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ZOOLOGICAL ARTICLES

3RATORY

LIBRARY

WOODS HOLE, MASS.
W. H. 0. I.

ZOOLOGICAL ARTICLES

CONTRIBUTED TO THE "ENCYCLOPEDIA BRITANNICA"

BY

E. RAY LANKESTER, M.A., LL.D., F.R.S.

DEPUTY LINACRE PROFESSOR IN THE UNIVERSITY OF OXFORD, AND HOK. FELLOW OF EXETER COLLEGE
PRESIDENT OF THE MARINE BIOLOGICAL ASSOCIATION OF THE UNITED KINGDOM ;

HON. MEMBER OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY;
CORRESPONDING MEMBER OF THE ACADEMY OF SCIENCES OF PHILADELPHIA.

TO WHICH ARE ADDED KINDRED ARTICLES BY

W. JOHNSON SOLLAS, LL.D., F.R.S.

PROFESSOR OF GEOLOGY IN TRINITY COLLEGE, DUBLIN.

LUDWIG VON GRAFF, PH.D.

I'ROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF GRAZ, AUSTRIA.

A. A. AV. HUBRECHT, PH.D., LL.D.

PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF UTRECHT.

A. G. BOURNE, D.Sc.

PROFESSOR OF BIOLOGY IN THE PItESIDENCY COLLKGF-, MADRAS.

W. A. HERDMAN, D.Sc.

PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY COLLEGE, LIVERPOOL.

EDINBURGH: ADAM & CHAELES BLACK
NEW YORK: CHARLES SCRIBNER'S SONS

MDCCCXCI

I

PREFACE.

T HAVE been anxious to render the articles on various groups of Animals written by
me for the Encyclopaedia Britannica more readily accessible to the University
student than they are when bound up in the large volumes of that great work. The
Publishers have very kindly met my wishes in this respect by consenting to issue the
present reprint. With my articles on Protozoa, Hydrozoa, Mollusca, Polyzoa, and
Vertebrata, are here included, by the kind consent of the authors, the article on Sponges
by Professor Sollas, that on Planarians by Professor von Graff, that on Nemertines by
Professor Hubrecht, that on Rotifera by Professor Bourne, and that on Tunicata by
Professor Herdman. The volume thus forms a treatise on a considerable section of the
animal kingdom. Obviously it does not profess to be a complete handbook. Since the
articles are reprinted from the original plates, and issued at a low price, it has not been
possible to introduce any large additions into the text. Here and there an error, due to
oversight, has been corrected, and one or two new figures have been added, rendering the
work more complete. The chief additions are — the woodcut illustrating recent discoveries
concerning the Dinoflagellata (p. 37) ; the note by Professor Sollas on the classification of
Monaxonida (p. 39); the woodcut of Scyphomedusre from the Deep-Sea (p. 57); the
woodcut fig. 19 on p. 107, which replaces a similar but incorrect figure in the original
article, and the woodcut, fig. IA on p. 159, showing forms connecting the Eupolyzoa and
other Gephyrsea.

There are one or two matters, by way of addition to or correction of my own articles,
which this preface gives me the opportunity of mentioning.

In regard to the Protozoa, the reader should note that Professor Butschli's treatise in
Bronn's Thicrreich is now completed. He has rejected the classification of the Ciliata.
which we owe to Stein, and adopts the following — Branch A. Gymnostoma ( = Holotricha
with chitiuised pharynx, Prorodon, Trachelius, &c.) ; Branch B. Trichostoma ( = the
remaining Ciliata, all of which have the pharynx ciliated, if present). The Trichostoma
are divided into two classes — the Aspirotricha, and the Spirotricha. The Aspirotricha
are the rest of the Holotricha of Stein, not comprised in the Gymnostoma of this classifi-

VI PREFACE

cation. The Spirotricha are characterised by all possessing the adoral " heterotrichal " band
of large cilia ; they are divided into the sub-classes Heterotricha, Hypotricha, Peritricha,
and Oligotricha. The two first of these groups correspond with Stein's groups of the same
names, whilst the Peritricha of Stein are now divided into Peritricha and Oligotricha, the
latter sub-class being formed for such genera as Halteria, Strombidium, and Tintinnus. I
consider Blitschli's classification an improvement upon Stein's, with the doubtful exception
of the distinct position assigned to the Oligotricha.

In regard to the Hydrozoa, the most important additions to knowledge since the date
of the article are to be found in the large and richly-illustrated monographs by Haeckel
(System der Medusen, Jena, 1879-1880 ; "Report on the Deep-Sea Medusse," Challenger
Reports, vol. iv., 1882; "Reports on the Deep-Sea Siphonophora," Challenger Reports^
vol. xxviii., 1888), and in the remarkable researches of Weissman on the origin of the
sexual products (Enstelmng der Sexnahellen bei der Hydromedusen, Jena, 1883). The
student who takes in hand the actual examination of a specimen of Aurelia aurita by
aid of the description given of it in the article Hydrozoa, should also refer to the plates of
Ehrenberg's account of this animal (Physikalische Abhandlungen der Konigl. Alxtd. d.
Wissensch., Berlin, 1835), and Mr Min chin's brief but valuable paper on the enclosure of
the embryos in minute brood pouches formed by sacculation of the grooves of the oral
lobes (Proc. Zool. Soc., 1889, No. xxxix.).

If I were rewriting the article Mollusca, I should adopt the conclusion of my friend
and former pupil, Dr Paul Pelseueer, of Ghent, and remove the Pteropoda from association
with the Cephalopoda, not to maintain them as a distinct class, but to place them, as he
has done, among the Palliate or Tectibranchiate Opisthobranchiate Gastropoda, to which, it
seems, they bear the same relation as do the Natantia to the Azygobranchiate Streptoneura.
It appears that the Thecosomate Pteropods are nearly related to the Bullidse and Torna-
tellidse, whilst the Gymnosomate forms are derivable from the Aplysiidse. A careful study
of the nervous system convinced Dr Pelseneer that the sucker-bearing lobes of such
Gymnosomate Pteropods as Pneumodermon are really cephalic in nature, and innervated
from the cerebral ganglion, whilst the sucker-bearing lobes of the Cephalopoda are produc-
tions of the foot, and are convincingly demonstrated by Pelseneer (as maintained by me in
the article " Mollusca ") to be innervated by the pedal ganglia. The remarkable coincidence
in the Pteropoda and Cephalopoda of adoral appendages provided with suckers which had
been, to my mind, the chief ground for supposing a genetic relationship between these
two sets of forms, proves to be a case of homoplasy.1 It is, indeed, a very striking case of
the parallelism of genetically distinct organs. The whole of this question is ably treated
by Pelseneer in Part III. of his " Report on the Pteropoda," published in vol. xxiii. of the
Challenger Reports, 1888. The student of molluscan anatomy should not fail to read this

1 The reader is referred for an explanation of this term, and a discussion of the phenomena concerned, to my article
" On the use of the term Homology in Modern Zoology, and the distinction between Homogenetic and Homoplastic
Agreements," Ann. and May. Nat. Hint., 1870.

PREFACE. Vll

clear and well-illustrated discussion of the structure of the Pteropoda, and of the inferences
which may be drawn therefrom as to their affinities.

Iii regard to the article Polyzoa, I may mention that I think it preferable to make use
of the established term "Gephyrsea" in place of that introduced in this article, viz.,
" Podaxonia," The Gephyreea, then, include the Sternaspidomorpha, Echiuromorpha,
Sipunculomorpha, Phoronidomorpha, Polyzoa (Eupolyzoa of the article), Brachiopoda, and
Pterobranchia, Concerning the affinities of the first four of these classes with one another,
there is little doubt : as to the affinities of the last three with one another, and with the
first four we are still in a very uncertain state, and are likely to remain so for some time,
owing to the absence of satisfactory embryological data and the difficulty of obtaining such.

The subject matter of the article Vertebra ta is much more extensive than that of the
other chapters, and, owing to limited space, is treated in a much more general way than is
the case with the latter. In regard to the Craniata, the intention was to give only a sketch
of leading features which should be supplemented by the study of such works as Gegenbaur's
Comparative Anatomy, Wiedersheim's Anatomy of Vertebrates, and the special articles on
Fishes, Eeptiles, Birds, and Mammals, written for the Encyclopaedia by eminent autho-
rities on those groups. The treatment of the Cephalochorda (Arnphioxus) and its relations
to the Urochorda is a little more complete, and I therefore take occasion to refer the reader
to recent publications, in which our knowledge of this most interesting member of the
Vertebrate group has been largely extended. They are — Contributions to the Know-
ledge of Rhabdopleura and Amphioxus (ubique citata), by E. Ray Lankester (London:
J. & A. Churchill, 1889); "The Development of the Atrial Chamber of Amphioxus," by
E. Ray Lankester and Arthur Willey, in the Quart. Jour, of Mic. Sci., vol. xxxi., 1890 ;
"The Later Larval Development of Amphioxus," by Arthur Willey, B.Sc., in the same
Journal, vol. xxxii. ; and "The Excretory Organs of Amphioxus," by F. E. Weiss, B.Sc.,
also in the Quart. Jour, of Mic. Sci., vol. xxxi.

The article " Sponges," by Professor Sollas, contains the only summary account of the
Porifera written since the recent extraordinary advances in our knowledge of this group.
Its incorporation in the present volume cannot fail to be welcome to students. In
Professor Bourne's article on Rotifera are given the only extant woodcuts of the important
genus Pedalion. This most important form is not figured or discussed in any other general
treatise accessible to students. The articles on Planarians and Nemertiues, by Professor
von Graff and Professor Hubrecht respectively, are brief summaries of what is known,
written by the chief living authority on each group.

E. RAY LANKESTER.

OXFORD, December 1890.

CONTENTS.

PAGE

PROTOZOA, . . . !

SPONGES, 39

HYDROZOA, . 57

PLANARIANS, 77

NEMERTINES, 83

ROTIFERA, . 89

MOLLUSCA, . 95

POLYZOA, 169

VERTEBRATA, 173

TUNICATA, 185

ZOOLOGICAL ARTICLES.

PKOTOZOA

PROTOZOA is the name applied to the lowest grade of
the animal kingdom, and originated as a translation
of the German term "Urthiere." Whilst at first used
some forty years ago in a vague sense, without any strict
definition, so as to include on the one hand some simple
organisms which are now regarded as plants and on the
other some animals which are now assigned a higher place
in the animal series, the term has within the last twenty
years acquired a very clear signification.

The Protozoa are sharply and definitely distinguished
from all the rest of the animal kingdom, which are known
by the names " Metazoa " or " Enterozoa." They are
those animals which are structurally single "cells" or
single corpuscles of protoplasm, whereas the Enterozoa
consist of many such units arranged definitely (in the first
instance) in two layers — an endoderin or enteric cell-layer
and an ectoderm or deric cell-layer — around a central
cavity, the enteron or common digestive cavity, which is
in open communication with the exterior by a mouth.

The Protozoa are then essentially unicellular animals.
The individual or person in this grade of the animal king-
dom is a single cell ; and, although we find Protozoa which
consist of aggregates of such cells, and are entitled to be
called " multicellular," yet an examination of the details
of structure of these cell-aggregates and of their life-
history establishes the fact that the cohesion of the cells
in these instances is not an essential feature of the life of
such multicellular Protozoa but a secondary and non-essen-
tial arrangement. Like the budded " persons " forming,
when coherent to one another, undifferentiated " colonies "
among the Polyps and Corals, the coherent cells of a com-
pound Protozoon can be separated from one another and
live independently ; their cohesion has no economic signifi-
cance. Each cell is precisely the counterpart of its neigh-
bour ; there is no common life, no distribution of function
among special groups of the associated cells, and no cor-
responding differentiation of structure. As a contrast to
this we find even in the simplest Enterozoa that the cells
are functionally and structurally distinguishable into two
groups — those which line the enteron or digestive cavity
and those which form the outer body wall. The cells of
these two layers are not interchangeable ; they are funda-
mentally different in properties and structure from one
another. The individual Enterozoon is not a single cell ;
it is an aggregate of a higher order consisting essentially
of a digestive cavity around which two layers of cells are

disposed. The individual Protozoon is a single cell; a
number of these individuals may, as the result of the pro-
cess of fission (cell-division), remain in contact with one
another, but the compound individual which they thus
originate has not a strong character. The constituent
cells are still the more important individualities ; they
never become differentiated and grouped in distinct layers
differing from one another in properties and structure ;
they never become subordinated to the individuality of
the aggregate produced by their cohesion ; hence we are
justified in calling even these exceptional aggregated
Protozoa unicellular.

By far the larger number of Protozoa are absolutely
single isolated cells, which, whenever they duplicate them-
selves by that process of division common to these units
of structure (whether existing as isolated organisms or as
constituents of the tissues of plants or of animals), separ-
ate at once into two distinct individuals which move away
from one another and are thenceforward strangers.

Whilst it is easy to draw the line between the Protozoa
and the Enterozoa or Metazoa which lie above them, on
account of the perfectly definite differentiation of the cells
of the latter into two primary tissues, it is more difficult to
separate the Protozoa from the parallel group of unicellular
plants.

Theoretically there is no difficulty about this distinction.
There is no doubt that organisms present themselves to us
in two great series starting in both cases from simple
unicellular forms. The one series, the plants, can take up
the carbon, hydrogen, oxygen, and nitrogen necessary to
build up their growing protoplasm from mineral com-
pounds soluble in water, compounds which constitute the
resting stage of those elements in the present physical
conditions of our planet. Plants can take their nitrogen
in the form of ammonia or in the form of nitrates and
their carbon in the form of carbonic acid. Accordingly
they require no mouths, no digestive apparatus ; their
food being soluble in water and diffusible, they absorb at
all or many points of their surface. The spreading diffuse
form of plants is definitely related to this fact. On the
other hand the series of organisms which we distinguish
as animals cannot take the nitrogen, necessary to build up
their protoplasm, in a lower state of combination than it
presents in the class of compounds known as albumens ;
nor can they take carbon in a lower state of combination
i than it presents when united with hydrogen or with

A

PROTOZOA

hydrogen and oxygen to form fat, sugar, and starch.
Albumens and fats are not soluble in water and diffusible ;
they have to be seized by the animal in the condition
of more or less solid particles, and by chemical processes
superinduced in the living protoplasm of the animal by
the contact of these particles they are acted upon, chemic-
ally modified, and rendered diffusible. Hence the animal
is provided with a mouth and a digestive cavity, and with
organs of locomotion and prehension by which it may search
out and appropriate its scattered nutriment. Further the
albumens, fats, sugars, and starch which are the necessary
food of an animal are not found in nature excepting as
the products of the life of plants or of animals ; accord-
ingly all animals are in a certain sense parasitic upon
either plants or other animals. It would therefore seem
to be easy to draw the line between even the most minute
unicellular plants and the similarly minute unicellular
animals — assigning those which feed on the albumens, &c.,
of other organisms by means of a mouth and digestive
apparatus to the animal series, and those which can appro-
priate the elements of ammonia, nitrates, and carbonates
to the plants.

Such absolute distinctions lending themselves to sharp
definitions have, however, no place in the organic world ;
and this is found to be equally true whether we attempt
to categorically define smaller groups in the classification
of plants and animals or to indicate the boundaries of the
great primary division which those familiar names imply.
Closely allied to plants which are highly and specially
developed as plants, and feed exclusively upon ammonia,
nitrates, and carbonates, we find exceptionally modified
kinds which are known as " insectivorous plants " and are
provided with digestive cavities (the pitchers of pitcher-
plants, &c.), and actually feed by acting chemically upon
the albumens of insects which they catch in these diges-
tive receptacles. No one would entertain for a moment
the notion that these insectivorous plants should be con-
sidered as animals. The physiological definition separat-
ing plant from animal breaks down in their case ; but the
consideration of the probable history of their evolution as
indicated by their various details of structure suffices at
once to convince the most sceptical observer that they
actually belong to the vegetable line of descent or family
tree, though they have lost the leading physiological char-
acteristic which has dominated the structure of other
plants. In this extreme case it is made very obvious that
in grouping organisms as plants or as animals we are not
called upon to apply a definition but to consider the
multifarious evidences of historical evolution. And we
find in the case of the Protozoa and the Protophyta that
the same principle holds good, although, when dealing
with extremely simple forms, it becomes much more diffi-
cult to judge of the genetic relationship of an organism in
proportion as the number of detailed points of possible
agreement with and divergence from other forms to which
it may be supposed to be related are few.

The feeding of plants upon carbonic acid is invariably
accompanied by the presence of a peculiar green-colouring
matter — chlorophyll. In virtue of some direct or indirect
action of this chlorophyll the protoplasm of the plant is
enabled to seize the carbon of the mineral world — the car-
bon which has sunk to the lowest resting stage of combina-
tion— and to raise it into combination with hydrogen and
oxygen and ultimately with nitrogen. There are plants
which have no chlorophyll and are thus unable to feed
upon carbonic acid. They are none the less plants since
they agree closely with particular chlorophyll-bearing
plants in details of form and structure, mode of growth
and reproduction. A large series of these are termed
Fungi. Though unable to feed on carbonic acid, they do

not feed as do animals. They can take their carbon from
acetates and tartrates, which animals cannot do, and their
nitrogen from ammonia. Even when it is admitted that
some of these colourless plants, such as the Bacteria
(Schizomycetes), can act upon albumens so as to digest
them and thus nourish themselves, it is not reasonable to
place the Bacteria among animals, any more than it would
be reasonable so to place Nepenthes, Sarracenia, and
Drosera (insectivorous Phanerogams). For the structure
and mode of growth of the Bacteria is like that of well-
known chlorophylligerous minute Algaj from which they
undoubtedly differ only in having secondarily acquired
this peculiar mode of nutrition, distinct from that which
has dominated and determined the typical structure of
plants.

So we find in a less striking series of instances amongst
animals that here and there the nutritional arrangements
which we have no hesitation in affirming to be the leading
characteristic of animals, and to have directly and perhaps
solely determined the great structural features of the
animal line of descent, are largely modified or even alto-
gether revolutionized. The green Hydra, the freshwater
Sponge, and some Planarian worms produce chlorophyll
corpuscles in the protoplasm of their tissues just as green
plants do, and are able in consequence to do what animals
usually cannot do — namely, feed upon carbonic acid. The
possibilities of the protoplasm of the plant and of the
animal are, we are thus reminded, the same. The fact
that characteristically and typically plant protoplasm ex-
hibits one mode of activity and animal protoplasm another
does not prevent the protoplasm of even a highly developed
plant from asserting itself in the animal direction, or of a
thoroughly characterized animal, such as the green Hydra,
from putting forth its chlorophylligenous powers as though
it belonged to a plant.

Hence it is not surprising that we find among the
Protozoa, notwithstanding that they are characterized by
the animal method of nutrition and their forms determined
by the exigencies of that method, occasional instances of
partial vegetable nutrition such as is implied by the deve-
lopment of chlorophyll in the protoplasm of a few members
of the group. It would not be inconsistent with what is
observed in other groups should we find that there are
some unicellular organisms which must, on account of
their structural resemblances to other organisms, be con-
sidered as Protozoa and yet have absolutely given up alto-
gether the animal mode of nutrition (by the ingestion of
solid albumens) and have acquired the vegetable mode of
absorbing ammonia, nitrates, and carbonic acid. Experi-
ment in this matter is extremely difficult, but such " veget-
able" or "holophytic nutrition " appears to obtain in the
case of many of the green Flagellata, of the Dinoflagellata,
and possibly of other Protozoa.

On the other hand there is no doubt that we may fall
into an error in including in the animal line of descent all
unicellular organisms which nourish themselves by the
inception of solid nutriment. It is conceivable that some
of these are exceptional creophagous Protophytes parallel
at a lower level of structure to the insectivorous Phanero-
gams. In all cases we have to balance the whole of the
evidence and to consider probabilities as indicated by a
widely-reaching consideration of numerous facts.

The mere automatic motility of unicellular organisms
was at one time considered sufficient indication that such
organisms were animals rather than plants. We now know
that not only are the male reproductive cells of ferns and
similar plants propelled by vibratile protoplasm, but such
locomotive particles are recognized as common products
(" swarm-spores " and " zoospores ") of the lowest plants.

The danger of dogmatizing erroneously in distinguish-

PROTOZOA

ing Protozoa from Protophyta, and the insuperable diffi-
culty in really accomplishing the feat satisfactorily, has led
at various times to the suggestion that the effort should be
abandoned and a group constituted confessedly containing
both unicellular plants and unicellular animals and those
organisms which may be one or the other. Hacckel has
proposed to call this group the Protista (I).1 On the
whole, it is more satisfactory to make the attempt to dis-
criminate those unicellular forms which belong to the
animal line of descent from those belonging to the veget-
able line. It is, after all, not a matter of much conse-
quence if the botanist should mistakenly claim a few
Protozoa as plants and the zoologist a few Protophyta
as animals. The evil which we have to avoid is that some
small group of unattractive character should be rejected
both by botanist and zoologist and thus our knowledge of
it should unduly lag. Bearing this in mind the zoologist
should accord recognition as Protozoa to as wide a range
of unicellular organisms as he can without doing violence
to his conception., of probability.

A very interesting and very difficult subject of speculation forces
itself on our attention when we attempt to draw the line between
the lowest plants and the lowest animals, and even comes again
before us when we pass in review the different forms of Protozoa.

That subject is the nature of the first protoplasm which was
evolved from not-living matter on the earth's surface. Was that
first protoplasm more like animal or more like vegetable proto-
plasm as we know it to-day ? By what steps was it brought into
existence ?

Briefly stated the present writer's view is that the earliest proto-
plasm did not possess chlorophyll and therefore did not possess the
power of feeding on carbonic acid. A conceivable state of things
is that a vast amount of albuminoids and other such compounds
had been brought into existence by those processes which cul-
minated in the development of the first protoplasm, and it seems
therefore likely enough that the first protoplasm f«l upon these
antecedent steps in its own evolution just as animals feed on
organic compounds at the present day, more especially as the
large creeping plasmodia of some Mycetozoa feed on vegetable
refuse. It indeed seems not at all improbable that, apart from their
elaborate fructification, the Mycetozoa represent more closely than
any other living forms the original ancestors of the whole organic
world. At subsequent stages in the history of this archaic living
matter chlorophyll was evolved and the power of taking carbon
from carbonic acid. The "green" plants were rendered possible
by the evolution of chlorophyll, but through what ancestral forms
they took origin or whether more than once, i.e., by more than
one branch, it is difficult even to guess. The green Flagellate Pro-
tozoa (Volvocineaj) certainly furnish a connecting point by which
it is possible to link on the pedigree of green plants to the primi-
tive protoplasm ; it is noteworthy that they cannot be considered
as very primitive and are indeed highly specialized forms as com-
pared with the naked protoplasm of the Mycetozoou's plasmodium.

Thus then we are led to entertain the paradox that though the
animal is dependent on the plant for its food yet the animal
preceded the plant in evolution, and we look among the lower
Protozoa and not among the lower Protophyta for the nearest
representatives of that first protoplasm which was the result of a
long and gradual evolution of chemical structure and the starting
point of the development of organic form.

The Protozoan Cell-Individual compared with the Typical
Cell of Animal and Vegetable Tissues.

MORPHOLOGY.

The Protozoon individual is a single corpuscle of proto-
plasm, varying in size when adult from less than the
TU1ujJth of an inch in diameter (some Sporozoa and Flagel-
lata) up to a diameter of an inch (Nummulites), and even
much larger size in the plasmodia of Mycetozoa. The sub-
stance of the Protozoa exhibits the same general properties
— irritability, movement, assimilation, growth, and division
— and the same irremediable chemical alteration as the result
of exposure to a moderate heat, which are observed in
the protoplasm constituting the corpuscles known as cells
which build up the tissues of the larger animals and

1 These numbers refer to the bibliography at p. 866.

plants. There is therefore no longer any occasion to make
use of the word " sarcode " which before this identity was
established was very usefully applied by Dujardin (2) to
the substance which mainly forms the bodies of the
Protozoa. Like the protoplasm which constitutes the
" cells " of the Enterozoa and of the higher plants, that
of the Protozoon body is capable of producing, by chemical
processes which take place in its substance (over and above
those related merely to its nutrition), a variety of distinct
chemical compounds, which may form a deposit in or
beyond the superficial protoplasm of the corpuscle or may
accumulate centrally. These products are therefore either
ectoplastic or entoplastic. The chemical capacities of
protoplasm thus exhibited are very diverse, ranging from
the production of a denser variety of protoplasm, probably
as the result of dehydration, such as we see in the nucleus
and in the cortical substance of many cells, to the chemical
separation and deposition of membranes of pure chitin or
of cellulose or of shells of pure calcium carbonate or quasi-
crystalline needles of silica.

NUCLEUS. — The nucleus is probably universally present in
the Protozoon cell, although it may have a very simple struc-
ture and be of very small size in some cases. The presence
of a nucleus has recently been demonstrated by means of
appropriate staining reagents in some Protozoa (shell-
bearing Reticularia or Foraminifera and many Mycetozoa)
where it had been supposed to be wanting, but we are not
yet justified in concluding absolutely that there are not
some few Protozoa in which this central differentiation of
the protoplasm does not exist ; it is also a fact that in the
young forms of some Protozoa which result from the
breaking up of the body of the parent into many small
" spores " there is often no nucleus present.

In contrast to this it is the fact that the cells which
build up the tissues of the Enterozoa are all derived from
the division of a nucleated egg-cell and the repeated
division of its nucleated products, and are invariably
nucleated. The same is true of tissue-forming plants, —
though there are a few of the lowest plants, such as the
Bacteria, the protoplasm of which presents no nucleus. In
spite of recent statements (3) it cannot be asserted that
the cells or protoplasmic corpuscles of the yeast-plant
(Saccharomyces) and of the hyphs of many simple moulds
contain a true nucleus. We are here brought to the
question " What is a true nucleus 1 " The nucleus which
is handed on from the egg-cell of higher plants and
Enterozoa to the cells derived from it by fission has lately
been shown to possess in a wide variety of instances such
very striking characteristics that we may well question
whether every more or less distinctly outlined mass or
spherule of protoplasm which can be brought into view by
colouring or other reagents, within the protoplasmic body
of a Protozoon or a Protophyte, is necessarily to be con-
sidered as quite the same thing as the nucleus of tissue-
forming egg-cell-derived cells.

Kesearches, chiefly due to Flemming (4), have shown
that the nucleus in very many tissues of higher plants
and animals consists of a capsule containing a plasma of
" achromatin " not deeply stained by reagents, ramifying
in which is a reticulum of "chromatin" consisting of fibres
which readily take a deep stain (Fig. I., A). Further it is
demonstrated that, when the cell is about to divide into
two, definite and very remarkable movements take place
in the nucleus, resulting in the disappearance of the
capsule and in an arrangement of its fibres first in the
form of a wreath (Fig. I., D) and subsequently (by the
breaking of the loops formed by the fibres) in the form of a
star (E). A further movement within the nucleus leads to
an arrangement of the broken loops in two groups (F), the
position of the open ends of the broken loops being reversed

PROTOZOA

as compared with what previously obtained. Now the
two groups diverge, and in many cases a striated appear-
ance of the achromatin substance between the two groups
of loops of chromatin is observable (H). In some cases
(especially egg-cells) this striated arrangement of the
achromatin substance precedes the separation of the loops
(G). The striated achromatin is then termed a " nucleus-
spindle," and the group of chromatin loops (Fig. I., G, a)

FIG. I.— Karyokinesis of a typical tissue-cell (epithelium of Salamander) alter
Flemming and Klein. The series from A to I represent the successive stag es
in the movement of the chromatin fibres during division, excepting G. which
represents the " nucleus-spindle "of an egg-cell. A, resting nucleus; D, wreath-
form; E, single star, the loops of the wreath being broken; F, separation of the
star into two groups of U-shaped fibres; H", diaster or double star; I, comple-
tion of the cell-division and formation of two resting nuclei. In G the
chromatin fibres are marked a, and correspond to the phase shown in F ; they
are in this case called the "equatorial plate"; b, achromatin fibres forming the
nucleus-spindle; c, granules of the cell-protoplasm forming a "polar star."
Such a polar star is seen at each end of the nucleus-spindle, and is not to be
confused with the diaster H.

is known as "the equatorial plate." At each end of
the nucleus-spindle in these cases there is often seen a
star consisting of granules belonging to the general proto-
plasm of the cell (G, c). These are known as " polar stars."
After the separation of the two sets of loops (H) the
protoplasm of the general substance of the cell becomes
constricted, and division occurs, so as to include a group of
chromatin loops in each of the two fission products. Each
of these then rearranges itself together with the associated
achromatin into a nucleus such as was present in the
mother-cell to commence with. This phenomenon is termed
" karyokinesis," and has been observed, as stated above,
in a large variety of cells constituting tissues in the higher
animals and plants.

There is a tendency among histologists to assume that
this process is carried out in all its details in the division
of all cells in the higher plants and animals, and accordingly
to assume that the structural differentiation of achromatin
plasma and chromatin nucleus-fibres exists in the normal
nucleus of every such cell. If this be true, it is necessary
to note very distinctly that the nucleus of the Protozoon
cell-individual by no means conforms universally to this
model. As will be seen in the sequel, we find cases in
which a close approach is made by the nucleus of Protozoa
to this structure and to this definite series of movements
during division (Fig. VIII. 3 to 12, and Fig. XXV.); and
a knowledge of these phenomena has thrown light upon
some appearances (conjugation of the Ciliata) which were
previously misinterpreted. But there are Protozoa with a
deeply-placed nucleus-like structure which does not pre-
sent the typical structure above described nor the typical
changes during division, but in which on the contrary the
nucleus is a very simple homogeneous corpuscle or vesicle
of more readily stainable protoplasm.

The difficulties of observation in this matter are great,
and it is proportionately rash to generalize ; but it appears
that we are justified at the present moment in asserting
that not all the cells even of higher plants and animals

exhibit in full detail the structure and movement of the
typical cell-nucleus above figured and described; and accord-
ingly the fact that such structure and movement cannot
always be detected in the Protozoon cell-nucleus must not
be regarded as either an isolated phenomenon peculiar to
such Protozoon cells, nor must it be concluded that we have
only to improve our means of analysis and observation in
order to detect this particular structure in all nuclei. It
seems quite possible and even probable that nuclei may
vary in these details and yet be true nuclei. Some nuclei
which are observed in Protozoon cell-bodies may be regarded
as being at a lower stage of differentiation and specializa-
tion than are those of the epithelial and embryonic cells
of higher animals which exhibit typical karyokinesis.
Others on the contrary, such as the nuclei of some
liadiolaria (ride infra), are probably to be regarded as
more highly developed than any tissue cell-nuclei, and will
be found by further study to present special phenomena
peculiar to themselves. In some of the highest Protozoa
(the Ciliata) it has lately been shown that the nucleus
may have no existence as such, but is actually dispersed
throughout the protoplasm in the form of fine particles of
chromatin-substance which stain on treatment with car-
mine but are in life invisible (84). This diffuse condition
of the nuclear matter has no parallel, at present known, in
tissue-cells, and curiously enough occurs in certain genera
of Ciliata whilst in others closely allied to them a solid
single nucleus is found. The new results of histological
research have necessitated a careful study of the nucleus
in its various stages of growth and division in the cell-
bodies of Protozoa and a comparison of the features there
observed with those established as " typical " in tissue-cells.
Accordingly we have placed the figure and explanation of
the typical cell-nucleus in the first place in this article for
subsequent reference and comparison.

CORTICAL SUBSTANCE. — The superficial protoplasm of
an embryonic cell of an Enterozoon in the course of its
development into a muscular cell undergoes a change
which is paralleled in many Protozoa. The cortical layer
becomes dense and highly refringent as compared with the
more liquid and granular medullary substance. Probably
this is essentially a change in the degree of hydration of
the protoplasm itself, although it may be accompanied by
the deposition of metamorphic products of the protoplasm
which are not chemically to be regarded as protoplasm.
The differentiation of this cortical substance (which is not
a frequent or striking phenomenon in tissue-cells) may be
regarded as an ectoplastic (i.e., peripheral) modification
of the protoplasm, comparable to the entoplastic (central)
modification which produces a nucleus.

The formation of " cortical substance " in the Protozoa
furnishes the basis for the most important division into
lower and higher forms, in this assemblage of simplest
animals. A large number (the Gymnomyxa) form no
cortical substance ; their protoplasm is practically (except-
ing the nucleus) of the same character throughout. A
nearly equally large number (the Corticata) develop a
complete cortical layer of denser protoplasm, which is
distinct from the deeper medullary protoplasm. This
layer is permanent, and gives to the body a definite shape
and entails physiological consequences of great moment.
The cortical protoplasm may exhibit further specialization of
structure in connexion with contractile functions (muscular).

ECTOPLASTIC PRODUCTS CHEMICALLY DISTINCT FROM
PROTOPLASM. — The protoplasm of all cells may throw down
as a molecular precipitate distinct from itself chemical
compounds, such as chitin and horny matter and other
nitrogenized bodies, or again non-nitrogenous compounds,
such as cellulose. Very usually these substances are
deposited not external to but in the superficial proto-

PROTOZOA

plasm. They are then spoken of as cell-cuticle if the cell
bounds the free surface of a tissue, or as matrix or cell-wall
in other cases. The Protozoon cell-body frequently forms
such "cuticles," sometimes of the most delicate and
evanescent character (as in some Amcebse), at other times
thicker and more permanent. They may give indications
(though proper chemical examination is difficult) of being
allied in composition to chitin or gelatin, in other instances
to cellulose, which is rare in animals and usual in plants.
These cuticular deposits may be absent, or may form thin
envelopes or in other cases jelly-like substance intimately
mixed with the protoplasm (Radiolaria). They may take
the form of hooks, tubercles, or long spines, in their
older and more peripheral parts free from permeation by
protoplasm, though deeply formed in and interpenetrated
by it. Such pellicles and cuticles, the deeper layers (if not
the whole) of which are permeated by protoplasm, lead
insensibly to another category of ectoplastic products in
which the material produced by the protoplasm is separated
from it and can be detached from or deserted by the proto-
plasm without any rupture of the latter. These are —

Shells and Cysts. — Such separable investments are
formed by the cell-bodies of many Protozoa, a phenomenon
not exhibited by tissue-cells. Even the cell-walls of the
protoplasmic corpuscles of plant tissues are permeated by
that protoplasm, and could not be stripped off without
rupture of the protoplasm. The shell and the cyst of the
Protozoon are, on the contrary, quite free from the cell-
protoplasm. The shell may be of soft chitin-like sub-
stance (Gromia, &c.), of cellulose (Labyrintlmla, Dino-
flagellata), of calcium carbonate (Globigerina, etc.), or of
silica (Clathrulina, Codonella). The term "cyst" is ap-
plied to completely closed investments (" shells " having
one or more apertures), which are temporarily produced
either as a protection against adverse external conditions
or during the breaking up of the parent-cell into spores.
Such cysts are usually horny.

Stalks. — By a localization of the products of ectoplastic
activity the Protozoon cell can produce a fibre or stalk of
ever-increasing length, comparable to the seta of a
Chsetopod worm produced on the surface of a single cell.

ENTOPLASTIC PRODUCTS DISTINCT FROM PROTOPLASM. —
Without pausing here to discuss the nature of the finest
granules which are embedded as a dust-cloud in the hyaline
matrix of the purest protoplasm alike of Protozoa and of
the cells of higher animals and plants, and leaving aside
the discussion of the generalization that all protoplasm
presents a reticular structure, denser trabeculte of extreme
minuteness traversing more liquid material, it is intended
here merely to point to some of the coarser features of
structure and chemical differentiation, characteristic of the
cell-body of Protozoa.

With regard to the ultimate reticular structure of
protoplasm it will suffice to state that such structure has
been shown to obtain in not a few instances (e.g., Lith-
amceba, Fig. V.), whilst in most Protozoa the methods of
microscopy at present applied have not yielded evidence
of it, although it is not improbable that a recticular
differentiation of the general protoplasm similar to that of
the nucleus may be found to exist in all cells.

Most vegetable cells and many cells of animal tissues
exhibit vacuolation of the protoplasm ; i.e., large spaces are
present in the protoplasm occupied by a liquid which is not
protoplasm and is little more than water with diffusible
salts in solution. Such vacuoles are common in Protozoa.
They are either permanent, gastric, or contractile.

Permanent vacuoles containing a watery fluid are some-
times so abundant as to give the protoplasm a "bubbly"
structure (Thalamophora, Radiolaria, etc.), or may merely
give to it a trabecular character (Trachelius, Fig. XXIV.

14, and Noctiluca, Fig. XXVI. 18). Such vacuoles may
contain other matters than water, namely, special chemical
secretions of the protoplasm. Of this nature are oil-drops,
and from these we are led to those deposits within the
cell-protoplasm which are of solid consistence (see below).

Gastric vacuoles occur in the protoplasm of most Proto-
zoa in consequence of the taking in of a certain quantity
of water with each solid particle of food, such ingestion of
solid food-particles being a characteristic process bound up
with their animal nature.

Contractile vacuoles are frequently but not universally
observed in the protoplasm of Protozoa. They are not
observed in the protoplasm of tissue-cells. The contrac-
tile vacuole whilst under observation may be seen to
burst, breaking the surface of the Protozoon and discharg-
ing its liquid contents to the exterior ; its walls, formed of
undifferentiated protoplasm, then collapse and fuse. After
a short interval it re-forms by slow accumulation of liquid
at the same or a neighbouring spot in the protoplasm.
The liquid is separated at this point by an active process
taking place in the protoplasm which probably is of an
excretory nature, the separated water carrying with it
nitrogenous waste-products. A similar active formation
of vacuoles containing fluid is observed in a few instances
(Arcella, some Amosbas) where the protoplasm separates a
gas instead of liquid, and the gas vacuole so produced ap-
pears to serve a hydrostatic function.

Corpuscular and Amorphous Entoplastic Solids. — Con-
cretions of undetermined nature are occasionally formed
within the protoplasm of Protozoon cells, as are starch and
nitrogenized concretions in tissue-cells (Lithamceba, Fig.
V. cone.). But the most important corpuscular products
after the nucleus, which we have already discussed, are
chlorophyll corpuscles. These are (as in plants) concavo-
convex or spherical corpuscles of dense protoplasm resem-
bling that of the nucleus, which are impregnated superfi-
cially with the green-coloured substance known as chloro-
phyll. They multiply by fission, usually tetraschistic,
independently of the general protoplasm. They occur in
representatives of many different groups of Protozoa (Pro-
teomyxa, Heliozoa, Labyrinthulidea, Flagellata, Ciliata),
but are confined to a few species. Similar corpuscles or
band-like structures coloured by other pigments are occa-
sionally met with (Dinoflagellata).

Recently it has been maintained (Brandt, 5) that the
chlorophyll corpuscles of Protozoa and other animals are
parasitic Algae. But, though it is true that parasitic Algre
occur in animal tissues, and that probably this is the nature
of the yellow cells of Radiolaria, yet there seems to be no
more justification for regarding the chlorophyll corpuscles
of animal tissue-cells and of Protozoa as parasites than
there is for so regarding the chlorophyll corpuscles of the
leaves of an ordinary green plant.

Corpuscles of starch, paramylum, and other amyloid
substances are commonly formed in the Flagellata, whose
nutrition is to a large extent plant-like.

Entoplastic Fibres. — A fibrillation of the protoplasm of
the Protozoon cell-body may be produced by differentia-
tion of less and more dense tracts of the protoplasm itself.
But as distinct from this we find horny fibres occasionally
produced within the protoplasm (Heliozoa) having definite
skeletal functions. The threads produced in little cavities
in the superficial protoplasm of many Ciliate Protozoa,
known as trichocysts, may be mentioned here.

Entoplastic Spicules. m — Needle-like bodies consisting
either of silica or of a horny substance (acanthin) are
produced in the protoplasm of many Protozoa (Heliozoa,
Radiolaria). These are known as spicules ; they may be
free or held together in groups and arranged either radially
or tangentially in reference to the more or less spherical

6

PROTOZOA

body of the Protozoon. A similar production of siliceous
spicules is observed in the tissue-cells of Sponges. Crys-
tals of various chemical nature (silica, calcium carbonate,
oxalate, <fcc.) are also frequently deposited in the protoplasm
of the Protozoa, differing essentially from spicules in that
their shape is due purely to crystallization.

GENERAL FORM OF THE PROTOZOON CELL. — Those Proto-
zoa which have not a differentiated cortical substance, and
are known as Gymnomyxa, present very generally an
extreme irregularity of contour. Their protoplasm, being
liquid rather than viscous, flows into the most irregular
shapes. Their fundamental form when at rest is in many
cases that of the sphere ; others are discoidal or may be
monaxial, that is to say, show a differentiation of one
region or " end " of the body from the other. Frequently
the protoplasm is drawn out into long threads or filaments
which radiate uniformly from all parts of the spherical or
discoidal cell-body or originate from one region to the
exclusion of other parts of the surface.

These non-corticate Protozoa can take solid particles of
food into their protoplasm, there to be digested in an
extemporized "gastric vacuole," at any part or most parts
of their superficies. They have no permanent cell-mouth
leading into the soft protoplasm since that soft protoplasm
is everywhere freely exposed.

The corticate Protozoa have (with the exception of some
parasites) one, and in the Acinetaria more than one, de-
finite aperture in the cortical substance leading into the
softer medullary protoplasm. This is the cell-mouth, —
morphologically as distinct from the mouth of an Entero-
zoon as is the hole in a drain pipe from the front door of
a house, but physiologically subserving the same distinc-
tively animal function as does the mouth of multicellular
animals. The general form of the body is in these Proto-
zoa oblong, with either monaxial symmetry, when the
mouth is terminal, or bilateral symmetry, when the body
is oblong and flattened and the mouth is towards one end
of what becomes by its presence the " ventral " surface.
Though the protoplasm is not nakedly exposed in irregular
lobes and long filaments in these corticate Protozoa so as
to pick up at all points such food-particles as may fall in
its way, yet the protoplasm does in most Corticata project
in one or more peculiarly modified fine hair-like processes
from the otherwise smooth surface of the cell-body.
These processes are vibratile. cilia, identical in character
with the vibratile cilia of epithelial tissue-cells of Entero-
zoa. They are essentially locomotor and current-produc-
ing (therefore prehensile) organs, and, whilst unable to
ingest solid food-particles themselves, serve to propel the
organism in search of food and to bring food into the cell-
mouth by the currents which they excite. Either a single
vibratile filament is present, when it is called a flagellum,
or a row or many rows of cilia are developed.

Constituent cells of the Enterozoa are well known which
closely resemble some of the Gymnomyxa or non-corticate
Protozoa in their general form. These are the colourless
blood corpuscles or lymph corpuscles or phagocytes (Mecz-
nikow, 6) which float freely in the blood and ingest solid
particles at any part of their surface as do non-corticated
Protozoa ; they exhibit a similar irregularity and muta-
bility of outline, and actually digest the particles which
they take in. The endodermal digestive cells of some
Enterozoa (Coslentera and Planarians) are also naked proto-
plasmic corpuscles and can take in solid food-particles.

No tissue-cells are known which present any close
parallel to the mouth-bearing corticate Protozoa. The
differentiation of the structure of a single cell has in these
forms reached a very high degree, which it is not surpris-
ing to find without parallel among the units which build
up the individual of a higher order known as an Entfiro-

zoon. Cilia are developed on such cell-units (ciliated
epithelium), but not used for the introduction of food-
particles into the cell. In rare cases (the ciliated " pots "
of the vascular fluid of Sipunculus) they act so as to freely
propel the ciliated cell through the liquid " blood " of the
Enterozoon, as the cilia of a Protozoon propel it through
water. An aperture in the cortical substance (or in
the cuticular product) of a tissue-cell is sometimes to be
observed, but is never (?) used for the ingestion of food-
particles. Such an aperture occurs in unicellular glands,
where it serves as the outlet of the secretion.

PHYSIOLOGY.

Hution. — As has just been hinted, the movement of
protoplasm, which in the tissue -cells of Enterozoa and
higher plants is combined and directed so as to produce
effects in relation to the whole organism built up of
countless cells, is seen in the Protozoa in a different
relation, namely, as subserving the needs of the individual
cell of which the moving protoplasm is the main sub-
stance. The phenomena known in tissue-cells as " stream-
ing" (e.g., in the cells of the hairs of Tradescantia),
as local contraction and change of form (e.g., in the
corpuscles of the cornea), as muscular contraction, and as
ciliary movement are all exhibited by the protoplasm of
the cell-body of Protozoa, with more or less constancy,
and are intimately related to the processes of hunting,
seizing, and ingesting food, and of the intercourse of the
individuals of a species with one another and their evasion
of hostile agencies. Granule streaming and the implied
movement of currents in the protoplasm are seen in the
filamentous protoplasm of the Heliozoa, Radiolaria, Reti-
cularia, and Noctiluca, and in the cyclosis of the gastric
vacuoles of Ciliata. Local contraction and change of form
is seen best in the Amcebze and some Flagellata, where it
results in locomotion. Definite muscular contraction is
exhibited by the protoplasmic band in the stalk of Vorti-
cella, by the leg-like processes of the Hypotrichous Ciliata,
and by the cortical substance of some large Ciliata. Cili-
ary movement ranging from the vibration of filaments of
protoplasm temporarily evolved, up to the rhythmic beat
of groups of specialized cilia, is observed in all groups of
Protozoa in the young condition if not in the adult, and
special varieties of ciliary movement and of cilia-like
organs will be noted below. For an account of the con-
ditions and character of protoplasmic movement generally
which cannot be discussed in the present article the reader
is referred to Engelmann (7).

The protoplasm of the cell-body of the Protozoa is drawn
out into lobes and threads which are motile and are used
as locomotive and prehensile organs. These processes are
of two kinds, which are not present on the same cell and
are not capable of transmutation, though there are excep-
tions to both of these statements. The one kind are
termed " pseudopodia," and are either lobose or filamentous
or branched and even reticular (Figs. IV.and IX.). The Pro-
tozoa which exhibit them are sometimes termed Myxopods.
The other kind are cilia and flagella, and are simple threads
which are alternately bent and straightened almost inces-
santly during the life of the organism. These Protozoa
are termed Mastigopods. Whilst the cilia and flagella are
permanent organs, the pseudopodia vary greatly in char-
acter ; they are in some cases rapidly expanded and with-
drawn in irregular form, and can hardly be said to be more
than lobose protuberances of the flowing moving mass of
protoplasm. In other cases they are comparatively per-
manent stiff threads of protoplasm which can be contracted
and can fuse with one another but rarely do so (Heliozoa,
Eadiolaria). Between these extreme forms of "pseudo-
podia " there are numerous intermediate varieties, and the

PROTOZOA

whole protoplasmic body of the Protozoon may even
assume the form of a slowly changing network of threads
of greater or less tenuity (Chlamydomyxa, Fig. VI.).

Nutrition. — Typically — that is to say, by determinate
hereditary tendency — the Protozoa take solid food-particles
into their protoplasm which form and occupy with the water
surrounding them " gastric vacuoles " in the protoplasm.
The food-particle is digested in this vacuole, by what
chemical processes is not ascertained. It has been shown
that the contents of the gastric vacuole give in some cases
an acid reaction, and it is not improbable that free acid is
secreted by the surrounding protoplasm. It is not known
whether any ferment1 is separated by the protoplasm,
but it is probable frcm observations made on the digestive
process of Ccclentera (Actinia) that the ferment is not
separated, but that actual contact of the food-particle with
the protoplasm is necessary for a " ferment influence " to be
exerted. The digestion of a food-particle by a Protozoon
is intra-cellular, and has been contrasted with the cavitary
digestion of higher animals. In the latter, ferments and
acids are poured out by the cells bounding the enteric
cavity into that space, and digestion is extra-cellular. In
the lowest Enterozoa (many Ccelentera and some Planarian
worms) it has been shown that food-particles are actually
taken up in a solid state by the soft protoplasm of the
enteric cells and thus subjected to intra-cellular digestion.
There appears to be a gradual transition from this process,
in which close contact with living protoplasm is necessary
that the solution of an albuminous food-particle may be
effected, onwards to the perfectly free cavitary digestion
by means of secretions accumulated in the enteron.

We have not yet any satisfactory observations on the
chemistry of intra-cellular digestion either of Protozoa or
of Ccelentera.

Certain Protozoa which are parasitic do not take solid
food particles ; they (like higher parasites, such as the
Tapeworms) live in the nutritious juices of other animals
and absorb these by their general surface in a liquid state.
The Gregarinse (Sporozoa), many Ciliata, &c., are in this
case. Other Protozoa are known which are provided with
chlorophyll corpuscles and do not take in solid food, but,
apparently as a result of exceptional adaptation in which
they differ from closely-allied forms, nourish themselves
as do green plants. Such are the Volvocinean Flagellata
and some of the Dinoflagellata. It has also been asserted
that other Protozoa (viz., some Ciliata) — even some which
possess a well-developed mouth — can (and experimentally
have been made to) nourish themselves on nitrogenous
compounds of a lower grade than albumens — such, for
instance, as ammonium tartrate. Any such assertions
must be viewed with the keenest scepticism, since experi-
mental demonstration of the absence of minute albuminous
particles (e.g., Bacteria) from a solution of ammonium
tartrate in which Ciliate Protozoa are flourishing is a
matter of extreme difficulty and has not yet been effected.

Undigested food-remnants are expelled by the protoplasm
of the Protozoon cell either at any point of the surface or
by the cell-mouth or by a special cell-anus (some Ciliata,
see Fig. XXIV. 22).

Respiration and Excretion. — The protoplasm of the
Protozoa respires, that is, takes up oxygen and liberates
carbonic acid, and can readily be shown experimentally
to require a supply of oxygen for tlie manifestation of its
activity. No special respiratory structures are developed
in any Protozoa, and as a rule also the products of oxida-
tion appear to be washed out and removed from the proto-
plasm without the existence of any special apparatus.

1 The digestive ferment pepsin has been detected by Krukenln-vi,' in
the plasmodium of the Mycetozooa Fuligo (flowers of tan). See on
this subject Zopf (13), p. 88.

The contractile vacuole which exists in so many Protozoa
Mppoars, however, to be an excretory organ. It has been
shown to rapidly excrete in a state of solution colouring
matters (anilin blue) which have been administered with
food particles (8). No evidence has been adduced to show
whether traces of nitrogenous waste-products are present
in the water expelled by the contractile vacuole.

Chemical Metamorphosis. — The form which the various
products of the activity of the Protozoon's protoplasm may
assume has been noted above. It will be sufficient here
to point out that the range of chemical capacities is quite
as great as in the cells of the higher Enterozoa. Chitin,
cellulose, silicon, calcium carbonate, fats, pigments, and
gases can be both deposited and absorbed by it. Owing
to the minuteness of the Protozoa, we are at present unable
to recognize and do justice to the variety of chemical bodies
which undoubtedly must play a part in their economy as
the result of the manufacturing activity of their pro-
toplasm. See, however, Zopf (13), p. 71.

Growth and Reproduction. — The Protozoon cell follows
the same course as tissue-cells, in that by assimilation of
nutriment its protoplasm increases in volume and reaches
a certain bulk, when its cohesion fails and the viscid
droplet divides into two. The coefficient of cohesion
varies in different genera and species, but sooner or later
the disrupting forces lead to division, and thus to multi-
plication of individuals or reproduction. The phenomena
connected with the division of the nucleus (already alluded
to) will be noticed in particular cases below.

Whilst simple binary division is almost without excep-
tion a chief method of reproduction among the Protozoa,
it is also very usual, and probably this would be found if
our knowledge were complete to have few exceptions, that
under given conditions the Protozoon breaks up rapidly
into many (from ten to a hundred or more) little pieces,
each of which leads an independent life and grows to the
form and size of its parent. It will then multiply by
binary division, some of the products of which division
will in their turn divide into small fragments. The small
fragments are called " spores." Usually the Protozoon
before breaking up into spores forms a " cyst " (see above)
around itself. Frequently, but not as a necessary rule,
two (rarely three or more) Protozoon cell-individuals come
together and fuse into one mass before breaking up into
spores. This process is known as "conjugation;" and
there can be no doubt that the physiological significance
of the process is similar to that of sexual fertilization,
namely, that the new spores are not merely fragments of
an old individual but are something totally new inasmuch
as they consist of a combination of the substance of indi-
viduals who have had different life experiences.

Whilst spore-formation is not necessarily preceded by
conjugation, conjugation is not necessarily followed by
spore-formation. Among the Mycetozoa the young indi-
viduals produced from spores conjugate at a very early
period of growth in numbers and form "plasmodia," and
after a considerable interval of feeding and growth the
formation of spores takes place. Still more remarkable is
the fact observed among the Ciliata where two individuals
conjugate and after a brief fusion and mixture of their
respective protoplasm separate, neither individual (as far
as certain genera at least are concerned) breaking up into
spores, but simply resuming the process of growth and
recurrent binary division with increased vigour.

There is certainly no marked line to be drawn between
reproduction by simple fission and reproduction by spore-
formation ; both are a more or less complete dividing of
the parent protoplasm into separate masses ; whether the
products of the first fission are allowed to nourish them-
selves and grow before further fission is carried out or not

8

PROTOZOA

does not constitute an essential difference. The fission of
the Ciliate Protozoon, Opalina (see below Fig. XXIV. 4-8),
is a step from the ordinary process of delayed binary divi-
sion towards spore-formation. In some Protozoa spores are
produced after encystation by a perfectly regular process
of cleavage (comparable to the cleavage of the egg-cell
of Enterozoa) —first two, then four, than eight, sixteen,
and thirty-two fission products being the result (see
Fig. XX. 24, 25, &c.).

But more usually there is a hastening of the process,
and in these cases it is by no means clear what part the
parent cell-nucleus takes. An encysted Gregarina (or two
conjugated Gregarinse) suddenly breaks up into a number
of equal-sized spores, which do not increase in number by
binary division and have not been formed by any such
process. This multicentral segregation of the parent pro-
toplasm is a marked development of the phenomenon of
sporulatiou and remote from ordinary cell-division. How
it is related to ordinary cell-division is not known, inas-
much as the changes undergone by the nucleus in this
rapid multicentral segregation of the parent protoplasm
have not been determined. The spores of Protozoa may
be naked or encased singly or in groups in little en-
velopes, usually of a firm horny substance (see Fig.
XX. 23 to 26, and Fig. XXIV. 15 to 18). Whenever
the whole or a part of a Protozoon cell divides rapidly
into a number of equal-sized pieces which are simultane-
ously set free and are destined to reproduce the adult
form, the term spore is applied to such pieces, but the
details of their formation may vary and also those of their
subsequent history. In typical cases each spore produced
as the result of the fission of an encysted Protozoon (con-
jugated or single) has its own protective envelope, as in
the Mycetozoa (Fig. III.) and the Sporozoa (Fig. XVIII.),
from which the contained protoplasm escapes by " ger-
mination " as a naked corpuscle either flagellate or amcebi-
form. In some terminologies the word " spore " is limited
to such a "coated" spore, but usually the naked proto-
plasmic particles which issue from such " coated " spores,
or are formed directly by the rapid fission of the parent
Protozoon, are also called " spores." The former condition
is distinguished as a " chlamydospore, " whilst the latter are
termed " gymnospores." Many Protozoa produce gymno-
spores directly by the breaking up of their protoplasm,
and these are either " ftagellulse " (swarm-spores) or "amce-
buke " (creeping spores). The production of coated spores
is more usual among the lower plants than it is among
Protozoa, but is nevertheless a characteristic feature of
the Gregarinse (Sporozoa) and of the Mycetozoa. The
term " gemma " or " bud-spore " is applied to cases, few
in number, where (as in Acinetaria, Fig. XXVI., Spiro-
chona, Fig. XXIII. 10, and Reticularia, Fig. X. 8) the
spores are gradually nipped off from the parent-cell one
or more at a time. This process differs from ordinary
cell-division only in the facts (1) that the products of
division are of unequal size — the parent-cell being distin-
guishable as the larger and more complete in structure,
and (2) that usually the division is not binary, but more
than one bud-spore is produced at a time.

Whilst in the binary cell-division of the Protozoa the
two products are usually complete in structure at the
period of separation, spores and spore-buds are not only of
small size and therefore subject to growth before attaining
the likeness of the parent, but they are also very often of
simple and incomplete structure. The gap in this respect
between the young spore and its parent necessarily varies
according to the complexity of the parental form.

In the case of the Eadiolaria, of the Gregarinse, of
Noctiluca, and of the Acinetaria, for instance, the spore
has before it a considerable process of development in

structure and not merely of growth, before attaining the
adult characters. Hence there is a possible embryology
of the Protozoa, to the study of which the same prin-
ciples are applicable as are recognized in the study of the
embryology of Enterozoa. Embryonic forms of great sim-
plicity of structure, often devoid of nucleus, and consist-
ing of simple elongate particles of protoplasm, are hatched
from the spore-cases of the Gregarime (Fig. XVII. 13, 14).
These gradually acquire a differentiated cortical protoplasm
and a nucleus. A very large number of Gymnomyxa pro-
duce spores which are termed "monadiform," that is, have
a single or sometimes two filaments of vibratile protoplasm
extended from their otherwise structureless bodies. By
the lashing of these flagella the spores (swarm-spores or
zoospores) are propelled through the water. The resem-
blance of these monadiform young (best called " flagel-
lulce ") to the adult forms known as Flagella ta has led to
the suggestion that we have in them a case of recapitula-
tive development, and that the ancestors of the Gymno-
myxa were Protozoa similar to the Flagellata. Again the
Acinetaria produce spores which are uniformly clothed
with numerous vibratile cilia (Fig. XXVI.), although the
adults are entirely devoid of such structures ; this is
accounted for by the supposition that the Aciuetaria
have been developed from ancestors like the Ciliata, whose
characters are thus perpetuated in their embryonic stages.
There can be little doubt that these embryological sugges-
tions are on the whole justified, and that the nucleated
Protozoa are the descendants of non-nucleated forms simi-
lar to the spores of Gymnomyxa and Sporozoa, whilst it
seems also extremely probable that the ancestral Protozoa
were neither exclusively amoeboid in the movement of
their protoplasm nor provided with permanent vibratile
filaments (flagella and cilia) ; they were neither Myxopods
nor Mastigopods (to use the terms which have been intro-
duced to express this difference in the character of the
locomotor processes), but the same individuals were capable
of throwing out their protoplasm sometimes in the form
of flowing lobes and networks, sometimes in the form of
vibratile flagella. A few such undifferentiated forms exist
at the present day among the Proteomyxa and in a little
more advanced condition among the lowest Flagellata, e.g.,
Ciliophrys.

Death. — It results from the constitution of the Proto-
zoon body as a single cell and its method of multiplication
by fission that death has no place as a natural recurrent
phenomenon among these organisms. Among the Entero-
zoa certain cells are separated from the rest of the consti-
tuent units of the body as egg-cells and sperm-cells ; these
conjugate and continue to live, whilst the remaining cells,
the mere carriers as it were of the immortal reproductive
cells, die and disintegrate. There being no carrying cells
which surround, feed, and nurse the reproductive cells of
Protozoa, but the reproductive cell being itself and alone
the individual Protozoon, there is nothing to die, nothing
to be cast off by the reproductive cell when entering on a
new career of fission. The bodies of the higher animals
which die may from this point of view be regarded as
something temporary and non-essential, destined merely to
carry for a time, to nurse, and to nourish the more import-
ant and deathless fission-products of the unicellular egg.
Some of these fission-products of the new individual de-
veloped from an egg-cell — namely, the egg-cells and sperm-
cells — are as immortal as the unicellular Protozoon. This
method of comparing the unicellular and the multicellular
organism is exceedingly suggestive, and the conception we
thus gain of the individuality of the Enterozoon throws
light upon the phenomena of reproduction and heredity in
those higher organisms.

Experiment and observation in this matter are extremely

PROTOZOA

9

difficult ; but we have no reason to suppose that there is
any inherent limit to the process of nutrition, growth, and
fission, by which continuously the Protozoa are propagated.
The act of conjugation from time to time confers upon
the protoplasm of a given line of descent new properties,
and apparently new vigour. Where it is not followed by
a breaking up of the conjugated cells into spores, but by
separation and renewed binary fission (Ciliata), the result
is described simply as " rejuvenescence." The protoplasm
originated by the successive division of substance traceable
to one parent cell has become specialized, and in fact too
closely adapted to one series of life-conditions ; a fusion
of substance with another mass of protoplasm equally
specialized, but by experience of a somewhat differing
character, imparts to the resulting mixture a new com-
bination of properties, and the conjugated individuals on
separation start once more on their deathless career with
renewed youth.

CLASSIFICATION OF THE PROTOZOA.

In attempting a scheme of classification it would be most in
accordance with the accepted probabilities of the ancestral history
of the Protozoa to separate altogether those forms devoid of a
nucleus from those which possess one, and to regard them as a
lower "grade" of evolution or differentiation of structure.

By some systematists, notably Butschli (9), the presence or
absence of a nucleus has not been admitted as a basis of elassifica-
tory distinction, whilst on the other hand both Haeckel (1) ami
Huxley (10) have insisted on its importance.

The fact is that during recent years many of those Protozoa
which were at one time supposed to be devoid of nucleus even in a
rudimentary form, and furnished therefore the tangible basis for a
lowest group of "Protozoa Homogenea" or "Monera," have been
shown by the application of improved methods of microscopic
investigation to possess a nucleus, that is to say, a differentiated
corpuscle of denser protoplasm lying within the general protoplasm,
and capable when the organism is killed by alcohol or weak acids
of taking up the colour of various dyes (such as carmine and
heematoxylin) more readily and permanently than is the general
protoplasm. In such cases the nucleus may be very small and
exhibit none of the typical structure of larger nuclei. It is usually
surrounded by a clear (i.e., non-granular) halo of the general
protoplasm which assists the observer in its detection. Nuclei
have been discovered in many Reticularia (Foraminifera), a group
in which they were supposed to be wanting, by Schultze (11) and
the Hertwigs (12) and more recently in the Mycetozoa and in
Vampyrella and Protomonas (Zopf, 13), where so excellent an
observer as C'ienkowski had missed them.

It seems therefore not improbable that a nucleus is present
though not observed in Protomyxa, Myxastrum, and other similar
forms which have been by Haeckel and others classed as " Monera "
or "Homogenea." The recently described (14) Archerina (Fig. II.
8, 11) certainly possesses no nucleus in the usual sense of that term,
but it is possible that the chlorophyll-coloured corpuscles of that
organism should be considered as actually representing the nucleus.
Whilst then refraining from asserting that there are no existing
Protozoa devoid of nucleus corresponding in this character with
non-nucleate Protophyta, such as the Bacteria, we shall not in our
scheme of classification institute a group of Homogenea, but shall
leave the taking of that step until it has been shown after critical
examination that those forms now regarded by some observers as
Homogenea are really so. In the meantime these forms will find
their places alongside of the Nueleata most nearly allied to them
in other characters.

The Protozoa with a definite permanent cortical substance of
differentiated protoplasm are undoubtedly to be regarded as evolved
from forms devoid of such differentiation of their substance, and
we accordingly take this feature as the indication of a primary
division of the Protozoa.1 The lower grade, the Gymnomyxa,
afford in other respects evidence of their being nearly related to
the ancestral forms from which the Corticata (the higher grade)
have developed. The Gymnomyxa all or nearly all, whilst
exhibiting amoeboid movement and the flowing of their protoplasm
into " pseudopodia " of very varied shapes, produce spores which
swim by means of one or two flagella of vibratile protoplasm
(monadiform young or flagelluhe). These flagellate young forms

1 The " exoplasm " and "endoplasm" described in Amceboe, &c. ,
by some authors are not distinct layers but one and the same con-
tinuous substance— what was internal at one moment becoming ex-
ternal at another, no really structural difference existing between
them.

are closely related to the Flagellata, a group of the Corticata from
which it seems probable that the Dinodagellata, the Ciliata, and
the Acinetaria have been derived. The Gymnomyxa themselves
cannot, on account of the small number of structural features
which they offer as indications of affinity and divergence in genetic
relationships inter sc, be classified with anything like confidence in
a genealogical system. We are obliged frankly to abandon the
attempt to associate some of the simpler forms with their nearest
genetic allies and to content ourselves with a more or less artificial
system, which is not, however, artificial in so far as its main
groups are concerned. Thus the genetic solidarity of each of the
large classes Heliozoa, Reticularia, Mycetozoa, and liadiolaria is
not open to question. The Lobosa on the other hand appear to
be a more artificial assemblage, and it is difficult to say that
genetically there is any wide separation between them and the
Mycetozoa or between the Mycetozoa and some of the simpler
forms which we bring together under the class Proteornyxa.
The scheme of classification which we adopt is the following : —

PROTOZOA.

G r. ADE A. G YMNOM YXA.
( Class I. PROTEOMYXA.
j Ex. Vampyrella, Protomyxa, Anlierina.
Class II. MYCETOZOA.

Ex. The Eu-mycetozoa of Zopf.
Class III. LOBOSA.

Ex. Amceba, Arcclla, Pclomyxa.
r Class IV. LABYRINTHULIDBA.

Ex. Labyrinthula, Chlamydomyxa.
j Class V. HKLIOZOA.
!>•!„„ I Ex. Actinovhnis, RapJiidiophrus, Clatltrnlina.

\ Class VI. BETICULAnii.

Ex. Gromift, Lituola, Astrorhiza, Globigcrina.
\ Class VII. RADIOLARIA.
L Ex. Thalassicolla, Eucyrtidiuin, Acanthometra.

GRADE B. CORTICATA.
\ Class I. SPOROZOA.
\ Ex. Gregarina, Coccidium.
( Class II. FLAGELLATA.

Ex. Monas, Salpingceca, Eitglena, Volvox.

Class III. DlNOFLAGELLATA.

Ex. Proroccntrum, Ceratium.
Stomato- Class IV. RHYNCHOFLAGELLATA.
phora. Ex. Nodiluca.

Class V. CILIATA.

Ex. Vorticclla, Paramtzcium, Slcntor.
Class VI. ACINETARIA.

Ex. Acineta, Dendrosoma.
The genetic relationships which probably obtain among theso
groups may be indicated by the following diagram : —

Sections.
Proteana.

Plasmodiata.
Lobosa.

Lipostoma.

Class Acinetaria.

Class Ciliata.

Claaa
Rhyncho-flagellata,

Class
Di no- flagella ta

Class Sporozoa.

Class
Myceujzoa.

Class \
Labyriuthulidea

(Protonucleata.J

I
Homogenea,

Literature. — Certain works of an older date dealing with micro-
scopic organisms, and therefore including many Protozoa, have
historical interest. Among these we may cite 0. F. Muller,
Animalcula Infusoria, 1786; Ehrenberg, Infusionsthicrchen, 1838;

B

10

PROTOZOA

Dujardin, Histoire naturclle des Infusoires, 1841 ; Pritchard, In-
fusoria, 1857.

The general questions relating to protoplasm and to the consti-
tution of the Protozoon body as a single cell are dealt with in the
following more recent treatises :— Max Schultze, Ueber dm Organ-
ismus dcr Pohjthalamicn, 1854, and Ueber das Protoplasma der
Rhizopoden wid Pflanxcnzellen, 1863; and Engelmann, article "Pro-
toplasma" in Hermann's ffandworterbuch der Physiologic, 1880.

Special works of recent date in which the whole or large groups
of Protozoa are dealt with in a systematic manner with illustra-
tions of the chief known forms are the following : — Biitschli, "Pro-
tozoa," in Bronn's Classen und Ordnungen des Thierreichs, a
comprehensive and richly illustrated treatise now in course of
publication, forming the most exhaustive account of the subject
matter of the present article which has been attempted (the writer
desires to express his obligation to this work, from the plates of
which a large proportion of the woodcut figures here introduced
have been selected); W. S. Kent, Manual of the Infusoria, 1882 —
an exhaustive treatise including figures and descriptions of all
species of Flagellata, Dinoflagellata, Ciliata, and Acinetaria ; Stein,
Lter Organismus der Infusionsthicre, 1867-1882; Haeckel, Die
Radiolarien, 1862; Archer, "Resume of recent contributions to
our knowledge of freshwater Rhizopoda, " Quart. Jour, of Micro-
scopical Science, 1876-77; Zopf, " Pilzthiere '' (Mycetozoa), in
Encyhlopadie der Naturwissenschaftcn, Breslau, 1884.

We shall now proceed to consider the classes and orders of
Protozoa in detail.

PEOTOZOA.

Characters. — Organisms consisting of a single cell or of a group
of cells not differentiated into two or more tissues ; incapable of
assimilating nitrogen in its diffusible compounds (ammonia or
nitrates) or carbon in the form of carbonates, except in special
instances which there is reason to regard as directly derived from
allied forms not possessing this capacity. The food of the Protozoa
is in consequence as a rule taken in the form of particles into the
protoplasm either by a specialized mouth or by any part of the
naked cell-substance, there to be digested and rendered diffusible.

GRADE A. GYMNOMYXA, Lankester, 1878 (64).

Characters. — Protozoa in which the cell-protoplasm is entirely or
partially exposed to the surrounding medium, during the active
vegetative phase of the life-history, as a naked undifferentiated
slime or viscous fluid, which throws itself into processes or
' ' pseudopodia " of various form either rapidly changing or
relatively constant. Food can be taken into the protoplasm in the
form of solid particles at any point of its surface or at any point
of a large exposed area. The distinction into so-called "exoplasm"
and "endoplasm " recognized by some authors, is not founded on a
permanent differentiation of substance corresponding to the cortical
and medullary substance of Corticata, but is merely due to the
centripetal aggregation of granules lying in a uniform undiffer-
entiated protoplasm. The cell-individual exhibits itself under
four phases of growth and development — (1) as a swarm-spore
(monadiform young or flagellula) ; (2) as an amoeba form ; (3) as
constituent of a plasmodium or cell-fusion or conjugation ; (4) as a
cyst, which may be a flagellula(Schwarme)-producing cyst, an
amcebula-producing cyst, a covered-spore(chlamydospore)-producing
cyst (sporoeyst sens, stric., Zopf), or a simple resting cyst which
does not exhibit any fission of its contents (hypnocyst). Any one
of these phases may be greatly predominant and specialized whilst
the others are relatively unimportant and rapidly passed through.

CLASS I. PROTEOMYXA, Lankester.

Characters. — Gymnomyxa which exhibit in the amoeba phase
various forms of pseudopodia often changing in the same individual,
and do not produce elaborate spore cysts ; hence they are not re-
ferable to any one of the subsequent six classes. Mostly minute
forms, with small inconspicuous nucleus (absent in some?).

A division into orders and families is not desirable, the group
being confessedly an assemblage of negatively characterized or
insufficiently known forms.

Genera. — Vampyrclla, Cienkowski (15); Fampyrcllulium, Zopf
(13); Spirophora, Zopf ( — Amoeba radiosa, Perty) ; Haplococcus,
Zopf; Leptophrys, Hertwig and Lesser (16); Endyomena, Zopf;
BursuUa, Sorokin (17) ; Mysastrum, Haeckel (1) ; Entcromyxa,
Cienkowski (18) ; Colpudella, Cienkowski (19); Pseudospora, Cien-
kowski (20) ; Protomonas, Cienkowski (15) ; Diplophysalis, Zopf
(13); Gt/mnococcus, Zopf; Aphclidium, Zopf; Pseudosporidium,
Zopf ; Protomyxa, Haeckel (1) ; Plasmodiophora, Woronin (21) ;
Tetramyxa, Gbbel (22) ; Gloidium, Sorokin (23) ; Gymnoplirys,
Cienkowski (24) ; Myxodictyum, Haeckel (1) ; Boderia, Wright
(25) ; Biomyxa, Leidy (92) ; Protogenes, Haeckel (1) ; Protama-ba,
Haeckel (1); Nuclearia, Cienkowski (26); Monobia, Aim. Schneider
(27) ; Archerina, Lankestcr (14).

The forms here brought together include several genera (the

first nineteen) referred by Zopf to the Mycetozoa, some again
(Vampyrella, Myxastrum, Nuclearia, Monobia) which are by
Biitschli associated with the Heliozoa, others (Protamceba, Gloidium)
referred by the same authority to the Lobosa (Amcebsea) and others
(Colpodella, Protomonas) which might be grouped with the lower
Flagellata. By grouping them in the manner here adopted we
are enabled to characterize those higher groups more satisfactorily
and to give a just expression to our present want of that knowledge
of the life-history both of these forms and of the higher Gymnomyxa
which when it is obtained may enable us to disperse this hetero-
geneous class of Proteomyxa. The group lias the same function
in relation to the other classes of Gymnomyxa which the group
Vermes has been made to discharge in relation to the better denned
phyla of the Metazoa ; it is a lumber-room in which obscure, lowly-
developed, and insufficiently known forms may be kept until they
can be otherwise dealt with.

It is true that, thanks to the researches of Continental botanists
(especially Cienkowski and Zopf), we know the life-history of
several of these organisms; but we are none the less unable to con-
nect them by tangible characteristics with other Gymnomyxa.

Nearly all of the above-named genera are parasitic rather than
" voracious," that is to say, they feed on the organized products of
larger organisms both plants and animals (Haplococcus is parasitic
in the muscles of the pig), into whose tissues they penetrate, and
do not, except in a few cases (Protomyxa, Vampyrella), engulph
whole organisms, such as Diatoms, &c. , in their protoplasm. Many
live upon and among the putrefying debris of other organisms
(e.g., rotting vegetable steins and leaves, excrements of animals),
and like the Mycetozoa exert a digestive action upon the substances
with which they come in contact comparable to the putrefying and
fermentative activity of the. Schizomycetes (Bacteria).

Fig. II. illustrates four chief genera of Proteomyxa.

Protomyxa aurantiaea was described by Haeekel (1), who found
it on shells of Spirula on the coast of the Canary Islands, in the
form of orange yellow flakes consisting of branching and reticular
protoplasm nourishing itself by the ingestion of Diatoms and
Peridinia. This condition is not a simple amceba phase but a
"plasmodium" formed by the union of several young amcebte. The
plasmodium under certain conditions draws itself together into a
spherical form and secretes a clear membranous cyst around itself,
and then breaks up into some hundreds of flagellula: or swarm-
spores (Fig. IT. 2). The diameter of the cyst is '12 to '2 millimetre.
The flagellulai subsequently escape (Fig. II. 3) and swim by the
vibratile movement of one end which is drawn out in the form of a
coarse flagellum. The swarm-spore now passes into the amoeba
phase (Fig. II. 4). Several of the small amcebae creeping on the
surface of the spirula-shell then unite with one another and form
a plasmodium which continues to nourish itself by "voracious"
inception of Diatoms and other small organisms. The plasmodia
may attain a diameter of one millimetre and be visible by the
naked eye.

A nucleus was not observed by Haeckel in the spores nor in the
amceba phase, nor scattered nuclei in the plasmodium, but it is not
improbable that they exist and escaped detection in the living con-
dition, in consequence of their not being searched for by methods
of staining, &c. , which have since come into use. A contractile
vacuole does not exist.

Vampyrclla spirogyrse, Cienkowski (Fig. II. 5, 6, 7), is one of
several species assigned to the genus Vampyrella, all of which
feed upon the living cells of plants. The nucleus previously stated
to be absent has been detected by Zopf (13). There is no con-
tractile vacuole. The amceba phase has an actinophryd character
(i.e., exhibits h'ne radiating pseudopodia resembling those of the
sun-animalcule, Actinophrys, one of the Heliozoa). This species
feeds exclusively upon the contents of the cells of Spirogyra, effect-
ing an entrance through the cell-wall (Fig. II. 5), sucking out the
contents, and then creeping on to the next cell. In some species
of Vampyrella as many as four amoeba-individuals have been
observed to fuse to form a small plasmodium. Cysts are formed
which enclose in this species a single amoeba-individual. The cyst
often acquires a second or third inner cyst membrane by the
shrinking of the protoplasmic body after the first encystment and
the subsequent formation of a new membrane. The encysted pro-
toplasm sometimes merely divides into four parts each of which
creeps out of the cyst as an Actinophrys-like amoeba (Fig. II. 7) ; in
other instances it forms a dense spore, the product of which is not
known.

Protogenes primordial is is the name given by Haeckel to a
very simple form with radiating filamentous pseudopodia which
he observed in sea-water. It appears to be the same organism as
that described and figured by Max Schultze as Amceba porrccta.
Schultze's figure is copied in Fig. II. 12. No nucleus and no con-
tractile vacuole is observed in this form. It feeds voraciously on
smaller organisms. Its life-history has not been followed over even
a few steps. Hence we must for the present doubt altogether as to
its true affinities. Possibly it is only a detached portion of the
protoplasm of a larger nucleate Gymnomyxon. The same kind of

PROTOZOA

11

doubt is justified in regard to Haeckcl's Fi-ii/niim-lit. /n-imitiva, which
was uhservi'd liy liim in pond water and ilill'ers from Protogenes in
having lobose pseudopodia, wliilst agreeing with it in absence of
nuclei, contractile vacuoles, and other differentiation of structure.

FIB. II.— Various Proteomyxa. 1. Protnmyxa aurantiaca, HaecM, plas-
modium phase. The naked protoplasm shows blanched, reticulate processes
(pseudopodia), and numerous non-contractile vacuoles. It is in the act of en-
pulphing a Ctratium. Shells of engulphed Ciliatu (Tintinnabula) arc embedded
deeply in the protoplasm a. 2. Cyst phase of 1'rotomyxa. a, transparent cyst-
wall ; 6, protoplasm brok'-n up into spores. 3. Flagellula phase of Protomyxa,
the form assumed by the spores on their escape from the cyst. 4. Anicebula
phase of the same, tile form assumed after a short period by the flagellula?. 5.
Vampyrella spirogyrx, Cienk., amoeba phase penetrating a cell of Spirogyra 6,
by a process of its protoplasm c, and taking up the substance of the Spirbgyra
cell, some of which is seen within the Vampyiella a. C. Large individual of
Vampyrella, showing pseudopodia p, and food particles a. The nucleus (though
present) Is not shown in this rtiawing. 7. Cyst phase of Vampyrella. The
contents of the cyst have divided into four equal parts, of which three are
visible. One is commencing to break its way through the cyst-wall /; a, food
particles. 8. Archerina Boltoni, Lankester, showing lobose and filamentous
protoplasm, and three groups of chlorophyll corpuscles. The protoplasm g is
engulphing a Bacterium i. 9. Cyst phase of Archerina. a, spinous cyst-wall ;
6, green-coloured contents. 10. Chlorophyll corpuscle of Archerina showing
tetraschistic division. 11. Actinophryd form of Archerina. b, chlorophyll cor-
puscles. 12. Proloyenes primordial^, Haeckel (Amteba porrecCa, M. Schultze),
from Schultze's figure.

The structureless protoplasmic network described by Haeckel

from spirit-preserved specimens of Atlantic ooze and identified by
him with Huxley's (28) Bathybiux, ;is also the similar network
described by Besscls (29) as Protobathybillfl, must be regarded for
tlie present as insufficiently known.

It is possible that these appearances observed in the ooze dredged
from great depths in the Atlantic are really due to simple Protozoa.
On the other hand it lias been asserted by Sir Wyville Thomson,
who at one time believed in the independent organic nature of
Bathybius, that the substance taken for protoplasm by both Huxley
and Haeckel is in reality a gelatinous precipitate of calcium
sulphate thrown down by the action of alcohol upon sea-water.
Other naturalists have pointed to the possibility of the protoplasmic
network which Bessels studied in the living condition on board
ship being detached portions of the protoplasm of Keticularia and
Radiolaria. The matter is one which requires further investigation.

Archerina Boltoni is the name given by Lankester (14) to a very
simple Gymnomyxon inhabiting freshwater ponds in company
with Desmids and other simple green Alga; (Fig. II. 8 to 11).
Archerina exhibits an amoeba phase in which the protoplasm is
thrown into long stiff filaments (Fig. II. 11), surrounding a spherical
central mass about inrVcth inch in diameter (actinophryd form).
A large vacuole (non-contractile) is present, or two or three small
ones. No nucleus can be detected by careful use of reagents in
this or other phases. The protoplasm has been seen to ingest solid
food particles (Bacteria) and to assume a lobose form. The most
striking characteristic of Archerina is the possession of chlorophyll
corpuscles. In the actinophryd form two oval green-coloured
bodies (b, 6) are seen. As the protoplasm increases by nutrition the
chlorophyll corpuscles multiply by quaternary division (Fig. II. 10)
and form groups of four or of four sets of four symmetrically
arranged. The division of the chlorophyll corpuscles is not
necessarily followed by that of the protoplasm, and accordingly
specimens are found with many chlorophyll corpuscles embedded
in a large growth of protoplasm (Fig. II. 8) ; the growth may increase
to a considerable size, numbering some hundreds of chlorophyll
corpuscles, and a proportionate development of protoplasm. Such
a growth is not a plasmodium, that is to say, is not formed by
fusion of independent amoeba forms, but is due to continuous
growth. When nutrition fails the individual chlorophyll corpuscles
separate, each carrying with it an investment of protoplasm, and
then each such amoeba form forms a cyst around itself which is
covered with short spines (Fig. II. 9). The cysts are not known
to give rise to spores, but appear to be merely hypnocysts.

The domination of the protoplasm by the chlorophyll corpuscles
is very remarkable and unlike anything known in any other
organism. Possibly the chlorophyll corpuscles are to be regarded
as nuclei, since it is known that there are distinct points of affinity
between the dense protoplasm of ordinary nuclei and the similarly
dense protoplasm of normal chlorophyll corpuscles.

CLASS II. MYCETOZOA, De Bary.

Characters. — Gymnomyxa which, as an exception to all other
Protozoa, are not 'inhabitants of water but occur on damp surfaces
exposed to the air. They are never parasitic, as are some of the
Proteomyxa most nearly allied to them (Plasmodiophora, &c.), but
feed on organic debris. They are structurally characterized by the
fact that the amoeba forms, which develop either directly or through
flagellulse from their spores, always form large, sometimes very
large, i.e., of several square inches area, fusion plasmodia (or
rarely aggregation plasmodia), and that the spores are always
chlamydospores (i.e., provided with a coat) and are formed either
in naked groups of definite shape (sori) or on the surface of peculiar
columns (conidiophors) or in large fruit-like cysts which enclose the
whole or a part of the plasmodium and develop besides the spores
definite sustentacular structures (capillitium) holding the spores in
a mesh- work.

Three orders of Mycetozoa are distinguishable according to the
arrangement of the spores in more or less complex spore-fruits.

ORDER 1. SOROPHORA, Zopf.

Characters. — Mycetozoa which never exhibit a vibratile (mouadi-
form) swarmspore or ttagellula phase, but hatch from the spore as
amoeba;. A true fusion plasmodium is not formed, but an aggrega-
gation plasmodium by the contact without fusion of numerous
amceba forms. The spore fruit is a naked aggregation of definitely
arranged encysted amoeba; called a sorus, not enclosed in a common
capsule ; each encysted amceba has the value of a single spore and
sets free on germination a single amcebula. They inhabit the dung
of various animals.

Genera.— Copromyxa, Zopf; Cynlliulina, Cicnk. ; Dictyostelium,
Brefeld ; Acrasis, Van Tieghem ; Polyspondylium, Brefeld.

ORDER 2. ENDOSPOREA, Zopf.

Characters. — Mycetozoa always passing through the flagellula
phase and always forming true plasmodia by fusion of amceba
forms. The spore-fruit is in the form of a large cyst which encloses
a quantity of the plasmodium ; the latter then breaks up into (a)

12

PROTOZOA

spores (one corresponding to each nucleus of the enclosed plas-
modium) each of which has a cellulose coat, and (b) a capillitium
of threads which hold the spores together. Each spore (chlamydo-
spore) liberates on germination a single mieleated flagellula, which
develops into an amcebula, which in turn fuses with other amcebulie
to form the plasmodium. The Endosporea are essentially dwellers
on rotten wood and such vegetable refuse.

FIG. III.— Mycetozoa (after DeBary). 1-6. Germination of spore(l) of Trichta
varia, showing the emerging "flagellula" (4, 5), and its conversion into an
"amcebula" (6). 7-18. Series leadintr from spore to plasmodium phase of
Chondrioderma difforme: — 7, spore; 10, flagellula; 1'2, amcebula; 14, apposi-
tion of two amcebulaB ; 15-17, fusions ; 18, plasmodium. 19, 20, Spore-fruit
(cyst) of Ptiysarum JcueopfuEum, Fr. (x 25), the former from the surface, the
latter in section with the spores removed to show the sustentacular network or
capillitium. 21. Section of the spore-cyst of Didymium squaintilosum,viith the
spores removed to show the radiating capillitium x and the stalk.

Sub-order 1. PERITHICHEA, Zopf.

Fain. 1. CLATHROPTYCHIACEJE, Rostafinski.

Genera. —Clathroptyckium, Rost. ; Enteridium, Ehr.
Fam. 2. CniBRARiACE^;.

Genera. — Dictydium, Pers. ; Cribraria, Pers.

Sub-order 2. EXDOTRICHEA, Zopf.

Fam. 1. PHYSAREA.

Genera. — Physarum, Pers.; Craterium, Trentepol; Eadliamia,
Berkeley ; Leocarpiis, Link. ; Tilmadochc, Fr. ; Fuligo
(slSthalium), Hall ; ^Ethaliopsis, Z.
Fam. 2. DIDYMIACE.E.

Genera. — Didymium; Lepidodcrma, DeBary.
Fam. 3. SpUMAKIACEai.

Genera. — Spuniaria, Pers. ; Diachca, Fries.
Fam. 4. STEMONITEA.

Genera. — Stemonitis, Gleditsch ; Comatricfta, Preuss ; Lam-

prodcrma, Rost.
Fam. 5. ENERTHKNEMEA.

Genera. — Enerthema, Bowman.
Fam. 6. RETICULARIACE.E, Zopf.

Genera. —Amauroch&te, Rost; Selwularia, Bull.
Fam. 7. TRICHINACE^;.

Genera.— ffemiarcyria, Rost. ; Trichia, Hall.
Fam. 8. ARCYRIACEJE.

Genera. — Arcyria, Hall; Cornuvia, Rost.; Lycor/ala, Ehr.
Fam. 9. PERicH^NACEi;.

Genera. — Perich&na, Fries. ; Lachnobolus, Fries.
Fam. 10. LICEACE^E.

Genera. — Licca, Schrader ; Tiibulina, Pers.: Lindbladia,
Fries. ; TubuUfcra, Zopf.

ORDER 3. EXOSPOREA, Zopf.

Characters. — The chlamydospore liberates an amcebula iu the
first instance, which develops into a flagellula. This subsequently
returns to the amceba form, and by fusion with other amxBbulse it
forms a true fusion plasmodium. The spores are not produced
within a cyst but upon the surface of column-like up-growths of the
plasmodium, each spore (conidum) forming as a little spherical out-
growth attached to the column (conidiophor) by a distinct pedicle.

Sole Genus. — Ceratium, [This name must be changed, since it
was already applied to a genus of Dinoflagellata, when Famintziu
and Worouin gave it to this Mycetozoon.]

Further Remarks on Mycetozoa. — About two hundred species of
Mycetozoa have been described. Botanists, and especially those who
occupy themselves with Fungi, have accumulated the very large

mass of facts now known in reference to these organisms ; never-
theless the most eminent botanist who has clone more than any
other to advance our knowledge of Myeetozoa, namely, De Bary, has
expressed the view that they are to be regarded rather as animals
than as plants. The fact is that, once the question is raised, it
becomes as reasonable to relegate all the Gymnomyxa without
exception to the vegetable kingdom as to do so with the Mycetozoa.
Whatever course we take with the latter, we must take also with
the Heliozoa, the Radiolaria, and the Reticularia.

The formation of plasmodia, for which the Mycetozoa are conspicu-
ous, appears to be a particular instance of the general phenomenon
of cell-conjugation. Small plasmodia are formed by some of the
Proteomyxa ; but among the other Gymnomyxa, excepting Myceto-
zoa, and among Corticate Protozoa, the fusion of two individuals
(conjugation scum stricto) is more usual than the fusion of several.
Zopf (13) has attempted to distinguish arbitrarily between conjuga-
tion and plasmodium formation by asserting that in the former
the nuclei of the cells which fuse are also fused, whereas in the
latter process the nuclei retain their independence. Both state-
ments are questionable. What happens to the nucleus in such
conjugations as those of the Gregarinre has not yet been made out,
whilst it is only quite recently that Strasburger (30) has shown
that the plasmodia of Mycetozoa contain numerous scattered nuclei,
and it is not known that fusion does not occur between some of
these. There is no doubt that the nuclei of plasmodia multiply
by fission, though we have no detailed account of the process.

The Sorophora are exceptional in that the amoeba? which unite to
form a cell-colony iu their case do not actually fuse but only remain
in close contact ; with this goes the fact that there are no large
spore-cysts, but an identification of spore and spore-cyst. The
amcebie arrange themselves in stalked clusters (sori), and each be-
comes encysted : one may, in this case, consider the cyst equally as
a spore or as a spore-cyst which produces but a single spore. The
amceba; described by various writers as inhabiting the alimentary
canal and the dung of higher animals (including man) belong to
this group. The form described by Cunningham in the Quart.
Jour. Micr. Sci., 1881, as Protomyxomyces coprinarius is appa-
rently related to the Copromyxa (Gitttulina) protca of Fayod (31).

The spore-fruits of the Endosporese occur in various degrees of
elaboration. Usually they are (1) spherical or pear-shaped cysts
with or without an obvious stalk (Fig. III. 19, 20, 21), and often
have a brilliant colour, and are of a size readily observed by the
naked eye, the plasmodia which give rise to them being by no
means microscopic. But they may present themselves (2) as
irregular ridges growing up from the plasmodium, when they are
termed serpula forms. Lastly, the cysts may be united side by
side in larger or smaller groups instead of forming at various sepa-
rate points of the plasmodium. These composite bodies are termed
"fruit-cakes" or sthalia," in view of the fact that the spore-cysts
of Fuligo, also called jEthalium — the well-known "flowers of tan"
— form a cake of this description.

The capillitium or network of threads which lies between the
spores in the spore-cysts of Endosporeie is a remarkable structure
which exhibits special elaborations in detail in different genera, here
not to be noticed for want of space. Although definite in form and
structure, these threads are not built up by cells but are formed
by a residual protoplasm (cf. Sporozoa) which is left in the cyst
after the spores have been segregated and enclosed each in its
special coat. They are often impregnated by calcium carbonate,
and exhibit crystalline masses of it, as does also the cyst-wall.

The spores of the Mycetozoa are as a rule about the T-sVoth inch
in diameter. They are produced by millions in the large fruit-
cakes of such forms as Fuligo. Often the spore-coat is coloured ; it
always consists of a substance which gives the cellulose reaction
with iodine and sulphuric acid. This has been sometimes con-
sidered an indication of the vegetable nature of the Mycetozoa, but
cannot be so regarded since many animals (especially the Tunieata
and various Protozoa) produce substances giving this same reaction.

Dryness, low temperature, and want of nutriment lead to a dor-
mant condition of the protoplasm of the plasmodium of many
Mycetozoa and to its enclosure in cyst-like growths known as
"sclerotia," which do not give rise to spores, but from which the
protoplasm creeps forth unaltered when temperature, nutrition, and
moisture are again favourable. The sclerotia are similar in nature
to the hypnocysts of other Protozoa.

The physiological properties — chemical composition, digestive
action, reaction to moisture, heat, light, and other physical influ-
ences— of the plasmodia of Mycetozoa have been made the subject
of important investigations ; they furnish the largest masses of
undirTerentiated protoplasm available for such study. The reader
is referred to Zopf's admirable treatise (13) as to these matters, and
also for a detailed account of the genera and species.

CLASS III. LOBOSA, Carpenter.

Characters. — Gymnomyxa in which (as in the succeeding four
classes) the amoeba-phase predominates over the others iu perma-
nence, size attained, and physiological importance. The pseudo-

PROTOZOA

podia are loboso, ranging in form from mere wave-like bulgings
of the surface to blunt finger-like processes, but never having the
character of filaments either simple, arborescent, or reticulate.
Fusions of two individuals (conjugation) have been observed in a

Fio. IV.— Various Lobosa. 1, 2, 3. Daclylosphxra (Ainoeba) polypodia, M.
Schultze, in three successive stages of division; the changes indicated
occupied fifteen minutes, a, nucleus ; b, contractile vacuole (copied from
F. E. Schultze, in Archivf. Milcratk. Anat.). 4. Amoeba princeps, Ehr.

(after Auerbach). a, nucleus ; 6, c, vacuoles (one or more contractile ; the
shaded granules are food-particles). 5. Pelomyxa palustns, Greeff

(after Greeff), an example with comparatively few food-particles (natural
size ^,th inch in length). 6. Portion of a Pelomyxa more highly magni-

fied, a, clear superficial zone of protoplasm (so-called " exoplasm ") ; b,
vacuoles, extremely numerous ; c, lobose pseudopodium ; d, a similar
pseudopodium ; e, nuclei ; /, " refractive bodies " (reproductive ?) ; scattered
about in the protoplasm are seen numerous cylindrical crystals. 7.

Arcella vulgaris, Ehr. a, shell; b, protoplasm within the shell ; c, extended
protoplasm in the form of lobose pseudopodia ; d, nuclei; e, contractile
vacuole ; the dark bodies unlettered are gas vacuoles. 8. Cochlio-

podium pellucidum, Hert. and Less, a, nucleus surrounded by a hyaline
halo sometimes mistaken for the nucleus, whilst the latter is termed
nucleolus.

few cases, but not fusions of many individuals so as to form
plasmodia ; nevertheless the size attained by the naked protoplasm
by pure growth is in some cases considerable, forming masses readily
visible by the naked eye (Pelomyxa). The presence of more than.

one nucleus is a frequent character. A contractile vacuole may or
may not be present. The formation of sporocysts and of chlamydo-
sporcs (coated spores) has not been observed in any species, but
naked spores (llagellulo; or amccbulae) have been with more or
less certainty observed as the product of the breaking up of some
species (Amosba? Pelomyxa). The cyst phase is not unusual, but
the cyst appears usually to be a hypnocyst and not a sporocyst.
In the best observed case of spore-production (Pelomyxa) the spores
were apparently produced without the formation of a cyst. Repro-
duction is undoubtedly most freely effected by simple fission
(Amoeba) and by a modified kind of bud-fission (Arcella). Fresh-
water and marine. Two orders of the Lobosa are distinguished in
accordance with the presence or absence of a shell.

ORDER 1. NUDA.

Characters. — Lobosa devoid of a shell.

Genera. — Ama>ba, Auct. (Fig. IV. 4); Ouramtxba, Leidy (with a
villous tuft at one end, Wallich's A. villosa) ; Cort/cia, Duj. (low,
ridge-like pseudopodia); Lithamceba, Lankester (Fig. V.); Dina-
mceba, Leidy (92) (covered with short stitf processes) ; Hyalodiscus,
H. and L. ; Plakojms, F. E. Schultze ; Dactylosph&m, H. and L.
(Fig. IV. 1, 2, 3); Pelomyxa, Greeff (Fig. IV. 5, 6) ; Amphizonella,
Greeff (forms a gelatinous case which is broken through by the
pseudopodia).

ORDER 2. TESTACEA.

Characters. — Lobosa which secrete a shell provided with an
aperture from which the naked protoplasm can be protruded. The
shell is either soft and membranous, or strengthened by the in-
clusion of sand-particles, or is hard and firm.

Genera. — CochHopodimn (Fig. IV. 8), H. and L. ; Pyxidicula,
Ehr. ; Arcella, Ehr. (Fig. IV. 7) ; Hyalosphenia, Stein ; Quad-
rula, F. E. Schultze (shell membraneous, areolated) ; Difflugia,
Leclerc (shell with adventitious particles).

Further remarks on the Lobosa. — The Lobosa do not form a very
numerous nor a very natural assemblage. Undoubtedly some of
the forms which have been described as species of Amreba are
amoeba forms of Mycetozoa ; this appears to be most probably the
case in parasitic and stercoricolous forms. But when these are
removed, as also those Proteomyxa which have pseudopodia of
varying character, at one time lobose and at another filamentous,
we have left a certain small number of independent lobose
Gymnomyxa which it is most convenient to associate in a
separate group. We know very little of the production of spores
(whether it actually obtains or not) or of developmental phases
among these Lobosa. The common Amcelxe are referable to the
species A. princeps, A. lobosa, Dactylosp/tasra polypodia, Ouramcela
villosa. Of none of these do we know certainly any reproductive
phenomena excepting that of fission (see Fig. IV. 1, 2, 3). Various
statements have been made pointing to a peculiar change in the
nucleus and a production of spores having the form of minute
Amceba?, arising from that body ; but they cannot be considered
as established. Whilst the observed cases of supposed reproduc-
tive phenomena are very few, it must be remembered that we have
always to guard (as the history of the Ciliata has shown, see
below) against the liability to mistake parasitic amcebulje and
flagellulse for the young forms of organisms in which they are
merely parasitic. The remarkable Pelomyxa palustns of Greeff (32)
was seen by him to set free (without forming a cyst) a number of
amcebulie which he considers as probably its young. Mr Weldon
of St John's College, Cambridge, lias observed the same pheno-
menon in specimens of Pelomyxa which made their appearance in
abundance in an aquarium in the Morphological Laboratory,
Cambridge. It seems probable that the amcebulffi in this case are
not parasites but spore-like young, and this is the best observed
case of such reproduction as yet recorded in the group.

Arcella is remarkable for the production of bud-spores, which
may be considered as a process intermediate between simple fission
and the complete breaking up of the parent body into spores. As
many as nine globular processes are simultaneously pinched off from
the protoplasm extruded from the shell of the Arcella ; the nuclei
(present in the parent Arcella to the number of two or three) have
not been traced in connexion with this process. The buds then be-
come nipped off, and acquire a shell and a contractile vacuole (33).

The presence of more than one nucleus is not unusual in Lobosa,
and is not due to a fusion of two or more uninuclear individuals,
but to a multiplication of the original nucleus. This has been
observed in some Amcebas (A. princeps'!) as well as Arcella.
Pelomyxa (Fig. IV. 6) has a great number of nuclei like the Helio-
zoon, Actinosphserium (Fig. VIII.).

Pelomyxa is the most highly differentiated of the Lobosa. The
highly vacuolated character of its protoplasm is exhibited in a less
degree by Lithamreba and resembles that of Heliozoa and Radiolaria.
Besides the numerous nuclei there are scattered in the protoplasm
strongly refringent bodies (Fig. IV. 6,/), the significance of which
has not been ascertained. The superficial protoplasm is free from
vacuoles, hyaline, and extremely mobile. Occasionally it is drawn

14

PROTOZOA

out into very short fane filaments. Scattered in the protoplasm are
a number of minute cylindrical crystals, of unascertained composi-
tion. Pelomyxa is of very large size for a Protozoou, attaining a
diameter of iVth of an inch. It takes into its substance a quantity
of foreign particles, both nutrient organic matter such as Rotifera
and Diatoms and sand particles. It occurs not uncommonly in old

FIG. ~V.— Lithamceba, discus, Lank, (after Lankester, 34). A, quiescent ; B,
throwing out pseudopodia. c.v., contractile vacuole, overlying which the
vacuolated protoplasm is seen ; cone, concretions insoluble in dilute HC1
and dilute KHO, hut soluble in strong HC1 ; n, nucleus.

muddy ponds (such as duck-ponds), creeping upon the bottom, and
has a white appearance to the naked eye. Lithamceba (Fig. V.) is
distinguished by its large size, disk-like form, the disk-like shape of
its pseudopodia, the presence of specific concretions, the vacuolation
of its protoplasm, and the block-like form and peculiar tessellated
appearance of its large nucleus, which has a very definite capsule.
In Lithamceba it is easy to recognize a distinct pellicle or temporary
cuticle which is formed upon the surface of the protoplasm, and
bursts when a pseudopodium is formed. In fact it is the rupture of
this pellicle which appears to be the proximate cause of the outflow
of protoplasm as a pseudopodium. Probably a still more delicate
pellicle always forms on the surface of naked protoplasm, and in the
way just indicated determines by its rapture the form and the
direction of the "flow" of protoplasm which is described as the "pro-
trusion" of a pseudopodium.

The shells of Lobosa Testacea are not very complex. That of
Arcella is remarkable for its hexagonal areolation, dark colour, and
firm consistence ; it consists of a substance resembling chitin.
That of Diftlugia has a delicate membranous basis, but includes
foreign particles, so as to resemble the built-up case of a Caddis
worm.

Arcella is remarkable among all Protozoa for its power of secret-
ing gas-vacuoles (observed also in an Amoeba by Biitschli), which
serve a hydrostatic function, causing the Arcella to float. The gas
can be rapidly absorbed by the protoplasm, when the vacuole neces-
sarily disappears and the Arcella sinks.

CLASS IV. LABYEINTHULIDEA.

Characters. — Gymnomyxa forming irregular heaps of ovoid
nucleated cells, the protoplasm of which extends itself as a branching
network or labyrinth of line threads. The oval (spindle-shaped)
corpuscles, consisting of dense protoplasm, and possessing each a
well-marked nucleus (not observed in Chlamydomyxa), travel regu-
larly and continuously along the network of filaments. The oval
corpuscles multiply by fission ; they also occasionally become
encysted and divide into four spherical spores. The young forms
developed from these spores presumably develop into colonies, but
have not been observed.

Genera. — Two genera only of Labyrinthulidca are known : —
I&byrinthula, Cicnkowski ; Chlamydomyxa, Archer.

Cienkowski (35) discovered Labyrinthula on green Alga; growing
on wooden piles in the harbour of Odessa (marine). It has an
orange colour and forms patches visible to the naked eye. Chlamy-
domyxa was discovered by Archer of Dublin (36) in the cells of
Sphagnum and crawling on its surface ; hence it is a freshwater
form. Unlike Labyrinthula, the latter forms a laminated shell of
cellulose (Fig. VI. 2, c), in which it is frequently completely
enclosed, and indeed has rarely been seen in the expanded
labyrinthine condition. The laminated cellulose shells are very
freely secreted, the organism frequently deserting one and forming
another within or adherent to that previously occupied. The
network of Chlamydomyxa appears to consist of hyaline threads of
streaming protoplasm, whilst that of Labyrinthula has a more
horny consistence, and is not regarded by Cienkowski as protoplasm.

The spindle-shaped cells are much alike in form and size in the
two genera; but no nucleus was detected by Archer in those of
Chlamydomyxa. The encysting of the spindle-cells and their
fission into spores has been seen only in Labyrinthula. Chlamy-
domyxa is often of a brilliant green colour owing to the presence of
chlorophyll corpuscles, and may exhibit a red or mottled red and
green appearance owing to the chemical change of the chlorophyll.

It has been observed to take in solid nourishment, though Labyiin-
thula has not.

The Labyrinthulidea present strong resemblances to the Myceto-
zoa. The genus Daetylostelium (Sorophora) would come very close
to Labyrinthula were the amcebfe of its aggregation plasmodium

Fjrt. VI.— Labyrinthulidea. 1. A colony or "cell-heap" of Labi/riui/niln
ritfllhia, Cirnk., cniwlin^ upon an Alga. 2. A colony or " cell-heap"

of Chlauii/iliitiii/xn labyrinthuloides, Archer, with fully expanded network
of thread's on which tlie oat-shaped corpuscles (cells) are moving, o is an
invested food particle ; at c a portion of the general protoplasm has
detached itself and become encysted. 3. A portion of the network of

Labyrinthula ritdlina, Cienk., more highly magnified, p, protoplasmic
mass apparently produced by fusion of several filaments ; p', fusion of
several cells which have lost their definite spindle-shaped contour ; s,
corpuscles which have become spherical and are no longer moving (perhaps
about to be encysted). 4. A single spindle cell and threads of Labii-

rintltuln macrocosm's, Cienk. n, nucleus. 5. A group of encysted cells

of L. maerocystis. embedded in a tough secretion. (i, 7. Encysted cells

of L. macrocystis, with enclosed protoplasm divided into four spores.
8. 9. Transverse division of a non-encysted spindle-cell of L. macrocystis.

set upon a network of threads. Such a network, whether in the
condition of soft protoplasm or hardened and horny, is represented
in the higher Mycetozoa by the capillitium of the sporocysts.

The most important difference between Archer's Chlamydomyxa
and Cienkowski's Labyrinthula is that in the former the threads

PROTOZOA

15

of the network appear to consist of contractile protoplasm, whilst
in the latter they are described as firm horny threads exuded by
the spindle-cells. Neither form has been re-examined since its
discovery ; and it is possible that this apparent difference will be
removed by further study.

FIG. VII.— Heliozoa. 1. Aetinophrys sol, Ehrli. ; x SOO. a. food-particle
lying in a large food-vacuole ; b, deep-lying finely granular protoplasm ; c,
axial filament of a pseudopodium extended inwards to the nucleus; d, the
central nucleus ; e, contractile vacuole ; /, superficial much-vacuolated
protoplasm. 2. Clathndina clegaiis, Cienk. ; X 200. 3. Hetcr-

ophrys marina, H. and L. x 660. a, nucleus ; b, clearer protoplasm
surrounding the nucleus; c, the peculiar felted envelope. 4. /Vrc//A<'-

diophrys pallida, F. E. Schultze ; x 430. a, food-particle ; b, the nucleus ;
c, contractile vacuole ; it, central granule in which all the axis-filaments of
the pseudopodia meet. The tangentially disposed spicules are seen
arranged in masses on the surface. 5. Acanthoc'/stis turfacea, Carter;

X 240. a, probaldy the central nucleus ; b, clear protoplasm around the
nucleus ; c, more superficial protoplasm with vaeuoles and chlorophyll
corpuscles ; d, coarser siliceous spicules ; e, finer forked siliceous spicules ;
/, finely granular layer of protoplasm. The long pseudopodia reaching
beyond the spicules are not lettered. C. Hi-flagellate "flagellula" of

Acunthacystis aculeata. a, nucleus. 7. Ditto of Clathrulina elegans.

a, nucleus. 8. Astroilisculus ruber, Green* ; x 320. a, red-coloured

central sphere (? nucleus) ; b, peripheral homogeneous envelope.

CLASS V. HELIOZOA, Haeckel, 1866.

Characters. — Gymnomyxa in which the dominating amoeba phase
has the form of a spherical body from the surface of which radiate

numerous isolated filamentous pseudopodia which exhibit very little
movement <>r change <>( form, except when engaged in the inception
of food-particles. The protoplasm of the spherical body is richly
vacuolated ; it may exhibit one or more contractile vaeuoles and
either a single central nucleus or many nuclei (Nuclearia, Aetino-
sphterium). Skeletal products may or may not be present. Flagi-1-
lulaj have been observed as the young forms of some species (Acan-
thocystis, Clathrulina), but very little has been as yet ascertained
as to spore-formation or conjugation in this group, though isolated
facts of importance have been observed. Mostly freshwater forms.

II

FIG. Till.— Heliozoa. 1. AcKnospluerium EicliJiornii, Ehr. ; x 200. a,
nuclei ; b, deeper protoplasm with smaller vaeuoles and numerous nuclei ;

c, contractile vaeuoles ; rf, peripheral protoplasm with larger vaeuoles.
2. A portion of the same specimen more highly magnified and seen in
optical section, a, nuclei ; b, deeper protoplasm (so-called endosarc);

d, peripheral protoplasm (so-called eetosarc); e, pseudopodia showing the
granular protoplasm streaming over the stitf axial filament ; /, food-
particle in a food-vacuole. 3, 4. Nuclei of Actinosphrcrium in the
resting condition. 5-13. .Successive stages in the division of a
nucleus of Actinosphrcriuni, showing fibrillation, and in 7 and 8 formation
of an equatorial plate of cliroinatin substance (after Hertwig). 14.
Cyst-phase of Actum-i'ti^'i-hnii BicAAomii, showing the protoplasm
divided into twelve chlamyduspores, each of which has a siliceous coat ;
a, nucleus of the spore ; g, gelatinous wall of the cyst ; ft, siliceous coat of
the spore.

16

PROTOZOA

ORDER 1. APHROTHORACA, Hertwig (56).
Characters. — Heliozoa devoid of a spicular or gelatinous envelope,
excepting in some a temporary membranous cyst.

Genera. — Nuclearia, Cienk. (37) (many nuclei ; many contractile
vacuoles ; body not permanently spherical, but amreboid) ; Actin-
ophrys, Ehr. (Fig. VII. 1 ; body spberical ; pseudopodia with an
axial skeletal filament ; central nucleus ; one large contractile
vacuole; often forming colonies ; A. sol, the Sun -animalcule);
Actinosph&rium, Stein (Fig. VIII. ; spherical body ; pseudopodia
with axial filament ; nuclei very numerous ; contractile vacuoles 2
to 14) ; Actinoloplius, F. E. Schulze (stalked).

ORDER 2. CHLAMYDOPHORA, Archer (57).

Characters. — Heliozoa with a soft jelly-like or felted fibrous
envelope.

Genera. — Hcterophrys, Archer (Fig. VII. 3); Sphxtiastrum,
Greeff; Astrodiscuhis, Greeff(Fig. VII. 8).

ORDER 3. CHALAROTHORACA, Hertw. and Lesser (58).

Characters. — Heliozoa with a loose envelope consisting of isolated
siliceous spieules.

Genera. — Raphidiophrys, Archer (Fig. VII. 4 ; skeleton in the
form of numerous slightly curved spieules placed tangeutially in
the superficial protoplasm) ; Pompholyxophrys, Archer; Pinacocystis
H. and L. ; Pinaciophora, Greeff ; Acanthoeystis, Carter (skeleton
in the form of radially disposed siliceous needles ; encysted con-
dition observed, and flagellula young, Fig. VII. 6) ; WagnerMa,
Meresch.

ORDER 4. DESMOTHORACA, Hertw. and Less.

Characters. — Heliozoa with a skeletal envelope in the form of a
spherical or nearly spherical shell of silica preforated by numerous
large holes.

Genera. — Orbulinella, Entz (without a stalk) ; Clalhrulina,
Cienk. (with a stalk, Fig. VII. 2).

Further remarks mi the ffeliozoa. — The Sun-animalcules, Actino-
phrys and Actinosphaerium, were the only known members of this
group when Carter discovered in 1863 Acanthoeystis. Our further
knowledge of them is chiefly due to Archer of Dublin, who dis-
covered the most important forms, and figured them in the Quart.
Jour. Micr. Sci. in 1867.

Some of the Proteomyxa (e.g., Vampyrella) exhibit " heliozoon-
like" or " nctinophryd " forms, but are separated from the true
Heliozoa by the fact that their radiant pseudopodia are not main-
tained for long in the stiff isolated condition characteristic of this
group. It is questionable whether Nuclearia should not be relegated
to the Proteomyxa on account of the mobility of its body, which in
all other Heliozoa has a constant spherical form.

Actinophrys sol is often seen to form groups or colonies (by
fission), and so also is Raphidiophrys. It is probable from the
little that is known that reproduction takes place not only by
simple fission but by multiple fission, producing flagellate spores
which may or may not be preceded by eucystment. Only Clath-
rulina, Acanthoeystis, Actinosphserium, and Aetinophrys have
been observed in the encysted state, and only the first two have
been credited with the production of flagellated young. The two
latter genera form covered spores within their cysts, those of Actino-
sphferium being remarkable for their siliceous coats (Fig. VIII.
14), but their further development has not been seen.

CLASS VI. EETICULARIA, Carpenter, 1862.
(Foraminifera, Auct. , Thalamophom. Hertwig).

Characters. — Gymnomyxa in which the dominating amoeba-
phase, often of great size (an inch in diameter), has an irregular
form, and a tendency to throw out great trunks of branching and
often anastomosing filamentous pseudopodia, and an equally strong
tendency to form a shell of secreted membrane or secreted lime or of
agglutinated sand particles (only in one genus of secreted silex) into
Which the protoplasm (not in all ?) can be drawn and out of and
over which it usually streams in widely spreading lobes and
branches. One nucleus is present, or there are many. A contrac-
tile vacuole is sometimes, but not as a rule, present (or at any rate
not described). Reproduction is by fission and (as in some other
Protozoa) by the formation of peculiar bud-spores which remain
for a time after their formation embedded in the parental proto-
plasm. No multiple breaking up into spores after or independent
of the formation of a cyst is known. Marine and freshwater.

The Keticularia are divisible into several orders. The marked
peculiarity of the shell structure in certain of these orders is only
fitly emphasized by grouping them together as a sub-class Per-
forata, in contrast to which the remaining orders stand as a
sub-class Imperforata. The distinction, however, is not an ab-
solute one, for a few of the Lituolidea are perforate, that is, are
sandy isomorphs of perforate genera such as Globigerina and
Rotalia.

Fio. IX.— Gromiidea (Eeticularia membranosa)- l.
Archeri, Barker, a, nucleus; b, contractile vacuoles; c, the yellow oil-like
Ijotly. Moor pools, Ireland. 2. Gromia ovtformis, Duj. a, the

numerous nuclei ; near these the elongated bodies represent ingested
Diatoms. Freshwater. 3. Shcpheardella ttvnuformis, Siddall (yitart.
Jour. Micr. Sci., 1880); X 30 diameters. Marine. The protoplasm is
retracted at both ends into the tubular case. CT, nucleus. 5. Skep-

heardella taniiformis; x 15; with pseudopodia fully expanded.
6-10. Varying appearance of the nucleus as it is carried along in the
streaming protoplasm within the tube. 11. Amphitrema Wrightianum,
Archer, showing membranous shell encrusted with foreign particles.
Moor pools, Ireland. 12. Diaplwrophodon mobile, Archer, a, nucleus.
Moor pools, Ireland.

SUB-CLASS A. Imperforata.

Characters.— Shell-substance not perforated by numerous aper-
tures through which the protoplasm can issue, but provided with
only one or two large apertures, or in branched forms with a few
such apertures.

ORDER 1. GROMIIDEA, Brady.

Characters. — Shell or test membranous, in the form of a simple
sac with a pseudopodial aperture either at one extremity or at both.
Pseudopodia thread-like, long, branching, reticulated. Marine and
freshwater.

Fam. 1. MONOSTOMINA, with a single aperture to the shell.

PROTOZOA

17

Genera. — licbcrkulinia, Clap, and Lach. ; Gromin. Tin']. (Fig.
IX. 2) ; MikfiHiniiiiin, Hert\v. ; EughjpJia, Duj. (shell built up of
hexagonal siliceous plates) ; Diajthuni/ilwilim, Archer (38) (many
foreign (articles cemented to form shell ; small pseudopodia issue
between these, hence resembling Perforata, and large long ones from
the proper mouth of the shell, Fig. IX. 12).

FlG. X.— Imperforata. 1. Spirolocullnaplamilata, Lamarck, showing five
"coils"; porcellanous. 2. Young ditto, with shell dissolved and

protoplasm stained so as to show the seven nuclei n. 3. Spirolina (Pene-
roplis) ; a sculptured imperfectly coiled shell ; porcellanous. 4.

Vertebralina, a simple shell consisting of chambers succeeding one another
in a straight line; porcellanous. 5, 6. Thiirammfna papillata, Brady, a
sandy form. 5 is broken open so as to show an inner chamber; recent,
x 25. 7. Lituola (Haplophragmium) canarietms, a sandy form;

recent, 8. Nucleated reproductive bodies (bud-spores) of Haliphysema.
9. S'/itammitlfna Ixrfs, M. Schultze ; x 40 ; a simple porcellnnous
Miliolkle. 10. Protoplasmic core removed after treatment with weak

chromic acid from the shell of Haliphysema Tumanovitzii, Bow. n,
vesicular nuclei, stained with hrematoxylin (after Lankester). 11.

Haliphysana TmiKtnnn'tzii ; x 25 diam. ; living specimen, showing the
wine-glass-shaped slvll built up of sand-grains and sponge-spicules, and
the abundant protoplasm p, issuing from the mouth of the shell and
spreading partly over its projecting constituents. 12. Shell of Astro-

rhiza, Ituricvla, Sand.; x J; showing the branching of the test on some of
the rays usually broken away in preserved specimens (original). 13.

Section of the shell of irarsipella, showing thick walls built of sand-
grains.

Fam. 2. AMrmsTOMiNA.with an aperture at each end of the shell.

Genera. — }}i/ihi/>liri/.<i, Barker (Fig. IX. 1); Jii/i-fimi, Archer;
Ampliitrcma, Archer (Fig. IX. 11); ,S'/ic/.//c.//v/,7/«, Siddall (39)
(membranous shell very lung and cylindrical so as to be actually
tubular, narrowed to a apout at each end, Fig. IX. 3; protoplasm
extended from either aperture, Fig. IX. 5, and rapidly circulating
within the tubular test during life, carrying with it the nucleus
which itself exhibits peculiar movements of rotation, Fig. IX. 6, 7,
8, 9, 10).

ORDER 2. ASTRORHIZIDEA, Brady.

Characters. — Test invariably consisting of foreign particles ; it ia
usually of large size and single-chambered, often branched or radiatu
with a pseudopodial aperture to each branch, the test often con-
tinued on to the finer branches of the pscudopodia (Fig. X. 12) ;
never symmetrical. All marine.

Fam. 1. ASTRORHIZINA, Brady. Walls thick, composed of loose
sand or mud very slightly cemented.

Genera. — Astrorhizct, Sandahl (Fig. X. 12, very little enlarged);
Pclosina, Brady ; Storthosphaera, Brady ; Dcndrophrya, St. Wright ;
Syrmgammina, Brady.

Fam. 2. PILULININA. Test single-chambered ; walls thick,
composed chiefly of felted sponge -spicules and fine sand, without
calcareous or other cement.

Genera. — Pilulina, Carpenter; TcchniteUa, Norman ; Bathy-

ilifni, Sars.

Fam. 3. SACOAMMININA. Chambers nearly spherical ; walls thill,
composed of firmly cemented sand grains.

Genera. — Psammosph&ra, Schultze; Sorosphxra, Brady ; Saccam-
mina, M . Sars.

Fam. 4. RHABDAMMINIXA. Test composed of firmly cemented
sand - grains, often with sponge - spicules intermixed ; tubular ;
straight, radiate, branched or irregular ; free or adherent ; with one,
two, or more apertures ; rarely segmented.

Genera. — Jaculella, Brady; Marsipclla, Norman (Fig. X. 13) ;
Rhalidammina, M. Sars ; Aschemonclla, Brady ; Bhizammina,
Brady ; Sagenella, Brady ; Botcllina, Carp. ; Haliphysema, Bower-
bank (test wine-glass-shaped, rarely branched, attached by a disk-
like base ; generally beset with sponge-spicules, Fig. X. 11 ; pseudo-
podial aperture at the free extremity). This and Astrorhiza are
the only members of this order in which the living protoplasm has
been observed ; in the latter it has the appearance of a yellowish
cream, and its microscopic structure is imperfectly unknown (61).
In Haliphysema the network of expanded pseudopodia has been
observed by Saville Kent as drawn in Fig. X. 11. Lankester (59)
discovered numerous vesicular nuclei scattered in the protoplasm
(Fig. X. 10, n), and also near the mouth of the shell reproductive
bodies (probably bud-spores) embedded in the protoplasm (Fig. X.
8). Haliphysema was described by Bowerbank as a Sponge, and mis-
taken by Haeckel (60) for a very simple two-cell-layered animal
(Enterozoon), to which he assigned the class name of Physemaria.

ORDER 3. MILIOLIDEA, Brady.

Characters. — Test impcrforate ; normally calcareous and porcel-
lanous, sometimes encrusted with sand ; under starved conditions
(e.g., in brackish water) becoming chitinous or chitino-arenaceous ;
at abyssal depths occasionally consisting of a thin homogeneous,
imperforate, siliceous film. The test has usually a chambered
structure, being divided by septa (each with a hole in it) into a
series of loculi which may follow one another in a straight line
(Fig. X. 4) or the series may be variously coiled (Fig. X. 1 and 3).
The chambering of the test does not express a corresponding cell-
segmentation of the protoplasm ; the latter, although growing in
volume as the new shell-chambers are formed, remains one continuous
cell-unit with many irregularly scattered nuclei (Fig. X. 2). The
chambered and septate structure results in this group and in the other
orders from the fact that the protoplasm, expanded beyond the
last-formed chamber, forms a new test upon itself whilst it lies and
rests upon the surface of the old test. The variations in such a
formation are shown in Fig. XII. 1, 2, 3, 4.

Fam. 1. NUBECULARINA. Test free or adherent, taking various
irregular asymmetrical forms, with variable aperture or apertures.

Genera. — Squ'ininiiiliiifi, Schultze (Fig. X. 9, showing the ex-
panded pseudopodia) ; yuln-nlni-in, Defrance.

Fam. 2. MIUOLIXA. Shell coiled on an elongated axis, either
symmetrically or in a single plane or inequilaterally ; two cham-
bers in each convolution. Shell aperture alternately during growth
(addition of new chambers) at either end of the shell.

Genera.— Biloculina, D'Orb. ; Fabularia, Defrance ; Spirolocu-
lina, D'Orb. (Fig. X. 1, 2) ; MiUoIina, Williamson (Fig. XL).

Fam. 3. HAUERININA. Shell dimorphous ; chambers partly
milioline, partly spiral or rectilinear.

Genera.— Artieulina, D'Orb.; Vertebralvna, D'Orb. (Fig X. 4);
Oplilhdliiiidhun. Ktibler ; Ifaufrina, D'( >rb. ; Planix/iii-inn, Seguenza.

Fam. 4. PENEROPUDINA. Shell planospiral or cyclical, some-
times crosier-shaped, bilaterally symmetrical.

Genera. —Cornuspira, Schultze ; Pencrup!i«, Jlontfort (Fig. X. 3) ;

C

18

PROTOZOA

Orbiculina, Lamarck ; Orbitolites, Lamarck (by a division of the
chambers regularly into chamberlets, and a cyclical mode of growth
which results in shells of the size of a shilling, a very elaborate-
looking structure is produced which has been admirably analysed
by Carpenter (40), to whose memoir the reader is specially referred).

FIO. XI.—MilioKna (Triloculina) tenera. Young living animal with ex-
panded pseudopoJia (after Max Schultze). A single nucleus is seen ill the
innermost chamber.

Fam. 5. ALVEOLININA. Shell spiral, elongated in the line of
the axis of the convolution ; chambers divided into chamberlets.
Genxis. — Alreolina, D'Orb.

Fam. 6. KERAMOSPHJERINA. Shell spherical ; chambers in con-
centric layers.

Genus. — Keramosphaera, Brady.

ORDER 4. LITUOLIDEA, Brady.

Characters. — Test arenaceous, usually regular in contour ; septa-
tion of the many-chambered forms often imperfect, the cavity being
labyrinthic. This order consists of sandy isomorphs of the simpler
Miliolidea, and also of the simpler Perforata (Lagena, Nodosaria,
Cristellaria, Globigerina, Rotalia, Nonionina, &c. ) ; it also contains
some peculiar adherent species.

Fam. 1. LITUOLINA. Test composed of coarse sand-grains, rough
externally ; often labyrinthic.

Genera. — Reophax, Montfort ; Haplophragmium, Reuss (Fig.
X. 7) ; CosTcinolina, Stache ; Placopsiliiia, D'Orb. ; Haplostiche,
Reuss ; Lituola, Lamarck ; Bdclloidina, Carter.

Fam. 2. TROCHAMMININA. Test thin, composed of minute
sand-grains incorporated with calcareous and other organic cement,
or embedded in a chitinous membrane ; exterior smooth, often
polished ; interior smooth or rarely reticulated ; never labyriuthic.

Genera. — Thurammina, Brady (test consisting typically of a
single spherical chamber with several mammillate apertures, Fig.
X. 5, 6) ; Hippocrepina, Parker ; Hormosina, Brady ; Ammo-
discus, Reuss ; Trochammina, Parker and Jones ; Carterina,
Brady; Webbina, D'Orb.

Fam. 3. ENDOTHYRINA. Test more calcareous and less sandy
than in the other groups of Lituolidea ; sometimes perforate ;
septation distinct.

Genera. — Nodosinella, Brady ; Polyphragma, Reuss ; Inrolutina,
Terij. ; Endothyra, Phillips ; Bradyina, Moll. ; Stachcia, Brady.

Fam. 4. LOFTUSINA. Test of relatively large size ; lenticular,
spherical, or fusiform ; constructed either on a spiral plan or in
concentric layers, the chamber cavities occupied to a large extent
by the excessive development of the finely arenaceous cancellated
walls.

Genera. — Cydammina, Brady ; Loftusia, Brady ; Parkeria,
Carpenter.

SUB-CLASS B. Perforata.

Characters. — Shell substance perforated by numerous minute
apertures, through which as well as from the main aperture the
protoplasm can issue.

ORDERS. TEXTULARIDEA, Brady.

Characters. — Tests of the larger species arenaceous, either with
or without a perforate calcareous basis ; smaller forms hyaline and
conspicuously perforated. Chambers arranged in two or more
alternating series, or spiral or confused ; often dimorphous.

Fam. 1. TEXTL-LAUINA. Typically bi- or tri-serial ; often bi-
rarely tri-morphous.

Genera. — Texlularia Defrance ; Cuneolina, D'Orb. ; Verneiul-
ina, D'Orb. ; Tritaxia, Reuss ; Chrysalidina, D'Orb. ; Bigenerma,
D'Orb.; Pawnina, D'Orb.; Spiroplecta, Ehr. ; Gaudryina, D'Orb.;
Valvulina, D'Orb.; Clamdina, D'Orb.

Fam. 2. BULIMININA. Typically spiral ; weaker forms more 01
less regularly biserial ; aperture oblique, comma-shaped or some
modification of that form.

Genera. — Bulimina, D'Orb. ; Virgulina, D'Orb. ; Bifarina,
Parker and Jones ; Bolivina, D'Orb. ; Plcurostomella, Reuss.

Fam. 3. CASSIDULINA. Test consisting of a Textularia-like series
of alternating segments more or less coiled upon itself.

Genera. — Cassidulina, D'Orb.; Ehrenbcrgina, Reuss.

ORDER 6. CHILOSTOMELLIDEA, Brady.

Characters. — Test calcareous, finely perforate, many-chambered.
Segments following each other from the same end of the long axis,
or alternately at the two ends, or in cycles of three, more or less
embracing. Aperture a curved slit at the end or margin of the final
segment.

Genera. — EllipsoiduM, Seguenza; Chilostomclla, Reuss; Allo-
morphiiia, Reuss.

ORDER?. LAGENIDEA, Brady.

Characters. — Test calcareous, very finely perforated ; either
single-chambered, or consisting of a number of chambers joined in
a straight, curved, spiral, alternating, or (rarely) branching series.
Aperture simple or radiate, terminal. No interseptal skeleton nor
canal system.

Fam. 1. LAGENINA. Shell single-chambered.

Genera. — Lagena, Walker and Boys; Nodosaria, Lamk. ; Lin-
gulina, D'Orb. ; Frondicularia, Defrance ; Rhabdogonium, Reuss ;
Marginulina, D'Orb. ; Vaginulina, D'Orb. ; Bimulina, D'Orb. ;
Cristellaria, Lamk. ; Amphicoryne, ScUumb. ; Lina/ulinopsis,'Re\iss;
Flabellina, D'Orb. ; Amphimorphina, Neugeb. ; Dentalinopsis,
Reuss.

Fam. 2. POLYMORPHININA. Segments arranged spirally or
irregularly around the long axis ; rarely biserial and alternate.

Genera. — Polymorphina, D'Orb. ; Dimorphiiia, D'Orb. ; Uviger-
ina, D'Orb. ; Sagrina, P. and J.

Fam. 3. RAMULININA. Shell branching, composed of spherical
or pyriform chambers connected by long stoloniferous tubes.

Genus. — Ramulina, Rupert Jones.

ORDER 8. GLOBIGERINIDEA, Brady.

Characters. — Test free, calcareous, perforate ; chambers few,
inflated, arranged spirally ; aperture single or multiple, con-
spicuous. No supplementary skeleton nor canal system. All the
larger species pelagic in habit.

Genera.— Globigerina, D'Orb. (Fig. XII. 6) : Orbulina, D'Orb
(Fig. XII. 8) ; Hastigerina, Wy. Thomson (Fig. XII. 5) ; Pul-
Unia, P. and J. ; Sphxroidina, D'Orb. ; Candeina, D'Orb.

ORDER 9. ROTALIDEA, Brady.

Characters. — Test calcareous, perforate ; free or adherent. Typi-
cally spiral and "rotaliform" (Fig. XII. 2), that is to say, coiled
in such a manner that the whole of the segments are visible on the
superior surface, those of the last convolution only on the Inferior
or apertural side, sometimes one face being more convex sometimes
the other. Aberrant forms evolute, outspread, acervuline, or
irregular. Some of the higher modifications with double chamber-
walls, supplemental skeleton, and a system of canals. The nature
of this supplemental skeleton is shown in Fig. XII. 2 and 10.

Fam. 1. SPIRILLINIXA. Test a complanate, plauospiral, non-
septate tube ; free or attached.

Genus. — Spirillina, Ehr.

Fam. 2. ROTALINA. Test spiral, rotaliform, rarely evolute, very
rarely irregular or acervuline.

Genera. — Patcllina, Williamson ; Cymbalopora, Hay ; Discorbina,
P. and J. ; Planorbulina, D'Orb. ; Truncatulina, D'Orb. ; Anomal-
ina, P. and J. ; Carpentaria, Gray (adherent) ; Rupcrtia,
Wallick ; Pulvinulina, P. and J. ; Rotalia, Lamk. ; Calcarina,
D'Orb. [Shell rotaliform ; periphery furnished with radiating
spines ; supplemental skeleton and canal system largely developed.
This form is shown in a dissected condition in Fig. XII. 10. Outside
and between the successive chambers with finely perforated walls
a2, a3, a* a secondary shell-substance is deposited by the proto-
plasm which has a different structure. Whilst the successive
chambers with their finely perforate walls (resembling dentine in
structure) are formed by the mass of protoplasm issuing from the
mouth of the last-formed chamber, the secondary or supplemental-
shell substance is formed by the protoplasm which issues through
the fine perforations of the primary shell substance ; it is not
finely canaliculated, but is of denser substance than the primary
shell and traversed by coarse canals (occupied by the protoplasm)
which make their way to the surface of the test (c't c"). In Cal-
carina a large bulk of this secondary shell-substance is deposited
around each chamber and also forms the heavy club-like spines.]

Fam. 3. TINOPORINA. Test consisting of irregularly heaped
chambers with (or sometimes without) a more or less distinctly
spiral primordial portion ; for the most part without any general
pseudopodial aperture.

PROTOZOA

Genera. — Tinaporns, Carpenter; Gijpsina, Carter ; Ajihrosina,
Carter ; Tlialamojwnt, Rocmcr ; /W////v,««, Risso. [Sliell para-
sitic, encrusting, or arborescent ; surfarr uivnl.itol, cnliiiiiTil pink
or white, Fig. XII. 9. Interior partly occupied by small chambers,
arranged in more or less regular layers, and partly by non-
segmented canal-like spaces, often crowded with sponge-spicules
No true canal system. This is one of the most important types as
exhibiting the arborescent and encrusting form of growth. It is
fairly abundant]

10

Fio. XII.— Perforata. 1. Spiral arrangement of simple chambers of a
Reticularian shell. 2. Ditto, with double septal walls, ami supple-

mental shell-substance (shaded). 3. Diagram to show the mode in

which successively-formed chambers may completely embrace their pre-
decessors. 4. Diagram of a simple straight series of non-embracing
chambers. 5. Hastigerina (Globitjerina) Murraiji, Wyv. Thomson.
a, bubbly (vacuolated) protoplasm, enclosing 6, the perforated Globi-
gerina-like shell (couf. central capsule of -Radiolaria). From the peripheral
protoplasm project, not only fine pseudopodia, but hollow spines of
calcareous matter, which are set on the shell, and have an axis of active
protoplasm. Pelagic ; drawn in the living state. 6. GlMgeraui
Indloides, D'Orb., showing the punctiform perforations of tho she'll aiul
the main aperture. 7. Fragment of the shell of Globigerina, seen
from within, and highly magnified, a, fine perforations in the inner shell
substances ; b, outer (secondary) shell substance. Two coarser perfora-
tions are seen in section, and one lying among the smaller. 8. Or-
bnlina universa, D'Orb. Pelagic example, with adherent radiating

. , -, , ,

oils in section, showing the thin primary

OKDEU 10. NUMMULINIDEA, Brady.

Characters. — Test calcareous and finely tubulated ; typically
free, many-chambered, and symmetrically spiral. The higher
modifications all possess a supplemental skeleton, and canal system
of greater or less complexity.

Fam. 1. FUSULININA. Shell bilaterally symmetrical ; chambers
extending from pole to pole; each convolution completely enclosing
the previous whorls. Shell-wall finely tubulated. Septa single or
rarely double ; no true interseptal canals. Aperture a single
elongated slit, or a row of small rounded pores, at the inner edge
of the final segment.

Genera. — Fusulina, Fischer ; Schivagcrina, Holler.

Fam. 2. POLYSTOMELLINA. Shell bilaterally symmetrical, nauti-
loid. Lower forms without supplemental skeleton or interseptal
canals ; higher types with canals opening at regular intervals along
the external septal depressions.

Genera. — Nonionina, D'Orb. ; Polystomella, Lamarck.

Fam. 3. NUMMULITINA. Shell lenticular or complanate ; lower
forms with thickened and finely tubulated shell-wall, but no inter-
mediate skeleton ; higher forms with interseptal skeleton and com-
plex canal system.

Genera. — Archseodiscus, Brady ; Amphistcgiim, D'Orb. ; Oper-
culina, D'Orb. ; Hctcrostcijina, D'Orb. ; Nummulites, Lamarck ;
Assilina, D'Orb.

Fam. 4. CYCLOCLYPEINA. Shell complanate, with thickened
centre, or lenticular ; consisting of a disk of chambers arranged
in concentric annuli, with more or less lateral thickening of lami-
nated shell substance, or acervuline layers of chamberlets. Septa
double and furnished with a system of interseptal canals.

Genera. — Cycloclypeus, Carpenter ; Orbitoidcs, D'Orb.

Fam. 5. EOZOONINA. Test forming irregular, adherent, acervu-
line masses.

Genus. — Eo:oon Dawson.

Further remarks on the Ecticularia, — The name Thalamophora,
pointing to the peculiar tendency which the larger members of
the group have to form chamber after chamber and so to build up
a complex shell, has been proposed by Hertwig (56) and adopted by
many writers. The old name Foraminifera (which did not refer
to the fine perforations of the Perforata but to the large pseudo-
podial aperture leading from chamber to chamber) has also been
extended by some so as to include the simpler Gromia-like forms.
On the whole Carpenter's term Reticularia (62) seems most suitable
for the group, since they all present the character indicated. It
has been objected that the Radiolaria are also reticular in their
pseudopodia, but if we except the pelagic forms of Reticularia
(Globigerina, Orbulina, &c.), we find that the Radiolaria are really
distinguishable by their stiller, straighter, radiating pseudopodia.
No doubt the Labyrinthulid Chlamydomyxa and the plasmodia of
some Mycetozoa are as reticular in their pseudopodia as the
Reticularia, but they possess other distinctive features which
serve, at any rate in an artificial system, to separate them.

The protoplasm of the majority of the Reticularia is unknown,
or only very superficially observed ; hence we have made a point of
introducing among our figures as many as possible which show this
essential part of the organism. It is only recently (1876) that
nuclei have been detected in the calcareous-shelled members of the
group, and they have only been seen in a few cases.

The protoplasm of the larger shell-making forms is known to be
often strongly coloured, opaque, and creamy, but its minute struc-
ture remains for future investigation. Referring the reader to the
figures and their explanation, we would draw especial attention to
the structure of the protoplasmic body of Hastigeriua (one of the
Globigerinidea) as detected by the "Challenger" naturalists. It
will be seen from Fig. XII. 5 that the protoplasm extends as a rela-
tively enormous "bubbly "mass around the shell which is sunk
within it ; from the surface of this " bubbly " (vacuolated or alveol-
atcd) mass the pseudopodia radiate.

The reader is requested to compare this with Fig. XIII., repre-
senting the "bubbly " protoplasmic body of Thalassicolla. It then
becomes obvious that the perforated central capsule CK of the latter
holds the same relation to the mass of the protoplasm as does the
central perforated shell of Globigerina (Hastigerina). The extreme
vacuolation of the protoplasm in both cases (the vacuoles being

20

PROTOZOA

filled with sea- water accumulated by endosmosis) and the stifl' radiat-
ing pseudopodia are directly correlated with the floatiug pelagic life of
the two organisms. All the Eadiolaria are pelagic, and many exhibit
this vacuolatiori ; only a few of the Reticularia are so, and their struc-
tural correlation to that habit has only lately been ascertained.

The Reticularia are almost exclusively known by their shells,
which otter a most interesting field for study on account of the very
great complexity of form attained by some of them, notwithstand-
ing the fact -that the animal which produces them is a simple uni-
cellular Protozoon. Space does not permit the exposition here of
the results obtained by Carpenter in the study of the complex shells
of Orbitolites, Operculina, Nummulites, &c. ; it is essential that his
work Introduction to the Study of the Foraminifcra (Ray Society,
1862) should be consulted, and in reference to the sandy-shelled
forms the monograph by Brady, in the Challenger Reports, vol. ix. ,
1883 ; and it must be sufficient here to point out the general prin-
ciples of the shell-architecture of the Reticularia. Let us suppose
that we have an ever-growing protoplasmic body which tends to
produce a calcareous shell on its surface, leaving an aperture for the
exit of its pseudopodia. It will grow too large for its shell and
accumulate outside the shell. The accumulated external mass may
then secrete a second chamber, resting on the first as chamber 1
rests on chamber 0 in Fig. XII. 4. By further growth a new
chamber is necessitated, and so is produced a series following one
another in a straight line, each chamber communicating with the
newer one in front of it by the narrow pseudopodial aperture
(a, a1, a2, as). Now it is possible for these chambers to be very
variously arranged instead of simply as in Fig. XII. 4. For instance,
each new chamber may completely enclose the last, as in Fig. XII.
3, supposing the protoplasm to spread all over the outside of the
old chamber before making a new deposit. Again the chambers
need not succeed one another in a straight line, but may be dis-
posed in a spiral (Fig. XII. 1). And this spiral may be a flat coil,
or it may be a helicine spiral with a rising axis ; further it may be
close or open. All these forms in various degrees of elaboration
are exhibited by lliliolidea and various Perforata.

But the Perforata in virtue of their perforate shell-walls introduce
a new complication. The protoplasm issues not only from the
mouth of the last-formed chamber, but from the numerous pores in
the wall itself. This latter protoplasm exerts its lime-secreting
functions ; it gathers itself into coarse branching threads which
remain uucalcified, whilst all around a dense deposit of secondary
or supplemental shell-substance is thrown down, thus producing a
coarsely canalicular structure. The thickness and amount of this
secondary shell and the position it may occupy between and around
the chambers of primitive shell-substance vary necessarily in dif-
ferent genera according to the mode in which the primitive cham-
bers are arranged and connected with one another. Cak-arina is a
fairly typical instance of •an abundant secondary shell-deposit (Fig.
XII. 10), audit is the existence of structure resembling the chambers
of Calcarina with their surrounding primary and secondary shell-
substances which has rendered it necessary to regard Eozoon (41) as
the metamorphosed encrusting shell of a pre-Cambrian Reticularian.

The division of the Reticularia iuto Imperforata and Perforata
which is here maintained has no longer the significance which was
once attributed to it. It appears, according to the researches of
Brady, that it is not possible to draw a sharp line between these
sub-classes, since there are sandy forms which it is difficult to
separate from impertbrate Lituolidca and are nevertheless perforate,
in fact are "sandy isomorphs of Lagena, Nodosaria, Globigerina,
and Kotalia. " It does not appear to the present writer that there
can be any insurmountable difficulty in separating the Lituolidca
into two groups — those which are sandy isomorphs of the porcel-
lanous Miliolidea, and those which are sandy isomorphs of the
hyaline Perforata. The two groups of Lituolidea thus formed
might be placed in their natural association respectively with the
Imperforata and the Perforata.

The attempt to do this has not been made here, but the classifi-
cation of Brady has been adopted. In Biitschli's large work on the
Protozoa (9) the breaking up of the Lituolidea is carried out to a
logical conclusion, and its members dispersed among the Milinlidi a
on the one hand and the various orders of Perforata on the other hand.

The calcareous shell-substance of the Miliolidea being opaque
and white has led to their being called " Porcellana," whilst the
transparent calcareous shells of the smaller Perforata has gained
for that group the synonym of "Hyalina. "

Tin; shells of the calcareous Reticularia and of some of the
larger arenaceous forms are found in stratified rocks, from the
Palaeozoic strata onwards. The Chalk is in places largely com-
posed of their shells, and the Eocene Nuimmilitic limestone is
mainly a cemented mass of the shells of Nummulites often as
large each as a shilling. The Atlantic ooze is a chalky deposit
consisting largely of the shells of Globigerina, &c.

CLASS VII. RADIOLAEIA, Ilacckel, 1862 (63) (Polyajstina, Ehr.).

Characters. — Gymnomyxa in which the protoplasmic body of

the dominant amreba phase has the form of a sphere or cone from

the surface of which radiate filamentous pseudopodia, occasionally
anastomosing, and encloses a spherical (homaxonic) or cone-shaped
(monaxonic) perforated shell of membranous consistence known as
the central capsule, and probably homologous with the perforated
shell of a Globigerina. The protoplasm within the capsule (intra-
capsular protoplasm) is continuous through the pores or apertures
of the capsule with the outer protoplasm. Embedded in the former
lies the large and specialized nucleus (one or more). Gelatinous
substance is frequently formed peripherally by the extracapsular
protoplasm, constituting a kind of soft mantle which is penetrated
by the pseudopodia. A contractile vacuole is never present.

Usually an abundant skeleton, consisting of spicules of silica or
of a peculiar substance called acanthin arranged radially or tangen-
tially, loose or united into a basket-work, is present. Oil globules,
pigment, and crystals are found in greater or less abundance in
the protoplasm.

In most but not all Radiolaria peculiar nucleated yellow cor-
puscles are abundantly present, usually regarded as parasitic Alga?.
Reproduction by fission has been observed, and also in some few
species a peculiar formation of swarm-spores (flagelluke) within the
central capsule, in which the nucleus takes an important part.
All the Radiolaria are marine. The Radiolaria are divided into
two sub-classes according to the chemical nature of their spicular
skeleton, and into orders according to the nature and the disposi-
tion of the apertures in the wall of the central capsule.

EP

FIG. XIII.— Thalassicollapelagica, Haeckel; x 25. C'K, central capsule ;
EP, extracapsular protoplasm ; al, alveoli, liquid-holding vacuoles in the
protoplasm similar to those of Heliozoa, Pelomyxa, Hastigerina, &c.; ps,
pseudopodia. The minute unlettered dots are the "yellow cells."

SUB-CLASS I. Silico-Skeleta, Lankcster.

Characters. — A more or less elaborate basket-work of tangential
and radial elements consisting of secreted silica is present ; in rare
exceptions no skeleton is developed.

ORDER 1. PEKIPYLYEA, Hertwig.

Characters. — Silico-skeletal Radiolaria in which the central cap-
sule is uniformly perforated all over by fine pore-canals ; its form is
that of a sphere (homaxonic), and to this form the siliceous skeleton
primarily conforms, though it may become discoid, rhabdoid, or
irregular. The nucleus is usually single, but numerous nuclei are
present in each central capsule of the Polycyttaria.

Fain. 1. SniJEitiDA, Haeck. Spherical Peripylwa with a spheri-
cal basket-work skeleton, sometimes surrounded by a spongy outer
skeleton, sometimes simple, sometimes composed of many concentric
spheres (never discoid, flattened, or irregular). The central capsule
sometimes encloses a part of the spherical skeleton, and often is
penetrated by radiating elements.

Genera (selected). — Ethmosphmra, Haeck. ; Xi/ihosjJm-ra, Haeck. ;
Staurosphiera, Haeck. ; 7/r/i'<«///i.Tra, Haeck. (Fig. XIV. 14) ; As-
trifinma, Haeck. ; Ilaliominn, Haeck. ; Aftinommn, Haeck. (Fig.
XIV. 17 ; note the sphere within sphere, the smallest lying in the
nucleus, and the whole series of spherical shells connected by radial
spines) ; Arai'lniw/ilm'i'it, Haeck. ; Plcrjmospheera, Haeck. ; Spunyo-
sphara, Haeck. (Fig. XVI. 8).

Fam. 2. DISCIPA, Hueck. Discoid Peripylrea ; both skeleton
and central capsule flattened.

Genera (selected). — I'hteodiscns, Haeck. ; lldiodiscus, Haeck. ;
Spongodiscus, Haeck. ; Sponcjums, Haeck.

PROTOZOA

21

Fain. 3. TIIALASSICOLUDA. lYripyhea devoid of a skeleton, or
with a skeleton composed of loose siliceous spicules only. Nucleus
single ; central capsule and general protoplasm s|ilierieal.

Ccncra (selected). — Thal,if:*i<vU,i, Huxley (Fig. XIII., Fig.
XIV. 1) ; Thalassospfitsra, Haeck. ; J'/ti/- nmiii'm, llaeek.

Fain. 4. POLYCYTTARIA. Peripvhea consisting of colonies of
many central capsules united by their eztracapsulai protoplasm.
Central capsules multiplying Ijy fission. Nuclei in each central
capsule numerous. Siliceous skeleton cither absent, or of loose
spicules, or having the form of a spherical fcncstrated shell sur-
rounding each central capsule.

Genera (selected). — Collosphssra, Mnller (with fenestrated globular
skeleton); SjiJuerozoum, Haeck. (skeleton of numerous loose spicules
which are branched); J!:i/i!iii!ii:uitin, Hacck. (spicules simple); Col-
faouin, Miiller (devoid of skeleton, Fig. XIV. 2, 3, 4, 5).

1

each encloses a crystalline rod. c, yellow cells hing in tin- extraeapsnlar
protoplasm. 5. A small colony of Colluzott at. iu< riu<', magnified -J5

diameters. a, alveoli (vaeuolus) of tlie extraeapsular prntdjilasni ; 6,
central ea]isub s. each eontainiiiL.' besiilrs ].rotupla>Tn a hove oil-globule.
6-13. Yellow cells of various Kailiolai ia :— (J, normal yellow cell; 7, 8.
division \\itb I.M ination of transverse septum ; it, a modilieil ronililiim
aeeonling to llraii'H ; in, division of a yellow cell into four ; 11, amieboid
condition of a yellow cell from the bu.ly of a dead Sphicro/.onn ; 1:2, a
similar cull in process of division ; 13, a yellow cell the protoplasm of
which is ereeping out of its cellulose envelope. 14. //'V/n..,y,/, < i -".

in, •riiiii, Hacck., living example; x 400. a, nucleus; i>, central capsule ;
c, siliceous basket-work skeleton. IS. Two swarm-spore* (tla^-llnhe)

of Coltozoitiii in<'fn>i', set fl'cu trom such a central capsule as that drawn in
4 ; each contains a crystal b and a nucleus a. n;. Two swarm-spores

of Collozoum iiicrmc, of the second kind, viz., devoid of crystals, and of
two sizes, a macrospore and a microspore. They have been set free

from central capsules with c tents of a dilferent appearance from that

drawn in 4. a, nucleus. 17. Acttnomma asteracanthion, Haeck ; x 200;
one of the i'eripyloca. Entire animal in optical section, a, nucleus;
6, wall of the central capsule ; c, innermost siliceous shell enclosed iu the
nucleus; c\ middle shell lying within the central capsule ; c2, outer shell
lying in the extraeapsular protoplasm. Four radial siliceous spines, hold-
ing the three spherical shells together arc Been. The radial fibrillation of
the protoplasm and the fine extraeapsular pseudopodia are to be noted
18. Amphilonche M<'.«(IH> H.«S, Haeck; x 200; one of the Acanthometridea.
Entire animal as seen living.

ORDER 2. MONOPYL&A, Hertwig.

Characters. — Silico-skeletal Kadiolaria in which the central cap-
sule is not spherical but monaxonic (cone-shaped), with a single per-
forate area (pore-plate) placed on the basal face of the cone ; the
membrane of the capsule is simple, the nucleus single ; the skeleton
is extraeapsular, and forms a scaffold-like or bee-hive-like structure
of mouaxonic form.

FIQ. XIV.— Eadiolaria. 1. Central capsule of Thalamcolla nucleata,
Huxley, iu radial section, a, the large nucleus (Binnenblaschen); b,
corpuscular structures of the intracapsular protoplasm containing con-
cretions; c, wall of the capsule (membranous shell), showing the fine
radial pore-canals ; d, nucleolar fibres (chromatiu substance) of the
nucleus. 2,3. Collozoinn i/ien/ie, J. Miiller, two dilferent forms of

colonies, of the natural size. 4. Central capsule from a colony of

Collozoum iturme, showing the intracapsular protoplasm and nucleus,
broken up into a number of spores, the germs of swarm-spores or flagellulc; ;

one of the Monopyhea.
central capsule ia
ch it is lodged.

Fam. 1. PLECTIDA, Haeck. Skeleton formed of siliceous spines
loosely conjoined.

Genera (selected).— Pliiiiufimtlta, Haeck. ; Plcgmatium, Haeck.

Fam. 2. CYRTIDA, Haeck. Skeleton a monaxonic or triradiate
shell, or continuous piece (bee-hive-shaped).

Genera (selected).— JL>1 hili/jilra, Haeck. ; Eueyrtidium, Haeck.
(Fig. XV.); CarpocanlttM. Haeck. (Fig. XVI. 3).

Fam. 3. BOTIUDA, Haeck. Irregular forms ; the shell composed
of several chambers agglomerated without definite order ; a single
central capsule.

Genera. — Botryocyrtia, Haeck. ; Lithobotrys, Haeck.

Fam. 4. SPYKIDA, Haeck. Gcmmiuate forms, with shell con-
sisting of two conjoined chambers ; a single central capsule.

Fam. 5. STEPHIDA, Hacck. Skeleton cricoid, forming a single
siliceous ring or several conjoined rings.

Genera (selected).— Acanthodesmia, Haeck.; Zygostcpkanus,
Haeck. ; Lithocirms, Haeck. (Fig. XVI. 1).

ORDERS. PHJIODARIA, Haeck. (Triinjlasa, Hertwig).
Characters.— Silico-skeletal Kadiolaria iu which the central

22

PROTOZOA

Fia. XVI. — Radiolaria, 1. Lithocircus annularis, Hertwig ; one of the
Monopylica. Whole animal in the living state (optical section), a, nucleus ;
6, wall of the central capsule ; c, yellow cells ; d, perforated area of the
central capsule (Monopyliea). 2. Cystidiuiu incniie, Hertwig ; one of the
Monopyltea. Living animal. An example of a Monopylaeon destitute of
skeleton, fl, nucleus ; 6, capsule-wall ; c, yellow cells in the extraeapsular
protoplasm. 3. Carjincaniuni diadema, Haeck. ; optical section of the bee-
hive-shaped shell to show the form and position of the protoplasmic body.
a, the tri-lobed nucleus ; b, the siliceous shell ; c, oil-globules ; d, the per-
forate area (pore-plate) of the central capsule. 4. Caelodendrum
gracillimum, Haeck. ; living animal, complete ; one of the Tripylrca. a, the
characteristic dark pigment (pheeodmm) surrounding the central capsule ft.
The peculiar branched siliceous skeleton, consisting of hollow fibres, and
the expanded pseudopodia are seen. 5. Central capsule of one of the
Tripytea, isolated, showing a, the nucleus ; b,c, the inner and the outer
lamina? of the capsule-wall ; d, the chief or polar aperture ; c,c, the two
secondary apertures. 6, 7. Acanthmmt rn Claimredei, Haeck. 7 shows
the animal in optical section, so as to exhibit the characteristic meeting of
the spines at the central point as in all Acanthometridea ; 6 shows the
transition from the uninuclear to the niultinnclear condition by the
breaking up of tile large nucleus, a, small nuclei ; ft, large fragments of
the single nucleus ; c, wall of the central capsule ; d, extracapsular jelly
(not protoplasm); c, peculiar intracapsular yellow cells. 8. Spongo-
spkirra streptacantlia, Haeck. ; one of the Peripylrca. .Siliceous skeleton
not quite completely drawn on the right side, a, the spherical extra-
capsular shell (compare Fig. XIV. 17), supporting very large radial spines
which are connected by a spongy network of siliceous "fibres. 9.
Aulosphxra eUgantousiTMt, Haeck.; one of the 1'hajodaria . Half of the
spherical siliceous skeleton.

capsule has a double membrane and more than one perforate area,
viz., one chief " polar aperture," and one, two, or more accessory
apertures (Fig. XVI. 5). The nucleus is single. Around the
central capsule is an abundant dark brown pigment (phieodium of
Haeckel). The siliceous skeleton exhibits various shapes regular
and irregular, but is often remarkable for the fact that it is built
up of hollow tubes.

Fam. 1. PH^EOCYSTIDA, Haeck. The siliceous skeleton is either
entirely absent or consists of hollow needles which are disposed
outside the central capsule, regularly or irregularly.

Genera (selected). — Aulacantha, Haeck. ; Thalassoplancta, Haeck.

Fam. 2. PH^EOGROMIDA, Haeck. The siliceous skeleton consists
of a single fenestrated shell, which may be spherical, ovoid, or often
dipk'uric, but always has one or more large openings.

Genera (selected). — Challcngeria, Wy. Thomson; Litliogromia,
Haeck.

Fam. 3. PH«OSPH.ERIDA. The siliceous skeleton consists of
numerous hollow tubes which are united in a peculiar way to form,
a large spherical or polyhedral basket-work.

Genera (selected). — Aulosplmra, Haeck. (Fig. XVI. 9); Aulo-
plegma, Haeck. ; C'annacantha, Haeck.

Fam. 4. PH^OCONOHIDA. The siliceous skeleton consists of two
separate fenestrated valves, similar to a mussel's shells ; often there
are attached to the valves simple or branched hollow tubes of silex.

Genera (selected). — Gonchidimn, Haeck. ; Ccelodcndrum, Haeck.
(Fig. XVI. 4).

SUB-CLASS II. Acanthometridea, Lankester ( = AcantUno-skeUta).

Characters. — Radiolaria in which the skeleton is composed of a
peculiar horny substance known as acauthin (rarely of silica).
The central capsule is uniformly perforate (Peripylsea type). A
divided or multiple nucleus is present in the capsule ; the capsule-
wall is single. The skeleton always has the form of spines which
radiate from a central point within the capsule where they are all
fitted to one another. Karely a fenestrated tangential skeleton is
also formed.

Fam. I. ACANTHONIDA, Haeck. Skeleton consisting of twenty
spines of acanthin disposed in five parallel zones of four spines each,
meeting one another at the central point of the organism ; nevei'
forming a fenestrated shell.

Genera (selected). — AcanOiometra, J. Miiller (Fig. XVI. 6, 7) ;
Astrolonclie, Haeck. ; AmpJiilonche, Haeck. (Fig. XIV. 18).

Fam. 2. DIPLOCONIDA, Haeck. Skeleton a double cone.

Genus unieum. — Diploeonus, Haeck.

Fam. 3. DORATASPIDA, Haeck. The twenty acanthin spines of
the skeleton form by transverse outgrowths a spherical fenestrated
shell.

Genera (selected). — Stauraspis, Haeck. ; Dorataspis, Haeck.

Fam. 4. SPHJEROCAPSIDA, Haeek. The twenty acanthin spines
are joined together at their free apices by a simple perforate shell
of acanthin.

Genus unieum. — Sph&rocapsa.

Fam. 5. LITHOLOPHIDA. Skeleton of many needles of acanthin
radiating from a single point without definite number or order.

Genera. — Litholuphus, Haeck. ; Astrolophus, Haeck.

Further remarks mi the Radiolaria. — It has not been possible in
the systematic summary above given to enumerate the immense
number of genera which have been distinguished by Haeckel (42) as
the result of the study of the skeletons of this group. The important
differences in the structure of the central capsule of different Radio-
laria were first shown by Hertwig, who also discovered that the spines
of the Acanthometridea consist not of silica but of an organic com-
pound. In view of this latter fact and of the peculiar numerical
and architectural features of the Acanthomctrid skeleton, it seems
proper to separate them altogether from the other Radiolaria. The
PeripyLea may be regarded as the starting point of the Radiolarian
pedigree, and have given rise on the one hand to the Acantho-
metridea, which retain the archaic structure of the central capsule
whilst developing a peculiar skeleton, and on the other hand to
the Monopyliea and Phajodaria which have modified the capsule
but retained the siliceous skeleton.

Phffiodaria.

PcripyUea.

Monopytea.

Acanthometridea.

Archi-peripyliea.
RADIOLARIA.

The occasional total absence of any siliceous or acanthiuous
skeleton docs not appear to be a matter of classiticatory importance,
since skeletal elements occur in close allies of those very few forms

PROTOZOA

23

which are totally devoid of skeleton. Similarly it does not appear
to be a matter of great significance that some forms (Polyeyttaria)
form colonies, instead of the central capsules separating from one
another after fission has occurred.

It is important to note that the skeleton of silex or acaiithin
does not correspond to the shell of other Gymnomyxa, which
appears rather to be represented by the membranous central cap-
sule. The skeleton does, however, appear to correspond to the
spiculcs of Heliozoa, and there is an undeniable affinity between
such a form as rlathrulina (Fig. VII. 2) and the Splwrid 1'eripyla-a
(such as lleliuspluera, Fig. XIV. 14). The Kadiolaria are, however,
a very strongly marked group, definitely separated from all other
Gymnoniyxa by the membranous central capsule sunk. 'in their proto-
plasm. Their differences inter sc do not affect their essential struc-
ture. The variations ill the chemical composition of the skeleton and
in the perforation of the capsule do not appear superficially. The
most obvious features in which they differ from one another relate to
tho form and complexity of the skeleton, a part of the organism so
little characteristic of the group that it may be wanting altogether.
It is not known how far the form-species and form-genera which
have been distinguished in such profusion by Haeckel as the
result of a study of the skeletons are permanent (i.e., relatively
permanent) physiological species. There is no doubt that very
many are local and conditional varieties of a single Protean species.
The same remark applies to the species discriminated among the
shell-bearing Reticularia. It must not be supposed, however, that
less importance is to be attached to the distinguishing and record-
ing of such forms because we are not able to assert that they are
permanent species.

The yellow cells (of spherical form, '005 to 0'15 of a millimetre
in diameter) which occur very generally scattered in the extra-
capsular protoplasm of Radiolaria were at one time regarded as
essential components of the Radiolarian body. Their parasitic
nature is now rendered probable by the observations of Cien-
kowski (43), Brandt (44), and Geddes (45), who have established
that each cell has a cellulose wall and a nucleus (Fig. XIV. 6 to 13),
that the protoplasm is impregnated by chlorophyll which, as in
Diatoms, is obscured by the yellow pigment, and that a starch-
like substance is present (giving the violet reaction with iodine).
Further, Cienkowski showed, not only that the yellow cells multiply
by fission during the life of the Radiolarian, but that when isolated
they continue to live ; the cellulose envelope becomes softened ;
the protoplasm exhibits amoeboid movements and escapes from the
envelope altogether (Fig. XIV. 13) and multiplies by fission.
Brandt has given the name Zooxanthdlti nutricola to the parasitic
unicellular Alga thus indicated. He and Geddes have shown that a
similar organism infests the endoderm cells of Anthozoa and of
some Siphonophora in enormous quantities, and the former has been
led, it seems erroneously, to regard the chlorophyll corpuscles of
Hi/'lra viritlis, Spongilla, and Ciliata as also parasitic Alga?, for
which he has coined the name Zoochlorella. The same arguments
which Brandt has used to justify this view as to animal chlorophyll
would warrant the creation of a genus " Phytochlorella " for the
hypothetical Alga which has hitherto been described as the
"chlorophyll corpuscles" of the cells of ordinary green plants.

Zoomnthclla nutricola does not, for some unknown reason, infest
the Acanthometridea, and it is by no means so universally present
in the bodies of the Silico-skeleta as was supposed before its
parasitic nature was recognized.

The streaming of the granules of the protoplasm has been observed
in the pseudopodia of Radiolaria as in those of Heliozoa and
Reticularia ; it has also been seen in the deeper protoplasm ; and
granules have been definitely seen to pass through the pores of the
central capsule from the intracapsular to the extracapsular pro-
toplasm. A feeble vibrating movement of the pseudopodia has
been occasionally noticed.

The proa'Ktion of swarm-spores has been observed only in
Acanthometra and in the Polycyttaria and Thalassicollidse, and
only in the two latter groups have any detailed observations been
made. Two distinct processes of swarm-spore production have
been observed by Cienkowski (43), confirmed by Hertwig (46) — dis-
tinguished by the character of the resulting spores which are
called " crystalligerous " (Fig. XIV. 15) in the one case, and "di-
morphous" in the other (Fig. XIV. 16). In both processes the
nucleated protoplasm within the central capsule breaks up by a
more or less regular cell-division into small pieces, the details of
the process differing a little in the two eases. In those individuals
which produce crystalligerous swarm-spores, each spore encloses a
small crystal (Fig. XIV. 15). On the other hand, in those indi-
viduals which produce dimorphous swarm-spores, the contents of
the capsule (which in both instances are set free by its natural
rupture) are seen to consist of individuals of two sizes " macro-
spores" and "microspores," neither of which contain crystals
(Fig. XIV. 16). The further development of the spores has not
been observed in either case. Both processes have been observed
in the same species, and it is suggested that there is an alternation
of sexual and asexual generations, the crystalligerous spores

developing directly into adults, which in their turn produce in
their central capsules dimorphous swarm-spores (inaerosporcs and
mierospores), which in a manner analogous to that observed in the
Volvocinnaii Flagellata ru]ni]:ite (permanently fuse) with one
another (the larger with the smaller) before proceeding to develop.
The adults resulting fnun this process would, it is suggested, pro-
duce in their turn crystalligerous swarm-spores. Unfortunately
we have no observations to support this hypothetical scheme of a
life-history.

Fusion or conjugation of adult Radiolaria, whether preliminary
to swarni-spore-productiou or independently of it, has not been
observed — this affording a distinction between them and Ilcliozoa,
and an agreement, though of a negative character, with the Reticu-
laria.

Simple fission of the central capsule of adult individuals and
subsequently of the whole protoplasmic mass has been observed in
several instances, and is probably a general method of reproduction
in the group.

The siliceous shells of the Radiolaria are found abundantly in
certain rocks. They furnish, together with Diatoms and .Sponge-
spicules, the silica which has been segregated as flint in the Chalk
formation. They are present in quantity (as much as 10 per cent.)
in the Atlantic ooze, and in the celebrated "Barbados earth" (a
Tertiary deposit) are the chief components.

GRADE B. CORTICATA, Lankester, 1878(64).

Characters. — Protozoa in which the protoplasm of the cell-body,
in its adult condition, is permanently differentiated into two layers,
an outer denser cortical substance and an inner more fluid medul-
lary substance (not to be confused with the merely temporary
distinction of exoplasm and endoplasm sometimes noted in
Gymnomyxa. which is not structural but due to the gravitation and
self-attraction of the coarser granules often embedded in the
uniformly fluid protoplasm).

Since the Corticata have developed from simple Gymnomyxa
exhibiting both amceboid and flagellate phases of form and activity,
it results (1) that the forms of the body of many Corticata are
traceable to modifications of these primitive forms ; (2) that the
young stages of the Corticata are in the lower classes of that group
typical flagellulfe or amcebulrc ; and (3) that there are certain
archaic forms included in those lower classes whose position there
is doubtful, and which might be with almost equal propriety assigned
to the Gymnomyxa, since they are transitional from that lower grade
to the higher grade of Corticata.

CLASS I. SPOROZOA, Leuckart (47) ; Syn. Gregarinida, Auct.

Characters. — Corticata parasitic in almost all classes and orders of
animals, imbibing nutriment from the diffusible albuminoids of
their hosts and therefore mouthless. In typical cases there is
hatched from a chlamydospore one or more modified nucleate or
non-nucleate flagcllulas (falciform young, drepanidium phase).
The flagellula increases in size and differentiates cortical and
medullary substance. Fission is common in the younger stages of
growth. The movements now become neither vibratile nor amoe-
boid but definitely restrained, and are best described as " eugle-
noid" (c/. Flagellata, Fig. XX. 27, 28). The nucleus is single,
large, and spherical. No contractile vacuole and rarely any vacuole
is present. A size of T\jth inch may be attained in this phase,
which may be definitely spoken of as the euglena phase corre-
sponding to the amreba phase of Gymnomyxa. It is usually of
oblong form, with sac-like contractile wall of cortical substance,
but may be spherical (Coceidiidea) or even amceboid (Myxosporidia).

Conjugation, followed directly or after an interval by sporulation,
may now ensue. The conjugated individuals (two), or sometimes a
single individual, become encysted. The contents of the cysts now
rapidly divide (by a process the details of which are unknown) into
minute ovoid nucleated (?) bodies ; sometimes a portion of the
protoplasm is not converted into spores but may form sporoducts
(cf. capillitium of Mycetozoa). Each piece acquires a special
cliitin-like colourless coat, and is then a chlamydospore. Rarely
one spore only is formed from the whole contents of a cyst. The
spore-coat is usually thick, and remarkable for processes and other
accessory developments. The included protoplasm of the chlamydo-
spore frequently divides into several pieces before hatching. These
usually, when set free from the spore-coat, have the form of modified
nucleated flagelluls, i.e., flagellula' in which the protoplasm is not
drawn out into a thread-like rlagellum but exhibits an elongate form,
uniformly endowed with vibratile activity. With few (if any) excep-
tions, the falciform young thus characterized penetrates a cell of some
tissue of its host and there undergoes the first stages of its growth
(hence called Cytozoa). In some forms the pre-cystic phase never
escapes from its'cell host. In other cases it remains connected with
the hospitable cell long after it has by growth exceeded by many
hundred times the bulk of its quondam entertainer ; often it loses
all connexion with its cell host and is carried away to some other
part of the infested animal before completing its growth and
encysting.

PROTOZOA

The Sporozoa are divided into four sub-classes, differing from one
another according to the form and development attained by the
euglena phase. We shall place the most highly developed first, not
only because our knowledge about it is most complete, but because
it is possible that one at least of the other sub-classes is derived by
degeneration from it.

SUB-CLASS I. Gregarinidea, Butschli (9).

Characters. — Sporozoa in which the euglena phase is dominant,
being relatively of large size, elongate in form, definitely shaped,
having contractile but not viscid cortex, and exhibiting often active
nutritional and locomotor phenomena. Though usually if not
invariably cell-parasites in early youth, they become free before
attaining adult growth, and inhabit either the body-cavity or the
intestine of their hosts. Many spores are produced in the encysted
phase. The spores have an oblong, sometimes caudate coat, and
produce each one or several falciform young. At present only
known as parasites of Invertebrata.

1'jo. XVII.— Sporozoa. 1, ••. .l/,,,,,,,-.,.,.v „ nil , Sb in ; 250; from the testis
of the Earthworm. Two phases of movement— a ring-like contraction
passing along the body from one end to the other. 3. Individual of the

same species which has penetrated in the young stage a sperm -cell of the
Earthworm, and is now clothed as it were with spermatoblasts. 4.

Monocyitis inagna, A. Schmidt, from the testia of the Earthworm (L. tcrres-

tris, L.). Two individuals, which are implanted hy one extremity at 6 in
two epithelial cells of the rosette of the spermatic duct, a, nucleus of the
Monocystis. 5. Tailed chlamydospores of Monocystis sxmtridis,

Roll. 0. Two M. ayilis encysted, spores forming on the surface of the

protoplasm. 7. A similar cyst further advanced in spore-formation (see

Fig. XVIII.). 8. Spore of M. agilis, now elongated but still naked,
a, nucleus, x 1400. 9. The spore has now encased itself in a navicula-

shaped coat, a, nucleus. 10. The spore protoplasm has now divided

into several falciform swarm-spores, leaving a portion of the protoplasm
unused, b, Schneider's residual core. 11. Optical transverse section of
a completed spore, b, Schneider's residual core. 12. Chlamydospore

of Eluxsia c/iilonis, nov. sp., from the liver of Chiton (original.) 13,

14. Chlamydospore of Monocystis nemerlis, Koll., liberating falciform
young, b, Schneider's residue. 15. Monocystis pellucida, Kbll. (from

Nereis) ; x 150 ; to show the very thick cortical substance and its fibrilla-
tion (after Lankester, 54). 16. Monocystis sxnuridis, Kbll., two indivi-
duals adhering to one another (a syzygium). For spores see 5. 17. Mona-
cystis aphroditie, Lankester (55); x 00; remarkable among Monocystids
for its long proboscis resembling the epimerite of some Septata. 18.

Klossia lieiicinn, Aim. Schn., from the kidney of Helix Iwrtensis. A single
cell of the renul epithelium in which a full-grown Klossia is embedded.
a, nucleus of the Klossia ; a', nucleus of the renal cell. 19. Cyst of

Klossia ht'l/cina., the contents broken up into spherical chlamydo-
spores. 20. Single spore from the last, showing falciform young and a
Schneider's residue b. 21. The contents of the same spore. 22. A small
renal cell of Helix containing two of the youngest stage of Klossia. 23.
Monocystis sagittate, Lenck., from the intestine of Cajjitclla capitata ;
X 100. 24 to 31. Coeeidium oviforme, Leuck., fromtheliver of the Rabbit :
—24, adult individual encysted ; 25, the protoplasm contracted— a,
nucleus ; 26, 27, division into four spores, as yet naked ; 28, 29, the spores
have acquired a covering, i.e. , are chlamydospores, and each contains a single
falciform young ; 30, 31, two views of a chlamydospore more highly magni-
fied so as to show the single falciform young (from Leuckart). 32. Klossia
octopiana, Aim. Schn., from Cephalopoda, a, nucleus; b, cyst-membrane,
x 200 diam. 33. Single spherical spore of the same ; x 1400 diam ;
showing numerous falciform young, and b, Schneider's residue. 34.
M<f.ridium Lieberkuhiiii, Biitschli, one of the Myxosporidia, from the
bladder of the Pike (Esox) ; creeping euglena phase, showing strongly
lobed amoeboid character (pseudopodia and undifferentiated (?) cortex) ;
x CO diam. 35-39. Eimeriafalc/formis, Eimer sp., from the Mouse : —
35, an adult non-encysted individual inhabiting an epithelial cell of the
intestine of the mouse ; 36, encysted phase ; 37, clear corpuscles appear
in the encysted protoplasm ; 38, the protoplasm now forms a single
spore containing several falciform young; b, Schneider's residue; 39,
isolated spore showing falciform young, and 6, Schneider's residue.
40. Chlamydospore of Myxobolus IHiUleri, Butschli, one of the Myxo-
sporidia from the gills of Cyprinoid Fishes, a, nucleus ; 6, refringent
corpuscle ; o, polar body or thread-capsule. 41. A similar chlamydo-
spore which has ejected the filaments from its thread capsules. 42.
Chlamydospore of a Myxosporidium infesting the kidney of Lotctmilgaris.
c, polar body (psorosperm of authors). 43, 44. Chlamydospores of
a Myxosporidium from the gills of Perca (psorosperm of authors).
Compare with the tailed chlamydospore of Monocystis sxnuridis, 5. 45
-47. Drepanidium ranarum, Lankester, the falciform young of an
unascertained Coccidiide infesting the Frog (supposed by Gaule to be pro-
duced by the blood corpuscles): — 45, specimen stained by iodine ; 46, red-
blood corpuscle of Fl'Og, showing b, two contained Drepanidia, and a, the
nucleus of the blood corpuscle ; 47, living Drepanidinm. 48. Chlamy-
dospore of Lieherkuhn's Coeeidium of the Frog's kidney, perhaps belong-
ing to the life-cycle of Drepanidium ranarum.. The spore contains
two falciform young (Drepanidia?) and a Schneider's residue. 49.
Chlamydospore of Sfonocystis thatassemes, Lankester, containing nume-
rous falciform young. 50, 51. Sarcoci/stis Mie*cli>ri, Lankester: — 50,
falciform young escaped from chlamydospores ; 51, adult englena phase
inhabiting a striated muscle fibre of the Pig.

OEDER 1. HAPLOCYTA, Lankester.

Characters. — Gregarinidea in which there is never at any time a
partition of the medullary substance into two or more chambers.
The eugleuoid is always a single contractile sac with one mass of
medullary substance in which floats the large vesicular transparent
nucleus. Spores larger than in the next group, each producing
several falciform young.

Genus unicum. — Monocystis, Stein, 1848. The various generic
subdivisions proposed by Aim. Schneider (48), and accepted by
Butschli, appear to the present writer to have insufficient characters,
and serve to complicate rather than to organize our knowledge of
the subject. We do not yet know enough of the sporulatiou and
subsequent development of the various monocystic Gregarinides to
justify the erection of distinct genera.

Monocystis ngilis, Stein, Fig. XVII. 1, 2, 3, 6, 7, 8, 9, 10, 11,
and Fig. XVIII. is the type. The other species of Monocystis
occur chiefly (and very commonly) in marine Annelids, Platyhel-
mintlics, Gephyiwa, and Tnnicata ; not in Arthropoda, Mollusca,
nor Vertebrata. The only definite differences which they present
of possibly more than specific worth, as compared with 3f. ngilis,
are in the form of the chlamydospores, which are sometimes tailed,
as in M. mrnnridis (Fig. XVII. 5), and in 31. ncmerlis (Fig. XVII.
13) and M. sipMTMStZi, and further also certain differences in the
general form, as for instance the anchor-like M. sagittata (Fig.
XVII. 231, and the prpboseidifercms .'/. iijiln;i<litx (Fig. XVII. 17).
The fine parallel striation of the cuticule in some species (M.
serjiulse, &c. ) might also be made the basis of a generic or sub-
generic group.

On the whole it seems best to leave all the species for the present
in the one genus Monocystis, pending further knowledge. It seems
probable that more than one species (at least two, .17. ngili.i and M.
niayiin) infest the common Earthworm.

OKIIEII 2. SEPTATA, Lankester.

Characters. — Gregarinidea in which in the adult the medullary
substance is separated into two chambers — a smaller anterior (the

PROTOZOA

protomerite) and a larger posterior (the ilputomoritc), in which lies
the nucleus. There is frequently if not always present, either in
early growth or more persistently, an anterior proboscis-like appen-
dage (the epimcrite) growing from the proti.merite. Tlie rpinn ni,
SCUTS to attach the parasite to its host, and may fur that purpose
carry booklets. It is always shed sooner or later. The phase in
which it is present is called a "cephalont," the phase after it has
broken oil' a "sporont " (see Fig. XIX. 2'2, 23). The spores are
smaller than in the preceding group, often very minute, and some-
times the cyst is complicated by the, formation of spcimdncts, and
by a kind of "capillitium " of residual protoplasm (Fig. XIX. 2).
Spores producing each only a single (?) falciform young.

Genera. — Orcgarina, Dufour ; Uoplorhynchus, Yon Cams.

[The numerous genera which have been proposed at different
times by Ilammersehmidt and others, and more recently by Aime
Schneider, appear to the present writer to be unserviceable, owing
to the fact that our knowledge is as yet very incomplete. A
good basis for generic, or family distinctions might probably be
found in the greater or less elaboration of the cyst and the forma-
tion or not of sporoducts. But of the majority of Septata we do
not know the cysts or the history of sporulation ; we merely know
that some have simple cysts with complete spornlation leaving no
residue of protoplasm, and that others form cysts with double walls
and elaborate tubular ducts, whilst a part of the protoplasm is not
sporulated but forms a capillitium (Fig. XIX. 2).

Another possible basis for generic division of the Septata may
be found in the characters of the epimerite. This may be present
or absent altogether. It may exist only in the young condition or
persist until growth is completed. It may be simple, short,
elongate, or provided with booklets. The presence of booklets on
the epimerite is the only character which at present seems to serve
conveniently for generic distinction. With regard to the other
points mentioned we are not sufficiently informed, since we know
the complete history of development from the young form set free
from the spore in only one or two cases.]

The Septata are found exclusively in the alimentary canals of
Arthropoda (Insects, Myriapods, Crustacea, not Arachnida). See
Fig. XIX. for various examples of the group.

Flo. XVIII.— Cyst of ilonoeystis agilis, the common Gregarinide of the
Earthworm ; x "50 diam. ; showing ripe chlamydospores and complete
absence of any residual protoplasm or other material in the cyst
(original).

SUB-CLASS II. Coccidiidea, Biitschli (9).

Sporozoa in which the euglena phase remains of relatively
minute size, of spherical shape and simple egg-cell-like structure.
It is not locomotive, but continues, until the cyst is formed, to
inhabit a single cell of the host. Many, few, or one single chlamy-
dospore are formed in the cyst. One or more falciform young
escape from each spore, and exhibit active movements (flagellula-
Hke) leading to a penetration of a tissue-cell by the young form as
in Gregarinidea. Many are parasites of Vertebrata.

OKIIER 1. MONOSPOEEA, Aim. Schn.

Characters. — The whole content of the cyst forms but a single
epore.

Genus unicum. — Einn'rla (in the intestinal epithelium of Triton,
Frog, Sparrow, Mouse, and the Myriapods Litliobius and Glomeris,
Fig. XVII. 35 to 39).

FIG. XIX.— Sporozoa (.Septata). 1. Gregarina Uattarum, Siebold, from
the intestine of ISlatta orientalis; x 80. A syzygiltm of two individuals.
Each animal consists of a small anterior chamber, the protomerite, and a
large posterior chamber, the deutonierite, in which is the nucleus a. 2.
Over-ripe cyst of Gregarina Mattaruai, with thick gelatinous envelope e,
and projecting sporoducts d. The spores have been nearly all discharged,
but amass of them still lies in the centre of the cyst b. The specimen has
been treated with dilute KHO, and the granular contents of the cyst
dissolved. Around the central mass of spores is rendered visible the net-
work of protoplasmic origin in which the ejected spores were embedded.
This distinctly resembles in origin and function the capillitium of
Mycetozoa (Fig. III.), a, the plasmatic channels leading to the everted
sporoducts ; b, the still remaining spores ; c, the proper cyst-wall ; d, the
everted sporoducts ; e, the gelatinous envelope. 3. A ripe spore

(chlamydo spore) of Grfgarina blallarum, a long time after its escape
from the cyst; x 1600 diam. 4. Commencing encystment of a syzy-

gium of G. blatlannn. a, protomerite of one individual ; b, gelatinous
envelope ; c, protomerite of the second individual. 5. Three epithelial
cells of the mid-gut of Eliitta orientalis, into the end of each of which an
extremely young Gregarina blaltantin lias made its way. 6. Further

development of the young Gregarina ; only the epimerite a is now buried
in the substance of the epithelial cell, and this will soon break off and set the
Gregariua free. It is now a " cc]jbalont " ; it will then become a "sporont."
7. Basal part of an everted sporoduct of Gregarina blattcrinn. a, granu-
lar-fibrous mass investing the base of the duct; b, commencement of the
plasmatic channel in the interior of which the sporoduct was produced as
au invaginated cuticular formation before its eversion. 8. Gregarina

yigantea, E. Van Ben., fmm the iutestiiieof the Lobster ; x 150. a, nucleus.

26

PROTOZOA

a
o

!> Anterior end of the same more highly magnified, a, protomerite ; b, layer
of circular nbrilbe lying below'the cuticle ; c, cortical substance of the
deutomerite • d, medullary substance of the deutomerite. 10. Two

spores of Gregarina mgantea (after Butschli), showing the very thick coat of
the spore. 11-15. Stayes in the development of Gregarina mgantea:— 11,
recently escaped from the spore-coat, no nucleus; 12, still no nucleus,
one vibratile anil one motionless process ; 13, the two processes have
divided; one here drawn has developed a nucleus; 14, further growth;
15, the deutomerite commences to develop. 16. Cysts of Gregarina

igantea, from the rectum of the Lobster. The double contents are
jelieved by Ed. Van Beneden to be due not to conjugation previous to
encystment but to subsequent fission. 17, 18. Gregarina longicoltis,

Stein, from the intestine of Blaps nutrtitaga, :— 17, cephalont phase, with a
long proboscis-like epimerite a, attached to the protomerite b ; 18,
sporont phase, the epimerite having been cast preliminarily to syzygy and
encystment. 19. Gregarina Manieri, Aim. Schneider, from the

intestine of Timarcha tenelirieosa, to show the network of anastomosing
fibres beneath the cuticle, similar to the annular fibrilUe of G. gigantea
shown in 9. 20. Gregarina (Iloylnrhynchus) obligacanthus, Stein,

from the intestine of the larva of Agrion. Cephalont witli spine-crowned
epimerite a. 21. Spores of Gregarina oligacanthus. 22, 23. Grega-

rina (Hoplorhynchus) Dujardini, Aim. Schneider, from the intestine of
Litkobius fnrticatus : — 22, specimen with epimerite a, therefore a "cepha-
lont"; 23, specimen losing its epimerite by rupture and becoming a
"sporont."

ORDER 2. OLIGOSPOREA, Aim. Schn.

Characters. — The cyst-content develops itself into a definite and
constant bxit small number of spores.

Genus unicum. — Coccidium, Leuck. (in intestinal epithelium and
liver of Mammals, and some Invertebrates, Figs. XVII. 24 to 31).

ORDER 3. POLYSPOREA.

Characters, — The cyst-content develops itself into a great num-
ber of spores (sixty or more).

Genus uuieum. — Klossia, Aim. Schn. Three species of Klossia
are found in Mollusca — viz., in Helix, in Cephalopoda, and in
Chiton. Schneider's genus, Adelea, from Lithobius, appears to
belong here. Kloss (49) discovered the parasite of the renal cells of
Helix hortensis represented in Fig. XVII. 18, 19, 20, 21, and 22;
Schneider that of Cephalopods, Fig. XVII. 32, 33. In Chiton Dr
Tovey has discovered a third species with very remarkable spores,
which are here figured for the first time (Fig. XVII. 12).

The Drcpnnidium Ranarum (Fig. XVII. 45, 46, 47), discovered
by Lankester (50) in the Frog's blood, is probably the falciform young
of a Coccidium parasitic in the Frog's kidney, and discovered there
by Lieberkiihn (51). A spore of this Coccidium is shown in Fig.
XVII. 48; whilst in 46 two Drepanidia which have penetrated a
red-blood corpuscle of the Frog are represented.

The Polysporous Coccidiidea come very close to the Gregarinidc
genus Mouocystis, from which they may be considered as being
derived by an arrest of development. The spores ami falciform
young of the Coceidiidea are closely similar to those of Monocystis,
and the young in both cases penetrate the tissue-cells of their host ;
but in Monocystis this is only a temporary condition, and growth
leads to the cessation of such "cell-parasitism." On the other
hand, growth is arrested in the Coccidiidea, and the organism is
permanently a cell-parasite.

Since the parasitism is more developed in the case of a cell-para-
site than in the case of a parasite which wanders in the body cavity,
it seems probable that the Coccidiidea have been derived from the
Gregarinidea rather than that the reverse process has taken place.

SUB-CLASS III. Myxosporidia, Butschli.

Characters. — Sporozoa in which the euglena-phase is a large
multinucleate amceba-like organism (Fig. XVII. 34). The cysts
are imperfectly known, but appear to be simple ; some attain a
diameter of two lines. The spores are highly characteristic, having
each a thick coat which is usually provided with a bifurcate process
or may have thread capsules (like nematocysts) in its substance
(Fig. XVII. 40, 41, 42, 43, 44).

The spores contain a single nucleus, and are not known to produce
falciform young, but in one case have been seen to liberate an
amcebula. The further development is unknown. The Myxo-
sporidia are parasitic beneath the epidermis of the gills and fins, and
in the gall-bladder and urinary bladder of Fishes, both freshwater
and marine.

Genera.— Myxidium, Butschli (Pike, Fig. XVII. 34); MyxoMii::,
Butschli (Cyprinoids) ; Lithocystis, Giard (the Lamellibranch Echino-
cardium).

The Myxosporidia are very imperfectly known. They present
very close affinities to the Mycetozoa, and are to be regarded as a
connecting link between the lower Gymnomyxa and the typical
Sporozoa. Possibly their large multinucleate amoeba phase is a
plasmodium formed by fusion of amrebula? set free from spores,
though it is possible that the many nuclei are the result of a division
of an original single nucleus, preparatory to spornlation.

Their spores arc more elaborate in structure than those of any
other Protozoa, and are more nearly paralleled by those of some
species of Monocystis than by those of Mycetozoa. The thread-
capsules of the spores are identical in structure with those of
Hydrozoa, and probably serve as organs of attachment, as do the
furcate processes of the spore-case. It is not certain that a definite

cyst is always or ever formed, but as occurs rarely in some" Gregari-
nidea, the spores may be formed in a non encysted amoeba form.

Although pseudopodia, sometimes short and thread-like, have been
observed in the amceba phase, yet it is also stated that a distinction
of cortical and medullary substance obtains.

The " psorosperms " of J. Miiller are the spores of Myxosporidia.

SUB-CLASS IV. Sarcocystidia, Butschli.

(This division is formed by Butschli for the reception of Sarco-
cystis, parasitic in the muscular fibres of Mammals, and of Amcebi-
diuin, parasitic in Crustacea. Both are very insufficiently known,
hut have the form of tubular protoplasmic bodies in which numer-
ous ovoid spores are formed from which falciform young escape.)

Genera. — Sarcocystis, Lankester; Aincebidium, Cienkowski (52).
Sarcocystis(Fig. XVII. 50, 51, S. Micschcri, Lank. ), was first observed
by Miescher in the striated muscle-fibres of the Mouse ; then by
Rainey in a similar position in the Pig, and taken by him for the
youngest stage in the development of the cysts of Teenia solium ;
subsequently studied by Beale and others in connexion with the
cattle-plague epidemic, and erroneously supposed to have a causal
connexion with that disease. It is common in healthy butcher's
meat. See Leuckart (47).

Further remarks on the Sporozoa. — The Sporozoa contrast
strongly with the large classes of Gymnomyxa, the Heliozoa,
Retieularia, and Radiolaria, as also with the Ciliate and Tentaculi-
ferous Corticata, by their abundant and rapidly recurrent forma-
tion of spores, and agree in this respect with some Proteomyxa,
with Mycetozoa, and some Flagellata. Their spores are remark-
able for the firm, chitin-like spore-coat and its varied shapes,
contrasting with the cellulose spherical spore-coat of Mycetozoa
and with the naked spores of Radiolaria and Flagellata.

The protoplasm of the more highly developed forms (Gregarini-
dea) in the euglenoid phase exhibits considerable differentiation.
Externally a distinct cuticle may be present, marked by parallel
ruga; (Monocystis serrmlx) or by fine tubercles (Monocystis sipun-
i-uli). A circlet of hooks may be formed by the cuticle at one end
of the body. Below the cuticle is sometimes developed a layer of
fibrils running transversely to the long axis of the body (Fig.
XIX. 9 and 19), which have been regarded as contractile, but are
probably cuticular. The cortical layer of protoplasm below these
cuticular structures is dense and ref'ringent and sometimes fibril-
lated (Monocystis pellurida, Fig. XVII. 15). It is the contractile
substance of the organism, and encloses the finely granular more
liquid medullary substance. The granules of the latter have been
shown by Butschli (9) to give a starch-like reaction with iodine,
&c. Probably the protoplasm in which they lie is finely reticulate
or vacuolar, and when the granules are few it is actually seen to be
so. No contractile vacuole is ever present. In Myxosporidia the
medullary protoplasm is coloured yellow by hrematoidin derived
from the blood of its host or by absorbed bile-pigment, and also
contains small crystals.

The nucleus of the Gregarinidea is a large clear capsule, with a
few or no nueleolar granules. It has never been seen in a state
of division, and it is not known what becomes of it during sporula-
tion, though sporulating Gregarinidea have been observed with
many minute nuclei scattered in their protoplasm, presumably
formed by a breaking up of the single nucleus.

The habit of attaching themselves in pairs which is common in
Gregarinidea is perhaps a reminiscence of a more extensive forma-
tion of aggregation plasmodia (compare Mycetozoa). The term
"syzygium" is applied to such a conjunction of two Gregarinidea ;
it is not accompanied by fusion of substance. The formation of
cysts is not connected with this pairing, since the latter occurs in
young individuals long before encystment. Also cysts are formed
by single Gregarinidea, as is always the case in the non-motile
Coccidiidea.

The encystment always leads to the formation of spores, but in
rare cases sporulation has been observed in unencysted Gregariui-
dea, and it occurs perhaps normally without true cyst-formation in
the Myxosporidia.

The cell-parasitism of the young Sporozoa, and their fiagellula-
like (falciform) young and active vibratile movement, are points
indicating affinity with the lower Gymnomyxa, and especially with
those Proteomyxa, such as Vampyiella and Plasmodiophora, which
are cell-parasites. Indeed it is probable that we have in this fact
of cell-parasitism, and especially of parasitism in animal cells, a
basis for the theoretical association of several unicellular organisms.
The Haplococcus of Zopf (regarded by him as a Mycetozoon) is
parasitic in the muscular cells of the Pig, and is probably related
to Sarcocystis. Recently Von Lendenfeld (53) has described in
Australia an ama-ba-like organism as parasitic in the skin of Sheep,
which will probably be found to be either a Sporozoon or referable
to those parasitic spore-producing Proteomyxa which are separated
from Sporozoa only by their negative characters (see previous
remarks on the negative characters of Proteomyxa).

The application of the name "Gregarines" has sometimes been

PROTOZOA

27

in. nl,: erroneously to external parasitic organisms, which have
nothing in common with the Sporozoa. This w;is tlie case in regard
to a fungoid growth in human hair — the so-called "chignon
ilregariiK'. " Tne Silk-worm disease known as "pebrine" lias also
been attributed to a Gregarine. It seems probable that the parasitic
organism which causes that disease is (as is also the distinct parasite
causing the disease known as "flaccidezza" in the same animals)
one of the Schizomycetes (Bacteria). No disease is known at
present as due to Sporozoa, although (e.g., the A7><«/</ I'/iitonis)
they may lead to atrophy of the organs of the animals which they
infest, in consequence of their enormous numbers. Coccidia and
Sarcocystis are stated to occur in Man.

CLASS II. FLAGELLATA,1 Ehrenberg.

Characters. — Corticata in which the dominant phase in the life-
history is a corticate flagellula, that is. a nucleated cell-body pro-
vided with one or a few large processes of vibratile protoplasm.
Very commonly solid food particles are ingested through a distinct
cell mouth or aperture in the cortical protoplasm, though in some
an imbibition of nutritive matter by the whole surface and a nutri-
tional process chemically resembling that of plants (holophytic),
chlorophyll being present, seems to occur.

Conjugation followed by a breaking up into very numerous minute
naked spores is frequent in some ; as also a division into small
individuals (microgonidia), which is followed by their conjugation
with one another or with big individuals (macrogonidia) and subse-
quent normal growth and binary fission.

Many have a well-developed cuticle, which may form collar-like
outgrowths or stalk-like processes. Many product either gelatinous
or ehitin-like shells (cups or coanrecia), which are connected so as to
form spherical or arborescent colonies ; in these colonies the proto-
plasmic organisms themselves produce new individuals by fission,
which separate entirely from one another but are held together by
the continuity, with those already existing, of the new shells or
jelly-houses or stalk -like supports produced by the new individuals.
A single well-marked spherical nucleus, and one or more contractile
vacuoles, are always present in the full-grown form.

Often, besides ingested food-particles, the protoplasm contains
starch granules (amylon nucleus), paramylum corpuscles, chromato-
phors and chlorophyll corpuscles, some of which may be so abundant
as to obscure the nucleus. One or two pigment spots (stigmata or
so-called eye-spots) are often present at the anterior end of the body.

SUB-CLASS I. Lissoflagellata, Lankester.

Never provided with a collar-like outgrowth around the oral
pole.

ORDER 1. MONADIDEA, Biitschli.

Characters. — Lissoflagellata of small or very small size and
simple structure; often naked and more or less aiiKeboid, sometimes
forming tests. Usually colourless, seldom with chromatophors.
With a single anterior large flagellum or sometimes with two
additional paraflagella. A special mouth-area is often wanting,
sometimes is present, but is never produced into a well-developed
pharynx.

Fam. 1. RHIZOMASTIGINA, Biitschli. Simple rnouthless forms
•with 1 to 2 flagclla; either permanently exhibiting a Gymnomyxa-
like development of pseudopodia or capable of passing suddenly
from a firm-walled into a Gymnomyxa-like condition, when the
flagclla may remain or be drawn in. Ingestion of food by aid of
the pseudopodia.

Genera. — Mrtstiyamceba, F. E. Schultze; Ciliophrys, Cienkowski
(65); Dimorpha, Gruber; Actinomonas, Kent; Trypanosoma, Gruby
(parasitic in the blood of Frogs and other Amphibia and Reptiles,
Fig. XX. 21, 22). The Rhizomastigina might all be assigned to
the Proteomyxa, with which they closely connect the group of
Flagellata. The choice of the position to be assigned to such a
form as Ciliophrys must be arbitrary.

Fam. 2. CERUOMOXADINA, Kent. Minute oblong cell-body
which posteriorly may exhibit amoeboid changes. One large
anterior flagellum. Mouth at the base of this organ. Reproduc-
tion by longitudinal fission and by multiple fission producing
spores in the encysted resting state.

Genera. — Ccrcomonns, Duj. (Fig. XX. 32, 33); Hcrpetomonas, S.
Kent; Oikmnonas, Kent ( = j)/cmas, James Clark; Pscudospora,
Cienkowski, Fig. XX. 29, 3d, 31) ; Ancijrononas, S. K.

Fam. 3. CODOXCECINA, Kent. Small colourless monads similar
to Oikotnonas in structure, which secrete a fixed gelatinous or
membranous envelope or cup.

Genera. — Cudonceca, James Clark; Plritythacca, Stein.

Fain. 4. BIKCECIXA, Stein. Distinguished from the last family
by the fact that the monad is fixed in its cup by a contractile
thread-like stalk ; cup usually raised on a delicate stalk.

Genera. — Bicosaiai, J. Cl. ; Poteriodendron, Stein.

1 Butschli's work (9) luis been pretty closely followed in tlte diagnosis of the
groups of Flaijellata and the enumeration of genera here given.

FIG. XX.— Flagellata. 1. Chlamydomonas vulciteulus, Ehr. (=Zygoselmis,
From.) ; one of the Phytomaetigoda ; free-swimming individual, a, nucleus ;
&, contractile vacuole ; c, starch corpuscle ; rf, cellulose investment ;
e, stigma (eye-spot). 2. Resting stage of the same, with fourfold

division of the cell-contents. Letters as before. 3. Breaking up of

the cell-contents into minute bithigellate swarm -spores, which escape,
and whose history is not further known. 4. Syjicrypta volvox, Ehr. ;

one of the Phytomastigoda. A colouy enclosed by a common gelatinous
test c. a, stigma; b, vacuole (non-contractile). 5. Cwfjlena volvox,

Ehr. ; one of the Monadidea. Half of a large colony, the flagellates
embedded in a conmiun jelly. 6. Chlaroyonium co'-h^n-n in, Ehr. ;

one of the Phytomastigoda. a, nucleus; 6, contractile vacuole; c, starch
grain; £/, eye-spot. 7. Chlorogonium curhlorum, Ehr., one of the

Phytomastigoda. Copulation of two liberated microgonidia. a, nucleus;
b, contractile vacuole ; rf, eye-spot (so-called). S. Colony of Dinobryon

sertitla/'ia, Ehr.; x 200; one of the Monadidea. 1*. Bxmato-

coccus palustris, Girod (= Chlamydococcus, Braun, Protoeoccits Cohn),
one of the Phytomastigoda ; ordinary individual with widely separated
test, a, nucleus; b, contractile vacuole; c, amylnn nucleus (pyrenoid).
10. Dividing resting stage of the same, with eight fission products in
the common test e. 11. A microgonidium of the same. 12.

Phalansteriitjit cnn^n-intnm, Cienk., one of the Choanofiagellata ;
X 325. Disk-like o>l"iiv. 13. Eu-jlcna nVn/iX Ehr.; x 300 ; one of

the Euglenoidea. a, pigment spot (stigma) ; 6, clear space ; c, paramylum
granules ; df chromatopnor (endocbrome plate). 14. Gvnium pectorals,
0. F. Mtiller ; one of the Phytomastigoda. Colony seen from the flat side.
X 300. a, nucleus ; b, contractile vacuole ; c, amylon nucleus. 15.

Dinobryon sertularia, Ehr. ; one of the Monadidea. a, nucleus ; b, con-

28

PROTOZOA

tractile vacuole ; c, amylon-nucleus ; rf, free colourless flagellates, probably
not belonging to Dinobryon ; e, stigma (eye-spot); /, chromatophors.
16. Peranema trichophorum, Ehr., (one ol the Euglenoidea), creeping
individual seen from the back ; x 140. a, nucleus ; b, contractile
vacuoles ; c, pharynx ; d, mouth. 17. Anterior end of Euglena acus,

Ehr., in profile, a, mouth ; b, contractile vacuoles ; c, pharynx ; rf, stigma
(eye-spot); e, paramylum-body ; /, chlorophyll corpuscles. 18. Part of

the surface of a colony of Volvox globatur, L. (Phytomastigoda), showing
the intercellular connective fthrils. a, nucleus ; b, contractile vacuole ;
c, amylura granule. 19. Two microgonidia of Volvox ylobator, L. a,

nucleus ; b, contractile vacuole. 20. Ripe asexually produced

daughter-individual of Volvox minor, Stein, still enclosed in the cyst
of the partheno-gonidium. a, young parthenp-gonidia. 21, 22.

Trypanosoma. sanguinis, Gruby ; one of the Rhizomastigina, from the
blood of Jtana esculenta. a, nucleus. x 500. 23-26. Repro-

duction of Boclo caudatus, Duj. (one of the Heteromastigoda), after Dallin-
ger and Drvsdale : — 23, fusion of several individuals (plasmodium) ; 24,
encysted fusion-product dividing into four ; 25, later into eight ; 26, cyst
filled with swarm-spores. 27. Astasia tenax, O. F. Mull. (Proteus) ; one of
the Euglenoidea ; x 440. Individual with the two flagella, and strongly
contracting hinder region of the body, a, nucleus ; b, contractile vacuole,
close to the pharynx. 28. The same devoid of flagella. a, nucleus ;

c, c, the two dark pigment spots (so-called eyes) near the mouth. 29.

Oikoinonas tenno (Nonas tenno) Ehr. ; one of the Alonadidea. a, nucleus ;
b, contractile vacuole ; c, food-ingesting vacuole ; d, food-particle, x 440.
30. The food-particle d has now been ingested by the vacuole. 31.

Oikomonas mutabilis, Kent (Monadidea), with adherent stalk, a, nucleus ;
b, contractile vacuole ; c, food-particle in food vacuole. 32, 33. Cerco-
monas crassicauda, Duj. (Monadidea), showing two conditions of the
pseudopodium-protrudiug tail. a, nucleus ; b, contractile vacuoles ; c,
mouth.

Fam. 5. HETERCMONADINA, Biitschli. Small colourless or green
monads which possess, besides one chief flagelliim, one or two smaller
paraflagella attached near it, often forming colonies secreting a
common stalk.

Genera. — Monas (Ehr.), Stein; Dendromonas. Stein; Cepludo-
thammum, Stein ; Anthophysa, Bory d. Vine. (Fig. XXI. 12, 13);
Dinobri/on, Ehr. (Fig. XX. 8 and 15) ; Epipyxis, Ehr. ; Uroglena,
Ehr. (Fig. XX. 5).

ORDER 2. EUGLENOIDEA, Biitschli.

Characters. — Generally somewhat large and highly developed
monoflagellate forms, of mouaxonic or slightly asymmetrical
build. Cuticle present ; cortical substance firm, contractile, and
elastic ; some forms quite stiff, others capable of definite annular
contraction and worm-like elongation. At the base of the flagellum
a small or large mouth leading into a more or less distinct
pharyngeal tube. Near this is always the contractile vacuole.
Rarely a pair of flagella instead of one.

Fam. 1. CffiLOMOXADlNA. Coloured Euglenoidea, with numer-
ous small chlorophyll corpuscles or 1 to 2 large plate-like green or
brown chromatophors. Mouth and pharynx inconspicuous ; nutri-
tion probably largely vegetal (holophytic).

Genera. — Ccelomonas, Stein ; Gonyostomum, Dies. ; Vacuolaria,
Cienk. ; Alicroglena, Ehr. ; Chromulina, Cienk. ; Cryptoglena, Ehr.

Fam. 2. EUGLENINA, Stein. Body monaxonic, elongated, hinder
end pointed. Spirally striated cuticle. A fine mouth-aperture
leads into the well-developed tubular pharynx. Flagellum usually
single, sometimes paired, often cast off. Near the pharynx is the
' ' reservoir " of the contractile vacuoles and several of the latter.
A single (sometimes two) stigma or colour-speck near the same
spot. Chromatophors nearly always present, generally bright
green. A large nucleus in the middle of the body. Multiplication
by longitudinal fission. Encysted condition and attendant fission
imperfectly studied. Copulation doubtful.

Genera. — (a) With flexible cuticle -.—EugUiM, Ehr. (Fig. XX. 13,
17; this is probably Priestley's "green matter," from which he
obtained oxygen gas ; though one of the very commonest of all
Protozoa, its life-history has yet to be worked out) ; Colacium,
Ehr. ; Eutrcptia, Perty.

(b) With stiff, shell-like cuticle : — Ascoglcna, Stein ; Trachclo-
monas, Ehr. ; Lcpocinclis, Perty ; Pkacus, Nitzsch.

Fam. 3. MENOIDINA, Biitschli. Similar to the Euglenina, but
devoid of chlorophyll, a deficiency connected with the saprophytic
mode of life. Stigma always absent.

Genera. — (a) With flexible cuticle : — Astasiopsis, Biitschli ; Asia-
siodcs, Biitschli.

(b) With stiff cuticle and non-contractile body : — Monoidium,
Perty ; Atraclotiema, Stein ; lihabdumonas, Fresenius.

Fam. 4. PERANEMINA. Very contractile (metabolic) colourless
Euglenoids. Mouth and pharynx large ; inception of solid nutri-
ment certainly observed.

Genera. — Peranema, Duj. (Fig. XX. 16) ; Urccolus, Meresch.

Fam. 5. PETALOMONADINA. Colourless, non-metabolic forms.
Mouth opening at the base of the single large flagellum.

Genera. — Petalomonas, Stein.

Fam. 6. ASTASINA. Colourless, metabolic, or stiff Euglenoids,
differing from the rest in having a small or large paraflagelluni in
addition to the chief one. Nutrition partly saprophytic partly
animal.

Genera.— Astasia, Ehr. emend. Stein (Fig. XX. 27, 28) ; Setcro-
ncma, Duj. ; Zygoselmis, Duj. ; Sphenomuiias, Stein ; Tropido-
scyphus, Stein.

ORDER 3. HETEROMASTIGODA, Biitschli.

Characters. — Small and large monads. Naked and even amo?boid
or with stiff cuticle. Two flagella at the anterior end differing in
size : the smaller directed forwards subserves the usual locomotor
function ; the larger is directed backwards and trailed, without
movement. Sometimes two backwardly directed Hagella are present.
Always a mouth and animal nutrition. Always colourless.

Fain. 1. BODONINA, Biitschli. Size of the two flagella not very
different.

Genera.— Bodo, Ehb. emend. Stein (Fig. XX. 23 to 26, and Fig.
XXI. 10 ; the hooked monad and the springing monad of Dai-
linger and Drysdale (66) ; Heteromita of Dujardin and Kent);
1'hyllomitus, Stein ; Colponcma, Stein ; Dallingeria, Kent ; Tri-
mastix, Kent.

Fam. 2. ANISONEMINA, Kent. Large forms with cuticle ; differ-
ence of the two flagella considerable. Mouth, pharynx, and animal
nutrition.

Genera. — Anisonana, Duj. ; Entosiphon, Stein.

ORDER 4. ISOMASTIGODA, Biitschli.

Characters. — Small and middle-sized forms of monaxonic rarely
bilateral shape. Fore-end with 2, 4, or seldom 5 equal-sized and
similar flagella. Some are coloured, some colourless ; naked or
with strong cuticle or secreting an envelope. Mouth and pharynx
seldom observed ; nutrition generally holophytic (i.e., like a green
plant), but in some cases, nevertheless, holozoie (i.e., like a typical
animal).

Fam. 1. AMPHIMONADINA. Small, colourless, biflagellate Iso-
mastigoda.

Genera. — Amphimonas, Duj. ( ? Pseudospora, Cienk.).

Fam. 2. SPONGOMONADINA, Stein. Small colourless oval forms
with two closely contiguous flagella. Chief character in the union
of numerous individuals in a common jelly or in branched gelatinous
tubes, the end of each of which is inhabited by a single and distinct
individual.

Genera. — Spongomonas, Stein; Cladomonas, Stein; Ehipido-
monas, Stein.

[Group Phytomastigoda, Biitschli. The following three families,
viz., Chrysomonadina, Chlamydomouadina, and Volvocina, are so
closely related to one another as to warrant their union as a sub-
order. They are typical Isornastigoda, but have chlorophyll
corpuscles and holophytic nutrition with correlated deficient
mouth and pharynx, they are usually regarded. by botanists as
belonging to the unicellular Algfe.]

Fam. 3. CHRY.SOJIONADINA, Biitschli. Single or colony-forming ;
seldom an envelope. Spherical free-swimming colonies may be
formed by grouping of numerous individuals around a centre.
With two or rarely one brown or greenish brown chromatophor;
a stigma (eye-speck) at the base of the flagella.

Genera. — Style/chrysalis, Stein; Chrysupyxis, Stein ; Nephrosel-
mis, Stein ; Synura, Ehr. ; Syncrypta, Ehr. (Fig. XX. 4).

Fam. 4. CHLAMYDOMONADINA. Fore-end of the body with two
or four (seldom five) flagella. Almost always green in consequence
of the presence of a very large single chromatophor. Generally a
delicate shell-like envelope of membranous consistence. 1 to 2
contractile vacuoles at the base of the flagella. Usually one eye-
sprrk. Division of the protoplasm within the envelope may pro-
duce four, eight, or more new individuals. This may occur in the
swimming or in a resting stage. Also by more continuous fission
microgonidia of various sizes are formed. Copulation is frequent.

Genera. — Hymcaomonas, Stein; L'hlorawjium, Stein; C/iloro-
ffonium, Ehr. (Fig. XX. 6, 7) ; Pulyloma, Ehr. ; Chlamydomonas,
Ehr. (Fig. XX. 1, '2, 3); fftematoeoenus, Agardh ( = Chlamydo-
COCCHS, A. Braun, Stein ; Protococcus, Colin, Huxley and Martin ;
Chlamydomonas, Cienkowski); Carlcria, Diesing ; Spondylomomm,
Ehr. ; Coccomonas, Stein ; Phacotus, Perty.

Fam. 5. VOLVOCINA. Colony-building Phytomastigoda, the cell-
individuals standing in structure between Chlamydomonas and
Hsematoeoceus, and always biflagellate. The number of individuals
united to form a colony varies very much, as does the shape of the
colony. Reproduction by the continuous division of all or of only
certain individuals of the colony, resulting in the production of a
daughter colony (from each such individual). In some, probably
in all, at certain times copulation of the individuals of distinct
sexual colonies takes place, without or with a differentiation of the
colonies and of the copulating cells as male and female. The
result of the copulation is a resting zygospore (also called zygote or
oo-spermospore or fertilized egg-cell), which after a time develops
itself into one or more new colonies.

Genera. — Gonium, 0. F. Muller (Fig. XX. 14) ; Stephanosph&ra,
Colin; Pandorina, Bory de Vine.; Eiulorina, Ehr. ; I'blwx,
Ehr. (Fig. XX. 18, 20).

[The sexual reproduction of the colonies of the Volvociua is one
of the most important phenomena presented by the Protozoa. In
some families of Flagelliita full-grown individuals become amoeboid,
fuse, encyst, and then break up into flagellate spores which develop

PROTOZOA

29

simply to the parental form (Fig. XX. 23 to 26). In the
Chlamydomonadina a single adult individual by division produces
small individuals, so-called "microgonidin." These copulate with
one another or with similar mierogonidia formed by other adults
(as in Chlorogonium, Fig. XX. 7) ; or more rarely in certain
genera a mierogonidium copulates with an ordinary individual
(macrogonidium). The result in either case is a " zygute, " a cell
formed by fusion of two which divides in the usual way to produce
new individuals. The microgonidium in this ease is the male
element and equivalent to a spermatozoon ; the niacrogonidiuni is
the female and equivalent to an egg-cell. The zygote is a fertilized
egg-cell, or oo-spermosporc. In the colony-building forms we find
that only certain cells produce by division mierogonidia ; and,
regarding the colony as a multicellular individual, we may consider
these cells as testis-cells and their mierogonidia as spermatozoa.
In some colony-building forms the mierogonidia copulate with
ordinary cells of the colony which, when thus fertilized, become
encysted as zygotes, and subsequently separate and develop by
division into new colonies. In Volvox the macrogonidia are also
specially-formed cells (not merely any of the ordinary vegetative
cells), so that in a sexually ripe colony we can distinguish egg-
cells as well as sperm mother-cells. Not only so, but in some
instances (Eudorina and some species of Volvox) the colonies which
produce sexual cells can not merely be distinguished from the
asexual colonies (which reproduce parthenogenetically), hut can be
distinguished also inter se into male colonies, which produce from
certain of their constituent cell-units spermatozoa or mierogonidia
only, and female colonies which produce no male cells, bnt only
macrogonidia or egg-cells which are destined to he fertilized by
the mierogonidia or spermatozoa of the male colonies.

The differentiation of the cell-units of the colony into neutral or
merely carrying cells of the general body on the one hand and
special sexual cells on the other is extremely important. It places
these cell-colonies on a level with the Euterozoa (Metazoa) in
regard to reproduction, and it cannot be doubted that the same
process of specialization of the reproductive function, at first com-
mon to all the cells of the cell-complex, has gone on in both
cases. The perishable body which carries the reproductive cells is
nevertheless essentially different in the two cases, in the Volvocina
being composed of equipollent units, in the Enterozoa being com-
posed of units distributed in two physiologically and morphologi-
cally distinct layers or tissues, the ectoderm and the endodenn.

The sexual reproduction of the Vorticellidae may be instructively
compared with that of the Phytomastigoda ; see below.]

Fam. 6. TETRAMITIXA. Symmetrical, naked, colourless, some-
what amoeboid forms, with four flagella or three and an undulating
membrane. Nutrition animal, but mouth rarely seen.

Genera. — Collodidyon, Carter; Tetramitus, Perty (Fig. XXI.
11, 14 ; calycine monad of Dallingcr and Drysdale (66)) ; Monoccrco-
monaa, Grassi ; Trichomonas, Donne ; Trichomastix, Blochmann.

Fam. 7. POLYMASTIGINA. Small, colourless, symmetrical forms.
Two flagella at the hinder end of the body and two or three on each
side in front. Nutrition animal or saprophytic.

Genera. — Hexamitus, Duj. (Fig. XXI. 5) ; Majastoma, Grassi ;
Polymastix, Biitschli.

Fam. 8. TREPOMONADINA, Kent. As Polymastigina, but the
lateral anterior flagella are placed far back on the sides.

Genera. — Trepomonas, Duj., described recently without name by
Dallinger (67).

Fam. 9. CRTPTOMONTADINA. Coloured or colourless, laterally
compressed, asymmetrical forms ; with two very long anterior
flagella, placed a little on one side springing from a deep atrium-
like groove or furrow (cf. Dinoflagellata and Noctiluca, to which
these forms lead).

Genera. — Oyathomonas, From. ; Chilomonas, Ehr. ; Cryptomonas,
Ehr. ; Oxyrrhis, Duj.

Fam. 10. LOPHOMONADINA. A tuft of numerous flagella anteriorly.

Genus. — Lophomonas, Stein (Fig. XXI. 9, connects the Flagel-
lata with the Peritrichous Ciliata).

Sub-class II. Choanoflagellata, Saville Kent.
Flagellata provided with an upstanding collar surrounding the
anterior pole of the cell from which the single flagellum springs,
identical in essential structure with the "collared cells" of Sponges.
Single or colony-building. Individuals naked (Codosit/a), orinhabit-
Ing each a cup (Salpingceca), or embedded in a gelatinous common
investment (Protcrosjxmgia).

ORDER 1. NUDA, Lankester.

Characters. — Individuals naked, secreting neither a lorica (cup)
nor a gelatinous envelope.

Genera. — Monosiya, S. Kent (solitary stalked or sessile) ; Codo-
siya, James Clark (united socially on a common stalk or pedicle,
Fig. XXI. 3, 4) ; Astrosiga, S. Kent ; Desmarella, S. Kent.

ORDER 2. LORICATA, Lankester.

Characters. — Each individual collared-cell unit secretes a horny
cup or shell.

FIG. XXI.— Flagellata. 1. Salpinyceca fustforwi*, S. Kent ; one of the
Choanoflagellata. The protoplasmic body is drawn together within the
goblet-shapeil shell, and divided into numerous spores, x 1500. 2.

Escape of the spores of the same as monoflagellate and swarm-spores.
3. Codosiga umbeliata, Tatem; one of the Choanoflagellata ; adult colony
formed by dichotomous growth ; x 625. 4. A single zooid of the same ;
x 1250. a, nucleus ; b, contractile vaeuole ; c, the characteristic "collar"
formed by cuticle on the inner face of which is a most delicate network of
naked streaming protoplasm. 5. Hcxamita injlatn, Duj, ; one of the

Isomastigoda ; x 650; normal adult; showinga, nucleus, and b, contrac-
tile vaeuole. 6, 7. Salfimja'ca itrcculata, S. Kent ; one of theCboano-
flagellata ;— 6, with collar extended; 7, with collar retracted within the
stalked cup. a, nucleus; b, contractile vaeuole. 8. Polytoma nvt'lla,
Mull. sp. ; one of tbe Phytomastigoda. n, nucleus ; 6, contractile vaeuole.
X 800. 9. Lophonwnas i>lat/tiniin, Stein; one of the Isomastitroda,
from the intestine of islattn ur/V«/(^/V. a, nucleus. 10. Bodolens, Mull. ;
one of the Heterornastlgoda ; x 800. a, nucleus; 6, contractile vaeuole;
the wavy filament is a flagellum, the straight one is an immobile trailing
thread. 11. Tetramitus sulcatus, Stein; one of the Isomastigoda; x430.
a, nucleus ; b, contractile vaeuole. 12. Anthophysa vcgetans, O. F.
Miiller; oneof the Monaditlea; x 300. A typical, erect, shortly-branching
colony stock with four terminal monad-clusters. 13. Monad cluster of
the same in optical section (x 800), showing the relation of the
individual monads or flagellate zonids to the stem a. 14. Tetramitus
rostratus, Perty; one of the Isom&etigoda ; x 1000. a, nucleus; b, con-
tractile vaeuole. 15. Protero&pongia Haeckeli, Saville Kent ; one of
the Choanoflagellata ; x 800. A social colony of about forty flagellate
zooids. a, nucleus; b, contractile vaeuole; c, anibiciform zooid sunk

30

PROTOZOA

within the common jelly or test (compared by S. Kent to the mesoderm-
cells of a sponge-colony) ; d, similar zooid multiplying by transverse
fission ; e, normal zooids with their collars contracted ; /, hyaline mucila-
ginous common test or zoothecium ; g, individual contracted and dividing
into minute flagellate spores (microgonidia) comparable to the spermato-
zoa of a Sponge.

Genera. — Salpingceca, James Clark (sedentary, Fig. XXI. 6, 7) ;
Lagenoeca, S. Kent (free swimming) ; Polyceca, S. Kent (cups united
socially to form a branching zocecium as in Dinobryon).

ORDER 3. GELATINIGERA, Lankester.

The cell-units secrete a copious gelatinous investment and form
large colonies.

Genera. — P/ialanstcrium, Cienk. (Fig. XX. 12) ; Proterospongia,
Saville Kent (Fig. XXI. 15).

[The Choanoflagelhita were practically discovered by the Ameri-
can naturalist James Clark (68), who also discovered that the ciliated
chambers of Sponges are lined by collared cells of the same peculiar
structure as the individual Choanoflagellata, and hence was led to
regard the Sponges as colonies of Choanoflagellata. Saville Kent
(69) has added much to our knowledge of the group, and by his
discovery of Proterospongia (sue Fig. XXI. 15, and description)
has rendered the derivation of the Sponges from the Flagellata a
tenable hypothesis.]

Further remarks an the Flagellata. — Increased attention has
been directed of late years to the Flagellata in consequence of the
researches of Cienkowski, Butsehli, James Clark, Saville Kent, and
Stein. They present a very wide range of structure, from the
simple amoeboid forms to the elaborate colonies of Volvox and
Proterospongia. By some they are regarded as the parent-group
of the whole of the Protozoa ; but, whilst not conceding to them
this position, but removing to the Proteomyxa those Flagellata
which would justify such a view, we hold it probable that they are
the ancestral group of the mouth-bearing Cortieata, and that the
Ciliata and Dinonagellata have been derived from them. One
general topic of importance in relation to them may be touched on
here, and that is the nature of the flagellum and" its movements.
Speaking roughly, a flagellum may be said to be an isolated filament
of vibratile protoplasm, whilst a cilium is one of mail}' associated
filaments of the kind. The movement, however, of a flagellum is
not the same as that of any cilium ; and the movement of all
flagella is not identical. A cilium is simply bent and straightened
alternately, its substance probably containing, side by side, a con-
tractile and an elastic fibril. A flagellum exhibits lashing move-
ments to and fro, and is thrown into serpentine waves during these
movements. But two totally distinct kinds of flagella are to be
distinguished, viz., (a) the pulsellum, and (J) the tractellum. An
example of the pulsellum is seen in the tail of a spermatozoon which
drives the body in front of it, as does the tadpole's tail. Such
a "pulsellum" is the cause of the movement of the Bacteria, It
is never found in the Flagellata. So little attention has been paid
to this fact that affinities are declared by recent writers to exist
between Bacteria and Flagellata. The flagellum of the Flagellata
is totally distinct from the pulsellum of the Bacteria. It is carried
in front of the body and draws the body after it, being used as a
man uses his arm and hand when swimming on his side. Hence
it may be distinguished as a "tractellum." Its action may be
best studied in some of the large Euglenoidea, such as Astasia.
Here it is stiff at the base and is carried rigidly in front of the
animal, hut its terminal third is reflected and exhibits in this
reflected condition swinging and undulatory movements tending to
propel the reflected part of the flagellum forward, and so exerting a
traction in that direction upon the whole animal. It is in this way
(by reflexion of its extremity) that the flagellum or tractellum of
the Flagellata also acts so as to impel food-particles against the base
of the flagellum where the oral aperture is situated.

Many of the Flagellata are parasitic (some hcematozoic, see Lewis,
70); the majority live in the midst of putrefying organic matter in
sea and fresh waters, but are not known to be active as agents of
putrefaction. Dallinger and Drysdale have shown that the spores
of Bodo and others will survive an exposure to a higher tempera-
ture than do any known Schizomyuetes (Bacteria), viz., 250° to
300° Fahr., for ten minutes, although the adults are killed at 180°.

CLASS III. DINOFLAGELLATA, Butsehli.

Characters. — Corticate Protozoa of a bilaterally asymmetrical form,
sometimes flattened from back to ventral surface (Diplopsalis,
Glenodiniiim), .sometimes from the front to the hinder region
(Ceratium, Peridininn, . sometimes from right to left (Dinophysis,
Amphidinium, Proroeentrum) — tlie anterior region and ventral
surface being determined by the presence of a longitudinal groove
and a large flagellum • "jertiug from it. In all except the genus
Proroceutrum (Fig. XXII. 6) there is as well as a longitudinal
groove a transverse groove (hence Dinifera) in which lies horizon-
tally a second (lagellum (Klebs and Butsehli), hitherto mistaken for
a girdle of cilia. The transverse groove lies either at the anterior
end of the body (Dinophysis, Fig. XXII. 3, 4 ; Amphidiiiium) or

at the middle. In Gymnodinium it takes a spiral course. In
Polykrikos (a compound metameric form) there are eight indepen-
dent transverse grooves.

The Dinoflagellata are either enclosed in a cuticular shell
(Ceratium, Peridinium, Dinophysis, Diplopsalis, Glenodinium,
Prorocentrum, &c. ) or are naked (Gymnodinium and Polykrikos).
The cuticular membrane (or shell) consists of cellulose or of a
similar substance (cf. Labyrinthulidea) and not, as has been sup-
posed, of silica, nor of chitin-like substance ; it is either a simple
cyst or perforated by pores, and may be built up of separate plates
(Fig. XXII. 10).

The cortical protoplasm contains triehocysts in Polykrikos.

The medullary protoplasm contains often chlorophyll and also
diatomin and starch or other amyloid substance. In these cases
(Ceratium, some species of Peridinium, Glenodinium, Prorocentrum,
Dinophysis acuta) nutrition appears to be holophytic. But in
others (Gymnodinium and Polykrikos) these substances are absent
and food-particles are found in the medullary protoplasm which
have been taken in from the exterior through a mouth ; in these
nutrition is holozoic. In others which are devoid of chlorophyll
and diatomin, &c., there is found a vesicle and an orilice connected
with the exterior near the base of the flngellum (cf. Flagellata) by
which water and dissolved or minutely granular food-matter is
introduced into the medullary protoplasm (Protoperidinium pellu-
cidum, Peridinium divcrgcns, Diplopsalis lenticula, Dinophysis
lie.ris). It is important to note that these divergent methods of
nutrition are exhibited by different species of one and the same
genus, and possibly by individuals of one species in successive
phases of growth (?).

No contractile vacuole has been observed in Dinoflagellata.

The nucleus is usually single and very large, and has a peculiar
labyrinthine arrangement of chromatin substance.

Transverse binary fission is the only reproductive process as yet
ascertained. It occurs either in the free condition (Fig. XXII. 2)
or in peculiar horned cysts (Fig. XXII. 8). Conjugation has been
observed iu some cases (by Stein in Gymnodinium).

Mostly marine, some freshwater. Many are phosphorescent.

The Dinoflagellata are divisible into two orders, according to the
presence or absence of the transverse groove.

ORDER 1. ADINIDA, Bergh.

Characters. — Body compressed laterally ; both longitudinal and
transverse flagellum placed at the anterior pole ; a transverse groove
is wanting ; a cuticular shell is present.

Genera. — Prorocentrum, Ehr. (Fig. XXII. 6, 7); Exuviella,
.( = Dinopy,iis, Stein; C'njptomonas, Ehr.).

ORDEE 2. DINIFERA, Bergh.

Characters. — A transverse groove is present and usually a longi-
tudinal groove. The animals are either naked or loricate.

Fam. 1. DINOPHYIDA, Bergh. Body compressed ; the transverse
groove at the anterior pole ; the longitudinal groove present ;
longitudinal flagellum directed backwards ; loricate.

Genera. — Dinophysis, Ehr. (Fig. XXII. 3, 4) ; Amphidinium,
Cl. & L. ; Ainphisolcnia, Stein; Histioneis, Stein; C itharistes,
Stein ; Ornithoccrcus, Stein.

Fam. 2. PERIDINIDA, Bergh. Body cither globular or flattened ;
transverse groove nearly equatorial ; longitudinal groove narrow or
broad ; loricate.

Genera. — Protoperidinium, Bergh; Peridinium (Ehr.), Stein
(Fig. XXII. 1, 2); Protoceratium, Bergh; Ceratium, Schrank (Fig.
XXII. 15); Diplopsalis, Bergh; Glenodinium, Ehr. ; Hclcroai/isa,
Stein ; Gonyaulax, Diesiug ; Goniodama, Stein ; Blspharocysta,
Ehr. ; Podolampas, Stein ; Amphidoma, Stein ; Oxytoxum, Stein ;
Plychodiscus, Stein ; Pyrvjijtacus, Stein ; Cemtucorys, Stein.

Fam. 3. GYMNODINIDA, Bergh. As Peridinida but no lorica
(cuticular shell).

Genera. — Gymnodinium (Fig. XXII. 5), Stein ; Hcmidinium,
Bergh.

Fam. 4. POLYDINIDA, Biitschli. As Gymnodinida, but with
several independent transverse grooves.

Genus. — Polykrikos, Biitschli.

Further Remarks on the Dinoflayellata.—1\\\s small group is at
the moment of the printing of the present article receiving a large
amount of attention from Bergh (81), Klebs (83), and Biitschli (82),
ami lias recently hern greatly extended by the discoveries of Stein
(80), — the last work of the great illustrator of the Ciliate Protozoa
before his death. The constitution of the cell-wall or cuticle from
cellulose, as well as the presence of chlorophyll ami diatomin, and
the holophytic nutrition of many forms recently demonstrated by
Bergh, has' led to the suggcstiim that the Dinoflagellata are to be
regarded as plants, and allied to the Diatomacere and Desmidiaeese.
Physiological grounds of this kind have, however, as has been
pointed out above, little importance in determining the affinities
of Protozoa. Biitschli (82) in a recent very important article has
shown in confirmation of Klebs that the Diuoflagellata do not

PROTOZOA

31

possess a girdle of cilia as previously supposed, but that the struc-
ture mistaken for cilia is a second flagellum which lies horizontally
in the transverse groove. Hence the name Ciliollagellata is super-
seded by Dinoflagellata (Gr. dinos, the round area where oxen tread
out on a threshing floor).

19

FIG. XXII.— Dinoflagellata and Rhynchodagellata. jv.c. In all these
figures the apparent girdle of cilia is, accodrinf; to Klebs and Butschli's
recent discovery, to be interpreted as an encircling flagellum lying in the
transverse groove. 1. Peridinium uberrimum, Allman ; x 300 (fresh-

water ponds, Dublin). Probably (according to Butschli) the processes on
the surface are not cilia nor flagellum. Both the longitudinal and the
transverse groove are well seen. 2. The same species in transverse

fission. 3. Dinophysis ovata, Cl. and L; x 350 (salt water, Norwegian

coast). 4. Dinppnysis acuininata, Cl. and L. ; X350 (salt water,

Norwegian coast). 5. Synmodinium, sp. ; xOOO. 6. Prorocen-

IniM micans, Ehr.; X300 (salt water). 7. Dorsal aspect of the

same species. 8, 9. Cysts of Peridinia; the contents of 8 divided

into eight minute naked Peridinia; X300. 10. Empty cuirass of

Cfratium divergens. Cl. and L. ; X500 ; showing the form and disposition
of its component plates. 11. The same species with the animal con-

tracted into a spherical form. The transverse groove well seen. 12.

The same species in the normal state. The apparent girdle of cilia is
really an undulating flagellum lying in the transverse groove. 13, 14.

Young stages of Noctiluca miliaris. n, nucleus; s, the so-called spine
(superficial ridge of the adult); n, the big flagellum ; the unlettered filament
(8 a flagellum which becomes the oral flagellum of the adult. 15. Cera-

tium tripm, mitt. The transverse groove well seen. The cilia really are
a single horizontal llagcllum. 10, 17. Two stages in the transverse

lission of Nootiluca miliiirix, Suriray. H, nucleus; N, fond-particles- ( tho
muscular flagellum. IS. NoetUum tnilinrix, viewed from tin: aboral

side (after Allman, Quint. ./..»/•. Mi,: Met., Is"'.!), a, the entrance to the
atrium or Bagellar fossa (=longitudlnal grooic of liinnibgc llata); c the
superllcial ridge; il. the l.ig flagellum (=the llagellum of the transverse
groove of Dinoflagellata); li, tin- nucleus. i». The animal acted upon
by iodine solution, showing the protoplasm like the "primordial utricle"
of a vegetable cell shrunk away frum the structureless linn .sli. II or
cuirass. 20. Lateral view of Noctiluca, showing «. the entrain •'• f» Hi.-

groove-like atrium or llagcllar fossa in which (/ is placed ; c, the superficial
ridge; rf, the big llagellum; <•, the mouth and gullet, in which is seen
Krohns oral llagellum (-the chief llagellum or Hagellum of the longitu-
dinal groove of Diuo-fiagcllata) ; /, broad process of protoplasm extending
from the superllcial ridge c to the central protoplasm; ./, duplicating o(
the shell in connexion with the superlicial ridge ; /<, nucleus.

Butschli further suggests that the Dinoflagellata with their
two flagella and their i-shapcd combination of longitudinal and
transverse grooves may be derived from the Cryptomonadina (see
p. 858). In the latter a groove-like recess is present in connexion
with the origin of the two llagclla. Butschli thinks the large pro-
boscis-like Hagellum of Noctiluca (Rhynchoflagellata) represents
the horizontal flagellum of Dinoflagellata, whilst the prominent
longitudinal tlagelium of the Dinoflagellata is represented in that
animal by the small flagellum discovered by Krohn within the
gullet (see Fig. XXII. 20, c). The young form of Noctiluea (Fig.
XXII. 14) has the longitudinal flagellum still of large size.

The phosphorescence of many Dinoflagellata is a further point
of resemblance between them and Noetilnca.

Bergh has shown that there is a considerable range of form in
various species of Dinoflagellata (Ceratium, &c.), and has also drawn
attention to the curious fact that the mode of nutrition (whether
holophytic or holozoic) differs in allied species. Possibly it may be
found to differ according to the conditions of life in individuals of
one and the same species.

The drawings in Fig. XXII. were engraved before the publication
of Butschli's confirmation of Klebs's discovery as to the non-existence
of cilia in the transverse groove. The hair-like processes figured
by Allmau (91) external to the transverse groove in his Peridinium
uberrimum (Fig. XXII. 1, 2) cannot, however, be explained as a
flagellum. Biitsclili inclines to the opinion that their nature was
misinterpreted by Allman, although the latter especially calls
attention to them as cilia, and as rendering his P. ulerrimum
unlike the Peridinium of Ehrenberg, in which the cilia (horizontal
flagellum) are confined to the transverse groove.

y.B.—Sfe Fig. XXVII., and explanation, p. 37.

CLASS IV. RHYNCHOFLAGELLATA. Lankester.

Characters. — Corticate Protozoa of large size (^Vth inch) and
globular or lenticular form, with a firm cuticular membrane and
highly vacuolated (reticnlar) protoplasm. In Noetiluca a deep
groove is formed on one side of the spherical body, from the bottom
of which springs the thick transversely striated proboscis or
"big flagellum." Near this is the oral aperture and a cylin-
drical pharynx in which is placed the second or smaller flagellum
(corresponding to the longitudinal flagellum of Dinoflagellata).

Nutrition is holozoic. No contractile vacuole is present ; granule-
streaming is observed in the protoplasm. An alimentary tract and
anus have been erroneously described. The nucleus is spherical
and not proportionately large (see for details Fig. XXII. 18 to 20).

Reproduction by transverse fission occurs, also conjugation and,
either subsequently to that process or independently of it, a forma-
tion of spores (Cienkowsld, 87), the protoplasm gathering itself,
within the shell-likacuticnlar membrane, into a cake which divides
rapidly into numerous flagellated spores (flagellulas). These escape
and gradually develop into the adult form (Fig. XXII. 13, 14).

The proboscis-like large flagellum is transversely striated, and
exhibits energetic but not very rapid lashing movements.

Noctiluca is phosphorescent, the seat of phosphorescence being,
as determined by Allman (86), the cortical layer of protoplasm
underlying the cuticular shell or cell-wall as the primordial cuticle
of a vacuolated vegetable cell underlies the vegetable cell-wall.

Genera. — Only two genera (both marine) are known : — Noctiluca,
Suriray (90) (Fig. XXII. 17-20) ; Leptodiscus, Hertwig (88).

'

Further Eemarks on the EhiinrhafliiycUatit. — The peculiar and
characteristic feature of Noctiluca appears to be found in its large
transversely-striated flagellum, which, according to Butschli, is not
the same as the longitudinal flagellum of the Dinoflagellata, but
probably represents the horizontal flagellum of those organisms in
a modified condition ; hence the name here proposed — Rhyncho-
flagellata.

Noctiluca is further remarkable for its large size and cyst-like
form, and the reticular arrangement of its protoplasm, like that of
a vegetable cell. This is paralleled in Trachelius mum among the
Ciliata (Fig. XXIV. 14), where the same stiffening of the cuticle
allows the vacuolation of the subjacent protoplasm to take place.
The remarkable Lcptodiscus mcdusoidcs of R. Hertwig (88) appears
to be closely related to Noctiluca.

It would no doubt be not unreasonable to associate the Dino-

32

PROTOZOA

flagellata and the Rhynchoflagellata with the true Flagellata in one
class. But the peculiarities of the organization of the two former
groups is best emphasized by treating them as separate classes de-
rived from the Flagellata. Neither group leads on to the Ciliata or
to any other group, but they must be regarded as forming a lateral
branch of the family tree of "Corticata. The relationship of Nocti-
luca to Peridinium was first insisted upon by Allman, but has quite
recently been put in a new light by Biitschli, who identifies the
atrial recess of Noctiluca (Fig. XXII. 20, 6) with the longitudinal
furrow or groove of the Dinoflagellata, and the large and minute
flagella of the former with the transverse and longitudinal flagella
respectively of the latter. The superficial ridge c of Noctiluca
appears to represent the continuation of the longitudinal groove.

The phosphorescence of the sea, especially on northern coasts, is
largely caused by Noctiluca, but by no means exclusively, since
Medusre, Crustaceans, Annelids, and various Protozoa often take part
in the phenomenon. Not unfrequently, however, the phosphor-
escence on the British coasts seems to be solely due to Noetiluca,
which then occurs in millions in the littoral waters.

FIG. XXIII. Ciliata. — i. Bpiroitomum ambiguum, Ehr.; one of the Hetero-
, tricha; x 120. Observe on the right side the oral groove and special hetero-

trichous band of long cilin. ft, moniliform nucleus ; b, contractile vacuole.

2. Stentor polymorphic, Miiller ; one of the Heterotricha ; x 50 ; group of

individuals with the area fringed by the heterotrichous cilia expanded
trumpet-wise. 3. Tintinnns lagenula, C. and L. ; one of the Hetero

tricha; x 300. 4. Strombidium Claparedii, S. K.; one of the Peritricha;
x 200. 5. Empty shell of Codonella campanella, Haeck.; one of the

Heterotricha ; x 180. 6, 7. Torquatella typica, Lankester. p, the supra-
oral lobe seen through the membranous collar. 8, 9. View of the
base and of the side of Trichodina pediculus, Ehr.; one of the Peritricha;
x 300. a, nucleus ; c, corneous collar ; d, mouth. 10. Sjiirochona
gemmipara, Stein ; one of the Peritricha ; x 350. a, nucleus ; g, bud. 11.
Vorticella citrina, Ehr.; x 150 (Peritricha). At d multiple fission of an
individual cell to form "microgonidia." 12. Vorticella -uricrostoma,
-Ehr. (Peritricha); x 300. At e eight "microgonidia" formed by fission
of a single normal individual. 13. Same species, binary fission, a,
elongated nucleus. 14. Vorticella nebidifira, Ehr. ; free-swimming
zooid resulting from fission in the act of detaching itself and swimming
away, possessing a posterior circlet of cilia, e, ciliated disk ; /,
pharynx. 15. Vorticella mtcrostoma, Ehr.; normal zooid with two
microgonidia (or microzooids) c,d, in the act of conjugation, o, nucleus ;
6, contractile vacuole ; e, ciliated disk ; /, pharynx. 16, Vorticella
microstoma, Ehr., with stalk contracted and body enclosed in a cyst, a,
nucleus. 17. Vorticella ncbitli/fra, Ehr. a, nucleus ; 6, contractile
vacuole ; c, 'muscular region of the body continuous with the muscle of the
stalk ; rf, pharynx (the basal continuation of the oral vestibule which
receives at a higher point the fcecal excreta and the ejected liquid from
the contractile vacuole). 18. Carchesittm spectabile, Ehr. ; retractile
colony; X 50. 19 Trichocysts of Epistylis Jlavicans, Ehr., as figured
by Greeff. 20. Opercularia stenostoma, Stein ; x 200 ; a small colony.
Observe the ciliation of the oral vestibule and the upstanding ciliate disk
(opercular-like). 21, 22. Pyzicola affinis, S. K.; one of the stalked
loricate Peritricha, in expanded and retracted states, x, the true oper-
culum. 23, 24. Gyrocoris oxyura, Stein ; one of the free-swimming
Peritricha, with spiral equatorial cilia-band ; x 250. b, contractile
vacuole. 25, 26. Ttiuricola valvata, Str. AVright ; one of the sessile
tubicolous Peritricha. Two individuals are as a result of fission tempo-
rarily occupying one tube ; tt, the valve attached to the tube, like the door
of the trap-door spider's nest and the valve of the Gasteropod Clausilium.

CLASS V. CILIATA, Ehrenherg (Infusoria seusu stricto).

Characters. — Corticata of relatively large size, provided with
either a single band of cilia surrounding the anteriorly placed oral
aperture or with cilia disposed more numerously over the whole
surface of the body. The cilia are distinguished from the flagella
of Flagellata by their smaller size and simple movements of
alternate flexion and erection ; they serve always at some period of
growth as locomotor organs, and also very usually as organs for
the introduction of food particles into the mouth. Besides one
larger oblong nucleus a second (the paranucleus) is invariably (?)
present (Fig. XXV. 2), or the nucleus may be dispersed in small
fragments. Conjugation of equal-sized individuals, not resulting
in permanent fusion, is frequent. The conjugated animals separate
and their nuclei and paranuclei undergo peculiar changes ; but no
formation of spores, either at this or other periods, lias been de-
cisively observed (Fig. XXV. 8 to 15). Multiplication by transverse
fission is invariably observed in full-grown individuals (Fig. XXV.
16), and conjugation appears to take place merely as an interlude
in the fissiparous process ; consequently young or small Ciliata are
(with few exceptions) unknown. Possibly spore-formation may
hereafter be found to occur at rare intervals more generally than is
at present supposed (Fig. XXIV. 15, 18). A production of micro-
gonidia by rapid fission occurs in some Peritricha (Fig. XXIII.
11, 12, 14, 15), the liberated microgonidia conjugating with the
normal individuals, which also can conjugate with one another.

The Ciliata, with rare exceptions (parasites), possess one or more
contractile vacuoles (Fig. XXV. 3). They always possess a delicate
cuticle and a body-wall which, although constant, in form is elastic.
They may be naked and free-swimming, or they may form horny
(Fig. XXIII. 21, 25) or siliceous cup-like shells or gelatinous
envelopes, and may be stalked and form colonies like those of
Choanoflagellata, sometimes with organic connexion of the con-
stituent units of the colony by a branching muscular cord (Vorti-
cellidoe). Many are parasitic in higher animals, and of these some
are mouthless. All are holozoic in their nutrition, though some are
said to combine with this sapropliytic and holophytic nutrition.

The Ciliata are divisible into four orders according to the
distribution and character of their cilia. The lowest group (the
Peritricha) may possibly be connected through some of its members,
such as Strombidium (Fig. XXIII. 4), with the Flagellata through
such a form as Lophomonas (Fig. XXI. 9).

In the following synopsis, chiefly derived from Saville Kent's
valuable treatise (71), the characters of the families and the names
of genera are not given at length owing to the limitation of our
space.

ORDER 1. PEKITRICHA, Stein (79).

Characters. — Ciliata with the cilia arranged in one anterior
circlet or in two, an anterior and a posterior ; the general surface of
the body is destitute of cilia.

Sub-order 1. NATANTIA (animals never attached).

Fam. 1. TOKQUATELLID.E.

Genns. — Torquatclla, Lankester, like Slrombidium, but the^cilia
adherent so as to form a vibratile membranous collar (Fig. XXIII.
6, 7).

Fam. 2. DICTYOCYSTID.E. Animals loricate.

Fam. 3. ACTINOBOLIIXE. Illoricate, with retractile tentacula.

PROTOZOA

33

Fam. 4. HALTEium.E.

Genera.— Slrombidium, Cl. & L. (Fig. XXIII. 4) ; ITaltcria,
Dujnrd., with a supplementary girdle of springing hairs ; Didinium,
Stein, (Fig. XXIV. 19).

Fam. 5. GYROCOKID/E.

Genera. — Oyracoris, Stein, with an equatorial ciliary girdle spirally
disposed (Fig. XXIII. 23, 24); Uroccutrum, Nitzsch, girdle annular.

Flo. XXIV. Ciliata- — 1. Ophaltnopsis sepiolse, Foett. ; a parasitic Holo-
trichous mouthless Ciliate from the liver of the Squid, a, nuclei ; b,
vacuoles (nou-contractile). 2. A similar specimen treated with picro-

carmine, showing a remarkably branched and twisted nucleus; a, in
place of several nuclei. 3. Trichonympha ay His, Leidy ; parasitic

in the intestine of the Termites (White Ants); x 600. a, nucleus; 6,
granules (food?). 4. Opalina ranarum, Purkinje ; a Holotrichrms

mouthless Ciliate parasitic in the Frog's rectum ; adult ; x 100. a, a, the
numerous regularly dispersed nuclei. 5. The same; an individual in pro-
cess of binary fission, a, nuclei. 6. The same; the process of fission has
now reduced the individuals to a relatively small size. 7. Smallest fission-
produced fragment encysted, expelled from the Frog in this state and
swallowed by Tadpoles. 8. Young uninucleate individual which has

emerged from the cyst within the Tadpole, and will now multiply its
nuclei and grow to full size before in turn undergoing retrogressive
fission. 9. Anoplophrya naidos, Duj. ; a mouthless Holotrichous

Ciliate parasitic in the worm Nais; x 200. a, the large axial nucleus; 6,
contractile vacuoles. 10. Anoplophriia prolifera, C. and L.; from the

intestine of Clitellio. Remarkable for the adhesion in a metameric series

of incomplete fission-products, a, nucleus. 11. Amphileptui gigas,

C. and L. ; oncof the Bolotricha; x 100. It, contractile vacuoles ; c, tricho-
cysts (see Fig XXIII. Ill); ''. nurlriis ; e, pharynx. 12, 13. TVoiW.,,,

nioeus, Ehr.; one of the lliiliitricha; x 75. a, nucleus; b, contra- hi.
vacuole ; c, pharynx with horny fascicular lining. 12. The fasciculate
cuticle of the pharynx isolated. 14. 2'r».7,, /,,,,. ,,/«,», Ehr. (llolo-

tricha) ; x SO ; showing the reticulate arrangement of the medullary pro-
toplasm, b, contractile vacuoles; c, the cuticle-lined pharynx. IT., 10,
17, is. rcthyophthinus iimHi/ilms, Fouquet ; one of the Holotriilia;
X 120. Free individual ami successive stages of division to fonu spores.
a, nucleus; b, contractile vacuoli's. 1!». Didintum ItiUUtuan, .Miill. ;

one of the Pcritriclia ; x 200. The pharynx is everted and has seized a
Paramo2cium as food, a, nucleus; 6, contractile vacuole; c, everted
pharynx. 20. Euplotet charon, Miill.; one of the Hypotricha ; lateral

view of the animal when using its great hypotrichous processes, £, as
ambulatory organs. 21. Euplotes Itarpa, Stein (Hypotricha); x 150.

h, mouth; x, hypotrichous processes (limbs). 22. Nyctotkcnu cordi-

funiiis, Stein ; a Hetei'otrichous Ciliate parasitic in the intestine of the
Frog, a, nucleus ; b, contractile vacuole ; c, food particle ; d, anus ; e,
heterotrichous band of large cilia ; /, </, mouth ; h, pharnyx ; it small cilia.

Fam. 6. UROEOI.ARUD^E.

Genera. — Trichodina, Ehr. ; two ciliate girdles ; body shaped as a
pyramid with circular sucker-like bnse, on which is a toothed corneous
ring (Fig. XXIII. 8, 9); Licnophora, Clap.; Cvdochata, Hat. JacKa.

FIG. XXV. Ciliata (conjugation, Ac.). 1. surface view of Holotrichous
Ciliate, showing the disposition of the cilia in longitudinal rows. 2.

34

PROTOZOA

Diagrammatic optical section of a Ciliate Protozoon, showing all structures
except the contractile vacuoles. a, nucleus; 6, paranucleus (so-called
nucleolus) ; c, cortical substance ; D, extremely delicate cuticle ; E,
medullary (more fluid) protoplasm ; /, cilia; (/, trichocysts ; h, filaments
ejected from the trichocysts ; i, oral aperture ; If, drop of water contain-
ing food-particles, about to sink into the medullary substance and form

lary protoplasm ; p, pharynx. 3. Outline of a Ciliate (Paramcecium), to
show the form and position of the contractile vactioles. 4-7.

Successive stages in the periodic formation of the contractile vacuoles.
The ray-like vacuoles discharge their contents into the central vacuole,
which then itself bursts to the exterior. 8-15. Diagrams of the changes

undergone by the nucleus and paranucleus of a typical Ciliate during
and immediately after conjugation :— N, nucleus; jm, paranucleus; S,
condition before conjugation ; 9, conjugation effected ; both nucleus
and paranucleus in each animal elongate and become flbrillated ; 10,
two spherical paranuclei pit? in each, two dividing or divided nuclei
N4 ; 11, the spherical paranuclei have become fusiform ; 12, there
are now four paranuclei in each (pn* and pn*), and a nucleus
broken into four or even more fragments ; 13, the two paranuclei
marked pn* in 12 have united in each animal to form the new nucleus
pn* ; the nuclear fragments are still numerous ; 14, after cessation
of conjugation the nuclear fragments N and the two unfused paranudear
pieces pn* are still present ; 15, from a part or all of the fragments
the new paranucleus is in process of formation, the new nucleus (pn1 = N)
is large and elongated. 10. Diagram of a Ciliate in process of trans-

verse fission. 17. Condition of the nucleus N, and of the paranucleus

pn in Paramceciitm aurclia after cessation of conjugation as observed
by Biitschli. 18. Stylvnichia mytilus (one of the Hypotricha),

showing endoparasitic unicellular organisms b, formerly mistaken for
spores ; a, nuclei (after conjugation and breaking up).

Fam. 7. OPHRYOSCOLEflDJE.

Genera. — Astylozoon, Engelm. ; Ophryoscolcx, Stein.

Sub-order 2. SEDENTARTA, animals always attached or sedentary
during the chief part of the life-history.

Fam. 1. VORTICELLID.E. Animals ovate, campanulate, or sub-
cylindrical ; oral aperture terminal, eccentric, associated with a
spiral fringe of adoral cilia, the right limb of which descends into
tlie oral aperture, the left limb encircling a more or less elevated
protrusible and retractile ciliary disk.

Sub-family 1. Vortieelliuse : animalcules naked.

a. — Solitary forms.

Genera. — Gcrda, Cl. andL. ; Scyphidia, Dujnrd. ; Spirochona, Stein
(sessile with peristome in the form of a spirally convolute mem-
branous expansion, Fig. XXIII. 10) ; Pyxidium, Kent (with a
non-retractile stalk) ; Vorticella, Linn, (with a hollow stalk in
which is a contractile muscular filament).

0. — Forming dendriform colonies.

Genera. — Carchcsium, Ehr. (Fig. XXIII. IS, with contractile
stalks) ; Zoothammiiim, Ehr. (contractile stalks) ; Epistylis, Ehr.
(stalk rigid) ; Opercularia, Stein (stalk rigid, ciliated disk oblique ;
an elongated peristomial collar, Fig. XX 11 1. 20).

Sub-family 2. Vaginicolirue : animalcules secreting firm cup-like
or tube-like membranous shells.

Genera. — Vaginicola, Lamarck (no internal valve); Thtmcola,
Kent (with a door-like valve to the tube, Fig. XXIII. 25, 26) ;
Cothurina, Ehr. (lorica or shell pediculate ; no operculum); Pijxicola,
Kent (lorica pedunculate, animal carrying dorsally a horny oper-
culum, Fig. XXIII. 21, 22).

Sub-family 3. Ophrydina : animalcules secreting a soft gelatinous
envelope.

Genera. — OpUionella, Kent; Ophrydium, Ehr.

ORDER 2. HETEROTRICHA, Stein.

Characters, — A band or spiral or circlet of long cilia is
developed in relation to the mouth (the heterotrichous band)
corresponding to the adoral circlet of Peritricha ; the rest of the
body is uniformly beset with short cilia.

a. — Heterotrichal band circular.

Genera (selected).— Tintinnus, Schranck (Fig. XXIII. 3); Tri-
chodinopsis, Cl. and L. ; Codonella, Haeck. (with a peri-oral fringe
of lappet-like processes) ; Calccolus, Diesing.

ff. — Heterotrichal band spiral.

Genera (selected). —Stcntor, Oken (Fig. XXIII. 2); BUpharisma,
Perty (with an undulating membrane along the oral groove);
Spirostomum, Ehr. (oral groove linear and elongate, Fig. XXI 11.
1); Lcucophrys, Ehr. (oral groove very short).

y. — Heterotrichal band in the form of a simple straight or oblique
adoral fringe of long cilia.

Genera (selected). — Bursaria, Mtiller ; Nyctothcrus, Leidy (with
well-developed alimentary tract and anus, Fig. XXIV. 22) ; Jlalan-
tidium, Cl. and L. (V. culi parasitic in the human intestine).

ORDER 3. HOLOTRICHA, Stein.

Characters. — There is no special adoral fringe of larger cilia, nor
a band-like arrangement of cilia upon any part of the body ; short
cilia of nearly equal size are uniformly disposed all over the surface.
The adoral cilia sometimes a little longer than the rest.

o. — With no membraniform expansion of the body wall.

Genera. — Paramcecium, Ehr. (Fig. XXV. 1, 2) ; Prorodon, Ehr.

(Fig. XXIV. 13); Coleps, Ehr. ; Enchclys, Ehr. ; Trachelocerea, Ehr. ;
Tracliclius, Ehr.; Amphileptus, Ehr.; Ictiiyopkthirius, Fouquet
(Fig. XXIV. 15).

£. — Body with a projecting membrane, often vibratile.

Genera. — Ophryoglena, Ehr.; Colpidium, Stein; Lcmbus, Cohn ;
Trichonympha, Leidy (an exceptionally modified form, parasitic,
Fig. XXIV. 3).

7. — Isolated parasitic forms, devoid of a mouth.

Genera. — Opalina, Purkinje (nuclei numerous, no contractile
vacuole, Fig. XXIV. 4 to 8) ; Bcnedcnia, Foett. ; Opalinopsis,
Foett. (Fig. XXIV. 1, 2); Anoplophrya, Stein (large axial nucleus,
numerous contractile vacuoles in two linear series, Fig. XXIV. 9
10) ; Haptophrya, Stein ; Hoplitoplmja, Stein.

OKDEE 4. HYPOTK1CHA, Stein.

Characters. — Ciliata in which the body is flattened and the
locomotive cilia are confined to the ventral surface, and are often
modified and enlarged to the condition of muscular appendages
(seta; so-called). Usually an adoral baud of cilia, like that of
Heterotricha. Dorsal surface smooth or provided with tactile
hairs only. Mouth and anus conspicuously developed.

a. — Cilia of the ventral surface uniform, fine, and vibratile.

Genera. — Chilodon, Ehr. ; Loxodcs, Ehr. ; Dystcria, Huxl. ;
Ifuxlcya, Cl. and L.

j3. — Cilia of the ventral surface variously modified as- setae
(muscular appendages), styles, or uncini.

Genera.— <S7j/Zo)uc7n'«, Ehr. (Fig. XXV. 18); Oxytricha, Ehr.;
Euplotes, Ehr. (Fig. XXIV. 20, 21).

Further remarks on the Ciliata. — The Ciliata have recently
formed the subject of an exhaustive treatise by Mr Saville Kent (71)
which is accessible to English readers. On the other hand Prof.
Biitschli has not yet dealt with them in his admirable critical
treatise on the Protozoa. Hence a large space has not been devoted
in this article to the systematic classification and enumeration of
their genera. See (79) and (93).

One of the most interesting features presented by the group is
the presence in many of a cell anus as well as a cell mouth (Fig.
XXIV. 22, d). In those devoid of an anus the undigested
remnants of food are expelled either by a temporary aperture on
the body-surface or by one opening into the base of the pharynx.
In many parasitic Ciliata, as in higher animal parasites, such as
the Cestoid worms, a mouth is dispensed with, nutriment being
taken by general imbibition and not in the solid form. Many
Ciliata develop chlorophyll corpuscles of definite biconcave shape,
and presumably have so'far a capacity for vegetal nutrition. In
Vorticella viridis the chlorophyll is uniformly diffused in the pro-
toplasm and is not in the form of corpuscles (72).

The formation of tubes or shells and in connexion therewith of
colonies is common among the Peritricha and Heterotricha. The
cuticle may give rise to structures of some solidity in the form of
hooks or tooth-like processes, or as a lining to the pharynx (Fig.
XXIV. 12).

The phenomena connected with conjugation and reproduction
are very remarkable, and have given rise to numerous misconcep-
tions. They are not yet sufficiently understood. It cannot be
surely asserted that any Ciliate is at the present time known to
break up, after encystment or otherwise, into a number of spores,
although this was at one time supposed to be the rule. Icthyoph-
thirius (Fig. XXIV. 15 to 18) and some Vorticellre (76) have been
stated, even recently, to present this phenomenon ; but it is not
impossible that the observations are defective. The only approach
to a rapid breaking up into spores is the multiple formation (eight)
of microgonidia or microzooids in Vortieellidte (Fig. XXIII. 11,
12); otherwise the result of the most recent observations appears to
lie that the Ciliata multiply only by binary fission, which is very
frequent among them (longitudinal in the Peritricha, transverse
to the long axis in the others).

Several cases of supposed formation of spores within an adult
Ciliate and of the production eiidogeuously of numerous "acineti-
form young " have been shown to be cases of parasitism, minute
unicellular parasites, c.y., parasitic Arineta' (such as Sphierophrya
described and figured in Fig. XXVI.) being mistaken for the young.

The phenomenon of conjugation is frequent in the Ciliata, and is
either temporary, followed by a separation of the fused individuals,
as in most cases, or permanent, as in the case of the fertilization
of normal individuals by the microgonidia of Vorticellidre.

Since the process of conjugation or copulation is not followed
by a formation of spores, it is supposed to have merely a fertilizing
effect on the temporarily conjoined individuals, which nourish
themselves and multiply by binary fission more actively after the
prut-ess than 1'cl'ure (In-nee termed "rejuvenescence)."

Remarkable changes have been from time to time observed in
the nuclei of Ciliata during m- subsequently to conjugation, and
these were erroneously interpreted by Balbiani (73) as indicating
the formation of spennatozoa and ova. The nuclei exhibit at one
period great elongation and a distinct fibrillation, as in the dividing

PROTOZOA

35

nuclei of tissue cells (compare Fig. I. and Fig. XXV. 9, 11, 17).
The tibrilhe were supposed to lie apermatozoids, and this erroneous
view was confirmed by the observation of rod-like Bacteria
(Schizomycetes) which in some instances infest the deeper proto-
plasm of large L'iliata.

The true history of the changes which occur in the nuclei of
conjugating Ciliata has been determined by Butschli (74) in some
typir.'il instances, but the matter is by no means completely under-
stood. The phenomena present very great obstacles to satis-
factory examination on account of their not recurring very fre-
quently and passing very rapidly from one phase to another.
They have not been closely observed in a sufficiently varied
number of genera to warrant a secure generalization. The follow-
ing scheme of the changes passed through by the nuclei must be
regarded as necessarily referring to only a few of the larger
Heterotricha, Holotricha, and Hypotricha, and is only probably
true in so far as details are concerned, even for them. It is at
the same time certain that some such series of changes occurs in
all Ciliata as the sequence of conjugation.

In most of the Ciliata by the side of the large oblong nucleus is a
second smaller body (or even two such bodies) which has been very
objectionably termed the nucleolus (Fig. XXV. 8), but is better
called the " parauucleus " since it has nothing to do with the nucle-
olus of a typical tissue-cell. When conjugation occurs and a
"syzygium" is formed, both nucleus and paranucleus in each con-
jugated animal elongate and show fibrillar structure (Fig. XXV.
10). Each nucleus and paranncleus now divides into two, so that
we get two nuclei and two paranuclei in each animal. Elongation
and fibrillation are then exhibited by each of these new elements
and subsequently fission, so that we get four nuclei and four para-
nuclei in each animal (11, 12). The fragments of the original
nucleus (marked N in the figures) now become more dispersed and
broken into further irregular fragments. Possibly some of them
are ejected (so-called " cell excrement "); possibly some pass over
from one animal to the other. Two of the pieces of the four-times-
divided paranucleus now reunite (Fig. XXV. 13), and form a
largish body which is the new nucleus. The remaining fragments
of paranucleus and the broken down nucleus now gradually dis-
appear, and probably as a remnant of them we get finally a few cor-
puscles which unite to form the new paranucleus (14, 15). The
conjugated animals which have separated from one another before
the later stages of this process are thus reconstituted as normal
Ciliata, each with its nucleus and paranucleus. They take food
and divide by binary fission until a new period of conjugation
arrives, when the same history is supposed to recur.

The significance of the phenomena is entirely obscure. It is not
known why there should be a paranucleus or what it may correspond
to in other cells — whether it is to be regarded simply as a second
nucleus or as a structurally and locally differentiated part of an
ordinary cell-nucleus, the nucleus and the paranucleus together
being the complete equivalent of such an ordinary nucleus. An
attempt has been made to draw a parallel between this process and
the essential features of the process of fertilization (fusion of the
spermatic and ovicell nuclei) in higher animals ; but it is the fact
that concerning neither of the phenomena compared have we as yet
sufficiently detailed knowledge to enable us to judge conclusively as
to how far any comparison is possible. Whilst there is no doubt
as to the temporary fusion and admixture of the protoplasm of the
conjugating Ciliata, it does not appear to be established that there
is any transference of nuclear or paranuclear matter from one indi-
vidual to the other in the form of solid formed particles.

Conjugation resulting merely in rejuvenescence and ordinary fis-
sive activity is observed in many Flagellata as well as in the Ciliata.

A noteworthy variation of the process of binary fission occurring
in the parasite Opalina deserves distinct notice here, since it is inter-
mediate in character between ordinary binary fission and that
multiple fission which so commonly in Protozoa is known as spore-
formation. In Opalina (Fig. XXIV. 4) the nucleus divides as the
animal grows ; and we find a great number of regularly disposed
separate nuclei in its protoplasm. (The nuclei of many other
Ciliata have recently been shown to exhibit extraordinary branched
and even "fragmented "forms; compare Fig. XXIV. 2.) At a certain
stage of growth binary fission of the whole animal sets in, and growth
ceases. Consequently the products of fission become smaller and
smaller (Fig. XXIV. 6). At last the fragments contain each but
two, three, or four nuclei. Each fragment now becomes encased
in a spherical cyst (Fig. XXIV. 7). If this process had occurred
rapidly, we should have had a uninucleate Opalina breaking up
at once into fragments (as a Gregarina does), each fragment being
a spore and enclosing itself in a spore-case. The Opalina ranarum
lives in the rectum of the Frog, and the encysted spores are
formed in the early part of the year. They pass out into the
water and undergo no change unless swallowed by a Tadpole, in
the intestine of which they forthwith develop. From each spore-
case escapes a uninucleate embryo (Fig. XXIV. 8), which absorbs
nourishment and grows. As it grows its nucleus divides, and so
the large multinucleate form from which we started is reattained.

This history has important bearings, not only on the nature of
sporulatiou, but also on the question of the significance of the
multinucleate condition of cells. Here it would seem that the
formation of many nuclei is merely an anticipation of the retarded
fissive process.

It is questionable how far we are justified in closely associating
Opalina, in view of its peculiar nuclei, with the other Ciliata. It
seems certain that the worm-parasites sometimes called Opalinre, but
more correctly Anaplophrya, &c., have no special affinity with the
true Opalina. They not only differ from it in having one large
nucleus, but in having numerous very active contractile racuoles
(75).

Recently it has been shown, more especially by Gruber (84), that
many Ciliata are multinucleate, and do not possess merely a single
nucleus and a paranucleus. In Oxytricha the nuclei are large and
numerous (about forty), scattered through the protoplasm, whilst
in other cases the nucleus is so finely divided as to appear like a
powder or dust diffused uniformly through the medullary proto-
plasm (Trachelocerca, Choenia). Carmine staining, after treatment
with absolute alcohol, has led to this remarkable discovery. The
condition described by Foettinger (85) in his Opalinopsis (Fig.
XXIV. 1, 2) is an example of this pulverization of the nucleus. The
condition of pulverization had led in some cases to a total failure
to detect any nucleus in the living animal, and it was only by the
use of reagents that the actual state of the case was revealed.
Curiously enough, the pulverized nucleus appears periodically to
form itself by a union of the scattered particles into one solid
nucleus just before binary fission of the animal takes place ; and
on the completion of fission the nuclei in the two new individuals
break up into little fragments as before. The significance of this
observation in relation to the explanation of the proceedings of the
nuclei during conjugation cannot be overlooked. It also leads to
the suggestion that the animal cell may at one time in the history
of evolution have possessed not a single solid nucleus but a finely
molecular powder of chromatin-substance scattered uniformly
through its protoplasm, as we find actually in the living Trachelo-
cerca.

Some of the Ciliata (notably the common Vorticella;) have been
observed to enclose themselves in cysts ; but it does not appear that
these are anything more than " hypnocysts " from which the animal
emerges unchanged after a period of drought or deficiency of food.
At the same time there are observations which seem to indicate that
in some instances a process of spore-formation may occur within
such cysts (76).

The differentiation of the protoplasm into cortical and medul-
lary substance is very strongly marked in the larger Ciliata.
The food-particle is carried down the gullet by ciliary currents
and is forced together with an adherent drop of water into the
medullary protoplasm. Here a slow rotation of the successively
formed food-vacuoles is observed (Fig. XXV. 2, I, m, n, o), the
water being gradually removed as the vacuole advances in position.
It was the presence of numerous successively formed vacuoles which
led Ehrenberg to apply to the Ciliata the not altogether inappro-
priate name " Polygastrica. " The chemistry of the digestive pro-
cess has not been successfully studied, but A. G. Bourne (8) has
shown that, when particles stained with water-soluble aniliu blue
are introduced as food into a Vorticella, the colouring matter is
rapidly excreted by the contractile vacuole in a somewhat concen-
trated condition.

The differentiation of the protoplasm of Ciliata in some special
cases as "muscular" fibre cannot be denied. The contractile
filament in the stalk of Vorticella is a muscular fibre and not
simple undifferentiated contractile protoplasm ; that is to say, its
change of dimensions is definite and recurrent, and is not rhythmic,
as is the flexion of a cilium. (Perhaps in ultimate analysis it is
impossible to draw a sharp line between the contraction of one side
of a cilium which causes its flexion and the rhythmical contraction
of some muscular fibres. ) The movements of the so-called " setae "
of the Hypotricha are also entitled to be called "muscular," as
are also the general contractile movements of the cortical substance
of large Ciliata. Haeckel (77) has endeavoured to distinguish
various layers in the cortical substance; but, whilst admitting that,
as in the Gregarinre, there is sometimes a distinct fibrillation of
parts of this layer, we cannot assent to the general distinction of a
" myophane" layer as a component of the cortical substance.

Beneath the very delicate cuticle which, as a mere superficial
pellicle of extreme tenuity, appears to exist in all Ciliata we
frequently find a layer of minute oval sacs which contain a spiral
thread ; the threads are everted from the sacs when irritant
reagents are applied to the animal (Fig. XXV. 2, g, h). These
were discovered by Allman (78), and by him were termed " tricho-
cysts." They appear to be identical in structure and mode of
formation with the nematocysts of the Ccelentera and Platyhelmia.
Similar trichocysts (two only in number) are found in the spores
of the Myxosporidia (see ante, page 855).

The comparative forms of the nucleus and of the contractile
vacuoles, as well as of the general body-form, &c., of Ciliata may

36

PROTOZOA

be learnt from an examination of Figs. XXIII., XXIV., XXV.,

and the explanations appended to them.

CLASS VI. ACINETARIA, Lankester (Tentaculifera, Huxley).

Characters.— Highly specialized Corticate Protozoa, probably
derived from Ciliata, since their young forms are provided with a
more or less complete investment of cilia. They are distinguished
by having no vibratile processes on the surface of the body iu the
adult condition, whilst they have few or many delicate but firm

Fro. XXVI.— Acinetaria. 1. Rhynclieta cydopum, Zenker. a, nucleus;
6, contractile vacuole ; only a single tentiicle, ami that suctorial ; x 150.
Parasitic on Cyclops. 2. Sphterophrya •uroxttjlm, Maupas ; normal

adult; x 200. a. nucleus ; 6, contractile vacuole. Parasitic in ri.istyhi.
3. The same dividing by transverse fission, the anterior moiety with tem-
porarily developed t-ilia. «, nucleus; b, contractile vacuole. 4, 5, 6.
Spharophrya xti'iit'irea, Maupas; x 200. Parasitic in Stentor, and at one
time mistaken for its young. 7. Trichnphrya epistylidis, Cl. and L. ;
x 150. ft, nucleus; b, contractile vacimlr. 8. ffemiovhrya <j>'i/iuii-
para, Hertwig; x 400. Example with six buds, into each of which a
branch of the nucleus a is extended. 9. Tile same species, showing
the two kinds of tentacles (the suctorial ami the pointed), and the con-
tniL'tile vacuoles l>. 10. Cilmted embryo of Podnpl/rya Sleinii, ('1. and
L.; x 300. 11. Arini'tii iiriiinlia, Saville Kent; X 100 ; showing pedun-
culated lorica, ami animal with two hunches of entirely suctorial tentacles.
a, nucleus. 12. Sphurophrya mayna, Maupas ; x 300. It has seized

with its tentacles, and is in the act of sucking out the juices of six examples
of the ciliate Cvlpoda paruifrons. 13. Podophrya elongata, Cl. and L. ;

x 150. a, nucleus; b, contractile vacuole. 14. Hemiophrya Benedenii,
Fraip.; x 200; the suctorial tentacles retracted. 15. Vendrocometes

paradoxus, Stein; x 350. Parasitic on Gammarus pulcx. a, nucleus;
b, contractile vacuole ; c, captured prey. 16. A single tentacle of

Podophrya; x 800. (Saville Kent.) 17-20. Dendrosoma radians, Ehr. :
• — 17, free-swimming ciliated embryo, X 600 ; 18, earliest fixed condition of
the embryo, x 600 ; 19, later stage, a single tentaculiferous process now
developed, X 600; 20, adult colony; c, enclosed ciliated embryos ; d,
branching stolon ; e, more minute reproductive (!) bodies. 21. Ophryo-
dendron pedicellatum, Hincks ; x 300.

tentacle-like processes, which are either simply adhesive or tubular
and suctorial. In the latter case they are provided at their ex-
tremity with a sucker-disk and have contractile walls, whereas in
the former case they have more or less pointed extremities. The
Acinetaria are sedentary in habit, even if not, as is usual, per-
manently fixed by a stalk. The nucleus is frequently arboriform.
Reproduction is effected by simple binary fission, and by a modified
fission (bud-fission) by which (as in Reticularia and Arcella) a
number of small bud-like warts containing a portion of the branched
parental nucleus are nipped off from the parent, often simul-
taneously (Fig. XXVI. 8). These do not become altogether dis-
tinct, but are for a time enclosed by the parental cell each in a
sort of vacuole or brood-chamber, where the young Acinetarian
develops a coat or band of cilia and then escapes from the body of
its parent (Fig. XXVI. 10, 17). After a brief locomotive existence,
it becomes sedentary, develops its tentacles, and loses its cilia.

The Acinetaria have one or more contractile vacuoles. Their
nutrition is holozoic.

The surface of the body in some crises is covered only by a
delicate cuticle, but in other cases a definite membranous shell or cup
(often stalked) is produced. Freshwater and marine. See Fraipont
(89).

ORDER 1. SUCTORIA, Kent.

A greater or less proportion or often all of the tentacles are
suctorial and terminated with sucker-like expansions.

Genera. — Ehyncheta, Zenker (stalkless, naked, with only one
tentacle ; epizoic on Cyclops ; Fig. XXVI. 1) ; Urnula, C. and L. ;
Sphszrophrya, C. and L. (naked, spherical, with distinctly capitate
tentacles only ; never with a pedicle ; parasitic within Ciliata,
supposed young ; Fig. XXVI. 2-6, 12) ; Trichophrya, C. and L. (as
Sphxrophrya, but oblong and temporarily fixed without a pedicle);
Podophryn, Ehr. (naked, solitary, globose, ovate or elongate, fixed
by a pedicle ; tentacles all suctorial, united in fascicles or distri-
buted irregularly; Fig. XXVI. 10, 13, 16) ; Hemiophrya, S. Kent (as
Podophrya, but the tentacles are of the two kinds indicated in the
definition of the group ; Fig. XXVI. 8, 9, 14); Podocyathus, S. Kent
(secreting and inhabiting stalked membranous cups or loricse ; ten-
tacles of the two kinds) ; Solcnophrya, C. and L. (with a sessile
lorica ; tentacles only suctorial) ; Acineta, Ehr. (as Solcnophrya,
but the lorica is supported on a pedicle; Fig. XXVI. 11); Dendro-
cometcs, Stein (cuticle indurated ; solitary, sessile, discoid ; tentacles
peculiar, viz., not contractile, more or less branched, root-like, and
perforated at the extremities and suctorial in function ; Fig.
XXVI. 15). Dendrosoma, Ehr. (forming colonies of intimate!)'
fused individuals, with a basal adherent protoplasmic stolon and
upstanding branches the termination of which bear numerous capi-
tate suctorial tentacles only ; Fig. XXVI. 17-20).

ORDERS. NON-SUCTORIA, Lankester (*= Actinaria, Kent).

Characters. — Tentacles filiform, prehensile, not provided with a
sucker.

Genera. — Ephdota, Str. Wright (solitary, naked, pedunculate,
with many flexible inversible tentacles) ; Adinocyathus, S. Kent ;
Ophryodcndron, C. and L. (sessile, with a long, extensile, anterior
proboscis bearing numerous flexible tentacles at its distal extremity ;
Fig. XXVI. 21) ; Acinctopsis, Robin (ovate, solitary, secreting a
stalked lorica ; from the anterior extremity of the animal is deve-
loped a proboscis-like organ which does not bear tentacles).

Further remarks on the Acinetaria, — The independence of the
Acinetaria was threatened some years ago by the erroneous view of
Stein (79) that they were phases in the life-history of VorticelHdae.
Small parasitic forms (Sphserophrya) were also until recently
regarded erroneously as the " aciuetiform young " of Ciliata.

They now must be regarded as an extreme modification of the
Protozoon series, in which the differentiation of organs in a
unicellular animal reaches its highest point. The sucker-tentacles
of the Suctoria are very elaborately constructed organs (see Fig.
XXVI. 16). They are efficient means of seizing and extracting the
juices of another Protozoon which serves as food to the Acinetarian.
The structure of Dendrosoma is remarkable on account of its
multieellular character and the elaborate differentiation of the
reproductive bodies.

The dilation of the embryos or young forms developed from the
buds of Acinetaria is an indication of their ancestral connexion
with the Ciliata. The cilia are differently disposed on the young
of the various genera (see Fig. XXVI. 10, 17).

PROTOZOA

37

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(39) SIDDALL (Shepheardella), Quart. Jour. Alicr. Sci., vol. xx., 1880, p. 130.

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(41) CARPENTER (Eoozoon)j Quart. Jour. Geol. Soc., vols. xxi. and xxii. ; and
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Lcipsic, 1R76. (47) LEFCKAPT (Spornznn), Die mrnxchlichcn Paraxiten, 2d ed.t
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p. 515, ls;r,, p. l:;-j ami p. 41CI, Ihsl, p. :;s7. (49) KUMB(Coccidiiik- of Helix),
A?>!i<i>t>i. >i. jS'-vjX1. ni>< r<j. n-iturf. (jfSfltxi'h., 1., ISijii. (50) LANKESTKIC, E. UAV
(Drepanidhun), Quart. JOUT. 2tffcr. .S«-/., vol. xxii., 1S8L', p. 5a. (51) Lir;iiEi:KUHN
(Coccidlam of ITO^'S ki'liu-y), Archiv /. Annt. and J'hysioloy., 18J4. (52)
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LENDENKBLD (parasitic aimcboid oi^unisin). in i'r<><-<-«lin<j* of Linnean Xociely of
New South Wales, 1885. (54) LANKKSTER, E. RAY (Alanocystis pelfucida), Quart.
Jour. Aficr. Sci. (IR-W series), vol. vi., isiifi. (55) LANKISTER, E. RAY (Mono-
cyst is aphroditie), Quart. Jour. Aficr. AVi. (new series), vol. iii., 1863. (56)
HEICTWIO, R. (Aphrothoi'aca), in (trintniiinii* dfr Radiolarien, Jtna, 1879. (57)
ARCHER (Chlareydophora), " Ke'sunn'. iVc.," (^nart. Jour. Ahcr. Sci., vol. xvi.,
1876. (58) HERTWIG, It., und LESSEE (Chalarothoraca), Archiv/. Mikrosk. Anat.t
x., Supplement, 1874. (59) LANKKSTER, E. KAY (Haliphysema), Quart. Jour.
Aficr, Sci. (new series), vol. xix., 187'J. (60) HAKCKKL (I'liysemaria), Jt-naischc
Zfitschr., x. (61) BESSELS (Astrnrhiza), J<:nni$fhc Z> ttschr., ix. (62) CARPENTER
(classification of Reticularia), "Researches on the Fovaminifera," Phil. Trans.,
185(i-59-(iO. (63) HAECKEI, (Hadiolaria), JJie liadw/arien, Berlin, 1862. (64)
LANKESTER, E. RAY (term Corticata), Preface to the English edition of Gegenbaur's
Elements of Comparative Anatomy, 1878. (65) CIENKOWSKI (Ciliophrys), Archie
f. Afikrosk. Anat., xii., 1876, p. 15-50. (66) DALUNGER and DRYSDALE (hooked
and springing Monads), a serk-s of papers in the Monthly Aficroscopical Journal,
1873-74-75. (67) IULLIXGKR (Trcpomonas), President's Address, Jour, of the
Hoy. Aficr. Soc., April 1885. (68) JAMES CLARK (Choanoflagellata, Alemoirs of
the Boston Society of A'at. Hist., 18U7, vol. i. (69) SAVILLE KENT (Choano-
flagellata), Monthly Aficroscopical Journal, vol. vi., 1871. (70) LEWIS, T. R.
(Hrematoznic Flagellatu), Quart. Jour. Alicr. S<'i., vol. xxiv., 1884, and vol. xix.,
1879. (71) SAVILLE KENT, Manual of the Jnfusoria, London, 1882. (72)
SALLITT, J. (chlorophyll of Ciliata), Quai t. Jour. Alicr . Sci.t 1884. (73) BALBIANI
(sexuality of Ciliata), Journal de la Physiologic, i., iii., and iv., and Archives de
Zool. Experim.i ii., 1873. (74) BUTSCHLI (conjugation of Ciliata), Abhand. d.
Senkenberg. naturf. Gesetlschaft., x., 1876. (75) LANKESTER (Opalina=Anaplo-
phyra), Quart. Jcur. Aficr. Sci. (new series), vol. x., 1870. (76) ALLMAN (encysted
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(73) ALLMAN (trichocysts of Ciliatn), Quart. Jour. Aficr. Sci., vol. iii., is;,-,.
(79) STEIN (relations of Acinetx to Ciliata) Der Organismus der Infusionstliitret
Abth. i., Leipsic, 1S59. (80) STEIN (Dlnonagellata), Der Organismus, &c.t Abth.
iii., Ltipsic, 1883. (81) BERGU (Dinoflagellata), Morpholoy. Jahrb., vii., 1881.
(82) BUTSCHLI (Dinonagellata), Morpholog. Jahrb., x., 1885. (83) KLEBS (Dino-
flagellata), Botan. Ztitung, 1884, pp. 7L'2, 737. (84) GKUBER (nuclei of Ciliiita),
Zeitschr. f. wiss. Zoologie, xi., 1884. (85) FOETTINGER (Opalinopsis, &c.),
Archives de Biologie, vol. ii. (86) ALLMAN (Noctiluca), Quart. Jour. Aficr. Sci.
(new series), vol. xii., 1872, p. 326. (87) CIENKOWSKI (Noctiluca spores), Arch,
f. Afikrosk. Anat., vii., 1871. (88) HERTWIG (Leptodiscus), Jenaische Zeitachr^
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Bulletins de VAcad. Roy. BruxeUes, 1877-78. (90) SURIRAY, Magasin de Zoologie,
1836. (91) ALLHAN (Peridinium), Quart. Jour. Aficr. Sci., iii., 1855. (92) LEIDY,
U.S. Geological Survey of the Territories, vol. xii. (93)CLAi-AREDiiandLACRSiANN,
Etudes sur les Infusoires et les Rhisopodest Geneva, 1858-61. (E. B.L.)

FIG. XXVII.

FIG. XXVII. — Dinoflagellata. This figure is not contained in the
article as published in the Encydopsedia Britannica. It presents
the recent discoveries of Klebs, Biitschli, and Stein.

1. Diagram of the Dinoflagellate Hemidinium. n, nucleus ; /,

flagellum of the transverse groove ; h, flagellum of the vertical
groove.

2. Diagram of the Cryptomonadine Oxyrrhis (to compare with the

preceding). «, nucleus; g, the deep fossa or pit in which the
two flagella are affixed; t, the origin of the flagellum which
corresponds with that of the transverse groove of Dino-
flagellata. The second flagellum is seen to be attached near
the mouth of the fossa.

3. Olenodinium cinctum, Ehr., seen from the ventral surface, a,

amyloid granules ; b, eye-spot ; c, chromatophores ; d, flagellum
of the transverse groove; c, flagellum of the vertical groove;
v, vacuole.

4. The same, seen from the hinder pole (letters as in 3).

5. Cuticle of Histioneis cymbalaria, Stein, from the Atlantic, i,

ventral process ; k, cuticular collar ; I, posterior process.

6. The same, seen from the dorsal surface, m, cephalic funnel (k

and I as iu 5).

7. Cuticle of Amphisolcnia globifcra, Stein, from the Atlantic, seen

from the left side, i, narrow ventral processes ; m, cephalic
funnel ; o, the mouth ; ]>, pharynx ; q, the shrunken proto-
plasm.

8. Cuticle of Ornithoccrcus magnificus, Stein, from the Atlantic.

mm', the cephalic funnel ; rr', the two largu rilis of the
cuticular collar (the collar itself similar to k in No. 5 is not
drawn); s, the two rows of dorsal cuticular teeth.

9. Cuticle of Ceratocorys horrula, Stein, from the Southern

Ocean, i, the large frontal plate ; pp' the outgrown margins
of the transverse groove ; v, if, basal plates ; w, one of the/
four frontal horns ; y, the dorsal horn ; y, the ventral horn.

39

rS P 0 N CT E S

(By ][r. Johnson tiollas, LL.D., F.It.S., Professor of Geology, Trinity College, Dublin.)

THE great advance which has been made during the
past fifteen years in our knowledge of the sponges
is due partly to the vivifying influence of the evolutional
hypothesis, but still more to the opportunities afforded by
novel methods of technique. To the strength and weak-
ness of the deductive method Haeckel's work on the Kalk-
schwamme (6) l is a standing testimony, while the slow but
sure progress which accompanies the scientific method is
equally illustrated by the works of Schulze (20), who by
a masterly application of the new processes has more
than any one else reconstructed on a sure basis the general
morphology of the sponges. In the general progress the
fossil sponges have been involved, and the application of
Nicol's method of studying fossil organisms in thin slices
has led, in the hands of Zittel and others (24, jj), to a
complete overthrow of those older classifications which
relegated every obscure petrifaction to the fossil sponges,
and consigned them all to orders no longer existing.
But, whilst many problems have been solved, still more
have been suggested. An almost endless diversity in
details differentiates the sponges into a vast number
of specific forms; the exclusive possession in common of
a few simple characters closely unites them into a compact
group, sharply marked off from the rest of the animal
kingdom.2

1 These italic numbers refer to the bibliography which will be
found at page 54.

2 Since this was written, in 1887, four large monographs, includ-
ing considerably over 2000 pages of letterpress, have been published on
the Sponges. Three of these, viz. : — Schulze on the Hexactinellidu,
Ridley and Dendy on the Monaxonida, and Sollas on the Tetractind-
lida appear as Reports of the "Challenger" Expedition, the fourth by
Von Leudenfeld on the " Horny Sponges " as a special volume issued
by the Royal Society. With this addition to our knowledge a longer
preface than this would be possible, but for the general student the
following amended classification of the Monaxonida will probably be
found sufficient.

Order. — Monaxonida.
Sub-order 1. ASEMOPHORA, Sollas.

Family 1. HOMORAPHTM, Ridley and Dendy. — Megascleres either
oxeas or strongyles. No microscleres. Ex. : Halichondria.

Sub-order 2. MENISCOPHORA, Sollas.

The microscleres when present are sigmaspires, sigmas, or cymbas.

Family 1. HETERORAPHEDJE, Ridley and Dendy. — Megascleres of
various forms, microscleres never cymbas. Ex.: lihizochalina, O.S.

Family 2. DESMACIDONIM:, O.S. — Megascleres usually mouactiual,
microscleres cymbas. Ex.: Desmacidon, O.S.

Sub-order 3. SPIXTHAROPHORA, Sollas.

The microsclere when present is some form of aster.

Group 1. HOMOSCLERA. — The spicules are all microscleres.

Family 1. ASTROPEPLID.E. — The microscleres are microxeas and
asters. Ex. : Astropeplus, Soil.

Group 2. HKTEROSCLERA, Soil. — Megascleres are always present,
and sometimes microscleres.

DEHDS 1. CENTROSPINTHARA, Soil. — The microsclere when present
is a euaster.

Family 1. AsiNELLiDa;, O.S. — Non-corticate, mesoderm collen-
chymatous, chamber system eurypylous. The skeleton consists of
axial and radial spicular fibres. Ex.: Axinella, O.S.

Family 2. DOUYFLKRID.E, Soil. — Non-corticate, mesoderm colleu-

Structure and Form.

Description of a Simple Sponge. — As an example of Simple
one of the simplest known sponges we select Ascetta sP°nSe-
primordialis (fig. 1), Haeckel. This is a hollow vase-like
sac closed at the lower end, by which it is attached,
opening above by a comparatively large aperture, the
osculum or vent, and at the sides by numerous smaller
apertures or pores, which perforate the walls. Except for
the absence of tentacles and the presence of pores it offers
a general resemblance to some simple form of Hydrozoon.
Histologically, however, it presents considerable dif-
ferences, since, in addition to an endoderm and an

FIG. 1. — Ascetta primordialis, Hacckel.
After Uaeckel.

ectoderm, a third or mesodermic layer contributes to
the structure of the walls ; and the endoderm consists of
cells (see fig. 21, g) each of which resembles in all essential
features those complicated unicellular organisms known
as choanoflagellate Infusoria (see PROTOZOA, vol. xix. p.
858). With this positive character is associated a nega-
tive one : nematocysts are entirely absent. The activity

divmatous. Skeleton consisting of oxeas arranged without order.
Ex. : Doryplercs, Sol).

Family 3. TETHYID^E, Vosm. — Corticate. Skeleton consisting of
radially arranged oxeas. The microsclere is a spheraster. Ex.:
Tftliyn, Lam.

DEMUS 2. SPIRASPINTHARA, Soil. — The microsclere is a spiraster.

Family 1. ScoLOPlDJE, Soil. — The cortex is thin and fibrous, with
radially arranged closely-packed microxeas and oxeas. The skeleton
consists of oxeas collected into radially disposed fibres. The micro-
sclere when present is an amphiaster. Ex. : Sculojitts, Soil.

Family 2. SOBHRITIDJE, O.S.— Cortex with a skeleton of radially
arranged styles. Microscleres usually absent. The megascleres are
tylostyles. Ex. : Suberites, Nardo .

Family 3. SPIRASTRELLID.E, Ridley and Deudy. — The megascleres
are rhabdi or styles. The microscleres are spirasters or discasters.
Ex.: Spiraslrella, O.S.

40

SPONGES

of the Ascetta, as of all sponges, is most obviously mani-
fested, as Grant (j) first observed, by a rapid outflow of
water from the oscule and a gentle instreaming through
the pores, — a movement brought about by the energetic
action of the flagella of the
endoderrnic cells. The in-
streaming currents bear with
them into the cavity of the
sac (paragastric cavity) both
protoplasmic particles (such as
Infusoria, diatoms, and other
small organisms) and dissolved
oxygen, which are ingested by
the flagellated cells of the en-
doderm. The presence of one
or more contractile vacuoles in
these cells suggests that they
extricate water, urea, and car-
bonic acid. The insoluble re-
sidue of the introduced food,
together with the fluid excreta,
is carried out through the os-
cule by the excurrent water.
New individuals are produced
from the union of ova and
spermatozoa, which develop
from wandering amoeboid cells
in the mesoderm. The walls
of Ascetta are strengthened by
calcareous scleres, more especi-
ally designated as spicules, FIG. 2.— Homoderma sycamim, Lfd.
which have the form of tri- mediana^ectlonaWAfte/va Lend'en!

radiate needles. If we make feid(xabout 6).

abstraction of these we obtain an ideal sponge, which

Haeckel has called Olynthus (6), and which may be re-

Canal System. — We shall now trace the several modifi-
cations which the Olynthus has undergone as expressed in
the different types of canal system.

The simple paragaster of Ascetta may become compli- Ascon
cated in a variety of ways, such as by the budding off tJT6-
from a parent form of stolon-like extensions, which then
give rise to fresh individuals, or by the branching of the
Ascon sac and the subsequent anastomosis of the branches;
but in no case, so long as the sponge remains within the
Ascon type, does the endoderm become differentiated into
different histological elements. The most interesting
modification of the Ascon form occurs in Homoderma sy-
candra (i2\ in which from the walls of a simple Ascon
csecal processes grow out radiately in close regular whorls,
each process reproducing the structure of the parent
sponge (figs. 2, 3). From this it is but a short step to
the important departure which gives rise to the Sycons.

In the simplest examples of this type the characters of Sycon
Homoderma sycandra are reproduced, with the important type,
exception that the endoderm lining the paragastric cavity
of the original Ascon form loses its primitive character,

FIG. 3. — Humodcrma syctiiulm, Lfd. Transverse section, showing radial tubes opening
into central naragustric cavity. After V. Lendeiiteld (xabout !-)•

garded as the ancestral form from which all other sponges
have been derived. To give greater exactness to our ab-
straction we should perhaps stipulate for the Olynthus a
somewhat thicker mesoderm and more spherical form than
a decalcified Ascon presents.

FIG. 4. — Heteropegnwnodus-gordii, Pol. Part of a transverse section. The straight
lines indicate spicules ; the poriferous surface is uppermost; the branching
radial tubes are rendered dark by numerous small circles representing
clioauocytes. After PolejaefT, ' ' Challenger " Report ( X 50).

and from a layer of flagellated cells becomes converted
into a pavement epithelium, not in any distinguishable
feature different from that of the ectoderm. The
flagellated cells are thus restricted to the caecal
outgrowths or radial tubes. Concurrently with
this differentiation of the endoderm a more abun-
dant development of mesoderm occurs. In some
Sycons (Sycaltis, Hk.) the radial tubes remain
separate and free; in others they lie close together
and are united by trabeculte, or by a trabecular
network, consisting of mesodermic strands sur-
rounded by ectoderm (fig. 4). The spaces between
the contiguous radial tubes thus become converted
into narrow canals, through which water passes
from the exterior to enter the pores in the walls
of the radial tubes. These canals are the " inter-
canals " of Haeckel, now generally known by their
older name of incurrent canals. The openings of
the incurrent canals to the exterior are called
pores, a term which we have also applied to the
openings which lead directly into the radial tubes
or paragastric cavity; to avoid ambiguity we shall
for the future distinguish the latter kind of open-
ing as a prosopyle. The term "pore" will then be

restricted to the sense in which it was originally used by
Grant. The mouth by which a radial tube opens into the
paragaster is known as a gastric ostimn. In the higher
forms of Sycons the radial tubes nolonger arise as simple out-
growths of the whole sponge- wall, but rather as outgrowths

SPONGES

41

of the emloderm into the mesoderm, \vliirh, together with
the ectoderm, exhibits an independent growth of its own;
and this results in the formation of a thick investment,
known as the cortex (fig. 5), to the whole exterior of the

'

Kb. agon
type.

Fio. 5. — Utc Argcnted, Pol. Fart of a transverse section. The concentric circles,
indicating transviTsr sections of spicules, lie within the cortex. After Pule-
jaeff, " Challenger" Report ( x 100).

sponge. The radial tubes may branch, Heteropegma (fig.
4). If the branches are given off regularly, as the radial
tubes were in the first plan, and if at the same time the
original radial tube exchanges its flagellated for a pave-
ment epithelium, a structure as shown in fig. 6 (Polejna

Fio. 6.— Polejna connexiva, Pol. Part of a transverse section. E, excurrent
canals, into which the flagellated chambers open. After Poleiaeff, " Challenger"
Effort (KM).

connexiva, Pol.) will result. This form might also be
brought about by unequal growth of the gastral endoderm
leading to a folding of the inner part of the sponge-wall.
Very little direct evidence exists as to which of these two
plans has actually been followed. Phylogenetically the
transition from a simple Ascon to the most complicated
Sycon can be traced step by step ; and ontogeny shows
that such a Sycon form as Grant in r<iphanus passes through
an Ascon phase in the course of its larval development.

Returning to the ancestral form of sponge, Olynthus,
let us conceive the endoderm growing out into a number
of approximately spherical chambers, each of which com-
municates with the exterior by a prosopyle and with the
paragastric cavity by a comparatively large aperture, which
we may term for distinction an apapyle; at the same time
let the endoderm lose its flagellated character and become

converted into a pavement epithelium, except in the
spherical chambers. Such a form, called by llaeckel
"dyssyciis," maybe more briefly named a ]{lni<i<,ii from
the grape-like form of its flagellated chambers, which differ
from those of a Sycon both by their form and their smaller
dimensions. The Khagon occurs as a stage in the early
development of Plakiiia monoloji/ui (Sehulxe) and llt'iiiera,
fcrtitis (9) (fig. 7); a calcareous sponge which appears to

FIG. 7. — Vertical section of a Rhagon, partly diagrammatic, o, oseulc ; p,
paragaster. After Keller (X about 100).

approach it somewhat is Leucopsis pedunndatn, Lfd. By
the folding of the wall of a Ehagon, or by its outgrowth
into lobes, a complicated structure such as that of Plakina
monolopha (20) (see fig. 26 /) results. This is character-

Fio. 8. — Transverse section across an excurrent. canal and surrounding choano-
some of Cydoniitm eosuster, Soil, e, excurreut canal ; /, flagellated chambers
communicating with it. by aphodal canals ; ;., an incurrent canal cut across ; 5,
a sterraster ; o, an oxea cut across. After SoUas, " Challemjcr " .Report (x 125).

ized by the chambers retaining their immediate communi-
cation with the incurrent and excurrent canals, opening
into the latter by the widely open apopyle and receiving
the former by one or
several prosopyles. This ','
may be termed the e ury-
pylous type of Ehagon
canal system. The fold-
ing of the sponge -wall
may be simple, as in the
example given, or too , ~<°},
complex to unravel. In
higher forms of sponges
(Geodinidx, Sttlhttidx) 0
the chambers cease to
open abruptly into the
excurrent canals : each is
prolonged into a narrow
canal, apfiodus, or alii(»a,
which usually directly,
sometimes after uniting
with one or more of its PIQ. 9.— mplodaTcanal system in Cort;?;,n«.

fellows Opens into an candelabrum, O.S. e, excutrent canal ; the
. —,, incurrent canal is ><li"\ni <*n the k-ft-hand
excurrent canal. llie side, near its commencement in the cortex,
prosopyles, now. restrict- After F- E- Scllulze <x :
ed to one for each chamber, may remain unchanged in
character, or at the most be prolonged into very short

F

42

SPONGES

tubes, each a prosodus or aditus (fig. 8). This may be
termed the aplwdal or racemose type of Rhagon system,
since the chambers at the ends of the aphodi radiating
from the excurrent canal look like grapes on a bunch. As
Haeckel, however, has used "racemose" in a different sense,
we shall adopt here the alternative term. By the exten-
sion of the prosodal or adital canals into long tubes a still
higher differentiation is reached (fig. 9). This, which from
the marked presence of both prosodal and aphodal canals
may be termed the diplodal type of the Rhagon canal
system, occurs but rarely. Chondrosia is an example.

The following scheme will render clear the foregoing
distinctions : —

1. Ascon type : simple, ex. Ascetta, Hk. ; strobiloid, ex. Homo-

derma, Lfd.

2. Sycon type : simple radial tubes, ex. Sijcctta, Hk. ; branched

radial tubes (cylindrical chambers), ex. Hctcropegma, Fl. ;
chamber-layer folded, ex. Polcjna, Pol.

3. Rhagon type : eurypylous, with several prosopyles to each

chamber, ex. S/jtmyelia ; with a single prosopyle to eacli
chamber, ex. Oscarctta, Thenca ; aphodal, aphodal canals well
developed, ex. Gcodia, Link. : diplodal, with both aphodal
and prosodal canals well developed, ex. Chondrosia, O.S.

In the case of the calcareous sponges Polejaeff has argued
forcibly that the eurypylous type arises directly from the
Sycon and not from the Rhagon. It is therefore doubtful
how far the Rhagon in other sponges is a primitive form
derived directly from an Olynihus, or whether it may not
be a secondary larval state resulting from the abbreviated
development of a former Sycon predecessor. Whatever
may have been its past history, the Rhagon serves now at
all events as a starting-point for the development of the
higher forms of canal system.

Subder- In the higher Rhagons, as in the Sycons, further com-
mal plications ensue, owing to an independent growth of the
cavities. externai ectoderm and the adjacent mesoderm. While the
endoderm, with its associated mesoderm, is growing out
or folding to form the excurrent canal system, the super-
ficial mesoderm increases in thickness, and the ectoderm,
extending laterally from the sides of the incurrent sinuses,
burrows into it, parallel to the surface of the sponge.
Thus it forms beneath the skin (i.e., the layer of superficial
mesoderm and investing ectoderm) cavities which may be
either simple and spacious or be broken up into a number
of labyrinthine passages by a network of mesoblastic
strands (invested with ectoderm) which extend irregularly
from roof to floor of the chamber. These cavities are
known as subdermal chambers.

With the appearance of subdermal chambers the sponge
Ecto- becomes differentiated into two almost independent regions,
some, all outer or ectosome and an inner or choanosome, which is
some"0" characterized by the presence of flagellated chambers.
The ectosome forms the roof and walls of the subdermal
chambers, and is in its simplest form merely an investing
skin ; but in a large number of sponges it acquires con-
siderable thickness and a very complicated histological
structure. It is then known as a cortex. The thickening
which gives rise to a cortex takes place chiefly beneath
those parts of the skin which are not furnished with pores.
Beneath the pores — in this case collected into sieve-like
areas — dome-like cavities are left in the cortex ; they open
freely into the subdermal cavities below and their roof is
formed by the cribriform pore membrane above. In many
sponges (Geodia, 8/rl/et/a) the cortical domes are constricted
near their communication with the subdermal cavity (sub-
cortical crypt) by a transverse muscular sphincter, which
defines an outer division or ectnchone from an inner or
endochone (fig. 10), the whole structure being a chone.
Chone. The endochone is frequently absent (fig. 10). The early
development of the cortex has scarcely yet been studied.
In Stdlettaphrissens($o\\,), one of the "Challenger" Stel-

lettidx, an early form of the sponge (fig. 11), shows the
choanosome already characteristically folded within the
cortex, which forms a com-
plete not-folded envelope
around it. The roots of
the incurrent sinuses form ,
widely open spaces imme-
diately beneath the cortex
and are the rudiments of ;
subcortical crypts. Again,
in some sponges a part of i
the endoderm and asso-
ciated mesoderm may like- 1 j(
wise develop independ- 1| /
ently of the rest of the /J
sponge, as in the Hexac-
tine/lida, where the choa-
nosome forms a middle
layer between a reticula-
tion of ectosome on
one side and of endoderm ;
and mesoderm, i.e., em.li-
some, on the other. Fin-
ally, the attached or lower Fl(
half of a Rhagon may de-
velop in an altogether dif-
ferent manner from the ,

The dotted circles in the cortex are sterr-
Otner or upper hall, the asters connected by fibrous strands.

endoderm not producing After Soiias1"CtaZto«ff«-»Bepor«(x 73).

any flagellated chambers. In this case the upper portion
alone is characterized by the flagellated chambers, which
are the distinctive mark of a sponge, and hence may be

Uj

10.— Section through tho cortex of (> Endo-
Jtnnuiti eososfer, SolL, showing the pore- some,
sieve overlying the chone, which com-
municates through a sphinctrate aperture
with the subcortical crypt, lying in the
choanosome with its flagellated chambers.

Fin. 11.— Young sponge of stelletta pTirissena, Soil. Longitudinal median sec-
tion, showing the clnaiuisomi' lol.U'd within the cortex, o, oscnle. After
Sollas, " Cltatlenijtr" Rc^n-t (x50).

called the spongomere ; the lower half, which consists of
all three fundamental layers, may be called the hypomtre.
The form and general composition of sponges are ex-
ceedingly various and often difficult to analyse, presenting,
along with some important differences, a remarkable general
resemblance to the Co-lentem in these respects. Like Oscule.
them, some sponges are simple, and others, through
asexual multiplication, compound. The only criterion by
which the individual sponge can be recognized is the oscu-
lum ; and, as it is frequently difficult, and in many cases
impossible, to distinguish this from the gastric opening of
a large excurrent canal, there are many cases in which the
simple or compound nature of the sponge must remain
open to doubt. The oscule may also fail (lipostomosis),
and so may the paragastric cavity (lijn.il/as/rrisiif) ; the
problem then becomes insoluble. The loss of the oscule

SPONGES

Mineral
spicules.

may in some cases be duo to the continued growth of
several endodermal folds towards the exterior, with a
corresponding absorption of the mcsoderm and ectoderm
which lie in the way, till the folds penetrate to the ecto-
derm and open at the exterior, thus giving rise to exeurrent
openings, which are not readily distinguishable from pores.
At the same time the original osculum closes up and
entirely disappears. Lipogastrosis, on the other hand,
may be produced by the growing together of the roots of
the choanosomal folds, thus reducing the paragastric cavity
to a labyrinth of canals, which may easily be confounded
with the usual form of exeurrent canals. While in some
sponges the original oscule is lost, in others secondary
independent openings, deceptively like oscules, are added.
This pseudostomosis is due to a folding of the entire sponge,
so as to produce secondary canals or cavities, which may
be incurrent (vestibular) or exeurrent (cloacal), the opening
of the latter to the exterior being termed a false oscule
or pse-udostvme. The faulty use of the term oscule for
what is neither functionally nor morphologically a mouth
is here obvious, for in one sense the oscule is always a
pseudostome ; it would be better if the term pseudojjroct
could be substituted.

Skeleton. Skeleton. — All sponges, except three or four genera be-
longing to the Myxospongise, possess some kind of skeletal
structures. They may be either calcareous or silicious or
horny scleres, the latter usually having the form of fibres,
which sometimes enclose silicious needles (spicules) or
foreign bodies introduced from without. Foreign bodies
also contribute to the formation of the skeleton of some
silicious sponges, and occasionally form the entire skeleton,
no other hard parts being present.

Mineral scleres usually occur in the form of spicules.
The spicules of calcareous sponges consist of carbonate of
lime, having the crystalline structure and other properties
of calcite (29). Each spicule, so far as its mineral com-
ponent is concerned, is a single crystal, all the molecules
of calcite of which it is built up being similarly oriented.
On the other hand, its form and general structure are
purely organic. Its surfaces are always curved, and usually
it has the form of a cone or combination of cones, each of
which consists of concentric layers of calcite surrounding
an axial fibre of organic matter, — probably of the same
nature as spongiolin or spongin, the chief constituent of
the fibres of horny sponges. A thin layer of organic matter,
known as the spiaile sheath, forms an outer investment to
the spicule and is best rendered visible as a residue by
removing the calcite with weak acid. Silicious spicules
consist of colloid silica or opal, and hence can be distin-
guished from calcareous by having no influence upon polar-
ized light. Structurally the two kinds of spicules present
no important difference. The spicules of different sponges
differ greatly both in form and in size. They may be
conveniently divided into two groups, — minute or flesh
spicules, which usually serve as the support of a single cell
only (microaderes), and larger or skeletal spicules, which
usually contribute to the formation of a more or less con-
sistent skeleton (meffasclereii). The distinction is not one
that can be exactly defined, and must so far be regarded
as of a provisional nature. There is usually but little diffi-
culty in applying it in practice, except in some doubtful
cases where large spicules do not form a continuous skeleton,
or in others where flesh spicules appear to be passing into
those of larger size. It is indeed highly probable that all
large spicules have originated from flesh spicules (/.?).

(1) Monaxon Biradiate Type (rhabdus). — By far the
commonest form is the oxea, a needle-shaped form pointed
at both ends and produced by growth from a centre at the
same rate in opposite directions along the same axis. It
is therefore uniaxial and eguibiradiate (fig. 12«). (2) Mon-

Type (stylus). — By the suppression of one
of the rays of an oxea, an acuate spicule or stylus results
(fig. 12 I). (3) Triaxon Trira<l!nte Ti/jie. — Linear growth

\

V

FIG. 12. — Typical megascleres. «, rhabdus (moiiaxon diactine); b, stylus
(monaxon iiionactim-) ; i', trind (tri;u\on triactine); d, call limps (trtraxon
U'tractinO ; i', triaxnli hexartiiR-; /, <li-snia nf an aiioiiiockuline Lithistid
(polyaxon) ; ff, sterraster (iiolyaxon) ; /(, radial section through the outer
part of g, showing two actine.s soldered togrtlnT l>y inlrrvnilng silica, tin-
in • ends terminating in recurved spines ana the axis traversed by a eentral
fibre.

from a centre in three directions inclined at an angle of
120° to each other gives rise to the primitive form of tri-
radiate spicule so eminently characteristic of the calcareous
sponges, but by no means confined to them (fig. 12 <•). (4)
Tetraxon Quadriradiate Type (Culthi-ups). — Growth from a
centre in four directions inclined at about 110" to each
other produces the primitive quadriradiate form of the
Tetractintllida and of some calcareous sponges (fig. 1 2 d).
(5) Sexradiate Type. — Growth in six directions along three
rectangular axes produces the primitive sexradiate spicule
of the Hexactindlida sponges (fig. 12 e). (G) Multiradiate
Type. — Extensions radiating in many directions from a
centre produce a stellate form (fig. 12/). (7) Spherical
S'-l'Ti-f. — Concentric growth of silica about an organic
particle produces the sphere, which occurs as a reduction
of the rhabdus in some species of Peecillfisti-a, or as an
overgrown globule (flesh spicule) in Caminus.

Usually conical, the spicular rays often become cylindrical ; usu- Uniaxial
ally pointed (oxeatc) at the ends, they are also frequently rounded type,
off (strongylate), or thickened into 'knobs (tylotatc), or branched
(cladosc). Their growth is not always rigorously confined to a

A

Mega-
scleres.

FIG. 13.— Modifications of monaxon type, a, strongyle ; 6, tylote ; r, oxea ; rf,
tylotoxea ; e, tylostyle ; /, style ; g, spined tylostyle ; h, sagittal tried (a
triaxon form derived from the nioiiaxon) ; j, oxytylote ; k, anatriaene ; ?, pro-
trijcne; m, orthotrirene ; u, dichotriitnc ; o, centrotrkene ; ji, amphitneene
(this is trichocladow) ; ry, crcpiilwl strongyle (basis of Khabdoorepid I.iihislid
di-sma); /•, young form of Rhabdocrepid desnia, showing crepidial strongyle
coated with successive layers of silica ; », Rhahdncrepiil di'sma fully gro\vn.
The dotted liue through the upper figures marks the origin of the actiues.

straight line : frequently they are curved or even undulating. They
are also liable to become spined, either by mere superficial thicken-
ing or by a definite outgrowth involving the axial fibre (fig. 13 g, h).
The rhabdus if pointed at both cuds is known as an oxea (fig.
13 c) ; if rounded at both ends as a slrongylc (fig. 13 a) ; if knobbed

44

SPONGES

at both ends as a tylote (fig. 13 b) ; the tylote if pointed at one end
is a tylotoxea (fig. 13 d) ; the strongyle similarly becomes a stronyyl-
oxea. These last two forms are with difficulty distinguished from
the stylus, which is usually pointed at the end, and strongylate (fig.
13/) or tylotate (fig. 13 e) about the origin. A particular case of
the cladose rhabdus, but one of the most frequent occurrence, is
the trixnc ; in this form one ray of a rhabdus ends in three branches,
which diverge at equal angles from each other. The rhabdus then
becomes known as the shaft or rhabdomc, and the secondary rays
are the amis or cladi, collectively the head or cladomc of the spicule.
The arms make different angles with the shaft : when recurved a
grapnel or anatrissne is produced (fig. 13 k), when projecting forwards
a protrixne (fig. 13 I), and when extended at right angles an ortho-
tri&ne (fig. 13 in). The arms of a triiene may bifurcate (dichotriirnc)
once (fig. 13 n), twice, or oftener, or they may trifurcate. Again,
they may extend laterally into undulating lamellae, or unite to form
a disk, the tri;ene character of which is indicated by the included
axial fibre. The shaft may also become trifid at both ends, amjihi-
trixnc (fig. 13 p), and the resulting rays all bifurcate, or the elndome
may arise from the centre of the rhabdome, centrotriaine (fig. 13 o).
Amongst one group of Lithistid sponges (Mhabdocrcpida) the normal
growth of a strongyle is arrested at an early stage ; it then serves
as a nucleus upon which further silica is deposited, and in such a
manner as to produce a very irregularly branching sclere or dcsina
(fig. 13 s), within which the fundamental strongyle can be seen en-
closed. In such a desina no axial fibre besides that of the enclosed
strongyle is formed.

Triradi- The chief modification of the triradiate spicule is due to an elonga-

ate type, tion of one ray, distinguished as apical, the shorter paired rays
being termed basal, and the whole spicule a sagittal triradiate. The
angle included by the basal rays is usually over 120° (fig. 14 a).

Quadri- Some or all of the rays of the primitive calthrops (fig. 14 b) may

radiate

type.

FIG. 14.— Modifications of the triaxon and tetraxon types, n, sagittal triradiate
or tri"d ; fr, calthro]is ; c, candelabra (a polycladose microcaltlirops) ; d, a
spiued microealthrops ; c, Tctradadiue Lithistid desina.

subdivide into a number of terminal spines candelabra (fig. 14 c} ;

or some or all of them may bifurcate once or twice and finally

terminate by subdividing into numerous variously shaped processes ;

such a trtmcludinc desnia (fig. 14 c) characterizes one division of the

Lithistid sponges.

Sexradi- By the excess or defect of one or more rays a series of forms such
ate type, as are represented in fig. 15 arise. In the oxea, which results from

FIG. 15. — Modifications of the triaxon hexactine type, a, dagger ; 6, c, two
varieties of pinnulus ; d, ainphidisk ; c, pentactine ; f, staurus ; g, dermal
rhabdus. After Schulze.

the suppression of all raj's but two, the sexradiate character is some-
times preserved by the axial fibre, which gives off two or four pro-
cesses in the middle of the spicule where the defective arms would
arise. Let fig. 12 c represent a regular sexradiate spicule with its
four horizontal arms extended beneath the dermis of its sponge ;
the over-development of the proximal ray and a reduction of the
distal ray produce a form known as the dagger (fig. 15 a) ; the
suppression of the proximal ray and the development of spines pro-
jecting forwards on the distal ray produce the piimulns (fig. 15 b, c) ;
the suppression of both proximal and distal rays gives the stmini*
(fig. IS/), and the suppression of two of the remaining horizontal
rays a dermal rhabdus (fig. 15 g). The suppression of a distal ray,
excessive development of a proximal ray, and recurved growth of
the remaining rays produce an anchor. In Hijalonema (glass rope
sponge:) anchors over a foot long occur, but their arms or teeth are
not restricted to four, and the axial fibre gives off its processes
before reaching the head of the spicule. Such a grapnel helps to
support the sponge in the ooze of the sea-bed. Other character-

"yj
FIG. 10. — a, uncinaria ; b, clavula ; c, scopularia. After Schulze.

istic spicules belonging to sponges distinguished by sexradiate
spicules are the following ; — the uncinaria (fig. 16 a), a spiuose

oxea with the spines all pointing one way ; the clavula, a tylotate
form with a toothed margin to the head (fig. 16 b); the scopularia
(fig. 16 c), a besom-shaped spicule with tylotate rays, which vary
in number from two to eight ; the amphldisk (fig. 15 d), a shaft
terminating at each end in a number of recurved rays. "When the
sexradiate spicules of the Hcxactincllida unite together in a manner
to be described later, the rays may be bent in a variety of ways
out of the triaxial type, so that the sexradiate character alone
remains.

Mitltiradiatc Type. — The rays of an aster as of other spicules Multi-
may be spined or tylotate. In one remarkable form known as a radiate
sterraster (fig. 12 g, h), and characteristic of the family Gcodinidx, type,
the rays are almost infinite in number, and coalesced for the greater
part of their length ; the distal ends, however, remain separate,
and, becoming slightly tylotate, are produced into four or live re-
curved spines, which give attachment to connective tissue fibres
by which adjacent sterrasters are united together.

In one aberrant group of Lithistid sponges (Anomocladina) the
skeleton is formed of desmas, which are multiradiate, each present-
ing a massive centrum (with an included cavity) produced into a
variable number (4 to 8) of rays, which rays terminate in expanded
ends (fig. 12/).

It is doubtful whether a distinction between megascleres and Micro-
microscleres can be maintained in the calcareous sponges, unless scleres
the minute oxeas which occur in Eilhardia, schuhci, Pol. (16), are
to be referred to this group. They are widely distributed through-
out the silicious sponges, and by their different forms afford charac-
ters of the highest importance in classification.

One of the simplest forms is the sigmaspire (fig. 17 a, b) • it looks
like the letter C or S, according to the direction in which it is

FIG. 17— Microscleres. n, !>, sigmaspire viewed in different directions,— n, along

, , ,—

axis, and I, obliquely; c, toxaspire ; d, spiraster ; e, saniclaster; /, amplii-
aster ; g, sigina or cymba ; h, cynilia, with three pteraat each end,— the central
one a proral pteron aud the lateral, pleural ptera ; j, one end of another form
of cymba, showing seven ptera ; 1;, monopteral cymba,— proral ptera only,
developed at ends, tropidial ptera much enlarged ; /, oocymba, in which proral

tylote ; s, microxea (q, r, and s arc reduced asters) ; (, rosette,
viewed, its actual form being that of a single turn of a cylindrical
spiral. A turn and a part of a turn of a spiral of somewhat higher
pitch than that of a sigmaspire gives the toxaspire (fig. 17 c) ; a con-
tinued spiral growth through several revolutions gives the poly-
spii-c. The sigmaspire becoming spined produces the spiraster or
spinispinda (fig. 17 d) ; this, by losing its curvature, becomes the
sanidastcr (fig. 17 <•), and by simultaneous concentration of its spines
into a whorl at each end, the amphiastcr (fig. 17/). By reduction
of the spire the spiraster passes into the stellate or aster (fig. 17 n).
A thickening about the centre of the aster produces the sphcrastcr
(fig. 17m), allied to which is the sterraster. By a reduction in the
number of its rays the aster becomes a minute calthrops, from which,
by increased growth, the skeletal calthrops may very well be derived ;
by further reduction to two rays a little rhabdus or microrabd re-
sults, and of this numerous varieties exist, of which the oxeate
microrabd is the most interesting, since it only differs in size from
the commonest of all skeletal spicules, the oxeate or acerate rhab-
dus. The sigmaspire is formed as a superficial spiral thickening
in the wall of a spicule cell or scleroblast ; as superficial deposits
also the next group of spicnles, the so-called anchorates, arise.
Take a hen's egg as the model of a scleroblast, draw round it a
broad meridional band, interrupted only on one side, for 30° above
and below the equator ; this will represent a truly C-sliMpcd spicule,
which differs from a sigmaspire by the absence of spiral twist.
It may be termed a cimilifi (fig. 17 .7). The back of the "C" is the
keel or tropis; the points are the prows or prorae. Now broaden out
the prora on the eggshell into oval lobes (proral plercs) ; and from
each pole draw a lobe midway between the prora and the tropis
[pleural ji/n-f/i), and a common form of auchorate, the

SPONGES

45

results (tig. 17 /(). The pterocymlia is subject to considerable modi-
fications: tin; prows may I"' simil;ir i/inmo/i/vi/w/) ar dissimilar
(hetcroproral) ; the ptcivs may In1 lamellar or lingual ; additional
lainclhc (ti'iijiii/iiil jiti-i'e.t'i may be produced by a lateral nut growth
of the keel (tig. 17 £) ; and by growing towards tin1 equal or tin1
opposed proral ami pleura! ptcres may conjoin, producing a spicnlo
of two meridional bands (oocymfes ; iig. 170- A curious group of
flesh spieulrs are the //'/r/i/Y.x In this group silica, instead of being
deposited in concentric coatings around an axial lilire, forms within
the sclcroblast a sheaf of immeasurably line librilhc or trichites,
which maybe straight (fig. 17 »») or twisted. The triehite sheaf
may be regarded as a tibrjllated spicule. Triehite sheaves form in
some sponges, as Dragmastra (zj), a dense aecumulation within
the cortex. In Hexactinellid sponges the rays of the aster are
limited to six, arranged as in a primitive sexradiate spicule, but
divided at the ends into an indefinite number of slender filaments,
which may or may not be tylotate, rosettes (fig. 17 0.
Spongiu Spongin is a horny substance, most similar to silk in
Bcleres. chemical composition, from which it differs in being in-
soluble in an ammoniacal solution of copper sulphate
(cuproso-ammonium sulphate). In Darwinc/la awta, F.
Miiller, it occurs in forms somewhat resembling tri-,
quadri-, and sex-radiate spicules. But usually the spongin
skeleton takes the form of fibres, consisting of a central
core of soft granular substance around which the spongin
is disposed in concentric layers, forming a hollow cylinder
(fig. 23 I). The relative diameters of the soft core and
of the spongin cylinder differ greatly in different sponges.
The fibres branch so as to form antler-like twigs or bushy
tree-like growths, or anastomose to form a continuous net-
work, as in the bath sponge (Suspongia qfflcinalis). The
detailed characters of the network differ with the species,
and are useful in classification. In lanthdla certain cells
(sponginblasts) become included between the successive
layers of the spongin cylinder, and their deep violet colour,
contrasting with the amber tint of the spongin, renders
them very conspicuous.

Union of In some sponges the scleres are simply scattered through the
scleres luesoderm and do not give rise to a continuous skeleton, — Carl i,' in ,,i,
into a ChomlfiUfi, Th i-ouilus. In the Calcarca and many silicious sponges
skeleton, they are dispersed through the mesoderm, but so numerously that
by the overlapping of their rays a loosely felted skeleton is pro-
duced. In the calcareous sponges the spicules are frequently regu-
larly disposed ; and in the Sycons in particular a definite arrange-

FIG. IS. — Articulate ami inarticulate tubar skeletons of calcisponges. a, articu-
late ; !), inarticulate skeleton. After Haeckd.

incnt, on two plans, the articulate and iunrticidnlc, can be traced
in the skeleton of the radial tubes. On the latter plan the triradi-
ate or quadriradiate spicules, the apical rays of which are of con-
siderable length, are arranged in two sets, one having the basal
rays lying in the mesoderm of the paragastral wall and the other
with the corresponding rays in the dermal mesoderm. The apical
rays of each set lie in the mesoderm of the radial tubes parallel to
their length, but pointing in opposite directions (fig. IS I). In the
articulate division numerous spicules, small in comparison with
the size of the radial tubes, form a series of rows round the tubes,
their basal rays lying parallel to the paragastric surface and the
apical pointing towards the ends of the radial tubes (fig. 18 o).

In the Siliciajimiyia sheaves of long oxeate spicules radiate from
the base of the sponge if of a plate-like form, or from the centre if
globular, and extend to the surface. If triaeues are present their
arms usually extend within the mesoderm immediately below the

dermal surface dig. 10). Single spicules reach from centre to sur-
face only ill small sponges. As the sponge increases in sixe, the
spicules must either correspondingly lengthen, or fresh spicules
mii-t be added, if a
continuous skeleton is
to be formed. The
latter is the plan fol-
lowed in fact : the ad-
ditional spicules over-
lap the ends of those
first formed like the
fusiform cells in a
woody fibre. YVitli the
formation of a fibre,
often strengthened by
spongin or bound to-
gether with connective
tissue, there appears to
lie a tendency for the
constituent spicules to
diminish in size, and
the length of each in
the most markedly Fie. 10. — Morle of arrangement of spicules in a
fibrous sponges is ill- >•"""!! St.-llottid sponge, Dragmnstra nonaani,
significant when com- *'M' Altw Sollas-

pared with the length of the fibre. The spiculnr fibre thus
formed may lie simple or echinated by spicules either similar to
those which form its mass or different. More usually they are
different, and generally styles, often spiuose about their origin.
The spongiu which sometimes cements together the spicules of a
fibre may progressively increase in quantity and the spicules di-
minish in number, till a horny fibre containing one or more rows
of small oxeas results. In an echinated fibre the axial spicules
may disappear and the echinating spicules persist. Finally all
spicnles may be suppressed and the horny fibre of the Ceratose
sponges results. The horny fibres may next acquire the habit of
embedding foreign bodies in their substance, though foreign en-
closures are not confined to the ('ernlo.vt, but occur in some Silici-
sponyiai as well. The included foreign bodies may increase in
quantity out of all proportion to the horny fibres ; and finally the
skeleton may consist of them alone, all spongiu matter having
disappeared.

In the Lithistid sponges a skeleton is produced by the articula-
tion of desmas into a network. The rays of the desrnas (figs. 12/,
13 s, 14 c) terminate in apophyses, which apply themselves to some
part of adjacent desmas, either to the centrum, shaft, arms, or
similar apophyses, ami then, growing round them like a saddle on
a horse's back, clasp them firmly without anchylosis. Thus they
give rise to a rigid network, in conjunction with which fibres com-
posed of rhabdus spicules may exist. In the HcxactincUida both
spicular felts and fibres occur, and in one division (Diclyonina) a. rigid
network is produced, not, however, by a mere clasping of apophyses,
but by a. true fusion. The rays of adjacent spicules overlap and a
common investment of silica grows over them.

Histology.

The ectoderm usually consists of simple pavement Ecto-
epithelial cells (pinnococytes), the margins of which can derm,
be readily rendered visible by treatment with silver nitrate,
best by Banner's method.1 The nucleus and nucleolus
are usually visible in preparations made from spirit speci-
mens, the nucleus being often readily recognizable by its
characteristic bulging beyond the general surface. In some
sponges (ThecKjJtorct) the epithelium, may be replaced
locally by columnar epithelium, and the cells of both pave-
ment and columnar epithelium may bear flagella (AplysUla
violacect, Oscarella lobitlaris). The endoderm presents the Endo-
sarue characters as the ectoderm, except in the Ascons and derm,
the flagellated chambers of all other sponges, where it is
formed of collared flagellated cells or choanocytes, — cells
with a nearly spherical body in which a nucleus and nucleo-
lus can be distinguished and one or more contractile vacu-
oles. The endoderm extends distally in a cylindrical neck
or collum, which terminates in a long flagellum surrounded
by a delicate protoplasmic frill or collar (fig. 21r/). In
Tetractinellida, and probably in many other sponges — cer-
tainly in some — the collars of contiguous choanocytes
coalesce at their margins so as to produce a fenestrated
membrane, which forms a second inner lining to the flagel-

1 S. F. Manner, " On a Method for the Silver Staining of Marine
Objects," Mitth. Zooloy. Staliun ~i> Xaijtcl, 1SS4, p. 445.

46

SPONGES

lated chamber (fig. 20, ii.). The presence of this membrane
enables us readily to distinguish the excurrent from the

Fio. 20.— Choanocytes with coalesced collars, (i.) Longitudinal section through
two flagellated chambers of Anthastra <:n/n/nunis, Soil.; j, prosopyles; c,
aphodal canals leading from the flagellated chambers ; e, excnrrent canal ;
the tissue surrounding the chambers is sarcenchyme (X360). (ii.) Diagram
showing the fenestrated membrane (in) produced by coalesced collars of
Choanocytes. After Sollas, " Challenger" Report.

incurrent face of the chamber, since its convex surface is
always turned towards the prosopyle. In sponges with an

FIG. 21.— Histological elements, a, collcncytes, from Tlmicn mnricata; b,
chondrenchyrnr, from enrtrx <il' I'm-lir'nuu fi/uli'tuhn/M (lln1 nnsliadrd bodies
are niicrosch-ivs); r, pv.strni-liynir. from r>i>1i//tiiiitt<nt<i ji>lni*t,>ui (partly dia-
grair.inatiV) ; d. deswacyte, from /v<i[/»m.</m tun-muni i >', ii.vorytcs in con-
nexion with coilrnryl.-s, tViim Oinachyn Imrhiin; f, thesocyte, from Thrum
mitricatfi\ gt choanocyte, from Xii<-im<lr« m^/m/Mi* ; ft-fi, scleroblasts — /<, of

a young oxra, Ii i an iMnbiyo of ( 'ruini lln enmiwm ; i, ol'a fully grown oxea,

fi'oin an adult <'. rnniiuin ; j, urthotriM-m-, with associated scleroblast from
x/rll</i"; /,,Mta tetracladine desma, trom Tlirourll" *tr;,,i,<>/i ; /, of a sigma-
sjthv, tViuii rrnuirU'i cranium; m, of an orthodragma, 1'rom Disyrtnga '/is.
similis ; n, of a stcrrast.er, from Gcudia burretti. Figs, b and ;/ al'lrr Schulze,
the others after Sollas.

aphodal canal system the flagellated chambers usually pass
gradually into the aphodal canal, but the incurrent canal

enters abruptly. This abrupt termination of the incurrent
canal appears to mark the termination of the ectoderm
and the commencement of the endoderm. The flagellated
chambers differ greatly in size in different sponges, and
evidently manifest a tendency to become smaller as the
canal system increases in complexity; thus Sycon are always
larger than Ehagon chambers, and eurypylous than aphodal
Rhagon chambers. In most sponges except the Ascons the
mesoderm is largely developed, and in many it undergoes Meso-
a highly complex histological differentiation. In its com- derm.
monest and simplest form it consists of a clear, colourless,
gelatinous matrix in which irregularly branching stellate
cells or connective tissue corpuscles are embedded ; these
may be termed collenc-ytes (fig. 21 a) and the tissue collen-
chyme. In the higher sponges (Geodia, Stelletta) it consists
of small polygonal granular cells either closely contiguous
or separated by a very small quantity of structureless jelly,
and in this form may be termed sarcenchyme (fig. 20).
Collenchyme does not originate through the transformation
of sarcenchyme, as one might expect, for it precedes the
latter in development. Schulze (20), who has compared
collenchyme to the gelatinous tissue which forms the chief
part of the umbrella of "jelly-fish," describes it as becoming
granular immediately in the neighbourhood of the flagel-
lated chambers in the bath sponge, the granules becoming
more numerous in sponges in which the canal system
acquires a higher differentiation, till at length the collen-
cytes are concealed by them. According to this view,
sarcenchyme would appear to originate from a densely
granular collenchyme. Amoeboid wandering cells or arc/ue-
ocytes (fig. 22) are scattered through the matrix of the
collenchyme. They evidently serve very different purposes :
some appear to act as carriers of nourishment or as
scavengers of useless or irritant foreign matter ; others
may possibly contribute to the formation of higher tissues,
some certainly becoming converted into sexual products.
Their parentage and early history are unknown.

A tissue (cystencTiyme) which in some respects resembles certain
forms of vegetable parenchyme occurs in some sponges, particularly
Geodinidas and other Tetrad iiu'lliilo. It consists of closely ad-
jacent large oval cells, with thin well -defined walls and fluid
contents. Somewhere about the middle of the cell is the nucleus
with its nucleolus, supported by protoplasm, which extends from
it in fine threads to the inner side of the wall, where it spreads out
in a thin investing film (fig. 21 c). Cystenchyme very commonly
forms a layer just below the skin of some Gcodimdie, particularly of
Pacln/matisma, and, as on teasing the cortex of this sponge a large
number of refringent fluid globules immiscible with water are set
free, it is just possible that it is sometimes a fatty tissue, and if so
the contained oil must be soluble in alcohol, for alcoholic prepara-
tions show no trace of it. A tissue resembling cartilage, chondren-
ckymc, occurs in Corticidie (fig. 21 b).

Connective-tissue cells or desmacytes are present in most Desma
sponges ; they are usually long fusiform bodies, consisting cytes.
of a clear, colourless, often minutely fibrillated sheath,
surrounding a highly refringent axial fibre, which stains
deeply with reagents (fig. 21 d). In other cases the des-
macyte is simply a fusiform granular cell, with a nucleus
in the interior and a fibrillated appearance towards the
ends. The desmacytes are gathered together, their ends
overlapping, into fibrous strands or felted sheets, which in
the ectosome of some sponges may acquire a considerable
thickness, often constituting the greater part of the cortex.
The spicules of the sponge often furnish them with a sur-
face of attachment, especially in the Geoiliifid.v, where each
sterraster of the cortex is united to its neighbours by des-
macytes, in the manner shown in fig. 10.

Contractile fibre cells or myocytes occur in all the higher Myo-
npungL's. They appear to be of more than one kind. Host cytes.
usually they are fine granular fusiform cells with long
filiform terminations, and with an enclosed nucleus and
nucleolus (fig. 21 e). In the majority of sponges both ex-
current and incurrent canals art- constricted at intervals

SPONGES

47

cytes.

by transverse diaphragms or vela, which contain myocytes
concentrically and sometimes radiatcly arranged. The
excessive development of myocytcs in such a velum gives
rise to muscular sphincters such as those which close the
chones of many corticate sponges, such as PachymatismoL.
In this spoimv, which occurs on the llritish shores, the
function of the oscular sphincters can be readily demon-
strated, since irritation of the margin of the oscule is
invariably followed after a short interval by a slow closure
of the sphincter.

(Supposed sense-cells or xstkucytes (fig. 22) were first
observed by Stewart and have since been described by
Von Lendenfeld (/.?). According to the latter, they are
spindle-shaped cells, O'Ol mm. long by 0'002 thick; the
distal end projects beyond the ectodermal epithelium in a
fine hair or palpocil ; the body is granular and contains a
large oval nucleus ; and the inner end is produced into
fine threads, which extend into the collenchyme and are
supposed — though this is not proved — to become con-
tinuous with large multiradiate collencytes, which Von
Lendenfeld regards as multipolar ganglion cells (fig. 22).

Proto-
plasmic
con-
tiuuity.

FIG. 22. — Transverse section through the edge of a pore in Demh-iUa cavernosa,
Lfd. ; cells in the middle to the right, archsocytes ; fusiform cells on
each side of them, myocytes ; g, above and below these, with processes
terminating against the epithelium, gland cells ; fusiform cells terminating
against the epithelium at s, :t- sthacytes ; at their inner ends these are con-
tinuous with ganglion cells. After Von Lendenfeld ( x SOO).

More recently he has described an arrangement of these
cells curiously suggestive of a sense-organ. Numerous
sesthacytes are collected over a small area, and at their
inner ends pass into a granular mass of cells with well-
marked nuclei, but with boundaries not so evident ; these
he regards as ganglion cells. From the sides of the gan-
glion other slender fusiform cells, which Von Lendenfeld
regards as nerves, pass into the mesoderm, running tan-
gentially beneath the skin. The inner end of the ganglion
is in communication with a membrane formed of fusiform
cells which Von Lendenfeld regards as muscular. If his
observations and inferences are confirmed, it is obvious
that we have here a complete apparatus for the conversion
of external impressions into muscular movements.

In most sponges a direct connexion can be traced by
means of their branching processes between the collen-
cytes of the mesoderm and the cells of the ectodermal
and endodermal epithelium and the choanocytes of the
flagellated chambers. As the collencytes are also united
amongst themselves, they place the various histological
constituents of the sponge in true protoplasmic continuity.
Hence we may with considerable probability regard the
collencytes as furnishing a means for the transmission of
impulses : in other words, we may attribute to them a
rudimentary nervous function. In this case the modifica-
tion of some of the collencytes in communication with the
ectoderm might readily follow and special eesthacytes arise.
Fusiform collencytes perpendicular to the ectoderm, and
with one end touching it, are common in a variety of
sponges ; but it is difficult to trace the inner end into
connexion with the stellate collencytes, so that precisely in

those cases in which it -would be most interesting to find
such a connexion absolute proof of it is wanting.

The colour of sponges usually depends on tin; presence Pigment
of cells containini; granules of pigment ; though dispersed cclls-
generally through the mesoderm, these cclls are most richly
developed in the ectosome. Pigment granules also occur
in the choanocytes of some sponges, — Oscaret/a lolmhiris
and Aplysina aerophoba, for instance. In the latter the
pigment undergoes a remarkable change of colour when
the sponge is exposed to the air, and finally fades away.
In many cases sponges borrow their colours from parasitic
alga: (Otcillatoria and Nostoc) with which they are infested
The colours of sponge-pigments are very various. They
have been examined by Krukenberg and Merejknovsky.
Zoonerythin, a red pigment of the lipochrome series, is one
of the most widely diffused ; it is regarded as having a
respiratory function. Reserve cells or thesoeytes (fig. 21 /)
have been described in several sponges as well as amylin
and oil-bearing cells.

Each spicule of a sponge originates in a single cell Sclero-
(fig. 21 h-n), within which it probably remains enclosed blasts,
until it has completed its full growth ; the cell then prob-
ably atrophies. During its growth the spicule slowly
passes from the interior to the exterior of the sponge, and
is finally (in at least some sponges, Geodia, Stelletta) cast
out as an effete product. The sponge is thus constantly
producing and disengaging spicules; and in this way we
may account for the extraordinary profusion of these struc-
tures in some modern marine deposits and in the ancient
stratified rocks. Within the latter these deciduous spicules
have furnished silica for the formation of flints, which have
been produced by a silicious replacement of carbonate of
lime (26).

The horny fibres of the Ceratosn are produced as a
secretion of cells known as sponginblasts, which surround
as a continuous mantle the sides of each growing fibre, and
cover in a thick cap each growing point (fig. 23). The

Fin. 23.— Section through the horny fibre and associated tissues of a horny
sponge (Dcndrilla). A, longitudinal section ; s, layers of spongin, surrounded
at the sides by the lateral mantle of sponginhlasts, and at the ends by the
terminal cap. A desmachymatous sheath, a, surrounds the whole (x!50).
B, transverse section; in the centre is the soft core, surrounded by wavy

spongin layers, the outermost being surrounded by s] ginblssts, and these

by a fibrous sheath : i, part of an incurrent canal lined by llagt-lluti-il epi-
thelium ; e, part of an excurreut canal ; /, part of a flagellated chamber ( X 150).
After Von Lendenfeld.

lateral sponginblasts are elongated radially to the fibre ;
the terminal cells are polygonal and depressed. The latter
give rise to the soft granular core and the former to the
spongin- walls of the fibre. Cells similar to the lateral
sponginblasts, and regarded as homologous with them,
occur in a single layer just below the outer epithelium of
some horny sponges (Apfyxi/lit and DenJriUa), and under
certain circumstances secrete a large quantity of slimy
mucus (//).

48

SPONGES

Classification.

Class!- The phylum Parazoa or Spongix consists of two main
fication. branches, as follows : —

Branch A. — MEGAMASTIC- Branch B. — MICROMASTIC-

TORA. TOR A.

Class CALCAKEA, Grant. Class I. — MYXOSPOXGM:,

Order 1. — llonwcaila, Pol. Haeckel.

Order 2. — Heterocosla, Pol. Order 1. — Halisardna.

Order 2. — Chondrosiiui.

ClaSS II. — SlLICISPONGLE.

Sub-class i. — HEXACTINELLIDA,

0. Schmidt.

Order 1. — Li/wciim, Zittel.
Order 2. — Dic/i/uiiinn, Zittel.

Suh- class ii. — DEMOSI'ONOI^E,

Sollas.
Tribe a. — Monaxonida.

Order 1. — Mi>iinj;n>ii.

Order 2. — Ceratosa, Grant.

Tribe 1. — Tetractinellida,

Marshall.

Order 1. — Choristida, Sollas.
Order Z.—Lithistida, O.S.

Position By the possession of both sexual elements and a complex histo-

in animal logical structure, and in the character of their embryological devel-

kingdom. opment, the sponges are clearly separated from the Protozoa ; on

the other hand, the choanoflagellate character of the endoderm,

which it retains in the flagellated chambers throughout the group

without a single exception, as clearly marks them off from the

JUctazoa. They may therefore be regarded as a separate phylum

derived from the choanoflagellate Infusoria, but pursuing for a

certain distance a course of development parallel with that of the

Mctazoa.

Different views have been propounded by other authors. Savile
Kent regards the sponges as Protozoa (10) ; Balfour suggested that
they branched off from the Metazoau phylum at a point below the
Ccelcntera, and considered them as intermediate between Protozoa
and Mctazoa ; Schulze regards them as derived from a simple
ancestral form of Ccelcntera (23) ; Marshall advocates the view that
they are degraded forms derived from Ccelenterates which were
already in possession of tentacles and mesenteric pouches (14).
Subdivi- As a phylum the Spongiie are certainly divisible into two branches,
siou in one including the Calcarea and the other the remaining sponges,
groups, which Yosmaer has termed Non- Calearca, and others Plethospongise.
Since, however, the choanocytes of the Calearca are usually, if not
universally, larger than those of other sponges, we may make use
of this difference in our nomenclature, and distinguish one branch
as the Megamastictora (fiaffriKTup, "scourger") and the other as
the Micromastictora.

Branch A. —MEGA MA STIC'TORA .

Sponges in which the ehoanocytes are of comparatively large
size, 0'005 to 0'009 mm. in diameter (Haeckel, 6).

Class CALCAREA.

Calcarea. Megamastictora in which the skeleton is composed of calcareous
spicules.

Order 1. HOMOCCELA. — Calcarea in which the endoderm consists
•wholly of choanocytes. Examples : Lcucosolcnia, Bwk. ; Homo-
derma, Lfd.

Order 2. HETEKOCCSLA. — Calcarea in which the endoderm is dif-
ferentiated into pinnacocytes, which line the paragastric cavity
and excurrent canals, and choanocytes, which are restricted to special
recesses (radial tubes or flagellated chambers). Examples : Si/con,
0. S. ; Grantia, Fl. ; Leuconia, Bwk.

Branch B.— MICROMASTICTORA.

(Non-Calcarca, Vosmaer ; riclltosjioniiiie, Sollas.) Sponges in
which the choauocytcs are comparatively small, O'OOS mm. in
diameter.

Class I. MYXOSPONGI5!.

Myxo- Micromastictora in which a skeleton or scleres are absent.

. Order 1. HAUSAIICIXA. — Myxosponqies in which the canal system
is simple, with simple or branched Sycon or enrypylous Rhagon
chambers. An ectosome sometimes and a cortex always absent.
Examples: Halisarca, Duj.; Oscarella, Vosin. ; Bajnlus, Lfd.

Order 2. CHOXDROSIXA. — .l/iMvw//ii//;//,r in which the canal
system is complicated, with diplodal Rhagon chambers and a
well-developed cortex. Example : Chondrosia, O.S.

The ffalifairciii't are evidently survivals from an ancient and
primitive type. The simplicity of the canal system is opposed to
the view that they are degraded forms ; we may therefore regard
the absence of scleres as a persistent primary and not a secondary
acquired character. They are as interesting, therefore, from out-

point of view (absence of scleres) as the Ascons are from another
(uudifferentiated endoderm). With the Chondrosina the case is
different ; they differ only from Chondrilla and its allies by the
absence of asters ; these differ only from the Tcthyidsz by the
absence of strougyloxeas ; and we may very reasonably assume that
in these three groups we have a series due to loss of characters, the
Chondrill& being reduced Tcthyidss and the Chondrosina reduced
Clumdrillss. Still, as Huxley has well remarked, " classification
should express not assumptions but facts " ; and therefore till we
are in possession of more direct evidence it will be well to exclude
the Chondrosina from the Silicisponcjix.

Class II. SmCISPONSI-E.

Micromaslidora possessing a skeleton or scleres which are not
calcareous.

Sub-class i. HEXACTINELLIDA.

Silicispongise characterized by sexradiate silicious spicules. Hexacti-
Canal system usually simple, with Sycou chambers. Sponge nellida.
differentiated into ecto-, choano-, and endo-some.

Order 1. LYSSACIXA. — HcxactincUula in which the skeleton is
formed of separate spicules, or, if united, then by a subsequent not a
contemporaneous deposit of silica. Examples : Eiiplcdella, Owen ;
Asconcma, S. Kent ; Hyaloncma, Gray ; Rossclla, Crtr.

Order 2. DlCTTONlNA. — Hexactincllida in which sexradiate
spicules are cemented together by a silicious deposit into a con-
tinuous network pan passu with their formation. Examples :
Ftimct, Bwk. ; Eurete, Marshall ; Aphrocallistcs, Gray ; Myliusia,
Gray ; Daclylocabix^ Stutchbury.

The Hexactinellida are a very sharply defined group, impressed
with marked archaic features. No other Silicisponyiie possess, so
far as is known, so simple a syconate canal system. The oldest
known fossil sponge is a member of the Lyssacina (7 and 24), viz.,
Protospongia, Salter, from the Menevian beds, Lower Cambrian,
St David's Head, Wales. The group is almost world-wide in distri-
bution, chiefly affecting deep water, from 100 to 300 fathoms, but
often extending into abyssal depths ; occasionally, however, though
rarely, it frequents shallow water (Cystispongia supcrstcs dredged off
Yucatan in 18 fathoms).

Sub-class ii. DEMOSPONGLSI.

SilicisponrjiK in which sexradiate spicules are absent. Demo-

Tribe a. MOXAXOXIDA. spongia

Dcmosjionyiie in which the skeleton consists either of silicious
spicules which are not quadriradiate, or of horny scleres or in-
cluded foreign bodies, or of one or more of these constituents ill
conjunction.

Order 1. MOXAXONA. — The skeleton is characterized by either
uniaxial or polyaxial spicules. Examples : Amorpkina, 0. S.
("crumb of bread" sponge); Sponrjilla, Link, ("freshwater"
sponge) ; Chalina, Bwk. ; Trthija, Link.

Order 2. CEKATOSA. — The skeleton consists of horny scleres
which never include "proper" spicules, or of introduced foreign
bodies, or of both these in conjunction. Examples : Darwinella,
F. Muller ; Euspongia, Broun (the " bath " sponge).

Tribe 5. TETRACTINELLIDA.

Demospongiss possessing quadriradiate or triame spicules or
Lithistid scleres (desmas).

Order 1. CHORISTIDA. — Tetrad incllida with quadriradiate or
triajne spicules, which are never articulated together into a rigid
network. Examples: Tctilla, 0. S. ; Thenea, Gray ; Gcodia, Lmk. ;
Dcrcitus, Gray.

Order 2. LITHISTIDA. — Tetrad incllida with branching scleres
(desmas), which may or may not be modified tetrad spicules, arti-
culated together to form a rigid skeleton. Tria>ne spicules may or
may not be present in addition. Examples: Thecmdla, Gray ; Coral-
Uslfn, O.S. ; Azorica, Crtr.; Vctulina, O.S.

This large sub-class embraces the great majority of existing sponges.
Its external boundaries are fairly well defined, its internal divisions
iiiurh less so, as its various orders and families pass into each other
at many points of contact. Although there does not appear to be
much resemblance between a Lithistid sponge, such as Thconclla,
a Monaxonid such as Amurjihina, and an ordinary "bath" sponge
(EuxiHiinjiii), yet between these extremes a long series of inter-
mediate forms exists, so nicely graduated as to render their dis-
ruption into groups by no means an easy task. If the delimitation
of orders is difficult, that of genera is often impossible, so that
they are reduced to assemblages depending on the tact or taste of
the author. Thus Polcjaeff states that with a single exception
" none of the genera of Ceratosa are separable by absolute charac-
ters." The chief spicules of Monaxona are uniaxial, often accom-
panied by characteristic microseleres. Although distinguished as a
group by the absence of quadriradiate or truene spicules, two ex-
ceptions are known in which these occur ( Triccntrion, Ehlers, and
Acarnus, Gray) ; these, however, present unusual characters which
suggest an independent origin. The canal system of Monaxona has
not yet been fully investigated ; it appears usually to follow the

SPONGES

49

(Mirypylous Rhagon typo, Imt the aphodal is not unknown. The
(•.•t-iiliixn. contain all sponges with a horny skeleton, except those
iu which the horny fibres are cored or spined with silicious spicules
secreted liy the sponge (" proper " spic-uk-s) ; these are arbitrarily
assigned to the .Mniiiij-iniii. There is convenience in this proceed-
ing, lor horny matter is widely disseminated throughout the Demo-
li/mnitia!, occurriug even in the Ltlhistidn, and it frequently serves
to cement the oxeate spicules of the Moniivuiin into a fibre, without
at the same time forming a preponderant part of the skeleton. It
would be wellnigh impossible to say where the line should be drawn

one
coin-

between a fibre composed of Bpicules cemented by spougin and
consisting of spongin with embedded spicules, while there is c
parativcly no difficulty in distinguishing between fibres containin^,
spicules and fibres devoid of them. That the distinction, however,
is entirely artificial is shown by the fact that, after spicules have
disappeared from the horny fibre, they may still persist in the
mesoderm ; thus Von Lendenfeld announces the discovery of micro-
scleres (eymba) in an Aplysillid sponge and of strongyles in a
Cocospongia, both horny sponges. (A form intermediate between
this Aplysillid and the Desmaddonidm would appear to be To;co-
chalinn, Ridley.) The Ccratosa frequently enclose sand, Fora-
mini/era, deciduous spicules of other sponges and of compound
Ascidiaus, and other foreign bodies within the horny fibres of their
skeleton ; they also sometimes attach this material, probably by a
secretion of spongin, to their outer surface, and thus invest them-
selves iu a thick protective crust; In some Ccratosa no other
skeleton than that provided by foreign enclosures is present. The
canal system is syconate or eurypylous in the simpler forms and
diplodal in the higher. The Mamaxonida make their earliest ap-
pearance in the Silurian rocks (Clfmacospongia, Hinde), and are
now found iu all seas at all depths. The only sponges inhabiting
fresh water belong to this group. The TctreuAincli ida adhere to
the Monaxonida at more than one point, and one of these groups
has probably been a fruitful parent to the other, but which is
offspring and which parent is still a subject for discussion. The
Choristida iu its simplest forms presents a eurypylous Rhagon
system, in the higher an aphodal system. It is in this group that
the most highly complex cortex is met with; in the Geodinidsc,
for instance, it consists usually of at least five distinct layers.
Thus, proceeding outwards, next to the choanosome is a layer of
thickly felted desmachyme, passing into collenchyme on its inner
face ; then follows a thick stratum of sterrasters united together
by desmacytes ; this is succeeded by a layer of cystenchyme or
other tissue of variable thickness ; external to this is a single layer
of small granular cells and associated dermal asters; and finally,
the surface is invested by a layer of pavement epithelium. The
Lithistida, like the Ccratosa, are possibly of polyphylitic origin ;
iu one group (Tdracladina) the articulated scleres are evidently
modified calthrops spicules (see fig. 14 c), and associated with them
are free triaenes, which support the dermis and resemble precisely
the tritenes of the Choristida. In another group (FJialidocrqrida)
the scleres are moulded on a Monaxonid base (see fig. 13 q-s) ; but,
associated with them, triivnes sometimes occur similar to those of
the Tctracladlna. Both these groups are in all probability derived
from the ClwristiJa, and a distinct passage can be traced from the
Tetracladose to the Rhabdocrepid group. In the Rhaldocrepida
we find forms without trirenes ; these may possibly be degenerate
forms. The third group of Lithistids is derived from the Rhabdo-
crepida, the Anomocladine desma being derivable from the Rhabdo-
crepid by a shortening of the main axis into a centrum. The
thick centrum, from which the arms, variable in number, ori-
ginate, is hollowed out by a cavity, which appears during life to
have been occupied by a large nucleus, like that of a scleroblast,
and it is quite conceivable that the scleroblast, which in the
Tetracladine Lithistids lies in an angle between the arms, may
have become enclosed in an overgrowth of silica, from which addi-
tional arms were produced. The constancy with which spicules
in other sponges maintain their independence is very striking.
When once a persistent character like this is disturbed, excessive
variability may be predicted, as in the Anomocladine scleres.

Classifi- The classification of the sponges into families is shown in the

cation in following scheme.

Class CALCAREA.
Order 1. HOMOCCELA, Pol.

Family 1. AsroxiME, Hk. — Humoccela which are simple or com-
posite, but never develop radial tubes. Examples : Ascctla, Hk.
(fig. 1) ; Lcucosolenia, Bwk.

Family 2. HOMODERMID.E, Lfd. — Homoccela with radial tubes.
Example : Hbmoderma, Lfd. (figs. 3, 4).

Order 2. HETEROCCELA, Pol.

Tribe a. tSTCONARlA.1

The flagellated chambers are either radial tubes or cylindrical
sacs.

Family 1. SYCONID^.— The radial tubes open directly into the
paragastiic cavity.

Sub-family a. Sycpnina.— The radial tubes are free for their whole)
length, or at least, distally. Examples : Synttu, Hk.; Sycon, O.S.

Sub-family 1>. Uteina, Lfd. — The radial tubes are simple :md
entirely united. The ec.tosome is differentiated from thrrlia-inosnme.
and sometimes develops into a cortex. Examples: Grantixnn, U'd. ;
Utc, O.S. (fig. 5); Sycortusa, Hk.; Ainplwriscus, Pol.
^ Sub-family c. Grantina, Lfd.— The radial tubes are branched.
The incurrent canal system is consequently complicated. An ecto-
some is present. Examples : Grantia, Fl. ; Hdcropajma, Pol. (fig.
4); Anamaxilla, Pol.

Family 2. SYLLEIBID.E, Lfd.— The choanosome is folded. The
flagellated chambers (which are partly rhagose in Vosmaeria)
communicate with the paragastric cavity by excurrent canals
Examples : Pofy'na, Lfd. (fig. 6) ; Vosmaeria, Lfd.

Family 3. TEICHONELLIOE, Carter.— Composite Sylleibidee with
the oseules and pores occurring on different parts of the surface.
Example : Tcickonclla, Crtr.

Tribe i. tLEucoNARiA.

The canal system belongs to the eurypylous Rhagon type.

Family 1. LEUCONIM, Hk.— The outer surface is not differentiated
into osculiferous and poriferous areas. Examples : Leucetta, Hk. ;
Leucaltis, Ilk.; Lcucortis, Hk.

Family 2. EILHARDID.E, Pol. —Composite Lcuconaria, with the
outer surface differentiated into special osculiferous and poriferous
areas. Example : Eilhardia, Pol.

The arrangement adopted above is founded on Von Lendenfeld's
revision (ft) of the classification propounded by Polejaeff(/6), who
in a masterly survey has thrown an unexpected light on the struc-
ture and inter-relationships of a group which Haeckel has rendered
famous. It should not be overlooked that Vosmaer (j/) had pre-
viously explained the structure of the Leucones. However errone-
ous in detail, Haeckel's views are confirmed in their broad outlines,
and it was with true insight that he pronounced the Calcarca to
offer one of the most luminous expositions of the evolutional theory.
In this single group the development in general of the canal system
of the sponges is revealed from its starting-point in the simple
Ascon to its almost completed stage in the Leucon, with a complete-
ness that leaves little further to be hoped for, unless it be the re-
quisite physiological explanation.

Class MYXOSPONGI^E.
Order 1. HALISARCINA.

Family 1. HALISAKCIDJB, Lfd. — The flagellated chambers are
syconate. Examples: HaUsarca, Dnj. (with branched chambers) ;
Bajalus, Lfd. (with simple chambers).

Family 2. OSCAKELLIDJB, Lfd. — The flagellated chambers are
eurypylous and rhagose. Example : Oscardla, Vosm.

Order 2. CHONDROSINA.

Family 1. CHONDROSIID.E.— With the characters of the order.
Example : Chondrosia, O.S.

Class SILICISPONGIJ3.

Sub-class I. HEXACTINELLIDA.

Order 1. +LYSSACINA.

Family 1. EupLECTELLiDiE. — The spicules of the dermal mem-
brane are "daggers" (fig. 15 a). Examples : Euplectclla, Owen;
Holascus, E. Sch. ; Habrodictyum, W.T.

Family 2. ASCONEMATID^:.— The dermal spicules are " pinnuli "
(fig. 15 b,c). Examples: Asamema, S. Kent; Sympagclla, O.S.;
Cauloph&us, Schulze.

Family 3. HYALONEMATIDJE.— The dermal spicules are pinnuli
and amphidisks (fig. 15 d). Example : Hyaloncma, Gray.

Family 4. tRossELiM). — The dermal spicules are gomphi, stauri
(fig. 15/), and oxeas. Examples: Rossella, Crtr.; CTdteromorpha,
Gray ; Aulochona, E. Sch.

Family 5. *RECEPTACULID^E, Hinde. —The distal ray of the
dermal spicules is expanded horizontally into a polygonal plate.
Example : *Receptaculitea, Defr.

Order 2. tDlCTTONINA.
Sub-order 1. UXCIXITAPJA.
Uncinate spicules are present.

Tribe a. CLAVULAEIA.
Clavulue (fig. 16 c) are present.

Family 1. FABKBmoL — Characters those of the tribe. Example :
Furi-ffi, Bwk.

Tribe b. SCOPULAKIA.

The dermal spicules are scopulariae (fig. 16 b).
Family 1. tEr/BETID^. — Branched anastomosing tubes, or goblet-
shaped, with lateral outlets. Examples : Eunte, Marshall ; Peri-
phniyella, Marshall ; Lcfroyclla, Schulze.

Family 2. tMELLiTTONlD^:. — Tubular or goblet-shaped, with
honeycomb-like walls. Example : Aphrocallistcs, Gray.

1 An * indicates that the group is only known in the fossil state, a t that it
is both recent and fossil.

G

50

SPONGES

Family 3. tCHOXELASMATiDa;. — Flat or beaker-shaped ; straight
funnel-shaped canals perforating the wall perpendicularly and
opening laterally on each side. Example : Chonelasma, Schulze.

Family 4. tVoLVULiNiM;.— Tubular, goblet-shaped, or massive ;
crooked canals more or less irregular in their course. Examples :
Volvulina, Schulze ; Ficldingia, S. Kent.

Family 5. SCLEROTHAMNID*. — Arborescent body ; perforated at
the ends and sides by round narrow radiating canals. Example :
Sclerothamn us, Marshall.

Sub-order 2. INERMIA.

Dictyonina without uncinati, clavulte, or scopulariae.
Family 1. tMYLlusiD.E.— Depressed cup-shaped; a complex
folding of the wall produces lateral excurrent tubes. Example :
Mylvusia, Gray.

_Family 2. tDACTYLOCALYCiDj:.— Goblet -shaped or pateriform,
with a thick wall consisting of numerous parallel anastomosing
tubes, of uniform breadth, which terminate at the same level
within and without. Examples: Dactylocah/x,Gra.v; Sderonlccima
O.S. ; Margaritdla, O.S.

Family 3. tEuiiYjpLEOMATiD^;. — Goblet-shaped or resembling
ear-shaped saucers ; the wall deeply folded longitudinally so as to
produce a number of dichotomously branched canals or covered-in
grooves. Example : Euryplegma, Schulze.

Family 4. tAuLOCYSTiM;.— Of massive rounded form, with an
axial cavity ; wall consisting of a system of obscurely radiating
anastomosing tubes and intervening inter-canals ; both inter-canals
and the external terminations of the tubes are covered by a thin
membrane, which is perforated by slit -like openings over the
lumina of the tubes, and thus assumes a sieve -like character.
Examples : Aulocystis, Schulze ; Cysti<spongia, Roemer.

This arrangement of the Hexactincllida is taken from the latest
work on the subject, Schulze's Preliminary Report on the "Challen-
ger " Hexactincllida. The reference of fossil forms to the families
here instituted is rendered difficult by the disappearance of the
requisite "guiding" spicules in the process of mineralization. A
revision of the fossil families to bring them into harmony with the
recent has certainly been rendered necessary, but this is too large
a task to undertake in this place.

Sub-class II. DEMOSPONGI.E.
Tribe a. MONAXONIDA,
Order 1. MONAXONA.

Family 1. TETHYIDJE.— Skeleton consisting of radiately arranged
strongyloxeas (except in the genus Chondrilla, which is without
megascleres) and large spherasters. The ectosome is a thick fibrous
cortex. Example: Tethya, Link. ; Chondrilla, O.S.

Family 2. POLYMASTIM.— Skeleton consisting of styles radiately
arranged and cortical tylostyles. Tbe oscules in many cases open
at the ends of long papilla;. Examples : Polyinastia, Bwk. ; Theca-
phora, O.S. ; Trichostcmma, Sars.

Family 3. SUBERITID-E.— Skeleton consisting of strongylate or
tylotate styles, arranged to form a felt. The flesh spicules when
present are usually mierorabds or spirasters. Examples : Subcrites.
Nardo ; C'liona, Grant ; Poterimi, Schlegel.

Family 4. DESMACIDOJUD.E.— The flesh spicules are cymbas.
Examples : Espmlla, Vosm. ; Desmacidon, Bwk. ; Cladorhiza, Sars

Family 5. tHALlCHONDRiOE.— The flesh spicules when present
are never cymbas. Examples : Ealkhondria, Fl. ; Rcnicra, O.S. •
Chalina, Bwk. ; * Pharetrospuugia, Soil.

Family 6. ECTTONIDA— The skeleton consists of fibres echinated
by projecting spicules. Examples : I'locamia, O.S. ; Ectyon. Gray •
Clathria, O.S.

Family 7. tSpONOiLLiD^.— EaliAondridss which are reproduced
both sexually and by statoblasts. Habitat freshwater. Examples :
SpongUla, Lmk. ; Ephydatia, Link. ; Parmula, Crtr. ; Patamolems.
Marshall.1

The foregoing classification is purely provisional ; the group re-
quires a complete revision.

Order 2. CERATOSA.

Family 1. DARWINELIID.E.— Canal system of the eurypylous
Rhagon type. Flagellated chambers, pouch-shaped, large ; the sur-
rounding collenchyme not granular. Horny fibres with a thick
core. Examples : DurwineUa, Fritz Miiller ; Aplysilla, F.E.S •
lanthcUa, Gray.

Family 2. SPQNGEUM.— Canal system as in the Darwincllidx,
but the flagellated chambers more or less spherical. Horny fibres
with a thin core, and usually containing foreign enclosures.
Examples : Vclinca, Vosm. ; Spongdia, Nardo ; Psammochui,,,
Marshall ; Psammopemma, Marshall.

Family 3. SI-ONOIDJE.— Canal system aphodal. Chambers small
and spherical ; surrounding collenchyme granular. Fibres with a
thin core. Examples : Hfuspongia, Broun ; Casciitodcrma, Crtr. •
Phyllosponyia, Ehlers.

1 Freshwater sponges without statoblasts are excluded from this family and
lelt rur distribution amongst allied marine genera.

Family 4. APLTSINID^.— Canal system diplodal ; collenchyme
surrounding the flagellated chambers densely granular. Fibres
with a thick core. Examples : Luffaria, Duch. and Mich. ; Vcron-
rjia, Bwk. ; Aplysina, Nardo.

The species of sponge in common use are three, — Euspongia
officinalis (Linn.), the fine Turkey or Levant sponge; E. zimocca
(O.S.}, the hard Zimocca sponge ; and Hippospongia cquina (O.S.),
the horse sponge or common bath sponge. The genus Euspongia
is distinguished by the regular development of the skeletal network
throughout the body, its narrow meshes, scarcely or not at all
visible to the naked eye, and the regular radiate arrangement of
its chief fibres. Hippospongia is distinguished by the thinness of
its fibres and the labyriuthic character of the choanosome beneath
the skin. As a consequence its chief fibres have no regular radiate
arrangement. The species of Euspongia are distinguished as fol-
lows. In E. officinalis the chief fibres are of different thicknesses,
irregularly swollen at intervals, without exception cored by sand
grains ; in E. simocea they are thinner, more regular, and almost
free from sand. In E. oj/kiualis, again, the uniting fibres are soft,
thin, and elastic ; whilst in E. simocea they are denser and thicker,
to which difference the latter sponge owes its characteristic hard-
ness. Finally, the skeleton of E. officinalis is of a lighter colour than
that of E. simocea. The common bath sponge (Hippospongia
equina) has almost always a thick cake-like form ; but its specific
characters are not yet further defined.

Tribe b. TETRACTINELLIDA.

Order 1. CHORISTIDA.
Sub-order 1. SIGMATOPHORA.
The microsclere is a sigmaspire.

Family 1. TETILLIDJL — The characteristic megasclere is a pro-
triune. Canal system in the lower forms eurypylous, in the higher
aphodal. The ectosome in the simpler forms is a dermal membrane,
in the higher a highly differentiated cortex. Examples : Tetilla
O.S.; Craniclla, O.S. (fig. 21 h, I).

Family 2. SAMIDJE. — The characteristic megasclere is an amphi-
tri^ne. Example : Samus, Gray.

Sub-order 2. ASTEROP30RA.
The microsclere is an aster.

Group 1. SPIKASTKOSA.— A spiraster is usually present.
Family 1. THENEIM;, Carter.— The flesh spicule is a spiraster.
Canal system eurypylous. Ectosome not differentiated to form a
cortex. Examples : Thcnea, Gray (fig. 21 a, /) ; Pcecillastra (Nor-
mania), Bwk.

Family 2. tPACHASTRELLiM. —Canal system eurypylous in the
lower, aphodal in the higher forms. Examples : Plakortis, F.E.S. •
Dereitus, Gray.

Group 2. EUASTROSA. — Spirasters are absent.
Family 1. tSTELLETTiDJi.— Canal system aphodal, but approach-
ing the eurypylous in the lower forms. The cortex chiefly consists
of collenchyme in the lower forms ; in the higher it is highly differ-
entiated. Example: Stcllctta, O.S. (fig. 11); Ancorina, O.S. ;
Alyriastra, Soil.

Family 2. TETHYID.E.— Although this family has been placed
in the Manaxonida, this seems to be its more natural position.

Group 3. STERRASTROSA. — A sterraster is present, usually in
addition to a simple aster.

Family 1. tGEODiNiDJE. — The megascleres are partly trirenes.
Canal system always aphodal. Cortex highly differentiated. Ex-
amples : Gcodia, Lmk. (fig. 21 ?i.); Padiymatisma, Bwk. (fig. 21 c);
Cydonium, Miiller (fig. 10); Erylus, Gray.

Family 2. PLACOSPONGID.E. — The megasclere is a tylostyle.
Tritenes are absent. Example : Placospongia, Gray.

Sub-order 3. MICROSCLEROPIIORA.
Microscleres only are present.

Family 1. PLAKINID.E, Schulze.— Canal system very simple,
belonging to eurypylous Rhagon type. Characteristic spicules
candelabra. Examples: Plaldna, F.E.S. (fig. 26).

Family 2. CORTICIM. — Canal system aphodal or diplodal.
Mesoderm a collenchyme crowded with oval granular cells; the
spicules either candelabra, amphitrirraes, or triienes irregularly
rMc""~<"1 ;" ;* Example: Cor/ieium, O.S. (figs. 9, 21 b).

dispersed in it.

Family 3. THROMBID.E.— Canal system diplodal. Spicules tricho-
tria-nes. Example : Thrombus, Soil.

The Pachastrettidie or the Corticidae. are probably the families
from which the Tetracladine Lithistids have been derived. In the
Tr/illiiliE the characteristic microsclere may occasionally fail, but
there is never any difficulty in identifying the sponge in this case,
as the trifenes are of a very characteristic form : the arms of the
protriaenes are slender, simple, and directed very much forwards,
making a very large angle with the shaft. Microscleres, having the
form of little globules, are sometimes present with the sigmaspires.

Order 2. LITHISTIDA, O.S.
Sub-order 1. TETRACLADIXA, Zittel.
The desmas are modified calthrops spicules.

SPONGES

51

granules ; at first they exhibit lively amoeboid movements,
but later pass into a resting stage. The cavity of the
mesodenn within which they are situated becomes lined

Fia. 24. — Spermatozoa. a-h, Development of spermatozoa in Sycarulra raph-
tmw, highly magnified ; *, mature spermatozoa. After Polejaeff(x792). j,

A sperm ball in lin-itrdla (u(n(iaris(x500) ; fc, an isolated mature spermatozoon.
After Schulze(xSOO).

by a layer of epithelium, which may not appear, how :ver,
till a late stage of segmentation. In Euspongia offia alis
the ova occur congregated in groups within the mesod rm,
thus presenting an early form of ovary. The spermatozoa,
which also develop from wandering amoeboid cells, are
minute bodies with an oval or pear-shaped head an-l a
long vibratile tail (fig. 24 I). Each amoeboid cell produces
a large number of spermatozoa, which occur in spkeiical
clusters or sperm-balls. The heads of the spermatozoa,
as in the Metazoa, are produced from the nucleus of the
mother-cell, the tails from the surrounding protopi sm.
The development in detail is upon two plans. In Grantia,

Family 1. TETRACLAmn/E. — With the characters of the sub-
order. Examples: Tlicondla, Gray (fig. 21 k); .Dworfcnuw, Bocagc;
"SipJumia, rarkinsnn.

Sub-order 2. RHABDOCRKPIDA.

The desmas are of various forms, produced by the growth of silica
over a uniaxial spicule.

Family 1. MEGAMORINIP.!;. — The desmas are comparatively
large. Trurnes, usually dichotri;i/ncs, help to support the ccto-
some. Microscleres usually spirastcrs. Examples : Corn/I 7.s-A\«,
O.S. ; *Hyalotragos, Zittel; Lyidium, O.S. ; *Dorijdcrmin, Zittel.

Family 2. MICKOMORINID/E. — The desmas are comparatively
small. Trionies and microscleres are both absent. Examples :
Azorica, Crtr. ; *TcrrutUiM, Zittel.

Sub-order 3. ANOMOCLADINA.

Desmas with a massive nucleated centrum, from which a variable
number of arms (zS) extend radiately (see fig. 12/). Examples:
Vctulina, 0. S. ; *Astylosponrjia, Roemer.

Reproduction and Embryology.
Fresh individuals arise by asexual gemmation, both

external and internal, by fission, and by true sexual repro-
duction.

Asexual Fission is probably one of the processes by which com-
multipli- pound sponges are produced from simple individuals,
cation. Artificial fission has been practised with success in the

cultivation of commercial sponges for the market. Ex-
ternal gemmation has been observed in Tlienna, Tethya,

Polymastia, and Oscarella. A mass of indifferent sponge-
cells accumulates at some point beneath the skin, bulges

out, drops off, and gives rise to a new individual. Internal

gemmation, which results in the formation of a statoblast,

is only known to occur in the freshwater Spongillidx,

The statoblasts consist of a mass of yolk -bearing

mesoderm cells, invested by a capsule, which in

Ephydatia fluviatilis is composed of an inner

cuticle of spongin separated from a similar outer

layer by an intermediate zone of amphidisks and

interspersed protoplasmic cells. On one side of

the capsule is a hilum which leads into the interior.
Their development has recently been studied by Giitte,

with results that confirm the conclusions of Carter (j)

and Lieberkiihn (fj). The process commences with an

accumulation of amoeboid cells within the mesoderm to

form a globular cluster ; yolk grannies develop within

them, especially in those that lie nearer the centre. The

external cells give rise to the investing capsule ; some

resemble sponginblasts and secrete the inner and outer

horny cuticle ; others give rise to the amphidisks and

interspersed cells of the middle layer. Under favourable

conditions the interior cells creep out through the pore

of the capsule, and form a spreading heap, which by

subsequent differentiation gives rise to a youug Spongilla.

Since the freshwater sponges can only be regarded as

modified descendants of ancient marine species (prob-
ably of the family Ifttlichondridse), we may consider the

internal gemmules, like the similar statoblasts of the

freshwater Poli/:oa, as special adaptations to a changed

mode of life. They appear primarily to serve a protective

purpose, ensuring the persistence of the race, since they

only appear in extreme climates on the approach of

drought, and in cold ones on the approach of winter.

As a secondary function they serve for the dispersal of

the species ; some are light enough to float down a

stream, but not too far, so that there is no danger of

their being carried to sea ; others, which are character-
ized by large air-chambers, are possibly distributed by

the wind.

Sexual Both sexual elements may be formed in the
repro- same individual, e.ff., Oscarella lobularis, Grantia
on. raphanus, and many others ; but even in herm-
aphrodites one or other element usually occurs to

excess in different individuals, SO that Some are F[G 25_ Development of a calcareous sponge (Symndra raphanw). a, ovum ; 6, c, ovum seg-
predominantly male and Others predominantly mented,-&, as seen from above, c lateral view; rf blastosphere; f, ampl.il.lastula; /corn-
•if:7 J meneement nf the invagination of the flagellated cells of the amphiblastula ; </, gastrula

female, rolejaeff found Only one such male form attached by its oval face; h,j, young sponge (Ascon stagi-),— ft, lateral vi™-, j, as seen from

to 100 female forms in Grantia raphanus. In above- After Schuize.

Other sponges— Beniera fertilis, Euspon,,i<i otfr.-itialis—ihe raphanus (/j) the nucleus of the mother-cell divides into two

,? . mi i (fi" 24 b) • one of the resulting daughter nuclei undergoes no

sexes are distinct. The ova develop from archaeocytes or ^^ ^ but with a gmall ^^ of ri heral protopiasm

wandering amoeboid cells, which increase in size and ac- forms a "cover-cell" to the other or primitive sperm nucleus and its
quire a store of reserve nourishment in the form of yolk associated protoplasm. The sperm nucleus repeatedly divides, with-

52

SPONGES

out involving the surrounding protoplasm (fig. 24 c-f). The result-
ing nuclei at length cease to exhibit a nucleolus, and become directly
transformed into the heads of spermatozoa; the tails are appropriated
by each head from the common protoplasmic residue. The mother-
cell in this case undergoes no increase in volume as development
proceeds, and it is not enclosed within an " endothelial " layer. In
the second and apparently more usual case (20) no "cover-
cell " is formed, but the mother-cell divides and subdivides,
protoplasm as well as nuclei, till a vast number of minute
cells results ; the nucleus of each becomes the head of a
spermatozoon and the protoplasm its tail. In this case the
sperm-ball does increase in bulk : it grows as it develops,
and the cavity containing it becomes lined by epithelium,
or so-called "endothelium" (fig. 24/). No doubt (75) the
development of the epithelium stands in direct physiological
connexion with the growth of the sperm-ball.

Ccelenterate history as exemplified in the last two events will furnish
an explanation of the remarkable divergencies which distinguish
the two phyla. The history of the second or planula type has been
thoroughly worked out by Schulze (20) in a little incrusting Tetrac-
tinellid sponge (Plakina monolopha, Sclmlze). The ovum by regu-
lar segmentation produces a blastosphere, the blastomeres of which

^ *?«t «*

9&W*

-«,««isJtf^g»^4S
8Sgpftfis£s?asJ9:

Embryo- Obscure as are the details of this subject, suffi-
loS7- cient is known to enable us to make out two chief

types of development. One, common amongst the

calcareous sponges, and possibly occurring in a single

genus (Gwnmina) of the Micromastictora, is char-
acterized by what is known as the " amphiblastula "

stage; the other, widely spread amongst the

Micromastictora (Seniera, Desmacidon, Eiispongia,

Spongelia, Aplysilla, Oscarella), is characterized by

a " planula " stage.

The first has been most thoroughly investigated in
Grantia raphanus by Schulze (20). The ovum by repeated
segmentation gives rise to a hollow vesicle, the wall of
which is formed by a single layer of cells — blastosphere
(fig. 25 d). Eight cells at one pole of the blastosphere
now become differentiated from the rest ; they remain ^j
rounded in form, comparatively large, and become filled
with granules (stored nutriment), while the others, rapidly
multiplying by division, become small, clear, columnar,
and flagellated. By further change the embryo becomes
egg-shaped; the granular cells, now increased in number
to thirty-two, form the broader end, and the numerous ^
small flagellated cells the smaller end. Of the granular
cells sixteen are arranged in au equatorial girdle adjoin-
ing the flagellate cells. A blastosphere thus differen-
tiated into two halves composed of different cells is "
known as an amphiblastula. The amphiblastula (fig. 25 c]
now perforates the maternal tissue, and is borne along an i— =
excurrent canal to the oscule, where it is discharged to FIG. 26.— Development of a Dcmespmv}ia.(riaki,ia monoloplia). n, planula (the central part

the exterior and swims about in a whirling lively dance.
It then assumes a more spherical form, a change premoni-
tory of the next most remarkable phase of its career. In
this the flagellated layer becomes flattened, depressed, and
finally invaginated within the hemisphere of granular cells,
to the inner face of which it applies itself, thus entirely obliterating
the cleavage cavity, but by the same process originating another
(the invagination cavity) at its expense (fig. 25/). The two-layered
sac thus produced is a paragastrula ; its outer layer, known as the
epiblast, gives rise to the ectoderm, the inner layer or hypoblast to
the endoderm. The paragastrula next becomes somewhat beehive-
shaped, and the mouth of the paragastric cavity is diminished in
size by an ingrowth of the granular cells around its margin. The
larva now settles mouth downwards on some fixed object, and ex-
changes a free for a fixed and stationary existence (fig. 25 3). The
granular cells completely obliterate the original mouth, and grow
along their outer edge over the surface of attachment in irregular
pseudopodial processes, which secure the young sponge firmly to
its seat (fig. 25 h). The granular cells now become almost trans-
parent, owing to the exhaustion of the yolk granules, and allow
the hypoblast within to be readily seen ; a layer of jelly-like
material, the rudimentary mesoderm, is also to be discerned between
the two layers. The spicules then become visible ; slender oxeas
appear first, and afterwards tri- and quadra-radiate spicules. The
larva now elongates into a somewhat cylindrical form ; the distal
end flattens ; and an oscule opens in its midst. Pores open in the
walls ; the endodermal cells, which had temporarily lost their
flagella, reacquire them, at the same time extending the character-
istic collar. In this stage (fig. 25 h, j) the young sponge corresponds
to a true Ascon, no trace of radial tubes being visible ; but as they
characterize the parent sponge they must arise Inter, and thus we
have clear evidence through ontogeny of the development of a
Sycon sponge from an Ascon.

The three most striking features in the history of this larva are,
first, the amphiblastula stage ; next the invagination of the llagi'l-
late cells within the granular, instead of invagination in the reverse
order ; and third the attachment of the larva by the oral instead of
the aboral surface. Should Srhulze be corrsct in deriving the
sponges from the Cuilcntcra, it is probable that the reversal of the

should be shaded), b, Section through side of planula ; ec, flagellated cells ; ft, their
flagella ; col, crenoblast. c, Attached gastrula (the paragaster is formed by fission), d,
Section across the foregoing, e, Young sponge (Rhagon). /, Part of a section through
fully grown sponge ; the attached basal layer is the hypomere ; the spongomere is fnM.il
so as to produce incuirent and excurrent canals ; the canal system is eurypylous ; ov, ova
(a segmented ovum lies between two of them) ; bl, blastospheres. After Schulze.

increase in number by further subdivision till they become con-
verted into hyaline cylindrical flagellated cells (fig. 26/). Thus a
blastosphere is produced consisting wholly of similar flagellated cells.
It becomes egg-shaped, and, hitherto colourless, assumes a rose-red
tint, which is deepest over the smaller end. The larva (now a
planula, fig. 26 a, by the filling in of the central cavity) escapes from
the parent and swims aliout broad end foremost. In this stage
thin sections show that the cleavage cavity is obliterated, its place
being occupied by a mass of granular gelatinous material contain-
ing nuclei (fig. 26 b). In from one to three days after hatching the
larva becomes attached. It then spreads out into a convex mass,
and a cavity is produced within it by the splitting of the central
jelly (fig. 26 c, d ; compare Eucope and others amongst the Cceleu-
terates). This cavity becomes lined by short cylindrical cells (endo-
derm), while the flagellated cells of the exterior lose their flagella
and become converted into piiiiiaeorvtes (ectoderm). The gelatin-
ous material left between the two layers now formed acquires the
characters of true collenchyme and thus becomes the mesoderm.
The endoderm then sends off into the mesoderm, as buds, rounded
chambers, which communicate with the paragastric cavity by a
wide mouth and with the exterior by small pores (fig. 26 c). An
oscule is formed Inter, and the sponge enters upon the lihngon phase.
Subsequent foldings of the sponge-wall give rise to a very simple
canal system (fig. 26/). In addition to these two well-ascertained
modes of development others have been described which at present
appear aberrant. In Oscarella lobularis, O.S. (sf), a curious series
of early developmental changes results in the formation of an
irregular paragastrula, the walls of which become folded (while still
within the parent sponge) in a complex fashion, so as to produce a
form in which the ineurrent and excurrent canals appear to be
already sketched out before the flagellated chambers are diflcrenti-
atecl off. In Spougilla Gotte describes the ectoderm as becoming
entirely lost on the attachment of the larva, so that the future
sponge proceeds from the endoderm alone. As Hfmujilla, however,

SPONGES

53

Homo-

jilasy.

Physio-
logy-

is a freshwater form, anomalies in its development (which remind
us of those in the development of the freshwater Ilydm) might
almost lie expected.

Probably in no other single group is the doctrine of
homoplasy enunciated by Lankcstcr more tellingly illus-
trated than in the sponges. The independent develop-
ment of similar types of canal system in different groups,
sometimes within the limits of a single family, is a remark-
able fact. In the following table the sign x shows inde-
pendent evolution of similar types of canal system in
different groups: —

Ascon.

Sycon.

Rb&gon.

Eury-
pylous.

Aphodal.

Diplodal

X

X
X

X
X
X
X

X
X
X

X
X

X

X
X

X
X

Order Jfulisarcina

X

Sub-order Microsclcro-
phora

Family I'dillida

In the gross anatomy of the canal system similar homo-
plasy obtains ; thus, to cite one case amongst many, a
peculiar type of canal system characteristic of Siphnnia
(Lithistid) occurs also in£mploca(S.6xa,c.tm&]lid),Sckmidtia
(Monaxonid), and other apparently unrelated genera. The
development of a cortex has likewise taken place inde-
pendently, but on parallel lines, in the Syconidte, L< u-
conidfc, Monaxona, TetiUidx, and Slellettidx. Calcareous
and silicious spicules have evidently an independent his-
tory, and yet all the chief forms of the former are repeated
in the latter. Quite as remarkable is the similarity of
the independently evolved horny spicules of Din-ii'un-ll<t
aurea to the quadri- and sex-radiate silicious spicules. We
have now sufficient knowledge of the morphology and evolu-
tion of the sponge to furnish the physicist with data for an
explanation of the skeleton, at least in its main outlines.
The obvious conclusion from this is that variation does not
depend upon accident, but on the operation of physical
laws as mechanical in their action here as in the mineral
world. Another important consequence follows: if homo-
plasy— i.e., the independent evolution of similar structures
— is of such certain and quite common occurrence in the
case of the sponges, it is also to be looked for in other
groups, and polyphylitic origin, so far from being improb-
able, is as likely an occurrence as monophylitic origin.

Physiology and ^Etiology.

Under the head of "physiology" we have almost a
blank. At present we do not even know what cells of the
sponge are primarily concerned in the ingestion of food.
If a living sponge, such as Sponr/illa, be fed with carmine
for a few minutes, then immersed in dilute osmic acid, and
examined in thin sections, its flagellated chambers are
found to be all marked out as red circular patches, and a
closer investigation shows that the choanocytes, and they
alone, have ingested the carmine. In this way we con-
firm the earlier observations of Carter made by teasing
carmine -fed sponges. This might be thought to decide
the question ; but, though, it effectually disposes of Pole-
jaeff's argument that the choanoeytes do not ingest nutri-
ment because mechanical disadvantages (conceived a jn-iwi)
make it impossible, it has not proved a final solution. Yon
Lendenfekl, by feeding sponges such as Aji///«i//ii. with
carmine for a longer interval — a quarter of an hour — finds
that amceboid cells crowd about the sides and particularly
the floor of the subdermal cavities, and are soon loaded
with carmine granules ; after a time they wander away to
the flagellated chambers and there cast out into the ex-
curreut canals the carmine they have absorbed, apparently

in an altered state. On the other hand, the choanocytes,
though they at first absorb the carmine, soon thrust it out,
:i|'|i.-irently in an unaltered state. Hence Von Lendenfeld
concludes that it is the epithelium of the subdermal cavities
which is charged with the function of ingestion, and that
the ama'boid cells subsequently digest and distribute it,
and finally cast out the worthless residues. There may be
much truth in this view, but it requires to be supported
by further evidence. (1) Sufficient proof is not adduced
to show that the carmine granules expelled from the amce-
boid cells are really more decomposed than those rejected
by the choanocytes. (2) There is at present no proof that
carmine is a food, or that if it is sponges will readily feed
upon it. In either case one would expect the amoeboid
cells to play tire part which they perform in other organisms
and to remove as soon as possible useless or irritant matter
from the surface which it encumbers ; at the same time
the choanocytes, not having found the food to their liking,
would naturally eject it. (3) If the choanocytes do not
ingest food, how does the Ascon feed, since in this sponge
all the pinnacocytes are external ? It is, however, a very
noticeable fact that, as the organization of a sponge
increases in complexity, the choanocytal layers become
reduced in volume relative to the whole bulk of the
individual ; and it is quite possible that as histological
differentiation proceeds it may be accompanied by physio-
logical differentiation which relieves the choanocytes to
some extent of the ingestive part of their labours.

The origin of the sponges is to be sought for among m\o-
the choanoflagellate Infusoria ; and Savile Kent has de- logJ'-
scribed a colonial form of this group which is suggestively
similar to a sponge. Its differences, however, are as
marked as its resemblances, and have been sufficiently
pointed out by Schulze (23). Kent has called this form
Protospongia, a name already made use of, and fortunately,
as the organism is not in any sense a true sponge ; the
present writer proposes, therefore, to call it Sarillia, in
honour of its discoverer. It consists of choanoflagellate
Infusoria (see PROTOZOA, vol. xix. p. 858, fig. XXL, 15),
half projecting from and half embedded in a structureless
jelly or blastema, within which other cells of an amceboid
character and reproductive function are immersed. Pro-
fessor Haddon arrives at the generalization that conjuga-
tion amongst the Protozoa always takes place between
individuals of the same order : flagellate cells conjugate
with flagellate, amceboid with amoeboid, but never with
flagellate ; while in true sexual reproduction the conjuga-
tion occurs between two individual cells in different stages
of their life cycle : a flagellate cell conjugates with a resting
amceboid cell. Now Sanllia would appear to be extremely
near such a true sexual process, since the simultaneous
coexistence of cells in two different stages of life and
within easy reach of each other — a necessary preliminary,
one would think, to the union — has already been brought
about. That coalescence between two different histological
elements should result in products similarly histologically
differentiated (compare amphiblastula stage of Calcarea)
has in it a certain fitness, which, however, has still to be
explained. The mode by which an organism like Rarilli<t
might become transformed into an Ascon cannot be sug-
gestively outlined with any satisfactory results till our
knowledge of the embryology of sponges is more advanced.
The minute characters of the flagellate cells of the amphi-
blastula and other sponge larvEe are still a subject for
research. They often possess a neck or collurn ; but the
existence of a frill or collar is disputed. Kent asserts
that it is present in several embryos which he figures ;
and Barrois makes the same assertion in respect to the
larva of Of«r<ll<i, and illustrates his description with a
figure. On the other hand, Schulze and Marshall both

54

SPONGES

deny its existence, and the former attributes Kent's
observations to error. One constant character they do
possess : they are provided with rlagella at some stage of
their existence, but never with cilia. Ciliated cells, in-
deed, are unknown amongst the sponges, and, when pinna-
cpcytes exceptionally acquire vibratile filaments, as in
Oscarella and other sponges, these are invariably flagella,
never cilia. An Ascon stage having been reached at some
point in the history of the sponges, the Sycon tubes and
Khagon chambers would arise from it by the active pro-
liferation of choanocytes about regularly distributed centres,
possibly as a result of generous feeding. Vosmaer recog-
nized as the physiological cause of Sycon an extension of
the choanocytal layer. Polejaeff, relying on Von Lenden-
fekl's experiments, which seem to prove that it is the
pinnacocytes and not the choanocytes which are concerned
in the ingestion of nutriment, argues that, as in Sycon
the pinnacocytal layer is increased relatively to the choano-
4 cytal, we have in this a' true explanation of the transition.
The existence of Homoderma, Lfcl., however, shows that
in the\ first stage there was not a replacement of choano-
cytes by pinnacocytes, but that this was a secondary
change, following the development of radial tubes, and
therefore cannot be relied upon to explain them. The
radial tubes having been formed by a proliferation of
choanocytal cells, the reduction of those. lining the para-
gastric cavity to pinnacocytes would follow in consequence
qf the poisonous character of the water delivered from the
radial tubes to the central cavity, since this water not
only parts with its dissolved oxygen to the choanocytes
it first encounters, but receives from them in exchange
urea, carbonic acid, and fascal residues. The development
of subdermal cavities is explicable on Von Lendenfeld's
hypothesis.

Distribution.

Distribu- Our knowledge of this subject is at present but frag-
tiou m rnentary ; we await fuller information in the remaining
reports on the sponges obtained by the " Challenger." The
sponges are widely distributed through existing seas, and
freshwater forms are found in the rivers and lakes of all
continents except Australia, and in numerous islands, in-
cluding New Zealand. Many genera and several species
are cosmopolitan, and so are most orders.

As instances of the same species occurring in widely remote
localities we take the following from Polejaeff : — Si/con arclicum is
found at the Bermudas and in the Philippine Islands, as also are
Leuconia multiformis a-nd Leucilla uter ; Sycon raphaniis occurs at
Tristan da Cunha and. the Philippines ; Hctcropcgma nodus-gordii
and Leuconia dura at the Bermudas and Torres Straits. We do not
know, however, whether these species are isolated in their distribu-
tion or connected by intermediate localities. Of the Calcarea about
eighty-one species have been obtained from the Atlantic, twenty-
two from the Pacific, and twenty-two from the Indian Ocean ; but
these numbers no doubt depend largely on the extent to which the
several oceans have been investigated, for the largest number of
species has been found in the ocean nearest home. Schulze states
that the ffexattinellida brought home by the "Challenger" were
obtained at seventeen Atlantic stations, twenty-seven Pacific, and
nineteen in the South Seas.' In the last the number of species
was greatest, in the Atlantic least. They flourish best on a
bottom of diatomaceous mud. The Calcarea and Ccmtosa are
most abundant in shallow water and down to 40 fathoms, but
they descend to from 400 to 450 fathoms. The ff.'.i'.i<iini'!!ida are
most numerous over continental depths, i.e., 100 to 200 fathoms;
but they extend downwards to over 2500 fathoms and upwards
into shallow water (10 to 20 fathoms). The Li'lltintii/n are not such
deep-water forms as the HcxactincUida, being most numerous from
10 to 150 fathoms. Only one or two species have been dredged
from depths greater than 400 fathoms, and none from 1000 fathoms.
The Clwristida range from shallow water to abyssal depths. A
characteristic deep-sea Choristid genus is Tlicnm, Gray (= U'yrill?
Thompsonia, Wright ; Dorvillia, Kent). This is most frequently
dredged from depths of from 1000 to 2000 fathoms ; but it extends
to 2700 fathoms on the one hand and to 100 on the other,
in time. Until about 1876 one of the chief obstacles to the inter-

space ;

^relation of fossil sponges arose from a singular mineral
replacement which most of them have undergone, leading
to the substitution of calcite for the silica of which their
skeletons were originally composed. This change was de-
monstrated by Zittel (jj) and Sollas (24), and, though it
was at first pronounced impossible, owing to objections
founded on the chemical nature of silica, it has since be-
come generally recognized. These observers also showed
that the fossil sponges do not belong to extinct types, but
are assignable to existing orders. Zittel in addition sub-
jected large collections to a careful analysis and marshalled
them into order with remarkable success. Since then
several palaeontologists have worked at the subject, — Pocta,
Dunikowski, and Hinde (/), who has published a Cata-
logue— which is much more than a catalogue — of the
sponges preserved in the British Museum. The result of
their labours is in general terms as follows. Fossil sponges
are chiefly such as from the coarseness or consistency of
their skeletons would be capable of preservation in a miner-
alized state. Thus the majority are Hexactinellida, chiefly
Dk-tyonina • Tetractinellida, chiefly Lithistida ; and Cal-
carea, chiefly Leuconaria. Monaxonid sponges rarely occur;
the most ancient is Climacospongia, Hinde, found in Sil-
urian rocks. A very common Halichondroid sponge of this
group (Pharetrospongia strakani, Soil.) occurs in the Cam-
bridge greensand; it owes its preservation to the collection
of its small oxeate spicules into dense fibres. The Clwristida,
though not so common as the Lithistids, are commoner
than the Monaxonids, particularly in Mesozoic strata. •

The distribution of fossil sponges in the stratified systems may
be summarized as follows. CALCAREA. — Homoccela, none. Hetcro-
ccela, a Syconid, in the Jurassic system. Numerous Leuconaria
from the Devonian upwards. MYXOSPONGM!. — None; not fitted
for preservation. HEXACTINELLIDA. — Lyssacina, from the Lower
Cambrian upwards. Dictyonina, commencing in the Silurian ; most
numerous in the Mesozoic group ; still existing. MOXAXONIDA. —
Moiiaxona, from the Silurian upwards. Ccratostt, none ; few are
fitted for preservation. TETRACTINELLIDA. — Choristida, from the
Carboniferous upwards ; most numerous in the Cretaceous system.
Lithistida, from the Silurian upwards ; most numerous in the
Mesozoic group. In ancient times the Hexactinellids and Lithistids
seem not to have been so comparatively uncommon in shallow
water as they are at the present day. Thus, in the Lower Jurassic
strata of the south-west of England we find Dictyoniue Hexactinel-
lids, Lithistids, and Leuconarian Calcarea associated together in a
shelly breccia and in company with littoral shells, such as Patella
and Trochus. Several Palfeozoic Hexactinellids actually occur in a
fine-grained sandstone. Of the Chalk, which is the great mine of
fossil sponges, we must speak with caution, owing to the insufficient
evidence as to the depth at which it was deposited.

As shown by Protospongia, the phylum of the sponges was in
existence in very early Cambrian times, and probably much earlier.
Before the end of the Silurian period its main branches had spread
themselves out, and, developing fresh shoots since then, they have
extended to the present day. Of the offshoots none of higher value
than families are known to have become extinct, and of these
decayed branches there are very few. The existence in modern
seas of the Asconidse, which must surely have branched off very
near the base of the stem, is another curious instance of the per-
sistence of simple types, which would thus appear not to be so vastly
worse off in the struggle for existence than their more highly
organized descendants.

Biblloaraphti.— A fairly complete list, "f works on sponges published before
1SS2 will be found in Vosinaer's article "Fcirifcni"," in Bronn's Klassen und
Onlnnngcn, vol. ii. D'Arcy Thompson's Catalogue of Papers on Piotozoa and
Ccdenterata, a still more complete list, extends to 1884.

The following is a list of works, including those referred to in the preceding
pages :— (/) C. Barrois, Embryologtt tt. qn,l,i,,rs Sponges d. I. Miim-h?, I':iris, is7ii.
(?) Bowei-tank, .-I Monograph nf n,ilisli Spongiartre, vols. i.-iv., 1SU4-S2 (vol.
iv. is posthumous, edited by Dr Norman), (j) Carter, a series of papers in the
A a a. ii ail .W,i:i. KnI. His/., from 1847 to the present time (18ST). (./) J. Clark, On
tlif Bponeiffi ciliatn> a* Infusoria flngcllata, ISii.j. (5) Grant, Eilin. Pliil. Jmn-n.,
1S2.1. (6) Haeckel, jlfontwmplu il. Kiillcsi-lni'iimme, 1S71. (-) Ilindt, .4 I'ulii-

logutaftheSpongeainthe British Museum, 1883. (flld., "On Hie Li-iTjjui-nliHil.-it."
in Quart. Journ. Gcol. Snr., xl. 705, 1884. (0) Keller, "Stndien ii. Organisation
u. Entwickelung d. Chalinecn," in Ztschr. f. wiss. Zoo}., xxxiii., 1S79. (10) Kent,
"Notes on the Embryology of the Sponges," in Ann. inul Mart. Nat. Hat.,
1S7S, ii. 139. (//) Von Lendenfeld, "On Aptmimtda," in Ztschr. f. wiss. Zool.,
xxxviii. (/?)Id., "A Monograph of Australian Sponges," in Proc. Linn. Snc.,N.S.
JlWra, vols. ix., x. (other papers by Von Lendenfeld will lie found under this

and Spermatogenesis in Sycandra raphanus," in Site. -Her. Acad. wiss. Zool.,

SPONGES

55

rf. Uniffniti'il f7m.-. (16) Id., "Ctinjlftiyrr" fif]inrt mi tlir Calcnrea, 188.1.
(/?) hi., Ditto on the Ctfratasa, lss-i. (/^) KMl«-y, «^ r//. /NM/. r(i//n7M>» M/ //*,

".-Ifrrt," 1SS4. (/a) Schmi.lt, N,i-in;|.'s M/ »,,' .I,/',-,, i/,',. ,s.,i, 1M1-J, with Suplilr-

iiinit, i in iM'i-i.aml supplnii.'iii •_• in ix>;r, ; Spongu of On < oast "/ ALgltrs, l*iw;
Spongt-Fauna of the Allantii; Isro; Sj»m,/,.« ,./ M,. tiulf nf Mrrim, 1S79. (20)
K. K. Schulze, investigations into ihc stnu'tmv ami di'vlopinmt <>f spnnurs,
in Ztschr. f. wiss. Zool., — " On llnli^ircn," vnl. xxviii., l<-77 ; " On -

xxix., 1S77; "On AlJiisinidir," xxx., 1S7X ; "On M.-ta rphusis of ftiinunlra

rajitianus," xxxi., 1S7S ; "on S^uu^/m," xxxii., ^7* ; "On .**["<, xri, I:,'," [ft.;
"On Ilin-iiiin and OligoctTOi," xxxiii., 1S70; "On J'luldniiln-," xxxiv., ISM);
"On Cortichtm candelabrum," xxxv., 1SS1. (if) III., "On Soft Parts of
Kni'leeteUa asperyin/i/n," in Trans. Roy. Soe. Etlin , xxix., 1SSO. (-V) l«l.,
I'/, It /i/iiiiu-ii Iteport on the "Challenger" Ilexari inrllHa. (2?) Id., "on llir
Relationship of the Sponges to (he ChoanoJJwjflltiln," in St^.-7>Vr. d. k.-p/rn.^.
Ahul.f. viss. Z., Berlin, 1SS5, translated in Ann. «n>l Mmj. Nnt. His!., iss.',.
(34) Sollas, "On Sfourcmemo, in Ann. and M<«j. Nut. Hist., xix., 1877; "On
I'li'ii-etroftpongta," in Quart. Journ. Geol. Soc., xxxiii., 1877; "On £)/>/«>»/","
ib.; "On Protosptmgia," (''»., xxxvi., 1SSO. (25) Id., "The Sponjre-Fauna of
Norway," in Ann. nii.l Mny. Kat. Hist., 1SSO-S2. (26) Id., "The Flint-Nodules of
the Trimmingham Chalk," ili., vi., 1S79. (27) Id., " Development "f /Mi'*r/vu
hbltltirix," in {'itart. ./num. Mier. ,sv/., xxiv., 1N84. (^J) Id., " On I'etitlina and
the Aiini/nu'nii/i mi," in /'roc. .ft. /risk vtcarf., iv., 1885. (29) Id., "Physical
Characters of Bponge-Spicules," in Proc. R. Dub. Soc., 1885. 'jo) Vejdovsky,
"The I'VshwatiT Sponges of Bohemia," in Abh. d. k. Hnhm. Aland, d. Wlss., ser.
vi., vol. xii., 1SS3. (ji) Vosmai-r, f'» Leurandra aspera (doctor's diss., Leyden,
1S80). (j^) Id., "On thr /'r -//M< n/c((;(/a?," in Notes from the Leyden Museum,
vol. ii. (jy) Sponges of the H'illem Barents Expedition, 1SS4. (?^) "Poriferje," in
Bronn's Klnssen und Onlnungen, vol. ii., 1SS2, and still in progress. Cy) Zittel,
studies of fossil sponges, in Abh. d. k. bayer. Akad^—llejactindlidu, 1877;
Litltistida, 1S7S; Monaftinellida and Calwrea, 1878.

Commerce.

When the living matter is removed from a Ceratose
sponge a network of elastic horny fibres, the skeleton of
the animal, remains behind. This is the sponge of com-
merce. Of such sponges the softest, finest in texture, and
most valued is the Turkey or Levant sponge, Euspongia
officinalis, Lin. The other two varieties are the Hippo-
spnngia equina, O. Schmidt, and the Zimocca sponge,
Euspongia zimocca, O.S., which is not so soft as the others
Distribu- (see p. 423 above). All three species are found at from 2
tion. to 100 fathoms along the whole Mediterranean coast, includ-
ing its bays, gulfs, and islands, except the western half of
its northern shores as far as Venice and the Balearic Isles,
Corsica, Sardinia, and Sicily. Bath sponges occur around
the shores of the Bahamas, and less abundantly on the north
coast of Cuba. They are of several kinds, one not dis-
tinguishable from the fine Levant sponge ; others, the
"yellow" and "hardhead" varieties, resemble the Zimocca
sponge ; and of horse sponges there appear to be several
varieties, such as the " lamb's-wool " and the "velvet"
sponge (Hippospongia gossypina and //. meandriformis).
The fine bath sponge occurs on the shores of Australia
(Torres Straits, the west coast, and Port Phillip on the
south coast). A sponge eminently adapted for bathing
purposes (Coscinoilerma lanuginosum, Crtr. ; Euspongia
mathewsii, Lfd.), but not yet brought into the market,
occurs about the South Caroline Islands, where it is actu-
ally in use, and at Port Phillip in Australia. The fine
bath sponge occurs in the North Pacific, South Atlantic,
and Indian Oceans, so that its distribution is world-wide.
Fishing. The methods employed to get sponges from the bottom
of the sea, where they grow attached to rocks, stones, and
other objects, depend on the depths from which they are
to be brought. In comparatively shallow water they may
be loosened and hooked up by a harpoon ; at greater
depths, down to 30 or 40 fathoms, they are dived for; and
at depths of from 50 to 100 fathoms they are dredged
with a net. The method of harpooning was the earliest
practised, and is still carried on in probably its most
primitive form by the Dalmatian fishermen. Small boats
are used, manned by a single ^harpooner with a boy to
steer ; when, however, the expedition is to extend over
night the crew is doubled. The harpoon is a five-pronged
fork with a long wooden handle, and if this is not long
enough another harpoon is lashed on to it. The Greek
fishers use a large boat furnished with two or three smaller
ones, from which the actual harpooning is carried on ; the
crew numbers seven or eight. One of the chief difficulties
is to see the bottom distinctly through a troubled surface.
The Dalmatian fishers throw a smooth stone dipped in oil

a yard or so in front of the boat ; the stone scatters drops
of oil as it flies and so makes a smooth track for the " look-
out." The Greeks use a zinc-plate cylinder about I.1, feet
long and 1 foot wide, closed at the lower end by a plate of
glass, which is immersed below the surface of the si'a ; on
looking through this the bottom may be clearly seen even
in 30 fathoms. This plan is also adopted in the Bahamas,
where harpooning carried on after the Greek system gives
employment to over 5000 men and boys.

The primitive method of diving-with no other apparatus
than a slab of stone to serve as a sinker and a cord to
communicate with the surface is still practised in the
Mediterranean. The diver carries a net round his neck
to hold the sponges. On reaching the bottom he hastily
snatches up whatever spojige he sees. After staying down
as long as he is able — an interval which varies from two
to at the most three minutes — he tugs violently at the
cord and is rapidly drawn up. On entering the boat from
depths of 25 fathoms he quickly recovers from the effects
of his plunge after a few powerful respirations ; but after
working at depths of 30 to 40 fathoms or more he reaches
the surface in a swooning state. At the beginning of .the
season blood usually flows from the mouth and nose after a
descent ; this is regarded as a symptom of good condition ;
should it be wanting the diver will scarcely venture a second
plunge for the rest of the season. The work is severe, and
frequently the diver returns -empty -handed to the boat.
Diving is usually carried on in the summer months; in
winter it is too cold, at all events without a diving-dress.
The ordinary diver's dress with ^pumping apparatus is
largely used by the Greeks. The diving is carried on
from a ship manned by eight or nine men, including one,
or rarely two, divers. At a depth of from 10 to 15 fathoms
the diver can remain under for an hour, at greater depths
up to 20 fathoms only a few minutes ; the consequences of
a longer stay are palsy of the lower extremities, stricture,
and other complaints. Dredging is chiefly carried on along
the west coast of Asia Minor, principally in winter after
the autumn storms have torn up the seaweeds covering
the bottom. The mouth of the dredge is 6 yards wide
and 1 yard high ; the net is made of camel-hair cords of
the thickness of a finger, with meshes 4 inches square. It
is drawn along the bottom by a tow-line attached to the
bowsprit of a sailing vessel or hauled in from the shore.

Prompted by a suggestion made by Oscar Schmidt, that Cultiva-
sponges might be artificially propagated from cuttings, ti°n-
the Italian Government supplied funds for experiments to
determine the feasibility of cultivating sponges as an in-
dustrial pursuit. A station was established on the island
of Lesina, off the Dalmatian coast, and experiments were
carried on there for six years (18G7-72) under the super-
intendence of Von Buccich. The results were on the whole
successful, but all expectations of creating a new source
of income for the sponge-fishers of Dalmatia were defeated
by the hostility of the fishers themselves.

The details of the method of sponge-farming as practised
by Von Buccich are briefly as follows. The selected speci-
mens, which should be obtained in as uninjured a state as
possible, are placed on a board moistened with sea water
and cut with a knife or fine saw into pieces about 1 inch
square, care being taken to preserve the outer skin as in-
tact as possible. The operation is best performed in winter,
as exposure to the air is then far less fatal than in summer.
The sponge cuttings are then trepanned and skewered on
bamboo rods ; the rods, each bearing three cuttings, are
secured in an upright position between two parallel boards,
which are then sunk to the bottom of the sea and weighted
with stones. In choosing a spot for the sponge-farm the
mouths of rivers and proximity to submarine springs must
be avoided ; mud in this case, as in that of reef-building

SPONGES

Prepara-
tion for
market.

corals, is fatal. A favourable situation is a sheltered bay
with a rocky bottom overgrown with green seaweed and
freshened by gentle waves and currents. So favoured,
the cuttings grow to a sponge two or three times their
original size in one year, and at the end of five to seven
years are large enough for the market. Similar experi-
ments with similar results have more recently been carried
on in Florida. The chief drawback to successful sponge-
farming would appear to be the long interval which the
cultivator has to wait for his first crop.

After the sponge has been taken from the sea, it is
exposed to the air till signs of decomposition set in, and
then without delay either beaten with a thick stick or
trodden by the feet in a stream of flowing water till the
skin and other soft tissues are completely removed. If
this process is postponed for only a few hours after the
sponge has been exposed a whole day to the air it is almost
impossible to completely purify it. After cleaning it is
hung up in the air to dry, and then with others finally
pressed into bales. If not completely dried before pack-
ing the sponges " heat," orange yellow spots appearing on
the parts attacked. The only remedy for this is to unpack
the bale and remove the affected sponges. The orange-
coloured spots produced by this "pest," or "cholera" as
the Levant fishermen term it, must not be confounded
with the brownish red colour which many sponges natu-
rally possess, especially near their base. The sponges on
reaching the wholesale houses are cut to a symmetrical
shape and further cleaned. The light-coloured sponges
often seen in chemists' shops have been bleached by
chemical means which impair their durability. Sponges
are sold by weight; sand is used as an "adulteration."

It is difficult to obtain recent statistics as to the extent
of the sponge trade ; the following tables gives a summary
of the sponges sold in Trieste, the great European sponge
market, in the year 1871 :—

TABLE I.

Description of Sponge.

For Export.

Value in £.

Mean price
per pound.

Horse spon ^e

£60,000
20,000
20,000
2,000

6s.
6s.
14s.
Ss.

TABLE II.

Description cf Sponge.

For Home Consumption.

Value in £.

Mean price
per pound.

£4400
550
950

6s.
6s.
14s.

(W. J. S.)

HYDROZOA

HYDEOZOA form one of the three classes into
JL wliichthe Ccelentera nematophora (distinguished from
the Coelentera porifera, or Sponges) have been divided.
It results from observations made by Ernst Haeckel that
the Ctenophora should not be regarded as a class equi-

valent to the Hydrozoa and Actinozoa, nor as a subdivision
of the latter class, but that they must be considered as a
peculiar modification of the medusiform Hydrozoa (see
final paragraph). If this conclusion be accepted, it will
be necessary to divide the Hydrozoa into two primary

B

Scyphomedus.ns from the Deep Sea. (After Haeckel, Challenger Reports, vol. iv. 1882).

A. PeriphyUa mirabUis, Haeck., one of the Peromedusw, one-third the natural size, a, one of the four interradial tentaculocysts (sensoiy organs) sunk
between its lappets ; b, one of the sixteen subnulial coronal lobes. The twelve tentacles (four peiTadi.il, eight adradial) are M-en.

13. Perradial section through Lucernarui bathyphila, Haeck., nat. size, or, perradial gastral pouch ; 6, gastral axial cavity ; c, ovary (fouv); d, gash-al filaments;

e, peiTadial gastral pouch ; /, manubvium and mouth ; g, the bundles of tentacles (eight, adradial).
The eight principal tentacles (four perradial and four interradial) are not in this species converted into adhesive anchors as in L. auricula, hut arc

altogether suppressed.

groups or grades, for which the names Polypomorpha and
Ctenophora are proposed.

The Hydrozoa correspond to the Linnajan genera Hydra,
Tuljidaria, Sertularia, and Medusa. The name was applied
by Huxley in 1856 to a group corresponding to that termed
Hydromedmx by Vogt (1851) and Medusse. by Leuckart
(1853), and embracing the forms placed by Gegenbaur in
his Elements of C cm/par alive Anatomy (1878) in four classes,
viz., Hydromedmx, Calycozoa, Tliecomedusx, and Medusse.
Our knowledge of the structure and life-history of the
Hydrozoa, many of which, on account of their delicacy and
oceanic habits, are excessively difficult to obtain in a state
fit for investigation, has greatly extended within the last
five years. Whilst in the two decades preceding this period
the admirable researches of Huxley, Gegenbaur, Agassiz,
and Allman had brought to light and systematized a vast
mass of information with regard to these organisms, the
later observations of Glaus, the Hertwigs, Haeckel, and
Metschnikoff, have corrected, extended, and added to
their history, especially in respect of embryological and
histological detail. An epitome of the present condition
of our knowledge of the group is afforded by the subjoined
tabular classification of its families, orders, and sub-classes.

The definition and synonymy of the divisions recognized

will be entered into, after a sketch has been given of the
common structural features of typical Hydrozoa.

CLASS HYDROZOA.
Sub-Class I. Scyphomedusae (syn. Epliymmcdusw).

Examples,

f Lncernaria (fig. 19).
'( Halidystus.
< Craterolophus.
"i Manama.

Order 1. LUCEBSASIS.

Fam. 1. EleuthcrocarpMa1 .

„ 2. Cleistocarpidze

Order 2. DISCOMEDUS.E (Haeckel).
Sub-Order 1. Cubostoma'.
Fam. 1. Protcphyridse.

„ 2. Nausitholdse Nausithoc.

,, 3. Ephyrellidaj.
,, 4. Atollidaj.
,, 5. Cyclorchida?.
Sub-Order 2. Semostoma;.

I Chrvsnnrn (fig. 24, l>).
Fum. 1. Pelagidffi i l'CllKla.

, 2. Cyana?id!c Cyamra.

, 3. Sthenonidse SOienonte.

„ 4. Aurclldn Aurelia (figs. 26-31).

Sub-Order 3. Rhizostomre.

Fain. 1. Tetragameliae

2. Monogamelia?

Order 3. COKOMEDCSJE (Haeckel).

Fam. 1. Charybdeidse

,, 2. liursarida1.
,, 3. Chiropsalmid.T.
UnkT 4. I'i;unMKi'rp.E (Hiieckel).
Fam. 1. Pcriphyllidje.
,, 2. Pericrj'pHdse.

Cephca.
C'nssidpcia.
Hhizostoma (fig. 24, n).
CiamlKSBa.

s. 20-2;i).

58

HYDROZOA

Sab-Class II. Hydromedusse.

Order 1. GrsiNOBLASiEA-A.

(Tubnlaria (fig. 35).

Fam. 1. Tubularidce •! Hybocodon.

( Curymorpha (fig. 34).

„ 2. Pennaridai { Vorticlava

3. Eudendridas .

4. Cladonemidse .

( Bougainvillia (figs. 36, 37).
. -! Perigonium.
( Lizzia (fig. 44).
t Cladonema.
" ( Ciavatella.

I Garveia.
5. Bimeridas ...................... } stylactis.

G. Dicorynidaj .................. Dicorync (fig. 40).

( Sarsiadse (fig. 45).

7. Corynids .................... < Coryne.

( Syncoryne (figs. 41, 46).
I Hydractiuia (tig. 39).

8. Hydrachmd* ............... \ Podocovyne.

,,10-Hydnd* ........................

Order 2. CALTPTOBLASTI;A-LEPTOME|>I;S«.

Fam.l.Pmmularid* ..................

{

/ Eucopidie.
3. Campanularid* j °5£S£*

4

4. Thauraantiadse

Laomedea.

Obelia.
Tlmumantias.
Lafoca.
Meliccrtum.
Tima.
jEquorea.
Zygodactyla.
_ Rhegmatodes.
Order 3. TRACHOMEDUS^: (Haeckel).

Fam. 1. rctasiche Petasus.

„ 2. Trachynemidse Rhopalonema.

„ 3. Aglauridse Aglaura.

5. .lEcuioridse

4. Geryonidse

{ carularina (figs. 48, 49).
Order 4. NARCOMEDCS^E (Haeckel).

Fam. 1. Cunanthidse .................. Cunina (figs 50,51).

,, 2. Peganthidie .................. Polyxenia.

Order 5. HYDROCORM.LINJE (Moseley).

Fam. 1. Milleporida: ................... Millepora (figs. 52, 53).

I Sporadopora.
„ 2. Stylasteridse .................. \ Distichopora.

(Astylus (fig. 54).
Order 6. S:PHONOPHORA.

Sub-Order 1. Pliysophoridse.

Fam. 1. Athorybiadas ................. Athorybia.

„ 2. Physophoridse ............... Physophora (fig. 57, C)

C l-'orskallia.
, 3. Agalmidffi ..................... < Halistemma.

(Agalma(fig 57. E)
„ 4. Apolemiadfe .................. Apolemia.

„ 5. lihizopliysidaj .............. Rhizopliysa.

Sub-Order 2. Physnlidoe.

Fam 1. Physalida? ..................... Physalia.

Sub-Order 3. Calycophoridse.

Fam. 1. Hippopodiido: ................ Gleba.

C Praya.
„ 2.Dipbyidse ..................... 1. Dipllyes (fig. 57. A).

I Abyla.
„ 3. Monophyidaj .................. Spliicvonectea.

Sub-Order 4. Discoidea;.

( Velella.
"

Fam. 1. Velelllds

.
• "( Poipita.

The ITi/drosoa present a greater simplicity of ultimate
structure than do any animal organisms possessed of as
great a complexity of external form. As in all Metazoa or
Enterozoa, the life cycle of a hydrozoon starts with an egg
which is at first a single cell or unit of protoplasm, but
proceeds after fertilization to multiply by transverse fission
in such a way that the resulting cells or units are arranged
in two layers, each one cell deep, disposed around a central
cavity — the enteron or archenteron. The sac thus formed
is known as a diblastula (figs. 1, 2, and 25). By the forma-
tion1 of a mouth to the sac, the enteron acquires the functions
of a digestive retort in which food matters taken in at
the mouth are brought into a chemical condition suitable
for the nutrition of the surrounding cells. The two layers
of cells (of which the outer only acquires additional layers"

1 In Ili/ilivmedusec the inner layer of cells forms by delamiuation,
in Scyphomeduscc by imagination. In the latter case the sac closes
up, ami the mouth is formed by a new opening.

- It is probable that the numerous rows of cells described in the
eudoderm of Tuliuliifin and f.'uri/iiiiirjjha by Allman, in his great mono-
graph of the Tithularuin Ifydroicls, are due to a plication of the

by the division of the primary cells, and that by no
means in all cases) received from Allmau (Phil. Trans.,
1855) the names respectively of the
ectoderm and the endoderm, having
previously been shown by Huxley
(1849) to be the fundamental mem-
branous constituents of which the
most varied parts of the more com-
plex Hydrozoa — such as tentacles,
swimming bells, and air-bladders —
are built up in the adult condition.
Huxley also pointed out the iden-
tity of these membranes with the
two primary layers of the vertebrate

•L, J , , j,i FIG. 1.— Diagram of a Di-

embryo. The endoderm and the
ectoderm, which present themselves,
as is now known, in the diblastula (or

, , , r 11 ET .

gastrula) phase of all anterozoa, re-
main in//y<7co^oa(and also in the allied
groups of Ccflentera) as permanently distinguishable ele-
ments of structure. This important disposition is associ-
ated with and dependent on the simple character which the
archenteron or primitive digestive space retains. Into what-
ever lobes or processes the sac-like body may be, so to

blastula. a, orifice of in-
vagination (blastopore) ;
fr, aichenteric cavity ; c,
endoderm ; rf, ectoderm.
(From Gegenbaur's Ele-
ments of Comparative
Anatomy )

FIG. 2.— Formation of trie Diblastnla of Ewape (one of the Calyptoblastic Hydro-
meduste) by delatnination. (From lialfour, after Kowalewsky.) A, B, C, three
successive stages. ept ectoderm; hit, endoilerm; at, enteric cavity.

speak, moulded, whether tentacles3 or broader expansions,
into these the cavity of the archenteron is extended in the
first instance; and where the actual cavity is obliterated
the endodermic cell-layer remains to represent it (Gefiiss-
platte or endoderm-lamella, see figs. 7 and 10).

Conversely, whatever canals or spaces are discovered in
the substance of a hydrozoon (excepting only the cavity of
ectodermal otocysts) are simple and direct continuations
of the one original enteric cavity of the diblastula, and all
such spaces are permanently in free communication with
one another.4

The whole of the IIydro:oa seem to present a lower grade
of structure than the Adinozoa, in so far as the latter,
whilst retaining permanently free communication between
all parts of the archenteric space, yet exhibit a differentia-
tion of this space into an axial and a periaxial portion — a
digestive tube and a body cavity. The differentiation has
only to proceed a step further, namely, to the closure or
shutting off of the axial from the periaxiul portion of
the archenteric space, and we obtain the condition which
characterizes the adult forms of the Ccelomata, or animals

original endodermal cell-layer. The two kinds of cells in two layers
figured by the same authority in the emloderm of Gfmmcllaria implexa,
pi. vii. tig. 5, cannot, however, be thus explained.

3 Some solid tentacles, with a single axial row of endodermal cells,
form au exception to this statrim-nt.

4 The observations of Eilhard Schulze cited in the article CCELENTEKA
do not form any real exception to this statement.

HYDROZOA

59

with blood-lymph space distinct from digestive canal.1
With the attainment of the ccclomate condition, the two
fundamental cell-layers, ectoderm and endodenn, which still
appear in the embryo, become so far interwoven, and their
products so highly differentiated, that it is no longer possible
to recognize them as anatomical structures in the adult.

The only deep-seated distinction between Hydrozoa and
Antkozoa (the Actinozoa being thus termed when the
Cteiiopkora are detached from them) appears to be the
particular differentiation of the archenteric space iu Anthozoa
which has just been noted. It is no longer possible to
separate the two groups from one another as Exoarii and
Endoarli, as was proposed by Kapp ( Ueber die Polyptn im
Allyemeinen und die Aetinien insbesondere, Weimar, 1&29)
— the first term indicating the Hydrozoa as possessed of
external generative organs, whilst by the latter term the
Anthozoa are pointed to as having internal generative
organs.2 This distinction breaks down completely in the
case of Lucernaria, and even in that of the so-called phanero-
carpous and some other medusce which discharge their
genital products by the mouth, and quite rarely by rupture of
the outer body-wall. The tendency to form calcareous
deposits in the deep layers of the ectoderm, or mesoderm,
as it has been termed, exhibited almost universally by the
Anthozoa (whence the name Coralligena applied to them),
is distinctive of them, though it has been shown first by
Louis Agassiz, and more fully and recently by Moseley, to
be paralleled among Hydrozoa, by the external calcareous
deposits of the abundant and widely distributed Millepores
and Stylasterids. A minute distinction between Hydrozoa
and Anthozoa, which does not, however, hold good uni-
versally, is found in the form of the barbed threads ejected
by the nematocysts. Instead of the complicated forms
present in the latter group, the Hydrozoa are usually pro-
vided with either an unbarbed thread or one in which the
barbs are confined to three at the base and a few minute
barblets (fig. 5).

Fundamental Forms of the Hydrozoa. — The diblastula
derived from the egg of a hydrozoon, when provided with
a mouth, may be spoken of (as are the equivalent forms
in other animals groups) as a person. Either this person
elongates and develops tentacles in a circlet around or near
the mouth, and usually becomes fixed by the aboral pole of
the sac-like body, or the sac gradually assumes the form
of a clapper-bell or of an umbrella with greatly thickened
handle, the mouth being placed at the free end of the handle
or of the clapper, and the animal freely swimming by the
contractions and expansions of the dome of the bell (disc
of the umbrella). The two forms of persons are known,
—the former as the "hydriform" (2, 3 in fig. 16), the
latter as the " medusiform " (4, 5, 6 in fig. 16).

The HYDRIFORM PERSONS usually occur as fixed branching
colonies or trees (figs. 36 and 37) produced by lateral budding
from an original hydra-form developed from a diblastula.

The hydriform person in its most fully developed state
is seen in the colonies of T-ubularia. In such a colony a
number of hydriform persons are united like the flowers of
a plant on its branches (whence Allman's terms hyclranth,
hydrophyton). Each hydriform person (fig. 35) has an
elongated body with oral and aboral pole. The mouth is
placed centrally at the oral pole, which is souiewhat enlarged
and conical. At the apex of the cone, immediately around
the mouth, is a circlet of small tentacles ; at the base of
the cone is a second circlet of larger tentacles; the surface
of the oral cone is termed the hypostome. In other genera

1 The Enterozoa or Metazoa admit of division into two grades — (1)
tlie Ccdentera, including sponges, polyps, jelly-fish, and corals, and
(2) the Cceloinata, including all remaining forms.

* See, however, note to the paragraph headed Definition of the
llijdro-.oa, p. 555.

(f.ff., Hydra, fig. 42) the smaller circle of tentacles is
wanting ; in others, again, the tentacles are irregularly
placed and not concentrated into one circlet (fig. 38).
We regard the former as the typical condition. In the
hydriform persons of the Scyphomedusce (figs. 20 and 27)
the vertical axis is much shortened, the hypostome is flat,
and the whole body cup-like or hemispherical

The tentacles of the hydriform person are sometimes
hollow (Hydra, Garveia nutans, Hydrocorallince), being
mere prolongations of the sac-like body ; but usually,
though the endodermal cell-layer is continued into them,
they are solid (2 in fig. 16). Very generally the tentacles
of the hydra-form are indefinite in number, but in those
belonging to the group of Scyphomedusae a primary series
indicating four radii (perradial) can be distinguished, to
which are added four intermediate to these, marking four
secondary radii (interraclial), whilst eight more placed
between the eight of the perradial and interradial series
are known as adradial tentacles. The surface of the hydra-
form may be entirely naked, or encased in a horny tube
(perisarc) formed by the ectoderm : this may be confined
to the aboral portion of the hydranth and to the common
stem which unites the persons of a colony, or it may rise
up and form a cup (or hydrotheca) around the oral region
of the hydranth (figs. 32 and 33).

The bodies of all hydriform persons, as well as the ten-
tacles, are excessively contractile, and when hydrothecse are
present can be withdrawn into them.

The ectoderm or outer cell-layer furnishes the protective
and contractile tissues of the hydra-form. Very usually
it is not more than one or
two cells deep, and is sepa-
rated from the eudoderni by
a structureless lamella of
firm consistence. In Hydra
large cells of the ectoderm
(neuro-muscular cells of
Kleinenberg) bound the
external surface (fig. 3) and give off horizontal muscular
processes which lie side by side on the structureless lamella —
forming thus a deep muscular coat, the fibrous elements of

FIG. 3. — Epidermo-muscular cells of Hydra.
m, muscular- fibre processes. (After
Klcinenberg, from Gegenbaur.)

FIG 4— Portion of the body-wall of Hydm, showing ectoderm cells above,
separated by " structureless lamella" from throe flagellate cndoderm cells
below The latter are vacunlated, and contain each a nucleus and several dark
granules In the middle ectoderm cell are seen a nucleus and three nemato-
cysts, with trigger hairs projecting beyond the cuticle. A large nematocyst,
with everted thread, is seen in the right-hand ectudennaJ cell. (After t. b.

Schulze.)

which are not independent cells. In larger species some of
the fibres may become separated from the tegumentary or
superficial cells, and acquire the character of independent
nucleated corpuscles (Hydractinia, Van Beneden). No
nervous elements nor sense-organs occur in any hydra-form
(except perhaps the Lucernarice). In Antmnularia some
ectoderm cells are amcebiform, and project processes which
change shape (nematophors). Tactile hairs (palpocils),

60

HYDROZOA

however, occur on the ectodermal cells, and the solid ten-
tacles are essentially tactile organs. Placed in and between
the large cells of the ecto-
derm (Hydra, Cordylophora,
Allman, Kleinenberg, F. E.
Schulze) are small nucleated
cells which become con-
verted into vesicles contain-
ing a three-barbed (figs. 4
and 5) or simple filament
(nematocysts). These are
frequently grouped on the
surface in wart-like pro-
cesses or " batteries." Ne-
matocysts also are found in
the endoderm; but it is prob-
able that their presence
there is due to their having
been swallowed.

The endoderm is usually
but one cell deep, and lines FIG. s.— xematocyst of xgjra, showing

the entire Cavity of the body cell-substance and nucleus, cyst trig-
" * ger nuir, rtiiu evert uu tln'catl. (Alter

starting from the margin of r. E. sciraize.)
the mouth. In the region of the body proper, and in hollow
tentacles, the cells are ciliated (fig. 4). In this region they are
concerned in the secretion of digestive fluids and in absorp-
tion, and sometimes contain coloured granules (hepatic?). All-
man found in Myriothela (Phil. Trans., 1875) that the endo-
dsrni cells project processes
like the pseudopodia of Pro-
tozoa, and suggests that solid
food particles are incepted
by them. T. J. Parker has
published similar observa-
tion on Hydra (1880). In
the solid tentacles the en-
dodermal cells are greatly
modified, forming a kind
of skeletal tissue, each cell recalling by its vacuolation
and firm cell-wall the characters of vegetable parenchyma
(fig. 6). In the stems of Siphonophora endoderm cells give
origin to muscular processes like those of the ectoderm
(Glaus). This latter fact has a morphological significance
which cannot be too gravely estimated.

Generative products are not developed by any hydriform
persons (excepting the Lucernarice), the sexual process being
carried on by a distinct set of buds developed on the sides
of hydriform persons. These buds either become medusi-
form persons, or are degenerated representatives of such
persons (sporosacs) (figs. 17 and 18). Even the fresh- water
Hydra (fig. 42) does nmj appear to be an exception to this
generalization. The single egg-cell of Hydra projects at
the breeding season in an ectodermal covering, as a wart,
from the lower part of the body. A conical eminence or
two nearer the mouth contains the spermatozoa. Each
ovarium and each spermarium represents an aborted gene-
rative person. According to Kleinenberg the egg-cell and
the sperm-cells are both derived from the ectoderm. The
Lucernarice develop internal generative organs (fig. 19)
which correspond closely with those of the medusiform
persons of the group Scypkomedusce (see below), with which
they are classified. Both ova and testis are endodermal in
origin in Lueernaria and in the medusiform persons of the
Scyphomednsa', whilst they appear to be ectodermal in
origin in the complete medusiform persons of Hydro-
r/iedusce, though in the degenerate medusiform persons
known as sporosacs they may either or both have an
endodermal origin.

MEDUSIFORM PERSONS usually present themselves as
isolated free-swimming individuals, but like hydriform

FIG. 6. — Vacuolated endoderm cells of carti-
laginous consistence from the axis of the
tentacle of a Medusa (Cunina). (From
Gegenbaur's Elements of Comparative
Anatomy.)

persons they have the power of producing new persons by
budding (figs. 44, 45, and 46), which may become detached
or may remain connected with the primary person (fig. 57)
to form a freely swimming colony (Siphonopkora) compar-
able to the fixed colonies of hydriform persons. Medusi-
form persons are often produced as the immediate result of
the development of the diblastula without any intermediate
hydriform phase (Pdagia among Scyphomedusce, Tracho-
medusce, Narcomeditsce, and probably som&Anihomeditsce and
Leptomedusix), but quite as frequently originate as lateral
buds upon the body-walls of hydriform persons (figs. 34,
37, and 43), or of other medusiform persons (see below), or
as metameric fission-products of hydra-forms. The typical
medusa-form is a hemispherical cup (the nectocalyx, or
umbrella, or disc), from the centre of which rises up a
cylindrical or conical process (the manubrium, erroneously
polypitf) at the summit of which is the mouth (4, 5 in fig.
16). Four perradial (see above for use of this term) ten-
tacle-like lobes very commonly surround the mouth, or
numerous small tentacles (fig. 58), whilst the margin of
the disc is beset with tentacles four in number, or a mul-
tiple of four (sometimes six, or one only, or indefinite).
Theaboral pole is dome-like, and is never attached except
in those forms which take their origin as buds on a hydri-
form colony when the connexion exists at this point. The
tentacles are, as in the hydriform persons, some solid, some
hollow : both occur in the same individual.

el

™

FIG. 7. — Portions of sections through the disc of medusa?, — the upper one of Lizzia,
the lower of Aurelia. el, endodmn lamella, or vascular lamella; jn, muscular
processes of the ectoderm cells in cross section ; rf, ectoderm ; e», endodenn
lining the enteric cavity; c, wandering endoderm cells of the gelatinous sub-
stance. (After Hertwig.)

The body is not so completely hollowed out as in the
hydriform persons. The mouth leads into a straight tube
(the stomach) which occupies the axis of the manubrium,
and expands at its insertion into the disc. The disc, even
when thick and fleshy, is not fully excavated by the enteric
cavity. In young forms the cavity does occupy it right up
to the margin, but gradually the lumen disappears (fig. 29),
leaving a series of canals and a continuous plate of endo-
derm (fig. 7) formed by the coalesced walls of the space (the
endodenn-lamella of the Hertwigs, see Organivmus der
Medmen, 1878; the vascular-lamella of Glaus, " Polypen
und Quallen der Adria," Wiener JDenhc/i., 1878). The
peripheral portion of the lumen of the original enteric cavity
forms the ring-canal, which runs all round the margin of
the disc, and is continued into the hollow tentacles. The
lumen is further retained at intervals in the form of radiat-
ing canals connecting the axial enteric cavity with the ring-
canal. These may be perradial, interradial, and adradial
(see above as to tentacles of hydra-form), and may branch
dichotomously in the disc or form networks.

The medusas are thicker and more fleshy to the touch
than are the hydra-forms, and are at the same time trans-
parent. This is entirely due to the enormous development
of a structureless substance between ectoderm and endoderm,
corresponding to the " Stlitz-lamella" or structureless lamella
of the hydra-forms. (See figs. 49 and 51, representing
sections of Carmarina and of Cunina.)

HYDROZOA

fil

The remarkable development of this substance in a hyaline con-
dition has led to the description of canals ami spares where none
exist — the supposed spaees being really occupied by this hyaline
substance. F. E. Schulze's statements as to extra-enteric spaces in
Xni-!./ir are thus explained — and more decidedly the supposed circular
and longitudinal canals attributed by some authors to the scyphi-
stoimi phase of I>i*ivninlusa:. In the same manner (according to
Clans) Allman's observations on StephanoseypMa are reconciled
with those of F. E. Schulze on Spotigicola — clearly the same form.
Stephanoscyphua is devoid of either circular or longitudinal canals,
ami though it has four remarkable ridges on the enteric wall like
those of the scyphistoma of Scyphomedusce (see fig. 26) stands in all
probability very close indeed to the Tubularian genus, Perigonimus.

In a large number of medusa-forms the hyaline gelatinous
substance is structureless, but in many of the larger Scy-
phomedusce it is occupied by in- wandering amoeboid cells de-
rived from the endoderui and by fibrous trabecute (fig. 8).

Flo. 8. — Gelatinous substance of the disc of Aurelia, showing — a, fibrous tra-
beculffi, and b, wandering endoderm cells, with amoeboid movements. (From
Gegenbaur.)

The wandering endodermal cells are nutrient in function,
and represent so far isolated elements of the enteric canal
system.

The medusiform person is fundamentally adapted to
swimming movements. The muscular fibres are mostly
transversely striated, and are as a rule outgrowths of super-

FIG. 0. — Muscular cells of medusrc (Li:zia). The uppermost is a purely muscular
cell fiom the sub-umbrella; the two lower are epidermo-musculur cells from
the base of a tentacle; the upstanding nucleated portion forms part of the
epidermal mosaic on the free surface of the body. (After Hertwig.)

ficial ectoderm cells as in Hydra (fig. 9), (though in some
cases distinct cells) ; they are confined to a sheet spread on
the oral face only of the disc or swimming-bell (sometimes
called sub-umbrella), to the extensile manubriurn and
tentacles, and to an inwardly directed flap of the margin uf
the disc known as the velum (Ve in 4 of fig. 16), which is
present in those medusae that are not flattened but conical
(bell-like). The muscular fibres on the oral face of the disc
and on the velum have a circular direction, interrupted
in some cases by radial tracts. The direction of the swim-
ming movements is obvious from this arrangement.

The velum is not a constant element in the medusa's
disc ; it serves to contract the space by which water is
expelled frurn beneath the bell in the act of swimming.

All fully-developed llt/dromedusce possess the velum, but
only a few of the Scypkomedwce (CharyMaia). In the
former the endoderm plate (vascular lamella) is not con-
tinued into it; in the latter vessels of the enteric system are
present in it (fig. 21), and, being probably morphologically
distinct, it has been here termed the "pseudo-velum."

Unlike the hydra-forms, the medusa-forms of Hydrozoa
possess iu addition to the tentacles highly-developed sense-
organs and gangliouic nerve-centres and nerves. The sense-
organs appear to be either eye-spots, or else otocysts, or
to combine the functions of both. In addition to these
are olfactory tracts or pits connected with the preceding.
The sense-organs are placed along the margin of the disc
(hence called marginal bodies), and are of three kinds: —
(1) ocelli — rounded pigment spots, rarely provided with a

Fig. 10. Fig. 11.

FIG. 10.— Ocellusof amedusa (LiiziaKoellitert). oc, pigmentcd ectodermal cells;
/, lens. (After Hertwig.)

FIG. 11.— Otocyst (formed entirely by ectoderm) of Phialidium, one of the
vesiculate mcdusce. rfl, superficial layer of ectoderm; d2, deep layer of ecto-
derm; fi, auditory cells of ectoderm; hh, auditory liairs; ?*/>, nerve body;
nrl, upper nerve-ring ; r, endoderm cells of the circular canal. The otolilh
cavity is seen above h.

\vas(Lizzia) (fig. 10), always placed at the base of a tentacle
or in the radius of one on the oral surface (Lizzia), entirely
ectodermal in origin ; (2) vesiculi or otocysts — formed (as
discovered by the Hertwigs, 1878) by an invagination of the
ectoderm (fig. 11) containing concretions and hair cells;
either open or entirely closed, generally numerous, and
placed between tentacles, sometimes at the bases of tentacles
(Obelia) ; (3) tentaculocysts — which are reduced and modi-
fied tentacles; into them alone of the three kinds of mar-

ginal bodies do the endoderm and, in the more complex,
the enteric canal system enter (figs. 12, 13, and 30). The
endodermal sac forms the axis of the tentaculocyst, its cells
secrete crystalline concretions, and it functions as an otocyst;
pigment spots, which may have cornea, lens, and retina
well developed, are formed sometimes to the number of
six (Charybdtea) on the ectoderm of the tentaculocyst (fig.
13). The olfactory sense-epithelium (fig. 14) is either dis-
tributed in a continuous band on the margin of the disc
(Hydromedusas, discovered here by the Hertwigs), or it 1£

62

HYDROZOA

confined to deep pits (fovese nervosee) from each of which
a tentaculocyst arises (discovered in the Scyphomedusce in-
dependently by Schafer and Claus). With some exceptions,
medusae provided with ocelli are destitute of vesiculi, which
alone occur in the vesiculate Leptomedusce. Tentaculocysts

Fig. 13.

Fio. 13. — Tentacnlocysts of medusae (^4, of Pelagia ; B, at Clianibdrea).
a, the free tentacle hanging in the notch of the disc; &, stalk; c, enteric
canal continued into it; d, enlarged portion of the canal; e, concretions
on endodermal cells; /, pigmented ectoderm ; g, lens. (From Gegenbaur.)

FIG, 14. — Cells from the olfactory pits (foveiE nervosffi)of ^wre/m. (After Schafer.)

characterize to the exclusion of the ocelli and vesiculi the
Trachomedusce and Narcomedusce among Hydromedusce and
all the Scyphomedusce, except Lucernaria, where they are
replaced by '' colleto-cystophors."

The nervous system has only recently been correctly
recognized in medusse, though seen by Agassiz as long ago
as 1849, and described both by Fritz Miiller and Haeckel
in certain forms (Geryonidw) more recently (1860). It
differs remarkably in the two great groups into which the
ITydrozoa are divisible. In the Scyphomedusce there is
no continuous nerve-centre, but around and about each
tentaculocyst nerve-fibres and cells are grouped in such a
way as to divide the disc into zones of nerve supply corre-
sponding to the number of tentaculocysts (usually eight).

FIG. 15.— Scattered nerve ganglion cells, c, from the sub-umbrella of Aurelia
aurita. (After Schafer.)

Both the Hertwigs (Nerven-System der Mcduscn, 1878) and
Eirner (Die Medusen, 1879) entirely missed in their re-
searches the large nerve-fibres and prominent ganglion cells
(fig. 15) which were discovered by Professor Schafer of
University College, London (Phil. Trans., 1879), in the
Scyphomedusce. The writer can confirm Schiifer's observa-
tion of the existence of such fibres and ganglion cells in
the region of the circular muscular zone on the oral face
of the disc of Aure/in, immediately beneath the flattened
epithelium of the ectoderm. Professor Claus of Vienna
has independently described (" Polypen und Quallen der
Adria," 1878) similar nerve-cells and fibres in Ghry-
saora and Charybdcea. Professor Schafer failed to ascer-
tain satisfactorily the origin and termination of the fibres,
which appear, however, to originate in superficial ecto-

dermal cells ("sense-epithelium") in the neighbourhood
of the tentaculocysts and in the cells of those organs,
and to terminate without any plexiform connexion with
one another in the muscular fibres. Eimer has described
very abundant and excessively fine fibres, often moniliform,
which extend from epithelial cells in the neighbourhood of
tentaculocysts and form a network traversing the gelatinous
substance of the disc in every direction. This observation,
though supported by the fact that such fibres ars indi-
cated by the extended experimental investigation of Eimer
and of Eomanes (Eimer, Die Medusen ; Komanes, Phil.
Trans., 1876, et seq.), is not confirmed by other observers,
and the fibres described are regarded as skeletal tissue. If
Einier's fibres do not exist, the muscular tissue of the
medusas must be regarded as acting to a large extent inde-
pendently of nerve-control; and this is borne out by Claus's
observation of the absence of sense-organs and nerve-fibres
from the swimming-bells of the Siphonophora (compound
medusse). In the Hydromedusce the nerve ganglion cells
arc grouped in a continuous ring around the margin of the
disc, separated horizontally into an inferior and superior
portion by the insertion of the velum. The difference in
the form of the nervous system has led Eimer to propose
the names Cycloneura for the Hydromedusae and Toponeura
for the Scyphomedusce. Amongst the latter, however,
Charybdcea, having a continuous velum like ffydromedusce,
has also a continuous nerve-ring.

Comparison and delations of Ilyclriform and Medusiform
Persons. — A simple shortening of the vertical axis, and a
widening of the hypostome, with obliteration of the lumen
(but not of the cells) of the endoderm over a considerable
region of the disc thus produced, suffice to convert the hydra-
form into the medusa-form.1 This change of proportion
made (fig. 16), the sense-organs of the medusiform person
have to be added, and the change is complete. Thus it be-
comes clear that we have to deal with one fundamental form,
appearing in a lower, fixed, nutritive phase and a higher,
locomotor, generative phase in the two cases respectively.

The phylogeny of the Hydrozoa and the historical relation-
ship of the two phases (hydriform and medusiform) appears
to be as follows.

A two-cell-layered sac-like form, with mouth and with or
without tentacles, was the common ancestor of Hydrozoa,
Anthozoa, and Sponges. The particular form which the
proximate ancestor of the Hydrozoa took (1 in fig. 16) is
most nearly exhibited at the present day in Lucernaria
and in the scyphistoma larva (hydra-tuba) of Discomedusce.
It was a hemispherical cup-like polyp with tentacles in
multiples of four, with four lobes to the wide enteric
chamber. This polyp, after passing a portion of its life fixed
by the aboral pole, loosened itself and swam freely by the
contractions of the circular muscular fibres of its hypostome
(sub-umbrella), and developed its ovaria and spermaria on
the inner walls of the enteric chamber. This ancestor
possessed, like its descendants, a very marked power of
multiplication, either by buds or by detached fragments of
its body. Accordingly it acquired definitely the character
of multiplying by bud-formation during the earlier period
of its life ; each of the buds so formed completed in the
course of time its growth into a free swimming person.
We must suppose that the peculiarities of the two phases
of development became more and more distinctly developed,
the earlier budding phase exhibiting a more elongated form
and simple enteric cavity (hydra-form), which subsequently

1 Tliis relationship, demonstrated by the Hertwigs' discovery of the
cndiidiTtn layer of the medusa's disc, differs from that supposed to
obtain by Professor Allman. He supposed the medusa's disc to
represent the coalesced tentacles of a hydra-form, and cited the webbed
tentacles of Laomcdca Jlcxuosa in support of the identification, which
had at the time very much to commend it.

HYDROZOA

63

became changed in the course of the ontogeny (develop-
ment of the individual) into the umbrella or disc-like
form, with coalesced enteric walls and radial and circular
surviving spaces (medusa-form). And now the ancestry
took two distinct lines, which have given rise respectively
to the two great groups into which the IIi/<lrn:na. are divi-
sible— the ScyphomedwscB and the I/i/</>'i»iK'<liniit. In the
one set the hydriform persons of a colony, instead of each
becoming metamorphosed into a medusiform person, pro-
ceeded each to break up into a series of transverse divisions ;
each division became a medusiform person, and was
liberated in its turn as a free swimming organism (figs.
26 and 27). We must suppose that this process began
historically by the outgrowth of new tentacles around the
point where the disc of a person fully transformed from the

'

Ge

FIG. 16. — Diagrams to exhibit the plan of structure of hydriform and mednsiform
persons (all except 5 are vertical sections). A, base of tentacles, margin of the
di?c; B, oral margin; J/a, manubiium; Te, tentacle; CV, circular vessel;
EnL, endoderm lamella; ot, otocyst; or, ocellus off, olfactory pit; //, hood of
tentaculocyst; nnj, genitalia developing in mimubrium; dfj, genitalia develop-
ing in the disc (wall of a radiating canal) ; QP, sub-genital pits of the sub-
umbrcllii; Ol'\ gastra) filaments; IV, velum. 1, Form intermediate between
medusa-form and hydra-form. 2, Hydra-form with wide disc, matmbrium,
and solid tentacles (Tiibularian). 3, Hydra-form with narrower disc, and
hollow tentacles (Hydra). 4, Medusa-form with endodcrm lamella on the
left, the section passing through a radiating canal on the right; a velum, two
possible positions of the genitalia, and two kinds of sense-organs are shown
(ffydromedtutE). 5, A similar medusa-form seen from the surface. G, Section
of Aitrc/iii inifita, to show especially the nature of the sub-genital pits, (77J,
outside the genital frills, and the position of the gastral filaments GF, as well
as the flattened form of the disc.

hydriform to the medusiform phase was loosened in its
attachment and about to separate from the colony. The
" hastening of events," a well-known feature of organic
growth-sequences, would complete the development of the
newly sprouting person before the loosened medusa had
got well away, and so on with a third, fourth, and even
with twenty such successive buds. The separation of the
adult form from its fixed larva by fission has been justly
compared by Louis Agassiz to the separation of the
Comatula from its pentacriuoid larval stalk. If the stalk
could only produce new Comatula', the analogy would be
complete, Lttcernariais in the same way comparable with
the stalked crinoids, being an adult form which retains the
characters exhibited by the immature phases of its congeners.
The Scyphqmedusce do not, however, all exhibit a
hydriform phase, and a production of medusa- by the

"strobilation" or " metamerizing" of a scyphistoma.
Some of them (Pelayia) " hasten events " so far that the
diblastula never fixes itself, but becomes at once a single
medusa, the hydriform phase of the ontogeny being alto-
gether omitted. Certain peculiarities of the medusa's struc-
ture, above all the possession of gastral filaments (solid
filaments like tentacles projecting in four interradial groups
near the genitalia into the enteric cavity), serve to unite
Pelagia, which has no larval stage, and Litcernaria (which
is always of intermediate character between hydra-form
and medusa-form) with the numerous species which develop
by the strobilation of hydriform larvse.

The second line of descent which has given rise to those
Ilydrozoa known as Hydromedusce not only acquired at
the start a different mode of producing medusiform persons,
but the medusiform persons acquired characters differing
from those of the Scyphomedwce in important (but not
fundamental) features. The larval stage in this series
developed the property of budding to a very great degree,
so as often to form fixed tree-like colonies of considerable
size. Then the transformation of the identical colony-
forming persons into free-swimming persons was finally and
definitively abandoned, and only a late-appearing set of buds
proceeded to complete the typical changes and to become
medusas. The earlier-produced buds were thus arrested
in development, and became specially modified for the
purposes of a fixed life as members of a colony. Thus
they acquired the elongate form and the sporadic position
of the tentacles which we see in some hydriform persons of
the Hydromedusce group (figs. 38 and 40), and were adapted
to nutrition solely (hence the term trophosome applied by
Allman to such colonies). The characters of the mature
generative person, with its power of detachment and free
locomotion, being confined to the later buds borne on the
sides of the hydriform persons or on special portions of the
colony, we find that the former became more and more
specialized as sexual medusiform persons in proportion as
the latter became specialized as asexual hydriform persons,
and thus it is that we have the remarkable phenomenon of
hydriform colonies, developed from the eggs of medusae,
producing as it were crops of medusas (figs. 34 and 37)
which detach themselves and swim away to deposit their
eggs (alternation of generations). The Hydromedusa? never
produce medusae by strobilation or transverse division of a
hydriform person, although in rare cases the cicatrix left
by a detached medusa-bud has been observed to sprout
and produce a hydriform person. Neither medusiform
nor hydriform persons of the Hydromedusce series ever
have gastral filaments (unless they are represented by the
"villi" of the Siphonophora described by Huxley, Oceanic
Ilydrozoa}, whilst the medusa-forms always possess a velum
and a comparatively simple set (four, six, or eight) of radi-
ating canals in the disc, the remains of the enteric lumen.

The complete differentiation of hydriform and medusi-
form persons existing on one and the same colony having
been attained in the Hydromedusce, further changes of a
most remarkable character were brought about in some of
the descendants of these forms. The condition which we
have so far noted is perpetuated at the present day in
Bougainvillia (Eudendrium), Campanularia, and a vast
number of the so-called hydroid polyps; others have
undergone further adaptational changes. We have to
notice at least four important additional modifications
independent of one another.

(1.) The hydriform stage was suppressed altogether,
and, as in some Scyphomedueoe, so here too the diblastula
developed directly into a medusa (Trachomedusos, Narco-
meJusce, and probably some Leptomedusce like Tliaumantias
and ^Eifiiorea, and some Ant homed lisa; like Oceania and
Turritopsis).

64

HYDROZOA

(2.) The medusiform persons being early produced did not
separate themselves from the colony, but the whole colony
became free (if it ever were fixed), the medusiform persons
carrying the hydriform persons away with them. Thus the
highly differentiated swimming and floating colonies of the
Siphonophora originated.

(3.) The medusiform persons ceased to detach themselves
from the fixed hydriform persons or colonies, and developed
the ova and sperm within themselves, whilst still small in
size and attached to the hydriform stock. Having once
abandoned the detached, free-swimming life, the medusae
underwent in different genera a varying amount of degene-
ration and atrophy, of which we have in existence all

FIG. 17.— Diagrams illustrating the gradual degeneration of the medusa bud
into the form of a sporosac. The black represents the enteric cavity and its con-
tinuations; the lighter shading represents the genital products (ova or sperm).
A, medusiform person still attached by 11 stalk at trie aboral pole to a colony
(plianeroeodonic gonophor of Altaian) ; B, modified medusiform person, with
margin of the disc (umbrella) united above and imperforate(mouthless) manu-
brium (adelocodonic gonuphor of Allman) ; C. sporosac, with incomplete
extension of the enteric cuvity into the umbrella, — rudimentary invagination
above to form the sub-umbrella cav'ty; D, sporosac with manubrial portion
only of the enteric cavity ; E, sporosac without any trace of manubrium.

possible degrees, leading from the fixed " plianeroeodonic
gonophors " (Allman, bell-like genital buds) of many
Siphonophora through the " adelocodonic gonophors "
(genital buds with the bell no longer open but closed by the
union of the margins of the disc) of Cordylophora to the
sporosacs of Hydractinia, and even to the simple genital
warts of the little degenerate Hydra viridis of fresh waters
(see fig. 17, and explanation). By this process a large num-

Fir.. 18.— Two female sporosacs (degenerate medusa?) of fft/dractiiria echinata.
(From Gegenbaur, after Van Beneden.) a, ectoderm; 6, endoderm; o, egg-
cells; (/, enteric cavity. In A an invagination of the ectoderm, which is
more complete in B, represents the rudiment of the sub-umbrella space.

ber of HydromediiscK (figs. 35, 38, 3D, 40, and 42) have lost
all evidence of the real characters of their medusa-forms, just
as others have suppressed the evidence of their hydra-forms
by direct development from the egg ; and inasmuch as both
these processes take place in genera having the closest affinity
with genera in which both hydra-form and medusa-form are
fully preserved, it is not possible to erect groups similar to
the Haplojnorpha of Gurus or the Jf'iiiopin.-<t of Allman for
their reception. The difficulty of classification is, however,
rendered very great, fora double system becomes necessary,
which shall deal with the characters of hydriform and
medusiform persons in parallel equivalent series. The
difficulty is considerably enhanced when we find that iden-
tical medusa-forms may spring from unlike hydra-forms,
and, conversely, that closely allied hydra-forms may give
rise to very different medusa-forms. The character first
noticed by Rapp as distinguishing the hydroid polyps from
the coral-polyps, namely, that of developing their geuitalia
as external bodies (Exoarii) instead of internally (Endoarii),

is seen by the considerations just adduced to be fallacious.
The Hydromedusce, it is true, often (not always) develop
their generative products from the ectoderm, and the geni-
talia frequently project as ridges and discharge themselves
directly to the exterior in this division. The Hydromedusce
contrast in this respect with the Scyphomedusce and An-
thozoa, which develop their genitalia from the endoderm,
and are (to use Rapp's terms) Endoarii whilst the former
are Exoarii. But the bodies mistaken for external generative
organs by Rapp and other early observers in many hyclroids,
and in Htjdra itself, are aborted degenerate medusae.

(4.) A further set of changes, which have affected the
original hydriform colonies and their medusa-buds so as to
produce new complications of structure among the Hydro-
medusce, are summed up under the head of " polymorphism."
The differentiation of hydriform and medusiform persons is
a case of dimorphism ; a further distribution of functions,
with corresponding modification of form, gives us "polymor-
phism." Polymorphism is unknown in the Scyphomedusce,
and it is chiefly confined to two groups of Hydromedusa? (the
Hydrocorallinae and the Siphonophora). In the hydriform
colonies of Ilt/dractinia (one of the Gymnoblasfea-Ant/tome-
dusce) the outer hydriform persons of the colony (fig. 39)
differ in form from the rest, and have wart-like tentacles. In
the same genus, and also in many Calyptoblastea, the hydri-
form persons which are destined especially to give origin
to medusa-buds are devoid of tentacles and mouth, and
are known as blastostyles (Allman), (fig. 43). In Hydro-
corallince (fig. 53) elongated hydriform persons (dacty-
lozooids) with no mouth and sporadic tentacles are set in
series around a central short mouth-bearing person (gastro-
zooids) forming the " cyclo-systems " of Mr Moseley (figs.
52 and 55). In the Siphonophora, in addition to nutritive
(hydriform) persons and generative (medusiform) persons,
there may be rows of swimming-bells (medusae devoid of
mouth and of genitalia), covering-pieces (flattened medusae),
and tentacle -bearers (hydriform persons with one long highly-
developed tentacle), (see figs. 56 and 57).

Hypothesis of the Individuation of Organs. — The building
up of complex individualities, such as a hydrozoon colony,
a flowering plant, or a segmented worm or arthropod — in
any one of which a number of common units are repeated,
but with varied form and function in each part of the com-
pound body — is generally admitted to be explicable in two
ways, and which of the two explanations may be adopted
in any one case must depend on the ultimate inference
from a wide series of observations. The first hypothesis,
which undoubtedly applies to the ordinary hydriform
colonies of Hydrozoa, to the segments of Tcenia, and to
plants formed by the repetition of phyllomes, is that an
original unit like those which constitute the composite
organism has freely budded, and repeated its own structure
in the well-marked units which remain conjoined to form an
aborescent or linear aggregate. This is " eumerogenesis,"
and such aggregates may be termed eumeristic. By a
division of labour and consequent modification of form
among the units of a eumeristic aggregate, such an aggregate
may (in the course of phylogeny) acquire varied shape and
definite grouping of its constituent units, and a high speci-
alization as an individual. The high degree of individua-
tion which may be thus attained is due to the more or
less complete synthesis of a eumeristic colony. The more
highly individuated Chajtopods and Arthropods are syn-
thesized linear colonies. The cyclo-systems of the Hydro-
corallines are undoubted examples of synthesized colonies.
The second hypothesis is one which is applicable to cases
which, in the absence of special evidence to the contrary,
might be regarded as highly synthesized colonies. Accord-
ing to this second hypothesis, such highly individuated
composite organisms have not (in their phylogeny) passed

HYDROZOA

65

through a eumeristic phase in which the units were well
developed and alike, but the tendency to bud-formation
(whether lateral, linear, or radial) has all along acted con-
currently with a powerful synthetic tendency, so that new
units have from the first made but a gradual and disguised
appearance. This is "dysmerogenesis," and such aggregates
as exhibit it may be called dysmeristic. In dysmeristic
forms the individuality of the primary unit dominates from
the first, and the merogenesis (segmentation or bud-forma-
tion) can only show itself by partially here and more com-
pletely there compelling (as it were) the organs or regions
of the body of the primary unit to assume the form of new
units. The arms of star-fishes are, when we consider them
as derived from the antimera of a Holothurian, explained
as examples of dysmerogenesis. So, too, the series of
segments constituting a leech, and probably also the
segments of a vertebrate. Eutnerogenesis and dysmero-
genesis are only variations of one process, merogenesis, and
no sharp line can be drawn between them. Individuation
may appear at any period in the phylogeny of a eumeristic
aggregate and synthesize its units. On the other hand, in-
dividuation is more or less completely dominant throughout
the history of a dysmeristic aggregate, and is gradually
broken down as a more and more complete analysis of the
primary unit into new units is effected. It will be observed,
however, that in dysmerogeuesis, tiie/orm which individua-
tion tends to preserve is that of the primary unit (notably
the case in leeches as compared with the ameristic flukes),
whereas when we have eumerogenesis followed by synthesis
the resulting form-individuality is something absolutely
new. Thus, using the terms eumeromorph and dysmero-
morph, we have — (1) synthesized eumeromorph simulates
normal dysmeromorph ; (2) aualysized dysmeromorph
simulates normal eumeromorph.

Whether the fixed hydriform colonies of the Hydrozoa,
with their more or less complete medusiform buds, and
further, the floating colonies of Siphrmophora, with their
polymorphous units, are to be regarded as synthesized
eumeroinorphs or as dysmeromorphs, more or less analysed,
is perhaps still open to discussion. The former view (that
adopted here) is that held by Allrnan (Monograph of the
Tubtdarian Hydroids, 1874), by Leuckart (1851), by
Gegenbaur (Grundriss, 1874), by Glaus (Grundziige der
Zoolof/ie, 1876), and by the Hertwigs (Organismus der
Medusen, 1878). On the other hand, Huxley (Oceanic
Hydro-Mi, 1856), formerly Gegenbaur (Zur Lehre der Gene-
rations -IVechsel, 1854), and, more recently, Ed. Van Beneden
(" De la distinction originelle du testicule et de 1'ovaire,"
Hull. Antd. Roy. Bely., 1874) have held that the medusi-
form person is a generative wart which has gradually
assumed the characters of a bud, and that the various
phases presented by it in different genera are so many more
or less successful strivings after complete assumption of the
hydra-form (from which the medusa-form is thus secondarily
derived). Similarly the variously modified units of the
siphonophorous colony have been regarded as the organs of
a parent unit which have each more or less completely
acquired the form of that parent unit, or, in other words,
the colonies in question have been held to be dysmero-
morphs. Recently ascertained facts as to the polymorphism
of HydrocoralliruB, but more especially the demonstration
of the identity of structure of the medusae of the Scypho-
medusan and Hydromedusan groups, and, further, the mode
of development of the Scypkomedusoe from the scyphistoma
and the relations of the generative products to the enteric
cavity, combine to render the view that the polymorphous
and dimorphous colonies of Hydrozoa are synthesized
eumeromorphs more probable, in the judgment of the
present writer, than that which would explain them as
dysmeromorphs.

The term "merogenesis," and its subordinate terms,
"eumerogenesis, dysmerogenesis," &c., are applicable to
units of the first order, namely, cells, as well as to the
" persons " which are built up by them. Ordinary cell-
division is an example of eumerogenesis; free-formation of
nuclei, as in the fertilized ovum of Arthropods, is dysmero-
genesis. A syncytium is usually a synthesized eumero-
morph, but may be a dysmeromorph.

Definition of the Hydrozoa. — The Hydrozoa are Ccelentera
nematophom, distinguished from the fellow-group Anthozua
(the name applied to Actinozoa when the Ctenophora are
removed from them) by not possessing the latter's constant
and sharp differentiation of the arch-enteric cavity into
axial digestive and periaxial septate portions, usually by a
simpler form of nematocyst, and generally by lower histo
logical differentiation.1

The following is a brief summary of the chief characters
of the larger divisions of the Hydrozoa: —

Sub-class I. SCYPHOMEDITS^E. — These are Hydrozoa which
in the adult condition al-
ways have four or eight
interradial groups of
gastral filaments ("pha-
cell£e"ofHaeckel)(figs. 16
(6),23,and26). Thegeni-
talia (ovaria and sper-
maria) are developed from
endoderm, and are always
iuterradial (in the four

radii formed after the first FlG. 19.— Diagrammatic vertical section of a

four). The hydra-form
is not a " hydroid," but a
short polyp with broad
hypostome — the "scyphi-
stoma," which gives rise to
medusa-forms by trans-
verse fission (strobilation),
or itself develops genitalia
(Lucernarice). Combined visual and auditory organs in
the form of modified tentacles (tentaculocysts) to the
number of four, eight, or more occur on the edge of the
disc (except in Lucerne/rice, where they are represented
by the "colleto-cystophors''), The medusa-form in some
cases develops from the egg without the intermediate
scyphistoma-stage (Pelagia, GJiarybdoea 1). The edge of
its disc is provided with lappets, which cover the sensorial
tentaculocysts (hence Steganophtluilmia of Forbes), and is
not provided with a velum (hence "Acraspeda" of Gegen-
baur), excepting the rudimentary velum of Amelia (fig. 31)
and the well-developed vascular velum (pseudo-velum) of
Charybdcea (fig. 21). There is no continuous marginal
nerve-ring (except in Charybdcea), but several separate
marginal nerve centres (hence Toponeura of Eimer). The

1 Quite recently the Hertwigs (Jenaische Zeitschr., bd. vi., new
series, 1879) have insisted that in the ffydromedusas the genitalia
(both ova and testes) are developed from the ectoderm, whilst in the
Scyphomedusce and in the Anthozoa they develop from the endoderm.
On this account they propose to abandon the grouping into Hydrozoa
and Anthozoa of Ccelentera ncmatophora, and suggest two groups, the
Ectocarpece and the Endocarpece — the former equivalent to Hydro-
medusce, the latter embracing Scyphomedusce and Anthozoa. The
Anthozoa exhibit a further predominance of the endoderm in its ex-
tensive origination in them of muscular fibre, which but rarely and in
small quantity develops from endoderm in the Hydromedvsa: or in the
Scyphomedusce. The Hertwigs base their generalization on their own
studies of medusas, but they have ignored the observations of Van
Beneden on Hydractinia and of Ciamician on vaiious Tubularians, in
which the origin of either sperm or ova from endoderm is established.
Recently Fraipont has repeated an observation of Van Beneden's on
Cainpanitlaria, and shown conclusively that the ova in that form arise
from endoderm. Weismann (Zoologischer Aiizcii/p/', May 1SSO) shows
the same for Plumularidce and Scrtulnrida:; the reader is referred to
his paper. 1

Lucernai'ia in the plane of an intervuclius.
a, one of the inttM-radial angles of the
disc, giving rise at a' to two groups of
tentacles adradial in position; ft, axial en-
teric cavity; f, endoderm; d, band-like
genital gland (ovary or tcstis). adradial in
position, and attached to the inten-ndial
septum which runs along the angular pro-
cess of the disc, to which the letters c, d
point; p, aboral region or "foot"; zt the
interradial gastral filaments or phacelia?.
(After Allrnan.)

66

HYDROZOA

diblastula in all cases, as yet observed, is formed by in-
vagination, the blastopore closing up (Balfour).

. 23.

FIG. 20. — Chanjbdiea marsvf>f<ilis (natural size, after Clans). The four annulated
tentacles are seen denending from the four lappets placed at the four corners of
the quadrangular umbrella. These are interradial. Two of the four perradial
enteric pouches of the umbrella, representing radiating canals, are seen of a pale
tint. Fg, gastral filaments (interradial); R. the modified perradial tentacles
forming tentaculocysts; (?, corner ridge facing the observer and dividing
adjacent pouches of the umbrella; OF, position of one of the genital bands.

FIG. 21. — View of the margin of the umbrella of Cfiari/lxJcFti n/di-.oipi'tlis (natural
size, after Glaus). At the four comers are seen the lappets which support the
long tentacles, and in the middle of each of the four sides is seen a tentaculo-
cyst. Vet, the vascular velum or pseudo-velum, with its branched vessels.

FIG. 22. — Horizontal section through the umbrella and manubrium of Cliartibdtea
marsupidUs (modified from Glaus). Ma, mnmibriuin; 8R, side ridge (perradial);
CR, corner ridges, separated by CO, the interradial corner groove; On, the
genital lamella in section, projecting from the interradial angles on each side
into UE, the enteric pouches of the umbrella; SU, the sub- umbrella space.

FIG. 23. — Vertical sections of Chftriibdwa marstii'nitix, to the left in the plane of
an interradius, to the right in the plane of a pcrradius. Ma, manubrium;
EAx. axial enteron; Gh, gastral filaments (/t/iacellce); CO, corner groove;
SR, side ridge ; EnL, endndenn lamelli (line of concrescence of the walls of
the enteric cavity of the umbrella, whereby its single chamber is broken up into
four pouches) ; He, line of attachment of a genital band; EU, enteric nouch of
the umbrella, in the Irft-haiid figure, points to the cavity uniting neighbouring
pouches near the. margin of the umbrella and giving origin to TCa, the tentacular
canal; 7%, velum; Fi\ freuum of the velum ; 7V, tcntaculocyst.

The binary division of the Hydrozoa was established by Esch-
scholtz (System dcr Acfilcphcn, 1829) whose Discophorce phanero-
carjicc correspond to the Hniiilwiiu-dusir, whilst his Discophorce
cryptocarpcc represent the Hijdromcihisce. The terms point to dis-
tinctions which are not valid. In 1853 Kolliker used the term Dis-

cophora for the Scyphomedusce alone, an illegitimate limitation ol
the term which was followed by Louis Agassiz in I860. Nichol-
son has used the term in the reverse sense for a heterogeneous

abandonment necessary. The teim Discomcduscc was used for the
ScypTumedusce by Haeckel in his Generclle Morpholoijie (exclud-
ing Charybdcea) — whilst Carus (Handlmch, 1867) confines the term
" Medusa: " to them alone, which is objectionable, since it belongs
as justly to the Hijdromcduscr. Forbes's term for them, Stcyanojih-
thahnia, indicates a true characteristic, failing only in the Luccr-
naricc, but its complementary term Gfymnophtkalmia is inaccurate.
Similarly tlie terms Acraspeda and its complement OmspcMa are
inaeceptable. Eimer his proposed to use the terms Tujmncura and
C'yclonctira for the two divisions — but Charybd(ea appears to break
down this division as so many others. The old term Acalcphce,
which is retained by Gegenbaur in its proper sense for all the
Ccr-lcntera iicmatophom, is used as the designation of the Scypho-
medusa: alone by Claus (Grundzuge dcr ZooL, 1878), which cannot
fail to produce confusion. The term Luccrnaridec, proposed so long
ago as 1856 by Huxley (Med. Times and Gazette), most truly indi-
cates the relationships of these organisms which he was the first to
recognize, but it seems desirable to restrict this term to the limited
order in which Lucernaria is placed, and to employ for the larger
group — Scyphomediisa: — a term which is the true complement of
the convenient name assigned to the other division ot Hydrozoa,
viz., ITydromedusce.1

Order 1. Lucemariae, — SeyphomedustB devoid of tenta-
culocysts, with the aboral pole of the body produced into
an adhesive disc by which the organism (which possesses
the power of swimming by contraction of the circular
muscular zone of the hypostome) usually affixes itself. The
enteric cavity is divided into four perradial chambers by
four delicate interradial2 septa. The genitalia are developed
as four-paired ridges at the sides of the interradial septa
on the oral wall of the chambers (fig. 19). No reproduc-
tion by fission nor "alternation of generations" is known
in the group. At the edges of the disc capitate tentacles
are developed in eight adradial 2 groups ; between these are
modified tentacles in some genera, — the marginal anchors
or colleto-cystophors. The canal system which has sometimes
been described in them is a product of erroneous observation.
A very few genera and species of this order are known.
They may be justly called the ccenotype of the medusae
(James Clark), and their relationship to the free swimming
forms may be compared, as was done by L. Agassiz, to the
relationship of the stalked Crinoids to such forms as Coma-
tula. Three species are not uncommon on the British coasts.

By Milne Edwards the animals forming this group were termed
PodacthiariaaMd associated with the Antho~oa. By Leuckart they
were termed Calycozoa ; it is only of late that the closeness of their
relationship to the Scyphomedusce has been fully recognized, though
long since insisted on by Huxley and by James Clark. Haeckel in
his new system of the medusa; (ititzmirisbcr. dcr Jenaischc Grxcllschafl
fur Medicin und Naturwiss. , July 26, 1878) adopts for them the
term Scypkomcduscu in allusion to their permanently maintaining the
distinctive features of the scyphistoma larval form of the Acraspcdce,
the term which he adopts from Gegenbaur for our Scyphomedusas.

Order 2. Discomedusce. — These are Scyphomedusce de-
veloping as sexual medusiform persons by transverse fission
from a scyphistoma, or else directly from the egg. They
have eight tentaculocysts, four perradial, four interradial,
and sometimes accessory ones (adradial). Four or eight
genital lobes (ovaria or spermaria or hermaphrodite) are
developed from the endoderm forming the oral floor of the
central region of the enteric cavity, which is produced into
a corresponding number of pouches. The mouth is either
a simple opening at the termination of a rudimentary
manubrium (sub-order Cubostomce), or it is provided with
four or eight arm-like processes (sub-orders Semostoma? and
Rhizostomce). In the sub-order Rhizostomce (fig. 24, «), the

1 Scyphomeduscr (<rKu<t>os, a cup) are medusae which are related by
strobilation to Scyphistoma, — a wide-mouthed polyp with four gastral
ridges. HydffotnedustK are medusn? related to a Jlydra, — a narrower
polyp, devoid of gastral ridges, — by lateral gemmation,

a For use of these terms see paragraphs ou Amelia below.

HYDROZOA

edges of the oral opening fuse together at an early age
and leave several sucker-like secondary mouths, which were
formerly mistaken for independent persons. The central
enteric chamber is continued through the disc by a com-
plicated often reticulate system of radiating canals, which
excavate the endoderm lamella.

FIG. 24. — ScyphomedusK. a, Rhizostoma p ulmo- b, Chrysaora hyosceita

In the Semostoma? and Rhizostomce (not in the Cubostomit)
four remarkable (respiratory) sub-genital pits (fig, 28) are
hollowed out in the gelatinous substance of the sub-umbrella
(oral face of the umbrella). These do not communicate, as

development have recently formed the subject of investiga-
tion by Claus, Eimer, and others. As the current accounts

FIG. 25.— Four stages in the development of Chrysaora. A, Diblastula stage ;
B, stage after closure ol blastopore; C. fixeil larva with commencing stomodieum
or oral ingrowth ; D, fixed larva with mouth, short tentacles, Ac. ; 'ep, ectoderm ;
hy, endoderm ; st, stomodjeum ; m, mouth ; &/, blastopore. (From balfour, after
Claus.)

has been erroneously supposed, with the genital organs, the
products of which normally are evacuated by the mouth.
In the Tetragamelian Rhizostomce these pits remain distinct
from one another as in Semostomce, but in the Monogamelian
Rhizostomce they unite to form one continuous sub-genital
cavity placed between the wall of the enteric cavity and
the polystomous oral disc. The common English forms,
Aurelia, Chrysaora, and Cyancea, are types of the Seino-
stonue, the somewhat less common Rhizostoma of the
Monogamelian Rhizostomw, whilst Nausithoe and Disco-
medusa represent the simple C'ubostomce.

The writer has adopted the term used by Haeckel for this order,
and is indebted to his preliminary notices of a large work on the
Medusa:, now in the press, for outlines of the classification and de-
finitions which have been introduced with modifications in relation
to these and the other Medusa:. The term Discophora is used by
Glaus (Gnuulzilije) for the Discomcduscc. It is quite clear from the
varied and inconsistent use by different authors of that term, and
also of the terms Acalephce and Medusa, that they must be ejected
altogether from use in systematic treatises.

The structure of the common Aurelia aurita and its

FIG. 21!.— Later development, of Chrysaora and Aurelia (after Claus). A, Scyphi-
stoma of Cltrysaora, with four perradial tentacles and horny basal perisarc.
B, Oral surface of later stage of scyphistoma of Am elia, with commencement
of four intervadial tentacles. The quadrangular mouth is seen in the centre ;
the outline of the stomach wall, seen by transparency around it, is nipped in
four places interradially to form the four gastric ridges. C, Oral surface of
a sixteen-tentacled scyphistoma of Aurelia. The four gastric interradial
ridges are seen through the mouth. I>, First constriction of the Aurelia
[ scyphistoma to form the pile of ephyrje or young medusie (see fig 27). The
single ephyra carries the sixteen scyphistoma tentacles, which will atrophy
and disappear The four longitudinal gastric ridges are seen by tiansparency.
E, Young epliyra just liberated, showing the eight bifurcate arms of the disc
and the internidial single gastral filaments. F, Ephyra developing into a
medusa by the growth of the adradial regions. The gastrul filaments have
Increased to three in each of the four sets. ^4, margin of the mouth; Ad,
adradial radius; F, gastral filament; //;, interradial radius; JG, adradrial
gastral canal ; Jfi=R3, adradial lobe of the disc ; K, lappet of a perradial arm ;
M, stomach wall; Afst, muscle of the gastral ridge; J/(P. gastral ridge;
J/s, mesoderm ; 0, tentaculocyst ; P, perradial radius ; A'-, interradial radius ;
R*, adradial radius; SO, commencement of lateral vessel.

in text-books are very inadequate, a short sketch of the

morphology of that form is appended here.
From the egg, according

to the researches of Claus

(whose figures, here repro-

duced, refer more especially \ /

to the closely allied genus \ /

Chrysaora, up to the compl e- \L
tion of the scyphistoma), a
single-cell-layered blastula de-
velops which forms a diblastula
by invagination (fig. 25, A, B,
C). The orifice of invagination
closes up, and the ciliated
" planula " (as this stage used
to be termed in all Ceelentera),
after swimming around for a
time, fixes itself, probably by FlG 27._DcveIopment of Aurelia.
the blastoporal pole. The true Above to left, young scyphistoma
mouth then forms by inruption
at the opposite pole. Two ten-
tacles now grow out near the
mouth opposite to one another
(fig. 25, D), and are followed
by two more (fig. 26), these
indicating the four primary
radii of the body which pass
through the angles of the four-
sided mouth, and are termed perradial. Meanwhile
the aboral pole narrows and forms a distinct stalk,
which in Chrysaora secretes a horny perisarc (fig. 25,

with four pel radial tentacles Be-
low to left, scyphistoma with six-
teen tentacles and first constiiction.
To the light, stiobila condition of
the scyphistoma, consisting of thir-
teen metameric segments ; the up-
permost Still pn-^^-l-s Illr Mxtrcn

tentacles of the scyphi>toma; the
remainder have no tentacles, but
are ephyrje, each with eight bifid
arms (processes of the disc). Each
segment when detached becomes
an ephyra, such as that drawn in
tig. 26, E, F. (From Gegenbaur )

68

HYDROZOA

D). Four new tentacles, those of the intermediate or
secondary radii, now appeaf between the first four, and
are termed interradial. At the same time four longi-
tudinal ridges grow forward on the wall of the enteric
cavity (fig. 26). These interradial ridges have sometimes

Tc

FIG. 28. — Surface view of the sub-umbrella or oral aspect of Aurelia aurita, to
show the position of the openings of the sub-genital pits, GP. In the centre
is the mouth, with four pel-radial arms corresponding to its angles (compare
fig. 26). The four sub-genital pits are seen to be interracial, x indicates tlie
outline of the roof (uboral limit) of a sub-genital pit; y, the outline of its floor
or oral limit, in which is the opening (compare 6 of fig. 16).

been erroneously described as containing each a longitudinal
canal connected with a circular canal at the base of the
tentacles. They are in reality solid, as is the margin of the
hypostome from which the tentacles spring. It is in con-
nexion with these four
ridges that the gastral
filaments will subse-
quently appear, as also
the genital organs either
along their middle line
or adradially to them.
The ridges correspond
tu the mesenteries of
the Authozoa. Eight
additional tentacles
placed one on each side
of the perradial ten-
tacles (or of the inter-
radial, according as we
may choose to regard
the matter) next appear,
and are distinguished as
iiiln.iifial. All the ten-
tacles reaching an equal
size, we obtain the ap-
pearance seen in fig. 26,

when the vouno- scvnhi F'°- 2!l-— Half of '»e lower surface of Aurelia
1L /Ul' ,iurila. Tlu- transparent tissues allow the

stoma is looked at from
above. Looked at from
the side, with its wide
hypostome and short

vertical axis, the scy- are not d""™- (from G<.-genbaur.)
phistoma differs widely from an ordinary hydra-form, and
approaches the medusa-form, to which its fnur longitudinal
gastral ridges further assimilate it. The little creature is
now about an eighth of an inch in height ; in other genera,
but not in Chrysaora, it may now multiply by the produc-
tion of a few buds frum its fixed basal disc. After nourish-
ing itself for a period, and increasing to four or five times
the size just noted, the vertical axis elongates and a series
of transverse constrictions appear on the surface, marking
off the body of the scyphistoma into a series of discs
(figs. 26 and 27), each of which by the development

ispan

enteric cavities and canals to be seen through
tin-Mi. '/, marginal lappets hiding tentaculo-
cysts; b, oral arms; r, axial or gastric portion
of the enteric cavity; gr, radiating and ana-
stomosing canals of the enteric system; ov,
ovaries. The gastral filaments near to these
are not drawn.

of tentacles and completion of the constriction will become
a separate medusa (in its young state called " ephyra ").
The tentacles of the Aurelia and the structure of the
margin of its hypostome are very different from those of
the scyphistoma. They are exhibited in their earliest
condition (when the vl«?Wia-medusa is first liberated from
its attachment and is an ephyra) in fig. 26, E, F. The
margin of the hypostome is drawn out into eight arms
(which are not to be confused with tentacles) ; the end of
each arm is bifid, carrying a pair of lappets — the marginal
lappets which persist in the adult (see figs. 30 and 31). Be-
tween the lappets is placed a short and peculiar tentacle, the
tentaculocyst or sense-organ. The eight arms of the disc
and their tentaculocysts are perradial and interradial. As
the organism grows, a set of eight arlradial tentacles appear
in the notches between the eight arms, but never attain any
relatively large size in Aurelia. The asteroid arm-bearing

End

FIG. 30.— Tentaculocyst and marginal lappets of Amelia aurita. In the left-
hand figure — ML, marginal lappets; T, tentaculocyst; ^4, superior or aboral
olfactory pit ; MT, marginal tentacles of the disc. The view is from the aboral
surface, magnified about r>0 diameters. In the right-hand figure — A, superior
or aboral olfactnry pit; 11, inferior or adoral olfactory pit ; //, bridge between
the two marginal lappets forming the hood ; T, tentaculucyst ; En<l. cndoderm ;
Ent, canal of the enteric system continued into the tentaculocyst; Can, endo-
dermal concretion (auditory); oc, ectodermal pigment (ocellus). The drawim;
represents a section, taken in a radial veitical plane so as to pass through the
long axis of the tentaculocyst. (After Eimer.)

character of the margin of the disc is soon obliterated by

the relative growth of the intermediate adradial areas, which

become quite filled up, so that in the adult the tentaculocyst

is carried in a notch instead of on a prominence, and is

concealed by the two lappets

(figs. 28 and 30). The margin

of the disc between adjacent

pairs of lappets gives rise to

a fold which grows inwards

(toward the mouth) during

an early stage (fig. 31), and

numerous small tentacles (the

fringe) appear along the

margin of the disc, which

soon equal in size the first

adradial tentacle. The in-

, . . . , , FIG. 31.— Part of the margin of the disc

growing told IS the velum or of a young Aunlia. to show therudi-

" nspndo velum " incl never ™enta>y velum, r,/, extending from
pseuao-veium, dim never the ma,.gi,Uil laI,pL.tSi 1!L< „„ eitllcl.

increases in size, SO that in

the adult it is not observ-
able. The tentacles also remain very small and fine in
Aurelia, forming a continuous fringe along the edge of
the disc, interrupted only by the eight notches for the
tentaculocysts (fig. 29).

The sixteen tentacles of the scyphistoma are necessarily
attached to the most anterior of the pile of medusa; ; they
atrophy, but to what extent they may be metamorphosed
to form the parts of the ephyra or young medusa has not
been determined. The scyphistoma, having given rise to
its pile of ephyr», may (in some genera, Awelial)
redevelop its own kind of tentacles below the constriction
marking off the last ephyra. Hence scyphistoma tentacles
appear sometimes at the top and sometimes at the bottom

side; T', the small tentacles fringing

HYDROZOA

69

of the pile, which has led to diverse accounts of the mode
of development of the ephyrre.

Whilst changes are going on in the configuration of the
margin of the disc of an ephyra on its way to the perfect
form of the adult Aurelia, the enteric cavity has also under-
gone most important changes. Foremost in importance is
the development of a single gastral filament on each of the
four gastral ridges which necessarily are present in the
transverse slice (so to call it) of a scyphistoma, which
becomes an ephyra (fig. 26). These rapidly increase in
number as the ephyra grows. Further, the enteric cavity
at first follows the outline of the ephyra, sending a process
into each arm, but then by adhesion of its walls is converted
into a four-lobed central chamber, a marginal canal, and an
endoderm lamella. A system of canals, the arrangement of
which is seen in figs. 29 and 31, subsequently opens out again
certain lines and tracts of the conjoined endoderm walls.

In the adult Aurelia we find the mouth surrounded by
four large arm-like perradial processes (figs. 25 and 29)
(not tentacles), and leading through a short manubrium
into a flattened four-lobed chamber, the lobes being inter-
radial, and having on their oral floor numerous gastral
filaments (rich in thread cells) (6 in fig. 16). Each pouch
or lobe gives off a canal, which runs towards the circular
canal at the margin of the disc, but breaks up into three or
four secondary canals on its way. Between the pouches
come off eight other " radiating " canals (adradial), which
do not branch, but go straight to the circular canal.

The oral lloor of the concavity of each lobe of the enteric
cavity is occupied by a horse-shoe-shaped frill (fig. 29, ov),
either testis or ovary (the sexes being in separate indi-
viduals). The open arms of the horse-shoe are turned
towards the centre of the disc, and the folds of the genital
frill are so deep as to show themselves on the outer ecto-
derrnal wall of the disc. Here, however, there is a very
remarkable arrangement, which has rarely, if ever, been
correctly described and figured in our common Aurelia.
The gelatinous substance of the disc is hollowed out on
that part of the oral face corresponding to the position of
the genital frills, so as to form four separate extensive pits
or chambers. Each of these sub-genital pits has in Aurelia
a small round opening on the oral face of the disc (fig. 28,
GP), but is otherwise entirely closed, having no com-
munication with the genital tissues, from which it is
separated by a delicate layer of ectoderm (6 in fig. 16).
The pits probably serve to admit water for respiratory pur-
poses into close proximity with the genital tissues.

The whole enteric surface, including canals, is ciliated,
whilst the ectoderm is not ciliated, but provided with
groups of nematocysts.

The tentaculocyst in the adult Aurelia is relatively an
extremely minute body, completely hidden by the two large
marginal lappets (fig. 30, T). Above it (that is, on the
aboral surface, as the Aurelia swims) is a deep pit (A),
Schafer's fovea nervosa superior, sunk in a sort of bridge
which connects the two lappets and overhangs the tenta-
culocyst. A similar pit (the fovea inferior) exists on the
oral surface. These have been recognized by Clans, Einier,
and the Hertwigs as olfactory organs. The tentaculocyst
is seen in section in fig. 30 (right-hand figure), which ex-
hibits its central cavity continuous with the enteric cavity,
its ectodermal pigment spot (eye), and its endodermal mass
of concretions (auditory organ).

The chief muscular mass of Aurelia, except that of the
oral arms, is a circular zone on the oral face of the disc.
The muscular fibres are not distinct cells, but transversely-
striated processes of the epidermic cells (epidermo-muscular
cells) (fig. 9). In the " arms " of other medusae, and pre-
sumably of Aurelia, the muscular fibre is formed by inde-
pendent nucleated cells (fig. 9).

The nerve-epithelium from the olfactory pits of Aurelia is
drawn in lig. 1 4. Starting from this and from the cells of
the teutaculocysts are nerve-fibres, which spread themselves
on the surface of the circular muscular zone in the neigh-
bourhood of the tentaculocysts, and these are connected each
and separately with large isolated nerve-ganglion cells (fig.
15). The nerve-fibre is continued beyond the cell, and
in some instances has been traced into a broadened ex-
pansion lying on a muscular fibre (Schiifer). The nerve-
ganglion cells lie very superficially immediately below the
Hat epithelium of the body surface and between it and its
muscular processes.

The ova and spermatozoa of Aurelia develop in the
genital frills from endoderm cells in separate individuals.
They pass to the exterior through the mouth.

Order 3. Conomedusce, — Scyphomedusce with only four
tentaculocysts, and these perradial. A broad velum (so-
called pseudo-velum) of complete circular form is present,
differing from that of the HyJromeJnst.i' in the fact that it
is penetrated by canals of the enteric system (Charybdcea).
The whole umbrella is bell-shaped. The genital organs are
four pairs of lamellifonn ridges (fig. 22) which are attached
to the four narrow interradial septa that divide the large
enteric cavity of the umbrella into four perradial gastro-
canal pouches. The lanielliform genital glands hang freely
in these pouches. At the edge of the umbrella are four
interradial lappet-like prolongations of the gelatinous sub-
stance of the disc, which support each a long tentacle (fig.
20). The nerve-ring is complete, like that of the Hijdro-
meduscv.

There is now no doubt that Charybdcea, which has been placed in
each of the two large divisions of the Hydrozoa, must be classed
with the Scyphomedusce. The recent investigations of Glaus
(Arbcitcn aim <A m Zool. Jnstititt :.u Il'icn, Bd. i. Hf't. ii., 1878), as
well as those of Haeckel and Fritz Miiller, lead to this conclusion.
The term Conomcdusic is adopted from Haeckel, who places here,
besides Charybdim and Tamoya, other forms, a fuller description of
which may be expected in his forthcoming System tier Meduscn. In
many respects — its quadrangular form, its marginal lappets, its
broad enteric pouches in place of fine canals, its vascular velum, and
its highly complicated tentaculocysts (fig. 13, ty—Cltni-yMaa
is peculiar. The simplicity of the enteric system and the arrange-
ment of the genital glands bring it near to Luccrnaria. The ex-
istence of four interradial groups of gastral filaments, and the dis-
position of the paired genital glands at the sides of the interradial
septa, determine its position to be among the Scyphomedusce. Its
development is not known. Figs. 20 to 23 illustrate the structure
of ChntrybdaBd,

Order 4. Peromedusae, — Seyphomedusce with four inter-
radial tentaculocysts. The enteric system consists of three
divisions, — an aboral main stomach with four interradial
gastral ridges and filament groups; a mid-stomach, which
communicates by means of four perradial slits with a very
large ring-sinus (occupying two-thirds of the umbrella) ; and
thirdly, an oral portion or pharynx, with four wide per-
radial pouches. The genital organs are four pairs of
sausage-shaped interradial ridges lying on the oral floor of
the ring-sinus.

This is a new group founded by Haeckel, of which we have at
present no further details.

Sub-class II. HYDROMEDUS.E. — These are Hydrozoa devoid
of gastral filaments ; the sexual persons are always medusi-
form, the genital glands are developed sometimes from ecto-
dermal cells, sometimes from endoderm, and are always per-
radial (in the radii of the first order). The medusiform per-
sons always possess a muscular non-vascular velum (hence
Craspedota) and a complete nerve-ring (hence Cycloneura of
Eimer). The marginal sense-organs are either ocelli or oto-
cysts or tentaculocysts. The diblastula, in all cases as yet
observed, is formed by delamination (Balfour). The sexual
medusiform persons may develop directly from the egg, but
more usually the etrg gives rise to a hydriform person— the
hydroid — which difl'ers from a scyphistoma in its elongate

70

HYDROZOA

vertical axis, the indefinite number (often also position) of
its tentacles, and its frequent formation of a colony of large
size by lateral budding. By lateral budding (not by

?IG. 32. — Diagram showing possible modifications of persons of a gymnoblastic
ffydromedusa. a, hydrocaulus (stem); 6, hydrorhiza (root); c, enteric cavity;
d, endoderm; c, ectoderm; f, perisarc (horny case); (/, hydranrh (hydriform
person) expanded; g', hydra'nth (hydriform person) contracted; h, hypostome,
bearing mouth at its extremity; A-, sacciform gonophor (sporosac) springing
from the hydrocaulus; k', sporosac springing from m, a modified hydriform
person (blas'tostyle): the genitalia are seen surrounding the spadix or manu-
brium; /, medusiform person or medusa; m, blastostyle. (Alter Allman.)

metameric fission) medusiform persons which alone develop
sexual glands are produced on the hydriform colonies;

FIG. 33. — Diagram showing possible modifications of the persons of a Calypto-
blastic HydromeduRa. Letters a to A same as in fig. 32. I, the horny cup or
hydrothe.ca of the hydriform persons; /. medusiform person springing from in,
a modified hydriform person (blastostyle); 71, the horny case or gonangittm
enclosing the blastostyle and its buds. This and the hydrotheca t give origin
to the name Calyptoblastea. (After Allnian.)

these may separate from the colony, or may be retained in
a more or less degenerate form adherent to it, as generative
bucls or warts.

The medusiform persons of this group are the DismphariK crypto-
carpcc of Eschscholtz, the Craspcdota of Gegenbaur (1854), and the
Hydromedusida of Kolliker (1853) — the last two authors at that
time separating the hydriform persons as Hydroidca. Louis
Agassiz (1860) includes both sets of persons under the term

Fig. 34.

Fig. 35.

PIG. 34. — Diagram of Corymorpha. A, a hydriform person giving rise to
medusiform persons by budding from the margin of the disc; B, free swim-
ming medusa (Steenstrupia of Forbes) detached from the same, with manu-
brial genitalia (Anthomedustp) and only one tentacle. (After Allmun.)

FIG. 35. — Diagram of Tubutaria irtilivisa. A single hydriform person a bearing a
stalk carrying numerous degenerate medusiform persons or sporosacs 6. (After
Allnian.)

Hijdroida (together with Lucernaria), which also is the term adopted
byAllnian in his beautiful monograph) 1871-74). J.V. Cams, amend-
ing the limitations given by Carl Vogt, was the first to use the term
Hydromcdusce in the sense here adopted (Handbuch der Zoologie,
1863), and it is now employed in the same sense by Gegenbaur
(Elements of Comparative Anatomy, London, 1878), namely, to em-
brace both the cryptocarpous medusae of Eschscholtz and the

FIG. 36.— Colony of Bougainvillea fivtieosa, natural size, attached to the
underside of a piece of floating timber. (After Allman.)

hydroids related to them. The term Hydromcdusos is used unwisely
by Claus (Gnindziigc d. Z.) for the whole group of Hydrozoa. _ It
has been the practice of some authors to give a double classification
of the group — one based on the characters of the medusiform per-
sons, the other on that of the hydriform persons. In the present
article a double name will in some cases be assigned to a group-
but the attempt is made to bring both sets of persons under one
system.

Order 1. GymnoUastea-Anthomedusof. — These are Hydro-
medusa?, which all, as far as is known, pass through a
hydriform phase, but in which the medusiform persons
may either reach full development or exhibit the extremest
degeneration (Hydra). The ectoderm of the hydriform

HYDROZOA

71

persons may secrete a horny tubular protective case
(perisarc), hut this does not form cups for the reception of
the tentacular crown nor cases enclosing groups of medusi-
form buds (gonangia). The fully-developed medusiform

FIG. 37. — Portion of colony of BougainriUea (Eiidendrhtu>)frtttico!;a (Anttiomeduscp-
catyptoblaslea) more magnified. (From Lubbnck, alter Aliman.)

persons never possess otocysts nor tentaculocysts, but always
ocelli at the base of the tentacles. The latter are usually
four or six, corresponding to the same number of simple
radial enteric canals, but may be more numerous or reduced
to one or to two ; rarely they are branched (Cladonema).

This is a very well defined group, since tlie GymnoWastea of
Allman, liased on the eh.-iraetcrs of the liydrifnrm ]iursous — also
known as Tiiliiilm-in- ;\m\ iti/mnotoka — correspond exactly with the
Antliniiii'iliimr of Ilaeekid'.s new system, llijdra is included here,
though placed in a separate order liy Aliman. Some of the leading
forms of hydriform and inedusiform persons are given in the cuts
(figs. 34 to 42). The greatest range in the amount of degenera-
tion of the medusiform persons is seen even in genera of the same
family — e.g. , Turris and Clava — the former producing free
medusse, the latter sessile .sporosacs. The (Jccanitlcc of Gegenliaur
(excluding the Williadit, which Haeekel assigns to the next group)
correspond on the whole to the medusa-forms of this order.

Fig. 41.

Fig. 42.

FIG. 41. — Hydriform person of Simcorime, witlimedusifnmi persons budding from
it. and shoun in various stages of development, a, 6, c, d, e. (From Gegenbaur,
after Desor.)

FIG. 42. — Hiidra riridis. ot», ovary; te, testis.

Order 2. Calyptoblastea-Leptomedmce. — These are Hydro-
of which the hydriform phase is known in a
large number of cases, whilst of others only the medusa-
forms are known ; none are known to develop directly from
the egg to the medusa-form. As in the preceding group,
the medusiform persons may reach full development or

Fig. 38.

Fig. 39.

Fig. 40.

FIG. 3S.— Diagram of Clara, showing a hydriform person surrounded by a
verticil of degenerate medusiform persons (sporosacs). (After Aliman.)

FIG. 39.— Diagram of a colony of JJijdraclinia, showing four forms of persons, a,
hydriform person ; 6, modified hydriform person,' or blastnstyle bearing r
degi-nerate medusiform persons or sporosacs; d, mndifled hydriform person
situated at the margin of the colony (rtactylozooid). (After Aliman )

FIG. 40.— Diagram of a colony of Diconjne, showing three forms of persons a,
normal hydriform person ; 6, modified bud-bearing hydriform person (blasto-
stylc); c, degenerate medusiform persons (sporosacs).' (After Aliman.)

The sexual glands are placed in the wall of the manubrium,
either equally distributed all round it or in four separate
perradial groups, which are often divided into eight ad-
radial groups by the perradial longitudinal muscles.

Fig. 43. Fig. 44.

FIG. 43. —Diagram of a colony of f'ampiinularia. showing four forms of per-
sons. A, portion of a fixed colony ; a. hydrifonn person ; fc, bud-bearing
hydrifoi-m person (blastostyle) ; B, flee -swimming colony, being a sexless
medusiform person (blastocheme of Aliman), with modified medusiform persons
budding from its radiating canals, as sporosacs. (After Allmim.)

FIG. 44. — Medusiform person (Li::ia), one of the Antliomeditsts, detached from a
hydroid colony of the fnmily iWi'WnW.r. ( icelli are seen at the base of the
tentacles, and two medusiform buds on the sides of the iu:inubrium. (After
Aliman.)

exhibit themselves as degenerate sexual sacs on the hydri-
form colonies. The ectoderm of the hydra-forms always
secretes a perisarc which forms a cup-like protection (hydro-
theca) to the tentacle-crown, and which also encloses the
group of medusa-buds in peculiar horny cases (gonangia),
The fully-developed medusiform persons (fig. 47) either

72

HYDKOZOA

have no otocysts, but only ocelli (Ocellatce), or they have
otocysts (fig. 11) (ectodermal sacs), four, eight, or over a
hundred, not homologous with tentacles, and sometimes in
addition ocelli ( Yesiculatce). The radial enteric canals are
usually four or eight in number, but may be more numerous,
whilst the marginal tentacles of the disc are either few or

a

Fig. 45. Fig. 4(3.

Fin. 45.— Medusiform person (Sarsia), one of the Anthomedusce, detached from a

hydroid colony of the family Cori/nitlie. I:, the long manubrium, bearing (as

an exception) medusiform bu.ls; a. mouth.
FIG. 46. — Medusiform person, nne of the Anthomediisfe, detached from a hydro.^

colony of Syncoryne. Ocelli are seen at the base of the tentacles, and also (as

an exception) groups of medusiform buds.

very numerous. The genital glands always are placed in
the course of the radial canals of the disc (not in the manu-
brium), and stand out as groups of wart-like processes on
the sub-umbrellar surface (fig. 43). Their mode of dis-
charge is uncertain.

FIG. 47. — View of the oral surface nf one of the Lrptomediitup (1,-,-nc pelliici<l,t,
Haeckel), to show the numerous tentacles and the otocysts. ge, genital glands;
M, mmmbrimn; ot, otocysts; re, the four radiating canals; V"f, the velum.

The (:'rili//itiili/nxti'ii of Allman, Skr.notoka of Cams, and Campanu-
Inrirr of authors form a well-marked group of hydroids which, when

according to Allman, gives rise to a i/m'n-likp medusa (Antlw-
inciliisa;), is the only recorded exception to this correspondence.
They4'i/Hori'rf«and other medusa> of similar structure have not been
traced into connexion with any hydriform trophosome, but we are
Hot justified therefore in concluding that they develop directly from
the egg without hydriform phase. The chief point distinguishing
the LeptomediisiB as a lot from the Anthomcdtmrr is the development
nf the generative bodies in the radial canals. This position is simi-
lar to that occupied by the same organs in Trui'lmMfiliiMc and
Sci/phomcduscc. Allman, however, considers the genital glands of
the L/'/i/i'i/K-i/iiwr, not as mere glands like those of Aurclia or
I'/mri/M.cca, but, as a series of buds— a generation of aborted
ine.lusM- or sporosacs. In consequence he terms the medusa of the
LeptomediiscEa.'bla.aioeh.eme (or bud-producer), as distinguished from
a gonocheme (or genital-producer). In support of this view,

AJlmaii (Monograph, 1874) adduces the various remarkable cases
of production of buds by medusge which have been recorded (fig.
44, 45, 46), and, further, the very striking similarity between the
structure of a lobe of the genital gland of Obelia and a sporosac
such as we find in Hydractinia. It seems necessary to accept
Allman's view on this matter, unless we are prepared to abandon
the homology of sporosacs with medusae in the case of hydriform
persons.

The colonies of hydriform persons of the present group differ inter
sc according to the arrangement of the cups or hydrothecas. In
Phnnularidce they are sessile, and all on one side of a branch ; in
K-i-liihtridcB they are sessile, and alternately placed on either side ; in
Cam/panularidcK each cup is raised on a pedicel or stalk. The
medusiform persons sometimes remain abortive and sessile in their
gonangia.

- h

FIG. 48. — Cai-j}iarin{i(Geryonia)lnist<jta,oneoftheTracliomedu5(?. (After Haeckel.)
a, nerve-ring; a', radial nerve ; 6, tcntaculocyst ; r, circular canal ; ^radiating
canal; g", ovary ; ft, pen.nia or cartilaginous process ascending from the carti-
laginous margin of the disc centripetally in the outer surface of the jelly-like
disc; six of these are perradial, six interradial. coi responding to the twelve solid
larval tenhicles, resembling those of Ciiniim; k\ dilatation (stomach) of the
manubrium; /, jelly of tlie disc; p, manubrium; /, tentacle (hollow and
tertiary, 2.e., preceded by six perradial and six interradial solid larval tentacles) ;
«, cartilaginous margin of the disc covered by tliread-cells ; r, velum.

Order 3. Trqckomedusce, — Ilydromedtiftz which have as
sense-organs tentaculocysts. The otoliths (fig. 12) are

FIG. 49. — Diagram of a vertical section of Cui-intiiiita hnstttta, passing on the,
right through the whole length of a radiating canal, and on the left through
the outspread lobe of an ovary. I, gelatinous substance of the disc and gjtstric
stalk (manubrium); r, radiating canal; rs, its outer, rl, its inner wall; g,
ovaries; A% stomach (dilatation of the manubrium); Z, tonglie-like process of
the gelatinous substance; /(, cartilaginous piocess ascendingfrom the marginal
ring at the sile of a tentaculocyst ; c, circular canal; ft, tentaculocyst; r,
velum; «£, carlilaginous marginal ling. (From Gegenbaur.)

formed by endodermic cells as in Sct/p/iomedtisce, and
ocelli may or may not be present on the tentaculocyst.

HYDROZOA

73

The genital glands have the form of wide outgrowths or
lamelliform enlargements in the course of the radial canals
(figs. 48, 49). No hydriform phase is known in any
member of this group, and one at least (Geryonia) has been
observed to develop from the egg directly into the medusa-
form.

Order 4. NarcomedusoE. — These have the same characters
as the TnichomediKce, excepting that the genital glands are
in the wall of the manubriuin or in pocket-like radial out-
growths thereof (figs. 50 and 51). Further, the marginal
tentacles of the disc possess peculiar " roots," which can be
traced upwards into the gelatinous substance of the body.
No hydriform phase has been observed in this group,
whilst sEfjina and ^ginopsis have been shown to develop
directly from the egg to the medusa-form.

FIG. 50. — Cunina rhododactyla, one of the Narcomeduste. c, circular canal; /i,
"otoporpffi" (ear-rivets) or centripetal process of the marginal cartilaginous
ring connected withtentaculocyst; £, stomach; I, jelly of the disc; /-, ruiluit-
ing canal (pouch of stomach); ^.tentacles; tie, tentacle root. (After HacckeL)
The lappets of the margin of the disc, separated by deep notches, above
which (nearer the aboral pole) the tentacles project from the disc (not mar-
ginal therefore), are characteristic of many Narcoiitedusie and Trachoiiiedttsce.
Cartilaginous strands (the mantle rivets or peroniae) connect the tentacle root
with the solid marginal ring.

The two orders Trackomedasce and Narcoincduscc are established
by Haeckel in his new " system " for the peculiar forms classed by
Cams as Haplomoi-pha, and by Allman as Monopsca. These latter
names have reference to the fact that no hydriform phase is known
to occur in the life-history of these organisms, a fact which is not
peculiar to them, and, if it should prove to be not universal amongst
them, would by no means invalidate their claim to a distinct posi-
tion on the grounds afforded by the characters above given. They
are remarkable for a certain hardness and stiffness of the gelatinous
substance of the disc, or at any rate of the cellular axis of the
tentacles, on accout of which the orders are contrasted by Haeckel
as Trachyliiicc with Anthomcdusce and Leptomcdusa:, which are

FIG. 51. — Diagram of a vertical section through a young Cunina rhoilodactyla,
passing on the right side through a radiating pouch. 6, tentaculocyst; <•,
circular canal ; g, ovary ; h, marginal cartilage and connecting process
springing from a tentaculocyst (ntoporpa) ; £, stomach; /, jelly of the disc;
r, radiating canal or pouch; tt, tentacle (solid, cartilaginous); tw, tentacle
root ; v, velum. (From Gegenbaur.)

termed Leplolince ; a curious parallelism as to the position of the
genitalia exists between Anthonudusw and Narcoincduscc on the
one hand and LeptofiiedustB and Trachomcdusce, on the other.
The orders present a very high degree of development, both in
coarser and histological differentiation. At one time it was sup-
posed, in accordance with Haeckel's observations, that Geryonia
(Carmarina, fig. 48), one of the Trachomcdusce, gave rise by buds
from its enteric walls to young Cunina; (ATarcomcduscc, fig. 50),
but this lias been explained by the observations of Franz Schulze
and of Uljanin as due to parasitism, ycmugCunince in the condition
of ciliated Planitlte entering the mouth and enteric chamber of the
Carmarina. The same explanation probably applies (Claus) to the
supposed internal buds of Cunina observed by Gegenbaur, Fritz
Miiller, and Metschnikow. The process is sufficiently remarkable
according to the last observer, for the first generation of buds pro-
duce a second generation by external gemmation, before attaining
the characters of the parent Cuniiut. The anatomy of these forms

is fully given in Haeckel's memoirs in the Jcnai.irhc Zeitschrift, vols.
i. ami ii., 1864-60 ; also further details as to Car marina are given in
Knurr's Mcdit.icii, 1878.

Order 5. Hydrocorallina'. — These are Hydromedmce in
which the hydriform phase forms large colcmies, presenting
a copious calcareous deposit
in the ectodermal tissue (coe-
nosteum of Moseley), leav-
ing only the hydranths or ten-
tacular region free from such
hardening. The tnedusiform
persons are, at present, only
known in the degenerate
form of sporosacs, which
occupy cavities (ampulke
of Moseley) in the har-
dened base of the colony
(Stylasteridce). No such
cavities have been detected FlG- ^•-Portion of the calcareous

. . , ,... • T \ 1-1 corallum of Mmepora nodosa, show-

in others (Mllleporidai), which ing the cyclical arrangement of the

nnv triprpfnrp m'vp ri«p tr> pores occupied by the "persons" or
may, inere[0ie, give rise to hydranths. Twice the natural size.

complete niedusiform persons. (From Moseley.)
In all a marked polymorphism has been observed (fig. 53),
consisting in the differentiation of longer tentacle-like
persons (dactylozooids) and shorter mouth-bearing persons
(gastrozooids). The persons of both kinds are either
scattered irregularly or the dactylozooids are arranged
around the gastrozooids in cyclosystems of greater or less
definiteness, or in distinct rows (fig. 55). The position 01
these two kinds of hydriform persons is marked by definite
groups of pits (cyclosystems) in the dried calcareous skeleton
of the colonies, which simulate the calycles of the stony
corals (Anthozoa).

FIG. 53. — Enlarged view of the surface of a living itilltpora, showing five
dactylozooids surrounding a central gastrozooid. (From Moseley.)

Louis Agassiz was the first to recognize the true nature of the
Millcporidcc, and his imperfect observations have been fully con-
firmed and greatly extended by Mr Moseley (Phil. Trans., 1878) who
added the Stylasteridos previously regarded as Anthozoa to the
category of calcigenous hydroids, and founded the order of
HydrocoralUnae. The Stylasterida: differ from the Millcporidcc in
possessing a calcified axial style at the base of the dilated portion of
each gastrozooid, and further in the ascertained development of
sporosacs, and in the greater complication of their cyclosystems.
These forms are abundant in tropical seas, and contribute with the
Anthozoa and Corallines to the formation of coral reefs. Allopora
and Slylaslcr occur off the Norwegian coast. The woodcuts illus-
trating* the structure of this group are borrowed from Mr Moseley 's
Notes of a Naturalist on the " Challenger."

The nearest allies of the Jfydrocorallina: are such polymorphic
Gymnoblastcii as Hi/.lni,iiin:i (lig. 39) ; the definite division of labour
and the polymorphism in the former, together with their calci-
genous peculiarity, entitle them to rank as a distinct onler.

K

74

HYDROZOA

Order 6. Siphoiiopftora. — These are II ydromedusce in
which hydriform persons alone (Velelhi) or hydriform
persons and sterile medusiform persons are united, under
many special modifications of form, to constitute floating
colonies of very definite shape and constitution. In
addition to these are developed medusiform sexual persons
which usually are spurosacs and
only exceptionally attain full de-
velopment so as to be liberated
from the colony as free-swimming
medusa; (Velella, as Chrysvmitra;
Physalia, only liberating female
medusce). The medusiform persons,
where sufficiently developed, exhibit
the velum characteristic of Hydro-
mechtsie; the larger mouth-bearing
nydriform persons, which are some-
times the only representatives of
their kind, are remarkable for
differentiation into four regions, —
a proboscis, a stomach, a basal ring,
and a short stalk on 'which the
single tentacle of great length is
situated (fig. 56, /). In the sub-
order Physophoridce (fig. 57, C) the
persons are united by a short or
long and spiral stem, terminated
at one end by a flask-like air-sac

(pneumatocyst); below the air-sac a FlG 54 _Portion of tl,conral.
bisenal or multiserial range of swim-
ming-bells (nectocalyces = medusee
with suppression of manubriuui,
tentacles, and sense-orgaus) are
placed. Covering pieces (hydro-
phyllia, reduced medusas) and dactylozooids are affixed
to the succeeding region of the stem, and alternate in
definite order with the mouth-bearing hydriform persons
(polyps or nutritive persons) and generative medusiform
persons. In the sub-order Physalidtx the stem is con-
verted into an air-sac, enormously enlarged, and the necto-

In in Df Astylits subviri'.lit
(one of the Stiilasta •/././ 1,
showing cyclosyst.unis placed
at intervals on the branches,
each with a central jj;astro-
pory and zone of slit-like dac-
tyloporcs. (After Moseley.)

Ftfj. 55. — Diagrams illustrating the .successive stages in the development of the
cyclnsysteins nf thr .S7v/".^c/-('Ar. 1, .S/""'"1'"/""'" <tic/i»t»/it,i. L', ::, J//H/'NJV/
tiobilis. 4, Allofxini in'vfinidft. 5, Alliyo/'d ini:ini>'i n. li, Astyliu tubviridis.
7, Ditlichopont COCCiliea. -s st\lr; <!/>. lUirtylupnre ; <//'. urastr<ipol-e; 6,illfig.(J.

inner horseshoe-shaped month of gastropore. (After Must-ley.)

calyces and hydrophyllia are absent. In the sub-order
Cafycophoridce the air-sac is not developed, the nectocalyces
are in a biserial group, or reduced to two or to one.
Dactylozooids are wanting. The modified persons (append-
ages, Huxley) arise from the stem in groups, and can be
withdrawn into the cavity of a swimming-bell (fig. 57, B).

Each group consists of a nutritive person, with long ten-
tacle, of generative medusoids, and usually also an umbrella-
shaped or funnel-like covering piece. The latter separate
in some Diphyida; and lead an independent life as
Eudoxice.

In the suborder Discoidic the stem is converted into a
flattened disc with a system of caualicular cavities. Above
this lies the air sac, a flattened reservoir of cartilaginous
consistence. The hydriform persons depend from the disc,
centrally a large nutritive person surrounded by smaller
similar' persons carrying at their bases the generative
medusoids ; near the edge of the disc are dactylozooids.
The medusoids develop into complete medusiform persons,
and develop the genital products after liberation from the
colonj, when they are known as Chrysomitra.

FIG. 5f>. — Diagram showing possible modifications of medusiform and hydri.
form persons of a colony of Siphonophoro. n, pneumatocyst; k, necto.
calyces (swimming bells); /, hydiupliyllium (covering-piece); /, generative,
medusiform person; p, dsctylozooid with attached tentacle, k ; e, nutritive
hydriform pcrsnn, with branched grappling tentacle, /; wt, stem. The thick
black line represents endoderm, the thinner line ectoderm. (After Allman.)

The Sijilioiwpfwra alone, amongst the colonies formed by Hijdrozoa,
exhibit a high degree of division of labour and consequent individua-
tinn. The mode of origin of such colonies has been discussed above.
The locomotive habit, as contrasted with the sessile habit of other
colonies, is no doubt correlated with the sharply denned individuality
\\liii-h they attain (compare Cristatdla among Poli/zoa). VdeUa
and Pliiimiliu are occasionally seen on the southern and western
shores of England, but as a rule tin- SiphonopJiora are met with only
in the open oeean and in the Mediterranean. By some authorities
the Siphonopkora are assigned n distinct position among the Hydro-
zoa, side by side with the l[ijdromcdusa'-o.m\ Scyphomcdusce ; their
interpretation as floating colonies of Hijdromednsa', an interpre-
tation necessitated by the structure of their medusiform persons,
forbids their separation from that group.

FOSSIL HYDKOZOA. — Tlicresearuhesof Moseley have neces-
sitated a redistribution of the group of Aitthosoa known as
the Tubulate. Among these appear to be a few llydro-
coralliitti', which occur in the fossil state. The Paheozoic
forms known as graptolites are by some authors assigned
to the IfydroMt, but the grounds for placing them in this
position are very slight, owing to the imperfect nature of
the remains. A discussion of the small amount of structure
which they present would be out of place here.

Remarkable ScyjihomeJiisw have been obtained from the
Rolcnhofen slatus (Jurassic); excepting these, no noteworthy
extinct Hydruiua are known (see Haeckel in Zeitsch. iviss.

HYDROZOA

75

Zool., vols. xv., xix., and Jennische Zeitsch., vol. viii.,
1874).

Relationship of the Ctenophora to the HyJrosoa. — The
remarkable medusa-form recently described by Haeckel
(Sitssungsber. Jenaische Gesel/sch., 1878) as Ctenaria ctena-
phora, and classed by him amongst the Anthomedusce, seems
to furnish a very direct transition from the structure of a
medusa to that of such a ctenophor as Cydippe (Pleuro-

FIG 57. — Floating colonies of Siphonophora. A, Diphyes campanulata B, A
group of appendages from the stem of the same Diphyes, C, Physopliora
hydrostatica. D, Separate nectocalyx of the same. E, Cluster of female
sporosacs (aborted medusa?) of Agalina sarsii. a, stem or axis of the colony;
a', pneumatocyst (air-bladder); m, nectocalyx; c, sub-umbrcllar cavitv of
nectocalyx; r, radiating canals of the umbrella of the nectocalyx: o, orifice
formed by the margin of the umbrella; (, hydrophyllia in B, dactylozooids in
C; n, stomach; i, tentacles; g, sporosacs. (From Gegenhaur.)

brachia). The woodcut and appended explanation (fig. 58)
copied from Haeckel's memoir will render the relations
of the two forms clear. Ctenaria has the margin of its

disc narrowed so ns to give the organism a spherical form.
The approximated margins bound an orifice leading to the
Bab-umbrella space. This orifice corresponds to the so-
called mouth of a Cydippe. Further, Ctnuu'iu has two,
and only two, long-fringed tentacles, like those of Cydippe,
and each springing from a pocket as in that genus, nnd
on the surface of its spheroidal umbrella eight rows of
differentiated ectodermal cells, which though not ciliated

FIG. 58. — Ctenaria Ctenophora (Haeckel), one of the Antliomeduscp, connecting
thut group with the Ctt'iwphora. A, lateral view of the entire medusa ; B, two
horizontal views, that to the left representing the surface of the abural hemi-
sphere, that to the right a section pnssing nearly equatorially. «, Hie eight
(ciliated?) rows of thread-cells, adradial in position, and corresponding to the
eight ctenophoral zones of Plcurobracliia; {>, jelly of the umbrella; c, circular
muscle of the sub-umbrella ; rf, longitudinal muscles of the sub-umbrella; e,
stomachal dilatation of the enteric cavity; /, the sixteen oral tentacles; g, the
four perradial generative glands in the stomneh wall (roannbrrnm) ; ft, the
four perradial primary radiating canals; ?, the eight adradial bifurcations of
the preceding; k, ring canal in the margin of the umbrella; /, velum; m, the
two lateral tentacle pouches ; n, the two lateral unilaterally fringed tentacles ;
o, the apical cavity (infundibulum) above the stomach. The canal system,
with its four primary and eight secondary rami agrees in Clcnaria and Plturo-
lirachia. The mouth of the latter is homologous with the margin of the
umbrella of the former. The mouth of Ct'iifnia is homologous with the
junction of the so-called funnel of l'l>'iir:>l'r<i<'liin with its so-called digestive
cavity. This last is the homologue of the snb-umbrellar cavity of Ctenaria.
The apical opening or openings of the funnel of Cti'nopliora is paralleled .by
the stalk canal of medusffi, whilst thy agreement between the tentacles and
their pouches in Ctenaria and Plcurobrachia is complete,

correspond closely in position with the eight ctenophoral
ambulacra of Cydippe. The disposition of the enteric canal-
system of Ctenaria is, as shown in the cut, also transitional
in the direction of Cydippe. Apart from the existence of
Ctenaria, the homologies suggested by Haeckel between
IlydromeduscK. and Ctenophora are such as to commend
themselves very stronglv to acceptance (E. E. L.)

PLANARIANS

(By Prof. Luduriy -von Graff, University of Graz, Austria.)

rPHE name Planaria was first applied by O. F. Miiller
in his Frodromus Zoologix Danicx (1776) to a
group of worms, inhabitants of fresh and salt water,
characterized, so far as was then known, by a flattened
leaf-like form. Ehrenberg in 1831 changed this name to
Tvrbellaria on account of the cilia with which the body is
furnished, by means of which the worms create a whirl-
pool in the surrounding water. The extent of this
group was subsequently more restricted, and at present
the name Twlellaria is applied to all those (mainly
free-swimming) Platyhelminths whose body is clothed exter-
nally with a ciliated epidermis (fig. 9), and which possess
a mouth and (with the exception of one division) an
alimentary canal, but are without an anus. The Tur-
bellarians, excluding the NEMEETINES (q.v.), which until
recently were classed with them, form an order of the
class Platyhelminthes, and the old name Planaria is now
confined to a group of the freshwater representatives of
this order.

Size and External Characters. — Many forms of the
Turbellarians are so minute as to be hardly visible with
the naked eye, while others attain to a length of several
inches, and a land Planarian of no less than 9 inches in
length has been described by Moseley. The freshwater
forms are generally small, the largest representatives
of the order being marine or terrestrial. The smaller
species are mostly cylindrical, or convex dorsally and
flat ventrally ; the anterior extremity is commonly trun-
cated and the posterior extremity pointed (fig. 1, a, I).
The larger aquatic forms are thinner in proportion to
the increasing surface of the body, so that they come to
resemble thin leaf-like lameVJaa (d), while the large land
I'lanariana instead of increasing in superficies grow in
length (e and /), so that they may be best compared to
leeches. The larger aquatic forms are frequently provided
with tentacles in the shape of paired finger-like processes
or ear-like folds of the anterior part of the body (d and
g) ; sometimes the tentacles are papillary outgrowths of
the dorsal surface ; the land Planarians are often to be
distinguished by a crescent-shaped area at the fore end
of the body, which is separated off from the rest (/').
In many cases the whole dorsal surface is beset with
papilUe (d). The aperture of the month varies greatly
in its position ; sometimes it is situated at the anterior
extremity, sometimes in the middle of the ventral surface
of the body, occasionally quite close to the posterior ex-
tremity ; the single common or distinct male and female
generative apertures are also situated upon the ventral
surface of the body, and the former in rare cases open in
common with the mouth ; the genital apertures always lie
behind the mouth. Many Turbellarians have a sucker
which serves to attach the animal to surrounding objects,
or to another individual during copulation.

Intiyitmcnt. — The integument is composed of a single
layer of ciliated epithelium ; between the cilia there are
often long flagella and stiff tactile hairs and even (in a
single instance) chitinous spines ; these latter must be
regarded as local thicken-
ings of the firm cuticle
which covers the epi-
dermic cells. The epi-
dermic cells are flat or
columnar, and are united
to each other by smooth
opposed margins or by
denticulate processes
which fit into similar
processes in the adjacent
cells (fig. 2). Sometimes
the epidermic cells are
separated by an inter-
stitial nucleated tissue.
The structure and func-
tions of the cells of the
epidermis differ, and four
varieties are to be found :
— (a) indifferent ciliated
cells ; (I) cells containing
certain definite structures
(rhabdites, nematocysts) ;
(c) gland cells ; and (>!)
glutinous cells (Kleb-
zellen). The rhabdites
are refracting homogene-
ous rod-like bodies, of a
firm consistency, which
are met with in most

ph.

,

, M. Sch.: <-, Mmetus fuscus,
Off.; d, Tliysanozoon trochii, Gr., with
elevated anterior extremity (after Joh.
Schmidt); e, Rhynchodemus terrestrit, O.
F. Miiller (after Kennel); /, Bipalium
ceres, Mos. (after Hoseley) ; ;/, I'uiy.-i Us
cornuta, O. Sch., attached by the iilum-nx
(pA)to a dead worm (after Johnson). All
the figures of natural size, and viewed

Turbellaria, and often f™m the dorsal surface,
fill all the cells of the epidermis ; they are not always found
entirely within the cells, but the extremity often projects
freely on to the exterior of the body. They are readily
extruded from the cells by pressure, and are often found in
great abundance in the mucus secreted by the glandular
cells (many Turbellarians, like snails, deposit threads of
mucus along their track) ; in this case the epidermic cells
become perforated like a sieve. In many Turbellarians the
rhabdites are chiefly massed in the anterior part of the
budy; frequently there are several varieties of rhabdites
in one and the same species, — some being pointed at
both ends, others cylindrical with truncated extremities.
These structures are either formed directly in the ordinary
epidermis cells as a kind of secreted product of the cell, or
in .special formative cells which lie beneath the integument
and are connected with the epidermis cells by protoplasmic
filaments, by means of which the rhabdites reach the surface
of the body. These cells must be regarded as epidermic

78

PLANARIANS

cells which have become disconnected with the epidermis
itself, and wandered into the subjacent parenchyma. The
function of the rhabdites seems to be to support the
tactile sense. In rare instances nematocysts are present
which in structure and development entirely resemble
those of the Culentera (see vol. xii. p. 550). Very com-
monly structures known as pseudo-rhabdites are present;
these have a rod-like form, but instead of being homo-
geneous are finely granular ; they are an intermediate step
between the rhabdites proper and a granulated secretion
occasionally thrown off by the gland cells. The unicellu-
lar glands are either situated among the epidermic cells or
in the parenchyma, in which case they are connected with
the exterior only by the excretory duct. A peculiar modi-
fication of the epidermic cells are the so-called " glutinous
cells," which occur on the ventral surface or at the hinder
end of the body of many Turbellarians, and compensate
for the suckers ; the surface of these cells is furnished
with numerous minute processes by means of which and a
sticky secretion the animals can attach themselves to sur-
rounding objects. Sometimes the epidermic cells contain
calcareous concretions, and very commonly pigment is
found either in the cells themselves or within the inter-
stitial tissue. The colours of Turbellarians are, however,
not always due to the pigment of the epidermis but to
pigment contained in the parenchyma. Beneath the
epidermis is a basement membrane (fig. 2, bm) which is in

FIG. 2. — Integument of Mesostoma linqua, O. Sell. On the right hand is the
epidermis (2) with perforations (/) through which the rhabdites (si) project.
Beneath this the basement membrane (bin), and beneath this again the muscular
layers consisting of circular (/•/»), diagonal (sm), unu longitudinal (Im) fibres.

some cases very delicate and structureless, and in other
cases much thicker and enclosing branched cells ; this
membrane is attached more firmly to the subjacent tissue
than to the epidermis. Since this tissue is the strongest in
the body, and serves as a surface of attachment for the
muscles, it has been termed by Lang a skeletal membrane.
The third section of the integument is formed by the
muscular layers. These form a continuous covering to
the rest of the body, but their arrangement and thickness
are very different in different forms. In the smaller species
(RJiabdocoelida) there are two layers, an outer circular and
an inner longitudinal, only in a few cases the circular layer
is external to the longitudinal ; sometimes there are three
distinct layers, as in fig 2, where a diagonal layer is inter-
posed. The larger forms (Dendrocoelida) have a much
more complicated muscular system : in the most differen-
tiated forms there are six separate layers (two circular,
two diagonal, and two longitudinal), which are, however,
always less developed upon the dorsal than upon the
ventral surface in that the thickest layer of the ventral
surface (the innermost longitudinal) is absent or very
feebly developed upon the dorsal side. Besides the

integumentary muscular system, there are also found dorso-
ventral muscular bands which traverse the whole body
from the dorsal to the ventral basement membrane, being
branched at both extremities, and the special muscles of
the pharynx, genital organs, and suckers.

The perivisceral cavity, bounded by the integument and
traversed by the dorso-ventral muscles, contains the
organs of the body — alimentary canal, excretory system,
nervous system, and genital glands. The space left
between these organs is filled with parenchyma ; the latter
varies much in appearance and is very difficult to study.
Generally it consists of a network of fibres and trabeculaj,
which contain nuclei, and between which is a system of
cavities filled during life with the perivisceral fluid. These
cavities are generally but few in number and vary with
the stronger or feebler development of the reticulum ;
they occasionally contain free cells.

Alimentary Canal. — All Turbellarians are furnished
with a mouth, which, as there is no anus, serves both
to take in nutriment and expel the undigested remains
of food. The alimentary canal consists of a muscular
pharynx and an intestine. The pharynx (figs 3, 5 to 8, ph)
is cylindrical in form, rather complicated in structure, and
surrounded by a muscular sheath, which opens on to the
exterior by the mouth (in). Often the pharynx consists
merely of a circular fold lying within the pharyngeal
pouch (fig. 8) ; it can be protruded through the mouth
and acts like a sucker, so that the animal can fasten itself
upon its prey and draw it into the intestine by suction.
At the junction of the pharynx with the intestine open the
salivary glands, which are frequently large and well-
developed (fig. 5, s). The intestine (i) has a very
characteristic form in the different sections, and has long
served to divide the Tiirbellaria into two groups: — (1)
Rhabdoccelida, with a straight unbranched intestine (figs. 5,
6), and (2) Dendroccelida, with a branched intestine (figs. 7,
8). In the latter group Lang has recently called attention
to further differences that exist in the form of the intestine :
in the Tricladida (fig. 7) there is no central "stomach,"
but three equally-sized intestinal branches (which have
secondary ramifications) unite together to open into the
pharynx ; in the second group, the Polydadida (fig. 8),
there is a median stomach (st), from which numerous
intestinal branches arise ; this stomach communicates
directly with the pharynx ; the branches of the intestine
are much ramified and often form an anastomosing net-
work. The epithelium of the intestine is a single layer of
cells generally not ciliated, capable of protruding amoeboid
processes by which the food is absorbed ; the digestion of
these animals is intracellular. Sometimes a muscular
coat surrounds the intestine, the lumen of which is thus
capable of being totally or partially contracted. To the
above-mentioned divisions of the group, distinguished
from each other by the varying form of the alimentary
tract, another has been added, viz., the Accrta (Ulianin),
which are characterized by the entire absence of any
intestine. In these forms (fig. 4) the mouth leads directly
into the parenchyma of the body by a short tube which is
merely an invagination of the integument ; the paren-
chyma is a syncytium, consisting of a soft protoplasmic
mass with scattered nuclei, which represents the elements
of the intestine and the body parenchyma (ento- and
mesoderm) completely fused and without any traces of
differentiation. This fact, as well as the disappearance of
a nervous and excretory system, reduces the Accel u to the
lowest position not only among the Tiirbellaria, but among
the whole group of the Vermes.

Ej-<-r<lnr// System. — The excretory system of the Turbel-
larians is quite similar to that of the Trematodes and
Cestoids; it consists of (1) the main trunks with their

PLANARIANS

79

external aperture, (2) the secondary branches of these, and
(3) the excretory cells with the line tubules leading from
them. Karely is there but a single main excretory trunk
present opening at the hinder end of the body (.SV, im
stoma); generally there are a pair of such trunks which
open in common at the hinder end of the body, or
separately (most l{li<ili,l<u;da), or by the mouth (fig. 3).
In the Tridndiili there are two or
four lateral trunks present which
open by a number of pores arranged
in pairs upon the dorsal surface of
the body ; the .same appears to be
the case in the Polydadida. The
main trunks of the excretory sys-
tem are generally much twisted in
their course, and anastomose with
each other ; they receive the fine
tubules either directly or, as in the
Rhabdocosla, there is a network of
secondary tubules interposed. The
excretory cells are pear-shaped ;
they are branched and furnished
with a nucleus and a large vacuole
which is directly continuous with
the lumen of the tubule ; from the
boundary wall of the vacuole springs
a single flagellum, which depends
into the lumen of the tubule and is
capable of active movement. Lang
discovered in a marine form of the

Tri'-ladida (Gltiulu) similar vacUO- FIG. 3.— Main trunks of the ex-
1,1 ii -.1 « I n ,1 crctory system of Itfesostoi/ia

lated cells with a single flagellum ehmiberja, o. Sen. open

among the epithelial cells of the °"c £„«£ <",.«'
intestine, and came to the conclu-
sion that the excretory cells were on that account derived
from the epithelium of the intestine. The movements of
the excretory fluid towards the external pore are directed
by this flagellum as well as by cilia developed upon the
walls of the fine tubules ; the motion of all these cilia is
such as to drive the contents of the tubules towards the
excretory pore. The main trunks of the excretory system
are either sparsely (Tridadida according to Jijima) or com-
pletely (Polydadida. according to Lang) lined with cilia.

Xci't'ous System. — The central organ of the nervous
system, the brain (en), is a double ganglion at the anterior
end of the body, and has been noticed in all the known
forms with the exception of the Acida. It is situated in
front of or above the pharynx ; in those species in which
a process of the intestine extends beyond the region of the
brain ((/. figs. 7 and 8 viewed from the ventral surface) it is
placed below this. In such cases there is sometimes a com-
missure encircling the prolongations of the intestine. Each
of the two ganglia gives off a strong longitudinal nerve
cord (figs. 5-8, In) from which arise branches going to the
various organs of the body. The structure of the nervous
system is somewhat different in the E/tabdoc<*la, Trida-
dida, and Polydadida. In the first group (figs. 5, 6) the
two longitudinal cords and their branches are the most
feebly developed, and there is but rarely (Mesostoma,
Mo/wtus) a transverse commissure uniting the longitudinal
cords. These cords are very large in the Tridadidn,
where the brain is to be regarded as a simple thickening of
them ; in this group there are numerous transverse com-
missures between the longitudinal nerve cords (fig. 7), and
the nerves arising from them and passing to the periphery
form a subcutaneous nerve plexus within the muscular
coat. Lang has observed a similar nerve plexus in the
Polydadida, the central nervous system of which differs
from that of the Tridadida in that a number of stout
nerve cords radiate outwards from the brain as well as the

two longitudinal cords; they arc all united together by
-m

-y

Fig. 4. Fig. 5.

FIG. 4.— Plan of an Accelous Turbellarian. e, eye; »i, mouth; ol, otolith; ov,
ovary; p, digesling parenchyma ; t, testicular follicles ; cs, vesicula seminalis
J , m;ile organ of copulation ; $ 9 , common sexual aperture.

FIG. 5. — Plan of a i!habdocii.'lou.s Turbellarian. 6c, bursa copulatrix ; m, brain ;
c, eye ; (j, germarium ; i, intestine ; fit, longitudinal nerve trunk ; m, mouth ;
;</;, pharynx; rs, reccptaculum scminis ; s, salivary gland; /, testis; w, uterus
(containing an egg); r, yelk gland; us, vesicula semimilis; J, chitinous
copulatory organ; 3 9 • common sexual aperture; be, bursa copulatrix.

numerous commissures, and a network is thus formed
which extends throughout the body.

tec

Fig. 6. Fig. 7.

FIG. 6.— Plan of an Alloioccelous Turbellarian. Lettering as in fig. 5.
FIG. 7. — Plan of a Tricladid. *',, anterior, and i.2, t3, paired posterior branches of

intestine; ad, oviduct; it, tentacle; rd, \as dcferens ; $ .male, and 9, female

copulatory organ. Other letters as in fig. 5.

Sense Organs. — These are represented by tactile organs,

80

PLANARIANS

auditory organs (otoliths), and eyes. The whole surface of
the body is very sensitive and (e.ff., in the Polydadida) con-
tains cells which end in tufts of fine hairs, so that certain
regions thus become specially sensitive and serve as tactile
organs. The anterior pointed extremity of the body in
the Rhabdocaila is characterized by an abundant develop-
ment of rhabdites and tactile hairs, and thus becomes a
special tactile organ ; in other cases this region of the body
is transformed into a conical tactile proboscis which can be
retracted into a sheath (Proboscida). In the freshwater
Tridadida the anterior margin of the head is richly inner-
vated, and is beset with a special row of tactile cells which
contain no rhabdites ; in the terrestrial forms of the same
family (Bipalium) Moseley has described a row of papillte
along the crescent-shaped anterior extremity which can be

ov-

extended and form tactile organs ; between the papilke are
peculiar ciliated grooves connected with nerves. In the
Piilydndidn there are tactile cells with stiff hair-like pro-
cesses on the summit of the dorsal papilla; and the various
tentacular structures ; the tentacles in this family also
serve to support the eyes.

The majority of the Turbellarians possess eyes ; the
Rhabdocoelida commonly have two or four, as also have the
Tridndidn ; the latter, however, are in some instances
furnished with a greater number arranged in a continuous
row round the anterior end of the body ; in the Poly-
cladida there are from fourteen to several hundred eyes
arranged in two symmetrical groups round the brain or

scattered over the whole of the anterior margin of the
body and upon the tentacles. The eyes are always situ
ated beneath the integument within the parenchyma,
sometimes directly upon the brain or connected with it by
special optic nerves. In its simplest form the eye is a
pigmented spot with or without a refractory lens-like
body ; the more complicated eyes consist of a pigmented
sheath containing a number of refracting rods which are
connected at their outer extremity with a series of retinal
cells, one to each rod ; the retinal cells are prolonged into
a nerve thread running to the brain ; the arrangement of
the visual elements is therefore precisely the same as in the
vertebrate eye. Of great interest is the fact that in the
Polycladida the number of eyes increases with the growth
of the animal, and Lang has shown that the eyes increase
in number by actual division. On the other hand Carriere
has discovered by experimenting with certain freshwater
Tridadida that the compound eyes (those containing a
number of rods) are formed by the coalescence of several
simple eyes. Only a single eye is found in the Monotida,
which has the form of a simple pigment spot in front of
the otolith.

Auditory organs are found in the shape of vesicles filled
with fluid and containing circular lenticular or spindle-
shaped otoliths formed of carbonate of lime. Otolithic
vesicles of this kind are found in many Rhabdocoelida
(Acoela, Monotida, fig. 4, ot) embedded in a depression on
the anterior surface of the brain. In the Dendrocoelida
these organs are but rarely present.

As a sensory organ of unknown function must be men-
tioned the paired lateral ciliated grooves which are met
with on either side of the brain in many Rhabdoccela (fig.
9, c) ; they are also found commonly in NEMERTINES (q. v.),
but are here more complicated in structure.

Reproductive Organs. — With a few exceptions all the
Turbellarians are hermaphrodite, and reproduce themselves
sexually. Only among the Microstomida is there an
asexual as well as a sexual reproduction. The male and
female organs open to the exterior, either through a com-
mon cloaca (atrium genitale) on the ventral surface (most
Rhabdocoelida and all Tridadida, figs. 4-7), or there are
separate male and female apertures. In this case the male
aperture is generally placed in front of the female aperture
(some Rhabdocoelida and all Polycladida, fig. 8), but
occasionally the positions are reversed (certain Rhabdo-
coelida). The genital glands display a primitive condition
in being paired, though frequently the gerrnarium (fig. 5,
</) of the Rhabdoceela, and occasionally also the testis, is
developed only upon one side of the body.

The structure of the female organs varies. In some
cases there are simple ovaries (ov in figs. 4, 8) in which
the ova originate and become fully mature without being
furnished with the secretion of a second gland ; in other
cases there is a division into germarium (fig. 5-7, y) and
yelk gland (c) ; the primordial ova or germs originate in
the former, and absorb the products of the yelk gland in
the atrium, where they become ready for fertilization.
An intermediate condition is seen in those forms where
there is but a simple gland present which produces germs
in its upper portion and yelk in the lower portion. The
ovaries are generally compact round or tubular glands
(fig. 4) ; sometimes they are formed of a number of pear-
shaped follicles (fig. 8) ; there is usually a simple or paired
uterus (u) which retains the ova for some time before
they are deposited ; sometimes, however, the ova undergo
their development within the uterus and are completely
developed before expulsion ; in some cases the egg-shell is
detached within the uterus so that the young are produced
alive.

In Turbellariuns without a yelk gland the uterus is a

PLANARIANS

81

simple widening of the oviduct (fig. 8) ; in those forms
which possess additional yelk glands the uterus is a simple
or paired cliverticulum of the atrium genitale (figs. 5, 7).
The ova are either surrounded by a more or less hard
chitinous shell, or one shell contains a number of ova
("cocoon" of Trtrfiii/ii/ii and many Polycladida). The
Polycladida deposit an egg-string which like that of the
(liistropodtt consists of a number of eggs bound together
by a transparent albumen-like mass. Many Rhabdoccel
Turbellarians (e.g., Mesostoma ehrenbergii) produce two
sorts of ova, thin-shelled summer ova and thick-shelled
winter ova ; the latter are capable of withstanding a
considerable amount of dessication, and are deposited in
the autumn. The accessory female organs of reproduction
are represented by bursze seminales, which receive the
semen during copulation and retain it until fertilization
is accomplished. A further division of labour is brought
about by the presence of two diverticula of the atrium
genitale, one of which serves as a bursa copulatrix (fig. 5,
be) and the other as a receptaculum seminis (>•«) in the
same sense as the equivalent organs of insect?. In the
place of a special receptaculum seminis the efferent duct
of the ovary is often (Me&ostomida) metamorphosed into a
chamber to contain the semen. In the Tridadida and
Polycladida the female efferent duct is often differentiated
into a muscular vagina which closely resembles the penis
(figs. 7, 8, $ ).

Finally, the female generative apparatus is furnished
with a number of glands which have been termed cement
glands, albuminiparous glands, and shell glands.

The male sexual glands (figs. 4-8, t) resemble the ovaries
in being either compact tubular (fig. 5) or follicular
(figs. 4, 6, 7, 8) structures. The vasa deferentia (vd) are
often widened out into vesicute seminales (figs. 4, 6, rs) ;
or there are special vesiculse seminales present, formed by
a portion of the penis (fig. 5, i's). In the male organ of
copulation there is frequently found in addition to the
spermatozoa an accessory granulated secretion produced
by special glands, but of unknown function.

The muscular penis, especially in the Skabdocoelct, has
a number of chitinous spines and hooks which serve to
assist the animal in maintaining a firm hold during
copulation, but also in capturing and retaining its prey.
In Macrorhynchus helgolandmts, Gff., there is a peculiar
poison dart connected with the male copulatory organ
which only serves the latter purpose. Very remarkable
is the opening of the penis into the mouth cavity in
Stylostomum (Polycladida) and Prorhynchiis (RJmbdocasla),
and also the existence of several (2-15) pairs of male
copulatory organs and genital apertures in certain Poly-
cladida.

The spermatozoa vary much in form, especially in the
Rhabdoc&lida, where frequently the species of one and the
same genus are distinguished by the different form of the
spermatozoa. Copulation in the Turbellarians is generally
reciprocal ; only in those cases where both summer and
winter ova (see above) are formed do the former arise from
self-fertilization ; the latter are the result of the copulation
of two individuals. The fertilization of the ova always
takes place in the atrium genitale. Many Turbellarians,
especially the Accela, display the phenomenon known as
" successive hermaphroditism," the male organs of an
individual attain to maturity first, and the female organs
become ripe subsequently. During copulation, therefore,
one individual is physiologically a male and the other a
female.

Asexual generation is met with only in the j!//c/w..-
mida ; it takes the form of transverse division accompanied
by budding. The posterior third of the body becomes
separated off by a septum running from the gut to the

integument and an external furrow corresponding to this ;
this part of the body grows in length until it equals the
anterior portion. Jiy further repetition of this double pro-
cedure of separation and equalization there, chains of 4,
then 8, 16, and 32 buds are formed, which remain attached
(tig- 9), and, although fresh mouth apertures (m1, m", m'")
have been formed, are still in communication by the
intestinal lumen ; this becomes closed before or after the
several buds break off from their connexion with each
other. Throughout the whole summer chains of zooids
are met with ; in autumn this asexual division probably
ceases to occur ; the several individuals become sexually
mature, separate from each other, and lay eggs which

Fig. 10.

FIG. 9. — Microstoma lineare, Oe., undergoing division. There are 16 individuals,
8 with mouth apertures, showing the buds of the first (m\ second (HI'), third
(in"), and fourth (in"") generation. The fifth gent-ration has not yet acquired
a mouth aperture, <-, ciliati-d grooves ; e, eye spots ; i, intestine.

FIG. 10. — Larva of Yungta aurantica, L. (Polycladida), with provisional ciliated
processes (after A. Lang).

remain quiescent during the winter and in the spring
develop into fresh individuals reproducing asexually.

Development. — The study of the development of the
Turbellariaus is unfortunately not very far advanced,
particularly among the small Bhabdocaelida, which are
extremely difficult to investigate, and about which hardly
any developmental facts are known. The larger fresh-
water Tridndidn and the Polycladida on the contrary have
been recently very fully investigated. The Rhabdocxla and
the Tridadida appear to develop directly without any meta-
morphosis, while a great part of the Polycladida undergo
a metamorphosis and pass through a larval condition,
during which they are furnished with provisional ciliated
processes (fig. 10) ; the Acula, have also a free larval form ;
pelagic larvae with a coat of long cilia apparently belonging
to this group have been observed by Ulianin. The seg-
mentation of the ovum is total, but unequal ; an epibolic
gastrula is formed and the aperture of invagination
becomes the permanent mouth of the adult.

Systematic Arrangement and Mode of Life. — Order
Turbcllaria. — Platyhelminths with a ciliated integument,
a mouth and pharynx, but no anus ; with paired cerebral
ganglia and two lateral nerve cords ; sexual organs her-
maphrodite ; chiefly free-swimming.

82

PLANARIANS

Sub-order A. Rhabdocoelida. — Of small size ; body cylin-
drical or depressed ; without an intestine, or with a simple
unbranched intestine ; the female genital glands always
compact, not follicular ; genital apertures single or distinct.

Tribe I. Acoela (fig. 1, «). — With a digestive paren-
chyma not differentiated into intestine and parenchyma
proper; with no nervous system or excretory organs;
sexual organs hermaphrodite, with follicular testes and
paired ovaries ; generally without a pharynx, but having
otoliths ; all the forms marine. Many quite flat, with
the lateral margins bent down towards the ventral surface
(Convohtta), frequently with brown or green parasitic alga;
in the parenchyma.

Tribe II. Rhabdoceela (fig. 1, b). — Intestinal tract and
parenchyma separate ; nervous system and excretory
organs present ; with compact testes and female genera-
tive glands (ovaries or separated germarium and yelk
glands) ; with a complicated pharynx, but generally without
otoliths. Numerous forms, freshwater and marine ; the
genus Prorhynckus (two species) also in damp earth. The
Microstomida (fig. 9) propagate asexually. Freshwater
forms mostly belong to the families Mesostomida and
Vorticida, some of which contain green parasitic algse.
Marine forms include representatives of these two families
and of the Proboscida (with a tactile proboscis). Of the
family Vorticida, the genera Graffilla and Anoplodium are
parasitic, the former in Gastropods the latter in Echino-
derms (Holothurians).

Tribe III. Alloiocoela (fig. 1, c). — Intestinal tract and
parenchyma separate ; nervous system and excretory
organs present ; with follicular testes and compact female
glands (as in the Rhabdocceld) ; pharynx similarly
developed as a shorter or longer sac. One family (Mono-
tid(i), with otoliths. All the species marine, with one
exception, Plagiostoma lemani, which lives in the deep
water of the Alpine lakes.

Sub-order B. Dendroccelida. — Large forms, with a
flattened body, branched intestine, follicular testes and
follicular yelk glands or ovaries ; without otoliths.

Tribe I. Tricladida. — Body elongate ; intestine with.
three main branches uniting to open into a cylindrical
retractile pharynx ; with follicular testes, two round
germariums, and numerous yelk follicles, with a single
sexual aperture. Planaria, Dendroccdum, Polycelis (fig.
1, g) are inhabitants of fresh water (with great power of
reproduction). Terrestrial forms (fig. 1, e, f) of leech-like
shape, especially met with in the tropics (only two
European species Rhynchodemus terrestris and Geodesmus
bilineatus) ; marine forms Gunda (characterized by a
metameric structure), Bdelloura (external parasite of
Limulm}.

Tribe II. Polydadida (fig. 1, </).— Body leaf-like, thin,
and broad, with numerous branched or retiform intestinal
cceca which unite to form a central tube (stomach) ; with
follicular testes and follicular ovaries, with two separated
genital apertures, the male in front of the female ; without
(Acotylea) or with (Cotylea) a sucker situated behind the
female generative opening. All marine.

Literature. — The most recent works, which also contain a full
account of what has gone before, are the following : — Rhabdoceela. —
L. v. Graff, Monographic dcr Turbcllarien : 1. Rhabdoccelida, Leip-
sic, 1882, with 20 plates. Marine and Freshwater Tricladida. —
A. Lang, " Der Ban von Gunda segmentata nnd die Verwandtschaft
der Platyhelniinthen mit Ccelenteraten und Hiradiueen," in Mitth.
Zool. Stat. Ncapcl, vol. Hi., 1881; El. Metschnikoff, "DieEm-
bryologie von Planaria polychroa," in Zeitschr. f. iiriss. Zool., vol.
xxxviii., 1883; Isao Jijima, " Untersuchungen liber den Ban und
die Entwickelungsgesehichte der Susswasser-Dendrocoelen," iu
Zcitschr. f. wiss. Zool., vol. xl., 1884. Land Planarians. — H. N.
Moseley, "On the Anatomy and Histology of the Land Planarians
of Ceylon, with some Account of their Habits, and with a Descrip-
tion of Two New Species, and with Notes oil the Anatomy of some
European Aquatic Species," in Phil, Trans. (London, 1874), and
"Notes on the Structure of several Forms of Land Planari.ms,
with a Description of Two New Genera and Several New Species,
and a List of all Species at present known," in Quart. Jour. Micr.
Sci., vol. xlvii., 1877 ; J. v. Kennel, "Die in Deutsehland gefun-
denen Landplanarien Rhyacliodemus terrestris und Geodesmus
bilineatus," in Arbeit. Zool.-Zootom. Instil. IF-iirzburg, v., 1879.
Polydadida. — A. Lang, "Die Polycladen," in Fauna und Flora
des Golfcs von Neapel, No. 11, 39 plates, Leipsic, 1884-85.

(L. V. G.)

NEMEKTINES

(By A. A. W. Hubrecht, Ph.D., LL.D., Professor of Zoology, University of Utrecht.)

Classifi-
cation.

"VTEMERTIXES, or NEMEETEANS (Nemertea), is the
_1A name of a subdivision of worms,1 characterized by
the ciliation of the skin, by the presence of a retractile
proboscis, by the simple arrangement of the generative
apparatus, and in certain cases by a peculiar pelagic larval
stage to which the name "[liliilium" has been given. Many
of them are long thread-shaped or ribbon-shaped animals,
more or less cylindrical in transverse section. Even the
comparatively shortest species and genera can always be
termed elongate, the broadest and shortest of all being
the parasitic JIalacobdclla and the pelagic Pelagonemertes.
There are no exterior appendages of any kind. The colours
are often very bright and varied. They live in the sea,
some being common amongst the corals and algse, others
hiding in the muddy or sandy bottom, and secreting gelatin-
ous tubes ^Yhich ehsheath the body along its whole length.
Formerly, they were generally arranged amongst the
Platyelminthes as a suborder in the order of the Turbel-
larians, to which the name of Rhynckocosla was applied, the
other suborders being the Dendrocoela and the Rliabdocxla.
With the advance of our knowledge of these lower
worms it has been found desirable to separate them
from the Turbellarians and to look upon the Nemertea as
a seperate phylum of Platyelminthes. Lately the interest
in their morphology has increased since it has been
advanced (6, 8)2 that certain points in their organization
appear to indicate a remote degree of relationship to the
ancestral forms which must have preceded the Chardata (to
which the vertebrate animals also belong), and that this
relationship is closer than that which exists between those
Protochordata and any other group of invertebrate animals.
CLASSIFICATION. — The Nemertines are subdivided into
three suborders : — Hoplonemertea, Schizonemertea, and
Palseonemertea (5). The (1) Hoplonemertea embrace all
the species with a stylet in the proboscis, and also
Malacobdella, which has an unarmed proboscis, but
which, by the details of its organization and its develop-
ment, must certainly be placed here (parasitism may
be the cause of its incipient degeneration). The special
characters of this suborder may be gathered from the
anatomical descriptions hereafter to be given. In those
species of which the embryology has been investigated
the development was direct. The more common or more
important genera are Amphiporus (A. pulcher, British
coasts, Mediterranean; A.splendidm, Atlantic), which is
comparatively short, Nemertes (N. gracilis, Atlantic and
Mediterranean; Jf. antonina, Mediterranean; N. echino-
ilinna, Mediterranean), which is long and thread-like,
Tetrastemma, Drepanop/iorus (with more complicate arma-
ture in the proboscis), Akrostomum, Malacobdella, (2) In
the Schizonmertea all those genera and species are united
which have deep, longitudinal, lateral cephalic fissures.
The development of some (Linens) is characterized by the

1 Nemertes was a sea nymph, daughter of Nereus and Doris. One
of the genera was named Nemertes by Cuvier.

3 These figures refer to the bibliography at page 88.

so-called larva of Desor, of others (Cerebratulus) by the
curious and characteristic pilidium-Lirva. The princi]>:il
genera are Linens (L. longissimus, Atlantic ; L. obscurus),
Cerebratulus (C. marginatus, C. bilinentus, both Atlantic
and Mediterranean; C.urticans, Mediterranean ; C'.fascio-
latus and aurantiacus, C. hepaticus and dohrnii, Medi-
terranean ; C. macintoshii, Madeira), Langia (L. formosa),
Borlasia (B. elizabethx). (3) Of the Palxonemertea the
most typical and most characteristic genera are Cari-
nella and Cephalothrix. They differ considerably both

Fig. 1.

Fig. 2.

FIGS. 1, •.!.— Diagiamsof the organs of a Ncmcrliue, fig. 1 from below, fig. 2 from
above. m, mouth ; div, intestinal diverticula ; a, anus; or. ovaries; ", neph-
ridia; Br, brain-lobes; In, longitudinal nerve stems; pr, proboscis; ps, pro-
boscidian sheath ; p.o., opening for proboscis.

from Hoplo- and from Schizonemertines, and evidently
belong to a lower stage of differentiation. The genera
Polia(P. ddineata and P. curta, Mediterranean) and I V> /«-
cinia are provisionally arranged in this order because,
though less primitive, they are not typical representatives
of the other two suborders. The development of these
species is not at all, or only very superficially, known. For
the further characters of the last two suborders see the
anatomical description below.

Another subdivision generally current is that into the
Enopla and the Anopla (14). This does not, however,
take into sufficient account the primitive and diverging

84

NEMERTINES

P.O.

ior portion of tlie
body of a Nemertine. Br,
brain-lobes; N, lateral nerves;
PS, proboscidian sheath ; Pi;
proboscis; P.O., exterior open-
ing through which the probos-
cis is everted. (Esophagus ;nul
mouth shown by dotted lines.

characters disclosed by the very important less highly
organized genera.

Ana- ANATOMY. — («) Proboscis and Proboscidian Sheath. — The

tomy organ most characteristic of a Nemertine is without doubt
the proboscis. With very few exceptions (Malacobdella,
Akrostommn, where it has fused with the mouth to a single
exterior opening), there is a terminal opening (subterminal
in Valencinia) at the foremost tip of the body, out of which
the proboscis is seen shooting backwards and forwards,
sometimes with so much force that both its interior
attachments are severed and it is entirely expelled from
the body. It then often retains its vitality for a long
time, apparently crawling about as if it were itself a worm,
a phenomenon which is at least
partially explained by the extra-
ordinary development of nervous
tissue, equally distributed all
through the walls of the proboscis,
and either united (10) into nu-
merous longitudinal nerve-stems
(Drepanophorus, Amphipoms) or
spread out into a uniform and
comparatively thick layer (C'erc-
bratulus, sp.). This very effective
and elaborate innervation, which
has been directly traced (6) to the
brain, whence strong nerves (gene-
rally two) enter the proboscis,

•C ./ T i i i i fIG- 3.— Anted

renders it exceedingly probable
that the most important functions
of the proboscis are of a sensi-
ferous, tactile nature, a supposi-
tion which is again strengthened
by the fact that amongst the llhabdocoel Turbellarians an
organ which may be called the forerunner of the Nemertean
proboscis has been proved (3) to be the morphological
equivalent of the foremost tip of the body, which, as an
organ of delicate touch, has acquired the property of
folding inwards. In Nemertines the everted proboscis is
retracted in the same way
as the tip of a glove finger
would be if it were pulled
backwards by a thread
situated in the axis and
attached to the tip. The
comparison may be car-
ried still further. The
central thread just alluded
to is represented in the
Nemertean proboscis by
that portion which is
never everted, and the
tip of the glove by the
boundary between the
evertible and non-evert- FlGS _4_ 5._Pl.ob08Ci8 witn styiet, "reserve ••

ible portion of the pro- S1CS. iind muscular bulb of n Iloplonemei'-
, .*• , , . , tine. Fig. * retracted ; tig. 5 everted.

boscis — a boundary which

in the Hoplonemertini is marked by the presence of a
pointed or serrated stylet. This stylet is thus situated
terminally when the proboscis has reached its maximum
aversion. It adds a decisively aggressive character to an
organ the original significance of which, as we have seen,
was tactile. This aggressive character has a different
aspect in several genera which are destitute of a central
stylet, but in which the surface that is turned outwards
upon eversion of the proboscis is largely provided with
nematocysts, sending the urticating rods of different sizes
in all directions. In others this surface is beset with
thick, glandular, adhesive papillae.

The comparison with the glove-finger is in so far

Fig. 4.

Fig. 5.

insufficient as the greater portion of the nou-evertible half
of the proboscis is also hollow and clothed by glandular
walls. Only at the very hindermost end does it pass into
the so-called retractor-muscle (fig. 2), which is attached to
the wall of the space (proboscidian sheath) in which the
proboscis moves about. This retractor-muscle, indeed,
serves to pull back with great rapidity the extruded
proboscis, and is aided in its action by the musculature of
the head. The extrusion itself depends entirely upon
contraction of the muscular walls of the space just
mentioned (proboscidian sheath). As it is (1) closed on
all sides, and (2) filled with a corpuscular fluid, the
contractions alluded to send this fluid to impinge against
the anterior portion, where the proboscis, floating in its
sheath, is attached with it to the muscular tissue of the
head (fig. 3). Partial extrusion lessening the resistance
in this region inevitably follows, and when further con-
tractions of the walls of the sheath ensue total extrusion
is the consequence. It is worthy of notice that in those
Nemertines which make a very free use of their proboscis,
and in which it is seen to be continually protruded and
retracted, the walls of the proboscidian sheath are enor-
mously muscular. On the other hand, they are much less
considerably or even insignificantly so in the genera that
are known to make a rather sparing use of their proboscis.

The proboscis, which is thus an eminently muscular
organ, is composed of two or three, sometimes powerful,
layers of muscles — one of longitudinal and one or two of
circular fibres. In the posterior retractor the longitudinal
fibres become united into one bundle, which, as noticed
above, is inserted in the wall of the sheath. At the
circular insertion of the proboscis in front of the brain the
muscular fibres belonging to the anterior extremity of the
body and those connected with the proboscis are very
intimately interwoven, forming a strong attachment.

The proboscis broken off and expelled is generally
reproduced, the posterior ribbon-like end of this reproduced
portion again fusing with the walls of the
sheath (11). There is reason to suppose that,
when a wound is inflicted by the central
stylet, it is envenomed by the fluid secreted
in the posterior proboscidian region being at
the same time expelled. A reservoir, a duct,
and a muscular bulb in the region (fig. 4) FlG (, _T]1L. ar.

where the stylet is attached serve for this pur- mature from
ml * . .f. f ^ ,. the proboscis

pose. The significance of two or more (in Of Drepana-
Drepanophorus very numerous) small sacs con- i''""'"s-
taining so-called " reserve " stylets resembling in shape that
of the central dart is insufficiently known.

The proboscidian sheath, which by its transverse con-
tractions serves to bring about eversion of the proboscis in
the way above traced, and the muscular walls of which
were similarly noticed, is attached to the musculature of
the head just in front of the ganglionic commissures
(fig. 3). In nearly all Nemertines it extends backwards
as far as the posterior extremity, just above the anus; in
CnrtM'lla it is limited to the anterior body-region. The
corpuscles floating in the fluid it contains are of definite
shape, and in Cerebratwliis iirtifnitx they arc deep red from
the presence of haemoglobin. Internally the muscular
layers are lined by an epithelium. In the posterior
portion this epithelium in certain 8c//i:n//i m< rli-u has a
more glandular appearance, and sometimes the interior
cavity is obliterated by cell-proliferation in this region.
Superiorly the sheath either closely adheres to the muscular
body-wall, with which it may even be partly interwoven,
or it hangs freely in the connective tissue which, tills the
space between the intestine and the muscular body-wall.

(li) Cutaneous System. — Externally in all species a layer
of ciliated cells forms the outer investment. In it are,

NEMERTINES

85

moreover, enclosed unicellular glands pouring their highly
refracting contents, of a more or less rod-like shape, directly
to the exterior. They appear to be the principal source
of the- mucus these animals secrete. In Schizonemertines
these elements are separated by a thin homogeneous base-
ment membrane (fig. 8) from the following, — that is, from
a layer in which longitudinal muscular fibres are largely
intermixed with tortuous glands, which by reason of their
deeper situation communicate with the exterior by a much
longer and generally very narrow duct. The pigment is
also principally localized in this layer, although sometimes
it is present even deeper down within the musculature.
The passage from this tegumentary layer to the subjacent
longitudinal muscular one is gradual, no membrane
separating them. In Cnrinella, Cephalothrix, Polia, and
the Hoplonemertines the two tegumentary layers with
their different glandular elements are fused into one ; a
thick layer of connective tissue is situated beneath them
(instead of between them) and keeps the entire cutaneous
system more definitely separate from the muscular (fiX's.
7,8).

(r) Musculature and Connective Tissue. — The muscular
layers by which the body-wall is constituted have been
very differently and to some extent confusingly described
by the successive authors on Nemertean anatomy. There
is sufficient reason for this confusion. The fact is that not
only have the larger subdivisions a different arrangement
and even number of the muscular layers, but even within
the same genus, nay, in the same species, well-marked
differences occur. Increase in size appears sometimes to
be accompanied by the development of a new layer of
fibres, whereas a difference in the method of preparation
may give to a layer which appeared homogeneous in one
specimen a decidedly fibrous aspect in another. Never-
theless there are three principal types under which the
different modifications can be arranged. One of them is
found in the two most primitively organized genera,
Carindla and Cephalothrix, i.e., an outer circular, a longi-
tudinal, and an inner circular layer of muscular fibres

Fig. 7. Fig. 8. Fig. 9.

FIGS. 7-9.— The layers of tlie body-wall in Carinella (fig. 7), the Iloplonemertea
(tic. 8), anil the Schizonemertea (fig. 9). c, cellular tissue of the integument ;
/lut, basement membrane ; cire. 1, outer circular, and long, longitudinal layer
of muscular tissue; circ. 2, long. 1, additional circular and longitudinal layers
of the same ; nl, nervous layer.

(fig. 7). The second is common to all the Schizonemer-
tines as well as to Polia and Valencinia, and also compre-
hends three layers, of which, however, two are longitudinal,
viz., the external and the internal one, there being a strong
circular layer between tnern (fig. 9). To the third type
all the Ifaplonemertea correspond ; their muscular layers
are only two, an external circular and an internal longi-
tudinal one (fig. 8).

The Schizonemertea thus appear to have developed an
extra layer of longitudinal fibres internally to those which
they inherited from more primitive ancestors, whereas the
Hoplonemertea are no longer in possession of the internal
circular layer, but have on the contrary largely developed
the external circular one, which has dwindled away in
the Schizonemertea. In only one instance has the present
writer met with a thin exterior circular layer in a very
large specimen of Gerebratulus ; younger specimens of the
same species did not show it. It is noticeable that Kefer-

stein (9) also observed four layers similarly arranged in
one of the specimens of C<'r<l>i;iliiliix which he investi-
gated. The situation of the lateral nerve-stems in the
different genera with respect to the muscular layers lends
definite support to the interpretation of their homologies
here given.

In C'ariiifl/ii, Cephalothrix, and 1'ulin, as well as in all
Hoplonemertines, the basement membrane of the skin
already above alluded to is particularly strong and immedi-
ately applied upon the muscular layers. In the Schizo-
nemertines there is a layer in which the cutaneous elements
are largely represented below the thin basement membrane
(fig. 8), between it and the bulk of the outer longitudinal
muscles. The difference in the appearance of the base-
ment membrane — sometimes wholly homogeneous, some-
times eminently fibrillar — can more especially be observed
in differently preserved specimens of the genus Polia.

The connective tissue of the integument and basement
membrane imperceptibly merges into that which surrounds
the muscular bundles as they are united into denser and
definite layers, and this is especially marked in those forms
(Akrostomum) where the density of the muscular body-
wall has considerably diminished, and the connective tissue
has thus become much more prominent. It can then at
the same time be observed, too, that the compact mass of
connective tissue (" reticulum," Barrels) which lies between
the muscular body-wall and the intestine (1) is directly
continuous with that in which the muscular layers are
imbedded. Nuclei are everywhere present. The omni-
presence of this connective tissue excludes the idea of any
true body cavity in Nemertiues.

In Polia the connective tissue enclosed in the external
muscular layer is eminently vacuolar, — all the interme-
diate stages between such cells in which the vacuole pre-
dominates and the nucleus is peripheral and those in which
the granular protoplasm still entirely fills them being
moreover present.

In addition to the musculature of the proboscis and
proboscidian sheath, longitudinal muscular fibres are
found in the walls of the resophagus, whilst transverse
ones are numerous and united into vertical dissepiments
between the successive intestinal caeca, thus bringing about
a very regular internal metamerization (4). The genital
products develop in intermediate spaces similarly limited
by these dissepiments and alternating with the digestive
caeca.

(d) Nervous System and Sense Organs. — The nervous system of
Nemertiues presents several interesting peculiarities. As central
organs we have to note the brain-lobes anil the longitudinal lateral
cords which form one continuous unsegmented mass of fibrous and
cellular nerve-tissue. The fibrous nerve-tissue is more dense in the
higher differentiated, more loose and spongy in the lower organized
forms ; the cellular nerve-tissue is similarly less compact in the
forms that are at the base of the
scale. No ganglionic swellings
whatever occur in the course of
the longitudinal cords. The
brain must be looked upon as
the anterior thickening of these
ords, and at the same time as

the spot where the two halves
of the central nerve system

_. J* . T."ii

Fig. 10.

Fig. 11.

iiitmvnmmiirnVatp Ttii* is FlGS- 10' H.— Brain and lateral organ of

1JUS IS H schizoncmertine (tig. 10) and a Hoplo-

brought about by a double com- nemerUne(flg.ll). eo, exterior opening;
missure, of which the ventral ".'., superior brain-lobe; p.l., posterior
portion is considerably thicker brain-lobe.

than the dorsal, and which, together with the brain-lobes, consti-
tutes a ring through which both proboscis and proboscidian sheath
pass. The brain-lobes are generally four in number, a ventral and
a dorsal pair, respectively united together by the above-mentioned
commissures, and moreover anteriorly interfusing with each other,
right and left. In C'arinella this separation into lobes of the
anterior thickenings of the cords has not yet commenced, the ven-
tral commissure at the same time being extremely bulky. There
is great probability that the central stems, together with the brain,

86

NEMERTINES

must be looked upon as local longitudinal accumulations of nervous
tissue in what was in more primitive ancestors a less highly dif-
ferentiated nervous plexus, situated ill the body-wall in a similar
way to that which still is found in the less highly organized
Ccelenterates. Such a nervous plexus indeed occurs in the body-
wall of all Schizonemertines (7), sometimes even as a compara-
tively thick layer, situated, as are the nerve stems, between the
external longitudinal and the circular muscles (fig. 9). In Cari-
iiella, where the longitudinal nerve-stems are situated exteriorly to
the muscular layers,
this plexus, although
present, is much less
dense, and can more
fitly be compared to
a network with wide
meshes. In both
cases it can be shown
to be in immediate
continuity with the
coating of nerve-cells
forming part of the
longitudinal cords.
It stretches forward
as far as the brain,

and in Oarinclla is FIG. 12.— The brain of a Nemertine, with its lobes an.l
a^ain continued in commissures. S.N., nerves to sensory apparatus;
front of it whereas P-N-' nc'rvcs for proboscis; fag. nerves for oeso-

, "'. * "l phaKus ; L.S., lateral nerve stems,

in the Schizonemer-
tines the iunervation of the anterior extremity of the head, in
front of the brain, takes the form of more definite and less numer-
ous branching stems. The presence of this plexus in connexion
with the central stems, sending out nervous filaments amongst
the muscles, explains the absence, both in Falseo- and Schizo-
nemertines, of separate and distinct peripheral nerve stems spring-
ing from the central stems innervating the different organs and
body-regions, the only exceptions being the nerves for the pro-
boscis, those for the sense organs in the head, and the strong
nerve pair (n. vagus) for the oesophagus. At the same time it
renders more intelligible the extreme sensitiveness of the body-
wall of the Nemertiues, a local and instantaneous irritation
often resulting in spasmodic rupture of the animal at the point
touched.

In the Hoploiiemcrtea, where the longitudinal stems lie inside
the muscular body-wall, definite and mutamerically placed nerve
branches spring from them and divide dichotomously in the
different tissues they innervate. A definite plexus can here no
longer be traced. In certain Hoplonemertines the lateral stems
have been noticed to unite posteriorly by a terminal commissure,
situated above the anus, the whole of the central nervous system
being in this way virtually situated above the intestine. In others
there is an approximation of the lateral stems towards the median
ventral line (DrcpanopJtorus) ; in a genus of Schizonemertines
(Langia), on the other hand, an arrangement occurs by which the
longitudinal steins are no longer lateral, but have more or less
approached each other dorsally (6).

In addition to the nerves starting from the brain-lobes just now
especially mentioned, there is a double apparatus which can hardly
be treated of in conjunction with the sense organs, because its
sensory functions have not been sufficiently made out, and which
will therefore rather be considered along with the brain and central
nervous system. This apparatus is usually known under the name
of the lateral organs. To it belong (a) superficial grooves or deeper
slits situated on the integument near the tip of the head, (b) nerve
lobes in immediate connexion with the nervous tissue of the brain,
and (c) ciliated ducts penetrating into the latter and communicating
with the former. Embryology shows that originally these different
parts are separately started, and only ultimately become united
into one. Two lateral outgrowths of the foremost portion of the
cesophagus, afterwards becoming constricted off, as well as two
ingrowths from the epiblast, contribute towards its formation, at
least as far as both Hoplo- and Schizonemertines are conrerni'il.
As to the Palssoncmcrtca, their embryology has not yet been studied,
and in the most primitive genus, Carinclla, we do not find any
lateral organs answering to the description above given. What we
do find is a slight transverse furrow on each side of the head, close
to the tip, but the most careful examination of sections made
through the tissues of the head and brain shows the absence of any
further apparatus comparable to that described above. Only in one
species, Carinclla incxpcckita, a step in advance has been made, in
so far as in connexion with the furrow just mentioned, which is
here also somewhat more complicated in its arrangement, a ciliated
tube leads into the brain, there to end blindly amidst the nerve-
cells. No other intermediate stages have as yet been notirrd
between this arrangement and that of the Schizonemertea, in which
a separate posterior brain-lobe receives a similar ciliated canal, and
in which the cesophageal outgrowths have made their appearance
and are coalesced with the nerve-tissue in the organ of the adult

animal. The histologieal elements of this portion remain distinct
both by transmitted light and in actual sections.

These posterior brain-lobes, which in all Schizoncmertiues are in
direct continuity of tissue with the upper pair of principal lobes,
cease to have this intimate connexion in the Hoploncmcrtea ; and,
although still constituted of (1 ) a ciliated duct, opening out exter-
nally, (2) nervous tissue surrounding it, and (3) histologieal ele
ments distinctly different from the nervous, and most probably
directly derived from the cesophageal outgrowths, they are never-
theless here no longer constantly situated behind the upper brain
lobes and directly connected with them, but are found sometimes
behind, sometimes beside, and sometimes before the brain-lobes.
Furthermore, they are here severed from the principal lobes and
connected with them by one or more rather thick strings of nerve-
fibres. In some cases, especially when the lobes lie before the brain,
their distance from it, as well as the length of these nervous con-
nexions, has considerably increased.

With the significance of these parts we are still insufficiently
acquainted. There appear to be analogous organs amongst
Platyelminthes, but a careful comparative study is wanted. A
partial comparison has been hazarded (8) with the anterior
cesophageal outgrowths in linlintuiilossux and Amjihioxus, and for
the Schizouemertines arguments have been adduced (6) to prove
that here they have the physiological significance of a special
respiratory apparatus for the central nervous tissue, which in all
these forms is strongly charged with haemoglobin. The haemoglobin
would, by its pre-eminent properties of fixing oxygen, serve to fur-
nish the nerve system, which more than any other requires a
constant supply, with the necessary oxygen. Such could hardly
be obtained in any other way by those worms that have no special
respiratory apparatus or delicately ramifying blood-vessels, and that
live in mud and under stones, where the natural supply of freshly
oxygenated sea-water is practically limited. Whether in the Hoplo-
nemertines, where the blood fluid is often provided with haenio-
globiniferous disks, the chief functions of the side organs may not
rather be a sensory one must be further investigated.

The exterior opening of the duct has been several times alluded
to. In the Hoplonemertines it is generally situated towards the
middle of a lateral transverse groove on either side of the head, as
was noticed for Carinella, and as is also
present in Polia. Generally a row7 of
shorter grooves perpendicular to the first,
and similarly provided with strong cilia,
enlarges the surface of these furrows (tig.
14). In Valcncinia there is nothing but
a circular opening without furrow. In all
Schizonemertines there is on each side of
the head a longitudinal slit of varying
length but generally considerable depth,

in the bottom of which the dark red brain *'";*• ,13'.i4--Late™i.!!i.c.ws
is very plainly visible by transparency.
These slits are continued into the ciliated
duct, being at the same time themselves
very strongly ciliated. In life they are
commonly rhythmically opened and shut
by a wavy movement. They are the head slits (cephalic fissures,
" Kopfspalten ") so characteristic of this subdivision (figs. 10
and 13).

With respect to the sense organs of the Nemertiiies, we find that
' lit occurrence, although many Schizonemer-
d appear to be blind. The more highly
often very numerous eyes (Amphiporus,

Drepanophorus), which are provided with a spherical refracting
anterior portion, with a cellular "vitreous body," with a layer of
delicate radially arranged rods, with an outer sheath of dark
pigment, and with a separate nerve-twig each, springing from a
common or double pair of branches which leave the brain as
ji. optici, for the innervatiou of the eyes. Besides these more
highly differentiated organs of vision, more primitive eyes are
present in others down to simple stellate pigment specks without
any refracting apparatus.

Organs of hearing in the form of capsules containing otoliths
have only been very rarely observed, apparently only in
Hoploncmcrtea.

As to the organ of touch, the great sensitiveness of the body has
already been noticed, as well as the probable primary significance
of the proboscis. Small tufts of tactile hairs or papillae are some-
times observed in small number at the tip of the head (11) ; some-
times longer hairs, apparently rather stiff, are seen on the surface,
very sparingly distributed between the cilia, and hitherto only in
a very limited number of small specimens. They may perhaps be
considered as sensory.

(c) Digestive System. — The anterior opening, the mouth, is
situated ventrally, close to the tip of the head and ill front of the
brain in the jffoplonemertea, somewhat more backward and behind
the brain in the other Nemertiues. In most Schizonemertines it is
found to be -ill elongated slit with corrugated borders ; in the

of head of a Schizone-
mertine (fig. 13) with
longitudinal slit, and of
a Hoplonemertine (fig. 14)
with transverse groove
and furrows.

eyes are of rather constant oeeurrei
tines living in the mud
organized species have often

NEMEETINES

Hoplonemcrtinrs it is smaller and rounded ; in Mnlamlntclln and
Akrostomum it, moreover, serves lor the extrusion of the proboscis,
which emerges by a separate dorsal opening just inside the mouth.
The oesophagus is the anterior pmlimi ..| the digestifs canal; its
walls are folded longitudinally, comparatively thick, and provided
with longitudinal muscular fibres. Two layers are specially obvious
in its walls, — the inner

layer bordering the lu-
men being composed of
smaller ciliated cells,
the outer thicker nnc
containing numerous
granular cells and hav-
ing a more glandular
character. Outside the
wall of the (.esophagus a
vascular space lias been
detected (11) which is
in direct continuity
with the longitudinal
blood-vessels. In cer-
tain eases, however, the
walls of the oesophagus
appear to be very closely
applied to the muscular
body-wall, and this vas-
cular space thereby con-
siderably reduced.

The posterior portion
of the intestine is speci-
ally characterized by the
appearance of the intes-
tinal divertieula hori-
zontally and symmetric-
ally placed right and
left and opposite to each
other. Sometimes this re-
gion, into which the oeso-
phagus leads, stretches
forwards under the
oesophagus (Hoplone-
mertines) for a certain
distance, anteriorly ter-
minating by a cul-de-
sac. Cases of asym-
metry or irregularity in
the arrangement of the
caeca, though sometimes
occurring, are not nor-
mal. At the tip of the

PS

ZBv

Fie. 16.

CT

place, the caeca are al-
ways eminently regular.
So they ore throughout
the whole body in most
of the Hoplonemertines.
In Carinella they are
generally deficient and
the intestine straight ;

LBu

Fig. 17.

tail, where the growth FIGS. 15-17.— Diagrammatic sections to show dis-
of the animal takes position of internal organs in Carinella (1'alxo-
nemertea), fig. 1.5, Schizonemcrtea, fig. 16, and
Hoplonemertea,' fig. '17. C, cellular portion of
integument ; B, basement membrane ; A, circu-
lar muscular layer; .-I', longitudinal do.; A",
second circular (in Cfii'itn'llfi) ; A'", second longi-
tudinal (in Schizonemertea) ; N, nervous layer ;
LN, lateral nerves ; PS, cavity of proboscidian
sheath (the sheath itself of varying thickness);
P, proboscis ; /, intestine ; LBv, latenil blood-
vessel; DBv, dorsal do. ; CT, connective tissue.

iii young specimens ^if this species, however, they .occur, though less
regular and more in the form of incipient foldings by which the
digestive surface is increased. The inner surface of the intestinal
cseca is ciliated, the caeca themselves are sometimes — especially in
the hindermost portion of the body— of a considerably smaller lumen
than the intermediate genital spaces ; sometimes, however, the
reverse is the case, and in both cases it is the smaller lumen that
appears enclosed between and suspended by the transverse fibres
constituting the muscular dissepiments above mentioned.

The anus is situated terminally, the muscular body-wall through
which the intestine must find its way outwards probably acting in
this region the part of a sphincter. The lateral nerve stems mostly
terminate on both sides in closest proximity to the anus ; in certain
species, however, they interfuse by a transverse connexion above
the anus. The longitudinal blood-vessels do the same. The
question has been raised whether the regular intestinal caeca of
Nemertines might not be compared with those intestinal divertieula
of the embryo Amphioxiis which ultimately become the mesoblastic
somites of the adult (8). This view would he a further extension
of the views concerning the ccelom first propounded by Huxley.

(/) Circulatory Apparatus. — This consists of three longitudinal
trunks, a median and two lateral ones. They are in direct con-
nexion with each other both at the posterior and at the anterior
end of the body. At the posterior end they communicate together
ny a T-shaped connexion in a simple and uniform way. Anteri< irly
there is a certain amount of difference in the arrangement. Whereas
in the Hoplonemertines an arrangement prevails as represented in

Fin. lS._r>i»gram of
the circulatory ap-
paratus in the ante-
rior body-region of
a Hoplonemcrtine.

lig. 18, the lateral steins in the Schizonemcrtincs, while entirely
unil'oriii all through the posterior portion of the body, no longer
individually exist in the icsophageal region, I nit «~

here dissolve themselves into a network of vascu-
lar spaces surrounding this portion of the di-
gestive tract (11). The median dorsal vessel,
however, remains distinct, but instead of con-
tinuing its course beneath the proboscidian
sheath it is first enclosed by the ventral muscu-
lature of this organ, and still farther forwards
it even bulges out longitudinally into the cavity
of the sheath. Anteriorly it finally communi-
cates with the lacunae just mentioned, which
surround the oesophagus, bathe the posterior
lobes of the brain, pass through the nerve ring
together with the proboscidian sheath, ami are
generally continued in front of the brain as a
lacunar space in the muscular tissue, one on each side.

Special mention must be made of the delicate transverse vessels
regularly connecting the longitudinal and the lateral ones. They
are metamerically placed, and belong to the same metamer as the
digestive cceca, thus alternating with the generative sacs. The
blood fluid does not flow in any definite direction ; its movements
are largely influenced by those of the muscular body-wall. It is
colourless, and contains definite corpuscles, which are round or
elliptical, and in many Hoplonemertines are coloured red by haemo-
globin, being colourless in other species. The circulatory system
of Carinella is considerably different, being more lacunar and less
restricted to definite vascular channels. Two lateral longitudinal
lacunae form, so to say, the forerunners of the lateral vessels. A
median longitudinal vessel and transverse connecting trunks have
not as yet been detected. There are large lacuna; in the head in
front of the ganglia.

(g) Nephridia. — Although these organs were already very well
known to Max Schultze (14), their presence in Nemertines was
repeatedly and seriously disputed until Von Kennel (10) definitely
proved their existence and gave details concerning their histology.
With the exception of a few genera where they have not as yet been
discovered (Carinella), one pair of nephridia appears to be very
generally present. They essentially consist of a complex coiled
tube, one on each side of the oesophagus (fig. 1), communicating
with the exterior by a duct piercing the body-wall. The two
openings of the nephridia are situated sometimes more towards the
ventral, at other times more towards the dorsal side. Even in the
larger Schizonemertines these pores are only a few millimetres
behind the mouth region. Internal funnel-shaped openings,
although sought for, have as yet not been detected. The coiled
tubes extend both forwards and backwards of the external opening,
by far the greater portion being situated backwards. The anterior
coils reach forwards till in the immediate vicinity of the posterior
brain-lobes. The coils are tubiform, with an internal lumen, only
one layer of rather large cells constituting the walls. These cells
are ciliated ; in some transparent species the internal ciliary move-
ment can be observed during life. In transverse sections the
nephridia can be shown to be generally situated in the region
limited by (1) the proboscidian sheath, (2) the upper wall of the
intestine, (3) the muscular body-wall. No trace of nephridia is
found posterior to the oesophagus.

(h) Generative System. — In the Nemertines the sexes are separate,
with only very few exceptions (12) (Tctrastcinma hermapliroditica,
Marion). The generative products are contained in separate
pouches placed metamerically in the way noticed above in treating
of the digestive system. They are conveyed outwards along narrow
canals, one pair for each metamer piercing the muscular body-wall,
and visible on the outside in mature individuals as minute light-
coloured specks. The ova and spermatozoa, when mature, present
no peculiarities. As the ova are in many species deposited in a
gelatinous tube secreted by the body-walls, in which they are
arranged (three or more together) in flask-shaped cavities, impreg-
nation must probably take place either before or at the very moment
of their being deposited. The exact mode has not yet been noticed.
Another point not yet sufficiently settled is the oogenesis in
Nemertines. In several cases the ova appear to originate directly
as the lining of the generative pouches, but the exact part which
the mesoblastic connective tissue plays, both with regard to these
pouches and to the generative products themselves, remains yet to
be settled.

Pro.wrliochmus claparedii is a viviparous form.

DEVELOPMENT. — The embryologyof the Nemertines offers Develop,
some very remarkable peculiarities. Our knowledge of i«ent.
the development of the most primitive forms is very scanty.
Of that of Carinella absolutely nothing is known. On
< '< /ilialothrix we have observations, in certain respects con-
tradictory. Both Bchizri- and Hoplonemertea have been
more exhaustively studied, the first, as was noticed above,

NEMER TINES

being characterized by peculiar larval forms, the second
developing without metamorphosis.

The larva of Cerebratulus is called the pilidium. In
exterior shape it resembles a helmet with spike and ear-
lobes, the spike being a strong and long flagellum or a tuft
of long cilia, the ear-lobes lateral ciliated appendages
(fig. 19). It encloses the primitive alimentary tract.
Two pairs of invaginations of the skin, which originally

FIG. 19. — Pilidium larva. B, bunch of cilia or flacellum ; <K, oesophagus; st,
stomach; cs, resopliageal outgrowth for lateral organ; am, amnion ; pi'.d. ,
prostomial disk; po.d., metastomial disk.

are called the prostomial and metastomial disks, grow
round the intestine, finally fuse tog-ther, and form the
skin and muscular body-wall of the future Nemertine,
which afterwards becomes ciliated, frees itself from the
pilidium investment, and developes into the adult worm
without further metamorphosis (2, 13).

The eggs of these species are not enveloped by such
massive gelatinous strings as are those of the genus Lineus.
In the latter we find the young Nemertines crawling about
after a period of from six to eight weeks, and probably
feeding upon a portion of this gelatinous substance, which
is found to diminish in bulk. In accordance with these
more sedentary habits during the first phases of life, the
characteristic pilidium larva, which is so eminently adapted
for a pelagic existence, appears to have been reduced to a
close-fitting exterior layer of cells, which is striped oS
after the definite body-wall of the Neniertiue has similarly

originated out of four ingrowths from the primary epiblast.
To this reduced and sedentary pilidium the name of " larva
of Desor " has been given (1).

In the Hoplonemertea, as far as they have been investi-
gated, a direct development without metamorphosis has
been observed. It appears probable that this is only a
further simplification of the more complicated metamor-
phosis described above.

As to the development of the different organs, there is
still much that remains doubtful. The hypoblast in some
forms originates by invagination, in others by delamina-
tion. The proboscis is an invagination from the epiblast ;
the proboscidian sheath appears in the mesoblast, but is
perhaps originally derived from the hypoblast. The origin
of the lateral organs has already been noticed ; that of the
nerve system is essentially epiblastic.

Literature.

(1) J. Barrels, " Reclierchcs sur 1'embryologie ties Nemertes,"
Annales dcs Se. -AT«/i//v//rs. vi., 1877.

(2) 0. Blitschli, " Einige Bemerkungen zur Metamorphose des
Pilidium," Archio fur NaturgcschieJitc, 1873.

(3) L. von Gralf, Monographic der TurleUarien, 1882.

(4) A. A. W. Hubrecht, " Untersuchungen tiber Neinertinen
a. d. Golf von Neapel," Niederl. Archivfiir Zoologic, ii.

(5) Id. , " The Genera of European Nemerteans critically revised,"
Notes from the Leijclen Museum, 1879.

(6) Id.? "Zur Anatomie u. Physiologic d. Nervensystems d.
Nemertinen," ]rcrh. Kon. Al'ad. r. H'ctcnsch., Amsterdam, 1880,
vol. xx.

(7) Id., "The Peripheral Nervous System of the Pal;eo- and
Schizonemertini, one of the layers of the body-wall," Quart.
Journal of Mic r. Science, vol. xx.

(8) Id., "On the Ancestral Forms of the Choi-data," II., July
1883.

(9) W. Keferstein, " Untersuclmngen iiber niedere Seetliiere,"
Zcitsehr. f. iri-ixfHSch. Zoo!., vol. xii., 1863.

(10) J. von Kennel, " Beitrage zur Kenntniss der Nemertinen,"
Arbeilen a. d. zool.-zoot. Instit., ii., Wiirzburg, 1878.

(11) AV. C. Macintosh, A Monograph of British AnixUlda : I.
Nfniertcans, Ray Society, 1873-74.

(12) A. F. Marion, " Eecherches snr les animaux inferienrs du
Golfe do Marseille," Ann. dcs Se. Nat., 1873.

(13) E. Metscbnikoff, " Stiulien iiber die Entwickelung der
Echinodermen und Nemertiuen," Mdm. rlc I'Acad. Imp. du St.
Peterst., xiv., 1869.

(14) Max Sehultze, Beitrage zur Naturycsfhichtc der TurMJuricn,
Greifswald, 1851. and Zcitschr. fiir wissensch. Zaol., iv., 1852.

(A. A. W. H.)

ROTIFERA

(By Prof. A. G. Bourne, Presidency College, Madras.)

THE Rut if era or Rotator'ni form a small, in many
respects well-defined, but somewhat isolated class of
the animal kingdom. Tliey are here treated of separately,
partly on account of the difficulty of placing them in one
of the large phyla, partly on account of their special
interest to microscopists.

Now familiarly known as "wheel animalcules" from
the wheel-like motion produced by the rings of cilia which
generally occur in the head region, the so-called rotatory
organs, they were first discovered by Leeuwenhoek (I),1 to
whom we also owe the discovery of Bacteria and ciliate
Infusoria. Leeuwenhoek described the Rutifer milgaris in
1702, and he subsequently described Melicerta ringens and
other species. A great variety of forms were described
by other observers, but they were not separated as a class
from the unicellular organisms (Protozoa) with which
they usually occur until the appearance of Ehrenberg's
great monograph (2), which contained a mass of detail
regarding their structure. The classification there put
forward by Ehrenberg is still widely adopted, but numer-
ous observers have since added to our knowledge of the
anatomy of the group (3). At the present day few groups
of the animal kingdom are so well known to the micro-
scopist, few groups present more interesting affinities to
the morphologist, and few multicellular animals such a
low physiological condition.

General Anatomy. — The Rotifera are multicellular
animals of microscopic size which present a ccelom. They
are bilaterally symmetrical and present no true metameric
segmentation. A head region is generally well marked,
and most forms present a definite tail region. This tail
region has been termed the "pseudopodium." It varies
very much in the extent to which it is developed. It
attains its highest development in forms like Philodina,
which affect a leech-like method of progression and use it
as a means of attachment. We may pass from this through
a series of forms where it becomes less and less highly
developed. In such forms as Brachionus it' serves as a
directive organ in swimming, while in a large number of
other forms it is only represented by a pair of terminal
styles or flaps. In the sessile forms it becomes a con-
tractile pedicle with a suctorial extremity. A pseudo-
podium is entirely absent in Asplanchna, Triarthra,
Polyarthra, and a few other genera. The pseudopodium,
when well developed, is a very muscular organ, and it may
contain a pair of glands (fig. 2, A, gl) which secrete an
adhesive material.

The surface of the body is covered by a firm homogeneous
structureless cuticle. This cuticle may become hardened
by a further development of chitin, but no calcareous
deposits ever take place in it. The cuticle remains softest
in those forms which live in tubes. Among the free-living
forms the degree of hardening varies considerably. In
some cases contraction of the body merely throws the
cuticle into wrinkles (Notommata, Asplanchna) ; in others
definite ring-like joints are produced which telescope into
one another during contraction ; while in others again it
becomes quite firm and rigid and resembles the carapace
of one of the Entomostraca ; it is then termed a " lorica."
The lorica may be prolonged at various points into spines,
which may attain a considerable length. The surface may
be variously modified, being in some cases smooth, in others
marked, dotted, ridged, or sculptured in various ways (fig.
1, K). The curved spines of Philodina aculeata (fig. 1, G)
and the long rigid spines of Triarthra are further develop-

1 These numbers refer to the bibliography at p. 93.

ments in this direction. The so-called setoc of Polyarthra on
the other hand are more complex in nature, and are moved
by muscles, and thus approach the " limbs " of Pedaliun.

FIG. 1. — A, Flosctilaria campannlata, an adult male, drawn from a dead specimen
(after Hudson): t, testis; oc, eye-spots. B, Floscularia appendiculata, on
adult female (after Gegenbaur): a, the ciliated flexible proboscis. C, Stfphano-
ceros eichhoriiii : a, the urceolus. D, Microcodon clavus, ventral view (after
Grenacher) : m, mouth; a, bristles; x, arcliitroch ; s, lateral sense-organs. E,
Polyarthra platyptera: oc, eye-spot; x' ', isolated tufts representing a cephalo-
troch; T, brancbiotrocb ; a, 6, and c, three pairs of appendages which are
moved by the muscles m. F, another figure of Pvlyarthru, to show the position
which the appendages may take up. G, Philodina aculeata: oc, eye-spots; s,
calcar. H, Actinunis nrplunius: oc, eye-spots; s, calcar. I, Asplanchrut .-(<-
boldii, male, viewed from the abdominal surface; a, anterior short arms; o,

rterior longer arms : in, mouth; x' , cephalotrochic tufts; j-, brancliiotroch.
Asplanchna siebohlii, female; letters as before. K, Xoteus ijttuilricortu's,
to show the extent to which the lorica may become sculptured. (AH, except
where otherwise stated, from Pritchard.)

Several genera present an external casing or sheath or
tube which is termed an "urceolus." In Floscularia and
Stephanoceros the urceolus is gelatinous and perfectly
hyaline ; in Conochifus numerous individuals live in such a
hyaline urceolus arranged in a radiating manner. The
urceolus, which is secreted by the animal itself, may
become covered with foreign particles, and in one species,
the well-known Melictrta ritiyens, the animal builds up its
urceolus with pellets which it manufactures from foreign

M

90

ROTIFERA

particles, and deposits in a regular oblique or spiral series,
and which are cemented together by a special secretion.
The urceolus serves as a defence, as the animal can by con-
tracting its stalk withdraw itself entirely within the tube.
Locomotor Organs. — While, as mentioned above, several
genera or individual species present long spines, these
become movable, and may be spoken of as appendages, in
two genera only. Jin Polyarthra (fig. 1, E, r) there are
four groups of processes or plumes placed at the sides of

A

Flo. 2. — Floscularia appendiculata. A and B represent the same animal, some of
the organs being shown in one figure and snme in the other, oc, eye-spots ; g,
nerve ganglion ; p, pharynx (the mouth should be shown opening opposite the
letter); ma, the mastax ; e, oesophagus; st, stomach; a, anus, opening the
cloaca; gl, mucous glands in the pseudopodium; 71, nephridia; /, flame-cells;
&/, contractile vesicle ; m, m, muscles.

the body, each of which groups can be separately moved
up and down by means of muscular fibres attached to their
bases, which project into the body. The processes them-
selves are unjointed and rigid. In Pedaliou (fig. 3), a
remarkable form discovered by Dr C. J. Hudson in 1871
(12, 13, 14, and 15), and found in numbers several times
since, these appendages have acquired a new and. .quite
special development. They are six in number, file largest
is placed ventrally at some distance below the mouth. Its
free extremity is a plumose fan-like expansion (fig. 3,
A, «, and H). It is (in common with the others) a hollow'
process into which run two pairs of broad, coarsely trans-
versely striated muscles. Each pair has a single insertion
on the inner wall — -the one pair near the free extremity of
the limb, the other near its attachment ; the bands run
up, one of each pair on each side and run right round
the body forming an incomplete muscular girdle, the ends
approximating in the median dorsal line. Below this
point springs the large median dorsal limb, which termin-
ates in groups of long seta?. It presents a single pair of
muscles attached along its inner wall which run up and
form a muscular girdle round the body in its posterior
third. On each side is attached a superior dorso-lateral
and an inferior ventro-lateral appendage, each with a fan-
like plumose termination consisting of compound hairs,
found elsewhere only among the Crustacea ; each of these

is moved by muscles running upwards towards the neck
and arising immediately under the trochal disk, the inferior
ventro-lateral pair also presenting muscles which form a
girdle in the hind region of the body. Various other
muscles are present : there are two complete girdles in the
neck region immediately behind the mouth; there are also
muscles which move the hinder region of the body. In
addition to these the body presents various processes
which are perhaps some of them unrepresented in other
Piotifers. In the median dorsal line immediately below
the trochal disk there is a short conical process presenting
a pair of muscles which render it capable of slight move-
ment. From a recess at the extremity of this process
spring a group of long setose hairs the bases of which are
connected with a filament probably nervous in nature.
This doubtless represents a structure found in many
Kotifers, and varkrasly known as the "calcar," "siphon,"
" tentaculuin," or "antenna." This calcar is double in
Tubieolaria and Melicerta. It is very well developed in
the genera Rotifer, Philodina, and others, and is, when so
developed, slightly retractile. It appears to be repre-
sented in many forms by a pit or depression set with hairs.
The calcar has been considered both as an intromittent
organ and a respiratory tube for the admission of water.
It is now, however, universally considered to be sensory
in nature. Various forms present processes in other parts

Wff. *

FIG. 3. — Pedalion mira. A, Lateral surface view of an adult female : a, median
ventral appendage; b, mrtlhm dnr.sal appendage; c, inferior ventro-laterul
appeudagr : d, superior dorso-lateral appendage ; /, dorsal sense-organ (calcar) ;
g, "chin;" .r, cephalotroch. B, lateral view, showing the viscera: oc, eye-
spots; n, nephridia; e, ciliated processes, prohably serving fur attachment;
other letters as above. C, ventral view: x' , cephalotroch; x, branclilotroch;
other letters as above. D, ventral view, showing the musculature (cf. text).
E, dorsal view of a male: a, lateral appendages; It, dorsal appendage. F,
lateral view of a male. G, enlarged view of the sense-organ marked/. H,
enlarged view of the median ventral appendage. (All after Hudson.)

of the body which have doubtless a similar function, e.g.,
J/iVr«r,«/o» (fig. 1, D, s) with its pair of lateral organs.
Pedalwn presents a pair of ciliated processes in the
posterior region of the body (fig. 3, B, c, and D, e), which
it can apparently use as a means of attachment ; Dr
Hudson states that he has seen it anchored by these and
swimming round and round in a circle. They possibly re-

ROTIFERA

present the flaps found on the tail of other forms.
alsu liiis a small ciliated muscular process (lii;-. .'!, A, <i) placed
immediately below the mouth, and termed a "chin," which
appears to bo merely a greater development of a sort of
lower lip which occurs in many Rotifers.

Siisli-in. — All the Soti/t-ni, prrsent a muscular system
which is grnrrally very well developed. Transverse striation occurs
among the iibres to a varying extent, being well marked in cases
whore the nuisrle is much used. The muscles which move the
hody as a whole are arranged as circular and longitudinal series,
Imt they are arranged in special groups and do not form a com-
plete layer of the body-wall as in the various worms. Some of the
longitudinal muscles are specially developed in connexion with the
tail or pedicle. Other muscles are developed in connexion with
special systems of organs, — the trochal disks, the jaw apparatus,
and the. reproductive system. The muscles ill connexion with the
trochal disk serve to protrude or withdraw it, and to move it about,
when extruded, in various directions. The protrusion is probably,
however, generally effected by the elasticity of the integument
coming into play during the relaxation of the retractor muscles, and
by a general contraction of the body wall. The tentaeuliferous
apparatus of Polyzoa and Gephyrca is protruded in the same manner.

Trochal Disk. — This structure is the peculiar characteristic of
the class. It is homologous with the ciliated bands of the larva?
of Echinoderms, Chrttopods, Molluscs, &c. , and with the tenta-
culiferuus apparatus of 2'ofy;oa and Gephyrca, and has been termed
in common with these a "velum." This velum presents itself in
various stages of complexity. It is found as a single circum-oral
ring (pilidium), as a single prx-oral ring (Chetopod larva;), or as
a single prre-oral ring coexisting with one or more post-oral rings
(Cluetopod larva;, Holothuriau larva?). "We may here assume that
the ancestral condition was a single circum-oral ring associated
with a terminal mouth and the absence of an anus, and that the exist-
ence of other rings posterior to this is an expression of metameric
segmentation, i.e., a repetition of similar parts. With the develop-
ment of a prostomiate condition a certain change necessarily takes
place in the position of this band: a portion of it comes to lie
longitudinally; but it may still remain a single band, as in the
larva of many Echinoderms. How have the other above-mentioned
conditions of the velum come about ? How has the prte-oral band
been developed ? Two views have been held with regard to this
question. According to the one view, the fact whether the single
band is a pros-oral or a post-oral one depends upon the position in
which the anus is about to develop. If the anus develops in such
a position that mouth and anus lie on one and the same side of the
band, the latter becomes pne-oral ; if, however, the anus develops
so that the mouth and anus lie upon opposite sides of the band,
the band becomes post-oral. If we hold this view we must consider
any second band, whether pr<je- or post-oral, to arise as a new
development. The other view premises that the anus always forms
so as to leave the primitive ring or "architroch" post-oral, i.e. ,
between mouth and anus. Concurrently with the development of
a prostomium this architroch somewhat changes its position and
the two lateral portions come to lie longitudinally ; these may be
supposed to have met in the median dorsal line and to have
coalesced so as to leave two rings — the one pros-oral (a " cephalo-
troch"), the other post-oral (a " branchiotroch"); this latter may
atrophy, leaving the single prte-oral ring, or it may become further
developed and thrown into more or less elaborate folds. The exist-
ing condition of the trochal disk or velum in the Rotifcra seems to
the writer of this article to bear out the latter view as to the way
in which modifications of the velum may have come about.

In its simplest condition it forms a single circum-oral ring, as in
Mii'focoilon (tig. 1, D). The structures at the sides of the mouth
in this form are stated to be bristles, and have therefore nothing
to do with the velum (fig. 4, A, p). This simple ring may become
thrown into folds, so forming a series of processes standing up
around the mouth; this is the condition in Steplianoceros (fig. 4, B,;;).
There are, however, but few forms presenting this simple condi-
tion ; and it must be remembered that the evidence for the assump-
tion here made, that this is a persistent architroch and not a bran-
chiotroch persisting where a cephalotroeh has vanished, is not at
present conclusive. This band, may, while remaining single and
perfectly continuous, become prolonged around a lobe overhanging
the mouth — a prostomium. This condition occurs in Philu/im/
(fig. 4, E, P, p); the two sides of the post-oral ring do not inert
dorsally, but are carried up and are continuous with the row of
cilia lining the "wheels." There is thus one continuous ciliated
baud, a portion of which runs up in front of the mouth. This
condition corresponds to that of the Auricularian larva. The fold-
ing of the band has become already somewhat complicated ; a
hypothetical intermediate condition is shown in fig. 4, c, D. The
next stage in the advancing complexity is that the prostomial por-
tion of the baud (fig. 4, G, H, p') becomes separated as a distinct
ring, a eephalotroch ; we find such a stage in Lacinularitt (fig. 4,

o, H), where both cephalotroeh and branchiotroch remain fairly
simple in shape. In Mclia-rta (fig. 4, I, j) both cephalotroeh and
branchiotroch are thrown into folds. Lastly, we find that in surh
forms as Brachiomts the cephalotroeh becomes first convoluted ami

p'

FIG. 4.— Diagrams of the Trochal Disk. A, Mitrocochn. B, StepJiatiocens ; the
mouth lies in the centre of a group of tentacles. C, hypothetical intermediate
form between Aficrocodoii and I'hifodina, showing the development of a pro-
stomial portion of the velum. D, dorsal view of the same. E, Philodina. F,
dorsal view of the same. G, Lacinularia; the dotted line represents the por-
tion of the velum which has become separated as a special ring — a cephalotroeh.
H, dorsal view of the same. I, Melicerta ; the dotted line represents the
cephalotroeh; both this and the branchiotroch have become thrown into folds.
J, dorsal view of the same. K^Brachtonus ; there is a large prze-oial lobe
with three ciliated regions, shown by the dotted lines c, c, a discontinuous
cephalotroeh. L, dorsal view of the same.

in, mouth ; p, f>', velum ; p, architroch ; p', portion of the architroch which
becomes carried forward to line the prostomial region, but does not become
separated ; c, cephalotroeh. (Original.)

then discontinuous (fig. 4, K, L, c), and further it may become so
reduced as to be represented only by a few isolated tufts, as in
Asplanehna (fig. 1, I, x and x'); in such a form as Lindia (fig. 6, c)
the branchiotroch has vanished and the cephalotroeh has become
reduced to the two small patches at the sides of the head.

The trochal apparatus serves the Rotifcra as a locomotive organ
and to bring the food particles to the mouth ; the cilia work so as
to produce currents towards the mouth.

Digestive System. — This consists of the following regions: — (I)
the oral cavity; (2) the pharynx; (3) the oesophagus; (4) the
stomach ; (5) the intestine, which terminates in an anus. The
anus is absent in one group.

The pharynx contains the maslax with its teeth ; these are
calcareous structures, and are known as the trophi. In a typical
mastax (8, 9) (Bra-
chionus, fig. 5, A)
there are a median
anvil or incus and
two hammer-like
portions, mallei.
The incus consists
of two rami (e)
resting upon a cen-
tral fulcrum (f] ;
each malleus con-
sists of a handle or
marmbrium (c) and
a head or uncus
(d), which often
presents a comb-
like structure. Fig.
5 shows some of

the most important Flo ,,._T hi of various forms . A, BraManus. B,
niodincationswhicll piglena forcipata ; C, Asplanehna ; D, Pliilalina. f,
the apparatus may fulcrum, ande,<?, rami, forming the incus; r, nianubrium,
exhibit The parts ant* <A uncus' forming the malleus. (After Hudson.)
may become very slender, as in Diylcna forclpata (fig. 5, B) ; the
mallei may be absent, as in Asplanehna (fig. 5, c), the rami being
highly developed into curved forceps and movable one on the other ;
or, the manubria being absent and the fulcrum rudimentary, the
rami may become massive and Bubquadratie, as in Philodina (fig.
5, D). All the true Rotifers possess a mastax. Ehrenberg's group
of the Affomphia consisted of a heterogeneous collection of forms,
— IcUthijdium and Ch&tonolus being Gastrotricha, and Cyphonaulef

92

ROTIFERA

a Polyzoan larva, while Eutcropka is probably a male Rotifer, and,
like the other males, in a reduced condition. There is no reason for
considering this mastax as the homologue of either the gastric mill of
Crustaceans on the one hand or the teeth in the Chajtopods' pharynx
on the other ; it is merely homoplastic with these structures, but has
attained a specialized degree of development. Both the pharynx
and the cesophagus which follows it are lined with chitin. The
oesophagus varies in length and in some genera is absent (Philo-
elinadae), the stomach following immediately upon the pharynx.
The stomach is generally large ; its wall consists of a layer of very
large ciliated cells, which often contain fat globules and yellowish-
green or brown particles, and outside these a connective tissue
membrane ; muscular fibrillre have also been described. Very
constantly a pair of glands open into the stomach, and probably
represent the hepato-pancreatic glands of other Invertebrates.

Following upon the stomach there is a longer or shorter intestine,
which ends in the cloaca. The intestine is lined by ciliated cells.
In forms living in an urceolus the iutestine turns round and runs
forward, the cloaca being placed so as to debouch over the margin
of the urceolus. The cloaca is often very large ; the uephridia and
oviducts may open into it, and the eggs lodge there on their way
outwards ; they are thrown out, as are the fecal masses, by an
eversion of the cloaca. Asplanchna, Notominata sicboldii, and cer-
tain species of Ascomarplia are said to be devoid of intestine or
anus, excrementitious matters being ejected through the mouth (11).

Nephridia. — The ccelom contains a fluid in which very minute
corpuscles have been detected. There is no trace of a true vascular
system. The nephridia (fig. 2, B, n) present a very interesting
stage of development. They consist of a pair of tubules with an
iutracellular lumen running up the sides of the body, at times
merely sinuous, at others considerably convoluted. From these
are given off at irregular intervals short lateral branches, each of
which terminates in a flame-cell precisely similar in structure to
the flame-cells found in Planarians, Trematodes, and Cestodes ;
here as there the question whether they are open to the ccelom or
not must remain at present undecided. At the base these tubes
open either into a permanent bladder which communicates with the
cloaca or into a structure presenting apparently no advance in its
development upon the contractile vacuole of a ciliate Infusoriau.

Nervmw System and Sense-Organs. — Various structures have been
spoken of as nervous which are now acknowledged to have been
erroneously so described (18). There is a supra-cesophageal gang-
lion which often attains considerable dimensions, and presents a
lobed appearance (fig. 2, A and B, g). Connected with this are the
eye-spots, which are seldom absent. AVhere these are most highly
developed a lens-like structure is present, produced by a thicken-
ing of the cuticle. In the genus Rotifer and other forms these are
placed upon the protrusible portion of the head, and so appear to
have different positions at different moments. The number of eye-
spots varies from one to twelve or more. They are usually red, red-
dish-brown, violet, or black in colour. Other structures are found
which doubtless act as sense-organs. The calcar above-mentioned
generally bears at its extremity stiff hairs which have been demon-
strated to be in connexion with a nerve fibril. On the ventral sur-
face of the body just below the mouth a somewhat similar structure
is often developed — the chin. There are besides at times special
organs, like the two lateral organs in Microcodon (fig. 1, D, s), which
no doubt in common with the calcar and chin have a tactile function.

Reproductive Organs and Development. — The Eotifera were
formerly considered to be hermaphrodite, but, while the ovary was
always clear and distinct, there was always some difficulty about
the testis, and various structures were put forward as representing
that organ. One by one, however, small organisms have been dis-
covered and described as the males of certain species of Rotifers,
until at the present time degenerated males are known to occur in
all the families except that of the Philodiiiadss. The male Rotifers
are provided with a single circlet of cilia (a peritroeh), a nerve
ganglion, eye-spots, muscles, and nephridial tubules all in a some-
what reduced condition, but there is usually no trace of mouth or
stomach, the main portion of the body being occupied by the testi-
cular sac. There is an aperture corresponding with the cloaca of the
female, where the testis opens into the base of an eversihle penis.
The males of Floscularia are shown in fig. 1. The male of Pedalion
mira possesses rudimentary appendages, The ovary is usually a
large gland lying beside the stomach connected with a short oviduct
which opens into the cloaca. The ova often present a reddish hue
(Philodina roseola, Brachionus rubens), due doubtless, like the red
colour of many Crustacean ova, to the presence of tetrouerythrin.

Up to the present our embryological knowledge of the group is
very incomplete. Many Rotifers are known to lay winter and
summer eggs of different character. The winter eggs are provided
with a thick shell and probably require fertilization. Two or three
of them are often carried about attached to the parent (Brachionus,
Notominata), but they are usually laid and fall into the mud, there
to remain till the following spring. The summer eggs are of two
kinds, the so-called male and female ova, both of which are stated
to develop parthenogenetically. They may be carried about in

large numbers in the cloaca or oviduct or attached to the body of
the parent. The female ova give rise to female and the male ova
to male individuals. Male individuals are only formed in the
autumn in time to fertilize the winter ova.

Habitat and Mode of Life. — The Rotifera are distri-
buted all over the earth's surface, inhabiting both fresh
and salt water. The greater number of species inhabit
fresh water, occurring in pools, ditches, and streams. A
few species will appear in countless numbers in infusions
of leaves, <tc., but their appearance is generally delayed
until the putrefaction is nearly over. Species of Rotifer
and Philodina appear in this way. A few marine forms
only have been described — Brachionus mulleri, B. hepta-
tomis, Synchxta baltica, and others.

A few forms are parasitic. Albertia lives in the intestine
of the earthworm ; a form has been described as occurring
in the body-cavity of Synapta ; a small form was also
observed to constantly occur in the velar and radial canals
of the freshwater jelly-fish, Limnocodium. Notommata
parasitica leads a parasitic existence within the hollow
spheres of Volvox globator, sufficient oxygen being given
off by the Volvox for its respiration.

Many Rotifers exhibit an extraordinary power of resist-
ing drought. Various observers have dried certain species
upon the slide, kept them dry for a certain length of time,
and then watched them come to life very shortly after the
addition of a drop of water. The animal draws itself to-
gether, so that the cuticle completely protects all the softer
parts and prevents the animal itself from being thoroughly
dried. This process is not without parallel in higher
groups ; e.g., many land snails will draw themselves far into
the shell, and secrete a complete operculum, and can remain
in this condition for an almost indefinite amount of time.
The eggs are also able to withstand drying, and are pro-
bably blown about from place to place. The Rotifera can
bear great variations of temperature without injury.

Since their removal from among the Protozoa various
attempts have been made to associate the Rotifera with
one or other large phylum of the animal kingdom.
Huxley, insisting upon the importance of the trochal disk,
put forward the view that they were " permanent Echino
derm larvas," and formed the connecting link between
the Nemertidx and the Nematoid worms. Ray Lankester
proposed to associate them with the Chxtopoda and
Arthropoda in a group Apptndicidata, the peculiarities in
the structure of Pedalion forming the chief reason for
such a classification. There is, however, no proof that we
thus express any genetic relationship. The well-developed
ccelom, absence of metameric segmentation, persistence of
the trochal disk in varying stages of development, and the
structure of the nephridia are all characters which point to
the Rotifera as very near representatives of the common
ancestors of at any rate the Mollusca, Arthropoda, and
Chxtopoda. But the high development of the mastax,
the specialized character of the lorica in many forms, the
movable spines of Polyarthm, the limbs of Pedalion, and
the lateral appendages of Asplanchna, the existence of a
diminutive male, the formation of two varieties of ova, all
point to a specialization in the direction of one or other of
the above mentioned groups. Such specialization is at
most a slight one, and does not justify the definite associa-
tion of the Rotifera in a single phylum with any of them.

Classification. — The following classification has been
recently put forward by Dr C. T. Hudson (19).

CLASS ROTIFERA.
Order I. — Rhizota.

Fixed forms ; foot attached, transversely wrinkled, non-retractile
truncate.

Fam. 1. FLOSCULARIADJE. Floscularia, Stcphanoceros.
Fain. 2. MELICERTADJE. , Meliccrta, Ccphalosiphon, Mcgalo-
troc/ta, Limnias, ^Ecistes, Lacinularia, Conochilus.

R 0 T T F E R A

93

Order II.--Bdelloida.

and . civrp like a leech ;
'

foot retractile

Forms which swim
jiintrd, Iclfsrupir, termination I'mvati

Fam. 3. PHILODINADJE. Philodina, Rotifer, C<tllii/iiin.

Order III.— Ploi'ma.

Forms which swim only.

Grade A. ILLOKICATA.

Fnm. 4. HYDATINAD.E. ffi/fln/inn, filiinojis.
Fain. 5. SYNCHATADA Synf/i.r/ii, I'li/ifurtlira.
Fan). 6. NOTOMMATAD-B. Xolom main , l>i'//rnti, Fu fcnlti fi,l t

Scaritliam, Pleurotrocha, Distcmma.
Fam. 7. TRIARTHHAD.E. Triartkra.
Fam. 8. ASPLANCHNAD^E. Asplanclm

Grade B. LORIQATA.
Fam. 9. BRACIIIONID^:. Bntdiionus, Nolens, Anurxa, Sac-

culiis. '

l''ain. 10. PrERODlNADte. Ptcrodiim, PompJiolyx.
Fam. 11. EUCHLANID.K. Eueh.lanis, Salpina, Diplax, Mono-

s/!/lit, Colurus, Monura, Mclopodia, Stcphanops, Monoccrca,

MasMgocerca, Dinocharis.

Oriler IV.— Scirtopoda.

Forms which swim with their ciliary wreath, and skip by means
of hollow limbs with internal locomotor muscles.

Fam. 12. PEDALIONIDJH. Pcdalion.

The above list includes only the principal genera. There are,
however, a number of forms which could not be placed in any of
the above families.

ABERRANT FORMS,

Trochosphxra xquatorialis (fig. 6, G), found by Semper
in the Philippine Islands, closely resembles a monotrochal

Fit;. G. — Various aberrant fnrms. A, BfJatro cah'us (after Claparedc): a, mastjix.
B, Seiwn neljfttiiK (after Claus): mt mouth; rrf, position of the aperture of the
vas acffivns. C, Lindia torulosa: a, ciliated processes at the sidts of the head
representing: cephalotroch ; or, eye-spots. D, E, and F, Apsilus lentiformia
(after Mcr/jiik'iw). D, adult female Avith expanded proboscis: /», position of
the mouth ; s, lateral scnsc-ortrans. E. youn? free-swimming female. F, adult
male. G, 7r<iclt<i*jili:rr*t ;t_-<iniit'ii'ii<lix (after Semper): m, mouth ; </, ganglion;
(?, anus; b, velum; oc, eye-spot; c, muscles.

polychsstoufl larva while possessing undoubtedly Rotiferal
characters. Mecznikow h;is described a remarkable form,

A/ixi/i/.t l< nliformis (fig. G, D, E, and F), tho adult I'ennl''
of which is entirely devoid of cilia but possesses a sort of
retractile hood ; the young female and the males are not
thus modified. Claparede discovered fixed to the bodies
of small Oligochsetes a curious non-ciliated form, Bnl'iiro
calvus (fig. 6, A), which has a worm-like very contractile
body and a well-developed mastax. As mentioned above,
the ciliation is reduced to a minimum in the curious worm-
like form Liiulia (fig. 6, c). Sfison nelalite (fig. 6, B),
living on the surface of Nebalix, which was described
originally by Grube, is the same form as the 8m-r,,/,,/,//,t
nAalix, which was supposed by Van Beneden and Hesse
to be a leech. It has been shown by Claus to be merely
an aberrant Rotifer.

Of the curious aquatic forms Idkydium, Cheetnun/n.-i,
Twbanella, Dasyditis, Gepkalidium, Ckxtura, and Ilemi-
dasys, which Mecznikow and Claparede included under
the name Gastrotricha, no further account can be given
here. They are possibly allied to the Rotifera, but are
devoid of mastax and trochal disk.

Bibliography.

The following are some of the more important memoirs, &c. , on
the Rotifera.

(1) Leeuwenhoek, Phil. Trans., 1701-1704.

(2) EhTeii\)erg,DicI>ifiiswiist!iicjrhciialsvollko»imencOrgfinismcn,

1838.

(3) M. F. Dujardin, Hist. Nat. dcs Zoophytes: Infusoircs, 1841.

(4) W. C. Williamson, "On Mcliccrta ringens," Quart. Join:

Mict. Sci., 1853.

(5) Ph. H. Gosse, "On Mcliccrta ringens," Quart. Jum:

Sci., 1853.

(6) T. H.Huxley, "QnLacinularia sodalis," Truns. M

1853.

(7) Fr. Leydig, " Ueber den Ban und die systematisclie Stellung

der Raderthiere," Zcit. f. w. Zool., vi., 1854.

(8) Ph. H. Gosse, Phil. Trans., 1856.

(9) F. Cohn, Zcit.f, w. Zool, vii., ix., and xii.

(10) Ph. H. Gosse, Phil. Trans., 1S58.

(11) Pritchard, Infusoria, 1861.

(12, 13, 14) C. T. Hudson, "On Pcdalion," Quart. Jour. Mi,-,-.
Sci., 1872, and Monthly Micr. Jour., 1871 and 1872.

(15) E. Ray Lankester, "On Pcdalion," Quart. Jour. Sci., 1872.

(16) El. Mecznikow, "On Apsilus Icntiformis," Zeit. f. w. Zool.,

1872.

(17) C. Semper, "On Trochosnhxm" Zeit. f. w. Zool., xxii..

1872.

(18) K. Eckstein, "Die Rotatorieu der Umgegend von Giessen,"

Zcit.f. w. Zool., 1883.

(19) C. T. Hudson, "On an Attempt to reclassify Rotifers,"

Quart. Jour. Micr. Sci., 1884.

(A. G. B.)

Mia:
i: Soc.,

MOLLUSCA

milE Mollusca form one of the great " phyla," or sub-
JL kingdoms of the Animal Pedigree or Kingdom.

Literary History of the Group. — The shell-bearing forms
belonging to this group which were kiiown to Linuiuus were
placed by him (in 1748) in the third order of his class
Vermes under the name "Testacea," whilst the Echino-
derms, Hydroids, and Annelids, with the naked Molluscs,
formed his second order, termed " Zoophyta." Ten years
later he replaced the name "Zoophyta" by "Mollusca,"
which was thus in the first instance applied, not to the
Mollusca at present so termed, but to a group consisting
chiefly of other organisms. Gradually, however, the term
Mollusca became used to include those Mollusca formerly
placed among the "Testacea," as well as the naked Mollusca.

It is important to observe that the term /uaAa/aa, of which
Mollusca is merely a Latinized form, was used by Aristotle
to indicate a group consisting of the Cuttle-fishes only.

The definite erection of the Mollusca into the position
of one of the great primary groups of the animal kingdom
is due to George Cuvier (1788-1800), who largely occupied
himself with the dissection of representatives of this type (I).1
An independent anatomical investigation of the Mollusca
had been carried on by the remarkable Neapolitan natur-
alist Poli (1791), whose researches (2) were not published
until after his death (1817), and were followed by the
beautiful works of another Neapolitan zoologist, the illus-
trious Delle Chiaje (3).

The " embranchement " or sub-kingdom Mollusca, as de-
fined by Cuvier, included the folio wing classes of shell-fish : —
1, the cuttles or poulps, under the name CEPHALOPODA; 2,
the snails, whelks, and slugs, both terrestrial and marine,
under the name GASTROPODA ; 3, the sea-butterflies or
winged-snails, under the name PTEROPODA ; 4, the clams,
mussels, and oysters, under the name ACEPHALA ; 5, the
lamp-shells, under the name BRACHIOPODA ; 6, the sea-
squirts or ascidians, under the name NUDA ; and 7, the
barnacles and sea-acorns, under the name CIRRHOPODA.

The main limitations of the sub-kingdom or phylum
Mollusca, as laid down by Cuvier, and the chief divisions
thus recognized within its limits by him, held good to the
present day. At the same time, three of the classes con-
sidered by him as Mollusca have been one by one removed
from that association in consequence of improved know-
ledge, and one additional class, incorporated since his day
with the Mollusca with general approval, has, after more
than forty years, been again detached and assigned an
independent position owing to newly-acquired knowledge.

The first of Cuvier's classes to be removed from the Mol-
lusca was that of the Cirrhopoda. Their affinities with the
lower Crustacea were recognized by Cuvier and his contem-
poraries, but it was one of the brilliant discoveries of that
remarkable and too-little-honoured naturalist, J. Vaughan
Thompson of Cork, which decided their position as Crus-
tacea. The metamorphoses of the Cirrhopoda were described
and figured by him in 1830 in a very complete manner,
and the legitimate conclusion as to their affinities was for-
mulated by him (4). Thus it is to Thompson (1830), and
not to Burmeister (1834), as erroneously stated by Kefer-
stein, that the merit of this discovery belongs. The next
class to be removed from Cuvier's Mollusca was that of the
Nuda, better known as Tunicata. In 1866 the Russian
embryologist Kowalewsky startled the zoological world with
a minute account of the developmental changes of Ascidia,
one of the Tunicata (5), and it became evident that the

1 These figures refer to the bibliography at the end of the article,
p. 695.

affinities of that class were with the Vertebrata, whilst their
structural agreements witli Mollusca were only superficial.
The last class which has been removed from the (Juviurhm
Mollusca is that of the Lamp-shells or Brachiopoda. The
history of its dissociation is connected with that of the
class, viz., the Polyzoa or Bryozoa, which has been both
added to and again removed from the Mollusca between
Cuvier's date and the present day. The name of J.
Vaughan Thompson is again that which is primarily con-
nected with the history of a Mollusean class. In 1830
he pointed out that among the numerous kinds of " polyps"
at that time associated by naturalists with the Hyclroids,
there were many which had a peculiar and more elaborate
type of organization, and for these he proposed the name
Polyzoa (6). Subsequently (7) they were termed Bryozoa
by Ehrenberg (1831).

Henri Milne-Edwards in 1844 demonstrated (8) the affi-
nities of the Polyzoa with the Mollusean class Brachiopoda,
and proposed to associate the three classes Brachiopoda,
Polyzoa, and Tunicata in a large group " Molluscoidea,"
coordinate with the remaining classes of Cuvier's Mollusca,
which formed a group retaining the name Mollusca. By
subsequent writers the Polyzoa have in some cases been kept
apart from the Mollusca and classed with the "Vermes;"
whilst by others (including the present writer) they have,
together with the Brachiopoda, been regarded as true Mol-
lusca. The recent investigation by Mr. Caldwell (1882)
of the developmental history of Phoronis (9), together
with other increase of knowledge, has now, however, estab-
lished the conclusion that the agreement of structure
supposed to obtain between Polyzoa and true Mollusca is
delusive ; and accordingly they, together with the Brachi-
opoda, have to be removed from the Mollusean phylum.
Further details in regard to this, the last revolution in Mol-
lusean classification, will be found in the article POLYZOA.

As thus finally purified by successive advances of em-
bryological research, the Mollusca are reduced to the
Cuvierian classes of Cephalopoda, Pteropoda, Gastropoda,
and Acephala. Certain modifications in the disposition of
these classes are naturally enough rendered necessary by
the vast accumulation of knowledge as to the anatomy and
embryology of the forms comprised in them during fifty
years. Foremost amongst those who have within that
period laboured in this group are the French zoologists
Henri Milne-Edwards (20) and Lacaze Duthiers (21), to
the latter of whom we owe the most accurate dissections
and beautiful illustrations of a number of different types.
To Kolliker (22), Gegenbaur (23), and more recently Spengel
(24), amongst German anatomists, we are indebted for
epoch-making researches of the same kind. In England,
Owen's anatomy of the Pearly Nautilus (10), Husky's dis-
cussion of the general morphology of the Mollusca (11),
and Lankester's embryological investigations (12), have
aided in advancing our knowledge of the group. Two
remarkable works of a systematic character dealing with
the Mollusca deserve mention here — the JAinunl of f/ic
Mollusca by the late Dr. S. P. Woodward, a model of clear
systematic exposition, and the exhaustive treatise on the
Malacozoa or Weichthiere by the late Professor Keferstein
of Gottingen, published as part of Bronn's dawn unil
Ordnungen des Thier-Rri<:hs. The latter work is the most
completely illustrated and most exhaustive survey of exist-
ing knowledge of a large division of the animal kingdom
which has ever been produced, and, whilst forming a monu-
ment to its lamented author, places the student of Mol-
lusean morphology in a peculiarly favourable position.

96

MOLLUSCA

Classes of the Mollusca. — The classes of the Mollusca
which we recognize are as follows : —
Phylum MOLLUSCA.
BRANCH A. — Glossophora. BRANCH B. — Lipocephala

( = Acepkala, L'uvicr).

Class 1. — GASTROPODA. Class 1. — LAMELLIBKANCHIA

Br. a. — Isoplcura. (Syn. Conchifera).

Examples — Chiton, Neo- Examples — Oyster, Mussel,

menia. Clam, Cockle.
Br. b. — Anisopleura.

Examples — Limpet, Whelk,
Snail, Slug.

Class 2. — SCAPHOPODA.
Example — Tooth-shell .

Class 3. — CEPHALOPODA.
Br. a. — Pteropoda.

Examples — Hyalrea, Pneu-

modernion.
Br. b. — Sipkonopoda.

Examples — Nautilus, Cut-
tles, Poulp.

General Characters of the Mollusca. — The forms com-
prised in the above groups, whilst exhibiting an extreme
range of variety in shape, as may be seen on comparing
an Oyster, a Cuttle-fish, and a Sea-slug such as Doris ;
whilst adapted, some to life on dry land, others to the
depths of the sea, others to rushing streams ; whilst capable,
some of swimming, others of burrowing, crawling, or jump-
ing, some, on the other hand, fixed and immobile ; some
amongst the most formidable of carnivores, others feed-
ing on vegetable mud, or on the minutest of microscopic
organisms — yet all agree in possessing in common a very
considerable number of structural details which are not
possessed in common by any other animals.

The structural features which the Mollusca do possess
in common with other animals belonging to other great
phyla of the animal kingdom are those characteristic of
the Ccelomata, one of the two great grades (the other and
lower being that of the Coelentera) into which the higher
animals, or Enterozoa as distinguished from the Protozoa,
are divided (13). The Enterozoa all commence their indivi-
dual existence as a single cell or plastid, which multiplies
itself by transverse division. Unlike the cells of the Proto-
zoa, these embryonic cells of the Enterozoa do not remain each
like its neighbour and capable of independent life, but pro-
ceed to arrange themselves in two layers, taking the form
of a sac. The cavity of the two-cell-layered sac or Diblas-
tula thus formed is the primitive gut or ARCH-ENTERON.
In the Ccelentera, whatever subsequent changes of shape
the little sac may undergo as it grows up to be Polyp or
Jelly-fish, the original arch-enteron remains as the one
cavity pervading all regions of the body. In the Coelomata
the arch-enteron becomes in the course of development
divided into two totally distinct cavities shut off from one
another — an axial cavity, the MET-ENTERON, which retains
the function of a digestive gut ; and a peri-axial cavity,
the CCELOM or body-cavity, which is essentially the blood-
space, and receives the nutritive products of digestion and
the waste products of tissue-change by osmosis. The
Mollusca agree in being Coelomate with the phyla Verte-
brata, Platyhelmia (Flat-worms), Echinoderma, Appendicu-
lata (Insects, Ringed-worms, <fec.), and others, — in fact,
with all the Enturozoa except the Sponges, Corals, Polyps,
and Medusae.

In common with all other Coelomata, the Mollusca
are at one period of life possessed of a PEOSTOMIUM
or region in front of the mouth, which is the essential
portion of the "head," and is connected with the property
of forward locomotion in a definite direction and the steady
carriage of the body (as opposed to rotation of the body
on its long axis). As a result, the Cojlomata, and with
them the Mollusca, present (in the first instance) the general

condition of body known as BTLATEEAL SYMMETEY; the
dorsal is differentiated from the ventral surface, whilst a
right and a left side similar to, or rather the complements
of, one another are permanently established. In common
with all other Ccelomata, the Mollusca have the mouth and
first part of the alimentary canal which leads into the
met-enteron formed by a special invagiuation of the outer
layer of the primitive body-wall, not to be confounded with
that which often, but not always, accompanies the ante-
cedent formation of the arch-enteron ; this invagination
is termed the STOMOM:UM. Similarly, an anal aperture is
formed in connexion with a special invagination which
meets the hinder part of the met-enteron, and is termed

the PEOCTOD.ETTM.

In common with many (if not all) Ccelomata, the Mol-
lusca are provided with at least one pair of tube-like organs,
which open each by one end into the ccelom or body cavity,
and by the other end to the exterior, usually in the neigh-
bourhood of the anus. These are the NEPHRIDIA.

Like all other Coelomata, the Mollusca are also provided
with special groups of cells forming usually paired or median
growths upon the walls of the ccelomic cavity, the cells
being specially possessed of reproductive power, and dif-
ferentiated as egg-cells and sperm-cells. These are the
GONADS. As in other Ccelomata, the cells of the gonads
may escape to the exterior in one of two ways — either
through the nephridia, or, on the other hand, by special
apertures.

As in all other Ccelomata, the cells, which build up
respectively the primary outer layer of the body, the
lining layer of the met-enteron, and the lining layer of the
ccelom, are multiplied and differentiated in a variety of
ways in the course of growth from the early embryonic
condition. TISSUES are formed by the adhesion of a num-
ber of similarly modified cells in definite tracts. As in all
Ccelomata, there is a considerable variety of tissues char-
acterized by, and differentiated in relation to, particular
physiological activities of the organism. Not only the
Ccelomata but also many Ccelentera show, in addition to
the EPITHELIA (the name given to tissue which bounds a
free surface, whether it be that of the outer body-wall, of
the gut, or of a blood-space), also deeper lying tissues,
of -which the first to appear is MUSCULAE tissue, and the
second NEEVOUS tissue.

The epithelia are active in throwing off their constituent
cells (blood-corpuscles from the wall of the ccelom), or in
producing secretions (glands of body-wall and of gut), or
in forming horny or calcareous plates, spines, and pro-
cesses, known as CUTICULAK PEODUCTS (shells and bristles
of the body- wall, teeth of the tongue, gizzard, tic.).

In the Mollusca, as in all other Ccelomata, in correspond-
ence with the primary bilateral symmetry and in relation
to the special mechanical conditions of the prostomium,
the nervous tissue which is in Ccelentera, and even in Flat-
worms, diffused over the whole body in networks, tends
to concentrate in paired lateral tracts, having a special
enlargement in the prostomium. The earlier plexiform
arrangement is retained in the nervous tissue of the walls
of the alimentary canal of many Coelomata, whilst a con-
centration to form large nerve-masses (GANGLIA), to which
numerous afferent and efferent fibres are attached, affects
the nervous tissue of the body-wall.

In all Ccelomata, including Mollusca, muscular tissue is
developed in two chief layers, one subjacent to the cleric or
outer epithelium (SOMATIC MUSCULATURE), and a second sur-
rounding the alimentary canal (SPLANCHNIC MUSCULATURE).
Thus, primarily, in Ccelomata the body has the character of
two muscular sacs or tubes, placed one within the other
and separated from one another by the cculomic space.
The somatic musculature is the more copious and develops

MOLLUSCA

J)7

very generally an outer circular layer (/.... a layr in which
the muscular lilircs run in a direction transverse to the
long axis of tho body) and a deeper longitudinal layer ;
to these oblique and radiating fibres may be added. The
splanchnic musculature, though more, delicate, exhibits a
circular layer nearer the enteric epithelium, and a longi-
tudinal layer nearer the ccelomic surface.

In Ccclomata and in many C'eelentera there are found
distributed between the tracts of muscular tissue, bounding
them and giving strength, and consistency also to the walls
of the body, of the alimentary canal, of the ccelom, and of
the various organs and tissue-masses (such as nerve-centres,
gonads, &c.) connected with these, tracts of tissue the
function of which is skeletal. The SKELETAL TISSUE of
Mollusca, in common with that of other Ccelomata, exhibits
a wide range of minute structure, and is of differing density
in various parts ; it may be fibrous, membranous, or carti-
laginous. The Mollusca, in common with the other Coelo-
mata, exhibit a remarkable kind of association between the
various forms of skeletal tissue and the epithelium which
lines the ccelomic cavity. The ccelomic cavity contains a
liquid which is albuminous in chemical composition (BLOOD-
LYMPH or H.EMOLYMPH), and into this liquid cells are shed
from the ccelomic epithelium. They float therein and are
known as BLOOD CORPUSCLES or LYMPH CORPUSCLES. The
ccelomic space with its contained hsemolymph is not usually
in Ccelomata, and is not in Mollusca, a simple even-walled
cavity, but is broken up into numerous passages and re-
cesses by the outgrowths, both of the aliiiientary canal and
of its own walls. By the adhesion of its opposite walls,
and by an irregularity in the process of increase of its area
during growth, the ccelom becomes to a very large extent a
spongy system of intercommunicating LACUN.E or irregular
spaces, filled with the ccelomic fluid. At the same time, the
ccelomic space has a tendency to push its way in the form of
narrow canals and sinuses between the layers of skeletal tissue,
and thus to permeate together with the skeletal tissue in
the form of a spongy, or it may be a tubular, network all
the apparently solid portions of the animal body. This
association of the nutritive and skeletal functions is accom-
panied by a complete identity of the tissues concerned in
these functions. Not only is there complete gradation
from one variety of skeletal tissue to another (e.g., from
membranous to fibrous, and from fibrous to cartilaginous)
even in respect of the form of the cells and their intercellular
substance, but the ccelomic epithelium, and consequently
the hseniolymph with its floating corpuscles derived from
that epithelium, is brought into the same continuity. The
skeletal and blood-containing and -producing tissues in fact
form one widely- varying but continuous whole, which may
be called the SKELETO-TROPHIC SYSTEM OF TISSUES.

In many Coeloniata not only do the skeletal tissues
allow the ccelomic space with its fluid and corpuscles to
penetrate between their layers, but a special mode of
extension of that space is found, which consists in the
hollowing out of the solid substance of elongated cells
having the form of fibres, which thus become tubular,
and, admitting the nutritive fluid, serve as channels for
its distribution. These are " capillary vessels," and it has
yet to be shown that such are formed in the Mollusca.
Larger vessels, however, concerned in guiding the move-
ment of the coelomic fluid in special directions are very
usually developed in the Mollusca, as in other Ccelomata,
by the growth of skeletal tissue around what are at first
ill-defined extensions of the ccelomic space. In this way
a portion of the ccelomic space becomes converted into
vessels, whilst a large part remains with irregular walls
extending in every direction between the skeletal tissues
and freely communicating with the system of vessels. As
in many other C'cclomata, muscular tissue grows around

the largest vessel fnnnnl from the primitive cnclom, which
thus becomes a contractile organ for propelling the blood-
lymph fluid. This "III;\I:T" has in Mollusca, as in most
other Ccelomata in which it is developed, a dorsal position.
A communication of the blood-lymph space with the
exterior by means of a pore situated in the foot or else-
where has been very generally asserted to be characteristic
of Mollusca. It has been maintained that water is intro-
duced by such a pore into the blood, or admitted into a
special series of water-vessels. It has also been asserted
that the blood-fluid is expelled by the Mollusca from these
same pores. Recent investigation (14) has, however, made
it probable that the pores are the pores of secreting glands,
and do not lead into the vascular system. There is, it there-
fore appears, no admission or expulsion of water through
such pores in connexion with the blood, although in some
other Coelomata it is established that water is taken into
the coelomic space through a pore (Echinoderms), whilst in
some others there is no doubt that the ccelomic hsemolymph
is occasionally discharged in quantity through pores of defi-
nite size and character (Earthworm, <tc.).

We have thus seen that the Mollusca possess, in common
with the other Ccelomata — 1, a body composed of a vast
number of cells or plastids, arranged so as to form a sac-
like body-wall, and within that a second sac, the met-enteron,
the wall of which is separated from the first by a axiom or
blood-lymph space ; 2, a stomodxum and a proctodxum ;
3, a prostomium, together with a differentiated dorsal and
ventral surface, and consequently right and left sides, i.e.,
bilateral symmetry ; 4, a pair of nephridia ; 5, gonads
developed on the wall of the ccelom ; 6, deric epithelium
(producing horny and calcareous deposits on its surface),
enteric epithelium, and coelomic epithelium ; 7, laterally
paired masses of nerve-tissue, especially large in the pro-
stoniial region (nerve-centres or ganglia) ; 8, muscular
tissue, forming a somatic tunic and a splanchnic tunic ; 9,
skeleto-trophic tissues, consisting of membranous, fibrous, and
cartilaginous supporting tissries, and of blood-vessels and the
walls of blood-spaces, the ccelomic epithelium, and the liquid
tissue known as hzcmolymph (commonly blood).

Schematic Mollusc. — Starting from this basis of structural
features common to them and the rest of the Ccelomata,
we may now point out what are the peculiar developments
of structure which characterize the Mollusca and lead to
the inference that they are members of one peculiar branch
or phylum of the animal pedigree. In attempting thus to
set forth the dominating structural attributes of a great
group of organisms it is not possible to make use of arbi-
trary definitions. Of Mollusca, as of other great phyla, it
is not possible categorically to enunciate a series of struc-
tural peculiarities which will be found to be true in refer-
ence to every member of the group. We have to remember
that the process of adaptation in the course of long ages
of development has removed in some cases one, in other
cases another, of the original features characteristic of the
ancestors from which the whole group may be supposed to
have taken origin, and that it is possible (and actually is
realized in fact) that some organisms may have lost all the
/'/•iiiKtry characteristics of Molluscan organization, and yet
be beyond all doubt definitely stamped as Mollusca by
the retention of some secondary characteristic which is so
peculiar as to prove their relationship with other Mollusca.
An example in point is found in the curious fish-like form
Phyllirhoe (fig. 58), which has none of the primary char-
acteristics of a Mollusc, and yet is indisputably proved to
belong to the Molluscan phylum by possessing the peculiar
and elaborate lingual apparatus present in one branch of
the phylum, the Gllossophora.

In order to exhibit concisely the peculiarities of organi-
zation which characterize the Mollusca, we find it most

N

MOLLUSCA

convenient to construct a schematic Mollusc, which shall
possess in an unexaggerated form the various structural
arrangements which are more or less specialized, exagger-
ated, or even suppressed in particular members of the group.
Such a schematic Mollusc is not to be regarded as an arche-

Fin. I.— Schematic Mollusc. A. Dorsal aspect. B. Ventral aspect. C. The
heart, pericardium, gonads, and nephridia shmvn in position. D. The nervous
system ; the reader is requested to note that the cord passing backwards
from g.pe lies beneath, and docs not in any way unite with the cord which
passes from g.ab to g.pl. E. Diagram in which the body-wall is represented
as cut in the median antero-posterior plane, so as to show organs in position,
— the shell-sac is seen in section, but the shell is omitted.

Letters in all the figures as follows :— n, cephalic tentacle ; fc, head ; c, edge
of the mantle-skirt or linibus pallialis ; d, dotted line indicating the line of
origin of the free mantle-skirt from the sides of the visceral hump ; c, outline
of the foot seen through the mantle-skirt in A, which is supposed to be trans-
parent, allowing the position of this and of the various parts h, i, k, I, m, to
be seen through its substance ; f, edge of the shell-follicle ; 17, the shell ; A,
the osphradiuin, paired (Spengel's olfactory organ) ; »', the ctenidium, paired
(gill-plume) ; k, aperture of the gonad, paired ; 1, aperture of one of the two
nephridia ; m, anus ; 71, posterior region of the foot reaching farther back
than the mass of viscera (dorsal hump) which it carries ; o, mouth ; p, plantar
surface of the foot ; <;, cut edge of the body- wall of the dorsal region ; r,
crelomic space (blood-lymph space or body-cavity), mostly occupied by liver,
but to some extent retained as blood-channels and lacunte ; s, pericardia!
cavity ; t, gonad (ovary or spermary), paired ; ?(, nephridium, paired ; r, ven-
tricle of the heart receiving the right and the left auricles at its sides, and
sending off anteriorly a large vessel, posteriorly a small one ; n\ the cephalic
eye, paired ; x, dotted ring to show the position occupied by the resophagus
in relation to the nerve ganglia and cords ; y, the otocyst, paired ; z.l, the
digestive gland (so-called "liver") of the left side ; z.g, duct of the digestive
gland of the right side ; g.c, cerebral ganglion united by the cerebral com-
missure to its fellow; ct.pl, plenral ganglion united by the cerebro-pleural
connective to the cerebral ganglion, and by the plenro-pi'ibn enimivtive to
the pedal gan^liun ; <t.pc, the pedal ganglion united to its IVllnu' liy tin- peihl
commissure — the two sending otr posteriorly tin- long ladder-like pair "I1 pedal
nerves; g.v, the visceral ganglion (or the lelt side) united by the visceral
loop or commissure to the similar ganglion on the right side, and by Ilie
viscero-pleural connective to the pleuraJ ganglion ; ii.ab, abdominal ganglion
developed on the course of the visceral loop ; g.nlf, olfactory ganglion placed
near the osphradiuin on a nerve taking its origin from the visceral ganglion.

type, in the sense which has been attributed to that word,
nor as the embodiment of an idea present to a creating mind,
nor even as an epitome of developmental laws. Were know-
ledge sufficient, we should wish to make this schematic

Mollusc the representation of the actual Molluscan ancestor
from which the various living forms have sprung. To defi-
nitely claim for our schematic form any such significance
in the present state of knowledge would be premature,
but it may be taken as more or less coinciding with what
we are justified, under present conditions, in picturing to
ourselves as the original Mollusc or archi-Mollusc (more
correctly Archimalakion). After describing this schematic
form, we shall proceed to show how far it is realized or
justified in each class and order of Mollusca successively.

The schematic Mollusc (fig. 1, A to E) is oblong in
shape, bilaterally symmetrical, with strongly differentiated
dorsal and ventral surface, and has a well-marked HEAD,
consisting of the prostomium (5) and the region imme-
diately behind the mouth. Upon the head we place a
pair of short CEPHALIC TENTACLES (a). The mouth is
placed in the median line anteriorly, and is overhung by
the prostomium (B, o) ; the amis is placed in the median
line posteriorly, well raised on the dorsal surface (A, m).
The apertures of a pair of NEPHRIDIA are seen in the
neighbourhood of the anus right and left (A, /). Near
the nephridial apertures, and in front of them, right and
.left, are the pair of apertures (k) appropriate to the ducts
of the GONADS (generative pores).

The most permanent and distinctive Molluscan organ
is the FOOT (Podium). This is formed by an excessive
development of the somatic musculature along the ventral
surface, distinctly ceasing at the region of the head, below
which it suddenly projects as a powerful muscular mass
(B, p ; E, p). It may be compared, and is probably genetic-
ally identical, with the muscular ventral surface of the
Planarians and with the suckers of Trematoda, but is more
extensively developed than are those corresponding struc-
tures. The muscular tissue of the foot, and of all other
parts of the body of all Mollusca, is cellular and unstriated,
as distinguished from the cr-jposite muscular fibre (con-
sisting of cell-fusions inste" 1 of separable cells) which
occurs in Arthropoda and in Vertebrata, and which has
the further distinction of being composed of alternating
bands of substance of differing refractive power (hence
" striated "). The appearance of cross striation seen in
the muscular cells of some Molluscs (odontophore of
Haliotis, Patella, &c.) requires further investigation. It
is by no means altogether the same thing as the mark-
ing characteristic of striated muscular fibre.

Contrasting with the ventral foot is the thin-walled
dorsal region of the body, which may be termed the anti-
podial region. This thin-walled region is formed by soft
viscera covered in by the comparatively delicate and non-
muscular body- wall (fig. 1, E). As the ventral foot is
clearly separate from the projecting head, so is this dorsal
region, and it is conveniently spoken of as the VISCERAL
HUMP or " dome " (cupola). Protecting the visceral dome
is a SHELL (conchylium) consisting of a horny basis impreg-
nated with carbonate of lime,1 and secreted by the deric
epithelium of this region of the body (g). The shell
in our schematic Mollusc is single, cap-shaped, and sym-
metrical. It does not lie entirely naked upon the surface
of the visceral dome, but is embedded all round its margin,
to a large extent in the body-wall. In fact, the integu-
ment of the visceral dome forms an open flattened sac
in which the shell lies. This is the PRIMARY SHELL-
SAC, or FOLLICLE (A and E, /). The wall of the body pro-
jects all round the visceral dome in the form of a flap or
skirt, so as to overhang and conceal to some extent the
head and the sides of the foot. This skirt, really an out-

1 As to the minute structure of the shell in various classes, see
Carpenter's article " Shell '' in the Cyc'op. of Anal, and Physiol. The
limits of our space do uot permit us to deal with this or other histo-
logical topics.

MOLLUSCA

99

growth of the dorsal body-wall, is called the MANTLE-FLAP
(liinbus pallialis), or more shortly the MANTLE or PALLIUM
('•). The space between the overhanging mantle-flap and the
sides and neck of the animal which it overhangs is called
the SUB-PALLIAL SPACE or CHAMBER. Posteriorly in this
space are placed the anus and the pair of nephridial aper-
tures (see tig. 1, E).

The development of the mantle-skirt and its sub-pallial
space appears to have a causal relation, in the way of pro-
tection, to a pair of processes of the body-wall which
spring, one on the right and one on the left, from the sides
of the body, nearer the anus than the mouth, and are
concealed by the mantle-flap to some extent (A, B, i).
These processes have an axis in which are two blood-vessels,
and are beset with two rows of flattened filaments, like the
teeth of a comb in double series. These are the CTENIDIA
or gill-combs. Usually, as will be seen in the sequel, they
play the part of gills, but since in many Molluscs (Lamelli-
branchs) their function is not mainly respiratory, and since
also other completely-formed gills are developed as special
organs in some Molluscs to the exclusion of these pro-
cesses, it is well not to speak of them simply as "gills " or
"branchise," but to give them a non-physiological name,
such as that here proposed. Near the base of the stem of
each ctenidium is a patch of the epithelium of the body-
wall, peculiarly modified and supplied with a special nerve
and ganglion. This is Spengel's olfactory organ, which
tests the respiratory fluid, and is persistent in its position
and nerve -supply throughout the group Mollusca. We
propose to call it the OSPHRADIUM.

Passing now to the internal organs, our schematic
Mollusc is found to possess an ALIMENTARY CANAL, which
passes from mouth to anus in the middle line, leaving
between it and the muscular body-wall a more or less
spongy, in parts a spacious, CCELOM. The stommfaum is
large and muscular, the proctodxum short ; the bulk of
the alimentary canal is therefore developed from the met-
enteron or remnant of the arch-enteron after the ccelom
has been pinched off from it. A paired outgrowth of the
met-enteron forms the glandular diverticulum known as
the digestive gland or (commonly) liver (E, y, zl).

Dorsally to the alimentary tract the ccelom is spacious.
The space (C, E, s) is termed the -PEEICARDHJM, since it is
traversed by a vessel running fore and aft in the median
line, which has contractile muscular walls and serves as a
heart to propel the coelomic blood-fluid. This pericardial
space, although apparently derived from the original ccelom,
is not in communication with the other spaces and blood-
vessels derived from the ccelom ; it never (or perhaps in a
very few instances) contains in the adult the Molluscan blood
or hsemolymph, and is always in free communication with
the exterior through the tubes called nephridia (renal
organs). The HEART receives symmetrically on each side,
right and left, a dilated vessel bringing aerated blood from
the ctenidia. These dilated vessels are termed the aurieles
of the heart, whilst the median portion itself, at the point
where these vessels join it, is termed the ventricle of the
heart (C, v). The vessel passing fore and aft from the
ventricle gives off a few trunks which open into spaces
of the ccelom, the so-called lacuna? ; these are excavated in
every direction between the viscera and the various bundles
of fibrous and muscular tissue, and- may assume more or
less the character of tube-like vessels with definite walls.
Right and left opening into the pericardial ccelom is a
coiled tube, the farther extremity of which opens to the
exterior by the side of the anus. These two tubes (C, «)
are the symmetrically disposed NEPHRIDIA (renal organs).

The GONADS (ovaries or spermaries) are placed in the
mid-dorsal region of the ccelom (C, t), and have their own
apertures in the immediate neighbourhood of those of the

nrphridia. The apertures ruv paired right and left, and so
are the ducts into which they lead; but at present we have
no ground for determining whether the gonad itself was
primarily in Molluscs a paired organ or a median organ,
nor have we any well-founded conception as to the nature
of the ducts when present, and their original relationship

'

•6r

Fm 2 — Ctenidia of various Mollusca (original). A. Of Chiton ; /.(., fibrous
tissue; a.b.v., afferent blood-vessel; e.b.v., efferent blood-vessel; g.L, later-
ally paired lamella;. B. Of Sepia ; letters as in A. C. Of Fissurella ; letters
as in A. D. Of Nucula ; d, position of axis with blood-vessels ; fl, inner ;
6 and c, outer row of lamelhv. E. Of Paludina ; i, intestine running parallel
•with the axis of the ctenidium and ending in the anus a ; br, rows of elongate
processes corresponding to the two series of lamellte of the upper figures.

to the gonads. The genital ducts of some organisms are
modified nephridia, but the nature of those of Mollusca,
of Arthropocla, of Echinoderma, of Nematoidea, and of
some Vertebrata has yet to be elucidated.

The disposition of the nerve-centres is highly character-
istic. There are four long cords composed of both nerve-
fibres and nerve-cells which are disposed in pairs, two right
and left of the pedal area or foot, two more dorsally and
tending to a deeper position than that occupied by the
pedal cords, so as to lie freely within the coelomic space
unattached to the body-wall. These are respectively the
•SEDAL NERVE-CORDS and the VISCERAL NERVE-CORDS. The
latter meet and join one another posteriorly. A right and
left (D, ff.v), and a median abdominal (g.aV) ganglion are
placed on these cords, and from them are given off the
osphradial nerves which have special ganglia (y.olf). In the
region of the prostomium the pedal nerve-cords are enlarged
behind the mouth, forming they>f«W i^tnylin (g.pe), and
are united by nerve-fibres to one another. From this spot
they are continued furward into the prostomium, where
they enlarge to form the right and left cerAml f/aiiylia (g.c),
which are united to one another by nerve-fibres in front of

100

MOLLUSCA

the mouth, just as the pedal ganglia are behind it. The
right and left pedal ganglia are joined by transverse cords
to the right and left visceral cords respectively, the point
of union being marked on either side by a swelling (ff.pl)
known as the pleural ganglion. The visceral nerve-cord
can also be traced up on each side beyond the pleural
ganglion to the cerebral ganglion. Thus we have a
nearly complete double nerve-ring formed around the oeso-
phagus by the two pairs of nerve-cords which are in this
region drawn, as it were, towards each other and away
from their lateral position both behind and before the
stomodseal invagination. Whilst the swollen parts of the
nerve-tracts are termed ganglia, the connecting cords
are conveniently distinguished either as connectives or as
commissures. Commissures connect two. ganglia of the
same pair We have a cerebral commissure, a pedal com-
missure and a visceral commissure. Connectives connect
ganglia of dissimilar pairs, and we speak accordingly of
the cerebro- pedal connective, the cerebro- pleural con-
nective, the pleuro- pedal connective, and the viscero-
pleural connective.

An ENTERIC NERVOUS SYSTEM forming a plexus on the
walls of the alimentary canal exists, but does not exhibit
cords and ganglia visible to the naked eye except in the
large Dibranchiate Cephalopoda.

Our schematic Mollusc is provided with certain ORGANS
OF SPECIAL SENSE. Tactile organs occur on the head in the
form of short CEPHALIC TENTACLES (a). Deeply placed are

ch

Fi'.. 0. — Development of the Pond-Snail, Limnpeus stagnatis (after Lankester,
15). dc, directive corpuscles (pra?seminal outcast cells) ; ch, egg-envelope
or chorion ; or, oral end of the blastopore ; r, anal end of the blast<ip..rr.
A. Formation of the Diblastula by the invagination of larger cells into the
area of smaller cells (optical section). B. View of the same specimen from
the surface of invagination ; the smaller cells are seen at the periphery ; by
division they will multiply and extend themselves over the four larger cells.
C. Fully-formed Diblastula, surface view to show the elongated form of the
orifice of invagination or blastopore ; its middle portion closes up and coin-
cides with the region of the foot ; the extremity, or, coincides with the mouth
and stomodfeum, the opposite extremity, r, with the anus. D. Optical section
of an embryo a little older than A. E. Surface view of the same embryo.

a pair of closed vesicles containing each a calcareous con-
cretion and acting as auditory organs ; these are known as
OCTOCYSTS (D, y). They are situated behind the mouth
in the foremost portion of the foot. At the base of each
cephalic tentacle is a pigmented eye-spot — the CEPHALIC
EYE (D, IP). The OSPHRADIUM (k), or peculiar patch of
olfactory epithelium at the base of the ctenidium, has
y already been mentioned.

To the scheme thus exhibited of the possible organization
of the ancestral Mollusc we shall now add a sketch of
the mode in which this form of body and series of internal
organs are developed from the egg.

The egg-cell of Mollusca is either free from food material
--a simple protoplasmic corpuscle — or charged with food

material to a greater or less extent. Those cases which
appear to be most typical — that is to say, which adhere to a

FIG. 4. — Development of the Pond-Snail, Limnxus stagnaUs (after Lankester,
15). r, directive corpuscle ; &/, blastopore ; en, endoderm or enteric cell
layer ; tc, ectoderm or deric cell-layer ; i>, velum ; m, mouth ; /, foot ; t, ten-
tacles ; fp, pore in the foot (belonging to the pedal gland?) ; mf, the mantle-
flap or limbus pallialis ; sk, the shell ; /, the sub-pallial space, here destined
to become the lung. A. First four cells resulting from the cleavage of the
original egg-cell. B. Side view of the same. C. Diblastula stage (see fig. 3),
showing the two cell-layers and the blastopore. D, E, F. Trochosphere
stage, D older than E orF. G. Three-quarter view of a Diblastula, to show
the orifice of invagination of the endoderm or blastopore, U. H, I. Veliger
stage later than D. (Compare fig. 70 and fig. 72***).

procedure which was probably common at one time to all
then existing Mollusca, and which has been departed from
A B ^3^ 0

A*

Fm. 5. — Early stages of division of the fertilized egg-cell in Nassa mittalnlis
(in nn Bulii.ur, after liobretzky). A. The egg-cell has divided into two
splirrcs, uf which the lower contains more food-material, whilst the upper is
again incompletely divided into two smaller spheres. Resting on the divid-
ing upper sphere are the ri^ht shaped "directive corpuscles," better called
" pnvseminal outcast cells or apoblasts," since they are the result of a cell-
division which affects the egg-cell before it is impregnated, and are mere
refuse, destined to disappear. B. One of the two smaller spheres is reunited
to the linger sphere. C. The single small sphere has divided into two, and
the reunited mass has divided into two, of which one is oblong and practi-
cally double, as in B. D. Each of the fuur segment-cells gives rise by divi-
sinii to a small pellucid cell. E. The cap of small cells has increased in
number by repeat fd tni-mal inn of pellucid cells in the same way, and by
division nf thnsr lirst, forim-d. The cap will spread over and enclose the four

Si-lnrlll-rrlls, as 111 fig. 3, A, B.

only in later and special lines of descent — show approxi-

MOLLUSCA

101

mately the following history. By division of the egg-cell
(fig. 3, A, B ; fig. 4, A, B ; and fig. 5) a mulberry-mass of
embryonic-cells is formed (Morula), which dilates, forming
a one-cell-layered sac (Blastula). By invagination one

A

Fio. 6. — Development of the Oyster, Ostrea echdis (modified from Horst, 16).
A. Blastula stage (one-cell-layered sac), with commencing invagination of
the wall of the sac at W, the blastopore. B. Optical section of a somewhat
later stage, in which a second invagiuation has commenced — namely, that
nf the shell-gland sk ; W, blastopore ; en, invaginated endoderm (wall of the
future arch-enterou) ; ec, ectoderm. C. Similar optical section at a little
later stage. The invagination connected with the blastopore is now more
contracted, d; and cells, me, forming the mesoblast from which the coelom
and muscular and skeleto-trophic tissues develop, are separated. D. Similar
section of a later stage. The blastopore, bl, has closed ; the anus will sub-
sequently perforate the corresponding area. A new aperture, ?n, the mouth,
has eaten its way into the invaginated endodermal sac, and the cells pushed
in with it constitute the stomodseum. The shell-gland, $k, is flattened out,
and a delicate shell, s, appears on its surface. The ciliated velar ring is cut
in the section, as shown by the two projecting cilia on the upper part of the
figure. The embryo is now a Trochosphere. E. Surface view of an embryo

ors , rom wom our rawngs o e oyst .

The account given by the American naturalist Brooks (19) differs greatly as
to matter of fact from that of Horst, and appears to be erroneous in some
respects.

portion of this sphere becomes tucked into the other — as
in the preparation of a woven night-cap for the head (fig.
6, B ; fig. 7, A). The orifice of invagination (blastopore)
narrows, and we now have a two-cell-layered sac, — the
Diblastula. The invaginated layer is the enteric cell-layer
or endoderm ; the outer cell-layer is the deric cell-layer or
ectoderm. The cavity communicating with the blastopore
and lined by the endoderm is the arch-enteron. The blas-
topore, together with the whole embryo, now elongates.
The blastopore then closes along the middle portion of its
extent, which corresponds with the later developed foot.
At the same time the stomodaeum or oral invagination
forms around the anterior remnant of the blastopore, and
the proctodaeuui or anal invagination forms around the
posterior remnant of the blastopore. There are, however,

variations in regard to the relation of the blastopore to the
mouth and to the anus which are probably modifications of
the original process described above. An examination of
figs. 3, 4, 5, 6, 7, and of others illustrative of the embryo-
logy of particular forms which occur later in this article,
is now recommended to the reader. The explanation of
the figures has been made very full so as to avoid the

E

FIG. ".—Development of the River-Snail, Paluclina rieipara (after Lankester,
17). dc, directive corpuscle (outcast cell) ; ac, arch-enteron or cavity lined
by the enteric cell-layer or endoderm ; &/, blastopore ; vrt velum or circlet
of ciliated cells ; dv, velar area or cephalic dome ; sm, site of the as yet un-
formed mouth ; /, foot ; mes, rudiments of the skeleto-trophic tissues ; pi,
the pedicle of invagination, the future rectum ; shgl, the primitive shell-sac
or shell-gland ; m, mouth; an, anus. A. Diblastnla phase (optical section).
B. The Diblastula has become a Trochosphere by the development of the
ciliated ring vr (optical section). C. Side view of the Trochosphere with
commencing formation of the foot. D. Further advanced Trochosphere
(optical section). E. The Trochosphere passing to the Veliger stage, dorsal
view showing the formation of the primitive shell-sac. F. Side view of the
same, showing foot, shell-sac (s/iyO, velum (IT), mouth, and anus.

N.B. — In this development the blastopore is not elongated ; it persists as
the anus. The mouth and stomoda-um form independently of the blastopore.

necessity of special descriptions in the text. Internally, by
the nipping off of a pair of lateral outgrowths (forming
part of the indefinable " mesoblast ") from the enteric cell-
layer the foundations of the ccelomic cavity are laid. In
some Ccelomata these outgrowths are hollow and of large
size. In Mollusca they are not hollow and large, which is
probably the archaic condition, but they consist at first of
a few cells only, adherent to one another ; these cells then
diverge, applying themselves to the body-wall and to the
gut-wall so as to form the lining layer of the ccelomic
cavity. Muscular tissue develops from deep-lying cells, and
the rudiments of the paired nerve-tracts from thickenings
of the deric-cell layer or ectoderm.

The external form meanwhile passes through highly char-
acteristic changes, which are on the whole fairly constant
throughout the Mollusca. A circlet of cilia forms when the
embryo is still nearly spherical (fig. 4, F ; fig. 6, E ; fig. 7,

102

MOLLUSCA

B), in an equatorial position. As growth proceeds, one
hemisphere remains relatively small, the other elongates and
enlarges. Both mouth and anus are placed in the larger
area ; the smaller area is the prostomium simply ; the cili-
ated band is therefore in front of the mouth. The larval
form thus produced is known as the Trochosphere. It
exactly agrees with the larval form of many Chcetopod
worms and other Ccelomata. Most remarkable is its
agreement with the adult form of the Wheel animalcules
or Eotifera, which retain the prse-oral ciliated band as their
chief organ of locomotion and prehension throughout life.
So far the young Mollusc has not reached a definitely
Molluscan stage of development, being only in a condition
common to it and other Ccelomata. It now passes to the
veliger phase, a definitely Molluscan form, in which the
disproportion between the area in front of the ciliated
circlet and that behind it is very greatly increased, so
that the former is now simply an emarginated region of
the head fringed with cilia (fig. 8 ; fig. 6, F ; fig. 7, F ;
and fig. 60, A). I It is termed the " velum," and is fre-
quently drawn out into lobes and processes. As in the
Rotifera, it serves the veliger larva as an organ of loco-

Fie. 8.— "Veliger" embryonic form of Mollusca (from Gegcnbanr). '', velum;
c, visceral dome with dependent mantle-skirt ; p, foot ; t, cephalic tentacles ;
op, operculum. A. Earlier, and B, later, Veliger of a Gastropod. C. Veli-
PT of a Pteropod showing lobe-like processes of the velum and the great
paired outgrowths of the foot.

motion. In a very few Molluscs, but notably in the Com-
mon Pond-Snail, the emarginated bilobed velum is re-
tained in full proportions in adult life (fig. 70), having
lost its marginal fringe of specially long cilia and its
locomotor function. The body of the Veliger is char-
acterized by the development of the visceral hump on
one surface, and by that of the foot on the other. Growth
is greater in the vertical dorso-ventral axis than in the
longitudinal oro-anal axis ; consequently the foot is rela-
tively small and projects as a blunt process between mouth
and anus, which are not widely distant from one another,
whilst the antipodal area projects in the form of a great
hump or dome. In the centre of this antipodal area there
has appeared (often at a very early period) a gland-like
depression or follicle of the integument (fig. 6, C, sk ; fig. 7,
E, F, shgl ; fig. 60, B ; fig. 68, s/is ; fig. 72*** .«)• This is
the primitive shell-sac discovered by Lankester (18) in 1871,
and shown by him to precede the development of 'the perma-
nent shell in a variety of Molluscan types. The cavity of
this small sac becomes filled by a horny substance, and then
it very usually disappears, whilst a delicate shell, commenc-

ing from this spot as a centre, forms and spreads upon the
surface of the visceral dome.

The embryonic shell-sac or shell-gland represents in a
transient form, in the individual development of most
Mollusca, that condition of the shell-forming area which
we have sketched above in the schematic Mollusc. In
very few instances (in Chiton, and probably in Limax), as
we shall see below, the primitive shell-sac is retained and
enlarged as the permanent shell-forming area. It is sup-
planted in other Molluscs by a secondary shell-forming
area, namely, that afforded by the free surface of the
visceral hump, the shell-forming activity of which extends
even to the surface of the depending mantle-skirt. Accord-
ingly, in most Mollusca the primitive shell is represented
only by the horny plug of the primitive shell-sac. The
permanent shell is a new formation on a new area, and
should be distinguished as a secondary shell.

The ctenidia, it will be observed, have not yet been
mentioned, and they are indeed the last of the charac-
teristic Molluscan organs to make their appearance. Their
possible relation to the prte-oral and post-oral ciliated bands
of embryos similar to the Trochosphere are discussed by
the writer in the Quart. Jour. Micr. Sci., vol. xvii. p.
423. The Veliger, as soon as its shell begins to assume
definite shape, is no longer of a form common to various
classes of Mollusca, but acquires characters peculiar to its
class. At this point, therefore, we shall for the present
leave it.

SYSTEMATIC REVIEW OF THE CLASSES AND OEDEES OF
MOLLUSCA.

We are now in a position to pass systematically in
review the various groups of Mollusca, showing in what
way they conform to the organization of our schematic
Mollusc, and in what special ways they have modified or
even suppressed parts present in it, or phases in the repre-
sentative embryonic history which has just been sketched.
It will be found that the foot, the shell, the mantle-skirt,
and the ctenidia, undergo the most remarkable changes of
form and proportionate development in the various classes
— changes which are correlated with extreme changes and
elaboration in the respective functions of those parts.

Division of the Phylum into two lintnches. — The Mollusca
are sharply divided into two great lines of descent or
branches, according as the prostomial region is atrophied
on the one hand, or largely developed on the other.

The probabilities are in favour of any ancestral form —
the hypothetical archi-Mollusc which connected the Mol-
lusca with their non-Molluscan forefathers — having pos-
sessed, as do all the more primitive forms of Coelomata, a
well-marked prostomium, and consequently a head. The
one series of Mollusea descended from the primitive head-
bearing Molluscs have acquired an organization in which
the Molluscan characteristics have become modified in
definite relation to a sessile inactive life. As the most
prominent result of the adaptation to such sessile life they
exhibit an atrophy of the cephalic region. They form the
branch LIPOCEPHALA — the mussels, oysters, cockles, and
clams. The other series have retained an active, in many
cases a highly aggressive, mode of life ; they have, corre-
spondingly, not only retained a well-developed head, but
have developed a special aggressive organ in connexion
with the mouth, which, on account of its remarkable nature
and the peculiarities of the details of its mechanism, serves
to indicate a very close genetic connexion between all such
animals as possess it. This remarkable organ is the odon-
tophoiv, consisting of a lingual ribbon, rasp, or radula,
with its cushion and muscles. On account of the pos-
session of this organ this great branch of the Molluscan
phylum may be best designated GLOSSOPHOEA. Any term

MOLLUSCA

103

which merely points to the possession of a lirail is objcc-
tionable, since this is common to them and the hypotheti-
cal arehi-Mollusea from which they descend. The term
Odontophora, which has been applied to them, is also un-
suitable, since the organ which characterizes them is not a
tooth, but a tongue.

Fio. !).— Odontophore of Glossophorous Jlollusca.
A.

wall of the cesophagus behind the radular sac («);/, anterior termina-
tion of the radula anil its lied, the point at which it wears away ; 17,
base of the rathilar sac or recess of the pharynx ; h, cartilaginous piece
developed in the floor of the pharynx beneath the radula, and si-rvinn
for the attachment of numerous muscles, and for the support of the
r;iilula ; i, anterior muscles ; A-, posterinr muscles attached to the carti-
];L^I' ; ', muscle acting as a retractor of the buccal mass; m, muscle
attached to the lower lip ; n, posterior extremity of the radular sac ;
n, the bed of the radnla or layer of cells by which its lower surface is
funned ; 71, the homy radula or lingual ribbon ; q, opening of the radular
sac into the pharynx or buccal cavity ; r, cells at tin- extreme end of
the inner surface of the radular sac which produce as a "cuticular
secretion" the rows of teeth of the upper surf.'ir>- nf the rndula.

B. Radula or lingual ribbon of Paliidina vivipara, stripped from its bed,— a

horny, cuticular product.

C. A single row of teeth from the radula of Trochtis cinerarias. Rhipido

plossate ; formula, x.S.l.o.x.

D. A single row of teeth from the radula of Faultimafrarjilis. Ptcnoglossate ;

formula, x.O.x.

E. A single row of teeth from the radula of Cliiton cinercus. Too elaborate

for formulation.

F. A single row of teeth from the radula of Patella ni!'r<>". Formula, 3.1.4.1.3.

G. A single row of teeth from the radula of Cyprtca hflvola. Tfpnioglossate :

formula, 3.1.3.

H. Asi!i(,'l<- rn\v nf trrth frnm the rnilula rif X<i**i anmilata. Rachiglossate ;
formula, 1.1.1. The Common Whelk is similar In this.

The general structure of the odontophore ( = tooth-
bearer, in allusion to the rasp-like ribbon) of the glosso-
phorous Mollusca may be conveniently described at once.
Essentially it is a tube-like outgrowth — the r<i<lnl<n- .v/c (fig.
9, A, y, n) — in the median line of the ventral floor of the
stomodieum, upon the inner surface of which is formed a
chitinous band (the radula) beset with minute teeth like a

rasp (ji). Anteriorly the ventral wall of the diverticulum
is converted into cartilage (//), to which protractor and re-
Inn-tor muscles are atlm-lied (/•, /), so that by the action of
the former the cartilage, with the anterior end of the ribbon
resting firmly upon it, may be brought forward into the
space between the lips of the oral aperture (ait, «/), and
made to exert there a backward and forward rasping action
by the alternate contraction of retractor and protractor
muscles attaclied to the cartilage. But in many Glosso-
phora (<'.;/., the Whelk) the apparatus is complicated by the
fact that the diverticulum itself, with its contained radula,
rests but loosely on the cartilage, and has special muscles
attached to each end of it, arising from the body wall ;
these muscles pull the whole diverticulum or radular sac
alternately backwards and forwards over the surface of the
cartilage. This action, which is quite distinct from the
movement of the cartilage itself, may be witnessed in a
Whelk if the pharynx be opened whilst it is alive. It has
also been seen in living transparent Gastropods. The chi-
tinous ribbon is continuously growing forward from the
tube-like diverticulum as a finger-nail does on its bed, and
thus the wearing away of the part which rests on the car-
tilage and is brought into active use, is made up for by
the advance of the ribbon in the same way as the wearing
down of the finger-nail is counterbalanced by its own for-
ward growth. And, just as the new substance of the
finger-nail is formed in the concealed part, sunk posteriorly
below a fold of skin, and yet is continually carried forward
with the forward movement of the bed on which it rests,
and which forms its undermost layers, so is the new sub-
stance of the radula formed in the compressed extremity
of the radular sac (n), and carried forward by the forward
movement of the bed (o) on which it rests, and by which
is formed its undermost layer. This forward-moving bed
is not merely the ventral wall of the radular diverticulum,
but includes also that portion of the floor of the oral cavity
to which the radula adheres (as far forward as the point /
in fig. 9, A). At the spot where the radula ceases, the for-
ward growth-movement of the floor also ceases, just as in
the case of the finger-nail the similar growth-movement
ceases at the line where the nail becomes free.

The radula or cuticular product of the slowly-moving
bed can be stripped off, and is then found to consist of a
ribbon-like area, upon which are set numerous tooth-like
processes of various form in transverse rows, which follow
one another closely, and exactly resemble one another in the
form of their teeth (fig. 9, B). The tooth-like processes in a
single transverse row are of very different shape and num-
ber in different members of the Glossophora, and it is pos-
sible to use a formula for their description. Thus, when
in each row there is a single median tooth with three teeth
on each side of it more or less closely resembling one
another, as in fig. 9, G, we write the formula 3.1.3. When
there are additional lateral pieces of a different shape to
those immediately adjoining the central tooth, we indi-
cate them by the figure 0, repeated to represent their
number, thus 0000.1.1.1.0000 is the formula for the
lingual teeth of Chiton Stelleri. A single median tooth,
an admedian series, and a lateral series may be thus dis-
tinguished. In some Glossophora only median teeth are
present, or large median teeth with a single small ad-
median tooth on each side of it (fig. 9, H) ; these are
termed Eachiglossa (formula, — .1. — or 1.1.1). In a large
number of Glossophora we have three admedian on each
side and one median, no lateral pieces (fig. 9, G) ; these
are termed Tzenioglossa (formula, 3. 1.3). Those with nume-
rous lateral pieces, four to six or more admedian pieces,
and a median piece or tooth (fig. 9, C) are termed Ehipi
doglossa (formula, x.G.l.G.x, where x stands for an inde-
finite number of lateral pieces). The Toxoglossa have

104

MOLLUSCA

1.0.1, the central tooth being absent and the lateral teeth
peculiarly long and connected with muscles. The term
Ptenoglossa (fig. 9, D) is applied to those Glossophora
in which the radula presents no median tooth, but an
indefinite and large number of admedian teeth, giving
the formula x.O.x. When the admedian teeth are inde-
finite (forty to fifty), and a median tooth is present, the
term Myriaglossa is applied (formula, x.l.x). It must be
understood that the pieces or teeth thus formulated may
themselves vary much in form, being either flat plates, or
denticulated, hooked, or spine-like bodies. We shall revert
to the terms thus explained in the systematic descriptions
of the groups of Glossophora.

The muscular development in connexion with the whole
buccal mass, and with each part of the radular apparatus,
is exceedingly complicated, — as many as twenty distinct
muscles having been enumerated in connexion with this
organ. In addition to the radula, and correlated with its
development, we find almost universally present in the
Glossophora a pair of horny jaws (usually calcified) de-
veloped as cuticular productions upon the epidermis of the
lips (fig. 9, A, I). The radula and the shelly jaws of the
Glossophora enable their possessors not only to voraciously
attack vegetable food, but the radula is used in some in-
stances for boring the shells of other Mollusca, and the
jaws for crushing the shells of Crustacea, and for wound-
ing even Vertebrata.

PHYLUM MOLLUSCA.
BRANCH A.— GLOSSOPHORA.

Characters. — Mollusca with head-region more or less
prominently developed ; always provided with a peculiar
rasping-tongue — the odontophore — rising from the floor of
the buccal cavity.

The Glossophora comprise three classes, chiefly distin-
guished from one another by the modifications of the foot.

Class I.— GASTROPODA.

Characters. — Glossophora in which (with special excep-
tion of swimming forms) the FOOT is simple, median in
position, and flattened so as to form a broad sole-like sur-
face, by the contractions of which the animal crawls, often
divided into three successive regions — the pro-, rneso-, and
meta-podium — by lateral constrictions.

The Gastropoda exhibit two divergent lines of descent
indicated by the term sub-class (see p. 649).

Sub-class 1.— GASTEOPODA ISO-PLEURA.

Characters. — Gastropoda in which not only the head
and foot but also the visceral dome with its contents and
the mantle retain the primitive BILATERAL SYMMETRY of
the archi-Mollusc. The anus retains its position in the
median line at the posterior end of the body. The whole
visceral mass together with the foot is elongated, so that
the axis joining mouth and anus is relatively long, whilst
the dorso-pedal axis at right angles to it is short. The

CTENIDIA, the NEPHRIDIA, GENITAL DUCTS, and CIRCULA-
TORY ORGANS are paired and bilaterally symmetrical. The
pedal and visceral NERVE-CORDS are straight, parallel with
one another, and all extend the whole length of the body ;
the ganglionic enlargements are feebly or not at all deve-
loped. The Isopleura comprise three orders.

Order 1. — Polyplacophora (the Chitons).

Characters. — Gastropoda Isopleura with a metameric re-
petition of the shell to the number of eight. The shells of
the primitive type are partially or wholly concealed in shell-
sacs comparable to the single embryonic shell-sac of other
Mollusca. On the surface of the mantle-flap numerous

calcified spines and knobs are frequently developed. The
ctenidia are of the typical form, small in size and meta-
merically repeated along the sides of the body to the

B C " ^ N>c A

Fio. 10.— Three views of Chiton. A. Dorsal view of Chiton Wosnessenksii,
Jlidil., showing the eight shells. (After Jliddendorf.) B. View from the
pedal surface of a species of Chiton from the Indian Ocean, p, foot ; o,
mouth (at the other end of the foot is seen the anus raised on a papilla) ; kr,
oral fringe ; br, the numerous ctenidia ("branchial plumes) ; spreading beyond
these, and all round the animal, is the mantle-skirt. (After Cuvier.) C. The
same species of Chiton, with the shells removed and the dorsal integument
reflected, b, buccal mass ; m, retractor muscles of the buccal mass ; ov,
ovary ; od, oviduct ; i, coils of intestines ; ao, aorta ; c1, left auricle ; c,
ventricle.

number of sixteen or more ; an osphradium or area of
" olfactory epithelium " (Spengel) is found at the base of
each ctenidium. The other organs are not subject to
metameric repetition. The odontophore is highly devel-
oped; the teeth of the lingual ribbon are varied in form, —
several in each transverse row (fig. 9, E). Paired genital
ducts distinct from the paired nephridia are present.

The order Polyplacophora contains but one family, the
Chitonidee, with the genera: Chiton, Lin. (figs. 10, 15, (Sic.);
Cri/ptochiton, Midd., 1847 ; and Cryptoplax ( = Chitonellus),
Blainv., 1818.

Order 2. — Neomenise.

Characters. — Gastropoda Isopleura devoid of a shell,
which is replaced by innumerable microscopic calcified
plates or spicules set in the dorsal epidermis ; mantle-flap
not lateral, but reduced to a small collar surrounding the

A BCD

FIG. ll.—Neomeiiia carinata, Tullberg (after Tnllberg). A. Lateral view. B.
Ventral view. C. Dorsal view. D. Ventral view of a more extended speci-
men, a, anterior; b, posterior extremity; c, furrow, in \vhich the narrow
foot is concealed.

anus ; ctenidia represented by a symmetrical group of bran-
chial filaments on either side of the anus ; foot very narrow,
sunk in a groove; odontophore feebly developed, but the
radula many-toothed ; gonads placed in the pericardium
discharging by the nephridia ; no special generative ducts.
The order Neomeniaj contains the two genera Neomenia,
Tullberg (Solenopus, Sars) (fig. 11); and Proneomenia,
Hubrecht.

Order 3. — Chsetoderma.

Characters. — Gastropoda Isopleura devoid of a shell,
which is replaced by numerous minute calcareous spines

FIG. 12.— Chirtotlerma nitiilulum, Loven (after Graff). The cephalic enlarge,
ment is to the left, the anal chamber (reduced pallial chamber, containing
the concealed pair of ctenidia) to the right.

standing up like hairs on the surface of the body; body

MOLLUSCA

much elongated so as to be vermiform; jmntlo-llnp as in
Neomeniw ; ctenidia in the form of a pair of branchial
plumes, one on each side of the anus ; foot aborted, its
position being indicated by a longitudinal furrow ; odonto-
phore greatly reduced, the radula only represented by a
single tooth ; gonads and nephridia as in Neomenia.

The order Chretoderma contains the single genus Chx-
foderma (fig. 12).

Further remarks on the Isopleurous Gastropods. — The
union of the Chitons with the remarkable worm-like forms
Neomenia and Chretoderma was rendered necessary by
Hubrecht's discovery (25) in 1881 of a definitely consti-
tuted radula and odontophore in his new genus Proneo-
menia, founded on two specimens brought from the arctic
regions by the Barents Dutch expedition.

By some writers — e.g., Keferstein — the Chitons have been
too intimately associated with the other Gastropoda, whilst,
on the other hand, Gegenbaur seems to have gone a great
deal too far in separating them altogether from the other
Mollusca as a primary subdivision of that phylum, inas-
much as they are inti-
mately bound to the
other Glossophora by
the possession of a
thoroughly typical
and well - developed
odontophore. They
undoubtedly stand
I nearer to the archi-
Mollusca than any
other Glossophora in
having retained a com-
plete bilateral sym-
metry and the primi-
tive shell-sac, though
the metameric repe-
tition of this organ
and of the ctenidia is
a complication of, and
departure from, the
primitive character.
It is not improbable
that in the calcareous
spines and plates of
the dorsal integument
of Neomenia andChfe-
toderrna, which occur

FIG. 13.— Diagrams of the alimentary canal of B^O On the part of
isopleura (from Hubrecht). o, mouth ; a, the dorsurn uncovered

anus ; d, alimentary canal ; /, liver (digestive , in* ^vi •

gland). A. Neomenia and Proneomenia. B. by shell 111 Chiton, we

chstoderma. c. chiton. jjave the retention of

a condition preceding the development of the solid Mol-
luscan shell, or a reversion to it. The minute calcareous
bodies may have the same relation to a compact shell which
the shagreen denticles of the sharks have to a continuous
dermal bone.

The anatomy of the Gastropoda Isopleura has been largely
elucidated within the past year by the researches of
Hubrecht and of Sedgwick, who have been the first to
apply the method of sections to the study of this group.

The leading points in the modifications of mantle-flap,
foot, and ctenidia are set forth in the preceding summaries,
and in the accompanying references to the figures. With
regard to other organs, we have to note the form of
the alimentary canal (fig. 13), which is simplest in
Chsetoderma, symmetrically sacculated in Neomenia, and
wound upon itself, forming a few coils, in Chiton. The
latter has a compact liver with arborescent duct, which is
represented by the sacculi in Neomenia and by a single

105

in Chsetodenna, Salivary glands are present in
Chiton and in Proneomenia. The raclula is highly devel-
oped in Chiton, and, though present in Proneomenia, lias
not been described in Neonienia. A single tooth in Choe-
toderma appears to represent the radula in a reduced state.
The circulatory organs of Chiton alone are known with
any degree of detail (fig. 10, C). There is a median dorsal
blood-vessel — the aorta — which is enlarged to form a
ventricle in the posterior region of the body. On either
side the ventricle is connected to a well-developed auricle,
which pours into it the aerated blood from the gills
(ctenidia). The extent to which vascular trunks are
developed has not been determined, but vessels to and
from the ctenidia, and in the mid-line of the foot, are
known. As in other Mollusca, the vessels do not extend
far, but lead into lacunas between the organs and tissues.
Dorsal and ventral vessels have been detected in Neomenia
and Cluetoderma, but no specialized heart.

A I!

Fin. 14.— Diagrams of the excretory and reproductive organs of Isnpleura (after
Uubrecht). 0, ovary ; P, pericardium ; N, nephridium ; v, external aperture
of nephridium ; j/, external aperture of the genital duct of Chiton ; r, rectum ;
Cl, cloacal or pallial chamber of Neomenire and ChEetoderma ; Dr, ctenidia
(branchial plumes). A. Chajtoderma. B. Neomenia. C. Proneomenia. 1>.
Chiton.

The heart of Chiton lies in a space which is to be
regarded as a specialized part of the ccelom, and, as in
other Molluscs, is termed the pericardium. In front of
this space in Chiton lies the ovary (fig. 1-t, D). In the
other Isopleura the genital bodies (gonads) lie in the peri-
cardium, which has a longer form and extends dorsally
above the intestine. Opening into the pericardium equally
in all the Isopleura (fig. 14) is a pair of bent tubes which
lead to the exterior. These are the nephridia, which in
Chiton are essentially renal in function. Their disposition
has been determined by Sedgwiek (26), who has shown that
each nephridium is much bent on itself, so that, as in the

0

106

MOLLUSCA

F-

nephridia of Conchifera (organ of Bojanus), the internal
aperture lies near the external. From the folded stem of
the nephridium very numerous secreting caeca are given off,
— omitted in the dia-
gram (fig. 14, D), but
accurately drawn in
fig. 15. The sexes in
Chiton are distinct,
and the ovary or testis,
as the case may be,
though lying in and
filling a chamber of
the original ccelom,
does not discharge into
the pericardium, but
has its own ducts,
which pass to the ex-
terior just in front of
those of the nephridia
(fig. 14, D, g, and fig. nl-
16). In this respect
Chiton is less primi- ng
tive than the other Iso- ff~\~
pleura, and even than
some other Gastropods
(the Zygobranchia),
and some Conchifera
(Spondylus, &c.), which
have no special genital
apertures, but make use
of the nephridia for

this purpose. In Chiton Fln- 15.— Dissection of the renal organs (neph-
, . ' . ,., ridia) of Chiton siculits, after HMer (Arbcitcn,

O&SCrepanS, in wnicn Zml. Instil., Vienna, 18S2). F, foot ; L, edge of

there are sixteen pairs $}| SSen^s™™oaopteignse- r™anus"Vf

of ctenidia, the orifices genital duct; go, external opining of the same;

nf t>ip nprilirirlia nrp W. stem of the nepliridium leading to no, its
• external aperture ; nk, reflected portion of the

coincident with the six- nephridial stem ; ng, tine Cieca of the nephri-
dium, which are seen ramifying transversely
over the whole inner surface of the pedal mus-
cular mass.

nk

of-

teenth pair of ctenidia,

those of the genital

ducts with a point between the thirteenth and fourteenth

ctenidia.

In the Neomenise and Chaetoderma the nephridia are
short and wide (JV in fig. 14, A,
B, C), and function as excretory
ducts for the genital products, the
gonads being lodged in the long
pericardium. Their separate or
united apertures open near the anus
into the small chamber formed by
the restriction of the mantle-skirt
to the immediate neighbourhood of
the anus.

The nervous system of the Gas-
tropoda Isopleura is represented in
the diagram fig. 17. In all it is
important to observe that nerve-
ganglion cells are by no means
a limited to special swellings — the
ganglia — but are abundant along
the whole course of the four great
longitudinal trunks. This is a pri-
mitive character comparable to that

hi-

FIO. 16.— Ovary and oviducts presented by the nerve-cords of Ne-

of CAtfoii sicuhis (after Ilal- „.„,.)-.•,„ wn,.ms nilr] nf <-!,„ Arrlirn
ler, loc. ctt.). »&, M>, anterior m ins, am 10 Altlllo-

an.i postiTii.r Nusiic-nsor of pod Peripatus. Higher dift'eren-

the ovary; t/, uterus (en- I- ,• • .-> -mr ,|- , T

larged part of oviduct); „, tiation in other Mollusca leads to

UVKluct- predominance if not an exclusive

presence of nerve-fibres in the cords, and of nerve-gun;// i<>n

cells in the specialized ganglia. The numerous transverse

connexions of the pedal nerve-cords in Chiton and Neo-

menia .(seen also in Fissurella (fig. 36) and some other
Gastropods) are comparable to the transverse connexions

of the ventral nerve-

A E

o

cords

of Chsetopod

worms and Arthro-
pods. In the abund-
ance of the nervous
network connected
with its longitudinal
nerve-tracts, Chiton
appears to retain some-
thing of the early con-
dition of the Ccelo-
mate nervous system
when it had the form
of a sub-epidermic net-
work or nerve-tunic
(seen more clearly in
Planarians and some
Nemertines), and when
the concentration into
definitely compacted
cords had not set in.

Ganglia are, how-
ever, distinguishable
upon the nervous cords
of Chiton (fig. 18). The
cerebral ganglia are
not distinguishable as
such, but a pair of
buccal ganglia (B in
fig. 18) are developed
on two connectives
which pass forward
from the cerebral re-
gion to the great mus-
cular mass of the
mouth. These buccal
ganglia are special de-
velopments connected PJ •

with the special niUS- Fl°- IT.— Diagrams of the nervous system of

, r T i- i Isopleura (alter Hubrecht, loc. ctt.). c, cere-

Clllarity OI the lips and bral ganglia ; s, snlilingual ganglia ; v, pedal

nrlni-itnnlinvp anrl QVO (ventral) nerve-cord ;l, visceral (lateral) nerve-

lontopnore, ana are cord . ^ ,,ost.anal j,mctiun of the visceral

found in all GloSSO- nerve-cords. A. Proneomenia. B. Neomenia.
i , , , • ,1 C. Cha-todi'i-ma. D. Chiton.

phora, but not in the

Lipocephala. Such special ganglia related to special
organs (and not introduced in our schematic Mollusc, fig.

1) we find in connexion with
the siphons of the Lipoce-
phala, and in various posi-
tions upon the visceral nerve-
cords of other Mollusca, both
Glossophora and Lipocephala.
A pair of pedal ganglia but
little developed (p in fig. 18),
and a special group of sub-
lingual ganglia are present in
Chiton. On the whole, the
nervous system of the Iso-
pleura is exceedingly simple
and archaic, whilst it does not
well serve as a type with
which to compare that of
systfiui.n7,,(,Mi tun-mis, in mnre de- other Mollusca on account of

tail(fromGe''enliaiir, /-.7. im-»fs<iRYmi;>. . .,

Anatomy). B, i.umii -a.miia (eon- the small amount of concen-

S!™^iwi*^,^l.0p*^0^)ia£ trati°n of its nerve-ganglion
"i pedal cells into ganglia, such as we
'uotT't- find well developed in other

forms.
The development of Neomenia and Cha;toderrna from

Fro. 18.-Anterior partof the nervou

''

an.i

trivd.

MOLLUSCA

107

the egg i.s entirely unknown, that of Chiton only par-
tially. Impregnation is effected when the. eggs have lieeii
discharged ami are lying beneath the mantle-skirt. A
trochosphere larva is developed from the I >i Ma- tula of
Chiton (Loven).

The Chitons are found in the littoral zone in all parts of
the world, and are exclusively marine. Neonienia, Proneo-
menia, and Cluetoderma have hitherto been dredged from
considerable depths (100 fathoms and upwards) in the
North Sea, Proneomenia also in the Mediterranean (Marion).

Sub-class 2.— GASTEOPODA ANISOPLEURA.

C/tar(U'ters. — Gastropoda in which, whilst the head and
foot retain the bilateral symmetry of the archi-Mollusca,
the visceral dome, including the mantle-flap dependent from
it, and the region on which are placed the ctenidia, anus,
generative and nephridiar apertures, have been subjected
to a ROTATION tending to bring the anus from its posterior
median position, by a movement along the right side,
forwards to a position above the right side of the animal's
neck, or even to the middle line above the neck. This
torsion is connected mechanically with the excessive vertical
growth of the visceral hump and the development upon
its surface of a heavy shell. The SHELL is not a plate en-
closed in a shell-sac, but the primitive shell-sac appears
and disappears in the course of embryonic development, and
a relatively large nautiloid shell (with rare exceptions)
develops over the whole surface of the visceral hump and
mantle-skirt. Whilst such a shell might retain its median
position in a swimming animal, it and the visceral hump
necessarily fall to one side in a creeping animal which
carries them uppermost.

The shell and visceral hump in the Anisopleura incline

FIG. 19. — Sketch of a model designed so as to show the effect of torsion or relation
of the visceral hump in Streptoneurous Gastropoda; A, unrotatcd ancestral
condition, £, quarter-rotation ; C, complete semi-rotation (the limit) ; rtn,anus;
In, r?(, primarily left nephridium and primarily right nephridium ; li'ij, primarily
left (subsequently the sub-intestinal) visceral ganglion; rvg, primarily right
(subsequently the sub-intestinal) visceral ganglion ; cerg, cerebral ganglion ;
pig, pleural ganglion ; ptdg, pedal ganglion ; abg, abdominal ganglion ; bucc,
buccal mass ; IK, wooden arc representing the base-line of the wall of the
visceral hump; x, xf, pins fastening the elastic cord (representing the visceral
nerve loop) to W.

normally to the right side of the animal. As mechanical
results, there arise a one-sided pressure and a one-sided
strain, together with a one-sided development of the
muscular masses which are related to the shell and foot.
Both the TORSION THROUGH A SEMICIRCLE of the base of the
visceral dome and the continued leiotropic spiral growth
of the visceral dome itself, which is very usual in the
Anisopleura, appear to be traceable to these mechanical
conditions. ATROPHY of the representatives on one side
of the body of paired organs is very usual. Those placed
primitively on the left side of the rectum, which in virtue
of the torsion becomes the right side, are the set which suffer
(see fig. 19). Some Anisopleura, after having thus acquired
a strongly-marked inequilateral character in regard to such
organs as the ctenidia, nephridia, genital ducts, heart, and
rectum, appear by further change of conditions of growth to
have acquired a superficial bilateral symmetry, the second-

ary nature of which is revealed by anatomical examination

(Opisthobranchia, Xatantia).

In all groups of Anisopleura examples are numerous in

which the shell is greatly developed, forming a "house7'
into which the whole animal can be with-
drawn, the entrance being often cl" • d
by a second shelly piece carried upon
the foot (the operculum). The power of
rapidly extending and of again contract-
ing large regions of the body to an
enormous degree is
usual, as in the Li-
pocephalous Mol-
lusca. In spite of
the theories which
have been held on
this matter, it ap-
pears highly prob-
able that no fluid
from without is in-
troduced into the
blood, nor is any ex-
pelled during these
changes of form.
A large mucous
gland with a med-
ian pore is usually Fl°f
developed on the the Streptoneurous "con-

06.1

ventral surface of
the foot, compar-
able to the similar

dition. B, buccal (sub.
a-sophageal) ganglion ; t\
cerebral ganglion ; Co,

Sleural ganglion ; P, pe-
al ganglion with otocyst

attached; p, pedal nerve;

A, - abdominal ganglion
thelong'lo'n'p'cdEutliy- LipOCephala, and in at the extremity of the
neurons condition. The / p,, twisted visceral "loop" ;

untwisted visceral loop some Cases (e.fj. , cy - sp, supra-intestinal visce-
is lightly shaded, cc, mla fio- 37 BHhis ral ganglion on the course
cerebral ganglion ; pi, . '. °' "'>."' , of the right visceral cord ;

pleural ganglion : pe, has been mistaken si>,sub-mtestmalganglion
pedal ganglion ; a!-, sp, fo - wate_ .,„,.„ on the course of the left

al«l.,minal ganglion, 1O1 a "aierpore. visceral cord. (FromGe-

which represents also The leiotropic genbaur, after Jhenng.)

gangiionprof "strepto- torsion of the visceral dome has had
neura and gives off the less deep -seated effect in one series of

nerve to the osphra- . . , , . , -, . i

dium (olfactory organ) Anisopleura than in another. Accord-
ietterednso^uiedI''Re" ^"S^' as *^e 1°°P formed by the two

nital" ganglion. The VISCERAL NERVES (fig. 19) is Or is not

giiaare omitted!1 (A?ter caught, as it were, in the twist, we are

Spengei.) abie t0 distinguish one branch or line of

descent with straight visceral nerves — the EUTHYNEDRA

Fio. 22.— Nervous system of the Pond-Snail, LimnKns stagnalis, as a type of
the short-looped Euthynetlrous condition. The short visceral "loop" with
its three ganglia i.s lightly-shaded, cf, cerebral ganglion ; pe, pedal ganglion ;
j'l, pleural ganglion; ab, abdominal ganglion; sp, visceral ganglion of the
left side ; opposite to it is the visceral ganglion of the right side, which
gives off the long nerve to the olfactmy ganvlimi and ospnradiom o. In
Planorbis and in Auricula (Pulmonata, allied to Limn;cus) the olfactory organ
is on the left side and receives its nerve from the left visceral ganglion.
(After Spengei.)

(fig. 20) — from a second branch with the visceral nerves

108

MOLLUSCA

twisted into a figure-of-eight — the STREPTONETJRA (fig.
21). Probably the Euthyneura and the Streptoneura
have developed independently from the ancestral bilater-
ally symmetrical Gastropods. The escape of the visceral
nerve-loop from the torsion depends on its having acquired
a somewhat deeper position and shorter extent, previously
to the commencement of the phenomenon of torsion,
in the ancestors of the Euthyneura than in those of
the Streptoneura. In the ancestral Streptoneura the
visceral loop was lateral and superficial as in the living
Isopleura.

Branch a.— STREPTONEURA (Spengel, 1881).

Characters.— Gastropoda Anisopleura in which the
visceral " loop " (the conterminous visceral nerves) was
adherent to the body-wall and so shared in the torsion of
the visceral hump, the right cord crossing above the left
so as to form a figure-of-eight (see fig. 19).

The Streptoneura. comprise two orders — the Zygo-
branchia and the Azygobranchia.

Order 1. — Zygoforanchia.

Characters. — Streptoneura in which, whilst the visceral
torsion is very complete so as to bring the anus into the
middle line anteriorly or nearly so, the atrophy of the
primitively left-side organs is not carried out. The right
and left ctenidia, which have now become left and right
respectively, are of equal size, and are placed symmetrically
on either side of the neck in the pallial space. Kelated
to them is a simple pair of osphradial patches. Both right

FIG. 23.—Haliotis tuberculata. d, foot; i, tentacular processes of the mantle.
(From Owen, after Cuvier.)

and left nephridia are present, the actual right one being
much larger than the left. Two auricles may be present
right and left of a median ventricle (Haliotis), or only one
(Patella). The Zygobranchia are further very definitely
characterized by the archaic character of absence of special
genital ducts. The generative products escape by the
larger nephridium. The sexes are distinct, and there is
no copulatory or other accessory generative apparatus.
The teeth of the lingual ribbon are highly differentiated
(Rhipidoglossate). The visceral dome lies close upon the
oval sucker-like foot, and is coextensive with its prolonga-
tion in the aboral direction.

The Zygobraiicliia comprise three families, arranged in two sub-
orders.

Sub-order 1. Ctcnidiobranchia.
Character. — Large paired ctenidia acting as gills.
Family 1. — Ifaliotidse.
Genera : Haliotis (Ear-Shell, Ornicr in Guernsey) ; mostly tropical ;

'/'• inotis.

Family 2. — Fiaxin-rlliiln-.

Genera: Fissurdla (Key-hole Limpet) (figs. 24, 36), Emaiyiniiln,
us (fig. 25) ; mostly tropical.

Sub-order 2. P/iyllidiobranchia.
.— Ctenidia reduced to wart-like papilla:; special sub-

pallial lamella;, similar to those of the Opisthobranch Pleuro-
phyllidia, perform the function of gills.
Family 3. — PatcUidse.

Genera: Patella (Limpet, figs. 26, &c.), Xacclla (Bonnet-Limpet),
Lottia.

Further Remarks on Zygobranchia. — The Common Limpet
is a specially interesting and abundant example of the
remarkable order Zygobranchia. A complete and accurate
account of its anatomy has yet to be •written. Here we
have only space for a brief outline. The foot of the
Limpet is a nearly circular disc of muscular tissue; in
front, projecting from and
raised above it, are the head
and neck (figs. 26, 30). The
visceral hump forms a low
conical dome above the sub-
circular foot, and standing out
all round the base of this dome
so as to completely overlap the
head and foot, is the circular
mantle-skirt. The depth of
free mantle-skirt is greatest in
front, where the head and neck
are covered in by it. Upon
the surface of the visceral
dome, and extending to the
edge of the free mantle-skirt,
is the conical shell. When
the shell is taken away (best
effected by immersion in hot
water) the surface of the vis-
ceral dome is found to be
covered by a black -coloured
epithelium, which may be re-
moved, enabling the observer
to note the position of some
organs lying below the trans-
parent integument (fig. 27).

Fio. 24.— Dorsal aspect of a specimen
of Fissurella from which the shell
has been removed, whilst the ante-
rior area of the mantle-skirt has
been longitudinally slit and its sides
reflected, a, cephalic tentacle ; b,
foot ; d, left (archaic right) gill-
plume ; c, reflected mantle-flap ; fi,
the fissure or hole in the mantle-flap
traversed liy the longitudinal inci-
sion ; /, right (archaic left) nephri-
dium's aperture ; </, anus ; ft, left
(archaic right) aperture of uephri-
dium ; p, snout. (Original.)

The muscular columns (c)

attaching the foot to the shell form a ring incomplete in
front, external to which is the free
man tie -skirt. The limits of the
large area formed by the flap over
the head and neck (ecr) can be traced,
_ and we note the anal papilla show-
ing through and opening on the right
shoulder, so to speak, of the animal
into the large anterior region of the
sub-pallial space. Close to this the
small renal organ (i, mediad) and the
larger renal organ (/•, to the right
and posteriorly) are seen, also the
pericardium (/) and a coil of the in-
testine (int) embedded in the com-
pact liver.

On cutting away the anterior part
of the mantle-skirt so as to expose

Vthe sub-pallial chamber in the region
/ of the neck, we find the right and
left renal papilla* (discovered by Lan-

XffiSSSSSa."tk«ter-(«) in 1867) on either side
mouth; T, cephalic ten- the anal rial lilla (fig. 28), but no gills.

tacle; br, one of the two _, . i.,1 v °. '. ,

symmetrical gills placed ou If a similar examination be made
the neck. (Original.) of t]le all;cd genug FissureHa (fig.

24, d), we find right and left of the two renal apertures
a right and left gill-plume or ctenidium, which by their
presence here and in Haliotis furnish the distinctive char-
acter to which the name Zygobranchia refers. In Patella
no such plumes exist, but right and left of the neck are
seen a pair of minute oblong yellow bodies (fig. 28, d),
which were originally described by Lankester as orifices
possibly connected with the evacuation of the generative

MOLLUSCA

109

products. On account of their position they were termed
by him the "eapito-pedal orifices," being placed near the
junction of head and foot. Spengel (24) has, however, in
a most ingenious way shown that these bodies are the repre-
sentatives of the typical pair of ctenidia, here reduced to a
mere rudiment. Near to each rudimentary ctenidium Spengel

FIG. 26.— The Common Limpet (Patella vulgata) in its shell, seen from the pedal
surface, tf, y, the median antero-posterior axis ; ft, cephalic tentacle ; b,
plantar surface of the foot ; c, free edge of the shell ; rf, the branchial effe-
rent vessel carrying aerated blood to the auricle, and here interrupting the
circlet of gill lamella; ; e, margin of the mantle-skirt ; f, gill lamellse (not
ctenidia, but special pallial growths, comparable to those of Pleurophyllidia);
g, the branchial efferent vessel ; h, factor of the branchial advehent vessel ;
i, interspaces between the muscular bundles of the root of the foot, causing
the separate areas seen in fig. 27, c. (Original.)

has discovered an olfactory patch or osphradium (consisting

of modified epithelium) and an olfactory nerve-ganglion

(fig. 32). It will be remembered that, according to

Spengel, the osphra-

dium of Mollusca is

definitely and inti-

mately related to the

gill - plume or cteni-

dium, being always

placed near the base

of that organ ; further,

Spengel has shown

that the nerve-supply

of this olfactory organ

is always derived from

the visceral loop. Ac-

cordingly, the nerve-

supply affords a means

of testing the conclu-

sion that we have in

Lankester's

, . , , .

pedal bodies the rudl-

nonf-irv ctpnirlin flip
.aryci lOia. J .

accompanying dia-

/c 11 OK\ {
grams (tig!,, d-t, 60) Ot

the nervous systems Of
T-, , n , jy-.. ,. .

ratella and OI JbtallOtlS,

int

capitO- Fl°- 27.— Dorsal surface of the Limpet removed
* , . from its shell and deprived of its black pig-
mented epithelium ; the internal organs are
seen through the transparent body-wall, c,
muscular blm(Ues forming the root of the foot,
and adherent to the shell ; c, free mantle-
skirt; em, tentaculiferous margin of the same;
^ smaller (left) nephridium ; i-, larger (right)
nephridium ; I, pericardium ; te, fibrous septum,
behind the pericardium; n, liver; int, intes-
tine; ecr, anterior area of the mantle-skirt ,ovcr-
as determined by Spen- ^nging the head (cephalic hood). (Original.)

gel, show the identity in the origin of the nerves passing
from the visceral loop to Spengel's olfactory ganglion of
the Limpet, and that of the nerves which pass from the
visceral loop of Haliotis to the olfactory patch or osphra-
dium, which lies in immediate relation on the right and
on the left side to the right and the left gill-plumes
(ctenidia) respectively. The same diagrams serve to de-

monstrate the Strcptoneurous condition of the visceral loop
in Zygobranchia.

Thus, then, we find that the Limpet possesses a sym-
metrically-disposed pair of ctenidia in a rudimentary con-
dition, and justifies
its position among
Zygobranchia. At
the same time it pos-
sesses a totally dis-
tinct series of func-
tional gills, which
are not derived from
the modification of
the typical Mollus-
can ctenidium.
These gills are in

the form of delicate Fio. 28.— Anterior portion of the same Limpet, with
/K or f\ tne uverlianning cephalic hood removed, a, ce-
(ng. Jb, J ), phalic tentacle; 6, foot; c, muscular substance
forming the root of the foot ; </, the capito-peda]
organs of Lankester (=rudimentary ctenidia); e,
mantle-skirt ; /, papilla of the larger nupliridium ;
f/, anus; /j, papilla of the smaller nepliridium ; i,
smaller nephndinm; k, larger nephridium ; (, peri-
cardium ; tnt cut edge of the mantle-skirt ; /',
liver ; p, snout. (Original.)

which form a series
extending com-
pletely round the
inner face of the

depending mantle-
skirt. This circlet of gill-lamellae led Cuvier to class the
Limpets as Cyclobranchiata, and, by erroneous identifica-
7- tion of them with

the series of meta-
merically repeated
ctenidia of Chiton,
to associate the
latter Mollusc
with the former.
The gill-lamelke
of Patella are
processes of the
mantle compar-
able to the plait-

c * c like folds often

Fio. 29.— The same specimen viewed from the left observed on the
front, so as to show the sub-anal tract (ff~) of the f f ,1 Viran
larger nephridium, by which it communicates with 1 OC
the pericardium, o, mouth; other letters as in fig. 28. dual chamber in

other Gastropoda (e.y., Buccinum and Haliotis). They are
termed pallial gills. The only other Molluscs in which
they are exactly represented
are the curious Opistho-
branchs Phyllidia and
Pleurophyllidia (fig. 57).
In these, as in Patella, the
typical ctenidia are aborted,
and the branchial function I
is assumed by close -set
lamelliform processes ar-
ranged in a series beneath
the mantle-skirt on either
side of the foot. In fig. 26,
d the large branchial vein of
Patella bringing blood from

flip dill «prip« tn flip V,porf Fia. 30.— Diagram of the two renal organs
tlie gill-Series , (nepnrjdia), to show their relation to the

is seen ; where it crosses
the series of lamelliB there
is a short interval devoid
of lamellae.

The heart in Patella con-
sists of a single auricle (not
two as in Haliotis and Fis-
surella) and a ventricle ; the
former receives the blood

ksl

rectum and to the pericardium. /, pa-
pilla of the larger nephridium ; g, anal
papilla with rectum leading from it ; h,
papilla of the smaller nephridium, which
is i >nly represented by dotted outlines ;
I, pericardium indicated by a dotted out-
line,—at its right side are seen tin1 t « • >
reno-pericardial pores ; /, the sub-anal
tract of the large nephridium given off
near its papilla and seen through the
unshaded smaller nephridium ; ks.a, an-
terior superior lube of the large ne-
phridium ; ks.lt left lobe of same ; ks.p,
posterior 1< 'be of same ; fcs. i, inferior
sub- visceral lobe of same. (Original.)

from the branchial vein, the

latter distributes it through a large aorta which soon leads
into irregular blood-lacunse.

110

MOLLUSCA

The existence of two renal organs in Patella, and their
relation to the pericardium (a portion of the ccelom), is

cor I

oclm

1.— Diagram of a vertical antero-postcro median section of a Limpet,
ers as in figs. 28, 29, with following additions : q, intestine in transverse

Fie. 31.

Lettei

section ; r, lingual sac (radular sac) ; rd, radula ; s, larnellated stomach ;
salivary gland ; 11, duct of same ; v, buccal cavity ; «•, gonad ; br.a, branchial
advehent vessel (artery) ; br.v, branchial efferent vessel (vein) ; bv, blood-
vessel ; odm, muscles and cartilage of the odontophore ; cor, heart within the
pericardium. (Original.)

important. Each renal organ is a sac lined with glandular
epithelium (ciliated cells with concretions) communicating

B

Fio. 32.— A. Section in a plane vertical to the surface of the neck of Patella
through a, the rudimentary ctenidium (Lankester's organ), and &, the ol-
factory epithelium (osphradium) ; c, the olfactory (osphradial) ganglion.
(After Spengel.) B. Surface view of a rudimentary ctenidium of Patella,
excised and viewed as a transparent object. (Original.)

with the exterior by its papilla, and by a narrow passage
with the pericardium. The connexion with the pericar-

Fio. 33.— Vertical section in a plane running right ami left through the
anterior part of the visceral hump of Patella, to show the two renal organs
and their openings into the pericardium, a, large or external or right renal
organ ; ab, narrow process of the same running below the intestine and lead-
ing by A: into the pericardium; b, small or median renal organ; c, prr-i-
cardium ; d, rectum ; e, liver ; /, manyplies ; y, epithelium of the dorsal sur-
face ; /(., renal epithelium lining the renal sacs; i, aperture connecting \\\>-
small sac with the pericardium; /,-, aperture cnuni'i'ting the lar^c sac \\itli
the pericardium. (From an original drawing by Mr J. T. Cunningham, Fellow
of University College, Oxford.)

dium of the smaller of the two renal organs was demon-
strated by Lankester in 1867, at a time when the fact

that the renal organ of the Mollusca, as a rule, opens into
the pericardium, and is therefore a typical nephridium,
was not known. Subsequent investigations (27) carried on
under the direction of the same - ,

naturalist have shown that, the
larger as well as the smaller renal
sac is in communication with the
pericardium. The walls of the
renal sacs are deeply plaited and
thrown into ridges. Below the
surface these walls are excavated
with blood-vessels, so that the sac
is practically a series of blood-ves-
sels covered with renal epithelium, s
and forming a mesh-work within
a space communicating with the
exterior. The larger renal sac (re-
markably enough, that which is
aborted in other Anisopleura) ex-
tends between the liver and the

integument of the visceral dome FIG. 34.— Nervous system of Pa-
very widely. It also bends round
the liver as shown in fig. 30, and
forms a large sac on half of the
upper surface of the muscular mass
of the foot. Here it lies close

niinn flip o-pnitfll bnrlv (nvarv nr tory S™glion> connected by

upon me genital ooay ^ovaiy or nerve to the streptoueurous
testis), and in such intimate rela- visceral loop. (After SpengeL)
tionship with it that, when ripe, the gonad bursts into the
renal sac, and its products are carried to the exterior by
the papilla on the right side of the anus (Robin, Ball).
This fact led Cuvier erroneously to the belief that a duct
existed leading from the gouad to this papilla. The
position of the gonad, best seen in the diagrammatic

ce.pe

tella ; the visceral loop is
lightly shaded ; the buccal
ganglia are omitted, ce, cere-
bral ganglia ; c'f,cerebral com-
missure ; pi, pleural ganglion ;
pe, pedal ganglion ; p'e, pedal
nerve ; s, s', nerves (right and
left) to the mantle ; o, olfac-

FIG. 35. — Nervous system of Haliotis ; the visceral loop is lightly shaded ;
the buccal ganglia are omitted, cc, cerebral ganglion ; j'/.jx1, thr fused pirn nil
and pedal ganglia ; pet the right pedal nerve ; ce.pl, the cereliru-pleural con-
nective ; Cf.pe, the cerebro-pedal connective ; s, s', right and left mantle
nrnvs ; alt, abdominal ganglion or site of same ; o, o, right and left olfactory
ganglia and ospliradia receiving nerve from visceral loop. (After JSpeugel.)

section (fig. 31), is, as in other Zygobranchia, devoid of
a special duct communicating with the exterior. This
condition, probably an archaic one, distinguishes the Zygo-
branchia among all Glossophorous Mollusca.

The digestive tract of Patella offers some interesting
features. The odontophore is powerfully developed ; the
radular sac is extraordinarily long, lying coiled in a space

MOLLUSCA

111

between the mass of the liver and the mnsrnlnr foot. The
radula has 160 rows of teeth with twelve teeth in each row.
Two pairs of salivary duets, each leading from a salivary
gland, open into the buccal chamber. The oesophagus leads
into a remarkable stomach, plaited like the manyplics of a
sheep, and after this the intestine takes a very large num-
ber of turns embedded in the yellow liver, until at last it
passes between the two renal sacs to the anal papilla. A
curious ridge (spiral 1 valve) which secretes a slimy cord is
found upon the inner wall of the intestine. The general
structure of the Molluscan intestine has not been suffi-
ciently investigated to render any comparison of this struc-
ture of Patella with that of other Mollusca possible. The
eyes of the Limpet (28) deserve mention as examples of
the most primitive kind of eye in the Molluscan series.
They are found one on each cephalic tentacle, and are
simply minute open pits
or depressions of the
epidermis, the epidermic
cells lining them being
pigment ed and connected
with nerves (compare fig.
118).

The Limpet breeds
upon the southern Eng-
lish coast in the early
part of April, but its de-
velopment has not been
followed. It has simply
been traced as far as the
formation of a Diblastula
which acquires a ciliated
band, and becomes a
nearly spherical Trocho-
sphere. It is probable
that the Limpet takes
several years to attain
full growth, and during
that period it frequents
the same spot, which
becomes gradually sunk
below the surrounding Fio
surface, especially if the
rock be carbonate of lime.
At low tide the Limpet
(being a strictly inter-
tidal organism) is ex-
posed to the air, and is
to be found upon its spot of fixation ; but when the water
again covers it, it (according to trustworthy observers)
quits its attachment and walks away in search of food
(minute encrusting alg£e), and then once more as the tide
falls returns to the identical spot, not an inch in diameter,
which belongs, as it were, to it. Several million Limpets
— twelve million in Berwickshire alone — are annually used
on the east coast of Britain as bait.

Order 2. — Azygobranchia.

Characters. — Strep toneura which, as a sequel to the
torsion of the visceral hump, have lost by atrophy the
originally left ctenidium and the originally left nephridium,
retaining the right ctenidium as a comb-like gill-plume to
the actual left of the rectum, and the right nephridium
(that which is the smaller in the Zygobranchia) also to the
actual left of the rectum, between it and the gill-plume.
The right olfactory organ only is retained, and may assume
the form of a comb-like ridge to the actual left of the
ctenidium or branchial plume. It has been erroneously
described as the second gill, and is known as the para-
branchia. The rectum itself lies on the animal's right

-Nervous system of Fissurella. pi,
pallial nerve ; p, pedal nerve ; .4, abdomi-
nal ganglia in the Stri'ptoneurotis visceral
commissure, with supra- and sub-intestine
ganglion on each side ; B, bnccal ganglia ;
C, C, cerebral ganglia ; es, cerebral commis-
sure ; o, otoeysts attached to the cerebro-
pedal connectives. (From Gegenbaur, after
Jhering.)

shoulder. The presence of glandular plication of the surface
of the mantle-Hap (fig. 46, x) and an adrectal gland (purple-
.'.'l;nid, fig. 47, ;/p) are frequently observed. The sexes are
always distinct; a special genital duct (oviduct or sperm
duct) unpaired is present, opening either by the side of the
anus or, in the males, on the right side of the neck in con-
nexion with a large penis. The shell is usually large and
spiral; often an operculum is developed on the upper sur-
face of the hinder part of the foot. The dentition of the
lingual ribbon is very varied. In most cases the visceral
hump and the foot increase along axes at right angles to
one another, so that the foot is extended far behind the
visceral hump in the ab-oral direction, whilst the visceral
hump is lofty and spirally twisted.

This is a very large group, and is conveniently divided
into two sections, the Reptantia and the Natantia. The
former, containing the immense majority of the group,
breaks up into three sub-orders, the Holochlaniyda, Pneu-
monochlamyda, and Siphonochlamyda, characterized by the
presence or absence of a trough-like prolongation of the
margin of the mantle-flap, which conducts water to the
respiratory chamber (sub-pallial space where the gill, anus,
&c., are placed), and notches the mouth of the shell by
its presence, or again by adaptation to aerial respira-
tion. The sub-orders are divided into groups according to
the characters of the lingual dentition. In some Azygo-
branchia the mouth is placed at the end of a more or less
elongated snout or rostrum which is not capable of intro-
version (Rostrifera) ; in the others (Proboscidifera) the
rostrum is partly invaginated and is often of great length.
It is only everted when the animal is feeding, and is with-
drawn (introverted) by the action of special muscles ; the
over- worked term "proboscis" is applied to the retractile
form of snout. The term " introversible snout," or simply
"introvert," would be preferable. The presence or absence
of this arrangement does not seem to furnish so natural a
division of the Reptant Azygobranchia as that afforded by
the characters of the mantle-skirt.

Section a.— REPTANTIA.

Characters. — Azygobranchia adapted to a creeping life ; foot either
wholly or only the mesopodium in the form of a creeping disc.

Sub-order 1 . — Holoclilamyda.

Cliaraclers. — Reptant Azygobranchia with a simple margin to the
mantle-skirt, and, accordingly, the lip of the shell unnotched ;
mostly Rostrifera (i.e., with a non-iutroversible snout), and vege-
tarian ; marine, brackish, fresh -water, terrestrial.

a. Wiipidoylossa (x.4 to 7.1.4 to 7.x).

Family 1. — Trochidai.
Genera : Turbo, Lin. ; Phosicmcllrt,, Lam. ; Imperator, Montf. ;

Trochus, Lin. ; Eotclla, Lam. ; Eiiompkalus, Low.
Family 2. — Neritidee.
Genera : Nerita, L. ; Ncrilina, Lam. ; Pilcolus, Low ; Navicella,

Lam.

Family 3. — Pleurotornarulse.

Genera : Pleurotomaria, Defr. ; Analomiis. Montf. ; Stomatia,
Helbing.

(3. Ptcnof/lossa (x.O.x).
Family 4. — Scalaridie.

Genus : Smlaria, Lam.
Family 5. — JantJtinUlie.
Genera : Jant/tina, Lam. (fig. 44) ; Rccluzia, Petit.

y. T/enioglossa (3.1.3).
Family 6. — Ccrithidfe.

Genera : Cerithium, Brug. ; Potamidcs, Brong. ; Ncrinaia, Defr.
Family 7. — Melanidse.

Genera : Melanin, Lam. ; Mdanopsis, Fer. ; Ancylohis, Lay.
Family 8. — Pyramided I ill

Genera: Pi/ramirlrlln, Lam.; Sty! inn, Flem. ; Aclis, Loven.
Family 9. — TnuritdUda .

Genera: Turritclln, Lam.; Civcitm, Flem.; Vcrmchis. Adans. ;

Siliquariit, Brug.
Family 10. — Xenophuriilte.

Genus : Phorus, Montf. (fig. 39).

112

MOLLUSCA

TABULAE VIEW OF THE SUBDIVISIONS OF THE CLASS GASTROPODA, ARRANGED so AS TO SHOW THEIR SUPPOSED GENETIC

RELATIONSHIPS.

Class.— GASTROPODA.
(Archisoplcurum. )

Sub-class 1. — Iso PLEURA,

Sub-class 2. — ANISOPLEURA.
(Archieuihyneivrum.)

Branch a. — STREPTONEURA.
(Archizyyobranchium. )

O

Order 1. — ZYGOBRANCHIA.

y

o*

Order 2. — AZYGOBRANCHIA.
(Archiholochlamydum.)

Sect. a.
licptantia.

Sect. 6.
Natantia.

co to

S §> S*
i 1- I

\ \

•

Branch b. — EUTHYNEURA.
(ArchiopisthobrancJiium. )

Order 1. — OPISTIIOBRANCHIA.
(Archipalliatum. )

Order 2. — PULMONATA.
(Archibasommatum.)

Sect. a.
Palliata.

co

r
I

Sect. J.
Non-Palliata.

!

8-

Family 11. — Naticidte.

Genera : Nalica, Lam. ; Sigarctus, Lam. ; Ncritopsis, Gratel.
Family 12. — Entoconchidie.

The single genus and species EntoconcJia mirabilis, discovered by
Joh. Muller in 1851, parasitic in Syna.pla digitata. The adult
form is not known.
Family 13. — Marsenidse.

Genera : Marsenia, Leach ; Onchidiopsis, Beck.
Family 1 4. — Acmieidie.
Genera : Acmtea, Eschsch. ; Lottia, Gr. ; (probably these will be

found to belong to the Zygobranchia).
Family 15. — Capulidse.
Genera : Capulus, Montf. ; Calyptreea, Lam. (fig. 40) ; Trochita,

Sebum.
Family 16. — Littorinidss.

Genera : Littorina (the Periwinkles, fig. 46) ; Modulus, Gray ;
Lacuna, Turt. ; Rissoa, Frem. ; Hydrobia, Hartm. ; Assiminia,
Leaeh.

Faniily 17. — Paludinidas.
Genera: Paludina (River-Snail) (figs. 7, 21); Bithyuia, Gray;

Tanalia, Gray.
Family 18. — I'alvatidie.

Genus : Vahvta (fig. 45), fresh-water.
Family 19. — AmpullarUUe.

Genus : Ampitllaria (can breathe air by means of the walls of
the pallial chamber as well as water by the gill ; fresh-waters
of tropical America, Africa, and East Indies).

Sub-order 2. — Pncumonochlamyda.

Characters. — Pallial chamber a lung-sac ; no gill ; mouth on a
rostrum, not a retractile proboscis; terrestrial habit.

Family 20. — Ctjclostomidie.

Genera : Ci/clostoma, Lam. ; Cyclnphortis, Montf. ; Fcnissina,

Gratol. ; Pujrina, Vignard.
Family 21. — Hclicinidie (radula rhipidoglossate rather than tanio-

glossate).
Genera : Sloastoma, Adams ; Trochatdla, Swains. ; LTcltciiia,

Lam. ; Proscrpiiia, Guild.
Family 22. — AdculUlee.
Genera : Acicula, Hartm. ; Gcomclania, Pfr.

Sub-order 3. — Siphonochlamyda.

Characters. — Reptant Azygobranchia with the margin of the
mantle drawn out to form a trough-like siphon which notches the
lip of the shell ; shell always spiral ; usually an operculum, horny
or lamelliform ; either a rostrum or a retractile proboscis ; exclusively
marine ; mostly carnivorous.

* Tsenioglossa (3.1.3).
Family 1. — Slrmnbidie.

Genera : Strombus, L. ; Ptcroccras, Lam. ; Rostcllaria., Lam.

(fig. 43).
Family 2. — Aporrhaidm.

Genus : Aporrhais, Da Costa.
Faniily 3. — Pedicularidss.

Genus : Pcdicularia, Swains.
Family 4. — DoUdse.

Genera: Cassis, Lam.; Cassidaria, Lam.; Doliuin, Lam.; Ficiilrt,

Swains.
Family 5. — Triton ill IF.

Genera : Tritonium, Cuv. (fig. 42) ; JSanclla., Lam.
Family 6. — Ct/pra'idtv (the Cowries).

Genera : Cypriea, L. ; Ovulum, Brug. (fig. 41) ; Erato, Risso.

* Toxiglosm (1.0.1).
Family 7. — Conidas.
Genus : Comts, L.
Family S. — Tcrcbridas.

Genus : Tercbra, Adans.
Family 9. — Fleurotomida.

Genus : Pleurotoma, Lam.
Family 10. — Canccllaridai.
Genus : Cancellaria, Lam.

* Bachiijlossa (1.1.1 or .1.).
Family 11. — Muricidss.
Genera : Murcx, L. ; TrojiJton, Montf. ; Fusus, Rrug. ; Pyrula,

Lam. (lig. 38); Turbiiidlu, Lam.
Family 12. — BuctinitJt?.

Genera : Buccinum, L. ; Nassa, Lam. (fig. 5) ; Purpura, Pirug.

(fig. 47); Concholepas, Lam.; Maijiltis, Montt.
Faniily 13. — AlUridas.
Genus : Mitra, Lam.

MOLLUSOA

113

Family 14. —

(ii nrra : tiliiii, Ilnij,'. ; Aneilla, Lam.; Ifurjui, Lam.
Family 1">. — 1'u/iiHi/n;

(icm/ra : J'olutu, L. ; Cymbium, Montf. ; Mn/yiinil!ti, Lam.;
Volvariu Lam.

Further Remark* on. the. R<'j>t«nt . I ://i/n/ii-<tn< -li'm. — The
very large assemblage of forms coining under this order
comprise the most highly developed predaceous sea-snails,
numerous vegetarian species, a considerable number of

Fin. ?.". — A. Tritmi varlegatum, to show the proboscis or fouccal introvert (e)
in a state of eversion. n, siphunal notch of the shell occupied by the siphonai
fold of the mantle-skirt (Siphonoehlamyda) ; h, ed^e of the mantle-skirt rest-
ing i in (hr shell ; f. cephalic eye ; rf, cephalic tentacle; e, everted buccal
inrrm-rrf (proboscis); / foot; g, operculuin ; 7i, penis; i, under snrface of
the mantle-skirt forming the roof of the sub-pallinl chamber. B. Sole of the
I,.,,! MI* l',ir/:!" hit"*, lu show i'. tin- pi ire usually s;iid to be "aquiferous"
but probably the orifice of a gland ; b, median line nf t'uut.

fresh-water, and some terrestrial forms. The partial dis-
section of a male specimen of the Common Periwinkle,
Littorina littorafis, drawn in fig. 46, will serve to exhibit
the disposition of viscera which prevails in the group.

retractor muscle of the foot, which clings to the spiral
column or columella of the shell (see fig. 42). This col-
umella muscle is the same thing us the muscular surface
marked c in the figures of Patella, marked k in fig. 91 of
Nautilus, and the posterior adductor of Lamellibranchs
(fig- 131).

The surface of the neck is covered by integument forming
the floor of the branchial cavity. It lias not been cut into.

FIG. 30.— Animal and shell of nnnis r.nilns. a, snout (nut introversible) ; ft,
cephalic tentacles; c, right eye ; ,1, pro- and mi'so-podium,— to the right of
this is seen the ineta podium bearing the sculptured opereulum.

Of the organs lying on the reflected mantle-skirt, that which
in the natural state lay nearest to the vas deferens on the
right side of the median line of
the roof of the branchial chamber
is the rectum i', ending in the
anus «. It can be traced back to
the intestine i near the surface of
the visceral hump, and it is found
that the apex of the coil formed
by the hump is occupied by the
liver h and the stomach v. Pha-
rynx and oesophagus are con-
cealed in the head. The enlarged
glandular structure of the walls
of the rectum is frequent in the
Azygobranchia, as is also though
not universally the gland marked
y, next to the rectum. It is the
adrectal gland, and in the genera
Murex and Purpura secretes a
colourless liquid which turns

Fin. M.— Animal and shell of T'ljrvjf, l.rriqata. o, siphon ; b, head-tentacles ; C, head, the letter placed near the rMit purple Upon exposure to the at-
bot, expanded as in crawling ; h, the mantle-skirt reflected over the sides of the shell. (From Owen. mOSphere, and was used by the

The branchial chamber formed by the mantle-skirt over-
hanging the head has been exposed by cutting along a line
extending backward from the letters vd to the base of the
columella muscle me, and the whole roof of the chamber
thus detached from the right side of the animal's neck has
been thrown over to the left, showing the organs which lie
upon the roof. No opening into the body-cavity has been
made ; the organs which lie in the coiled visceral hump
show through its transparent walls. The head is seen in
front resting on the foot and carrying a median non-retractile
snout or rostrum, and a pair of cephalic tentacles at the
base of each of which is an eye. In many Gastropoda the
eyes are not thus sessile but raised upon special eye-tentacles
(figs. 43, 69). To the right of the head is seen the muscular
penis p close to the termination of the vas deferens (sper-
matic duct) rd. The testis t occupies a median position in
the coiled visceral mass. Behind the penis on the same
side is the hooklike columella muscle, a development of the

ancients as a dye.
Near this, and less
advanced into the
branchial chamber,
is the single renal
organ or nephri-
dium r with it~
opening to the ex-
terior r. Internally
this glandular sac
presents a second
slit or aperture
which leads into the
pericardium (as is
now found to be
the case in all Mol-
lusca). The heart
c lying in the

. 40. — Shell of Calyptrrea, seen from below so as

tn slinw thr imi'T \\hnrl h, e< nieralfd by the cap-
like oilier \vhurl o.

ericardinm is seen in close proximity to

114

MOLLUSCA

o. 41. — Animal and shell of Oviilum. h, cephalic
tentacles; d, foot; *, mantle-skirt, which is ntitn-

the renal organ, and consists of a single auricle receiving

blood from the gill, and of a single ventricle which pumps

it through the body by an anterior and posterior aorta

(see fig. 105). The

surface x of the

mantle between the

rectum and the gill-

plume is thrown

into folds which

in many sea-snails

(Whelks, &c.) are

very strongly deve-

l J rrnT ' 1, 1

loped. ine Whole

of this surface ap- rally carried in a reflected condition so as to
. * cover in the sides of the shell.

pears to be active

in the secretion of a mucous-like substance. The single

gill-plume br lies to the left of the median line in natural

position. It corresponds to the

right of the two primitive cten-

idia in the untwisted archaic

condition of the Molluscan body,

and does not project freely into

the branchial cavity, but its

axis is attached (by concres-

cence) to the mantle-skirt (roof

of the branchial chamber). It

is rare for the gill-plume of an

Anisopleurous Gastropod to

stand out freely as a plume,

but occasionally this more ar-

chaic condition is exhibited, as

in Valvata (fig. 45). Next be-

yond (to the left of) the gill-

plume we find the so-called para-

branchia, which is here simple,

but sometimes lamellated as in

Purpura (fig. 47). This organ

has, without reason, been sup-

posed to represent the second Fm_ 42 ___ <* thsheii of

ctemCUUm Of the typical MolrUSC, Ttitonium, Cuv. «, apex;nc, si-

which it cannot do on account
of its position. It should be

phonal notch of the mouth of the
shell ; «c to pc, mouth of the shell;
*<•. f, \v]iorls of the shell ; s, s, sn-

,1 - . i , r ,1 tares. Occupying the axis, and

to the right Ot the anus were exposed by the section, is seen the

-

this the case. Recently Spengel ^jp^STfiSfiSS, «™
has shown that the parabran- to be divided into separate cham-
chia of Gastropods is the typical ^X^'"^ "(S
olfactory organ or osphradium Owen.)
in a highly-developed condition The minute structure
of the epithelium which clothes it, as well as the origin of

f

Fio. 43. — Animal ami shell of finsttlhtria rcctirnstris. a, snout or rostrum;
t, cephalic tent-acle ; c, eye ; </, pi-Mimdmm and nu-snpodimn ; e, metapodinm ;
/, operculum ; }i\ proltm-ril sl|ili»n;il jmk-h nf the shell occupied by the
siphon, or trough-like process of the mantle-skirt. (From Owen.)

the nerve which is distributed to the parabranchia, proves
it to be the same organ which is found universally in Mol-

luscs at the base of each gill-plume, and tests the indrawn
current of water by the sense of smell. The nerve to this

Fio. 44. — Female Janthina, with egg-float (a) attached to the foot; ?>, egg-
capsules ; c, ctenidium (gill-plnine) ; rf, cephalic tentacles.

organ is given off from the superior (original right, see
fig. 19) visceral ganglion.

The figures which are here given of various Azygo-
branchia are in most cases suffi-
ciently explained by the refer-
ences attached to them. As an
excellent general type of the
nervous system, attention may
be directed to that of Paludina
drawn in fig. 21. On the whole,
the ganglia are strongly indivi-
dualized in the Azygobranchia,
nerve-cell tissue being concen-
trated in the ganglia and absent Flc. 45._ra,mta „„•„,„,„, M«n.
from the cords (contrast with Zy- °. mouth ; op, operculum ; f>r,

i -, . i T i \ * ctenidium (branchial plume) : a*

gobranchia and Isopleura). At filiform appendage (? rudiment

the Same time, the junction of aryctenidium). The freely pro-

j xi/i^u v,i )ect,mg etenulmm of typical form

the Visceral loop above the in- not having its axis fused to the.
tpstinp nrpvpnra in ill t!trov,t,-> roof of the branchial chamber is

•,lme pi events m all btieptO- the notable character of this

neura the shortening of the vis- genus,
ceral loop, and it is rare to find a fusion of the visceral
ganglia with either pleural, pedal, or cerebral — a fusion
which can and does
take place where the
visceral loop is not
above but below the
intestine, e.g., in the
Euthyneura (fig. 67),
Cephalopoda(fig. 112),
and Lamellibranchia
(fig. 144). As con-
trasted with the Zygo-
branchia and the Iso-
pleura, we find that in
the Azygobranchia the
pedal nerves are dis-
tinctly nerves given off
from the pedal ganglia,
rather than cord-like
nerve-tracts contain-
ing both nerve -cells
or ganglionic elements
and nerve-fibres. Yet
in some Azygobran-
chia (Paludina) a lad- Fin. 46.— Male of Littarina littoralin, Lin., re-
T |.n moved fromits shell; the mantle-skirt cut along

der-llke arrangement jts right line of attachment and thrown over
of the two pedal to the left side of the animal so as*to expose the
organs on its inner face, a, anus ; i, intestine;
r, nephridium (kidney) ; r', aperture of the
nephridium ; c, heart ; 6?', ctenidium (gill-
plume) ; j'/'r, parabraneliia ( = the osphradium
or olfactory patch) ; x, glandular htmrlUe of
r -,, the inner face of the mantle-skirt; y, adrectal

tolOgy Ot the nerVOUS (purpuripanms) gland; (, lestis; nV, vaa de-
ferens; j),penis; mf.colmnella muscle(imiscula

nerves and their lateral
branches has been de-
tected (30). The his-

process grasping the slu'll); p, stomach ; h, liver.
.A'./*'. Note the simple snout or rostrum not in-
troverted as a " proboscis."

system of Mollusca
has yet to be seri-
ously inquired into.

The alimentary canal of the Azygobranchia presents
little diversity of character, except in so far as the buccal
region is concerned. Salivary glands are present, and in
some carnivorous forms (Dolium) these secrete free sul-

MOLLUSCA

115

FIG. 47.— Female of P
piU-ns removed from its shell ;
the mantle-skirt cut along its
left Hue of attachment and
thrown over to the right side
of the animal so as to expose
the organs on its inner face,
n, anus ; w/, vagina ; <ip, adrec-
tal purpuriparous gland ; r't
aperture of the nephridium(kid-
ney) ; br, ctenidium (branchial
plume); br', parabranchia( = the
comb-like osphrudium or olfac-
tory organ).

phurie acid (ns much as two per cent is present in the
secretion), which assists the animal in boring holes by
nu'ans of its rasping tongue through the shells of other
Molluscs upon which it preys. A crop-like dilatation of
the gut and a recurved intestine, embedded in the com-
pact yellowish-brown liver, the ducts of which open into it,
form the rest of the digestive tract and occupy a large
bulk of the visceral hump. The buccal region presents a
pair of shelly jaws placed laterally upon the lips, and a
wide range of variation in the form of the denticles of the
lingual ribbon or radula, the nature of which will be un-
derstood by a reference to fig. 9, whilst the systematic list
of families given above shows the particular form of den-
tition characteristic of each division of the order.

The modification in the form of the snout upon which
the month is placed, leading to the
distinction of " proboscidiferous "
and " rostriferous " Gastropods, re-
quires further notice. The condi-
tion usually spoken of as a "pro-
boscis " appears to be derived from
the condition of a simple rostrum
(having the mouth at its extrem-
ity) by the process of incomplete
introversion of that simple rostrum.
There is no reason in the actual
significance of the word why the
term "proboscis" should be applied
to an alternately introversible and
eversible tube connected with an
animal's body, and yet such is a
very customary use of the term.
The introversible tube may be
completely closed, as in the "pro-
boscis " of Nemertean worms, or
it may have a passage in it leading into a non-eversible
oesophagus, as in the present case, and in the case of the
eversible pharynx of the predatory Chsetopod worms. The
diagrams here introduced (fig. 48) are intended to show
certain important distinctions which obtain amongst the
various " introverts," or intro- and e-versible tubes so fre-
quently met with in animal bodies. Supposing the tube
to be completely introverted and to commence its ever-
sion, we then find that eversion may take place, either
by a forward movement of the side of the tube near its
attached base, as in the proboscis of the Nemertine worms,
the pharynx of Chaetopods, and the eye-tentacle of Gastro-
pods, or, by a forward movement of the inverted apex
of the tube, as in the proboscis of the Ehabdoccel Planar-
ians, and in that of Gastropods here under consideration.
The former case we call " pleurecbolic " (fig. 48, A, B, C,
H, I, K), the latter " acrecbolic " tubes or introverts (fig.
48, D, E, F, G). It is clear that, if we start from the
condition of full eversion of the tube and watch the pro-
cess of introversion, we shall find that the pleurecbolic
variety is introverted by the apex of the tube sinking in-
wards ; it may be called acrembolic, whilst conversely the
acrecbolic tubes are pleurembolic. Further, it is obvious
enoiigh that the process either of introversion or of eversion
of the tube may be arrested at any point, by the develop-
ment of fibres connecting the wall of the introverted tube
with the wall of the body, or with an axial structure such
as the oesophagus ; on the other hand, the range of move-
ment of the tubular introvert may be unlimited or complete.
The acrembolic proboscis or frontal introvert of the Nemer-
tine worms has a complete range. So has the acrembolic
pharynx of Chwtopods, if we consider the organ as ter-
minating at that point where the jaws are placed and the
oesophagus commences. So too the acrembolic eye-tentacle
of the snail has a complete range of movement, and also the

pleuremliolir proboscis of the Tihabdoccel prostoma. The
introverted rostrum of the Azygobranch Gastropods pre-
sents in contrast to these a limited range of movement.
The " introvert " in these Gastropods is not the pharynx as
in the Chsetopod worms, but a pne-oral structure, its apiV:il
limit being formed by the true lips and jaws, whilst the
apical limit of the Cluvtopod's introvert is formed by the
jaws placed at the junction of pharynx and oesophagus, .so
that the Cluetopod's introvert is part of the stomod;eum
or fore-gut, whilst that of the Gastropod is external to the
alimentary canal altogether, being in front of the mouth,
not behind it, as is the Ch&topod's. Further, the Gastro-
pod's introvert is pleurembolic (and therefore acrecbolic),
and is limited both in eversion and in introversion ; it can-

f\

BJ

C D* &VB

or a a\\

/— '

— ^

/" ' A

C\ r~\ /-\\ \/~\

c •

„

d

b

fl

iU u

^

i

.1

e*/

V

4

c

6 j.

a

a

n

fS

fl

H

I >

X

c

L

a

1

K

\

b

,!

A

**•<.

c

d

«7-

a/-

al-

aZ-

c
al

FIG. 48. — Diagrams explanatory of the nature of so-called proboscides or "intro-
verts." A. Simple introvert completely introverted. B. The same, partially
everted by eversion of the sides, as in the Nemertine proboscis and Gastropod
eye-tentacle=pleurecbolic. C. The same, fully everted. D, E. A similar
simple introvert in course of eversion by the forward movement, not of its
sides, but of its apex, as in the proboscidean Rhabdoco?ls = acrecbolic. F.
Acrecbolic ( = pleurembolic) introvert, formed by the snout of the proboscidi-
ferous Gastropod, al, alimentary canal ; d, the true mouth. The introvert
is not a simple one with complete range both in eversion and introversion,
but is arrested in introversion by the fibrous bands at c, and similarly in
eversion by the fibrous bands at b. G. The acrecbolic snout of a probos-
cidiferous Gastropod, arrested short of complete eversion by the fibrous band
b. H. The acrembolic ( = pleurecbolic) pharynx of a Chsetopod fully intro-
verted, al, alimentary canal ; at d, the jaws ; at a, the mouth ; therefore a.
to d is stomodjeum, whereas in the Gastropod (F) ft to d is inverted body-
surface. I. Partial eversion of II. K. Complete eversion of H. (Original.)

not be completely everted owing to the muscular bands
(fig. 48, G), nor can it be fully introverted owing to the bands
(fig. 48, F) which tie the axial pharynx to the adjacent
wall of the apical part of the introvert. As in all such
intro- and e-versible organs, eversion of the Gastropod
proboscis is effected by pressure communicated by the
muscular body-wall to the liquid contents (blood) of the
body-space, accompanied by the relaxation of the muscles
which directly pull upon either the sides or the apex of
the tubular organ. The inversion of the proboscis is effected
directly by the contraction of these muscles. In various
members of the Azygobranchia the mouth-bearing cylinder
is introversible (i.e., is a -jD-nliiwix) — with rare exceptions
these forms have a siphonate mantle-skirt. On the other
hand, many which have a siphonate mantle-skirt are not
provided with an introversible mouth-bearing cylinder, but
have a simple non-introversible rostrum, as it lias been

11(5

MOLLUSCA

termed, which is also the condition presented by the mouth-
bearing region in nearly all other Gastropoda. One of the
best examples of the introversible mouth-cylinder or pro-
boscis which can be found is that of the Common Whelk
and its immediate allies. In fig. 37 the proboscis is seen
in an everted state ; it is only so carried when feeding,
being withdrawn when the animal is at rest. Probably
its use is to enable the animal to introduce its rasping
and licking apparatus into very narrow apertures for the
purpose of feeding, e.y., into a small hole bored in the shell
of another Mollusc.

The foot of the Azygobranchia, unlike the simple mus-
cular disc of the Isopleura and Zygobranchia, is very often
divided into lobes, a fore, middle, and hind lobe (pro-,
ineso-, and meta-podium, see figs. 39 and 43). Very usually,
but not universally, the meta-podium carries an operculum.
The division of the foot into lobes is a simple case of that
much greater elaboration or breaking up into processes and
regions which it undergoes in the class Cephalopoda. Even
among some Gastropoda (viz., the Opisthobranchia), we
find the lobation of the foot still further carried out by
the development of lateral lobes, the epipodia, whilst there
are many Azygobranchia, on the other hand, in which the
foot has a simple oblong form without any trace of lobes.

The development of the Azygobranchia from the egg has
been followed in several examples, e.g., Paludina, Purpura,
Nassa, Vermetus, Neritina. As in other Molluscan groups,
we find a wide variation in the early process of the forma-
tion of the first embryonic cells, and their arrangement as
a Diblastula dependent on the greater or less amount of
food-yelk which is present in the egg-cell when it com-
mences its embryonic changes. In fig. 7, the early stages
of Paludina vivipara are represented. There is but
very little food-material in the egg of this Azygobranch,
and consequently the Diblastula forms by invagination ;
the blastopore or orifice of invagination coincides with the
anus, and never closes entirely. A well-marked Troeho-
sphere is formed by the development of an equatorial
ciliated band ; and subsequently, by the disproportionate
growth of the lower hemisphere, the Trochosphere becomes
a Veliger. The primitive shell-sac or shell-gland is well
marked at this stage, and the pharynx is seen as a new
ingrowth (the stomodreum), about to fuse with and open
into the primitively invaginated arch-enteron (fig. 7, F).

In other Azygobranchs (and such variations are repre-
sentative for all Mollusca, and not characteristic only of
Azygobranchia), we find that there is a very unequal
division of the egg-cell at the commencement of embryonic
development, as in Nassa (fig. 5). Consequently there is
strictly speaking no invagination (emboly), but an over-
growth (epiboly) of the smaller cells to enclose the larger.
The general features of this process and of the relation of
the blastopore to mouth and anus have been explained
above in treating of the development of Mollusca generally.
In such cases the blastopore may entirely close, and both
mouth and anus develop as new ingrowths (stomodasum
and proctodcBum), whilst, according to the observations of
Bobretzky, the closed blastopore may coincide in position
with the mouth in some instances (Nassa, &c.), instead of
with the anus. But in these epibolic forms, just as in the
embolic Paludina, the embryo proceeds to develop its cili-
ated band and shell-gland, passing through the earlier con-
dition of a Trochosphere to that of the Veliger. In the
veliger stage many Azygobranchia (Purpura, Nassa, Ac.)
exhibit, in the dorsal region behind the head, a contractile
area of the body-wall. This acts as a larval heart, but
ceases to pulsate after a time. Similar rhythmically con-
tractile areas are found on the foot of the embryo Pulmo-
nate Limax and on the yelk-sac (distended foot-surface)
of the Cephalopod Loligo (see fig. 72**).

The history of the shell in the development of Azygo-
branchia (and other Gastropods) is important. Just as
the primitive shell-sac aborts and gives place to a cap-like
or boat-like shell, so in some cases (Marsenia, Krohn) has
this first shell been observed to be shed, and a second shell
of different shape is formed beneath it.

A detailed treatment of what is known of the histo-
genesis in relation to the cell-layers in these Mollusca would
take us far beyond the limits of this article, which aims at
exposing only the well-ascertained characteristic features
of the Mollusca and the various subordinate groups. There
is still a great deficiency in our knowledge of the develop-
ment of the Gastropoda, as indeed of all classes of animals.
The development of the gill (ctenidium) as well as of the
renal organ, and details as to the process of torsion of the
visceral hump, are still quite insufficiently known.

One further feature of the development of the Azygobran-
chia deserves special mention. Many Gastropoda deposit
their eggs, after fertilization, enclosed in capsules ; others, as
Paludina, are viviparous ; others, again, as the Zygobranchia,
agree with the Lamellibranch Conchifera (the Bivalves) in
having simple exits for the ova without glandular walls,
and therefore discharge their eggs unenclosed in capsules
freely into the sea- water ; such unencapsuled eggs are
merely enclosed each in its own delicate chorion. When
egg-capsules are formed they are often of large size, have
tough walls, and in each capsule are several eggs floating
in a viscid fluid. In some cases all the eggs in a capsule
develop ; in other cases one egg only in a capsule (Neri-
tina), or a small proportion (Purpura, Buccinum), advance
in development ; the rest are arrested either after the first
process of cell-division (cleavage) or before that process.
The arrested embryos or eggs are then swallowed and
digested by those in the same capsule which have advanced
in development. The details of this history require renewed
study, our present knowledge of it being derived from the
works of Koren and Danielssen, Carpenter and Claparede.
In any case it is clearly the same process in essence as that
of the formation of a vitellogenous gland from part of the
primitive ovary, or of the feeding of an ovarian egg by
the absorption of neighbouring potential eggs ; but here
the period at which the sacrifice of one egg to another
takes place is somewhat late. What it is that determines
the arrest of some eggs and the progressive development
of others in the same capsule is at present unknown.

Section b (of the Azygobranchia).— NA TANTIA.

Characters. — Azygobranchiate Streptoneura which have the
form and texture of the body adapted to a free-swimming pelagic
habit. They appear to be derived from holoehlamydic forms of
Keptant Azygobranchia. The foot takes the form of a swimming
organ. The nervous system and sense-organs (eyes, otocysts, and
osphradium) are highly developed. The odontophore also is re-
markably developed, its admedian teeth being mobile, and it serves
as an efficient organ for attacking other pelagic forms upon which
the Natantia prey. The sexes are distinct as in all Streptoneura ;
and genital duets and accessory glands and pouches are present as
in all Azygobranchia. The Natantia exhibit a series of modifica-
tions of the form and proportions of the visceral mass and foot,
leading from a condition readily comparable with that of a typical
Azygobranch such as Rostellaria, with the three regions of the foot
(pro-, meso-, and meta-podium) strongly marked, and a coiled
visceral hump of the usual proportions, up to a condition in which
the whole body is of a tapering cylindrical shape, the foot a plate-
like vertical tin, and the visceral hump almost completely atrophied.
Three steps of this modification may be distinguished as three sub-
orders, the Atlantacca, the Carinariacca, and the Pterotrackeacea.

Sub-order 1. —

Cliaractcrs. — Katantia with a large' spirally-wound visceral hump,
covered by a hyaline spiral shell ; mantle-skirt large, overhanging
a well-developed sub-pallial branchial chamber as in Azygobranchia,
to the wall of which is attached the branchial ctenidium; foot
well developed, divisible into a mobile propodium, a mesopodium
on which is formed a sucker, and a metapodium which, when the
animal is expanded, extends backwards beyond the shell and viscera]

MOLLUSCA

117

hump ; 11(1011 the upper surface cit' tin1 metapiidiimi is developed an

operculum.

Genera: Atluntu, Oj-i/i/nnis. Probably hero belong the Palaeozoic
fossils Bellcruplion.

Sub-order 2. —

Characters. — Visceral hump greatly reduced in relative size;
shell small, eap-like, hyaline ;
ctenidium (branchial plume)
projecting from the small sub-
pallial chamber; body cylin-
drical ; of the foot-lobes only
the mesopodiuni is prominent,
provided with a sucker, and
compressed laterally so as to
form a vertical plate -like I'm
projecting from the ventral
surface; the propodium forms
simply the ventral surface of
the anterior region of the cy-
lindrical body whilst the me-
tapodium forms its posterior
region.

Genera : Carinaria, Cardio-
poda.

Sub-order 3. — P/crotracheacea.

Characters. — Visceral hump
still further reduced, forming
a mere oval sac embedded in
the posterior dorsal region of
the cylindrical body ; no shell ;
foot as in Carinariacea, except
that the sucker is absent from
the mesopodiuni in the females.

Genera ; Ptcrotrachea, Firu- FlG- SO.-Carinara meilitermnm.
laides.

the visceral loop of the Natantia is Streptoneurous. Special
to the Natantia is the hiirh elaboration of the lingual
rililion, and, as an agreement witli sonic of the Opistho-
branchiate Euthyneura but as a iliH'i'ivmv from the A/.VL'O-
branchia, we find the otocysts closely attached to the cerebral
ganglia. This is, however, less of a difference than it was

A. The animal. B. The .shell removed. C, I). Two views ui' the shell of Car<li»|.n<]a

«, mouth and odontophore; ft, cephalic tentacles; r, eye; </, tin- tin-like mesopoditun ; <l', its sucker; e, metapudium
/, salivary glands ; h, border of the mantle-flap ; i", ctenidium (gill-plume) ; '", stomach ; n, intestine ; o, anus ; p, liver
t, aorta, springing from the ventricle ; ut cerebral ganglion ; v, pleural and pedal ganglion ; ic, testis ; x, visceral ganglion
y, vesicula seminalis ; ?. penis. (From Owen.)

, Further BemarTcs on the

Natantia Aryt/obranchia. —

Logically the Natantia should stand as we have placed them,

viz., as a special branch or section of the Azygobranchia,

related to them somewhat as are the Birds to the Reptiles.

They are true Azygobranchia which have taken to a pelagic

life, and the peculiarities of structure which they exhibit

Flo. 49.— Atlanta (Orygnra) Kerniulrenii (magnified 'Jn diameters), n, mouth
and odontophore ; /', cephalic tentacles ; f, eye ; i/, pi-npodium (II) and meso-
podium ; e, inetapodium ; /, operculum; //, mantle-chamber; (.ctenidium
(gill-plume) ; k, retractor muscle of foot ; /, optic tentacle ; m. stomach ; it,
dorsal surface overhung by the mantle-skirt, the letter is close to the salivary
gland ; o, rectum and anus ; p, liver ; 5, renal organ (nephridium)]; s, ven-
tricle ; u, the otocyst attached to the cerebral ganglion ; w, testis ; J~, auricle
of the heart ; y, vesicle on genital duct; z, penis. (From Owen.)

are strictly adaptations of the structure common to them
and the Azygobranchia consequent upon their changed
mode of life. Such adaptations are the transparency and
colourlessness of the tissues, and the modifications of the
foot, which still shows in Atlanta the form common in
Azygobranchia (compare fig. 49 and fig. 39).

The cylindrical body of Pterotracheacea is paralleled by
the slug-like forms of Euthyneura. Spengel has shown that

at one time supposed to be, for it has been shown by Lacaze
Duthiers, and also by Leydig, that the otocysts of Azygo-
branchia even when lying close upon the pedal ganglion
(as in fig. 21) yet receive their special nerve (which can
sometimes be readily isolated) from the cerebral ganglion (see
fig. 36). Accordingly the difference is one of position of the
otocyst and not of its nerve-supply. The Natantia are further
remarkable for the high development of their cephalic eyes,
and for the typical character of their osphradium (Spengel's
olfactory organ). This is a groove, the edges of which are
raised and ciliated, lying near the branchial plume in
the genera which possess that organ, whilst in Firuloides,
which has no branchial plume, the osphradium occupies a
corresponding position. Beneath the ciliated groove is

ml ___

Fio. 51. — Pterotrachw vuitic« seen from the right side. ", pouch for reception
of the snout when retracted : c, pericardium ; pli, pliannx ; or. cephalic eye ;
g, cerebral ganglion; </', pieuro-jiedal gangliun ; ]•/-, Inot (mesopodlnm) ; v,
stomach ; t, intestine ; n, so-called nucleus ; br, branchial plume (ctenidium);
t/;, osphradium ; mtt foot (metapodium) ; z, caudal appendage. (After Kefer-
stein.)

placed an elongated ganglion (olfactory ganglion) connected
by a nerve to the supra-intestinal (therefore the primitively
dextral) ganglion of the long visceral nerve-loop, the strands
of which cross one another, — this being characteristic of
Streptoneura (Spengel).

The Natantia belong to the " pelagic fauna " occurring
near the surface in the Mediterranean and great oceans in
company with the Pteropoda, the Siphonophorous Hydrozoa,
Salp;e, Leptocephali, and other specially-modified trans-
parent swimming representatives of various groups of the
animal kingdom. In development they pass through the
typical trochosphcre and veliger stages provided with boat-
like shell.

118

MOLLUSCA

Branch b.—EUTHYNEURA (Spengel, 1881).

Characters. — Gastropoda Anisopleura in which the
visceral loop (the conterminous visceral nerves) does not
share in the torsion of the visceral hump, but, being suuk
entirely below the body-wall, remains straight and un-
twisted. Although the anus is not brought so far forward

FIG. 52. — Bulla re.rillnin. (Chemnitz), as seen crawlinp. a, oral hood (compare
with Tethys, fig. 62, B), possibly a continuation of the epipodia; b, b' , cephalic
tentacles. (From Owen.)

by the visceral torsion as in the Streptoneura, and may even

by secondary growth assume a posterior median position, yet,

as fully developed, an asymmetry has resulted as in the

Azygobranchia, only the original right renal organ, right

ctenidiuro (if any), right osphradium, right side of the heart,

and right genital ducts being retained. All the Euthy-

neura are hermaphrodite. The lingual ribbon has very

usually numerous fine denticles

undifferentiated into series in

each row. The shell is light

and little calcified ; often it is

not developed in the adult,

though present in the embryo.

An operculum, often found in d

the embryo, is never present in FIG. 53.— Tornatella. ft,
.-, a ij_ / j. • m ,.11 °ral hood; rf, foot: /, o

the adult (except in Tornatella,

fig. 53). Many Euthyneura show a tendency to, or a
complete accomplishment of, the suppression of the mantle-
skirt as well as of the shell, also of the ctenidium, and ac-
quire at the same time a more or less cylindrical (slug-like)
form of body.

The Euthyneura comprise two orders, the Opistho-
branchia and the Pulmonata.

Order 1. — Opisthobranchia.

Marine Euthyneura the more archaic forms of which
have a relatively large foot and a small visceral hump,
from the base of which projects on the right side a short
mantle-skirt. The anus is placed in such forms far back

iliell; b,
perculum.

Fjo. 54.— Umbrella mnliterrunm. », l ith; h, n-phnlic tentacle; ft, Kill

(cteuidium). The free edge of the mantle is si'cn just below the margin of
the shell (compare with Aplysia, fig. 63). (From Owen.)

beyond the mantle-skirt. In front of the anus, and only
partially covered by the mantle-skirt, is the ctenidium with
its free end turned backwards. The heart lies in front of,
instead of to the side of, the attachment of the ctenidium,
— hence Opisthobranchia as opposed to " Prosobranchia,"

Fio. 55. — Umbrella meditcrrancn, seen
from above, ft, mouth ; it cephalic
tentacles ; A', penis-sheath. (After
Keferstein.)

which correspond to the Streptoneura. A shell is possessed
in the adult state by but few Opisthobranchia, but all pass
through a veliger larval stage with a nautiloid shell (fig. 60).
Many Opisthobranchia have
by a process of atrophy lost
the typical ctenidium and the
mantle-skirt, and have deve-
loped other organs in their
place. As in some Azygo-
branchia, the free margin of
the mantle-skirt is frequently
reflected over the shell when
a shell exists; and, as in some
Azygobranchia, broad lateral
outgrowths of the foot (epi-
podia) are often developed,
which, as does not, occur in Azy-
gobranchia, may be thrown
over the shell or naked dorsal F
surface of the body.

The variety of special deve-
lopments of structure accom-
panying the atrophy of typical organs in the Opisthobranchia
and general degeneration of organization is very great, and
renders their classification difficult. Two sections of the
order may be distinguished, according as the typical
Molluscan mantle-skirt (limbus pallialis) is or is not atro-
phied, and within each section certain sub-orders.

Section a.— PALLIA TA (=Teclibranchiata, Woodward)— the
typical Molluscau mantle-skirt or pallium retained.

Sub-order 1. — Ctenidiobranchia.

Characters. — Palliata in which the ctenidium is retained as the
branchial organ ; with rare exceptions a delicate shell, which may
be very small or completely enclosed by the reflected margin of the
mantle; epipodia (lateral outgrowths of the foot) fi equently present.
Family 1. — Tornatellidse.

Genera : Tornatella, Lam. (fig. 53) ; CinuUa, Gray, &c.
Family 1.—Bullidse.

Genera : Bulla, Lam. (fig. 52) ; Acera, Miiller ; Scaphander,
Montf. ; Sullied, Lain. ; Doridium, Meckel ; Gastropteron,
Meckel, &c.
Family 3. — Aplysiidse.
Genera: Aplysia, Gmelin (the Sea-Hare) (figs. 20, 56, &c.) ;

Dolobdla, Lam.; Lobigcr, Krohn, &c.
Family 4. — Plciirobranchidte.

Genera : Plciirobranchus, Cuvier ; Umbrella, Chemnitz (figs. 54,
55); Euncina, Forbes, &c.

Sub-order 2. — Phyllidiobrancli la.

CTiaracto-s.— Palliata in which the cteuidia have atrophied ; much
as in Patellidje among the Zygobranchiate Streptoneura their place
is taken by laterally-placed lamella:, developed from the inner surface
of the bilaterally-disposed mantle-skirt in two lateral rows.
FarnDy 5. — Ph ul/iifimlu'.
Genera : Phijllidia, Cuiver ; Pleurophyllidia, Meek. (fig. 57).

Section b.— NON-PALLIA TA.

Characters. — The typical Molluscan mantle-skirt is atrophied in
the adult. No shell is present in the adult, though the dorsal
integument may be strengthened by calcareous spicules (Doris) . The
otocysts are not sessile on the pedal ganglia as in other Gastropods,
1 nit, as in the Natantia Azygobranchia, lie close to the cerebral ganglia.
In one sub-order (Pygobrauchia) the typical ctenidium appears to
be retained in a modified form ; in the others special developments
of the body-wall take its place, or no special respiratory processes
exist at all. The general form of the body is slug-like, the foot
and visceral hump being coextensive, and a secondary bilatc-ral
symmetry is asserted by the usually median (sometimes right-sided)
dorsal position of the anus on the hinder part of the body.
Sub-order 1. — Pygobraiich in.

Characters. — The ctenidium assumes the form of acirclet of pinnate
processes surrounding the median dorsal anus ; a strongly-marked
iMiipodial fold may occur all round the foot and simulate a mantle-
skirt (see fig. 62, C, Doris) ; papilla; or " cerata " of the dorsal integu-
ment may occur as well as the true ctenidium (tig. 61).
Family 6. — Doridides.

Genera : Doris, L. ; Oonwdoris, Forbes ; Triopa, Johnst. ; Mgina,
Loven ; Tliecam-a, Fleming ; Polyccra, Cuvier ; Idalia, Leuck-
art ; Ancula, Loven; Ccratosoma, Adams; Onchidoris, Blainv.

MOLLUSCA

119

Sub-order 2. — Ccrntonota.

Oinractfrs. — The typical Mollnsran rtriiiilium is not <lcv<']n|>r,l ;
upon the dorsal area is developed a more or less niniii-rous scries <>t
e\ lindrieal or liraneheil processes ^Ihe eera(a) into eaeli of which 111*'
intestine usually sends a process ; anus dorsal, inudian, or rifjht-sidcd.
Family 7. — Trilmiitn/ir.
Genera: Tritonia, Cuvier; ScyUma, L.; Tcthys, L. (fig. 62, B);

lii'in/ronotus, A. and H. ; Dolo, Oken.
Family 8. — Eolidse.

Genera : En! is, Cuvier (fig. 62, A) ; Olaticus, Forstcr ; Fiona, A.
and H. (tig. 67); Embletonia, A. and H. ; Proc/onatits, A. and
H. ; Antiopa, A. and H.; Hcrimrn, Lovi-n ; Alilcria, Alhuan.

Sub-order 3. — Haplumorji/ia.

Characters. — No ctenidia, cerafoi, mantle-skirt, or other processes
of the body-wall ; degenerate forms of small size.
Family 9. — PhyllirJiuiiltr.

Genera : Plii/llirhoe, Peron and Lesucnr (fig. 58) ; Acura, Adams.
Family 10. — Elysiadie.

Genera: Elysia, Risso (fig. 62, 1), E) ; Action in, Quatref. ; Ccnia,
A. and H. ; Limapontin, Johnston ; Ehodopc, Koll.

Further Remarks on the Opisthobranchia. — The Opis-
thohranchia present the same wide range of superficial
appearance as do the Azygobranchiate Streptoneura, forms

Flo. 56. — Three views of Aplysia sp., in various conditions of expansion and
retraction, t, anterior cephalic tentacles ; t"t posterior cephalic tentacles ;
e, eyes ; f, metapodium ; ep, epipodiuiu ; g, gill-plume (ctenidium) ; m, mantle-
flap reflected over the thin oval shell ; os, s, orifice formed by the unclosed
In mier of the reflected man tie- skirt, allowing the shell to show ; pet the sper-
matic groove. (After Cuvier.)

carrying well-developed spiral shells and large mantle-
skirts being included in the group, together with flattened
or cylindrical slug-
like forms. But in
respect of the substi-
tution of other parts
for the mantle -skirt
and for the gill which
the more degenerate
Opisthobranchia ex-
hibit, this Order
stands alone. Some
Opisthobranchia are
striking examples of
degeneration (some
Haplomorpha), hav-
ing none of those re-
gions or processes of Fm 5T"Dorsal and ventral view of phvTorlnf,H.
the body developed dioZin«ato(Otfco\oneofthePhyllidiobranchiate
, . , A' t oli Palliate Opistholjranchs. ft, the mouth ; I. the

Which distinguish ]amPnifnnn sub-pullial Rills, which (as in Patella)
the archaic MollllSCa replace the typical Molluscan ctenidium. (After
, , ,, Keferstein.)

from such flat-worms

as the Denclroccel Planarians. Indeed, were it not for their
retention of the characteristic oclontophore we should have
little or no indication that such forms as Phyllirlioe and

B

3-!

Limapontia really belong to the Mollusca at all. The inter-
esting little Ji'/Kii/H/ii' Viriini/ii, \vliieh has no odontophore,
lias been associated by systematists both with these simpli-
fied Opistlicil>niiiehs and with Khabdcieiel I'lanarians (29).

In many respects
the Sea-Hare (Aply-
sia) of which several
species are known
(some occurring on
the English coast),
serves as a conven-
ient example of the
fullest development

brunch. The intemai organs are shown as SITU
by transmitted light, o, mouth ; (/, radular sac ;
c, oesophagus ; '/, stomach ; c', inteslinf ; ;'. .inn , ;
.". :' • ''". if", the turn loljes of tl» liver; /;, tin-
heart, (auricle and vi-ntricle) ; t, the renal sac (ne-
phridinm) ; /', the ciliated communication of the
renal sae \\ilh Hie prrieanlkiiii ; in, the --xlnn;il
opening of the renal sac ; n, the cerebral ganglion ;
o, the cephalic ti-nlaeles ; / the genital pore;
y, the ovo-testes ; »•, the parasitic l!y<lr"jm'<lnsa
V'c'.^r'f.usual^1 found attached in this position by

Fio. 08. — rJujllirhoi' }»ir,}>}tftl(f.f twice the nntnrnl
size, a transparent pisdfnrm pelagic

thcaboral jxileofits umbrella. (After Keferstein.)

of the organization
characteristic of
Opisthobranchia.
The woodcut (fig. "'i!)
gives a faithful repre-
sentation of the great
mobility of the vari-
ous parts of thebody.
The head is well marked and joined to the body by a some-
what constricted neck. It carries two pairs of cephalic
tentacles and a pair of sessile eyes. The visceral hump is
low and not drawn out into a spire. The foot is long,
carrying the oblong visceral mass upon it, and projecting
(as metapodium) a little beyond it (/). Laterally the
foot gives rise to a pair of mobile fleshy lobes, the epipodia
(ep), which can be thrown up so as to cover in the dorsal

Fie. M.—AceTalullata. A single row of teeth of the radula. (Formula, x.l.x.)

surface of the animal. Such epipodia are common, though
by no means universal, among Opisthobranchia. The
torsion of the visceral hump is not carried out very fully,

Fin. GO.— A. Veliger-iarva of an Opisthobranch (Pnlycera). /, foot ; op, oper-

pft, the commencing bi^nnjv^,*... ~. D

pg, r.-d-1-oloureil pij-'inetit sput. C. liilil.istula of an Opisthobranch (Poly-
cera) with elongated blastopore oi. (All from LankeBter.)

the consequence being that the anus has a posterior posi-
tion a little to the right of the median line above the
metapodium, whilst the branchial chamber formed by the
overhanging mantle-skirt faces the right side of the body
instead of lying well to the front as in Streptoneura and
as in Pulmonate Euthyneura. The gill-plume which in
Aplysia is the typical Molluscan ctenidium is seen in fig.

120

MOLLUSCA

63 projecting from the branchial sub-pallial space. The
relation of the delicate shell to the mantle is peculiar,
since it occupies an oval area upon the visceral hump,
the extent of which is indicated in fig.
56, C, but may be better understood
by a glance at the figures of the allied
genus Umbrella (figs. 54, 55), in which
the margin of the mantle-skirt coin-
cides, just as it does in the Limpet,
with the margin of the shell. But in
Aplysia the mantle is reflected over
the edge of the shell, and grows over
its upper surface so as to completely
enclose it, excepting at the small cen-
tral area s where the naked shell is
exposed. This enclosure of the shell
is a permanent development of the
arrangement seen in many Strepto-
neura (e.g., Pyrula, Ovulum, see figs.
38 and 41), where the border of the
mantle can be, and usually is, drawn
over the shell, though it is withdrawn
(as it cannot be in Aplysia) when they
are irritated. From the fact that
Aplysia commences its life as a free-
swimming Veliger with a nautiloid
shell not enclosed in any way by the
border of the mantle, it is clear that
the enclosure of the shell in the adult
is a secondary process. Accordingly,
the shell of Aplysia must not be con-
founded with a primitive shell in its
shell-sac, such as we find realized in
the shells of Chiton and in the plugs
which form in the remarkable tran-
sitory "shell-sac" or "shell-gland" of Molluscan embryos

[G. 61. — Pnlycrra cristatd,
one of the Pygobranchi-
ate Opisthobranchs (dor-
sal view), o, anus ; br,
the ctenidium peculiarly
modified so as to encircle
the anus; *, cephalic ten-
tacles. External to the
branchial ctenidium are
seen ten club-like pro-
cesses of the dorsal wall,
these are the "cerata"

sub - order of Opistho-
branchs, the Ceratonota
(see fig. 62, A). (From
Gegenbaur, after Alder
and Hancock.)

Fin. 62.

A. Eolis papillosa (Lin.), dorsal view. (/, b, posterior and :nil. ii,,r cephalic

tentacles ; c, the dorsal "cerata " (hence Ceratohranchia).

B. Ti'tli;/* !>/>«>'ii/<i, dorsal virw. a, the cephalic hood ; b, cephalic tentacles ;

c, neck ; </, genital pore ; e, anus ; /, large cerata ; p, smaller cerata ;
ft, margin of the fi>ot.

C. Z>on'.s (At'lhi/ifii'lii.*) tubercutohu (Ouv.), seen from the pedal surface, m,

mouth ; '', margin of the head ; / sole of the fout ; .«y, tin.1 mantle-like

r|H J" "lilUIL.

D. E. Dorsal and lateral view of Elysia (Jc/jron) I'iridis. ep, epipodial out-

growths. (After KeferstiMii )

(see figs. 7, 68, and 72***). Aplysia, like other Mollusca,

develops a primitive shell-sac in its trochosphere stage of

development (fig. 68), which disappears and is succeeded

by a nautiloid shell (fig. 60). This forms the nucleus of

the adult shell,

and, as the ani-

mal grows, be-

comes enclosed

by a reflexion of

the mantle-skirt.

In reference to

the possible com-

parison of the

enclosed shell of

Aplysia and its

allies with those

of some Slugs and

of Cuttle-fishes,

the reader is re-

ferred to the para-

graphs dealing

especially with

those Molluscs.

When the shell

of an Aplysia

enclosed in its FIG. 63. — Aplysi(thporina(camehts, Cuv.), with epipodia
and mantle reflected away from the mid-line, a, an-
terior cephalic tentacle; &, posterior do.; between a
and b, the eyes ; c, right epipodium ; d, left epipo-
dium ; e, hinder part of visceral hump ; ff, posterior
extremity of the foot ; fa, anterior part of the foot
f ,, underlying the head; 17, the ctenidinm (branchial

space IS ttUly ex- plume) ; A, the mantle-skirt tightly spread over the
horny shell and pushed with it towards the left side ;
i, the spermatic groove ; k, the common genital pore

mantle is pushed
well to the left,
the sub-pallial

POSed as in ficf

^' ,

OO, and the van- (male and female) ; /, orifice of the grape-shaped (sup-
Pused Poisonous) gland ; ?n, the osphradium (olfac-

OUS apertures of lory organ of Spengel) ; w, outline ofpart of the" renal

the body are Seen. sac (nephridium) below the surface ; o, external aper-
• i ture of the uephridium ; p. anus. (Original.)

Posteriorly we

have the anus, in front of this the lobate gill-plume, be-
tween the two (hence corresponding in position to that of
the Azygobranchia) we have the aperture of the renal
organ. In front, near the anterior attachment of the gill-
plume, is the osphradium (olfactory organ) discovered by
Spengel, yellowish in colour, in
the typical position, and overly-
ing an olfactory ganglion with
typical nerve-connexion (see fig.
20). To the right of Spengel's
osphradium is the opening of a
peculiar gland which has, when
dissected out, the form of a bunch
of grapes ; its secretion is said to J~
be poisonous. On the under side h
of the free edge of the mantle are
situated the numerous small cu-
taneous glands which, in the large
A]i! i/sin ciiu/eh/s (not in other
species), form the purple secretion
which was known to the ancients.
In front of the osphradium is the
single genital pore, the aperture

,., Fin. 64.— Gonad, and accessory

of the common or hermaphrodite ^lamis and duets of Aplysia.

duct. From this point there \> ovo-testis ; A, hermaphrodite

stretches forward to the right
side of the head a groove — the

duct; (7, albuniiniparous gland ;
/, vesicula seminalis ; A-, open-
ingof the alliuminiparous uland
into the hermaphrodite duct;

Spermatic groove down Which <•, hermaphrodite diiH (ill, , ine

„ . . T portion); 6. vaginal port

the spermatic fluid passes. In

other Euthyneura this groove may

close up and form a canal. At

its termination by the side of the head is the muscular

introverted penis. In the hinder part of the foot (not

shown in any of the diagrams) is the opening of a large

mucous-forming gland very often found in the Molluscan

foot.

,

the uterine duct; c, sperni.'L-
tln-ra; <l, its duct; a, gmitul
pi 'iv. (original.)

MOLLUSCA

121

With regard to internal organization wo may commence
with the disposition of the renal organ (nephridinm), the
external opening of which has already been noted. The
position of this opening and other features of the venal
organ have. l>een determined recently by Mr. J. T. Cunning-
ham, Fellow of University College, Oxford, who writes as
follows from Naples, February 1883 : —

"There is considerable uncertainty with respect to the names of
tlie species of Aplysia. There are two forms \vhirh are very common
in the Gulf of Naples, and which I have used in studying the ana-
fumy cil' tin1 renal organ in the genus. One is <|uite Mark in colour,
and measures when outstretched eight or nine inches in length.
The other is light brown and somewhat smaller, its length usually
not exceeding seven inches. The tirst is flaccid and sluggish in its
movements, and lias not much power of contraction ; its epipoilial
lobes are enormously developed and extend far forward along the
hinlv ; it gives out when handled an abundance of purple liquid,
which is derived from cutaneous glands situated on the under side
of the free edge of the mantle. In the Zoological Station this form
is known as AJI. It'fiorltia ; but according to Blochmann it is iden-
tical with A. Camel us of Cuvier. The other species is -•/. </• /ii/nnx ;
it is firm to the touch, and contracts forcibly when irritated ; the
secretion of the mantle-glands is not abundant, and is milky white
in appearance. The kidney has similar relations in both genera,
and is identical with the organ spoken of by many authors as the
triangular gland. Its superficial extent is seen when the folds
covering the shell are cut away and the shell removed ; the external
surface forms a triangle with its base bordering the pericardium and
its apex directed posteriorly and reaching to the left-hand posterior
corner of the shell-chamber. The dorsal surface of the kidney
extends to the left beyond the shell-chamber beneath the skin in
tin1 space between the shell-chamber and the left epipodium.

" When the animal is turned on its left-hand side ami the mantle-
chamber widely opened, the gill being turned over to the left, a
part of the kidney is seen beneath the skin between the attachment
of the gill ami the right epipodium (fig. 63). On examination
this is found to be the under surface of the posterior limb of the
gland, the upper surface of which has just been described as lying
beneath the shell. In the posterior third of this portion, close to
that edge which is adjacent to the base of the gill, is the external
opening (tig. 63, o).

" When the pericardium is cut open from above in an animal
otherwise entire, the anterior face of the kidney is seen forming
the posterior wall of the pericardial chamber ; on the deep edge of
this face, a little to the left of the attachment of the auricle to the
floor of the pericardium, is seen a depression ; this depression con-
tains the opening from the pericardium into the kidney.

"To complete the account of the relations of the organ : the right
anterior corner can be. seen superficially in the wall of the mantle-
chamber above the gill. Thus the base of the gill passes in a slant-
ing direction across the right-hand side of the kidney, the posterior
end being dorsal to the apex of the gland, and the anterior end
ventral to the right-hand corner.

"As so great a part of the whole surface of the kidney lies adjacent
to external surfaces of the body, the remaining part which faces
the internal organs is small ; it consists of the left part of the under
surface ; it is level with the floor of the pericardium, and lies over
the globular mass formed by the liver and convoluted intestine.

"Mere dissection does not give sufficient evidence concerning such
communications as these of the kidney in Aplysia. I studied the
external opening by taking a series of sections through the sur-
rounding region of the gland ; to demonstrate the internal aperture
injected a solution of Berlin blue into the pericardium ; it did not
fill the whole kidney easily, but ran down iuto the part adjacent to
the base of the gill."

Thus the renal organ of Aplysia is shown to conform to
the Molluscan type. The heart lying within the adjacent
pericardium has the usual form, a single auricle and ven-
tricle. The vascular system is not extensive, the arteries
soon ending in the well-marked spongy tissue which builds
up the muscular foot, epipodia, and dorsal body-wall.

The alimentary canal commences with the usual buccal
mass ; the lips are cartilaginous, but not armed with horny
.jaws, though these are common in other Opisthobranchs ;
the lingual ribbon is rnultidenticulate, and a pair of salivary
glands pour in their secretion. The oesophagus expands
into a curious gizzard, which is armed internally with large
horny processes, some broad and thick, others spinous, fitted
to act as crushing instruments. From this we pass to a
stomach and a coil of intestine embedded in the lobes of a
voluminous liver ; a caecum of large size is given off near

Ihi' cotiuiH mi MM nt .if tin- inlestlne. The livrr opens by
two ducts into flu- digestive tract.

The generative organs lie close to the foil of intestine
and liver, a little to the leftside. When dissected out" they
appear as represented in fig. 04. The essential reproductive,

FIG. 65. — Follicles of the hermaphrodite monads of Euthynenrous Anisopleura.
— At of Helix ; P>, of Eolidia. o, ova ; ft, developing spennatozoids ; -•, com-
mon efferent duct.

organ or gonad consists of both ovarian and testicular
cells (see fig. 65). It is an ovo-testis. From it passes a
common or hermaphrodite duct, which very soon becomes
entwined in the spire of a gland — the albuminiparous gland.
The latter opens into the common duct at the point x, and
here also is a small diverticulum of the duct y. Passing
on, we find not far from the genital pore a glandular spherical
body (the spermatheca a) opening by means of a longish
duct into the common duct, and
then we reach the pore (fig. 6.3,
I'). Here the female apparatus
terminates. But when the male
secretion of the ovo-testis is
active, the seminal fluid passes
from the genital pore along the
spermatic groove (fig. 63,) to
the penis, and is by the aid of
that eversible muscular organ
introduced into the genital pore
of a second Aplysia, whence it
passes into the spermatheca, there
to await the activity of the fe-
male element of the ovo-testis of
this second Aplysia. After an
interval of some days — possibly
weeks — the ova of the second
Aplysia commence to descend
the hermaphrodite duct ; they
become enclosed in a viscid secre- FIG. 66.
tion at the point where the al- {
buminiparous gland opens into S, all of which are not figured ;
,, , * . ' . , * . , ., e, hind gut; on, anus. (From

the duct intertwined With it ; Gegenliaur, after Alder and Han-

and on reaching the point where COL'k-)
the spermathecal duct debouches they are impregnated by
the spermatozoa which escape now from the spermatheca
and meet the ova.

The development of Aplysia from the egg presents many
points of interest from the point of view of comparative
embryology, but in relation to the morphology of the
Opisthobranchia it is sufficient to point to the occurrence
of a trochosphere and a veliger stage (fig. 60), and of a
shell-gland or primitive shell-sac (fig. OS, «•/*.<), which is suc-
ceeded by a nautiloid shell.

The nervous system of Aplysia will be found on com-
parison of fig. 20, which represents it, with our schematic
Mollusc (fig. 1, D) to present but little modification. It is
in fact a nervous system in which the great ganglion-pairs
are well developed and distinct. The Euthyneurous visceral
loop is long, and presents only one ganglion (in Aplysia
but two distinct ganglia joined to one another in

122

MOLLUSCA

Aplysia kyhrida of the English coast), placed at its extreme
limit, representing both the right and left visceral ganglia
and the third or abdominal ganglion, which are so often
separately present. The diagram (fig. 20) shows the nerve
connecting this abdomino-
visceral ganglion with the
olfactory ganglion of Spen- A .._ "*
gel. It is also seen to be
connected with a more re-
mote ganglion — the genital. »
Such special irregularities
in the development of gan-
glia upon the visceral loop,
and on one or more of the
main nerves connected with
it, are, as the figures of
Molluscan nervous systems
given in this article show,
very frequent. Our figure
of the nervous system of
Aplysia does not give the
small pair of buccal ganglia
which are, as in all Glosso-
phorous Molluscs, present
upon the nerves passing from the cerebral region to the
odontophore.

For a comparison of various Opisthobranchs, Aplysia will
be found to present, a convenient starting-point. It is
one of the more typical Opisthobranchs, that is to say,
it belongs to the section Palliata, but other members of the
Palliata, namely, Bulla and Tornatella (figs. 52 and 53),
are less abnormal than Aplysia in regard to their shells and
the form of the visceral hump. They have naked spirally-
twisted shells which may be concealed from view in the
living animal by the expansion and reflexion of the epipodia,

FIG. 67. — Central nervous system of Fiona
(one of the Ceratonotous Opistho-
branchs), showing a tendency to fusion
of the great ganglia. A, cerebral, pleu-
ral, ami visceral ganglia united ; II, pe-
dal ganglion ; I', buccal ganglion ; D,
(esophageal ganglion connected with the
buccal ; a, nerve to superior cephalic
tentacle ; b, nerves to inferior cephalic
tentacles ; r, nerve to generative organs;
rf, pedal nerve ; e, pedal commissure ; t?,
visceral loop or commissure (?). (From
Gegenbaur, after Bergh.)

Flc.. 68.— Young veliger l.irva of an Opisthnriraneh (Flenrn-hv.ineliidiuni). m,
mouth ; r, ciliated kiml marking off the velum ; ny, cerebral ganglion de-
veloping; from epibhist, witliin tin.- vrl;tr ;uv;i ; vt, iitnryst also developing
from epililast; /, foot; /, intestine; T\I/, residual nutritive yelk; sks, primi-
tive shell -sac or shell-gland. (From Lankester.)

but are not enclosed by the mantle, whilst Tornatella is
remarkable amongst all Euthyneura for possessing an oper-
culuin like that of so many Streptoneura.

The great development of the epipodia seen in Aplysia
is usual in Palliate Opisthobranchs; it occurs also in Elysia
(fig. 62, D) among Non-Palliata ; in Doris it seems pmb-
alile that the mantle-like fold overhanging the foot is to
be interpreted as epipodium, the mantle-skirt being alto-
gether absent, as shown by the naked position of the gills
and anus on the dorsal surface (figs. 61 and 62, C). The
whole surface of the body becomes greatly modified in
those Non-Palliate forms which have lost, not only the
mantle-skirt and the shell, but also the ctenidinm. Many
of these. (Ceratonota) have peculiar processes developed
on the dorsal surface (fig. 62, A, B), or retain purely

negative characters (fig. 62, D). The chief modification of
internal organization presented by these forms, as compared
with Aplysia, is found in the condition of the alimentary
canal. The liver is no longer a compact organ opening
by a pair of ducts into the median digestive tract, but we
find very numerous hepatic diverticula on a shortened
axial tract (fig. 66). These diverticula extend usually one
into each of the dorsal papillae or "cerata" when these are
present. They are not merely digestive glands, but are
sufficiently wide to act as receptacles of food, and in them
the digestion of food proceeds just as in the axial portion
of the canal. A precisely similar modification of the liver
or great digestive gland is found in the Scorpions, where
the axial portion of the digestive canal is short and straight,
and the lateral ducts sufficiently wide to admit food into
the ramifications of the gland there to be digested ; whilst
in the Spiders the gland is reduced to a series of simple
c<eca.

The typical character is retained by the heart, peri-
cardium, and the communicating nephridium or renal organ
in all Opisthobranchs. An interesting example of this is
furnished by the fish-like transparent Phyllirhoe (fig. 58),
in which it is possible most satisfactorily to study in the
living animal, by means of the microscope, the course of
the blood-stream, and also the reno-pericardial communi-
cation. With reference to the existence of pores placing
the vascular system in open communication with the
surrounding water, see the paragraph as to Mollusca gener-
ally. In a form closely allied to Aplysia (Pleurobranchus)
such a pore leading outwards from the branchial vein 1ms
been precisely described by Lacaze Duthiers. No such pore
has been detected in Aplysia. In many of the Non-Palliate
Opisthobranchs the nervous system presents a concentra-
tion of the ganglia (fig. 67), contrasting greatly with what
we have seen in Aplysia. Not only are the pleural ganglia
fused to the cerebral, but also the visceral to these (see in
further illustration the condition attained by the Pulmonate
Limnanis, fig. 22), and the visceral loop is astonishingly short
and insignificant (fig. 67, e). That the parts are rightly thus
identified is probable from Spengel's observation of the os-
phradium and its nerve-supply in these forms ; the nerve to
that organ, which is placed somewhat anteriorly — on the dor-
sal surface — being given off from the hinder part (visceral) of
the right compound ganglion — the fellow to that marked A in
fig. 67. The Ceratonotous Opisthobranchs, amongst other
specialities of structure, are stated to possess (in some cases
at any rate) apertures at the apices of the "cerata" or
dorsal papillae, which lead from the exterior into the hepatic
creca. This requires confirmation. Some amongst thuin
(Tergipes, Eolis) are also remarkable for possessing
peculiarly modified epidermic cells placed in sacs at the
apices of these same papilla;, which resemble the "thread-
cells" of the Planarian Flatworms and of the Ciulentcra.
The existence of these thread-cells is sufficiently remark-
able, seeing that the Non-Palliate Opisthobranchs resemble
in general form and habit the Planarian worms, many of
which also possess thread-cells. But it is not conceivable
that theirpresence is an indication of genetic affinity between
the two groups, rather they are instances of homoplasy.
The development of many Opisthobranchia has been
examined — ?.//., Aplysia, Pleurobranchidium, Elysia, Poly-
cera, Doris, Tergipes. All pass through trorhosphiTe and
veliger stages, and in all a nautiloid or boat-like shell is
developed, preceded 1 >y a well-marked "shell-gland " (see figs.
60 and 68). The transition from the free-swimming veliger
larva with its nautiloid shell (fig. 60) to the adult form has
not been properly observed, and many interesting points as
to the true nature of folds (whether epipodia or mantle or
velum) have yet to be cleared up by a knowledge of such
development iu forms like Tethys, Doris, Phyllidia, &c.

MOLLUSCA

123

As in other Molluscan groups, wo find even in closely-
allied genera (for instance, in Aplysia and. Pleurobran-
chidium, and other genera observed by Lankester) the
greatest differences as to the aiiiiiunt of food-material by
which the egg-shell is encumbered. Sonic form their
Diblastula by emboly (fig. 7), otliers by cpiboly (fig. 5);
and in the later history of the further development of the
enclosed cells (arch-enteron) very marked variations occur
in closely-allied forms, due to the influence of a greater or
less abundance. of food-material mixed with the protoplasm
of the egg.

Order 2 (of the Euthyncura). — Pulmonata.

Characters. — Euthyneurous Anisopleurous Gastropoda,
probably derived from ancestral forms similar to the
Palliate Opisthobranchia by adaptation to a terrestrial life.
The ctenidium is atrophied, and the edge of the mantle-skirt
is fused to the dorsal integument by concrescence, except at
one point which forms the aperture of the mantle-chamber,
thus converted into a nearly closed sac. Air is admitted
to this sac for respiratory and hydrostatic purposes, and it
thus becomes a lung. An operculum is never present ; a
contrast being thus afforded with the operculate Pulmonate
Streptoneura (Cyclostoma, etc.), which differ in other
essential features of structure from the Pulmonata. The
Pulmonata are, like the other Euthyneura, hermaphrodite,
with elaborately-developed copulatory organs and accessory
glands. Like other Euthyneura, they have very numerous
small denticles on the lingual ribbon. The ancestral
Pulmonata appear to have retained both the right and the
left osphradia (Spengel's olfactory organs), since in some
(Planorbis, Auricularia) we find the single osphradium to
be that of the original left side, whilst in others (Limnteus)
it is that of the original right side.

In some Pulmonata (Snails) the foot is extended at right
angles to the visceral hump, which rises from it in the
form of a coil as in Streptoneura ; in others the visceral
hump is not elevated, but is extended with the foot, and
the shell is small or absent (Slugs).

The Pulmonata are divided into two sub-orders according to the
position of the cephalic eyes.

Sub-order 1 . — Basommatophora.

Characters. — Eyes placed mediad of the cephalic tentacles at their
base ; the embryonic velar area retained in adult life as a pair of
cephalic lobes (tig. 70, v) ; male and female generative apertures
separate, placed (as is typical in Anisopleura) on tbe right side of
the neck ; visceral hump well developed, with a well-developed
shell ; aquatic in habit.
Family 1. — Limnseidse.

Genera: Limnieus, Lam. (figs. 3, 4, &e. ); Chilinia, Gray; Fhysa,

Draparn. ; Aiicylns, Geotf. ; Planorbis, Mull., &c.
Family 2. — Auriculidee.

Genera : Auricula, Lam. ; Conovulus, Lam. ; Pitkurclla, Wood.
&c.

Sub-order 2. — Stylommatoplwra.

Characters. — Eyes placed on tiie summit of two hollow tentacles ;
visceral hump well or not at all developed ; shell large and coiled,
or minute or absent ; almost exclusively terrestrial.
Family 1. — Hclicidie.

Genera : Helix, L. (figs. 69, A; 72*) ; Vitrinn, Draparn. ; Suc-
cinca, Draparn. ; Bulimus, Scopoli ; Achatina, Lam. ; Pupa,
Lam. ; Clausilia, Draparu., &c.
Family 2. — Limacidse (Slugs).

Genera : Umax, L. ; Indiana, Benson ; Arion, Ferussac (fig.
69, D) ; Parmacclla, Cuviei ; Tcstacella, Cuvier (fig. 69, C), &c.
Family 3. — Oncidiadee.

Genera : Oncidium, Buchanan ; Peronia, Blainv. (fig. 72) ;
Vaginulus, Ferussac, &c.

Further Remarks on Pulmonata. — The land-snails and
slugs forming the group Pulmonata are widely distinguished
from a small set of terrestrial Azygobranchia, the Pneumo-
nochlamyda (see above), at one time associated with them
on account of their mantle-chamber being converted, as in

Pulmonata, into a lung, and the ctenidium or branchial
plume aborted. The Pneumonochlamyda (represented in
England by the common genus Cyclostoma) have a twisted

Fia. 69. — A series of Stylnmmatophorous Pulmonata, showing transitional forms
between snail and slug.

A. Helix pomatia (from KrtVrstein).

B. Helicophaiita brevipes (from Keferstein, after Pfeiffer).

C. Testacella haliotidca (from Keferstein).

D. Arion ater, the great Black Slug (from Keferstein).

a, Shell in A, B, C, shell-sac (closed) in D ; b, orifice leading iuto tbe
subpallial chamber (1UI1S)-

visceral nerve-loop, an operculum on the foot, a complex
rhipidoglossate or ttenioglossate radula, and are of distinct
sexes ; they are, in fact, Azygobranchiate Streptoneura.
The Pulmonata have a straight visceral nerve-loop, never
an operculum (even in the embryo), and a multidenticulate

Flo. 70. — A, B, C. Three views of Limnxus stagnates, in order to show the
persistence of the larval velar area r, as the circum-oral lobes of the adult.
m, mouth ; f, foot ; r, velar area, the margin v corresponding with the
ciliated band which demarcates the velar area or velum of the embryo Gas-
tropod (see fig. 4, D, E, F, H, I, r). (Original.)

radula, the teeth being equi-formal ; and they are hermaphro-
dite. Some Pulmonata (Limnams, itc.) live in fresh-waters
although breathing air. The remarkable discovery has
been made that in deep lakes such Limnaei do not breathe
air, but admit water to the lung-sac and live at the bottom.
The lung-sac serves undoubtedly as a hydrostatic apparatus
in the aquatic Pulmonata, as well as assisting respiration.
It is not improbable that here, and in other air-breathing
animals, the hydrostatic function was the primary one, and
the respiratory a later development.

124

MOLLUSCA

The same general range of body-form in shown in Pul-
inonata as in the Natant Azygobranchia and in the Opis-
thobranchia ; at one extreme we have Snails with coiled
visceral hump, at the other cylindrical or flattened Slugs
(see fig. 69). Limpet-like forms are also
found (fig. 71, Ancylns). The foot is al-
ways simple, with its flat crawling .surface
extending from end to end, but in the
embryo Lmimeus (fig. 4, H) it shows a
bilobed character, which leads on to the
condition characteristic of Pteropoda.

The adaptation of the Pulmonata to ter-
restrial life has entailed little modification
of the internal organization. The vascular system appears
to be more complete in them than in other Gastropoda,
fine vessels and even capillaries being present in place of
lacuna?, in which arteries and veins find their meeting-
point. The subject has not, however, been investigated
by the proper methods of recent histology, and our know-

»rm aquatic

>T*,~»W ** j* • iii: .lia°SS* " *wl° VT~!"' rtr.<7?04.

FIG. 72. — Peronia Tongfe, a littoral Pulmonate, found on the shores of the Indian
and Pacific Oceans (Mauritius, Japan).

ledge of it, as of the vascular system of Molluscs generally,
is most unsatisfactory. In one genus (Planorbis) the
plasma of the blood is coloured red by haemoglobin, this
being the only instance of the pre-
sence of this body in the blood of
Glossophorous Mollusca, though it
occurs in corpuscles in the blood
of the bivalves Area and Solen
(Lankester, 31).

The generative apparatus of the
Snail (Helix) may serve as an ex-
ample of the hermaphrodite appa-
ratus common to the Pulmonata
and Opisthobranchia (fig. 72*).
From the ovo-testis, which lies
near the apex of the visceral coil,
a common hermaphrodite duct v.e
proceeds, which receives the duct
of the compact white albuniini-
parous gland E.d., and then be-
comes much enlarged, the addi-
tional width being due to the
development of glandular folds,
which are regarded as forming a
uterus u. Where these folds cease
the common duct splits into two

portions, a male and a female. Fio.72*. — Hermaphrodite repro-
The male duct v.d becomes fleshy
and muscular near its termination
at the genital pore, forming the
penis p. Attached to it is a diver-

d active apparatus of tin1 Gar-
den Snail (llclii- hortrnsix). ~,
ovo-testis ; v.e, herniaphrn-
dito duct ; E.d., allmminipar-
ous gland ; «., uterine dilata-
tion of the hermaphrodite
duct ; ct, digitate accessory
tlClllum jL) m Which the Sperma- glands on the female duct;

tozoa which have descended from ^c ^ttTfOTili^diicf -S/

.siH-rinathcca or receptacle of
the sperm in copulation, open-
ing into the female duct : v.d,
male duct (vas di-IVrens); p,
penis ; fl., Dagellum.

the ovo-testis are stored and mo-
delled into sperm ropes or sperma-
tophores. The female portion of
the duct is more complex. Soon
after quitting the uterus it is joined by a long duct leading
from a glandular sac, the spermatheca (K.f). In this duct
and sac the spermatophores received in copulation from
another snail are lodged. In Helix kortensis the sperma-

theca is simple. In other species of Helix a second duct
(as large in Hdix nx/icrxa as the chief one) is given off from
the spermathecal duct, and in the natural state is closely
adherent to the wall of the uterus. This second duct has
normally no spermathecal gland at its termination, which
is simple and blunt. But in rare cases in Hi-li.e (ix/nrsa a
second spermatheca is found at the end of this second duct.
Tracing the widening female duct onwards we now come
to the openings of the digitate accessory glands d, d, which
probably assist in the formation of the egg-capsule. Close
to them is the remarkable dart>sac ps, a thick-walled sac,
in the lumen of which a crystalline four-fluted rod or dart
consisting of carbonate of lime is found. It is supposed
to act in some way as a stimulant in copulation, but pos-
sibly has to do with the calcareous covering of the egg-
capsule. Other Pulmonata exhibit variations of secondary
importance in the details of this hermaphrodite apparatus.

The nervous system of Helix is not favourable as an
example on account of the fusion of the ganglia to form
an almost uniform ring of nervous matter around the
oesophagus. The Pond-Snail (Limnseus) furnishes, on the
other hand, a very beautiful case of distinct ganglia and
connecting cords (fig. 22). The demonstration which it
affords of the extreme shortening of the Euthyneurous vis-
ceral nerve-loop is most instructive and valuable for com-
parison with and explanation of the condition of the nervous
centres in Cephalopoda, as also of some Opisthobranchia.
The figure (fig. 22) is sufficiently described in the letter-
press attached to it ; the pair of buccal ganglia joined by
the connectives to the cerebrals are, as in most of our figures,
omitted. Here we need only further draw attention to the
osphradium, discovered by Lacaze Duthiers (32), and shown
by Spengel to agree in its innervation with that organ in all
other Gastropoda. On account of the shortness of the
visceral loop and the proximity of the right visceral
ganglion to the cesophageal nerve-ring, the nerve to the
osphradium and olfactory ganglion is very long. The posi-
tion of the osphradium corresponds more or less closely
with that of the vanished right ctenidium, with which it is
normally associated. In Helix and Limax the osphradium
has not been described, and possibly its discovery might
clear up the doubts which have been raised as to the nature
of the mantle-chamber of those genera. In Planorbis, which
is dexiotropic (as are a few other genera or exceptional
varieties of various Anisopleurous Gastropods) instead of
being leiotropic, the osphradium is on the left side, and
receives its nerve from the left visceral ganglion, the whole
series of unilateral organs being reversed. This is, as might
be expected, what is found to be the case in all " reversed "
Gastropods. It is also the case in the Pulmonate Auricula,
which is leiotropic.

The shell of the Pulmonata, though always light and
delicate, is in many cases a well-developed spiral " house,"
into which the creature can withdraw itself ; and, although
the foot possesses no operculum, yet in Helix the aperture
of the shell is closed in the winter by a complete lid, the
"hybernaculum," more or less calcareous in nature, which
is secreted by the foot. In Clausilia a peculiar modifica-
tion of this lid exists permanently in the adult, attached
by an elastic stalk to the mouth of the shell, and known as
the " clausilium." In Limmvus the permanent shell is
preceded in the embryo by a well-marked shell-gland or
primitive shell-sac (fig. 72***), at one time supposed to be
the developing anus, but shown by Lankester to be identical
with the " shell-gland " discovered by him in other Mol-
lusca (Pisidium, Pleurobranchidium, Neritina, <tc.). As in
other Gastropoda Anisopleura, this shell-sac may abnorm-
ally develop a plug of chitonous matter, but normally it
flattens out and disappears, whilst the cap-like rudiment of
the permanent shell is shed out from the dome-like surface

MOLLUSCA

1 2/>

of the visceral hump, in the centre of which the i-.hull-.sac
existed for a brief period.

In Clausilia, according to the observations of Gegenbaur,
the primitive shell-sac docs not flatten out and disappear,
but takes the form of a flattened closed sac. Within this
dosed sac a plate of calcareous matter is developed, and
after a time the upper wall of the sac disappears, and the
calcareous plate continues to grow as the nucleus of the
permanent shell. In the slug Testacella (fig. 69, C) the
shell-plate never attains a large size, though naked. Li
oilier slugs, namely, Limax and Arion, the shell-sac remains
permanently closed over the shell-plate, which in the latter
genus consists of a granular mass of carbonate of lime.
The permanence of the primitive shell-sac in these slugs is
a point of considerable interest. It is clear enough that
the sac is of a different origin from that of Aplysia (described
in the section treating of Opisthobranchia), being primitive
instead of secondary. It seems probable that it is identical
with one of the open sacs in which each shell-plate of a
Chiton is formed, and the series of plate-like imbrications
which are placed behind the single shell-sac on the dorsum
of the curious slug, Plectrophorus, suggest the possibility
of the formation of a series of shell-sacs on the back of
that animal similar to those which we find in Chiton.
Whether the closed primitive shell-sac of the slugs (and
with it the transient embryonic shell-gland of all other
Mollusca) is precisely the same thing as the closed sac in
which the calcareous pen or shell of the Cephalopod Sepia

cnn

Fio. 72"*. —Comparative diagrams of an embryo Slug, Limax (left), and an
embryo Cuttle-fish, Loligo (right), sh, internal shell ; pk, embryonic renal
organ (Stiebel's canal) in Limax ; ml, edge of the mantle-flap in Loligo ; of,
cephalic eye ; (, cephalic tentacle ; m, position of the mouth ; Ft, the foot ;
Fu, the hinder part of the foot drawn out to form the funnel of Loligo ; con,
the contractile yelk-sac or hernia-like protrusion of the mid-region of the foot,
corresponding to the line, of closure of the blastopore in Linma>us. N.B.—
The blastopore in the embryo of Loligo, which, like that of a bird, is much
distorted by excess of food-yelk, does close at the extremity of the yelk-sac
con. (Original.)

and its allies is formed, is a further question, which we
shall consider when dealing with the Cephalopoda. It
is important here to note that Clausilia furnishes us
with an exceptional instance of the continuity of the shell
or secreted product of the primitive shell-sac with the
adult shell. In most other Mollusca (Anisopleurous
Gastropods, Pteropods, and Conchifera) there is a want of
such continuity ; the primitive shell-sac contributes no
factor to the permanent shell, or only a very minute knob-
like particle (Neritina and Paludina). It flattens out and
disappears before the work of forming the permanent shell
commences. And just as there is a break at this stage,
so (as observed by Krohn in Marsenia = Echinospira) there
may be a break at a later stage, the nautiloid shell formed
on the larva being cast, and a new shell of a different form
being formed afresh on the surface of the visceral hump.
It is, then, in this sense that we may speak of primary,
secondary, and tertiary shells in Mollusca, recognizing the
fact that they may be merely phases fused by continuity
of growth so as to form but one shell, or that in other
cases they may be presented to us as separate individual
things, in virtue of the non-development of the later phases,

or in virtue of sudden changrs in the activity of the mantlc-
surface musing the shedding or disappearance of one phase
of shell-formation before a later one is entered upon.

The development of the aquatic I'uhnonata from the
egg offers considerable facilities for study, and that of
Limna-us has been elucidated by Lankester, whilst Rabl
has with remarkable skill applied the method of sections
to the study of the minute embryos of Planorbis. The
chief features in the development of Liinnams are exhibited
in the woodcuts (figs. 3, 4, and 72***). There is not a
very large amount of food-material present in the egg of
this snail, and accordingly the cells resulting from division
are not so unequal as in many other cases. The four cells
first formed are of equal size, and then four smaller cells
are formed by division of these four so as to lie at
one end of the first four (the pole corresponding to
that at which the "directive corpuscles" <lr are extruded
and remain). The smaller cells now divide and spread
over the four larger cells (fig. 3) ; at the same time a space

r

FIG. T2V'\ — Embryo of Limmvxs still/unlit, at a stage when the Trochosphere
is developing foot and shell-gland and becoming a V<-liger, seen as a transparent
object under slight pressure, pit, pharynx (stomotUeal in vagi nation); v, )',
the ciliated band marking out the velum ; m;, cerebral nerve-ganglion ; re,
Stiebel's caual (left side), probably an evanescent embryonic nephridium ; sh,
the primitive shell-sac or shell-gland ; pi, the rectal peduncle or pedicle of
invagiuation, its attachment tu the ectoderm is coincident with the hindmost
extremity of the elongated blastopore of Jig. 3, C ; tge, mesoblastic (skeleto-
trophic and muscular) cells investing fjs, the bilobed arch-enteron or lateral
vesicles of invagiuated endoderm, which will develop into liver ; /, the foot.
(Original.)

— the cleavage cavity or blastoccel — forms in the centre
of the mulberry-like mass. Then the large cells recom-
mence the process of division and sink into the hollow
of the sphere, leaving an elongated groove, the blastopore,
on the surface (fig. 3, C, and fig, 4, G). The invaginated
cells (derived from the division of the four big cells) form
the endoderm or arch-enteron ; the outer cells are the ecto-
derm. The blastopore now closes along the middle part of
its course, which coincides in position with the future "foot."
One end of the blastopore becomes nearly closed, and an
ingrowth of ectoderm takes place around it to form the
stomodasum or fore-gut and mouth. The other extreme
end closes, but the invaginated endoderm cells remain in
continuity with this extremity of the blastopore, and form
the " rectal peduncle " or " pedicle of invagination " of
Lankester (see also the account and figures (fig. 151, A) of
the development of the bivalve Pisidium), although the
endoderm cells retain no contact with the middle region
of the now closed-up blastopore. The anal opening forms
at a late period by a very short ingrowth or proctodseum
coinciding with the blind termination of the rectal peduncle
(fig. 72***, pi).

The body-cavity and the muscular, fibrous, and vascular
tissues are traced partly to two symmetrically-disposed

126

MOLLUSCA

"mesoblasts," which bud off from the invaginated arch-
enteron, partly to cells derived from the ectoderm, which
at a very early stage is connected by long processes with
the invaginated endoderm, as shown in fig. 3, D. The ex-
ternal form of the embryo goes through the same changes
as in other Gastropods, and is not, as was held previously
to Lankester's observations, exceptional. When the middle
and hinder regions of the blastopore are closing in, an
equatorial ridge of ciliated cells is formed, converting the
embryo into a typical " Trochosphere " (fig. 4, E, F).

The foot now protrudes below the mouth (fig. 4), and the
post-oral hemisphere of the Trochosphere grows more rapidly
than the anterior or velar area. The young foot shows a
bilobed form (fig. 4, D, / ). Within the velar area the eyes
and the cephalic tentacles commence to rise up (fig. 4, D, t),
and on the surface of the post-oral region is formed a cap-
like shell and an encircling ridge, which gradually increases
in prominence and becomes the freely depending mantle-
skirt. The outline of the velar area becomes strongly
emarginated and can be traced through the more mature
embryos to the cephalic lobes or labial processes of the
adult Limnanis (fig. 70).

This permanence of the distinction of the part known
as the velar area through embryonic life to the adult state
is exceptional among Mollusca, and is therefore a point of
especial interest in Limnasus. None of the figures of
adult Limmeus in recent works on Zoology show properly
the form of the head and these velar lobes, and accordingly
the figures here given have been specially sketched for the
present article. The increase of the visceral dome, its
spiral twisting, and the gradual closure of the space over-
hung by the mantle-skirt so as to convert it into a lung-sac
with a small contractile aperture, belong to stages in the
development later than any represented in our figures.

We may now revert briefly to the internal organization
at a period when the Trochosphere is beginning to show a
prominent foot growing out from the area where the mid-
region of the elongated blastopore was situated, and having
therefore at one end of it the mouth and at the other the
anus. Fig. 72*** represents such an embryo under slight
compression as seen by transmitted light. The ciliated
band of the left side of the velar area is indicated by a
line extending from v to v ; the foot /is seen between the
pharynx ph and the pedicle of invagination pi. The mass
of the arch-enteron or invaginated endodermal sac has
taken on a bilobed form (compare Pisidium, fig. 151), and
its cells are swollen (</s and tr/e). This bilobed sac becomes
entirely the liver in the adult ; the intestine and stomach
are formed from the pedicle of invagination, whilst the
pharynx, oesophagus, and crop form from the stomodseal
invagination ph. To the right (in the figure) of the
rectal peduncle is seen the deeply invaginated shell-gland
ss, with a secretion s/t protruding from it. The shell-gland
is destined in Limmuus to become very rapidly stretched
out, and to disappear. Farther up, within the velar area,
the rudiments of the cerebral nerve-ganglion ny are seen
separating from the ectoderm. A remarkable cord of cells
having a position just below the integument occurs on each
side of the head. In the figure the cord of the left side is
seen, marked re. This paired organ consists of a string of
cells which are perforated by a duct. The opening of the
duct at either end is not known. Such cannulated cells
are characteristic of the nephridia of many worms, and it
is held that the organs thus formed in the embryo Limna'iis
are embryonic nephridia. The most important fact about
them is that they disappear, and are in no way connected
with the typical nephridium of the adult. In reference
to their first observer they are conveniently called "Stiebel's
canals." Other Fulmonata possess, when embryos, Stiebel's
canals in a more fully-developed state, for instance, the

common slug Limax (fig. 72**, pk). Here too they disap-
pear during embryonic life. Further knowledge concern-
ing them is greatly needed. It is not clear whether there
is anything equivalent to them in the embryos of marine
Gastropoda or other Mollusca, the ectodermal cells called
" embryonic renal organs" in some Gastropod embryos hav-
ing only a remote resemblance to them. The three pairs
of transient embryonic nephridia of the medicinal leech,
the ciliated cephalic pits of Nemertines, and the anterior
nephridia of Gephyrajans, all suggest themselves for com-
parison with these enigmatical canals.

Marine Pulmonata. — Whilst the Pulmonata are essen-
tially a terrestrial and fresh-water group, there is one
genus of slug-like Pulmonates which frequent the sea-
coast (Peronia, fig. 72), whilst their immediate congeners
(Onchidium) are found in marshes of brackish water. Sem-
per (33) has shown that these slugs have, in addition to
the usual pair of cephalic eyes, a number of eyes developed
upon the dorsal integument. These dorsal eyes are very
perfect in elaboration, possessing lens, retinal nerve-end
cells, retinal pigment, and optic nerve. Curiously enough,
however, they differ from the cephalic Molluscan eye (for
an account of which see fig. lib) in the fact that, as in
the vertebrate eye, the filaments of the optic nerve pene-
trate the retina, and are connected with the surfaces of the
nerve-end cells nearer the lens instead of with the opposite
end. The significance of this arrangement is not known,
but it is important to note, as shown by Hensen, Hickson,
and others, that in the bivalves Pecten and Spondylus,
which also have eyes upon the mantle quite distinct from
typical cephalic eyes, there is the same relationship as in
Onchidiad;e of the optic nerve to the retinal cells (fig. 145).
In both Onchidiadto and Pecten the pallial eyes have prob-
ably been developed by the modification of tentacles, such
as coexist in an unmodified form with the eyes. The
Ouchidiadre are, according to Semper, pursued as food
by the leaping fish Periophthalmus, and the dorsal eyes
are of especial value to them in aiding them to escape
from this enemy.

Class II— SCAPHOPODA.

Characters. — Mollusca Glossophorawith the FOOT adapted
to a BURROWING life in sand (figs. 73, 74, /). The body,

Fin. 73. — DcntaUnm VV.lga.r6, Da C. (after Laca/c Duthiers). A. Ventral view
Of the animal removed from its shell. IS. Dorsal view of the same. C. Late-
ral view of the same. D. The shell in section, E. Surface view of the shell
with Kill-tentacles cxsvrreil as in life. «, mantle; a', longitudinal muscle;
a", fringe surrounding the anterior opening of the mantle-chamber ; a"', the
]>osterior appendix of the mantle ; It, anterior circular muscle of the mantle ;
l»', posterior do. ; c, c', longitudinal muscle, of mantle ; c, liver ; /, K°nad ; /;,
buccal mass (showing; through the 111.111 tie) ; <i, left nephridium ; s', club-shaped
extremity of the foot ; •»', w't longitudinal blood-sinus of the mantle.

and to a much greater extent the mantle-skirt and the foot,
are elongated along the primitive antero-posterior (oro-anal)

MOLLUSCA

127

axis, and retain, both externally and in the disposition of
internal organs, the archi-Molluscan BILATERAL SYMMETRY.
The margins of the mantle-skirt of opposite sides (right
and left) meet below the foot and fuse by concrescence ;
only a small extent in front and a small extent behind of
the mantle-margin is left unfused. Thus a CYLINDRICAL
FOEM is attained by the mantle, and on its surface a TUBU-
LAR shell (incomplete along the ventral line in the youngest
stages) is secreted (fig. 73, D). The FOOT is greatly elon-
gated, and can be protruded from the anterior mantle-
aperture. It has a characteristic clavate form (fig. 74, /).
The pair of typical CTENIDIA are symmetrically deve-
loped in the form of numerous gill-filaments (fig. 74, A, g]

FIG. 74.— Diagrams of the anatomy of Dentalinm. A. The anterior portion of
the tubular mantle is slit open along the median dorsal line, and its cut
margins (a) reflected so as to expose the foot, snout, and gills. B. Lateral
view with organs showing as though by transparency. C'. Similar lateral
view to show the number and position of the nerve-ganglia and cords, a,
the mantle-skirt ; l>, anterior free margin of the same ; c, hinder extension of
tin- mantle-skirt; (i, the appendix of the mantle-skirt separated liyavalve
from the peri-anal portion of the sub-pallia! chamber, h ; c, the snout or oral
process ; f, the foot ; (i, the ctenidial filaments ; h, the peri-anal part of the
sub-pallia! chamber ; ?", the peri-oral part of the same chamber ; /,-, the anus ;
/, the left nephridium ; ?», the mouth surrounded by pinnate tentacles ; n,
the bnccal mass and odontophore ; o, oesophagus; p, the left lobe of the
liver; g.p, pedal ganglion-pair; f/.c, cerebral ganglion-pair; g.pl, plenrul
ganglion-pair; g.v, visceral ganglion-pair. Possibly further research will
show that >i.p! is the typieal viseeral ganglion-pair, and that g.v is a pair 'it
olfactory ganglia placed on the visceral loop as in the Lipocepnala according
to Spengel.

placed at the base of the cylindrical cephalic prominence
or snout (fig. 74, e). A pair of NEPHEIDIA (fig. 74, I) are
present, opening near the anus (fig. 74, /•). The right
serves as a genital duct, the left is apparently renal in
function. The LIVER (;)) is large and bilobed, the lobes
divided into parallel lobules. The NERVE-GANGLIA are
present (fig. 74, C) as well-marked cerebral, pleural, pedal,
and visceral pairs, the typical pleural pair being closely
joined to the cerebral. The visceral loop or commissure is
untwisted, that is to say, the Seaphopoda are EUTHYNEUR-
ous. HEART and distinct VESSELS are not developed ; a
colourless blood is contained in the sinuses and networks
formed by the body-cavity. The GONADS are either male
or female, the sexes being distinct.

The embyro is remarkable for developing five ciliated
rings posterior to the ciliated ring and tuft characteristic
of the trochosphere larval condition of Molluscs generally.
These rings are comparable to those of the larva of Pneu-
modermon (fig. 84), and like them disappear.

The class Scaphopoda is not divisible into orders or
families. It contains only three genera : Dentnlium, L. (figs.
73, 74) ; Siphonodentalium, Sars. ; and Entaiium, Dfr.

They inhabit exclusively the sand on the sea-coast in
depths of from 10 to 100 fathoms.

It is worthy of remark that the Scaphopoda constitute
among the Glossophora a parallel to the sand-boring forms
so common among the Lipocephala (such as Solen and Mya).
This parallelism is seen in the special mode of elongation
of the body, in the form of the foot, and in the tubular
form of the mantle brought about by the concrescence of
its ventral margins, as in the Lipocephala mentioned.
The cylindrical shell of Dentalium is also comparable to
the two semi-cylindrical valves of the shell of Solen ; or,
better, to the tubular shell of Aspergillum and Teredo.
Nevertheless, it is necessary to consider the Scaphopoda as
standing far apart from the Lipocephala, and as having no
special genetic but only a homoplastic relationship to them,
in consequence of their possessing a well-developed odonto-
phore, the characteristic organ of the Glossophora never
possessed by any Lipocephala.

Class III— CEPHALOPODA.

Characters. — Mollusca Glossophora with the FOOT prim-
arily adapted to a FREE-SWIMMING mode of life. The
archi-Molluscan BILATERAL SYMMETRY predominates both
in the external and internal organs generally, though in
many cases (especially the smaller forms) a one-sided dis-
placement of primitively median organs and a suppression
of one of the primitively paired organs is to be noted.

An ANTERIOR, MEDIAN, and POSTERIOR region of the
FOOT can be distinguished (fig. 75, (4), (5), (6)), corre-
sponding to but probably not derived from the pro-, meso-,

FIG. 75. — Diagrams of a series of Molluscs to show the form of the foot and its
regions, and the relation of the visceral hump to the antero-posterior and

and meta-podium of Gastropoda. The fore-foot invariably
has the HEAD MERGED into it, and grows up on each side
(right and left) of that part so as to surround the mouth,
the two upgrowths of the fore-foot meeting on the dorsal
aspect of the snout, — whence the name Cephalopoda. In
the more typical forms of both branches of the class, the
peri-oral portion of the foot is drawn out into paired arm-

128

MOLLUSCA

like processes, either very short and conical (Clio, Euryliia),
or lengthy (Pneumodennon, Octopus) ; these may be beset
with suckers or hooks, or both. The mid-foot (fig. 75, mf)
is expanded into a pair of muscular lobes right and left,
which either are used for striking the water like the wings
of a butterfly (Pteropoda), or are bent round towards one
another so that their free margins meet and constitute a
short tube, — the siphon or funnel (Siphonopoda). The hind
foot is either very small or absent.

A distinctive feature of the Cephalopoda is the ABSENCE
of anything like the TORSION of the visceral mass seen in
the Anisopleurous Gastropoda, although as an exception
this torsion occurs in one family (the Limacinidae).

The ANUS, although it may be a little displaced from
the median line, is (except in Limacinidffi) approximately
median and posterior. The MANTLE-SKIRT may be aborted
(Gymnosomatous Pteropoda) ; when present it is deeply
produced posteriorly, forming a large sub-pallial chamber
around the anus. As in our schematic Mollusc, by the side
of the anus are placed the single or paired apertures of the
NEPHEIDIA, the GENITAL APERTURES (paired only in Nau-
tilus, in female Octopoda, female Ommastrephes, and male
Eledone), and the paired CTENIDIA (absent in all Pteropoda).
The VISCERAL HUMP or dome is elevated, and may be very
much elongated (see fig. 75, (4), (5), (6)) in a direction
almost at right angles to the primary horizontal axis (A, P
in fig. 75) of the foot.

A SHELL is frequently, but not invariably, secreted on
the visceral hump and mantle-skirt of Cephalopoda ; but
there are both Pteropoda and Siphonopoda devoid of any
shell. The shell is usually light in substance or lightened
by air-chambers in correlation with the free-swimming
habits of the Cephalopoda. It may be external, when it is
box-like or boat-like, or internal, when it is plate-like. Very
numerous minute pigmented sacs capable of expansion and
contraction, and known as CHROMATOPHORES, are usually
present in the integument in both branches of the class. The
GONADS of both sexes are developed in one individual in some
Cephalopoda (Pteropoda), in others the sexes are separate.

SENSE-ORGANS, especially the cephalic eyes and the oto-
cysts, are very highly developed in the higher Cephalopoda.
The osphradia have the typical form and position in the
lower forms, but appear to be more or less completely
replaced by other olfactory organs in the higher. The
normal NERVE-<;AN<;LIA are present, but the connectives are
shortened, and the ganglia concentrated and fused in the
cephalic region. Large special ganglia (optic, stellate, and
supra-buccal) are developed in the higher forms (Siphono-
poda).

The Cephalopoda exhibit a greater range from low to
high organization than any other Molluscan class, and hence
they are difficult to characterize in regard to several groups
of organs ; but they are definitely held together by the
existence in all of the encroachment of the fore-foot so as

Fig. 76.

Fig. 77.

Flo. 7G.—Sp!rht!ifi ln<li,ii<H<t<'*, Sun]., one of the Limaciniila? enlarged (from
Owen). C t\ ptrroj.i.dial lubes of the mid-foot; /, operculum carried on the
hind-foot. ; <i, spiral slirll.

FIG. 77. — Operculum of Spirialis enlarged.

to surround the head, and by the functionally important

1ULOBATION OF THE MID-FOOT.

Two very distinct branches of the Cephalopoda are to
be recognized : the one, the Pteropoda, more archaic in
the condition of its bi-
lobed mid-foot, including
a number of minute, and
in all probability degen-
erate, oceanic forms of
simplified and obscure
organization ; the other,
the Siphonopoda, con-
taining the Pearly Nau-
tilus and the Cuttles,
which have for ages (as
their fossil remains show)
dominated among the in-
habitants of the sea, be-
ing more highly gifted
in special sense, more
varied in movement,
more powerful in pro-
portion to size, and more

.,-, Flo. 77«. — Cymlmlia I'ernnii, Cuvier (from
With Owen).

C, C, the expanded pteropodial
lobes or wing-like tins of the mid-foot.

heavily equipped

destructive weapons of

offence than any other marine organisms.

Branch a,— PTEROPODA.

Characters. — Cephalopoda in which the mid-region of
the foot is (as compared with the Siphonopoda) in its more
primitive condition, being
relatively largely developed
and drawn out into a pair
of wing-like muscular lobes
(identical with the two halves
of the siphon of the Siphon-
opoda) which are used as
paddles (see figs. 76-86). The
hind -region of the foot is
often aborted, but may carry
an operculum (figs. 76, 77).
The fore-region of the foot
(that embracing the head) is
also often rudimentary, but
may be drawn out into one
or more pairs of tentaclrs.
simulating cephalic tentacles,
and provided with suckers
(figs. 84, 85).

Though the visceral hump
is not twisted except in the
Limacinidae (fig. 76), there is
a very general tendency to
one-sided development of the
viscera, and of their external
apertures (as contrasted with
Siphonopoda). The ctenidia
are aborted, with the possible
exception of the processes (fig.
85, c) at the end of the body
of Pneumodennon. The vas-
cular system resembles that
of the Gastropoda. The ne-
phridium is a single tubular FIO. 78.— siyiioia actcvia, Rang. sp. en

horlv cori-pqnonrlinrr to flip larged (from Owen). C, C, the wing-
like lobes of the mid-foot ; d, mediae

right neiihridiuill of the typi- fold of same ; e, copulatory organ ; h,

i . * ,-, i • -m- ii pointed extremity of the shell ; i, an-

CaJ pair ot the arclll-JMollusC. {erior margin of tie shell; n, stomach-

The anal aperture is usually °- >'ver; "• hermaphrodite gonad.
placed a little to the left of the median line, more rarely
to the right, In the Limacinidie it has an exceptional
position, owing to the torsion of the visceral mass, as in
Anisopleurous Gastropoda.

MOLLUSCA

129

Jaws and a lingual ribbon are present as in typica
Glossophora, the dentition of the ribbon and the number oi
jaw-pieces presenting a certain range of variation. Sense-

C

't, C

Fig- 79. Fig. 80.

Fio. 79.— Cawlinia tridentata, Forsk. from the Mediterranean, magnified two

Ot

closing the visceral hump ; h, the median spine of the shell.
Fio. SO.— Shell of Cavolinia trideniata, seen from the side. /, postero-dorsal
surface ; g, antero-ventral surface ; h. median dorsal spine : i, mouth of the
shell.

organs are present in the form of cephalic eyes in very few
forms (Cavolinia, Clione, and in an undescribed form dis-
covered by Suhm during the "Challenger" Expedition); oto-
cysts are universally present. The osphradia are present
in typical form, although the ctenidia are aborted ; only
one osphradium (the
right of the typical
pair) is present (fig.
87). The gonads are
both male and female
in the same individual.
The genital aperture is
single. Copulatory or-
gans, often of consider-
able size, are present
(fig- 86, z). xnJLU-L

The mantle -skirt is
present in one divi-
sion of the Pteropoda
(Thecosomata), and in
these an extensive sub-
pallial chamber is de-
veloped, the walls of
which in the absence
of ctenidia have a
branchial function. In ^_ ^

asecoild division (Gym- Flo. si.-Embryo of CW^ridmtato (from

nosomata), which COD!- Balfour, after Fol.). a, anus; f, median portion

t 1-11 i °f the foot ; pn, pteropodial lobe of the foot •

prises forms highly de- ft, heart ; i, intestine ; ot, otocyst ; g, shell ; r,

Veloned in rpo-qrH to nephridium ; s, resophagus ; a, sac containing

1 nutritive yelk; mb, mantle-skirt; me sub°

the processes Of the pallial chamber ; Kn, contractile sinus.

fore-foot, the mantle-skirt is aborted. A shell is developed
on the surface of the visceral hump and mantle-skirt of the
Thecosomata, whilst in the Gymnosomata, which have no
mantle-skirt, there is in the adult animal no shell. The
embryo passes through a trochosphere and a veliger stage
(fig- 81), provided with boat-like shell, except in some
Gymnosomata in which the Trochosphere with its single
velar ciliated band becomes metamorphosed into a larva
which has three additional ciliated bands but no velum
(resembling the larva of the Scaphopod Dentalium) ; this
banded larva does not form a larval shell (fig. 84).
The Pteropoda are divided into two orders.

Order 1. — Thecosomata.

Characters. — Pteropoda provided with a -man tie -skirt,

and with a delicate hyaline shell developed on the surface
of the visceral hump and mantle-skirt ; visceral hump, and
consequently the shell,
spirally twisted in one
family, the Limacmidiv ;
shell often with con-
tracted mouth and di-
lated body, its walls
sometimes drawn out
into spine-likeprocesses,
which are covered by
reflexions of the free
margin of the mantle
(Cavolinia, figs. 79, 80).

Family 1. — Cymliiliidse.
Genera : Tiedeinannia,
Chj.; Haloj:isiji:he, Thc-
ceurybia (figs. 82, 83),
Cymbulia, P. and L.
(fig- 77a).
Family 2. — Conulariidx

(fossil).

Genus : Conularia, Mill.
Family 3. — Tentaculiltdie

(fossil). FIG. S3. — TJieceimjbia Gandichaudii, Soul.,

Genera : Tentaculitcs, (fr°ni Owen). Much enlarged ; the body-wall
Schlth. ; Cornulitcs,
Schlth. ; Cokoprion,
Sandb.

Family 4. — Hijalcidss.

Genera : Triptcra, Q. and

G. ; StyUola, Les. (fig.

78) ; Balanlium, Lch. ; Vagitulla, Dand. ; Clcodora, P. and

L. ; Diacria, Gr. ; Plcuropus, Esch. ; Cavolinia, Gioni. (figs.

79, 80, 81).

Family 5.—T/iccidie.

Genera : Thr.ca, Low ; Pterothcca, Salt.
Family 6. — Limacinidaz.

Genera : Eccyliomphalus, Porti. ; Hderofusus, Fig. ;
Spirialis, E. S. (fig. 76) ; Limacina, Cuv.

Order 2. — Gymnosomata.

Characters. — Pteropoda devoid of man tie -
skirt and shell ; tentacular processes of the
fore-foot well developed and provided with
suckers.

Family 1. — Pterocymodoceidse.
Genus : Pterocymodoce, Kef.
Family 2. — Clionidse.
Genera : Cliodita, Q. and G. ; Clionopais, Trosch. ;

Clione, Pall. (fig. 86).
Family 3. — Pneumodcrmidss.

Genera : Trichocydus, Esch. ; Spongobranchia,

d'Orb. ; Pneumodermopsis, Kef. ; Pncumodcrmon, Cuv. (fig. 85).

removed. a, the mouth ; c, the pteropodial
lobes of the foot; /, the centrally-placed
hind-foot ; (1, Z, e, three pairs of tentacle-like
processes placed at the sides of the mouth,
and developed (in all probability) from the
fore-foot ; o', anus ; y, genital pore ; k, retractor
muscles ; o and p, the liver ; H, v, w, genitalia.

FIG. 83.— Shell
of Thmury-

nttosVbt

lower ' figure
shows the na-

tural size'

Branch b.—SIPHONOPODA.

Cephalopoda in which the two primarily divergent right
and left lobes of the mid-region of the foot have their free
borders recurved towards the middle line, where they are
either held in apposition (Tetrabranchiata), or fused with
one another to form a complete cylinder open at each end
(Dibranchiata). This fissured or completely closed tube is
the siphon (fig. 75, (6), mf) characteristic of the Siphono-
poda, and is used to guide the stream of water expelled
by the contractions of the walls of the branchial chamber.
The pallial skirt is accordingly well developed and muscular,
subserving by its contractions not only respiration but
ocomotion. The visceral hump is never twisted, and ac-
cordingly the main development of the pallial skirt and
chamber is posterior, the excretory apertures, anus, and
gills having a posterior position, as in the archi-Mollusc.
At the same time the visceral hump is usually much elon-
gated in a direction corresponding to an oblique line be-
:wet-n the vertical dorso-ventral and the horizontal antero-
msterior axes (see fig. 75, (6)).

B

130

MOLLUSCA

The fore-part of the foot which surrounds the mouth, as
in all Cephalopoda, is drawn out into four or five pairs of
lobes, sometimes short, but usually elongated and even fili-

Fig. 84. Fig. 85.

FIG. 84.— Larvse of Pneumodermon (from Balfour, after Gegenbaur). The
prce-oral ciliated band of the trochosphere stage (velum) has atrophied. In
A three post-oral circlets of cilia are present. The otocysts are seen, and
the rudiments of a pair of processes growing from the head. In B the fore-
most ciliated ring has disappeared ; the cephalic region is greatly developed,
and, as compared with the adult (fig. 85), is large and free ; the pair of hook-
bearing processes on each side of the mouth are retractile, probably part of
the fore-foot. At the base of the cephalic snout are seen the pair of arm-
like processes (fore-foot) provided with suckers, and behind these the broad
pteropodial lobes or wing-like fins of the mid-font.

FIG. 85. — Pneumodermon inolaccinii, d'Orb. ; magnified five diameters, a, the
sucker-bearing arms ; b, the tins of the mid-foot (in the middle line, between
these, is seen the sucker -like median portion of the foot, by means of which
the animal can crawl as a Gastropod) ; c, the four branchial processes. (After
Kefersteiu.)

form. These lobes either carry peculiar sheathed tentacles

(Nautilus), or, on the other hand, acetabulif orm suckers, which

may be associated with claw-like hooks (Dibranchiata).

The hind-foot is probably represented by the valve which

depends from the inner

wall of the siphon in

many cases.

A shell (figs. 89, 100)

is very generally present, c • * a

affording protection to

the visceral mass and

attachment for muscles.

It may be external or en-
closed in dorsal llpgrow- FIG. 86.— Clione boreaUs, L. ; magnified two
f , , j. , i . i diameters,— postero-ventral aspect, o, the

ing tOldS 01 tne mantle, cephalic region carrying o'— three pairs of
cephalic cones provided each with very nu-
merous minute sucker-like processes, and
surrounded by a hood-like upgrowth, —
and i), the more elongated tentacles (the

retractile eye-tentacles are not seen, being
placed dorsally) ; c, the pteropodial fins ;
d, the median portion of the foot ; o, the
anus ; ?/, the vagina ; z, the penis. (From
Uwen, after Eschricat.)

CcJi

which (except in Spirilla)
close up at an early period
of development, so as to
form a shut sac in which
the shell is secreted. The
cteuidia are well deve-
loped as paired gill-plumes, serving as the efficient bran-
chial organs (figs. 101, 103,
and fig. 2, B).

The vascular system is
very highly developed ; the
heart consists of a pair of
auricles and a ventricle (figs.
104, 105). Branchial hearts
are formed on the advehent
vessels of the branchiae. It
is not known to what extent
the minute subdivision of
the arteries extends, or
whether there is a true
capillary system.

The pericardium is ex-
tended so as to form a very
large sac passing among
the viscera dorsal wards and
sometimes containing the
ovary or testis — the viscero-
pericardial sac — which opens to the exterior either directly

Vis.i-

FIG. 87. —Enlarged diagram nf the nerve-
centres of Pneumodrrmon (from ^pen-
gel, after Souleyef). Cell, right cere-
bral ganglion ; Pl.R, right plcnral
ganglinii; l'':, right pedal ganglion;
J'/NJ.'., right visceral ganglion; 1'i's. /,.,
left visceral ganglion; c;w, right cere-
bro-pedal connective; e/./, right cere-
bro-ph'iiral cnnni-ctive ; ds/i., osphra-
dium connected by a nerve with the
right visceral ganglion.

or through the nephridia. It has no connexion with the
vascular system. The nephridia are always paired sacs,
the walls of which invest . the branchial advehent vessels
(figs. 104, 108). They open each by a pore into the viscero-

ff

FIG. SS. — Male (upper) and female (lower) specimens of Nautilus poinpilu/s as
seen in the expanded condition, the observer looking down on to the buccal
cone e ; one-third the natural size linear. The drawings have been made
from actual specimens by A. G. Bourne, B.Sc., and serve to show the
natural disposition of the tentaculiferous lobes and tentacles of the circum-
oral portion of the foot in the living state, as well as the great differences
1 « t ween the two sexes, a, the shell ; 6, the outer ring-like expansion (annular
lobe) of the circnm-oral muscular mass of the fore-foot, carrying nineteen
tentacles on each side — posteriorly this is enlarged to form the "hood"
(marked v in fig. 89 and m. in figs. 90 and 91), giving off the pair of tentacles
marked g in the present figure ; c, the right and left inner lobes of the fore-
foot, each carrying twelve tentacles in the female, in the male subdivided
intop, the "spadix" or hectocotylus on the left side, and 7, the "anti-spadix,"
a group of four tentacles on the right side,— it is thus seen that the subdivided
right and left inner lul.es nf the male correspond to the undivided right and
left inner lobes of the female ; il, the inner inferior lobe of the fore-foot, a
bilateral structure in the female carrying two groups, each of fourteen tenta-
cles, separated from one another by a lamellated organ n, supposed to be
olfactory in function — in the male the inner inferior lobe of the fore-foot is

papilhe ; /, the tentacles of the outer circum-oral lobe or annular lobe of the
fore-foot projecting fmm their sheaths ; g, the two most posterior tentacles
nf this series belonging to that part of the annular lobe which forms the
hood (m. in figs. 90 and 91) ; i, superior ophthalmic tentacle ; fr, inferior
nphthalmic tentacle ; ?, eye ; ?», paired laminated organ on each side of the
base of the inner inferior lobe ('') of the female, probably olfactory in func-
tion ; n, olfactory lamella; upon the inner inferior lobe (in the female); o,
the siphon (mid-foot) ; ;), the spadix (in the male), the hectocotylized portion
of the left inner lobe of the fore-foot representing four modified tentacles,
right being left unmodified; q, the anti-spadix (in the male), being four of
111.- twelve tentacles of the right inner lobe of the fore-foot isolated from
the rein.'iiniiiL' eight, and representing on the right side the differentiated
spadix of the left side. The four tentacles of the anti-spadix are set, three
on one base and one on a separate base.

There are thus in the female, where they are most numerous, ninety-four
l.'iilael.'s, fhniy eight on the outer annular lube, four ophthalmic (a pair to
each eye), t \\chv on each of the right and left inner lobes, and twenty-eight
on tlie inner int'rrior lobe.

pcricarclial sac except in Nautilus. The anal aperture is
median and raised on a papilla. Jaws (fig. 88, e) and a lin-
gual ribbon (fig. 107) are well developed. The jaws have
the form of a pair of powerful beaks, either horny or calcified
(Nautilus), and are capable of inflicting severe wounds.

MOLLUSCA

131

Sense-organs .ire highly developed ; the eye exhibits a
very .special elaboration of structure in the Dlbranchiata,

and a ivmurkable archaic form in the Nautilus. Otocysts
are present in all. The typical osphradiuni is not present,

Fic SO.-Lateral view of the female Pearly Nautilus, contracted by spirit and lying in its shell,
the right half of which is cut away (from Gegenbaur, after Owen), o, visceral Simp; &,por- pericardial

term liectocotylization is applied to this modification (see
tigs. ss, '.!.">, !ii>). Elaborate spennatophores or sperm-ropes
are formed by all Siphonopoda, and very usually the female
possesses special capsule-forming and nidamental glands for
providing envelopes to the eggs (fig. 101, y.n,).
The egg of all Siphouopoda is large, and the
development is much modified by the presence
of an excessive amount of food-material diffused
in the protoplasm of the egg-cell. Trochosphere
and veliger stages of development are conse-
quently not recognizable.

The Siphonopoda are divisible into two
orders, the names of which (due to Owen) de-
scribe the number of gill-plumes present ; but
in fact there are several characters of as great
importance as those derived from the gills by
which the members of these two orders are
separated from one another.

Order 1. — Tetrabranchiata ( = Schizosiphona,
Tentaculifera).

Characters. — Siphonopodous Cephalopods
in which the inrolled lateral margins of the
mid-foot are not fused, but form a siphon by
apposition (fig. 101). The circum-oral lobes
of the fore-foot carry numerous sheathed ten-
tacles (not suckers) (fig. 88). There are two
pairs of ctenidial gills (hence Tetrabranchiata),
and two pairs of nephridia, consequently four

nePhrid>al artures (fig. 101). The viscero-
Opens by two independent

the male, the left duct in both

fore-loot,— tin- jointed tentacles are seen protruding a little from their long cylindrical sheaths' » S6X6S being rudimentary.

the dorsal "hood" formed by an enlargement in this region of the annular Inbe of the fore- A larcrp p-rtprna] slip]] pitlipr rni'Wl nr strfli'crhr
' A S6 external Bn( el COUed Or Straight

is present, and is not enclosed by reflexions of

ne o e or
foot (m. in figs. 00, 01) ; I', a swelling of tin., mantle-skirt, indicating the position on its inn

f the nidamental gland (see fig. ioi, j/.ii.).

except in Nautilus, but other organs are present in the

•i.

71.

Fir.. 00.— Spirit specimen of female Pearly Nautilus, removed from its shell,
and seen from the antero-dorsal aspect (drawn from nature by A. G.
Bourne). 7H., the dorsal "hood " formed by the enlargement of the outer i >r
annular lobe of the fore-foot, and corresponding to the sheaths of two tenta-
cles (ff, g in fig. 88) ; •»., tentacular sheaths of lateral portion of the annular
lobe ; w., the left eye ; 6., the nuchal plate, continuous at its right and Irll,
posterior angles with the root of the mid-foot, and convspunilhig to tin-
nuchal cartilage of Sepia; c., visceral hump; it., the free margin of the
mantle-skirt, the middle letter d. points to that portion of the mantle-skirt
which is reflected over a part of the shell as seen in fig. 89, b ; the cup-like
fossa to which b. and d. point in the present figure is occupied by the coil of
the shell; «/.«. points to the lateral continuation of the nuchal plate '-. t<>
join the root of the mid-foot or siphon.

cephalic region, to which an olfactory function is ascribed
both in Nautilus and in the other Siphonopoda.

The gonads are always separated in male and female
individuals. The genital aperture and duct is sometimes
single, when it is the left ; sometimes the typical pair is
developed right and left of the anus. The males of nearly
all Siphonopoda have been shown to be characterized by a
peculiar modification of the arm-like processes or lobes of
the fore-foot, connected with the copulative function. The

the mantle-skirt, except such narrow-mouthed shells as
that of Gomphoceras, which were probably enclosed by the

Flo. 91. — Lateral view of the same specimen as that drawn in fig. 90. Letters
as in that figure with the following additions — e points to the concave margin
of the mantle-skirt leading into the sub- pallia! chamber ; g, the mid-foot or
siphon; fc, the superficial origin of its retractor muscles closely applied to
tli.- shell and serving to hold the animal in its place ; 1, the siphuncular pedicle
of the visceral hump broken off short ; i', r, the superior and inferior ophthal-
mic tentacles.

mantle as in the Dibranch Spirilla. The shell consists of
a series of chambers, the last formed of which is occupied
by the body of the animal, the hinder ones (successively
deserted) containing gas (fig. 89).

The pair of cephalic eyes are hollow chambers (fig. 118,
A) opening to the exterior by minute orifices (pinhole
- HIM i-a), and devoid of refractive structures. A pair of
osphradia are present at the base of the gills (fig. 101, off).
Salivary glands are wanting. An ink-sac is not present.
Branchial hearts are not developed on the branchial adve-
hent vessels.

132

MOLLUSCA

Family 1. — Nautilidse.
Genera : [Orthuccms], Breyn. ; [Cyrtoceras], Goklfuss ; [Gompho-

ccras], Minister ; [Phragmoceras], Brod. ; [Gyroccras], Meyer ;

[Ascoeeras], Barraude ; [Oncoccras], Hall ; [Lituitcs], Breyn. ;

[Troehoccras], Barraude ; Nautilus, L. (figs. 88, 89, 90, &c.) ;

[Clymenia], Munst. ; [Nothoceras], Barraude.
Family 2. — Ammonitidas.

Genera : [Boclrilcs], Sanderg. ; \Goniatitcs], de Haan ; [EJiaMo-

ccras], Hauer ; [Clydonitcs], Hauer ; [Cochloccras], Bauer ;

[Baculina], d'Orb. ; [Ccratites], de Haau ; [Saculites], Lam. ;

[Toxoceras], d'Orb.; [Crioceras], Leveille ; [P/i/choecms],<i'Orb.;

[Hamitcs], Parkinson ; [Ancyloceras], d'Orb. ; [Scaphitcs],

Parkinson; [Ammonites], Breyn.; [Turrilitcs], Lam. ; \_Hclio-

ccrasj, d'Orb. ; [Hctcroccras], d'Orb.

N.B. — The names in brackets are those of extinct genera.

Order 2. — Dibranchiata ( = Holosiphona, Acetabulifera).
Characters. — Siplionopodous Cephalopods in which the
inflected lateral margins of the mid-foot are fused so as to
form a complete tubular siphon (fig. 96, i). The circum-
oral lobes of the fore-foot carry suckers disposed upon them
in rows (as in the Pteropod Pneumodermon), not tentacles
(see figs. 92, 95, 96). There is a single pair of typical
ctenidia (fig. 103) acting as gills (hence Dibranchiata), and

FIG. 02. — Sepia officinalis, L., half the natural size, as seen when dead, the long
prehensile arms being withdrawn from the pouches at the side of the head,
in which they are carried during life when not actually in use. a, neck ;
b, lateral fin of the mantle-sac ; c, the eight shorter arms of the fore-foot ; t?,
the two long prehensile arms ; c, the eyes.

a single pair of nephridia opening by apertures right and
left of the median anus (fig. 103, r), and by similar internal
pores into the pericardial chamber, which consequently does
not open directly to the surface as in Nautilus. The ovi-
ducts are sometimes paired right and left (Octopoda),
sometimes that of one side only is developed (Decapoda,
except Ommastrephes). The sperm-duct is always single
except, according to Keferstein, in Eledone moschata.

A plate-like shell is developed in a closed sac formed by
the mantle (figs. 98, 99), except in the Octopoda, which have
none, and in Spirula (fig. 100, D) and the extinct Belemni-
tidte, which have a small chambered shell resembling that

of Nautilus with or without the addition of plate-like and
cylindrical accessory developments (fig. 100, C).

The pair of cephalic eyes are highly-developed- vesicles
with a refractive lens (fig. 120), cornea, and lid-folds, — the
vesicle being in the embryo an open sac like that of Nautilus
(fig. 119). Osphradia are not present, but cephalic olfac-
tory organs are recognized. One or two pairs of large
salivary glands with long ducts are present. An ink-sac
formed as a diverticulum of the rectum and opening near
the anus is present in all Dibranchiata (fig. 103, t), and has
been detected even in the fossil Belemnitidic. Branchial
hearts are developed on the two branchial advehent blood-
vessels (fig. 104, vc, vi).

The Dibranchiata are divisible into two sub-orders, accord-
ing to the number and character of the arm-like sucker-
bearing processes of the fore-foot.

Fin. 93. — Pepapodous Siphonopods ; one-fourth the natural size linear. A.
L'liciniteiitliis 'J'l-ranyi, d'Orli. (from the Mediterranean). B. TlnjMnoteutliis
rhombus, Troschel (from Messina). C. Loligopsis cydura, Fer. and d'Orb.
(from the Atlantic Ocean).

Sub-order 1. — Decapoda.

Clmradcrs. — Dibranchiata with the fore-foot drawn out into
eight shorter and two longer arms (prehensile arms), the latter being
placed right and left between the third and fourth shorter arms.
The suckers are stalked and strengthened by a horny ring. The
eyes are large and have a horizontal in place of a sphincter-like lid.
The body is elongated and provided with lateral fins (lamelliform
expansions of the mantle). The month has a buccal membrane.
The mantle-margin is locked to the base of the siphon by a specially-
developed cartilaginous apparatus. Numerous water-pores are pre-
sent in the head and anterior region of the body, leading into re-
cesses of the integument of unknown significance. The oviduct is
single ; large nidamental glands are present. The viscero-pericar-
dial space is large, and lodges the ovary (Sejiia). There is always
a shell present which is enclosed by the upgrowth of the mantle,
so as to become " internal."

Section a. — Decapoda Calciphora.
Character. — Internal shell calcareous.
Family 1. — Sfirulidse.

Genus : Spirula, Lam. (fig. 100, D).
Family 2. — Belcmnitidse.

Genera: [SpimiUrostra,], d'Orb. (fig. 100, C); [Bclaptcrn], Desh. ;
[Bclcmnnsis], Edw. ; [Coiwtc.ut!i.is], d'Orb. (fig. 100, A) ; [Amn-
thotcuthis], R. Wag. ; [Belemnites], Lister, 1678; [Selemnitella],
d'Orb.; [Xiphoteiithis], Huxley.
Family 3. — Scpiadss.

Genera: Sfpia, L. (figs. 92, 98, &c.); [Bclosepia], Voltz ; Cocco-
teitthis, Owen.

MOLLUSCA

133

Section b. — Decnpoda Cliondroplmrti.
CJtaractcr.- — Internal shell horny.

Sub-section o. — Myopsidic (d'Orb.).

Eye with closed cornea, so that the surrounding water does not
tiiueli the lens; mostly frequenters of tlie eoast.
Family 1. — Loligiihr.

(uneni: Luliija, Sehueid. (figs. 99, &.c.); LaliiiJus, Steenstrup;
Sepioteuthis, I51v. ; [Teuthopsis], Desl. ; [Lcptotctithis], Meyer;
[Selemnosepia], Ag. ; [Eelotcuthis], Mimst.
Family 2. — Sepiolidse,
Genera : Scpiola., Schneid. ; Rossia, Owen.

Sub-section /3. — Oiijopsidss (d'Orli.).

Eye with open cornea, so that the surrounding water bathes the
anterior surface of the lens ; mostly pelagic animals.
Family 3. — Cranckiadx.

Genus: Cranthia, Leach (fig. 94, C).
Family 4. — Loligopsidx.

Genus : Loligopsis, Lara. (fig. 93, C).
Family 5. — Chfiroteutkidse.

Genera : Chcirotcuthis, d'Orb. (fig. 93. A) ; Histiotculhis, d'Orb.
Family 6. — Thysanoteuthidee.

Genus : Thijsanoleuthis, Trosehcl (fig. 93, B).
Family 7. — Omjcliotcuthidss.

Genera : Gonatus, Gray ; Onychoteitthis, Lichtenst. (fig. 97) ; Ony-
chia, Lesueur ; Enoplotcuthis, d'Orb. , Veranya, Krohn ; \flcsio-
teuthis], A. Wag. ; [Ccl&no], Mimst. ; Dosidicus, Steeustrup ;
Ommastrcphes, d'Orb.

Sub-order 1. — Octopoda.

Characters. — Dibrauchiata with the fore-foot drawn out into eight
arms only; suckers sessile, devoid of horny ring; eyes small, the

D

FIG. 94.— Octopodous Siphonopods ; one-fourth the natural size linear. A.
Pinnoctopus cordiformis, Quoy and Gain (from New Zealand). B. Tremoc-
topns violaaus, Ver. (from the Mediterranean). C. Cmnckia scabra, Owen
(from the Atlantic Ocean ; one of the Decapoda). D. Cirrkotciitliis Miilleri,
Each, (from the Greenland coast).

outer skin can be closed over them by a sphincter-like movement.
The body is short and rounded ; the mantle has no cartilaginous
locking apparatus, and is always fused to the head dorsally by a
broad nuchal baud. No buccal membrane surrounds the mouth.
The siphon is devoid of valves. The oviducts are paired ; there are
no nidameutal glands. The viscero-pericardial space is reduced to
two narrow canals, passing from the uephridia to the capsule of the
genital gland. There is no shell on or in the visceral hump.
Family 1 . — Cirrhoteuthidx.

Genus: Clfflifitculhis, Esch. (Sciadcpliorus, Eeinh.) (fig. 94, D).
Family 2. — Octojtodidse.

Genera : Pinnoctopus, d'Orb. (fig. 94, A) ; Octopus, Lam. (fig. 95) ;
Sc&urgus, Trosch. ; Eledone, Leach ; Bolit&ua, Steenstrup.

Family 3. — Fliihncxidse.

<.'eiicra: Tniniii-liiiiii.t, Pelln <'1na]Y (r/ii!iinc.ri.i, d'Orb.) (fig. 94,
B) ; ParOSira, .Stccnstrup (Or-tu/iiix nil, miltitiix, Fer. , is the
female, and Orln/mx riin-ini, Ver., is the male of the one species
of this genus :irrnrdiiig tu Stc-eii.sti'U]) (tig. 96)) ; Arii»i><iiilii, L.
(the shell of tliis genus is formed only in the female by the
expanded ends of the two large "arms " of the fore-foot).

FIG. 95.— A. Male specimen of Octopus yrmilanrlicus, with the third aim of the
right side hectocotylized. B. Enlarged view of the hectocotylized arm of
Sepia.

Further Remarks on the Cephalopoda. — In order to give
a more precise conception of the organization of the Cephalo-
poda in a concrete form we select the Pearly Nautilus for
further description, and in pass-
ing its structure in review we
shall take the opportunity of
comparing here and there the
peculiarities presented by that
animal with those obtaining in
allied forms. In the last edition
of this work the Pearly Nautilus
was made the subject of a de-
tailed exposition by Professor
Owen, and it has seemed accord-
ingly appropriate that it should
be somewhat fully treated on
the present occasion also. The
figures which illustrate the pre-
sent description are (excepting
fig. 89) original, and prepared
from dissections (made under the
direction of the writer) of a male
and female Nautilus pom/>i//irx,
lately purchased for the Museum
of University College, London.

Visceral Hump and Shell. —
The visceral hump of Nautilus
(if we exclude from considera-
tion the fine siphuncular pedicle FIO. oe.— Male of Parosira catenu-

wliifh it trails ns it ivprp VipVn'nrl '«'"> Steenstrup (Octopus mrena,
Wllicn It trails, as It were, Deninci Var.), showing the Jiectoeotylized

it) is very little, if at all, affected ami. ft, v, &, t*, the nrst, second,
by the coiled form of the shell
which it carries, since the animal
always slips forward in the shell
as it grows, and inhabits a cham-
ber which is practically cylindri-
cal^'. 89). Were the deserted chambers thrown off instead
of being accumulated behind the inhabited chamber as a
coiled series of air-chambers, we should have a more correct
indication in the shell of the extent and form of the animal's

third, and fourth arms or pro-
cesses of the fore-foot; h, the
third arm of the right side hecto-
cut ylized ; z, the apical sac of the
hectocotylized arm ; y, the fila-
ment which issues from the sac
wlirii development is complete;
i, the siphon. (From Gegenbaur.)

134

MOLLUSCA

-rf— .

body. Amongst Gastropods it is not very unusual to find
the animal slipping forward in its shell as growth advances
and leaving an unoccupied chamber in the apex of the shell.
This may indeed become shut off from the occupied cavity
by a transverse septum, and a series of such septa may be
formed (fig. 42), but in no Gastropod are these apical
chambers known to contain a
gas during the life of the
animal in whose shell they
occur. A further peculiarity
of the Nautilus shell and of
that of the allied extinct Am-
monites, Scaphites, Orthoceras,
&c., and of the living Spirula,
is that the series of deserted
air-chambers are traversed by
a cord -like pedicle extending
from the centro-dorsal area of
the visceral hump to the small-
est and first-formed chamber of
the series. No structure com-
parable to this siphuncular
pedicle is known in any other
Mollusca. Its closest repre-
sentative is found in the so-
called "contractile cord" of
the remarkable form Rhabdo-
pleura, referred according to
present knowledge to the Poly-
zoa. There appears to be no
doubt that the deserted cham-
bers of the Nautilus shell con-
tain in the healthy living
animal a gas which serves to
lessen the specific gravity of
the whole organism. The gas
is said to bo of the same com-
position as the atmosphere,
with a larger proportion of
nitrogen. With regard to its
origin we have only conjec-
tures. Each septum shutting
off an air-containing chamber
is formed during a period of
quiescence, probably after the
reproductive act, when the vis-
ceral mass of the Nautilus may
be slightly shrunk, and gas is
secreted from the dorsal inte-
gument so as to fill up the
space previously occupied by
the animal. A certain stage
is reached in the growth of
the animal when no new cham-
bers are formed. The whole
process of the loosening of the
animal in its chamber and of
its slipping forward when a
new septum is formed, as well as the mode in which the
air-chambers may be used as a hydrostatic apparatus, and
the relation to this use, if any, of the siphuncular pedicle,
is involved in obscurity, and is the subject of much in-
genious speculation. In connexion with the secretion of
gas by the animal, besides the parallel cases ranging from
the Protozoon Arcella to the Physoclistic Fishes, from
the Hydroid Siphonophora to the insect-larva Corethra,
we have the identical phenomenon observed in the closely-
allied Sepia when recently hatched. Here, in the pores
of the internal rudimentary shell, gas is observable, which
has necessarily been liberated by the tissues which secrete

d-

07.— Head and eirotvm-oral pro-
cesses of the fore-foot of Ouycho-
teuthis (frnm Owen), «, neck ; 6,
eye ; c, the eight short arms ; d, long
prehensile arms, the clavate extre-
mities of which are provided with
suckers at e, and with a double row
of hooks beyond at/. The tempo-
rary conjunction of the arms by
means of the suckers enables them
to act in combination.

the shell, and not derived from any external source
(Huxley).

The coiled shell of Nautilus, and by analogy that of the
Ammonites, is peculiar in its relation to the body of the
animal, inasmuch as the curvature of the coil proceeding

Fig. 98. Fig. 99.

Fin. 98.— The calcareous internal shell of Sepia officinalis, the so-called cuttle-
bone, a, lateral expansion ; b, anterior cancellated region ; r, laminated
region, the laminte enclosing air.

FIG. 99.— The horny internal shell or gladius or pen of Loligo.

from the centro-dorsal area is towards the head or forward,
instead of away from the head and backwards as in other
discoid coiled shells such as Planorbis ; the coil is in fact
absolutely reversed in the two cases. Amongst the extinct
allies of the Nauti-
lus (Tetrabranch-
iata) we find shells
of a variety of
shapes, open coils
such as Scaphites,
leading on to per-
fectly cylindrical
shells with chamber
succeeding cham-
ber in a straight
line (Orthoceras),
whence again wo
may pass to the
cork-screw spires KHT^ D
formed by the shell
of Turrilites.

Whilst the Tetra-
branchiata, so far as
we can recognize
their remains, are
characterized by
these large chambered shells, which, as in Nautilus, were
with the exception of some narrow-mouthed forms such
as Gomphoccras but very partially covered by reflexions
of the mantle-skirt (fig. 89, b), the Dibranchiata present
an interesting series of gradations, in which we trace —
(«) the diminution in relative size of the chambered
shell ; (b) its complete investiture by reflected folds of
the mantle (Spirula, fig. 100, D) ; (c) the concrescence

FIG. TOO. — Internal shells of Cephalopoda Sipliono-
poda. A. Shell of Conotnitliis diipiniaiia, d'Orb.
(from the Neocomian of France). B. Shell of
Si-i'ili uiliitniaiitt'FfT. (Mediterranean). C. Shell
nf SpiruliTOStra /.v//<mfi'i, d'Urb. (from the Mio-
cene of Turin). The specimen is cut so as to show
in section the chambered shell and the laminated
"guard" deposited upon its surface. D. Shell of
Spirilla ten's, Gray (New Zealand).

MOLLUSCA

135

of the folds of the mantle to form a definitely-closed
shell-sac ; (il) the secretion by these mantle-folds or walls
of the shell-sac of additional laminre of calcareous shell-
substiince, which invest the original shell and completely
alter its appearance (Spirulirostra, fig. 100, C; Belemnites) ;
(c) the gradual dwindling and total disappearance of the
original chambered shell, and survival alone of the calcare-
ous laminre deposited by the inner walls of the sac (Sepia,
fig. 100, B) ; (/) the disappearance of all calcareous sub-
stance from the pen or plate which now represents the
contents of the shell-sac, and its persistence as a horny
body simply (Loligo, fig. 99); (</) the total disappearance
of the shell-sac itself, and consequently of its pen or plate,
nevertheless the rudiments of the shell-sac appearing in
the embryo and then evanescing (Octopus). The early
appearance of the sac of the mantle in which the shell is
enclosed, in Dibrauchiata, has led to an erroneous identifi-
cation of this sac with the primitive shell-sac of the archi-
Mollusc (fig. 1), of Chiton (fig. 10, A), of Arion (fig. 69,
D, a), and of the normally-developing Molluscan embryo
(figs. 68 and 72***, sfi). The first appearance of the shell-
sac of Dibranchiata is seen in figs. 121 and 122, its forma-
tion as an open upgrowth of the centro-dorsal area of the
embryo having been demonstrated by Lankester (34) in
1873, who subsequently showed (35) that the same shell-sac
appears and disappears without closing up in Argonauta
and Octopus, and pointed out the distinctness of this sac
and the primitive shell-gland. The shell of the female
Argonauta is not formed by the visceral hump, but by the
enlarged arms of the foot, which are in life always closely
applied to it.

The shell of such Pteropoda as have shells (the Thecoso-
mata) is excessively light, and fits close to the animal, no
air-chambers being formed. It is important to note that
in this division of the Cephalopoda there is the same tend-
ency, which is carried so far in the Dibranchiate Siphono-
pods, for the mantle-skirt to be reflected over and closely
applied to the shell (e.g., Cavolinia, figs. 79 and 80). But
in Pteropoda there is no complete formation of a closed
sac by the reflected mantle, no thickening of the enclosed
shell, no dwindling of the original shell and substitution
for it of a laminated plate. The variety of form of
the glass-like shells of Pteropoda is a peculiarity of that
group.

Head, Foot, Mantle-skirt; and Sub-pallial Chrimlier. — In
the Pearly Nautilus the ovoid visceral hump is completely
encircled by the free flap of integument known as mantle-
skirt (fig. 91, d, e). In the antero-dorsal region this flap
is enlarged so as to be reflected a little over the coil of the
shell which rests on it. In the postero-ventral region the
flap is deepest, forming an extensive sub-pallial chamber,
at the entrance of which e is placed in fig. 91. A view of
the interior of the sub-pallial chamber, as seen when the
mantle-skirt is retroverted and the observer faces in the
direction indicated by the reference line passing from e in
fig. 91, is given in fig. 101. With this should be com-
pared the similar view of the sub-pallial chamber of the
Dibranchiate Sepia (fig. 103). It should be noted as a
difference between Nautilus and the Dibranchiates that in
the former the nidamental gland (in the female) lies on
that surface of the pallial chamber formed by the dependent
mantle-flap (figs. 101, g.n. ; 89, V), whilst in the latter it lies
on the surface formed by the body-wall ; in fact in the
former the base of the fold forming the mantle-skirt com-
prises in its area a part of what is unreflected visceral
hump in the latter.

The apertures of the two pairs of nephridia, of the vis-
cero-pericardial sac, of the genital ducts, and of the anus
are shown in position on the body-wall of the pallial cham-
ber of Nautilus in figs. 101, 102. There are nine apertures

in all, one median (the anus), and four paired. Besides
these apertures we notice tico pairs of gill-plumes which
are undoubtedly typical ctcnidia, and a short papilla (the

Fio. 101.— View of the postero-ventral surface of a female Pearly Nautilus, the
mantle-skirt (c) being completely reflected so as to show the inner wall of
the sub-pallial chamber (drawn from nature by A. G. Bourne). «, muscu-
lar band passing from the mid-foot to the integument ; b, the valve on the
surface of the funnel-like mid-foot, partially concealed by the inrolled lateral
margin of the latter ; c, the mantle-skirt retroverted ; a»., the median anus ;
x, post-anal papilla of unknown significance ; 17.71., nidamental gland ; r.ov.,
aperture of the right oviduct ; l.oi'., aperture of the rudimentary left oviduct
(pyriform sac of Owen); neph.a., aperture of the left anterior nephridium;
neph.p, aperture of the left posterior nephridium ; riscper., left aperture of
the viscero-pericardial sac ; olf, the left osphradium placed near the base of
the anterior gill-pluine. The four gill-plumes (cteuidia) are not lettered.

osphradium) between each anterior and posterior gill-plume
(see figs. 101, 102, and explanation). As compared with
this in a Dibranchiate, we find (fig. 103) only four aper-

me suo-pamai cnamiier, ana Tin- lour cieniuia ;iliu uie loot cut snort tuiaun
from nature by A. G. Bourne). j>c., penis, being the enlarged termination
of the right spermatic duct ; l.sp., aperture of the rudimentary left spermatic
duct (pyriforni sac of Owen). Other letters as ill fig. 101.

tures, viz., the median anus with adjacent orifice of the
ink-sac, the single pair of uephridial apertures, and one
asymmetrical genital aperture (on the left side), except in
female Octopoda and a few others where the genital
duets and their apertures are paired. No viscero-peri-
cardial pores are present on the surface of the pallial
chamber, since .in the Dibranchiata the viscero-pericardial

136

MOLLUSCA

sac opens by a pore into each nephridium instead of
directly to the surface. A single pair of ctenidia (gill-
plumes) is present instead of the two pairs in Nautilus.
The existence of two pairs of ctenidia and of two pairs
of nephridia in Nautilus, placed one behind the other, is
highly remarkable. The interest of this arrangement is in
relation to the general morphology of the Mollusca, for
it is impossible to view this repetition of organs in a linear
series as anything else than an instance of metameric seg-
mentation, comparable to the segmentation of the ringed
worms and Arthropods. The only other example which
we have of this metamerism in the Mollusca is presented
by the Chitons. There we find not two pairs of ctenidia
merely, but sixteen pairs (in some species more) accom-

t— '

FIG. 103. — View of the pnstero-ventral surface of a male Sepia, obtained by
cutting longitudinally the firm mantle-skirt and drawing the divided halves
apart. This figure is strictly comparable with fig. 101. t', the head ; J, the
mid-foot or siphon, which has been cut open so as to display the valve i ; Rt
the glandular tissue of the left nephridium or renal-sac, which has been cut
open (see fig. 108); P, P, the lateral tins of the mantle-skirt; Br, the single
pair of branchise (ctenidia) ; o, the anus, — immediately below it is the open-
ing of the ink-bag ; c, cartilaginous socket in the siphon to receive c', the
cartilaginous knob of the mantle-skirt,— the two constituting the " pallial
binge apparatus " characteristic of Decapoda, not found in Octopoda ; g, the
azygos genital papilla and aperture ; 't, valve of the siphon (possibly the rudi-
mentary hind-foot) ; m, muscular band connected with the fore-foot and
mid-foot (siphon) and identical with the muscular mass k in fig. 91 ; r, renal
papilla, carrying the apertures of the nephridia ; r.lu\ branchial efi'erent
blood-vessel ; r.br', bulbous enlargements of the branchial blood-vessels (see
figs. 104, 108); /, ink-bag. (From Gegenbaur.)

panied by a similar metamerism of the dorsal integument,
which carries eight shells. In Chiton the nephridia are
not affected by the metamerism as they are in Nautilus.
It is impossible on the present occasion to discuss in the
way which their importance demands the significance of
these two instances among Mollusca of incomplete or partial
metamerism ; but it would be wrong to pass them by with-
out insisting upon the great importance which the occur-
rence of these isolated instances of metameric segmentation
in a group of otherwise unsegmented organisms possesses,
and the light which they may be made to throw upon the
nature of metameric segmentation in general.

The foot and head of Nautilus are in the adult inex-
tricably grown together, the eye being the only part belong-
ing primarily to the head which projects from the all-
embracing foot. The fore-foot or front portion of the foot

in Nautilus has the form of a number of lobes carrying
tentacles and completely surrounding the mouth (figs. 88,
89, 91). The mid-foot is a broad median muscular process
which exhibits in the most interesting manner a curling in
of its margins so as to form an incomplete siphon (fig.
101), a condition which is completed and rendered per-
manent in the tubular funnel, which is the form presented
by the corresponding part of Dibranchiata (fig. 96). The
hind-foot possibly is represented by the valvular fold on the
surface of the siphon-like mid-foot. In the Pteropoda the
wing-like swimming lobes (epipodia or pteropodia) corre-
spond to the two halves of the siphon, and are much the
largest element of the foot. The fore-foot surrounding
the head is often quite small, but in Clione and Pneumo-
dermon carries lobes and suckers. A hind-foot is in Ptero-
poda often distinctly present ; it is open to doubt as to
whether the corresponding region of the foot in Siphono-
poda is developed at all.

The lobes of the fore-foot of Nautilus and of the other
Siphonopoda require further description. It has been
doubted whether these lobes were rightly referred (by
Huxley) to the fore-foot, and it has been maintained by some
zoologists (Grenacher, Jhering) that they are truly processes
of the head. It appears to the present writer to be im-
possible to doubt that the lobes in question are the fore-
portion of the foot when their development is examined
(see fig. 121, and especially fig. 72**), further, when the fact
is considered that they are innervated by the pedal ganglion,
and, lastly, when the comparison of such a Siphonopod as
Sepia is made with such a Pteropod as Pneumodermon in its
larval (fig. 84) aswell as in its adult condition (fig. 85). The

FIG. 104. — Circulatory and excretory organs of Sepia (from Gegenbaur, after
John Hunter). &r, branchite (ctenidia) ; c, ventricle of the heart ; a, anterior
artery (aorta) ; a', posterior artery ; vt the right and left auricles (enlarge-
ments of the efferent branchial veins) ; v', efferent branchial vein on the free
face of the gill-plume ; v.c, vena cava ; vi, vc', advehent branchial vessels
(branches of the vena cava, see tig. 108) ; vc", abdominal veins ; x, branchial
hearts and appendages ; re, e, glandular substance of the nephridia developed
on the wall of the great veins on their way to the gills. The arrows indicate
the direction of the blood-current.

larval rneumodermon shows clearly that the sucker-bearing
processes of that Mollusc are originally far removed from
the head and close in position to the pteropodial lobes of
the foot. By differential growth they gradually embrace
and obliterate the head, as do the similar sucker-bearing
processes of Sepia. In both cases the sucker-bearing pro-
cesses are "fore-foot." The fore-foot of Nautilus completely
surrounds the buccal cone (fig. 88, e), so as to present an
appearance with its expanded tentacles similar to that of the
disc of a sea-anemone (Actinia). No figure has hitherto
been published exhibiting this circum-oral disc with its
tentacles in natural position as when the animal is alive and
swimming, the small figure of Valenciennes being deficient
in detail. All the published figures represent the actual
appearance of the contracted spirit-specimens. Mr A. G.

MOLLUSCA

137

Bourne, B.Sc., of University College, has prepared from
actual specimens the drawings of this part in the male and
female Nautilus reproduced in fig. 88, and has restored the
parts to their natural form when expanded. The drawings
show very strikingly the difference between male and female.
In the female (lower figure), we observe in the centre of
the disc the buccal cone e carrying the beak-like pair of
jaws which project from the finely papillate buccal membrane.
Three tentaculiferous lobes of the fore-foot are in immediate
contact with this buccal cone ; they are the right and left
(c, <•) inner lobes, as we propose to call them, and the in-
ferior inner lobe (d), — called inferior because it really lies
ventralwards of the mouth. This inner inferior lobe is
clearly a double one, representing a right and left inner
inferior lobe fused into one. A lamellated organ on its sur-
face, probably olfactory in function (•»), marks the separation
of the constituent halves of this double lobe. Each half
carries a group of fourteen tentacles. The right and the
left inner lobes (c, c) each carry twelve tentacles. Y^-

FIG. 105.— Diagram to show the relations of the heart in the Mollusca (from
Gegenbaur). A. Part of the dorsal vascular trunk and transverse trunks of
a worm. B. Ventricle and auricles of Nautilus. C. Of a LanieUibraneh, of
Chiton, or of Loligo. D. Of Octopus. £'. Of a Gastropod, n, auricle ; )',
ventricle ; ar, arteria cephalica (aurta) ; ni, arteria abdominalis. The arrows
show the direction of the blood-current.

ternal to these three lobes the muscular substance of the
mouth-embracing foot is raised into a wide ring, which
becomes especially thick and large in the dorsal region
where it is notably modified in form, offering a concavity
into which the coil of the shell is received, and furnish-
ing a protective roof to the retracted mass of tentacles.
This part of the external annular lobe of the fore-foot is
called the "hood" (figs. 90, 91, in.). The median antero-
posterior line traversing this hood exactly corresponds to
the line of concrescence of the two halves of the fore-foot,
which primitively grew forward one on each side of the
head, and finally fused together along this line in front of
the mouth. The tentacles carried by the great annular
lobe are nineteen on each side, thirty-eight in all. They
are somewhat larger than the tentacles carried on the three
inner lobes. The dorsalmost pair of tentacles (marked
g in fig. 88) are the only ones which actually belong to
that part of the disc which forms the great dorsal hood in.
The hood is, in fact, to a large extent formed by the enlarged
sheaths of these two tentacles. In the Ammonites (fossil
Tetrabranchiata allied to Nautilus) the dorsal surface of
the hood secreted a shelly plate in two pieces, known to
palaeontologists as Trigonellites and Aptychus. Possibly,
however, this double plate was carried on the surface of
the bilobed nidamental gland with the form and sculptur-
ing of which, in Nautilus, it closely agrees. All the ten-
tacles of the circum-oral disc are set in remarkable tubular
sheaths, into which they can be drawn. The sheaths of
some of those belonging to the external or annular lobe are
seen in fig. 91, marked n. The sheaths are muscular as
well as the tentacles, and are simply tubes from the base
of which the solid tentacle grows. The functional signifi-
cance of this sheathing arrangement is as obscure as its
morphological origin. With reference to the latter, it
appears highly probable that the tubular sheath represents
the cup of a sucker such as is found on the fore-foot of the

Dibranchiata. In any case, it seems to the writer impos-
sible to doubt that each tentacle, and its sheath on a lobe
of the circum-oral disc of Nautilus, corresponds to a sucker
on such a lobe of a Dibranchiate. Keferstuin follows Owen
in strongly opposing this identification, and in regarding
such tentacle as the equivalent of a whole lobe or arm of a
Decapod or Octopod Dibranch. We find in the details of
these structures, especially in the facts concerning the
hectocotylus and spadix, the most conclusive reasons for
dissenting from Owen's view. We have so far enumer-
ated in the female Nautilus ninety tentacles. Four more
remain which have a very peculiar position, and almost
lead to the suggestion that the eye itself is a modified
tentacle. These remaining tentacles are placed one above
(before) and one below (behind) each eye, and bring up
the total to ninety-four (fig. 91, v, v). They must be con-
sidered as also belonging to the fore-foot which thus sur-
rounds the eye.

In the adult male Nautilus we find the following im-
portant differences in the tentaculiferous disc as compared
with the female (see upper drawing in fig. 88). The
inner inferior lobe is rudimentary, and carries no tentacles.
It is represented by three groups of lameike (d), which are
not fully exposed in the drawing. The right and left inner
lobes are subdivided each into two portions. The right
shows a larger portion carrying eight tentacles, and smaller
detached groups (q) of four tentacles, of which three have
their sheaths united whilst one stands alone. These four
tentacles may be called the " anti-spadix." The left inner
lobe shows a similar larger portion carrying eight tentacles,
and a curious conical body in front of it corresponding to
the anti-spadix. This is the " spadix " of Van der Hoeven
(36). It carries no tentacles, but is terminated by imbri-
cated lamellae. These lamellae appear to represent the four
tentacles of the anti-spadix of the right internal lobe, and
are generally regarded as corresponding to that modification
of the sucker-bearing arms of male Dibranchiate Siphono-
pods to which the name " hectocotylus " is applied. The
spadix is in fact the hectocotylized portion of the fore-
foot of the male Nautilus. The hectocotylized arm or lobe
of male Dibranchiata is connected with the process of copu-
lation, and in the male Nautilus the spadix has probably a
similar significance, though it is not possible to suggest
how it acts in this relation. It is important to observe
that the modification of the fore-foot in the male as com-
pared with the female Nautilus is not confined to the
existence of the spadix. The anti-spadix and the reduction
of the inner inferior lobe are also male peculiarities. The
external annular lobe in the male does not differ from that
of the female ; it carries nineteen tentacles on each side.
The four ophthalmic tentacles are also present. Thus in
the male Nautilus we find altogether sixty-two tentacles,
the thirty-two additional tentacles of the female being repre-
sented by lamelliform structures.

If we now compare the fore-foot of the Dibranchiata with
that of Nautilus, we find in the first place a more simple
arrangement of its lobes, which are either four or five pairs
of tapering processes (called " arms ") arranged in a series
around the buccal cone, and a substitution of suckers for
tentacles on the surface of these lobes (figs. 92, 95, 96).
The most dorsally-placed pair of arms, corresponding to the
two sides of the hood of Nautilus, are in reality the most
anterior (see fig. 75, (6) ), and are termed the first pair. In
the Octopoda there are four pairs of these arms (figs. 94,
95), in the Decapoda five pairs, of which the fourth is
greatly elongated (figs. 92, 93). In Sepia and other Deca-
poda (not all) each of these long arms is withdrawn into a
pouch beside the head, and is only ejected for the purpose
of prehension. The figures referred to show some of the
variations in form which these arms may assume. In tho

S

138

MOLLUSCA

Octopoda they are not unfrequently connected by a web,
and form an efficient swimming-bell. The suckers are placed
on the ad-oral surface of the arms, and may be in one,
two, or four rows, and very numerous. In place of suckers
in some genera we find on certain arms or parts of the
arms horny hooks ; in other cases a hook rises from the
centre of each sucker. The hooks on the long arms of
Onychoteuthis are drawn in fig. 97. The fore-foot, with
its apparatus of suckers and hooks, is in the Dibranchiata
essentially a prehensile apparatus, though the whole series
of arms in the Octopoda serve as swimming organs, and in
many (e.g., the Common Octopus or Poulp) the sucker-
bearing surface is used as a crawling organ.

In the males of the Dibranchiata one of the arms is
more or less modified in connexion with the reproductive
function, and is called the " hectocotylized arm." This
name is derived from the condition assumed by the arm
in those cases in which its modification is carried out to
the greatest extent. These cases are those of the Octo-
pods Argonauta argo and Parasira catenvlata (fig. 96).
In the males of these the third arm (on the left side in
Argonauta, on the right side in Parasira) is found before
the breeding season to be represented by a globular sac of
integument. This sac bursts, and from it issues an arm
larger than its neighbours, having a small sac at its extremity
in Parasira (fig. 96, x), from which subsequently a long
filament issues. Before copulation the male charges this
arm with the spermatophores or packets of spermatozoa
removed from its generative orifice beneath the mantle-skirt,
and during coitus the arm becomes detached and is left
adhering to the female by means of its suckers. A new arm
is formed at the cicatrix before the next breeding season.
The female, being much larger than the male, swims away
with the detached arm lodged beneath her mantle-skirt.
There, in a way which is not understood, the fertilization
of the eggs is effected. Specimens of the female Parasira
with the detached arm adherent were examined by Cuvier,
who mistook the arm for a parasitic worm and gave to it
the name Hectocotylus. Accordingly, the correspondingly
modified arms of other Siphonopoda are said to be hecto-
cotylized. Steenstrup has determined the hectocotylized
condition of one or other of the arms in a number of male
Dibranchs as follows : — in all, excepting Argonauta and
Parasira, the modification of the arm is slight, consisting in
a small enlargement of part or the whole of the arm, and
the obliteration of some of its suckers, as shown in fig. 95,
A, B ; in Octopus and Eledone the third right arm is
hectocotylized ; in Rossia the first left arm is hectocotylized
along its whole length, and the first right arm also in the
middle only ; in Sepiola only the first left arm along its
whole length ; in Sepia it is the fourth left arm which is
modified, and at its base only ; in Sepioteuthis, the same at
its apex ; in Loligo, the same also at its apex; in Loliolus,
the same along its whole length ; in Ommastrephes,
Onychoteuthis, and Loligopsis no hectocotylized arm has
hitherto been observed.

In the females of several Dibranchs (Sepia, &c.) the
packets of spermatozoa or spermatophores received from
the male have been observed adhering to the smaller arms.
How they are passed in this case by the female to the ova
in order to fertilize them is unknown.

Mvsntlatvre, Fins, mid Ga/rtHaginous Skeleton. — Without
entering into a detailed account of the musculature of
Nautilus, we may point out that the great muscular masses
of the fore-foot and of the mid-foot (siphon) are ultimately
traceable to a large transverse mass of muscular tissue,
the ends of which are visible through the integument on
the right and left surfaces of the body dorsal of the
free flap of the mantle-skirt (fig. 89, !, !, and fig. 91, £).
These muscular are;e have a certain adhesion to the shell,

and serve both to hold the animal in its shell and as the
fixed supports for the various movements of the tentaculi-
ferous lobes and the siphon. They are to be identified
with the ring-like area of adhesion by which the foot-muscle
of the Limpet is attached to the shell of that animal (see
fig. 27). In the Dibranchs a similar origin of the muscular
masses of the fore-foot and mid-foot from the sides of the
shell — modified, as this is, in position and relations — can be
traced.

In Nautilus there are no fin-like expansions of the integu-
ment, whereas such occur in the Decapod Dibranchs along
the sides of the visceral hump (figs. 92, 93). As an excep-
tion among Octopoda lateral fins occur in Pinnoctopus (fig.
94, A), and in Cirrhoteuthis (fig. 94, D). In the Ptero-
podous division of the Cephalopoda such fin-like expansions
of the dorsal integument do not occur, which is to be con-
nected with the fact that another region, the mid-foot, which
in Siphonopods is converted into a siphon, is in them
expanded as a pair of fins.

In Nautilus there is a curious plate-like expansion of
integument in the mid-dorsal region just behind the hood,
lying between that structure and the portion of mantle-
skirt which is reflected over the shell. This is shown in
fig. 90, b. If we trace out the margin of this plate we
find that it becomes continuous on each side with the
sides of the siphon or mid-foot. In Sepia and other Deca-
pods (not in Octopods) a closely similar plate exists in an
exactly corresponding position (see b in figs. 110, 111). In
Sepia a cartilaginous development occurs here immediately
below the integument forming the so-called " nuchal plate,"
drawn in fig. 116, D. The morphological significance of
this nuchal lamella, as seen both in Nautilus and in Sepia,
is not obvious. Cartilage having the structure shown in
fig. 117 occurs in various regions of the body of Siphono-
poda. In all Glossophorous Mollusca the lingual apparatus
is supported by internal skeletal pieces, having the char-
acter of cartilage ; but in the Siphonopodous Cephalopoda
such cartilage has a wider range.

In Nautilus a large H-shaped piece of cartilage is found
forming the axis of the mid-foot or siphon (fig. 116, A,
B). Its hinder part extends up into the head and supports
the peri-cesophageal nerve-mass (a), whilst its two anterior
rami extend into the tongue-like siphon. In Sepia, and
Dibranchs generally, the cartilage takes a different form,
as shown in fig. 116, C. The processes of this cartilage
cannot be identified in any way with those of the capito-
pedal cartilage of Nautilus. The lower larger portion of
this cartilage in Sepia is called the cephalic cartilage, and
forms a complete ring round the oesophagus ; it completely
invests also the ganglionic nerve-collar, so that all the
nerves from the latter have to pass through foramina in
the cartilage. The outer angles of this cartilage spread
out on each side so as to form a cup-like receptacle for the
eyes. The two processes springing right and left from this
large cartilage in the median line (fig. 116, C) are the
" pra>orbital cartilages ;" in front of these, again, there is
seen a piece like an inverted T, which forms a support to
the base of the "arms" of the fore-foot, and is the "basi-
brachial " cartilage. The Decapod Dibranchs have, further,
the "nuchal cartilage" already mentioned, and in Sepia, a
thin plate-like " sub-ostracal " or (so-called) dorsal cartilage,
the anterior end of which rests on and fits into the concave
nuchal cartilage. In Octopoda there is no nuchal cartilage,
but two band-like " dorsal cartilages." In Decapods there
are also two cartilaginous sockets on the sides of the funnel
— " siphon-hinge cartilages " — into which fleshy knobs of
the mantle-skirt are loosely fitted. In Sepia, along the
whole base-line of each lateral fin of the mantle (fig. 92),
is a " basi-pterygial cartilage." It is worthy of remark that
we have, thus developed, in Dibranch Siphonopods a more

MOLLUSCA

139

complete internal cartilaginous skeleton than is to lie found
in s<>ine of the lower Vertebrates. There are other instamvs
of cartilaginous endo-skeleton in groups other than the
Vertebrata. Thus in some eapito-branchiate C'luL'topnds
cartilage forms a skeletal support for the gill-plumes, whilst
in the Arachnids (Mygale, Scorpio) and in Limulus a large
internal cartilaginous plate — the ento-sternite — is devel-
oped as a support for a large series of muscles.

Alimentary Tract. — The buccal cone of Nautilus is ter-
minated by a villous margin (buccal membrane) surround-
ing the pair of beak-like jaws. These are very strong and
dense in Nautilus, being calcified. Fossilized beaks of Tetra-
branchiata are known under the name of Rhyncholites.
In Dibranchs the beaks are horny, but similar in shape to
those of Nautilus. They resemble in general those of a
parrot, the lower beak being the
larger, and overlapping the upper or
dorsal beak. The lingual ribbon and
odontophoral apparatus has the struc-
ture which is typical for Glosso-
phorous Mollusca. In fig. 107, A is
represented a single row of teeth
from the lingual ribbon of Nautilus,
and in fig. 107, B, C, of other Si-
phonopoda.

In Nautilus a long and wide crop
or dilated oesophagus (<•>•, fig. 110)
passes from the muscular buccal mass,
and at the apex of the visceral hump
passes into a highly muscular stom-
ach, resembling the gizzard of a bird
((fizz, fig. 110). A nearly straight
intestine passes from the muscular
stomach to the anus, near which it
develops a small CEecurn. In other

Siphonopods the ceSOphagUS is usually FIG. 106.— Alimentary canal

narrower (fig. 106, oe), and the mus-
cular stomach more capacious (fig.
106, v), whilst a very important
feature in the alimentary tract is
formed by the caecum. In all but
Nautilus the csecum lies near the
stomach, and may be very capacious
— much larger than the stomach in Loligo vulf/aris — or
elongated into a spiral coil, as in fig. 106, e. The simple

of Loliqo sa(]ittata(fTom Ge-
genbaur). The buccal mass
is omitted, oe, oesophagus ;
»',the stomach opened long-
itudinally ; x, probe passed
through the pylorus ; c,
commencement of the cee-
cum ; e, its spiral portion ;
i, intestine ; a, ink-bag ; b,
itsopening into the rectum.

Fin. 10".— Lingnal dentition of Siphonopoda. A. A single row of lingual teeth
of Nautilus pompilins (after Keferstein). B. Two rows of lingual teeth of
Sfpia qflicitialis (after Troschel). C. Lingual teeth of Eludonc cirrliosa (after
Loven).

U-shaped flexure of the alimentary tract as seen in fig.
106, and in fig. 110, is the only important one which it
exhibits in the Cephalopoda, — the Pteropoda (except the
Limacinida) agreeing with the Siphonopoda in this sim-

plicity in consequence of their visceral hump being un-
twisted. The acini of the large liver of Nautilus are
compacted into a solid reddish -brown mass by a firm
membrane, as also is the case in the Dibranchiata.
The liver has four paired lobes in Nautilus, which open
by two bile-ducts into the alimentary canal at the com-
mencement of the intestine. The bile-ducts unite before
entering the intestine. In Dibranchiata the two large
lobes of the liver are placed antero-dorsally (beneath
the shell in Decapoda), and the bile-ducts open into the
cascum. Upon the bile-ducts in Dibranchiata are deve-
loped yellowish glandular diverticula, which are known
as " pancreas," though neither physiologically nor morpho-
logically is there any ground for considering either the so-
called liver or the so-called pancreas as strictly equivalent
to the glands so denominated in the Vertebrata. In Nauti-
lus the equivalents of the pancreatic diverticula of the
Dibranchs can be traced upon the relatively shorter bile-
ducts.

Salivary Glands are not developed in Nautilus unless a
pair of glandular masses lying on the buccal cavity are to
be considered as such. In the Dibranchs, on the contrary,
one (Sepia, Loligo) or two pairs of large salivary glands
are present, an anterior and a posterior (Octopus, Eledone,
Onychoteuthis). Each pair of salivary glands has its
paired ducts united to form a single duct, which runs
forward from the glands and opens into the buccal cavity

FIG. 108. — Diagram of the nephridial sacs, and the veins which run through
them, in Sepia offcinaUs (after Vigelius). The nephridial sacs are supposed
to have their upper walls removed, v.c, vena cava ; r.d.v.c, right descending
branch of the same ; r.s.v.c, left descending branch of the same ; v.o.a., vein
from the ink-bag; v.m, mesenteric vein; v.g, genital vein; v.a.d, right
abdominal vein ; r.a.s, left abdominal vein ; v.p.d, right pallial vein ; v.p.s,
left pallial vein ; c.b, branchial heart ; x, appendage of the same ; c.r, capsule
of the branchial heart ; up, external aperture of the right nephridial sac ; y,
reno-pericardial orifice placing the left renal sac or nephridium in communi-
cation with the viscero-pericai'dial sac, the course of which below the nephri-
dial sac is indicated by dotted lines ; 7/, the similar orifice of the right side ;
o.r, glandular renal outgrowths ; w.k, viscero-pericardial sac (dotted outline).

near the radula. The anterior pair of glands when present
lie in the head near the buccal mass, the posterior pair lie
much farther back beneath the liver, at the sides of the
oesophagus. It is the posterior pair which alone are pre-
sent in Sepia and Loligo. The ink-bag is to be considered
as an appendage of the rectum. It is not developed in
Nautilus, nor in the Pteropoda; in all Dibranchiata (even in
the fossil Belemnites) it is present (fig. 106, a ; fig. 103, t),
and has been observed to develop as a diverticulum of the
rectum, with spirally plaited walls which very early secrete
a black pigment. The spiral plaitings of the walls diminish

14-0

MOLLUSCA

in relative size as the volume of the sac increases. Its
outer surface acquires a metallic iridescence similar to that
of the integuments of many fishes. The opening of the
ink-sac is in the adult sometimes distinct from but near to

•vent

FIG. 109.— Diagram to show the relations of the four nephridial sacs, the viscero-
pericardial sac, and the heart and large vessels in Nautilus (drawn by A. G.
Bourne), neph,, neph, on the right side point to the two nephridia of that
side (the two of the opposite side are not lettered),— each is seen to have an
independent aperture ; x is the viscero-pericardial sac, the dotted line indicat-
ing its backward extension ; viac.per.apert marks an arrow introduced into
the right aperture of the viscero-pericardial sac ; r.e., r.e., point to the
glandular enlarged walls of the advehent branchial vessels, — two small
glandular bodies of the kind are seen to pro.ject into each nephridial sac,
whilst a larger body of the same kind depends from each of the four branchial
advehent vessels into the viscero-pericardial sac; v.c., vena cava ; vent,
ventricle of the heart ; at>., cephalic aorta (the small abdominal aorta not
drawn) ; a.b.i', advehent branchial vessel ; e.v.b., efferent branchial vessel.

the anus (Sepia) ; in other cases it opens into the rectum
near the anus. The ink-bag of Dibranch Siphonopoda is
possibly to be identified with the adrectal (purpuriparous)
gland of some Gastropoda.

Ccelom, Blood-vascular System, and Excretory Organs. —
Nautilus and the other Siphonopoda conform to the

> ,
a b.v.

FIG. 110. — Diagram representing a vertical approximately median antero
posterior section of Naittilus jw/npilins (frum a drawing by A. G. Bourne).
The parts which are quite black are the cut muscular surfaces of the foot and
buccal mass, a, the shell ; b, the nuchal plate identical with the nuchal
cartilage of Sepia (see fig. 90, 6) ; c, the integument covering the visceral
hump ; d, the mantle flap or skirt in the dorsal region where it rests against
the coil of the shell ; e, the inferior margin of the mantle-skirt resting on the
lip of the shell represented by the dotted line ; /, the pallial chamber with
two of the four 'gills ; g, the vertically cut median pcrtion of the mid-foot
(siphon); h, the capito-pedal cartilage (see fig. 116); i, the valve of the
siphon ; I, the siphuncular pedicle (cut short) ; m, the hood or dorsal enlarge-
ment of the annular lobe of the fore-foot ; 71, tentacles of the annular lobe ;
p, tentacles of the inner inferior lobe ; q, bnccal membrane ; r, upper jaw or
beak ; s, lower jaw or beak ; t, lingual ribbon ; x, the viscero-pericardial sac ;
n.c, nerve-collar ; oe, cesophagus ; cr, crop ; qizz, gizzard ; int, intestine ; an,
anus ; ni, nidamental gland ; ncpt, aperture of a nephridial sac ; r.e, renal
glandular masses on the walls of the afferent branchial veins (see lig. 109);
0.6.11., afferent branchial vessel ; e.b.v, efferent branchial vessel ; vt, ventricle
of the heart.

general Molluscan characters in regard to these organs.
Whilst the general body-cavity or co;lom forms a lacunar

blood-system or series of narrow spaces, connected with
the trunks of a well-developed vascular system, that part
of the original ccelom surrounding the heart and known
as the Molluscan pericardium becomes shut off from this
general blood-lymph system, and communicates, directly in
Nautilus, in the rest through the nephridia, with the exte-
rior. In the Siphonopoda this specialized pericardial cavity
is particularly large, and has been recognised as distinct
from the blood-carrying spaces, even by anatomists who
have not considered the pericardial space of other Mollusca
to be thus isolated. The enlarged pericardium, which may
even take the form of a pair of sacs, has been variously
named, but is best known as the viscero-pericardial sac or
chamber. In Nautilus this sac occupies the whole of the
postero-dorsal surface and a part of the antero-dorsal (see
fig. 110, .r), investing the genital and other viscera which
lie below it, and having the ventricle of the heart sus-
pended in it. Certain membranes forming incomplete
septa, and a curious muscular band — the pallio-cardiac
band — traverse the sac. The four branchial advehent veins,
which in traversing the walls of the four nephridial sacs
give off, as it were, glandular diverticula into those sacs,
also give off at the same points four much larger glandular

app.

Vr.b.

f

FIG. 111.— Diagra

pi isterior secti

>ram representing a vertical approximately median antero
.ion of tie-pia oflrcinolis (from a drawing by A. G. Bourne). Thi

The

i i ep cns o . . .

lettering corresponds with that of fig. 110, with which this drawing is intended
to be compared, a, shell (here enclosed by a growth of the mantle) ; 6, the
nuchal plate (here a cartilage) ; c (the reference line should be continued

maiine-sim ij loriiiiug me sac uinea encloses me alien , t, me jmeiiui in ^
of the mantle-skirt (mouth of the pallial chamber) ; /, the pallial chamber ;
17, the vertically cut median portion of the mid-foot (siphon); i, the valve of
the siphon ; m, the two upper lobes of the fore-foot ; 11, the long prehensile
arms of the same ; o, the fifth or lowermost lobe of the fore-foot ; p, the third
lobe of the fore-foot ; q, the buccal membrane ; v, the upper beak or jaw ; s,
the lower beak or jaw ; t, the lingual ribbon ; x, the viscero-pericardial sac ;
7i.c, the nerve-collar ; cr., the crop ; aizz., the gizzard ; an, the anus ; c.t., the
left ctenidium or gill-plume; vent, ventricle of the heart; a.b.v., afferent
branchial vessel ; e.b.v, efferent branchial vessel ; re, renal glandular mass ;
n.n.a, left uephridial aperture; visc.per.apert., viscero-pericardial aperture
(see fig. 108); Ir.ti., branchial heart; app., appendage of the same; i.s., ink-
masses, which hang freely into the viscero-pericardial
chamber (fig. 109, r.e). In Nautilus the viscero-pericardial
sac opens to the exterior directly by a pair of apertures, one
placed close to the right and one close to the left posterior
nephridial aperture (fig. 101, viscjier.). This direct opening
of the pericardial sac to the exterior is an exception to what
occurs in all other Mollusca. In all other Molluscs the
pericardial sac opens into the nephridia, and through them
or the one nephridium to the exterior. In Nautilus there
is no opening from the viscero-pericardial sac into the
nephridia. Therefore the external pore of the viscero-peri-
cardial sac may possibly be regarded as a shifting of the
reno-pericardial orifice from the actual wall of the nephridial
sac to a position alongside of its orifice. Parallel cases
of such shifting are seen in the varying position of the
orifice of the ink-bag in Dibranchiata, and in the orifice
of the genital ducts of Mollusca, which in some few cases
(e.ff., Spondylus) open into the nephridia, whilst in other
cases they open close by the side of the nephridia on the
surface of the body. The viscero-pericardial sac of the

MOLLUSCA

141

I >il>ranchs is very large also, and extends into the dorsal
ividon. It varies in shape — that is to say, in the extensions
of its area right and left between the various viscera — in
different genera, but in the Decapods is largest. In an ex-
tension of this chamber is placed the ovary of Hepia, whilst
the ventricle of the heart and the branchial hearts and their
appendages also lie in it. It is probable that water is
drawn into this chamber through the nephridia, since sand
and other foreign matters are found in it. In all it opens
into the pair of nephridial sacs by an orifice on the wall of
each, not far from the external orifice (fig. 108, y, y').
There does not seem any room for doubting that each orifice
corresponds to the reno-pericardial orifice which we have
seen in the Gastropoda, and shall find again in the Lamelli-
branchia. The single tube-like nephridium and the peri-
cardium of the Pteropoda also communicate by an aperture.

The circulatory organs, blood-vessels, and blood of Nauti-
lus do not differ greatly from those of Gastropoda. The
ventricle of the heart is a four-cornered body, receiving a
dilated branchial efferent vessel (auricle) at each corner
(fig. 109). It gives off a cephalic aorta anteriorly, and
a smaller abdominal aorta posteriorly. The diagram, fig.
105, serves to show how this simple form of heart is related
to the dorsal vessel of a worm or of an Arthropod, and how
by a simple flexure of the ventricle (D) and a subsequent
suppression of one auricle, following on the suppression of
one branckia, one may obtain the form of keart charac-
teristic of the Anisopleurous Gastropoda (excepting tke
Zygobranchia). Tke flexed condition of the heart is seen
in Octopus, and is to some extent approached by Nautilus,
the median vessels not presenting that perfect parallelism
wkick is shown in tke figure (B). The most remarkable
feature presented by the heart of Nautilus is the possession
of four instead of two auricles, a feature which is simply
related to the metamerism of the branchiae. By the left
side of the keart of Nautilus, attached to it by a membrane,
and hanging loosely in tke viscero-pericardial chamber, is
the pyriform sac of Owen. This has recently been shown
to be the rudimentary left oviduct or sperm-duct, as the
case may be (Lankester and Bourne, 37), the functional
right ovi-sac and its duct being attacked by a membrane
to the opposite side of the heart.

The cephalic and abdominal aortas of Nautilus appear,
after running to the anterior and posterior extremes of the
animal respectively, to open into sinus-like spaces surround-
ing the viscera, muscular masses, &c. These spaces are
not large, but confined and shallow. Capillaries are stated
to occur in tke integument. In the Dibranchs the arterial
system is very much more complete ; it appears in some
cases to end in irregular lacunae or sinuses, in other cases
in true capillaries which lead on into veins. An investiga-
tion of these capillaries in the light of modern histological
knowledge is much needed. From tke sinuses and capil-
laries tke veins take origin, collecting into a large median
trunk (the vena cava), which in the Dibranchs as well as in
Nautilus has a ventral (postero-ventral) position, and runs
parallel to the long axis of the animal. In Nautilus this
vena cava gives off at the level of the gills four branchial
advehent veins (fig. 109, r.e.), which pass into the four
gills without dilating. In the Dibranchs at a similar posi-
tion tke vena cava gives off a right and a left branchial
advehent vein (fig. 108, r.s.v.c, r.d.e.c), each of wkick,
traversing the wall of the corresponding nephridial sac and
receiving additional factors (fig. 108, v.g, v.p.d, v.a.d, v.b.a),
dilates at the base of the corresponding branchial plume,
forming there a pulsating sac — the branchial keart (fig. 104,
x; and fig 108, c.b). Attacked to eack branchial keart is a
curious glandular body, wkich may possibly be related to
the larger masses (r.e in fig. 109) wkich depend into the
viscero-pericardial cavity from the branchial advehent veins

of Nautilus. From the dilated branchial heart the bran-
chial advehent vessel proceeds, running up the ad-pallial
face of the gill-plume (w, n1', tig. 104). From each gill-
plume the blood passes by the branchial efferent vessels
(c', fig. 104) to tke keart, tke two auricles bring formed
by tke dilatation of these vessels (v, v in fig. 104).

The blood of Siphonopoda contains the usual amceboid cor-
puscles, and a diffused colouring matter — the kamiocyanin
of Fredericque — which has been found also in the blood of
Helix, and in that of the Arthropods Homarus and Limulus.
It is colourless in the oxidized, blue in the deoxidized state,
and contains copper as a chemical constituent.

The nephridial sacs and renal glandular tissue are closely
connected with tke branchial advehent vessels in Nautilus
and in the other Siphonopoda, Tke arrangement is such
as to render the typical relations and form of a nephridium
difficult to trace. In accordance with the metamerism of
Nautilus already noticed, there are two pairs of nephridia.
Each nephridium assumes the form of a sac opening by a
pore to the exterior. As is usual in nephridia, a glandular
and a non-glandular -portion are distinguished in each sac ;
these portions, however, are not successive parts of a tube, as
happens in other cases, but they are localized areae of the wall
of the sac. The glandular renal tissue is, in fact, confined
to a tract extending along that part of the sac's wall which
immediately invests the great branchial advehent vein.
The vein in this region gives off directly from its wall a
complete herbage of little venules, which branch and ana-
stomose with one another, and are clothed by the glandular
epithelium of tke nephridial sac. The secretion is accumu-
lated in the sac and passed by its aperture to the exterior.
Probably tke nitrogenous excretory product is very rapidly
discharged ; in Nautilus a pink-coloured powder is found
accumulated in the nephridial sacs, consisting of calcium
phosphate. The presence of this
phosphatic calculus by no means
proves that such was the sole ex-
cretion of the renal glandular tis-
sue. In Nautilus a glandular
growth like that rising from the
wall of the branchial vessel into
its corresponding nephridial sac,
but larger in size, depends from
each branchial advehent vessel into
the viscero-pericardial sac, — prob-
ably identical with the "append-
age" of the branchial hearts of
Dibranchs.

The chief difference, other than
that of number between tke ne-
phridia of the Dibranchs and those
of Nautilus, is tke absence of the
accessory growths depending into
the viscero-pericardial space just
mentioned, and, of more import-
ance, the presence in tke former of Flo n2._Nervous' system of
a pore leading from tke nephridial
sac into tke viscero-pericardial sac
(y, y in fig. 108). Tke external
orifices of the nephridia are also
more prominent in Dibranchs than
in Nautilus, being raised on papillie
(np in fig. 108 ; r in fig. 103). In
Sepia, according to Vigelius (38),
the two nephridia give off each
a diverticulum dorsalwards, which
unites with its fellows and forms
a great median renal chamber,
lying between the ventral portions of tke nephridia and
the viscero-pericardial chamber. In Loligo the fusion

Nautilus pnmpiluis (from Ge-
genbaur, after Owen). /, tt
ganglion-like enlargements on
nerves passing from the pedal
ganglion to the inner series of
tentacles; C, nerves to the ten-
tacles of tlie outer or annular
lobe ; &, pedal ganglion-pair ;
a, cerebral ganglion -pair) ; c,
pleuro - visceral gan_r'i"mr
band (fused pleural and visce-
ral ganglion-pairs) ; rf, genital
pm-liim placed on the course
of the large visceral nerve, just
before it gives off its branchial
and its osphradial branches ;
m, nerves from the pl-'iiral
ganglion to the mantle-skirt.

142

MOLLUSCA

of the two nephridia to form one sac is still more obvious,
since the ventral portions are united. In Octopus the
nephridia are quite separate.

Tegumental pores have not been described in Nautilus,
but exist in Dibranchiata, and have been (probably
erroneously, but further investigation is needed) supposed
to introduce water into the vascular system. A pair of

Fig. 114.

FIGS. 113, 114.— Nerve-centres of Octopus. Figure 113 gives a view from the
dorsal aspect, figure 114 one from the ventral aspect, bnc, the bnccal mass ;
ped, pedal ganglion ; opt, optic ganglion ; txr, cerebral ganglion ; pi, pleural
ganglion ; vise, visceral ganglion ; as, oesophagus ; /, foramen in the nerve-
mass formed by pedal, pleural, and visceral ganglion-pairs, traversed by a
blood-vessel.

such pores leading into sub-tegumental spaces of consider-
able area, the nature of which is imperfectly known, exist
on the back of the head in Philonexis, Tremoctopus, and
Argonauta. At the base of the arms and mouth four such
pores are found in Histioteuthis and Ommastrephes, six
in Sepia, Loligo, Onychoteuthis. Lastly, a pair of such
pores are found in the
Decapoda at the base
of the long arms, lead-
ing into an extensive
sub-tegumental pouch
on each side of the head
into which the long
arms can be, and usually
are, withdrawn. In
(Sepia, Sepiola, and Ros-
sia the whole arm is
coiled up in these sacs ;
in Loligo only a part
of it is so ; in Histio-
teuthis, Ommastrephes,
and Onychoteuthis, the
sacs are quite small
and do not admit the
arms.

Nervous System. —
Nautilus, like the other
Cephalopoda (e.y., Pneu-
modermon, fig. 87 ;
Octopus, fig. 113), ex-
hibits a great concentra-
tion of the typical Mol-

, i- i_ Fra. 115.— Lateral view of the nervous centres

lUSCan ganglia, as Shown and nerves of the right side of Octopus i'ul-
The °"an- g^ris (from a drawing by A. G. Bourne). fy/»

in fig. 112.

glia take on a band-like
form, and are but little
differentiated from their
commissures and con-
nectives, — an archaic
condition reminding us of Chiton. The special optic out-
growth of the cerebral ganglion, the optic ganglion (fig.
112, o), is characteristic of the big-eyed Siphonopoda.
The cerebral ganglion-pair (<*) lying above the oesophagus

buccal ganglion; ccr., cerebral ganglion;
ped., pedal ganglion ; pi., pleural, and risi\,
visceral region of the pleuro-viBceral ganglion;

gang, stcll., the. right stellate ganglion of the
mantle connected by a nrnr !•> Ihr plrmM
portion; ti.visc., the right visceral nerve;
n.olf., its (probably) olfactory branches ;
n.br., its branchial branches.

is connected with two sub-cesophageal ganglion -pairs of
band-like form. The anterior of these is the pedal b, b,
and supplies the fore-foot with nerves t', t, as also the
mid-foot (siphon). The hinder band is the visceral and
pleural pair fused (compare fig. 112 with fig. 87, and
especially with the typical Euthyneurous nervous system
of LimiiiBiis, fig. 22) ; from its pleural portion nerves pass
to the mantle, from its visceral portion nerves to the
branchite and genital ganglion (d in fig. 112), and in
immediate connexion with the latter is a nerve to the
osphradium or olfactory papilla. No buccal ganglia have
been observed in Nautilus, nor has an enteric nervous system
been described in this animal, though both attain a special
development in the Dibranchiata. The figures (114 and
115) representing the nerve-centres of Octopus serve to
exhibit the disposition of these parts in the Dibranchiata.
The ganglia are more distinctly swollen than in Nautilus.
In Octopus an infra-buccal ganglion-pair are present cor-
responding to the buccal ganglion-pair of Gastropoda. In
Decapoda a supra-buccal ganglion -pair connected with
these are also developed. Instead of the numerous radi-
ating pallial nerves of Nautilus, we have in the Dibran-
chiata on each side (right and left) a large pleural
erve passing from the pleural portion of the pleuro-
visceral ganglion to the mantle, where it enlarges to
form the stellate ganglion. From each stellate ganglion
nerves radiate to supply the powerful muscles of the
mantle-skirt. The nerves from the visceral portion of the
pleuro- visceral ganglion have the same course as in Nautilus,
but no osphradial papilla is present. An enteric nervous
system is richly developed in the Dibranchiata, connected
with the somatic nervous centres through the buccal
ganglia, as in the Arthropoda through the stomato-gastric
ganglia, and anastomosing with deep branches of the vis-
ceral nerves of the viscero-pleural ganglion-pair. It has
been especially described by Hancock (39) in Omma-
strephes. Upon the stomach it forms a single large and
readily-detected gastric ganglion. It is questionable as to
how far this and the "caval ganglion" formed in some
Decapoda by branches of the visceral nerves which accom-
pany the vena cava are to be considered as the equivalents
of the "abdominal ganglion," which in a typical Gastropod
nervous system lies in the middle of the visceral nerve-loop
or commissure, having the right and left visceral ganglia
on either side of it, separated by a greater or less length
of visceral nerve-cord (see figs. 20, 21, 22). There can be
little doubt that the enteric nervous system is much more
developed in the Dibranchiata than in other Mollusca, and
that it effects a fusion with the typical " visceral " cords
more extensive than obtains even in Gastropoda, where
such a fusion no doubt must also be admitted.

Special Sense-Organs. — Nautilus possesses a pair of
osphradial papilke (fig. 101, off) corresponding in position
and innervation to Spengel's organ placed at the base of the
ctenidia (branchiae) in all classes of Mollusca. This organ
has not been detected in other Siphonopoda. In Ptero-
poda it is well developed as a single ciliated pit, although
the ctenidia are in that group aborted (fig. 87, Osp.).
Nautilus possesses other olfactory organs in the region
of the head. Just below the eye is a small triangular
process (not seen in our figures), having the structure of a
shortened and highly-modified tentacle and sheath. By
Valenciennes, who is followed by Keferstein, this is regarded
as an olfactory organ. The large nerve which runs to this
oriran originates from the point of juncture of the pedal
with the optic ganglion. The lamclliform organ upon the
inner inferior tentacular lobe of Nautilus is possibly also
olfactory in function. In Dibranchs behind the eye is a
pit or open canal supplied by a nerve corresponding in
origin to the olfactory nerve of Nautilus above mentioned.

MOLLUSCA

143

I'ossilily (lie sense of taste resides in certain processes
within the mouth of Nautilus and other Siphonopoda.

B

Flo. llfi. — Cartilaginous skeleton of Siphonopoda (after Keferstein). A. Capito-
pedal cartilage of Nautilus p&mpilius ; a points to the ridge which supports
the pedal portion of the nerve-centre. B. Lateral view of the same,— the
large anterior processes are sunk in the muscular substance of the siphon.
C. Cephalic cartilages of Sepia officinalis. D. Nuchal cartilage of Sepia affici-
•italis.

The otocysts of Nautilus were discovered by Macclonald
(40). Each lies at the side of the head, ventral of
the eye, resting on the capito-pedal cartilage, and supported
by the large auditory
nerve which arises
from the pedal gan-
glion. It has the
form of a small sac,
1 to 2 mm. in dia-
meter, and contains
whetstone - shaped
crystals, such as are
known to form the
otoliths of other Mol-
lusca. The otocysts
of Dibranchiata are
larger and deeply
sunk in .the cephalic

cartilage. It has FIO. ll".— Minute structure of the cartilage of

been shown bv Lan- Loligo (from Gegenbaur, after Furbringer). a,

simple, 6, dividing, cells : r, ranaliculi ; rf, an

kester that they de- empty cartilage capsule witli its pores; c, cauali-

velop as open pits cuU in section-

(fig. 121, (5), (6), o), which gradually close up, the com-
munication with the exterior becoming narrowed into a
fine canal, which is reflected over one end of the sac, and
finally has its external opening obliterated. A single
otolith only is found in all Dibranchiata.

The eye of Nautilus is among the most interesting struc-
tures of that remarkable animal. No other animal which
has the same bulk and general elaboration of organization
has so simple an eye as that of Nautilus. When looked
at from the surface no metallic lustre, no transparent
coverings, are presented by it. It is simply a slightly pro-
jecting hemispherical box like a kettle-drum, half an inch
in diameter, its surface looking like that of the surrounding
integument, whilst in the middle of the drum-membrane is
a minute hole (fig. 91, «). Owen very naturally thought
that some membrane had covered this hole in life, and had
been ruptured in the specimen studied by him. It, how-
ever, appears from the researches of Hensen (41) that the
hole is a normal aperture leading into the globe of the eye,
which is accordingly filled by sea-water during life. There
is no dioptric apparatus in Nautilus, and in place of refract-
ing lens and cornea we have actually here an arrangement
for forming an image on the principle of "the pin-hole
camera." There is no other eye known in the whole animal
kingdom which is so constructed. The wall of the eye-

globe is tough, and the cavity is lined solely by the naked
retina, which is lathed by sea-water on one surface and
receives the fibres of the optic nerve on the other (see fig.
118, A). As in other Siphonopoda (<•.</., tig. 120, AV, Jlc,
/>), the retina consists of two layers of cells separated by a
layer of dark pigment The most interesting consideration
connected with this eye of Nautilus is found when the
further facts are noted — (1) that the elaborate lens-bearing
eyes of Dibranchiata pass through a stage of development
in which they have the same structure as the eye of Nautilus
— namely, are open sacs (fig. 119) ; and (2), that amongst
other Mollusca examples of cephalic eyes can be found which
in the adult condition are, like the eye of Nautilus and the
developing eye of Dibranchs, simple pits of the integument,
the cells of which are surrounded by pigment and connected
with the filaments of an optic nerve. Such is the structure

C

Int*

Coep

Int

Pal.

Ir

Int3

Jut

Jf.Cif

G.ep

N.op

Fro. 118.— Diagrams of sections of the eyes of Mollusca. A. Nautilus (and
Patella). B. Gastropod (Limax or Helix). C. Dibranchiate Siphonopod
(Oigopsid). Pa/, eyelid (outermost fold) ; Co, cornea (second fold) ; Ir, iris
(third fold) ; /»(!, 2, 3, 4, different parts of the integument ; I, deep portion
of the lens ; l\ outer portion of the lens ; Co.ep, ciliary body ; /;, retina ;
N.op, optic nerve ; G.op, optic ganglion ; ar, inner layer of the retina ; Ar..S,
nervous stratum of the retina. (From Balfour, after Grenacher.)

of the eye of the Limpet (Patella); and in such a simple eye
we obtain the clearest demonstration of the fact that the
retina of the Molluscan cephalic eye, like that of the
Arthropod cephalic eye and unlike that of the Vertebrate
myelonic eye, is essentially a modified area of the general
epiderm, and that the sensitiveness of its cells to the action
of light and their relation to nerve-filaments is only a
specialization and intensifying of a property common to the
whole epiderm of the surface of the body. What, however,
strikes us as especially remarkable is that the simple form
of a pit, which in Patella serves to accumulate a secretion
which acts as a refractive body, should in Nautilus be
glorified and raised to the dignity of an efficient optical
apparatus. Natural selection has had an altogether excep-
tional opportunity in the ancestors of Nautilus. In all other
Mollusca, starting as we may suppose from the follicular or
pit-like condition, the eye has proceeded to acquire the form
of a closed sac, the cavity of the closed vesicle being then
filled partially or completely by a refractive body (lens)
secreted by its walls (fig. 118, B). This is the condition
attained in most Gastropoda. It presents a striking contrast
to the simple Arthropod eye, where, in consequence of the
existence of a dense exterior cuticle, the eye does not form
a vesicle, and the lens is always part of that cuticle.

In the Dibranchiate division of the Siphonopoda the
greatest elaboration of the dioptric apparatus of the eye
is attained, so that we have in one sub-class the extremes
of the two lines of development of the Molluscan eye, those
two lines being the punctigerous and the lentigerous. The
structure of the Dibranchiate's eye is shown in section in fig.
118, C, and in fig. 120, and its development in fig. 119 and
fig. 123. The open sac which forms the retina of the young
Dibranchiate closes up, and constitutes the posterior chamber
of the eye, or primitive optic vesicle (fig. 123, A,poc). The

144

MOLLUSCA

lens forms as a structureless growth, projecting inwards from
the front wall of this vesicle (fig. 1 23, B, I). The integument
around the primitive optic vesicle which has sunk below

A

FIG. 119. — Diagrams of sections showing the early stage of development of the
eye of Loligo when it is, like the permanent eye of Nautilus and of Patella,
an open sac. A. First appearance of the eye as a ring-like upgrowth. B.
Ingrowth of the ring-like wall so as to form a sac, the primitive optic vesicle
of Loligo. (From Lankester.)

the surface now rises up and forms firstly nearest the axis
of the eye the iridian folds (if in B, fig. 123 ; ik in fig. 120 ;
Ir in fig. 118), and then secondly an outer circular fold
grows up like a wall and completely closes over the iridian
folds and the axis of the primitive vesicle (fig. 120, C).
This covering is transparent, and is the cornea. In the
oceanic Decapoda the cornea does not completely close,
but leaves a central aperture traversed by the optic axis.
These forms are termed Oigopsidse by d'Orbigny (42), whilst
the Decapoda with closed cornea are termed Myopsidse.
In the Octopoda the cornea is closed, and there is yet
another fold thrown over the eye. The skin surrounding
the cornea presents a free circular margin, and can be drawn
over the surface of the cornea by a sphincter muscle. It
thus acts as an adjustable diaphragm, exactly similar in

ft—

t- l

Fin. 120. — Horizontal section of the eye of Sepia (Jlyopsid). KK, cephalic
cartilages (see lig. 110); O, cornea (closer!); L, lens; ct, ciliary body; Ri,
internal layer of the retina ; Re, external layer of the retina ; p, pigment
between these ; o, optic nerve ; <7<s optic gan^liun ; frand k', capsular cartilage ;
ik, cartilage of the iris ; v>, white body ; (tt1, argentine integument. (From
Gegenbaur, after Uensi-n.)

movement to the iris of Vertebrates. Sepia and allied Deca-
pods have a horizontal lower eyelid, that is to say, only
one-half of the sphincter-like fold of integument is movable.
The exact history of the later growth of the lens in the
Dibranehs' eye is not clear. As seen in fig. 120, it appears,
after attaining a certain size, to push through the front
wall of the primitive optic vesicle at the point correspond-
ing to its centre of closure, and to project a little into the
anterior chamber formed by the cornea. The wall of the

primitive optic vesicle adjacent to the embedded lens (L)
now becomes modified, forming a so-called "ciliary body,"
in which muscular tissue is present, serving to regulate the
focus of the lens (a in fig. 120). Bobretzky (43) differs
from Lankester, whose view is above given, in assigning a
distinct origin to the protruding anterior segment of the
lens (I1 in fig. 118). The optic ganglion, as well as the
other large ganglia of the Dibranchiata, originate in the
mesoblast of the embryo. The connexion between the cells
of the retina and the nerve- fibres proceeding from the optic
ganglion must therefore be a secondary one.

Chromatophores. — In Nautilus these remarkable struc-
tures, which we mention here as being intimately asso-
ciated with the nervous system, appear to be absent. In
Dibranchiata they play an important part in the economy,
enabling their possessor, in conjunction with the discharge
of the contents of the ink-bag, to elude the observation of
either prey or foe. They consist of large vesicular /ells
(true nucleated cells converted into vesicles), arranged in
a layer immediately below the epidermis. Each chroma-
tophore-cell has from six to ten muscular bands attached
to its walls, radiating from it star-wise. The contraction
of these fibres causes the chromatophore-cell to widen
out; it returns to its spherical resting state by its own
elasticity. In the spherical resting state such a cell may
measure '01 mm., whilst when fully stretched by its radiat-
ing muscles it covers an area of "5 mm. The substance
of the chromatophore-cells is intensely coloured with one
of the following colours — scarlet, yellow, blue, brown —
which are usually of the greatest purity and brilliance. The
action of the chromatophores may be watched most readily
in young Loligo, either under the microscope or with the
naked eye. The chromatophores are suddenly expanded,
and more slowly retracted with rapidly-recurring alter-
nation. All the blue, or all the red, or all the yellow
may be expanded and the other colours left quiescent.
Thus the animal can assume any particular hue, and
change its appearance in a dazzling way with extraordinary
rapidity. There is a definite adaptation of the colour
assumed in the case of Sepia and others to the colour of
the surrounding rock and bottom.

Gonads and Genital Ducts. — In Nautilus it has recently
been shown by Lankester and Bourne (37) that the genital
ducts of both sexes are paired right and left, the left duct
being rudimentary and forming the " pyriform appendage,"
described by Owen as adhering by membranous attach-
ment to the ventricle of the heart, and shown by Kefer-
stein to communicate by a pore with the exterior. Thus
the Cephalopoda agree with our archi-Mollusc in having
bilaterally symmetrical genital ducts in the case of the
most archaic member of the class. The ovary (female
gonad) or the testis (male gonad) lies in Nautilus as in
the Dibranchs in a distinct cavity walled off from the
other viscera, near the centro-dorsal region. This chamber
is formed by the cceloniic or peritoneal wall ; the space
enclosed is originally part of the coelom, and in Sepia
and Loligo is, in the adult, part of the viscero-pericardial
chamber. In Octopus it is this genital chamber which
communicates by a right and a left canal with the ncphrid-
ium, and is the only representative of pericardium. The
ovary or testis is itself a growth from the inner wall of this
chamber, which it only partly fills. In Nautilus the right
genital duct, which is functional, is a simple continuation
to the pore on the postero-dorsal surface of the membran-
ous walls of the capsule in which lies the ovary or the
testis, as the case may be. The gonad itself appears to
represent a single median or bilateral organ.

The true morphological nature of the genital ducts of the
Cephalopoda and of other Mollusca is a sul pject which invites
speculation and inquiry. In all the cases in which such

MOLLUSCA

1 45

ducts continuous with the tunic of the gonad itself occur —
viz., in Ncmatoid worms, in Arthropocla, and in Teleostean
fishes, besides Mollusca — there is an absence of definite
knowledge as to the mode of development of the duct.
It seems, however, fn>m such facts as have been ascer-
tained that the gonad lies at first freely in the ccelom,
and that the duct develops in connexion with the genital
pore, and attaches itself to the embryonic gonad, or to the
ciipsule which grows around it. The question then arises
as to the nature of the pore. In other groups of animals
we find that the pore, and funnel or tube connected with
it by which the genital products are conveyed to the
exterior, is a modified nephridium (usually a pair, one
mht and one left). Is it possible that this is also the
case where the duct very early becomes united to the
gonad, and even gives rise to the appearance of a tubular
ovary or testis ? Probably this is the case in Teleostean
fishes (see Huxley's observations on the oviducts of the
.smelt, 44) ; but it seems to be a tenable position that in other
cases, including the Mollusca, the genital pore is a simple
opening in the body-wall leading into the body-cavity
or cceloni, such as we find on the dorsal surface of the
earth-worm, which has become specialized for the extrusion
of the genital products. Possibly, as in Nemertine and
Chietopod worms, the condition preceding the development
of these definite genital pores was one in which a temporary
rupture of the body-wall occurred at the breeding season,
and this temporary aperture has gradually become perma-
nent. The absence of genital pores in Patella, and some
Lamellibranchs which make use of the nephridia for the
extrusion of their genital products, suggests that the very
earliest Mollusca or their forefathers were devoid of genital
ducts and pores. In no Mollusca, however, is the nephrid-
ium used in the same way as a genital duct as it is in the
Chaetopoda, the Gephyrsa, and the Vertebrata ; for the
open mouth of the nephridium in Mollusca leads into the
pericardial space, and it is not through this space and this
mouth that the genital products of any Mollusca enter
the nephridium (except perhaps in Neomenia), although
it is by this mouth that the genital products enter the
nephridium in the former classes above named. Hence
the arrangement in Patella, &c., in to be looked upon as a
special development from the simpler condition when the
Mollusca brought forth by rupture ( = schizodinic, from oiSi's,
travail), and not as derived from the common arrangement
of adaptation of a nephridium to the genital efferent func-
tion ( = nephrodinic).1

The functional oviduct of Nautilus forms an albumin i-
parous gland as a diverticulum, which appears to corre-
spond to a dilatation in the male duct, which succeeds the
testis itself, and is called the " accessory gland." The male
duct has a second dilatation (Needham's sac), and then is
produced in the form of a large papilla. In Dibranchs
the genital ducts are but little more elaborated. They are
ciliated internally. In female Octopoda, in Ommastrephes,
and in one male Octopod (Eledone moschatu) the genital
ducts are paired, opening right and left of the anus. But
in all other Dibranchs a single genital duct only is deve-
loped, viz., that of the left side, and leads from the genital
capsule or chamber of the gonad to an as}'mmetrically-placed
pore. In the male Dibranchs the genital duct is coiled
and provided with a series of glandular dilatations and

1 Ceelomate aDimals are, according to this nomenclature, either
Schizodinic or Porodinic. The Porodinic group is divisible into Ne-
phrodiuic and Idiodinic, in the former the uephridium serving as a
pore, in the latter a special (i'Sios) pore being developed. In each of
these latter groups the pore maybe — (1) devoid of a duct, (2) provided
with a duct which is unattached to the gonad and opens into the body-
cavity, (3) provided with a duct which fuses with the gouad. The
genital ducts of Idiodinic forms may be called Idiogonaducts, as dis-
tinguished from the Nephrogonaducts of nephrodinic forms.

receptacles. These are connected with the formation of
the spennatophores. In the Siphonopoda the ,spi-nn:iiir
fluid does not flow as a liquid from the genital pore, but
the spermatozoa are made up into little packets hi-fnro
extrusion. In other Mollusca (I'ulmonata) and in other
animals (Chaitopoda) this formation of " sperm-ropes " is
known, but in the Siphonopoda it attains its highest
development. Exceedingly complicated structures of a
cylindrical form (sometimes an inch in length) are funned
in the male genital duct by a secretion which embeds and
cements together the spermatozoa. They are formed in
Nautilus as well as in Dibranchs, the actual manner in which
their complicated structure is produced being not easily con-
jectured. Accessory glands not forming part of the oviduct,
but furnishing the material for enclosing the eggs in an elastic
envelope, are found as paired structures, opening some way
behind the anus in Nautilus (101, ff.n.) and in the Di-
branchs. They are known as the nidamental glands. In
the female Sepia they are particularly large and prominent,
and are accompanied by a second smaller pair.

Reproduction and Development. — The details of sexual
congress and of the actual fertilization of the egg are quite
unknown in Nautilus, and imperfectly in the Dibranchs
and the Pteropoda. Allusion has already been made to
the subject in connexion with the hectocotylized arm. The
mature eggs of Nautilus are unknown, as well as the appear-
ance which they present when deposited. In the Dibranchs
the eggs are always very large ; in some cases the amount
of food-yelk infused into the original egg-cell is so great as
to give it the size of a large pea. This results in that
mode of development which is only known outside this
class among the Vertebrata ; it is discoblastic. The proto-
plasm of the fertilized egg-cell segregates to one pole of
the egg, and there undergoes cell-division, resulting in the
formation of a disc of cleavage cells (fig. 121, (1)) resem-
bling the cicatricula of the hen's egg, which subsequently
spreads over and invests the whole egg (fig. 121, (2)). For
details of this process we must refer the reader to other
works (45, 46); but it may here be noted that in addition to
the layer of cleavage cells, which consists of more than one
stratum of cells in the future embryonic area as opposed
to the yelk-sac area, additional cells are formed in the
mass of residual yelk apparently by an independent process
of segregation, each cell having a separate origin, whence
they are termed " autoplasts." The autoplasts eventually
form a layer of fusiform cells (fig. 121, (7), h; fig. 122, m;
and fig. 123, ps), — the "yelk-membrane" which everywhere
rests upon and encloses the residual yelk. The cleavage
cells form a single layer on the yelk-sac area and two layers
on the embryonic area, an outer layer one cell deep (fig. 122,
ep), and an inner — the middle layer of the three — which
is often thick and many cells deep (fig. 122, m). There is
great difficulty here in identifying the layers with the three
typical layers of other animal embryos, except in regard
to the outermost, which corresponds with the epiblast of
Vertebrates in many respects. The middle layer, however,
gives rise to the nerve-ganglia as well as to the muscles,
ccelom, and skeleto-trophic tissues, and to the mid-portion
of the alimentary canal with its hepatic diverticula, the
liver (see fig. 121, (7) and explanation, where the origin of
the mid-gut as a vesicle r is seen). It is clearly, therefore,
something more than the mesoblast of the Vertebrate,
giving rise, as it does, to important organs formed both by
epiblast and hypoblast in other animals. Lastly, the yelk-
membrane, though corresponding to the Vertebrate hypo-
blast in position and structure, furnishes no part of the
alimentary tract, but disappears when the yelk is com-
pletely absorbed. In fact, the developmental phenomena
in Sepia, Loligo, and Octopus are profoundly perturbed by
the excessive proportion of food-yelk. Balfour has shown

T

146

MOLLUSCA

that in the chick the orifice of closure of the overspreading
blastoderm does not represent the whole of the blastopore,

(1)

(7)

FIG. 121.— Development of Loligo. (1) View of the cleavage of the egg during
the first formation of embryonic cells. (2) Lateral view of the egg at a little

ce protrusion of the median portion of tl

the yelk or food-material lying within it. (Original.)

but that this is in part to be sought in the widely-separated
primitive streak. The present writer has little doubt that

a structure corresponding to the primitive streak of the
chick, and lying near the klastic pole, will be found in
Sepia and Loligo, and the strange vesicular origin of the
mid-gut will be traced to and explained by it.

Leaving this difficult question of the cell-layers of the
embryo, we would draw the reader's attention to the series
of sketches representing the semi-transparent embryo of
Loligo, drawn in fig. 121. When the cleavage cells have
nearly enclosed the yelk, the upper or embryonic area
shows the rudiments of the centro-dorsal mantle-sac or
pen-sac, the mouth, the paired optic pits, and the paired

Fir:. 122. — Section through the still open shell-sac occupying the centro-rlorsal
area of an embryo of Loligo ; the position is inverted as compared with fig.
}'2l (3) and (7). fp, outer cell-layer; m, middle cell-layer; in', deep cell-
layer of fusiform cells ; i/, the granular yelk or food-material of the egg ;
shs, the still open shell-sac. (From Laukester.)

otic pits (fig. 121, (3), (5)). The eye-pits close up (fig.
119), the orifice of the mantle-sac narrows, and its margin
becomes raised and freely produced as mantle-skirt ;
at the same time an hour-glass-like pinching in of the
whole embryo commences, separating the embryo proper
from the so-called yelk-sac (fig. 121, (4) ). Around the
" waist " of constriction, pair by pair, ten lobes arise (fig.
121, (8)), — the arms of the fore-foot. It now becomes
obvious that the yelk-sac is but the median surface of the
fore-foot bulged out inordinately by food-yelk, just as the
hind region of the foot is in the embryo slug (see fig. 72**,
and explanation). Just as in the slug, this dilated yelk-
holding foot is rhythmically contractile, and pulsates
steadily over the area g in fig. 121, (4). At this stage,
and long subsequently, the mouth of the young C'ephalopod
is in no way surrounded by the fore-foot, but lies well
above its nascent lobes (e in fig. 121). Subsequently it
sinks, as it were, between the right and left most anterior
pair of the series, which grow towards one another and
fuse above it, and leave no trace of their original position
and relations. Fig. 121, (6) gives a view of the postero-
dorsal surface of an embryo, in which the important fact
is seen of the formation of the funnel or siphon by the
union of two pieces ('/), which grow1 up each independently,
one right and one left, like the sides of the siphon of
Nautilus or the swimming lobes of a Pteropod, and subse-
quently come together, as shown in (9), where the same
letter q indicates the same part. The explanations of figs.
121 and 123 are given very full, and here, therefore, we
shall only allude to two additional points. A curious mass
of tissue of unknown significance occurs in the orbit of
Bibranehs, known as the white body (w in fig. 120). A
strongly-marked invagination just above the orbit is a very
prominent feature in the embryo of Loligo, Sepia, and
Octopus, and appears to give rise to this so-called white
body. This invaginated portion of the outer cell-layer is
seen in fig. 121, (8) and (9), lettered c ; in fig. 123, A and
B, it is lettered wb.

Lastly, in fig. 123, A, the origin of the optic nerve-
ganglion ny from the cells of the middle layer should be
especially noticed. In some other Molluscs the nerve-
ganglia have been definitely traced to the outer cell-layer,

MOLLUSCA

147

whilst in some Gastropods, according to Bobretzky, they
originate, as here shown, for Loligo.

'I'll,. ,. ^-coverings of the Diliranchiato arc very coniiilete.
Argouauta and Octopus deposit each egg in a firm oval
case, thin and transparent, which has a long stalk by
which (in Octopus) the egg is fixed in company with two
or three hundred others to some foreign object. Sepia
encloses each egg in a thick envelope of many layers
resembling india-rubber. Loligo encloses many rows of
eggs in a copious tough jelly, and affixes a dozen or twenty
such egg-strings to one spot. Sepia and Loligo desert
their eggs when laid. The female Octopus most jealously

of

mtf

FIG. 123.— Right and left sections tkrongh embryos of Loligo. A. Same stage
as tig. 121 (4). B. Same stage as fig. 121 (S) ; only the left side of the sections
is drawn, and the food-material which occupies the space internal to the
membrane yin is omitted, al, rectum ; t>, ink-sac ; ep, outer cell-layer ; mes,
middle cell-layer; y?», deep cell-layer of fusiform cells (yelk-membrane); ngt
optic nerve-ganglion ; ot, otocyst ; wbt the " white body" of the adult ocular
capsule forming as an imagination of the outer cell-layer ; nttf, mantle-skill ;
p, gill ; ps, pen-sac or shell-sac, now closed ; cf j/, dorsal groove ; poc, primitive
optic vesicle, now closed (see fig. 119) ; ?, lens ; r, retina ; soc, second or anterior
optic chamber still open ; if, iridean folds. C. The primitive imagination to
form one of the otocysts, as seen in fig. 121 (5) and (6). (After Lankester.)

guards them, building a nest of stones and incubating.
Argonauta carries hers with her in a special brood-holding
shell.

The development of the Pteropoda, so far as is known,
presents no points of contact with that of the Siphonopodu
rather than with that of the Gastropoda, owing to the fact
that in them the egg has not an excess of food-yelk. Con-
sequently, we find typical trochosphere and veliger larva?
among the Thecosomata (fig. 8, C, and fig. 81), whilst the
isolated observation of Gegenbaur has made known very
remarkable larvre referable to the Gymnosomata, and with
little doubt to Pneumodennon (fig. 84). The former set of
larva.- are sufficient to demolish once for all the view which
has been entertained by some zoologists, viz., that the velar
disc of the veliger larva is the same thing as the ptero-
podial lobes of the mid-foot of Pteropoda. The latter
larva: are of importance in showing that, as in embryo
Siphonopods so in embryo Pteropods, the sucker-bearing
lobes of the fore-foot are truly podial structures, and only
embrace the head and surround the mouth as the result of
late embryonic growth.

BRANCH ¥,.— LIPOCEPHALA.

Characters. — Mollusca with the head region undeveloped.
No cephalic eyes are present ; the buccal cavity is devoid

of biting, rasping, or prehensile organs. The animal is
sessile, or endowed with very feeble locomotive powers.
The Lipocephala comprise, but one class, the Lamelli-
branehia, also known as Elatobranchia and Conchifera.

Class LAMELLIBRANCHIA.

Characters. — Lipocephala in which the archaic IULA-
TERAL SYMMETRY of the Mollusca is usually fully retained,
and raised to a dominant feature of the organization by the
lateral compression of the body and the development of the
shell as two bilaterally symmetrical plates or valves cover-
ing each one side of the animal. The FOOT is commonly a
simple cylindrical or ploughshare-shaped organ, used for
boring in sand and mud, and more rarely presents a crawl-
ing disc similar to that of Gastropoda ; in some forms it is
aborted. The paired CTENIDIA are very greatly developed
right and left of the elongated body, and form the most
prominent organ of the group. Their function is chiefly
not respiratory but nutritive, since it is by the currents
produced by their ciliated surface that food -particles are
brought to the feebly-developed mouth and buccal cavity.

The Lamellibranchia present as a whole a somewhat
uniform structure, so that, although they are very numerous,
it is not possible to divide them into well-marked sub-classes
or sections, and orders. The chief points in which they
vary are — (1) in the structure of the ctenidia or branchial
plates ; (2) in the presence of one or of two chief muscles,
the fibres of which run across the animal's body from one
valve of the shell to the other (adductors) ; (3) in the greater
or less elaboration of the posterior portion of the mantle-
skirt so as to form a pair of tubes, by one of which water
is introduced into the sub-pallial chamber, whilst by the
other it is expelled ; (4) in the perfect or deficient symmetry
of the two valves of the shell and the connected soft parts,
as compared with one another ; (5) in the development of
the foot as a disc-like crawling organ (Area, Nucula, Pectun-
culus, Trigonia, Lepton, Galeomma), as a simple plough-
like or tongue-shaped organ (Unionacea, ifec.), as a re-curved
saltatory organ (C'ardium, itc.), as a long burrowing cylin-
der (Solenacea, &c.), or its partial (Mytilacea) or even com-
plete abortion (Ostracea).

The essential Molluscan organs are, with these excep-
tions, uniformly well developed. The MANTLE-SKIRT is
always long, and hides the rest of the animal from view, its
dependent margins meeting in the middle line below the
ventral surface when the animal is retracted ; it is, as it
were, slit in the median line before and behind so as to
form two flaps, a right and a left ; on these the right and
the left calcareous valves of the shell are borne respectively,
connected by an uncalcified part of the shell called the
ligament. In many embryo Lamellibranchs a centro-dorsal
PRIMITIVE SHELL-GLAND or follicle has been detected (figs.
8 and 151). The MOUTH lies in the median line anteriorly,
the ANUS in the median line posteriorly.

Both CTENIDIA right and left are invariably present, the
axis of each taking origin from the side of the body as in
the schematic archi-Mollusc (see fig. 1 and fig. 131). A
pair of NEPHRIDIA opening right and left, rather far forward
on the sides of the body, are always present. Each opens
by its internal extremity into the pericardium. A pair c <(
GENITAL APERTURES, connected by genital ducts with the
paired gonads, are found right and left near the nephridial
pores, except in a few cases where the genital duct joins
that of the nephridium (Spondylus). The sexes are often,
but not always, distinct. No accessory glands or copulatory
organs are ever present in Lamellibranchs. The ctenidia
often act as brood-pouches.

A dorsal contractile HEART, with symmetrical right and
left auricles (fig. 143, A) receiving aerated blood from the
ctenidia and mantle-skirt, is present, being unequally de-

148

MOLLUSCA

veloped only in those few forms which are inequivalve.
The typical PERICARDIUM is well developed. It appears,
as in other Mollusca, not to be a blood-space although
developed from the cceloin, and it communicates with the
exterior by the pair of nephridia. As in Cephalopoda (and
possibly other Mollusca) water can be introduced through
the nephridia into this space. The ALIMENTARY CANAL
keeps very nearly to the median vertical plane whilst ex-
hibiting a number of flexures and loopings in this plane.
A pair of large glandular outgrowths, the so-called "liver"
or great digestive gland, exists as in other Molluscs. A
pair of pedal OTOCYSTS, and a pair of OSPHEADIA at the
base of the gills, appear to be always present. A typical
NERVOUS SYSTEM is present (fig. 144), consisting of a
cerebro-pleuro-visceral ganglion-pair, united by connectives
to a pedal ganglion-pair and an osphradial ganglion-pair
(parieto-splanchnic).

A special cascum connected with the pharynx is some-
times found, containing a tough flexible cylinder of trans-
parent cartilaginous appearance and unknown significance,
called the "crystalline style" (Mactra), which possibly
represents the radular sac of Glossophora. In many Lamelli-
branchs a gland is found on the hinder surface of the foot
in the mid line, which secretes a substance which sets into
the form of threads — the so-called " byssus " — by means of
which the animal can fix itself. Sometimes this gland is
found in the young and not in the adult (Anodon, Unio,
Cyclas). In some Lamellibranchs (Pecteu, Spondylus,
Pholas, Mactra, TeUina, Pectunculus, Galeomma, &c.),
although cephalic eyes are always absent, special eyes
are developed on the free margin of the mantle-skirt,
apparently by the modification of tentacles which are
commonly found there (fig. 145). The existence of pores
in the foot and elsewhere in Lamellibranchia by which liquid
can pass into and out of the vascular system, although
asserted as in the case of other Mollusca, appears to be
improbable. It has yet to be shown by satisfactory micro-
scopic sections that the supposed pores are anything but
epidermal glands.

The Lamellibranchia live chiefly in the sea, some in
fresh waters. A very few have the power of swimming by
opening and shutting the valves of the shell (Pecten, Lima) ;
most can slowly crawl or rapidly burrow ; others are, when
adult, permanently fixed to stones or rocks either by the
shell or the byssus. In development some Lamellibranchia
pass through a free-swimming trochosphere stage with pros-
oral ciliated band ; other fresh-water forms which carry the
young in brood-pouches formed by the ctenidia have sup-
pressed this larval phase.

The following classification and enumeration of genera
are based primarily upon the characters of the adductor
muscles. The Heteromya and Monomya must be conceived
of as derived from forms resembling such Gastropodous
Isomya as Nucula and Trigonia, which undoubtedly are
the nearest living representatives of the ancestral Lipo-
cephala, and bring us nearest to the other branch of the
Mollusca, the Glossophora.

Order 1. — Isomya.

Character. — Anterior and posterior adductor muscles of approxi-
mately eijual size.

Sub-order 1. — Integripallia.

Characters. — Marginal attachment of the mantle to the shell not
inflected to form a sinus ; siphons not developed in some, present
in most.
Family 1. — Arcacea.

Genera: Area, L. (fig. 132) ; Cuctillxa, Lam. ; Pcctunculus, Lam. ;
Limopsis, Sassi ; Nueufa, Lam. (fig. '134) ; Isoarca, Minister :
Lcda., Scliu. ; 1'ulilin, Mull. ; Sulcnella, Sowerby, &c.
Family 2. — Trii/on iacea.
Genera : Trigonia, Brug. ; Axinus, Sow. ; Lyrodcsma, Conrad.

Family 3. — Unionacea.
Genera : Unio, Retz. ; Castalia, Lam. ; Anodon, Cuv. (figs. 124,

&c. ); Iridina, Lain. ; Mycctopus, d'Orb., &c.
Family 4. — Lucinaeea.

Genera : Lucina, Brug. ; Corlris, Cuv. ; Diplodonta, Brown ;
K 'Mia, Turtou ; Montacuta, Turtou ; Lepton, Turton ; Gale-
omma, Turton ; Astartc, Sow. ; C'mssatclla, Lam. ; Cardinia,
Ag. ; Cardita, Brug., &c.
Family 5. — Cyprinacca.

Genera : Tridacna, Da C. ; Chama, L. ; Dimya, Ron. ; Diccras,
Lk. ; Isocardia, Lam. ; Eippopodium, Sow. ; Cardium, L. ;
Corbicula, Meg. ; Cyrena, Lk. ; Cydas, Brug. (fig. 146) ; Pisid-
ium, Pfr. (figs. 148-153) ; Cyprina, Lam., &c.

Sub-order 2. — Sinupallia.

Cliaraetcrs. — Marginal attachment of the mantle to the shell in-
flected so as to form a sinus into which the pallial siphons can be
withdrawn ; siphons always present, and large.
Family 6. — Vencracca.

Genera : Cypricardia, Lam. ; Tapes, Megl. ; Cyclina, Desh. ;
Cythcrca, Lam. (figs. 125, &c.) ; Chioiic, Megl. ; Venus, L. ;
Lucinopsis, F. H. ; Sanijuinolaria, Lam. ; Psammobia, Lani.
(fig. 130) ; Tcllina, L. ; Donax, L. ; Scrobiciilaria, Schu. ;
Cumingia, Sow. ; Raiirjia, Dsml. ; Mactra, L. (fig. 140) ; Trigo-
nella, Da C. ; Vaganclla, Gr. ; Lutraria, Lam.
Family 7. — Myacca.

Genera : Myochama, Stb. ; Chamostrca, Rois ; Pandora, Sol. ;
Thraeia, Leach ; Thetis, Sow. ; Pholadomya, Sow. ; Corlntla,
Brug. ; Mya, Lam. ; Saxicava, Fleur ; Panopsea, Ad. ; Glyci-
mcris, Lam. ; Siliqua, Mhlf., &c. ; Sokn, L.
Family 8. —Pholadacca.

Genera : Clavagella, Lam. ; Aspcrgillum, Lam. (figs. 128, 129) ;
Humphrcyia, Gr. ; Pholas, L. ; Pholadidea, Turt. ; Teredo, L. ;
Teredina, Lam. ; Furcella, Okeu, &c.

Order 2. — Heteromya.

Characters. — Anterior adductor (pallial adductor) much smaller
than the posterior adductor (pedal adductor) ; siphons rarely present.
Family 1. — Mytilaeea.

Genera : Mytilus, L. (fig. 133) ; Modiola, Lam. ; Crenella, Brown ;
Lithodomus, Cuv. ; Drcissena, Ben. (fig. 136) : Modiolarca,
Gr., &c.

Family 2. — Miitteriacea.
Genera : Acthcria,, Lam. ; Mullcria, Fer.

Order 3. — Monomya.

Characters. — Anterior adductor absent in the adult; siphons
never developed.
Family 1. — Avieulacea.

Genera : Cardiola, Brdp. ; Avicula, III. ; Malleus, Lam. ; Ino-

ceramiis, Sow. ; Crcnatula, Lam. ; Pcrna, Brug., &c.
Family 2. — Ostracea.

Genera: Ostrca, L. (fig. 6); Anmnia, L. ; Spondylus, L. ; Plicatula,
Lam. ; Vulsella, Lam. ; Lima, Brug. ; Pecten, L. ; Hiunitcs,
Dfr., &c.

Further Remarks on the Lamellibranchia. — The Lamelli-
branchia are the only members of the Lipocephalous branch
of Mollusca existing at the present day; and we must
suppose that, whilst on the one hand the earliest Glosso-
phorous forms were developing from the archi-Mollusca by
the elaboration of the buccal apparatus, the bivalved sessile
Lamellibranchs were developing in another direction from
univalve cephalophoroua ancestors. The large bilobed
mantle-flap with its pair of shells covering in the whole
animal, the current-producing largely-expanded ctenidia,
and the reduced cephalic region are characters which go
hand in hand, and were simultaneously acquired, each being
related to the development of the others. Unless the
" crystalline style " of Lamellibranchs is to be considered
as the rudiment of the " radular sac " of Glossophora, as
suggested by Balfour, there is no indication whatever that
the ancestors of the Lamellibranchia had acquired a repre-
sentative of the buccal apparatus — so highly developed in
Glossophora — before diverging from the archi-Mollusca ;
that is to say, the common ancestors of the two great
branches of Mollusca presented the distinctive character
of neither branch — they had not an aborted cephalic region,
and they had not a lingual ribbon.

As an example of the organization of a Lamellibranch,
we shall review the structure of the Common Pond-Mussel
(Anodunta tygnea), comparing its structure with those of

MOLLUSOA

14!)

other Lamellibranchia. The Swan Mussel lias superficially
a perfectly-developed bilateral symmetry. The leftside of
the animal is seen as when removed from its .shell in fig.
124 (1). The valves of the shell have been removed liy
severing their adhesions to the muscular area1 //, i, k, I, in, u.

(I)

(6)

FIG. 124. — Diagrams of the external form and anatomy of A nodoiita. ci/tjnca, the
Pond- Mussel ; in all the figures the animal is seen from the left side, the
centro-dorsal region uppermost, as in the drawings of fig. 75, which compare.
(1) Animal removed from its shell, a probe ff passed into the sub-pallial
chamber through the excurrent siphoiial notch. (2) View from the ventral
surface of an Auodon with its foot expanded and issuing from between the
gaping shells. (3) The left mantle-flap reflected upwards so as to expose the
sides of the hi>dy. (4) Diagrammatic section of Anodon to show the course of
the alimentary canal. (5) The two gill-plates of the left side reflected upwards
so as to expose the fissure between foot and gill where the probe </ ]i;is>rs.
(a) Diagram to show the positions of the nerve-ganglia, hear!, ami aepnridia.
Letters in all the figures as follows : — cr, centro-dorsal area ; bt margin of
the left mantle-nap ; c, margin of the right mantle-flap ; d, excurrent siphoual
notch of the mantle margin ; e, incurreut siphonal notch of the mantle
margin ; / foot ; g, probe passed into the superior division of the sub-pallial
chamber through the excurrent siphonal notch, and issuing by the side of
the foot into the inferior division of the sub-pallial chamber ; h, anterior
(pallia!) adductor muscle of the shells ; i, anterior retractor muscle of the
foot; k, protractor muscle of the foot; ?, posterior (pedal) adductor muscle
of the shells ; m, posterior retractor muscle of the foot ; «, anterior labial
tentacle ; o, posterior labial tentacle ; p, base-line of origin of the reflected
mantle-flap from the side of the body; </, left external gill-plate; r, left in-
ternal gill-plate ; rr, inner lamella of the right inner gill-plate ; rg, right outer
gill-plate ; s, line of concrescence of the outer lamella of the left outer gill-
plate with the left mantle-flap ; t, pallial tentacles ; v, the thickened mus-
cular pallial margin which adheres to the shell and forms the pallial line of
the left side ; v, that of the right side ; w, the mouth ; a-, aperture of the left
organ of Bojanus (nepliridium) exposed by cutting the attachment of the
inner lamella of the inner gill-plate ; y, aperture of the genital duct ; z, fissure

inner gill-plate with the inner lamella of the left inner gill-plate ; ab, nc, ud,
three pit-like depressions in the median line of the foot supposed by some
writers to be pores admitting water into the vascular system ; at, left shell
valve ; a/, space occupied by liver ; ag, space occupied by gonad ; ah, muscular
substance of the foot ; tci, duct of the liver on the wall of the stomach ; ok,
stomach ; al, rectum traversing the ventricle of the heart ; am, pericardium ;
on, glandular portion of the left nephridium ; nji, ventricle of the In-art ; «./,
aperture by which the left auricle joined the ventricle ; ar, non-glandular por-
tion of the left nephridium ; «s, anus ; at, pore leading from the pericardium into
the glandular sac of the left nephridium ; o », pore leading from the glandular
into the non-glandular portion of the left nephridium ; a r, internal pore lead-
ing from the non-glandular portion of the left nephriiliuin tn the exti-i-nal
pore x; aw, left cerebro-pleuro-visceral ganglion; ax, left, pedal ganglion;
ay, left otocyst ; ar, left olfactory ganglion (parieto-splanclmic) ; hh, floor of
the pericardium separating that space from the non-glandular portion of the
nephridia.

The free edge of the left half of the mantle-skirt li is repre-
sented as a little contracted in order to show the exactly simi-
lar free edge of the right half of the mantle-skirt c. These
edges are not attached to, although they touch, one another ;
each flap (right or left) can be freely thrown back in the way
which has been carried out in fig. 124, (3) for that of the
left side. This is not always the case with Lamellibranchs ;
there is in the group a tendency for the corresponding
edges of the mantle-skirt to fuse together by concrescence,

and so to form a. more or less completely dosed bag, as in
the Scaphopoda (J>i-iil;iliimi). In this way the notches
</, <: of the. hinder part of the mantle-skirt of Anodon are in
the Siphonate forms converted into two separate holes, the
edges of the mantle being elsewhere fused together along
this hinder margin. Further than this, the part of the
mantle-skirt bounding the two holes is frequently drawn out
so as to form a pair of tubes which project from the shell (figs.
130, 141). In such Lamellibranchs as the oysters, scallops,
and many others which have the edges of the mantle-skirt
quite free, there are numerous tentacles upon those edges.
In Anodon these pallial tentacles are confined to a small area
surrounding the inferior siphonal notch (fig. 124, (3), t).

The centro-dorsal point a of the animal of Anodonta
(fig. 124, (1)) is called the umbonal area ; the great anterior
muscular surface h is that of the anterior adductor muscle,
the posterior similar surface i is that of the posterior
adductor muscle ; the long line of attachment u is the
simple "pallial muscle," — a thickened ridge which is seen
to run parallel to the margin of the mantle-skirt in this
Lamellibraneh. In some of the Siphonate Isomya, which
are hence termed " Sinupallia," the pallial muscle is not
simple but deeply incurved at the posterior region so as to
allow of the large pallial siphons being retracted within the
shell or expanded at will (fig. 127, and figs. 140, 141).

It is the approximate equality
in the size of the anterior and
posterior adductor muscles which
has led to the name Isoyma for
the group to which Anodon be-
longs. The hinder adductor
muscle may be considered as re-
presenting morphologically the
transverse fibres of the root of
the foot of Nautilus by which it-
adheres to its shell (fig. 91, /.•), the
annular muscular area of Patella
(fig. '27, c), and the columella muscle
of the Gastropods generally. It
is always large in Lamellibranchs,
but the anterior adductor may
be very small (Heteromya), or FIG- 125.— View of the two valves

.J /IT <>f the shell of Cytherea(oneof

absent altogether (Monomya). the Sinupalliate Isomya), from

The anterior adductor muscle is the dorsal aspect,
in front of the mouth and alimentary tract altogether,
and must be regarded as a special and peculiar deve-
lopment of the median anterior part of the mantle-flap

umbo

hmalc

w.idlli

liyament-

FIG. 120.— Right valve of the same shell from the outer face.

in Heteromya and Isomya. Amongst those Lamelli-
branchs which have only a posterior adductor (Monomya),
it is remarkable that the oyster has been found (by
Huxley) to possess, when the young shells and muscles
first develop, a well-marked anterior adductor as well as a
posterior one. Accordingly there is ground for supposing

150

MOLLUSCA

that the Monomya have been developed from Isomya-
like ancestors, and have lost by atrophy their anterior
adductor. The single adductor muscle of the Monomya
is separated by a
difference of fibre
into two portions,
but neither of these
can be regarded as
possibly represent
ing the anterior
adductor of the
other Lamelli-
branchs. One of
these portions is
more ligamentous,
and serves to keep
the two shells con-
stantly attached to

one another, whilst Flo. 127.— Left valve of the same shell from the inner

the more fleshypor-

tion serves to close the shell rapidly when it has been gaping.
In removing the valves of the shell from an Anodon, it
is necessary not only to cut through the muscular attach-
ments of the body-wall to the shell but to sever also a
strong elastic ligament, or spring resembling india-rubber,
joining the two shells about the umbonal area. The shell
of Anodon does not present these parts in the most strongly
marked condition, and accordingly our figures (figs. 125,
126, 127) represent the valves of the Sinupalliate genus
Cytherea. The corresponding parts are recognizable in
Anodon. Eeferring to the figures (125, 126) for an ex-
planation of terms applicable to the parts of the valve and
the markings on its inner surface — corresponding to the
muscular area which we have already noted on the surface
of the animal's body — we must specially note here the posi-
tion of that denticulated thickening of the dorsal margin
of the valve which is called the hinge (fig. 127). By this
hinge one valve is closely fitted to the other. Below this
hinge each shell becomes concave, above it each shell rises a
little to form the umbo, and it is into this ridge-like upgrowth
of each valve that the elastic ligament or spring is fixed (fig.
127). As shown in the diagram (fig.
127:f) representing a transverse sec-
tion of the two valves of a Lamelli-
branch, the two shells form a double
lever, of which the toothed-hinged is
the fulcrum. The adductor muscles
placed in the concavity of the shells a
act upon the long arms of the lever
at a mechanical advantage ; their con-
traction keeps the shells shut, and
stretches the ligament or spring h.
On the other hand, the ligament /(
acts upon the short arm formed by
the umbonal ridge of the shells ; when-
ever the adductors relax, the elastic
substance of the ligament contracts,
and the shells gape. It is on this
account that the valves of a dead La-
mellibranch always gape ; the elastic
ligament is no longer counteracted by
the effort of the adductors. The state
of closure of the valves of the shell is
not, therefore, one of rest ; when it is
at rest — that is, when there is no
muscular effort — the valves of a Lamellibranch are slightly
gaping, and are closed by the action of the adductors when
the animal is disturbed. The ligament is simple in Anodon ;
in many Lamellibranchs it is separated into two layers, an
outer and an inner (thicker and denser). That the condition

shells, ligament, and ad-
ductor muscle, a, 6, right
and left valves of the
shell ; c, (I, the ombonos
or short ai ms of the lever ;
e, f, the long arms of the
lever; fj, the hin^r ; h, Ihc
]i-:uiir!it ; i, the adductor
muscle.

of gaping of the shell-valves is essential to the life of the
Lamellibranch appears from the fact that food to nourish
it, water to aerate its blood, and spermatozoa to fertilize
its eggs, are all introduced into this gaping chamber by
currents of water, which are set going by the highly-
developed ctenidia. The current of water enters into the
sub-pallial space at the spot marked e in fig. 124, (1),
and, after passing as far forward as the mouth w in fig. 1 24,
(5), takes an outward course and leaves the sub-pallial
space by the upper notch d. These notches are known
in Anodon as the afferent and efferent siphonal notches
respectively, and correspond to the long tube-like afferent
inferior and efferent superior " siphons " formed by the
mantle in many other Lamellibranchs (fig. 130).

Whilst the valves of the shell are equal in Anodon we
find in many Lamellibranchs (Ostrasa, Chama, Corbula, etc.)
one valve larger, and the other smaller and sometimes flat,
whilst the larger shell may be fixed to rock or to stones
(Ostrsea, &c.). A further variation consists in the develop-
ment of additional shelly plates upon the dorsal line be-
tween the two large valves (Pholadidaj). In P/iolas dadylus
we find a pair of umbonal plates, a dors-umbonal plate and
a dorsal plate. It is to be remembered that the whole of the
cuticular hard product produced on the dorsal surface and
on the mantle-flaps is to be regarded as the "shell," of
which a median band-like area, the ligament, usually remains
uncalcified, so as to result in the production of two valves
united by the elastic ligament. But the shelly substance
does not always in boring forms adhere to this form after
its first growth. In Aspergillum the whole of the tubular
mantle area secretes a continuous shelly
tube, although in the young condition two
valves were present. These are seen (fig.
129) set in the firm substance of the adult
tubular shell, which has even replaced the
ligament, so that the tube is complete. In
Teredo a similar tube is formed as the animal
elongates (boring in wood), the original shell-
valves not adhering to it but remaining mov-
able and provided with a special muscular
apparatus in place of a ligament.

Let us now examine the organs which lie
beneath the mantle-skirt of Anodon, and are
bathed by the current of water which cir-

Fig. 12S. Fig. 129.

Fin. 128.— Shell of Aspergillum mginijeritm(trum Owen).

Fio. 12'.'. — Shell of Asi:<'ryilhim v<njintfcrinn to show the original valves n, now
embedded in a continuous calcification of tubular form (from Owen).

culates through it. This can be done by lifting up and
throwing back the left half of the mantle-skirt as is re-
presented in fig. 124, (3). We thus expose the plough-
like foot ( /'), the two left labial tentacles, and the two
left gill-plates or left ctenidium. In fig. 124, (5), one of
the labial tentacles n is also thrown back so as to show

MOLLUSCA

151

the mouth w, and the two left gill-plates are reflected
so as to show the gill-plates of the right side (rr, nj) pro-
jecting behind the foot, the inner or median plate of each
side being united by concrescence to its fellow of the
opposite side along a continuous line (no). The left inner
gill-plate is also snipped so as to show the subjacent orifices
of the left nephridium r, and of the genital gland (testis or
ovary) >/. The foot thus exposed in Anodon is a simple
muscular tongue-like organ. It can be protruded between
the flaps of the mantle (fig. 124, (1), (2)) so as to issue
from the shell, and by its action the Anodon can slowly
crawl, or burrow in soft mud or sand. It has been sup-
posed that water is taken into the blood-vessels of the
Anodon through pores in the foot, and in spite of opposi-
tion this view is still maintained (Griesbach, 47). In fig.
124, (-) the letters ab, ac, ad, point to three pit-like depres-
sions, supposed by Griesbach to be pores leading into the
blood-system. According to Carriere (48) these pits are
nothing but irregularities of the surface ; in some cases

they are the entrances to ramified glands. Other Lamelli-
branchs may have a larger foot relatively than has Anodon.
In Area it has a sole-like surface. In Area too and many
others it carries a byssus-forming gland and a byssus-
cementing gland. In the Cockles, in Cardium, and in
Trigonia, it is capable of a sudden stroke, which causes
the animal to jump when out of the water, in the latter

and the posterior a continuation of the inner gill-plate.
There is no embryological evidence to support this sug-
gested connexion, and, as will appear immediately, the
history of the gill -plates in various forms of Lamelli-
branchs does not directly favour it. Yet it is very prob-
able that the labial tentacles and gill-plates are modi-
fications of a double horseshoe -shaped area of ciliated
filamentous processes which existed in ancestral Mollusca
much as in Phoronis and the Polyzoa, and is to be com-
pared with the continuous pra> and post-oral ciliated band
of the Echinid larva Pluteus and of Tornaria (49).

The gill-plates have a structure very different from that
of the labial tentacles, and one which in Anodon is singu-
larly complicated as compared with the condition presented
by these organs in some other Lamellibranchs, and with
what must have been their original condition in the ances-
tors of the whole series of living Lamellibranchia. The
phenomenon of " concrescence " which we have already had
to note as showing itself so importantly in regard to the

free edges of the mantle-skirt and the formation of the
siphons, is what, above all things, has complicated the
structure of the Lamellibranch ctenidium. Our present
knowledge of the interesting series of modifications through
which the Lamellibranch gill-plates have developed to their
most complicated form is due to E. Holman Peck (50)
and to Mitsukuri (51). The Molluscan ctenidium is typi-
cally, as shown in fig. 2, a plume-like struc-
ture, consisting of a vascular axis, on each
side of which is set a row of numerous la-
melliform or filamentous processes. These
processes are hollow, and receive the venous
blood from, and return it again aerated into,
the hollow axis, in which an afferent and an
efferent blood-vessel may be differentiated.
In the genus Nucula (fig. 134), one of the

Fro 130. — Psammobia florida, right side, showing expanded foot c, and g incmrent and g' excurrent Arcace<£ we have an example of a Lamelli-

siphons (from Owen). , . . , . , 1... . . ...

branch retaining this plume-like form of gill,
genus to a height of four feet. In Mytilus the foot is
reduced to little more than a tubercle carrying the aper-
tures of these glands. In the Oyster it is absent alto-
gether.

The labial tentacles of Anodon (n, o in fig. 124, (3), (5) )

are highly vascular
flat processes richly
supplied with nerves.
The left anterior ten-
tacle (seen in the
figure) is joined at
its base in front of
the mouth («>) to the
right anterior ten-
tacle, and similarly
the left (0) and right
posterior tentacles
are joined behind the
mouth. Those of

. ,. . . „ .

Area (l, K in tig. loJ) the animal of Anixlon/a ci/gniea, from which the

«lmw tin's rplatinn tn mantle-skirt, the labial tentacles, and the gill-

filaments have been entirely removed so as to

the inuUth (a). These show the relations of the axis of the gill-plumes

/ , - -
OrgailS are cnaracter-

istic of all Lamelli-

e axs o enum ; , xs o

branchs; they do not right ctenidium; I-, portion of the axis of the
left ctenidium which is fused with the base of

_

FIG. 131.— Diagram of a view from the left side of

or ctenidia g, h. a, centro-dorsal area ; ft, ante-
rjor adductor muscle ; c, posterior adductor
muscle ; d, mouth ; e, anus ; f, foot ; g, free por-
tion of the axis of ieft ctenidium ; ft, axis of

vary except in size,
being sometimes
drawn out to
streamer-like dimen-
sions. Their appear-
ance and position suggest that they are in some way
related morphologically to the gill-plates, the anterior
labial tentacle being a continuation of the outer gill-plate,

the foot, the two dotted lines indicating the
origins of the two rows of gill-filaments ; m, line
of origin of the anterior labial tentacle ; n, ne-
phridial aperture ; o, genital aperture ; T, line
of origin of the posterior labial tentacle. (Ori-
ginal.)

In other Arcaceae (e.g., Area and Pectunculus) the lateral
processes which are set on the axis of the ctenidium are not
lamellae, but are slightly-flattened, very long tubes or hol-

low filaments. These fila-
ments are so fine and are
set so closely together
that they appear to form
a continuous membrane
until examined with a
lens. The microscope
shows that the neighbour-
ing filaments are held to-
gether by patches of cilia,
called " ciliated junc-
tions," which interlock
with one another just as
two brushes may be made
to do. In fig. 133, A a
portion of four filaments
of a ctenidium of the Sea-
Mussel (Mytilus) is repre-
sented, having precisely }
the same structure as
those of Area. The fila-

5. 132. — View from the ventral (pedal) as-
lect of the animal of ,4rcn Note, the mantle-
japand gill-li laments having been cut away,
a, mouth ; 6, anus ; c, free spirally turned

of the right side ; d, do. of the left side ;
c, / anterior portions of these axes fused
by concrescence to the wall of the body ;
g, anterior adductor muscle; h, posterior
adductor ; i, anterior labial tentacle ; k,
posterior labial tentacle ; I, base line of the
foot; m, sole of the foot; u, callosity.
(Original.)

ments of the gill (cteni- extremity of the gill-axis or ctenidial axis

° . * nf tho riirVlf clHp • ll flu nf thp Ipft «Tllo -

dium) of Mytilus and

Area thus form two

closely set rows which

depend from the axis of

the gill like two parallel

plates. Further, their structure is profoundly modified by

the curious condition of the free ends of the depending

filaments. These are actually reflected at a sharp angle —

152

MOLLUSCA

'

doubled on themselves in fact — and thus form an additional
row of filaments (see fig. 133, B). Consequently, each primi-
tive filament has a descending and an ascending ramus, and
instead of each row forming a simple plate, the plate is
double, consisting of a descending and an ascending lamella.
As the axis of the ctenidium lies by the side of the body,
and is very frequently connate with the body, as so often
happens in Gastropods also, we find it convenient to speak
of the two plate-like structures formed on each ctenidial
axis as the outer and the inner gill-plate ; each of these is

A

J|

II

ep.-

ep:

Base

Y

epr

nc
V"

lac.

Fir:. 133. — Filaments of the ctenidium of Mytihis eflttlis (after Holman Peck).
A. Part of four filaments seen from the outer face in order to show the ciliated
junctions c.j. B. Diagram of the posterior face of a single complete filament
with descending ramus and ascending ramus ending in a hook-like process.
ep., ep., the ciliated junctions ; il.j., inter-lamellar junction. C. Transverse
section of a filament taken so as to cut neither a ciliated junction nor an
inter-lamellar junction. f.?., frontal epithelium ; l.f.e'.t l.f.e"., the two rows
of latero-frontal epithelial ct-lls with long cilia ; ch, chitonous tubular lining
of the filament ; lac., blood lacuna traversed by a few processes of connective
tissue cells ; b.c., blood-corpuscle.

composed of two lamellse, an outer (the reflected) and an
adaxial in the case of the outer gill-plate, and an adaxial and
an inner (the reflected) in the case of the inner gill-plate.
This is the condition seen in Area and Mytilus, the so-
called plates dividing upon the slightest touch into their
constituent filaments, which are but loosely conjoined by
their "ciliated junctions." Complications follow upon
this in other forms. Even in Mytilus and Area a con-
nexion is here and there formed between the ascending
and descending rami of a filament by hollow extensible
outgrowths called " interlamellar junctions " (il.j in B, fig.
133). Nevertheless the filament is a complete tube formed
of chitonous substance and clothed externally by ciliated
epithelium, internally by endothelium and lacunar tissue —
a form of connective tissue — as shown in fig. 133, C.
Now let us suppose, as happens in the genus Dreissena —
a genus not far removed from Mytilus — that the ciliated
inter-filamentar junctions (fig. 136) give place to solid
permanent inter-filamentar junctions, so that the filaments
are converted, as it were, into a trellis-work. Then let us
suppose that the inter-lamellar junctions which we have
already noted in Mytilus become very numerous, large,
and irregular ; by them the two trellis-works of filaments
would be united so as to leave only a sponge-like set
of spaces between them. Within the trabeculse of the
sponge-work blood circulates, and between the trabeculre
the water passes, having entered by the apertures left

in the trellis-work formed by the united gill-filaments
(fig. 138, A, B). The larger the intra-lamellar spongy

FIG. 134. — Structure of the cteuidia of Nucula (after Mitsukuri) ; see also
fig. 2. A. Section across the axis of a ctenidinm with a pair of plates —
flattened and shortened filaments— attached, i, j, fc, g are placed on or near
the membrane which attaches the axis of the ctenidium to the side of the
body ; a, b, free extremities of the plates (filaments) ; rt, mid-line of the
inferior border ; e, surface of the plate ; t, its upper border ; h, chitonous
lining of the plate ; r, dilated blood-space ; K, fibrous tract ; o, upper blood-
vessel of the axis ; n, lower blood-vessel of the axis ; s, chitonous framework
of the axis ; cp, canal in the same ; A, B, line along which the cross-section '
C of the plate is taken. B. Animal of a male Nucula proximo,, Say, as seen
when the left valve of the shell and the left half of the mantle-skirt are re-
moved, a.a., anterior adductor muscle ; p.a., posterior adductor muscle ;
v,m, visceral mass; /, foot; g, gill; I, labial tentacle; l.a., filamentous
appendage of the labial tentacle ; Ib, hood-like appendage of the labial 1m-
tacle : m, membrane suspending the gill and attached to the body along the
line x, y, z, w ; p, posterior end of the gill (ctenidium). C. Section am iss
one of the gill-plates (-4, B, in A) comparable with fig. 133, C. i.a., outer
border ; d.a., axial border ; ?./., latero-frontal epithelium ; r, epithelium of
general surface ; r, dilated blood-space ; 7i, chitonous lining (compare A).

growth becomes, the more do the original gill-filaments
lose the character of blood-holding tubes and tend to
become dense elastic rods for the simple purpose of sup-
porting the spongy growth. This is seen both in the
section of Dreissena gill (fig. 136) and in those of Anodon
(fig. 137, A, B, C). In the drawing of Dreissena the
individual filaments/, /, / are cut across in one lamella at
the horizon of an inter-filamentar junction, in the other
(lower in the figure) at a point where they are free. The
chitonous substance ch is observed to be greatly thickened
as compared with what it is in fig. 133, C, tending in
fact to obliterate altogether the lumen of the filament.
And in Anodon (fig. 1 37, C) this obliteration is effected. In
Anodon, besides being thickened, the skeletal substance of
the filament develops a specially dense rod-like body on
each side of each filament. Although the structure of the
ctenidium is thus highly complicated in Anodon, it is yet
more so in some of the Siphonate genera of Lamellibranchs.
The filaments take on a secondary grouping, the surface of
the lamella being thrown into a series of half-cylindrical
ridges, each consisting of ten or twenty filaments; a filament

MOLLUSCA

153

of much greater strength and thickness than the others may
be placed between each pair of groups. In Anodon, as in

K j' Y "• p.

ii

Fin. 135.— Diagrams of transverse sections of a Lamellibraneh to show the
adhesion, by concrescence, of the gill-lamelh« to the mantle-Haps, to the toot,
and to one another. A shows two conditions with free gill-axis ; B, con-
dition at foremost region in Anodon ; C, hind region of foot in Anodon ; D,
region altogether posterior to the foot in Anodon. a, visceral mass ; 6, foot ;
c, mantle flap ; d, axis of gill -or ctenidium ; c, adaxial lamella of outer gill-
plate ; er, reflected lamella of outer gill-plate ; /, adaxial lamella of inner
gill-plate ; fr, reflected lamella of inner gill-plate ; (/, line of concrescence of
the reflected lamelke of the two inner gill-plates ; A, rectum ; i, supra-brauchial
space of the sub-pallial chamber. (Original.)

many other Laniellibranchs, the ova and hatched embryos
are carried for a time in the ctenidia or gill apparatus, and
in this particular case the space between the two lamellae

Vv

SPfcW

FIG. 136.— Transverse, section of the outer gill-plate of Drtissena poliimnrpTia
(after Holman Perk). / constituent gill-filaments ; /, libr-ms sub-epidermic
tissue; rh, cliitonmis substance of the filaments; nch, cells relate,! to tin-
chitonous substance; lac, lacunar tissue; piy, pigment-cells; '«-, blnod-
corpuscles ; Jc, frontal epithelium; lfc',lfe', tw.i r,.ws of hti-r.i-tii.ntal e\A-
thelial cells with long cilia ; Irf, librous, possibly muscular, substance of the
in ter-filamenter junctions.

of the outer gill-plate is that which serves to receive the
ova (fig. 137, A). The young are nourished by a substance

formed by the cells which cover the spongy inter-lamellar
outgrowths.

There arc certain other points in the modification of the
typical ctenidium which must lie noted in order to under-
stand the ctenidium of Anodon. The axis of each ctenid-
ium, right and left, starts from a point well forward near
the labial tentacles, but it is at first only a ridge, and does
not project as a free cylindrical axis until the back part of

ch

ol

FIG. 137.— Transverse sections of gill-plates of Anodon (after Peck). A. Outer
-

gill-plate. B. Inner gill-plate. C. A portion of B more highly magnified.
o.l, outer lamella ; i.l, inner lamella ; i1, blood-vessel ; f, constituent fila-
ments ; lac, lacunar tissue ; ch, chitonotis substance of the filament ; chrt
chitonous rod embedded in the softer substance ch.

the foot is reached. This is difficult to see at all in Ano-
don, but if the mantle-skirt be entirely cleared away, and
if the dependent lamellse which spring from the ctenidial
axis be carefully cropped away so as to leave the axis itself
intact, we obtain the form shown in fig. 131, where y and
h are respectively the left and the right ctenidial axes pro-
jecting freely beyond the body. In Area this can be seen
with far less trouble, for the filaments are more easily re-
moved than are the consolidated lamella? formed by the
filaments of Anodon, and in Area the free axes of the
ctenidia are large and firm in texture (fig. 132, c, </).

If we were to make a vertical section across the long
axis of a Lamellibranch which had the axis of its ctenidium
free from its origin onwards, we should find such relations
as are shown in the diagram fig. 135, A. The gill axis d
is seen lying in the sub-pallial chamber between the foot
6 and the mantle c. From it depend the gill-filaments or
lamellae — formed by united filaments — drawn as black lines
/. On the left side these lamella; are represented as hav-
ing only a small reflected growth, on the right side the
reflected ramus or lamella is complete (/>• and er). The
actual condition in Anodon at the region where the gills
commence anteriorly is shown in fig. 135, B. The axis of
the ctenidium is seen to be adherent to, or fused by con-
crescence with, the body-wall, and moreover on each side
the outer lamella of the outer gill-plate is fused to the
mantle, whilst the inner lamella of the inner gill-plate is
fused to the foot. If we pass a little backwards and take
another section nearer the hinder margin of the foot, we

U

154

MOLLUSCA

get the arrangement shown diagrammatically in fig. 135,
C, and more correctly in fig. 142. In this region the inner
lamellae of the inner gill-plates are no longer affixed to the
foot. Passing still further back behind the foot, we find
,fe

concres-
g. In

FIG. 13S. — Gill-lamellx of Anorlon (after Peck). A. Fragment of the outer
lamella of an inner gill-plate torn from the connected inner lamella, the sub-
nlamentar tissue also partly cut away round the edges so as to expose the
filaments, their transverse junctions Ir, and the "windows" left in the lattice-
work ; s/e, internal surface of the lamella ; v, vessel. B. Diagram of a block
cut from the outer lamella of the outer gill-plate and seen from the inter-
lamellar surface (after Peck). /, constituent filaments ; trf, fibrous tissue of
the transverse inter-filamentar junctions ; v, blood-vessel; ;'//, inter-lamellar
junction. The series of oval holes on the back of the lamella are the water-
pores which open between the filaments in irregular rows separated horizon-
tally by the transverse inter-filamentar junctions.

in Anodon the condition shor.rn in the section D, fig. 135.
The axes i are now free ; the outer lamellae of the outer
gill-plates (er) still adhere by concrescence to the mantle-
skirt, whilst the inner lamell* of the inner gill-plates meet
one another and

r i

fuse by

cence at

the lateral view of

the animal with

reflected mantle -

skirt and gill-

plates, the line of

concrescence of the

inner

tlio innpr frill

plates is readily
seen; it is marked an in fig. 124, (5). In the same
figure the free part of the inner lamella of the inner
gill-plate resting on the foot is marked z, whilst the
attached part — the most anterior — has been snipped
with scissors so as to show the genital and nephridial
apertures x and ?/. The concrescence, then, of the
free edge of the reflected lamella? of the gill-plates of
Anodon is very extensive. It is important, because such
a concrescence is by no means universal, and does not
occur, for example, in Mytilus or in Area ; further, because

, ,, -Flo. 130.— Transverse sections of A, a Lampllibranch,

lamelUB OI and £, an Isopleurous Gastropod (Chiton), to show
the relations of p, the foot ; 1>, -tin- brancliui.' : and
m, the mantle, frrom Gegenbaui.)

when its occurrence is once appreciated, the reduction of
the gill-plates of Anodon to the plume-type of the simplest
ctenidium presents no difficulty ; and, lastly, it has import-
ance in reference to its physio-
logical significance. The me-
chanical result of the concres-
cence of the outer lamellae to
the mantle-flap, and of the
inner lamella? to one another
as shown in section D, fig.
135, is that the sub-pallial
space is divided into two

spaces by a horizontal Sep- Fro. 140.— Lateral view of a Mactra,

turn. The upper space (i)
communicates with the outer
world by the excurrent or su-
perior siphonal notch of the
mantle (fig. 124, rf); the lower
space communicates by the
lower siphonal notch (e in fig. 124). The only communica-
tion between the two spaces, excepting through the trellis-
work of the gill-plates, is by the slit (z in fig. 124, (5)) left
1 iy the non-concrescence of a part of the inner lamella of the
inner gill-plate with the foot. A probe (</) is introduced
through this slit-like passage, and it is seen to pass out by
the excurrent siphonal notch. It is through this passage,
or indirectly through the pores of the gill-plates, that the
water introduced into the lower sub-pallial space must pass
on its way to the excurrent siphonal notch. Such a
subdivision of the pallial chamber, and direction of the

the right valve of the shell and right
mantle -flap removed, and the si-
phons retracted, br, br', outer and
inner gill-plates ; t, labial tentacle ;
ta, tr, upper and lower siphons ; ?HS,
siphonal muscle of the mantle-flap ;
ma, anterior adductor muscle ; vip,
posterior adductor muscle ; p, foot ;
c, umbo. (From "

FIG. 141. — The same animal as fig. 140, with its foot and siphons expanded.
Letters as in fig. 140. (From Gegeubaur.)

currents set up within it do not exist in a number of
Lamellibranchs which have the gill-lamellae comparatively
free (Mytilus, Area, Trigonia, &c.), and it is in these forms
that there is least modification by concrescence of the pri-
mary filamentous elements of the lamelte. Probably the
gill -structure of Lamellibranchs will ultimately furnish
some classificatory characters of value when they have
been thoroughly investigated throughout the class.

The alimentary canal of Anodon is shown in fig. 124, (4).
The mouth is placed between the anterior adductor and
the foot ; the anus opens on a median papilla overlying
the posterior adductor, and discharges into the superior
pallial chamber along which the excurrent stream passes.
The coil of the intestine in Anodon is similar to that of
other Lamellibranchs, but the crystalline style and its
diverticulum are not present here. The rectum traverses
the pericardium, and has the ventricle of the heart wrapped,
as it were, around it. This is not an unusual arrangement
in Lamellibranchs, and a similar disposition occurs in some
Gastropoda (Haliotis). A pair of ducts (<«') lead from the
first enlargement of the alimentary tract called stomach
into a pair of large digestive glands, the so-called liver,
the branches of which are closely packed in this region
('if). The food of the Anodon, as of other Lamellibranchs,
consists of microscopic animal and vegetable organisms,
which are brought to the mouth by the stream which sets
into the sub-pallial chamber at the lower siphonal notch
(e in fig. 124). Probably a straining of water from solid

MOLLUSCA

155

particles is effected by the lattice-work of the ctenidia or
gill-plates.

The heart of Anodon consists of a median ventricle em-
bracing the rectum (rig. 143, A), and giving oil' an anterior
and a posterior artery, and of two auricles which open into
the ventricle by orifices protected by valves.

The blood is colourless, and has colourless amoeboid
corpuscles floating in it. In two Lamellibranchs, /S'./A »
(Ceriitixnlfii) /Kjiiintn and Area Nooe, the blood is crimson,
owing to the presence of corpuscles impregnated with
hivmoglobin (Lankester, 31). In Anodon the blood is
driven by the ventricle through the arteries into vessel-
like spaces, which soon become irregular lacuna; surround-
ing the viscera, but in parts — e.g., the labial tentacles and
walls of the gut — very fine vessels with endothelial cell-
lining are found. The blood makes its way by large
veins to a venous sinus which lies in the middle line be-
low the heart, having the paired renal organs (nephridia)
placed between it and that organ. Hence it passes
through the vessels of the glandular walls of the nephridia
right and left into the gill-lamellfe, whence it returns
through many openings into the widely-stretched auricles.
A great deal more pre-
cision has been given to
accounts of the structure
of arteries, veins, and
capillaries in Anodon
than the facts warrant.
The course of the blood- X-
stream can only be some-
what vaguely inferred ex-
cept in its largest out-
lines. Distinct arterial
and venous channels can-
not be distinguished in
the gill-lamellae, in spite
of what Langer (52) has
written on the subject,
though it is highly prob-
able that there is some

, . , , . . . . , FIG. 142.— Vertical section through an Ano-
kllld of Circulation 111 the donta, about the mid-region of the foot, ra,
(Jills Tn flip filimpiit* mantle-flap ; br, outer, bY, inner gill-plate
MS -each composed of two lamella? ; /, foot ; r,
of the gill of Mytllus the ventricle of the heart ; «, auricle ; p, p',

tubular cavity is divided Perirardil11 •»«<* : «• ""
by a more or less complete fibrous septum into two
channels, presumably for an ascending and a descend-
ing blood-current. The ventricle and auricles of Anodon
lie in a pericardium which is clothed with a pave-
ment endothelium (d, fig. 143). Veins are said by Keber
and others to open anteriorly into it, but this appears to
be an error. It does not contain blood or communicate
directly with the blood-system ; this isolation of the peri-
cardium we have noted already in Gastropods and Cephalo-
poda. A good case for the examination of the question as
to whether blood enters the pericardium of Lamellibranchs,
or escapes from the foot, or by the renal organs when the
animal suddenly contracts, is furnished by the Solen legu-
men, which has red blood-corpuscles. According to ob-
servations made by Penrose (53) on an uninjured A'»A/<
lei/umen, no red corpuscles are to be seen in the pericardial
space, although the heart is filled with them, and no such
corpuscles are ever discharged by the animal when it is
irritated.

The pair of nephridia of Anodon, called in Lamelli-
branchs the organ of Bojanus, lie below the membranous
floor of the pericardium, and open into it by two well-
marked apertures (e and / in fig. 1 43). Each nephridium,
after being bent upon itself as shown in fig. 143, C, D,
opens to the exterior by a pore placed at the point marked
x in fig. 124, (5), (6). It is no doubt possible, as in the

Gastropoda nnd Cephalopoda, for water to enter from the
exterior by the nephridia into the pericardium, but that
it ever does so is as yet not proved. What is certain
from the set of the ciliary currents is that liquid generally

CJ

D

u .7 * **•

FIG. 143. — Diagrams showing the relations of pericardium and nephridia in a
Lamellibranch such as Anodon. A. Pericardium opened dorsally so as to
expose the heart and the floor of the pericardial chamber d. B. Heart
removed and floor of the pericardium cut away on the left side so as to open
the non-glandular sac of the nephridium, exposing the glandular sac 6,
which is also cut into so as to show the probe /. C. Ideal pericardium and
nephridium viewed laterally. D. Lateral view showing the actual relation
of the glandular and non-glandular sacs of the nephridium. The arrows
indicate the course of fluid from the pericardium outwards. «, ventricle of
the heart ; l>, auricle ; bb, cut remnant of the auricle ; c, dorsal wall of the
pericardium cut and reflected; c, reno-pericardial orifice; /, probe iiitm-
duced into the left reno-pericardial orifice ; g, non-glandular sac of the left
nephridium ; ft, glandular sac of the left nephridium ; i, pore leading from
the glandular into the non-glandular sac of the left nephridium ; k, pore
leading from the non-glandular sac to the exterior ; ac, anterior, ab, posterior,
cut remnants of the intestine and ventricle.

passes out of the pericardium by the nephridia. One half
of each nephridium is of a dark-green colour and glandular
(Ii in fig. 143). This opens into the reflected portion which
overlies it as shown in the diagram fig. 143, D, i ; the latter
has non-glandular walls, and opens by the pore k to the
exterior. The nephridia may be more ramified in other
Lamellibranchs than they are in Anodon. In some they
are difficult to discover. That of the common oyster
has recently (1882) been detected by Hoek (54). Each
nephridium in the oyster is a pyriform sac, which commu-
nicates by a narrow canal with the urino-genital groove
placed to the front of the great adductor muscle ; by a
second narrow canal it communicates with the pericardium.
From all parts of the pyriform sac narrow stalk-like tubes
are given off, ending in abundant widely-spread branching
glandular cseca, which form the essential renal secreting
apparatus. The genital duct opens by a pore into the
urino-genital groove of the oyster (the same arrangement
being repeated on each side of the body) close to but distinct
from the aperture of the nephridial canal. Hence, except
for the formation of a urino-genital groove, the apertures
are placed as they are in Anodon. Previously to Hoek's
discovery a brown-coloured investment of the auricles of
the heart of the oyster had been supposed to represent
the nephridia in a rudimentary state. This investment,
which occurs also in Mytilus but not in Anodon, may pos-
sibly consist of secreting cells, and may be comparable to
the pericardial accessory glandular growths of Cephalopoda.
A'ermus System and Sense-organs. — In Anodon there are
three well-developed pairs of nerve-ganglia (fig. 144, B and
fig. 1 24, (6)). An anterior pair, lying one on each side of the

156

MOLLUSCA

mouth (fig. 144, B, a) and connected in front of it by a
commissure, are the representatives of the cerebral, pleural,
and visceral ganglia of the typical Mollusc, which are not
here differentiated as they are in Gastropods (compare,
however, fig. 67). A pair placed close together in the foot
(fig. 144, B, 6, and fig.
124, (6), ax] are the typ-
ical pedal ganglia ; they
are joined to the cerebro-
pleuro - visceral ganglia
by connectives.

Posteriorly beneath
the posterior adductors,
and covered only by a
thin layer of elongated
epidermal cells, are the
olfactory ganglia, their
epidermal clothing con-
stituting the pair of os-
phradia, which are thus
seen in Lamellibranchs
to occupy their typical
position and to have the
typical innervation, — the

nprvp tn p

nerve tp e ,

itim being given off by Teredo; 2?, of Anodonta; C, of Pecten. n,

i , ,. J cerebral ganglion-pair ( = cerebro-pleuro-

tne Visceral ganglion - visceral) ; b, pedal ganglion-pair ; c, olfac-

that is to say, by the tory (osphradial) ganglion-pair.

undifferentiated cerebro-pleuro- visceral ganglion of its
proper side. This identification of the posterior ganglion-
pair of Lamellibranchs is clue to Spengel (11). Other

n«r.Tii»»rl FIG. 144.— Nerve-ganglia and corcls o
Ospliracl- Lamellibranchs (from Gegenbaur): A, of

of three

FIG. 145.— Pallia! eye of Spondylus (from Hickson). a, prse-corneal epithe-
lium ; &, cellular lens ; c, retinal body ; (1, tapetum ; c, pigment ; / retinal
nerve; g, complementary nerve ; li, epithelial cells tilled with pigment; k,
tentacle

anatomists have considered this ganglion-pair as corre-
sponding to either the pleural or the visceral of Gastropoda,
or to both, and very usually it is termed " the parieto-
splanchnic " (Huxley).

The sense-organs of Anodon other than the osphradia
consist of a pair of otocysts attached to the pedal ganglia
(fig. 124, (6), ay). The otocysts of Cyclas are peculiarly
favourable for study on account of the transparency of the
small foot in which they lie, and may be taken as typical
of those of Lamellibranchs generally. The structure of
one is exhibited in fig. 146. A single otolith is present
as in the veliger embryos of Opisthobranchia. In adult
Gastropoda there are frequently a large number of rod-like
otoliths instead of one.

Anodon has no eyes of any sort, and the tentacles on the
. mantle edge are limited to its posterior border. This
duficiency is very usual in the class; at the same time, many
Lamellibranchs have tentacles on the edge of the mantle
supplied by a pair of large well-developed nerves, which
are given off from the cerebro-pleuro-visceral ganglion-pair,

and very frequently some of these tentacles have undergone
a special metamorphosis converting them into highly-
organized eyes. Such eyes on the mantle-edge are found
in Pecten, Spondylus, Lima, Ostrea (1), Pinna, Pectunculus,
Modiola, Mytilus (?), Cardium, Tellina,
Mactra, Venus, Solen, Pholas, and Ga-
leomma. They are totally distinct from
the cephalic eyes of typical Mollusca, and
have a different structure and historical de-
velopment. They have not originated as
pits but as tentacles. _ They agree with the FlQ_ m_otncyst
dorsal eyes of Onchidium (Pulmonata) in of Cyclas (from
the curious fact that the optic nerve pene- ^^"ciiiated
trates the capsule of the eye and passes in ceils lining the

c f .1.1 i- i i i /i> -i *K\ ±1 , same; o, otulith.

front of the retinal body (fig. 145), so that
its fibres join the anterior faces of the nerve-end cells as
in Vertebrates, instead of their posterior faces as in the
cephalic eyes of Mollusca and Arthropoda ; moreover, the
lens is not a cuticular product but a cellular structure,
which, again, is a feature of agreement with the Vertebrate
eye. It must, however, be distinctly borne in mind that
there is a fundamental difference between the eye of Verte-
brates and of all other groups in the fact that in the
Vertebrata the retinal body is itself a part of the central
nervous system, and not a separate modification of the
epidermis— myelonic as opposed to epidermic. The struc-
ture of the reputed eyes of several of the above-named
genera has not been carefully examined. In Pecten and
Spondylus, however, they have been fully studied (see fig.
145, and explanation).

The gonads of Anodon are placed in distinct male and
female individuals. In some Lamellibranchs — for instance,
the European Oyster and the Pisidium pusiflum — the sexes
are united in the same individual; but here, as in most
hermaphrodite animals, the two sexual elements are not
ripe in the same individual at the same moment. It has
been conclusively shown that the Ostrea edulis does not
fertilize itself. The American Oyster (0. virginiana) and
the Portuguese Oyster (0. angulata) have the sexes sepa-
rate, and fertilization is effected in the open water after
the discharge of the ova and the spermatozoa from the
females and males respectively. In the Ostrea edulis fertil-
ization of the eggs is effected at the moment of their escape
from the uro-genital groove, or even before, by means of
spermatozoa drawn into the sub-pallial chamber by the in-
current ciliary stream, and the embryos pass through the
early stages of development whilst entangled between the
gill-lamellse of the female parent (fig. 6). In Anodon the
eggs pass into the space between the two lamellre of the
outer gill-plate, and are there fertilized, and advance whilst
A ,.. B

ml

-pad

Fin. 147. — Two stages in the development of Anodonta (from Balfour). Both
figures represent the glnehidium stage. A, when free swimming, shows the
two dentigerous valves widely open. 15, a lati-r stage, after fixture to the fin
of a fish, ah, -sln-ll ; ic/, adductor muscle ; s, teeth of the shell ; by, byssus ;
"."</, anterior adduetor ; ;>.<«/, posterior adductor; mt, mantle-flap; ft foot;
l>f, branchial filaments; au.v, otocyst ; «/, alimentary canal.

still in this position to the glochidium phase of develop-
ment (fig. 147). They may be found here in thousands
in the summer and autumn months. The gonads theni-
srlvi's are extremely simple arborescent glands which open
to the exterior by two simple ducts, one right and one

MOLLUSCA

Io7

left, continuous with the wall of the tubular branches of
the gland (fig. 124, (5), (6), y). In no Lamellibranch is
there a divergence from this structure, excepting that in
some (Ostrea) the contiguous nephridial and the genital
aperture are sunk in a urino-genital groove, which in other
cases (Spondylus 1) may partially close up so as to con-
stitute a single pore for the nephridial and genital ducts.
No accessory genital glands are present.

The development of Anodon is remarkable for the curious
larval form known as Glochidium (fig. 147). TheGlochidium

B

Fig. 148.— Embryos of Pisiilinm pnsiUum (after Lankester). A. Only four
embryonic cells are present, still enclosed in the egg envelope. B. The cells
have multiplied and commenced to iuvaginate, forming a blastopore or orifice
of invagination, bl.

quits the gill-pouch of its parent and swims by alternate
opening and shutting of the valves of its shell, as do
adult Pecten and Lima, trailing at the same time a long

Fig. 150.

Fin. 140.— Embryo of Pisidium pusiUum in the diblastula stage, surface view
(after Lankester). The embryo has increased in size by accumulation of
liquid between the outer and the invaginated cells. The blastopore has
closed.

FIG. 150. — B. Same embryo as fig. 149, in optical median section, showing the
invaginated cells )iy which form the arch-enU-nm, and the mesoblastic cells
7?i£ which are budded oft' from the surface of the mass ky, and apply them-
selves to the inner surface of the deric or epiblastic cell-layer ep. C. The
same embryo focused so as to show the mesublastic cells which immediately
underlie the outer cell-layer.

byssus thread. By this it is brought into contact with the
fin of a fish, such as Perch, Stickleback, or others, and effects

a hold thereon by means of the toothed edge of its shells.
Here it becomes encysted, and is nourished by the exuda-
tions of the fish. A distinct development of its internal
organs has been traced by the late Professor P.alfmir, but no
one has followed it to the moment at which it drops from
the fish's fin and assumes the form of shell characteristic of
the parent. Other Lamellibranrhs exhibit either a trocho-
sphcre larva which becomes a Veliger, differing only from
the Gastropod's and Pteropod's Veliger in having bilateral
shell-calcifications instead of a single central one ; or, like
Anodon, they may develop within the gill-plates of the
mother, though without presenting such a specialized larva
as the Glochidium. An example of the former is seen in the

Fio. 151.— Further stages in the development of Pisidivm pusiUum (after
Lankester). A. Optical section of an embryo in which the foot lias begun to
develop. B. The same embryo focused to its surface plane to show the
mouth o. C. Later embryo, showing the shell-gland s7i. D. Lateral view of
the same embryo. E. Later stage, with rudiments of the mantle-flap, lateral
view. F. Still later stage, with shell-valves and branchial filaments. IT,
epiblast ; mp, mesoblast ; al, met-enteron ; rp, rectal peduncle or pedicle of
invagination connecting the met-enteron with the cicatrix of the blastopore ;
o, mouth; ph, pharynx; sh, shell-gland; mn, mantle-Hap; br, branchial
lilameuts ; y, granular cells of doubtful sigiiilicance ; r, vesicular structure
of unknown significance.

development of the European Oyster, to the figure of which
and its explanation the reader is specially referred (fig. 6).
An example of the latter is seen in a common little
fresh-water bivalve, the Pisidimn puxiUum, which has been
studied by Lankester (12). The successive stages of the
development of this Lamellibranch are illustrated in the
woodcuts figs. 148 to 153 inclusive. These should be
compared with the figures of Gastropod development
(figs. 3, 4, 5, 7, and 72***). Fig. 148 shows the cleavage
of the egg-cell into four (A), and at a later stage the tucking
in of some of the cells to form an invaginated series (B).

158

MOLLUSCA

The embryonic cells continue to divide, and form an oval
vesicle containing liquid (fig. 149); within this, at one pole,
is seen the mass of in vagina ted cells (fig. 150, fit/). These
invaginated cells are the arch-enteron ; they proliferate and
give off branching cells, which apply themselves (fig. 1 50,
C) to the inner face of the vesicle, thus forming the meso-

Fm. 152.— Diagram of embryo of Pisidium in the same stage as E in fig. 151.
m, mouth ; /, foot ; ph, pharynx ; gs, met-enteron ; pi, rectal peduncle or
pedicle of invagination ; shst shell-gland. (From Lankester.)

blast or ccelomic outgrowths. The outer single layer of
cells which constitutes the surface of the vesicle (fig.
147) is the ectoderm or epiblast or cleric cell-layer. The
little mass of hypoblast or
enteric cell -mass now en-
larges, but remains con-
nected with the cicatrix of
the blastopore or orifice of
invagination by a stalk, the
rectal peduncle (fig. 151, A,
rp). The enteron itself be-
comes bilobed and is joined
by a new invagination, that
of the mouth and stomo-
dreum, ph. Fig. 151, B
shows the origin of the
mouth o, being a deeper

view of the Same specimen Fio. 153.— Diagram of embryo of Pisidium,

,, ... *• , . , in same stasje as F in fig. 151 (after Lan-

m the Same position wllicn kester). m, month ; x, anus ; /, foot ; br,

ic /li-TnTn in firr 1^1 A branchial filaments ; ma, margin of the

ng. J -J i , .rt.. mantle.skirt . BI organ of Bojanus (ne-

The mesoblast multiplies phridium). The unshaded area gives
its cells, which become the position of the shell-valve.

partly muscular and partly skeleto-trophic. Centro-dor-
sally now appears the embryonic shell-gland (fig. 151,
C, sh). The pharynx or stomodoeum is still small, the
foot not yet prominent. A later stage is seen in fig.
152, where the pharynx is widely open and the foot pro-
minent. No ciliated velum or pne-oral (cephalic) lobe
ever develops. The shell-gland disappears, the mantle-
skirt is raised as a ridge (fig. 151, E, mn), the paired
shell-valves are secreted, the anus opens by a proctodoeal
ingrowth into the rectal peduncle, and the rudiments of
the gills (In-) and of the nephridia (B) appear (figs. 151,
F, and 153, dorsal and lateral views of same stage), and
thus the chief organs and general form of the adult are

acquired. Later changes, not drawn here, consist in the
growth of the shell-valves over the whole area of the
mantle-flaps, and in the multiplication of the gill-fila-
ments and their consolidation to form gill -plates. It
is important to note that the gill-filaments are formed
one by one posteriorly. The labial tentacles are formed
late. In the allied genus Cyclas, a byssus gland is formed
in the foot and subsequently disappears, but no such gland
occurs in Pisidium. The nerve-ganglia and the otocysts
probably form from thickenings of the epiblast, but detailed
observation on this and other points of histogenesis in the
Lamellibranchia is still wanting.

List of Memoirs, £c., referred to by numbers in the preceding article. — (1) G.
Cuvier, Memoires pour servir a I'histoire et a I'auatomie des Mollusques, Paris,
1816. (2) J. Poli, Tcstacea utriusque Sieitix, eorumque historia et anatome,
tabulis aeneis 49 illuitrata, vols. i.-iii., fol., Parma, 1791-1795 and 1820-1827.

(3) St delle Chiaje, Memorie sulla storia e notomia degli animali senza verttbre
ilel regno di Napoli, Naples, 1823-1829: new edit, with 172 plates, fol., 1S43.

(4) J. Vaughan Thompson, Zoological Researches, Cork, 1830 ; memoir iv., "On
the Cirripedes or Barnacles, demonstrating their deceptive character." (5) A.
Kowalewsky, " Entwickelungsgeschichte der einfachen Ascidien," in Mem. de
VAcad. dcs Sciences de St Petersbourg, I860, and "Entwickelungsgeschichte
des Amphioxits lanceolalus," ibid., ISO". (6) J. Vaughan Thompson, Zoological
Researches, Cork, 1830 ; memoir v., " Polyzoa, a new animal discovered as
an inhabitant of some Zoophytes." (7) C. G. Ehrenberg, Die Korallenthiere dcs
Ruthen Meeres, Berlin, 1834 (Abhand. d. k. Akad. d. Wisscnschaften in Berlin,
1S32). (8) H. Milne-Edwards, Recherches anatomiques physiologist's et zoolo-
giques sur ks Polypiers de France, Paris, 1841-1844. (9) W. H. Caldwell, "On
the development of Phoronis," Proc. Roy. Soc., 1SS2. (10) Richard Owen, Memoir
on the Pearly Nautilus, London, 1832. (11) T. H. Huxley, " On the morphology
of the cephalous Mollusca," Phil. Trans., 1853. (12) E. Ray Lankester, "Con-
tributions to the developmental history of the Mollusca," Phil. Trans., 1S75.
(13) E. Ray Lankester, "Notes on Embryology and Classification," Quart.
Journ. Microsc. Sc., 1877. (14) J. Carriere, " Das Wassorgefiiss-System d. Lamelli-
branchiaten u. Gastropoden," Zoolog. Anzeiger, 1881, No. 90. (15) E. Ray
Lankester, "Development of the Pond-Snail," Quart. Journ. Microsc. Sc., 1874,
and " Shell-gland of Cyclas and Planula of Limna;us," ibid., 1876. (16) R.
Horst, "Development of the European Oyster," Quart. Journ. Microsc. Sc.,
1S82, p. 341. (17) E. Ray Lankester, "Coincidence of the blastopore and
anus in Paludina," Quart. Journ. Microsc. Sc., 1876. (18) Id., "Zoological
Observations made at Naples," Annals and Mag. Kat. Hist., February, 1S73.
(19) W. K. Brooks, "Development of the American Oyster," Report of the
Commissioners of Fisheries of Maryland, I860. (20) Henri Milne-Edwards,
Papers in the Annales des Sciences Katurelles, 1S41-1S60. (21) H. de Lacaze
Duthiers, Papers in the Annales des Sciences Naturdhs, e.g., " Anomia " (1854),
" Mytilus"(1856), " Dentalium " (1856, 1857), " Purpura " (1859), " Haliotis "(1859),
" Vermetua " (1860). (22) A. Kolliker, EnturickelungsgeschicMt der Cephalopoden,
Zurich, 1844. (23) C. Gegenbaur, Untersuchungen uber Pteropoden und Hetero-
poden, Leipsic, 1855. (24) J. W. Spengel, " Die Geruchsorgane und das Nerven-
system der Mollusken," Zeitschr. f. wiss. Zool, 1881. (26) A. A. W. Hubrecht,
"On Proneomenia Shuteri nov. gen. et sp., Tvith remarks upon the anatomy
and histology of the Amphineura," Niederldndisches Archivfiir Zooloyie, supple-
ment volume, 1SS1. (26) Adam Sedgwick, "On certain points in the anatomy
of Chiton," Proc. Roy. Soc. Land., 1881. (27) E. Ray Lankester, "On some
undescribed points in the anatomy of the Limpet," Annals and Mag. Nat.
History, 1867; J. T. Cunningham, "The Renal Organs of Patella," Quart. Journ.
Microsc. Sc., 1883. (28) P. Fraisse, " Ueber Molluskenaugen mit embryonaleiu
Typus," Zeitschr. f. wiss. Zool., 1881. (29) L. v. Gran", " Ueber Rhodope Vcranil,
Roll.," Morpholog. Jahrb., vol. viii. (30) H. Simroth, " Das Fussnervensystem
der Paludina vivipara," Zeitschr. f. wiss. Zool., 1881. (31) E. Ray Lankester,
"A contribution to the knowledge of Hemoglobin," Proc. Xoy. Soc. Land.,
1873. (32) H. de Lacaze Duthiers, " Du systtme nerveux des Mollusques
Gasteropodes Pulmones aquatiques et d'un nouvel organe d'innervation," Arch,
de Zoologie erpcrimentale, vol. i. (33) C. Semper, Animal Life (for eye of
Onchidium, p. 371), International Scientific series, 1881. (34) Same as number

A". Akad. r. ll'et. Katurk., Amsterdam, 1856. (37) E. Kay Lankester and A. G.
Bourne, "On the existence of Spengel's olfactory organ and of paired genital
ducts in the Pearly Nautilus," Quart. Journ. Microsc. Sc., 1883. (38) J. W.
Vigelius, "Ueber das Excretions-System der Cephalopodeu," Niederldndisches
Archiv fiir Zoologie. bd. v., 1880. (39) Albany Hancock, "On the nervous
system of Ommaslrephes todarus," Annals and Mag. Nat. }Iist., 1852. (40)
J. D. Macdonald, "On the anatomy of Nautilus umbilicatns," Phil. Trans, oj
Hoy. Soc. Lond., 1855. (41) V. Hensen, " Ufber das Augeeiniger Cephalopoden,"
Zeitschr. f. wiss. Zool, 1865. (42) A. d'Orbigny, Mollusques rimnts et fossiles,

Smelt," Proc. Zool. Soc. Loml, 1883. (45) Same as 35. (46) F. M. Balfour,
I'omimrativc Embryology, vols. i. and ii., London, 1881-1882. (47) H. Grics-
baeh, " Ueber das Gefass-System und die Wasseraufnahme bei den N.i.jaden und
Mytilidcn," Zeitschr. f. wiss. Znal., lxS3. (48) Same as 14. (4a) Same as 13.
(50) R. Holman Peek, "The structure of the Lamellibranchiate gill," Quart.
Journ. of Microsc. Sc., 1870. (51) K. Mitsukuri, "Structure and significance
of some aberrant forms of Lamellibranehiate gills," Quart. Journ. Microsc. Sc.,
issi. (52) K. Langer, "Das Gefass-System der Teichmussd," Deal;, kais.
Akail. d. ll'isscnsch., Vienna, 1805-1850. (53) J. Penrose, in "Report of the
OiimmitU'.' on the Zoological Station of Naples," British Assoc. Report, 1882.
(54) P. P. C. Hock, " Les orgaues de la generation de 1'huitrc," Journ. de la Soc.
Ncerlutulaise de Zool, 1883. (E. R. L.)

P 0 L Y Z 0 A

POLYZOA is the name applied by J. Vaughan Thompson
in!730 (I)1 to a group of minute polyp-like organisms
which were subsequently (1834) termed "Bryozoa" by
Ehrenberg (2). The forms included in this group were
stated by Thompson to be "in a general way the whole
of the Flustraceas, in many of which I have clearly ascer-
tained the animals to be Polyzoae," they having been pre-
viously considered by zoologists to be allied to the Hydra-
like polyps. These organisms had previously been known
by the hard corneous " cells " or chambers which are formed
by the animals on the surface of their bodies, and build up,

in consequence of the formation of dense colonies by bud-
ding, complex aggregates known as "sea mats" and "sea
mosses." Thompson expressly stated the opinion that the
organization of the animals detected by him led to the
conclusion that "they must be considered as a new type of
the Mollusca Acephala."

Subsequently (1844) Henri Milne-Edwards (3) pointed
out the relationship of Thompson's Polyzoa to the Brachio-
poda, and, adopting the latter's view as to their Molluscan
affinities, proposed to unite these two classes with the
Tunicata in a group to be called " Molluscoidea." Recent

FIG. IA. — Forms connecting the Eupolyzoa and the Grphyrxa.

1. Phormis auitralit, Harwell.

2. One of the two nephridia of the same; ext, external aperture ; int, in?, the two internal funnel-like apertures.

3. View of the tentacular area of Phoronis australis — the tentacles cut to their bases, ft, outer line of tentacles ; it, inner line of tentacles ; m, mouth ;

ep, epistome ; x, gap in the inner series of tentacles ; nepft, nephridio-pores ; an, anus ; gl, glandular pit.
(After Benham, Quart. Journ. ilicr. Sri., vol. 30, 1889.)

4. Oolftngia urintnshii, Lankcster. Specimen in which the introvert is telescoped into the body, a, the sclerorliynchus, which with b, the scleropyge,

represents the hard zncecium of Eupolyzoa ; c, anus.

5. View of the same in an expanded condition, a, sclerorhynchus ; 6, scleropyge ; d, the soft introvert carrying mouth, surrounded by six pinnate tentacles.

(After Lankcster, Trans. Linn. /Soc., 2nd ser., "Zoology," vol. ii., IHH.X)

G. Aspi'li'siphon >Sf< enstmpii, Diesing. a, anterior corneous plate ; b, terminal posterior plate ; d, introvert.
(After Selenka, Die Sipunculidm, 1S83.)

researches have entirely separated the Tunicata from this
association, and have demonstrated that they belong to
the great phylum of Vertebrata. On the other hand, the
association of the Polyzoa with the Brachiopoda appears
at present to be confirmed, though the relationship of
these two classes to the Mollusca has been shown to rest

1 These numbers refer to the bibliography which will be found in
page 171.

on mistaken identification of parts; see, however, Harmer
(18).

The Polyzoa appear to be related to the Sipunculoid
Gephyrsean worms (Gephyraea inermia) more nearly than
to any other class of the animal kingdom. The study and
interpretation of the facts of their ontogeny (growth from
the egg) presents such extreme difficulty that in the pre-
sent state of our knowledge it is necessary to regard them

160

P O L Y Z 0 A

ad interim as forming with the Brachiopoda and Sipuncu-
loids an isolated group, to which the name " Podaxonia"
may be applied, pending the decision of their affinities by
the increase of our knowledge of the embryology of import-
ant members of the group.1

The forms included at the present day in Thompson's
class of " Polyzoa " may then be thus classified : —

PHYLUM PODAXONIA.
CLASS l.—SIPUNCULOIDEA.
CLASS II.— BRACHIOPODA.
CLASS III.— POLYZOA.

Section 1.— VERMIFORMIA.

Sole genus : Phoronis (figs. 4 and 5).
Section 2.— PTEROBRANCHIA.
Genus 1 : Ehcibdopleura (fig. 7).
Genus 2 : Ccphalodiscus (figs. 8. 9, 10).
Section 3.— EUPOLYZOA.
Sub-class 1. — Ectoprocta.

Order 1. — PHYLACTOL^MA.

Examples : Lophopus, Plumatclla (fig. 2, B), Crutatclla

(fig. 3), Frcdcricclla.
Order 2. — GYMNOLJEMA.
Sub-order 1. — Cyclostoma.
Examples : Crisia (fig. 13, A), Horncra, Tubulipora,

HiscojMrella.

Sub-order 2. — Ctenostoma.

Examples: Alcyonidium, Vcsicularia, Scrialaria, Bower-
lankia (fig. 1, A), Paludicella (fig. 1, E and fig. 2, A).
Sub-order 3. — Chilostoma.

Examples : Cellularici , Scrupocellaria, Kinetoskias (fig.
14), Biigula, Biccllaria, Flustra (fig. 1, G), Mucro-
nella (fig. 1, C, D, F), Menibranipora, Lcpralia,
Eschara, Ccllepora, lietepora.
Sub-class 2.— Entoprocta.

Genera: Pcdiccllina (fig. 15), Loxosoma (fig. 16), Urna-

tclla, Ascopodaria.

We shall most readily arrive at a conception of the
essential structure of a Polyzoon, and of the variations to
which that essential structure is subject within the class,
by first examining one member of the group in detail and
subsequently reviewing the characters presented by the
divergent sub-classes, orders, &c., above indicated.

The most convenient form for our purpose is Paludicella
Ehrenbergii (fig. 2, A), belonging to the typical section of
the class (the Eupolyzoa) and to the order Gymnolaema.
The organism occurs as minute tree-like growths (figs. 2,
A and 1, E) attached to stones in freshwater streams and
canals. The branches of the little tree are rarely more than
an inch in length, and are regularly swollen and jointed at
intervals. Each of the very numerous joints is about one-
fifth of an inch long, and is in reality a tubular horny box
attached above and below to the preceding and succeeding
joints, and having on one side of it a spout-like aperture
from which a crown of tentacles can be protruded. Each
joint is thus inhabited by a distinct animal which is more
or less completely shut off from the one in front of it and
the one behind it, although it originated from the hinder
and has given rise to the fore-lying individual by a process
of budding, and retains a continuity of substance with both.
A single cell or joint with its contained animal is repre-
sented in fig. 2, A.

Paludicella produces an arboriform colony, the main
trunk or stolon being adherent to some stone or piece of
wood. The substance of the wall of the cells is formed
by a chemical body allied to chitin. Other Polyzoa may
form mat-like expansions — the cells being placed in one
plane, side by side (fig. 1, C, D, F, G), as well as in linear
series ; others again form solid masses, whilst many agree
with Paludicella in the simple linear arrangement of their
units. Phoronis and Loxosoma, on the other hand, do
not form colonies at all — the former because it does not

1 The research of Harmer (18) on Loxosoma is published too late
for due notice in tins article. Jt tends to the conclusion that the
Eupolyzoa are after all degraded Mollusca, and have no connexion
with the Vermit'ormia, Pterobrauchia, Brachiopoda, and Sipunculoidea.
The reader is referred to Mr Runner's memoir.

bud, the latter because the buds become detached from
their parent as soon as formed, as do the buds of the
Hydrozoon Hydra.

On the whole Paludicella presents us with a very simple
form of Polyzoon-colony (technically termed a "zoarium"),
in which the aggregate of budded persons, each of which

FIG. 1. — Various forms of zoaria of Eupolyzoa.
A. Bowbankia pustulosa, one of the Ctenostoma ; natural size.
15. A cluster of polypides of Bowerbankia pusttilosa, sonic with expanded
tentacles ; more highly magnified.

C. Zncecia of Afucroni'Ila pavonella (Chilostoma); highly magnified.

D. Zoarium of tlacronella paroiiella, forming a disk-like encrustation on a

piece of stone; natural size.

E. Zoarium of Paludicella Ehrenbergii (Ctenostoma), natural size.

F. Zocucia of Mucronelta I'eacliii ; highly magnified. Compare with C in Older

to note specific characters.

G. Zoarium of Ftustra securifrons; natural size.

is called a " polypide," does not exhibit any marked indi-
viduation, but is irregular and tree-like. But, just as in
the Hydrozoa we find the Siphonophora presenting us with
a very definite shape and individuality of the aggregate or
colony, so in the Polyzoa we find instances of high indi-

P 0 L Y Z 0 A

161

viduation of the zoarium of a similar kind. The most
remarkable example is afforded by the locomotive zoarium
or colony of Cristati-lla (tig. 3) ; and another very striking
instance is that of the stalked zoaria of Kinetoskias
(tig. 14) and Adeona.

The horny consistence of the cells which are produced
by Paludicella is very usual in other Polyzoa ; but we find
frequently that the substance which forms the cells is
gelatinous and soft instead of being horny, or again may
be strongly calcareous. The term ccenoscium is applied to
the mass of cells belonging to a colony or zoarium when
considered apart from the living polypides which form it.
Often such ccencecia are found retaining form and structure
when the soft living polypides have decomposed and dis-
appeared. A single cell of the ccencecium, corresponding
to a single polypide, is called by the special students of
the Polyzoa a zooecium.

If we examine a single cell or zocecium of Paludicella
more carefully whilst
its polypide is alive,
we discover that the
horny cell is nothing
more than the cuticle
of the polypide itself,
to which it is absol-
utely adherent. At
the so-called " mouth "
or spout of the cell
the cuticle suddenly
changes its character
and becomes a very
delicate and soft pel-
licle instead of being
thick and horny. There
is no real discontinuity
of the cuticle at this
region, but merely a
change in its qualities.
This gives to that por-
tion of the body of the
polypide which lies
beyond the spout a
mobility and capacity
for folding and pleat-
ing which is entirely
denied to that part
where the cuticle is
more dense (fig. 2, A).

A rnnrrl i n 0-1 v wp find Fjo- - ~ A- Polypideof PaJudicella Ehrrnbergii,
ui um^iji seen as a tT.ansplll.ent object ;„ optical section

and highly magnified (from Gegenbaur, after
Allman). For natural size see fig. 1. E. a,
anus; li\ peristomial circlet of ciliated ten-
tacles; *', thickened cuticle of the body-wall,
forming the horny cell or zocecium ; m,
median retractor muscle of the introversible
part of the body ; /•'. anterior retractor of the
same ; iiir. great retractor muscle of the same;
o, ov;u-y, passing from which to the stomach is
the anterior mesentery or funiculus ; t, testis ;
a1, oesophagus ; r, stomach ; x, posterior mes-
'.-iitrry ur fmiiculus; x', anterior mesentery or
funiculus. Observe at the light upper corner
of the figure the base of a second polypkle and
the " rosette-plate " of separation.

B. Diagram of a polypide of Plumatella.
Letters as above.

rnent is universal in the Ectoproctous Eupolyzoa, but does
not obtain either in the Entoprocta, the Pterobranchia, or
the Vermiformia. In Phoronis, Bhabdopleura, and Cepha-
lodiscus the anterior part of the body can not be tucked or
telescoped into the hinder part as it can in typical Eu-
polyzoa. On the other hand it is very important to note
that the Sipunculoid Gephyraans are all pre-eminently
characterized by possessing identically this arrangement.
The introversion is effected in Paludicella (as in other Eu-
polyzoa) by a series of long detached retractor muscles of

that the anterior por-
tion of the body of the
polypide can be pulled
into the hinder part as
the finger of a glove
may be tucked into the
hand. It is, in fact, an
" introvert " (for the
use of this term see
MOLLUSCA, vol. xvi.
p. 652). Thisarrange-

considerable power (fig. 2, A, mr, r', m) ; the same is true
of Ripunculus.

The view has been advanced by Allman (4) that the re-
tractile part of the polypide is to be considered as a distinct
individual budded from the basal portion, which is regarded
as an equivalent individual. It does not appear to the
present writer that such a theoretical conception tends to
facilitate the understanding of the structure and relations
of these animals.

An " ectocyst " and "endocyst" have also been distin-
guished in former treatises, and these terms form part of a
special "polyzoarial " nomenclature, but do not appear to
be any longer needful. Equally undesirable is the misap-
plied term "endosarc " lately introduced by Jolliet (5) to
denote a certain portion of the Polyzoon structure which
will not be referred to here by that name.

The retractile or introversible portion of the body of the
polypide of Paludicella is terminated by a crown of sixteen
stifi non-contractile tentacles (fig. 2, A, Ir) which form a
circle around a central aperture — the animal's mouth.
These tentacles are hollow and beset with vibratile cilia.
The beating of the cilia causes a powerful current in the
water by which food is brought to the animal's mouth.
Each tentacle is also muscular, and can be bent and
straightened at will. The tentacles not only serve to
bring food into the mouth, but they are efficient as gill-
filaments, being possibly homologous with (as well as func-
tionally similar to) the gill-filaments of Lamellibranch
Molluscs. They also serve as delicate tactile organs, and
are the only sense organs possessed by the Eupolyzoa.

In Paludicella the platform around the mouth from
which the tentacles arise, or lophophore, as it is termed, is
circular. This is the case in all members of the large
group of Gymnolrema and in the Entoprocta. But in
the Phylactolasma the lophophore is drawn out on each
side, right and left, so as to present a horse-shoe shape
(fig. 2, B), and in some forms, notably Lophopus and
Alcyonella, the two arms or diverging rami of the horse-
shoe are very strongly developed.

In the Pterobranchia the tentacles are confined in one
genus (Rhabdopleura) to the two arm-like outgrowths of
the lophophore, and are not simply hollow but contain a
well-developed cartilaginoid skeleton (fig. 7). In the allied
genus Cephalodiscus there are not merely a single pair of
such arm-like processes, each bearing two rows of tentacles,
but the lophophore is developed into twelve arm-like pro-
cesses (fig. 9), which form a dense tuft of filaments around
the anterior extremity of the animal.

In the Vermiformia (Phoronis) we again meet with a
very perfect horse-shoe-shaped lophophore (fig. 4). The
tentacles upon the crescentic or otherwise lobed circumoral
region of the Sipunculoids are the representatives of the
tentacles of the Polyzoa; whilst the tentaculiferous
" arms " of the Brachiopoda appear to be the equivalents
of the Polyzoon's lophophore much drawn out and in most
cases spirally rolled.

Just below the circular crown of tentacles in Paludicella
we find an aperture which the study of internal anatomy
proves to be the anus. In all Polyzoa the anus has this
position near the mouth ; and in this respect we again
note an agreement with Sipunculus and the other so-called
Gephyrasa inermia. In one division of the Polyzoa alone
is there any noteworthy variation in the position of the
anus, namely, in the Entoprocta (sub-class of the section
Eupolyzoa). In these forms the anus, instead of lying
just below the lophophore or platform from which the
tentacles spring, is included like the mouth within its
area (fig. 15, C).

Passing now to the deeper structure of Paludicella, we
find that it is a Ccelomate animal ; that is to say, there

162

P O L Y Z O A

exists between the body-wall and the wall of the aliment-
ary tract a distinct space termed " perigastric space,"
"body-cavity," or "coelom." This is true of all Polyzoa,
though it has been erroneously stated by G. 0. Sars that
Rhabdopleura does not possess such a coelom. In Eu-
polyzoa (excepting the Entoprocta) the coelom is very
capacious ; it is occupied by a coagulable hremolymph in
which float cellular corpuscles, and also the generative
products, detached, as is usual in Ccelomata, from definite
"gonads" developed on its lining membrane (fig. 2, A, o, t).
This lining membrane or " ccelomic epithelium " is ciliated
in the Phylactokema, but its characters appear not to have
been definitely determined in other Eupolyzoa. The
coelomic space and the tissues bounding it are continuous
throughout the colony or zoarium of a Polyzoon — either
directly without any constriction marking off one polypide
from another, or through perforate septum-like structures
as in Paludicella (see right-hand upper process of fig. 2, A),
which form incomplete barriers between juxtaposed zooecia,
and are termed " rosette-plates " or " communication-plates. "
The coelomic cavity is continued in Paludicella and probably
in all Polyzoa into the tentacles, so that these organs expose
the hcemolymph fluid to a respiratory action, and hence
may be called branchial.

The body-wall of Paludicella consists, alike in the
anterior introversible region and in the posterior region, of
an outer cuticle which has already been spoken of as
thickened around the base of the polypide so as to become
there the hard tube-like zocecium. Beneath this is the
delicate layer of living epidermic cells which are the
mother-cells or matrix of that cuticle. Beneath this again
are a few scattered annuli of muscular fibre-cells arranged
ring - wise around the cylindrical body ; more deeply
placed than these are five large bundles of longitudinally
placed muscular fibre-cells which are attached at three
different levels to the soft introversible portion of the
body, and by their retraction pull it in three folds or tele-
scopic joints into the capacious hinder part of the body.
In some Polyzoa the muscular fibre-cells present trans-
verse striations. These folds are shown in fig. 2, A ;

FIG. 3.— The locomotive zoarium of the freshwater Phylactol.T.mous Polyzoon
Cristatella mucedo; magnified six times linear (after Allman). a, individual
polypides with their horse-shoe-shaped crown of tentacles exserted; 6, srato-
blasts seen through the transparent tissues ; c, the muscular foot or base of the
colony by means of which it crawls; dt portion of water-weed upon which the
Cristatella is crawling.

but when the longitudinal muscles are completely con-
tracted the tentacular crown would be pulled down far out
of sight into the midst of the body by the great longitu-
dinal muscle mr. Deeper than the longitudinal muscles,
and clothing them and everything else which projects into
the ccelom, is the ccelomic epithelium, not easily observed,
and sufficiently known only in the Phylactolaema. Part of it
gives rise to the generative products (fig. 2 A, u, t).
Other Eupolyzoa have a similar but not identical arrange-
ment of the longitudinal muscles — acting essentially as
retractors of the " introvert " or soft anterior region of the
body — and a similar structure of the body -wall which is in

essential features identical with that of the Sipunculoid
worms, the Chaetopod worms, and other typical Ccelomate
animals.

The alimentary canal of Paludicella forms a closely com-
pressed U-shaped loop depending from the closely approxi-
mated mouth and anus into the capacious coelom. It is
clothed on its ccelomic surface (in Phylactolasma at any
rate) with ccelomic epithelium, and beneath this are
extremely delicate muscular layers. Within it is lined,
except in the immediate region of the mouth (which is
lined by the in-pushed outer cell-layer), by the enteric cell-
layer — the digestive cells derived from the archenteron of
the embryo. We can distinguish in Paludicella a contrac-
tile pharyngo-cesophagus (fig. 2, A, of), a digestive stomach
v (the lining cells of which have a yellow colour), and an
intestine which forms that arm of the loop connected with
the anus. This simple form of alimentary canal is uni-
formly present in Polyzoa. In Bowerbankia and its allies
a muscular gizzard with horny teeth is interposed between
oesophagus and digestive stomach.

The alimentary canal of Paludicella does not hang quite
freely in the coelomic cavity, but, as is usually the case in
other classes where the ccelom is large, mesenteries are
present in the form of fibrous (muscular 1) bands clothed
with coelomic epithelium and suspending the gut to the
body-wall. In Paludicella there are two of these mesen-
teries, an anterior (x) and a posterior (x). The presence
of two mesenteric bands is exceptional. Usually in the
Eupolyzoa we find one such mesentery only, corresponding
to the hinder of the two in Paludicella. The special name
funiculus (Huxley) is applied to this mesenteric band, and
it is noteworthy that the cells of the ccelomic epithelium,
either upon its surface or at its point of insertion into the
body-wall, are modified as reproductive elements, forming
either the testis or ovary; in the Phylactol;ema they form
here also special asexual reproductive bodies, the stato-
blasts. The nervous tissue and organs of Paludicella have
not been specially investigated, but in many Eupolyzoa
an oval mass of nerve-ganglion cells is found lying between
the mouth and anus, and there is no doubt that it is
present in this case. In Plumatella nerve-fibres have
been traced from this ganglion to the tentacles and other
parts around the mouth (fig. 1 1 , w, A; y). A " colonial
nervous system " was described some years ago by Fr.
Miiller in Serialaria ; but modern histologists do not
admit that the tissue so named by Miiller is nerve-tissue.
The ganglion above mentioned is the only nervous tissue
at present known in Polyzoa (but see fig. 17, x).

No heart or blood-vessels of any kind exist in Paludi-
cella nor in any of the Eupolyzoa or Pterobranchia. On
the other hand the isolated vermiform genus Phoronis
presents a closed contractile system of longitudinal vessels
(dorsal and ventral) which contain nucleated corpuscles
coloured red by haemoglobin (figs. 4, 5).

No excretory organs (nephridia) or genital ducts have
been observed in Paludicella, nor have such organs been
detected in the majority of the Polyzoa which have been
studied. In the Entoprocta, however, a pair of minute
ciliated canals are found in the nearly obliterated body-
cavity opening to the exterior near the tentacular crown in
both Pedicellina and Loxosoma, which represent the cephalic
nephridia of worms. A definite pair of nephridia occur in
Phoronis. A similar significance is perhaps to be attributed
to the " intertentacular organ " of Farre — a ciliated pas-
sage opening between two tentacles of the lophophore in
Membranipora, Alcyonidium, and other forms — through
which Hincks has observed the spermatozoa to escape in
large numbers. This organ occurs equally in female speci-
mens of Membranipora, and is not therefore simply a sper-
matic duct.

P O L Y Z O A

163

Paludicella, as we have seen, develops both ova and
spermatozoa in one and the same polypidc. The details
of impregnation and development have not been followed
in this instance, but in some of the marine Eupolyzoa
(Gymnolsema) remarkable bud-like structures termed unvin
are developed for the special reception of the ova, and in
these organs fertilization takes place. In the Entoprocta
there is a peculiar brood-pouch. The spermatozoa of one
polypide probably in all cases fertilize the ova of another,
but we have not yet in many cases a knowledge of how
the spermatozoa get to the eggs, or how the eggs escape
from the body-cavity of the parent. In the hippocrepian
freshwater Polyzoa (Phylactohuma) the ova appear to be
fertilized and undergo the early stages of development
within the body-cavity of the parent or in a hernia-like
protrusion of it. Probably in such cases the embryos
escape by the death of the parent and rupture of the
parental tissues, as do also the peculiar asexual internal
buds or statoblasts of these forms.

The embryo Polyzoon or "larva" swims freely in its
early condition by means of cilia, and is in this condition
a single polypide or "person." The forms assumed by
these ciliated larvje in different Polyzoa are very various
and exceedingly difficult of interpretation. We shall have
more to say with regard to them below (see figs. 19, 20,
21). The ciliated larva then fixes itself and commences
to produce polypides by a process of budding, the buds
remaining not merely in contact but in organic continuity,
and increasing continually in number so as to form a large
colony or zoarium. In Paludicella we have seen that this
colony has a simple tree-like form. The new buds form
as wart-like growths, usually one, sometimes two in number,
at the free end of a cell or zooecium near the spout-like
process from which the tentacular crown is everted. In
Paludicella all the polypides of a colony are alike ; there
is no differentiation of form or distribution of function
amongst the members of the colony. In many Eupolyzoa
this simplicity is by no means maintained, but a great
variety of form and function is assumed by various
members of the aggregate. The only approach to a
differentiation of the polypides in Paludicella is in the
arrest of growth of some of the buds of a colony in
autumn, which, instead of advancing to maturity, become
conical and invested with a dark-coloured cuticle. They
are termed Jiybernacula. Should the rest of the poly-
pides die down in winter, these arrested buds survive
and go on to complete development on the return of
spring.

In Paludicella we have thus seen a fairly simple and
central example of Polyzoon structure and life-history.
The variations upon this theme presented in different
groups of Polyzoa have been to some small extent noted
in the preceding account, but we shall now be able to
indicate them more precisely by considering the various
groups of Polyzoa in succession. The limit assigned to
this article necessitates very large omissions. The reader
who wishes to have the fullest information on the many
difficult and uncertain matters connected with this subject
is referred to Allman, Freshwater Polyzoa (Ray Society,
1856) ; Hincks, British Marine Polyzoa (Van Voorst,
1880) ; Haddon, " Budding in Polyzoa," Quart. Jmtrn.
Micr. Sci., 1883 ; Balfour, Embryology, vol. i. p. 242 ; and
the original memoirs cited by these writers.

THE VERMIFORMIA.

The first section of the Polyzoa comprises but a single
genus, Phoronis. It differs from all other Polyzoa first
in its greater size (species 2 inches long are known)
and elaboration of organization, and correlatively with
that in the fact that it does not produce buds. Further,

it does not produce a closely adherent cuticular zoueeium
as do Paludicella and the Eupolyzoa generally, but a
leathery tube in
which the animal
freely moves, resem-
bling that of some
Chsetopods (Sabel-
la). Like some
Sabella;, Phoronis
forms closely packed
aggregates of indi-
viduals not brought
together by any
process of budding,
but each separately
developed from an
egg. Phoronis has
an elongate, worm-
like, unsegmented
body, with a conical
posterior termina-
tion (like Sipuncu-
lus), and anteriorly
provided with a
horse - shoe - shaped
crown of tentacles
surrounding the
mouth (figs. 4, 5).
There is an inter-
tentacular " web "
between the bases
of the tentacles as
in the Phylactolaj-
ma. Caldwell (6)
has recently shown
that the tentacles
are supported by a
mesoblastic skele- *
ton, as is also the
case in Rhabdo-
pleura, but appar-
ently not the case
in any other Polyzoa. Close to the mouth, as in all
Polyzoa, is placed the anus, outside the horse-shoe-shaped
lophophore or tenta-
cular platform (fig.
11, i). The tenta-
cular crown is not
introversible ; in this
point Phoronis differs
from Paludicella and
the Ectoproctous Eu-
polyzoa, and agrees a_
with the Entoprocta
and the Pterobranchia.
Overhanging the

mouth is a small praa-
oral lobe or " epi-
stome " (figs. 4, 5, c).
This organ is aborted
in Paludicella, and in-
deed in all the Gym-
noliema, but is present
in the other Polyzoa,

•t . *ni FIG. 5. — Lateral view of the anterior region of

and IS especially large Phoronis. The tentacles of the right arm of the

and well developed in lophophore are cut short in order to expose clearly

-, ,-. the mouth b and the overhanging "epistomc"

lihaodopleuraand Le- or pra-ural lobe c. e, intestine ; A, dorsal vessel,
phalodiscus. It has Otter letters aslnflg. 4.
been compared to the Molluscan foot, but undoubtedly in
Phoronis it is the persistent representative of the prse-oral

G. 4. — Phoronis Jiippocrepia, Wright ; magnified
six times linear (from Allman). a, horse-shoe-
shaped lophophore with tentacles; c, epistome
(prfe-oral lobe or prostomium); d, oesophagus; /,
ventral vessel; 17, g, two anterior vessels which
unite to form /; i, longitudinal muscular coat
of the body-wall ; k, intertentacular membrane.

164

P O L Y Z O A

lobe of the larva (fig. 6), and therefore cannot be compared
to the Molluscan foot. If we are right in associating
Phoronis with the Polyzoa, this fact is sufficient to show
that the epistome of the Phylactotema (fig. 11, e) and the
buccal shield of Rhabdopleura (fig. 7, d) and of Cephalodis-
cus (fig. 9, b) are also cephalic in nature, and cannot rightly
be identified with the post-oral and ventral muscular lobe
known as the foot in Mollusca. A circum-oral nerve ring
occurs at the base of the tentacles and sends off a cord
which runs along the left side of the body. The alimen-
tary canal presents the same general form and regions as
in Paludicella. It hangs in the body-cavity, to the walls
of which it is suspended by definite mesenteries.

Phoronis presents a closed contractile vascular system
containing red-coloured blood-corpuscles (figs. 4, 5, /, g,
K). A. pair of ciliated canals acting as genital pores is
found near the anus ; these have been shown by Caldwell
to be typical nephridia.

The development of Phoronis is remarkable. The egg

gives rise (after the usual phases of cleavage and gastrula-

tion) to the larval form known as Actinotrwhn (fig.

6). This larva possesses a hood-like region overhanging

A. A

^ -*^

(3)

V— 4—

C' • B

FIG. 6.— Development of Phoronis and typical ciliatc larvos. (1), (2), (3), (8).
(9), (10), stages in the development of Phoronis— (1), earliest .larva; (2), lateral
view of the Actinotrocha; (3), ventral view of the same; (8), the ventral in-
vagination iv is formed; (9), the ventral invagination is everted, carrying with

arclu'troth divided into a pra?-oral cephalotroch (Molluscan and Rotifer's velum),
and a post-oral branchiotroch. (7), Chsetopod trochosphere larva with cephalo-
troch only, and elongation and segmentation of the oro-anal axis, a, anus; o,
mouth; pt\ prostomiuni ; iv, vential invagiuation of Phoronis larva. A B, oro-
unal axis; V D, dorso-ventral axis.

the mouth and a numbef of ciliated post-oral processes
or tentacles. The anus is placed at the extremity of the
elongate body opposite to that bearing the mouth and

prfe-oral hood. The pra?-oral hood becomes the epistome,
and the tentacles, by further development (new tentacles
replacing the larval ones), become the horse-shoe-shaped
group of tentacles of the adult. A very curious process
of growth changes the long axis of the body and results
in the anus assuming its permanent position near the
mouth. An invagination appears on the ventral face of
the larva between the anus and mouth, and attains con-
siderable size. At a definite moment in the course of
growth this invagination is suddenly everted, carrying
with it in its cavity the intestine in the form of a loop.
Thus a new long axis is suddenly established at right
angles to the original oro-anal axis, and continues to de-
velop as the main portion of the body. The short area
extending from the prse-oral hood to the anus is thus the
true dorsal surface of Phoronis, whilst the elongated body
is an outgrowth of the ventral surface perpendicular to
the primary oro-anal axis, as conversely in many Mollusca
we find a short ventral area (the foot) between mouth and
anus, and an outgrowth of the dorsal surface (the visceral
hump) perpendicular to the primary oro-anal axis, forming
the chief body of the animal. In these relations Phoronis
(and with it the other Polyzoa) agrees with Sipunculus.
On the other hand Echiurus, the Chastopods, Nemertine
worms, and some other groups which start from a simple
larval form not unlike that of Phoronis, present a continual
elongation of the original oro-anal axis, and no transference
of the long axis by the perpendicular or angular growth of
either the ventral or the dorsal surface of the larva.

Phoronis was discovered originally in the Firth of Forth
by Dr Strethill Wright. It occurs in the Mediterranean
and in Australian seas (Port Jackson).

THE PTEROBRANCHIA.

This section of the Polyzoa also comprises forms which
differ very widely from Paludicella. Inasmuch as their
development from the egg is at present quite iinknown,
it may possibly prove that they have other affinities.
Only two genera are known, Rhabdopleura (Allman) and
Cephalodiscus (M'Intosh), the former dredged by Dr
Norman in deep water off the Shetlands (and subse-
quently in Norway), the latter taken by the " Challenger "
expedition in 250 fathoms off the coast of Patagonia.

The Pterobranchia have the mouth and anus closely
approximated, and immediately below the mouth are given
off a series of ciliated tentacles, but these do not form a
complete circle as in Paludicella, nor is the lophophore (the
platform of their origin) horse-shoe-shaped as in Phoronis.
The lophophore is drawn out into a right and a left arm in
Rhabdopleura (fig. 7), upon each of which are two rows
of ciliated tentacles ; no tentacles are developed centrally
in the region between the two arms, so that the mouth is
not completely surrounded by these processes. The horse-
shoe-shaped lophophore of Phoronis could be modified so as
to represent the tentaculiferous arms of Rhabdopleura by
suppressing both rows of tentacles at the curve of the
horse-shoe, and leaving only those which occur on the
arms or rami of the horse shoe (see fig. 4). The lopho-
phore of Cephalodiscus presents us with twelve processes,
each carrying two rows of ciliated tentacles ; in fact we
have six pairs of tantaculiferous arms instead of a single
pair, and each of these arms is precisely similar to one
of the arms of Rhabdopleura (fig. 9), excepting that it
terminates in a knob instead of tapering. There is no
arrangement for introverting the anterior portion of the
body into the hinder portion in the Pterobranchia.

The little epistome or pnu-oral lobe of Phoronis is repre-
sented in the Pterobranchia by a large muscular shield or
disk-like structure (fig. 7, d and fig. 9, 6) which over-
hangs the mouth and has an actively secreting glandular

P 0 L Y Z O A

165

surface by which the tube or case (tubarium) in which the
polypide is enclosed is secreted.

Both llhabdopleura and Cephalodiscus produce colonies
by • budding ; but the colonies of the former are large,
definite, and arborescent, whilst those of Cephalodiscus
are remarkable for the fact that the buds do not remain
long in organic continuity with their parent, but become
detached and nevertheless continue to be enclosed by the
same common envelope or secretion. The bud-formation
of Khabdopleura recalls that of Paludicella in the fact that
it leads to the formation of continuous arboriform com-
munities. That of Cephalodiscus resembles the budding
of Loxosoma, since no two fully-formed individuals remain

9

FIG. 7.— Rhabdopleura A'ormnni, Allman (original drawings, Lankcster). A.
A single polypide removed from its tube and greatly magnified. a, mouth; b,
anus ; c, polypide-stalk or gynraocauliu, the " contractile cord " of Sars ; rf, the
pros-oral lobe (buccal shield or disk of Allman); e, intestine; /, thoracic
region of the polypide; jr. one of the ciliated tentacles. B. Lateral view to
show the form of the buccal shield and its pigment spot, g, ciliated tentacle
(in outline); A, basal ridge of the right arm of the lophophore. C. Lateral vic'w
of a polypide. f, ciliated patch (Sars's organ) at the base of the lophophore-arm.
Other letters as above. D. Part of a lophophore-arm. with soft tissues rubbed
off to show the cartilaginoid skeleton, a, epithelium and soft tissues still
adherent at the tip of a tentacle ; b, skeleton of tentacle; c, skeleton of axis.
E. Portion of a colony of Rhabckipleura Normani, showing the branched tube-
like cases formed by the polypides. The black line within the tubes represents
the retracted polypides connected together by their common stalk, the pecto-
caulus. Magnified to three times the size of nature.

in organic continuity. Both Khabdopleura and Cephalo-
discus (like Phoronis) produce cases or investments in
which they dwell. These are free secretions of the organ-
ism, and are not, like the ccencecia of Eupolyzoa, cuticular
structures adherent to and part of the polypide's integu-
ment. The dwelling of Rhabdopleura is a branched
system of annulated tubes of a delicate membranous con-
sistency, each tube corresponding to a single polypide, the
rings of which it is built being successively produced at
the termination of the tube by the secreting activity of the
prse-oral disk (fig. 7, E). The polypides freely ascend and
descend in these tubes owing to the contractility of their
stalks. On the other hand the dwelling of Cephalodiscus

is a gelatinous, irregularly branched, and fimbriated mass
(fig. 8), excavated by numerous cavities which communicate
with the exterior. In these
cavities are found the nu-
merous detached small
colonies of Cephalodiscus
(fig. 9), or we should rather
say the isolated budding
polypides. The remaining
important feature in the
organization of the Ptero-
branchia, namely, the parts
connected with the forma-
tion of buds, are best un-
derstood by first examining
Cephalodiscus. The body
of Cephalodiscus is seen
(fig. 9) to be an oval sac ;
in this is suspended the
U-shaped alimentary canal,
and from the walls of its
cavity (ccelom) the ova and
the spermatozoa are de-
veloped. Projecting from
the ventral face of this
oval sac is a muscular cy-
lindrical stalk, into which
the viscera do not pass,
though the ccelom is con-
tinued into it (fig. 9, c).
This stalk is merely the
outdrawn termination of
the body.

long as the whole of the
rest of the animal, and it
is from its extremity that
the buds are produced (fig.
9, a). Before the buds have attained half the size of their
parent they become detached, but continue to occupy some
portion of the common gelatinous dwelling.

It is about as FlG' S.— Dwelling of gelatinous consistence
and brown colour formed by the polypides
of Cephalodiscus dodecalophltt, M'lntosh;
natural size (from an original drawing
kindly supplied by Prof. Jl'Intosh, F.R.S.).
o, polypide within the jelly ; p, cavity once
occupied by polypides.

FIG. 9. — A polypide of C?i>hatv<U*cus (Jotffcaiopkus removed from the gelatinous
house (from an original drawing liy Prof. M'lntosh). No organic connexioq
has been severed in thus i^nhiling this polypide with its attached buds a, a.
The figure represents the furthest point to which colony-formation attains in
this form, a, buds growing from the bas ; of the polypide-stalk ; b, the prse-
oral lobe (buccal shield or disk); c, the polypide-stalk; d, the ciliated tentacles
of the twelve lophophore arms (six pairs, each like the single pair of Khab-
dopleura) inextricably matted and confused ; e, anterior margin of the prse-
oral lobe ; /, posterior margin of the same. Magnified about fifty times linear.

Turning to Rhabdopleura, we find that each polypide
has a body of similar shape and character to that described
for Cephalodiscus, and a similar ventrally developed
" stalk " (fig. 7, A, c). But, inasmuch as the buds deve-

166

P O L Y Z 0 A

loped on the stalk of a Rhabdopleura polypide do not
detach themselves, we find that we can trace the stalk of
each polypide of a colony into connexion with the stalk of
the polypide from which it was originally budded, which
may now be considered as a " branch " bearing many-
stalked polypides upon its greatly extended length, and
such a " branch-stalk " may be further traced to its junc-
tion with the " stem-stalk " of the whole colony. The
stem-stalk was at one time the simple terminal stalk of a
single polypide, but by lateral budding it gave rise to
other polypides, and so became a gemmiferous " branch " ;
and further, when some of these in their turn budded and
became branches, it became the main " stem " of a copious
colony.

A serious error has been made in comparing the contrac-
tile stalk of the Pterobranchiate polypide to the "funi-
culus " or cord-like mesentery of Eupo-
lyzoa. With this it has morphologi-
cally nothing in common, since it is
not an internal organ, but simply the
elongated termination or stalk of the
body, comparable to the stalk of Pedi-
cellina (fig. 15) and Loxosoma (fig. 16),
or to the hydrocaulus of such a Hydro-
zoon colony as Cordylophora. The
stalk where it bears only very young
buds, or none at all, as is always its
condition in Cephalodiscus and in many
polypides of a Rhabdopleura colony,
may be called a " gymnocaulus"; when
once its buds have devel-
oped into full grown poly-
pides, and it has elongated
proportionally with their

growth, it becomes a "pec- F!a, IO.-A polypide of cephaiodiscus *>-
tocaulus"; that is to say, it
is to that part of it which
bears such polypides that
this term may be conveni-
ently applied. The pecto-
caulus of Khabdopleura, both in the form of branch and
stem, undergoes remarkable change of appearance as com-
pared with the gymnocaulus. It loses its contractility,
shrinks, and develops on its surface a hard, dark, horny
cuticle (whence its name), comparable precisely in its nature
'to the hardened cuticle which forms the zocecia of Eupo-
lyzoa. It now has the appearance of a black cord or
rod-like body lying within and adherent to the inner face
of the much wider tubular stem, and branches formed by
the gradual building up and arborescent extension of the
annulated tubarium secreted by the individual polypides.
It has been regarded both by Allman and by Sars as a
special structure, and called by the former " the chitinous
rod" or " blastophore," by the latter "the axial cord."

In reality it is the black-coloured pectocaulus of
Rhabdopleura which corresponds to the coancecium of an
ordinary Polyzoon; whilst the term "ccenojcium" is
totally inapplicable morphologically to the annulated
branched tube in which the Rhabdopleura colony lives,
this having absolutely no parallel in the Eupolyzoa.

A sac-like testis has been discovered in Rhabdopleura
opening by the side of the anus (Lankester, 7) ; but the
ova have not yet been seen, nor is anything known of its
development. Similarly the eggs of Cephalodiscus are
observed within the body of the parent in the " Chal-
lenger " specimens, but nothing further is known of its
life-history.

A body-cavity is present (Lankester), though its exist-
ence has been denied by Sars and by M'Intosh. Neph-
ridia and nerve ganglia are not described. Cephalodkcus

decatophiis, from which the lophophore-
tentades and buccal shield have been
removed in order to show the remark-
jible eyes, a, buds; c, stalk; g, eyes;
h, post-oral collar, hidden by the buccal
shield in fig. 9. (Original drawing by
Prof. M'Intosh F.R.S.)

has two remarkable eye spots dorsal to the cephalic disk
(fig. 10, </).

THE EUPOLYZOA.

Whilst it is necessary to include in the group Polyzoa
the forms we have already noticed as Vermiformia and
Pterobranchia, there can be no doubt that those organisms
to which we assign the name Eupolyzoa are primarily
those upon which naturalists have framed their concep-
tion of the group, and that they constitute a very con-
sistent assemblage, held together by well-defined characters,
and yet presenting an immense number of varied forms
showing a wide range of modifications.

All the Eupolyzoa have closely approximated mouth
and anus, and, like Paludicella, a complete range of hollow
ciliate tentacles, describing either a circle or a horse shoe,
surrounding the mouth. The anus as well as the mouth
is included in this area in a few exceptional forms (the
Entoprocta) ; it lies near but outside the lophophore (as the
area is termed) in the vast majority (the Ectoprocta).
Except in the Entoprocta, where the movement is limited,
the whole anterior portion of the body bearing the
lophophore can be invaginated into the hinder part (as
described above for the typical Eupolyzoon Paludicella).
This character distinguishes the Eupolyzoa from both
Vermiformia and Pterobranchia. The polypides of all the
Eupolyzoa are minute, but all produce buds which remain
in organic continuity with their parent (except in Loxo-
soma) and build up very considerable and sometimes
massive colonies.

In all Eupolyzoa the cuticle of the hinder part of each
polypide is thick and dense, thus forming a hard-walled
sac, the zocecium. This is peculiar to and universal in
the Eupolyzoa (except Loxosoma), and is not to be
confounded with the non-adherent tubes of Phoronis and
Rhabdopleura or the jelly-house of Cephalodiscus. The
connected zorecia of a colony of Eupolyzoa constitute a
ccencecium. A simple nerve ganglion between mouth and
anus, a large body-cavity (except in Entoprocta), simple
gonads without accessory glands or ducts, usually testis
and ovary in the same polypide, absence of a blood-vascular
system, of any but the most rudimentary nephridia, and
of eyes, otocysts, or other special sense-organs, are features
characterizing all adult Eupolyzoa.

The section Eupolyzoa, with its vast number of species
and genera, requires a somewhat elaborate classification.
The forms in which the anus is enclosed within the
tentacular circle are very few, and are peculiar in other
respects. We follow Nitsche (8) in separating them as
the sub-class Entoprocta from the majority of Eupolyzoa
forming the sub-class Ectoprocta.

Sub-class 1. Ectoprocta, Nitsche.

Eupolyzoa with the anus not included within the area
of the lophophore. Anterior portion of the body of the
normal polypide introversible. Tentacles not individually
capable of being coiled or flexed.

Order 1. PHYLACTOL.EMA, Allman.
Ectoproctous Eupolyzoa in which the polypide possesses
a praj-oral lobe or epistome, similar to that of Phoronis,
and comparable to the more highly developed buccal
shield or disk of the Pterobranchia. Lophophore (except in
Fredericella, where it is nearly circular) horse-shoe-shaped
(hippocrepian). Polypides of a colony equi-fonnal, that is,
not differentiated in structure and function. Neighbouring
zocecia are in free and open communication, the bud never
becoming shut off by a perforated cuticular plate from its
parent. Cuticle of the zocecia either gelatinous or horny,
forming massive or else arborescent coencecia, in one genus

P 0 L Y Z O A

167

(Cristatella) having the form of a plano-convex ellipse and
locomotive (fig. 3). In addition to the multiplication
of polypides in a colony by budding, and to the annual
production of new individuals from fertilized eggs which
initiate new colonies, a reproduction by internal buds
called " statoblasts," comparable to tho gemmre of Spon-
gilla, has been observed in all the genera (fig. 3, I). The
statoblasts are developed from the funiculus (mesentery),
and are enclosed in ornate lenticular capsules of chitinous
substance, characteristic in form in each species.

The fertilized egg of the Phylactolwma does not give
rise to a zonociliate larva, but to a uniformly ciliate cyst-
like diblastula, which develops directly and produces
polypides by budding. The Phylactolieuia are all inhabit-
ants of fresh water (lacustrine).

Fio. 11.— Semi-Weal view of paitof the lophophore of Lophopus and its tentacles,
— intended to show the nerve-ganglion, nerves, and parts around the mouth.
The tentacles have been cut away all along the right arm of the lophophore and
from the inner margin of the left arm. c, foramen .placing the cavity of the
epistome in communication with the body-cavity; c', body-wall ; d, mouth; e,
the epistome or pra-oral lobe ; /, wall (if the pharynx ; A, wall of the intestine ;
t, anus ; k, lophophore ; I, a ciliated tentacle ; r, elevator muscle of the epistome;
IT, the nerve-ganglion ; x, x't nerves to lophophore and tentacles • y, nerve to
pharynx.

The Phylactola?ma include the genera Lophopus, Cristatella,
Alcyonella, Plumatella, and Fredericella, which have been beauti-
fully figured and described in Allman's classical Freshwater Polyzoa,
Ray Society, 1856. The colonies of Lophopus are small, consist-
ing of half a dozen polypides embedded in a massive glass-like
ccencecium. Cristatella (fig. 3) is remarkable amongst all Polyzoa
for its locomotive zoarium. Alcyonella forms massive ccencecia of
many hundred polypides, as large as a man's fist. Plumatella and
Fredericella are delicate arborescent forms commonly encrusting
stones and the leaves of water-plants. All the genera known are
British.

The Phylactolrema furnish a remarkable instance of a well-
marked zoological group being confined to fresh water. Their
reproduction by statoblasts (not known in the marine Polyzoa)
appears to be related to the special conditions of lacustrine life,
since it is also observed under the same exceptional conditions in
the single freshwater genus of another great group of animals, viz.,
Spongilla. Also related to their non-marine conditions of life is
the development of the fertilized egg, which, as in so many similar
cases, does not produce the remarkable banded forms of locomotive
larvce which are characteristic of their marine congeners.

Order 2. GYMNOL^EMA, Allman.

Ectoproctous Eupolyzoa in which the polypide is devoid
any trace of the prre-oral lobe or epistome, whilst the

lophophore is perfectly circular. The polypides of a colony
are frequently highly differentiated as avicularia, vibracu-
laria, ocecia (egg-receptacles), and even as root and stem
segments. The neighbouring polypides of a colony
communicate (?) with one another by " rosette-plates " or
" communication-plates " — perforated areas in the walls of
contiguous zooecia. The greatest variety in the character
of the cuticle forming the zooecia (gelatinous, horny,
calcareous) and in the grouping of the polypides, as well as
in the shape of their zooecia, is observed in different
sub-orders and families. In addition to the ordinary
sexual reproduction, there are various modifications of the
process of budding, the full exposition of which would
necessitate more space than is here allotted, and is not
yet indeed within the possibilities of present knowledge.
The fertilized egg of the Gymnokema gives rise to
remarkable ciliate larvse of various
forms (figs. 19, 20, 21), from which
the first polypide of a colony is
developed l>y an extraordinary and
unexplained series of changes.
The Gymnola?ma are, with the
single exception of the genus Palu-
dicella, inhabitants of the sea.

The Gynmolffima are divided, accord-
ing to the system of Busk, into three
sub-orders characterized by the shape
of their zocecia, and the nature of the
mouth-like margin which it presents
when the exsertile portion of the poly-
pide is withdrawn within it. The
Cyclostoma have long tubular zocecia,
often of large size and often calcified,

§ laced side by side in cylindrical bun-
les, or in other definite grouping ; the
mouth of the zocecium is circular and
devoid of processes. There is little or
no differentiation of the polypides con-
stituting a colony. Host of this group
are fossil, and the living genera belong
mostly to southern seas. The genera
Crisia (fig. 13, A), Diastopora, Tubuli-
pora, and Hornera are typical. The
Ctenostoma have usually a soft zoce-
cium ; its orifice is closed by the folds
of the retracted polypide or by a
circlet of bristles which surround it.
Alcyonidium gdatinosum is the com-
monest representative of this group
on the British coasts. Bowerbankia
(fig. 1, A) and Paludicella (fig. 1, E)
also belong here. The ChSostoma
form the largest and most varied sub-
order of Gymnolfema. The zocecia are
horny or calcified ; their orifices can be
closed by a projecting lip in the form
of an operculum. The operculum is
a separable plate developed on the
cuticle of the retractile part of the
polypide, and lias muscles attached
to it (fig. 13, B, C, D). The surface
of the zooecia is frequently sculptured,
and its orifice provided with processes
and spines (fig. 1, C, F). Very usually
some of the polypides of a colony are
modified asavicularia,vibracularia, radi-
cal fibres, and ocecia. The avicularitnn
is a polypide reduced to a simple muscu-
lar apparatus workingupon the modified
operculum and zocecium so as to cause these hard parts to act as a
snapping apparatus comparable to a bird's head (fig. 12, o). They
are frequently found regularly distributed among the normal cells
of a colony, and probably have a cleansing function similar to that
attributed to the Pedicellaria?. of the Echinoderms. "Vibracularia"
are even more simplified polypides, being little more than motile
filaments, probably tactile in function. The opercula of zocecia,
ocecia, and avicularia have recently been used by Busk in character-
izing genera and species, in a systematic way. Stem-building and
root-forming polypides are frequently found, being closed pulvpidcs
which subserve anchoring or supporting functions for the benefit of
the whole colony. The stem of Kinetoskias (fig. 14) is produced

Fio. 12.— Two zooecia of Ata-
marchis (Bugula) avicularia,
Lmx. (Chiiostoma), of which
the anterior contains a living
polypide, whilst the posterior
is empty. To each is attached
one of the characteristical-
ly modified polypides known
as an " aviculaiinm" o\ the
hinder of these hiis grasped
and holds in its beak a small
worm. «, anus; i, intestine;
t>, stomach ; r, body-cavity
(coelom) ; >, tentacular crown
surrounding the mouth ; tet
testis cells developed on the
surface of the terminal mesen-
tery or " funiculus " ; o, o, avi-
cularia.

168

P O L Y Z O A

in this way. The Chilostoraa include a large series of genera
arranged in the sections Cellularina, Flustrina, Escharina, and

FIG. 13. — A. Crencecium of Ct'isia ebnrnea, Lin., one of the Cyclostoma ; g, g,
tubular zocecia with circular terminal mouths; .r, ocecium, being a zocecium
modified to serve as a brood-chamber.

B. Diagram of a single polypide of one of the Chilostoma in a state of expansion,
in order to show the position and action of the operculum. a, operculum,
a plate of thickened cuticle hinged or jointed to b, the main area of dense
cuticle of the antitentacular region known as the zocecium ; c, the soft-walled
portion of the polypide in expansion.

C. The same zooecinm with the polypiile invaginated (telescoped) and the
operculum a shut down over the mouth of the zocecium.

D. Operculum detached, and seen from its inner face, to show the occlusor
muscles d d.

Celleporina. For the systematic description of the highly complex
and very varied colonial skeletons or ccencecia of the Gymnola;uia,

Fro. U.—KinelosHas (ITaresia) ci/nllnis (from Sir Wyville Thomson). The poly-
pidea and zocecia are allied toSuffttla, but the zoarium as a whole is remarkable
for its definite shape, consisting of a number of slightly branched gracefully
bending filamenls supported like the leaves of a palm on along transparent
stalk. (See Husk, in Quart. Joarn. jlficr. »•<., 1X81, for further details.)

the reader is referred to the works of Busk (9), Hincks (10), Sinitt
(11), and Heller (12). See also Ehlers (13) on Hypophorella.

Sub-class 2. Entoprocta, Nitsche.

Eupolyzoa in which the anal aperture lies close to the
mouth within the tentacular area or lophophore. Lopho-
phore sunk within a shallow basin formed by the inversion
of the broad truncated extremity of the cup-shaped body.
Tentacular crown not further introversible, the individual
tentacles (as in the Pterobranchia and unlike the Ecto-
procta) capable of being flexed and partially rolled up so
as to overhang the mouth (see fig. 15, B and C). Body-
cavity (ccelom) almost completely obliterated. The anti-
tentacular region of the polypide's body is drawn out to
form a stalk similar to the gymnocaulus of the Pterobran-
chia. The extremity of this stalk is provided with a
cement gland in the young condition which persists in the
adult of some species (Loxosoma neapolitanum, fig. 16,
dis). Cuticular investment (zocecium) of the polypides
feebly developed. A pair of small nephridia are present.

The Entoprocta consist of the marine genera Pedi-
celliua (fig. 15), Loxosoma (fig. 16), and probably the

FIG. 15. — A. Two polypides and buds of Pedicellina belgica, Van B. (after
Van Beneden); greatly magnified, a, the polypide-stalk of a fully developed
polypide ; c, that of a less mature individual; 6, a bud. All are connected by
a common stalk or stolon. B. and C. Two views of the body of the polypide
of Pedicellina (after Alhnan). a, cuticle ; b, body-wall ; c, permanently in-
troverted anterior region of the body; </, margin of the tentacular cup or
calyx thus formed; e, mouth; /, pharynx; tj, stomach; A, intestine; i, anus;
k, epistome or prse-oral lobe ; /, nerve-ganglion ; m, gonad ; «, retractor
muscle of the lophophore ; o, lophophore.

insufficiently known freshwater American genus Urnatella
of Leidy. To these must be added P>usk's new genus
Ascopodaria, as yet undescribed, based on a specimen
dredged by the " Challenger," showing a number of Pedi-
cellina-like polypides, carried as an umbel on a common
stalk of very peculiar structure. Pedicellina is found at-
tached to alg;e, shells, zoophytes, etc., and to the integu-
ment of some Gephyrrean worms (Xijnt/iat/i/n pimi'/atus)
and Annelids (Aphrodite) ; Loxosoma occurs on carious
worms, &c. Whilst the buds of Pedicellina remain connected
so as to constitute a colony, those produced by Loxosoma
are continually detached, so that the polypide is solitary.
Further, the cup-like body of Pedicellina is deciduous, and
frequently falls from the stalk and is replaced by new
growth. There is less distinction between body and stalk
in Loxosoma, and the former does not become detached.
Apparently a very important feature in the structure of
the Entoprocta is the absence of a body-cavity. This is,
however, more apparent than real. The Entoprocta are
true Ccelomata, but the ca-lom is partially obliterated by
the growth of mesoblastic tissue. The nephridia presum-
ably lie in a space which, small as it is, represents the
ccelom. See Harmer (18) for details.

P 0 L Y Z O A

Genealogical Relations/lips nf the flrmtps of Polyzoa.

It is necessary that we should try to form some opinion
as to which of the various groups of Polyzoa are most like
the ancestral form from which they have all sprung, and
what are the probable lines of descent within the group.
Any attempt of the kind is speculative, but it is absolutely
needful since zoology has become a science — that is to say,
an investigation of causes and not merely a record of
unexplained observations — to enter upon such questions.
Colonial organisms have necessarily descended from soli-
tary ancestors, and it is probable that the ancestral form
of Polyzoa was not only solitary, as are Phoronis and
Loxosoma at the present day, but of relatively large size
and more elaborately organized than the majority of living
Polyzoa. Whilst the polypides have dwindled in size and

FIG. 16. — Diagram nf Li.rrtsoma Xfapolitamtm (after Kowalewsky). A single
polypide devoid of buds, in, mouth ; stt stomach ; s/is, basal gland of the
polypide-stalk.

lost some of their internal organs, the modern Polyzoa
have developed pari passu with this degeneration an
elaborate system of bud-production and colony-formation.
The new individuality (the tertiary aggregate) attains a
high degree of development (Cristatella, Kinetoskias) in
proportion as the constituent units merged in this new
individuality have suffered a degeneration. The prae-oral
lobe (epistome, buccal disk) present in all Polyzoa except
the most minute and most elaborately colonial forms —
namely, the Gymnolasma — is to be regarded as an ancestral
structure which has been lost by the GymnoUema. The
horse-shoe-shaped lophophore, such as we see it in Phoronis
and in Lophopus, is probably the ancestral form, and has
given rise to the two other extreme forms of lophophore, —
namely, the " pterobranehiate," associated with a great
development of the epistome, and the " circular," associated
with a complete suppression of the epistome. The ento-
proctous lophophore is a special modification of the horse-
shoe-shaped, as shown in the diagram fig. 15, C. The
formation of zocecia, and so of an elaborate colonial

skeleton, was not a primary feature of the Polyzoa. Even
after budding and colony-formation had been established
zoa'cia were not at once produced, but possibly dwellings
of another kind (Pterobranchia). Wo aro thus led to look
upon the GymnoUema as the extreme modification of the
Polyzoon type. Starting with an organism similar to
Phoronis, we may suppose the following branchings in the
pedigree to have occurred.

VEUMIFOUMIA

A. The complete hippocrepian
lophophore becomes specialized
in the, form of etenidia or gill-
phnnes ; the epistome enlarged.

= PTEIIOBRANCHIA.
o. The anti-tentacular region of
the body elongated as a stalk
gives rise to one or two
rapidly detached buds (IV-
phalodiseus).

B. The stalk gives rise to Imds
which do not detach them-
selves, but remain in con-
tinuity so as to form a
colony of a hundred or
more individuals (Khabdo-
pleura).

15. The complete hippocrepian
lophophore retains its form, but
acquires a gradually increasing
power of being telescoped into
the hinder part of the body.
= The Pro-Eupolyzoon.

A. The anti-tentacular region
of the body becomes stalk iikf,
and develops buds which either
detach themselves as they form
(Luxosoma) or remain to form a
small colony (Pedicellina). The
telescopic introversibility of the
lophophore docs not advance be-
yond an initial stage. The arms
of the lophophore grow round so
as to embrace the anus.
= Sub-class 1 (of the Eupoly-
zoa) Entoprocta.

15. The complete hippocrepian
lophophore remains in its origi-
nal form, and also the prse-orul
epistome, but the telescopic in-
troversibility of the anterior
region of the bod}' is greatly de-
veloped at the same time that
the cuticle of the hinder part of
the body is increased in thickness
and toughness. Bud production,
not from a stalk-like pedicle, but
from all parts of tho body, now
becomes characteristic, the buds,
which were at first deciduous,
now remaining in permanent
continuity so as to form colonies.
= Tlir Pro-Ectoprocton.

A. The polypides acquire the
property of carrying their young
so as to avoid the disastrous
influences of fluviatile currents,
and also the property of produc-
ing resistent statoblasts, and
thus are enabled to become
isolated and to persist in the
peculiar conditions of fresh
waters.

= The 1st order (of Eetoprocta)
Phvlactolcema.

B. The polypides forming
relatively larger colonies, and
themselves becoming relatively
more minute, loso by atrophy the
prie-oral epistome ; and simul-
taneously the arms of tho hippo-
crepian lophophore dwindle, and
a simple circnm-oral circlet of
tentacles is the result. The
cuticle of the hinder part of the
polypide becomes more and more
specialized as the cell or zoce-
cium, and in different polypides
in various parts of the colony
acquires special forms — as egg-
cases, snappers (avicularia), ten-
tacles, stalk and root segments.
=-Thc 2d order (of Eetoprocta)
Gymnolajma.

Distinctive Characters of the Poly:<i«.

From all that has preceded it appears that the really
distinctive characters common to all the Polyzoa may be
summed up as follows : —

Ciulomata with closely approximated mouth and anus,
the bulk of the body forming a more or less elongate
growth at right angles to the original (ancestral) oro-anal
axis, and starting from the original ventral (i.e., oral) sur-
face. A variously modified group of ciliated tentacles is
disposed around the mouth, being essentially tho develop-
ment by digitiform upgrowth of a post-oral ciliated band.

170

P O L Y Z 0 A

As negative characters it is important to note the absence
of all trace of metameric segmentation, of setse, and of
paired lateral (parapodia of Appendiculata) or median
ventral (podium of Mollusca) outgrowths of the body-wall.
Larval Forms of Polyzoa.

In the consideration of the probable pedigree and affinities of the
Polyzoa, we are not at present able to make use of the facts of
development from the egg, on account of the extreme difficulty
which the study of the young stages of these organisms presents.
In the case of Phoronis we have the only readily intelligible his-
tory. The larva, to start with, is of that form known as an aivhi-
troch (see Lankester, "Notes on Embryology and Classification,"
Quart. Journ. Micr. Sci., 1876), having a pros-oral ciliated area
(velum or cephalotroch) continuous with a post-oral ciliated band
(the branchiotroch), which latter becomes developed into the ten-
tacular crown of the adult.

The actmotrocha (Phoronis) larva is readily comparable with the
trochosphere larva? of Echinoderms, Chsetopods, Gephyrreans, and
Molluscs. Its special character consists in the strong develop-
ment of the post-oral ciliated band, whereas the prse-oral ciliated
band is in most other classes (the Sipunculoids excepted) the
predominant one. The Fhoronis larva exhibits first of all an oro-
anal long axis, and this is suddenly abandoned for a new long axis
by the growth of the ventral surface of the larva at right angles to
the primary axis (hence the term Podaxonia).

In the other Polyzoa we do not at present know of any larva
which retains even in its earliest phases the original oro-anal long
axis. They all appear to start at once with the peculiar and
secondary long axis of the adult Phoronis, so that Balfour has
diagrammatically represented the Polyzoon larva by the sketch
given in fig. 19. This diagram applies, however, more especially to
the Entoprocta, since the anus is represented as included in the area
of the post-oral ciliated ring. The development of Pedicellina has
been very carefully followed by Hatschek, and may be said to be

Fig. 17. Fig. 18.

FIG. 17. — Larva of Pedicellina (from Balfour. after Hatschek). t', vestibule
(the cup-like depression of the tentaculiferous end of the body) ; in, mouth ;
/, digestive gland ; uu.i, anal invagination ; fg, the ciliated disk (corresponding
to the cement gland of Loxosoma (fig. 16, s/is) ; x, so-called "dorsal organ,"
supposed by llalfour to be a bud, by Haimer (18) regarded as the cephalic
ganglion.

FIG. 18. — Later stage of the same larva as fig. 17. Letters as before, with the
addition of nph, duct of the right nephridium ; a, anus ; hg, hind-gut.

the only instance among the Eupolyzoa in which the growth of
the different organs and the consequent relation of the form of the
larva to the form of the adult is understood (see figs. 17 and 18).

In the other Polyzoa, in spite of the painstaking and minute
studies of Barrois (14), the fact is that we do
not know what face of the larva corresponds to
the tentacular area, what to the stalk or anti-
tentacular extremity, what to the anterior and
what to the posterior surface. The conversion
of the larva into the first polypide has not st
been observed in the case of these free-swim-
ming forms, and it is even probable that no
such conversion ever takes place, but that the
first polypide forms as a bud upon the body-
wall of the larva. FIG. 19.— Diagram of an

Two of the most remarkable forms of free-
swimming larva; of Gynmolffima are repre-
sented in figs. 20 and 21. In both, in addition
to the chief post-oral ciliated band, a smaller
ciliated ring is observed, which is identified
by Balfour with that which is found at the anti-tentacular extremity
(base of the stalk) in the Pedicellina larva.

(from Balfoui).
anus ; m, mouth ; st,
stomach ; 5, ciliated
; in figs. 17, IS,

_ It does not seem justifiable, in the face of the existing uncertain-
ties as to identification of parts, and in view of the high probability
that the Gymno-
Isema are extremely
modified and degen-
erate forms (a con-
sideration which
applies in some re-
spects even more
strongly to the En-
toprocta), to assume
that the larval form

schematized in fig. -,._„„, . .

3 F:G. 20. — Larva of Ahyomdium mytih (from Balfour
iy represents an an- after Barrois). m 1, problematic "structure; st, oral
cestral condition of invngination (?) = Harmer's cephalic ganglion; s, cili-
the Polyzoa. Pro- ated disk (corresponding to/.? in figs. 17, 18, and 21).
fessor Balfour (15) was, however, led to entertain such a view ; and,
assuming that the chief ciliated band (drawn as a broad black line)
corresponds to the single
pive-oral ciliated band of
the trochosphere larva of
Eehiurus, Polygordius,
Cluetopods, and Mollus-
ca, he pointed out that in
both eases the ciliated
girdle divides the larva
into a hemisphere in
which month and anus
lie and a hemisphere
which is the complement
of this ; in most classes
the first hemisphere
elongates and forms the
bulk of the body, whilst
the second hemisphere
forms the prostomium or
nrsp-nval Inhp Rut in IGl 21-~Lal'va cf MemDrampora (known as Cy
>oe. But, ac- plKmautcs) m> mouth ; a', amis j /?, ciliated
cording to Balfour s body; a-, problematical body, supposed by Bal'
theory, in Polyzoa it is f°ur *° ^e a Dutl> similar to the dorsal organ 2
the second hemisphere in "e9- 17. 18- ™4 '» either s/ or m in flg. 20.
which enlarges and becomes the stalk-like body of the adult, whilst
the first hemisphere remains small and insignificant. Thus the
Polyzoa would fix themselves in
later growth by what corresponds
to the head or prostomium of
other animals, as do the Bar-
nacles and the Ascidians. In-
genious as this speculation is, we
must remember that it takes nn
account of the facts known as
to Phoronis, nor of the Ptero-
branchia, and that it is con-
fessedly based upon the assump-
tion that the larva; of extremely
degenerate and peculiar members
of the group are not adaptive and
modified, but retain primary and
archaic characters. Further, it
is to be distinctly borne in mind
that the interpretation of parts
upon which this speculation
rests is, except in the case of
Pedicellina, altogether hypo-
thetical.

Riiationa of the Po!y:oa to the
Brachiopoda.

The Polyzoa were first asso-
ciated with the Brachiopoda by
H. Milne-Edwards. The inves-
tigation of the development of
Terebratulina by Morse (16) led
to a further perception of the
points of agreement in struc-
ture between these two groups.
Lastly, Caldwell (6) has shown
that the mesenteries of Phoronis
have precisely similar relations
to the lophophore, the nephridia,
ami the termination of the intes-
tine as have the gastro-parietal
and ilio-parietal bands or mes-
enteries of the Terebratulidse.
The young Terebratulina (fig. 22)
may be readily compared with Loxosoma (fig. 16), — the peduncle
with its cement glands in the former being identical with the stalk
and basal gland of the latter. The form of the alimentary canal

fa

Jf, seta.- at the margin of the calyx ; p,
stalk comparable to the stalk ot Pedi-
cellina, Loxosoma, Cephalodiscus, and
Rhabdopleura;pa, cement gland ut the
apex of the stalk (after Moise).

P 0 L Y Z 0 A

171

niul tlio disposition uf tin' tentacular arms (fig. 23) is the same in
Braohiopoda and Polyzoa. Tin1 nephriiiia (oviducts) of Terebratula

have a position anil relations similar to those of the nephridia (geni-
tal ducts) of 1'horonis. The chief
difference between Polyzoa and
Braehiopoda consists in the special
development of the margin of the
cupped end of the body, into which
the lophophoro is sunk, as in IVdi-
cellina (see fig. 15, 13, e). This
up-standing margin is enormously

Fig. 23.

FIG. 23.— Lophoplinre and epistome of young Terebratulina, showing the her
shoe shape; the aims are turned in the direction the reverse of that taken

the lophophore-arms in Polyzoa (see fig. 4). In later growth they will become
spirally coiled. (After Sloise.)

FIG. 24.— Larv.i of the Brachiopod Argiope (from Gegcnbaur, after Kowalewsky).
m, setigerous lobe; 6, setre; </, enteron.

increased in the Braehiopoda, so as to form a voluminous hood or
collar, which surrounds the large tentacular arms, and forms a pro-
tective chamber for them, it is notched right and left so as to be
divided into two lobes, and on its surface is developed a horny or a
calcareous shell in two corresponding moieties. Until recently it
was held (see Lankester, 17) that both Braehiopoda and Polyzoa
were modifications of the Molluscan type, and the Brachiopods'
collar was identified with the pallial fold of Mollusca. The resem-
blance of the two structures must now be considered as purely
homoplastic, and not as having any real morphological (homo-
genetic) significance.

The larvae of the Braehiopoda (figs. 24, 25) are as exceptional and
difficult of interpretation as those of Polyzoa, but no attempt has
been yet made to show that the one can be reduced to a common
form with the other. The three segments presented by some
Brachiopod larva? (fig. 25) have been compared to the segments of
Cha;topod worms by some writers ; and these, together with the

presence of seta1, have been regarded as indicative of aflinity between
the Braehiopoda and ChtBtopoda (Morse). Hut it is sullieient, in
order to dispose of this suggestion, to point out that the segments
of the Chrctopoda follow one another along the primary oro-aiial
:ixis, whilst those of Brachiopoda are developed along an axis at
right angles to this (C'aldwell).
The Brachiopoda must be classified together with the Polyzoa

FIG. 25. — Surface views of ten stages in the development of Terebratulina. showing
the free-s\vimming larva and its mode of fixation (after Morse), c, lopliophoral
segment; tfi, thoracic segment; p, peduncular segment; rf.s deciduous seise.

and Sipunculoidea in a phylum (Podaxonia) characterized by the
development of this secondary axis.

_,„_ ., ..._ me rvyzoa tn ifie llrmsli Museum (J^-'J, anu loyage

of the " Challenger," "Report on (he Polyzoa," vol. x.; (10) Hincks, British
Marine Polyioa, London, 1880; (11) Smitt, Ki-itist FSrttclcning ofvtr Standi-
' ffafs Bryozoa, 1864-68; (12) Heller, Die Bryozocn d. Jdriatisclim

soma," Quart. Jour. Alicr. Set., April 18S5,

VERTEBEATA

TTERTEBRATA, the name of a great branch or phylum

V of the Animal Kingdom which comprises those ani-

La- mals having bony " vertebra ", or pieces of bone jointed

marck's go as to form a spinal column. The first recognition of

*ncl. , the group is due to Lamarck (1797), who united the four

Cuvier s , . , ° . . , . ,

classifi- highest classes of Linnreus s system as ammaux a verte-

cations. bres," whilst distinguishing the rest of the animal world as
"animaux sans vertebres." The same union of the four
Linnsean classes had been previously made by Katsch in
1788, who, however, proposed for the great division thus
constituted the name " Knochenthiere." The significance
of Lamarck's classification was materially altered, and the
foundation laid of our present attempts to represent by
our classifications the pedigree of the animal kingdom,
when Cuvier propounded his doctrine of " types," and re-
cognized the Vertebrata as one of four great types or plans
of structure to be distinguished in the animal world (i).1

The Vertebrata of Lamarck and Cuvier included beasts,
birds, reptiles, and fishes, and until recently the group
was considered as one of the most sharply limited in the
animal kingdom. The progress of anatomical studies
very soon rendered it clear that all Vertebrata did not
possess bony vertebras ; for, besides the commoner sharks
and skates, with their purely cartilaginous skeletons, natu-
ralists became acquainted with the structure of fishes, such
as the sturgeons and the lampreys, which possess no verte-
brae at all, but merely a continuous elastic rod (the noto-
chord) in the place of the jointed spinal column. The
muscles and their skeletal septa were seen in these fishes
to be arranged in a series of segments attached to the sides
of this continuous rod ; and hence the structural character
of bony vertebra;, as distinguishing the Vertebrata, gave
place to the character of segmental arrangement of the
muscles of the body-wall, such muscles being supported
by a skeletal axis which might be itself unsegmented
(notochord), or replaced by segmental cartilaginous or bony
Essential vertebra. The studies of embryologists furnished a sound
struc- foundation for this conception by demonstrating that in
hires of tjie eulbryos of Vertebrata with true vertebrae these struc-
Verte" tures are preceded by an unsegmented continuous noto-
teS' chord. The inquiry into the structural characteristics of
Vertebrata led further to the recognition of several addi-
tional points of structure, the combination of which was
present only in the group which had been recognized by
Lamarck on superficial grounds^Jt was found that all
T These numerals refer to the bibliography at the end of the artiek-.

Vertebrata possess laterally-placed passages leading from
the pharynx to the exterior, serving in the aquatic forms
as the exits for water taken in by the mouth, and provided
with vascular branchial processes, whilst in the embryos
of the higher air-breathing classes they appear only as
temporary structures. It was further established that the
great mass of nervous tissue lying dorsally above the
spinal column, and known as the cerebro- spinal nerve-
centre or brain and spinal cord, is in all cases a tube,
and originates as part of the dorsal surface of the embryo,
which becomes depressed in the form of a long groove and
finally closed in by the adhesion of its opposite edges, thus
forming a tube or canal. The three structures, — noto-
chord, gill-slits, and tubular dorsal nerve-cord, — were more
than twenty years ago recognized as characterizing, together
with the metameric segmentation of the musculature of
the body-wall, all Vertebrata at some one or other period
of their existence.

The establishment by Darwin of the doctrine of organic Question
evolution in 1859 led naturalists consciously to make the?' Verte-
attempt to determine the genetic affinities and the probable eestry'
ancestry of the various groups of animals, and enabled
them to recognize in the classifications by " type ", and
other such conceptions of earlier systematists, the uncon-
scious striving after genealogical representation of the
relationships of organic beings. The question naturally
arose in regard to the Vertebrata, as in regard to other
great divisions of the animal kingdom, What were the
characters of the earliest forms, the ancestors of those now
living1? Then came the further questions as to whether
any surviving Vertebrata closely resemble the ancestral
form, and whether any animals are still in existence which
retain the general characters of those primeval forms
which were the common ancestors at once of Vertebrates
and of other large and equally well-marked phyla or
branches of the animal kingdom, such as the Molluscs, the
Annulates, &c. This fascinating subject of inquiry received
its most important impulse from the embryological investi-
gations of the Russian naturalist Kowalewsky, and has
been for nearly a quarter of a century the fertile source of
speculation and its indispensable accompaniments, new
observation and research. Kowalewsky published in 1866 Kowa-
an account of the embryology of the lowest and simplest '"vxkv's
of then recognized Vertebrates, the lancelet (Amphioj-ns laljours'
lanceolatus), in which he attempted to trace, cell for cell
from the fertilized egg-cell, the origin of the characteristic

174

VERTEBRATA

Vertebrate organs of this animal (2). This work alone
would not have acquired historic importance, although it
is the starting-point of what may be called strict cellular
embryology, as compared with the less severely histological
works of previous students. But it was accompanied by
an account (3) of the development of Ascidia mamittata,
one of the so-called Tunicate Molluscs, in which it was
demonstrated by Kowalewsky, not only that this supposed
Mollusc possesses when first hatched from its egg-envelope
a notochord, pharyngeal gill-slits, and a tubular dorsal
nerve-cord and brain, but that these three characteristic-
ally Vertebrate features of organization originate from the
same cell-layers of the embryo, and in essentially the same
way as in Amphioxus, whilst the cell-layers themselves
originate from the egg-cell in the two animals by precisely
A 7 B A C D

FIG. 1. — Early stages of Ascidia and A mphionis. a, blastopore ; &, neural
groove; c, neural folds; d, closed portion of neural tube; e, commencing
oral inragination (stomorla'um) of Ascidian tadpole ; /, right and left cloaca!
imaginations of Ascidian tadpole; g, anterior opening of neural tubr of
Amphtoseitt, coincident with the later developed olfactory pit ; /i, wall of one
of the series of paired outgrowths of archenteron or c<elomic pouches of
Amphioxus; i, ectoderm; fc, endoderm (of diblastula phase); /, notochord,
derived from endoderm; m, cavity of gut; 11, cavity of nerve-tube; o, wall
of nerve-tube, formed by upgrowth and union of neural folds ; p, mesoblast
of tail of Ascidian tadpole, derived from eudoderm ; 9, lumen of coelomic
pouches of Aitiphioxiis, ouce continuous with m, but nipped oil1 in the course
of development.

A, B, C, D. Four stages in development of ,4snVm, surface views showing
Rradu.il enclosure of neural area. E, F, G. Three similar stages in develop-
inentof Amphioins. AA. Vertical antero-posterior median section of A. BB.
Similar section of B. DD. Similar section of a stage a little earlier than D.
EE. Similar section of E( Amphioxus). GG. Similar section of embryo repre-
sented in G. H. Transverse section of dihlastnla stage of Amphioxus, with
widely open blastopore (earlier than A or E). I. Transverse (right and left)
section about the middle of F, showing neural area. K. Transverse section
about middle of G, showing nerve-tube, notochord, and ccelomic pouches or
inesoblastic somites q. L. Transverse section of a much further advanced
embryo of Ainphi".iii*, showing nrrvr.tubf, notochord, and gut; the walls
of the ccelomic pouches are now converted into muscular tissue ami the
pouch ca\ ity (<f) compressed. (All the figures after Kowalewsky, 77, IS.)

similar movements of cell division and invagination (see
figs. 1 and 2). Kowalewsky's discoveries established once
for all that the Ascidian tadpole is identical in three very
special and distinct features of structure with the frog's

tadpole. No classification which pretended to set forth
the genetic affinities of animals could henceforth separate

HOTOCHOBD

POSITION OESEIBACLC C|LL.VSLLTSI;
SPINAITCHORO

FIG. 2.— Diagram illustrating relationship of tadpoles of Frog and Aseidian.
The two upper figures represent surface views of the tadpoles ; the two lower
ones show in place the chief Vertebrate organs, viz., notochord, gill-slits,
nerve-tube, and myelonic eye. (From Lankester's Degeneration.)

the Ascidian from the Vertebi-ata, and with it the Ascidian
brought the whole series of Twiicata.

The admission of Tunicata as a group of Vertebrate was Atlmis-
proposed by the present writer as long ago as 1877 (•/) j ™on .of
but it required the intermediate proposition by Balfour of Jv^,,'!
a group Chordata, to comprise the two divisions Tunicata brates.
and Vertebrata, in order to render the final admission of
Tunicata to their proper association with the Vertebrata
of Cuvier palatable to systematists. As an objection to
the simple inclusion of Tunicata in the great phylum Verte-
brata it has been urged that Tunicata do not possess verte-
brae,— a proposition which is equally true of Amphioxus
and of some Fishes. Shifting the objection, some writers
have maintained that the vertebration of the Vertebrata
may be understood as having reference to the segmenta-
tion of the muscles of the body-wall, which is exhibited
by all Cuvier's Vertebrata without exception, inclusive of
Amphioxus, though not by Tunicata. To this it may be
replied that the Ascidian tadpole, and more clearly the
free-swimming Tunicate AppeneHcularia (see fig. 9), do ex-
hibit a segmentation of the muscles of the hinder part of
the body-wall similar to and identical with that of Amphi-
oj'xs, whilst no such strict application of a name in its
original descriptive sense is desirable in systematic nomen-
clature. All Gastropoda (it has been pointed out) are not
gastropodous ; all Art/irujxiJa. are not arthropodous ; and
many Echinoderma are not echinodermous. It is, in the
present writer's opinion, better to retain an historic and
familiar name for the great branch of the animal pedigree
to which it has become necessary to admit forms whose
affinities therewith were at one time unsuspected rather
than to sacrifice historical significance to a futile striving
after etymological accuracy.

The admission of Tunicata to association with Cuvier's Inclu-
Yertebrata has been followed by a further innovation. si°n of
The remarkable marine worm Balanoglossus — originally ^'J^,
described by Delia Chiaje at the end of the 18th century — ';„ yerte
was shown in 1866 by Kowalewsky (j) to possess a series brate
of pharyngeal gill-slits similar to those of Tunicata and phylum.
Awphioxu*. Later researches by Bateson (6) have de-
monstrated that Balanoglossus develops in embryonic life
a short notochord, whilst its nerve-cord is, in part at least,
tubular, and similar in position and relations to the median

VERTEBRATA

175

epidermal tract by the infolding of which the nerve-tube

of Tiuii'-'t/ii and the other Verti-ln-n/n is formed, Hence
it seems impossible to exclude /.'.//<i/t«</i''«,s-»s from a place
in the phylum ]', liihmla. The possession of pharyngcal
gill slits alone might not justify the association; but, when
this is combined with the presence of the notoehord, though
rudimentary and with a special condition and position of
the main nerve-centre, it becomes impossible to ignore
Jin/iiiin'//'i.i!!K.i in our conception of the Vertebrate phylum.
The recent discoveries of Manner (/) with regard to Cephalo-
il/'si-us will hereafter render it necessary to associate that
form, and in all probability RhiiMn[>lcnr<i also, with I>«/</no-
gtosstts in the Vertebrate phylum. The further conse-
quences of such an association in regard especially to the
affinities of Polytna and of Gej.>/iyrxa open up a large field
of speculation and of consequent embryological and ana-
tomical research.

Ances- The Cuvierian Vtrtelrata, AmpJtiojfus, T-unicata, and
'''•>! IIT»I jjn/itnoi'/ltHisMS being thus indisputably connected by a re-
' 1'iarkable combination of structural points, which admit of
no explanation consistent with the principles of evolutional
morphology except that of the genetic relationship of the
forms thus enumerated, we are at once confronted by those
questions as to the ancestral history of Vertebra/a which
have been already mentioned above as stimulated by
Kowalewsky's discoveries. Undoubtedly Ampkioxus is
lower and simpler in structure than any Fish, Tunicata as
low as or lower than Ampkioxus, and Balanoglossus, in
some respects, more archaic than either Ampkioxus or the
. \-cicliau tadpole. The first tendency arising from the
discovery of the affinities of these simpler forms with the
Cuvierian Vertelrata was to see in them the representatives
of the ancestors of all Vertebrate. Ampkioxus has been
pointed to by authorities in morphology as the living
presentation of our common Vertebrate ancestor ; a similar
position corresponding to an earlier stage of development
has been admitted by no less an authority than Darwin
for the Ascidian. It appears, nevertheless, that all such
simple solutions of the problem of Vertebrate ancestry are
without warrant. They arise from a very common tend-
ency of the mind, against which the naturalist has to
guard himself, — a tendency which finds expression in the
very widespread notion that the existing anthropoid apes,
and more especially the gorilla, must be looked upon as
the ancestors of mankind, if once the doctrine of the descent
of man from ape-like forefathers is admitted. A little re-
flexion suffices to show that any given living form, such as
the gorilla, cannot possibly be the ancestral form from
which man was derived, since ex hypothesi that ancestral
form underwent modification and development, and in so
doing ceased to exist. The same considerations apply to
the question of the ancestry of Vertehrata. Probably no
existing low form of Vertebrate closely represents the an-
cestral form by the modification of which higher forms
have been developed. We have no justification for assum-
ing that such low forms do more than present to us a col-
lateral branch of the family, and that collateral branch
must, in all probability, have experienced its own special
Arju- series of modifications of structure. Not only this, but
nieuts we have no sufficient ground for assuming that, even in
from sim- ecj. Qf ^g s;mplicity of their structure, any given
plioityof • , « .. • .• i -, •, • i

struc- animal forms at present existing exhibit a mere survival

ture. of a corresponding degree of simplicity in their remote
ancestors. Such an assumption was almost universally
made, until a more correct view was pressed on the atten-
tion of naturalists by Dr Anton Dohrn, the founder of the
zoological station of Naples ($}. So far from its being
the case that simplicity of organization necessarily implies
the continuous hereditary transmission of a low stage of
structural development from remote ancestors, there are

numerous instances in which it is certain that the existing
simplicity of structure is due to a process of degeneration,
and that an existing form of simple structure is thus de-
scended from ancestors of far higher complexity of organi-
zation than itself. Such are various parasitic worms and
Crtix/itcrtt. The evidence iu favour of the occurrence of
progressive simplification of structure or degeneration, in
place of progressive elaboration, depends (1) upon the com-
parison of the adult structure of the degenerate organism
with that of its nearest allies, by which it is often rendered
clear that the ensemble of the organization of the simpler
organism cannot be explained on the hypothesis that it
represents an ancestral or archaic condition common to it
and its more elaborate congeners, and (2) on the direct
evidence of individual development or life-history. The
latter evidence is conclusive, when we find, as in the case
of Cirrhipede Crustaceans and of Ascidian Tunicates, that
the embryo on its way to the adult condition passes through
stages of development presenting a higher degree of or-
ganization than that ultimately reached, so that, as in the
Cirrhipede larva and the Ascidian tadpole, the young form
resembles allied organisms of a higher stage of develop-
ment, and subsequently degenerates from the point of
progressive elaboration to which it had attained, and be-
comes greatly simplified in the final stage of its growth.
Conclusive as such evidence is, there is no law of develop-
ment which necessitates its preservation. If it be an
advantage to the organism, the full force of heredity has
play, and what are called the " recapitulative phases " of
ancestral development are passed through by the indi-
vidual in the course of development from the egg. But
with remorseless thoroughness all such hereditary tenden-
cies may be removed when such removal is an advantage
to the organism, and the development from the egg may
proceed directly to the adult degenerate form. Such is
the case with many Tunicata, the young of which never
exhibit notoehord and tadpole form ; indeed, were it not
for the preservation of a few exceptional cases, like that
of the Ascidian section of the group, we should have no
direct evidence of the degeneration of Tunicata from tad-
pole-like ancestry.

The general result of the considerations which have Hypo-
been urged with regard to degeneration is this, that it is tln'sis nf
prima facie as legitimate an hypothesis, that any existing ^™]
animal has developed by progressive simplification from
more elaborate ancestors, as it is that such an animal has
developed by a continuous and unbroken progress in
elaboration from simpler ancestors ; and we are specially
called upon to apply the hypothesis of degeneration where
the animal under consideration is likely from its mode of
life to have undergone that process. Such modes of life,
tending to degeneration, are parasitism, sessile or adherent
habit, burrowing in the sea-bottom, and diffuse feeding.
The animal which pursues living prey, and contends with
other organisms for the dominion of the regions of earth and
water that are flooded with light and richly supplied with
oxygen gas, is the animal which represents the outcome of
a longer or shorter period of progressive elaboration. It
is worth while noting in parenthesis that in all cases the
" whirligig of time " has probably brought its revenges,
and that the ancestry of a form evolved through a long
period of progressive elaboration was at an antecedent
period subject to simplification and degeneration, whilst in
the past records of the present exemplars of the latter
process there must certainly have been long stretches of
continuous elaboration.

Applying these considerations to the construction of the Genea-
genealogical tree of Verielrata, we find that the task is by logical
no means simplified. We cannot with the earliest evolu- tree-
tionists adopt a scale or ladder-like series, placing the

176

VERTEBRATA

simplest form on the lowest step ; nor can we be satisfied
with a tree-like arrangement, in which the forms at the
ends of the branches are always more elaborate than those
nearer the trunk. Our genealogical tree will more strictly
conform to that of a parvenu human family, if we take
worldly prosperity in the latter case as corresponding to
elaboration of structure in the former. The strict family
genealogist will include in the successive ramifications of
the tree the five sons of the founder of the family, one of
whom remained an agricultural labourer, whilst two be-
came brewers and two emigrated. The cousins in the
next generation will be set forth in place, the sons of one
brewer becoming paupers, whilst those of the other advance
to the position of Government employes, and one to the
peerage. Thus in successive branchings of PHARTNOEAr,
the family history there may be alternate pro- SACS & SLITS.
gress and degeneration. And so it must be R Masai:
in the genealogical trees constructed by the R. Eye.-
naturalist : the fact that a branch is later in
origin will not imply that it is higher in ela-
boration than those below it, and accordingly
we must not expect to draw our tree so as to
be able to trace all simpler forms to lower — — — •'

off-sets of the tree. -<-'-h B»- —

Divisions The structural features of those animals _sth_ 5?.-__

of Verte- >vhich must be admitted to the Vertebrate _««, DO.

phylum in consequence of possessing noto- nh Do

chord, pharyngeal gill-slits, and dorsal nerve- ~

plate, tubular or unrolled, are such as enable -~— — Ss= —

us very readily to group them in four great

divisions, which appear to be equally distinct

from one another. As to what may be the

laterally so as to enclose and protect the brain (hence Craniata}.
Cartilage is developed in other parts of the body as a skeletal sub-
stance, though it may be subsequently replaced in the cranium, as
elsewhere, by bone. The longitudinal muscles of the body- wall are
divided by transverse fibrous septa into a series of segments, vary-
ing in the adnlt from ten to one hundred or more in number.
Cartilaginous neural arches, corresponding in number and position
to the fibrous septa, and resting on the notochord, are developed
so as to protect the nerve-cord. Cartilaginous bars also pass out-
wards, with a direction at first horizontal and then ventral, from
the sides of the notochord into the mtermuscular fibrous septa.
Very generally, but not always, a tubular cartilaginous sheath
forms round the notochord ; this sheath with rare exceptions be-
comes segmented to form a series of vertebral bodies, which lie in
the planes of the fibrous intersegmental septa, and, increasing in
thickness by encroaching upon the substance of the notochord,
finally obliterate it almost entirely.

VASCULAR AKCBES.

brate
phylum.

,. i .. r ,1 p FIG. 3. — Three diagrams showin

genetic relations to one another ot these lour arches, and farcal nerves in

Charac-
ters of
Crani-
ata.

groups we will inquire subsequently; for the
present we term these groups " branches."
They are as follows : —

I'hyhim VERTEBRATA.

Branch a — Craniata (Cuvicrian J'crtrbratti).
,, !i — Ccphalochordn. (AmpMoxus).
,, c — Urochorda (Tun icatn ).
,, d — Hfiiiifhnrda (Balanoglossus}.

The Vfrteljrnla thus limited may be defined as C'relomate
Enterozoa (ilfrtfi-na) with well-developed coelom. In all,
with the exception of the more degenerate members of
UrocJinrJit, an elastic skeletal rod — the notochord — is de-
veloped dorsally by an outgrowth of cells forming the wall
of the primitive archenteron ; the notochord may or may
not persist in adult life. Pharyngeal gill-slits, which may
or may not persist in adult life, are developed in all Verte-
brates. In all, except in certain Urochorda, the chief nerve-
centre has the form of a dorsal, median, elongate tract,
derived from the epiblast, which becomes sunk below the
surface and invaginated so as to form a tube. In all there
is a tendency to metameric repetition of parts, which may
find its expression in a strongly-marked segmentation of
the musculature of the body-wall and its skeleton, or may
be recognizable only in a limited degree, as exhibited by
the successive gill-slits or successive gonads.

We shall now examine the distinctive features of each
of these large groups, and form an estimate of their rela-
tions to one another, and of their probable ancestry, this
being the task to which we must limit ourselves in the
brief space here afforded.

THE CKAXIATA.

The Craniata are Vcrtcbrata in which the tubular cerebro-spinal
nerve-mass is swollen anteriorly to form a brain, consisting primarily
of three successive vesicles, in connexion with the anterior of which
the special nerves of the olfactory organs and of the eyes originate.
The notochord, whilst extending posteriorly to the extremity of the
body, does not reach quite so far forward anteriorly as the termina-
tion of the nerve-tube. A cartilaginous cranium or brain-case de-
velops round the anterior extremity of the nerve-cord, and rises up

characteristic disposition of gill-slits, skeletal arches, vascular
primitive ideal Craniate. The following abbreviations require

explanation : — pros., proseneephalon ; thai., thalaineneephalon ; 7»es., mesencephalon ; me/., met-
ciieephalon ; tuuL, otocyst or auditory sac ; tni. art., truticus arteriosus. (Original.)

The pharyngeal slits follow closely upon the mouth, and in ex-
isting Craniata never number more than eight pairs (see fig. 3).
They are separated from one another and their apertures strengthened
by a series of cartilaginous hoops, the first of which, that between
the mouth and the first gill-slit, forms the primitive upper and
lower jaw in all but the small and degenerate group Cyclus/u>nn.
The gill-slits when functional are generally protected by an opercular
fold of the body-wall, which overhangs them and corresponds to
the epipleural i'uld of Amp/tio.ru.'i, the collar of Balanoglossus, and
doubtfully to the wall of the atrial chamber of Urochorda. The
extension of this fold along the sides of the middle third of the
body (between the pharyngeal region and the anus) acquired in
ancestral Craniata the function of a continuous right and left lateral
fin (see tig. 4). At the same time a continuous median fin, corre-

BrF

Bt' An

Fin. 4. — A. Hypothetical primitive Fish, with continuous lateral fins S, S (paired

ri^hl: anil Irt'l) ntlnent with median azygos fin (An), the post-anal part of

which is marked S', whilst ils dorsal part is marked IX B. Actual Fish,
showing relation of isolated lateral and median (ins to original hypothetical
tins of the upper figure. UrF, left pectoral tin (paired) ; liF, left pelvic fin
(paired) ; AF, anal (pn.st.anal) azypos fin; SF, caudal azygos fin; RF and
FF, anterior and posterior azygos dorsal lins ; An, anus. (From Wiedersheim.)

spending to the dorsal, caudal, and anal fins of existing Fishes, was
developed. In both lateral and median fins a cartilaginous
skeleton was developed, consisting of a basal longitudinal bar,
supporting a number of rods like the teeth of a comb. The
primitive form of fin skeleton is retained in the median fins of
some sharks ; the primitive lateral fin has in all cases either

VERTEBRA! A

177

entirely disappeared (Cydostoma) (as has their anterior extension,
the opcrculiim, in many cases) or it has become, together with its
skeletal elciiirnts, concent raled in two regions — forming the pectoral
and the pelvic paired appendages or limbs, with their respective
girdles.

The cerebro-spinal nerve-centre and the disposition of the nerves
issuing from it present a remarkable complexity, and at the same
time uniformity, of structure in all Cmniata (see fig. 5). The fore-
most of the three prim-
ary cerebral vesicles
gives rise to paired an-
terior outgrowths, the
proscncephala, to a
median dorsal out-
growth, the
stalk of the
pineal eye
(rudimentary
in all exist-
ingCraniata),
and to a me-
dian ventral
outgrowth,
which is met
by an inva-

eiuation of Fm. 5.— Diagrams of Craniate brain. A. Embryonic condition
. of neural tube. G, Cerebral portion ; R, spinal cord ; I,
1 IF III, three primary cerebral vesicles. B. Longitudinal
ot the oral section of adult brain, applicable to any and every Craniate
Vertebrate. J3c, Floor of skull ; C/i, notochord ; SD, roof of
skull; NH1, nasal cavity ; VH, fore-brain, probfliceplialoi],
or cerebrum ; Olf, olfactory lobe ; Cs, corpus striatum ; ZH,
thalamencephalon, corresponding to primary anterior vesicle,
from which the prosencephalon has grown out as well as Z,
theepiphysis or pineal body; I, infundibulum, with attached
hypophysis or pituitary body H ; Opt, optic nerve ; Tho,
thalanms optie.us ; HC, posterior commissure; MH, mesen-
cephalon (corpora yuadrigemina or optio lobes or mid-brain);
HH, cerebellum or metencephalon ; NH, medulla oblongata
or epencephalon (the reference line touches the membranous
roof of the so-called "fourth ventricle" of the brain); Co,
canal of the spinal cord or myelon. (From Wiedersheim.)

oral

cavity form-
ing the pitu-
itary body,
further, to a
pair of lateral
outgrowths,
which be-
come the
right and left
optic, nerves
and retina;

respectively. The modifications of the hindmost of the three prim-
ary vesicles are also extremely definite and persistent throughout
the group : its anterior dorsal surface enlarges and becomes the
cerebellum (the metencephalon), whilst the cavity of its hinder
part (the medulla oblougata or the epencephalon) becomes compara-
tively wide, and is covered dorsally by a thin membrane only, in
which nervous tissue does not take a part. The intermediate
primary cerebral vesicle (the meseucephalon) does not give rise to
outgrowths.

In all Craniata nerves are given oft' from the cerebro-spinal cord
or tube with great regularity, one right and left in each success-
ive myomere or segment of the body-wall. Each nerve has two
roots, a dorsal (sensory) and a ventral (motor). A commissure
between each successive pair of deep or intestinal branches of the
spinal nerves forms the so-called sympathetic nerve-cords, one on
each side of the vertebral column. Nerves similar to the spinal
nerves, but not identical with them, are given off from the brain,
and perforate the cranial box right and left. In all Cmniata there
are ten pairs of nerves which originate thus, and in the higher
forms two more pairs (elsewhere spinal) are included amongst those
which thus perforate the cranium (spinal accessory and hypoglossal).
The order and character of the cranial nerves are the same in all
Cmniata. The first (olfactory) and the second (optic) are unlike
spinal nerves in both distribution and origin. As we pass back-
wards along the series, the cranial nerves are found to resemble more
and more the ordinary spinal nerves. Hence it has been inferred
that the cranial region consisted at one time of a number of distinct
myomeres (as many as nine), which have become fused and modi-
fied to form the typical craniate "head." The oculo-motor or 3d
nerve indicates the first of these segments, the trochlear or 4th
nerve the second, the abducens or 6th the third, the facial and
auditory (7th and 8th) the fourth, the glosso-pharyngeal or 9th the
fifth, and the vagus or 10th, with certain of its branches supposed
to have been originally distinct nerves, the sixth, seventh, eighth,
and ninth. It is probable that in Cmniata the metamerism of the
gill-slits does not correspond to the metamerism of the body-wall.
The mouth and each successive gill-slit are related to a bifurcate
branch of a cranial nerve (furcal nerve) in lower Craniata (see fig.
3) ; these furcal nerves do not correspond, so far as we can at present
judge, with cranial myomeres.

The lateral pair of eyes (as opposed to the rudimentary pineal or
parietal eye) present a striking uniformity of origin and structure
throughout Craniata. Not only are they uniformly developed from
three elements, viz., the retinal cup which grows out from the
anterior of the primary cerebral vesicles, the epidermal lens which
grows inwards from the surface of the skin, and the connective
tissue between these two, but we find that the muscles attached to
the eyeball are identical throughout the series : that is, the superior,

infericii1, and internal rectus, and the inferior oblique muscles repre-
sent the lirst cranial myomere, the superior oblique represent
the second, and the external rectus represent the third cranial
mypmeie.

The olfactory sacs are paired in all except Cyclosloma, in which
they arc represented by a single sar which may or may not bo
archaic in its azyg"s character. Tho auditory sacs are paired
organs which develop as invaginations from the surface, — the orifice
of invagination closing up, — at the hinder part of the cranial region.
They present a gradually increasing complexity of form as we pass
from aquatic to terrestrial forms, but are identical in essential struc-
ture throughout.

All Craniata, except some Fishes, possess a muscular process on
the floor of the oral cavity which may carry teeth, or act as a lick-
ing organ, or assist in suction. This is the tongue.

All Craniata, with degradational exceptions, possess an outgrowth,
single or paired, of the post-pharyngeal region of the alimentary
canal, which is filled with gas. In many Fishes this becomes shut
oil' from the gut ; in others it remains in communication with the
gut by an open duct. In Fishes it functions as a hydrostatic appar-
atus. In terrestrial Craniata it is subservient to the gas-exchange
of the blood and becomes the lungs.

All Cmniata have a large and compact liver ; and a pancreas is
also uniformly present, except in Cydostoma, some bony Fishes, and
the lower Amphibia.

All C'raniitta have a thick- walled muscular heart, which appears
first as an "atrium," receiving the great veins, attached to a
"ventricle" by which the blood received from the atrium is pro-
pelled through a number of arteries, right and left, corresponding in
number to the pharyngeal gill-slits between which they pass.
A prseatrial chamber (the sinus) and an extra-ventricular chamber
(the conus) are added to the primitive chambers ; but the most
important modifications arise in consequence of the development
of pulmonary respiration and the gradual separation of the cavities
of the heart by median septa into a double series, a right and a left.
The plan of the great arteries in all Craniata is in origin the same,
and is determined by tli3 primitive existence of a branchial circula-
tion in the gill-slits, which is obliterated in higher forms. Simi-
larly the plan of the great veins is identical, the primitive posterior
vertebral veins of lower Craniata, though persistent in higher
members of the group, having their function gradually usurped by
the excessive development of the renal vein, and of renal-portal and
ultimately of iliac veins.

All Craniata have a lymphatic system or series of channels by
which the exudation from the capillary blood-vessels is returned
to the vascular system. It includes in its space-system the ccelom
and a variety of irregular and canalicular spaces in the connective
tissues. Masses of spongy tissue (adenoid tissue, lymphatic glands)
exist, through which the lymph filters, and there acquires corpus-
cular elements as well as chemical elaboration. At various points
in various Craniata pulsating or simple communications are estab-
lished between the lymphatic system and the veins. A special
and characteristic communication is established in the spleen, an
organ which is found in all Craniata, either as a single mass or as
scattered masses of spongy tissue in which blood-vessels and lymph-
atics unite.

The renal organs of Craniata are primitively a series of nephridia
corresponding in number to the myotomes of the mid-region of the
body in which they exist. They are connected in the simplest
Cmniata by a right and a left archinephrie duct, which appear to
be in origin lateral grooves of the epidermal surface. This primi-
tive renal system has been modified in some lower forms (Cydostoma
and Teleosteau Fishes) by the atrophy of its anterior portion. But
in all other Craniata it acquires relations to the gonads or ovary
and testes, so that an anterior portion of the archinephros and a
corresponding longitudinal tract of the duct become separated to
serve as oviduct, a middle portion to serve as sperm-duct, while a
posterior portion retains exclusively or shares with the middle
portion the function of urinary excretion. The male and female
gonads are, with the rarest exceptions, developed in distinct indi-
viduals, though the rudiments of the suppressed gonad may in
some cases (Amphibia) be traceable in either sex.

The group of Craniate Vcrtcbrata thus anatomically described, Sub-
whilst retaining the essential unity indicated, presents an immense divisions
variety of modifications. The chief modifications are distinctly of Crani-
traceable to and accounted for by mechanical and physiological ala.
adaptation to a terrestrial and air-breathing life, as opposed to the
earlier aquatic and branchial condition. The existing forms of
Craniata have been arrested at several points, in the progress towards
the most extreme adaptation to terrestrial conditions, which is
presented by those forms that can not only breath air and live oil
dry ground but fly habitually in the air. The organs most obviously
affected by this progressive adaptation are the skin, the skeleton,
especially of the limbs, the pharyngeal gills, and the air-bladder.
This fact will appear most clearly in the subjoined classification of
Craniata • for space does not permit us to pursue further the
history of these modifications.

178

VERTEBRATA

Classification

Grade. A. CYCLOSTOMA.

Class I. Myxinoidca.

II. PetTomy&mtia.
Grade B. GNATHOSTOMA.

Grade a. Branchiata heterodactyla.
Class I. Pisces.
II. Dipnoi.
Grade b. Branchiata pentadactyla.

Class. Amphibia.

Grade c. Pentadactyla lipobranchia.
Branch a. Monocondyla. Branch b. Amphicondyla.

Class I. Meptilia. Class. Mannnalia.

II. Aves.

Ances- If we now briefly consider what must have been the common
tral form ancestral form from which these Craniata have proceeded, making
ofCVani-nse of such internal evidence as their structure affords, we find
ata. that we get no further back than such an animal as would fit the
description given above, with the exception that we should be
warranted in substituting in the ancestor a pair of continuous
lateral fins, with comb-like cartilaginous skeleton, in place of the
two pairs of fins, or their total defect, seen in living Craniata. We
get no clear suggestion from the study of Craniata themselves as
to the meaning of the curious shape of the brain and its outgrowths
(though the pineal outgrowth has recently been explained as an
eye), nor as to the original genesis of the notochord. We should,
however, be justified in representing that region which now cor-
responds to the hinder part of the skull and brain as more fully
developed aud segmented, so as to give a series of separate myotomes
and perhaps separate nerves corresponding to the several furcal
branches of the vagus ; and we may very well suppose that the num-
ber of pharyngeal gill-slits was larger in the ancestral than in any
living form, though it seems improbable that in any true Craniate
did each gill-slit correspond to a distinct muscular segment.

An attempt to go further than this has been made by Dr Anton
Dohrn by the method of hypothesis and subsequent corroborative
inquiry into facts of minute structure and embryological history.
Making use of the principle of degeneration, Dohrn started with
the legitimate hypothesis that the branches of Vertcbrata other
than Craniata — viz. , Ccphalochorda, Urochorda, and (though at the
time he commenced his work their structure was not fully under-
stood) Hcmichorda — were not to be regarded as permanent records
of steps in the evolution of Craniata, but rather as greatly de-
generate offshoots from the ancestors of that group, which could
throw but little light on the character of their non - degenerate
ancestors. A second fundamental assumption which led Dohrn to
his position was that the segmentation of the Craniates' body-wall
is a primitive and essential feature in their structure, and becomes
more and more fully expressed instead of less developed the further
we go back in their ancestry. Dohrn, in fact, assumes that what
is called metameric segmentation is a phenomenon of structure
which lias occurred once only in the history of animal form, aud
that all segmented animals are genetically related aud descended
from a common segmented ancestor. Assuming this, he pointed
to the existing Chittopod Worms as most nearly representing at
the present day the common ancestor of segmented animals. They
have, as he pointed out, a high organization, little inferior to that
of the lowest Craniata; they possess a well -developed ccelom,
blood-vessels with red blood, a segmental series of nephridia
(modified in some as gonaducts), segmental branchiae, and lateral
locomotive organs ; not a few develop cartilage as a skeletal sup-
port ; and many show a concentration and fusion of segments to
form a complex head, which resembles, so far, that of Craniata.
The ventral in place of the dorsal position of the nerve-cord led
Dohrn to accept De Blainville's conception that the dorsal and
ventral surfaces are reversed in Vertebrata, as compared with
Annelids, Crustaceans, and Insects, so that the Vertebrate is com-

}iared to an Insect walking with its ventral surface upward. This
ed further to the notion that the mouth of the Chivtopod or
Annelid, which penetrates the nerve-cord, or rather passes between
its two divaricated lateral constituents in those animals, has in
Craniata disappeared, its place being taken by a new mouth de-
rived from the modification of a pair of gill-slits. The remnant of
the old month, which should, if the comparison instituted holds
good, lie in Craniata somewhere on the dorsal surface of the cranial
region, was sought by Dohrn in some of the peculiar and hitherto
unexplained median structures of the brain: at one time the fourth
ventricle with its deficient roof was suggested as thus to be explained,
whilst subsequently the curious median structures, — the pineal and
pituitary bodies, — were called in as possibly thus significant.

Without pursuing further the elaboration of Dohrn's views, it
must be at once noted that, whilst the legitimacy of the assumption
of degeneration must be admitted, the second assumption, viz.,
that metameric segmentation is a character bringing all forms
showing it into a special genetic continuity, cannot be accepted.
1 Tin' ••his s IHTC enumerated arc descvibpd in separate articles, whilst
Cyclostoma and Dipnoi are included in the article ICHTHYOLOGY.

The property of repeating units of structure, so as to build up a
complex of many similar parts united to form one individual, is a
very general one in organic forms, and is exhibited in various con-
ditions by both animals and plants. Its simplest expression is
found in cell-structure and the binary division of cells. It shows
itself as affecting larger masses of structure in the arborescent
colonies of Cwlentcra, in the radial or antimeric composition of
Echinoderms and of Compound Ascidians, and in the linear or
metameric segmentation of Worms, Arthropods, and Vertebrates.
There is abundant evidence that this property is a general one,
which may assert itself at any period in the history of a group of
animals, and does not imply special unity of origin in forms which
exhibit it. As pointed out in the article HYDKOZOA, merogenesis
— the name applicable to this phenomenon generally — may take
an extreme and complete character, leading to the separation and
independence of the units of structure produced ; in that case it
may be termed eumerogenesis. Or the process may be very partial,
occurring only during a period of embryonic growth, and subse-
quently ceasing, so that later growth obscures or obliterates it
altogether (dysmerogenesis). There is no ground for assuming
that either one of these extremes is fundamental or original. Any
mechanical or nutritional condition may lead to merogenesis in an
organism in which the tissues have a certain reproductive capacity,
or have not acquired final differentiation ; and it will depend upon
the balance of advantage, determined by natural selection, whether
the segmentation (supposing the merogenesis to take the linear
form) results in the separation of segment-buds, or in the formation
of an annulate body, or leaves traces of its occurrence only in certain
tissues and organs. The Cestoid Worms present within the range
of a single group almost every grade of eumerogenesis and dysmero-
genesis (Canjophyllxus, Liyula, Tienia). In the otherwise amero-
genetic Molfusca, Chiton and the pearly Nautilus show dysmero-
genesis in certain organs, whilst the Planariau Worms frequently
exhibit eumerogenesis in their bud-segmentation (to be compared
with that of the Annelid Ctcnodrilus described by Zeppelin, g) and
the elongated Nemertines only slight traces of dysmerogenesis.

If we deny Dohrn's assumption with reference to segmentation,
we are no longer led in the direction of the Annelids (Chretopods)
in our search for the ancestry of the Craniate Vertebrata.

The fact that the notochord is the forerunner of the segmented
vertebral column, and is itself never segmeutal, instead of being a
difficulty, acquires directive significance. The fact that the nerve
tube is dorsal, and not ventral, no longer requires the large assump-
tion that animals have reversed their habitual carriage, but suggests
that the Craniates' ancestor had a dorsal median nerve, which has
increased in size and importance so as to become the nerve-tube
of existing forms.
The explanation of
the curious struc-
ture of the brain
will have to be /
found otherwise (
than in the assump-
tion of a perforating
pharynx, — an as-
sumption which the
recent discovery of
the true nature of
the pineal bod}1 has
rendered untenable / 4
in the latest form \
advocated by its '
ingenious author,
whose speculations,
nevertheless, de-
serve the fullest re-
cognition as having
stimulated inquiry
and guided observa-
tion.

Balfour (10) in
1878 refused to a-

dopt Dohrn's views, -j?j— AMPHIOXUS

and considered it Fm.e.-Comparison of nervous systems of a Nemertine,
probable that the a primitive Craniate, and .4 TOpfciortiS. m, Median dor-
dorsal position of sal nerve, which becomes the myelon in the Craniate
flip Tiprvp-cnvH in a"1' -1>«J''< <»•"«, acquiring an anterior enlargement in
1 the former ; I, lateral nerve (right and left), absent by
degeneration in Ampftioxus ; ?</, ganglia of lateral
nerve, forming a single large lobe on each side in
the Nemcrtine, anil broken into a metameric series in
the Craniate ; v, roots of vagus nerve of the Craniate;
rfr, dorsal roots of nerves given off from myelon or
median dorsal nerve ; vr, ventral roots of these nerves,
here represented as separate nerves; 0, mouth. (After
Hnbreoht.)

Ti'f/i limtct could be
accounted for, with-
out any assumption
of a substitution of
a pair of gill-slits for
the original mouth,

by

assuming that

primitively the nerve-rord consisted of two lateral cords, as seen at
the present time in the Nemertine Worms, and that these cords
have coalesced dorsallij in Vertcbrata, just as it is clearly demon

VERTEBRA T A

179

strablc that two originally lateral cords have coalesced rnilnil/y tu
form the Annelid's ventral nerve-chain.

The comparison of the Verteln-ates' nervous system with that of
the Nemertiues lunl already been made tiy tile present writer, as
cited hy Hubrecht (//) ill connexion witli the latter's discovery ,,f
a complete sub-epidermal nerve-tume in those worms. Hubrecht
has more recently on two occasions (12 and ij) di-vclnped an in-
teresting and important comparison of Xi'imTliiie and Vertebrate
structure. He has in the first place suggested that the notorhord
of Vertebrate is nothing more than a modified survival of the pro-
boscidean sheath of the Ncmertines, whilst the oral invaginatiou of
the epidermis, in connexion with the hypophysis cercbri of the
Vertebrate, may be a last remnant of the proboscis itself. More
conclusively he has drawn attention to the median dorsal nerve of
Nemertines as corresponding to the Vertebrate ecrebro-spinal nerve-
cord, whilst the great lateral nerve-cords of Nemertines, and the
lateral ganglia in which they expand anteriorly, are compared to
the lateral ganglia of the cephalic region of Craniate Vcrtclrata
and the nerve of the lateral line (see tig. 6). The comparison is
strengthened by the existence of a metameric series of transverse
nerves in the Nemertine, which correspond in respect of their meta-
_ merism and their connexion with a dorsal median trunk, with the
spinal nerves of Craniate. Hubrecht is careful to insist that he
does not regard the Nemertines as representing the direct ancestry
of Vcrtclimta, ; but he points out that from the primitive condition
of an elongate animal, with a plexiform nerve-tunic, it is readily
conceivable that a form was developed in which the nervous tissue
was concentrated in three cords, — a median dorsal and two lateral, —
and from such a form we can derive the Craniates' condition by
excessive development of the median tract and relatively small
development of the lateral cords, whilst the Nemertines' condition
would be attained by the converse process. The tubular condition
of the cerebro-spiual nerve-cord of lrcrlcbrala, it may here be re-
marked, is now very generally regarded as being in its origin a
purely developmental feature. It was primitively separated from
the epidermis by delamination and in-sinking, and the mode of
formation by inv agination of a canal has been substituted in accord-
ance with a general embryological law of growth, which is that
bulky structures originating beneath a surface from the cells form-
ing that surface take up their position in embryonic growth by in-
vagination of the parent surface. The tubular form, having thus
started, seems to have been utilized during one phase of Vertebrate
evolution for the respiration of the nervous tissue, by the introduc-
tion through an anterior unclosed pore of a current of water, which
escaped by the neuranal canal (as in larval Amphioxus).

There is a wide gap between any form presenting an approach to
a Nemertine Worm and the most simple Craniate Vertebrate which
can be imagined still provided with the organization characteristic
of all Craniata. To pass from such a Worm-like animal to a Craniate,
we have to account for and introduce, amongst other new develop-
ments, (1) a greatly increased metamerism, showing itself in the
segmentation of the muscles of the body-wall and in the repetition
of the nephridia ; (2) the characteristic sense organs; (3) the lateral
and median longitudinal folds or continuous fins ; (4) the carti-
laginous rods and bars of the skeleton ; (5) the gill-slits, even if
we admit the notochord to be represented by the proboscidean
sheath.

It remains to inquire whether the structure of the other Verte-
trata throws light on this long hypothetical passage from the
simple Worm phase to the elaborate Craniate, or suggests any other
ancestry.

THE CEPHALOCHORDA.

Char- Ccphalochorda are Vertcbrata in which there is no anterior

acters of dilatation of the nerve-tube to form a brain (see fig. 6) and no
Cephalo- specialized skeletal brain-case. The notochord extends from one
extremity of the elongate body to the other as a tapering uncon-
stricted rod, passing anteriorly some distance in front of the
nerve-cord. The longitudinal muscles of the body- wall are divided
by transverse fibrous septa into a series of segments (sixty-two in
Amphioxus lanccolatus), the more anterior of which are 'in front
of the mouth and not in any way fused to form a head or cranial
structure. Dense connective tissue (differing but little from car-
tilage) forms an unsegmented sheath to the notochord and an
unbroken neural canal above it, in which the nerve-cord lies.
The same tissue forms a series of metamerically repeated fin rays,
which support the base of a median fin extending along the entire
dorsal surface. The fin is continued veutrally from the caudal ex-
tremity as far forward as the anus, hut without fin rays. Two
lateral up-growths of the bod'y-wall (the epipleura) extend one on
either side from the head as far back as the anus. Each of these
is divided into three regions, — (1) an anterior, which forms the
prieoral hood ; (2) a median, which forms the wall of the great
branchial chamber, the two folds meeting one another and coales-
cing in the ventral mid-line, excepting where they leave a posterior
median aperture, the atriopore ; and (3) the post-atrioporal preanal
ventral fin (extending between atriopore and anus), which is formed
by the complete coalescence of the two folds behind the atriopore.

chorda,

FIG. 7. — Amphioriis lanceolatus, Yarrell (Brancliiostoma hibricum, Coste).
(Original drawings.) (1) Lateral view of adult, to show general form, the
myomeres, fin rays, and gonads. A, oral tentacles (28 to 32 in full-grown
animals, 20 to 24 in half-grown specimens) ; B, prjeoral hood or prpeoral epi-
pleur ; C, plicated ventral surface of atrial chamber ; D1, D1", D-6, gonads,
twenty-six pairs, coincident with myotomes 10 to 36 ; E, metapleur or lateral
ridge on atrial epipleur ; F, atriopore, coincident with myotome 36 ; G1, G1^,
G34, double ventral tin rays, extending from myotomes 37 to 52, but hav-
ing no numerical relation to them ; H, position of anus, between myotomes
51 and 52 ; I, notoehord, projecting beyond myotomes ; K", K2?, K62, myo-
tomes or muscular segments of body- wall, 02 in number ; Lioo, L230, L253, dor-
sal fin rays, about 250 in number, the hard substance of the ray being absent
at the extreme ends of the body (these have no constant numerical relation
t'i the myomeres); M. notochord as seen through the transparent myotomes,
the thin double-lined spaces being the connective-tissue septa and the
broader spaces the muscular tissue of the inyotomes ; N, position of brown
funnel of left side (atrin-ccelomic canal) ; O, nerve-tube resting on notochord.
(2) Dissection of Amphioxus. By a horizontal incision on each side of the
body a large ventral area has bern scprtraU-d and turned over, as it were on
a hinge, to the animal's left side. The perforated pharyngeal region has then
been detached from the adherent epipleura or opercular folds (wall of atrial
or branchial chamber) by cutting the fluted pharyngo-pleural membrane d,
and separated by a vertical cut from the intestinal region. «, Edge of groove
formed by adhesion of median dorsal surface of alimentary canal to sheath
of notochord ; b, median dorsal surface of alimentary canal ; c, left dorsal
aorta ; cc, single dorsal aorta, formed by union of the two anterior vessels ; tf,
same vessel resting on intestine ; d, cut edge of pharyngo-pleural folds of
atrial tunic, really the original outer body. wall before the downgrowth of
epipleura ; if, atrial tunic (original body-wall) at non-perforate region, cut
and turned back so as to expose peri-enteric cctlom and intestine r ', <?, up-
standing folds of body-wall (pharyngo-pleural folds) on alternate bars of per-
forate region of body ; /, atrio-co-lomic canals or brown funnels (collar-pores
of Balanoglossiis) ; g, cavity of a gonad-sac ; ??i, cut musculature of body-
wall ; n, anus ; a, post-atrioporal extension of atrial chamber in form of a

180

VERTEBRATA

tubular cfecum ; j>, atriopore ; 7, hepatic cxcnm ; r, intestine ; s, ccx-lom ; t,
area of adhesion between alimentary canal and sheath of notochord ; r, atrial
chamber or branchial cavity; w, post-atrioporal portion of intestine; x,
canals of metapleura exposed by cutting ; B, probe passing through atrio-
pore into atrial or branchial chamber; FF', probe passed from ccelom, where
it expands behind the atriopore, into narrower perienteric coelom of pree-
atrioporal region.

(3) Portion of (2) enlarged to show atrio-co?lomic canals ("brown funnels
of Lankester). Lettering as in (2).

(4) Section taken transversely through praoral region near termination of
nerve-tube, a, Olfactory ciliated pit on animal's left side, its wall confluent
with substance of nerve-tube ; 6, pigment spot (rudimentary eye) on anterior
termination of nerve-tube ; c, first pair of nerves in section ; d, fin ray ; e,
myotome ; /, notochord; g, space round myotome (? artifact or ccelom) ; It,
subchordal canal (? blood-vessel); i, a symmetrical epipleura of praoral hood.

The originally double character of this part of the ventral fin is
indicated by the double series of metameric fin rays which support
it. It is probable that these "epipleural" folds of Amphioxus
correspond to the opercular folds and lateral fins of Craniala. No
cartilaginous fin rays are developed in the atrio-pleural (opercular)
region of the epipleura ; but a longitudinal tmsegmented bar of
cartilaginous consistency strengthens its side and bounds a lymph-
holding canal (x in fig. 8).

The gill-slits in A mphio.ms are very numerous (one hundred or
more), and have no numerical relation to the metameres of the
muscular body -wall, though the first few which appear in the
embryo correspond at the time to successive myomeres, — a relation
which they subsequently lose. The sides of the gill-slits are sup-
ported by chitiuous (?) bars, and each slit is divided into two equal
portions by a longitudinal tongue or bar, which grows out from
the dorsal margin of the slit soon after its first formation. The
number of gill-slits increases continually throughout the life of
Amphioxus by the formation of new ones at the posterior border of
the pharynx, whilst the myomeres do not increase in number after
early embryonic life.

The nerves given off from the dorsal nerve-cord of Amphioxus are
of two kinds, — dorsal and ventral. The dorsal nerves correspond
in number and position to the myomeres, right and left, except in
the most anterior region of the body, where two larger pairs of
dorsal nerves are given off from near the extremity of the nerve-
cord, and pass forward, supplying the region which lies in front of
the termination of the musculature. The ventral nerves are
minute, and are given off numerously, right and left, from the
nerve-tube throughout its leugth. The dorsal and ventral nerves
of a single inyomere appear to correspond, respectively, to the
dorsal and ventral roots of a spinal nerve of a Craniate.

There is a single olfactory pit in Amphioxus, which rests upon
the left side of the anterior termination of the nerve-cord (see fig.
7, 4). Within the cavity of the nerve-cord at the same point a
patch of brown pigment is present (eye-spot). There are no repre-
sentatives of the lateral eyes of Craniata and no otocysts.

There is no representative of the Craniates' swim-bladder in
Amphioxus. A single wide diverticulura of the alimentary canal
represents the liver of Craniata ; the pancreas is unrepresented.

The vascular system is singularly incomplete : large trunks
exist, but few branches and no heart, whilst the blood itself is
colourless, and communicates (as in Craniata by the lymphatic
"hearts ") with the coelomic fluid at various points. A contractile
ventral trunk runs along the lower face of the slit pharynx, and
sends vessels right and left up the successive bars ; these vessels
unite above, as in Craniata, to form a double "dorsal aorta," which
posteriorly becomes a single vessel. A portal system of veins can
be traced in connexion with the hepatic oecum.

No system of lymphatic vessels, nor lymphatic "glands," nor a
spleen exist ; but the crelom, and certain other spaces in the
connective tissue, contain coagulable lymph, and correspond to the
lymph spaces of Craniata.

There is no series of nephridia, nor a renal organ formed by the
coalescence of nephridia, nor arc gonaducts present. The "brown
funnels," a pair of funnel-shaped tubes discovered by Lankester
(14), place the ccelomic space of the opercular (epipleural) down-
growths of the body-wall in communication with the spa™ which
these folds enclose. They appear to be identical with the "collar-
pores" of Balanoglossus, and it is doubtful whether they represent
nephridia.

In the larval Amphioxus there is developed from the left anterior
ccelomic pouch a glandular tube and a sense-organ, which are re-
presented in the adult by the structures marked/ in fig. 8, B. This
tube is probably the same thing as the subneural gland of Ascidians
and as the proboscidean gland and pore of Balanoglossus. Quite
distinct from the foregoing is a nephridial tube lying on the left
side behind the mouth of the larval Amphioxus. All are probably
of the nature of nephridia. In the adult Amphioxus the nephridial
tube is in an atrophied condition, though largo and active in a lute.
larval stage, when the olfactory pit opens into the neural canal.
Hatschck (/j) describes this condition in "ausyebildcte" but not
in adult examples.

The gonads are distinct ovaries and testes ; they are developed
in distinct male and female individuals in corresponding positions,
viz., in that part of the coelom which is carried downwards in the

ft/GHT

Fia 8.— Transverse sections of Amphioxrts. (Original.) A. Section through re-
gion of atrio-coelomic canals, v. B. Section in front of mouth ; the right and
left sides are transposed, a, Cavity surrounding fin ray^ a', fin ray; b,
muscular tissue of myotome ; c, nerve-cord ; d, notochord ; c, left aorta ; /,
thickened ridges of epithelium of pra-oral chamber (Under organ); g, coiled
tnl if lying in a ccelomic space on right side of prteoral hood, apparently an
artery ; ht cuticle of notochord ; i, connective-tissue sheath of notochord ; k,
median ridge of skeletal canal of nerve-cord ; I, skeletal canal protecting
nerve-cord ; m, inter-seg men fail skeletal septum of myotome ; n, subcutaneous
skeletal connective tissue; o, ditto of iiH-tapk'ur (this should be relatively
thicker than it is) ; q, subcutaneous connective tissue of ventral surface of
atrial wall (not a canal, as supposed by & tic-da and others) ; r, epiblastic epi-
thelium ; s, gonad-sac containing ova ; t, pharyngeal bar in section, one of
the " tongue " bars alternating with the main bars and devoid of pliaryngo-
pleura! fold and coelom; r, atrio-ccelomie funnel ; w, so-called "dorsal "coelom;
;r, lymphatic space or cannl of metaplcur; V-, sub-pharyngeal vascular trunk; z,
blood-vessel (portal vein) on wall of hepatic caicum; «a, space of atrial ov
branchial chamber; bl>, ventral groove of pharynx (anteriorly this takes the
form of a ridge) ; cr, liypcrbranchial groove of pharynx ; dd, lumen or space of
hepatic cwcum; ee, narrow coelomic space surrounding hepatic ctecum ; ff,
lining cell-layer of hepatic cfficum ; <;£/, inner face of a pharyngeal bar clothed
with hypobla'st, the outer face covered with epililast (represented black); M, a
main pharyngeal bar with projecting pharyngeal fold (<m which the reference
line rests) in section, showing cttloiMic sp:icn ln-neath the blank epiblast ; ii,
transverse ventral muscle of r pi pleura ; /.'A', raphe or plane of fusion of two
down-grown epipleura; U, space and nucleated cells on dorsal face of noto-
chord ; mm, similar space anil cells on its ventral face.

descending right ami k-ft outgrowth (epiplrura, opercula) of the
body-wall, which encloses the atrial or branchial chamber. The
gonads are twenty-six pairs in number, corresponding to the 10-36
myomeres. They are devoid of ducts, and discharge their pro-
duets by deluscence into the atrial chamber, whence they pass to
the exterior, either by the atriopore or by entering the pharynx
through its slits, when they are ejected by the mouth.

In many respects Amphioxus, the only representative of Ccphalo-
chorda, bears evidence of being derived from a more highly organized

VERTEBRATA

181

brate
ancestry,

Relation- ancestry. Its mode of life (burrowing in tho sand in shallow water,
ship cif whilst its general build is that of a swimming animal) and the
Ci'/i/iitlo- nature of its food (diatoms, &c., carried into the pharynx by ciliary
rlim-il'i to currents) ill themselves suggest such a history. The vascular
Cfuiiintii system is elaborate in plan yrt incomplete in detail, suggesting an
and the, atrophy of its liner branches, which is consistent with the small
Verte- size of Amphiarus and the general principle that a complex vascular
system can only be developed in an animal which has attained to
a" certain bulk. The absence of well-developed sense organs and of
"cephalization" in an animal which has attained to such elabora-
tion of structure as is shown by the pharynx and atrial chamber,
and which has such well-developed muscles to the body-wall, is an
inconsistency best explicable by degeneration ; so, too, the existence
of the elaborate series of fin rays, which are out of proportion to
the mechanical requirements of so small a form.

Degenerate though Ampliioxus must be, the ancestor from
which it started on its retrogressive course was probably a long
way behind any living Craniate. There is no reason to suppose
that this ancestor had a cranium, or that the muscular segments
and segmental nerves in its cephalic region were fused and
welded. Ampliioxus has probably lost, as compared with that
ancestor, lateral eyes and otocysts, nephridia, and, above all, size.
The epipleural folds which now form oral hood, branchial opercula,
and coalesced ventral fin were probably originally less developed
lateral ridges, protecting the gill-slits anteriorly and- posteriorly,
serving by their undulations to assist in locomotion, whilst the
median fin and its rays were large and functional.

One of the most curious features in the structure of AmpTiioxus
is its asymmetry. The anus is on the animal's left side ; the nasal
pit upon its left ; the myomeres on the two sides of the noto-
chord do not coincide ; and the right and left dorsal spinal nerves
do not arise vis-a-vis to one another. There is no conclusive reason
for regarding this as an ancestral feature, although the early larval
form is as curiously asymmetrical as the adult. Amphioxug habitu-
ally rests upon the sand, lyiug upon one side of the body, and it is
possible that the distortion is related to this habit, as in the case
of the Pleuronectid Fishes.

However we may estimate Ampliioxus, we are not led by it, though
its muscular metamerism is so well marked, a single step in the
direction of the Annelids, neither are we led directly, it is true, in
the direction of Ncmertina in connexion with those points, as to
relationship of notochord with proboscis sheath and nerve-cord
with median dorsal nerve, insisted on by Hubrecht. But it will be
seen below that, by the agreement of Ampliioxus with Balanoglossus
in the structure of the perforations of the pharynx, in the possession
of collar pores, and in the pneoral glandular body, we do arrive at
an important connexion with Nemertine-like forms.

THE UKOCHOKDA.

Char- Urochorda are Vertebrata which, with the exception of the group

acters Larvalia (Appcndicularia, Fritillaria, Oikoplcura), have receded
of Uro- very far indeed from the characteristic Vertebrate structure, show-
chorda. ing neither notochord nor nerve-cord, and gill-slits only of the
most highly modified and aberrant form ; some, however (certain
Ascidians), pass through a larval condition in which these struc-
tures are present in the normal form. It is necessary for the pur-
poses of the present article to confine our attention to Larvalia and
to the larval forms which retain ancestral characters. (For a de-
scription of the whole group, see the article TUNICATA. ) In Uro-
cliorda thus signalized the notochord never reaches forward into the
anterior part of the body, but is confined to the tail (hence Uro-
chorda). The longitudinal muscles of the region traversed by the
notochord show traces of metameric segmentation, which are prob-
ably survivals of a more complete development of myomeres in
ancestral forms (iff). There is no trace in Larvalia of fin rays
or other skeletal structure. Corresponding to the opercular folds
and epipleura of Craniata and Ccplialochorda are ridges of the
body-wall, which protect the pharyngeal gill-slits, and may give
rise, as in Ceplialochorda, to an enclosed atrial chamber with
atriopore. The gill-slits in these larval forms are few in number
(one or two pairs), but in many of the aberrant Urochorda (by far
the majority of the group) they become excessively numerous and
complicated in structure, and are supported by a chitinous (?)
framework, as in Cephalochorda. It has been suggested that the
fenestrated structure of the pharyngeal wall in Tunicata docs not
represent a series of gill-slits, but a single pair of slits subdivided.
This suggestion is worthy of further consideration.

The cerebro-spinal nerve-cord is tubular and presents itself as a
dilated cerebral vesicle in front of the notochord, and as a narrower
part running along the whole length of the notochord.

Sense organs are present — a single eye with pigment and lens,
a single otocyst, and an olfactory pit (Larvalia). The mouth is
dorsal in position in the Ascidian tadpole, but subterminal in Lar-
valia. The pharynx is wide, and is followed by a narrow oeso-
phagus, stomach, and intestine, which does not open ventrally but
turns upwards to the anus. The Larvalia have a rudimentary
heart and no vascular system, — a fact connected with their diminu-

tive size. For the same reason no vascular system develops in tho
Ascidian tadpole until it has ceased to he locomotive and has entered
upon its later development ; but in the larger adult Urochorda a
contractile heart and a well-developed vascular system are present.

No undeniable nephridia are present in Larvalia nor in the larval
Ascidian, and no structure comparable to the collar pores of lialano-
f/fc.sHs or the atrio-etelomic funnels ofAmphioxus is known in them.

The subneural gland, however, a glandular tube opening anteriorly
near the mouth of the pharynx, appears to be identical with tho
prreoral larval gland of Amphiuxiis ami the proboscis pore and gland
of Balanoglossus. It is probably to be regarded as a nephridium,
and has been compared by Jnlin and Van Beneden to the pituitary
body of Craniata, with which it corresponds in position and de-
velopment.

The gonads of Larvalia are developed in irregular masses on the
walls of the coslom, ovaries and testes in the same individual.

As above indicated, there is a small section of Urochorda which Classi-
retain in adult life the tadpole-like form and the essential Vcrte- fication
brate organs which are exhibited by the larvse only of other Uro- of Uro-
chorda, and by a few only of these. This necessitates a primary chorda.
division of the branch into two grades.

Grade A. — LARVALIA (Appcndicularia, Fritillaria, Oikopleura).

Grade B. — SACCATA.
Class I. — Ascidiss (Simplices, Sociales, Cornpositx, Pyroso-

miidca).
Class II. — Salpiformia (Salpiidea, Dolioliidca).

Fio. 9.— Fritillaria (Appmdicularm) furcata, one of the Urochorda. (Original
drawings.) A. Lateral surface view, showing habitual carriage of "body"
at right angles to " tail." B. Organs of body as seen by transparency. C.
Lateral view of body with tail in morphological position, showing organs by
transparency. D. Surface view of animal from below to show apertures, a,
Otocyst in connexion with brain ; &, olfactory pit ; c, dorsal hood ; <?, nerve-
tube passing from enlarged brain to caudal region, where it forms one true
ganglion and a series of minor ciilai'iTi'iiii-nts, corresponding to the rudimentary
"myotomes" or "myomeres" of the tail; c, stomach ;/, ovary ; g, testis ; K,
notochord (urochord) ; i, nerve-tube or myelon in tail ; t, fifth myomere of
tail ; I, anus ; m, heart ; n, gill-slit ; o, endostyle or hypobranchial groove ;
p, mouth.

Urochorda are so extremely aberrant, and show so little more Relations
than a transient developmental indication of the essential Verte- of Uro-
brate organs, that we cannot hope to get much positive information diordaio
from them on the subject of Vertebrate ancestry. Only the minute Verte-
Appcndiculnriie (Larvalia) retain the Vertebrate structure through brate
life, and they are obviously, on account of their minute size, ex- ancestry,
tremely degenerate. It is possible to make hypotheses as to the

182

VERTEBRATA

greater or less elaboration of the ancestors of Urocliorda, and to
maintain even that their ancestry had reached as high a condition
as that shown by Craniata ; on the other hand, it does not seem
likely that their point of divergence from the main ancestral line
leading to Craniata was lower than, or even so low as, that at
which Amphioxus branched off. The differentiation of trunk and
tail by the limitation of the notochord anteriorly is a nearer ap-
proach to Craniate structure than that shown by Amphioxus,
whilst the definite development of a brain of considerable relative
size places Urocliorda nearer to the Craniates than is Ampliioxus.
The metameric myomeres so strongly developed in this last are
not absent in Urocliorda, as is often maintained, but exist in a
rudimentary form, indicating that they had once a fuller develop-
ment.

THE HEMICHOEDA.

Char- Hemichorda, comprise the single genus Balanoglossus —

acters of formerly classified by Gegenbaur as Enteropneusta, an
Hemi-
chorda.

fe-

FIG. 10. — Balanorjlossus, anatomy and development. (Modified from Uateson.)
A. #rt/a;("!/'"ws k"ir<t!i'irskn, Bateson; from the coast of Virginia, U..S.;
natural size; fl, proboscis; 6, collar; c, perforate region; d, flattened diges-
tive region; e, cylindrical hind region. B. Diagram of dorsal view, showing
certain organs as though the body-wall were transparent. C. Diagram of a
vertical antero-posterior section. D. Diagram of a dorsal view to show
vessels and nerves by transparency. E. Diagram of a transverse section
tlinmgh the collar. F. Larva of It. koictileirskn; diagram of horizontal sec-
tion. G. Vertical longitudinal section of an older larva of the same. Letter-
ing B-G : — «, proboscis; 6, collar ; f. nerve-tunic of proboscis; f/, proboscis
pore (ciliated orifice) ; fi, notochord (limited to a small tract of modified tissue
derived from pra.-oral extension of alimentary canal); ?, dorsal nerve-plate:
A, collar-pore (right and left), opening to exterior fmm collar co?lomjust bc-
neuth the collar;/, continuation of dorsal nerve-plate as a nerve-cord: m,
pharyngeal perforations (gill-slits) ; n', coelom of proboscis (anterior azygos
primitive ca-lomic pouch); ?;2, collar ca-lom (right and left middle ccelomic
pouches of embryo) ; w3, body co?lom (right and left posterior ccelomic pouches
of embryo) ; o, mouth; />, ventral nerve-tunic of body-wall; x, proboscis
gland; r, strands connecting dorsal nerve-plate with outer wall of collar; s,
cavity of pharynx in front of pel fiu:iti- rruinu ; t, dilated part (heart) of dorsal
vessels within proboscis-gland ; *', dorsal vessel; jt. blood-vessels of body-wall
in section ; w>, paired nerves of collar region in transverse section ; ar, pcri-
h:rmal cu.'lmii, surrounding dorsal vessel in collar region ; y, digestive region of
gut (in embryo); s, mcsolilast. II. Larva of another species of fialuiwrjlossiis,
Knn\vn a.s llic T"nniria larva of Johann Miiller, :ind resembling an KchiniMlrnn
larva, aa, piworal ciliated band of Turnarin ; l>b, post-oral ditto; re, terminal
ditto; ddt mouth: ee, apical plate and sense organ;.//, canal system and
pure ; gg, gut ; hh, anus.

independent phylum of the animal kingdom. They are
Vertebrata of worm-like form, elongate and somewhat

flattened from above downwards. In front of the mouth
is a long cylindrical proboscis, and behind it a collar, the
free margin of which is turned backwards, and corresponds
to the opercular epipleural folds of Cephctlochorda and
Craniata. This agreement is supported by the existence
of a pair of collar pores opening into the coelom of the
collar, as the "brown funnels" of Amphioxus open into
the epipleural ccelom of that animal. A proboscis pore,
opening on the left side into the prasoral ccelom of the
proboscis (paired in B. kupjferi), is exactly representative
of the similarly placed pore which in the young Amphioxus
(according to Hatschek, jj) leads into the tubular organ
derived from the left ccelomic chamber of the prseoral
lobe of that animal. The whole surface of the body is
ciliated, as in Nemertines and Echinoderms, and as in no
other Vertebrates. Following the collar is a perforated
region of the body, — gill-slits opening from the outer
surface into the pharynx. In the young form there is for
a time, as in Appendicularix and the Ascidian tadpole,
only one pair of gill-slits, but they subsequently increase
in number as the animal grows in length. They resemble
in form and structure those of Amphioxus. The notochord
(h in fig. 10) arises at the anterior end of the hypoblast
in the young, and grows forward, forming a support for
the base of the proboscis. It is limited to this very small
region. The cerebro-spinal nerve-cord originates by a
clelarnination of a solid cord of epiblast in the mid-
dorsal line of the middle third of the body; then by
invaginatiou of its two ends it extends as a tube both
anteriorly and posteriorly. A general network of nerve-
fibres (and cells'!) exists beneath the epidermis all over
the body. The blood-system is peculiar, consisting of an
anterior heart and a dorsal and ventral vessel ; these are
united by a plexus of subcutaneous vessels. The muscu-
lature of the body-wall is not broken into successive
myomeres ; but, on the other hand, the gonads (ovaries or
testes) are sac-like, and, as in Amphioxus, are repeated in
a series throughout a great length of the body. In the
pharyngeal region the gonad sacs agree in number with
the gill-slits. There are no nephridia (unless proboscis
pore and collar pores are to be so regarded) ; but the con-
nective-tissue cells of the body-cavity are active as excret-
ing agents, as in Echinoderms and in Urochorda, and a
large glandular organ in the proboscis attached to the end
of the notochord appears to have to do with this function.
Not the least remarkable fact about Hemichorda, is the
nature of their Iarva3. No other Vertebrata, present larval
forms which indicate the nature of the early ancestral
history in what we may call pras-chordal times ; however
interesting the Ascidian larva, or the young Amphioxus,
and the embryo dog-fish, they do not take us out of the
Vertebrate area. Some species of Balanoglossus (1 B. minu-
tus), however, pass through a banded ciliate larval con-
dition, which was known as Tnrnnria, and was considered
to be an Echinoderrn larva allied to Bipinnaria, before
its relation to Balanoglossus was discovered. It is not
possible to view the Tomaria larva of Balanoglossus as
otherwise than identical with Echinoderm larva;, and it
results that Balanoglossus and the Echinoderms have
remote genetic affinities of a special kind.

No classification of Hemichorda is possible beyond an Species
enumeration of the species : —

1. Balanoglossus claviycrus (Delia Chiaje), Naples.

1. JJ. miniitus (Kowalewsky), ,,

3. B. Jcowalewslrii (Al. Agassi/), east coast, United States.

4. H. l/ruu/,-f.ii ( liati'son), ,, „

5. 11. siiliniiiifus (Giard), Brittany.

6. P. rvliinii (Giard), „

It seems that in Balanoylossus we at last find a form
which, though no doubt specialized for its burrowing

chorihi.

VEBTEBEATA

183

Relations
of llcmi-
thorda
to Verte-
brate
ancestry.

sand-life, and possibly to some extent degenerate, yet has
not to any large extent fallen from an ancestral eminence.
Tho ciliated epidermis, the long Worm-like form, and tlie
complete absence of segmentation of the body-muscles lead

us to forms like the Nemertines. The great proboscis of
Balanoglossus may well be compared to the invaginable
organ similarly placed in the Nemertines. The collar is
the first commencement of a structure destined to assume
importance in Cephalochorda and Craniata, and

great

perhaps protective of a single gill-slit in /!ti/iiii"t//i>ssus
before the number of those apertures had been extended.
Borrowing, as we may, the nephridia from the Nemertines,
and the lateral in addition to the dorsal nerve, we find
that Balanogloews gives the most hopeful hypothetical
solution of the pedigree of Vertebrates. Space has not
permitted us to go so fully into pros and cons as the
speculative nature of the subject requires ; but we give
the final conclusion to which our consideration of the
structure of the four great branches of the Vertelrata leads
in the form of the accompanying genealogical tree.

Bibliography. — (/) Cuvier, " Sur un Nouvcau Rapjirochemeut,"
IK., in Ann. du Musec, 1812, vol. xix. p. 73; (2) Kowaluwsky,
"Devel. of Amphioxus," in Mem. dc VAcad. Imp., St Petersburg,
7th series, vol. xvi., No. 12, 1866 ; \j) Id., "Devel. of Ascidia,"
ibid,; (4) Lankester (E. Ray), " Notes on Embryology and Classi-
lication," in Quart. Journ. Micr. Sci., vol. xvii., 1877; (5) Kowa-
lewsky, " Balanoglossus, " in Mem. dc VAcad. Imp., St Petersburg,
vol. x., No. 3, 1886; (6) Bateson, "Balanoglossus," in Quart.
Journ. Mia: Sci., April 1884; ibid., supplement number 1885,
ibid., June 1886; (7) Harmer, " Ccphalodiscus," in Challenger
Reports, vol. xviii. ; (3) Dohrn, Ursprung dcr U'irlcllhicrc, 1875 ;
(9) Zepplin, "Budding in Ctcnodrilus," in Zcitschi: iciss. Zoologic,
vol. xxxix. ; (10) Baltbur, Monograph of Development of Elasmo-
branch Fishes, 1878; (//) Hubrecht, "Nerve-Tunic of Nemer-
tines," in Quart. Journ. Micr. Sci., vol. xx., 1880 ; (12) and (/j)
Id., "Comparison of Nemertines and Vertebrates," in Quart. Journ.
Micr. Sci., vol. xxiii., 1883, and vol. xxvii., 1887 ; (14) Lankester
(E. Ray), "Brown Funnels of Amphioxus," in Quart. Journ. Micr.
Sci., vol. xv. p. 257, 1S75 ; (/5) Hatschek, "Subneural Gland of
Amphioxus and Olfactory Pit," in Zool. Anzcigcr, p. 517, 1884 ;
(16) Lankester (E. Ray), " Vertebration of the Tail of Appcndicu-
laria," in Quart. Journ. Micr. Science, vol. xxii., 1882 ; (77)
Kowalesvsky, "Later Researches on Ascidian Development," in
Archiv fur mikroskopischc Anatomic, vol. vii., 1871 ; (rS) Id.,
"Later Researches on Amphioxus Development," in Archill fur
mikroskopischs Anatomic, vol. xiii., 1876. (E. R. L. )

TUNICATA

(Hi/ H'. A. Ilerdman, D.Sc., Professor of Natural History, University College, Liverpool.)

Simple

Ascid-
iuus.

Com-

IlOtlllil

Ascid-
iaus.

Cuvier

and

Savigny.

r;roup of animals was formerly regarded as con-
J_ stituting along with the Pi>/>/:»a and the Ili-iii-lti"
jHiitu the invertebrate class M < ill iixcoidea. It is now known
to be a degenerate branch of the Clmi-ilntn, and to be more
nearly related to the VerU'lu-titn than to any group of the
Invertebrate.

HISTORY.1

More than two thousand years ago Aristotle gave a
short account of a Simple Ascidian under the name of
Tethyum. He described the appearance and some of the
more important points in the anatomy of the animal.
From that time onwards to little more than a century ago,
although various forms of Ascidians had been briefly de-
scribed by writers on marine zoology, comparatively little
advance was made upon the knowledge of Aristotle.
Schlosser and Ellis, in a paper containing a description of
Jiotryllus, published in the Philosophical Transactions of
the Koyal Society for 1756, first brought the Compound
Ascidians into notice ; but it was not until the commence-
ment of the 19th century, as a result of the careful ana-
tomical investigations of Cuvier (i) upon the Simple
Ascidians and of Savigny (2) upon the Compound, that
the close relationship between these two groups of the
Tunicata was conclusively demonstrated. Up to 1816,
the date of publication of Savigny's great work (2), the
few Compound Ascidians then known had been generally
regarded as Alcyonaria or as Sponges; and, although
many new Simple Ascidians had been described by 0. F
Miiller (4) and others, their internal structure had not
Lamarck, been investigated. Lamarck (3) in 1816, chiefly as the
result of the anatomical discoveries of Savigny and Cuvier,
instituted the class Tunicata, which he placed between the
Radiata and the Vermes in his system of classification.
The Tunicata included at that time, besides the Simple
and the Compound Ascidians, the pelagic forms Pyrosoma,
which had been first made known by Peron in 1804, and
Safpa, described by Forskal in 1775.
Cham- Chamisso in 1820 made the important discovery that
isso and S(,lpa in its life-history passes through the series of changes
which were afterwards more fully described by Steenstrup
in 1842 as "alternation of generations"; and a few years
later Kuhl and Van Hasselt's investigations upon the same
animal resulted in the discovery of the alternation in the
directions in which the wave of contraction passes along
the heart and in which the blood circulates through the
body. It has since been found that this observation holds
good for all groups of the Tunicata. In 1826 H. Milne-
Edwards and Audouiu made a series of observations on
living Compound Ascidians, and amongst other discoveries
they found the free-swimming tailed larva, and traced its
development into the young Ascidian. Milne -Ed wards
(5) also founded the group of "Social" Ascidians, now
known as the Clavelinidx, and gave a classification of the
Compound Ascidians which was universally accepted foi
many years. From the year 1826 onwards a number o
new and remarkable forms were discovered, as, for instance
some of the Boltminx (Macleay), Chetyosoma (Broderip anc
Sowerby, and afterwards Eschricht), Oikopleura (Mertens)
Perophora (Lister), Pelrmaia (Forbesand Goodsir), C/imnlro
stachys and Diplosoma (Denis Macdonald), Diazona (Forbes
and Goodsir), and Rhodosoma (Ehrenberg, and afterwards
Lacaze-Duthiers).

In 1845 Carl Schmidt (6) first announced the present

alterna-
tion of
geuura-
tions.
Circula
tiou.

Milne-
Edwards.

Carl
Schmidt.

1 Only the more important works can be mentioned here. For
more detailed account of the history of the group and a full biblio
graph y, see (77) in the list of works at the end of this article.

n the test of some Aseidians of " tunicinc," a substance
cry similar to cellulose, and in the following year Lb'wig
ind Kollikcr (/) confirmed the discovery and niaile some
.dditimial observations upon this substance and upon the
structure of the test in general. Huxley (S), in an im- Huxley.
wrtant series of papers published in the Transactions of
lie lloyal and Linnean Societies of London from 1851 on-
wards, discussed the structure, embryology, and affinities
of the pelagic Tunicates Pyrosoma, ti<i//j(t, Dohohim, and
Appendioularia. These important forms were also investi-
jated about the same time by Gegenbaur, Vogt, H. Miiller,
Krohn, and Leuckart. The most important epoch in the
listory of the Tunicata is the date of the publication of
Kowalevsky's celebrated memoir upon the development of Kowa-
a Simple Ascidian (<?). The tailed larva had been previously levsky.
discovered and investigated by several naturalists — notably jr;u
H. Milne-Edwards (j), J. P. van Beneden (10), and Krohn al
(//) ; but its minute structure had not been sufficiently
examined, and the meaning of what was known of it had
not been understood. It was reserved for Kowalevsky in Relation-
1866 to demonstrate the striking similarity in structure ship to
and in development between the larval Ascidian and the ^
vertebrate embryo. He showed that the relations between
the nervous system, the notochord, and the alimentary
canal are much the same in the two forms, and have been
brought about by a very similar course of embryonic de-
velopment. This discovery clearly indicated that the
Tunicata are closely allied to Amphioxus and the Verte-
brata, and that the tailed larva represents the primitive
or ancestral form from which the adult Ascidian has been
evolved by degeneration, and this led naturally to the view
usually accepted at the present day, that the group is a
degenerate side-branch from the lower end of the phylum
Chordata, which includes the Tunicata (Urochorda), Amphi-
oxus (Cephalochorda), and the Vertebrata. Kowalevsky's
great discovery has since been confirmed and extended to
all other groups of the Tunicata by Kupffer (12), Giard Kupffer,
(13 and fj), and others. Important observations upon Giard, &c.
the process of gemmation and the formation of colonies in Gemma-
various forms of Compound Ascidians have been made by tion.
Krohn, Metschnikoff, Kowalevsky, Ganin, Giard, Delia
Valle, and others, and have gradually led to the establish-
ment of the general principle, that all the more important
layers of the bud are derived more or less directly from
the corresponding regions in the body of the parent.

In 1872 Fol (14) added largely to the knowledge of the Fol, &c.
Appendiculariidse, and Giard (/j) to that of the Compound
Ascidians. The latter author described a number of new
forms and remodelled the classification of the group. The
most important additions which have been made to the
Compound Ascidians since Giard's work have been those
described by Von Drasche (16) from the Adriatic and
those discovered by the "Challenger" expedition (/;).
The structure and the systematic arrangement of the Simple
Ascidians have been mainly discussed of recent years by
Alder and Hancock (iS), Heller (19), Lacaze-Duthiers
(20), Traustedt (21), and Herdmau (17, 22). In 1874
Ussoff (2j) investigated the minute structure of the nervous Sub-
system and of the underlying gland, which was first dis- neural
covered by Hancock, and showed that the gland has aj^"1
duct which communicates with the front of the branchial ^OTSa}
sac or pharynx by an aperture in the dorsal (or "olfactory ") tubercle,
tubercle. In an important paper published in 1880 Julin
(24) drew attention to the similarity in structure and rela-
tions between this gland and the hypophysis cen-lri of the
vertebrate brain, and insisted upon their homology. He
suggests that they perform a renal function. The Thaliacea

2A

186

TUNICATA

Julin,

have of late years been the subject of several very import-
ant memoirs. The researches of Todaro, Brooks (-?j),
Salensky (26), and others have elucidated the embryology,
the gemmation, and the life-history of the Salpidte ; and
Grobben, Barrels (27), and more especially Uljanin (28)
have elaborately worked out the structure and the details
of the complicated life-history of the Ddiolidx. Finally,
in an important work published in 1886 on the morpho-
Van logy of the Tunicata, E. van Beneden and Julin (jo) have,
Beneden mainly as the result of a close comparison of the embryo-
j0gy Of Ascidians with that of Amphioxus and other
Chordata, added considerably to our knowledge of the
position and affinities of the Tunicata, and of the exact
relations of their organs to the corresponding parts of the
body in the Vertebrata.

ANATOMY.

Ascidia As a type of the Tunicata, Ascidia mentula, one of the
mmiula. iarger species of the Simple Ascidians, may be taken. This
species is found in most of the European seas, generally in
External shallow water on a muddy bottom. It has an irregularly
charac- ovate form, and is of a dull grey colour. It is attached to
ters' some foreign object by one end (fig. 1). The opposite end
of the body is usually nar-
row, and it has a terminal
opening surrounded by eight
rounded lobes. This is the
mouth or branchial aperture,
and it always indicates the
anterior end J of the animal.
About half-way back from
the anterior end, and on a
rounded projection, is the
atrial or cloacal aperture —
an opening surrounded by
six lobes — which is always
placed upon the dorsal re-
gion. When the Ascidian
is living and undisturbed,
water is being constantly
drawn in through the
branchial aperture and
passed out through the
atrial. If coloured par-
ticles be placed in the
water near the apertures,
they are seen to be sucked
into the body through the
branchial aperture, and after
a short time some of them
are ejected with consider-
able force through the atrial
aperture. The current of
water passing in is for re-
spiratory purposes, and it
also conveys food into the animal. The atrial current is
mainly the water which has been used in respiration, but
it also contains all excretions from the body, and at times
the ova and spermatozoa or the embryos.

The test. The outer grey part of the body, which is attached at
or near its posterior end and penetrated by the two aper-
tures, is the "test." This is a firm gelatinous cuticular
secretion from the outer surface of the ectoderm, which is
a layer of flat cells lining its inner surface. Although at
first produced as a cuticle, the test soon becomes organized
by the migration into it of cells derived from the ectoderm
(see fig. 2). These test cells may remain as rounded or
fusiform or stellate cells imbedded in the gelatinous matrix,
to which they are constantly adding by secretions on their

1 Some writers use a different nomenclature of regions ; see (77).

surfaces ; or they may develop vacuoles in their proto-
plasm, which become larger and fuse to form a huge ovate

clear cavity

t,c

(a " bladder

cell "), sur-

|rW--^.;-7-.--^;

rounded by a

blc ^4. _A

NK CT

delicate film

10 '^ ,M

of protoplasm
and having

ra^-i

t A -vr _SQ5

the nucleus

? x JjHP

still visible at

V I * **

one point ; or

Ms

they may form

Aji j W I
A " j, -*§•-

pigment gran-
ules in the pro-

'Q <**<£

toplasm ; or,

__X^. -V--

lastly, they
may deposit

A 77?- f

FIG. 2. — Diagrammatic secti
of an Asculian, showing t

carbonate of

the structure of the test.

•tp^t. r>f>11 • tin. mnti'iv ' lilr..

[0. 1. — Asclilin mcntulu from the right
side, at, atrial aperture ; br, branchial
aperture ; t, test. (Original.)

m, mantle ; e, ectoderm ; (c,

, tm, matrix ; We, bladder cell ; s, s", blood sinus

lime SO that in mantle being drawn out into test ; me, mantle cells ;
i y. septum of vessel. (From Henlman, Challenger Report.

one or several

of them together produce a calcareous spicule in the test.
Only the unmodified test cells and the bladder cells are
found in Ascidia mentula. Calcareous spicules are found
chiefly in the Didemnidse, amongst Compound Ascidians ;
but pigmented cells may occur in the test of almost all
groups of Tunicata. The matrix in which these structures
are imbedded is usually clear and apparently homogeneous;
but in some cases it becomes finely fibrillated, especially
in the family Cynthiidse. It is this matrix which contains
tunicine. At one point on the left side near the posterior
end a tube enters the test, and then splits up into a num-
ber of branches, which extend in all directions and finally
' terminate in rounded enlargements or bulbs, situated chiefly
in the outer layer of the
test. These tubes are
known as the " vessels " of
the test, and they contain
blood. Each vessel is
bounded by a layer of ec-
toderm cells lined by con-
nective tissue (fig. 3, .B),
and is divided into two

tubes by a septum of con- s~ ''ft

nective tissue. The septum Flo s__At a vcssel fromthe test. B.dia-

does not extend into the grammatic transverse section of a ves-

, gel. ec, ectoderm ; ct, connective tissue ;

terminal bulb, and conse- S) ^ the two tubes ; y, septum ; (fc, ter-

quently the two tubes com- "»"al bulb- ("ria»'a1-)
municate at their ends (fig. 3, A). The vessels are formed
by an outgrowth of a blood sinus (derived originally from
the blastoccele of the embryo) from the body wall (mantle)
into the test, the wall of the sinus being formed by con-
nective tissue and pushing out a covering of ectoderm in
front of it (fig. 2, «'). The test is turned inwards at the
branchial and atrial apertures to line two funnel-like tubes,
—the branchial siphon leading to the branchial sac and the
atrial siphon leading to the atrial or peribranchial cavity.

The body wall, inside the test and the ectoderm, is formed Mantle,
of a layer (the somatic layer of mesoderm) of connective
tissue, inclosing muscle fibres, blood sinuses, and nerves.
This layer (the mantle) has very much the shape of the test
outside it, but at the two apertures it is drawn out to form
the branchial and atrial siphons (fig. 4). In the walls of
these siphons the muscle fibres form powerful circular
bands, the sphincter muscles. Throughout the rest of the
mantle the bands of muscle fibres form a rude irregular
network. They are numerous on the right side of the body,
and almost totally absent on the left. The muscles are all
formed of very long fusiform non-striped fibres. The con-
nective tissue of the mantle is chiefly a clear gelatinous

TUNICA T A

187

matrix, containing cells «S various shapes ; it is frequently
pigmcntcd anil is penetrated by numerous lacuna1, in which
the blood flows. In-
side the mantle, in
all parts of the body,
except along the ven-
tral edge, there is a
cavity, — the atrial or
peribrancliial cavity,
— which opens to the
exterior by the atrial
aperture. This cavity
is lined by a layer of
cells derived origin-
ally from the ecto-
derm l and directly
continuous with that
layer through the vd —
atrial aperture (fig.
5) ; consequently the

mantle is covered both g,a '

externally and inter-
nally by ectodermal
cells. <*--

Bran- The branchial aper- p lr—

chial sac ture (mouth) leads in-

iei<-h ^° branchial si- ^r

bourine phon (buccal Cavity Fio. 4.— Diagrammatic dissection of A. mentula to

i \ j show the anatomy, at, atrial aiierture ; br,

organs, or StoniOdasum), and branchial aperture; n, anus; Irs, branchial

this Opens into the sac • <*'> dorsal lamina ; dt, dorsal tubercle

• j » ' end, endostyle ; h, lieart ; i, intestine ; in

anterior end Ot a very mantle ; ng, nerve ganglion ; a-, (Esophagus

laro-p r-nvirWtlip ln-an a'a> cesophageal aperture; OP, ovary; plr

large Cavity (IK peribrancliial cavity ; r, rectum ; st, st.,,,,a,-l,

chial Sac) which ex- t, test ; /«, tentacles ; vd, vas deferens ; ngl,

tends nearly to the subneural slani1' (Urisil
posterior end of the body (see figs. 4 and 5). This branchial
sac is an enlarged and modified pharynx, and is therefore
properly a part of the ali-
mentary canal. The oeso-
phagus opens from it far
back on the dorsal edge (see
below, p. 612). The wall of
the branchial sac is pierced
by a large number of ver-
tical slits, — the stigmata,
— placed in numerous trans-
verse rows. These slits
place the branchial sac in
communication with the

d I--...

:i .— end

—B

FIG. 5.— Diagrammatic longitudinal (A) and transverse (B) sections thrtmgh
Aseidia to show the position of the ectoderm and the relations of the bran-
chial and peribranchial cavities. The lettering is the same as fur fig. 4. li
represents a section taken along the dotted line A-B in A. (Original.)

peribranchial or atrial cavity, -which lies outside it (fig. 5,
B}. Between the stigmata the wall of the branchial sac
is traversed by blood-vessels, which are arranged in three
regular series (fig. 6), — (1) the transverse vessels, which
run horizontally round the wall and open at their dor>al
and ventral ends into large longitudinal vessels, the dorsal
and ventral sinuses ; (2) the fine longitudinal vessels, whirli
run vertically between adjacent transverse vessels and open
into them, and which bound the stigmata; and (3) the
internal longitudinal bars, which run vertically in a plane

1 According to E. van Beneden and Julin's recent investigations (jo)
only the outer wall of the atrium is lined with epiblast, the inner wall
being derived from the hyiioblast of the primitive branchial sac.

Trough

Fin. Ci.—A. Part of branchial sac of Aseidia from inside.
Jl. Transverse section of same, tr, transverse vessel ;
cd, connecting duct ; Tim, horizontal membrane ; il,
internal longitudinal bar ; Ic, fine longitudinal vessels;
p, p', papillfe ; sg, stigmata. A and B are drawn to
different scales. (From Herdmau, Challenger llcport.)

internal to that of the transverse and fine longitudinal
vessels. Th<-se liars communicate with the transverse
vessels by short side
branches where they
cross, and at these. /.
points are prolonged
into the lumen of the
sac in the form of
hollow papilla?. The
edges of the stigmata n
are richly set with
cilia, which drive the
water from the bran-
chial sac into the
peribranchial ca- h ,"_
vity, and so cause
the currents that
flow in through
the branchial
aperture and
out through the
atrial.

Along its vent-
ral edge the wall
of the branchial
sac is continu-
ous externally with the mantle (fig. 5, B), while internally
it is thickened to form two parallel longitudinal folds
bounding a groove, the " endostyle," Lypobranchial groove, Endo-
or ventral furrow (figs. 4, 5, end). The endoderm cells style,
which line the endostyle are greatly enlarged at the
bottom and on parts of the sides of the furrow so as to
form projecting pads, which bear very long cilia. It is
generally supposed that this organ is a gland for the pro-
duction of the mucous secretion which is spread round the
edges of the branchial sac and catches the food particles in
the passing current of water ; but it lias recently been
pointed out that there are comparatively few gland cells in
the epithelium of the endostyle, and that it is more prob-
able that this furrow is merely a ciliated path along which
the mucous secretion (produced possibly by the subneural
gland) is conveyed posteriorly along the ventral edge of
the branchial sac. At its anterior end the edges of the Peri-
endostyle become continuous with the right and left halves pharyn-
of the posterior of two circular ciliated ridges, — the peri-p ,
pharyngeal bands, — which run parallel to one another
round the front of the branchial sac. The dorsal ends of
the posterior peripharyngeal band bend posteriorly (en- Dorsal
closing the epibranchial groove), and then join to form lamina,
the anterior end of a fold which runs along the dorsal edge
of the branchial sac as far as the cesophageal aperture.
This fold is the dorsal lamina (figs. 4, 5, dl). It probably
serves to direct the stream of food particles entangled in
a string of mucus from the anterior part of the dorsal
lamina to the oesophagus. In many Ascidians this organ, Dorsal
instead of being a continuous membranous fold as in A. languets.
mentula, is represented by a series of elongated triangular
processes — the dorsal languets, — one attached in the dorsal
median line opposite to each transverse vessel of the
branchial sac. The anterior peripharyngeal band is a
complete circular ridge, having no connexion with either
the endostyle or the dorsal lamina. In front of it lies the
prebranchial zone, which separates the branchial sac behind
from the branchial siphon in front. The prebranchial
zone is bounded anteriorly by a muscular band — the pos-
terior edge of the sphincter muscle, — which bears a circle
of long delicate processes, the tentacles (figs. 4, 7, 8, in). Ten-
These project inwards at right angles so as to form a net-tac'es-
work across the entrance to the branchial sac. Each
tentacle consists of connective tissue covered with epithe-

188

TUNICATA

Hum (endoderm), and contains two or more cavities which
are continuous with blood sinuses in the mantle. In the

Subneur- dorsal median line near the anterior end of the body, and

al gland, imbedded in the mantle on the ventral surface of the nerve
ganglion, there lies a small glandular mass — the subneural
gland — which, as Julin has shown (^y), there is reason to
regard as the homologue of the hypo-
physis cerebri of the vertebrate brain.
Julin and E. van Beneden have sug-
gested that the function of this organ
may possibly be renal.1 The sub-
neural gland, which was first noticed
by Hancock, communicates anteriorly,
as Ussoff (23) pointed out, by means
of a narrow duct with the front of
the branchial sac (pharynx). The
opening of the duct is enlarged to
form a funnel-shaped cavity, which
may be folded upon itself, convoluted,
or even broken up into a number of

Dorsal smaller openings, so as to form a

tubercle, complicated projection, called the
dorsal tubercle, situated in the dorsal
part of the prebranchial zone (fig. 7).
The dorsal tubercle in A. mentula is I
somewhat horse-shoe-shaped (fig. 8) ; I
it varies in form in most Ascidians '
according to the genus and species, '
and in some cases in the individual J
also. Possibly, besides being the FK' 7.-Diagrammatic sec-
opening of the duct from the sub-
neural gland, it may be a sense-organ
for testing the quality of -the water
entering the branchial sac.

Nervous The single elongated ganglion in

system. jjjg median dorsal line of the mantle
between the branchial and atrial si-
phons is the only nerve-centre in A. mentula and most other
Tunicata. It is the degenerate remains of the anterior

tion through anterior dor-
sal part of A. •mentula,
showing the relations of
the nerve ganglion, sub-
neural gland, &c. Letter-
ing as for fig. 4 ; 7i, nerve ;
ri , myelon ; pp, perii'lia-
ryngeal band ; sgl, sub-
neural gland ; sgdt its
duct ; t1, test lining branch-
ial siphon. (Original.)

Flo. 8. — Dorsal tubercle ami nrigMjmiring organs of A. maitulii.
as before ; cyr, epibranchial groove ; z, prubraucliial zone. (Original.)

part of the cerebro-spinal nervous system of the tailed
larval Ascidian (see below, p. 614). The posterior or
spinal part has entirely disappeared in most Tunicata.
It persists, however, in the Appendiculariidx, and traces of
it are found in some Ascidians (e.g., Clavelina ; see Julin).
The ganglion gives off distributory nerves at both ends,

1 See also Ilerdman, Nature, vol. xxviii. p. 284.

which run through the mantle to the neighbourhood of the Sense
apertures, where they divide and subdivide. The onlyorgans-
sense-organs are the pigment spots between the branchial
and atrial lobes, the tentacles at the base of the branchial
siphon, and possibly the dorsal tubercle and the languets
or dorsal lamina. These are all in a lowly developed con-
dition. The larval Ascidians on the other hand have well-
developed intra-cerebral optic and auditory sense-organs ;
and in some of the pelagic Tunicata otocysts and pigment
spots are found in connexion with the ganglion.

The mouth and the pharynx (branchial sac) have already Aliment-
been described. The remainder of the alimentary canal air
is a bent tube which in A. mentula and most other Ascid- canal-
ians lies imbedded in the mantle on the left side of the
body, and projects into the peribranchial cavity. The
oesophagus leaves the branchial sac in the dorsal middle
line near the posterior end of the dorsal lamina (see fig.
4, <xa). It is a short curved tube which leads ventrally
to the large fusiform thick-walled stomach. The intestine
emerges from the ventral end of the stomach, and soon
turns anteriorly, then dorsally, and then posteriorly so as
to form a curve — the intestinal loop — open posteriorly.
The intestine now curves anteriorly again, and from this
point runs nearly straight forward as the rectum, thus com-
pleting a second curve — the rectal loop — open anteriorly
(see fig. 4). The wall of the intestine is thickened inter-
nally, to form the typhlosole, a pad which runs along its
entire length. The anus opens into the dorsal part of the
peribranchial cavity near to the atrial aperture. The walls
of the stomach are glandular ; and a system of delicate
tubules with dilated ends, which ramifies over the outer wall
of the intestine and communicates with the cavity of the
stomach by means of a duct, is probably a digestive gland.

A mass of large clear vesicles which occupies the rectal Excre-
loop, and may extend over the adjacent walls of the in-tory
testine, is a renal organ without a duct. Each vesicle is Or6aus-
the modified remains of a part of the primitive ccelom or
body-cavity, and is formed of cells which eliminate nitro-
genous waste matters from the blood circulating in the
neighbouring blood-lacunas and deposit them in the cavity
of the vesicle, where they form a concentrically laminated
concretion of a yellowish or brown colour. These concre-
tions contain uric acid, and in a large Ascidian are very
numerous. The nitrogenous waste products are thus de-
posited and stored up in the renal vesicles in place of
being excreted from the body. In other Ascidians the
renal organ may differ from the above in its position and
structure ; but in no case has it an excretory duct, unless
the subneural gland is to be regarded as a renal organ.

The heart is an elongated fusiform tube placed on the Blood-
ventral and posterior edge of the stomach, in a space (the vascular
pericardium) which is part of the original cceloni or body- SVI*tem
cavity, the rest of which exists merely in the form of lacuna?
and of the cavities of the reproductive organs and renal
vesicles in the adult Ascidian. The wall of the heart is
formed of a layer of epithelio-muscular cells, the inner
ends of which are cross-striated ; and waves of contraction
pass along it from end to end, first for a certain number of
beats in one direction and then in the other, so as to reverse
the course of circulation periodically. At each end the
heart is continued into a vessel (see fig. 9), a large sinus
or lacuna lined with a delicate endothelial layer. The
sinus leaving the ventral end of the heart is called the
branchio-cardiac vessel,- and the heart itself is merely the
differentiated posterior part of this sinus and is therefore
a ventral vessel. The branchio-cardiac vessel, after giving
off a branch which, along with a corresponding branch from
the cardio-visceral vessel, goes to the test, runs along the

2 On account of the periodic reversal of the circulation none of the
vessels can be called arteries or veins.

TUNICATA

IH'.I

tion.

. 9. — Diagram of circulation in Ascidia. Lettering as "he-
fore ; be, branchio-cardiac or ventral vessel ; cv, cardio-
visceral vessels; vb, viscero-branchial or dorsal vessel;
vt> vessels to test. (Original.)

ventral edge of the branchial sac externally to the endostyle,
ami communicates laterally with the ventral ends of all the
transverse vessels <>!' the branchial sae. The sinus leaving
tlio dorsal end of the heart is called the carilio-visceral
vessel, and this, after giving off to tho test the branch
above mentioned, breaks up into a number of sinuses,
which ramify over the alimentary canal and tho other
viscera. These visceral lacun;r finally communicate with
a third great sinus, the viscero-branchial vessel, which runs
forward along tho dorsal edge of tho branchial sac exter-
nally to the dorsal lamina and joins the dorsal ends of all
tho transverse vessels of the branchial sac. Besides these
three chief systems there are numerous lacuna) in all parts
of the body, by means of which anastomoses are established
between the different currents of blood. All these blood
spaces and lacunas are to be regarded as derived from the
blastoccele of tho embryo, and not, as has been usually
Course of supposed, from the ccelom (jo). When the heart contracts
circula- ventro-dorsally, the course of the circulation is as follows :
the blood which is flowing through the vessels of the
branchial sac is collected in an oxygenated condition in
the branchio-cardiac vessel, and, after receiving a stream
of blood from
the test, en-
ters the

heart. It is
then pro-
pelled from
the dorsal

end of the
i , • ,

JiearC into

the cardio-

visceral vessels, and so reaches the test and digestive and
other organs ; then, after circulating in the visceral lacunae,
it passes into the viscero-branchial vessel in an impure
condition, and is distributed to the branchial vessels to be
purified again. When the heart on the other hand contracts
dorso-ventrally, this course of circulation is reversed. As
the test receives a branch from each end of the heart, it
follows that it has afferent and efferent vessels which-
ever way the blood is flowing. In some Ascidians the
vessels in the test become very numerous and their end
branches terminate in swollen bulbs close under the outer
surface of the test. In this way an accessory respiratory
organ l is probably formed in the superficial layer of the
test. The blood corpuscles are chiefly colourless and
amoeboid ; but in most if not all Ascidians there are also
some pigmented corpuscles in the blood. These are gener-
ally of an orange or reddish brown tint, but may be opaque
white, dark indigo-blue, or of intermediate colours. Pre-
cisely similarly pigmented cells are found throughout the
connective tissue of the mantle and other parts of the body.
A. mentula is hermaphrodite, and the reproductive organs
lie, with the alimentary canal, on the left side of the body.
The ovary is a ramified gland which occupies the greater
part of the intestinal loop (see fig. 4). It contains a cavity
which, along with the cavities of the testis, is derived from
a part of the original ccelom, and the ova are formed from
its walls and fall when mature into the cavity. The
oviduct is continuous with the cavity of the ovary and
leads forwards alongside the rectum, finally opening near
the anus into the peribranchial cavity. The testis is com-
posed of a great number of delicate branched tubules,
which ramify over the ovary and the adjacent parts of the
intestinal wall. Those tubules terminate in ovate swell-
ings. Near the commencement of the rectum the larger
tubules unite to form the vas deferens, a tube of consider-
able size, which runs forwards alongside the rectum, and,
like the oviduct, terminates by opening into the peri-
1 See Herdman, Nature, vol. xxxi. p. 247.

Repro-
ductive
organs.

branchial cavity close to the anus. Tho lumen of the
tubiilcs of the testis, like the cavity of the ovary, is a part
of the original ccelom, and the spermatozoa arc formed
from the cells lining tho wall. In some Ascidian.s repro-
ductive organs are present on both sides of the body, and
in others (Pdyca/rpa) there are many complete sets of both
male and female systems, attached to the inner surface of
the mantle on both sides of tho body and projecting into
tho peribranchial cavity.2

EMBRTOLOOY 3 AND LIFE HISTORY.

In most Ascidians the eggs are fertilized in the peribranchial Embryc-

cavity, and undergo most of their development before leaving the logy,
parent ; in some cases, however, the eggs are laid, and fertilization
takes place in the surrounding water. The segmentation is com-
plete and regular (fig. 10, A) and results in the formation of a
spherical blastula, which then undergoes invagination (fig. 10, B).
The embryo elongates, and the blastopore or invagination opening
comes to be placed on the dorsal edge near the posterior end (fig.
10, C). The hypoblast cells lining the archenteron are columnar
in form, while the epiblast cells are more cubical (fig. 10, B, C, D).
The dorsal surface of tho embryo now becomes flattened and then
depressed to form a longitudinal groove, extending forwards from
the blastopore to near the front of the body. This "medullary
groove" now becomes converted into a closed canal by its side
walls growing up, arching over, and coalescing in the median dorsal

Fio. 10.— Stages in the embryology of a Simple Ascidian (after Kowalevsky).
A to F. Longitudinal vertical sections of embryos, all placed with the dorsal
surface uppermost and the anterior end at the right. A. Early blastula
stage, during segmentation. B. Early gastrula stage. C. stage after gas-
trula, showing commencement of notochord. D. Later stage, showing forma-
tion of notochord and of neural canal. E. Embryo showing body and tail
and completely formed neural canal. F. Larva just hatched; end of tail
cut off. G. Transverse section of tail of larva.

adf, adhering papilte of larva ; at, epiblastic (atrial) involution; au, auditory
organ of larva ; ar, archenteron ; be, blastoccele ; bp, blastopore ; cTi, nnto-
chord ; cp, epiblast ; liy, hypoblast ; nc, neural canal ; -HPC, ncnrenti-ric
canal; oc, ocular organ of larva; g, gelatinous investment of embryo; >u,
muscle fells of tail; ntcs, mesenteron ; me, mesoderm cells; ni; cerebral
vesicle at anterior end of neural canal.

line (fig. 10, D). This union of the lamiuse dorsalcs to form the
neural canal commences at the posterior end behind the blastopore
and gradually extends forwards. Consequently the blastopore
comes to open into the posterior end of the neural canal (fig. 10,
D), while the anterior end of that cavity remains open to the
extrvior. In this way tho archenteron communicates indirectly
with the exterior. The short canal leading from the neural canal
to the archenteron is known as the neurenteric canal (fig. 10,

2 For structure of other forms, see p. 614 sq. below.

3 For reproduction by gemmation, see under "Classification," p.
614 sy. below.

190

TUNICA T A

Larval
stage.

D, nee). Previous to this stage some of the hypoblast cells at tlie
front edge of the blastnpore and forming part of the dorsal wall of
the archenteron (fig. 10, C, A) have become separated off, and then
arranged to form an elongated band, two cells wide, underlying
the posterior half of the neural canal (fig. 10, D, E, <*.). This
is the origin of the notochord. Outgrowths from the sides of the
archenteron give rise to laterally placed masses of cells, which are
the origin of the mesoblast. These masses show no trace of meta-
meric segmentation. The cavities (reproductive and renal vesicles)
which are formed later in the mesoblast represent the ccelom.
Consequently the body-cavity of the Twiimta, is a modified form
of enteroccele. The anterior part of the embryo, in front of the
notochord, now becomes enlarged to form the trunk, while the
posterior part elongates to form the tail (tig. 10, E). In the trunk
the anterior part of the archenteron dilates to form the mescnteron,
the greater part of which becomes the branchial sac ; at the same
time the anterior part of the neural canal enlarges to form the
cerebral vesicle, and the opening to the exterior at the front end of
the canal now closes. In the tail part of the embryo the neural
canal remains as a narrow tube, while the remains of the wall of the
archenteron— the dorsal part of which becomes the notochord— are
converted into lateral muscle bauds (fig. 10, G) and a ventral cord
of cells, which eventually breaks up to form blood corpuscles. As
the tail grows longer, it becomes bent round the trunk of the embryo
inside the egg-membrane. About this period the epiblast cells
begin to form the test as a cuticular deposit upon their outer surface.
The test is at first devoid of cells and forms a delicate gelatinous
investment, but it shortly afterwards becomes cellular by the
migration into it of test cells formed by proliferation from the epi-
bla'st. '

The embryo is hatched about two or three days after fertilization,
in the form of a tadpole-like larva, which swims actively through
the sea by vibrating its long tail. The anterior end of the body
is provided with three adhering papillte (fig. 10, F, adp) in the
form of epiblastic thickenings. In the free-swimming tailed larva
the nervous system, formed from the walls of the neural canal,
becomes considerably differentiated. The anterior part of the
cerebral vesicle remains thin-walled (fig. 10, F), and two unpaired
sense organs develop from its wall and project into the cavity.
These are a dorsally and posteriorly placed optic organ, provided
with retina, pigment layer, lens, and cornea, and a ventrally placed
auditory organ, consisting of a large spherical partially pigmented
otolith, attached by delicate hair-like processes to the summit of a
hollow crista acoustica (fig. 10, F, ait). The posterior part of the
cerebral vesicle thickens to form a solid ganglionic mass traversed
by a narrow central canal. The wall of the neural canal behind the
cerebral vesicle becomes differentiated into an anterior thicker
region, placed in the posterior part of the trunk and having a
superficial layer of nerve fibres, and a posterior narrower part which
traverses the tail, lying on the dorsal surface of the notochord, and
gives off several pairs of nerves to the muscles of the tail. Just in
front of the anterior end of the nervous system a dorsal involution
of the epiblast breaks through into the upturned anterior end of
the mesenteron and thus forms the mouth opening. Along the
ventral edge of the mesenteron, which becomes the branchial sac,
the endostyle is formed as a narrow groove with thickened side
walls. It probably corresponds to the median portion of the thyroid
body of Tertebrata. A curved outgrowth from the posterior end
of the mesenteron forms the alimentary canal (oesophagus, stomach,
and intestine), which at first ends blindly. An anus is formed
later by the intestine opening into the left of two lateral epiblastic
involutions (the atria), which rapidly become larger and fuse dorsally
to form the peribranchial cavity. Outgrowths from the wall of the
branchial sac meet these epiblastic involutions and fuse with them
to give rise to the first formed pair of stigmata, which thus come
to open into the peribranchial cavity ; and these alone correspond
to the gill clefts of Amphioxiis and the J'ci-td'i-nln.

Metamor- After a short free-swimming existence the fully developed tailed
phosis larva fixes itself by its anterior adhering papilla: to some foreign
to adult object, and then undergoes a remarkable series of retrogressive,
form. changes, which convert it into the adult Ascidian. The tail atro-
phies, until nothing is left but some fatty cells in the posterioi
part of the trunk. The adhering papilla? disappear and are replaced
functionally by a growth of the test over neighbouring objects.
The nervous system with its sense organs atrophies until it is re-
duced to thr single small ganglion, placed on the dorsal edge of the
pharynx, and a slight nerve curd miming for some distance pos-
teriorly (Van Beneden and Jnlin). Slight change's in the shape
of the body and a further growth and differentiation of the branehia
sac, peribranchial cavity, and other organs now produce gradually
the structure found in the adult Aseidian.

The most important points in connexion with this process o
development and metamorphosis are the following. (1) In tin.
Ascidiau embryo all the more important organs (e.g., notochord
neural canal, archenteron) are formed in essentially the same

1 Some of the lirst test cells are also probably derived from the epitlieliun
of the egg follicle.

lanner as they are in Amphioxus and other Chordata. (2) The
fee-swimming tailed larva possesses the essential characters of the
'Ihordata, inasmuch as it has a longitudinal skeletal axis (the noto-
hord) separating a dorsally placed nervous system (the neural
anal) from a ventral alimentary canal (the archenteron) ; and
herefore during this period of its life-history the animal belongs
o the Chordata. (3) The Chordate larva is more highly organized
ban the adult Ascidian, and therefore the changes by which the
atter is produced from the former may be regarded as a process of
legeneration (j>/). The important conclusion drawn from all this
s that the Titnicata are the degenerate descendants of a group of
he primitive Chordata (see below' p. 618).

CLASSIFICATION AND CHARACTERS OF GROUPS.

Order I.— LARVACEA.

Free-swimming pelagic forms provided with a large locomotory Char-
appendage (the tail), in which there is a skeletal axis (the urochord I. acters of
_ A relatively large test (the "Haus") is formed with Larvacea-
great rapidity as a secretion from the ectoderm ; it is
merely a temporary structure, which is cast off and
replaced by another. The branchial sac is simply an
enlarged pharnyx with two ventral ciliated openings
(stigmata) leading to the exterior. There is no se-
parate peribranchial cavity. The nervous
system consists of a large dorsally placed
ganglion and a long nerve cord, which
stretches backwards over the alimentary
canal to reach the tail, along which it
runs on the left side of the
urochord. The anus opens
ventrally on the surface of
the body in front of the stig-
mata. No reproduction by
gemmation or metamorphosis
is known in the life-history.
This is one of the most in-
teresting groups of the Tuni-
cata, as it shows more com-
pletely than any of the rest
the characters of the original
ancestral forms. It has un-
dergone little or no degen-
eration, and consequently
corresponds more nearly to
the tailed -larval condition
than to the adult forms of
the other groups. The order

Fio. 11.—

ips,
.. includes a single family, the

ir. ll.— oikrwnm cnplioccrca in "Haus • -.,' ,

(after Fol), seen from right side, magnified Al'FENPICULARIIDjE, all tlie
six times. The arrows indicate the course members of which are minute
of the water ; x, lateral reticulated parts of an(j flee . swimming. They

occur on the surface of the

sea in most parts of the world. They possess the power to form Struc-
with great rapidity an enormously large investing gelatinous layer tare of
(fig. 11), which corresponds to the test of other groups. This was Appen-

ol
'-It

dicularia.

les

Fio. 12.— Semi-diagrammatic view of Ai>i«-i>il!ri'liirM from the right, n, anus ;
ni one of the atrlol apertures; afp,ia\\; &r, branchial aperture; brs, branchial
sue ; ill, ill irsal tubercle ; end, endostyle ; h, heart ; i, intestine ; m, muscle

caudal ganglion ; ng, enargement nf nerve cor n a ; so, se
(tactile) on Imver lip ; sij, ciliated aperture iu pharynx; st, stomach ; te, testis
«, urochord ; «', its cut end. (Original.)

It

first described by Von Mertens and by him named "Haus."
is only loosely attached to the body and is frequently thrown off
soon after its formation. The tail in tlie AppmcUffulwriidss is at-
tached to the ventral surface of the body (fig. 12), and usually

TUNIC ATA

191

Thali-
acea.

Charac-
ters of
Cycle-
myaria.

Struc-
ture of
Dolio-
lum.

points more or less anteriorly. It shows di.-tinet traces .of im-tu-
inoric segmentation, having its muscle bunds broken up into myo-
tomes, while the nerve conl presents a series of enlargements from
which distributary nerves arc given oil' (lig. 12, tig"). Near
the base of the tail there is a distinct elongated gunglimi
i lig. 12, ;<;/'). The anterior (cerebral) ganglion has connected
with it an otocyst, a pigment spot, and a tubular process
opening into the branchial sac and representing tlic dorsal
tubercle and associated parts of an ordinary Aseidian. Tim
branchial aperture or mouth leads into the branchial sac or
pharynx. There are no tentacles. The endostyle is short.
There is no dorsal lamina, and the peripharyngeal bands run
dorsally and posteriorly. The wall of the branchial sue has <"
only two ciliated apertures. They are homologous with the,
primary stigmata of the typical Ascidians and the gill
clefts of Vertebrates. They are placed far back on the -ven-
tral surface, one on each side of the middle line, and lead
into short funnel-shaped tubes which open on the surface of so
the body behind the anus (fig. 12, at). These tubes corre-
spond to the right ami left atrial involutions which, in an
ordinary Aseidian, fuse to form the peribranchial cavity. The
heart, according to Lankester, is formed of two cells, which
are placed at the opposite ends and connected by delicate con-
tractile protoplasmic fibrils. The large ovary and testis are placed
at the posterior end of the body. The remainder of the structural
details can be made out from fig. 12.

The family Ajijmnliciiinriiila; comprises the genera, — Oikoplnim
(Mertens), and Appcndicularia (Cham.), in both which the body is
short and compact and the tail relatively long, while the endostyle
is straight ; FritHlm-ia (Q. and G.), in which the body is long a'nd
composed of anterior and posterior regions, the tail relatively short,
the endostyle recurved, ami an ectodermal hood is formed over the
front of the body ; and Koimlcvskia (Fol), a remarkable form de-
scribed by Fol (rj), in which the heart, endostyle, and intestine
are said to be absent, while the branchial sac is provided with four
rows of ciliated tooth-like processes.

Order II.— THALIACEA.

Free-swimming pelagic forms which may be either simple or
compound, and the adult of which is never provided with a tail or a
notochord. The test is permanent and may be either well developed
or very slight. The musculature of the mantle is in the form of
more or less complete circular bands, by the contraction of which
locomotion is effected. The branchial sac has either two large or
many small apertures, leading to a single peribranchial cavity, into
which the anus opens. Alternation of generations occurs in the life-
history, and may be complicated by polymorphism. The Tlialiacca
comprises two groups, Cyclomyaria and Hemimyaria.

Sub-order 1. — Cyclomyaria.

Free-swimming pelagic forms which exhibit alternation of genera-
tions in their life-history but never form permanent colonies. The
hody is cask-shaped, with the branchial and atrial apertures at the
opposite ends. The test is more or less well developed. The
mantle has its musculature in the form of circular bands surrounding
the hody. The branchial sac is fairly large, occupying the anterior
half or more of the body. Stigmata are usually present in its
posterior part only. The peribranehial cavity is mainly posterior
to the branchial sac. The alimentary canal is placed ventrally
close to the posterior end of the branchial sac. Hermaphrodite
reproductive organs are placed ventrally near the intestine.

This group forms one family, the DOLIOLIM, including two
genera, Doliolum (Quoy and Gaimard) and Anchinia (C. Vogt).

Doliolum, of which several species are known from various
seas, has a cask-shaped body, usually from 1 to 2 cm. in length.
The terminal branchial and atrial apertures (fig. 13) are lobed,
and the lobes are provided with sense organs. The test is very
slightly developed and contains no cells. The mantle has eight or
nine circular muscle bands surrounding the body. The most
anterior and posterior of these form the branchial and atrial
sphincters. The wide branchial and atrial apertures lead into
large branchial and peribranehial cavities, separated by the pos-
terior wall of the branchial sac, which is pierced by stigmata ; con-
sequently there is a free passage for the water through the body
along its long axis, and the animal swims by contracting its ring-
like muscle-bands, so as to force out the contained water posteriorly.
Stigmata may also be found on the lateral walls of the branchial
sac, and in that case there are corresponding anteriorly directed
diverticula of the peribranehial cavity. There is a distinct endo-
style on the ventral edge of the branchial sac and a peripharyngeal
band surrounding its anterior end, but there is no representative
of the dorsal lamina on its dorsal edge. The oesophagus com-
mences rather on the ventral edge of the posterior end of the
branchial sac, and runs backwards to open into the stomach, which
is followed by a curved intestine opening into the perihranchial
cavity. The alimentary canal as a whole is to the right of the
middle line. The hermaphrodite reproductive organs are to the
left of the middle line alongside the alimentary canal. They open

into the pcribranohial cavity. The ovary is nearly spherical, while
the Icstis is elongated, and may be continued anteriorly for a long
distance. The heart is placed in the middle line ventrally, be-

/ bi-

nt I

end

brs

Fir,. W.—DoUolmndenticvlaliim, sexual generation, frnm tlic left side. Letter-
ing as fur tig. 12 ; ml—msy muscle bands ; mj, nerve ganglion ; 53, stigmata ;
s;//, Bubneural gland; pbr, pexibranchial cavity; alt, atrial lubes: so, sense
organs ; Itrl, branchial lobes. (Original.)

twecn the posterior end of the endostyle and the cesophageal aperture.
The nerve ganglion lies about the middle of the dorsal edge of the
hody, and gives off many nerves. Under it is placed the suhueural
gland, the duct of which runs forward and opens into the anterior
end of the branchial sac by a simple aperture, surrounded by the
spirally twisted dorsal end of the peripharyngeal band (fig. 13, dt).

The ova of the sexual generation produce tailed larvse ; these Develop-
devclop into forms known as " nurses " (blastozooids), which are ment of
asexual, and are characterized by the possession of nine muscle Dolio-
bands, an auditory sac on the left side of the body, a ventrally- lum.
placed stolon near the heart, upon which buds are produced, and
a dorsal outgrowth near the posterior end of the body. The buds
give rise eventually to the sexual generation, which is polymor-
phous, having three distinct forms, in two of which the reproduc-
tive organs remain undeveloped. The buds while still very young
migrate from their place of origin on the stolon, divide by fission,
and become attached to the dorsal outgrowth of the body of the
nurse, where they develop. The three forms produced are as follows.
(1) Nutritive forms (trophozooids), which remain permanently at-
tached to the nurse and serve to provide it with 1'ood ; they have
the hody elongated dorso- ventrally, and the musculature is very
slightly developed. (2) Foster forms (phorozooids), which, like the
preceding, do not become sexually mature, but, unlike them, are
set free as cask-shaped bodies with eight muscle bands and a ventral
outgrowth, which is formed of the stalk by which the hody was
formerly united to the nurse. On this outgrowth the (3) forms
(gonozooids) which become sexually mature are attached while still
young buds, and after the foster forms are set free these reproductive
forms gradually attain their complete development, and are event-
ually set free and lose all trace of their connexion with the foster
forms. They resemble the foster forms in having a cask -shaped
body with eight muscle bands, but differ in having no outgrowth
or process, and in having the reproductive organs fully developed.1

Anchinia, of which only one species is known, A. rubra, from Anchinia.
the Mediterranean, has the sexual forms permanently attached
to portions of the dorsal outgrowth from the body of the unknown
nurse. The body is elongated dorso-ventrally. The test is well
developed and contains branched cells. The musculature is not
so well developed as in Doliolttm. There are two circular bands
at the anterior end and two at the posterior, and two on the
middle of the body. The stigmata are confined to the obliquely
placed posterior end of the branchial sac. The alimentary canal
forms a U-shaped curve. The reproductive organs are placed on
the right side of the body. The life-history is still imperfectly
known. As in the ease of Doliulmn the sexual generation is
polymorphous, and has three forms, two of which remain in a
rudimentary condition so far as the reproductive organs are con-
cerned. In Anchinia, however, the three forms do not occur to-
gether on one stolon or outgrowth, but are produced successively,
the reproductive forms of the sexual generation being independent
of the "foster forms" (see Barrois, ^7).

Sub-order 2. — Hemimyaria.

Free-swimming pelagic forms which exhibit alternation of genera- Charac-
tions in their life-history and in tin' sexual condition form colonies, ters of
The body is more or less fusiform, with the long axis antero-posterior, llemi-
and the branchial and atrial apertures nearly terminal. The teat my aria.
is well developed. The musculature of the mantle is in the form
of a series of transversely-running bands, which do not form com-
plete independent rings as in the Ci/i-lunn/itrin. The branchial and

1 For further details see Uljanin (.jj1).

192

TUNICATA

*»-.

peribranchial cavities form a continuous space in the interior of the
body, opening externally by the branchial and atrial apertures, and
traversed obliquely from the dorsal and anterior end to the ventral
and posterior by a long narrow vascular band, which represents the
dorsal lamina, the dorsal blood-vessel, and the neighbouring part
of the dorsal edge of the branchial sac of an ordinary Ascidian.
The alimentary canal is placed ventrally. It may either be stretched
out so as to extend for some distance anteriorly, or — as is more
usual — be concentrated to form along with the reproductive organs
a rounded opaque mass near the posterior end of the body, known
as the visceral mass or ' ' nucleus. " The embryonic development
is direct, no tailed larva being formed.

This sub-order contains two very distinct families, the SALPIM;,
which are the typical members, and the OCTAONEMID^E, including a
single very remarkable form (Oetacnemus bythiiis), which in some
respects does not conform with the characters given above.
Salpidte. The Salpidee, includes the single genus Salpa (Forskal), which,
however, may be divided into two well-marked groups of species, — ( 1 )
those, such as S. pinnala, in which the alimentary canal is stretched
out along the ventral surface of the body, and (2) those, such as
8. fusiformis (fig. 14, A), in which the aliment-
ary canal forms a compact globular mass, the
" nucleus," near the posterior end of the body.
About fifteen species altogether are known ;
they are all pelagic forms and are found in
nearly all seas. Each species occurs in two
forms — the solitary asex-
ual (proles soli/aria) and
the aggregated sexual
(proles grcgaria) — which
are usually quite unlike
one another. The soli-
tary form (fig. 14, B)
gives rise by internal
---ill gemmation to a complex
---- tubular stolon, which
contains processes from
all the more important
organs of the parent body
and which becomes seg-
mented into a series of
Fio. 14.— Salpa runcimita.-fuslform.ls. A. Aggre- buds or embryos. As
gated form. B. Solitary form. Lettering as the stolon elongates, the
before ; 1-9, muscle bands ; cm, embryo ; gem, emT,rvos near the free
cemminarous st"lon; M, mantle; t't'so, visceral , •> , . . , ,

ruass (nucleus), (original.) end which have become

advanced in their deve-
lopment are set free in groups, which remain attached together by
processes of the test, each enclosing a diverticulum from the mantle
so as to form "chains" (fig. 15). k
Each memberof the chain isa Salpa R
of the sexual or aggregated form, I
and when mature may— either still /
attached to its neighbours or so- j/
parated from them (fig. 14, A) — |
produce one or several embryos,
which develop into the solitary
Salpa. Thus the two forms altcr-
Struc- nate regularly. The more import-
ture of ant points in the structure of a
Salpa. typical Salpa are shown in fig. 16.
The branchial and atrial apertures
are at opposite ends of the body,
and each leads into a large cavity,
the branchial and peribranchial Fin. 15. — Posterior part of solitary
sacs, which are in free communica- ^'"whig aa ?&ti\\™ci\Ai"yn"m^y
tion at the sides of the obliquely- ready to be st-t free, gem, young
running dorsal lamina or "gill." aggregjatedSaZjw forming the chain;
The test is well developed and s(' Sy',on JQ™ ' jn'ai'\°le lja"
adheres closely to the surface of

the mantle. The muscle bands of the mantle do not completely
encircle the
body. They are
present dorsally
and laterally,
but the major-
ity do not reach
the ventral sur-
face. In many
cases neigh-
bouring bauds

join in the med- ^ i *--

iau dorsal line, Fjo 1(J__ gemj.fiiagrammatic representation of fialpa from
(fig. 14). 1 he leftside. Lettering as before ; em&. embryo ; m, mantle ;
anterior end of I, languet; sgil, duct of subneural gland; 1-11, muscle
tlin ,lr,K.«l In bands of mantle ; f, thickening of test over nucleus ; i/(,
the dorsal la- m or bram.lliai (original.)
mina is pro-
longed to form a prominent tentacular organ, the languet, pro-

jectiug into the branchial sac. The nerve ganglion, subneural
gland, dorsal lamina, peripharyngeal bands, and endostyle are
placed in the usual positions. A pigment spot and an otocyst
are found in connection with the ganglion. The hirge spaces at
the sides of the dorsal lamina (often called the gill or branchia
of Salpa), by means of which the cavity of the branchial sac is
placed in free communication with the peribranchial cavity, are to
be regarded as gigantic stigmata formed by the suppression of the
lateral walls of the branchial sac. Fig. 16 represents an aggre-
gated or sexual Salpa which was once a member of a chain, since
it shows a testis and a developing embryo. The ova (always few
in number, usually only one) appear at a very early period in the
developing chain Salpa, while it is still a part of the gemmiparous
stolon in the body of the solitary Salpa. This gave rise to the
view put forward by Brooks (23), that the ovary really belongs to
the solitary Salpa, which is therefore a female producing a series
of males by asexual gemmation, and depositing in each of these an
ovum, which will afterwards, when fertilized, develop in the body
of the male into a solitary or female Salpa. This idea would of
course entirely destroy the view that Salpa is an example of alterna-
tion of generations. The sexual or chain Salpa, although really
hermaphrodite, is always protogynous : i.e., the female elements or
ova are produced at an earlier period than the male organ or testis.
This prevents self-fertilization. The ovum is fertilized by the Develop-
spermatozoa of an older Salpa belonging to another chain, and ment of '
the embryo is far advanced in its development before the testis is Salpa.
formed. At an early period in its development a part of the embryo
becomes separated off, along with a part of the wall of the cavity
in which it lies, to form the " placenta," in which the embryonic and
the maternal blood streams circulate in close proximity (or actually
coalesce during one period) and so allow of the passage of nutriment
to the developing embryo. At a somewhat later stage a number of
cells placed at the posterior end of the body alongside the future
nucleus become filled up with oil-globules to form a mass of nutrient
material — the elseoblast — which is used up later on in the develop-
ment. Many suggestions have been made as to the homology of
the elfeoblast. The most probable is that it is the disappearing
rudiment of the tail found in the larval condition of most Ascidians.

The family Oclacncmidss includes the single remarkable form Octa-
Octamcmus bythius, found during the " Challenger " expedition, and cnemidee.
first described by
Moseley (29). It is
apparently a deep-
sea representative
of the pelagic Sal-
pidfe, and may pos-
sibly be fixed. The
body is somewhat
discoid, with its
margin prolonged
to form eight taper- Fl0'
ing processes, on to
which the muscle
bands of the
mantle are con-
tinued. The ali-

17. — Diagrammatic vertical longitudinal section
of Oftacneinus bi/thius (after Moseley). &r, branclual
aperture ; m, opening of oesophagus ; r, rectum ; at,
atrial aperture ; rm, rm, radiating muscles ; n, nucleus ;
mh, muscles of nucleus; (7, respiratory membrane;
b, thickened margin of base of test; pa, pedicle of
attachment.

mentary canal forms a compact nucleus (fig. 17) ; the endostyle is
very short ; and the dorsal lamina is apparently absent. The re-
production and life-history are entirely unknown.

Order III.— ASCIDIACEA.

Fixed or free-swimming Simple or Compound Ascidians which in Ascid-
the adult are never provided with a tail and have no trace of a iacea.
notochord. The free-swimming forms are colonies, the Simple
Ascidians being always fixed. The test is permanent and well
developed ; as a rule it increases with the age of the individual.
The branchial sac is large and well developed. Its walls arc per-
forated by numerous slits (stigmata) opening into the peribranchial
cavity, which communicates with the exterior by the atrial aperture.
Many of the forms reproduce by gemmation, and in most of them
the sexually-produced embryo develops into a tailed larva.

The Ascidiacca includes three groups, — the Simple Ascidians,
the Compound Ascidians, and the free-swimming colonial Pyrosoma.

Sub-order 1. — Ascidise Simplices.

Fixed Ascidians which are solitary and very rarely reproduce by Simple
gemmation ; if colonies are formed, the members are not buried in Ascid-
a common investing mass, but each has a distinct test of its own. ians.
No strict line of demarcation can be drawn between the Simple and
the Compound Ascidians, and one of the families of the former
group, the C'A/fv//«/</.i; (the Social Ascidians), forms a transition
from the typical Simple forms, which never reproduce by gemmation,
to the Compound forms, which always do (see p. 618 below). The
*lf<-i<li;c Xim/'/ii'i.'i may be divided into the following families: —

Family I. — CLAVKI.INIH.IO. Simple Ascidians which reproduce by
gemmation to form small colonies in which each ascidiozooid has
a distinct test, but all are connected by a common blood-system.

TUNICATA

193

Com-

pouud

Aseid-

IHids formed on stolons which are vascular outgrowths from the pos-
terior end of the body, containing prolongations from the ectoderm,
mesoderm, and endoderm of the ascidiozooid. Branchial sac not
folded ; internal longitudinal bars usually absent; stigmata straight ;
tentacles simple. This family contains three genera: Ectciiuisct'li"
(Herdman), with internal longitudinal bars in branchial sac ; Clavd-
ina (Savigny), with intestine extending behind branchial sac ;
and Perophora (Wiegnuini), with intestine alongside branchial sac.

Family II. — ASCIDIIDJE. Solitary fixed Ascidians with gelatinous
test ; branchial aperture usually eight-lotted, atrial aperture usually
six-lobed. Branchial sac not folded; internal longitudinal bars
usually present ; stigmata straight or curved ; tentacles simple.
This family is divided into three sections : —

Sub-family 1.— HYI-OBYTHISJE. Branchial sac with no internal
longitudinal bars. One genus, Hypobythius (Moseley).

Sub-family 2. — ASCIDIX.E. Stigmata straight. Many genera, of
which the following are the more important : — dona (Fleming),
dorsal languets present; Ascidia (Lim\iuus,=Phallusia, Saviguy),
dorsal lamina present (see figs. 1 to 10) ; Ekodosoma (Ehrenberg),
anterior part of test modified to form operculum ; Abyssascidia
(Herdman), intestine on right side of branchial sac.

Sub-family 3. — CORELLIN*^;. Stigmata curved. Three genera : —
Cordla (Alder and Hancock), test gelatinous, body sessile ; £'"/•/•,;-
ascidia (Herdman), test gelatinous, body pedunculated ; Chdyo-
soma (Brod. and Sow.), test modified into horny plates.

Family III. — CYNTHIID.E. Solitary fixed Ascidians, usually with
leathery test ; branchial and atrial apertures usually both four-lobed.
Branchial sac longitudinally folded ; stigmata straight ; tentacles
simple or compound. This family is divided into three sections : —

Sub-family 1. — STYELIN^E, not more than four folds on each side
of branchial sac ; tentacles simple. The more important genera are
— Styda (JIacleay), stigmata normal, and Bathyoncus (Herdmau),
stigmata absent or modified.

Sub-family 2. — CYNTHISJE, more than eight folds in branchial
sac; tentacles compound;
body sessile. The chief
genus is Cynthia, (Sa-
vigny), with a large
number of species.

Sub-family 3. — BOL-
TEXIX.E, more than eight
folds in branchial sac ;
tentacles compound ;
body pedunculated (fig.
18, A). The chief genera
are — Boltenia (Savigny),
branchial aperture four-
lobed, stigmata normal ;
and Culcohus (Herd- FIG.
man), branchial aper-
ture with less than four
lobes, stigmata absent or
modified (fig. 18, B).
This last is a deep-sea
genus discovered by the "Challenger" expedition (see !j}.

Family IV. — MOLGULID.E. Solitary Ascidians, sometimes not
fixed ; branchial aperture six-lobed, atrial four-lobed. Test usually
iucrusted with sand. Branchial sac longitudinally folded ; stigmata
more or less curved, usually arranged in spirals; tentacles compound.
The chief genera are — Mo'lrjida (Forbes), with distinct folds in the
branchial sac, and Eugyra (Aid. and Hanc.), with no distinct folds,
but merely broad internal longitudinal bars in the branchial sac.
In some of the Mol'jitlidx (genus Anurdla, Lacaze-Duthiers, 20)
the embryo does not become converted into a tailed larva, the
development being direct, without metamorphosis. The embryo
when hatched assumes gradually the adult structure, and never
shows the features characteristic of larval Ascidiaus, such as the
urochord and the median sense-organs.

Sub-order 2. — Ascidise Composite.

Fixed Ascidians which reproduce by gemmation, so as to form
colonies in which the ascidiozooids are buried in a common invest-
ing mass and have no separate tests. This is probably a somewhat
artificial assemblage formed of two or three groups of Ascidians
which produce colonies in which the ascidiozooids are so intimately
united that they possess a common test or investing mass. This
is the only character which distinguishes them from the Clavclinidse,
but the property of reproducing by gemmation separates them
from the rest of the Ascidix Simpliccs. The Ascidiss Composites
may be divided into the following families : —

Family I. — DISTOMID.E. Ascidiozooids divided into two regions,
thorax and abdomen ; testes numerous ; vas deferens not spirally
coiled. The chief genera are — Distoma (Gaertner) ; Distaplia (Delia
Valle) ; Coldla, (Herdman), forming a pedunculated colony (see fig.
19, A) in which the ascidiozooids develop incubatory pouches,
connected with the peribranchial cavity, in which the embryos
undergo their development (/7) ; and Chondrostadiys (Maedonald).

i»-f i I

S. — Cnlcolus willcm&si. A. Entire body,
natural size. B. Part of branchial sac mag-
nified, at, atrial aperture ; 6r, branchial aper-
ture ; ped, peduncle ; or /, slight fold of braneh-
ial sac ; i I, internal longitudinal bar ; mh, mesh ;
sp, calcareous spicules in vessels ; tr, transv. r^-
vessels. (After Herdman, Challenger Report.)

Family II. — CiEi.nroKMin.n. Colony not fixed, having a large axial
cavity with a terminal aperture. liranehial apci'lines Ihv-hibrd.
'I'll is includes one S]ieeies, Cii'hmii'inii:: l:ii.: -I, i/i (Jli-nlni.in i, which 13 a
transition form between the ordinary < ''impound AscMians (c.y.,
Jiix/n>nii/a'} and the Asi'iili:>: ,W;./M/'»i<'\ ( I'l/rnsuma).

Family III. — DiOBMNIDA Colony usually thin and incrusting
Test containing .stel-
late calcareous spi-
cules. Tcstis single,
large ; vas deferens
spirally coiled. The
chief genera are — /'/-
dtmnum (Savigny),
in which the colony
is thick and fleshy
and there are only FIG. 19.— Colonies of Ax'ulitc Composite (natural size).

A. Colclla q-ttoyi. B. Leptodimtm itF'jlcctuvi. C. I'lta-
ryngodictyon mircibilc. D. Botrj/tlvs, showing ar-
rangement of ascidiozooids in circular systems each
of which has a central common cloaca. (Alter HLi-d-
man, Challenger Report.)

three rows of stig-

mata on each side of

the branchial sac ;

and Leptodinum

(Milne-Edwards), in

which the colony is thin and incrusting (fig. 19, B) and there are

four rows of stigmata on each side of the branchial sac.

Family IV. — BIPLOSOMID.E. Test reduced in amount, rarely con
taining spicules. Vas deferens not spirally coiled. In iiiplosoma
(Macdonald), the most important genus, the larva is gemmiparous.

Family V. — POLYCLINID.E. Ascidiozooids divided into three
regions, — thorax, abdomen, and post-abdomen. Testes numerous ;
vas deferens not spirally coiled. The chief genera are — Phari/ngo-
dictyon (Herdman), with stigmata absent or modified, containing
one species, Ph. mirabile (fig. 19, C), the only Compound Ascidian
known from a depth of 1000 fathoms ; Poh/dinum (Savigny), with
a smooth-walled stomach; AjiUdiuin (Savigny), with the stomach
wall longitudinally folded ; and Aiiinrvin'iinii (Milne-Edwards), in
which the ascidiozooid has a long post-abdomen and a large atrial
languet.

Family VI. — BOTRYLLID.E. Ascidiozooids having the intestine
and reproductive organs alongside the branchial sac. Dorsal lamina
present ; internal longitudinal bars present in branchial sac. The
chief genera are — Botryllus (Gaertn. and Pall. ), with simple stellate
systems (fig. 19, D), and Botrylloides (Milne -Edwards), with
elongated or ramified systems.

Family VII. — POLYSTYELID.E. Ascidiozooids not grouped in
systems. Branchial and atrial apertures four-lobed. Branchial
sac may be folded ; internal longitudinal bars present. The chief
genera are — ThijJaeium (Carus), with ascidiozooids projecting above
general surface of colony ; Goodsiria (Cun-
ningham), with ascidiozooids completely
imbedded in investing mass ; and Chorizo-
cormus (Herdman), with ascidiozooids
united in little groups which are connected
by stolons. The last genus contains one
species, Cli. rcliculattis, a transition form
between the other Poli/stydidx and tin-
StyfUnse amongst Simple Ascidiaus.

The methods of reproduction by gemma-
tion differ in their details in the various
groups of Compound Ascidians ; but in all
cases the process is essentially a giving off
from the parent body of groups of cells re-
presenting the ectoderm, the mesnderm,
and the cndoderm, vjhich develop into the
corresponding layers of the bud. The first
ascidiozooid of the colony produced by the
tailed larva does not form sexual repro-
ductive organs, but reproduces by gemma-
tion so as to make a colony. Thus there
is alternation of generations in the life-
history. In the most completely formed
colonies (e.g. , BotryUus) the ascidiozooids
are arranged in groups (systems or cceno-
bii), and in each system are placed with
their atrial apertures towards one another,
and all communicating with a common
cloacal cavity wliieU opens to the exterior
in the centre of the system (fig. 19 D).

Sub-order 3. — Ascidiaa Salpifonnes.

Free-swimming pelagic colonies having
the form of a hollow cylinder closed at one

end. The ascidiozooids forming the colony FlG 20.— rurosnma Otg<uas
are imbedded in the common test in such a natural si, :< . -\. Si le'view
manner that the branchial apertures open of entire colony. B. End
on the outer surface and theatrial aperturea
on the inner surface next to the central
cavity of the colony. The ascidiozooids arc produced by gemmation
from "a rudimentary larva (the cyathozooid) developed sexually.

2B

Eepro-
ductiou
by gem-
niation

"*, <-!

Ascidise

Salpi-

/ormet-

. .
extremlt>''

194

TUNICATA

Struc-
ture of
Pyro-
soma.

Tliis sub-order includes a single family, the PYBOSOMID.E, con-
taining one well-marked genus, Pyrosoma (Perou), with several
species. They are found swimming near the surface of the sea,
chiefly in tropical latitudes, and are brilliantly phosphorescent.
A fully developed Pyrosoma colony may be from an inch or two to
upwards of four feet in length. The shape of the colony is seen in
fig. 20. It tapers slightly towards the closed end, which is rounded.
The opening at the opposite end is reduced in size by the presence
of a membranous prolongation of the common test (fig. 20, B).
The branchial apertures of the ascidiozooids are placed upon short
papilla? projecting from the general surface, and most of the ascidio-
zooids have long conical processes of the test projecting outwards
beyond their branchial apertures (figs. 20, 21, and 22). There is
only a single layer of ascidiozooids in the Pyrosoma colony, as all
the fully developed ascidiozooids are placed with their antero-
posterior axes at right angles to the surface and communicate by
their atrial apertures with the central cavity of the colony (fig. 21).

t.f

FIG. 21. — Part of a longitudinal section through wall of Pyrosoma, showing
arrangement of ascidiozooids, magnified (partly after Savigny). at, atrial
apertures ; br, branchial apertures ; asc, young ascidiozooid of a future colony
produced by budding from cy, cyathozooid •, em, embryos in various stages ;
t, test ; tp, processes of test ; brs, branchial sac ; yas, young ascidiozooid.

Their dorsal surfaces are turned towards the c-pen end of the colony.
The more important points in the structure of the ascidiozooid o-f
Pyrosoma, are shown
in fig. 22. A circle of
tentacles, of which
one, placed ventrally
(fig. 22, tn), is larger
than the rest, is
found just inside the
branchial aperture.
From this point a
wide cavity, with a
few circularly-placed
muscle bauds run-
ning round its walls,
leads back to the
large branchial sac,
which occupies the
greater part of the
body. The stigmata
are elongated trans-
versely and crossed
by internal longitu-
dinal bars. The dor-
sal lamina is repre-
sented by a series of
fight languets (I).
The inTve ganglion
(on which is placed
a small pigniented^0'
sense organ), the sub-
neural gland, the dor- ov. —
sal tubercle, the pcri-
pharyngcal bands,
and the endostyle are

placed in the usual ,,

-Inna On or.,,1 Flo. 22.— Mature ascidiozooid of Pyrosoma, from left

positions. _ un eacn 6ide (partlv uft,er Keferstein). Lettering as before ;
cm, cellular mass, the seat of phosphorescence ;
cm', posterior cellular mass ; gs, gemmiparous
stolon ; mb, muscle band ; ngl, subneural gland ;

tes.

pig, pigment spot on ganglion ; tp, process of test.

side of the anterior
end of the branchial
sac, close to the peri-
pharyngeal bands, is
a mass of rounded gland cells which are the source of the phosphores-
cence. The alimentary canal is placed posteriorly to the branchial
eac, and the anus opens into a large peribranchial (or atrial) cavity,
of which only the median posterior part is shown (plr) in fig. 22.
The reproductive organs are developed in a divcrticulum of the peri-

branchial cavity, and consist of a lobed testis and a single ovum at
a time. The development takes place in a part of the peribranchial Devclop-
cavity (fig. 21, em). The segmentation is meroblastic, and an merit of
elongated embryo is formed on the surface of a mass of yolk. The Pyro-
embryo, after the formation of an alimentary cavity, a tubular soma.
nervous system, and a pair of laterally placed atrial tubes, divides
into an anterior and a posterior part. The anterior part then
segments into four pieces, which afterwards develop into the first
ascidiozooids of the colony, while the posterior part remains in a
rudimentary condition, and was called by Huxley the "cyatho-
zooid " ; it eventually atrophies. As the four ascidiozooids increase
in size, they grow round the cyathozooid and soon encircle it (fig. 21,
asc and cy). The cyathozooid absorbs the nourishing yolk upon
which it lies, and distributes it to the ascidiozooids by means of a
heart and system of vessels which have been meanwhile formed.
When the cyathozooid atrophies and is absorbed, its original atrial
aperture remains and deepens to become the central cavity of the
young colony, which now consists of four ascidiozooids placed in a
ring, around where the cyathozooid was, and enveloped in a common
test. The colony gradually increases by the formation of buds from
these four original ascidiozooids.

PHYLOGENY.

The accompanying diagram shows graphically the pro- Phylo-
bable origin and course of evolution of the various groups geny.
of Tunicata, and therefore exhibits their relations to one
another much more correctly than any system of linear
classification can do. The ancestral Proto-Tunicata are
here regarded J as an offshoot from the Proto-Chordata —
the common ancestors of the Tunicata (Urochordd), Am-

phioxus (Cephalochorda), and the Vertebrata. The ances-
tral Tunicata were probably free-swimming forms, not
very unlike the existing Appendiculariids, and are repre-
sented in the life-history of nearly all sections of the
Tunicata by the tailed larval stage. The Larvacea are the
first offshoot from the ancestral forms which gave rise to
the two lines of descendants, the Proto-Thaliacea and the
Proto-Ascidiacea. The Proto-Thaliacea then split into the
ancestors of the existing Cydomyaria and Hemimyaria.
The Proto-Ascidiacea gave up their pelagic mode of life
and became fixed. This ancestral process is repeated at
the present day when the free-swimming larva of the
Simple and Compound Ascidians becomes attached. The
Proto-Ascidiacea, after the change, are probably most
nearly represented by the existing genus Clavelina. They
have given rise directly or indirectly to the various groups
of Simple and Compound Ascidians and the Pyrosomidss.
These groups form two lineb, which appear to have diverged
close to the position of the family Clavelinidx. The
one line leads to the more typical Compound Ascidians,
and includes the Polydinidx, Distomidx, Didemmdse,
Diplosomidx, Caslocormidx, and finally the Ascidice Sal-
pi/ormes. The second line gave rise to the Simple
Ascidians, and to the Botryllidx and Polystyelidx, which

1 By Dohrn and others their point of origin is placed considerably
farther up on the stem of the Chordata, thus causing the Tunicata
to be regarded as very degenerate Vertebrata (see 32).

TUNIOATA

195

are, therefore, not closely allied to the other Compound
Ascidiaus. The later Proto-Ascidiacea were probably
colonial forms, and gemmation was retained by the Clave-
liniilx and by the typical Compound Ascidians (Distomida,
&c.) derived from them. The power of forming colonies
by budding was lost, however, by the primitive Simple
Ascidians, and must, therefore, have been regained inde-
pendently by the ancestral forms of the Botryllidx and
the Polystyelidss. If this is a correct interpretation of the
course of evolution of the Tunicata, we arrive at the
following important conclusions. (1) The Tunicata, as a
whole, form a degenerate branch of the Proto-Chordata ;
(2) the Ascidix Saipiformes (Pyrosomd) are much more
closely related to the typical Compound Ascidians than
to the other pelagic Tunicata, viz., the Larvacea and
the Thaliacea ; and (3) the Ascidix Compositx form a
polyphyletic group, the sections of which have arisen
at several distinct points from the ancestral Simple
Ascidians.

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(W. A. HE.)

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