ZOOLOGY AND PHYSIOLOGY.

BUMPUS' INVERTEBRATE ZOOLOGY. A Lab-
oratory Course. Revised Edition. 15? pp. 12mo.

HO WELL'S DISSECTION OF THE DOG. 100 pp.

8vo.

MARTIN'S THE HUMAN BODY. (American
Science Series.)

Advanced Course, xvi + 621 + 34 pp. 8vo.
Briefer Course, xii + 377 pp. 12mo.
Elementary Course. vi + 261pp. I2mo.

The Human Body and the Effects of Narcotics,
viii + 39U pp. 12mo.

McMURRICH'S INVERTEBRATE MORPHOL-
OGY. vii+661pp. 8vo.

PACKARD'S ZOOLOGIES. (American Science Series.)
Advanced Course, vii + 722 pp. 8vo.
Briefer Course, viii +338 pp. 12ino.
Elementary Course.' viii + 290 pp. 12mo.

PACKARD'S ENTOMOLOGY FOR BEGIN-
NERS, xvi + 367 pp. 12mo.

PACKARD'S GUIDE TO THE STUDY OF

INSECTS, xii + 715 pp. 8vo.
SCUDDER'S BUTTERFLIES. x+322pp. 12mo.

SCUDDER'S BRIEF GUIDE TO BUTTERFLIES.

xi + 20(5 pp. 12m< >.

SCUDDER'S THE LIFE OF A BUTTERFLY.
(For General Readers. ) 186pp. 16mo.

SEDGEWICK AND WILSON'S GENERAL BI-
OLOGY. (American Science Series.) viii + U)3pp. 8vo.

The Publishers' Educational Catalogue with descriptions
and prices, free.

HENRY HOLT & CO.,
29 WEST 28o STREET, NEW YORK.

A TEXT-BOOK

:

OF

INVERTEBRATE MORPHOLOGY

BY

J. PLAYFAIR McMURRICH, M.A., PH.D.

Professor in the University of Michigan

NEW YORK

HENRY HOLT AND COMPANY

1894

*? ^

Copyright, 1894,

BY

HENRY HOLT & Co.

ROBERT DRUMMOND, ELECTROTYPER AND PRINTER, NEW YORK.

PREFACE.

THE Morphology of Invertebrate Animals may be treated
either from the standpoint of Comparative Anatomy or from
the zoological side, and either method of treatment has much
to recommend it. In my experience, however, the zoological
method has proved most satisfactory for the presentation of
the subject to students, inasmuch as it is necessarily the
method employed in the laboratory, and accordingly in the
present work that plan of presenting the facts of morphology
has been followed. A bare statement of the structural
peculiarities of the various groups, however, is simply collect-
ing the bricks and stones without the mortar necessary to
unite them together into a substantial edifice, and where the
opportunity has presented itself attention has been directed
to the comparative significance of various organs and to the
affinities of the various groups.

A word is perhaps necessary in regard to the classification
adopted, which presents many radical changes from the schemes
usually employed. For the larger groups, following the ex-
ample of Glaus, the term type has been employed, and no less
than twelve of these types are adopted. This increased num-
ber has resulted from a division of two groups usually recog-
nized, namely, the Vernies and the Arthropoda. As regards the
former it has long been acknowledged to be a heterogeneous
collection, and its retention is to be regarded as a survival.
It is true that the forms assigned to it do present certain
phylogenetic affinities; but if this is to be the reason for its
retention, then the Mollusca and Prosopygia (Molluscoidea)
should also be assigned to it. It has seemed more satisfac-
tory to retain the Mollusca and Prosopygia as distinct groups,
and to divide the Vermes into several types, such as the
Platyhelminthes, Nemathelmiuthes, and Annelida, each of the
same rank as the Mollusca, and presenting approximately
similar degrees of affinity among themselves.

iii

IV PREFACE.

As to the Artliropoda, its right to exist as a group coordi-
nate with, for instance, the Mollusca has been questioned by
several authors. Undoubtedly in this case also many similar
structural features obtain among the various members of the
group, but embryology has indicated a probability of a more
or less independent origin of two Arthropodan groups usually
regarded as closely related, namely, the Arachnida and the
Tracheata proper. Apparently the former have originated
from Crustacean ancestors, while, if the supposed significance
of Peripatus be accepted, the Tracheates are to be traced back
to Annelidan forebears, and for the purpose of calling the
attention of the student to this probable phylogeny the
Crustacea, Arachnida and Tracheata have been regarded as
distinct types coordinate with the Annelida and Mollusca.

A book of this kind must necessarily be highly tinged with
the individual opinions of the writer, and for these indulgence
must be craved. So far as the facts are concerned every
care has been taken that they should be accurate and as
far as possible up to date with the most recent investigations.
Errors have no doubt crept in, a misfortune almost inevitable
for the mass of material which must pass under consideration
during the progress of the work, and for these again indul-
gence must be asked.

Refraining from further apologies, the more pleasant duty
remains of thanking the many friends who have so kindly
aided the work by suggestion or otherwise, and especially
those who have permitted the use of figures taken from
special papers. A large number of the figures employed
are original and the great majority have been especially
drawn for this work, the attempt being made to diagramma-
tize them to a greater or less extent for the sake of clearness.
In all cases where figures have been borrowed the original
authorship has been duly acknowledged.

Finally, I desire to make public recognition of my indebt-
edness to my wife for the invaluable assistance she has ren-
dered in many ways during the progress of the work.

J. PLAYFAIE McMuEKicn.

UNIVERSITY OP MICHIGAN,
September, 1894.

TABLE OF CONTENTS.

PAGES

CHAPTER I. PROTOPLASM AND THE CELL 1-12

Composition of Protoplasm, pp. 1-3. Structure of the Cell,
pp. 4-8. Cell-division, pp. 9-12. Literature, p. 12.

CHAPTER II. THE SUBKINGDOM PROTOZOA 13-40

The Class Rhizopoda, pp. 14-24. The Class Sporozoa, pp. 24-
28. The Class Flagellata, pp. 28-33. The Class Infusoria,
pp. 33-38. Synoptical Classification, pp. 38, 39. Literature,
pp. 39, 40.

CHAPTER III. THE SUBKINGDOM METAZOA 41-62

Individuality of the Metazoa, pp. 41-42. Sexual Reproduction,
pp. 42-51. The Segmentation and Early Development of
the Ovum, pp. 51-58. Non-sexual Reproduction of the
Metazoa, pp. 58-60. Alternation of Generations, pp fi^ 62.
Literature, p. 62.

CHAPTER IV. TRICHOPLAX, THE DICYEMID.E AND THE

ORTHONECTIDA 63-67

Trichoplax, pp. 63, 64. The Dicyemidse, pp. 64, 65. The Or-
thouectida, pp. 65-67. Literature, p. 67.

CHAPTER V. THE TYPE CGELENTERA 68-119

The Subtype Porifera, pp. 69-76. The Subtype Cnidaria, pp.
76-115. The Class Hydromedusse, pp. 78-97. The Class
Scyphomedusae, pp. 97-104. The Class Anthozoa. pp. 104-
115. Synoptical Classification, pp. 115-117. Literature,
pp. 118, 119.

CHAPTER VI. THE CTENOPHORA 120-126

Description of the Group, pp. 120-126. Synoptical Classifica-
tion, p. 126. Literature, p. 126.

v

VI TABLE OF CONTENTS.

PAGES

CHAPTER VII. THE TYPE PLATYHELMINTHES 127-171

The Class Turbellaria, pp. 130-143. The Class Trematoda, pp.
143-152. The Class Cestoda, pp. 152-161. The Class
Nemertiua, pp. 162-169. Synoptical Classification, pp.
169, 170. Literature, pp. 170, 171.

CHAPTER VIII. THE TYPE NEMATHELMINTHES 172-183

The Class Nematoda, pp. 173-179. The Class Acauthocephala,
pp. 179-182. Synoptical Classification, pp. 182, 183. Lit-
erature, p. 183.

CHAPTER IX. THE ORDER ECHINODERA; THE CLASS
CH^TOGNATHA ; THE CLASS EOTIFERA ; THE
ORDER GASTROTRICHA ; AND DINOPHILUS 184-201

The Order Echinodera, pp. 184-186. The Class Choetoguatha,
pp. 186-189. The Class Rotifera, pp. 189-195. The Order
Gastrotricha, pp. 195-198. The Genus Dinophilus, pp.
198-200. Synoptical Classification, p. 200. Literature, p.
201.

CHAPTER X. THE TYPE ANNELIDA 202-253

The Class Chsetopoda, pp. 204-227. The Class Hirudinea, pp.
228-237. The Class Gephyrea, pp. 237-243. The Class
Myzostornese, pp. 244-246. The Class Phorouidse, pp. 247-
251. Synoptical Classification, pp. 251, 252. Literature,
pp. 252, 253.

CHAPTER XI. THE TYPE PROSOPYGIA 254-275

The Class Polyzoa, pp. 255-268. The Class Brachiopoda, pp.
268-274. Synoptical Classification, p. 274. Literature, p.
275.

CHAPTER XII. THE TYPE MOLLTJSCA 276-367

The General Characteristics of the Type, pp. 276-284. The
Class Amphineura, pp. 284-293. The Class Gasteropoda,
pp. 293-322. The Class Scaphopoda, pp. 322-326. The
Class Pelecypoda, pp. 326-340. The Class Cephalopoda,
pp. 340-362. The Affinities of the Mollusca, pp. 362, 363.
Synoptical Classification, pp. 363-365. Literature, pp.
365-367.

CHAPTER XIII. THE TYPE CRUSTACEA 368-434

The General Characteristics of tlie Type, pp. 368-385. The
Class Entomostraca, pp. 385-403. The Class Malacostraca,
pp. 403-417. The Development and Affinities of the Crus-
tacea, pp. 417-423. Synoptical Classification, pp. 423, 424.
Literature, pp. 424-427. The Order Xiphosura, pp. 427-
434. Literature, p. 434.

TABLE OF CONTENTS. Vll

PAGES

CHAPTER XIV. THE TYPE ARACHNIDA 435-468

The General Characteristics of the Type, pp. 435-441. Descrip-
tion of the Various Orders, pp. 441-456. The Phylogeuy
of the Arachnida, pp. 456-458. Synoptical Classification,
pp. 458, 459. Literature, pp. 459, 460. The Order Peuta-
stomidae, pp. 461-463. The Order Pycuogonida, pp. 483-
466. The Order Tardigrada, pp. 466-468. Literature, p.
468.

CHAPTER XV. THE TYPE TRACHEATA 469-530

The General Characteristics of the Type, pp. 469-474. The
Class Protracheata, pp. 474-480. The Class Myriapoda,
pp. 480-487. The Class Insecta, pp. 487-522. The Phy-
logeuy of the Tracheata, pp. 523-525. Synoptical Classifi-
cation, pp. 525-528. Literature, pp. 528-530.

CHAPTER XVI. THE TYPE ECHINODERMA 531-595

The General Characteristics of the Type, pp. 531-540. The
Class Crinoidea, pp. 541-551. The Class Asteroidea, pp.
552-560. The Class Ophiuroidea, pp. 561-570. The Class
Echinoidea, pp. 570-584. The Class Holothuroidea, pp.
584-590. The Phylogeny of the Echinoderma, pp. 590-
592. Synoptical Classification, pp. 592, 593. Literature,
pp. 593-595.

CHAPTER XVII. THE TYPE PROTOCHORDATA 596-641

The Class Hemichorda, pp. 596-608. The Class Cephalochorda,
pp. 608-618. The Class Urochorda, pp. 618-639. Synop-
tical Classification, pp. 639, 640. Literature, pp. 640, 641.

INDEX OF PROPER NAMES 643-653

INDEX OF SUBJECTS., ... 654-661

INVERTEBRATE MORPHOLOGY

CHAPTER I.

PROTOPLASM AND THE CELL.

IN the examination of organisms presenting the series of
phenomena which we term life, the invariable presence of a
peculiar semi-fluid transparent or hyaline substance becomes
quickly apparent. Whether the organism be a plant or an
animal, whether it be of the simplest or of the most complex
organization, it is still composed of this substance, which is
known as protoplasm, and it may be said that so far as our
knowledge extends life never exists except in association with
this material. Protoplasm is " the physical basis of Life,"
and it becomes of great importance that its nature should be
fullv understood, in order that the results of its activities,
Life, may become more intelligible.

Much has yet to be accomplished, however, before an accu-
rate knowledge of the structural and chemical characters of
this substance is obtained, and indeed it is incorrect to regard
it as a substance, since it is rather the aggregate of a large
number of exceedingly complex chemical compounds, none
of which are sufficiently known. From the very nature of
things it is impossible at present to get a correct idea of these
substances and the relations which they bear to one another,
since our present analytical methods are not capable of deter-
mining and isolating them in living protoplasm and the mere
act of subjecting protoplasm to analysis destroys those very
relationships which are the cause of the vital manifestations.

2 INVERTEBRATE MORPHOLOGY.

Dead protoplasm is something very different from living pro-
toplasm, and our present knowledge only imperfectly extends
to this much-altered material.

Furthermore even in the dead material the chemist has to
deal not only with the complex substances which constitute
protoplasm proper, but also with numerous secondary prod-
ucts either in the process of being built up into protoplasmic
molecules or else resulting from the destruction of these
molecules. For both these processes are continually going
on, the living organism continually uniting simple chemical
compounds to form new complex molecules, a process known
as anabolism, and resulting in growth ; and just as continually
it is resolving into simpler compounds the complex mole-
cules already formed, a process known as catabolisin, and
resulting in the manifestation of energy in its various forms,
such as heat, motion, electricity, and even light. Growth and
the manifestation of energy are then two most important
phenomena exhibited by living organisms, standing in oppo-
sition to one another and determining the general condition
of the organism. If anabolic changes are the more active,
then the animal or plant grows, as we express it, adds new
protoplasm and increases in size ; if the anabolic and catabolic
changes are practically equal in amount, stability results ;
while the preponderance of catabolism leads to a lessening
of material, and finally to what we term death. These
changes constitute a cycle occurring in the life-history of
probably every organism and causing the periods which we
denote as youth, maturity, and old age.

Dead protoplasm then, together with the anabolic and
catabolic constituents which are inextricably associated with
it, will be found on analysis to consist to a large extent of the
chemical elements Carbon, Hydrogen, Oxygen, and Nitrogen,
together with Sulphur and Phosphorus, as well as a number
of substances present in varying amounts, such as Chlorine,
Potassium, Sodium, Iron, Calcium, and Magnesium. Exactly
how these various elements are united together it is difficult
to determine, but especial importance has been assigned to
the C, H, O, N, and S compounds which occur and which form
a group of chemical compounds known as Proteids. Of such

PROTOPLASM AND THE CELL.

compounds several, such as Albumin, Globulin, Fibrin, Plastiu,
Nuclein, have been isolated from protoplasm, some being
probably secondary products resulting from the alteration of
the protoplasmic molecules proper, but others, such as
Plastiu and Nucleiu, are especially constant, and seem to be
important constituents of the protoplasmic complex.

Plastin forms when isolated a sticky fibrous mass, insoluble in concen-
trated alkaline solutions and unaffected by the peptic and tryptic ferments,
and consists of C, H, O, N, S, and P. Nuclein is more especially charac-
teristic of a special portion or modification of protoplasm termed the
nucleus, of which more will be said hereafter, and resembles plastin very
closely, being, however, less insoluble than it, and consists of the same
chemical elements. Analyses of these substances, however, differ greatly,
the nuclein from spermatozoa, for instance, containing no sulphur ; and it
seems probable not only that they differ materially according to the source
from which they are obtained, but also that they are not reaHy chemical
compounds, but a mixture of several highly complex substances.

With these proteids, then, there exist in protoplasm vari-
ous salts, such as Potassium, Sodium, and Calcium phosphate,
Potassium and Sodium chloride, Magnesium sulphate, and
other such salts, the exact significance of which it is difficult
to estimate. How living protoplasm differs chemically from
dead has not up to the present been accurately determined.

As regards its general structure protoplasm appears as a
moderately consistent jelly-like substance, usually colorless
and more or less granular in appearance. As a rule the
peripheral portion of a mass of protoplasm is less granular
than the central, appearing therefore clearer, and is espe-
cially distinguished as the ectoplasm from the more opaque
endoplasm. Imbedded in the endoplasm are to be found
usually various bodies, the products of the activities of the
protoplasm, such as large, clear spaces occupied by fluid
and known as vacuoles, food-particles of various kinds in the
simpler organisms, starch granules and crystals in plant-pro-
toplasm, and depositions of pigment. One particular struc-
ture, the nucleus, however, seems to be invariably present,
occupying the central portion of the mass, and, as will be
seen later, playing a very important role in the life of the
protoplasm. It is indeed a specially modified portion of the
protoplasm and cannot, therefore, be placed in the same

4 INVERTEBRATE MORPHOLOGY.

category as the vacuoles and other accidental or secondaiy
constituents which have been mentioned, and eveiy mass of
protoplasm may be considered as consisting of two essential
parts, the protoplasm proper or cytoplasm and the special
modification of it, the nucleus, which for convenience is
termed the caryoplasm. Such a combination of cytoplasm
aud caryoplasm forms what is technically known as a cell, and
all living organisms are composed of one or more such struc-
tures, which are to be regarded therefore as morphological
units.

If the more intimate structure of the cytoplasm of such a
unit or cell (Fig. 1) be examined, disregarding the various

nm

. 1.— DlAGKAM SHOWING THE STRUCTURE OF AN ANIMAL CELL.

c = centrosome. m = microsome.

cl — cytolymph. nl = nucleolus.

cr — chromatic, nm = nuclear membrane.

r = reticulum.

secondary constituents it may enclose, it will be found to
consist of a network of exceedingly fine fibrils, along which,
and more especially at the points where two or more of them
meet, are to be found minute granules which stain deeply with
the ordinary microscopical staining reagents. The fibrils
constitute the reticulum (Fig. 1, r) of the cytoplasm, and the
granules are termed the microsomes (m). The reticulum
seems to be formed principally of the proteid substance
already mentioned as plastin, and its meshes are occupied by
a more fluid substance which has been termed the cyto-
lymph (cl).

PROTOPLASM AND THE CELL. 5

Several opinions have been given in regard to the structure of the cyto-
plasm, in addition to that here presented, according to which it may be
compared to a sponge the meshes of whose network are occupied by the
cytolymph. According to another view it is composed of a number of
fibrils of varying lengths imbedded in a matrix, the fibrils corresponding to
the reticulum of the reticular theory and the matrix to the cytolymph.
According to still another theory which rests on the appearance produced
in the cytoplasm by a special method of treatment, there is present a color-
less matrix in which are imbedded numerous exceedingly small granules
sometimes scattered and sometimes united together into chains. Indeed
the upholder of this granular theory has carried his view to the extent of
regarding the granules as structural units of which the cell is composed,
its structure being comparable to that of a zoogloea of micrococci. It
seems probable, however, that the granules are to a large extent secondary
products of the activities of the cytoplasm and have therefore but a sub-
ordinate value in its composition. The reticular theory seems to stand
more nearly in harmony with the majority of observations, though it
must be admitted that some observers do not seem to have perceived the
true reticulum, confining their attention to the coarser network produced
in some cases by extensive vacuolization of the cell.

An imitation of the cytoplasm has been recently obtained by the mix-
ture of thickened olive-oil with a solution of potassium carbonate or of
chloridre of sodium, the watery solution taking the form of polyhedral
globules each surrounded by a thin layer of oil which from its higher
refractive index gives the appearance of the plastin reticulum surrounding
the cytolymph. Solid particles finely divided and mixed with the oil tend
to collect at the points where the oil-films of three of the globules come
together, and resemble the microsomes, while it is further noticeable that
under certain conditions the superficial globules of the emulsion take on a
columnar form and maybe compared with the ectoplasm of the cell. It
is possible that the cytoplasm may have this structure, in which case the
reticular theory would require to be modified, since there would no longer
be a spongy structure, but rather an emulsion in which the cytolymph is
divided into a number of globules each surrounded by a thin layer of
plastin. At present, however, the reticular theory seems to correspond
most accurately with the actual appearances, and therefore may be pro-
visionally accepted.

The caryoplasm or nucleus, as already stated, lies usually
about the middle of the cytoplasm and to a certain extent re-
sembles it, though it presents certain peculiar features. It
is usually round or oval, though occasionally it may assume
elongated, horseshoe-shaped, moniliform, or even branching
forms, and is as a rule clearly marked off from the cytoplasm by
a membrane (Fig. 1, mm), which, however, at certain periods of

6 INVERTEBRATE MORPHOLOGY.

nuclear activity seems to disappear, a new one subsequently
forming. Traversing the space enclosed by the membrane,
so as to form a network, are fibres which do not stain very
deeply with the usual staining fluids and which are composed
of a substance termed limn, which does not, however, appear
to differ essentially from the plastin of the cytoplasm. In-
deed it is not improbable that the linin network is con-
tinuous through the nuclear membrane with the plastin
reticulum and that both are identical, as is also the caryo-
lymph contained in the meshes of the linin with the cyto-
lyinph.

A more characteristic substance is the chromatin (Fig.
1, cr), so called from the strong affinities it shows for many
staining fluids, such as carmine, hieinatoxylin solutions, and
certain aniline stains. It seems to consist of the substance
nuclein, already alluded to, and in the resting nucleus forms
a reticulum intimately associated with the linin network,
which it usually to a considerable extent obscures. Where
the various strands of the network meet, thickenings of the
chromatin sometimes occur, producing densely staining
'bodies (nl) to which the term uucleoli is given, though it is
probable that bodies of a somewhat different composition
are also included under this name ; for there are usually to
be found in the nucleus, imbedded in the substance of the
network, one or more spherical bodies whose chemical re-
actions differ noticeably from those of the chromatin nucleoli,
the substance of which they are composed being termed
paranuclein or pyrenin.

There are then in the cell the following structural con-
stituents :

( membrane (cell-wall),
I. C}rtoplasm : -/ reticulum (plastin),
( caryolymph.

membrane,
reticulum (limn),
II. Caryoplasm : ^ caryolymph,

| chromatin network (nuclein),
nucleoli (nuclein and parauucleiu).

PROTOPLASM AND THE CELL. 7

In addition to these there is, however, still another body to
be mentioned which is especially evident in cells which are
undergoing multiplication, but which has also been found in
various resting cells, especially in lymph-corpuscles, various
kinds of endothelial cells, and in pigment-cells. This is the
structure known as the centrosome (Fig. 1, c). It is usually
an exceedingly small spherical body which does not readily
stain with the reagents which place the chromatiu in evi-
deuce, but has a strong affinity for certain acid aniline stains,
such as safraniu, fuchsiu, or orange. Usually but a single
centrosome is present in each cell, though occasionally two or
even more may occur, and it is situated in the cytoplasm in
the neighborhood of the nucleus, sometimes resting in a
slight concavity on the surface of that structure. Surround-
ing the centrosome there is frequently to be seen, more
especially in dividing cells, a radial arrangement of the cyto-
plasmic reticulum, the centrosome being comparable to a
star from which rays pass out in all directions, whence the
term aster which is applied to the combination of the cen-
trosome and the cytoplasniic rays.

The significance of the centrosome will be seen later when the phe-
nomena of cell-division are under consideration, but its origin may be
inquired into at this place. Two views are current in regard to this
matter, according to one of which the centrosome has its origin in the
nucleus and at a certain period of the cell's existence is extruded from it.
In favor of this view the intimate association of the centrosome and the
nucleus are pointed out, an association which becomes especially pro-
nounced during cell-division, the astral rays connected with the cen-
trosome appearing to penetrate the nucleus and in fact to bring about its
division into two parts. According to the other theory, however, the cen-
trosome is a constituent of the cytoplasm and in its origin has nothing to
do with the nucleus. Quite recently an interesting amplification of this
idea has been suggested to the effect that the centrosome is nothing more
or less than an aggregation of cytoplasm ic microsomes. The astral rays
are cytoplasmic fibres converging from all sides, and since microsomes
occur along their course an aggregation of these bodies might be found
where the fibres meet. This idea cannot be discussed in detail here, but
it may be pointed out that the absence of a centrosome in cells which have
lost their powers of reproduction is readily explicable on this idea, the ag-
gregated microsomes having scattered in such cells, and furthermore
that the theory explains certain peculiar arrangements replacing the
typical aster during the division of some cells.

IN VERTEBRA TE MORPHOL OGT.

Such a combination of cytoplasm and caryoplasm consti-
tutes a morphological element capable of carrying on all the
functions of life. It is not only a morphological but also a
physiological element. It is capable of assimilating the
necessary substances and building up protoplasm ; metabol-
ism and the consequent evolution of energy goes on in it ; it
excretes waste products ; it is contractile and may therefore
be capable of motion ; it responds to stimuli of various kinds,
or in other words it is irritable ; and, finally, it is capable
of reproduction. The question naturally arises, however,
whether this combination of the two substances mentioned
is essential — whether, that is to say, organisms without nuclei
do not exist and manifest all the phenomena of life. At
one time the existence of unicellular organisms destitute of
a nucleus was recognized, the term cytode being applied to
them to distinguish them from nucleated cells. Within re-
cent years, however, a growing skepticism has- come into
existence as to the non-nucleate character of these organ-
isms, the recent improvements of the microscope and the
application of modern staining reagents having revealed the
existence of nuclei in many of the forms at one time regarded
as typical cytodes. It would perhaps be going too far to
state that cytodes do not exist, but the evidence at hand indi-
cates that their existence is highly problematical.

This conclusion is strengthened by the results which have
been obtained from the observations of artificially produced
cytodes. Some of the larger unicellular organisms have
been cut into fragments some of which can be definitely
shown to be destitute of nuclear or caryoplasmatic substance.
In such cases it is found that the nucleated fragments if
placed under proper conditions will regenerate and carry on
their existence as before, while the cytode fragments, though
manifesting signs of life for a considerable length of time,
will not regenerate and do not possess the power of repro-
duction. The nucleus seems to possess a marked regulating
or coordinating action upon the cytoplasm, coordinating the
anabolic and catabolic activities upon which the continuance
of life depends.

It would be beyond the scope of the present work to enter

PROTOPLASM AND THE CELL.

into a discussion of the various forms of physiological activity
of the cell, but one of its physiological functions, reproduc-
tion, must receive special attention in connection with the
remarkable structural changes which accompany it. Since
the disproval of the doctrine of spontaneous generation the
epigrammatic statement Omnis cellula e celluld has been the
watchword of modern histology and embryology, and to-day
it linds its complement in a corresponding epigram, Omnis
nucleus e nucleo. Every cell at present in existence may be
assumed to have descended from some previously existing
cell, and the nucleus it contains to be a portion of the nucleus
of the ancestral cell. New cells arise by the division of
previously existing cells, and each division of the cytoplasm
is accompanied by a division of the nucleus. Not but that
under certain conditions a division of the nucleus may
occur without a corresponding division of the cytoplasm,
niultinucleated cells thus arising, and conversely a division
of the cytoplasm may possibly in certain cases be inaugu-
rated without entailing a division of the caryoplasm ; but, as
might be expected from the relation which exists between
the nucleus and the cytoplasm, the division of the latter
is usually preceded by a division of the caryoplasm.

This latter process may take place in two ways. It may
begin as a simple constriction of the nucleus which, becoming
deeper and deeper, finally separates off a portion of it, a divis-
ion of the cytoplasm in a similar manner then occurring, so
that each of the new cells thus formed contains a portion of
the original nucleus. This method of nuclear division, which
is rather rare, occurs for instance in the embryonic mem-
branes of the Scorpion and is termed direct or amitotic divis-
ion, to distinguish it from the more usual indirect or mitotic
method which is accompanied by a series of complicated
phenomena to which the general term karyoldnesis or mitosis
is applied.

Starting with a t}rpical cell, consisting of the various parts
mentioned above, the karyokinetic phenomena may be re-
garded as affecting two constituents, i.e. the centrosome and
the nuclear chromatiu. The centrosome which lies at one
pole of the nucleus first divides, the two resulting portions

10

INVERTEBRATE MORPHOLOGY.

gradually separating from one another (Fig. 2, A] until they
lie at opposite poles of the nucleus, usually taking up a posi-
tion ninety degrees distant from the point at which the origi-
nal centrosome lay. During this process the radiating fila-
ments which surround the centrosome become especially
distinct and may be divided into two portions, those which
come in contact with the nucleus and which from their
appearance in later stages are termed the spindle-fibres, and

FIG. 2. — DIAGRAM SHOWING THE PHENOMENA OF CELL-DIVISION.

A, separation of the ceutrosomes ; cbromatin iu skein-stage.

B, fully formed spindle ; cbroniatiu loops formed.

C, longitudinal division of the cbroniatiu loops.

I), separation of cbroniatiu loops and commencement of the division of the
cytoplasm.

those which radiate outwards and are lost in the cytoplasmic
network and form the aster. In the meantime, however, im-
portant changes have been taking place Avithin the nucleus.
The chromatin substance, Avhich original!}- Avas scattered in
a reticulum, begins to arrange itself in a baud (Fig. 2, ^1)
which Avith many turns traverses the nuclear substance, the
nucleoli Avhich Avere present at the same time gradually van-
ishing. This stage of the process is termed the skein stage.

PROTOPLASM AND THE CELL. 11

The spindle-fibres of the centrosorue then appear to penetrate
through the nuclear membrane, which sooner or later disap-
pears, and by their growth push the chromatin skein towards
the equator of the nucleus, the skein at the same time break-
ing into a number of fragments, termed chromosomes. The
number of these chromosomes is practically constant for the
cells of any species of animal, and though there is consider-
able variation in different species, yet in the majority of ob-
served cases the number belongs either to the series 2, 4, 8,
16, 32, or to that of 6, 12, 24. They vary considerably in size
in different forms, being in some cases V-shaped or in others
dumbbell-shaped, and arrange themselves finally in a more or
less definite ring surrounding the equator of the nucleus. At
this stage, which is known as the equatorial-plate stage, the
appearance presented in Figure 2, B, is found. At each pole
of the nucleus is a ceutrosonie surrounded by the astral rays
and with the spindle-fibres extending towards and coming
in contact with the chromosomes lying at the equator of the
nucleus, and to "the entire complex the term amphiaster is
sometimes applied.

In the next stage the V-shaped chromosomes, to take this
as a typical shape, which are arranged with the apex of the
V towards the nuclear axis, divide longitudinally. Assuming
that there were originally six chromosomes in the equatorial
plate, as the result of the division there are now twelve ar-
ranged in pairs (Fig. 2, C). One of each pair now proceeds
to move towards one of the poles of the nucleus and the other
to the other, so that eventually near each pole there is a
group of six chromosomes, and between the two groups there
may be seen stretched a number of connecting fibres identical
in appearance with the original spindle-fibres, while in some
cases at the equator of the egg there is to be seen on these
fibres a number of darkly staining dots which may be termed
the intermediate bodies (Fig. 2, D}. At about this stage the
cytoplasm begins to divide, the plane of its division passing
through the equator of the nucleus, and there are thus formed
two cells, each containing a nucleus composed of six chromo-
somes and a centrosome. The chromosomes now begin to
become irregular in shape, they gradually fuse and are finally

12 INVERTEBRATE MORPHOLOGY.

scattered in the form of a chromatic reticulum through the
substance of the nucleus, which thus passes again into the
resting stage, developing a new nuclear membrane.

Our knowledge of many of the details of karyokinesis is yet imperfect,
and especially is this the case with regard to the mode in which the cen-
trosome exerts its influence. It has been regarded as a simple centre of
attraction, similar to the pole of a magnet, but the spindle-fibres seem to
be more than passive in the phenomena. A comparison of the centrosome
•with an aggregation of microsomes has already been referred to, and if
this idea be extended some light may be thrown upon the spindle-fibres.
They would then naturally be regarded as reticular fibres, i.e. fibres of
plastin to which a certain amount of contractility and extensibility may be
ascribed. During the earlier stages of karyokinesis their extensibility is
more manifest, and extending into the nucleus they compress its chromatic
substance, the contractility manifesting itself later and determining the
migration of the chromatin loops or chromosomes towards the poles of the
nucleus. Furthermore, since the linin reticulum of the nucleus is probably
continuous with the plastin reticulum of the cytoplasm, it is conceivable
that the activities of the centrosomes may call out in it changes of contrac-
tion or extension which may suffice to bring about the characteristic skein
formation of the chromatin and the subsequent fragmentation of the skein
into the chromosomes, as well as the formation of the connective fibres in
later stages, the intermediate bodies upon these being regarded as micro-
somes. These views, which have been but recently suggested, require -con-
firmation, however ; if true they afford a new basis from which to attack the
problems involved in the phenomena of karyokinesis, and even at present
throw no little light upon the structural details associated with the process.
It must be mentioned, however, that certain recent observations have been
held to prove that the centrosome has a nuclear origin, and for the present
the important question of its significance must be considered as open.

LITERATURE.

0. Hertwig. Die Zelle und die Oewebe. Jena, 1892.

W. Flemming. Zellsubstanz, Kern und Zetttheilung. Leipzig, 1882.

0. Biitschli. Untersuchungen uber mikroskopische Schdume und das Proto-

plitsma. Leipzig, 1892.

C. Rabl. Ueber Zdltheilung. Morpliolog. Jalirbuch, x. 1884.
G. Platner. Beitrdye zur Kenntniss der Zelle und Hirer Theilung. Archiv fur

mikrosk. Anatomie, XXXIII. 1889.

M. Heidenhain. Uber Kern und Protoplasma. Leipzig, 1892.
H. P. Johnson. Amitosis in the Embryonal Envelopes of the Scorpion. Bulletin

of the Museum of Comp. Zoology, xxn. 1892.

8UBKINGDOM PROTOZOA. 13

CHAPTER II.

SUBKINGDOM PKOTOZOA.

A SIMPLE cell, as has already been stated, possesses the
power of performing all the functions of life, and conse-
quently the existence of unicellular organisms is possible.
Such organisms, together with those which consist of a
number of cells grouped together, each cell, however, retain-
ing to a greater or less extent its own individuality, are
grouped together in a subkiugdom and are collectively
termed PKOTOZOA. In its simplest form a Protozoou may
show but little differentiation of its protoplasm, but in the
majority of cases various portions of the cell-substance take
upon themselves special functions, and in accordance with
this physiological differentiation undergo various structural
modifications. Loconiotor and prehensile structures of vari-
ous forms may be developed, excretory pulsating vacuoles, a
permanent mouth and pharynx, special contractile bands,
and even pigment spots presumably connected with light
absorption may occur, and in addition the power of secreting
horny, calcareous, or siliceous skeletons, serving either as
protective or supportive structures, is frequently present. A
high degree of complexity may therefore occur in a unicel-
lular organism, a complexity produced by a differentiation of
various portions of the protoplasm composing the individual.
For the most part the organisms are simple, but occasionally
they associate together to form colonies. The individuals of
the colonies are as a rule all alike, each carrying on all the
functions of existence for itself, and there is no division of
labor among the various individuals. The complexity which
exists is individual and not colonial. A few forms, however,
such as Volvox, do present a certain amount of colonial differ-
entiation ; all the cells composing the colony are not perfectly
identical physiologically, some becoming, for instance, spe-

14 INVERTEBRATE MORPHOLOGY.

cialized for reproductive purposes, while the rest take but
little part in this process. Such a colony presents indica-
tions of a passage towards a higher grade of individuality,
some of the various cell-individuals merging to a certain ex-
tent their individualities in that of the entire colony, aud
becoming somewhat dependent for existence on the coopera-
tion of their fellows. This dependence, however, never
reaches a high degree of development in the Protozoa and is
for the most part entirely absent. It is in this respect that
colonial Protozoa differ from the higher organisms, but the
difference is one of degree, not of kind.

Four well-marked classes may be distinguished among
the Protozoa :

I. 01. Ilhizopoda.
I.I. 01. Sporozoa.

III. 01. Flagellata.

IV. 01. Infusoria.

I. CLASS RHIZOPODA.

The simplest Rkizopods present an approach to the least
complicated condition under which protoplasm is known to
us. They are simply small masses of protoplasm, more or
less granular towards the centre, clearer towards the periph-

cv

FlG. 3. — Amoeba proteuK (after GRUBER).
cv = contractile vacuole. n = nucleus. ps = pseudopodium.

ery, and continually alter their shape by pushing out lobe-
or thread-like processes known as pseudopodia (Fig. 3, ^>,v).

SUBKINGDOM PUOTOZOA. 15

By throwing out such a process and flowing after it, as it
were, locomotion is performed, which from a well-known
genus of the class is termed amoeboid. Food is simply en-
gulfed by the protoplasm flowing around it, after it has come
in contact with a pseudopodium, and the digestion of the
food-substance takes place within the protoplasm, being thus
iutracellular. Uudigestible material is discarded at any
part of the body ; respiration and excretion are carried on by
the general surface ; and reproduction is limited to the sim-
ple process of division.

It is rare, however, to find such a simple condition as
this ; even among the simpler forms a certain differentiation
of the protoplasm exists, and it is doubtful if it is really
absent in any of the forms known to us. The structural dif-
ferentiations most usually occurring are the nucleus (Fig. 3, n)
and the contractile vacuole (Fig. 3, cv). The former, as was
noticed in the preceding chapter, is of great importance to
the cell, and it is questionable whether it is really absent
even in those Rhizopods in which it has not yet been dis-
covered. It is presumable, of course, that it is a structure
which has gradually become elaborated, that has evolved,
and that in the simplest conceivable organism it may have
been undifferentiated, but whether such an organism now
exists is questionable. The contractile vacuole is excretory
in its function, fluid containing products of metabolism in
solution accumulating at one or more definite regions of the
protoplasm to form it, and being by the sudden and rhythmi-
cal contraction of the surrounding protoplasm periodically
expelled from the body.

Various degrees of complexity are, however, found among
the Rhizopods, the higher forms presenting a considerable
degree of differentiation both in structure and in the modes of
reproduction, and three orders based upon structural charac-
teristics may be distinguished.

1. Order Foraminifera.

The Foramiuifera contains the simpler members of the
class. In the genus Amoeba (Fig. 3) are organisms presenting
the simple characters above alluded to, being simple naked

16

INVERTEBRATE MORPHOLOGY.

FIG. 4. — Arcella mitrata

(after LEIDYJ

masses of protoplasm containing a nucleus and a contractile
vesicle and presenting a slight differentiation into a peripheral
more transparent ectoplasm and a central more granular
endoplasm in which the nucleus is imbedded. The pseudo-
podia are as a rule blunt lobose processes, though in some

species they are more or less fila-
mentous and may even be some-
what permanent. The majority of
forms, however, secrete a protective
shell of varying composition and
complexity. In Arcella (Fig. 4) it
is chitinous and smooth, and len-
ticular in shape, completely sur-
rounding the protoplasm, the pseu-
dopodia projecting from the cir-
cular opening on the flat surface ;
in Euglypka it is similar in composition, but sculptured on
the convex surface ; in Difflugia the shell is flask-shaped and
composed of particles of sand and similar foreign bodies
cemented together, while in a large number of forms, es-
pecially those which are marine in habitat, the shell is
calcareous in composition.

It is in these forms with calcareous shells that the great-
est complexity of structure occurs. In some, such as Gromia,
the shell is simple and flask-shaped, the protoplasm pro-
truding from the mouth of the shell and covering its entire
surface as a delicate layer, from which the long, slender, and
frequently anastomosing pseudopodia take their origin. Al-
though the pseudopodia are practically permanent in form
their protoplasm is continually changing, currents streaming
from the body towards the tips of the pseudopodia and re-
turning again to the central mass, a constant circulation being
thus maintained, and food-particles caught by the delicate
pseudopodia conveyed to the central mass, there to be di-
gested. A simple shell is, however, comparatively rare
among these calcareous forms , more frequently it consists of
several chambers, as in Miliola, the chambers varying in size,
the first-formed one being the smallest, and, in addition, in
very many forms the shell is perforated by minute pores

8UBKINODOM PROTOZOA.

17

through which the pseudopodia are emitted. The successive
chambers are arranged in various ways, sometimes end to end
as in Nodosaria, sometimes alternately on opposite sides of an
axis as in Textularia, sometimes as a spiral as in Globigerina,
sometimes as a helix as in Rotalia (Fig. 5), and sometimes
more or less irregularly as in Acervularia.

• ' /, • •• • '/'I •• ' • ' •

'- •
••• .

FIG. 5. — Rotalia venata (after M. SCHULTZE from HATSCHEK).

Notwithstanding the complexity of the shell, however, the
protoplasm retains throughout the order its simple structure,
and though in the more complicated forms the single nucleus
may be replaced by several, yet beyond this they present no
more marked differentiation than is found in the simpler
genera.

2. Order Heliozoa.

In the second order, the Heliozoa, the pseudopodia are
slender as in the calcareous Foraminifera and are permanent
and somewhat rigid, the central protoplasm of each one
being differentiated into an elastic axial support. The ani-
mals are usually globular in shape, the slender pseudopodia
radiating out from the central mass, an appearance being
thus produced which is sufficient cause for the popular term
"sun-animalcule ".which is applied to several of the genera,
such as Actinophrys and Actinosplirerium (Fig. 6). Currents

18

INVERTEBRATE MORPHOLOGY.

of protoplasm traverse the pseuclopoclia as in the Foramiuifera
and carry the food-particles to the body proper. This has a
delicate ectoplasm and a central endoplasm which is fre-
quently highly vacuolated and contains one or more nuclei

if

cv

ef

cv

FIG. 6. — Actinosph(erium Eiclihornii (after LEIDY).
cv and cv = contractile vacuoles. if= iugested food,

ef= egested food. ps = pseudopodium.

and contractile vacuoles. In some forms also a skeleton is
developed ; it reaches its most perfect form in the stalked
Clathrulina, in which it consists of a delicate fenestrated
siliceous sphere.

3. Order Radiolaria.

The Radiolaria are exclusively marine and are the most
complicated of all the Rhizopods. Their pseudopodia re-
semble closely those of the Heliozoa, being slender and pos-
sessing an axial support. The body varies in shape somewhat
in accordance with the shape of the siliceous shell with which
almost all the forms are provided. In those forms in which
the shell is simplest, as in Thalassicolla (Fig. 7), where it is in
reality absent, the body is spherical and is clearly differen-
tiated into two regions, not, however, corresponding to the
ectoplasm and endoplasm. of other Rhizopods. The centre of

SUBK1NGDOM PROTOZOA. 19

the body is occupied by a spherical mass surrounded by a
firm cbitiuous covering and forming the central capsule. This
contains usually many nuclei as well as vacuoles, oil-globules,
and in some cases crystals and pigment-granules. The wall
of the capsule is probably comparable to the shell of the
Foraminifera, being perforated as in those forms by minute
pores through which the intracapsular protoplasm becomes

\ v

v- V, \ •-.- ili/ijj.///, /• .'

i

- . " :-V'"-K-''^>r:'.i-r' ''.'^&p''- •!•• - '':^ ' •' ".-'"

':-.2!&^*&3*ffJS& i',: ''d »**..• /^v".'.1' ••."' '

-y

1 • ft :. ^^$$l^mM?§ s '

/ X '•' -X " ft • % ' ' -" IN I"' ?^^*?-' S.

r // £ /. ' . r3 •>! -* . • -. --.V \ vx

'/P^IKiiS?

FlG. 7. — Thalassicolld pelagica (after HAECKEL from HATSCHEK).

continuous with the extracapsular. This latter portion on
this supposition, notwithstanding its greater relative thick-
ness, is equivalent to that portion of the protoplasm of the
Foraminifera which is outside the shell and from which the
pseudopodia arise. It is usually richly vacuolated and pig-
uiented, but contains no nuclei ; the axial supports of the
pseudopodia traverse it and take their origin from the inner
layers which immediately surround the central capsule and
are more homogeneous than the outer portions.

The shell is very various in form in the different genera,
reaching a high degree of differentiation in some forms, such
as Heliosphcwa (Fig. 8), where it consists of a fenestrated
globe traversed by radiating spines. Its greatest simplicity
is seen in Sphwrozoum, in which it is represented by scattered

20

IN VERTEBRA TE MORPHOL OGY.

spicules, while in Thalassicolla, already alluded to (Fig. 7), it
is entirely absent. As stated, it is usually siliceous in char-
acter, though in Acanthometra it is composed of a peculiar
horny material termed acauthiu.

Scattered through the protoplasm of the Eadiolarians
there are usually to be seen numbers of small yellowish
bodies long known as the "yellow cells." They are not con-
stant, however, individuals of any species frequently being
destitute of them, a peculiarity due to the "yellow cells"
not being really constituent parts of the Eadiolarian, but

FlG. 8. — Heliosphcera actinota (after HAECKEL from HATSCHEK).

foreign bodies, in fact unicellular plants, for which the term
Zooxanthellce has been proposed. They cannot be consid-
ered parasites, since they do not appear to exist at the ex-
pense of the host, but, on the contrary, their presence seems
actually to be beneficial. Mutual benefits are conferred by
the plant and the Radiolarian, the coexistence constituting
an example of the phenomenon known as Symbiosis.

Reproduction in the Rluzopods. — Throughout all the groups
the simplest form of reproduction, fission, is probably preva-
lent (Fig. 9), though it is not yet definitely known to occur in

SUBKINGDOM PROTOZOA.

21

the marine Foraminifera nor among the Radiolaria. In the
fresh-water Foraminifera and Heliozoa it is, however, the
usual method in genera both with and without shells.
Where the -shell is thin it may be divided during the process,
but where it is thicker the protoplasm divides within it, one
of the new individuals retaining the old shell, while the other
wanders forth and constructs a new house for itself. This is
the case, for instance, in Arcella, in which the wandering indi-

FlG. 9. — Division of Am&ba (after SCHULZE).

vidual protrudes from the mouth of the parent shell until it
forms its new shell, only separating when this is accom-
plished.

Colonies, produced by repeated divisions and the imper-
fect separation of the forms so produced, are occasionally
formed, but they are simply aggregations of similar individ-
uals, no differentiation or iudividualizatiou of the colony as a
whole occurring. Among the fresh-water Rhizopods this is
the case with Microgromia, a shelled form, numerous individ-

22 INVERTEBRATE MORPHOLOGY.

uals of which may remain iu connection with one another by
means of their profusely-branching pseudopodia. Colonies
of Actinophrys are also formed in a similar manner, and among
the Kadiolaria the forms with rudimentary shells — such as
Sphcerozoum, produce, apparently by the division of the cen-
tral capsule, numerous individuals which remain in contact.

A modification of fission known as budding or gemmation
also occurs in some forms. It differs from fission only in
that the products of the division differ in size, so that it is
possible to regard the larger individual as the parent and
the one or more smaller ones formed from it by budding as
the progeny. The process is, however, fundamentally the
same as fission and is a derivative of that process. In Arcella
bud-like processes arise from the periphery of the parent
protoplasm, separate, and assume amoeboid movement leav-
ing the shell in an Amoeba-like condition, and it seems prob-
able that the marine Foramiuifera and certain Heliozoa re-
produce in a similar manner.

Spore-formation also occurs, the parent protoplasm break-
ing up more or less completely into a number of small por-
tions termed spores, which later increase in size and assume
the characters of the parent. This process is sometimes pre-
ceded by encystment, a phenomenon not, however, in its origin
connected with reproduction. It is more prevalent among
fresh-water than among marine forms, and seems to have
been originally developed as a protection from injurious ex-
ternal conditions, such as the drying up of the pools in which
the organisms live. When about to encyst an Amoeba, for
iustauce, withdraws its pseudopodia and assumes a spherical
shape, and then secretes a more or less dense chitinous case
or cyst which completely encloses it. In virtue of the resist-
ent and non-conductive nature of the cyst the organism may,
while in this state, suffer uninjured prolonged exposure to
conditions which would quickly entail the death of the non-
encysted individual, and on the return of favorable condi-
tions may leave the cyst and reassume its active life. Occa-
sionally, too, eucystnient may occur as the result of good
nutrition, an individual which has engulfed a number of
diatoms, for instance, secreting a c}-st around itself within

SUBKINGDOM PROTOZOA.

23

which it remains until the food-matter has been thoroughly
digested, when the cyst is thrown off together with the empty
diatom shells and the animal again becomes active.

Plentiful nutrition and reproduction by division (including
under this term the various modifications of fission) are
related to a certain extent, and it is easy to understand why
the two processes of encystment and spore-formation should
be associated together. The Heliozoan Vampyrella (Fig.
10, A) feeds in its active condition on diatoms, and especially
on a stalked form, Gomphonema. After having digested the
contents of the diatom frustules which it engulfs it pushes

A

FlG. 10.— Vampyrella (from HAECKEL after BUTSCHLI).

A. Vampyrellu feeding upon the stalked diatom Gomphonema.

B. Vampyrella eucysted upon the stalk of the diatom.

them aside and encysts itself upon the stalk previously occu-
pied by them. Within the cyst the animal divides into four
spores (Fig. 10, B}, each of which escaping from the cyst
becomes a new Vampyrella.

Among the Radiolaria spore-formation seems to be the
most usual method of reproduction, and a complication occurs
among them in that spores of two kinds may be formed. In
some cases the spores, which are formed from the intracap-
sular protoplasm, are all equal in size (isospores], while in
others some of the spores may be large (macrospores) and
others small (microspores). Both macrospores and micro-
spores may be formed in the same individual, or each indi-
vidual may produce only one of the two forms. In such cases
it is easy to determine whether one has to do with macro-
spores or isospores, which closely resemble each other in size,

24 INVERTEBRATE MORPHOLOGY.

from the fact that the isospores are spherical in shape and
each possesses a peculiar whetstone-like crystal, wanting in
the macrospores. All the spores are provided with single
whip-like processes, flagella, by which they are propelled
through the water when set free from the parent.

The various processes so far mentioned concern a single
individual only and are therefore non-sexual. Whether sex-
ual reproduction, the union of two individuals (conjugation},
occurs among the Khizopods is uncertain, although the fusion
of two individuals preceding spore-formation has been ob-
served in several instances. That the fusion, however, is the
predisposing cause of the spore-formation seems probable,
but cannot be positively asserted until the behavior of the
nuclei of the two fused individuals is ascertained. It seems
exceedingly probable, also, that the macrospores and micro-
spores of the Eadiolaria are sexual cells, their further de-
velopment depending on the conjugation of a micro- with a
macrospore, but the fate of these spores has not as yet been
ascertained, and their conjugation can only be imagined from
analogy with other forms.

II. CLASS SPOROZOA.

The Sporozoa, which constitute the second class of Proto-
zoa, are all parasitic, living in the cavities, cells, or tissues of
other animals and deriving their nutrition from their hosts.
At present much is lacking to an adequate knowledge of the
various members of the group, but at least three orders are
to be recognized.

1. Order Gregarinida.

The Gregarinida include some of the largest Sporozoa,
and are parasitic either in the body-cavity, intestine, or
organs of various Invertebrata (especially in Annelids and
Tracheata), or in the cells especially of Vertebrated Animals,
these iutracellular parasites being .usually known as the
Coccidia in contradistinction to the former, the Gregariuida
proper. The members of both groups show a marked differ-
entiation of their protoplasm into ectoplasm and eudoplasm,

SUBKIKGLOM PROTOZOA.

FlG-

C?IUS oliy acanthus
(after SCHNEIDER).

a relatively large nucleus lying in tlie latter, and none are

known to possess pseudopodia. Indeed in

many Gregarinida a well-marked cuticle

covers the exterior of the body (Fig. 11),

sometimes distinctly striated or occasionally

tuberculated. The Coccidia and many Gre-

gariuida show little differentiation beyond

what has been mentioned, but the Gregari-

nida which inhabit Tracheate hosts usually

present the appearance of being composed

of two cells, owing to the anterior portion

of the body being separated by a partition

of ectoplasm from the posterior part, and in

addition to this the anterior moiety in some

cases is furnished with hooks, bristles, or

finger-like processes (Fig. 11) of use in fixing
,-, . T , , -i 11 \. , i •, i-i

the animal to the walls ol the cavity in which

it lives. Even in these cases, however, but

a single nucleus is present and the organism is unicellu-

lar.

Reproduction is carried on by spore-formation, preceded
in some cases by conjugation (Fig. 12), but simple division
or gemmation is not known to occur, apparent instances of
division being more probably cases of conjugation. In spore-
formation, preceded or not by conjugation, the animal as-
sumes a spherical shape and forms a cyst about itself, the
greater portion of the protoplasm splitting up into usually
a number of nucleated spores, a small portion of it, how-
ever, remaining undivided (residual body} (Fig. 12). When
mature the spores are usually spindle- or boat-shaped and
have received the name of pseudonavicetlce. They do not,
however, develop directly into Gregariues, but their proto-
plasmic contents break up into 2, 8, or more crescentic
spores (Fig. 12), a residual body being again formed as in
the formation of pseudonavicellae. The further history of
these crescentic spores is not thoroughly known, but in some
cases (Po/'ospora from the intestine of the lobster) each
seems to become converted into an amoaboid structure which
later elongates to an actively moving thread-like organism,

26

IN VERTEBRA TE MORPHOLOG T.

the pseudqfilaria, and this, gradually losing its motility, de-
velops into the adult form.

B

PIG. 12. — REPRODUCTION OP GREGARINE (from

1. Clepsidrinablattarum in conjugation; ck = ectosarc, en = endosarc,c?f =

cuticula, pm = anterior portion, dm = posterior portion, n = nucleus.

2. Cysts in transformation into pseudonavicelke; pn =- pseudouavicelloe;

rk = residual protoplasm .

3. A, a pseudouavicella strongly magnified; B, the same divided into spores,

sk; n = nucleus, rk = residual protoplasm.

2. Order Myxosporidia.

The Myxosporidia are found almost exclusively parasitic
in Fishes, affecting principally the skin, but also occurring
in the internal organs, such as kidneys, spleen, and urinary
bladder. They consist of irregularly-shaped masses of pro-
toplasm, sometimes reaching a length 0.1 mm., but usually
falling considerably short of this size. Frequently they are
enclosed in cysts developed from the tissues of the host, but

SUBKINGDOM PROTOZOA. 27

when not so enclosed seem to possess the power of slow
auiosboid movement. The endoplasni is usually well differ-
entiated from the ectoplasm and contains in the adult condi-
tion a large number of minute nuclei.

Reproduction by division is not known to occur, spore-
formation being the only method as yet observed. In the
Myxosporidium occurring in the urinary bladder of the
Pike the protoplasm breaks up into a number of spherical
masses each containing a number of nuclei. The fate of
all of these masses is not known, but some, containing only
six nuclei, form a wall about themselves and divide into
two portions each of which contains three nuclei. These
triuucleated bodies elongate, develop a wall, and become
pseudonavicella-like spores, one of the three nuclei per-
sisting as the spore-nucleus, while the other two, situated
at the extremities of the spore, seem to give rise to a sac-like
structure containing within its interior a spirally rolled rila-
roent which is emitted when the spore is subjected to press-
ure and probably serves for the fixation of the spore to the
body of a host. The further history of the spores is not
thoroughly known, but it seems probable that the contents
escape as amoeboid masses which develop into adult Myxo-
sporidia.

In many respects the Myxosporidia resemble closely the Gregariuida,
but the possibility of their being in reality not of an animal but of a
plant nature must not be overlooked. By some authors their nearest re-
lations have been found in the Myxomycetous and Chytridiaceous fungi, a
view which certainly has not a little to recommend it.

3. Order Sarcosporidia.

The Sarcosporidia are, with a single exception, parasites
in the muscle-tissue of warm7blooded animals, especially
of Mammalia, being found in the interior of the primitive
fibrils of the striated muscles, whose contents they more or
less destroy.

They form somewhat elongated sacs 1-2 mm. in length,
the wall of the sac being formed of a distinct membrane
which has the appearance of being covered with fine bristles.
The contents of the sac consist of a protoplasmic ground-

IN VER TEBRA TE MORPHOL OGY.

substance in which a large number of nuclei are imbedded,
sometimes aggregated into masses each of which is sur-
rounded by a delicate membrane. It seems probable that
these masses represent a process of spore-formation, but as
yet nothing is known regarding the further development of
the spores.

III. CLASS FLAGELLATA.

The Flagellates are characterized by the possession of
one or more long filamentous processes of protoplasm, known
&sfl,agella, which, by whip-like movements, propel the organ-
isms through the water in which they live, and at the same
time by the production of currents in the water bring food-
particles within their reach. Some forms possess pseudo-
podia in addition to the flagella, which are indeed simply at-
tenuated and mobile pseudopodia, but the majority have a
more or less permanent body-form. This in many species
is accompanied by the formation at the exterior of the body
of a skin or cuticle which in some cases, as in the Dino-
flagellata, may assume a sufficient density and thickness to
entitle it to be termed a shell.

1. Order Autoflagellata.

In the Autoflagellata the body is usually more or less
oval, and while in many forms it is naked and capable of
changing form (Fig. 13, A], yet in others special cuticular in-
vestments may be present, taking the form in some cases of
a simple cuticular covering, as in Euglena (Fig. 13, 12), in
others forming a stalk by which the organism is attached
to a foreign body ; in some forms, as in Codosiga (Fig. 13, 6'),
a cuticular collar surrounding the base of the flagellum is
present, while in others, such as Dinobryon, a cup is formed,
within which the organism lives.

Usually but one or two whip-like flagella are present,
though occasionally a larger number (6 or 8) may occur, and
in some instances one or more may assume a firmer character
and serve for fixation of the organism. All forms possess
a single nucleus and a contractile vacuole. In the simpler

SUBKINGDOM PROTOZOA.

29

forms, such as Monas, in which no cuticle is developed, no
special mouth-orifice is present, though the in-
gestion of food takes place at a more or less
definitely localized region at the base of the
flagellum, the food-particles drawn to the
organism by the currents established by the
flagellum usually impinging at this point ;
where, however, a definite cuticle or shell is
developed a definite mouth occurs, and in
some cases, as Euglena (Fig. 13, B], this leads
into a distinct tubular pharynx projecting some
distance into the interior. No hollow digestive
tract is, however, present, but the food-parti-
cles, after traversing the gullet, are received
directly into the protoplasm of the body, and
are digested there as in Amoeba. A localized
egestive region, situated usually towards the
posterior end of the body, has been ascer-
tained to occur in some species, but in no
instance is it a permanent orifice, as is the
case with the mouth. In addition to the nucleus, contractile
vacuole, and food-particles, other definitely organized particles,
such as starch-like granules and pigment-granules, may be
imbedded in the protoplasm. In Euglena the pigment is
green and resembles plant-chlorophyll, probably too possess-
ing a similar function. A red pigment-spot (stigma) is also
present in this and other genera at the base of the flagellum
and is supposed to be concerned in light-perception.

The typical Flagellate is a free-swimming single organism,
but many forms are fixed, developing a stalk by which they
are fastened to foreign bodies ; the stalk may be very much
branched, each terminal branch supporting an individual, the
whole thus forming a colony, without, however, any differ-
entiation among the individuals. Free-swimming colonies
also exist, such for example as Volvox, in which a large
number of individuals are grouped together to form a spheri-
cal hollow colony. Each individual contains chlorophyll-
grannies and a red stigma, and is provided with two fla-
gella by the action of which the entire colony is propelled

0

FIG. 13.
A, Oikomonas
(after BUTSCHLI).
B, Euglena acus.
0, Codosiya
(after BUTSCHLI).

30 INVERTEBRATE MORPHOLOGY.

through the water with a rotatory motion. The rotation is
around a definite axis, one portion of the spherical colony
always being in front in progression, and it is noticeable that
the stigmata of the individuals of this anterior hemisphere
are slightly larger than those of the cells of the posterior
hemisphere, a slight differentiation of the individuals being
thus present.

2. Order Linoflagellata.

The Diuoflagellata are distinguished from the members of
the preceding order by the almost general occurrence of a
rather dense shell composed of plates of a substance resem-
bling closely vegetable cellulose. Some of the forms, such as

Ceratium (Fig. 14), present a rather
bizarre shape on account of the shell
being prolonged into horns, and in the
majority the shell-plates are delicately
sculptured, while around the equator
of the shell runs a furrow, and from
an opening in the line of the furrow
two flagella protrude, one of which
possesses the ordinary whip-like char-
acter, while the other lies in the fur-
row and in some cases has the form
FIG. 14.— Ceratium tripos of a delicate undulating baud. Chlo-
(a combination of two figures rOphyll-Hke pigment is almost invari-

byBuTSCHLi). l J ' .

ably present, as is also the red stigma.

Peculiar cysts are also present in the protoplasm of many
forms, consisting of a hollow capsule having rolled up within
it a hollow thread, which on occasion may be rapidly evagi-
nated and no doubt has a protective function, resembling very
closely in its structure the neniatocysts of the Coalenterates.

3. Order Cystoflagellata.

The order of the Cystoflagellata includes only two genera,
Noctiluca and Leptodiscus. The latter is a somewhat disk-like
structure nearly 2 mm. in diameter, while Noctiluca (Fig. 15)
is almost globular with a slight depression at one point where

SUBKINGDOM PROTOZOA. 31

the flagella are situated, and at the botton of which is situated
the mouth-opening. Noctiluca has the form of a cyst, pos-
sessing an external thin membrane-
like outer wall, to which branching
strands of protoplasm extend from
the central mass containing the
nucleus and lying slightly below the
depression which contains the fla-
gella. These are two in number,
one being short and whip-like, while
the other, usually known as the
" tentacle " (Fig. 15, t), is a highly
contractile, somewhat flattened, and, FlG- ^- -Noctiluca

(after BUTSCHLI).

relatively to the flagellum, thick pro- t = tentacle n = nucleus.
cess of the internal protoplasm.

This structure is unrepresented in Leptodiscus, which other-
wise closely resembles Noctiluca.

Noctiluca is of considerable physiological interest, since it is one of the
forms to which the phosphorescence of the ocean is due. The cause of the
light and its character are, however, as yet unknown.

Reproduction in the Flagellata. — The most frequent method
of reproduction in all the orders of the Flagellates is simple
division, either transverse or longitudinal. Encystment, fol-
lowed or not as the case may be by spore-formation, is also
common, and when accompanied by spore-formation may be
preceded by the conjugation and fusion of two individuals,
la Cercomonas the spores are exceedingly abundant and small,
presenting the appearance of minute granules even under the
highest powers of the microscope, but in other forms, as
Chlamydomonas, the spores are larger and much fewer in num-
ber, being only 4 or 8 in this particular case. An interesting
modification occurs in closely-related species (Fig. 16), some
individuals of which divide into a number of small spores
(microspores), while others undergo a more restricted division
and give rise to a few large spores (macrospores). The latter
develop directly into the adult forms, but the microspores
show a tendency to conjugate in pairs before undergoing
further development. This differentiation of two kinds of
spores is carried still farther in other forms where neither

32

INVERTEBRATE MORPHOLOGY.

macro- nor microspores develop directly but further develop-
ment is contingent upon the conjugation of a micro- with a
macrospore.

In this respect considerable interest attaches to Volvox ;
certain cells, usually those situated in the posterior hemi-
sphere, enlarge and project into the interior cavity, dividing

FIG. 16. — 1. Phacotus lenticularis ; 2. MACROSPORES AND MICROSPORES OP

THE. SAME SPECIES (after BUTSCHLI).
sch = shell. n = nucleus.

when they have reached their full growth into a number of
cells which arrange themselves in a hollow sphere forming
daughter colonies in the interior of the parent. In addition
to this a sexual process occurs ushered in by certain indi-
viduals gradually enlarging, and leaving their position at the
surface of the colony. In the interior some of them continue
to enlarge, forming ova (niacrospores), Avhile others divide
frequently, forming packets of elongated cells furnished with
flagella; these may be termed spermatozoa (microspores). The
ova develop into colonies similar to the parent after conjuga-
tion with spermatozoa. Since many of the cells of the parent
colony do not participate in this reproductive act, but disin-
tegrate and die on the development of the daughter colonies,
it is clear that we have in this form a rather marked differ-
entiation of the individuals of the colony, the individualities
of the constituent cells being to a slight extent merged in the
individuality of the colony.

In Noctilnca in addition to simple division a process of reproduction
occurs which partakes of the character of budding. It is apparently pro-

SUBKINGDOM PROTOZOA. 33

ceded by the conjugation of two individuals, the combined central proto-
plasms coming to the surface of the cyst where they form a protuberance.
Repeated division of the nucleus into 2, 4, 8, etc., up to 256 or more now
takes place accompanied by only a partial division of the protoplasm, so
that the surface of the protuberance is covered by a large number of bud-
like structures. Eventually these separate, develop a flagellum, and take
on the character of motile spores. Their further development into the
adult Noctiluca has, however, not yet been followed.

IV. CLASS INFUSORIA.

The Infusoria are tlie most highly specialized of all the
Protozoa, showing a differentiation of the protoplasm unat-
tained by other members of the group. They are character-
ized by the possession during the whole or part of their lives
of numerous delicate short motile hair-like processes termed
cilia by means of which locomotion is performed and food
procured. In one of the orders into which the class may be
divided, the CILIATA, these structures are present during the
adult life of the organisms, while in the other, the SUCTOEIA,
though present in the young stages they are replaced later by
immovable processes of the body, which extract the nourish-
ment from the food-particles which come into contact with
them.

1. Order Ciliata.

The Ciliata are for the most part free-swimming organ-
isms, though some, e.g. Vorti-cella (Fig. 17, C), adhere to foreign
bodies by means of a stalk, similar to that found in Flagel-
lates, and colonial stalked forms also occur as in that class.
In these stalked forms the body is enveloped in a chitinous
case, of which the stalk is a prolongation, the surface oppo-
site the stalk being, however, left naked and being surrounded
by cilia which are absent on the portions of the body pro-
tected by the chitiu (Peritrichous forms, Fig. 17, C). In the
free-swimming forms, however, the cilia are more universally
distributed, covering either the entire surface (Holotrichous
forms, Fig. 17, A) or else one surface of the flattened body,
some of them in this case being modified into stout movable

34

IN VERTEBRA TE MOKPHOLOG T.

bristles upon which the animal creeps (Hypotrichous forms,
Fig. 17, D}.

A definitely localized mouth-opening is always present,
situated frequently at the extremity of a peristomial groove
and leading into a gullet of variable extent, usually lined by
cilia, though sometimes furnished with a chitiuous support

CF.-

FIG. 17.— A, Paramcecium ; B, Stentor ; C, Vorticella ; D, Euplotes.

cv = contractile vacuole. n -- nucleus.

m = mouth. n' — micron ucleus.

my = myopkatie. tr = trichocyst.

(Chilodon). There is, however, no special digestive tract, the
food-particles after traversing the gullet being received into
the body-protoplasm, where they are digested. Usually there
is a localized cgestive region, and in a few cases there is a
definite anal opening. The food is procured as in the Flag-
ellates by the currents set up in the water by the cilia carry-

SUBKINGDOM PROTOZOA. 35

ing minute organisms to the neighborhood of the mouth, the
cilia surrounding this opening directing them to the gullet.

The body-protoplasm is usually very granular in its cen-
tral part, and tilled with food-vacuoles and products of diges-
tion. Pigment-granules are sometimes present and may con-
sist of Chlorophyll, as in Stentor, and one or more excretory
contractile vacuoles are always present. The nucleus is usu-
ally single, though occasionally two are present, and in the
genus Opalina, which occurs in the intestine of the Frog, they
are numerous in the adult condition. When single the nu-
cleus may be very large and either spherical, elongated,
horseshoe-shaped as in Vorticella (Fig. 17, c), moniliform as in
Stentor (Fig. 17, B), or otherwise shaped. In addition to the
nucleus there are one or two minute structures usually to be
found in its vicinity which play an important part in repro-
duction and are known as micronuclei (Fig. 17, A, n'}. Other
differentiations of the protoplasm are also found in certain
forms, as, for instance, special bauds differentiated so as to be
specially contractile and therefore corresponding in function
to the muscles of the higher animals, and hence termed myo-
plianes. In Vorticella a more striking differentiation of spe-
cially contractile protoplasm occurs (Fig. 17, C, my] ; running
in an open spiral through the centre of the supporting stalk
of this organism is a strong myophane terminating above in
the protoplasm of the animal. When the latter is stimulated
the myophane contracts, coiling the stalk into a close spiral
and withdrawing the animal from the source of irritation. In
some of the Holotricha, such as Paramcecium, numerous mi-
nute rod-like structures occur imbedded in the protoplasm
near the surface of the body (Fig. 17, A, tr). They are appar-
ently defensive in function, since when stimulated they sud-
denly, as if by an explosive action, become transformed into
long threads or needle-like structures projecting beyond the
cilia. These trichocysts also occur in some Flagellates.

2. Order Suctoria.

The Suctoria lack the active movements of the Ciliata,
being destitute in the adult stage of cilia, and many of the

36

INVERTEBRA TE MORPHOL OGY.

FIG. IS.—Acineta
grandis (after
SAVILLE-KENT).

forms, e.g. Acineta (Fig. 18), are attached to foreign bodies by
a stalk. They do not possess any mouth, but a number of
simple or branched stiff processes project
from the body which serve for the prehension
and digestion of the organisms upon which
they feed. A contractile vacuole, nucleus, and
micronucleus are always present, the nucleus
having sometimes a very complicated shape.
It seems pretty clear that they have been de-
rived from the Ciliata, since in their young
stages they are free-swimming ciliated struc-
tures ; the tentacular processes have been
compared to the pseudopodia of the Rhizo-
pods, but good reasons for such an homology
do not exist, and it is more probable that they
are structures peculiar to the group.

The Reproduction of the Infusoria. — In the
Infusoria the reproductive processes reach a
much higher grade of complication than occurs
in other Protozoa, though the simple pro-
cesses of fission and spore-formation likewise occur. The
former occurs in the majority of forms, and may be the only
mode of reproduction occurring- throughout a number of gen-
erations. Long-continued fission seems, however, to lead in
many cases to structural and physiological derangements,
unless the process of conjugation be interposed.

Eucystment is also of frequent occurrence and may occur
under various conditions. In Colpoda, in which the process
has been most thoroughly studied, encystment may or may
not be followed by reproduction. In the latter case the cyst,
a resting cyst, is perfectly closed, and the walls are thick and
resistent so as to withstand unfavorable conditions, such as
insufficient aeration or dryuess. When reproduction is asso-
ciated with encystment it may be either fission or spore-forma-
tion. The division cyst is thin- walled and is not completely
closed, and within it the animal undergoes division into two
or four parts. In spore-formation a thin cyst is first formed,
within which the animal slowly rotates, at the same time
gradually growing smaller by the expulsion of fluid. Finally

SUBKINGDOM PROTOZOA. 37

it contracts to a round mass and surrounds itself with a sec-
ond cyst within the first. At the surface of the encysted
animal from eight to thirty minute spherical and highly re-
fractive bodies appear which are the spores, and by the
bursting of the cyst they, with the remains of the protoplasm
in which they arose, escape to the exterior and soon begin to
develop. Losing its spherical shape each spore becomes
amoeboid ; then, drawing in all the pseudopodia but one,
which elongates and becomes a flagellum, it passes from the
Rhizopod to the Flagellate stage ; and finally the flagellum is
withdrawn, cilia appear, and the animal gradually assumes
the adult form. Spore-development somewhat similar to this
has been observed also in Vorticella, and special interest at-
taches to it as probably indicating the line of descent of the
Infusoria.

Conjugation is a frequent process among the Infusoria,
where it seems to have a rejuvenating rather than a strictly
reproductive function. If prevented, and fission goes on
through a number of generations, marked degeneration en-
sues ; while if it be allowed, the same number of generations
may be produced without any signs of degeneration. The
process consists of a renewal of the nuclei and micronuclei of
the conjugating forms, and the process as it occurs in Colpid-
ium colpoda may be described thus. Two individuals come
into contact by the anterior portions of their body, actual
fusion of the two protoplasms taking place at the point of
contact. The micronucleus in each individual then enlarges
and divides, the two thus formed subsequently dividing again,
so that each of the conjugating individuals contains four
micronuclei and one nucleus. One of the four micronuclei
iu each individual now divides, and one of the two thus
formed (the male pronucleus) crosses over to the other indi-
vidual and unites with the other product of the division, the
female pronucleus, there being thus a mutual interchange of
micronuclei. The individuals now separate and resume their
independent existences, and a rearrangement of the nuclear
structures accompanied by fission takes place. The three
micronuclei which did not take part in the formation of the
pronuclei of conjugation degenerate, as does also the original

38 INVERTEBRATE MORPHOLOGY.

nucleus. The conjugation micronucleus, formed by the fusion
of the male and female pronuclei, divides twice, forming four
microuuclei, and this is followed by a fission of the entire
Infusorian, each of the daughter forms so produced possessing
two inicronuclei. One of these, enlarging, becomes the new
nucleus, while the other remains as the microiiucleus. This
complicated process may perhaps be better followed in the
accompanying diagram (Fig. 19).

OV Vo

FIG. 19. — DIAGRAM TO ILLUSTRATE THE BEHAVIOR OF THE NUCLEI AND

MlCRONUCLEI DURING CONJUGATION IN INFUSORIA (after MAUPAS).

In the majority of forms the conjugation is a temporary
process, the two individuals separating after the exchange of
pronuclei. In Vorticello,, however, a permanent fusion occurs.
By repeated longitudinal fission a Vorticella becomes divided
into a number of small individuals which leave their stalks
and swim about freely in the water. Should one of them
come into contact with a large individual a complete and
permanent fusion of the small with the large one occurs.

SUBKINGDOM PROTOZOA.

I. Class RHIZOPODA. — Protozoa with lobe-like or filamentous psetidopodia.
1. Order Foraminifera. — Pseudopodia without axial support; shell
when present horny or calcareous,
(a) Shell absent. Amoeba.
(6) Shell horny. Arcella, Euglypha.

(c) Shell of foreign particles cemented together. Difflugia. '

(d) Shell calcareous, imperforate. Gromia.

(e) Shell calcareous, perforate. Miliola, Nodosaria, Textn-

laria, Qlobiyerina, Rotida,

SUBK1NGDOM PROTOZOA. M

2. Order Heliozoa. — Pseudopodia slender, with axial support; shell if

present siliceous ; no central capsule.

(a) Shell wanting. Actinophrys, Actinosphcerium, Vampy-

rella, Microgromia.

(b) Shell present. Clatlirulina.

3. Order Radiolaria. — Pseudopodia slender with axial support; shell

usually present and siliceous (rarely horny) ; central capsule
present.

(a) Sliell wanting. TTialassicotta, Sphcerozoon.

(b) Shell siliceous. Actinomma, Heliosphcera.

(c) Shell horny. Acanthometra.

II. Class SPOROZOA. — Parasitic ; without pseudopodia, flagella or cilia.

1. Order G-regarinida. — Parasitic in cavities of the body especially of

Invertebrates or in the cells especially of Vertebrates.

2. Order Myxosporidia. — Parasitic usually in the skin, sometimes in

internal organs of fishes.

3. Order Sarcosporidia. — Parasitic in the muscle-fibres of Mammalia.

III. Class FLAGELLATA. Provided with one or more flagella.

1. .Order Autoflagellata. — Without shell, protoplasm not especially

vacuolated.

(a) Without collar. — Monas, Cercomonas, Chlamydomonas,
Euglena, Volvox.

(b) With collar. — Codosiga, Dindbryon.

2. Order DinoflageUata. — With shell composed of cellulose. Cera-

tium.

3. Order Cystoflagellata. — Without shell, protoplasm highly vacuo-

lated, marine. Noetiluea, Leptodiscus.

IV. Class INFUSORIA. — Provided with cilia or immovable processes.

1. Order Ciliata. — Provided with cilia in adult stage.

(a) Cilia of nearly uniform length all over the body (Holo-
tricha). Paramcetium, Colpoda, Colpidium, Chilodon,
Opalina,

(b) Cilia around anterior end of body longer than the rest

(Heterotriclia) . Stentor.

(c) Cilia limited to anterior end of body ( Peritrichd) . Vorti-

cella.

(d) Cilia or setae only on ventral surface of the body (Hypo-
tricha). Stylonychia.

2. Order Suctoria. — With cilia only in the young stages, in the adult

with immovable processes. Podophrya, Acineta.

LITERATURE.

0. Biitschli. Protozoa. Bronn's Klassen u. Ordnungen des Thierreichs. Leip-
zig u. Heidelberg, 1883-87.
W. S. Kent. A Manual of the Infusoria. London, 1880-82.

40 INVERTEBRATE MORPHOLOGY.

J. Leidy. Fresh-water Rhizopods of North America. U. S. Geological Sur-
vey of the Territories, xii. 1879.

E. Hertwig. Bemerkungen zur Organisation u/id systematischen Stellung der
Foraminiferen. Jenaiscbe Zeitschr. , x. 1876.

E. Haeckel. Die Radiolarien. Mne Monographie. Berlin, 1862-88.

K. Brandt. Die Kolonie-bildendeti Radiolarien (Sphanrozoeti) des Golfes von
Neapel. Fauna u. Flora des Golfes von Neapel. Monographie, xin. 1885.

F. Stein. Der Organismus der Infusionsthiere. Leipzig, 1859-79.

E. Maupas. La rajeunissement karyogamique chez les Cities. Archives de Zool.

experimeutale, 2me Ser. vn. 1889.
L. Ehumbler. Die verschiedenen Cystenbildungen und die Entwicklungsge-

schichte der holotrichen Infusoriengattung Colpoda. Zeitschr. fiir wis-

sensch. Zoologie, XLVI. 1888.
R. Hertwig. Ueber Podophrya gemmipara, etc. Morpholog. Jahrbuch, i.

1876.

SUBK1NGDOM METAZOA. 41

OHAPTEE III.

SUBKINGDOM METAZOA.

THE Metazoa are equivalent to colonies of Protozoa, the
individual cells of which have differentiated in various direc-
tions, some being more especially contractile, others nutritive,
others irritable, others reproductive, etc., instead of each one
for itself performing equally all the functions necessary for
existence. A physiological division of labor of a more or less
perfect kind is introduced among the individuals composing
the colony, and the welfare of each individual becomes de-
pendent upon the proper performance by its colleagues of
their special functions ; in short, the individualities of the
component cells are merged in the higher individuality of
the whole organism.

Physiologically a Metazoon is equivalent to a Protozoon,
but morphologically it is the equivalent of a large number of
them. Each is physiologically an individual, but morpholog-
ically the Metazoon is a colony of Protozoan individuals. To
harmonize the physiological and morphological conceptions
of an individual it is necessary to recognize several grades
of morphological individuality of which the cell may be as-
sumed to be the lowest. In the Metazoa the physiological
differentiations of the cell-individuals are accompanied by
structural differentiations, so that it is possible, as a rule, to
determine from its structure what the function of a cell may
be ; aggregates of similar cells are termed tissues or tissue-indi-
viduals, and as the simplest Metazoa are complexes of various
tissues, such a complex forms the third grade of individu-
ality and may be termed an Organ-individual. A complex of
organ-individuals united to form a physiological unit consti-
tutes an individual of the third grade, the Metamere-individual,
while the fourth grade, the Cormus, is formed by a similar
union of a number of metameres, as, for instance, in the
Earthworm, each joint or segment of which is a rnetamere.

42 INVERTEBRATE MORPHOLOGY.

It has been pointed out that the Flagellate Volvox presents
a tendency towards a higher individuality, being somewhat
higher than a mere colony of cell-individuals and yet not
quite reaching the dignity of an organ -individual ; similarly
intermediate conditions between the other grades may occur.
In certain worms, for instance, considerable independence of
the constituent metameres exists, any one of them, when de-
tached, being capable of carrying on an independent exist-
ence, and of developing into an organism similar to that of
which it was originally a part. In the Earthworm the depend-
ence of the various segments or metameres upon one another
is greater than this, but in it, too, a certain amount of inde-
pendence is shown by the power it possesses of regenerating
lost metameres. In other cormi, as, for instance, in the
Lobster, the interdependence of the component metameres
proceeds still farther, and a differentiation of the various meta-
meres occurs, a process carried to its greatest extent in the
higher Vertebrates. A physiological division of labor among
the metameres develops, some of them losing, for instance,
their excretory organs, while in others these organs lose their
excretory functions and serve as ducts by which the repro-
ductive elements may pass to the exterior. The subordina-
tion of the metameres proceeds most rapidly and is most
complete at the anterior extremity of the organism, leading
to the formation of a head bearing highly developed sense-
organs and containing a complex nervous system, which rep-
resents originally distinct metamere nervous systems, now
fused and destitute of all independence.

Sexual Reproduction in the Metazoa. — In cell-individuals it
has been seen that fission is the most frequent and simplest
mode of reproduction ; in the Metazoa this method and its
modification, budding, also occurs, but, as a rule, only in
forms of a low grade of individuality or in a transition stage
between a lower and a higher grade. In organ-individuals
it is of frequent occurrence, the imperfect separation of the
individuals so produced leading, in many cases, to the forma-
tion of colonies, and in cormi in which the integration of the
constituent metameres is but slight it also occurs.

In the Protozoa cell-division naturally entails reproduc-

SUBKINODOM METAZOA. 43

tion, but in organ-individuals reproduction of the constitu-
ent cell-individuals is not necessarily connected with the
reproduction of the entire individual, but may simply increase
the number of lower-grade individuals of which it is com-
posed. Similarly multiplication of the organ-individuals of a
metainere, or of the metamere-individuals of a cormus may
occur without producing reproduction of the whole ; it is
simply growth. From growth to reproduction by budding
the path is short, and various intermediate stages connecting
the two processes can be found. Hence reproduction has been
aptly denned as " discontinuous growth," though perhaps it
would be even more apt to define growth as reproduction with-
out discontinuity, growth in a Metazoon depending on the
reproduction of the lower-grade individuals of which it is
composed.

It is possible to carry this idea still farther back and refer the growth
of a cell to the reproduction of the constituent elements, plasomes, of which,
it may be imagined, it is composed. In the simplest cells the various
forms of plasomes are distributed throughout the cell, but in the higher
Protozoa, for instance, an aggregation of similar plasomes occurs, giving
rise to such structures as the myophanes. In a similar manner in the
lower Metazoa, although a division of labor and structural differentiation
has taken place among the constituent cells, yet the cells possessing similar
functions, as, for instance, the nerve-cells, are more or less irregularly
scattered throughout the body, only becoming aggregated in the higher
forms into distinct tissues, and giving rise to the most perfect type of an
organ-individual. Likewise in a metamere-individual a multiplication of
the organs leads to a transition form with discretely arranged parts, the
definite aggregation of which produces a cormus, composed in the simpler
forms of distinct metameres, which become more and more integrated and
subordinated to the individuality of the cormus in higher types of that
grade of individual.

According to this view the segmentation or metamerism of the higher
Metazoa is the result of the multiplication and subsequent integration of
the organ-individuals of an ancestral metamere-individual, and explains
the occurrence of imperfect metamerism in certain forms of that grade of
individuality (Turbellaria). Some authors have considered metamerism to
have arisen by the reproduction by budding of an ancestral metamere, an
idea which fails to explain satisfactorily the condition just referred to.
The view presented here considers metamerism to be the result of growth.
It has not arisen by the reproduction of the metamere, but by that of its
organs, just as a typical organ-individual has arisen by the reproduction

and integration of its constituent cell-individuals.

44 INVERTEBRATE MORPHOLOGY.

As a mode of reproduction in the Metazoa division plays
but a secondary part, the sexual process being the character-
istic method. Attention has already been called to the par-
tial specialization in Volvox of reproductive cells which serve
to perpetuate the species, the remaining cells of the colony
perishing. This condition is a premonition of the more per-
fect specialization found in the Metazoa of reproductive or germ
cells and non-reproductive or somatic cells, the latter serving
for the nutrition and protection of the germ-cells, to which
the perpetuation of the species is entrusted. Comparatively
early in the development of an individual certain cells differ-
entiate from the others, not undergoing like them a physi-
ological and structural specialization, but retaining a general-
ized character. These are the germ-cells usually grouped
together to form the reproductive organs.

In describing the methods of reproduction occurring in the
Flagellata, the manner of the development of sexual repro-
duction was indicated. It appears to have been originally a
more or less accidental fusion of two similar cells or spores,
and from being accidental this fusion gradually became the
rule on account of the greater vitality which the conjugate in-
dividual possessed over cells which did not conjugate. The
next step was the differentiation of rnicrospores and macro-
spores, which reaches a high development in Volvox, where
it is associated also with a differentiation into somatic and
germ cells. In the Metazoa both these differentiations are
carried to a higher degree, the niacrospores being known
as ova and the microspores as spermatozoa, while the aggre-
gates of these cells are termed respectively ovaries and testes.

In a young embryo a mass of germ-cells which is to give
rise to spermatozoa cannot be distinguished from one which
is destined to be converted into ova. Fundamentally both
are the same, and occasionally a portion of a mass of germ-
cells may be differentiated into ova, while the rest of it devel-
ops into spermatozoa. This has not unfrequeutly been seen
in fishes in which there is normally a separation of the sexual
elements in distinct individuals, and throws considerable
light upon the occurrence of forms which normally possess
both elements. This condition of hermaphroditism, which oc-

SUBKINODOM METAZOA.

45

g

curs in many parasitic forms and in certain sponges, Flat-
worms, Mollusks, and Crustacea, seems to have been second-
arily acquired. It is probable that the ancestral Metazoa
were unisexual, possessing reproductive elements of only one
kind, a supposition borne out by the frequent association of
hermaphroditism with a parasitic or sessile mode of life, such
conditions being what may be termed abnormal, and usually
accompanied by marked structural characters which are to
be regarded as secondary modifications. On the other hand,
it is noticeable that the lowest free Metazoa (such as the free-
swimming Cnidaria) are unisexual.

An ovum is a single cell, and in its typical form consists
of a mass of protoplasm containing a nucleus, and may or
may not be surrounded by a membrane.
Seldom, however, does such a simple ovum
occur ; usually more or less yolk, consisting
of fatty and albuminous globules, is distrib-
uted throughout the protoplasm, and fre-
quently the amount of yolk far overbalances
the amount of protoplasm. Other structures,
such as albumen and one or more enveloping
membranes, may be added, the ova of different
species differing greatly in this respect.
Among the lower forms the ova are usually
extruded freely from the body of the parent,
but in many of the higher Metazoa they are
enclosed within protective cases (cocoons), as
in the Earthworm, or imbedded in jelly-like
masses, as in the common Pond-snails.

In the ovary of a young individual all the FIG.SU.-OVARIAL
germ-cells are alike, and all are potentially TUBE OF A BEE-
reproductive cells; very frequently, however,
many of the primitive germ-cells relinquish _ _ germjnai re.
their reproductive function and serve as pur- o = ova. [gion.
veyors of nutrition to certain of their com- o'— mature ovum.

rades which enlarge and become mature ova. ^ ~

. . /= follicle-cells.

This is well seen in insects, in which each ovary

(Fig. 20) consists of a number of tubes tapering to a point at

one end, while at the other they open into a common duct,

46

INVERTEBRA TE MORPHOL 0 G T.

the oviduct, leading to the exterior. At the tip of each tube
the primitive germ-cells (Fig. 20, g] are located, and lower
down ova (o) in various stages of development towards matu-
rity are to be found, each surrounded by a number of small
undeveloped germ-cells, known as follicle-cells (/), whose func-
tion it is to transfer food-yolk (y) to the growing ovum. As
the latter approaches maturity the follicle-cells secrete around
it a thick, sometimes highly sculptured shell and finally
degenerate.

As a rule, conjugation with a spermatozoon, i.e. fertiliza-
tion, is necessary as an antecedent to further development.
Before this takes place, however, certain modifications of the
ovum are necessary, the phenomena which accompany them
being known as the maturation of the ovum. In this process

A B

FIG. 21.— DIAGRAMS ILLUSTRATING THE MATURATION OF THE OVUM.

A = formation of the first polar globule (pg).

B = formation of the second polar globule and entrance of the sperm-nucleus
(sp).

(Fig. 21, A) the nucleus approaches the surface of the ovum
and there undergoes a karyokinetic division which is peculiar
in that in the equatorial-plate stage twice as many chromo-
somes are formed as are typical for the species. These do not
undergo longitudinal division, and by the karyokinesis their
number is reduced to the typical number, a small cell, the
polar globule (pg), being separated from the ovum with half
the chromosomes, while the others are retained within the
ovum. The nucleus of the ovum, instead of now returning
to the resting stage, divides again (Fig. 21, B\ a second polar
globule being formed and receiving half the chromosomes

SUBKINGDOM METAZOA. 47

which remain, so that the nucleus of the ovum now possesses
only half the number of chromosomes which are character-
istic for the species. At the time of the formation of the
second polar globule the first frequently divides without its
nucleus passing into a resting stage, so that as the result of
this maturation process four cells have been formed, three of
which are small, while the third is relatively very large and
will alone undergo further development. When these divi-
sions have been completed and the chromosomes have been
reduced to one-half their proper number the nucleus of the
ovum passes into the resting stage, migrates back towards
the centre of the ovum, and is ready for conjugation with the
nucleus of a spermatozoon.

The spermatozoa are always much smaller than the ova,
and are, as a rule, capable of active motion, though in certain
Crustacea, for instance, they lack this power. The ova and
spermatozoa have specialized in opposite directions in this
respect. The ova of the Metazoa are specialized as the
nutritive cells of conjugation, possessing abundant protoplasm
and usually a considerable amount of yolk for the nutrition
of the young embryo. They consequently have lost their
motility, and in order that conjugation may be made prob-
able the spermatozoa lack all unnecessary material which
would interfere with their motility, no yolk being stored up
and the protoplasm even being reduced to the smallest
amount consistent with the development of a locomotor
organ. The nuclei, as will be seen later, are essential ele-
ments in conjugation, and the spermatozoa are to all intents
locomotor nuclei, the ova supplying the protoplasmic nidus
necessary for the growth and division of the nucleus formed
by conjugation.

In their typical form spermatozoa are composed of a
globular or pyriform head consisting of a nucleus' surrounded
by a small amount of protoplasm, and a long filamentous tail
continuous with the protoplasm and frequently provided with
a delicate fringe-like membrane (Fig. 22, F). By the rapid
whipping movements of the tail the organism is propelled
through the water, or other fluid in which it may find itself,
and so may come into contact with an ovum.

48

INVERTEBRATE MORPHOLOGY.

The transformation of the germ-cells present in an era
bryo into spermatozoa is usually a somewhat complicated
process. In the Round-worm Ascaris, in which it retains
somewhat primitive characters, the process closely resem-
bles what takes place during the maturation of the ovum.

PlG. 22. — DlAGKAMS TO ILLUSTRATE THE MATURATION OF THE SPEKM-CELL-

A = division of the spermogone.
B — division of the two spermocytes.
C = the four spermatids.

D, E — conversion of a sperinatid into a spermatozoon.
F — fully developed spermatozoon.

The embryonic germ-cells (spermatogones, Fig. 22, A) undergo
karyokinetic division, the number of chromosomes being, as
in the ovum in the division which results in the formation of
the first polar globule, twice that which is characteristic for
the species. They do not undergo longitudinal division, and
one half of them passes into one of the daughter cells (sper-
matocytes) and the other half into the other, so that these two
cells possess the number of chromosomes characteristic for
the species. A division of these daughter cells (Fig. 22, B)
immediately takes place without a return to the resting stage,
and unaccompanied by a longitudinal division of the chromo-
somes, so that four cells (spermatids, Fig. 22, 6') are formed,
each of which contains only half the typical number of chro-
mosomes, and each one of these cells becomes a spermato-
zoon. This process is comparable step by step with the

SUBKJNGDOM METAZOA. ±9

maturation of the ovum and seems to indicate tliat the polar
globules are to be regarded as abortive ova.

The conversion of the spermatids into spermatozoa is
simply a differentiation of structures already present. In
the air-breathing Mollusca, for instance, the spermatids consist
of a mass of cytoplasm containing a nucleus, in close proxim-
ity to which may be found the centrosome, while an irregular
mass of filaments represents the remains of the spindle-fila-
ments. In the differentiation which follows (Fig. 22, D, E,
and F) the nucleus elongates and its chromatin-filaments
fuse to form a homogeneous mass ; the cytoplasm likewise
elongates, and in it appears an axial filament which later will
form the tail-filament. The origin of this filament is doubt-
ful, some authors maintaining that it is a differentiation of
the cytoplasm, while others believe it to be a prolongation of
the nuclear substance ; but, however that may be, the spiral
fringe which surrounds the axial filament is certainly the
remains of the cytoplasm of the spermatid. The remains of
the spindle-filaments disappear, while the centrosome prob-
ably persists as a structure lying behind the head and termed
the " Mittelstiick."

In some cases, as the insect Pyrrhocoris and the crustacean Diaptomus,
the doubling of the chromosomes previous to division into spermatocytes
does not take place. In Pyrrhocoris twenty-four chromosomes are typi-
cally present and twelve of these pass into each of the spermatocytes, and
in the division of these to form the spermatids each of the twelve chromo-
somes divides so that each spermatid possesses half the typical number.
In Diaptomus the same result is brought about somewhat differently.
The spermatogoues possess eight chromosomes which assume a dumbbell
shape and divide transversely, so that each spermatocyte has the typical
number of chromosomes ; the spermatocytes divide without passing
through a resting stage, and each spermatid thus contains four chromo-
somes, i.e. half the typical number.

Fertilization of the Ovum. — So. soon as the formation of the
polar globules has been completed, the nucleus of the ovum
migrates towards the centre of the protoplasm and is the
female pronucleus (Fig. 23, fp) of conjugation. The penetra-
tion of the spermatozoon may occur at any portion of the sur-
face of the ovum and may take place before, during (Fig. 21,
B, sp), or after the formation of the polar globules, a single

50

INVERTEBRATE MORPHOLOGY.

spermatozoon, as a rule, in healthy ova, penetrating and tak-
ing part in the conjugation, though apparently in some cases
polyspermy, or the penetration of several spermatozoa, may
occur. The head of the spermatozoon comes into contact
with the protoplasm of the ovum, which in some cases rises
up to meet it, and is rapidly engulfed. The tail likewise of

. oc

mp

FIG. 23. —DIAGRAMS TO ILLUSTRATE THE PHENOMENA OF FERTILIZATION.

A, the approximation of the male and female nuclei.

B, division of the ceutrosomes.

C, rotation of the centrosomes.

D, fusion of the ceutrosomes aud nuclei, arid formation of the segmentation

spindle.

cc = compound ceutrosome. oc — ovum centrosome.

fp = female nucleus. sc = sperm centrosome.

mp = male nucleus. sn = segmentation nucleus.

the spermatozoon is taken into the ovum and seems to be com-
pletely absorbed, the head alone being visible in later stages ;
it constitutes the male pronucleus (Fig. 23, mp) and moves
towards the centre of the egg until it comes into contact with
the female pronucleus, without, however, fusing with it. A
spindle now makes its appearance, and the two pronuclei p;iss
through the various karyokiuetic stages, forming equatorial
plates each with half the typical number of chromosomes,

SUBKINGDOM METAZOA. 51

which divide longitudinally in the usual uiauiier, one half the
chromosomes of each nucleus passing towards one of the cen-
trosomes. The ovum then divides into two cells and the
compound nucleus of each passes into the resting stage, the
chromosomes now uniting to form a single chromatic net-
work.

It will be seen from this that the conjugation or fertiliza-
tion process consists of the iiniou of two distinct nuclei, whose
complete fusion does not necessarily occur until after the first
division or segmentation of the ovum.

A conjugation of ceutrosomes to form those of the first
segmentation-spindle also occurs. A ceutrosorne accompanies
each of the conjugating nuclei (Fig. 23, A], and before the
formation of the spindle each divides into two (Fig. 23, B, oc
and sc), which conjugate in pairs (Fig. 23, C and D), forming
the ceutrosomes of the spindle, each of which thus contains
elements of both the original ceutrosomes.

Furthermore, in some cases at least, it is possible to dis-
tinguish the nuclear elements derived from the male and
female prouuclei respectively in stages later than the first
segmentation, owing to a slightly different behavior to certain
staining reagents which characterizes them. The prouuclei
undergo a morphological fusion during the first cleavage of
the ovum, but a physiological differentiation persists.

Segmentation and Early Development of the Ovum. — The
development of the ovum into the embryo consists in its divi-
sion into a number of cells, which gradually undergo a phy-
siological and morphological differentiation resulting in the
formation of tissues, organs, etc. These divisions constitute
the segmentation of the ovum.

The first division has already been described ; it bears a
definite relation to the formation of the polar globules, the
plane of the division passing through the point at which they
were separated from the ovum. Considering this point to
represent one pole of the ovum, the first division is meridio-
nal, and the second division likewise, though its plane is at
right angles to that of the first division (Fig. 24, A). The
third division is, on the other hand, equatorial, its plane cutting
the planes of previous divisions at right angles (Fig. 24, B).

52 INVERTEBRATE MORPHOLOGY.

Eight segmentation-cells are thus formed which remain in
contact with each other and enclose a small cavity, the seg-
mentation-cavity or blustocoel. The further division of the
cells (Fig. 24, C) results in the formation of an oval or spheri-
cal organism (Fig. 24, D] which may be compared to Volvox,
consisting of a single layer of cells enclosing a more or less
voluminous blastocoel. This embryonic stage is known as
the blostula. In its simplest form it shows no special differ-
entiation into tissues, its cells being uniformly ciliated, and

FIG. 24 — DIAGRAMS ILLUSTRATING THE SEGMENTATION OP THE OVUM.

A, four-celled stage.

B, eight-celled stage of a telolecithal ovum.

C, sixteen-celled stage.

D, blastula.

The arrows iudicate the mode of division.

the organism free-swimming, moving through the water with
a rotatory movement about a definite axis, one and the same
end of which is always anterior. In many blastulas, however,
especially in those which for one reason or another are not
free-swimming, an early differentiation of the cells takes place,
especially at the extremity which is posterior in the free-
swimming forms or which corresponds to that pole in the
non-motile embryos. These posterior cells are usually some-
what larger than those at the anterior pole, and if much food-
yolk is present in the embryo it is especially concentrated in

SUBKINGDOM NETAZOA, 53

these cells, which iu the later development will assume the
vegetative functions of the organism.

In many ova the processes just described are modified to a greater or
less extent, but from the frequency of their occurrence they must be re-
garded as fundamental and the modifications as secondary.

Ova which contain but little yolk usually follow more or less closely
the typical processes, but where the yolk is abundant, being an inert sub-
stance, it acts as a drag upon the protoplasmic activity and produces modi-
fication of the segmentation-processes. Two methods of arrangement of
the yolk may be recognized : (a) it may be aggregated more or less com-
pletely at one pole of the ovum, such ova being termed telolecUhal, or (6)
it may be distributed in the meshes of a protoplasmic network, a small
quantity of yolkless protoplasm being concentrated around the nucleus of
the ovum, while another portion of it forms a thin peripheral layer sur-
rounding the yolk, this arrangement being termed centrolecithal.

In telolecithal ova the third segmentation-division results in the forma-
tion of four cells containing very little yolk at one pole of the ovum, while
nearly all the yolk is concentrated in the four cells at the other pole (Fig.
24, B). This arrangement, which occurs in many Mollusca, constitutes
what is termed a total irregular segmentation, in which, owing to the large
size of the yolk-containing vegetative cells, the blastocffil is usually com-
paratively small. In the Squids the amount of yolk present at the vegetative
pole is very great and the protoplasm of the ovum collects upon its surface,
there undergoing division and producing a plate of cells, the blastoderm,
which by further division gradually extends and finally encloses the inert
yolk. This partial segmentation is the result of the presence of a very
large quantity of yolk and its telolecithal arrangement, and necessarily
obscures greatly the blastula stage.

In centrolecithal ova which occur in Crustacea and Insects, the division
of the nucleus is accompanied by a division of the central yolkless proto-
plasm only, the yolk-containing reticulum and the peripheral layer not tak-
ing part in the process. As the divisions continue the nuclei gradually
approach the surface and finally come to lie in the peripheral protoplasm,
which then takes part in the division, a greater or less portion of the inert
undivided yolk occupying the blastocoel of the resulting blastula. Many
intermediate gradations occur between such a typical centrolecithal and a
total regular segmentation, from which both the centrolecithal and telo-
lecithal methods are to be derived.

The blastula is a single layer of cells surrounding a large
blastocoal in typical cases, and is a stagerquickly passed over
in the Metazoa. It is succeeded by a stage in which the em-
bryo consists of a double-walled sac open at one end, the gas-
trula (Fig. 25). This is most frequently produced from the

INVERTEBRATE MORPHOLOGY.

FIG. 25. — DIAGRAM OF A GASTRULA.

blastula by the pushing in or invagination of the cells of one
pole (the posterior in free-swimniiug blastulas) into the blas-
tocoel, which thus becomes more or less perfectly obliterate J.

The cavity lined by the iuvagi-
nated cells is the primitive di-
gestive tract or archenteron, its
opening to the exterior being
the gastrula mouth or blastopore.
The gastrula is a two-layered
organism or is diploblastic, and
the cell-layers of which it is
composed are the primitive germ-
layers. The outer layer in the
higher Metazoa gives rise to the
integument, nervous system, and
sense-organs of the adult and
is known as the ectoderm, while
the inner one, from which the digestive tract and its glands,
such as the liver, will develop, is termed the endoderm.

Just as the presence of yolk in the ovum may modify the
segmentation, so too it may produce decided modifications in
the formation of the gastrula. The method just described,
which occurs in embryos containing little food-yolk, is distin-
guished as embolic from the epibolic method occurring in telo-
lecithal ova which undergo a markedly irregular segmentation.
In such ova, as has been stated, one pole is occupied by inert
yolk-laden spherules, while at the other are almost yolkless
active cells. These latter divide rapidly and extend as a cap
over the yolk-laden cells and finally completely enclose them.
The result is practically the same as in the embolic method,
the yolk-laden endoderm cells being enclosed within the yolk-
less ectoderm.

Among the lower Metazoa especially, another method oc-
curs by which the diploblastic embryo is formed. Instead of
certain cells invaginating, each cell of the blastula divides in
a plane parallel to the surface of the organism, one of the two
cells thus produced becoming ectoderm, while the other is a
portion of the eudoderm. A diploblastic closed sac thus re-
sults, the blastopore appearing later and placing the archeu-

&UBX1NGDOM METAZOA.

55

terou, which in this case is identical with the blastocoel, in
communication with the exterior. This process is known as
delamination (Fig. 26, A).

A third method also exists, occurring like delamination in
its most typical form among the lower Metazoa. This is the
immigration method (Fig. 26, B), certain cells of the blastula
leaving their position at the surface and passing into the
blastocoel. Here they undergo division, and, by the addition
of other cells by immigration, the blastocoel gradually be-
comes filled iip and a solid organism, consisting of an exter-
nal layer of cells surrounding a central more or less solid

JJ

FIG. 26.— DIAGRAM ILLUSTRATING THE FORMATION OF THE DIPLOBLASTIC
STAGE (A) BY DELAMINATION, (B) BY IMMIGRATION.

mass, results. This is known as the parenchymella or sterrula.
Later a cavity appears in the centre of the solid mass, whose
cells gradually are pushed towards the periphery, where they
form eventually a single layer, the endoderm. Finally a blas-
topore is formed and the embryo becomes a gastrula.

It does not seem easy to bring the delamination and invagination
methods of gastrulation into direct relation with each other, or to derive
one from the other, but it is probable that both must be referred back to
the immigration method. In typical cases of immigration the cells which
migrate are situated irregularly at any part of the blastula, but frequently,
especially in free-swimming blastulas, the migrating cells are all located at
the posterior extremity. If in such cases of polar immigration the migrat-
ing cells were to pass into the blastoccel en masse instead of individually,
invagination would result. On the other hand, if a considerable amount
of yolk were present in all the cells of a blastula, it might happen that, in-
stead of migrating, the cell might undergo division, cutting off the yolk-
containing protoplasm from the yolkless, delamination thus taking place.

56 INVERTEBRATE MORPHOLOGY.

The fact that in some cases both immigration and delamination may occur
simultaneously, leading to the formation of a sterrula, bears out the idea
that the latter process has arisen from the former.

Furthermore, it may be pointed out that the occurrence of immigration
in such colonial Flagellates as Volvox indicates the primitive character of
immigration in the Metazoan blastulas, as well as the manner in which
diploblastic organisms have arisen from the more primitive single-layered
organisms.

It is only in the lowest Metazoa, however, that the adult
organism is diploblastic. In all others a triploblastic (Fig.

dm

,ec

/:*&^M ^^K bm

FIG. 27.— DIAGRAMMATIC TRANSVERSE SECTION OF AN EARTHWORM TO
SHOW THE TRIPLOBLASTIC CONDITION.

bm == basement membrane. en — emloderm.

C =• ccelom. sm = somatic mesoderm.

dm — dorsal mesentery. spin = splanchnic mesoderm.

ec = ectoderm. vm = ventral mesentery.

27) condition supervenes during embryonic life, by the devel-
opment of a third layer, primitively separated from the eudo-
derm, and occupying the space which may remain between
the two primitive layers. This is the secondary germ-layer
or mesoderm. From it there arise the muscular, excretory,
circulatory, and reproductive systems in the triploblastic ani-
mals, the first and last of these being derived in diploblastic
forms from either one or both of the primary layers, while
the excretory and circulatory systems are not differentiated.

The manner of formation of the mesoderm in the embryo
varies greatly. In some cases it arises as bilateral pouch-like
outgrowths of the archeuterou, which later form closed sacks
completely surrounding the digestive tract, the sack of either

SUBKINGDOM METAZOA. 57

side coming into contact above and below, the united walls
forming the dorsal and ventral mesenteries which suspend the
intestine (Fig. '27, am and bin}. That wall of each sack which
surrounds the digestive tract is termed the splanchnic layer of
the mesoderin (Fig. 27, spm), while that lying immediately
below the ectoderm is the somatic layer (sin), and the enclosed
'cavity is the ccelom (C) or body-cavity. In other cases the
protoplasm destined to give rise to the mesoderrn segregates
into a small number of cells, or sometimes even into a single
cell, at an early period of the development, frequently while
the embryo is still in what may be considered the blastula
stage. These cells, known as mesoblasts, give rise by repeated
division in one direction, and by the subsequent division of
the daughter cells so formed, to bauds of mesoderrnic tissue
extending along the ventral surface of the embryo (see Fig.
105), and later growing dorsally so as to enclose the diges-
tive tract. The ccelom forms by the hollowing out of the
mesodermic bands, and when fully developed presents the
same appearance as in the former case.

In many animals, such as some Turbellarian worms, a
well-developed ccelom is not present, the only traces of it
being minute scattered cavities in a mass of mesodermic tissue
which fills up the space between the endoderm and ectoderm.
A strict demarcation of this form of ccelom (schizocoel) from
the other variety (enterocoeT) does not, however, exist, grada-
tions occurring in various groups of animals and both varie-
ties sometimes being coexistent in the same form, as for
instance in bivalve Mollusca, where the pericardial cavity is
to be regarded as an enterocoel, while the spaces existing else-
where in the rnesoderrn are schizoccels.

If the conditions <whieh exist in the lowest triploblastic animals known
to us, the Turbellarian worms, throw any light upon the origin of the meso-
derm, it would seem that primitively it was a solid tissue, not completely
marked otf from the endoderm, and that any ccelom that it contained was
of the nature of a schizoccel. From this condition it became more and
more differentiated from the endoderm proper, and either tended to appear
as a separate germ-layer at an early stage of development in the form of
the mesoblasts, or was delayed in its development until after the formation
of the primitive digestive tract, from which it then separated in the meso-
dermic pouches. According to this view the mesoderni is a secondary

58

IN VERTEBRA TE MORPHOLOG T.

derivative of the endoderm, and the endoderm of the diploblastic organ-
isms is equivalent to the endoderm plus mesoderm of the triploblastic
forms. The apparent derivation of the mesoderm from the ectoderm in
some of the latter (e.g. Annelida) is to be regarded as resulting from the
precocious segregation of the mesoderm at an early period of development
and is not to be regarded as indicating its original derivation.

Non-sexual Reproduction in the Metazoa.— Reproduction by
division and by budding, though playing by no means so im-
portant a part as in the Protozoa, is neverthe-
less of frequent occurrence in the Metazoa,
im. especially in certain groups. In certain Tur-
bellarian worms (Microatoma) division is the
Di'" usual mode of reproduction, replacing almost
Hi,, completely the sexual method, and, the indi-
viduals so produced remaining in connection
m with one another, longitudinal chains are
produced, consisting of individuals in various
Q» degrees of separation (Fig. 28). In certain
Annelids also (Naidida?) division frequently
•'" takes place, occasionally each metarnere being
capable of developing into a new animal, as in
Ctenodrilus.

n Budding, however, is a rather more fre-

quent method and is characteristic of certain
groups, such as the Hydroids, Anthozoa, and
FIG. 28. - D i A- Bryozoa. In some cases, as in Hydra and

OWING some me(}US8e the buds separate from the
NON-SEXUAL RE-
PRODUCTION OF Parent an(3 lead an independent existence ;

A TURBELLARIAN but frequently the separation is not complete,

WORM Microsto- resulting in the formation of colonies the

individual components of which are in or-

GRAFF).

gauic connection with each 'other. In such
colonies a physiological division of labor among the con-
stituent individuals may take place, as in the Hydroid
Hydractinia (see p. 87) where some of the individuals devote
themselves to the nutrition of the colony, others to its
reproduction, and others again to the protection of their
weaker companions. The assemblages produced by bud-
ding may assume very complicated shapes, though occa-

SUBK1NGDOM METAZOA. 59

sionallj linear colonies are formed which are with difficulty
to be distinguished from those formed by division. Indeed
a definite distinction between budding and division is not
possible, though where an alternation of older and younger
individuals occurs in a linear colony division is indicated,
while in one produced by budding there is a regular succes-
sion of gradually older individuals from before backwards.

Closely related to budding is the power of regeneration
of parts. The higher Crustacea possess an extraordinary
power of regenerating lost limbs, and provision is present in.
crabs and the lobster for the self-amputation of a limb when
such a mutilation seems to be demanded by the exigencies
of the situation. In the lower forms, however, the extent to
which such regeneration may be carried is much greater, ex-
tending even to the reproduction of the whole by a compara-
tively small part. A Starfish is not only able to regenerate
an arm which has been accidentally lost, but from an arm and
a portion of the disk all the missing parts may be developed ;
and Hydra or a Sponge may be divided into a large number
of pieces each of which is capable of developing into an entire
animal. Such phenomena, as well as budding and division,
depend either upon a low degree of differentiation of the
tissues, as in such a form as a Sponge or in Hydra, or else to
the persistence of a certain amount of tissue in an embryonic
or undifferentiated condition. In a Bryozoau bud, for in-
stance, as its tissues gradually differentiate into the adult
condition, a number of cells lag behind and do not take part
in the differentiation, and later give rise to a new bud ; and
similarly in the Annelid worms the tissues of a regenerating
part show an appearance and mode of differentiation similar
to what they present in the development from the ovum.
Conversely, the greater the degree of differentiation and in-
tegration of the tissues and organs of an animal the less
is the power of regenerating lost parts or of reproducing by
budding.

As a general rule ova are incapable of developing into the
adult form unless fertilized by a spermatozoon. In a number
of forms, however, a development of unfertilized ova occurs
constituting a mode of reproduction known as partheno-

60

INVERTEBRATE MORPUOLOG Y.

genesis. Examples of this phenomenon are to be met with in
Insects, a familiar one being the common Hive Bee, the
queens of which species deposit large numbers of eggs,
those last deposited, which give rise to drones, being unfer-
tilized and developing parthenogeuetically. In certain flies
(Cecidomyia) this parthenogenetic development of the ova
may occur while the insect is still in tbe larval or maggot
stage, a phenomenon which is known as pcedogenesis (Fig. 29).
Alternation of Generations. — The majority of forms which
possess the power of non-sexual reproduction also repro-
duce by the sexual method, no definite relation
existing, however, between the two processes.
In some cases, however, a definite relation is
established, the one method succeeding the
other with rhythmic regularity, the individuals
also which reproduce sexually differing materi-
ally in form and organization from those which
gave rise to them by a non-sexual method ;
such a condition of affairs is termed Alter-
nation of Generations, a generation of in-
dividuals reproducing only by a non-sexual
method alternating with a second generation
reproducing exclusively or almost so in the
sexual manner. Typical examples of this
process are afforded by the Discomedusse,
in many of which the individual produced
by the development of the ovum is a fixed,
FIG °9 - P < cyliudrical organism of simple structure, known
GENETIC Cecido- as a Potyp> possessing the power of non-sexual
myia LARVA (after reproduction (see Fig. 55). By a series of
PAGKNSTECHER from transverse divisions it gives rise to a linear

HATSCHEK). °m

colouv of individuals which in the course of

•/

development assume a form very different from that of the
parent polyp, becoming more complicated in structure, more
highly organized, and free-swimming. These organisms,
known as Medusce, are the sexual generation, producing sper-
matozoa and ova, the latter after fertilization developing a
non-sexual generation, a polyp, witli which the cycle begins

again.

SUBKINGDOM METAZOA. 61

Schematically such an arrangement may be represented thus, A repre-
senting the non-sexual and B the sexual generation :

/B— A, etc.
A//B— A, etc.
A\\ B— A, etc.

\B— A, etc.

Among the Hydromedusse, in which group alternation of generations
likewise occurs, the process is usually complicated by a number of non-
sexual generations succeeding one another before the intervention of the
Medusa, thus :

A//A"

./A\A" =B = A, etc.
A\.,/A" = B = A, etc.

And in some cases the succession is still further complicated by non-sexual
reproduction on the part of the medusa, thus :

A, etc.

/A'\A''-B/B'Z=A'^-
./ D\B=A, etc.

\ /A" = B/B/ = A' etc-
XA< .„ ' ,1 \B' = A, etc.
\A II

A, etc.

But such complications do not interfere with the general alternation
which invariably occurs in such forms before the completion of the repro-
ductive cycle.

Such a phenomenon as this where a true non-sexual gen-
eration alternates with a sexual one presenting a different
structure is usually distinguished as metagenesis from another
form of alternation of generations known as heterogony, in
which the first generation reproduces parthenogenetically,
giving rise to a second generation differing in form from the
first and reproducing by the sexual method. Typical exam-
ples of this process are to be found among the Trematode
worms (q.v.), where the sexual worm gives rise to a sporocyst
in the interior of which ova, developing parthenogenetically,
give rise to a larva which later on transforms to the adult
worm. In a less perfect form heterogony occurs in many
lower Crustacea (Daphnia), which throughout the warmer
portion of the year produce " summer eggs " which develop
parthenogeuetically, male animals appearing only for a short
period in the autumn, as a rule, when the females produce

62 INVERTEBRATE MORPHOLOGY.

" winter eggs " which develop after fertilization. Here no
difference of form exists betwe.en the two generations, but
such cases, as well as those in which two sexual generations
unlike in form and habitat alternate with each other, are
usually associated with the more typical examples as in-
stances of heterogony.

LITERATURE.

0. Hertwig. Beitrage zur Kenntniss der Bildung, Befruchtung und Theilung
des thierischen Eies. Morpliolog. Jahrbuch, i., in. and iv. 1875-78.

E. van Beneden and A. Neyt. Noiivelles recherches sur la fecondation et la

division mitosique chez FAscaride megalocephale. Bulletin de la Societe

royale Belgique, xiv. 1887.

Th. Boveri. Zellenstudien. Jenaische Zeitschrift, xxn. 1888 and xxiv. 1890.
H. Fol. Le quadrille des centres. Archives des Sciences pliys. et naturelles

Geneve, xxv. 1891.
0. Hertwig. Vergleich der Ei- und Samenbildung bei Nematoden. Arcliiv

fiir mikrosk. Anatomic, xxxvi. 1890.

F. M. Balfour. A Treatise on Comparative Embryology. London, 1880.
E. Metschnikoff Embryologische Studien an Mcdiiu n. Vienna, 1886.

E. Haeckel. Die Gastrula und die Eifurchung der Thieve. Jenaische Zeit-
schr., ix. 1875.

TRWHOPLAX, THE JJICYEMID^! AND ORTHONEVTW^E. 63

CHAPTER IV.

IRICHOPLAX, THE DICYEMID^E AND ORTHONECTID^E.

BEFORE passing on to a description of the first type of
Metazoa, it will be necessary to consider a few forms wliich
can hardly be assigned to it and yet present too great a dif-
ferentiation of their component cells to warrant their reference
to the Protozoa. A third subkiugdoro, the Mesozoa, has been
proposed for them, but until more is known of the relations
of some of them at least to other forms the establishment of
such a subkingdom seems inadvisable.

Triclioplax adhcerens.

FIG. 30. — A, SURFACE VIEW AND B TRANSVERSE SECTION THROUGH Tricho-

plax (after SCHULZE).
b = botryoidal slructure. r = refractive bodies.

In the marine aquaria at Gratz, Vienna, and Berlin there
has been found a small organism (Fig. 30, A) measuring from

64 INVERTEBRATE MORPHOLOGY.

1.5 to 4 mm., but capable of great alteration of form. It is
flattened, and creeps about upon the walls of the aquaria in
an amoeboid manner. It consists, however, of numerous cells
(Fig. 30, £), the upper surface being covered by a flattened
ciliated epithelium, and the lower formed by a layer of
columnar cells also ciliated, while the space between the two
surfaces is occupied by a network of branching cells, the
branches appearing to unite with those of adjacent cells and
with prolongations from both the upper and the lower epithe-
lium. The arrangement suggests the three germ-layers ecto-
derm, endoderm, and mesoderm, but until more is known con-
cerning the reproductive processes such an homology is
unwarranted. At present the organism is only known to re-
produce by division, and no structures have been discovered
which may be identified as ova or spermatozoa. Beneath the
upper epithelium, imbedded in the cells of the middle tissue,
•large refractive spheres (Fig. 30, B, r) and yellowish-green
botryoidal masses (1>) occur, but they have apparently no con-
nection with reproduction.

THE DICYEMID^:.

The Dicyemid?e are elongated vermiform organisms which
are parasitic in the renal organs of the Cephalopods. The
various species of Dicyema (Fig. 31) vary in length from
0.5-7 mm. and are all very simple in structure, consisting of
a single elongated central cell (Fig. 31, C) extending from one
end of the body to the other and covered by a number of
ciliated cells arranged in a single layer. Some of these, situ-
ated at one end of the body, are smaller than the others and
mark off the anterior extremity ; there is no mouth or diges-
tive tract and no sense-organs.

Reproduction is carried on by the development of germ-
cells (g] produced by the division of the nucleus of the central
cell and the concentration around the nuclei so produced of a
portion of its protoplasm. The development of these germ-
cells is apparently parthenogenetic and no male Dicyema is as
yet known. In young individuals the germ-cells segment in
the interior of the central cell and give rise to " vermiform "

TRICHOPLAX, THE DICYEMIDS AND ORTHONECTID^E. 65

g

enibiyos (Fig. 31, V) similar to and developing directly into the
adult form. Another form of embryo is, however, produced
by older individuals, its formation being
accompanied by a peculiar behavior of the
germ-cells. The nucleus of each one first
divides into two unequal parts, the smaller
part separating as a parauucleus and under-
going no further development. The germ-
cell now segments, and an embryo (Fig.
31, e) consisting of a single large cell
partially surrounded by smaller cells re-
sults. The smaller cells are now thrown
off and separate somewhat from each other,
and the larger cell repeats the segmentation-
process, the smaller cells being again thrown
off; and this may happen three or four
times, the result being the production of
three or four concentric layers of small
cells surrounding a single larger one, all
lying in the central cell of the parent. The
large cell undergoes no further develop-
ment, but the smaller ones, except those
of the last generation, develop into " infu-
soriform " embryos of a peculiar and com-
plicated structure. The cells of the last
generation develop into " vermiform " em- FlG. si.-Dicyema ty-
bryos similar to those found in young pus (combined from

Dicyemids several figures by

The fate of the " infusoriform " embryos c = central cell,
has not been determined. Since they are e, = embryos.

ciliated it seems not improbable that they (J = »erm-cells

PI-IT ,- e ,1 . 7i = nucleus of central

serve lor the dissemination ol the species ,,

and its transference from one Cephalopod y= vermiform em-
host to another. It has, however, been bryo.
suggested that they may develop into males.

THE ORTHONECTIDA.

The Orthouectids are parasitic on Echiuoderms and
Nernertean worms and resemble in structure the Dicyemids,

66

IN VERTEBRA TE MORPHOLOG T.

the ectoderm consisting of a number of ciliated cells arranged
in a sin-gle layer and enclosing a mass of germ-cells which
correspond to the central cell of JDicyema. Between the germ-
cells and the ectoderm fine nucleated fibres occur which are
presumably muscular.

Three forms of individual are known to occur in the genus
Rhopalura, one being a male, and the other two females.

B

C

FIG. 32. — Rhopalura Oiardii (after JULIN).
A, male ; B, round female ; C, flat female.

The male (Fig. 32, A) is about half the size of the females,
which measure about 0.25 mm. in length, and presents a niet-
americ arrangement of the ectoderm which does not extend to
the internal cells. The cells of the anterior segment have
their cilia directed anteriorly, and are succeeded by a segment
consisting of several rows of small non-ciliated cells each
containing a refractive body, and behind this there follow three
or four segments formed of cells provided with cilia directed
backwards. One of the female forms (Fig. 32, B) is elon-
gated, and is segmented like the male except that the segments
are more numerous and the second non-ciliated segment con-
sists of a single row of cells destitute of refractive bodies.

TEICHOPLAX, THE DlCYEMJD^ AND ORTUONEGTID^l. 67

The other female (Fig. 32, C] is, on the contrary, ovoitl, flat-
tened, and unsegmented, being ciliated all over ; it differs
furthermore from the elongated female in possessing on one
side near the anterior 'extremity a granular mass containing a
large nucleus whose significance is entirely problematical.

Associated with the difference of form of the two females
there is a difference of function. In the elongated form when
the ova are mature the anterior two segments split off as a
cap and allow the ova to escape, and, on fertilization, these
give rise to males. In the ovoid form, however, the ova are
imbedded in a gelatinous mass, and are liberated by the
breaking up of the parent into a number of fragments ; from
the ova females of both forms develop.

The systematic position and affinities of the Dicyemidae and Orthonec-
titla- is a matter of uncertainty. They have been held by some authors to
possess affinities with the Gregarinida and by others to be degenerate flat
worms, while others have sought to trace resemblances to the Rotifers.
The granular mass with the large nucleus which occurs in the ovoid
Rhop<tliira has been supposed to represent, a rudiment of a digestive tract,
while the superficial metamerism of the male and elongate female of the
Orthonectida may possibly point to a derivation from more highly organ-
ized ancestral forms. There can be but little doubt that the Dicyeraidse and
Orthonectida are closely related, but at present sufficient evidence is want-
ing to warrant any definite conclusions as to their relationships to other
forms.

LITERATURE.

TRICHOPLAX ADH^ERENS.

F. E. Schulze Uber Trichoplax adharens. Abhandl. Akad. Wiss. Berlin,
1891 (see also Zoolog. Anzeiger, vr. 1883).

DICYEMIDAE.

C. 0. Whitman. A Contribution to the Embryology, Life-history, and Classi-
fication of the Dicyemids. Mitth. a. d. Zool. Station zu Neapel, iv. 1882.

ORTHONECTIDA.

C. Julin. Contributions d Vhistoire des Mesozoaires. Archives de Biologie in

1882.

68

INVERTEBRATE MORPHOLOGY.

CHAPTER V.

TYPE CCELENTERA.

The Coelentera include the diploblastic Metazoa, only
two germ-layers, the ectoderm and endoderm, being repre-
sented in their organization (Fig. 33). Between these two

layers, however, a third (Fig. 33, mg}
is invariably present, which in its
primitive condition is not cellular, but
consists of a gelatinous or fibrous sub-
stance secreted by one of the two
cellular layers. Usually, however,
cells from the endoderm or ectoderm
wander into it, and sometimes are so
numerous as to give it the appearance
of a cellular layer. Even in such
cases, however, the gelatinous matrix
is the fundamental substance of the
FIG. 33.— DIAGRAM OF Hy- layer, which it seems preferable to
dra TO SHOW THE GEN- term ^}ie mesogloea, rather than to imply

ERAL STRUCTURE OF a i • i i • ± -i "

an homology which does not exist by

designating it the mesoderm.

In consequence of the absence
of the mesoderm the Ccelenterates

en

CCELENTERATE.

ec — ectoderm.

en = endoderm.

mg — niesoglceu.

present in the interior only a single cavity. Consequently
it may be said that the coelom is not represented in the
Coelentera, though their central cavity is usually regarded
as equivalent to both ccelorn and enteron of the higher
forms. The so-called endoderm, however, seems to be homol-
ogous with their mesoderm pins endoderm, and may be more
accurately termed the mes-endoderm, and it seems preferable
to regard the ccelom as not yet differentiated.

Another feature which obtains throughout the group is
the radiate ground-form. In many but one axis can be de-

TYPE CCELENTERA. 69

teriniued, and in the Sponges the form may become so irregu-
lar that thay may be considered to be destitute of axes. In
such forms as the Medusae, however, a typical radiate form
occurs, there being two or more similar axes at right angles
to the vertical one, and throughout the higher members of
the group this radiate symmetry is more or less apparent,
though it becomes decidedly obscured in certain Anthozoa by
a pronounced tendency towards bilaterality, which in a few
forms (Cerianthidce) actually replaces it.

In correspondence with their low grade of general struc-
ture there is no very extensive differentiation of tissues. A
considerable degree of division of labor of course occurs
among the cells, and cells having the same function may be
aggregated together so as to form a somewhat definite tissue,
as in the case of the nerve, muscle, and reproductive cells, but
even in these tissues there seems to be a considerable amount
of individuality retained by the constituent cells, and the tis-
sues can only be regarded as exceedingly diffuse. Of organs,
except in some colonial forms with division of labor among
the constituent individuals, it is hardly correct to speak, the
Ccelenterates not having progress 3d beyond the organ stage
of individuality.

The type Coalentera may be divided into two subtypes,
the Porifera, or Sponges, and the Cnidaria.

I. SUBTYPE Porifera.

The Sponges, on account of their fixed life and irregular
form, were long regarded as plants, and it is only within com-
paratively recent times that their true relationships have been
ascertained. They are almost exclusively marine in habitat,
occurring in large numbers in the warmer seas, and inhabit
the ocean depths as well as the shallower waters. A few
genera, e.g. Spongitta, Ephyatia, represented by numerous
species, are inhabitants of fresh water.

The simplest Sponges (Fig. 34) have the form of a hollow
cylinder fixed at one end, while at the other is an opening,
the osculuin, and scattered over the surface of the cylinder
are a number of smaller openings, the pores. Through these

70

INVERTEBRA TE MORPHOLOG T.

water passes into the central cavity, the ccelenteron, and
escapes by the osculum. The exterior of the body is covered
by a layer of flat cells, the ectoderm, and the coelenteron is
lined by collared cells provided with a single nagelluin and
resembling greatly Autofiagellata belonging to the genus
Codosiga. These cells constitute the endoderni, and between

Os

FIG 34.— AN ASCON FIG. 35. —DIAGRAM TO SHOW THE GENERAL
SPONGE, Asceitapri- STRUCTURE OF A SYCON SPONGE.

mordiaiis (after HAE- The upper portion represents the simplest con-

cKELfrom SOLLAS). ditiou, the complexity increasing downwards.

cc = ciliated chamber. ic — iuhalent canal.

Os = osculum. p = iuhaleut pore.

pr — prosopyle

it and the ectoderm is the rnesogloea, in which are imbedded
large numbers of cells, giving it almost the appearance of a
cellular layer.

In such simple Sponges the mesoglcea is comparatively
thin and the pores open almost directly into the creleuteron
lined by the collared cells. This arrangement constitutes the
first or Ascon type of structure. In the majority of forms a
much greater complexity arises from the walls of the simple
c}"linder being, as it were, drawn out into a number of finger-
like processes, each of which communicates by a wide open-

TYPE COSLENTERA. 71

ing with the cavity of the original cylinder (Fig. 35). The
cells lining the central cavity become flattened, the collared
cells being fou^d onl}T in the interior of the secondary cylin-
ders which radiate from the central chamber (cc). Pores,
termed prosopyles (pr), occur in the walls of the secondary
cylinders, which are closed at their free ends. Through these
prosopyles water passes into the interior of the radiating cyl-
inders, thenoe into the central cavity and so to the exterior
b}* the osculum. Further complication occurs by the walls
of the radiating cylinders coming in contact with each other
and fusing in a more or less irregular manner, the space be-
tween the various cylinders being thus divided into a series
of more or less well-defined inhalent canals (ic) into which the
water passes through pores (p) which lie, morphologically, be-
tween the extremities of the radiating cylinders. The cavities
of these cylinders now form the ciliated chambers, and Sponges
in which they possess the cylindrical form are said to belong
to the Sycon type. The annexed diagram (Fig. 35) illustrates
the different stages of complexity met with in Sycon Sponges.

The next complication consists of the branching of the
ciliated chambers, though they still retain a cylindrical shape,
and their separation from the central cavity by a tract lined
with flattened cells (Fig. 36, A) ; and finally the collared
cells become limited to a portion of the radial chambers,
the ciliated canals thus becoming circular in shape and united
with the central chamber by long and rather slender canals
lined with flattened cells (Fig. 36, B], This constitutes what
is termed the Leucon type of structure. In this type the
pores upon the surface of the Sponge frequently do not open
directly into the canals leading to the ciliated chambers,
but into a wide lucunar cavity, the subdermal space, lying be-
low the cortical layers of the sponge, and with this the canals
communicate.

To these complications of arrangement further complexity
is added by the occurrence in many Sponges of what may be
considered budding, in some cases leading to the formation
of definite branches, or in others producing only a number
of oscula, each, however, with its own canal system.

The general characteristics of the ectoderm and eudoderni

72 INVERTEBRATE MORPHOLOGY.

have been indicated in the preceding description of the canal
systems ; the tnesoglcea requires, however, further notice. It
consists of a gelatinous matrix which, however, contains large
numbers of cells presenting a considerable amount of dif-
ferentiation. Some are amoaboid in form, others contain pig-
ment, others again are elongated and spindle-shaped, forming
the contractile cells, others form the reproductive elements,
ova and spermatozoa, while others again are skeletogenous
in function, well-developed skeletal structures being present

FIG. 36. — Two FIGURES SHOWING DIFFERENCES IN THE COMPLEXITY OF

STRUCTURE OF A LEUCON SPONGE.

A. Leucilla ater (after DENDY); B. Oscarella lobularis (after SCHUJ.ZEK
cc = ciliated chamber. p = inbalent pore. sp = spicule.

in almost all Sponges. In some forms the skeleton presents
the form of siliceous spicules either of a simple needle-like
form, or presenting modifications of a four- or six-rayed
ground-form, or finally assuming the form of hooks, anchors,
or spiny spheres. In another group there is, associated usu-
ally with needle-like siliceous spicules, a network of a horny
material termed spongiolin which forms a supportive scaffold-
ing for the soft parts of the Sponge, and lastly, in another
group the spicules are composed of carbonate of lime and
present a variety of forms (Fig. 36, A).

TYPE CCELEXTEKA. 73

Nerve-cells Lave also been described, though a definite
nervous system cannot be said to exist. Elongated retractile
processes have been observed projecting from the surface of
certain sponges, and at the base of each is a group of stellate
cells each of which sends a long slender prolongation into
the process. To these cells a nervous function has been at-
tributed and the processes have been considered sensory ;
with the exception of these structures, however, no sense-
organs or nerve-elements have been yet observed.

The Sponges may be arranged according to the nature of
their skeleton, in four orders.

1. Order Calcarea.

In the Calcarea the skeleton is always present and is
formed of spicules consisting of carbonate of lime. The
group contains forms of various complexity of structure, from
the simple cylindrical Leucosolenia constructed upon the
Ascon type through Sycon forms such as Grantia to repre-
sentatives of the third and fourth types. Indeed it is only in
this group that the Ascon and Sycou types of structure are
found. All the known species are marine and live at only
slight depths.

2. Order Cornacuspongise.

The skeleton of the Cornacuspougise consists either of
siliceous, needle-like spicules, freqiiently more or less united
by spongiolin, or else entirely of a network of fibres com-
posed of the latter substance. Like the Calcarea they are
inhabitants of shallow water and are for the most part marine,
though some forms (Spongilla, Fig. 37, Ephydatia) live in fresh
water. These fresh-water forms are of a green color due to
chlorophyll, the presence of this pigment being supposed by
some observers to depend on numerous unicellular algse liv-
ing in the substance of the Sponges. To this group belongs
also the Sponge of commerce (Euspongia), whose value de-
pends upon the entire absence of siliceous spicules, and
which is found in the shallow waters of the eastern portion
of the Mediterranean, in the Bed Sea, and in the Western
Hemisphere in the waters surrounding the Bahama Islands.

74

IN VERTEBRA TE MORPUOLOO 7.

3. Order Spiculispongise.

The skeleton in the Spiculispongise is occasionally entirely
wanting, as in the genus llalisarca, but usually consists of
siliceous spicules usually tetraxial or rod- or club-shaped,
sometimes interlocking with one another so as to form a firm

^f

^\

^^^fL^^W^ffi^^ '
c l^^f^xV: ^/w.if-^%'--i^^-<

FIG. 37. — A SMALL Spongilla WITH ONLY A SINGLE OSCULXJM (from HUXLEY).

a — inhalant pore. c — ciliated chamber seeu through

d = osculum. the tissues.

skeleton. One of the members of the group is the " boring
sponge," Cliona, which excavates channels in and assists in
the disintegration of oyster-shells, frequently attacking the
shells of living animals and contributing to their destruction.

•1. Order Hyalospongiee.

The Hyalospongise are essentially deep-sea forms, and are
characterized by the possession of six-rayed siliceous spicules
as skeletal elements. The spicules m&y become fused to-
gether to form a firm siliceous network having the appear-
ance of spun glass, as in the genus EuplecteUa, commonly
known as Venus' Flower-basket.

Reproduction of the Porifera. — Sexual reproduction occurs
probably throughout the entire group of the Sponges, the re-

TYPE COSLENTERA. 75

productive elements, ova and spermatozoa, differentiating
from mesogloaal cells. Many Sponges are hermaphrodite, the
spermatozoa developing usually somewhat in advance of the
ova, but some forms seem to have separate sexes. The ova
are fertilized while still within the tissues of the parent and
undergo a portion of their development there, later breaking
through into a canal and so passing to the exterior as a cili-
ated free-swimming structure. The segmentation of the
ovum in typical cases results in the formation of a blastnla,
which becomes converted into a solid ciliated sterrula by
immigration. After swimming about for a time the sterrula
loses its cilia and settles doAvu, a cavity appearing in its in-
terior, which later, in forms which possess ciliated chambers,
gives rise to these structures as a series of pouches in connec-
tion with which canals arise. The pores and the osculum
finally break through, imaginations from the exterior giving
rise to the former.

In some forms, however, the blastula stage undergoes
imagination, usually of a rather peculiar form, in which case
a gastrula results instead of a sterrula. The gastrula settles
down and becomes fixed by the pole at which the blastopore
occurs, the further development being similar to that found
in the immigration types.

In addition to the sexual method most Sponges also pos-
sess the power of non-sexual reproduction, dependent on their
capabilities for regeneration. A detached portion of a Sponge
will, under favorable conditions, regenerate into a new indi-
vidual, and this power has been applied to the artificial re-
production of the commercial Sponges. In some forms in
addition to this a process of internal budding occurs, a num-
ber of the mesogloeal cells aggregating together and develop-
ing into an oval, ciliated, sterrula-like structure which, leaving
the parent, develops into an adult Sponge (Esperella). In the
fresh-water Spongilla this process is carried to the greatest
extent, and towards the approach of winter in temperate
latitudes completely replaces the sexual method. The in-
ternal buds of Spongilla are known as gemmules and are
especially adapted for tiding the species over unfavorable
conditions, such as cold or dryness, which the vegetative in-

76 INVERTEBRATE MORPHOLOGY.

dividual cannot withstand. They are spherical bodies con-
sisting of a mass of cells richly laden with food-matter, and
enclosed in a double chitinous wall, with an opening at one
point, a number of siliceous spicules, in the allied genus
Epliydatia of a very characteristic form and known as amphi-
discs, being arranged between the two layers of the wall. On
the approach of cold weather the Sponge dies down and the
gemmules thus fall to the bottom of the ponds or streams,
where they remain unchanged until the approach of warmer
weather, when the internal cellular mass flows out through
the pore (which is closed only by a thin membrane) and de-
velops into a new Spongilla.

The relationships of the Sponges have long been a matter of discussion.
For a long time they were regarded as plants and later as colonies of Pro-
tozoa, but the discovery of sexual reproduction in them and of their mode
of development demonstrated that they were to be considered Metazoa.
At present the question as to whether they are to be associated with the
Cnidaria among the Coelenterates or regarded as a distinct type is still
open, though the weight of evidence and authority is in favor of their
Ccelenterate character. Such simple forms as Leucosolenia certainly point
in that direction, and, if the occurrence of a sterrula formed by immigra-
tion prove the typical mode of development, the embryology of the Sponges
presents stages up to the formation of the ciliated chambers which are
step by step comparable to what occurs in the Cnidaria.

II. SUBTYPE Cnidaria.

The Cnidaria, like the Sponges, have in their simplest
forms the general form of a hollow cylinder open at one end
and consisting of but two cellular layers, the ectoderm and
endoderm, between which is interposed a fibrous or gelatinous
rnesogloaa which may or may not contain cells. Differences
from the Sponges are found in the occurrence, except in one
or two forms, of a number of elongated, contractile processes
or tentacles around the mouth of the cylinder (see Fig. 33),
and in the absence of iuhalent pores upon its surface. Such
simple forms are known as polyps, and they are usually
attached organisms with little or no power of locomotion. A
large number of Cnidaria present a very different form, how-
ever, being disk- or bell-shaped, a process comparable to the
clapper of a bell hanging down from the centre and hav-

TYPE CCELENTERA. 77

ing at its extremity the mouth-opening. This leads into
a central cavity, the coelenteron, lying in the substance of
the bell, and from this pouches or tine canals radiate out
towards the rim, where, in some cases, they are united by a
circular canal which runs completely round the bell at this
region. To the margin of the bell tentacles are usually
attached, and sense-organs, presenting frequently consider-
able complexity of structure, are found in the intervals be-
tween the tentacles or at their bases. These forms, known
as medusce, are, as a rule, free-swimming, propelling them-
selves through the water by vigorous contractions of the bell.
All Cuidaria, whether of the polyp or medusa form, pos-
sess in their tissues peculiar elements altogether unrepre-
sented in the Sponges. These are the
nematocysts or so-called thread-cells.
Each consists of an oval or spherical
cyst (Fig. 38, c) with a membranous wall
and fluid contents, the wall being pro- i
longed at one end into a long, exceed-
ingly delicate, hollow, thread-like fila-
ment sometimes furnished with spines at
its base, and, in an undisturbed cyst, is
iuvaginated, and coiled up in the interior
(Fig. 38, t). These nematocysts are pro-
duced by and enclosed within special FIG. 38. — NEMATOCYST
cells known as cnidoblasts (en) lying CELL OF Physalia.

principally in the ectoderm, and in their "a °cys

cl = cnidbcil.

most highly developed form differen- cn _ cnidoblast.
tiating below into a supporting stalk (s) n — nerve prolongation,
which rests upon the outer surface of * = supporting process,
the mesogloaa. From the outer extremi-
ty there projects beyond the surface of the ectoderm a short
hair-like process, the cnidocil (cl), and, in addition, an exceeding-
ly fine process of some length (n) is given off at the junction of
the stalk with the cyst-containing portion of the cell. These
two processes are supposed to be sensitive, the longer one per-
haps bringing the cnidoblast into connection with nerve-cells
lying elsewhere in the ectoderm. In some way not yet prop-
erly understood, a stimulation, such as a touch by some foreign

78 INVERTEBRATE MORPHOLOGY.

body, produces an evagination of the coiled thread from the cyst,
in the interior of which it has been bathed by the fluid contents.
The evagination is of sufficient force apparently to puncture the
skin of many animals and so inoculate the contents of the
cyst, which are of a poisonous nature, and produce inflam-
matory disturbances, and in minuter organisms paralysis or
death. To the presence of these structures jelly-fishes owe
their stinging powers, and they form efficient weapons both
for obtaining food and for warding off enemies.

The Cuidaria may convenient!}' be divided into three
classes :

I. Class -Uydromedusce.
II. " Scyphomedusce.
III. " Anthozoa.

1. CLASS HYDROMEDUSJE.

The Hydromedusae present both polyp and medusa forms,
the members of some of the orders contained within the
class being only of the medusa form, while in another order
only the polyp form occurs. Usually, however, both forms
occur in more or less perfect development, representing two
stages in the life-history of an individual and succeeding one
another in the definite manner which has been already •
described as an alternation of generations. The polyps pos-
sess the power of non-sexual reproduction by budding and
give rise by this method not only to medusae, but also to
other polyp individuals which may remain in connection with
one another and thus give rise to branching colonies. As a
rule, the medusa individuals separate from the polyp and
lead a free life, but in the order Siphonopkorce they may
remain in connection with each other and with polyp individ-
uals, undergoing various adaptations of form in accordance
with different functions which they assume, the whole form-
ing a colony presenting in a high degree a division of labor
among the component individuals.

As has already been pointed out the structure of the polyp
form differs considerably- from that of the medusa. The
polyp is more or less cylindrical in form, tapering off above

TYPE COSLENTERA.

79

m

— go

so that the free end has a somewhat conical shape, the month
being situated at the end of the cone. At the base of the
cone, which is termed the Jtypo-
stome, there is usually a circle of
tentacles, though occasionally
they are scattered irregularly over
the surface of the hypostome, and
a second circle of tentacles may
be present at the base of the
polyp (Pennaria). In colonial
forms each individual (hydranth)
(Fig. 39, liy] is situated at the ex-
tremity of a stalk or hydrocauius
(he), the various stalks either
uniting together to form a branch-
ing colony or else arising from a
network, the hydrorhiza, which
covers the surface upon which
the colony grows ; or occasion-
ally each hydranth arises, with-
out the intervention of a hydro- PIG 39._PoRTION OF A CoLONY
caulus, from a flat plate-like OP THE CAMPANULARIAN HY-
expansion common to all. In
either case the creleuterou is
continuous throughout the entire
colony, the fleshy substance of
the hydrocauius and hydrorhiza,
voenosarc (co), being tubular and in
direct continuity with the body-
walls of the hydranths. . The
coeuosarc is enclosed within a chitinous substance termed the
perisarc (p) secreted by the ectoderrnal cells and sometimes
prolonged at the end of each hydrocauius into a cup-like
structure, the hydrotheca (ht), into which the hydranth may
be retracted ; occasionally this ectoderrnal secretion takes
the form of carbonate of lime. '

The ectoderm generally shows a considerable amount of
differentiation of its constituent cells. In addition to the
cnidoblasts, which have been already described (p. 77), epithe-

DROID Clylia

co — cceuosarc.
he — hydrocauius.
ht = hydrotheca.
hy — hydruuth.
go — gouotheca.
m = nioulh.

p = perisarc.

t = tentacle.

80 INVERTEBRATE MORPHOLOGY.

lio-rnuscular, muscular, glandular, and nerve cells are gener-
ally to be found in it. The epithelio-muscular cells (Fig. 40,
A) are the most numerous and consist of columnar cells, one

extremity of which bears a cilium and
helps to form the outer surface of the
body, while the other is prolonged into
a somewhat spindle-shaped process of
highly contractile muscular substance.
The muscle-cells (Fig. 40, £) are modi-
fications of these, having lost their

FIG. 40.- = Epithelio- connection with the surface of the body,
muscular cell of Ccelen-

terate • B = muscular the cell-protoplasm and nucleus form-
cell ; C = sensory cell. ing a small elevation on the muscle-
fibre. The muscle-fibres rest upon the

outer surface of the mesoglcea, and in the ectoderm are, as a
rule, arranged longitudinally, so that by their contraction
they cause a shortening or retraction of the polyp. The nerve-
cells are of two kinds: (1) sensory cells (Fig. 40, 6*), which are
slender cells whose free end bears a single cilium, while
the inner end is produced into one or more slender nerve-proc-
esses which are supposed to place these cells in connection
with (2) the ganglion-cells. These are stellate cells lying in
the deeper layers of the ectoderm, just external to the muscle-
cells, and sending off delicate processes in various directions
so as to form a plexus of nerve-fibres ramifying through the
ectoderm.

The mesogloaa is thin and more or less fibrous in structure,
and rarely contains cells. The eudoderm-cells are large and
are of the epithelio-muscular variety, the muscle-fibres hav-
ing a circular arrangement producing by their contraction a
diminution of the diameter of the polyp. The protoplasmic
portion of each cell is furnished with a single flagellum and
is digestive in function, taking food-particles into its sub-
stance and there digesting them. In addition to these, gland-
ular cells also occur in the eudoderm, especially in the region
of the hypostome.

The medusa forms have the shape of a bell (Fig. 41)
which may be either shallow or deep, the mouth of the
bell in all cases being partially closed by a fold of tissue

TYPE C(ELENTERA.

81

which projects from the rim, and is known as the velum ;
from the presence of this structure these medusae have been
termed craspedote medusae. The cavity between this and the
bell is the subumbrellar cavity, into which projects a process
corresponding to the clapper of the bell and termed the
manubrium. At the free end of this is the mouth, which
opens into a canal traversing the mauubrium and communi-
cating at its base with the gastric cavity lying in the sub-
stance of the bell. From this four (in some cases more)
canals or pouches radiate out towards the rim and com^
niunicate there with a circular canal which extends coni-

FIG. 41. — Liriope scutigera (after BROOKS).

pletely round the margin of the bell, just where the velum
joins it. The canal of the manubrium, the gastric cavity,
and the radiating and circular canals together constitute the
ccelenterou. To the margin of the bell tentacles, varying in
number, are generally attached, on or between the bases of
which sense-organs are to be found.

The medusae differ not only in form but also in habit from
the polyps, being free-swimming organisms, propelling them-
selves through the water by expelling the water from the
subumbrellar cavity through the velar opening by sudden
and rhythmical contractions of the bell. In accordance with
this free mode of life sense-organs are present, and a higher

IN VERTEBRA TE MORPHOLOG T.

development of the nervous system tliaii is found in the
polyps obtains. The sense-organs vary both in structure and
function in different rnedusse, in some being spots of pigment
sometimes provided with a refractive lens and functioning as
light-percipient structures. Such medusae (Fig. 45) are
termed ocellate, or, since the eyes are on the surface of the
bell and are not enclosed within a chamber formed by the
growth around them of the adjoining tissues, they are some-
times termed gyinnoplithalmatous. In other forms again the
sense-organs consist of a group of cells containing in their
interior crystals of carbonate of lime and having a somewhat
vesicular appearance, and in the neighborhood of these are
sensory cells wdth long stiff cilia to which are imparted any
vibrations which the crystals may manifest. Such crystal-
containing cells are known as otocysts, on the supposition that
they form auditory organs, but it seems more probable
that their function is that of equilibrium organs, informing
the medusae of their position. Medusae possessing such
organs (Fig. 43) are termed, to distinguish them from ocellate
forms, vesiculate medusae.

The ectoderm which covers the outer surface of the bell is
much flattened, but near the rim it becomes columnar and
is ciliated, some of the cells assuming the form of sensory
cells similar in appearance to those of the polyps, their
slender nerve-filaments forming a delicate plexus in the
deeper layers of the ectoderm, in which are here and there
imbedded stellate ganglion-cells. A special nerve-ring sur-
rounds the bell at its margin. The ectoderm-cells which
cover both surfaces of the velum and the subumbrellar surface
are of the epithelio-muscular and muscular types, the mus-
cle-fibres being for the most part arranged circularly. In
the deeper layers of the subumbrellar ectoderm numerous
ganglion-cells with long prolongations occur forming a net-
work immediately external to the muscle-processes, and at
certain definite regions collections of reproductive cells occur.

The mesoglcea of the subumbrellar surface and of the
velum is thin, resembling that of the polyps, but in the convex
portion of the bell it is very thick and gelatinous in texture, and
may in some cases contain cells. The endoderni is throughout

TYPE C(ELENTERA. 83

the ccslenteron ciliated ; in the tentacles, which may be either
hollow or solid, it may retain its ciliated character, or else in
the solid tentacles form a solid axis similar to that of the
tentacles of the polyp forms.

Such are the general structural features common to all
the Hydromedusse. It remains to add here certain points by
which especially they are distinguished from other groups of
Cnidaria. These are : (a) the ectoderm and eudoclerm meet at
the mouth-opening ; (b) the sense-organs when present are
never modified tentacles : (c) a velum is always present in the
medusa forms ; and (d) the reproductive elements have their
origin in the ectoderm.

1. Order Hydrarise.

To this order belongs the Hydra, a common inhabitant of
fresh-water ponds and streams in all parts of the world. It
is a simple cylindrical organism, adherent by one extremity
to foreign bodies, though not fixed, being able slowly to
change its position. Below the short hypostome is a single
row of exceedingly extensible tentacles, which are hollow, a
peculiarity found in no other Hydromedusan polyps. So long
as conditions are favorable and nutrition abundant Hydra
reproduces by budding, the buds separating from the parent
form after a certain period of growth. Sexual reproduc-
tion also occurs, the spermatozoa developing in the ectoderm
a little below the ring of tentacles, while the ova form in the
same layer somewhat lower down on the bod}T, the animals
being hermaphrodites. The ova after fertilization remain
imbedded in the ectoderm of the parent for some time, but
later, developing around them a cyst, they sink to the bottom
of the water, and there remain usually for some time without
developing further. In this condition they are able to with-
stand a considerable amount of cold and dryness, and so
may tide the species over unfavorable conditions. No medusa
stage occurs in Hydra.

Hydra grisea L. is a brown form, relatively large in size, while H.
ririiUs L. is smaller and of a dark-green color due to chlorophyll-contain-
ing corpuscles imbedded in the ectoderm-cells and supposed by some
authors to be unicellular Algae of a symbiotic habit. (See p. 20.)

84 INVERTEBRATE MORPHOLOGY.

2. Order Narcomedusse.

In the Hydrarise the polyp form occurs without a medusa,
in the order Narcomedusw the reverse is the case, since in it
the medusa form is the only one present in a typical state of
development. The medusse are usually somewhat lens-
shaped structures (Fig. 42), with a lobed margin, the velum
(v), instead of being horizontal, being pendent from the niar-

FIG. 42. — Cunoctantlin ocionaria, HAECK. (After BROOKS).
m = margin of bell. ot = otocyst. v — velum.

gin (m) and extending up in the intervals between the lobes.
At the apex of each one of these intervals is situated a short,
stiff, solid tentacle which is usually bent backward over the
exumbrellar surface and is tipped by a knob of neniatocysts.
The cavity of the short manubriurn leads into a gastric
chamber which is prolonged out towards the margin into broad
pouches which lie opposite the tentacles and the intervals be-
tween the lobes, and around the margin, following the edge of
the lobes and therefore having a festooned arrangement, runs
a narrow circular canal which communicates with each radial
pouch at the apex of each interlobular interval. This struc-
ture is, however, absent in the American species Cunoctantka
octonaria. The reproductive organs develop in the subuni-
brellar ectoderm covering the pouches and sometimes extend
on to the mauubrium. Around the margin of the lobes are
seated club-shaped projecting otocysts (ot) composed of an
external layer of ectoderm surrounding a number of endo-
dermal cells, one or more of which contain a crystal of
carbonate of lime. The ectoderm cells in the neighborhood
are largely sensory and provided with long cilia, their inner
ends contributing to the formation of the marginal nerve-

ring.

TYPE CCELENTERA. 85

3. Order Trachymedusae.

In this order only the medusa form is present and it re-
sembles in many respects that of the Narcomedusse. The
bell is somewhat flattened, but is not lobed at tue margin ;
while the velum has the usual horizontal position and the
long slender manubrium projects some distance outside of the
subumbrellar cavity. From the gastric cavity four, six, or
eight radiating canals arise, which join at the margin the cir-
cular canal, and in the ectoderm of the subumbrellar surface
over a small area along the line of these cauals the reproduc-
tive cells develop. At the margin a number of sense-organs,
otocysts, are present, agreeing essentially in structure with
those of the Narcomedusae and are sometimes projecting or
in other cases more or less enclosed in a cavity formed by
the growing up around them of the adjacent substance of the
bell. The tentacles vary somewhat in shape and structure in
different genera. In JShopalonema they are all solid, eight
being somewhat longer than the other eight at whose bases
are the otocysts ; in Liriope (Fig. 41) four, situated opposite
the radiating canals, are hollow and extensible, while the
other four are solid and bent back over the exumbrellar sur-
face as in the Narcomedusse ; while in Geryonia all the six
tentacles are hollow.

4. Order Campanulariae or Leptomedusse.

In this order both the polyp and the medusa form occur
in the life-history of the individual, alternation of generations
being the rule. The polyp generation takes the form of a
branching colony, which generally shows a certain amount of
division of labor among the component individuals. The
nutritive hydrauths or trophopolyps present the typical form
already described (p. 78), and each is seated in a cup of per-
isarc, the hydrotheca, into which the polyp ma}r retract. Such
polyps reproduce only polyp forms, but in addition others
destitute of mouth and tentacles, and known as gonopolyps,
occur, which produce by budding numerous medusse or medu-
soid buds, the polyp and its progeny being together enclosed
in a cup of perisarc, differing in shape from the hydrotheca

86 INVERTEBRATE MORPHOLOGY.

and known as the gonotheca (Fig. 39, go). On account of the
presence of these cups the polyps of this order are sometimes
termed calyptoblastic. In the typical Campanularian polyp
colonies, such as those of Eucope or Ubelia, no further differ-
entiation of the polyps is found, but in the family Plumulari-
dae in the neighborhood of each of the small hydrothecse there
are one or more slender extensible polyps lacking mouth and
tentacles whose eudoderm is a solid axial cord, while the
ectodernial cells send off long, streaming, pseudopodia-like
processes, these polyps apparently playing the part of food-
providers for the colony.

The medusae (Fig. 43) are usually very shallow bells, with
numerous hollow tentacles depending from the margin, and

resemble the Trachyniedusse in
that the reproductive organs
develop on the line of the ra-
diating canals. Of these there
are in the majority of cases
four, though occasionally, as in
jffiquorea, they may be very
numerous. Sense-organs are
always present at the margin of
the bell and, as in the Trachy-
FIG. 43.—fflegmatodes tennis, Ag. medussB and Narcomedusa?, are

always otocysts, the rnedusse

belonging to the vesiculate category. A marked difference
obtains between the otocysts of the Leptomedusfe and those
of the two preceding orders in that the calcareous crystals
are in the former developed in ectoderm cells. The otocysts
furthermore primitively occur on the inner surface of the
velum, where they are lodged in a slight depression, which
may, however, deepen so much that the otoc}Tsts appear to be
imbedded in the substance of the bell.

In the typical Campanularians free-swimming medusre are
developed, and according as the polyps or the medusae attract
especial attention the order may be termed that of the Cam-
pauularise or that of the Leptomedusa3. In a few forms, such
as Rhegmatodes, up to the present no polyp generation is
known to occur, and conversely in certain genera, such as Ser-

TYPE CCELENTERA. 87

tulariaand Ilaleciuni, and in the Plumularidae, e.g. Aglaoplieuia,
it would appear at first sight that no medusa generation oc-
curred. This, however, is not strictly accurate, the appear-
ance depending on the medusa-buds in these forms never
becoming free-swimming but remaining in a more or less
undeveloped condition, the ova and spermatozoa becoming
mature notwithstanding the imperfect development of the
medusae. Alternation of generations, however, exists in these
forms just as much as in the typical Campanularians, for, no
matter how degenerate the buds of the gonopolyps may be,
they must still be regarded as the medusa generation.

5. Order Tubulariae or Anthomedusae.

In this order, as in the preceding one, a well-marked
alternation of polyp and medusa generations occurs. The
polyps are united to form colonies, the individual hydrauths
being constructed on the Tubulariau type, i.e. the perisarc
ceases at the base of each hydrauth so that there is no
hydrotheca. The tentacles show a greater variet}- of form
and structure than in the Campauulariau forms, and, though
sometimes filiform and arranged in a single cycle at the base
of the hypostorne (Margelis), are yet in other forms scattered
irregularly over the surface of the hypostome (Clavd), or may
be club-shaped (Coryne), or in addition to scattered club-
shaped tentacles a circle of filiform ones may occur at the
base of the hydranth as in Pennaria. A division of labor is
not the rule as in the Campanularians, but nevertheless this
phenomenon is in some cases carried to a much greater ex-
tent than in that group. In some forms special gonopolyps
are present from which all the medusae arise, but more fre-
quently any of the hydranths may produce these structures.
The gouopolyps when they occur are never enclosed within a
gonotheca, and hence the term gymnoblastic, frequently applied
to the polyps of this order. In the genus Hydractinia (Fig.
44) a complicated division of labor occurs. The various
hydranths composing the colony arise from a flat expansion
common to all and formed by the fusion of an original net-
work of ccenosarcal tubes, and on the surface of which numer-

88

INVERTEBRATE MORPHOLOGY.

ous stout spines occur. Some of the hydranths are typical
trophopolyps (Fig. 44, tp) with filiform tentacles and a mouth,
but among them are gonopolyps (gp) with short-knobbed
tentacles below which the medusa-buds arise. In addition
there are also towards the periphery of the colony much
longer slender hydranths without a mouth and like the gono-

mp

FIG. 44. — PORTION OP COLONY OF Hydractinia echinata (adapted from figure by

HlNCKS).

gp = gouopolyp. mp — offensive polyp. tp =. trophopolyp.

polyps in having short-knobbed tentacles, but differing from
them in tkeir greater length and in never producing medusa-
buds. Finally, in a European species of the genus a fourth
exceedingly long polyp (mp), destitute of both mouth and
tentacles, but furnished at its free end with numerous neniato-
cysts, occurs. These third and fourth varieties of polyp
probabl}' are offensive and defensive in function, procuring
food for the colony and warding off some predatory enemies.
The AuthoineduscTe, as the medusa forms are termed, have
much deeper bells than the Leptomedusse, and differ from
the latter in that the sense-organs are light-percipient in their

TYPE COSLENTEKA.

89

function, whence the medusae are frequently termed ocellate,
and, secondly, in that the re-
productive organs develop in
the wall of the manubrium
instead of on the line of the
four radiating canals. Ten-
tacles as a rule occur at the
margin of the bell, and it is at
their bases that the eyes are
formed. Occasionally, as in
Margelis (Fig. 45), tentacles
also arise from the end of the
manubrium. As a rule, the
sexual generation is composed
of free-swimming medusae, as
in Coryne, Margelis, and Pen-
naria, but not infrequently the
medusa-buds become retarded
in their development, as in FlG- 45-— MEDUSA OP Margelis caroli-
Hydractinia, Clava, Tubularia,

and to an extreme extent in Eudendrium, and all intermediate
stages between the two extremes are to be found.

6. Order Hydrocorallinee.

The Hydrocoralliuge are colonial marine forms represented
by the Stag's-horn Coral, Millepora, and characterized by the
densely ramified coenosarcal tubes being enclosed in a mass
of carbonate of lime secreted by the coanosarcal ectoderm
and taking the place of the perisarc. From minute pores on
the surface of this calcareous mass, the corallum, the hydrauths
protrude and present a well-marked polymorphism. The
pores are arranged in groups consisting of a central one sur-
rounded by a varying number of smaller ones. From the
central pore protrudes a hydranth with a circle of short ten-
tacles tipped with knobs containing numerous nematocysts ;
this is the trophopolyp or gasterozoid (Fig. 46, g). From the
smaller surrounding pores more elongated hydranths protrude,
destitute of a mouth, and with short scattered tentacles also

90

INVERTEBRATE MORPHOLOGY.

knobbed ; these are probably offensive polyps, of use to the
colony in obtaining food, and are known as dactylozoids (Fig.
46, d). The cavities in the corallum in which the gasterozoids
live are divided by transverse partitions into chambers into
the outermost of which the hydranth may be retracted, the
arrangement recalling what occurs in the corallum of the fossil
Tabulate Corals. In the genus Stylaster, however, which
forms a rose-red branching corallum, these partitions are

<1

rr

FIG. 46. — PORTION OF COLONY OF Millepora (after MOSELEY).
co = corallum. d = dactylozoid. g = gasterozoid.

wanting, a calcareous cone, the columella, projecting upwards
from the floor of the cavity.

In one species at least of Millepora a well-marked and typi-
cal alternation of generations occurs, medusae being formed
which are set free and develop the reproductive elements in
the ectoderm of the maimbriuin. In the majority of the
members of the order, however, the medusa-buds are never
set free, and are usually much degenerated, and indeed in
Millepora alcicornis they may be said to have completely dis-
appeared, and with them the alternation of generations. The
medusa-buds develop on the walls of the coenosarcal tubes,
and lie in cavities in the corallum which open to the exterior
by a pore through which the egg-embryos escape.

TYPE CGELENTEEA.

91

-P

7. Order Siphonophorae.

The Siphonopbores are free-swimming Hydromedusan
colonies, the constituent individuals of which show a high
degree of division of labor. The
various forms of individuals which
are to be found in different members
of the order are usually not present
in any one colony, some genera pos-
sessing some forms which others
lack ; it will be convenient, therefore,
to consider an ideal form in which the
various modifications are present.
Each colony (Fig. 47) consists of an
axis or stolon on which the various
individuals are seated and which
places them in connection with each
other ; it is usually long and slender,
but in some cases, as in Porpita, may
be reduced to a disk. At the ex-
tremity of the stolon may be found
a float or pneumatophore (Fig. 47, p),
a double-walled sac containing air in
the interior, and which is to be re-
garded as a modified medusa form.
Next to it come usually several medusa
forms lacking manubrium, tentacles,
and sense-organs, and possessing a
locomotor function ; these are the
swim-bells or nedocalyces (n). At in-
tervals along the rest of the stolon
are situated groups of individuals,
each group covered over by one or
more scale-like structures (cs), which
are again highly modified medusae, and
in each group is to be found a tropho-
polyp (tr\ a vase-like polyp form with
a wide trumpet-shaped mouth, and
having near its base a single tentacle
(t) which bears along one side a row of numerous secondary

47. — DlAGKAM OF A Sl-

PHONOPHORE COLONY.

cs — covering scale.
n = uectocalyx.
p — pneumatocyst.
r = reproductive polyp.
s = sensory polyp.
Ir — nutritive polyp.
t = tentacle.

92 INVERTEBRATE MORPHOLOGY.

brandies, each richly provided with nematocysts. Associated
with this nutritive individual is usually a reproductive form,
which in some cases may take the form of an Anthomedusa,
separating from the colony and leading a free life, as in
Velella, or may be medusoid, presenting a medusa form, but
lacking a mouth and tentacles and never separating from the
colony, or finally a gonopolyp (r) may occur which bears
numerous much-degenerated medusoid buds. In some forms
there is still another form of individual (s), resembling a tro-
phopolyp, but being destitute of a mouth and having a sim-
ple tentacle without the secondary branches. From its great
sensibility to stimuli this is supposed to be a sensory polyp.

In some forms, such as Diphyes, no pneumatophore occurs,
but nectocalyces are present ; in others, as Agalma, both occur
and the colonies resemble somewhat the diagrammatic form
described ; while in a third group, including the Portuguese
man-of-war Caravella, the pneumatophore becomes largely
developed and nectocalyces are wanting, the stolon at the
same time being contracted to a disk lying on the lower sur-
face of the pneumatophore. In Velella and Porpita the stolon
is reduced to a disk, but the pneumatophore is wanting.

Alternation of generations of a typical form, complicated,
however, by the polymorphism, occurs in such forms as
Velella, which possess a free-swimming medusa ; in the ma-
jority, however, it is obscured, as in many Tubularian hy-
droids, by the greater or less degeneration of the medusa
An alternation of another kind, however, occurs in some
forms, the bunches of individuals separating from the stolon
and leading for a time an independent existence, during which
their medusoid reproductive individuals become mature.

The complicated polymorphism of the Siphonophore colonies leads to a
merging of the individualities of the component individuals in that of the
entire colony, a process which reaches its highest pitch in such forms as
Velella. The various polyp and medusa forms of the colony may be con-
sidered as organ-individuals, and by their integration an individuality of a
higher grade — a metamere-individual — is produced.

Development of the Hydromedusce. — It has been mentioned
as one of the characteristic features of the Hvdrornedusne that

fj

the reproductive elements arise in the ectoderm. They reach

TYPE CCELENTERA. 93

their maturity in the medusae or medusoid buds except in the
Hydrarise, in which this stage is entirely wanting, and in cer-
tain Hydrocorallinse, in which it has disappeared, and may
likewise first become differentiated in the medusa. In many
forms, however, in which an alternation of generations occurs
they arise in the polyp, sometimes at a point far distant from
where the medusa-buds will arise, and reach these structures
only after, it may be, a rather extensive series of wanderings.
Thus, to take an extreme case, in Eudendrium the ova arise
in the ectoderm of the main stem of the pinnately branching
colony a short distance below the terminal hydranth ; as new
branches are formed in this same region the young ova
migrate into them, passing through the supporting layer and
wandering among the endoderm cells. Later on when the
gonopolyps arise on the lateral branches the ova wander into
them, still in the endoderm, and finally when the medusa-
buds develop on the gonopolyps the ova continue their endo
dermic course into them and eventually, again passing through
the mesogloea, take up their final position in the medusoid
ectoderm. Gradations between such an extensive migration
and cases in which none occurs are to be found, and it may be
stated as a general rule that the more the medusoid buds
depart from the medusa form the greater is the migration
undergone by the reproductive cells.

As a rule the Hydromedusae are of separate sexes, the sepa-
ration affecting the entire colonies — or, to put it slightly dif-
ferently, the medusae are always unisexual, and a polyp colony
when it occurs gives rise to medusae or medusoid buds all of
the same sex. The Hydrariae, however, are exceptions to the
rule, being hermaphroditic.

A blastula results from the segmentation of the ovum, and
this is converted into either a sterrula by immigration or a
diblastula (i.e. a hollow two-layered organism without mouth
or tentacles) by delamination (see p. 55). If a sterrula be
formed, it assumes the diblastula condition by a hollowing out
of the central mass, and after swimming about for some time
in this condition, if a polyp is to be formed, it settles down
upon some foreign body, a mouth breaks through and ten-
tacles appear, producing the first hydrauth.

94 INVERTEBRATE MORPHOLOGY.

In some forms, such as Hydractinia, the free-swimming embryo when
it settles down becomes converted into a flat plate-like expansion without
mouth or tentacles, from which, as a bud, the first hydranth arises.

If, however, the ovum develops directly into a medusa, as
in the Tracliymedusee and Narcomedusse, the breaking through
of the mouth and the formation of tentacles takes place
while the embryo is still free-swimming, and the stage so
produced may resemble closely a free-swimming hydranth, as
in Cunoctantlm, or may, by the great development of mesogloaa
at the extremity opposite the mouth, assume a rather globular
form, as in Liriope. As the tentacles develop and the bell
becomes differentiated by the extension laterally, as it were,
of the embryo, the velum arises at the margin of the bell.
At this time the coelenteron is a flattened cavity extending
to the margins of the bell, but later it becomes obliterated
along four lines, and the obliteration of the cavity extending,
the radiating and circular canals and the gastric cavity alone
persist, a layer of eudoderrn-cells sometimes joining them
and representing the obliterated portion of the coelenteron,
though often this also disappears.

In the Anthomedusse and Leptomedusae, in which the medu-
sae arise by budding from the polyps, the buds are at first
tubular outgrowths of the body-wall (Fig. 48, A). The ecto-
derm at the tip of the bud thickens, depressing the central
portion of the endoderm (Fig. 48, _B), and on the appearance
of a cavity in the thickened ectoderm, the subumbrellar
cavity, the central endoderm pushes out into the cavity, carry-
ing with it the ectoderm covering it and forming the rnanu-
brium (Fig. 48, C}. In this stage the bud, though still lacking
mouth and tentacles, is comparable to the polyp stage of the
medusa of direct development at least so far as the coalen-
terou is concerned, and by processes identical with those
occurring in the directly-developing embryo the radiating and
circular canals are formed (Fig. 48, JE1}, and on the formation
of a mouth at the extremity of the manubrium and the de-
velopment of tentacles the medusa is perfectly formed.

In many cases, however, as already stated, the medusa-
buds never reach their complete development, but become
sexually mature while still imperfect in form. The stage at

TYPE C(ELENTEKA.

95

which development ceases varies in different forms ; in Tubu-
laria, for instance, the medusoid bud resembles a medusa ex-
cept that it lacks tentacles, sense-organs and mouth, and is
not free-swimming ; in Clava not only does development
cease at an earlier stage, but a certain amount of degeneration

m

FIG. 48. — DIAGRAMS SHOWING THE DEVELOPMENT OP A MEDUSA-BUD.

A, outpushing of body-wall of polyp ; B, thickening of ectoderm ; C, forma-
tion of subumbrellar cavity ; D, transverse section through G, along the
Hue indicated by ab ; E, formation of radial canals ; F, transverse section
through E, along the line indicated by ab.

cc — circular canal. rs = radial pouch.

m = cavity of manubriurn. su = subumbrellar cavity.

re = radial canal. v = velum.

occurs, the rudimentary subumbrellar cavity never commu-
nicating with the exterior and the radiating and circular
canals being entirely obliterated ; and finally in Eudendrium
the bud never develops beyond the earliest stage in which it
is a simple tubular outgrowth of the body-wall.

TJ\e Relationships of the various Orders to one Another. — Since the
Hydrariae show such a simple type of organization it is generally supposed
that they represent more or less closely a primitive ancestral form, though
their usual habitat in fresh water suggests the possibility of their having
undergone some degradation. If they do represent the primitive Hydro-

96

INVERTEBRATE MORPHOLOGY:

medusan, then, it is evident that the medusa form is a secondary modifica-
tion— a specialized reproductive organ, which in the Narcomedusas and
Trachymedusas has become so important that the ancestral polyp form is
practically suppressed in the life-history. On this view it must be sup-
posed that organisms so similar as the rueduscb of the Tubularian and
Campanulariau polyps have been developed entirely independently of one
another, a view which carries with it many difficulties, and that the medu-
soid buds represent stages in their evolution.

It seems more probable, however, that, leaving the Hydrariie out of the
question, in all the other groups the medusa was the parent form. This
is borne out by the fact that in the Narco medusae, which, with their broad
pouch-like extensions of the gastric cavity, are the most primitive of all

the craspedote medusae, there is no fixed
polyp form. It has been shown, how-
ever, that the Narcomedusae and Trachy-
medusae in their development pass
through a stage which may be considered
to represent the polyp form, and if, while
in this form, non-sexual reproduction
should have taken place, the buds re-
sembling the immature form which gave
rise to them, a polyp colony would result,
some of the buds of which might con-
tinue their development and become
medusae. By this view the difficulties
presented by the similarity of the medusae,
throughout all the groups where they
occur are overcome and the medusoid
buds are regarded as imperfectly devel-
oped or degenerate medusae. Further-
more this view is rendered more than
probable by the development of Ctinoc-
tantha aud the allied Cunina. The
former while in the embryonic polyp
form actually does bud (Fig. 49), the
buds resembling the original embryo which gave rise to them, and all the
buds, the parent embryo included, later develop into medusa?. In Cunina,
however, the parent embryo which gives rise to buds undergoes no further
development, only the buds continuing on their course of growth to medu-
sae. In this case a true aud typical alternation of generations occurs and
points out a simple explanation of the alternation which is found in the
Anthomedusae and Leptomedusae. In these the polyp colonies are the re-
sults of non-sexual reproduction of a larval medusa, and some only of the
individuals so formed continue their development to medusae.

The relationships of the Hydrocorallinae to the other groups a:-e not yet
quite demonstrated. It would seem, however, from the medusa-bud which

FIG. 49. — BUDDING LARVA OF

CunoctanUia octonaria (after
McCEKADY, from BROOKS).
a = egg larva.
bb = budded larvae.

TYPE CCELENTERA. 97

occurs in one species of Millepora having its reproductive organs in the
walls of the manubrium that the affinities of the group are with the Antho-
medusae, and that an exceptional amount of degeneration of the medusa
had occurred in correspondence with the development of the calcareous
corallum.

The Siphonophores are evidently allied to the Anthomedusae, judging
from the characters of their medusae. The colony, however, contains both
medusa and polyp individuals, the former not being in all cases reproduc-
tive as in the Anthomedusae. The embryology of those members of the
order which have been studied in this particular indicates that they too must
be regarded as produced by budding in embryonic stages, some of the buds
developing to medusae, others remaining in the polyp stage. A further
differentiation of the medusae took place by which the pneumatophore,
nectocalyces, and covering scales have been specialized from medusae origi-
nally reproductive, the pneumatophore probably representing the parent
individual of the colony. It is interesting to note in this connection that
in some forms the reproductive rnedusaa after having expelled their ova or
spermatozoa become converted into nectocalyces.

II. CLASS SCYPHOMEDUSJE.

Iii the Scyphomedusse the medusa form is preeminent, the
polyp form being placed in the background and occurring
only as a larval stage, though in some forms it assumes
somewhat greater importance on account of the power it may
possess of reproduction by transverse division.

The medusae are usually free-swimming, though a few of
the more lowly organized forms are attached throughout their
lives by a prolongation of the exunibrellar surface (Fig. 52),
forming a connecting link between the free-swimming forms
and the polyp. As a rule they reach a much greater size
than do the Hydromedusae, from which they are further dis-
tinguished by (a) the absence of a velum, (6) by the sense-
organs when present being modified tentacles, and (c) by the
reproductive cells always arising in the endoderm. On ac-
count of the first of these characters the Scyphomedusse are
sometimes termed the Acraspeda.

They are all more or less bell-shaped, a number of ten-
tacles usually hanging from the margin of the bell (Fig. 53).
These, however, are frequently secondarily developed, there
being in the simpler forms eight primary tentacles which may
persist as tentacles, or four or all of them may be converted

98

INVERTEBRA TE MORPHOLOO T.

iiito sense-organs (Fig. 50, s) situated near the margin of the
bell and more or less enclosed in special chambers by the
growth around them of folds of the bell substance (?). From
the centre of the subunibrella there hangs the manubriurn
(TO), the extremity of which is frequently prolonged into four
elongated niouth-lobes, and above it communicates with the
gastric cavity, g, which in simple forms extends to the
margin of the bell, being obliterated only at four interradial
points or lines. The four broad radial pouches thus produced
correspond with the radiating canals of the craspedote medu-
sae, and the line of fusion being imperfect at the margin of the

FIG. 50. — DIAGRAM OF SCYPHOMEDUSA.

g = gastric cavity. r = reproductive organs.

I = lobe covering the sense-organ. s = sense-organ.

m = month. sg = subgenital cavity.

mf = uieseuterial filaments. t = tentacle.

bell a communication between adjacent pouches is present
comparable to the craspedote circular canal. This condition
is, however, only retained in the simpler forms ; in the higher
Scyphomedusse the lines or points of obliteration may be
omitted, and by secondary obliterations taking place over
various areas of the coelenteron, and by its irregular extension
towards the rim of the bell as this grows in diameter, a com-
plicated branching arrangement of the peripheral portion of
the ccelenteron is produced, in which, however, the original
4-radial arrangement is as a rule distinctly indicated. Along
the iuterradial axes there are four deep depressions of the
subumbrellar surface, the funnels or siibgenital chambers (Fig.
50, sg), above which lie the horseshoe-shaped reproductive
organs, r, developed in the ccelenteric eudoderm, one limb of
each horseshoe lying in each of the adjacent radial pouches.

TYPE CCELENTERA. 99

Finally, iu the line of each interradius there project into the
ccelenteric cavity a number of coarse thread-like filaments,
the mesenterial filaments (mf\ which are unrepresented in the
Hydromedusse.

Such is the general structure of the Scyphomedusse ; the
modifications will be better described in connection with the
various orders into which the class may be divided. In histo-
logical structure the resemblance to the Hydromedusse is so
great as to do away with the necessity of a detailed account,
except as regards the sense-organs. As already stated, these
when present are modified tentacles and partake of the char-
acters of both eyes and otocysts. They are usually short
finger-like stalks, lying in a notch of the rim of the bell
and covered over by folds (covering plates, Fig. 51, cp) arising
from the adjacent substance of the bell on either side of the

ec

o

FIG. 51. — MARGINAL SENSE-ORGAN OF Rhopalonema Cafter HERTWIG).
cc = ccelenteric cavity. en = endoderm.

cp = coveriug plate. o — eye.

ec = ectoderm. ot = otocyst.

niche and frequently uniting so that the stalks lie in pouch--
like cavities. The ectoderm of the finger-like stalks contains
numerous sensory and ganglion cells, and at one or more
regions pigment-cells are associated with these to form the
eye (o), which may be further perfected by the addition of a
cuticular lens. The stalks are hollow, containing a prolonga-
tion of the ccelenteric cavity (cc) lined by endoderm, and at the
tip of the stalk the endoderm-cells are filled with crystals of
carbonate of lime, the whole mass of crystals forming a rather
large otocyst (ot). The covering plates, furthermore, above the
sensory stalks are usually grooved, the bottom of the groove

100

INVERTEBRA TE MORPHOLOG T.

being lined by sensory cells to which an olfactory function is
attributed. The marginal sense-organs of the Scyphoinedusse
are thus much more complicated in structure and in addition
have a different mode of origin than those of the Hydrome-
dusse.

1. Order Stauromedusse.

In this order some of the forms are fixed throughout their
adult life, e.g. Lucernaria (Fig. 52) ; while others are free-
swimming when mature, as Tessera. The deep bell has at
the margin eight tentacles — the primary tentacles, none of
which become modified into sense-organs, these structures
being wanting in the group. In some species of Lucernaria

FIG. 52.—Haliclystus auricula DIVIDED LONGITUDINALLY (after H. JAMES-CLARK).

/ = funnel. mf = mesenterial filaments.

go = reproductive organs. rp = radial pouch.

ia = iiiterradial adhesion. t — modified primary tentacle.

these primary tentacles (t) are somewhat altered, and in all
the margin of the bell is produced into eight knob-tipped
lobes, to each of which a bunch of secondary tentacles
may be attached. The ccelenteron extends out to the margin
of the bell, and is interrupted along the interradii by a point

TYPE CCELENTERA.

101

(Tessera) or Hue (Lucernaria) of adhesion. The depressions
of the subumbrellar surface, the funnels (/), are very deep in
Lucernaria, extending almost to the summit of the bell.

2. Order Peromedusae.

In this order the adult medusae are always free- swimming,
and are characterized by the bell being pointed in shape and
about its middle marked with a distinct constriction. The
coelenterou is obliterated at only four points, as in Tessera,
and they differ from the Staurornedusse
by possessing four sense-organs, the
four interradial tentacles of the primary
series of eight becoming modified to
form these structures, while the radial
ones retain their original character.

3. Order Cubomedusse.

This order, of which Charybdea (Fig.
53) is a typical example, is characterized
by the bell being of a cubical shape.
The interradial obliterations of the
coelenteron are linear, and, as in the pre-
ceding order, four of the primary ten-
tacles, these being the only ones which
develop, are modified to form sense-
organs. In this order, however, it is FIG. 53.— Charybdea mar-
the four radial tentacles which form the supialis (after GLAUS).
sense-organs (so), the four interradial per-
sisting as tentacles (£).

4. Order Discomedusae.

In this order, which includes the majority of the known
Scyphomedusse, all the eight primary tentacles are converted
into sense-organs, a number of secondary tentacles usually
developing at the margin (Fig. 54). The primary iuterradial
obliterations of the coelenteron do not develop, but on the
other hand secondary obliterations frequently make their

102

INVERTEBRATE MORPHOLOGY.

appearance, which, combined with irregularities of growth of
the coeleuteron, give its peripheral portions an irregular out-
line (Cyanea), or convert them into a series of anastomosing
canals, e.g. Aurelia. The margin of the bell is usually more
or less lobed, eight of these lobes being especially distinct
and carrying the sense-organs, the intervals between them
being usually occupied by the secondary tentacles when these
are present. The depressions of the subumbrellar surface are
no longer deep funnels, but form rather shallow subgenital

FIG. 54. — Pelagia cyanella (after AGASSIZ).

chambers with thin roofs, into the cavity of which the repro-
ductive organs bulge out.

In many forms the margins of the mouth are prolonged
into long fringed lobes, and in one family, the fihizostomidce
(e.g. Stomolophus), the margins of these lobes may fuse to-
gether, leaving, however, a large number of minute openings
along the line of fusion. These lead into canals traversing

TYPE C(ELENTERA. 103

the substance of the lobes, and uniting finally in a larger
central canal at the upper extremity of which lies the original
mouth.

Development of the Scypliomedusce. — The segmentation of
the ovum leads as usual to a blastula form which may be-
come solid by immigration and subsequently hollow out, or
may abbreviate these processes by a typical iuvaginatiou.
The gastrula resulting passes in some forms gradually into the
adult condition (Pelagia), without ever relinquishing its free-
swimming habits, though in the majority of cases in which the
life-history is known, cases confined entirely to the Discome-
dusse, the embryo settles down and leads for a time a fixed
sessile existence, resembling very much a polyp. This polyp
form (Fig. 55, A), known as the Scyphostoma, differs mate-
rially from the hydroid polyp ; in the first place, from the
body-wall there project into the ccelenteron four linear
ridges (mesenteries) which extend from the neighborhood of
the mouth to the posterior end of the cceleuterou, and later
give rise to the meseuterial filaments of the medusae ; and in
the second place, a deep funnel-like depression extends into
each one of these mesenteries from the oral surface of the
Scyphostoma, and produce later the subgenital chambers of
the medusae.

The Scyphostoma may develop directly into the free-
swimming medusae, but in a number of forms it undergoes a
series of transverse divisions (Fig. 55, B) into a number of
saucer-like structures produced at the edges into eight blunt
notched prolongations. These separate from the parent Scy-
phostoma and are known as Ephyrce. (Fig. 55, C), developing
into the adult Discomedusan by the intervals between the
eight lobes, which carry the sense-organs, filling out, by the
development of tentacles, and by the growth of the coelenteron.
A typical alternation of generations is thus brought about.

There can be little doubt but that the ancestral Scyphomedusan was a
sessile organism with much resemblance to Lucernaria. From this two
lines of descent arose, both marked by a development of sense-organs from
tentacles and terminating in the Peromeduste and Cubomedusae respec-
tively, the Discomedusse resulting in the culmination of this sense-organ
development. So far as the Discomedusse themselves are concerned the

104

INVERTEBRATE MORPHOLOGY.

Epliyra seems to represent an ancestral stage, since some mature medusae
resemble this stage very closely and it occurs in the life-history of all, and
earlier than this is the Scyphostoma representing the Lucernaria stage of
evolution. The Scyphostoma has superficial resemblance to a hydroid

FIG. 55. — A, Scyphostoni of Aurelia ; B, Stromla of Aurelia ; C, Ephyra of

Pelayia (all after AGASSIZ).

polyp, which resemblance is almost an identity in the earlier stages of the
Scyphostoma before the development of the mesenteries and funnels. This
suggests a relationship of the Scyphomedusae to the Hydromedusse only
through the polyp, the separation of the two classes having occurred be-
fore the appearance of the medusae on the scene.

III. CLASS ANTHOZOA.

The Anthozoa never assume the medusa form, but are ses-
sile, usually colony-producing polyps of the Scyphostoma
type. Typically they are cylindrical structures (Fig. 56) at-
tached at one extremity, the base, and bearing at the other
extremity the mouth in the centre of a flat surface, the disk,
around the margins of which are a number of hollow tenta-
cles. The ccelenteron is imperfectly divided into a number
of chambers by longitudinal partitions arising from the body-
wall, the mesenteries (Fig. 57, me), the various iutermeseuterial

TYPE C(ELENTEKA.

105

chambers communicating freely with a central space. The
mouth does not open directly into the coeleuteron, as in the
hydroid polyps, but into a tube, the stomatodceum (stj, lined
by ectoderm and communicating freely below with the central
coelenteric space. Certain of the mesenteries in their upper
portions are attached to the endoderuiic surface of the stoma-
todreum, but below its lower end all have free edges. Along
this free edge there runs, in most of the mesenteries, a cylindri-

me

FIG. 56. — Metridium margina-
tum, LES.

rm

FIG. 57. — DIAGRAMMATIC TRANS-
VERSE SECTION THROUGH Edward-
sia IN REGION OF STOMATODCEUM.
me = meseulery.
rm — retractor muscle.
si — siphonoglyphe.
st = stoinalodiEuin.
I-IV = mesenteries in the order of
their development.

cal thickening, the mesenterial filament, composed in its lower
portion of cnidoblasts and gland cells, and usually longer than
the mesentery, on the edge of which it forms a complicated
system of coils and twists. In its lower part the filament in
some forms separates from the mesentery as a bunch of fine
filaments richly provided with nematocysts and capable of
being protruded from the mouth or through pores in the
body-wall. The upper part of the filament is usually of dif-
ferent structure, bearing on each side a baud of elongated
ciliated cells whose function it is to produce a circulation of
the fluids in the coelenterou.

The reproductive cells develop in the endoderm of the

106 INVERTEBRATE MORPHOLOGY.

mesenteries, whence they are shed into the intermesenterial
chamber and make their exit by the month. The nervous
system is well developed especially in the ectoderm of the
disk and tentacles, though it also occurs in the endoderm.
It possesses the general character of the Coelenterate nervous
tissue consisting of sensory cells, nerve-fibres, and ganglion-
cells. The muscular system is very well developed both in
the ectoderm and endoderm, the muscle-fibres being generally
longitudinal in the former layer, and in the latter circular in
their direction. At certain regions of the body the muscle-
fibres are especially abundantly developed, the mesogloaa
being thrown into complicated folds for their support, so that
it is possible to distinguish certain definite muscles. One of
these is developed upon one face of each mesentery, and, its
fibres being directed longitudinally, it forms a strong retractor
(Fig. 57, rm) for the disk and tentacles ; a second is developed
in the endoderm of the body- wall a short distance below its
junction with the disk, and its fibres may, by the growth of
the inesogloea around them, become imbedded in that layer ;
it forms a more or less powerful sphincter, serving to cover in
the disk and tentacles when these have been retracted by the
mesenterial retractors.

The Anthozoa are constructed upon a radial symmetry,
as are the other Coslentera, this symmetry appearing in the
arrangement of the mesenteries and tentacles and in the cylin-
drical form of the body. Nevertheless it is always possible
to divide the Anthozoan by a single plane into two similar
halves, that is, a bilateral symmetry is also present which is
produced by the arrangement of the retractor muscles on only
one face of each mesentery and by the flattening of the sto-
matodaeum. This latter feature is furthermore usually made
more pronounced by the occurrence, at one or both ends of
the longer transverse axis of the stornatoda3um, of a distinct
groove lined by high columnar cells with long cilia, these
grooves forming the siplionogly plies (Fig. 57, si), and by the
mesenteries which are attached to the stomatodaeum in the
neighborhood of the siphonoglyphes usually having their
retractor muscles on different faces from those on which they
occur in the other mesenteries.

TYPE CCELENTERA.

107

Frequently the ectoderm of the anthozoan polyps secretes
a, skeletal substance which may either be carbonate of lime, or
else an organic substance of a horny consistency. In the
corals the secretion takes the form of carbonate of lime and
forms a cup (Fig. 58, ap) in which the polyp is seated, ridges

pap \hs .ss-

~.ap

FIG. 58. — DIAGRAM OF THE STRUCTURE OF A CORAL (after VON KOCH, from LANG).

ap — exotbeca. hs — mesentery.

fp = basal plate. ss = septa.

Calcareous skeleton, white; ectoderm, shaded; inesogloea, black; endoderm,

dotted.

(septa, ss), over which the soft tissues of the animal are
moulded, projecting up from the bottom of the cup. The
septa may be united by delicate tangential bars, synapticulce,
and from the bottom of the cup a somewhat cylindrical colu-
mella may project, other upright rods, the pali, intervening be-
tween the free edges of the septa and the columella. The
upper portion of the body-wall of the polyp is reflected over
the rim of the calcareous cup and may produce ridges, costce,
on its outer surface corresponding in position with the septa,
and inasmuch as the cup is continually increasing in depth so
long as the polyp lives, and the polyp only occupies the upper
portion, the lower part may from time to time be separated
off by a transverse partition or dissepiment.

In other forms, such as the Alcyonarians, however, the
skeleton is secreted only b}r the basal ectoderm and the colony

108 INVERTEBRATE MORPHOLOGY.

becomes moulded over it (Fig. 59), so that it forms a central
horny or more or less calcified axial support, and in addition

the mesogloeal cells secrete scattered
particles of carbonate of lime, having a
more or less definite form for each spe-
cies, but not uniting together to form a
firm skeleton.

The development of the Authozoa is
always direct, and the diploblastic
FIG. 59. —DIAGRAM OF condition is produced by delamiua-
YOTTNG GORGONIAN (af- tion of the cells of the blastula. Many

ter VON KOCH). i i* r j.i e

adult lorms possess the power of

division, either transverse or longitudinal, the latter giving
rise to complex colonies in many cases. In other forms the
primary polyp may develop a stolon from which other indi-
viduals may bud, producing a diffuse colony, or the intervals
between the individuals may be filled up by a growth of
mesogloaa traversed by a network of canals, forming a tissue,
the coenenchyme (Fig. 60), in which the various individuals are
imbedded.

The class Anthozoa may be divided into a number of
orders, whose existence depends mainly on the arrangement
of the mesenteries.

1. Order Alcyonariae.

The majority of the Alcyonariaus produce colonies by bud-
ding. In some the individuals are scattered on stolons, in
others imbedded in a coenenchyme (Alcyonium, Fig. 60), or in
others united to form fleshy colonies of a feather or reuiform
shape (Eenilld), the whole being imbedded in the sand by a
fleshy stalk. In some of the groups a horny or calcareous
skeleton is present in addition to the calcareous spicules im-
bedded in the mesogloea and may form a central axis enclosed
by the coeneuchyme (Fig. 59) and of a horny consistency, as in
Gorgonia and Leptogorgia, or more or less calcareous, as in
Isis and Corallium, the skeleton of the latter constituting the
red coral of commerce. In the Organ-pipe Coral, Tubipora,
each individual lives in a calcareous tube, the various tubes

TYPE CCELENTERA.

109

being united by transverse plates, and in Heliopora the skeleton
becomes very massive, resembling that of the ordinary corals
even to the occurrence of septa projecting into the interior of
the cups which contain the individual polyps.

Notwithstanding these manifold variations of the skeleton
and of the colony form, the individual polyps present through-
out a great similarity of structure. They possess only eight
pinnate tentacles and eight mesenteries whose retractor mus-
cles are arranged in the manner shown in the annexed dia-
grammatic cross-section of a polyp (Fig. 61). In £enitta and

rm— f / -

FIG. 60. — DIAGRAM OP YOUNG
COLONY OF Alcyonium (after
VON KOCH.

FIG. 61.— DIAGRAMMATIC TRANSVERSE
SECTION OF AN ALCYONARIAN.
rm = retractor muscle.

si = siphonoglyphe.
I-IV — mesenteries.

allied forms, such as Pennatula, a slight polymorphism occurs,
certain polyps possessing no tentacles and functioning as
inhalent zooids through which currents of water pass into the
coelenteric cavities of the colony through which they circulate.

2. Order Edwardsiae.

The Edwardsiae never produce colonies nor do they possess
a skeleton, though frequently the exte ior of the body is en-
crusted by foreign particles. They live usually imbedded in
sand, the base being rounded and not adhesive, and possess
eight (sometimes sixteen or thirty-two) simple tentacles and
eight mesenteries, differing from those of the Alcyonarians in

110

INVERTEBRATE MORPHOLOGY.

the arrangement of the retractor muscles as shown in Figure
57.

3. Order Ceriantheae.

The Cerianthepe are, like the Edwardsiae, solitary forms
destitute of a skeleton, and live imbedded in sand or mud.
The basal region is rounded and not adhesive, having at the
centre a pore which communicates with the coeleuteron. In
Cerianthus a fibrous investment surrounds the body as a tube,
secreted by the ectoderm, this layer of the body being further
characterized by an enormous development of muscle-fibres
arranged longitudinally and supported upon slender processes
of the mesogloea of the body-wall. The tentacles are simple
and very numerous, being arranged in two sets, one surround-
ing the margin of the funnel-
shaped disk and the other im-
mediately surrounding the
mouth. The mesenteries are
also very numerous and are
distinguished by the absence in
the adult of retractor muscles,
theectodermal muscles playing
the part of the retractors, and
the characteristic Anthozoan
sphincter is also absent. The
arrangement of the mesenteries

(Fig. 62) is peculiar to the
FIG. 62. — DIAGRAMMATIC TRANS-
VERSE SECTION OP A YOUNG Ceri-
anthus (according to CARLGREN).
si = siphonosrlyphe.

SrouP' *ew ones continuing to
form during the entire life of
the animal and making their

I-IV -- the Edwardsian mesenteries, appearance one on each side

1-4 = seccmdary mesenteries. ,. ,-, ... -,

of the sagittal plane between

the two which immediately preceded them. The older mes-
enteries are thus crowded to one surface of the body, the
dorsal surface, at which the single siphouoglyphe (si) occurs
in the stomatodaeum, and the four on either side of the
mid-dorsal line (T-TV} are the homologues of the eight
mesenteries of the Edwardsije, the rest being secondary
structures not represented in that group.

TYPE CCELENTERA.

Ill

4. Order Antipatharise.

The members of this order are all colonial and secrete a
branching axial skeleton of a black horny material. The
polyps possess usually but six simple tentacles, and as a rule
only six mesenteries are present, of which only the two lying
in the transverse axis bear reproductive organs and mesen-
terial filaments ; in some forms four or six additional imper-
fectly-developed mesenteries are present, but six seems to be
the number typical for the group.

5. Order Protactinise.

This order includes a group of forms, all simple and with
simple tentacles, but showing considerable variation in the
number of the tentacles. They all
agree in this particular, however,
that there are twelve mesenteries
arranged in pairs (Fig. 63, I-V1\
the two pairs attached to the si-
phonoglyphe region of the stoma-
todieum having their retractor
muscles on the faces turned away
from each other, while in the other
four pairs they are on adjacent
faces. The two former pairs are ,

termed the directive mesenteries FlG . GS.-DIAGRAMMATIC TRANS-
(Fig. 63, D and D'}, their constit- VERSE SECTION OP Gonactinia.
uent mesenteries lying one on each ®' D> = directive mesenteries,
side of the sagittal plane, and to- Z~IV '- the Edwardsian niesen-

gether with one mesentery (//and v> VI = ries forming

/) from each of the other pairs pairs with //and I.

represent the eight Edwardsian 7 ~ secondary pair of mes-
inesenteries. To these six primary

pairs a varying number is added in the different forms ; it
may be, on each side, one between one of the pairs of direc-
tives and the adjacent lateral pair (Scytopliorus), or a pair in
the same locality (Gonactinia, Fig. 63, 7), or two pairs one of
which corresponds to the pair of Gonactinia, the second pair
lying between the two lateral primary pairs (Oractis).

112

INVERTEBRATE MORPHOLOGY.

In all these forms there is a strictly bilateral arrangement
of the mesenteries, and a tendency for them to arrange them-
selves in pairs.

6. Order Zoantheae.

The Zoanthege form very frequently colonial aggregates
either of a diffuse stoloniferons character (Zoanthus] or of a
more compact form, the individuals being imbedded in a cce-
nenchyme (Palythoa). No skeleton is present, though many
forms have a dense crust on the outside of the body formed
of particles of sand, sponge-spicules, radiolarian antl fora-

ininii'eran shells, etc., imbedded
in the outer portion of the rneso-
gloea. They possess a varying
number of simple tentacles, and
there is only a single siphono-
glyphe which marks the ventral
surface of the body. The mes-

10 • j -

entenes are arranged in pairs,
six of which (Fig. 64, /- V, II- VI,
///and IV) correspond with the
six primary pairs of the Protac-
tinise ; of these the dorsal direc-
FIG. 64.— DIAGRAMMATIC TUANS- tives (D) are never united to the
VERSE SECTION OP ZonntJius. stornatodaeum and the dorsal
D D' = directive mesenteries. ^^ [r (// y ^ consigts of
I- VI as in preceding figure.

1-4 = pairs of secondary mesen- ono perfect and one imperfect
teries. mesentery, the latter being ventral

to the former. The ventral lat-
eral primary pair may consist of two perfect mesenteries or
may have the same arrangement as the dorsal lateral pair.
To these six pairs a varying number of secondary pairs (1— t)
may be added, the new pair always arising immediately on
either side of the ventral directives. Each of the new pairs
consists of a perfect and an imperfect mesentery, the latter
being the dorsal one of the two, these secondary pairs thus
differing from the lateral primary mesenteries.

TYPE C(ELKNTERA.

113

7. Order Hexactiuiae.

In the Hexactinise the six primary pairs of mesenteries
described as occurring in the two preceding orders are again
found (Fig- 65, /, D, and D), and in a few forms (Halcampa)
may be the only ones present. As a rule, however, a varying

D

a

i

I.T

D

FIG. 65. —DIAGRAMMATIC TRANSVERSE SECTION OF AN HEXACTINIAN, Aip-
tasia, with only the mesenteries of the first cycle perfect.

D = directive mesenteries. / — mesenteries of the first cycle.

g = reproductive region of mesentery. // = " " second "

mf = meseuterial filament. 111= " "third "

number of secondary pairs develop, each of these appearing
in the interval between two primary pairs, so that two cycles
of mesenteries (/ and //) may be distinguished. Usually,
however, the process of mesentery formation does not stop
here, tertiary (///), quaternary, etc., cycles being developed,
the pairs of each new cycle appearing in the intervals between
the pairs of the cycles already present. Consequently, since
there are six primary pairs, the second cycle will consist also
of six pairs, the third of twelve, the fourth of twenty-four, and
so on. In a few forms, owing to the precocious development
of one or two of the secondary pairs on each side, the
symmetry becomes converted from an hexanierous one to an
octamerous (Aiptasia annulata) or a decamerous one (Tealia).
Since the tentacles develop in connection with the spaces
between the mesenteries, they are arranged in cycles corre-

11-t INVERTEBRATE MORPHOLOGY.

spending to the mesenteries. Usually but a single tentacle
communicates with each space, but in some forms a series
may arise on the roof of each space so that the tentacles have
a radiating arrangement (Discosoma) or may appear to be
irregularly scattered, as in some corals (fungia). They are
usually simple in form, though they may be in some cases
pinnate (PlnjuxndJius, Thalassiantlms] or even branched.
The order is usually divided into two suborders :

1. Suborder Malacodermata.

This includes the Sea-anemones or Actinians, all simple
forms, not producing colonies, and usually attached by an
adhesive base. They never form a skeleton of any kind,
though they may develop an enveloping cuticle, usually very
thin and in some cases encrusted with foreign matter ; this
is more especially the case with deep-water forms, the shallow-
water forms, such as Metridium, jBunodes, etc., lacking a
cuticle. Many forms possess the power of division, the in-
dividuals so produced separating completely and not forming
colonies ; furthermore some forms reproduce non-sexually by
separating off portions of the tissue at the margin of the base,
each portion eventually developing into an adult Actiuian.

2. Suborder Sclerodermata.

This suborder includes the ordinary corals, which secrete
a calcareous skeleton of the character already described
(p. 107). A few forms are simple, but the majority produce
complex colonies by longitudinal division and by budding,
while in others the division is only carried to the extent of
the formation of an individual with a number of mouths, as in
Fungia and Manicina. In most of the forms the coralluin is
tolerably dense and may be either branching, as in Oculina, or
form massive blocks, as the Brain-stone Coral (Mwandrina),
but in Madrepora it is more or less porous.

The Corals are most abundant in tropical seas and in shallower water,
the Madrepores forming under such conditions large reefs, in the lagoons
of which the Fungius, Manicimis, and Masandrinas are found. In colder seas
but few forms (Astrangia) are found in shallow water, but in the greater

TYPE CCELENTERA. 115

depths of the ocean the simple forms which do not produce colonies are
frequently found.

Relationships of the Anthozoa. — As has been pointed out, it seems prob-
able that the Anthozoa are to be traced back to a Scyphostoma-like polyp
lacking interradial funnels. Nofour-mesenteriedfonn, however, is known,
a large gap existing between the Scyphostoma and the Alcyonaria, which
are probably the simplest Anthozoa known to us. The primitive Alcyonaria
were undoubtedly simple forms, and from them to the Edwardsise was not
a very great step. By the formation of four additional mesenteries the
Ivhvardsian condition became converted into the twelve-mesenteried con-
dition which forms the ground-form of the Protactinia?, Zoanthea?, and
Hexactinia3, the various stages seen in the Protactinise indicating the
manner in which, the Hexactinian condition has been brought about.
The Cerianthese seem to be offsets from the Edwardsian condition, but it is
difficult in the present state of our knowledge to conjecture the affinities of
the Antipatharia.

It is noticeable that the members of all the orders except the Hexac-
tmiae have a strictly bilateral arrangement and development of the mesen-
teries ; this arrangement becomes gradually modified, first, by the ten-
dency of the mesenteries to arrange themselves in pairs ; second, by the
formation of secondary mesenteries ; third, by a tendency for these to
appear in pairs ; fourth, by a tendency for such pairs to appear in all the
intervals between the primary pairs. Thus the Anthozoa are forms which
are gradually specializing away from the radial symmetry characteristic
of all Coelenterales towards a bilateral symmetry, and the more pronounced
radiality of the Hexactiniae is a secondary condition.

SUBKINGDOM METAZOA.
TYPE CCELENTERA.

I. Subtype PORIFERA. — With pores in the walls and without nemato-
cysts.

1. Order Calcarea. — Skeleton calcareous.

(a) Ascon type. Leucosolenia.

(b) Sycon type. Grantia.

2. Order Cornacuspongice. — Skeleton of spongiolin, usually with

simple siliceous spicules.

(a) With spicules ; fresh water. Sponyilla, Ephydatia.

(b) Without spicules ; marine. Euspongia.

3. Order Spicidispongm.— Skeleton of uniaxial or tetraxial sili-

ceous spicules. Sometimes entirely wanting.

(a) Skeleton wanting. Halisarca.

(b) Skeleton present. Cliona, Esperella.

4. Order Hyalospongice. — Skeleton of 6-rayed siliceous spicules.

Euplectella.

116 INVERTEBRATE MORPHOLOGY.

II. Subtype CNIDARIA. Without pores in walls and with nematocysts.
I. Class HYDROMEDUS^E. — Ectoderm and endoderm meet at mouth. Re-
productive organs develop in ectoderm. Medusa with velum ; sense-
organs not modified tentacles.

1. Order HydraricE. — No medusa form ; tentacles hollow. Hydra.

2. Order Narcomedusae . —No hydroid form ; sense-organs otocysts

of endoderuial origin ; radiating canals represented by broad
pouches. Cunoetautha, Cuniua.

3. Order Tr a cliy medusae. — No hydroid form ; sense-organs otocysts

of endodermal origin ; radiating canals narrow. Liriope, Ger-
youia, Rhopalonema.

4. Order Leptomedusce or Campanularice. — With both hydroid and

medusoid forms, the latter frequently degenerate. Hydrantlis
with hydrothecae ; gonangia present. Medusa with otocysts of
ectodermal origin ; reproductive organs on radial canals.

(//) Hydroid and medusoid forms both well developed. Eu-

cope, Obelia.
(6) Hydroid form not well developed. ^Equorea, Rhegma-

todes.

(c) Medusoid form degenerate. Sertularia, Halecium, Aglao-
phenia.

5. Order Anthomedusce or Tubularice. With both hydroid and me-

dusoid forms, the latter frequently degenerate. Hydroid forms
without hydrothecse or gonangia. Medusoid forms with eye-
spots ; reproductive organs developed in wall of mauubrium.

(a) Medusoid form well developed. Margelis, Coryne, Pen-

naria.

(b) Medusoid form degenerate. Clava, Hydractinia, Tubu-

laria, Eiidendrium.

6. Order Hydrocorallince. — Hydroid forms polymorphic ; secreting

calcareous skeleton by ectoderm. Medusoid forms usually de-
generate. Millepora, Stylaster.

7. Order Sipkoiwpliora. — Free-swimming, pelagic, polymorphic col-

onies.

(a) Nectocalyces present, without pneumatophore. Dipfiyes.
(6) With both nectocalyces and pneumatophore. Agalma.

(c) With pneumatophore only. Caravella.

(d) Discoidal forms without nectocalyces. Velella, Porpita.
II. Class SCYPHOMEDUS^:. — With medusoid form only in adult stage.

Velum not present ; sense-organs are modified tentacles. Repro-
ductive organs develop in endoderm.

1. Order Stauromedusce. — With the eight primary tentacles not at

all or but slightly modified. Tessera, Lucernaria.

2. Order Peromedusce. — With the four iuterradial primary tentacles

transformed into sense-organs.

TYPE C(ELENTERA. 117

3. Order Cubomednscc. — With the four radial primary tentacles

transformed into sense-organs. Charybdea.

4. Order DiscomeduscB.—^Wiih all eight primary tentacles trans-

formed into sense-organs.

(a) Mouth-lobes not fused. Cyanea, Aurelia, Pelagia.

(b) With mouth-lobes fused. Stomolophus.

III. Class ANTHOZOA.— Without medusoid forms. With ectodermal storna-
todoeum ; coelenteron divided into chambers by vertical mesenteries ;
reproductive organs developed in the endoderm.

1. Order Alcyonaria. — Colonial forms with eight mesenteries not

arranged in pairs ; tentacles pinnate.

(a) Without axial skeleton. Renilla, Alcyonium, Pennatula.
(6) With axial skeleton. Gorgonia, Leptogorgia, Isis, Coral-
Hum.

(c) With tubular calcareous skeleton. Tubipora.

2. Order Edwardsice. — Simple forms with eight mesenteries not ar-

ranged in pairs ; tentacles simple. Edwardsia.

3. Order CenatttJiece. — Simple forms with numerous mesenteries

not arranged in pairs ; new mesenteries formed on each side of
dorsal mid-line. Cerianthus.

4. Order Antipatharice. — Colonial forms with axial horny support ;

with six simple tentacles ; mesenteries not arranged in pairs.
Antipatharia.

5. Order Protactinice.— Simple forms \vith twelve primary mesenter-

ies arranged in pairs, and in addition one unpaired mesentery
on each side, or one or two pairs. Scytophorus, Gonactinia,
Oractis.

6. Order Zoanthece. — Simple or colonial forms with twelve primary

mesenteries arranged in pairs, and in addition a varying num-
ber of secondary pairs developed on each side of the primary
pair occupying the mid-ventral line. Zoanthus, Palythoa.

7. Order Hexactinice. — Simple or colonial forms with twelve pri-

mary mesenteries arranged in pairs, and in addition a variable
number of secondary pairs arranged in cycles, the newer pairs
developing in the intervals between the pairs already present.
An external calcareous skeleton present in many forms.

(a) Without calcareous skeleton (Malacodermata). Hal-

campa, Aiptasia, Tealia, Metridiinn, Bunodes.

(b) With a calcareous skeleton (Sclerodermata). Madrepora,

Fungia, Manieina, Mceandrina, Astrangia.

118 INVERTEBRATE MORPHOLOGY.

LITERATURE.

A. PORIFERA.

Vosmaer. Porifera. Bronn's Klassen und Ordnungen des Thierreichs. Leip-
zig u. Heidelberg, 1887.

E. Haeckel. Die Kalkscliwamme. Berlin, 1872.

F. E. Schulze. Untersuchungen iiber Ban und Entwicklung der Spongien.

Zeitschr. filr wissensch. Zoologie, xxv-xxxv. 1876-81.

A. Dendy. Observations on the Structure and Classification of the Calcarea
heti i-ni'irln. Quarterly Journal of Microsc. Science, xxxv. 1893.

E. von Lendenfeld. A Monograph of the Horny Sponges. London, 1889.

F. E. Schulze. Report on the Hexactinellidcv. Reports on the Scientific Results

of the Voyage of H.M.S. Challenger. Zoology, xxi. 1887.

B. CNIDARIA.

GENERAL.

L. Agassiz. Contributions to the Natural History of the United States. Vols.

in and iv. Boston, 1860-62.
A. Agassiz. North American Acalephce. Illustr. Catalogue of the Museum of

Comp. Zoology, n. Cambridge, Mass., 1865.

E. MetschnikofF. Embryologische Studien an Medusen. Wien, 1886.
0. & R. Hertwig. Das Nervensysttm und die Sinnesorgane der Medusen.

Leipzig, 1878.

HYDROMEDUS^E.

G. J. Allman. A Monograph of the Gymnoblastic Hydroids. Ray Society.

London, 1871-72.

E. Haeckel. System der Medusen. 1. Craspedoten. Jena, 1879.

H. N. Moseley. Report on certain Hydroid, Alcyonarian, and Madreporanan
Corals. Reports of the Scientific Results of the Voyage of H.M.S. Chal-
lenger. Zoology, ii. 1881.

W. K Brooks. The Life-history of the Hydromedusce. Memoirs Boston Soc.
Nat, Hist., in. 1886.

A. Weismann. Di> Entstehung der Sexualzellen bei den Hydromedusen. Jena,
1883.

H. V. Wilson. T/ic Structure of Cunoctantha octonaria in the Adult and Lar-
rul Stages. Studies from the Biol. Laboratory Johns Hopkins University,
iv. 1886.

E. Haeckel. Report on the Siphon ophora>. Reports of the Scientific Results of
the Voyage of H.M.S. Challenger. Zoology, xxvili. 1889.

A. Agassiz. The Porpiti<ln and Velellidw of the Gulf Stream. Memoirs Mu-
seum of Comp. Zoology, vni. 1883.

SCYPHOMEDUS^E.

E. Haeckel. System der Medusen. II. Acraspeda. Jena, 1880.

TYPE CCELENTERA. 119

ANTHOZOA.

G. von Koch. Die Gorgonidcn. Fauna u. Flora des Golfes von Neapel

Monogr. xv. 1887.
A. von Kblliker. Die Pennatuliden. Abhandl. Senckenburg. Nat. Gesellsch. ,

vii and vni. 1872.
G. Brook. Report on the Antipatharia. Reports of the Scientific Results of

tlie Voyage of H.M.S. Challenger. Zoology, xxxn. 1889.
A. Andres. Le Attinie. Fauna und Flora des Golfes von Neapel. Monogr

ix. 1883.
0. & R. Hertwig. Die Actinien anatomisch und histologisch untersucht. Jena-

isclie Zeitschr., xiv. 1879.
J. P. McMurrich. The Phytogeny of the Authozoa. Journal of Morphology, v,

1891.

120

INVERTEBRATE MORPHOLOGY.

CHAPTER VI.

THE CTENOPHORA.

THE group of forms known as the Ctenophora, to which
the systematic value of a class may be given, present no little
general resemblance to the Coalentera, but at the same time
depart so widely in structural and histological characters
from the Ciiidaria and Porifera that it seems advisable,
until further evidence is forthcoming, to consider them as a
group apart.

All the Cteuophores are pelagic and are of great transpa-
rency and delicacy, due to the nature of the mesogloeal tissue.

s

tc

FIG. 66. — Bolina Jiydatina (after CHUN).

cp = ciliated plates. tc = funnel-canal.

g = stomach or funnel.

I = lobe.

me = stomodseal canal.

In form they vary greatly, some being almost spherical or
pyriform (Pleurobrachia], sometimes with broad lobes project-
ing from near the oral extremity (Fig. 66, 1) (Bolina, Mnemi-

THE CTENOPHORA. 121

ops is), others being ribbon-like, as Cestum, the Venus' girdle,
others sac-like, as Beroe and Idyia. Indications of a radial
symmetry are seen in the eight longitudinal bauds of cilia-
plates (cp) which serve as locomotor organs, but this is thrown
into the background by the more pronounced bilaterality.
The stomodaeum is flattened in one plane and the gastric
cavity (g) in a plane at right angles to this, two tentacles, one
on each side of the body, lying also in this plane. It is possi-
ble accordingly to recognize a sagittal plane, that of the
stomodseurn, and a transverse plane, that of the gastric cavity,
and corresponding axes.

The mouth lies at the extremity of the vertical axis which
is directed backwards in locomotion, and opens into the ecto-
dermal stomodseum, which is flattened parallel to the sagittal
plane. At its upper end the stomodseum opens into the en-
dodermal gastric cavity (g) or so-called "funnel," from which
five canals arise ; one of these (tc) passes directly upwards to
the aboral surface, where it branches and opens to the exterior
by usually two openings ; two others pass downwards parallel
with the broad surface of the stoniodseuin (me) and end
blindly ; while the other two (Fig. 67, re) pass directly out-
wards in the transverse axis of the body and end at the base
of the tentacles, a short distance before their termination
giving off two branches, one on each side. These branches
soon divide and give rise each to two canals which commu-
nicate peripherally with canals running longitudinally below
the rows of cilia-plate*.

These plates, which constitute the locomotor organs, are
arranged one above the other and are composed of fused cilia
arising from ectodermal thickenings. There are eight merid-
ional rows of these ciliated plates, and from the upper end of
each row a delicate groove lined with ciliated cells extends
towards the aboral pole, each groove uniting with an adjacent
one as they approach the pole, so that their number is
reduced to four (Fig. 67, eg}. These pass in upon the floor of
a dome- shaped cavity enclosed by fused cilia which arch
together at the centre but do not quite meet. The cavity thus
enclosed is somewhat broader in the sagittal than in the
transverse axis and contains the aboral sense-organ. The floor

122

INVEETEBRATE MORPHOLOGY.

of the cavity is formed of high ciliated cells probabl}r nervous
in function, and above them is a mass of otoliths supported
on four incurved rods of fused cilia, one of which forms the
termination of each of the four meridional grooves.

In addition to this sense-organ, which is to be regarded
as of the same function as the otocysts of the medusae, there

csr

re

FIG. 67. — Pleurobraclda SEEN FOR THE ABOKAL POLE (after AGASSIZ).
eg — ciliated groove. p = polar urea.

ot = otocysl. re = radial caual.

t = tentacle.

lies at each end of the sagittal axis of the sensory dome a so-
called pole-area (Fig. 67, p), the cells of which are furnished
with small plates of fused cilia, each area being surrounded
by a thickened ciliated rim. These structures are from their
form and situation supposed to be sensory, and an olfactory
function has been attributed to them.

The tentacles (Fig. 67, t), of which there are two, situated
at the extremities of the transverse axis, are present in all
forms except the Beroids. Each tentacle lies in a deep
depression termed the tentacle-sheath and consists of a princi-
pal axis which gives rise to a large number of secondary
tentacles arranged upon one side only. Both the primary
axis and the secondary tentacles are solid, being composed
mainly of muscle-cells and containing no prolongation of the
tentacular vessel. In Mnemiopsis and its allies and in Cesium

THE CTENOPHORA.

123

c

FIG. 68.— ADHESIVE CELLS
FROM TENTACLE OP A
CTENOPHORE (after CHUN).
c = adhesive cap.
m = contractile stalk

the primary axis is practically wanting, the secondary tentacles
arising directly from the bottom of
the tentacle-sheath. The ectoderm of
the secondary tentacles contains nu-
merous cells supposed to be sensory,
and also so-called adhesive cells, which
in this group replace the uematocysts.
They consist of a slender spirall}"-
coiled muscular fibre (Fig. 68, m) at-
tached at one extremity to the subja-
cent tissue and terminating at the
other on the under surface of a hemi-
spherical cap (c), whose surface is
covered by small spherical masses of
a sticky secretion. A small animal
coming into contact with these caps

is held by the adhesive secretion, the muscle-fibre being suf-
ficiently elastic to yield to the struggles of the victim and to
bring it in contact with the general ectoderm by contracting
when its struggles cease.

The reproductive organs lie in the outer walls of the
canals which lie beneath the meridional rows of plates, but
apparently are originally derived from the ectoderm. All the
Ctenophores are hermaphrodites, the ova being arranged on
one side of each canal and the spermatozoa on the other, in
such a manner that the adjacent sides of any two canals bear
the same kind of sexual cells. A peculiar phenomenon
termed Dissogony has been observed in certain forms, consist-
ing of the occurrence of two periods of sexual maturity in the
life-history of the individual, the reproductive organs ripening
first while it is still in a larval stage and again when it has
reached its adult form.

The main bulk of the body of a Ctenophore is made up by
a gelatinous tissue intervening between the endoderm and
ectoderm and which ma}^ be termed the mesoglcea, though it
is not improbable that its cellular elements are in great part
derived from embryonic cells corresponding to the mesoderm-
cells of higher forms. It consists of a gelatinous matrix
through which are scattered branched cells and fibres. Some

124 INVERTEBRATE MORPHOLOGY.

of the latter extend throughout the entire thickness of the
inesogloea and are inserted by their branched extremities
into the ectoderm on the one side and the endoderm or
stomodaeal ectoderm on the other. They are contractile in
function, consisting of a central protoplasmic axis containing
a nucleus and of a peripheral contractile substance. In addi-
tion to these there are other much finer fibres which have
been supposed to be nervous, and on the outer surface of the
mesogloaa, between it and the bases of the ectoderm-cells, is
a network of stellate ganglion-cells whose processes overlap
but do not unite with each other. They are especially abun-
dant in the region of the meridional rows of plates ; just as
the slender fibres of the mesogloaa are especially abundant
below the aboral sense-organ and the meridional grooves.
Further information is, however, required as to the nervous
system of the Ctenophores.

The class may be divided into two orders :

1. Order Tentaculata,

The members of this order possess tentacles either with
or without the primary axis. The simple forms, such as
Pleurobraohia, belong to this order, as well as the lobate and
ribbon-shaped forms. In the lobate forms, such as Bdina
(Fig. 66) and Mnemiopsis, there is at each end of the sagittal
axis a large lobe developed into which four of the meridional
canals are continued ; two of the canals, those nearest the ex-
tremities of the transverse axis, pass around the edge of the
lobe and unite with each other, while the other two, which
also unite, are thrown into arabesque-like twistiugs. The
Venus'-girdle, Cestum, is ribbon-shaped, being flattened in the
transverse plane and much drawn out in the sagittal plane ;
the result being the great extension of four of the meridional
plate-rows and the almost complete disappearance of the
other four. In its young stages, however, Cestum is a spher-
ical form closely resembling the simple genus Mertensia.

THE CTENOPHORA. 125

2. Order Eurystomeae.

This order, which includes the Beroid forms, is character-
ized by the entire absence of tentacles and by the wide bell-
like stomodseum. The meridional canals send off along
their course numerous branching processes into the niesogloea
and are united around the mouth by a circular canal. To
this order belongs the Mediterranean genus Beroe, and the
genus Idyia of the northwest Atlantic.

Relationships of the Ctenophora. — The Ctenophores have been by most
authors assigned to the type Coelentera, on account of their jelly-like con-
sistency and the presence of gastro-vascular canals, and of indications of a
radiate symmetry. It is possible, however, that these characters are sim-
ply superficial, and that the group possesses but very remote affinities to
the Cu3lenterates. They have furthermore been regarded by some as
connecting links between the Coelenterates and the Turbellarians, and in
connection with this idea two aberrant forms may be briefly described.
One, Ctenoplana, is a flattened form, on the middle of whose dorsal surface
lies the otolith sac, and at a short distance from this are eight short rows of
cilia-plates, each in a slight depression. Two tentacles lie in the transverse
axis, and the mouth is situated at the centre of the lower surface and leads
into a cavity from which numerous branching gastric pouches arise with-
out any definite arrangement. The other form, Cceloplana, is also flat-
tened and creeping ; the mouth lies on the under surface and opens into a
wide cavity, which, as in Ctenoplana, gives origin to a number of pouches
which branch and give rise to a network towards the periphery of the
body ; a canal passes from the gastric cavity towards the dorsal surface of
the body, where it divides into two branches which end blindly, and lying
between them is a vesicle containing otoliths ; two tentacles similar to
those of Pleurobrachia lie in the transverse axis. In both these forms the
general surface of the body is ciliated, and they seem to represent inter-
mediate forms between the Ctenophores and Turbellaria, Ctenoplana being
more closely allied to the former and Cceloplana to the latter.

There are various important structural differences, however, between
the Co3lentera and the Ctenophores. Among these may be mentioned
the structure and position of the sense-organ, the structure and position of
the niesogloeal muscle-fibres, the structure of the tentacles, the presence of
the adhesive cells which cannot possibly be homologized with nematocyst-
cells, and finally the early differentiation in the embryo of cells, resembling
the mesoderm-cells of triploblastic animals, which give rise to the muscles of
the tentacles and perhaps to some of the mesogloeal elements.

It seems not improbable that the affinities of the Ctenophores would be
more accurately indicated in the classification if they were entirely removed

126 INVERTEBRATE MORPHOLOGY.

from the Coelentera and associated with the Turbellaria, being regarded
as highly modified forms, adapted for pelagic life, descended from Tur-
bellarian ancestors. The evidence which has been brought forward in
favor of a relationship of the Turbellaria to the Coelentera through the
Ctenophores would support this view as well as that it was intended to
support, and to this may be added the fact that while the peculiar adhesive
cells of the Ctenophores cannot be homologized with any of the histological
elements of the Cnidaria, they may readily have been evolved from the
adhesive cells which occur in the ectoderm of many Turbellarians.

SUBKINGDOM METAZOA.

Class CTENOPHORA. — Pelagic organisms provided with eight meridional rows
of plates formed by the fusion of cilia.

1. Order Tentaculata. — Ctenophora provided with tentacles.

(a) Without lobes ; more or less oval in shape. PleurobracMa,

Mertensia.

(6) Lateral lobes occurring at oral pole. Bolivia, Mnemiopsis.
(c) Ribbon-like form. Cestum.

2. Order Ettrystomece. — Without tentacles ; stomodseum wide and

bell-like. Beroe, Idyia.

LITERATURE.

VSL

R. Hertwig. Ueber den Ban der CtenopJwren. Jenaisclie Zeitschr., ^r. 1880.
C. Chun. Die Ctenophoren des Golfcs von Neapel. Fauna und Flora des Golfes
von Neapel. Monogr. i. 1880.

TYPE PLATYUELMINTHES.

127

CHAPTER VII.

TYPE PLATYHELMINTHES.

THE Platyhelmiuths constitute a group which, though
presenting a much higher grade of organization than the
Coelentera, nevertheless show certain general structural
similarities to the representatives of that type. Thus upon
the exterior of the body there is a thin ectoderm (Fig. 69,

vd

ec

t»m

TL

bin vd -a.

FIG. 69. — DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH VARIOUS TURBEL-
LAP.IA. A, au Accelan ; B, au Alloiocoelan ; C, a Rhabdoccelau ; D, a
Triclad.

bm — basement-membrane. o = ovary.

d — intestine. od = oviduct.

ec — ectoderm. p = pareuchyme.

in = muscle-layer. t = testis

n = nerve. « = vitellarium.

vd — vas defereus.

D, ec), below which is a basement -membrane (bm) sometimes
thin, structureless, and destitute of cells, sometimes thicker
and enclosing branched cells, and strictly comparable to the
mesoglo3a of the Ccelenterates. Within the basement-mem-
brane there is a compact mass of tissue surrounding, in the

128 INVERTEBRATE MORPHOLOGY.

majority of forms, a cavity, the enteron (d), the cells lining
the walls of this being differentiated into a digestive epithe-
lium or endoderm. The space between the euteron and the
basement-membrane is occupied by the mesoderm, consisting
peripherally of compact layers of circular and longitudinal
muscle-fibres (TO), while below these it forms a mass of nu-
cleated cells, usually vacuolated so as to resemble a network
of fibres enclosing spaces and constituting the parenchyma (p).
It is traversed by dor so- ventral muscle-fibres and has im-
bedded in it various organs most of which are further dif-
ferentiations of this middle germ-layer. These two layers,
the endoderm and mesoderm, are together comparable with
the inner layer of the Coelenterates, the mes-eudoderm, and
when the enterou exists it communicates with the exterior, as
in that group, by a single opening, the mouth, the Nemer-
teaiis only, the most highly organized class of the Platyhel-
niinths, possessing a second opening, the anus.

These homologies are, however, associated with a com-
plexity of organization unrepresented in the Coelenterates.
The Platyhelmiuths all present a typical bilaterality of form,
and show furthermore a well-marked antero-posterior as well
as, in most cases, a dorso-ventral differentiation. The body is
usually flattened and more or less vermiform, whence the name
of the group, and is adapted to a creeping habit, certain para-
sitic forms, and some Nemerteaus which live buried in sand,
being the only forms not presenting such a mode of life.

The greatest contrast to what occurs in the Coelenterates
however, is presented by the development of compact organs.
The nervous system is no longer an altogether diffuse tissue,
scattered in a thin layer throughout the body, but a large
number of ganglion-cells are aggregated into a compact mass,
the brain, embedded in the mesoderm parenchyma near the
anterior end of the body, and from this there pass backwards
two or more longitudinal cords of nerve-fibres which give off
branches extending to all parts of the body and forming a
network below the basement-membrane from which the pe-
ripheral muscles derive their nerve-supply. In some cases
nerves have been observed to pass from this network through
the basement-membrane to come into connection apparently

TYPE PLATYUELMINTHES.

129

with nerve-cells lying between the inner ends of the ectoderm-
cells as well as with sensory cells resembling in general form
those already described as occurring in the Cnidaria. It must
not be understood, however, that the ganglion-cells are limited
in their distribution to the lower layer of the ectoderm and
the brain ; on the other hand, they are scattered along the
nerve-cords which arise from the brain, the Platyhelmmths
presenting in the structure of their nervous system a condi-
tion intermediate between the diffuse arrangement of the
ganglion-cells seen in the Cnidaria and the more perfect ag-
gregation occurring in higher types.

An excretory system of branching tubes traversing the
mesoderrn parenchyma and opening to the exterior is also
present. It consists usually of two main tubes, nephridia,
from which numerous branches arise, terminating in blind
funnel-like extremities (Fig. 70, f) lying in the meshes of the
parenchyma. Each funnel (Fig.
78, J3] is closed by a single cell
(tc), from which there projects
into the tube a bundle of cilia
(fl), and which, from the resem-
blance of the motion of these cilia
to a flame flickering in the wind, is
known as aflame-cell. The larger
tubes are lined by a layer of
cells which seem, in certain cases
at least, to be ciliated, but the

smaller branches consist of a_

„ JIG. 70.— EXCRETORY SYSTEM OF

series of cells succeeding one THE ANTERIOR PORTION OF THE

another in a single row, the canal BODY OF Planaria montana (after

running through the centres of CHICHKOFF>-

the cells and being thus iutra-

cellular. The tubes throughout the entire system contain

a fluid in which particles resembling guanin in their behavior

to reagents have been seen, and there is little room for doubt

but that the tubes have an excretory function.

Finally, a complicated reproductive apparatus (see Figs.
68-70) is present, the Platyhelmmths being for the most part
hermaphrodite. The testes consist of from two to many

130 INVERTEBRATE MORPHOLOGY.

globular bodies whose ducts unite to form two vasa deferentia
opening to the exterior through a muscular intromittent organ,
and sometimes dilating to form reservoirs, the seminal vesicles,
in which spermatozoa may be stored up until required for
fertilization. The female apparatus is somewhat more com-
plicated. The ovaries are usually two in number and their
products pass to the exterior through special tubes, the ovi-
ducts, which may be exceedingly long and with the terminal
portion dilated to form a uterus in which the ova may pass
through certain stages of their development. Connected with
the oviducts there is usually a pouch-like structure, the semi-
nal receptacle, for the reception of spermatozoa, and further-
more they may receive the products of two other glands
which supply the yolk and the shell for the ova. The yolk-
glands are in some cases very voluminous, forming what is
termed the vitellarium, and have been apparently developed
by the separation of a portion of the original ovary, their
cells, which manufacture the yolk material, being accordingly
equivalent to germ-cells. The evidence for this supposition
is derived from the arrangement found in some Turbellaria
and will be pointed out, together with the variations which
the complex of organs presents, in the descriptions of the
various groups.

I. CLASS TURBELLARIA.

The Turbellaria derive their name from the fact that the
ectoderm is furnished with cilia, which form the locomotor
organs of the animals, whose gliding motion over tbe sur-
face of the objects among which they live is very charac-
teristic. The majority of the members of the class lead a
free life, some in fresh and some in salt water, and some even
on laud, creeping about on the under surfaces of stones or
weeds. A few, however, are parasitic either upon the outside
of the bodies of their hosts (Bdellurd) or in a few cases living
in the body-cavity or even being imbedded in the tissues.

In addition to the ordinary ciliated cells the ectoderm con-
tains numerous sensory as well as gland cells. Special
glands secrete in most of the groups peculiar rod-like bodies

TYPE PLATYHELM1NTIIES. 131

which lie scattered about iu the ectoderm between its compo-
nent cells or may project more or less beyond its surface.
These rhabdites, as they are termed, are produced as a secre-
tion by cells lying usually in the mesoderm and connected
with the exterior by a slender neck passing through the base-
ment-membrane, the rhabdites thus making their way to the
exterior. The rhabdite-cells are ectodermal, their position in
the mesoderm being quite secondary, and in fact in one groiip
they are confined to the ectodermal layer. The function and
nature of the rhabdites have been variously interpreted, some
authors considering them equivalent to the Cnidariau nernato-
cysts, but it seems more probable that they are the condensed
secretion of cells which originally produced a mucous sub-
stance and by slowly dissolving in water produce a viscid
slime of sufficient tenacity to retain organisms coming in con-
tact with it.

In addition to these structures many forms possess adhe-
sive cells, columnar cells which produce a strongly adhesive
secretion which is poured out in drops upon the free ex-
tremity of the cell, recalling in this respect the adhesive cells
of the Cteuophores. These cells seem to be of use mainly iu
enabling the worms to adhere to the surface on which they
are creeping, and are especially developed towards the hinder
end of the body. Another organ of adhesion in the form of a
muscular sucker, situated usually about the middle of the
ventral surface, is present in certain marine Turbellaria, but
the majority of the members of the group lack such struc-
tures.

The nervous system consists of a brain from which a num-
ber of nerve-cords arise, varying somewhat in their arrange-
ment in the different orders. Sense-organs of one kind or
another are usually present in addition to the widely-distrib-
uted sensory cells of the ectoderm. A large number of forms
possess eyes, which in some Polyclads may be exceedingly
numerous, and usually consist of a patch of pigment lying in
the mesoderm and upon which a refractive lens-like structure
lies. In a few cases, as in Microstoma, the eye is simply a patch
of pigment in the ectoderm near the anterior end of the body.
An otocyst, consisting of a spherical vesicle filled with fluid

132 INVERTEBRATE MORPHOLOGY.

and containing an otolith of carbonate of lime, is present in
some of the lower Turbellaria, as Monotus, and rests directly
upon the surface of the brain ; these structures probably, as
in the Cnidaria, are sense-organs of equilibrium rather than
of audition. In the Polyclads tentacles are frequently pres-
ent, sometimes capable of being retracted and serving as
organs of touch, and in certain Rhabdoccels there is a ciliated
depression on each side of the head richly supplied with
nerves forming what has been considered an olfactory organ.

1. Order Acoela.

The Acoela form a group of lowly-organized Turbellaria
exclusively marine in habitat and leading an active and free
existence. They all possess a mouth (Fig. 71, ni) situated on
the ventral surface and leading into a short pharynx, though
in some forms this may be absent; but beyond this there is
no trace of a digestive tract, the food passing from the
pharynx into the parenchyma (p), where it is digested. Ow-
ing to the lack of a digestive tract these forms are strictly
two-layered (Fig. 69, A), only the ectoderm and mes-endoderm
being represented, and consequently are exceedingly interest-
ing as indicating the manner in which the differentiation of
the triploblastic condition has been derived from the diplo-
blastic.

The nervous system has been described in Convoluta as
consisting of a bilobed ganglion surrounding the otocyst, and
in front of this and united to it by commissures is a second
pair of ganglia. From the anterior ganglia there arise by a
common stem two nerves on each side which pass backwards,
one along the edge of the body and the other a little internal
to it, while the posterior gangliouic mass gives rise to two
nerves which pass backwards, one on each side of the median
line. All six nerves send ofl' numerous transverse branches
which unite to form with the nerve-cords a square-meshed
network. In addition to the single otocyst (Fig. 71, ot] two
pigment-spots lying in the ectoderm and representing light-
percipient organs (e) are present, as well as a peculiar refrac-
tive highly-movable organ, lying in the median line on the

TYPE PLATYHELMINTHES.

133

anterior margin of the body, which is supposed to be tactile
in function.

No excretory apparatus has as yet been described for the
Accela, but a reproductive system
with some interesting peculiarities
occurs. The male apparatus consists
of numerous spherical testes (£) whose
ducts unite to two vasa deferentia,
dilating below to form the seminal
vesicles (vs) and uniting in the mus-
cular iutroniittent organ. The female
organ is, however, relatively simple,
consisting of two club-shaped ovaries
(ov) whose short oviducts open almost
directly to the exterior near the pos-
terior end of the body by a pore ($ 9 )
common to both male and female ap-
paratus. There is no vitellarium, no
shell-gland, no seminal receptacle,
and no special uterus, a state of
affairs indicating great simplicity of
structure compared with what is
found in the other orders.

2. Order Alloioeoela.

FIG. 71.— DIAGRAM OF AN
ACCELOUS TURBELLARIAN

(after VON GRAFF).

The members of this order are
marine with the single exception of
Plagiostoma lemani, which is found in
the deep waters of the Swiss lakes.
They present a distinct advance upon
the Acoela in that a well-defined diges-
tive tract is present (Fig. 69, !>}, the
interval between it and the peripheral
musculature being completely filled up by the usual paren-
chyma and the organs imbedded in it. These forms are then
triploblastic, possessing well-defined ectoderm, rnesoderrn, and
endoderm, a condition found in all the higher orders.

The mouth varies somewhat in position, lying either near

e = eye.
m = mouth.
ot = otocyst.
ov = ovary.
p = parenchyma.

t = testis.

vs = vesicula seminalis.
$ = reproductive orifice.

134 INVERTEBRATE MORPHOLOGY.

the anterior or the posterior end of the body, and opens into
a pharyngeal pouch, whose walls are thickened by muscle-fibres
in such a way as to form a somewhat bulbous mass sharply
marked off from the parenchyma which surrounds it. In
Monotus, however, the pharynx is more developed, projecting
as a strong circular fold into the pharyngeal pouch and form-
ing what is termed a plicated pharynx. This at its inner
extremity communicates with the sac-like intestine, usually
quite simple but occasionally somewhat pouched, and ter-
minating, as in all the Turbellaria, blindly.

The nervous system consists of a bilobed ganglionic brain-
mass from which pass backwards two nerve-cords which may
(Monotus) or may not present transverse anastomosing
branches, and in addition a number of smaller branches pass
forward to be distributed to the anterior end of the body.
Eyes, consisting of pigment-spots seated upon the brain, are
frequently present, and in Monotus an otocyst is found, while
lateral ciliated depressions on each side of the head occur in
Plagiostoma.

The excretory system is present, but presents no notable
departures from the typical arrangement. As regards the
reproductive organs, the testes resemble those of the Acoela,
but the ovaries are comparatively small and the separate
vitellaria are large and sometimes branched, opening into a
cavity, the genital atrium, common to them, the oviducts
and the intromittent organ, and communicating with the ex-
terior by a single median pore situated near the posterior end
of the body. In a few forms the vitellaria are not differen-
tiated from the ovaries, presenting a condition similar to that
found in the Accela.

3. Order Rhabdoccela.

The Khabdocoela are found both in fresh and salt water and
are usually small. They possess a distinct tubular digestive
tract (Fig. 69, C, d) without lateral pouches or branches, but
the principal characteristic lies in the presence in the paren-
chyma of large spaces resembling the coelomic cavities of
higher types, a feature not repeated in any other Turbellaria.

TYPE PLATYHELMINTHES.

135

-VI

ov

The mouth is situated at various regions of the body in
different forms, being anterior in Microstoma, while in J/eso-
stoma (Fig. 72) it is situated at the middle of the ventral sur-
face. The walls of the pharyn-
geal pouch (ph) may be quite
simple, as in the Accela which
possess a pharynx, or may pre-
sent a muscular thickening forin-
iug a bulbous pharynx, but no
further complexity occurs, al-
though in certain forms, such as
Prorhynchus, the pharynx is capa-
ble of being protruded from the
mouth, acting probably as a
delicate tactile organ.

The nervous system (n) is
essentially similar to that of the ^
Alloiocoela ; two or four eyes (oc)
frequently occur, though otocysts
are wanting, while the ciliated
depressions on the side of the
head supposed to be olfactory in
function occur in Microstoma and
Prorhync/ius and allied forms.

The excretory system consists .

J • . FIG. 72. — A RHABDOCCEI.OUS TUR-

occasionally ol a single uephri-

diuin with numerous branches
which open near the posterior
end of the body, but more usually
two main tubes are present open-
ing near the middle of the body
either directly to the exterior or
into the pharyngeal pouch (Meso-
stoma), though in some cases they
uuite near the posterior end of the
body into a single tube which vi = yolk-gland,
opens to the exterior by a single median pore ( Vortex).

The reproductive system presents considerable variation
in the structure of the female apparatus, but the testes (t) are

sr

at

atg

BELLAKIAN, Mesostoma splendi-
dum (after VON GRAFF).

at = atrium.
atg = atrial gland.
d = iutestine.

n = brain.

oc = eye.

ov = ovary.

p = penis.

ph = pharynx.

sr = receptaculum seminis.
t = testis.

136 INVERTEBRATE MORPHOLOGY.

uniformly two simple club-shaped bodies uniting below to
form a common seminal vesicle. The female apparatus may
consist of a single ovary (ov) combined with a vitellarium or
of two such structures, but usually there is a separation of the
vitellarium (vi). In the more complicated cases there is but
a single small ovary opening almost directly into the genital
atrium, which receives also in addition to the introuiitteut
organ the ducts of the two vitellaria. Its walls are further-
more pouched out into a seminal receptacle and a sac-like
cavity which serves as a uterus, while a peculiar muscular
sac, lined by a strong cuticle, the bursa copulatrix, serves for
the reception of the intromittent organ during copulation.
As stated, however, numerous variations from such a condition
occur, and it is not possible to describe any one arrange-
ment characteristic of all the Rhabdocosls.

4. Order Tricladea.

The Triclads constitute a group of forms with very definite
structural peculiarities, occurring principally in fresh water
(Planaria, Dendrocodurn, Phagocata), though a few forms are
terrestrial (Bipalium), and a still smaller number marine
(Gunda, Bdelloura*). As a rule they are elongated in form,
one of the terrestrial species reaching a length of 2 cm., and
are for the most part free-living, though Bdelloura and Syncce-
lidium are ectoparasites of the King-crab (Limulits). The
mouth is situated in all cases behind the middle of the body
and leads into a somewhat capacious pharyugeal pouch (Figs.
69, D, and 73, ph) in which lies a muscular cylindrical pharynx
capable of protrusion from the mouth-opening. The diges-
tive tract at the base of the pharynx divides into three
branches, one of which passes forward in the median line,
giving off simple or branched diverticula on both sides, while
the other two pass backwards on either side of the pharyngeal
pouch, giving off diverticula only from the outer side. The
intestinal branches, whose number has suggested the name of
the order, and their diverticula are imbedded in a compact
parenchyma, no well-marked ccelomic spaces being present
(Fig. 69, Z>).

TYPE PLATYHULMINTUES.

137

VI

ov

ex

The nervous system consists of a bilobed brain lying in
the anterior part of the body and
from which two nerve-cords pass
backwards, united at intervals by
cross-commissures and giving off
on their outer sides branches
which anastomose with one an-
other, forming a network.

In Griinda seyintittata the transverse
commissures agree in number and ar-
rangement with the lateral branches on
the one hand and with the diverticula of
the intestine on the other, the arrange-
ment of the two systems producing an
appearance of metamerization which is
most striking, especially as it affects as
well the excretory and reproductive sys-
tems. In this form an indication is
afforded of the manner in which the
more pronounced and typical metameri-
zation of the higher types has been pro-
duced by the more or less completed
multiplication of the organs and the
integration of the parts so formed into
a metamere (see p. 43).

Eyes are usually present, fre-
quently provided with lenses, and,
though usually two in number,
may be very numerous and situ- FIG. 73.— A TRICLAB TURBEI.LARI-
ate'd along the margin of the body. AN' Sync^lidium peUucidum (after

No otocysts occur, and the sides
of the anterior end of the body
are in some forms produced into
more or less elongated processes
which may possibly be mainly
sensory in function, while behind
them are areas of strongly ciliated
cells richly supplied with nerves
and presumably corresponding
with the ciliated depressions oc-
curring in the same region in the Alloiocoela and Ehabdocoela.

WHEELER).

ex = excretory system.
g = accessory glaud.
ggl = vaginal glands.
n = nervous system,
od = oviduct.
ov = ovary.

p = reproductive orifice.
ph =- pharynx.
t = testis.
u = uterus.
•vd = vas deferens.
m = yolk-gland.

138 INVERTEBRATE MORPHOLOGY.

The excretory system differs from that of the lower orders
in that the two longitudinal uephridia open on the dorsal sur-
face of the body by numerous pores, which in Gunda corre-
spond in number with the intestinal diverticula and nerve-
commissures. The reproductive apparatus consists of nu-
merous testes (Fig. 73, t\ as in the Accela (arranged rnetameri-
cally in Gunda), whose ducts unite to vasa deferentia (vd)
uniting in the muscular iutromittent organ which projects into
the genital atrium. Two small ovaries (ov) occur in the ante-
rior end of the body, their large oviducts passing backwards to
unite in a muscular bursa copulatrix, and receiving at inter-
vals the secretion of numerous lateral diverticula which con-
stitute the vitellarium (vi). A pouch-like diverticulum of the
atrium serves as a uterus, and the single median orifice (p) of
the atrium lies near the posterior end of the body behind the
mouth-opening.

5. Order Polycladea.

The Polyclads are exclusively marine and assume various
forms, some being quite elongated while others are flat leaf-
like expansions. Compared with the members of the other
orders they may be said to be as a rule Jarge, though few
reach the length which has been mentioned for some land
Triclads. The mouth varies greatly in position, as in the
Rhabdoccels, and opens into a spacious pharyngeal pouch
containing a plicated pharynx (Fig. 7% pli). The intestine
consists of a central cavity, into which the pharynx opens at
its inner end and from which numerous branches (hence the
name of the order) pass off into the compact parenchyma,
where they branch and may anastomose with one another to
form a network. The nervous system presents a somewhat
similar condition, the bilobed brain (ce), usually situated near
the anterior end of the body, giving off a number, usualty six,
of nerve-cords which become lost in a wide-meshed network
ramifying through the body-tissues. Eyes are usually pres-
ent, frequently in enormous numbers, and furthermore in many
forms (Planocera] tentacles arise from the dorsal surface or
else from the margin near the anterior end of the body. As

TYPE PLATYHELMINTHES.

139

ce-jL_

ov

in the Triclads otocysts are wanting, nor have ciliated lateral
depressions been described as occurring in the order.

Little is known concerning the
excretory system. The reproduc-
tive system differs from that of
the other orders in that the male
and the female apparatus each
possess a separate opening ( $ and
? ), there being no genital atrium
common to both. Both apertures
lie behind the mouth-opening, near
the posterior end of the body,
the male apparatus opening an-
teriorly to the female. The former
is similar in structure to what has
been described for the Triclads.
The female apparatus possesses
no vitellariurn, and the ovaries (ov)
are very numerous, lying in the
lateral parts of the body, their
various ducts uniting to form wide
canals which serve as uteri (ut).
These open into a single tube, the
vagina, which receives the secretion
of the numerous glands (sg) which
form the shell-gland

In some forms there is situated
about the middle of the ventral
surface of the body a muscular
sucker which serves as an organ of
adhesion. Since the presence or
absence of this organ is in either
case associated with the occurrence
of other important structural pecu-
liarities, the order has been divided
into two suborders — the C'otylea,
provided with a sucker (Thysano-
zoon, Eurylepta), and the Acotylea, in which it is absent
(Planocera, Leptoplanci).

FIG. 74.— A POLYCLAD TUKBEL-
LAEIAN, Leptoplaiia alcinvi
(.after LANG).

ag = accessory gland.

ce = cerebral ganglion.

ov = ovary.
ph -- pharynx.

sg = shell-gland.

te = testis.

ut = uterus.

vd = vas deferens.

vs = vesicula seminalis.
|,2 = male and female orifices.

140 INVERTEBRATE MORPHOLOGY.

Reproduction of the Turbellaria. — Non-sexual reproduction
is not characteristic of the Turbellaria, though it occurs in cer-
tain Rhabdocoals. In Microstoma a transverse partition, con-
sisting of two closely-applied lamellae, forms, extending
from the outer wall of the body to the wall of the digestive
tract, which it constricts slightly without dividing. Later a
constriction of the outer surface of the body appears, the
two lamelhe of the partition separate slightly, and the indi-
vidual lying behind the partition develops a new mouth and
pharynx and a new brain, so that it resembles exactly the
anterior individual with which it is directly connected by the
uninterrupted digestive tract. Before these processes are
complete, however, they are repeated in each of the two indi-
viduals, so that a chain of four imperfectly separated individ-
uals results, and by further repetitious of the process chains
of 8, 16, or 32 individuals may arise, each provided with
mouth, pharynx, and brain, the anterior individual possessing
the original structures, and all connected by the digestive
canal which runs uninterruptedly through the entire chain
(see Fig. 28). Eventually the various individuals separate
from one another and become sexually mature.

The sexual method, however, plays a much more important
part in the life-histories of the Turbellaria. The development
of the three lower groups has not as yet been as thoroughly
investigated as is desirable, but the phenomena which occur
in the Triclads, and especially in the Polyclads, have been fol-
lowed. The Triclads deposit their ova in chitiuous cocoons,
which contain, besides the ova proper, large numbers of
amceboid cells, originating in the vitellarium-pouches of the
parent, and serving as food for the young embryo. In asso-
ciation with this condition of affairs many peculiarities of
segmentation and growth occur in the Triclad embryos, all of
which must be considered as secondary adaptations.

In the Polyclads, however, a more primitive state of affairs
occurs, the food-yolk being incorporated with the protoplasm
of the ovum, a more or less distinct irregular segmentation
resulting from its telolecithal arrangement (p. 53). The
diploblastic condition arises by an invagiuation either of the
embolic or epibolic type, but at an early period of the segmen

TYPE PLATYEELMINTHES. 141

tation the cells which are to form the mesoderm are separated
off from those from which the ectoderm and endoderm are to
be derived, so that even before the iuvaginatiou all the three
layers are represented. This, however, is to be regarded as a
precocious segregation of the germ-layers, and even within the
limits of the few forms whose embrology is known consider-
able variations in the time and manner of the differentiation
of the mesoderm occur. The result of the iuvaginatiou is in
some cases a solid, bilaterally symmetrical, ciliated embryo
consisting of a layer of ectoderm enclosing a central mass of
endoderm and mesoderm, in the interior of which a cavity ap-
pears surrounded by the endoderm. A depression appears on
the ventral surface, which, deepening, finally unites with the
enteron and forms the pharyngeal pouch, and gradually the
characters of the adult are assumed.

In some forms whose ova are provided with comparatively
little yolk the embryo leads from an early period a free-swim-
ming existence, and in accordance with this a specialized
form has been acquired and a slight metamorphosis is neces-
sary for the conversion of this larva into the adult condition.
In Stylochus the embryo develops into what is known as
Goettes larva, a bilateral ciliated structure with an anterior
and posterior tuft of strong sensory hairs, while from the ven-
tral surface on either side of the mouth
there hang down two ciliated ear-like lobes
or lappets. In another form (Thysanozoori)
these lappets are much more developed,
passing round to the dorsal surface of the
body, and their edges are drawn out into
four or eight lobes, one of which lies in
front of the mouth and another on the
dorsal surface, the other two or six lying
at the sides of the body and being arranged
symmetrically on either side. It seems j-JG
probable that this larva, known as Midlers Thymnozoon, Miil-
larva (Fig. 75), may be traced back to a con- ler>s Larva (after
dition such as that described in Goette's
larva, the two lappets of that form having united in front of
the mouth, while their lines of attachment have become more

142 INVERTEBHATK MORPHOLOGY.

and more oblique until wliat were originally the posterior
edges of the* lappets meet ou the dorsal surface. The edges
of the lobes of the lappets are fringed with long cilia, and
consequently a lobed prseoral band of cilia is produced.
These larvae pass into the adult form by gradually becoming
more and more flattened dorsoventrally, the ciliated lappets
or lobes at the same time growing smaller and smaller until
they finally disappear.

Relationships of the Turbellaria. — A relationship of the Turbellaria,
especially of the Polyclads, with the Ctenophores has been advocated
within recent years, and through this relationship genetic affinities with
the Cnidaria have been sought. The question of the affinities of the Cteno-
phores has already been discussed, and it has been pointed out that it is
probable that, instead of being a connecting link between the Cnidaria and
the Turbellaria, they are rather highly modified Turbellaria adapted to a
pelagic life. In this sense the idea of a genetic affinity between the Turbel-
laria and Ctenophores may be correct , though it seems probable that the
Polyclad affinity should be given up and the relationship sought for among
Alloioccelan forms.

The Ctenophore-Polyclad theory necessarily viewed the Polyclads as the
most primitive Turbellaria, and came into contact in this way with the
more simple organization of the Accela, Alloiocoela, and Rhabdocoela, a
difficulty which was avoided by assuming that these were degenerate
groups derived from Polycladan ancestors. No good grounds for such an
assumption exist however, nothing in the mode of life suggesting a cause
for degeneration ; and until embryological evidence of degeneration is
obtained, it is preferable to consider their simplicity primitive.

This latter view is strengthened if it harmonizes with a probable phy-
logeny. It has already been pointed out that the solid embryo or sterrula
is to be recognized as an ancestral form of the Cnidaria, With such an
ancestral form the Accela show affinities in the absence of a differentiation
of the central mass into well-defined endoderm and mesoderm. The local-
ization of a definite region for the ingestion of nutrition would lead to the
formation of a mouth in. the Sterrula, just as it has done in the riayellata.
The differentiation of muscle-fibres from the mesendodermal cells would
naturally f >llow the assumption of a creeping habit, so that it is only the
possession of a definite nervous system imbedded in the mesoglcea (in which
tissue, however, Cnidarian characteristics are yet discernible, as already
pointed out) and the occurrence of a complicated reproductive apparatus
that render a close comparison witli the Sterrula difficult; but even the
explanation of the presence of these structures makes fewer demands upon
our ideas of developmental possibilities than does the assumption that the
Acoela owe their peculiarities to degeneration.

Upon this view of the phylogeny the Accela are united with the Coelcn-

TYPE PLATYHELMINTHES. 143

tera only through the Sterrula ancestor common to both, or more probably
through an ancestor in which the mouth had developed, as well as a slight
differentiation of muscle-fibres, but in which no hollowing out of an enteron
had yet occurred. This appearing in a primitive acoelan form gave rise to
the Alloiocoda from which two divergent lines of descent arose, one leading
to the Rhabdoccels and the other to the Triclads and Polyclads.

If this be the true phylogeny of the class, some evidence of it ought to
be found in the embryological history of some of the higher members of
the group in accordance with what is termed the Biogenetic law, which is
to the effect that an individual in its development recapitulates more or
less accurately its phylogenetic development, or, to put it more briefly, the
ontogeny is a recapitulation of the phylogeny. Secondary modifications,
especially in the form of the abbreviation or omission of certain stages,
may intervene in the individual development, forming what are termed
cenogenetic modifications, but notwithstanding exceptions produced in this
way the law is of general application.

In StylocJms the young larva is a solid body without any enteron and
represents, therefore, an Acoslan stage of development ; later the central
mass becomes hollowed out to form an enteron who.se walls are not at first
clearly marked off from the surrounding parenchyma, and a representa-
tion of the Alloiocoelan condition results, from which the Polyclad condi-
tion gradually develops. Consequently in Stylochus the ontogeny indi-
cates a primitive nature for the Accela, and agrees with the phylogeny
which has been outlined above. It must be recognized, however, that all
reconstructions of the phylogeny of the Turbellaria and all views as to
their affinities to the Cniduria must be accepted with much reservation,
until the much-needed facts as to the developmental history of the Acoda
and Alloiocozla are available.

II. CLASS TREMATODA.

The Trematodes or Fluke-worms are throughout parasitic
either upon the exterior of their hosts or in the cavities of
their body, and in correspondence with this mode of life
structures are developed by means of which they adhere to
their hosts. These structures are of two kinds ; in all suckers
are present consisting of cup- like depressions whose walls are
richly supplied with muscle-cells, by the contraction of which
a vacuum is formed, and in many forms, in addition to these,
chitiuous hooks occur. The suckers vary in number from one
(Monostomum) or two (Distomum, Fig. 76,) to several (Poly-
stomum), and at the bottom of one situated at the anterior
extremity of the body is the mouth-opening. This leads

144

IN VERTEBRA TE MORPHOLOG Y.

into a tubular oesophagus wliose walls are thickened near
its anterior end to form a muscular plmryngeal bulb which

functions as a pump for the ingestiou
of the nutritive fluids of the host. At
its posterior extremity the oesophagus
branches into two limbs which are con-
tinued backwards, in some cases giv-
ing off secondary branches, to near
the posterior end of the body, where
they either end blindly or unite together
in the middle line (Polyatomum) to form
a loop.

The body is covered by a distinct
cuticle secreted by the ectoderrnal cells,
which in the adult may undergo a con-
siderable amount of degeneration, or
probably in some cases the cuticle is
formed in part by the transformation
into chitin of the ectoderm. Spiny ele-
vations of the cuticle are present in

noides (from a drawin/by niany forms, and the large chitiuous
c. LANOENBECK). hooks which occur in many ectopara-

sitic forms are but further developments of these structures.
Below the ectoderm lies the usually thin basement-membrane,
below which again lie the circular and longitudinal peripheral
muscle-sheets, and between the intestine and these muscles is
the parenchyma traversed by dorsoventral muscle-bundles
and having imbedded in it the various organs.

The nervous system (Fig. 77) consists of a transversely
elongated ganglion lying dorsal to the oesophagus — usually
between the bottom of the anterior sucker and the pharyngeal
bulb. The ganglion is somewhat swollen at each extremity,
indicating its origin by the approximation of two gauglionic
masses, and from these thickenings nerves arise which pass
both forward and backward. The anterior nerves are short
and slender, and supply the musculature of the anterior
sucker and the sides of the anterior end of the body, while
the posterior nerves are much stronger and longer and vary
from two to six in number ; in the latter case four run along

FIG. 7o. — Disk/mum

TYPE PLATYHELMINTHES.

145

the ventral surface of the body, two on each side of the mid-
dle line, the other two having a more dorsal position, while
when only two are present they correspond to the two more
median ventral nerves of this
arrangement. Sense-organs are
but feebly developed as a rule,
especially among the endopar-
asitic forms, but in some ecto-
parasites eyes are present con-
sisting usually of four spots of
pigment seated upon the brain-
ganglion and sometimes provid-
ed with a lens-like structure.

FIG. 77.— NERVOUS SYSTEM OP TRE-
MATODE, Trixtomum molce (after
LANG).

A

FIG. 78. — EXCRETORY SYSTEM
OP TREMATODE, Distomum
divergens (after FRAIPONT).
A, eutire system; B, terminal

funnels.
/ = funnel.
fl = flame of cilia.
n — main trunk.
ph — pharyngeal bulb.
s = tiiiterior sucker.
tc = terminal cell.
vs = ventral sucker.
tit = contractile vesicle.

The excretory apparatus (Fig. 78) consists, as is usual in
the Platyhelininths, of two longitudinal, more or less irregu-
larly twisted tubes (n) from which arise the funnel-bearing
branches (/). A peculiarity of the Trematodes is, however,
the union of the two longitudinal tubes in a terminal vesicle
(vf) which opens to the exterior at the hinder end of the body
by a single pore.

The reproductive system is exceedingly complicated, though
essentially similar to that of the higher Turbellaria. It opens

146 INVERTEBRATE MORPHOLOGY.

to the exterior by two pores lying close together on the ventral
surface rather nearer the anterior than the posterior end.
The male apparatus consists in the Polystomece of numerous
closely-aggregated testes, or else, as in the Distomece (Fig. 76),
of only two situated in the posterior half of the body ; the
ducts from the testes pass forwards towards the genital pore,
near which they unite to form a sac-like seminal vesicle,
from whose anterior end the single vas deferens is continued
on towards the pore, passing in the latter part of its course
through a muscular protrusible intromittent organ, the cirrus.
The ovary is single, and its duct shortly after leaving it
receives the ducts coming from two yolk-glands situated one
on either side of the body, and is surrounded at about the
same region by a shell-gland, consisting of a number of uni-
cellular glands arranged in a radiating manner around the
oviduct. Beyond its union with these ducts the oviduct
either runs almost directly to the genital atrium, opening
into it in close proximity to the cirrus, or else pursues a
winding contorted course through the parenchyma and serves
as a iiterus or oott/p, within which the ova undergo a portion
of their development.

From the oviduct in the region where the ducts from the vitellaria and
shell-gland open into it one or more canals may arise whose significance is to
a certain extent problematical. In the Distomese one such canal occurs, and
when a seminal receptacle is present it stands in more or less close relations
to this canal, known as Laurer's canal, which, after a short course, opens to
the exterior on the dorsal surface of the body. In some Polystomece two
canals arise from the yolk-ducts and pass forwards parallel to the uterus
to open by a number of pores situated on the margin of the body. These
canals have been termed the vagina, and in some forms are represented by
a single canal. In addition to the vagina, however, another canal is pres-
ent which has been shown in Polystomum and Spltyramira to open into
the digestive tract, and has been homologized with Laurer's canal of the
Distomese.

It seems pretty certain that the vagina of the Polystomeas functions in
copulation, the genital orifice of one Polystomum having been observed to
come into contact with the vaginal openings of the other during that act.
But the Laurer canals do not seem to have any such function, and it has
been suggested that they may serve for the removal of surplus yolk-
material produced in accordance with the favorable conditions for nutri-
tion offered by the parasitic mode of existence of the Trematodes.

TYPE PLATYUELMINTHES. 147

Two orders may be recognized as occurring in the Tre-
nt) atoda.

1. Order Polystomeae.

The Polystomese are for the most part ectoparasites and
present fewer signs of degeneration than do the endopara-
sitic members of the class. The apparatus for adhering to
their hosts is usually strongly developed, several suckers
usually being present, as, for instance, three in Tristomum
and seven in Polystomum integerrimum (the latter parasitic in
the urinary bladder of the Frog), and in addition a number of
chitinous hooks may occur, as in Gyrodadylus and Sphyranura
(the latter parasitic on the skin of Menobranchus). In accord-
ance, too, with their mode of life, sense-organs in the form of
eyes and probably of tactile papillae on the skin occur, and
furthermore the processes of development are much simpler
than in the endoparasites, as will be seen later.

Some peculiar anomalies occur in the life-histories of some of the Poly-
stomeae, as, for instance, in the GyrodaetyliLS, which lives upon the gills of
the Carp. It is a viviparous form, and the young while still within the
body of the parent may already have become mature and contain young
likewise, which again may contain ova in course of development, four gen-
erations being thus enclosed one within the other. Diplozoon, which lives
likewise on the gills of Cypriuoid fishes, is peculiar in that at the time of
sexual maturity two individuals become fused with one another in the
form of an X, the fertilized ova giving rise to a single form formerly
known as Diporpa.

2. Order Distomeae.

This order includes endoparasites which show a more
marked degeneration than do the members of the preceding
order. Eyes may be present in the young but are absent
in the adult, and furthermore a very complicated metamor-
phosis is passed through in the development. The suckers
for adhesion to the host are either one (Monostomwti} or
two (Distomum), and as a rule no chitinous hooks are pres-
ent.

Among the more interesting members of this order are Distomum
hepaticum, a large form measuring 2-3 cm. in length and inhabiting the

148 INVERTEBRATE MORPHOLOGY.

bile-ducts of Sheep, in which it produces what is termed the " Kot," which,
in the low-lying pastures of England and the Continent, is frequently the
cause of the destruction of large numbers of sheep. In exceptional cases
it has been known to occur in man. In Egypt, however, the Fellaheen
are not unfrequently attacked by another form, Distoimim hcematobium,
which is peculiar in that, contrary to the rule, the sexes are separated in
different individuals. The margins of the body of the male are rolled
inwards on the ventral surface, forming a tube within which the more slen-
der female lives. Associated in pairs in this way, they are found in the
blood of the portal vein and its connections and pass to the ureters and
bladder, in whose mucous membrane they deposit their ova, thus pro-
ducing an inflammation, accompanied by suppuration, of these organs.

Development of the Trematodes. — The ova of Trematoda
consist of two distinct parts, a germ-cell, the product of the
ovary, surrounded by a mass of food-material, the secretion
of the vitellaria, the whole being enclosed in a shell formed
by the shell-gland. In the Polystomese the development, as
a rule, is entirely carried on outside the body of the parent,
the stalked ova being attached to the body of the host, though
Gyrodactylus is viviparous. In the Distomese, however, the
reverse is the rule, the ova undergoing a certain part of their
development in the uterus of the parent, and leaving the egg
shortly after its extrusion as a larva, sometimes ciliated,
sometimes provided in the place of the cilia with a structure-
less cuticle, and furthermore in these endoparasites there
occurs a remarkable alternation of generations of the kind
already referred to as heterogony (see p. 61).

The heterogony may be of various degrees of complexity.
It begins, however, in all cases wTith the embryo (Fig. 79, A),
which may be a free-swimming ciliated organism provided
with a short pouch-shaped intestine and with a mouth, and
frequently possessing also a nervous system and pigment
eye-spot as well as excretory tubes ; in other cases, however,
as stated, the embryo is destitute of cilia, usually in this case
being provided with one or more spines at the mouth-end
of the body, and all gradations of degeneration of the eye-
spot and nervous system, as well as of the excretory tubes
and digestive system, may be observed. In all, however, the
space between the more or less developed digestive, tract and
the body-wall is occupied by numerous unspecialized cells

TYPE PLATTHELMINTHES.

149

(gc\ which are in reality germ-cells or ova capable of under-
going a partheiiogenetic development. Eventually this larva
makes its way into the interior of an animal of some kind,
usually a Mollusk, and there undergoes a further develop-
ment, either retaining its digestive apparatus and elongating
somewhat to form a Redia (Fig. 79, B], or becoming an oval

m

ii

FIG. 79. — A, Ciliated larva, and B, Redia of Distomum hepaticum (after LEUCK-

ART).

d = intestine. m = mouth.

gc = germ-cells. ?•' = second generation of Rediae.

sac without mouth or digestive tract, the Sporocyst. The
Redia is a much more highly organized form than the Sporo-
cyst and is frequently capable of motion, two blunt projec-
tions near the hinder end of the body serving as supports in
a somewhat similar manner to the sucker-like feet of cater-
pillars. It adheres to the wall of a cavity of its host, from
which by energetic action of its muscular pharynx it is able
to absorb nutrition.

From this stage onwards the development varies in com-
plexity in various forms. It is simplest in Monostomum muta-
bile, whose ciliated embryo, while still free-swimming, contains
within it a small sexually immature Monostomum, and after it
has made its way into the interior of its Molluscan host the
young Monostomum becomes encapsuled in the tissues of its
host. The mode of origin of this immature form has not as

150

INVERTEBRATE MORPHOLOGY.

sp

yet been observed, but there is no reason for doubting that
it is the result of the parthenogeuetic development of one of
the germ-cells which occur in the body-cavity of the embryo.
So long, however, as it remains in the tissues of the Mollusk
it undergoes no further development; it can only reach ma-
turity in a second host, in this case some water-bird which
swallows the Mollusk ami its eucapsuled parasite, when the
latter, its capsule being dissolved by the digestive juices of
the bird, is set free, fastens itself to the wall of some of the
cavities of its host and becomes sexually mature.

In this species of Trematode but two hosts are required

in the life-history ; in the majority
of the Distornese a third occurs, an
additional stage of development in-
tervening between the Redia or Spo-
rocyst and the encapsuled immature
worm. The germ-cells of the Iledia
or Sporocyst while in the interior of
the Mollusk develop into a form
resembling an immature Distome,
but provided with a mobile muscular
tail whose axis is formed by a fibrous
rod resembling somewhat in appear-
ance the Vertebrate notochord. Such
an organism is known as a Cercaria
(Fig. 80), and when fully developed
the Cercaria brood leaves the body
of the parent Eedia or Sporocyst,
makes its exit from the tissues of
armata the Mollusk and leads for a time
a free-swimming existence. Eventu-
ally the Cercaria makes its way into
the body of a second host, usually
like the first a Mollusk, and there
becomes encapsuled in the tissues,
losing at the same time its tail, and
it reaches its maturity only after the
Mollusk has been swallowed by the definitive host, as was
the case in Monostomum.

FIG. 80.— Cercaria
(after SCHWARZEI.
us = anterior sucker.
cv = contractile vesicle

d — intestine.

n = uephridial tube.
ph = pbarynx.
sp = spine.
vs = ventral sucker.

TYPE PLATTHELMINTHES.

151

A still further complexity is found in the Liver Fluke,
Distomum hepaticum. In this form the free-swimming embryo
makes its way into the tissues of a small snail and there be-
comes converted into a Sporocyst. The germ-cells of the
Sporocyst give rise by their development, not to Oercarise,
as in the usual cases, but to Bedise, and these may give rise
under certain conditions to a second brood of Bedise (Fig. 79,
£, r'\ During the summer, however, the Bediae produce Cer-
carise, which, leaving their host, swim about for a short time,
and finally encyst themselves, not necessarily in a second
Mollusk, but on grass or any other object with which they
may come in contact, the tail at the same time being lost.
If, now, these encysted forms are swallowed by a sheep, the
young Distome makes its way to the bile-ducts of the host,
where it becomes mature.

The following schema by K. Hertwig will show the relationships of these
different methods of development :

SIMPLE MODE.

USUAL MODE.

COMPLICATED MODE.

u
1 -

§1

°*

h- *

Embryo.

Water.

la
5.2

°*

M

Embryo.

Water.

si*
<& -

§1

°s

M

Embryo.

Water.

Sporocyst
or Redia

I. Host
(Mollusk)

Sporocyst
or Redia

I. Host
(Mollusk)

Sporocyst

I. Host
(Mollusk)

.£§

a£i

0*

Rediae

cc

II. Generation

II. Generation

Cercarias.

Water.

III. Generation

Cercariae.

Water.

Encapsuled
Distome

I. Host

Encapsuled
Distome

II. Host,

Encapsuled
Distome

II. Host or
on grass, etc.

Mature
Distome

II. Host

Mature
Distome

III. Host

Mature
Distome

III. Host

In a few Distomeae a simplification more extensive than that repre-
sented in the first columns of the schema occurs, as for instance in the
genus Holostomum, whose embryo, after making its way into the body of
the first host, seems to be gradually metamorphosed into the immature
Distome, without any alternation of generations.

A very peculiar life-history is found in Distomum macrostomum, which
is parasitic in insect-eating birds. The Sporocyst is found in a snail, and
is peculiar in that it assumes a branching form, the branches forming a
network among the tissues of the host. In the ends of filaments of the net-
work young Distomes develop without the intervention of a Cercaria stage,
and by their development and its own growth the terminal branches be-
come of a considerable size, two of them extending into the tentacles of

152 INVERTEBRATE MORPHOLOGY.

the snail, which thus become .enormously distended. The club-shaped
structures so formed are abundantly supplied with muscle-fibres, and by
rigorous movements finally burst the distended wall of the tentacle, and
separating from the Sporocyst fall to the ground. There they move
about, resembling an insect larva in general appearance, a resemblance
increased by banded markings of green and white, which render them very
conspicuous, and they are apt finally to be snapped up by some bird, in
whose digestive tract the young Distomes are set free and become mature.
There can be little question but that the simple metamorphosis of the
Polystomece represents the original method of development of the Trematoda,
the heterogony characteristic of most Distomece being a secondary acquisi-
tion developed in accordance with their endoparasitism. An idea of the
mode in which this alternation of generations has been brought about is
furnished by such forms as Gfyrodaetyltts, in which the development of the
ovum takes place within the body of the parent, the young in their turn
developing embryos before being born (see p. 147). This acceleration of
sexual maturity, accompanied by parthenogenesis, has brought about the
condition seen in the Sporocyst or Redia, which are embryos provided with
ova capable of parthenogenetic development. Thus fundamentally the
heterogony is a paBdogenesis (see p. 60), and may be compared, in a gen-
eral way, with the formation of a h.ydroid colony by the budding of a
medusa larva.

III. CLASS CESTODA.

Like the Trematoda the members of this class are para-
sites, but are throughout endoparasites, and present a much
greater degeneration of structure than is found in the Disto-
mese, accompanied by peculiarities of development differing
somewhat from what occurs in these forms. The Cestodes
or Tapeworms lack all trace of a digestive tract and of a
mouth, living in their mature state attached to the wall of the
digestive tract of their host, and immersed in the nutritive
fluids contained in the intestine.

In some forms, such as Caryopliyllwus (Fig. 81, A), para-
sitic in the intestine of Cyprinoid fishes, the similarity to a
Trematode is very striking, except in the absence of suckers
for adhesion and of a digestive tract. The worm consists of a
somewhat dilated head, succeeded by a narrower portion which
may be termed a neck and gradually enlarges to the rather
cylindrical body, which contains a single set of reproductive
organs. In Ligula, which is found in the intestine of aquatic
birds, there is likewise an absence of suckers, but the repro-

TYPE PLATYHELMINTHES.

lf)3

d active organs are present in several sets succeeding one an-
other, without any external indications of a reduplication of
parts. In Tricenophorus the multiplication of the sets of re-
productive organs is indicated, however, externally by indis-
tinct constrictions of the body, an indication of a tendency

od-

FIG. 81. — A, Caryophyllceus tnutabilis (after STEIN) ; B, Tcenia saginata (after

LEUCKART) ; C, anterior eud of T. saginata (after LEUCK.ART).

od = oviduct. vd — vas deferens.

ov = ovary. vi — yolk-glands.

t = testis. vid = yolk-duct.

t)S = vesicula seuiinalis.

for the individual to separate into a number of parts, each
possessing a certain amount of individuality. This tendency
reaches its highest development in such forms as Bothrio-
cephalus and Tcenia (Fig. 81, B}, which consist of an anterior
portion, the Scolex (Fig. 81, 6r), provided with organs of adhe-
sion in the form of suckers, accompanied or not by chitiuous

154 INVERTERRATE MORPHOLOGY.

hooks and followed by a varying number of segments or pro-
glottides, each possessing a set of reproductive organs and cap-
able of separating from its fellows, maintaining for a time an
independent life. The progiottides towards the hinder end
of the chain or strobila are the most advanced in development,
and one after another drop off and pass to the exterior of the
host's body with the faeces; more anteriorly the progiottides
are sexually immature, and still nearer the scolex they are to
be found in various stages of formation. In fact the hinder
end of the scolex rna}* be regarded as a zone of growth, new
progiottides being successively formed at this region. The
process of proglottid formation resembles not a little what
has been described as the non-sexual reproduction of the Dis-
comedusse, the scolex corresponding to the parent Scyphos-
toma and the progiottides to the Ephyne, the entire aggre-
gation in both cases being termed the Strobila.

The exterior of the body of a Cestode is formed by a cuti-
cle without any trace of cellular structure, and is perhaps to
be regarded as a basement-membrane, the ectoderm, originally
present, having disappeared. The cuticle varies much in
thickness, and is throughout traversed by fine pores which
allow of the absorption into the body substance of the nutri-
tive fluids in which the Tapeworm lives, either directly or
by permitting the passage to the exterior of fine protoplasmic
processes from the subjacent tissue. Special developments
of the cuticle in the form of chitinous hooks are frequently
present, arranged in some Tsenias, for example, in a double
circle upon a prominence, the rostellum, at the apex of the
scolex, and forming a very efficient means of attaching the
worm to the wall of the intestine of its host. Beneath the
cuticula there is to be found a very thin muscular layer, the
peripheral musculature, but the main bulk of the muscula-
ture consists of those fibres which traverse the parenchyma.
These, especially the longitudinal and transverse ones, are
massed into strong bands, the former lying usually exterior
to the latter, and both enclosing a central mass which is trav-
ersed by weaker bundles of dorse-ventral muscles, and con-
tains the reproductive apparatus.

In connection with the muscular system may be men-

TYPE PLAT7HELMINTHE8. 155

tioned the suckers which frequently occur upon the scolex,
and serve with the hooks, when these are present, to attach
the parasite to its host. In Tcenia these suckers are four in
number, and have the form of circular depressions whose
walls are richly supplied with muscle-fibres, while in Bothrio-
cephalus they have the form of elongated grooves, situated on
the edges of the somewhat flattened scolex.

As might be expected from the great development of the
muscles, a well-defined nervous sy stem is present. It consists
of a brain lying imbedded in the tissues of the anterior por-
tion of the scolex, evidently composed by the union of two
ganglionic masses and giving rise to two main nerve-cords,
which pass backwards through the entire length of the
strobila without interruption (Fig. 82, n). So, too, the excre-
tory system (Fig. 82, ne) extends through the entire strobila
uninterruptedly. It consists of two nephridial tubes, which
in the anterior part of the scolex may be united by a cross
branch, as they are at the posterior edge of each proglottid,
and open to the exterior by a pore situated at the centre of
the posterior edge of the last proglottid. As each proglottid
separates from the chain, a new pore forms in the one pre-
ceding it, which becomes the terminal one, so that an opening
for the system is always present.

The reproductive system (Fig. 82) possesses a complexity
similar to what has been described for the Trematoda, and
hermaphroditism prevails throughout the class. In the stro-
bilar Cestodes each proglottid contains a complete set of
organs, both male and female ; the testes (Fig. 82, te) are
usually very numerous, consisting of small spherical masses
scattered through the parenchyma, each being provided with
a small duct, which after a short course unites with similar
ducts coming from other testes, all finally uniting to a
single vas deferens (vd), which opens to the exterior after
passing through a muscular organ, the cirrus-sac, by the con-
traction of which its terminal portion, often provided on its
inner surface with barbed hooks, is protruded to the exterior
as an intromittent organ or cirrus (c). The female apparatus
varies somewhat in its arrangement. In the majority of forms
the ovary is a bilobed organ (ov\ lying near the posterior

156

INVERTEBRATE MORPHOLOGY.

end of the proglottid. The oviduct soon after leaving
the ovary unites with the yolk-duct (vid) coming from the
albuminous vitellarium (vi), which consists of a number of
glands scattered through the parenchyma similarly to the
testes. At the point of union with the yolk-duct the oviduct

vld

va

ov

FIG. 82. — PROGLOTTID OF Tcenia filicollis (after KRAEMER).

c = cirrus. te = testis.

n = nerve. ut = uterus.

ne = excretory canal. va = vagiua.

ov = ovary. vd — vas deferens.

sg = shell-gland. m = yolk-gland.

md = yolk-duct.

enlarges, receiving at the enlargement the secretion of a num-
ber of unicellular glands composing the shell-gland (sg).
From this enlargement two tubes arise : one, the vagina (va),
runs almost directly forward to open into a chamber, the
genital atrium, which contains also the cirrus-sac and com-
municates directly with the exterior; while the other, the
uterus (ut), after a somewhat convoluted course opens iude-

TYPE PLATYHELMINTHES. 157

pendently to the exterior a little behind the genital pore.
The vagina serves as a duct for the spermatozoa during
copulation, and corresponds with the canals opening at the
sides of the anterior end of the body in the Polystomese (see
p. 146), while the uterus serves for the retention of fertilized
and mature ova. In Botliriocephalus the opening of the genital
pore is in the middle line of one of the surfaces of the pro-
glottid ; in T<xnia, however, it occurs on the lateral margin of
the proglottid, and in some cases each proglottid may have an
opening in each of the lateral margins, there being a duplica-
tion in such cases of the genital ducts. Furthermore, in the
Ttenias the vitellarium is much less voluminous than in
.Bothriocephalus, and produces an albumen-like secretion in-
stead of yolk-cells, and in addition the uterus has no special
opening to the exterior and is relatively small, though it may
become fairly voluminous by its walls being pushed out into
pouch-like sacculations by the contained ova.

Development of the Cestoda. — Accompanying the differences
in the arrangement of the reproductive apparatus differences
in the development are found in the two groups. In Botlirio-
cephalus (taking this form again as the example of the one
group) the egg is richly provided with yolk-cells, among
which the germ-cell lies imbedded. The embryo leaves the
egg in the form of a spherical ciliated body, provided with
six chitiuous hooks, arranged more or less distinctly in pairs.
After swimming about for a time the cilia and their cells are
thrown off, and the six-hooked embryo makes its way into the
body of the first host, where it becomes enclosed in a thin
cyst, within which it develops directly to a scolex. If this
be swallowed by the second host, the worm fastens itself to
the walls of the digestive tract, and soon develops to the
sexuallv mature strobila.

•>

In the Taeuias, however, the ova are much smaller, the
yolk-cells being replaced by an albumen-like substance,
relatively small in amount, and the embryos when they hatch
out are destitute of cilia, resembling the six-hooked embryo
of Bothriocephalm after it has lost its ciliated covering (Fig.
83, A]. In this condition it makes its way into the primary
host, in whose tissues it becomes encysted, and develops in

158

INVERTEBRATE MORPHOLOGY.

some forms, such as the Tcenia cucumerina of the dog, whose
primary host is the Dog-tick, into a Cysticercoid. This resem-
bles a scolex, whose head has been withdrawn into and enclosed
by the body, and when it is swallowed by the secondary host, the
dog in the instance cited, the head is pushed out, fastens itself
to the wall of the digestive tract, and begins to grow and form
proglottides. In other cases, however, the posterior part of the
scolex into which the head is retracted becomes enormously
swollen by the accumulation of fluid within it, forming a large
vesicle, into the interior of which the head projects, having
become completely invaginated. Such a form as this is

FIG. 83. — A, six-hooked embryo of Taenia ; B, diagram of Cysticercus ; C,
diagram of Camv/rus; D, diagram of Echinoeoccus.

termed a C'ysticercits (Fig. 83, B) ; when it is swallowed by the
secondary host the head evaginates, and the cyst remains for
some time attached to the hinder end of the scolex, but later
disappears, and the formation of the proglottides occurs.
Further modifications arise by the formation in the wall of the
cyst of not only one but several invagiuated heads, forming
the Ccenurus (Fig. 83, O) ; or even secondary cysts may arise
from the inner wall of the original vesicle, and each of them
may develop several heads, forming what is known as the
Eciiinococcus (Fig. 83, D}.

Several of the Cestoda are especially interesting from a medical stand-
point, inasmuch as they are parasitic in man either during the adult or the

TYPE PLATYHELMINTHE8. 159

larval stage. Among these may be mentioned Bothriocephaliis latus, which
occurs in the human intestine, where it may reach a length of as much as
13 metres, in such cases consisting of many thousand proglottids. These
may readily be recognized by the convoluted uterus, and by the open-
ings of the reproductive organs on the median line of one of the flat sur-
faces, wliile the head is characterized by being flattened, and provided
on the margin with two elongated suckers. The ova give rise to a ciliated
larva which becomes transformed into the six-hooked embryo, this latter
making its way into the tissues of certain fish, which serve as the first host.
Man becomes infected with the worm by eating improperly cooked or salted
fish, the Pike being the more usual primary host, though this part may also
be played by other forms.

The genus Tcenia furnishes two human parasites. The genus is char-
acterized by the head being provided with four circular suckers, as well as
in some cases with one or more crowns of hooks ; the genital pore is situ-
ated upon the margin of the proglottids, and the uterus is a straight tube
with a varying number of lateral transverse pouches.

Tceitia solium is by far the most frequent tapeworm of man, and may
reach a length of 3-3£ metres, and consist of 800-900 proglottides. The
head, in addition to the four suckers, is provided with a rostellum bearing
a double crown of from 26-28 hooks. The proglottids are about 5 mm.
broad and 10-12 mm. long, and the uterus has 7-9 stout lateral pouches.
The Cysticercus state of this worm occurs in the muscles of pigs, whence
man becomes infected by eating improperly cooked or salted pork. It
measures 8-10 mm., and possesses when imbedded in the muscles an ellip-
tical shape, its long axis being parallel with the long axis of the muscle
fibres. In addition to its occurrence in swine muscle, however, it has also
been found occasionally in man, encysted in the muscles, brain, or eye.
The source of the infection of man is, in many cases at least, cress, lettuce,
and such articles of food which have been watered with liquid manure
containing the fertilized ova of the worm. The six-hooked embryo encysts
itself in the tissues named, and man becomes the intermediate host of the
worm.

TcEnia sayinata, also known as T. mediocanellata, is of less frequent
occurrence than T. solium, from which it is easily distinguished by its
greater length, 7-8 metres, and by the greater number of proglottids,
1200-1300. The head has no rostellum or crown of hooks, and the pro-
glottids are recognizable by their size, measuring 5-7 mm. in breadth and
18-20 mm. in length, and also by the lateral branches of the uterus being
slender and 20 or 30 in number. The Cysticercus occurs in the muscles or
occasionally in other organs of cattle, improperly cooked beef being the
source of infection for man.

In addition to being occasionally the intermediate host of T. solium,
man may also be the host of the Echinococcus of T. echinococcus, a small
worm about 4 mm. in length and with only three proglottids, which occurs
in its adult state in the intestine of the dog. The ova may be received into

160 INVERTEBRATE MORPHOLOGY.

the human digestive tract by fondling, and especially by kissing, infected pet
dogs, and the six-hooked embryo makes its way to the liver, lungs, brain,
or other organs, where it becomes encysted, producing tumors which,
especially in the liver, may reach a great size and a weight of from 10 to,
in some cases, 15 kilogrammes.

Domestic animals are also apt to be infected with Cestodes in addition
to those already mentioned, occasionally with fatal results. This is es-
pecially the case with sheep, in whose intestine T. expanse/ may develop in
such numbers as to occlude the lumen, and cause death, especially in young
iambs. A Coenurus also occurs occasionally encysted in the brain of sheep,
producing a disease known from its symptoms as the "staggers," which
may likewise result fatally.

The Relationships of the Cestodes.— In considering the affinities of the
Cestoda, the nature of the strobila, so far as its individuality is concerned,
must be inquired into. Two views upon this point are open. The older
one regards the Cestode as a colony, considering each proglottid an indi-
vidual equivalent to the scolex, and the process of strobilation one of
reproduction by budding. On this view the strobila is exactly comparable
to the Scyphostoma strobila, the scolex corresponding to the Scyphostoma
base and the proglottids to the Ephyrae. There is undoubtedly much to be
said in favor of such a view which regards the reproduction of the Cestoda
as a process of alternation of generations, but at the same time it must be
recognized that the buds or proglottids are not reduplications of the parent
bud as is the case with Microstoma, where the budding individual has the
adult form. In the Scyphostoma strobila the buds do differ from the
parent which gives rise to them ; but the Scyphostoma is a larva which
gives rise by budding to the adult form, and is comparable rather to the
Cysticercus than to the scolex of the Cestode. Non-strobilating Scyphos-
tomas become medusae, but the scolex never becomes a proglottid, and
the latter cannot be considered the terminal stage of the life-history in the
same sense as a medusa is. The nervous system of the entire Cestode
strobila centres in the brain of the scolex, the various proglottids never
developing independent brains, the reproductive organs being practically
the only organs which are reduplicated in successive buds.

According to the second view the strobila is an individual, and the
strobilation is regarded as a culmination of the reduplication of organs
seen in many forms, but more especially in the Nemerteans (q.v.). This
view receives strong support from the occurrence of such forms as Canj-
ophylloeus, Ligula, and Tricenophorus, described on a preceding page, in
which may be seen successive gradations of strobilation, beginning with a
simple reduplication of the reproductive apparatus in Ligula, this redupli-
cation being accompanied in Tricenophorus by a tendency for the body to
constrict into parts, each of which contains one of the sets of reproductive

organs.

The choice between these two views hinges upon the question of indi-
viduality. The individuality of either Liytda or Tricenoplionis can hardly

TYPE PLATYHELMINTHES. 161

be questioned, and there is no reason for regarding a Tcenia for instance
as an individual belonging to a higher grade than either of these — a view
which the first and older theory implies, since it regards a Tcenia as a
colony of equivalent individuals. Such a form as Caryophyllceus is an
aggregate of individualities of a lower grade, organ-individuals ; and just
as the cell-individuals composing these may divide, so the organs, or
rather the embryonic masses of cells destined to give rise to them, may
bud, producing a reduplication of organs. This reduplication may occur
in one or more organs ; in the Accela among the Turbellaria it affects only
the testes, in the Alloiocoela it affects both ovaries (the vitellaria being
originally parts of the embryonic mass which gives rise to the ovaries) and
testes, and in the Rhabdocoela it affects only the ovaries. In the Cestodes
the entire reproductive apparatus is reduplicated in this manner, a series
being produced, and secondarily a tendency for each member of the
series to be capable of separation from its fellows has come about owing to
the greater certainty it gives for the perpetuation of the species. A cer-
tain amount of individuality of the proglottids is thus brought about, but
at the same time the process of strobilation cannot accurately be termed a
process of non-sexual reproduction by budding, since the proglottid indi-
viduals are not quite of the same grade of morphological individuality as
CaryophyllmsSj which the scolex represents. Both views are correct to a
certain extent : the strobilation is a budding off of individuals from the
scolex, but of individuals of a lower grade ; and the entire strobila is in
reality an individual comparable to Caryophyllceus or a Trematode.

Considering, then, the strobila as a metamere-individual, what are the
affinities of the Cestodes ? They seem to have been derived from Trema-
todes, the simpler forms without reduplication of the reproductive organs
being capable of being regarded either as Trematodes without a digestive
tract or as Cestodes without any indications of strobilation. If this be true,
indications of their affinity should appear in the life-history in accordance
with the biogenetic law. One interesting form deserves mention in this
respect — Archigetes, which occurs in certain Annelids. It is a Cestode
without reduplication of organs and provided with a tail, similar in a gen-
eral way to that of a Cercaria. Certain facts in its life-history seem to
indicate that Archigetes is comparable, not to an adult Cestode, but to a
Cysticercus which has become sexually mature, and it might be expected
that similarities to the Trematode Cercaria might be found in Cysticerd.
Recently such similarities have been shown to exist in certain Cysticer-
coids : a tail-like appendage, which later separates and degenerates, has
been described as occurring at this stage of the development ; and further-
more it has been suggested that the cavity of the Cysticercoid into which
the head is invaginated may be equivalent to the Trematode intestine, later
on becoming obliterated by the growth of the parenchyma. The evidence
at present available points, then, to a derivation of the Cestode from the
Trematodes, and from Trematodes in which the Cercaria-stage had already
been established.

162

INVERTEBRATE MORPHOLOGY.

pr

oc

IV. CLASS NEMERTINA.

The three preceding classes show marked evidences of
genetic affinity, the characteristic differences of structure in
the Trernatodes and Cestodes being due to the parasitic
habits of these forms. The Nernerteans, on the other hand,
though apparently tracing descent from a Turbellarian-like
ancestor, show a marked advance in structure, and must be
regarded as organisms of a considerably higher grade than
the other Platyhelrninths.

They are for the most part marine, though a few forms
inhabit fresh water or even damp earth,
and are usually elongated ribbon-like
forms, reaching a length, in some cases,
of several centimetres. The body is ex-
ce ternally unsegmeuted, though a more or
less marked metamerism of the internal
organs, due to their repetition at definite
intervals, may be present. The ectoderm
of the body resembles that of the Tur-
bellaria in being throughout ciliated, and
rests upon a basement membrane, which
in some cases contains cells. Within
the membrane are a varying number of
muscle layers, differentiations of the
outermost portions of the mesodermal
tissue, which in the form of a parenchyma
occupies the interval between them and
Fn;.S4.-ANTERioRPoK- the digestive tract. This (Fig. 84, d, is
TION OF NEMEUTEAN. an almost straight tube, except in the

genus Malacobdella, and is pushed out on
each side into sac-like pouches, which
are arranged in some cases with a
regularity of succession almost rneta-
meric. It opens to the exterior at the
rm ---- retractor muscle of anterior end of tjie body bv the mouth,
proboscis. , ",

a short non-muscular oesophagus inter-
vening between the intestine proper and that opening; and
at the other end of the body is a second communication with

ov

ce = cerebral ganglion.
eg = ciliated funnel.
d = intestine.
oc = eyes.
ov — ovary.
pr = proboscis.

TYPE PLATYHELMINTHES. 163

the exterior, the amis, an opening unrepresented in other
Platyhelminths. The digestive tract is no longer a blind
sac, but has the form of a tube, as in all the higher types.

In the anterior end of the body, above the digestive tract,
is a structure, the proboscis (Fig. 84, pr), essentially peculiar
to the Nemerteans, although indications of such an organ are
to be found in the Rliabdocoda. It consists of a closed tube,
the proboscis sheath, with muscular walls, imbedded in the
body parenchyma and extending backwards in some cases
almost to the end of the body, and within it lies the proboscis,
also a tube, united to the wall of the sheath near its anterior
end and in fact closing it at that region. From this line of
attachment the proboscis stretches back in the cavity of the
sheath, the space between it and the walls of the sheath being
filled with fluid. It is a simple imagination into the cavity
of the sheath of the external body-wall, whose musculature
as well as ectoderm are continuous with that of the proboscis.
From the tip of the invaginatiou a band of muscle fibres,
forming the retractor muscle (rm) of the proboscis, passes to
the wall ol the body. By the contraction of the muscular
walls of the sheath the fluid contained in its cavity forces the
proboscis to be evaginated sometimes with sufficient force
to tear itself loose from its line of attachment ; but should
this accident not happen, the proboscis can be reinvaginated
by the contraction of its retractor muscle. The function of
this organ is doubtful. In some cases it is undoubtedly a
weapon of offence and defence ; but it seems not improbable,
from its rich nerve-supply and from the probable function of
its prototype in the Khabdoccela, that in some cases at least
it may be a tactile organ.

A well-developed nervous system is always present, though
it may show in some cases, as Carinella, the primitive character
of being still imbedded in the ectoderm or else lying immedi
ately beneath it. In other cases, however, as Cerebratulus, it
is enclosed in the muscles of the body-wall or may even be
completely within them, imbedded in the parenchyma. It
consists in its most usual form of two ganglionic masses
(Fig. 84, ce) from which short nerves pass forwards and which
are united by two transverse commissures, one of which passes

164 INVERTEBRATE MORPHOLOGY.

over or in front of the resophagus, while the other arches
from one gauglionic mass to the other over the proboscis
sheath. Each ganglion is bilobed, the smaller posterior
lobe being in some cases united to the larger one by a rela-
tively thin baud of nerve-tissue so that it appears to be
almost independent. From the larger lobe of each side a
nerve-cord passes towards the posterior end of the body,
where the two may unite to form an arch passing over the
posterior part of the intestine. In addition to these a third
nerve originating from the commissure passing over the pro-
boscis sheath and running backwards in the median dorsal
line is frequently present as well as, in some forms, another
nerve running along the dorsal wall of the proboscis sheath,
to which it sends branches. In many forms, such as Cerebrat-
ulus, a fine plexus of nerve-tissue, lying between the muscle
layers of the body-wall, unites the three main nerve-cords,
some of the strands of the plexus being sometimes larger
than the others and forming circular commissures between the
nerve-cords ; in Tetrastemma and Amphipoms, for example,
these circular commissures may be strongly developed and
have an almost metameric arrangement, the general plexus
being in such cases wanting.

Eyes (Fig. 84, oc) are present in some forms occasionally
in Considerable numbers, but are frequently wanting, and oto-
cysts occur but seldom. The lateral ciliated grooves which
occur 011 the sides of the head of some Khabdoccela reach in
the Nemerteans a high development (eg), in some forms, e.g.
Cerebratulus and Tetrastemma, becoming ciliated funnels of some
length, whose inner ends are imbedded in the substance of the
posterior lobe of the brain. An olfactory function has been
assigned to these organs, though some authors have consid-
ered them mainly respiratory.

The excretory system consists of a longitudinal canal on
each side in the anterior portion of the body, sometimes re-
placed by a network of canals, which opens to the exterior by
one or more ducts leading to pores situated on the margin of
the body. In some cases these lateral ducts and the pores
may be quite numerous and, like the intestinal pouches and the
circular nerve-commissures, may have a somewhat metameric

TYPE PLATYHELMINTHES. 165

arrangement. The various terminal branches of the nephridial
tubes are club-shaped and closed, a flame of cilia projecting
from the closed end into the lumen of the tube. The canals
and tubes are lined with ciliated cells, and are therefore inter-
cellular and not iutracellular, i.e., do not perforate cells, differ-
ing in this respect from the nephridia of other Platyhel-
minths.

The blood-vascular system is peculiar to the Nemertea
among Platyhelrninths, and consists in the simple forms, such
as Carinella, of two lateral vessels which anteriorly open into
lacuuar spaces without definite walls. In the more highly
organized forms, however, three longitudinal trunks, two
lateral and one dorso-median, are present with definite and
sometimes muscular walls, and unite in a J_-shaped manner
at the posterior end of the body, while in front they may
either open into a system of lacuna?, or, as in Tetrastemma,
unite with each other as they do posteriorly, a perfectly closed
system thus resulting. Transverse connecting branches be-
tween the dorsal and lateral vessels occur in regular succes-
sion, a metamerism being again suggested. The blood-vessels
and lacijnee contain a fluid in which float round or elliptical
corpuscles, which in some of the higher forms have a red
color, due to the presence of haemoglobin. No heart or
special contractile organ is present, the blood being driven
through the vessels, without an}^ definite direction, by the
movements of the body.

The occurrence of a blood- vascular system in the Nemerteans and its
character in the lowest members of the group suggests a mode of origin for
the system which agrees well with what may be deduced from embryological
observations on other forms. It may be supposed that in the primitive
Nemerteans a system of spaces filled with fluid existed, in which cells derived
from the parenchyma floated. These spaces would represent a simple
•oelom, and were lacunar in character, lacking definite walls, the cir-
"ilation of the fluid they contained being very irregular. In time the
•aces along the sides of the body might arrange themselves in a linear
. ianner, and might acquire definite walls, the rest of the spaces remaining
lacunar, when a condition resembling that in Carinella would ensue, the
arrangement found in higher forms resulting from the conversion of the
remaining lacunar spaces into vessels with definite walls.

According to this view the blood-vascular system is to be regarded as

166 INVERTEBRATE MORPHOLOGY.

in reality a portion of the coelom separated off for a special purpose, and
other instances bearing the same significance will be noticed later.

The reproductive system differs from that of the other
Platyhelminths iu its much greater simplicity, no vitellaria or
shell-gland being present, and furthermore the Nemerteaus
are almost without exception of separate sexes. The ovaries
(Fig. 84, ov) or testes are present in considerable numbers,
one lying in each interval between two lateral diverticula of the
intestine, so that they partake in their arrangement of the
more or less pronounced metamerism of that organ. Between
the intestine and the genital masses there is in some forms a
distinct cavity, or coaloinic space, and at the time of maturity
a separate communication with the exterior forms for each
ovary or testis.

The class Nemertina may be divided into four orders,
whose chief characteristics may be briefly stated, having been
for the most part already described.

1. Order Palseonemertini.

To this order belong the genus Carinella and allied forms,
all characterized by structural peculiarities which are to be
regarded as primitive. The lateral ciliated organs are, as in
the Khabclocoela, mere grooves, not being continued inwards
to the brain in the form of a funnel ; and furthermore the
nervous system is either imbedded in the ectoderm or lies
immediately below it. To these characters may be added the
more or less lacuuar nature of the blood vascular system, and
the communication, in some cases, of the nephridia with it.

2. Order Schizonemertini.

In the Schizonernertmi the ciliated funnels are well devel-
oped, and the nervous system is imbedded in the muscular
layers of the body-wall ; and though the nerve-cords are still,
as in the preceding order, united by a plexus, nevertheless
there are indications of a development of coinmissural con-
necting nerves. The blood vascular system is still lacuuar
anteriorly, though posteriorly three well-defined vessels are
present. The genus Cerebratulus belongs here.

TYPE PLATYHELMINTHES. 167

3. Order Hoplonemertini.

This order, wliicli includes tlie genera Tetrastemma and
Amphiporus mentioned above, has, like the preceding order,
ciliated funnels as lateral organs, and the nervous system lies
completely within the muscular layer of the body-wall and
the nerve-cords are united by transverse commissures, the
plexus being wanting. The blood vascular system is a closed
series of tubes, not communicating with lacunar spaces. The
most striking characteristic of the order is, however, the
structure of the proboscis, which is armed near its posterior
(that is, while invagiuated) end by one or more dagger-like
spines or stylets. The most posterior portion is not capable
of being evagiuated, and its Avails are glandular, secreting a
poisonous fluid which is poured into the more anterior por-
tion of the tube, bathing the stylets and thus being carried
into the wound which may be made by the forcibly evaginated
proboscis with the stylets coming into contact with the body
of the prey or enemy.

4. Order Malacobdellina.

This order contains a single genus, Mcdacobdetta, which is
found in the mantle-cavity of marine Lamellibranchs, such as
the common Mussel and Clam. It resembles the Hoplonemer-
tini in many particulars, but is destitute of lateral ciliated
organs, and its proboscis possesses no stylets. The intestine
is a convoluted tube without lateral diverticula, and the hind
end of the body is provided with a sucker.

Development of the Nemertina. — In some Nemerteans, such as Tetra-
stemma and Malacobdella, the young worm leaves the egg in the form of a
cylindrical ciliated larva, usually provided at the extremities of the body
with bunches of longer cilia, which may possibly be sensory in function,
and gradually changes without any marked metamorphosis into the adult
form. The mouth opens upon the ventral surface of the body into a re-
tort-shaped digestive tract which in early stages possesses no anus — this
structure only appearing much later. In many forms, however, a peculiar
metamorphosis occurs during the transformation of the larva, known from
its first describer as Desor's larva, into the adult. On the ventral surface
of the body there appear four invagmatioiisof the ectoderm, two situated in

168

INVERTEBRATE MORPHOLOGY.

front of the mouth and two behind it, which gradually separate from the
ectoderm to form four single-layered plates lying immediately beneath it.
By a subsequent growth and fusion of these plates a new ectodermal cover--
ing is formed enclosing the internal organs, and on its completion the orig-
inal larval ectoderm is thrown off. In some species a somewhat more com-
plicated process occurs. The larva, known as the Pilidium (Fig. 85), has
the shape of a helmet from whose rim two ear-like lappets hang down, be-

ap

FlG. 85. — Pilidium LAKVA (after SALENSKY).
ap = apicul plate. m = mouth. s = digestive sac.

tween which lies the mouth-opening (m), while at the apex of the helmet
there is an ectodermal thickening (ap), nervous in character, from which
projects a bunch of strong sensory cilia. As in the Desor larva four iuvagi-
nations of the ectoderm of the ventral surface occur, which, however, sepa-
rate from the larval ectoderm as four hollow sacs which unite together,
their inner walls thickening to form the ectoderm of the young Nemertean,
while the outer walls become thin and form what is termed the amnion sur-
rounding a cavity within which lies the young worm. During the process
of fusion of the four sacs the enteron ( 8) and a portion of the mesoderm
of the Pilidium are enclosed and give rise to the digestive tract and meso-
derm of the young worm, which later breaks through the amuion and
Pilidium wall to become free.

The significance of this metamorphosis is decidedly obscure. Some
authors regard it as more primitive than the direct method of development,
on the ground that the Pilidium with its lappets presents general simi-
larities to the Mullerian larva of the Polyclades and is derived phylogeneti-
cally from such a form, being therefore more ancestral in its characters
than the simpler larvae. It must be recognized, however, that there is no
indication of metamorphosis in the Polyclad larvae, and furthermore that

TYPE PLATYHELMINTHES. 169

the Nemerteans perhaps show greater similarities to Alloiocoelan Turbel-
laria than to Polyclads. Perhaps an explanation of the process is to be
found in the sloughing of the ectoderm and the formation of new ciliated
cells which is seen in the larva of a Palseonemertean, Cephalothrix, the
metamorphosis of Desor's larva and of the Pilidium being a greater and
more complicated ecdysis derived from the simpler one.

Some interesting evidence as to the morphological significance of the
anus is to be derived from a study of its development in the Nemerteans.
It is an opening which has been considered by some to have arisen by the
closure in the middle of an elongated slit-like blastopore — the two ends, how-
ever, remaining open to form respectively the mouth and anus; and it has
been thought that the direct transformation of the blastopore into the per-
manent mouth in some cases, and in others into the permanent anus,
receives on this theory an explanation. The phenomenon of the closure of
the blastopore in the middle does actually occur in the Annelid-like Tra-
cheate Peripatus, and in many forms both mouth and anus stand in close
ontogenetic relationship to the blastopore. In the Nemerteans are repre-
sented the most lowly organized animals which possess both mouth and
anus, and accordingly it might be expected that in them the original rela-
tionships will be most clearly seen. The young Nemertean possesses no
anus. It resembles, so far as its digestive tract is concerned, an Alloiocoelan;
it is only relatively very late in its life-history that the anus appears, and
then in a region of the body which has no relation whatever to the original
blastopore. This fact should carry considerable weight with it, especially
as in the majority of forms the anus is, in comparison with the mouth,
of relatively late formation. It seems not improbable that primitively it
had no relation with the blastopore, and where such relations do occur
they are entirely secondary.

The indication of metamerism seen in the Nemerteans needs no further
discussion after what has been said on p. 43 with reference to similar pecul-
iarities in the Turbellarians.

SUBKINGDOM METAZOA.

TYPE PLATYHELMINTHES.

I. Class TURBELLARIA. — Ectoderm ciliated; no anal opening.

1. Order Acoda. — Mouth present, but no digestive tract. Convoluta.

2. Order Alloioccela. — Digestive tract present; space between it and

body-wall occupied by parenchyma. Monotus, Playiostoma.

3. Order Rhabdoccela. — Digestive tract straight rod- or sac-like;

space between it and body-wall not filled with parenchyma.
Microstoma, Mesostoma, Prorhynchus, Vortex.

4. Order Tridadea. — Digestive tract branched, three principal limbs

giving rise to secondary branches; male and female reproductive

170 INVERTEBRATE MORPHOLOGY.

organs with common opening. Gfunda, Planaria, Phagocata,
Dendrocoelum, Bdelloura, Bipalium, Syncoslidium.
5. Order Polycladea. — Digestive tract branched, the primary branches
being numerous; male and female organs having separate open-
ings.

(a) With terminal sucker (Cotylea). Thysanozoon, Eurylepta.
(6) Without sucker (Acotylea). Planocera, Leptoplana, 8ty-

lochus.

II. Class TREMATODA. — Ecto- or endoparasites; ectoderm not ciliated;
with digestive tract and suckers.

1. Order Polystomece. — Suckers more than two; development direct;

usually ectoparasites. Polystomum, Sphyranura, Tristomum,
Oyrodactylns.

2. Order Distomece.— Suckers one or two; development indirect; usu-

ally endoparasitic. Distomum, Monostomum.

III. Class CESTODA. — Endoparasites; ectoderm without cilia; no digestive

tract or mouth; usually strobilated. Tcenia, Bothriocephalus,
CaryophyllcBus, Ligula, Tricenophorus, Archigetes.

IV. Class NEMERTINA. — Ectoderm ciliated; not parasitic; anus present;

with protrusible proboscis.

1. Order Palceonemertini. — Lateral ciliated funnels shallow; nervous

system imbedded in ectoderm; proboscis without stylets. Cari-
nella.

2. Order Schizonemertini. — Lateral ciliated funnels deep; nervous

systetn imbedded in muscle-layer; proboscis without stylets.
Oerebratulus.

3. Order Hoplonemertini. — Lateral ciliated funnels deep; nervous

system within muscle-layer; proboscis with stylets. Tetra-
stemma, Amphiporiis,

4. Order Malacobdellina. — No lateral ciliated funnels; proboscis

without stylets. Malacobdella.

LITERATURE.

TURBELLAKIA.

L von Graff. Monographic der Turbellarien .: 1. RhftMocwliden. Leipzig, 1882

L. von Graff. Die Organisation der Turbdlaria accela. Leipzig, 1891.

L. Bb'hmig. Untersuchungen fiber rhabdoca'len Turbellarien, II. Zeitschr. fiir
wissensch. Zoologie, LI, 1890.

W. M. Woodworth. Contributions to the Morphology of the Turbdlaria, I. Bul-
letin of the Museum of Comp. Zool., xxi, 1891.

A. Lang. Die Polycladen. Fauna u. Flora des Golfes von Neapel. Monogr.,
xi, 1884.

W. M. Wheeler. Syitea'lidium pellucid um, a new marine Tridad. Jour, of
Morjjhology, IX, 1894.

TYPE PLATYHELMINTHES. 171

TREMATODA AND CESTODA.

R. Leuckart. Die Parasiten des Menschen. 2te Aufl. Bd. I. Leipzig u.
Heidelberg, 1879-1889.

A. Lang. Untersuchiiugen zur vergleichenden Anatomie und Histologie des
Nervensystems der TurbeUarien, II u. III. Mittheilungen a. d. Zool. Sta-
tion zu Neapel, n, 1881.

C. Claus. Zur morphologischen und phylogenetischen Beurtheilung des Band
wurmkorpers. Arbeiten a. d. Zool. Institut Wien, vin, 1889.

NEMERTINA.

W. C. Mclntosh. A Monograph of the British Annelids. Part I. Nemerteans.

London, 1872-74.
0. Burger. Untersuchunge/t iiber die Anatomic und Histulogie der Nemertinen.

Zeitschr. fiir wissensch. Zoologie, L, 1890.

172 INVERTEBRATE MORPHOLOGY.

CHAPTEE VIII.

TYPE NEMATHELMINTHES.

THE Nemathelminths are, like the members of the preced-
ing type, characterized by the form of the body, which is
cylindrical and usually elongated or even thread-like, whence
the popular terms Round-worms or Thread-worms which
are frequently applied to them. The ectoderm is covered by
a thick layer of cuticle which it secretes, and in connection
with which spines, bristles, or hooks may be developed at
various parts of the body. There is no trace of segmentation
or reduplication of organs, with the exception that in some
forms the circular nerve-commissures uniting the longitudinal
cords may succeed each other with tolerable regularity ; the
cuticle, it is true, especially when thick, is ringed by numer-
ous grooves succeeding one another at short intervals, but
this cannot be interpreted as an indication of metamerism,
but is more probably a provision to counteract the rigidity of
the cuticle and to give a considerable amount of mobility to
the body. The Nemathelmiuths accordingly have the same
grade of individuality as a simple Platyhelminth, such as an
Alloioccelaii, and are to be regarded as metamere individuals.

One important difference of structure which these worms
show from the Platyhelminths is the presence of a capacious
coelom, the interval between the digestive tract and the mus-
culature of the body-wall not being tilled up by parenchyina-
tous mesoderm, but being a simple undivided cavity in which
lie the reproductive organs. These hitter are simple, the
animals being as a rule bisexual, and there is no separation
of the female organ into ovary and vitellarium. Structures
of an excretory nature occur in one of the two classes into
which the type is divisible, but a blood vascular system is
entirely wanting.

The habit of life varies greatly in the various members of

TYPE NEMATHELMINTHE8.

173

the group. In the class Nematoda many forms live freely in
the sea, fresh water, or damp earth, while others are parasitic
during a part of their lives, and others again are parasites
practically throughout their whole existence. The Acantho-
cephala are without exception parasitic.

I. CLASS NEMATODA.

The Nematodes are distinguished from the members of
the second class by the presence in nearly all cases of a dis-
tinct digestive tract, usually with mouth and anus, and by
the absence of a retractile proboscis furnished with hooks
at the anterior end of the body. The arrangement of the
muscles of the body-wall are also peculiar inasmuch as longi-
tudinal muscles only are present (Fig. 86, m), which instead of
forming a closed sheath are interrupted along four longitudi-

:d

FIG. 86. — TRANSVERSE SECTION OF Ascaris lumbricoides AT THE LEVEL OP

PHARYNX (from HERTWIG).

c = cuticle. m = longitudinal muscles.

d = dorsal line. s = lateral Hue.

h = hypodermis. » = ventral line.

w = nephridiuin.

nal lines (d, v and s), or in some cases along a single ventral
line, in the former case there being four longitudinal bundles
of muscles extending the length of the body. In the struc-
ture of most of the organs, however, considerable variation is
found, and it will be most convenient to describe them as

174

INVERTEBRATE MORPHOLOGY.

they are found in each of the two orders into which the class
may be divided.

-ph

1. Order Eunematoda.

This order contains the majority of the Nematoda, and
all its members are furnished with a mouth and anus and a
functional digestive tract. The mouth is in
some cases at the bottom of a funnel-like
depression which may be armed with spines,
special developments of the cuticula which
covers the body. This is throughout cylin-
drical in shape, except that in the males of
some species it expands at the posterior ex-
tremity into a relatively large funnel-shaped
structure with thin walls, the bursa (Fig. 87),
at the bottom of which lies the opening of the
cloaca, a cavity into which the intestine and
the male reproductive organ open. Beneath
the ringed cuticle lies the ectoderm (liypo-
dermis] which secretes it, and beneath this
the muscular layer Avhich consists only of
longitudinal muscle-fibres, differentiations of
the outer ends of large cells whose undiffer-
entiated inner ends project into the coelom, so
as almost to obliterate it in some cases. The
muscle-fibres do not, however, form a com-
plete continuous sheath surrounding the
coelom, but are interrupted along four longi-
tudinal lines, two lateral, one dorsal, and
one ventral (Fig. 88). The coelom contains
the intestine and reproductive organs, and
is peculiar in that it is not bounded by a
limiting cellular membrane or peritoneal
lining, being simply a space comparable to
the coelomio cavities of the Rhabdoccela or
the blood-sinuses of the Nemerteans.

The digestive tract is a straight tube traversing the body
from one extremit}r to the other, opening posteriorly in the

W.

FIG. 87. — Ascaris

(after LEUCKAUT).
i =- iutestiue.
ph — pharynx.
sp = spicules.
te = testis.

TYPE NEMATHELMIXTHES. 175

female directly to the exterior, in the males into a cloaca
common to it and to the male organ of reproduction. Its
anterior part is a muscular oesophagus lined with cuticle di-
rectly continuous with that covering the surface of the body,
while posteriorly it is a delicate tube composed of a single
layer of cells, not being surrounded by any rnesodermal mus-
cular tissue.

The excretory system is not as yet fully understood. It
appears to consist of a pair of tubes, for which no cellular
lining has as yet been made out, which lie, one on each side,
in the thickened hypodermis of the lateral lines. In the an-
terior portion of the body they unite to form a single tube
which opens to the exterior in the median ventral line not far
behind the brain (Fig. 88, B).

This latter consists of a ring or nerve-collar surrounding
the anterior part of the oesophagus on which lateral masses of
ganglion-cells occur and which gives rise to two main nerves,
one of which runs back in the median dorsal line, while the
other, which in some forms appears to be double, lies in the
median ventral line. Other nerves pass forwards from the
nerve-ring to the anterior part of the body, and in addition to
the dorsal and ventral nerve-cords two lateral nerves pass
backwards a short distance, while circular commissures con-
nect the two main nerve-cords, those of the two sides of the
body not, however, being opposite each other, so that they do
not suggest a pseudo-metamerism so strongly as the similar
commissures of the Hoplouemertiui. Special sense-organs
are as a rule absent, though a few forms possess eyes.

The reproductive organs are exceedingly simple. In the
male they are represented by a single convoluted tube, lined
in its upper part by the mother-cells of the spermatozoa and
dilating below into a seminal vesicle, to which succeeds a
short ejaculatory duct which opens into the cloaca. The walls
of this latter cavity are frequently invagiuated to form two
small sacs in each of which lies a chitiuous spicule capa-
ble of being protruded from the cloacal opening and serving,
with the bursa, as copulatory organs. The female organs, on
the other hand, consist of a pair of convoluted tubes, each of
which dilates into a uterus and unites with its fellow to form a

INVERTEBRATE MORPHOLOGY.

single tube, the vagina, sometimes with muscular walls, which
opens to the exterior in the ventral mid-line some distance in
front of the anus. As a rule the sexes are separate, her-
maphroclitism occurring only in a few isolated cases.

Many Nematodes are free throughout their entire exist-
ence, living in the sea, fresh water, or damp earth, and fre-
quently possessing eyes. Others are found in some domestic
products, such as the vinegar-eel (Anguillula), found in vine-
gar and sour paste ; while others, again, are parasitic on
plants, such as Tylenclius, which lives upon the young grains
of wheat and in some cases produces very serious damage to
crops, and Heterodera, which is quite as injurious to root-
crops. More interesting, however, are a number of forms
occurring as parasites in animals, many affecting man, in some
cases producing serious results.

Life-histories of the Eunematoda. — The free-living forms show no
peculiarities of development, the immature animal developing directly
from the egg. Among the parasitic forms, however, interesting variations
from direct development, due to a change of host, occur, a well-marked
heterogony occasionally being found. An example of this is seen in Rhab-
ditis niyrovenosa, which at one stage of its existence lives in damp earth,
the females being viviparous and producing young which make their way
into the lungs of frogs, where they assume a form which led them to be
assigned to the genus Ascaris, and where they become mature. At this
stage they differ from the Rhabditis forms in being hermaphrodites, and
from the eggs deposited by them the Khabditis generation again results.

From a medical standpoint one of the most important forms is Trichina
spimlis, which occurs encapsuled in the muscles of various warm-blooded
animals, such as man, the pig, rat, mouse, and occasionally in the fox, cat,
and rabbit. The capsules are oval and about 0.6 mm. in length, and occa-
sionally have a white color, due to the deposition of calcareous matter in
the wall. In the interior of the capsule lies coiled up an immature
Trichina, which may retain its vitality in this condition apparently during
the lifetime of its host. Should, for instance, improperly cooked or salted
pork which contains such capsules be eaten by man, the capsule becomes
dissolved by the digestive juices and the young Trichina is set free in the
small intestine and in the course of a few days becomes sexually mature.
Each female may deposit in the intestine as many as 1000 eggs, from
which, in the second or third week after infection, young Trichina meas-
uring about 0.01 mm. hatch out and at once proceed to bore through the
walls of the intestine, producing a more or less violent inflammation
according to the degree of infection. They wander through the connective

TYPE NEMATIIELMINTHES.

177

tissue and finally reach the muscles, especially of the neck and diaphragm,
into which they bore, producing a degeneration of the tissue upon which
they feed. In the course of the third month after infection they encyst
themselves in the muscle-tissue, and inflammatory changes produced in the
connective tissue in their immediate vicinity result in the formation of a
second cyst-wall around them (Fig. 88, A). If the intestinal inflammation
and the succeeding muscular inflammation have not proved fatal to the
host, the danger is past, the encapsuled Trichince undergoing no further
development in the muscles.

Other forms which occur in man are Ascaris lumbricoides, the round-
worm (Fig. 88, B), a large form, of which the female measures 40 cm. in
length and the male 25 cm., and which bears some resemblance in shape to
an earth-worm, Oxyurisvermicularis, asmaller form, 1 cm. in length, which
inhabits the rectum especially of young children, and Trichocephalus dispar
(Fig. 88), which measures 3-5 cm. in length and is characterized by the
anterior half of the body being exceedingly slender, the worm boring into
the intestinal wall, especially in the neighborhood of the cajcum, by this
slender portion, the hinder thicker por-
tion hanging freely in the wall of the
intestine. The presence of these three
forms may be recognized, independently
of the finding of the actual worm, by
their ova, whose respective characters dif-
fer very greatly. So far as is known the
development of these forms is direct and
there is no intermediate host, but the ova
are taken into the body with the food.
The exact manner of infection is, howrever,
obscure.

In addition to the forms which have
been mentioned there are a few which are
more especially frequent in tropical cli-
mates. Dochmius duodenalis is a small FIG. 88. A, TrichintT^ucysted

form about 1-2 cm. in length, with strong in muscle; B, anterior extrem-
teeth or blunt spines in the mouth region,
which fastens itself to the wall of the
small intestine and lives upon the blood
of its host, producing auasmia. Its ova
develop in stagnant water or damp earth,
and probably man becomes directly in-
fected. It has long been known in the tropics, producing the disease
known as Chlorosis (egyptiaca, but may also affect miners or workers in
tunnels, having appeared endemically in the workers on the St. Gothard
tunnel, whence it has since spread somewhat in Germany, especially among
workers in clay. Filaria medinensis is limited entirely to the tropics and
is a very slender worm nearly 1 metre in length which lives in the connec-

ity of Ascaris lumbricoides
from theveiitral surface, show-
ing the two ventral oral papillae
and the excretory pore (both
after LEUCKART) ; C, Trichocepk-
alus dispar (after OWEN).

178

IN VERTEBRA TE MORPHO LOGY.

tive tissue beneath the skin, producing ulcers, at the bottom of which the
worm lies coiled up. The ova develop in water and the embryos pass
probably into small Crustacea, which are swallowed with drinking-water.
Filaria sanguinis ho minis, also solely tropical in its distribution,
receives its name from the fact that it lays its ova in the blood of man,
which may thus swarm with countless numbers of small worms. These
make their way to the exterior of the body by the kidneys, producing
haemorrhages or minute abscesses in that organ and, as the result of
these, milky or bloody urine.

2. Order Gordiacea.

This order includes the families of the Gordildce and Mer-
mithidce, long slender thread-like worms, which differ from the
Euuematoda in several important respects. They occur in
their mature state in fresh water ; in their immature stages,
however, they are parasitic in insects. In the adult Gordius
the mouth is usually closed by an overgrowth of the cuticle,
and the anus is lacking in Merm-t's. The musculature of the
body-wall consists only of longitudinal fibres (Fig. 89, m), which

ov

cu

01

FIG. 89. —TRANSVERSE SECTION OF Gordius (after VEJDOVSKY).
cu = cuticle. n — nerve-cord.

d = intestine. pe — peritoneum.

hy = hypodermis. ut = uterus.

m = longitudinal muscles. ov — oviduct.

differ in their arrangement from those of the Eunematoda in
being interrupted only in the mid-ventral line. The coeloui
is lined by a peritoneal epithelium (pe) lying beneath the

TYPE NEMATIIELM1NTHES. 179

muscle-cells, and is divided into two lateral chambers by a
mesentery (m) running the entire length of the body and con-
sisting of two layers surrounding the intestine (d), and inserted
into the body-wall dorsally and veutrally, their outer surfaces
being lined by a continuation upon them of the peritoneal
epithelium.

No excretory system has been as yet discovered. The
nervous system consists of a gangliouic ring surrounding the
oesophagus, from which a number of nerves pass forward,
while a single nerve-cord (w) passes backwards in the mid-
ventral line, dilating at the posterior end of the body into
a gauglionic mass.

The reproductive organs consist in the female of a series
of ovaries (ov) attached one behind the other to each mesen-
tery above the intestine. In the mesenteries two tubes (ut)
pass backwards which receive some of the ova and function
as uteri, near the hind end of the body bending veutrally to
open into the cloaca, whose Avail is iuvagiuated to form a
single seminal receptacle. The testes have not yet been
found, but two seminal vesicles, corresponding to the uteri of
the female, occur and open likewise into the cloaca, which in
the male is evertible and serves as a copulatory organ.

The Affinities of the Nematodes. — The relationships of the Nematodes are
exceedingly obscure. Their unsegmented character and the character of
the nervous system seem to ally them more closely with the Platyhelminths
than with higher forms, but the relationships to any of the known Platy-
helminths must be exceedingly remote. The parasitism which occurs so
frequently in the group is to be considered as secondary, since so many
forms lead a free life and peculiarities of structure can hardly be attributed
to degeneration. The Gordiacea stand on a .higher plane than the Eu-
nematoda, as shown by the possession of a mesentery and the arrange-
ment of the reproductive organs and nervous system, which bear some sim-
ilarities to those of the Annelids, but their Nematode characteristics are
most pronounced. Perhaps the ancestors of the Nematodes are to be found
in the yet unknown intermediate forms between the Platyhelminths and
Annelids, a view which would account for their similarities in certain
respects to both these groups.

II. CLASS ACANTHOCEPHALA.

This class contains a number of parasitic forms which
occur more especially in tho digestive tract of fishes, though

180

INVERTEBRATE MORPHOLOGY.

also found in Mammalia and in exceptional cases in man. ( A
great uniformity of structure exists throughout all the species,
so that they are all referable to a single genus, Echinorh(/nckus.\

rm

The body (Fig. 90) is cylindrical and as a rule not very long,

and a marked distinction from the
Nematodes is found in the retractile
proboscis (pr) occurring at the anterior
end of the body. It is a cylindrical
prolongation of the anterior portion of
the body and is provided with a number
of chitinous hooks by means of which
it adheres to the intestinal wall of its
host. The proboscis may be iuvagi-
nated into a double-walled muscular
proboscis-sheath by whose contraction
it may again be protruded, a strong
retractor muscle, extending from the
tip of the proboscis to the base of the
sheath, serving for the invagiuation ;
and from the base of the sheath re-
tractor muscles (rm) pass to the body-
walls and serve to hold the sheath in
position. No traces of a digestive tract
occur.

The body is covered upon the out-
side by a thick cuticle secreted by the
subjacent hypoderrnis, which is a rather

FIG. 90.— MALE Echinorhyn- thick layer consisting of a protoplasmic

matrix in which nuclei are scattered
but in which no cell-outlines are to be
distinguished. Beneath the cuticle the
matrix has a fibrillar character, and
near its inner surface it is hollowed out
into a network of anastomosing canals
of which mention will be made later.
Beneath the hypodermis lies a basement-membrane within
which are two layers of muscle-cells, having the same epi-
thelio-muscular character as those of the Nematodes, the
fibres of the external layer having a circular direction, while

diUS (after LEUCKART).
g — glands.
I = lemniscus.
p = penis.

pff =- proboscis ganglion
pr = proboscis.
rm — retractor muscle.
t = testis.

TYPE NEMATHELMINTHES. 181

those of the irmer layer have a longitudinal course. The
body-wall encloses a well-marked ccelorn, riot lined by a
special peritoneal epithelium, but which contains the repro-
ductive organs and is traversed by the retractor muscles of
the proboscis-sheath.

The nervous system consists of a gangliouic mass (pg)
lying within the proboscis-sheath which sends forward nerves
for the supply of the walls of the sheath and of the retractor
muscle of the proboscis. Posteriorly two lateral nerve-cords
extend backwards along the sides of the body, and in male
individuals are connected near the posterior extremity with a
ganglion lying beneath the reproductive ducts and from which
nerves pass to the genital apparatus.

The system of lacunar canals which form a network in the
lower layers of the hypoderrnis is probably excretory in
function. The canals are found throughout the entire hypo-
dermis, both in the proboscis and in the body-wall, in the
latter there being indications of two larger lateral trunks.
From the point of junction of the proboscis with the body-
wall two muscular sacs, the lemnisci (1} , hang down into the
coaloni. The cavity which they contain communicates with a
circular lacuna which surrounds the base of the proboscis
and with which the lacunse of the proboscis-hypodermis like-
wise communicate, this system of the proboscis-lacunae and
the lemnisci being shut off from the system of the body-wall
by a partition extending from the basement-membrane to the
cuticle. The lemnisci have been regarded, as possible repre-
sentatives of a digestive tract, but it seems more probable
that they are reservoirs for the reception of the fluid con-
tained in the lacunae of the proboscis when it is driven from
them during invaginatiou.

The reproductive organs are much more complicated than
those of the Nematodes. The sexes are separate, the male
individuals being usually smaller than the females. The
ovaries are paired bodies enclosed within a muscular ligament
attached anteriorly to the base of the proboscis-sheath and
posteriorly to the reproductive duct. At an early stage of
their development, however, the ovaries split up into masses
which float about in the coelom together with large numbers

182 INVERTEBRATE MORPHOLOGY.

of separated ova. They pass to the exterior by a complicated
system of ducts, the most anterior portion of which is a wide
funnel-shaped structure, the bell, to whose wall the ligament is
attached and which, by a rhythmical expansion and contrac-
tion, engulfs the ova and ova-masses floating about in the
coeloni. From the lower end of the bell they escape, the ova-
masses to be returned to the coeloin, while the fertilized sepa-
rate ova pass into a short tube, the oviduct, which opens below
into a muscular uterus, which finally communicates with the
exterior at the posterior end of the body.

The male apparatus consists of usually two testes (Fig.
90, t) enclosed within the ligament, which is attached below
to the wall of the evertible bursa. From each testis a duct
passes backwards, the two soon uniting to the single vas de-
ferens, which, after receiving the ducts of some unicellular
glands (gl), opens into the bursa at the tip of a muscular penis
(p). The bursa when everted is a somewhat funnel-shaped
structure at the bottom of which is the penis, the edge being
furnished in some forms with hooks by means of which it
serves as a copulatory organ.

The life-history of the Amnthocephala includes a change of host. The
larvae are found in the body-cavity of Crustacea or insects, and reach ma-
turity only when the intermediate hosts are swallowed by the proper final
host. The largest species of Echinorhynchus is the E. gigas, which occurs
in the intestine of the pig ; the intermediate host of this form is the June
bug (Melolontha).

Nothing can as yet be stated with any certainty concerning the relation-
ships of the AcanthocepJiala. They are usually associated with the Nema-
todes, to which they certainly present similarities, but no intermediate
forms bridging the gap between the two classes are yet known, and the
embryological history throws little light upon the question.

SUBKINGDOM METAZOA.

TYPE NEMA THELMINTHES.

I. Class NEMATODA. — With digestive tract ; without proboscis furnished

with chitinous hooks.

1. Order Eiineniatoda. — Musculature of body-wall interrupted along
the lateral line ; no mesentery ; no peritoneal epithelium. An-
<j n ill iila, TylemJius, Heterodera, Trichina, Ascaris, Oxyuris,
Trickocephalus, Dochmius, Filariu.

TYPE NEMATHELMINTHE8. 183

2. Order Qordiacea. — Musculature of body-wall not interrupted along
the lateral line ; with mesentery and peritoneal epithelium.
Gordius, Mermis.

II. Class ACANTHOCEPHALA. — Without digestive tract ; with proboscis
armed with recurved chitinous hooks ; parasitic throughout.
Echinorhynchus.

LITERATURE.

R. Leuckart. — Die Parasiten des Menschen. 2te Aufl. (In course of publica-
tion.)

A. Schneider. — MonograpJde der Nematoden. Berlin, 1866.

L. Orley. — MonograpJde der Anguilluliden. Buda-Pesth, 1880.

J. G. de Man. — Die einheimischen, frei i>i der reinen Erde und in smsen Wasser
lebenden Nematoden. Tijschr. d. Nederland. Dierkund. Vereen, v, 1880.

T. Vejdovsky. — Studien uber Gordiideii. Zeitschr. fiir wissenscli. Zoologie,
XLVI, 1888.

0. Hamann. — Monographie der Acanthocephalen (Echinorhynchen). Jenaische
Zeitschr., xxv, 1890.

184

IN VERTEBRA TE MORPHOLOQ Y.

CHAPTEK IX.

ORDER ECHINODERA; CLASS CHyETOGNATHA ; CLASS
ROTIFERA ; ORDER GASTROTRICHA ; DINOPHILUS.

THIS chapter includes a description of a number of forms
whose affinities are at present rather doubtful and which
show similarities sometimes to the Neniathelmmths and
sometimes to the Annelida. Instead, however, of assigning

them to one or the other of these types,
it has been thought advisable to consider
them in a separate chapter and each
group independently, indicating briefly
their most probable affinities.

Order Echinodera.

The order Echinodera includes a
number of small organisms all marine in
habitat, and all referable to a single
genus, Echinoderes (Fig. 91). The body
varies in length from somewhat less than
1 mm. to almost 0.1 mm. according to the
species, and tapers somewhat posteriorly,
terminating in one or two prolongations
or cerci, while anteriorly there is a pro-
FIG. 91. -- Echinoderes boscis armed with strong setse which
Dujardinii (after CLA- m ^ invap-iDated within the anterior

PAREDE from HATSCHKK).

portion of the body, and serves as an or-
gan of locomotion as well as for the prehension of food.
The outer surface is covered by a layer of chitin which is
divided into distinct metameric rings, the number of which,
eleven, is constant for all known species, and which are pro-
vided in some species with definitely-arranged setae. No
cilia are present. Beneath the chitinous rings lies the ecto-
derm, which shows indications of metamerism also, being

ORDER ECIIINODERA. 185

thickened beneath the interval between two successive rings ;
it consists of a granular layer of protoplasm in which scat-
tered nuclei occur. Beneath the ectoderm lies a somewhat
incomplete layer of longitudinal muscles, which become spe-
cialized anteriorly into separate bundles for the retraction of
the proboscis ; in each metamere two dorso-veutral muscle-
bundles, one on each side of the middle line, are also found.
A relatively spacious body-cavity in which various organs
lie occurs, but no lining peritoneal epithelium or mesenteries
have been observed.

The digestive tract begins with the mouth, which lies at
the bottom of the invaginated proboscis and opens by the in-
tervention of a short tube into a muscular pharynx into the
anterior portion of which, four glands, either salivary or poi-
sonous in function, pour their secretion. The pharynx com-
municates posteriorly with a sac-like stomach, upon which
follows a short straight intestine opening to the exterior at
the posterior end of the body between the terminal cerci.

Two elongated pear-shaped bodies lying in the coelom in
about the middle region of the body have been described as
excretory organs. They are closed at the free end, their
cavity is ciliated, and they open to the exterior on the dorsal
surface near the margin of the body. The reproductive
organs are cylindrical sacs which are provided with ducts
opening to the exterior on the terminal segment ; all the
species whose reproductive organs have been studied are
bisexual.

Four cellular masses lying above the pharynx seem to
represent the nervous system, though no nerves passing from
them have been discovered ; nor do any special sense-organs
exist.

The affinities of these forms is highly problematical, especially since
nothing is known of their development. The metamerism indicated by the
chitinous rings, the thickenings of the ectoderm, and the dorso-ventral
muscles suggest an affinity with the Annelids, while, on the other hand, in
the chitinous covering, and the occurrence of a longitudinal musculature
only, similarities to the Nematodes may be found. The excretory organs
may perhaps be compared with the larval nephridia of the Annelids, and
the existence of but a single pair of them, together with the absence of
any metameric arrangement of nerve-ganglia, favors the idea that the

186

IN VERTEBRA TE MORPHOLOG Y.

ce

Echinodera are not to be considered as being truly nietameric, indications
of metamerism which are found being altogether secondary and without
phylogenetic significance. Until, however, something has been ascertained

regarding their embryological history nothing can
be positively stated as to their affinities. It is
worthy of notice, however, that in some particulars
they resemble the Gastrotricha, and it is not im-
probable that their nearest allies are to be found
in that order, which on its part is related to the
Kotifera (see p. 189).

CLASS CHJETOGNATHA.

V

ov

The Chsetognatha constitute a small
group of forms separable into two genera,
Sagitta and Spadella. All the members
of the group are marine, and are elonga-
ted in form, the sides of the body being
furnished with one or two pairs of lateral
expansions or fins, to which is added a
caudal fin. The anterior portion of the
body is somewhat enlarged so as to form
a head, and on either side of the mouth
are a number of strong chitiuous bristles
movable by means of special muscles
and serving the purpose of jaws.

The ectoderm consists of several
layers of flattened cells giving rise in the
head region by secretion to chitinous
FIG. 92.— Spadella ceph- plates which serve for the attachment
(after HERT- of the muscles which move the jaw-
bristles. Both the lateral and the caudal
fins are ectodernial expansions consisting
of a homogeneous lamella covered by
one or two layers of ectodernial cells.
They possess no muscle-fibres and are
passive in locomotion, which is per-
formed by the contraction of the longitudinal muscles
producing rapid lateral movements of the posterior part of the
body. The genus Sagitta possesses two lateral fins, while in
Spadella (Fig. 92) but one large one is present. Below the

aloptera

WIG).
ce = cerebral ganglion.

i = intestine.

o - olfactory organ.
oc = eye.
ov — ovary.

t = testis.

CLASS CH^ETOGNATHA. 187

ectoderm lies a well-defined basement-membrane, and below
this are the muscles of the body-wall, which are, as a rule,
longitudinal in their direction, and are interrupted, as in the
Nematodes, along four longitudinal lines, one dorsal, one ven-
tral, and two lateral. In one species of Spaddla there is on
the inner side of the longitudinal muscles a thin layer of trans-
verse muscles, but usually only longitudinal fibres are present,
except in the head, when there are a number of special muscle-
bundles for the movement of the jaw-bristles.

Within the musculature of the body-wall is the spacious
coaloin, lined throughout by a delicate layer of cells constitut-
ing the peritoneum, and divided into three chambers by
transverse partitions, one of which lies just behind the head,
while the other is towards the hind end of the body. The
peritoneal epithelium lines the surfaces of these dissepiments,
and in the trunk and tail regions is reflected in the mid-
dorsal and ventral lines towards the centre of the body, form-
ing a mesentery, surrounding the intestine and dividing the
crelom into lateral compartments.

The mouth lies on the ventral surface of the head and
opens into an oesophagus surrounded by a single layer of
muscle-fibres having a dorso-ventral direction and passing
above and below into the general musculature of the head.
After being narrowed in passing through the anterior dissepi-
ment the digestive tube again expands (Fig. 92, i), and is sup-
ported throughout the trunk region by the mesentery. In
this region it is a simple straight tube, unprovided with mus-
cle-fibres, and terminates in an anal opening situated ven-
trally at the junction of the trunk and tail regions, not being
continued into the latter.

Neither an excretory nor a blood vascular S37stem is pres-
ent. The nervous system lies for the most part imbedded in
the ectoderm, and consists of two principal ganglionic masses,
of which one, the cerebral or supracesophageal ganglion (ce),
lying in the head region, is situated in the ectoderm of the
dorsal surface of the body and has a somewhat hexagonal
outline, giving off five pairs of nerves, one pair passing back-
wards as commissures to unite with the ventral or sub-
O3sophageal ganglion, lying also in the ectoderm a little in

188 INVERTEBRATE MORPHOLOGY.

front of the middle of the trunk region of the body. This
ganglion gives off numerous nerves, among which are two
principal nerve-cords passing backwards and giving off along
their entire length finer nerves which branch and finally lose
themselves in a fine ectodernial nerve-plexus throughout
which ganglion-cells are scattered. In addition to these
ectodernial portions three pairs of ganglia are found in the
head region at the sides of the oesophagus, the largest gan-
glion on either side being united with the supracesophageal
ganglion by a commissure. From the supracesophageal
ganglia, behind the commissures to the ventral ganglion, a pair
of nerves pass backwards to the two eyes (oc), which lie com-
pletely imbedded in the ectoderm of the dorsal surface of the
head, each consisting of three biconvex lenses imbedded in a
central pigment mass and surrounded on their outer surfaces
by a retina composed of an outer layer of cubical cells, a
middle layer of cylindrical cells with large nuclei, and an
inner layer of rod-like structures arranged perpendicularly to
the surface of the lenses. Behind the eyes lies a circular band
of fine columnar ciliated cells (o), which is supplied by a pair
of nerves arising from the supracesophageal ganglion be-
tween the optic nerves. The function of this organ is doubt-
ful, though it has been considered olfactory. Scattered some-
what regularly over the body are numerous round or oval
eminences consisting of a number of central spherical cells
arranged in two rows and bearing rod-like bristles. These
are enclosed in a sheath of cylindrical cells and below come
into contact or are continuous with terminal nerve-branches.
These sensory hillocks are supposed to be tactile in function
and resemble not a little the lateral sense-organs of certain
Annelids (see p. 210).

The Chsetagnatha are without exception hermaphrodite.
The ovaries (ov) are cylindrical bodies lying in the trunk re-
gion of the body, one on each side of the digestive tract, and
upon the outer side of each is a tubular oviduct which ends
blindly anteriorly and opens posteriorly at the sides of the
body near the dissepiment between the trunk and tail regions
of the body. There is no communication apparently between
the cavity of the oviduct and the ovary or crelom, and the

CLASS ROTIFER A. 189

manner iu which the ova make their escape is yet unknown.
Both ovaries and oviducts are enclosed within a fold of peri-
toneum (mesentery) extending from the sides of the body.
The testes (t) are situated behind the posterior dissepiment,
i.e., in the tail region of the body, and consist of a streak of
cells on each side in the peritoneal covering of the body-wall.
From these streaks masses of immature spermatozoa separate
and float about in the coelorn of the tail segment, and when
mature make their escape through canals, each of which com-
municates with the coelorn by means of a fine ciliated opening,
and near its opening to the exterior at the side of the body is
dilated into a seminal vesicle.

The embryological history of Sagitta throws no light upon the affinities
of these forms. In structure they recall, especially in the arrangement of
their musculature, the Nematodes, and especially the Gordiaceae, but at the
same time show many similarities to the lower marine Annelids, as for in-
stance in the origin of the spermatozoa from the wall of the coelom, and in
the similarity of the vasa deferentia to nephridial canals. The occurrence
of dissepiments also suggests affinities to the Annelids, but it does not
seem that these structures indicate a segmentation of the body, since the
arrangement of the nervous system points to the conclusion that the Chae-
tognaths consist of a single segment. From the evidence at present open
to us it would seem that the Chcctognatha are more nearly related to the
Annelids than to the Nematodes, but the relationship must be regarded as
a rather remote one, and it seems hardly fitting to include Sagitta and its
allies among the Annelida.

CLASS ROTIFEEA.

.

The Rotifers or " Wheel-animalcules " are microscopic
Metazoa which are widely distributed both in salt and fresh
water. They are unsegmented forms with a well-developed
coslom, and are somewhat oval in form as a rule, the an-
terior end of the body being surrounded by one or two
bands of cilia whose rapid movement produces the appear-
ance of a wheel, and has suggested the popular name for
the group. The posterior end of the body is frequently pro-
longed into a usually extensible so-called foot, which in some
cases (Lacinularia) is furnished with adhesive glands, and is
used as a point of fixation, though the majority of forms swim
about freely or attach themselves only temporarily, the foot

190

INVERTEBRATE MORPHOLOGY.

M

having in such cases the form of a sucker (Pkilodina), or ter-
minating in two movable lamellae, Brachionus (Fig. 93), or else
being entirely wanting (Asplanclma).

The body, with the exception of the anterior portion or
trochal disk, Avhich bears the bands of cilia, is enclosed in a

chitinous cuticle, occasionally
comparatively thick and firm,
forming a case, the lorica, into
which the softer parts may be
withdrawn, and frequently pre-
senting a delicate sculpturing

GJ or prolongations into spines.
A few forms (Floscularia) se-
crete a gelatinous case within

QV which the}' live, foreign parti-
cles being sometimes added
to the secretion ; a species of
Melicerta, for instance, building
a case for itself of pellets man-
ufactured from foreign bodies
and arranged in oblique or
spiral rows and cemented to-
gether by the gelatinous secre-
tion.

The trochal disk which oc-
FIG. 93. — Brachionus urceolaris (after

ECKSTEIN*.

Br = nerve-ganglion.
cv — contractile vesicle.
Gl = digestive glaud.
M = muscle.
Met = mastax.
N — nephridial canal.
0 = eye.
Oc = ocellus.

Ov = ovary.
Sp = calcar.

cupies the anterior end of the
body is but rarely circular in
outline ; more usually it is
lobed at its margins and may
even be separated into two
parts. The margin of the disk
is surrounded by one or two
bauds of cilia Avhich follow the
lobations, when two bands are

present one being entirely
prseoral and the other postoral in its position, so that the
mouth lies between the two on the ventral side of the disk.
Various differences of arrangement of the bands are found
in different species, one of them, the pmeoral, being some-

CLASS ROTIFERA. 191

times discontinuous, as in Brachionus, or reduced to a few
isolated patches, as in Asplanchna.

It is a question whether the forms with a double band of cilia or those
in which it is single represent the more primitive arrangement. It may be
supposed that originally there was but a single band which later became
double, but it seems more probable that the double condition is the more
primitive, from the fact of its frequent occurrence and also since, when
a single band is present, it seems to represent in some cases the prge-
oral band and in others (Floscidaria) the postoral one. Such a condition
of affairs can be most plausibly explained on the assumption that originally
two bands were present, and that in some forms the praeoral one gradually
gained pre-eminence in its development, the postoral one disappearing pari
passu, while in the Flosculariidae the reverse was the case.

Beueatli the cuticle lies the ectoderm, consisting of a layer
of cells whose outlines cannot be distinguished, and within
this comes the musculature of the body, which does not, how-
ever, form a more or less continuous layer beneath the skin,
but consists of aggregations of muscle-fibres into bundles
which traverse the body-cavity in various directions, some
running longitudinally and forming retractors of the foot and
of the trochal disk, while others have a circular direction. The
ccelom, in which the muscle-bundles and the various organs lie,
is not lined by a special peritoneal layer of cells, but may be
traversed by a greater or less number of delicate fibrils
arising from amoeboid cells and representing uudifferentiated
mesoderm.

The mouth lies near the ventral. border of the trochal disk,
the ciliated bauds serving to produce currents which con-
verge toward the mouth-opening, and so carry to it food-parti-
cles, which are then carried through the ciliated oesophagus
to the pharynx, whose walls contain a somewhat complicated
comminuting apparatus, the mastax (Fig. 93, ma), consisting
of two calcareous bodies, the mallei, of varying shape, and
sometimes also of a median body, the incus. By the action of
muscles attached to the mallei, these parts of the apparatus
can be brought into contact with each other, and with the
incus when this is present, the food-particles being thus
comminuted. From the pharynx the food passes through a
shorter or longer tube lined with chitiu, which is to be
regarded as a continuation of the pharynx, to the stomach,

192 INVERTEBRATE MORPHOLOGY,

usually a globular cavity, whose wall is formed by a layer of
ciliated cells containing fat-globules and various other par-
ticles, probably absorbed food-particles, these cells being
covered externally by a layer of connective tissue. Into the
stomach there opens from either side the duct of a gland (gl),
whose secretion is probably digestive in function and which,
may be termed a digestive gland from its resemblance to
similarly located glands in other invertebrates. The stomach
opens below into the shorter or longer intestine, whose walls
are lined by ciliated cells ; and this in turn communicates with
the terminal cloaca, which receives in some cases the termi-
nations of the excretory tubules and may be contractile.
The cloaca opens to the exterior, usually on the dorsal sur-
face, near the base of the foot, though in some forms which
live within a case the intestine bends forward upon itself, so
that the cloacal opening lies further forward.

The nervous system consists of a relatively large ganglionio
mass (Br) lying on the dorsal side of the oesophagus, from
which nerves pass anteriorly to the trochal disk, and posteriorly
to supply a dorsal sensory papilla, the calcar (Sp). In addi-
tion to this, two pairs of posterior nerves have been described,
one of which passes to a sense-organ situated on each side of
the body in its posterior third, while the other pair runs back-
wards on each side of the middle line to near the posterior
end of the body, giving off branches to the musculature as
it goes. Among the sense-organs eyes (O) are very generally
present, varying in number from one to several, and situated in
the region of the supraoesophageal ganglion, with which they
are connected. They consist of patches of red, brown, or
black pigment with which sensory or retinal cells are asso-
ciated, and which are in some cases covered by a refracting
lens formed as a special cuticular thickening. Other sense-
organs to which a tactile function has been ascribed con-
sist in their simplest form of one or several cells bearing
stiff cilia. A pair of such organs is usually present, one on
each side immediately above the ganglion of the lateral
nerves, and anteriorly in the mid-dorsal line just behind the
trochal disk a third occurs, the calcar (Sp), which frequently
is situated upon the extremity of a tubular extensible process

CLASS ROTIFERA. 193

of the body-wall, supplied with muscles for its retraction, and
to which nerve-fibres pass from the supraoesophageal ganglion.
In a few forms, such as Melicerta, the calcar is double.

No blood vascular system exists, but a well-developed
excretory apparatus (JV), resembling that of the Turbellaria,
is present. It consists of two longitudinal tubes, one on each
side of the body, from which arise a varying number of finer
lateral branches, each of which terminates in a funnel closed
by a flame-cell, as in the Turbellaria. Anteriorly the two
tubes may be united by a transverse connecting tube, and
posteriorly they may unite together to form a contractile
bladder which opens into the cloaca, or in some cases m&y
open directly to the exterior.

The female reproductive apparatus consists of a relatively
large ovary (Ov) which in some cases at least consists of a
vitellariuni portion and an ovary proper, the whole being
surrounded by a thin membrane a backward prolongation of
which forms an oviduct opening into the cloaca.

The preceding description of the structure of a Rotifer is
that of such a form as is most frequently met with. It was
for a long time believed that these were hermaphrodite, but
no trace of a testis could be found. It was later found, how-
ever, that they were all females, and the males of several
species have been discovered, differing decidedly in size and
structure from the females, and besides being usually rather
rare in their occurrence. They are considerably smaller than
the female, and possess like it eyes, nerve-ganglion, muscles,
and excretory system ; but the ciliated band of the trochal
disk is single, and the digestive tract, with the exception of the
cloaca, is reduced to a solid band of tissue. The single testis
occupies the greater portion of the body-cavity, and the short
vas deferens opens into the cloaca, passing through an avertible
intrornitteut organ. This marked difference of form of the
male and female individuals of the same species constitutes
a phenomenon known as sexual dimorphism. An explanation
of the usual numerical preponderance of the females over the
males is to be found in the fact that under favorable condi-
tions the females produce ova capable of developing parthe-
nogeuetically, and giving rise in all cases to females. A series

194

INVERTEBRATE MORPHOLOGY.

of generations reproducing by these so-called " summer ova "
may thus succeed each other without the intervention of a
male. Under certain conditions, however, certain females pro-
duce " summer ova " of a smaller size than usual, which, devel-
oping parthenogenetically, give rise to the males. In addition
to these two forms of " summer ova," some species produce a
third kind of egg, the so-called " winter ovum," which differs
from the summer ova in containing more yolk and in being
enclosed within a stout resistant shell. It seems probable
that these ova develop only after fertilization.

There are two Rotifers which deserve a special description on account of
their having served as a basis for phylogenetic speculation. One of these,

Trochosphcera (Fig. 94), is spherical
in shape ; a band of cilia runs round
the equator of the sphere, not encirc-
ling it completely, however, but leav-
ing an unciliated region on the dorsal
surface. Anteriorly this band passes
above the mouth-opening, which is
bounded below by a very small post-
oral band and opens into a pharynx
provided with a mastax (Ma), from
which the stomach, with digestive
glands, passes towards the centre of
the body and there bends at right
FIG. 94. — TrocfwspTmra wquatorialis angles to open through the intestine

11) U

(after SEMPER).

A — anus.

Cl = cloaca.
Ex — excretory tube.

M = mouth.
Ma = mastax.
mu — muscle.

N = uerve-ganglion.

n = nerve.
o = ovary.

So = sense-organ.

into a cloaca (Cl) which receives the
excretory tubules (Ex) and the oviducts
and opens to the exterior at the lower
pole of the sphere (A). The brain (N)
lies above the pharynx and sends
nerves to the two eyes situated, one
on each side, below the equatorial
band of cilia, and also to a small
sensory papilla (So), probably the
calcar, lying on the dorsal surface.

this nerve («) encircling the anterior
half of the sphere, and running in a plane at right angles to that in which
the ciliated band lies.

The other form belongs to the genus Hexarthra and differs from other
Rotifers principally in the occurrence of six hollow processes of the body,
arising from the ventral surface and arranged in pairs diminishing in size
from before backwards. Each is terminated by a bunch of stiff bristles or
setae, and all are supplied with muscles whereby they can be rapidly swept

ORDER GASTROTRICHA. 195

backwards in the manner of a paddle and so serve as locornotor organs, pro-
ducing a quick jerky movement quite different from the steady progression
caused by the cilia of the trochal disk. In another nearly-related form,
Pedalion, six processes are also present, but are arranged somewhat differ-
ently from those of Hexarthm, the largest one arising from the ventral
and another from the dorsal surface, while the other four are lateral in
position, two occurring on each side.

The Affinities of the Rotifera. — Several views have been advanced as
to the affinities of the Rotifers, especially as regards their relationships to
higher forms ; these opinions will not, however, be fully considered here,
but merely indicated, attention being directed first to the relationships in
which the Rotifers stand to organisms lower in the scale. In this connec-
tion the excretory system becomes of no little importance on account of its
resemblance to that of the Turbellaria, a resemblance which is further
emphasized by the nervous system, — consisting of the simple brain, from
which posteriorly-directed nerve-cords arise, — by the combined ovary and
vitellarium, and by the absence of a blood vascular system. Here, how-
ever, the resemblance ceases, and the presence of an anal opening to the
digestive tube marks the Rotifers as standing on a higher level than the
Turbellaria. It seems probable, however, that the similarities do indicate
the ancestry, and that the Rotifera have been derived from the Turbellariau
type.

Another possibility which has been suggested is to the effect that they
are derived from the form represented by the Trochophore larva of the
Annelida (see p. 213). The principal argument for this view is found in the
arrangement of the trochal cilia, which, in the occurrence in many cases of
both praeoral and postoral bauds, certainly resembles not a little that of
the Trochophore larva. It must be remembered, however, that the similar-
ity in the arrangement of the cilia is not quite perfect, and that it may be
without phylogenetic significance, having been acquired independently in
the Rotifers and in the Trochophore larva ; and furthermore it is noticeable
that in one important character at least a marked difference is found, the
nervous ganglion lying in the Rotifers behind instead of before the praeoral
band of cilia. The most that can be said at present is that the RotifVis
show closer structural affinities to the Turbellaria than to any other group,
and that it is probable that they represent the culmination of a line <>!'
development originating in that group, and furthermore that it is possible
that they represent the ancestral annelid form indicated by the Trocho-
phore larva.

Order Gastrotricha.

The Gastrotricha are minute forms, few exceeding 0.2 mm.
in length, which occur in fresh water and have an elongated
form flattened somewhat on the ventral surface, tapering

196

INVERTEBRATE MORPHOLOGY.

nephr

posteriorly to eiid usually in one or two cereal processes,
and anteriorly show a dilatation succeeded by a more or
less well-marked narrow region, the two giving rise to a
bead and neck. The body is covered upon the outside by a-
cuticle, which may be smooth as in the genus Ichthydium, or
take the form of overlapping scales as in Chcetonotus (Fig. 95),
sometimes bearing spine-like prolongations. Along the ven-
tral surface two bands of cilia run from
the posterior part of the head region
almost to the hind end of the body,

*/ •*

and in addition to these patches of
cilia are found upon the ventral surface
and on the sides of the head, some of
which are undoubtedly sensory in func-
tion. Beneath the cuticular covering
lies the ectoderm in the form of a layer
of protoplasm in which no cell outlines
can be perceived, but which contains
numerous scattered nuclei. A pair
of longitudinal muscle-bands lie be-
neath the ectoderm on the dorsal sur-
face, and other bands traverse the
coalom in an autero-posterior direction.
Transverse and circular muscles are,
however, absent. A distinct coeloin is
present, the greater portion of which
FIG. 95.-Ch«tonotu8 maxi- ig occupieci} however, by the internal

organs ; it is not lined with a peri-
toneal epithelium, nor are any mesen-
teries present.

The mouth is situated at the an-
terior extremity of the body and opens
into a muscular oesophagus (a?), which
opens in turn into the cylindrical
stomach (t). To this succeeds a short intestine opening to
the exterior at the posterior extremity of the body.

No blood vascular system is present, but the excretory sys-
tem consists of a single pair of much-convoluted tubes (nephr)
which terminate at one end in a closed ciliated " funnel,"

mus (after ZKLINKA).
eg = nerve-ganglion.
dr = glands.

i = intestine.
in = longitudinal

muscles.

neplir •=. nepliridia.
oe — oesophagus.
ov = ovary.

ORDER OASTROTRICHA. 197

while at the other they open 011 the ventral side of the body
to the exterior. The reproductive system (pv) consists of two
groups of germ-cells lying in the posterior part of the body,
one on each side of the digestive tract, but no oviduct has been
definitely made out to exist. With regard to the testes some
uncertainty exists, an oval body lying in the same region of
the body as the ovaries, but beneath the intestine, having been
described as such an organ, though the identification is open
to question. If, however, the body in question be the testes,
the animals are hermaphrodite. As in the case of the female
organ no ducts have been observed leading from the testes,
and nothing is known as to the method by which the sexual
products are extruded.

The nervous system (ri) consists of a large ganglionic mass
which lies above the oasophagus in the head region, and from
the posterior border of which two processes, one on each side
of the middle line, are directed posteriorly and dorsally,
perhaps representing the origin of a pair of nerves, while the
postero-exterual angles of the gangliouic mass are continued
backwards to near the posterior extremity of the body to
form the lateral nerves. Certain of the elongated cilia found
on the head no doubt function as sense-organs, coming into
intimate connection at their bases with the cells of the supra-
03sophageal ganglion ; in addition to these sense-organs eyes
have also been described as occurring in some species, either
in the form of simple patches of pigment lying in the integu-
ment above the brain, or else of such patches provided with
lens-like structures.

The affinities of the Gastrotricha seem almost certainly to be with the
Kotifera, many of the structural features being exceedingly similar in the
two groups. The principal differences are to be found in the arrangement
of the cilia and in the structure of the nephridia. With regard to the
former it seems not improbable that in the arrangement seen in the Gastro-
tricha a relic of a more primitive uniform ciliation is presented, and that
in this particular as well as in the greater simplicity of the digestive tract,
and in the general form of the body and life-habits, the Gastrotricha
approach more nearly an ancestral Turbellarian form than do the Rotifera.
The nephridia depart much more widely, however, from the Turbellarian
condition than do those of the Rotifera — a fact which argues against the
more primitive character of the ffastrotricha, as does likewise the absence
of ducts for the reproductive organs. Whether, therefore, the Gastrotricha

198

INVERTEBRATE MORPHOLOGY.

are to be considered as representing the ancestral form from which, both
they and the Rotifera have descended more nearly than the latter group,
or whether they are modifications of the Rotifer type of structure and have
had for their ancestors forms which were Rotifer-like in structure, it is
difficult to say ; though the balance of evidence seems to tip in favor of
the former view.

Attention should be called, however, to a possible affiliation of the Gas-
trotricha with the Echinodera. If, as has been suggested (p. 186), the seg-
mentation of the latter has no phylogenetic significance, it is not difficult
to trace similarities of structure in the two groups, the principal differ-
ences being connected with external parts. It is by no means improbable
that the Gastrotricha, Rotifera, and Echinodera form a series, each of the
groups being of equivalent rank, and related to each other somewhat as
are the Turbellaria, Trematoda, and Cestoda.

Genus Dinophilus.

The genus Dinophilus includes some small marine organ-
isms all of which are referable to a small number of species.
The body (Fig. 96) is cylindrical and consists of a head segment

followed by from 5-7 trunk segments (the
number varying according to the species),
each of which bears a ring of cilia, inter-
rupted ventrally by a uniform ciliation which
covers the entire ventral surface. The
head is likewise provided with a ring of
cilia which is usually double, one of the
constituent bands pa.ssing in front of the
mouth and the other behind it, the area in-
tervening between these two bauds being,
in one species at least, occupied by smaller
cilia. The musculature of the body-wall
is but weakly developed, though both an
external layer of circular fibres and an in-
ternal one of longitudinal fibres may be
foundj botb l s bei absent in one

. -mi

species in the dorsal region. The ccelom

is traversed by a network of branching cells,
there being no special peritoneal layer, and
no musculature in the walls of the intestine.
The mouth is situated on the ventral surface at the junc-
tion of the head and first trunk segments, and leads into a

gyrociliatus (after
MAYER\

ne = nepbridium.
ovary.

GENUS D1NOPHILUS. 199

wide ciliated oesophagus, beneath which lies a muscular pro-
boscis contained in a special sheath and protrusible through
the mouth-opening. Behind the oesophagus is a proventriculus,
a small thick-walled ciliated cavity, into which, at its junction
with the oesophagus, a pair of salivary glands (sg) pour their
secretion. Behind, the proventriculus communicates with a
cylindrical stomach, upon which follows the short straight in-
testine, terminating in the anus at the posterior end of the
body.

There is no blood vascular system. An excretory system
is present, consisting in I), gyrociliatus and D. tceniatus of five
pairs of nephridia (ne) which open externally on the sides of
the body and terminate in the ccelom-spaces in a funnel con-
taining a flame-like bunch of cilia. Whether a direct commu-
nication between the lumen of the nephridial tubes and the
coelom exists in all cases has not been definitely ascertained,
but a similarity of structure to the Platyhelmiuth type of
nephridium is shown by the flame-like bunch of cilia and by
each nephridium being composed of a series of perforated cells.
The reproductive organs are separated in different individu-
als ; and in one species, D. gyrociliatus, a marked sexual dimor-
phism similar to that occurring in the Rotifera exists, the
male being much smaller than the female and possessing only
the ciliated ring of the head and the ventral ciliation ; and
furthermore the digestive tract and the principal sense-
organs are entirely wanting. The reproductive elements (ov)
are shed into the ccelom-spaces and find their way to the ex-
terior in some species at least by means of the most posterior
pair of nephridia, which in the male of D. tceniatus are trans-
formed into seminal vesicles and are connected with an intro-
mittent organ situated in the posterior segment.

The nervous system consists of a brain or supracesophageal
ganglionic mass which occupies the greater portion of the
head segment and from which two nerve-cords pass back-
wards in the lateral region of the body, and in D. tceniatus
possess ganglionic enlargements equal in number to the pairs
of nephridia and the trunk segments and are connected by
transverse commissures. In other species, however, these
structures have not been observed. Eyes occur imbedded

200 INVERTEBRATE MORPHOLOGY..

in the substance of the supraoesophageal ganglion, and tactile
hairs occur at various regions of the body.

Affinities of Dinophilus. — The descriptions given of the various known
species of Dinophilus indicate a considerable variation in the structure of
certain parts, more especially of the nervous system, which in D. tceniatus
partakes of the metamerism shown by the nephridia and the bands of cilia,
while in other forms it is apparently non-metameric. This would indicate
either that the metamerism has been acquired within the limits of the
genus, or else that those forms lacking it are degraded in this respect and
have descended from metanieric ancestors. There is little justification to
be found, however, for the calling in of degradation to explain obscure re-
lationships unless there is sufficient collateral evidence to support such an
appeal ; in the present case this seems to be absent, and the marked simi-
larity of the non-metameric nervous system to that of the Turbellaria sug-
gests an origin from these forms and favors the first hypothesis as to the
origin of the metamerism. The nephridia also and the character of the
ccelom strengthen the probability of a Turbellarian ancestry.

A close relationship to the Rotifera has also been suggested and is not
debarred by the supposition of a descent from Turbellarian forms ; but it
seems doubtful if such a relationship can be other than a very distant one.
The position of the supraoesophageal ganglion relatively to the cephalic
cilia or prototroch, and the paired arrangement of the nephridia as well as
the occurrence of circular fibres in the subepidermal musculature, stand
in opposition to the view, and the most that can be said is that both
Dinophilus and the Rotifera are to be referred back to closely-similar
ancestors.

The affinities of Dinophilus and the Rotifers to the Annelida will be
discussed in connection with the latter group (p. 217).

SUBKINGDOM METAZOA.

Order Echinodera. — Body cylindrical, with 11 rings ; no cilia ; with pro-
boscis ; minute forms ; marine. Echinoderes.

Class CH^TOGNATHA. — Marine ; body divided into three segments ; with
lateral and tail fins ; mouth with chitinous jaws composed of
series of strong bristles. Sagitta, Spadella.

Class ROTIFERA. — Anterior end provided with a retractile crown of cilia ;
minute forms both aquatic and marine. Flosctilaria, Melicerta,
Lacinularia, Philodina, Brachionus, Asplanchna, Trocho-
sphcera, Pedalion, Hexarthra.

Order Gastrotricha. —Minute forms both marine and aquatic ; ventral sur-
face of body ciliated ; no anterior crown of cilia. Ichthydium,
Chwtonotus.

Genus Dinophilus. — Small marine forms ; body with 5-7 segments, each
with a ring of cilia.

GENUS DINOPHILUS. 201

/

LITERATURE.

ECHINODERA.

W. Reinhard. Kinorhyncha (Echinoderes), ihr anatomiscJier Bau und ihre
Stellung im System. Zeitschr. filr wissensch. Zoologie, XLV, 1887.

Ctt/ETOGNATHA.

0. Hertwig. Die Ghcetognathen. Mne Monographic. Jenaische Zeitschr. , xiv,

1880.

ROTIFERA.

C. T. Hudson and P. H. Gosse. TJie Rotifera or Wheel-animalcules. London,

1889.
L. Plate. Beitrage zur Naturgeschichte der Rotatorien. Jenaische Zeitsclir.,

xix, 1885.
C. Zelinka. Studien uber Rdderthiere. Zeitschr. fur wissensch. Zoologie, XLIV

1886 ; XLVII, 1888; LIII, 1891.

GASTROTRICHA.
C. Zelinka. Die Gastrotrichen. Zeitschr. filr wissensch. Zoologie, XLIX, 1889.

DINOPHILU8.

E. Korschelt. Ueber Bau und Entwicklung des Dinophilus apatris. Zeitschr.

filr wissensch. Zoologie, xxxvn, 1882.

W. F. R. Weldon. Dinophilus gigas. Quarterly Journ. of Microscop. Science,
xxvn, 1886.

202

INVERTEBRATE MORPHOLOGY.

ce-

CHAPTEE X.

TYPE ANNELIDA

THE type Annelida includes a series of forms among
which metamerism reaches a high grade of development. In.

what may be considered a typical Annelid
(Fig. 97) a number of segments or meta-
meres succeed one another from the
head to the tail, each one resembling its
predecessor and its successor in all its
parts ; the uephridia, reproductive organs,
nerve-ganglia (?i), and appendages, when
present, are repeated in each successive
segment, and each metamere is marked
off from its fellows, externally by a groove
surrounding the body and internally by
a partition or dissepiment extending
transversely across the coelom from the
body-wall to the digestive tube. This
latter structure and the blood vascular
tubes cannot well from the nature of
things be divided metamerically, but are
continuous from one end of the body to
the other, showing, however, in the meta-
meric pouches and intermetaineric con-
strictions of the digestive tract, and in
FIG. 97.— DIAGRAM OF

GENERAL PLAN OF AN the metamerically arranged lateral vessels
ANNELID. of the blood vascular system which encir-

a = anus. cle the digestive tube, indications of the di-

ce = cerebral ganglion, vision which has affected the other organs.

ra = mouth. Two segments, however, the head

n = ventral nerve-cord. . . -in T«T

pr = prostoiniuin. (Pr) and tlie tai1' llsuallJ Present differ-

ences from the rest in their structure ;
the head or anterior metamere bears sense-organs when

TYPE ANNELIDA. 203

these are developed, is destitute of uepbridia in the adult,
and contains primarily the supracesophageal ganglion of the
nervous system (Fig. 97, ce), the ganglia of the trunk meta-
meres (ri) lying ventrally to the digestive tube ; while the tail
segment bears the anal opening and usually presents other
characteristics which distinguish it from the preceding seg-
ments. It is rare, however, leaving aside this antero-pos-
terior differentiation, that a perfect metarneric condition is
found in any Annelid. Secondary changes may interfere
with the similarity of all the metameres ; a suppression of
parts usually present in some of the segments may occur, as,
for instance, where the reproductive organs are coutiued to
one or two metameres, or again there may occur a differentia-
tion of the anterior appendages for a special function where-
by a marked dissimilarity between the anterior and posterior
metameres is produced. Finally, owing to peculiar habits of
life, the metamerism may be almost or completely lost, being
indicated only, perhaps, by one set of organs, such as the
nerve-ganglia, or else only evident in the larval stages. Para-
sitism or a fixed or tubicolous habit of life are among the
principal causes of this degeneration, examples of which will
be seen later.

•

In consequence of this degeneration some Annelids pre-
sent a metamerism of a lower grade than that found in such
forms as the Nemerteans. Other peculiarities of structure
occur, however, which serve, together with the indications of
metamerism, to mark out the Annelid type. One of these
peculiarities is the occurrence in nearly all forms of a series
of nerve-ganglia along the ventral nerve-cords ; this feature
is of course a part of the metamerism, but it is not usually
marked in the metamerism of the nervous system seen in
lower forms. In these scattered ganglion-cells occur all along
the nerve-cords, which extend backwards from the brain,
while in the Annelids these scattered cells are associated
together to form metameric ganglia. Another peculiarity is
found in the structure of the nephridia. These are no longer
in all cases rows of perforated cells closed at the inner end
by a flame-cell, but may consist of more or less convoluted
tubes lined by ciliated epithelium and open as a rule into

204 INVERTEBRATE MORPHOLOGY.

the coeloni by a wide funnel-like extremity. Provisional
kidneys of the Turbellarian type occur in the larvae of many
Annelids, but the nephridia of the adult are, as a rule, of the
character just indicated and depart widely from the Turbel-
larian character. In the third place the reproductive organs
are developed in the peritoneal lining of the coelom and are
not usually (except in the Hirudiuea) provided with special
ducts. When mature the ova or spermatozoa are simply
shed into the ccelornic cavity and make their way to the
exterior through the ordinary nephridia, or through nephridia
specially modified for the purpose. Finally it may be mem-
tioned that a blood vascular system is usually present.

I. CLASS Ghtetopoda.

The Chaetopoda are Annelids in which the external seg-
mentation of the body corresponds with the internal seg-
mentation of the organs, and which bear along the sides of the
body two rows of pouches, the seta-sacs, the cells lining which
secrete chitinous spicules or setce of various shapes, which
serve for the purpose of locomotion or in some cases consti-
tute a defensive armament.

The class is conveniently divisible into two subclasses.

Subclass I. POLYCH2ETA.

The forms included in this subclass are exclusively ma-
rine, and are characterized by the presence on the sides of a
greater or less number of the metameres of a pair of hollow
processes of the body-wall upon which the seta-sacs occur
and which are known as parapodia. In a few forms (Serpida)
the parapodia, and indeed the setae as well (Polygordius), may
be absent, and in others, such as Clymenella, they may be very
much reduced in size, but as a rule they possess a high de-
gree of development. In its typical form a parapodium
(Fig. 98) consists of a dorsal and a ventral lobe each of which
bears seta-sacs and setae (s). Towards the base of each lobe
there may frequently be found a slender hollow process, the
dorsal and ventral cirrus (dc and vc), and plate-like or more
or less dendritic appendages, the branchice (br), either modifi-

TYPE ANNELIDA.

205

\ c

FIG.

98.— PARAPODIUM OP
Aferets virens.

br = bi-Niiclim.
dc = (i()rsai ciri.us.

s = setae.
vc : : ventral cirrus.

cations of the cirri or brandies arising from them, uml

respiratory in function, also occur. Muscles pass from the

body-wall to the parapoclia, which thus

become important organs of locomo-

tion and in some of the actively swim-

ming species assume a more or less

flattened plate-like form.

The head segment is generally well
differentiated from those which succeed
it, being destitute of parapodia and
setfle, and as a rule carrying a number
of appendages sensory in function, and

being likewise usually provided with

mi TV i

eyes. The cephalic appendages may

•* x , . '

be short and rather stout, forming

'

what are termed palpi (Fig. 100, p),
or somewhat longer and more slender,
forming the cirri (c), or even still more
slender, being then known as tentacles (£).

The body is enclosed in a chitinous covering secreted by
the subjacent ectoderm, here known as the hypodermis (Fig.
99, liy}. The musculature of the body-wall which lies below
the hypodermis is separated from this by a basement-mem-
brane and consists of an external layer of circular fibres (cm)
and a subjacent layer of longitudinal fibres (Im) which is, as
a rule, interrupted in the mid-dorsal and ventral lines and
also in the region of the two lobes of the parapodia so as to
form four bundles. Special muscles extend from the body-
wall to the base of the seta-sacs, and furthermore a pair of
muscle-bauds cross the cavity of each metamere, in typical
cases passing from the lateral regions of the dorsal surface
downwards and inwards to be inserted into the ventral bod}--
wall on each side of the median line. The inner surface of
the longitudinal muscle-layers is lined with a layer of peri-
toneal cells which completely enclose the coelom (cd) of each
metamere, being reflected upton the surfaces of the dissepi-
ments which form the internal partitions between adjacent
metameres. The separation of the coalomic cavities of the
rnetameres is, however, rarely perfect, openings occurring here

206 INVERTEBRATE MORPHOLOGY.

and there in the dissepiments, and in some forms, such as Capi-
tella, a number of the dissepiments may at the breeding season
completely degenerate so that the cavities of the various
metameres concerned become perfectly continuous. The
ccelom of each metamere consists in reality of two sacs which
are folded around the digestive tract, which they enclose,
and come into contact with each other above and below the
intestine, forming the dorsal and ventral mesenteries (Fig. 99,
dm and urn). That wall of each sac which lines the muscula-
ture of the body-wall is termed the somatic layer of the peri-
toneum, while that surrounding the digestive tract is the
splanchnic layer.

The blood vascular system consists of a dorsal vessel
(Fig. 99, db) which runs along the mid-dorsal line of the diges-
tive tract and which is frequently contractile in portions of
its course, serving as a heart, and a ventral vessel (ub) lying
below the digestive tract, and being connected with the dor-
sal vessel by lateral trunks, arranged metamerically. From
these vessels branches are distributed to the various regions
of the body. The blood is frequently colored, usually red,
and contains colorless corpuscles, the coloring-matter being
dissolved in the plasma in which the corpuscles float. The
blood vascular system is completely closed throughout its
entire course, never opening into sinuses without definite
walls. In addition to the blood which circulates within this
definite system of tubes the ccelom also contains a corpuscu-
lated fluid, frequently colored and approaching blood very
closely in its characters. This hcemolympTi contains corpus-
cles, usually amoaboid in form, and may circulate through the
body from one metamere to another through openings in the
dissepiments. In a few forms, such as Capitella, it may fulfil
the functions of the blood, a true blood vascular system being
wanting, and in this case contains, in addition to the colorless
amoeboid corpuscles, others which are disk-shaped and pig-
meuted. It seems probable, however, that the absence of a
true blood vascular system is a purely secondary phenome-
non, and accordingly does not indicate a primitive condition.

The mouth is situated on the ventral surface of the body,

«/ *

at the junction of the head metamere with the first trunk

TYPE ANNELIDA.

207

metamere, and leads in many forms into a strongly muscular,
usually protrusible pharynx provided with chitiuous teeth.
Upon the pharynx follows the usually straight intestine which
opens to the exterior at the posterior extremity of the body.
In Capitella and the allied genera, as well as in certain mem-
bers of the family Eunicidse, an accessory intestine lies ven-
trally to the principal one, into which it opens either both
anteriorly and posteriorly or else anteriorly alone. This ac-

Uy dm-db

un urn ub c

FIG. 99. — DIAGRAM OF TRANSVERSE SECTION OF ANNELID (combination of

figures by LANG and EHLERS).

br = brancliia.

c = cirrus.
cm = circular muscles.

co = cosloni.

db = dorsal blood-vessel.
dm = dorsal mesentery.
hy = hypodermis.

i = intestine.

Im = longitudinal muscles.
ne = nephridium.
ov = ovary.
p = parapodium.
ub = ventral blood-vessel.
um = ventral mesentery.

un — ventral nerve-cord.

cessory intestine is ciliated and seems never to contain food-
matter ; it has been considered to be respiratory in function
and seems to be a special development of a ciliated groove
which runs along the ventral surface of the intestine in cer-
tain other forms. Ponch-like outgrowths of the intestine are
frequently present and may sometimes become essentially
glandular in function. In Hesione and in certain species of
Syllis pouches communicating with the anterior part of the

208 INVERTEBRATE MORPHOLOGY.

digestive tract occur, which normally are filled with air and
are richly supplied with blood-vessels ; they may be respira-
tory in function, and have been compared to the swim-bladder
of fishes.

The nervous system is well developed in all Polychseta,
and consists of a supravesopliageal ganglionic mass situated in
the head segment, frequently presenting a division into sev-
eral lobes. From it various nerves arise passing to the an-
terior segment, and in addition a strong cord passes from it
ventrally on either side of the oesophagus to unite with a
ganglion lying below the oesophagus in the second metamere,
forming the circumcesophageal commissure. To the subcesopha-
geal ganglion of the second metamere there succeeds a
pair of ganglia in each metamere, each pair being united
with the preceding and succeeding pairs by two longitudinal
cords of nerve-fibres, the connectives, the whole constituting
the ventral nerve-chain, and furthermore the ganglia of each
pair are united by a transverse commissure. The ventral
nerve-chain has therefore a distinctly ladder-like arrange-
ment, frequently somewhat obscured, however, by the approxi-
mation of the ganglia of each pair and a consequent shorten-
ing of the transver.se commissures. From the various ganglia
nerves arise which pass to the musculature of the metarneres
and to the hypodermis and its sense-organs. In the major-
ity of forms the nervous system lies freely in the ccelom
surrounded by a special sheath, but occasionally in various
forms widely separated genetically from one another, such as
Polygordius and the Opheliaceee, it presents a primitive char-
acter in being completely -imbedded in the hypodermis,
recalling the condition in certain Nemerteans and in the
Cnidaria. Special nerves arising from the supracesophageal
ganglion are supplied to the walls of the digestive tract, form-
ing the so-called stomatogastric nerves.

Sense-organs of various kinds are of frequent occurrence
at different portions of the bod}- of the Polychaeta. In addi-
tion to the cephalic and caudal cirri which are richly supplied
with nerves and are presumably tactile in function, eyes are
of very general occurrence. They are usually situated on the
head, sometimes in connection with the hypodermis and

TYPE ANNELIDA. 209

sometimes imbedded in the dorsal surface of the braiu. For
the most part they consist of a cup of pigment-cells, in which
numerous sensory cells are present — a lens being in some
instances developed above each eye. Occasionally, however,
as in the pelagic genus Alciope, the eyes reach a high grade
of development. In some forms they are not confined to the
region of the head, as for instance in the genus Polyophthalmus

—so named from the fact that pairs of eyes are found on the
sides of a number of the trunk metameres ; in the majority
of tubicolous Annelids eyes are found in considerable num-
bers upon the branchial lobes of the head segment, the genus

Vermilia possessing somewhere in the neighborhood of 11,000
separate ocelli in this region. These eyes are simply differ-
entiations of the ectoderm, and in many cases are still situated
in the hypodermis ; they consist of a number of cells which
are prolonged at their inner ends into a nerve filament, while
peripherally their protoplasm is converted into a refractive
substance, each of these cells being separated from its neigh-
bors by pigment deposited in its peripheral layers, as well as
by a number of smaller pigment-cells. On account of this
pigment-sheath it is presumable that each of these optic
elements or ommatidia functions more or less independently
of the rest, and the eyes are to be considered as compound,
composed of a number of independent parts each of which is
physiologically an eye.

Auditory organs or otocysts also occur in certain forms,
but cannot be considered as typical of the Polychseta. In
Arenicola they consist of two sacs lying in close proximity to
the circumoesophageal commissures and connected with the
exterior by a narrow canal, indicative of their origin as invag-
inations of the hypodermis. The walls of the sack are formed
by columnar cells terminating below in a plexus of nerve-
fibrils and covered on the surface turned towards the cavity
of the otocyst with a firm homogeneous cuticle, and not pos-
sessing any terminal hairs. In the cavity a varying number
of spherical particles of carbonate of lime, the otoliths, are
found. In some forms a number of such otocysts are present,
as in Aricia, where four or five pairs have been found in adult

210 INVERTEBRATE MORPHOLOGY.

individuals; but in the majority of species they do not seeni
to be developed.

Ciliated depressions which have been supposed to be
olfactory have been described as occurring in the anterior
region of the body in various species, reaching a high devel-
opment in the Capitellidse, where they form club-shaped sacks
capable of being evaginated. In addition there are to be
found scattered on the surface of the body minute beaker-shaped
depressions, at the bottom of which are cells bearing long
hairs and presumably sensory in function ; and furthermore
in a few forms, such as the Capitellidae and Polyophthalmus, a
series of sensory hillocks occur along the sides of the body—
a pair in each metamere, forming the sense-organs of the lateral
line. In the Capitellidse these organs are in the anterior inet-
ameres contained in depressions, but more posteriorly they
project slightly from the surface. The central part of each
projection is retractile and is formed of a number of hair-cells,
each of which is in connection at its inner end with a nerve-
fibril. No little interest attaches to these organs, which
forcibly recall, both in their structure and distribution, the
lateral line organs of the lower Vertebrates.

The nephridia (Fig. 99, ne), in typical adult forms, occur
as a single pair in each metamere except the two terminal
ones. Each consists of a usually contorted or coiled tube
lined with cells opening by a funnel-shaped mouth into the
ccelom of the metamere, perforating the dissepiment between
it and the next metamere in which the greater portion of it
lies and in which it opens to the exterior by a small pore sit-
uated on the ventral surface of the body at the base of the
parapodium. It is rare, however, that any such metarneric
regularity of arrangement occurs, and very frequently they
become reduced to a small number, or even to two pairs ; in
the tubicolous forms a few pairs are frequently found in the
anterior portion of the body much larger than any of the rest.
In addition to their original excretory function they may also
serve as outlets for the reproductive elements, and in some
cases become specially modified for this purpose and lose
their original function.

TYPE ANNELIDA. 211

In Capitella only one pair, that of the eighth metamere, becomes con-
verted into a genital duct; and it is interesting to note that in this same
segment a true excretory nephridium is also present. Whether this indi-
cates or not the occurrence originally of more than one pair of nephridia in
each metaniere remains to be seen, but it is interesting in connection with
what occurs in the Oligochseta (see p. 223).

The reproductive organs consist of local thickenings of the
peritoneal epithelium (Fig. 99, ov) in more or fewer of the
segments. The ova or spermatozoa fall when ripe into the
ccelomic cavity and pass to the exterior by the nephridia.
With very few exceptions the Polychaeta are bisexual.

The classification into smaller groups is to a certain ex-
tent artificial at- present, and does not profess to have any
phylogenetic significance. Three orders may be recognized.

1. Order Archiannelida.

This order includes a few forms which are supposed to
present more primitive structural characteristics than the
remaining Polychaets. They show as a rule but indistinct
traces of an external segmentation, and are entirely devoid of
either parapodia or setae. Tentacles occur at the anterior
extremity of the head metamere ; but no other appendages,
such as cirri or branchiae, occur. The nervous system is im-
bedded in the hypodermis, and the nephridia are short tubes,
a single pair occurring in nearly every segment. To this
group belong the genera Polygordius and Protodrilus.

2. Order Errantia.

In this order are placed the free-swimming or creep-
ing Polychaeta, in which a considerable similarity of the
various trunk rnetameres occurs. The parapodia are as a
rule well developed, and occasionally are broad and plate-like
in adaptation to a free-swimming existence. Branchiae are
usually found on the dorsal lobes of a considerable number
of parapodia ; the head is distinctly marked off from the
trunk and may bear eyes ; while the anterior portion of
the digestive tract is converted into a protrusible pharynx,
usually armed with chitinous teeth. To this order belong
the genera Nereis (Fig. 100), usually found lurking beneath

212

INVERTEBRATE MORPHOLOGY.

stoiies during the day-time, but becoming, in some species at
least, free-swimming at night ; Lepidonotus, characterized by
the possession of elytra arranged in overlapping series on the
dorsal surface ; Diopatra, which forms tubes for itself by glu-
ing together particles of foreign matter ; and Autolytus and
Syllis, which are peculiarly pelagic in habit, as is also Alciope,
characterized by its large highly-organized eyes.

3. Order Sedentaria.

This order includes a number of forms which manufacture
for themselves tubes of various substances — some being
merely composed of particles of sand glued together by an
adhesive secretion, while others consist of a chitiuous sub-
stance, to which foreign bodies may be added, or even of car-

FIG. 100. — ANTERIOR
END OP Nereis virens.
c — cirrus.
p = parapodium.
pi = palp.
t = tentacle.

FIG. 101. — Amphitrite ornata (after

VERRILL).

bonate of lime. Within these tubes the animals permanently
reside, and in conformity with this mode of life numerous
adaptations of structure are found. The head is usually pro-
vided with a number of long cirri and the branchiae are for the
most part confined to the head region. In some forms, such

TYPE ANNELIDA. 213

as Serpula and Sabella, pluine-like branchiae supported by ail
axial cartilage-like skeleton occur upon the sides of the head,
and numerous eyes may be found in the hypoderinis of these
structures. Parapodia are as a rule but slightly developed,
sometimes being entirely wanting though the setae persist,
those of the lower parapodial lobe being usually hook-like.
The protrusible pharynx with chitinous teeth does not for the
most part occur. Amphitrite (Fig. 101) lives in tubes in sand,
while Terebetta composes tubes by gluing together particles
of sand. In /Sabella the tubes are membranous in character,
while Serpula manufactures more or less contorted tubes of
carbonate of lime.

Development of the Polychceta. - - An important feature in
the development of the Polychseta is the occurrence of the
Trochophore larva. A typical example of this larva is to be
found in the development of Polygordius ; it is a transparent
organism, having the form of two low cones united by their
bases (Fig. 102). Just below the junction of the two cones is
the mouth (M], leading by a short stomodceum or oesophagus
into a retort-shaped stomach, the intestine opening at the
apex of the lower cone. Above the mouth, along the line
where the two cones are united, lies a band of strong cilia
arranged in two rows and forming an almost complete girdle
for the body, being wanting, however, in the mid-dorsal region.
This is the prototroch (pro) or praeoral band of cilia, and par-
allel to it is a second weaker band which passes behind the
mouth — the paratroch (po) or postoral band. The slight
groove between the two bands is lined by fine cilia, the adoral
cilia, and in some Trochophores a band of fine cilia extends
backwards along the ventral surface of the body towards the
apex of the lower cone.

At the apex of the upper cone is a strong thickening of
the ectoderm, the apical plate (ap), which is nervous in function
and bears a number of strong cilia and may also have imbedded
in it pigment-spots which function as light-percipient organs.
From the apical thickening four nerve-cords (n) extend back-
wards, one being dorsal, the other ventral, and the remaining
two, stronger than the others, lateral. A series of fine nerve-
rings arranged concentrically about the apical thickening unite

214

INVERTEBRA TE MORPHOLOG Y.

these cords at regular intervals, the lower ring being con-
nected with the cells which bear the protrochal cilia and
forming the prototroch nerve.

At the apex of the lower cone and ventral to the intestine
lie two cells, or two masses of small cells, which constitute the
mesoblasts and give rise to two longitudinal mesoblast bands
(mb). A few scattered cells are also found between the ecto-

m

M

mb

FIG. 102. — TKOCHOPHORE OF POLYGORDIUS (after HATSCHKK).

A ~ anus. mb = mesoblast-band.
ap = apical plate. ne = uephridium.

M == mouth. pro — prseoral baud of cilia.
m, m', m" - muscles. po = postoral band of cilia.

derm and the digestive tract, some of which elongate and be-
come muscle-fibres (m}, and which have been thrown off from
the mesoblast bands. In some forms a band of muscle-fibres
underlies the prototroch cells. In the neighborhood also of
the mesoblast bauds in the posterior cone there occurs on
either side of the digestive tract a small, sometimes branched,
tubular body, the head-kidney (ne). Each kidney consists of
a row of perforated cells, terminating in a funnel-shaped
structure closed at its mouth by a cell, the whole structure
thus agreeing closely with the uephridia of the Platyhelmiuths.
From such a larva the adult condition is derived by the
gradual elongation of the posterior part of the body — an
elongation with which the mesoblast bands keep pace, the
mesoblasts retaining their position at the posterior extremity,

TYPE ANNELIDA. 215

and continually adding to the bauds by the formation of new
cells. The bands as growth proceeds break up into a number
of masses, the mesoblastic somites, in the interior of which
cavities appear, and adjacent pairs of masses growing dorsally
and veutrally finally come into contact above and below the
digestive tract, the dorsal and ventral mesenteries of the
intestine being thus formed, and later a metamerization of the
body-wall, corresponding with that of the mesoderm, also takes
place. The anterior cone of the larva, which at first sur-
passed in size the posterior one, gradually becomes smaller,
and the prototrochal cilia, and in some cases the cells also, are
thrown off. The apical plate takes part in the formation of
the supraoesophageal ganglion of the adult, and the lateral
nerve-cords arising from it form the circumoesophageal com-
missure, becoming connected with thickenings of the ventral
hypodermis arranged nietamerically and representing the ven-
tral chain of ganglia. The head-kidneys gradually disappear,
being merely provisional larval structures, and new nephridia
of the Annelid type develop from the mesoderm of the truiik-
metameres.

Although the Trochophore larva occurs in the life-history of many of
the Annelids, as well as in other groups as will be seen later, yet never-
theless it is not invariably present. In some forms a single band of large
cilia runs around the middle of the body, which is elsewhere uniformly
ciliated, while in others the cylindrical larva is surrounded by several
bands of cilia succeeding one another at definite intervals. In certain
species the larva is provided with very long setae which are thrown off
during larval life, and are interesting on account of similar setae having
been found in fossil forms, though absent in recent adult species.

It is also worthy of note that in some forms the Trochophore larva is
succeeded by a well-marked stage in which, in addition to the head seg-
ment, three trunk segments are developed. It is possible that this may
represent an ancestral form from which certain other groups have taken
their origin.

In what may be considered exceptional cases a non-sexual repro-
duction by budding also occurs. In the genus Protnla a zone of growth
occurs in the sixteenth segment, and at this point later separation takes
place, a new head developing for the posterior individual from the original
seventeenth segment. In one species of Syllis the new individuals
arise not only in a linear series but also as lateral buds, so that a
branching colony is produced ramifying through the canal system of the
Hyalosponge in which the form lives. The buds eventually separate as

216 INVERTEBRATE MORPHOLOGY.

sexually mature male and female individuals which, since they differ from
the parent form in possessing more highly developed eyes as well as a
more perfect adaptation of the parapodia for swimming, probably leave the
sponge and swim about freely in the ocean distributing their sexual elements.
If this be the case, this species presents both colony formation and alterna-
tion of generations, the latter phenomenon being also manifested by other
species of Syllis and by Autolytus, in which buds are produced linearly,
differing from the parent in the structure of the parapodia and separating
to lead a free existence as male and female individuals. A modification of
this process is found in certain species of Nereis, in which at the time of
sexual maturity the posterior segments of the body develop setae more
perfectly adapted for active locomotion by swimming than were those of
the immature form ; these sexually mature forms were at one time
referred to a separate genus, Heteronereis.

The Phylogeny of tlte Polychceta. — The origin of the Polychseta has been
within recent years the subject of considerable discussion. The discovery
of the wide distribution of the Trochophore larva led to the supposition
that it was an ancestral form, from which the Polychseta had been de-
veloped by a process of linear budding, each metamere of the Polychaet
body being equivalent to the original Trochophore and the adult organism
being a co-ordinated succession of Trochophore individuals. Other authors,
however, who do not see iu metamerism the result of a budding of the
individual, but rather the multiplication of its subordinate parts, are in-
clined to refer the Annelids to a Nemertean-like ancestor and to consider
the Trochophore larva a purely secondary adaptation. Between these two
views it is difficult to decide, and it is possible that in their plain statement
neither is quite correct, though each may contain certain elements of
truth.

It seems exceedingly probable that a larval form which is met with in
the life-history of the Annelids and Mollusca, as well as in a modified form
in other groups, has some ancestral significance. It is difficult on any
other hypothesis to explain its occurrence in widely different groups, since
it seems hardly probable that it could have arisen independently in several
instances. Convergent evolution could hardly be carried to such an ex-
tent as to produce in the Mollusca, quite independently of any genetic
relationships, a larva resembling in all its structure that of an Annelid.
If the Trochophore occurred only in the Annelids, it might be quite possible
that it had made its appearance in the life-history of some primitive
Annelid as a secondary modification of a more primitive larva, and had
reappeared subsequently in the life-history of all forms descended from this
Annelid ancestor, but this would not explain its occurrence also in the
Mollusca, unless it be supposed that the members of this group have been
derived from the primitive segmented Annelid, a view that has little to
recommend it, The working of the biogenetic law (see p. 143) is interfered
with in innumerable instances, and the distinguishing between examples of
its action and secondary modifications is the most difficult task of the

TYPE ANNELIDA. 217

embryologist. The evidence at present available seems, however, to point,
in the case of the Trochophore, to its being an example of the law, and to
this extent the first of the two views stated above is probably correct.

But, on the other hand, this may not be the case with the second part
of the theory. If the view as to the origin of metamerism which is advo-
cated in this work be correct, then the Annelid caiiiioi be regarded as
having arisen directly by a process of reproduction by budding of the
Trochophore. It is not a colony of Trochophore individuals, but a single
elongated Trochophore whose organs have undergone repetition, producing
a high grade of metamerism. To this extent the second of the views may
be correct, but this does not necessarily imply that the Annelids are to be
derived from a Nemertean-like form in which the metamerism is not quite
so perfect. Metamerism, as here explained, is simply the following out
into the higher individualities of the phenomenon of discontinuous growth
or reproduction by division which characterizes the cell, and it is quite
possible that there may be no more genetic connection between the meta-
merism of the Nemertean and that of the Annelid than there is between
that of the Cestode and that of the Nemertean. It may have arisen quite
independently in the two forms, and in fact when the details of
metamerism are examined in the two groups considerable differences are
to be seen.

The view here advocated in regard to the origin of the Polychseta may
be briefly expressed as follows : The Polychasta — and with them the
Annelida in general — have had for their ancestor a non-metameric form
of which the Trochophore is the larval representative, and this in the
course of its development elongated, the elongation being accompanied by
the repetition by a budding process of certain organs, a high grade of
metamerization being thus produced.

The relationships of the Trochophore seem to be with the Turbellaria.
The nervous system, consisting of the apical thickening and lateral nerve-
cords, is very similar to that of Turbellaria, and also it is interesting to
notice the similarity of structure of the head-kidney with the Turbellarian
nephridium. The exceedingly small development of the parenchyma is
probably a secondary condition, and the presence of an anus is an im-
portant advance upon the Turbellaria. An undoubted similarity in many
respects exists between the Rotifera and the Trochophore, and the former
have been regarded as persistent Trochophores or else as forms descended
from the Trochophore. This latter view in one of its phases has already
been considered (p. 195), and an important difference in the relation of the
supraoesophageal ganglion to the prototroch mentioned. Reasons have also
been given for the belief that the Rotifera are descended from Turbellarian
ancestors, and it seems probable that the line of descent of the Rotifers
was identical for a time with that followed by the Trochophore, the
former group branching off from it shortly before the Trochophore ancestor
made its appearance on the scene. In this respect the Rotifers and the

218

INVERTEBRATE MORPHOLOGY.

0(1

Trochophore are related to each other, but hardly with so close au affinity
as would be implied by a statement that they are persistent Trochophores.

II. Subclass OLIGOCH.ETA.

The OKgochaeta are with few exceptions fresh-water or ter-
restrial Chaetopods, arid present a much simpler body form

than do the Polychaets. The first
segment or prostomiiDii (Fig. 103,
pr} is devoid of tentacles or cirri,
and only in a few forms are eyes
present upon it. The body is divided
into well-marked segments, but
parapodia are lacking, though in
the majority of forms setse. arranged
in a more dorsal (s') and a more
ventral (s) group, occur on the sides
of each metamere ; in a few forms,
however, a single series of groups
only is present, while in Perichceta
the setae are arranged in a ring
around each metamere, and in Ana-
chceta they are wanting, their place
being indicated only by the sacks
in which in other forms they are

FIG. 103,-ANTERioKENDOF cleveloped and which project into

r.iiiu/it'jr u v

the coelom as large hypodermal

glands. As a rule, too, no branchiae
are present, the blood being aerated
through the walls of the body, mi-
nute branches of the blood-vessels
penetrating into the hypodermis in
vd = opening of vas defereus. the terrestrial forms ; a few aberrant

forms, however, possess either dorsal

or ventral (Clmtobrancltus) appendages, which are probably
respiratory in function, on many of the segments, while in
the genus Dero the finger-like processes of the terminal
metamere are probably branchiae.

The exterior of the body is covered by a well-marked cuti-
cle, and beneath it lies the ectoderm or hypodermis, usually

cus.

C = clitellmn.
gl = glands.

m = mouth.

od = opening of oviduct.

pr = prostomium.

s = ventral setae.

s' = lateral setae.

TYPE ANNELIDA. 219

rich in gland-cells and also containing numerous sensory
cells connected at their inner ends with slender fibrils which
pass centrally and unite with others to form nerve-cords pass-
ing to the ventral nerve-cord. In some of the segments near
the anterior portion of the body the hypodermis is usually
thicker than elsewhere and richer in gland-cells, forming a
structure known as the clitellum (Fig. 103, c). Within the hypo-
dermis is a usually thick layer of circular muscles, and, internal
to this, longitudinal muscles whose continuity as a layer is in-
terrupted at the dorsal and ventral mid-lines as well as at the
sides. Muscular dissepiments divide the ccelom into compart-
ments corresponding to the external segmentation of the
body, and the various compartments are lined by a layer of
peritoneal cells which along the dorsal and ventral lines is
reflected towards the digestive tract, which it surrounds, form-
ing a dorsal and ventral mesentery. The former of these fre-
quently disappears, as may also the ventral one. The dissepi-
ments are rarely perfect, being usually perforated so that the
various ccelomic compartments are placed in communication
with each other, and in rare cases a number of the dissepi-
ments may be wanting, as in jEolosoma, where but a single
one separating the head from the trunk ccelomic spaces oc-
curs. On the median dorsal line of more or fewer metameres
towards their posterior edge a small opening usually occurs,
the dorsal pore, and similar pores are found in the head
segment of some forms. They place the ccelomic cavitj in
communication with the exterior, but do not seem under ordi-
nary circumstances to be the means of any extensive inter-
change between the external water and the ccelomic luerno-
lymph, though occasionally this latter fluid may find exit
through them. Among the peritoneal cells are usually to be
found some which enclose greenish-brown particles and may
detach themselves from the peritoneal layer and float about
in the hsemolymph, eventually dying and disintegrating. Oc-
casionally these chloragogm cells are specially aggregated in a
furrow which runs along the dorsal surface of the intestine,
and immediately surround the dorsal blood-vessel. They
seem to be excretory in function, performing perhaps to a cer-
tain extent the part of the liver-cells of the Vertebrata.

220 INVERTEBRATE MORPHOLOGY.

A circulatory system is always present, and consists of a
dorsal longitudinal vessel lying on the dorsal surface of the
digestive tract and a ventral one lying below it, the two
being united in more or fewer of the uietanieres by one or
two lateral vessels on each side. The dorsal vessel is con-
tractile, the blood in it flowing towards the anterior extremity,
and the lateral vessels are also usually contractile. Branches
are given off to the muscles and to the various organs in reg-
ular metameric succession, and additional longitudinal vessels
are also found accompanying the ventral nerve-cord, two
lateral and one ventral. In the terrestrial forms fine branches
penetrate into the hypodermis, the aeration of the blood being
thus effected in the absence of special branchiae. The blood
in the majority of forms is red from the presence of haemo-
globin dissolved in the plasma and contains colorless cor-
puscles. As in the Polychaeta the coeloni contains a haerno-
lymph in which corpuscles float.

The digestive tract forms a straight tube, extending from
the mouth, situated on the ventral surface at the junction of
the prostoiniurn and first trunk metamere, to the terminal
anus. The mouth opens into a short mouth-cavity, and this
into a more or less muscular pharynx, which in most cases
can be protruded from the mouth and is slung to the body-
wall by numerous radiating muscular bauds. To it succeeds
a smaller oesophagus, which communicates posteriorly in ter-
restrial forms, after in some cases dilating to form a sack-like
thin-walled crop, with a muscular gizzard. The intestine
which succeeds this is usually somewhat pouched, being con-
stricted in the region of the dissepiments and bulging out
into the intervening coelomic cavities. In the terrestrial Oli-
gochffits its absorptive surface is increased by the projection
into it along the dorsal surface of a longitudinal fold, the
typhlosole, the chloragogue cells lying in the furrow produced
by the fold. Various glands open into the digestive tract at
different regions, as, for instance, salivary glands which open
into the anterior part of the oesophagus in some forms, and
calciferous glands (Morreu's glands) Avhich contain particles
of carbonate of lime and are found opening into the oesopha-
gus in terrestrial forms.

TYPE ANNELIDA. 221

The nervous system consists of a supraoesophageal gau-
glioii (Fig. 104, ce) of a somewhat complicated structure lying
in the anterior portion of the body. In jEolosoma it is con-
nected with the hypodermis (i.e., the ectoderm), but in most
forms it lies upon the anterior part of the digestive tract, quite
separate from the hypodermis. It differs in position from
the corresponding ganglion of the Polychseta in that it is not
usually situated in the anterior nietamere or prostomium, but
has passed farther back and may lie in the second, third, or
fourth metarnere or even more posteriorly. It sends off nerves
to the sensitive prostomium and gives rise to two commis-
sures which pass backwards and downwards on either side of
the pharynx to unite with the subcesophageal ganglion (so],
which, like the brain, is formed of two more or less fused lat-
eral masses, each of which in many forms shows indications of
being compound and formed by the fusion of two or more
ganglia (Lumbricus}. To this there succeeds in each nieta-
mere a pair of ganglia each of which is united to its prede-
cessor and successor by a pair of connecting cords, the whole
ventral cord (ri) so produced having a characteristic ladder-
like arrangement. Usually the connecting cords are closely
approximated, and the same may be the case with the gan-
glion pairs, the whole being ensheathed in connective tissue
so that the cord seems to be single. From each pair of gan-
glia in Lumbricus three nerves pass out on each side, the two
posterior ones being closely related so as to appear to be one.
Nerves especially connected with the digestive tract, the
stomatogastric nerves, seem to be present, but their distribu-
tion and connections have not yet been thoroughly studied.
In some aquatic forms a lateral nerve imbedded in the hypo-
dermis and united anteriorly with the supraoesophageal
ganglion runs along the lateral line between the two rows of
sefoe, recalling the lateral-line nerve of the CapitelUdce among
the Polychseta.

Sense-organs of various kinds are present. Tentacles are
absent throughout the group, and in only a few forms (Nais)
do eyes, consisting of pigment-spots imbedded in the hypo-
dermis, occur. Ciliated depressions at the side of the pro-
stomial segment occur in ^EoLosoma and a few other genera,

222 INVERTEBRATE MORPHOLOGY.

while tactile setae or papill* are scattered over the body. In
those forms which possess a lateral nerve sense-organs re-
sembling those of Capitella occur rnetamerically along it, and
in the genus Slavina are increased in number so as to form a
circle of from fifteen to twenty papillae surrounding each
metaniere and innervated by a branch from the lateral nerve.
Cup-shaped organs, supposed to be gustatory, occur especially
abundantly on the prostomial metaniere.

The excretory system has usually a typically nietameric
arrangement — a single pair of coiled tubules lying in each
inetarnere. Each tube opens by a ciliated funnel into one
coelomic compartment and then passes backwards, perforating
the dissepiment, into the next succeeding compartment in
which the coiled portion lies, and opens to the exterior in
this metaniere between the dorsal and ventral rows of setae.
The lumen of the coiled portion of the tubule is intracellular,
the tubule consisting in this region of a series of perforated
cells recalling the condition found in the Platyhelminths. In
a certain number of anterior metameres the uephridia may
be wanting in the adult condition, though in younger stages
provisional uephridia are to be found in these metameres
later disappearing.

Considerable variation is to be found in the nephridial system of the
Oligochaeta, some of the variations suggesting important theoretical con-
siderations. A head-kidney similar to that described as occurring in the
Trochophore larva persists in the adult stage in some Oligochaeta, and in
Ctenodrttus appears to be the only nephridium which exists. In other
forms, such as Chcetogaster, the entire nephridial system is composed of
tubules having a decided similarity to the head-kidney in the intracellular
character of the lumen, and in the absence of any ciliated funnel, the
inner end of the tubule being closed. Furthermore the cells enclosing the
canal are in addition perforated by numerous minute branching canals
which open into the central lumen. This fact suggests the complete
homology of the nephridial system throughout the entire body notwith-
standing the usual marked histological distinction between the head-
kidneys and the nephridia of the trunk metameres. There seems little
reason to doubt that the Oligoclueta have been derived from the Poly-
chreta, and the nephridial system in the two forms is, therefore, homol-
ogous. It must, therefore, be possible for a nephridium with an iutra-
cellular canal to be transformed into one in which the canal is intercellular.
The nephridia of Chatoijaster are unquestionably homologous with those

TYPE ANNELIDA.

of Lumbricns, for instance, which possess a terminal funnel, and are like-
wise similar in structure to and may be regarded as repetitions of the
head-kidney, thus establishing the homology between the two forms of
nephridium.

In carrying the homology of the Annelid nephridium back to that of
the Platyhelmiuth the question arises whether it is equivalent to the whole
branching system of the Turbellariau or only to a part of it. Whichever
view of metamerism be taken the various nephridia of the Annelids are to
be regarded as bud-products, and each, therefore, equivalent to a branching
Turbellarian nephridium. It has been suggested that the Annelid uephrid-
ial system has been produced by the fragmentation of an originally con-
tinuous system, but for this there is no embryological evidence. Each
uephridium being a bud from an undifferentiated nephridial blastema is
just as much an organ-individual as is the branched nephridium of a Tur-
bellarian—just as much an individual, though of a lower grade, as is the
bud of a Polyzoon developed from an undifferentiated blastema. It
might be supposed, then, that the Annelid nephridium might show a
branched structure in certain primitive forms, and indeed a branched
head-kidney occurs in Polychset Trochophores. A branched condition is,
however, rare in the trunk nephridia, though it does occur in certain ter-
restrial Oligochaeta in which, however, it must be regarded as a purely
secondary phenomenon without any phylogenetic significance, since in the
development of such nephridia a single tube is first formed which later on
becomes solid and then gives off the branches, the various nephridial
branches of successive segments becoming sometimes united. This
branched condition passes into one in which the various branches separate
and acquire independent openings ; several pairs of nephridia, four iu the
anterior segments of a species of Periclmta and a greater number in other
forms, occurring in a single segment. This branching and multiplication
of nephridia is confined to terrestrial forms which in their conditions of
existence are farthest removed from the primitive state, and it is not
improbable that the multiplication bears some relation to the assumption
of a terrestrial mode of life. In the genus Lumbricus, in which the
nephridia are simple coiled tubes, a duplication of the nephridia in some
segments is to be found. The reproductive ducts are probably modified
nephridia, and in all aquatic forms other nephridia are absent in the
metameres in which they occur. In Lnmbricus, however, in, for instance,
the metamere which contains the oviducts, two pairs are present, one of
which retains its original excretory function, while the other has been
modified to form a duct for the reproductive elements.

The reproductive organs have a very different arrangement
from what is found in the Polychaeta, being limited to a
comparatively few metameres ; and furthermore the Oli-
gochpeta are throughout hermaphroditic, the male and female

224

IX VERTEBRA TE MORPHOLOO Y.

ce
so

n

orgaiis both lying iii the auterior portion of the body, usually
between the ninth and fourteenth inetameres, or sometimes

even farther forward. There are
either one or two pairs of testes
(Fig. 104, t)y which arise like the
ovaries from the peritoneal epithe-
lium and early break down to a
greater or less extent, their cells
passing into seminal vesicles (W),
where they undergo further de-
velopment into spermatozoa. The
vasa deferentia (vd) are modified
nephridia, and are either two or
four in number according as there
are one or two pairs of testes.
When four are present they may
open separately, or may unite in
pairs on either side in a common
atrium, through which they open

to the exterior, or finally those of
game ^ ^ & ^^

•>
distance below the tunnels, forming

vs

FIG. 104 NERVOUS SYSTEM

AND REPRODUCTIVE ORGANS

OF Lumbricus.
ce = supracesophageal ganglion, for the greater part of their course

n = ventral nerve-cord.
od = oviduct.
ov = ovary.

rs — receptaculum seniinis.
so = subcesophageal ganglion.
t = testis.
vd = vas deferens.
•cs = vesicula semiualis.

a single tube. There is only a
single pair of ovaries (ov), to which
in some forms ovarian receptacles
similar to the seminal vesicles are
added ; and in all but some of the
lower forms oviducts (od), which
are modified nephridia, are pres-
ent. In front of the metameres which bear the testes one,
two, or occasionally three pairs of invaginations of the body-
wall occur, producing pouches projecting into the body-cavity
—the seminal receptacles (rs) — which receive the seminal
fluid during the mutual interchange of it which takes place on
copulation.

A satisfactory subdivision of the Oligochseta into orders has not yet
been possible ; indeed the various families are so related to one another
that such a subdivision seems unnecessary. Formerly it was the custom

TYPE ANNELIDA. 225

to recognize two orders, Limicolce and Terricolce, aquatic forms being
referred to the former, and terrestrial ones to the latter — a division, how-
ever, which is decidedly artificial. Less so, but still unsatisfactory, is a
division into Naidoinorp/ia, reproducing nou-sexually, and Lunibrieo-
iHu/'p/ia, reproducing by the sexual method only. It seems on the whole
better to omit a subdivision into larger groups, and recognize one into
families only.

Development of the Oligochceta.-I.ii the development of the
Oligochseta there is practically no larval stage, 'but a sufficient
amount of nutrition is supplied to the embryo, either in the
form of yolk in the egg itself or as an albuminous substance
stored up in the interior of a cocoon in which the ova are
contained, to enable it to pass through all its early stages
while still within the egg-shell or cocoon, and to assume a
free life only when it has reached the form of the adult. The

»t

Trochophore larva under such conditions is useless, and is
suppressed in the ontogeny, the development becoming thus
direct or of the foetal type. This mode of development has
been acquired as an adaptation to the aquatic or terrestrial
life, in which, for obvious reasons, the occurrence of a free-
swimming larva would be an inconvenience rather than an
advantage.

In the Polychseta it was stated that usually at a very early
stage of development one cell, later dividing into two, differ-
entiates from the rest as the primary mesoblast, and gives
rise to all the mesodermal tissues of the adult worm. This is
an example of a precocious segregation of the mesodermal
material into a single cell. It is to be presumed that in more
primitive forms the mesoderm separated off from the endo-
derm only at a relatively late period of development ; the
tendency, however, for the appearance of an important struc-
ture to be thrown farther and farther back in the individual
development, to appear at successively earlier stages in the
development, has asserted itself to such an extent that the
mesoderm in the Polychseta makes its appearance while the
embryo is still composed of but a few cells, becoming there-
fore segregated in a single cell. Such a process has further-
more the advantage of permitting a rapid growth, the original
embryonic mesoblasts retaining their position at the posterior

226

INVERTEBRATE MORPHOLOGY.

end of the body and giving rise by division in a transverse
plane to rows of cells, the mesoblast bands. Such a pre-
cocious segregation of the inesoderm also occurs in the Oli-
gocliseta. In a Lumbricus embryo there may be seen near
the posterior extremity of the body the two mesoblasts (Fig.

105, ra), lying one on each side
of the middle line, with the meso-
blast-bauds (mb) extending for-
wards from them. A little in
front of them and on either side
may be seen another cell (nb),
giving rise to a band extending
anteriorly, which later on will
become differentiated into the
ventral nerve-cord, the cells which
give rise to it being neuroblasts j
while a little behind and exter-
nally to these, on either side, two
TKUIOK EXTREMITY OF JLMBRYO

OF Lumbricus (after E. B. WILSON), other cells (we and a;) occur, giving

rise likewise to germ-bands, whose
further fate is undecided, though
it seems probable that the inner of
the two bands gives rise to the
nephridia, the cells being nephro-
blasts. Thus from a small number

of cells the entire nervous system, with the exception of the
suprao3sophageal ganglion (which arises as a local thickening
of the ectoderm, comparable to the apical thickening of the Tro-
chophore), the nephridia and all the other niesodermal tissues
arise, the precocious segregation of these organs being carried
to an extent only equalled in the Hirudinea. Indications of
it, however, are found in the Polychseta, not only in the meso-
blasts but also in a layer of cells occupying the ventral
surface of the embryo, and forming the so-called ventral plate,
from which the ventral nerve-cord, the nephridia, and some of
the musculature seem to arise. A reduction of the number
of cells constituting this ventral plate to the smallest number
consistent with a bilateral symmetry, that is, to two for each

FIG. 105.— SURFACE VIEW OF Pos-

ec = ectoderm.
TO = niesoblnst.
mb — mesoblast-band.
nb = lieu rob last.
ne — nepbroblast.
x = lateral teloblast.

TYPE ANNELIDA. 227

set of structures formed from it, would give rise to a condition
such as is found in the Oligochseta.

A number of the simpler Oligochaets, in addition to repro-
ducing in a sexual manner, also reproduce by division, and in
some forms it plays a much more important part than the
sexual method, which in jEolosoma is not yet known to occur.
In the simplest form of this method of reproduction the
animal simply divides at the middle, each portion after sepa-
rating regenerating the parts which are wanting. In one spe-
cies of Ctenodrilus each metamere except the anterior one
may separate and become a new individual ; a phenomenon
which might be regarded as illustrating the bud theory of
metamerism, but which seems more properly to be a case in
which the gradual integration of the multiplied organs has
reached its highest development — the case standing as the
culmination of the process of metamerization rather than as
an example of its mode of origin. In Nais a division of the
new individuals may begin before they have separated, and
chains may thus be produced composed of individuals vary-
ing in the stage of regeneration which they have reached,
but which eventually separate and may later become sexually
mature.

As might be expected from the occurrence of this mode of
reproduction, the power of regeneration of lost parts is pos-
sessed in a high degree by the Oligochaeta ; and not only in
those forms which habitually reproduce by division, but also
in forms like Lumbricus, in which under normal conditions
this method of reproduction is unknown.

Affinities of the Oligocli ceta.- -There is little reason to doubt that the
Oligochseta have been derived from the Polychoeta, and represent members
of that subclass which have become specially adapted to aquatic or terrestrial
modes of life. A few Oligochaats, such as Halodrttu-s, are marine, living
below stones between tides; but they are undoubtedly derived from aquatic
forms, and cannot be regarded as having any ancestral significance. As re-
gards the more definite affinities of the group little can at present be stated
with certainty. They have been referred to forms like the Capitellidae,
in some of which the parapodia are very much reduced, as is likewise
the distinctness of the head, while, as in the Naids, lateral-line sense-organs
are present. A more remote relationship through the Archiannelida has
also been suggested, but at present no definite evidence is forthcoming as
to which view is to be preferred.

228 INVERTEBRATE MORPHOLOGY.

II. CLASS Hirudinea.

The Hirudinea differ from the Chaetopoda iu their external
form, being destitute either of parapodia or setae, and possess-
ing at the anterior end of the body a muscular sucker at the
bottom of which the mouth is situated, while a second larger
sucker used for attachment occurs at the posterior extremity
of the body. The outer surface of the body is distinctly
ringed, but a comparison of the rings with the internal organs
shows that they have not a inetarneric value, but that a num-
ber of them, varying in different forms, are included in each
true segment of the body. In Brancliellion and Clepsine three
such rings correspond to a rnetarnere, in Ichthyobdella and
Pontobdella six, in Piscicola twelve, and in all the group of
the Gnathobdellidae five. Towards the anterior and posterior
ends of the body a reduction of the number of rings corre-
sponding to a metamere is found, as for instance in the genus
Macrobdella (Fig. 106), which has in the middle region five rings
to a segment. The first two metanieres consist of but one ring
each, the third of two rings, the fourth, fifth, and sixth each
of three rings. At the posterior end of the body the twenty-
third metamere consists of four rings, the twenty-fourth,
twenty-fifth, and twenty sixth of two rings each ; while prob-
ably no less than seven metanieres whose rings are not
readily distinguishable are represented in the posterior sucker.
The entire animal consists, therefore, of thirty-three rnetaineres,
and this number is characteristic for all the Hirudinea — a
defiuiteness of number which contrasts strongly with the wide
variations found in the Chaetopoda. This number does not
include a small lobe in front of the most anterior metamere,
which may be equivalent to the prostomial lobe of the Oligo-
chaeta, and may represent another metamere.

As in the Oligochaeta the gland-cells of the hypodermis at
about the time of reproduction become enlarged and more
abundant in a definite region of the body, forming a clitellum
which is usually in the neighborhood of the tenth, eleventh,
and twelfth metameres. As a rule no branchiae occur, though
an exception is found in the marine genus Brancliellion, in

TYPE ANNELIDA.

229

which each ring of the middle region of the body bears an
appendage which functions as a gill. In another marine form,
Pontobdella, large warts occur on certain rings, which prob-

f I/"

Efv." ... ;,^24

X

1

1

[a n J ,~»rl

r v ~ -i^J29

XI

— s — -4 - mo

- !« -

"" ••<-{" f

^ -to

H^-^^^439

gc-

XE('-^

(TTM:;i-.''i

^E^44

an

FlG. 106. —ANTERIOR AND POS-
TERIOR EXTREMITIES OF Ma-

crobdellu sestertia (after WHITMAN).

an — auus.

fo — opening of oviduct.

gc — copulatory glands.
mo — openiug of vasa deferentia

oc — eyes.

p = nephridial pores.

sp = sense-papilla.
1-100 = anuuli.
i-xxv = uietauieres.

lv

ne

FIG. 107. — DIAGRAMS TO SHOW AR-
EANGEMENT OF BLOOD-SINUSES OF (A)
Hirudo, (B) Clepsine, AJSID (C) Nephelis
(after BOURNE).

al = digestive tract.
c = coelom.
ds = dorsal sinus.
Is and lv = lateral sinus or vessel.
n and nc — ventral uerve-cord.
ne = uepbridium.
ov = ovary.
te = testis.
vs — ventral sinus.

ably are mainly respiratory in function, being richly supplied
with blood-vessels.

The exterior of the body is covered by a cuticle, beneath
which lies the hypodermis. The muscular tissue which un-
derlies the hypodermis consists, as in other Annelida, of layers

230 INVERTEBRATE MORPHOLOGY.

of longitudinal and circular fibres ; and in addition, between
these, a layer composed of fibres which cross one another
obliquely is usually present. A marked distinction from what
occurs in the Chsetopoda is found in the ccelom, which in the
Hirudinea is traversed by a parenchyma, recalling that of the
Platyhelrninths, so that the actual cavity is to a great extent
obliterated, and the dissepiments only to be distinguished with
difficulty. Those portions of the ccelom which persist (Fig.
107, Cc) are occupied by a red or colorless fluid containing
corpuscles and identical and continuous with that found in the
blood-vessels. The ccelom is in fact represented by a number
of blood-sinuses, which in some forms are lined by an epithe-
lium, while in others such a lining is wanting. On account of
the manner in which the blood-vessels anastomose with the
sinuses it is exceedingly difficult to distinguish which spaces
should be considered as belonging to the circulatory system
proper and which to the ccelorn — if, indeed, the two are to be
considered fundamentally distinct. As a rule four main lon-
gitudinal vessels or sinuses are to be found — viz., one dorsal
(Fig. 107, ds), which may be wanting (Nephelis, Fig. 107, C) and
which probably corresponds to the dorsal vessel of the Chre-
topoda ; one ventral (vs), sinus-like in character and frequent-
ly destitute of an epithelial lining, which surrounds the ventral
nerve-cord ; and two lateral vessels (Iv and Is} unrepresented
in the Choetopods, and perhaps also to be regarded as rem-
nants of the coelomic cavit}-. Communications between these
longitudinal vessels occur through the medium of smaller
vessels ; and in some forms, such as Nephelis, the connection
between the lateral and ventral vessels takes place through
ampullae, globular vesicles arranged in two pairs on each
side of a number of metanieres and receiving blood-vessels
from the ventral sinus, while other vessels passing to the
main lateral vessels arise from them. In many forms, espe-
cially among the Guathobdellidse, a rich plexus of capillary
vessels penetrates the hypoderinis.

The union of the blood vascular system with sinuses which most prob-
ably represent portions of the coelomic cavity suggests an intimate relation,
so far as its origin is concerned, of the vascular system with the coalom ;
and this view is borne out by what has already been seen to occur in the

TYPE ANNELIDA. 231

Nemerteans (p. 165), the lowest forms that possess a distinct blood vascular
system. In this group the coelom, so far as it exists, consists of small spaces
without any definite walls scattered through the parenchyma. In some
forms the blood vascular system communicates with these spaces through
which the blood circulates, it being only in the most highly differentiated
Nemerteans that the vascular system is closed. It might be supposed from
this that the blood-vessels were simply ccelomic spaces which had acquired
definite walls; and it seems probable that such has been their origin. In the
Annelida a somewhat different state of affairs occurs. Here, as a rule, there
is a definite coelom lined with peritoneum and completely separated from the
cavity of the blood-vessels, which seem to represent rather the remains
of an original cavity, the so-called blastoccel (see p. 52), which has been
almost obliterated by the growth of the mesodermal segments by the hollow-
ing out of which the coelomic cavities have been formed (see p. 56). It
seems certain that the coelomic spaces of the Nemerteans are likewise the
remains of a primitive blastoccel, so that to this extent the homology of the
blood-vessels holds in the two groups.

In the Hirudinea, however, the blood-sinuses, if they are coelomic, cor-
respond with the coelom of the Polychaeta ; and furthermore, in the Oligo-
chsets and Polychsets, as well as in the Gephyrea, as will be seen later, the
haemolymph contained in the ccelorn is very nearly if not quite identical in
composition with the blood contained in the blood-vessels. These facts
would seem to indicate a close relationship between the Annelid coelom and
the more primitive blastoccel; or, in other words, would lead us to suppose
that the coelom of the Annelids lined with peritoneum is not something
apart and distinct from the blastoccel cavity, as has usually been supposed.
The view which maintains the distinctness of the two forms of ccelom has
its origin in the fact that in some forms, such as Sagitta, a ccelom lined with
peritoneum is formed as an outgrowth from the primitive digestive tract;
and it was supposed that all coelomic cavities with definite walls were pri-
marily of a similar origin, and hence were termed enteroccels in contradis-
tinction to the schizocads or simple spaces in the mesoderm without defi-
nite walls, which are in reality remnants of the blastoccel. The significance
of true enteroccels will be discussed later. In the mean time it may be
pointed out that there is no embryological evidence in favor of the Annelid
ccelom having arisen as a series of pouch-like outgrowths from the primi-
tive digestive tract. It is rather to be regarded as a schizoccel whose char-
acter has been altered by metamerization, and by the manner of its forma-
tion from mesoblasts. On this view the union of the cavity of the blood-
vessels with the ccelom in the leeches, and the similarity of the ha3molymph
to the blood in other forms, cease to be morphological puzzles.

The mouth lies at the bottom of the anterior sucker and
opens into a muscular pharynx, which in some forms (e.g. Clep-
sine] is folded similarly to that of some Turbellaria (see p. 134)
so as to form a protrusible tube, while in others (e.g. Hirudo,

232

IN VERTEBRA TE MORPHOL OGT.

ce

n

FIG. 108.— DIAGRAM OF
THE EXCRETORY RE-
PRODUCTIVE AND NER-
VOUS SYSTEMS OF Hirudo
(after BOURNE).

ce = cerebral ganglion.
ep = epididymus.
gl — oviducul gland.
lv = lateral blood-vessel.
n = nephridia.
ov = ovary.
pe = penis.
te = testis.

masses. Six of these

Macrobdella) it is thrown into three
longitudinal muscular ridges whose
edges may become converted into
chitiu, thus forming teeth. Salivary
glands open into the pharynx in some
forms. The large stomach into which
the pharynx opens behind gives off a
number of lateral pouches (eleven
pairs in Hirudo, seven in Clepsine),
sometimes branched and increasing in
size from before backwards, the most
posterior pair being usually quite long
and directed backwards parallel to the
straight narrow intestine which opens
to the exterior on the dorsal surface of
the body, just anterior to the posterior
sucker. Occasionally only the poste-
rior pair of pouches is present, and in
a few forms they are entirely wanting.
The nervous system (Fig. 108) is
constructed on the typical Annelid
plan. It consists of a circunioesopha-
geal ring and a ventral nerve-cord
composed of fibres which have their
origin in ganglion-cells grouped to-
gether at definite intervals into gan-
gliouic masses. Several of these gan-
' gliouic masses correspond to single
segments, but at the anterior and
posterior extremities a considerable
amount of fusion of the inetameric
groups of ganglia has occurred. In
Clepsine plana the portion of the ner-
vous system which lies above the
oesophagus consists of a transverse
band of fibres passing lateral!}' into
the circumoasophageal commissures
and of a number of ganglionic
latter lie in front of the baud of fibres

TYPE ANNELIDA. 233

and correspond to the metainere formed by the prostomial
lobe. Behind the transverse band are four additional gaugli-
onic masses, apparently forming with the other six the supra-
cesophageal ganglion, but in reality forming together with two
additional masses on the ventral side of the nerve-cord below
the oesophagus the ganglion of the second somite. Immedi-
ately posterior to the two ventral masses is a chain of eight
ganglia lying one behind the other on the mid-ventral line of
the cord, and corresponding to these there occur on each side
along the dorsal surface of the cord other eight masses,
between each successive pair of which a nerve passes out.
There are therefore four metameric ganglia represented in
this complex structure, each consisting of six gauglionic
masses and each giving rise to a pair of nerves. The sub-
O3sophageal ganglion accordingly consists of the ganglia of
four metameres, to which must be added the two ventral
masses of a fifth metamere, the supra- and subo3sophageal
ganglia representing together six metameric ganglia. Behind
the subcesophageal gangliouic aggregate there lie twenty-one
ganglia separated at some distance from each other, especially
anteriorly, each one representing a metamere ; and finally at
the posterior end of the body is another ganglionic aggregate,
representing, to judge from the number of nerves arising from
it, seven metameric ganglia. Thus there are in all thirt}T-
three, or, counting the ganglion which innervates the prosto-
mium, thirty-four, metameric ganglia — numbers exactly corre-
sponding with those obtained by counting the rings.

The sense-organs of the Hirudinea have especial interest
as showing an adaptation of what may be considered tactile
sense-organs to a different purpose. On each metamere of
the body in all Hirudiuea, with a few possible exceptions,
small sensory papillae (Fig. 106, sp) are to be seen, arranged
in definite lines. They occur in the majority of forms on the
first ring of each segment, though in some species of Nephelis
they occur on all the rings. On the dorsal surface of each
sensory ring there are three papillae on each side of the
middle line, and the same arrangement occurs on the ventral
surface, and in addition a single papilla is found at the
margin of the ring on each side. There are thus fourteen

234 INVERTEBRATE MORPHOLOGY.

longitudinal rows of papillae, six on the dorsal surface, six on
the ventral surface, and two marginal. In the anterior and
posterior segments whose width is reduced the marginal pa-
pillae may be wanting, but throughout the rest of the body
the number of rows is constant. In structure these papillae
are . somewhat complicated, consisting of an axial bunch of
elongated sensory cells bearing fine cilia at their outer ends,
and lying in the connective tissue in their immediate vicinity
is a varying number of large cells, each containing a large
watery vacuole in the interior, the nucleus, in consequence,
being pushed to one side. A strong nerve runs to each pa-
pilla and is supplied to the large vacuolated cells as well as
to the axial sensory cells.

Slight differences are to be found in various forms in the structure of
these organs. In Clepsine there is an axial bunch of hair-bearing cells to
which the terminal fibres of the nerve run, and posteriorly and below the
nerve are found the large vacuolated cells. In Hirudo and Neplielis no
hair-bearing cells occur, the nerve occupying the axis of the organ and the
vacuolated cells being arranged symmetrically around it.

It is probable, in view of the two kinds of constituent elements in Clep-
sine, that in this and similar genera a double function is possessed by the
sensory papillae, the hair-bearing cells having perhaps a tactile function,
while the vacuolated cells are visual. It seems probable also that primarily
the papillae were similar in structure and function to the organs of the
lateral line of certain Polychaeta, such as the Capitellidae, or perhaps it would
be better to compare them with the tactile papillae of certain aquatic
Oligocha3ta, which in the genus Slavina have an arrangement on each meta-
mere recalling that found in the Hirudinea.

Towards the anterior extremity of most of the Hirudinea
a varying number of eyes are found. In some species of
Clepsine but two such organs occur, while in others there are
six, and in Ilirudo, Macrobdella (Fig. 106, oc), and allied forms
there are always ten. In the latter forms the eyes are always
arranged in a definite manner: one pair is situated on the
anterior ring (when more than one ring occurs) of each of the
five metameres immediately following the prostomial lobe,
and if their position be determined it will be found that they
occupy the place of one of the dorsal sense-papillae, the eyes
being serially homologous with the sense-papillae of one of
the dorsal rows. This conclusion is verified by their struc-

TYPE ANNELIDA. 235

ture, since they differ from the sensory papillae only in the
greater number of the large vacuolated cells and in the pres-
ence of a quantity of black pigment in the surrounding tissues.

Other sense-organs somewhat beaker-shaped in character
are found upon the prostomium and have been regarded as
gustatory in function.

Nephridia occur in a number of the metaineres of the
middle portion of the body, there being in Hirudo (Fig. 108, n)
and its allies seventeen pairs. Each nephridium has a ter-
minal funnel, which in Clepsine has the typical Annelidan
structure, but in Hirudo has been modified so that the inner
extremity of each nephridium is constituted by a lobed spongy
ciliated mass without any definite central lumen. The funnel
lies in a blood-sinus, either the ventral one as in Clepsine
(Fig. 107, B] or the dorsal as in PontoMella, or in a sinus
which surrounds the testes as in Hirudo (Fig. 107, A), or in
a special sinus which is to be regarded as a ccelomic space as
in Nephelis (Fig. 107, 0). The canal which traverses each
nephridium is intracellular as in the Oligochneta, and in some
forms minute canals traverse the substance of each cell, open-
ing into the central lumen. As a rule the various nephridia
are quite separate and distinct from each other, but in Pontob-
della and one or two other genera they unite to form a net-
work of intracellular canals traversing several metameres.
Immediately before their exit to the exterior the canals
enlarge in some forms to bladder-like vesicles, from which
a short tube leads to the exterior, the opening being situated
either upon the anterior (Clepsine) or the posterior (Hirudo)
ring of the metamere to which the nephridia belong.

The reproductive organs differ from those of the Chasto-
poda in possessing ducts which do not seem to be modified
nephridia and which are continuous with the walls of the
ovaries or testes. All the Hirudiuea are hermaphroditic.
The ovaries constitute in Clepsine two elongated organs which
lie in the middle region of the body, extending through several
metameres, but in Hirudo (Fig. 108, ov) they are small oval
or spherical bodies ; their ducts dilate to form a uterus and
finally unite to open on the inid-ventral line usually in the
eleventh metamere (Fig. 106, /o). The testes (Fig. 108, te)

236 INVERTEBRATE MORPHOLOGY.

consist of a number of pairs, varying from twelve or more to
six (Clepsine}, of spherical bodies lying in the same region of
the body as the ovaries. Each testis has its own duct, which
opens into a longitudinal vas deferens common to all the
testes of the same side of the body. Anteriorly the two vasa
deferentia unite to open in the mid-ventral line of usually the
tenth metamere (Tig. 106, mo), frequently through a strong
muscular penis (Fig. 108, pe). In many forms special glan-
dular thickenings, supposed to be useful in copulation, occur
'on the ventral surface of one of the metameres behind that
bearing the opening of the oviduct (Fig. 106, gc).

The Hirudinea are at present usually divided into two
orders, though it seems probable that further division of one
of them will be necessary later.

1. Order Gnathobdellidse.

In this order are included the leeches which are provided
with chitinous jaws in the walls of the muscular pharynx.
In addition to this all the members of the order are charac-
terized by possessing five rings to each fully developed meta-
mere. To this order belong the Hirudinidse, characterized by
possessing ten eyes arranged in pairs on the five anterior
metameres behind the prostomium, and including Hintdo, the
medicinal leech, a native of Europe, instead of which Macro-
Mella is sometimes used in America. The Nephelidge, with
the genus Nephelis, differ in possessing fewer eyes (four pairs),
and in having distinct segmental sense-organs either wanting
or occurring on all the rings of each segment.

2. Order Rhynchobdellidse.

The Bhynchobdellidse are characterized by possessing a
protrusible pharynx, as well as by possessing three, six, or
twelve rings to a metamere. In the Ichthyobdellidse, or fish-
leeches, the larger numbers are found, the number six being
characteristic of Pontobdetta, while twelve occurs in Piscicola.
In the Clepsiuidie but three rings are found to each meta-
mere, and the eyes are either two or six in number. To this
family belongs the genus Clepsine, a common fresh-water
form, as well as the tropical laud-leech, Hcementeria.

TYPE ANNELIDA. 237

Development of the Hirudinea. — The Gnathobdellidae deposit their eggs
in chitinous COCOOLIS, as do the Oligochaeta, and the development is of the
foetal type, ill contradistinction to the larval, the ova containing as a rule a
considerable amount of yolk. Tiie mode of oviposition of the majority of
the Rhynchobdeilidae is unknown ; but in the genus Clepsine the eggs are
fastened to the ventral surface of the body of the parent, where they un-
dergo development. This resembles closely the development of Lumbricus,
allowing for the greater amount of yolk which is usually present. The
same precocious segregation of inesoderin, nervous system, and nephridia
in special budding cells, the tuesoblasts, neuroblasts, and nephroblasts, is
likewise found, and in later stages the mesoblast is distinctly segmented
and ccelomic cavities are present, which later become to a great extent
obliterated.

The Affinities of the Hirudinea. — It is exceedingly probable that the
ancestors of the Hirudiuea were to be found in the Oligochasta, the two
groups having not a few structural features in common. The embryologi-
cal peculiarities found in the two groups are strikingly similar ; and fur-
thermore the aquatic or terrestrial habits are not a little suggestive, for
although some leeches are marine, nevertheless the majority are aquatic
and a few terrestrial. The complete disappearance of parapodia may be
considered a further development of the tendency towards their oblitera-
tion in the Oligochaeta, where only the setae are present, these even having
disappeared in the Hirudinea in consequence of the development of the
suckers and a new mode of locomotion. The suggestive arrangement of
the sense-papillae of the Oligoclutte Slavina has already been mentioned.

It must not be forgotten, however, that the differences between the two
groups are many and important. Such are, for instance, the disappearance
of the original coelomic spaces, the communication of the blood vascular
system with sinuses, and the occurrence of special ducts for the reproduc-
tive organs. These differences have, however, equal or even greater im-
portance when the attempt is made to trace the Hirudinea directly to the
Polychaeta, and it seems more satisfactory at present to refer them back to
the Oligochaeta.

III. CLASS Gepliyrea.

The Gephyreans constitute a group of marine worms which
differ from the Chaetopoda principally in the more or less
complete absence of metamerization. All trace of it is ab-
sent upon the outside of the body ; for although the thick
cuticle may be marked by distinct rings, these bear no relation
to the internal parts and are, as in the Nematoda, due simply
to the thickness of the cuticle. All traces of parapodia are
lacking in many forms, while in others they are represented

238 INVERTEBRATE MORPHOLOGY.

only by a pair of seta? situated on the ventral surface of the
body, nearer the anterior than the posterior end. The body-
wall presents a close similarity in its structure to that of the
Chsetopods — differing, however, in the occurrence of a more or
less pronounced layer of fibres having an oblique direction.
The coelom is lined by a layer of flat peritoneal cells, but
shows no division into more or less distinct compartments, no
trace of metamerism, but, as in the Chaetopods, the peritoneal
lining is reflected upon the walls of the digestive tract, form-
ing mesenteries suspending the intestine. As a rule the dor-
sal mesentery disappears, and in some cases the ventral one
is almost wanting, the intestine being slung only by a
number of irregular strands of connective tissue extending
from it to the body-wall. In some forms (Sipunculus) the sur-
face of the peritoneum, especially that covering the intestine,
is dotted with numerous irregularly scattered minute depres-
sions, whose openings are guarded each by a peculiar ciliated
cell, and which contain cells comparable in function to the
chloragogue cells of the Chgetopoda. The ccelomic cavity is
occupied by a haamolymph, which in some cases is colored,
and contains numerous cell-elements, some of which may be
circular in outline and colored by hemoglobin, while others
are amoeboid and colorless.

A blood vascular system, principally developed in the an-
terior portion of the body, is present and appears to be com-
pletely closed, though connections with the coelom are said to
exist in some forms. In Sipunculus, for instance, the system
consists of a collar surrounding the oesophagus, sending
branches into the tentacles which surround the mouth, and
dorsally dilating into a wide sinus lying just below the brain ;
and from this sinus a dorsal vessel (Fig. 109, Bs] passes
backwards along the digestive tract for a short distance, end-
ing blindly where the oesophagus joins the stomach. In Echiu-
rus a ventral vessel runs the entire length of the body just
above the nerve-cord, and it is united with the dorsal vessel
by lateral vessels at its anterior and posterior extremities.

The digestive tract may be either straight (Priapulus) or
considerably convoluted (Ecliiurus and Sipunculus, Fig. 109,
Int), and the anus is in some forms terminal (Ecliiurus), while

TYPE ANNELIDA.

239

Oe

in others the intestine bends upon itself and passes forward to
open on the dorsal surface near the anterior end of the body
(Fig. 109, A). Throughout the greater extent of the intestine
there runs along its ventral surface
a ciliated groove which is no doubt
homologous with the accessory in-
testine of certain Polychseta (see
p. 207).

The nervous system partakes of
the absence of distinct metamerism
which characterizes the other parts.
It consists of a brain lying in the
anterior portion of the body above
the oesophagus and sending a com-
missure downwards and backwards
on each side to form the circuni-
cesophageal collar. These two com-
missures unite to form a single
nerve-cord (Fig. 109, ri) extending
the entire length of the body in the
ventral median line, differing from
the ventral cord of the Chsetopoda
in the absence of ganglia. Nerve-
cells are scattered along the entire
length of the cord and are not
aggregated into special ganglia,
though slight indications of such
an aggregation are found in Priapu-
lus. Nerves are given off at more
or less regular intervals on either
side, a somewhat metameric ap-
pearance being thus produced, but
the corresponding nerves of op-

posite sides do not invariably arise ^ = veniral retractor muscle,
from the cord opposite each other.

One, two or three pairs of nephridia (Fig. 109, ne) are as
a rule present and form conspicuous brown tubes, which com-
municate by a funnel with the body-cavity at one extremity
and with the exterior of the body at the other. They are

FIG. 109. — STRUCTURE OF Si-
punculus Oouldii (after AN-
DREWS).
A = anus.
Bs = blood vessel.
dR = dorsal retractor muscle.
Int = intestine.
N — nerve- coid.
ne = nephridium.
Oe = O3soplmgus.
Ov = ovary.

240 INVERTEBRATE MORPHOLOGY.

undoubtedly homologous with the nephridia of the Chsetop-
oda, possessing the same relations. In a few forms (Bonellia,
Phascolion) a single nephridium only is present. In addition
to these in EcMurns, Thalassema, and allied genera there is a
usually much-branched organ on either side lying in the body-
cavity and opening into the terminal portion of the intestine.
Numerous ciliated funnels occur upon the branches placing
the organ in communication with the body-cavity. This so-
called " respiratory tree " (so named from a supposed homol-
ogy with the similarly named organs of the Holothuria (q. v.)
are probably nephridia, though whether or not they per-
form excretory functions is not quite clear. In Priapidus
these organs are represented by branched tubes, the branches
of which terminate blindly in flame-cells, resembling thus the
excretory organs of the Platyhelminths, and in Sipunculus
rudiments of these organs have been described as short tubes.

The Gephyrea are bisexual, the reproductive organs (oc)
forming small digitate, elongate, or ovoid processes arising
from the peritoneal lining of the body-cavity ; but in some
forms (Sipunculus} their products early escape into the cce-
lomic cavity, in which they float. The exact manner in which
the ova and spermatozoa escape to the exterior has not been
definitely ascertained for the majority of forms, but it seems
probable that the nephridia serve as the generative ducts.
In Priapulus the " respiratory trees " are said to give rise to
the reproductive organs, and also to serve as the reproductive
ducts — a behavior which would render exceedingly probable
the supposition that they are modified nephridia.

Two orders are recognizable in the Gephyrea.

1. Order Echiureae.

The Echiureae, sometimes known as the Gephyrea armata,
are characterized by the presence on the ventral surface ui
the body, in front of the openings of the uephridia, of a pair
of setse — the genus Echiurus possessing, in addition to these,
two circles of set* at the posterior extremity of the body.
The anus is terminal in all the known species, and the ter-
minal portion of the intestine has opening into it the
branched respiratory trees. The anterior end of the bodv is

TYPE ANNELIDA.

211

prolonged into a prostomiuni of considerable size overlying
the mouth ; it may be short and broad as in Echiurus, more
elongated and slender as in Thalassema, or deeply bifurcated
at the extremity as in JSonellia.

B

d

FIG. 110. — Bonellia viridis A, ADULT FEMALE OPENED so AS TO SHOW THE
PRINCIPAL ORGANS ; B, male much enlarged in proportion to the female
(from HEBTWIG).

c — cloaca w = muscles.

d — rudimentary intestine. s — proboscis.

g — respiratory trees. s (in Fig. B) — spermatozoa.

i — intestine. 'od = vas deferens.

u — single uephridium which serves also as the oviduct.

The last-named genus is interesting as affording an exam-
ple of sexual dimorphism, the males being small Turbellariau-
like organisms which live parasitically in the anterior portion
of the digestive tract of the female, only coming to the exterior
for the purpose of copulation.

2. Order Sipunculacea.

The Sipunculacea, to which the term Gephyrea inermes is
also applied, is an order including forms which lack all traces

242 INVERTEBRATE MORPHOLOGY.

of setae. In Priapulus the intestine is almost straight and the
anus terminal ; but in Sipunculus and the allied genera, such as
Phascolosoma and Phascolion, the digestive tract is convoluted
and bent back upon itself, so that the anus lies on the dorsal
surface near the anterior extremity of the body. A " respira-
tory tree " is absent or rudimentary as a rule except in Pria-
pulus and allied genera, and the large prostomial lobe char-
acteristic of the Echiurese is lacking. The anterior por-
tion of the body, however, is capable of being iuvaginated by
means of strong retractor muscles (Fig. 109, dR and vR) into
the fore part of the digestive tract, forming the so-called in-
trovert. The extremity of this is provided with a circle of
finger-like or branched tentacles in the centre of which lies
the mouth, and which are supposed to have a respiratory
function, being richly supplied with blood. In Priapulus
these are absent, but at the posterior end of the body there
is a prolongation which bears papilla-like processes which
probably function as respiratory organs.

Development and Affinities of the Gephyrea. — The early development
of the Gephyrea resembles closely that of the Polychaeta, more especially in
the Echiureae. In this order a Trochophore larva is formed resembling very
closely the typical Polygordius trochophore, the similarity extending even
to a segmentation of the primitive niesoderm bands. In later stages this
metamerism of the mesoderm disappears, no trace«of it being found in the
adult forms. In the Sipunculacea the larva differs from the Trochophore
in lacking the typical praeoral baud of cilia, though this may be weakly
developed in some forms, such as Phascolosoma. The postoral cilia are, on
the other hand, strong. A further difference is found in the absence of
metamerization of the mesoderm, which at a very early stage of develop-
ment forms a layer lining the interior surface of the body-wall, and also
covering the digestive tract and enclosing a coalomic cavity continuous
through the entire body.

Notwithstanding these important differences there seems little room for
doubt but that the Kip/un'/ihis larva has arisen us an adaptation of the
typical Annelidan Trochophore still represented in the development of the
Echiureae. By these forms a close relationship is shown to the PolychaMa ;
and the Gephyrea are to be regarded as Polychseta which have secondarily
lost a metamerization originally present in the adult ancestors and still
represented in the Echiurus larva, but lost even in the larval stages of the
Sipunculacea.

Since the discovery of the larval forms of certain Echiurid and Sipun-
culid forms there has been a tendency to regard these two orders as being

TYPE ANNELIDA. 243

much less closely related than they are here supposed to be. The Echiureae
are still held to have Anuelidan affinities, while the Sipuuculacea are as-
signed to tiie next type to be described. This tendency has its origin in
the attachment of too great importance to the metamerism which is indi-
cated in the Echiurid trochophore but lacking in the Sipunculid larva.
There seems no good ground for supposing lhat its absence in the latter
group may not be sufficiently explained by the assumption that it repre-
sents the final stage of the reduction of metamerism of which the transient
segmentation of the Echiurid is a stage. In their anatomical character-
istics the adult forms of the two groups are too much alike to be assigned
to different types and the similarities of detail too numerous to warrant the
belief that they have been independently acquired. It seems much more
probable that both orders have descended from segmented ancestors — the
degeneration, if degeneration it can be called, having been carried to a
greater extent in the Sipuuculacea than in the Echiureae, and having in
consequence been thrown back upon the larval stages and so obscuring the
developmental evidences of the phylogeny.

A connecting link between the Echiurege and the Polychaeta has been
traced by some authors in the genus Sternaspis, at one time associated with
the Gephyrea but now universally assigned to the Polychaeta. In this
genus the metamerization, though to a certain extent reduced, is still pro-
nounced, 8. arcuata consisting of from twenty to twenty-two metameres,
of which the anterior seven, together with the head-lobes, may be invagi-
nated— the introvert of the Sipunculacea being thus recalled. On the ven-
tral surface near the posterior extremity of the body are two shield-like
plates armed with setae, and at the posterior extremity, as in Priapulus,
are a number of filamentous appendages which are regarded as branchiae.
Setae are present on all the metameres except the fifth, sixth, and seventh •
those of the eighth to the sixteenth metameres being, however, concealed
beneath the hypodermis. The digestive tract is somewhat convoluted, but
opens terminally ; the ventral nerve-cord shows traces of ganglionic swell-
ings, and at the posterior end of the body possesses a marked enlargement ;
and only two nephridia are present. The musculature and the vascular
system resemble those of the Polychaeta rather than those of the Gephyrea,
while the reproductive organs are peculiar in possessing special ducts,
which, it has been held, show no indications of being modified nephridia.

In many respects., accordingly, Sternaspis does hold a position interme-
diate between the Echiurese and the Polychaeta, and it seems not improb-
able that it may represent an offshoot from near the base of the line along
which the Gephyrea have been differentiated. Whether this be the case or
not, it is exceedingly probable that the Gephyrea have been derived from
the Polychseta, the Echiureae preserving more numerous traces of their an-
cestry than do the Sipunculaceae.

244 INVERTEBRATE MORPHOLOGY,

IV. CLASS Myzostomeae.

The Myzostomege constitute a group of Annelids which pre-
sent but few traces of a typical metameric form, being much
modified by their parasitic habit. All the known forms are
parasitic upon Crinoids, some producing malformations of the
pinnules of their host in the form of c}Tsts in the interior of
which they live. The body of Myzostomum (Fig. Ill) is
flattened and oval, a number of finger-like processes or cirri
(c) projecting around the margin. There is no trace of external
segmentation, although five pairs of parapodia (p), each with
an axial supporting chitiuous rod and a single hooked seta,
occur on the ventral surface. On the same surface too, near
the margin, are to be found in most species three or four
sucker-like depressions (su) on each side, which have been
supposed to represent highly-modified nephridia.

The body is covered by a thick cuticle beneath which lie
the hypodermis and the musculature of the body-wall, which
has the characteristic Aunelidan arrangement. A body-cavity
can hardly be said to exist (unless it be indicated by the space
occupied by the ova), the interior of the body being cornpletely
filled up by the internal organs and by numerous muscle-
bands passing both dorso-veutrally and from side to side,
these latter in some forms being arranged in such a way as
to represent incomplete dissepiments. There is no blood
vascular system.

The mouth is situated near the anterior end of the body on
the ventral surface and opens into the proboscis-sheath,
within which lies the proboscis (ph\ constructed upon the
same plan as that of the Ehynchobdellid Hirudinea. Around
the extremity of the proboscis are arranged a number of
short tentacles, and its walls are very muscular ; behind it
opens through a short oesophagus into the wide intestine (s}
from which three (or two) branched pouches project on either
side towards the margin of the body. The short and relatively
narrow rectum (r) opens near the posterior end of the body,
uniting shortly before its termination with the oviduct.

The nervous system consists of a circumoesophageal coni-
missural ring upon which lie numerous scattered ganglion cells

TYPE ANNELIDA.

245

likewise surrounding the oesophagus and apparently represent-
ing the supracesophageal ganglion. Numerous longitudinal
nerves pass forward from the ring to unite with another ring
around the base of the proboscis from which nerves pass to
the tentacles. Below the intestine lies a large ganglionic mass
with which the circumo3sophageal commissures unite and
which gives off a number of peripheral nerves. This mass is

mo

ov

fo-

do

FlG. 111. —Myzostomum (after VON GRAFF).

c = cirrus. v = rectum.

do = cloacal opening.
fo = opening of uterus iuto cloaca.

o = opening of male reproductive organs

p = parapodium.

pJi = proboscis.

s = stomach.
su = sucker.
t = testes.
u = uterus.

composed of several (probably 6) united ganglia and represents
the ventral nerve-cord of other Annelids. Nerves pass pre-
sumably from the supracesophageal ganglion-cells along the
dorsal wall of the intestine and seem to constitute a sympa-
thetic system. The only structures which can be considered
sense-organs are the marginal cirri and the tentacles of the
proboscis, which probably have a tactile function. No traces
of eyes have yet been observed.

Nephridia, unless they be represented by the sucker-like
depressions aud the oviducts, are wanting. The Myzostomeje

246 INVERTEBRATE MORPHOLOGY.

are as a rule hermaphrodite. It seems doubtful if the ovaries
have actually beeu made out, the large masses of ova (ov)
lying between the branches of the intestinal pouches, which
have been considered ovaries, being more probably original
coalomic spaces which have become filled with ova set free
from the ovaries ; while the so-called uterus (u\ tying im-
mediately above the intestine, and which iu mature animals
is closely packed with ova, is probably of the same nature.
Three oviducts, one dorsal and two lateral, pass from the uterus
to open (fo) into the rectum near its termination, though the
dorsal one in some forms may open directly to the exterior
near the anus.

If the uterus is correctly identified as a ccelomic space, then it seems
not improbable that the oviducts may represent modified nephridia.
Their opening into the rectum is a secondary condition and does not
necessarily stand in opposition to their nephridial character, since practi-
cally the same conditions obtain in some Kotifera.

The testes (t) are branched organs lying for the most part
between the intestine and the nervous system, though isolated
masses occur in some forms near the margin of the body. On
each side two vasa deferentia, one anterior and one posterior,
convey the spermatozoa to a muscular sperm-vesicle opening
to the exterior at the margin nearly opposite the centre of
the body (mo).

In some species, notwithstanding their hermaphroditism, " comple-
mental males," small individuals which possess ripe spermatozoa while
lacking ova, have been described as occurring. Further observations have
not, however, tended to confirm this idea iu its original sense, since these
small individuals have been found to be, like the larger ones, hermaphro-
dites, being secondary adaptations from the prevailing hermaphroditic
condition, and not having, therefore, the same significance as the " com-
plemental males" of the Cirrhipedia (q. v).

There can be little room for doubt but that the Myzostomese are Annelida
degenerated by parasitism, and that they are most closely related to the
Polychaeta. It is interesting to note in this connection the effect their
parasitic and sessile mode of life under equable external conditions has had
in producing indications of a radial symmetry.

TYPE ANNELIDA. 247

APPENDIX TO THE TYPE ANNELIDA.

CLASS PHORONID.E.

The class Phoroniclae includes a single genus, Phoronis, of
which but a few species are known. They are all marine
forms of comparatively small size, reaching in some cases a
length of 50 mm. Each individual is contained within a
chitinous tube to which particles of sand are in some cases
agglutinated, and is worm-like and cylindrical in form, the
anterior extremity of the body being provided with a horse-
shoe-shaped fold, termed the lophophore (Fig. 112, a), bearing
a number of tentacles arranged around its margins. Between
the two circles of tentacles is situated the mouth (&), over
which hangs a fold known as the epistome, representing the
prostornium or prgeoral lobe of the larva. Outside the area
enclosed by the tentacles is the anus, on either side of which
a pore, the opening of a nephridium, is found.

The ectoderm of the body-wall is separated by a distinct
basement-membrane from a layer of circular muscles, within
which is a second layer of longitudinal muscles (i) — an arrange-
ment resembling that found in the body-wall of the Annelids.
Internally the longitudinal muscle-layer is lined by a layer
of peritoneal cells enclosing a spacious ccelom. Near the
anterior end of the body there is a transverse septum sepa-
rating off, more or less perfectly, an anterior chamber, with
which the cavity of the epistome and of the lophophore com-
municates, from a larger posterior chamber in which lie the
intestine and reproductive organs, and which is divided lon-
gitudinally by three mesenteries extending from the intestine
to the body-wall. One of these mesenteries accompanies the
intestine throughout its entire extent, while the other two
lateral mesenteries are in connection only with the sides of
the descending limb of the intestine.

The tentacles are processes of the body-wall, with a
ciliated ectoderm, and contain a chitinous axial supporting
tissue.

A completely closed blood vascular system is present,
consisting below the transverse septum of two longitudinal

248

INVERTEBRATE MORPHOLOGY.

vessels (h and /). One of these (/) divides (g) near the an-
terior extremity of the body, the two branches passing into a
circular vessel lying at the bases of the tentacles and sending

branches up into them. The
vessels which return the
blood from the tentacles open
into a second ring external
to the first, and from it two
vessels pass backwards and
unite to form the second
longitudinal trunk from which
numerous csecal pouches
arise. All the vessels have
contractile walls, and the
blood which they contain
possesses nucleated red cor-
puscles.

The digestive tract is bent
FIG. 112.— LATERAL VIEW OF AN- upon itself (d and e), the
TERIOR REGION OF Phoronis (after mouth and anus, as already

described, lying in close prox-
imity at the anterior extrem-
ity of the body. Several
regions, such as oesophagus,
first stomach, second stom-
ach, and intestine, are to be
distinguished, and along one

BOURNE).

a = lophophore.

b = mouth, surrounded by epitome.

c = lophopboral disk.

d = O3sopliagus.

f = intestine.

/ = ventral blood-vessel.

g = brancb of/.

h = dorsal blood-vessel.

i =- longitudinal muscle of body-wall, surface of the oesophagus and

, intertentacular membrane. firgt stomach runs in P.

architecta a ridge, becoming a groove in the stomach region,
of ciliated gland-cells, which recalls the accessory intestine
of the Gephyrea. There are no special digestive glands.

The nervous system is completely imbedded in the ecto-
derm. It consists of a nerve-ring, following the outline of
the lophophore at the bases of the tentacles and surround-
ing, therefore, the mouth but not the anus. From it a nerve
runs backward asymmetrically «upou one side of the body
to near the posterior extremity. It contains a large clear
rod-like structure which seems to be a colossal nerve-fibre.

TYPE ANNELIDA,

249

The only sense-organs which have been described are a pair
of ciliated depressions lying one on each side in the concavity
of the lophophore ; no definite statement can be made as to
their function.

A single pair of nephridia is present, opening into the
posterior chamber of the coaloin by funnel-like mouths, and to
the exterior on each side of the anus. They serve not only
for excretion, but also as ducts for the reproductive elements.
The various species of Phoronis, with the possible exception
of P. arclriteda, are hermaphrodite, the ova and spermatozoa
developing from cells of the peritoneum lying in the vicinity
of the pouched longitudinal blood-vessel. They are shed
from their place of formation into the ccelomic cavity and
thence pass to the exterior through the nephridia.

. 113. — METAMORPHOSIS op Actinotrocha (after METSCHNIKOFF from BALFOUR).

in = mviigination.

Development of the Phoro7iidce. — In their development the
various species of Phoronis so far as known undergo a very
remarkable metamorphosis. The larva which develops from
the ovum, is known as Actinotrocha (Fig. 113, A] and is a some-
what elongated structure possessing at the anterior end a
large hood which overhangs the mouth, its edge bearing

250 INVERTEBRATE MORPHOLOGY.

strong cilia. Behind the mouth are a number of ciliated
tentacle-like processes arranged in a horseshoe-shaped
curve, their cilia, together with those of the edge of the
prostomial hood, forming a band encircling the mouth. The
digestive tract opens to the exterior at the posterior ex-
tremity of the body, and the axis of the body is the axis
passing through the anus and the centre of the prostomial
lobe. A little later (B) an invagiuation (in) of the body-wall
into the crelorn of the larva develops on the ventral surface
behind the band of ciliated processes and becomes of a con-
siderable size. At the time of the metamorphosis this in-
vagiuation is suddenly everted (Fig. 113, C and D), the intes-
tine being carried with it as a loop, and entirely new axial
relations are thus brought about. The long axis of the body
is now (D) almost at right angles to what it was in the
Actinotrocha, and since the imagination originally formed
on the ventral surface of the larva, the body of the adult
Phoronis must be regarded as formed by an excessive de-
velopment of the ventral surface, the dorsal surface being
represented only by the short interval between the moiith, or
rather the epistome, and the anus. The epistome represents
the prostomial lobe of the larva, and the ciliated processes
represent the lophophoric region, though they themselves are
afterwards replaced by the permanent tentacles.

There can of course be no question but that this remarkable metamor-
phosis is a secondary phenomenon, and it seems probable that its acquisi-
tion stands in relation to the tubicolous habits of the adult which neces-
sitate the change of the principal axis of the body. The metamorphosis is
the means of avoiding a slow and tedious change necessitated by the differ-
ent habits of the larva and the adult, just as the occurrence of the chry-
salis stage in the development of the butterfly is required on account of
the great differences between the mouth-parts of the larval caterpillar and
the adult butterfly.

The affinities of Phoronis cannot be considered to be finally settled as
yet, though there has been a tendency of late years to associate them with
the Polyzoa. They also seem to show affinities to the Gephyrea, and by
some authors are considered more correctly referable to that group. The
tendency to develop the ventral surface of the body at the expense of the
dorsal and so to form a new body-axis is seen in Sipunculus and carried
to its culmination in Phoronis, and further similarities between the two
forms are to be found in the character of the nephridia and in the occur-

TYPE ANNELIDA. 251

rence of a closed blood vascular system. The lophophore of Phoronis, and
the epistome, are on the other hand characteristic Polyzoan features, and it
seems not at all improbable that Phoronis occupies an intermediate posi-
tion between the Gephyrea and the Polyzoa. There is this at all events to
be noted concerning the Prosopygia (see following chapter), and that is
that they are certainly closely related to the Annelida. If the supposition
advanced on p. 243 to the effect that the Sipunculacea are to be regarded
as Annelida which have secondarily lost their metamerism be correct, and
if Phoronis really indicates a derivation of the Prosopygia from Gephyrean-
like ancestors, then the Prosopygia too must be regarded as Annelida in
which all traces of metamerism have been lost. This view seems preferable
to that which would refer the Polyzoa, for instance, back to unsegmeuted
ancestors — back, that is to say, to the non-segmented ancestors of the
Annelida.

SUBKINGDOM METAZOA.

TYPE ANNELIDA.

I. Class CHjETOPODA. — Metamerism usually well marked ; with dorsal

and ventral rows of setae along the sides of the body.
I. Subclass POLYCILETA. — Marine forms ; with the setae usually borne
upon lateral lobes of the body (parapodia).

1. Order Archiannelida. — Without parapodia. Poly gor dins.

2. Order Errantia. — Elongated swimming or creeping forms;

metauieres more or less similar. Nereis, Lepidonotus,
Diopatra, Autolytus, Hesione, Syllis, Alciope, Capitella,
Polyophtlialmus, Arenicola, Aricia.

3. Order Sedentaria. — Usually tubicolous ; anterior metameres

more or less different from the rest. Amphitrite, Serpula,
ISabella, Terebella.

II. Subclass OLIGOCH.ETA. — Aquatic or terrestrial forms ; with setae,
but without parapodia.

1. Order Naidomorpha. — For the most part aquatic ; fre-

quently reproducing non-sexually ; nephridia serve as re-
productive ducts. Nais, Dero, Chcetobranchus, *3?olo-
soma, dialog aster, Ctenodrilus, Tubifex.

2. Order Lumbricomorpha. — For the most part terrestrial ;

not reproducing non-sexually ; special reproductive ducts.
Lumbricus, PericTiceta, Anachceta.
II. Class HIRUDINEA. — Metamerism well marked ; without setae ; with

anterior and posterior suckers.

1. Order Gfnathobdellidce. — Mouth with three more or less well
developed teeth ; pharynx not protrusible. Hirudo, Ma-
crobdella, Nephelis.

252 INVERTEBRATE MORPHOLOGY.

2. Order Rhynchobdellidce. — Without teeth and with protrusible
pharynx. Clepsine, Pontobdella, Piscicola, Branchellion.

III. Class GEPHYKEA. — Metamerism indistinct ; without parapodia.

1. Order Echiurece. — With seise. Echiurus, Thalassema, Bo-

nellia.

2. Order Sipunculacece. — Without setae. Sipunculus, Priapu-

lus, Phascolosoma, Phascolion.

IV. Class MYZOSTOMEvE. — Parasitic on Crinoids ; approximating a radial

symmetry. Myzostoma.

APPENDIX.

Class PHORONID^;. — Without metamerism ; tubicolous ; with lophopnore.
Phoronis.

LITERATURE.

CH^ETOPODA.

A. de Quatrefages. Histoire naturelle des Anneles marins et d'eau douce.

Paris, 1865.

E. Ehlers. Die Borstenwurmer. Leipzig, 1864-68.
E. Meyer. Studien iiber den Korperbau der Anneliden. Mittli. a. d. zoolog.

Station zu Neapel, vn, 1887 ; vm, 1888.
H. Eisig. Die Capitelliden des Oolfes von Neapel. Fauna und Flora des Qolfes

von Neapel. Monogr., xvi, 1887.
J. Fraipont. Le genre Polygordius. Fauna und Flora des Golfes von Neapel.

Monogr., xvi, 1887.

E. Claparede. Recherches anatomiques sur les Oligocfietes. Geneve, 1862.

F. Vejdovsky. Monographic der Enchytrmden. Prag, 1879.

- System und Morphologie der Oligochccten. Prag, 1884.
Numerous papers on Oligockseta by G. E. Beddard and W. B. Benham in
Quarterly Journ. Microscop. Science.

HIRUDINEA.

A. Moquin-Tandon. Monographic de lafamille de Hirudinees. Paris, 1846.
C. 0. Whitman. The Leeches of Japan. Quarterly Journ. of Microscop. Science,

xxvi. 1886.

— The Metamerism of Clepsine. Festsclir. zum siebenzigsten Geburtstage
Rudolf Leuckarts. Leipzig, 1892.
A. G. Bourne. Contributions to the Anatomy of the Hirudinea. Quarterly

Journ. of Microscop. Science, xxiv, 1884.

GEPHYREA.

R. Greef. Die Echiuren (Qephyrea armata). Nova Acta Leopol. Carol. Akad.,

XLI, 1879.

E. Selenka. Die Sipunc.uliden. Wiesbaden, 1883.
E. A. Andrews. Notes on the Anatomy of Sipunculus Gouldii, Pourtales.

Studies from the Biolog. Labor. Johns Hopkins Univ., iv, 1890.

TYPE ANNELIDA. 253

MYZOSTOMIDA.

L. von Graff. Das Genus Myzostoma. Leipzig, 1877.

Report on the Myzostomidce. Sclent. Results of the Voyage of H.M.S.

Challenger. Zool., x, 1884.
J. Beard. On the ? Life-history and Development of the Genus Myzostoma.

Mitth. a. d. zool. Station Neapel, v, 1884.

PHORONIDA.

E. B. Wilson. The Origin and Significance of the Metamorphosis of Actinotrocha.

Quarterly Journ. Microscop. Science, XXI, 1881.
C. J. Cori. Untersuchungen iiber die Anatomie und Histologie der Gattung

Phorords. Zeitschr. fur wissensch. Zoologie, LI, 1891.

254 INVERTEBRATE MORPHOLOGY.

CHAPTEE XL

TYPE PPvGSOPYGIA.

THE members of the type Prosopygia are compact, soli-
tary, or colonial organisms destitute of a true metamerism
and having the digestive tract usually bent upon itself, so
that the anus lies in more or less close proximity to the
mouth and therefore near the anterior end of the body. A
chitinous or more or less calcareous investment is formed
about the exterior of the body, and in some cases assumes
the form of a calcareous bivalve shell, similar to that of the
Pelecypoda in its general appearance, although in the rela-
tions of the valves to the body-surfaces and in other particu-
lars (see p. 327) there are very decided differences, the simi-
larity being simply an analogy.

A more characteristic feature, however, is the presence at
the anterior end of the body of a circular or horseshoe-shaped
fold, or else of two armlike lateral processes, forming what
is termed the lophophore, upon which are borne a number of
tentacles which play important roles not only in obtaining
food, but also in the process of respiration, no branchial or
other special respiratory organs being present.

A more or less spacious coeloni is usually present, trav-
ersed by muscle-fibres and some specially developed muscle-
bauds, though the muscular system is on the whole poorly
developed. The ccelom contains a hsemolymph, but a sepa-
rate blood vascular system and heart is entirely wanting.
The nervous system, in accordance with the absence of met-
amerism, is exceedingly simple, consisting either of a single
ganglion, lying between the mouth and anus and sending off
nerves to the various regions of the body, or else of a nerve-
ring surrounding the oesophagus, with more or less distinct
supra- and sub-cesophageal ganglionic enlargements. Special
sense-organs are wanting.

TYPE PHOSOPTG1A. 255

A pair of simply-constructed nepliridia are present in
some forms, but iu many a special excretory organ seems to
be entirely wanting. Bisexuality is the usual arrangement,
although in the Polyzoa hermaphroditisrn is of not unfre-
quent occurrence.

» The great majority of the Prosopygia are marine in habi-
tat, though a number of Polyzoa are inhabitants of fresh
water. The type may be divided into two well-marked

•/ X */

classes, the Polyzoa and the Brachiopoda.

I. CLASS POLYZOA.

The Polyzoa, a group usually spoken of by German zool-
ogists as the Bryozoa, are almost without exception colonial
organisms, forming encrusting, massive, or more or less den-
dritic masses composed of a large number of small individuals
or polypides, each of which is enclosed within a chitiuous or
in some cases partially calcified investment, the zooecium, from
the mouth of which the anterior portion of the body bearing
the lophophore may be protruded. This outer investment or
ectocyst (Figs. 115 and 116, ec) is lined upon its interior sur-
face by a layer of ectoderm-cells, within which is a delicate
peritoneal lining, these two layers together constituting the
true body-wall or endocyst (Fig. 115, en] practically destitute
of muscle-tissue, though a sphincter is usually present at
the mouth of the cup, which may thus be closed over the re-
tracted polypide.

A more or less spacious ccelom (Fig. 115, co) is present in
the majority of forms, containing a hsemolymph and tra-
versed by a number of muscle-strands (m) which may be
aggregated into special retractor bands ; but in one order, the
Endoprocta (Fig. 114), these are wanting and indeed the
coslom is reduced to a very small space between the body-
wall and the digestive tract. This latter structure has the
characteristic U- or Y-shaped form and presents but little
differentiation into special parts, though an oesophagus,
stomach, lined with glandular so-called liver-cells, and rectum
may be distinguished. An anus (Figs. 115 and 116, «) is
always present and may be situated either within or Avithout

256

INVERTEBRATE MORPHOLOGY.

the area enclosed by the lophophore. The nervous system
is exceedingly simple, consisting of a single ganglionic mass
(Figs. 114 and 115, ce) lying between the mouth and anus,
nerves ramifying from it to the various parts of the body.
The only sense-organs that have been detected are situated
upon the free portions of the body, more especially on the"1
lophophoral tentacles (t), and are represented by scattered
ectoderm al cells each of which bears a strong ciliuin and is
in connection with a nerve-fibre ; they have been assigned a
tactile function, though it seems probable that they react to
stimuli of various kinds and have a much more generalized
function.

The arrangement of the excretory and reproductive organs
varies considerably in different forms and may be more con-
veniently described in connection with the various orders.

1. Order Endoprocta.

This order contains but a small number of forms, which,
with one exception, Urnatella, are marine in habitat. They

all possess the power of reproducing
by budding, colonies being thus formed,
as in Pedicellina, Ascopodaria, and other
genera with the exception of Loscosoma,
in which the buds separate completely
from the parent at a relatively early
stage of their development. Each in-
dividual (Fig. 114) is a cup-shaped
structure, prolonged posteriorly into
a stalk (st) and upon the rim of the
cup, which represents the lophophoral
fold, or slightly below it on the inner
FIG. IU.-SINGLE INDI- surface, the tentacles (t) are arranged
VIDUAL OP Pedicellina in a circle surrounding a depression,

the vestibule, into which open both
the mouth and the anus, the situation
of the latter opening within the circle
of tentacles having suggested the name
given to the order. The tentacles can
be coiled in a circulate manner, so as to lie completely

-ce

-st

(after NITSCHE).
ce = ganglion.
I = so-called liver,
s = stomach.
st — stalk.
t = tentacles.

TYPE PROSOP7GIA. 257

within the vestibule, and the rirn of the cup can be closed
over them, owing to the presence in it of a circular band of
muscle-fibres.

The entire body is covered by a delicate cuticular ectocyst
similar to the cuticle of the Annelids, below which is the
ectoderm containing numerous gland-cells, as well as scat-
tered sensory hair-bearing cells which, however, have not been
found to exist in some genera (Ascopodaria). Scattered mus-
cular fibres occur in the body-wall, but they do not as a rule
reach an extensive development.

The co3lom is of very slight extent and in Loxosoma is re-
placed by a gelatinous matrix enclosing branching cells and
muscle-fibres and recalling the parenchyma of the Turbel-
laria. Imbedded in this parenchyma is the U-shaped diges-
tive tract, beginning with the mouth situated in the vestibule
and overhung by a well-marked epistome. The mouth leads
into a narrow oesophagus lined by ciliated columnar cells,
and opening below into a saclike stomach (Fig. 114, s)
which forms the lower transverse portion of the U. The
cells of its anterior (vestibular) wall (?) are large and destitute
of cilia, and contain numerous granules, on which account
they have been termed " liver-cells." The intestine forms
the ascending limb of the U, and like the oesophagus is cili-
ated, opening into the vestibule at the summit of a well-
marked papilla.

The nerve-ganglion lies below the floor of the vestibule
between the epistome and the anal papilla and is a single
dumb-bell-shaped structure from which from one to three
nerves arise on each side, branching to be distributed to the
tentacles and muscles of the body.

A single pair of nephridia occurs, opening into the vesti-
bule, and each is composed of a number of perforated cells,
the lumen being ciliated. It is doubtful whether a flame-cell
occurs at the inner extremity as in the Annelid head-kidneys,
which otherwise they resemble. Most of the Endoprocta seem
to be bisexual, though Pedicellina is perhaps hermaphrodite.
The reproductive organs arise from the mesoderm of the body-
wall, forming masses projecting into the parenchyma, and are
provided with special ducts which either remain independent

258 INVERTEBRATE MORPHOLOGY.

of each other (Pedicellinci), or unite together to form a single
tube and open into the vestibule, between the epistonie and
the ganglion.

2. Order Ectoprocta.

•

The order Ectoprocta includes the great majority of forms
which are referable to the class Polyzoa. They are without
exception colonial forms of small size in which the tentacles
are arranged either in a circle or in the form of a horseshoe
surrounding the mouth, the anus being invariably situated,
contrary to the arrangement in the Endoprocta, outside the
limits of the lophophore. The tentacles, too, when retracted
are not flexed or coiled as in the Endoprocta, but are simply
approximated to form a bunch, each tentacle being straight
and parallel to its fellows.

The most characteristic peculiarity of the Ectoprocta,
however, is the power which they possess of withdrawing or
retracting the anterior portion of the body with its crown of
tentacles within the posterior part (Fig. 115). This latter
portion is enclosed in the ectocyst (ec) to which the body- wall
is closely adherent and which forms a chitinous or, in some
cases, more or less calcareous cell, termed a zooecium. At the
mouth of the cell the cuticle becomes suddenly exceediugly
thin, so that the anterior portion of the body is quite mobile,
and by means of special retractor muscles (TO) may be with-
drawn within the zooacium. The retraction is a process of in-
vagination, similar to what occurs in the withdrawal of the
pharynx of the Annelida, the most anterior lophophoric part
of the retracted portion not, however, sharing in the invagina-
tion : the whole process indeed is similar to what may be
obtained when one finger of a glove is caught from within
somewhat less than half way from the tip and drawn down
towards the palm ; half of the lower portion will thus be in-
vagiuated within the other half, while the tip of the finger
remains uninvagiuated.

The portion of the body-wall enclosed by the ectocyst is
thin, its longitudinal muscles being for the most part sepa-
rated in the form of bands traversing the coslom and function-

TYPE PROSOPYGIA.

259

ing as retractors, while the circular muscles are specially de-
veloped as a rule only around the mouth of the cup, which
may by their action be closed over the retracted tentacles.
A relatively spacious coalom, containing a colorless corpuscu-
lated haeinolyrnph is present, and is lined by flattened perito-

ec

en

FIG. 115. — DIAGRAM OF THE STRUCTURE OF Alcyonidium albidum (after

PROUHO).

a = aims. io = interteutacular organ.

ce = ganglion. m = retractor muscle.

co = cuilom. oi) = ovary

ec = ectocyst. t = tentacles.

en = endocyst. te = testis.

neal cells, some of which bear tufts of cilia. The intestine
has a characteristic Y-shape (Fig. 116), its posterior portion
being prolonged backwards to form a c?ecal pouch, from the
extremity of which a band or plate, the funicidus (Fig. 116, /),
containing in some cases muscular fibres, and lined by perito-
neal cells, arises, and passes backwards to be inserted into
the ectocyst posteriorly. The anus (a), as • already stated,
opens on the anterior surface of the body, outside the limits
of the lophophore, and between it and the mouth lies the
nerve-ganglion, which is frequently hollow and sends off
nerves to the various portions of the body

260 INVERTEBRATE MORPHOLOGY.

As in the Eiidoprocta, a heart and a blood vascular system
is entirely wanting. Special excretory organs seern to "be
wanting in the marine Ectoprocta, excretion being performed
apparently by the hrernolymph-corpuscles and other meso-
derin- cells, especially those of the funiculus, as well as by
the granular cells of the stomach and csecal pouch. In Cris-
taiella, a fresh- water form, however, a pair of ciliated canals
opening into the coeloin by ciliated funnels have been de-
scribed, and presumably are excretory in function, the two
canals uniting together to open to the exterior by a single
pore situated between the mouth and anus. Up to the pres-
ent, however, these structures have not been observed in
other forms and apparently they do not exist in the marine
forms.

In some of these latter (Alcyonidium, etc.), however, a cili-
ated tubular structure, which communicates at one end with
the ccelom and opens to the exterior between the tentacles
at the other, occurs and has been termed the intertentacular
organ (Fig. 115, io). It suggests a nephridium in its relations,
but apparently does not possess an excretory function, but
serves as an exit for the reproductive elements to the exterior.
In other marine forms and in all the fresh-water genera such
special reproductive ducts have not been observed, and the
mode of escape of the sexual products in these forms is still
unknown. Many of the Ectoprocta are hermaphrodite, the
ova and spermatozoa (Fig. 115, ov, te) arising from the peri-
toneal rnesoderm, frequently from that surrounding the
funiculus. Whether, however, herniaphroditisin is a charac-
teristic of the order or not is a point as yet undecided.

1. Suborder PliylactolcBinata.
The members of this suborder are exclusively inhabitants

•/

of fresh water and are characterized by the tentacles being
arranged in a horseshoe-shaped manner (except in the genus
Fredericdla, where they form a circle), and by the occurrence
of a well-developed lobe or epistome overlapping the mouth.

The colonies assume various shapes in different genera,
being sometimes dendritic and incrustiug stones or other

TYPE PROSOPTOIA. 261

bodies, as in FredericeUa, or formiug compact masses, as in
Alcyonella arid Lophopus, or even being capable of motion, as
in Cristatetta. In some forms, e.g. Lophopus, the ectocjst pos-
sesses a gelatinous consistency, though usually it is chitinous,
and the various zooecia are in free communication with one
another, not being separated by transverse partitions.

In addition to multiplying by the usual processes of bud-
ding and by ova, the Phylactolsemata develop upon the
fuuiculus special internal buds, termed statoblasts, which are
enclosed within dense chitiuous capsules. These are set free
by the dying and disintegration of the parent and, being pro-
tected by the resistant capsule, retain their vitality under
conditions, such as cold and dryness, that destroy the adult
individual. They are evidently a special provision for the
perpetuation of the species developed in accordance with the
fresh-water habitat, in which the organisms are exposed to
various conditions not apt to be met with in the ocean ; it is
interesting to note in this connection the occurrence of gem-
mules in the fresh-water sponges which are strictly compara-
ble to the statoblasts and have a similar significance.

2. Suborder Gymnolcemata.

The Gymnolaemata are distinguished from the Phylactolre-
mata by being, with the single exception of the genus Paludi-
cella, marine in habitat, by the tentacles being arranged in
the form of a circle, and by the invariable absence of an epi-
stome.

As in the Phylactolsemata, the colonies vary greatly in
form, being in some cases encrusting, Membranipora, Flus-
t/'ii, in others branching, Scrupocettaria, or in others again
massive, Alcyomdium ; and furthermore the ectocyst pre-
sents varying degrees of consistency, being frequently chitiu-
ous, but occasionally somewhat gelatinous or calcified to a
greater or less extent. The zooecia are not in free communi-
cation with each other, but each is closed below or posteri-
orly by a transverse chitiuous plate in which perforations are
said to be present, though doubt has recently been thrown
upon their existence as perforations. In shape, too, the
zooecia like the colonies vary greatly, especially so far as their

262

INVERTEBRATE MORPHOLOGY.

- rm

t

av

mouths are concerned, and it is possible to divide the Gyrnuo-
Iseinata into three groups or tribes, based upon these differ-
ences. In the tribe Cyclostomata the zooecia are usually
cylindrical, and the mouth is circular and destitute of any
appendages ; in the Ctenostomata the mouth is closed during
retraction by a series of bristles which surround it (Alcyonid-
iiini) ; while in the Ckilostomata, in which the ectocyst is
usually firm and frequently calcified, the mouth is closed by a

lid, the operculum, furnished with spe-
cial muscles (Bugula, Membranipora}.
In this last-named tribe a poly-
morphism of the individuals coinpos-
_ av ing a colony is frequently found.
Scattered among the ordinary indi-
viduals otheus, the Avicularia (Fig.
116, av), may be found having the
appearance of a bird's head, the
lower beak being fastened to the head
by a hinge and having inserted into
it strong muscles ; bunches of sensory
hairs are also present, and when these
are stimulated the lower beak is
rapidly snapped against the upper
and the stimulating organism thus
caught. There can be little doubt
but that these Avicularia are specially
modified individuals whose head and
upper beak represent the ordinary
individual, while the lower beak inav
possibly be the equivalent of the
operculum ; physiologically they have
been usually regarded as specialized
for the purpose of catching food for
the ordinary individuals, but it is not
improbable that their services may
rather be of a cleansing nature, re-
moving from the colony particles of

dirt and the excreta, which by accumulating might interfere
with the proper function of the tentacles. Another polymor-.

1- OC:

bb

FIG. 116. — PORTION
COLONY OF Bugula.

OF A

a = amis.
av — aviculavia.
bb = brown body.
ec = ectocyst,

/ =- fmriculus.
oc = ovicell.
rm = retractor muscle.

t — tentacles.

TYPE PROSOPYGIA.

263

phic form is known as the Vibracula, and consists of a slender
filament movably articulated to a rounded structure corre-
sponding to the head of the Avicularia ; the filaments wave
continually to and fro and are probably tactile in function.
In many forms, too, in the neighborhood of the mouths of the
ordinary individuals sac-like pouches occur, in which the
ova undergo their development. These structures, known as
ovicells (Fig. 116, <>c], or ocecia, have also been considered modi-
fied individuals, but seem rather to be organs of the ordinary
individuals, arising as a pouching of their walls. Finally, not
infrequently certain individuals relinquish their nutritive

cal

B

FIG. 117. — A, LAUVA OF Pedicellina (after HATSCHEK) ; B, Cyphonautes (after

PROUHO).

ad = adhesive urguii. s = stomach.

cal — calotte. sh = shell.

cor = corona. pyr = pyriform organ.

function and serve as root-like anchors or stem-like supports
for the entire colony.

As regards the internal structure of the Gymnolaemata it
is unnecessary to add anything to what has already been
stated in describing the general characteristics of the order
Ectoprocta.

Development of the Polyzoa. — The larva of Pcdi«'Uut,a (Fig.
117, A), which may be taken as a type of the Endoprocta, is a
somewhat dome-shaped organism, the summit of the dome
being occupied by an apical thickening (cal) bearing a tuft of
cilia, while at the margin there is a stout ciliary baud, the

264 INVERTEBRATE MORPHOLOGY.

corona (cor). The cavity of the dome is occupied by the U-
shaped digestive tract (s), the mouth and anus both opening
within the circle formed by the corona, a deep depression,
the vestibule, lying between the two. In the ccelom above
the floor of the vestibular depression are a number of meso-
derm-cells, and also a ciliated canal composed of a single row
of perforated cells and probably excretory in function. Upon
one surface of the larva between the marginal ciliated baud
and the apical thickening is a peculiar glandular organ termed
the cement-gland (pyr), around the mouth of which are situ-
ated a number of strong cilia.

The development of this larva into the adult form is accompanied by a
number of remarkable changes, which in their details and significance are
not yet thoroughly understood. The larva settles upon the ventral or oral
face and shortly thereafter one wall of the vestibule becomes pushed over
towards the other and eventually unites with it, the original vestibular
cavity becoming divided into two portions, one of which remains in con-
nection with the surface of fixation and later degenerates, while the other
has opening into it the mouth and anus, though the former opening at
about this period becomes closed. Later a remarkable rotation through
180° of the digestive tract, together with the portion of the vestibule in
connection with it, occurs, the portion of the body immediately above the
point of fixation elongating to form the stalk of the adult, becoming at the
same time filled with mesodermal tissue. The mouth opens again into the
vestibular cavity, the tentacles arise from the wall of the cavity which
later opens to the exterior, the adult form being thus assumed. The fate
of the apical thickening and of the cement-gland is uncertain ; they have
been described as degenerating without taking any part in the formation of
the adult organs, though it has been suggested that the apical thickening
may give rise to the nervous system of the adult.

The form of the larvee in the Ectoprocta is subject to con-
siderable modification. In Membranipora and some other
genera the larva is kno\vn as the Cyphonautes (Fig. 117, />">,
having been so designated before its life-history was eluci-
dated. It has a somewhat triangular outline and is character-
ized by being enclosed in a bivalved chitiuous shell (sh). At
the apex of the triangle is the apical thickening (cal), with its
elongated cilia, while around the base there is to be found the
corona (cor). A well-developed digestive tract is present, both
the mouth and anus opening upon the basal surface of the tri-
angle, and therefore within the area enclosed by the corona.

TYPE PROSOPJGIA.

265

This vestibule is a deep depression of the oral surface of the
larva, differing from that of the Pedicellina larva only in hav-
ing an arch-like thickening of its walls (only one side of the
arch is represented in the figure) which imperfectly separates
an oral portion of the vestibule from a posterior or anal
portion, a glandular depression situated in the roof of this
latter portion constituting the adhesive organ (ad}. In front
of the oral vestibule is situated a ciliated depression from
which projects a tuft of long cilia and which appears to
correspond to the cement-gland of the Pedicellina larva and
to a glandular structure in the more modified Ectoprocta
larvae, known as the pyriform organ (pyr), by which name it
may be known here. The similarity of this larva to that of
Pedicellina is clear, the details of organization of the two forms
agreeing part for part ; in other Ectoprocta, however, great
differences are to be found. In the genus Uugitla, for exam-
ple, the larva (Fig. 118) is a barrel-shaped organism at the one
extremity of which is a thickening,
the calotte (col), which appears to
correspond, in part at any rate, to
the apical thickening or dorsal organ,
as it is sometimes termed, of Pedicel-
lina and Cyplionautes. The sides of
the barrel are formed by a circle
of elongated cells forming the corona
and equivalent to the marginal
corona of the other larvae ; it does
not, however, form a simple band in FIG 118 _LARVA OF Bugula

Bligula, but its Cells are much flabellata (after BABBOIS).

shorter on one of the faces of the Letters as in Fig. 117.
embryo than elsewhere, producing

a well-marked groove at the apex of which lies the pyriform
organ (pyr} whose homologies in Cyphonautes have already
been pointed out. A peculiarit}' of this larva is the entire
absence of a digestive tract, the lower end of the barrel being
occupied by a depression, the adhesive organ (ad).

Between such a larva as that just described, entirely des-
titute of a digestive tract, and that of Cyplionautes intermediate
stages occur, as for instance in the larvae of the Cyclostomata,

266 INVERTEBRATE MORPHOLOGY.

in which the digestive tract is represented by a yolk-laden
mass of tissue, having little resemblance, however, to the
differentiated eiidoderinal tube of Pedicellina. The occur-
rence of such forms, however, shows that the absence of the
tract in Jjugula, etc., is the result of progressive degeneration,
such larvae as those of Pedicellina and Cyphonautes represent-
ing the primitive condition more nearly than the remarkable
larva of JJugula.

The transformation of the Ectoproctous larvae into the adult is even
more remarkable than that of Pedicellina. Fixation takes place by the
oral surface, the adhesive organ being evaginated for the purpose, and is
succeeded by a degeneration of the corona and pyriform body. In Cypho-
nautes the digestive system completely degenerates likewise, a new one
being formed later, the tissue in the neighborhood of the apical thickening
taking a prominent part in its formation. In those larvae which are desti-
tute of a digestive tract one, corresponding to the second one of Cyplio-
nautes, develops after fixation, likewise from the tissue of the apical thick-
ening. The exact method of this regeneration, for so it may be con-
sidered, can hardly be described here without leading to a recapitulation
of details too minute for the scope of this work. It may be remarked,
however, that the phenomena do not seem to merit the designation of an
alternation of generations, as might at first sight be supposed, but are
rather simply a metamorphosis the significance of which is at present
decidedly obscure.

Budding of the Polyzoa. — As already stated, colony formation by bud-
ding is a characteristic feature of the Polyzoa, Loxosoma alone not pre-
senting this method of growth, though like other forms it reproduces by
budding, the buds, however, separating at an early stage from the parent.
In the Endoprocta a stolon arises from near the point of fixation of the
primary individual which develops from the ovum, mesoderm tissue from
the stalk of this individual migrating into the stolon, but there is no pro-
longation into it of the parental endoderm. At a more or less definite part
of the stolon the ectodermal cells thicken and later on invaginate towards
the centre of the stolon. This invagination becomes surrounded by meso-
derm already present in the stolon, and later differentiates into two cavi-
ties, one of which retains connection with the exterior and forms the
vestibular chamber, from the walls of which the tentacles develop, while
the other becomes the digestive tract, its original connection with the ves-
tibular cavity becoming the anus, the mouth developing later as a depres-
sion of the floor of the vestibular cavity which joins the stomach. It
is interesting to note that from the ectodermal invagination the nervous
system as well as the digestive tract develops.

In the Ectoprocta practically the same method obtains in the budding,
though the stolon is represented by the tip of a branch or even by the

TYPE PROSOPTGIA. 267

tissue in the neighborhood of the mouth of the zooecium. The colony
resulting from continued budding becomes accordingly as a rule much more
compact than in the Endoprocta, each polypide being more or less approxi-
mated to its predecessor.

Closely related to the process of budding is that of regeneration, also
of frequent occurrence among the Polyzoa. Among the Endoprocta Pedi-
cellina shows the process in a periodical though not simultaneous moult-
ing of the polypides, new ones developing from the tip of the stalk which
bore the amputated polypide. Here, as in ordinary budding, the tissues
concerned appear to be ectoderm and mesoderm, the stalk containing no
prolongation of the original endoderm.

In the Ectoprocta, however, regeneration is carried to a greater extent.
In examining any colony of Biujula, for example, in some of the zocecia in
addition to the polypide a brown mass may be seen, the so-called " brown
body" (Fig. 116, bb) ; in others the brown body may be seen without
any distinct polypide. This body is the result of the degeneration of the
digestive tract and other organs of the original polypide, only its body-
wall or endocyst persisting, from which new organs are developed and the
polypide regenerated. The significance of this process is not clear, but it
has been suggested that it stands in relation to the process of excretion,
the formation of the brown body occurring in forms which do not possess
any special excretory organs. It is now known that in the marine Ecto-
procta the excretory products are taken up in part by the cells of the
stomach and caecal pouch, a fact which seems to harmonize with the sug-
gested significance of the brown body.

The formation of a new polypide from ectoderm and mesoderm appar-
ently is a difficult fact to explain on the theory of the germ-layers. It is
possible, however, to regard the tissue from which buds arise as undiffer-
entiated embryonic tissue passed on from polypide to polypide and trace-
able back to the embryonic tissue of the ovum. In the formation of each
polypide a certain amount of the tissue becomes differentiated, but some
still retains its embryonic character, a continuation of the budding process
being thus possible.

Affinities of the Polyzoa. — There seems to be little room for doubt but
that the Endoprocta represent more nearly the original Polyzoa than do
the Ectoprocta. Their colony formation is of a more simple form than
that of the other group, they possess nephridia which are wanting in the
majority of the Ectoprocta, and their development is much simpler, the
highly modified larva of the mai'ine Ectoprocta having undoubtedly been
derived from one approximating in structure that of Pedicellina, Cypho-
nautes representing a stage in the evolution.

Similarities have been traced between the Pedicellina larva and the
Annelid Trochophore, and it is not improbable that this may have been the
true derivation of the group, in which case the Polyzoa are to be regarded
as forms which have never possessed any traces of metamerism, but stand
in about the same relationship to the Annelida as do the Rotifera.

268 INVERTEBRATE MORPHOLOGY.

Another view, however, which has had ardent supporters is that
which recognizes a relationship between the Polyzoa and Phoronis. There
is a lophophore in both, likewise a U-like bending of the digestive
tract, and the nephridia of Phoronis may be considered comparable to
those of the Eudoprocta. But here the similarity ceases. The anus in
Phoronis is outside the limits oi' the lophophore and is comparable in
position with that of the Ectoprocta, a point which tells against this
phylogeny since these forms have been shown to be less primitive than the
Endoprocta. If, however, this phylogeny should prove to be correct, it
will show a descent for the Polyzoa from metameric Annelids, through the
Gephyrea, since it is to this group that Phoronis seems to be most nearly
related.

CLASS BRACHIOPODA.

The Brachiopoda constitute a very well-defined group
whose present poverty in species is in striking contrast to its
great development during Palaeozoic times. Like the Poly-
zoa they possess a tentaculate lophophore (Fig. 120, Ip) which
usually takes the form of two exceedingly elongated, some-
times spirally-coiled, arm-like processes projecting, one on
either side, from the anterior portion of the body, and fur-
nished upon their outer or posterior border with tentacles.
The bod}r is usually somewhat short and stout, and prolonged
posteriorly into a peduncle (pe) or stalk which is in some
cases at least provided with adhesive papillae and serves as
an anchor.

The most characteristic feature of the Brachiopoda is,
however, the presence of a bivalved shell (Fig. 119) similar
to that of a bivalve Mollusk, with which forms the Brachio-
pods were until comparatively recently associated. From
near the base of the peduncle, upon the dorsal and ventral
surfaces of the body, a fold of the body-wall is found, which
contains a cavity in communication with and indeed in reality
a portion of the coalomic cavity. These two folds are of suffi-
cient size to enwrap or enclose the body and the lophophore
and are termed the mantle-lobes (Fig. 120, m), the space between
them and the bod}r being known as the mantle-cavity. They
subserve largely if not entirely the function of respiration,
the portion of the coelom which they contain being more or
less divided up into a system of lacume through which tbe

TYPE PKOSOPYGIA.

269

hseinolyniph circulates. Upon the outer surface of each
inaiitle-lobe, and formed by it, is a valve of the shell, composed
of a certain amount of organic matter, but largely of carbonate
of lime. Since the mantle-lobes are dorsal and ventral in
position, so too are the valves of the shell, and consequently
their hinge-line is posterior and their mouth anterior. In
a number of forms, which may be grouped together as the
suborder Testicar dines, the shells along the hinge-line are
provided with interlocking teeth, a true hinge being present,
the peduncle in these cases perforating a backward prolonga-
tion or beak of the lower valve. In a few genera, however,
forming the suborder J^cardines, no such hinge is present,

FIG. 119. — DORSAL VALVE OF Spirifer, SHOWING ARM SKELETON (after LEUNIS).

the peduncle passing out between the two valves of the shell.
Special muscles are present extending from one valve of the
shell to the other and are necessary both for the opening and
the closing of the shell, and furthermore it should be noted
that except for a slight difference in concavity both valves of
the shell are similar and symmetrical.

It will be seen by comparing the various facts mentioned
here with what is said on p. 327 regarding the shell of the
bivalve Mollusks that the structures in the two groups are
very different. This difference is emphasized by the presence,
in the majority of the Testicardiues, of a calcareous support
for the coiled lophophore attached to the inner surface of the
dorsal valve (Fig. 119). It consists of a pair of calcareous
rods which project downwards and forwards, uniting to form
a transverse arch, and may give rise on each side to a spirally-
wound process upon which the coils of the lophophore rest.

The body-wall is composed of an outer layer of ectoderm

210 INVERTEBRATE MORPHOLOGY.

from which numerous papillse or in some cases branching
processes arise, projecting into corresponding cavities or
tubes in the substance of the sholl-valves. Below the ecto-
derm is a more or less homogeneous connective tissue con-
taining cells and recalling the mesogkea tissue of the Coelen-
terates. Scattered muscle-fibres, arranged transversely and
longitudinally, occur in the mantle-lobes and in the body-wall,
but there are no definite muscular layers such as are found in
the Annelida, though the longitudinal muscles of the peduncle
are well developed. Special muscles, which cannot be con-
sidered differentiations of the musculature of the body-wall,
traverse the ccelom from one valve of the shell to the other,
one pair, the divaricators, being inserted in such a way as to
cause by their contraction a separation of the two valves,
while another pair, the adductors (Fig. 120, am), approximate
them. Other muscles also occur, such as the adjustores,
which produce lateral movements of the shell-valves, and pro-
tractors and retractors (Fig. 120, rm) of the peduncle.

The coelom is lined by a peritoneal epithelium and con-
tains a corpusculated hsemolymph which is driven about
through the coelomic spaces, and the lacmife in the mantle-
folds and the lophophore which communicate writh them, by
the contractions of the body-wall and the musculature, there
being no distinct heart or blood-vessels. A dorso-ventral
mesentery which slings the intestine is present and divides
the body-coelom more or less completely into two lateral
chambers, and furthermore two transverse partitions or dis-
sepiments occur in several forms and divide the coelom into
anterior, middle, and posterior compartments, an arrangement
recalling the metamerism of such a form as Sagitta (p. 187).

The mouth opens at the anterior end of the body between
the two lophophoric arms and leads into a short, somewhat
muscular oesophagus, which posteriorly communicates with a
stomach-like dilatation (Fig. 120, i) into which open the
ducts of one or more pairs of branching tubular glands — the
so-called liver or digestive glands (I). Behind the stomach lies
the intestine, which, in most of the Ecardines, such as Lingula,
bends upon itself and opens into the mantle-cavity in the
mid dorsal line near the anterior end of the body. In Crania,

TYPE PEOSOPYGIA.

271

however, it opens posteriorly, while in Terebratuliaa, Argiope,
and Waldlieimia, in fact in all the Testicardines, it ends
blindly, the anus being wanting.

The nervous system consists of an oesophageal ring lying
in the connective tissue substance, the lower portion being in
connection with the ectoderm and slightly swollen, represent-
ing probably the subcesophageal ganglion of the Annelida ;

ov

ue

am

pe

FIG. 120. — STRUCTURE OF Terebratulina septentrionalis.

am = adductor muscle. ne = nephridium.

i = intestine. ov = ovary.

I = liver-lobes. pe = peduncle.

Ip = lophopbore. rm — retractor muscle.
m = mantle. s = shell.

a similar swelling in the dorsal portion of the ring represents
the suprao3sophageal ganglion, and in addition there are usu-
ally two further lateral ganglion-like swellings. Nerves pass
off from the upper ganglion to the lophophore and other
regions, and from the lower one to the mantle, muscles, etc.,
both sets terminating in the superficial layers of the lopho-
phore-tentacles or of the mantle in a network of ganglion
cells and fibres. No trace of a ventral nerve-cord in addition
to the suboesophageal ganglion is present.

Sense-organs are but poorly developed, neither eyes nor
auditory organs occurring. The tentacles on the lophophoric
arms are in all probability sensory, as indicated by their rich
nerve-supply, and the papillae of the mantle-ectoderm which

272 INVERTEBRATE MORPHOLOGY.

project into the canals of the shell have been stated to be
sensory, containing an axial nerve-fibre terminating in a sen-
sory cell.

The nephridia (Fig. 120, ne) are represented by two or
four (Rhynchonella) funnel-shaped short tubes which open by
a fimbriated mouth at one extremity into the coelomic cavity
and, rapidly narrowing towards the outer end, open by a small
pore into the mantle-cavity. In addition to their probable
excretory function, these structures, as in some of the Anne-
lida, serve also as ducts for the passage to the exterior of the
reproductive elements. These are derived from the coelomic
peritoneum and form branching masses (Fig. 120, ov) lying in
some cases in the coalomic spaces of the mantle, or in addition
extending into the body, as in most Ecardiues, or, as in Tere-
bratulina, confined to this region. Most of the Brachiopods
are bisexual apparently, though it is possible that Lingula
and perhaps some other forms may be hermaphroditic, the
male and female elements maturing at different times.

Development and Affinities of the Brachiopods. — The Testi-
cardines are characterized by the occurrence of a free larval
stage destitute of a shell. In Argiope (Fig. 121) it is appar-
ently divided into four segments, the most
anterior of which bears two eye-spots and
assumes an umbrella-like form, long cilia
projecting from the margin. The third
segment develops two folds which enclose
the posterior segment and bear on their
margin bunches of setae inserted in seta-
sacs and recalling the setae of certain Annelid
larvse. After swimming about for a time
the larva settles down and fastens itself by
the posterior segment and the mantle-lobes
FIG. 121. --LARVA turn forward to enclose the anterior seg-

OF Argiope (after ml

KOWALEWSKI). ments. 1 he posterior segment becomes the

peduncle of the adult, and the shell de-
velops on the surface of the mantle-lobes, whose bunches of
setae are thrown off. The mouth makes its appearance only
after fixation just ventral to the eye-spots, and around it there
develops a ring of tentacles placed somewhat obliquely, and

TYPE PROSOPYGIA. 273

later elongating laterally to form the coiled lophophore with
its numerous tentacles.

The early stages of the development of the Ecardines is
not known, but in Lingula the larva is free-swimming long
after the shell has formed, the peduncle being late in develop-
ing. In this form also the lophophore arises as a circle of
tentacles surrounding the mouth and subsequently elongates
laterally.

The affinities of the Brachiopods have long been an open question.
They were by early writers regarded as Mollusca, later as Annelida or
closely related to that group, but are now usually considered to be more
nearly related to the Polyzoa than to any other forms and to be most
properly associated with them, the general likeness of a young Lingula, for
instance, to a Polyzoan being very striking. The presence of the mantle-
lobes and the shell seem to mark the Brachiopoda as something far removed
from the other members of the type Prosopygia, but it must be remembered
that in the larval Ectoproctous Polyzoa the corona behaves in a manner
closely similar to the Brachiopod mantle and it is not impossible that the
two structures may have something in common.

Another distinguishing feature of the Brachiopods is the indication of
a segmentation. The presence of two dissepiments and in Rhynchonelta of
two pairs of nephridia certainly suggests metamerism, but objection has
been raised to the dissepiments having any metameric significance, on the
ground that they do not bear the proper relationships to the body axis to
be regarded as comparable to the dissepiments of the Annelida. It has
been stated by some authors as a characteristic of the Prosopygia that
their body axis is bent upon itself so that the two ends are approximated
and one surface, the dorsal, is almost obliterated, while the other, the
ventral, is very much enlarged, as seems to be actually the case in
Phoronis. It must be remembered, however, that the terms dorsal and
ventral are not to be defined by reference to the digestive tract alone, but
other structures have also to be taken into consideration. Thus it is quite
possible that in the Polyzoa the approximation of the mouth and anus indi-
cates simply a bending of the digestive tract and a migration forwards of
the anus and not necessarily a bending of the body axis; and the varying
position of the anus in the Ecardinate Brachiopods tends to support this
idea, the body axis in Crania with a terminal anus certainly being similar
to that of Lingula, in which the anus lies far forwards. In this connec-
tion, too, the arrangement in Sipunculus is of interest, the nerve-cord show-
ing the usual relations to the body axis, while the digestive tract is bent
upon itself and the anus opens far in front of the posterior extremity. In
the Sipunculacea there can be no question of a difference of the body axis
in the various forms, and it seems probable that the supposed bending of
the body axis in the Prosopygia has not really occurred, but that there

274 INVERTEBRATE MORPHOLOGY.

has been simply a bending of the digestive tract and a migration forwards
of the anus.

If this be the correct way of regarding the matter, then there is no
reason for disputing the homology of the dissepiments of the Brachiopoda
with those of the Annelida, and the idea that they represent a metamerism
is borne out by the arrangement of the two pairs of nephridia of Rhyn-
chonella. The question is then, Does the metamerism of the Brachiopods
indicate a descent of the Prosopygia from metameric ancestors, i.e., from
Annelids through Gephyrean-like forms, or is it a structural feature inde-
pendently acquired by the Brachiopods ? The evidence at our disposal is
not sufficient for the solution of this problem, and all that can be main-
tained is that a very close relationship exists between the Polyzoa and the
Brachiopoda.

SUBKINGDOM METAZOA.
TYPE PROSOPYGIA.

I. Class POLYZOA. — Small, usually colonial forms ; lophophore circular or
horseshoe-shaped ; no bivalve shell ; no mantle-lobes.

1. Order Endoprocta. — Mouth and anus both within the area enclosed

by the lophophore. Loxosoma, Pedicellina, Ascopodaria,
Urnatella.

2. Order Ectoprocta. — Anus outside the area enclosed by the lopho-

phore.

1. Suborder Phylactolcemata. — Fresh-water forms ; lophophore

usually horseshoe shaped ; epistome present. Frederi-
cella, Alcyonella, Lopliopus, Cristatella.

2. Suborder Gymnolamata.— Usually marine ; lophophore cir-

cular ; no epistome.
Mouth of zoceciuin without bristles or operculum (Cydo-

stomata). Crisia.
Mouth of zocecium usually surrounded by bristles which

close over it (Ctenostomatd). Paludicella, Alcyonidium.
Mouth of zocecium provided with an operculum (Chilo-

stomata). Membranipora, Buyula, Flnstra, Scnipocel-

laria.
II. Class BRACHIOPODA. — Non-colonial and of moderate size ; lophophore

usually arm-like and coiled into a spiral ; mantle-lobes

and bivalve shell present.

1. Order Ecardines. — Shell-valves not hinged ; peduncle protrudes

between the valves ; anus present. Lingnla, Crania.

2. Order Testicardines. — Shell-valves hinged ; peduncle when present

protruding through perforation in the ventral shell ; anus
wanting. Terebralula, Waldheimia, Argiope, Rltynclio-
nella.

TYPE PROSOPYG1A. 275

LITERATURE.

POLYZOA.

G J. Allman. A Monograph of the Fresh-water Polyzoa. London, 1856.

T. Hincks. ..-I History of the British Marine Polyzoa. London, 1880.

H. Nitsche. Bdtrage zur Kenntniss tier Bryvzot-n. Zeitschr. fur vvissensch.

Zoologie, xx, 1869 ; xxi, 1871 ; xxv, Suppl., 1875.
A. Hyatt. Observations on Polyzoa. Suborder Phylactolannata. Proceedings

Essex Institute, iv and v, 1866-68.
S. F. Harmer. On the Structure and Development of Loxosoma. Quarterly

Journ. Microscop. Science, xxv, 1885.
C. B. Davenport. Papers in Bulletin of the Museum of Comp. Zoology, xx,

1890 ; xxn, 1891 ; and xxiv, 1893.

BRACHIOPODA.

E. S. Morse. On the Systematic Position of the Brachiopoda. Proceed. Boston

Soc. Nat. History, xv, 1893.
W. K. Brooks. The Development of Lingula. The Chesapeake Zoolog. Labo-

• ratory. Scient. Results of the Session of 1878.
J. F. van Bemmelen. Untersuchungen uber den anatomischen und ?iistologischen

Ban der Brachiopoda testicardinia. Jenaische Zeitschr., xvi, 1883.
M. A. Schulgin. Argiope Kowalewskii. Ein Beitrag zur Kenntniss der Brachio-

poden. Zeitschr. fur wissensch. Zoologie, XLI, 1884.
H. G. Beyer. A Study of the Structure of Lingula (Glottidia) pyramidata.

Studies from the Biolog. Labor. Johns Hopkins University, ill, 1886.

F. Blochmann. Untersucliungen, liber den Bau der Brachiopoden. Jena, 1892.

276 INVERTEBRATE MORPHOLOGY.

CHAPTER XII.

TYPE MOLLUSC A.

WHILE the Annelida are characterized by an elongated
form of body, the Mollnsca present the opposite condition,
being compact, non-metameric organisms, though at the same
time primitively bilateral in the arrangement of their organs.
Upon the external surface of the body a cuticular secretion
is formed in which usually particles of carbonate of lime
are deposited, a calcareous shell being thus developed, which
encloses more or less perfectly the soft body, assuming,
however, very different forms in the various groups. It is
essentially a dorsal structure developed in the majority of
forms from a depression on the dorsal surface of the body
—the shell-gland (Fig. 122, /) — and in some forms maybe
entirely confined to this area. Usually, however, a circular
or bilateral fold of the body, the mantle (c), arises peripheral
to the margins of the shell-gland and extends downwards
towards the ventral surface, and the growth of the shell may
accompany that of the mantle-fold, so that the entire body is
enclosed by or may be retracted within the greatly-developed
shell. Even in cases, however, in which the shell is but
slightly developed the mantle-folds retain their development,
forming a marked structural feature of the Mollusca, and en-
closing a more or less spacious cavity, the mantle-cavity, in
^vhich lie the respiratory organs and into which the intestine
and nephridia and reproductive ducts open.

The body-wall is formed of an external layer of ectoderm,
below which a more or less thick layer of muscle-tissue is
found whose fibres sometimes show the arrangement in cir-
cular and longitudinal layers characteristic of the Annelida,
but usually the simplicity of this arrangement is interfered
with by a development of connective tissiie in which irregu-
larly-arranged muscle-bundles lie. Upon the ventral surface

TYPE MOLLUSC A.

277

of the body there is a special thickening of the muscle-tissue
to form a "foot" (Fig. 122, p), which assumes a great variety
of forms, and special muscles are developed for its protrac-

!/pe zl k 7

FIG. 122. — DIAGRAMS SHOWING THE ARRANGEMENT OF THE ORGANS IN AN

IDEAL MoLLUSK (after LANKESTEK).

a — tentacle. i = ctenidium.

b = head. k = reproductive pore.

c = margin of mantle I = nephridial pore.

d = margin of shell. in = anus.

e = edge of body. n and p — foot.

/= edge of shell depression. r = coelom.

g = shell. s = pericardium.
gc = cerebral ganglion. t = testis.

gpe = pedal ganglion. u = nephridium.

gpl = pleural ganglion. v = ventricle of heart.

h = osphradinm. zl = liver.

tion or retraction when this is necessary, as well as for the
closure of the shell in those forms (Pelecypoda) in which it is
a bilateral structure.

The ccelom is in some forms a relatively spacious cavity,
traversed, however, even in these cases by thin bands of con-

278 INVERTEBRATE MORPHOLOGY.

nective tissue, but more usually it is reduced to a system of
lacunar spaces (a so-called schizocoel) by the development of
muscle-bundles traversing it in various directions. A special
portion of it (the so-called euterocosl) is, however, always en-
closed in definite walls lined by a peritoneal layer of cells,
forming a cavit}', the pericardium (Fig. 122, s), which lies nor-
mally near the dorsal surface of the body, containing the
heart and having the inner ends of the nephridia (u) opening
into it. The blood vascular system consists of a primitively
three-chambered heart (Fig. 122, r) enclosed within the
pericardium and composed of a tubular muscular ventricle
and two wing-like auricles which open into the ventricle,
their openings being guarded by valves which prevent regur-
gitatiou. From the anterior and posterior extremities of the
ventricle aortae arise, which, however, as a rule soon lose
themselves in the coeloniic lacunae. There is thus no distinc-
tion between the blood and pseud-haemal fluids in the Mol-
lusca, since the blood vascular system is not closed. The
blood is a colorless fluid in which numerous amoeboid cells
float and which holds in solution a substance, hsemocyauiu,
which subserves a respiratory function in a manner similar
to the haemoglobin of the Vertebrata.

The heart is a systemic heart, as is usual in the Invertebrata, and con-
tains only aerated blood, which it propels through the lacunae of the body.
Returning from these, the blood passes either directly to the respiratory
organs or branchiae, or else a greater or less portion of it traverses first the
walls of the nephridia and then passes to the branchia?. From these, in
which it is aerated, it is received into the auricles, and on their contraction
is forced into the ventricle.

In some Mollusca respiration is carried on by the general
surface of the body, but such an arrangement must be re-
garded as the exception. As a rule special respiratory or-
gans are present in the form of one or more pairs of plume-
like processes (ctenidia) of the body-wall (Fig. 122, i) lying
free in the mantle-cavity. They have various forms in the
different groups, but consist essentially of a central axis con-
taining an afferent and efferent canal for the blood and bear-
ing a single or double series of filaments whose walls are thin
and whose ectoderm is ciliated, an interchange of the gases

TYPE MOLLUSC A.

279

of the blood with those of the water contained in the mantle-
cavity being thus readily effected, a renewal of the water con-
stantly taking place in consequence of the action of the ecto-
dermal cilia. The thin-walled niautle-fold is, however, a very
efficient adjunct to the branchiae in respiration, the spaces
within the fold being portions of the lacuuar ccelom and con-
seqiieutly containing blood ; indeed in some cases the mantle
assumes completely the respiratory function, the cteuidia
becoming rudimentary.

The digestive tract is a usually more or less coiled or con
voluted tube in which various regions may be distinguished.
In a few forms, characterized either by the slight develop-
ment of the mantle or its development as two lateral folds,
the anus is terminal in position, but when an extensive cir-
cular mantle-fold is developed the intestine bends upon itself
and opens upon the side of the body, more or less anteriorly,
into the mantle-cavity. Immediately behind the mouth chi-
tinous teeth (Fig. 123, Jij ) are usually developed in the wall of

st

FIG. 123. — BUCCAL MASS AND RADULA OF Helix (after HOWES).

ce = cerebral ganglion. re = radular cartilage.

lij = horny jaw. rd = radula.

im = intrinsic muscles. rd' = radular sac.

= pedal gland. st — opening of salivary gland.

the pharynx, and behind these a large muscular thickening
generally occurs, the buccal mass, in connection with which
is developed a characteristic Molluscan structure, the lingual
ribbon or radula (rd). The sides and floor of the phaiynx in
this region are largely thickened by the development in them
of muscular tissue (im). The thickening of the floor is usually
so extensive as to project into the pharyngeal cavity, forming

280 INVERTEBRATE MORPHOLOGY.

the so-called tongue, and in addition to the muscular tissue
two or more pieces of cartilage, the radular cartilages (re), are
frequently found in it. Covering the tongue is a stout chi-
tinous membrane, the basal membrane, which bears upon
its surface a usually enormous number of chitinous teeth
arranged in transverse rows, so that the basal membrane
and the teeth, together constituting the radula (rd), recall
somewhat the appearance of a flat tile. Behind the tongue
the floor of the pharynx is produced downwards and back-
wards into a pouch, the radula-sac (rd'), sometimes of con-
siderable length, into which the radula is continued on
the ventral wall, the cells (odontoblasts) which form the teeth
as a cuticular secretion, lying at the bottom of the sac. The
tongue, with its radula, can be protruded to a greater or less
extent from the mouth by special protractor muscles, and its
intrinsic muscles serve to give it a slow licking movement,
whereby the radula acts as in the manner of a file or rasp
upon the object with which it is in contact. Owing to this
action the radula is continually being worn away at its anterior
end, but is also continually being pushed forward upon the
tongue by the addition of new teeth to its posterior portion
at the base of the radula-sac.

In connection with the digestive tract various glands are
usually present, of which the most constant are the salivary
glands and the " digestive glands." The former open into
the pharynx and in some cases reach extensive development ;
their function for the most part is but little understood, but
in some predaceous Gasteropods their secretion has been
found to contain a considerable amount of free sulphuric acid
which probably serves to soften the calcareous shell of
Echinoderrns and other Mollusca which serve these forms as
food. The " digestive glands " open into a dilated portion of
the intestine, usually termed the stomach, and are usually
paired, voluminous, much-branched tubular glands wrhose
function is indicated by the name applied to them. They
seem to be the physiological representatives of the pancreas
of the Vertebrata, and to secrete digestive ferments which
are brought into contact with the food in the stomach.

The nervous sj'stem of the Mollusca (Fig. 124), in accord-

TYPE MOLLUSC A.

281

ot

pa

auce with the absence of metamerism, lacks the ladder-like
arrangement which characterizes the Annelida. Nevertheless
there are two gaugliouic masses, each in typical cases com-
posed of two ganglia which may be homologized with the su-
praoesophageal and the most anterior suboesophageal ganglia
of the metameric forms, and are known respectively as the
cerebral (Fig. 124, ce) and pedal (pe) ganglia. The former lies
above the oesophagus behind the buccal
mass and is connected by nerve-cords
termed connectives, surrounding the oeso-
phagus, with the pedal ganglion. The
cerebral ganglion gives off nerves which
pass to the eyes and otocysts (ot) and to
the tentacular structures of the head,
while the pedal ganglion receives its
name from the fact that it sends nerves
to the muscular mass forming the foot.
In addition to this system of nerves and
ganglia there is another system highly FIG 124>_DlAGRAM
developed in the Mollusca which would
seem to correspond to the visceral system
found in some other forms. It consists bu = buccal ganglia,
typically of a pair of pleural ganglia ( pi), ce == cerebral ganglion.

«,.,,. -,, ot = otocyst.

one of which lies upon either side of the pa = parietal gangliou

pharynx, being united by connectives pe - pedal ganglion,
with both the cerebral and pedal ganglia, pi =- pie n ml ganglion.
From each pleural ganglion a nerve-cord vi ^ visceral g^g^on.
passes backwards to unite with one or

more visceral ganglia (vi), situated below the intestine near its
posterior termination, and on each of these visceral cords a
ganglion occurs, the parietal ganglion (pa), from which nerves
pass to the gills, or rather to the sense-organ which is in con-
nection with them. The pleural ganglia innervate especially
the mantle and the body-wall behind the head, the visceral
ganglia send branches principally to the various viscera, while
the parietal ganglia, in addition to the branches which go to
the gills and their sense-organs, also assist in the inuervation
of the mantle.

Besides these principal ganglia, however, others connected

OF

NERVOUS SYSTEM OF
MOLLUSK.

282 INVERTEBRATE MORPHOLOGY.

with either the cerebro-pedal or pleuro-visceral system may
be developed, the most constant of which are the buccal gan-
glia (bu] which lie at the sides of or more usually below the
buccal mass which they innervate and are united by commis-
sures with the cerebral ganglia. Two nerve-rings in such
cases surround the oesophagus, i.e., that formed by the cere-
bro-pedal and that of the cerebro-buccal connectives.

This description has reference only to what may be con-
sidered a typical condition, and it must be remembered that
frequent modifications of it may occur. In the Gasteropods,
for example, in which, in accordance with the development of
a circular mantle-fold, the anus comes to lie on the anterior
portion of the body-wall, a peculiar crossing of the pleuro-
visceral commissures occurs in some cases, and as a result
what was originally the right parietal ganglion conies to lie
upon the left side of the body and the original left ganglion
upon the right side. Further consideration of this arrange-
ment may, however, be postponed until the Gasteropods are
under discussion. Mention should, however, be made here
of another not unfrequent modification of the typical arrange-
ment of the nervous system, which consists in the concentra-
tion of the ganglia and the shortening of the various connec-
tives. This may affect only the cerebral, pedal, and pleural
ganglia, bringing them into close approximation, or, as in some
Cephalopods, the visceral ganglia may also be carried forward
so that all the principal ganglia are united into a single lobed
mass closely surrounding the oesophagus behind the pharynx.
This condition constitutes of course the culmination of the
concentration process, but various gradations of it are to be
found in the different groups.

Sense-organs are as a rule well developed in the Mollusca,
and descriptions of many of them may be more conveniently
given in connection with the detailed account of the various
groups. The general ectoderm of the mantle and body-wall
has scattered in it numerous sensory cells which may become
specially aggregated at certain points to form definite sense-
organs. Thus tentacles are frequently borne upon the head
which are tactile or in some cases olfactory in nature, and at
the bases of the gills special aggregations of sensory cells are

TYPE MOLLUSC A. 283

to be found forming the osphradia, also supposed to have an

olfactory function. Otocysts (Fig. 125) are present in nearly

all the groups, consisting of a vesicle with a membranous wall,

the interior of which is lined by sensory cells bearing bunches

of hairs projecting into the vesicle

which contains one or more calca-

reous otoliths. An auditory func-

tion has usually been attributed to

these organs, but it seems probable

that, as in the lower forms (see p.

82), they are rather to be regarded

as organs of an equilibrium-sense,

and in fact that they subserve such

a function in part at least has been

experimental^ determined in the

Cephalopods.

T-, , ,, , FIG. 125.- OTOCYST OF Ptero-

Eyes are very frequently present

iafter CLAUS)

and in the Cephalopods reach an

exceedingly high development. They occur usually upon the
head, but may also be found, as in the Pelecypoda, upon the
edge of the mantle, or even on the dorsal surface of the body,
as in the Pulmonate Oncliidium. They vary, however, so
much in structure in different groups that an account of the
various modifications which they present may be postponed.

Excretory organs in the form of a pair of nephridia are
present, each nephridium consisting of a tube which opens at
one extremity into the mantle-cavity, while at the other it
communicates^ with the cavity of the pericardium, which, as
has been seen, is a portion of the coeloru. The relationships
of these structures are therefore the same as those of the
nephridia of the Annelids, and, as in those forms, they receive
a rich supply of blood, most of the venous blood returning from
the tissues passing through their walls on its way to the
branchiae. The reproductive organs are unpaired in the
majority of forms and in some cases come into relation with
the nephridia, which serve as reproductive ducts. More
usually, however, they open directly to the exterior, a con-
dition which is probably a secondary one. The majority of
the Mollusca are bisexual, but hermaphroditism is by no

284 INVERTEBRATE MORPHOLOGY.

means uncommon, the single reproductive gland producing
both ova and spermatozoa and being therefore an ovo-testis.
Accessory structures are frequently added to the essential
parts of the reproductive apparatus, such, for instance, as al-
buminiparous glands, intromittent organs, spermatophore-
sacs, etc., so that a relatively complicated arrangement may
occur.

I. CLASS AMPHINEURA.

The Aniphineura are Mollusca in which the primitive bi-
lateral symmetry is fully retained and which seem to approach
most nearly to what may be considered the primitive Molluscan
condition. All the known members of the group are marine
in habitat and are more or less elongated forms in which the
elongation of the ventral surface or foot is accompanied by a
corresponding elongation of the visceral complex, which ac-
cordingly is not elevated at right angles to the long axis of
the foot to form a visceral dome. In a general way, therefore, in
the form of their body the Amphiueura may be compared to the
Platyhelminths, especially to such forms, sometimes flattened,
sometimes more or less cylindrical and elongated, as are found
among the Poly clad Turbellaria. The mouth and anus are situ-
ated at the extremities of the body, and to either side of the anus
are situated the one or more pairs of plumelike branchiae and the
openings of the single pair of nephridia. The shell may con-
sist either of a number of scattered calcareous spicules im-
bedded in or projecting from a thick cuticle, or else may take
the form of a number of plates arranged in a longitudinal
series upon the dorsal surface of the body, and as a rule the
mantle-fold is but slightly developed and may be in some forms
almost rudimentary. The foot, too, which is so characteristic
for the Mollusca, may in some forms be practically un-
developed, but in other forms is a broad flat muscular surface,
showing no differentiation into special regions such as are
found in the higher Mollusca.

Little need be said here as to the internal organs except to
emphasize the fact that both the heart and the nephridia have
a perfectly bilateral arrangement. The nervous system is
characterized by the absence of a definite aggregation of the

TYPE MOLLUSCA. 285

nerve-cells into concrete ganglia ; they are scattered along the
longitudinal nerve-cords, of which there are two pairs, i.e., the
pleuro-visceral cords, which run along the lateral portions of
the body, and the pedal cords, which are situated more
ventrally and which, as well as the pleuro-visceral, are fre-
quently united by cross-commissures which suggest an imper-
fect metamerism. In front these cords unite together to form
the circuinoasophageal ring in which the ganglion-cells are
somewhat more numerous than elsewhere, without, however,
forming distinct ganglia. Sense-organs are but slightly de-
veloped throughout the group, which is divisible into two well-
marked orders.

1. Order Solenogastres.

The members of this order are for the most part elongated
worm-like animals, though some forms are short (Fig. 126)
and more nearly approach the typical
Molluscan form. The exterior of the
body is covered by an exceptionally
thick cuticle traversed by bands of
cells extending into it from the ecto- FlG 12G^Neomenia ca-
dernial layer of the body and terrnina- rinata (after NANSEN).
ting in cup-shaped groups of cells ci _ cteuidium.
which secrete the calcareous spicules m = mouth.
which are scattered through the cuti- ^ = ventral groove.
cle (Proneomenia) or may project upon

its surface (Chcetoderma), and which are the sole repre-
sentatives of the shell of the higher Mollusca. Upon the
ventral surface of the body is a longitudinal furrow (Fig. 126,
vg) at the bottom of which lies the but slightly developed
foot, represented by a small ciliated longitudinal ridge, which in
CJwetoderma may be quite undeveloped, the furrow being in this
form also barely indicated or entirely absent. The lips of the
furrow which enclose the foot probably represent the mantle-
folds of higher forms, here very much reduced, though more
extensively developed at the posterior end of the body, where
they project to form a funnel-like structure (Fig. 127) whose
cavity — the cloaca — receives the openings of the digestive

286

IN VERTEBRA TE MORPHOL OOT.

FIG. 127. — DIAGRAM OF AR-
RANGEMENT OF ORGANS AT
HIND END OF Chceloderma
(after HUBRECHT from LANKESTER).

ct = ctenidium.
n = nephridium.
o = ovary.
p — pericardium.
?• = rectum.

tract (r) and the nephridia (?i) and contains the branchiae (ct).

These last are either a single pair of structures each consisting

of a central axis with piunately-
arranged lateral appendages or in
some cases are represented by
bunches of ciliated filaments.

The ectoderm rests upon a
layer of muscular tissue in which
both circular and longitudinal
layers can be distinguished, and

•^ CJ

numerous bauds of transverse
fibres, in some cases arranged to
form septa placed at regular
intervals, traverse the body-
cavity. A fairly-capacious peri-
cardium is present, lying dorsally
to the posterior portion of the digestive tract and into its
upper portion the heart projects slightly, not being, however,
completely enclosed by the pericardium. No auricles seem
to be developed, nor are any definite blood-vessels present,
the circulation being throughout lacunar.

This condition of the heart in relation to the pericardium is interesting
as showing its original independence of that portion of the body-cavity.
Its enclosure in the pericardium in the higher Mollusca is a secondary con-
dition, the heart and its cavity belonging to the schizoccelic structures
rather than to the so-called enterocoelic pericardium. This agrees perfectly
with the relationships of the blood vascular system of the Nemerteans and
Annelids. (See pp. 165 and 231.)

The mouth is a longitudinal slit upon the ventral surface
of the body and opens into a pharynx provided usuall}T with a
radula and with salivary glands, though both these structures
are absent in Neomenia. The intestine pursues a straight
course towards the anal opening, being, however, in some
forms pouched, owing to its constriction at more or less regu-
lar and close intervals by muscular transverse septa. The
walls of the pouches thus formed are glandular and represent
the digestive gland of other Mollusca, though in Chcetoifcnmi
there is a simple outgrowth of the digestive tract which rep-
resents it more perfectly.

TYPE MOLLUSC A.

287

The nervous system varies in the details of its -arrange-
ment in the different species, but is characterized in general
by a tendency to form ganglia, although nerve-cells are scat-
tered along the nerve-cords throughout their entire length.
In Promomenia there is present a well-developed and closely-
approximated pair of cerebral ganglia from which arise the
pleuro-visceral cords which extend backward along the sides
of the body and possess a number of gaugliouic swellings
near their posterior extremity. Two nerve-rings surround the
oesophagus: (1) the cerebro-pedal connectives, which end
below in the pedal ganglia, from which two pedal cords extend
backward along the foot, in some forms (Dondersia) connected
at regular intervals by transverse commissures in an almost
metameric manner, gauglionic enlargements of the cords being
developed in connection with the commissures ; and (2) the
cerebro-buccal connectives, which pass to two buccal ganglia
lying below the pharynx. Special sense-organs have not yet
been discovered in the Solenogastres.

The uephridia consist of a pair of tubes which communi-
cate internally with the pericardial cavity and, bending around

FIG. 128.— DIAGRAMMATIC LONGITUDINAL SECTION OP Chiton (after HALLER).

c = perivisceral coelom. n -- nerve.

h = heart. p — pericardium.

m — mouth. ro = reproductive organ.

1-8 = shell-plates.

the posterior part of the digestive tract, unite to open into the
cloaca ventral to the anus by a common orifice. The walls of
the tubes are glandular and probably, therefore, excretory in
function, but the uephridia also serve as the ducts for the
reproductive elements. With the exception of Chcetoderma the
Soleuogastres are hermaphrodite, the single reproductive

288 INVERTEBRATE MORPHOLOGY.

gland producing both ova and testes. This hermaphrodite
glaud is a hollow sac divided into two principal compart-
ments by a longitudinal partition and lies above the digestive
tract. It is a hollow structure (Fig. 128, ro), the reproductive
elements developing from the cells lining its walls and pass-
ing from its cavity into that of the pericardium (p), with
which the reproductive sacs communicate. They are in fact
simply prolongations of the pericardial body-cavity, and the
epithelium lining them is continuous with that of the pericar-
dium. From the pericardial cavity the ova and spermatozoa
pass to the exterior by the uephridia.

The Solenogastres are especially interesting on account of the many
structural peculiarities of a primitive character which they present and in
consequence of which they have been regarded as representatives of ances-
tral Molluscan forms. By others, however, this important position is
denied them on the ground that many of their peculiarities are due to
degeneration produced in accordance with their life in the mud at the bot-
tom of the ocean. The absence of a shell, the reduction of the mantle-
lobes, foot, and radula may with plausibility be accounted for in this
manner, but there are other peculiarities that are certainly primitive
which are not thus explicable. The relation of the heart to the pericardium
is one of these, and others are the communication of the hermaphrodite
gland with the pericardium, and the functioning of the nephridia as ducts
for the reproductive organs. The Solenogastres are unquestionably primi-
tive Mollusca; the only question which is yet to be settled is to what extent,
if any, degeneration is responsible for their external peculiarities, such as
the absence of a shell, the reduction of the mantle-lobes and of the foot.
It must be noted in this connection that one form belonging to the genus
Dondersia has been described as passing through in its development a
stage in which indications of a shell consisting of several plates and simi-
lar to that of the Polyplacophora was present, a condition which would
seem to indicate the derivation of the members of this group from forms
provided with a distinct shell.

2. Order Polyplacophora.

The Polyplacophora, like the preceding order, contains
only marine forms. For the most part they are somewhat flat-
tened animals with a rather broad foot occupying the ventral
surface, while from the sides of the body a slight fold, the
mantle-fold, projects. In one genus, Ckitondlus, the form of
the body is more cylindrical and the foot is rather narrow

TYPE MOLLUSC A.

289

FIG. 129. — CJiceto-
pleuru apiculata.

and situated, as in the Solenogastres, at the bottom of a
median ventral furrow, the lips of which correspond to the
more dorsally situated inantle-folds of such forms as Chiton,
Tracliydermon (Fig. 129), etc. In all cases, in the groove be-
tween the mantle-folds and the foot a number of gills, pinnate
processes of the body-wall, are to be found, in some cases
occurring at definite intervals along the entire side of the
body, in others (Chitonellus) limited to the posterior part only.

One of the most characteristic features of the
Polyplacophora is, however, the shell, which
consists of eight calcareous plates arranged in
a longitudinal series along the dorsal surface
of the body so that the posterior border of one
overlaps the anterior border of the other.
The series covers only the median portion of
the surface, the more peripheral portions and
the outer surface of the mantle-lobes possess-
ing a large number of scattered spicules,
plates or granules imbedded in their Avail.

The body-wall has not so definite an arrangement of the
muscle-fibres lying below the ectoderm as is the case in the
Solenogastres, but, on the other hand, the body-cavity is well
developed. Indeed the schjzoccelic lacunae play a rather sub-
ordinate part in the Chitouidte, as the order is sometimes
termed, the enterocoalic cavity (Fig. 128) being very large and
divisible into three usually separated parts united by bauds,
which indicate the original continuity. One of the parts (c)
surrounds the intestine and the digestive gland ; another, l}Ting
rather towards the anterior end of the dorsal portion of the
body, contains the reproductive cells (ro); while the third part
(p), lying dorsally and posteriorly, is the so-called pericar-
dium.

The two auricles of the heart are elongated tubes which
open about the middle of their length into the single ventricle
and also unite together posteriori}-, the ventricle, also an
elongated tube, again communicating with this united portion.
Anteriorly the ventricle is continued into a short aorta from
which the blood passes to the lacuuar spaces of the schizocoel.
Two vessels with distinct walls run longitudinally in the foot,

290 INVERTEBRATE MORPHOLOGY.

and presumably receive the blood which they contain more
or less directly from the aorta and distribute it to the lacuuar
spaces of the foot.

The mouth lies on the ventral surface, in front of the
anterior end of the foot, and leads into a pharynx provided
with a well-developed radula characterized by a somewhat
complex arrangement of the teeth. Into the oesophagus
a pair of glands opens in Chiton whose secretion contains an
amylolytic ferment, and in addition a pair of small glands
open into the mouth-cavity. The oesophagus communicates
with a sac-like stomach, into which open the ducts of the
paired digestive gland, and the intestine, being considerably
longer than the body, is thrown into numerous coils, and
terminates by a short rectum which opens at the posterior
extremity of the body.

The nervous system is characterized by the diffuse
arrangement of the nerve-cells, no well-defined ganglia oc-
curring on the principal nerve-cords. These consist of a
strong circumcesophageal ring (Fig. 130), the upper part of
which gives off numerous nerves and evidently corresponds
to the cerebral ganglia of other Mollusca, while the lower
part, corresponding to the pedal ganglia, gives rise to two
nerve-cords (pc), the pedal nerves, which pursue a parallel
course throughout the foot, giving off a number of nerves
laterally and being connected by a number of somewhat irreg-
ularly arranged transverse commissures, which almost suggest
a metameric arrangement. From the sides of the circum-
oesophageal ring two other strong nerves, the pleuro-visceral
nerve-cords, arise and pass backwards along the sides of the
body, uniting with each other posteriorly above the terminal
portion of the digestive tract. These cords (pi], like the cir-
cumoesophageal ring, present no distinct gaugliouic enlarge-
ments, but contain the elements of the pleural, visceral, and
parietal ganglia, sending off numerous nerves to the branchiae,
the mantle, and probably also to the heart and nephridia.

In addition to these principal nerve-cords others of smaller'
size also arise from the circumo3sophageal ring. One pair of
these pass to a pair of ganglia, the buccal ganglia, lying
beneath the buccal mass and send nerves to the oesophagus

TYPE MOLLUSCA.

•Jill

while another pair pass to a pair of ganglia lying below the
radula and in intimate connection with a peculiar subradular
organ, probably sensory, lying in this region.

FIG. 130.— DIAGKAM OF NERVOUS AND EXCRETORY SYSTEMS OP Ghiton siculus

(combination of two figures by HALLER).

no = nephridial orifice.

an — auus.

Br = cteuidia.
go = genital orifice.
n = nephridiuin.

oe = oesophagus.

pe = pedal nerve cord.

pi = pleural nerve-cord.

As regards sense-organs, in addition to this subradular
organ whose function is entirely problematical, ridges of
sensory epithelium exist along the sides of the body in the
mantle-cavity. One such ridge runs along the inner wall of
the mantle-fold, while the other is found at the bottom of
the mantle-cavity passing over the bases of the branchial
plumes and sending a short prolongation outwards upon each
of these structures and seeming thus to correspond with the
osphradia of other Mollusca.

292 INVERTEBRATE MORPHOLOGY.

A mucli more peculiar series of organs, found, however, in
their perfect form only in certain species, is developed in
connection with the shell of the Chitouidse. They consist of
club-shaped structures contained in pores which traverse the
shell-plates and possess a definite arrangement, being ar-
ranged in groups of larger and smaller organs (meg alee stlietes
and micrcesthetev). Each group is in connection Avith a num-
ber of large glandlike cells, which terminate in the megal-
sesthete, covered externally by a cup-shaped layer of chitin,
and from this cell-mass more or less numerous branches
arise, the micrsesthetes, which terminate in club-shaped
swellings likewise covered by a chitinous layer. Below the
group of cells is in connection with fibrils which unite to
form a nerve probably passing to the pleuro-visceral nerve-
cords, and it thus seems tolerably certain that these struc-
tures are sensory and perhaps tactile in function. In some
species the megalaesthetes become modified into eyes consist-
ing of an external convex chitiuous cap, the cornea, below
which is a lens and below this a layer of retina-cells con-
nected with nerve-fibrils and surrounded by a cup of pig-
ment-cells. No eyes other than these occur in the Polypla-
cophora, nor are tactile tentacles or otocysts, of such fre-
quent occurrence in other Mollusca, found.

The nephridia (Fig. 130, n) are paired, one lying on each
side of the body and consisting of a long tube giving rise to
numerous dendritic branches. Posteriorly the tube branches,
one of the branches opening into the mantle-cavity in its
posterior part, while the other communicates with the peri-
cardial portion of the enteroccel. In function these organs
of the Chitouidse differs from the corresponding ones of the
Soleuogastres in being excretory only and in not serving
as ducts for the reproductive elements. These are developed
in a portion of the euterocoel which lies anteriorly to the
pericardium and make their way to the mantle-cavity and so
to the exterior by special ducts arising one on each side from
near the posterior part of the reproductive euterocoel and
ending (go] 011 the sides of the body slightly in front of the
openings of the nephridia (no). The Polyplacophora are with-
out exception bisexual.

TJPE MOLLUSC A. 293

The structural peculiarities of the Polyplacophora point strongly to
their primitive character, though in many respects they are less primitive
than the Soleuogastres. Thus they possess special reproductive ducts, in all
probability a secondary acquisition, and furthermore the reproductive and
pericardial moieties of the enterocoel no longer communicate freely. If
the Solenogastres have been derived from forms with Chiton-like shells
(see p. 288), then it must be supposed that the two groups represent di-
verging lines of development from a common ancestor whose character-
istics have been partly retained in the one group and partly in the other.

II. CLASS GASTEROPODA.

The Gasteropoda form a very complex group, the various
members differing so much in the details of their organiza-
tion that it is difficult to give a general description which
will apply to all the forms. Certain features may, how-
ever, be considered typical of the class, and these may be
mentioned here, reserving notice of the more important varia-
tions until the various subdivisions are being considered.

One of the most characteristic features is the occurrence of
what may be termed the " visceral hump " whose presence is
responsible for many of the peculiarities of Gasteropod struc-
ture. It consists of an elevation into a dome-like structure
of the dorsal region of the body, the digestive tract and
gland being contained within the elevation. The mantle
arises as a circular fold surrounding the hump, but usually is
more highly developed, and therefore encloses a deeper
cavity, upon the right side or anterior surface of the hump,
and in the cavity so arranged lie the structures which usually
are associated with the mantle-cavity, namely, the branchiae
and the openings of the digestive tube and of the nephridia.
There is thus a very decided asymmetry in most Gasteropods,
usually emphasized by the visceral hump being coiled into a
spiral, a coiling which is shared by the shell, usually present
and consisting of a single tubular structure surrounding the
visceral hump, but usually sufficiently ample to permit of the
retraction within it of the rest of the body.

In a number of forms the visceral hump may be very
much reduced, and with this reduction there is generally con-
comitant a reduction of the shell, but such conditions are

294 INVERTEBRATE MORPHOLOGY.

plainly secondary inasmuch as the primitive asymmetry is
indicated in certain of the organs in all cases. In order to
understand the exact nature of this asymmetry it will be nec-
essary to consider what may have been the original form of
the Gasteropoda. Judging from what is known of the Amphi-
neura, it may be supposed that in the primitive Gasteropod
(Fig. 131, A) the anus (a) was terminal and opened into a
mantle-cavity, the mantle being, except posteriorly, only a
slight fold. In this mantle-cavity there was present also a
single pair of branchial plumes (cfy, and into it the two
uephridia opened («), passing from the posteriorly-situated
pericardium which contained the heart provided with two
auricles.

It may be imagined now that in such a form the visceral
hump enclosed by a dome-like shell became elevated to such
an extent that it could no longer be retained in an erect posi-
tion, but fell over to one side — it may be supposed the left
side. The result of this would be an interference with the
development of the mantle- cavity towards the left side, and
a prevention of the perfect growth of the left branchia and of
the proper functioning of the left uephridium. There would
be a tendency then for the mantle-cavity, and with it the anus
and indeed the entire posterior region of the body with the
heart and nephridia, to be pushed over towards the right side
(Fig. 131, B), and this process might in some cases be con-
tinued until the mantle-cavity and the organs associated with
it had been pushed round through 180° (Fig. 131, C, D) and
had come to lie apparently in front of the visceral hump (D).
The anus in such a case would open into the mantle-cavity in
the mid line, dorsal to the mouth, and what was originally the
right brauchia would lie upon the left side of the body ; the
digestive tube, which may originally have been practical!}- a
straight tube, would now be bent upon itself, and furthermore
the original right parietal nerve-ganglion would have passed
over to the left side of the body and the original left ganglion
to the right side, a crossing of the pleuro-parietal connectives
(vc) being thus brought about.

The original pressure of the shell upon the left half
of the mantle-cavity would, however, as pointed out, have

TYPE MOLLUSCA.

295

tended to produce a retardation in the. growth or even the
complete abortion of the organs lying in that region. Accord-
ingly the original left nephridium is in many Gasteropoda com-
pletely suppressed as well as the original left brauchia, and
in accordance with the disappearance of this latter structure

V

I)

FIG. 131. — DIAGRAMS TO ILLUSTRATE THE ROTATION OF THE MANTLE-CAVITY
AND ITS ORGANS IN A GASTEROPOD (after figures by BUTSCHLI and LANG from
KOESCHELT aud HEIDER).

a — anus. in — mouth.

ao = aorta. n == uephridial pore.

ct = cteuidium. peg = pedul ganglion.

eg = cerebral ganglion. pig = pleural ganglion.

vc = visceral connective.

there is a disappearance also of the left auricle of the heart
which receives blood from it.

The visceral hump does not, however, retain its original
conical form, but, owing perhaps to unequal pressure, grows
more rapidly upon one surface, the anterior, and so becomes
coiled into a right-handed spiral, the shell covering the
hump naturally assuming a similar form. In the majority of

296 INVERTEBRATE MORPHOLOGY.

Gasteropods consequently a shell coiled in a right-handed
spiral occurs, but this rule has not a few exceptions. "Where
the shell forms a left-handed spiral it is to be explained by
supposing that in such cases the visceral hump tended towards
the right side of the body rather than the left, and this is
confirmed by the fact that in most left-handed forms it is the
left branchia and nephridium that have persisted.

It must be pointed out, however, that the extent to which
the rotation of the mantle-cavity, the abortion of the organs
of either the left or right side of the body, and the crossing
of the pleuro-parietal nerve-cords has been carried varies in
different forms. In some the rotation has been carried so far
that the original right branchia, etc., has passed the median
line in front so as to lie on the left side of the body, and in
such cases the crossing of the nerve-cords (chiastoueurisrn) is
completed. Many forms, however, stop short of this, and
numerous gradations are to be found. The rotation, however,
is present in all forms to some extent and forms a character-
istic feature of Gasteropod morphology.

The anterior portion of the body (Fig. 132) is usually well
marked off by a more or less distinct constriction or neck, and
consequently it is possible in the Gasteropods to speak of a
head in contradistinction to the trunk region of the body ; in-
deed so prevalent is this character that the term Cephalo-
pliora has been applied to the group. Tentacles, either one or
two pairs, are borne by the head, and furthermore eyes are
usually present upon it either at the bases of one of the pairs
of tentacles or else borne at the tips of these structures.

The foot is generally well developed and usually has a flat
creeping sole. It undergoes many modifications, however,
sometimes becoming more keel-like, or becoming differentiated
into three regions differing in form, the propodium, mesopo-
diuni, and metapodium, the last-named portion frequently
secreting a chitinous plate, the operculuni (Fig. 132, op), which
serves to close the mouth of the shell when the animal is
withdrawn within it. In addition to these portions an epi po-
dium is frequently highly developed, consisting in its primi-
tive form of a fold arising from the sides of the foot where it
passes into the body-wall. In many cases, however, it loses

TYPE MOLLUSC A.

297

this simple form, its margin becoming fringed or tentaculate,
or else it may be reduced to one or more separate lobes or
tentacular processes on either side of the bod}". Opening
upon the surface of the foot is frequently to be found a so-
called "foot-gland " which secretes a sticky mucous fluid and
is comparable to the byssus-glaud of the Pelecypoda (q. v.).

FIG. 132. — Buccinum undatum.
op = operculum. si — sipho.

The respiratory organs (Fig. 133, ct) consist in typical cases
of a single pair of pinnate branchial plumes lying in the
mantle-cavity, but, as has already been mentioned in connec-
tion with the rotation of that cavity, one of these structures is
very frequently aborted. Other changes, however, also occur,
such, for example, as the fusion of the central axis of the
branchial plume throughout its entire length to the inner
surface of the mantle (Haliotis), or the disappearance of the
pinnae from one side of the plume in connection with such a
fusion (Sycotypus, Fig. 133). In some forms accessory bran-
chiae may be produced as folds of the mantle, richly supplied
with blood, and their development may be carried to such an
extent that they may entirely supplant the branchiae proper
(Patella). From such a condition as this a passage is not dif-
ficult to such a condition as is found in the air-breathing Gas-
teropods (Pulmonata) in which the entire inner surface of the
mantle serves a respiratory purpose, an interchange of gases
taking place between the air contained in the mantle-cavity
and the blood which is richly supplied to the mantle.

The musculature of the body-wall does not as a rule pre-
sent the Aunelidan arrangement in layers, as in some Amphi-
neura, but usually are irregularly arranged as dorso-ventral
and oblique bands traversing the schizoccel. Special muscles,

298 INVERTEBRATE MORPHOLOGY.

however, are developed iii many forms, the most important
being those connected with the foot and serving for locomotion,
retractor muscles in connection with the head, proboscis, and
tentacles, and the spindle-muscle, which has a general vertical
direction running along the right side of the visceral hump
from its insertion into the shell to the foot in whose wall its
fibres spread out, interlacing as it were with the horizontal
and transverse muscles there developed ; it serves to retract
the entire animal within the shell, and its development is
naturally in proportion to that of the shell, those forms in
which the shell is rudimentary or absent frequently lacking it.

The euterocoelic portion of the coalom is much reduced
in comparison with what occurs in the Amphiueura, being dis-
tinctly represented only by a comparatively small pericar-
dium surrounding the heart, the auricle in some cases not
being enclosed by it. From analogy with the Amphiueura,
however, the reproductive organs must be regarded as repre-
senting a portion of the enteroccel whose connection with the
pericardium has been completely severed. A glandular struc-
ture, the pericardial gland, is in some Gasteropods developed
by the folding of the pericardial walls, and has apparently
an excretory function acting as an accessory nephridium ; it
is not, however, as highly developed as in some of the other
Molluscan groups.

The circulatory organ possesses in some forms the charac-
teristic Molluscau structure, consisting of an unpaired ven-
tricle lying in the pericardium and receiving the blood from
two lateral wing-like auricles. In many cases, however, as al-
ready pointed out, the asymmetry produced by the develop-
ment of the visceral hump affects the heart, resulting in the
suppression of one of the auricles, that of the left (or right)
side (Fig. 133). In such cases the persisting auricle may
secondarily assume a terminal position with regard to the
ventricle, and the latter, instead of being continued into an ar-
tery at either extremity, gives off a single artery at the end op-
posite to that at which the blood enters from the auricle, this
artery dividing into two main trunks which distribute the
blood to the various regions of the body. These arteries may
be continued as distinct tubes with definite walls for some

TYPE MOLLUSC A.

299

distance from the heart, but sooner or later the blood passes
into the system of lacunar spaces constituting the schizocoel,
whence it is again returned to the auricle through a series of
Teius. The position of the single auricle with reference to the
body axis differs in different orders of Gasteropoda, in accord-

pe

i -

FIG. 133. — STRUCTURE OP Sycotypus canaliculatus. The mantle is divided in
the middle Hue and turued aside, exposing the mantle-cavity.

os = osphnidium.
p = pericardial cavity.
pe = penis.
pr — proboscis.
si = sipho.

I ~ tentacle.
te = testis.

an — anus.

ct = cteuidium.
dg — digestive gland.
i = intestine.

n = nephridium.
no — nephridial opening.

o = eye.
op = operculum.

i} = ventricle.
vd = vas defereus.

The arrows show the openings of nephridium to the mantle-chamber and to

the pericardium.

ance with the varying position of the branchia. In those
forms in which the branchia lies in front of the heart the
auricle lies at the anterior end of the ventricle, while when
the branchia is posteriorly situated the auricle lies behind
the ventricle.

The mouth lies in all Gasteropoda at the anterior ex-
tremity of the body, towards the ventral surface of the head,

300 INVERTEBRATE MORPHOLOGY.

and opens into a mouth-cavity frequently provided with two
or more chitinous teeth. The pharynx usually receives the
ducts of a pair of salivary glands, contains a well-developed
radular organ in practically all cases, and communicates
posteriorly with a tubular oesophagus. In many cases the
anterior portion of the digestive tract is capable of being
protruded as a proboscis (Fig. 133, pr), which lies when re-
tracted within a proboscis-sheath, formed by a circular infold-
ing of the body-wall around the mouth. The intestine (i) is
usually more or less coiled, extending into the visceral hump,
and presents a stomach-like enlargement which receives
the ducts of the digestive gland (dg), a structure usually wrell
developed and forming the greater portion of the visceral
hump. The intestine terminates in a straight portion, the
rectum (r), which passes forward to the anus (an), which, as
has already been indicated, lies in the mantle-cavity, slightly
to the right, but occasionally to the left, of the middle
line, its position depending upon the amount of rotation
which the mantle-cavity and the associated organs have
undergone. It should be mentioned that in one suborder of
Gasteropods the pericardium and ventricle have wrapped
themselves around the rectum in such a way that the diges-
tive tube seems to have penetrated the ventricle, a feature
which will later be seen to be characteristic of one of the
other groups of Mollusca.

The nervous system has the arrangement which has been
described as characteristic of the Mollusca (Fig. 124), the
peculiar feature being the crossing of the pleuro-parietal con-
nectives which is found in many forms. Numerous modifica-
tions of the typical condition are -to be found, consisting
principally in (1) the concentration of the ganglia, more
especially the cerebral, pleural, and pedal, or the pedal,
pleural, parietal, and visceral (Fig. 137), to form a single
mass ; (2) in the suppression in some cases of one of the
parietal ganglia ; and (3) in the occurrence of several visceral
ganglia. In accordance with the flat elongated form of the
foot in many species, the nerve-cords passing backward from
the pedal ganglia may be of considerable size, and further-
more may be connected by regularly-arranged transverse

TYPE MOLLUSC A.

301

commissures, recalling the condition seen in the Chitons, as
well as the ladder-like arrangement of the ventral nerve-cords
of the Annelida, though there cannot in the Gasteropods be
any question of metamerism in this connection.

Special sense-organs are very generally well developed in
the Gasteropods. The tentacles so usually found upon the
head have probably a tactile function as well as the tentacular
or winglike processes sometimes found in connection with
the anterior extremity of the foot, and the epipodial ten-
tacles which occur in some forms (Haliotis). On the ventral
side of the bases of the epipodial tentacles of some forms
special sensory thickenings have been found which have
suggested a comparison with the sense-organs of the lateral
line of the Annelida, a comparison which, however, at present
seems rather strained ; it seems probable, notwithstanding
their inuervation from the pedal ganglia, that these sensory
patches are to be placed in the same category as the osphra-
dia and the sensory ridges of the mantle-cavity of the Chi-
tons. The osphradia (Fig. 133, os) in all Gasteropods which
are provided with branchiae are associated with these organs ;
and even where one or both
branchiae have been suppressed
the osphradia may still persist.
Eyes (Fig. 134) are very generally
present in the Gasteropods, being
situated at the base of the ten-
tacles, or at their summit in some
forms. They present a very uni-
form structure throughout the
group and arise as a depression
of the integument, the lips of
the cavity fusing and giving rise
to a globular sac lying beneath
the epidermis, which remains thin and transparent, forming an
outer cornea (co). The cells of the outer wall of the sac
likewise remain clear, forming the inner cornea, while over
the remainder of the wall of the sac they are sensory in
function, piginented cells being scattered among them, the
two together forming the retina (ret). The nerve-fibres pass-

FIG. 134.— EYE OF Hnliotis (after
PATTEN).

co = cornea.
I = lens.
rt = retina.

302 INVERTEBRATE MORPHOLOGY.

ing to the eye from the cerebral ganglia pass through an optic
ganglion lying beneath the optic sac and are distributed to
the sensory cells, and the centre of the sac is filled up by a
cuticular mass which serves as a refractive lens (I). In some
forms (Patella, etc.) the development of the eye ceases Avhile
it is still in the cup form, there being then no formation of
corueal layers and no central lens, though the retina is usu-
ally covered by a thin cuticular layer. In some species of a
peculiar genus of the air-breathing Gasteropods, OncMdium,
eyes are developed upon the dorsal surface of the body, the
shell being lacking and the visceral hump undeveloped. In
structure these eyes differ very materially from those usually
occurring in the Gasteropoda and will be described later
(p. 318).

Otocysts are usually imbedded in the tissues of the foot
close to the pedal ganglia, though in all cases they receive
their iuuervation from the cerebral ganglia ; they have the
usual sac-like form and are lined with sensory hair-bearing
cells and contain otoliths.

The uephridia of the Gasteropods are in nearly all cases
modified from the original typical condition in accordance
with the asymmetry of the body (Fig. 133, re). In only a few
forms, so far as known (Fissurella, Patella\ are two functional
uephridia, opening on the one hand into the pericardia!
cavity and on the other to the exterior through the mantle-
cavity, perfectly developed. In other forms, such as Hali-
otis, Turbo, etc., both uephridia are present and are struct-
urally perfect, though the left* one has lost its secretory
function, but in the majority of cases the left (or, in forms
with a left-handed coiling of the visceral hump, the right)
uephridium is completely aborted.

The Gasteropods are in some cases bisexual, in others
hermaphrodite. The reproductive sac (Fig. 133, t) is quite
unconnected with the pericardial enterocoel and is an un-
paired structure lying in the visceral hump. The ova and
spermatozoa in most cases reach the exterior by a special

* The terms left and right refer to the position of the nephridia as they are
supposed to have been arranged in the primitive symmetrical Gasteropod.

TYPE MOLLUSC A.

303

duct (vd}, having apparently no relation to the nephridia and
opening into the mantle-cavity to the right side of the anus.
In the more primitive Gasteropoda, however, such as Haliotis,
Fissurella, and Patella, the nephridia, as in the Solenogastres,
serve as reproductive ducts ; and it has been suggested that
the special reproductive duct of the remaining Gasteropods
may represent the left uephridium, which is usually described
as having disappeared. The reproductive duct, especially in
hermaphrodite forms, has developed in connection with it
accessory glandular structures as well as external copulatory
organs, the whole reproductive system becoming highly com-
plicated. An account of the more important arrangements
will be more satisfactorily given in connection with the
various orders.

1. Order Prosobranchia.

The Prosobranchia are, with very few exceptions, marine
Gasteropods, provided with well-developed shells, which are
usually spirally coiled, the height of
the spiral varying, however, in different
forms. In some, such as Patella and
Fissurella, the shell has a simple
conical form, without any indication
of a spiral ; and since these forms in
many respects show primitive charac-
ters, it might be supposed that this
type of shell was also primitive. These
very forms, however, show also that
asymmetry of parts, which is character- Fio.135.— SHELLS OF PROSO-
istic for the Gasteropods, and which BRANCH GASTEROPODS.
accompanies the rotation of the mantle- Acm(*a ^udinaiis

rl. 77 (after GOULD) ; B, Haliotis

cavity, and furthermore, m Fissurella (&tler LKVyia}. 0, Turriiella
at least, a distinct indication of a (after LEUNIS).
spiral coiling, is present in the shells of

young animals. It seems more probable, accordingly, that
these conical shells are to be regarded as secondary modifi-
cations of an originally spirally-coiled shell.

The mantle-cavity is situated in front of the well-devel-
oped visceral hump, and is usually somewhat capacious, com-

304 INVERTEBRATE MORPHOLOGY.

municatiug with the exterior freely. In some forms the
mantle is slit from its margin upwards and backwards, a
corresponding slit occurring in the shell (Emarginula) . In
Haliotis and Pleurotomaria the slit in the shell becomes
closed at regular intervals, producing a row of round perfora-
tions, beneath which lies the mantle-slit, and through which
water finds a ready exit from the. mantle-cavity, and, in Fis-
surella, in which at an early stage the margin of the shell
possesses a slit, by the subsequent growth and obliteration of
the spiral coiling the slit becomes converted into an aperture
which lies almost at the apex of the conical shell and leads
into the mantle-cavity, functioning as a means of exit of the
water and excrementa from that cavity. In the greater num-
ber of forms, however, such slits or apertures do not exist ;
but one finds frequently the margin of the mantle produced
at one point on the left side into a projecting narrow lobe
whose edges may be brought into opposition, thus produciug
a tube or siphon through which water may pass into the
mantle-cavity. Where this siphon is well developed a dis-
tinct notch is found in the margin of the shell, through which
it may be protruded, or else the lips of the notch are pro-
longed so as to form a grooved process, the siphonal canal, in
which the siphon lies, being by these arrangements able to
function even when the mouth of the shell is closed \>j the
operculum. In many forms the mantle-folds are sufficiently
large to allow of their being reflected over the outer surface
of the shell when the body is fully protruded.

The foot is as a rule adapted for creeping, but in many
cases is differentiated into pro-, meso- and inetapodium, the
last usually bearing a chitinous or more or less calcified
operculum. In certain forms belonging to a group of pelagic
forms, however, which were formerly associated together as a
distinct order, the Heteropoda (Fig. 138), the pro- and nieso-
podium are modified into a keel-like structure and bear a
peculiar sucker. The epipodium is frequently developed in
the Prosobrauchia, especially in the more primitive species-
most frequently, however, being reduced to tentacle- or lobe-
like processes arising from the sides of the foot.

In the majority (Fig. 1 33) of forms there is but a single

TYPE MOLLUSC A. 305

brauchia which lies in front of the heart, whence the name of
the order, but in a few genera the original left gill also per-
sists. In many forms a gland is developed in the floor of the
mantle-cavity close to the rectum — hence called the adrectal
gland — which in some forms, e.g. Murex secretes a purple
pigment. The rotation of the mantle-cavity and the associated
organs has called forth a crossing of the pleuro-parietal nerve-
cords, a feature which is lacking in the other orders and
therefore forms a characteristic of the Prosobranchs.

In all but a few cases the members of the order are bisex-
ual, the unpaired reproductive gland lying in the visceral
hump. The oviduct has in connection with it one or more
receptacula serninis and dilates into a glandular uterus in
which the eggs are supplied with the albumen in which they
are .usually imbedded and also surrounded by a shell. In
the males, except in the more primitive forms, there is present
a well-developed iutromittent organ or penis (Fig. 133, pe),
situated upon the right side of the head or neck and there-
fore removed at some distance from the opening of the vas
deferens into the mantle-cavity. A groove or tube extends,
however, from the reproductive orifice to the grooved or tubu-
lar penis, and along this groove or tube, by the ciliary action
of the cells lining it, the seminal fluid is carried.

1. Suborder Diotocardia.

This suborder includes the more primitive Prosobranchs,
in which, although a considerable rotation has occurred, yet
nevertheless the abortion of the organs of the original left
side of the body has not been carried very far. Thus, except
in Patella and some allied forms, there are two auricles to the
heart, although in Turbo, Troclius, Neritina, and allied genera
that of the right side (i.e., the original left one) does not com-
municate with the ventricle. Attention may again be called
to the fact that in those forms which possess two functional
auricles the ventricle and pericardium have wrapped them-
selves round the rectum which seems to perforate the ventri-
cle. Such forms as Haliotis, Fissurella, and Pleurotomaria pos-
sess two branchia3, but in the majority of the members of the

306 INVERTEBRATE MORPHOLOGY.

group only one is present, while in Patella both have disap-
peared, their place having been taken by respiratory folds
of the mantle. Both kidneys are invariably present.

The primitive character of the suborder is further shown
in the absence of certain structures found in more specialized
forms. Thus the foot is flat and uudifferentiated into pro-,
meso-, and metapodiuni ; the anterior part of the digestive tract
is not evertible as a proboscis ; there is no siphonal prolonga-
tion of the mantle, and no notch or siphonal groove on the
margin of the shell ; and there is no penis. On the other hand
the epipodium is usually well developed, as are also the pedal
nerve-cords, which are connected by numerous cross-commis-
sures.

A further distinguishing feature of the suborder is the
arrangement of the teeth of the radula. Each transverse row
of teeth presents an indefinite number of marginal teeth, usu-
ally a single lateral, a single median, and a varying number
of admedian teeth, an arrangement known as rhipidoglossate.
Thus in Haliotis the arrangement is indicated by the formula

FlG. 1 36. —DENTITION OF Trochus (after LANKESTER).

x, 1, 5, 1, 5, 1, x ; in Fissurdla by as, 1, 4, 1, 4, 1, x ; and in Tro-
cJius (Fig. 136) and Turbo by x, 0, 5, 1, 5, 0. x, the single lateral
tooth being absent in these forms. In Patella, however, an-
other arrangement is found characterized by the occurrence
of only a small number of marginal teeth and by the absence
of the median, the formula being 3, 1, 2, 0, 2, 1, 3 ; this ar-
rangement is termed docoglossate.

2. Suborder Monotocardia.

In this suborder the effect of the pressure of the visceral
hump on the organs of the left side of the mantle-cavity is more
pronounced than in the Diotocardia. The heart possesses a

TYPE MOLLUSC A. 307

single auricle only, except in Cyprcea, where the rudiment of a
second occurs, and throughout the group but a single nephrid-
ium is present. There is never more than a single gill, which
is usually more or less united to the mantle-wall and bears
lateral branches only upon one side.

The foot is in some cases flat and broad, as in the Dioto-
cardia, and in such cases may possess the parallel pedal nerve-
cords with transverse commissures as in Cyprcea and Paludina,
but usually it becomes more or less differentiated, a propodi-
um being in many cases well defined (titrombus, Naticct), while
a chitiuous or calcareous operculum is usually carried by the
metapodium, and the pedal nerve-cords are very much re-
duced or wanting, the pedal ganglia being on the other hand
more highly developed than in the Diotocardia. The epipo-
diuin is usually entirely wanting, and when present is but
slightly developed, reaching its fullest development as a con-
tinuous fold upon the sides of the foot only in lanthina. In
Paludina it is represented by two anteriorly-situated tentacle-
like lobes, and in Calyptrcea by a semicircular fold on each
side of the neck region.

The Mouotocardia are further distinguished by the fre-
quent occurrence of a well-developed siphon and a more or
less developed siphon groove at the margin of the shell, and
furthermore a well-developed penis is usually present.

The anterior portion of the digestive tract is in many forms
capable of being protruded as a proboscis. The arrangement
of the teeth of the radula varies considerably in different
forms, but the rhipidoglossate arrangement is not represented.
In one group, including the genera Cyprcea, Natica (Fig. 137,
A), Littorina (the periwinkles), Calyptrcea, Strombus, etc., the
tseuioglossate arrangement is found, represented by the for-
mula 2 or 3, 1, 1, 1, 2 or 3, the admedian teeth, however, being
very similar to the lateral. In other cases but a single median
tooth or the median with a single admedian on each side is
found, as in Fusus, Buccinum (the whelks), Nassa (Fig. 137, B),
Jlurex, Purpura, Olivet, Maryinella, etc., forming the rachi-
glossate arrangement represented by the formulas — , 1, — , or
1, 1, 1. In Terebra, Conus, Pleurotoma (Fig. 137, C), and allied
genera the median tooth is absent, and the single admediau

308

IN VERTEBRA TE MORPROLOG T.

tooth on either side peculiarly long, forming the toxiglossate
arrangement with a formula 1, 0, 1 ; and finally certain forms,
such as lanthina, Scalaria (Fig. 137, U), Solarium, etc., have a
ptenoglossate arrangement in which the median is wanting
but in which there are a large number of admedians, x, 0, x.

The suborder is relatively very rich in species, and conse-
quently considerable variety of form is found. The majority
are marine, but a few are fresh-water or even terrestrial in
habitat. In these latter adaptations to their mode of life are
found in modifications of the respiratory processes. In Am-

FIG. 137.— A, DENTITION OF Natica ; B, OF Nassa ; G, OP Pleurotoma ; D,

OP Scalaria (from BRONN).

pullaria the single branchia persists, but in addition a com-
paratively capacious " lung-cavity " is formed by a fold of the
mantle, its walls being richly supplied with blood-vessels and
its cavity being in communication with the exterior, so that
air can be taken into and expelled from it. The species of
this genus live partly in fresh water and partly are terrestrial,
but in other forms, such as Cyclostoma, which are purely ter-
restrial, the branchia has entirely aborted, respiration being
aerial and performed by the highly vascular wall of the
mantle-cavity.

The majority of the marine Monotocardia have a creep-
ing habit, but a number are pelagic and form a group pre-
senting many adaptive peculiarities which obtained for it the
dignity of an order in older classifications. The members of
this group, HETEROPODA, are more or less transparent animals,
some of which, Avith this exception, present few differences

TYPE MOLLUSCA.

309

from the other Monotocardia, while others are extensively
modified. The germs Atalanta possesses a large transparent
shell within which the animal can be completely retracted.
The foot is no longer adapted for creeping, but is differenti-
ated into a laterally flattened keel-like pro- and mesopodium
which bears a sucker on its posterior surface, and a metapo-
dium provided with an operculurn. In Carinaria (Fig. 138)

h

te

FIG. 138. — STRUCTURE OF Carinaria mediterranea (after OWEN).

ao = aorta.

b = buccal mass.
eg •=. cerebral ganglion.
ct — ctenidium.

h = heart.

i = intestine.

I — liver.
mp = mesopodium.

o = eye.
p = penis.

peg = pedal ganglion.
s = salivary gland.
su = sucker.
te = testis.
i)d = vas deferens.
vg = visceral ganglion.

vs = seminal vesicle.

the visceral hump is reduced to a comparatively small mass
upon the dorsal surface of the elongated body and is enclosed
in a transparent shell shaped like a liberty-cap. The pro- and
mesopodium have the form of a plate hanging doAvn from
about the middle of the under-surface of the body, and the
metapodium is directed backwards, forming in reality the
posterior portion of the body. The same relationships of the
foot are found in Pterotrachea, which presents the extreme of
modification found in this group ; in this form the visceral
hump is still more reduced than in Carinaria, forming only a
small oval mass imbedded in the dorsal surface of the body
and being destitute of any shell. Considering these two forms,

310 INVERTEBRATE MORPHOLOGY.

Pterotrachea and Carinaria, by themselves, the formation of
a separate order for their reception would perhaps be justi-
fiable, but Atalanta shows their close relationships with the
Prosobranchia and indicates their true position as Mouoto-
cardia.

2. Order Opisthobranchia.

The Opisthobrauchs are exclusively marine forms, present-
ing numerous modifications of shape and structure, but all
agreeing in certain important particulars. The rotation of
the mantle-cavity has not proceeded quite so far as in the
Prosobranchs, the cavity and its organs lying upon the right
side of the body, but at the same time the abortion of the
organs of the primitively left side of the body has occurred.
Thus in those forms which possess respirator}* organs homol-
ogous with the branchiae of the Prosobrauchs, but one (that
of the right side) is present, and with this character is associ-
ated the occurrence in the heart of but a single auricle,
which lies behind the ventricle. Only one uephridium occurs,
and a distinction from the Mouotocardiate Prosobrauchs is
found in the fact that the branchia when present lies as in-
dicated by the position of the auricle, behind the heart — the
name bestowed upon the order being suggested by this pecul-
iarity.

A more important distinguishing character perhaps is,
however, to be found in the arrangement of the nerve-cords.
The rotation of the mantle-cavity and its associated parts has
not been carried to such an extent as to produce a crossing of
the pleuro-visceral connectives, which run more or less par-
allel with one another and present- what is termed an ortho-
neurous arrangement in contradistinction to the chiastoueu-
risin of the Prosobranchs. In addition to this character a
tendency towards an aggregation of the various ganglia to
a complex mass lying behind the pharynx may also be con-
sidered a characteristic of the Opisthobranchs. One or both
parietal ganglia may disappear, and in some cases where
there is a marked concentration of the ganglia the visceral
ganglion may also be unrepresented, though usually from one

TYPE MOLLUSC A. 311

to three suck ganglia may be distinguished. In the figure of
the nervous system of Fiona (Fig. 139) the concentration of
the ganglia is well marked, but a
decided asymmetry is made evi-
dent in some forms by the exist-
ence of a single parietal ganglion
and of three visceral ganglia. In
Fiona, however, the gauglionic
concentration has been carried
still farther, and at the same time
by the suppression of the parietal FJG 139 _NERVOUS SYSTEM OP
ganglion as a distinct mass of Fiona atlantica (after BERGH from
cells an apparent symmetry has GEGENBAUE).
resulted ^ = cereki'o-pleiiro-visceral gan-

B = pedal ganglion. [glion.

With regard to many other c = £uccaf gaitg]iou.

features of their anatomy con- jy - gastro-cesophageal ganglion,
siderable variations are to be e = pedal commissure,
found. Thus in some forms a ^ - visceral commissure,
well-developed spirally-coiled visceral hump is developed,
while in others it loses its spiral arrangement, and in others
again is elongated in the direction of the foot and can hardly
be said to exist. So, too, with the occurrence of the shell,
mantle, and branchiae ; all are well developed in some forms,
but entirely absent in others. These peculiarities will be more
conveniently referred to in connection with the various groups,
and it is only necessary here to refer to another feature in
addition to those already given, which is common to all the
members of the order — i.e., the hermaphroditic character of
the reproductive gland.

This forms part of the visceral mass and is usuall}T com-
posed of numerous lobes, these again being divided into
secondary lobes or acini, the lining epithelium of which
give rise to both ova and spermatozoa. In some forms,
such as Sulla and Aplysia, both elements are formed in ml
the acini ; but in others, such as Doris, Janus, Pteropoda, etc.,
the epithelium of the terminal acini gives rise to ova only ; the
epithelium of the lobes, i.e., the central portions of the gland,
producing spermatozoa. Whether or not, however, there be
such a separation of the epithelium into, male and female

312 INVERTEBRATE MORPHOLOGY.

areas, the reproductive elements make their way into a com-
mou hermaphrodite duct, which presents variations of structure
in different forms and receives the secretion of certain acces-
sory glands. In its simplest form, as seen for instance in
Aplysia, the duct runs forward, pursuing a somewhat tortuous
course and becomes surrounded by an albuminiparous gland,
from which it receives a viscid secretion, within which the
ova become imbedded just in front of the point where the
gland opens into the duct. The latter has attached to it a
pouch-like structure, the vesicula seminalis, and is continued
on as a somewhat wider tube to open to the exterior at the
genital pore situated on the right side of the body, shortly
before reaching the pore, however, receiving a duct from a
globular sac, the spermatheca. From the anterior edge of the
pore a groove, the seminal groove, extends along the right
side of the body to the neck region, where it ends in a mus-
cular evertible penis, situated near the anterior right tentacle.
It seems probable that the spermatozoa mature before the
ova, and passing to the vesicula are stored up there. During
copulation the seminal fluid is transferred through the penis
to the spermatheca of another individual (perhaps the trans-
ference is a mutual interchange), and when later the ova pass
along the duct they are impregnated by the spermatozoa
so stored away, a cross-fertilization being thus brought

about.

This arrangement of the reproductive duct is found in the
more primitive Opisthobranchs, i.e., in those in which the
mantle-lobe still persists, and in the group Pteropoda-, in
the more highly-modified forms, such as Doris, JEolis, etc.,
and, among the more simple forms, in Pleurobranchwa the
hermaphrodite duct divides into an oviduct and a vas deferens.
The former after receiving the spermathecal duct opens into
a genital atrium, with which communicate also the albumiuipa-
rous gland and a nidamental gland, which manufactures the
outer shell-like investment of the ova. The vas defereus, after
a more or less tortuous course, enters the muscular saclike
penis-sheath which communicates with the genital atrium; the
enlarged termination of the vas, the penis, being thus capable
of eversion through the pore by which the atrium communicates

TYPE MOLLUSC A. 313

with the exterior. This couditiou seems to be a secondary
modification of one in which the oviduct and vas deferens
open independently at widely separated points — a condition
which is represented by a few Opisthobrauchs.

Suborder Tectibranchia.

The Tectibrauchiates are those Opisthobrauchs which
present the smallest amount of modification from what has
been considered the typical Gasteropod structure. A more
or less developed mantle-fold is usually present, sometimes
sufficiently voluminous to cover in the single branchia which
persists (Bulla), but frequently represented only by a slight
fold, which leaves the branchia exposed (Aplysia, Gasterop-
terori). A shell is very generally present, sometimes well de-
veloped (Bullet), but in other cases reduced to a plate-like
structure enclosed within the mantle which has been reflected
over it and the lips of the reflected portion meeting and fusing
(Aplysia, Pleurobranchus). The visceral hump, however, is as
a rule low and elongated in the direction of the long axis of
the body instead of at right angles to it, as in the majority of
Prosobrauchs. In many members of the group the foot pos-
sesses a broad creeping surface, but its margins are prolonged
into broad thin wings, the parapodia, which may be bent up-
wards, as in Aplysia, so as almost to enclose the body.

The Tectibranchiates are divisible into two groups accord-
ing to their habits, in accordance with which the form of the
foot and especially of the parapodia is modified. Those forms
which possess a broad flat sole to the foot have a creeping
habit ; but there are many forms which are pelagic in habit
and present many modifications of structure in adaptation to
this mode of life, and were consequently classified at one time
as a distinct order, the PTEROPODA, and consequently call for
special mention. One of the most characteristic features of
this group is the foot, which is limited to the anterior portion
of the body and consists of a small median portion and two
lateral wing-like parapodia arising from the sides of the
median portion, and by the rapid flapping of wdiich the

314

INVERTEBRATE MORPHOLOGY.

animals are propelled through the water. In their general
form much diversity is observable. In accordance with their
pelagic habits the majority are more or less transparent ; and
some, the Gymnosomata, e.g. Pneumoderma, Clione, etc., are
entirely destitute of a shell, mantle, and, except in Pneuino-
derma and its allies, of a brauchia. Others, the Thecosomata,
possess these structures, however — the shell in Limacina being
spirally coiled, the mantle-cavity situated in front of the

FlG. 140. — Hyalea complanata (after GECENBAUR, from HERTWIG).

m = mantle.

a = anus.

br = branchiae.
c = heart.

O = reproductive organs.
Jt — digestive gland.

oe = oesophagus.
re = nephridia.
v = stomach.
II = pedal ganglion.

visceral hump being without a brauchia ; iii Sfyliola the shell
is not coiled, but is cone-shaped and bilaterally symmetrical,
the mantle- cavity containing a gill ; while in Cyuibuliopsis the
original shell is replaced by a cartilaginous case formed by
the subepidermal tissues of the mantle, and the voluminous
mantle-cavity contains no gill. The head of the Gynmoso-
mata carries a non-retractile proboscis, at the extremity
of which is situated the mouth, and it may furthermore bear
in addition to the tentacles usually present peculiar tentacle-
like processes, sometimes provided with suckers and perhaps

TYPE MOLLUSC A. 315

modifications of portions of the foot, being innervated from
the pedal ganglia. In these forms also fringed or simple pro-
cesses of the posterior portion of the body occur which serve
as respiratory organs, though they are not homologous with
the true branchia which in Pneumoderma coexist with them.

Suborder Nudibranchia,

In the Nudibranchs the visceral hump has undergone
elongation parallel with the long axis of the foot, from which it
is not distinctly marked off, and an apparent bilateral sym-
metry is manifested by the body. This condition, however,
is evidently entirely secondary, as is shown by the structure
of the heart and nephridium, in which the usual asymmetry is
well marked. There is no shell, mantle, or ctenidia. Adaptive
branchiae are, however, frequently developed, as in Pleurophyl-
lidia, where they form a series of folds
which lie in a groove at the side of the
body and recall somewhat the arrange-
ment in the Chitonidse, or in Doris, where
they surround the anus, which has a
dorsal position, and form a circle of pin-
nate processes. In the pelagic Phyllirhoe
and in the creeping Limapontia, however,
there is no trace of respiratory organs.
Many forms (Fig. 141), such as ^Jolis,
Facellina, and their allies, bear upon the
dorsal surface of the body numbers of
finger-like processes usually arranged in
bunches, and frequently brightly colored.
These cerata frequently enclose branches
from the intestine which correspond to
the digestive gland of other forms, and pia 141 _ NUDIRRAN.
bear at their extremities a sac in which CHIATE OPISTHO-
are developed nematocysts. These organs BRANCH (^Eolid).
are usually richly provided with blood-
vessels, and are probably respiratory in function, though
the presence of nematocysts renders it probable that they
are also protective — an idea which is confirmed by their

316

INVERTEBRA TE MORPHOLOG T.

usually brilliant coloration. The foot in the pelagic Pliyl-
lirhoe has entirely disappeared, but is usually elongated and
provided with a broad flat surface, in accordance with the
creeping habits of the Nudibranchs. Parapodial folds, such
as occur in the Tectibranchs, are never developed.

Order Pulmonata.

The Pulmouates differ from all the other groups of Gas-
^teropods in that they are, with the exception of a single genus,
Onchidium, either terrestrial or aquatic ; and in adaptation to
this assumed habit certain well-defined changes have occurred.
In some genera, more especially the aquatic forms, such as
Limncea, Physa, and Planorbis, the visceral hump has its typi-
cal Gasteropod development, and is spirally coiled ; but in
many terrestrial forms, such as Limax (Fig. 142, A], Arion,

and Fagimda, it is low and elon-
gated parallel to the long axis
of the foot with which it is
fused. The mantle is in all
forms well developed, but pre-
sents the peculiarity that it is
fused by its edges to the body-
wall except at one point upon
the right side, where an open-
ing is left by which the other-
wise completely-closed mautle-
' cavity communicates with the
exterior and through which air

may be taken into the cavity. The position of the mantle-
cavity, when not interfered with by secondary changes, is
upon the right side of the body and somewhat in front of
the visceral hump when this is present. A spirally-coiled
shell is present in all forms in which the visceral hump
is well developed, as in Limncva, Physa, Helix (Fig. 142, 13],
and Planorbis, but in the elongated terrestrial forms a
rudimentatiou of the shell accompanies the diminution of
the visceral hump. Thus in Daudebardia, in which only
a slight trace of the hump persists, the shell has become

FIG. 142. — A, Limax maximum,
Helix (after HOWES).

TYPE MOLLUSC A. 317

quite small, tlicmgli still showing plainly a spiral form ; but in
Limax it is represented only by a partially calcified plate, im-
bedded in the roof of the mantle- cavity by the closure over
it of a fold of the mantle. In Arion only a few isolated parti-
cles of carbonate of lime persist, while in Vaginula and Onchi-
dium all trace of it has disappeared.

A marked characteristic of the Pulmonata is found in the
character of their respiratory organ. A cteuidium is entirely
wanting, the only trace of its existence being the occurrence
in some of the aquatic forms (Limncea, Pliysa, etc.), of an os-
phradium near the mantle-pore. Its place is taken by the
roof of the mantle-cavity, which receives a rich vascular net-
work and functions as a lung, the mantle-cavity containing
air which can be renewed through the mantle-pore. The
heart is situated far back in the mantle-cavity, its auricle
lying in front of the ventricle and receiving the blood from
the more anteriorly-situated lung, so that the relation of the
~ respiratory organ to the heart is the same as obtains in the
ivYlK Opityfckobranchs. In the immediate neighborhood of the
heart lies the single uephridium, opening into the mantle-
cavity or else into the terminal portion of the rectum (Helix),
this structure opening on the right side of the body in close
proximity to the mantle-pore.

As in the Opisthobrauchs, the rotation of the mantle-cavity
and its organs, as indicated by its position on the side of the
body, has not extended as far as in the Prosobranchs, and
consequently there is no crossing of the pleuro-visceral con-
nectives. The Pulmonates are orthoueurous. The ganglia
are present in typical number, and are massed together, as in
some Opisthobranchs and Prosobranchs, behind the buccal
mass.

Special visual organs are invariably present with the struct-
ure which has already been described. In some forms they
are situated, as in the Prosobranchs, at the bases of the ten-
tacles ; while in others they are found at the tips of these
structures — the Pulmouates being divisible, according to the
situation of the eyes, into the Basommatopliora, including such
forms as Limncea, Physa, Planorbis, and in general the aqua-

318 INVERTEBRATE MORPHOLOGY.

tic forms, and the Stylommatophora, which includes the ter-
restrial forms, and Onchidium.

This last genus in addition to the usual eyes borne upon the tentacles
is in some species further provided with a number of eyes situated upon
the back and differing from the typical eye in the arrangement of the retinal
cells. As has been seen, the optic nerve in typical eyes on entering the eye
spreads out in a layer to form the retina, the terminal optic cells being
situated on that surface of the retina which is turned towards the light.
The dorsal eyes of Onchidium, however, present a somewhat different
arrangement, the cells in which the nerve-fibres tei initiate having their
distal ends turned away from the light, which to affect them must pass
through the layer of nerve-fibres formed by the spreading out of the optic
nerve. Compared with the retinae of typical eyes, those of the dorsal eyes
of Onchidium are inverted and have assumed an arrangement exceedingly
rare in Invertebrates, but typical for the lateral eyes of the Vertebrata.

Otocysts are always present, and the tentacles borne by the
head are probably tactile in function. In the Stylornmatoph-
orous Pulmonates there are in some cases (Helix) two pairs
of such tentacles, the eyes being situated upon the posterior
pair, both pairs furthermore being capable of being invagi-
nated for protection into the body-cavity, a peculiarity not
presented by the tentacles of the Basomrnatophora. As
stated above, the osphradium is represented in certain aquatic
forms, but in the Stylommatophora it has disappeared with
the suppression of the ctenidiuni.

The Pulmouata are hermaphrodite, the epithelium of the
reproductive gland (Fig. 143, hg) differentiating into both
spermatozoa and ova, there being no localization of the for-
mation of either one or the other in a special portion of the
gland, as happens in some Opisthobranchs. In the Basom-
matophora and certain terrestrial Pulmonates, such as Vagi-
nula and Onchidium, the common duct (hd) for the spermatozoa
and ova divides and passes to the exterior by two distinct
and separate apertures. Thus in Limncea the hermaphro-
dite duct shortly after leaving the gland divides, and into one
of the branches immediately after the division there opens a
well-developed albuminiparous gland (cd\ and it then becomes
somewhat folded, forming what is termed the uterus (w£).
Beyond this structure the duct, now known as the oviduct (od),
receives the duct of a nidameutal gland and dilates into a

TYPE MOLLUSC A.

319

large pyriform structure, which tapers somewhat to form a
vagina opening to the exterior and receives a duct from the
receptaculuni semiuis. The vas deferens (vd) shortly after
its separation from the hermaphrodite duct dilates into a
glandular structure, the prostate gland, from which the nar-
row duct passes onward to terminate in an enlarged penis-
sheath (pe) which contains the
muscular protrusible penis
and opens to the exterior
quite independent of the
opening of the vagina.

In the majority of the
Stylommatophora (Fig. 1-13),
however, the two ducts open
into a common atrium so that
only one genital orifice occurs,
as in some of the Opistho-
branchs (see p. 312). Other-
wise the arrangement is simi-
lar to what has been de-
scribed for the Basornmato-
phora, except that in some
forms, as Helix, one or two FIG. 143.— REPRODUCTIVE ORGANS OF
additional accessory struc- Limax maximus <aftel
tures are added. Thus the
atrium has communicating
with it a sac which contains
a sharp calcareous rod, the
"dart," which serves as a
stimulus during copulation,
being plunged into the body
of the. other party to the act;
and again just at the point where the vas defereus opens
into the penis it has communicating with it an elongated
tubular structure, the "flagellum," which perhaps furnishes
the material of which the capsule of the spermatophores is
composed.

Development and Affinities of the Gasteropods. — The devel-
opment of the Gasteropods is made interesting on account of

al = albumiuipavous glaud.
hd = hermaphrodite duct.
kg — hermaphrodite gland.
li — ligament.
od = oviduct.
pe = penis-sheath.
rs = receptaculum seminis.
at = uterus.
vd = vas defereus.
vs = vesicula semiualis.

320

INVERTEBRATE MORPHOLOGY.

the occurrence in the majority of forms of a larva known as
the Veliger (Fig. 144) which presents many interesting affin-
ities to the Annelid Trochophore. In the early stages of de-
velopment the embryo is strictly
bilateral, with the mouth and
anus at the extremities of the
longitudinal axis. Upon the dor-
sal surface posteriorly is a de-
pression lined with columnar cells
which secrete the larval shell (Sh),
and in front of this is an area
enclosed by two rows of cells
bearing stout cilia and forming
the velum (V}. This band of
cilia is praeoral (Pro) in position,
and in addition to it a second
band of smaller cilia is to be
found which passes ventrally to
the mouth and constitutes a post-
oral baud (Poo), the groove be-
t-ween it and the pmeoral baud
being occupied by the adoral cilia. On the ventral surface
is found a prominence which represents the foot.

In later stages the lateral edges of the velum are drawn
out so as to form a broad lobe, sometimes divided into two
arms, projecting on each side of the head ; the prseoral and
postoral bands of cilia extending round the margin of the
fold, not, however, completely enclosing the velar area, but re-
maining open on the dorsal surface. The shell area increases
markedly in size, the shell becoming spirally coiled, the vis-
ceral hump which develops in the shell area likewise assum-
ing the coiled form. At the margins of the shell area a fold
appears, the rudiment of the mantle, which gradually increases
in size as the shell area extends, and at the same time the
anus becomes rotated forwards from its original terminal posi-
tion along the right side of the body to a greater or less ex-
tent. As these changes progress, the embryo gradually ap-
proaches more and more to the adult form, differing from it
mainly in the existence of the velum, by means of which it

FIG. 144.— VELIGER LAKVA.
F~ foot.
M = mouth.
Oc = eye.

Poo = postoral band of cilia.
Pro = praoral baud of cilia.
Sh = shell.
T = tentacle.
V — velum.

TYPE MOLLU8CA. 321

leads a free-swimming pelagic existence, assuming the adult
habit only after a further growth which is accompanied by a
reduction of the velum.

Such a Veliger larva occurs in the life-history of the majority of the
Gasteropoda, though, as might be expected, it undergoes certain modifica-
tions more especially in terrestrial forms, though even in these there are
ample indications of its existence. Indeed the Veliger is so frequent in its
occurrence that the conclusion is almost unavoidable that it has an ances-
tral significance and represents in a more or less modified condition a
primitive form from which the Mollusca have descended. A comparison of
the Veliger with the Annelid Trochophore brings out, as already men-
tioned, numerous similarities. These are especially noticeable in the ar-
rangement of the ciliary bauds, which resemble those of the Trochophore
part for part, even to the dorsal break in their continuity. It is difficult
to believe that such marked similarities should have been acquired inde-
pendently in the larva} of two different groups of animals and have become
so characteristic, a difficulty rendered all the greater by the occurrence of
other points of similarity, such as the development of the mesoderm, in
some forms at least, from a pair of mesoblasts situated at the posterior
extremity of the blastocoel ; the existence of a thickening of the ectoderm
in the centre of the velar area in some forms, corresponding to the apical
plate of the Trocophore ; and the occurrence of a larval excretory organ or
nephridium in some Veligers which may be compared to the larval ne-
phridium or head-kidney of the Trochophore. The probable significance of
this larval form will be more suitably discussed at the conclusion of this
chapter ; it remains to be said here regarding it that the occurrence among
the Pteropods of larvae with several bands of cilia surrounding the visceral
hump is probably to be explained as a secondary adaptation, just as the
mesotrochal Annelid larvae are probably secondary modifications of a Tro-
chophore.

As regards the relationships of the various groups of Gasteropods
among themselves, there is little doubt but that the Diotocardiate Proso-
branchs are, on the whole, the most primitive of all the groups and stand
nearest to the Amphineura, and from them the Monotocardia have devel-
oped. The Opisthobranchs and Pulmonates are apparently closely re-
lated, the latter group having been derived from Tectibranchiate ancestors
somewhat more generalized probably than any Opisthobranch now living.
The orthoneurous character of the nervous system and the structure of the
reproductive system in the two groups indicates their affinity, and it seems
probable that the Pulmonates are to be regarded as Opisthobranchs which
have accommodated themselves at first to an amphibious life, somewhat
similar to that now led by Onchidinm, and later to one purely terrestrial,
at the same time differentiating an organ for aerial respiration. Such an
origin would imply that the aquatic species have secondarily taken to fresh
water as a habitat, having originally been terrestrial, an idea which on

322 INVERTEBRATE MORPHOLOGY.

a priori grounds seems improbable ; but there seems to be no good reason,
if the aquatic forms are derived directly from marine ancestors, why their
ctenidia should have become replaced by a lung, since in the aquatic Pro-
sobranch Paludina the ctenidium is still retained. On the other hand, it
may be again mentioned that the terrestrial Prosobraucbs such as Cyclo-
stoma, Acicula, etc., have lost their ctenidium and resemble a Pulmonate
in their mode of respiration.

III. CLASS SCAPHOPODA.

The class Scaphopoda contains a small number of closely-
related genera of marine Mollusca, Dentalium, Siplionodenta-
lium, Cadulus, etc., living imbedded in the sand in depths of
from 10 to 100 fathoms and possessing but slight powers of
locomotion. They resemble the Gasteropoda in possessing a
visceral hump which is relatively enormously elongated but
does not undergo a spiral twisting, nor has it fallen over to
the right or left side of the body. Consequently the Scaph-
opods are bilaterally symmetrical and stand in marked con-
trast in this respect to the Gasteropods.

The mantle-folds are two in number, arising from the
anterior surface of the visceral hump and extending around
the body so as to completely enclose it, meeting posteriorly
and fusing together, except for a short extent, dorsally and
ventrally, and forming thus a tube to the anterior wall of
which the body is as it were attached. This tube is open at
either end, the ventral opening being somewhat larger than
the dorsal one, and the whole is enclosed within a tubular
shell (Fig. 145, sli) whose shape corresponds essentially to that
of the mantle. From the ventral opening the foot (/) pro-
jects to a greater or less extent, being in Dentalium a cylin-
drical structure, terminating in a conical process provided
with two lateral lobes.

The mouth (ni) is situated at the extremity of a cylindrical
proboscis (not to be confounded with the protrusible proboscis
of a Gasteropod) and is surrounded by a number of leaflike
tentacles, while at the base of the snout there is upon each
side a bunch of long filamentous tentacles (t) capable of being
protruded from the mouth of the shell and of being withdrawn
within it. Each tentacle terminates in a spoon-shaped struct-

TYPE MOLLUSC A.

323

l-sh

sn - ••-

VI

me -

pe

ure whose concave surface is furnished with ciliated cells and
also towards the margin with unicellular glands. These
structures have been supposed to
represent the cteuidia of the other
Mollusca, but this view cannot, in the
present condition of our information
concerning their structure and devel-
opment, be accepted without reser-
vation. The mouth opens into a
short oesophagus provided with a
single chitinous jaw-tooth apparently
formed by a fusion of two chitinous

masses, and behind this there is a m J

pharynx provided with a radula and
opening posteriorly into the some-
what U-shaped more or less convolu-
ted intestine (i) which terminates in
the anus (a) lying in the mid-ventral
line behind the foot. Into the intes-
tine at the turn of the U there open
the ducts of the digestive gland (I),

and into the posterior portion of the FIG. 145. — STRUCTURE OF
intestine, the rectum, there open in
Dentalium several ducts from a rectal
gland which surrounds this portion
of the digestive tract and whose sig-
nificance is quite obscure.

The nervous system presents the
majority of the ganglia characteristic
of the Gasteropoda, and the pleuro-
visceral connectives do not cross one
another. The cerebral ganglia (ce)
lie at the base of the proboscis an-
terior to the oesophagus and have
closely associated with them the pleural ganglia, the
cerebro-pedal and pleuro-pedal connectives fusing with one
another to pass downwards and forwards to the pedal ganglion
(pe) situated in the foot. Posteriorly in the vicinity of the
rectum lie the two visceral ganglia (vi) from which long

Dentalium (after LEUCKART).
a = aims.

ce = cerebral ganglion.
/ = foot.
i = intestine.
I = liver.
m = mouth.
me = mantle-cavity.
pe — pedal ganglion.
r = reproductive organ.
rn = right nephridium.
s7i = shell.

t = tentacle.
m = visceral ganglion.

324 INVERTEBRATE MORPHOLOGY.

nerves pass dorsally, but no special parietal ganglia occur.
Two pairs of buccal ganglia are also present. Otocysts are
present imbedded in the foot in tlie neighborhood of the
pedal ganglia, but no other special organs of sense, unless
the bunches of tentacles be considered such, occur.

No special respiratory organs are developed, the mantle
probably subserving the respiratory function. The heart, a
simple invagination of the wall of the pericardia! cavity, lies
in the posterior region of the body, on the dorsal surface of
the intestine. It possesses no auricle, but receives the blood
through small slits in its walls. There are no special blood-
vessels, but the blood circulates through a series of sinuses
traversing the body in various directions.

A pair of uephridia occurs in the posterior region of the
body, opening to the exterior by a pore on either side of the
anus, but a communication with the pericardial cavity is said
to be wanting. However this may be, the right uephridiuni
(rri) serves for the exit of the reproductive elements, though
the exact method by which these latter make their way into
the duct is unknown. Between each nephridial pore and the
anus there is a pore which seems to be the opening of a short
tube which communicates directly with the schizocoelic sinus
surrounding the terminal portion of the intestine and places
it in communication with the surrounding water, a peculiar
arrangement which recalls the dorsal pores of the oligo-
cha3tous Annelids. The Scaphopods are bisexual, and the
reproductive organs, ovaries or testes, are single, consisting of
long completely closed sacs with lateral diverticula, Ij'ing
along the posterior wall of the visceral hump. As already
stated, the reproductive elements after the rupture of the wall
of the reproductive gland make their way to the exterior
through the right nephridium.

Development and Affinities of the Scaphopoda. — The larva of Denta-
lium, though presenting considerable resemblance to the Trochophore,
differs from it nevertheless in several points of detail. It possesses a dis-
tinct apical tuft of cilia and the prototroch is present, though represented
by three or more circles of cilia-bearing cells. The mantle-folds develop at
a relatively early stage as two lateral folds, quite separate along the ventral
line, the fusion characteristic of the adult only appearing later. It is this
early development of the mantle-lobes and the multiplication of the proto-

TYPE MOLLUSC A.

325

troch bauds which obscure the Trochophore characters, a still earlier larva
preseutiug greater similarities to the annelid larva.

By the earlier writers the Scaphophods were considered more closely
related to the Pelecypoda than to the other Molluscau groups, this relation-
ship being indicated more especially by the symmetrical form, the apparent
lateral arrangement of the mantle-folds and the absence of eyes. On the
other hand, there are a large number of differences between the members
of the two groups, as for instance the univalve character of the shell, and
especially the occurrence of a radula and jaw. This latter feature suggests

Fie. 146. — DIAGRAMS TO SHOW THE ORIGIN OP THE SCAPHOPODS FROM A Fis-

suretla-Uke ANCESTOR (after PLATE).
ct = ctenidium. go = reproductive organ.

/ = foot. ra = uiouth.

sh = shell.

the Gasteropods, and it seems most probable that it is to this group that
the Scaphopods should be considered as related. They must, however, be
referred to the more primitive Gasteropods, those in which the rotation of
the mantle-chamber had not occurred. An elongation of the dorsal hump
of a Fissuretta-like ancestor unaccompanied by a twisting to one side, as
represented in Fig. 146, would bring about a condition from which it does
not seem a great step to reach the Scaphopods.

Among recent Gasteropods it is with the Diotocardiates that the
Scaphopods seem to be most affiliated, and, as we shall later see, it is from
the primitive members of this order that the Pelecypods have probably
been derived, and thus any similarities which may exist between the
Scaphopods and Pelecypods is readily explicable on the basis of a similar
ancestry, both groups being derived from Prosobranch-like forms. The
absence of a larva corresponding closely to the Gasteropod Veliger would
seem to oppose such a view, but it must be remembered that the Veliger is
characteristic only of the more highly-differentiated Prosobranchs — such

326 INVERTEBRATE MORPHOLOGY.

forms as Patella, for instance, having a larva destitute of some of the
more characteristic Veliger features and more closely resembling the
Annelid Trochophore and the Scaphopod larva.

IV. CLASS PELECYPODA.

The class Pelecypoda, also known as the Lamdlibrancliia,
contains a number of fresh- water genera, though the majority
are marine, and all its members retain the primitive bilateral
symmetry of form, no visceral hump being developed. The
body is more or less laterally compressed and two large
mantle-folds (Fig. 149, m) are developed, arising one on each
side a short distance ventrad of the dorsal mid-line and extend-
ing downward so as to meet below. They thus enclose a wide
space, the mantle-cavity, between their inner surfaces and the
body-wall, within which lie the cteuidia (Fig. 148, ct) and the
foot (p). Upon the mantle-edge in many forms tentacles,
papillae, glands, and eyes are developed, and in many cases
the edges of the two lobes may fuse more or less completely,
openings being, howrever, left for the entrance and exit of
water into the mantle-cavity, and also for the protrusion of
the foot. All gradations of fusion are represented : thus in
Niicula, Ostrea, etc., there is no fusion whatever; in Unio
(Fig. 149) and other forms the posterior edges of the mantle-
folds are modified, so that while the edges of the folds are in
contact throughout the greater portion of their extent two
openings are left, through the uppermost of which, the exha-
lent opening (eo), water carrying with it the excreta and the
reproductive elements finds an exit, while through the lower
one, the inhalent opening (io\ fresh water passes in ; in the
next gradation the point of separation between these two
openings, which in Unio was simply formed by the contact of
the mantle-edges, becomes permanent by the fusion of these
latter parts, and a further stage, seen in Venus for example,
is formed by the fusion of the mantle-edges ventral to the
branchial opening, a fusion which may extend forward a con-
siderable distance. In this last condition there are three
openings which place the mantle-cavity in communication
with the exterior, one anterior, through which the foot is pro-

TYPE MOLLUSC A.

327

truded, aud two posterior, the braucliial aud anal openings.
The mantle around these latter frequently becomes prolonged
so that two tubes, or siphons as they are termed, ai;e formed,
sometimes in contact with one another (Pholas), sometimes
quite separate ( Venus), sometimes capable of retraction within
the shell, sometimes so large as to be incapable of retraction
(Mya}.

FIG. 147. — A, Mya arenariavrhk the siphons slightly expanded; B, iuner sur-
face of the right valve of the shell of Mya.

aa = impression of anterior ad- pa = impression of posterior ad-
ductor muscle. ductor muscle.

I = ligament. pi = pallial line.
m = mantle edge. s = siphon.

p = foot. si = siphoual impression.

In conformity with the form of the mantle-lobes the shell
consists of two similar portions or valves, lying on the sides
of the body and united along the dorsal mid-line by a hinge.
The hinge is formed by a ligament, as it is termed, which is
really a portion of the shell substance, and consists of an
external portion continuous with the epidermis of the shell
and an internal elastic portion, frequently calcined to a cer-
tain extent, and continuous with the middle layer (prismatic
layer) of the shell. When at rest the two valves of the shell
are kept apart along the ventral line by the elasticity of the
hinge-ligament, and it is only by the application of force that

328 INVERTEBRATE MORPHOLOGY,

the two valves can be brought together, the ligament being
then compressed. The hinge is frequently complicated by
the development of tooth-like processes and corresponding
sockets so that the two valves may be firmly locked together.
Upon the inner surface of the valves are certain impressions
produced by the softer parts and of considerable value in
systematic couchology. A short distance from the margin of
each valve and parallel to it is a distinct line, the pallial
impression (Fig. 147, pi), produced by the attachment of the
muscle-fibres which bind the mantle-lobes to the shell. In
some forms, such as Anodon, this pallial line follows the shell
margin throughout its entire course, but in those genera
which possess well-developed and retractile siphons it is
deeply incurved in the posterior portion of its course. Other
markings of the shells are produced by the insertion into
them of a number of muscles. The largest and most import-
ant of these are the adductor muscles of the shell (aa), large
muscles passing from one valve to the other, by their contrac-
tion overcoming the elasticity of the hinge-ligament and clos-
ing the shell. In the majority of forms there are two such
muscles, situated towards the anterior and posterior portion
of the bod}', but not imfrequently, as in Ostrea and Pecten,
but one, corresponding to the posterior adductor of other
forms, is present. In the immediate vicinity of the adductor-
impressions other smaller muscle-impressions are usually
observable, produced b}T the protractor and retractor muscles
of the foot and siphons.

Although in the Pelecypod shell the two valves are typically similar
and symmetrical, yet in a number of cases a marked dissimilarity is found
in their shape. Thus in Ostrea the valve upon which the animal rests,
usually the left valve, is large and concave, while the other is smaller
and flattened, and a similar relation is found in other forms which be-
come temporarily fastened to rocks, etc. Occasionally additional cal-
careous plates are added to the usual shell, as in the boring mollusk Pholas,
in which three accessory calcareous plates are developed on the dorsal sur-
face of the body. In the Ship-worm, or Teredo, which bores extensively
into timber and is in some cases exceedingly destructive, the true shell-
valves are very small and situated at the anterior end of the body, and the
mantle projects backwards far beyond them and secretes a thin calcareous
tube which lines the interior of the passages excavated by the animal. A

TYPE MOLLUSC A. 829

similar peculiarity is found in the Aspergillum. Here, too, the true shell-
valves are exceedingly small and are united together by and imbedded in
a calcareous tube secreted by the mantle, which projects far beyond the
shell proper and is fused throughout the greater portion of its extent.
The calcareous tube is open behind for the passage of the two siphons,
but anteriorly is closed by a perforated plate, the margins of the perfora-
tions being sometimes prolonged into tubes which may branch dichoto-
mousiy. The animal lives imbedded in the sand, the posterior ex-
tremity of the shell being directed upwards, and seems to have been
derived from forms originally possessing a boring habit, such as is seen in
Teredo.

The foot of the Pelecypoda is as a rule very simple. In
tlie most primitive members of the group, such as Nucula
(Fig. 151), it is a flat disk-like structure, recalling somewhat
the foot of the Gasteropoda, but more usually it is a keel-
shaped structure (Fig. 149, p). The modifications in shape
which it undergoes are, however, numerous and it may even
in some cases be almost absent, as in the Oyster (Ostrea), but
special developments, such as epipodia, are never found in
connection with it. A " byssus-gland " is a characteristic
development of the Pelecypod foot, consisting of a cavity
with usually greatly folded walls lying in the tissues of the
foot and connected with the exterior by a canal opening on
the sole of the foot. By the cells lining the cavity threads of
a horny consistency are secreted by means of which the
animal is enabled to fasten itself to stones, etc., or even in
some cases, as Mytilus, to move about in the absence of a
well-developed foot, throwing out byssus filaments, attaching
them, and then drawing itself forward towards them.

The respiratory organs (Fig. 149, br) of the Pelecypoda
consist of a pair of platelike structures situated on each side of
the body, and being attached along their dorsal margins hang
down between the mantle and the body-wall. Notwithstand-
ing their platelike form they are modifications of the plumose
ctenidium of the Gasteropods. If the typical b.ipiuuate
cteuidiuni be imagined to be directed parallel to the long
axis of the body and the median axis to have fused with the
body-wall, so that the two rows of pinnae are bent down so
as to lie parallel to one another, the simplest form of the
Pelecypod ctenidium, such as occurs in Nucula (Fig. 151), will

330

INVERTEBRATE MORPHOLOGY.

be obtained. lu the majority of forms, however, the arrange-
ment is much more complicated than this. Thus in Mytilus
it will be found that the various pinnre composing each
plate are held together by a series of patches of strong cilia

F/G. 148. — A, diagrammatic section through Pecten, and B, through Anodon;

C, section through gill-lamella of Pecten, and D, of Anodon.
au = auricle. ol = outer lamella of outer gill.

/ = foot. pc = pericardia! cavity.

gf = gill-filament. p = pore.

iil = inner lamella of inner gill. s = blood-sinus.

il — interlamellar junction. sbr = suprabranchial chamber.

ne = nephridium. s/t = shell.

which interlock forming the " ciliated junctions," and further-
more the pinnae are at their free ends bent abruptly upon
themselves, those of the outer row outwards and those of the
inner row inwards, so that each gill-plate is composed of two
lamellae (Fig. 148, A). This condition may be regarded as the
next step in the modification, which is continued even
further by the permanent union of the outer and inner limbs

TYPE MOLLUSCA. 331

of the pinnae, or gill-filaments as they may be called, by hollow
processes, the " iuterlamellar junctions " (Fig. 148, C, il).
A still greater departure from the primitive condition
is found, however, in the greater number of existing Pelecy-
pods, consisting of a fusion of all the filaments of each lamella
into a plate (Fig. 148, Z>), small openings (p) only being left here
and there between adjacent filaments ; furthermore the inter-
lamellar junctions become very well developed, so that the
two lamellae of each gill become firmly united together to form
a plate, containing in the interior a cavity, the interlamellar
space.

In addition to these various modifications which lead to
the formation of a true lamellate gill, the edge of the external
lamella of the outer plate fuses with the inner surface of the
mantle, and the internal lamella of the inner plate fuses sim-
ilarly with the side of the foot (Fig. 148, B], and the mantle-
cavity thus becomes divided into two chambers. Into the ven-
tral chamber the iuhalent siphon opens, and the water which
enters by it passes through the openings left between the
filaments and so reaches the iuterlamellar spaces which com-
municate above with the dorsal or suprabranchial chamber
(sbr)t whence it passes to the exterior through the exhalent
siphon. In the region of the foot the suprabranchial cham-
ber is of course divided into two portions, one of which lies
011 each side of the base of the foot, and each of these is again
divided longitudinally into an inner and an outer portion by
the line attachment of the gills to what may be considered
the roof of the mantle-cavity. Behind the foot the inner cavities
of the two sides unite and in some forms open ventrally into
the mantle-cavity proper ; in others, however, the inner lamellae
of the inner gill-plates fuse with one another along the middle
line so that a distinct partition, formed by the gills, sepa-
rates the suprabrauchial chamber from the ventral mantle-
chamber throughout its entire length. In a few forms, such
as Cuspidaria, the gills become reduced to simple muscular
partitions perforated by pores and separating the two cham-
bers, practically all indication of the original cteuidium
characters having disappeared.

The muscular system of the Peleeypoda reaches a some-

332 INVERTEBRATE MORPHOLOGY.

what extensive development in connection with the presence
of the bivalved shell. The mantle-folds are as a rule some-
what richly provided with muscle-fibres especially near the
margin ; and where siphons are developed some of the fibres
are specialized into retractors for these organs. For the closure
of the shell- valves, however, more extensive muscular bands are
present which seem, like the siphoual retractors, to be special-
ized portions of the mantle musculature. Of these shell-
adductors there may be one, as in Ostrea and Pecten, or two, as in
Anodon (Fig. 149, cm and pa], which pass transversely across
the body from one shell-valve to the other, in the form of
stout compact muscular bands. In connection with the foot
special bauds are also developed which function as protrac-
tors (pp), retractors (rp), and elevators arranged in pairs and
extending from the inner surfaces of the shell-valves to spread
out below in the foot. These various bundles seem to cor-
respond to the spindle-muscle of the Gasteropods.

The ccelom presents an arrangement similar to that of
other Mollusca, both schizoccelic and enteroccelic portions
being distinguishable. To the former portion belong the
numerous lacuuar spaces which traverse the body and mantle-
folds, and to the latter the pericardial cavity (Fig. 149, p) and
the cavity of the reproductive glands. The blood-vascular
S}Tstem consists of a heart provided with two lateral auricles
and lying in the pericardium. In the majority of forms the ven-
tricle (y) seems to be traversed by the terminal portion of the
digestive tract, a condition produced by its having folded itself
longitudinally around the rectum, and which recalls what
occurs in certain Diotocardiate Gasteropods (see p. 305). This
arrangement does not, however, obtain in all forms, some of
the more primitive (Nucula, Area] having the ventricle
entirely dorsal to the intestine, as it is in the Amphineura, for
example, while in a few others (Ostrea) it has assumed a
secondary position ventral to the intestine. From both the
anterior (ao) and posterior extremities of the ventricle arteries
arise which, after branching a number of times, pour the blood
into the schizoccelic lacunar system. Traversing this the
venous blood is returned to a longitudinal sinus lying in the
middle line of the body just below the pericardium (Fig.

TYPE MOLLUSC A. 333

148, B, s), whence the greater portion passes into the compli-
cated network of the nephridia and thence to a blood-vessel,
the branchial artery, running along the base of the gill of each
side. After traversing the gill-filaments it becomes arterial
and is returned to the branchial veins which run parallel to
the branchial arteries and thence is returned to the auricles
of the heart.

The digestive tract has a much simpler structure than in
the majority of the Mollusca, lacking all trace of a radula
and muscular pharynx. On each side of the mouth are two
usually triangular plates, the so-called labial palps, the upper-
most of which meet above the mouth forming a sort of upper
lip, while the lower ones similarly form a lower lip. At the
bottom of the space separating the two palps of each side is
a groove which, starting at the sides of the mouth, runs back-
wards along the sides of the body to the gills. This groove
serves for the conduction to the mouth of the particles of food
brought into the mantle-cavity by the action of the cilia of
the gills, the food of the Pelecypods consisting of diatoms
and other minute organisms capable of being captured in this
manner. The oesophagus opens into a stomach (Fig. 149, s)
which receives by numerous openings the secretion of the
usually voluminous digestive gland (7), the so-called liver, and
passes posteriorly into the intestine ({), which, usually in sev-
eral convolutions, lies imbedded in the tissues of the base of
the foot. In the Avail of the anterior portion of the intestine
is a groove, frequently converted into a canal, which may
open into the stomach by an independent opening ; the epi-
thelium of this groove or canal secretes a substance which
forms a transparent glass-like rod lying in the canal and pro-
jectiug into the lumen of the intestine. The function of this
crystalline style, as it is termed, has been the subject of much
speculation, the most plausible theory being that the secre-
tion serves to surround sharp-edged particles of sand or simi-
lar substances, taken into the intestine with food, with a jelly-
like coating which will prevent them from injuring the delicate
walls of the intestine. Towards its posterior end the intes-
tine bends upwards, i.e. dorsally, to a point in front of the heart
and then passes directly backwards to terminate in the anus

334

INVERTEBRA TE MORPHOLOG T.

(a) which opens into the suprabranchial chamber (sbr) in the
vicinity of the exhalent siphon. The relations of this rectum
to the heart have already been noted (p. 332).

The nervous system of the Pelecypoda differs somewhat
apparently from that of the Gasteropods, a smaller number

VI

eo/

sbt

Sb

br

m

FIG. 149. — STRUCTURE

a

aa
ao

br

eo
f

go

10

I

m
ne
npl

anus. np2 =
anterior adductor.

aorta. p =

gill. pa =

cerebral ganglion. pg -

exhaleut orifice of siphon. pp =

foot. r =

genital orifice. rp =

intestine. s =

inhalent orifice of siphon. sbr -

liver. sh =

mantle. v =

nephridium. m =
uephridial opening into pericar-
dial cavity.

OF Anodon.

uepbridiul opening into supra-
branchial chamber,
pericardia! cavity,
posterior adductor,
pedal ganglion,
protractor pedis.
reproductive organ,
retractor pedis.
stomach.

suprabranchial chamber,
shell,
ventricle,
visceral ganglion.

of ganglia being discernible. Above the oesophagus a short
distance behind the mouth is on either side a well-marked
ganglion (Fig. 149, eg] connected with its fellow of the oppo-
site side by a transverse commissure. In the more primitive
forms (Nucula) two ganglia are found on either side, of which
one evidently corresponds to the cerebral and the other to the
pleural ganglion of the Gasteropods. Where, therefore, as in
the majority of the Pelecypods, but a single ganglion occurs on

TYPE MOLLUSC A, 335

each side, it is to be regarded as a cerebro-pleural ganglion.
From each of these a pedal connective passes downwards into
the foot to terminate in a paired pedal ganglion (pg), and a
second strong connective passes backwards on each side of
the base of the foot to terminate in a large ganglion (vi), sit-
uated below the rectal portion of the intestine and frequently
in close proximity to the posterior adductor muscle, and which
from its relations is evidently to be regarded as representing
both the parietal and the visceral ganglia of the Gasteropods
and hence may be termed the viscero-parietal ganglion.

The sense-organs are of essentially the same nature as in
the Gasteropods. Tactile cells exist scattered over the sur-
face of the body, and are especially numerous in certain lo-
calities, as upon the siphons when these are present. A pair
of osphradia are also present situated above the viscero-parie-
tal ganglion close to the insertion of the bases of the gill-
plates into the side of the body ; and imbedded in the tissues
of the foot, usually in close proximity to the pedal ganglia,
though innervated by the cerebro-pleural, are a pair of oto-
cysts having the usual structure (see p. 283). In a number of
forms paired elevations, evidently of a sensory nature, have
been found in the neighborhood of the inner ends of the
siphons, or on the sides of the body a little in front of the
anus ; the function of these is doubtful, though it has been
suggested that they are olfactory.

Eyes are present in a number of forms and present various
degrees of complexity. In some cases a perception of sudden
variations in the intensity of light is present, as in the siphons
of some forms, without any distinct optic sense-organs being
developed. Sensory and pigment cells are present, however,
and may be regarded as forming a diffuse optic organ. No
eyes occur upon the head, nor are tentacles developed in any
of the Pelecypods, but large numbers of eyes are developed
upon the edge of the mantle of many forms, such as Pecten
and Spondylus. These eyes may be simple depressions of the
mantle-margin, the bottom of the depression being lined with
pigmented and sensory cells, a cuticle of varying thickness cov-
ering this retinal surface. Another form of eye (Fig. 150) also
occurs upon tentacular processes which presents an arrange-

336

INVERTEBRATE MORPHOLOGY,

inent unusual for Invertebrates. The extremity of the pro-
cess is occupied by a number of clear transparent cells which
serve as a cornea (co) and which are continuous with a zone of
pigmeuted cells (pg) analogous to an iris, and which pass grad-
ually over into ordinary ectodernial cells. Upon the inner
surface of the cornea is a mass of transparent cells constitut-
ing a lens (I), and below this lies the sensory portion of the
eye. The optic nerve as it conies towards the eye branches ;

1

co

pg

rt

FlG. 150. — EYE OP Pecten (modified slightly from PATTEN).
co = cornea. op, op' = optic nerve.

I = lens. pg = pigment-cells.

la = blood-lacuna. rt = retina.

tl = tapetum lucid um.

one branch (op'}, passing to one side of the eye, bends inwards
towards the axis of the eye between the retina-cells (rt) and
the lens. The sensory portion of the eye consequently is in-
verted, the retina-cells being turned away from the light which
must pass through the fibres of the optic nerve to reach them.
Below the retina and separated from it by a space is a layer
of tissue, the tapetum lucidum (tl), which serves as a reflector
and gives the metallic lustre which is characteristic of the

TYPE MOLLUSC A. 337

eye of Pecten, and below this again comes a pigment-layer

In a small number of forms, e.g. Area, peculiar compound
eyes are also found on the edge of the mantle. They form
slight rounded elevations and consist of a number of conical
retinal cells, each surrounded by a sheath of six cylindrical
pigment-cells. Each of these groups of retinal and pigment
cells is known as an ommatidium and is separated from the
adjoining ones by slender intermediate cells, so that on sur-
face view the composite character of the eye is very distinct.

The nephridia (Fig. 149, ne) of the Pelecypoda are always
paired, and each consists of a tube bent upon itself lying im-
mediately beneath the pericardia! cavity into which one of
the limbs opens («/>')' while the other communicates with the
suprabrauchial chamber (wp2), and so with the exterior. In
the simplest forms the entire extent of both limbs is glandu-
lar, but in the majority the limb which opens to the exterior
loses its glandular character and surrounds to a certain ex-
tent the glandular or proximal limb. In addition to these
nephridia, frequently known as the organs of Bojauus, peri-
cardial glands are of common occurrence in all but the
simplest Pelecypods, and apparently assist the nephridia in
their excretory function. They are known also as Keber's
organs and consist either of outpouchiugs of the anterior
portion of the pericardial wall into the space between the
two walls of the mantle (Unio, Venus) or of similar evagiua-
tions of the walls of the auricles into the pericardial cavity
(Mytilus), both methods of formation usually being associated.

The reproductive organs (Fig. 149, r) are paired, lying usu-
ally in the tissue forming the base of the foot, though extend-
ing in some cases into the lacuuar spaces between the walls
of the mantle (Mytilus). They are very richly branched and
usually contain in any one individual only ova or sperma-
tozoa, as the case may be, though a number of forms are
hermaphrodite — such, for example, as the members of the
genus Ct/clas and some species of the genera Ostrea and
Pecten. The ducts which convey the reproductive elements
to the exterior open into the nephridia near their proximal
ends in Nucula and a few other primitive genera, but more

338 INVERTEBRATE MORPHOLOGY,

usually open directly into the suprabrancliial chamber quite
near the openings of the nephridia (Fig. 149, go) ; conditions
connecting these two extremes are to be found, as in Pecten,
where the reproductive ducts communicate with the nephridia
near their distal ends, and in Cyclas and Ostrea, where both
uephridial and reproductive openings are contained in a
common groove. No complex accessory structures are de-
veloped in connection with the reproductive organs, as in
some of the Gasteropods, nor is there an intromittent organ
in the male, the ova and spermatozoa being usually extruded
to the exterior, where fertilization takes place, or else the ova
pass from the suprabrauchial chamber into the iuterlamellar
spaces of the gill-plates and are fertilized there.

The structure of the gills forms a suitable character for a
classification of the Pelecypoda.

1. Order Protobranchia.

The gill is a true ctenidium attached by its axis to the
roof of the mantle-cavity in its posterior part. In addition to

f
FIG. 151. — Nucula nucleus FOR THE LEFT SIDE AFTER THE REMOVAL OF

THE LEFT SHELL, AND LEFT MANTLE-LOBE (after PELSENEEB).
aa = anterior adductor. / = foot.

ar - anterior retractor pedis. g — reproductive organ.

c = cteuidium. p - labial pulp.

cp = levator pedis. pa == posterior adductor.

pr = posterior retractor.

this primitive feature the foot has a creeping surface, the
pleural ganglia are not completely united with the cerebral,

TYPE MOLLUSC A. 339

and the reproductive ducts communicate with the proximal
portion of the nephridia. To this order, which represents the
most primitive Pelecypods, belong the genera Nucula (Fig.
151), Yoldia, and others.

2. Order Filibranchia.

In this group the gill-filaments have elongated consider-
ably and commenced to bend upwards at their ends to form
the outer and inner lamellae (Anomia; Mytilus, Modiolaria, the
mussels ; Area).

3. Order Pseudolamellibranchia.

In this the gill-filaments show a tendency to become
united together and the inner and outer lamellse are united
(Pecten, the Scallop ; Ostrea, the Oyster).

4. Order Eulamellibranchia.

In which the gill-filaments are united to form continuous
lamellse. To this order belong the majority of forms, such
as the fresh-water mussels Unio and Anodon, the small fresh-
water Cydas, the hard-shell clam or Quahog Venus, the soft-
shell clam Mi/a, the razor-shell Ensatella, the boring-shell
Pholas, the ship- worm Teredo, and a very large number of
other genera.

5. Order Septibranchia.

A small group in which the gills are reduced to a muscu-
lar perforated septum dividing the suprabrauchial chamber
from the more ventral mantle-chamber (Cuspidaria).

Development and Affinities of the Pelecypoda .— The larva which is
characteristic of the Pelecypods resembles a Trochophore very closely in-
deed and may be described as a Trochophore provided with a bivalved
shell. In certain forms the characteristic ciliary bands may, however,
be very much reduced, and in the fresh- water mussels (Unio, Anodnn,)
a remarkable secondary larva known as the Glochidium is developed.
The ova undergo their development in the interlamellar spaces of the gill
plates, and the shell-valves assume a somewhat triangular shape, the apex
usually constituting a somewhat curved tooth, while smaller teeth may
also be present on the edges. Each mantle-lobe is provided witli four
tactile papillae on each side, the slightest stimulation of which causes the

340 INVERTEBRATE MORPHOLOGY.

shell-valves to close with considerable force and the teeth to adhere to any
soft object they come in contact with. By these arrangements the Glo-
chidia are able to fasten themselves to the skin of the fins or to the gills of
fishes, where, setting up an inflammation, they become enclosed in a cyst
within which the organs assume the adult form, the embryo assuming a
truly parasitic habit and drawing nourishment from the tissues of its host.
When sufficiently developed the young mussel makes its way out of the
cyst and assumes a free mode of life.

As regards the affinities of the Pelecypoda there can be no doubt that
between the Protobranchiate forms and the Diotocardiate Gasteropoda
there are not a few resemblances. They possess a creeping foot, true
ctenidia function as gills, the heart is traversed by the rectum, the pleural
and cerebral ganglia are distinct, and the nephridia serve to transmit the
reproductive elements to the exterior, all of which are also to be found in
certain of the Diotocardiates, such as Haliotis and Fissnrella. It may be
supposed that the Pelecypods arose from the Gasteropod stem before the
asymmetry became developed, and subsequently, by assuming a fixed or
limicolous mode of life, a certain amount of degeneration, such as the loss
of the radula and of tentacles and cephalic eyes, supervened. The plough-
share-shaped foot is undoubtedly an adaptation to the limicolous habit,
and the great development of the gills stands in relation to their mode of
obtaining food, the cilia of the gills being responsible for the production of
currents sufficiently strong to carry with them diatoms and other small
organisms upon which the Pelecypods feed.

V. CLASS CEPHALOPODA.

The Cephalopoda are in some respects the most specialized
of all the Mollusca, but nevertheless present the primitive
bilateral symmetry and arrangement of the structures associ-
ated with the mantle-cavity. The visceral hump is enor-
mously developed (Fig. 152), so that the true anterior and
posterior surfaces of the body are very long, whereas the
ventral surface is comparatively short, the general form of
the body being not unlike that of the Scaphopoda. Unlike
the members of this latter class the Cephalopods lead, how-
ever, a somewhat active existence, some, such as the Squids,
swimming actively about in the sea to which they are exclu-
sively confined, while others, such as the Cuttlefishes, have a
more creeping habit, though capable of swimming freely.
While swimming the animals assume a position in which their
longest axis, i.e. the dorse-ventral axis, is more or less hori-
zontal, the true morphological anterior surface thus becoming

TYPE MOLLUSC A.

341

physiologically tlie dorsal, arid the posterior the ventral sur-
face. In the following description the surfaces will be con-
sidered in their morphological relations.

The head is usually somewhat distinctly marked off from
the body proper by a neck constriction and bears two usually
remarkably-developed and highly-specialized eyes. A pecul-
iar feature of the Cephalopoda, and the one which has sug-

coe

FIG. 152. — DIAGRAM TO SHOW GENERAL PLAN OF STRUCTURE OF A CEPHA-

LOPOD (slightly modified from LANG).

an — anus.

6 = bnccal mass.
cce = csecuin of stomach,
ct = ctenidium.

e = eye.
go = reproductive organ.

i = iuk-bag.
m = mantle.

me = mantle-cavity.
ne = nephridium.

ce = oesophagus.

« = stomach.
sh = shell.

si = funnel.
t = tentacle.

v = valve of funnel.

gested the name applied to the class, is the fusion with the
head of a portion of the foot. The mouth thus becomes situ-
ated at the bottom of a funnel-like depression, formed by the
foot, whose margins are drawn out into a number of tentacle-
bearing lobes or into eight or ten long armlike processes
(Fig. 152, t) provided with suckers, and serving as powerful

342 INVERTEBRATE MORPHOLOGY.

organs of prehension. A second portion of the foot lies in
the neck region on the ventral surface and has the form of
two folds (si), whose edges may be approximated or even fused
to form a tube, through which the water contained in the
mantle-cavity may be violently expelled, the animal being
thereby propelled through the water in a direction of their
long dorse-ventral axis. This portion of the foot is termed
the funnel and is perhaps equivalent to the epipodium of the
Gasteropods. In the majority of forms there projects into
the lumen of the funnel a fold (v) arising from the body-wall
and termed the valve of the funnel. It is probably homolo-
gous with the posterior portion of the foot, the metapodium,
of the Gasteropoda, so that all portions of the Gasteropod
foot are represented in the Cephalopods, the propodium and
mesopodium by the arm bearing portion, the metapodium by
the valve just mentioned, and the epipodium by the funnel.
In many forms two depressions are to be found on the outer
surface of the funnel, which receive two corresponding eleva-
tions on the inner surface of the mantle, which thus becomes
locked as it were to the funnel during the expulsion of water
from the mantle-cavity.

The mantle (ra) forms a circular fold surrounding the vis-
ceral hump, but upon the anterior surface it has usually only
a very slight development, while posteriorly there is a wide
space, the mantle-cavity (me), between it and the body-wall.
Within this space lie the ctenidia (ct), and into it the nephri-
dia (ne) and the digestive tract open, the excreta being ex-
pelled from it during the expulsion of water from the funnel.
The mantle-fold is rather thick as a rule, owing to the pres-
ence in it of abundant muscle-fibres, by the contraction of
which the mantle-cavity may be considerably reduced in size,
and frequently there is a special muscular thickening around
the edge of the mantle whereby the mouth of the cavity,
widely open during the intaking of water, may be firmly ap-
pressed upon the funnel during the expulsive act. In the
majority of Cephalopods the integument covering the outer
surface of the mantle and of the visceral hump is provided
with abundant pigment-cells or chromatophores each of
which is provided with a muscular arrangement by which its

TYPE MOLLUSCA. 343

size may be rapidly diminished, remarkable flushes of color
passing over the surface of the living animal.

In the Nautilus (Fig. 160) a chambered calcareous shell is
present having a rather complicated structure which will be
described later, and in one or two other living forms, such
as Argonauta and Spirula, an external shell also exists, but
in the majority of forms the edges of the mantle close over
the shell, which thus becomes internal and takes the form of a
plate lying along the anterior surface of the body, being some-
times calcareous as in the common Cuttlefish bone of com-
merce obtained from the Sepia (Fig. 152, sh), or else chitiuous
as in the common Squid, Loligo. In connection with the
mantle there are also frequently developed finlike expansions
with a cartilaginous support and provided with muscles,
sometimes running along the sides of the visceral hump or
in other cases situated near its dorsal extremity.

The respiratory organs or cteuidia (Fig. 154, ct] are present
as either one or two (Nautilus] pairs of pinnate structures
lying in the mantle-cavity. Each consists of a central
axis attached throughout its entire length to the body-wall,
forming a rather high ridge upon it and containing near its
outer edge two blood-vessels running throughout its entire
length. The vessel nearer the summit of the ridge is the
branchial vein carrying the aerated blood back to the body,
and between it and the branchial artery is a cavity or canal
which communicates with the mantle-cavity between each
pair of branchial pinnre. These structures arise from near
the free edge of the axial ridge, but each is bound to the ridge
by a thin triangular membrane so that they possess the form
of lamellse rather than of pinnae. Near the line of attachment
of the axial ridge to the body-wall is a cord of cellular tissue
richly supplied with blood coming from the branchial artery,
forming what is termed a blood-gland, from which the blood
is collected into two longitudinal canals which conduct it back
to the heart.

The coeloni of the Cephalopods is characterized by the
great development of the pericardial cavity (Fig. 153, pc),
which recalls the condition found in the Amphineura, and
may perhaps be better termed the viscero-pericardial cavity.

344

INVERTEBRATE MORPHOLOGY.

In the majority of forms it is a large sac occupying a con-
siderable portion of the apex of the visceral hump and ex-

— -f

Ov

FIG. 153. — DIAGRAM OP BODY CAVITY OP Sepia (after GROBBEN).
bh = branchial heart. od = oviduct.

/ = funnel. Ov — ovary.

go = reproductive opening into ccelom. p = pancreas.
H = heart. pa = partition partially dividing coe-

i = intestine. loin.

Ib = ink-bag. pc = coelora.

I — liver. s = stomach.

Id — liver-duct. Sh = shell.

me = mantle cavity. u = external opening of nephridium.

_ZV = nephridium. W == opening of nephridium into cce-

lorn.

tending ventrally a considerable distance, the more ventral
portion being incompletely separated from the more capacious

TYPE MOLLUSC A. 345

dorsal portion by a transverse fold or partial partition (pa).
In Nautilus it is placed in direct communication with the man-
tle-cavity by two minute pores, but in other forms such direct
communications do not occur. With a ventral prolongation
of the ventral cavity the nephridia (N) communicate, and the
walls of the cavity fold themselves around the heart (H] in
the usual manner, and in addition also enclose the branchial
hearts (M), becoming thickened and considerably folded in
this region so as to form the so-called appendages of the
branchial hearts, which are homologous with the pericardial
glands of the Lamellibranchs. The wall of the dorsal cavity
is in a similar manner folded over the viscera present in that
region, and more or less completely encloses the reproductive
organs (ov) so as to form around them a capsule, sometimes
with muscular walls, into the cavity of which the reproduc-
tive elements are shed when mature. In one group of Ce-
phalopods, however, the Octopoda, the arrangement departs
slightly from this owing to the reduction of the viscero-peri-
cardial cavity to a number of comparatively small canals
which constitute the so-called water vascular canals of the
older authors. Three of these canals are found on either side
of the body, meeting together in a common centre, the ne-
phridia communicating with one, another passing to the
branchial heart of its side to form the pericardial gland, while
the third extends dorsally to dilate with its fellow of the
opposite side into the capsule surrounding the reproductive
organs (Fig. 158, we). The general relationships of these
canals are evidently comparable with those of the viscero-
pericardial cavity of the majority of the Cephalopoda, but
they differ in one very marked peculiarity, i.e., the heart is
not enclosed within their lumen. The tubelike condition of
the cavity is evidently a secondary condition, and the exclu-
sion of the heart can be understood as a result of the diminu-
tion of the extent of the cavity, when the manner in which it
is enclosed, as exemplified by the Solenogastres, is considered.
The schizoccelic portion of the coalom takes the form partly
of lacunar spaces, but partly of blood-vessels with definite
walls. To a certain extent the blood system is completely
closed, an unusual condition among Mollusca ; well-defined

346

INVERTEBRATE MORPHOLOGY.

veins return the blood carried by the arteries to various por-
tions of the body, definite capillaries connecting the two sets
of vessels. A lacunar system also exists, however, so that,
while showing a much greater differentiation than the other
Mollusca, the Cephalopods yet retain indications of the more
primitive arrangement.

The heart consists of a tubular ventricle (Fig. 154, v),
usually arranged with its long axis directed dorso-ventrally,

va

FIG. 154. — CIRCULATORY APPARATUS OP Sepia (after HUNTER from OWEN).
ao = anterior aorta. Iv — lateral vein.

ao' -- abdominal aorta. ne — excretory appendage.

au = auricle. pg = pericardial gland.

bh = branchial heart. v = ventricle.

ct = ctenidium. va — abdominal vein.

vc = cephalic vein.

though in the Octopoda it is transverse, and has opening into
it at each side one or two (Nautilus') auricles (ati) which re-
ceive the blood from the branchiae (ct). Two principal arte-
ries arise from the ventricle, a larger one running ventrally
(ao), and a smaller one which runs towards the tip of the
visceral hump and supplies the viscera of that region (ao').
As already stated, these arteries pass into a fine capillary net-
work from which the veins arise, sinuses, however, interven-
ing in some cases in the course of the latter, and possibly
some arterial branches may terminate in such sinuses. The

TYPE MOLLUSCA. 347

principal venous trunk is the cephalic vein (w), which lies on
the posterior side of the oesophagus, and passing dorsally
divides into two branches, the venae cavse, with each of which
an abdominal vein (oa) unites, the conjoined trunk on each
side passing into a contractile dilatation, the branchial heart
(bh), at the base of the cteuidium of that side. The venae cavse
and the abdominal veins are covered by a much-folded mass
of tissue, the venous appendages (tie), which are portions of
the nephridia and will be considered in the description of
those organs. Mention may also be made here of the peri-
cardial glands (pg) attached to the branchial hearts, which
have already been described in connection with the viscero-
pericardial cavity.

Slight variations from the arrangement here described
may be found in various forms, of which the most important
is that found in Nautilus, in which, in accordance with the
presence of two pairs of cteuidia, each vena cava divides into
two branches, one passing to each ctenidium. No branchial
hearts occur, and, as has been already mentioned, the ventri-
cle has opening into it two pairs of auricles instead of the
single pair usually present.

In the mesodermal tissue of the Cephalopods in various
portions of the body there are developed plates and nodules
of a consistency resembling cartilage and like it consisting of
a hyaline or partly fibrous matrix through which numerous
cells with branching processes are scattered. These cartilagi-
nous structures resemble the tissue which is developed in the
pharynx of the Gasteropoda below the radula, but reach a
much more extensive development in the Cephalopods, serving
as a protection for some of the more important organs, and
also as a point d'appui for the various muscles, and therefore
constituting a true endoskeleton. In the Nautilus there is but
a single cartilage which lies on the posterior surface of the
ossophagus," being deeply grooved for the reception of the
brain and optic ganglia. In other forms, however, the carti-
lages are more numerous. There is a well-developed cephalic
cartilage forming a deeply-concave disk perforated by the
oasophagus, and partially enclosing the brain, being also ex-
panded at the sides and hollowed out so as to form a support

348 INVERTEBRATE MORPHOLOGY.

for the eyes, which are further covered by a pair of plates
which project anteriorly and laterally from the anterior margin
of the disk. At the base of the arms a brachial cartilage,
sometimes united with the cephalic mass, is found which
serves for the origin of the brachial musculature, and further-
more a nuchal plate is present lying below the anterior sur-
face of the body just behind the head. In connection with
the infundibuluni plates and nodules are developed, the most
important of which is the infundibular cartilage on the pos-
terior (strictly speaking the ventral) surface of the body in
the floor of infundibuluni, nodules being found below the de-
pressions on the side of the infundibuluui and the corre-
sponding elevations of the mantle which have already been
described as interlocking during the expulsion of water
through the funnel. Finally, it may be mentioned that the
centre of each fin is occupied by a cartilaginous plate which
serves for the origin of the muscles which move the fin.

In harmony with the peculiar modification of the foot
there is a considerable amount of differentiation of special
muscles in the Cephalopods, which pass from cartilage to
cartilage or from the shell to the various cartilages. Leaving
aside the general musculature of the mantle and of the arms,
mention may be made of the three or four strong retractor
muscles, which pass from the shell to the cephalic cartilage
and are sometimes fused together to form a single strong
muscle which serves to retract the head ; the collaris, which
runs on either side of the neck from the infundibular cartilage
to be inserted into the sides of the nuchal cartilage ; and
finally the adductors and depressor of the funnel, which pass
respectively from the cephalic cartilage and the shell to be
inserted into the infundibular cartilage. Considerable varia-
tion is to be found in the arrangement of muscles in various
forms, but the typical arrangement may be regarded as being
somewhat as described.

Like the other organs the digestive system presents a con-
siderable amount of differentiation. The mouth opens in the
centre of the disk which bears the arms or tentaculiferous
lobes, and is guarded by two strong chitinous or partly calca-
reous (Nautilus] jaws resembling in form the beak of a parrot.

TYPE MOLLUSCA. 349

It leads into a muscular pharynx (Fig. 152, Z>), upon the floor
of which lies the characteristic inolluscan radula, while into
its cavity the ducts of one or two pairs of salivary glands
open. Succeeding the pharynx is a tubular oesophagus (ce)
which in some forms is provided with a lateral diverticulurn,
the crop, and which terminates below in the large pyriform
stomach (s). The intestine leaves the stomach close to the
entrance of the oesophagus, and a pouchlike structure, in some
forms prolonged into a spiral csecuin (cce), is to be found
either communicating with the stomach close to this point or
else opening into the proximal portion of the intestine (Ncmti-
lus). Into this caecum the two ducts from the large digestive
glands, or so-called liver, open, their walls being in the ma-
jority of cases provided with sacculations arranged in bunches
and constituting the pancreas, a structure which in Loligo
(Fig. 153, p) is imbedded in the thickened walls of the ducts or
else, as in Octopus, attached to the digestive gland in the region
where its ducts arise. From its origin in the stomach the
intestine passes ventrally, the entire tract having thus a
Y-shaped arrangement, and opens into the mantle-cavity on
the summit of a papilla situated a short distance from the
dorsal end of the infundibuluni. From each side of the anal
papilla a fleshy appendage arises, the anal valve, which in
some forms may be drawn down so as to completely close the
anal opening.

In connection with the posterior portion of the digestive
tract there is found in all Cephalopods except Nautilus a sac-
like gland (Fig. 152, i) which secretes a dark pigment and is
known as the ink-bag, the animal discharging the ink into
the surrounding water to conceal its retreat when alarmed.
It arises as a saclike diverticulum of the rectum close to its
termination and, elongating, becomes differentiated into a duct
of considerable length opening into the terminal portion of
the rectum and closed by a circular band of muscle-fibres
which surround it near its opening. The more or less globu-
lar extremity of the diverticulum becomes differentiated into
(1) a cavity which serves as a reservoir for the inky secretion
manufactured in (2) a special glandular region, traversed by a
series of trabeculse lined by the secreting cells.

350

INVERTEBRATE MORPHOLOGY.

The nervous system of the Cephalopods shows a high
degree of concentration, the various ganglia being more or less
fused with one another to form a mass surrounding the oeso-
phagus just behind the pharyngeal mass. In Nautilus this
mass takes the form of two rings surrounding the oesophagus,
united in front but widely divergent behind, and in which the
various ganglia are but indistinctly indicated, the condition
which occurs in Chiton in this respect being recalled. That
portion of the ring which lies in front of the oesophagus rep-
resents the cerebral ganglia ; the
lateral portions of the more ventral
of the two rings found on the pos-
terior surface are the pedal ganglia,
giving rise to the nerves to the pedal
lobes and the infundibulum ; while
the more dorsal posterior ring rep-
resents the combined visceral, pa-
rietal, and pleural ganglia. In other
forms the ganglia become more
perfectly marked off and at the same
time more concentrated. A cerebral
ganglion (Fig. 155, c) is always dis-
tinguishable, and with it are con-
nected pleuro-parieto-visceral (pi
and v) and pedal (p and p') ganglia ;
the latter, however, are usually divi-
ded into two portions — a more ven-
tral mass (pf) which sends branches
to the armlike prolongations of the pro- and mesopodium and
which is hence termed the brachial ganglion, and a more dorsal
one (p) which supplies the infuudibulurn and is known as the
pedal ganglion proper. A study of a number of different
forms shows clearly that the brachial ganglion is merely a
separated portion of the pedal, and that the arms are to be con-
sidered portions of the foot and are not cephalic appendages.
At the sides of the cerebral ganglia there are to be found a
pair of large ganglia (op) which stand in relation to the eye and
are termed the optic ganglia ; they are undoubtedly spe-
cializations of the cerebral ganglia, owing their separate exist-

FIG. 155. — NERVOUS GANGLIA

OF (A) Loligo AND (B) Octopus
(after PELSENEER).
b = buccal ganglion.
c = cerebral ganglion.
p = pedal ganglion.
p' = brachial ganglion.
op = optic ganglion.
v = pleuro - parieto - visceral
ganglion.

TYPE MOLLUSC A.

351

ence to the remarkable development and differentiation of
the eye which is found in the majority of the Cephalopods.

A sympathetic system of nerves is well developed and con-
sists of one or two pairs of buccal ganglia (&) innervating the
large pharyugeal mass and united to the cerebral ganglia by
connectives and giving rise to a strong nerve which runs
dorsally along the oesophagus to end in a large gastric
ganglion from which nerves pass to the viscera. Mention
should also be made of two other ganglia, the ganglia stellata,
which belong to the central system and are situated in the
lateral portions of the mantle, being united with the pleuro-
visceral ganglia by strong nerves ; they correspond probably
with tlie parietal ganglia of the Gasteropods, sending branches
to the tissues of the mantle.

The special sense-organs are exceedingly well developed,
and especially is this the case with the eyes. In Nautilus,
however, the eye (Fig. 156, A] stands on a much lower grade

FIG. 156. — A, Eye of Nautilus (modified from HENSEN); and B, of Loligo.
c =. cartilage. I = lens.

co = cornea, n == nerve-layer.

g = layer of ganglion-cells. op = optic nerve and retinal ganglion.

ir = iris. pg = pigment-layer.

r = layer of rods.

of organization than that of the other Cephalopods, con-
sisting of a cup lined by a retina composed of several
layers and richly supplied with nerves. The outermost
layer consists of rodlike bodies (r) below which is a
layer of pigment (pg}, below which again lies a layer of
ganglion-cells (g). No refractive structures are, however,
present, the cavity of the cup communicating freely with the

352 INVERTEBEATE MORPHOLOGY.

external water through a small circular opening in the front
flattened wall of the cup. The eye is a camera constructed on
the " pin-hole " type, the image being defined by the exclusion
of all the more divergent rays of light which pass in from the
object towards the eye.

In the remaining forms the eyes (Fig. 156, I>) are large
globes imbedded in an orbit formed by the lateral portions
of the cephalic cartilage and its processes. The retinal por-
tion of the eye closely resembles that of Nautilus, consisting
of an external layer of rods (/•) bounded beneath by a pigment-
layer (pg) beneath which is a nerve-layer (ri) enclosed within
a connective tissue-sheath in which cartilage (c) is developed.
The optic nerve dilates into a retinal ganglion before being
distributed to the retina, the rods of which, it will be noted,
are turned towards the source of light. The eye-cup differs,
however, from that of Nautilus in being completely closed,
and the cells which form the outer and inner layers of the
outer wall of the cup secrete chitinous material which acts
as a lens (I), forming a powerful biconvex condenser. In ad-
dition to this the eye is further complicated by the develop-
ment of a series of folds from the skin in its neighborhood.
One such fold is developed from the front wall of the optic
sac, surrounding the region occupied by the lens and form-
ing an iris (ir\ the circular opening in its centre correspond-
ing to the pupil of the Vertebrate eye. A second likewise
forms nearer the base of the optic sac and, growing forward,
may enclose a space bounded behind by the iris and lens,
resembling the anterior chamber of the Vertebrate eye, the
portion of the fold immediately in front of the lens becoming
transparent and forming a cornea (co). The anterior chamber
is not, however, closed in all forms, but remains in communi-
cation with the exterior by an aperture produced by a failure
of the edges of the fold to unite completely. Finally, in some
forms other folds, which from analogy have been termed eye-
lids, develop.

The resemblance of such an eye to that found in the Vertebrates is ex-
ceedingly striking, but a detailed study of the structure and mode of origin
of the various parts demonstrates conclusively that the similarities are ana-
logical only and not homological. One of the most important of the differ-

TYPE MOLLUSCA. 353

ences is found in the arrangement of the layers of the retina, the rods being
turned towards the light as is usual in Invertebrate eyes, while in the Verte-
brates they are reversed, the nerve-fibre layer lying above them, the light
of necessity penetrating it before reaching the rods. The structure of the
lens is again very different, being cellular and formed as an invagination of
the ectoderm in the Vertebrates, while in the Cephalopods it is a cuticular
structure. These are fundamental differences and may suffice to show
what is meant, but many other dissimilarities may readily be found.

Otocysts also occur imbedded in a capsule forming part
of the cephalic cartilage. They have the characteristic Mol-
luscan form and receive a large nerve arising from the cere-
bral ganglion. Osphradia occur only in Nautilus, where they
form a pair of sensory papillae one of which lies at the base
of each of the more ventral ctenidia. Other Cephalopods,
though lacking these structures, are yet provided with special
olfactory organs in the form of a pair of fossae or grooves
lined by ciliated and sensory cells and situated above the eye
in the position occupied by the eye-tentacles of Nautilus
(see p. 358), from which they may possibly have been derived.

The excretory organs consist of two comparatively large
sac-like nephridia except in Nautilus, in which, in harmony
with the number of ctenidia and auricles, there are four. In
Octopus and the other members of the group Octopoda the
two nephridia are quite separate from one another, but in the
group Decapoda, to which Loligo and Sepia belong, they are
placed in communication with one another by transverse
canals one of which may be produced dorsally into a large
sac occupying a great portion of the anterior region of the
body. The venae cavse and branchial veins lie between the
walls of this anterior sac and the paired posterior nephridia,
and along the course of the veins the walls of the excretory
sacs are richly folded (Fig. 154, ne), constituting the venous
appendages, for a long time considered to be the excretory
organs in their entirety. The posterior paired nephridia
present the same relations to the exterior and to the entero-
003! which exist in other Mollusca, opening by two distinct
apertures into the mantle-cavity on the one hand, and on the
other communicating with the large euteroccel which has
been shown to be the equivalent of the pericardial cavity of
the Gasteropods and Pelecypods.

354

INVERTEBRATE MORPHOLOGY.

The reproductive organs are situated near the dorsal ex-
tremity of the visceral hump. The sexes are always sepa-
rated in different individuals, there being occasionally well-
marked differences between the two sexes of the same species,
as in Argonauta, the female of which possesses a well-devel-
oped shell which the male lacks. The ovary (Fig. 157, ov) is
single and is enclosed in a capsule (c) formed by the walls of
the enterocoel or viscero-pericardial cavity, into which the
organ seems to project, though morphologically it is entirely
outside it.

The germ-producing region is nearly always the anterior
surface of the organ, the stalked ova surrounded by their
follicle-cells projecting forward into the capsule, into the
cavity of which, i.e. into the viscero-pericardial cavity, they
burst when mature. In some forms the germ-producing sur-

ov

°g

rs

FIG. 157. — FEMALE REPRODUCTIVE ORGANS OP Tremoctopus molaceus

(after BROCK).

c = capsule. ov = ovary.

od = oviduct. rs = seminal receptacle.

og = oviducal gland. we — coelouiic canal.

face becomes more highly folded and more or less dendritic
in form, the area over which the ova are formed becoming
thus much greater. The ova reach the exterior after they
have passed into the cavity of the capsule by means of one
or two complicated ducts (od) opening into the mantle-cavity.
In Nautilus two ducts are present, that of the left side, however,
being non-functional, and in the Octopoda and some Decapods,
such as Ommastrephes, both ducts are present. In other forms
but a single duct persists, which, contrary to what occurs in

TYPE MOLLUSC A. 355

Nautilus, is that of the left side. The oviduct opens into the
mantle-cavity at the extremity of a well-marked papilla, its
terminal portion being richly supplied with glands, and in
addition in some forms two small pear-shaped glands are
attached to it in this region. In connection with the female
ducts there should be mentioned a pair of glands which take
part in the formation of the investments of the ova, but which
open quite separate from the oviduct into the mantle-cavity.
These are the nidameutal glands which are present in the
majority of forms, excluding the Octopoda, and consist of a
pair of large pyriforni structures lying on the posterior sur-
face of the visceral mass ; in connection with them in some
forms are developed accessory nidameutal glands consisting
of a central and two lateral portions whose ducts open into
the mantle-cavity in close proximity to those of the nidamen-
tal glands proper. As stated, the gelatinous mass within
which the ova are imbedded is probably manufactured by
these glands.

The testis in its general relations resembles the ovary,
being single and enclosed in a capsule which is a portion of the
viscero-pericardial cavity. The organ is attached to the wall
of the capsule by a thin band of tissue and is in most cases
almost completely surrounded by the capsule, into the cavity
of which the spermatozoa are shed when mature. From the
wall of the capsule the vas defereus arises and is usually a
single tube opening upon the left side of the body into the
mantle-cavity. In Nautilus there are, as in the female, two
ducts, the right, however, being fuuctionless, but in other
forms a paired arrangement is very rare. The proximal por-
tion of the duct is a coiled vas deferens, which opens into a
thick-walled glandular seminal vesicle which on its part by
means of a narrow duct passes into a saclike structure known
as Needham's pouch which finally passes into the muscular
penis. In most forms the duct connecting the seminal vesicle
with Needham's pouch receives the secretion of a special gland
known as the prostate.

The majority of the accessory structures connected with
the male ducts are concerned in the formation of cases or
spermatophores in which a number of spermatozoa are en-

356

INVERTEBRA TE MORPHOLOG T.

closed. Suck cases are cylindrical structures with a double
wall, and are provided at one extremity with a somewhat
complicated apparatus for the ejection of the spermatozoa.
The exact method of their formation is not understood, but
apparently the seminal vesicles and the prostate play an im-
portant part in the process, the Needham's pouches being a
reservoir in which they may be stored up until required for
fertilization.

Since the genital capsule is a portion of the viscero-pericardial cavity,
and the reproductive ducts are continuations of its walls, these structures
must also be regarded as prolongations of the enteroccel; and indeed second-
ary communications may exist between them and the viscero-pericardial
cavity proper. The genital capsule is not completely separated off from
the rest of the enterocoel, so that it might be possible for the reproductive
elements to pass from its cavity into the viscero-pericardial cavity proper,
and so to the exterior through the nephridia, though this method of exit
does not seem to be made use of.

A remarkable modification of one of the armlike processes of the foot
occurs in the males of certain species in connection with reproduction.
The arm — in Tremoctopus and PJiilonexis the third arm of the right side
of the body counting from the anterior mid line, in Argonauta (Fig. 158)

A B

FIG. 158.— MALE OF Argonauta WITH HECTOCOTYLJZED ARM
(after H. MULLER from HATSCHEK).

A = arm still enclosed within a membranous sac.
B = arm freed from the sac.

the third of the left side — is at first enclosed within a sac, by the
bursting of which it becomes free, the walls of the sac being reflected
back so as to form a pouch which in some unexplained manner receives a

TYPE MOLL USC A. 357

sperrnatophore. The terminal portion of the arm, which is traversed
throughout its entire length by a canal, is developed into a long terminal
filament through which the spermatozoa may pass. During copulation
the arm is probably thrown off and passes into the mantle-cavity of the
female, the manner in which the spermatozoa reach the ova being, however,
not yet understood. When first discovered in the mantle-cavity of a female
the arm was regarded as a parasitic worm, and the name Hectocotylus was
applied to it — a term which is still retained on account of its convenience.
In other genera of Cephalopods one arm is generally peculiarly modified in
the male — in the Decapoda usually the fourth of the left side and in the
Octopoda usually the third of the right side, though frequent exceptions are
found. This arm is termed the hectocotylized arm, though it is doubtful
whether it takes any part in copulation.

As will be seen from the above description the genus
Nautilus differs in many important particulars from the re-
maining genera of Cephalopods, and the class is therefore
divided into two orders.

1. Order Tetrabranchia.

This order, of which the genus Nautilus (Fig. 159) is the
sole living representative, was in former periods of the earth's
history the dominant group of the Cephalopods — the Ortho-
cerites of the Palaeozoic and the Ammonites of the Mesozoic
being extinct members of it. It is characterized by its mem-
bers possessing four ctenidia, four auricles to the heart, and four
uephridia; and in addition there maybe mentioned, as further
peculiarities, the presence of paired reproductive ducts, of
which the right one alone is functional, and also of direct
communication of the viscero-pericardial cavity with the ex-
terior by two pores, and by the occurrence of a single pair of
osphradia. For a more detailed account of the peculiarities
of Nautilus the preceding general description may be con-
sulted. It remains to discuss here the shell and the structure
of the foot-lobes — structures which, with the other characters
mentioned, serve to distinguish Nautilus from all its living
congeners.

The shell is voluminous, coiled, and calcareous, its cavity
being divided by a series of transverse partitions into a num-
ber of chambers, in the last — that is to say, the youngest — of
which the animal lives, while the remaining ones are filled with

358 INVERTEBRATE MORPHOLOGY.

gas. The centre of each partition is perforated, arid through
the opening there extends to the tip of the shell a prolongation
of the body of the animal, termed the sipuncle.

The foot of Nautilus, or at least that portion of it which
fuses with the head, has already been described as forming a
number of tentaculiferous lobes. These lobes are arranged
in the female in two series- — one ventral, consisting of three

FIG. 159. — Nautilus pompilius, — FEMALE. WITH THE SHELL SECTIONED LONGI-
TUDINALLY TO SHOW ITS INTERNAL STRUCTURE (after LECNIS from HKRTWIG).

1 = mantle. 7 = uidameutal gland.

2 = dorsal lobe of mantle. 8 = shell-muscle.

3 = tentacles. 9 = terminal chamber of shell.

4 = head-cap. 10 = partitions between the various

5 = eye. chambers.

6 = funnel. 11 -= sipuucle.

lobes which immediately abut upon the mouth, and a more
dorsal riuglike lobe the anterior portion of which is de-
veloped into a hood (4) which arches over and protects the re-
tracted tentacles. Around the margins of both the ventral
and dorsal lobes are arranged the tentacles, each of which
is filiform and capable of being withdrawn into the basal por-
tion, which thus serves as a sheath. In addition to these
tentacles two other tentacles are found in close proximity to
the eye, one being on its ventral side and the other on its
dorsal. In the male the arrangement is very similar, except
that the median lobe of the ventral series is transformed into
a lamellated structure and does not bear tentacles, while a
portion of each of the lateral lobes of the inner series is sepa-

TYPE MOLLUSC A,

359

rated from the rest of the lobe — that of the left side becoming
modified into a couical structure, lamellated at the extremity
and destitute of tentacles, forming what is termed the spadix,
probably homologous with the hectocotylized arm of the
male Octopods and Decapods.

2. Order Dibranchia.

The members of thin order, which includes the majority
of living Cephalopods, possess but a single pair of cteuidia,
uephridia, and auricles, and lack
the direct communication of the
viscero-pericardial cavity with the
exterior as well as the osphradia
which occur in Nautilus. The
portion of the foot which is fused
with the head is drawn out into
a number of arms provided with
suckers, which seem to represent
the tentacles and their sheaths
found in Nautilus. The suckers
are very numerous and may be
arranged in from one to four rows
on the ventral surface of the
arms, the margin of each sucker
being in some forms strengthened
by a horny ring, which may be
toothed. The number of the
arms varies, being either eight
or ten ; and, since, other struc-
tural differences are associated
with this difference, the order
may be divided into two suborders FIG. WO.—Loligo pallidu, DORSAL

—the Octopcda with eight arms, VIEW (after EMERTON from VERRILL).

including the genera Octopus, Tremoctopus, and Argonauta
(Fig. 158), and the Decapoda with ten arms, the genera Spi-
rula, Ommastrephes, Sepia, and Loligo (Fig. 160) belonging to
this group.

In the Decapoda the ten arms are not of equal size, one

360 INVERTEBRATE MORPHOLOGY.

on each side of the head, the fourth, counting from the ante-
rior mid-line, being longer than the rest, usually destitute of
suckers except towards the tip, and in most species kept
retracted within a groove on each side of the head except
when required for prehension. They are all good swimmers,
and the body is elongated and provided with lateral fins of
greater or less extent.

A shell is present in the Decapoda, but shows a great
reduction in size and complexity from that of the Tetra-
brauchiates ; and in order to understand its homologies in the
different genera it will be necessary to obtain an idea of its
form in the fossil members of the group which occur in the
Mesozoic rocks forming the family Belemnitidee. In the
genus Beleinnites (Fig. 161, B\ for instance, the shell consisted
of a terminal conical solid portion, termed the rostrum (r),
the base of which was hollow and contained a chambered
shell, the phragmacone (pli), corresponding to the Nautilus
shell, the anterior portion of the last chamber of this being
elongated into a broad flat process termed the proostracon
(p?1). By various modifications of this structure the shells of
the different living Decapods have been developed. In Spirula
the shell is coiled into a spiral and is partly enclosed by the
mantle, the rostral and proostracal portions having disap-
peared. In all other forms the shell has a more or less flat-
tened form and becomes completely enclosed within the
mantle, folds of which grow up around it. In Sepia (Fig.
161, A) the proostracon becomes almost obliterated and the
rostrum (r) is exceedingly small, the phragmacone (ph) form-
ing the principal bulk of the shell. This, however, has become
very much modified — that portion of it which lies posterior to
the sipuncle (s) ceasing to develop, or rather becoming exceed-
ingly compact by the various partitions lying in close con-
tact with one another without any intervening air-chambers.
These chambers are, however, developed in the portion ante-
rior to the sipuncle, but are comparatively flat and traversed
by calcareous spicules, so that the shell has a somewhat
spongy appearance and is exceedingly light. In other forms,
however, the proostracon is the portion that persists, the
rostrum and phragmacone both disappearing (Fig. 161, C), so

TYPE MOLLUSC A.

361

that finally nothing is left but a plate imbedded in the man-
tle, formed entirely of chitinous material and destitute of any
calcareous substance. This forms the so-called " pen " of
such forms as Loligo and Ommastrephes (Fig. 161, D], a slight

FIG. 161.— DIAGRAMS OF SHELL OF A, Sepia; B, Belemnites (fossil); C, Os-

tracoteuthis (fossil); and D, Ommastrephes (after LANG).
ph = phragmocone. r = rostrum.

pr = proostracon. s = sipuncle.

thickening of the dorsal end of it in some forms representing
the remains of the phragmacone.

In the Octopoda the eight arms are practically equal in
length, and the body is more massive than in the Decapods
and less suited for active swimming. The visceral hump is a
more or less globular mass, destitute as a rule of lateral fins,
and in all forms a shell comparable to that of the Decapods
is wanting. In the females of Argonauta, however, a non-
chambered calcareous shell is present, to which the body of
the animal does not closely adhere, but which is held in posi-
tion by the broad plate-like anterior arms which embrace it.

362 INVERTEBRATE MORPHOLOGY.

It seems to be a secretion of the ectoderm of the mantle and
visceral hump, the anterior arms contributing only a thin
external layer.

Development and Affinities of the Cephalopods. - - The ova of those
Cephalopoda which have been studied are richly provided with yolk, in
consequence of which the development becomes considerably modified,
definite traces of the Veliger condition being entirely lost. It seems clear,
however, from the marked development of the head, the presence of a
radula, and the general arrangement of the viscera, that the ancestry of the
group is to be sought for among the primitive Gasteropods, but in forms
more primitive than any existing forms. The symmetrical shape of the
body and the character of the viscero-pericardial cavity suggests forms
intermediate in development between the Amphineuraandthe Diotocardiate
Prosobranchs.

So far as the various groups are concerned there can be little doubt but
that the Tetrabranchs are the more primitive forms, showing as they do
less specialization of the foot, what must be considered a more primitive
shell, and a more general tendency towards a paired arrangement of the
organs than is found in the Dibranchs. The duplication of the ctenidia
and uephridia must, however, be considered a secondary acquisition. The
Decapods, again, seem to be on the whole more primitive than the Octo-
pods, the character of the co3lom and the presence of a shell in the former
being points to which attention may be called in this connection.

The Affinities of the Mollusca. — Attention has already been called to
the similarity of the typical Gasteropod and Pelecypod larvas to the An-
nelid Trochophore, and the evident conclusion has been pointed out that
the Annelids and Mollusca are to be traced back to a common ancestor
represented by the Trochophore. It is difficult otherwise to understand
the remarkable similarity which exists between the two larvae— similarities,
including not only the general arrangement of the locomotor cilia, but ex-
tending as well to internal organs, such as the nephridia. In two respects,
however, the Molluscan Veliger differs from the Annelid .Trochophore ; it
possesses a shell and a foot. These features are, however, readily ex-
plicable as a throwing back in the ontogeny of important structures origi-
nally developing at a much later period in the life of the animal — a phe-
nomenon of by no means unfrequent occurrence. It must be admitted,
however, that frequent modifications of the Trochophore arrangement are
to be found, as has been indicated in the descriptions of the Amphineura,
and these become especially interesting from the fact that in the former a
primitive arrangement of the parts of the body must be recognized. If
the Trochophore represents an ancestor, then it might be expected that it
would be found more perfectly represented in the Amphineura than in the
highly specialized Gasteropods, or even than in the Pelecypods.

It is important, then, that the possibility of some of the similar struct-
ures of the Trochophore and Veliger having been independently acquired

TYPE MOLLUSC A.

363

should be kept in mind, especially in view of another idea as to the gene-
alogy of the Mollusca which has been advocated by certain authors. Ac-
cording to this theory they have sprung from Turbellarian-like ancestors, the
creeping surface of the worm having become more muscular, and so having
given rise to the foot of the Mollusk, the dorsal region of the body elevat-
ing into the visceral hump. The nervous system of the Amphineura, with
its ladder-like arrangement, might readily be deduced from the arrange-
ment found in the Platyhelminths, and thus many points on which the Tro-
chophore theory throws no light become intelligible.

This theory concerns itself mainly with the adult forms, yet it is not
impossible that a reconciliation between it and the Trochophore theory may
be possible. It has already been pointed out that the Trochophore may
possibly be the representative of a Turbellarian larva, and the same idea
may be applied to the Veliger. In other words, it is possible that the
Mollusca may have been derived from the Turbellaria, and that the ances-
tral worms possessed in their life-history a larva which, independently of
the adults, underwent a series of modifications leading to the Veliger. The
Veliger would then be the descendant of a Turbellarian larva, while the
adult Mollusk would be directly descended from the Turbellarian. This
view may be contrasted with that which regards the Trochophore (includ-
ing the Veliger under this term for convenience) as the ancestor by means
of the following scheme :

TUKBELLARIAN THEORY.

Turbellarian larva -Turbellarian
(ancestor! \

"Trochophore

TROCHOPHORE THEORY.

Annelid

Mollusk

Turbellarian
(ancestor)

//Turbellarian

Trochophore

/^Annelid
//

'C<

Mollusk

SUBKINGDOM METAZOA.
TJPE MOLLUSCA.

I. Class AMPHINEURA. — Visceral hump not developed; bilaterally sym-
metrical ; shell represented by scattered spicules or by a series
of calcareous plates; anus terminal.

364 INVERTEBRATE MORPHOLOGY.

1. Order Solenogastres. — Shell represented by scattered calcareous

spicules. Neomenia, Proneornenia, Chcetodenna, Dondersia.

2. Order Polyplacophora. — Shell formed by eight plates on dorsal

surface of body. Chiton, Trachydermon, Chitonellus.
II. Class GASTEROPODA. — Visceral hunip usually well developed; body
asymmetrical; shell univalved and usually spirally coiled, some-
times absent; anus not terminal.

1. Order Prosobrancliia.— Ctenidia present, situated in front of the

heart; auricle in front of ventricle; mantle edge not fused with
body.

Heart with two auricles; two nephridia (Diotocardia). Haliotis,
Turbo, Troehus, Neritina, Pleurotomaria, Fissurella, Patella,
Acmcea.

Heart with a single auricle and a single nephridium (Monotocar-
dia).
Dentition taenioglossate.

With creeping habit. Cyprcea, Paludina, Natica, Ampul-

laria, Littorina, Cydostoma, Calyptrcea, Strombus.
With pelagic habit (Heteropoda). Atalanta, Carinaria,

Pterotracliea.

Dentition ptenoglossate. lanthina, Scalaria, Solarium.
Dentition rachiglossate. Fusus, Buccinum, Nassa, Murex,

Pttrpura, Olivet,, Marginella.
Dentition toxiglossate. Terebra, Conus, Pleurotoma.

2. Order Opisthobranchia. — Ctenidia frequently absent, when pres-

ent behind the heart; auricle behind ventricle; mantle when

present not fused by its edges to body- wall; shell frequently

absent.
Mantle present ( Tectibrancliia) .

Foot with broad flat sole; with creeping habit. Bulla, Janus,
Aplysia, PUurobranchus.

Foot with winglike parapodia, pelagic (Pteropoda).

Wi th shell (Thecosomata). Limacina, Styliola, Cymbuiiopsis,
Without shell (Gymnosomata). Pneumoderma, Clione.
Mantle not developed (Nudibramhia). PlKurophyllidia, Phyl-

ItrJtoe, Limapontia, Doris, JEolis, FaceUina.

3. Order Ptdmonata. — Ctenidia wanting; mantle fused by its edges

to body- wall; terrestrial or aquatic.
Eyes at base of tentacles (Basommatophord). Limncza, Physa,

Planorbis.
Eyes at tip of tentacles (Stylomniatophord). Helix, Limax, Arion,

Vayinula, Daudebardia, Onchidium.

III. Class SCAPHOPODA. — Visceral hump developed; bilaterally symmetrical;
shell cylindrical, open at both ends. Dentalmm, Siphodenta-
liiim, Cadnlus.

TYPE MOLLUSC A. 365

IV. Class PELECYPODA. — Visceral bump not developed; bilaterally sym-
metrical; mantle forms two lateral folds; shell bivalved; anus
terminal.

1. Order Protobranchia. — Gill a true ctenidium; pleural ganglia not

united to cerebral. Nucula, Yoldia.

2. Order Filibranchia. — Gill-filaments elongated and bent upwards

at ends; cerebral and pleural ganglia fused. Anomia, Mytilus,
Modiolaria, Area.

3. Order Pseudolamellibranchia. — Gill-filaments turned up at ends

and with interlamellar junctions; cerebral and pleural ganglia
united. Pecteu, Ostrea.

4. Order Eulamellibranchia.—GriM filaments united to form a plate-

like gill; cerebral and pleural ganglia united. Venus, Hya,
Ensatella, Pholas, Teredo, Unio, Anodon, Cyclas.

5. Order Septibranchia. — Gill reduced to a muscular perforated sep-

tum between the mantle and suprabranchial chambers. Cus-
pidaria.

V. Class CEPHALOPODA — Visceral hump developed; bilaterally symmetri-
cal; mantle a circular fold; foot (propodium and mesopodiutri)
forming arm-like structures provided with suckers and sur-
rounding the mouth.

1. Order Tetrabranchia. — With four ctenidia and with external

chambered shell. Nautilus.

2. Order Dibranchia. — With two ctenidia ; shell if external not

chambered, usually internal.
With eight arms to foot (Octopoda). Argonauta, Octopus, Trem-

octopus, Plrilonexis.
With ten arms to foot (Decapoda). Spirula, Sepia, Loligo,

Ommastrephes.

LITERATURE.

GENERAL.

Gould and W. G. Binney. The Invertebrata of North America. Boston, 1870.

G. W. Tryon and H. A. Pilsbry. Manual of Concholoyy. In course of publi-
cation. Philadelphia.

S P Woodward. A Manual of the Mollmca. 3d ed. London, 1878.

E. R. Lankester. Article " Mollusca." Encyclopaedia Britannica, 9th Ed.
London, 1883.

W. Patten. The Eyes of Mollusks and Arthropods. Mitth. a. d. Zool. Station
Neapel, vi, 1886.

AMPHINETJRA.

B. Haller. Die Organisation der Chitonen der Adria. Arbeiten a. d. Zoolog.

Institut Wien, iv, 1882 ; v, 1883.
H. N. Moseley. On the Presence of Eyes in the Shells of certain Chitonidw and

366 INVERTEBRATE MORPHOLOGY.

on the Structure of these Organs. Quarterly Journ. Microscop. Science,
xxv, 1885.

H. J. Pruvot. L 'organisation de quelques Neomeniens des Cotes de France. Ar-
chives de Zool. exper., 2me Ser., rx, 1891.

S. Wiren. Studien fiber die Solenogastres. I. MonograpMe des Ghcetoderma
nitidulum. Svensk Vetenskap. Akad. Handl., xxiv, 1892.

GASTEROPODA.

Alder and Hancock. A Monograph of the British Nudibrauchiate Mollusca.
London, 1850-51.

J. W. Spengel. Die Gemchsorgane und das Nervensystem der Mollusken.
Zeitschr. fiir wissensch. Zoologie, xxxv, 1881.

H. L. Osborn. On the Gill in some Forms of Prosobranchiate Mollusca. Studies
from the Biolog. Labor. Johns Hopkins Univ., in, 1884.

B. Haller. Beitrdge zur Kenntniss der Niere der Prosobranchier. Morpholog.
Jahrbuch, xi, 1885.

H. Simroth. Versuch einer Naturgeschichte der deutschen NacktscJmecken und
ihrer europaischen Verwandten. Zeitschr. fiir wissensch. Zoologie, XLII,
1885.

W. G. Binney. A Manual of American Land-shells. Bulletin U. S. National
Museum, No. 28, 1885.

Bela Haller. Untersuchungen fiber marinen Rhipidoglossen. Morpholog. Jahr-
buch, ix, 1883 ; xi, 1886.

E. L. Bouvier. Systeme nerveux, morphologic generate, et classification des Gas-
teropodes prosobranches. Aunales Sciences Nat., Zool., Ser. 7me, in, 1887.

B. Haller. Die Morphologic der Prosbranchier gesammelt auf einer Erdumse-
gelung durch die koiiigl. itnl. Corvette " Vettor Pisani." Morpholog. Jahr-
buch, xiv, 1888 ; xvi, 1890 ; xvni, 1892.

P. Pelseneer. Report on the Pteropoda. Scient. Results of the Voyage of
H. M. S. Challenger. Zoology. LVIII, 1887 ; LIX, 1888 ; LXV, 1888.

J. I. Peck. On the Anatomy and Histology of Cymbuliopsis calceola. Studies
from the Biol. Labor. Johns Hopkins Univ., iv, 1889.

A. Lang. Versuch einer Erkldrung der Asymmetric der Gasteropoda. Viertel-
jahresschr. Naturf. Gesellsch. Zurich, xxxvi, 1891.

R. von Erlanger. Zur Entwickeliing von Pahidina mvipara. Morpholog. Jahr-
buch, xvn, 1891.

H. von Ihering. Morphologic und Systematik des G enitalapparates wn Helix.
Zeitschr. fiir wissensch. Zoologie, LIV, 1892.

SCAPHOPODA.

H. de Lacaze-Duthiers. Histoire de ^organisation et du developpement du Dentale.

Annales des Sci. Nat., Zool., 4me Ser., vi, 1856 ; vn, 1857 ; vm, 1858.
L. H. Plate. TTeber den Bau und die Verwandtschaftsbeziehungen der Soleno-

conchen. Zoolog. Jahrbiicher, Anat. Abth., v, 1892.

PELECYPODA.

R. H. Peck. The Structure of the Lamellibranch Gill. Quarterly Journ. Mi-
croscop. Science, xxvi, 1876.

TYPE MOLLUSC A. 367

C. Grobben. TTeber die Pericardialdruse der Lamelhbraiichiaten. Ein Beitrag
zur Kenntniss der Anatomie dieser Molluskenklasse. Arbeiteu a. d. Zool.
lustitut Wien, vn, 1888.

W. M. Rankiu. Ueber das Bojanus" schen Organ der Teichmuschd (Anodonta
Cygnea, Lam.). Jenaiscbe Zeitschr., xxiv, 1890.

B. Rawitz. Der Mantelrand der Acephalen. Jenaisclie Zeitschr., xxn, 1888 ;

xxiv, 1890 ; xxvn, 1892.

P. Pelseneer. Contributions d I' etude des Lamellibr (inches. Archives de Bio-
logic, xi, 1891.

CEPHALOPODA.

R. Owen. Article " Cephalopoda." Todd's Cyclopaedia of Anatomy and Phy-
siology, I. London, 1836.

J. Brock. Zur A natomie und Systematik der Cephalopoden. Zeitschr. f ilr vvis-
sensch. Zoologie, xxxvi, 1882.

C. Grobben. Morpliologische Studien iiber den Harn- und Geschlecfitsappa rat

sowie die Leibeshohle der Cephalopoden. Arbeiten a. d. Zoolog. Institut Wien,
v, 1884.

P. Pelseneer. Bar la valeur morphologique des bras et la composition d>/ systeme
nerveux central des Cephalopodes. Archives de Biologie, vm, 1888.

368 INVERTEBRATE MORPHOLOGY.

CHAPTEK XIII.

TYPE CKUSTACEA.

THE group Crustacea includes a very large number of
forms, most of which are marine, though many are found in
fresh water and a few are even terrestrial. A great diversity
of form is found in the various members of the group, but at
the same time the general structure, except in. forms degen-
erated by parasitism, shows comparatively close similarity
throughout.

The body is enclosed in a thick chitiuous cuticle which
not infrequently becomes hardened by the deposition of
calcareous matter in it, producing what may almost be con-
sidered a shell and giving origin to the name applied to the
group. This covering serves not only for protection, but also
as a point d'appui for the insertion and origin of muscles.
Where it reaches a considerable thickness it becomes more or
less regularly divided into segments, separated by intervals in
which the cuticle remains thin, so that movement of the
various segments upon each other are possible.

As a rule there is attached to the sides of each of these
segments an appendage, also inclosed in a more or less thick
cuticle and jointed, this jointed character having suggested
the reference of the Crustacea together with the Araclmida
and Tracheata to a single group termed the Arthropod a. An
examination of the internal parts, especially of the nervous
system, shows that these various body-segments are in reality
metameres, and that the Crustacean is, like the Annelid, a
metameric organism. A characteristic of the Crustacea, how-
ever, is a tendency towards a greater differentatiou and con-
solidation of the metameres than is found in the Annelida, a
tendency especially well marked in the anterior region of the
body, where a varying number of the metameres fuse more or
less perfectly together to form a distinct head, bearing the

TYPE CRUSTACEA.

369

principal sense-organs and the organs of mastication (Fig.
162), there being behind this head, more or less perfectly dis-
tinguishable, a thorax and an abdomen. Judging from the
number of pairs of appendages arising from this head region
it seems that the typical number of rnetameres consolidated
to form it is five, but to these there must be added an anterior
segment which does not bear appendages but upon which the
eyes are developed. To these six somites there are added,
especially in the more highly-differentiated forms, a number
of additional metameres which properly belong to the thorax,
the apparent extent of the head region being thus increased.

FIG. 162. — A DECAPOD CRUSTACEAN, Cambarus.

There is indeed throughout the Crustacea a tendency towards
what has been called " cephalization," i.e., a condensation of
the anterior metameres, and as a rule the higher the form the
greater is this condensation and the greater the apparent ex-
tent of the head region. The number of segments composing
the thorax and abdomen is exceedingly variable in the lower
forms, but in the higher there are constantly eight thoracic
and seven abdominal segments, the posterior one, termed the
telson, being alone destitute of appendages. Frequently,
especially in the higher forms (Fig. 162), the thoracic seg-
ments consolidate to a greater or less extent, the segmentation
of this region of the body being indicated in some forms only
by the appendages and the nerve-ganglia, and furthermore
lateral folds of the body-wall may project backwards from the
sides and dorsum of the head or anterior thoracic regions,
enclosing the thorax or even the entire body in a firm cara-
pace or else in a bivalved shell, sometimes provided with
adductor muscles.

370

INVERT EBB A TE MORPHOLOG T.

The study of the embryology of some of the higher Crustacea has
brought out the fact that in these there are indications of a segment desti-
tute of appendages but represented by a pair of nerve-ganglia, immedi-
ately succeeding the eye-bearing anterior segment. In these cases, then,
the head really consists altogether of seven segments. Whether this seg-
ment represents the first appendage-bearing segment of the lower forms

D

FIG. 163.— CRUSTACEAN APPENDAGES.

A, antennule of Crayfish, Cambarus; B, antennule of Copepod, Oiihona (after
GIESBRECHT); (7, antenna of Cambarus; D, antenna of Phyllopod, Eulimnadia
(after PACKARD) ; ss, sensory hairs.

or whether in these also it exists in a degenerate condition has not yet been
determined ; for convenience at present the six fully-developed head-
segments may be considered homologous throughout the group.

The appendages vary much in form in different parts of
the body and in different forms. Those of the head region

TYPE CRUSTACEA. 371

are modified to serve as sensory organs and organs of masti-
cation. The first pair, termed the first antennw or antennules,
are usually sensory in function, though occasionally also loco-
motor (Ostracoda and some Copepoda), and are frequently
supplied with peculiar seta3 supposed to be olfactory in addi-
tion to others probably tactile in function (Fig. 163, B). They
consist in their typical form of a basal portion composed of
three or four joints, the terminal one bearing one (Fig. 163, B)
or two (Fig. 163, A} many-jointed flagella. The second pair, the
second antenna3 termed also simply antenna? (Fig, 163, C and D],
are also principally sensory and consist typically of a two-joint-
ed basal portion, bearing two many-jointed branches. One of
these, that upon the outer side, frequently becomes reduced to
a scalelike rudiment (Fig. 163, C), the inner branch persisting as
the flagellum. The third pair, the mandibles, serve as mas-
ticatory organs and are generally much modified in correspon-
dence with this function. Typically (Fig. 164, A] they consist
of a two-jointed basal portion bearing two branches. The
proximal joint of the basal portion, however, becomes much
indurated by the thickening of the chitinous cuticle and also
toothed, forming the mandible proper, while the remaining joint
and the two branches undergo reduction even to disappear-
ance, being known when present as the mandibular palps
(Fig. 164, C, nip). The fourth pair are the first maxillce (Fig.
164, B and U) and serve like the mandibles for mastication, un-
dergoing a somewhat similar modification. They do not, how-
ever, become so indurated, though one or both of the basal
joints may be provided with stiff setae and serve as a jaw, and
the two branches more frequently persist than in the mandi-
bles. The fifth pair, the second maxilla', are also masticatory
and resemble the first in the modifications which they
undergo.

The thoracic and abdominal appendages in all but the
lowest forms can be reduced to a typical appendage consisting
of a two-jointed basal portion tipped by two branches also
jointed. In appendages employed for swimming both
branches persist (Fig. 165, A), and may possess a broad
platelike form, but when modified for walking the outer
branch disappears. From limbs modified in this latter

372

INVERTEBRATE MORPHOLOGY.

FIG. 164.— CRUSTACEAN APPENDAGES.

A, mandible of Copepod, Notodelphys (from BRONN); B, first maxilla of Noto-
delphys (from BRONN); G, mandible of Cambarus; D, first maxilla of Cam-
barus.
en = endopodite. ex = exopodite. mp = maudibular palp.

,ex

FIG. 165. — CRUSTACEAN APPENDAGES.

A, second thoracic appendage of Mysis (after SARS); B, second thoracic appen-
dage of an Amphipod.

TYPE CRUSTACEA,

373

manner the grasping claws (Fig. 165, B] or chelae are devel-
oped by the flexion of the terminal joint on the subterininal or
by the elongation of the angles of the latter into a more or
less strong process against which the terminal joint may be
approximated.

The description given above of the various appendages is
of course general, the modifications found in the various
forms being almost endless. Indeed in parasitic forms the ap-
pendages, except those concerned in mastication, may entirely
disappear, all gradations between fully-developed append-
ages and the merest rudiments being found in various forms.

B

FIG. 166. — SIXTH (A) AND SECOND (B) THORACIC APPENDAGES OF BKANCHIO-

POD, Apus (after ZADDACH from BRONN).
br = bract. fl = flabellum. 1-6 = inuer lobes.

From what has been said, however, it may be seen that typi-
cally the Crustacean appendage may be considered a birainous
structure, consisting of a two-jointed basal portion termed
the protopodite and two jointed branches termed the exopodite
and endopodite (Fig. 165, ex, en) according to their relation to
the median axis of the body. Additional rarni are frequently
developed upon the protopodite — such, for example, as that
termed the epipodite (Fig. 167, ep) and the branchia (br). How-
ever, although such a limb may be considered typical, it is
not necessarily also the most primitive. Indeed when the
simplest forms, such as the Phyllopoda, are examined it will
be found that the more posterior appendages have a very
different composition. Thus in the genus Apus the sixth
thoracic appendage (Fig. 166, A) consists of a central two-

374

INVERTEBRATE MORPHOLOGY.

jointed axis ending in a rounded lobe and bearing upon its
inner edge six lobes (1-6), some of which are united to the
axis by a joint. On the outer side are two large lobes, the
distal one being termed the nabellum (fi\ while the proximal
one is the bract (br) and serves respiratory purposes. The
entire appendage has thus a leaflike form. In one of the
more anterior appendages, however, an interesting modifica-
tion of this will be found. Thus in the second thoracic ap-
pendage (Fig. 166, B) the axis
will be found to be more dis-
tinctly divided into two joints,
each bearing two of the internal
lobes somewhat reduced in
size, while the terminal one in
addition carries two other lobes,
the fifth and sixth, which have
become somewhat elongated.
The flabellum and bract remain
nearly the same as in the
first post - genital appendage.
If now such an appendage be
compared with the second tho-
racic appendage of the Shrimp
Palcemonetes (Fig. 167), a direct
homology of the parts may be
discovered. The axis of the
Phyllopod limb is represented
by the protopodite while the
exopodite (ex) and endopodite
(en) represent the two terminal
inner lobes, the others having disappeared; the flabel-
luni is represented by the epipodite (ep) and the bract per-
haps by the branchia (br), attached to the epipodite in this par-
ticular limb, but free on the more posterior ones.

It would appear probable from these facts that the bira-
mous limb is really a derivative from the more complicated
foliate appendage possessed by the Phyllopods ; the foliate
condition, however, has given place to such a great extent to

FIG. 167.— THE SECOND
FED OF Palcemonetes.
br — braucbia.
en = endopodite.
ep = epipodite.
ex — exopodite.

TYPE CRUSTACEA. 375

the biramous that it is most convenient to regard the latter as
the typical condition in the Crustacea.

Respiratory organs are not always present, but when they
are they take the form of thin-walled outgrowths of the body-
wall. In some forms in which the surface of the body-wall is
increased by the development of a bivalved shell or carapace
the lining-surface of the fold serves for respiration, and may
be thrown into a number of folds so as to increase the extent
of surface, as in the Gasteropod Patella. In the majority of
cases, however, more or less branched hollow processes are
seated upon the sides of the body or on a greater or less
number of the appendages, their cavities communicating with
the lacuuar spaces of the body, so that the blood can circulate
through them and receive aeration through their thin walls.
In the Isopoda a certain number of the appendages are de-
voted to the respiratory function, both the exopodite and
endopodite being lamellar and thin-walled, or else the endo-
podite alone may ha\re this function, the exopodite serving as
a covering-plate for the protection of the inner respiratory
ramus.

As already stated, the body is covered by a chitinous or
more or less calcareous cuticle. This is secreted by the cells
of the hypodermis, as it is termed, which correspond to the
ectoderm of other forms and rest below on a more or less
well-developed layer of connective tissue. A dermal muscu-
lar system is entirely unrepresented in the Crustacea, owing
no doubt to the development of the thick cuticula ; but never-
theless muscles are well-developed. These take the form in
the body of four longitudinal bands, two situated dorsally and
two ventrally, giving off slips to be inserted into the cuticle
of each metamere, flexion and extension of the various meta-
meres upon one another being thus permitted. In addition
muscles extend from the body-wall to the various appendages
and between the various joints of these structures, being in
all cases, it is needless to say, situated within the body and
the appendages. In some cases, more especially in those
forms in which the appendages are adapted for walking,
special chitinous plates or processes project into the body-
cavity from the ventral surface forming the endophragmal sys-

376 INVERTEBRATE MORPHOLOGY.

tern and serving for the attachment of the muscles passing to
the appendages. In forms furnished with a bivalved shell
special adductor muscles for its closure are frequently devel-
oped ; and in the higher Crustacea, in which the so-called
stomach is usually provided with a series of chitinous teeth,
special muscles are developed for their movement.

The coelom of the Crustacea consists for the most part of
a series of cavities, without definite walls, between the viscera
and the muscle-bundles and extending out into the append-
ages and the branchiae. One of these occupies the mid-line
below the dorsal surface of the body, and contains the heart,
whence it is known as the pericardial sinus. It is bounded
below by a .distinct partition, the pericardial septum, but
seems to be a schizocoelic space, since it contains blood, and is
therefore not comparable to the pericardial cavity of the
Mollusca. A true enteroccel does exist, however, in some of
the higher forms (e.g., Palcemonetes), consisting of a sac lying
in the anterior thoracic region. It surrounds the anterior
aorta as a narrow cavity and behind expands so as to cover
the anterior portion of the reproductive organs, and then
passes veutrally into the schizocoelic cavity which surrounds
the intestine. It is a perfectly closed sac, having no com-
munication with the pericardial sinus beneath which it lies,
and contains a coagulable fluid in which no corpuscles have
been observed.

The saclike cavity into which the antenuary gland, to be described
later, opens is also to be regarded as a true enterocosl; but attention must
again be called to the inadvisability of maintaining a wide distinction be-
tween schizoccelic and enterocoelic spaces. (See p. 231.)

The circulatory system is comparatively simple. In many
forms a heart and distinct blood-vessels are entirely wanting,
the blood circulating through the lacunar coelom by the
movements of the appendages. In the majority of forms,
howrever, a pulsatile heart (Fig. 168, /*) is present, lying near
the dorsal surface of the body in the pericardial sinus, ex-
tending in some forms throughout the entire thoracic and
abdominal regions of the body. More usually, however, the
heart is limited to the thoracic region, or occasionally is
almost entirety confined to the abdomen, its anterior ex-

TYPE CRUSTACEA.

377

trenrity encroaching but slightly upon the thorax (Isopoda).
It is provided with a varying number of openings along its
sides, through which the blood gains entrance to its cavity
from the pericardial sinus — these openings, termed ostia,
being guarded by valves opening inwards and preventing
regurgitatiou of the blood during systole. From either end
of the heart arteries arise which, after a longer or shorter
course and many or few branchings, open widely into the
lacunar spaces. From these the blood passes in some forms
into a venous sinus situated on the ventral surface of the

sa

Vd vn

t,K an

FIG. 168.— DIAGRAM OF STRUCTURE OP CRUSTACEAN (Cambarus).

ne = nephridium.

s = stomach.
sa = sternal artery.
te — testis.
tl = telson. t
tid = vas deferens.
vn = ventral nerve.
1-6 = abdominal segments.

an — auus.

ca — carapace.

ce = cerebral ganglion.

h = heart.
i — intestine.
I — digestive gland.
TO — mouth.
mp = opening of vas deferens.

body, and thence is distributed to the branchiae, passing from
them back to the pericardial sinus, and so to the heart again.
The blood is usually colorless, though occasionally greenish,
in which case it contains a respiratory copper-containing pig-
ment termed hsemocyauin, or reddish, in which case the pig-
ment is haemoglobin. It consists of a plasma in which float
amoeboid nucleated corpuscles.

The digestive system consists of an almost straight tube
extending from mouth (Fig. 168, m) to anus (an) and divisible
into three regions. The mouth is bounded in front by an
overhanging lip, and behind by a lower lip which arises as
two separate parts, which by some writers have been regarded

378 INVERTEBRATE MORPHOLOGY.

as appendages, though the absence of a corresponding nerve-
ganglion tells very strongly against such an idea. The ante-
rior portion of the digestive tract arises in the embryo as an
ectoderinal invagination, and is frequently lined throughout
by a chitiuous cuticle. In the higher forms (Malacostraca)
the posterior portion of this foregut is enlarged to form a so-
called stomach (Fig. 168, s), in which the chitiuous lining
thickens to form a complicated arrangement of teeth, which,
moved by special muscles extending from the stomach to the
walls of the body, serve for the comminution of the food.
No salivary glands occur. The midgut is frequently of very
small extent, and has usually connected with it a digestive
gland (Fig. 168, I) consisting either of from one to four pairs
of simple or but slightly branched coecal tubes, or else of a
much-branched compact gland opening into the intestine by
two or more ducts. The hiudgut (i), like the foregut, arises
as an ectoderinal invagination, and is usually lined with chitin
and unprovided with special glands.

The nervous system presents a typically metameric condi-
tion throughout the greater portion of the body, a pair of
ganglia occurring in each segment, united by paired connec-
tives with the ganglia of the preceding and succeeding meta-
meres (Fig. 168, vn). In the anterior portion of the body,
however, as well as posteriorly, a certain amount of concen-
tration and fusion of the various ganglia occurs. An ideal
condition in which no fusion has taken place would show a
pair of cerebral ganglia (Fig. 169, ce) with which more or less
complicated optic ganglia are connected. From the cerebral
ganglia connectives pass backward and unite with a pair of
ganglia (g1), clearly indicated in the embryos of many of the
higher forms, but not yet definitely known iri the Entomo-
straca, though it seems probable that they occur in these
also. The metamere and appendages which should properly
be associated with them seem to have disappeared ; that is
to say, they are the sole representatives of a metamere inter-
vening between the cerebral and antennulary segments. These
ganglia are united by a pair of connectives with a third pair
sending nerves to the autennules ((f), and these again with a
fourth pair belonging to the anteunary metamere (g3), and so

TYPE CRUSTACEA.

379

on, a pair of ganglia occurring in each metaniere throughout
the body. Such a condition as this is found only in em-
bryonic stages, and even there not always perfectly. The
ganglia representing the preauteunulary metaniere fuse with
the cerebral, as do also the antenuulary, and in higher forms
the antennary ganglia, there being thus formed a complex

FIG. 169.— DIAGRAM OF NERVOUS
SYSTEM OF CRUSTACEAN.
ce = cerebral ganglion.
g] = second
<72 = antennulary "
<73 = anteuuary "
mn — rnandibular "
mxl, mx- = maxillary ganglia.
<K = oesophagus.
thl == first thoracic ganglion.

tab

FIG. 170. — NERVOUS SYSTEM OP

(A) AN ISOPOD, Asellus, AND

(B) A BRACHYURAN DECAPOD,

Maja (after MILNE-EDWABDS).

cerebrum, which, in contrast to the simple cerebrum (arclii-
cerebrum) of the Annelida, may be known as a syncerebrum.
The remaining ganglia may remain perfectly separate, the
connectives joining the more anterior ones usually being
much shortened, or a greater or less number of them may
fuse. Thus in the Crayfish the ganglia of the three posterior

380 INVERTEBRATE MORPHOLOGY.

liead-metameres unite witli those of the two anterior thoracic
segments to form a single gaugliouic mass lying behind the
oasophagus and sending nerves to the appendages of the
somites represented in the fusion. Similarly, in the posterior
region of the body of the Isopoda all the ganglia of the
abdominal region may fuse to a more or less simple mass
(Fig. 170, A, ub), and an extreme condition of fusion is to be
found in some Crabs (Fig. 170, B], in which all the ganglia
behind the antenuary segment fuse to a single mass (tab),
lying in the thorax — a condition standing in relation to the
reduction of the abdomen and the extensive concentration of
the head and thoracic regions which are characteristic of
these forms.

A sympathetic nervous system seems to be generally
present, consisting in its most complete condition of an un-
paired nerve arising from the syncerebrum and passing back-
wards to be distributed to the stomach, and of a median
nerve (Fig. 170, A, tii) extending from one pair of postoasoph-
ageal ganglia to the other, lying between the two connectives.

Sense-organs reach a high degree of development in the
group. Hairs occur in abundance on the appendages and
body, the majority no doubt having merely a mechanical
function ; but among them will be found some beneath which
lie one or more ganglion-cells, giving rise to a nerve which
passes into the hair. These hairs are supposed to be tactile
in function. On the anteuuules of many forms and more
rarely upon the antennae, hairs of special forms occur, usually
in bunches or in rows. They may be club-shaped or cylin-
drical, and each has a nerve-fibre extending into it- without
dilating into a ganglion-cell beneath its base. To these hairs
an olfactory function has been assigned, and it is noticeable
that they are usually more abundant upon the antenuules of
the males than on those of the females — an arrangement
which suggests a probable service as guides in finding the
latter.

Eyes are very generally present in the Crustacea, and
reach usually a high degree of efficiency. Two forms of eye
are known — a median unpaired one, frequently spoken of as
the simple eye, and the lateral or compound eyes. The un-

TYPE CRUSTACEA.

381

paired eye is present iii the larval stages of probably all
Crustacea, arid persists iu a more or less perfect form in the
adults of most Eutomostraca, — a group which contains the
more primitive forms, — and has even been detected in those
of some of the higher forms (e.g. Crangori). It consists when
well developed of three patches of pigment, forming cups in
each of which lies a group of clear cells from which nerve-
tibres arise passing to the optic nerve.

The lateral eyes are composed of a number of units each
of which possesses all the parts of a visual organ and is
termed an ommatidium, and consequent-
ly these eyes have been regarded as an
aggregation of a number of individual
eyes, whence the term compound usually
applied to them. Each ommatidium is
a complicated structure consisting of
several parts (Fig. 171). The outermost
layer of each is a transparent cornea
which is continuous with the general
cuticle of the body, and in some forms
is only distinguished from this by its
transparency. More frequently, how-
ever, this cuticle becomes more or less
perfectly divided into a series of corneas
of an hexagonal or tetragonal shape,
one corresponding to each ommatidium,
the surface of the eye thus acquiring
a faceted appearance. Below the cuti-
cle

FIG. 171. — DIAGRAM OF
CRUSTACEAN OMMATI-
DIUM.

C = cone-cell,
come the hypodermal cells (Off) Cff = coraeal hypodermis.

which secrete it, arranged irregularly £^= crystalline cone.

.*?... / DR = distal retinula.
without reference to the ommatidia m PR = proximal relinula

the simpler non-faceted eyes, but in Rh •.= rhabdom.
the faceted eyes with two hypodermal cells lying beneath
each cornea and constituting the corneal hypodermis.
Below these come the cone-cells (6'), two to four in number
as a rule ; these are elongated cells a portion of whose pro-
toplasm becomes converted into a refractive translucent body,
the crystalline cone ( CC), composed of as many segments as
there are cone-cells taking part in its formation, and sur-

382 INVERTEBRATE MORPHOLOGY.

rounded upon the outside by a delicate layer of protoplasm
placing the part of each cell above the cone in continuity with
the part lying below it. In the higher forms there occur,
partially surrounding the cone-cells, two pigmeuted cells
which seem to be sensory in function and are termed the
distal retinular cells (DR). They are, however, unrepresented
in the lower forms, in which the sensory portion or retiuula is
represented by a single circle of usually 5 cells (PR) lying
proximally to the cone-cells and surrounding a chitiuous rod
which is manufactured as a secretion from their approximated
surfaces, and is termed the rhabdom (Eli). These cells are
also pigmented and are prolonged below into nerve-fibres,
which, piercing the basement-membrane upon which the om-
niatidia rest, pass to the optic ganglia. In the higher Crus-
tacea, in which a distal retiuula is present, the rhabdom is
formed by a circle of eight cells (one of which is almost
aborted, so that there appear to be only seven). These con-
stitute the proximal retinula, and appear to correspond to the
single retinula of the simpler forms. Finally, a number of
accessory cells, usually pigmented, may surround each om-
matidium, separating it from its neighbors, but not appearing
to be essential constituents of the eye.

The view according to which these lateral eyes are regarded as an ag-
gregation of a number of independent eyes has already been referred to.
It seems questionable, however, if this be the correct interpretation of
them in view of the occurrence of so-called compound eyes in the Mollusca
(Area) and the Polyehaetous Annelida. It seems more probable that, as
in these forms, the Crustacean eye is to be regarded as a separation into a
number of more or less isolated parts of an originally continuous retina,
a corresponding division of the originally simple refractive apparatus
also taking place. This view seems to harmonize most satisfactorily with
the facts of development.

Occasional departures from the usual arrangement of the
eyes are to be found — as for instance in Phronima, one of the
Amphipoda, in which two pairs of compound eyes occur on
the head. Mention may also be made here of the peculiar
eyelike structures occurring in Euphausia, one of the Schizo-
poda. They occur on the basal joints of the appendages of
certain of the thoracic metameres, as well as upon the ventral

TYPE CRUSTACEA.

surface of the abdomen, and appear to be rather phospho-
rescent than optic organs.

Otocysts occur throughout the group Decapoda, to which
the Crayfish, Lobster, and Crab belong, and consist of sacs
lined by sensory setse and containing otoliths. They are
situated on the basal joint of each of the autennules and in
some forms are completely closed, though usually their cavity
communicates with the exterior, being guarded by a number
of closely-approximated bristles. In the Schizopoda similar
otocysts occur in the endopodite of the last pair of abdominal
appendages, and in the Amphipod Oxyceplialus two lie above
the syncerebrum. These structures, which are usually spoken
of as auditory organs, seem to be rather sense-organs of equi-
librium.

In the larvae of many forms and in the adults of some
Eutomostraca one or two papilla-like processes project from
the anterior surface of the head and are supposed to be sen-
sory in function, though what purpose they may subserve is
unknown. Strong nerves pass to these frontal sense-organs
which appear to be of considerable importance.

The excretory system consists of two pairs of uephridia,
one or other of which may be absent in many forms. One of
these develops in connection with the antennary segment and
opens to the exterior on the basal joint of the antennae (Fig.
168, ne), whence it is known as the antennary gland, some-
times, however, receiving the name of the green gland. It
reaches its highest development among the Malacostraca,
occurring in many Entornostraca only in larval stages, later
on degenerating. In its simplest condition it consists of a
coiled tube whose lumen appears in some cases to be iiitra-
cellular, though in others it is undoubtedly intercellular, and
which terminates internally in a saclike dilatation whose
wall is richly supplied with blood-lacunse. In the higher
forms (Eig. 172, A) a great complexity is brought about by
the development of lateral branches from the tubular portion,
and the terminal sac (s) may enlarge and fuse with that of
the opposite side to form a cavity of considerable size lying
in the anterior portion of the thorax and termed the uephro-
peritoueal sac.

384

IKVERTEERA TE MORPHOLOG T.

The second nephriclium (Fig. 172, B] develops in connec-
tion with the second maxillary segment, and opens usually
upon the appendage of that segment. It is especially devel-
oped in the Entomostraca, in which it may lie in the folds of
the body-wall which form the shell, and hence is usually
known as the shell-gland. It occurs also in the larval stages
of many Malacostraca, and may possibly persist in a degen-
erated condition in the adults of some forms. In structure it

sa-A

B

FIG. 172. — A, DIAGRAM OF NEPHRIDIUM (GREEN-GLAND) OF Astacus (after

MARCHAL); B, SHELL-GLAND OF Eulimnadia.
s = terminal sac. sa = saccule.

resembles closely the antennary gland, but does not present
the complexity frequently found in that gland.

The majority of the Crustacea are bisexual, hermaphro-
ditism occurring only in forms which have a parasitic habit
and in some which are sessile in adult life (Cirrhipedia). The
ovaries or testes (Figs. 173, A and B} are paired organs lying
alongside of the intestine or slightly dorsal to that organ,
transverse connecting bars in some cases passing from the
organ of one side to that of the other. Each organ may be
regarded as a tube, sometimes simple, sometimes branched,
and lined on its interior by an epithelium which gives rise
to the germ-cells. Special germ-producing regions are fre-
quently developed, as, for instance, at the extremities of the
tubes or along one side (Isopoda), the cells in other regions
ceasing to give rise to ova or spermatozoa. The reproductive
elements pass to the exterior by special ducts, oviducts

TYPE CRUSTACEA.

or vasa deferentia (vd), connected with each organ, and open-
ing usually upon the ventral surface of the body at or near
the junction of the thoracic and abdominal regions. The
origin of these ducts has not yet been discovered, but it has
been suggested that they may represent a third pair of ue-
phridia. Accessory structures, such as receptacula semiuis
and cement-glands, for the attachment of the ova in the fe-
males, and spermatophore-sacs, in which the spermatozoa are
eucapsuled in spermatophores in the males, are frequently de-

ov

FIG. 173. — OVARY AND TESTIS OF My sis (after SARS).
od = oviduct. tb = transverse bar of ovary.

ov = ovary. te = testis.

vd = vas deferens.

veloped in connection with the ducts, and in the Malacostraca
certain of the appendages in the neighborhood of the genital
openings are, especially in the males, modified so as to serve
as copulatory organs.

Owing to the great variety of form and structure met
with in the various species of Crustacea the group is separa-
ble into a large number of subdivisions. Two principal classes
are, however, readily discernible, of which the first is

I. CLASS Eiitomostraca.

In this class the number of segments of which the body is
composed varies greatly in the various groups and even in
closety-related genera. The abdominal region is in some
forms very much abreviated and is destitute of appendages, a
rule which, however, finds exception in certain Phyllopods in

386 INVERTEBRATE MORPHOLOGY.

which some of the segments behind the genital 'openings,
which may be taken as indicating the line of separation be-
tween the two regions, are provided with appendages. Folds
arising from the head region and forming either a carapace
or a bivalved shell are frequently present and the animals are
for the most part small, the largest reaching a length of about
eight centimetres, while the majority measure less than a milli-
metre. The unpaired eye usually persists in the adult, as
does also the shell-gland, the antennary gland, on the other
hand, being usually rudimentary or absent. A masticatory
stomach is never present, and a further characteristic is found
in the fact that the larva which hatches from the egg is almost
invariably a Nauplius (see p. 417).

1. Order Phyllopoda.

The Phyllopoda are principally confined to fresh water,
the genus Artemia, however, being found in salt lakes, while
a few Cladocera are marine. They seem to be the most
primitive of all the Crustacea and present the greatest variation
in the number of metameres composing the body, some spe-
cies possessing over forty pairs of appendages, while in others
again the number is reduced to nine. All the thoracic ap-
pendages, however, as a rule bear branchial lobes, and in some
cases (Apus} present the many-lobed and imperfectly-jointed
condition which has been considered the most primitive form of
the Crustacean limb (see p. 373). The auteunules are usually
small and abundantly provided with olfactory hairs, while
the antennae (except in Apus, in which they entirely disappear)
are long and serve as locomotor organs. The mandibles are
reduced to simple masticatory plates without palps, and the
maxillae undergo likewise considerable reduction. A heart is
always present, but no blood-vessels exist, the blood passing
from the heart into lacunar spaces.

1. Suborder Brancliiopoda.

The Brancliiopoda have all a plainly-segmented body con-
sisting of many segments, and, with the exception of Brand i-
pus and Artemia, are provided with a fold of the body-wall

TYPE CRUSTACEA.

387

which may form a dorsal carapace, as in Apus, or a bivalved
shell, as in Limnadia, Limnetis, and Esfheria (Fig. 174), an
adductor muscle being developed for the closure of the shell
within which the entire body may be withdrawn. The auten-
nules are as a rule small and are provided with olfactory
hairs ; the anteunse, on the other hand, are well developed ex-
cept in Apus, in which they are in some species quite small
and in others entirely wanting. In the shelled forms they are
birarnous, consisting of a several-jointed protopodite termi-
nated by two many-jointed flagella, and serve as oarlike loco-
motor organs, but in Branchipus they are short strong struc-

FIG. 174. — Estheria compleximanus (after PACKARD).
at1 = auteunule. at- = antenna. m = shell-muscle.

tures without any locomotor function, serving in the males
as clasping organs of use in copulation. The mandibles are
reduced to toothed plates, lacking a palp, and the first max-
illte show an almost similar reduction, while the second are
entirely wanting in some genera, such as Limnetis. The suc-
ceeding appendages are not limited to the thoracic region of
the body, taking the genital opening as the limit between the
two regions. Thus in Apus cancriformis there are eleven
thoracic appendages, while behind the genital ring there are
no less than over fifty locomotor limbs, and in such forms as
Limnetis and Estlieria (Fig. 174) it is difficult to distinguish
between the thorax and the anterior abdominal segments.

The heart of the Brauchiopoda is a more or less elongated
organ with several ostia and is usually limited to the anterior
portion of the thoracic cavity, though in Brancliipus it extends

388 INVERTEBRATE MORPHOLOGY.

into the anterior abdominal region. Lateral eyes are present
in addition to the unpaired median eye. In Branchipus they
are situated upon the sides of the head upon well-defined
stalks, but in Apus they are closely approximated on the
dorsal surface of the cephalo-thoracic carapace, while in the
shelled forms they are united together to form a single eye
whose double nature is revealed only by a study of the details
in its arrangement.

A peculiar feature in the life-history of the members of this group is the
comparative iufrequency of males, their proportion to females being so
small that for some time they were not known to exist. The females are
able to reproduce parthenogenetically — males appearing only under certain
conditions which are not as yet satisfactorily understood. The eggs de-
velop generally in brood-pouches situated upon certain of the thoracic ap-
pendages (Apus, Limnadia) or else are affixed to filamentar processes of
these appendages (Mstheria).

2. Suborder Cladocera.

TheCladocera are distinguished from the Branchiopoda by
the segmentation of the body being much less clearly defined
and by the small and more definite number of appendages,
there being only from four to six pairs of thoracic limbs. A
bivalved shell arising from the maxillary segments and pro-
vided with an adductor muscle is always present ; it does not
enclose the head, but the rest of the body may be completely
withdrawn within it except in some genera, such as Evadne
and Polyphemus, in which it is transformed into a brood-
chamber, leaving the body almost unprotected.

The antennules are always small unjoiuted structures pro-
vided with a bunch of olfactory hairs usually terminal in po-
sition, and the antennae are strong biramous locomotor organs.
The mandibles are simple toothed plates without palps, and
the second maxillae are usually entirely wanting in the adults.
The thoracic limbs are six in number in the genus Sida and
are all lamellate and abundantly supplied with marginal setae,
but, in Daphnia (Fig. 175), Moina, and allied forms the number
is reduced to five, and the more anterior ones are more or less
modified towards simple cylindrical jointed appendages, a
condition found in all the four thoracic appendages of Evadne

TYPE CRUSTACEA.

389

FlG. 175. — Daphnia pulex (from HERTWIG).

& = brood-pouch.
e = immature ova.
g = cerebral ganglion.
go — optic ganglion.
h = heart.
)c = germinal region of ovary.

0 = ovary.

s = shell gland.

1 = antennule.

2 = antenna.

3 = mandible.

5-9 = thoracic limbs.

390 INVERTEBRATE MORPHOLOGY.

and Polyphemus, the branchial lobes being at the same time
rudimentary or entirely wanting. The abdomen, which is
composed of four segments, possesses on its dorsal surface
elevations for the closure behind of the brood-chamber, and
on its terminal segment setse are usually developed ; it does
not, however, bear any appendages.

The heart is an oval structure situated in the thoracic re-
gion and possesses but a single pair of ostia. The lateral
eyes are in all cases fused to form a double eye situated in
the median line of the head and capable of movement within
a socket by means of muscles which are attached to it.

The majority of the Cladocera are fresh-water forms,
though some, such as Evadne, are marine. The ova undergo
development in a brood-chamber formed by the space in-
cluded between the shell-valves and the dorsal surface of the
abdomen, and in Evadne and Polyphemus, as already stated, the
entire shell, which is somewhat reduced in size, is adapted to
serve as walls for the chamber.

As in the Branchiopoda, collections of Cladocera, es-
pecially if made during the spring or summer, will show an
enormous preponderance of females, and several generations
may be reared without a single male making its appearance.
The eggs, which have a thin egg-membrane and little yolk,
develop parthenogeuetically and produce females, and this
method of reproduction will continue so long as the condi-
tions, such as temperature and food, remain satisfactory ;
hence the eggs of this kind are generally known as " summer
eggs." Towards autumn, however, or whenever the condi-
tions tend to become unfavorable, males, distinguishable by
their smaller size, the absence of a brood-pouch, and their
more highly-developed sense-organs, as well as by the de-
velopment of hooked setae on the anterior appendages which
serve as clasping organs, make their appearance, and at the
same time the females begin to deposit ova much larger in
size than the summer eggs and containing a considerable
amount of yolk. These " winter eggs " develop apparently
only after fertilization. In Polyphemus they possess a thick
shell, but in other forms special arrangements occur to render
them resisteut to cold, drying, etc. In some forms the ma-

TYPE CRUSTACEA. 391

ternal shell is sloughed and serves as a protecting case, but
more usually, as iu Daphnia, Moina. and others, a saddle-
shaped thickening, the epliippium, appears on the dorsal wall
of the brood-pouch at the time of the passage of the winter
egg into it, and this thickening is thrown off with the egg and
forms a protective covering for it.

2. Order Ostracoda.

The Ostracoda resemble the Cladocera in the segmentation
of the body being but slightly marked and in possessing a
bivalved shell provided with an adductor muscle. The shell,
however, encloses the head as well as the thoracic and ab-
dominal regions, and furthermore but two thoracic limbs
exist.

The antennules and antennae are both uniramous append-
ages and serve for creeping, though the former are also pro-
vided with olfactory hairs. The mandible consists of a tooth-
bearing plate and a strong jointed palp which in some forms
also functions as a creeping limb, and behind it are two well-
developed maxillfe. The first of these is distinguished bv the
development of the jaw portion and the reduction of the palp,
and in Cypris and Cythere bears a large plate with numerous
marginal setee which is usually termed a branchial lobe. The
second maxilla, on the other hand, shows considerable modi-
fication in different genera. In Cypridina (Fig. 176, MX*} it is
jaw like and bears a large branchial lobe (wanting on the first
pair), and in Cypris is adapted for the same function, but bears
in addition to the rudimentary branchial lobe a short two-
jointed palp, which in Halocypris becomes enlarged to form a
three- or four-jointed limb, while finally in Cy there the append-
age is practically a walking limb, its jaw function not being-
developed. The first thoracic appendage is an elongated
many-jointed limb except in Cypridina (Fig. 176, t'\ where it
possesses a jaw function, and the second is also limblike. In
Halocypris this latter appendage is, however, rudimentary, and
iu Cypris and Cypridina (Fig. 176, T") it is dorsally directed
and serves for cleansing the inner surface of the shell from
foreign bodies, in the latter genus arising some distance up

392

INVERTEBRA TE MORPHOLOO T.

upon the sides of the body and forming a long cylindrical
uujoiuted appendage.

Respiration is usually effected by the general surface of
the body and the inner walls of the shell duplicature, though
in certain Cypridiuidse a double row of respiratory processes
are situated upon the dorsal surface of the body near the
second thoracic appendage. The so-called branchial lobes on
the maxillae probably subserve the respiratory function only
by renewing the water in contact with the body surface. A

H

Sm

1)

FIG 176. — Cypridina tnediterranea, FEMALE (after GLADS).

At1 = auteuuule.
At1 — antenna.

h — heart.

Mnp — mandibular palp.
Mx\ MX- = first and second maxilia.

o — simple eye.
Oc = compound eye.
Pr = frontal organ.
Sm — shell-muscle.
tl, T* = first and second thoracic
appendages.

single median eye alone is present in Cypris and Cythere, but
in addition a pair of lateral compound eyes occurs in Cypri-
dina. The frontal sense-organ is a single strong process, in
certain forms lying slightly above and between the autemmles.
A heart is present in Cypridina and Halocypris as a saclike
organ with two lateral ostia and is not prolonged into arteries.
In Cypris and Cythere it is entirely wanting.

The Ostracoda occur both in fresh water and in the ocean.
The genus Cypris and its allies are for the most part aquatic,
while the other genera mentioned are exclusively marine.

TYPE CRUSTACEA. 393

3. Order Copepoda.

The members of the order Copepoda present great varia-
tions in form, due to the fact that there are a number of para-
sitic forms belonging to it some of Avhich show so much de-
generation that their relationships to the non-parasitic forms
only become apparent by a study of their development.
Typically, however, the body is generally elongated (Fig. 177)
and consists of ten segments in addition to those of the head,
the five anterior ones usually bearing appendages and con-
stituting the thorax, while the five posterior lack appendages
and form the abdomen, the terminal segment of which bears
a pair of caudal furcse provided with setpe. In female indi-
viduals the two anterior abdominal segments fuse together to
form a genital double segment, and in all cases the head seg-
ments fuse together, while the anterior thoracic segment
usually fuses with this consolidated mass. No shell-duplica-
ture occurs. In the parasitic forms there is a tendency for
the various segments to become indistinct and all trace of
them may vanish, the abdomen in some cases becoming also
extremely reduced in size. Add to this that lobes and pro-
cesses are frequently developed upon the body and it will be
understood how far these degenerate forms depart from the
typical arrangement.

The anteimules (Fig. 177, at1) in all free-swimming Cope-
poda form long many-jointed swimming-organs used in an
oarlike manner. They consist of a certain number of stout
basal joints, terminated by a single long multiarticulate
flagellum, no trace of a biramous condition being apparent. In
addition to their locomotor function they also, as in other
forms, serve as sense-organs, olfactory hairs being scattered
along the flagellum, and in male individuals they are specially
modified to form clasping organs for use in copulation. The
antennae (at9) are much smaller and are frequently biramous,
and the mandible (mri) has usually a palp, while the first
maxillse (mxl), bearing strong masticatory bristles on their
basal joints, also show more or less indication of a biramous
condition. The second maxillae (mx*), sometimes termed the
maxillipeds, show a peculiar arrangement in that each.

394

INVERT EBRA TE MORI >I10L OGT.

appendage separates into two portions inserted separately

into the body-wall. The an-
terior one is a comparatively
small plate provided with
numerous masticatory setae on
its inner edge, while the
posterior is an elongated
limblike structure. It is this
combination of a maxillary
and limblike portion that has
gained for this appendage the
term maxilliped, though it
must be recognized that it is
a true cephalic appendage and
not comparable to the inaxil-
lipeds of the higher forms.
The five thoracic appendages
(tl — £5) are typically biramous
and serve for swimming.

This description refers to
the free-swimming forms ; in
parasitic species much modi-
fication of the appendages
ensues. The antennules lose
their long oarlike character
and may even be degenerated
to strong hooks which serve
to fasten the animal to its
host, a degeneration which
the antennae may also under-
go. The mouth-parts become

FIG. ITI.—Calanus hyperboreus (after adapted to a piercing func-

GlESBRECHT).

an = anus.
at[ = auteunule.
at* = antenna.
mn = mandible.

mxl, mx1 — first and second maxillae.
P-p = thoracic appendages.

thus produced. The first

tiou, and the mandibles are
represented b}^ sharp stylet-
like structures, sometimes en-
closed in a tube formed by the
union of the upper and lower
lips, a sucking-organ being
maxillae undergo considerable

TYPE CRUSTACEA. 395

reduction, while the second pair is frequently adapted to form
organs for adhering to the host, and finally the thoracic
appendages may undergo various stages of degeneration, in
some forms entirely disappearing.

Branchial organs are entirely wanting throughout the
order, respiration taking place over the entire body surface.
A heart is present in a few forms (Calauidse) consisting of a
saclike organ with but a single pair of ostia, but in the
majority of cases it is wanting. A single median eye is gen-
erally present, and in a few forms, Pontella, Corycceus, and
Argulus, lateral eyes are also present, though absent as a
rule throughout the group. Each lateral eye in Corycceus
consists of a single ommatidium, but in Argulus is compound
and similar to the lateral eyes of the Branchiopoda.

The Copepoda are throughout bisexual even in the cases
of the parasitic forms. The vasa deferentia are provided with
an enlargement in which the spermatozoa are included within
a capsule, forming a spermatophore which during copulation
is deposited in the neighborhood of the female genital open-
ing. The spermatozoa being discharged from the spermato-
phore-capsule, by a special discharging apparatus with which
it is provided, make their way into a receptaculum semiuis
which communicates with each oviduct, the ova being fertilized
during their passage to the exterior. These are usually
carried in one or two masses attached to the first abdominal
segment of the female, though in some forms, such as Notodel-
phys, they undergo their development in a brood-chamber
formed by the duplication of the integument of the dorsal
surfaces of the fourth and fifth thoracic segments. A peculiar
dimorphism of the sexes occurs in some of the most highly
modified parasites, such as Cliondr acanthus, Achtheres, and
others, the male being very much smaller than the female
and showing much less degradation, frequently presenting
well-developed eyes and more or less perfectly-developed
appendages, so that it is able to lead for a time a free exist,
ence. It is to be regarded as a larval stage sexually mature,
since it resembles closely the female when in the stage immedi-
ately before fixation to its host, the greater part of the degen-
eration taking place after that has been accomplished.

Two suborders are recognizable.

396

INVERTEBRATE MORPHOLOGY.

1. Suborder Eucopepoda.

This suborder includes the majority of the Copepoda, and
its members are characterized by having only the first thoracic
segment fused with the head and by possessing usually a
well-developed abclomeu. Many are free-swimming, some in-
habiting fresh water, as Cyclops and Canthocamptus, while
others are more especially marine, such as Harpacticus, Calanus

B

FIG. 178. — A, Philichfhys xiphice SEEN FROM THE DORSAL SURFACE (after
CLAUS); B, Achtheres percarum (from BRONN).

(Fig. 177), and Cetochilus, the latter sometimes occurring in
enormous schools, and forming an important food-supply for
fish and the baleen whales. Some, on the other hand, lead a
commensalistic life, occurring in the branchial chamber of
Tuuicates, e.g. Notodelphys, while a large number of forms are
parasitic. The degree of parasitism varies greatly in different
forms ; thus many are capable of free existence, becoming
parasitic only occasionally, such asG'on/coews and the brilliantly-
colored Sapphirina, while others, such as Ergasilus, parasitic
on the gills of fishes, and Caligus and Pandarus, though essen-

TYPE CRUSTACEA.

397

tially parasitic, still retain more or less perfectly the segmen-
tation and general appearance of free-swimming forms, the
modifications which they have undergone affecting principally
the antennae, which are modified for purposes of adhesion to
the host, the mandibles, which are piercing organs, and in some
cases the maxillae, which may, like the antennae, become hook-
like. Frequently, however, the body assumes aberrant forms,
as in Pldlichthys (Fig. 178, A), and the segmentation may en-
tirely disappear, as in Penella, Lerncea, Chondracanthus, Acldlieres
(Fig. 178, B}, and Anchorella, these last two forms presenting
a peculiar modification of the second maxillae in the females,
the two appendages fusing at their tips to form a chitiuous
adhesive disk which serves as an organ of adhesion. In the
majority of these forms, as
already noted, the thoracic
appendages may become more
or less rudimentary; indeed
even in the less modified
forms, such as Ergasilus, the
appendages of the fifth thora-
cic segment may be Avantiug.

oc

2. Suborder BrancMura.

In the BrancMura the

cephalic and thoracic seg-
ments are fused together to
form a shield-shaped cephalo-
thorax, while the abdomen is
small and divided into two
platelike halves which have
a rich blood-supply, appar- FlG-
ently serving respiratory pur-
poses, and in the males
contain the testes.

The basal joint of the an-
teunules (Fig. 179, at'} is devel-
oped into a strong hooked process, and the mandibles and
first maxillae, which are stylet-like, are enclosed in, a tube

foliaceus (after

CLAUS).

af = antennules.

I = digestive gland
mx = second maxilla.

oc= eye;
t — tcstis

398

INVERTEBRATE MORPHOLOGY.

formed by the fusion of the upper and lower lips. The
second maxillae (mx) develop at their bases large suckers,
while the first thoracic appendages, here termed inaxilli-
peds, are limblike and have also hooked processes upon the
basal joints. These are succeeded by four pairs of birarnous
swimming appendages.

A well-developed heart is present, giving rise to arteries
extending throughout the length of the body. A pair of
lateral compound eyes (oc) are also present, and a further
difference from the majority of the Eucopepoda lies in the
fact that the eggs are not carried by the female, but are de-
posited on foreign bodies.

All the forms are parasitic, in some cases, as Argulus,
upon fresh-water fishes, but they also possess the power of
swimming actively.

4. Order Cirrhipedia.

The Cirrhipedia or Barnacles are without exception ma-
rine forms, and in the adult condition either adhere to foreign

P

FIG. 180. — CYPUIS LARVA OF Lepas (after GLAUS).
Ab = abdomen. Oc — eye.

At1 = autennules. Ov = ovary.

Cg = duct of cement-gland. p = penis.

o = opening of oviduct. T3 — third thoracic foot

bodies, leading a perfectly sessile life, or olse bore in the shells
of certain Mollusca, or finally are parasitic. It will be con-
venient to describe first of all the organization of the sessile
and boring forms, later considering briefly the parasitic forms
which show many peculiarities due to degeneration.

During the course of development the Cirrhipedia all pass
through a larval stage similar in general appearance to an

TYPE CRUSTACEA. 399

Ostracode and hence termed the Cypris-stage (Fig. 180).
The body is enclosed in extensive folds of the body-wall
termed the mantle, and the antennules (at1) are characterized
by being directed forwards and terminating in an adhesive
disk upon which open the ducts of cement-glands. Adher-
ing to a foreign body by these disks, the adhesion being
made permanent by the secretion of the glands, a rotation of
the body upon the antennae through 90° takes place, so that
the animal comes to lie upon its back, the ventral surface
looking away from the point of fixation. The antennules
persist as rudimentary structures, and the adult animal really
seems to be fixed by the dorsal surface of the head, which
may elongate to form a stalk bearing the body proper at its
extremity (Lepas, Fig. 181).

The body shows no indication of segmentation, but a head
region may be distinguished from the thorax and this from a
short abdomen by means of the appendages. The character
of the anteuuules has already been mentioned ; the antennae
are wanting in adults, and the mandibles and first maxillae
are simple toothed plates destitute of palps, while the second
maxillae are small and fused together to form a kind of lower
lip. The thoracic appendages (Fig. 181, -5) are biramous, the
basal portion supporting two long multiarticulate and usually
setose filaments. In typical cases six pairs of these append-
ages occur, but they may be reduced to four (Alcippe) or
three pairs (Cryptophialus). In the living animal flexions of
these appendages towards the ventral surface of the body
take place almost rhythmically, currents of water being thus
impelled towards the mouth together with any food-particles
they may contain. The abdomen does not bear appendages,
but from it arises a long slender cirrus (Fig. 181, B, cir) which
contains the terminal portions of the vasa deferentia.

The mantle-folds which occur in the Cypris-larva persist
in the adult, and calcification of their walls takes place, giving
rise to a calcareous shell, composed of several pieces, which
encloses the animal. In the genus Lepas, the goose-barnacle,
this shell consists of five pieces. On the dorsal side there is
a single unpaired piece which receives the name of the caritnt
(Fig. 181, A, car) ; at the sides and resting below on the upper-

400

IN VERTEBRATE MORPHOLOG 7.

most part of the stalk are the two scuta (sc), while above
these are the terga (te), also paired, the opening into the in-
terior l}'ing between the terga and the scuta of opposite sides.
In Scalpellum between the two scuta a sixth, unpaired, piece,
the rostrum, is inserted, and in the same genus between the
scuta, terga, and cariua and the summit of the stalk small
accessory pieces occur; and if one imagines a disappearance

T

Cir

Car

B

FIG. ISl.—Lepasfascmilaris. A, exterior; B, structure.
ag = auteunary gland. Ov = ovary.

Car = carina. pe = peduncle.

Cir = cirrus Sc = scutum.

M = shell-muscle. t = testis.

Od = oviduct. Te = tergum,

Vd — vas deferens.

of the stalk of such a form, an enlargement of these accessory
pieces, usually six in number, and their articulation to form
a wall-like circle around the body of the animal, the scuta
and terga closing it in and forming as it were a roof, an idea
of the arrangement of the shell of Bcdanus, the acorn-barnacle,
will be obtained.

No special respiratory organs exist, the entire surface of
the body probably performing this function, nor does a heart
seem to occur in any member of the group. The nervous

TYPE CRUSTACEA. 401

system consists in Lepas of a syiicerebrum and five or six
ventral ganglia, — of which the last is probably composed of at
least two fused ganglia, and a certain amount of fusion has
also probably occurred in the first. In Balanus the fusion
has reached its greatest extent, the entire ventral chain of
ganglia having fused to a single mass. The median unpaired
eye is usually represented, and in some forms rudimentary
lateral eyes are present, showing, however, a marked degen-
eration from the large compound eyes which occur in the
Cypris-like larva.

As a rule the Cirrhipedia are hermaphrodite in accord-
ance with their sessile or parasitic life. The testes (Fig.
181, J3, t) lie one on each side of the digestive tract, and the
vasa defereutia (vd) after dilating into seminal vesicles pass
to the long cirrus (ciY) borne by the abdomen, at the tip of
which they open by a short common duct. The ovaries lie
in Lepas (Fig. 181, B, ov) in the stalk, and in stalkless forms,
such as Balanus, in the basal fold which corresponds to the
stalk, and the oviducts (od) passing upwards and then back-
wards open on the basal joints of the anterior thoracic ap-
pendages. Although hermaphroditism is the rule throughout
the order, yet in some cases small males have been found
which have received the name of " complemental " males.
These occur in the genus lUa and in some species of Scalpel-
lum and live like parasites in folds of the mantle of the her-
maphrodite forms. In form they do not advance greatly
beyond the Cypris stage, and possess in addition to the auten-
uules only four pair of small thoracic limbs, the mandibles and
maxillse as well as the mouth being entirely wanting, while
the digestive tract is rudimentary. In other species of Scal-
peUum, and in the genera Alcippc. and Crypt opliialus, these
pigmy males are also present, but the forms in which they
live are no longer hermaphrodites but females, so that bi-
sexuality with sexual dimorphism occurs in these forms,

It might be supposed from the general occurrence of bisexuality among
the Crustacea that these last cases represented the first stage in the dis-
appearance of the males, leading finally to hermaphroditism. Since, how-
ever, Alcippe and Cryptophialus are the most degenerate of the Cirrhi-
peds so far discussed, it would seem that this is not the case, but rather

402 INVERTEBRATE MORPHOLOGY.

that, on the assumption of a sessile life hermaphroditism became character-
istic of the order, the bisexualism of these boring forms being secondarily
acquired. The fact that the pigmy males present larval characters sug-
gests the idea that their occurrence may be an extreme case of proterandry.
If in the hermaphrodite forms it is a rule that the spermatozoa mature
earlier than the ova, thus preventing self-fertilization, it is conceivable that
this early maturation of the testes might be carried back almost to the
Cypris stage and pigmy males be thus developed.

Not unfrequently barnacles choose the bodies of other
animals upon which to fasten, as for instance upon the cara-
pace of Limulus, or on the skin of whales, and the genus
Anelasma fastens itself upon the surface of the body of a
Shark, its stalk penetrating into the tissues and developing
rootlike processes and so enabling it to lead a parasitic life.
As a result of this the calcareous plates cease to develop,
the mantle having merely a leathery consistency and the
mandibles and maxillae remain rudimentary. This degenera-
tion is carried still further in Proteolepas (Fig. 182), which

lives as a parasite in the
mantle-cavity of other Cir-
-vs rhipecjs ailc| uas a maggotlike

appearance, the body being
distinctly divided into eleven
segments and lacking all
traces of a mantle. The
mouth-parts are modified so

as to be suctorial, and the
FIG. 182. —Proteolepas (from BRONN). , . . , . ,

thoracic teet are entirely
m = muscle. .. -, •-, -,• ,. "

wanting, while the digestive
ov = ov;iry.

vs = vesicula seminalis. tract becomes rudimentary.

Finally, a group of forms,

known as the IZhizocephala, fasten themselves to the abdomen
of crabs and become transformed into cylindrical or saclike
structures entirely destitute of digestive tract and appendages,
rootlike processes arising from the anterior end of the body
and traversing the body of the host, by whose juices the
parasite is nourished. The genus Sacculina consists of an an-
terior short cylindrical portion from the extremity of which
the rootlike processes arise and which perforates the integu-
ment of the host. From the base of this a circular fold arises

TYPE CRUSTACEA. 403

which encloses between its walls and the wall of the body a
cavity which serves as a brood-pouch and communicates with
the exterior by a terminal opening capable of being closed by
a sphincter. The body proper contains only the nervous
system, reduced to a single ganglion, and the ovaries and the
paired testes, as well as a pair of cement-glands .connected
with the female genital openings.

The development of Sacculina presents some extraordinary features.
It resembles in its early stages the development of the other Cirrhipeds
and reaches a typical Cypris stage during which it fastens itself by the
antennules to the body of a crab. The tissues of the larva then retract
themselves from the cuticle, arid a remarkable degeneration of the body
together with an amputation of the entire thoracic and abdominal regions
then ensues, leaving an oval mass of tissue, richly pigmented, attached to
the body of the crab by the empty cuticle of the antennules. At the
anterior end of this mass a hollow dartlike process arises which is
pushed forward through the hollow cuticle of the antennules and pierces
the body-wall of the host, the parasite apparently flowing then through the
dart and so becoming an endoparasite. Within the body of the crab the
development of the Sacculina takes place from the apparently undiffer-
entiated mass of tissue by which it is represented, and growing rapidly
produces an absorption of the ventral integument of the host, which allows
the saclike body to protrude to the exterior. It is to be noted that para-
sitic Cirrhipeds (Laura) have been found in the stem of a Gorgonian and
also in the body-cavity of Echinoderms (Deiidrogaster). These forms
show many peculiarities of structure and have been grouped together in
the suborder Ascothoracida.

II. CLASS Malacostraca.

The Malacostraca are distinguished from the Entomo-
straca by the definiteness throughout the entire class of the
number of inetameres entering into the composition of the
body. The head consists of five segments which are invari-
ably fused, and the thorax is composed of eight, of which the
anterior one, or indeed all, may unite with the head to form
a perfect or imperfect cephalothorax. The abdomen is the
only region in which variation of number takes place, and
this variation is confined to a single group of forms (Lepto-
straca). In these the abdomen is composed of eight segments,
while in all other forms it possesses only seven, counting in
both these cases the terminal segment which bears the anus

404 INVERTEBRATE MORPHOLOGY.

and is known as the telson. All these segments with the
exception of the telsou, and in the Leptostraca of the seg-
ment immediately in front of it, bear appendages. Folds of
the integument forming a cephalothoracic carapace are fre-
quently present, but it is rare that a bivalved shell occurs.

The stomach is always provided with chitinous teeth and
forms an efficient masticatory organ; and lateral eyes are
present except in some Cumacea and in some forms belong-
ing to other groups which inhabit caves or the depths of the
ocean, under which conditions the eyes become rudimentary.
The openings of the female reproductive organs are always
situated on the basal joints of the appendages of the sixth
thoracic segment, and the male openings on the appendages
of the eighth segment. The antennary gland is usually well
developed, while the shell-gland is either rudimentary or
wanting in the adult.

Although numerous rather small forms belong to this
class, yet on the whole they much surpass in size the Ento-
rnostraca, some forms even reaching a length of over 50 cm.
A few forms, such as Euphausia and Pence-its, leave the egg as a
Nauplius, but in the majority this stage is passed before
hatching, the embryo first leading a free existence at a later
stage in the larval form known as the Zdea, though in some
cases hatching may be retarded until later stages, in fact
sometimes until the adult form is acquired.

I. SUBCLASS LEPTOSTRACA.

The Leptostraca are exceedingly interesting forms, present-
ing similarities to the Eutoinostraca on the one hand and to
the Malacostraca on the other, thus connecting the two
classes. They are exclusively marine in habitat and possess
a thin bivalved shell- duplicature which is provided with an
adductor muscle and is prolonged in front into an unpaired
plate which covers the dorsal surface of the head.

The anteuuules (Fig. 183, at1) consist of a three-jointed
basal portion bearing in addition to the multiarticulate flagel-
lurn a scalelike exopodite, a structure wanting in the antennae
(of), which otherwise have a similar form. The mandibles

TYPE CRUSTACEA.

405

bear a palp, as do also the first maxillae, it being iii these latter
appendages prolonged into a long slender limblike (ma?)
structure which is directed dorsally and serves for cleansing
the inner surface of the shell. The second maxillae are
biramous foliate structures, as are also the eight thoracic
appendages (£), each of which bears upon its basal joints a
platelike epipodite which is respiratory in function. The
four anterior abdominal appendages («&") are strong biramous
swimming-legs, while the two posterior are small and uuira-
mous. Behind the last appendage- bearing segment are two

adr

FIG. 183. — Nebalia Geoff royi, MALE (after GLAUS).
ab* = abdominal appendage. = heart.

adr = autenuary glaud mx = process of first maxilla.

at} = anteunule. sm = shell-muscle.

at2 = antenna. t = thoracic appendage.

te = testis.

others without appendages, the terminal one being the telson,
the Leptostraca possessing one more rnetamere than the rest
of the Malacostraca.

The heart is an elongated organ extending from the
maxillary region as far back as the fourth abdominal seg-
ment ; it possesses several ostia, and is prolonged anteriorly
and posteriorly into aortre. The antennary gland is present
and a rudimentary shell-gland also persists. The lateral eyes
are borne upon short stalks.

The group contains but few species, the majority belong-
ing to the genus Nebalia (Fig. 183).

406 INVERTEBRATE MORPHOLOGY.

II. SUBCLASS THORACOSTRACA.

The Thoracostraca are characterized by the occurrence
throughout the group of a well-developed duplicature of the
body-wall, arising from the posterior head-segments and
covering in a greater or less number of the thoracic segments,
constituting what is termed a carapace. On the dorsal sur-
face it fuses with the body-wall, but, at the sides encloses a
respiratory chamber in which the branchiae, when present, lie.
According as the carapace extends over all or ouly over the
anterior thoracic segments a more or less perfect cephalo-
thorax is formed, a fusion of the covered thoracic segments
with each other and with the head-segments occurring, the
abdominal segments remaining in all cases distinct.

Branchiae, consisting of bunches of hollow thin-walled
processes whose cavities communicate with the lacunar spaces
of the body, are borne by certain of the appendages except in
the Mysidese. The lateral eyes except in the Cumacea are
stalked and the antennary gland is usually well developed.

1. Order Schizopoda.

The carapace in the Schizopoda covers in the entire
thorax, but a certain number of the posterior thoracic seg-
ments remain ununited with it. The antenuules are bira-
rnous, as are also the antennae (Fig. 184), the exopodite in the
latter case being represented by a scalelike structure. The
thoracic appendages are all similar and are biramous, the
endopodites being limblike structures tipped by claws,
while the exopodites are multiarticulate Hagella. In the
genus Eupliausia the two last pairs are quite rudimentary,
their branchiae remaining, however, well developed. The two
anterior pairs in the genus Mysis have their basal joints en-
larged to form jaws and consequently are distinguished as
maxillipeds, but in Euphausia this distinction does not occur.
The abdominal appendages in the females are generally small
with the exception of the sixth pair, and in the genus Mysis
are quite rudimentary. In the males of all genera they are,
however, well-developed biramous swimming-feet, and the
sixth pair in both sexes forms with the telson a tail-fin.

TYPE CRUSTACEA.

407

Branchiae are present in Mysis only in the form of small
epipodial elevations of the thoracic appendages, and in Siriella
as coiled tubular structures on the protopodites of the abdom-
inal appendages of the males. In Euphausia, however, they
form large ramified bunches attached to the protopodites of
the thoracic limbs and are present even on the rudiments of
the seventh and eighth pairs ; they are not, however, enclosed

ot

FIG. 184. — Mysis relicta (after SARS).
bp — brood-pouch. ot = otocyst.

within a chamber formed by the lateral portions of the cara-
pace, but project freely to the exterior.

Otocysts occur in the inner lamellae of the sixth abdominal
appendages (Fig. 184, ot), and in Euphausia a number of eye-
like phosphorescent organs occur on the basal joints of the
second and seventh thoracic appendages as well as upon the
ventral surface of the four anterior abdominal segments.
They are spherical in shape and each consists of a cup of
cells containing red pigment covered in by a lens.

The Schizopoda are essentially marine, though some
species of the genus Mysis (Fig. 184) occur in fresh and
brackish water.

2. Order Cumacea.

In this order the carapace covers only the anterior three
or four thoracic segments, five or four of them remaining dis-
tinct. The antennules are short and in the male biramous,
while the antennae, though in the female almost rudimentary,
may be as long as the entire body. The two anterior thoracic
appendages form maxillipeds, their basal joints serving for
mastication while the succeeding six pairs are limblike, all
but the last or three last possessing small exopodites. The

408

IN VERTEBRA TE MORPUOLOG Y.

sixth abdominal segment
bears a pair of biramous ap-
pendages with a long single-
jointed protopodite, the re-
maining segments being in
the female destitute of ap-
pendages, but in the male the
anterior 2 (Diastyliu), 3, or 5
(Campylaspis) segments may
bear biramous swimming-feet.

The lateral eyes are never
stalked and may be closely
approximated or even fused
on the dorsal surface of the
cephalothorax. They are
generally composed of but
few ornmatidia and in some
species are entirely wanting.

The Cuinacea are exclu-
sively marine and are more
especially characteristic of
the colder seas.

3. Order Stomatopoda.

As in the Cumacea the
carapace covers only some of
the anterior thoracic seg-
ments, the last three or four
remaining distinct, but the
abdomen, instead of being
slender, is even stouter than
the thorax and ends in a ter-
minal tail-fin. The anterior

portion of the head, bearing
FIG. l85.—Diastylis stygia, MALE (after ]

SARS from LANG). the eyes and the two pairs

«I=antennule. en=endopodite. of anteunse, is separated from
«2 = antenna. ea^exopodite. , , , ,, ,.

JUabdominal ap p= abdominal sip- and movable upon the rest of
pcudages. pendages. the cephalo-thorax, and only

IV- VIII = thoracic

segments the more anterior thoracic

ctft = carapace.

TYPE CRUSTACEA. 409

segments are fused with the carapace, though it covers iu
several others.

The antermules consist of an elongated three-jointed basal
portion bearing three rnauy-jointed flagella, while the anten-
nae are generally shorter, the exopodite being represented by
a large scale. The maxillae are comparatively small, and the
appendages of the five anterior thoracic appendages are
crowded forwards and are termed maxillipeds, being limb-
like structures destitute of exopodites, but possessing well-
developed epipodites, and with the terminal joint capable of
flexion upon the next succeeding one. The second maxilli-
ped is especially long and large, and with its strong terminal
and penultimate joints forms a very efficient weapon for secur-
ing prey. The three posterior appendages of the thorax are

OC

FIG. 1^6. — Squilla mantis (from LEUNIS).
a1 = autenuules. pm = maxillipeds.

a2 — antennae. p = thoracic limbs.

oc = compound eyes. pa, = abdominal limbs.

slender birarnous structures, the somewhat stronger abdomi-
nal appendages being also biramous and somewhat lamellar
swimming-feet. The last pair are especially enlarged and di-
rected backwards, forming with the telson the strong tail-fin.

Bunches of branchial filaments occur upon the outer lamel-
lae of the abdominal appendages with the exception of the last
pair. The heart is much elongated, extending from the ante-
rior thoracic region as far back as the fifth abdominal seg-
ment and possessing numerous pairs of ostia. It is prolonged
anteriorly and posteriorly into aortae and gives off laterally
in each segment a pair of arteries.

The Stomatopods are all marine and pass through a com-
plicated series of metamorphoses during development. Some
of the principal genera are Squilla (Fig. 186), Lysiosquilla, ami
Gonodadylus.

410 INVERTEBRATE MORPHOLOGY.

4. Order Decapoda.

In the Decapods the carapace is well developed, covering
iu the thorax completely (Fig. 162), the segments of that region
of the body fusing with it dorsally, so that a perfect cephalo-
thorax is present. The autennules generally possess two
terminal inultiarticulate flagella, and the autenme frequently
lack the scalelike exopodite which occurs in other groups
(e.g., Schizopoda). In the second maxillae the exopodite is
transformed into a platelike structure which, swinging to and
fro, serves to renew the water in the branchial chamber lying
between the lateral portions of the carapace and the body-
walls. On account of this action this appendage is usually
spoken of as the scaphognathite. The three anterior thoracic
appendages are maxillipeds, the third one frequently becom-
ing almost limblike, a characteristic which distinguishes the
five posterior pairs of appendages which are adapted for walk-
ing and are hence termed the pereiopods. They lack all
traces of exopodites, though usually bearing epipodites and
branchiae, and a certain number of the anterior ones are fre-
quently chelate, thus serving for the prehension of food. The
number of the pereiopods has suggested the name given to
the order. The abdominal appendages are sometimes want-
ing or very rudimentary, but when present are biramous swim-
ming-feet and are hence termed pleopods — a term equally
applicable in some other groups.

The branchiae lie entirely within the branchial chamber
and are developed in connection with the thoracic append-
ages. They may be seated upon the basal joints of the ap-
pendages (podobranchia), or upon the joint between the ap-
pendage and the body-wall (arthrobranchia), or finally upon
the body-wall itself (pleurobranchia). All three kinds may
occur on the same segment, so that the entire number of gills
may be much greater than that of the appendages, amount-
ing in the Lobster to no less than twenty in each branchial
chamber.

The heart is a short saclike organ lying in the thorax and
possessing as a rule three pairs of ostia, one pair being situ-
ated on the dorsal surface, one upon the sides, and the third on

TYPE CRUSTACEA.

411

the ventral surface. Arteries pass off from both ends of the
heart. Otocysts are always, developed in the basal joints of
the autennules.

1. Suborder Macrura.

In the Macrura the abdomen is well developed and usu-
ally as long as the cephalothorax, and is provided with its
full complement of appendages, the sixth pair forming with
the telson a tail-tin. Exceptions to these arrangements oc-
cur ; in the Hermit-crabs, Eupagurus, which inhabit the empty
shells of Gasteropod Mol-
lusks, the abdomen is gener-
ally soft and unsymmetrical,
since it is coiled around the
columella of the shell, but
terminates in a movable tail-
fin which serves, together with
the remaining pleopods and
the last (and sometimes also
the penultimate) pereiopod,
which is bent dorsally, to re-
tain the animal in the shell.
The chelae of the anterior
pereiopods are generally un-
equal in size, serving to oc-
clude the mouth of the shell,
and occasionally the abdomi-

J ^rr ^/ ~^^pS D

nal appendages of only one
side are developed. In the
genus Hippa too the abdomen,
though with a well-developed
and calcified cuticle, is short,
the terminal half being bent
up under the thorax, the
condition characteristic of
the Crabs being thus ap-
proached. In some forms, such as Sergestes and Lucifer,
tLe fourth and fifth pereiopods may be rudimentary or even ab-
sent, but more usually all these appendages are well devel-

FIG. 187. — A, A YOUNG Lucifer (adapted
from BROOKS); B, Eupagurus bern
hardns (after LECNIS).

412 INVERTEBRATE MORPHOLOGY.

oped, the anterior ones becoming chelate. In the Crayfish,
Cambariis, and the Lobster, Homarus, the first pereiopod is an
exceedingly strong chela, and the same arrangement is found
in Alpheus, while in the Shrimp, Pcdcemonetes, the second pe-
reiopod is somewhat longer than the first.

The branchiae are usually numerous and are for the most
part bunches of cylindrical processes, but in Pcdcemonetes and
the shrimps and prawns in general, which form the family Ca-
rididse, and in the Hermit-crabs they are lamellate. In Lucifer
branchiae are entirely wanting. The'Macrura are essentially
marine, a few forms, such as Cambarus and some species of
Pcdcemon, occurring in fresh water. The genus Birgus, one of
the Hermit-crabs, commonly known as the robber-crab, is
almost entirely terrestrial, living in holes in the ground and
climbing cocoa-nut palms for the sake of the nuts, on which
it lives. In harmony with its terrestrial life the inner surface
of the branchial chamber is thrown into folds richly supplied
with blood-lacunae, a luuglike structure, recalling the lungs of
the Pulmonate Gasteropods, being thus developed.

2. Suborder Bracliyura.

In the Brachyura the body is exceedingly compact, the
abdomen being very much reduced in size and usually desti-
tute of a tail-fin, and in addition
is bent up so as to lie in a groove
upon the ventral surface of the
cephalothorax. In some cases
the cephalothorax is almost glo-
bular, though prolonged anterior-
ly into a strong rostral spine, as
in Libinia, the spider-crab ; while
in other cases it is more flattened
and triangular in shape and lacks

a distinct rostrum, as in the
FIG. 188. — PanopcKus depressus -,., -, , /-, 77-

(after EMKBTON from VKBRA^ edlble crab> CaUmectes, the lady-

crab, Platyonychus, and the com-
mon crab, Cancer, and in others again is more or less
quadrangular and thicker, as in Pinnotheres, the oj^ster-crab,
Ocypoda, the sand-crab, and Gelasimus, the tiddler-crab. The

TYPE CRUSTACEA. 413

antermules are small and they and the eyes can be partially
concealed in a groove on the anterior edge of the carapace.
The abdominal appendages, with the exception of the anterior
one or two pairs which are adapted for copulation, are ab-
sent in the males, while the females generally possess four
pairs, to which the ova are attached.

The gills are generally few in number, except in Porcel-
lana and some allied forms, and are usually lamellate in form.
While essentially marine in habit, the Brachyura are fre-
quently more or less terrestrial, the sand-crabs, Ocyjjoda, and
the fiddler-crabs, Gelasimus, living in holes in the sand just
above high-tide mark, while the land-crabs, Gecarcinus, of the
tropics may live some distance from the sea, migrating to it
in armies during the breeding-season. A few forms, such as
the genus Telphusa, are aquatic.

III. SUBCLASS ARTHROSTRACA.

The Arthrostraca, with the exception of the small group
of the Anisopoda, are destitute of a carapace, and the tho-
racic appendages, with the exception of the first pair, are
jointed walking-limbs lacking an exopodite. The anterior,
or in some cases the anterior two thoracic segments fuse
with the head, the appendages of these segments differing
from those of the free segments, being modified to assist in
the process of mastication, whence they are termed maxilli-
peds. The abdomen is composed of six segments provided
with appendages, and of a terminal telsou ; occasionally the va-
rious segments fuse together, and in Some forms the abdomen
is reduced to a small unsegmeuted structure. Platelike ap-
pendages attached to the basal joints of some of the thoracic
limbs form by their meeting and overlapping a brood-pouch
in which the ova undergo their development.

The lateral compound eyes are not, except in Tanais, sup-
ported on stalks, a characteristic which has suggested the
term Edriophtlialmata sometimes applied to the group.

1. Order Anisopoda.

The Anisopoda are exclusively marine forms in which the
two anterior thoracic segments are fused with the head and

414 INVERTEBRATE MORPHOLOGY.

covered in at the sides by duplicatures of the body-wall,
which enclose a small respiratory cavity.

The antennules and antennae are uuiramous except in
Apseudes in which the autenuules carry two terminal flagella.
The palps of the anterior maxillae project into the respiratory
chamber and serve for cleansing it, and the first thoracic
limbs, the maxillipeds, bear each an epipodial branchial ap-
pendage lying in the respiratory chamber. This limb and
the succeeding one are chelate, the inner angle of the penul-
timate joint being prolonged into a process against which the
terminal joint may be apposed. The abdominal appendages
are biramous swimming-feet in Tanais and Apseudes, the last
pair being in Anthura especially enlarged to form with the
telson a terminal finlike structure.

2. Order Isopoda.

The majority of the Isopoda are marine, the genus Asellus
(Fig. 189), however, occurring in fresh water, while Onisciis,
Porcellio, and Armadillidium are terrestrial, being commonly
known as Wood-lice or Sow-bugs. The body in all forms is
more or less flattened dorso-veutrally and only the anterior
thoracic segment is fused with the head, the remaining seven
remaining perfectly distinct. There is no trace in the adult
of a carapace, and the abdominal segments are usually small
and may be fused more or less completely.

The maxillae are destitute of palps and the maxillipeds
(mxp) usually fuse together to form a sort of lower lip. The
remaining thoracic appendages are limblike and do not bear
any respiratory appendages, though lamellae are attached to
the basal joints of several of them in female individuals,
serving to form a brood-pouch. The five posterior abdominal
appendages are biramous and lamellar (ab), serving both for
swimming and for respiration, the anterior pair (op) usually
becoming hard and forming an operculum which covers in
the posterior more delicate appendages and in the terrestrial
forms may have branching spaces containing air (tracheae)
ramifying through them.

The heart (Jit), in conformity to the position of the respira-
tory organs, is situated principally in the abdomen, extending

TYPE CRUSTACEA.

415

forwards only a short distance into the thorax segment. It
possesses one or two pairs of ostia and is closed behind,
giving off in front and at the sides numerous aortse. A shell-
gland has been observed in some Isopoda, but the auteunary
gland is wanting.

Although the majority of the marine forms, such as Idotea
and Spha&roma, lead a free existence, nevertheless there are
certain parasitic forms. Thus the genera Cymothoa and JEga
are parasitic on the skin or in the mouth of fishes, but also
retain the power of swimming and consequently are not much

FIG. 189. — Asellus communis, DIAGRAM op STRUCTURE.

ab = abdominal appendages. I = liver-coeca.

ao = aorta. mnp = inaudibular palp.

at1 = antennule. mxp = maxilliped.

at1 = antenna. r = rectum.

ce = cerebral ganglion. s = stomach.

ch = chelate limb. t = thoracic appendage.

fit = heart. vn = ventral nerve-cord.

modified. The genus Bopyrus, which lives in the branchial
cavity of shrimps, becomes in the female somewhat distorted
in shape and asymmetrical, and the mouth-parts become
transformed into a suctorial proboscis and the eyes disap-
pear. The male, however, which is much smaller than the
female, retains the eyes and does not depart from the usual
symmetrical body form. The degeneration of the female
proceeds much farther in the genus Entoniscus, which lives
either partly or wholly included within the body-cavity of
other Crustacea and assumes a saclike unsymmetrical form,
recalling to a certain extent that of some of the parasitic
Copepoda. At the time of pairing both sexes are alike fully
segmented and with an almost full complement of appendages.

416

INVERTEBRATE MORPHOLOGY.

After copulation, however, the female assumes the degener-
ated form, while the male dies.

3. Order Amphipoda.

Like the Isopoda these are essentially marine forms,
though the genus Gammarus is aquatic and Orchestia
(Fig. 190) partly terrestrial, living among the wrack on sea-
beaches just beyond the reach of the waves. The body in

ht

ro

ru

ce

mt

FIG. 190. — DIAGRAM OF STRUCTURE OF Orchestia cammana (after NEBESKI).

at1 = antenuule. m — mouth.

at2 = antenna. mt — Malpighian tubule.

br = bran chia. oc = eye.

ce = cerebral ganglion. r = rectum.

ch = chelate limb. rd = reproductive duct.

ht = heart. ro =- reproductive organ.

I = liver-caeca. vn = ventral nerve-cord.

the Amphipoda is laterally flattened and presents therefore a
very different appearance from that of the Isopoda, though,
as in that group, lacking all traces of a carapace. The first
thoracic segment is fused with the head, and in Caprella and
. Cyamus the second segment likewise. The appendages of the
head and the maxillipeds resemble those of the Isopoda, and
the remaining thoracic appendages are liinblike, a certain
number of the anterior ones frequently possessing a terminal
joint capable of flexion upon the succeeding one, or even
being chelate The five posterior limbs or the third and
fourth only bear epipodial lobes which serve as branchi;i>, and

TYPE CRUSTACEA. 417

a number of the limbs also in females bear lamellae Avhich
may enclose a brood-pouch. The three anterior abdominal
limbs are biranious and serve for swimming, while the three
posterior ones, also biramous, are frequently directed back-
wards and serve as springing organs, the springing powers of
Orchestia having gained for it the name of the Beach-flea. In
Caprella, which crawls about over colonies of Hydroids and
Polyzoa, and Cyamus, which is parasitic upon the skin of
whales, the abdomen becomes almost rudimentary and is des-
titute of appendages.

The heart (Jit) lies in the thoracic region in the anterior
five or six segments and possesses from one (Corophium) to
three ostia. It is prolonged into an aorta at either end. In
connection with the mid-gut portion of the digestive tract, in
addition to the four so-called Irver-cseca (I) is a pair of gland-
ular cseca which seem to be excretory in function and have
been termed Malpighiau tubules (nit). An antennary gland
occurs, but the shell-gland is apparently unrepresented in
adults.

Development of the Crustacea. — The majority of the Crus-
tacea pass through a more or less complicated series of
metamorphoses, the larval forms being highly suggestive
when studied from the phylogeuetic standpoint. A few forms,
especially those inhabiting fresh water, abbreviate their de-
velopment considerably, so that the young animal when it
leaves the egg practically may differ from the parent only in
size (Cambarns), and among the higher forms the development
is generally abbreviated to the extent that a greater or less
number of the larval stages, characteristic of lower forms, are
passed through while the young animal is still within the egg-
membrane, only the final stages being free-swimming.

Throughout the Entomostraca the first larval form which
hatches from the egg is termed the Nauplius (Fig. 191) and
differs markedly from the adult, chiefly, however, in the small
number of appendages it possesses. The body in typical
forms shows no trace of segmentation and possesses a single
1 median eye generally x-shaped. But three pairs of limbs
are present, which become transformed later into the anten-
nules, antennae, and the mandibles of the adult. The Naupliar

418 INVERTEBRATE MORPHOLOGY.

autennules are unirarnous and, like the other limbs, but indis-
tinctly jointed, the anteunules and mandibles being, however,
biramous and possessing strong sefce at their bases which
function as jaws, though both pairs of appendages are essen-
tially locoinotor. Judging from the appendages, therefore, the
Nauplius may be regarded as consisting of five segments, one
corresponding to the prostomial lobe of Annelids and contain-
ing the primitive cerebral ganglion (archicerebrum), one cor-

, v,

J^-^-

FIG. 191. — NATJPLIUS OF CeiocJnlus septentrionalis (after GBOBBEN).

responding to each pair of appendages and one to the region
of the body behind the mandibles.

A Nauplius of this simple form may be regarded as typical
and is that which is found in the majority of the Copepoda and
in the Cirrhipedia as well as in some Brauchiopoda (Esther i«,
Limnadia). In the Ostracoda the arrangement of the limbs
and segments is the same, but the bivalved shell characteristic
of the adult is already developed, giving the Nauplius an ap-
pearance very different from that of the Copepoda. Not un-
frequently, however, as for instance in Apus among the
Branchiopoda, and Leptodora among the Cladocera (the re-
maining Cladocera, so far as is known, leave the egg with the
adult form), the Nauplius, though possessing only the three
pairs of appendages, yet shows indications in the post-mandib-
ular region of a varying number of additional segments, and
to this form it is convenient to apply the name Metanauplius,

As a rule in the Eutomostraca further development con-
sists of a series of moults (ecdyses), an increase in the number
of segments and appendages and modifications of the hitter
taking place at each ecdysis, until the adult form is attained.
No special larval forms beyond the Nauplius are common to

TYPE CRUSTACEA.

419

all the members of the class, and it is only in the Cirrhipedia
that a second definite larval form can be distinguished, the
Cjpris-larva, to which attention has already been called (p. 399).
In the Malacostraca the occurrence of a free-swimming
Nauplius is the exception rather than the rule, and indeed
larval forms are practically wanting in some groups, such as
the Leptostraca and Arthrostraca, and in certain species or
families of other groups (e.g. Mysidce, Gambarus). In the
genus Penceus among the Decapods, and in Euphausia among
the Schizopods, a typical free-swimming Nauplius occurs, and
in Lucifer the embryo leaves the egg in the form of the Meta-
nauplius. In the majority of forms these stages are passed
over while the embryo is still within the egg-shell, and it
hatches only when it has acquired a greater degree of develop-
ment. In such forms as Penceus, Eupliausia, and Lucifer the
Metanauplius stages pass into what is termed the Protozoea
(Fig. 192, A) a stage also passed over within the egg by the

FIG. 192.— A, PROTOZOKA OF Lucifer (after BROOKS); B, ZOEA OF Palcemonetes

(after FAXON).

At1 = autenuule. m = mandible.

Af2 = antenna. mxl, mx* = maxillae.

c — cerebral ganglion. mp1, mp* = maxillipeds.

E = compound eye. oc = simple eye.

h = heart. r = rostrum.

s = stomach.

majority of Malacostracaus, though occurring as the first
larval stage of some Stomatopods. It is characterized by the
development of two maxillse and the two or three anterior

420 INVERTEBRATE MORPHOLOGY.

thoracic appendages in addition to those already present in
the Nauplius, and furthermore by the distinct separation of
the body into an anterior cephalo-thoracic portion covered by
a carapace and a posterior abdomen which is usually but
imperfectly segmented. This stage is succeeded sometimes
after two or more ecdyses by the Zo'ca (Fig. 192, £), a stage in
which the majority of Decapoda leave the egg. It is distin-
guished from the Protozoea principally by the perfect segmen-
tation of the abdominal region, though it still possesses no
appendages, unless it be rudiments of the sixth pair, and it
is furthermore characterized by the compound eyes being
stalked, a feature but slightly indicated in the Protozoea, in
which stage they make their appearance. The Zoea stage in
the Brachyura is generally characterized by the development
of spines, sometimes of enormous length (Porcellana), upon the
dorsuin and sides of the carapace.

In such a form as Euphausia the next stage is the adult,
but in the Decapods other larval stages intervene before the
adult condition is reached. The first of these is characterized
in the majority of the Macrura by the appearance of the re-
maining thoracic appendages which were unrepresented in
the Zoea, in the form of biramous structures closely resem-
bling the thoracic appendages of the Schizopoda, whence the
stage is generally termed the Mysis stage (Fig. 193). The

FIG. 198. - MYSIS STAGE OP LOBSTER, Homarus americanus (after S. I. SMITH).

abdominal appendages also develop during this stage.
Among the Hermit-crabs (Paguridae) and the Brachyura the
development is to a certain extent abbreviated, the pereiopods
never being represented by biramous appendages, but being
from the first uniramous, and in these forms therefore a
true Mysis stage never occurs. To the corresponding stage,

TYPE CRUSTACEA.

421

or rather to one in which the pereiopods are indicated but
not fully developed, the term Metazoea is applied. Further-
more in certain Macrura, such as Scyllarus and Palinurus, the
Mysis stage is represented by peculiarly-shaped transparent
larvae which have been termed Phyllosoma, or glass-crabs.
The carapace is divided into two portions, of which the an-
terior or larger covers in the head region and the posterior
the thorax, the body being throughout flat and the ab-
domen very small. The pereiopods, of which in the earliest
stages there are but three, are birarnous, and the first maxil-

FIG. 194. — MEGALOPA-STAGE OF Cancer irroratus (after EMERTON from VERRILL).

lipeds are either entirely wanting or very rudimentary. Dur-
ing successive ecdyses the missing appendages are gradually
developed, though the actual transformation of the Pliyllosoma
into the youngest Scyllarus or Palinurus stage (which is de-
cidedly smaller than the oldest Pliyllosoma) has not yet been
observed.

The change from the Mysis stage to the adult is usually
gradual, and no specially definite larval forms are to be found
as a rule among the Macrura. In the Brachyura, however,

422 INVERTEBRATE MORPHOLOGY,

the Metazoea becomes transformed into a well-marked form,
the Megalopa (Fig. 194), so called from the usually large size
of the cephalothorax. It resembles closely a Macruran,
differing only in the abdomen being relatively small, and
becomes converted into the adult form by the doubling of
the abdomen beneath the thorax. A Megalopa stage occurs
also in the Hermit-crabs, but is not so well marked off
from the young fully -formed animals as in the Brachyura.

Affinities of the Crustacea. — The relationships of the higher groups of
the Malacostraca to one another are clearly shown by their larval forms,
the Megalopa showing the origin of the Brachyura from Macruran forms,
and the Mysis stage that of the latter from Schizopod ancestors. When
attempts are made to go still further difficulties stand in the way. As
regards the Stomatopoda it is to be noted that they^pass through a stage,
the Erichthus, in which the thoracic appendages which are present are
biramous, and it seems probable that both they and the Cumacea are re-
ferable back to Schizopod ancestors. The Arthrostraca, on the other hand,
are probably traceable to Curnacealike ancestors, while the Leptostraca
represent more nearly the Entomostracan ancestors than any other group,
though widely differentiated from them in certain particulars. It is even
still more difficult to trace out relationships of the various Entomostracan
orders, but it seems fairly clear that Phyllopodau forms such as Apus are
to be considered as representing more nearly than any others the primitive
Crustacea.

As regards the affinities with other groups very interesting questions
arise, two possibilities seeming to be open. According to one the Crustacea
have been derived directly from segmented Annelids, through forms repre-
sented in a modified condition to-day by Apus. The lobed appendage of
Apus is a modified parapodium, and the segmentation of the body has been
inherited. What then as to the Nauplius '! According to this view it has
practically no ancestral significance, or at best can be considered only as
representing a Trochophore larva highly modified and with many adult
characters thrown back upon it. This latter idea does not seem, however,
to agree with the facts, since the Trochophore is an nnsegmented structure
and can be comparable only to the prostomial and first appendage-bearing
segments of the Nauplius. In other words, the Nauplius is comparable,
if comparable at all, to a Trochophore pints certain additional segments.
It has recently been suggested that possibly the Nauplius may represent
not the Trochophore but the larval Annelid with three parapodia, which, as
indicated (p. 215), is a well-marked stage in the development of many
Polychseta. The number of segments is apparently similar in the two
forms, and the idea is plausible. If, however, in all Crustacea a ganglion,
representing a segment, intervenes between the archicerebral ganglia and
the antennulary (see p. 378), then the Muinlius ha* potentially one seg-

TYPE CRUSTACEA. 423

ment more than the Annelid larva and the comparison will not hold. If
the direct Annelid origin is to be accepted, it seems most satisfactory at
present to regard the Nauplius as a secondarily acquired larval stage
without any ancestral significance.

Another suggestion has, however, been made which gives the Nauplius
a significance and traces the Crustacea back to unsegmented ancestors. It
is to the effect that the Nauplius can be referred to Rotiferlike ancestors,
the remarkable Hexarthra with its six processes being supposed to indi-
cate the line of descent. It is exceedingly doubtful, however, whether
this similarity can be regarded as anything more than a superficial one

TYPE CRUSTACEA.

I. Class ENTOMOSTRACA. — Number of segments varies ; abdomen without
appendages ; larva a Nauplius.

1. Order Phyllopoda. — Number of segments variable ; appendages

with branchiae.

1. Suborder Branchiopoda. — Body plainly segmented and seg-
ments of thorax more numerous than six. Apus, Branchipus,
Estheria, Limnadia, Limnet-is.

2. Suborder Cladocem. — Body indistinctly segmented ; with bi-
valved shell ; four to six thoracic appendages. Daphnia,
Moina, Sida, Evadne, Polyphemus.

2. Order Ostracoda. — With bivalved shell ; body indistinctly seg-

mented ; two thoracic appendages. Cypris, Cythere, Cypri-
dina, Halocypris.

3. Order Copepoda. — Without shell ; five pairs of thoracic limbs ;

many forms parasitic and degenerate.

1. Suborder Encopepoda. — First thoracic segment only fused with
head ; abdomen cylindrical and segmented except in highly
degenerated forms. Cyclops, Canthocamptus, Harpacticus,
Calanus, Cetochilus (free-swimming) ; Notodelphys (commen-
salistic) ; Corycceus, Sapp/iirina, Eryasilus, Caligus, Panda-
rus (partly parasitic) ; Philichthys, Penella, Lerncea, Chon-
dracantnus, Ac7itheres, Anchorella (parasitic).

2. Suborder Branchiura. — All thoracic segments fused with head ;
abdomen small and lamellar, partly parasitic. Argulus.

4. Order Cirrhipedia. — Sessile or parasitic ; segmentation indis-

tinct ; six pairs of thoracic appendages ; pass through Cypris
stage. Lepas, Scalpellum, Ibla, Balanus (sessile) ; Alcippe,
Cryptophialus (boring) ; Proteolepas, Sacculina, Laura,
Dendrogast&r (parasitic) .

II. Class MALACOSTRACA. — Number of segments constant ; thoracic seg-
ments eight, abdominal seven or eight.

1. Subclass Leptostraca. — With bivalved shell; abdomen with eight
segments. Nebalia.

424 INVERTEBRATE MORPHOLOGY.

2. Subclass Thoracostraca. — With carapace covering the whole or a

part of the thorax ; abdominal segments seven.

1. Order Schizopoda. — Thorax completely covered ; thoracic append-

ages biranious. Mysis, Euphausia, Siriella.

2. Order Cumacea. — Last four or five thoracic segments not covered

by the carapace ; eyes sessile or rudimentary. Diastylis,
Campylaspis.

3. Order Stomatopoda. — Last three or four thoracic segments not

covered by the carapace ; eyes stalked ; five maxillipeds.
Squilla, Lysiosquilla, Gfonodaetylus.

4. Order Decapoda. - - Thorax completely covered ; five posterior

appendages uniramous and three maxillipeds ; otocysts in
antennules.

1. Suborder Macrura. — Abdomen usually well developed. Ser-
gestes, Lucifer, Penceus, Palcemonetes, Alpheus, Cambarus,
Homarus, Eupagurus, Birgus, Hippa.

2. Suborder Brachyara. — Abdomen small and concealed beneath
cephalothorax more or less perfectly. Porcellana, Libinia,
Callinectes, Platyonychus, Cancer, Pinnotheres, Ocypoda,
Gelasimus, Gecarcinus.

3. Subclass Art7irostraca. — No shell or carapace as a rule ; with seven

(or six) walking-limbs ; eyes sessile.

1. Order Anisopoda, — Carapace slightly developed : fii'st two tho-

racic segments fused with head ; branchiae on anterior maxilla?.
A seudes, Tanais, Anthura.

2. Order Isopoda.—No carapace ; first thoracic segment fused with

head ; body flattened dorso-ventrally ; branchiae on abdomi-
nal appendages. Asellus, Oniscus, Porcellio, Armadillid-
ium. Idotea, Sphceroma (free) ; Cymothoa, JEga, Bopyriis,
Entoniscus (parasitic).

3. Order Amphipoda. — No carapace, first thoracic segment fused

with head ; body flattened laterally ; branchi® on thoracic
appendages. Gammarus, Orcliestia, Corophiiim, Cyamus,
Caprella.

LITERATURE.

GENERAL.

H. Milne-Edwards. Histoire Naturelle des Crustaces. Paris, 1834-1840.

F. Miiller. Fiir Darwin. Leipzig, 1864.

H. Gerstaecker. ArlJiropoda. Bronn's Klassen und Ordnuugen des Thier-

reichs, Bd. v. Abth. 1. 1866- (not yet completed).
C. Glaus. Untersuchungen zur Erforschung der genealogischen Grundlage des

Crustaceen-Systems. Wien, 1876.
C. Grobben. Die Anlennendrilse der Orustaceen. Arbeiten a. d. Zoolog. lust.

Wien, in. 1880.

TYPE CRUSTACEA. 425

J. E. V. Boas. Studien iiber die Vencandtschaftsbeziehungen der Malakostraken.

Morpholog. Jahrbuch, vm, 1883.
J. Carriere. Die Sehorgane der Thieve, vergleichend-anatomisch dargeslellt.

Munich and Leipzig, 1885.
C. Glaus. Neue Beitrdge zur MorpJiologie der Crustaceen. Arbeiten a. d.

Zoolog. Inst. Wicu, vi, 1886.
S. Watase. On the Morphology of the Compound Eyes of Arthropoda. Studies

from the Biolog. Laboratory, Johns Hopkins Univ., iv, 1890.
G. H. Parker. The Compound Eyes in Crustaceans. Bulletin Museum of

Coiiip. Zoology, xxi, 1891.
C. Grobben. Zur Kenntniss des Stamba nines und des Systems der Crustaceen.

Sitzungsber. Akad. wissensch. Wit-n, ci, 1892.

PHYLLOPODA.

A. S. Packard. A Monograph of North American Phyllopod Crustacea.

Twelfth Annual Report U. S. Geolog. Survey. Washington, 1883.
C. L. Herrick. A Final Report on the Crustacea of Minnesota. Twelfth An-
nual Report of the Geolog. and Natural History Survey of Minnesota.

Minneapolis, 1884.
C. Claus. Zur Kenntniss des Baues und der Entwicklung von Branchipus stag-

nalis und Apus cancriformis. Abhandl. k. Akad. wissensch. Gottiugen,

xviii, 1873.
A. Weismann. Ueber Bau und Lebenserscheinungen von Leptodora hyalina.

Zeitschr. fiir wissensch. Zoologie, xxiv, 1874.
- Beitrdge zur Naturgeschichte der Daphnoiden. Zeitschr. fiir wissensch.

Zoologie, xxvii-xxxiii, 1876-1879.
E. Ray Lankester. Observations and Reflections on the Appendages and on the

Nervous System of Apus cancriformis. Quarterly Journ. Microscop.

Science, xxi, 1881.
C. Claus. Untersuchungen iiber die Organisation und Entwicklung von

Branchipus und Artemia. Arbeiten a. d. Zoolog. Inst. Wien, vi, 1886.
P. Pelseneer. Observations on the Nerwus System of Apus. Quarterly Journ.

of Microscop. Science, xxv, 1885.
P. Samassa. Untersuchungen fiber das centrale Nervensystem der Crustaceen.

Archiv fiir mikr. Anat., xxxvur, 1891.

G. M. Bernard. The Apodidce, a Morphological Study. London, 1892.
C. Grobben. Die Entwicklungsgeschichte der Moina rectirostris, etc. Arbeiten

a. d. Zool. Inst. Wien, n, 1879.

P. Samassa. Die Keimblatterbildung bei den Cladoceren. I. Moina rectirostris
Brady. Archiv. ftir rnikrosk. Anat., XLI, 1893.

OSTRACODA.

G. E. Brady. A Monograph of the Receno British Ostracoda. Transactions

Linnaean Soc. London, xxvi.
C. Claus. Ueber die Organisation der Cypridinen. Zeitschr. fiir wissensch.

Zoologie, XT, 1865.
Beitrage zur Kenntniss der smmasser Oslracoden. Arbeiten a. d.

zoolog. Inst. Wien, xn, 1892.

426 INVERTEBRATE MORPHOLOGY.

COPEPODA.

C. L. Herrick. A Final Report on the Crustacea of Minnesota. Twelfth An-
nual Report of the Geol. and Nat. Hist. Survey of Minnesota. Min-
neapolis, 1884.

W. Giesbrecht. Pelagisclie Copepoden. Fauna und Flora des Golfes von
Neapel. Monogr., xix, 1892.

C. Claus. Ueber die Entwickiung, Organisation und systematische Stellung der
Arguliden. Zeitschr. fur wisseusch. Zoologie, xxv, 1875.

C. Heider. Die Gattung Lernanthropus. Arbeiten a. d. zoolog. Inst. Wieu,
n, 1879.

M. Hartog. The Morphology of Cyclops and the Relations of the Copepoda.
Trans. Linnaean Soc. London, 3d Series, v, 1888.

F. Leydig. Ueber Argulus foliaceus. Archiv. fiir mikr. Anat. , xxxin, 1889.
C. Grobben. Die Entwicklungsgeschichte von Cetochilus septentrionalis, Goodsir.

Arbeiten a. d. zoolog Inst. Wien, in, 1881.

CIKRHIPEDIA.

C. Darwin. A Monograph of the Subclass Cirrhipedia. London, 1851-1854.
H. de Lucaze-Duthiers, Histoire de la Laura Gerardice. Archives de Zool.

exper. et. gen., vui, 1880.
P. P. C. Hoek. Report OK the Cirrhipedia. Scientific Results of Voyage of

H.M.S. Challenger. Zool., vm, 1883; x, 1884.
Yves Delage. Evolution de la Sacculine. Archives de Zool. exper. et. gen.,

2me ser., n, 1884.

LEPTOSTRACA.

C. Claus. Ueber den Organismus der Nebaliiden, und die systenatischen
Stellung der Leptostraken. Arbeiteu a. d. Zool. Inst. Wieu, vm, 1888.

SCHIZOPODA.

G. 0. Sars. Report on the Schisopoda. Scientific Results of the Voyage of

H.M.S. Challenger, xm, 1885.

CTJMACEA.

A. Dohrn. Ueber Bau und Entwickiung der Cumaceen. Jenaische Zeitschr.

filr Naturwiss., v, 1870.
G. 0. Sars. Report on the Cumacea. Scientific Results of the Voyage of

H.M.S. Challenger, xix, 1887.

STOMATOPODA.

C. Claus. Die Kreislaiifsorgane und Bluthewegung der Stomatopoden. Arbeiten

a. d. Zoolog. Inst. Wien, v, 1883.
W. K. Brooks. Report on the Slomatopoda. Scientific Results of the Voyage

of H.M.S Challenger, xvi, 1886.

DECAPODA.

8. I. Smith. Various Papers in Trans. Connecticut Academy and in Reports
of the U S. Commissioner of Fish and Fisheries.

TYPE CRUSTACEA. 427

V. Hensen. Studien iiber das OehOrorgan der Decapoden. Zeitschr. fur

wissensch. Zool., xin, 1863.
C. Grobben. Beitrdge zur Kenntniss der mdnnlichen Geschlechtsorgane der

Decapoden. Arbeiteu a. d. Zoolog. Inst. Wieu, i, 1878.
T. H. Huxley. Tlie Crayfish. London and New York, 1881.
W. K. Brooks. Lucifer: a Study in Morphology . Philosoph. Trans. Royal

Soc. London, CLXXIII, 1882.
H. Reichenbacb.. Studien zur Entwicklungsgeschichte des Flusskrebses. Ab-

handl. Senckenburg. Gesellsch. Frankfurt, xiv., 1886.
W. F. E. Weldon. Ccelom and Nephridia of Palcemon serralus. Journal

Marine Biolog. Assoc., i, 1889.
G. H. Parker. The Histology and Development of the Eye in the Lobster.

Bulletin Museum Cornp. Zoolog., xx, 1890.
P. Marchal. Recherches anatomiques et physiologiques sur I'appareil excreteur

des Crustaces decapodes. Archives de Zool. exper. et gen., 2ine ser., x,

1892.
W. K. Brooks and F. H. Herrick. The Embryology and Metamorphosis of the

Macroura. Proc. U. S. National Acacl., v, 1892.
Z. J. Allen. Nephridia and Body-cavity of some Decapod Crustacea. Quarterly

Jouru. Microscop. Science, xxxiv, 1893.

ARTHKOSTRACA.

0. Harger. Report on the Marine Isopoda of New England and Adjacent

Waters. Report of the U. S. Commissioner of Fish and Fisheries for

1878.
A. Delle Valle. Gammarini del Golfo diNapoli. Fauna und Flora des Golfes

von Neapel. Monogr., xx, 1893.
P. Mayer. Caprelliden. Fauna und Flora des Golfes von Neapel. Monogr.,

vi and xvi, 1882, 1890.
0. Nebeski. Beilrlige zur Kenntniss der Amphipoden der Adria. Arbeiten a.

. d. Zoolog. lust. Wien, in, 1890.
E. Kossmann. Studien uber Bopyriden. Zeitschr. fiir wissensch. Zoologie,

xxxv, 1881 ; Mitth. a. d. Zoolog. Station zu Neapel, in, 1882.
Yves Delage. Contribution d I' etude de I'appareil circulaloire des Crustaces

edriophthalmes marins. Archives de zool. exper. et gen., ix, 1881.

APPENDIX TO THE TYPE CRUSTACEA.

Order Xiphosura.

The Xiphosura is a group which possesses many Crus-
tacean peculiarities, and also many foreign to that group and
more especially characteristic of the Arachnida ; consequently
it is advisable to consider it as an order by itself, intermediate
between the two types.

A single genus, Limulus (Fig. 195), with few species consti-
tutes the order, the members of which are popularly known

428

INVERTEBRATE MORPHOLOGY.

as King-crabs or Horseshoe-crabs. They are large forms
measuring a foot or so in diameter, and the body is composed
of three portions. The anterior is a broad semicircular ceph-

oh

FIG. 195. — Limulus polyphemus, FEMALE, FROM THE VENTRAL SURFACE.

ab = abdomen cp = cephalothorax,

an = amis. ol = olfactory organ.

ch = cbelicera. op = operculum.

chi = cbilarium. sp = spiue.

alothorax (cp), prolonged backwards into sharp points at its
posterior angles and bearing upon its dorsal surface a pair of
compound eyes towards the sides and near the median line
two simple eyes. The middle region is the abdomen (ab),

TYPE CRUSTACEA. 429

showing but faint indication of segmentation, and bearing on
its terminal segment the anus, behind which is a long mova-
ble spine (sp), the post-abdomen, forming the third region
and to be regarded probably as a movable prolongation of
the dorsum of the last abdominal segment.

The cephalothorax bears seven pairs of appendages. The
first pair, the chelicerai (Fig. 195, c/n, which lie in front of the
mouth, are small and, like the following four pairs, are chelate.
These together with the sixth are much longer and surround
the mouth, their basal joints being provided with strong bris-
tles and serving as jaws. The sixth pair of appendages differ
from their predecessors in not being chelate and in possessing
upon their basal joints a peculiar process which has been
termed the flabelluni and by some is regarded as representing
an exopodite The seventh pair of appendages is very differ-
ent from the others, forming a broad flat plate, the two
appendages of the opposite sides meeting in the middle line.
This plate covers in the abdominal appendages to a certain
extent and hence is termed the operculum (op). The abdom-
inal appendages, of which there are five pairs, resemble the
operculum in form, and like it allow an external larger exopo-
dite and an inner smaller endopodite to be distinguished.
They carry upon their posterior surfaces series of large leaf-
like, thin-walled folds which function as branchiae.

The heart (Fig. 196, Tit) is an elongated tubular organ lying
in the posterior part of the cephalothorax and the anterior
part of the abdomen, and possesses eight ostia. Arteries
arise from it which carry the blood to various parts of the
body, eventually, however, opening into the general lacunar
system. The blood has a distinct bluish color which deepens
on exposure to the air and is due to a copper-containing
respiratory pigment, hseinocyauin.

The body is enclosed in a hard chitinous cuticle, and in
addition a peculiar fibro-cartilaginous plate, the endosteruite,
is found in the cephalothorax between the intestine and the
nervous system. It is formed by the fusion of a number of
tendons and may be regarded as an endoskeleton.

The mouth is an elongated opening lying between the
bases of the anterior cephalothoracic appendages and is

430

INVERTEBRATE MORPHOLOGY.

bounded behind by a pair of processes which represent a
lower lip and are known as the chilaria (Fig. 195, chi). The
O3sophagus passes upwards and forwards and dilates into a
large proventriculus (Fig. 196, pr~) in the front part of the
cephalothoracic shield, and this, bending upon itself and
constricting again, opens into the stomach (s), from which the
intestine (i) passes straight back to open on the ventral sur-
face of the body at the base of the terminal spine. The inner
wall of the hind-gut, oesophagus, and proventriculus is lined
by chitiu, which in the last-named structure is thrown into

ht

es

A Xx^agBs-JL. • -t-^T^^^

Jffi-3?S<. $Sv± ^ --=- i.-.-AL .•„„•. .-.--fci-'- --•»•• ;=s^-t-s=s

Vll

FIG. 196. — LONGITUDINAL SECTION THROUGH A YOUNG Limulus polyphemus,

DIAGRAMMATIC (after PACKARD).
ce = cerebral ganglion. I = liver.

Jit = heart. pi' = proventriculus.

i — iutestiue. s — stomach.

vn = ventral nerve-cord.

folds and recalls the masticatory apparatus in the stomach
of the Decapodous Crustacea. Into the stomach there open
the ducts of two pairs of voluminous digestive glands (I)
which occupy the greater portion of the cephalothorax and
are much branched greenish structures.

The nervous system consists of a syncerebrum (ce) com-
posed apparently of three pairs of ganglia. It lies in front of
the oesophagus, sending branches to the compound and simple
eyes. Behind the oesophagus and united with the syncere-
brum by circumcesophageal connectives comes a series of
seven pairs of ganglia closely approximated, the first pair
innervating the chelicerse and the remaining six the other
thoracic limbs in succession. A chain of six pairs of ganglia
lying in the abdomen is connected with the cephalothoracic
series and innervate the abdominal appendages.

TYPE CRUSTACEA.

431

As already noticed, a pair of simple eyes are borne upon the
dorsal surface of the carapace, one on each side of the median
line, while a pair of larger compound eyes are situated lat-
erally. The structure of these compound eyes is peculiar
(Fig. 197). Over their surface the cuticle is considerably
thickened and shows upon the outer surface no indication of
corneal facets, but its inner surface is prolonged into a num-
ber of papillae (I) each one of which projects into a depression
of the ectoderm. At the bottom of each depression is a bulb-

Tt

opn

FIG. 197. — COMPOUND EYE OF Limulus polyphemus, Two OMMATIDIA (after

WATASE).

c = central cell. ms - mesoderm.

I = leus. opn -- optic nerve.

rt = retiuula.

like structure composed of a number of cells arranged in a
circle and constituting a retinula (rt), the lower ends of the
cells being continued inwards to form part of the optic nerves
(opn). Upon the face which is turned towards its fellow each
retinular cell secretes a layer of chitin, and these various chit-
inous rods being in contact there is formed a structure com
parable to the rhabdorn of the Crustacean eye. In the centre
of the retiuular cells and below the rhabdorn is a single clear
cell (c) whose lower end is also prolonged into a nerve-fibre.
Each depression with its retiuula and the chitinous papilla
which fits into it and represents its cornea is an ommatidium,
and the development shows that the ommatidia arise as

432 INVERTEBRATE MORPHOLOGY.

number of separate iuvaginations of tlie ectoderm, the sides
of the retinular cells which secrete the rhabdom being in
reality those sides which before iuvaginatiou were at the
surface of the body, and the rhabdom may therefore be
regarded as composed of portions of the general cuticle which
have been separated by the invaginatiou.

On the under surface of the carapace in the median line
in front of the cheliceree is a small tubercle (Fig. 195, ol)
which contains an organ supposed to be olfactory in function,
and probably some of the setae upon the basal joints of the
limbs may also possess a similar function.

Nephridia are represented by a single pair of large
reddish bodies lying at the sides of the cephalothorax. They
have no communication with the exterior in the adult, but in
the early stages of development open upon the basal joint of
the fifth appendage, and are at first tubular organs and
nephridialike, later becoming much contorted and complex.
"What their function in the adult may be is uncertain, and to
avoid possible misconceptions it seems preferable to speak
of them as coxal glands, a term indicating their original point
of opening on the basal joints (coxae) of one of the pairs of
limbs.

The Xiphosura are bisexual, the genital ducts opening on
both males and females on the posterior surface of the oper-
culum near its base. The ovaries are much branched paired
structures, the various branches frequently anastomosing
even across the median line. The testes are numerous
spherical bodies scattered through the body and situated on
branching and anastomosing vasa deferentia.

Development and Affinities of the Xiphosura. — When the
young Limulus leaves the egg it presents a remarkable resem-
blance to a Trilobite and suggests a possible affinity with
these forms which are known to occur only in the Palaeozoic
rocks. In these same rocks there occur also the remains of
forms known as the Eurypteridw which seem to have been
even more nearly related to Limulus than were the Trilobites.
In them the cephalothorax bore apparently only six pairs of
appendages which resembled more or less closely those of
Limulus, except that the sixth pair was broad and oarlike,

TYPE CRUSTACEA.

433

probably serving for swimming. The abdomen was com-
posed of twelve segments, the anterior six of which were
much more massive than the others and bore five pairs of
platelike appendages on whose posterior surface were the
branchiae. The terminal segment bore a spine or fiulike
structure. Such a form as this, represented by the genus
Eurypterus (Fig. 198), presents strong similarities to Limulus
and also to the Scorpions, bearing out
the numerous similarities of structure
occurring between Limulus and those
forms. This side of the affinity may
be postponed, however, until the
next chapter, and the comparison of
Limulus with the Crustacea discussed
here. Its chitiuous cuticle, its jointed
and biramous appendages, and its
branchial respiration show similari-
ties to the Crustacea, as do also the
form of the heart and the compound
eyes. Whether or not the coxal
gland is comparable to the shell-
gland is at present uncertain, but the
other similarities are sufficient to
justify the recognition of a Crusta-
cean origin for Limulus. It forms FIG. 198 — Eurypterus remipes
indeed a connecting link between the (from ^HOLSON).

Crustacea and the Arachnida, presenting probably on the
whole more affinities with this latter group than with the
former.

Since, however, a Crustacean ancestry is probable, a com-
parisiou of the appendages of Limulus with those of a repre-
sentative of the ancestral group ought to be possible. It has
already been noticed that the brain of Limulus is a syncere-
brum composed of three segments ; it represents, therefore,
two segments of which the appendages and other parts have
disappeared. Furthermore, recalling that, in the higher
Crustacea at least, a ganglion occurring between the cerebral
autennary ganglia in the embryo indicates a lost pair of

434 INVERTEBRATE MORPHOLOGY.

appendages in these forms, the following table may represent
the honiologies of the appendages of the two groups.

Crustacean. Liniulus.

1 segment no appendage no appendage

o << << " << <<

3 " antennules • " "

4 " antennae chelicerae

5 " mandibles 1st pair of legs

6 " 1st maxillse 2d " " "

7 " 2d 3d " " "

8 " 1st thoracic appendages 4th " " "

9 2d " " 5th " " "

10 3d " " operculum

LITERATUKE.

A. Gerstaecker. Crustaceen. Bronn's Klassen und Ordnungen des Tkierreichs,

Bd. V. 1. Abth., 1866-79.
A. S. Packard. The Anatomy, Histology, and Embryology of Limulus poly-

phemus. Memoirs Boston Soc. Nat. History, 1880.
E. R. Lankester. Limulus an Arachnid. Quarterly Journ. Microscopical

Science, xxi, 1881.
S. Watase. On the Morphology of the Compound Eyes of Arthropods. Studies

from the Biol. L:ib. Johns Hopkins Univ., iv, 1890.
W. Patten. On the Origin of Vertebrates from Arachnids. Quarterly Journ.

Microscop. Science, xxxi, 1890.
J. S. Kingsley. TJie Embryology of Limulus. Journ. of Morphology, vn,

1892 ; Tin, 1893.
W. Patten. On the Morphology and Physiology of the Brain and Sense-organs

of Limulus. Quarterly Jouru. Microscop. Science, xxxv, 1893.

TYPE ARACHNID A. 435

CHAPTEK XIV.

TYPE ARACHNIDA.

THE Arachnida are essentially terrestrial forms, for though
a few species lead an aquatic or marine life, they are evi-
dently descendants of forms which led a terrestrial existence
and have only secondarily acquired the power of living under
water. In all members of the group the body is covered by
a more or less thick chitiuous cuticle and the appendages are
as a rule jointed.

A characteristic feature of the group is the fusion of the
head and thorax to form an unsegmented cephalothorax bear-
ing usually six pairs of limbs. The first pair of these are
the chelicercB (Fig. 201, cli), composed of one to three joints
and terminated either by a claw or a chela ; they lie in front
of the mouth, which is bounded at the sides by the basal
joints of the second pair of appendages, the pedipalps (pe\
which may be long and limblike, or chelate, or in some cases
clawlike, their basal joints serving in all cases as mandibles.
Behind these follow four pairs of legs composed of six or seven
joints, the basal joint being termed the coxa, the next, usually
short, the trochanter, the third the femur, the next two to-
gether form the tibia, then follows in some forms a metatarsus,
while the terminal one, provided with two claws, termed
ungues, and in some mites also with a suctorial disk, consti-
tutes the tarsus. Variations from this structure of course
occur, the chelicene, for example, in some mites being re-
duced to short stylets, and in others the two posterior pairs of
legs may be quite rudimentary (Phytoptus). The most impor-
tant variation is, however, that found in the members of the
order Solifugse, in which a head is distinctly marked off from
a thorax composed of three segments.

The abdomen in some forms is segmented, in others all
trace of the segmentation is lost, and, finally, in the Mites it

436

INVERTEBRATE MORPHOLOGY.

may be iinited with the cephalothorax. In the Scorpions it
is divisible into an anterior portion, the prseabdomen, much
broader and stouter than the posterior postabdomen, an ar-
rangement also indicated in certain other forms. In the
adults the abdomen is usually destitute of appendages, though
they may be present in the embryos ; the Scorpions, however,
possess two highly-modified pairs, and it seems probable that
the four or six papillae upon which the ducts of the sphmiug-
glauds open in the Spiders represent also modified append-
ages.

A special respiratory system is entirely wanting in a few
forms. In the majority there occur on the sides of the body
from one to four pairs of pores termed stigmata (Fig. 201, s^~4).
In the Scorpions and some other forms each stigma opens

into a cavity lined with chitiu
continuous with that which covers
/ the general surface of the body,
and into this cavity there project
• a number of lamellae arranged
like the leaves of a book (Fig.
199), whence the term lung-books
frequently applied to them. Each
lamella is hollow, trabeculse ex-
tending across the cavity from
one wall to the other, and the
cavities communicate with the
coelomic lacunes, so that blood can
readily flow into them and so

change its gases through the thin
FIG. 199.— TRANSVERSE SECTION

THROUGH THE LUNG-BOOK OP A walls of tne lamellae. In other

SPIDER (after MCLEOD). cases there occurs in connection

= chitinogenous tissue. w^n the lung-book apparatus, or

else entirely replacing it, a tra-
lp = pulmonary lamella. r. .

st = stio-ma. cheal system consisting of a uum-

t = last compartment of lurg- ber of tubes ramifying through

book, trachealike in char- tlie body. In some cases a strong

tube or trachea arises at each

stigma and traverses the body, giving off branches to all parts
as it goes ; in others there is in connection with each stigma

TYPE ARACHNID A. 437

a bunch of unbranched tracheae, and all gradations between
these two conditions occur. The tracheae are lined with
chitin, which is sometimes thickened to form rings or spiral
bands which serve to keep the lumen of the tubes open and
thus permit a free passage of air into them.

The co3lom is filled for the most part with the various
organs and is reduced to a series of lacunar spaces containing
blood, sometimes rich in hgemocyauin and assuming a blue
color when oxygenated. A heart is wanting in some Mites,
but is present in the majority of forms, varying from a saclike
organ with a single pair of ostia guarded by valves to an
elongated cylinder with as many as eight pairs of ostia (Scorpi-
ons). It is for the most part situated in the abdominal region,
and in the Spiders is enclosed within a space with definite
walls which is termed the pericardium, though it cannot be
considered homologous with the pericardium of the Mollusca,
since it contains blood ; muscle-bands extend from it to the
walls of the body and by their contraction cause its expan-
sion, fibres in its wall diminishing its cavity and forcing the
blood through the ostia into the heart. Arteries in many
forms arise from the heart, but after usually a short course
open into the lacunar ccelom.

The digestive tract pursues a more or less straight course
through the body, but shows a tendency to develop coacal out-
growths which sometimes reach a considerable size. The
anterior and posterior portions of the tract are ectodermal,
while the middle region or mid-gut is endodermal and is the
portion with which the cceca are connected. In the Scorpi-
ons the ducts of a digestive gland open into the mid-gut, and
in many forms there is connected with the posterior portion
of this same region a pair of tubular Malpighian vessels
which are presumably excretory in function and recall the
similar structures of the Amphipoda. The end-gut is fre-
quently dilated into a large bladderlike structure, the rectal
bladder.

The nervous system consists of a suprao3sophageal syn-
cerebrum composed of three pairs of ganglia fused together,
and in some forms even four pairs may be included, since the
chelicerse may be innervated from the mass, their ganglia in

438 INVERTEBRATE MORPHOLOGY.

embryonic life being, however, distinct and postoral in posi-
tion, only later moving forward. The succeeding ganglia are
generally more or less fused, and indeed in some forms all
the ganglia of the limb-bearing segments of the cephalothorax
may be united with those of the abdominal region to form a
single gangliouic mass. In some forms a single ganglion
occurs behind this mass at the junction of the cephalothorax
and the abdomen, and in the Scorpions there is posteriorly a
ventral nerve-cord with seven pairs of ganglia, the anterior
pair corresponding with the fifth abdominal segment. A
sympathetic nervous system occurs in the Scorpions, Spiders,
and Harvest-spiders, consisting of a nerve arising from the
syncerebrum and passing to the digestive tract.

Hairs situated upon the body and appendages serve as
sense-organs of touch and apparently also of audition, since
Spiders are sensitive to air-vibrations and possess no definite
auditory organs. Eyes are very generally present and vary
considerably in number, there being in the Spiders three or
four pairs ; in the Scorpions, in which there are from two to
six pairs, one pair become closely approximated on each side
of the mid-line and recall the median eyes of Limulus, while
the remaining pairs are situated more to the sides of the
cephalothorax. In structure the median eyes differ from the
lateral ones ; the chitiuous cuticle is thickened over them to
form a simple unfaceted lens below which lies a layer of
transparent cells continuous with the general ectoderm (hypo-
dermis) of the body and which may be termed the cornea!
hypoderinis, though more usually they are known as the
vitreous cells. Below them comes the retina, consisting of a
single layer of elongated cells with their nuclei situated
toward their inner ends, with which the fibres of the optic
nerve come into connection. The various retinal cells are
arranged in groups of five (retinulse), which secrete a thin
chitinous rod upon their contigiious faces, producing thus a
rhabdom composed of five parts. Behind the retina is a thin
layer of cells, the postretiuular layer, and numerous pigment-
cells occur between the various retiuulse. The lateral eyes
are constructed upon a very different plan, lacking a corneal
hypodermis between the retina and the cornea. They are

TYPE ARACHNIDA.

439

cup-shaped structures, the cavity of the cup being filled by
the cuticular cornea and its wall transformed into the retina,
which is continuous at the margins of the cup with the gen-
eral hypodermis. The retinal cells are of two kinds, viz.,
large sensory cells and smaller interstitial cells. Each sen-
sory cell is surrounded by pigment and bears upon its lateral
walls a chitinous secretion which, with the corresponding
secretion of contiguous cells, forms a rhabdom. The nuclei
of the cells are situated nearer their inner than their outer
ends, and behind them in Euscorpius highly refractive spheres
occur imbedded in the cells, constituting what have been
termed the phaospheres. Upon its inner wall the retina is
lined by a basement-membrane continuous with that lying
below the general hypodermis and perforated by the fibres of
the optic nerves which come into contact with the inner ends
of the sensory cells.

In the Spiders, in which there are six or eight eyes arranged
upon the dorsum and sides of the cephalothorax, the ante-
rior dorsal pair differs in structure from the remaining ones.
In both forms of eyes (Fig. 200) the cuticular cornea (c) rests

_V

B

FIG. 200. — EYES OF SPIDEK. A, ANTERIOR, AND B, POSTERIOR EYE (after

BKRTKAU from KORSCHELT and HEIDER).
b = rods. r = retina.

I = lens. t = tapetum lucidum.

v = vitreous layer.

upon a corneal hypodermis, (the vitreous cells, v\ but the
arrangement of the retina differs greatly. In the anterior
dorsal pair (A] it is composed of a layer of elongated cells (r)
whose nuclei are situated towards their inner ends, while near

440 INVERTEBRATE MORPHOLOGY.

the outer ends are situated a number of rodlike bodies (rhab-
dorns, 6), wheuce these eyes have been termed prebacillar ;
the nerve-fibres are continuous with the inner ends of the cells.
In the posterior dorsal and lateral eyes (B) an inversion of
the retina (r) has taken place, so that the rods (b) are situated
at the apparently inner ends of the cells and the nuclei at
their outer ends, whence the term postbacillar applied to
these eyes. The optic nerve-fibres enter at the sides of the
eye and are distributed to the nuclear ends of the retinal
cells, recalling the arrangement occurring in Pecten among the
Mollusca. The innermost layer of the eye upon which the
ends of the rods rest is cellular, numerous minute crystals
being deposited in the cells, whence it has the function of a
reflector and is termed the tapetum (£). It is quite wanting in
the prebacillar eyes.

The significance of the structure of the Arachnid eye may be under-
stood by supposing it to have been derived from a compound eye similar to
that of Limulus (see p. 431), the individualities of the various ommatidia
being more or less subordinated. The cuticular cornea in Limulus is
smooth upon its outer surface, the inner surface being produced into
papilla?, one of which corresponds to each ommatidium. In the Arachnids
even these papillae are wanting, the cornea showing no evidence of the
presence of ommatidia. The lateral eyes of the Scorpions approach more
nearly in their general structure the eyes of Limulus, though the conden-
sation of the ommatidia has been carried further than in the median eyes
of that form, or in the posterior dorsal and lateral eyes of the Spiders. But
in these eyes the condensation is associated with an invagination of the en-
tire eye, a process which, it may be remarked, is indicated in the median
eyes of Limulus. This invagination has been regarded as a pushing in,
under and parallel to the hypodermis, of a pouch of that layer, a process
which gives in cross-section the appearance of an S-shaped fold. The
outermost layer of the fold forms the vitreous cells or corneal hypodermis,
the middle layer the retina, the. inversion of which is plainly seen in the
posterior dorsal and lateral eyes of the Spiders, while the innermost layer
forms the postretinal layer in the Scorpions and the tapetum of the
Spiders. The ommatidial retinulae are more or less retained in these eyes,
as is shown by the structure of the rhabdom, which in the Scorpions is
composed of five parts, in the Spiders of two, and in the Harvest -spiders
of three. The anterior dorsal eyes of the Spiders do not seem to have
undergone an invagination, hence the absence of a tapetum and the preba-
cillar structure of the retina; a corneal hypodermis is, however, present,
and would seem to indicate an invagination, but its mode of origin seems
at present but imperfectly understood. If a generalization is to be made, it

TYPE ARACHN1DA. 441

will be to the effect that the eyes of the Arachnids have been derived from
compound eyes similar to those of Limulus, and that in the median eyes of
the Scorpions, and the posterior dorsal and lateral eyes of the Spiders the
eutii'e optic area has been invaginated, making them comparable to the
median eyes of Limulus, while the lateral eyes of the Scorpions and the
anterior dorsal eyes of the Spiders have not undergone invagination and
hence are comparable to the lateral eyes of Limulus. Whether the com-
parability indicates also the homology from a phylogenetic standpoint of
eye to eye must remain at present uncertain (see p. 457).

Iii addition to the Malpfiighian tubules already mentioned //
as excretory organs occurring in connection with the digestive
tract of the Spiders, there exist in many forms additional
glands which probably are also excretory in function or sig-
nificance. These are the coxal glands, so called on account of
their openings when present being on the basal joints (coxae)
of one of the pairs of legs. In the Scorpions and Spiders
the ducts of the glands open on the third pair of legs (i.e., the
fifth pair of appendages) in the embryo, but are usually
wanting in the adults. In the Solifugse and Harvest-spiders
coxal glands also occur in connection with the fourth pair
of legs, and similar glands have also been observed in several
genera of Mites, opening, however, at varying points.

Glands are also of frequent occurrence in connection with
the pedipalps, having apparently varying functions in differ-
ent genera. They do not, however, seem to belong to the same
category as the coxal glands and are in no case excretory.

The Arachnida are bisexual throughout. The ovaries not
infrequently fuse to form a single mass or a circular band,
and in connection with the oviducts, which are in direct com-
munication with them, there is usually developed a receptac-
uluin seminis, and in the Harvest-spiders, an elongated ovi-
positor. The testes are also frequently fused, and the vasa
defereutia are provided with vesiculse seminales and usually
terminate in a copulatory organ. The majority of forms are
oviparous, exceptions to the rule being found, however, as in
the genus Phrynus and in the Scorpions, which are viviparous.

1. Order Scorpionida.

In the Scorpions (Fig. 201) the body is composed of an
unsegmented cephalothorax and an elongated segmented

442

INVERTEBRATE MORPHOLOGY.

abdomen. The seven anterior segments (the prseabdomen)
of the abdomen are broader and thicker than the remaining
five segments (the postabdonien), the last one of which ter-
minates in a curved stout spine which bears at its extremity
the openings of two ducts leading from a pair of glands lying
in the twelfth abdominal segment and secreting a poisonous
fluid.

The chelicerae (ch) are small chelate appendages situated
in front of the mouth, while the pedipalps (pe) are long and

provided with strong chelse, their
basal joints and those of the two
succeeding appendages surround-
ing the mouth and serving as jaws.
The four pairs of appendages be-
hind the pedipalps are all similar
in form, being six-jointed walking-
limbs. Upon the abdomen modi-
fied appendages are also found,
the second abdominal segment
bearing a pair, each member of
which consists of a single joint
whose posterior edge is beset with
a number of processes which give
it the appearance of a comb,
whence the name pectines (pt)
applied to these appendages. In
front of the pectiues lies the geni-
tal opening, protected by a small
genital operculum (op) which may
possibly represent another pair of
appendages belonging to the first
abdominal segment.

Upon the ventral surfaces of
third, fourth, fifth, and sixth abdominal segments elongated
pores are to be found which are stigmata (st1-*) leading into
the respiratory cavities containing the lung-books, of which
there are in all four pairs in this group. No tracheae occur.
The intestine is quite straight in the Scorpions and lacks
csecal outgrowths excepting the two Malpighiau tubules sit-

FlG. 201.— SCORPION (after OWEN).
cli — cbelicerse.
op — geuital operculum.
pe = pedipalp.
pt = pecten.
= stigmata.

TYPE ARACHNID A. 443

uated at the posterior end of the mid-gut. The digestive
gland is a large five-lobed structure which empties through
several ducts into the mid-gut.

The nervous system consists of a syncerebrurn lying
above the oasophagus and giving rise to nerves for the e3res
and for the chelicerse. It is connected with a suboesophageal
mass from which the pedipalps and the three anterior legs are
innervated, the fourth pair of legs receiving its nerves from a
pair of distinct ganglia separated only by a short distance
from the subcesophageal mass. Behind this in the abdomen
is a chain consisting of seven pairs of ganglia united by long
connectives. The eyes vary in number from two to six pairs,
one pair being situated on or near the median line, while the
others are lateral.

Coxal glands occur in connection with the third pair of
legs, and the heart is an elongated structure lying in the an-
terior portion of the abdomen and possessing eight pairs of
ostia.

The Scorpions are viviparous. The ovaries are situated
in the anterior abdominal region and are elongated, that of
one side of the body being united with the other by several
transverse connections. The oviducts, which are short, serve
as uteri, and open to the exterior by a single median opening
situated on the ventral surface of the first abdominal segment.
The testes consist of four tubes, those of the same side being
connected by transverse anastomoses, and unite together to
open into a protrusible penis, accessory glands, vesiculae sem-
inales, occurring in connection with each vas deferens. The
single genital orifice occupies the same position as in the
female.

The Scorpions are confined to the warmer regions of the
globe, but few genera being known. Of these the genera Eu-
scorpius and Buthus are perhaps the commonest.

2. Order Pseudoscorpionida.

This order includes a number of small forms which are
found under the bark of trees or among dead leaves or moss,
one genus, Chelifer (Fig. 202), occurring occasionally between

444 1NVEHTEBUATE MORPHOLOGY.

the pages of books, and lieuce being known popularly as the
Book-scorpion. The cephalothorax is unsegmented, and is
followed by a broad flattened abdomen composed of eleven
segments. A prseabdomen and a postabdonien, such as can
be distinguished in the Scorpiouida, does not occur, nor is
there a terminal poison-spine nor a poison-gland.

The cheliceme and pedipalps resemble those of the Scor-
pions, being chelate, and the four succeeding appendages are

walking-legs, while the abdomen pos-
sesses no appendages in the adult. Both
the second and third abdominal seg-
ments bear upon their ventral surfaces a
pair of stigmata which are the openings
of tubular tracheae which extend through
the body sending off branches, except
in Cliernes, in which bunches of uu-
branched tracheae arise from each stig-
ma. A heart is present, but consists of
FIG. 2Q2.—Ckelifer caret- a simple tube with either a single pair
noides (fro,,, CUVIER). of ostia near its posterior extremity

(Obisium) or with four ostia (Chernes).

The eudoderrnal portion of the digestive tract gives rise
to a pair of lateral caecal diverticula branched at the apex and
to one unpaired ventral one. Two eyes are present in Chelifer
and four in OMsium, while they are entirely wanting in
Chernes. The reproductive organs open upon the ventral sur-
face of the second abdominal segment, and the opening is
surrounded with glands which secreie a fluid which quickly
hardens to silky filaments and serves to fasten the eggs to
the abdomen of the parent. These glands are hypoderrnal in
origin and correspond to the spinning-glands of the Spiders.

3. Order Solifugae.

The members of this order are characterized by the head-
region being separated from a thorax consisting of three seg-
ments and bearing the three posterior pairs of legs. The
abdomen is also segmented, its ten segments showing no dif-
ferentiation into prseabdomeu and postabdomen, nor does it

TYPE ARACHNIDA.

445

possess any sting or poison-gland. The chelicerae are chelate,
but the pedipalps are long and leglike and possess glands
which in Galeodes have been supposed to be poisonous. The
anterior pair of legs lacks the terminal uugues found on the
others, and functions as a second
pedipalp rather than a walking-leg.
No appendages occur on the abdo-
men.

Three pairs of stigmata occur
on the ventral surface of the body,
the most anterior pair being situ-
ated on the first thoracic segment,
while the other two are on the
second and third abdominal seg-
ments. The anterior position of the
first pair is probably to be regarded
as secondary, and produced by a
forward migration of the pair which
should occur upon the first abdomi-
nal sement. The stigmata lead

FlG' 203.-<7«fe^ spinipalpus

(from CUVIER).

into tubular tracheae which branch

extensively. A comparatively simple heart is situated in the

abdomen.

The mid-gut possesses numerous branched diverticula as
well as Malpighian tubiiles. The nervous system consists of
a syncerebrurn connected with a subcesophageal mass which
represents all the thoracic and abdominal ganglia fused to-
gether. Two eyes are present, situated on a common eleva-
tion at the front edge of the head.

The reproductive organs resemble those of the Scorpions
except that transverse anastomoses do not occur, and the gen-
ital opening is situated upon the ventral surface of the first
abdominal segment.

The Solifugse is a small order living more especially in
warm sandy regions. They are usually, on rather insufficient
grounds, supposed to be capable of inflicting a poisoned
wound. Only two genera, Solpuga and Galeodes, belong to
the order.

446 INVERTEBRATE MORPHOLOGY.

4. Order Pedipalpi.

The order Pedipalpi includes two genera, Phrynus and
Thelyphonus, both of which are inhabitants of the warmer
regions of the earth. The cephalothorax is uusegmeuted ;
the abdomen in Phrynus is elongated and oval, and composed
of eleven segments showing little differentiation of form,
while in Thelyphonus there are twelve segments, the last

three of which are much smaller than
the others and bear a long, many-
jointed terminal filament. The cheli-
cera3 are not chelate, but the terminal
joint may be flexed upon the basal
one and contains the duct of a poison-
gland which opens at its extremity.
The pedipalps in Phrynus are long
and leglike, though richly provided
with spines, and terminate with uii-
gues, but in Thelyphonus they are rel-
atively short and stout with a flexible
terminal joint as in the chelicerse ;
f in both genera the basal joints of the

Fm.2M.-TMyphonU8cau- fWQ pepipa]ps are fused. The first

datus (from CUVIER). / ,

leg is long and slender and termi-
nates in a filament-like structure, the other three pairs being
typical walking-legs.

Four stigmata occur, one pair situated in the second and
another in the third abdominal segment, and they open into
cavities containing lung-books. The digestive tract is com-
paratively simple, but the nervous system shows a concentra-
tion of the postoasophageal ganglia similar to that described
for the Solifugas, except that a single pair of ganglia occurs
in the abomiual region united by long connectives with the
cephalothoracic mass. Eight eyes are present, two of which
are larger than the others and situated at the anterior edge
of the dorsal surface of the cephalothorax, while the other
three pairs are situated laterally.

The reproductive organs are paired and open by a median

TYPE ARACHNID A. 447

orifice situated on the ventral surface of the first abdominal
segment. Plirynus is viviparous.

5. Order Phalaugida.

The Phalaugida (Fig. 205), popularly known as the Harvest
spiders, possess an unsegineuted cephalothorax (ct) and have
from six to nine segments composing the abdomen (ab). The
chelicerse are chelate, while the pedipalps (pe) are long and
leglike, with terminal ungues. The eight walking-legs are
usually exceedingly long, though in the genera Cyphoplithalmus
and Grwbocellum they are shorter. A single pair of stigmata
are usually all that occur ; they are situated upon the first

FIG. 205. — Leiobunum.
ab = abdomen. ct = cephalothorax. pe = pedipalps.

abdominal segment and open into branching tracheae. In
Gibbocellum, however, two pairs occur, situated upon the
second and third abdominal segments, the anterior pair open-
ing into branched tracheae, while a bunch of simple unbranched
tracheae arises from each of the posterior ones. The heart
is somewhat elongated and possesses three pairs of ostia ; ar-
teries are entirely wanting, the blood passing from the heart
directly into the lacunar spaces.

The digestive tract dilates into a sac-like stomach from
which numerous much-branched csecal diverticula pass off.
Malpighian vessels, two in number, are found in Cyphophfhal-
mus and Gibbocellum, and have been described as occurring

448 INVERTEBRATE MORPHOLOGY.

in other forms also, though it is probable that two glandular
tubes which open to the exterior on the sides of the cephalo-
thorax have in some forms been mistaken for these organs.
Odoriferous glands are also found in the abdomen of some
forms, and so-called salivary glands occur in connection with
the pedipalps.

The nervous system shows a marked concentration of the
postoral ganglia, a single pair only remaining separate from
the fused mass formed of the remainder. The majority of
forms possess but a single pair of eyes on the dorsum of the
cephalothorax, but in Gibbocettum two lateral pairs are
found.

Coxal glands have been described in connection with the
coxal joints of the third pair of legs and have been observed
to communicate with the exterior, differing therefore from
those of other Arachnoids in being functional in the adult.
The reproductive organs are unpaired, a condition which
results from the fusion of originally paired structures, and
the genital pore lies in both sexes at the junction of the
cephalothorax and abdomen or on the first abdominal seg-
ment. The vasa deferentia and oviducts are paired, each of
the former communicating with a protrusible penis, while
similarly each oviduct unites with a long protrusible ovipositor.

Certain genera such as Leiobunum (Fig. 205), Plialangium,
and Opilio, are exceedingly common, and to them the terms
Harvest-men, Harvest-spiders, or Daddy Longlegs are popu-
larly applied. Other forms, such as Gonyleptus, with spiuose
pedipalps, are tropical in habitat, while OyphopMhcHmus and
Gibbocellum have a limited distribution, and on account of the
many differences of structure which they present when com-
pared with other forms are sometimes grouped together to
form a separate order. It is to be noted especially that these
two forms possess upon the second abdominal segment a pair
of wartlike elevations at the summit of which the ducts of
numerous spinning-glands open.

6. Order Aranese.

The order Aranese includes a large number of forms
possessing very definite characteristics. The cephalothorax

TYPE ARACHNID A.

449

is unseginented, as is also the abdomen, which is an oval,
spherical, or sometimes irregularly-shaped region which
narrows suddenly anteriorly so as to be much narrower than
the cephalothorax. The chelicene project somewhat in front
of the cephalothorax and each consists of a broad basal joint
and a terminal strong claw which may be flexed upon the
basal joint, and has opening at its tip the duct of a poison-
gland (Fig. 206, pg) which lies in the cephalothorax. The

ht

oc

ce

sd

rb

spg

tr

FIG. 206.— DIAGRAM OF STRUCTURE OF A SPIDER (after LET-CHART).

ao = aorta.

ce = cerebral gauglkm.
ch = chelicera.
dg = digestive gland.
gp — genital pore.
Jit = heart.
Ib = lung-book.
mt = Mulpigbian tubule.
oc = eye.
ov = ovary.

pe = pedipalp.
pg = poison-gland.

rb = rectal bladder.

rs = receptacuhim semiuis.
s — stomach.

sd = stomach diverticulum.

sp = spinneret.
spg — spinning-glands.

tg = thoracic ganglion.

tr = trachea.

pedipalps of the females are leglike structures usually with a
terminal unguis, but in the male are more or less swollen to
serve as accessory organs in copulation. The four pairs of
seven-jointed legs are all similar in structure and serve for
walking, differing in relative length in different genera. In
the embryo the abdomen is distinctly segmented and bears
five or six pairs of rudimentary appendages, the more ante-
rior of which later disappear, while the two or three posterior
pairs persist as the spinnerets (sp), so called from the occur-
rence on them of the openings of the ducts of the spinning-
glands (spg}.

These are very numerous and open at the apices of the
spinnerets, each gland producing a fluid secretion which
quickl}r hardens on exposure to the air to form a silken

450 INVERTEBRATE MORPHOLOGY.

thread. The thickness of the thread may be modified by
uniting together the secretions of a greater or less number of
the glands, which, moreover, differ among themselves, some
producing, for instance, a sticky secretion with which certain
of the threads may be covered. In some forms there is situ-
ated upon the abdomen just in front of the -m-immcrctc a $ I
chitinous plate, the cribeUum, which is perforated, like the
spinnerets, by the ducts of numerous spinning-glands. Its
presence is associated with that of a calamistrum, a peculiar
modification of the metatarsus of the last pair of legs, it being
furnished with a double row of bristles which are rapidly
waved over the cribellum and draw from its glands their
secretion. The threads are used for several purposes, as, for
example, to fasten the ova to the body of the parent or to
form a cocoon for them, or else to form a snare by which
insects may be caught to serve as food. These snares in
some cases are composed of an irregular network of threads
arranged without any definite pattern, as in Theridium, but
some other forms show a certain amount of architectural
skill, weaving a platform of felted threads which terminates
in a tubelike place of concealment for the spider (e.g., Age-
lena, Tegenaria) or webs composed of threads radiating from
a central point and united by other threads arranged in a
spiral or in concentric circles (e.g., Epeira, the common garden-
spider), or else using the threads to form a hinged trap-door
covering in a burrow in the earth which serves as a domicile
as in the Trap-door Spider.

The digestive tract expands in the thoracic region into a
saclike structure (s) from each side of which three or more
usually five csecal diverticula (sd) arise, the anterior pair
sometimes anastomosing so as to form a ring, while in some
cases (Epeira) secondary diverticula extend from the more
posterior ones into the coxal joints of the legs. In the abdo-
men the intestine is more cylindrical, giving rise to much-
branched lateral diverticula which together form the so-called
liver (dg), and having connected with it, just as it joins the
end-gut, two elongated Malpighian tubules (mt). The end-
gut itself dilates into a large rectal bladder (rb) which a short

TYPE ARACHNID A. 451

rectum connects with the anus situated at the posterior ex-
tremity of the body.

In the genus My gale and allied forms two pairs of stig-
mata are found near the anterior portion of the abdomen,
both of which lead into cavities containing lung-books.
In the majority of forms, however, but one pair of lung-books
(Ib) occurs, the second pair of stigmata opening into a tracheal
tube (tr) extending into the cephalothorax and terminating in
a bunch of unbranched tracheae, a similar bunch arising near
its base and extending backwards into the abdomen (Seges-
tria). In some forms the second or tracheal stigmata may
be situated far back upon the abdomen, and may be united
to a single median transversely-elongated cleft, from which a
bunch of unbranched (Attus) or branched tracheae arises.

The heart (Jit), which lies in the abdomen, is enclosed
within a so-called pericardium and possesses three pairs of
ostia. It is continued anteriorly and posteriorly into aortse,
and gives off also lateral arteries, all of which open after rel-
atively short courses into the lacunar spaces. The blood is
returned to the pericardia! cavity, whence it passes into the
heart, the greater portion on its way to the pericardium pass-
ing through the lung-books.

The nervous system consists of a syncerebrum (ce) and a
large cephalothoracic ganglionic mass (fa/). In addition to
the nerves to the appendages, a posterior nerve arises from
this mass and passes backwards towards the abdomen, in
Mygale dilating at the junction of that region with the cephalo-
thorax into a pair of small ganglia. A sympathetic or visceral
system, consisting of a nerve arising by paired trunks from
the brain, is distributed to the anterior portion of the diges-
tive tract. The eyes are usually numerous, three or four pairs
occurring on the anterior portion of the cephalothorax, their
arrangement varying in different genera.

Coxal glands have been found in several forms in connec-
tion with the third pair of legs, but have not been found to
open to the exterior in the adult. The reproductive organs
open in both sexes by a single opening situated near the
anterior end of the abdomen between the anterior stigmata.
The ovaries (ov) are paired, or may unite to form a ring, and

452 INVERTEBRATE MORPHOLOGY.

the two short oviducts unite to form a vagina with which
may be associated receptacula seminis (rs\ though more usu-
ally these structures open independently in front of the
genital orifice and may be single, or paired, or in some cases

even three in number. The testes are cy-
lindrical structures whose long, slender, and
frequently-contorted vasa deferentia unite
just before opening to the exterior. A
remarkable copulatory organ is formed by
the terminal joint of the pedipalp of the
male (Fig. 207), which bears upon its inner
surface a process containing a spirally-
coiled tube. This tube opens at the ex-
tremity of the process, and is filled by the
FIG. 207.— PEDIPALP
OP MALE SPIDEK spider with spermatozoa, and during copu-

(after BERTKAU). latiou is inserted into the receptacula semi-
nis of the female.

The males are usually smaller than the females, and their
approaches are frequently resisted by the latter, who en-
deavor to capture and destroy the persistent swains. In the
Attidse a process of courtship has been observed to occur, the
male posturing before the female and displaying to their best
advantage the highly-colored hairs with which the body is
covered. The ova are in many forms (Lycosa} attached to
the under surface of the abdomen, while in others they are
enclosed in a silken cocoon which may either be carried
about by the female or suspended in the webs or deposited in
protected situations.

Two suborders are recognized, according as there are two
pairs of lung-books or only one. The Tetrapnei-imones in-
clude the forms with two pairs of lung-books, among which
are the Trap-door Spiders, Cteniza, already mentioned, and the
Tarantula, Mygale, the largest of all the spiders and reputed
to attack even small birds. The Dipneumones have but a
single pair of lung-books, the majority of living spiders be-
longing to the suborder. Some, such as Epeira, Agelena, Tege-
naria, Theridium, and Segestria, spin webs, while others catch
their prey by their rapid movements (Lycosa) or by suddenly
springing upon it (Attus).

TYPE ARACHNID A.

453

7. Order Acarina.

The Acarina are for the most part srnall forms, many
being almost microscopic, while the largest, the Ticks (lxodes\
do not when at their greatest size exceed a centimeter in
length, the males being much smaller. Some forms, such as
Onbates and Nothrus, live among moss and in similar situa-
tions, while others, such as Hydrachna and Atax, are aquatic.
Many forms are, however, parasitic either upon plants (Te-
tranyclius and Pliytoptus) or on animals, the genus Sarcoptes
being the cause of the disease termed the Itch in man, the
symptoms being produced by the Mites burrowing beneath
the skin. Other forms affect various animals and birds, the
genera Dermaleichus, Analges, etc., feeding upon the feathers

FIG. 208. — A, Sarcoptes scabiei ; B, Demodex phylloides (after CSOKOR from

WRIGHT).

of various birds, while others, such as Demodex (Fig. 208, B\
live ia the hair-follicles or sebaceous glands of the skin, pro-
ducing acnelike pustules. The larvse of many forms which
are non-parasitic in adult life have a parasitic habit, as for
instance the larvae of many of the Water-mites and of the
Harvest-mites (Trombidiuni), while other forms live upon
organic matter of various kinds, as does the Cheese-mite,
Tyroglyphus.

A distinguishing characteristic of the Acarina is the ab-
sence of any segmentation and the fusion of head-thorax and
abdomen to a single mass (Fig. 208, A). The form of the
appendages varies greatly in different genera according to the
use to which they are put. The cheliceree (Fig. 209, Md) are

454 INVERTEBRATE MORPHOLOGY.

frequently chelate, but in parasitic forms are reduced to
stylets enclosed by the fused basal joints of the pedipalps, a
proboscis being thus produced which can pierce the integu-
ment and thus render the juices of the host available as food.
The pedipalps (Mxp) undergo various modifications, being
sometimes long and limblike, sometimes chelate, while their
basal joints may or may not be fused. The four pairs of legs
are generally adapted for walking, and terminate in ungues or
bunches of hairs or, in some parasitic forms, in suctorial
disks, while in the Water-mites they are provided with usually
long bristles along the sides, serviceable swimming-organs
being thus produced. In the genus Demodex the four legs
are reduced to short unjointed structures each provided with
four ungues, while in the Leaf-mites, Phytoptus, which pro-
duce galls on the leaves of various plants, the two pairs of
posterior limbs are reduced to wartlike elevations bearing
bristles, the two anterior pairs being on the other hand five-
jointed.

The chitiuous covering of the body is usually thick and
delicately wrinkled. It usually bears numerous setae and
occasionally also plates or lateral prolongations, as in Ori-
bates and its allies. Dermal glands also frequently occur,
producing oily fluids and sometimes odoriferous secretions.
Spinning-glands opening on the pedipalps occur in Tetrany-
chus, frequently parasitic on the leaves of the Hose, but as a
rule they are not developed.

A pair of stigmata (Fig. 209, st) occurs in many forms,
situated usually near the coxae of the last pair of legs, but
not unfrequeutly they are much further forward, lying near
the basal joints of the pedipalps or even of the chelicerse.
They open into tracheae which branch once, bunches of lateral
tracheae being situated at intervals upon the two branches.
Frequently, however, especially in parasitic and aquatic forms,
both tracheae and stigmata are wanting, as is usually also the
heart. When present (Gamasus, Ixodes) this latter structure
is small, with but a single pair of ostia, and is prolonged an-
teriorly into a slender aorta.

The digestive tract is frequently provided with glands
opening into its anterior portion and supposed to be salivary.

TYPE ARACHNIDA.

455

The mid-gut usually sends off a number of csecal diverticula
which may branch at the ends, and Malpighian vessels,
sometimes one, sometimes a pair, and sometimes many are
usually present, while in addition, in some forms, a rectal
bladder, similar to that occurring in the Aranese, is found.

M"d

an

Fie. 209. — MALE OF Gamasiis tergipes (after WINKLER).

an = anus. st = stigma.

<xe = caecal pouches of intestine. stc = stigma-canal.

go = genital orifice. T = testis.

Md = chelicera. t = tongue.

mg = Malpighian tubules. Vd = vas deferens.

Mxp = pedipalps. I-IV - limbs.

The nervous system, as might be supposed from the con-
centration of the body regions, is composed of a supraoesoph-
geal syncerebrurn and a larger suboesophageal gangliouic
mass from which numerous nerves are given off. Eyes are
usually wanting, or may be present in the form of one
(Ixodes) or two pairs of small apparently simple ocelli.

Coxal glands have been described as occurring at the
bases of the second pair of legs (Oribatidse). The repro-

456 INVERTEBRATE MORPHOLOGY.

ductive organs show much variety iu their arrangement,
being sometimes paired and sometimes united to a single
mass. The single genital orifice is situated far forward, in
some cases even between the basal joints of the second pair
of legs. Numerous accessory structures may be associated
with the ducts, the receptacula semiuis in some forms open-
ing to the exterior quite independently of the oviduct, and
protrusible organs serving for copulation in the male and for
ovipositiou in the female may occur. The Acarina are as a
rule oviparous, though a few forms are viviparous.

Development of the Acarina. — Most of the Acarina whose development
has been traced pass through a series of larval stages. While the young
embryo is still within the egg and sometimes before the appendages have
developed, a cuticular membrane is secreted around it lying between the
embryo and the egg-shell. This is the deutovum, and within it further
development proceeds. In those forms in which it does not appear until
after the appendages are formed a degeneration of these structures takes
place, and the egg-shell may also be thrown off leaving the embryo sur-
rounded only by the deutovum (Trombidium). New appendages now
appear, and the larva hatches out from the deutovum as a six-legged
form, sometimes showing traces of segmentation either in the thoracic
region or in the abdomen. After a certain time a certain amount of de-
generation of the tissues occurs (histolysis) and the appendages again dis-
appear, a chitinous membrane forming around the now almost spherical
body of the larva. A regeneration of the limbs and tissues takes place
within this lafval membrane, and the nymph is formed, resembling the
adult in the number of appendages, but lacking fully-developed repro-
ductive organs. A period of rest, and histolysis again occurs, accom-
panied by the formation of a third cuticular membrane within which the
nymph becomes transformed into the fully-developed and sexually-mature
adult or imago, which finally issues from the membrane.

This complicated process, it is needless to say, has no phylogenetic sig-
nificance, the deutovum indeed being absent in certain forms (Tetrany-
cluis), nor does it seem likely that even the six-legged larva is anything
but a secondary stage which has been developed within the group of the
Acarina. There is no question but that the order represents the culmina-
tion of a divergent line of evolution, perhaps from the Pseudoscorpionida,
and since the separation many of the peculiarities characteristic of the
group have been developed.

Phytogeny of the Arachnida. — There seems little room for doubt but
that the Scorpions among living forms represent most closely the ancestral
Arachnoids, their segmentation being most perfect and their appendages
more numerous than those of other forms. It is through the Scorpions

TYPE AEACHNIDA.

457

accordingly, that relationships to other forms must be looked for, and a
comparison of them with Limulus reveals similarities of structures so
numerous and so detailed that the conclusion is unavoidable that both are
to be traced back to a common ancestor. Thus the cephalothoracic ap-
pendages in both are identical in number, and, so far as the first two pairs
are concerned, in general structure also, while the genital opercula of the
.Scorpions are comparable in their relation to the genital orifices to the
opercula of Limulus, and the pectines to the first pair of abdominal ap-
pendages. The remaining abdominal appendages of Limulus, which are
branchiate, seem at first sight to be unrepresented, but the embryo-
logical investigation of the Scorpions appears to indicate that they are
represented by the lung-books, which bear no little resemblance to the
branchial lamellae of Limulus, and the
conversion of one set of organs into the
other may be supposed to have been brought
about by the formation behind each pair
of abdominal appendages of an invagina-
tion, which, deepening, has carried in with
it the branchial lamellae, the original an-
terior surface of the appendage forming
the ventral wall of the body beneath the
lung-sac, while the lamellae project into the
sac for its ventral surface (Fig. 210). In
the general form of the body Limulus
corresponds fairly well with the Scorpions,
the cephalothoracic regions being strictly FJG_ m_DlAOKA^;F

OP LUNG-BOOKS (after KINGS-
LEY).

/= indifferent stage.

L = Limulus stage.

A — Arachuiclau stage.

comparable, as is also the terminal spine

with the sting ; the abdomen, however, in

the branchiate form has a smaller number

of segments which are all fused, a difference

readily explained by the probable derivation

of both forms from Eurypterus-like ancestors in which the abdomen

possessed a relatively large number of distinct segments, and even showed

indications of a differentiation into a pra?abdomen and a postabdomen

(see Fig. 198).

In the internal structure quite as striking similarities are to be found
in the presence of an endosternite in both groups and of coxal glands in
connection with the fifth pair of appendages, in the tendency towards the
concentration of the postoral ganglia, and in the .invaginate origin of the
median eyes, to mention but a few points.

The Arachnida are accordingly to be traced back to Limulus or Euryp-
terus-\\V.Q ancestors, and through these finally to the Entomostraca, perhaps,
a Crustacean ancestry being clearly indicated. As to the relationships of
the various orders little that is definite can be said, differentiations having
taken place along different lines in the various orders, so that while the

458 INVERTEBRATE MORPHOLOGY.

Pedipalpi are more primitive as regards the number of abdominal seg-
ments and their distinctness than the Araneas, yet the latter and especially
the Tetrapneumones show a much more primitive condition of the respira-
tory organs. With regard to these organs it may be stated that the con-
dition in which they are represented by bunches of uubrauched tracheae
is more primitive than that in which they are branching tubes, the
bunched condition being probably derived by a modification of original
lung-books.

TYPE ARACHNIDA.

1 . Order Scorpionida. — Abdomen segmented and differentiated into prae-

abdomen and postabdomen ; postabdomen terminating in poison-
spine ; pedipalps chelate ; two pairs of abdominal appendages ;
four of stigmata and lung-books. Euscorpius, £ nth us.

2. Order Pseudoscorpionida. — Abdomen segmented but not differentiated ;

no terminal spine ; pedipalps chelate ; no abdominal appen-
dages ; two pairs of stigmata opening into tracheae ; first pair
of legs adapted for locomotion. Chelifer, Obisium, Chernes.

3. Order Solifiigce. — Head separated from thorax with three segments ;

abdomen segmented but undifferentiated ; no terminal spine ;
pedipalps palplike ; three pairs of stigmata leading into tracheae.
Galeodes, Solpuga.

4. Order Pedipalpi. — No distinction of head and thorax ; abdomen seg-

mented, and either undifferentiated or with three small segments
terminated by a multiarticulate flagellum ; pedipalps leglike or
subchelate ; two pairs of stigmata and lung-books ; first pair of
legs elongated and palplike. Phrynus, Thelyphonns.

5. Order Phalangida. — Abdomen segmented but undifferentiated and

without appendages or terminal spine ; pedipalps leglike ; one
pair of stigmata leading into tracheae ; no spinning-glands.
Leiobii'iium, Phalangium, Opilio, Gonyleptus, Cyphophthalmus,
Gibbocellam.

6. Order Araneoe.— Abdomen unsegmented and with two or three pairs of

rudimentary papillalike appendages bearing the openings of
ducts of numerous spinning-glands ; abdomen not fused with
cephalothorax ; pedipalps long and palplike or leglike.

1. Suborder Tetrapneumones. — With four stigmata opening into

sacs containing lung-books. Mygale, Cteniza.

2. Suborder Dipneumones. — With four or three stigmata, the anterior

pair opening into sacs with lung-books, the posterior one or
two with tracheae. Epeira, Agelena, Tegenaria, Theridium,
Seyestria, Attus, Lycosa.

7. Order Acarina. — Abdomen unsegmented, without appendages, and

fused with the cephalothorax ; pedipalps sometimes long and
leglike, sometimes chelate ; stigmata wanting or present as a

TYPE ARACHNID A. 459

single pair leading into tracheae ; many forms parasitic ; fre-
quently with complicated metamorphoses.

Nonparasitic, or parasitic only iii larval stage ; terrestrial. Ori-
bates, Nothrus, Trombidium.

Aquatic. Hydrachna, Atax.

Living on organic matter. Tyroglyphus.

Parasitic on animals. Demodex, Sarcoptes, Dermaleichus, Anal-
ges, Gamasus, Ixodes.

Parasitic on plants. Tetranyclms, Phytoptus.

LITERATURE.

GENERAL.

H. Grenacher. Untersuchungen ilber das Sehorgan der Arthropoden. Gottingen,
187(J.

E. R. Lankester. Limulus an Arachnoid. Quarterly Jouru. Microscop. Sci-
ence, xxi, 1881.

J. Macleod. RecJierches sur la structure et la signification de I'appareil respira-
toire des Arachnides. Archives de Biologic, v, 1884.

R. Sturany. Die Coxaldrilsen der Arachnoiden. Arbeiten a. d. Zoolog. Inst.
Wien, ix, 1891.

J. S. Kingsley. The Embryology of Limulus. Part II. Journ. of Mor-
phology, vni, 1893.

SCORPIONIDA.

L. Dufour. Histoire anatomique et physiologique des Scorpions. Memoirs
Acad. Sciences. Paris, xiv, 1856.

E. R. Lankester. On the Coxal Glands of Scorpio, etc., and the Brick-red
Glands of Limulus. Proceedings of the Royal Society, xxxiv, 1884.

G. H. Parker. The Eyes in Scorpions. Bulletin of the Museum of Compar-
ative Zoology, xni, 1879.

W. Patten The Origin of Vertebrates from Arachnoids. Quarterly Journal
of Microscop. Scieuce, xxxi, 1890.

M. Laurie. The Embryology of a Scorpion (Euscorpius italicus). Quarterly
Journ. Microscop. Science, xxxi, 1890.

PSEUDOSCORPIONIDA.

A. Croneberg. Beitrag zur Kenntniss des Baues der Pseudoscorpione. Bulletin

Soc. Imp. Naturalistes Moscou, 11, 1888.

SOLIFUG.E.

L. Dufour. Anatomie, pJiysiologie, et histoire naturelle des Galeodes. Me-
moires Acad. Sciences, Paris, xvii, 1858.

PHALANGIDA.

B. Rossler. Beitrage zur Anatomie der Phalangiden. Zeitschrift fur
wissensch. Zool., xxxvi, 1882.

460 INVERTEBRATE MORPHOLOGY.

C. M. Weed. A Descriptive Catalogue of the Harvest -spiders ( Phalangiida) of
Ohio. Proceedings United States National Museum, xvi, 1893.

ARANE^E.

E. Keyserling. Die Spinnen Amerikas. Niiruberg, 1880-91.

G. W. aiid E. G. Peckham. North American Spiders of the Family Attidce.

Transactions of the Wisconsin Acad. Sciences, 1888.
H. C. McCook. American Spiders and their Spinning Work. Philadelphia,

1889-90.
J. H. Emerton. Papers in the Transactions of the Connecticut Academy, vn,

1889, aud vui, 1891.
G. Marx. Papers in the Proceedings of the Entomological Society of Wash-

iugtou, 1891, and in the Proceedings of the U. S. National Museum, xn,

1890.
W. Schimkewitsch. Etude sur Vanatomie de L'Epeire. Auuales des Sciences

Naturelles, 6me ser., xvii, 1884.
Ph. Bertkau. Ueber den Verdauungsapparat bei Spinnen. Archiv. fiir mi-

kroskop. Anatomie, xxiv, 1885

- Beitrage znr Kenntniss der Sinnesorgane bei Spinnen. 1. Die Augen.
Archiv fiir mikroskop. Anatomie, xxvn, 1886.

E. L. Mark. Simple Eyes in Arthropods. Bulletin Museum of Comparative

Zoology, xm, 1887.
A. T. Bruce. Observations on the Embryology of Insects and Arachnids.

Baltimore, 1887.

ACARINA.

G. Haller. Zar Kenntniss der Tyroglyphen und Verwandten. Zeitschr. fiir

wissensch. Zoologie, xxxiv, ]880.
H. Henking. Beitrage zur Anatomie, EntwicJclungsgescliichte und Biologie von

Trombidium fuligiuosum. Zeitschr. fiir wissensch. Zoologie, xxxvn,

1882.

A. D. Michael. British Oribatidce. London, 1884.
A Nalepa. Anatomie der Phytopten. Sitzungsber. Akad. wissensch. Wien,

xcv, 1887.
W. Winkler. Das Herz der Acarinen nebst vergleichenden Bemerkungen iiber

das Herz der Phalangiden und Chernetiden. Arbeiten a. d. zool. Ins.

Wien, vii, 1888.

- Anatomie der Gamasiden. Arbeiten a. d. zoolog. Inst. Wien, vn,
1888.

APPENDIX TO THE ARACHNIDA.

There are three orders which show a certain amount of
affinity to the Arachuida, but which are not so closely related
as to warrant the actual association of them with the orders
which have been assigned to that type. They will be de-

TYPE ARACHNIDA. 461

scribed here, and are the orders of the Pentastomidce, the
Pycnogonida, and the Tardigrada.

Order Pentastomidae.

The Pentastomidse are all parasitic, living in the adult
stage in the lungs or nasal cavities of various animals, one
species, Pentastomum tcenioides, occurring in the nasal cavities
or sinuses of dogs and wolves, while several species have been
found in the lungs of different species of snakes (Fig. 211).
They are all elongated worrnlike forms, some-
times slightly flattened and usually distinctly
annulated, the annuli, however, not repre-
senting a metamerism. The anterior end of
the body is rounded and bears on the ventral
surface the mouth, upon each side of which
is situated a pair of strongly-recurved hooks
(h] supplied with special muscles and serving
for the attachment of the animal to the tissues
of the host. "With the exception of these
hooks no appendages are present.

The body is covered by a cuticle secreted
by the ectodermal cells (hypoclermis), be-
neath which lies a layer of circular niuscle-
fibres, and beneath these again a layer of
longitudinal muscles. The coelom is ample
and is traversed by dorso-ventral muscle-
bands, which divide it into a central com-
partment containing the various organs, sus- IG' ~ i. — rena-

-

pended by mesenteries, and two lateral ones. lum pEMALE (after
There is no heart or circulatory apparatus, SPENCER).

and tracheae or other respiratory organs are r nooks.

... go = genital orifice.

also wanting.

The digestive tract is a straight tube extending through
the body from the mouth to the terminal anus, giving off no
lateral diverticula throughout its course. The nervous sys-
tem (Fig. 212, no] consists of a ganglionic mass lying below
the oesophagus, a comparatively small commissural ring
passing round that portion of the digestive tract, without,

462

INVERTEBRA TE MORPHOLOG T.

however, possessing any ganglionic enlargement which can
be termed a cerebrum. Various nerves are given off from
the mass, two of which extend backwards throughout nearly
the entire length of the body. The only sense-organs pres-
ent are a number of small papillse on the anterior portion of
the body, which are probably tactile in function.

Glandular organs are highly developed. Scattered over
the surface of the body are numerous flask-shaped glands,
apparently ectodermal in origin, while lying in the co3lom on
each side of the mid-gut and extending back almost to the
posterior end of the body are two long csecal tubes, a glan-
dular structure being also connected with them anteriorly.
These glands open in the vicinity of the hooks and have
hence been termed the hook-glands (Fig. 211, kg), and it has
been suggested that they secrete a fluid which serves to keep
the blood which the parasite ingests from coagulating, being
thus similar to the glands in the pharynx of the Leeches

FIG. 212. — DIAGRAM OF STRUCTURE OF FEMALE Pentastomum (after SPENKCR).
go — genital orifice. od — oviduct.

hg — hook-glaud. ov = ovary.

i — intestine. rs = seminal receptacle.

ng — nerve-ganglion. ut = uterus.

which serve the same purpose. Unless the ectodermal glands
are excretory, no special organs for the carrying on of that
function occur.

The Peutastomidse are bisexual, the male being smaller
than the female, and recognizable by the situation of the geni-
tal orifice (Fig. 211, go), which is near the anterior end of the
body, while in the female it is near the posterior end. The
ovary and testis are both unpaired organs situated beneath
the dorsal surface of the body and extending almost its entire
length. Anteriorly a pair of oviducts (Fig. 212, od) arise
from the extremity of the ovary (ov) and pass downwards and

TYPE ARACHNID A. 463

forwards towards the ventral surface, ou nearing which they
unite to form a long coiled tube, the uterus (ut), which passes
backwards to the genital orifice, and just where the two ducts
unite they have opening into them a pair of pyriform seminal
receptacles (rs). The vasa deferentia are also paired, and
arise at the anterior end of the testis, passing veutrally
towards the genital pore, uniting before they reach it and
dilating to form a complicated iutromittent organ, from which
two tubes with muscular walls and containing spermatozoa
project backwards and serve as ejaculatory ducts for the ex-
pulsion of the spermatozoa through the intromittent organ.
The only genus belonging to the order is Pentastomum.

Development of the Pentastomidce. — During the life-history of & Pen-
tastomum it passes through a marked metamorphosis associated with a
change of hosts, recalling what occurs in the Cestoda. The ova are passed
to the exterior with the excreta of the host, or, in the case of the clog,
with the mucous discharge from the nasal passages, and the embryo which
hatches out is a decidedly Mitelike form, possessing, however, only two
pairs of legs terminating in uugues. No other appendages are present,
but the embryo is provided anteriorly with a boring apparatus. If this
larva of P. tcunioides, the parasite of the dog, succeeds in gaining entrance
to the digestive tract of a rabbit or cat, for instance, it bores through the
wall of the intestine and, reaching the liver, encysts itself. Within the
cyst it undergoes several moults, finally assuming a condition similar to
the adult except that each annulus bears a circle of hooks. Leaving the
cyst, then, it wanders through the tissues of the host, and if while it is in
this condition the host is eaten by a dog, it adheres to the mucous mem-
brane of the mouth of the latter, and makes its way into the nasal passages,
there moulting again, losing the ring of hooks and assuming the adult form.

The principal reason for supposing Pentastomum to be related to the
Arachnids is the occurrence of the four-legged larva, which resembles, so
far as its external form is concerned, a Mite. The internal structure is
very different, however, although certain Arachnid features are indicated ;
but it is evident that these forms must have undergone an enormous de-
parture from the ancestral form during which the remarkable life-history
and peculiar structure have been acquired. The parasitic habits of many
Mites, and the general similarity of the body form of Demodex to that of
Pentastomum, suggest the Mites as the ancestors of the latter, a theory
which is as plausible as any other which can at present be suggested.

Order Pycnogonida.

The Pycnogonida are exclusively marine in habitat, and
vary considerably in size, the smaller forms, such as Tanysty-

464

INVERTEBRATE MORPHOLOGY.

lum, being only about a millimetre iu breadth, while the
purple Pkoxichilidium measures over three millimetres from
tip to tip of the legs, and the deep-sea form Collossendeis has
a span of over sixty centimetres. The body proper is compar-
atively small, the four pairs of long legs which arise from the
thorax being exceedingly conspicuous, a feature which has

FIG. 213. — Phoxichilidiiim maxillare (after MORGAN).

suggested the term Pantopoda sometimes applied to the
group. Anteriorly there is a well-marked proboscis carrying
the mouth at its anterior end, and at the base of this there
arise the cheliceme, which are rather short chelate limbs.
The next segment of the body succeeding that which bears
the chelicerae bears upon its dorsal surface the eyes, and may
be regarded as a fusion of three segments since it bears three
pairs of appendages. The most anterior of these are slender

TYPE ARACHNID A. 465

jointed palps; the second pair, wanting in the females of
some species, but always present in the males, arise from the
ventral surface of the segment, and are curved jointed struc-
tures serving to carry the ova ; while the third pair are ex-
ceeding long jointed walking-legs. The next three segments
also bear long walking-legs, the last o'ne having attached to
it the usually unsegmeuted rudimentary abdomen.

The body and the appendages are encased in a well-de-
veloped chitinous cuticle, and there are no indications of
special respiratory organs. The heart lies immediately be-
neath the dorsal integument and is a simple tubular organ
with from two to three pairs of ostia.

The portion of the digestive tract which lies within the
proboscis is lined with chitin and opens behind into an
elongated mid-gut, from which long diverticula extend out into
the chelicerse and the proboscis and into the walking-legs,
sometimes reaching even into the terminal joints of the latter.
A short hind-gut leads to the anus at the tip of the abdomen.

The nervous system consists of a supracesophageal gan-
glionic mass, from which arise the optic nerves and those for
the chelicerse, as well as certain nerves passing to the pro-
boscis. Connected with this brain by circumoesophageal com-
missures is a ventral chain consisting of five pairs of ganglia,
the first pair of which is really formed by the fusion of two
pairs, distinct in the embryo, and innervates the palps and the
ovigerous legs, while the four pairs of walking-legs are sup-
plied by the remaining four pairs. Finally one or two small
ganglia also occur, innervating the abdomen. The eyes are
four in number, situated at equal intervals upon a small
domelike elevation on the dorsum of the first thoracic seg-
ment, which, it is to be remembered, is compound. Each eye
is covered by cuticle, sometimes thickened so as to form a
lens, below which is a layer of cells forming the corneal or
cuticular hypodermis. Below this comes a thick layer com-
posed of retinal elements with nuclei in their outer portions
and rodlike bodies towards the inner ends where the}r rest
upon a layer of pigment.

These eyes recall the postbacillar eyes of the Arachnids by their struct-
ure, but show one remarkable peculiarity, i.e., a distinctly bilateral ar-

466 INVERTEBRATE MORPHOLOGY.

rangement both of the corneal hypodermis and of the retinal elements, a
distinct raphe being observable upon the inner surface of the eye, the reti-
nal elements being arranged on either side of it. Such a condition as this
cannot readily be explained by a simple unilateral inversion such as was
described as probably occurring in Arachnidan eyes; it suggests rather an
inversion of two sides of a primitive optic cup, the posterior wall at the
same time forming the pigmented layer of the eye. Whether the Arachnid
eye is not also traceable to such an arrangement, all traces of the original
raphe being lost, is a question, though at present it seems more probable
that it has been produced by a suppression of the inversion of one side of
the cup.

Glands, occurring in the palps and ovigerous legs, have
been regarded as excretory in function, but no Malpighian
tubules or coxal glands seem to exist, though an homology of
the excretory glands just mentioned and of glands occurring in
the walking-legs of the males with the latter is not impossible.
The Pycuogonids are bisexual, the reproductive organs lying
in the thorax and sending out branches into the walking-legs,
on the fourth joints of one or more of which they open. As
already stated, the male carries the eggs upon his ovigerous
legs, fastening them as they are extruded by the female by
means of the excretion of the glands occurring upon the
walking-legs.

Development and Affinities of the Pycnogonida. — The young Pycno-
gonid leaves the egg as a six-limbed embryo, which recalls, in a general way,
the nauplius of the Crustacea, and indeed has suggested a derivation of
the Pycnogouids from that group. The resemblance is, however, but
superficial, important differences being found in the structure of the eyes
and in the absence of an anus, to say nothing concerning the details of the
early development. On the other hand these last, as well as the structure
of the eyes, recall the Arachnids, and it seems most probable that the
Pycnogouids are to be regarded as having descended from ancestors which
might have been included in the type Arachnida.

Order Tardigrada.

The Tardigrada are small forms not exceeding a milli-
metre in length, with an un segmented body provided with
four pairs of short conical appendages tipped with claws, the
last pair being situated at the posterior extremity of the body.
The body is covered by a cuticle secreted by the subjacent
hypodermis, below which and traversing the ccelom is a well-

TYPE ARACHNID A.

467

ce

developed system of muscle-bands. There are no special
organs either for respiration or circulation.

The mouth, surrounded by papillae (Fig. 214, p), lies at the
anterior extremity of the body, and leads into a tubular
mouth-cavity containing, imbedded in
its walls, a pair of chitiuous or partly
calcareous teeth, and receiving the
ducts of two glands (sg) which have
been regarded as salivary or perhaps
poisonous in function. Behind, this
cavity opens into a muscular pharynx
(pli) which is connected by a short
oesophagus with the mid-gut. At the
junction of this with the rectum or
hind-gut is a pair of caeca! diverticula
(?(/), probably Malpighian tubules, and
into the hind-gut there also open the
ducts of the reproductive organs, the
hind- gut thus serving as a cloaca. It
opens on the ventral surface of the
body a short distance from the pos-
terior extremity and therefore in front
of the last pair of appendages.

The nervous system consists of a
supraoesophageal ganglion (ce) united
with a chain of four pairs of ventral
ganglia. No special sense-organs occur
except two eyes situated at the sides
of the head. The sexes are distinct,
the reproductive organ being unpaired
and opening into the cloaca, into which

opens also in both sexes an unpaired ph ''

sg — sahvary(?) glands.

accessory gland.

The Tardigrada occur in water usually, especially in such
locations as the gutters on the roofs of houses, though some-
times found also among moss. The group contains but a
small number of genera, of which Macrobiotus is perhaps the
most common.

FIG. 214. — DIAGRAM OF
STRUCTURE OP Macro-
biotus Hufelandii (com-
bination of figures by
PLATE).
an = anus.

ce = cerebral ganglion.
dg = dorsal gland.
Ig = lateral gland or Mal-
pighian tubule.
m = muscle.
ov = ovary.
p = papillae.

468 INVERTEBRATE MORPHOLOGY.

Affinities of the Tardigrada. —The embryological history of these
forms has not yet been sufficiently studied to allow of any definite conclu-
sions as to their affinities. The presence of four pairs of limbs has usually
been regarded as pointing to a relationship with the Acarina, but the ab-
sence of all mouth-appendages, the structure of the legs, and the position
of the last pair with regard to the anal opening, not to mention the peculi-
arities of the internal organization, are opposed to any close relationship
with the Arachnida. The Tardigrada must be considered as holding an
independent position, without distinct indications of relationship with any
of the types, until further information as to their developmental phenomena
has been secured.

LITERATURE.

PENTASTOMIDA.

R. Leuckart. Ban und EntwicklungsgescJiichte der Pentastomen. Leipzig uud
Heidelberg, 1860.

C. W. Stiles Ban und Entwicklungsgeschichte wn Pentastomum proboscideum
und P. subcylindricum. Zeitsckr. filr wissensch. Zoologie, LII, 1891.

A. B. Spencer. T7ie Anatomy of Pentastomum teretiusculum (Baird). Quar-
terly Journal of Microscop. Science, xxxiv, 1892.

PYCNOGONIDA.

E. B. Wilson. TJie Pycnogonida of New England and Adjacent Waters. Re-
port of the U. S. Commissioner of Fish aud Fisheries for 1878. Wash-
ington, 1880.

A. Dohrn. Die Panlopoden des Golfes von Neapel. Fauua und Flora des
Golfes von Neapel. Monographic, in, 1881.

P. P. C. Hoek. Report on the Pycnogonida. Scientific Results of the Voyage
of H.M.S. Challenger. Zoology, in, 1881.

T. H. Morgan. A Contribution to t7te Embryology and Phytogeny of the Pycno
gonids. Studies for the Biol. Laboratory, Johns Hopkius University, v,
1891.

TARDIGRADA.

L. Plate. Beitrdge zur Naturgeschichte der Tardigraden. Zoolog. Jahrbiicher,
Anatom. Abtheilung, in, 1888.

TYPE TRACHEATA. 469

CHAPTEK XV.

TYPE TRACHEATA.

THE Tracheata are, like the Arachnida, essentially terres-
trial forms, for, though a few Iiisects have adapted themselves
to an aquatic mode of life, they are nevertheless air-breathers,
living either at the surface of the water or coming to the sur-
face from time to time to renew the air contained in the
tracheae which ramify through the body and serve as respira-
tory organs. However, a few Insect-larvae have acquired the
power of extracting oxygen from the water by branch ia-like
processes of the body, but, even in these cases, tracheae form
the organs by which the respiration is carried on, the branchiae
being richly supplied with them.

The body is distinctly segmented (except in Peripatus,)
and is covered by a chitiuous cuticle secreted by the ecto-
derrnal cells, which constitute the so-called hypodermis. The
appendages are usually uniramous, and with few exceptions
(Peripatus) are jointed. The anterior pair in all cases are
more or less elongated multiarticulate structures provided
with sense-hairs, and are situated prseorally, while of the
remaining pairs, varying in number in different groups, the
most anterior pair is specialized to serve as mandibles, while
the succeeding one or two pairs usually form maxillae. Numer-
ous glands of varying function are developed in the hypo-
dermis, the most interesting of which are the crural glands,
well developed in Peripatus, and represented more or less
perfectly in certain other forms. In addition to these, glands
which secrete an acrid or offensive fluid (repugnatorial glands)
are frequently present, as well as others which secrete waxy
substances, or even in some cases silk.

The ccelom except in Peripatus is lacunar throughout, pos-
sessing no definite walls, and is traversed in various directions
by muscles, serving to flex or extend the body and to move

470 INVERTEBRATE MORPHOLOGY.

the appendages. A marked difference between the Tracheata
and the Arachnida is the universal absence of an eudo-
sternite, a structure of considerable phylogenetic significance
in the latter group. A heart is invariably present, lying
above the intestine, and situated in a pericarclial sinus incom-
pletely partitioned off. In the majority of forms the parti-
tion is composed of a varying number of triangular muscles,
the alar muscles, which are attached by their bases to the
walls of the heart, and by their apices to the body-wall.
While at rest they are somewhat vaulted, the convexity being
dorsal, and on contraction flatten down, thus enlarging the
sinus and causing a flow of blood into it. The heart (Fig.
227, A) is elongated and imperfectly divided into a series of
chambers, separated by pairs of valves which allow the blood
to flow from behind forwards but not in the reverse direction,
the heart being closed behind. Ostia are present in the
lateral walls to allow of the entrance of blood into the heart-
chambers, whence it is propelled through very short arteries
which open widely into the lacuuar spaces of the coslom. In
many forms a ventral sinus surrounds the ventral ganglionic
nerve-chain, the blood flowing in it from before backwards,
but with this exception definite vessels are wanting. This is
compensated for by the rich branching of the tracheae, which,
as stated, serve as respiratory organs and convey air to all
parts of the body ; the air is in fact brought directly to the
tissues, instead of being carried to them by the blood from
limited portions of the surface of the body. The blood is
usually colorless, but in some cases is of a bright yellow or
green color, owing to pigment contained in the plasma, and it
contains in all cases colorless amoeboid corpuscles.

The tracheae (Fig. 215, tr) communicate with the exterior
along the sides of the body by a varying number of pairs of
stigmata (st), and may either consist of bunches of uubranched
tubes connected with each stigma, or of a number of richly-
branching tubes, each one arising from a separate stigma and
anastomosing in some cases through some of its branches
with the tubes from other stigmata. Each stigma is usually
provided with an apparatus by which it may be closed, and
in the Insects the air is expired from the tracheae by the con-

TYPE TRACHEATA.

471

At

traction of certain dorso-ventral muscles of the abdomen,
which cause a compression of the organs in that region of the
body, inspiration following on their relaxation and the conse-
quent re-expansion of the abdomen. In structure the tracheae
are simply to be regarded as invaginations of the body-wall,
and consist of a single layer of cells continuous with the hypo-
dermis of the body, lined within

—that is to say, on the surface
with which the air is in contact

—with chitin, which is thick-
ened in such a way as to form
a spiral baud extending along
the tube and serving as a spring
to keep its walls apart.

The digestive tract is in
most groups a straight tube,
but in Insects (Fig. 227) it may
be coiled in a more or less com-
plex manner and differentiated
into several parts. Glands of
various kinds are usually asso-
ciated with it, salivary glands
(Fig. 227, sg) opening into the
anterior portion and Malpighiau
tubules (mv), in connection with
the posterior portion, being
the most constant in occur-
rence. It is to be noted that
the fore-gut and hind-gut are
ectodermal in origin, and that
the Malpighiau bodies arising
as outgrowths from the hind-
gut are also ectodermal, differing
thus in origin from the similarly-
named organs of the Arachnida, which are apparently of
endodermal origin, arising from the mid-gut. In function
both organs are similar, the Malpighian bodies of Tracheates
being excretory.

The nervous system in the less-differentiated members of

FIG. 215. — FIGURE SHOWING THE
DISTRIBUTION OF TRACHEAE IN

Aphis pelargonii (after WITLACZIL).

At = autennse.
g = gland-duct.
st = stigma.
tr — trachea.
1, 2, 3 = thoracic appendages.

472 INVERTEBRATE MORPHOLOGY.

the type consists of a supraoesophageal gangliouic mass, con-
nected by circumcesophageal commissures with a chain of
ventral ganglia, a pair of ganglia corresponding typically with
each segment. In the Insecta (Fig. 228) more especially,
however, considerable concentration occurs, a number of the
postoral ganglia, or, in some cases, all of them, fusing to a
single mass. A well-developed stoinatogastric or sympathetic
nervous system occurs in all forms, arising from the supra-
cesophageal gauglionic mass by two trunks, which unite to
form a single nerve, passing to the digestive tract, and in
some cases provided with gaugliouic enlargements both
paired and unpaired.

Sense-organs of various kinds are well developed in the
Tracheata, with the exception of Peripatus, in which the
only definite organs of special sense are the eyes. In other
forms the antennae and other portions of the body are pro-
vided with hairs connected with nerves and serving as tactile
organs, and setae situated upon the mouth-parts and associated
with peculiar nerve-endings have been supposed to represent
organs of taste, and others again, on the antennae, olfactory
organs. 'Eyes are very generally present. In Peripatus and
most Myriapoda simple ej^es or ocelli are alone present ; iii
Peripatus they resemble closely in structure the eyes of the
Annelids or Mollusca (e.g. Haliotis, see Fig. 127), but in the
Myriapods and Insects they are usually more complicated.
Thus in a young larva of Acilius (Fig. 216, A), a water-
beetle, the chitiu is thickened to form a cornea (?) which lies
over a depression of the hypoderinis, the cells at the bottom
of which are modified to form a retina, each being continuous
at its inner end with the optic nerve (n), while at its outer
end it bears a layer of chitiu (r). The cells of the lip of the
depression have converged together so as to meet beneath the
cornea, which is indeed formed by these cells, and a cavity is
thus enclosed into which there protrude from among the
retinal cells large cells (HKJC) with chitiu deposited on their
adjacent surfaces. In a later stage (Fig. 216, B) the lips of
the depression have united, a continuous corneal hypodermis
(vb) being thus produced ; pigment has been deposited in the
lateral cells, and the retinal cells, pigmeuted near their outer

TYPE TRACHEATA.

473

ends arid in continuation with tlie optic nerve, have developed
distinct rods (r) at their outer ends.

In the Insecta and occasionally in the Myriapotla (Scuti-
gera) there are in addition to these simple ocelli compound
eyes, situated at the sides of the head and similar in structure
to the compound eyes of the Crustacea. Each of the oni-
matidia of which the eye is composed, and there may be

FIG. 216. — SECTIONS THROUGH AN OCELLUS OF A LARVA OF AciMus IN (A) A
VERY YOUNG AND IN (B) AN OLDER SPECIMEN (after PATTEN).
ir — inverted rods. n = nerve.

I = cornea. pg = pigment,

mgc = median giunt cells. r = rods.

vb = vitreous body.

several thousand of them in each eye, consists of an external
cornea (Fig. 217, co), usually more or less hexagonal in outline,
giving the eye a faceted appearance. Beneath the cornea
are two cells which secrete it and form the corueal hypoder-
mis, and below these again come four cells, the crystalline-
cone cells, which may (euconous eyes) or may not (aconous
eyes) manufacture a crystalline cone (c), and finally beneath
these is a circle of seven retinular cells (four in Lepisma),
each one of which is pigmented and manufactures a portion
of the chitinlike rhabdoni (rh) which they surround ; these
cells are probably continuous at their inner ends with the
optic nerve. Additional pigment-cells (pg) separate the

474

INVERTEBRATE MORPHOLOGY.

various ommatidia.

co-

L-Rh

Other sense-organs occur in the various
groups, but may more satisfactorily be
considered in the special descriptions
of these groups.

True uephridia similar to those of
the Annelids occur in Peripatus, but in
the Myriapods and Insects they are en-
tirely wanting, their place as excretory
organs being taken by the Malpighian
tubules. The Tracheata are bisexual,
the reproductive organs being typically
paired and opening to the exterior by
ducts, which may unite before reaching
the genital orifice. Accessory struc-
tures, such as a bursa copulatrix for
the reception of the penis and a recep-
taculum seininis occur in the female, and
vesiculse serniuales and accessory glands
in the male. The region of the body-
wall in the vicinity of the reproductive
orifice is in the Insects frequently in-
vaginated, adding a still greater com-
plication, and furthermore the terminal
portion of the duct in the male is
frequently capable of being evagiuated
and thus serving as a penis, while

FIG. 217. — OMMATIDIUM . ..

OF EYE OF Musca (after integumentary elevations or processes

of the last abdominal segment form
ovipositors in the females.

c = crystalline cone.
co = coruea.
Pff = pigment cells.

r = retinula-cells.
Rk = rhabdom.
T = trachea.
tv = tracheal dilatation

I. CLASS

This interesting group contains but a
single genus, Peripahts, which has, how-
ever, a wide distribution, species being

found in the West Indies and South America, at the Cape of
Good Hope and in New Zealand, thus indicating an original
wide distribution of the genus which has become extinct
except in these few widely-separated regions.

Peripatus is an elongated c}Tlmdrical form, measuring in

TYPE TE ACHE AT A. 475

the Cape species (Fig. 218) from about five to six and a half
centimetres in length, and is found beneath stones or bark or
amongst decaying wood. The body-wall is finely anuulated,
the aunuli not, however, corresponding to segments, and the
cuticle is thin, small papillae being scattered all over the
surface of the body, each terminating in a short bristle.
The head is but poorly marked off from the rest of the body
and bears a pair of many-jointed antennae, at the base of each
of which towards the sides of the head is situated an eye. The
mouth lies in the middle of the ventral side of the head and
is surrounded by numerous papillae, and within its cavity is
situated a pair of jaws furnished with strong chitinous sickle-
shaped teeth. These jaws really represent the second pair of

FIG. 218. — Peripatus capensis (after MOSELEY from BALFOUB).

appendages, the third pair being represented by two short
papillae lying at the sides of the head and having at their tips
the openings of a pair of glands which extend far back into
the body-cavity and from which, when the animal is irritated,
there is violently emitted a sticky fluid, whence the glands
have been termed the slime-glands (Fig. 219, sd). There is
no division of the trunk into thoracic and abdominal regions,
and it bears a number, varying according to the species from
seven to twenty-one pairs, of ambulatory appendages, each of
which consists of a proximal stouter and somewhat conical
portion bearing rings of papillae and a more slender short
distal portion which bears at its tip a pair of claws (ung-ues).
These limbs are unsegmented, differing in this respect from
those of the Myriapods and Insects, and are also soft owing
to the thinness of the cuticle, a feature which has suggested
the name Malacopoda formerly applied to the type, as the
presence of the terminal ungues has suggested the term
Onychophora. The anus is situated at the posterior extremity
of the body and has on either side of it the anal papillae
which represent the last pair of limbs.

476

INVERTEBRATE MORPHOLOGY.

m bundles, two of
are situated dorsally,

ven-

Beneath the thin cuticle is situated the hypodermis, and

beneath this a well- developed
dermal muscular system re-
calling that of the Annelida,
being composed of an outer
layer of circular muscles, be-
low which are diagonal fibres,
and below these again strongly-
developed longitudinal muscles

bundles,
which

two laterally, and three
trally. In addition to these,
dorso - ventral bands occur
passing across the ccelom and
dividing it into three chambers,
one median and two lateral,
and special muscles are also
present for moving the limbs.

The coelom is tolerably
capacious and consists of two
portions. The larger portion
is divided by partitions into
several subordinate cavities
(Fig. 220), and is lined through-
out by a peritoneal epithelium
which covers the various or-
gans. From it are, however,
separated certain cavities with
definite walls, Avhich stand in
relation to the uephridia and
the reproductive organs and
will be spoken of in connection
with these organs. A heart
(Fig. 220, Ji) of a tubular form
extends throughout almost the
entire length of the body, lying
in a pericardial space (Fig. 220, pc] incompletely separated
from the rest of the coelorn by a fenestrated transverse par-

FIG. 219. — FIGURE SHOWING THE
STRUCTURE OF A FEMALE Peripatus

(from HERTWIG).

rt = anus.
at = antennae.
bm = ventral nerve-cord.

d = intestine.
go = opening of reproductive organ.

o = ovary.
og = brain.

p = pharynx.
sd = "slime-glands.

so = nephridia.

sp — salivary glands.

tr = tracheae.

u = uterus.

TYPE TRACHEATA. 477

tition. A pair of ostia are situated ou the dorsal surface of
the heart in each inetaroere, and pass the blood into the heart
from the pericardia! space. Respiration is performed by
tracheae (Fig. 219, tr) consisting of slender unbranched tubes
which arise in bunches from stigmata, either scattered ir-
regularly over the surface of the body in considerable
numbers or else arranged, as in P. capensis, somewhat imper-
fectly in two rows upon the dorsal and two on the ventral
surface of the body.

The mouth opens into the mouth-cavity containing the
mandibles, and this communicates posteriorly with a muscu-
lar pharynx, and has opening into it the ducts of two long
tubular salivary glands (sp) which extend through more than
half the length of the body. The pharynx (p) communicates
by a short oesophagus with the stomach, which extends as a
straight tube almost to the extremity of the body, where a
short rectum places it in connection with the anus. The
pharynx and oesophagus and the rectum are lined with chitin
and represent the fore-gut and hind-gut of other Tracheates,
the stomach being the mid-gut. No Malpighiau tubules or
other diverticula of the intestine occur.

The nervous system shows several highly-interesting
features. There is, as is usual in metameric animals, a supra-
cesophageal ganglion-mass (Fig. 219, og) composed of at least
two and probably three pairs of ganglia, of which the first
supplies the antennae and the second the mandibles, while a
third pair lies at the sides in close contact with the second
pair and sends nerves to the oral papillae. These latter are,
however, postoral and ventral in position, and from them
there extend back two ventral cords (bni) which in each
metamere dilate into a gaugliouic swelling. The two ventral
cords are, however, widely separated, lying in the lateral
chambers of the coelom (Fig. 220), and are connected by a
large number of cross-commissures — a condition which recalls
the arrangement in the Amphiueurous Mollusca (see Fig.
124), the similarity being further increased by the facts that
the two cords unite behind and above the rectum, as in the
Solenogastres, and that the ganglion-cells are not confined to
the enlargements but are scattered all along the cords. The

478

IN VERTEBRA TE MORPHOLOG T.

eyes are the only special organs of sense ; their structure has
been already indicated (p. 472).

One of the most interesting features of Peripatus is the
occurrence in it of typical nephridia (Fig. 219, so}. Upon the
under surface of the proximal portion of each limb, with the
exception of the penultimate or last pair, there is a slitlike
opening which leads into a more or less coiled tube lying in
the coelomic compartment, which extends into the limb and

mg

N

ne

FIG. 220. — TRANSVERSE SECTION OP A Peripatus (after SEDGWICK).

c = central compartment of coelom.
g = reproductive organ.
A = heajt.
/ = intestine.

t -- lateral compartment of coelom.
in — muscles.

mg = slime-glands.
If = ventral nerve-cords.
ne = uephridia.
p = compartment of coelom which

extends into the limb.
pc = pericardial compartment of the
coelom.

terminates in a thin-walled vesicle. These tubes are ne-
phridia, the terminal vesicles (Fig. 220, ne) representing por-
tions of the ccelom into which the nephridia open — a fact
indicated by their embryological history. The uephridia are
thus exactly comparable in every respect with the nephridia
of Annelids, communicating at one extremity with the exterior,
and at the other with the coelomic cavity. It is interesting to
note that the development of the salivary glands shows that

TYPE TRAC11EATA. 479

they are the modified nephridia of the third segment of the
body, that which bears the oral papillae, and furthermore it is
to be noted that in the last or next to last (according to the
species) limb-bearing segment, in which nephridia are wanting,
are found the ducts of the reproductive organs — a fact which
suggests that these are also modified nephridia. This idea is
confirmed by the development of the genital ducts, and car-
ries with it the corollary that the cavities of the reproductive
organs (Fig. 220, g) are portions of the crelom, just as they
were shown to be in the Mollusca (see p. 288).

In addition to the nephridia there are associated with cer-
tain of the appendages glands which open on the under sur-
face of their basal moiety and are termed the crural glands.
In P. capensis they are present in all the appendages except
the more anterior one, and the slime-glands are simply the
highly-modified crural glands of the oral papillae, those of
the last pair of appendages in the males of this species being
similarly elongated though possessing a different function.
In P. Edwardsii, however, crural glands occur only in the
males, and in these only in a few segments immediately in
front of that bearing the reproductive opening.

The Protracheata are bisexual, the female usually being
somewhat larger than the male. The ovaries are paired,
though included within a common capsule, and lie in the pos-
terior part of the coelom. They are continuous with two
nteri, which immediately at their origin are united by a trans-
verse tube, and each bears a receptaculum ovorum and a
receptaculum seminis. Beyond this each continues its course
along the side of the body, passing backwards to finally
unite at the common orifice, lying a short distance in front of
the anus on the ventral surface of the body. The testes are
slender paired structures which are continuous with a slender
vas deferens. This dilates a short distance from the testis
into a vesicula seminalis and then unites with its fellow of the
opposite side to form a slender somewhat coiled tube, the
ductus ejaculatorius, in the terminal portion of which the
spermatozoa are united together into a spermatophore. The
Protracheata are viviparous.

480 INVERTEBRATE MORPHOLOGY.

Affinities of the Protracheata.— Peripatus is a highly-suggestive form
on account of possessing both Annelidau and Tracheate characteristics, so
that it has been generally regarded as indicating a descent of the Tracheate
forms from the Annelids. Its Annelidan features are, first, the presence of
a distinct dermal muscular system; second, the occurrence of crural glands
which seem to be homologues of the glands which secrete the setas in the
Annelida; third, the possession of nephridia corresponding closely to those
of the Annelids; and fourth, the structure of the eyes. On the other
hand, its Tracheate affinities are shown by the claw-tipped feet, by the
adaptation of the feet (mandibles) for masticatory purposes, by the ten-
dency towards a concentration of the anterior segments to form a head,
and by the occurrence of tracheas. Both these sets of features are highly
important, and, taken with the wide distribution of Peripatus, point
strongly to its being the representative of a connecting link between Tra-
cheates and Annelida, a phylogeny which may be considered more in de-
tail at the close of this chapter.

II. CLASS Myriapoda.

The Myriapoda possess a distinct head composed of a num-
ber of fused segments and followed by a distinctly-segmented
bod)7 formed of a varying number of segments, all of which
are more or less similar, there being no differentiation of a
thorax and abdomen. A single pair of appendages as a rule
is borne by each segment, with the exception, in some cases,
of the last. The most anterior pair are usually long multi-
articulate antennye, the second pair mandibles, and the third
and fourth, or the third alone, are modified to form niaxillre;
the succeeding pairs, with one or two exceptions, are ambula-
tory, and are jointed and tipped by a claw.

The chitiuous cuticle is generally thick, and consequently
no definite system of dermal muscles is developed, a number
of separate muscles occurring in each segment for moving the
appendages and the various segments upon one another.
Glands of various kinds opening upon the surface of the body
occur, the most important being glands or protrusible gland-
ular sacs situated upon the basal joints of a number of the
appendages and apparently homologous with the crural
glands of Peripatus.

The heart is in all forms very long, extending through the
entire length of the body behind the head, and possessing

TYPE TRACHEATA. 481

just as many chambers and pairs of alar muscles as there are
trunk-segments. The number of stigmata vary, in some
forms only a single pair occurring, while in others there is a
pair on each segment of the trunk; and the form of the
tracheae varies also, as they are sometimes branched and
sometimes arranged in bunches composed of a number of un-
brauched tubes.

The digestive tract is almost always a straight tube, ex-
tending through the body to the terminal anus. The mouth
is guarded in front by a well-developed upper lip or labrum,
while the fusion of the maxillae behind it in many forms pro-
duces a lower lip. It leads into an ectodermal fore-gut, and
this into an endodermal mid-gut, which is usually provided
with a number of unbranclied diverticula termed hepatic caeca.
One or two pairs of Malpighian tubules open into the ante-
rior end of the ectodermal rectum, and serve as excretory
organs.

The nervous system except in the head region shows but
little trace of concentration, there being as a rule in each
segment of the trunk a pair of ganglia. The antennal ganglia
are fused with the supracesophageal gaugliouic mass which
sends off branches to the ocelli ; these may be quite numer-
ous, though compound eyes do not as a rule occur. A sympa-
thetic system is present as in other Tracheates.

There are no nephridia so far as known in the group, the
excretion being performed by the Malpighiau tubules. The
reproductive organs are paired, and open to the exterior in
some cases by paired orifices, but more usually by a single
opening, which may be situated either far forwards, or else
in other cases near the posterior extremity of the body.

1. Order Pauropoda.

The order Pauropoda contains a few small forms in which
the trunk possesses but ten metameres and nine pairs of ap-
pendages, which, with the exception of the first pair, are six-
jointed and terminate in a claw. In some species, when viewed
from the dorsal surface, the segments appear to be less nu-
merous than the appendages, a condition which results from

482 INVERTEBRATE MORPHOLOGY.

the fusion of certain metameres in pairs, so that two pairs of
appendages appear to belong to some of the segments, the

double nature of which is further shown by
the occurrence in them of two pairs of
nerve-ganglia. The antennae (Fig. 221) are
remarkable in form, consisting of a four-
jointed basal portion which bifurcates at
the tip, one of the bifurcations bearing two
long flagella and a peculiar spherical
stalked body, while the other one bears a
single flagellum. Mandibles are present,
and there is also a single pair of but poorly-
developed maxillae.
FIG. 221. — Pauropus Tracheae or other respiratory organs are
Huxieyi (from I.KUIII&). not yet known to exist. A large simple eye
occurs on each side of the head near the
base of the antennae, and the reproductive opening is situa-
ted upon the second trunk-segment.

Further study of this group is much needed to elucidate
its characteristics. The genus Pauropus has ten trunk-seg-
ments, of which the first nine bear appendages ; while in
Eurypauropus there are only six trunk-segments, and eyes are
wanting.

2. Order Diplopoda.

The Diplopoda, sometimes termed the Chilognatha, are
popularly known as the Millipedes on account of the commoner
forms possessing an unusually large number of appendages.
The body is usually cylindrical and provided with a hard
cuticle, and many forms are in the habit of rolling themselves
when disturbed into a ball or a helixlike coil, thus protect-
ing the more delicate ventral surface of the bod}\ The an-
tennae (Fig. 222, at] are generally seven-jointed and are never
very long, and the mandibles are strong jaws without palps.
In front of the mouth is a well-developed upper lip (id), while
behind it is a lower lip formed by a fusion of the maxillae (ma;).
According to some authors this lower lip represents two pairs
of appendages, but its iunervation and embryological history
seem to be opposed to this view. The segments behind the

TYPE TRACHEATA.

483

head vary in number in different genera from eleven (Glo-
meris) to over one hundred, and the number of appendages is
much greater still, since the majority of the segments bear
two pairs of limbs and in reality represent each two meta-
meres. The four or five anterior trunk-segments are, however,
single, bearing but a single pair of limbs (Fig. 222), and one
of them — in some cases the
first, in others the second, but
more usually the third (lulus)—
is entirely destitute of appen-
dages. The last few segments
also carry but a single pair of
appendages, as does also the
seventh segment in the males,
the appendages of which are
usually modified to serve as
copulatory organs.

Stigmata occur on each of
the trunk-segments, the double
segments bearing two pairs,
situated on the ventral surface
near the coxal joints of the
limbs. Each one has in connec-
tion with it a bunch of un-
brauched tracheae, a condition
recalling somewhat that of Peri-
patus, although the location of
the stigmata is much more regular and definite. Upon the
dorsal surface of the bod}7 there is in most species a row of
pores which have been mistaken for stigmata, but are really
the openings of glands (gl. repugnatoria) secreting an oily
evil-smelling fluid which serves as a means of defence. In
the genus Polydesmus the secretion contains hydrocyanic
acid. Crural glands do not as a rule occur, but protrusible
warts occurring on the coxal joints of a number of legs in
some genera (Lysiopetalum) have been regarded as homolo-
gous structures.

The nervous system has the characteristic Myriapodau
arrangement, each of the double segments possessing two

FIG. 222.— ANTERIOR PORTION OF A
DIPLOPOD.

at = anteuna.

cop = copulatory appendages.
mx = maxilla.

ul = upper lip. [segments.

1, 2, 3, 4 = the four anterior trunk-

484 INVERTEBRATE MORPHOLOGY.

pairs of ganglia. Eyes are usually present and are always
simple, varying in number from two to as many as eighty.

The Diplopoda are bisexual, and the ovaries or testes form
a single mass from which two ducts, or one which later
divides into two, arise and pass forward to open on the ven-
tral surface of the body between the second and third trunk-
segments. The embryos when first hatched out possess but
three pairs of legs, situated upon the first, third, and fourth
segments in Strongylosoma, and on the first, second, and fourth
in lulus, one or more segments without appendages lying
behind the fourth pair. By successive moults new segments
and appendages are added and the form of the adult gradu-
ally acquired.

The Diplopoda live for the most part under stones, etc.,
or among dead leaves, and find their food in decaying vege-
table matter, though some forms will attack living vegetation
and may prove thereby injurious to gardens. The commonest
form, lulus, may readily be obtained under stones or boards
all through the summer.

3. Order Chilopoda.

The Chilopoda, or Centipedes, are very different in their
habits from the Millipedes, being carnivorous and provided
with poison-glands which render the larger forms of iScolo-
pendra dangerous even to man. The body is as a rule some-
what flattened and less hard than that of the Diplopoda.
The antennae (Fig. 223, at) are usually long, with at least
twelve joints, and may be as long as the body, while the
mouth-parts are much more complicated than in the Dip-
lopoda. The mandibles and upper lip resemble the corre-
sponding parts in that group, but the maxillfe (mx) are jaw-
like, are not fused together, and in some forms (Geopliilns)
bear a palp. Behind the maxillse conies a pair of second
maxillae (mx"], which, however, do not serve as jaws but are
reduced to a pair of palplike structures, and behind these again
is a pair of maxillipeds (mxp\ the appendages of the first
trunk- segment, with their basal joints fused to form a lower
lip supporting a four-jointed palp, the last joint of which is

TYPE TRACER AT A.

485

FIG. 223.— ANTERIOR POR-
TION OF A CHILOPOD.

at = antennas.
mx} = maxilla.
ms? — second maxilla.
mxp — maxilliped.

clawlike and is perforated by the duct of a poison-gland.
Each trunk-segment, of which there may be over a hundred,
bears but a single pair of appendages,
there being no compound segments as
in the Diplopoda. Each leg is as a
rule seven-jointed, the coxal joints of
those of the same segment being widely
separated, and there is no modifica-
tion of the seventh pair to serve as
copulatory organs, though the pair of
the penultimate segment are much
reduced in size and lie at the sides of
the reproductive orifice.

Stigmata are usually wanting in the
first three trunk-segments, but occur in
a certain number of the others, lying
usually laterally between the segments
except in Scutigera, in which they have a dorsal position.
They open into branched tracheal trunks which usually an-
astomose with one another, though in Scutigera they open
into sacs from which a large number of simple unbranched
tracheal tubes arise arranged in a bunch as in the Diplopoda.
Crural glands occur on the coxal joints of several of the
posterior appendages.

The nervous system is arranged as in other Myriapods, and
simple eyes are usually present, in Scutigera only being closely
aggregated together to form a faceted eye. This, however, is
not a compound eye exactly similar to that of the Insects, but
is to be regarded simply as a close aggregation of simple
eyes.

The reproductive organs are usually paired, and the sexes
separate. The ducts unite before opening to the exterior, so
that there is but a single opening situated on the antepenulti-
mate segment of the trunk, the appendages of which are
greatly reduced in size. The embryos of Scolopendra and
Geopliilus leave the egg with almost the same number of ap-
pendages as the adult, while those of Scutigera and LitJiobius
possess but seven pairs of legs (in addition to the maxillipeds)
and gradually acquire others by successive moults.

486 INVERTEBRATE MORPHOLOGY.

In Scutigera, a form which frequents the warmer parts of
the world, the dorsal surface of the body is covered in by
eight shieldlike folds which conceal a certain number of the
segments, which are about fifteen in number. Lithobius has
the same number of segments and is common under stones,
etc., as is also Geophilus and Scolopendra, both elongated forms,
the former usually without eyes, while the latter usually
possesses them but has only some nine or ten pairs of stig-
mata. Some of the species of tScolopendra, especially those
living in warm countries, grow to a considerable size and are
capable of inflicting a dangerous wound.

4. Order Symphyla.

The order Symphyla contains a number of small forms
referable to one or two genera, of which the best known is the
genus Scolopendrdla (Fig. 224). Unfortunately the details of
the structure of the members of the group are by no means
well known, a circumstance all the more to be regretted since
/Scolopendrdla seems to possess certain Insect-like features.

The body is elongated, and on the dorsal
surface possesses a number of plates
which overlap slightly, but which do
not correspond in number with the ap-
pendages. The head bears a pair of
long many-jointed antennae, and behind
these, in the region of the mouth, is a
pair of mandibles and a single pair of
maxillae, both these last-named ap-
pendages being deeply imbedded as it
were in the tissues of the head, their

FIG. 224.- Scolopendrella tiPs onl-y projecting. The first pair of
immaculata (from LEUMS). trunk appendages is not transformed into

rnaxillipeds as in the Chilopoda, but is

ambulatory in function, and most, but not all, of the succeed-
ing segments, of which there are apparently fourteen, bear a
pair of five-jointed legs terminated by two claws. Attached
to the coxal joints of most of these appendages is a peculiar
spuiiike process, internal to which is situated a protrusible

TYPE TRACES AT A. 487

glandular sac which is probably to be regarded as a crural
gland. The last pair of appendages may be unjointed, each
bearing a tactile seta, and attached to the last segment is a
pair of conical processes each of which has opening at its tip
the duct of a spinning-gland.

Two stigmata, situated at the base of the antennae, are the
only ones which occur, their position being very remarkable.
They open into bunches of branched tracheae which extend
throughout the greater portion of the body, leaving only the
appendages and the posterior part of the trunk destitute of
an air-supply. The Malpighiau tubules attached to the
hind-gut are very long, and salivary glands opening upon the
maxillae are present.

Eyes do not occur. The reproductive organs are situated
in the fourth trunk-segment and are paired. The oviducts
and vas defereus unite together and open to the exterior by a
single pore situated also upon the fourth segment, though it
has been described by some authors as situated at 'the pos-
terior extremity of the body. Very little is as yet known
concerning the development of Scolopendrella, whose similarity
to the Insects is shown in the antennae and the mouth-parts ;
indeed it has been advocated by some writers that the genus
should be associated with the Thysanura among the Insecta,
and the possibility of its being a connecting link between that
group and Myriapodous forms is indicated in the name applied
to the order.

III. CLASS Insecta.

The class Insecta is far richer in species than any other
class of animals, some two hundred thousand species belonging
to it being known to exist, and of these about eighty thousand
are beetles. A very large number are provided with organs
of flight and may be termed aerial ; others are terrestrial,
living either upon the surface of the earth or excavating bur-
rows beneath its surface ; while some have adapted them-
selves to an aquatic mode of life, and others are even marine,
members of the genus Halobates being found on the surface
of the ocean many miles from land. Many species, living as

488 INVERTEBRATE MORPHOLOGY.

they do upon vegetable food either in the adult or larval
stages, and occurring occasionally in enormous numbers, form
powerful enemies to the horticulturist and agriculturist, the
Kocky Mountain Locust, for example, devastating at times
the crops, while fruit and forest trees are injured by the at-
tacks of other forms.

The Insecta differ from other Tracheata in having the
body divided into three well-marked regions. The most an-
terior of these is the unsegmented head, bearing the antennae
and the masticatory appendages, and immediately following
it is the thorax, composed of three segments, the prothorax,
mesothorax, and metathorax, each of the last two bearing
usually a pair of wings upon its dorsal surface, while pos-
teriorly is the segmented abdomen composed typically of ten
segments, sometimes as broad as the thorax at the junction
with that region, sometimes contracted to a narrow stalk. In
many cases, however, the apparent number of segments falls
below ten owing to the fusion of certain of the posterior seg-
ments or the union of the anterior segment with the thorax,
and in the Butterflies and two-winged Flies the thoracic seg-
ments seem to be reduced to two owing to the close associa-
tion of the metathorax with the first abdominal segment.

four pairs of appendages are borne by the head. The
antennae, and indeed all the appendages, vary greatly in
shape in the various groups, but are usually long slender
niultiarticulate structures provided with sensory hairs. The
masticatory appendages are a pair of mandibles and two
pairs of maxilhe, which are variously specialized for biting,
piercing, or sucking. The most typical condition is that in
\vhich the entire apparatus is adapted for biting and that may
be described here, leaving special modifications to be con-
sidered in connection with the orders in which the}' occur.
The mandibles (Fig. 225, 67) are strong unjoiuted toothed
plates which meet together in the middle line and are pro-
vided with strong muscles. The first maxillae, or, as they are
usually termed, the maxillae (Fig. 225, B], on the other hand
are distinctly jointed, and consist of a basal joint, or cardo,
succeeded by a second joint, or stipes, which bears on its
outer side a multiarticulate palpus (p) and terminates in one

TYPE TRACHEATA.

489

or two unsegineiited plates of which the innermost is usually
toothed. The second maxillae (Fig. 225, A) are also jointed
and are fused together to form a Iow7er lip, or labium. The
fused basal joints form the
submenlum (smj, the second \1
joints the mentum (m], which \
bears, as does the stipes of the
maxillae, a jointed palp (p) and
terminates frequently in two
uujointed plate-like processes.
The three pairs of appendages
of the thorax are typically
ambulatory, but are modified
for clasping, swimming, digging,
etc., according to the habits of
the insect. They typically con-
sist of a basal joint, the coxa,
succeeded by one or two small
joints, the trockanter, upon
which follow a femur, a tibia,
and a tarsus, the last consisting

of live (occasionally four) short joints, the terminal one bear-
ing two claws or ungues. The abdomen in the adult forms is
as a rule, destitute of appendages, except in the Thysanuraus
(Fig. 231), the lowest of all the orders of Insects. In these a
number of the segments are provided with a pair of spurlike
processes which recall the spurs upon the basal joints of the
trunk appendages of Scolopendrella, and are apparently ho-
mologous with them. In the embryos of probably all forms
rudimentary appendages are found on some of the abdominal
segments, but they later disappear, showing, however, a de-
scent of the Insecta from forms in which abdominal appen-
dages were functional in the adult. Processes of various
kinds, such as cerci, ovipositors, and copulatory organs, are
frequently borne by the posterior abdominal segments, but
these do not seem to be equivalent to appendages.

As stated, a pair of wings is usually borne by the meso-
and metathoracic segments. These structures are entirely
wanting in the lowest insects, the Thysanura and Collembola,

B

FIG. 225. — MOUTH-PARTS OF A BEE-
TLE, Monohammus,
A = labium.
B = maxilla.
G = mandible.

490 INVERTEBRATE MORPHOLOGY.

as well as in certain forms belonging to other groups which
have lost them through parasitism (Mallophaga, Pulex, Melo-
plmyus) or other causes (Worker and Soldier Termites, Neuter
Ants, the females of some Moths). They are, when their
possessors are first hatched out, saclike structures, pro-
cesses of the body-wall, tracheae enclosed within blood-lacunae
extending from the body into their cavities. Later, however,
the Avails of the sac come into contact, the cavity being ob-
literated, the tracheae with the blood-lacunae in which they
are situated remaining enclosed within the flat plates so
formed and constituting the so-called veins of the wring, which
have in most species a characteristic and constant arrange-
ment. In certain forms the anterior wings become more
or less thickened by the deposition in them of additional
chitin, and may form hard plates (elytra, Fig. 239) which
serve as a cover and protection for the posterior wings, which
in such cases are alone used in flight. In the two-winged
Flies (Fig. 244) the posterior wings are very much reduced,
being represented only by two small club-shaped structures
termed "balancers," attached to the sides of the metathorax.

The body is enclosed in a chitinous cuticle, usually of
some firmness and frequently bearing numerous hairlike pro-
cesses, certain of which serve as sense-organs. Glands open-
ing on the surface of the body also occur in connection with
the integument ; for example, peculiar protrusible glandular
sacs are situated, two or four on each segment, on the ab-
dominal segments of the Thysauura and Collembola close to
the spurlike abdominal appendages present in those forms,
and are in all probability homologous with the similar struc-
tures of ScolopendreUa and therefore presumably represent
crural glands. These glands appear, however, to be wanting
in other insects. Many genera of Hemiptera possess glands
which produce a malodorous secretion, and wax-glands occur
in the Plant-lice (Coccidce) and Bees, the latter also possess-
ing poison-glands in connection with a complicated stinging-
apparatus, which i's a modified ovipositor.

The respiratory stigmata vary greatly in number in dif-
ferent groups of Insects. In the wingless Thysanura and
Collembola there are usually ten stigmata on each side of the

TYPE TRACHEATA.

491

body, two being situated on the sides of the thorax and eight
on the abdomen, but in Campodea the number is reduced to
three pairs, which occur upon the thorax. In the winged
forms the number also varies somewhat, but there are again
typically ten pairs, arranged as in the Thysanura. They lead
into short trunks, which, in Campodea, ramify through the
body without anastomosing, but more usually they are united
on each side of the body by a longitudinal tube, from which
pass off numerous branches penetrating to all parts of the
body, and transverse connecting tubes passing between the
systems of the two sides (see Fig. 215). In certain forms
which are active flyers the longitudinal tubes are frequently
dilated to form air-sacs, as in the
Bees, or numerous air-sacs may occur
which may be more or less emptied
or expanded according to the will of
the insect, the specific gravity of the
body being thus altered. In the
aquatic larvae which occur in some
forms, such as the May-flies (Epheme-
ridae, Fig. 226), adaptations occur for
the breathing of air dissolved in the
water, the sides of the body in the
abdominal region being prolonged into
a number of pairs of platelike process-
es, into which branches of the tracheae
project, an interchange of the gases
contained in the tracheae for those
dissolved in the water taking place

through the walls of the plates, simi-

111 . , . FIG. 226.— LARVA OF AN

larly to what occurs in the branchiae ™DI

•' JiiPHEMERIU (horn LEUNIS).

of the Crustacea, though in these forms a = tracheal bmuchise.
the exchange is directly with the gases

of the blood. These structures are consequently termed
tracheal branchiae, and while they are functional, the stigmata
are closed, only opening when the adult stage is reached and a
terrestrial life adopted. As a rule the tracheal branchia3 are
thrown off at the moult bv which the adult form is reached,

«y

but in a few forms they persist throughout life.

492 INVERTEBRATE MORPHOLOGY.

A dermal muscular system does not exist, but complicated
and well-developed muscles are present for the movement of
the various parts of the body, those occurring in the thorax
being especially well developed and serving for the movement
of the limbs and wings. As in other Tracheates the coelom
is lacuuar, and the heart lies in a pericardial sinus below the
dorsal surface of the body, alar muscles extending from it to
the walls of the body and partly dividing the sinus into a dor-
sal and a ventral chamber. In the Thysanura the heart
extends from the posterior thoracic region throughout the
greater part of the abdomen, and consists of nine chambers
separated by valves and each provided with a pair of ostia and
a pair of alar muscles. In the majority of forms (Fig. 227, li),
however, the heart is entirely confined to the abdominal
region, and the number of chambers, though frequently as
high as eight, may be greatly reduced. An aorta extends
forwards from the anterior chamber into the head, in the
Butterflies (Fig. 227) dilating in the thorax to form a second-
ary heart (all), and sends off branches which quickly empty
into the lacunar spaces.

The greater portion of the abdomen is occupied by a
peculiar tissue, termed the fat-body, in which the various
organs are more or less imbedded, and which receives its
name from the fact that its cells contain globules of fatty
matter, and in the adult insect usually also concretions of uric
acid. It is not necessarily confined to the abdomen, but may
extend into the thorax or even into the head. In certain
Beetles — the Fireflies (Lampyridse) and Pyrophorus of the
West Indies — certain regions of the body, especially the
abdomen, and, in Pyrophorus, two spots upon the thorax, give
out under certain conditions, apparently under control of the
animal, a very bright light, usually spoken of as a phosphores-
cence. The tissue which produces the light is the fat-body,
or special portions of it abundantly supplied by tracheae, and
the process seems to be one of oxidation of phosphorus-
containing substances. The exact nature of the phenomenon
is but poorly understood at present, and it is not possible by
any means at 'our disposal to produce in the laboratory a

TYPE TRACHEATA.

493

light equal in intensity to that of the Firefly with the expendi-
ture of as little energy.

The digestive tract is as a rule much more complicated
than in other classes of Tracheates and is generally more or
less twisted or contorted in the abdominal region, so that
usually it is longer than the body. The mouth is bounded
in front by a usually large upper lip or labrum, generally
described with the mouth-parts, but distinguished from them
in not representing a pair of appendages. The anterior
portion of the intestine, the fore-gut, is ectodermal in origin

FIG. 227. — STRUCTURE OF BUTTERFLY, Danais archippits (after BDROESS).

a = antenna.
ag = accessory gland.
ah = accessory heart.
an = abdominal ganglion.

be = bursa copulatrix.
c — crop.

cc — canal uniting be and oviduct.

ce = cerebral ganglion.

h — heart.
I = thoracic limbs.

mv = Malpighian tubules.
od = oviduct.
ov = ovary.
ph = pharynx.
pi = palp.
s = stomach.
sg = salivary gland.
tg = thoracic ganglion.
7-7/7 = thoracic segments.
1-9 = abdominal segments.

as in other Tracheates and consists of a mouth-cavity
into which, or in its neighborhood, the ducts of one or
more generally well-developed salivary glands (Fig. 227, sg)
open. The secretion of these glands varies considerably in
different forms, one of the pairs present in the larvae of the
Butterflies and certain Moths being transformed into silk-
spinning glands, the silk of the Silkworm being a product of
their activity. When digestive the secretion seems to have
a peptonizing effect as well as the power of transforming
starch into sugar, and is consequently of considerable diges-
tive importance. The mouth-cavity opens behind into an

494

INVERTEBRATE MORPHOLOGY.

oesophagus, whose posterior region is frequently dilated into a
crop (Fig. 227, c) which in some Beetles is lined with chitiuous
teeth or bars and whose walls are muscular, the apparatus
probably serving for a further mastication of the food. The
mid-gut which succeeds the crop is usually dilated into
a stomach (s), lined in some cases by glandular cells, or,
in others, having opening into it numerous glandular diver-
ticula, the so-called liver-pouches. The hind-gut, like the
fore-gut of ectoderrnal origin, has opening into its anterior
extremity the Malpighian tubules (mv). which vary consider-
ably in number, amounting to nearly one hundred in some
Hymeuopteraus, though more usually limited to from four to
eight. They are excretory in function, and are apparently the

o

\

A B \ //

FIG. 228.— DIFFERENT ARRANGEMENTS OF THE NERVOUS SYSTEM IN INSECTS
(from GEGENBAUR). A, Termes; B, Dytiscus; C, a fly.

only excretory organs which occur. The anus is situated at
the extremity of the body, and in close proximity to it odor-
iferous glands frequently open into the hind-gut, serving as
organs of defence. In some cases they secrete an acrid fluid
which, as in the Bombardier beetle (BracUnus), can be ex-
pelled with almost explosive force.

The nervous system in forms where it shows the least
amount of modification (Fig. 228. A) consists (1) of a supra-
cesophageal mass composed apparently of three pairs of
ganglia and supplying the eyes and the antennae ; (2) of a sub-
cesophageal mass composed also of three pairs of ganglia
supplying the segments indicated by the mandibles, the

TYPE TE ACHE AT A. 495

maxillae, and the labium ; (3) of three pairs of ganglia in the
corresponding thoracic segments ; and (4) of a chain of ab-
dominal ganglia, a pair occurring in each segment except
usually the last two or three, in which a compound ganglion
occurs. Frequently, however, this typical condition is modi-
fied by a greater or less concentration of the various ganglia,
the thoracic ganglia fusing to a single mass, as may also, more
or less perfectly, the ganglia of the abdominal chain (Fig. 228,
B\ and the latter may even unite with the thoracic ganglia to
form a single mass situated in the thorax, as in certain two-
winged flies (Fig. 228, C). A visceral system is usually pres-
ent arising from the supraoesophageal (cerebral) mass and
beiug distributed to the walls of the digestive tract.

The antennae of insects seem to act as sense-organs, and
serve apparently to control the flight, since when removed the
insect is not able to fly with its accustomed ability. So too
it seems probable that in the Ants and Koaches these appen-
dages are the seat of the olfactory sense, and in the Mosquito
it seems that certain hairs upon them may be auditory in
function. Compound eyes, frequently consisting of several
thousand oininatidia, are usually present, as well as a small
number of simple eyes (ocelli) situated upon the dorsal sur-
face of the head. Special organs, which have usually been
considered auditory, also occur in many forms, varying con-
siderably in complexity. In its simple form such an organ
consists of a single nerve-fibre which dilates into a ganglion-
cell, prolonged into a terminal hair which is enclosed within
a sheath fastened at one end to the wall of the body. This
whole apparatus is termed a chordotonal organ, and there is
usually attached to the sheath just where the hair arises from
the ganglion a ligament, which is also inserted into the body-
wall. In the majority of cases a number of ganglion-cells and
hairs are associated to form a chordotonal organ (Fig. 229),
the various hairs sometimes being grouped within a single
sheath, — sometimes, however, spreading out in a faulike man-
ner, each possessing its own sheath. These organs occur in vari
ous parts of the body, on the antennae or on the limbs. In
the grasshoppers (Acridiidae) the first abdominal segment
bears on either side a thin tense membrane, a thinned portion

496

IN VER TEBRA TE MORPHOLO G T.

of the cuticle, recalling the tympanic membrane of the human
ear, beneath and in connection with which is a chordotonal
apparatus, further improved by the occurrence in close prox-
imity to it of a saclike enlargement of a trachea which serves
as a resonator. Similar organs occur in pairs on opposite
sides of the tibne of the first pair of legs in the Crickets
(Gryllidse), and seem from their structure to be auditory

bk

-tr

FIG. 229. — SUBGENUAI, CHORDOTONAL ORGAN OF THE TIBIA OF THE SECOND

THORACIC APPENDAGE OF Isopteryx (after GHABER from LANG).
bk = blood -corpuscles. gz — nerve-cells.

c — integment. tr — trachea.

es = terminal ligament.

sc = terminal hairs and sheaths.

organs, whence the conclusion that the more simple chordo-
tonal organs also subserve this function.

It is interesting to note that the males of the forms provided with a
tympanal organ possess the power of making a harsh or sharp chirping
noise, produced in the Grasshopper by rubbing the femora of the hind legs,
which are furnished on their inner surfaces with a row of fine teeth, over
the strong marginal veins of the anterior pair of wings ; and in the Crickets
and Locustidae by rubbing together the two anterior wings, a row of teeth
upon a vein of one wing working upon a projecting smooth vein of the
other. The male Cicadas also make a similar noise, the stridulating appa-
ratus resembling that of the Grasshoppers, and in all cases it seems to be a
sexual characteristic serving to attract the females.

The sexes are separate, and a more or less distinct sexual
dimorphism occurs, the males being usually smaller and more
slender than the females. In some cases, as in the Tussock-
moth (Orgyia), the female lacks wings and has a very different

TYPE TRACHEATA. 497

appearance from the males, and in man}- Beetles the male is
adorned with spines and tubercles upon the head which are
but rudimentary or absent in the female. Differences in the
shape of the antennae and the presence or absence of stridu-
lating organs also serve to distinguish the two sexes in some
of the groups. In a few forms a polymorphism is produced
by the failure of certain individuals to reach sexual maturity
and by the assumption by them of certain special structural
characteristics. Examples of such cases are afforded by the
Bees, Ants, and Termites, the workers of the first two groups
being immature females, wrhile in the Termites (Fig. 237) the
so-called neuters may be either males or females, always,
however, immature.

The ovaries (Fig. 227, ov} are paired and consist of a
varying but usually rather large number of tubes, which start
from a common basis. At the extremit}- of each tube is the
germ-producing region, the rest of the tube being divided into
a series of chambers each of which contains an ovum sur-
rounded by a layer of follicle-cells. Not uufrequeutly the
chambers are arranged more or less distinctly in pairs, the
lower one of each pair containing an ovum, while the upper
contains a number of small cells similar in appearance to the
primary germ-cells, but which serve as nutrition for the ovum
which gradually absorbs them (see Fig. 20). From each
ovary an oviduct arises, the two, however, soon uniting, and
receiving, usually not far from the unpaired orifice, the ducts
of various glands (ag) whose secretion serves to cause an
adhesion of the ova to the structures on which they are
deposited. A receptaculum semiuis is usually present, and
there is frequently a large pouch, partially separated from the
oviducts, which receives the male organ during copulation and
is termed the bursa copulatrix (be). The genital orifice is situ-
ated on the ventral surface of the ninth abdominal segment
and is usually surrounded by a number of papillae, or some-
times by long processes, which serve as ovipositors and are
to be regarded simply as processes of the segments from which
they arise and not as modified limbs.

The testes are also paired, each being composed of a
number of separate spherical or tubular portions. The ducts

498 INVERTEBRATE MORPHOLOGY,

from these various portions on each side unite to form a vas
defereus which may dilate into a vesicula seminalis and then,
uniting with its fellow of the opposite side, forms the ductus
ejaculatorius. Occasionally the vesicula is unpaired arising
from the point of union of the two vasa deferentia, and very
frequently accessory glands occur. The ductus ejaculatorius
opens usually on the ventral surface of the tenth abdominal
segment, and projections of the body-wall in the vicinity of
the orifice form a groove or tube through which the sperma-
tozoa, usually united into sperrnatophores, are introduced
into the bursa copulatrix of the female.

Parthenogenesis occurs as a normal process in certain
Insects, though always associated with true sexual reproduc-

FIG. 230. — Aphis mail, WINGED AND WINGLESS FORMS (from Packard).

tiou. Examples of it are found in certain Coccidse (Aspidio-
tus] and in some of the Gall-wasps (Cynipidse), the fertilized
ova producing both males and females, while in the Bees, for
example, in which both fertilized and unfertilized ova are de-
posited, the latter give rise to drones or males alone, while
workers or queens, i.e. the females, develop from the fertilized
ova. Occasionally heterogouy occurs, as in the Plant-lice
(Aphid;ie). These forms under favorable conditions of tem-
perature and food produce viviparously usually wingless, indi-
viduals, not, however, from true ova, but by a process which
may rather be compared to internal biuldiug, as in the Kedne
of certain Trematoda. Generation after generation of such
individuals may be produced during the summer, but on the

TYPK TRACHEATA. 499

approach of cold weather or on the exhaustion of the food-
supply males aud females appear by which true fertilized ova
are produced, and from these, surviving the winter, viviparous
heterogonous females develop.

In the genus Phylloxera, which has played such havoc on grape-vines
in France, a greater complication of generations occurs. A winter egg,
which has survived beneath the bark of the vine, gives rise to wingless
forms which migrate to the roots, and there produce numerous genera-
tions. After a time winged forms appear which ascend from the roots,
and, reproducing parthenogenetically, increase rapidly in number and serve
to distribute the species over wider areas. Certain of these produce small
ova from which males develop, and others larger ones which give rise to
females, both sexes being destitute of both wings and digestive tract, and
by these forms the fertilized winter eggs are produced.

In certain flies (Miastor, (Jecidomyia) psedogenesis occurs,
the female reproductive organs becoming mature while the
insect is still in the larval stage, and the ova, developing par-
thenogeuetically within the body, give rise to another gen-
eration of larvae. This process may be repeated several
times, the last generation of larvae developing into the adult
form (see Fig. 29).

The more primitive Insects, the Thysanura and Collem-
bola, leave the egg in a form resembling the adult, differing
from it only in size and in the immaturity of the reproduc-
tive organs, and pass through no marked metamorphosis
during their post-embryonic development. Such forms are
termed ametabolic. A similar absence of metamorphosis is
found in certain forms degenerated by parasitism and lacking
wings, but these have evidently descended from winged forms
which passed through a certain amount of metamorphism,
so that the ametabolism is secondary and should be distin-
guished from the primitive ametabolism of the Thysanura.
In the majority of winged forms, however, a more or less
pronounced metamorphosis occurs. In the simpler cases the
young are distinguishable from the adults by the absence or
but slight development of the wings, which become larger
after successive moults, the adult form being thus gradually
acquired. In these cases of gradual metamorphosis the
habits of the adult and larva are similar, but where they

500

INVERTEBRATE MORPHOLOGY.

differ greater changes result, leading to hemimetabolism. This
occurs, for instance, in the Fish-flies (Ephemeridae) and
Dragon-flies (Libellula), in which the larvae are adapted for an
aquatic life and possess tracheal branchiae (Ephenieridse) and
other features which are lost, either gradually by successive
moults or suddenly at the last moult, the adult winged Dragon-
fly, for instance, issuing from a peculiar aquatic larva with
the merest rudiments of wings.

Finally, a large number of forms are holometabolic. Ill
such cases the habits of the larvae are different from those of
the adults ; for instance, the larvae of the Butterflies, the
caterpillars (Fig. 231), are wormlike creatures with power-

FIG. 231. — LARVA, PUPA, AND IMAGO OF Pitris oleracea (from RILEY).

ful jaws feeding on plant-tissues, while in the adults the mouth-
parts are adapted for sucking. The transformation from the
larva to the adult is accomplished by the intervention of a
resting stage or pupa, during which no nutrition is taken, and
when the transformation takes place the fully-developed
insect or imago issues from the ruptured skin of the pupa.
The pupa varies considerably in form in different groups,
in some being enclosed in a silken case manufactured by
the larva before the last moult and termed a cocoon. In
some cases the adult appendages project from the bod}r of
the pupa (pupa, libera), but in other cases they are united
with the surface of the body and but indistinctly visible
(pupa obtecta), an arrangement usually found in the Butter-
flies, whose pupae, owing to their frequent brilliant colora-
tion, are termed chryscdids. a term which has been somewhat
incorrectly extended to the mummy-like pupae of other forms.

TYPE TRACES AT A. 501

Filially, in some of the two-winged flies the pupa is enclosed
within the last larval skin, possessing then a cylindrical form
without any indication of the adult limbs (pupa coarctata).
A metamorphosis in which a distinct pupa-stage occurs is said
to be "complete" in contradistinction to the hemimetabolic
form frequently spoken of as "incomplete."

Mention should be made here of the dimorphism or polymorphism which
occurs in certain adult Insects. It has already received passing mention
(p. 497), but in addition to the frequently-occurring sexual dimorphism
there occurs in forms which live together in colonies a polymorphism asso-
ciated with a division of labor on the parts of the members of the colony.
Thus in the Bees there are found the drones or males with heavy bodies,
the queen or female, as large as the drones but with a much more slender
body, and the workers, which are sterile females distinguished by their
smaller size and by other features, such as a peculiar modification of the
tibias of the last pair of legs which adapt them for the collection of pollen
from the flowers which they visit. Among the Ants a similar trimorphism
occurs, males, females, and neuters or workers constituting the colony; and
in some tropical forms the workers are of two kinds, namely, ordinary
workers with small heads and mandibles, and soldiers with large heads and
strong prominent mandibles, whose functions are indicated by the popular
name applied to them, though guards would perhaps be more appropriate.
Finally, among the Termites, popularly known as the White Ants, four
forms, i.e., males, females, workers, and soldiers, also occur.

In certain Butterflies a peculiar form of dimorphism or trimorphism
termed "seasonal dimorphism" occurs, an excellent example of it being
offered by the American Papilla AJax, of which there have been described
three distinct varieties, differing markedly in coloration both in the males
and the females, and distinguished as the varieties Walshii, Telamonides
and Marcellus. From chrysalids which have passed the winter there hatch
out in the early days of spring forms belonging to the variety Walshii, and
somewhat later, from those whose development has been retarded, the
Telamonides forms. During the early part of summer the Walshii forms
die out and a little later the Telamonides also disappear, both forms pre-
viously, however, depositing ova, most of which develop into larvae and
chrysalids and hatch out in the later months of summer as the Marcellus
form, whose ova, again developing into chrysalids, pass the winter in that
state, and give rise in the following spring successively to the Walshii and
Telamonides forms. The three varieties are evidently produced by influ-
ences acting upon the chrysalis and differing according to the season, per-
haps according to temperature, whence the distinguishing name applied to
this form of dimorphism, which is also said to occur in certain Spiders.

502 INVERTEBRATE MORPHOLOGY.

1. SUBCLASS APTERYGOTA.

Tlie members of this subclass are all small and do not
possess wings, the absence of these structures being a primi-
tive feature and not due to degeneration resulting from para-
sitism or other causes. In some forms rudiments of abdomi-
nal limbs are present in the adults, and there is no meta-
morphosis in the post embryonic development (primary
ainetabolisin).

1. Order Thysanura.

The Thysanura or Bristle-tails possess ten abdominal
segments, the terminal one bearing two- or three-jointed hair-
like processes, whence the name applied to the order. The

body in some forms (Lepisma) is covered
with scalelike hairs giving it a silvery-gray
appearance, but in other cases these are
wanting. The antennae vary in length, but
are always simple cylindrical structures,
the terminal joint in some forms (Campoded)
bearing a peculiar bilobed structure sup-
posed to be sensory, and the mouth-parts
are adapted for biting purposes and are
usually well developed. The first abdom-
inal segment in some forms bears a pair of
indistinctly-jointed appendages, probably
rudimentary limbs, and a number of the
succeeding segments in Campodea bear spur-
like processes, also supposed to be limbs
and recalling the spurs of Scolopend.rella,
especially as protrusible glandular struc-
tures, comparable perhaps to crural glands,
FIG. zw.-Campodea occm> in associatiOn with them in some

staphyhnus (after

LUBBOCK from Hux- lOriHS.

LEY) The nervous system shows but little

concentration, eight abdominal ganglia occurring in Lepisma,
and eyes are usually present, being in some cases compound.
The stigmata vary in number, being usually ten, though in
Campodea they are reduced to three, and the tracheae in this

TYPE TR ACHE ATA. 503

same form are interesting iu being destitute of longitudinal
and transverse anastomoses.

Lepisma is frequently found in houses, in attics and similar
places, feeding upon woollen, linen, and other fabrics, but also
on meal or sugar. Campodea (Fig. 232), on the other hand, is
to be found under stones or dried leaves and is a small white
form, by no means uncommon.

2. Order Coliembola.

The Coliembola are distinguished from the Thysauura by
the abdomen consisting usually of but six segments, and in
some cases the number is even smaller. The body in Podura
is covered with scales, and the terminal segment of the body
is usually provided with two processes which may be bent up
underneath the abdomen and then suddenly extended, pro-
pelling the insect to a considerable distance. These structures
are absent in the adult Anurida, but occur in young speci-
mens, and their occurrence and function have suggested the
popular name of Spring-tails applied to the order. Neither
abdominal appendages nor coxal glands occur, but the first
segment bears a peculiar organ, having in Anurida the form
of a saccular protrusion, which is probably adhesive in func-
tion. The antennae are usually short, and bear iu some forms
an antennal sense-organ similar to that of Campodea; the
mouth-parts are biting, but frequently much reduced in size.

The nervous system is usually much concentrated, there
being in Anurida but three postoral ganglia situated in the
thorax, the abdominal ganglia having evidently fused with
the last thoracic. Simple eyes are present in varying numbers,
but compound eyes never occur. A peculiar organ lying be-
hind the bases of the antennae, and hence termed the post-
antennal organ, occurs, and has been supposed to be a sense-
organ, but further information is required concerning it.
Tracheae are usually present, though quite wanting in Anurida.

The genus Podura is to be found, sometimes in consider-
able numbers on the surface of standing water in the early
spring, while other forms occur in damp earth or under bark.
Anurida is found upon the seashore underneath stones just
above tide -mark.

504 INVERTEBRATE MORPHOLOGY.

II. SUBCLASS PTERYGOTA.

The members of the subclass Pterygota are, as the name
indicates, typically provided with wings, though in a compar-
atively few cases these structures may have disappeared
through degeneration due to parasitic habits, or through special
adaptation to certain conditions of life, as in the neuters of
the Ants and Termites. In nearly all cases the larvae differ
in form from the adults, and various grades of metamorphosis
are found.

1. Order Dermaptera.

The Dermaptera or Earwigs (Fig. 233) are usually small
insects which resemble not a little the Thysanura. The
abdomen terminates in a pair of forceplike
processes termed cerci, their shape suggesting
the generic name Forficula, applied to certain
members of the order. The anterior wings are
small and chitiuous and serve as covers for
the protection of the posterior pair, which are
larger, membranous and veined, and when at
rest are folded longitudinally like a fan, and in
FIG. 233.— Zato addition twice transversely, so that they are
almost completely hidden by the scalelike an-
terior pair. The antennae are long and filiform,
and the mouth-parts adapted for biting. The Earwigs are
terrestrial forms and pass through a gradual metamorphosis.
In many respects they approach nearer the Thysauura than
any other insects, and are related rather closely to the suc-
ceeding order.

2. Order Orthoptera.

In this order, which includes the Locusts, Grasshoppers,
(Caloptenus), Crickets (Gryllus), Cockroaches (Periplaneta),
and other forms, the mouth-parts are adapted for biting and
the last segment of the abdomen bears two-jointed cerci. The
anterior wings form, as in the Dermaptera, covers for the
posterior pair and are chitinous plates; the posterior ones are,

TYPE TRACES AT A. 505

on the other hand, membranous and the veins are for the
most part arranged longitudinally, so that when at rest the
wings are folded like a fan, though in some forms, such as
the Crickets, in which the anterior wings are short, a trans-
verse fold also occurs. In the female Cockroaches the ante-
rior wings are very small, and the posterior ones wanting, and
in the Walking Stick (Diapheromera) — so named from its resem-
blance to a green or dead twig — both pairs are entirely want-
ing.

The antenDse are usually long and filiform, and the legs
strong and adapted to a terrestrial life, some forms, such as
the Cockroach, being exceedingly active. In the Grasshop-
pers, Locusts, and Crickets the femora of the last pair of legs
are greatly enlarged and very muscular, serving for jumping,
while in the Mole-cricket (Gryllotalpa), which burrows in the
ground, the anterior pair is greatly enlarged and adapted for
digging.

As in the Earwigs, the metamorphosis is gradual.

3. Order Ephemeridae.

The Ephemeridse, or May-flies (Fig. 234), are characterized
by the remarkable brevity of their existence in the imago-
stage, some forms existing but for a few
hours, while others live for several days,
the existence being merely long enough
to ensure the accomplishment of the re-
productive acts. The body is elongated
and terminates in two or three elongated
hairlike cerci, and on the thorax there are
borne usually two pairs of wings, of which
the anterior pair is considerably larger FIG. 231.—Potamanthus
than the posterior. The antennae are marginatus (from PACK-
short, and the mouth-parts adapted for

biting, though usually much reduced, since the imago takes
no nutrition during its short existence. The first pair of legs
is usually slender and directed forwards, being of little use in
locomotion. An interesting structural peculiarity is the oc-
currence of paired reproductive ducts which open by separate

506

INVER TEBRA TE MORPHOLOG T.

pores instead of uniting as they do in the majority of In-
sects.

The larvae are aquatic and provided with tracheal bran-
chiae (see Fig. 226), recalling, except for these structures, the
Thysauura. By a series of moults the adult stage is gradu-
ally acquired, the wings appearing in what is termed the sub-
imago stage, a final moult being necessary before maturity is
reached. The metamorphosis is thus incomplete.

The genus Ephemera is of frequent occurrence in the
neighborhood of lakes and ponds, sometimes occurring in
enormous numbers.

4. Order Odonata.

The members of this order, the Dragon-flies, are elongated
forms with two pairs of nearly equal, abundantly-veined wings
of usually large size, all the forms being excellent fliers and
seeking their prey in the air. The head is united to the
thorax by a narrow stalk which permits extensive rotation of
the head, and the abdomen, terminating in two uusegmeuted
platelike cerci, is long, and in the large Dragon-flies, ^Eschna
and Diplax (Fig. 235), and in the brightly-colored Agrion very
slender, though somewhat stouter in the genus Libellula. The
antenna} are very small and the mouth-parts adapted for biting,
while the legs are slender, the anterior pair being directed
somewhat forwards so as to serve for grasping the prey.
The lateral compound eyes are very large, meeting on the

dorsum of the head, and
in front of them are situ-
ated a pair of small ocelli.
The larvaa are aquatic
and are characterized by
the remarkable develop-
ment of the labiuin, which
is very much enlarged,
terminating in two power-
FIG. M5.-Diplaz elisa (from PACKARD). ful jawg and proviaed with

a hinge, so that it can be flexed so as to lie beneath the head
or suddenly thrust out to capture the unwary prey. This

TYPE TRACHEATA. 507

apparatus is termed the " mask." Kespiration is carried on
by traclieal gills, consisting in Agrion of three leaflike pro-
cesses situated at the posterior end of the body, and also by
the terminal portion of the intestine, into which water is
taken and which is abundantly supplied with tracheae. The
water can be forcibly expelled from the intestine, serving to
propel the insect through the water if it so desires. The
metamorphosis is incomplete.

5. Order Plecoptera.

The Plecoptera, or Stone-flies (Fig. 236), are found in the
vicinity of water and have a somewhat elongated body, fre-
quently terminating in two long
cerci (Perla}. The antennae are long
and filiform and the mouth-parts
adapted for biting, while the legs
are strong and used for walking.
Two pairs of wings occur almost
equal in size, but lacking the com-
plicated venation found in the Odo-
uata, and when at rest lie flat upon FlG- 236-- STONE-FLY, Perla.
the abdomen, completely concealing it. The larvae are
aquatic, and are usually to be found in considerable numbers
under stones in swiftly-running streams. They recall the
Thysauura in their appearance, and possess traclieal branchiae
on the under surface of the thorax, which in some forms are
retained in the adult. The metamorphosis is gradual or in-
complete according as these structures are or are not retained
in the imago.

6. Order Corrodentia.

The members of this group possess biting mouth-parts
and are sometimes destitute of wings. The Termites, or
White Ants, live in colonies and show a polymorphism. The
males and females, termed kings and queens (Fig. 237, A, B),
are at first provided with large wings resembling those of the
Plecoptera, but after the marriage flight settle to the ground
and become wingless. The workers select from the many pairs

508

INVERTEBRA TE MORPHOL OGT.

one for each nest, the remaining un selected ones soon dying.
The neuters are of two sorts : the workers (Fig. 237, C), pale in
color and with comparatively small heads and mandibles, and
the soldiers (Fig. 237, D\ in which the head is very large and
dark colored and carries a pair of large mandibles. Both
these forms are destitute of eyes, and are to be regarded as in-
dividuals which have not passed beyond the larval stage, being
potentially either males or females with the reproductive
organs, however, undeveloped. The young larvse resemble
Thysanura in their general form and are cared for and fed by

A B 0

FIG. 237. — Termes lucifugus (from LEUNIS).
A, winged male; B, female after loss of wiugs; C, worker; D, soldier.

the workers. Those forms which are destined to become
kings and queens are nursed for a longer time than the others,
and progress further in their development, being really the
only members of the colony which reach the imago state.

The Termites shun the light, and the American species are
chiefly found in rotten wood, upon which they feed, excavating
burrows within it. In some cases they prove very destructive
to the woodwork in houses, eating away the interior of the
wood and leaving eventually only a thin shell in place of the
originally solid beam. The African species builds large clay
mounds from three to four metres in height, tunnelled by a

TYPE TEACHEATA. 509

somewhat complicated system of chambers, galleries, and
storehouses.

To this group belong also the Psocidce and the Mallophaga.
The former are small forms found upon the leaves of various
trees and occasionally in houses. They do not show poly-
morphism and are usually provided with wings, though the
genus Atropos, not uncommon in books which have remained
long undisturbed, lacks them. The Mallophaga are all
destitute of wings and are parasitic, living upon the bodies of
birds (Liotlieum], whence they are usually termed the Bird-lice.
They feed upon the feathers and are comparatively active in
their movements. A few forms occur on mammals, e.g.
Trichodectes on the dog.

The larvae of all these forms resemble the adults except in
size and in the absence of wings, and the metamorphosis is
gradual. Since the Mallophaga are destitute of wings in the
adult condition they may properly be said to be secondarily
ametabolic.

7. Order Thysanoptera.

The Thysanoptera are small Insects which live upon the
leaves of various plants, which they pierce for the purpose of
obtaining nutrition, and sometimes are very injurious to
wheat, clover, and other cultivated plants. The wings are
narrow, but imperfectly veined, and with the edges fringed
with numerous slender hairs ; they are, however, occasionally
wanting. The antennae are short and filiform and the mouth-
parts intermediate between the biting and the sucking type.
The mandibles are reduced to styletlike piercing-organs and
are enclosed within a tubular proboscis formed by the fusion
of the labrum with the maxillae and labium, both of these last
appendages retaining their palps and showing usually their
typical parts. The legs are adapted for rapid locomotion and
are peculiar in that the terminal joint of the tarsus, instead of
bearing ungues, is provided with a protrusible sac which
serves for adhesion ; on account of this peculiarity the order
is sometimes known as the Physapoda.

The larvae except for the absence of wings are closely

510 INVERTEBRATE MORPHOLOGY.

similar to the adults and the metamorphosis is gradual,
though the tendency towards the development of a distinct
pupal stage is shown by the fact that the last larval stage
takes no nourishment. The genus Phlceotkrips is character-
ized by the last abdominal segment being tubular in form,
while Tlirips possesses in the female forms an ovipositor com-
posed of four valvelike pieces.

8. Order Rhynchota.

The .members of this order are divisible into two groups,
the Heteroptera and ffomoptera, according to the character of
the anterior wings. In the Heteroptera, which includes the
majority of forms popularly known as Bugs, the basal partions
of the anterior wings are chitiuous, while the tips are mem-
branous, the posterior wings being entirely membranous. A
typical member of this group is the common Squash-bug
(Anasa, Fig. 238, A), and other examples are the Water-boat-
man (Notonecta), the large Water-scorpion (Belostoma), and the
slender Water-scorpion (JKanatra), all of which are of frequent
occurrence in ponds, swimming powerfully beneath the water
by means of the flattened posterior legs which serve as oars,
the anterior pair being directed forwards and serving for
grasping the prey. The Water- measurer or Water-spider
(Hydrometra) is also veiy common in ponds, darting about
upon the surface in search of prey, a habit which also char-
acterizes the genus Halobates, which lives upon the surface of
the ocean and is found many miles from land. Some mem-
bers of the group are entirely destitute of wings, as for ex-
ample the Bedbug (Gimex) and the Louse (Pediculuti).

In the Homoptera the wings are both membranous, the
anterior pair being larger than the posterior, and, as in the
other group, are sometimes wanting. The Cicada is a member
of this group, as are also the Aphidse, or Plant-lice (Fig. 230),
so frequent in green-houses and upon various uncultivated
plants whose juices they suck, a habit also shared by the
nearly -allied Coccidse, including the scale-insects (Aspidiotus)
and the Mealy-bugs (Dactylopius), both of frequent occurrence
on cultivated plants, the former sometimes doing no little

TYPE TRACHEATA.

511

ra = mandibles.
mx = maxilliE.

damage to apple-trees. The remarkable lieterogony of these
forms has already been described (p. 498).

In both the suborders the mouth-parts are adapted for
piercing and suckiug. The
labium (Fig. 238, B, 11} is
prolonged into a slender,
usually four-jointed process,
grooved upon its upper sur-
face, the groove being con-
vertible into a tube by the
closure over it of the long

slender mandibles (in) and

.„ , -i • i c FIG. 238.— .4, Anasa tristis; B, MOUTH-

maxillae (mx} which form long pARTg QF Nepa c .^.^ (aftep SAVIQNY

slender needlelike piercers, from OWEN).

The antennas are usually ^ == labium.

short and filiform, though

in some of the Heteroptera they may be almost as long as

the body.

Many of the Rhynchota are provided with glands which
secrete an offensive fluid, e.g. in Cimex and Anasa, and in the
Coccidse wax-glands are also abundantly present, producing
a secretion which may cover the body with waxen scales, or
in some cases form a wool-like mass covering the greater
part of the abdomen (Pemphigus). The Aphidae also possess
as a rule upon the antepenultimate abdominal segment a pair
of tubular elevations or papillae from which a sweet secretion
issues, the so-called "Honey-dew," which covers the leaves
and stems of the plants upon which the Insects live, and is
eagerly sought for by various Insects, more especially by
Ants.

The larvae of the Ehynchota as a rule resemble the adults
even to the structure of the mouth-parts, and the metamor-
phosis is consequently gradual. The Cicada forms, however,
an exception to this rule, the larva occurring beneath the
surface of the ground and living upon the roots of trees.
It becomes transformed into a pupa, which, however, con-
tinues to lead an active existence, becoming quiescent only a
short time before the moult which results in the formation of

512 INVERTEBRATE MORPHOLOGY.

the imago, very different in appearance from the pupa. The
metamorphosis here approaches the complete type.

9. Order Coleoptera.

The order Coleoptera includes the Beetles and is richer
in species than any other order of animals. The members of
the group are characterized by the anterior wings being con-
verted into hard chitinous plates, the elytra, which cover in
and protect the posterior membranous wings and the abdo-
den, being short only in a few forms, such as the Buryiug-
beetles (Necrophorus), in which the tip of the abdomen remains
exposed, and the Staphylinidse, or Rove-beetles, and Meloe, in
which they cover only the more anterior portions of the ab-
domen, the posterior wings in the last-named form being
wanting, as they may also be in some of the Weevils. Occa-
sionally, as in the Fireflies (Lampyris), the elytra are but
slightly thickened, and in some forms they may be completely
fused together.

The antennae vary greatly in shape, being usually filiform
and sometimes very long, as in the Boring-beetles (Monoham-

FIG. 239. — Cotalpa lanigera AND ITS LARVA (a) (from PACKARD).

mus, Clytus, Saperda, etc.), though occasionally, as in the
Lamellicorn beetles (Melolontlia — -the June Bugs and Cotalpa,
Fig. 239), the terminal joints are flattened and folded together
like the leaves of a book. The mouth-parts (Fig. 225) are in
all cases adapted for biting, and the legs for locomotion. In
the Lady-bugs (Coccinella) the tarsus consists of but four
joints, one of which is rudimentary, while in the Weevils
(Curculionidne), in which the anterior part of the head is pro-

TYPE TRACHEATA. 513

longed into a cylindrical snoutlike process at the extremity
of which is the mouth, in the Boring-beetles, and in the Po-
tato-beetle (Doryphora) it is formed of five joints, one of which
is exceedingly small. In other forms, such as Meloe and the
Blister-beetles (Lytta), the tarsi of the two anterior pairs of
legs are five-jointed and those of the last pair four-jointed,
and in others again, such as the Fireflies, the Click-beetles,
(Elateridae), the Lamellicornes, the Burying-beetles and
Staphylinidae, the Water-beetles (Gyrinus, Hydrophilus, etc.),
the Carabidne (Calosoma, Carabus, Harpalus, Bracliinus, etc.),
and the Tiger-beetles (Oicindela), all the tarsi are five-jointed,
and all the joints approximately equally developed.

The larvae vary greatly in form in the different genera.
In the Lady-bugs and some other forms they are Thysanuri-
form, the three anterior trunk-segments (corresponding to the
thoracic segments of the imago) possessing each a pair of limbs,
while the abdomen terminates in a pair of cerci. In some
Water-beetles (Gyrinus] tracheal gills are present, and the
larvae of the Lamellicorns (Fig. 239, «) are soft-bodied eyeless
white forms, characterized by a saclike dilatation of the last
abdominal segment, and live beneath the surface of the
ground feeding upon the roots of grasses. In the Click-bee-
tles (Elateridse) the body of the larva is elongated and slen-
der and very hard, these forms being known as the wire-
worms and feeding, like the Lamellicorn larvae, upon the roots
of plants. In the Boring-beetles, the larvae, which excavate
burrows beneath the bark or in the wood of various trees,
have the limbs almost or quite rudimentary, while maggot-
like larvae are characteristic of the Weevils.

The larva, whose life may be prolonged through several
years, passes finally into a resting pupa stage of the libera
form, resembling in the body form and the mouth-parts the
imago which sooner or later issues from it. The metamor-
phosis is thus complete.

In the peculiar Meloid form Sitaris an interesting phenomenon known
as hypermetamorpJiosis occurs. The first larva is Thysauuriform, and is
parasitic upon the males of certain bees, passing to the female bee during
copulation, and then, during the deposition of the ova in the cells filled
with honey, the parasite slips upon the egg, which it consumes. It then

514 INVERTEBRATE MORPHOLOGY.

transforms into a maggotlike second larva which lives upon the honey on
the surface of which it floats, and after a time passes into a resting pseudo-
chrysalis stage, from which a larva similar to the second one emerges, and
this finally transforms into a pupa which gives rise to the adult.

10. Order Neuroptera.

The Neuroptera are characterized by the abundant and
rich venation of their wings, in which numerous cross-veins
extend between the longitudinal ones. The mouth-parts are
adapted for biting, the mandibles being in some forms (Cory-
dalis) very large. The lace-winged flies (Chrysopct) also be-
long to this group, as does also the Ant-lion (Myrmdeon, Fig.
240), whose larva excavates a funnel in loose sand, and
buries itself at the bottom with only the head and powerful

FIG. 240. — Myrmeleon obsoletus (from PACKARD).

mandibles projecting, ready to snap up any insect which slips
down the yielding sides of the trap. The larvae are usually
Thysanuriform, those of Chrysopa attacking Aphides, whence
they are frequently termed Aphis-lions, while those of C'ory-
dalis are aquatic and possess tracheal branchiae upon the
abdomen. This larva is familiar to anglers as the Hell-
gramite. The metamorphosis is complete.

11. Order Panorpata.

This order contains a small number of forms, the majority
of which possess membranous wings resembling those of the
Neuroptera, except that the cross-veins are not so numerous.
The anterior part of the head is produced into a downwardly
projecting snout, at the extremity of which are the small
biting mouth-parts, the arrangement recalling that found in
the Curculionidae among the Coleoptera. In the genus Pa-

TYPE Tit ACHE ATA. 515

norpa, the Scorpion-fly, the abdomen terminates in a pair of
forceplike processes similar to those of the Dermaptera.

The metamorphosis is complete, the larvae differing from,
those of the orders already described in possessing in addi-
tion to the three pairs of thoracic legs eight pairs of abdom-
inal proplike appendages.

12. Order Trichoptera.

The Trichoptera, also a small order, includes the Caddis-
flies (Phryganea, Anabolia). They possess two pairs of wings,
the anterior pair usually differing slightly in appearance
from the posterior, which are larger and folded when at rest
in a faulike manner, the venation consisting principally of
longitudinal veins, with but few transverse ones. The body
and the wings are generally abundantly covered with hairs,
which in some forms are scalelike. The antennae are
usually long and filiform, and the mandibles rudimentary, the
maxillae and labiuin forming a short sucking proboscis.

The metamorphosis is complete, the larvae being aquatic
and provided with spinning-glands with which they bind to-
gether small twigs and particles of sand to form cases within
which they live. They possess tracheal branchiae upon the
sides of the abdominal segments, and the last segment bears
a pair of short but stout processes which are provided with
hooks. The pupa is formed within the larval case, but before
transforming into the imago it leaves the case and crawls to
laud, where the imago emerges.

13. Order Lepidoptera.

This is a large order, including the Butterflies and Moths,
all of which, with the exception of the females of a few forms
(Orgyia), possess two pairs of wings covered with overlapping
scalelike hairs, and Avith but few transverse veins. When at
rest the wings are rarely folded, but are either held erect, as
in the Butterflies, or lie one over the other, resting upon the
abdomen. The body, like the wings, is covered with hairs or
scales.

The antennae differ considerably in shape in different

516

INVERTEBRATE MORPHOLOGY.

forms, being in the Butterflies usually club-shaped, while in
male moths they are frequently featherlike, though more sim-
ple or filiform in the females. The mouth-parts are adapted
for sucking, forming in most cases a long tube, which, when
not in use, is coiled into a helix. In the smaller members of
the group (Microlepidoptera), which are in many respects the

most primitive and include such
forms as the Clothes-moth (Tinea),
the moth of the Apple-maggot (Car-
pocapsa), the leaf-rollers (Pyralidae),
etc., the sucking arrangement is by
no means perfect, the mandibles being
present, and the maxillae and labium
resembling in structure the corre-
sponding parts in biting insects, ex-
cept that the two inner terminal
plates of the labium are united to
form a short tube. In the higher
forms (Macrolepidoptera), however,
the mandibles (Fig. 241, mri) are quite
rudimentary and the labium is much
reduced in size, though its palps (Ip)
are frequently large and well de-
veloped. The sucking-tube is com-
posed of the two maxillae (m) which are produced into two
long filaments grooved on their mesal surfaces, and by their
apposition the tube is formed.

The metamorphosis is in all cases complete, the larvae
being wormlike structures known as caterpillars. Their
mouth-parts are adapted for biting, and they live for the
most part upon the leaves of various plants, frequently ac-
complishing much destruction. This is especially the case
with the Tent-caterpillar (Clisiocampa), which lives in colonies
enclosed within a web which is extended from twig to twig as
the leaves are gradually eaten ; various kinds of trees suffer-
ing from its ravages. The shade-trees in cities, especially
the Horse-chestnut, are sometimes greatly injured by the
caterpillar of the Tussock-moth (Orgy id), and the larvae of the
common white Cabbage-butterfly (Pieris) feed upon the leaves

ran

FIG. 241.— HEAD OF MOTH.
Spkinx liguslri, SHOWING

JVIOUTH-PABTS (after NEW-
PORT).

a — antennae.
I — labrum.
Ip — labial palp ; that of

left side removed.
m = maxillae.
mn = mandible.
o = eye.

TYPE TRACHEATA. 517

of the Cabbage ; many other similar examples might be
given. A few of the Microlepicloptera possess aquatic larvae,
but they form exceptions. In the typical caterpillar there
are, in addition to the three pairs of thoracic legs, five pairs of
short stout prop-legs situated upon the third, fourth, fifth,
sixth, and tenth abdominal segments, and the body may be
covered with hairs of various lengths, as in the larvae of many
moths (e.g. the Woolly Bear, Spilosoma), or may possess
spiny processes, as in the larvae of the Mourning-cloak Butter-
fly ( Vanessa) -which feeds on the "Willow, or variously-
shaped tubercles, as in the American silkworm (Telea) and
the Cecropia larva. In one group of moths, the Geometridre,
but two or three pairs of prop-legs occur, situated on the
more posterior segments, and in progression these forms
draw these legs up close to the thoracic limbs, throwing the
intervening portion of the body into a loop, whence the terms
"measuring-worms" or "loopers" often applied to them.
In rare cases, as in a few Microlepidoptera, the larva is
without feet and maggotlike.

The pupa or chrysalis is of the obteda variety, and is fre-
quently enclosed within a silken case termed the cocoon,
spun by the larva whose salivary glands are converted into
spinning-glands. A cocoon is more generally present in the
Moths than in the Butterflies, whose chrysalids are suspended
by a patch of silk to which the hind end of the pupa is at-
tached or may be in addition slung in a silken loop passing
round the body near the middle (Fig. 231).

14. Order Hymenoptera.

The Hvmenoptera possess four membranous wings, with
comparatively few veins and not covered with scales or hairs
but transparent, the anterior pair being usually larger than
the posterior. The abdomen is sometimes broadly attached
to the thorax, as in the Saw-flies (Tenthredinidae), but more
usually the anterior one (Bees) or two (Ants) abdominal seg-
ments are very narrow, so that the abdomen seems to be at-
tached by a stalk. The females possess ovipositors which
may be retractile and provided with a poison-gland, forming

518

INVERTEBRATE MORPHOLOGY.

mxp —

Pg

efficient organs of offence and defence, as in the Ants, Bees,
aiid Wasps, or else long and slender and but partially retrac-
tile and destitute of a poison-gland, as in the Saw-flies, Gall-
flies, and Ichneumonidae.

The mouth-parts are adapted partly for biting and partly

for licking. The mandibles (Fig. 242,
/////) are well developed and fitted for
biting in all forms, and in the Ten-
thrediuidae the maxillae are also like
those of biting insects, while the
inner of the two terminal plates of
the labium are united to form a tube,
the outer plates remaining separate.
In the Bees and Wasps the maxillae
(nix) become elongated and are no
longer adapted for biting, and the
inner terminal plates of the labium
are fused together to form a long
tonguelike structure, the ylossa (I),
the outer plates forming what are
termed the paraglossae (pg). The
entire apparatus is adapted for biting
and also for licking up the honey
contained in the nectaries of flowers.
The great majority of forms are
solitary, but a few Bees (^pis, Bom-
bus] and Wasps (Vespa, Sphex) and the Ants (Formica,
Camponotus] form social aggregations with more or less
pronounced polymorphism, to which reference has already
been made. The Gall-flies (Cynips) lay their eggs upon the
leaves or stems of plants, at the same time injecting a poison
which causes a proliferation of the plant-tissues, forming a
gall in the interior of which is the larva of the insect ; while
many forms, such as the Ichneumon-flies, Proctotrupes, Ptero-
malus, Microgaster, etc., are parasitic in their larval stage, the
eggs being deposited in or upon the bodies of the larvae of
other insects, a very decided check being exerted upon the
larvie of injurious insects, such as the Cabbage-butterfly, by
these forms.

FIG. 242 — MOUTH-PARTS OF
BEE, Anilwpliora (after
NEWPORT from GEGENBAUR).

I = glossa.
Ip = labial palp.
mn = inaudible.
mx = maxilla.
mxp = maxillary palp.
pg = paraglossa.

TYPE TRACHEATA.

519

The larvae of the Tenthredinidae, for example that of the
Pear-slug (Selandria), which feeds upon the leaves of the pear-
tree, resemble the caterpillars iu possessing prop-legs, of
which there are as many as eight pairs. In the majority of
forms, however, owing to parasitism or to being in contact

FIG. 243. — Sphex icfineumonea (from PACKARD).

FTG 244. — Hypoderma boms (from PACKARD).

with an abundant supply of nutrition stored up by the parents
(Bees, Wasps) or to being fed and cared for by the workers
among the Ants, the larvae are maggotlike and almost or
entirely destitute of legs. The metamorphosis is complete,
the pupa being a pupa libera.

15. Order Diptera.

In this order, as the name indicates, but two wings are
present (Fig. 244), which are those of the mesothorax, the

520

IN VERTEBRA TE MORPHOLOG 71

..wd

metathoracic pair being usually represented by a pair of club-
shaped bodies on the sides of the segment, termed halteres or

balancers. The wings are always trans-
parent and the veins by no means
abundant. In a few forms, such as the
Sheep-tick (Mdophagus) and the Fleas
(Pulex), the wings are entirely wanting
in harmony with the parasitic habits
which these forms possess, but they
form exceptions to the general rule.

The mouth-parts are adapted for
sucking and also for piercing ; the
labruin (Fig. 245, Ir) and labium (la)
are prolonged into grooved processes,
forming together a tube within which
lie, in the female Mosquitoes (Oulex)
and Gadflies (Tabanus), two pairs of
elongated needlelike rods which repre-
sent the mandibles (md) and maxillae
(ma;), to which a fifth unpaired stylet
may be added which arises as a growth
from the lower wall of the pharynx
(hy). In other forms the maxillae only

OF A GNAT, Culex, THE have the acicular form, the mandibles
LABRUM TURNED TO fusing with the labrum, and in all cases
ONE SIDE (from HERT- the maxiHary palps are present, while

hy*= hypopharynx, a the labial PalPs are undeveloped. In
process of labium. the ordinary House-fly (Musca) the ex-
tremity of the sucking-tube is expanded
into a disklike structure, and in all
forms the salivary glands open near the
extremity of the tube.
The larvae are usually maggotlike (Fig. 244), entirely
destitute of feet, and in some forms the head even is indis-
tinguishable. The metamorphosis is complete, the pupa being
in the Mosquitoes active, swimming about in water, though
more usually it is incapable of motion, and enclosed within
the last larval skin, thus belonging to the coarctata variety.

FIG. 245.— MOUTH-PARTS

la = labium.

Ir = labrum.
md = mandible.
tnx = maxillae.

p = maxillary palp.

TYPE TKACUEATA,

Development and Affinities of the Inseata .—The early stages of Insect
development cannot be discussed here, belonging more properly to text-
books of Embryology, but mention should be made of the remarkable phe-
nomenon which occurs during the transformation from the pupal to the
iinaginal conditions in those forms whose metamorphosis is complete. In
describing the development of the Acarina it was pointed out that during
the transition from one stage to the next a histolysis and subsequent regen-
eration of certain parts of the body occurred. In the holometabolic Insects
the same process occurs during the pupa stage, the larval hypodermis,
the majority of the muscles, and the entire digestive tract and its appen-
dages undergoing degeneration, and being absorbed and digested by the
blood-corpuscles, the parts being formed anew from patches of cells present
in the larva and known as iinaginal disks. The histolysis and regenera-
tion proceed pari passu, so that the identity of the various organs is pre-
served throughout the process. The imaginal discs are to be regarded as
portions of the original anlagen of the various organs which have re-
mained during larval life in an embryonic condition, springing into activity
and completing their development during the pupal stage.

As regards the affinities of the various orders of the Pterygota, it may
be pointed out that the frequent occurrence of Thysanuriform larvse indi-
cates a descent from Apterygote ancestors, and those orders which present
larvae of a wormlike or maggotlike form are in all probability the most
highly specialized. It is in these cases that the complete metamorphosis
occurs, and it is self-evident that the gradual and incomplete metamor-
phoses are more primitive than the complete. Indeed all metamorphosis
depends upon the differences in habit and structure of the larva and imago,
and becomes more and more complete according as the larvse and imagines
depart more and more widely from the Thysanuriform type of structure.
Consequently it may be concluded that those forms are the most primitive
which retain most perfectly both in the larva and imago the Thysauurid
characters. These are found most perfectly in the Dermaptera, to which
both in the adult and larval stages the Corrodentia (so far as they have
not become modified by parasitism) and the Orthoptera seem closely re-
lated, and it is interesting to note in this connection that the earliest In-
sects known from the Palseozoic rocks seem to have been closely related to
the recent group of the Orthoptera.

Another order which has retained Thysanuran characters in the larva,
though the imagines are more highly specialized than are those of the
Dermaptera, is that of the Thysanoptera, whose habits and mouth-parts
indicate affinities with the Rhynehota, these two orders together forming
a second group traceable back to the primitive Pterygota.

A third group starts with the Epherneridae, which lead up to the Odo-
nata, the larvse of the latter having, however, become greatly specialized,
the resemblances being most marked in the adults, and are indicated by
the character of the wings and by the mouth-parts. More distantly re-

522 INVERTEBRATE MORPHOLOGY.

lated are the Neuroptera with Thysanuriform larvae, probably to be re-
garded as a group which has undergone a development parallel to that of
the Ephemeridae and Odonata, the relationship being traceable back to an
ancestor common to it and the Ephemeridae. To this group may also be
referred the Plecoptera.

A fourth group includes those forms in which the larvae are provided
with prop-legs, secondary forms in which all indications of the Thysanurid
ancestors have disappeared. Of such forms the Panorpata show relation-
ships on the one hand with the Ephemerid group, and somewhat closely
related are the Trichoptera, whose entire organization points to a close
affinity with the Microlepidoptera. From the primitive Microlepidoptera
two lines of descent are probably to be traced, one leading to the Macro-
lepidoptera and the other to the primitive Hymenoptera, the resemblance
between the larvae and the mouth-parts of the Tenthredinidae, and those
of the Microlepidoptera being very striking.

The two remaining orders, the Coleoptera and Diptera, are very highly
specialized, both being holometabolic, and the temptation is to look for
their ancestors in forms with a similar metamorphosis. This temptation
may be justified in the case of the Diptera, whose larvae are the most
modified of all, and it is not impossible that they have descended from
primitive Hymenopteran ancestors, their nearest existing relatives being
the Tenthredinidae, whose sluglike larvae, suggest not a little the least
modified Dipterau maggots. With the Coleoptera, however, the case is
different, and it seems more probable that their holometabolism has been
acquired quite independently of that of the other holometabolic orders.
The larvae of some beetles, notably those of the Coccinellidae, are markedly
Thysanuriform, and prop-legs do not occur in the order. To which of the
groups they are to be referred it is very difficult to say, though the mouth
parts and the arrangement and structure of the wings in the adults point
to an affinity with the Orthoptera.

Granting a descent of the Pterygota from wingless ancestors, it becomes
an interesting problem to discover the origin of the wings. Attempts have
been made to show that they are modified tracheal branchiae, a theory
whicli necessitates the derivation of the Pterygota from aquatic ancestors.
Such a derivation, however, is unsupported by any evidence at present at
our disposal, it being much more probable that the immediate ancestors of
the Pterygota were terrestrial, just as Campodea is to-day. The wings
arise in the embryo as dorsal outpouchings of the meso- and metathorax,
tracheae later pushing out into them, and transient indications of out-
l><mchings of the prothorax also occur in some embryos. It has been sug-
gested that primarily the wings were platelike outgrowths of the thoracic
segments which served to break the fall and increased the distance trav-
ersed by jumping Insects, and in support of this view the fact may be
mentioned that many Apterygota are saltatorial. The limitation of the
wings to the meso- and metathorax may stand in some relation to the
centre of gravity of the body.

TYPE TR AGUE ATA. 523

The Phylogeny of the Tracheata. — It has been the custom to unite
together the Crustacea, Arachnida, and Tracheata in a single group, the
Arthropoda, characterized by the possession of a chitinous cuticle, by the
occurrence of jointed limbs, and by the masticatory organs being modified
limbs ; and furthermore it has been customary to consider the Arachnida
and the Tracheata as closely related on account of the occurrence in both
groups of tracheae. The early processes of development in the three
groups also show many points of similarity, though a closer examination
shows decided differences in the details. How far convergent evolution
may have operated to produce the similarities is the problem to be settled,
and it can be settled only by a consideration of all the facts at our disposal
which indicate the phylogeny of the various groups, a discussion which
would prove entirely beyond the limits of a text-book.

It has been pointed out that the probable ancestry of the Crustacea is
to be found in the Annelida, and that the Arachnida have in all probability
descended from Eurypterus-like ancestors, which were certainly Crustacean
in their affinities. Are the Tracheata then also descended from the Crus-
tacea and from forms which possessed tracheae? Our present knowledge
of the group negatives any such supposition ; it seems impossible that the
Tracheata should, like the spiders, have descended from Earypterns-\\\iQ
ancestors ; it must rather be concluded that the similarities between them
and the Arachnida are due to convergent evolution, the embryonic simi-
larities to the acquisition of comparatively large amounts of food-yolk in
the ova, distributed in a similar manner, and the similarities of the adult
to the exigencies of a terrestrial life. The occurrence of tracheae in both
groups seems at first an important point of similarity to be accounted for
only by a community of descent, but, when it is considered that in the
terrestrial Isopoda tracheae also occur in the branchial opercula, and that
their occurrence in these forms is a purely secondary adaptation, without
any phylogenetic significance, it is evident that their importance as indi-
cations of affinity is much reduced. It may also be pointed out that the
Malpighian tubules of the Arachnida and Crustacea are endodermal, whereas
in the Tracheata they are ectodermal, arising from the ectodermal hind-
gut.

There is little room for doubt but that Peripatus is closely related to
the Annelida, and its relationships to the Myriapoda are also pronounced,
so that the conclusion seems inevitable that the Tracheata have been de-
rived from Annelid-forms, and have therefore a phylogeny practically inde-
pendent of that of the Arachnida. However, it is possible that the Anne-
lid ancestors of Peripatus and those of the Crustacea were more or less
closely related, and that certain of the general similarities of all the three
groups are thus to be accounted for, though to what extent we are not at
present in a position to judge. One point, namely, the occurrence of com-
pound eyes of similar structure in both groups, seems worthy of considera-
tion, since it seems to be unexplainable by this hypothesis and to be a re-

524 INVERTEBRATE MORPHOLOGY.

markable instance of convergent evolution. It is to be noticed that the
most primitive Insects, the forms through which affinities to the Crustacea
if they exist must be traced, are as a rule provided only with simple eyes,
a condition repeated in the eyes of Insect larvaB — a fact which indicates that
the compound eyes are structures which were not characteristic of the
primitive Insects, but have developed within the limits of the group and
can therefore have no phyletic connection with the compound eyes of the
Crustacea. Adding to this fact the independently-developed tendency to
form compound eyes seen in certain Annelida and Pelecypod Mollusks, it
seems probable that notwithstanding their remarkable structural similari-
ties the compound eyes of Crustacea and Insects have been independently
acquired. Instead, therefore, of uniting the three groups together as a type
Arthropoda equivalent to the other types, it seems preferable to separate
them as distinct, just as is done with the Annelida and Mollusca, and the
Annelida and Prosopygia.

Starting, then, with the supposition that Peripatus has descended from
Annelid ancestors and represents the ancestors of the Myriapoda, the rela-
tionships of the various orders of this class and of the Insects remains to
be traced. Unfortunately a large gap exists between Peripatus and any
recent Myriapods, and it is possible that this class is a heterogeneous group ;
indeed by some recent authors it has been suggested that it should be
done away with as a class, the Chilopods being united with the Insecta to
form one class, while the Diplopocls (perhaps with the Pauropoda associated
with them) should form a second. There is no doubt but that Peripatus
possesses many tracheate peculiarities, but its affinities to the remaining
Tracheates are much more remote than those which exist between the vari-
ous groups of Myriapoda, or between any of these groups and the Insecta.
The character of the various appendages considered in relation with the
nervous system seems to afford an admirable means of indicating the rela-
tionships of the various groups. The brain of Peripatus seems to be
formed by the fusion of three pairs of ganglia ; the most anterior and dor-
sal of these gives rise to the antennal nerve and the most posterior inner-
vates the mandibles, while upon the middle one, which is closely related to
the rnandibular ganglion, the eye seems to be placed. It may be assumed
that the ganglia with which the eyes are associated represent the Annelid
supracesophageal or cerebral ganglia and may therefore be termed the pro-
tocerebrum, while the antennary ganglia form the deutocerebrum, and the
mandibular the tritocerebrum. In the Myriapods and Insects the brain is
also composed of three parts to which the same names are applied, the
antenna being innervated from the deutocerebrum, while the tritocerebrum
lacks a corresponding appendage, though in certain Insects transient indi-
cations of a tritocerebral appendage have been seen. Bearing these facts
in mind, the ganglia and appendages of the various groups may thus be
tabulated, and to make the comparison complete the Crustacea are also in-
cluded.

TYPE THACHEATA.

525

Ganglion.

Crustacea.

Peripatus.

Diplopoda.

Chilopoda.

Insecta.

Deutocere-
bral

Antennae ?

Antennae

Antennae

Antennae

Tritocere-

A.nt6nnul6S

Mandibles

1 st postoral
','d
3d
4th
5th
6th

Aniennte
Maudibles
1st maxillae
2d
1st thoracic limb
2d

Oral papillae
1st legs
3d "
3d "
4th "
5th "

Mandibles
Maxillae
1st legs
2d "
3d "
4th "

Mandibles
1st maxillae
2d
Maxillipedes

1 st legs
3d "

Mandibles
1st maxillae

3d
1st legs
2d "
3d "

It .will be seen from this that in the Diplopoda the arrangement is
intermediate between that found in Peripatus and that of the Chilopoda,
while these latter approach closely the Insecta, and this seems to be the ac-
tual relationship, Scolopendrella forming an intermediate link between the
Chilopods and the Insecta, approaching the Thysanura closely in the ar-
rangement of the mouth-parts and in the number of segments of which the
body is composed. The Diplopoda, it is true, pass through a larval stage in
which but six legs are present, and it might at first sight be supposed that
this indicates an affinity with the Insecta, but these legs do not belong to
the same segments as do those of the Insects, and furthermore the occur-
rence of rudimentary abdominal appendages in some Thysanura, as well as
in the embryonic stages of probably all Pterygota, indicates that the In-
secta have been derived immediately from forms with many pairs of appen-
dages, and these forms seem to be represented most accurately by the exist-
ing Scolopendrella.

SUBKINGDOM METAZOA.
TYPE TRACHEATA.

I. Class PROTRACHEATA. — Annelid-like forms ; trunk not differentiated

into thorax and abdomen ; with nephridia. Peripatus.
II. Class MYRIAPODA. — Elongated forms; trunk not differentiated into
thorax and abdomen ; posterior trunk-segments with appen-
dages in the adult.

1. Order Pauropoda. — Small forms ; with only one pair of maxillae ;

antennae ending in three flagella ; reproductive orifices at basis
of second pair of trunk-appendages. Pauropus, Enrypauropiis.

2. Order Diplopoda. — With only one pair of maxillae ; antennas

simple ; reproductive orifice on second or between second and
third trunk-segments ; most of the trunk-segments with two
pairs of legs. lulus, Lysiopetalum, Polydesmus, Strongylo-
so ma, Glomeris.

3. Order Chilopoda. — With two pairs of maxillae and with maxillipeds;

antennae simple; reproductive orifice on the antepenultimate
segment ; each trunk-segment with a single pair of legs.
Cfeophiliis, ticolupendra, Lit/iobius, Scutiyera.

526 INVERTEBRATE MORPHOLOGY.

4. Order Symphyla. — With only one pair of maxillae and no maxilli-
peds ; antennas simple ; most of the trunk-segments with a
single pair of legs. Scolopendrella.

III. Class INSECTA. — Trunk differentiated into thorax composed of three
rings and an abdomen with typically ten segments.

1. Subclass Apterygota.— Thorax without wings; abdominal segments

sometimes with rudimentary limbs in the adult.

1. Order Tltysamira. — Abdomen with ten segments, terminating in

three cerci ; abdominal appendages frequently present. Le-
pisma, Campodea.

2. Order Colleinbola. — Abdomen with six segments terminating in

two spriugiug-organs ; abdominal appendages wanting. Podura,
Anurida.

2. Subclass Pterygota. — With usually two pairs of wings situated on

the meso- and metathoracic segments ; abdominal appendages
wanting in adults.

1. Order Dermaptera. — Abdomen with forceplike cerci ; anterior

wings small and chitinous, posterior folded like a fan and also
transversely ; mouth-parts biting ; metamorphosis gradual.
Forficula, Labia.

2. Order Orthoptera. — Abdomen usually with cerci ; anterior wings

chitinous, covering the posterior, which fold fanlike and some-
times also transversely ; mouth-parts biting ; metamorphosis
gradual. Caloptemis, Gryllus, Gryllotalpa, Periplaneta, Dia-
pheromera.

3. Order Epltemeridce. — Abdomen with two long cerci ; wings mem-

branous and richly veined, the anterior larger ; not folded when
at rest ; mouth-parts biting, but reduced ; metamorphosis in-
complete. Ephemera.

4. Order Odnnata.— Abdomen with two platelike cerci ; wings mem-

branous and richly veined, not folded when at rest ; mouth-
parts biting; metamorphosis incomplete, sometimes approaching
completeness. Libelltda, jEschna, Ayrion, Diplax.

5. Order Plecoptera. — Abdomen usually with cerci ; wings membra-

nous, moderately veined with few cross-veins ; the anterior cov-
ering the posterior when at rest ; mouth-parts biting ; meta-
morphosis incomplete. Perla.

6. Order Corrodentia. —Abdomen without cerci ; wings sometimes

wanting (parasites and neuters), membranous, the anterior cov-
ering the posterior when at rest ; mouth-parts biting ; meta-
morphosis incomplete or wanting. Termes (with polymorphism),
Atropos, Liotheum, Trichodectes.

7. Order Thysanoptera. — Abdomen without cerci ; wings sometimes

wanting, narrow, poorly veined, fringed with hairs ; the anterior
pair covering the posterior when at rest ; mouth-parts piercing
and sucking; metamorphosis incomplete. Thrips, Phlwotl trips.

TYPE TRACUEATA. 527

8. Order Rhynchota. — Abdomen without cerci ; basal portion of

anterior wings chitinous, posterior wings and tips of anterior
membranous, or else both membranous, the anterior the larger,
or both wanting ; mouth-parts piercing and sucking ; metamor-
phosis incomplete.

Anterior wings chitinous at base (Hemiptera). Anasa, Notonecta,
Belostoma, Ranatra, Hydrometra, Halobates, Cimex, Pediculus
(wings wanting in the last two).

Anterior wings both membranous (Homoptera). Cicada, Aspidi-
otus, Daetylopius, Pemphigus, Aphis (wings may be wanting
in the last three).

9. Order Coleoptera. — Abdomen without cerci ; anterior wings chi-

tinous, covering in the posterior when at rest ; mouth-parts
biting ; metamorphosis complete.

(a) Tarsi of four joints, one of them very small (Cryptote-

tramera). Coccinella.

(b) Tarsi of five joints, one of which is very small (Cryptopen-

tamera). Curculionida?, Clytus, Saperda, Monohainmits,
Doryphora.

(c) Tarsi of posterior legs four-jointed, of two anterior pairs

five-jointed (Heteromera). Meloe, Lytta.

(d) Tarsi all five-jointed and all the joints of equal size

(Pentamera). Lampyris, Elateridae, Melolontha, Necro-
phorus, Staphylinidae, Hydrophilus, Gyrinus, Bra-
chinus, Harpalus, Carabus, Calosoma, Cicindela.

10. Order Neuroptera. — Abdomen without cerci; wings membranous,

richly veined with numerous cross-veins ; mouth-parts biting ;
metamorphosis complete. Corydalis, Chrysopa, Nt/rmeleon.

11. Order Panorpata. — Abdomen sometimes with cerci ; wings mem-

branous with few cross- veins ; mouth-parts biting, at end of
cylindrical rostrum ; metamorphosis complete. Panorpa.

12. Order Trichoptera. — Abdomen without cerci ; wings covered with

hairs or scales, posterior ones larger and folded fanlike when
at rest ; mouth-parts sucking ; metamorphosis complete. Phry-
ganea, Anabolia.

13. Order Lepidoptera. — Abdomen without cerci ; wings covered with

scales, not folded when at rest, though they may overlap; mouth-
parts usually sucking ; metamorphosis complete.

Small forms (Mirrolepidoptera). Tinea, Carpocapsa, Pyralidse.

Larger forms (Macrolepidoptera). Geometridae, Clisiocampa,
Orgyia, Telea, Pieris, Vanessa, Papilio.

14. Order Hymenoptera. — Abdomen without cerci ; wings membra-

nous, without scales, not folded ; mouth-parts biting and lap-
ping ; metamorphosis complete.

Ovipositor retractile with poison-gland (Aculeata}. Apis, Bom-
bus, Vespa, Cainponotus, Formica.

528 INVERTEBRATE MORPHOLOGY.

Ovipositor non-retractile, without poison-gland (Terebrantid).
Ichneumon, Proctotrupes, Pteromalus, Microgaster, Cynips,
Selandria.

15. Order Diptera.— -Abdomen without cerci ; wings sometimes want-
ing, only the anterior pair ever present, posterior pair repre-
sented by halteres ; mouth-parts piercing and sucking ; meta-
morphosis complete.

With wings. Culex, Tabanus, Musea.

"Without wings. Pulex, Melophagus.

LITERATURE.

PROTRACHEATA.

H. N. Moseley. On the Structure and Development of Peripatus capensis.
Philosophical Transactions Royal Society, London, CLXIV, 1874.

F. M. Balfour. The Anatomy and Development of Peripatus capensis. Quar-
terly Journ. Microscop. Science, xxm, 1883.

E. GaiFron. Beitriige zur Atiatomie und Histologie des Peripatus. Zoologische

Beitrage, i, 1885.

J. von Kennel. Entwicklungsgeschichte wn Peripatus Edwardsii und P. torqua-
tus. Arbeiten des Zool. Instituts Wiirzburg, vn, 1885 ; vm, 1886.

A. Sedgwick. The Development of the Gape Species of Peripatus. Quarterly
Jouru. Microscop. Science, xxv-xxvin, 18^5-1888.

A. Sedgwick. A Monograph of the Species and Distribution of the Genus Peri-
patus. Quarterly Journal of Microscop. Science, xxvui, 1888.

MYRIAPODA.

H. C. Wood. The Myriapoda of North America. Trans. American Philosoph.

Soc., xm, 1865.
C. H. Bollman. The Myriapoda of North America. Bulletin U. S. National

Museum, No. 46, 1893.
R. Lalzel. Die Myriapoden der osterreichisch-ungarischen Monarchic. Wien,

1880-1884.
J. A. Ryder. The Structure, Affinities, and Species of Scolopendrella. Proc.

Academy Nat. Sciences, Philadelphia, 1881.
J. Bode. Polyxenus lagurus, de Oeer. Ein Beltrag zur Anatomie, Morphologic

und Entwicklitngsgeschichte der Chilognathen. Zeitschr. fiir d. gesammte

Naturwisseusch., XLIX, 1877. •

0 vom Rath. Beitrage zur Kenntniss der Chilognathen. Bonn, 1886.
C. Herbst. Beitriige zur Kenntniss der CJdlopoden. Bibliotheca Zoologica,

Heft ix, 1891.

F. G. Heathcote. The Early Development of lulus terrestris. Quarterly Journ.

Microsc. Science, xxvi, 1886.

F. G. Heathcote. The Post-embryonic Development of lulus lerrestns. Philo-
sophical Trans. Royal Soc.. London, ci.xxix, 1888.

TYPE TRACUEATA. 529

INSECTA.

SYSTEMATIC.

A. S. Packard. A Guide to the Study of Insects. New York, 1889.

Sir John Lubbock. A Monograph of the Collembola and TJiysanura. London,

1893.
H. Hagen. Synopsis of the Neuroptera of NortJi America. Smithsonian Inst.

Miscellaneous Collection, iv, 1861.

C. H. Fernald. 2 fie Orthoptera of New England. Boston, 1888.
J. L. Leconte. Classification of the Coleoptera of North America. Smithsonian

Inst. Miscellaneous Collection, in, 1892 ; xi, 1873.
J. L. Leconte. Numerous Papers on Coleoptera in Proceedings Acad. Nat.

Sciences, Philadelphia.
S. H. Scudder. The Butterflies of the Eastern United States and Canada, with

Special Reference to New England. Cambridge, 1889.
J. B. Smith. Contributions towards a Monograph of the Noctuidce of North

America. Proc. and Bulletin U. S. Nat. Museum.
A. S. Packard. Monograph of the Oeometrid Moths. Report U. S. Geological

Survey of the Territories, x, 1876.
E. T. Cresson. Synopsis of the Families and Genera of the Hymenoptera of

America north of Mexico, together with a Catalogue of Described Species and

Bibliography. Trans American Entomological Society, 1887.
H Loew and Baron Osten Sacken. Monograph of the Diptera of North America.

Smithsonian Institution, Miscellaneous Collection, vi, 1862 ; vi, 1864 ;

VITI, 1869 ; xi, 1873.
S. H. Scudder. The Fossil Insects of North America. New York, 1890.

See also the publications of the U. S. Entomological Commission and the
Annual Reports of the U. S. Department of Agriculture for numerous papers
by C. V. Riley, A. S. Packard, and others.

STRUCTURAL.

H. Grenacher. Untersuch^ngen uber das Sehorgan der Arthropoden. Gottin-

gen, 1879.
E. Burgess. Contributions to the Anatomy of the Milkweed Butterfly (Danais

archippus). Anniversary Memoirs Boston Soc. Nat. History, 1880.
H. C McCook. The Honey-ants of the Garden of the Gods, etc. Philadelphia,

1881.
V. Graber. Die chordotonal Sinnesorgane und das Gehor der Insekten. Archiv

fur mikrosk. Anat., xx, 1882.
E. Witlaczil. Zur Anatomie der Aphiden. Arbeiten a. d. zool. Inst. Wien,

iv, 1882.
Sir John Lubbock. The Origin und Metamorphosis of Insects. London, 1883.

Ants, Bees, and Wasps. London and New York, 1883.

J. A. Palmen. Zur vergleichenden Anatomie der Ausfiirhrungsgdnge der Sex-

ualorgane bei den Insekten. Morpholog. Jahrbuch, ix, 1883.
J.Carriere. Die Sehorgane der Thiere vergleichend-anatomisch dargestellt.

Munchen, 1885.

530 INVERTEBRATE MORPHOLOGY.

B. Grassi. Analomia comparata dei Tisanuri e comiderazioni generali sull'

organizzaziane degli Insetti. Atti della R. Accad. Lincei, Serie IV. rv,

1887.
K. Heider. Die embryonale Entwicklung wn Hydrophilus piceus L. Jeua,

1889.
J. Van Rees. Beitrdge zur Kenntniss der inneren Metamorphose wn MU.&CO,

vomiloria. Zoolog. Jahrbiicber, in, 1888.
V. Graber. Vergleichende Studien am Keimstreif der Insekten. Denksckr.

Acad. Wisseusch. Wieu, LVIII, 1890.
H. T. Fernald. The Relationships of Arthropods. Studies from the Biolog.

Labor. Jobns Hopkins Univ., iv, 1890.

B. T. Lowne. Anatomy, Physiology, Morphology, and Development of the Blow-
fly. London, 1890-91.
W. M. Wheeler. A Contribution to Insect Embryology. Journ. of Morphology,

vni, 1893.
T. H. Huxley. Anatomy of Cockroach. Text-book of Anatomy of Invertebrate

Animals. New York, 1878.
W. K. Brooks. Anatomy of Grasshopper. Hand-book of Invertebrate Zoology.

Boston, 1890.

TYPE ECHINODERMA. 531

CHAPTER XVI.

TYPE ECHINODEKMA.

THE Echinoderms are exclusively marine organisms and
vary considerably in shape, some forms being elongated and
vermiform, others stellate, and others again almost spherical.
Whatever may be the shape, however, a well-marked radial
symmetry can be distinguished, which suggested to the older
zoologists the association of the members of this group with
the Coslentera in a type Radiata. The radii in the Echino-
dernia are, however, almost invariably five, instead of four or
six or some multiple of these numbers as in the Coelentera ;
and, furthermore, while in the Coeleutera the radial symmetry
represents a primitive condition and any departure from it
towards bilaterality, as in the Authozoa, is secondary, the
reverse is the case with the Echinoderma. The larval forms
of this group are strictly bilateral, and even in the adults
certain organs or parts of organs interfere with the regularity
of the pentamerous arrangement and bring about a more or
less pronounced bilaterality.

This may be clearly seen if one of the stellate forms, such
for instance as the ordinary five-rayed Starfish (Fig. 246 \
be examined. This animal consists of a central disk, at the
centre of one surface of which, the oral surface, the mouth is
found, while the anus occupies a somewhat excentric position
on the other surface, which may be termed the aboral or
apical surface. From the edge of the disk the five arms or
rays project outwards, and along the median line of the oral
surface of each arm there extend outwards from rings around
the mouth a nerve-cord and a hydroccel canal, this latter
forming a part of a peculiar system of vessels characteristic
of the Echinoderms. In consequence of this radiation of
these structures out along the arms, and the arrangement of

532

INVERTEBRA TE MORPHOL OGT.

the other organs for the most part in conformity with the
radiation, the arms may be regarded as representing the
radial axes of the body, the interradial axes lying in the in-
tervals between them. If now the aboral surface of the disk
be examined, there will be found upon it, in one of the inter-
radii, a peculiar tubercle, known as the inadreporiforrn tuber-

Uti

rail -.--sSlS

""r»'f>'U>

^^

sSHiJSGffilA" ^Eft*!*""*

•r *••

j^

^w

$r

FIG. 246. — .4s£mas arenicola (after AGAssizfrom VEKRILL).

cle, which serves to place the hydrocoel system of canals in
communication with the exterior water. There is but one
such tubercle, and but one canal leading down from it to the
hydrocoel ring which surrounds the mouth, and consequently
there can be but one plane in which the animal can be di-
vided into two similar parts. Therefore the Starfish, though
superficially appearing to possess a radial symmetry, is funda-
mentally bilateral — a statement which applies equally well to
any member of the Echinoderm type.

It does not necessarily follow, however, that the plane
which passes through the madreporiform tubercle is the
median plane of the body. The larvae of the Echiuoderms
are strictly bilateral organisms, no sign of radiality being
found in them in an early stage of development, and it would
seem more satisfactory to take as the median plane of the

TYPE ECHINODERMA. 533

adult animal one which corresponds as closely as possible
with the larval median plane. The madreporiform tubercle,
or rather the pore which corresponds to it, and the tube
which leads from it to the rudiment of the hydroccel system
can readily be made out in the larvae of most forms, and it
can be seen that it lies to the left of the median plane of the
body. Indeed in the larvae of some Starfishes two pores
occur at an early stage of development, one to the left and
the other to the right of the median plane, the latter subse-
quently disappearing. The madreporiform tubercle might
therefore be regarded as lying to the left of the median plane,
which wrill accordingly pass through the radius to the right
of the tubercle.

However, it is impossible to tell how much modification
has taken place during the transition from the bilateral to
the radial condition, and it is not impossible that the greater
portion of the adult represents one of the halves of the
embryo, the other half remaining more or less undeveloped.
Furthermore a secondary bilaterality supervenes in certain
forms of Echinoidea and Holothuroidea which does not agree
with that indicated in the preceding paragraph, and is not
indeed the same in the two groups. It seems therefore pref-
erable to assume a perfectly arbitrary method of indicating
the radii of the body, calling that radius which lies opposite
the madreporiform tubercle A, that which lies to the left of
this when the animal is held with the oral surface upwards
By and so on C, D, E, following the direction of the hands of
a watch. The interradii may be indicated by combining the
letters of adjacent radii, the interradius between A and B
being denoted by AB.

The body-wall in the Echinoderma is covered on the out-
side by a usually delicate, and in some cases ciliated, ecto-
derm, which may, however, be indistinguishable from the sub-
jacent mesodermal tissues in certain parts of the body.
Below this ectoderm, when present, comes a layer of meso-
dermal connective tissue consisting of relatively few cells
imbedded in a more or less fibrillar matrix, and in this con-
nective tissue there are imbedded numerous calcareous plates,
in some forms, such as the Holothuriaus, somewhat widely

534

INVERTEBRA TE MORPHOLOO T.

separated from one another so that the body-wall has a more
or less leathery consistency, but more frequently placed
almost or quite in contact with each other, and uniting in
most of the Echinoids or Sea-urchins to form a firm test en-
closing the principal vegetative organs, a small area or peri-
stome around the mouth alone remaining but partially calci-
fied and retaining a leathery consistency. Spinous elevations
are frequently developed upon these dermal plates (whence
the name of the type) and may assume various forms, being
in some cases quite long, movably articulated with the plates,
and supplied with muscles so that they may aid in locomo-
tion.

The arrangement of the calcareous plates differs greatly

in the different classes which com-
pose the type, but certain of them,
distinguishable by their position
and relative arrangement, reappear
in the majority of the classes.
These plates are situated at the oral
and aboral surfaces of the body.
The oral plates are not so constant
nor so numerous as the aboral or
apical, and show a tendency, even in
those groups in which they are most
highly developed, to undergo a
greater or less amount of resorption
during development, being frequent-
ly more pronounced in larval than
in adult life. Typically the oral

Zoroaster, SHOWING THE system consists of a central oral
SYSTEM OF PLATES plate> the orocentra^ untnown iu

recent forms, but occurring in cer-
tain fossil genera, and this is sur-
rounded by a ring of five plates,
which may be termed the oral plates,
and which have an interradial posi-
tion. The apical system has as a
central plate the so-called centro-
dorsal (Fig. 247, CD), which in some forms is replaced by a

FIG. 247.— DISK AND ARM OF

(after SLADEN).
an — au us.
cd = centro-dorsal.
mt =r madreporite.
T -= terminal plate.

2 = under-basals.

3 = basals.

4 == radials.

TYPE ECIIINODERMA. 535

number of small plates between which the anal opening of
the digestive tract is to be found. Forming a ring around
this are frequently five plates possessing a radial position
which are termed the under-basals (Fig. 247, 2) and are un-
represented in certain forms ; next to these comes a second
circle of five plates, the basals (Fig. 247, 3), which are inter-
radial and correspond to the oral plates, while next to these
again is a third cycle, also of five plates, the radials (Fig. 247,
4), whose name denotes their position. Numerous other
plates may intervene in the various groups betAveen the radials
and the orals, but their number and arrangement is not suffi-
ciently constant to permit of homologies ; the oral and apical
systems are, however, represented more or less perfectly in
all but one of the classes, and consequently deserve special
mention.

A well-developed dermal muscular system occurs in the
Holotlmrians in which the calcareous plates are scattered
and the body-wall consequently capable of considerable con-
traction and expansion, but in other forms it is very much
reduced. In those forms in which the calcareous plates are
simply in apposition strands of muscular tissue pass from
plate to plate, a considerable amount of movement being pos-
sible, but in the Sea-urchins, for example, the dermal muscu-
lature is almost wanting, being reduced to bands passing to
the bases of the movable spines and to the complicated mas-
ticatory apparatus.

The ccelom is somewhat complicated in its relations, which
vary cousiderabl}' in the different groups. In all enteroccelic
and schizocoelic portions are distinguishable, the former in
the embiyo arising as pouchlike diverticula from the primi-
tive intestine or enterou, and later becoming completely con-
stricted off from it. Much variation occurs in the later his-
tory of the pouches in the various groups, but in general it
may be stated that one of them, the left, has a portion con-
stricted off from it, which forms the adult water vascular sys-
tem or hydroccel, a structure characteristic of the Echiuoderms ;
and furthermore this same left enteroccel communicates with
the exterior by a dorsal pore, situated in the iuterradius CD,
and represented in the adult by one or many pores opening

536 INVERTEBRATE MORPHOLOGY.

upon a sievelike calcareous plate known as the madreporiforni
tubercle or madreporite. The hydroccel in the adult communi-
cates with the left enterocoel by a tube, termed the stone-
cnnal from the deposition of calcareous matter which occa-
sionally takes place in its walls, and so indirectly opens to
the exterior through the madreporiform tubercle (see Fig.
265). The various departures from this arrangement which
occur will be more conveniently considered in connection
with the special descriptions of the various groups ; the con-
dition just mentioned may be provisionally accepted as rep-
resenting the typical arrangement.

After the separation of the hydrocoel from the left entero-
ccel, the latter and the enterocoel of the right side increase in
size and finally apply themselves closely to the inner surface
of the body-wall and to the outer surface of the digestive
tract, forming the peritoneal lining of these structures. Where
the two cceloniic sacs meet there are formed, of course, two
partitions extending from the body-wall to the intestine, and
suspending that structure between them. These partitions
are the mesenteries, but before the embryo reaches the
adult stage one of these mesenteries disappears, the other
persisting in a more or less perfect form. The coiling of the
intestine, which occurs frequently in the adult forms, brings
about complications of the course of the mesentery, compli-
cations further increased in most cases by the formation of
other partitions which may traverse a greater or less portion
of the coelom either longitudinally or transversely. One of
the transverse partitions, most frequently present, separates
off more or less completely from the rest of the coelom, a por-
tion of it surrounding the pharyngeal region of the digestive
tract and hence termed the peripharyngeal cavity, while in
some cases a periaual cavity may similarly be formed.

The hydroccel, whose origin has been described, develops
into a tubular ring (Fig. 248, cc) surrounding the oesophagus
quite close to the mouth. Upon this ring in the interradii
one or several saclike diverticula, termed Polian vesicles (p),
occur, and in one interradius a canal, the stone-canal (sc),
passes aborally to open into a thin-walled sac termed the
ampulla of the stone-canal, which is in reality a portion of

TYPE ECHINODEKMA.

537

the left enferoccel, partly or wholly separated off from the
rest of that cavity. This ampulla, as already mentioned,
communicates with the exterior through, the madreporite. In
the radii tubes (re) arise from the ring which extend out to
the aboral extremity of the body in the elongated and spheri-
cal forms, and to the ends of the rays in the brachiate forms,
terminating frequently in tentacular structures (t) which pro-
trude to the exterior, pushing the ectoderm before them.
Along the course of these tubes lateral branches are given off

FIG. 248. — DIAGRAM TO SHOW THE ARRANGEMENT OF THE HYDROCCEL OF

AN ECHINOUERM.

a = ampulla. p = Poliau vesicle.

as — axial siuus. re = radial canal.

cc = circular canal. sc — stone-canal.

M = madreporite. t = terminal tentacle.

if — tube-foot.

which terminate either in tentacular structures, or else in
tubes terminating in a sucker, which, since they play an im-
portant role in locomotion, are termed tube-feet (tf). In many
forms a globular reservoir or ampulla (a) is attached to each
tube-foot, and valves are found at the junction of the branch
passing to the foot with the radial canal, so that the foot can
be extended to a considerable distance by the contraction of
the muscles in the walls of the ampulla and the consequent
forcing of water into it. By means of the sucker they may
then adhere to foreign objects, and their contraction then
produces a movement of the body towards the point of fixa-
tion.

538 INVERTEBRATE MORPHOLOGY.

In connection with the stone-canal a peculiar body is
developed in most forms. Its function is a matter of ques-
tion, it having been at one time taken for the heart and at
another for a gland. It is generally termed the ovoid gland
(Fig. 265, og) and consists of a mass of cells, derived from the
peritoneal lining of the enterocoel, grouped together to form
a more or less solid mass. The oral end of the gland is pro-
longed into a cordlike structure which seems to enter into
close relationships with the oral lacunar ring (see below), while
at the other it is continued out to enter into close relationships
with the reproductive organs in a manner that will be de-
scribed when treating of those organs. Surrounding the
gland is a sinus — the axial sinus (Fig. 265, us) — separated off
from the enteroccel and, in some forms, in communication
with the ampulla of the stone-canal, and the portion of the
gland which passes off towards the reproductive gland is also
surrounded by a sinus, or rather lies in the wall of a sinus
which may or may not communicate with the axial sinus but
has, like it, origin from the general enteroccel.

What has been termed a blood system is usually present,
consisting of a tubular ring surrounding the oesophagus, and
lying between the hydroccel-ring and the nerve-ring. Five
branches may extend off from it along the radii, preserving
the same relations to adjacent structures as does the ring.
These spaces seem to be schizoccelic in their character, and
may be termed the schizoccelic ring and radial schizoccelic sinuses
in order to avoid confusion with another system of vessels
which sometimes lie within the sinuses and have also been
termed blood-vessels. This latter system may be termed the
liit'H/Hir system, and is composed of a network of vessels lying
in the walls of the intestine, and collecting usually into a
perioesophageal ring or plexus (Fig. 265, Ir), with which also
the ovoid gland comes into connection. In the Echiuoids,
as has just been indicated, prolongations of this perioesophageal
ring or plexus extend out in the radial schizoccelic sinuses.

The fluids contained in the sinuses, lacuuse, hydroccel, and
enteroccel are all very similar, consisting of a plasma contain-
ing amoeboid cells sometimes deeply pigineuted. In a few
forms haemoglobin is present ; in the Ophiuran Ophiactis it is

TYPE EC11INODERMA. 539

contained in flat non-nucleated disks, resembling Mammalian
red blood-corpuscles, floating in the plasma of the water
vascular system ; in the Holothuriaus, Thy one and Cucumaria,
it is, however, contained in amoeboid corpuscles, which are
most abundant in the coelomic fluid, though occurring also in
the water vascular tubes.

The digestive tract is generally more or less twisted into a
spiral ; and even when, as in some Holothurians, it appears to
be straight, it is to be regarded as a much-drawn-out spiral,
since the mesentery still retains a spiral arrangement. In the
Holothurians, Echiuoids, and Starfishes it opens on the aboral
surface of the bod}T, but in the Crinoids it is bent upon itself
so that the anus is on the oral surface. In some Starfishes
and in all Ophiuraus no anus is present. Various accessory
structures, masticatory apparatus, coecal pouches, etc., are
found in the various groups, but their description may be
deferred until later.

The nervous system may be regarded as being composed of
three portions, one of which has essentially the same arrange-
ment as the water vascular tubes, consisting of a circumoral
or a perioesophageal ring from which five radial nerves pass
off (Fig. 265, nr and rri). In the Starfishes and Crinoids the
entire system is imbedded in the ectoderm, but in other forms
it sinks within the body-cavity. From it branches pass in-
wards at the mouth to supply the walls of the oesophagus, and
other branches form a network covering the surface of the
body, supplying the sense-organs which may occur thereon.
The radial nerves, in addition to sending branches to join the
epidermal plexus, supply the ambulacral system. This por-
tion of the nervous system may be termed the epidermal
portion, and the second, inasmuch as it supplies the majority
of the muscles of the body, may be termed the muscular
system. This is not always developed, being absent in the
Crinoids, but when present accompanies in general the epi-
dermal portion in the form of delicate nerve-cords lying on
the inner surface of the circumoral ring and radial nerves, and
sending branches to the various muscles, including possibly
those of the tube-feet. The third or aboral portion appears
to be entirely wanting in the Holothuriaus, but when present

540 INVERTEBRATE MORPHOLOGY.

consists of a ring situated at the aboral surface of the body,
sending off branches to the reproductive organs as well as, in
some cases at least, forming anastomoses with the epidermal
system.

Sense-organs of various kinds are developed. Tactile
tentacles occur at the extremities of the radii of some forms
and round the mouth in others, while in the softer-skinned
Holothurians tactile papillae may occur. Eyes occur at the
extremities of the radial nerves of the Starfishes, and have
also been described as occurring in some Echinoids, while
otocysts occur in some Holothurians, sometimes in considera-
ble numbers.

No special excretory organs occur in the Echiuodermata,
the amoaboid cells of the coelomic fluids perhaps serving in
some cases to remove the waste substances. They have been
observed to pass through the body-wall, in regions where it is
thin, to the exterior and there degenerate. For the most
part, however, the waste products are deposited in the tissues,
or else pass to the exterior by osmosis. In the Holothurians
special branched appendages of the terminal portion of the
intestine appear to take some part in excretion, but such
organs do not occur in other groups.

The Echinoderms are almost invariably bisexual, and the
reproductive organs are usually situated in the interradii.
They are enclosed in a special coalomic sinus, the genital sinus,
in whose wall may be found the branches of the aboral nerves.
From each organ or mass of reproductive cells a cellular cord,
the genital rachis, surrounded by the sinus may be traced,
except in the Holothurians, to the ovoid gland, and it appears
probable that in some cases at least the reproductive cells
originate in a part of the ovoid gland and migrate to the
reproductive organ along the rachis, becoming mature in their
final position. The openings by which the reproductive ele-
ments pass to the exterior vary both in number and position
in the different groups, but are usually situated on the aboral
surface of the body.

TYPE ECHINODERMA.

541

I. CLASS CRINOIDEA.

The Crinoids, or Sea-lilies (Fig. 249), constitute a group of
forms which iu the earlier geological periods reached a high
grade of development, but to-day the class is represented by
comparatively few forms, for the most part confined to deep

FlG. 249. — Pentiicrinus maclearanus (after WYVILLE THOMPSON from HERTWIG).

water. One of the most characteristic features of the group
is the presence of a more or less elongated cylindrical stalk,
one end of which is attached to stones or other objects which
serve as supports for the animal, while at the other end is
the body proper, which has a more or less cuplike form. In
the peculiar genus Hotopus the stalk is thick and short, and
may be described rather as the prolonged apex of the body

542 INVERTEBRATE MORPHOLOGY.

than as a distinct stalk, while iii other forms, such as Antedon
aud Actinometra, the stalk, though present in young forms, is
entirely wanting in adult life, during which the animal is free-
swirnming, though having the power of anchoring itself
temporarily to solid objects by means of a number of slender
processes termed cirri which project from the apex of the cup

(Fig. 251, c).

The lower portion of the cup, or calyx, is formed by a num-
ber of series of calcareous plates united to each other by
sutures, while its mouth is covered in by a flat or dome-
shaped disk in which calcareous plates may or may not be
present. In the centre of the disk is the mouth of the animal,
while to one side is the anus, lying in the interradius CD.
From the mouth five grooves, known as the ambulacra! grooves,
extend outwards towards the margin of the cup, and, near the
margin, branch, being then continued outwards on the oral
surfaces of ten arms which arise from the junction of the disk
and calyx, frequently branching in their course, and bearing
along their sides a series of short processes resembling them
in structure, and termed the pinnules, upon which the ambu-
lacral grooves are also continued. These arms are capable of
considerable movement, being at one time extended out at
right angles to the body or even reflexed, and at another
coiled up circinnately over the disk, the pinnules being at the
same time bent inwards towards the median axis of the arm.

The stem when present consists of a number of disklike or
cylindrical calcareous plates, placed one on top of the other,
being held together by bauds of connective tissue, and is
traversed by a central canal containing prolongations of certain
of the visceral structures. The terminal plate serves as the
point of fixation, the plates immediately above it having
attached to them a number of cirri which assist in fixation
and are, like the stem, composed of calcareous plates contain-
ing prolongations of the central canal. In some forms, such
as Pentacrinus, whorls of cirri also occur at intervals all along
the stem, those plates from which they arise being termed
nodal plates, a varying number of plates destitute of cirri
occurring between two nodes in different genera. In certain
genera, however, such as Hyocrinus and Rhizocrinus, these

TYPE -ECHINODERMA.

543

stem cirri are entirely wanting except near the point of fixa-
tion.

The uppermost plate of the stem is usually regarded as
forming the apex of the calyx and is termed the centrodorsal.
Above this comes in most recent forms a series of usually five
(sometimes three) iuterradial plates, the basals (Fig. 250, b\ but
in one genus, Thaumatocrinus, there occurs between the ceu-
trodorsal and basal plates a series
of five radial plates which are
termed the parabasals or under-
basals, and which have also been
found to occur in the embryo of
Antedon, later on fusing with the
centrodorsal. Succeeding the ba-
sals are from two to seven circles
of radials (r), each circle being
also composed of five plates termed

the first, second, third, etc., radials FIG. 250.— THE APICAL SYSTEM
according to their succession
counting from the centrodorsal.
In Antedon and some of its allies
the number of cycles of radials
seen from the exterior is one
short of the actual number which

exists, the first radials being overlapped and covered in
by the second ; and furthermore in the same forms the
basals have also been pushed, as it were, within the calyx
and have fused to form a single plate, the so-called roxette
plate (Fig. 251, Ros), which rests upon the centrodorsal, par-
tially closing a cavity in that plate. The terminal radials
usually present two articulating facets in their distal sur-
faces and are generally known as the axillaries (Fig. 250, a),
since the arms articulate with them. In the genus Thauma-
tocrinus between each pair of first radials an mterradial plate
occurs, a condition frequently found in fossil genera (i), but
usually wanting in recent forms. These various plates which
constitute the apical system are united by sutures, the edges
of the various series of plates coming into contact, so that a
firm support is afforded for the arms.

OP Melocrinus (FOSSIL) (from
BROWN).

a = axillary plates.
b = basal plates.
i — interradial plates.
r = radial plates.

544 INVERTEBRATE MORPHOLOGY.

These are in reality continuations of the radial series of
plates ; in fact, in some forms certain of the radials appear to
enter into the formation of the arm. In most forms, however,
a series of arm-plates arises from each facet of the five axillary
plates, so that the arms are ten in number — a condition which
finds an exception in the remarkable genus Tliaumatocrinus,
which possesses but five. In some forms these ten arms
branch dichotomously ; the plates intervening between the
axillaries and the first branching are termed brackials, those
between the first and second branchings distichals, and those
between the second and third branchings palmars — terms
which are useful in systematic descriptions. These various
plates are united together by ligaments and muscles, or else
by ligaments alone (this last form of union being known as a
syzygy), the movements of the arms noted above being thus
rendered possible. The pinnules repeat the arm in their
structure, though usually on a much-reduced scale. They
are situated on the joints separating consecutive plates of the
arms, and are placed alternately on the right and left sides
of the arm which bears them. They appear at first to have
been produced by lateral budding from the joints, but closer
examination indicates that in reality they represent a branch-
ing, one of the branches remaining small, while the other in-
creases in size and places itself in the direction of the axis of
the arm. The whole arrangement is comparable to that form
of inflorescence termed by botanists a scorpioid cyme, the
pinnules representing the flower-pedicels. Owing to the
pinnules being in reality one of the branches of a dichotomy,
it is evident why, in those forms in which the arms branch,
there is no pinnule at the joint where the branching occurs ;
in addition, however, pinnules are also lacking on syzygial
joints, so that their regular succession may be somewhat dis-
turbed.

As regards the oral system of plates an oro-central is found
in some fossil forms, but is unrepresented in recent genera.
A circle of five interradial oral plates is found in Holopus,
RJiizocrinus, Ilyocrinus, Thaumatocrinus, and Calamocrinns,
and in the stalked larva of Antedon, but in the adults of this
hitter form and in other genera than those mentioned these

TYPE ECHINODERMA. 545

plates disappear during growth, the disk being either naked,
or covered by a number of small plates which are termed an-
ambulacrals, certain of which lying on either side of the ambu-
lacral grooves receive the special name of ddambulacrals, or
covering-plates.

The ectoderm cannot usually be distinguished over the
surface of the calyx or on the stem, but is present on the
disk and on the oral surfaces of the arms and pinnules, being
there non-ciliated except along the ambulacral grooves. It
rests upon a connective tissue in which the calcareous plates
are developed, and from which strands, frequently with cal-
careous spicules imbedded in them, usually traverse the body-
cavity. The ligaments which unite the plates of the arms
and stem are formed of this connective tissue, and contractile
fibres of a peculiar character are sparingly developed in it,
stretching across the non-syzygial joints of the arms, pinnules,
and cirri, and probably also reaching a slight development
in the stern.

The internal structure of the Criuoids is knoAvn princi-
pally from observations on Antedon, and the following account
represents what occurs in that form. The ccelom, as already
stated, is traversed by numerous strands of connective tissue,
and primarily consists of two cavities separated from each
other by a mesentery, each cavity being continued out into
the arms, forming the oral and aboral canals of these struc-
tures, at the extremities of which they unite. The mesentery
does not, however, long persist in its entirety, but the two
cavities fuse, new membranes, however, arising and dividing
them in some species of Antedon. One of these membranes
(Fig. 251, vs) surrounds the intestine and forms the visceral
sac, its presence rendering the evisceration of Antedon an
easily-accomplished process and one which is made use of
by the animal in unfavorable conditions, a new visceral mass
being later regenerated. The portion of the ccelom which
lies peripherally to this sac is termed the circumvisceral
portion (cc), and that within it the intervisceral (ic), the latter
containing an axial cavity (Axi) enclosed by a membrane sim-
ilar to the visceral sac and continuous with the oral coelomic
cavities (oc) of the arms, the aboral cavities (ac) communicat-

546

IN VERTEBRA TE MORPHOLOO T.

ing with the circumvisceral coeloni. A portion of one of the
ccelomic cavities at an early stage becomes cut off from the
rest of the ccelom and divided into five chambers whose walls
are formed of a dense fibrous membrane. This constitutes

wp.

FIG. 251. — VERTICAL, SECTION THROUGH Antedon (combination of figures by

LUDWIG and MARSHALL).

ac = aboral canal of arm.
an — aboral nerve.
Ax — axial sinus.
Br = bracbial plates.
C — cirri.

cc = circumvisceral cavity.
CD = centrodorsal plate.

co = central organ.
Do = dorsal organ.

gr •— genital rachis.
/ == intestine.

ic = intervisceral cavity.

M — moutb.

oc = oral cavity of arm.

on = oral nerve-ring.

B — radial plates.

rb — radial lacunar vessel.
rn = radial epitbelial nerve.
ros — rosette-plate.
rw = radial hydroccel-caual,

sc = stone-canal.

T — oral tentacle.

m = visceral septum.
wp = water-pore.

the chambered organ (co), which in Antedon lies in a cavity
in the centrodorsal plate and is roofed over by the rosette-
plate, but in other forms simply rests upon the centrodorsal ;
communicating with it is the lower end of a somewhat club-
shaped structure termed the dorsal organ (Do), which pro-
jects orally parallel to the axial ccelomic cavity.

The epithelium of the aboral ccelomic cavities of the arms
is differentiated here and there into peculiar organs the cili-

TYPE ECH1NODERMA. 547

ated cups, consisting of slight depressions lined by columnar
cells each of which bears a long cilium. These cups are
especially abundant in the pinnules, and serve to create a
circulation of the coelomic fluid, which, as in other Echiuo-
derms, contains numerous amoeboid cells floating freely in it.
The water vascular system, or hydrocoel, consists of a ring
surrounding the mouth, and sending outwards five radial
canals (rtv) which lie below the ambulacral grooves and are
continued along the arms and pinnules. Occasionally sub-
ambulacral calcareous plates are developed in the connective
tissue below the radial canals, and in some fossil forms these
plates assume a regular arrangement in two rows. At regu-
lar intervals along the arms are situated the ambulacral ten-
tacles, which are tiugerlike outpouchiugs of the radial canals
destitute of terminal suckers and are arranged in groups of
three, the canals being somewhat enlarged in the region where
they occur, an indication perhaps of the ampullae found in
other groups ; in some forms the cavities of the tentacles
seem to be united with those of the canals only by exceed-
ingly small orifices, which may be closed, since the tentacles
in their greatest contraction always remain filled with fluid.
In the neighborhood of the mouth are a number of oral ten-
tacles (Fig. 251, T) arising directly from the oral ring, and
differing from the ambulacral tentacles in not being arranged
in groups of three. From the oral ring there also arise in
Antedon a number of ciliated tubes (sc) which open into the
coelomic cavity, each one corresponding to a stone-canal of
the other Echiuoderins. In Antedon there are as many as
thirty of these canals in each interradius. and in Pentacrinus
an even greater number occurs ; but in other forms they may
be fewer, Rhizocrinus, for example, possessing onty five in all,
one being situated in each interradius. In the larva of Aiif<'-
don there is at an early stage only one, communicating with a
portion of one of the primary coelomic cavities, which on its
part opens to the exterior by a pore, an arrangement which
may be regarded as typical for the Echiuoderins. Later,
however, this portion of the coelomic cavity degenerates, and
the canal then opens directly into the general ccelom, and
this communicates with the exterior by the pore. In subse-

548 INVERTEBRATE MORPHOLOGY.

quent stages additional stone- canals develop from the oral
ring, and at the same time additional pores develop in the
walls of the body, forming what are called the calyx-pores
(wp). These may reach a considerable number, it being esti-
mated that in Antedon there are no less than fifteen hundred
of them scattered over the disk ; in Shizocrinus, Hyocrinus,
and Holopus, however, there are only five pores, one piercing
each of the oral plates present in these forms.

The schizocoelic system consists of five radial sinuses (Fig.
251, rb) lying between the radial Lydrocoel vessels and the
more superficial radial nerve, together with, according to some
authors, a circular sinus surrounding the mouth into which
the radial sinuses open. A plexus of lacunae occurs in the
walls of the intestine, and another surrounds the oesopha-
gus, this latter in part aggregating itself into a structure re-
sembling a lymphatic gland and known as the spongy body ;
the dorsal organ likewise contains a dense network of tubes
lined with epithelium. Along the sides of .the hydroccel-
canals, in the disk, arms, and pinnules, alternating in the two
last with the triads of tentacles, in the walls of the intestine,
and occasionally elsewhere, there are imbedded in the con-
nective tissue yellowish spherical bodies known as the sacculi.
The interior of each sacculus is lined with cells, and contains
a number of pyriform masses formed of small highly-refrac-
tive spherules, apparently of an albuminoid substance. The
function of these bodies is very obscure ; they have been re-
garded as organs for secreting carbonate of lime, as excretory
organs, as parasites, as mucous glands, and lately as organs
of reserve in which proteid matter may be stored up for
future use. At present, however, the question is an open
one, and a function cannot with certainty be assigned to them.

The mouth (Fig. 251, m) is usually situated at the centre
of the oral disk, and opens into a simple tubular intestine
which coils once round the ccelomic cavity in the direction of
the hands of a watch and then, bending upon itself, turns
orally to open in the iuterradius CD upon the disk. In Ac-
tinometra, a genus closely related to Antedon, the intestine
lies in four coils, but there is as a rule little variation from the

TYPE ECHINODERMA.

549

condition described, and 110 specially-marked differentiation
of the tube occurs.

The nervous system of the Crinoids is characterized by
the remarkable development of the aboral portion and by
the apparently entire absence of the muscular portion. The
epithelial portion consists of a ring and five radial nerves (rri)
which pass along the ambulacra! grooves and out upon the
arms and pinnules, imbedded throughout their entire course
in the lower layers of the ectoderm. The aboral system is,

FIG. 252. — DIAGRAM OF THE ARRANGEMENT OF THE ABORAL NERVOUS

SYSTEM OK Autedou (after MARSHALL).
a — arms. cd = ceiitrodorsal.

Br — bracbial plates. B = radials.

on the other hand, much more strongly developed, and stands
in intimate association with the chambered organ in the walls
of which it is imbedded. Its central portion is somewhat
complex, as is shown in Fig. 252, but may be said to consist
of a ring, or, more properly speaking, of a pentagon from
which five strong cords radiate out into the arms, perforating
the plates of which these are composed. Both fibres and
ganglion-cells enter into the composition of the cords and
ring, and a complicated system of commissures exists. From
the central portion branches are also sent to the cirri, and
probably in stalked forms a branch traverses the central
cavity of the stalk, accompanying prolongations of the cavities

550 INVERTEBRATE MORPHOLOGY.

of the chambered organ. The terminal branches of the radial
aboral nerves pass to the integument of the oral surfaces of
the arms and to the muscles which unite the various plates,
so that the system governs and coordinates the movements of
the arms and pinnules as well as of the cirri. The epithelial
system, on the other hand, controls the movements of the am-
bulacral and oral tentacles, stimulation of it causing move-
ment of these structures in the immediate vicinity of the
region to which the stimulus is applied.

Another system of nerve-fibres, consisting of a pericesoph-
ageal ring which sends off two branches to each arm, one
lying on each side of each of the ambulacral grooves, and
which is connected with nerve-fibres passing from the dorsal
organ, has been described as occurring, but its significance
has not yet been satisfactorily determined. No special sense-
organs occur in the Crinoids.

The reproductive organs are developed for the most part
in the pinnules, occasionally a slight development of them
appearing in the arms or even in the body proper ; in Holopus
alone they are confined to the arms. They consist of tubes
lined with germinal epithelium on their inner surfaces and
enclosed within a prolongation of the ccelom. They lie be-
tween the two coilomic prolongations of the arms already
mentioned, and though the reproductive organs are developed
only in the pinnules as a rule, nevertheless each genital tube
or rachis (Fig. 251, yr) can be traced through the arm to the
body, where it terminates in connection with the dorsal organ.
In their development indeed they grow out from this organ,
and it seems probable that the ova and spermatozoa mother-
cells migrate out from it along the rachides to reach maturity
in the pinnules. Comparing this with the condition in other
Echiuoderma, it seems clear that the so-called dorsal organ
of the Crinoids is homologous with the ovoid gland of the
other forms. The reproductive elements pass to the exterior
by one or two ducts connected with each reproductive mass ;
the origin of these ducts is unknown.

The Crinoids seem to have been closely related to two groups of forms
known only as fossils. These were the Cystoids, which appear in the Lower
Silurian rocks and die out in the Carboniferous, and the Blastoids, jvhich

TYPE ECHINODERMA.

551

appear in the Upper Silurian and also disappear in the Carboniferous.
For a description of these forms reference must be made to the standard
works on Palaeontology. On account of their similarity to the Crinoids they
have been associated with them in the class Pelmatozoa, of which each of
the groups formed an order; inasmuch, however, as the present work is
concerned only with recent forms; it has been thought more convenient to
regard the Crinoids as a class.

The class Crinoidea has been divided into two orders. The Palceocri-
nida, chiefly Palaeozoic forms, characterized principally by the presence of
under-basals and of a series of plates covering in the disk almost com-
pletely, to which may be added the usual presence of interradials and the
greater width of one of the interradii, that in which the anus occurred
(see Fig. 250). The Neocrinida, on the other hand, included the recent
forms, the group making its first appearance in the Mesozoic, and is char-
acterized by the disk being only imperfectly covered by plates, by the
under-basals and interradials being absent, and the interradii all equal in
width. Transition forms between the two groups occur, however, the
genera Hyocrinus and Calamocrinus, for example, presenting certain
Palteocrinid peculiarities combined with Neocrinid ones, and it seems more
satisfactory to divide the class into families only, leaving orders out of the
question.

Development of the Crinoids. — Antedon is the only Criuoid whose
development has been studied. The embryo leads
for a time a free-swimming existence, and possesses
a somewhat ovoidal form (Fig. 253) with a tuft of
cilia at the smaller anterior end and five rings of
cilia surrounding the body. Not far from the ante-
rior end is a slight groove, and lower down upon the
side is a much larger one. This larva settles down
upon the anterior end, the slight depression near
this end serving as an organ of fixation, and then
a rather remarkable rotation occurs, the large groove
shifting round together with the interior organs until
it comes to lie at the free end of the organism, and at
the same time its lips unite so as to enclose a cavity,
the vestibule. Calcareous plates have ere this de-
veloped in the connective tissue of the embryo and
outline a stalked Crinoid into which the larva is
gradually transformed, the larval skin shrinking as
it were, so as to closely surround the stalk and calyx,
while the vestibule opens to the exterior by the
gradual thinning and final disappearance of its roof,

its floor forming the ectoderm of the disk. After continuing its growth for
some time as a stalked Crinoid, the young Antedon finally separates from
its stalk, and thereafter leads a free existence.

FIG. 253.— LARVA OF

Antedon (combination
of figures by THOMPSON
AND GOETHE after KOR-

SCHELT AND HEIDER).

552 INVERTEBRATE MORPHOLOGY.

II. CLASS ASTEROIDEA.

The Asteroidea, or Starfishes, are all flattened forms, at
no period of their lives attached by a stalk, but creeping
about freely upon the oral surface. In some forms the body
is a flattened disk pentagonal in outline (Asterina), but more
usually (Fig. 246) the five radii are prolonged out into five
stout unbranched arms, and in some forms, such as Brisinga,
the arms may be long and slender and more than five in
number. The mouth is situated in the centre of the oral
surface, and the anus slightly eccentrically upon the aboral
surface, while the hydroccel system of tubes is confined, as
in the Crinoids, to the oral surface of the body, except that
the madreporiform tubercle by which the system communi-
cates with the exterior is upon the aboral surface in the in-
terradius CD.

The ectoderm is throughout ciliated, and contains usually
numerous mucous glands, while in its lower layers ganglion-
cells and nerve-fibrils form a plexus extending over the entire
surface of the body.

Calcareous matter is deposited in the connective tissue,
but in the majority of forms the primitive apical plates are
not recognizable in the adult ; more usually the aboral sur-
face is covered by a large number of small plates arranged
without any regularity, or else the calcareous matter forms
a reticulum composed of numerous fused bars, short spines
rising frequently from the points of union. Iii embryos,
however, and in some adult forms, such as Zoroaster (Fig.
247), the apical system can readily be made out, and con-
sists of a ceutrodorsal plate (Fig. 247, CD), sometimes
grooved upon the edge for the anus (an), surrounded by five
under-basals (2) usually small, alternating with which are
five basals (3). At the base of each arm is a radial (4), and
in embryos beyond this there is in each radius another plate,
which as growth takes place is carried further and further
from the radial and finally forms the terminal plate (T) of the
arm, by which name it is known.

Of the oral system the orals are possibly represented by

TYPE ECHINODERMA. 553

the so-called odontopliore plates, which are generally small
and in many cases covered over by other plates of the oral
surface, and lie in the immediate neighborhood of the
mouth. At the junction of the oral and aboral surfaces of
the disk and arms two series of plates are frequently to be
found which from their position are termed the supra and
infra-marginals, and, in addition to these, series of plates
with definite arrangement are developed in connection with
the water vascular system. Thus along each side of the mid-
dle line of each arm is a series of plates, which, sloping
aborally and towards the axis of the arm, meet to form the
floor of an ambulacral groove which extends outwards from
the mouth to the extremity of the arm. These are the am-
bulacral plates (Fig. 254, A), and each series of them is flanked
upon the outer side by a row of adambulacrals (Fig. 254, B}
whose number may or may not correspond with that of the
ambulacrals. Between the adambulacral series of adjacent
arms a series of plates may be interposed upon the oral sur-
face of the disk, and to these the name of inter ambulacrals
may be applied.

Spines are very frequently borne by the plates or reticu-
lum of the aboral surface, but are usually low and immova-
ble, though upon the marginal and adambulacral plates they
are very frequently longer, united to the plates by a rudi-
mentary articular surface and supplied with muscle-fibres by
which they can be moved. In addition to these appendages
of the dermal skeleton, others are to be found in the Star-
fishes, such, for example, as the ciliated spines found in a
few forms, such as Luidia, upon the marginal plates. These
spines are small and delicate, and grouped together, their
principal peculiarity being that they are covered by an
epithelium of high columnar cells which bear strong cilia. In
most Starfish also peculiar structures termed pedicellarice are
developed in connection with the skeleton, but their descrip-
tion may be deferred until the Echiuoids are under discus-
sion, in which group they reach a high grade of develop-
ment. Peculiar to certain genera of Starfishes, e.g. Luidia,
are the paxillw, found principally upon the aboral surface of
the body. They consist of small columns of carbonate of

554

INVERTEBRATE MORPHOLOGY.

lime imbedded iu the connective tissue, and bear upon their
free extremity a number of radiating spines, which vary in
the amount of movement of which they are capable in differ-
ent species. The paxillse are frequently found in groups
around the dermal branchiae, over which the spines may be
bent so as to serve for protection.

These dermal branchiae (Fig. 254, b) are pouchlike evagi-
natious of the coalomic cavity with thin walls composed of
ectoderm and a layer of ciliated cells continuous with the
peritoneal lining of the coeloni. between these two layers
there being but a slight development of connective tissue and

an

ec

am

w^,^ { "\y^^|piMiilP^

B A N tr

FIG. 254. — TRANSVERSE SECTION OP AUM OP A STARFISH (modified from

LUDWIG).

A -- ambulacra! plate.
am = ampulla.
an = aboral nerve.
B = adambulacral plate.
b = branchia.
c = digestive caecum.
ec = ectoderm.

I = schizoccelic sinus.
mn = muscular nervous system.
2? = epithelial nervous system.
o = ovum.

p = peritoneal epithelium.
pi = calcareous plate.
rh = radial hydrocoel-vessel.

tf — tube-foot.

circular and longitudinal muscle-fibres. These pouches are
scattered plentifully over the aboral surface, and in some
forms occur upon the oral surface also. Their thin walls and
the extent of surface they collectively represent leave little
room for doubt but that they possess respiratory functions,
though they may also serve indirectly in excretion, since it has
been asserted that the amoeboid cells of the coslomic haemo-

TYPE ECHINODERMA. 555

lymph, laden with excretory particles, migrate to the exterior
through the walls of the branchiae.

The calcareous plates are imbedded in a connective tissue
usually of considerable thickness and sometimes of a high
degree of consistency. Upon its inner side are to be found
circular and longitudinal bands of muscles, especially devel-
oped in the arms, which are capable of considerable move-
ment.

The general ccelom is traversed by mesenteries extending
from the body-wall to the digestive tract, which do not require,
however, a detailed description. Suffice it to say that each
radial caecum of the digestive tract which extends out into the
arm is suspended by two longitudinal mesenteries (Fig. 254)
which, with the body-wrall and csecurn, enclose a canal open-
ing proximally into the general co3lom. Extending vertically
through the c?elom in the iuterradius CD is a cavity with
strong walls which is in communication with the exterior by
the madreporiform tubercle and is the axial sinus. This is a
portion of the ccelom which is early separated from the rest,
and in the embryo opens to the exterior by the water-pore,
the stone-canal opening into it. In the adult the cavity of the
sinus is fairly spacious and contains the ovoid gland sus-
pended to its walls by a mesentery. Prolongations of the
sinus accompany the genital rachis and, enclosing the repro-
ductive organs, form the genital sinuses.

The schizoccelic system consists of an oral ring lying
between the nervous and water vascular rings, and of five
radial vessels which pass out from this along the axes of the
arms. The oral ring is divided by an oblique septum into two
portions, one of which lies upon the aboral surface of the
other, and enters into connection with the axial sinus, the
ovoid gland abutting upon the septum in one of the iuterradii.
The oral sinus also communicates with the cceloin by rive
interradial orifices. The radial sinuses are divided into two
cavities by a median longitudinal septum, and, like the oral
sinus, communicate with the crelom. A lacuuar system, such
as occurs in the Crinoids, is not developed in the Asteroideu,
though certain spaces in the wall of the ovoid gland and its
prolongations are perhaps representatives of it. The ovoid

556 INVERTEBRATE MORPHOLOGY.

gland is formed of loose connective-tissue trabeculre, which
are covered by cells frequently found in active division, and
are supposed to become the amoeboid corpuscles of the coslorn
and blood system.

The hydroccel consists as usual of an oral ring and five
radial canals, the latter lying at the bottom of the ambulacral
grooves and therefore external to the ambulacral plates (Fig.
254, rh). Between each pair of plates a branch passes upwards
(i.e., aborally), and dilates into a globular sac, the ampulla
(Figs. 254 and 255, am), which is occasionally double, and from
this a cylindrical tentaclelike process passes outwards again
between two plates forming extensible processes (Fig. 254, tfy
equivalent to the tentacles of the Criuoids. These processes
in some of the more primitive forms, such as Luidia and
Astropecten, and the terminal ones at the extremity of the arms
of all forms, are conical in shape, but more usually the great
majority of them are provided at their extremities with suck-
ing disks, whereby they can adhere to foreign bodies and serve
thus as locoinotor organs. Hence they are known as the
tube-feet or ambulacra. In some forms they are arranged in
two rows, one on each side of the axis of the arm, but in
others, as for example the common Starfish Asterias, the suc-
cessive feet of each row alternate with each other, so that they
have the appearance of being arranged in four rows. By
means of the muscles of the wall of the ampullae water can be
forced into the tube-feet, which may be thus extended, a
circular valve occurring in the branch which passes from the
radial canal to the ampulla preventing the water from passing
back into the canal. Contrary to what occurs in the Criuoids,
there are several appendages to the oral ring, in addition to
the stone-canal. This leaves the ring in the iuterradius CD
and, passing aborally, communicates with the axial sinus which,
as already stated, opens to the exterior by the madreporite.
This is a complicated calcareous sieve-plate of some thickness,
and the union of the canal and the sinus takes place within
its substance, so that in reality the canal seems to open to
the exterior. The embryonic history, and the fact that injec-
tions forced through the tubercle pass into both the sinus and
the canal, show that what has been described is the true

TYPE ECHINODERMA. 557

relationship. In the walls of the stone-canal calcareous
matter is deposited, whence the name applied to it, and its
walls form projections extending into the lumen of the canal,
the surface for the ciliated epithelium being thus increased.

The appendages of the oral ring are of two kinds, both
being situated in the interradii, that containing the stone-canal,
however, usually lacking any other appendage. In some
forms hollow saclike structures open into the ring by a
narrow neck, and are termed the Polian vesicles ; their walls
consist of connective tissue in which are situated muscle-
fibres, and their interior is lined by an epithelium which
appears to separate and give rise to the amoeboid cells of the
hydroccel fluid. The other kind of appendages occur generally
throughout the group and are known as Tiedemau's vesicles,
consisting of masses of hollow tubes arranged in pairs in one
or more of the interradii. The epithelium lining the walls of
these structures also seems to give rise to the amoeboid cells,
both kinds of organs being therefore comparable to lymphatic
glands, though the Poliau vesicles have also been regarded as
reservoirs for the hydroccel fluid.

The mouth is situated at the centre of the oral surface of
the disk, and opens into a short oesophagus which, in some
forms, has connected with it ten glandular pouches. The
oasophagus opens into a usually capacious cardiac stomach
which is frequently lobed (Fig. 255, c), is eversible and pro-
vided with special muscles for its retraction. Above this
comes the pyloric stomach which gives rise to five radial
pouches, which soon branch into a pair of sacculated pouches
extending out into the arms, and being termed the radial
cfeca (I). From the pyloric stomach a short rectum passes
aborally, interradial caeca being sometimes found close to its
origin from the pyloric stomach, and opens upon the dorsal
surface. In a few forms, such as Luidia, Astropecten, and their
allies, the anus is wanting, but more usually it is present in
the region indicated.

The epithelial nervous system consists of a plexus of
ganglion-cells and fibres imbedded in the ectoderm and cover-
ing the surface of the body, and of an oral ring and five radial
nerves (Fig. 254, JV) which, as in the Criuoids, are situated in

558

INVERTEBRATE MORPHOLOGY.

the lower layers of the ectoderm. Upon the aboral surface
of the oral ring aiid the radial nerves sections show distinct
bands of fibres separated from the ring and nerves by a
delicate layer of connective tissue ; these constitute the mus-
cular system of nerves (Fig. 254, mri), and their branches
appear to be supplied to the muscles of the body-wall and of
the ampullae and tube-feet. The aboral system is but feebly
developed when compared with that of the Criuoids. A trans-
verse section of an arm shows lying between the muscles of
the aboral surface and the peritoneal inesoderni a cord of

FIG. 255.— A STARFISH, Asteracantfiion, WITH THE INTEGUMENT OP THE DISK

AND RAYS REMOVED TO SHOW THE INTERNAL STRUCTURE.

a = anus. g — reproductive organ.

am = ampullae of tube-feet. I = liver cteca.

ao — ambulacral ossicles. M = inadrepori'te.

c — cardiac pouch of stomach. A-E = the five radii.

nerve-fibres (Fig. 254, an), the five cords converging towards
the centre of the aboral surface of the body, the entire system
forming thus a five-rayed star. In position and general rela-
tions this system of nerve-cords is directly comparable to the
aboral system of the Crinoids, and may be regarded as
homologous with it.

Special sense-organs are represented by the terminal ten-
tacles of the radial hydrocoel canals, which, as already stated,
retain a tentacle-like form and do not develop suckers at the
extremity. Their walls are richly supplied with nerves, and

TYPE ECUINODERMA. 559

they are surrounded and may be covered in by the movable
spines of the adarnbulacral and marginal plates. That they
have a sensory function seems clear, but what the exact
nature of the function may be is as yet uncertain. At the
base of the terminal tentacle of each arm is situated an eye,
consisting of a large number of conical depressions, lined by
an epithelium containing a red pigment, covered on the out-
side by a cuticle and richly supplied with nerve-filrils. There
do not seem to be present any refractive structures other than
the cuticle, and these eyes can only convey to the animal im-
pressions of changes in the intensity of the light falling upon
them ; they cannot form images of external objects.

The reproductive organs are ten in number, two being
situated in each arm (Fig. 255, g). Each consists of a mass
of reproductive cells, and is enclosed in a genital sinus (Fig.
254, ?), which, as already stated, communicates with the axial
sinus. The proximal end of each gland is connected with a
cordlike structure, the genital cord or rachis, the ten cords
uniting in a ring situated beneath the aboral surface of the
body, a cord passing orally from this ring to unite with the
tissue of the ovoid gland. The genital sinuses accompany the
cords, enclose the ring, and pass to the axial sinus along with
the descending cord. A connection therefore exists between
the reproductive organs and the ovoid gland, just as in the
Crinoids, and indeed the reproductive organs arise in the
embryo from an outgrowth of the ovoid gland. In reality the
genital cords are tubes containing in their interior immature
reproductive cells which seem to migrate from the ovoid gland
to the reproductive organs where they become mature. The
reproductive openings are usually placed upon the aboral
surface (Aster ina forming an exception) of the arms or disk in
the iuterradii ; a single pore usually exists for each gland,
and occasionally there may be several.

Development of the Asteroidea. — The larval forms of the Starfishes are
knowu as the Bipinnaria and Brachiolaria. The former has a somewhat
triangular shape, the apex of the triangle being the anterior extremity,
and in the middle of the ventral surface is a deep concavity in which the
mouth opens. The posterior border of the concavity is formed by a band
of cilia which is continued around the lobed sides of the body to the an-

560

INVEBTEBRA TE MORPHOLOG T.

terior extremity, forming a postoral ciliated band, the anus lying without
the area enclosed by it. In front of the mouth is a trilobed region also
surrounded by a band of cilia, the adoral band. In young embryos the
adoral and postoral bands are united at the apex, separation only super-
vening later. In later stages two additional arms are developed at the
sides of the apical lobe, which becomes like the new arms destitute of cilia,
and tipped with a group of wartlike elevations. This form of the larva is
known as the Brachiolaria.

A peculiar process, amounting almost to a metamorphosis, occurs during
the transformation of the larva into the Starfish. Calcareous plates of the
aboral system make their appearance on the dorsal surface of the stomach

A

FIG. 256. — BIPINNARIA OF Aster acnniliion (after AGASSIZ).
an = anus. Jiy = liydrocoel. m = mouth.

near the posterior end of the body, and oral plates on the ventral surface
of the same organ. These two systems, at first rather widely separated,
gradually approach each other, and at the same time the internal organ >
assume the adult form. Finally the two series of plates unite, enclosing
between them the hydroccel, a portion of the digestive tract and of the coe-
lom. The original mouth and anus are obliterated, and indeed the anter-
ior half of the larva takes no part in the formation of the adult animal,
but is gradually absorbed.

A highly-developed faculty for regeneration occurs in the Asteroidea,
the disk being able to regenerate lost arms ; and indeed an arm, with which
a small fragment of the disk is in connection, has the power of regenerat-
ing all the missing parts. Specimens of the common Starfish Axti'ritiN are
in consequence frequently found with one or more of the arms bifid at the
tip, or even with an abnormal number of arms.

TYPE ECHINODEEMA.

561

III. CLASS OPHIUKOIDEA.

The Ophiuroidea, or Brittle-stars, resemble the starfishes
closely in their general form, consisting of a central disk from
which five arms radiate (Fig. 257). The arms, however, are
in all cases slender and distinctly marked off from the disk,
and in Astropfiyton branch dichotomously. Closer examina-
tion reveals, however, considerable differences from the Star-
fishes ; there is no anus, the madreporite is on the oral sur-

FIG. 257. — Ophioglyplia aculeata FROM THE ABORAL SURFACE TO SHOW THE
PERSISTENT APICAL SYSTEM OF PLATES.

(The arms are cut off close to the disk).

1 = ceutrodorsal plate. 3 = basals.

2 = under basals. 4 = radials.

face, there are no visible ambulacral grooves on the arms,
which are more or less circular in section and do not con-
tain csecal processes of the digestive tract. Furthermore, on
each side of each of the five radii there is upon the edge of
the oral surface of the disk a slitlike opening, divided into
two parts in Ophioderma, and leading into a thin-walled cili-
ated sac, which is to be regarded as an imagination of the
wall of the body. There are thus ten of these genital bursce
as they are termed, two being situated in each interradius.
They seem to have a respiratory function, and serve also for
the exit of the reproductive elements, in some forms, e.g. Am-

562 INVERTEBRATE MORPHOLOGY.

phiura squamata, even serving as brood-pouches iii which the
young develop.

The ectoderm is indistinguishable over the greater portion
of the body in the adults, becoming, as in the Crinoids, con-
founded with the mesoderm. Calcareous plates are largely
developed in this tissue (except in Opliiomyxa and its allies),
giving to the disk and arms a brittleness which has suggested
the popular name for the group. The extent to which the
apical system of plates is distinguishable in the adults varies
considerably even in members of the same group, and while
in some forms (Fig. 257) all the plates represented in the
Starfish Zoroaster can be distinguished, in others only the
radials or the basals or both are visible. At the tip of each
arm is a plate comparable to the terminal of the Aste-
roidea, and in addition there are frequently present series of
interradials or interbrachials, the most aboral plates of
which separate the radials from each other and extend round
to the oral surface, abutting on five large plates known as the
buccal shields and corresponding to the orals of other forms.
On the aboral surface of the disk above the origin of each arm
there is a pair of plates termed the radial shields, which
must not, however, be confused with the radial plates extend-
ing along the aboral surfaces of the arms.

These latter form a complete series extending from the
disk to the terminal plates, and form the aboral wall of the
arms, their lateral walls being formed by another series of
plates, the adambulacrals (Fig. 258, Ad), while still another
series, the superambulacrals, form their oral walls. Between
each adambulacral plate and its successor is a pore (usually
bounded by a number of small plates) through which the
tube-feet are protruded, the radial water-vascular canals being
situated in the interior of the arm. The cavity of the arms
is occupied almost entirely by a linear series of calcareous
masses termed the vertebral or ambulacra! ossicles (Figs. 258
and 260, A), each of which consists of two halves, usually
firmly united by suture. The ossicles are united by well-de-
veloped articular surfaces, and have attached to them muscles,
whereby a considerable amount of motion is possible for the
arms as a whole, the motion being almost entirely in a hori-

TYPE ECHINODEHMA.

H63

zoutal plane, except in Astropliyton and its allies, in which
the anus may be coiled up over the oral surface, in a manner
similar to what is found in the Criuoids. These ambulacral
ossicles seem to correspond with the similarly-named plates
of the Asteroidea.

In the neighborhood of the mouth certain modifications
in the arrangement of some of these plates occur. The two
halves of each first ambulacral ossicle (Fig. 258, At) are widely
separated, and come into close relation with the similarly-
separated ossicles of adjacent radii, forming a buccal shield.
The plate so formed rests upon the aboral surface of the first
adambulacrals (AdJ, which unite in pairs in a similar manner,

Art,

r "^
! A<

FIG. 258. — DIAGRAM TO snow THE ARRANGEMENT OF THE CIRCTJMORAL

PLATES OF AN OpHIURAN (after LUDWIG).

A == ambulacral plates. p = pala angularis.

Ad = adambulacrals. T — torus.

/ == Intel-radial. t = oral tentacles.

wr = radial hydrocoel-vessel.

forming a triangular plate, termed an oral angle-piece, lying in
an interradius, and partly covered on its oral surface by a
buccal shield. At the sides of the buccal shield are the so-
called lateral buccal shields (AdJ, which are in reality the
second adambulacrals of adjacent arms, and cover in the
second ambulacrals (A^), which serve as supports for the
oral angle-piece. Along the margins of the oral surface of
this are a series of spines, the buccal papiUce, while, at the apex
of the triangle, are the dental papilla1. The vertical edge of
the piece is furnished with a number of stout projections, the

564 INVERTEBRATE MORPHOLOGY.

palce angulares (Fig. 260, p), whose bases generally fuse to
form a supporting plate, the torus angularis (T).

Spines developed in connection with the dermal skeleton,
leaving out of consideration the oral angle-pieces, may be en-
tirely wanting, but in many forms they are borne in vertical
rows upon the adarnbulacrals, and are usually movable. In
a few forms, especially those inhabiting rocky bottoms, pe-
culiar hooked spines are situated on the oral surface of the
arms towards their extremities, and seem to serve an adhe-
sive function. Pedicellarue are absent, except in Astropliyton
and allied genera.

The coelom (Fig. 260, c) is of comparatively slight extent,
the cavity of the disk being largely occupied by the digestive
tract, and that of the arms by the ambulacral ossicles. In the
disk the cavity is traversed by numerous bands which ex-
tend from the body-wall to the wall of the digestive sac,
and from the wall of the oesophagus a membrane extends
outwards and orally to be attached to the peribuccal plates,
forming a septum (Fig. 260, s), enclosing a cavity surround-
ing the oesophagus, the peripharyngeal space (ps), which is
completely separated from the rest of the coelom. In Opldo-
thrix and some other forms a second septum occurs parallel
to the one just mentioned, so that the peripharyngeal space
is double. The coelom of the arms consists of two portions,
one lying on the aboral and the other on the oral side of the
series of ambulacral ossicles. The aboral cavity is expanded
laterally so as to partially surround the ossicles, but this lat-
eral portion is traversed opposite each ossicle by a calcareous
lamella, and is thus separated into a series of chambers
which open into the undivided aboral portion, termed the
aboral or dorsal canal. An axial sinus, standing in close re-
lationship to the ovoid gland, exists, but presents some fea-
tures not found in the Asteroidea. It consists in Ampldura
sguamata (Fig. 259) of three distinct portions completely sep-
arated from one another ; one of these is the so-called am-
pulla (am) of the stone-canal ; the second (s) lies in close
relation to the ovoid gland, which is developed on its axial
wall ; while the third is comparatively small, and is associated
with the genital cords (f/r), and the mass of cells in the ovoid

TYPE ECHINODERMA.

565

gland from which these arise. These two last cavities are
said to be portions of the general cosloin which become sepa-
rated off during development, and are not simple extensions
of that portion of the coelom into which the stone-canal opens
in the embryo, and which persists as the ampulla.

mu
am pc

FIG. 259.— DIAGRAM SHOWING THE RELATIONSHIPS OF THE STONE-CANAL,

AXIAL SINUS, ETC., IN AmpMura squamata (after MACBRIDE).
am = ampulla of stone-canal. N = riug-uerve.

U ~ genital bursa. o = ovoid gland.

gr = origin of genital rachis. pc — pore-canal.

M — mouth. ps = peripharyngeal space.

mp = inadreporite. s = sinus.

mu = muscle. sc = stone-canal.

These differences from what occurs in the Asteroidea do not imply,
however, a want of homology between the axial sinuses of the two groups.
The entire sinus is after all a separated portion of the coelom, and it
makes little difference whether it he all separated off at au early stage
iu the development, as in the Asteroidea, or only a part of it, the rest de-
veloping later from, the general coelom as in the Ophiuroidea and Crinoidea,
the dorsal organ of the latter being homologous with the axial sinus
in inns the ampulla of the stone-canal of the Ophiuroidea and Asteroidea.

Lying 011 the aboral surface of each radial nerve-cord is
a radial schizocoalic sinus (Fig. 260, br], which communicates
with an oral sinus surrounding the mouth. The relations of
this system are similar to those of the schizocoalic system of
the Asteroidea, and numerous communications between it
and the ccelomic cavities occur. It contains, however, a
system of canals, which correspond to the lacunse occurring

566 INVERTEBRATE MORPHOLOGY.

in the walls of the oesophagus in the Crinoids. They have
been termed blood-vessels in the Ophiuroidea, the sinuses
which surround them being termed the perihaemal canals;
they follow the course of these latter, a process of the ovoid
gland coming into connection with the oral lacuuar ring.
This gland (Fig. 259, o) is, as in other groups, partly asso-
ciated with the lacuuar system and partly with the genital
apparatus. It lies in the wall of the axial sinus and projects
into it so as almost to fill it. At one extremity, as stated, it
comes into connection with the oral lacunar ring, and at one
point in its wall it contains a mass of cells from which the
genital cords pass out to the reproductive organs, accom-
panied by strands of the lacuuar tissue.

The hydro coal has the usual arrangement, and is confined
to the oral surface of the disk and arms. The radial canals
(Fig. 260, wr) lie on the oral surface of the ambulacral ossi-
cles, extending to the terminal plate, and ending, at least in
those forms which have simple arms, in a terminal tentacle.
At regular intervals, corresponding in number to the ambu-
lacral ossicles, the radial canals give off transverse branches,
which pass outwards in the substance of the ossicles (Fig. 258),
and make their exit through the ambulacral pores between
successive adambulacral plates to terminate as tube-feet. No
ampullae occur on these transverse branches, though a circu-
lar valve occurs just where each branch becomes continuous
with the tube-foot. The feet are simple conical structures
destitute of a terminal sucker, and do not therefore serve
for locomotion. Their walls are richly supplied with nerves,
and in some forms are provided with numerous papilla ap-
parently sensory in function. Surrounding the mouth are
ten buccal tentacles (Fig. 260, bt), which correspond to the
first two pairs of tube-feet of each radius of the Asteroids,
but arise by fine branches, which later divide, and are
directly connected with the oral ring-canal (Fig. 258, t).
These seem to be undoubtedly seusoiy and perhaps olfactory
in function. The oral ring-canal iisually has attached to it
in each interradius, except that in which the stone-canal lies,
a single Poliau vesicle (Fig. 260, PV\ though in Opliiactis
two, three, or even four vesicles may occur in each in terra-

TYPE ECHINODERMA. 567

dius. The stone-canal, as lias been noted, opens into a spe-
cial portion of the ccelom, the ampulla of the stone-canal,
and this again communicates with the exterior by a tube
opening by a pore placed in the adults on the oral surface
of the body in one of the buccal shields ; primitively the
opening is situated on the aboral surface, only later migrat-
ing to its final position. In Astrophyton, in which the buccal
shields are wanting, the madreporiform tubercle occurs on
the oral surface of the disk in one of the interradii, and in
some species there may be five tubercles, one in each interra-
dius, a multiplication of the pores and stone-canals occurring
also in other genera, such as Amphiura and Opliiolepis.

In consequence of the position of the pore or tubercle the position of
the stone-canal in the Ophiuroidea is very different from that which it
possesses in other groups. Whereas in these it passes aborally from the
water vascular ring, in the Brittle-stars it hangs down from the ring
towards the oral surface of the body. All those structui-es too, such as
the axial sinus and the ovoid gland, which are usually associated with the
canal, undergo a similar transformation of position, which is possible on
account of the distance from the oral surface at which the water vascular
ring is situated (see Fig. 259).

The digestive tract is very simple. The mouth guarded
by the oral angle-pieces opens into a short oesophagus (Fig.
260, 0], which communicates with a capacious saclike stomach,
slightly pouched out in each radius, but not extending into
the cavities of the arms. There is no anus in any member
of the group.

The epithelial nervous system of the Ophiuroidea is asso-
ciated with the general ectoderm in very young specimens,
but later sinks into the cavity of the bod}r by a process
which may be compared to an invagiuation. Consequently
a tube is formed lying within the body-wall on the oral sur-
face of the body, the radial nerves (Fig. 260, nr] being
situated in its aboral wall. This tube forms the epineurai
sinus, and the cavity it encloses seems to be in reality a por-
tion of the exterior, though it may be schizocoelic. The
oral ring of the nervous system is not enclosed in an epi-
neural canal, but remains in connection with the ectoderm
at the lower extremity of the oesophagus, being pushed thus

568

IN VERTEBRA TE MORPHOLOG T.

aborally by the development of the oral angle-pieces. The
radial nerves are, however, contained in the wall of the sinus,
coming to the surface of the body at the tips of the arms,
where they terminate by fusing with the general ectoderm.
The muscular nervous system is, as in the Asteroidea, closely
associated with the oral ring and radial nerves, l}*iug on their
aboral surface and separated from them only by a thin layer
of connective tissue. The aboral system consists of a ring
situated beneath the aboral surface of the body, from which
branches pass oft' towards the reproductive organs. Indeed
the entire system is intimately associated with the genital

FIG. 260.— SECTION THROUGH AN OFHIURAN SHOWING STRUCTURE (after

LUDWIG).

A — umbulacral ossicles. 0 = mouth.

br — schizocrelic siuus. p — pala ungularis.

bt — buccal teutacles. ps = periphuryngeal space.

C — cceioui. PV = Polian vesicle.
M — muscle. S = peripharyngeal septum

nr — radial nerve. T — torus angularis.

wr = hydroccel-vessel.

cords, and its course can be understood from a description
of these structures. No special sense-organs other than the
terminal and buccal tentacles and the tube-feet, already de-
scribed, occur in the Ophiuroidea.

As already stated, the genital cord arises from a group of
cells in the wall of the ovoid gland (Fig. 259, gr) and passes
in an interradius towards the aboral surface of the body,
carrying with it a portion of the axial sinus. Arrived at this
point the sinus ami cord form rings, the aboral nerve-ring
lying in the wall of the sinus, while the genital cord lies in its
interior, attached to its wall by a lamella of connective tissue.
From the genital-cord ring ten short branches are given off

TYPE ECUINODERMA.

569

which enlarge into ten saclike reproductive organs, lying
in close contact with the walls of the genital bursse. The
ova and spermatozoa migrate from their point of origin in
the ovoid gland along the genital cords, which also contain
prolongations of the lacuuar tissue of the gland, and mature
in the reproductive pouches. During the spawning-time the
lobes of the reproductive organs protrude into the bursse,
pushing before them the thin walls of these pouches, and the
reproductive elements when mature burst through into the
cavities of the bursse, whence they make their way to the
exterior, or else, as in Amphiura squamata, undergo their de-
velopment in the pouches.

From what has been said it may be seen that the genus
Astrophyton and its allies differ in many respects from the

st

FIG. 261. — PLUTEUS LARVA OP EcJiinarachnius parma (after FEWKES).
a = oesophagus. TO = mouth.

e =• rudimeut of adult. s = calcareous skeleton.

st = stomach.

other Ophiuroids, having arms sometimes branched and
capable of being curled in over the oral surface, possessing
pedicellarise and lacking buccal shields, not to mention other
peculiarities. Consequently the class Ophiuroidea may be
regarded as consisting of two orders, the EUKYALIDA, includ-
ing Astrophyton, commonly known as the Basket-star, Tri-
chaster, in which the arms do not branch, and other similar
forms, and the OPHIUEIDA, which includes the other genera,
such as OpJiiotlirix, Ophioderma, Ophiolepis, Amphiura, etc.

570 INVERTEBRATE MORPHOLOGY.

/

Development of the Ophiuroidea. — Except in a few cases, such as Am-
phinra squamata, whose habits have already been referred to, the devel-
opment of the young Ophiuran takes place outside the body of the parent
in the surrounding water and a typical larval form occurs. This is known
as the Pluteus (Fig. 261) and in its general form resembles the Bipinnaria
of the Starfish. The ciliated baud, however, does not divide into an
adoral and a postoral portion, but remains continuous, and the lateral
lobes become long armlike processes supported by a special skeleton,
formed of calcareous rods developed in their interior. The mode of devel-
opment of the young Ophiuran from this larva resembles closely that de-
scribed for the Asteroidea.

CLASS IV. ECHINOIDEA.

The Echinoidea present greater differences in shape than
are found in any of the other groups of Echinodernis, being
more or less spherical, oval, discoid, pentagonal, or heart-
shaped, but they are all characterized by the absence of arms,
by the calcareous plates being immovably united (except in a
few forms, such as Asthenosoma, where their edges overlap
and they are consequently movable) to form a firm test, and
by a great development of movable spines upon the plates,
whence the popular name of Sea-urchins usually applied to
members of the group. The test is covered by ciliated ecto-
derm, below which is a plexus of nerve-fibres and ganglion-
cells which coordinate the movements of the spines, to whose
bases muscle-fibres are attached.

The test presents certain variations in the different forms,
but there are also certain features which are to be considered
typical for the group. The apical system of plates is usually
well developed. A centrodorsal is present in the genus tia-
lenia, but in all other recent forms it is replaced by a series of
small plates which constitute the periproct in the simpler
forms, since in these they surround the anus. These plates
are surrounded by a circle of five basals (Fig. 262, g), usually
termed genitals on account of the reproductive ducts opening
by a pore upon them. The five radials (o) are also repre-
sented, alternating with the basals ; and since the terminal
tentacle of the radial hydroccel-cauals protrudes through a
pore situated upon them, and more especially since a pigment-
spot, supposed to be an eye, lies frequently at the base of

TYPE ECHINODERMA. 571

this tentacle, these plates are usually knowii in this group
as the oculars. Starting from each ocular and each genital,
rows of plates pass outwards and downwards towards the
mouth, the test being formed of twenty such rows extending
in meridional lines from the aboral to the vicinity of the oral
pole. The rows are grouped together in pairs, five of the pairs
starting from the genital plates and the other five from the
oculars ; and since a number of the plates in these latter rows
are perforated for the emission of tube-feet, they are generally
known as the ambulacral plates (Fig. 262, A], while those of

m

---' A

I

FIG. 262. — FIGURE SHOWING THE ARRANGEMENT OF THE APICAL SYSTEM OF

PLATES OF Strongylocentrotus.

A = ambulacral areas. / = interambulacral areas.

an = anus. m = madreporite.

g = genital plates. o = ocular plates.

the intervening pairs are termed the interambulacrals (/). The
pores for the tube-feet are almost always double, and are
situated usually near that side of a plate which abuts upon
the adjacent interambulacral ; in some forms but a single
pair of pores occurs on each plate, but more usually two or
more pairs are found (Fig. 262) — an arrangement which indi-
cates that such plates are formed by a fusion of several
smaller ones, each of which is represented by one of the pairs
of pores.

In the more primitive forms one of the genital plates is
transformed into the madreporiform tubercle (Fig. 262, m),
but in others the limits of the tubercle may extend so as to

572

INVERTEBRATE MORPHOLOGY.

include all the plates of the apical system, and at the same
time the anal opening may leave its position near the centre
of the apical system and become situated in the iuterradius AB,
either at the margin of the flattened disklike test, or even on
its oral surface. A marked bilaterality of form is thus de-
veloped, which may become still more pronounced by a mi-
gration of the mouth away from the centre of the oral surface
along the line of the radius D, which at the same time be-
comes more or less altered in size and form, and consequently
dissimilar to the other radii (Fig. 263). In these cases it is

possible to recognize in addi-
tion to oral and aboral surfaces
anterior and posterior poles
and a right and left side, the
median line of the body pass-
ing in front through the radius
D and posteriorly through the
iuterradius AB. Three of the
radii, C, D, and E, thus lie in
the anterior half of the body,
and for descriptive purposes
these have been termed the
trivium, while the two posterior

ones, A and B. constitute the
FIG. 263 — A PETALOSTICHOUS ECHI- . .

KOID, Brinsopsis lyrifera, FROM ^iviwm.

THE ABORAL SURFACE WITH THE The mouth, which is usual-
SPINES REMOVED (after A. AOASSIZ). ly situated in the centre of the
D = modified ambulacrum. y^j surfa is snrroTmded by

T ~ itisciolc

an area, the peristome, which

has imbedded in it only a few scattered calcareous plates and
consequently possesses a somewhat leathery consistency.
An oral system of plates cannot be distinguished in adult
Echinoids.

The marked bilateral symmetry referred to above as occurring in cer-
tain Echinoids is undoubtedly a secondary condition, those forms in which
the mouth is central and the anus approximately so, and whose bilaterality
is indicated only by the madreporiform tubercle, being, there is every
reason to believe, the most primitive. The bilaterality cannot be regarded
as a reversion to the more primitive symmetry of the larva, since in the

TYPE ECIIINODERMA. 573

latter the hydrocoel-pore lies to the left of the median line, while in the
bilateral Echinoids it is situated in the right anterior inter radius. Nor
can it be regarded as indicating a primitive adult symmetry, which, though
usually disguised, exists in all the Echinoderms, since, as stated, the forms
in which it is most pronounced are the most highly differentiated members
of their group. It should be mentioned that in the less differentiated
forms the radiality is disturbed by the arrangement of the plates bordering
upon the peristome, as well as by the unpaired madreporiform tubercle.
For if the rows of plates of each ambulacra! region be indicated alternately
a and &, proceeding contrary to the direction of the hands of a watch, so
that the posterior interambulacral region is bordered by the plates Ba and
Ab, then it will be found that the plates bordering the peristome in the
rows Ab, Ha, Ca, Db, and Ea are large and usually pierced by a double
pore, while those of the rows Aa, Bb, Cb, Da, and El are smaller and
pierced usually by only one pore. This arrangement does not, however,
necessarily point to any special plane of bilaterality, but is interesting on
account of its constant occurrence in both the bilateral and the more radial
forms, whence it furnishes a means of identifying the plane of bilaterality
in the latter.

Projecting inwards from the inner surface of the test in
the neighborhood of the peristome are frequently to be found
calcareous plate- or pillar-like processes termed auricula1 (Fig.
265, au), which may either be confined to the interambulacral
plates or occur also on the ambulacrals, uniting in some forms,
such as Strongylocentrotus, in pairs, so as to form arches
through which the radial hj'drocoel-canals and nerve-cords
pass. In the flattened disklike forms, such as Echinarachnius,
these pillars are much more numerous, extending from the
oral to the aboral surfaces of the test. Attached to the outer
surface of the test are numerous spines, each of which is
hollowed out at its base, the hollow fitting over the convexity
of a tubercle upon the test. This ball-and-socket articulation
allows of a free movement of the spines in any direction, a
movement which is effected by muscles extending from the
test to the base of each spine, and forming a sheath around
its base. The spines thus serve as efficient organs of locomo-
tion, usurping this function entirely in some forms, while in
others they are aided by the tube-feet. They also in some
forms serve as defensive structures, as in Diadema, where they
are long and slender and readily penetrate the skin of less-
protected animals, or in Asthenosoma , in which the larger

574

INVERTEBRATE MORPHOLOGY.

spines are somewhat enlarged towards the tip, the enlarge-
ment containing a poison-gland whose secretion is injected
into the wound produced by the spine. Pedicellarise, which
have already been noted as occurring in the Asteroidea and
the Euryalid Ophiuroidea, are richly developed in the Echi-
noids, more especially in the neighborhood of the mouth and
anus. They assume varying forms, in the typical one (Fig.
264) being composed of a stalk surmounted by three calca-
reous pieces or teeth, hinged upon the
stalk, and capable of being divaricated and
approximated by means of muscles. Each
tooth bears cushionlike elevations which
are tactile in function, so that the three
teeth are vigorously approximated when
touched by any foreign body. In some
pedicellariae the teeth are very much re-
duced in size, but in their place three
mucous glands are developed, structures
sometimes found also in association with
well-developed teeth. The functions of the
. 264.— PEDTCEI-- pedicellariae may be various ; they may
LAKIA FROM Doi'o serve for the prehension of prey or for
cidaris papillate protection and they have also been seen

(after KOEHLER).

m = muscle-fibres, to remove excreta from the surface of the

test in the neighborhood of the anus. In

the bilateral Echinoids a third form of spine is found, of

small size and covered by a richly-ciliated epidermis. These

davulce, as they are termed, are usually associated together
in groups of considerable extent termed Semites or fasdoles,
occurring especially in the neighborhood of the plates per-
forated for the emission of tube-feet (Eig. 263), and in the
vicinity of the anus. The clavulse have a rich supply of
nerve-fibres, and are on this account supposed to be sen-
sory in function, though they may also assist in renewing
the water in the vicinity of the tube-feet, which probably
assist to a greater or less extent in respiration. A fourth
variety of appendage to the test is formed by the spJtic-
ridia, which consist of a stalk surmounted by an oval
mass of carbonate of lime traversed in all directions by deli-

TYPE ECHINODERMA.

575

cate canals. These organs, which are usually quite small,
are situated in the vicinity of the ambulacra! pores or near
the mouth, and are supposed to have a sensory, perhaps
olfactory, function.

Owing to the presence of the firm test the muscular system

Gp pp an tt

oc

FIG.

Al

amp

an

as

au

br

Co

0

Gd

Gp

Or

lir

1

Ir

M

m

265. — DIAGRAM SHOWING THE
= Aristotle's lantern.
= ampulla.
= aboral nerve-ring.
= axial sinus.
= auricula.

= external branchia.

= coelom.

= reproductive organ.
= genital duct.

= genital pore.

= genital rachis.
= hydroccel-ring.
= ossophagus.
= lacunar ring.
= madreporite.

STRUCTURE OF AN ECHINOID.

nr = epithelial nerve-ring.
oc = ocular plate.
og — ovoid gland.
pa = perianal space.
pp = periproctal space.
pph -- peripharyngeal space.
pv == Polian vesicle.

R = rectum.

r?i = radial hydroccel-vessel.
rn = radial nerve.

sc = stone-canal.

si — siphon.

sp = spine.

tf = tube-foot.

tt = terminal tentacle.

is but feebly developed, being represented by the muscles
attached to the spines, pedicellariae, etc., and by those which
move the parts of the masticatory apparatus when it is
present.

The coelom (Fig. 265, Co) is comparatively spacious, though

576 INVERTEBRATE MORPHOLOGY.

traversed ill some forms by the calcareous pillars already
mentioned, as well as by a perforated mesentery extending
from the inner surface of the test to the intestine and follow-
ing the convolutions of the latter. In the bilateral Echinoids,
which as a rule swallow large quantities of sand, the mesen-
tery is much stronger than in other forms, and additional
meseuterial bands are added to assist in the support of the
intestine. As in the Ophiuroids, a circular partition extends
from the oesophagus outwards to be inserted into the test in
the neighborhood of the auriculae, enclosing a peripharyngeal
space (Fig. 265, ppJi), which has no communication with the
rest of the ccelorn, and contains the organs of mastication when
these are present. In many forms the partition is pouched
out into five radial diverticula which project into the general
coalom and are known as the organs of Stewart or as internal
branchiae. In those radial Echiuoids in which these structures
are absent, ten lobed diverticula of the floor of the peripharyn-
geal space project upon the outside of the body at the margin
of the peristome, a pair being situated in each interambula-
cral region ; these are termed external branchiae (or). At the
aboral surface of the body in the radial forms a partition
similar to that enclosing the peripharyugeal space is found,
surrounding the terminal portion of the intestine and enclos-
ing a subperiproctal cavity (pp), while within this occurs a
second partition shutting off a periaual space ( pa). Muscular
fibres occur in these partitions, and it has been suggested that
by contracting and thus compressing the fiuid contained in
the spaces they serve to close the lumen of the rectum and
the anus.

As regards the axial sinus (as) the Echiuoids resemble the
Asteroids and Ophiuroids in that the portion of the coelom
into which the larval stone-canal opens persists in the adult
and forms a pouch extending downwards towards the oral
surface parallel to the stone-canal, the ovoid gland (og) devel-
oping in its walls. Into the upper portion of it the adult
stone-canal (sc) opens, and it communicates with the exterior
through the madreporiform tubercle (M).

The so-called blood-vessels are, as in the Ophiuroidea,
portions of the lacuuar system, and are contained in peri-

TYPE ECU1NODERMA. 577

hseinal canals, as the radial schizocoelic sinuses have been
termed in this group, as in the Ophiurids. These canals con-
sist of five tubes lying between the radial hydroccel- canals
and the radial nerve-cords, and terminating blindly at their
oral extremities by coming into contact with the peripharyn-
geal partition ; they are not continued within the partition and
there is no circumoral sinus. The lacuuar system consists of
five radial lacunae, lying in the periluemal canals, penetrating
into the peripharyugeal space, where they unite into a circular
circumoral lacuna (lr), from which branches pass to the walls
of the digestive tract and which is in connection with the
lacunae of the ovoid gland. This structure, as stated, lies in
the wall of the axial sinus, and, as in other forms, stands in
close relationship to the reproductive organs, its lacunaa being
continued into the walls of the genital cords.

The hydroccel has the usual arrangement of a perioeso-
phageal circular canal (hr) from which five radial canals pass
off (r/i), each terminating in a tentacle (tt) perforating an
ocular plate. From the perioesophageal ring the stone-canal
(sc) passes aborally to open into the axial sinus close to the
madreporiform tubercle, and in addition in the radial Echi-
noids the ring has attached to it in each iuterradius a spongy
structure which is usually termed a Poliau vesicle (pv\
though these structures in other groups are saclike. The
tube-feet (tf) which perforate the ambulacral plates are in the
majority of forms, and especially in the radial ones, very
extensible and provided at the tip with a sucking-disk, and so
assist the spines in locomotion. Two pores as a rule exist
for each foot ; through one of these the branch issuing from
the radial canal passes, and through the other a branch passes
back from the foot into the interior of the body to terminate
in a saclike ampulla. The feet, however, near the aboral
surface are frequently branched and lack a sucker, serving a
respiratory function rather than a locomotor, and in the
bilateral Echinoids, in which frequently the tube-feet occur
only on the aboral surface of the test, nearly all the feet may
assume a tentaclelike or pinnate form and become respiratory.

The digestive tract in all those forms in which the mouth
occupies the centre of the oral surface is provided with a

578

INVERTEBRATE MORPHOLOGY.

a

pharynx surrounded by a complicated calcareous masticatory
apparatus usually termed Aristotle s lantern (Fig. 265, Al, and
Fig. 266). When most highly developed it has the form of a
pentagonal pyramid, whose apex is directed towards the
mouth and consists of five similar portions united together.
Each portion contains an elongated ribbonlike tooth (Fig.
266, £) lying in an interradius and projecting slightly beyond
the lips of the mouth, though for the greater portion of its
e length imbedded in a calcareous

socket or alveolus (a) composed of
a right and a left half united above
by epiphyses (e). Between each
pair of alveoli, at their basal ends
is another calcareous piece termed
a radius, and below each of these,
i.e. on its oral surfaces, lies another
piece, the radula (?•). Muscles pass
to this complicated apparatus from
the auriculas and from one piece to
the other, producing approximation
and divarication of the projecting
tips of the teeth. The presence
of this apparatus brings it about
that the circumoral hydrocoel and
lacunar rings are forced back some distance from the mouth,
surrounding the oesophagus just where it leaves the lantern.
It seems well accordingly to speak of these rings as being
perioesophageal rather than circumoral.

On leaving the lantern the digestive tract, starting in the
interradius 1)E, passes around the ccelomic cavity in the direc-
tion of the hands of a watch, until it reaches the interradius
CD, when it bends abruptly on itself and, on another plane,
nearer the aboral surface, retraces its course almost to its
point of starting, whence it passes to the anus. The portion
of the intestine immediately succeeding the pharynx is termed
the oesophagus and is succeeded by a slightly wider intestine,
the junction of the two parts being in some forms further
indicated by the occurrence at that point of a large ccecum.
As a rule, however, appendages to the digestive tract are rare,

t

FIG. 266 —ARISTOTLE'S LAN-
TERN FROM Arbacia.
a = alveolus.
e = epiphysis.
r = radula.
t — tooth.

TYPE ECHINODERMA. 579

the only one occurring with any marked degree of constancy
being the siphon (Fig. 265, si), a tube which arises from the
oesophagus and runs, closely applied to the intestine, to opeii
again into it at the extremity of the oral coil. The function of
this structure appears to be respiratory, but it is to be noted
that it is wanting in all the members of one of the orders (the
Clypeastroideci) into which the group may be divided.

The epithelial nervous system has the usual arrangement
consisting of a pericesophageal ring (Fig. 265, nr) and five
radial cords (rri). As in the Ophiuroidea, these latter struc-
tures have withdrawn themselves from the ectoderm and sunk
within the body-cavity, and accordingly there is to be found
an epineural sinus lying below the nerve-cords. Below the
nerve-ring, however, no sinus is to be found, and it seems
possible that it may have fused with the peripharyngeal space.
The extremity of each radial cord fuses with the ectoderm in
passing through the pore in the ocular plate, and is distrib-
uted to the Avails of the terminal tentacle. A muscular
nervous system is present, consisting of five masses lying on
the aboral surface of the radial nerve-cords just where they
join the ring, and apparently having no direct connection
with each other ; they send fibres to the muscles of the inas-
ticatorv apparatus, and are said to be wanting in those forms
which lack this organ. The visceral system consists of a
ring (an) lying near the margin of the subperiproctal cavity
and imbedded in its wall. From the ring five branches arise
which pass to the walls of the ducts of the reproductive
organs.

Sense-organs of various kinds have already been referred
to, such as the terminal tentacles of the hydroccel canals, the
fascioles, and the sphseridia. In addition to these, pigment-
spots occurring on the ocular plates have been regarded as
eyes, and somewhat complicated structures of a bright blue
color which occur abundantly over the surface of the test in
a species of Diadema have also been regarded as light-percip-
ient organs.

As in other forms, the reproductive system consists of the
genital cords and the reproductive organs. The former have
their origin from a single cord, which is a hollow tube

580 INVERTEBRATE MORPHOLOGY.

internally by immature germ-cells and is connected at its oral
extremity with the ovoid gland. It passes thence to the
aboral surface of the body, where it forms a ring (Fig. 266, gr)
from which in each iuterradius a branch passes outwards to
expand into a highly racemose sac, the reproductive organ
(G). In some forms the number of the organs may be re-
duced to four or even to two, though five is to be regarded as
the typical number. Each organ opens to the exterior by a
special duct (Gd), usually opening on a genital plate, but
sometimes in an interradius outside the genital plates.

As already noted, there is considerable variety in the rela-
tive positions occupied by the mouth and anus, and many
differences of structure are associated with these variations.
It is possible, in fact, to divide the Echinoidea into three
orders, which are marked out by the positions of the openings
of the digestive tract.

1. Order Desmosticha.

In these forms the mouth occupies the centre of the oral
surface, and the anus approximately that of the aboral sur-
face, the radial symmetry usual among Echiuoderms being
well marked. The body is usually more or less spherical in
form, though occasionally somewhat flattened ; all the arnbu-
lacral plates are perforated for the emission of tube-feet, and
all five ambulacral areas are equally developed (Fig. 262). In
the members of this order, consequently, the bilaterality is
marked externally only by the position of the madreporiform
tubercle.

The primary ambulacral plates frequently fuse to form
secondary plates each of which is perforated by several pairs
of pores, as many as six occurring on some plates in Strongy-
locentrotus. The spines are sometimes exceedingly long, as
in Diadema, and are usually well developed, being in Arbatia
equal in length to about half the diameter of the bod}^. The
auriculae are the only representatives of the calcareous plates
or bars which extend from the oral to the aboral surface, and
an Aristotle's lantern is always well developed, its alveoli
being much longer than broad. In this order external branchiae

TYPE ECHINODERMA. .r>Sl

are generally present, the genus Ci(l<u'is and its allies alone
lacking them and possessing Stewart's organs. It is custom-
ary, therefore, to divide the order into two sub-groups : the
ENTOBRANCHIATA, including the Cidaridce, and the ECTOBEANCHI-
ATA, including all other forms, such as Strongylocentrotus and
Arbacia.

2. Order Clypeastroidea.

In this order, named from one of the genera contained in
it, the mouth still occupies the centre of the oral pole, but the
anus is situated in the interradius AB, either at the margin
of the flattened test, as in Ecliinarachnius (Fig. 267), or on its
oral surface, as in Mellita. In
Clypeaster the body is but slightly
flattened, but in the two other
genera already mentioned the
flattening is carried to such an
extent that the test has a more
or less disklike shape, whence
the term Sand- dollars applied to
certain forms.

In accordance with the shift-
ing of the anus from the centre.

f . , . FIG. 267. —A CLYPEASTROID ECHI-

of the apical system certain NOID EcMnarachnius parmf,t

changes take place in it, the FROM THE ABORAL SURFACE
most marked being an extension WITH TnE SPINES FOR THE MOST
of the madreporiform tubercle **ART REMOVED (after A. AGA
until it includes all the genital and ocular plates, the genital
pores being forced outwards so as to lie on one of the iuter-
ambulacral plates, and in some cases the pore of the posterior
interradius AB and the corresponding reproductive organ is
wanting (Echinarachnius). The perforated ambulacral plates
are as a rule confined to the apical portion of the aboral
surface, the plates near the margin of the test and those upon
the oral surface being irn perforate. The perforated plates
are of course very narrow at the apical end of the series, and
gradually enlarge as they pass towards the edge of the test,
giving thus the appearance of five flower-petals, and hence

582 INVERTEBRATE MORPHOLOGY.

the ambulacra or areas occupied by perforated plates are
termed petaloid. In Echinarachnius, for instance, the plates
after having reached their greatest width retain it to their
abrupt termination (Fig. 267), the petals being then termed
open, but in other forms, e.g. Mellita, they contract again
peripherally, in which case the ambulacra are said to be
closed.

The pores belonging to each pair are generally united by
a groove, and are termed yoked pores, and in Mellita, for
example, in addition to the pair of yoked pores on each plate
there is a third one situated near the middle line of the am-
bulacrum. The tube-feet which project from the yoked pores
are frequently pinnate in form, while those emitted through
the single pores are simple and tentaclelike. The spines are
generally very small, though those of the oral surface serve
for locomotion.

In Mellita, towards the periphery of the test, the im per-
forate auibulacral plates of the radii A, B, C, and E do not
meet, leaving elongated holes passing through the test, and
the same thing also occurs with the plates in the interradius
AB, so that altogether five such holes exist. In other forms,
instead of holes, notches occur at the margin of the test, and
other interambulacra than that in which the hole occurs in
Mellita may be affected. Calcareous columns extend from the
oral to the aboral surfaces of the test, being especially abun-
dant towards the periphery, and calcareous plates uniting the
two surfaces occur on either side of each ambulacrum. An
Aristotle's lantern is present, but the alveoli are usually
broader than long.

3. Order Petalosticha.

In this order, as its name indicates, the ambulacra are
usually petaloid, and the bilaterality indicated in the Clypeas-
troids is more pronounced, since neither the mouth nor the
anus retains its original position at the centre of the oral or
apical surface. The anus lies in the posterior interradius
A B, while the mouth has moved forwards to a greater or less
extent along the radius D. The test is oval or, frequently,

TYPE ECH1NODERMA. 583

somewhat heart-shaped, owing to the anterior radius D being
more or less depressed so as to form a groove (Fig. 263).

The ruadreporiforrn tubercle extends usually through the
centre of the apical system into the posterior iuterradius,
thus dividing the apical system. The posterior genital pore
is obliterated in all members of the order, and the reproduc-
tive gland corresponding to it disappears, so that but four
reproductive organs and pores are present (Spatangus). In
some forms, however, the reduction of the reproductive organs
and pores is carried still further by their disappearance in the
right anterior iuterradius CD, and those of the left anterior
iuterradius DE may also disappear, the genital plate becom-
ing part of the madreporiforrn tubercle, so that the number of
reproductive organs and genital pores may be reduced to two
(Moira). The ambulacra are usually dissimilar, especially in
heart-shaped forms, in which the anterior ambulacrum be-
comes much modified. Fascicles are generally present, ar-
ranged in different manners in different species, in some sur-
rounding the ambulacra, in others forming a ring in the vicinity
of the anus, and in others arranged in numerous patches or
lines. Spines are abundantly present and are usually of a
moderate length. The mouth is bounded behind by a well-
marked lip or labrum, produced by the extension forwards
below the mouth-opening of the plates of the posterior inter-
ambulacruni AB. There is no Aristotle's lantern in the
Petalosticha.

Development of the Echinoidea. — The development is in its general
features very similar to that of the Ophiuroids, the free-swimming larvae
having a closely-similar form and being known by the same name. The
Eehiuoid Pluteus (Fig. 261) may, however, in some cases be distinguished
by the occurrence of two (Arbacid) or three (Spatangus) processes upon
the posterior portion of the body, which are wanting in the Ophiurid larvae;
and furthermore in some Echinoid Plutei (Arbacid) two earlike lobes fringed
with cilia occur upon the sides of the posterior portion of the body and
are known as ciliated epaulettes. The young Sea-urchin develops in the
posterior portion of the body of the larva as in other forms, the armlike
processes and the praeoral lobe of the larval body being gradually resorbed.

The relationships of the various groups have already been referred to
and need not be again discussed here, except to repeat the statement that
the evidence at our disposal indicates that the Desuiosticha are the most

584

1NVERTEBRA TE MORPHOLOG Y.

primitive, while the Clypeastroidea and Petalosticha are secondarily-de-
rived forms. The bilaterality of these latter forms is not to be regarded,
therefore, as having any phylogenetic significance.

CLASS V. HOLOTHUROIDEA.

The Holothurians (Fig. 268) are characterized so far as
their form is concerned by being elongated in the oral-aboral
axis, having thus a somewhat wormlike form, the mouth be-
ing at or near one extremity and the anus at the other, except
in the genus Wiopalodina, in which the two openings are ap-
proximated. As a rule the body is cylindrical, but in some

forms, such as Psolus and the Elasipoda,
there is a well-marked flattened ventral
surface. Three of the radial hydrocoel-
cauals lie upon this ventral surface, the
other two being dorsal, and it is usual to
apply the term trivium to the ventral radii
and bivium to the dorsal. It must be
recognized, however, that this use of the
terms does not imply a homology with
the radii similarly named in the Echin-
oidea, since in the latter the radii C, D,
and ^'constitute the trivium, whereas in
the Holothurians it is the radii A, B,
and E.

The mouth is surrounded by a circle
of tentacles varying in number from ten
to thirty. There are at first five primary
HOLOTHCRIAN. tentacles, interradial in position, which
are formed in connection with five ceecal

outgrowths of the hydrocoal-ring, and the tentacles subse-
quently formed receive branches from the five primary caeca.
In shape the tentacles vary considerably, being cylindrical
in some forms, arborescent or pinnate in others (Fig. 268),
and in others peltate, and in some forms they are retractile.

The exterior of the body is usually covered by an epithe-
lium over which a cuticle may be developed, but in some
forms the ectodermal cells sink into and become fused with
the subjacent connective tissue. The calcareous skeleton is

PIG. 268.— Pentacta fron

TYPK ECHINODERMA. 585

but feebly developed in the Holothurians, the body-walls
having as a rule a somewhat leathery consistency, though in
Elpidia interlocking spicules, and in Deima and P solus closely-
approximated or overlapping plates, give the skin considerable
rigidity. In the majority of forms, however, the calcareous
skeleton is represented by scattered plates of various shapes,
perforated, knobbed, sometimes wheel-shaped as in Chirodota,
or associated with an auchorlike spicules in Synapta, and
are not sufficiently numerous to give a rigidity to the integu-
ment, which in Synapta may even be thin and translucent.
There is no indication of an apical system of plates, though in
Miilleria a circle of five plates surrounds the anus, but the
oral system is represented in a species of Psolus by five
plates which may be closed over the mouth and tentacles.
In other parts of the body than the integument calcareous
matter is also frequently deposited, especially in the connec-
tive tissue of the wall of the peripharyngeal cavity, the pharyn-
geal ring (Fig. 269, Z>) so formed consisting typically of five
radial ossicles, grooved or perforated by the radial nerves
and hydrocoel-canals, and of five interradial ossicles alternat-
ing with them, though in those forms in which the number of
tentacles is greater than ten the number of the interradial
ossicles may be increased. Spicules are also found in the
mesentery, and in some forms plates or a calcareous network
develops in the wall of the pharynx. Spines are rarely
present, though the plates of Echinocucumis, Elpidia, and a few
other forms bear them, and pedicellarise are entirely wanting.
The coelom is traversed by several mesenteries uniting the
digestive tract to the body-wall, the most constant being the
so-called dorsal mesentery (Fig. 269, m), which lies in the
anterior portion of the interradius CD. The portion of the
ccelom which surrounds the oesophagus is separated from the
rest, as in the Echinoidea, and forms the peripharyngeal space,
and similarly in some forms (Cucumaria, Holothuria) a perianal
space surrounds the terminal portion of the digestive tract.
In /Synapta and its allies there are attached by slender pedun-
cles to the body-wall along the line of the mesenteries, and
hanging freely in the body-cavity, numerous ciliated urnlike
bodies, which probably function similarly to the ciliated cups

586 INVERTEBRATE MORPHOLOGY.

of the Crinoids iu maintaining a circulation of the coelomic
fluid.

So far as is known, the portion of the coelom which in the
embryo opens to the exterior by the water-pore and with which
the stone-canal communicates in the Asteroids and Echinoids
does not persist in the adult Holothurian, and consequently
there is no axial sinus, and it is doubtful if a structure com-
parable to the ovoid gland of other forms exists. Schizocoelic
sinuses corresponding to the perihsemal canals of the Echinoids
occur in their usual position between the nervous system and
the hydrocoel-canals, and consist of a ring accompanying the
nerve-ring and five radial canals which abut against the ring
at their oral ends but seem to be completely separated from
it by septa. A lacunar system is well developed, consisting
of a plexus in the walls of the intestine, the various branches
uniting to form a dorsal and a ventral intestinal vessel, which,
passing forwards, unite with a lacuiiar ring surrounding the
oesophagus at about the level of the hydrocosl-riug. From
this ring five radial lacunae extend backwards, lying in the
connective tissue between the radial perihgernal sinus and the
hydrocoel-cauals, and giving branches to the tentacles and the
tube-feet. A lacuna also extends from the pericesophageal
lacunar ring to the reproductive organs arising from a thick-
ened portion of the ring, and this thickening has been re-
garded as the rudiment of the ovoid gland.

The hydrocoel has the usual arrangement, consisting of a
ring (Fig. 269, c) surrounding the oesophagus behind the ring
of peripharyngeal ossicles, and having arising from it a stone-
canal which in the majority of forms hangs freely in the
ccelomic cavity, where it terminates in a madreporiform plate.
In the embryo it as usual opens upon the surface of the body,
and this condition is retained in many Nlasipoda, in which the
canal opens upon the dorsal surface of the body, probably
indirectly through the intervention of an ampulla, as in other
forms. In the majority of forms, however, the connection with
the exterior becomes lost, the ampulla which is present in the
embryo disappearing, and occasionally a number of secondary
canals develop. A single Poliau vesicle (e) is usually attached
to the ring, but in some cases the number of these structures

TYPE ECHINODERMA. 587

may be considerably increased. Five interradial canals arise
from the ring and pass forwards to the tentacles, branching if
these structures are more than five, and in some forms (Holo-
thuria, Chirodota] these tentacular canals are provided with
ampullae. Five radial canals also pass backwards from the
ring in all forms except the Syuaptidre, corresponding to the
radial hydrocoel-canals of other Echinoderms and bearing
tube-feet.

The distribution of these latter structures is peculiar in
many forms. In Molpadia although the canals are present
the tube-feet are entirely wanting, as they are also from the
dorsal canals of P solus and from the median canal of the
trivium of the Elasipoda ; when present they may be arranged
along the lines of the radial canals (Cucumaria, Pentacta, Fig.
268) or may be scattered irregularly over the surface of the
body (Tliijone). In form they also vary considerably, being
either simple fiugerlike processes or else tipped with a
sucker. Frequently the tube-feet are not retractile, and in
the Elasipoda they take the form of strong well-developed
conical processes arranged in pairs.

Owing to the absence of a firm test in the Holothurians
there is a much more extensive development of the muscular
system than in other Echinoderms. The inner surface of the
body-wall is formed by a layer of circular muscle-fibres, and
on each side of each radial hydrocoel-canal is a longitudinal
muscle-bundle (Fig. 269, p) from which in some forms special
bundles pass to the peripharyngeal ossicles and serve as
retractors of the tentacles and mouth-disk.

As stated the mouth is usually at the anterior end of the
body at the centre of a disk surrounded by the tentacles, but
in the Elasipoda it has a somewhat ventral position. The
digestive tract is a simple tube, which occasionally has a per-
fectty straight course from mouth to anus, but more frequently
it is bent twice upon itself, so that there is an anterior de-
scending limb (Fig. 269, /), an ascending (</), and a posterior
descending one (K). The terminal portion of the posterior de-
scending limb is dilated, forming a cloaca (i) from whose wall
muscle-bauds (q) radiate to the walls of the body. This cloaca,
is rhythmically contractile, and has opening into it except in

588

IN VERTEBRA TE MORPHOL OGY.

the Synaptidae two much-branched structures termed the re-
spiratory trees (k). As their name indicates, these structures
are supposed to have a respiratory function, but it is possi-
ble that they may also aid in excretion, the waste products
of metabolism collecting in the cells lining the interior of the

FIG.

269.— DIAGRAM REPRESENTING THE INTERNAL ANATOMY OF A HOLO-

THURIAN (after LUDWIG from LEUNIS).

i' • - cloacal opening.
k = respiratory trees.
I = Cuvieriuu organ.
m = dorsal mesentery.
n = duct of reproductive organ.
o = reproductive organ.
p = longitudinal muscles.

a = tentacles.

b = calcareous pharyngeal ring.
c = hydroccel ring.
d — stone-canals.
e -- Polian vesicle.
/, g, h — intestine.
i — cloaca.

q — radiating muscles of the cloaca.

tubular branches and being carried to the exterior by a
de squama tion of the cells. In addition in a small number of
forms (Holothuria) there occur upon one side of the cloaca a
large number of slender tubes (I), which, at the will of the
animal, can be evagiuated so as to project through the anal
opening. These constitute the organ of Cuvier, the function
of which is not as yet satisfactorily explained.

TYPE ECHINODERMA. 589

The epidermal nervous system consists of a perioesopha-
geal ring arid five radial nerves as in other forms, and iu addi-
tion fi\e interradial nerves pass from the ring to the tentacles.
The system is almost completely separated from the ectoderm,
with which in the embryo it is connected, and lies within the
tissues of the body, the radial nerves only at their posterior
extremities passing through the tissues of the body-wall to fuse
with the ectoderm. In accordance with this arrangement an
epineural sinus accompanies each radial nerve, though absent
from the perioesophageal ring. The muscular nervous sys-
tem has an arrangement similar to that of the sinuses, being
well defined in connection with the radial nerves, though
wanting in the pericesophageal ring. No trace of the aboral
nervous system has been discovered in the Holothurians.

The tentacles serve as tactile organs, and may have also
an olfactory function in addition to being respiratory, and in
the Syuaptids numerous ciliated depressions with a strong
nerve-supply are found upon them. No visual organs occur
in the Holothurians, but in the Syuaptids and Elasipoda
otocysts occur. In Synapta there are ten of these organs, im-
bedded in the connective tissue in the neighborhood of the
pericesophageal nerve-ring, one lying on each side of each
radial nerve close to its origin from the ring. Each otocyst
contains a number of otoliths, and is lined interiorly by cells
which are provided with cilia. In Elpidia the number of
otocysts is fourteen and in other Elasipoda the number may
be much greater, amounting to about thirty-six in a species of
Kolga.

The Synaptids and Molpadids are hermaphrodite, but all
other Holothurians are bisexual. The reproductive organs
(Fig. 269, o) consist of one, or in some cases two bundles of
csecal tubes which are attached to the left side (or on both
sides) of the dorsal mesentery, and open by usually a single
duct (n) upon the dorsal mediau line, sometimes within the
circle of tentacles, but usually behind it. The caeca are lined
interiorly with an epithelium from which the ova and sperma-
tozoa develop.

The arrangement of the reproductive organs in the Holothuriaus is de-
cidedly different from what occurs in other groups of Echinoderms, and the

r.tto

INVERTEBRATE MORPHOLOGY.

differences are associated with the absence or reduction of the ovoid gland
and of an aboral nervous system. The number of the organs is very much
reduced, and no genital cords have as yet been discovered. It is interest-
ing to note, however, the existence of a genital lacuna mentioned above,
in association with which the reproductive organs seem to develop, and it
may be, as stated, that the lacunar thickening from which it arises is to be
regarded as representing the ovoid gland, which, as has been seen, is
intimately connected with the lacunar system in other forms. It seems
probable that in harmony with the shortening of the stone-canal and its
separation from the body-wall, and with the abortion of the axial eutero-
coel, there has been a shortening of the genital cords so that the aboral
ring no longer exists, and the reproductive organs, reduced in number,
develop directly upon the wall of the genital lacuna. It must be remarked
that in some forms there is no distinct genital lacuna, but the reproductive
organs are associated with the intestinal lacunae, a condition which may
be secondary.

Development of the Holothuroidea. — The typical larva of the Holothu-
rians is known as the Auricularia (Fig. 270), and is distinguished from

that of the Asteroids, Ophiuroids, and
Echinoids by being destitute of armlike
processes. In later stages the ciliated
bands fuse in such a manner as to form
st a series of circular bands surrounding
the barrel-shaped larva and recalling the
*'P larva of the Crinoids. By the gradual
elongation of this larva and the disap-
pearance of the ciliated bands the adult
form is acquired, there being no absorp-
tion of any extensive portion of the larval
body as in the Brachiolaria and Pluteus.
The Pliijlotjeny of the Echinoderma.
-The Echinoderms form a well-defined
group showing little indication of afflui-
FIG. 270.— AURICULARIA LARVA OF ties with other forms, and the establish-

Syn<ipta (after SEMON).
dp = dorsal pore.
H =- hydroccel.
pi = primary tentacles.
st = secondary tentacles.

ment of a plausible phylogeny is an
unusually difficult task. One thing,
however, seems certain from their de-
velopmental history, and that is that they
have been derived from primitive bilat-
eral forms, and that the radiality charac-
teristic of the adults has been secondarily acquired. The larvae are strictly
bilateral, there being indications that originally two water-pores, situated
symmetrically upon the dorsal surface, existed. The first question to be
decided then is the cause of the radial symmetry seen in the adult.

Bilaterality in the animal kingdom is usually associated with an antero-
posterior differentiation, and this with a definite axis of progression. Thus

TYPE ECHINODERMA. 591

in an Annelid the most anterior metamere is that which is apt to first come
into contact with new conditions of environment, and consequently it has
become specially provided with sense-organs for the perception of these
new conditions whether favorable or unfavorable; similarly the conditions
affecting the dorsal and ventral surfaces are different and consequently a
dorso-veutral differentiation exists; while on the other hand the conditions
affecting the two sides of the body are apt to be alike, and consequently
the differentiations which occur on each side of the median line are usually
alike. Bilaterality is then associated with a free-mode of life.

Conversely, radiality is usually associated with indefinite axes of pro-
gression or with a fixed mode of life, and the idea suggests itself that the
radiality of the Eehiuoderms may be the result of a fixation of the bilateral
larva. The majority of recent Echinoderms are, it is true, free forms, the
Crinoids alone being sessile, but it will be found that geologically the free
forms are the latest to appear, and that the Pelmatozoa are especially char-
acteristic of the Pakeozoic rocks. This would imply that the Crinoids are
to be regarded as the most perfect representatives among recent forms of
the ancestral types, an idea which is borne out by certain points in Echino-
derm development. Thus the calyx of the Crinoid is developed in the
posterior portion of the oval larva, the anterior portion being occupied by
the stalk; in the Brachiolaria and Pluteus larvae the adult body is developed
in the posterior portion of the larva, the anterior portion undergoing
absorption, an arrangement which may be explained by supposing that
originally the adult forms which possess these larvse were stalked, but
secondarily acquired a free mode of life, and that as a result the stalk dis-
appeared, the body, corresponding to the calyx of the Criuoid, still develop-
ing in its accustomed position in the posterior portion of the larva. In
the Holothurians there is not apparently that distinction into anterior and
posterior regions in the larva which obtains in other forms, but it is to be
noted that in the young Cuciimaria, for example, there is a well-marked
prseoral lobe which later on disappears and may represent the partially-
aborted stalk region of other forms.

Attempts have been made to trace out a phylogeny taking the Holo-
thurians as a primitive type arid tracing them back to such forms as the
Gephyrean worms, the presence of respiratory trees in both forms suggest-
ing a possible affinity. Or again the later barrel-shaped larva with five
tentacles has been considered to represent a common ancestor for all the
various groups of Echinoderms, the term Pentactcea being given to the
hypothetical ancestor. Neither of these views, however, affords any clue
to the origin of the radiality, and the structure of the Holothurians departs
so widely from that of the other groups that it seems preferable to consider
them as most remote from the common ancestor. Furthermore as regards
the Pentactcea it is difficult to understand how there should be so much
similarity in structure of the different forms if they all differentiated in-
dependently from this common ancestor.

From the evidence at our disposal it seems far more logical to accept a

592 INVERTEBRATE MORPHOLOGY.

stalked form for an ancestor, and to consider the Crinoids as approaching
it more nearly than any other recent forms. It has been suggested, with
no little reason, that the Cystoidea were the ancestors of the Crinoids and
perhaps of most of the other groups as well. A full consideration of this
point, as well as of the details of the conversion of the bilateral remote
ancestor into the radial form, would carry us beyond the scope of this work,
and reference must be had to special works treating of these questions
(P. and F. Sarasin, O. Butschli).

As stated, the relationships of the Echinoderms to other groups is a
question which has not yet been satisfactorily settled. Attention may be
called, however, to the remarkable similarity of the jforacm'a-larva of
Balanoglossus (p. 606) to the Echinoderm larva, a similarity so great as to
suggest affinity. This suggestion may, however, be postponed until the
Tornaria has been described.

SUBKINGDOM METAZOA.
TYPE ECRINODERMA.

I. Class CRINOIDEA. — Usually stalked ; with ten (or five ) arms, provided
with lateral pinnules, arising from the margin of the cup-shaped
body. Dermal skeleton well developed.

In adult life free-swimming. Antedon, Actinometra.

Fixed throughout life, stalk with numerous whorls of cirri.

Pentacrinus.

Fixed throughout life, slender stalk with cirri either wanting
or only on distal joints. Rhizocrinus, Calamocrinus, Hyo-
crin us. Thaumatocrinus.

Fixed throughout life, stalk short and stout. Hulopus.
II. Class ASTEROIDEA.— Free forms ; stellate or pentagonal in shape ; arms
containing caecal processes of digestive tract ; ambulacra limited
to oral surfaces.
Dermal skeleton reticulate ; no paxilla3 ; anus present. Aste-

rias, Brisinga, Asterina, Zoroaster.

Dermal skeleton of separate plates ; paxillae present ; no anus.
Astropecten, Lnidia.

III. Class OPHIUROIDEA. — Free forms ; stellate in shape ; arms not contain-

ing cascal processes of the digestive tract ; ambulacra limited to
oral surface ; ambtilacral ossicles contained within the arms.

1. Order Ophiurida. — Arms unbranched ; madreporiform tubercle

on mouth-shield. Ophiura, Ophiolepis, Amphiura, Opltiactis,
oplriotlirix, Ophioderma, Ophiomyxa.

2. Order Euryalida. — Arms usually branched ; mouth-shields not

well developed ; usually several madreporiform tubercles. As-
trophyton, Trichaster.

IV. Class ECHINOIDEA.— Free forms; without arms; test composed of

TYPE ECHINODERMA. 593

twenty rows of plates ; ambulacra extending from oral to
aboral surfaces or else limited to aboral surface.

1. Order Desmosticha. — Ambulacra all similar, extending from oral

to aboral surfaces ; mouth and anus in centre of the oral and
aboral surfaces respectively ; masticatory apparatus present.

With internal branchiae. Cidaris.

With external branchiae. Asthenosoma, Salenia, Diadema,
A rbacia , Strotigylocentrotus.

2. Order Clypeastroidea. — Ambulacra all similar, limited to the

aboral surface ; mouth in centre of oral surface, anus excentric ;
masticatory apparatus present. Clypeaster, EcJiinarachnius,
Mellita.

3. Order Petalosticha. — Ambulacra more or less dissimilar, limited

to the aboral surface ; mouth and anus both excentric ; masti-
catory apparatus wanting. Moira, Spatangus, Brissopsis.
V. Class HOLOTHUROIDEA. — Free forms ; body mostly elongate and ver-
miform ; mouth surrounded by circle of 10-30 tentacles ; skele-
ton usually of small scattered plates.

1. Order Elasipoda.— Bilateral symmetry well marked ; stone-canal

frequently opens to exterior ; no respiratory trees. Deima, El-
pidia.

2. Order Pedata. — Bilateral symmetry as a rule not particularly

well marked ; respiratory trees present ; stone-canal not open-
ing to exterior ; tube-feet well developed.

Tentacles branched dendritically. Cucumaria, Psohts, Thy-
one.

Tentacles pinnate. Rhopalodina.

Tentacles peltate. Holothuria, Miilleria.

3. Order Apoda. — Bilateral symmetry not well marked ; stone-canal

not opening to exterior ; tube-feet wanting.
Kespiratory trees present. Molpadia
Respiratory trees wanting. Synapta, Chirodota.

LITERATURE.

GENERAL.

Johannes Muller. Ueber die Larven und die MetaphorpJiosen der Echinodermen.

Abhandl. KK. Acad. Wissensch. Berlin, 1848-55.
P. H. Carpenter. The Oral and Apical Systems of Eehinoderms. Quarterly

Journal Microscop. Science, xvm, 1878 ; xix, 1879.
W. P. Sladen. Tlie Homologies of the Primary Larval Plates in the Test of the

Brachiate Eehinoderms. Quarterly Journal Microscop. Science, xxiv,

1884.
0. Hamann. Die wandernden Urkeimzellen und ihre ReifungsstlMen bei den

Echinodermen. Zeitschr. fill1 wisseusch. Zoologie, XLVI, 1887.
P. & F. Sarasin. Ueber die Anatomic der Echinothuriden und die Phylogenie

594 INVERTEBRATE MORPHOLOGY.

der Echiriodermen. Ergebnisse uaturwiss. Forschungeu auf Ceyloii, i,
1888.

H. Bury. Studies in the Embryology of the Echinoderms. Quarterly Journ.
Microscop. Science, xxix, 1889.

M. Neumayr. Die Stamme des Thierreichs, etc. Wieii u. Prag, 1889.

L. Cuenot. Etudes morphologiques su,r les Echinodermes. Archives de Biolo-
gic, xi, 1891.

0. Butschli. Versuch der Ableitung des Echinoderms aus einer bilateralen Ur-
form. Zeitschr. fiir wisseuscb. Zoologie, LIII, Suppl., 1892.

H. Ludwig. Echinodermen. Brontis Klasseri uud Orduuugeu des Thierreichs,
in. (In course of publication.)

CRINOIDEA.

W. Carpenter. Memoir on the Structure, Physiology, and Development of Ante-
don rosaceus. Philosoph. Transactions Royal Society, London, CLVI,
1866.

H. Ludwig. Beitrage zur Anatomic der Crinoiden. Zeitschr. fiir wisseusch.
Zoologie, xxvin, 1877.

A. M. Marshall. On the Nervous System of Antedon rosaceus. Quarterly
Journ. Microscop. Science, xxiv, 1884.

E. Perrier. Memoire sur I' organisation et la developpement de la Comatule de la
Mediterranee. Nouvelles Archives du Museum d'Hist. Nat. de Paris, 2me
ser., ix, 1886 ; 3me ser., i, 1889 ; 11, 1890.

H. Bury. The Early Stages in the Development of Antedon rosacea. Philoso-
phical Transactions of the Royal Society, London, CLXXIX, 1888.

0. Hamann. Anatomie der Ophiuren und Crinoiden, Jenaische Zeitschr.,
xxiii, 1889.

ASTEEOIDEN.

A. Agassiz. Tlie Embryology of the Starfish. Memoirs from the Museum of
Comp. Zool., v, 1877.

H. Ludwig. Beitrage zur Anatomie der Astenden. Zeitschr. fiir wissensch.
Zoologie, xxx, 1878.

0. Hamann. Die Asteriden. Jena. 1885.

L. Cuenot. Contributions d V etude anaiomique des Aslerides. Archives de Zool.
exper. et. geu., 2me ser., v, 1887.

G. W. Field. The Larva of Asterias vulgaris. Quarterly Journal of Micro-
scop. Science, xxxiv, 1892.

OPHITJBOIDEA.

Th. Lyman. OphiuridoB and Astrophytida. Illustr. Catalogue Museum Comp.
Zool., 1865.

H. Ludwig. Beitrage zur Anatomie der Ophiuren Zeitschr. fiir wissensch.
Zool., xxxi, 1878.

H Ludwig. Das Mundskelett der Asterien und Ophiuren. Zeitschr. fiir wis-
sensch. Zool., xxxii, 1879.

H. Ludwig. Neue Beitrage zur Anatomie der Ophiuren. Zeitschr. fiir wis-
seusch. Zool., xxxiv, 1880.

TYPE ECH1NODERMA 595

P. H. Carpenter. On the Apical System of the Ophiurids. Quarterly Journ.
Microscop. Science, xxi, 1881.

L. Cuenot. Etudes anatomiques et morpkologiques sur les Ophiures. Ar-
chives tie Zool. exper. et gen., 2me ser., vi, 1888.

0. Hamann. Anatomic der Ophiuren und Crinoiden. Jenaiscbe Zeitschr.,
xxin. 1889.

E. W MacBride. The Development of the Genital Organs, Ovoid Gland, Axial
and Aboral Sinuses in Amphiura, etc. Quarterly Journ. Microscop. Sci-
ence, xxxiv, 1892.

ECHINOIDEA.

A. Agassiz. Revision of the Echini. Illustr. Catalogue of the Museum of

Comp. Zool., No. 7, 1872-74.
R. Koehler. Recherches sur les Echinides des cotes de Provence. Annales du

Musee d'Hist. Nat. de Marseille, i, 1883.
S. Loven. Etudes sur les Echinoidees. Kongl. Svensk. Vetensk. Akacl.

Handl., n, 1884.
H. Ayers. On the Structure and Function of the Sphceridia of the Echinoidea.

Quarterly Journ. of Microscop. Science, XXYI, 1886.
0. Hamann. Anatomic und Histologie der Echiniden und Spatangiden. Jena-

ische Zeitschr., xxi, 1887.

HOLOTHUROIDEA.

E. Selenka. Beilnige zur Anatomie und Systematik der Holothurien. Zeitschr.

fur wissensch. Zool., xvn, 1867 ; xvm. 1868.
H. Theel. Report on the Holothuroidea. Scientific Results of the Voyage of

H.M.S. Challenger. Zoology, iv, 1882.
0. Hamann. Die Holothurien. Jena, 1884.
R. Semon. Die Enticicklung der Synapla digitata und ihre Bedeutung fiir die

Phylogenie der Echinodermen. Jeuaische Zeitschr., xxn, 1888.
H. Ludwig und Ph. Barthels. Beitragezur Anatornie der Holothurien. Zeitschr.

fiir wisseusch. Zoologie, LIV, 1892.
Th. Mortensen Zur Anatomic und Entwic/clung des Cucumaria glacialis

(Ljungman). Zeitsclir. iiir wissensch. Zoologie, Lvn, 1894.

596 INVERTEBRATE MORPHOLOGY.

CHAPTEE XVII.

TYPE PROTOCHORDATA.

THE type Protochordata contains a number of forms which
present certain features of similarity to the Chordata (Verte-
brata), one member of the type, Amphioxus being frequently
considered as belonging to that group, which is to be regarded
as the most highly differentiated of all the types composing
the Animal Kingdom.

The various groups of the Protochordata differ greatly in
general appearance, but certain structural features of great
morphological importance are common to all of them. These
may be briefly stated as (1) a notochord, consisting of a more
or less well-developed rod, arising from the mid-dorsal line
of the digestive tract and either extending the entire length of
the body, or else limited to its anterior or its posterior part,
or even present only during larval life, as in the majority of
the Tunicata ; (2) branchial slits which place the cavity of the
pharynx in communication with the exterior and serve as
respiratory organs ; (3) a central nervous system, situated in the
mid-dorsal line of the body, and arising in some forms as ail
ectodermal iuvagination.

Metamerism is but feebly indicated in the majority of
cases, some forms possessing only three mesodermal somites,
while others, such as some of the Tunicata, show traces of
it only in the posterior region of the body, Amphioxus being
the only form in which it is at all well marked. Limbs
do not occur in any members of the group, nor are there any
special jaws or organs of mastication. All the members of
the group are marine, and the various classes possess a wide
geographical distribution.

I. CLASS HEMICHORDA.

The members of this class are characterized by the noto-
chord being a comparatively small diverticulum of the an-

TYPE PROTOCHORDATA.

597

terior portion of the digestive tract arid containing a more or
less perfect cavity which communicates with the oesophagus.
The body is divisible into three regions : an anterior prseoral
epistonie or proboscis, a median collar region, and a posterior
visceral sac or trunk region. The coelom is divided into
corresponding regions, a pair of pores placing the collar com-
partment in communication with the exterior, while one or
two additional pores perform a similar office for the epistome
or proboscis cavity. The nervous system remains in connec-
tion with the ectoderm, and its principal portion is situated
in the collar region.

1. Order Pterobranchia.

This order contains but two genera, Rhabdopleura and
Cephalodiscus, the former occurring in deep water off the coast
of Norway, while the latter was obtained by the " Challenger "
Expedition in the Straits of Magellan.

Rhabdopleura (Fig. 271) is a colonial form, consisting of a

FIG. 271. — COLONY OF Rhabdopleura (after LANKKSTER).

stolonlike system of tubes ramifying over the surface of
stones, etc., and giving off shorter lateral tubes each one of
which contains an individual. The tubes are composed of
chitiulike material and form a "house" for the colony, and

598

INVERTEBRATE MORPHOLOGY.

are traversed, except towards the extremities of the lateral
tubes where the individuals occur, by a chitinous rod which
results from the chitiuizatiou of what was once the stem of the
various polyps. Each of these is stalked (Fig. 272), the stalk
(G) becoming continuous below with the chitinous rod, and

E

FIG. 272. — INDIVIDUAL OF Rhabdopleura (slightly modified after LANKKSTER).
(N.B.— The tentacles of one side of one arm only are represented.)
B — anal papilla. G -- tentacle.

C = stalk. Oa = arm.

D = epistome. / = intestine.

E = trunk region. K == sensory papilla.

F — collar region. N -- notochord.

each consists of three well-marked regions. What may be
termed the anterior portion of the body is formed by a large
disklike epistome (D), beneath which on the ventral surface is

TYPE PROTOCHORDATA. 599

the mouth. Behind these is what is termed the collar region
(F), which bears upon its dorsal surface two long armlike
processes ( Go), each carrying a double row of ciliated tentacles
(G) arranged pinnately. The third region is the visceral sac
(£), from the posterior and ventral portion of which the stalk
arises, while dorsally and anteriorly it carries a stout papilla
(5), at the extremity of which the anus opens.

The digestive tract consists of a straight oesophagus trav-
ersing the collar, and having connected with it on the dorsal
surface a short blind process (N) whose cavity communicates
with that of the oesophagus. This is the rudimentary noto-
chord. The oesophagus opens into a large saclike stomach,
from the lower end of which the intestine (7) arises, and, bend-
ing upon itself, runs forwards to open on the anal papilla.

The nervous system consists of a thickening of the ecto-
derm on the dorsal surface of the collar region, where is also
found a small ciliated elevation (Ii) supposed to be sensory ;
no other special sense-organs occur. On each side of the
collar a pore occurs which, by a short ciliated canal which
perforates the wall of the body, places the coelom of the collar
in communication with the exterior, and may be regarded as
representing an excretory organ. No epistome-pore or branchial
slits have yet been observed.

Cephalodiscus resembles Rliabdopleura in being colonial, but
the house is gelatinous, and the various buds formed from the
short stalk do not remain in connection with each other, but
early separate from the parent. Each polyp (Fig. 273) con-
sists of three regions — an anterior prseoral portion which forms
a large epistonie, a middle collar region, and a posterior visceral
sac ; the body-cavity being divided into three corresponding
regions. Two epistome-pores occur, the canal leading from
the ccelom to them passing through the anterior part of the
nervous system (n). The dorsal ectoderm of the collar region
is thickened to form the central nervous system, and on each
side of this is a cluster of six tentacles (t), each ending in a
kuoblike dilatation and bearing numerous lateral pinnules
arranged in two rows. At the sides the collar is continued
backwards as a pair of lateral folds which slightly overlap the
anterior portion of the visceral sac and form the operculum,

600

IN VERTEBRA TE M ORPHOL OGY.

upon the inner surface of which there is on each side a collar-
pore.

The mouth (rri) opens beneath the epistorne into an oesoph-
agus, which in the collar region bears a dorsal diverticuluin,
the notochord (x), projecting forwards into the epistoine. In
this same region there is on each side a branchial slit (sp),
structures which are apparently wanting in Bliabdopleitra.
Behind the oesophagus opens into a saclike stomach from

FIG. 273. — DIAGRAMMATIC LONGITUDINAL SECTION THROUGH Cephalodiscus

(after EHLERS from KORSCHELT and HEIDER).
a = anus. n = nervous system.

ex = excretory organ. sp = branchial slit.

g = ovary. t = tentacles.

TO = mouth. x = notochord.

which the intestine, bending upon itself, passes forwards to
open (a) upon the dorsal surface of the visceral sac, a short
distance behind the collar.

The collar-pores probably serve as excretory organs, and
it has been stated that the epistome-pores open into well-
developed tubes (ex) terminating in the epistome cavity in a
dilatation ; they also have been regarded as excretory. The
reproductive organs (g) are paired sacs, which open on the
dorsal surface just in front of the anus. No circulatory system

TYPE PRO TO CHORD ATA. 601

occurs in the Pterobranchia, and nothing is yet known con-
cerning the embryonic development of either of the genera
contained in the order.

2. Order Enteropneusta.

The order Euteropneusta contains a small number of
closely-related forms which, until recently, have been grouped
together in a single genus, Balanoglossus, and which, notwith-
standing that they present in some respects a decided advance
in structure over the Pterobranchia, yet may properly be as-
sociated in a class with that order.

All the species live buried in sand, and the body is elon-
gated and vermiform, the digestive tract being practically
straight and the anus terminal. The anterior portion of the
body (Fig. 274, pr) has the form of a cylindrical proboscis,
corresponding to the epistome of the Pterobranchia and
united to the second region by a narrow neck, upon the dor-
sal surface of which is a pore (occasionally two) which places
the proboscis-coelom in communication with the exterior,
while upon its ventral surface, just where it joins the second
region, is the mouth.

The second region is comparatively short and forms the
collar (c), its posterior border being prolonged backwards for
a short distance in the form of a fold on each side, as in
Ceplndodiscus. These folds enclose between their inner walls
and the sides of the anterior part of the third portion of the
body a space which communicates behind freely with the
exterior and is known as the atrium. The posterior or trunk
region of the body is much longer than either the proboscis
or collar regions, and contains the greater portion of the
digestive tract and the reproductive organs. Anteriorly it is
somewhat flattened and presents on the dorsal surface a
ridge, on either side of which are to be found a varying num-
ber of U-shaped pores (br) with ciliated margins. These
pores lead into short tubes opening internally into the cavity
of the digestive tract and are the branchial slits. They seem
to increase in number during the life of the animal, the pos-
terior ones being merely circular openings, as the anterior

602

INVEBTEBRA TE MORPHOLOG Y.

ones are in the younger stages of development, a tonguelike
valve later growing down from the dorsal border of the pore
and giving it its U-shaped form. Water ilows in at the mouth
and passes out through the branchial slits, which thus pos-
sess a respiratory function. A few of the anterior slits open

FIG. 274. — Balanoglossus Kowalewsku (after MINOT from SPENGEL).
br — branchial slit. c = collar. pr — proboscis.

externally into the atrium, being covered over by the back-
wardly-projecting atrial folds of the collar, but the majority
are quite uncovered and are plainly visible from the exterior.

The ectoderm contains numerous mucous glands and is
ciliated throughout, no external cuticle or " house," such as
occurs in the Pterobraiichia, being developed. Below it rests
upon a thin basement-membrane.

The ccelom is clearly marked out, and consists of three
portions completely separated from, one another aud corre-
sponding to the three body regions. The proboscis-co3lom
(Fig. 275, A, pc) is, in its anterior portion, a simple unpaired
cavity lined with delicate cells and traversed by circular and
longitudinal (Im) muscle-fibres. Posteriorly it is prolonged
into two horns between which lies a mass of tissue consisting

TYPE PROTOCHORDATA. 603

of several different organs. The centre of the mass is occu-
pied by the notochord (nc), which is, as in the Pterobranchia,
a forwardly-directed diverticulum of the dorsal wall of the
anterior portion of the digestive tract and contains a more or
less distinct cavity communicating with that of the oesophagus.
Dorsal to the notochord lies a contractile heart (h), and
dorsal to this again, and surrounding it, a sac (ps) containing
a few, or in some cases many, cellular elements. This pro-
boscis-vesicle, as it has been termed, may possibly represent a
portion of the proboscis-coelom, which would then consist of
two separate cavities, originally right and left, one of which
becomes very large and fills the greater portion of the pro-
boscis, while the other remains quite small. Surrounding
these structures are a number of folds of the splanchnic
layers of the proboscis-ccelom (pg), loops of blood-vessels
lying in the folds, while the cells covering them frequently
contain yellow granules. It has been supposed that these
granules indicate a glandular function for the cells, and con-
sequently the entire mass of folds has been termed the pro-
boscis-gland.

The coelonric cavities of the collar (Fig. 275, B cop) and
trunk are much simpler and are paired, the cavities of the
right and left sides being separated from each other by dorsal
and ventral mesenteries. From the dorsal portion of the an-
terior end of the trunk-coslom two prolongations extend for-
ward into the collar, lying on each side of the dorsal blood-
vessel and forming the perihsBinal cavities in the interior of
which are longitudinal muscle-fibres. Two other similar for-
ward prolongation of the truuk-ccelom lie between the collar
coelom and the oesophagus, forming the peripharyngeal cavi-
ties.

In contradistinction to what occurs in the Pterobranchia,
a well-developed blood system is present, consisting in the
collar and trunk of a dorsal and ventral longitudinal vessel
with distinct muscular walls and lying in the mesenteries.
From the dorsal vessel branches pass to the tonguelike pro-
cess of the branchial slits, and the vessel itself is continued
forward into the proboscis to enter a space between the pro-
boscis-vesicle and the notochord which is termed the heart.

604

J^ VERTEBRA TE MORPHOLOG T.

The blood-spaces in tlie proboscis-gland communicate with
the heart, and the dorsal and ventral vessels of the collar and
trunk are united by a double set of fine lacuuar capillaries,
one set being situated in the body-wall, and the other in the

nv

FIG. 275. — TRANSVERSE SECTIONS THROUGH (A) THE PROBOSCIS AND (B) THE
BRANCHIAL REGION OP THE TRUNK OP Balanogiossus Kvwalewskii (B after

SPENGEL).

cop = collar-coelom.

g = tongue- bar of skeleton.

h — heart.

kd = branchial valve.
kh = branchial portion of oesophagus.
ks = branchial septum.
kp = branchial pore.

Im = longitudinal muscles.
nc = uotochord.
nd = dorsal nerve.
no = ventral nerve.

o — oesophagus.
pc = proboscis-cceloui.
pg = proboscis-gland.

ps — proboscis-vesicle.

wall of the intestine. The blood is a colorless coagulable
fluid, apparently destitute of corpuscles.

In the posterior portion of the proboscis is found a plate
of chitinlike material produced into two horns posteriorly,
and frequently somewhat hollowed out in front. It is evi-
dently supportive in function, and forms the proboscis-skele-
ton. In connection with the branchial slits a similar chitinous
skeleton is formed (Fig. 276) consisting of a series of trans-
verse bars placed over each septum between adjacent slits.
From the middle of each bar a rod (really double) passes
down each septum (sb\ and from the extremities a bar (tl)
passes into each of the adjacent tonguelike valves, each valve
thus possessing a bar from the arch lying in front of it and
another from that lying behind it. The septal bars and the

TYPE PROTOCIIORDATA.

<;on

tongue bars belonging to each arch are connected by trans-
verse bars (syuapticula, s) which extend across the valves of
the branchial slits.

The digestive tract is practically a straight tube extending
from the mouth, situated on the ventral surface of the neck
of the proboscis, to the terminal anus. From the dorsal wall

of the oesophagus a finger-
shaped diverticulum arises, the
notochord, which extends for-
wards into the proboscis ; its
lumen, in its terminal portion,
being practically obliterated by
the vacuolization and enlarge-
ment of the cells which line it
and which are continuous with
the endodermal lining of the
digestive tract. In the anterior
portion of the trunk region the
branchial slits already referred
to occur, arising as diverticula
from the dorso-lateral portions
of the intestine (Fig. 275, B,
Ui) and eventually opening to
the exterior (kp). More poste-
riorly in some species the wall of the intestine is pouched out
into sacculations which have been regarded as liver-sacs. In
Balanoglossus (Glandiceps) hacksi an accessory intestine occurs,
arising in the middle of the liver region from the dorsal sur-
face of the intestine in the form of a tube, which, more poste-
riorly, opens again into the intestine. It recalls the accessory
intestine of certain Annelids, Gephyreans and Echinoderms,
but is peculiar in being dorsal in position. In certain species
also paired or unpaired communications of the intestine with
the exterior have been found, usually arising in the liver
region, and opening upon the dorsal surface ; their signifi-
cance is at present imkuowu.

A well-developed nervous system is present in the form of
an elongated cord lying in the mid-dorsal line of the collar
region, with the ectoderm of which it is in connection at

FIG. 276. — DIAGRAM OF THE BRAN-
CHIAL SKELETON OF Balanoglossus
(after SPENGED.

s = syuapticula.
sb — septal bar.
tb = toiigue-bar.

606 INVERTEBRATE MORPHOLOGY.

either end, though free throughout the greater portion of its
length. It contains in young forms a central lumen, which
may be represented in adults by a series of separated cavities
and which results from its formation as an invagination of the
ectoderm. From this dorsal cord a plexus of nerve-fibres
extends all over the surface of the body, lying in the lower
layers of the ectoderm and being at certain regions specially
developed so as to form nervelike thickenings. One of these
surrounds the dorsal and lateral surfaces of the base of the
proboscis, being perforated by the proboscis-pore ; another
occurs at the posterior edge of the collar; while two others
occur in the trunk region, one in the dorsal (Fig. 275, B, nd)
and the other (nv) in the ventral mid-line, extending the entire
length of the trunk. No special optic, olfactory, or auditory
organs seem to be developed.

The short canal opening by the proboscis-pore has been
regarded as excretory, but the assignment of such a function
to it seems questionable. A similar function has been as-
signed to two short tubes with folded ciliated walls which
•communicate internally with the coelom of the collar, and
open to the exterior by the collar-pores, situated, one on each
side, on the edges of the atrial folds. More definite informa-
tion is required concerning these organs before they can
finally be accepted as excretory ; they evidently correspond
to the collar-pores of the Pterobranchia.

All the known species of Balanoglossus are bisexual, the
reproductive organs, ovaries or testes, consisting of simple or
branched pouches situated in the trunk, beginning in the
braucial region and extending some distance backwards.
Each pouch opens to the exterior by a special duct, upon the
dorso-lateral portions of the body.

Development of the Enteropneusta. — Some species of Balano-
gJossus (B. Koivalewskii) develop directly without the inter-
vention of a larval stage in the life-history, but the majority
possess a characteristic free-swimming larva known as the
Tornaria (Fig. 277). It is a barrel-shaped organism, bulged
out slightly at either pole, and possessing a locomotor appa-
ratus in the form of somewhat complicated bands of cilia.
One of these surrounds the posterior portion of the body as

TYPE PRO TOCUORDA TA.

607

a

a simple circular baud, while auother runs over the anterior
portion in an exceedingly tortuous course. It may be con-
sidered as consisting of two portions, one prseoral in position
and the other postoral, the two meeting, however, at the
apex of the body in an ectoder-
mal thickening, the apical plate
(a), which bears two eyes.

The mouth (J/) opens by a
short oesophagus into a capacious
stomach ($), separated by a per-
forated partition from the short
rectum (JR), which opens by the
terminal anus. In the anterior
portion of the body is a saclike
structure (pc), iinited to the apical
plate by a muscular baud and
opening to the exterior by a pore
(p) situated a little to the left of
the mid-dorsal line. This sac is
the proboscis-ccelom, and the pore
the proboscis-pore, and in connec-
tion with the sac is a smaller sac,
the so-called heart, which becomes
the proboscis-vesicle of the adult.
At a later stage of development
two other pairs of ccelomic sacs (cc and tc) make their ap-
pearance at the sides of the stomach and give rise to the
collar and trunk coelom. The adult form is acquired by the
gradual transformation of this larva, there being no sudden
metamorphosis.

The Affinities of the Hemicliordata. — There seems little room for doubt
but that the Pterobranchia and the Enteropneusta are closely related,
so many similar and at the same time peculiar structures being found in
both groups. Thus, to mention only some of the more striking features,
both groups possess a notochord of a similar character, proboscis-pores,
collar-pores and branchial slits, and have the body and coelom divided into
three strictly- comparable regions. These remarkable similarities can only
be explained on the assumption of a common ancestry for both groups.

In many respects, too, the Enteropneusta present similarities to the suc-
ceeding group, the Cephalochorda, but a discussion of this relationship may

FIG. 277.— TORNARIA LARVA OF
Balanoglosnus J&rtM-fowekii. S-p

a = apical plate.
cc = collar-ccelom.
M — mouth.

p — dorsal pore.
pc = proboscis-coslom.

R = rectum.

S = stomach.

tc — trunk-ccelom.

608 INVERTEBRATE MORPHOLOGY.

be postponed for the present, and attention called to the suggestive char-
acter of the Tornaria. Its first describer took it for an Echinoderm larva,
and the majority of succeeding authors have been inclined to regard it as
indicating affinities with that group. The arrangement of the prasoral and
postoral ciliated bands, and the occurrence of the proboscis-pore, suggest
the Echiuoderm larva without doubt, but it must still be regarded as a
decidedly open question whether or not these features indicate an affinity.
Further information is required both in regard to the ancestry of the
Echinoderms and as to the life-histories of the Pterobranchia, before the
question can be settled.

Another line of ancestry must also be mentioned, namely, one which
leads back to ancestors common to the Hemichordates and the Prosopygia.
The similarities of the Pterobranchia to the Polyzoa are striking, there
being the same bending of the intestine, similar lophophorelike tentacular
structures, and, what is of considerable importance, a dorsally situated
nervous system arising as an invagination of the ectoderm. A further
point perhaps of some importance may also be mentioned, i.e., the occur-
rence of three sections in the body-cavity of the Brachiopoda. In following
out the line of descent suggested by these similarities, we are, however,
quickly brought to a halt by the uncertainty connected with the origin of
the Prosopygia, and we are left standing between two lines, one leading
back to the Prosopygia and the other to the Echinoderms. Whether or not
these two lines converged to common ancestors in pre-Cambrian times can-
not be ascertained, and the solution of the problem must be left to future
embryological investigations.

II. CLASS CEPHALOCHORDA.

The class Cephalochorda contains a single genus, Amphi-
oxus (Branchiostoma), which is exclusively marine in habitat,
being found buried in an upright position in the sand, the
anterior end of the body alone showing at the surface.

The body in all species is elongated (Fig. 278) and some-
what flattened from side to side, and bears along the mid-
dorsal line an unpaired fin, formed as a fold of the body-wall,
and containing a cavity traversed by numerous skeletal rods
(see Fig. 279) which serve as a support for the fin. Poste-
riorly it becomes somewhat higher, and forms a caudal fin
surrounding the posterior end of the body, while on the ven-
tral surface two fins run forward a short distance, both these
and the caudal fin being supported by fin-rays. Some dis-
tance from the hind end of the body on the left side of the
caudal fin is situated the anal opening (Fig. 278, ct), while in

TYPE PROTOCHORDATA.

609

front of the anterior end of the paired ventral fin is the
atrial pore (p), which leads into a cavity termed the atriuin
(b). The outer walls of this cavity arise as folds of the
sides of the body, termed the epipleural folds, which, in-
creasing in size, fuse together below except at the atrial
pore, thus enclosing the atrial cavity (Fig. 279, b), which is
lined throughout by ectoderm and surrounds the sides and
ventral surface of the anterior two thirds of the body. Ante-
riorly the cavity is closed by the fusion of the folds with the
body-wall in the neighborhood of the larval mouth, but in
front of this is a hood-shaped fold which arises independ-
ently of the atrial folds and forms the oral hood, at the bot-

au

FlG. 278. — AmpJiioxus lanceolatus (after BOVERI from HERTWIG).

a — anus. m = muscles.

au = eye. n — uephridiura.

b = atrial chamber. o — mouth.

ch — notochord. p = atrial pore.

g = reproductive organ. r = dorsal nerve-cord.

I = liver. sp = branchial cleft.

torn of which lies the original mouth, the margins of the hood
surrounding what may be termed for distinction the adult
mouth (Fig. 278, o). These margins are produced into a
number of tentacular processes, termed cirrhi, each one of
which contains an axial supportive rod borne by a skeletal
ring surrounding the adult mouth, and each has its surface
raised into numerous sensory papillae.

The ectoderm is very simple in its character, consisting of
a single layer of cells resting below upon a well-developed
layer of connective tissue. The arrangement of the coelomic
cavities is by no means simple, however, and may be best
understood by considering their mode of development. In a
very young embryo a fold may be seen extending along each
side of the primitive digestive tract on its dorsal surface. In

610 INVERTEBRATE MORPHOLOGY.

time these folds are gradually constricted off from the intes-
tine, and at the same time are divided transversely into a
number of sacs lying one behind the other, their number in-
creasing as the folds are separated from before backwards
from the intestine, until in A. lanceolatus there may be as
many as sixty-one. These sacs are the primitive mesodermic
somites, and the cavities they contain are the primitive cce-
lornic cavities. At first entirely dorsal in position, the various
sacs later on extend ventrally, those of opposite sides meeting
below the intestine ; and still later the cavities of these ven-
tral extensions fuse to form a continuous ccelom extending the
entire length of the body on the ventral surface, and forming
what is termed the splanchnoccel. This becomes eventually
separated by a layer of connective tissue from the more dor-
sal portions of the somites, which remain distinct from each
other throughout life and are termed the myoccels. The future
history of the t\vo portions of the mesoderm thus formed
is very different. The walls of the splauchuocoel remain
thin, and the cavity well marked (Fig. 279, co), but in the
myoccels the cells forming the median walls become converted
into longitudinal muscle-fibres (ni) which traverse the entire
length of each myoccel, filling it almost completely, and are
inserted into plates of connective tissue which develop be-
tween the various myoccels and separate them from one an-
other. At the same time each myoccel becomes bent, so that
its dorsal portion is directed downwards and forwards and
its ventral portion downwards and backwards, each muscle-
plate having in a longitudinal section of the body a <-shaped
appearance and fitting into the one in front of it. When
the epipleural folds develop, both the splauchnoccel and
the muscle-plates are continued into them, the muscle-plates
lying to the outer side of the splanchnoccel, and their fibres
here having for the most part a transverse direction, in-
stead of a longitudinal one, as in their upper portions.
Owing to the myoccels being practically obliterated by the
muscle-plates, the ccelorn of the adult is principally formed
of the splanchuoccel, but other spaces also occur which are
probably schizoccelic in origin and form various lacunae
throughout the body.

TYPE PROTOCHOEDATA.

611

A blood system is present, probably communicating here
and there with these lacunae, and containing a colorless blood
in which float numerous colorless amoeboid corpuscles. The
system consists of a dorsal aorta, single throughout the
greater portion of its course, but divided into two trunks over

7,

FIG.

279. — TRANSVERSE SECTION THROUGH THE BRANCHIAL REGION OF

Amphioxus (after LANKESTER and BOVERI from HERTWIG).

a — descending aorta.

b = atrial cbamber.

c = notochord.
co — coelom.

e = bypobrauchial groove beneath
whfcu is the ascending aorta.

g = reproductive organ
kb = branchial arches.

kd = pharynx.

kp = branchial clefts.

I = liver.
m — muscles.
n = nephridium.

r = nerve-cord.
sn — nerves.

the branchial region of the intestine. It sends off branches to
various regions of the body which, after breaking up into
capillaries, unite again to form the subiutestinal or ventral
vein, which, passing forwards till it reaches a point in front of
the branchial region, becomes sinuous, and gives off a right
and left branch to the lips of the larval mouth (the velum),
and a third large vessel on the right side which passes dor-

612 INVERTEBRATE MORPHOLOGY.

sally to unite with the right aortic vessel. The blood which
passes from the dorsal aorta to the intestine is not, however,
returned directly to the ventral vein, but the intestinal capil-
laries unite to form a vena porta which passes to the liver
and there breaks up into a second set of capillaries, these
finally emptying through the hepatic vein into the ventral
vessel. An hepatic portal system, resembling that found in the
Vertebrata, thus occurs in Amphioxus. While passing be-
neath the branchial region of the intestine the ventral vein
gives off paired vessels, the branchial arteries, opposite each
branchial septum, and these passing dorsalwards in the sep-
tum open into the dorsal aortic trunks. There is no definite
heart, but certain of the vessels, notably the vena porta and
branchial arteries, seem to be contractile.

The notochord (Figs. 278, ch, and 279, c) has a much more
extensive development than in the Hemichorda, since it
traverses the entire length of the body. It arises from the
dorsal surface of the digestive tract, but early loses all con-
nection with the intestine ; and though in early stages it con-
tains traces of a lumen, this quickly disappears, the cells
becoming richly vacuolated, so that the uotochordal tissue
assumes a characteristic appearance. At either end it is
pointed, and throughout its entire length it is surrounded by
a sheath of dense connective tissue, which is continuous
below with the partitions separating the splauchuoccel from
the muscle-plates and these from one another. From each
side of the dorsal surface of the sheath a longitudinal lamella
extends dorsally, the two lamellae enclosing the central
nervous system and being continued above it as a strong-
neural ridge (Fig. 279).

As has been already stated, the adult mouth is formed by
the margins of the oral hood, the original larval mouth lying
at the bottom of the oral cavity enclosed by the hood and
being surrounded by a circular fold of tissue termed the
velum. A short tube leads from the mouth to the branchial
or pharyugeal region of the digestive tract, whose wralls are
here perforated by numerous slits (Fig. 278, sp) placing its
cavity in communication with the atrium (see Fig. 279). In
the adult the slits are elongated and are placed obliquely to

T YPE PR 0 TO CHORDA TA .

013

,sb

FIG.

the axes of the bod v ; they do not possess a metameric arrange-
ment, but are much more numerous than the muscle-plates of
the branchial region of the body, as many as one hundred
slits occurring in fully-developed individuals. In the parti-
tions between each pair of slits of
the same side of the body skeletal
rods occur which unite above with a
longitudinal bar lying above the
dorsal ends of the slits (Fig. 280).
A closer examination shows that
alternate bars differ in structure, a
condition due to the fact that each
pair of slits is primarily derived
from a single slit, which becomes
divided longitudinally by the growth
from its dorsal edge of a tonguelike
valve, which eventually fuses with
the ventral edge of the slit. The
condition of affairs then is almost
identical with what obtains in Ba-
lanoglossus, and, as in that form, the
skeletal bars are to be considered as
grouped together in threes, although
the two bars which in Balanog^ossus occur in each tongue are
fused together (#5) in Amphioxus, a continuity of the various
triads being thus produced. The similarity in the arrange-
ment of the branchial skeleton in the two forms may be seen
by comparing Fig. 280 with Fig. 276 (p. 605).

It follows from this that the actual number of branchial slits in
Amphioxus is half the apparent number ; but even with this reduction the
metamerism of the slits does not agree with that of the muscle-plates. In
the early stages of development eight pairs of slits are developed which do
correspond metamerically with the mesodermic somites, but later addi-
tional non-metaineric slits are formed, and the original ones are pushed
forward at the same time, so that the metameric correspondence is lost.

Blood-vessels occur both in the septa between the primi-
tive slits and in the tongues, and the edges of the slits are
ciliated, so that a current of water enters by the mouth, passes
through the slits into the atrial cavity, and thence to the

-DIAGRAM OP THE
BRANCHIAL SKELETON OP
Amphioxus (after SPENGEL).
s = syuapticula.
sb = septal bar.
tb = tongue bar.

614

IN VERTEBRA TE MORPHOL OOT.

exterior by the atrial pore. Along the dorsal and ventral
mid-lines of the branchial region is a distinct ciliated groove,
the ventral one having projecting from its floor a longitudinal
ridge, while ventral to it is a chitinous skeletal plate composed
of paired moieties having a metameric arrangement. This
ventral or hypopharyngeal groove (Fig. 279, e) is termed the
eudostyle, and from its anterior end a band of ciliated cells
passes dorsally on each lateral wall of the pharynx to unite
dorsally with the epithelium of the dorsal or hyperpharyngeal
groove.

From the digestive tract behind the branchial region a
diverticulum, termed the liver (Fig. 278, I) arises, and pro-
jects forwards, covered of course by the body-wall, into the
atrial cavity (Fig. 279, I), and behind this the intestine passes
straight back to open at the anus (Fig. 278, a), situated, as
already indicated, upon the left side of the body, some dis-
tance from the posterior end.

The nervous system consists of a thick-walled tube (Figs.
278, and 279, r) which lies immediately above the notochord
and is enclosed by the connective tissue lamellae which arise
from the uotochord-sheath. It extends throughout the entire
length of the body, tapering rather suddenly at either ex-
tremity. Throughout the greater part of its course the lumen

FIG. 281. — DIAGRAM OF THE ANTERIOR PORTION OF THE .NERVOUS SYSTEM

OF Ampliioxus (after HATSCHEKI.

c7t = notochord. N= hypophysis.

1, 2, 3 = are phiced over the three veutricles.

is very small, forming the central canal from which a well-
marked cleft, the dorsal fissure, extends to the dorsal surface.
At the anterior end of the tube, however, the lumen enlarges
to form an anterior ventricle (Fig. 281, 1) which has been
compared with the anterior of the three primary vesicles of
the Vertebrate brain, and behind this the lumen contracts

TYPE PROTOCHORDATA. 615

forming the aqueduct of Sylvius of the mid brain (2), while
behind this again an expansion of the dorsal portion of the
dorsal fissure forms a fossa rhomboidalis (3) similar to that of
the Vertebrate hind-brain, the resemblance to the embryonic
Vertebrate brain being thus very marked — a resemblance
which is increased by the occurrence of a funnel-like extension
(N) of the anterior ventricle towards the dorsal surface of the
body, where it abuts upon a ciliated depression of the ecto-
derm. This is the remains of a communication of the ventricle
with the exterior which exists in the embryo, and has been
compared with the hypophysis of the Vertebrate brain, the
difference of its position in the latter being due to the flexion
of the brain round the anterior end of the notochord.

From the brain region of the nerve-tube three pairs of
nerves are given off (Fig. 281). the first and second of which
come from the dorsal portion of the brain, while the third
pair on each side is double, consisting of a root arising from
the dorsal surface of the brain and another arising from the
ventral side, a condition which is repeated in the succeeding
metamerically-arranged nerves. The dorsal and ventral roots
never unite to form a common trunk as in the Vertebrata, nor
do the dorsal nerves bear a ganglion, but nevertheless they
are sensory in function, while the ventral nerves are motor,
supplying only the musculature of the body.

Of sense-organs the papillre upon the cirrhi and the
ciliated depression in connection with the hypophysis have
been already mentioned, the latter, on somewhat insufficient
grounds, having been supposed to be olfactory in function.
In addition to these a pigment-spot, which may represent
an exceedingly simple eye, occurs upon the anterior end of
the brain, and in the roof of the oral cavity a patch of cells
occurs, surrounding a depression lined with columnar cells
bearing long refractive hairs. This last structure seems to
be a sense-organ of some kind, but its exact function is un-
known.

Different structures have from time to time been con-
sidered excretory organs. In the first place an excretory
function has been assigned to a ciliated tube lying in the wall
of the oral cavity on the left side and communicating by a

616

INVERTEBRATE MORPHOLOGY.

funnel with the ccelorn just behind the level of the velum.
Secondly, although in all probability they are not nephridia,
the " brown canals " may be here mentioned. These lie in
the splauchuoccel at about the level of the twenty-seventh
muscle-plate in A. lanceolatus, and open by wide funnels into
the atrium, though it is uncertain whether the inner end lying
in the ccelom is perforated. Thirdly, in the pharyngeal

i n s i n

FIG. 282. — EXCRETORY ORGAN OF Amphioxus (after BOVERI).
nc = nephridium. np = nephridial pore.

nd — nephridial fuunels. s = synapticulum.

/= branchial septum. //= branchial tongue.

region a number of nephridial canals have lately been de-
scribed. They are situated above the upper ends of the
branchial slits (Fig. 282, nc), each opening into the atrium
opposite a tongue-valve (np), and from the short tube which
passes inward from this opening an anterior and a posterior
branch arises, each of which opens into the coslomic cavity
by a terminal funnel. Between these two funnels three or
four others may occur (nd), and around the mouth of each
funnel are a number of threadlike processes which end in
round strongly-refractive cells. That these structures are
nephridia seems indicated by their relations to the ccelom
and furthermore by the fact that in the neighborhood of each

TYPE PROTOCHORDATA. 617

of them the branchial blood-vessels form a small plexus which
may be regarded as a glomerulus such as occurs iu connec-
tion with the urinary tubules of the Vertebrate kidney.

Ampliioxus is bisexual. The reproductive organs, ovaries
or testes (Figs. 278 and 279, g), occur in the epipleural folds,
and are arranged metainerically, extending in A. lanceolatns
from the fifteenth to the thirty-fifth or thirty-sixth metamere.
They lie at the level of the junction of the lateral longitudinal
and the ventral transverse muscles, and are contained within
a coelomic cavity which is a portion of the original inyocosls
of the segments to which the reproductive masses belong.
They lie on the inner surface of the atrial folds, and are
covered on their inner surfaces by the thin body-wall, through
which the reproductive elements break Avhen mature, falling
into the atrium and thence passing to the exterior through
the atrial pore. There are no reproductive ducts.

TJie Affinities of Ampliioxus. — The Cephalochorda have usually been
considered the most primitive Vertebrates, — as representing, in other words,
in a more or less modified condition the stem from which the Vertebrata
have descended, — and there are many points of similarity between Amplii-
oxus and the larval Lampreys (Ammocetes) by which such a view may be
supported. The character and origin of the notochord and nervous
system, the arrangement of the nerves of the latter, the lateral muscle-
plates, the occurrence of an hepatic portal circulation and the character of
the early stages of development are all points of similarity which seem
explicable only on the supposition of a somewhat close affinity.

On the other hand, resemblances to the Enteropneusta are but slightly
less marked. It seems hardly possible that the marvellous similarities in
the arrangement of the branchial skeleton should have been acquired in-
dependently in the two forms, and the atrial folds of Amphioxus may be
regarded as the more extensively-developed atrial folds of Balanoglossus.
Amphioxus is undoubtedly much more advanced along the Vertebrate line
than Balanoglossus, and both are probably more or less widely divergent
from the direct line, but the general similarities, such as the occurrence of
branchial slits, of an endodermal uotochord, and of a dorsally-placed
central nervous system, which have led to the association of both forms in
the type Protochordata, speak strongly for a community of descent.

The principal difficulty in the way of the acceptance of Balanoglossns
as the representative of the ancestral forms from which the Vertebrates
are descended seems to lie in the supposed necessity for deriving highly-
organized metameric forms, such as the Vertebrates, from more lowly but
also metameric ancestors, and consequently most authors have sought for

618 INVERTEBRATE MORPHOLOGY.

indications of Vertebrate ancestry among the Annelida. This difficulty
depends upon the interpretation placed upon metamerism and the causes
assigned for its origin. If the ideas regarding these points advocated in
preceding pages (see pp. 43 and 217) of this book be accepted, the difficulty
seems to be practically done away with, since these ideas imply a possi-
bility of the independent origin of metamerism in different groups. And
indeed it has already been pointed out that such an independent origin has
probably occurred, the metamerism of the Annelids being probably entirely
unconnected with the metamerism found in the more highly-organized
Platyhelminths (see p. 217).

A full discussion of the intricate problem of the origin of the Vertebrata
would be out of place here, but one additional point may be referred to.
Difficulties have always stood in the way of an homology of the Vertebrate
nervous system with that of the Annelids. In the latter there is a supra-
cesophageal cerebrum and a ventral chain, while in the former the entire
central system is dorsal to the digestive tract. Various theories have been
advanced to account for this difference, none of which have, however,
proved entirely satisfactory. The acceptance of an ancestry leading back
to Hemichordalike forms obviates this difficulty, since in these the slightly
differentiated nervous cord is already entirely dorsal, a future extension of
it and a metameric arrangement of its elements and branches in correla-
tion with the metamerism of the mesoderm bringing about the Vertebrate
condition. Furthermore, the occurrence of a central lumen in the nerve-
cord and the mode of its origin are essentially the same in both Verte-
brates and Hemichordates, a fact which in itself must be given no little
weight in the final determination of the question.

III. CLASS UROCHORDA.

The Urochorcla, also known as the Tunicata or Ascidians,
are, like the other Protochordates, exclusively marine. At
first sight they appear to have little resemblance to such a
form as Amphioxus, the majority of them lacking in the adult
condition all trace of a notochord, though a branchial region
of the digestive tract is always present. In a few adult forms
(Appendicularians, Fig. 285), and in the larvre of all, a well-de-
veloped notochord is present, however, situated in a backward
prolongation of the body, resembling in appearance and
structure the tail of a young tadpole — an arrangement Avhich
has suggested the name applied to the class. In the majority
of forms this tail disappears at the close of larval life, and
with it the notochord also vanishes. The name Tunicate is
derived from the fact that the body is enclosed within an

TYPE PROTOCHORDATA. 619

external coat or tunic secreted by the ectoderm of the body
and composed, so far as its matrix is concerned, of a substance
which is closely related in its chemical characters to vege-
table cellulose.

Inasmuch as considerable variation in form occurs in the
class, it will be most convenient to describe in detail what
may be considered a typical Tunicate, pointing out subse-
quently the more important modifications which occur.

The simple Asciclians are for the most part ovate in form
and usually attached at one end to some support ; in the
genus Boltenia, however, the surface of attachment is at the
extremity of an elongated stalk, which at the other end passes
into the pyriform body. The test or tunic which encloses
the body presents usually towards the free end of the body
two openings, through one of which, the branchial aperture,
water passes into the interior of the body and is eventually
expelled through the second or atrial aperture. This test is
composed of a matrix sometimes almost homogeneous, some-
times fibrillar (Cynthia), and of varying consistency, secreted
by the ectoderm. It contains scattered through it numerous
cells, which have recently been shown to be mesoderrnal in
origin and to migrate into the test after it has reached a cer-
tain thickness. They may remain amoeboid in shape or may
develop vacuoles, becoming thereby swollen into bladderlike
structures, or may become the seat of pigment-depositions,
or, finally, may secrete spicules of carbonate of lime.

Extending through the test are numerous branching tubes
which communicate with the blood-vessels of the body.
They arise as outgrowths of the blood-vessels and push the
body-wall before them, being therefore lined externally by
ectoderm, beneath which is a layer of connective tissue, and
each is separated into two compartments by a longitudinal
partition which does not, however, extend into the bulblike
enlargement with which each branch ends, the two compart-
ments thus communicating in the bulb and allowing the
blood to circulate.

The shape of the body conforms to that of the test, and
opposite the branchial and atrial apertures of the latter is
drawn out into two tubular processes, the branchial and atrial

620

INVERTEBRATE MORPHOLOGY.

siphons, in whose walls circular muscles are developed to
serve as sphincters of the openings. The branchial siphon
opens posteriorly into the branchial region of the digestive

FIG. 283.— FIGURE OF A TUNICATE, Heterotrema, REMOVED PROM THE TEST

(after FIEDLER).

A — atrial pore. pc = peripharyugeal ciliated band.
an = anus. s = stomach.

CG = cerebral ganglion. sn — subneural gland.

en — endostyle. at — branchial stigrna.
ex = excretory organs. t = testis.

/ = intestine. vd = vas deferens.

tract, the opening being known as the mouth, and usually
being surrounded by a number of tentacles (Fig. 283) which
arch over it. The atrial siphon, on the other hand, does not
open into the body proper but into a cavity, lined probably

TYPE PROTOCHORDATA. 621

witli ectoderm, which almost completely surrounds the body,
being wanting only along the ventral wall and at the extreme
anterior and posterior ends. This cavity is the atrium and
its walls are termed the mantle. It is comparable to the
atrial cavity of Amphioxus, but has a somewhat more exten-
sive development and arises in the larva as a pair of dorsally-
situated imaginations of the body-wall which, gradually in-
creasing in size, enclose the greater portion of the body, and
their openings, gradually approximating, finally fuse to form
the atrial aperture (Fig. 283, A), the dorsal partition between
them at the same time disappearing, so that the two cavities
become continuous.

The external surface of both the mantle and the body
proper is covered with ectoderm, and rests below upon a
layer of mesodermal connective tissue which contains muscle-
fibres. The coelomic cavity consists of a number of lacunar
spaces which have, especially in the walls of the branchial
region, a more or less definite arrangement and serve as blood-
vessels. In a somewhat distinct space near the hind end of
the body is situated a tubular heart, whose walls are formed
of a single layer of cells, the inner ends of which are con-
verted into muscle-fibres. The contractions of the heart are
wavelike, starting from one end and passing gradually though
rather quickly towards the other ; but a remarkable peculiarity
is that after a certain number of beats, at each of which the
contraction-wave begins at one end, its course is reversed,
and for a similar number of beats it begins at the other end.
This change takes place with a certain amount of rhythm,
and at each change the course of the blood through a portion
at least of the body is reversed. At each end the heart opens
into a large lacuna, one of which runs along the ventral mid-
line of the branchial sac, while the other, giving off lacunar
branches to the intestine and test, runs along the dorsal mid-
line of the same region, smaller lacunae, traversing the bran-
chial bars, uniting the two vessels. The blood-plasma is
colorless and contains amceboicl corpuscles which are also
usually colorless, though a few colored ones, resembling the
pigmeuted cells of the test, are frequently found.

The mouth opens into a capacious pharyngeal or branchial

622 INVERTEBRATE MORPHOLOGY.

sac whose walls are perforated by numerous slits or pores
termed stigmata (Fig. 283, st), arranged in transverse or spiral
rows. The bars separating the stigmata enclose lacunae
which place the ventral and dorsal branchial lacuuse in com-
munication so that the walls of the sac are richly supplied
with blood, opportunities for its aeration being provided by
currents of water drawn by the cilia which border each stigma
through the mouth and out into the atrial cavity, whence it
escapes by the atrial aperture. The transverse bars which
separate the rows of stigmata are generally stouter than the
longitudinal ones, and in most species there is a second series
of longitudinal bars lying on the inner surface of the sac, less
numerous than the bars which separate adjacent stigmata,
united with each transverse bar by a short connecting branch,
and bearing opposite each junction a hollow papilla which
projects into the cavity of the branchial sac. Running along
the entire ventral mid-line of the sac is a ciliated groove (Fig.
283, Eri) bounded on each side by a distinct longitudinal ridge.
This is the endostyle, comparable to that of Amphioxus, and
from its anterior end a band of ciliated cells (pc] passes dor-
sally on each side of the pharyngeal wall to unite in the dorsal
mid-line. In front of these bauds another pair running par-
allel to them is usually found, the two pairs forming the
peripharyngeal ciliated bauds. From the dorsal point of
union of the two posterior bands a ridge, the dorsal lamina,
extends backwards in the dorsal mid-line of the branchial sac,
and in man}- species (Fig. 283) is produced into a number of pro-
cesses succeeding one another at intervals, and projecting into
the branchial cavit}^ ; these are termed the dorsal languets.

The remaining portions of the digestive tract is in the
simple Ascidiaus generally situated in the mantle on the left
side of the body, owing to the enormous development of the
branchial sac, but in other forms it constitutes a part of a
visceral mass lying immediately below the posterior end of
the sac. The ossophagus, beginning at the lower end of the
sac, forms a short tube which opens into a fusiform stomach,
from the further end of which the intestine (/) arises. This
is generally bent twice upon itself, forming thus two loops,
and ends in a straight piece, the rectum, which opens by the

TYPE PROTOCHORDATA. 623

anus (aw) into the atrial cavity. A thickening occurs along the
entire length of the inner surface of the intestinal wall, form-
ing the tjphlosole, and a number of branched tubules open-
ing into the stomach (s) represent a so-called " liver " or
digestive gland.

The nervous system consists of a single ganglionic mass
(ca) lying on the dorsal side of the body near the anterior
end and giving off nerves both anteriorly and posteriorly.
Immediately below it is a glandular structure, the subneural
gland (sn), from which a duct passes forwards to open into
the anterior part of the branchial sac, at the base of a well-
marked papilla which may possibly be sensory in function.
The gland, from its relation to the nervous system and the
branchial sac, has been compared to the pituitary body
(hypophysis cerebri) of the Vertebrates.

Sense-organs are but slightly developed, pigment-spots
situated in the neighborhood of the branchial and atrial
apertures perhaps representing rudimentary eyes, while
sensory functions have been attributed to the tentacles, the
dorsal lauguets, and the papilla at the opening of the duct of
the subneural gland.

An excretory function has been assigned to the subneural
gland, but in addition to this there are found in the visceral
mass a number of spherical bodies (ex) without ducts, in
whose cells concretions of uric acid are found. These seem
to represent excretory organs, the waste material instead,
however, of being passed out from the body is stored up in
the cells of the organs — a condition recalling what is found in
some Echiuoderins and, to a certain extent, the arrangement
in the Ectoproctous Polyzoa.

The Tunicates are for the most part hermaphrodites. The
ovary is a ramified structure lying in the loop of the intes-
tine, and contains a cavity lined with a germinal epithelium.
An oviduct is continuous with the ovar}^ and leads toAvards
the atrial cavity, opening into it in close proximity to the
anus. The testes (Fig. 283, t) are numerous spherical bodies,
also situated in the visceral mass, each portion being provided
with a duct which joins with others to form the single vas

624

INVERTEBRATE MORPHOLOGY.

deferens (vd\ opening into the atrial cavity near the opening
of the oviduct.

On account of the larval characters being more important

than the adult in indicating the
affinities of the Tunicates and in
justifying the term Urochorda applied
to the class, it seems convenient to
depart from the usual arrangement
and consider the development of the
simple Ascidians here, before passing
on to a description of the various
orders.

Development of the Simple Asci-
dians.— For an account of the early
development reference must be made
to embryological text-books, the
present description being confined to
the larva and the changes it under-
goes in transforming to the adult.
Suffice it to say that the early stages
resemble very closely those of Am-
phioxus, and they result in the forma-
tion of a remarkable structure usually
known as the Ascidian tadpole, a
term which indicates its general ap-
pearance. This larva is a free-swim-
ming organism and consists (Fig.
284) of an anterior somewhat globular
portion, the body, and a posterior
flattened region, the tail. The entire
body is enclosed writhin a continuous
case, the test (c), which, in the tail
region, is elevated into a dorsal and
ventral ridge, serving as fins. Upon
the anterior end of the body are
papillae (ap) which serve for fixation
when the larval life is completed, while in the interior of
the body region indications of the various adult organs
may be seen. Certain interesting modifications of these

FIG. 284. — DIAGRAM OF THE
TADPOLE LARVA OF A
TUNICATE.

a — anus.
ao = atrial orifice.
ap = adhesive papilla.
at = atrial cavity.

c = cellulose test.

ce = brain.
en = eudostyle.

h = heart.
m •= mouth.

n = nerve
nc = notochord.
ph = pharynx.
sg = subueural gland.

TTPE PROTOCHORDATA. 625

are, however, to be noticed ; the atrial cavity (at) is yet
quite small and the anus (a) may or may not communicate
with it, while the mouth (in) is not functional, owing to the
test being continuous over it. The nervous system is the
most remarkable structure, however, consisting of a large an-
terior saclike structure, the brain (ce), into the cavity of which
two sense-organs project. Upon the dorsal wall is the eye,
consisting of a cup-shaped group of cells whose inner ends
are pigmented, the hollow of the cup being occupied by a
lens above which is a membrane, the so-called cornea. Pro-
jecting dorsally from the ventral wall is the otolith, consist-
ing of a pear-shaped cell bearing a cap of pigment, its smaller
end extending between the cells of the lower wall of the
brain, which in its neighborhood are provided with stiff cells
forming a crista acustica.

Posteriorly the brain is prolonged into a thick-walled tube
(ri) which extends back almost to the tip of the tail, to the
muscles of which it sends off nerves. Beneath the nerve-
tube, throughout nearly its entire length, lies a notochord (nc)
which serves as a skeletal support to the tail, and on each
side of it is a plate of longitudinal muscle-fibres.

By means of energetic lateral movements of the tail this
larva swims about for some time, but when about to trans-
form itself into the adult it fastens to some solid object by
one of the adhesive papillae. The tail with its nervous sys-
tem, muscles, and notochord then undergoes degeneration
and is completely absorbed, the portion of the test covering
it being thrown off, and a rotation of the body takes place so
that the mouth comes to lie at the opposite end of the body
from the point of fixation. The branchial and atrial aper-
tures form, and the anterior saclike brain collapses, the
sense-organs degenerate, and the adult brain is gradually
developed. The extension of the atrium and the develop-
ment of additional stigmata complete the acquisition of the
adult characteristics.

It will be seen from this, then, that the larvae are of great
importance in estimating the systematic position and the
affinities of the Urochorda, and that the adults, except in the
Appendicularice, in which the tail and the free-swimming habit

626

INVERTEBRATE MORPHOLOGY.

are persistent, are to be regarded as degenerate. Several
orders of Tuuicates may be recognized.

1. Order Larvacea.

To this order belongs the genus Appendicularia which has
already been several times mentioned. It is throughout life
free-swimming and retains the larval tail, greatly resembling
in general appearance a tadpole larva. It secretes an exten-
sive test which is gelatinous in consistency and is but loosely
attached to the body, being frequently thrown off shortly
after its formation. The body (Fig. 285) is comparatively

FIG. 285. — AN APPENDICULARIAN, Oikopleura cophocerca (after Fol from

HERTWIG).

/= ciliated groove.
g — brain with auditory vesicle.
g' -.- first gauglion of tail.
h •= testis.

a = anus.

c = notochord.
d' = pharynx.
d" = stomach.
en — endostyle.

ov — ovary.
s = branchial cleft.

small, the tail being attached to its ventral surface, while its
posterior extremity is somewhat enlarged and contains the
reproductive organs (ov and //). The brauchial sac has but a
single pair of stigmata (s) which open to the exterior by a
pair of funnel-like tubes situated behind the anus. This ar-
rangement represents exactly a condition present in the larvae
of other Tunicates, two stigmata first forming and the atrial
sac arising as two separate invagiuations of the body-wall
into which the primary stigmata open, the invagiuations only

TYPE PROTOCHORDATA. 627

later fusing to form the extensive atrium. The endostyle (en]
is very short, and the nervous system is constructed upon the
larval plan, though the brain (g) is not the sensory sac so
characteristic of the larva. It is more gauglionlike and sends
off branches to an otocyst lying in its neighborhood, and also
gives rise to a tubular prolongation opening into the terminal
portion of the branchial sac. Pigment-spots, probably rep-
resenting rudimentary eyes, are also present, and from the
posterior end of the brain a tubular nerve- cord arises which
passes backwards and downwards into the tail, which it trav-
erses, dilating at intervals into ganglia (</'). The uotochord
(c) lies below the caudal nervous system, and the lateral
muscles of the tail show indications of metameric segmen-
tation.

2. Order Ascidiacese.

The members of this order present the structural features
which have been described as typical for the class. Many
forms possess the power of non-sexual reproduction by bud-
ding, colonies being thus produced some of which may be
free-swimming, the simple forms, however, being always fixed.
They differ from the Larvacese chiefly in the absence of the
tail in the adult and in the large development of the branchial
sac and the numerous stigmata.

Owing to the complexities produced by the methods of
budding it is customary to divide the order into subordinate
groups.

1. Suborder Ascidice simplices.

The simple Ascidians agree with the description given as
typical and do not require any further notice here, except to
mention the fact that there are included within the suborder
a few genera which reproduce by budding, e.g. Clavdlina and
Peropliora. The formation of new individuals takes place in
these cases from stolonlike outgrowths of the parent form,
and each bud remains seated upon the stolon surrounded by
its own test. The stolon (Fig. 286) arises from the lower por-
tion of the body of the parent and pushes before it a portion

628

IN VERTEBRA TE MORPHOLOG T.

br

of the test ; the cavity it contains is continuous with the body-
ccelom and is therefore lined by mesoderm, and is divided

into two compartments by a longitudinal
partition which may be traced back to
its origin from the posterior wall of the
branchial sac of the original individual.
Since ectodermal tissue lies between the
mesoderm and the inner surface of the
test, the stolon contains portions of all
three germ-layers, and a portion, of each
passes into each bud (6) as it arises.
The first indication of a bud is a slight
wartlike elevation of the wall of the
stolon which increases in size, its cavity
being a diverticulum of the stolon-ccelom.
FIG. 286.— PORTION OF A Into the elevation a process (en) of the
STOLON OF Perophora en(jodermal stolon-partition extends, and,

(after KOWALEWSKY from _ .

KORSCHELT and HEIDER). forming a hollow saclike body, gives

b = bu(1- rise to the digestive tract of the bud.

= branch of stolon. ri ^ ^ ^ to tbeir

ec = ectoderm. .

en = endoderm. respective organs with one exception,

and that is that the atrial walls, the man-
tle, arise from the endodermal branchial sac as diverticula
which unite together, the atrial cavity being thus lined
throughout with endoderm. Such anomalies are not infre-
quent in the Urochorda, and indicate a necessity for further
study of the nature of the germ-layers in these forms.

The simple non-budding forms are quite numerous. Com-
mon genera are Molgula, Cynthia in which the test has a
leathery consistency owing to the fibrillar character of the
matrix, and Boltenia, a stalked form.

2. Suborder Ascidice composites.

All the members of this order reproduce by budding in
some form or other, and differ from such forms as Clavdlina
in that all the individuals remain imbedded in a common test
whether or not they remain in organic connection with one
another. The group seems to be a somewhat composite one,

TYPE PROTOCHORDATA.

629

and it is probable that it is aii aggregation of forms with quite
distinct derivations. The varieties of budding which are
found are quite numerous. In some cases it closely resem-
bles that of Clavellina, some species of Distaplia, for example,
possessing a short stolon, essentially similar to that of the
former genus, from which buds arise which, however, early
separate from the stolon and remain imbedded in the thick

FIG. 287. — A, YOUNG SOLITARY Amarwcium DEVELOPED FROM EGG ; B, A
SLIGHTLY ULDEK FORM, IN WHICH THE POSTABDOMEN is SEGMENTED ; C,

THE NON-SEXUALLY PRODUCED FoRMS MIGRATING TOWARDS THE SURFACE
OF THE TEST (after KOWALEWSKY from KORSCHELT and HEIDEE).
a = parent individual. b = bud which has reached the surface.

c — migrating buds.

test of the parent, a repetition of this process through several
generations leading to the formation of massive colonies.

In Amarcecium the arrangement is a little different, but can
readily be traced back to the stolon form. The posterior eud
of the body in budding forms is continued backwards as a
long postabdomen (Fig. 287, A), having a structure similar to
the stolon of Clavellina, but instead of giving off buds the
stolon segments into a number of parts (Fig. 287, B) which,

630

INVERTEBRATE MORPHOLOGY.

separating, rise through the thick test of the parent (Fig. 287,
C) until they reach the surface, and develop to complete in-
dividuals, in which the process may be repeated. Amarcecium
thus forms massive colonies consisting of a number of quite
separate individuals all imbedded in a common test, and all
directly or indirectly the result of the budding of a single in-
dividual developed by the sexual method.

In other forms, however, the stolon is practically sup-
pressed and the buds arise directly from the body of the

b

FIG. 288. — A SYSTEM OP Six INDIVIDUALS PHOM A COMPOUND COLONY OP

Botryllus (after OKA).

a = adult individual. ecp = eclodermal processes extending

b = bud. into test from each individual.

cl = common cloaca. m = mouth of one of the individuals.

parent. This is the case in Didemnum and Tridemnum, for
instance, peculiar complications being also introduced into
the process. In the latter form the daughter individual arises
as two buds which later fuse. One arises from the upper end
of the oesophagus and gives rise to the intestine and neigh-
boring organs in the bud, while the other, arising from the
branchial sac, gives rise to that structure, the atrium, and
terminal portion of intestine. Usually the two buds arise
simultaneously, but occasionally one may fail, the result be-
ing the production of half individuals which remain united
with the parent, producing double monsters ; and since either

TYPE PROTO CHORD ATA. 631

bud may fail, these monsters may have either a double bran-
chial sac, atrium, and rectum, or a double digestive tract, heart,
and other organs of the visceral mass. If, however, both
develop simultaneously, in the course of time the perfect
daughter form separates from the parent. In Botryllus (Fig.
288) but a single bud (b) arises from each individual of the
colony, developing on one side of the body in the region of
the branchial sac, early separating from the parent except
for a slight stalk like union. After the formation of the
daughter individuals the parent forms die, and so a succes-
sion of generations occurs, the various individuals of each
generation being arranged in a circle around a common cloaca
(cl) into which the atrial aperture of each opens.

The origin of a colony of Botryllus takes place as follows : An individ-
ual developing from an egg fastens itself and gives rise to a single bud,
which remains imbedded in the test of the parent, which, on its part, dies
and disappears. The individual of this second generation gives rise to two
individuals of the third generation and then likewise dies, while each of
the members of the third generation gives rise to two more members of a
fourth generation and degenerates. The four individuals so formed ar-
range themselves so that they radiate from a common point at which a
depression of the test occurs, forming the cloaca. New generations then
succeed each other, the parents of each generation in turn degenerating,
and so the colony extends, and some of the individuals failing to form a
connection with the original cloaca become centres for a new radiating
colony, still, however, imbedded in the common test.

3. Suborder Pyrosomidm.

In this order there is but a single genus, Pyrosoma, which
includes floating pelagic colonies having the shape of a cylin-
der closed at one end, the individuals composing the colonies
being enclosed in a common test and arranged radially around
the central cavity or cloaca into which their atrial apertures
open, the branchial apertures opening on the exterior of the
cylinder. Each individual resembles in structure a simple
Ascidian, the principal difference being that the atrial aper-
ture, as in Botryllus, is at the posterior end of the body, and
that each individual has the power of reproducing by budding
and so assisting in the further growth of the colony, the
parent forms not, however, degenerating after giving rise to

632

IN VERTEBRA TE MORPHOL 06 T.

buds, as in Botryllus. The buds arise from the branchial
sacs behind the eudostyle, and, on separating from the parents,
place themselves between them and the opening of the com-
mon cloaca, so that the oldest members of the colony lie at
the closed end of the cylinder. On each side of the branchial
sac of each individual near the anterior end, or more precisely
near the peripharyngeal ciliated bauds, is a mass of cells
which are brightly phosphorescent, the entire colony, which
may reach a length of over a metre, emitting a brilliant light
when stimulated.

The development of Pyrosoma is exceedingly interesting
inasmuch as it presents an alternation of generations. From

el

N^ //

A B

FIG. 289.— LARVAL BUDDING OF Pyrosoma. A, embryo divided into the
cyathozooid aud four ascidiozooids; B, later stage showing the ascidiozo-
oids twisting to form the circle of four primary individuals (after KOWA-

LEWSKY).

cl = cloaca. en = endostyle.

el = elseoblast. h — heart.

n — nerve-ganglion of ascidiozooid.

the embryo which develops from the egg at a very early
stage a stolon develops (Fig. 289), containing a prolongation
of what corresponds to the embryonic branchial sac and also
of the embryonic mesoderm. The embryo itself never
reaches a full development and is termed the Cyathozooid,
serving to supply the individuals developed from the stolon
with nourishment until they have reached a certain stage of
development. This it is able to do on account of the ovum
being plentifully supplied with yolk, which the Cyathozooid
gradually absorbs. The stolon at an early stage divides into

TYPE PROTOCHOIWATA. 633

four parts (Fig. 289, A), which arrange themselves in a circle
around the Cyathozooid (Fig. 289, B], being enclosed with it
in a common test, and develop eventually into the four
primary individuals or Ascidiozooids, which occupy the closed
end of the cylindrical colony. As they develop the Cyatho-
zooid degenerates and finally completely disappears, the
cloacal cavity of the colony arising, as in Botryllus, as a de-
pression of the test, which grows deeper and deeper as new
individuals arise by budding.

The entire process shows a marked similarity to what occurs in Botryl-
lus, the principal differences being that the Cyathozooid, which represents
the first generation of Botryllus, buds while yet in an embryonic condition,
and that it alone degenerates, the parents of succeeding generations per-
sisting and forming parts of the fully-developed colony. In each Ascidio-
zooid there occurs behind the branchial sac a pair of peculiar masses of
cells termed the elceoblast (Fig. 289, B, el), whose function is uncertain,
though it has been suggested that they may represent the rudimentary
larval tail.

3. Order Thaliacea.

The Thaliacea are with a single exception pelagic organ-
isms, and present a life-history complicated by the occur-
rence of an alternation of generations. In the genus Salpa
(Fig. 290) a well-developed test is present, and the muscula-
ture of the mantle is arranged in bands which do not quite
surround the body and furthermore show a tendency to unite
together on the dorsal surface of the body. At one end of
the body is a large branchial aperture (m) leading into a
branchial sac, in the floor of which is the endostyle (en\
while in its roof is a ridge, the dorsal lamina, which anteriorly
is produced into a single languet projecting into the interior
of the sac. On each side of the dorsal lamina is a large
opening which represents an enormous stigma and communi-
cates with the atrial cavity, whose aperture is situated at the
posterior end of the body. The intestine and the other viscera
are grouped together to form a small mass termed the nucleus
(mi) lying behind the branchial sac and ventrally, though in
some forms the intestine is more elongated and projects some-
what from the nucleus. The nervous system (n) has its usual
form and position ; it has in connection with it three pigmentecl

634

INVERTEBRATE MORPHOLOGY.

spots probably representing eyes, and the subueural gland is
present as usual.

Each species of Salpa, however, presents two distinct
forms (A and B\ differing in shape and in the number of the
muscle-bands which are found in the mantle and having like-
wise a different origin. In the sexual form (A) reproductive
organs are developed, the ovary usually containing but a

em

cl

nu

h A

ma

FIG. 290. — A, Salpa mucronata, THE SEXUAL FORM, AND B, Salpa democra-

lica, THE NON-SEXUAL FORM OF Salpa democratica-mucronata (after CLAUS).

c, cl = cloaca, in = branchial pore (mouth).

cp — ciliated pit. ma = test.

em = embryo. n = nerve-ganglion.

en = endostyle. nu = nucleus.

U — heart. si — stolon.

single ovum. This when fertilized (em) is passed into the
atrial cavity, the follicle-cells with which it is surrounded
forming an adhesion to the wall of the cavity, and later
modifying to form a structure recalling the placenta of the
Mammalian Vertebrates by which nourishment is conveyed
from the parent to the embryo. As the result of the develop-
ment of this ovum the non-sexual form (.#) is produced, which
is characterized not only by its general form, but also by the
possession of a stolon (st) arising from the branchial sac just

TYPE PROTOCHOEDATA. 635

behind the posterior end of the endostyle. This stolon
eventually divides into a large number of parts, each of which,
after undergoing certain somewhat complicated shiftings of
position on the stolon, develops into the sexual individual.
The terminal portions of the stolon with the maturer individ-
uals from time to time break off and float about, forming
what are termed the Salpa-chains, and the constituent individ-
uals of the chain, the sexual individuals, becoming mature
either while still united with their fellows or after separation
from them, start again the life-cycle which may be represented
by the following scheme :

Ovum — Solitary Salpa Chain Salpa Ovum.

(Non-sexual) (Sexual)

In another genus belonging to the order, Doliolum (Fig.
291), the process is somewhat more complicated. The mem-
bers of this genus are barrel-shaped forms, the wide branchial
aperture being situated at one end and the atrial aperture at
the other ; the stigmata are fairly numerous and arranged in
two lateral rows, and the mantle muscle-bands form complete
circles around the body, resembling the hoops of the barrel.
From the ovum there develops a peculiarly-constructed tailed
embryo, which, with the loss of the tail, becomes converted
into the non-sexual form (Fig. 291, A) characterized by the
possession of nine circular muscle bands, an otocyst (ot) situ-
ated some distance from the nervous system on the side of
the body, a ventral stolon (st), and a dorsal posteriorly-
directed process (dp). From the stolon a number of buds
are produced, which, at an early stage, separate from the
stolon and migrate to the dorsal process to which they attach
themselves, the migration being accomplished, it is said, by
means of amoeboid cells, probably cells of the test, which
attach themselves in pairs to the base of each bud and serve
to convey it to the dorsal process. Upon this process the
buds arrange themselves in three rows, the individuals of the
lateral rows developing into forms quite different from those
resulting from the development of the buds of the median row.
The lateral buds when freely developed are characterized by
the possession of a large branchial aperture, which occupies

636

INVERTEBRATE MORPHOLOGY.

almost the entire length of one side of the body and leads into
a branchial sac whose stigmata open directly to the exterior,
the atrial cavity disappearing during the course of develop-
ment. The intestine is well developed, but the muscles are
but slightly indicated, while the reproductive organs, rudi-

ot en

st i

en

B

FIG. 291. — A, THE NON-SEXUAL, AND B, THE SEXUAL, FORM OF Doliolum

(after UUANIN).

cl = cloaca. i = intestine.

dp = dorsal process. n = nerve-gauglion.

en — endostyle. ot = otocyst.

g = reproductive organ. ph = pharynx.

h = heart. si = stolon.

ments of which were present in the young buds, completely
atrophy during the process of development. These buds
are incapable of leading a free existence, serving only as
nutritive and respiratory individuals for the median buds, as
well as for the parent, whose digestive tract degenerates, its
muscle-bands and nervous system at the same time under-

TYPE PROTOCHORDATA. 637

going enlargement, so that it serves eventually as a loconiotor
individual for the entire aggregation of individuals.

With regard to the median buds differences of opinion
occur. Certain of them, or according to one authority all,
being set free, develop into forms possessing but eight muscle-
bands, no otocyst or dorsal process, but having a ventral
median process upon which buds are found. It is in regard
to the origin of these buds that the difference of opinion
exists. According to one view they are produced from the
ventral process which is regarded as a stolon and represent a
third generation, while according to the other view they are
certain members of the median row of the second generation,
the forms bearing them being as it were their sisters and
serving as nurses for them. Whatever may be their origin,
however, the buds eventually are transformed into sexual
individuals (Fig. 291, B) with which the life-cycle was com-
menced. The two views as to the cycle in Ddiolum may be
schematically represented thus :

/ Nutritive individuals

Ovum — non-sexual form^—Nurses— —Sexual forms— Ova

^-Nutritive individuals

Nutritive individuals

x
Ovum = non-sexual former

X^-Sexual forms— -- Ova

x Nutritive individuals

Mention should be made of the genus Octacnemus, a deap-sea form be-
longing to this order which is apparently attached by a pedicle to foreign
bodies. The body proper is flattened and disklike, its margins being pro-
longed into eight tapering processes. The mouth lies on the surface of the
disk and leads into a shallow branchial sac. The atrium is comparatively
large, and the intestine and viscera are, as in Salpa, massed together in
the form of a nucleus. Nothing is as yet known as to the life-history of
this form.

Affinities of the Urodiorda.— There seems little room for doubt but
that the Urochorda are related to AmpTiioxus. The two groups pre-
sent too many common structural features to allow of their being re-
garded as distinct types, but at the same time it is noticeable that the
relationship is through the larval Tunicates rather than through the
adults. These latter are degenerated forms, the entire group forming a
degenerate offset from the main line of evolution represented by the Proto-
chordata and leading to the Vertebrata. The early stages of development

638 INVERTEBRATE MORPHOLOGY.

of the simple Tunicates (see text-books of embryology) are so very similar
to those of Amphioxus that it must be concluded that the evolution of the
Urochorda and Cephalochorda proceeded for some distance along similar
lines, and the general affinities of the Protochordata may possibly be indi-
cated by a scheme thus :

Vertebrata
Cephalochorda

Urochorda
Hemichorda

Ancestral Protochordata

Taking the larval Tunicates as a basis for comparison, we find as
features common to them and Amphioxus a dorsal nervous system aris-
ing as an invagination of the ectoderm and extending the entire length
of the body ; in the anterior portion the lumen of the nerve-cord expands
to form a brain which in Amphioxus opens in early stages to the exterior
and in the Tunicates into the anterior portion of the branchial sac, i.e.,
the ectodermal portion, the canal of communication in the latter forms
losing in later stages its connection with the brain and forming the sub-
neural gland. An atrial cavity occurs in both, which, though arising in a
somewhat different manner in the two groups, nevertheless seems quite
homologous, and homologies have also been pointed out between the
primary gill-slits. The increased number of stigmata and their arrange-
ment in the Tuuicates is a secondary character resulting probably from the
sessile existence ; and the development of the test and the limitation of the
notochord to the tail are also probably secondary characters. The resem-
blances are important ones, and when taken into consideration with the
embryonic development point very strongly to a close affinity.

As regards the relationships of the various groups of Urochorda to one
another considerable difference of opinion exists. The Appendicularians,
which at first sight seem to be the most primitive of all the orders, present
certain remarkable peculiarities, such as the separate openings of the atrial
cavities and the anus, and some authors are inclined to regard them not as
primitive forms, but as sexually-mature larvae of sessile forms in which a
test had already developed and degeneration far advanced. As regards
the remaining forms the simple Ascidians seem to be the most primitive,
the composite forms being derived from them by the acquisition of non-
sexual reproduction. The composite forms, however, seem really to
represent several groups originating independently, all the members not
having descended from an ancestral simple form, but some from one
ancestor and others from auother, and so on. The Thaliacea, finally, have

TYPE PROTOCHORDATA. 639

probably been derived from composite forms, Pyrosoma showing certain
affinities in its budding to Salpa, the fact of some individuals being
solitary not necessarily indicating a primitive character, since in the com-
posite forms no organic union exists between the various individuals of the
colony when they have reached maturity, so that the so-called colonies are
rather aggregations than colonies, and the forms more properly termed
social than colonial or even composite.

SUBKINGDOM METAZOA.
TYPE PROTOCHORDATA.

I. Class HEMICHORDA. — Body divided into three distinct regions ; noto-
chord a small fingerlike diverticulum projecting forwards
from anterior portion of digestive tract, with which it
retains connection.

1. Order Pterobranchia. -- Sessile colonial forms, secreting a

"house"; intestine bent upon itself; collar region with
lophophorelike, tentacle-bearing processes. Cephaloclis-
cus, RJiabdopleura.

2. Order Enteropneusta. — Free forms not forming colonies ; not

secreting a test ; intestine straight ; collar region without
lophophorelike processes. Balanoglossus.

II. Class CEPHALOCHORDA. — Free forms, not pelagic ; not secreting a test ;
body not divided into three distinct regions, with numer-
ous metameres ; notochord completely separated from
digestive tract and traversing the entire length of the
body. Ampkioxus.

III. Class UROCHORDA. — Sessile or free pelagic forms; secreting a test;
body in adult not divided into three distinct regions
and showing no indications of metamerism ; notochord
usually wanting in adult, present in tail-region of larvae
and entirely sepai-ate from digestive tract.

1. Order Larvacea. — Retaining the larval tail with notochord ; free

simple pelagic forms. Appendicularia.

2. Order Ascidiacece. — Simple and sessile or colonial and occasion-

ally pelagic ; tail and notochord wanting in adult.

1. Suborder Aacidice simplices. — Simple sessile forms or else

budding from stolons and forming somewhat straggling
. colonies, the various individuals not enclosed in a common

test. Simple forms, Jlolyula, Cynthia, Boltenia ; Co-
lonial, Clavellina, Perophora.

2. Suborder Ascidice composite. — Colonial forms, the various

individuals embedded in a common test; if the various
individuals open into a common cloaca the colony is not
pelagic. Ainarcecium, Didemnum, Tridemnum, Di-
staplia, Botryllus.

640 INVERTEBRATE MORPHOLOGY.

3. Suborder Pyrosomidce. — Colonial forms, the various in-
dividuals imbedded in a common test, and opening into
a common cloaca ; pelagic ; with alternation of genera-
tions. Pyrosoma.

3. Order Thaliacea. — Simple pelagic forms, with alternation of
generations. Salpa, Doliolum, Octacnemus.

LITERATURE.

HEMICHOKDA.

W. C. Mclntosh. Report on Cephalodiscus dodecalophus, Mclntosh, a New
Type of the Polyzoa. Scieut. Results of the Voyage of H.M.S. Chal-
leuger, xx, 1887.

S. F. Harmer. Appendix to Report on CepJialodiscus. Scient. Results of the
Voyage of H.M.S. Challenger, xx, 1«87.

E. R. Lankester. A Contribution to the Knowledge of Rhabdopleura. Quar-
terly Jouru. Microscop Science, xxiv, 1884.

G. H. Fowler. The Morphology of Rhabdopleura Normanni Allm. Festschrift
fiir R. Leuckart. Leipzig, 1892.

A. Agassiz. The History of Balanoglossus and Tornaria. Memoirs American

Acad. Arts aud Sci., ix, 1893.
W. Bateson. The Early Stages in the Development of Balanoglossus (sp. incert.).

Quarterly Journ. Microscop. Science, xxiv, 1884.
The Later Stages in the Development of Balanoglossus Kowalewskii,

with a Suggestion on the Affinities of the Enteropneusta. Quarterly Journ.

Microscop. Science, xxv, Suppl., 1885 ; xxvi, 1886.
- The Ancestry of the Chordata. Quarterly Jouru. Microscop. Science,

xxvi, 1886.

T. H. Morgan. The Growth and Metamorphosis of Tornaria. Journ. of Mor-
phology, v, 1891.
J. W. Spengel. Die Eateropneusten. Fauna und Flora des Golfes von Neapel.

Monogr., xvm, 1893.
T. H. Morgan. The Development of Balanoglossus. Jouru. of Morphology,

ix, 1894.

CEPHALOCHORDA.

W. Rolph. Untersuchungen fiber den Bau des Amphioxus lanceolatus. Mor.
phologisches Jahrbuch, 1876.

B. Hatschek. Studien fiber Entwickluny des Amphioxus. Arbeiten aus d.

Zool. Inst. Wieu, iv, 1881.

E. Ray Lankester. Contributions to the Knowledge of Amphioxus lanceolatus,

Yarrell. Quarterly Journ. Microscop. Science, xxix, 1889.
B. Lwoff. Ueber Bau und Entwickluny der Chorda von Amphioxus. Mitth. a.

d. Zoolog. Station /u Neapel, ix, 1889.
E R. Lankester and A. Willey. The Development of the Atrial Chamber of

Amphioxus. Quarterly Jouru. Microscop. Science, xxxi, 1890.

F. E. Weiss. Excretory Tubules in Amphioxus lanceolatus. Quarterly Journ.

Microscop. Science, xxxi, 1890.

TYPE PROTOCEORDATA. 641

A. Willeyo The Later Larval Development of Amphioxus. Quarterly Journ.

Microscop. Science, xxxn, 1891.

Th, Boveri. Die Nierenkanalchen des Amphioxus. Ein Beitrag zur Phylogenie
des Urogenitalsystems der Wirbelthiere. Zoolog. Jahrbiicber, v, 1892.

B. Hatschek. Die Metamerie des Amphioxus und des Atnmocetes. Auatom.

Anzeiger, Ergauzuugs-Heft, vu, 1892.

W. B. Benham. The Structure of the Pharyngeal Bars of Amphioxus. Quar-
terly Jouru. Microscop. Science, xxxv, 1893.

E. B. Wilson. Amphioxus and the Mosaic Theory of Development. Journal of
Morphology, vin, 1893.

UROCHORDA.

T. H. Huxley. Observations on the Anatomy and Physiology of Salpa and
Pyrosoma. Philosopli. Transactions Royal Society, London, 1851.

A. Kowalewsky. Entwicklungsgeschichte der einfachen Ascidien. Memoires
Acad. St. Petersbourg, 7me ser., x, 1866.

C. Kupffer. Die Stammescerwandtschfift zwischen Ascidien und Wirbelthieren.

Archiv fur mikr. Anatomie, vi, 1870.

A. Kowalewsky. Weitere Studien liber die Entwicklung der einfachen Asci-
dien. Archiv far mikr. Anatomie, vn, 1891.

H. Fol. Etudes sur les Appendiculaires du detroit de Messine. Memoires Soc.
de Physique et d'Hist. Nat. Geneve, xxi, 1872.

A. Giardo Recherches sur les Synascidies. Archives de Zool. exper. et generale,
I, 1872.

H. de Lacaze-Duthiers. Les Ascidies simples des cotes de France. Archives de
Zool. exper. et gen., in, 1874.

A. Kowalewsky. Ueber die Knospung der Ascidien. Archiv. fur mikr. Ana-

tomie, x, 1874.
W. G. Herdman. Report on the Tunicata. Scientific Results of the Voyage of

H.M.S. Challenger, vi, 1882; xiv, 1886; xxvn, 1888.
L. Roule. Recherches sur les Ascidies simples des cotes de Provence. Annales

Musee d'Hist. Nat. Marseille, n, 1884-5.

B. Ulianin. Doliolum. Fauna und Flora des Golfes von Neapel. Monogr.,

x, 1884.
E. van Beneden et C. Julin. Le systeme nerveux central des Ascidies adultes et

ses Rapports avec celui des larves Urodeles. Archives de Biol., v, 1884.
Recherches sur la Morphologic des Tuniciers. Archives de Biol., vi,

1887.

C. Maurice. Etude monographique d'une Espece d'Ascidie composee (Fragaroi-

des aurantiacum n. sp.). Archives de Biol., vin, 1888.

0. Seeliger. Zur Enlwicklungsgesc7dchte der Pyrosomen. Jenaische Zeitschr.
fur Naturwiss., xxni, 1889.

A. Oka. Ueber die Knospung der Botrylliden. Zeitschr. fur wissensch. Zoo-
logie, LIV, 1892.

A. Willey. Studies on the Protochordata. Quarterly Journ. Microscop. Sci-
ence, xxxiv, 1893 ; xxxv, 1893.

W. K. Brooks. The Genus Salpa : a Monograph. With a Supplementary
Paper by M. M. Metcalf. Memoirs Biolog. Labor. Johns Hopkins
Univ., n, 1893.

INDEX OF PROPER NAMES.

(The names applied to larvae are included in the Subject-index.)

Group names are in SMALL CAPITALS, generic names in italics, and popular names in
roman type.

ACANTHOCEPHALA, 179, 183

Acanthometra, 20, 39
ACARINA, 453, 458
Acervularia, 17, 38
Achtheres, 397, 423
Acicula, 322
Acineta, 36, 39
Acnum, 364
ACCELA, 132, 169
ACOTYLEA, 139, 169
ACRASPEDA, 97
Actinian, 114
Actinometra, 542, 592
Actinomma, 39
Actinophrys, 17, 39
Actinospkcerium, 20, 39
ACULEATA, 527
^•Ega, 415, 424
jEolis, 315, 364
^Eolosoma, 219, 251
jEquor&t, 86, 116
JSscJma. 506, 526
Agalma, 92, 116
Agclena, 450, 458
Aglaophenia, 87, 116
Agrion, 506, 526
Atptasia, 113. 117
Alciope. 212, 251
Alcippe, 399, 423
ALCYONAKI^E, 108, 117
Alcyonella, 261, 274
Alcyonidium, 261, 274
Alcyonium, 108, 117
ALLOIOCCELA, 133, 169
Alpheus, 412, 424

Amarcecium, 629, 639
Amceba, 15, 38
AMPHINEURA, 284, 363
Amphioxus, 608, 689
AMPHIPODA, 416, 424
Amphiporus, 167, 170
Amphitrite, 213, 251
Amphiura, 564, 592
Ampullaria, 308, 364
Anabolia, 515, 527
Anachata, 218, 251
Analges, 453, 459
Anasu, 510, 527
AncJwrella, 397, 423
Anelasma, 402
ANISOPODA, 413, 424
ANNELIDA, 202, 251
Anodon, 339, 365
Anomia, 339, 365
Antedon, 542, 592
ANTHOMEDUS^E, 87, 116
ANTHOZOA, 104, 117
Antipathariae, 111, 117
Ants, 518
Anthura, 414, 424
Anuridu, 503, 526
APIIID^, 510
Aphis, 527
.-1^/s, 518, 527
Aplysia, 313, 364
APODA, 593

Appendieularia, 626, 639
Apscudes, 414, 424
APTEUYGOTA, 501, 526
, 387, 423

643

644

INDEX OF PROPER NAMES.

ARACHNIDA, 435
ARANE.E, 448, 458
Arbacia, 580, 593
Area, 339 365
Arcella, 16, 38
ARCHIANNELIDA, 211, 251
Archigeles, 161, 170
Aricia, 209, 251
Arenicola, 209, 251
Argiope, 271, 274
Argonauta, 359, 365
Argulus, 397, 423
Arion, 316, 364
Armadillidium, 414, 424
Artemia, 386
ARTHROPOD A, 368, 523
ARTHROSTRACA, 413, 424
Ascaris, 48, 177, 182
ASCIDIACE.E, 627, 639
ASCIDI^; COMPOSITE, 627, 639
ASCIDI^E SIMPLICES, 627, 639
Ascidians, 617
Ascopodaria, 256, 274

ASCOTHORACIDA, 403

Asellus, 414, 424
AspergMum, 329
Aspidiotus, 510, 527
Asplanchna. 189, 200
Asterias, 556, 592
Asterina, 552, 592
ASTEROIDEA, 552, 592
Astkenosoma, 570, 593
Astrangia, 114, 117
Astfopeclen, 556, 592
A&tropliyton, 561, 592
Atalanta, 309, 364
Atax. 453, 459
Alropos, 509, 526
^4«MS, 451, 458
Aurelia, 101, 117
ArjTOFLAGELLATA, 28, 39

Autolytus, 212, 251

Balanoglossus, 601, 639
Balanus, 400, 423
Barnacles, 398
BASOMMATOPHORA, 317, 364
Bdellura, 136, 170

Beach-flea, 416
Bees, 518
Beetles, 512
Belemnites, 360
Belosloma, 510, 527
.Br/w, 121, 126
Bipalium, 136, 170
Birgus, 412, 424
Blastoids, 550
Bolina, 124, 126
Boltenia, 628, 639
Bombus, 518, 527
Bonellia, 241, 252
Book-scorpion, 444
Bopyrus, 415, 424
Bothriocephalus, 153, 170
Botryllus, 631, 639
Brachinus, 513, 527
Brachionus, 189, 200
BRACHIOPODA, 268, 274
BRACHYURA, 412, 424
Branchellion, 228, 252
BRANCHIOPODA, 385, 423
Brancliiostoma, 608
BrancMpus, 386, 423
BRANCHIURA, 397, 423
Brisinga, 552, 592
Bristle-tails, 501
Brittle-stars, 561
BRYOZOA, 255
Bugs, 510
Bugula, 262, 274
-B«#a, 313, 364
Bulhus, 443, 458

Caddis-flies, 515
Cadulus, 322, 364
Calamocrinits, 544, 592
Calanus, 396, 423
CALCAREA, 73, 115
Caligus, 396, 423
Callinectes, 412, 424
Caloptenus, 504, 526
Calosoma, 513, 527
Calyptrcea, 307, 364
Cambarus, 412, 424

'AMPANULARIvK, 85, 116

Campodea, 502, 526

INDEX OF PROPER NAMES.

645

Camponotus, 518, 527
Campy laspis, 408, 424
Cancer, 412, 424
Canthocamptus, 396, 423
Capitella, 206, 251
Caprella, 416, 424
Carabus, 513, 527
Caravella, 92, 116
Carinaria, 309, 364
Carinella, 106, 170
Carpocapsa, 516, 527
Gary ophy Hi PUS, 152, 176
Cecidomyia, 60
Centipedes, 484
CEPHALOCHORDA, 608, 639
Cephalodiscus, 597, 639
CEPHALOPODA, 340, 365
Ceplialothrix, 169
Ceratium, 30, 39
Cercomonas, 31, 39
Cerebratulus, 116, 170
CERIANTHE.E, 110, 117
Cerianthus, 110, 117
CESTODA, 152, 170
Cestum, 124, 126
CetocWus. 396, 423
Chcetobranchus, 218, 251
Cfuetoderma, 285, 364
Chcetogaster, 222, 251
CH.ETOGNATHA, 186, 200
Chcetonotus, 196, 200
CiLas-ropoDA, 204, 251
Chnrybden, 101, 117
Chelifer, 443, 458
Chernes, 444, 458
Clillodon, 34, 39
CHILOGNATHA, 482
CHILOPODA, 484, 525
CHILOSTOMATA, 262, 274
CMrodota, 585, 593
C%»to», 289, 364
Chitunellus, 288, 364
Chlamydomonas, 31, 39
Chondr acanthus, 397, 423
Chrysopa, 514, 527
Cicada, 510, 527
Cicindela, 513, 527
CfoZarw, 580, 593

CILIATA, 33, 39
CIRRHIPEDIA, 398, 423
CLADOCERA, 388, 423
Clam, 339
ClathruliiM, 18, 39
(7to»a, 87, 116
Clavellina, 627, 639
Clepsine, 236, 252
tftowa, 74, 115
C'foo/ie, 314, 364
Clisiocampa, 516, 527
Clymenelloi, 204
Clypeaster, 581, 593
CLYPEASTROIDEA, 581, 593
(7^«Ms, 512, 527
CNIDARIA, 76, 116
COCCID^;, 510
COCCIDIA, 24
Coccinella, 512, 527
Cockroach, 504
Codosiya, 28, 39

COZLENTERA, 68, 115

Coeloplana, 125
COLEOPTERA, 512, 527
COLLEMBOLA, 503, 526
Collossendeis, 464
Colpidium, 37, 39
Colpoda, 36, 39
Convolute, 132, 169
COPEPODA, 393, 423
Coral, 114
Corallium, 108, 117

CORNACUSPONGIA, 73, 115

Coi'ophium, 417, 424
CORRODENTIA, 507, 526
Corycceus, 396, 423
Corydalis, 514, 527
Coryne, 87, 116
COTYLEA, 139, 170
Crabs, 412
Grama, 271, 274
Crayfish, 412
Crickets, 504
CRINOIDEA, 541, 592
Crisia, 274
Cristatella, 260, 274
CRUSTACEA, 368
CRYPTOPENTAMERA, 527

646

INDEX OF PROPER NAMES.

CryptopJiialus, 399, 423
CRYPTOTETRAMEKA, 527
Cteniza, 452, 458
Ctenodrilus, 58, 222
CTENOPHORA, 120, 126
Ctenoplana, 125
CTENOSTOMATA, 262, 274
CUBOMEDUS.E, 101, 117
Cucumaria, 585, 593
Culex, 520, 528
CUMACEA, 407, 424
Cunina, 96, 116
Cunoctantha, 84, 116

CtJRCULIONID/E, 512, 527

Cuspidaria, 339, 365
Cyamus, 416, 424
('//K/ted, 101, 116
Cyclas, 339. 365
Cyclops, 396, 423
Cydostoma, 308, 364
CYCLOSTOMATA, 262, 274
Cymbuliopsis, 314, 364
Cymollioa, 415, 424
Cynips, 518, 528
Cynthia, 628, 639
Cyplioplitkalmus, 447, 458
Cypra'a, 307, 364
Cypridina, 391, 423
Cfypra, 391, 423
CYSTOPI.AGELLATA, 30, 39
Cystoids, 550
Cytliere, 391, 423

Dactylopius, 510. 527
DapJinia, 388, 423
Daudebardia, 316, 364
DECAPODA (Cephalopoda), 359, 365
DECAPODA (Crustacea), 410, 424
Deima, 585, 593
Demodex, 453, 458
Dendrocailum, 136, 170
Dendroy aster. 403, 423
DentaUum, 322, 364
Dermaieichus , 453, 458
DERMAPTERA, 504, 526
/ter0, 218, 251
DESMOSTICHA, 580, 593
Diadema, 580, 593

Diapheromera, 505, 526
Diastylis, 408, 424
DIBRANCHIA, 359, 365
Dicyema, 64
DICYEMID^;, 64
Didcmnum, 630, 639
Diffli/gia, 16,38
Dinobryon, 28, 39

DlNOFLAGELLATA, 30, 39

Dinophilus, 198, 200
Uiopatra, 212, 251
DIOTOCARDIA, 305, 364
Diphyes, 92, 116
ZJz^toa;, 506, 526
DIPLOPODA, 482, 525
Diplozoon, 147
DIPNEUMONES, 452, 458
Diporpa, 147
DIPTERA, 519, 528

DlSCOMEDUS^E, 101, 117

Discosoma, 114, 117
Distaplia, 629, 639
DISTOME^E, 147, 170
Distominn, 147, 170
Dochmius, 177, 182
Doliolum, 635, 640
Dondersia, 287, 364
Ztorw, 315, 364
Doryphora, 513, 527
Drugou-flies, 506

Earwigs, 504
ECARDINES, 269, 274
Echinarachnius, 581, 593
Echinocucumis, 585
ECHINODERA, 184, 200
Echinoderes, 184, 200
ECIIINODERMA, 531
ECHINOIDEA, 570, 592
Echinorhynchus, 180, 183
ECHIURE^;, 240, 252
Echiurus, 240, 252
ECTOPROCTA, 257, 274
EDRIOPHTHALMATA, 413
Edwardsia, 117
EmvARDsi^;, 109, 117
ELASIPODA, 585, 593
ELATERID/E, 513. 527

INDEX OF PROPER NAMES.

647

Elpidia, 585, 593
ENDOPROCTA, 256, 274
Ensatella, 339, 365
ENTEROPNEUSTA, 601, 639
ENTOMOSTRACA, 385, 423
Eiitoniscus, 415, 424
Epeira, 450, 458
Ephemera, 504, 526
Ephemeridce, 505, 526
Ephydatia, 73, 115
Ergasilus, 396, 423
ERRANTIA, 211, 251
Esperella, 75, 115
Estheria, 387, 423
Eucope, 86, 116
EUCOPEPODA, 396, 423
Eudendrium, 93, 116
Euglena, 28, 39
Euglypha, 16, 38

EULAMELLIBRANCHIA, 339, 365
EUNEMATODA, 174, 182

Eupagurus, 411, 424
Euphausia, 406, 424
Enplectella, 74, 115
EURYALIDA, 569, 592
Eurylepta, 139, 170
Eurypauropus, 482, 525

EURYPTERID^E, 432

Eurypterus, 433
EURYSTOME^E, 125, 126
Euscorpius, 443, 458
Euspongia, 73, 115
Evadne, 388, 423

Facellina, 315, 364
Filaria, 177, 182
FILIBRANCHIA, 339, 365
Fiona, 311
Fissurella, 305, 364
FLAGELLATA, 28, 39
Flies, 519

Floscularia, 189, 200
Flustra, 261, 274

FORA.MINIFERA, 15, 38

Forficula, 504, 526
Formica, 518, 527
FredericeMa, 260, 274
Fungia, 114, 117

Galeodes, 445, 458
Gamasus, 453, 459
Gammarus, 416, 424
GASTEROPODA, 293, 364
Gasteropteron, 313
Gecarcinus, 412, 424
Gelasimus, 412, 424
GEOMETRID.E, 517, 527
Geopkilus, 484, 525
GEPHYREA, 237, 252
Geryonia, 85, 1.16
Gibbocellum, 447, 458
Globigerina, 17, 38
Glomeris, 483, 525
GNATHOBDELLID^E, 236, 251
Gonactinia, 111, 117
Gonodactylus, 409, 424
Gonoleptus, 448, 458
GORDIACEA, 178, 183
Gordius, 178, 183
Gorgonia, 108, 117
Grantia, 73, 115
Grasshoppers, 504
GREGARINIDA, 24, 39
Gromia, 16, 38
Gryllotalpa, 504, 526
Gryllus, 504, 526
Gw«da, 136, 170
GYMNOL^MATA, 261, 274
GYMNOSOMATA, 314, 364
Gyrinns, 513, 527
Gyrodaclylus, 147, 170

Hccmenteria, 236
Halcampa, 113, 117
Halecium, 87, 116
Haliotis, 305, 364
Halisarca, 74, 115
Halobates, 510, 527
Halof.ypris, 891, 423
Halodrilus, 227
Harpacticus, 396, 423
Harpalus, 513, 527
Harvest-men, 448
Harvest-mite, 453
Harvest-spider, 447
Heliopora, 109
Heliosphara, 19, 39

648

INDEX OF PROPER NAMES.

HELIOZOA, 17, 39
Helix, 316, 364
HEMICHORDA, 596, 639
HEMIPTEKA, 510, 527
Hermit Crab, 411
Hesione, 207, 251
Heterodera, 176, 182
HETEKOMEHA, 527
Heteronereis, 216
HETEROPODA, 309, 364
HETEROTRICHA, 39
HEXACTINI^E, 113, 117
Hexarthra, 194, 200
Hippo, , 411, 424
HIRUDINEA, 228, 251
Hirudo, 236, 251
IIolopus, 541, 592
Holothuria, 585, 593
HOLOTHUROIDEA, 584, 593
HOLOTRICHA, 39
Homarus, 412, 424
HOMOPTERA, 510, 527

HOPLONEMERTINI, 167, 170

Horseshoe Crab, 428
HYALOSPONGI^E, 74, 115
Hydra, 58, 83, 116
Hydrachna, 453, 459
Hydractinia, 58 87, 116
HYDRARI.E, 83, 116
HYDROCORALLJN.E, 89, 116
HYDROMEDUS,E, 78, 116
llydrometra, 510, 527
Hydrophilus, 513, 527
HYMENOPTERA, 517, 527
Hyocrinus, 542, 592
HYPOTRICHA, 39

lantMna, 307, 364
Ibla, 401, 423
Ichneumon, 528
Ichtltydium, 196, 200
Ichthyobdella, 228, 252
Idoten, 415, 424
Idyia, 125, 126
INFUSORIA, 33, 39
INSECTA, 487, 526
Isis, 108, 117
]sopoda, 414, 424

lulus, 483, 525
Ixodes, 453, 459

Janus, 311, 364

King-crab, 428
Kolga, 589

Labia, 526

Lacinularia, 189, 200
LAMELLIBRANCHIA, 326
Lampyris, 513, 527
LARVACEA, 626, 639
Laura, 403, 423
Leiobunum, 448, 458
Lepas, 399, 423
Lepidonotus, 212, 251
LEPIDOPTERA, 515, 527
Lepisma, 502, 526
Leptodiscus, 30, 39
Leptodora, 418
Leplogorgia, 108, 117
LEPTOMEDUS^E, 85, 116
Leptoplana, 139, 170
LEPTOSTRACA, 404, 423
Lerncea, 397, 423
Leucosolenia, 73, 115
Libellula, 506, 526
Libinia, 412, 424
Ligula, 152, 170
Limacina, 314, 364
Limaponlia, 315, 364
Zmaa;, 316, 364
Limnadia, 387, 423
Limncea, 316, 364
Limnetis, 387, 423
Limulus, 427
Lingula, 271, 274
Liotheum, 509, 526
Liriope, 85, 116
Lithobius, 485, 525
Lobster, 412
£0Z«>70, 359, 365
Lopliopus, 261, 274
Loxosown, 256, 274
Lucernaria, 100, 116
Lucifer, 411, 424
ftZfo, 553, 592

INDEX OF PROPER NAMES.

649

LUMBRICOMORPHA, 251

Lumbricus, 223, 2.J1
Lycosa, 452, 458
Lyswpetalum, 483, 525
Lysiosquilla, 409, 424
Lytta, 513, 527

Macrobdella, 236, 252
Macrob'iotus, 467
MACROLEPIDOPTERA, 516, 527
MACKTJRA, 411, 424
Mudrepora, 114. 117
Malacobdella, 167, 170
MALACOBDELLINA, 167, 170
MALACODERMATA, 114, 117
MALACOPODA, 475
MALACOSTRACA, 403, 423
MALLOPHAGA, 509
Margelis, 87, 116
May-tiy, 505
Melicerta, 189, 200
MeUita, 581, 593
Meloe, 513, 527
Melolonthu, 512, 527
Melophagus, 520, 528
Membranipora, 261, 274
Mermis, 178. 183
Mertensia, 124, 126
Mesostoma, 135, 169
MESOZOA, 63
METAZOA, 41
Metridimn, 114, 117
Mierog aster, 518, 528
Microgramia, 21, 39

MlCROLEPIDOPTERA, 516, 527

Microstoma, 58, 135, 140, 169
Miliola, 16, 38
Millepora, 89, 116
Millipedes, 482
Mites, 453

Mnemiopsis, 124, 126
Modiolaria, 339, 365
Moina, 388, 423
1/O'iVa, 583, 593
Molgula, 628, 639
MOLLUSCA, 276
Molpadia, 593
Monas, 28, 39

Monostvmum, 147, 170
MONOTUCARDIA, 306, 364
Mo not us, 134, 169
MiiUeria, 585, 593
Jl/wsca, 520, 528
Mussel, 339
J/ytt, 339, 365
My gale, 451, 458
MYRIAPODA, 480, 525
Myrmeleon, 514, 527
Mysis, 406, 424
My til us, 339, 365
MYXOSPORIDIA, 26, 39
MYZOSTOME^E, 244, 252
Myzostomum, 244, 252

NAIDOMORPHA, 251
Nau, 227, 251
Narcomedusae, 84, 116
JWzfea, 307, 364
Nautilus, 357, 365
Nebalia, 405, 423
Necrophorus, 512, 527
NEMATHELMINTHES, 172, 182
NEMATODA, 173, 182
NEMERTINA, 162, 170
NEOCRINIDA, 551
Neomenia, 286, 364
Neplieiis, 236, 251
Nereis, 212, 251
Neritina, 305, 364
NEUROPTERA, 514, 527
Noctiluca, 30, 39
Nodosaria, 17, 38
Nothrus. 453, 459
Notodelphys, 396, 423
Notonecta, 510, 527
Nucula, 339, 365
NUDIBRANCHIA, 315, 364

, 86, 116
Obi&ium, 444, 458
Octacnemus, 637, 639
OCTOPODA, 359, 365
Octopus, 359, 365
Oculina, 114
Ocypoda, 412, 424
ODONATA, 506, 526

650

INDEX OF PROPER NAMES.

OLIGOCELETA, 218, 251
Ommastrephes, 359, 365
Onchidium, 316, 364
Oniscus, 414, 424
ONYCHOPHORA, 475
Opalina, 35, 39
Ophiactis, 566, 592
Ophioderma. 561, 592
Ophiolepis, 567, 592
Ophiomyxa, 561, 592
Ophwthrix, 564, 592
Ophiura, 592
OPHIURIDA, 569, 592
OPHIUROIDEA, 561, 592
Opilio, 448, 458
OPISTHOBRANCHIA, 310, 364
Oractis, 111, 117
Orgyia, 516, 527
Oribates, 453, 459
ORTHONECTIDA, 65
ORTHOPTERA, 504, 526
OSTRACODA, 391, 423
Oslrea, 339, 365
Oxyuris, 177, 182
Oyster, 339

Palcemon, 412
Palwmonetes, 412, 424

PALiEOCRINIDA, 551

PAL/EONEMERTINI, 166, 170
Pdlinurus, 421
Paludicella, 261, 274
Paludina, 307, 364
Palythoa, 112, 117
Pandarus, 396, 423
Panorpa, 514, 527
PANORPATA, 514, 527
Papilio, 527
Param&cium, 35, 39
Patella, 305, 364
PAUROPODA, 481, 525
Pauropus, 482, 525
Pecten, 339, 365
Pedalion, 195, 200
PEDATA, 593
Pedicellina, 256, 274
Pcdiculus, 510, 527
PEDIPALPI, 446, 458

Pelagia, 103, 117
PELECYPODA, 326, 365
PELMATOZOA, 551
Pemphigus, 511, 527
PentKus, 419, 424
Penella, 397, 423
Pennaria, 87, 116
Pennatula, 109, 117
Pentacrinus, 542, 592
PENTAMERA. 527
PENTASTOMID^, 461
Pentastomum, 461
PerichcRla, 218, 251
Peripatus, 474, 525
Periplanela, 504, 526
PERITRICHA, 39
/Jerto, 507, 526
PEROMEDUS^;, 101, 116
Perophora, 627, 639
PETALOSTICHA, 582, 593
Phagocata, 136, 170
PHALANGIDA, 447, 458
PJialangium, 448, 458
Phascolion, 242, 252
Phuscolosoma, 242, 252
Philichthys, 397, 423
Philodina, 189, 200
Phl(£olhrips, 510, 526
PAotas, 339, 365
PHORt)NiD^, 247, 252
Phoronis, 247, 252
Plwxichilidium, 464
Phryganea, 515, 527
Phrynm, 446, 458
PHYLACTOL^MATA, 261, 274
Phyllirhve, 315, 364
PHYLLOPODA., 385, 423
PJtymanthus, 114, 117
PAysa, 316, 364
PHYSAPODA, 509
Pltytoptus, 454, 458
P/erw, 516, 527
Pinnotheres, 412. 424
Piscicola, 236, 252
Plagiostoma, 133, 169
Ptanaria, 136, 170
Planocera, 136, 170
Planorbis, 316, 364

INDEX OF PROPER NAMES.

651

PLATYHELMINTHES, 127, 169
Platyonychus, 412, 424
PLECOPTERA, 507, 526
Pleurobrachia, 124, 126
Pleurobranchcea, 312
Pleurobranchus, 313, 364
Pleurophyllidia, 315, 364
Pleurotomaria, 305, 364
Pneumoderma, 314, 364
Podura, 503, 525
Porcellana, 413, 424
Porcellio, 414, 424
POLYCH^TA, 204, 251
POLYCLADEA, 138, 170
Polydesmus, 483, 525
Polygordius, 211, 251
Polyophthalmus, 209, 251
Polyphemus, 388, 423
POLYPLACOPHORA, 288, 364
POLYSTOME.E, 147, 170
Polystomum, 147, 170
POLYZOA, 255, 274
Pontobdella, 236, 252

PORIFERA, 69, 115

Porospora, 25
Porpila, 91, 116
Portuguese Man-of-war, 92
Priapulm, 242, 252
Proctotrupes, 518, 528
Proneomenia, 285, 364
Prorhynchus, 135, 169
PROSOBRANCHIA, 303, 364
PROSOPYGIA, 254
PROTACTINI^E, 111. 117
Proteolepas, 402, 423
PROTOBRANCHIA, 338, 365
PROTOCIIORBATA, 596
Protodrilus. 211
PROTOZOA, 13, 38
PROTRACHEATA, 474, 525
Protula, 215

PSEUDOLAMELLEBRANCHIA, 339, 365
PSEUDOSCORPIONIDA, 443, 458
PSOCID^E, 509
Psolus. 584, 593
PTEROBRANCHIA, 597, 639
Pteromalus, 518, 528
PTEROPODA, 313, 364

Pterotrachea, 309, 364
PTERYGOTA, 504, 526
Pulex, 520, 528
PULMONATA, 316, 364
PYCNOGONIDA, 463
PYRALID^S, 516, 527
Pyrosoma, 631, 640
PYROSOMID^E, 631, 640

RADIOLARIA, 18, 39
Ranatra, 510, 527
Razor-shell, 339
RenWa, 108, 117
Rhubditis, 176
RHABDOCOSLA, 134, 169
Rhabdopleura, 597, 639
Rhegmatodes, 86, 116
RHIZOCEPHALA, 402
Rhizocrinus, 542, 592
RHIZOPODA, 14, 38
RHIZOSTOMID^E, 101
Rhopalodina, 584, 593
Rhopalonema, 85, 116
Rhopalura, 66

RHYNCHOBDELLID^E, 236, 251
Rhynchonella, 272, 274
RHYNCHOTA, 510, 527
Rotalia, 17, 38
Rotifera, 189, 200

Sabella, 212, 251
Sacculina, 402, 423
Sagitta, 186, 200
Snlenia, 570, 593
Salpa, 633, 640
Saud- dollar, 581
Saperda, 512, 527
Sapphirina, 396. 423
Sarcoptes, 453, 459
SARCOSPORIDIA, 27, 39
Scalaria, 308, 364
Scallop, 339
Scalpellum, 400, 423
SCAPHOPODA, 322, 364
SCHIZONEMERTINI, 166, 170
SCHIZOPODA, 406, 424
SCLERODERMATA, 114, 117
Scolopendra, 484, 525

652

INDEX OF PROPER NAMES.

Scolopendrella, 486, 525
Scorpion, 441
SCORPIONIDA, 441, 458
Scrupocellaria, 261, 274
Scntigera, 485, 525
Scyllarus, 421

SCYPHOHEDL'S^E, 97, 116

Scytophorus, 111, 117
Sea Anemones, 114
Sea- lilies, 541
Sea-urchins, 570
SEDENTARIA, 212, 251
Segestria, 452, 458
Selandria, 519, 528
Sepia, 359, 365
SEPTIBRANCHIA, 339, 365
Sergestes, 411, 424
Serpula, 213, 251
Sertularia, 86, 116
Ship-worm, 339
Shrimp, 412
Sida, 388, 423
Siphonodenlalium, 322, 364

SlPHONOPHOR^E, 91, 116
SlPUNCULACEA, 241, 252

Sipunculus, 242, 252
Siriella, 407, 424
Sitaris, 513
Slamna, 222
Solarium, 308, 364
SOLENOGASTRES, 285, 364
SOLIFUG^E, 444. 458
Solpuga, 445. 458
Sow-bug, 414
Spadella, 186, 200
Spatangus, 583, 593
Spltaroma, 415, 424
SpJuvrozoon, 19, 39
Spliyranura, 147, 170
SPICULISPONGI.E, 74, 115
Spider, 448
Spirula, 359. 365
Spondylus, 335
Sponges, 69
Spongilla, 73, 115
SPOKOZOA , 24, 39
Spring-tails, 503
Sguilla, 409, 424

STAPHYLINID<E, 512, 527
Starfish, 052

STAUROMEDUS.<E, 100, 116
Stentor, 35, 39
Sternaspis, 243
STOMATOPODA, 408, 424
Stomolophus, 101, 117
Stone-flies, 507
Strombus, 307, 364
Strongylocentrotus, 580, 593
Strongylosoma, 484, 525
Stylaster, 90, 116
Styliola, 314, 364
Stylochus, 141, 170
STYLOMMATOPHORA, 318, 364
Stylonychia, 39
SUCTOHIA, 35, 39
Sun-animalcule, 17
Syllis, 212, 251
SYMPHYLA, 486, 526
Synapta, 633, 640
Synccelidium, 136, 170

Tabanus, 520, 528
Tcenia, 153, 170
Tanais, 414, 424
Tanystylum, 463
TARDIGRADA, 466
Tealia, 113, 117
TECTIBRANCHIA, 313, 364
Tegenaria, 450, 458
Tfefca, 517, 527
TENTACULATA, 124, 126
Terebella, 213, 251
TEREBRANTIA, 527
Terebratulina, 271, 274
Teredo, 339, 365
Termes, 508, 526
Termites, 507
Tessera, 100, 116
TKSTICAUDINES, 269, 274
TETRABRANCHIA, 357, 365
Tetranychus, 454, 459
TETRAPNEUMONES, 452, 458
Tetratitemma. 167, 170
Textularia, 17, 38
Thalassema, 241, 252
TJtalassianthus , 114, 117

INDEX OF PROPER NAMES.

TJialassicolla, 18, 39
THALIACEA, 633, 640
Thaumatoerinvs, 543, 592
THECASOMATA, 314, 364
Theli/phonns, 446, 458
Theridium, 450, 458
THORACOSTRACA, 406, 424
Thrips, 510, 526
Thyone, 587, 593
THYSANOPTEHA, 509, 526
Thysanozoon, 139, 170
THYSANURA, 501, 526
Ticks, 453
Tinea, 516, 527
TRACHEATA, 469
Trachydermon, 289, 364
TRACHYMEDUS^E, 85, 116
TREMATODA, 143, 170
Tremoctopus, 359, 365
Tricenopliorus, 153, 170
Trtc/iaster, 569, 592
Trichina, 176, 182
TricliocepJialus, 177, 182
Trichodectes, 509, 526
Trichoplax, 63
TRICHOPTERA, 515, 527
TRICLADEA, 136, 169
Tristomum, 147, 170
TrocJwsplicera, 194, 200
Trochus, 305, 364
Trombidium, 453, 459
lubipora, 108, 117
Tubularia. 89, 116
TUBTJLARI.E. 87, 116
TUNICATA, 617

TURBELLARIA, 130, 169

rwrfto, 305, 364
Tyroglyphus, 453, 459

Unto, 339, 365
Urnatella, 256, 274
UROCHORDA, 617, 639

Vaginula, 316, 364
Vampyrella, 23, 39

, 517, 527
, 92, 116
Venus, 339, 365
Venus' girdle, 121
Vermilia, 209
Vespa, 518
yofcox, 29, 32, 39
Vortex, 135, 169
Vorticella. 33, 39

Waldheimia, 271, 274
Walking Stick, 504
Wasps, 518

Wheel-animalcule, 189
White ants. 507
Wood louse, 414

XIPHOSURA, 427
Toldia, 339, 365

ZOANTHE^E, 112, 117

Zoanthus, 112, 117

ZoOXANTHELLuE, 20

Zoroaster, 552, 592

INDEX OF SUBJECTS.

Accessory intestine, Cephalocorda,

605; Echinoidea, 579; Gephyrea,

239; Polychseta, 207
Aconous eyes, 473
Actinotrocha, 249
adrectal gland, 305
adhesive cells, 123, 131
alar muscles, 470
albumiuiparous gland, 312
alternation of generations, 60; Cesto-

da, 158; Hydromedusse, 96; Neraa-

toda, 176; Scyphornedusse, 103;

Trematoda, 148; Urochorda, 633
auiitosis, 9

ametabolic insects, 499
amoeboid motion, 15
amphiaster, 11
amphidiscs, 76
ampulla, 537
antennary gland, 383
apical plate, 213, 607
archenteron, 54
archicerebrum, 379
Aristotle's lantern, 578
arthrobraucliia, 410
ascidiozooid, 632
Ascon, 70
aster, 7
atrium, 596; Cephalochorda, 612; En-

teropueusta, 601 ; Pterobranchia,

599, 600; Urochorda, 622
atrium (genital), 134
auriculae, 573
Auricularia, 590
avicularia, 262
axial sinus, 538

Basement-membrane, 127
biogeuetic law, 143

Bipinnaria, 559

bivium, 572, 584

blastocoel, 52

blastopore, 54

blastula, 52

blood-vascular system, Arachnida, 437;
Cephalochorda, 611; Cephalopoda,
346; Chyetopoda, 206, 220; Crusta-
cea, 376; Enteropueusta, 603; Geph-
yrea, 238; Hirudiuea, 230; Mollusca,
278; Nemertina, 165; Phoronidae,
247; Tracheata, 470; Urochorda,
621; Xiphosura, 429

Bo jauus, organ of, 337

Brachiolaria, 559

branchial heart, 347

branchial skeleton, Cephalochorda,
613; Enteropneusta, 609

brown body, 267

brown canal, 616

bursa copulatrix, Nematoda, 174, 182;
Tracheata, 497; Turbellaria, 136,
138

byssus gland, 329

Calamistrum, 449
calcar, 192

calciferous glands, 220
calyptoblastic, 86
caryolymph, 6
caryoplasm, 5
cell, 4,

cell-division, 9
cellulose, 30, 619
cenogenetic, 143
central capsule, 19
ceutrolecilhal, 53
centrosome. 7, 51
cephalization, 369.

655

656

INDEX OF SUBJECTS.

cerata, 315

Cercaria, 150

cerci, 489

chambered organ, 546

chela, 373

ckelicerse, 429, 435

chiastoneurism, 296

chilaria, 430

chloragogue-cells, 219, 238

chlorophyll, Flagellata, 29, 30; Hy-

drarise, 83; lufusoria, 35; Porifera,

73

chordotonal organ, 495
chvomatin, 6
chromosome, 11
chrysalis, 500
cilia, 33
cilia-plates, 121
cirrus, 146, 155
cirri, Cephalochorda, 609; Crinoidea,

542; Myzostorneae, 244; Polychaeta,

204, 205
clavulae, 574
clitellum, 219, 228
cloaca, Nematoda, 175, 179; Rotifera,

192; Urochorda, 631
cnidocil, 77
cnidoblast, 77
coelenterou, 77
coelom, 57
coenen chyme, 108
ccenosarc, 79
Coenurus, 158
colony-formation, 5, 8; Anthozoa, 108,

111, 112, 114; Flagellata, 29; Hydro-

medusae, 78, 85, 87, 91; Polyzoa,

255; Rhizopoda, 21; Urochorda,

628

columella, 90, 107
complemental males, 401
conjugation, 24. 25, 31, 37
contractile vacuole, 15
corallum, 89
cormus, 41
costae, 107
coxal glands, Arachnida, 441; Xi-

pbosura, 432
cribellum, 449

crural glands, Insecta, 502; Myriop-

oda, 485, 487; Protracheata, 479.
crystalline style, 333
ctenidium, 278
Cuvierian organs, 588
cyathozooid, 632
Cyphonautes, 264
Cysticercoid, 158
Cysticercus, 158
cytode, 8
eytolymph, 4
cytoplasm, 4

Dactylozoid, 90

delaiiiinatiou, 55

Desor's larva, 167

deutovum, 456

development, Acanthocephala, 182;
Acariua, 456; Asteroidea, 559;
B"rachiopoda, 272: Cephalopoda,
363; Cestoda, 157; Crinoidea, 551;
Crustacea, 417; Echinoidea, 583;
Enteropueusta, 605; Gasteropoda,
319; Gephyrea, 242; Hirudinea,
237; Holothuroidea, 590; Hydio-
medusa3, 92; Insecta, 521; Nema-
toda, 176; Nemertina, 167; Oli-
gochaeta, 225; Ophiuroidea, 570;
Pelecypoda, 339; Pentastomidae,
463; Phorouidae, 249; Polychaata,
213; Polyzoa, 263; Porifera, 74;
Pycnogouida, 466; Scapbopoda,
324; Scyphornedusse, 103; Troma-
toda, 148; Turbellaria, 140; Uro-
chorda, 624; Xiphosura, 432

digestive gland, Arachuida, 437;
Braehiopoda, 271; Crustacea, 378;
Mollusca, 280; Rotifera, 192;
Xiphosura, 430

digestive system, Amphineura, 286;
Arachuida, 437; Braehiopoda, 270;
Cephalocorda, 612; Cephalopoda,
348; Chsetognatha, 187; Cha-topo-
da, 206, 220; Crustacea, 377; Di-
itophilus, 198; Echiuodera, 185;
Echinoderma, 5li9; Enteropueusta,
605; Gasteropoda, 300; Gastrotrieha,
196; Gephyrea, 238; Hirudinci,

INDEX OF SUBJECTS.

657

231; Mollusca, 279; Myzostomeae,
244; Nematoda, 174; Nemertiua,
162; Pelecypoda, 383; Pentastomi-
dae, 461; Phorouidae, 247; Polyzoa,
255; Pycuogouida, 465; Rotifera,
191; Scaphopoda, 323; Tardigrada,
467; Tracheata, 471; Trematoda,
144; Turbellaria, 133, 135, 136, 138;
Urochorda, 622; Xiphosura, 430

dimorphism, sexual, 193, 199, 241,
395, 496

dimorphism, seasonal, 501.

dissepiment, 107, 187, 202, 270

dissogony, 123

divisiou of labor, 85, 87, 91 (see also
polymorphism)

docoglossate dentition, 306

dorsal organ, 546

dorsal pore, 219

Echinococcus, 158
ectocyst, 255
ectoderm, 54
ectoplasm, 3
elaeoblast, 633
elytra, 490
embole, 54
encystment, 22, 36
endocyst, 255
endodenn, 54
endophragmal system, 375
endoplasm, 3
eudopodite, 373
eudosteruite, 429, 457
endostyle, Cephalocorda, 614, Uro-
chorda, 622
enteroccel, 57
ephippium, 391
EpJiyra, 103
epibole, 54
epipleural folds, 609
epipodite, 373
epipodium, 291
epistome, 247, 260. 597
epithelio-muscular cells, 80
Erichthus, 422
euconous eyes, 473
excretory system, Acanthocephala,

181; Arn phi ue ura, 287, 292; Arach-
nida, 437; Brachiopoda. 272; Cepha-
locorda, 615; Cephalopoda, 353;
Cestoda, 155; Chsetopoda, 210, 222;
Crustacea, 38o; DinopMlus, 199;
Echiuodera, 185; Enteropneusta,
605; Gasteropoda, 302; Gasterotri-
cha, 196; Gephyrea, 239; Hirudinea,
235; Mollusca, 283; Myzostomeae,
245 ; Nematoda, 175; Nemertiua,
164; Pelecypoda, 337; Phorouidaj,
249; Platyhelminthes, 129; Poly-
zoa, 257, 260; Pterobranchia, 599,
600; Rotifera, 193; Scaphopoda,324;
Tracheata, 474; Trematoda, 145;
Turbellaria, 135, 138; Urochorda,
623; Xiphosura, 432

exopodite, 373

eyes— Amphineura, 292 ; Arachnida,
438; Asteroidea, 559; Cepualochor-
da, 615; Cephalopoda, 351 ; Chaeto-
guatha, 188; Chaetopoda, 208; Crus-
tacea, 380; Gasteropoda, 301, 318;
Hirudinea, 234; Hydromedusae, 82;
Nemertina, 164; Pelecypoda, 335 ;
Pycnogonida, 465; Rolifera, 192;
Scyphomedusae, 99; Tracheata, 472;
Turbellaria, 131; Urochorda, 625;
Xiphosura, 431

Fascioles, 574
fat-body, 492
flagellum, 28
flame-cell, 129
follicle-cells, 46
fossa rhomboidalis, 615
funiculus, 259

Gasterozooid, 89
gastrula, 53
gemmation, 22
gemmules, 75
genital bursae, 561
germ-cell, 44
germ-layer, 54
Qlochidium, 339
Goette's larva, 141
gonopolyp, 85

658

INDEX OF SUBJECTS.

gonotheca, 86
green-gland, 383
gymnoblastic, 87

Hsemocyanin, 278, 377, 429

hemoglobin, 877, 538

hsemolymph, 206

balteres, 520

bead-kidney, 214, 222

beart — Ampbiueura, 289; Arachnida,
437; Cepbalopoda, 346; Crustacea,
376; Gasteropoda, 298; Pelecypoda,
332 ; Pycnogouida, 465; Tracheata,
470 ; Urocborda, 621 ; Xipbosura,
429

Hectocotylus, 356

heuiimetabolic insects, 500

hermaphroditism, 44

beterogouy, 60, 148, 498

histolysis, 456

bolonietabolic insects, 500

hook-gland, 462

hydranth, 79

hydrocaulus, 79

hydroccel, 535

hydrorhiza, 79

bydrotheca, 79

hypermetamorphosis, 513

bypodermis, 174

hypophysis cerebri, 615, 623

hypostorae, 79

Imago, 500
immigration, 55
individuality, 41
ink-bag, 349

interteutacular organ, 260
invagiuation, 54

Karyokinesis, 9
Keber's organ, 337

Lacunar system, 538

languets, 622

lateral -line organs, 210, 222

Laurer's canal, 146

lemnisci, 181

Leucon, 71

liniu, 6
liver, 614

lopbophore, 247, 254
lung- books, 436, 457

Madreporiform tubercle, 536

madreporite, 536

malpighian tubules — Amphipoda,
417; Arachnida, 437 ; Tracheata,
474

mantle, 268, 276, 621

manubrium, 81

mastax, 191

Medusa, 77, 80 ; Oaspedote, 81 ;
Gyniuopbtbalmatous, 82 ; Ocel-
late, 82, 89; Vesiculate, 82

megaloesthetes, 292

Megalopu, 422

mesendoderm, 68/132

meseuterial filaments, 99, 105

mesentery, 57, 104, 179, 187, 206, 270

mesoblasts, 57, 214, 225, 237

mesoderm, 56

mesoglrea, 68

mesopodiurn, 296

mesotborax, 488

metagenesis, 60

metamere, 41

metamerism, 43

metamorphosis of insects, 499

Metanauplius, 418

metapodium, 296

metathorax, 488

Metazoea, 421

micrtestheles, 292

micronucleus, 35, 37

microsomes, 4

mitosis, 9

Morren's gland, 220

Milller's larva, 141

muscular system — Acanthocephala,
180; Amphineura, 286, 289; Antho-
zoa, 166; Brachiopoda, 270; Cepha-
lochorda, 610 ; Cephalopoda, 348;
Cestoda, 154 ; Chsetoguatha, 187 ;
Chaetopoda, 205, 219 ; Crustacea,
375; DinopMlus, 198 ; Ecbinodera,
185; Echinoderma, 535; Gasteropo-

INDEX OF SUBJECTS.

659

da, 298; Gastrotricha, 196; Hirudi-
nea, 229; lusecta, 492; Nematoda,
174, 178; Pelecypoda, 332; Tra-
cheata, 469

myoccel, 610

myophanes, 35

Nauplius, 417

uectocalyx, 91

Needham's pouch, 355

nematocyst, 77

uephridia (see Excretory System)

nephroblasts, 226, 237

nervous system — Acautbocephala,
181; Amphiueura, 287, 290; Arach-
nida, 437 ; Brachiopoda, 271 ; Ce-
pbalochorda. 614 ; Cepbalopoda,
350 ; Cestoda, 155 ; Chaetognatha,
187 ; Chaetopoda, 208, 221 ; Crusta-
cea, 378; Cteuophora, 124; Dinophi-
lus, 199 ; Echinodera, 185 ; Echiuo-
derma, 539 ; Enteropneusta, 605 ;
Gasteropoda, 300; Gastrotricha, 197;
Gepbyrea, 239; Hirudiuea, 232; Hy-
dromedusse, 80; Mollusca, 281 ; My-
zostomese,244; Nematoda, 175, 179;
Nemertina, 163 ; Pelecypoda, 334;
Pentastomidac, 461 ; Pborouidae,247;
P]atybelminthes,128; Polyzoa,257;
Porifera, 73 ; Pterobrauchia, 599,
600 ; Pycnogonida, 46o ; Kotifera,
192 ; Scapbopoda, 323 ; Tracbeata,
471 ; Trematoda, 144; Turbellaria,
131, 132, 137; Urocborda, 623; Xi
pbosura, 430

rieuroblasts, 226, 237

nidamental gland, 312. 355

notochord -- Cephalochorda, 612 ;
Enteropueusta, 603; Pterobranchia,
599, 600; Urochorda, 625

nucleiu, 3

nucleolus, 6

nucleus, 5

nympb, 456

Odontoblasts, 280

olfactory organ — Cepbalopoda, 353 ;
Chaatognatha, 188 ; Mollusk, 282 ;

Scypbomedusae, 100 ; Tracbeata,
472; Xipbosura, 432

ommatidium • Chaelopoda, 209 ;
Crustacea, 381; lusecta, 472; Pele-
cypoda, 337; Xipbosura, 431

ocecia, 263

ootyp, 146

operculuru— Gasteropoda, 296; Poly-
zoa, 262; Scorpiouida, 442; Xipbo-
sura, 429

organ, 41

organs of Bojanus, 337 ; of Cuvier,
588; of Stewart, 576

orthoueurisin, 310

osculum, 69

osphradium, 283

otocysts — Cbaelopoda, 209 ; Crusta-
cea, 383 ; Cteuopbora, 122 ; Holo-
tburoidea, 589 ; Hydromedusse, 82,
84, 85, 86; Mollusca, 283 ; Scypbo-
rnedusse, 99; Turbellaria, 131, 132,
134; Urocborda, 625

ovary, 44

ovicell, 263

ovum, 44 ; Fertilization of, 49 ; Ma-
turation of, 46; Segmentation of
51

Pasdogenesis, 60, 499

pali, 107

palpi, 205

parapodia, 204, 313

paratrocb, 213

pareucbyma, 128

Parencbymelia, 55

parthenogenesis, 60, 498

paxillae, 553

pectines, 442

pedicellarise, 574

pedi palps, 435

Pentactifa, 591

pereiopod, 410

pericardial glands, 298, 337, 345

pericardium, 278, 437

perisarc, 79

peritoneal cells, 205

pbaospbere, 439

phosphorescence, — Crustacea, 382;

660

INDEX OF SUBJECTS.

Cystoflagellata, 31; Insecta, 492;
Urochorda, 632
phragrnocone, 360
Phyllosoma, 421
Pilidium, 168
pimiules, 542
plasome, 43
plastic, 3
pleopod, 410
pleurobraucbia, 410
Pluteus, 570
pneumatophore, 91
podobranchia, 410
polar bodies, 46, 49
Polian vesicle, 536
polyp, 76, 78
polypide, 255
polymorpbism, — Alcyonarise, 109;

Insecta, 497, 500; Polyzoa, 262

(see also division of labor)
polyspermy, 50
portal system, 612
proboscis, — Acautbocephala, 180;

Gasteropoda, 300; Myzostomese,

244; Nemertina, 163
proglottid, 154
proostracou, 360
propodiura, 296
prosopyle, 71
prostomiuin, 218
prothorax, 488
protoplasm, 2
protopodite, 573
prototrocb, 213
Protozoin, 419
pseudofllaria, 25
pseudouavicella, 25
pseudopodium, 14
pteuoglossate dentition, 308
pupa, 500

Rachiglossate dentition, 307

radula, 279

Redia, 149

regeneration, 59

reproduction, — Flagellata, 31; Infuso-
ria, 36; Metazoa, 42; Myxosporidia,
27; Rhizopoda, 20; Sporozoa, 25

reproduction, — by budding, 22, 58,
71, 83, 96, 114, 215, 256, 266, 627;
by conjugation, 24, 25, 31, 37; by
division, 21, 36, 58, 114, 227; by
spore - formation, 22, 25, 31, 36;
sexual, 44

reproductive system, — Acanthoceph-
ala, 181; Amphiueura, 287; An-
tbozoa, 105; Aracbnida, 441; Brach-
iopoda, 272; Cepualochorda, 617;
Cephalopoda, 354; Cestoda, 155;
Clisetoguatha, 188; Chaetopoda,
211, 223; Crustacea, 384; Cteno-
phora, 123; Dinophilux, 199; Echi-
uodera, 185; Echinodenna, 540;
Gasteropoda, 302, bi./.">, 311, 318;
Gastrotricha, 197; Gephyrca. ~40;
Hirudiuea, 235; Hydromedusoe, 83,
85, 86; Myzostomese, 246; Kema-
toda, 175, 179; Jmnertiua, 166;
Pelecypoda, 337; Peulastouiidne,
462; Platyheluiiuthes, 129; Poly-
zoa, 257, 260; Pterobrancbia, 600;
Pycnogonida, 466; Rotifcra, 193;
Scyphouiedusse, 98; Tracbeata, 474;
Trematoda, 146; Turbellaria, 133,
134, 135, 137, 139; Urochorda, 623;
Xipbosura, 432

ropugnatorial glands, 483

respiratory system. — Annelida, 204;
Arachnida, 436; Asteroidea, 554;
Cephalochorda, 612; Cephalopoda,
343; Crustacea, 375; Echiuoidea,
576; Enteropneusta, 601; Gaster-
opoda, 297, 317; Mollusca, 278;
Pelecypoda, 329; Pterobrauchia,
599, 600; Trnolieata, 470; Uro-
chorda, 622; XiphoMira, 429

respiratory trees, 240, 588

Rhabditis, 131

rhipidoglossate dentition, 306

rostellum, 154

Sacculi, 548

salivary glands, 280, 493

scapbognathite, 410

scbizocoel, 57

scolex, 153

INDEX OF SUBJECTS.

661

Scyphostoma, 103

seasonal dimorphism, 501

Semites, 574

septa, 107

set*, 204, 218

seta-sacs, 204

sexual dimorphism, 193, 199, 241,
395, 496

shell, — Ainphineura, 289; Brachiop-
oda, 2(59; Cephalopoda, 343, 357,
360; Gasteropoda, 303, 316; Pele-
cypoda, 327; Scaphopoda, 322

shell-gland, 383

siphon, 304, 327, 493, 579

siphoiioglyphe, 10(5

sipuucle, 358

skeleton, — Cephalochorda, 613; Ente-
ropneusta, 609

somatic cells, 44

somatic mesoderm, 206

spermatid, 48

spermatocyte, 48

spermatogenesis, 48

spermatogoue, 48

spermatophore, 355

spermatozoa, 44, 47

sphseridia, 574

spiuniug-glauds, 449

splanchnic mesoderm, 206

splauchnocoel, 610

spongiolin, 72

sporocyst, 149

statoblast, 261

Sterrula, 55

Stewart, organs of, 576

stigma, 29

stigmata,— Arachuida, 436; Tra-
cheata, 470; Urochorda, 622

stomatodseum, 105

stomodaeum, 213

stone-canal, 536
strbbila, 104, 154
subueural gland, 623
Sycon, 71
symbiosis, 20, 83
syucerebrtim, 379

Tsenioglossate dentition, 307

tapetum lucidum, 336, 440

telolecithal, 53

telson, 369

testis, 44

thorax, 488

Tiedemauu's vesicles, 557

tissue, 41

Tornaria, 606

toxiglossate dentition, 308

tracheae, — Arac-huida, 436; Isopoda,

414; Tracheata, 470
tracheal branchiae, 491
trichocyst, 35
trivium, 572, 584
Trochophore, 213
trophopolyp, 85, 91
tube-feet, 537
tympanal organ, 496
typhlosole, 220, 623

Veliger, 320
velum, 81, 320, 612
ventral plate, 226
vibracula, 263
vitellarium, 130. 155, 193

Water- vascular system, 535
wax-glands, 490
wings, 489, 522

Zoea, 420
zooecium, 255
zooxanthellse, 20

7 /