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
{ OCTOPODA (p. 382).
{ { Phragmophora (p. 386).
{ =Dibranchiata= { { Sepiophora (p. 388).
{ { {
{ { DECAPODA { Chondrophora { _Myopsidae_ (p. 389).
{ { { { _Oigopsidae_ (p. 390).
=CEPHALOPODA= {
{ { NAUTILOIDEA { Retrosiphonata (p. 393).
{ { { Prosiphonata (p. 395).
{ =Tetrabranchiata= {
{ { AMMONOIDEA { Prosiphonata (p. 397).
{ { { Prosiphonata (p. 397).
{ =Amphineura= { POLYPLACOPHORA (p. 400).
{ { APLACOPHORA (p. 404).
{
{ { { Docoglossa (p. 405).
{ { DIOTOCARDIA { { _Zygobranchiata_
{ { { Rhipidoglossa { (p. 406).
{ { { { _Azygobranchiata_
{ =Prosobranchiata= { { { (p. 407).
{ {
{ { { Ptenoglossa (p. 411).
{ { { { _Platypoda_ (p. 411).
{ { { Taenioglossa { _Heteropoda_ (p. 420).
{ { MONOTOCARDIA {
=GASTEROPODA= { { { Gymnoglossa (p. 422).
{ { {
{ { { Toxoglossa (p. 426).
{
{ { { Bulloidea (p. 429).
{ { TECTIBRANCHIATA { 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).
=SCAPHOPODA=
(p. 444).
Class Order Suborder
{ =Protobranchiata= (p. 447).
{ { ANOMIACEA (p. 448).
{ =Filibranchiata= { ARCACEA (p. 448).
{ { MYTILACEA (p. 448).
{
{ =Pseudolamellibranchiata= (p. 449).
{
=PELECYPODA= { { SUBMYTILACEA (p. 451).
{ { TELLINACEA (p. 453).
{ { VENERACEA (p. 454).
{ =Eulamellibranchiata= { CARDIACEA (p. 454).
{ { MYACEA (p. 456).
{ { PHOLADACEA (p. 457).
{ { ANATINACEA (p. 458).
{ =Septibranchiata= (p. 459).
BRACHIOPODA
Order Family
{ { Lingulidae (pp. 487 and 503).
{ { Obolidae (p. 504).
{ ECARDINES { Discinidae (pp. 487 and 504).
=Brachiopoda= { { Craniidae (pp. 487 and 504).
{ { Trimerellidae (p. 504).
{
{ { Productidae (p. 504).
{ { Strophomenidae (p. 505).
{ { Koninckinidae (p. 505).
{ { Spiriferidae (p. 505).
{ { Atrypidae (p. 505).
{ TESTICARDINES { 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 INDIAN ARCHIPELAGO _Between pp. 308 and 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 INDIES _Between pp. 344 and 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.
[Illustration: 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
MOLLUSCA
|
+----------------+----------------------------------+
| |
=Glossophora= =Aglossa=
| |
+--------+----------+-------------------+ |
| | | |
_Cephalopoda_ _Gasteropoda_ _Scaphopoda_ _Pelecypoda_
=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_.
[Illustration: FIG. 2.--An example of the Polyplacophora: _Chiton
spinosus_ Brug.]
[Illustration: 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.
[Illustration: 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_.
[Illustration: 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.]
[Illustration: 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
{ _Amphineura_ { Polyplacophora
{ { Aplacophora
{
{ _Prosobranchiata_ { Diotocardia
=Gasteropoda= { { 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.
[Illustration: 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.
[Illustration: FIG. 8.--=A=, The common _Mytilus edulis_ L.,
a marine genus and species. =B=, _Dreissensia_, a fresh-water
genus, closely allied to _Mytilus_.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.]
[Illustration: 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).
[Illustration: 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.
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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).
[Illustration: 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]
[Illustration: 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.
[Illustration: 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).
[Illustration: 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.)]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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_.