolonies of Protozoa, 1 1
Pseudonavicellae, 95
Pseudophyllidea, 229
Pseudopodia, 14; of Amoebina, 68,
69; of Foraminifera, 72, 74; of
Heliozoa, 83; of Hydra, 147; of
Mastigamoeba, 60; of Mycetozoa,
86 ; of Radiolaria, 76, 80
Pseudopodiospores, see Amoebulae
Pseudotracheae, 506
Pseudolransverse fission, 29 ; 29, 45
Pseudovelum, 176
Psocoptera, 471
Pterobranchia, 662
Pteropoda, 569
Pterostichus, 457, 499
Pterotrachea, 566; 563, 566
Pterygota, 466
Pterygotus, 526; 525; osiliensis, 525
Ptilinum, 509, 511
Ptychomyia remota, 512
Pulex irritans, 513; 514
Pulmonata, 569; 554
Pulvillus, 428
Pupa, 456; 490, 508, 509, 511, 513
"Pupa", of Holothuroidea, 633; of
Lernea, 375
Pupae, Coarctate, Exarate and Ob-
tect, 456 : 509
Puparium, 456, 508, 509, 511
Pure lines, 36
Purpura, 563
Pycnogonum littorale, 539
Pygidium, 323
Pyloric caeca, of Asteroidea, 636 ; of
Insecta, 434
Pyloric chamber, 344; sac, 636
Pyralidae, 491
Pyrameis, 450
Pyrenoids, 47; 56, 57
Pyriform organ, 611
Pyrosoma, 685 ; 682, 682, 683
Pyrosomatida, 685; 681, 682
Pyrrhocoridae, 476
Quadrant, 281
Quartan ague, 93
Quartettes, 281
Rachiglossa, 563
Radial " blood vessel ", 629, 637, 645,
650, 658 ; nerve (aboral), 656 ; nerve ,
(ectoneural), 630, 637, 645, 646;
ossicles, 656; perihaemal vessel,
626, 637, 638, 645, 656; water
vessel, see Water vascular system
Radial cleavage, 283; 145, 631
Radial fission, 29; 29, 58
Radial symmetry, 633 ; 143 ; of Acti-
nozoa, 182; of Cnidaria, 151; of •
Echinodermata, 623, 633
Radii of Echinodermata, 623 ; 646,
649
Radiolaria, 76; 41, 43
Radula, 557; 544, 550, 565, 567, 569,
570, 572, 595; sac, 557
Receptaculum seminis, see Sper-
matheca
Receptors, 137
Rectal caeca, 636
Rectum, see Alimentary canal
Reduction division, 27, 37, 88
Reduvidae, 476
Reflex arc, 137
Reflexes, in Metazoa, 137; in Pro-
tozoa, 38
Regeneration, in Crustacea, 339; in
Turbellaria, 213
7i8
INDEX
Regular sea urchins, see Endocyclica
Relation of Protozoa to their En-
vironment, 40
Relicts, 393
Renal openings (apertures, papillae)
of Mollusca, 544, 549, 556, 565,
591
Renopericardial openings (apertures,
canals), 556; 572, 593
Repeated fission of Protozoa*, 28 ;
29, 45, 54, 57
Reproduction, 5. See also Asexual
reproduction. Sexual reproduc-
tion ; of Protozoa, see Fission
Reproductive aperture, organs, see
Generative organs
Reserve materials, 5, 20
Respiration, 4, 139; of Arthropoda,
314; of Crustacea, 348 ; of Cyclops,
373 ; of Lamellibranchiata, 576 ; of
Protozoa, 22 ; of Tubiculous Poly-
chaeta, 270. See also Respiratory
movements. Respiratory organs
Respiratory movements, of Aphro-
dite, 268; of Arachnida, 517; of
Branchiopoda, 355 ; of Carcinus,
409 ; of Crustacea, 348 ; of Cyclops,
373; of Gasteropoda, 556; of In-
secta, 441 ; of Mysis, 393 ; of
Pulmonata, 556, 572; of Tubicu-
lous Polychaeta, 270; of Tubifex,
293
Respiratory organs, 139; of Arach-
nida, 517; of Arthropoda, 314; of
Branchiopoda, 354; of Chaeto-
poda, 263 ; of Crustacea, 348 ; of
Echinodermata, 628 ; of Holo-
thuroidea, 650; of Lamellibran-
chiata, 576 ; of Ligia, 397. See also
Gills, Lung, Tracheae
Respiratory pigments, see Pigments,
Blood
Respiratory trees, 629, 650
Resting cysts, 22; eggs, 241, see also
Winter eggs; phase of Phyto-
mastigina, 47
Rete mirabile, 650
Retinaculum, 429, 489
Retinulae, 310
Retractor muscle(s), of penis, 561 ; of
proboscis, 233; of stomach, 636;
of tentacles, 649
Retral processes, 76
Retropharyngeal band, 672
Rhahdammina, 76; 12; ahyssorwn,
Rhabdites, 204
Rhabditis, 245 ; 247
Rhabditoid larva, 251
Rhabdocoelida, 214; 201
Rhabdoliths, 50
Rhabdom, 310
Rhabdomeres, 310
Rhabdopleura, 668 ; 660, 662 ; nor-
mani, 667
Rhipicephalus , 537
Rhipidoglossa, 563
Rhizocephala, 382; 330, 377
Rhizochrysis, 50
Rhizocrinus, 658; 657
Rhizomastigina, 60; 69
Rhizoplasts, 17
Rhizopoda, see Sarcodina
Rhizopodia, 14, 15
Rhizostoma, 179
Rhizostomeae, 179
Rhodites, 500
Rhodnius, 437, 440 n. ; prolixus, 476,
438
Rhombifera, 659
Rhyacophila, 487
Rhynchobdellidae, 300; 297, 298
Rhynchoccel, 233
Rhynchodaeum, 235
Rhynchodemus terrestris, 215
Rhynchota, see Hemiptera
Rhyssa, 500
Ring canal of Polyzoa, 606
Rings (Nervous, Water vascular,
etc.) of Echinodermata, 623 ; 626,
630. See also Nervous system,
W^ater vascular system, etc.
Ripe proglottis, 225 ; 227
Ripple-marking, 604
Rods of eyes of Arthropoda, 310
Rosette ossicle, 656
Rostellum, 227
Rostrum, of Cephalopoda, 600; of
Crustacea*, 334; 370, 390, 407,
415 ; of Hemiptera, 474, 476
Rotifer f 241
Rotifera, 237; 2, 197, 244
Rotulae, 645
Royal pair, 469;. 471
Sabella, 264
Saccharicida, 476
Saccocirrus, 295 ; 260, 295
INDEX
719
Sacculina, 382; 383, 384, 385
Sagitta, 622 ; 129, 620, see also Chae-
tognatha ; bipunctata, 620, 62 1 ;
hexaptera, 619
Salivary glands, of Arachnida*, 522,
530, 534, 540; of Helixy 558; of
Hirudinea, 297 ; of Insecta, 432 ; of
Lithobius, 421 ; of Onychophora,
320; of Sepia, 595
Salpa, 686; 196, 677, 678, 684;
deniocratica-mucronata, 682, 683
Salpida, 685 ; 679, 682
Saltatoria, 466
Sao hirsuta, 325
Saprophytic nutrition, 19 n.; of
Phytomastigina*, 46, 47, 49, 52,
54> 57 > of Protozoa, 20, 42
Saprophytic Protozoa, see Sapro-
phytic nutrition
Saprozoic, see Saprophytic
Sarcocystis, 102; li?idemanm, 103
Sarcodina, 68 ; 44, 49
Sarcophaga, 436
Sarcosporidia, 102
Sarsia, 169; 167
Saxicava, 586
Scallops, see Pec ten
Scalpellum, 380; vulgare, 381
Scaphites, 605
Scaphopoda, 572
Scarabaeus Thonisoni, 494
Scarabeidae, 494, 495
Scent scales, 488
Schellackia, 91 ; 90
Schistocephalus gasterostei, 228
Schistosoma, 222 ; 222
Schizocystis, 93 ; 94
Schizogony*, 88; 37, 89, 91, 93, 94
Schizogregarinaria, 93
Schizopod larva, 389
Schizopoda, 388
Schizozoites*, 88 ; 37, 89, 93
Sclerotium, 86
Scolex, 225 ; 227
Scolopendra, 306 n., 307, 421
Scorpio, 524; swammerdanii, 521
Scorpionidea, 520
Scutigera, 421
Scutum, 378
Scyphistoma, 179
Scyphomedusae, 172; 152
Scyphozoa, see Scyphomedusae
Scytomonas, see Copromonas
Secondary body cavity, see Coelom
Secondary embr>'os, 612
Segmental organs, 275. See also
Coelomoducts, Nephridia
Segmentation, 143 ; of Annelida, 260,
261; of Arthropoda, 308, 309; of
Cestoda, 225; 143; of Chordata,
660; of Vertebrata, 143 ; 661 ; sug-
gested by certain organs in Mol-
lusca, 549, 603
Segmentation of the ovum, see
Cleavage
Segments of the body, see Somites
Self-fertilization, 250, 561
Seminal groove, of Oligochaeta, 289 ;
of Opisthobranchiata, 567
Seminal receptacle of Sagitta, 619
Seminal vesicles (Vesiculae semi-
nales), of Chaetognatha, 619; of
Insecta, 447 ; of Oligochaeta, 286,
291 ; of Platyhelminthes, 21.0
Sense organs, of Araneida, 533 ; of
ascidian tadpole, 676 ; of Chaeto-
poda, 263, 265 ; of Coelenterata*.
147, 149, 156, 160, 161, 164, 177;
of Crustacea, 340; cf Echino-
dermata, 630; of Hirudinea, 300;
of Insecta, 449 ; 426 ; of Mollusca*,
549, 555 ; of Onychophora, 31? ; of
Platyhelminthes, 200 ; of Protozoa,
17 ; of Rotifera, 240. See also Eyes
Sensillae, 450, 451
Sepia, 589; 588, 590, 597, 599, 600,
601, 602; officinalis, 589; 592, 593,
594, 595
Sepioidea, 588
Sepiola, 588, 602
Septa, of Chaetopoda*, 261, 272, 285,
291, 294; of shell of Tetrabranchi-
ata, 602, 604; of Zoantharia, 190
Septibranchiata, 579
Sergestes, 353; arcticus, 335
Serpula intestinalis , 288
Sertularia, 164; 163
Sexual congress, see Sexual differ-
ences and sexual behaviour, Mutual
fertilization
Sexual differences and sexual be-
haviour, of Arachnida, 524, 529,
" 533, 537 1 of Archiannelida, 296 ; of
Balanoglossus, 668 ; of Bonellia,
302 ; of Cephalopoda, 598 ; of Coe-
lenterata, 152; of Crustacea*, 351,
356, 358, 362, 372, 373, 374, 375,
376, 381, 382, 383, 397, 402, 410;
720
INDEX
Sexual differences and sexual be-
haviour (cont.)
ofEchinodermata,629; oflnsecta*,
446, 469, 470, 472, 479 ; of Myria-
poda, 422, 424 ; of Nematoda, 250 ;
of Onychophora, 321 ; of Panto-
poda, 539; of Polychaeta, 278; of
Protozoa, 32 ; of Rotifera, 240.
See also Generative organs
Sexual reproduction, 6 ; of Metazoa,
see Generative organs ; of Protozoa,
30. See also Life history
Sexuparae, 480
Shape of body, see Body
Shell of Crustacea, see Carapace
Shell of Echinoidea, see Corona
Shell glands, of Crustacea, see Maxil-
lary glands; of Platyhelminthes,
210
Shell ligament, 573
Shell types, Arenaceous, 75 ; Imper-
forate, 75 ; Perforate, 75
Shells, of Brachiopoda, 613 ; of Crus-
tacea, see Carapace; of Foramini-
fera*, 13, 72, 74, 75, 75, 7^; of
Mollusca*, 545, 547, 548; 549-605
(passim); of Protozoa, 12
Shield-shaped tentacles, 649
Sialis, 485 ; lutaria, 485
Sicula, 170; 171
Sida, 363
Silicoflagellata, 50
Silicoflagellidae, see Silicoflagellata
Silver fish, see Lepisma saccharina
Simocephalus , 364 ; sima, 365
Simuliidae, 509
Simulium, 446
Sinus system of Hirudinea, 298
Sinuses, Haemal, of Arachnida*,
522, 531; of Arthropoda, 314; of
Crustacea, 348; of Helix, 556; of
Lamellibranchiata, 580 ; oiLumbri-
culus, 293, 294; of Pomatoceros,
etc., 263
Siphon, of Echinoidea, 646 ; of
Gasteropoda, 556; of suctorial
Crustacea, see Proboscis
Siphonoglyphes, 182
Siphonophora, 166; 154, 167, 168
Siphonozooids, 185
Siphons of Lamellibranchiata, 574
Siphuncle, 602 ; 600
Sipunculoidea, 304; 261, 301
Sipunculus, 303, 304
Sirex gigas, 500
Sitaris, 458
Size of Protozoa, 8
Skeletal plates, of Echinoidea, 641 ;
of Ophiuroidea, 638
Skeletogenous layer, 117
Skeleton, External, see Corals, Cuti-
cle, Perisarc, Shell ; Internal, see
Internal skeleton
Skin, 136
Skull of Cephalopoda, 596
Slimonia, 524; 525
Small intestine of Insecta, 434
Social life, of Hymenoptera, 498,
502 ; of Isoptera, 469
Soldiers of Isoptera, 469; 471
Solenia, 183
Solenocyte, 275; 134
Somatoblast, 282
Somite, First, of Arthropoda, 306-7,
308, 309, 318, 319, 332, 420, 425,
515
Somites (body segments). Series of,
in Arthropoda, 306-7, 308; in
Crustacea, 328-9; in Polychaeta*,
266, 268, 270, 272. See also
Tagmata
Somites, mesoblastic, see Mesoderm
segments
Spatangoida, 647
Spatangus, 648
Sperm pouch of Chaetognatha, 619
Sperm sacs, see Seminal vesicles
Spermatheca (Receptaculum sem-
inis), of Helix, 561 ; of Insecta, 447 ;
of Nematoda, 250; of Platyhel-
minthes, 210
Spermatic atrium, 291
Spermatophores, of Crustacea, 352;
of Helix, 561 ; of Peripatus, 321 ; of
Sepia, 598
Spermatozoa, 6; of Crustacea, 352;
of Hirudinea, 300; of Nematoda,
250
Spermiducal glands of Oligochaeta,
287
Sphaeractinomyxon, 102
Sphaerella, see Haematococcus
Sphaerophrya, 116; 115; sol, 108
Sphecoidea, 503
Spherularia, 257 ; 256
Sphex, 503
Sphingidae, 491
Sphinx, 439
INDEX
721
Spicules, of Alcyonaria*, 183, 184,
186; of Porifera, 118, 120, 122,
123; of Radiolaha*, 80, 81, 83
Spines, of Echinodermata, 626; of
Echinoidea, 641 ; of Ophiuroidea,
638; of Starfish, 634
Spinnerets, 530; 532
Spiracles, of Arthropoda, see Stig-
mata; of Blastoidea, 659
Spiral cleavage, 281 ; 145
Spirochona, 114; 43 ; ^emmi/)ara, II4
Spirographis, 264
Spirostomum, no; 40; ambiguum,
IIO
Spirula, 600; 588, 600
Spirulirostra, 600 ; 600
Sponges, see Porifera
Spongilla, 127; lacustris, 125
Spongillidae, 124
Spongki, 121
Spongioplasm, 12
Sporangium, 86
Spore cases, 38; 94, 98, 100
Spores, 37; 12, 38, 55, 70, 80, 86, 88,
89, 94, 96, 98, 100
Sporoblasts, 87; 38, 89, 93, 100
Sporocysts, 22 ; 38, 89, 96
Sporogony, 37, 89
Sporont, 36, 37, 92
Sporozoa, 87; 8, 43, 44
Sporozoites, 36; 87-100 (passim)
Spumellaria, see Peripylaea
Squilla, 392 ; mantis, 391
Stainers, 476
Stalk (Peduncle), of Brachiopoda,
613 ; of Cirripedia*, 377, 378, 379,
380, 382; of Pelmatozoa*, 625,
654, 655, 658, 659; of Protozoa*,
8, 65, 86, III, 115, 116; of Ptero-
branchia, 668 ; Proboscis, 662
Stalked gland organ, 211
Staphylinidae, 495
Staphylocystis , 229
Statoblasts of Polyzoa, 609
Statocysts (Otocysts), of Calypto-
blastea, 156; of Crustacea, 342; of
Turbellaria, 202
Statolith of ascidian tadpole, 676
Stauromedusae, 172
Stegomyia, 252, 509
Stenopodium, 334, 335
Stentor, no; 18, 104, 105; coeruleus,
23, no
Stephanoceros, 241
Sterna, 332; of Arachnida*, 522, 525,
537; of Crustacea, 332, 409; of
Myriapoda*, 421, 424
Sternites, see Sterna
Sternorhyncha, 476
Stewart's organs, 645
Stigma of Odonata, 472
Stigmata (Spiracles), of Arachnida,
519, 536, 537;of Insecta, 440, 441,
444, 508, 509; of Myriapoda, 421,
424; of Onychophora, 320
Stigmata of Tunicata, 672, 673
Stimuli, Effect of, on Protozoa, 38
Stipe, 170; 171
Stipes, 427
Stolon, of Alcyonaria, 183; of Hy-
dratuba, 178; of Hydrozoa, see
Hydrorhiza ; of Tunicata, 677, 678,
678, 680, 681, 683
Stomach, 130. See also Alimentary
canal
Stomatopoda, 391 ; 336, 353, 389
Stomodaeum (Fore gut), 130; of
Anthozoa*, 182, 187; of Arthro-
poda, 314; of Ciona, 669; of
Crustacea*, 344, 358, 379; of
Ctenophora, 195; of Insecta, 432;
of Nematoda, 248 ; of Onycho-
phora, 320; of Rotifera, 240; of
Tricladida (pharynx), 206; of
Trochosphere, 282
Stone canal, 626, 627, 637, 639, 646,
647, 658
Strepsiptera, 429
Streptoneura, 563; 552
Strobilation, of Aurelia, 178, 180;
of Cestoda, 143, 225
Stromatocystis, 658
S trombus, 563
Strongyloid larva, 251
Strongy hides stercoralis, 254
Structureless lamella (Mesogloea),
146, 150, 156, 161, 174, 179, 180-6
{passimi)
Stylaria, 291 ; 286; proboscidea, 292
Stylets, of Hemiptera, 474 ; of Nemer-
tea, 233 ; 234
Stylommatophora, 570
Stylonichia, 1 1 1 ; 42 ; mytilus, 112
Sty lops, 514
Subchela, 339
Subchelate limbs, 391, 401, 415, 515,
530
Subdermal cavities, 123
722
INDEX
Subgenital pits, 174
Subimago, 481
Submentum, 427
Subneural gland of Tunicata, 672
Suboesophageal ganglion, see Gan-
glion, Suboesophageal
Substratum, 3
Subtentacular canals, 656
Subumbral pit, 174
Subumbrellar cavity, 156, 158; ecto-
derm, of Medusa, 156; of trocho-
sphere, 282; musculature, 156
Suctoria, 114; 8, 19
Sulcus, 54
Summer eggs of Cladocera, 367; of
Mesostomum, 213
Superlinguae, 428 ; 306, 463
Supero-marginal ossicles, 634
Superposition image, 313
Supraoesophageal ganglia, see Gan-
glion, Cerebral
Surface, of Ciliata, 105 ; of Protozoa,
12, 20, 22. See also Ectoplasm
Suture line of ammonoid shell, 605
Swarm spores, 38
Swarming of Polychaeta, 278.
Sycon, 126; 119; raphanus, 125
Sycon grade, 120
SylliSy 268; 264, 266, 279; ramosa,
280 ; 279
Symbiosis, 44 n. ; 44, 47, 52, 68, 1 1 1 ,
193, 213, 470
Symmetry, 143, 633; of Actinozoa,
182; of Echinodermata, 623, 624,
633 ; of Metazoa, 143 ; of Protozoa,
8. See also Bilateral symmetry,
Radial symmetry
Sympathetic system, of Crustacea,
340; of Insecta, 448
Symphyta, 500
Symplasts, 10
Synagoga, 386; mira, 386
Synalpheus, 386
Synapse, 138, 149
Synapta, 652; 650, 653
Synaptida, 652
Syncarida, 392; 389
Syncytia, 10, 100, 102. See also
Plasmodia
Syngamy, 30 ; 6, 37 ; of Ciliophora*,
30 n., 33, 34, 35, 107, H4, 115; of
Dinoflagellata, 54; of Mastigo-
phora, 45 ; of Sarcodina*, 72, 80,
83, 85, 86; of Sporozoa*, 88-100
(passim) ; of Volvocina*, 31, 46, 56,
57> 58 ; of Zoomastigina, 59
Syracosphaera, 50 ; pulchra, 51
Syringopora, 186
Syrphus, 511
Syzygy, 88 and n. ; 89, 90, 94, 95
Tabanus, 506
Tachardia lacca, 478, 481
Tachinidae, 512
Tactile organs, see Sense organs
Taenia, 224, 230; coenurus, 228;
echinococciis , 229, 230; serrata,
228 ; solium, 226, 227
Taenioglossa, 563
Taeniothrips incovsequeiis, 485
Tagmata, 308 ; of Arachnida, 308 ; of
Crustacea*, 308, 332, 370, 387; of
Insecta*, 308, 483 ; of ?vIyriapoda,
308 ; of Onychophora, 308. See also
Abdomen, Cephalothorax, Head,
Mesosoma, Metasoma, Opistho-
soma, Prosoma, Pygidium, Thorax,
Trunk
Tail, 662 ; 622 ; of Chaetognatha, 618,
619, 620, 622; of Chordata, 662;
of Tunicata, 675, 677, 679, 686
Tail fan, 388 ; 391, 392, 393, 398, 404
Tanaidacea, 395
Tanais, 395
Tardigrada, 539
Tarsus, 428 ; 464, 483, 485, 493, 495,
512
Tealia, 193
Tectibranchiata, 567
Teeth, of Echinoidea, 645 ; of Ophiu-
roidea, 639. See also Radula
Tegenaria guyonii, 532
Tegmen, 655
Tegmentum, 549
Tegmina, 466
Telosporidia, 88 ; Reduction division
of, 27-8, 37
Telson, 306-7; of Arachnida*, 517,
524; of Crustacea*, 326, 328-9,
332, 355, 358, 368, 372, 386, 395,
400; of Lithobius, 421; of Trilo-
bita, 324
Te7nnocephala , 216; minor, 217
Temnocephalea, 216
Temperature, 4, 42
Tentacle sheath, 608
Tentacles, of polyp and medusa, 150,
151, 152, see also Tentaculocysts ;
INDEX
723
Tentacles (cont.)
of Ctenophora, 195, 196; of Gas-
teropoda, 544, 550, 555, 567, 569;
of Holothuroidea*, 649, 652; of
Hydrozoa*, 155-69 {passim); of
Nautilus, 603; of Polychaeta, 263,
265, 268, 270, 271 ;of Polyzoa, 606;
of Scyphomedusae, 173, 174, 178;
of Suctoria*, 19, 115, 116; of
Turbellaria, 202
Tentaculata, 196
Tentaculocysts, 176, 177
Terebella, 264
Terebratula , 613, 618; semiglobosa,
614
Teredo, 586; 560, 587, 587
Terga (Tergites), 332; 334, 415, 421,
424
Terga of Lepas, 378
Tergo-sternal muscles, 441
Termes, 470
Terminal arborization of axon, 137;
organ, nephridial, 197; tentacle
of Echinodermata, 630. 641
Terricoia, 214
Tertian ague, 93
Test of Tunicata*, 669; 675, 677,
679, 680, 686
Testacella, 570; 572
Testes, see Generative organs
Testicardines, 618
Tetrabranchiata, 602 ; 588
Tetractinellida, 127 n.
Tetragraptus, 171; 171, 172; denti-
culatus, 172 ; hicksi, 172 ; siniilis, 170
Tetraphyllidea, 229
Tetrarhynchidae, 229
Tetrastemma, 237
Textrix denticulata, 532
Thalassema, 304 ; misakiensis , 304 ;
neptuni, 304; taenioides, 304
Thalassicolla, 81 ; pelagica, 41
Thaliacea, 681 ; 677, 679
Theca, of corals, 190; of Pelmatozoa,
658
Thecoidea, 658
Theridium, 516
Thompsonia, 384; 386
Thoracic limbs, of Crustacea*, 328-
9, 330. 336, 337, 339, 35^, 362,
372, 379, 380, 387, 388, 389, 391,
392, 393, 397; of Eucarida and
Pericarida, 336. See also Legs,
Maxiilipeds
Thoracic membrane, 270
Thoracica, 377
Thorax, of Arthropoda, 308 ; of
Crustacea*, 333; 330, 353, 356,
370, 371-2, 387, 389, 390-400
(passim)', of Insecta*, 309, 428,
472, 476; of lulus, 424; of Poly-
chaeta, 270
"Thorax" of Tunicata, 674, 677
Thysanoptera, 485
Thysanozoon, 216
Thysanura, 463 ; 430
Tibia, 428
Ticks, 537
Tiedemann's bodies, 637, 646
Tinea biselliella, 436, 491
Tintinnidium, no; inquilinum, 108
Tintinnina, no
Tipula, 97; ochracea, 510
Tocophrya quadripartita, 108
Todarodes Sagittarius, 601
Tomoceros, 465
Topotaxis, 38
Tornaria larva, 668; 145, 666
Torsion of Gasteropoda, 551
Toxiglossa, 563
Trabeculae, of Ciona, 673 ; of
coelom of Crinoidea, 656
Tracheae, 314; 139; of Arachnida*,
517, 519, 531, 534, 537; of In-
secta, 439, 440, 443; of Myria-
poda*, 421, 424; of Onychophora,
320; of Velella, 169; of Woodlice,
314, 348. See also Stigmata,
Tracheal gills
Tracheal gills, 444, 471, 472, 481,
482, 486, 487
Trachelomonas , Flagellum of, 16
Tracheoles, 440
Trachomedusae, 164; 153
Trachylina, 154; 162, 164, 177
Transverse fission of Protozoa, 29
Trematoda, 216; 198, 220, 230, 232
Triaenophonis , 229
Triangle of odonate wing, 472
Triarthrus becki, 324
Triatoma, 64
Trichinella spiralis, 255
Trichobranchiae, 408
TricJtodina, in; pediculus, 108
Trichomonas, 65 ; 17 ; muris, 66
Trichonympha, 67; 435; campanula,
67
Trichoptera, 486 ; 444
724
INDEX
Trichosphaerium, 74; 72
Tricladida, 214; 211
Trilobita, 323
Trilobite stage of Xiphosura, 529
Triploblastic animals (Triploblas-
tica), I ; 129, 136
Tripylaea, 80
Tritocerebrum, 310, 340
Triungulin, 492
Trochal disc, 239
Trochanter, 428
Trochocystis, 658
Trochodiscus longispinus, 80
Trochosphere larva, 282; i, 2, 145,
618; of Annelida, i ; of Mollusca,
I. 547, 582; of Polychaeta, 263,
281, 282; oi Polygordius, 284, 294;
of Polyzoa, 2, 609, 610
Trochostoma, 652 ; 653
Trochus, 239
Trochus, 552
Trophallaxis, 498
Trophi, 240
Trophoch'romatin, 26
Trophozoite, 88
Trunk, of Arthropoda, 308 ; of
Crustacea*, 308, 327, 330, 331,
333, 360, 364: of Trilobita, 323.
See also Abdomen, Thorax
"Trunk" of ascidian tadpole, 675
"Trunk" ganglion, 676
Trunk limbs, of Crustacea*, 326,
327, 329-30, 362, 363, 364, 368,
370; of Onychophora, 319. See
also Abdominal limbs, Thoracic
limbs
Trunk segments of Polychaeta, 265.
See also Abdomen, Thorax
Trypanosoma, 64 ; brucei, 62, 65 ;
cruzi, 64; eqidperdum, 64; gam-
biense, 64 ; lewisi, 64 ; rhodesiense, 64
Trypanosomidae, 63
Trypanosyllis, 280 ; gemmipara, 279
Tryphaena pronuha, 491 ; 488, 497
Tube feet (Podia), 623; 625, 627,
630, 638, 639, 641, 643, 647, 648,
649, 650, 655
Tubifex, 293
Tubipora, 185
Tubularia, 158; 154, 159, 160, 162
Tunicata, 669; 661
Turbellaria, 213; 197, 198
Tylenchus devastatrix, 253 ; dispar,
256; tritici, 253
Tympana, 468
Typhlosole, 287
Tyroglyphus, 534; siro, 535
Umbo of Brachiopoda, 613
Umbrella of trochosphere, 282
Umbrellar surfaces, etc., of Medusae,
see Exumbrellar, Subumbrellar
UncJni, 270
Uncoiling of Cephalopoda, 605
Undulating membranes, of ciliates,
17, 104, 109; of flagellates, 17, 64,
66
Uniramous limbs of Crustacea, 338
Urochorda, see Tunicata
Uropods, 388; 395, 397, 398, 407,
415
Urosome, 372
Uterus, of Cestoda, 211, 225, 228,
232; of Chirocephalus, 359; of
Cyclops, 373 ; of Paludina, 566; of
Platyhelminthes, 211; of Rhab-
ditis, 250; of Rhabdocoelida, 211 ;
of Scorpionidea, 524; of Trema-
toda, 211, 230
Vacuolaria, 54
Vacuoles, 12; Contractile, 20, 21, 49,
52, 69, 70, 80, 83, 86, 107, hi;
Food, 19, 20, 65; Gas, 72, 74;
Hydrostatic, of ectoplasm, 40, 76,
80, 83 ; of Dinoflagellata, 54
Vagina, see Generative organs
Vahlkampfia, 69
Valves of shell, of Brachiopoda, 613,
615 ; of Conchostraca, 333, 354; of
Lamellibranchiata, 545, 585, 586;
of Lepas, 379; of Ostracoda, 333,
368
Vanadis, 276
Vas deferens, see Generative organs
Vasa efferentia, of Insecta, 447 ; of
Platyhelminthes, 210
Vascular system, 132; of Anostraca,
348; of Araneida, 530; of Arthro-
poda, 314; of Balanoglossus, 664,
666; 665; of Carcinus, 414; of
Chaetopoda, 263, 273; of Ciona,
674 ; of Crustacea, 348 ; of Insecta,
437 ; of Lamellibranchiata, 579 ; of
Lernanthropus, 349-5 1 ; of Limulus,
529; of Malacostraca, 348-9; of
Myriapoda*, 421, 424; of Nemer-
tea, 233, 235 ; of Scorpionidea, 522
INDEX
725
"Vascular" system of Echinoder-
mata, see Lacunar system ; of Scy-
phomedusae, 174
"Vascular" tissue of Echinodermata,
see Lacunar tissue
Vegetative phase, 36
Vegetative pole, 281
Vein, Abdominal, 594; Afferent
branchial, 581 ; Branchial, 594; Ef-
ferent branchial, 581 ; Genital,
594; Ink sac, 594; Longitudinal, of
kidney, 581 ; Pulmonary, 556. See
also Circulus venosus, Sinuses,
Vena cava
Velella, 168; 168
Veliger larva, 547
Velum, of Medusae, 156, 176; of
Rotifera, 239; of Veliger larva, 547
Vena cava, 582, 594
Ventilators, 504
Ventral, see Dorsal and ventral
Ventral blood vessel, of Balano-
glossus, 668; of Chaetopoda, 263,
275 ; of Rhynchobdellidae, 299
Ventral "blood vessels" of Echino-
dermata, 629, 643, 650
Ventral cirrus, 265 ; mesenteries, of
Alcyonaria,i82 ; of Polychaeta,285 ;
midline of Nematoda, 244; plate
of trochosphere, 282 ; siphon, 574 ;
tube of Collembola, 464
" Ventral " plates of Ophiuroidea, 639
Ventricle, see Heart
Venus' Girdle, see Cestus Veneris
Vermes, 198
Vertebrae of Ophiuroidea, 638
Vcrtebrata, i, 2, 660, 661, 662
Vesiculae seminales, see Generative
organs
Vesicular nuclei, 23, 24
Vespa, 496, 503, 504; crabro, 494,
503 ; germanica, 503 ; vulgaris, 503
Vespoidea, 502
Vestibulata, 109
Vestibule, 19; 104. See also Gullet
Vibracula, 608
Visceral clefts, 660
Visceral hump, 544, 545, 550, 551,
554. 555, 558, 565, 589. 591, 602
Visceral mass, 383
Visceral nerves, see Sympathetic
system
Vitellarium, of Platyhelminthes, 210;
of Rotifera, 240
Vitelline ducts, 210
Vitrellae, 310
Volvocina, 56; 43, 49; reduction
division of, 28, 37 ; syngamy of, 31,
46
Volvox, 58 ; 3 1 , 38, 42, 45, 46, 60, 61 ;
aureus, 59
Vorticella, 1 1 1 ; 8, 9, 22, 34, 104
Waldheimia, 613; 614, 616, 618
Water pore of Echinodermata, 628
Water vascular ring, see Water
vascular system
Water vascular system, 626 ; 627,
637, 646, 658
Wings, 429; 459, 460
Winter eggs, 213
Wire v/orm, see lulus terrestris
Workers, of Ants, 502; of Isontera,
469; 470, 471 ; of Wasps, 498
Xanthophyll, 47
Xanthoplasts, 47
Xenocoeloma, 376
Xenopsylla cheopis, 513
Xestobium, 435 ; rufovillosum, 493
Xiphosura, 526
Xylophaga, 586
Yellow cells of Chaetopoda*, 262,
273
"Yellow cells". Symbiotic, see Zoo-
xanthellae
Yolk gland, see Vitellarium
Young, see Life history
Yungia, 216
Zammara tympanum, 477
Zoaea larva, 389; 353, 389, 392, 403,
414, 414, 415, 417
Zoantharia, 186
Zones of fission, 291
Zoochlorellae, 47
Zooecium, 606
Zooids, of Coelenterata, see Hy-
dranth, Polyp; of Polyzoa, see
Polypide; of Protozoa, 8; 10; of
Rhabdopleura, 668; of Tunicata,
" 677; of Volvocina, 10, 57, 58
Zoomastigina, 58; 46, 62
Zooxanthellae, 47; 48, 50, 81, 83
Zygoptera, 472
Zygote*, 36; 37, 46, 52, 58, 85, 86,
87, 88-100 (passim), 107, 116
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