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Title: The Cambridge natural history, Vol. 03 (of 10)

Editor: S. F. Harmer

Author: A. H. Cooke
F. R. C. Reed

Editor: Sir A. E. Shipley

Release date: September 20, 2023 [eBook #71693]

Language: English

Original publication: New York: MacMillan and Co., 1895

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*** START OF THE PROJECT GUTENBERG EBOOK THE
CAMBRIDGE NATURAL HISTORY, VOL. 03 (OF 10) ***
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THE

CAMBRIDGE NATURAL HISTORY
EDITED BY

S. F. HARMER, M.A., Fellow of King’s College, Cambridge;
Superintendent
of the University Museum of Zoology
AND

A. E. SHIPLEY, M.A., Fellow of Christ’s College, Cambridge;
University Lecturer on the Morphology of Invertebrates

VOLUME III

Map to illustrate
THE GEOGRAPHICAL DISTRIBUTION
of the
LAND OPERCULATE MOLLUSCA
The figures indicate the number of known species.

MOLLUSCS
By the Rev. A. H. Cooke, M.A., Fellow and Tutor of King’s College, Cambridge

BRACHIOPODS (RECENT)
By A. E. Shipley, M.A., Fellow of Christ’s College, Cambridge

BRACHIOPODS (FOSSIL)
By F. R. C. Reed, M.A., Trinity College, Cambridge

New York
MACMILLAN AND CO.
AND LONDON

1895
All rights reserved

“Why, you might take to some light study: conchology, now; I always think that
must be a light study.”
George Eliot, Middlemarch.

Copyright, 1895,
By MACMILLAN AND CO.

Norwood Press:
J. S. Cushing & Co.—Berwick & Smith.
Norwood, Mass., U.S.A.

PREFACE TO THE MOLLUSCA
The general plan of classification adopted in this work is not that of any
single authority. It has been thought better to adopt the views of recognised
leading specialists in the various groups, and thus place before the reader
the combined results of recent investigation. This method may, perhaps,
occasion a certain number of small discrepancies, but it is believed that the
ultimate effect will be to the advantage of the student.
The classification adopted for the recent Cephalopoda is that of Hoyle
(‘Challenger’ Reports, Zoology, vol. xvi.), for the fossil Cephalopoda
(Nautiloidea) that of Foord (Catalogue of the Fossil Cephalopoda in the
British Museum, 1888–91), and (Ammonoidea) P. Fischer (Manuel de
Conchyliologie, 1887). In the Gasteropoda the outlines are those adopted by
Pelseneer (Mém. Soc. Malacol. Belg. xxvii. 1894), while the details are
derived, in the main, from P. Fischer. The Amphineura, however, have not
been regarded as a separate class. The grouping of the Nudibranchiata is
that of Bergh (Semper, Reisen im Archipel der Philippinen, ii. 3). The
Pelecypoda are classified according to Pelseneer’s most recent grouping.
Acknowledgment of the principal sources of information has been made
in footnotes, and a short list of leading authorities has been appended to the
chapters on anatomy, for the use of students desirous to pursue the subject
further. In the case of geographical distribution the authorities are too
numerous and scattered to admit of a list being given.
A special word of thanks is due to Mr. Edwin Wilson for his patient care
in preparing the illustrations, the majority of which are taken from
specimens in the University Museum of Zoology. Mr. Edgar Smith, besides
affording the kind help which visitors to the British Museum always
experience at his hands, has permitted me to use many specimens for the
purposes of illustration.
A. H. COOKE.
King’s College, Cambridge,
20th December 1894.

CONTENTS
Scheme of the Classification adopted in this Book.

MOLLUSCA
CHAPTER I
Introduction—Position of Mollusca in the Animal Kingdom—Classification—Origin
of Land and Fresh-water Mollusca 1
CHAPTER II
Land and Fresh-water Mollusca, their Habits and General Economy 23
CHAPTER III
Enemies of the Mollusca—Means of Defence—Mimicry and Protective Coloration—
Parasitic Mollusca—Commensalism—Variation 56
CHAPTER IV
Uses of Shells for Money, Ornament, and Food—Cultivation of the Oyster, Mussel,
and Snail—Snails as Medicine—Prices Given for Shells 96
CHAPTER V
Reproduction—Deposition of Eggs—Development of the Fertilised Ovum—
Differences of Sex—Dioecious and Hermaphrodite Mollusca—Development of
Fresh-water Bivalves 123
CHAPTER VI
Respiration and Circulation—The Mantle 150
CHAPTER VII
Organs of Sense: Touch, Sight, Smell, Hearing—The Foot—The Nervous System 177
CHAPTER VIII
The Digestive Organs, Jaw, and Radula: Excretory Organs 209
CHAPTER IX
The Shell, its Form, Composition, and Growth—Designation of its Various Parts 244
CHAPTER X
Geographical Distribution of Land and Fresh-water Mollusca—The Palaearctic,
Oriental, and Australasian Regions 277
CHAPTER XI
Geographical Distribution of Land Mollusca (continued)—The Ethiopian, Nearctic,
and Neotropical Regions 328
CHAPTER XII
Distribution of Marine Mollusca—Deep-sea Mollusca and their Characteristics 360
CHAPTER XIII

Class Cephalopoda 378
CHAPTER XIV
Class Gasteropoda—Amphineura and Prosobranchiata 400
CHAPTER XV
Class Gasteropoda (continued): Opisthobranchiata and Pulmonata 427
CHAPTER XVI
Classes Scaphopoda and Pelecypoda 444

BRACHIOPODA (RECENT)
CHAPTER XVII
Introduction—Shell—Body—Digestive System—Body Cavity—Circulatory System
—Excretory Organs—Muscles—Nervous System—Reproductive System—
Embryology—Habits—Distribution—Classification 463

BRACHIOPODA (FOSSIL)
CHAPTER XVIII
Introduction—Division I. Ecardines—External Characters—Internal Characters—
Division II. Testicardines—External Characters—Internal Characters—Synopsis
of Families—Stratigraphical Distribution—Phylogeny and Ontogeny 491

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

MOLLUSCA
Class Order Sub-order Section
CEPHALOPODA Dibranchiata Octopoda (p. Phragmophora (p.
382). 386).
Sepiophora (p.
388).
Decapoda Chondrophora Myopsidae (p.
389).
Oigopsidae (p.
390).

Tetrabranchiata Nautiloidea Retrosiphonata (p.
393).
Prosiphonata (p.
395).
Ammonoidea Retrosiphonata (p.
397).
Prosiphonata (p.
397).
GASTEROPODA Amphineura Polyplacophora
(p. 400).
Aplacophora (p.
404).
Prosobranchiata Diotocardia Docoglossa (p.
405).
Rhipidoglossa Zygobranchiata
(p. 406).
Azygobranchiata
(p. 407).
Monotocardia Ptenoglossa (p.
411).
Taenioglossa Platypoda (p.
411).
Heteropoda (p.
420).
Taenioglossa
Gymnoglossa (p.
422).
Toxoglossa (p.
426).
Tectibranchiata Bulloidea (p.
429).
Aplysioidea (p.
430).
Pleurobranchoidea
(p. 431).
Siphonarioidea (p.
431).
Opisthobranchiata Ascoglossa (p.
431).
Nudibranchiata Cladohepatica (p.
432).
Holohepatica (p.
433).
Pteropoda Thecosomata (p.
435).
Gymnosomata (p.
437).
Pulmonata Basommatophora
(p. 438).
Stylommatophora
(p. 439).

Class Order Suborder
SCAPHOPODA (p. 444).
PELECYPODA Protobranchiata (p. 447).
Filibranchiata Anomiacea (p. 448).
Arcacea (p. 448).
Mytilacea (p. 448).
Pseudolamellibranchiata (p. 449).
Eulamellibranchiata Submytilacea (p. 451).
Tellinacea (p. 453).
Veneracea (p. 454).
Cardiacea (p. 454).
Myacea (p. 456).
Pholadacea (p. 457).
Anatinacea (p. 458).
Septibranchiata (p. 459).

BRACHIOPODA
Order Family
Brachiopoda Ecardines Lingulidae (pp. 487 and 503).
Obolidae (p. 504).
Discinidae (pp. 487 and 504).
Craniidae (pp. 487 and 504).
Trimerellidae (p. 504).
Testicardines Productidae (p. 504).
Strophomenidae (p. 505).
Koninckinidae (p. 505).
Spiriferidae (p. 505).
Atrypidae (p. 505).
Rhynchonellidae (pp. 487 and 505).
Terebratulidae (pp. 487 and 506).
Argiopidae (p. 506).
Stringocephalidae (p. 506).
Thecidiidae (pp. 487 and 506).

LIST OF MAPS
The Geographical Distribution of the Land Operculate Mollusca Frontispiece
The Geographical Distribution of the Land Mollusca of the East Between pp. 308 and
Indian Archipelago 309
The Relations of the Land Mollusca of New Guinea with those of
North Australia To face p. 322
The Geographical Distribution of the Land Mollusca of the West Between pp. 344 and
Indies 345

MOLLUSCS
BY

REV. A. H. COOKE, M.A.
Fellow and Tutor of King’s College, Cambridge

CHAPTER I
INTRODUCTION—POSITION OF MOLLUSCA IN THE ANIMAL KINGDOM—
CLASSIFICATION—ORIGIN OF LAND AND FRESH-WATER MOLLUSCA

It is the generally accepted opinion among men of science that all life
originated in the sea. Not that all parts of the sea are equally favourable to
the development of forms of life. The ocean surface, with its entire absence
of shelter or resting-place, and the deep sea, whose abysses are always dark
and cold and changeless, offer little encouragement to plant or animal life,
as an original starting-point. True, both the surface and the depths of the sea
have become colonised by myriads of forms, Mollusca amongst them, but
these quarters are in the truest sense colonised, for the ancestors of those
who inhabit them in all probability migrated from elsewhere.
It was no doubt the littoral region and the shallow waters immediately
below it, a region of changeable currents, of light and shade, of variation,
within definite limits, of temperature and tide effects, which became the
scene of the original development of plant life, in other words, of the food-
supply which rendered possible its colonisation by higher animals. But the
littoral region, besides the advantages of tenancy which it offers to animal
life, has also its drawbacks. The violence of the surf may beat its inhabitants
in pieces, the retreat of the tide exposes them, not merely to innumerable
enemies in the shape of predatory birds and beasts, but also to a change in
the atmospheric medium by which they are surrounded. Hence, in all
probability, have arisen the various forms of adaptation which are
calculated to bring about the ‘survival of the fittest’; hence, to narrow our
point of view to the Mollusca, the development of hard shells, or
exoskeletons, hence the sand-burrowing, rock-boring, rock-clinging
instincts of various genera and species.[1]
What was the primitive form of molluscan life is little likely to be ever
positively known, although, on grounds of comparative anatomy, something
approaching to the archi-mollusc is often constructed, with more or less
probability, by careful observers. From one of the oldest known geological
strata, the Cambrian, nearly four hundred species of Mollusca are known,
which include representatives of nearly all the great Orders as they exist at

the present day, and without the slightest sign of approximation to one
another. With regard to the origin of the land and fresh-water Mollusca
some definite conclusions can be arrived at, which will be given in their
proper place.
Scarcely any portion of the coast-line of the world is destitute of
molluscan life, except in regions where extreme cold forbids its existence.
Thus along the shores of Northern Asia there is no proper littoral fauna, the
constant influence of travelling ice sweeping it all away; animal life begins
at about three fathoms. But in every coast region not positively hostile to
existence Mollusca make their home. Each description of habitat has its
own peculiar species, which there flourish best, and exist precariously, if at
all, elsewhere. Thus the sandy waste of estuaries, the loose and shingly
beaches, the slimy mud-flats beset with mangroves, the low stretches of
jagged rock, and even the precipitous cliffs, from whose base the sea never
recedes, have all their own special inhabitants. The same is true of the deep
sea, and of the ocean surface. And when we come to examine the land and
fresh-water Mollusca, it is found not merely that some Mollusca are
terrestrial and others fluviatile, but that certain species haunt the hills and
others the valleys, some the recesses of woods and others the open meadow
sides, some prefer the limestone rocks, others the sandy or clayey districts,
some live only in still or gently moving waters, while others are never
found except where the current is rapid and powerful.
It is within the tropics that the Mollusca become most numerous, and
assume their finest and quaintest forms. A tropical beach, especially where
there is a good tide-fall and considerable variety of station, abounds in
molluscan life to an extent which must literally be seen to be believed. The
beach at Panama, to select an instance familiar to the present writer, is
astonishingly rich in species, which probably amount in all to several
hundreds. This is due to the immense variety of habitat. On the rocks at
high-water mark, and even above them, occur Truncatella, Melampus,
Littorina, and Siphonaria; where a mangrove-swamp replaces the rock, on
the branches overhead are huge Littorina, while three species of Cerithidea
crawl on the mud, and Cyrena and Arca burrow into it. Lower down, in the
rock pools at half-tide mark are Cerithium, Purpura, Omphalius, Anachis (2
sp.), Nassa, and several Crepidula. At low-water mark of ordinary tides,
under stones half buried in clean sand, are Coecum and Vitrinella; under the

blocks which rest on solid rock are Cypraea (4 or 5 sp.), Cantharus, more
Anachis, Columbella (3 sp. including the graceful C. harpiformis), and
Nitidella. Where the blocks of rock are rather muddy, Conus lurks, and with
it Turritella and Latirus. Where the rocks form a flat-topped platform 2 or 3
feet high, with here and there a deep crack, huge Chitons 3 inches long
conceal themselves, with two species of Turbo, Purpura, and Clavella. At
extreme low-water mark of spring tides, on the isolated rocks are
Monoceros, Leucozonia, and Vermetus, in them are Pholas and a burrowing
Mytilus, under them are more Conus, Dolium, and huge frilled Murices.
Patches of clean gravelly sand here produce Strombus; on the operculum of
the great Str. galea is sure to be a Crepidula, exactly fitting its breadth. On
the liquid mud-flats to the north glide about Marginella, Nassa, and
Truncaria, in the clean sandstretch to the west Olivella ploughs about by
hundreds with several species of Natica, and Tellina and Donax bury
themselves deep, while farther down are Artemis, Chione, and, where mud
begins to mix with the sand, Mytilus and more Arca. Each of these species
has its own habitat, often circumscribed to a few square feet at the most,
and it would be utterly useless to seek for it anywhere except in its own
special domain.
Equally abundant are the land Mollusca of the tropics. Prof. C. B. Adams
relates that within the limits of a single parish in Jamaica, named
Manchester, which measures no more than four miles long and one mile
broad, he obtained no fewer than one hundred species. Mr. J. S. Gibbons, in
a description of the Mollusca he obtained near St. Ann’s, Curaçao, gives a
lively picture of their abundance in an exceptionally favoured locality:—[2]
“Near the outskirts of the town a waste piece of ground supplied me with
occupation for all the time I had to spare. Neither grass nor water was to be
seen, the only vegetation consisting of a few stunted cacti and still fewer
acacia bushes. This, however, was so rich in shells that of several species
enough specimens could have been collected in a few yards to supply, I
should suppose, all the shell cabinets in the world.... The stones, plants, and
ground were covered with Strophia uva L., Tudora megacheila, P. and M.,
was in equal abundance, suspended by its silk-like thread from acacia
boughs, or strewed thickly on the ground underneath. A Bulimulus (B.
multilineatus var. sisalensis) abounded on the smaller boughs, while under
masses of coral Macroceramus inermis Gundl., Pupa parraiana d’Orb, and

P. pellucida Pfr., were abundant. In the loose soil Cylindrella Raveni Bland,
Cistula Raveni Bland, and a curious Cionella were so numerous that a
spade would have been the best instrument with which to collect them. I
wasted a good deal of valuable time in separating them from the soil, when
by simply taking away a few handfuls of mould, I might have obtained a
larger number of specimens. A species of Stenogyra and a Succinea
complete a list, all of which might have been gathered from almost any
square yard of ground on the hillside.”

Position of Mollusca in the Animal Kingdom.—Up to very recent
times it was usual to regard the Mollusca as one of the four subdivisions of
a great family known as Malacozoa, the subdivisions being (1) Mollusca,
(2) Tunicata, (3) Brachiopoda, (4) Polyzoa or Bryozoa. This classification is
still retained in the leading modern manual on the subject.[3] The progress,
however, of investigation leads to the belief that the Mollusca are not so
closely related to these other groups as such a classification would seem to
imply. The Tunicata, for instance, appear, from the whole course of their
development, to occupy a position near to the Vertebrata. The relations of
the Brachiopoda and Polyzoa will be more particularly referred to in that
part of this History which deals especially with those groups. The position
of the Mollusca is, in many respects, one of considerable isolation. Any
attempt, therefore, definitely to relate them to one group or another, is, in all
probability, to go further than the present state of our knowledge warrants.
Especially to be deprecated are systems of classification which confidently
derive the Mollusca in general from this or that group. The first undisputed
traces of animal life, which appear in the Cambrian epoch, exhibit the same
phyletic distinctions as now exist. Sponges, Echinoderms, Mollusca, and
Worms, formed already, in those immeasurably remote ages, groups
apparently as generally distinct from one another as they are at the present
time. It would seem that any theory of development, which confidently
teaches the derivation of any one of these groups from any other, is, in the
present state of the evidence before us, hazardous in the extreme.
Some indications of relationship, which must not be pushed too far, may
be drawn from a consideration of embryonic resemblance. An especial
characteristic of the Mollusca is the possession of a particular form of larva,
which occurs in one of the stages of development, known as the

trochosphere (see p. 130). This form of larva is shared with two orders of
Annelida, the Chaetopoda and the Gephyrea armata, and, in all probability,
with the Polyzoa as well. It may also be significant that the adult form in
Rotifera bears a close resemblance to the trochosphere larva in those
groups.
Basis of Classification.—The Mollusca are divided into four great
Orders—Cephalopoda, Gasteropoda, Scaphopoda, and Pelecypoda.[4]
Each name, it will be noticed, bears reference to the ‘foot,’ i.e. to the organ
of motion which corresponds in function to the foot in the Vertebrata.
In the Cephalopoda the feet, or, as they are more frequently termed, the
‘arms,’ are arranged symmetrically round the head or mouth. The common
forms of ‘cuttle-fish’ (Octopus, Loligo) are familiar examples of
Cephalopods.
The Gasteropoda crawl on the flat under-surface or ‘sole’ of the foot.
Snails, slugs, sea-hares, whelks, periwinkles, and coats-of-mail or chitons
are examples of this Order.
The Scaphopoda possess a long tubular shell open at both ends; with
their small and elongated foot they are supposed to dig into the mud in
which they live. The common Dentalium or tusk-shell of our coasts is a
representative of this Order.

Fig. 1.—Examples of the four Orders: A,
Cephalopoda; B, Gasteropoda; C,
Scaphopoda, and D, Pelecypoda.
A, Ommastrephes sagittatus Lam., Naples: a,
a, arms surrounding the mouth; f, funnel; t, t, the
two ‘tentacular’ arms, × ⅖. B, Buccinum undatum
L., Britain: f, foot; pr, proboscis. × ½. C,
Dentalium entalis L., Norway: f, foot. D, Cardium
oblongum Chem., Naples: f, foot; s, efferent or
anal siphon; s’, efferent or branchial siphon, × ½.

The Pelecypoda[5] are enclosed in a bivalve shell fastened by a
muscular hinge, the adjacent part of the valves being generally more or less
toothed; the foot is as a rule roughly comparable to the shape of an axe-
head.
To these four Orders is frequently added a fifth, the Pteropoda, whose
exact position is at present not absolutely settled. The Pteropoda[6] are
‘pelagic,’ i.e. they live in the open waters of the ocean, rising to the surface

at night, and sinking into cooler water by day. They are provided with a pair
of wing-like appendages or ‘feet,’ on each side of the head, by means of
which they are enabled to swim. Some authorities regard the Pteropoda as a
subdivision of Gasteropoda, others as forming a separate Order, of
equivalent value to the other four. The question will be further discussed
below (see chap. xv.), but for the present it will be sufficient to state that the
weight of evidence appears to show that the Pteropoda are modified
Gasteropoda, with special adaptations to pelagic life, and are therefore not
entitled to rank as a separate Order.
Some writers conveniently group together the first three of these Orders,
the Cephalopoda, Gasteropoda, and Scaphopoda, under the title
Glossophora,[7] or Mollusca furnished with a radula or ribbon-shaped
‘tongue,’ set with rows of teeth and situated in something of the nature of a
head, as distinguished from the Aglossa (or Lipocephala),[8] i.e. those
Mollusca which have no radula and no head. To the latter belong only the
fourth Order, the Pelecypoda. This view postulates, for the primitive
ancestral Mollusc, a body with a more or less developed head, and possibly
the rudiments of an apparatus for grinding or triturating food. This form, it
is held, either developed or degenerated. In the former case, in consequence
of the more active mode of life upon which it may be supposed to have
entered, it gave rise to all the more highly organised forms which are
grouped under the three great Orders. When, on the other hand, the
ancestral form associated itself with an inactive or sedentary life, it was, we
may believe, modified accordingly, and either lost by atrophy or failed to
acquire those special points of organisation which characterise the highly-
developed form. Hence the Pelecypoda, or bivalves, whose characteristic is
the absence of any definite cephalic region or masticatory apparatus. It is a
remarkable fact in support of this theory of the origin of the Aglossa that
certain of their larvae are known to possess traces of higher organisation,
e.g. an external mouth and eyes, the former of which becomes covered by
the mantle lobes, while the latter disappear long before the adult stage is
reached.
Thus we have

Classification of Gasteropoda.—The Gasteropoda are numerically
very largely in excess of the two other Orders of the Glossophora, far more
complicated as regards classification, and contain a large proportion of
those examples of the Mollusca which are most familiar to the ordinary
observer. It will therefore be convenient to postpone for the present a fuller
discussion of the subdivisions of the Cephalopoda and Scaphopoda, as well
as of the Aglossa, returning to them again in special chapters (chaps. xiii.
and xvi.), and to devote a few introductory words to the classification and
relations of the Gasteropoda.
The Gasteropoda are divided into four Classes, Amphineura,
Prosobranchiata, Opisthobranchiata, and Pulmonata.

Fig. 2.—An example of the
Polyplacophora: Chiton spinosus
Brug.

Fig. 3.—An example of the
Aplacophora, Neomenia carinata
Tullb.: a, anus; gr, ventral groove;
m, mouth.

(1) The Amphineura[9] are bilaterally symmetrical Mollusca, i.e. with
organs either single and central, or paired and disposed on either side of the
longer axis of the animal. The shell, when present, is never spiral, but
consists of eight overlapping plates, kept together by an elliptical girdle.
The Amphineura are divided into (a) Polyplacophora,[10] or Chitons, and
(b) Aplacophora (Chaetoderma and Neomenia).
(2) The Prosobranchiata[11] are so named from the fact that the
breathing organ (branchia or ctenidium[12]) is as a rule situated in front of
the heart, the auricle at the same time being in front of the ventricle. They
are asymmetrical, almost always furnished with a shell, which is at some
time spiral, and with an operculum. The sexes are separate. They are either
marine animals, or can be shown to be more or less directly derived from
genera which are marine. They are divided into (a) Diotocardia[13]
(Haliotis, Fissurella, Trochus, Nerita, Patella), which have, or whose
immediate ancestors are believed to have had, two auricles to the heart, two
sets of breathing organs, two kidneys, but no proboscis, penis, or siphon,
and (b) Monotocardia,[14] in which the heart has only one auricle, the true
breathing organ is single, and there is a single kidney. To this division
belong the great majority of marine univalve Mollusca, e.g. Cypraea,
Buccinum, Murex, Littorina, Ianthina, all the land and fresh-water
operculates (Cyclostoma, Melania, Paludina, etc.), as well as the
Heteropoda, which are a group of Prosobranchiata which have betaken
themselves to a pelagic life.

Fig. 4.—Example of a Heteropod, Carinaria mediterranea Lam.,
Naples: a, anus; br, branchia; f, foot; i, intestine; m, mouth;
p, penis; s, sucker; sh, shell; t, tentacles. × ½. The animal
swims foot uppermost.

(3) In the Opisthobranchiata[5] the breathing organs (when present) are
behind the heart, and the auricle of the heart is consequently behind the
ventricle. They are asymmetrical marine animals; usually, but by no means
always, without a shell, scarcely ever with an operculum in the adult state.
The sexes are united in the same individual. The Opisthobranchiata fall into
two divisions: (a) Tectibranchiata, in which the breathing organ is more or
less covered by the mantle, and a shell is usually present, which is
sometimes rudimentary, e.g. Bulla, Aplysia, Umbrella, and the whole group
of Pteropoda; (b) Nudibranchiata, or sea slugs, which have no shell and no
true ctenidia, but breathe either by the skin, or by ‘cerata’ or papilliform
organs prominently developed on the back: e.g. Doris, Aeolis, Dendronotus.

Fig. 5.—A, A Tectibranchiate Opisthobranch, Umbrella mediterranea
Lam., Naples: a, anus; br, branchia; f, foot; m, mouth; rh,
rhinophores; sh, shell.
B, A Pteropod, Hyalaea tridentata Forsk., Naples: sh, shell; l, l,
swimming lobes of foot.
C, A Nudibranchiate Opisthobranch, Aeolis peregrina, Naples: f, foot;
c, cerata.

Fig. 6.—Examples of—A, Pulmonata Basommatophora, the common
Limnaea peregra Müll.: e, e, eyes; t, t, tentacles. B, Pulmonata
Stylommatophora, Helix hortensis Müll.: e, e, eyes; t, t, tentacles; p.
o, pulmonary orifice (the position of the pulmonary orifice in
Limnaea will be seen by reference to Fig. 101).

(4) The Pulmonata[15] are asymmetrical air-breathing non-marine
Mollusca, generally, but not always, furnished with a shell. The sexes are
always united in the same individual, and the operculum is always wanting,

except in Amphibola. They are conveniently divided into Stylommatophora,
[16]
in which the eyes are at the tip of the upper tentacles, which are
retractile (Helix, Limax, Bulimus, and all true land slugs and snails), and
Basommatophora, in which the eyes are at the base of the tentacles, which
are not retractile (Limnaea, Planorbis, Physa, and all the Auriculidae).
Thus we have
Gasteropoda Amphineura Polyplacophora
Aplacophora
Prosobranchiata Diotocardia
Monotocardia (incl. Heteropoda)
Opisthobranchiata Tectibranchiata (incl. Pteropoda)
Nudibranchiata[17]
Pulmonata Stylommatophora
Basommatophora

The relation of the four great Orders to one another will be better
discussed when we come to deal with each Order separately. The problem
of the origin and mutual relationship of the various forms of molluscan life
is of extreme subtlety, and its solution can only be approached after a
comprehensive survey of many complicated anatomical details. But there is
one branch of the Mollusca—the land and fresh-water genera—whose
origin is, comparatively speaking, of recent date, and whose relationships
are therefore less likely to have suffered complete obliteration.
Origin of the Land and Fresh-water Mollusca.—The ultimate
derivation of the whole of the land and fresh-water molluscan fauna must,
as has already been remarked, be looked for in the sea. In certain cases the
process of conversion, if it may be so termed, from a marine to a non-
marine genus, is still in progress, and can be definitely observed; in others
the conversion is complete, but the modification of form has been so slight,
or the date of its occurrence so recent, that the connexion is unmistakable,
or at least highly probable; in others again, the modification has been so
great, or the date of its occurrence so remote, that the actual line of
derivation is obscured or at best only conjectural.

Fig. 7.—A, the common cockle (Cardium edule L.). B,
Adacna plicata Eichw., Caspian Sea. C, Didacna
trigonoides Pall., Caspian Sea.

This passage from a marine to a non-marine life—in other words, this
direct derivation of non-marine from marine genera—is illustrated by the
faunal phenomena of an inland brackish-water sea like the Caspian, which
is known to have been originally in connexion with the Mediterranean, and
therefore originally supported a marine fauna. The Mollusca of the Caspian,
although without exception brackish- or fresh-water species, are in their
general facies distinctly marine. Of the 26 univalve species which inhabit it
19 belong to 4 peculiar genera (Micromelania, Caspia, Clessinia,
Nematurella), all of which are modified forms of the marine Rissoidae. The
characteristic bivalves belong to the genera Adacna, Didacna, and
Monodacna, all of which can be shown to be derived from the common
Cardium edule. We have here a case where complete isolation from the sea,
combined no doubt with a gradual freshening of the water, has resulted in
the development of a number of new genera. The singularly marine facies
of several of the fresh-water genera now inhabiting Lake Tanganyika, has
given rise to the belief, among some authorities, that that lake was at one
time an inlet of the Indian Ocean. In the upper waters of the Baltic, marine
and fresh-water Mollusca flourish side by side. So complete is the
intermixture, that an observer who had lived on no other shores would

probably be unable to separate the one set of species from the other.[18]
Thus between Dagö and Papen-Wiek[19] Mytilus edulis, Cardium edule,
Tellina balthica, Mya arenaria, Littorina rudis, and Hydrobia balthica are
the only true marine species; with these live Unio, Cyclas, Neritina,
Limnaea, and Bithynia. The marine species and Neritina live down to 15–
20 fath., the rest only down to 3 fath. Under stones close to the shore of the
Skärgård at Stockholm[20] are found young Cardium and Tellina, and at 3 to
6 fath. Limnaea peregra, and Physa fontinalis. Near Gothland Limnaea is
found in the open sea at 8–12 fath., and with it occur Cardium and Tellina.
At the Frisches Haff[21] Mya arenaria is the only marine species, and lives
in company with 6 sp. Limnaea, 1 Physa, 9 Planorbis, 1 Ancylus, 4 Valvata,
2 Sphaerium. Were the Sound to become closed, and the waters of the
Baltic perfectly fresh, it would be inevitable that Mya arenaria, and such
other marine species as continued to live under their changed conditions,
should in course of time submit to modifications similar in kind to those
experienced by the quondam marine species of the Caspian.
It seems probable, however, that the origin, at least in a great part, of the
land and fresh-water Mollusca need not be accounted for by such
involuntary changes of environment as the enclosure of arms of the sea, or
the possible drying up of inland lakes. These cases may be taken as
illustrations of the much more gradual processes of nature by which the
land and fresh-water fauna must have been developed. The ancestry of that
fauna must be looked for, as far as the Gasteropoda are concerned, in the
littoral and estuarine species; for the Pelecypoda, in the estuarine alone.
The effect of the recess of the tide, in the one case, and the effect of the
reduced percentage of salt, in the other, has tended to produce a gradual
adaptation to new surroundings, an adaptation which becomes more and
more perfect. It may be safely asserted that no marine species could pass
into a land or fresh-water species except after a period, more or less
prolonged, of littoral or estuarine existence. Thus we find no land or fresh-
water species exhibiting relationships with such deep-sea genera as the
Volutidae, Cancellariidae, Terebridae, or even with genera trenching on the
lowest part of the littoral zone, such as the Haliotidae, Conidae, Olividae,
Capulidae. The signs of connexion are rather with the Neritidae,
Cerithiidae, and above all the Littorinidae, which are accustomed to live for
hours, and in the case of Littorina for days or even weeks, without being

moistened by the tide. Similarly the fresh-water Pelecypoda exhibit
relationships, not with genera exclusively marine, but with genera known to
inhabit estuaries, such as the Mytilidae, Corbulidae, Cardiidae.
It would be natural to expect that we should find this process of
conversion still going on, and that we should be able to detect particular
species or groups of species in process of emigration from sea to land, or
from sea to fresh water. Such species will be intermediate between a marine
and a land or fresh-water species, and difficult to classify distinctly as one
or the other. Cases of Mollusca occupying this intermediate position occur
all over the world. They inhabit brackish swamps, damp places at high-
water mark, and rocks only at intervals visited by the tide. Such are
Potamides, Assiminea, Siphonaria, Melampus, Hydrobia, Truncatella,
among the univalves, and many species of Cyrena and Arca among the
bivalves.
Origin of the Fresh-water Fauna

(a) Pelecypoda.—Estuarine species, which have become accustomed to
a certain admixture of fresh water, have gradually ascended the streams or
been cut off from the sea, and have at last become habituated to water
which is perfectly fresh.

Fig. 8.—A, The common Mytilus edulis L., a
marine genus and species. B, Dreissensia,
a fresh-water genus, closely allied to
Mytilus.

Fig. 9.—A, Arca navicella Reeve, Philippines, a marine species. B,
Arca (Scaphula) pinna Bens., R. Tenasserim, a fresh-water
species which lives many miles above the tide-way.

Thus Dreissensia (rivers and canals throughout N. Europe and N.
America) and Mytilopsis (rivers of America) are scarcely modified Mytili
(Fig. 8); Scaphula is a modified Arca, and lives in the Ganges, the Jumna,
and the Tenasserim at a distance of 1600 miles from the sea (Fig. 9). Pholas
rivicola is found imbedded in floating wood on the R. Pantai many miles

from its mouth. Cyrena, Corbicula, and probably Sphaerium and Pisidium
are derived, in different degrees of removal, from the exclusively marine
Veneridae; Potamomya (rivers of S. America), and Himella (R. Amazon)
are forms of Corbula. The Caspian genera derived from Cardium (Adacna,
Didacna, Monodacna), have already been referred to. Nausitora is a form
of Teredo, which lives in fresh water in Bengal. Rangia, Fischeria, and
Galatea probably share the derivation of the Cyrenidae, while in Iphigenia
we have one of the Donacidae which has not yet mounted rivers, but is
confined to a strictly estuarine life. The familiar Scrobicularia piperata of
our own estuaries is a Tellina, which lives by preference in brackish water.

Fig. 10.—Trigonia pectinata Lam.,
Sydney, N.S.W.

The great family of the Unionidae is regarded by Neumayr[22] as derived
from Trigonia, the points of similarity being the development of a nacreous
shell, the presence of a strong epidermis, and the arrangement of the
muscular scars. It is remarkable, too, that on many Uniones of Pliocene
times there is found shell ornamentation of such a type as occurs elsewhere
among the Pelecypoda only on Trigonia.
The genera of fresh-water Pelecypoda are comparatively few in number,
and their origin is far more clearly discernible than that of any other group.
This is perhaps due to the fact that the essential changes of structure
required to convert a marine into a fresh-water bivalve are but slight. Both
animals “breathe water,” and both obtain their nutriment from matter

contained in water. Similar remarks apply to fresh-water operculate
Gasteropoda. But the passage from a marine to an aerial life involves much
profounder changes of environment, which have to be met by
correspondingly important changes in the organism. This may be in part the
reason why the ancestry of all Pulmonata, whether land or fresh-water, is so
difficult to trace.

Fig. 11.—A, Cominella, a marine
genus, which lives between tide
marks, and from which is
probably derived B, Clea, a
genus occurring only in fresh
water.

Fig. 12.—A, Cerithium columna Sowb. (marine). B, Potamides
microptera Kien. (brackish water). C, Io spinosa Lea, one of the
Pleuroceridae (fresh water).

(b) Gasteropoda.—(1) Operculate. Canidia and Clea are closely allied,
with but little modification, to the marine Cominella[23] (Fig. 11), as is also
Nassodonta to Nassa. They occur (in fresh water) in the rivers of India,
Indo-China, Java, and Borneo, associated with essentially fresh-water
species. Potamides, with its various sub-genera (Telescopium, Pyrazus,
Pirenella, Cerithidea, etc.), all of which inhabit swamps and mud-flats just
above high-water mark in all warm countries, are derived from Cerithium
(Fig. 12); Assiminea, Hydrobia, and perhaps Truncatella, from Rissoa. It is
a remarkable fact that in Geomelania (with its sub-genera Chittya and
Blandiella) we have a form of Truncatella which has entirely deserted the
neighbourhood of the sea, and lives in woody mountainous localities in
certain of the West Indies. Cremnoconchus, a remarkable shell occurring
only on wet cliffs in the ghâts of southern India, is a modified Littorina.
Neritina and Nerita form a very interesting case in illustration of the whole
process. Nerita is a purely marine genus, occurring on rocks in the littoral
zone; one species, however, (N. lineata, Chem.) ascends rivers as far as 25
miles from their mouth, and others haunt marshes of brackish water.
Neritina is the fresh-water form, some species of which are found in
brackish swamps or even creeping on wet mud between tide marks, while
the great majority are fluviatile, one group (Neritodryas) actually occurring
in the Philippines on trees of some height, at a distance of a quarter of a
mile from any water. Navicella is a still further modified form of Neritina,
occurring only on wet rocks, branches, etc., in non-tidal streams (Fig. 13).

Fig. 13.—Illustrating the development of the fresh-water genus Navicella, through
the brackish-water Neritina, from the marine Nerita, with corresponding
changes in the operculum. 1. Nerita; 2, 3. Neritina; 4. Neritina, intermediate
form; 5, 6. Navicella.

The great family of the Melaniidae, which occurs in the rivers of warm
countries all over the world, and that of the Pleuroceridae, which is
confined to North America, are, in all probability, derived from some form
or forms of Cerithium. The origin of the Paludinidae, Valvatidae, and
Ampullariidae is more doubtful. Their migration from the sea was probably
of an early date, since the first traces of all three appear in the lower
Cretaceous, while Melaniidae are not known until Tertiary times.
Ampullaria, however, shows distinct signs of relationship to Natica, while
the affinities of Paludina and Valvata cannot as yet be approximately
affirmed.
(2) Pulmonata.—Intermediate between the essentially fresh-water and
the essentially marine species come the group sometimes known as
Gehydrophila, consisting of the two families Auriculidae and Otinidae.
These may be regarded as Mollusca which, though definitely removed from
all marine species by the development of a true lung or lung cavity in the
place of a gill, have yet never become, in respect of habitat, genuine fresh-
water species. Like Potamides, they haunt salt marshes, mangrove swamps,
and the region about high-water mark. In some cases (Otina, Melampus,

Pedipes) they live on rocks which are moistened, or even bathed by the
spray, in others (Cassidula, Auricula) they are immersed in some depth of
brackish water at high tide, in others again (Scarabus) they are more
definitely terrestrial, and live under dead leaves in woods at some little
distance from water. Indeed one genus of diminutive size (Carychium) has
completely abandoned the neighbourhood of the sea, and inhabits swampy
ground almost all over the world.

Fig. 14.—Examples of the Auriculidae: A, Auricula
Judae Lam., Borneo; B, Scarabus Lessoni Blainv., E.
Indies; C, Cassidula mustelina Desh., N. Zealand; D,
Melampus castaneus Mühlf., S. Pacific; E, Pedipes
quadridens Pfr., Jamaica.

Fig. 15.—An example of
Amphibola (avellana Chem.),
the only true Pulmonate
which possesses an
operculum.

To this same section Gehydrophila have been assigned two remarkable
forms of air-breathing “limpet,” Siphonaria and Gadinia (see page 151),
and the aberrant Amphibola, a unique instance of a true operculated
pulmonate. Siphonaria possesses a pulmonary cavity as well as a gill, while
Gadinia and Amphibola are exclusively air-breathing. Siphonaria lives on
rocks at or above high-water mark, Gadinia between tide marks, Amphibola
(Fig. 15) in brackish water at the estuaries of rivers, half buried in the sand.
There can be little doubt that all these are marine forms which are gradually
becoming accustomed to a terrestrial existence. In Gadinia and Amphibola
the process is so far complete that they have exchanged gills for a
pulmonary cavity, while in Siphonaria we have an intermediate stage in
which both organs exist together. A curious parallel to this is found in the
case of Ampullaria, which is furnished with two gills and a pulmonary
chamber, and breathes indifferently air and water. It is a little remarkable
that Siphonaria, which lives at a higher tide level than Gadinia, should
retain the gill, while Gadinia has lost it.
The ultimate affinities of the essentially fresh-water groups, Limnaea,
Physa, Chilina, cannot be precisely affirmed. The form of shell in Latia,
Gundlachia, and perhaps Ancylus, may suggest to some a connexion with
the Otinidae, and in Chilina, a similar connexion with the Auriculidae. But,
in a question of derivation, similarities of shell alone are of little value. It is
not a little remarkable, for instance, that we should find a simple patelliform

shell in genera so completely distinct from one another in all anatomical
essentials as Ancylus, Patella, Siphonaria, Propilidium, Hipponyx,
Cocculina, and Umbrella.
Some recent authors, on grounds of general organisation, regard the
Limnaeidae and their allies as Opisthobranchs adapted to an aerial life. It is
held[24] that the Nudibranchiate Opisthobranchs have given birth to the
Pulmonata Stylommatophora or land snails, and the Tectibranchiate
Opisthobranchs to the Pulmonata Basommatophora or fresh-water snails.
Such a view seems at first sight open to some objection from other views
than those which deal simply with anatomy. The Opisthobranchiata are not,
to any marked extent, littoral genera, nor do they specially haunt the mouths
of rivers. On the contrary, they inhabit, as a rule, only the very lowest part
of the littoral zone, and are seldom found, except where the water is purely
salt. In other cases, when the derivation of land or fresh-water genera is
fairly well established, intermediate forms persist, which indicate, with
more or less clearness, the lines along which modification has proceeded. It
has, however, recently been shown that Siphonaria[25] and Gadinia,[26]
which have, as has been already mentioned, hitherto been classified as
Pulmonata, are in reality modified forms of Opisthobranchiata, which are in
process of adaptation to a life partly marine, partly on land. They may
therefore be regarded as supplying the link, hitherto missing, between the
land Pulmonata and the marine groups from one or other of which the latter
must have been derived. The general consensus of recent opinion inclines
towards accepting these views, some writers[27] being content to regard the
Pulmonata, as a whole, as derived from the Tectibranchiate Opisthobranchs,
while others[28] go further and regard the Stylommatophora as derived
directly from the Basommatophora.
Origin of the Land Fauna

Gasteropoda.—(1) Operculate. On a priori grounds, one might predict
a double origin for land operculates. Marine species might be imagined to
accustom themselves to a terrestrial existence, after a period, more or less
prolonged, of littoral probation. Or again, fresh-water species, themselves
ultimately derived from the sea, might submit to a similar transformation,
after a preliminary or intermediate stage of life on mudbanks, wet swamps,
branches overhanging the water, etc. Two great families in this group, and

two only, seem to have undergone these transformations, the Littorinidae
and the Neritidae. The derivation of almost all existing land operculates
may be referred to one or other of these groups.

Fig. 16.—Two rows of the radula of Littorina littorea L., × 72.

The power of the Littorinidae to live for days or even weeks without
being moistened by the sea may be verified by the most casual observer. In
the tropics this power seems even greater than on our own shores. I have
seen, in various parts of Jamaica, Littorina muricata living at the top of low
cliffs among grass and herbage. At Panama I have taken three large species
of Littorina (varia, fasciata, pulchra), on trees at and above high-water
mark. Cases have been recorded in which a number of L. muricata,
collected and put aside, have lived for three months, and L. irrorata for four
months.[29] These facts are significant, when we know that the land
operculates almost certainly originated in a tropical climate.
The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, as
contrasted with the other land operculates, form one group, have very close
relations, particularly in the length and formation of the radula, or lingual
ribbon, with the Littorinidae.

Fig. 17.—Two rows of the radula of Cyclophorus sp., India, × 40.

On the other hand, the Helicinidae, Hydrocenidae, and Proserpinidae are
equally closely related to Neritina. The Proserpinidae (restricted to the
Greater Antilles, Central America and Venezuela) may perhaps be regarded
as the ultimate term of the series. They have lost the characteristic
operculum, which in their case is replaced by a number of folds or lamellae
in the interior of the shell. It has already been noticed how one group of
Neritina (Neritodryas) occurs normally out of the water. This group
furnishes a link between the fresh-water and land forms. It is interesting to
notice that here we have the most perfect sequence of derivatives; Nerita in
the main a purely marine form, with certain species occurring also in
brackish water; Neritina in the main fresh-water, but some species
occurring on the muddy shore, others on dry land; Helicina the developed
land form; and finally Proserpina, an aberrant derivative which has lost the
operculum.[30]

Fig. 18.—A, Neritina reticularis Sowb., Calcutta (brackish water); B,
Helicina neritella Lam., Jamaica (land); C, Proserpina (Ceres) eolina
Ducl., Central America (land).

Gasteropoda.—(2) Pulmonata. The origin of these, the bulk of the land
fauna, must at present be regarded as a problem not yet finally solved.
Some authorities, as we have seen, regard them as derived from the
Nudibranchiate, others, probably more correctly, from the Tectibranchiate
Opisthobranchs.
The first known members of the land Pulmonata (Pupa [?], Hyalinia) are
from the Carboniferous of North America. Similar but new forms appear in
the Cretaceous, from which time to the present we have an unbroken series.
The characteristically modern forms, according to Simroth,[31] are Helices
with thick shells. According to the same author, Vitrina and Hyalinia are
ancestral types, which give origin not only to many modern genera with
shells, but to many shell-less genera also, e.g. Testacella is probably derived
through Daudebardia from Hyalinia, while from Vitrina came Limax and
Amalia. A consideration of the radulae of the genera concerned certainly
tends in favour of these views.
Godwin-Austen, speaking generally, considers[32] genera of land
Pulmonata with strongly developed mantle-lobes and rudimentary shell as
more advanced in development than genera in which the shell is large and
covers all or nearly all the animal.

CHAPTER II
LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND GENERAL ECONOMY

The majority of the Land Mollusca are probably more sensitive than is
usually believed. The humidity of the air must affect the surface of their
skin to a considerable extent. Every one has noticed how the snails ‘come
out’ on a damp evening, especially after rain. As a rule, they wait till rain is
over, probably objecting to the patter of the drops upon their delicate
tentacles. Snails kept in captivity under a bell-glass are acutely sensitive of
a damp atmosphere, and will bestir themselves after rain just as if they were
in the open air. Certain Helices which are accustomed to live in moist
places, will find their way to water, if removed from their usual haunts. A
case is recorded[33] of a specimen of H. arbustorum, kept in a kitchen,
which used to find its way directly under the cold water tap, and appeared
to enjoy the luxury of a douche. How delicately the conditions of life are
balanced in some of these creatures is seen in the case of Omalonyx, a
genus akin to Succinea, which is found in Brazil and the northern parts of
South America. It lives creeping on plants which overhang the margin of
water, but perishes equally, if placed in the water itself, or removed to a
distance from it for any length of time.[34]
Endurance of Heat and Cold.—The Mollusca are capable, at least as
far as some species are concerned, of enduring severe extremes both of cold
and heat. The most northern pulmonate yet observed is a fresh-water
species, Physa (Aplecta) hypnorum L. This hardy mollusc, whose shell is so
fragile as to need most careful handling, has been noticed on the peninsula
of Taimyr, North Siberia, in 73° 30’ N. lat, a region whose mean annual
temperature is below 10° F. with a range of from 40° F. in July to -30° F. in
January.
It is well known that the Limnaeidae, and probably most fresh-water
Mollusca of sub-temperate regions, can continue to live not merely under,
but enveloped in ice, and themselves frozen hard. Garnier relates[35] that,
during the winter of 1829–30, some large Limnaea auricularia, which had
been placed in a small basin, were frozen into a solid mass, experiencing a
cold of -2° F. He supposed they were dead, but, to his surprise, when the
basin thawed, the Limnaea gradually revived. Paludina vivipara and

Anodonta anatina have been known to resist a temperature of 23° F., and
the former has produced young shortly after being thawed out of the ice.[36]
As far north as Bodø in Norway (67° 37’ N. lat., well within the Arctic
circle) there are found no less than fourteen species of terrestrial Mollusca,
among them being Balea perversa and Clausilia rugosa.[37]
Vitrina is one of our most hardy molluscs, and may be observed crawling
on bright mornings over the frost-covered leaves of a wood or copse. V.
glacialis is said by Charpentier to live in the Alps at a height where the
stones are covered with snow from nine to ten months of the year. Many of
the Hyaliniae are very hardy. Arion, in spite of having no external shell to
protect it, is apparently less affected by the cold than Helix, and does not
commence hibernation till a later period in the autumn. The operculate land
Mollusca, in spite of the protection which their operculum may be supposed
to afford, are exceedingly sensitive to cold, and the whole group is without
doubt a product of tropical or semi-tropical regions (see map at
frontispiece). A species of Helicina which inhabits the southern States of
North America has been known to be almost exterminated from certain
districts by the occurrence of an unusually severe winter.
One of the highest altitudes at which a land shell is known to live
appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous
altitude, two species of Buliminus (arcuatus Hutt. and nivicola Bens.) live
on juniper bushes among patches of snow. An Anadenus is said to have
been found in a similar locality at 15,000 ft., while Limnaea Hookeri has
been taken from over 16,400 ft. in Landour. In the Andes of Peru and
Bolivia, five species of Bulimulus, one of Pupa, and one of Limax occur at
an elevation of 10,500 to 15,000 ft. Several fresh-water Mollusca inhabit
Lake Titicaca, which stands at a height of 12,550 ft. in the Bolivian table-
land.
In certain parts of the desert of Algeria, where there is not a trace of
vegetation to be seen, and the temperature at mid-day is 110° F., the ground
is sometimes so covered with Helix lactea as to appear perfectly white. Dr.
F. H. H. Guillemard has told me that he noticed, in somewhat similar
surroundings between Fez and Tangier, H. pisana in such extraordinary
abundance that they hung from the low scrub in bunches the size of a man’s
two fists. It is singular that Mollusca should live, and not only live, but
flourish, in localities apparently so unpromising. Shells which occur in the

Algerian Sahara are actually larger and altogether finer than the ordinary
European form of the same species. In order to protect themselves to some
extent against the scorching heat and consequent evaporation, desert species
are frequently modified in one of two ways; the shell becomes either white
or a light dusky brown, as in the familiar Helix desertorum, or else it gains
immensely in thickness. Specimens of H. pomatia, recently procured from
Fez, are of extraordinary thickness as compared with forms from our own
chalk downs of Kent and Surrey.
Fresh-water Mollusca are frequently found inhabiting hot springs. Thus
Neritina fluviatilis lives at Bagnères de Bigorre in water at about 68° F. In
another hot spring in the eastern Pyrenees a Bithynia lives at a temperature
of over 73° F.; while Blainville mentions another case of a Bithynia living
in water at 122° F.
Hibernation and Aestivation.—As autumn begins to draw on, and the
first frosts to nip vegetation, terrestrial species retire beneath stones, into
cracks in old walls, holes in tree trunks, deep fissures in rocks, and nooks
and crannies of every kind, or else bury themselves deeply in the earth or in
moss and heaps of leaves. They thus commence their period of hibernation,
which varies in length according to the duration of winter. Frequently
masses of Helices may be found attached to one another, probably not so
much for the sake of warmth, for their temperature is but low, as to share
the comforts of a cosy retreat in common. Slugs generally hibernate alone,
excavating a sort of nest in the earth, in which they encyst themselves,
contracting their bodies until they are almost round, and secreting a
covering of their own slime. The Helices usually close up the mouth of their
shell by the formation of a membranous or chalky epiphragm, which will be
further described below. Both snails and slugs take care to be in good
condition at the time their winter sleep begins, and for this reason the
former are said to be most esteemed by foreign epicures if captured just at
this period.[38]
During hibernation, the action of the heart in land Pulmonata ceases
almost entirely. This appears to be directly due to the effect of cold. Mr. C.
Ashford has related[39] some interesting experiments made upon H.
hortensis and Hyal. cellaria, with the view of ascertaining the effect of cold
upon their pulsations. His observations may be tabulated as follows:—

Number of pulsations per minute
Helix hortensis Hyal. cellaria At degrees Fahr.
22 21 52°
14 12 44°
10 11 38°
4 9 30°

At low temperatures the character, as well as the number of the
pulsations changed; they became imperfect and intermittent, although
exceptionally at 31° F. a H. rufescens gave five or six pulsations a minute,
very full and deliberate. The result of taking the Hyalinia suddenly into the
heat of a greenhouse was to bring on palpitations. Further experiments
resulted in evidence of a similar kind. Hyal. radiatula, placed upon a deal
table in a room, showed 52 pulsations per minute at 62° F. Placed upon the
palm of the hand, the action soon rose to 108. Hyal. alliaria, similarly
treated, rose from 72 pulsations to 110. Floated upon water, the action of the
heart of the latter suddenly fell to 29.
Fresh-water Pulmonata do not appear to hibernate. Unio and Anodonta,
however, bury themselves more deeply in the mud, and Dreissensia casts
off its byssus and retires under the mud in deeper water.[40] Limnaea and
Planorbis have often been noticed to crawl about under the lower surface of
a thick coating of ice. In periods of prolonged drought, when the water in
the ponds dries up, the majority of genera bury themselves in the mud. I
have known Limnaea peregra bury itself three inches deep, when surprised
by a sudden fall of the water in the ditch on Coe Fen, behind Peterhouse,
Cambridge. Physa hypnorum frequents by preference ditches which dry up
in summer, as does also Planorbis spirorbis, the latter often forming a sort
of epiphragm against evaporation. Ancylus has been observed to spend the
whole winter out of water, and P. spirorbis has been noticed alive after four
months’ desiccation.[41]
True aestivation, however, occurs mainly in the tropics, where there is no
winter, but only a period when it is not quite so hot as the rest of the year, or
on a coast like the Mediterranean, which is subject to sudden and severe
heat. This period is usually rainless, and the heat is therefore a dry heat. At
this season, which may last for three or four months, most of the land
Mollusca enter upon a period of inaction, either burying themselves deeply
in the ground, or else permanently attaching themselves to the stalks of
grass and other herbage, or the under sides of rocks. For instance, the large

and beautifully painted Orthalicus, Corona, and Porphyrobaphe, which
inhabit Brazil, Ecuador, and eastern Peru, bury themselves deeply in the
ground during the dry season, while in the rains they climb to the topmost
branches of the great forest trees.[42] Thus it may well happen that a visitor
to a tropical island, Ceylon for instance, or one of the Greater Antilles, if he
times his visit to coincide with the rainless season, may be grievously
disappointed at what seems its unaccountable poverty in land Mollusca. But
as soon as the weather breaks, and the moisture penetrates their retreats,
every bush and every stone, in favoured localities, will be alive with
interesting species.
The Epiphragm.—A considerable number of the land Pulmonata (and a
very few of the fresh-water) possess the power of closing the aperture of
their shell by means of what is known as an epiphragm or covering of
hardened mucus. This epiphragm is habitually formed by certain species
during hibernation or aestivation, or even during shorter periods of
inactivity and retirement, the object being, either to check evaporation of
the moisture of the body, or to secure the animal against the cold by
retaining a thin layer of slightly warm air immediately within the aperture
of the shell.
The epiphragm differs widely in character in different species,
sometimes (Clausilia, Pupa, Planorbis) consisting of the merest pellicle of
transparent membrane, while at others (Helix aperta, H. pomatia) it is a
thick chalky substance, with a considerable admixture of carbonate of lime,
with the consistency of a hardened layer of plaster of Paris. Within these
extremes every variety of thickness, solidity, and transparency occurs.
During long hibernation several epiphragms are not unfrequently formed by
the same individual snail, one within the other, at gradually lessening
distances. The epiphragm thus performs, to a certain extent, the part of an
operculum, but it must be remembered that it differs radically from an
operculum physiologically, in being only a temporary secretion, while the
operculum is actually a living part of the animal.
The actual mode of formation of the epiphragm would seem to differ in
different species. According to Fischer,[43] the mollusc withdraws into its
shell, completely blocking all passage of air into the interior, and closing
the pulmonary orifice. Then, from the middle part of the foot, which is held
exactly at the same plane as the aperture, is slowly secreted a transparent

pellicle, which gradually thickens, and in certain species becomes
calcareous. Dr. Binney, who kept a large number of Helix hortensis in
confinement, had frequently an opportunity of noticing the manner in which
the epiphragm was formed.[44] The aperture of the shell being upward, and
the collar of the animal having been brought to a level with it, a quantity of
gelatinous matter is thrown out [? where from]. The pulmonary orifice is
then opened, and a portion of the air within suddenly ejected, with such
force as to separate the viscid matter from the collar, and to project it, like a
bubble of air, from the aperture. The animal then quickly withdraws farther
into the shell, and the pressure of the external air forces back the vesicle to
a level with the aperture, when it hardens and forms the epiphragm. In some
of the European species in which the gelatinous secretion contains more
carbonate of lime, solidification seems to take place at the moment when
the air is expelled, and the epiphragm in these is in consequence strongly
convex.
Thread-spinning.—A considerable number of fresh-water Mollusca
possess the power of stretching a thread, which is no more than an
exceedingly elongated piece of mucus, to the surface of the water, and of
using it as a means of locomotion. This thread bears no analogy whatever to
the fibrous byssus of certain bivalves, being formed in an entirely different
manner, without the need of a special gland.
The threads are ‘spun’ by several species of Limnaea, Physa, and
Planorbis, by Bithynia tentaculata, and several of the Cycladidae. They are
anchored to the surface by a minute concavity at the upper end, which
appears to act like a small boat in keeping the thread steady. The longest
threads are those of the Physae, which have been noticed to attain a length,
in confinement, of 14 inches. They are always spun in the ascent, and as a
rule, when the animal descends, it rolls the thread up and carries it down as
it goes. A single thread is never spun on the descent, but occasionally, when
a thread has become more or less of a permanence, it becomes stronger by
the addition of more mucus each time it is used, whether for ascending or
descending purposes. Cyclas cornea appears to be an exception to the rule
that threads are only spun on the ascent. This species, which is particularly
fond of crawling along the under surface of the water, has been noticed to
spin a thread half an inch in length while on the surface, and to hang
suspended from it for a considerable time.

What the exact use of the thread may be, must to a certain extent be
matter of conjecture. The Limnaeidae are, in the great majority of cases,
compelled to make periodic visits to the surface in order to inspire oxygen.
It is also a favourite habit with them to float just under the surface, or crawl
about on its under side, perhaps in pursuit of tiny vegetable organisms.
Whatever may be the object of an excursion to the surface, a taut thread will
obviously be a nearer way up than any other which is likely to present
itself; indeed, without this thread-spinning power, which insures a tolerably
rapid arrival at the surface, the animal might find itself asphyxiated, or at
least seriously inconvenienced, before it could succeed in taking in the
desired supply of oxygen. With the Cycladidae, which do not breathe air,
such an explanation is out of place; in their case the thread seems to be a
convenient means of resting in one position in the intervals of the periods of
active exercise to which several of the species are so much addicted.
The power of suspension by a thread is also possessed by certain of the
Cyclostomatidae, by some Cerithidea, several Rissoa and other marine
genera, prominent among which is Litiopa bombyx, whose name expresses
its power of anchoring itself to the Sargasso weed by a silken thread of
mucus. Several species of slugs are known to be able to let themselves
down by threads from the branches of trees. Limax arborum is especially
noted for this property, and has been observed suspended in pairs during the
breeding time. According to Binney, all the American species of Limax,
besides those of Tebennophorus, possess this singular property. Limax
arborum appears to be the only slug which has been noticed to ascend, as
well as descend, its thread. It has also been observed[45] that when this
species is gorged with food, its slime is thin and watery, and unable to
sustain its weight, but that after the process of digestion has been
performed, the mucus again becomes thick and tenacious. It appears
therefore that when the animal is hungry and most in need of the power of
making distant excursions in search of food, its condition enables it to do
so, but that when no such necessity is pressing, the thread-forming mucus is
not secreted, or is perhaps held in suspense while the glands assist in
lubricating the food before digestion.[46]
Food of Land and Fresh-water Mollusca.—Arion ater, the great black
slug, although normally frugivorous, is unquestionably carnivorous as well,
feeding on all sorts of animal matter, whether decaying, freshly killed, or

even in a living state. It is frequently noticed feeding on earthworms; kept
in captivity, it will eat raw beef; it does not disdain the carcases of its own
dead brethren. An old man near Berwick-on-Tweed, going out one morning
to mow grass, found a black slug devouring, as he supposed, a dead mouse.
Being of an inquisitive turn, and wishing to ascertain if it were really thus
engaged, he drew the mouse a little back. When he returned in the evening,
the mouse was reduced almost to a skeleton, and the slug was still there.[47]
Indeed it would seem almost difficult to name anything which Arion ater
will not eat. Dr. Gray mentions[48] a case of a specimen which devoured
sand recently taken from the beach, which contained just enough animal
matter to render it luminous when trodden on in the dark; after a little time
the faeces of the slug were composed of pure sand, united together by a
little mucus. A specimen kept two days in captivity was turned out on a
newspaper, and commenced at once to devour it. The same specimen ate
dead bodies of five other species of slugs, a dead Unio, pupae of Adimonia
tanaceti, part of the abdomen of a dragon-fly, and Pears’ soap, the latter
reluctantly.[49]
According to Simroth[50] and Scharff[51] the food of several of our
British slugs, e.g. Limax maximus, L. flavus, Arion subfuscus, A.
intermedius, consists of non-chlorophyllaceous substances only, while
anything containing chlorophyll is as a rule refused. On the other hand L.
agrestis and Amalia carinata feed almost entirely on green food, and are
most destructive in gardens. The latter species lives several inches under
ground during the day, and comes to the surface only at night. It is largely
responsible for the disappearance of bulbs, to which it is extremely partial.
L. marginatus (= arborum Bouch.) feeds exclusively on lichens, and in
captivity absolutely refuses green leaves and a flesh diet. It follows
therefore, if these observations are correct, that the popular notions about
slugs must be revised, and that while we continue to exterminate from our
gardens those species which have a taste for chlorophyll, we ought to spare,
if not encourage those whose tastes lie in the opposite direction.
Limax agrestis has been seen devouring the crushed remains of Arion
ater. Five specimens of the same species were once noticed busily
devouring a May-fly each, and this in the middle of a large meadow, where
it may be presumed there was no lack of green food. The capture and eating
of insects by Mollusca seems very remarkable, but this story does not stand

alone. Mr. T. Vernon Wollaston once enclosed in a bottle at least three
dozen specimens of Coleoptera together with 4 Helix cantiana, 5 H.
hispida, and 1 H. virgata, together with an abundant supply of fresh leaves
and grass. About a fortnight afterwards, on the bottle being opened, it was
found that every single specimen of the Coleoptera had been devoured by
the snails.[52] Amalia marginata in captivity has been fed upon the larvae of
Euchelia jacobaeae, eating three in two hours.[53]
Limax maximus (Fig. 19) has been seen frequently to make its way into a
dairy and feed on raw beef.[54] Individuals kept in confinement are guilty of
cannibalism. Mr. W. A. Gain kept three specimens in a box together, and
found one of them two-thirds eaten, “the tail left clean cut off, reminding
one of that portion of a fish on a fishmonger’s stall.” That starvation did not
prompt the crime was proved by the fact that during the preceding night the
slug had been supplied with, and had eaten, a considerable quantity of its
favourite food. On two other occasions the same observer found one of his
slugs deprived of its slime and a portion of its skin, and in a dying
condition.[55] An adult L. maximus, kept for thirty-three days in captivity
with a young Arion ater, attacked it frequently, denuded it of its slime, and
gnawed numerous small pieces of skin off the body and mantle.[56] The
present writer has found no better bait for this species on a warm summer
night than the bodies of its brethren which were slain on the night
preceding; it will also devour dead Helix aspersa. Mr. Gain considers it a
very dainty feeder, preferring fungi to all other foods, and apparently doing
no harm in the garden.

Fig. 19.—Limax maximus L. PO, pulmonary orifice: × ⅔.

Limax flavus, which is fond of inhabiting the vicinity of cellars, makes
its presence most disagreeable by attacking articles of food, and especially
by insinuating itself into vessels containing meal and flour.[57] It is
particularly partial to cream.
Slugs will sometimes bite their captor’s hands. Mr. Kew relates that a
Limax agrestis, on being stopped with the finger, while endeavouring to
escape from the attack of a large Arion, attempted to bite fiercely, the
rasping action of its radula being plainly felt. According to the same
authority, probably all the slugs will rasp the skin of the finger, if it is held
out to them, and continue to do so for a considerable time, without however
actually drawing blood.[58] While Mr. Gain was handling a large Arion ater,
it at once seized one of the folds of skin between the fingers of the hand on
which it was placed; after the action of the radula had been allowed to
continue for about a minute, the skin was seen to be abraded.[59] Another
specimen of Arion ater, carried in the hand for a long time enclosed in a
dock leaf, began to rasp the skin. The operation was permitted until it
became too painful to bear. Examination with a lens showed the skin almost
rasped away, and the place remained tender and sore, like a slight burn, for
several days.[60]
Helix pisana, if freshly caught, and placed in a box with other species,
will set to work and devour them within twenty-four hours. The present

writer has noticed it, in this position, attack and kill large specimens of H.
ericetorum, cleaning them completely out, and inserting its elongated body
into the top whorls of its unfortunate victims in a most remarkable manner.
Amongst a large number of species bred in captivity by Miss F. M. Hele,[61]
was Hyalinia Draparnaldi. In the first summer the young offspring were
fed on cabbage, coltsfoot, and broadleafed docks. They would not hibernate
even in the severest frosts, and, no outdoor food being available, were fed
on chopped beef. This, Miss Hele thinks, must have degenerated their
appetites, for in the following spring and summer they constantly devoured
each other.
Zonites algirus feeds on decayed fruit and vegetables, and on stinking
flesh.[62] Achatina panthera has been known to eat meat, other snails (when
dead), vegetables, and paper.[63] The common Stenogyra decollata of the
South of Europe has a very bad character for flesh-eating habits, when kept
in captivity. Mr. Binney[64] kept a number for a long time as scavengers, to
clean the shells of other snails. As soon as a living Helix was placed in a
box with them, one would attack it, introduce itself into the upper whorls,
and completely remove the animal. One day a number of Succinea ovalis
were left with them for a short time, and disappeared entirely! The
Stenogyra had eaten shell as well as animal. This view of Stenogyra is quite
confirmed by Miss Hele, who has bred them in thousands. “I can keep,” she
writes,[65] “no small Helix or Bulimus with them, for they at once kill and
eat them. They will also eat raw meat.”
Even the common Limnaea stagnalis, which is usually regarded as
strictly herbivorous, will sometimes betake itself, apparently by preference,
to a diet of flesh. Karl Semper frequently observed the Limnaeae in his
aquarium suddenly attack healthy living specimens of the common large
water newt (Triton taeniatus), overcome them, and devour them, although
there was plenty of their favourite vegetable food growing within easy
reach.[66] The same species has also been noticed to devour its own ova, and
the larvae of Dytiscus. Limnaea peregra has been detected capturing and
partially devouring minnows in an aquarium, when deprived of other food,
and Dr. Jeffreys has seen the same species attack its own relatives under
similar circumstances, piercing the spire at its thinnest point near to the

apex.[67] L. stagnalis, kept in an aquarium, has succeeded in overpowering
and partially devouring healthy specimens of the common stickleback.[68]
Powers of Intelligence, Homing, and finding Food.—It is not easy to
discover whether land Mollusca possess any faculties which correspond to
what we call intelligence, as distinct from their capacities for smell, sight,
taste, and hearing. Darwin mentions[69] a remarkable case, communicated to
him by Mr. Lonsdale. A couple of Helix pomatia, one of which was sickly,
were placed in a small and ill-provided garden. The stronger of the two
soon disappeared over the wall into the next garden, which was well
furnished with food. It was concluded that the snail had deserted its weakly
mate, but after twenty-four hours it returned, and apparently communicated
the results of its expedition, for after a short time both started off along the
same track, and disappeared over the wall. According to Dr. W. H. Dall,[70]
a young girl who possessed a remarkable power over animals succeeded in
training a snail (H. albolabris) to come out of its lurking-place at her call. If
placed in a room, it would shrink into its shell at the sound of any other
voice, but it would always start off in the direction of hers.
Snails and slugs possess to a considerable extent the faculty of ‘homing,’
or returning to the same hiding-place day after day, after their night
excursions in search of food. Mr. C. Ashford once marked with a dab of
white paint seven Helix aspersa found lurking under a broken flagstone; at
10 p.m. the same evening three had disappeared on the forage; the next
morning all were ‘at home.’ The following night at 10 p.m. five were gone
out, two being discovered with some difficulty ‘in a small jungle’ six feet
away; the next morning six out of the seven were safely beneath the
flagstone. According to the same authority, Helix aspersa will find its way
across a cinder-path (which it specially detests) to get to its favourite food,
and will return by the same way to its old quarters, although it could easily
have found new lodgings nearer the food-supply. A snail has been observed
to occupy a hole in the brick wall of a kitchen-garden about four feet from
the ground. Leaning against the wall, and immediately under the hole, was a
piece of wood, the lower end of which rested in a bed of herbs. For months
the snail employed this ladder between its food and its home, coming down
as soon as it was dark, and retiring to rest during the day.

In greenhouses a slug will forage night after night—as gardeners know
to their cost—over the same beat, and will always return to the same
hiding-place. Limax flavus has been noticed crawling with great regularity
to a sink from a hole near the water-pipe, and keeping to a well-marked
circular track. In all probability the scent, either of the desired object of
food, or of the creature’s own trail, plays a considerable part in keeping it to
the same outward and homeward track, or at least in guiding it back to its
hiding-place. Yet even scent is occasionally at fault, for on one occasion a
Limax flavus was accustomed to make nightly excursions to some basins of
cream, which were kept in a cool cellar. When the basins were removed to a
distant shelf, the creature was found the next morning ‘wandering
disconsolately’ about in the place where the basins had formerly stood.[71]
A remarkable case of the power of smell, combined with great
perseverance on the part of a Helix, is recorded by Furtado.[72] He noticed a
Helix aspersa lodged between a column on a verandah and a flower-pot
containing a young banana plant, and threw it away into a little court below,
and six or seven yards distant. Next morning the snail was in precisely the
same place on the flower-pot. Again he threw it away, to the same distance,
and determined to notice what happened. Next morning at nine o’clock, the
snail was resting on the rail of a staircase leading up to the verandah from
the court; in the evening it started again, quickening its space as it
advanced, eventually attacking the banana in precisely the same place
where it had been gnawed before.
For further instances of the power of smell in snails, see chap. vii.
Slugs have been known to make their way into bee-hives, presumably
for the sake of the honey.[73] ‘Sugaring’ the trees at night for moths will
often attract a surprising concourse of slugs. Sometimes a particular plant in
a greenhouse will become the object of the slugs’ persistent attacks, and
they will neglect every other food in order to obtain it. Farfugium grande is
one of these favourite foods, “the young leaves and shoots being always
eaten in preference to all other plants growing in the houses; where no
Farfugiums were kept the slugs nibbled indiscriminately at many kinds.”[74]
The flowers of orchidaceous plants exercise a special attraction over slugs,
which appear to have some means of discovering when the plants are in
bloom. “I have often observed,” says Mr. T. Baines, “that a slug will travel

over the surface of a pot in which is growing a Dendrobium nobile, a
Cattleya, Vanda, or similar upright plant for a score of times without ever
attempting to ascend into the head of the plant unless it is in bloom, in
which case they are certain to find their way straight to the flowers; after
which they will descend, and return to some favourite hiding-place, often at
the opposite end of the house.”[75] Mr. R. Warner has “actually seen many
little slugs suspending themselves by slime-threads from the rafters and
descending on the spikes of the beautiful Odontoglossum alexandrae; and
thus many spikes, thickly wadded round with cotton wool (which the slugs
could not travel over), and growing in pots surrounded by water, had been
lost.”[76] Perhaps the most singular instance of a liking for a particular food
is that related by Mr. E. Step.[77] In a London publishing house, slugs were
observed, during a period of nearly twelve months, to have fed almost
nightly on the colouring matter in certain bookcovers, and though the trails
were often seen over the shelves, and cabbage and lettuce leaves laid down
to tempt the creatures, they continued their depredations with impunity for
the time above mentioned.
Limnaea peregra has been observed feeding on old fish-heads thrown
into a dirty stream, and a large gathering of Limnaea stagnalis has been
noticed feeding upon an old newspaper in a pond on Chislehurst Common,
‘so that for the space of about a square foot nothing else could be seen.’[78]
Tenacity of Life.—Land Mollusca have been known to exhibit, under
unusual conditions, remarkable tenacity of life. Some of the most
noteworthy and best authenticated instances of this faculty may be here
mentioned.
The well-known story of the British Museum snail is thus related by Mr.
Baird.[79] On the 25th March 1846 two specimens of Helix desertorum,
collected by Charles Lamb, Esq., in Egypt some time previously, were fixed
upon tablets and placed in the collection among the other Mollusca of the
Museum. There they remained fast gummed to the tablet. About the 15th
March 1850, having occasion to examine some shells in the same case, Mr.
Baird noticed a recently formed epiphragm over the mouth of one of these
snails. On removing the snails from the tablet and placing them in tepid
water, one of them came out of its shell, and the next day ate some cabbage

leaf. A month or two afterwards it began repairing the lip of its shell, which
was broken when it was first affixed to the tablet.
While resident in Porto Santo, from 27th April to 4th May 1848, Mr. S.
P. Woodward[80] collected a number of Helices and sorted them out into
separate pill-boxes. On returning home, these boxes were placed in empty
drawers in an insect cabinet, and on 19th October 1850, nearly two and a
half years afterwards, many of them were found to be still alive. A whole
bagful of H. turricula, collected on the Ilheo de Cima on 24th April 1849,
were all alive at the above-mentioned date.
In September 1858 Mr. Bryce Wright sent[81] to the British Museum two
specimens of H. desertorum which had been dormant for four years. They
were originally collected in Egypt by a Mr. Vernèdi, who, in May 1854,
while stopping at one of the stations in the desert, found a heap of thorn-
bushes lying in a corner of the building, rather thickly studded with the
snails. He picked off fifteen or twenty specimens, which he carried home
and locked up in a drawer, where they remained undisturbed until he gave
two to Mr. Wright in September 1858.
In June 1855 Dr. Woodward placed specimens of H. candidissima and H.
aperta in a glass box, to test their tenacity of life; he writes of their being
still alive in April 1859.
Mr. R. E. C. Stearns records[82] a case of Buliminus pallidior and H.
Veatchii from Cerros I. living without food from 1859 to March 1865.
H. Aucapitaine mentions[83] a case of H. lactea found in calcinated
ground in a part of the Sahara heated to 122° F., where no rain was said to
have fallen for five years. The specimen revived after being enclosed in a
bottle for three and a half years.
In August 1863, Mr. W. J. Sterland[84] put specimens of H. nemoralis in
a box and afterwards placed the box in his cabinet; in November 1866 one
specimen was discovered to be alive.
Gaskoin relates[85] a case in which specimens of H. lactea were
purchased from a dealer in whose drawer they had been for two years. This
dealer had them from a merchant at Mogador, who had kept them for more
than that time under similar conditions. One of these shells on being

immersed in water revived, and in April 1849 was placed quite alone under
a bell jar with earth and food. In the end of the following October about
thirty young H. lactea were found crawling on the glass.
Mr. R. D. Darbishire bought[86] some H. aperta in the market at Nice on
18th February 1885. Two specimens of these, placed with wool in a paper
box, were alive in December 1888. This is a very remarkable case, H.
aperta not being, like H. desertorum, H. lactea, H. Veatchii and Bul.
pallidior, a desert snail, and therefore not accustomed to fasting at all.
Age of Snails.—It would appear, from the existing evidence, which is
not too plentiful, that five years is about the average age of the common
garden snail. Mr. Gain has published[87] some interesting observations on
the life of a specimen from the cradle to the grave, which may be exhibited
in a tabular form.
Aug. 1882. Eggs hatched; one attained diameter of ⅜ in. before
winter; fed on coltsfoot and cabbage.
5th Oct. 1883. Shell 1 in. in diameter, no lip formed.
July 1884. Shell finished; diameter 1⅛ in., including perfect lip.
3rd May 1885. Left winter quarters; companion introduced, with
which it was seen in company on 5th August.
9th Aug. „ Laid eggs in soil, which were hatched on 10th
September, and feeding on 17th September; in
May 1886 the largest of these was 11/16 in.
diameter.
13th Oct. 1887. Old snail died, aged 5 years 2 months.

According to Clessin, the duration of life in Vitrina is one year, Cyclas 2
years; Hyalinia, Succinea, Limnaea, Planorbis, and Ancylus are full grown
in 2 to 3 years, Helix and Paludina in 2 to 4, and Anodonta in 12 to 14.
Hazay finds[88] that the duration of life in Hyalinia is 2 years, in Helix
pomatia 6 to 8, in Helix candicans 2 to 3, in Paludina 8 to 10, in Limnaea
and Planorbis 3 to 4.
Growth of the Shell.—Mr. E. J. Lowe, many years ago, conducted[89]
some interesting experiments on the growth of snails. The facts arrived at
were—
(1) The shells of Helicidae increase but little for a considerable period,
never arriving at maturity before the animal has once become dormant.
(2) Shells do not grow whilst the animal itself remains dormant.

(3) The growth of shells is very rapid when it does take place.
(4) Most species bury themselves in the ground to increase the
dimensions of their shells.
Six recently hatched H. pomatia were placed in a box and regularly fed
on lettuce and cabbage leaves from August until December, when they
buried themselves in the soil for winter; at this period they had gradually
increased in dimensions to the size of H. hispida. On the 1st April
following, the box was placed in the garden, and on the 3rd the Helices
reappeared on the surface, being no larger in size than they were in
December. Although regularly fed up to 20th June, they were not
perceptibly larger, but on that day five of them disappeared, having buried
themselves, with the mouth of the shell downwards, in the soil. After ten
days they reappeared, having in that short time grown so rapidly as to be
equal in size to H. pisana. On the 15th July they again buried themselves,
and reappeared on 1st August, having again increased in size. For three
months from this date they did not become perceptibly larger; on 2nd
November food was withheld for the winter and they became dormant.
A similar experiment, with similar results, was carried on with a number
of H. aspersa, hatched on 20th June. During the summer they grew but
little, buried themselves on 10th October with the head upwards, and rose
to the surface again on 5th April, not having grown during the winter. In
May they buried themselves with the head downwards, and appeared again
in a week double the size; this went on at about fortnightly intervals until
18th July, when they were almost fully grown.
Helix nemoralis, H. virgata, H. caperata, and H. hispida bury
themselves to grow; H. rotundata burrows into decayed wood; Hyalinia
radiatula appears to remain on decaying blades of grass; Pupa umbilicata,
Clausilia rugosa, and Buliminus obscurus bury their heads only.
The observations of Mr. W. E. Collinge[90] do not at all agree with those
of Mr. Lowe, with regard to the mode in which land Mollusca enlarge their
shells. He bred and reared most of the commoner forms of Helix and also
Clausilia rugosa, but never saw them bury any part of their shell when
enlarging it. While admitting that they may increase their shells when in
holes or burrows of earthworms, he thinks that the process of burying
would seriously interfere with the action of the mantle during deposition,

and in many cases damage the membranaceous film before the calcareous
portion was deposited. Mr. Collinge has found the following species under
the surface in winter: Arion ater (3–4 in.), Agriolimax agrestis, (6–8 in.),
Hyalinia cellaria and H. alliaria (6–8 in.), Hyalinia glabra (5 in.), Helix
aspersa (5–6 in.), H. rufescens (4–6 in.), H. rotundata (4–5 in.), H. hispida
(7 in.), Buliminus obscurus (4–6 in.), B. montanus[91] (24 in.), and the
following in summer, Hyalinia cellaria and alliaria (6–8 in.), Helix
rotundata (4–5 in.), Balea perversa (6–8 in.), Cyclostoma elegans (3–4 in.).
The same author has found the following species of fresh-water Mollusca
living in hard dry mud: Sphaerium corneum (3–14 in.), S. rivicola (5–6 in.),
S. lacustre (10–14 in.), all the British species of Pisidium (4–12 in.),
Limnaea truncatula (18 in., a single specimen). All our species of Unio,
Anodonta, Bithynia, and Paludina bury themselves habitually in fine or
thick wet mud, to a depth of from 4 to 14 inches.
This burying propensity on the part of Mollusca has been known to play
its part in detecting fraud. When my friend Mr. E. L. Layard was
administering justice in Ceylon, a native landowner on a small scale
complained to him of the conduct of his neighbour, who had, during his
absence from home, diverted a small watercourse, which ran between their
holdings, in such a way as to filch a certain portion of the land. The
offender had filled up and obliterated the ancient course of the stream, and
protested that it had never run but in its present bed. Mr. Layard promptly
had a trench sunk across what was said to be the old course, and the
discovery of numerous living Ampullaria, buried in the mud, confirmed the
story of one of the litigants and confounded the other.[92]
Depositing and Hatching of Eggs: Self-fertilisation.—There appears
to be no doubt that Helices, when once impregnated, can lay successive
batches of eggs, and possibly can continue laying for several years, without
a further act of union. A specimen of Helix aspersa was noticed in company
with another on 5th August; on 9th August it laid eggs in the soil, and early
in the following summer it laid a second batch of eggs, although its
companion had been removed directly after its first introduction. An Arion
received from a distance laid 30 eggs on 5th September, and 70 more on the
23rd of the same month, although quite isolated during the whole time.[93]
By far the most remarkable case of the kind is related by Gaskoin.[94] A
specimen of Helix lactea was kept in a drawer for about two years, and then

in another drawer for about two years more. It was then taken out, and
placed in water, when it revived, and was placed alone under a bell jar with
earth and food. Six months after, about 30 young H. lactea were found
crawling on the glass, the act of oviposition not having been observed.
The observations of Mr. F. W. Wotton,[95] with regard to the fertilisation
and egg-laying of Arion ater, are of extreme interest and value. A pair of
this species, kept in captivity, united on 10th September 1889, the act
lasting about 25 minutes. From that date until the eggs were laid, the
animals looked sickly, dull of colour, with a somewhat dry skin. Eggs were
deposited in batches, one, which we will call A, beginning three days before
B. On 10th October A laid 80 eggs; on the 16th, 110; on the 25th, 77; on 8th
November, 82; and on 17th November, 47; making a total of 396. Specimen
B, which began on 13th October, three days after A, made up for the delay
by laying 246 eggs in 40 hours; on 26th October it laid 9, on 10th
November, 121; and on 30th November, 101; a total of 477. These eggs
weighed 624 to the ounce, and, in excluding the batch of 246, B parted with
⅜ of its own weight in 40 hours, while the whole number laid were rather
over ¾ of its own weight!
While depositing the eggs, the slug remained throughout in the same
position on the surface of the ground, with the head drawn up underneath
the mantle, which was lifted just above the reproductive orifice. When
taken into the hand, it went on laying eggs without interruption or agitation
of any kind. After it had finished laying it ate half a raw potato and then
took a bath, remaining submerged for more than an hour. Bathing is a
favourite pastime at all periods. Specimens, says Mr. Wotton, have survived
a compulsory bath, with total submersion, of nearly three days’ duration.
Mr. Wotton’s account of the hatching of the eggs is equally interesting. It
is noticeable that the eggs of one batch do not hatch by any means
simultaneously; several days frequently intervene. The average period is
about 60 days, a damp and warm situation bringing out the young in 40
days, while cold and dryness extended the time to 74 days, extremes of any
kind proving fatal. Of the batch of eggs laid by B on 30th November, the
first 2 were hatched on the following 16th January, and 2 more on the 17th;
others, from 10 to 20, followed suit on the succeeding 5 days, until 82 in all
were hatched, the remaining 19 being unproductive.[96]

By placing the egg on a looking-glass the act of exclusion can be
perfectly observed. For several days the inmate can be seen in motion, until
at last a small crack appears in the surface of the shell: this gradually
enlarges, until the baby slug is able to crawl out, although it not
unfrequently backs into the shell again, as if unwilling to risk itself in the
world. When it once begins to crawl freely, it buries itself in the ground for
4 or 5 days without food, after which time it emerges, nearly double its
original size. At exclusion, the average length is 9 mm., increasing to 56
mm. after the end of 5 months. Full growth is attained about the middle of
the second year, and nearly all die at the end of this year or the beginning of
the next. Death from exhaustion frequently occurs after parturition. Death
from suffocation is sometimes the result of the formation of small blisters
on the margin of the respiratory aperture. The attacks of an internal parasite
cause death in a singular way. The upper tentacles swell at the base in such
a way as to prevent their extrusion; digestive troubles follow, with rigidity
and loss of moisture, and death ensues in 2 or 3 days.
Mr. Wotton isolated newly-hatched specimens, with the view of
experimenting on their power of self-fertilisation, if the opportunity of
fertilising and being fertilised by others was denied them. One of these,
after remaining in absolute solitude for 10½ months, began to lay, scantily
at first (11th January, 2; 25th January, 2; 11th February, 2), but more
abundantly afterwards (3rd April, 60; 15th and 16th, 70; 29th, 53, etc.), the
eggs being hatched out in 42–48 days. The precautions taken seem to have
been absolutely satisfactory, and the fact of the power of self-fertilisation
appears established as far as Arion ater is concerned.
Braun took young individuals of Limnaea auricularia on the day they
were hatched out, and placed them singly in separate vessels with differing
amounts of water. This was on 15th June 1887. In August 1888 specimen A
had only produced a little spawn, out of which three young were hatched;
specimen B had produced four pieces of spawn of different sizes, all of
which were hatched; specimen C, which happened to be living with three
Planorbis, produced five pieces of spawn distinctly Limnaeidan, but
nothing is recorded of their hatching. Self-impregnation, therefore, with a
fruitful result, appears established for this species of Limnaea.[97]
Reproduction of Lost Parts.—When deprived of their tentacles, eyes,
or portions of the foot, Mollusca do not seem to suffer severely, and

generally reproduce the lost parts in a short time. If, however, one of the
ganglia is injured, they perish. Certain of the Mollusca possess the curious
property of being able to amputate certain parts at will. When Prophysaon,
a species of Californian slug, is annoyed by being handled, an indented line
appears at a point about two-thirds of the length from the head, the line
deepens, and eventually the tail is shaken completely off. Sometimes the
Prophysaon only threatens this spontaneous dismemberment; this line
appears (always exactly in the same place), but it thinks better of it, and the
indentation proceeds no further.[98] According to Gundlach,[99] Helix
imperator and H. crenilabris, two large species from Cuba, possess the
same property, which is said to be also characteristic of the sub-genus
Stenopus (W. Indies). Amongst marine species, Harpa ventricosa and Solen
siliqua have been observed to act in a similar way, Harpa apparently cutting
off the end of the foot by pressure of the shell. Karl Semper, in commenting
on the same property in species of Helicarion from the Philippines (which
whisk their tail up and down with almost convulsive rapidity, until it drops
off), considers[100] it greatly to the advantage of the mollusc, since any
predacious bird which attempted to seize it, but only secured a fragment of
tail, would probably be discouraged from a second attack, especially as the
Helicarion would meanwhile have had time to conceal itself among the
foliage.
Strength and Muscular Force.—The muscular strength of snails is
surprisingly great. Sandford relates[101] an experiment on a Helix aspersa,
weighing ¼ oz. He found it could drag vertically a weight of 2¼ oz., or nine
times its own weight. Another snail, weighing ⅓ oz., was able to drag in a
horizontal direction along a smooth table twelve reels of cotton, a pair of
scissors, a screwdriver, a key, and a knife, weighing in all no less than 17
oz., or more than fifty times its own weight. This latter experiment was
much the same as asking a man of 12 stone to pull a load of over 3¾ tons.
If a snail be placed on a piece of glass and made to crawl, it will be seen
that a series of waves appear to pursue one another along the under surface
of the foot, travelling from back to front in the direction in which the animal
is moving. Simroth has shown that the sole of the foot is covered with a
dense network of muscular fibres, those which run longitudinally being
chiefly instrumental in producing the undulatory motion. By means of these
muscles the sole is first elongated in front, and then shortened behind to an

equal extent. Thus a snail slides, not on the ground, but on its own mucus,
which it deposits mechanically, and which serves the purpose of lubricating
the ground on which it travels. It has been calculated that an averaged sized
snail of moderate pace progresses at the rate of about a mile in 16 days 14
hours.[102]
Sudden Appearance of Mollusca.—It is very remarkable to notice how
suddenly Pulmonata seem to appear in certain districts where they have not
been noticed before. This sudden appearance is more common in the case of
fresh-water than of land Mollusca, and there can be little doubt that,
wherever a new pond happens to be formed, unless there is something in its
situation or nature which is absolutely hostile to molluscan life, Mollusca
are certain to be found in it sooner or later. “Some 23 years ago,” writes Mr.
W. Nelson,[103] “I was in the habit of collecting shells in a small pond near
to the Black Hills, Leeds. At that time the only molluscan forms found there
were a dwarf form of Sphaerium lacustre, Pisidium pusillum, Planorbis
nautileus, and Limnaea peregra. About 10 years ago I resumed my visits to
the locality, and found, in addition to the species already enumerated,
Planorbis corneus. These were the only species found there until this spring
[1883], when, during one of my frequent visits, I was surprised to find
Physa fontinalis and Planorbis vortex were added to the growing list of
species. Later on Pl. carinatus, Limnaea stagnalis, and Ancylus lacustris
turned up; and during June, Pl. contortus was found in this small but
prolific pond.” Limnaea glutinosa is prominent for these remarkable
appearances and disappearances. In 1822 this species suddenly appeared in
some small gravel pits at Bottisham, Cambs., in such numbers that they
might have been scooped out by handfuls. After that year they did not
appear numerous, and after three or four seasons they gradually
disappeared.[104] Physa (Aplecta) hypnorum is noted in a similar way. In
February 1852, for instance, after a wet month, the water stood in small
puddles about 3 feet by 2 in a particular part of Bottisham Park which was
sometimes a little swampy, though usually quite dry. One of these puddles
was found to contain immense numbers of the Aplecta, which up to that
time had not been noted as occurring in Cambridgeshire at all.[105] In a few
days the species entirely disappeared and was never again noticed in the
locality.[106]

Writing to the Zoological Society of London from New Caledonia, Mr.
E. L. Layard remarks:[107] “The West Indian species Stenogyra octona has
suddenly turned up here in thousands; how introduced, none can tell. They
are on a coffee estate at Kanala on the east coast. I have made inquiries, and
cannot find that the planter ever had seed coffee from the West Indies. All
he planted came from Bombay, and it would be interesting to find out
whether the species has appeared there also.”
Sometimes a very small event is sufficient to disturb the natural
equilibrium of a locality, and to become the cause either of the introduction
or of the destruction of a species. In 1883 a colony of Helix sericea
occupied a portion of a hedge bottom twenty yards long near Newark. It
scarcely occurred outside this limit, but within it was very plentiful, living
in company with H. nemoralis, H. hortensis, H. hispida, H. rotundata,
Hyalinia cellaria and Hy. nitidula, and Cochlicopa lubrica. In 1888 the
hedge was well trimmed, but the bottom was not touched, and the next year
a long and careful search was required to find even six specimens of the
sericea.[108]
Showers of Shells.—Helix virgata, H. caperata, and Cochlicella acuta
sometimes occur on downs near our sea-coasts in such extraordinary
profusion, that their sudden appearance out of their hiding-places at the
roots of the herbage after a shower of rain has led to the belief, amongst
credulous people, that they have actually descended with the rain. There
seems, however, no reason to doubt that Mollusca may be caught up by
whirlwinds into the air and subsequently deposited at some considerable
distance from their original habitat, in the same way as frogs and fishes. A
very recent instance of such a phenomenon occurred[109] at Paderborn, in
Westphalia, where, on 9th August 1892, a yellowish cloud suddenly
attracted attention from its colour and the rapidity of its motion. In a few
moments it burst, with thunder and a torrential rain, and immediately
afterwards the pavements were found to be covered with numbers of
Anodonta anatina, all of which had the shell broken by the violence of the
fall. It was clearly established that the shells could not have been washed
into the streets from any adjacent river or pond, and their true origin was
probably indicated when it was found that the funnel-shaped cloud which
burst over the town had passed across the one piece of water near
Paderborn, which was known to contain the Anodonta in abundance.

Cases of Singular Habitat.—Mollusca sometimes accustom themselves
to living in very strange localities, besides the extremes of heat and cold
mentioned above (pp. 23–24). In the year 1852, when some large
waterpipes in the City Road, near St. Luke’s Hospital, were being taken up
for repairs, they were found to be inhabited in considerable numbers by
Neritina fluviatilis and a species of Limnaea.[110] Dreissensia polymorpha
has been found in a similar situation in Oxford Street, and also in Hamburg,
and has even been known to block the pipes and cisterns of private houses.
In an engine cistern at Burnley, 60 feet above the canal from which the
water was pumped into the cistern, were found the following species:
Sphaerium corneum, S. lacustre; Valvata piscinalis, Bithynia tentaculata;
Limnaea peregra, very like Succinea in form and texture; Planorbis albus,
P. corneus, P. nitidus, P. glaber, and thousands of P. dilatatus, much larger
than the forms in the canal below, a fact probably due to the equable
temperature of the water in the cistern all the year round.[111] In certain
parts of southern Algeria the fresh-water genera Melania and Melanopsis
inhabit abundantly waters so surcharged with salt that the marine Cardium
edule has actually become extinct from excess of brine. The common
Mytilus edulis is sometimes found within the branchial chamber and
attached to the abdomen of crabs (Carcinus maenas), which are obliged to
carry about a burden of which they are powerless to rid themselves (see p.
78). A variety of the common Limnaea peregra lives in the hot water of
some of the geysers of Iceland, and has accordingly been named
geisericola.
Underground Snails.—Not only do many of the land Mollusca
aestivate, or hibernate, as the case may be, beneath the surface of the soil,
but a certain number of species live permanently underground, like the
mole, and scarcely ever appear in the light of day. Our own little
Caecilianella acicula lives habitually from 1 to 3 feet below ground,
appearing to prefer the vicinity of graveyards. Testacella, the carnivorous
slug, scarcely ever appears on the surface during the day, except when
driven by excessive rain, and even then it lurks awhile under some
protecting cover of leafage. There is a curious little Helix (tristis Pfr.),
peculiar to Corsica, which is of distinctly subterranean habits. It lives in
drifted sand above high-water mark, always at the roots of Genista
Saltzmanni, at a depth which varies with the temperature and dryness of the

air. In hot and very dry weather it buries itself nearly 2 feet below the
surface, only coming up during rain, and burying itself again immediately
the rain is over. Like a Solen, it often has a hole above its burrow, by which
it communicates with the air above, so as to avoid being stifled in the sand.
The animal, in spite of its dry habitat, is singularly soft and succulent, and
exudes a very glutinous mucus. It probably descends in its burrow until it
arrives at the humid stratum, the persistence of which is due to the
capillarity of the sand.[112] I am assured by Mr. E. L. Layard that precisely
similar underground habits are characteristic of Coeliaxis Layardi, which
lives exclusively in sand at the roots of scrub and coarse grass at East
London.
Rock-boring Snails.—Cases have sometimes been recorded, from
which it would appear that certain species of snails possess the power of
excavating holes in rocks to serve as hiding-places. At Les Bois des Roches,
ten miles from Boulogne, occur a number of solid calcareous rocks
scattered about in the wood. The sides of the rocks which face N.E. and E.
are covered with multitudes of funnel-shaped holes, 1½ inch in diameter at
the opening and contracting suddenly within to ½ inch. Sometimes the
holes are 6 inches deep, and terminate, after considerable windings, in a
cup-shaped cavity. Helix hortensis inhabits these holes, and has been
observed to excavate them at the rate of ½ inch each hibernation, choosing
always the side of the rock which is sheltered from the prevailing rains. It
does not form an epiphragm, but protrudes part of its body against the rock.
That the snails secrete an acid which acts as a solvent seems probable from
the fact that red litmus paper, on being applied to the place where the foot
has been, becomes stained with violet.[113] Helix aspersa is said to excavate
holes 10 to 12 cm. deep at Constantine,[114] and H. Mazzullii is recorded as
perforating limestone at Palermo.[115]
Snails as Barometers.—An American writer of more than thirty years
ago[116] gave his experience of Helices as weather-prophets. According to
him, H. alternata is never seen abroad except shortly before rain; it then
climbs on the bark of trees, and stations itself on leaves. Helix clausa, H.
ligera, H. pennsylvanica, and H. elevata climb trees two days before rain, if
it is to be abundant and continuous. Succinea does the same, and its body is
yellow before rain and bluish after it. Several of the Helices assume a

sombre colour after rain, when their bodies are exceedingly humid; after the
humidity has passed off they resume a clearer and lighter tint.
Production of Musical and other Sounds.—Certain molluscs are said
to be capable of producing musical sounds. Sir J. E. Tennent describes his
visit to a brackish-water lake at Batticaloa, in Ceylon, where the fishermen
give the name of the ‘crying shell’ to the animal supposed to produce the
sounds. “The sounds,” he says,[117] “came up from the water like the gentle
thrills of a musical chord, or the faint vibrations of a wineglass when its rim
is rubbed by a moistened finger. It was not one sustained note, but a
multitude of tiny sounds, each clear and distinct in itself; the sweetest treble
mingling with the lowest bass. On applying the ear to the woodwork of the
boat, the vibration was greatly increased in volume. The sounds varied
considerably at different points as we moved across the lake, and
occasionally we rowed out of hearing of them altogether.” According to the
fishermen, the shells were Pyrazus palustris and Littorina laevis. It appears
uncertain whether the sounds are really due to Mollusca. Fishermen in other
parts of India assert that the sounds are made by fish, and, like those in
Ceylon, produce the fish which they say ‘sings.’ The same, or a similar
sound, has also been noticed to issue from the water in certain parts of
Chili, and on the northern shores of the Gulf of Mexico. Dendronotus
arborescens, when confined in a glass jar of sea water, has been noticed[118]
to emit a sound like the clink of a steel wire. According to Lieut.-Col.
Portlock,[119] F.R.S., Helix aperta, a very common species in South Europe,
has the property of emitting sounds when irritated. When at Corfu, he
noticed that if the animal is irritated by a touch with a piece of straw or
other light material, it emits a noise, as if grumbling at being disturbed. He
kept a specimen in his house for a considerable time, which would make
this noise whenever it was touched.
The Rev. H. G. Barnacle describes the musical properties of Achatinella
in the following terms:[120] “When up the mountains of Oahu I heard the
grandest but wildest music, as from hundreds of Aeolian harps, wafted to
me on the breezes, and my companion (a native) told me it came from, as
he called them, the singing shells. It was sublime. I could not believe it, but
a tree close at hand proved it. On it were many of the Achatinella, the
animals drawing after them their shells, which grated against the wood and

so caused a sound; the multitude of sounds produced the fanciful music. On
this one tree I took 70 shells of all varieties.”
Habits of the Agnatha.—Not much is known of the habits and mode of
life of the Agnatha, or carnivorous Land Mollusca. In this country we have
only two, or at most three, of this group, belonging to the genus Testacella,
and, in all probability, not indigenous to our shores. There seems little
doubt, when all the circumstances of their discovery are taken into account,
that both Testacella haliotidea and T. Maugei have been imported, perhaps
from Spain or Portugal in the first instance, along with roots imbedded in
foreign earth, for their earliest appearances can almost invariably be traced
back to the neighbourhood of large nursery grounds, or else to gardens
supplied directly from such establishments.
The underground life of Testacella makes observation of its habits
difficult. It is believed to feed exclusively on earthworms, which it pursues
in their burrows. Continued wet weather drives it to the surface, for though
loving damp soil it is decidedly averse to too much moisture, and under
such circumstances it has even been noticed[121] in considerable numbers
crawling over a low wall. In the spring and autumn months, according to
Lacaze-Duthiers,[122] it comes to the surface at night, hiding itself under
stones and débris during the day. Earthworms are, at these periods, nearer
the surface, and the Testacella has been seen creeping down into their
burrows. The author has taken T. Maugei abundantly under clumps of the
common white pink in very wet weather, lying in a sort of open nest in the
moist earth. On the other hand, when the earth is baked dry by continued
drought, they either bury themselves deeper, sometimes at a depth of 3 feet,
in the ground, or else become encysted in a capsule of hardened mucus to
prevent evaporation from the skin. When first taken from the earth and
placed in a box, the Testacella invariably resents its capture by spitting up
the contents of its stomach in the shape of long fragments of half-digested
worms.

Fig. 20.—Testacella haliotidea Drap., protruding its pharynx (ph) and radula (r);
oe, oesophagus; p.o, pulmonary orifice; sh, shell; t, tentacles (after Lacaze-
Duthiers).

It appears not to bite the worm up before swallowing it, but contrives, in
the most remarkable manner, to take down whole worms apparently much
too large for its stomach. Mr. Butterell relates[123] that, after teasing a
specimen of T. Maugei, and making it emit a quantity of frothy mucus from
the respiratory aperture, he procured a worm of about three inches long, and
rubbed the worm gently across the head of the Testacella. The tongue was
rapidly extended, and the victim seized. The odontophore was then
withdrawn, carrying with it the struggling worm, which made every effort
to escape, but in vain; in about five minutes all had disappeared except the
head, which was rejected. This protrusion of the tongue (radula) and indeed
of the whole pharynx, is a very remarkable feature in the habits of the
animal. It appears, as it were, to harpoon its prey by a rapid thrust, and
when the victim is once pierced by a few of the powerful sickle-shaped
teeth (compare chap. viii.) it is slowly but surely drawn into the oesophagus
(Fig. 20).
Most gardeners are entirely ignorant of the character of Testacella, and
confuse it, if they happen to notice it at all, with the common enemies of
their tender nurslings. Cases have been known, however, when an
intelligent gardener has kept specimens on purpose to kill worms in
ferneries or conservatories. In some districts these slugs are very numerous;
Lacaze-Duthiers once dug 182 specimens from a good well-manured piece
of ground whose surface measured only ten square yards.
Towards the end of September or beginning of October the period of
hibernation begins. I infer this from the behaviour of specimens kept in
captivity, which, for about a fortnight before this time, gorged themselves
inordinately on as many worms as I chose to put into their box, and then
suddenly refused food, buried themselves deeply in the earth, and appeared

no more during the winter. The eggs are apparently much less numerous
than is the case with Limax or Helix, and very large, measuring about ⅙
inch in diameter. They are enveloped in a remarkably tough and elastic
membrane, and, if dropped upon any hard surface, rebound several inches,
just like an india-rubber ball.
The animal creeps rather rapidly, and has the power of elongating its
body to a remarkable extent. When placed on the surface of the ground, in
the full light of day, it soon betrays uneasiness, and endeavours to creep into
concealment. Its method of burying itself is very interesting to watch. It
first elongates its neck and inserts its head into the soil; gradually the body
begins to follow, while the tail tilts upwards into the air. No surface motion
of the skin, no writhing or wriggling motion of any kind occurs; the
creature simply works its way down in a stealthy and mysterious way, until
at last it is lost to view.
The great Glandina, which attain their maximum development in
Mexico and the southern United States, are a very noticeable family in this
group. According to Mr. Binney,[124] Glandina truncata Gmel., one of the
commonest species of the genus, is somewhat aquatic in its habits. It is
found in the sea islands of Georgia and around the keys and everglades of
Florida, where it attains a maximum length of 4 inches, while in less humid
situations it scarcely measures more than 1 inch. It occurs most abundantly
in the centre of clumps and tussocks of coarse grass in marshes close to the
sea-coast. By the action of the sharp, sickle-shaped teeth of its radula the
soft parts of its prey (which consists chiefly of living Helices) are rapidly
rasped away; sometimes they are swallowed whole. It has been known to
attack Limax when confined in the same box, rasping off large pieces of the
integument. In one case an individual was noticed to devour one of its own
species, thrusting its long neck into the interior of the shell, and removing
all the viscera.

Fig. 21.—Glandina sowerbyana Pfr. (Strebel).

The Glandinae of southern Europe, although scarcely rivalling those of
Central America in size or beauty, possess similar carnivorous propensities.
Glandina Poireti has been observed,[125] on Veglia Island, attacking a living
Cyclostoma elegans. By its powerful teeth it filed through two or three
whorls of the shell of its victim, and then proceeded to devour it, exactly in
the same manner as a Natica or Buccinum perforates the shell of a Tellina or
Mactra in order to get at its contents.
Few observations appear to have been made on the habits or food of
Streptaxis, Rhytida, Ennea, Daudebardia, Paryphanta, and other
carnivorous Mollusca. A specimen of Ennea sulcata, enclosed in the same
box as a Madagascar Helix (sepulchralis Fér.) many times its own size,
completely emptied the shell of its inhabitant.[126] Mr. E. L. Layard informs
me that certain Cape Rhytida, e.g. R. capsula Bens., R. dumeticola Bens.,
and R. vernicosa Kr., eat Cyclostoma affine, Helix capensis, H. cotyledonis,
etc. To Mr. Layard I am also indebted for the—perhaps apocryphal—
tradition that the best time to capture the great Aerope caffra Fér. in
numbers was after an engagement between the Kaffirs and Zulus, when
they might be observed streaming from all points of the compass towards
the field of slaughter. The Cuban Oleacina are known to secrete a very
bitter fluid which they emit; this perhaps produces a poisoning or
benumbing effect upon their victims when seized. They devour operculates,
e.g. Helicina regina and sagraiana.[127]

CHAPTER III
ENEMIES OF THE MOLLUSCA—MEANS OF DEFENCE—MIMICRY AND PROTECTIVE
COLORATION—PARASITIC MOLLUSCA—COMMENSALISM—VARIATION

Enemies of the Mollusca

The juicy flesh and defenceless condition of many of the Mollusca make
them the favourite food and often the easy prey of a host of enemies besides
man. Gulls are especially partial to bivalves, and may be noticed, in our
large sandy bays at the recess of the tide, busily devouring Tellina, Mactra,
Mya, Syndosmya, and Solen. On the Irish coast near Drogheda a herring
gull has been observed[128] to take a large mussel, fly up with it in the air
over some shingly ground and let it fall. On alighting and finding that the
shell was unbroken it again took it up and repeated the process a number of
times, flying higher and higher with it until the shell was broken. Hooded
crows, after many unavailing attempts to break open mussels with their
beak, have been seen to behave in a similar way.[129] Crows, vultures, and
aquatic birds carry thousands of mussels, etc., up to the top of the
mountains above Cape Town, where their empty shells lie in enormous
heaps about the cliffs.[130]
The common limpet is the favourite food of the oyster-catcher, whose
strong bill, with its flattened end, is admirably calculated to dislodge the
limpet from its seat on the rock. When the limpet is young, the bird
swallows shell and all, and it has been calculated that a single flock of
oyster-catchers, frequenting a small Scotch loch, must consume hundreds of
thousands of limpets in the course of a single year. Rats are exceedingly
fond of limpets, whose shells are frequently found in heaps at the mouth of
rat holes, especially where a cliff shelves gradually towards a rocky shore.
A rat jerks the limpet off with a sudden movement of his powerful jaw, and,
judging from the size of the empty shells about the holes, has no difficulty
in dislodging the largest specimens. ‘I once landed,’ relates a shepherd to
Mr. W. Anderson Smith,[131] ‘on the I. of Dunstaffnage to cut grass, and it
was so full of rats that I was afraid to go on; and the grass was so full of
limpets that I could scarcely use the scythe, and had to keep sharpening it
all the time.’ Sometimes, however, the limpet gets the better both of bird
and beast. The same writer mentions the case of a rat being caught by the

lip by a limpet shell, which it was trying to dislodge. A workman once
observed[132] a bird on Plymouth breakwater fluttering in rather an
extraordinary manner, and, on going to the spot, found that a ring dotterel
had somehow got its toe under a limpet, which, in closing instantly to the
rock, held it fast. Similar cases of the capture of ducks by powerful bivalves
are not uncommon, and it is said that on some parts of the American coasts,
where clams abound, it is impossible to keep ducks at all,[133] for they are
sure to be caught by the molluscs and drowned by the rising tide.
The Weekly Bulletin of San Francisco, 17th May 1893, contains an
account of the trapping of a coyote, or prairie wolf, at Punta Banda, San
Diego Co., by a Haliotis Cracherodii. The coyote had evidently been
hunting for a fish breakfast, and finding the Haliotis partially clinging to the
rock, had inserted his muzzle underneath to detach it, when the Haliotis
instantly closed down upon him and kept him fast prisoner.
Rats devour the ponderous Uniones of North America. When Unio
moves, the foot projects half an inch or more beyond the valves. If, when in
this condition, the valves are tightly pinched, the foot is caught, and if the
pinching is continued the animal becomes paralysed and unable to make use
of the adductor muscles, and consequently flies open even if the pressure is
relaxed. The musk-rat (Fiber zibethicus) seizes the Unio in his jaws, and by
the time he reaches his hole, the Unio is ready to gape.[134] Rats also eat
Vivipara, and even Limnaea, in every part of the world.
Every kind of slug and snail is eaten greedily by blackbirds, thrushes,
chaffinches, and in fact by many species of birds. A thrush will very often
have a special sacrificial stone, on which he dashes the shells of Helix
aspersa and nemoralis, holding them by the lip with his beak, until the
upper whorls are broken; heaps of empty shells will be found lying about
the place of slaughter. The bearded Titmouse (Parus biarmicus) consumes
quantities of Succinea putris and small Pupa, which are swallowed whole
and become triturated in the bird’s stomach by the aid of numerous angular
fragments of quartz.[135]
Frogs and toads are very partial to land Mollusca. A garden attached to
the Laboratory of Agricultural Chemistry at Rouen had been abandoned for
three years to weeds and slugs. The director introduced 100 toads and 90
frogs, and in less than a month all the slugs were destroyed, and all kinds of

vegetables and flowers, whose cultivation had until then been impossible,
were enabled to flourish.[136]
Certain Coleoptera are known to prey upon Helices and other land
Mollusca. Récluz noticed, near Agde, a beetle (Staphylinus olens) attack
Helix ericetorum when crawling among herbage, sticking its sharp
mandibles into its head. Every time the snail retreated into its shell the
beetle waited patiently for its reappearance, until at last the snail succumbed
to the repeated assaults. M. Lucas noticed, at Oran, the larva of a Drilus
attacking a Cyclostoma. The Drilus stood sentinel at the mouth of a shell,
which was closed by the operculum, until the animal began to issue forth.
The Drilus then with its mandibles cut the muscle which attaches the
operculum to the foot, disabling it sufficiently to prevent its being securely
closed, upon which it entered and took possession of the body of its
defenceless host, completing its metamorphosis inside the shell, after a
period of six weeks.[137] The female glow-worm (Lampyris noctiluca)
attacks and kills Helix nemoralis.
Among the Clavicornia, some species of Silpha carry on a determined
warfare against small Helices. They seize the shell in their mandibles, and
then, throwing their head backwards, break the shell by striking it against
their prothorax.
The common water beetle, Dytiscus marginalis, from its strength and
savage disposition, is a dangerous enemy to fresh-water Mollusca. One
Dytiscus, kept in an aquarium, has been noticed to kill and devour seven
Limnaea stagnalis in the course of one afternoon. The beetles also eat L.
peregra, but apparently prefer stagnalis, for when equal quantities of both
species were placed within their reach, they fixed on the latter species first.
[138]

In East Africa a species of Ichneumon (Herpestes fasciatus) devours
snails, lifting them up in its forepaws and dashing them down upon some
hard substance.[139] In certain islands off the south coasts of Burmah, flat
rocks covered with oysters are laid bare at low tide. A species of Monkey
(Macacus cynomolgus) has been noticed to furnish himself with a stone,
and knock the oysters open, always breaking the hinge-end first, and then
pulling out the mollusc with his fingers.[140]

The walrus is said to support himself almost entirely on two species of
Mya (truncata and arenaria), digging them out of the sand, in which they
live buried at a depth of about 1½ feet, with his powerful tusks. Whales
swallow enormous numbers of pelagic molluscs (Clio, Limacina), which
are at times so abundant in the Arctic seas, as to colour the surface for
miles. Many of the larger Cetacea subsist in great part on Cephalopoda; as
many as 18 lbs. of beaks of Teuthidae have been taken from the stomach of
a single Hyperoodon.
Fish are remarkably partial to Mollusca of various kinds. The cat-fish
(Chimaera) devours Pectunculus and Cyprina, crushing the stout shells
with its powerful jaws, while flounders and soles content themselves with
the smaller Tellina and Syndosmya which they swallow whole. As many as
from 30 to 40 specimens of Buccinum undatum have been taken from the
stomach of a single cod, and the same ‘habitat’ has been recorded for some
of the rarer whelks, e.g. Bucc. humphreysianum, Fusus fenestratus, the
latter also occurring as the food of the haddock and the red gurnard. No less
than 35,000 Turtonia minuta have been found in the stomach of a single
mullet. Nudibranchs are no doubt dainty morsels for fish, and hence have
developed, in many cases, special faculties for concealment, or, if
distasteful, special means of remaining conspicuous (see pp. 71–74).

Fig. 22.—Two valves of Mytilus
edulis L., representing
diagrammatically the
approximate position of the holes
bored by Purpura in about 100
specimens of Mytilus, gathered at
Newquay, Cornwall.

Besides the dangers to which they are exposed from other enemies,
many of the weaker forms of Mollusca fall a prey to their own brethren.
Nassa and Murex on this side of the Atlantic, and Urosalpinx on the other,
are the determined foes of the oyster. Purpura lapillus prefers Mytilus
edulis to any other food, piercing the shell in about two days’ time by its
powerful radula, which it appears to employ somewhat in gimlet fashion. If
Mytilus cannot be procured, it will eat Littorina or Trochus, but its attempts
on the hard shell of Patella are generally failures. The statement which is
sometimes made, that the Purpura makes its hole over the vital parts of the
Mytilus, appears, according to the evidence embodied in the annexed figure,
to be without foundation. The fact is that a hole in any part of its shell is
fatal to the Mytilus, since the long proboscis of the Purpura, having once
made an entrance, can reach from one end of the shell to the other. The
branchiae are first attacked, the adductor muscles and edges of the mantle
last. Natica and Nassa pierce in a similar way the shells of Mactra, Tellina,
Donax, and Venus. Murex fortispina is furnished with a powerful tooth at
the lower part of its outer lip. At Nouméa, in New Caledonia, its favourite
food is Arca pilosa, which lives half buried in coral refuse. The Murex has
been seen to drag the Arca from its place of concealment, and insert the
tooth between the valves, so as to prevent their closing, upon which it was
enabled to devour its prey at leisure.[141]
The carnivorous land Mollusca, with the exception of Testacella, appear
to feed by preference upon other snails (pp. 54, 55).
Parasitic Worms, Mites, etc.—A considerable number of the
Trematode worms pass one or more of the stages in the cycle of their
development within the bodies of Mollusca, attaining to the more perfect or
sexual form on reaching the interior of some vertebrate. Thus Distoma
endolabum Duj. finds its first intermediate host in Limnaea stagnalis and L.
ovata, its second in L. stagnalis, or in one of the fresh-water shrimps
(Gammarus pulex), or in the larvae of one of the Phryganeidae
(Limnophilus rhombicus), attaining to the sexual form in the common frog.
Distoma ascidia v. Ben. passes firstly through Limnaea stagnalis or
Planorbis corneus, secondly through certain flies and gnats (Ephemera,
Perla, Chironomus), and finally arrives within certain species of bats.
Distoma nodulosum Zed. inhabits firstly Paludina impura, secondly certain
fishes (Cyprinus Acerina), and lastly the common perch. The sporocyst of

Distoma macrostomum inhabits Succinea putris, pushing itself up into the
tentacles, which become unnaturally distended (Fig. 23). While in this
situation it is swallowed by various birds, such as the thrush, wagtail, and
blackbird, which are partial to Succinea, and thus obtains lodgment in their
bodies. Amphistoma subclavatum spends an early stage in Planorbis
contortus, after which it becomes encysted on the skin of a frog. When the
frog sheds its skin, it swallows it, and with it the Amphistoma, which thus
becomes established in the frog’s stomach.[142]

Fig. 23.—A Trematode worm
(Leucochloridium paradoxum Car.)
parasitic in the tentacles of Succinea
putris L. × 20 (after Baudon).

The common liver-fluke, which in the winter of 1879–1880 cost Great
Britain the lives of no less than three million sheep, is perhaps the best
known of these remarkable parasitic forms of life. Its history shows us, in
one important particular, how essential it is for the creature to meet, at
certain stages of its existence, with the exact host to which it is accustomed.
Unless the newly-hatched embryo finds a Limnaea truncatula within about
eight hours it becomes exhausted, sinks, and dies. It has been tried with all
the other common pond and river Mollusca, with Limnaea peregra,
palustris, auricularia, stagnalis, with Planorbis marginatus, carinatus,
vortex, and spirorbis, with Physa fontinalis, Bithynia tentaculata, Paludina
vivipara, as well as with Succinea putris, Limax agrestis and maximus,
Arion ater and hortensis. Not one of them would it touch, except
occasionally very young specimens of L. peregra, and in these its
development was arrested at an early stage. But on touching a L. truncatula

the embryo seems to know at once that it has got what it wants, and sets to
work immediately to bore its way into the tissues of its involuntary host,
making by preference for the branchial chamber; those which enter the foot
or other outlying parts of the Limnaea proceed no farther.[143]
Many similar cases occur, in which littoral Mollusca, such as Littorina
and Buccinum, form the intermediate host to a worm which eventually
arrives within some sea-bird.
Certain Nematode worms (Rhabditis) are known to inhabit the intestine
of Arion, and the salivary glands of Limax agrestis. Diptera habitually lay
their eggs within the eggs of Helix and Limax. Many species of mite
(Acarina) infest land Pulmonata. No adult Limax maximus is without at
least one specimen of Philodromus (?) limacum, and the same, or an allied
species, appears to occur on the larger of our Helices, retiring upon
occasion into the pulmonary chamber.
Several of the Crustacea live associated with certain molluscs.
Pinnotheres lives within the shell of Pinna, Ostrea, Astarte, Pectunculus,
and others. Apparently the females alone reside within the shell of their
host, while the males seize favourable opportunities to visit them there. A
specimen of the great pearl-oyster (Meleagrina margaritifera) was recently
observed which contained a male Pinnotheres encysted in nacre. It was
suggested that he had intruded at an unfortunate time, when no female of
his kind happened to be in, and that, having penetrated too far beneath the
mantle in the ardour of his search, was made prisoner before he could
escape.[144] Ostracotheres Tridacnae lives in the branchiae of the great
Tridacna. A little brachyurous crustacean inhabits the raft of Ianthina, and
assumes the brilliant blue colour of the mollusc.
Means of Defence

As a rule, among the Mollusca, the shell forms a passive mode of
resistance to the attacks of enemies. Bivalves are enabled, by closing their
valves, to baffle the assault of their smaller foes, and the operculum of
univalves, both marine and land, serves a similar purpose. Many land
Mollusca, especially Helix and Pupa, as well as a number of Auriculidae,
have the inside of the aperture beset with teeth, which are sometimes so
numerous and so large that it is puzzling to understand how the animal can

ever come out of its shell, or, having come out, can ever draw itself back
again. Several striking cases of these toothed apertures are given in Fig. 24.
Whatever may be the origin of these teeth, there can be little doubt that their
extreme development must have a protective result in opposing a barrier to
the entrance, predatory or simply inquisitive, of beetles and other insects.
Sometimes, it will be noticed (G), the aperture itself is fairly simple, but a
formidable array of obstacles is encountered a little way in. It is possible
that the froth emitted by many land snails has a similar effect in involving
an irritating intruder in a mass of sticky slime. The mucus of slugs and
snails, on the other hand, is more probably, besides its use in facilitating
locomotion, a contrivance for checking evaporation, by surrounding the
exposed parts of their bodies with a viscid medium.

Fig. 24.—Illustrating the elaborate arrangement of teeth in the aperture of
some land Pulmonata. A. Helix (Labyrinthus) bifurcata Desh., Equador.
B. H. (Pleurodonta) picturata Ad., Jamaica. C. H. (Dentellaria) nux
denticulata Chem., Demerara. D. Anostoma carinatum Pfr., Brazil; a,
tube communicating with interior of shell. E. H. (Stenotrema)
stenotrema Fér., Tennessee, × 3/2. F. H. (Polygyra) auriculata Say,
Florida, × 3/2. G. H. (Plectopylis) refuga Gld., Tenasserim (a and b ×
2).

Some species of Lima shelter themselves in a nest constructed of all
kinds of marine refuse, held together by byssiferous threads. Modiola

adriatica, M. barbata, and sometimes M. modiolus conceal themselves in a
similar way. Gastrochaena frequently encloses itself in a sort of half cocoon
of cement-like material. The singular genus Xenophora protects itself from
observation by gluing stones, shells, and various débris to the upper side of
its whorls (Fig. 25). Sometimes the selection is made with remarkable care;
the Challenger, for instance, obtained a specimen which had decorated its
body whorl exclusively with long and pointed shells (Fig. 26).

Fig. 25.—Xenophora (Phorus)
conchyliophora Born., concealed by
the stones which it glues to the
upper surface of its shell. (From a
British Museum specimen.)

Fig. 26.—Xenophora (Phorus) pallidula
Reeve. A mollusc which escapes
detection by covering itself with
dead shells of other species. (From a
Challenger specimen in the British
Museum, × ½.)

The formidable spines with which the shells, e.g. of the Murex family,
are furnished must contribute greatly to their protection against fishes, and
other predatory animals. Murex tenuispina, for instance (see chap. ix.),
would prove as dangerous a morsel in the mouth of a fish as a hedgehog in
that of a dog. Whether the singular tooth in the outer lip of Leucozonia (see
chap. xiv.), a feature which is repeated, to a less marked extent, in
Monoceros and several of the West Coast muricoids, is developed for
defensive purposes, cannot at present be decided.
The Strombidae possess the power of executing long leaps, which they
doubtless employ to escape from their foes. In their case alone this power is
combined with singular quickness of vision. On one occasion Mr. Cuming,
the celebrated collector, lost a beautiful specimen of Terebellum, by the
animal suddenly leaping into the water, as he was holding and admiring it in
his hand. Miss Saul has informed me that the first living specimen of
Trigonia that was ever obtained was lost in a similar way. It was dredged by
Mr. Stutchbury in Sydney Harbour, and placed on the thwart of a small
boat. He had just remarked to a companion that it must be a Trigonia, and
his companion had laughed at the idea, reminding him that all known
Trigonia were fossil, when the shell in question baffled their efforts to

discover its generic position by suddenly leaping into the sea, and it was
three months before Mr. Stutchbury succeeded in obtaining another.
Some genera possess more than merely passive means of defence. Many
Cephalopoda emit a cloud of inky fluid, which is of a somewhat viscous
nature, and perhaps, besides being a means of covering retreat, serves to
entangle or impede the pursuer. The formidable suckers and hooks
possessed by many genera in this Order are most dangerous weapons, both
for offence and defence. Aplysia, when irritated, ejects a purple fluid which
used to be considered dangerously venomous. Many of the Aeolididae,
including our own common Aeolis papillosa, possess stinging cells at the
end of their dorsal papillae, the effect of which is probably to render them
exceedingly distasteful to fish.
The common Vitrina pellucida has a curious habit which in all
probability serves for a defence against birds in the winter. When crawling
on the edge of a stone or twig it has the power of suddenly jerking its ‘tail,’
so as to throw itself on the ground, where it is probably lost to sight among
decaying leaves. At other times it rolls away a few inches and repeats the
jump. It also possesses the power of attaching to itself bits of leaves or soil,
which entirely cover and conceal both shell and animal.[145] The property of
parting with the tail altogether, a remarkable form of self-defence, has
already been noticed on p. 44.
The poisonous nature of the bite of certain species of Conus is well
authenticated. Surgeon Hinde, R.N., saw[146] a native on the I. of Matupi,
New Britain, who had been bitten by a Conus geographus, and who had at
once cut small incisions with a sharp stone all over his arm and shoulder.
The blood flowed freely, and the native explained that had he not taken
these precautions he would have died. Instances have been recorded of
poisonous wounds being inflicted by the bite of Conus aulicus, C. textile,
and C. tulipa. According to Mr. J. Macgillivray[147] C. textile at Aneitum (S.
Pacific) is called intrag, and the natives say it spits the poison upon them
from several inches off! Two cases of bites from C. textile occurred to this
gentleman’s notice, one of which terminated fatally by gangrene. Sir
Edward Belcher, when in command of the Samarang, was bitten[148] by a
Conus aulicus at a little island off Ternate in the Moluccas. As he took the
creature out of the water, it suddenly exserted its proboscis and inflicted a

wound, causing a sensation similar to that produced by the burning of
phosphorus under the skin. The wound was a small, deep, triangular mark,
succeeded by a watery vesicle. The natives of New Guinea have a
wholesome dread of the bite of Cones. Mr. C. Hedley relates[149] that while
collecting on a coral reef he once rolled over a boulder and exposed a living
C. textile. Before he could pick it up, one of the natives hastily snatched it
away, and explained, with vivid gesticulations, its hurtful qualities. On no
account would he permit Mr. Hedley to touch it, but insisted on himself
placing it in the bottle of spirits.

Fig. 27.—A tooth from the
radula of Conus imperialis
L., × 50, showing barb and
poison duct.

Mimicry and Protective Coloration.

Cases of Mimicry, or protective resemblance, when a species otherwise
defenceless adopts the outward appearance of a better protected species, are
rare among the Mollusca. Karl Semper[150] mentions an interesting case of
the mimicry of Helicarion tigrinus by Xesta Cumingii, in the Philippines. It
appears that all species of Helicarion possess the singular property of
shaking off the ‘tail’ or hinder part of the foot, when seized or irritated.
Specimens captured by collectors, Hel. tigrinus amongst them, have

succeeded in escaping from the hand, and concealing themselves, by a sort
of convulsive leap, among the dry leaves on the ground. This power of self-
amputation must be of great value to Helicarion, not only as enabling it to
escape from the clutch of its enemies, but also as tending to discourage
them from attempting to capture it at all. Now the genus Xesta is, in
anatomy, very far removed from Helicarion, and the majority of the species
are also, as far as the shell is concerned, equally distinct. Xesta Cumingii,
however, has, according to Semper, assumed the appearance of a
Helicarion, the thin shell, the long tail, and the mantle lobes reflected over
the shell; but it has not the power of parting with its tail at short notice. It
lives associated with Helicarion, and so close is the resemblance between
them that, until Semper pointed out its true position, it had always been
classified as a member of that group.
In the same passage Semper draws attention to two other cases of
apparent mimicry. The first is another species of Xesta (mindanaensis)
which closely resembles a species of Rhysota (Antonii), a genus not indeed
so far removed from Xesta as Helicarion, but, as far as the shell is
concerned, well distinguished from it. In this case, however, there is no
obvious advantage gained by the resemblance, since Rhysota as compared
with Xesta is not known to possess any definite point of superiority which it
would be worth while to counterfeit. A second case of resemblance between
certain species of the genus Chloraea and the characteristic Philippine
group Cochlostyla will not hold good as affording evidence of mimicry, for
Chloraea is now recognised as a sub-genus of Cochlostyla.
The Mollusca are not much mimicked by creatures of different
organisation. This appears at first sight strange, since it might have been
thought that the strong defensive house of a snail was worth imitating. Still
it is probably not easy for creatures bilaterally symmetrical to curl
themselves up into an elevated spiral for any length of time. One or two
instances, however, may be mentioned. The larva of a moth belonging to
the Psychidae, and occurring in France, Germany, the Tyrol, and Syria, coils
itself up into a sinistral spiral of three whorls, and is aptly named Psyche
helix, a kindred species from Italy being known as Ps. planorbis.
An insect larva (Cochlophora valvata) from E. Africa is said to resemble
a Valvata or young Cyclostoma. In this case the spiral is indifferently
dextral or sinistral, the ‘shell’ being formed of masticated vegetable matter,

united together by threads spun by the larva. Certain larvae of the
Phryganeidae (“Caddis-worms”) enclose themselves in houses which more
or less resemble a spiral shell, and have in some cases actually been
described as molluscan; such species, some of which belong to
Helicopsyche, have been noticed in S. Europe, Ceylon, Further India,
China, Tasmania, New Zealand, Tennessee, Mexico, Central America,
Venezuela, Brazil, and Argentina, and all[151] possess a dextral ‘shell.’ In all
these cases ‘mimicry’ is probably not so much to be thought of as the
practical advantages which accrue to the animal in question from the spiral
form, which gives it greater strength to resist external blows, and enables it
to occupy, during a very defenceless portion of its existence, a very small
amount of space.
The larva of some species of the Syrphidae (Diptera) fixes itself on the
under side of stones in the Tyrol, and closely resembles a small slug. The
naturalist Von Spix, in 1825, described to the Bavarian Academy as a new
genus of land Mollusca a somewhat similar larval form found in decaying
wood on the banks of a German lake.[152] Simroth mentions[153] a curious
case as occurring near Grimma. The caterpillars of certain Microlepidoptera
occur on slabs of porphyry, associated with a species of Clausilia. Besides
being of the same colour as the Clausiliae, the caterpillars have actually
developed cross lines on the back, i.e. on the side turned away from the
rock, in imitation of the suture of the mollusc.
It has been suggested[154] that there is mimicry between Aeolis papillosa
(a common British nudibranch) and Sagartia troglodytes (an Actinian), and
also between another species of Sagartia and Aeolidiella Alderi. The facts
observed are not sufficient to warrant a decided opinion, but it seems more
probable that the Actinian mimics the nudibranch than vice versâ, since
Aeolis is known to be unpalatable to fishes.

Fig. 28.—A, Strombus mauritianus Lam.,
which mimics Conus in shape. B,
Conus janus Hwass, Mauritius.

Certain species of Strombus (mauritianus L., luhuanus L.) show a
remarkable similarity in the shape of the shell to that of Conus, so much so,
that a tiro would be sure to mistake them, at first sight, for Cones. In the
case of S. luhuanus at least, this similarity is increased by the possession of
a remarkably stout brown epidermis. Now Conus is a flesh-eating genus,
armed with very powerful teeth which are capable of inflicting even on man
a poisonous and sometimes fatal wound (see p. 66). Strombus, on the other
hand, is probably frugivorous, and is furnished with weak and inoffensive
teeth. It is possible that this resemblance is a case of ‘mimicry.’ It is quite
conceivable that powerful fishes which would swallow a Strombus whole
and not suffer for it, might acquire a distaste for a Cone, which was capable
of lacerating their insides after being swallowed. And therefore the more
like a Cone the Strombus became, the better chance it would have of being
passed over as an ineligible article of food.
Protective coloration is not uncommon among the Mollusca. Littorina
obtusata is habitually found, on our own coasts, on Fucus vesiculosus, the
air-bladders of which it closely resembles in colour and shape. Littorina
pagodus, a large and showy species, resembles so closely the spongy
crumbling rocks of Timor, on which it lives, that it can hardly be discerned
a pace off. Helcion pellucidum, the common British ‘blue limpet,’ lives,
when young, almost exclusively on the iridescent leaves of the great

Laminariae, with the hues of which its own conspicuous blue lines
harmonise exactly. In mature life, when the Helcion invariably transfers its
place of abode to the lower parts of the stalk and finally to the root of the
Laminaria, which are quite destitute of iridescence, these blue lines
disappear or become much less marked.
The specimens of Purpura lapillus which occur at Newquay in Cornwall
are banded with rings of colour, especially with black and white, in a more
varied and striking way than any other specimens that have ever occurred to
my notice. I am inclined to refer this peculiarity to a tendency towards
protective coloration, since the rocks on which the Purpura occurs are often
banded with veins of white and colour, and variegated to a very marked
extent.
Ovula varies the colour of its shell from yellow to red, to match the
colour of the Gorgonia on which it lives. The same is the case with
Pedicularia, which occurs on red and yellow coral.
Helix desertorum, by its gray-brown colour, harmonises well with the
prevailing tint of the desert sands, among which it finds a home. Benson
observes that the gaudy H. haemastoma, which lives on the trunks of palm-
trees in Ceylon, daubs its shell with its excrement. Our own Buliminus
obscurus, which lives principally on the trunks of smooth-barked trees,
daubs its shell with mud, and must often escape the observation of its
enemies by its striking resemblance to the little knots on the bark, especially
of beech trees, its favourite haunt. Some species of Microphysa, from the
West Indies, habitually encrust their shells with dirt, and the same
peculiarity in Vitrina has already been mentioned. Ariophanta Dohertyi
Aldr., a recent discovery from Sumatra, is of a green colour, with a
singularly delicate epidermis; it is arboreal in its habits, and is almost
invisible amongst the foliage.[155] Many of our own slugs, according to
Scharff, are coloured protectively according to their surroundings. A claret-
coloured variety of Arion ater occurred to this observer only in pine woods,
where it harmonised with the general colouring of the ground and the pine-
needles, while young winter forms of the same species choose for hiding-
places the yellow fallen leaves, whose colour they closely resemble. Limax
marginatus (= arborum Bouch.) haunts tree trunks, and may easily be
mistaken for a piece of bark; Amalia carinata lives on and under the
ground, and in colour resembles the mould; Arion intermedius feeds almost

exclusively on fungi, to which its colour, which is white, gray, or light
yellow, tends to approximate it closely; Geomalacus maculosus conceals
itself by its striking resemblance to the lichens which grow on the surface of
rocks, and actually presumes on this resemblance so much as to expose
itself, contrary to the usual custom of its congeners, to the full light of the
afternoon sun.[156]
Several views have been advanced with regard to the dorsal papillae, or
cerata, in the Nudibranchs. Professor W. A. Herdman, who has examined a
considerable number of our own British species, in which these processes
occur, is of opinion[157] that they are of two quite distinct kinds. In the first
place, they may contain large offshoots, or diverticula, of the liver, and thus
be directly concerned in the work of digestion. This is the case with Aeolis
and Doto. In the second place, they may be simply lobes on the skin, with
no connexion with the liver, and no special function to perform. This is the
case with Tritonia, Ancula, and Dendronotus.
Professor Herdman is of opinion that although the cerata may in all cases
aid in respiration to a certain extent, yet that extent is so small as to be left
out of consideration altogether. He regards the cerata in both the two classes
mentioned above as “of primary importance in giving to the animals, by
their varied shapes and colours, appearances which are in some cases
protective, and in others conspicuous and warning.”
Thus, for instance, Tritonia plebeia, which is fairly abundant at Puffin
and Hilbre Is., appears always to be found creeping on the colonies of a
particular polyp, Alcyonium digitatum, and nowhere else. The specimens in
each colony of the polyp differ noticeably both in the matter of colour, and
of size, and of varied degrees of expansion. The Tritonia differs also, being
marked in varied tints of yellow, brown, blue, gray, black, and opaque
white, in such a way as to harmonise with the varied colours of the
Alcyonium upon which it lives. The cerata on the back of the Tritonia
contribute to this general resemblance. They are placed just at the right
distance apart, and are just the right size and colour, to resemble the crown
of tentacles on the half-expanded polyp.
Similarly, Doto coronata, which, when examined by itself, is a very
conspicuous animal, with showy, bright-coloured cerata, is found by
Professor Herdman to haunt no other situations but the under side of stones

and overhanging ledges of rock which are colonised by a hydroid, known as
Clava multicornis. The Doto is masked by the tentacles and clusters of
sporosacs on the zoophyte, with whose colouring and size its own cerata
singularly correspond. A similar and even more deceptive correspondence
with environment was noticed in the case of the very conspicuous
Dendronotus arborescens.
In these cases, the colouring and general shape of the cerata are
protective, i.e. they match their surroundings in such a way as to enable the
animal, in all probability, to escape the observation of its enemies.
According to Professor Herdman, however, the brilliant and showy
coloration of the cerata of Aeolis is not protective but ‘warning.’ Aeolis does
not hide itself away as if shunning observation, like Doto, Tritonia, and
Dendronotus; on the contrary, it seems perfectly fearless and indifferent to
being noticed. Its cerata are provided with sting-cells, like those of
Coelenterata, at their tips, and its very conspicuousness is a warning to its
enemies that they had better not try to attack it, just as the showy white tail
of the skunk acts as a sort of danger-signal to its own particular foes. It is
important for the Aeolis, not merely to be an unpalatable nettle in animal
shape, but also to be conspicuous enough to prevent its being experimented
upon as an article of food, in mistake for something less nasty.
Professor Herdman subsequently conducted some experiments[158] with
fishes, with the view of testing his theory that the shapes and colours of
Nudibranchs serve the purpose either of protection or warning, and bear
direct relation to the creature’s edibility. These experiments, on the whole,
distinctly tended to confirm the theory. Aeolis was evidently very nasty, and
probably stung the mouths of the fishes who tried it. For the complete
success of the theory, they ought to have let it severely alone, but the fish
were evidently accustomed to make a dash at anything that was dropped
into their tank. Another conspicuous mollusc, Ancula cristata, was
introduced, Professor Herdman and his collaborator each commencing
operations by eating a live specimen themselves. They found the taste
pleasant, distinctly like that of an oyster. The fish, however, when the
experiments were conducted under conditions which made the scene as
much like ‘real life’ as possible, did not agree with Professor Herdman. The
Ancula crawled over various parts of the tank for several days untouched by
the fish, who sometimes went close to them and looked at them, but never

attempted to taste them. Experiments with species whose colours were
protective, such as Dendronotus, were also conducted, and the decided
edibility of these species was established, the fish competing eagerly for
them, and tearing them rapidly to pieces.
Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological
Association, confirms[159] Professor Herdman’s views as to the shape and
colour of Opisthobranchs. Pleurobranchus membranaceus is known to
secrete, on the surface of the body, an acid which reddens blue litmus paper.
It is, therefore, no doubt distasteful to fish, which all abominate the taste of
acids, and is conspicuously marked with red-brown and yellowish
‘warning’ colours. Haminea and Philine, on the other hand, are good to eat,
and consequently possess ‘protective’ coloration. Runcina Hancocki, which
is of a brown colour, crawls over brown mud and weeds, but avoids green
weeds, on whose surface it would appear conspicuous. Elysia viridis varies
its colour according to its habitat, being green when on green weeds, and
dark olive, brown, or reddish brown, on pools among tufts of littoral algae.
Green specimens of Hermaea dendritica were kept in captivity, and placed
in a dish with green and red sea-weeds. They were never observed crawling
upon the red weed, upon which they would have been very conspicuous.
Archidoris flammea occurred on bright red sponges, to which its colour was
so closely assimilated that Mr. Garstang at first quite overlooked it.
Goniodoris castanea was found under stones, feeding on compound
Ascidians (Botryllus), which it sufficiently resembled to be very
inconspicuous in that position.
Again, Jorunna Johnstoni lives[160] upon stones on our southern coast,
associated with a certain sponge (Halichondria sp.), which it resembles so
closely in outline, in colour, in character of surface, and in its projecting
plumes, as to make it very difficult even for the careful observer to
distinguish the one from the other. And, since fishes, are known to be
distinctly averse to sponges of any kind as an article of food, this
resemblance must be decidedly to the advantage of the Jorunna. Another
Nudibranch (Calma glaucoides A. and H.) imitates the ova of certain fishes,
on which it feeds. Its elongated and depressed form of body, transparent
integuments, and silvery gray papillae combine to give it a strong
resemblance to the spawn of the fish, which is deposited on stones, the roots
of Laminaria, etc.[161]

The common Lamellaria perspicua appears to possess the power of
protectively assimilating its colour, markings, etc., to the Ascidians on
which it lives. A recent case, occurring off the Isle of Man, is thus described
by Professor Herdman.[162] “The mollusc was on a colony of Leptoclinum
maculatum, in which it had eaten a large hole. It lay in this cavity so as to
be flush with the general surface; and its dorsal integument was not only
whitish with small darker marks which exactly reproduced the appearance
of the Leptoclinum surface with the ascidiozooids scattered over it, but
there were also two larger elliptical clear marks which looked like the large
common cloacal apertures of the Ascidian colony.... Presumably the
Lamellaria escapes the observation of its enemies through being mistaken
for part of the Leptoclinum colony; and the Leptoclinum, being crowded
like a sponge with minute sharp-pointed spicules, is, I suppose, avoided as
inedible (if not actually noxious through some peculiar smell or taste) by
carnivorous animals which might devour such things as the soft unprotected
mollusc.”
Parasitic Mollusca

Various grades of parasitism occur among the Mollusca, from the true
parasite, living and nourishing itself on the tissues and secretions of its host,
to simple cases of commensalism. Some authors have divided these forms
into endo- and ectoparasites, according as they live inside or outside of their
host. Such a division, however, cannot be rigidly carried out, for certain
forms are indifferently endo- and ecto-parasitical, while others are ecto-
parasitic in the young form, and become endo-parasitic in the adult. It will
be convenient, therefore, simply to group the different forms according to
the home on which they find a lodgment.

Fig. 29.—Magilus antiquus L.: A, the
adult, imbedded in coral, which has
been broken away to show the tube;
B, the young (free) form.

On Sponges.—Vulsella and Crenatula almost invariably occur in large
masses of irregular shape, boring into sponges. They are especially
abundant on Porifera from the Red Sea. Corals form a favourite home of
many species, amongst which are several forms of Coralliophila,
Rhizochilus, Leptoconchus, and Sistrum. Rhizochilus is a very singular
creature, inhabiting branching corals. When adult, it forms irregular shelly
extensions of both the inner and outer lips, which adhere to the shafts of the
coral, or to the surface of neighbouring shells; at length the aperture
becomes completely closed with the exception of the siphonal tube, which
becomes long, and consists of the same shelly material. The common
Magilus (Fig. 29), from the Red Sea and Indian Ocean, in the young form is
shaped like a small Buccinum. As the coral (Meandrina) to which it attaches
itself grows, the Magilus develops at the mouth a long calcareous tube, the
aperture of which keeps pace with the growth of the coral, and prevents the
mollusc from being entombed. The animal lives at the free, or outer, end of
the tube, and is thus continually shifting its position, while the space it
abandons becomes completely closed by a mass of solid calcareous matter.
Certain species of Ovula inhabit Gorgonia, assuming the colour, yellow or
red, of their host, and, in certain cases, developing, probably for prehensile
purposes, a pointed extension of the two extremities of the shell.
Pedicularia, a form akin to Cypraea, but with a more patulous mouth,
inhabits the common Corallium rubrum of the Mediterranean, and another
species has been noticed by Graeffe[163] on Melithaea ochracea in Fiji.

On Echinodermata.—(a) Crinoidea. Stylina comatulicola lives on
Comatula mediterranea, fixed to the outer skin, which it penetrates by a
very long proboscis; the shell is quite transparent.[164] A curious case of a
fossil parasite has been noticed by Roberts.[165] A Calyptrea-shaped shell
named Platyceras always occurred on the ventral side of a crinoid,
encompassed by the arms. For some time this was thought to afford
conclusive proof of the rapacity and carnivorous habits of the echinoderm,
which had died in the act of seizing its prey. Subsequent investigations,
however, showed that in all the cases noticed (about 150) the Platyceras
covered the anal opening of the crinoid in such a way that the mouth of the
mollusc must have been directly over the orifice of the anus. (b) Asteroidea.
The comparatively soft texture of the skin of the starfishes renders them a
favourite home of various parasites. The brothers Sarasin noticed[166] a
species of Stilifer encysted on the rays of Linckia multiformis. Each shell
was enveloped up to the apex, which just projected from a hole at the top of
the cyst. The proboscis was long, and at its base was a kind of false mantle,
which appeared to possess a pumping action. On the under side of the rays
of the same starfish occurred a capuliform mollusc (Thyca ectoconcha),
furnished with a muscular plate, whose cuticular surface was indented in
such a way as to grip the skin of the Linckia. This plate was furnished with
a hole, through which the pharynx projected into the texture of the starfish,
acting as a proboscis and apparently furnished with a kind of pumping or
sucking action. Adams and Reeve[167] describe Pileopsis astericola as
living ‘on the tubercle of a starfish,’ and Stilifer astericola, from the coast
of Borneo, as ‘living in the body of a starfish.’ In the British Museum there
is a specimen of Pileopsis crystallina ‘in situ’ on the ray of a starfish, (c) On
the brittle starfishes (Ophiuroidea) occur several species of Stiliferina. (d)
Echinoidea. Various species of Stilifer occur on the ventral spines of
echinoids, where they probably subsist on the excreta, and are sometimes
found imbedded in the spines themselves. St. Turtoni occurs on the British
coasts on several species of Echinus, and Montacuta substriata frequents
Spatangus purpureus and certain species of Echinocardium, Cidaris, and
Brissus. Lepton parasiticum has been described from Kerguelen I. on a
Hemiaster, and a new genus, Robillardia, has recently been established[168]
for a Hyalinia-shaped shell, parasitic on an Echinus from Mauritius. (e)
Holothurioidea. The ‘sea-cucumbers’ afford lodgment to a variety of

curious forms, some of which have experienced such modifications that
their generic position is by no means established. Entoconcha occurs fixed
by its buccal end to the blood-vessels of certain Synapta in the
Mediterranean and the Philippines. Entocolax has been dredged from 180
fath. in Behring’s Straits, attached by its head to certain anterior muscles of
a Myriotrochus.[169] A curious case of parasitism is described by
Voeltzkow[170] as occurring on a Synapta found between tide-marks on the
I. of Zanzibar. In the oesophagus of the Synapta was found a small bivalve
(Entovalva), the animal of which was very large for its shell, and almost
entirely enveloped the valves by its mantle. As many as five specimens
occurred on a single Synapta. In the gut of the same Holothurian lived a
small univalve, not creeping freely, but fixed to a portion of the stomach
wall by a very long proboscis which pierced through it into the body cavity.
This proboscis was nearly three times as long as the animal, and the
forward portion of it was set with sharp thorns, no doubt in order to enable
it to retain its hold and resist evacuation. Various species of Eulima have
been noticed in every part of the world, from Norway to the Philippines,
both inside and outside Holothurians.[171] Stilifer also occurs on this section
of Echinoderms.[172]
On Annelida.—Cochliolepis parasiticus has been noticed under the
scales of Acoetes lupina (a kind of ‘sea-mouse’) in Charleston Harbour.[173]
On Crustacea.—A mussel, ⅜ in. long, has been found[174] living under
the carapace of the common shore-crab (Carcinus maenas), and one case
has been noticed[175] where two mussels, one of several months’ growth, the
other smaller, well secured by their byssi, were found under the abdomen of
the same species, in such a position as to force the appendages apart and
askew. These, however, are not so much cases of parasitism as of
involuntary habitat, the mussel no doubt having become involved in the
branchiae and the abdomen of the crab in the larval form.
On Mollusca.—A species of Odostomia (pallida Mont.) is found on our
own coasts on the ‘ears’ of Pecten maximus, and also[176] on the operculum
of Turritella communis. Another species (O. rissoides) frequently occurs in
hiding under beds of mussels, but it is not clear whether the habitat is due to
parasitism, or simply to the fact that the mass of mussels, knitted together
and to the rock by the byssi, affords the Odostomia a safe lurking-place. At

Panama the present writer found Crepidula (2 sp.) plentiful on the opercula
of the great Strombus galea and of Cerithium irroratum. In each case the
parasite exactly fitted the size of the operculum, and had assumed its colour,
dark brown or chestnut. Amalthea is very commonly found on Conus,
Turbo, and other large shells from the South Pacific, but this is probably not
a case of parasitism, but simply of convenience of habitat, just as young
oysters are frequently seen on the carapace and even on the legs of large
crabs.

Fig. 30.—Crepidula onyx Sowb.,
parasitic on the operculum
of Strombus galeatus
Swains., Panama.

On Tunicata.—Lamellaria deposits its eggs and lives on an Ascidian
(Leptoclinum), and the common Modiolaria marmorata lives in colonies
imbedded in the test of Ascidia mentula and other simple Ascidians.

Fig. 31.—Two species of Eulima: A is
sessile on the skin of a
Holothurian, through which it
plunges its sucking proboscis
(Pr); B creeps freely in the
stomach of a Holothurian. (After
K. Semper.)

Special points of interest with regard to parasitic Mollusca relate to (1)
Colour. This is in most cases absent, the shell being of a uniform hyaline or
milky white. This may be due, in the case of the endo-parasitic forms, to
absence of light, and possibly, in those living outside their host, to some
deficiency in the nutritive material. A colourless shell is not necessarily
protective, for though a transparent shell might evade detection, a milk-
white hue would probably be conspicuous. (2) Modifications of structure.
These are in many cases considerable. Entoconcha and Entocolax have no
respiratory or circulatory organs, and no known nervous system; Thyca and
certain Stilifer possess a curious suctorial apparatus; the foot in many cases
has aborted, since the necessity for locomotion is reduced to a minimum,
and its place is supplied by an enormous development of the proboscis,
which enables the creature to provide itself with nutriment without shifting
its position. K. Semper notices a case where a Eulima, whose habitat is the
stomach of a Holothurian, retains the foot unmodified, while a species
occurring on the outer skin, but provided with a long proboscis, has lost its
foot altogether.[177] Special provision for holding on is noticed in certain
cases, reminding us of similar provision in human parasites. Eyes are
frequently, but not always wanting, even in endo-parasitic forms. A

specially interesting modification of structure occurs in (3) the Radula or
ribbon-shaped arrangement of the teeth. In most cases of parasitism
(Eulima, Stilifer, Odostomia, Entoconcha, Entocolax, Magilus,
Coralliophila, Leptoconcha) it is absent altogether. In Ovula and
Pedicularia, genera which are in all other respects closely allied to
Cypraea, the radula exhibits marked differences from the typical radula of
the Cypraeidae. The formula (3·1·3) remains the same, but the laterals are
greatly produced and become fimbriated, sometimes at the extremity only,
sometimes along the whole length. A very similar modification occurs in
the radula of Sistrum spectrum Reeve, a species which is known to live
parasitically on one of the branching corals. Here the laterals differ from
those of the typical Purpuridae in being very long and curved at the
extremity. The general effect of these modifications appears to be the
production of a radula rather of the type of the vegetable-feeding Trochidae,
which may perhaps be regarded as a link in the chain of gradually-degraded
forms which eventually terminate in the absence of the organ altogether.
The softer the food, the less necessity there is for strong teeth to tear it; the
teeth either become smaller and more numerous, or else longer and more
slender, and eventually pass away altogether. It is curious, however, that the
same modified form of radula should appear in species of Ovula (e.g. ovum)
and that the same absence of radula should occur in species of Eulima (e.g.
polita) known to be not parasitic. This fact perhaps points back to a time
when the ancestral forms of each group are parasitic and whose radulae
were modified or wanting, the modification or absence of that organ being
continued in some of their non-parasitical descendants.
Commensalism

Mollusca are concerned in several interesting cases of commensalism, or
the habitual association of two organisms, as distinguished from parasitism,
where one form preys more or less upon the other.
Mr. J. T. Marshall has given[178] an interesting account of the association
of Montacuta ferruginosa with Echinocardium cordatum. The Echinoderm
lives in muddy sand in Torbay, at a depth of about 6 inches, and the
Montacuta lives in a burrow leading from its ventral end and running
irregularly in a sloping direction for 3 or 4 inches, the burrow, which is
made by a current from the Echinoderm, being almost exactly the width of

the Montacuta. The Montacuta were always arranged in the burrows in
order of size, the largest being close to the Echinoderm, and the smallest of
a string of about six at the other end of the burrow. In another part of S.
Devon, where the sand was soft and sloppy, the Echinocardia rise to the
surface and travel along the sand; in this case the Montacuta were attached
to their host by means of a byssus, and were dragged along as it travelled.
The Rev. Dr. Norman has noted[179] a somewhat similar habitat for
Lepton squamosum. This rare little British species was found at Salcombe,
living in the burrows of Gebia stellata, in all probability feeding upon the
secretions from the body of the crustacean. Dr. Norman suggests that the
extreme flatness of the shell of the Lepton is of great advantage in enabling
it not to get in the way of the Gebia as he scuttles up and down his burrow.
Another species of Lepton is found on the coast of Florida in a precisely
similar locality,[180] while a third species, occurring on the Oregon and
California coasts, actually attaches itself to the inner surface of the
abdomen of a Gebia.[181]

Fig. 32.—Ephippodonta Macdougalli Tate, S. Australia. A,
Burrow of prawn, the X indicating the position of the
mollusc; sp, sponge. B, Ventral view of Ephippodonta; by,
byssus; f, foot; m, mantle; mm, fused mantle borders. C,
View of interior of shells; h, hinge; m´m´, adductor
muscles. (A × ½; B and C × 2.)

A very singular case of commensalism has been recently discovered with
regard to a genus of Australian bivalve shells, Ephippodonta. This genus is
never found except in the burrow of a species of prawn (Axius
plectorhynchus Str.). For some reason at present unexplained, the burrow of
this particular prawn appears to be exceedingly popular as a habitat for
certain bivalves, for, besides two species of Ephippodonta, a Kellia and
three Mylitta are found there, and there alone. Sometimes the prawn, when
the rock is hard, builds a tunnel of mud upon it, at other times it excavates
the soft calciferous sandstone. “This burrow is lined with a tenacious brown
mud, composed of excrementitious matter; and, in addition to the mud
lining, there is always more or less present an orange-coloured sponge
which I have never found elsewhere. Upon the mud or sponge, and
adhering very closely, are found the Ephippodonta. They quickly form a
pit-like depression by means of their foot, and appear almost covered by the
mud.” During the winter months (March-July) the prawn appears to fill his

burrow, possibly as a provision against stormy weather, with large
quantities of minced seaweed, underneath which immense numbers of very
young Ephippodonta are found living.[182] The extreme flatness of the
Ephippodonta must be due to the same cause as the flatness of the Lepton
noticed above, namely, the necessity of not impeding or interfering with the
lively motions of the prawn. In the case of Lepton the two valves close
completely and the shell is still very flat; in Ephippodonta, on the other
hand, the same result is produced by the valves being opened to their widest
possible extent. As in Entovalva, a continuation of the mantle covers the
outer surface of the shell.
Variation

It is a familiar experience to the student, not only of the Mollusca, but of
every branch of animal or vegetable life, to come across examples which
exhibit certain slight deviations from the type form as usually understood.
These deviations may be more or less pronounced, but, as a rule, a series of
forms can be discovered, gradually leading up to or down from the type.
The definition of what constitutes a species,—and, still more, the rigid
application of such definition—will always remain a difficult task, so long
as the personal element persists in him who defines.[183] What seems to one
authority ample ground for distinction of species, another may regard as of
comparatively trivial importance. The practical outcome of these divergent
views is sufficiently illustrated by the attitude of Mr. F. P. Marrat on the one
hand, and of what may be called the modern French school of conchologists
on the other. Mr. Marrat holds, or held, that the great genus Nassa, of which
more than 150 species are generally recognised, is one shell (species) in an
endless variety of forms. The modern French school go to the other
extreme, and apparently proceed upon the view that almost any difference
in form, however slight, is sufficient to constitute a separate species.
It will be generally admitted, however, that some structural difference in
the organisation of the animal (as distinct from that of the shell alone) is
necessary for the permanent constitution of specific rank.[184] What amount
of structural difference is required, what particular organ or organs must
exhibit this difference, will depend largely upon the idiosyncrasy of the
observer. But if this, or something like this definition of a species be
accepted, it will follow that a so-called ‘variety’ will be a form which

exhibits differences from the type which do not amount to permanent
structural differences in the organisation of the animal. The final court of
appeal as to what affords sufficient evidence for ‘permanent structural
differences’ will have to be, as with Aristotle of old, the judgment of the
educated man.
It is, however, more to our present purpose to discuss the causes of
variation than to lay down definitions of what variation is. One of the most
obvious causes of variation lies in a change or changes in the environment.
If we may assume, for the moment, that the type form of a species is the
form which is the mean of all the extremes, and that this form is the
resultant of all the varied forces brought to bear upon it, whether of food,
climate, temperature, competition of numbers, soil, light, amount of water,
etc., it will follow that any change in one or more of these forces, if
continuous and considerable, any change, in other words, of the
environment, will produce its effect upon the organism in question. And this
effect will be for the better or for the worse, according to the particular
nature of the change itself as tending towards, or away from, the optimum
of environment for the species concerned. Hence may be produced
varieties, more or less marked according to the gravity of the change,
although it must be noted that at times a change apparently unimportant
from our point of view, will produce very marked results upon the species.
It is indeed scarcely possible to predict with any certainty, in the present
state of our knowledge (beyond certain broad results) what will be the
particular effect upon a species of any given change in its surroundings.
Effects of Change in the Environment as tending to produce
Variation.
(a) Changes in Climate, Temperature, Elevation, etc.—In the eastern
basin of the Baltic the marine Mollusca are much more stunted than in the
western.[185] For instance, Mytilus edulis near Kiel is 8–9 cm. long, while
near Gothland it only attains a length of 3–4 cm. Mollusca living at only a
shallow depth (e.g. Tellina balthica, Mya arenaria, Cardium edule) do not
differ much in size in different parts of the Baltic, but in the far eastern
basin the calcareous layers of the shells of Mya arenaria and Tellina
balthica are extraordinarily thin, and disappear very rapidly after death,
leaving only the cuticular membrane, still united by the ligament, in a
perfect state of preservation. These remarkable variations are no doubt to a

large extent due to the violent changes of temperature which are
experienced in the Baltic, and by which the steady development of the
animals in question is interrupted and thrown out of gear. The same species
occur on the coasts of Greenland and Iceland, where they attain a
considerably larger size than in the Baltic, in spite of the lower mean
temperature, probably because their development is not interrupted by any
sudden change from cold to heat or vice versâ.
Karl Semper has shown that Limnaea stagnalis is developed, lives and
feeds best in a mean temperature of about 20° C. (= 68° F.). This mean,
however, must not be the mean of two distant extremes, for the Limnaea
cannot digest its food and grow in a temperature which is less than 14° or
15° C. (= 57° or 59° F.), or more than 30° to 32° C. (= 86° to 90° F.). In
certain localities, therefore, the interruption to the growth of this species
must be serious and prolonged, and may tend towards the production of
more or less dwarfed varieties. Thus specimens from Malham Tarn, a lake
in Yorkshire 1250 feet above the sea, are permanently dwarfed, and have a
very thin and fragile shell. Limnaea peregra in the Pyrenees, Alps, and
Himalayas is generally of a very delicate form and dwarfed habit, while the
small variety known as lacustris occurs, according to Jeffreys, only in
mountain lakes in Zetland, Scotland, Ireland, and N. England. Specimens
brought by Mr. Bateson from lakes near the Sea of Aral, which are salt for
some months and comparatively fresh for others, exhibit clearly the effect
of changes in the environment (Figs. 33 and 34). Excess of heat produces
similar results to excess of cold. L. peregra var. thermalis, found in the
warm springs of the Pyrenees and the Vosges, and the var. geisericola, from
the hot water of the Iceland geysers, are alike thin and dwarfed forms.
Many instances may be given of ‘varieties due to locality.’ In some of
these, the cause which predisposes towards variation can be inferred with
some approach to certainty, in others we must be content to note the fact,
without at present being able to perceive its explanation.

Fig. 33.—Four examples of Limnaea peregra Müll., from salt
marshes near the Sea of Aral, showing different effects
produced by abnormal conditions of life.

Fig. 34.—Four examples of Limnaea stagnalis L., from marshes
in the Aral district which are salt for several months in the
year, illustrating variation produced by changes in the
environment. × ½.

Desert specimens of widely distributed species, e.g. Helix pomatia, H.
niciensis, H. pisana, Leucochroa candidissima are much thicker than the
type, and tend to lose all trace of coloured bands. These modifications are
clearly the means of preventing evaporation of moisture, the dull white or
grayish brown colour being calculated to absorb the smallest possible
amount of heat. Desert shells in all parts of the world (e.g. N. Africa,
Arabia, Central Asia, S. Africa, W. America) have been noticed to exhibit
these peculiarities.
A very singular case of the reverse process, i.e. the production of
darkened forms of shell through cold, has been noticed by Fischer as
characteristic of the marine shells of the west coast of South America.[186]
This melanism is especially noticeable in Trochus, Turbo, Chiton, Mitra,
and Pleurotoma, and is attested by the specific names, not merely
expressive of actual blackness (e.g. nigerrimus, ater, atramentarius,

maurus), but also of a generally lugubrious tone (e.g. moestus, funebralis,
tristis, lugubris, luctuosus). It is highly probable that this concurrence of
specific melanism (which stands quite alone in the world) is due to the cold
polar current which impinges on the Chilian coasts, for the same genera
occur on the opposite shores of the continent without exhibiting any trace
whatever of this mournful characteristic.
It is a well-known fact, attested by many observers, that our common
Limax agrestis as well as the young of Arion ater become decidedly darker
in summer than in winter. If these slugs were accustomed to disport
themselves in the sun, it might have been suggested that this increased
darkness of colour tended to absorb more of the heat rays. But since this is
not the case, the result is probably due to some unexplained effect of higher
temperature. According to Lessona and Pollonera, the length of the keel in
Limax arborum varies greatly in different parts of Italy, being shorter in
specimens from low ground, but much longer in those inhabiting more
elevated regions. The longer the keel, the more obscure the colouring
becomes, so that in the Upper Alps of Piedmont individuals are practically
black. Roebuck has observed that Scottish specimens of this same slug are
much darker and less translucent than English forms. According to Simroth,
our common black slug, Arion ater, is a northern type, which in more
southern latitudes assumes the form known as A. rufus. Similarly Limax
maximus “in its northern form cinereo-niger is almost wholly black, but in
the more genial climate of Italy develops a series of brilliantly coloured and
strikingly marked variations which have received numerous distinctive
names from Italian limacologists.”[187] According to Scharff, however[188]
(who regards the colours of slugs as in the main protective), these dark
forms are by no means exclusively northern, being found equally on the
parched plains of Spain and Portugal, and in the bleak climate of Norway.
The same authority observes that similar forms occur both in the dry
regions of E. Germany, and in the very humid district of western Ireland.
It appears unquestionable that marine genera from high northern
latitudes are provided with shells of uniform colour, or whitish with a pale
brown epidermis; spots, bands, or stripes seldom occur. The arctic forms of
Buccinum, Trophon, Chrysodomus, Margarita, Crenella, Leda, Yoldia,
Astarte illustrate this fact. In the more temperate seas of Europe, colours
tend on the whole to increase, although there are certain genera (e.g.

Pecten) which are not more brightly coloured in Mediterranean than in
Icelandic waters.
Land Mollusca inhabiting the mainland of a continent not unfrequently
become smaller when they have spread to adjacent islands where perhaps
the rainfall is less abundant or the soil and food-supply less nicely adjusted
to their wants. Orthalicus undatus is decidedly larger on the mainland of S.
America than on the adjacent islands of Trinidad and Grenada. Specimens
of Bulimulus exilis from Barbados are invariably broader and more obese
than those from S. Thomas, while those from the volcanic island of S.
Lucia, where lime is deficient, are small and very slender. Streptaxis
deformis, as occurring at Trinidad, is only half the size of specimens from
Georgetown, Demerara.[189]
Certain localities appear, for some unexplained reason, to be particularly
favourable to the production of albino varieties. The neighbourhood of
Lewes, in Sussex, has produced no fewer than fourteen of these forms of
land Mollusca and five of fresh-water.[190]
Our common Helix aspersa, as found near Bristol, is said to be ‘dark
coloured’; about Western-super-mare ‘brown, with black markings’; near
Bath ‘very pale and much mottled’; at Cheddar ‘very solid and large.’[191]
Sometimes the same kind of variation is exhibited by different species in
the same locality. Thus specimens of H. aspersa, H. nemoralis, and H.
hortensis, taken from the same bank at Torquay, presented a straw-coloured
tinge of ground colour, with red-brown bands or markings. Trochiform H.
nemoralis and H. arbustorum, sinistral H. hortensis and H. aspersa,
sinistral H. aspersa and H. virgata, and similarly banded forms of H.
caperata and H. virgata, have been taken together.[192]
The immediate neighbourhood of the sea appears frequently to have the
effect of dwarfing land Mollusca. Thus the var. conoidea of Helix aspersa,
which is small, conical, with a compressed mouth, occurs ‘on sandhills and
cliffs at the seaside.’ The varieties conica and nana of Helix hispida are
found ‘near the sea.’ Helix virgata is exceedingly small in similar localities,
and tends to become unicoloured. H. caperata var. Gigaxii, a small
depressed form, occurs at ‘Sandwich and Falmouth.’[193] Sometimes,
however, the exact opposite is the case, for H. nemoralis var. major, which
is ‘much larger’ than the type, occurs on ‘sandhills and downs’ and is

‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of
Limnaea peregra known as maritima appears to be confined to the
neighbourhood of the sea.
Dwarfing of the shell seems frequently to be the result of an elevated
locality, not perhaps so much as the direct consequence of purer air and less
barometric pressure, as of changes in the character of the food supply and in
the humidity of the air. Several species of Helix have a variety minor which
is characteristic of an Alpine habitat. Helix arbustorum var. alpestris, which
is scarcely two-thirds the size of the type, occurs on the Swiss Alps in the
region of perpetual snow. Sometimes a very slight elevation is sufficient to
produce the dwarfed form. At Tenby the type form of Helix pisana is
scattered in countless numbers over the sandhills just above high-water
mark. At the extreme western end of these sandhills rises abruptly to a
height of over 100 feet the promontory known as Giltar Head, the
vegetation of which is entirely distinct from that of the burrows below.
There is a colony of H. pisana at the end of Giltar, all of which are devoid
of the characteristic markings of the typical form, and most are dwarfed and
stunted in growth.
Occasionally the same variety will be found to be produced by
surroundings of very different nature. Thus the var. alpestris, of H.
arbustorum mentioned above, besides being characteristic of high Alpine
localities, also occurs abundantly in low marshes at Hoddesdon on the
River Lea. Helix pulchella var. costata, according to Jeffreys, is found in
dry and sandy places, often under loose stones and bricks on walls, while
other authorities have noticed it in wet and dry localities quite indifferently.
Sometimes the production of a variety may be traced to the intrusion of
some other organism. According to Brot, nine-tenths of the Limnaea
peregra inhabiting a certain pond near Geneva, were, during one season,
afflicted with a malformation of the base of the columella. This deformity
coincided with the appearance, in the same waters, of extraordinary
numbers of Hydra viridis. The next season, when the Hydra disappeared,
the next generation of Limnaea was found to have resumed its normal form.
It has been noticed that a form of Helix caperata with a flattened spire
and wide umbilicus is restricted to tilled fields, especially the borders of
clover fields, while a form with a more elevated spire and more compact

whorls occurs exclusively in open downs and uncultivated places. The Rev.
S. S. Pearce accounts[194] for this divergence by the explanation that the
flatter spire enables the shell of the fields to creep about more easily under
the leaves or matted weeds, seldom requiring to crawl up a stalk or stem,
while on the short turf of the downs and pastures the smaller and more
rounded shell enables the animal to manoeuvre in and out of the blades of
grass, and even to crawl up them with considerable activity. The same
writer endeavours to explain the causes which regulate the distribution of
H. caperata var. ornata. He found that this variety (dark bands on a white
ground) occurred almost exclusively on downs which were fed upon by
sheep, associated with the common or mottled form, while the latter form
alone occurred in localities where sheep were not accustomed to feed.
Assuming then, as is probably the case, that sheep, in the course of their
close pasturing, devour many small snails, he believes that individuals of
the more conspicuous form ornata were more likely to be noticed, and
therefore avoided, by the sheep, than the mottled form, which would more
easily escape their observation. Hence the var. ornata is due to the
advantage which strikingly coloured individuals obtained owing to their
conspicuous habit, as compared with the typical form, which would be less
readily detected.
(b) Changes in Soil, Station, Character of Water, etc.—A deficiency of
lime in the composition of the soil of any particular locality produces very
marked effects upon the shells of the Mollusca which inhabit it; they
become small and very thin, occasionally almost transparent. The well-
known var. tenuis of Helix aspersa occurs on downs in the Channel Islands
where calcareous material is scarce. For similar reasons, H. arbustorum
develops a var. fusca, which is depressed, very thin, and transparent, at
Scilly, and also at Lunna I., E. Zetland.

Fig. 35.—19 specimens of Purpura lapillus L., Great Britain,
illustrating variation.
(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined
and coloured rock; (4) Herm, rather exposed; (5) Solent, very
sheltered; (6) Land’s End, exposed rocks, small food supply; (7)
Scilly, exposed rocks, fair food supply; (8) St. Leonards, flat
mussel beds at extreme low water; (9) Robin Hood’s Bay,
sheltered under boulders, good food supply; (10) Rhoscolyn, on
oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed

rocks; (12) Estuary of Conway, very sheltered, abundant food
supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor
food supply; (14) slightly monstrous; (15), (16), (17) Morthoe,
rather exposed rocks, but abundant food supply; (18) St. Bride’s
Bay; (19) L. Swilly, sheltered, but small food supply. All from the
author’s collection, except (10).

The common dog-whelk (Purpura lapillus) of our own coasts is an
exceedingly variable species, and in many cases the variations may be
shown to bear a direct relation to the manner of life (Fig. 35). Forms
occurring in very exposed situations, e.g. Land’s End, outer rocks of the
Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a short spire
and relatively large mouth, the latter being developed in order to increase
the power of adherence to the rock and consequently of resistance to wave
force. On the other hand, shells occurring in sheltered situations, estuaries,
narrow straits, or even on open coasts where there is plenty of shelter from
the waves, are comparatively of great size, with a well-developed,
sometimes produced spire, and a mouth small in proportion to the area of
shell surface. In the accompanying figure, the specimens from the Conway
estuary and the Solent (12, 5) well illustrate this latter form of shell, while
that from exposed rocks is illustrated by the specimens from Robin Hood’s
Bay (13, 14). Had these specimens occurred alone, or had they been
brought from some distant and unexplored region, they must inevitably
have been described as two distinct species.

Fig. 36.—Valves of Cardium edule from the four upper terraces of
Shumish Kul, a dry salt lake adjacent to the Aral Sea. (After
Bateson.)

Mr. W. Bateson has made[195] some observations on the shells of
Cardium edule taken from a series of terraces on the border of certain salt
lakes which once formed a portion of the Sea of Aral. As these lakes
gradually became dry, the water they contained became salter, and thus the
successive layers of dead shells deposited on their borders form an

interesting record of the progressive variation of this species under
conditions which, in one respect at least, can be clearly appreciated. At the
same time the diminishing volume of water, and the increasing average
temperature, would not be without their effect. It was found that the
principal changes were as follows: the thickness, and consequently the
weight, of the shells became diminished, the size of the beaks was reduced,
the shell became highly coloured, and diminished considerably in size, and
the breadth of the shells increased in proportion to their length (Fig. 36).
Shells of the same species of Cardium, occurring in Lake Mareotis, were
found to exhibit very similar variations as regards colour, size, shape, and
thickness.
Unio pictorum var. compressa occurs near Norwich at two similar
localities six or seven miles distant from one another, under circumstances
which tend to show that similar conditions have produced similar results.
The form occurs where the river, by bending sharply in horse-shoe shape,
causes the current to rush across to the opposite side and form an eddy near
the bank on the outside of the bend. Just at the edge of the sharp current
next the eddy the shells are found, the peculiar form being probably due to
the current continually washing away the soft particles of mud and
compelling the shell to elongate itself in order to keep partly buried at the
bottom.[196]
The rivers Ouse and Foss, which unite just below York, are rivers of
strikingly different character, the Ouse being deep, rapid, with a bare, stony
bottom, and little vegetable growth, and receiving a good deal of drainage,
while the Foss is shallow, slow, muddy, full of weeds and with very little
drainage. In the Foss, fine specimens of Anodonta anatina occur, lustrous,
with beautifully rayed shells. A few yards off, in the Ouse, the same species
of Anodonta is dull brown in colour, its interior clouded, the beaks and
epidermis often deeply eroded. Precisely the same contrast is shown in
specimens of Unio tumidus, taken from the same rivers, Ouse specimens
being also slightly curved in form. Just above Yearsley Lock in the Foss,
Unio tumidus occurs, but always dwarfed and malformed, a result probably
due to the effect of rapidly running water upon a species accustomed to live
in still water.[197] Simroth records the occurrence of remarkably distorted
varieties in two species of Aetheria which lived in swift falls of the River
Congo.[198]

A variety of Limnaea peregra with a short spire and rather strong, stoutly
built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-
fach. It lives adhering to stones in places where there are very few weeds,
its shape enabling it to withstand the surf of these large lakes, to which the
ordinary form would probably succumb.[199]
Scalariform specimens of Planorbis are said to occur most commonly in
waters which are choked by vegetation, and it has been shown that this
form of shell is able to make its way through masses of dense weed much
more readily than specimens of normal shape.
Continental authorities have long considered Limnaea peregra and L.
ovata as two distinct species. Hazay, however, has succeeded in rearing
specimens of so-called peregra from the ova of ovata, and so-called ovata
from the ova of peregra, simply by placing one species in running water,
and the other in still water.
According to Mr. J. S. Gibbons[200] certain species of Littorina, in
tropical and sub-tropical regions, are confined to water more or less
brackish, being incapable of living in pure salt water. “I have met,” says Mr.
Gibbons, “with three of these species, and in each case they have been
distinguished from the truly marine species by the extreme (comparative)
thinness of their shells, and by their colouring being richer and more varied;
they are also usually more elaborately marked. They are to be met with
under three different conditions—(1) in harbours and bays where the water
is salt with but a slight admixture of fresh water; (2) in mangrove swamps
where salt and fresh water mix in pretty equal volume; (3) on dry land, but
near a marsh or the dry bed of one.
“L. intermedia Reeve, a widely diffused E. African shell, attaches itself
by a thin pellicle of dried mucus to grass growing by the margin of slightly
brackish marshes near the coast, resembling in its mode of suspension the
Old World Cyclostoma. I have found it in vast numbers in situations where,
during the greater part of the year, it is exposed to the full glare of an almost
vertical sun, its only source of moisture being a slight dew at night-time.
The W. Indian L. angulifera Lam., and a beautifully coloured E. African
species (? L. carinifera), are found in mangrove swamps; they are,
however, less independent of salt water than the last.”

Mr. Gibbons goes on to note that brackish water species (although not so
solid as truly marine species) tend to become more solid as the water they
inhabit becomes less salt. This is a curious fact, and the reverse of what one
would expect. Specimens of L. intermedia on stakes at the mouth of the
Lorenço Marques River, Delagoa Bay, are much smaller, darker, and more
fragile, than those living on grass a few hundred yards away. L. angulifera
is unusually solid and heavy at Puerto Plata (S. Domingo) among
mangroves, where the water is in a great measure fresh; at Havana and at
Colon, where it lives on stakes in water but slightly brackish, it is thinner
and smaller and also darker coloured.
(c) Changes in the Volume of Water.—It has long been known that the
largest specimens, e.g. of Limnaea stagnalis and Anodonta anatina, only
occurred in pieces of water of considerable size. Recent observation,
however, has shown conclusively that the volume of water in which certain
species live has a very close relation to the actual size of their shells,
besides producing other effects. Lymnaea megasoma, when kept in an
aquarium of limited size, deposited eggs which hatched out; this process
was continued in the same aquarium for four generations in all, the form of
the shell of the last generation having become such that an experienced
conchologist gave it as his opinion that the first and last terms of the series
could have no possible specific relation to one another. The size of the shell
became greatly diminished, and in particular the spire became very slender.
[201]

The same species being again kept in an aquarium under similar
conditions, it was found that the third generation had a shell only four-
sevenths the length of their great grandparents. It was noticed also that the
sexual capacities of the animals changed as well. The liver was greatly
reduced, and the male organs were entirely lost.[202]
K. Semper conducted some well-known experiments bearing on this
point. He separated[203] specimens of Limnaea stagnalis from the same
mass of eggs as soon as they were hatched, and placed them simultaneously
in bodies of water varying in volume from 100 to 2000 cubic centimetres.
All the other conditions of life, and especially the food supply, were kept at
the known optimum. He found, in the result, that the size of the shell varied
directly in proportion to the volume of the water in which it lived, and that

this was the case, whether an individual specimen was kept alone in a given
quantity of water, or shared it with several others. At the close of 65 days
the specimens raised in 100 cubic cm. of water were only 6 mm. long, those
in 250 cubic cm. were 9 mm. long, those in 600 cubic cm. were 12 mm.
long, while those kept in 2000 cubic cm. attained a length of 18 mm. (Fig.
37).
An interesting effect of a sudden fall of temperature was noticed by
Semper in connection with the above experiments. Vessels of unequal size,
containing specimens of the Limnaea, happened to stand before a window
at a time when the temperature suddenly fell to about 55° F. The sun, which
shone through the window, warmed the water in the smaller vessels, but had
no effect upon the temperature of the larger. The result was, that the
Limnaea in 2000 cubic cm., which ought to have been 10 mm. long when
25 days old, were scarcely longer, at the end of that period, than those
which had lived in the smaller vessels, but whose water had been
sufficiently warm.

Fig. 37.—Four equally old shells of
Limnaea stagnalis, hatched from the
same mass of ova, but reared in
different volumes of water: A in
100, B in 250, C in 600, and D in
2000 cubic centimetres. (After K.
Semper.)

CHAPTER IV
USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD—CULTIVATION OF THE
OYSTER, MUSSEL, AND SNAIL—SNAILS AS MEDICINE—PRICES GIVEN FOR SHELLS

The employment of shells as a medium of exchange was exceedingly
common amongst uncivilised tribes in all parts of the world, and has by no
means yet become obsolete. One of the commonest species thus employed
is the ‘money cowry’ (Cypraea moneta, L.), which stands almost alone in
being used entire, while nearly all the other forms of shell money are made
out of portions of shells, thus requiring a certain amount of labour in the
process of formation.
One of the earliest mentions of the cowry as money occurs in an ancient
Hindoo treatise on mathematics, written in the seventh century a.d. A
question is propounded thus: ‘the ¼ of 1/16 of ⅕ of ¾ of ⅔ of ½ a dramma
was given to a beggar by one from whom he asked an alms; tell me how
many cowry shells the miser gave.’ In British India about 4000 are said to
have passed for a shilling, but the value appears to differ according to their
condition, poor specimens being comparatively worthless. According to
Reeve[204] a gentleman residing at Cuttack is said to have paid for the
erection of his bungalow entirely in cowries. The building cost him 4000
Rs. sicca (about £400), and as 64 cowries = 1 pice, and 64 pice = 1 rupee
sicca, he paid over 16,000,000 cowries in all.
Cowries are imported to England from India and other places for the
purposes of exportation to West Africa, to be exchanged for native products.
The trade, however, appears to be greatly on the decrease. At the port of
Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]
A banded form of Nerita polita was used as money in certain parts of the
South Pacific. The sandal-wood imported into the China market is largely
obtained from the New Hebrides, being purchased of the natives in
exchange for Ovulum angulosum, which they especially esteem as an
ornament. Sometimes, as in the Duke of York group, the use of shell money
is specially restricted to certain kinds of purchase, being employed there
only in the buying of swine.

Among the tribes of the North-West coasts of America the common
Dentalium indianorum used to form the standard of value, until it was
superseded, under the auspices of the Hudson’s Bay Company, by blankets.
A slave was valued at a fathom of from 25 to 40 of these shells, strung
lengthwise. Inferior or broken specimens were strung together in a similar
way, but were less highly esteemed; they corresponded more to our silver
and copper coins, while the strings of the best shells represented gold.
The wampum of the eastern coast of North America differed from all
these forms of shell money, in that it required a laborious process for its
manufacture. Wampum consisted of strings of cylindrical beads, each about
a quarter of an inch in length and half that breadth. The beads were of two
colours, white and purple, the latter being the more valuable. Both were
formed from the common clam, Venus mercenaria, the valves of which are
often stained with purple at the lower margins, while the rest of the shell is
white. Cut small, ground down, and pierced, these shells were converted
into money, which appears to have been current along the whole sea-board
of North America from Maine to Florida, and on the Gulf Coast as far as
Central America, as well as among the inland tribes east of the Mississippi.
Another kind of wampum was made from the shells of Busycon carica and
B. perversum. By staining the wampum with various colours, and disposing
these colours in belts in various forms of arrangement, the Indians were
able to preserve records, send messages, and keep account of any kind of
event, treaty, or transaction.
Another common form of money in California was Olivella biplicata,
strung together by rubbing down the apex. Button-shaped disks cut from
Saxidomus arata and Pachydesma crassatelloides, as well as oblong pieces
of Haliotis, were employed for the same purpose, when strung together in
lengths of several yards.
“There is a curious old custom,” writes Mr. W. Anderson Smith,[206]
“that used formerly to be in use in this locality [the western coast of
Scotland], and no doubt was generally employed along the sea-board, as the
most simple and ready means of arrangement of bargains by a non-writing
population. That was, when a bargain was made, each party to the
transaction got one half of a bivalve shell—such as mussel, cockle, or
oyster—and when the bargain was implemented, the half that fitted exactly
was delivered up as a receipt! Thus a man who had a box full of unfitted

shells might be either a creditor or a debtor; but the box filled with fitted
shells represented receipted accounts. Those who know the difficulty of
fitting the valves of some classes of bivalves will readily acknowledge the
value of this arrangement.”
Shells are employed for use and for ornament by savage—and even by
civilised—tribes in all parts of the world. The natives of Fiji thread the large
Turbo argyrostoma and crenulatus as weights at the edge of their nets, and
also employ them as sinkers. A Cypraea tigris cut into two halves and
placed round a stone, with two or three showy Oliva at the sides, is used as
a bait for cuttles. Avicula margaritifera is cut into scrapers and knives by
this and several other tribes. Breast ornaments of Chama, grouped with
Solarium perspectivum and Terebra duplicata are common among the
Fijians, who also mount the Avicula on a backing of whales’ teeth sawn in
two, for the same purpose. The great Orange Cowry (Cypraea aurantiaca)
is used as a badge of high rank among the chieftains. One of the most
remarkable Fijian industries is the working of whales’ teeth to represent this
cowry, as well as the commoner C. talpa, which is more easily imitated.
Among the Solomon islanders, cowries are used to ornament their
shields on great field days, and split cowries are worn as a necklace, to
represent human teeth. Small bunches of Terebellum subulatum are worn as
earrings, and a large valve of Avicula is employed as a head ornament in the
centre of a fillet. The same islanders ornament the raised prows of their
canoes, as well as the inside of the stern-post, with a long row of single
Natica.
The native Papuans employ shells for an immense variety of purposes.
Circlets for the head are formed of rows of Nassa gibbosula, rubbed down
till little but the mouth remains. Necklaces are worn which consist of strings
of Oliva, young Avicula, Natica melanostoma, opercula of Turbo, and
valves of a rich brown species of Cardium, pendent at the end of strings of
the seeds known as Job’s tears. Struthiolaria is rubbed down until nothing
but the mouth is left, and worn in strings round the neck. This is
remarkable, since Struthiolaria is not a native Papuan shell, and indeed
occurs no nearer than New Zealand. Sections of Melo are also worn as a
breast ornament, dependent from a necklace of cornelian stones. Cypraea
erosa is used to ornament drinking bowls, and Ovulum ovum is attached to

the native drums, at the base of a bunch of cassowary feathers, as well as
being fastened to the handle of a sago-beater.
In the same island, the great Turbo and Conus millepunctatus are ground
down to form bracelets, which are worn on the biceps. The crimson lip of
Strombus luhuanus is cut into beads and perforated for necklaces. Village
elders are distinguished by a single Ovulum verrucosum, worn in the centre
of the forehead. The thick lip of Cassis cornuta is ground down to form
nose pieces, 4½ inches long. Fragments of a shell called Kaïma (probably
valves of a large Spondylus) are worn suspended from the ears, with little
wisps of hair twisted up and thrust through a hole in the centre. For
trumpets, Cassis cornuta, Triton tritonis, and Ranella lampas are used, with
a hole drilled as a mouthpiece in one of the upper whorls. Valves of Batissa,
Unio, and Mytilus are used as knives for peeling yams. Spoons for scooping
the white from the cocoa-nut are made from Avicula margaritifera. Melo
diadema is used as a baler in the canoes.[207]
In the Sandwich Islands Melampus luteus is worn as a necklace, as well
as in the Navigator Islands. A very striking necklace, in the latter group, is
formed of the apices of a Nautilus, rubbed down to show the nacre. The
New Zealanders use the green opercula of a Turbo, a small species of
Venus, and Cypraea asellus to form the eyes of their idols. Fish-hooks are
made throughout the Pacific of the shells of Avicula and Haliotis, and are
sometimes strengthened by a backing made of the columella of Cypraea
arabica. Small axe-heads are made from Terebra crenulata ground down
(Woodlark I.), and larger forms are fashioned from the giant Tridacna (Fiji).
Shells are used to ornament the elaborate cloaks worn by the women of
rank in the Indian tribes of South America. Specimens of Ampullaria,
Orthalicus, Labyrinthus, and Bulimulus depend from the bottom and back
of these garments, while great Bulimi, 6 inches long, are worn as a breast
ornament, and at the end of a string of beads and teeth.[208]
The chank-shell (Turbinella rapa) is of especial interest from its
connexion with the religion of the Hindoos. The god Vishnu is represented
as holding this shell in his hand, and the sinistral form of it, which is
excessively rare, is regarded with extraordinary veneration. The chank
appears as a symbol on the coins of some of the ancient Indian Empires,
and is still retained on the coinage of the Rajah of Travancore.

The chief fishery of the chank-shell is at Tuticorin, on the Gulf of
Manaar, and is conducted during the N. E. monsoon, October-May. In
1885–86 as many as 332,000 specimens were obtained, the net amount
realised being nearly Rs.24,000. In former days the trade was much more
lucrative, 4 or 5 millions of specimens being frequently shipped. The
government of Ceylon used to receive £4000 a year for licenses to fish, but
now the trade is free. The shells are brought up by divers from 2 or 3
fathoms of water. In 1887 a sinistral specimen was found at Jaffna, which
sold for Rs.700.[209] Nearly all the shells are sent to Dacca, where they are
sliced into bangles and anklets to be worn by the Hindoo women.
Perhaps the most important industry which deals only with the shells of
Mollusca is that connected with the ‘pearl-oyster.’ The history of the trade
forms a small literature in itself. It must be sufficient here to note that the
species in question is not an ‘oyster,’ properly so called, but an Avicula
(margaritifera Lam.). The ‘mother-of-pearl,’ which is extensively employed
for the manufacture of buttons, studs, knife-handles, fans, card-cases,
brooches, boxes, and every kind of inlaid work, is the internal nacreous
laminae of the shell of this species. The most important fisheries are those
of the Am Islands, the Soo-loo Archipelago, the Persian Gulf, the Red Sea,
Queensland, and the Pearl Islands in the Bay of Panama. The shell also
occurs in several of the groups of the South Pacific—the Paumotu, Gambier
and Navigator Islands, Tahiti being the centre of the trade—and also on the
coasts of Lower California.[210]
Pearls are the result of a disease in the animal of this species of Avicula
and probably in all other species within which they occur. When the Avicula
is large, well formed, and with ample space for individual development,
pearls scarcely occur at all, but when the shells are crowded together, and
become humped and distorted, as well as affording cover for all kinds of
marine worms and parasitic creatures, then pearls are sure to be found.
Pearls of inferior value and size are also produced by Placuna placenta,
many species of Pinna, the great Tridacna, the common Ostrea edulis, and
several other marine bivalves. They are not uncommon in Unio and
Anodonta, and the common Margaritana margaritifera of our rapid streams
is still said to be collected, in some parts of Wales, for the purpose of
extracting its small ‘seed-pearls.’ Pink pearls are obtained from the giant

conch-shell of the West Indies (Strombus gigas), as well as from certain
Turbinella.
In Canton, many houses are illuminated almost entirely by skylights and
windows made of shells, probably the semitransparent valves of Placuna
placenta. In China lime is commonly made of ground cockle-shells, and,
when mixed with oil, forms an excellent putty, used for cementing coffins,
and in forming a surface for the frescoes with which the gables of temples
and private houses are adorned. Those who suffer from cutaneous diseases,
and convalescents from small-pox, are washed in Canton with the water in
which cockles have been boiled.[211]
A recent issue of the Peking Gazette contains a report from the outgoing
Viceroy of Fukhien, stating that he had handed over the insignia of office to
his successor, including inter alia the conch-shell bestowed by the Throne.
A conch-shell with a whorl turning to the right, i.e. a sinistral specimen, is
supposed when blown to have the effect of stilling the waves, and hence is
bestowed by the Emperor upon high officers whose duties oblige them to
take voyages by sea. The Viceroy of Fukhien probably possesses one of
these shells in virtue of his jurisdiction over Formosa, to which island
periodical visits are supposed to be made.[212]
Shells appear to be used occasionally by other species besides man.
Oyster-catchers at breeding time prepare a number of imitation nests in the
gravel on the spit of land where they build, putting bits of white shell in
them to represent eggs.[213] This looks like a trick in order to conceal the
position of the true nest. According to Nordenskjöld, when the eider duck of
Spitzbergen has only one or two eggs in its nest, it places a shell of
Buccinum glaciale beside them. The appropriation of old shells by hermit-
crabs is a familiar sight all over the world. Perhaps it is most striking in the
tropics, where it is really startling, at first experience, to meet—as I have
done—a large Cassis or Turbo, walking about in a wood or on a hill side at
considerable distances from the sea. A Gephyrean (Phascolion strombi)
habitually establishes itself in the discarded shells of marine Mollusca.
Certain Hymenoptera make use of dead shells of Helix hortensis in which
they build their cells.[214] Magnus believes that in times when heavy rains
prevail, and the usual insects do not venture out, certain flowers are

fertilised by snails and slugs crawling over them, e.g. Leucanthemum
vulgare by Limax laevis.[215]
Mollusca as Food for Man.—Probably there are few countries in the
world in which less use is made of the Mollusca as a form of food than in
our own. There are scarcely ten native species which can be said to be at all
commonly employed for this purpose. Neighbouring countries show us an
example in this respect. The French, Italians, and Spanish eat Natica, Turbo,
Triton, and Murex, and, among bivalves, Donax, Venus, Lithodomus,
Pholas, Tapes, and Cardita, as well as the smaller Cephalopoda. Under the
general designation of clam the Americans eat Venus mercenaria, Mya
arenaria, and Mactra solidissima. In the Suez markets are exposed for sale
Strombus and Melongena, Avicula and Cytherea. At Panama Donax and
Solen are delicacies, while the natives also eat the great Murex and Pyrula,
and even the huge Arca grandis, which lives embedded in the liquid river
mud.
The common littoral bivalves seem to be eaten in nearly all countries
except our own, and it is therefore needless to enumerate them. The
Gasteropoda, whose habits are scarcely so cleanly, seem to require a bolder
spirit and less delicate palate to venture on their consumption.
The Malays of the East Indian islands eat Telescopium fuscum and
Pyrazus palustris, which abound in the mangrove swamps. They throw
them on their wood fires, and when they are sufficiently cooked, break off
the top of the spire and suck the animal out through the opening. Haliotis
they take out of the shell, string together, and dry in the sun. The lower
classes in the Philippines eat Arca inaequivalvis, boiling them as we do
mussels.[216] In the Corean islands a species of Monodonta and another of
Mytilus are quite peppery, and bite the tongue; our own Helix revelata, as I
can vouch from personal experience, has a similar flavour. Fusus colosseus,
Rapana bezoar, and Purpura luteostoma are eaten on the southern coasts of
China; Strombus luhuanus, Turbo chrysostomus, Trochus niloticus, and
Patella testudinaria, by the natives of New Caledonia; Strombus gigas and
Livona pica in the West Indies; Turbo niger and Concholepas peruvianus
on the Chilian coasts; four species of Strombus and Nerita, one each of
Purpura and Turbo, besides two Tridacna and one Hippopus, by the natives
of British New Guinea. West Indian negroes eat the large Chitons which are
abundant on their rocky coasts, cutting off and swallowing raw the fleshy

foot, which they call ‘beef,’ and rejecting the viscera. Dried cephalopods are
a favourite Chinese dish, and are regularly exported to San Francisco,
where the Chinamen make them into soup. The ‘Challenger’ obtained two
species of Sepia and two of Loligo from the market at Yokohama.
The insipidity of fresh-water Mollusca renders them much less desirable
as a form of food. Some species of Unionidae, however, are said to be eaten
in France. Anodonta edulis is specially cultivated for food in certain
districts of China, and the African Aetheriae are eaten by negroes. Navicella
and Neritina are eaten in Mauritius, Ampullaria and Neritina in
Guadeloupe, and Paludina in Cambodia.
The vast heaps of empty shells known as ‘kitchen-middens,’ occur in
almost every part of the world. They are found in Scotland, Denmark, the
east and west coasts of North America, Brazil, Tierra del Fuego, Australia
and New Zealand, and are sometimes several hundred yards in length. They
are invariably composed of the edible shells of the adjacent coast, mixed
with bones of Mammals, birds, and fish. From their great size, it is believed
that many of them must have taken centuries to form.
Pre-eminent among existing shell-fish industries stands the cultivation of
the oyster and the mussel, a more detailed account of which may prove
interesting.
The cultivation of the oyster[217] as a luxury of food dates at least from
the gastronomic age of Rome. Every one has heard of the epicure whose
taste was so educated that

“he could tell
At the first mouthful, if his oysters fed
On the Rutupian or the Lucrine bed
Or at Circeii.”[218]

The first artificial oyster-cultivator on a large scale appears to have been
a certain Roman named Sergius Orata, who lived about a century b.c. His
object, according to Pliny the elder,[219] was not to please his own appetite
so much as to make money by ministering to the appetites of others. His
vivaria were situated on the Lucrine Lake, near Baiae, and the Lucrine
oysters obtained under his cultivation a notoriety which they never entirely

lost, although British oysters eventually came to be more highly esteemed.
He must have been a great enthusiast in his trade, for on one occasion when
he became involved in a law-suit with one of the riparian proprietors, his
counsel declared that Orata’s opponent made a great mistake if he expected
to damp his ardour by expelling him from the lake, for, sooner than not
grow oysters at all, he would grow them upon the roof of his house.[220]
Orata’s successors in the business seem to have understood the secret of
planting young oysters in new beds, for we are told that specimens brought
from Brundisium and even from Britain were placed for a while in the
Lucrine Lake, to fatten after their long journey, and also to acquire the
esteemed “Lucrine flavour.”
Oysters are ‘in season’ whenever there is an ‘r’ in the month, in other
words, from September to April. ‘Mensibus erratis,’ as the poet has it, ‘vos
ostrea manducatis!’ It has been computed that the quantity annually
produced in Great Britain amounts to no less than sixteen hundred million,
while in America the number is estimated at five thousand five hundred
million, the value being over thirteen million dollars, and the number of
persons employed fifty thousand. Arcachon, one of the principal French
oyster-parks, has nearly 10,000 acres of oyster beds, the annual value being
from eight to ten million francs; in 1884–85, 178,359,000 oysters were
exported from this place alone. In the season 1889–90, 50,000 tons of
oysters were consumed in London.
Few will now be found to echo the poet Gay’s opinion:

“That man had sure a palate covered o’er
With brass or steel, that on the rocky shore
First broke the oozy oyster’s pearly coat,
And risq’d the living morsel down his throat.”

There were halcyon days in England once, when oysters were to be
procured at 8d. the bushel. Now it costs exactly that amount before a
bushel, brought up the Thames, can even be exposed for sale at Billingsgate
(4d. porterage, 4d. market toll), and prime Whitstable natives average from
3½d. to 4d. each. The principal causes of this rise in prices, apart from the
increased demand, are (1) over-dredging; (2) ignorant cultivation, and to
these may be added (3) the effect of bad seasons in destroying young
oysters, or preventing the spat from maturing. Our own principal beds are

those at Whitstable, Rochester, Colchester, Milton (famous for its ‘melting’
natives), Faversham, Queenborough, Burnham, Poole, and Carlingford in
Co. Down, and Newhaven, near Edinburgh.
The oyster-farms at Whitstable, public and private, extend over an area
of more than 27 square miles. The principal of these is a kind of joint-stock
company, with no other privilege of entrance except birth as a free
dredgeman of the town. When a holder dies, his interest dies with him.
Twelve directors, known as “the Jury,” manage the affairs of the company,
which finds employment for several thousand people, and sometimes turns
over as much as £200,000 a year. The term ‘Natives,’ as applied to these
Whitstable or to other English oysters, requires a word of explanation. A
‘Native’ oyster is simply an oyster which has been bred on or near the
Thames estuary, but very probably it may be developed from a brood which
came from Scotland or some other place at a distance. For some
unexplained reason, oysters bred on the London clay acquire a greater
delicacy of flavour than elsewhere. The company pay large sums for brood
to stock their own grounds, since there can be no certainty that the spat
from their own oysters will fall favourably, or even within their own
domains at all. Besides purchases from other beds, the parks are largely
stocked with small oysters picked up along the coast or dredged from
grounds public to all, sometimes as much as 50s. a bushel being paid for the
best brood. It is probably this system of transplanting, combined with
systematic working of the beds, which has made the Whitstable oyster so
excellent both as to quality and quantity of flesh. The whole surface of the
‘layings’ is explored every year by the dredge, successive portions of the
ground being gone over in regular rotation, and every provision being made
for the well-being of the crop, and the destruction of their enemies. For
three days of every week the men dredge for ‘planting,’ i.e. for the
transference of suitable specimens from one place to another, the separation
of adhering shells, the removal of odd valves and of every kind of refuse,
and the killing off of dangerous foes. On the other three days they dredge
for the market, taking care only to lift such a number as will match the
demand.
The Colne beds are natural beds, as opposed to the majority of the great
working beds, which are artificial. They are the property of the town of
Colchester, which appoints a water-bailiff to manage the concern. Under his

direction is a jury of twelve, who regulate the times of dredging, the price at
which sales are to be made, and are generally responsible for the practical
working of the trade. Here, and at Faversham, Queenborough, Rochester,
and other places, ‘natives’ are grown which rival those of Whitstable.
There can be no question, however, that the cultivation of oysters by the
French is far more complete and efficient than our own, and has reached a
higher degree of scientific perfection combined with economy and solid
profits. And yet, between 40 and 50 years ago, the French beds were utterly
exhausted and unproductive, and showed every sign of failure and decay. It
was in 1858 that the celebrated beds on the Ile de Ré, near Rochelle, were
first started. Their originator was a certain shrewd stone-mason, by name
Boeuf. He determined to try, entirely on his own account, whether oysters
could not be made to grow on the long muddy fore-shore which is left by
the ebb of the tide. Accordingly, he constructed with his own hands a small
basin enclosed by a low wall, and placed at the bottom a number of stones
picked out of the surrounding mud, stocking his ‘parc’ with a few bushels of
healthy young brood. The experiment was entirely successful, in spite of the
jeers of his neighbours, and Boeuf’s profits, which soon began to mount up
at an astonishing rate, induced others to start similar or more extensive
farms for themselves. The movement spread rapidly, and in a few years a
stretch of miles of unproductive mud banks was converted into the seat of a
most prosperous industry. The general interests of the trade appear to be
regulated in a similar manner to that at Whitstable; delegates are appointed
by the various communities to watch over the business as a whole, while
questions affecting the well-being of oyster-culture are discussed in a sort
of representative assembly.
At the same time as Boeuf was planting his first oysters on the shores of
the Ile de Ré, M. Coste had been reporting to the French government in
favour of such a system of ostreiculture as was then practised by the Italians
in the old classic Lakes Avernus and Lucrinus. The principle there adopted
was to prevent, as far as possible, the escape of the spat from the ground at
the time when it is first emitted by the breeding oyster. Stakes and fascines
of wood were placed in such a position as to catch the spat and give it a
chance of obtaining a hold before it perished or was carried away into the
open sea. The old oyster beds in the Bay of St. Brieuc were renewed on this
principle, banks being constructed and overlaid with bundles of wood to

prevent the escape of the new spat. The attempt was entirely successful, and
led to the establishment or re-establishment of those numerous parcs, with
which the French coast is studded from Brest to the Gironde. The principal
centres of the industry are Arcachon, Auray, Cancale, and la Teste.
It is at Marennes, in Normandy, that the production of the celebrated
‘green oyster’ is carried out, that especial luxury of the French epicure.
Green oysters are a peculiarly French taste, and, though they sometimes
occur on the Essex marshes, there is no market for them in England. The
preference for them, on the continent, may be traced back as early as 1713,
when we find a record of their having been served up at a supper given by
an ambassador at the Hague. Green oysters are not always green, it is only
after they are placed in the ‘claires,’ or fattening ponds, that they acquire the
hue; they never occur in the open sea. The green colour does not extend
over the whole animal, but is found only in the branchiae and labial
tentacles, which are of a deep blue-green. Various theories have been started
to explain the ‘greening’ of the mollusc; the presence of copper in the tanks,
the chlorophyll of marine algae, an overgrowth of some parasite, a disease
akin to liver complaint, have all found their advocates. Prof. Lankester
seems to have established[221] the fact,—which indeed had been observed
70 years before by a M. Gaillon,—that the greening is due to the growth of
a certain diatom (Navicula ostrearia) in the water of the tanks. This diatom,
which is of a deep blue-green colour, appears from April to June, and in
September. The oyster swallows quantities of the Navicula; the pigment
enters the blood in a condition of chemical modification, which makes it
colourless in all the other parts of the body, but when the blood reaches the
gills the action of the secretion cells causes the blue tint to be restored. The
fact that the colour is rather green than blue in the gills, which are yellowish
brown, is due to certain optical conditions.
Not till the young white oyster has been steeped for several years in the
muddy waters of the ‘claires’ does it acquire the proper tint to qualify it for
the Parisian restaurant. The ‘claires’ are each about 100 feet square,
surrounded by low broad banks of earth, about 3 feet high and 6 feet thick
at the base. Before the oysters are laid down, the gates which admit the tide
are carefully opened and shut a great many times, in order to collect a
sufficient amount of the Navicula. When this is done, the beds are formed,
and are not again overflowed by the sea, except at very high tides. The

oysters are shifted from one ‘claire’ to another, in order to perfect the
‘greening’ process. About fifty million of these ‘huitres de Marennes’ are
produced annually, yielding a revenue of 2,500,000 francs.
It appears, from the experience of one of the most enthusiastic of French
oyster-growers (Dr. Kemmerer), that oysters grow best in muddy water, and
breed best in clear water. Thus the open sea is the place where the spat
should fall and be secured, and, as soon as it is of a suitable size, it should
be transferred to the closed tank or reservoir, where it will find the quiet and
the food (confervae, infusoria, minute algae) which are so requisite for its
proper growth. In muddy ground the animal and phosphorous matter
increases, and the flesh becomes fatter and more oily. A sudden change
from the clear sea-water to the muddy tank is inadvisable, and thus a series
of shiftings through tanks with water of graduated degrees of nourishment
is the secret of proper oyster cultivation.
The American oyster trade is larger even than the French. The Baltimore
oyster beds in the Chesapeake River and its tributaries cover 3000 acres,
and produce an annual crop of 25 million bushels, as many as 100,000
bushels being sometimes taken from Chesapeake Bay in a single day.
Baltimore is the centre of the tinned oyster trade, while that in raw oysters
centres in New York. Most of the beds whose produce is carried to New
York are situated in New Jersey, Connecticut, Delaware, or Virginia. The
laws of these states do not allow the beds to be owned by any but resident
owners, and the New York dealers have consequently to form fictitious
partnerships with residents near the various oyster beds, supply them with
money to buy the beds and plant the oysters, and then give them a share in
the profits. It has been estimated that from the Virginia beds 4,000,000
bushels of oysters are carried every year to Fair Haven in New England,
4,000,000 to New York, 3,000,000 to Providence, and 2,000,000 each to
Boston, Philadelphia, and Baltimore. The American ‘native’ (O. virginica)
is a distinct species from our own, being much larger and longer in
proportion to its breadth; it is said to be also much more prolific.
According to Milne-Edwards,[222] in the great oyster parks on the coasts
of Calvados, the oysters are educated to keep their shells closed when out of
water, and so retain water enough inside to keep their gills moist, and arrive
at their destination in good condition. As soon as an oyster is taken out of
the sea, it closes its shells, and keeps them closed until the shock of removal

has passed away, or perhaps until the desirability of a fresh supply of water
suggests itself. The men take advantage of this to exercise the oysters,
removing them from the sea for longer and longer periods. In time this has
the desired effect; the well-educated mollusc learns that it is hopeless to
‘open’ when out of the water, and so keeps his shell closed and his gills
moist, and his general economy in good condition.
Oysters have been known to live entirely out of water for a considerable
time. Prof. Verrill once noticed[223] a large cluster of oysters attached to an
old boot, hanging outside a fish-shop in Washington. They had been taken
out of the water on about 10th December, and on 25th February following
some of the largest were still alive. It was noticed that all those which
survived had the hinge upward and the ventral edge downward, this being
the most favourable position possible for the retention of water within the
gill-cavity, since the edge of the mantle would pack against the margins of
the shell, and prevent the water from leaking away.
Such a succulent creature as the oyster has naturally many enemies. One
of the worst of these is the ravenous Starfish, or Five-finger. His
omnivorous capacities are well described by a clever writer and shrewd
observer of nature: “Here is one doubled up like a sea-urchin, brilliant of
hue, and when spread out quite 16 inches in diameter; where, and oh where,
can you obtain a prey? The hoe we carry is thrust out and the mass dragged
shorewards, when the rascal disgorges two large dog-whelks he has been in
the process of devouring. We feel a comfortable glow of satisfaction to
think that this enemy of our oyster-beds is also the enemy of our other
enemy, this carnivorous borer. Here, quite close alongside, is another, only
inferior in size, and we drag him ashore likewise, to find that the fellow has
actually had the courage and audacity to suck the contents out of a large
horse-mussel (Modiola), the strong muscle alone remaining undevoured.
We proceed along but a short way when we meet with still another in the
curled-up condition in which they gorge themselves, and as we drag it
shorewards the shell of a Tapes pullastra drops from the relaxing grasp of
the ogre. Slowly the extended stomach returns to its place, and the monster
settles back to an uncomfortable after-dinner siesta on an exposed boulder;
for the starfish wraps its turned-out stomach around the prey it has secured,
in place of attempting to devour the limey covering in which most of its
game is protected. Once the mouth of the shell is enclosed in the stomach of

the starfish, the creature soon sickens, the hinge-spring relaxes its hold, and
the shell opening permits the starfish to suck out the gelatinous contents,
and cast free the calcareous skeleton.”[224]
According to other observers the starfish seizes the oyster with two of
his fingers, while with the other three he files away the edge of the flat or
upper valve until the points of contact with the round valve are reduced
almost to nothing; then he can introduce an arm, and the rest is easy work.
Others suggest that the starfish suffocates the oyster by applying two of its
fingers so closely to the edge of the valves that the oyster is unable to open
them; after a while the vital powers relax and the shell gapes. The Rev. J. G.
Wood holds[225] that the starfish pours a secretion from its mouth which
“paralyses the hinge muscle and causes the shell to open.” Sometimes in a
single night a whole bed of oysters will be totally destroyed by an invasion
of starfish. Another dreaded enemy is the ‘whelk,’ a term which includes
Purpura lapillus, Murex erinaceus, Buccinum undatum, and probably also
Nassa reticulata. All these species perforate the shell with the end of their
radula, and then suck out the contents through the neatly drilled hole. Skate
fish are the cause of terrible destruction in the open beds, and a scarcely less
dangerous visitant is the octopus. Crabs crush the young shells with their
claws, and are said to gather in bands and scratch sand or mud over the
larger specimens, which makes them open their shells. Yet another, and
perhaps unconscious, foe is found in the common mussel, which takes up
room meant for the young oysters, grows over the larger individuals, and
harbours all sorts of refuse between and under its closely packed ranks.
Cliona, a parasitic sponge, bores in between the layers of the oyster’s shell,
pitting them with tiny holes (corresponding to its oscula), and disturbing the
inmate, who has constantly to construct new layers of shell from the inside.
Weed, annelids, ‘blubber,’ shifting sand or mud, sewage or any poisoning of
the water, are seriously harmful to the oyster’s best interests. A very severe
winter is often the cause of wholesale destruction in the beds. According to
the Daily News of 26th March 1891, the Whitstable oyster companies lost
property to the value of £30,000 in the exceptionally cold winter of 1890–
91, when, on the coast of Kent, the surface temperature of the sea sank
below 32°, and the advancing tide pushed a small ice-floe before it. Two
million oysters were laid down in one week of the following spring, to
make up for the loss. During the severe winter of 1892–93 extraordinary

efforts were made at Hayling I. to protect the oysters from the frost. Twenty
million oysters were placed in ponds for the winter, and a steam-engine was
for days employed to keep the ponds thawed and supplied with water, while
large coal and coke fires were kept burning at the edge of the ponds.[226] On
the other hand, the unusually warm and sunny summer of 1893 is said to
have resulted in the finest fall of spat known in Whitstable for fifty years.
[227]

The reproductive activity of the oyster is supposed to commence about
the third year. Careful research has shown[228] that the sexes in the English
oyster are not separate, but that each individual is male as well as female,
producing spermatozoa as well as ova in the same gland. Here, however,
two divergent views appear. Some authorities hold that the oyster does not
fecundate its own eggs, but that this operation is performed by spermatozoa
emitted by other specimens. It is believed that, in each individual, the
spermatozoa arrive at maturity first, and that the ova are not produced until
after the spermatozoa have been emitted; thus the oyster is first male and
then female, morphologically hermaphrodite, but physiologically unisexual.
Others are of opinion that the oyster does fecundate its own eggs, ova being
first produced, and passed into the infrabranchial chamber—the ‘white-
sick’ stage—and then, after an interval, spermatozoa being formed and
fecundating these ova—the ‘black-sick’ stage. In this latter view the oyster
is first female and then male, and is, both morphologically and
physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters are
the male, and ‘white-sick’ the female, is therefore quite incorrect.
The ova, in their earliest stage, consist of minute oval clusters of
globules floating in a transparent mucus. They pass from the ovary into the
gills and folds of the mantle, and are probably fecundated within the
excretory ducts of the ovary, before arriving in the mantle chamber. In this
stage the oyster is termed ‘white-sick.’ In about a fortnight, as the course of
development proceeds, the fertilised ova become ciliated at one end (the so-
called veliger stage, p. 131), and soon pigment appears in various parts of
the embryos, giving them a darker colour, which varies from grayish to
blue, and thus the white-sick oyster becomes ‘black-sick.’ When the black
spat emerge, they are still furnished with cilia for their free-swimming life.
This is of very short duration, for unless the embryo finds some suitable
ground on which to affix itself within forty-eight hours, it perishes. As the

spat escapes from the parent oyster, which slightly opens its valves and
blows the spat out in jets, it resembles a thick cloud in the water, and is
carried about at the mercy of wind and tide. April to August are the usual
spawning months, warm weather being apparently an absolute necessity to
secure the adhering of the spat. A temperature of 65° to 72° F. seems
requisite for their proper deposit. Thus on a fine, warm day, with little wind
or tide running, the spat will fall near the parents and be safely secured,
while in cold, blustering weather it will certainly be carried off to a
distance, and probably be altogether lost. The number of young produced
by each individual has been variously estimated at from 300,000 to
60,000,000. Either extreme seems enormous, but it must be remembered
that besides climatal dangers, hosts of enemies—other Mollusca, fish, and
Crustacea—beset the opening career of the young oyster.
As soon as the spat has safely ‘fallen,’ it adheres to some solid object,
and loses the cilia which were necessary for its swimming life. It begins to
grow rapidly, increasing from about 1/20 inch in diameter to about the size
of a threepenny piece in five or six months, and in a year to one inch in
diameter. Roughly speaking, the best guide to an oyster’s age is its size; it is
as many years old as it measures inches across.
The oyster is at its prime at the age of five; its natural life is supposed to
be about ten years. The rings, or ‘shoots’ on a shell are not—as is frequently
supposed—marks of annual growth; cases have been noticed where as
many as three ‘shoots’ were made during the year.
An oyster is furnished, on the protruding edges of the mantle, with
pigmented spots which may be termed ‘visual organs,’ though they hardly
rise to the capacities and organisation of real ‘eyes.’ But there is no doubt
that they are sufficiently sensitive to the action of light to enable the oyster
to apprehend the approach of danger, and close his doors accordingly. ‘How
sensitive,’ notes Mr. W. Anderson Smith,[229] ‘the creatures are to the light
above them; the shadow of the iron as it passes overhead is instantaneously
noted, and snap! the lips are firmly closed.’
The geographical distribution of Ostrea edulis extends from Tränen, in
Norway, close to the Arctic circle, to Gibraltar and certain parts of the
Mediterranean, Holland, and N. Germany to Heligoland, and the western
shores of Sleswick and Jutland. It occurs in Iceland, but does not enter the

Baltic, where attempts to colonise it have always failed. Some authorities
regard the Mediterranean form as a distinct species.
The literature of oyster-cookery may be passed over in silence. The
curious may care to refer to M. S. Lovell’s Edible British Mollusks, where
no less than thirty-nine different ways of dressing oysters are enumerated. It
may, however, be worth while to add a word on the subject of poisonous
oysters. Cases have been known where a particular batch of oysters has, for
some reason, been fatal to those who have partaken of them. It is possible
that this may have been due, in certain instances, to the presence of a
superabundance of copper in the oysters, and there is no doubt that the
symptoms detailed have often closely resembled those of copper poisoning.
Cases of poisoning have occurred at Rochefort through’ the importation of
‘green oysters’ from Falmouth. It would no doubt be dangerous ever to eat
oysters which had grown on the copper bottom of a ship. But copper is
present, in more or less minute quantities, in very many Mollusca, and it is
more probable that a certain form of slow decomposition in some shell-fish
develops an alkaloid poison which is more harmful to some people than to
others, just as some people can never digest any kind of shell-fish.[230]
These alkaloid developments from putrescence are called ptomaines. In
confirmation of this view, reference may be made to a case, taken from an
Indian Scientific Journal, in which an officer, his wife, and household ate
safely of a basket of oysters for three days at almost every meal. The basket
then passed out of their hands, not yet exhausted of its contents, and a man
who had already eaten of these oysters at the officer’s table was afterwards
poisoned by some from the same basketful.
The cultivation of the common mussel (Mytilus edulis L.) is not
practised in this country, although it is used as food in the natural state of
growth all round our coasts. The French appear to be the only nation who
go in for extensive mussel farming. The principal of these establishments is
at a little town called Esnaudes, not far from La Rochelle, and within sight
of the Ile de Ré and its celebrated oyster parks. The secret of the cultivation
consists in the employment of ‘bouchots,’ or tall hurdles, which are planted
in the mud of the fore-shore, and upon which the mussel (la moule, as the
French call it) grows. The method is said[231] to have been invented as long
ago as 1235 by a shipwrecked Irishman named Walton. He used to hang a
purse net to stakes, in the hope of capturing sea birds. He found, however,

that the mussels which attached themselves to his stakes were a much more
easily attainable source of food, and he accordingly multiplied his stakes,
out of which the present ‘bouchot’ system has developed. The shore is
simply a stretch of liquid mud, and the bouchots are arranged in shape like a
single or double V, with the opening looking towards the sea. The
fishermen, in visiting the bouchots, glide about over the mud in piroques or
light, flat-bottomed boats, propelling them by shoving the mud with their
feet. Each bouchot is now about 450 yards long, standing 6 feet out of the
mud, making a strong wall of solid basket-work, and as there are altogether
at least 500 bouchots, the total mussel-bearing length of wall is nearly 130
miles.
The mussel-spat affixes itself naturally to the bouchots nearest the sea, in
January and February. Towards May the planting begins. The young
mussels are scraped off these outermost bouchots, and placed in small bags
made of old canvas or netting, each bag holding a good handful of the
mussels. The bags are then fastened to some of the inner bouchots, and the
mussels soon attach themselves by their byssus, the bag rotting and falling
away. They hang in clusters, increasing rapidly in size, and at the proper
time are transplanted to bouchots farther and farther up the tide level, the
object being to bring the matured animal as near as possible to the land
when it is time for it to be gathered. This process, which aims at keeping the
mussel out of the mud, while at the same time giving it all the nutrition that
comes from such a habitat, extends over about a year in the case of each
individual. Quality, rather than quantity, is the aim of the Esnaudes
boucholiers. The element of quantity, however, seems to come in when we
are told that each yard of the bouchots is calculated to yield a cartload of
mussels, value 6 francs, and that the whole annual revenue is at least
£52,000.
In this country, and especially in Scotland, mussels are largely used as
bait for long-line fishing. Of late years other substances have rather tended
to take the place of mussels, but within the last twenty years, at Newhaven
on the Firth of Forth, three and a half million mussels were required
annually to supply bait for four deep-sea craft and sixteen smaller vessels.
According to Ad. Meyer,[232] boughs of trees are laid down in Kiel Bay, and
taken up again, after three, four, or five years, between December and
March, when they are found covered with fine mussels. The boughs are

then sold, just as they are, by weight, and the shell-fish sent into the interior
of Germany.
Mussels are very sensitive to cold weather. In 1874, during an easterly
gale, 195 acres of mussels at Boston, in Lincolnshire, were killed in a single
night. They soon affix themselves to the bottom of vessels that have lain for
any length of time in harbour or near the coast. The bottom of the Great
Eastern steamship was at one time so thickly coated with mussels that it
was estimated that a vessel of 200 tons could have been laden from her. In
some of our low-lying coast districts mussels are a valuable protection
against inundation. “An action for trespass was brought some time ago for
the purpose of establishing the right of the lord of the manor to prevent the
inhabitants of Heacham from taking mussels from the sea-shore. The
locality is the fore-shore of the sea, running from Lynn in a north-westerly
direction towards Hunstanton in Norfolk; and the nature of the shore is such
that it requires constant attention, and no little expenditure of money, to
maintain its integrity, and guard against the serious danger of inundations of
the sea. Beds of mussels extend for miles along the shore, attaching
themselves to artificial jetties running into the sea, thereby rendering them
firm, and thus acting as barriers against the sea [and as traps to catch the
silt, and thus constantly raise the level of the shore]. Therefore, while it is
important for the inhabitants, who claim a right by custom, to take mussels
and other shell-fish from the shore, it is equally important for the lord of the
manor to do his utmost to prevent these natural friends of his embankments
and jetties from being removed in large quantities.”[233]
The fable that Bideford Bridge is held together by the byssi of Mytilus,
which prevent the fabric from being carried away by the tide, has so often
been repeated that it is perhaps worth while to give the exact state of the
case, as ascertained from a Town Councillor. The mussels are supposed to
be of some advantage to the bridge, consequently there is a by-law
forbidding their removal, but the corporation have not, and never had, any
boat or men employed in any way with regard to them.
Poisoning by mussels is much more frequent than by oysters. At
Wilhelmshaven,[234] in Germany, in 1885, large numbers of persons were
poisoned, and some died, from eating mussels taken from the harbour. It
was found that when transferred to open water these mussels became
innocuous, while, on the other hand, mussels from outside, placed in the

harbour, became poisonous. The cause obviously lay in the stagnant and
corrupted waters of the harbour, which were rarely freshened by tides. It
was proved to demonstration that the poison was not due to decomposition;
the liver of the mussels was the poisonous part. In the persons affected, the
symptoms were of three kinds, exanthematous (skin eruptions), choleraic,
and paralytic. Cases of similar poisoning are not unfrequent in our own
country, and the circumstances tend to show that, besides the danger from
mussels bred in stagnant water, there is also risk in eating them when ‘out
of season’ in the spawning time.
Whelks are very largely employed for bait, especially in the cod fishery.
The whelk fishery in Whitstable Bay, both for bait and for human food,
yields £12,000 a year. Dr. Johnston, of Berwick, estimated that about 12
million limpets were annually consumed for bait in that district alone. The
cockle fishery in Carmarthen Bay employs from 500 to 600 families, and is
worth £15,000 a year, that in Morecambe Bay is worth £20,000.
Cultivation of Snails for Food; Use as Medicine.—It was a certain
Fulvius Hirpinus who, according to Pliny the elder,[235] first instituted snail
preserves at Tarquinium, about 50 b.c. He appears to have bred several
species in his ‘cochlearia,’ keeping them separate from one another. In one
division were the albulae, which came from Reate; in another the ‘very big
snails’ (probably H. lucorum), from Illyria; in a third the African snails,
whose characteristic was their fecundity; in a fourth those from Soletum,
noted for their ‘nobility.’ To increase the size of his snails, Hirpinus fed
them on a fattening mixture of meal and new wine, and, says the author in a
burst of enthusiasm, ‘the glory of this art was carried to such an extent that
a single snail-shell was capable of holding eighty sixpenny pieces.’
Varro[236] recommends that the snaileries be surrounded by a ditch, to save
the expense of a special slave to catch the runaways. Snails were not
regarded by the Romans as a particular luxury. Pliny the younger
reproaches[237] his friend Septicius Clarus for breaking a dinner engagement
with him, at which the menu was to have been a lettuce, three snails and
two eggs apiece, barley water, mead and snow, olives, beetroot, gourds and
truffles, and going off somewhere else where he got oysters, scallops, and
sea-urchins. In Horace’s time they were used as a gentle stimulant to the
appetite, for

“’Tis best with roasted shrimps and Afric snails
To rouse your drinker when his vigour fails.”[238]

Escargotières, or snail-gardens, still exist in many parts of Europe, e.g. at
Dijon, at Troyes and many other places in central and southern France, at
Brunswick, Copenhagen, and Ulm. The markets at Paris, Marseilles,
Bordeaux, Toulouse, Nantes, etc., are chiefly supplied by snails gathered
from the open country, and particularly from the vineyards, in some of
which Helix pomatia abounds. In the Morning Post of 8th May 1868 there
is an account of the operation of clearing the celebrated Clos de Vougeot
vineyard of these creatures. No less than 240 gallons were captured, at a
cost in labour of over 100 francs, it being estimated that these snails would
have damaged the vines to an extent represented by the value of 15 to 20
pipes of wine, against which may be set the price fetched by the snails when
sold in the market.
It is generally considered dangerous to eat snails at once which have
been gathered in the open country. Cases have occurred in which death by
poisoning has resulted from a neglect of this precaution, since snails feed on
all manner of noxious herbs. Before being sent to table at the restaurants in
the great towns, they are fattened by being fed with bran in the same way as
oysters.
The Roman Catholic Church permits the consumption of snails during
Lent. Very large numbers are eaten in France and Austria at this time. At the
village of Cauderon, near Bordeaux, it is the proper thing to end Carnival
with especial gaiety, but to temper the gaiety with a dish of snails, as a
foretaste of Lenten mortification.
The following species appear to be eaten in France at the present day: H.
pomatia, aspersa, nemoralis, hortensis, aperta, pisana, vermiculata, lactea.
According to Dr. Gray, the glassmen at Newcastle used to indulge in a snail
feast once a year, and a recent writer informs us that H. aspersa is still eaten
by working people in the vicinity of Pontefract and Knottingley.[239] But in
this country snails appear to be seldom consciously used as an article of
food; the limitation is necessary, for Lovell tells us that they are much
employed in the manufacture of cream, and that a retired (!) milkman
pronounced it the most successful imitation known.

Preparations made from snails used to be highly esteemed as a cure for
various kinds of diseases and injuries. Pliny the elder recommends them for
a cough and for a stomach-ache, but it is necessary “to take an uneven
number of them.”[240] Five African slugs, roasted and beaten to a powder,
with half a drachm of acacia, and taken with myrtle wine, is an excellent
remedy for dysentery. Treated in various ways, snails have been considered,
in modern times, a cure for ague, corns, web in the eye, scorbutic affections,
hectic fevers, pleurisy, asthma, obstructions, dropsy, swelling of the joints,
headache, an impostume (whitlow), and burns. One of Pliny’s remedies for
headache, which competes with the bones of a vulture’s head or the brain of
a crow or an owl, is a plaister made of slugs with their heads cut off, which
is to be applied to the forehead. He regards slugs as immature snails, whose
growth is not yet complete (nondum perfectae). Lovell states that “a large
trade in snails is carried on for Covent Garden market in the Lincolnshire
fens, and that they are sold at 6d. per quart, being much used for
consumptive patients and weakly children.”
The custom still seems to linger on in some parts of the country. Mr. E.
Rundle, of the Royal Cornwall Infirmary, gives his experience in the
following terms: “I well remember, some twelve years since, an individual
living in an adjoining parish [near Truro] being pointed out to me as ‘a snail
or slug eater.’ He was a delicate looking man, and said to be suffering from
consumption. Last summer I saw this man, and asked him whether the
statement that he was a ‘snail eater’ was true: he answered, ‘Yes, that he
was ordered small white slugs—not snails—and that up till recently he had
consumed a dozen or more every morning, and he believed they had done
him good.’ There is also another use to which the country people here put
snails, and that is as an eye application. I met with an instance a few weeks
since, and much good seemed to have followed the use.”[241]
A reverend Canon of the Church of England, whose name I am not
permitted to disclose, informs me that there was a belief among the youth of
his native town (Pontypool, in Monmouthshire) that young slugs were
‘good for consumption,’ and that they were so recommended by a doctor
who practised in the town. The slugs selected were about ¾ inch long,
“such as may be seen crawling on the turf of a hedge-bank after a shower of
rain.” They were “placed upon the tongue without any previous preparation,
and swallowed alive.” My informant himself indulged in this practice for

some time, “not on account of any gustatory pleasure it afforded, but from
some vague notion that it might do him good.”
A colleague of mine at King’s College tells me that the country people at
Ponteland, near Morpeth, habitually collect Limax agrestis and boil it in
milk as a prophylactic against consumption. He has himself frequently
devoured them alive, but they must be swallowed, not scrunched with the
teeth, or they taste somewhat bitter.
Snails have occasionally fallen, with other noxious creatures, under the
ban of the Church. In a prayer of the holy martyr Trypho of Lampsacus
(about 10th cent. a.d.) there is a form of exorcism given which may be used
as occasion requires. It runs as follows: “O ye Caterpillars, Worms, Beetles,
Locusts, Grasshoppers, Woolly-Bears, Wireworms, Longlegs, Ants, Lice,
Bugs, Skippers, Cankerworms, Palmerworms, Snails, Earwigs, and all other
creatures that cling to and wither the fruit of the grape and all other herbs, I
charge you by the many-eyed Cherubim, and by the six-winged Seraphim,
which fly round the throne, and by the holy Angels and all the Powers, etc.
etc., hurt not the vines nor the land nor the fruit of the trees nor the
vegetables of —— the servant of the Lord, but depart into the wild
mountains, into the unfruitful woods, in which God hath given you your
daily food.”
Prices given for Shells.—Very high prices have occasionally been given
for individual specimens, particularly about thirty or forty years ago, when
the mania for collecting was at its height. In those days certain families,
such as the Volutidae, Conidae, and Cypraeidae, were the especial objects
of a collector’s ardour, and he spared no expense to make his set of the
favourite genus as complete as possible. Thus at Stevens’ auction-rooms in
Covent Garden, on 21st July 1854, one specimen of Conus cedo nulli
fetched £9: 10s., and another £16, a C. omaicus 16 guineas, C. victor £10,
and C. gloria maris, the greatest prize of all, £43: 1s. At the Vernède sale,
on 14th Dec. 1859 two Conus omaicus fetched £15 and £22, and a C. gloria
maris £34. At the great Dennison sale, in April 1865, the Conidae fetched
extravagant prices, six specimens averaging over £20 apiece. Conus cedo
nulli went for £18 and £22, C. omaicus for £12, C. malaccanus for 10
guineas (this and one of the cedo nulli being the actual specimens figured in
Reeve’s Conchologia Iconica), C. cervus for £19 and C. gloria maris for
£42. On 9th May 1866 a Cypraea Broderipii was sold at Stevens’ auction-

rooms for £13, and at the Dennison sale a Cypraea princeps fetched £40,
and C. guttata £42. The Volutidae, although not quite touching these prices,
have yet done fairly well. Mr. Dennison’s Voluta fusiformis sold for £6:
15s., V. papillaris for £5, V. cymbiola for £5: 15s., V. reticulata for eight
guineas, and two specimens of the rarest of all Volutas, V. festiva, for £14
and £16, both being figured in the Conchologia. At the same sale, two
unique specimens of Oniscia Dennisoni fetched £17 and £18 respectively,
and, at the Vernède sale, Ancillaria Vernèdei was bought for £6: 10s., and
Voluta piperata for £7: 10s.
A unique specimen of a recent Pleurotomaria (quoyana F. and B.) was
purchased by Miss de Burgh in 1873 for 25 guineas, and another species of
the same genus (adansoniana Cr. and F.), of extraordinary size and beauty,
is now offered for sale for about £100.
Bivalves have never fetched quite such high prices as univalves, but
some of the favourite and showy genera have gone near to rival them. On
22nd June 1869, at Stevens’, Pecten solaris fetched £4: 5s., P. Reevii £4:
8s., and Cardita varia 5 guineas. Mr. Dennison’s specimens of Pecten
subnodosus sold for £7, of Corbula Sowerbyi for £10, of Pholadomya
candida for £8 and £13, while at the Vernède sale a Chama damicornis
fetched £7.

CHAPTER V
REPRODUCTION—DEPOSITION OF EGGS—DEVELOPMENT OF THE FERTILISED OVUM
—DIFFERENCES OF SEX—DIOECIOUS AND HERMAPHRODITE MOLLUSCA—
DEVELOPMENT OF FRESH-WATER BIVALVES

Reproduction in the Mollusca invariably takes place by means of eggs,
which, after being developed in the ovary of the female, are fertilised by the
spermatozoa of the male. As a rule, the eggs are ‘laid,’ and undergo their
subsequent development apart from the parent. This rule, however, has its
exceptions, both among univalve and bivalve Mollusca, a certain number of
which hatch their young from the egg before expelling them. Such
ovoviviparous genera are Melania, Paludina, Balea, and Coeliaxis among
land and fresh-water Mollusca, and Cymba and many Littorina amongst
marine. The young of Melania tuberculata, in Algeria, have been noticed to
return, as if for shelter, to the branchial cavity of the mother, some days
after first quitting it. Isolated species among Pulmonata are known to be
ovoviviparous, e.g. Patula Cooperi, P. Hemphilli, and P. rupestris,
Acanthinula harpa, Microphysa vortex, Pupa cylindracea and muscorum,
Clausilia ventricosa, Opeas dominicensis, Rhytida inaequalis, etc. All
fresh-water Pelecypoda yet examined, except Dreissensia, are
ovoviviparous.
The number of eggs varies greatly, being highest in the Pelecypoda. In
Ostrea edulis it has been estimated at from 300,000 to 60,000,000; in
Anodonta from 14,000 to 20,000; in Unio pictorum 200,000. The eggs of
Doris are reckoned at from 80,000 to 600,000, of Loligo and Sepia at about
30,000 to 40,000. Pulmonata lay comparatively few eggs. Arion ater has
been observed to lay 477 in forty-eight days (p. 42). Nests of Helix aspersa
have been noticed, in which the number of eggs varied from about 40 to
100. They are laid in little cup-shaped hollows at the roots of grass, with a
little loose earth spread over them. The eggs of Testacella are rather large,
and very elastic; if dropped on a stone floor they will rebound sharply
several inches. The Cochlostyla of the Philippines lay their eggs at the tops
of the great forest trees, folding a leaf together to serve as a protection.
The eggs of the great tropical Bulimus and Achatina, together with those
of the Macroön group of Helix (Helicophanta, Acavus, Panda) are
exceedingly large, and the number laid must be decidedly less than in the

smaller Pulmonata. Bulimus oblongus, for instance, from Barbados, lays an
egg about the size of a sparrow’s (Fig. 38), Achatina sinistrorsa as large as
a pigeon’s. The Cingalese Helix Waltoni when first hatched is about the size
of a full-grown H. hortensis. There is, in the British Museum, a specimen of
the egg of a Bulimus from S. America (probably maximus or popelairanus)
which measures exactly 1¾ inch in length.
The Limnaeidae deposit their eggs in irregular gelatinous masses on the
under side of the leaves of water-plants, and on all kinds of débris.

Fig. 38.—Newly-hatched young and egg of
Bulimus oblongus Müll., Barbados.
Natural size.

The Rachiglossa or marine carnivorous families lay their eggs in tough
leathery or bladdery capsules, which are frequently joined together in
shapes which differ with the genus. Each capsule contains a varying number
of ova. The cluster of egg-capsules of Buccinum undatum is a familiar
object on all our sandy coasts. The capsules of Purpura lapillus are like
delicate pink grains of rice, set on tiny stalks. They are not attached to one
another, but are set closely together in groups in sheltered nooks of the
rocks. A single Purpura has been observed to produce 245 capsules!
Busycon lays disc-shaped capsules which are all attached at a point in the
edge to a cartilaginous band nearly 3 feet in length, looking like a number
of coins tied to a string at equal distances from one another. In Murex
erinaceus the egg-capsules are triangular, with a short stalk. They are
deposited separately in clusters of from 15 to 150, there being about 20 ova
in each capsule. It appears that all the species of the same genus have by no
means the same method of depositing their eggs, nor do they always

produce eggs of at all similar size or shape. Thus, of two British species of
Nassa, N. reticulata lays egg-capsules in shape like flattened pouches with
a short stalk, and fastens them in rows to the leaves of Zostera; M.
incrassata, on the other hand, deposits solitary capsules, which are shaped
like rounded oil-flasks. Neptunea antiqua lays its eggs in bunched capsules,
like Bucc. undatum (Fig. 40), but the capsules of N. gracilis are solitary.

Fig. 39.—Various forms of spawn in Prosobranchiata:
A and D, Pyrula or Busycon; B, Conus; C, Voluta
musica; E, Ampullaria (from specimens in the
British Museum); all × ⅔.

In Natica the eggs are deposited in what looks like a thick piece of sand-
paper, curled in a spiral form (Fig. 41). The sand is agglutinated by copious
mucus into a sort of sheet, and the eggs are let into this, sometimes (N.
heros) in regular quincunx form. Ianthina attaches its eggs to the under side
of its float (Fig. 42). The Trochidae deposit their eggs on the under side of
stones and sea-weeds, each ovum being contained in a separate capsule, and

all the capsules glued together into an irregular mass of varying size. The
female of Galerus chinensis hatches her eggs by keeping them between her
foot and the stone she adheres to. They are laid in from 6 to 10 capsules,
connected by a pedicle and arranged like the petals of a rose, with 10 to 12
eggs in each capsule. Those Littorina which are not ovoviviparous deposit
their spawn on sea-weeds, rocks, and stones. The eggs are enveloped in a
glairy mass which is just firm enough to retain its shape in the water; each
egg has its own globule of jelly and is separated from the others by a very
thin transparent membrane.[242]

Fig. 40.—Egg-capsules of, A,
Nassa reticulata L. × ⅔; B,
Buccinum undatum L. × ⅔;
C, Neptunea antiqua L. ×
⅓.

Fig. 41.—Spawn of a species of
Natica (from a specimen in
the British Museum) × ½.

Fig. 42.—Ianthina fragilis Lam. FL, float; O, ova; Pr, proboscis; Br,
branchiae; F, foot. (Quoy and Gaimard.)

Chiton marginatus, when kept in captivity, has been noticed[243] to
elevate the posterior part of the girdle, and to pour out a continuous stream
of flaky white matter like a fleecy cloud, which proved to be of a glutinous
nature. It then discharged ova, at the rate of one or two every second, for at
least fifteen minutes, making a total of 1300 to 1500, each being about 1/100
inch diameter. The ova were shot into the glutinous cloud, which seemed to
serve as a sort of nidus to entangle the ova and prevent them being carried
away. The subsequent development was rapid, and in seven days the young
Chiton was hatched, being then about 1/20 inch long. Lovén has described
the same species as laying its eggs, loosely united in clusters of seven to
sixteen, upon small stones. There is probably some mistake about the
identification, but the observation illustrates the varying methods of
oviposition among allied forms.

Fig. 43.—Egg-capsules of A, Sepia elegans Orb., and
B, Octopus vulgaris Lam.

Not very much is known with regard to the ovipositing of the
Cephalopoda, especially those which inhabit deep water. Masses of ova
arranged in very various forms have occasionally been met with floating in
the ocean, but it is next to impossible to determine to what species, or even
genus, they belong.[244]
In Loligo punctata the ova are contained in small cylindrical cases
measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each case.
Hundreds of these cases are attached together like a bundle of sausages or
young carrots, and the movements of the embryos within can be distinctly
noted. Sepia officinalis lays large black pear-shaped capsules, each of which
is tied to some place of attachment by a kind of ribbon at the upper end of
the capsule, the whole forming a large group like a bunch of grapes.
Octopus vulgaris deposits thousands of small berry-shaped ova, attached to
a string which runs along the centre of the mass (Fig. 43).
The so-called shell of the female Argonauta is nothing more than a form
of protection for the ova, and is in no sense homologous to the ordinary
molluscan shell. The ova consist of a large granulated mass, attached to a
many branched stem; they are contained in the spire of the shell, in contact
with the posterior part of the body of the mother, but sometimes project
externally beyond the coil of the spire.

Certain species possess the curious property of laying their eggs on the
outside of their own shells. Buccinopsis Dalei is not unfrequently found
decorated with its own egg-capsules. Possibly this species, which lives on
oozy ground, finds this the only secure place of attachment for its progeny.
Neritina fluviatilis has a similar habit, and so have many other species of
Neritina and Navicella. It is not quite clear, in the latter cases, whether the
eggs are laid by the specimens on whose shell they are found, or whether
they are deposited by others. In either case, perhaps the shell is the safest
place for them in the rapid streams which both genera frequent. Specimens
of Hydrobia ulvae taken on the wet sands at the mouth of the Dee, are
found to have several little rounded excrescences scattered over the surface
of the shell. These, on examination, are found to be little masses of small
sand-grains, in the centre of which is a clear jelly containing segmenting
ova or young embryos. Here again, in all probability, the shell is the only
comparatively stable object, in the expanse of shifting sands, on which the
eggs can be laid.[245]
The pulmonate genus Libera, which occurs on a few of the island groups
in the Central Pacific, is remarkable for the habit of laying its eggs within
its own cavernous umbilicus, which is narrowed at the lower part. The eggs
number from four to six, or the same number of very young shells may be
seen closely packed in the cavity, each being in shape exactly like a young
Planorbis. This constriction of the umbilicus does not occur till the
formation of the last two whorls, i.e. till the animal is sexually mature.
Some species, but not all, provide for the safety of their eggs more
completely by forming a very thin shelly plate, which nearly closes the
umbilical region, and breaks away or is absorbed to facilitate the escape of
the young shells.[246]
Union of Limax.—With regard to the act of union itself, the method in
certain species of Limax deserves special notice. L. maximus has been
observed at midnight to ascend a wall or some perpendicular surface. A pair
then crawl round and round one another emitting a quantity of mucus which
at length forms a patch, 2 to 2½ inches in diameter. When this acquires
consistency the pair begin to twist round each other in corkscrew form, and
detach themselves from the wall, hanging by a cord of the thickened mucus,
about 8–15 inches long, and still twisting round each other. The external

generative organs are then protruded and copulation takes place, after which
the bodies untwist, separate, and crawl up the cord again to the wall.[247]
Periodicity in Breeding.—In the marine Mollusca, the winter months
appear to be the usual time for the deposition of eggs. Careful observations
have been made on the Mollusca occurring at Naples,[248] and the general
result seems to be that for all Orders alike the six winter months from
November to April, roughly speaking, are the breeding time. Scarcely any
forms appear to breed habitually in August, September, or October. On our
own coasts, Nudibranchiata come in shore to deposit their ova from January
to April. Purpura lapillus may be observed depositing ova all the year
round, but is most active from January to April. Buccinum undatum breeds
from October to May; Littorina all the year round.
The land Mollusca exhibit rather more periodicity than the marine. In
temperate climates they breed exclusively in the summer months. In the
tropics their periods are determined by the dry and rainy seasons, where
such occur, otherwise they cohabit all the year round. According to Karl
Semper, the snails of the warm Mediterranean region arrive at sexual
maturity when they are six months old, i.e. before they are fully grown.
After a rest of about three months during the heat of summer, a second
period of ovipositing occurs.[249] Helix hortensis and H. nemoralis ascend
trees, sometimes to a height of forty feet, when pairing.[250]
Hybridism as the result of union between different species of Mollusca
is exceedingly rare. Lecoq once noticed[251] on a wall at Anduze (Gard) as
many as twenty specimens of Pupa cinerea united with Clausilia papillaris.
No offspring seem to have resulted from what the professor calls ‘this
innocent error,’ for the wall was carefully scrutinised for a long time, and no
hybrid forms were ever detected.
The same observer noticed, in the Luxembourg garden at Paris, and M.
Gassies has noticed[252] at various occasions, union between Helix aspersa
and nemoralis, H. aspersa and vermiculata, between Stenogyra decollata
and a Helix (sp. not mentioned), H. variabilis and pisana, H. nemoralis and
hortensis. In the two latter cases a hybrid progeny was the result. It has
been noticed that these unions generally took place when the air was in a
very electric condition, and rain had fallen, or was about to fall, abundantly.

Of marine species Littorina rudis has been noticed[253] in union both
with L. obtusata and with L. littorea, but no definite facts are known as to
the result of such unions.
Self-impregnation (see p. 44).
Development of the Fertilised Ovum.—The first stages in the
development of the Mollusca are identical with those which occur in other
classes of animals. The fertilised ovum consists of a vitellus or yolk, which
is surrounded with albumen, and is either contained in a separate capsule, or
else several, sometimes many, ova are found in the same capsule, only a
small proportion of which ultimately develop. The germinal vesicle, which
is situated at one side of the vitellus, undergoes unequal segmentation, the
result of which is usually the formation of a layer of small ectoderm cells
overlying a few much larger cells which contain nearly the whole of the
yolk. The large cells are then invaginated, or are simply covered by the
growth of the ectoderm cells. The result in either case is the formation of an
area, the blastopore, where the inner cells are not covered by the ectoderm.
The blastopore gradually narrows to a circular opening, which, in the great
majority of cases, eventually becomes the mouth. The usual differentiation
of germinal layers takes place, the epiblast eventually giving rise to the
epidermis, nervous system, and special sense organs, the hypoblast to the
liver and to the middle region of the alimentary tract, the mesoblast to the
muscles, the body cavity, the vascular, the excretory and reproductive
systems. The next, or trochosphere (trochophora) stage, involves the
formation of a circlet of praeoral cilia, dividing the still nearly spherical
embryo into two unequal portions, the smaller of which consists simply of
the prostomium, or part in front of the mouth, the larger bearing the mouth
and anus.
So far the series of changes undergone by the embryo are not peculiar to
the Mollusca; we now come to those which are definitely characteristic of
that group. The stage next succeeding the development of the trochosphere
is the definitive formation of the velum, a process especially characteristic
of the Gasteropoda and Pelecypoda, but apparently not occurring in the
great majority of land Pulmonata.

Fig. 44.—Veligers of Dentalium entalis L.: A, longitudinal section of a larva 14 hours old, × 285;
B, larva of 37 hours, × 165; C, longitudinal section of larva of 34 hours, × 165; m, mouth; v,
v, velum. (After Kowalewsky.)

The circlet of cilia becomes pushed more and more towards the anterior
portion of the embryo, the cilia themselves become longer, while the
portion of the body from which they spring becomes elevated into a ridge or
ring, which, as a rule, develops on each side a more or less pronounced
lobe. The name velum is applied to this entire process of ciliated ring and
lobes, and to the area which they enclose.

Fig.45.—Veliger of Patella vulgata L.,
130 hours old: f, rudimentary foot;
op, operculum; sh, shell; v, v, velum.
(After Patten, highly magnified.)

Fig. 46.—Developed larva of Cyclas cornea
L.: br, rudimentary branchiae; by, byssus;
f, foot; m.e, mantle edge; sh, shell. (After
Ziegler, highly magnified.)

Fig. 47.—A, Advanced veliger of Dreissensia: f, foot; m, mouth; sh,
shell; v, v, velum. (After Korschelt and Heider, much enlarged.)
B, Veliger of a Pteropod (Tiedemannia): op, operculum; sh,
shell; v, velum. (After Krohn, much enlarged.)

In this so-called veliger stage, the velum serves, in the first place, to
cause rotation of the larva within the egg-capsules, and, after hatching, as
an organ of locomotion. As a rule, the velum disappears entirely in the adult
mollusc after the free-swimming stage is over, but in the common Limnaea
stagnalis it persists, losing its cilia, as the very prominent circum-oral lobes.
Simultaneously with the development of the velum, and in some cases
earlier, appear the rudiments of the shell-gland and of the foot, the latter
being situated on the ventral side, between the mouth and anus, the former
on the dorsal side, behind the velum, and above the surface of the eventual
visceral sac. Thus the prime characteristics of the veliger stage, subsequent
to the appearance of the velum itself, are the development of the visceral
sac and shell-gland on the upper, and of the foot on the under side.
According to Lankester the primitive shell-gland does not, as a rule, directly
give rise to the shell of the adult mollusc, but becomes filled up by a horny
substance, and eventually disappears; the permanent shell then forms over
the surface of the visceral hump from the original centre of the shell-gland.
It is only in Chiton, and possibly in Limax, that the primitive shell-sac is
retained and developed into the final shell-forming area, which is much
wider, and extends to the edges of the mantle. Within the velar area first
appear the rudiments of the tentacles and eyes; when these become
developed the velum atrophies and disappears.

Several of these veligers when captured in the open sea have been
mistaken for perfect forms, and have been described as such. Thus the larva
of Dolium has been described as Macgillivrayia, that of a Purpura as
Chelotropis and Sinusigera, that of Aporrhais pes pelecani as Chiropteron,
that of Marsenia conspicua as Brownia, Echinospira, and Calcarella.
Cephalopoda.—The embryonic development of the Cephalopoda is
entirely distinct from that of all other Mollusca. The segmentation of the
vitellus is partial, and the embryo is furnished with a vitelline sac, which is
very large in the majority of cases (Fig. 48). There is no free-swimming
stage, but the embryo emerges from the egg fully developed.
Differences of Sex.—In the Mollusca there are two main types of sexual
difference: (i) sexes separate (dioecious type), (ii) sexes united in the same
individual (hermaphrodite type).

Fig. 48.—Two stages in the development of Loligo
vulgaris Lam.: a1, a1, first, and a2, a2, second
pairs of arms; br, branchiae, seen through m,
mantle; e, e, eyes; fi, fins; fu, funnel; v.s, vitelline
sac. (After Kowalewsky.)

In some cases—e.g. certain Pelecypoda—what is practically a third type
occurs. The animal is hermaphrodite, but the male and female elements are

not developed simultaneously, i.e. the same individual is at one time female,
at another male.
1. The sexes are separate in
All Cephalopoda.
Gasteropoda Amphineura (except Neomeniidae).
Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia).
Scaphopoda.
Many Pelecypoda.

2. The sexes are united in
Gasteropoda Opisthobranchiata.
Gasteropoda Pulmonata.
Certain Pelecypoda.[254]

In the dioecious Mollusca, sexual union is the rule, but is by no means
universal. In some instances,—e.g. Vermetus, Magilus, Patella, Haliotis,
Crepidula, Chiton, the Scaphopoda—the form and habits of the animal do
not admit of it; in others (many Trochus) a male copulative organ is
wanting. When this is the case, the male scatters the spermatozoa freely; the
majority must perish, but some will be carried by currents in the direction
of the female.
When the sexes are separate, the female is frequently larger than the
male. This is markedly the case in Littorina, Buccinum, and all the
Cephalopoda; in Argonauta the difference is extreme, the male not being
more than ¼ the size of the female.
Those hermaphrodite Mollusca which are capable of sexual union
(Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently divided
into two sections, according as (1) there are separate orifices for the male
and female organs, or (2) one orifice serves for both. To the former section
(Digonopora[2]) belong the Limnaeidae, Vaginulidae, and Onchidiidae, and
many Opisthobranchiata, including all the Pteropoda; to the latter
(Monogonopora[255]) nearly all the Nudibranchiate Opisthobranchiata, and
all the rest of the Pulmonata. In the latter case during union, mutual
impregnation takes place, and each of the two individuals concerned has
been observed (compare p. 42) to deposit eggs. In the former, however, no
such reciprocal act can take place, but the same individual can play the part
of male to one and female to another, and we sometimes find a string of

Limnaea thus united, each being at once male and female to its two adjacent
neighbours.
The Reproductive System.—Broadly speaking, the complicated
arrangements which are found in Mollusca resolve themselves into
modifications of three important factors:—
(a) The gonads or germ-glands, in which are developed the ova and the
spermatozoa. These glands are generally known as the ovary in the female,
the sperm-gland or testis in the male.
(b) The channels which provide for the passage of the seminal products;
namely, the oviduct in the female, the vas deferens or sperm-duct in the
male.
(c) The external generative organs.

Fig. 49.—Generative and other organs of
Littorina obtusata L., female.
A, anus.
Br, branchia.
Buc, buccal mass.
H, heart.
Hep, hepatic duct.
I, continuation of oesophagus.
Ki, kidney.
Li, liver.
M, muscle of attachment.
O´, female orifice.
Od, oviduct.
Oes, oesophagus.
Ov, ovary.
Ra, radula.
St, stomach.
U, uterus.

(After Souleyet.)

Fig. 50.—Generative and other organs of
Littorina obtusata L., male.
A, anus.
Br, branchia.
H, heart.
I, intestine.
Li, liver.
M, muscle of attachment.
Pe, penis.
Te, testis.
VD, vas deferens.

(After Souleyet.)

Dioecious Mollusca.—The common Littorina obtusata will serve as a
typical instance of a dioecious prosobranchiate, exhibiting the simplest
form of organs. In the female the ovary, a lobe-shaped body, is embedded in
the liver. An oviduct with many convolutions conveys the ova into the
uterus, an oblong chamber which consists simply of a dilatation of the
oviduct. The ova descend into the uterus, which is sometimes furnished
with a seminal pouch. In this seminal pouch, or above it, in the oviduct, the

ova come into contact with the spermatozoa. The lower part of the uterus
secretes a gelatinous medium (or capsule, as the case may be) in which the
fertilised ova become enclosed previous to exclusion. In position the
oviduct abuts on the kidney, while the uterus is in close proximity to the
rectum, and the female external orifice is found close to the anus, within the
branchial cavity.
The male organs of Littorina are more simple. The testis is lodged, like
the ovary, in the liver; the vas deferens is, like the oviduct, convoluted, and
eventually traverses the right side of the neck, emerging near the right
tentacle, and terminating in the penis or external copulative organ (Fig. 50).
This system prevails, with but slight modifications in detail, throughout
the prosobranchiate Gasteropoda. The most important modification is the
passage of the seminal products in certain cases (many of the Diotocardia)
through the right kidney, with which the oviduct and vas deferens always
stand in close relation. The same arrangement occurs in the Scaphoda and
some Pelecypoda.
The penis varies greatly in form and size. In the Strombidae (see Fig. 99)
and Buccinidae (Fig. 62) it is very large and prominent; in Littorina it is
somewhat spinulose at one side; in Paludina a portion of it is lodged in the
right tentacle, which becomes atrophied and much more obtuse than the
tentacle on the left side.
Spermatozoa.—The shape of the spermatozoa and of the ova in
Mollusca is of the usual type. In Paludina Ampullaria, and certain species
of Murex two types of spermatozoa occur, one hair-like, the other worm-
like, three times as long as the former, and not tapering at one end. The
former type alone take part in fertilisation, and penetrate the ovum. It has
been suggested that these worm-like spermatozoa are a kind of incipient
ova, and indicate a possible stage in commencing hermaphroditism. And,
since the nearest allies of the Prosobranchiata (in which these types occur)
are hermaphrodite (i.e. the Opisthobranchiata and Pulmonata), it is not
unreasonable to suppose that the Prosobranchiata should show some
tendency towards hermaphroditism in their genital glands.[256]
Cephalopoda.—The special characteristic of the reproductive organs in
female Cephalopoda is the development of various glands, some of
considerable size, in connexion with the ovary and oviduct. Sepia, Loligo,

and Sepiola are furnished with two large nidamental glands, which open
into the mantle cavity independently of the oviduct. Their purpose is to
produce a viscid mucus, which envelops the ova at the moment of their
emission and eventually hardens into the egg-capsules. A pair of accessory
nidamental glands occur in Sepia, as well as a pair of smaller glands
situated on the oviduct itself.
In many of the male Cephalopoda the vas deferens is long and dilated at
its outer end into a glandular reservoir, within which are formed the
spermatophores, or narrow cylindrical packets which contain a very large
number of spermatozoa. When charged, the spermatophores pass into what
is known as Needham’s sac, where they remain until required for use. These
spermatophores are a very characteristic part of the reproductive
arrangements in the Cephalopoda. The male of Sepia has been noticed to
deposit them, during union, upon the buccal membrane of the female.
During the emission of the ova by the female, the spermatophores,
apparently through the agency of a kind of spring contained at one end,
burst, and scatter the spermatozoa over the ova.
The Hectocotylus Arm.—Perhaps the most remarkable feature in the
sexual relations of all the Mollusca is the so-called hectocotylus of the
Cephalopoda. In the great majority of the male Cephalopoda, one of the
‘arms,’ which is modified for the purpose in various ways and to a greater
or less extent, becomes charged with spermatophores, and sometimes,
during union, becomes detached and remains within the mantle of the
female, preserving for some considerable time its power of movement.
The hectocotylus is confined to the dibranchiate Cephalopoda, and its
typical form, i.e. when part of the arm becomes disengaged and left with the
female, occurs only in three genera of the Octopodidae, viz.[Argonauta,
Ocythoe (Philonexis), and Tremoctopus. In all of these, the male is many
sizes smaller than the female. In Argonauta the third arm on the left side
becomes hectocotylised. At first it is entirely enveloped in a kind of cyst, in
such a way that only a small portion of the tip projects; subsequently the
cyst parts asunder, and allows the arm to become expanded to its full length,
which considerably exceeds that of the other arms. At a certain point the
acetabula or suckers terminate, and the remainder of the arm consists of a
very long, tapering, sometimes thread-like filament, which is pointed at the
extreme tip. It is not yet known how the spermatophores find their way into

the hectocotylus, or how the hectocotylus impregnates the ova of the
female. The arm thus affected is not always the same. In Tremoctopus it is
the third of the right side, in the Decapoda the modification usually affects
the fourth of the left.

Fig. 51.—Male of Ocythoe tuberculata Raf.
(= Philonexis catenulatus Fér.),
Mediterranean, showing three stages, A,
B, and C, in the development of the
hectocotylus arm: h.cy, hectocotylus still
in the cyst; c´y´, spoon-shaped cyst at
the end of the arm when freed; th,
thread-like organ freed by the rupture of
c´y´. Natural size. From specimens in the
British Museum.

This singular property of the male Cephalopoda has only recently been
satisfactorily explained. It is true that Aristotle, more than twenty-two
centuries ago, distinctly stated that certain of the arms were modified for

sexual purposes. Speaking of what he calls the polypus (which appears to
represent the Octopus vulgaris of the Mediterranean), he says: ‘It differs
from the female in having what the fishermen call the white sexual organ on
its arm;’ again, ‘Some say that the male has something of a sexual nature
(αὶδοιῶδές τι) on one of its arms, that on which the largest suckers occur;
that this is a kind of muscular appendage attached to the middle of the arm,
and that it is entirely introduced within the funnel of the female’.
Unfortunately the word translated by introduced is corrupt, and can only be
restored conjecturally. He again remarks, ‘The last of the arms, which tapers
to a fine point and is the only whitish arm, it uses in sexual union.’[257]
The typical hectocotylus seems to have entirely escaped notice until
early in the present century, when both Delle Chiaje and Cuvier described
it, as detected within the female, as a parasite, the latter under the name of
Hectocotylus octopodis. Kölliker, in 1845–49, regarded the Hectocotylus of
Tremoctopus as the entire male animal, and went so far as to discern in it an
intestine, heart, and reproductive system. It was not until 1851 that the
investigations of Vérany and Filippi confirmed a suggestion of Dujardin,
[258]
while H. Müller, in 1853, completed the discovery by describing the
entire male of Argonauta.
In all genera of dibranchiate Cephalopoda except Argonauta, Ocythoe,
and Tremoctopus, one of the arms is sexually modified in various ways, but
never becomes so much prolonged, and is never detached and left with the
female. In Loligo Forbesii Stp. the fourth arm on the left has 23 pairs of
regularly developed acetabula, which then lessen in size and disappear,
being replaced by long pedunculated papillae, of which there are about 40
pairs. In Loligo vulgaris Lam. and L. Pleii Orb. 18 or 19 pairs of acetabula
are regularly formed, and then occur 40 pairs of papillae, as in Forbesii. In
other species of Loligo (gahi Orb., brevis Bl., brasiliensis Orb.) only the
outer row of suckers becomes modified into papillae after about the 20th to
the 22nd pair. In Sepioteuthis sepioides the modification is the same as in
the Loligo last mentioned, but the corresponding arm on the right side is so
covered with acetabula towards its extreme end, that it is thought that it in
some way co-operates with the hectocotylised left arm.
In Octopus, the third arm on the right side is subject to modification.
This arm is always shorter than the corresponding arm on the other side,
and carries fewer suckers, but is furnished at the extreme tip with a peculiar

kind of plate, which connects with the membrane at the base of the arm by a
channel of skin, which probably conveys the spermatophores up to the tip.
In Octopus vulgaris, the species referred to by Aristotle, the
hectocotylised arm is short, thin in its outer half and pointed at the
extremity, while the fold of skin is very white, and gives the arm an
appearance of being divided by a cleft at the side. At the same time, an
unusual development of one or two suckers on the arm is not uncommon.
[259]

Fig. 52.—Octopus lentus Baird, N. Atlantic, showing the
peculiar formation of the hectocotylus arm, h.a. (After
Verrill, × ½.)

It is believed that in the Tetrabranchiate Cephalopoda (Nautilus) a union
of the four inner ventral arms may correspond functionally to the
hectocotylising of the arm in the Dibranchiates.
Hermaphrodite Mollusca.—(a) Monogonopora.—The reproductive
system in the hermaphrodite Mollusca is far more complicated than in the
dioecious, from the union of the male and female organs in the same
individual. As a type of the Monogonopora, in which a single orifice serves
for both male and female organs, may be taken the common garden snail
(Helix aspersa), the accompanying figure of which is drawn from two
specimens found in the act of union (Fig. 53).

Fig. 53.—Genitalia of Helix aspersa Müller, drawn
from two individuals in the act of union, from a
dissection by F. B. Stead.

A.G, albumen gland.
C, coecum.
Cr, crop.
D.S, dart sac.
E, eye (retracted).
Fl, flagellum.
H.D, hermaphrodite duct.
H.DF, ditto, female portion.
H.DM, ditto, male portion.
H.G, hermaphrodite gland.
L, liver.
M.G, M.G, mucous glands.
Ov, oviduct.
P.S, penis sac.
R.M, retractor muscle of penis.
Sp, spermatheca.
V, vagina.
V.D, vas deferens.

Beginning from the inside and proceeding outwards we have firstly the
hermaphrodite gland or ovo-testis (h.g.), a yellowish white mass of
irregular shape, embedded in the liver (l.) and forming part of its spiral but
not reaching quite to the apex. Within this gland are developed the ova and
spermatozoa. The former are rather large round cells, produced within the
outer wall of the gland, while the spermatozoa, which are produced in the
more central part, are thread-like bodies, generally aggregated in small
bundles. From the hermaphrodite gland the ova and spermatozoa pass
through the upper part of the hermaphrodite duct (h.d.), which is always
more or less convoluted. Below the convoluted portion, the duct opens into
the albumen gland (a.g.), a large linguiform mass of tissue which becomes
dilated at the time of pairing, and secretes a thick viscid fluid which
probably serves to envelop the ova. Up to this point both the male and
female elements follow the same course, but on their exit from the albumen
gland they diverge. The hermaphrodite duct becomes greatly enlarged, and
is partially divided by a kind of septum into a male and female portion.
These run parallel to one another, the larger or female portion (h.df.),
through which the ova pass (and which is sometimes termed the uterus)
being dilated into a number of puckered folds, while the smaller or male
portion (h.dm.) is comparatively narrow, and not dilated. At their anterior
end, the two portions of the duct separate completely from one another, the
female portion being then termed the oviduct (ov.) and the male portion the
vas deferens (v.d.).

Following first the oviduct, we find that it soon widens into the vagina
(v.), which is furnished with a pair of mucous glands (m.g.), one on each
side. These are much branched, and resemble little bunches of whitish
seaweed. A little above the mucous glands a long tube diverges from the
vagina, which is furnished with a produced coecum (c.) and a pouch, the
spermatheca (sp.) at the extreme end. In this pouch, and in the duct leading
to it, is stored the spermatophore received in union with another snail. Just
below the mucous glands the vagina is joined by the dart sac (d.s.), which
is more fully described below. Finally, at its lower end the vagina unites
with the penis sac at a point just posterior to the common orifice.
Returning now to the male organs, we find that the vas deferens is the
continuation of the male portion of the hermaphrodite duct, after its final
separation from the female portion. It passes under the retractor muscle of
the upper right tentacle, which has been cut away in the specimen figured,
to dissect it out. Just before the vas deferens widens into the penis sac, it
branches off into a long and tapering tube, the flagellum, in which the
spermatozoa are stored and become massed together in the long packet
known as the spermatophore. The penis sac (p.s.) is the continuation of the
vas deferens beyond the point at which the flagellum diverges. It joins the
vagina at its extreme anterior end, uniting with it to form the common
genital aperture, which cannot be exactly represented in the figure. The
penis itself lies in the interior of the penis sac, and is a rather long muscular
tube which is protruded during union, but at other times remains retracted
within the sac.
In the Helicidae generally, the form of the generative organs varies with
each separate species, sometimes merely as regards the size of the different
parts, at others in the direction of greater simplicity or complication. The
mucous glands may be absent, and the flagellum greatly reduced in size, or
absent altogether.
The Dart Sac.—A remarkable part of the reproductive system in many
of the true Helicidae is the so-called dart, Liebespfeil, or telum veneris. It
consists of ‘a straight, or curved, sometimes slightly twisted tubular shaft of
carbonate of lime, tapering to a fine point above, and enlarging gradually,
more often somewhat abruptly, to the base.’ The sides of the shaft are
sometimes furnished with two or more blades; these are apparently not for
cutting purposes, but simply to brace the stem. The dart is contained in a

dart sac, which is attached as a sort of pocket to the vagina, at no great
distance from its orifice. There are four different forms of sac. It may be
single or double, and each of these divisions may be bilobed, each lobe
containing one dart at a time. In Helix aspersa the dart is about 5/16 in. in
length, and ⅛ in. in breadth at its base (see Fig. 54).
It appears most probable that the dart is employed as an adjunct to the
sexual act. Besides the fact of the position of the dart sac anatomically, we
find that the darts are extruded and become embedded in the flesh just
before or during the act of copulation. It may be regarded, then, as an organ
whose punctures induce excitement preparatory to sexual union. It only
occurs in well-grown specimens. When once it begins to form, it grows
very rapidly, perhaps not more than a week being required for its entire
formation.

Fig. 54.—Darts of British land snails: A, Hyalinia
excavata Bean; B, Helix hortensis Müll.; C,
Helix aspersa Müll. (After Ashford.)

The dart is almost confined to Helicidae, a certain number of exceptions
being known which border on Helix. Hyalinia nitida and excavata are the
only British species, not Helices, which are known to possess it. It has not
been noticed to occur in the slugs, except in the N. American genus
Tebennophorus. About one-third of the British Helices are destitute of the
dart.[260] H. rufescens possesses a double bilobed sac, but only two darts,
which lie in the lower lobes. It does not use the darts, and could not do so,
from the relative sizes of dart and sac; it has often been watched when
uniting, but the use of the darts has never been observed. From this it has

been inferred that the darts are degenerate weapons of defence, and that
they were in fact at one time much stronger organs and more often used.
[261]
This theory, however, does not seem consistent with the whole
circumstances of the occurrence, position, and present use of the darts.
Hermaphrodite Mollusca.—(b) Digonopora.—As an example of the
Digonopora or hermaphrodite Mollusca with separate generative apertures
for the male and female organs, we may take the common Limnaea
stagnalis (Fig. 55). It will be seen from the figure that the relative positions
of the hermaphrodite gland and duct, and of the albumen gland, are the
same as in Helix. When the oviduct parts company from the vas deferens, it
becomes furnished with several accessory glands, one of which (Gl.E.)
probably serves as a reservoir for the ova, and answers more or less to a
uterus. The tube leading to the spermatheca is short, and there is no
divergent caecum. The female orifice lies near to the external opening of the
branchial cavity. The vas deferens, which is very long, is furnished with a
large prostate gland. The penis sac is greatly dilated, and there is no
flagellum. The male orifice is behind the right tentacle, slightly in advance
of the female orifice (compare Fig. 102).

Fig. 55.—Genitalia of Limnaea stagnalis L. (from a
dissection by F. B. Stead), × 2.
A.G, albumen gland.
Ac.G, accessory gland.
F.O, female orifice.
Gl.E, glandular enlargement.
H.D, hermaphrodite duct.
H.G, hermaphrodite gland.
Li, liver.
M.O, male orifice.
P, penis sac.
Pr, prostate.
R.M, retractor muscle of penis.
Sp, spermatheca.
V.D, vas deferens.]

Most of the Opisthobranchiata, but not all, have separate sexual orifices.
Numerous variations from the type just described will be found to occur,
particularly in the direction of the development of accessory glands, which
are sometimes very large, and whose precise purpose has in many cases not
been satisfactorily determined.
Pelecypoda.—In the dioecious Pelecypoda, which form the great
majority, the reproductive system is simple, and closely parallel in both
sexes. It consists of a pair of gonads, which are either ovaries or testes, and
a pair of oviducts or sperm-ducts which lead to a genital aperture. The
gonads are usually placed symmetrically at the sides or base of the visceral
mass. The oviduct is short, and the genital aperture is usually within the
branchial chamber, thus securing the fertilisation of the ova by the
spermatozoa, which are carried into the branchial chamber with the water
which passes through the afferent siphon.
Hermaphrodite Pelecypoda are rare, the sexes being usually separate.
The following are assured instances: Pecten glaber, P. jacobaeus, P.
maximus, Ostrea edulis, Cardium norvegicum, Pisidium pusillum, Cyclas
cornea, Pandora rostrata, Aspergillum dichotomum, and perhaps
Clavagella. The greater number of these have only a single genital gland
(gonad) on each side, with a single efferent duct from each, but part of the
gland is male and part female, e.g. in the Pectens above mentioned.
Pandora and Aspergillum have two distinct glands, respectively male and
female, on each side, each of the two glands possessing its separate duct,
and the two ducts from each side eventually opening near one another. It
appears probable that the Septibranchiata (Cuspidaria, Poromya,
Lyonsiella, etc.) must also be added to the number of hermaphrodite
Pelecypoda which have separate male and female glands.
It is worthy of remark that all the hermaphrodite Pelecypoda belong to
forms decidedly specialised, while forms distinctly primitive, such as
Nucula, Solenomya, Arca, and Trigonia are all dioecious. In Gasteropoda
similarly, the least specialised forms (the Amphineura, with the exception of
the Neomeniidae, and the Rhipidoglossa) are dioecious. It is possible
therefore that in the ancestors of the Mollusca the separation of the sexes
had already become the normal type of things, and that hermaphroditism in
the group is, to a certain extent, a sign or accompaniment of specialisation.
[262]

Development of Fresh-water Bivalves.—The vast majority of fresh-
water bivalves either pass the larval stage entirely within the mother, and do
not quit her except in a perfectly developed form (Cyclas, Pisidium), or
assume a mode of development in which free larvae indeed occur, but are
specially modified for adaptation to special circumstances (Unio). Cyclas
and Pisidium, and no doubt all the kindred genera, preserve their ova in a
sort of brood-pouch within the gills, in which the ova pass the earlier stages
of their development. But, even so, the larva of these genera retains some
traces of its original free-swimming habits, for a rudimentary velum, which
is quite useless for its present form of development, has been detected in
Cyclas.
The larva of Dreissensia (see Fig. 47, A), so far as is at present known,
stands alone among fresh-water bivalves in being free-swimming, and to
this property has been attributed, no doubt with perfect justice, the fact of
the extraordinarily rapid spread of Dreissensia over the continent of Europe
(chap. xvi.). In expelling the ova, the parent slightly opens the shells and
then quickly closes them, shooting out a small point of white slime, which
is in fact a little ball of eggs. The general course of development is precisely
parallel to that of marine Pelecypoda, greatly resembling, so far as form is
concerned, certain stages in the growth of the larvae of Modiolaria and
Cardium, as figured by Lovén.[263]
In June and July the larvae appear in large numbers on the surface of the
water, when in spite of their exceedingly small size, they can be captured
with a fine hand-net. They pass about eight days on the surface, feeding
apparently on minute floating algae. During this time, the principal change
they undergo is in the formation of the foot, which first appears as a small
prominence midway between the mouth and anus, and gradually increases
in length and flexibility. When the larva sinks to the bottom, the velum soon
disappears entirely, the foot becomes exceedingly long and narrow, while
the shell is circular, strongly resembling a very young Cyclas.
Larvae of Unionidae.—The early stages of the development of Unio
and Anodonta (so far as the species of North America, Europe, and Asia are
concerned) is of extreme interest, from the remarkable fact that the young
live for some time parasitically attached to certain species of fresh-water
fishes. In order to secure this attachment, the larva, which is generally
known as Glochidium, develops a long filament which perhaps renders it

aware of the neighbourhood of a fish, and also a larval shell furnished with
strong hooks by which it fastens itself to the body of its unconscious host
(Fig. 56). According to some interesting observations made by Mr. O. H.
Latter,[264] the ova pass into the external gill of the mother, in which is
secreted a nutritive mucus on which they are sustained until they arrive at
maturity and a suitable opportunity occurs for their ‘being born.’ If this
opportunity is deferred, and the Glochidia mature, their so-called ‘byssus’
becomes developed, and by being entangled in the gill filaments of the
parent, prevents their escaping. It is interesting to notice that, when the
nutritive mucus of the parent is used up, it becomes, as it were, the turn of
the children to provide for themselves a secondary mode of attachment.

Fig. 56.—A, Glochidium immediately after it is hatched: ad, adductor muscle;
by, ‘byssus’ cord; s, sense organs; sh, shell. B, Glochidium after it has been
on the fish for some weeks: a.ad, p.ad, anterior and posterior adductors;
al, alimentary canal; au.v, auditory vesicle; br, branchiae; f, foot; mt,
mantle. (Balfour.)

The mother Anodonta does not always retain the Glochidium until fish
are in her neighbourhood. Gentle stirring of the water caused them to emit
Glochidium in large masses, if the movement was not so violent as to cause
alarm. The long slimy masses of Glochidium were observed to be drawn
back again within the shell of the mother, even after they had been ejected
to a distance of 2 or 3 inches.
It is a mistake to assert that the young Glochidium can swim. When they
finally quit the mother, they sink to the bottom, and there remain resting on
their dorsal side, with the valves gaping upwards and the so-called byssus
streaming up into the water above them. There they remain, until a

convenient ‘host’ comes within reach, and if no ‘host’ comes within a
certain time, they perish. They are evidently peculiarly sensitive to the
presence of fish, but whether they perceive them by smell or some other
sense is unknown. “The tail of a recently killed stickleback thrust into a
watch-glass containing Glochidium throws them all into the wildest
agitation for a few seconds; the valves are violently closed and again
opened with astonishing rapidity for 15–25 seconds, and then the animals
appear exhausted and lie placid with widely gaping shells—unless they
chance to have closed upon any object in the water (e.g. another
Glochidium), in which case the valves remain firmly closed.”
In about four weeks after the Glochidium has quitted its host, and the
permanent shell has made its appearance within the two valves of the
Glochidium, the projecting teeth of the latter press upon the ventral edge of
the permanent shell, at a point about half way in its lengthward
measurement, retarding the growth of the shell at that particular point, and
indenting its otherwise uninterrupted curve with an irregular notch or dent.
As growth proceeds, this dent becomes less and less perceptible on the
ventral margin of the shell itself, but its effects may be detected, in well-
preserved specimens, by the wavy turn in the lines of growth, especially
near the umbones of the young shell.
Mr. Latter found that all species of fish with which he experimented had
a strong dislike to Glochidium as an article of food. Sometimes a fish would
taste it “just to try,” but invariably spit it out again in a very decided
manner. The cause of unpleasantness seemed not to be the irritation
produced in the mouth of the fish by the attempt of the Glochidium to attach
itself, but was more probably due to what the fish considered a nasty taste
or odour in the object of his attentions.

The following works will be found useful for further study of this
portion of the subject:—
F. M. Balfour, Comparative Embryology, vol. i. pp. 186–241.
F. Blochmann, Ueber die Entwickelung von Neritina fluviatilis Müll.: Zeit. wiss.
Zool. xxxvi. (1881), pp. 125–174.

L. Boutan, Recherches sur l’anatomie et le développement de la Fissurelle: Arch.
Zool. exp. gén. (2) iii. suppl. (1885), 173 pp.
W. K. Brooks, The development of the Squid (Loligo Pealii Les.): Anniv. Mem.
Bost. Soc. Nat. Hist. 1880.
„ „ The development of the oyster: Studies Biol. Lab. Johns Hopk.
Univ. i. (1880), 80 pp.
R. von Erlanger, Zur Entwickelung von Paludina vivipara: Morph. Jahrb. xvii.
(1891), pp. 337–379, 636–680.
„ „ Zur Entwickelung von Bythinia tentaculata: Mitth. Zool. Stat. Neap,
x. (1892), pp. 376–406.
H. Fol, Sur le développement des Ptéropodes: Arch. Zool. exp. gén. iv. (1875), pp.
1–214.
„ Etudes sur le développement des Mollusques. Hétéropodes: ibid v. (1876),
pp. 105–158.
„ Etudes sur le développement des Gastéropodes pulmonés: ibid. viii.
(1880), pp. 103–232.
H. Grenacher, Zur Entwickelungsgeschichte der Cephalopoden: Zeit. wiss. Zool.
xxiv. (1874), pp. 419–498.
B. Hatschek, Ueber Entwickelungsgeschichte von Teredo: Arb. Zool. Inst. Univ.
Wien, iii. (1881), pp. 1–44.
R. Horst, On the development of the European oyster: Quart. Journ. Micr. Sc. xxii.
(1882), pp. 339–346.
E. Korschelt and K. Heider, Lehrbuch der vergleichenden
Entwickelungsgeschichte der wirbellosen Thiere, Heft iii. (1893), pp. 909–1177 (the
work is in process of translation into English).
A. Kowalewsky, Embryogénie du Chiton polii avec quelques remarques sur le
développement des autres Chitons: Ann. Mus. Hist. Nat. Mars. Zool. i. (1883), v.
E. Ray Lankester, Contributions to the developmental history of the Mollusca:
Phil. Trans. Roy. Soc. vol. 165 (1875), pp. 1–31.
„ „ Observations on the development of the pond-snail (Lymnaeus
stagnalis), and on the early stages of other Mollusca: Quart. Journ. Micr. Sc. xiv.
(1874), pp. 365–391.
„ „ Observations on the development of the Cephalopoda: ibid. xv.
(1875), pp. 37–47.
W. Patten, The embryology of Patella: Arb. Zool. Inst. Univ. Wien, vi. (1886), pp.
149–174.
M. Salensky, Études sur le développement du Vermet: Arch. Biol. vi. (1885), pp.
655–759.
L. Vialleton, Recherches sur les premières phases du développement de la Seiche
(Sepia officinalis): Ann. Sc. Nat. Zool. (7) vi. (1888), pp. 165–280.
S. Watase, Observations on the development of Cephalopods: Stud. Biol. Lab.
Johns Hopk. Univ. iv. (1888), pp. 163–183.

„ „ Studies on Cephalopods: Journ. Morph. iv. (1891), pp. 247–294.
E. Ziegler, Die Entwickelung von Cyclas cornea Lam.: Zeit. wiss. Zool. xli.
(1885), pp. 525–569.

CHAPTER VI
RESPIRATION AND CIRCULATION—THE MANTLE

The principle of respiration is the same in the Mollusca as in all other
animals. The blood is purified by being brought, in successive instalments,
into contact with pure air or pure water, the effect of which is to expel the
carbonic acid produced by animal combustion, and to take up fresh supplies
of oxygen. Whether the medium in which a mollusc lives be water or air,
the effect of the respiratory action is practically the same.
Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’
water, while those whose usual habitat is the land ‘breathe’ air. But this rule
has its exceptions on both sides. The great majority of the fresh-water
Mollusca which are not provided with an operculum (e.g. Limnaea, Physa,
Planorbis), breathe air, in spite of living in the water. They make periodic
visits to the surface, and take down a bubble of air, returning again for
another when it is exhausted. On the other hand many marine Mollusca
which live between tide-marks (e.g. Patella, Littorina, Purpura, many
species of Cerithium, Planaxis, and Nerita) are left out of the water,
through the bi-diurnal recess of the tide, for many hours together. Such
species invariably retain several drops of water in their branchiae, and,
aided by the moisture of the air, contrive to support life until the water
returns to them. Some species of Littorina (e.g. our own L. rudis and many
tropical species) live so near high-water mark that at neap-tides it must
frequently happen that they are untouched by the sea for several weeks
together, while they are frequently exposed to a burning sun, which beats
upon the rocks to which they cling. In this case it appears that the
respiratory organs will perform their functions if they can manage to retain
an extremely small amount of moisture.[265]
The important part which the respiratory organs play in the economy of
the Mollusca may be judged from the fact that the primary subdivision of
the Cephalopoda into Dibranchiata and Tetrabranchiata is based upon the
number of branchiae they possess. Further, the three great divisions of the
Gasteropoda have been named from the position or character of the
breathing apparatus, viz. Prosobranchiata, Opisthobranchiata and

Pulmonata, while the name Pelecypoda has hardly yet dispossessed
Lamellibranchiata, the more familiar name of the bivalves.
Respiration may be conducted by means of—(a) Branchiae or Gills, (b)
a Lung or Lung-cavity, (c) the outer skin.
In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of
the Gasteropoda, respiration is by means of branchiae, also known as
ctenidia[266], when they represent the primitive Molluscan gill and are not
‘secondary’ branchiae (pp. 156, 159).
In all non-operculate land and fresh-water Mollusca, in the Auriculidae,
and in one aberrant operculate (Amphibola), respiration is conducted by
means of a lung-cavity, or rarely by a true lung, whence the name
Pulmonata. The land operculates (Cyclophoridae, Cyclostomatidae,
Aciculidae, and Helicinidae) also breathe air, but are not classified as
Pulmonata, since other points in their organisation relate them more closely
to the marine Prosobranchiata. Both methods of respiration are united in
Ampullaria, which breathes indifferently air through a long siphon which it
can elevate above the surface of the water, and water through a branchia
(see p. 158). Siphonaria (Fig. 57) is also furnished with a lung-cavity as
well as a branchia. Both these genera may be regarded as in process of
change from an aqueous to a terrestrial life, and in Siphonaria the branchia
is to a great extent atrophied, since the animal is out of the water, on the
average, twenty-two hours out of the twenty-four. In the allied genus
Gadinia, where there is no trace of a branchia, but only a lung-cavity, and in
Cerithidea obtusa, which has a pulmonary organisation exactly analogous
to that of Cyclophorus,[267] this process may be regarded as practically
completed.

Fig. 57.—A, Siphonaria gigas
Sowb., Panama, the animal
contracted in spirit: gr,
siphonal groove on right side.
B, Gadinia peruviana, Sowb.,
Chili, shell only: gr, mark of
siphonal groove to right of
head.

Respiration by means of the skin, without the development of any
special organ, is the simplest method of breathing which occurs in the
Mollusca. In certain cases, e.g. Elysia, Limapontia, and Cenia among the
Nudibranchs, and the parasitic Entoconcha and Entocolax, none of which
possess breathing organs of any kind, the whole outer surface of the body
appears to perform respiratory functions. In others, the dorsal surface is
covered with papillae of varied size and number, which communicate with
the heart by an elaborate system of veins. This is the case with the greater
number of the Aeolididae (Fig. 58, compare Fig. 5, C), but it is curious that
when the animal is entirely deprived of these papillae, respiration appears to
be carried on without interruption through the skin.

Fig. 58.—Aeolis despecta Johnst.,
British coasts. (After Alder and
Hancock.)

In the development of a distinct breathing organ, it would seem as if
progress had been made along two definite lines, each resulting in the
exposure of a larger length of veins, i.e. of a larger amount of blood, to the
simultaneous operation of fresh air or fresh water. Either (a) the skin itself
may have developed, at more or less regular intervals, elevations, or folds,
which gradually took the form of papillae, or else (b) an inward folding, or
‘invagination,’ of the skin, or such a modification of the mantle-fold as is
described below (p. 172) may have taken place, resulting in the formation
of a cavity more or less surrounded by walls, within which the breathing
organs were ultimately developed. Sometimes a combination of both
processes seems to have occurred, and after a papilliform organ has been
produced, an extension or prolongation of the skin has taken place, in order
to afford a protection to it. Respiration by means of a lung-cavity is
certainly subsequent, in point of time, to respiration by means of branchiae.

Fig. 59.—Chiton squamosus L.,
Bermuda: A, anus; Br,
branchiae; M, mouth.

Fig. 60.—Fissurella virescens Sowb.,
Panama, showing position of the
branchiae: Br, branchiae: E, E, eyes;
F, foot; M, mantle; T, T, tentacles.

The branchiae seem to have been originally paired, and arranged
symmetrically on opposite sides of the body. It is not easy to decide
whether the multiple form of branchia which occurs in Chiton (Fig. 59), or
the simple form as in Fissurella (Fig. 60), is the more primitive. Some
authorities hold that the multiple branchia has gradually coalesced into the
simple, others that the simple form has grown, by serial repetition, into the
multiple. There appears to be no trace of any intermediate forms, and, as a

matter of fact, the multiple branchia is found only in the Amphineura, while
one or rarely two (never more) pairs of branchiae, occur, with various
important modifications, in the vast majority of the Mollusca.
Amphineura.—In Chiton the branchiae are external, forming a long row
of short plumes, placed symmetrically along each side of the foot. The
number of plumes, at the base of each of which lies an osphradial patch,
varies from about 70 to as few as 6 or 7. When the plumes are few, they are
confined to the posterior end, and thus approximate to the form and position
of the branchiae in the other Amphineura. In Chaetoderma, the branchiae
consist of two small feather-shaped bodies, placed symmetrically on either
side of the anus, which opens into a sort of cloaca within which the
branchiae are situated. In Neomenia the branchiae are still further degraded,
consisting of a single bunch of filaments lying within the cloaca, while in
Proneomenia there is no more than a few irregular folds on the cloaca-wall
(Fig. 61).

Fig. 61.—Terminal portions of the Amphineura, illustrating the gradual degradation of the branchiae,
and their grouping round the anus in that class. A, Chiton (Hemiarthrum) setulosus Carp., Torres
Str.; B, Chiton (Leptochiton) benthus Hadd., Torres Str.; C, Chaetoderma; D, Neomenia; a,
anus; br, br, branchiae; k, k, kidneys; p, pericardium. (A and B after Haddon, C and D after
Hubrecht.)

In the Prosobranchiata, symmetrically paired branchiae occur only in
the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former of which
two perfectly equal branchiae are situated on either side of the back of the
neck. These three families taken together form the group known as
Zygobranchiata.[268] In all other families the asymmetry of the body has
probably caused one of the branchiae, the right (originally left), to become
aborted, and consequently there is only one branchia, the left, in the vast

majority of marine Prosobranchiata, which have been accordingly grouped
as Azygobranchiata. Even in Haliotis the right branchia is rather smaller
than the left, while the great size of the attachment muscle causes the whole
branchial cavity to become pushed over towards the left side. In those forms
which in other respects most nearly approach the Zygobranchiata, namely,
the Trochidae, Neritidae, and Turbinidae, the branchia has two rows of
filaments, one on each side of the long axis, while in all other
Prosobranchiata there is but one row (see Fig. 79, p. 169).

Fig. 62.—Bullia laevissima Gmel.,
showing branchial siphon S;
F, F, F, foot; OP, operculum;
P, penis; Pr, proboscis; T, T,
tentacles. (After Quoy and
Gaimard.)

In the great majority of marine Prosobranchiata the branchia is securely
concealed within a chamber or pouch (the respiratory cavity), which is
placed on the left dorsal side of the animal, generally near the back of the
neck. For breathing purposes, water has to be conveyed into this chamber,
and again expelled after it has passed over the branchia. In the majority of
the vegetable-feeding molluscs (e.g. Littorina, Cerithium, Trochus) water is
carried into the chamber by a simple prolongation of one of the lobes or
lappets of the mantle, and makes its exit by the same way, the incoming and
outgoing currents being separated by a valve-like fringe depending from the

lobe. In the carnivorous molluscs, on the other hand, a regular tube, the
branchial siphon, which is more or less closed, has been developed from a
fold of the mantle surface, for the special purpose of conducting water to
the branchia. After performing its purpose there, the spent water does not
return through the siphon, but is conducted towards the anus by vibratile
cilia situated on the branchiae themselves. In a large number of cases, this
siphon is protected throughout its entire length by a special prolongation of
the shell called the canal. Sometimes, as in Buccinum and Purpura, this
canal is little more than a mere notch in the ‘mouth’ of the shell, but in
many of the Muricidae (e.g. M. haustellum, tenuispina, tribulus) the canal
becomes several inches long, and is set with formidable spines (see Fig.
164, p. 256). In Dolium and Cassis the canal is very short, but the siphon is
very long, and is reflected back over the shell.
The presence or absence of this siphonal notch or canal forms a fairly
accurate indication of the carnivorous or vegetarian tendencies of most
marine Prosobranchiata, which have been, on this basis, subdivided into
Siphonostomata and Holostomata. But this classification is of no particular
value, and is seriously weakened by the fact that Natica, which is markedly
‘holostomatous,’ is very carnivorous, while Cerithium, which has a distinct
siphonal notch, is of vegetarian tendencies.
In the Zygobranchiata the water, after having aerated the blood in the
branchiae, usually escapes by a special hole or holes in the shell, situated
either at the apex (Fissurella) or along the side of the last whorl (Haliotis).
In Pleurotomaria the slit answers a similar purpose, serving as a sluice for
the ejection of the spent water, and thus preventing the inward current from
becoming polluted before it reaches the branchiae (see Fig. 179, p. 266).
In Patella the breathing arrangements are very remarkable. In spite of
their apparent external similarity, this genus possesses no such
symmetrically paired plume-shaped branchiae as Fissurella, but we notice a
circlet of gill-lamellae, which extends completely round the edge of the
mantle. It has been shown by various authorities that these lamellae are in
no sense morphologically related to the paired branchiae in other Mollusca,
but only correspond to them functionally. The typical paired branchiae, as
has been shown by Spengel, exist in Patella in a most rudimentary form,
being reduced to a pair of minute yellow bodies on the right and left sides
of the back of the ‘neck.’ A precisely similar abortion of the true branchiae,

and special development of a new organ to perform their work, is shown in
Phyllidia and Pleurophyllidia (see below under Opisthobranchiata). This
circlet of functional gills in Patella has therefore little systematic value,
being only developed in an unusual position, like the eyes on the mantle in
certain Pelecypoda, to supply the place of the true organs which have fallen
into disuse. Accordingly Cuvier’s class of Cyclobranchiata, which included
Patella and Chiton, has no value, and has indeed long been discarded. In
Chiton the gills never extend completely round the animal, but are always
more or less interrupted at the head and anus. They are the true gills, the
plumes being serially repeated in the same way as the shell plates.

Fig. 63.—Patella vulgata L., seen from the
ventral side: f, foot; g.l, circlet of gill
lamellae; m.e, edge of the mantle; mu,
attachment muscle; sl, slits in the
same; sh, shell; v, vessel carrying
aerated blood to the heart; v´, vessel
carrying blood from the heart; ve,
small accessory vessels.

Fig. 64.—Patella vulgata L., seen from
the dorsal side after the removal of
the shell and the black pigment
covering the integument; the
anterior portion of the mantle is
cut away or turned back: a, anus;
br, br, remains of the true
branchiae (ctenidia); i, intestine; k,
k´, kidneys; k.ap, their apertures
on each side of the anus; l, liver;
m, m, mantle; mu, attachment
muscles, severed in removal of
shell; t, t, tentacles.

In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae,
Aciculidae, Helicinidae) which, having exchanged a marine for an aerial
life, breathe air instead of water, the branchia has completely disappeared,
and breathing is conducted, as in the Pulmonata, by a lung-cavity. In certain
genera of land operculates, e.g. Pupina, Cataulus, Pterocyclus, a slight
fissure or tube in the last whorl (see Fig. 180, p. 266) serves to introduce air
into the shell, which is perhaps otherwise closed to air by the operculum. In
Aulopoma, which has no tube, the operculum admits free circulation of air.
In certain other Cyclostomatidae the apex is truncated, and air can enter
there. De Folin closed with wax the aperture of Cycl. elegans, and found
that on placing it in a pneumatic machine, the shell gave off air through its
whole surface. On the other hand, Cylindrella and Stenogyra decollata, on
being submitted to the same test, showed that the truncated part alone was
permeable by air.

Fischer and Bouvier have made some interesting observations on the
breathing of a species of Ampullaria (insularum Orb.). The species has, in
common with all Ampullaria, two siphons, but while the right siphon is but
slightly developed, the left is very long, almost twice as long as the shell
(see Fig. 65). The animal, when under the water, lengthens its siphon,
brings the orifice to the surface, and by alternately raising and depressing its
head produces in the pulmonary sac movements of ex- and inspiration;
these are repeated about ten or fifteen times at regular intervals of from six
to eight seconds, a method of respiration strongly resembling that of the
Cetacea. At the same time, branchial respiration takes place. If powdered
carmine is added to water, the particles are seen to enter the branchial cavity
by the siphon and pass out by the short right siphon. Sometimes the animal
remains under water for hours without rising to the surface to inspire air. In
Valvata (Fig. 66) the branchia is very large, and projects like a leaf or fan
above the shell on the left side; on the corresponding position on the right
side is a long filiform appendage, which some have regarded as
representing the other branchia.

Fig. 65.—Ampullaria insularum Orb.:
A, breathing water; B, breathing
air; Si, siphon; T, upper; t, lower
tentacles; X, pallial expansion,
performing the part of excurrent
siphon. (After Fischer and
Bouvier, x ⅓.)

Opisthobranchiata.—A true branchia occurs only in the Tectibranchiata
and the Ascoglossa. It lies on the right side, and is usually more or less
external, being partly covered sometimes by the shell (as in Umbrella, Fig.
5), sometimes by a fold of the mantle. In the Pteropoda (which are probably
derived from the Tectibranchiata), all the Thecosomata, with the exception
of Cavolinia, have no specialised branchia, but probably respire through
portions or the whole of the integument. In the Gymnosomata an accessory
branchia has in many cases been developed at the posterior end of the body.
Pneumodermon alone has both lateral and posterior branchiae well
developed, Clione and Halopsyche are destitute of either, while the four
remaining families have one branchia, sometimes lateral, sometimes
posterior.[269]

Fig. 66.—Valvata piscinalis
Müll.: br, branchia; fi,
filament; f.l, foot lobes.
(After Boutan.)

Fig. 67.—Doris (Archidoris)
tuberculata L., Britain: a,
anus; br, branchiae,
surrounding the anus; m,
male organ; rh, rh,
rhinophores. × ⅔.

Fig. 68.—Pleurophyllidia
lineata Otto,
Mediterranean: a, anus;
br, secondary branchiae;
m, mouth; s.o, sexual
orifice.

Certain of the Nudibranchiata possess no special breathing organs, and
probably respire through the skin (Elysia, Limapontia, Cenia, Phyllirrhoë).
The majority, however, have developed secondary branchiae, in the form of
prominent lobes or leaf-like processes (the cerata), which are carried upon
the back, without any means of protection. These cerata are, as a rule, of
extreme beauty and variety of form, consisting sometimes of long whip-like
tentaculae, in other cases of arborescent plumes of fern-like leafage, in
others of curious bead-like appendages of every imaginable shape and
colour. In Doris they lie at the posterior end of the body, in a sort of rosette,
which is generally capable of retraction into a chamber. In Phyllidia and
Pleurophyllidia these secondary branchiae lie, as in Patella, on the lateral
portions of the mantle.
The Scaphopoda in all probability possess neither true nor secondary
branchiae.
Pulmonata.—When we use the term ‘lung,’ it must be remembered that
this organ in the Mollusca does not correspond, morphologically, with the
spongy, cellular lung of vertebrates; it simply performs the same functions.
The ‘lung,’ in the Mollusca, is a pouch or cavity, lined with blood-vessels

which are disposed over its vaulted surface in various patterns of network.
The pulmonary sac or cavity is therefore a better name by which to denote
this organ.

Fig. 69.—Geomalacus maculosus Allm., S. Ireland: P.O, pulmonary orifice.

It seems probable, as has been already shown (pp. 18–22), that all
Pulmonata are ultimately derived from marine forms which breathed water
by means of branchiae. Thus we find intermediate forms, such as
Siphonaria, possessed of both a branchia and a pulmonary sac, the former
being evanescent, while in Gadinia and Amphibola it has quite disappeared.
In the vast majority of Pulmonata no trace of a branchia remains; its
function is performed by a chamber, always situated at the right side of the
animal, and generally more or less anterior, admitting air by a narrow
aperture which is rhythmically opened and closed. In Arion and
Geomalacus (Fig. 69) this aperture is in the front of the right side of the
‘shield,’ in Limax (Fig. 71) in the hinder part, in Testacella (Fig. 20) it is
near the extremity of the tail, under the spire of the shell; in Janella it is on
the middle of the right edge of the shield (Fig. 70). If a specimen of Helix
aspersa, or better, of H. pomatia, is held up to the light, the beautiful
arborescent vessels, with which the upper part of the pulmonary chamber is
furnished, can be clearly seen by looking through the aperture as it dilates.
It is only in the Auriculidae that an actual spongy mass of lung material
appears to exist. When in motion, a Helix inspires air much more frequently
than when at rest. Temperature, too, seems to affect the number of
inspirations; it appears doubtful whether, during hibernation, a snail
breathes at all. In any case, the amount of air required to sustain life must be
small.

Fig. 70.—Janella hirudo Fisch.,
N. Caledonia: G, generative
orifice; P, pulmonary
orifice; T, T, tentacles.
(After Fischer.)

Fig. 71.—Limax maximus L.: PO, pulmonary orifice. ×
⅔.

With regard to the respiration of fresh-water Pulmonata there appears to
be some difference of opinion. It is held, on the one hand, that the
Limnaeidae only respire air, making periodic visits to the surface to procure

it, and that they perish, if prevented from doing so, by asphyxiation. If, we
are told,[270] as a Limnaea is floating on the surface of the water in a glass
jar, a morsel of common salt be dropped upon its outstretched foot, it will
sink heavily to the bottom, emitting a stream of air from its pulmonary
orifice. On recovering from the shock, it will anxiously endeavour to regain
the surface, but will have some difficulty in doing so, owing to its now
much greater specific gravity. When it succeeds, it creeps almost out of the
water, and exposes its respiratory orifice freely to the air. If the experiment
is repeated several times on the same individual, it becomes so much
weakened that it has to be taken out of the water to save its life. Moquin-
Tandon, on the other hand, is strongly of opinion[271] that there is no
absolute necessity for Limnaea to obtain air by rising to the surface, and
that, if prevented from emerging, it can obtain air from the water. When
covered in by a roof of ice, Limnaea has not been observed to suffer any
inconvenience. Moquin-Tandon kept L. glabra and Planorbis rotundatus in
good health under 20 mm. of water for eighteen and nineteen days, and
relates a case in which Physa was kept alive under water for four days, and
Planorbis for twelve. Young specimens, both of Limnaea and Planorbis, do
not rise to the surface for a supply of air; they are hatched with the
pulmonary cavity full of water.
It is probable, therefore, that Limnaeidae are capable, on occasion, of
respiration through the skin. Some authorities are of opinion that certain
long and narrow lamellae, situated within the pulmonary sac, are employed
for the purpose of aqueous respiration. Ancylus, which never makes
periodic excursions to the surface, perhaps respires by receiving into its
pulmonary chamber the minute quantities of oxygen given off by the
vegetation on which it feeds.
Limnaeidae taken from a great depth of water, e.g. from 130 fathoms in
the lake of Geneva, have been examined by Forel.[272] The pulmonary sac is
full of water, but there is no transformation of organs, no appearance of a
branchia, to meet the changed circumstances of their environment.
Doubtless a good deal of respiration is done by the skin; being soft and
vascular, it respires the air dissolved in the water. Forel cites cases of
Limnaea living at much shallower depths, which come to the surface once,
and then remain below for months. The oxygen of this supply must soon
have become exhausted, and the animals, discontinuing for a time the use of

the pulmonary chamber, must have respired through the skin. Shallow-
water Limnaea, according to the same authority, remain beneath the surface
during cold weather; when warm weather returns they rise to the surface to
take in a supply of air. Since the water at great depths is always very cold,
there is no need for the Limnaea living there to rise to the surface at all.
It is a curious fact that Limnaea, which have been respiring by the skin
for the whole winter, should suddenly, on the first warm days of summer,
take to rising to the surface and breathing air. But exactly the same
phenomenon is shown in the case of Limnaea from great depths. Placed in
an aquarium, they immediately begin rising to the surface and inspiring air;
in other words, they experience instantaneously a complete transformation
of their respiratory system.
In Onchidium, a land pulmonate which has retrogressed to an
amphibious or quasi-marine mode of life, there is no organ which represents
the pulmonary or branchial cavity, the so-called lung being only a cavity of
the kidney. Respiration is, however, conducted by the skin as well, and by
the dorsal papillae.[273]
Land Mollusca can sustain, for a considerable time, complete
deprivation of atmospheric air. Helices placed in an exhausted receiver
show no signs of being inconvenienced for about 20 hours, and are able to
survive for about two or three days. If detained under water, they are very
active for about 6 hours, then become motionless, the body swells, owing to
the water absorbed, and death ensues in about 36 hours. Immersion for only
24 hours is generally followed by recovery. In the latter case, the cause of
death is not so much deprivation of air as compulsory absorption of water
by the skin. The amount of water thus taken up is surprising. Spallanzani
found that a Helix which weighed 18 grammes increased in weight by 13½
grammes after a prolonged immersion. Even slugs enclosed in moist paper
gained more than 2 grammes in the course of half an hour. Experiment has
shown that the amount of carbonic acid gas produced by respiration stands
in direct relation to the amount of food consumed. Four pairs of snails were
taken which had recently awakened from their winter sleep and had eaten
heartily, and an equal number, under the same circumstances, which had
been prevented from eating. It was found that the first four pairs produced,
in consuming a given amount of oxygen, 11, 9, 10, and 13 parts
respectively of carbonic acid, while the second set produced, in consuming

the same amount of oxygen, only 4, 8, 7, and 9 parts of carbonic acid.[274]
Hibernating Helices, if weighed in December and again in April, will be
found to have lost weight, due to the expiration of carbonic acid. Owing to
the difficulty of experiment, opinions vary as to the absolute temperature of
snails. It appears to be established that several snails, if placed together in a
tube, raise the temperature one or two degrees C., but as a rule, the
temperature of a solitary Helix differs very slightly from that of the
surrounding air. Increased activity, whether in respiration or feeding, is
found to raise the temperature.

Fig. 72.—Cardium edule L.: A, anal; B, branchial siphon; F, foot.
(After Möbius.)

W. H. Dall, writing of the branchia in Pelecypoda, remarks[275] that there
can be no doubt that its original form was a simple pinched-up lamella or
fold of the skin or mantle. This, elongated, becomes a filament. Filaments
united by suitable tissue, trussed, propped, and stayed by a chitinous
skeleton, result in the forms, wonderful in number and complexity, which
puzzle the student to describe, much more to classify.

Fig. 73.—Scrobicularia piperata Gmel., in its natural position
in the sand: A, efferent or anal siphon; B, afferent or
branchial siphon. (After Möbius.)

In Pelecypoda the branchiae are placed on each side of the body,
between the mantle and the visceral mass. They lie in a chamber known as
the branchial cavity. Leading into this cavity, and behind it, are, as a rule,
two tubes or siphons, one of which conducts water to the branchiae, while
the other carries it away after it has passed over them. The lower is known
as the branchial or afferent siphon, the upper as the anal or efferent siphon
(see Figs. 72 and 73). The action of these siphons can readily be observed
by placing a little carmine in water, near to the siphonal apertures of an
Anodonta or Unio. In many cases (e.g. Psammobia, Tellina, Mya, genera
which burrow deeply in sand) both the siphons are exceedingly long,
sometimes considerably longer than the whole shell. In some cases the two
tubes are free throughout their entire length, in others they become fused
together before their entrance within the shell (Fig. 74). In other genera,
which do not burrow (e.g. Ostrea, Pecten, Arca, Mytilus), the siphons are
rudimentary or altogether absent (Fig. 75).

Fig. 74.—Solecurtus strigillatus
L., Naples: s.af, afferent
siphon; s.ef, efferent siphon,
the two uniting in SS
externally to the shell, × ½.

Fig. 75.—Mytilus edulis L., attached by its byssus (By) to a
piece of wood: F, foot; S, anal siphon, the branchial
siphon being below it and not closed. (After Möbius.)

The number and arrangement of the branchiae varies considerably. It
appears probable that the different degrees of complication of the gill
indicate degrees of specialisation in the different groups of Pelecypoda, in
other words, assuming that a simpler form of gill precedes, in point of
development, a more complicated form, the nature of the gill may be taken
as indicating different degrees of removal from the primitive form of
bivalve.
1. The simplest form of gill (Nucula, Leda, Solenomya, etc.) is that
which consists (Fig. 76, A, compare Fig. 100, p. 201) of two rows of very
short, broad, not reflected filaments, the rows being placed in such a way
that they incline at right angles to one another from a common longitudinal
axis. The filaments are not connected with one another, nor are the two
leaves of each gill united at any point. (Protobranchiata.)

Fig. 76.—Morphology of the branchiae of Pelecypoda, seen diagrammatically in
section: A, Protobranchiata; B, Filibranchiata; C, Eulamellibranchiata; D,
Septibranchiata; e, e, external row of filaments; i, i, internal row of filaments;
e´, external row or plate folded back; i´, internal row folded back; f, foot; m,
mantle; s, septum; v, visceral mass. (From A. Lang.)

Fig. 77.—Four gill filaments
of Mytilus, highly
magnified; cj, ciliary
junctions; f, filament.
(After Peck.)

2. In the Anomiidae, Arcadae, Trigoniidae, and Mytilidae each gill
consists of two plates or rows of much longer filaments, which
consequently occupy a much larger space in the mantle cavity (Fig. 76, B).
Unable to extend beyond the limits of the mantle, filaments are reflected or
doubled back upon one another, those of the external plate being reflected
towards the outside, those of the internal plate towards the inside. Each
separate filament is not connected with the filament next adjacent, except
by surface cilia situated on small projections on the sides of the filaments,

and interlocking with the cilia of the adjacent filament. The two superposed
plates or leaves of the gill may or may not be united by cords running
between the two parts of a filament. (Filibranchiata.)
3. In the Pectinidae, Aviculidae, and Ostreidae a further development
takes place. The filaments of each gill are reflected in the same way as in
the Filibranchiata, but the part thus reflected may become completely
united or ‘concresce’ with the mantle on the exterior and with the base of
the foot on the interior side. The leaves of each gill plate, which have thus
become doubled (the gills being apparently two instead of one on each
side), are folded or crumpled, and the filaments are modified at the re-
entrant angles of the fold. (Pseudolamellibranchiata.)
4. In all the remaining Pelecypoda, except class 5, in other words, in the
very large majority of families, the filaments are either reflected, as in (3),
or simple; but the process of concrescence is so far advanced that the
adjacent filaments are always intimately connected with one another in such
a way as to admit the passage of the blood; and the leaves of each gill-plate
(Fig. 76, C) are united by cross channels in a similar way.
(Eulamellibranchiata.)
5. In certain of the Anatinacea alone (Cuspidaria, Lyonsiella, Poromya,
Silenia) the gills are transformed into a more or less muscular partition,
extending from one adductor muscle to the other (Fig. 76, D), and
separating off the pallial chamber into two distinct divisions, which
communicate by means of narrow slits in the partition. (Septibranchiata.)

Fig. 78.—Transverse section of portion of an
outer gill plate of Anodonta, highly
magnified: il, inner lamella; il´, outer
lamella; ilj, interlamellar junctions; v,
large vertical vessels. (After Peck.)

Thus the process of gill development in the Pelecypoda appears to lead
up from a simple to a very complex type. In its original form, at all events
in the most primitive form known to us, the gill is a series of short
filaments, quite independent of one another, strung in two rows; then the
filaments become longer and double back, while at the same time they
begin to show signs of adhesion, as yet only superficial, to one another. In a
further stage, the reflected portions become fused to the adjacent surfaces of
the foot and mantle, while the interlamellar junctions serve to lock the two
gill-plates together; finally, the mere ciliary junction of adjacent filaments is
exchanged for intimate vascular connection, while the gill-plates as a whole
become closely fused together in a similar manner.
This theory of origin is strengthened by closer observation of the
phenomena of a single group. Taking the Septibranchiata as an instance, we
find that in Lyonsiella the branchiae unite with the mantle in such a way as
to form two large pallial chambers, the structure of the branchiae being
preserved, and their lamellae covering the partition. A further stage is
observed in Poromya. There, a similar partition exists, but it has become
muscular, preserving, however, on each side two groups of branchial
lamellae, separated one from the other by a series of slits, which form a
communication between the two pallial chambers. A further stage still is
seen in Silenia. There the same muscular partition exists, but the branchial
lamellae on either side have disappeared, the slits between the two
chambers, which occur in Poromya, still persisting, but separated into three
groups. Cuspidaria represents the last stage in the development. In the
ventral chamber there appears nothing at all corresponding to a branchia;
the surface of the partition appears perfectly uniform, but on careful
examination three little separate orifices, remains of the three groups of
orifices in Silenia, are observed.[276]
Relation between Branchiae and Heart.—The object of the branchiae
being, as has been already stated, to aerate the blood on its way to the heart,
we find that the heart and the branchiae stand in very important structural
relations to one another. When the branchiae are in pairs, we find that the
auricles of the heart are also paired, the auricle on the right and left sides
being supplied by the right and left branchiae respectively. This is the case
with the Dibranchiate Cephalopods (Argonauta, Octopus, Loligo, etc.), the
Zygobranchiate Prosobranchs (Fissurella, Haliotis), and all Pelecypoda. In

the Amphineura (Chiton, etc.) there are two auricles corresponding to the
two sets of multiple branchiae. In the case of the Tetrabranchiate
Cephalopods (Nautilus) there are four auricles corresponding to each of the
four branchiae. Compare Fig. 79, A, B, C, D, E.
On the other hand, when the branchia is single, or when both branchiae
are on the same side, and one is aborted and functionless, the auricle is
single too, and on the same side as the branchia. This is the case with the
Tectibranchiate Opisthobranchs (Philine, Scaphander, etc.), all the
Pectinibranchiate Prosobranchs (Rachiglossa, Taenioglossa, and
Ptenoglossa), and the other Azygobranchiate Prosobranchs (Trochidae,
Neritidae, etc.). In the last case the right auricle exists, as well as the left,
but is simply a closed sac, the coalescing of the two gills on the left side
having thrown all the work upon the left auricle. Compare Fig. 79, F, G, H.

Fig. 79.—Diagram illustrating the relations between branchiae, heart,
and aorta in the Mollusca: A, In Chiton; B, Pelecypoda; C,
Dibranchiate Cephalopoda; D, Tetrabranchiate Cephalopoda; E,
Prosobranchiata Zygobranchiata; F, Prosobranchiata
Azygobranchiata; G, Prosobranchiata Monotocardia; H,
Opisthobranchiata Tectibranchiata: 1, Ventricle; 2, Auricle; 3,
Aorta; 3a, Cephalic aorta; 3b, Visceral aorta; 3c, Posterior aorta.
(From A. Lang.)

Circulatory System

All Mollusca, without exception, possess a circulatory system of more or
less complexity. The centre of the system is the heart, which receives the
aerated blood from the breathing organs, and propels it to every part of the
body. In the Scaphopoda alone there appears to be no distinct heart.
The heart may consist simply of a single auricle and ventricle, and an
aorta opening out of the ventricle. From the aorta the blood is conveyed to
the various parts of the body by arteries. Veins convey the blood back to the
breathing organs, after passing over which it returns by the branchial or
pulmonary vein to the heart, thus completing the circuit.
As regards position, the heart is situated within the pericardium, a
separate chamber which in the Pelecypoda, Cephalopoda, and the bilaterally
symmetrical Gasteropoda lies on the median line, while in the asymmetrical
Gasteropoda it is on one or other of the sides of the body, usually the right.
The veins connected with the branchiae, and consequently the auricle into
which they open, are situated behind the ventricle in the Opisthobranchiata
(whence their name), while in the Prosobranchiata they are situated in front
of the ventricle.
The number of auricles corresponds to the number of branchiae. Thus
there is only one auricle in the great majority of Prosobranchiata (which are
accordingly classified as Monotocardia), and also in the Opisthobranchiata,
while the Pulmonata have a single auricle corresponding to the pulmonary
chamber. There are two auricles in the Amphineura, in a small group of
Gasteropoda, hence known as Diotocardia, in all Pelecypoda, and in the
Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda alone there
are four auricles corresponding to the four branchiae.
A single aorta occurs only in the Amphineura and in the Tetrabranchiate
Cephalopoda. In all the other groups there are two aortae, leading out of the
anterior and posterior ends of the ventricle in Pelecypoda and Dibranchiate
Cephalopoda, while a single aorta leads out of the posterior end alone, and
subsequently bifurcates, in most of the Gasteropoda. One aorta, the
cephalic, supplies the front part of the body, the oesophagus, stomach,
mantle, etc.; the other, the visceral aorta, supplies the posterior part, the
liver and sexual organs.

The general circulatory system in the Mollusca has not yet been
thoroughly investigated. As a general rule, the blood driven from the
ventricle through the aorta into the arteries, passes, on reaching the
alimentary canal and other adjacent organs, into a number of irregular
spaces called lacunae. These in their turn branch into sinuses, or narrow
tubes covered with muscular tissue, which penetrate the body in every
direction. In the Dibranchiate Cephalopoda true capillaries are said to occur,
which in some cases form a direct communication between the arteries and
veins. According to some authorities[277] capillaries and veins exist in
certain Pelecypoda in connexion with the intestinal lacunae, but this again
is regarded by others as not established. A similar difference of opinion
occurs with regard to the precise function of the foot-pore which occurs in
many Mollusca, some holding that it serves as a means for the introduction
of water into the blood-vascular system, while others regard it as a form of
secretion gland, the original purpose of which has perhaps become lost.
Blood.—As a rule, the blood of the Mollusca—i.e. not the corpuscles
but the liquor sanguinis—is colourless, or slightly tinged with blue on
exposure to the air. This is due to the presence of a pigment termed
haemocyanin, in which are found traces of copper and iron, the former
predominating. Haemoglobin, the colouring matter of the blood in
Vertebrates, is, according to Lankester,[278] of very restricted occurrence. It
is found—(1) in special corpuscles in the blood of Solen legumen (and Arca
Noae); (2) in the general blood system of Planorbis; (3) in the muscles of
the pharynx and jaws of certain Gasteropoda, e.g. Limnaea, Paludina,
Littorina, Chiton, Aplysia. This distribution of haemoglobin is explained by
Lankester in reference to its chemical activity; whenever increased facilities
for oxidisation are required, then it may be present to do the work. The
Mollusca, being as a rule otiose, do not possess it generally diffused in the
blood, as do the Vertebrata. The actively burrowing Solen possesses it, and
perhaps its presence in Planorbis is to be explained from its respiring the air
of stagnant marshes. Its occurrence in the pharyngeal muscles and jaws of
other genera may be due to the constant state of activity in which these
organs are kept.[279]
According to Tenison-Woods[280] a species of Arca (trapezia Desh.) and
two species of Solen, all Australian, have red blood. It is suggested that in
these cases the habits of the animal (the Solen burrowing deeply in sand, the

Arca in mud) require some highly oxidising element, surrounded as the
creature is by ooze. In Arca pexata (N. America) the blood is red, the
animal being familiarly known as the ‘bloody clam.’ Burrowing species,
however, are not all distinguished by this peculiarity. Tenison-Woods finds
red fluids in the buccal mass of many Gasteropoda, e.g. in species of
Patella, Acmaea, Littorina, Trochus, Turbo, giving the parts the appearance
of raw meat.
The Mantle

On the dorsal side of the typical molluscan body, between the visceral
sac and the shell, lies a duplicature of the integument, generally known as
the mantle. The depending sides of the mantle, which are usually somewhat
thickened, enclose between themselves and the body mass a chamber of
varying size and shape, called the mantle cavity, which communicates
freely with the external air or water, and encloses and furnishes a protection
for the organ or organs of respiration. On its upper or dorsal surface the
mantle is closely applied to the shell throughout its whole extent, the cells
with which it is furnished secreting the materials from which the shell is
formed (see p. 255). The whole mantle is capable, to some degree, of
secreting shelly matter, but the most active agent in its production is the
mantle edge or margin.
In the Prosobranchiata the mantle cavity, for reasons which have already
been explained, is found on the left side of the animal, its front portion
being in many cases produced into a tubular siphon. Within the mantle
cavity are found, besides the branchia, the anus, the apertures of the
kidneys, and the osphradium. In the pulmonata the mantle fold encloses a
so-called lung-cavity. The front edge of the mantle coalesces with the
integument of the neck in such a way as to enclose the cavity very
completely, the only communication with the outer air being by means of
the contractile breathing or pulmonary aperture on the right side. In the
Tectibranchiate Opisthobranchs the mantle fold is inconsiderable, and is
usually not of sufficient extent to cover the branchia, while in the
Nudibranchs, which have no true branchiae, it disappears altogether.
In the Pelecypoda the mantle cavity is equally developed on each side,
enclosing the two sets of branchiae. The mantle may thus be regarded as
consisting of two equal portions, which form a sort of lining to the two

valves. The lower or ventral portion of the mantle edges may be simple, or
provided with ocelli (Pecten, Arca), tentacles, cilia (Lima, Lepton), or
doubled folds. The two portions of the mantle touch one another along the
whole line of the edge of the two valves, and, although thus in contact, may
remain completely separate from one another, or else become permanently
united at one or more points. This fusion of the mantle edges corresponds to
important changes in the organisation of the animal as a whole. The anal
and branchial siphons are no more than prolongations of the mantle edges
on the posterior side into a tubular form. These ‘siphons’ exhibit the
siphonal form more distinctly according as the adjacent portions of the
mantle become more definitely fused together.

Fig. 80.—Diagram illustrating the various stages in the closing of the mantle in Pelecypoda: A,
mantle completely open; B, rudiments of siphons, mantle still completely open; C, mantle
closed at one point; D, mantle closed at two points, with complete formation of siphonal
apertures; E, development of siphons, ventral closure more extended; F, mantle closed at three
points, with fourth orifice: f, foot; s.a, s.b, anal and branchial siphons; 1, 2, 3, first, second, and
third points of closure of mantle. (After A. Lang.)

This progressive fusion of the mantle edges may be taken as indicating
definite stages in the development of the Pelecypoda. A perfectly free
mantle edge, joined at no point with the edge of the adjacent mantle, occurs
in Nucula, Arca, Anomia, and Trigonia (see Fig. 80, A, B). Here there is
nothing in the nature of a siphon, either anal or branchial; in other words,
no contrivance exists to prevent the spent water which has passed over the
branchiae from becoming mixed with the fresh water which is to reach
them. When the mantle edges are fused at one point only, this is invariably

on the middle part of the posterior side, thus separating off an anal opening
which may become prolonged into a tube-like form. At the same time the
adjacent underlying portions of the mantle edges draw together, without
actually coalescing, to form an opening for the incurrent stream of water,
the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the case with
most Mytilidae (see Fig. 75) with Cardita, Astarte, and Pisidium. In the
next stage the branchial opening is separated off by the concrescence of the
mantle edges beneath it, and we have the mantle united in two places, thus
forming three openings, the ventral of which is the opening for the
protrusion of the foot (Fig. 80, D). This is the case in Yoldia, Leda, the
majority of the Eulamellibranchiata (e.g. Lucina, Cyrena, Donax,
Psammobia, Tellina, Venus, Cardium, Mactra), and all Septibranchiata. In
Chama and Tridacna the fused portions of the mantle become more
extended, and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this
concrescence takes place over the greater length of the whole mantle edge,
so that the mantle may be regarded as closed, with the exception of the
three apertures for the foot and the two siphons (Fig. 80, E).
In certain genera there occurs, besides these three apertures, a fourth, in
the line of junction between the pedal and branchial orifices. It appears
probable that this fourth orifice (which has been regarded by some as an
inlet for water when the siphons are retracted), stands in relation to the
byssal apparatus (Fig. 80, F). In Lyonsia, for instance, a thick byssus
protrudes through the orifice, which is large and open. In Solen, Lutraria,
Glycimeris, Cochlodesma, Thracia, Aspergillum, and a few more genera,
which have no byssus, the orifice is very small and narrow. It is possible
that in these latter cases, the byssal apparatus having become atrophied, the
orifice has been correspondingly reduced in size.[281]
Mantle Reflected over the Shell.—It is sometimes the case that the
mantle edges tend to double back over the external surface of the shell, and
to enclose it to a greater or less extent. When this process is carried to an
extreme, the edges of the reflected mantle unite, and the shell becomes
completely internal. We see an incipient stage of this process in Cypraea
and Marginella, where the bright polish on the surface of the shell is due to
the protection afforded by the lobes of the mantle. A considerable portion of
the shell of Scutus is concealed in a similar way, while in Cryptochiton,
Lamellaria, and Aplysia the shell is more or less completely enclosed.

Among Pulmonata, it is possible that in forms like Vitrina, Parmacella,
Limax, and Arion, we have successive stages in a process which starts with
a shell completely external, as in Helix, and ends, not merely by enveloping
the shell in the mantle, but by effecting its disappearance altogether. In
Vitrina and some allied genera we have a type in which the mantle lobes are
partly reflected over the shell, which at the same time exhibits rather less of
a spiral form than in Helix. In the stage represented by Parmacella, the
mantle edges have coalesced over the whole of the shell, except for a small
aperture immediately over the spire; the nucleus alone of the shell is spiral,
the rest considerably flattened. In Limax the shell has become completely
internal, and is simply a flat and very thin plate, the spiral form being
entirely lost, and the nucleus represented by a simple thickening at one end
of the plate. In Arion, the final stage, we find that the shell, being no longer
needed as a protection to the vital organs, has either become resolved into a
number of independent granules, or else has entirely disappeared.
Some indications of a similar series of changes occur in the Pelecypoda.
The mantle edge of Lepton is prolonged beyond the area of the valves,
terminating in some cases in a number of filaments. In Galeomma and
Scintilla the valves are partially concealed by the reflected mantle lobes,
and in a remarkable form recently discovered by Dall[282]
(Chlamydoconcha) the shell is completely imbedded in the mantle, which is
perforated at the anterior end by an orifice for the mouth, and at the
posterior end by a similar orifice for the anus. In all these cases, except
Lepton, it is interesting to notice that the hinge teeth have completely
disappeared, the additional closing power gained by the external mantle
rendering the work done by a hinge unnecessary. It is quite possible, on the
analogy of the Gasteropoda mentioned above, and also, it may be added, of
the Cephalopoda and other groups, that we have here indicated the eventual
occurrence of a type of Pelecypoda altogether deprived of valves, a greatly
thickened mantle performing the part of a shell.[283]

The following works will be found useful for further study of this
portion of the subject:—

F. Bernard, Recherches sur les organes palléaux des Gastéropodes prosobranches:
Ann. Sc. Nat. Zool. (7) ix. (1890), pp. 89–404.
G. Cuvier, Le Régne animal (ed. V. Masson); Mollusca, Text and Atlas.
C. Grobben, Beiträge zur Kenntniss des Baues von Cuspidaria (Neaera) cuspidata
Olivi, nebst Betrachtungen über das System der Lamellibranchiaten: Arb. Zool. Inst.
Wien, x. (1893), pp. 101–146.
E. Ray Lankester, Encyclopaedia Britannica, 9th ed., vol. xvi. (1883), Art.
‘Mollusca.’
A. Ménégaux, Recherches sur la circulation des Lamellibranches marins:
Besançon, 1890.
K. Mitsukuri, On the structure and significance of some aberrant forms of
Lamellibranchiate gills: Q. Journ. Micr. Sc., N.S. xxi. (1881), pp. 595–608.
H. L. Osborn, On the gill in some forms of Prosobranchiate Mollusca: Stud. Biol.
Lab. Johns Hopk. Univ. iii. (1884), pp. 37–48.
R. Holman Peck, The structure of the Lamellibranchiate gill: Q. Journ. Micr. Sc.,
N.S. xvii. (1877), pp. 43–66.
P. Pelseneer, Contributions à l’étude des lamellibranches: Arch. Biol. xi. (1891),
pp. 147–312.

CHAPTER VII
ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING—THE FOOT—THE NERVOUS
SYSTEM

Organs of Sense: I. Touch

Tactile organs, although occurring in some of the Mollusca, do not
appear to attain special or marked development, except in a few cases. The
whole surface of the skin, and particularly of the foot, is very sensitive to
the slightest impression. Nearly all Gasteropoda are furnished with at least
two cephalic tentacles, projecting like horns from each side of the fore part
of the head. At or near the base of these are generally situated the eyes. In
the Helicidae the eyes are situated, not at the base, but at the apex of the
tentacles, and in that case—except in Vertigo—a second pair of shorter
tentacles appears beneath the longer pair. It frequently happens that several
senses are centred in a single organ; thus the upper tentacles of snails not
only carry the eyes and serve to a certain extent as tactile organs, but they
also carry the organs of smell.
The edges of the mantle, which are sometimes specialised into lobes,
appear to be keenly sensitive to touch in all Gasteropoda.
In Cypraea (Fig. 81) these lobes, or tentaculae, are a prominent feature
of the animal, and also in certain genera of the Trochidae (Fig. 82). In most
of the carnivorous land Pulmonata—e.g. Testacella, Rhytida, Ennea—there
are developed, under the lower pair of tentacles, and close to the mouth,
large labial palps or feelers. These are connected with the cerebral ganglion
by a very large nerve, and may therefore be supposed to be of extreme
sensitiveness. In some of the large carnivorous forms (Glandina, Aerope,
compare Fig. 21, p. 54) these palpae are of great size, and curl upwards like
an enormous pair of moustaches. When a Glandina seizes its prey, the
palpae (see Fig. 83) appear to enfold it and draw it in towards the mouth.

Fig. 81.—Cypraea moneta L., showing tentaculae at edge of
mantle, which partly envelops the shell: Si, siphon; M,
M, mantle; F, foot; T´, tentaculae at edge of mantle.
(After Quoy and Gaimard.) × 3/2.

Fig. 82.—Monodonta canalifera
Lam., New Ireland, showing
mantle lobes. (After Quoy and
Gaimard.)

Fig. 83.—Glandina seizing its
prey, with buccal papillae
turned back. (Strebel.)

It is in the Opisthobranchiata that the organs of touch attain their
maximum development. Many of this group are shell-less or possess a

small internal shell, and accordingly, in the absence of this special form of
defence, a multiplied sense of touch is probably of great service. Thus we
find, besides the ordinary cephalic tentacles, clusters or crowns of the same
above the head of many Nudibranchiata, with lobe-like prolongations of the
integument, and tentacular processes in the neighbourhood of, or
surrounding the branchiae (see Figs. 58 and Fig. 84, or even projecting from
the whole upper surface of the body (Fig. 5, C).
In the Pelecypoda, the chief organs of touch are the foot, which is always
remarkably sensitive, especially towards its point, the labial palps on each
side of the mouth, and the siphons. In certain cases the mantle border is
prolonged into a series of threads or filaments. These are particularly
noticeable in Pecten, Lepton, and Lima (Fig. 85), the mantle lobes of the
common L. hians of our own coasts being very numerous, and of a bright
orange colour. In many genera—e.g. Unio, Mactra—this sensibility to
touch appears to be shared by the whole mantle border, although it is not
furnished with any special fringing. The ‘arms’ of the Cephalopoda appear
to be keenly sensitive to touch, and this is particularly the case with the
front or tentacular pair of arms, which seem to be employed in an especial
degree for exploration and investigation of strange objects.

Fig. 84.—Idalia Leachii A. and H., British seas; br,
branchiae. (After Alder and Hancock.)

Fig. 85.—Lima squamosa Lam., Naples,
showing tentacular lobes of mantle (t, t);
a, anus; ad.m, adductor muscle; br, br,
branchiae; f, foot; sh, shell.

Taste.—The sense of taste is no doubt present, to a greater or less extent,
in all the head-bearing Mollusca. In many of these a special nerve or nerves
has been discovered in the pharynx, connecting with the cerebral ganglion;
this no doubt indicates the seat of the faculty of taste. The Mollusca vary
greatly in their likings for different kinds of food. Some seem to prefer
decaying and highly odoriferous animal matter (Buccinum, Nassa), others
apparently confine themselves to fresh meat (Purpura, Natica, Testacella),
others again, although naturally vegetarian, will not refuse flesh on
occasion (Limax, Helix).
Mr. W. A. Gain[284] has made some interesting experiments on the taste
of British land Mollusca, as evidenced by the acceptance or rejection of
various kinds of food. He kept twelve species of Arion and Limax, and eight
species of Helix in captivity for many months, and tried them with no less
than 197 different kinds of food, cannibalism included. Some curious points
came out in his table of results. Amalia gagates appears to be surprisingly
omnivorous, for out of 197 kinds of food it ate all but 25; Arion ater came
next, eating all but 40. Limax arborum, on the other hand, was dainty to a
fault, eating only seven kinds of food, and actually refusing Swedes, which
every other species took with some avidity. Certain food was rejected by all
alike, e.g. London Pride, Dog Rose, Beech and Chestnut leaves, Spruce Fir,

Common Rush, Liverwort, and Lichens; while all, or nearly all, ate greedily
of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries, Boletus edulis,
and common grasses. Few of our common weeds or hedgerow flowers were
altogether rejected. Arion and Limax were decidedly less particular in their
food than Helix, nearly all of them eating earthworms and puff-balls, which
no Helix would touch. Arion ater and Limax maximus ate the slime off one
another, and portions of skin. Cyclostoma elegans and Hyalinia nitida
preferred moist dead leaves to anything else.
II. Sight

Position of Eyes.—In the majority of the head-bearing Mollusca the
eyes are two in number, and are placed on, or in the immediate
neighbourhood of the head. Sometimes they are carried on projecting
tentacles or ‘ommatophores,’ which are either simple (as in
Prosobranchiata) or capable of retraction like the fingers of a glove (Helix,
etc.). Sometimes, as in a large number of the marine Gasteropoda, the eyes
are at the outer base of the cephalic tentacles, or are mounted on the
tentacles themselves, but never at the tip (compare Fig. 60, p. 153 and Fig.
98, p. 199). In other cases they are placed somewhat farther back, at the
sides of the neck. The Pulmonata are usually subdivided into two great
groups, Stylommatophora and Basommatophora (Fig. 86), according as the
eyes are carried on the tip of the large tentacles (Helix, and all non-
operculate land shells), or placed at the inner side of their base (Limnaea,
Physa, etc.). In land and fresh-water operculates, the eyes are situated at the
outer base of the tentacles.

Fig. 86.—A, Limnaea peregra Müll.; e, e, eyes; t, t, tentacles; B, Helix
nemoralis Müll.; e, e, eyes; t, t, tentacles; p.o, pulmonary orifice.

In the Helicidae, careful observation will show that the eyes are not
placed exactly in the centre of the end of the tentacle, but on its upper side,
inclining slightly outwards. The eye is probably pushed on one side, as it
were, by the development of the neighbouring olfactory bulb. The sense of
smell being far more important to these animals than the sense of sight, the
former sense develops at the expense of the latter.
Organisation of the Molluscan Eye.—The eye in Mollusca exhibits
almost every imaginable form, from the extremely simple to the elaborately
complex. It may be, as in certain bivalves, no more than a pigmented spot
on the mantle, or it may consist, as in some of the Cephalopoda, of a
cornea, a sclerotic, a choroid, an iris, a lens, an aqueous and vitreous
humour, a retina, and an optic nerve, or of some of these parts only.
In most land and fresh-water Mollusca the eye may be regarded, roughly
speaking, as a ball connected by an exceedingly fine thread (the optic
nerve) with a nerve centre (the cerebral ganglion). In Paludina this ball is
elliptic, in Planorbis and Neritina it is drawn out at the back into a conical
or pear shape. In Helix (Fig. 87) there is a structureless membrane,
surrounding the whole eye, a lens, and a retina, the latter consisting of a
nervous layer, a cellular layer, and a layer of rods containing pigment, this
innermost layer (that nearest the lens) being of the thickness of half the
whole retina.

Fig. 87.—Eye of Helix pomatia L.,
retracted within the tentacle; c,
cornea; ep, epithelial layer; l, lens;
op.n, optic nerve; r, retina. (After
Simroth.)

Comparing the eyes of different Gasteropoda together, we find that they
represent stages in a general course of development. Thus in Patella the eye
is scarcely more than an invagination or depression in the integument,
which is lined with pigmented and retinal cells. The next upward stage
occurs in Trochus, where the depression becomes deeper and bladder-
shaped, and is filled with a gelatinous or crystalline mass, but still is open at
the top, and therefore permits the eye to be bathed in water. Then, as in
Turbo, the bladder becomes closed by a thin epithelial layer, which finally,
as in some Murex, becomes much thicker, while the ‘eyeball’ encloses a
lens (Fig. 88), which probably corresponds with the ‘vitreous humour’ of
other types.

Fig. 88.—Eyes of Gasteropoda, showing arrest of development at
successive stages: A, Patella; B, Trochus; C, Turbo; D, Murex;
ep, epidermis; l, lens; op.n, optic nerve; r, retina; v.h, vitreous
humour. (After Hilger.)

In Nautilus the eye is of a very simple type. It consists of a cup-shaped
depression, with a small opening which is not quite closed by the
integument. The retina consists of cells which line the interior of the
depression, and which communicate directly with the branches of the optic
nerve, there being no iris or lens. This type of eye, it will be observed,
corresponds exactly with that which occurs in Patella. It appears also to
correspond to a stage in the development of eyes in the Dibranchiata (e.g.
Octopus, Sepia, Loligo). Lankester has shown[285] that in Loligo the eye
first appears as a ridge, enclosing an oval area in the integument. By
degrees the walls of this area close in, and eventually join, enclosing the
retinal cells within the chamber in which the lens is afterwards developed
(Fig. 89). It thus appears that in some cases the development of the eye is

arrested at a point which in other cases only forms a temporary stage
towards a higher type of organisation.

Fig. 89.—Three stages in the development of the eye of
Loligo; r, r, ridge, enclosing p.o.c, primitive optic
chamber; or, orifice between the closing ridges;
s.o.c, secondary optic chamber; ci, ci, ciliary body;
l, rudimentary lens; R, retina. (After Lankester.)

Fig. 90.—Eye in A, Loligo; B, Helix or Limax; C, Nautilus: a.o.c, anterior optic chamber; c,
cornea; int, integument; ir, iris; l, lens; l´, external portion of lens; op.n, optic nerve; op.g,
optic ganglion; p.o.c, posterior optic chamber; r, retina. (After Grenacher.)

The developed eye in the dibranchiate Cephalopods consists of a
transparent cornea, which may or may not be closed over the front of the
lens. Behind the cornea is a narrow chamber (the anterior optic chamber)
which is continued for three parts round the whole circle of the eye, and
into which project the front portion of the lens and the folds of the iris.
Throughout its whole extent, the anterior optic chamber is lined by the
integument, the portion of which on the inner side is the choroid. The lens is
divided into an outer and inner segment by a thin membrane, and is
supported by the ciliary body which forms a continuation of the retina. The
main portion of the lens lies within the posterior optic chamber, at the back
and sides of which is found the retina (Grenacher).
There can be no doubt that the Cephalopoda use their eyes to observe,
but there is nothing to show that any other Mollusca use their eyes for this
purpose, the sense of smell in their case largely taking the place of visual
observation. Madame Jeannette Power once saw[286] the Octopus in her
aquarium holding a fragment of rock in one of its arms, and watching a
Pinna which was opening its valves. As soon as they were perfectly open,
the Poulpe, with incredible address and promptitude, placed the stone
between the valves, preventing the Pinna from closing again, upon which it

set about devouring its victim. The next day the Poulpe was seen, after
crushing some Tellina, to stretch himself down close by a Triton nodiferus,
and watch it attentively. After four hours the Triton emerged from its shell,
when the Octopus sprang upon it, and surrounded it with its arms.
Powers of Vision in Land Mollusca.—The Helicidae are undoubtedly
very short-sighted. Seldom emerging from their retreats except in twilight
and darkness, they are naturally myopic, and see better in a subdued than in
a bright light. Experiment has shown that a Helix can perceive an object
better at 6 centimetres distance in a weak light than at 4 or 5 millimetres in
a strong one. Cyclostoma elegans and Paludina vivipara are comparatively
long-sighted, perceiving objects at a distance of 20 to 30 centimetres.[287]
The increased power of vision is due, in these two cases, to increased
elaboration in the construction of the eye, Paludina possessing a large and
almost spherical lens, to which the vitreous humour closely adheres, while
in Cyclostoma the lens is remarkably hard, and the aqueous humour very
abundant. According to V. Willem,[288] the Pulmonata are very sensitive to
the slightest movement of the air or jarring of the surface on which they
crawl, but are so short-sighted as only to perceive a confused image of a
large object at about 1 cm., and to distinguish the form of objects at not
more than 1 or 2 mm. The senses of touch and smell are far more active
than that of sight. A bean-pod enclosed in a narrow glass case and placed
before a hungry snail was not noticed, but when taken out of the case and
placed 8 cm. behind the snail, the latter at once turned towards it to devour
it.
Some interesting experiments were conducted by the same author with
the view of ascertaining whether snails avoid or court the light. He placed a
number of species in different wooden boxes, which were divided into a
light and a dark compartment, having previously well soaked the boxes in
water to secure a humid atmosphere and surface, and so induce the snails to
move about. The result showed that nearly all species have a marked
predilection one way or the other, but not all in the same way. Helix
aspersa, Arion empiricorum, six species of Limax, and three of Planorbis,
are lovers of darkness, while H. nemoralis, Succinea putris, and two species
of Limnaea are lovers of light. Physa fontinalis stands alone in being quite
indifferent.

M. Willem endeavoured further to discover whether any of the Mollusca
possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of
light by means of the skin alone. He accordingly repeated the above-
mentioned experiments, having previously extirpated the eyes in all cases.
The result was remarkable. In a few instances the experiment was not
conclusive, but H. aspersa, A. empiricorum, several species of Limax, and
one Limnaea shunned or sought the light just as they had done when their
eyes were present. A few marine Mollusca (Littorina littorea, Trochus
cinerarius, T. umbilicatus, Patella vulgata) were also shown to be
exceedingly sensitive to the impact of a shadow, whether with or without
their eyes.
Blind and Eyeless Mollusca.—In a large number of marine Mollusca
which habitually creep about half buried in wet sand (Bullia, Sigaretus,
Scaphander, Philine), eyes are altogether absent. In some species of Natica
and Sigaretus, and in Doris, eyes are developed, but are enclosed in a thick
layer of skin, through which they can probably do little more than faintly
appreciate different degrees of light and darkness. Chiton has cephalic eyes
in the embryo, but loses them in the adult stage. The two great Auricula, A.
auris Judae and A. auris Midae, which habitually creep about in the liquid
mud of mangrove swamps, have entirely lost their eyes. Certain pelagic
Mollusca seem to have a tendency, which is not easily explained, to lose
their eyes or the power of seeing with them. Thus Ianthina has no eyes at
all. Pteropoda as a rule have no eyes, and the few that have (Creseis,
Cavolinia) possess only certain pigmented spots placed near to the nervous
centres. In the Heteropoda, however, and the Cephalopoda, many of which
are pelagic, the eyes are unusually large.

Fig. 91.—Sigaretus laevigatus
Lam., a species frequenting
wet sand, and destitute of
external eyes: F, anterior
portion of foot. (After
Souleyet.)

Eyes in Deep-sea and Underground Mollusca.—Deep-sea Mollusca,
as a rule, possess no visual organs, or possess them only in a rudimentary
state, but this rule has its exceptions. Dr. Pelseneer found[289] no trace of
eyes in two species of Pleurotoma from 1850 and 1950 fath., none in a
Fossarus from 1400 f., none in a Puncturella from 1340 f. A remarkable
form of Voluta (Guivillea) from 1600 f. possessed eyes which could hardly
be functional, as they were destitute of pigment, and exhibited other
changes of structure. On the other hand, it is remarkable to notice that in
three different species of Trochus from 450 f., 565 f., and 1375 f., the eyes
were pigmented and well developed.
In land Mollusca which live beneath the surface of the ground or in
absolute darkness, the eyes are generally more or less modified. Thus in
Testacella, which usually burrows deeply in the soil, but occasionally
emerges into the open air, the eyes are very small, but distinct and
pigmented. Our little Caecilianella acicula, which is never seen above the
surface, is altogether destitute of eyes. A species of Zospeum, a Helix, and a
Bithynella from dark caves in Carniola have suffered a similar loss. On the
other hand, a small Hyalinia from a dark cave in Utah (probably a recent
addition to the cave fauna) has the eyes normally developed.

Eyes of Onchidium.—Many species of Onchidium, a naked land
pulmonate which creeps on rocks near high-water mark, are provided with
dorsal eyes of various degrees of organisation, and in numbers varying up
to nearly one hundred. The tropical Onchidium are the prey of a fish
(Periophthalmus) which skips along the beach by the aid of its large ventral
fins, and feeds principally on insects and Onchidium. Karl Semper
suggests[290] that the eyes are of service to Onchidium as enabling it to
apprehend the shadow of the approaching Periophthalmus, and defend itself
by suddenly contracting certain glands on the skin and expressing a liquid
secretion which flies into the air like shot and frightens the Periophthalmus
away. This theory for it is no more than theory—may or may not be true,
but it is remarkable that Onchidium with dorsal eyes have precisely the
same geographical distribution as Periophthalmus, and that where no
Periophthalmus exists, e.g. on our own S.W. coasts, the Onchidium are
entirely destitute of dorsal eyes. In those species of Onchidium which have
no dorsal eyes, the latter are on the tips of the tentacles, as in Helix. The
eyes are developed on the head, and afterwards ascend with the growth of
the ommatophores, while in Helix the ommatophores are formed first, and
the eyes developed upon them.[291]
Dorsal Eyes in the Chitonidae.—The remarkable discoveries of
Moseley with regard to the dorsal eyes of Chiton were first published in
1884.[292] He happened to notice, while examining a specimen of
Schizochiton incisus, a number of minute black dots on the outer surface of
the shell, which appeared to refract light as if composed of glass or crystal.
These ‘eyes,’ in all the species of Chiton yet examined, are restricted to the
outer surface of the exposed area of the shell, never being on the laminae of
insertion or on the girdle. In certain sub-genera of Chiton the eyes are
scattered irregularly over the surface, in others they are arranged
symmetrically in rows diverging from the apex of each plate, but in old
specimens the eyes towards the apices are generally rubbed off by erosion
or abrasion. Moseley regarded the occurrence of scattered eyes as indicating
an original stage of development, when the eyes were at first disposed
irregularly all over the surface of the shell; the gathering into regular rows
showing a later stage.

Fig. 92.—Dorsal eyes of Chitonidae, showing the various
forms of arrangement in the first and fourth valves of
1, 1a, Acanthopleura spinigera Sowb., E. Indies, × 2;
2, 2a, Tonicia suezensis Reeve, Suez, × 3; 3, 3a,
Acanthopleura granulata Gmel., W. Indies, × 2; 4, 4a,
Tonicia lineolata Fremb., Chili, × 2. From specimens
in the Museum of Zoology, Cambridge.

The eyes appear to be invariably more numerous on the anterior plate.
Thus in Corephium aculeatum there are about 12,000 in all, of which more
than 3000 are on the anterior plate. In Schizochiton they are arranged in
very symmetrical rows, six of which are situated on the anterior, and only
two, sometimes only one, on the central plates. In Tonicia marmorata the
eyes are sunk in little cup-shaped depressions of the shell, possibly to
escape abrasion. As regards shape and size, in Ch. incisus they are circular,
and about 1/35 inch in diameter, this being the largest size known; in Ch.
spiniger and Ch. aculeatus they are oval, measuring about 1/100 x 1/600 inch.
There are no eyes in Chiton proper, nor in Mopalia, Maugeria, Lorica, and
Ischnochiton.[293] None of our English species appear to possess them.[294]

Eyes in Bivalve Mollusca.—Some, possibly most, of the Pelecypoda
possess, in the larval state, true paired eyes at the oral end of the body.
These become aborted as the animal develops, since that part of the body
becomes entirely screened from the light by the growth of the shell. To
compensate for their loss, numerous ocelli, or pigmented spots sensitive to
the action of light, are in many cases developed on different parts of the
mantle, functionally corresponding to the ‘eyes’ of Chiton described above.
As in Chiton, too, we have here an interesting series of instances in which
true eyes have suffered total obliteration, through disuse, and, as if to
restore to the animal in some measure its lost sense, visual organs of a low
power have subsequently been developed and are now observed in various
stages of specialisation.
Concentration of Eyes in Special Parts of the Mantle.—Sharp has
shown[295] that in several species of Ostrea, Cardium, Anomia, Lima,
Avicula, Arca, and Tellina pigmented cells, with a highly refractive cuticle,
are scattered over a considerable portion of the mantle. Experiment has
proved the powers of ‘vision,’ i.e. of sensitiveness to different degrees of
light, possessed by these organs. In Dreissena polymorpha, Tapes
decussatus, and two species of Venus these cells are concentrated on that
particular part of the mantle which is not always covered by the shell, i.e.
the siphon, but since the siphon can be completely retracted within the
shell, there is no special provision for their protection. A further step is
shown in the case of Mya arenaria, where the siphon is scarcely capable of
complete retraction. Here, while some of the pigment cells are scattered
about over the surface of the siphon, the majority are placed in grooves at
the base of the siphonal tentacles, forming an intensely black band round
them. A higher stage still is shown in Solen vagina, S. ensis, and Mactra
solidissima, where the cells are situated only in the siphonal grooves, which
are more or less specialised in numbers and complexity.
Arca Noae, according to Patten, is very sensitive to any sudden change
in the amount of light falling upon its mantle-edge. A faint shadow cast
upon it by the hand is sufficient to cause it to close its valves quickly, but
always one or two seconds afterwards, the promptitude in all cases
depending upon the depth of the shadow. Sensitiveness in this direction was
found to depend greatly upon the vitality of the animals themselves, since it
always became less in those specimens which had been kept for long in

confinement. A shadow was not always necessary to make them close. An
ordinary black pencil, if approached within two or three inches with
extreme caution, produced the same result, while a glass rod brought within
the same distance, and even moved rapidly to and fro, appeared to cause no
alarm. Sensitiveness to change in intensity of light was experimentally
noticed by the same author in the case of Ostrea, Mactra, Avicula (to a
special extent), and Cardium. It is very remarkable to find that increased
elaboration in the structure of the eyes does not necessarily carry with it
increased sensitiveness, i.e. higher visual powers. Avicula, which is only
provided with a few scattered ommatidia, which would entirely escape the
notice of any one who had not seen them better developed elsewhere, was
considerably more sensitive to light and shade than Arca, with its eyes of
conspicuous size and much more perfect organisation, instantly contracting
the mantle upon the impact of a shadow so faint as to be invisible to the
experimenter.[296]
Visual Faculties of Solen and Ostrea.—The visual power of Solen may
be exemplified by any one who is walking along almost any of our sandy
bays at extreme low-water mark. If the day be warm and sunny, numbers of
Solen will be seen raising themselves an inch or two out of their holes; but
if you wish to catch them you must approach very cautiously, and on no
account allow your shadow to fall upon them, or they will pop down into
their burrows in an instant, and it is vain to attempt to dig them out. ‘How
sensitive,’ remarks Mr. W. Anderson Smith, with reference to oysters,[297]
‘the creatures are to the light above them; the shadow [of the boat] as it
passes overhead is instantaneously noted, and, snap! the lips are firmly
closed.’
Ocelli of Pecten.—In Pecten and Spondylus the ocelli are remarkably
large and prominent, shining like precious stones, and are placed along the
two edges of the mantle so as to receive the light when the shell gapes (Fig.
93). In Pecten opercularis, jacobaeus, and maximus their number varies
from 80 to 120. In Spondylus gaederopus, a very inequivalve shell, 60 have
been counted on the right or fixed valve, and 90 on the left or upper valve.
Each ocellus is connected, by means of its optic nerve, with the large
circumpalleal nerve, and so with the branchial ganglion. They possess a
cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298]
bear a considerable resemblance to the vertebrate type of eye. In spite of

this, the power of vision in these genera does not appear at all superior to
that of other Pelecypoda.

Fig. 93.—Pecten opercularis L., showing the ocelli
on the two edges of the mantle.

Fig. 94.—Compound eyes (c.e) of Arca barbata L.; m.l,
mantle fold; omm, ommatidia. (After Patten.)

According to the elaborate investigations of Patten, the ‘eyes’ in Arca
occur upon the middle or ‘ophthalmic’ fold of the mantle-edge, which is
thickened at the end to admit of their reception. Along this is ranged a row
of dark brown spots of various sizes, which are larger at the anterior and
posterior ends of the mantle-edge, but smaller and more numerous towards
the middle. These brown spots, or ‘eyes,’ are many of them compound,
being made up of the fusion of a number of ommatidia (from 10 to 80) into
one large round eye, which is generally elevated above the surface of the
surrounding epithelium. Sometimes these eyes themselves tend to fuse
together. In one specimen of Arca Noae, 133 of these faceted eyes were
counted in one mantle border, and 102 in the other.
There can be little doubt that the development of these functional eyes,
or sensitive spots, in bivalve Mollusca, is due to special needs. They appear
to be entirely absent in fresh-water bivalves (with the exception of

Dreissensia, which is obviously a marine genus recently become fresh-
water), while they are most abundant in genera living between tide marks
(Solen, Mya, Mactra), and most highly specialised in a genus that is, for a
bivalve, of singularly active habits (Pecten). Now genera living in sand
between tide marks, as the three above-mentioned genera are in the habit of
doing, and also protruding their siphons, and occasionally a considerable
portion of their shells, out of their burrow, are manifestly very much at the
mercy of their watchful enemies the gulls, and anything which would
enable them to apprehend the approach of their enemies would be greatly to
their advantage. Here, perhaps, lies the explanation of the greater
elaboration of these pigmented spots in littoral genera, as compared with
those inhabiting deeper water. Pecten, again, a genus distinguished by great
activity, which can ‘fly’ for considerable distances in the water by flapping
its valves together and expelling the water from the apertures at either side
of the hinge, may be greatly assisted by its ocelli in directing its flight so as
to escape its enemies.
III. Smell

The sense of smell—touch at a distance, as Moquin-Tandon has called it
—is probably the most important sense which the Mollusca possess, and is
unquestionably far more valuable to them than that of sight. Any one who
has ever enjoyed the fun of hauling up lobster pots will recollect that part of
the contents was generally a plentiful sprinkling of Buccinum, Nassa, and
Natica, attracted by the smell of the stinking piece of fish with which the
trap was baited. According to Mr. J. S. Gibbons,[299] Bullia rhodostoma
congregates in hundreds on gigantic medusae which are stranded on the
sandy bays near the Cape of Good Hope. Dr. J. G. Jeffreys says[300] that
quantities of the common Neptunea antiqua “are procured on the Cheshire
coast by the fishermen placing a dead dog on the sands at low-water mark
during spring tides. The bait is then completely covered with stones, which
are piled up like a cairn. On the next turn of the tide the heap of stones is
visited, and the whelks are found on the surface in great numbers, having
been apparently attracted by the smell of the bait, but unable to get at it.”
Mr. W. A. Lloyd kept specimens of Nassa reticulata in a tank in the sand, at
the bottom of which they usually remained buried. If a piece of meat of any
kind were drawn over the sand, the Nassa would appear above the surface
in a few minutes. Half-picked beef or mutton bones, if placed in the tank,

were covered in a few minutes. In fact, no animal matter, whether living or
dead, could be introduced without the Nassa smelling it, and coming up to
see what they could get.[301]
Any one can experiment for themselves on the olfactory powers of our
common snails or slugs. Moquin-Tandon records[302] two interesting cases,
one communicated to him by letter, the other occurring to himself. His
correspondent, a M. Parenteau, was one day walking along a dusty high-
road, when he noticed, near the middle of the road, an empty bean-pod and
two Arions eating it. Attributing the meeting of feeders and food to mere
chance, he was walking on, when he noticed a second bean-pod, and, about
two yards away from it, a third Arion, hurrying straight towards it. When
the Arion had yet more than a yard to traverse, M. Parenteau picked up the
bean and put it in his pocket. The Arion stopped, raised its head, and turned
in every direction, waving its tentacles, but without advancing. M.
Parenteau then carried the bean to the other side of the road, and put it in a
small hole behind a piece of stone. The Arion, after a moment’s indecision,
started off straight for the bean. Again the position of the precious morsel
was changed, and again the Arion made for it, this time without being
further tantalised. M. Moquin-Tandon noticed, one rainy day in the
botanical gardens at Toulouse, two Limax maximus approaching a rotten
apple from different directions. He changed the position of the apple several
times, placing it at a sufficient distance, to be sure they could not see it, but
they always hit it off correctly, after raising their heads and moving their
long tentacles in every direction. It then occurred to him to hold the apple in
the air, some centimetres above the head of the Limax. They perceived
where it was, raised their heads and lengthened their necks, endeavouring to
find some solid body on which to climb to their food.
Several of the land Mollusca have the power of exhaling a disagreeable
smell, Hyalinia alliaria smelling strongly of garlic, and Stenogyra decollata
of laudanum; but this need not be any argument for the sense of smell in the
creatures themselves.
Position of Olfactory Organs in Pulmonata.—Most authorities are of
opinion that the olfactory organs are situated in the tentacles. Moquin-
Tandon considered that in the Helicidae and Limacidae the sense of smell is
confined to the little knob or elevation at the end of the longer tentacles,
close to the eye. He found that when he cut off these tentacles both in Limax

and Arion, the creatures were quite unable to discover the whereabouts even
of strongly-scented food. The same author believed that in the
Basommatophora the sense of smell was present in the whole of the
tentacle, which is covered with an exceedingly sensitive ciliated epithelium.
Lacaze-Duthiers, however, places the olfactory sense in this group at the
outer side of the base of the tentacles, near to the eyes. Some authorities[303]
deny that the Helicidae have the olfactory organ at the tip of the tentacles,
and locate it in a pedal gland near the mouth, which contains conspicuous
sensitive cells. A Helix whose tentacles had been removed manifested its
repulsion to the smell of spirits of turpentine, while another Helix, which
was unmutilated, did not object to the turpentine being held between its
tentacles. Altogether, then, the exact position of the smell-organ in the
Helicidae must be considered as not yet thoroughly determined. Simroth
holds that the sense of smell is distributed over the whole soft integument,
and is especially concentrated in the feelers, and in the neighbourhood of
the respiratory orifice.[304]
In nearly all marine Mollusca yet examined, the organ of smell or
osphradium is in situation intimately connected with the breathing organs,
being generally placed near their base, with the object, apparently, of testing
the quality of the water before it passes over the branchiae. It consists of a
patch of the epithelium, modified in a special manner, and connected by its
own nerve with one of the visceral ganglia.
An osphradium does not necessarily occur in all genera; for instance, it
has not been detected in Fissurella. It is most highly specialised in the
Conidae, and in the carnivorous Gasteropoda generally. In Buccinum
undatum, for instance, it is very large indeed, and, from its plumed form,
has sometimes been mistaken for an accessory branchia (Fig. 95). In
Haliotis it is paired, one lying in close proximity to each of the two
branchiae, but in Turbo it is single, corresponding to the single branchia. In
Chiton there is an osphradium at the base of each separate gill filament,
making a total of twenty or more on each side. Its position in Physa and in
Cyclostoma will be seen by reference to Figs. 103 and 104 (p. 205). In the
Pelecypoda the osphradia are paired, and lie adjacent to the posterior
adductor muscle, close to the hinder end of the axis of the branchiae. In the
Tetrabranchiate Cephalopoda there are two osphradia, placed between the
bases of the two pairs of gills. In the Dibranchiates on the other hand, a

groove above the eyes has been regarded as the seat of the organ of smell.
This groove contains sensory and ciliated cells, and appears to be connected
with a special nerve centre of its own, which ultimately is derived from the
cerebral ganglion.

Fig. 95.—Buccinum undatum L., deprived of its
shell, showing the relative position of
branchia (br) and osphradium (os); m,
mucous glands; s, siphon. The portion of
the mantle covering the osphradium has
been removed.

Scarcely any instances of the exercise of the sense of smell on the part of
bivalve Mollusca have been recorded. Something of the sort, however,
seems to have been present in a case related by Mr. R. L. King.[305] A skull
of a fox had been placed in a small ditch in order to soak, and after a few
days, when taken out, was found to be covered with Pisidium pusillum to
the number of at least two hundred, which had been probably attracted from
the water in the immediate neighbourhood by the smell of the decaying
flesh.
IV. Hearing

Experiments made with a view to ascertain whether the Mollusca are
sensitive to noises have usually led to the conclusion that they are deaf to
very loud sounds. This is the more curious, because an undoubted auditory
apparatus has been discovered in a large number of genera. In the case of an
experiment, it is not easy to be sure that the animal is not affected, at least
in part, by the shock or jar, rather than by the actual sound. In some
experiments, however, conducted at the Plymouth Marine Biological
Laboratory, Mr. Bateson found[306] that Anomia could be made to shut its
shell by smearing the glass of the tank with the finger in such a way as to

make a creaking sound. It was evident that the cause of alarm was not the
jarring of the solid framework of the tank, for the same result occurred
when the object on which the Anomia were fixed was suspended in the
water by a thread. It was found that the sound had to be of a particular pitch
to excite the attention of the mollusc.
As a rule the organ of hearing is nothing more than a small vesicle or sac
(the otocyst), filled with a fluid secretion, in which are suspended one or
usually more calcareous concretions known as otoliths. By means of cilia,
which connect with sense-cells, these otoliths are given a peculiar
movement or oscillation in the medium in which they are suspended. The
number of the otoliths varies in different genera and species; there are
several hundreds in Arion and Limax, about a hundred in Helix pomatia,
nemoralis, hispida, arbustorum, rotundata, Succinea putris, and Limnaea
stagnalis; about fifty in Planorbis contortus and Physa fontinalis, only one
in Cyclostoma elegans. The number increases with age. In young specimens
of Limn. stagnalis as few as ten, nine, and seven have been noticed.[307]
The otocysts are always paired, and, in Gasteropoda, are placed close to
the pedal ganglia. The acoustic nerve, however, has been shown by Lacaze-
Duthiers to connect with the cerebral ganglia in certain cases. The otocysts
are never on the surface of the body and are rarely connected with it by any
passage or tube; it is probable therefore that sound reaches them simply
through the medium of the tissues.
In the Pelecypoda the otocyst is similarly situated near the pedal
ganglion, and is probably (though this has not yet been proved) similarly
connected with the cerebral. There is only a single otolith. Pelseneer
finds[308] in Nuculidae alone a free communication between the otocyst and
the exterior. Anodonta has been observed[309] to withdraw its foot into the
shell at the noise of an opening door, a loud voice, or a shrill whistle,
whether in a basin of water or lying on a study table.

Fig. 96.—Illustrating the otocyst in A, Anodonta, B, Cyclas; ot, otolith; a, b, c,
c´, cellular layers surrounding the chamber; ci, cilia on interior walls of
chamber: C, an otolith crushed. (After Simroth.)

Delage extirpated the otocysts in certain Octopoda, and obtained some
unexpected results. He found that remarkable effects were produced upon
the animal’s powers of locomotion, so that it was unable to preserve its
proper balance in the water when in rapid motion, but its body was forced
to undergo a form of rotation more or less pronounced. He concluded that
the otocysts must possess, besides their auditory functions, a power which
stands in some relation to the proper orientation of the body in locomotion,
a power which is not wholly supplied by sight and touch alone. The
otocysts may thus regulate locomotion by stimulating muscular acts which
tend to keep the body in the straight line during the process of movement.
[310]

The Foot

One of the most characteristic organs of the Mollusca is the foot, which,
under one form or another, occurs throughout the whole phylum. The foot is
a thickening, on the ventral side, of a portion of the integument of the
animal, modified to serve different forms of motion. It attains its maximum
relative area in the Chitonidae, many Nudibranchs, and the slugs generally,
in nearly all of which there is no portion of the body which is not subtended
by the foot. Here too it presents the form of a regular disc or ellipse, which

is more or less produced. In many cases, however, the foot becomes
modified in such a way that we are enabled to recognise well-marked
anterior and posterior portions, which have received the name of propodium
and metapodium respectively, while the intervening central portion is
termed the mesopodium.

Fig. 97.—Sigaretus laevigatus Lam., showing excessive development of the
propodium (pr) and metapodium (met) in a mollusc living in sand (the
shell, which covers only the liver and adjacent parts, has been removed);
l, liver; s.ap, aperture of proboscis, here deflected from the median line; t,
t, tentacles. (After Quoy and Gaimard.)

The propodium is most strongly developed in genera which crawl about
in wet sand, e.g. Natica, Sigaretus, Oliva, Harpa, Scaphander (Figs. 97 and
98, and compare Fig. 91). In such cases it seems to serve as a sort of fender
or snow-plough, to push the sand away on both sides of the path the animal
is traversing. In some species of Sigaretus the propodium becomes as it
were banked up against the head and proboscis, which are thus unnaturally
elevated, or tend to disappear altogether. Bullia (Fig. 62), which crawls
about rapidly on wet sand, appears to attain its object by a wide extension
of the foot on all sides, and so slides over the sand instead of ploughing
through it; the little lappets at the end of the ‘tail’ probably serve as rudders.
In Melampus and Pedipes the propodium is marked off by a groove
across the ventral surface. When the animal is in motion it first advances the
propodium and then pulls the rest of the foot after it with the looping gait of
certain caterpillars. In many Cyclostomatidae this groove, instead of being
transverse, is longitudinal, and the animal advances first the right and then
the left segment of the foot, which gives it a swaying motion from side to
side.

Upon the metapodium lies the operculum, when it occurs. As a rule the
metapodium is not sharply marked off from the rest of the foot. In Strombus
(Fig. 99) it becomes erected into a sort of hump or column, on the top of
which the operculum is situated.

Fig. 98.—Oliva textilina Lam., showing how the front part of the foot
(f) is developed into a sort of fender, the propodium (pr); e, e,
eyes; m.ap, front appendage of mantle; m.ap´, hinder appendage of
mantle, folded into the suture when the animal is at rest; si, siphon;
t, t, tentacles. (After Quoy and Gaimard.)

The epipodium is a prominent fold or border, which occurs upon the
upper edge of the foot in most Diotocardia. In Haliotis it is of considerable
breadth, and is covered by a number of lobes which spring from a moss-like
prolongation of the skin. From the epipodium are developed the lateral
tentaculae of Monodonta (Fig. 82, p. 178), and of other sub-genera of the
Trochidae.[311]
In the Opisthobranchiata the lateral edges of the foot (the parapodia) are
frequently produced into broad folds or wing-like extensions, which in
many cases tend to fold over the shell, and, in conjunction with the mantle,
eventually imbed it altogether. By the wavy motion of the parapodia the
animal is enabled to progress through the water. The paired natatory lobes
of the Pteropoda are simply the parapodia of the Tectibranchs modified for
swimming purposes.

Fig. 99.—Strombus lentiginosus Lam., showing the modified
form of the foot (f): e, e, eyes on their pedicels; mp,
metapodium; op, operculum; p, penis; pr, proboscis; t, t,
tentacles. (After Quoy and Gaimard.)

It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda,
groups which have, for the most part, exchanged a crawling for a swimming
life, that the modifications of the foot are most considerable. In Oxygyrus
and Atlanta, for instance, the propodium and metapodium are sharply
distinguished from the mesopodium, and no doubt have acquired, as a
means of propulsion, the power of separate movement, the animal
swimming with these portions of the foot uppermost. In Carinaria and
Pterotrachea the metapodium has probably become continuous with the
long axis of the body, while the so-called ‘foot’ with its sucker represents
only the original propodium. In the Cephalopoda the arms and funnel
represent the modified foot, the sides of which are prolonged into a number
of very long specialised tentaculae. In the adult Cephalopod some of the
arms have assumed a position in advance of the mouth, the latter being in
fact surrounded by a circle of arms. But in the Cephalopod embryo the
mouth opens as in the Gasteropoda, i.e. in advance of the arms, and it is

only gradually that it becomes encircled by them. Arms and funnel alike are
found to be innerved from the pedal ganglion.[312]
The pointed axe-shaped foot, which is characteristic of the majority of
Pelecypoda, is doubtless derived from a form more akin to the flattened
‘sole’ of the Gasteropoda. A foot with something of this disc-shaped base
actually occurs in some of the Nuculidae, the parapodia being furnished
with pleats which recall similar formations in other Orders (Fig. 100). The
principal modifications of the foot are due to its employment as a
burrowing organ. In genera which burrow but slightly it is small and feebly
developed, while in genera which habitually excavate, it becomes the
largest and strongest organ of the body. At the same time it has a tendency
to shift its position from the ventral to the anterior margin, accompanied by
a corresponding narrowing of the shell, until it arrives at the position seen
in Mollusca of the shape of Mya, Pholas, and Solen. In sedentary or
attached genera, e.g. Pecten, Chama, Ostrea, the foot tends to become
aborted.

Fig. 100.—Yoldia limatula Say, Greenland, showing the short
plumed branchiae (br, br), the gasteropodous foot (f), and the
large labial palps (l.p, l.p): A, as seen from the ventral margin;
B, from the left side, with the mantle turned back; a.m,
position of anterior adductor muscle; i, intestine; l, liver; m, m,
mantle.

The byssus gland, secreting a byssus of horny threads, is characteristic of
many Pelecypoda, and may be observed by any one in the common mussel.
It occurs in the larvae of many species which do not possess a byssus in the
adult stage. The pedal gland of many Prosobranchiates, which secretes a
tough and almost thread-like slime, is possibly homologous with the byssus
gland of bivalves.
Nervous System

The Mollusca possess a nervous system, which usually consists of a
number of nerve centres or ganglia, linked together by bands (the
commissures) and sending out thread-like nerves which ramify into the
various organs. The character of the nervous system varies greatly in
different groups, ranging as it does from a condition of extreme complexity,
in which the ganglia are numerous and the commissures equally so, to that
of considerable simplicity, in which ganglia are almost entirely absent.
The most important ganglia are (1) the cerebral,[313] which are always
placed above or on either side of the mouth, and from which proceed the
nerves of the eyes and tentacles; (2) the pedal, which in Gasteropoda are
situated below the oesophagus, in Pelecypoda at the base of the foot, and
from which the nerves of the foot and sometimes the acoustic nerve arise;
(3) the pleural,[314] whose position varies considerably, but is always below
the oesophagus and slightly above the pedal ganglia; these innervate the
mantle, branchiae, heart, and viscera generally.
Gasteropoda.—The simplest form of nerve system as thus understood
occurs in the Amphineura, and more particularly in the Chitons. Here we
find four longitudinal nerve-cords, parallel to one another for nearly the
whole length of the mollusc. The two exterior cords probably represent the
pleural, the two interior the pedal nervous system. There being no head or
tentacles, but simply a mouth at the anterior end, the cerebral ganglia do not
exist, but they are represented by the curved ring formed by the massing
together of the two nerve-cords on each side. The only distinct ganglia are a
pair of buccal ganglia (which are developed on a pair of commissures
which pass forward from the cerebral mass and innervate the lips and
buccal region), and a much smaller group, the sublingual. The two pedal
cords are united by a number of transverse parallel connectives, which

recall similar modes of connexion in the Chaetopod worms and in
Arthropoda.
This quadruple set of nerve-cords is characteristic of all the Amphineura,
but the absence of ganglia is most marked in the Chitons. In Proneomenia
and Neomenia there is a distinct cerebral ganglion, formed by the massing
of the two ganglia into one, while in Proneomenia the lateral cords are
joined to the pedal, as well as the pedal to one another, by connectives. In
Chaetoderma the cerebral ganglia, though adjacent, are distinct, and both
the pedal and lateral cords connect directly with them, while there are no
transverse connectives.
The remaining three great divisions of Gasteropoda, namely, the
Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as
comprising two distinct types of nervous condition, according as the loop
formed by the two visceral nerve-cords is twisted over itself, forming a
figure of 8, or continues straight and uncrossed. In the former case, we get
the condition known as streptoneurous, in the latter that as euthyneurous.
[315]
The Euthyneura include the whole of the Opisthobranchiata[316] and
Pulmonata, the Streptoneura all the Prosobranchiata.

Fig. 101.—Nervous system of the Amphineura: A,
Proneomenia; B, Neomenia; C, Chaetoderma; D,
Chiton; c, cerebral ganglia; l, l, lateral cords; pc,
posterior commissure; s, sublingual commissure or ring,
with ganglia; v, v, pedal cords. (Alter Hubrecht.)

The simplest form of nervous system in the euthyneurous Gasteropoda
occurs in the Opisthobranchiata. The cerebral, pleural, and pedal ganglia
tend to become concentrated in a ring-like form, united by short

commissures at the posterior end of the pharynx. The visceral loop is in
some cases long, and the two or three visceral ganglia are then situated at its
posterior extremity. The nervous system of the Pulmonata is of a similar
type, the visceral loop being often much shorter, and tending to draw in
towards the central group of ganglia. The tentacular and optic nerves are, as
usual, derived from the cerebral ganglion, with which also the octocysts are
probably connected by rather long nerves. A pair of buccal ganglia
innervate the buccal mass, and are united by commissures with the right and
left cerebral ganglia. The osphradial nerve springs from one of the ganglia
on the visceral loop, the osphradium itself being situated (in Limnaea)
immediately above the pulmonary orifice and adjacent to the anus (Fig.
102). This massing of the ganglia is still better illustrated by the
accompanying figure of Physa (Fig. 103), in which the animal is
represented as if transparent, so that the ganglia and nerves are seen through
the tissues.

Fig. 102.—I. Nervous system of Limnaea stagnalis L.
The oesophagus has been cut and pulled forwards
through the nerve-collar, so as to expose the lower
surface of the buccal mass(dissected by F. B.
Stead)
B.M, buccal mass.
B.G, buccal, C.G, cerebral, Os.G, osphradial,
Pe.G, pedal ganglia.
Pl.G, pleural ganglia.
Op.N, optic, Os.N, osphradial, Te.N, tentacular
nerve.
Ot, otocyst; V.L, visceral loop.
R, rectum, dotted in to show its position relative
to the osphiadium.

II. Right side of the head of Limnaea stagnalis. The
overhanging flap of the mantle has been cut in the
middle line, and the right half twisted back, so as
to expose the pulmonary orifice, etc. The points A
A on the mantle edge were continuous before the

mantle was cut; the line BA is part of the free edge
of the mantle.
An, anus; F, female generative orifice; J, portion of
jaw; M, male generative orifice under right tentacle;
Os, osphradium; P.O, pulmonary orifice.

Of the streptoneurous Gasteropoda, the nervous system of Fissurella and
Haliotis shows distinct points of similarity to that of the Amphineura. The
pedal nerves are united by transverse commissures throughout their entire
length, while a double commissure unites the cerebral ganglia to the mass
from which the pedal nerves proceed. In the great majority of the
Streptoneura the ganglia (except the visceral) are more concentrated and the
commissures are consequently much shorter. The accompanying figure of
Cyclostoma, in which the animal is represented as in that of Physa just
described, illustrates this grouping of the ganglia, the twist of the visceral
loop, and the position of the visceral ganglia at its posterior end.

Fig. 103.—Nervous system of Physa
acuta Drap., showing the massing
of the ganglia at the hinder end of
the pharynx: e, e, eyes; m, mouth;
m.l, m.l, mantle lappets; o.f, female
generative orifice; o.m, male
generative orifice; os, osphradium.
(After Lacaze-Duthiers.)

Fig. 104.—Example of a streptoneurous
Gasteropod (Cyclostoma elegans Drap.):
c.g, c.g, cerebral ganglia; e, e, eyes; os,
osphradium; ot, ot, otocysts; p.g, p.g, pedal
ganglia; pl.g, pl.g, pleural ganglia; sp.g,
supra-intestinal ganglion; sb.g, sub-
intestinal ganglion; t.n, tentacle nerve; v.g,
visceral ganglion. (After Lacaze-Duthiers.)

Scaphopoda.—In the Scaphopoda the nervous system resembles that of
the Pelecypoda. The cerebral and pleural ganglia lie close together, while
the pedal ganglia are placed in the anterior part of the foot, connected with
the cerebral ganglia by long commissures; the visceral loop is rather long,
and the two visceral ganglia are adjacent to the anus.
Pelecypoda.—The nervous system in the Pelecypoda is the simplest
type in which well-marked ganglionic centres occur. The ganglia are few,
symmetrically placed, and are usually at a considerable distance apart.
There are, as a rule, three distinct pairs of ganglia, the cerebral (cerebro-
pleural), pedal, and visceral. The cerebral are formed by the fusion of the
cerebral and pleural ganglia, which, however, in some cases
(Protobranchiata) continue distinct.[317] They lie above or on each side of
the mouth, united by a commissure of varying length. Another pair of

commissures unites them with the pedal ganglia, which are placed at the
base of the foot, and are usually very close together, sometimes (as in
Anodonta) becoming partially fused. The length of these commissures
depends upon the distance between mouth and foot; thus they are very long
in Mya and Modiola, and very short in Pecten. In cases where the foot is
rudimentary or becomes aborted through disuse (e.g. Ostrea), the pedal
ganglia may dwindle or disappear altogether. The visceral ganglia are on the
ventral side of the posterior adductor muscle, beneath the rectum, and
innervate the branchiae, osphradia, and the whole of the visceral sac. A pair
of cerebro-visceral commissures traverses the base of the foot, surrounding
it with a comparatively short loop (compare Fig. 106, c.v.c´), while a long
commissure, which runs round the entire edge of the mantle, and supplies
branching nerves to the mantle border and siphons (Fig. 106, c.v.c), may
also connect the visceral and cerebral ganglia.

Fig. 105.—Nervous system of
Pelecypoda: A, Teredo; B,
Anodonta; C, Pecten; a, a,
cerebral ganglia; b, pedal
ganglia; c, visceral ganglia.
(After Gegenbaur.)

Cephalopoda.—In the Cephalopoda the concentration of ganglia attains
its maximum, and may perhaps be regarded as approaching the point at
which a definite brain may be said to exist. Another point of distinction is

the formation of special small ganglia upon the nerve-cords in different
parts of the body. In the Tetrabranchiata (Nautilus) the cerebral and pedal
ganglia form a broad ring which surrounds the oesophagus, the former
giving out the optic nerves, with their special optic ganglion, and a pair
each of buccal and pharyngeal ganglia, the latter the nerves of the arms and
funnel. The visceral loop is still present in the form of a separate band,
which innervates the branchiae, osphradia, and viscera generally, forming a
special genital ganglion in connexion with the reproductive organs. The
principal ganglia of the Dibranchiata are still more concentrated, even the
visceral loop being possibly united with the rest in forming an unbroken
mass in which scarcely any trace of commissures can be detected. The
pedal ganglion becomes separated into two portions, one of which
innervates the arms, the other the funnel. Two peculiar ganglia (the stellate
ganglia) supply a number of branching nerves to the mantle.

Fig. 106.—Nervous system of Cardium edule L.: a.m, anterior adductor muscle; br, branchiae;
br.n, branchial nerve; c.g, c.g, cerebral ganglia; c.p.c, cerebro-pedal commissure; c.v.c’,
cerebro-visceral commissure; c.v.c, cerebro-visceral commissure of mantle; l.p, labial palps:
m, mouth; p.g, pedal ganglion; p.m, posterior adductor muscle; v.g, visceral ganglion. (After
Drost, × 3.)

E. L. Bouvier, Système nerveux, morphologie générale et classification des
Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), iii. 1887, pp. 1–510.
J. Brock, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool. xlviii. 1889, pp.
67–83.
O. Bütschli, Bemerkungen über die wahrscheinliche Herleitung der Asymmetrie
der Gasteropoda, etc.: Morph. Jahrb. xii. 1886, pp. 202–222.
B. Haller, Zur Kenntniss der Muriciden. I. Anatomie des Nervensystems:
Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp. 87–106.
„ Untersuchungen über marine Rhipidoglossen. II. Textur des„
Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885, pp. 319–436.
H. Grenadier, Abhandlungen zur vergleichenden Anatomie des Auges: Abh.
Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii. 1886, pp. 1–64.
A. P. Henchman, The Origin and Development of the Central Nervous System in
Limax maximus: Bull. Mus. C. Z. Harv. xx. 1890, pp. 169–208.
V. Hensen, Ueber das Auge einiger Cephalophoren: Zeit. wiss. Zool. xv. 1865, pp.
157–242.
C. Hilger, Beiträge zur Kenntniss des Gasteropodenauges: Morph. Jahrb. x. 1885,
pp. 352–371.
Lacaze-Duthiers, Otocystes ou Capsules auditives des Mollusques
(Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp. 97–166.
„ „ Du système nerveux des Mollusques gastéropodes pulmonés
aquatiques: ibid. pp. 437–500.
P. Pelseneer, Recherches sur le système nerveux des Ptéropodes: Arch. Biol. vii.
1887, pp. 93–130.
„ Sur la valeur morphologique des bras et la composition du système
nerveux central des Cephalopodes: Arch. Biol. viii. 1888, pp. 723–756.
H. Simroth, Ueber die Sinneswerkzeuge unserer einheimische Weichthiere: Zeit.
wiss. Zool. xxvi. 1876, pp. 227–348.
J. W. Spengel, Die Geruchsorgane und das Nervensystem der Mollusken: Zeit.
wiss. Zool. xxxv. 1881, pp. 333–383.

CHAPTER VIII
THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS

The digestive tract, or, as it is often termed, the alimentary canal or gut,
is a very important feature of the Mollusca. It may be regarded as consisting
of the following parts: (1) a mouth or oral aperture: (2) a throat or pharynx;
(3) an oesophagus, leading into (4) a stomach, (5) an intestine and rectum,
ending in (6) an anus.
The primitive positions of mouth and anus were presumably at the
anterior and posterior ends of the animal, as in the Amphineura and
symmetrical Mollusca generally. But the modifications of original
molluscan symmetry, which have already been referred to (p. 154, compare
pp. 245, 246), have resulted in the anus becoming, in the great majority of
Gasteropoda, twisted forward, and occupying a position on some point in
the right side in dextral, and in the left in sinistral species.
The process of digestion, as the food passes from one end of the tract to
the other, is performed by the aid of the secretions of various glands, which
open into the alimentary canal at different points in its course. The principal
of these are the salivary glands, situated on the pharynx and oesophagus,
and the liver, biliary or hepatic gland, connecting with the stomach. With
these may be considered the anal and ink-glands, which, in certain genera,
connect with the terminal portion of the rectum.
1. The mouth is generally, as in the common snail and periwinkle, placed
on the lower part of the head, and may be either a mere aperture, circular or
semicircular, in the head-mass, or, as is more usual, may be carried on a
blunt snout (compare Fig. 6, p. 10, and Fig. 68, p. 159), which is capable of
varying degrees of protrusion. From the retractile snout has doubtless been
derived the long proboscis which is so prominent a feature of many genera
(compare Figs. 1, B, and 99), and in some (e.g. Mitra, Dolium) attains a
length exceeding that of the whole body. As a rule, Mollusca provided with
a proboscis are carnivorous, while those whose mouth is on the surface of
the head are Vegetable feeders, but this rule is by no means invariable. The
mouth is thickened round the aperture into ‘lips,’ which are often extensile,
and appear capable of closing upon and grasping the food. In the
Pelecypoda the mouth is furnished, on each side, with a pair of special

external lobes, the ‘labial palps,’ which appear to be of a highly sensitive
nature, and whose object it is to collect, and possibly to taste, the food
before it passes into the mouth.
2. The Pharynx, Jaws, and Radula.—Immediately behind the lips the
mouth opens into the muscular throat, pharynx, or buccal mass. The
pharynx of the Glossophora, i.e. of the Gasteropoda, Scaphopoda, and
Cephalopoda, is distinguished from that of the Pelecypoda,[318] by the
possession of two very characteristic organs for the rasping or trituration of
food before it reaches the oesophagus and stomach. These are (a) the jaw or
jaws, and (b) the radula,[319] odontophore, or lingual ribbon. The jaws bite
the food, the radula tears it up small before it passes into the stomach to
undergo digestion. The jaws are not set with teeth like our own; roughly
speaking, the best idea of the relations of the molluscan jaw and radula may
be obtained by imagining our own teeth removed from our jaws and set in
parallel rows along a greatly prolonged tongue.[320]
In nearly all land Pulmonata the jaw is single, and is placed behind the
upper lip. If a common Helix aspersa be observed crawling up the inside of
a glass jar, or feeding on some succulent leaf, the position and action of the
jaw can be readily discerned. It shows very black when the creature opens
its mouth, and under its operation the edge of a lettuce leaf shows a regular
series of little curved indentations, in shape not unlike the semicircular bites
inflicted by a schoolboy upon his bread and butter. The jaw of Helix (Fig.
107, B) is arched in shape, and is strengthened by a number of projecting
vertical ribs. That of Limax (A) is straighter, and is slightly striated, without
vertical ribs. In Bulimulus (C) the arch of the jaw is very conspicuous, and
the upper edges are always denticulated; in Orthalicus there is a central
triangular plate with a number of overlapping plates on either side; in
Succinea (E) there is a large square accessory plate above the jaw proper.
The form of the jaw is peculiar not only to the genus but to the species as
well. Thus the jaw of H. aspersa is specifically distinct from that of H.
pomatia, and that of H. nemoralis is distinct from both. Wiegmann has
observed[321] that in young Arion, Limax, and Helix, the jaw consists of two
pieces, which coalesce by fusion in the adult, thus indicating a stage of
development in advance of the double jaw which is found in most of the
non-pulmonate Mollusca. In all fresh-water Pulmonata there are two small
accessory side plates besides the jaw proper (Fig. 107, F).

Fig. 107.—Jaws of various Pulmonata: A, Limax (gagates
Drap., Lancashire, × 15); B, Helix (acutissima Lam.,
Jamaica, × 15); C, Bulimulus (depictus Reeve,
Venezuela, × 20); D, Achatina (fulica Fér., Mauritius, ×
7); E, Succinea (elegans Riss., Aral District, × 30); F,
Limnaea (stagnalis L., Cambridge, × 30).

Nearly all the non-carnivorous Prosobranchiata, land, fresh-water, and
marine alike, are provided with two large lateral jaws. Many of these are
sculptured with the most elaborate patterns, and appear to be furnished with
raised teeth, like a file. In the Nudibranchiata the jaws are of great size and
beauty of ornamentation (Fig. 109).

Fig. 108.—Jaws of A, Triton australis Lam., Sydney; B, Ampullaria fasciata Reeve,
Demerara; C, Calliostoma punctulatum Mart., New Zealand; D, Cyclophorus
atramentarius Sowb., Sanghir; all × 15.

Fig. 109.—Jaws of A, Chromodoris gracilis Iher., × 15; B, Scyllaea
pelagica L., × 7; C, Pleurobranchus plumula Mont., × 10; D,
Pleurobranchaea Meckelii Lam., × 5/2.

The carnivorous genera, whether marine (e.g. Conus, Murex, Buccinum,
Nassa) or land (e.g. Testacella, Glandina, Streptaxis, Ennea), are entirely
destitute of jaws, the reason probably being that in all these cases the teeth
of the radula are sufficiently powerful to do the work of tearing up the food
without the aid of a masticatory organ as well. Jaws are also wanting in the
Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.
In the Cephalopoda the jaws, or ‘beaks,’ as they are called, are most
formidable weapons of attack. In shape they closely resemble the beaks of a
parrot, but the hook on the dorsal side of the mouth does not, as in birds,
close over the lower hook, but fits under it. Powerful muscles govern these
mandibles, which must operate with immense effect upon their prey (Fig.
110).

Fig. 110.—Jaws of Sepia: A, in
situ within the buccal mass,
several of the arms having
been cut away; B, removed
from the mouth and slightly
enlarged.

Fig. 111.—Patella vulgata L., showing
the normal position of the radula,
which is doubled back in a bow; the
shell has been removed, and the
whole visceral mass is turned
forward, exposing the dorsal surface
of the muscular foot: gr,
longitudinal groove on this surface;
i, i, intestine; l, liver; m, m, mantle
edge; mu, muscles (cut through)
fastening the visceral mass to the
upper sides of the foot; ov, ovary; r,
radula; u.f, upper or dorsal surface
of the foot.

The Radula.[322]—When the food has passed beyond the operation of the
jaw, it comes within the province of the radula, the front part of which
perhaps co-operates to a certain extent with the jaw in performing the biting
process. The function of the radula as a whole is to tear or scratch, not to
bite; the food passes over it and is carded small, the effect being very much
the same as if, instead of dragging a harrow over the surface of a field, we
were to turn the harrow points upwards, and then drag the field over the
harrow.
The radula itself is a band or ribbon of varying length and breadth,
formed of chitin, generally almost transparent, sometimes beautifully
coloured, especially at the front end, with red or yellow.[323] It lies

enveloped in a kind of membrane, in the floor of the mouth and throat,
being quite flat in the forward part, but usually curving up so as to line the
sides of the throat farther back, and in some cases eventually forming
almost a tube. The upper surface, i.e. the surface over which the food
passes, is covered with teeth of the most varied shape, size, number, and
disposition, which are almost invariably arranged in symmetrical rows.
These teeth are attached to the cartilage on which they work by muscles
which serve to erect or depress them; probably also the radula as a whole
can be given a forward or backward motion, so as to rasp or card the
substances which pass over it.
The teeth on the front part of the radula are often much worn (Fig. 112),
and probably fall away by degrees, their place being taken by others
successively pushed up from behind. At the extreme hinder end of the
radula the teeth are in a nascent condition, and there are often as many as a
dozen or more scarcely developed rows. Here, too, lie the cells from which
the teeth are originally formed.
The length and breadth of the radula vary greatly in different genera. In
Littorina it is very narrow, and several times the length of the whole animal.
It is kept coiled away like a watch-spring at the back of the throat, only a
small proportion of the whole being in use. I have counted as many as 480
rows in the common Littorina littorea. In Patella it is often longer than the
shell itself, and if the radula of a large specimen be freshly extracted and
drawn across the hand, the action of the hooks can be plainly felt. In
Aerope, the Turbinidae generally, and Haliotis it is very large. In Turritella,
Aporrhais, Cylichna, Struthiolaria, and the Cephalopoda it is small in
proportion to the size of the animal. In the Pulmonata generally it is very
broad, the length not exceeding, as a rule, thrice the breadth; in most other
groups the breadth is inconsiderable, as compared to the length.
The Radula is wanting in two families of Prosobranchiata, the Eulimidae
and Pyramidellidae, which are consequently grouped together as the section
Gymnoglossa. It is probable that in these cases the radula has aborted
through disuse, the animals having taken to a food which does not require
trituration. Thus several genera contained in both these families are known
to live parasitically upon various animals—Holothurians, Echinoderms, etc.
—nourishing themselves on the juices of their host. In some cases, the
development of a special suctorial proboscis compensates for the loss of

radula (see pp. 76–77). In Harpa there is no radula in the adult, though it is
present in the young form. No explanation of this fact has yet been given. It
is also absent in the Coralliophilidae, a family closely akin to Purpura, but
invariably parasitic on corals, and probably nourished by their exudations.
There is no radula in Entoconcha, an obscure form parasitic on the blood-
vessels of Synapta, or in Neomenia, a genus of very low organisation, or in
the Tethyidae, or sea-hares, or in one or two other genera of
Nudibranchiata.

Fig. 112.—Example of a front portion of
a radula (Cantharus ringens Reeve,
Panama), much worn by use. × 70.

The number of teeth in the radula varies greatly. When the teeth are very
large, they are usually few in number, when small, they are very numerous.
In the carnivorous forms, as a rule, the teeth are comparatively few and
powerful, while in the phytophagous genera they are many and small. Large
hooked and sickle-shaped teeth, sometimes furnished with barbs like an
arrow-head, and poison-glands, are characteristic of genera which feed on
flesh; vegetable feeders, on the contrary, have the teeth rounded, and blunter
at the apex, or, if long and narrow, so slender as to be of comparatively little
effect. Genera which are normally vegetarian, but which will, upon
occasion, eat flesh, e.g. Limax and Hyalinia, exhibit a form of teeth
intermediate between these two extremes (see Fig. 140, A).

In Chaetoderma there is but one tooth. In Aeolis coronata there are about
17, in A. papillosa and Elysia viridis about 19, in Glaucus atlanticus about
21, in Fiona nobilis about 28. In the common whelk (Buccinum undatum)
there are from 220 to 250, in the common periwinkle about 3500. As many
as 8343 have been counted in Limnaea stagnalis, about 15,000 in Helix
aspersa (that is, about 400,000 to the square inch), about 30,000 in Limax
maximus, and as many as 40,000 in Helix Ghiesbreghti, a large species from
Mexico; they are very numerous also in Nanina, Vitrina, Gadinia, and
Actaeon. But Umbrella stands far and away the first, as far as number of
teeth is concerned. In both U. mediterranea and U. indica they entirely
baffle calculation, possibly 750,000 may be somewhere near the truth.
The teeth on the radula are almost invariably disposed in a kind of
pattern, exactly like the longitudinal rows of colour in a piece of ribbon,
down the centre of which runs a narrow stripe, and every band of colour on
one side is repeated in the same relative position on the other side. The
middle tooth of each row—the rows being counted across the radula, not
longitudinally—is called the central or rachidian tooth; the teeth next
adjacent on each side are known as the laterals, while the outermost are
styled uncini or marginals. As a rule, the distinction between the laterals
and marginals is fairly well indicated, but in the Helicidae and some of the
Nudibranchiata it is not easy to perceive, and in these cases there is a very
gradual passage from one set to the other.
The central tooth is nearly always present. It is wanting in certain groups
of Opisthobranchiata, some of the carnivorous Pulmonata, and in the
Conidae and Terebridae, which have lost the laterals as well. Voluta has lost
both laterals and marginals in most of the species, and the same is the case
with Harpa. In Aeolis, Elysia, and some other Nudibranchiata the radula
consists of a single central row. Other peculiarities will be described below
in their proper order.
The extreme importance of a study of the radula depends upon the fact,
that in each species, and a fortiori in each genus and family, the radula is
characteristic. In closely allied species the differences exhibited are
naturally but slight, but in well-marked species the differences are
considerable. The radula, therefore, serves as a test for the distinction of
genera and species. For instance, in the four known recent genera of the
family Strombidae, viz. Strombus, Pteroceras, Rostellaria, and Terebellum,

the radula is of the same general type throughout, but with distinct
modifications for each genus; and the same is true, though to a lesser extent,
for all the species hitherto examined in each of the genera. These facts are
true for all known genera, differences of the radula corresponding to and
emphasising those other differences which have caused genera to be
constituted. The radula therefore forms a basis of classification, and it is
found especially useful in this respect in dealing with the largest class of all,
the Gasteropoda, and particularly with the chief section of this order, the
Prosobranchiata. Thus we have—
Prosobranchiata Monotocardia (a) Toxoglossa
(b) Rachiglossa
(c) Taenioglossa
(d) Ptenoglossa
(e) Gymnoglossa

Diotocardia (f) Rhipidonlossa
(g) Docoglossa[324]

(a) Toxoglossa.—Only three families, Terebridae, Conidae, and
Cancellariidae, belong to this section. There is no central tooth, and no
laterals, the radula consisting simply of large marginals on each side. In
Conus these are of great size, with a blunt base which contains a poison-
gland (see p. 66), the contents of which are carried to the point by a duct.
The point is always singly and sometimes doubly barbed (Fig. 116). When
extracted, the teeth resemble a small sheaf of arrows (Figs. 113, 115). A
remarkable form of radula, belonging to Spirotropis (a sub-genus of Drillia,
one of the Conidae), enables us to explain the true history of the radula in
the Toxoglossa. Here there are five teeth in a row, a central tooth, and one
lateral and one marginal on each side, the marginals being very similar in
shape to the characteristic shafts of the Conidae (Fig. 114). It is evident,
then, that the great mass of the Toxoglossa have lost both their central and
lateral teeth, and that those which remain are true uncini or marginals.
Spirotropis appears to be the solitary survival of a group retaining the
primitive form of radula.

Fig. 113.—Radula of Bela
turricula Mont. × 70.

Fig. 114.—Portion of radula of Spirotropis carinata
Phil., Norway. × 70.

Fig. 115.—Eight teeth from the
radula of Terebra
caerulescens Lam. × 60.

The arrangement of teeth in all these sections is expressed by a formula
applicable to each transverse row of the series. The central tooth, if present,
is represented by 1, and the laterals and marginals, according to their
number, on each side of the central figure. Thus the typical formula of the
Toxoglossa is 1.0.0.0.1, the middle 0 standing for the central tooth which is
absent, and the 0 on each side of it for the absent laterals; the 1 on each
extreme represents the one uncinus in each row. Thus the formula for
Spirotropis, which has also one lateral on each side and a rachidian or
central tooth, is 1.1.1.1.1. Often the formula is given thus:
where 30 and 42 stand for the average number of rows of
teeth in Conus and Spirotropis respectively; the same is sometimes
expressed thus: 1.0.0.0.1 × 30; 1.1.1.1.1 × 42.

Fig. 116.—A tooth from the
radula of Conus imperialis
L., S. Pacific, × 50,
showing barb and poison
duct.

Fig. 117.—Portion of the radula of Melongena vespertilio
Lam., Ceylon. × 30.

Fig. 118.—Portion of the radula of Eburna japonica Sowb., China. × 30.

Fig. 119.—Portion of the radula of Murex regius Lam., Panama. × 60.

(b) The Rachiglossa comprise the 12 families Olividae, Harpidae,
Marginellidae, Volutidae, Mitridae, Fasciolariidae, Turbinellidae,
Buccinidae, Nassidae, Columbellidae, Muricidae, and Coralliophilidae.
Certainly most and probably all of these families are or have been
carnivorous, the Coralliophilidae being a degraded group which have
become parasitic on corals, and have lost their teeth in consequence. The
characteristics of the group are the possession of a central tooth with from
one cusp (Boreofusus) to about fourteen (Bullia), and a single lateral more
or less cuspidate, the outer cusp of all being generally much the largest.
Thus in Melongena respertilio (Fig. 117) the central tooth is tricuspid, the
central cusp being the smallest, while the laterals are bicuspid; in Eburna
japonica (Fig. 118) the central tooth is 5-cusped, the two outer cusps being

much the smallest. The teeth, on the whole, are sharp and hooked, with a
broad base and formidable cutting edge. In the Olividae, Turricula,
Buccinopsis, and the Muricidae the laterals are unicuspid and somewhat
degraded (Fig. 119). In Mitra and the Fasciolariidae they are very broad and
finely equally toothed like a comb (Figs. 120, 121). The whole group is
destitute of marginals.

Fig. 120.—Portion of the radula of Imbricaria marmorata Swains. ×
80.

Fig. 121.—Three rows of teeth from the radula of Fasciolaria trapezium
Lam. × 40.

Fig. 122.—Six teeth from the
radula of Cymbium
diadema Lam., Torres
Strait. × 25.

Fig. 123.—Examples of degraded
forms of radula: A, Cantharus
pagodus Reeve, Panama
(nascent end), × 40; A´, same
radula, central and front
portion; B, Columbella varia
Sowb., Panama, × 50.

Fig. 124.—Three rows of the radula of Sistrum spectrum Reeve, Tonga, × 80. The laterals to
the right are not drawn in.

Several remarkable peculiarities occur. Harpa loses the radula altogether
in the adult. In the young it has lost only the laterals, and consists of nothing
but the central tooth. Marginella has no laterals; the central tooth is small
and comb-shaped, with blunt cusps. In Voluta the laterals are generally lost,
but in Volutomitra and one species of Voluta[325] they are retained. The
central tooth usually has three strong cusps, and is very thick and coloured a
deep red or orange (Fig. 122); in the sub-genus Amoria it is unicuspid, in
shape rather like a spear-head with broadened wings; in Volutolyria it is of a
different type, with numerous unequal denticulations, something like the
laterals of Mitra or Fasciolaria. Of the Mitridae, Cylindromitra has lost the
laterals. Among the Buccinidae, Buccinopsis possesses a curiously
degraded radula, the central tooth having no cusps, but being reduced to a
thin basal plate, while the laterals are also weakened. This degradation from
the type is a remarkable feature among radulae, and appears to be
characteristic, sometimes of a whole family, e.g. the Columbellidae (Fig.
123, B), sometimes of a genus, sometimes again of a single species. Thus in
Cantharus (a sub-genus of Buccinum) the radula is typical in the great
majority of species, but in C. pagodus Reeve, a large and well-grown
species, it is most remarkably degraded, both in the central and lateral teeth
(Fig. 123, A). This circumstance is the more singular since C. pagodus lives
at Panama side by side with C. ringeus and C. insignis, both of which have
perfectly typical radulae. It is probable that the nature of the food has
something to do with the phenomenon. Thus Sistrum spectrum Reeve was
found to possess a very aberrant radula, not of the common muricoid type,
but with very long reed-like laterals. This singularity was a standing puzzle
to the present writer, until he was fortunate enough to discover that S.

spectrum, unlike all other species of Sistrum, lives exclusively on a
branching coral.
The dental formula for the Rachiglossa is thus 1.1.1, except in those
cases where the laterals are absent, when it is 0.1.0.

Fig. 125.—Portion of the radula of Cassis sulcosa Born., × 40. The marginals to the right are not
fully drawn.

(c) The Taenioglossa comprise 46 families in all, of which the most
important are Tritonidae, Cassididae, Cypraeidae, Strombidae, Cerithiidae,
Turritellidae, Melaniidae, Littorinidae, Rissoidae, Paludinidae,
Ampullariidae, Cyclophoridae, Cyclostomatidae, and Naticidae. The radula
is characterised by a central tooth of very variable form, the prevailing type
being multicuspid, the central cusp the largest, on a rather broad base; a
single lateral, which is often a broad plate, more or less cusped, and two
uncini, rather narrow, with single hooks, or slightly cusped. The
accompanying figures of Cassis, Vermetus, and Cypraea, and those of
Littorina and Cyclophorus given on pp. 20, 21, are good examples of
typical taenioglossate radulae.

Fig. 126.—Four rows of teeth from the radula of Vermetus grandis
Gray, Andamans. × 40.

In Homalogyra the radula is much degraded, the central tooth is large
and triangular on a broad base, the lateral is represented only by a thin
oblong plate, and the uncini are absent. In some species of Jeffreysia the
uncini are said to be absent, while present in others. Lamellaria has lost
both its uncini, but the radula of the allied Velutina is quite typical. A
peculiar feature in this group is the tendency of the marginals to increase in
number. A stage in this direction is perhaps seen in Ovula, Pedicularia, and
the Cyclostomatidae. Here the outermost of the two marginals is by far the
larger and broader, and is strongly pectinated on its upper edge; in the
Cyclostomatidae the pectinations are rather superficial; in Ovula (where
both marginals are pectinated) they are decidedly deeper; in Pedicularia
they are deeper still, and make long slits in the tooth, tending to subdivide it
altogether. In Turritella the number of marginals is said to vary from none
(in T. acicula) to three (T. triplicata), but the fact wants confirmation.
Solarium is an aberrant form, possessing simply a number of long uncini,
which recall those of Conus or Pleurotoma, and is therefore hard to
classify; the allied Torinia has a radula which appears allied to Ovula or
Pedicularia. In Triforis the teeth are identical throughout, very small, about
27 in a row, tricuspid on a square base, cusps short.
The normal formula of the Taenioglossa is 2.1.1.1.2; in Lamellaria,
1.1.1; in Triforis, 13.1.13, or thereabouts.

Fig. 127.—Two rows of the radula of Cypraea tigris L. × 30.

Fig. 128.—Portion of the radula of Ianthina communis
Lam. × 40.

(d) Ptenoglossa.—This section consists of two families only, which
certainly appear remarkably dissimilar in general habits and appearance,
viz., the Ianthinidae and Scalariidae. In all probability their approximation is
only provisional. The radula, which in Ianthina is very large, and in
Scalaria very small, possesses an indefinite number of long hooked teeth,
of which the outermost are the largest. The central tooth, if present (it does
not occur in Ianthina), is the smallest in the series, and thus recalls the
arrangement in some of the carnivorous Pulmonata (p. 232). In Ianthina the
radula is formed of two large divisions, with a gap between them down the
middle.
The formula is ∞.1.∞ or ∞.O.∞ according as the central tooth in Scalaria
is or is not reckoned to exist.
(e) Gymnoglossa.—In the absence of both jaw and radula it is not easy to
classify the two families (Eulimidae and Pyramidellidae) which are grouped
under this section. Fischer regards them as modified Ptenoglossa; one
would think it more natural to approximate them to the Taenioglossa.

Fig. 129.—Portion of the radula of Margarita umbilicalis Brod.,
Labrador. × 75 and 300.

(f) Rhipidoglossa.—This section consists of seventeen families, the most
important being the Helicinidae, Neritidae, Turbinidae, Trochidae,
Haliotidae, Pleurotomariidae, and Fissurellidae. The radula is characterised
by—
(1) The extraordinary development of the uncini, of which there are so
many that they are always reckoned as indefinitely numerous. They are
long, narrow, hooked, and often cusped at the top, and crowded together
like the ribs of a fan, those at the extreme edge not being set straight in the
row, but curving away backwards as they become smaller; in Solariella
alone, where there are from five to ten, can they be counted.

Fig. 130.—Portion of the radula of Nerita albicilla L., Andaman Is., with central
tooth highly magnified: c, c, the capituliform tooth. × 40.

(2) The varying number of the laterals. The average number of these is
five on each side; in some cases (Livona) there are as many as nine, in some
(Neritopsis) only three. The lateral next to the uncini (which is specially
large in the Neritidae, and is then known as the capituliform tooth) is
regarded by some authorities as the first uncinus, by others as the sole
representative of the laterals, the teeth on the inner side of it being reckoned
as multiplied central teeth. According to this latter view, Livona will have as
many as seventeen central teeth. Taking five as the average number of
‘laterals,’ we shall have the following different ways of constituting the
rhipidoglossate formula, the first being that to which preference is given,
viz.:—
(1) ∞.5.1.5.∞, i.e. one central, five laterals, including the ‘last lateral’ tooth.
(2) (∞.1).4.1.4.(1.∞), regarding the ‘last lateral’ as first uncinus, but specialising it
by a number.
(3) ∞.1.(4.1.4).1.∞, regarding the ‘last lateral’ as the only lateral.

In the Neritidae and the derived fresh-water genera (Neritina, Navicella)
the first lateral, as well as the capituliform tooth, is very large, and in shape
rather like the blade bone of a shoulder of mutton; the intervening laterals
are very small. In Neritopsis (a degraded form) the central tooth and first
lateral are entirely wanting. In the neritiform land-shells (Helicina,
Proserpina) the first lateral is no larger than the others, while the
capituliform tooth is enormous. Hydrocena is a very aberrant and
apparently degraded form; the laterals between the first and the capituliform
tooth are all wanting. In Haliotis, Scissurella, and Pleurotomaria the five

laterals are of fairly equal size; in Fissurella we again meet with a large
capituliform tooth, with very small laterals.
(g) The Docoglossa are in direct contrast with the Rhipidoglossa in
possessing few and strong teeth, instead of many and weak. There are only
three families, Acmaeidae, Patellidae, and Lepetidae. In some of the
Acmaeidae there are not more than two teeth in a row, while in no species
are there more than twelve. The radula is, however, very long; there are as
many as 180 rows in Patella vulgata. The teeth are thick, generally of a
very deep red horn colour, rather opaque. The cartilage in which they are set
is remarkably thick, and in some species whose teeth are very few a
considerable portion of this cartilage is left quite bare.

Fig. 131.—Portion of the radula of Patella
cretacea Reeve, seen in half profile. × 40.

Although the teeth are so few, the arrangement is by no means simple.
The special feature of the group is the multiplication of identical centrals.
Of these there are two in Acmaea, and four, as a rule, in Patella. Thus in
these two genera there is seldom an absolutely central tooth. Either laterals
or marginals are liable to be lost, but there are never more than two of either
in Acmaea, and never more than two laterals and three marginals in Patella.
Thus the formula varies from 0.0.(1 + 0 + 1).0.0 in Pectinodonta, 2.2.(1 + 0
+ 1).2.2 in Collisellina (both Acmaeidae), to 3.2.(1 + 0 + 1).2.3 in Patinella,

and 3.1.(2 + 0 + 2).1.3 in Patella proper. In the Lepetidae there is an
absolutely central tooth, which appears to be made up of the coalescence of
several teeth, no laterals, and about two marginals; formula, 2.0.1.0.2.

Fig. 132.—Two rows of the radula of Pterotrachea mutica Les.,
Naples. × 60.

The radula of the Heteropoda is quite characteristic, and shows no sign
of affinity with any other Prosobranchiate. The central tooth is large, broad,
tricuspid, and denticulated on a broad base; the single lateral is strong, often
bicuspid; the two marginals simple, long, falciform; formula, 2.1.1.1.2 (Fig.
132).

Fig. 133.—A, Portion of the radula of Chiton (Acanthopleura)
spiniger] Sowb., Andamans, × 30; B, portion of the radula of
Dentalium entalis L., Clyde, × 50.

Amphineura.—(a) Polyplacophora.—The radula of the Chitonidae is
quite unique. It resembles that of the Docoglossa in being very long, and
composed of thick and dark horn-coloured teeth. The number of teeth,
however, is considerably greater, amounting almost invariably to seventeen
in each row. There are three rather small central teeth, the two outer of these
being similar; next comes a very large lateral (the major lateral), usually
tricuspid, which is followed by two much smaller laterals, which are
scarcely more than accessory plates; then a very large and arched marginal
(the major uncinus), at the outer side of which are three accessory plates.
Some consider there is only one central tooth, and count the two small teeth
on each side of it as laterals.
Thus the formula is either (3 + 1).(2 + 1).3.(1 + 2).(1 + 3) or (3 + 1).(2 +
1 + 1).1.(1 + 1 + 2).(1 + 3).
(b) Aplacophora.—Of this rather obscure order, Chaetoderma has a
single strong central tooth, Neomenia has no radula, Proneomenia and
Lepidomenia have about twenty falciform teeth, much larger at one end of
the radula than the other; formula, 0.1.0.
Opisthobranchiata.—The radula of the Opisthobranchiata is
exceedingly variable in shape, size, and number and character of teeth. Not
only do allied families differ greatly from one another, but allied genera
often possess radulae widely distinct in plan. Thus, among the Polyceridae,
Goniodoris has no central tooth, one large lateral and one marginal (form.
1.1.0.1.1); Doridunculus the same, with five marginals (form. 5.1.0.1.5);
Lamellidoris one each of median, laterals, and marginals (1.1.1.1.1); Idalia,
Ancula, and Thecacera the same as Goniodoris; Crimora several each of
laterals and marginals. Even species of the same genus may differ; thus the
formula for Aeolis papillosa is 0.1.0, but for Ae. Landsbergi 1.1.1; for
Philine aperta 1.0.1, but for Philine pruinosa 6.0.6.

Fig. 134.—Two teeth from the radula of Aeolis papillosa L. ×
55.

It must not be forgotten, however, that a simple repetition of the same
tooth, whether lateral or marginal, does not necessarily constitute an
important characteristic, nor does the presence or absence of a central tooth.
In most of the cases mentioned above, the difference in the number of
laterals and marginals is due to the multiplication of identical forms, while
the central tooth, when present, is often a mere plate or narrow block
without cusps, whose presence or absence makes little difference to the
character of the radula as a whole.
There appear to be three well-marked types of radula among the
Opisthobranchiata.
(a) Radula with a single strong central tooth, rows few. This form is
characteristic of the Aeolididae, Fionidae, Glaucidae, Dotoidae,
Hermaeidae, Elysiidae (Fig. 135), and Limapontiidae. In the Aeolididae it is
sometimes accompanied by a single lateral. The same type occurs in
Oxynoe, and in Lobiger (= Lophocercus).
(b) Radula with the first lateral very strongly developed. This type may
take the form of (1) a single lateral, no central or marginals, e.g.
Onchidoris, Scaphander (Fig. 137, A), Philine (certain species), Ringicula,
or (2) first lateral strongly developed, and repeated in succeeding laterals
(2–6) on a smaller scale, e.g. Philine (certain species). A few marginals are

sometimes added, e.g. in Polycera, Lamellidoris (where there is a degraded
central tooth, Fig. 137, B), Idalia, and Ancula.

Fig. 135.—Radula of Elysia
viridis Mont. × 40. Type
(a).

Fig. 136.—Portion of the radula of
Gadinia peruviana Sowb.,
Chili. × 250. Type (c).

(c) Radula with an indefinite number of marginals, laterals (if present)
merging into marginals, central tooth present or absent, inconspicuous, teeth
all very small. This type of radula, among the Nudibranchiata, is
characteristic of certain sub-genera of Doris (e.g. Chromodoris,
Aphelodoris, Casella, Centrodoris), of Hypobranchiaea and
Pleurophyllidia; among the Tectibranchiata, of Actaeon, many of the
Bullidae, Aplustrum, the Aplysiidae, Pleurobranchus, Umbrella and
Gadinia (Figs. 136 and 137, C).
In the Pteropoda there are two types of radula. The Gymnosomata,
which are in the main carnivorous, possess a radula with a varying number

(4–12) of sickle-shaped marginals, central tooth present or absent. In the
Thecosomata, which feed on a vegetable diet, there are never more than
three teeth, a central and a marginal on each side; teeth more or less cusped
on a square base.
Pulmonata.—The radula of the Testacellidae, or carnivorous land
Mollusca, is large, and consists of strong sickle-shaped teeth with very
sharp points, arranged in rows with or without a central tooth, in such a way
that the largest teeth are often on the outside, and the smallest on the inside
of the row (as in Rhytida, Fig. 139). The number and size of the teeth vary.
In Testacella and Glandina, they are numerous, consisting of from 30 to 70
in a row, with about 50 rows, the size throughout being fairly uniform. In
Aerope they are exceedingly large, and only eight in a row, the outermost
marginal being probably the largest single tooth in the whole of the
Mollusca. The central tooth is always obscure, being, when present, simply
a weaker form of the weakest lateral; in genera with only a few teeth in a
row it is generally absent altogether.

Fig. 137.—Portions of the radula of
Opisthobranchiata, illustrating types (b)
and (c); A, Scaphander lignarius L.; A´,
one of the teeth seen from the other side,
× 40; B, Lamellidoris bilamellata L.,
Torbay, × 60; C, Hydatina physis L., E.
Indies, × 75.

The first family of jaw-bearing snails, the Selenitidae, is distinctly
intermediate. The possession of a jaw relates it to the main body of
Helicidae, but the jaw is not strong, while the teeth are still, with the
exception of the central, thoroughly Testacellidan. The central tooth is quite
rudimentary, but it is something more than a mere weak reproduction of the
marginals. There are no true laterals. The Limacidae show a further stage in
the transition. Here the central tooth has a definite shape of its own,
tricuspid on a broad base, which is more or less repeated in the first laterals;
these, as they approach the marginals, gradually change in form, until the
outer marginals are again thoroughly Testacellidan.[326] This is the general
form of radula, varied more or less in different genera, which occurs in
Nanina, Helicarion, Limax, Parmacella, and all the sub-genera of Zonites.
It is certain that some, and probable that all of these genera will, on
occasion, eat flesh, although their usual food appears to be vegetable. The

jaw is more powerful than in the Selenitidae, but never so large or so
strongly ribbed as in Helix proper.

Fig. 138.—Portion of the radula of Glandina truncata Gmel. × 40.

Fig. 139.—Portion of the radula of Rhytida Kraussii Pfr., S.
Africa. × 25.

When we reach the Helicidae, we arrive at a type of radula in which the
aculeate form of tooth—so characteristic of the Agnatha—disappears even
in the marginals, and is replaced by teeth with a more or less quadrate base;
the laterals, which are always present, are intermediate in form between the
central and the marginals, and insensibly pass into the latter. In size and
number of cusps the first few laterals resemble the central tooth; in the
extreme marginals the cusps often become irregular or evanescent. As a
rule, the teeth are set squarely in the rows, with the exception of the

extreme marginals, which tend to slope away on either side. In some
Helicidae there is a slight approximation to the Zonitidae in the elongation
of the first marginals.
The above is the type of radula occurring in the great family Helicidae,
which includes not only Helix proper, with several thousand species, but
also Arion, Bulimus, Ariolimax, and other genera. The jaw is almost always
strongly transversely ribbed.
In the Orthalicidae (Fig. 140, C) the teeth of the radula, instead of being
in straight rows, slope back at an angle of about 45 degrees from the central
tooth. The central and laterals are very similar, with an obtuse cusp on
rather a long stem; the marginals become bicuspid.
In the Bulimulidae, which include the important genera Placostylus,
Amphidromus, Partula, Amphibulimus, and all the groups of South
American Bulimulus, the jaw is very characteristic, being thin, arched, and
denticulated at the edges, as if formed of numerous narrow folds
overlapping one another. The radula is like that of the Helicidae, but the
inner cusp of the laterals is usually lengthened and incurved. In Partula the
separation between laterals and marginals is very strongly marked.
The remaining families of Pulmonata must be more briefly described. In
the Cylindrellidae there are three distinct types of radula: (a) Central tooth a
narrow plate, laterals all very curiously incurved with a blunt cusp, no
marginals (Fig. 140, D); (b) radula long and narrow, central tooth as in (a),
two laterals, and about eight small marginals; (c) much more helicidan in
type, central and laterals obtusely unicuspid, marginals quite helicidan.
Type (c) is restricted to Central America, types (a) and (b) are West Indian.
Pupidae: Radula long and narrow; teeth of the helicidan type, centrals
and laterals tricuspid on a quadrate base, marginals very small, cusps
irregular and evanescent. This type includes Anostoma, Odontostomus,
Buliminus, Vertigo, Strophia, Holospira, Clausilia, and Balea.
Stenogyridae, including Achatina, Stenogyra, and all its sub-genera:
Central tooth small and narrow, laterals much larger, tricuspid, central cusp
long, marginals similar, but smaller.
Achatinellidae: Two types occur; (a) teeth in very oblique rows, central,
laterals, and marginals all of the same type, base narrow, head rather broad,

with numerous small denticles (Achatinella proper, with Auriculella and
Tornatellina, Fig. 140, E); (b) central tooth small and narrow, laterals
bicuspid, marginals as in Helix (Amastra and Carelia).

Fig. 140.—Portions of the radula of A, Hyalinia nitidula Drap., Yorkshire,
with central tooth, first lateral, and a marginal very highly magnified; B,
Helix pomatia L., Kent, showing central tooth, laterals, and one extreme
marginal, the two former also highly magnified; C, Orthalicus undatus
Brug., Trinidad, with three laterals highly magnified; D, Cylindrella
rosea Pfr., Jamaica, central tooth and laterals, the same very highly
magnified; E, Achatinella vulpina Fér., Oahu, central tooth (c) and
laterals, the same highly magnified.

Succineidae: Central and laterals helicidan, bi- or tricuspid on a quadrate
plate, marginals denticulate on a narrow base; jaw with an accessory oblong
plate.
Janellidae: Central tooth very small, laterals and marginals like
Achatinellidae (a).
Vaginulidae: Central, laterals, and marginals unicuspid throughout, on
same plan.
Onchidiidae: Rows oblique at the centre, straight near the edges; central
strong, tricuspid; laterals and marginals very long, falciform, arched,

unicuspid.
Auriculidae: Teeth very small; central narrow, tricuspid on rather a broad
base; laterals and marginals obscurely tricuspid on a base like Succinea.
Limnaeidae: Jaw composed of one upper and two lateral pieces; central
and lateral teeth resembling those of Helicidae; marginals much pectinated
and serriform (Fig. 141, A). In Ancylus proper the teeth are of a very
different type, base narrow, head rather blunt, with no sharp cusps, teeth
similar throughout, except that the marginals become somewhat pectinated
(Fig. 141, B); another type more resembles Limnaea.

Fig. 141.—Portions of the radula of A,
Limnaea stagnalis L., with the central
tooth and two first laterals, and two of
the marginals, very highly, magnified;
B, Ancylus fluviatilis Müll., with two of
the marginals very highly magnified;
C, Physa fontinalis L., with central
tooth and two of the marginals very
highly magnified.

Physidae: Jaw simple, but with a fibrous growth at its upper edge, which
may represent an accessory plate; radula with very oblique rows, central
tooth denticulate, laterals and marginals serriform, comb-like, with a wing-
like appendage at the superior outer edge (Fig. 141, C).
Chilinidae: Central tooth small, cusped on an excavated triangular base,
marginals five-cusped, with a projection as in Physa, laterals comb-like,
serrations not deep.

Amphibolidae: Central tooth five-cusped on a broad base, central cusp
very large; two laterals only, the first very small, thorn-like, the second like
the central tooth, but three-cusped; laterals simple, sabre-shaped.
Scaphopoda.—In the single family (Dentaliidae) the radula is large, and
quite unlike that of any other group. The central tooth is a simple broad
plate; the single lateral is strong, arched, and slightly cusped; the marginal a
very large quadrangular plate, quite simple; formula, 1.1.1.1.1 (Fig. 133, B).
Cephalopoda.—The radula of the Cephalopoda presents no special
feature of interest. Perhaps the most remarkable fact about it is its singular
uniformity of structure throughout a large number of genera. It is always
very small, as compared with the size of the animal, most of the work being
done by the powerful jaws, while the digestive powers of the stomach are
very considerable.
The general type of structure is a central tooth, a very few laterals, and
an occasional marginal or two; teeth of very uniform size and shape
throughout. In the Dibranchiata, marginals are entirely absent, their place
being always taken, in the Octopoda, by an accessory plate of varying shape
and size. This plate is generally absent in the Decapoda. The central tooth
is, in the Octopoda, very strong and characteristic; in Eledone and Octopus
it is five-cusped, central cusp strong; in Argonauta unicuspid, in
Tremoctopus tricuspid. The laterals are always three in number, the
innermost lateral having a tendency to assume the form of the central. In
Sepia the two inner laterals are exact reproductions of the central tooth; in
Eledone, Sepiola, Loligo, and Sepia, the third lateral is falciform and much
the largest.

Fig. 142.—Portion of the radula of Octopus tetracirrhus D. Ch.,
Naples, × 20.