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_.

Professor Herdman is of opinion that although the cerata may in all cases aid in respiration to a certain extent, yet that extent is so small as to be left out of consideration altogether. He regards the cerata in both the two classes mentioned above as “of primary importance in giving to the animals, by their varied shapes and colours, appearances which are in some cases protective, and in others conspicuous and warning.”

Thus, for instance, _Tritonia plebeia_, which is fairly abundant at Puffin and Hilbre Is., appears always to be found creeping on the colonies of a particular polyp, _Alcyonium digitatum_, and nowhere else. The specimens in each colony of the polyp differ noticeably both in the matter of colour, and of size, and of varied degrees of expansion. The _Tritonia_ differs also, being marked in varied tints of yellow, brown, blue, gray, black, and opaque white, in such a way as to harmonise with the varied colours of the _Alcyonium_ upon which it lives. The cerata on the back of the _Tritonia_ contribute to this general resemblance. They are placed just at the right distance apart, and are just the right size and colour, to resemble the crown of tentacles on the half-expanded polyp.

Similarly, _Doto coronata_, which, when examined by itself, is a very conspicuous animal, with showy, bright-coloured cerata, is found by Professor Herdman to haunt no other situations but the under side of stones and overhanging ledges of rock which are colonised by a hydroid, known as _Clava multicornis_. The _Doto_ is masked by the tentacles and clusters of sporosacs on the zoophyte, with whose colouring and size its own cerata singularly correspond. A similar and even more deceptive correspondence with environment was noticed in the case of the very conspicuous _Dendronotus arborescens_.

In these cases, the colouring and general shape of the cerata are protective, _i.e._ they match their surroundings in such a way as to enable the animal, in all probability, to escape the observation of its enemies. According to Professor Herdman, however, the brilliant and showy coloration of the cerata of _Aeolis_ is not protective but ‘warning.’ _Aeolis_ does not hide itself away as if shunning observation, like _Doto_, _Tritonia_, and _Dendronotus_; on the contrary, it seems perfectly fearless and indifferent to being noticed. Its cerata are provided with sting-cells, like those of Coelenterata, at their tips, and its very conspicuousness is a warning to its enemies that they had better not try to attack it, just as the showy white tail of the skunk acts as a sort of danger-signal to its own

## particular foes. It is important for the _Aeolis_, not merely to _be_

an unpalatable nettle in animal shape, but also to be conspicuous enough to prevent its being experimented upon as an article of food, in mistake for something less nasty.

Professor Herdman subsequently conducted some experiments[158] with fishes, with the view of testing his theory that the shapes and colours of Nudibranchs serve the purpose either of protection or warning, and bear direct relation to the creature’s edibility. These experiments, on the whole, distinctly tended to confirm the theory. _Aeolis_ was evidently very nasty, and probably stung the mouths of the fishes who tried it. For the complete success of the theory, they ought to have let it severely alone, but the fish were evidently accustomed to make a dash at anything that was dropped into their tank. Another conspicuous mollusc, _Ancula cristata_, was introduced, Professor Herdman and his collaborator each commencing operations by eating a live specimen themselves. They found the taste pleasant, distinctly like that of an oyster. The fish, however, when the experiments were conducted under conditions which made the scene as much like ‘real life’ as possible, did not agree with Professor Herdman. The _Ancula_ crawled over various parts of the tank for several days untouched by the fish, who sometimes went close to them and looked at them, but never attempted to taste them. Experiments with species whose colours were protective, such as _Dendronotus_, were also conducted, and the decided edibility of these species was established, the fish competing eagerly for them, and tearing them rapidly to pieces.

Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological Association, confirms[159] Professor Herdman’s views as to the shape and colour of Opisthobranchs. _Pleurobranchus membranaceus_ is known to secrete, on the surface of the body, an acid which reddens blue litmus paper. It is, therefore, no doubt distasteful to fish, which all abominate the taste of acids, and is conspicuously marked with red-brown and yellowish ‘warning’ colours. _Haminea_ and _Philine_, on the other hand, are good to eat, and consequently possess ‘protective’ coloration. _Runcina Hancocki_, which is of a brown colour, crawls over brown mud and weeds, but avoids green weeds, on whose surface it would appear conspicuous. _Elysia viridis_ varies its colour according to its habitat, being green when on green weeds, and dark olive, brown, or reddish brown, on pools among tufts of littoral algae. Green specimens of _Hermaea dendritica_ were kept in captivity, and placed in a dish with green and red sea-weeds. They were never observed crawling upon the red weed, upon which they would have been very conspicuous. _Archidoris flammea_ occurred on bright red sponges, to which its colour was so closely assimilated that Mr. Garstang at first quite overlooked it. _Goniodoris castanea_ was found under stones, feeding on compound Ascidians (_Botryllus_), which it sufficiently resembled to be very inconspicuous in that position.

Again, _Jorunna Johnstoni_ lives[160] upon stones on our southern coast, associated with a certain sponge (_Halichondria_ sp.), which it resembles so closely in outline, in colour, in character of surface, and in its projecting plumes, as to make it very difficult even for the careful observer to distinguish the one from the other. And, since fishes, are known to be distinctly averse to sponges of any kind as an article of food, this resemblance must be decidedly to the advantage of the _Jorunna_. Another Nudibranch (_Calma glaucoides_ A. and H.) imitates the ova of certain fishes, on which it feeds. Its elongated and depressed form of body, transparent integuments, and silvery gray papillae combine to give it a strong resemblance to the spawn of the fish, which is deposited on stones, the roots of _Laminaria_, etc.[161]

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

=Parasitic Mollusca=

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

[Illustration: FIG. 29.--_Magilus antiquus_ L.: =A=, the adult, imbedded in coral, which has been broken away to show the tube; =B=, the young (free) form.]

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

On _Echinodermata_.--(_a_) _Crinoidea._ _Stylina comatulicola_ lives on _Comatula mediterranea_, fixed to the outer skin, which it penetrates by a very long proboscis; the shell is quite transparent.[164] A curious case of a fossil parasite has been noticed by Roberts.[165] A _Calyptrea_-shaped shell named _Platyceras_ always occurred on the ventral side of a crinoid, encompassed by the arms. For some time this was thought to afford conclusive proof of the rapacity and carnivorous habits of the echinoderm, which had died in the act of seizing its prey. Subsequent investigations, however, showed that in all the cases noticed (about 150) the _Platyceras_ covered the anal opening of the crinoid in such a way that the mouth of the mollusc must have been directly over the orifice of the anus. (_b_) _Asteroidea._ The comparatively soft texture of the skin of the starfishes renders them a favourite home of various parasites. The brothers Sarasin noticed[166] a species of _Stilifer_ encysted on the rays of _Linckia multiformis_. Each shell was enveloped up to the apex, which just projected from a hole at the top of the cyst. The proboscis was long, and at its base was a kind of false mantle, which appeared to possess a pumping action. On the under side of the rays of the same starfish occurred a capuliform mollusc (_Thyca ectoconcha_), furnished with a muscular plate, whose cuticular surface was indented in such a way as to grip the skin of the _Linckia_. This plate was furnished with a hole, through which the pharynx projected into the texture of the starfish, acting as a proboscis and apparently furnished with a kind of pumping or sucking action. Adams and Reeve[167] describe _Pileopsis astericola_ as living ‘on the tubercle of a starfish,’ and _Stilifer astericola_, from the coast of Borneo, as ‘living in the body of a starfish.’ In the British Museum there is a specimen of _Pileopsis crystallina_ ‘in situ’ on the ray of a starfish, (_c_) On the brittle starfishes (_Ophiuroidea_) occur several species of _Stiliferina_. (_d_) _Echinoidea._ Various species of _Stilifer_ occur on the ventral spines of echinoids, where they probably subsist on the excreta, and are sometimes found imbedded in the spines themselves. _St. Turtoni_ occurs on the British coasts on several species of _Echinus_, and _Montacuta substriata_ frequents _Spatangus purpureus_ and certain species of _Echinocardium_, _Cidaris_, and _Brissus_. _Lepton parasiticum_ has been described from Kerguelen I. on a _Hemiaster_, and a new genus, _Robillardia_, has recently been established[168] for a _Hyalinia_-shaped shell, parasitic on an _Echinus_ from Mauritius. (_e_) _Holothurioidea._ The ‘sea-cucumbers’ afford lodgment to a variety of curious forms, some of which have experienced such modifications that their generic position is by no means established. _Entoconcha_ occurs fixed by its buccal end to the blood-vessels of certain _Synapta_ in the Mediterranean and the Philippines. _Entocolax_ has been dredged from 180 fath. in Behring’s Straits, attached by its head to certain anterior muscles of a _Myriotrochus_.[169] A curious case of parasitism is described by Voeltzkow[170] as occurring on a _Synapta_ found between tide-marks on the I. of Zanzibar. In the oesophagus of the _Synapta_ was found a small bivalve (_Entovalva_), the animal of which was very large for its shell, and almost entirely enveloped the valves by its mantle. As many as five specimens occurred on a single _Synapta_. In the gut of the same Holothurian lived a small univalve, not creeping freely, but fixed to a portion of the stomach wall by a very long proboscis which pierced through it into the body cavity. This proboscis was nearly three times as long as the animal, and the forward portion of it was set with sharp thorns, no doubt in order to enable it to retain its hold and resist evacuation. Various species of _Eulima_ have been noticed in every part of the world, from Norway to the Philippines, both inside and outside Holothurians.[171] _Stilifer_ also occurs on this section of Echinoderms.[172]

On _Annelida_.--_Cochliolepis parasiticus_ has been noticed under the scales of _Acoetes lupina_ (a kind of ‘sea-mouse’) in Charleston Harbour.[173]

On _Crustacea_.--A mussel, ⅜ in. long, has been found[174] living under the carapace of the common shore-crab (_Carcinus maenas_), and one case has been noticed[175] where two mussels, one of several months’ growth, the other smaller, well secured by their byssi, were found under the abdomen of the same species, in such a position as to force the appendages apart and askew. These, however, are not so much cases of parasitism as of involuntary habitat, the mussel no doubt having become involved in the branchiae and the abdomen of the crab in the larval form.

On _Mollusca_.--A species of _Odostomia_ (_pallida_ Mont.) is found on our own coasts on the ‘ears’ of _Pecten maximus_, and also[176] on the operculum of _Turritella communis_. Another species (_O. rissoides_) frequently occurs in hiding under beds of mussels, but it is not clear whether the habitat is due to parasitism, or simply to the fact that the mass of mussels, knitted together and to the rock by the byssi, affords the _Odostomia_ a safe lurking-place. At Panama the present writer found _Crepidula_ (2 sp.) plentiful on the opercula of the great _Strombus galea_ and of _Cerithium irroratum_. In each case the parasite exactly fitted the size of the operculum, and had assumed its colour, dark brown or chestnut. _Amalthea_ is very commonly found on _Conus_, _Turbo_, and other large shells from the South Pacific, but this is probably not a case of parasitism, but simply of convenience of habitat, just as young oysters are frequently seen on the carapace and even on the legs of large crabs.

[Illustration: FIG. 30.--_Crepidula onyx_ Sowb., parasitic on the operculum of _Strombus galeatus_ Swains., Panama.]

On _Tunicata_.--_Lamellaria_ deposits its eggs and lives on an Ascidian (_Leptoclinum_), and the common _Modiolaria marmorata_ lives in colonies imbedded in the test of _Ascidia mentula_ and other simple Ascidians.

[Illustration: FIG. 31.--Two species of _Eulima_: =A= is sessile on the skin of a Holothurian, through which it plunges its sucking proboscis (_Pr_); =B= creeps freely in the stomach of a Holothurian. (After K. Semper.)]

Special points of interest with regard to parasitic Mollusca relate to (1) _Colour_. This is in most cases absent, the shell being of a uniform hyaline or milky white. This may be due, in the case of the endo-parasitic forms, to absence of light, and possibly, in those living outside their host, to some deficiency in the nutritive material. A colourless shell is not necessarily protective, for though a transparent shell might evade detection, a milk-white hue would probably be conspicuous. (2) _Modifications of structure._ These are in many cases considerable. _Entoconcha_ and _Entocolax_ have no respiratory or circulatory organs, and no known nervous system; _Thyca_ and certain _Stilifer_ possess a curious suctorial apparatus; the foot in many cases has aborted, since the necessity for locomotion is reduced to a minimum, and its place is supplied by an enormous development of the proboscis, which enables the creature to provide itself with nutriment without shifting its position. K. Semper notices a case where a _Eulima_, whose habitat is the stomach of a Holothurian, retains the foot unmodified, while a species occurring on the outer skin, but provided with a long proboscis, has lost its foot altogether.[177] Special provision for holding on is noticed in certain cases, reminding us of similar provision in human parasites. Eyes are frequently, but not always wanting, even in endo-parasitic forms. A specially interesting modification of structure occurs in (3) the _Radula_ or ribbon-shaped arrangement of the teeth. In most cases of parasitism (_Eulima_, _Stilifer_, _Odostomia_, _Entoconcha_, _Entocolax_, _Magilus_, _Coralliophila_, _Leptoconcha_) it is absent altogether. In _Ovula_ and _Pedicularia_, genera which are in all other respects closely allied to _Cypraea_, the radula exhibits marked differences from the typical radula of the Cypraeidae. The formula (3·1·3) remains the same, but the laterals are greatly produced and become fimbriated, sometimes at the extremity only, sometimes along the whole length. A very similar modification occurs in the radula of _Sistrum spectrum_ Reeve, a species which is known to live parasitically on one of the branching corals. Here the laterals differ from those of the typical _Purpuridae_ in being very long and curved at the extremity. The general effect of these modifications appears to be the production of a radula rather of the type of the vegetable-feeding _Trochidae_, which may perhaps be regarded as a link in the chain of gradually-degraded forms which eventually terminate in the absence of the organ altogether. The softer the food, the less necessity there is for strong teeth to tear it; the teeth either become smaller and more numerous, or else longer and more slender, and eventually pass away altogether. It is curious, however, that the same modified form of radula should appear in species of _Ovula_ (_e.g._ _ovum_) and that the same absence of radula should occur in species of _Eulima_ (_e.g._ _polita_) known to be not parasitic. This fact perhaps points back to a time when the ancestral forms of each group are parasitic and whose radulae were modified or wanting, the modification or absence of that organ being continued in some of their non-parasitical descendants.

=Commensalism=

Mollusca are concerned in several interesting cases of commensalism, or the habitual association of two organisms, as distinguished from parasitism, where one form preys more or less upon the other.

Mr. J. T. Marshall has given[178] an interesting account of the association of _Montacuta ferruginosa_ with _Echinocardium cordatum_. The Echinoderm lives in muddy sand in Torbay, at a depth of about 6 inches, and the _Montacuta_ lives in a burrow leading from its ventral end and running irregularly in a sloping direction for 3 or 4 inches, the burrow, which is made by a current from the Echinoderm, being almost exactly the width of the _Montacuta_. The _Montacuta_ were always arranged in the burrows in order of size, the largest being close to the Echinoderm, and the smallest of a string of about six at the other end of the burrow. In another part of S. Devon, where the sand was soft and sloppy, the Echinocardia rise to the surface and travel along the sand; in this case the _Montacuta_ were attached to their host by means of a byssus, and were dragged along as it travelled.

The Rev. Dr. Norman has noted[179] a somewhat similar habitat for _Lepton squamosum_. This rare little British species was found at Salcombe, living in the burrows of _Gebia stellata_, in all probability feeding upon the secretions from the body of the crustacean. Dr. Norman suggests that the extreme flatness of the shell of the _Lepton_ is of great advantage in enabling it not to get in the way of the _Gebia_ as he scuttles up and down his burrow. Another species of _Lepton_ is found on the coast of Florida in a precisely similar locality,[180] while a third species, occurring on the Oregon and California coasts, actually attaches itself to the inner surface of the abdomen of a _Gebia_.[181]

[Illustration: FIG. 32.--_Ephippodonta Macdougalli_ Tate, S. Australia. =A=, Burrow of prawn, the X indicating the position of the mollusc; _sp_, sponge. =B=, Ventral view of _Ephippodonta_; _by_, byssus; _f_, foot; _m_, mantle; _mm_, fused mantle borders. =C=, View of interior of shells; _h_, hinge; m´m´, adductor muscles. (=A= × ½; =B= and =C= × 2.)]

A very singular case of commensalism has been recently discovered with regard to a genus of Australian bivalve shells, _Ephippodonta_. This genus is never found except in the burrow of a species of prawn (_Axius plectorhynchus_ Str.). For some reason at present unexplained, the burrow of this particular prawn appears to be exceedingly popular as a habitat for certain bivalves, for, besides two species of _Ephippodonta_, a _Kellia_ and three _Mylitta_ are found there, and there alone. Sometimes the prawn, when the rock is hard, builds a tunnel of mud upon it, at other times it excavates the soft calciferous sandstone. “This burrow is lined with a tenacious brown mud, composed of excrementitious matter; and, in addition to the mud lining, there is always more or less present an orange-coloured sponge which I have never found elsewhere. Upon the mud or sponge, and adhering very closely, are found the _Ephippodonta_. They quickly form a pit-like depression by means of their foot, and appear almost covered by the mud.” During the winter months (March-July) the prawn appears to fill his burrow, possibly as a provision against stormy weather, with large quantities of minced seaweed, underneath which immense numbers of very young _Ephippodonta_ are found living.[182] The extreme flatness of the _Ephippodonta_ must be due to the same cause as the flatness of the _Lepton_ noticed above, namely, the necessity of not impeding or interfering with the lively motions of the prawn. In the case of _Lepton_ the two valves close completely and the shell is still very flat; in _Ephippodonta_, on the other hand, the same result is produced by the valves being opened to their widest possible extent. As in _Entovalva_, a continuation of the mantle covers the outer surface of the shell.

=Variation=

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

It will be generally admitted, however, that some _structural difference_ in the organisation of the animal (as distinct from that of the shell alone) is necessary for the permanent constitution of specific rank.[184] What _amount_ of structural difference is required, what particular organ or organs must exhibit this difference, will depend largely upon the idiosyncrasy of the observer. But if this, or something like this definition of a species be accepted, it will follow that a so-called ‘variety’ will be a form which exhibits differences from the type which do not amount to permanent structural differences in the organisation of the animal. The final court of appeal as to what affords sufficient evidence for ‘permanent structural differences’ will have to be, as with Aristotle of old, the judgment of the educated man.

It is, however, more to our present purpose to discuss the _causes of variation_ than to lay down definitions of what variation is. One of the most obvious causes of variation lies in a change or changes in the environment. If we may assume, for the moment, that the type form of a species is the form which is the mean of all the extremes, and that this form is the resultant of all the varied forces brought to bear upon it, whether of food, climate, temperature, competition of numbers, soil, light, amount of water, etc., it will follow that any change in one or more of these forces, if continuous and considerable, any change, in other words, of the environment, will produce its effect upon the organism in question. And this effect will be for the better or for the worse, according to the particular nature of the change itself as tending towards, or away from, the _optimum_ of environment for the species concerned. Hence may be produced varieties, more or less marked according to the gravity of the change, although it must be noted that at times a change apparently unimportant from our point of view, will produce very marked results upon the species. It is indeed scarcely possible to predict with any certainty, in the present state of our knowledge (beyond certain broad results) what will be the particular effect upon a species of any given change in its surroundings.

=Effects of Change in the Environment as tending to produce Variation.=

(_a_) _Changes in Climate, Temperature, Elevation, etc._--In the eastern basin of the Baltic the marine Mollusca are much more stunted than in the western.[185] For instance, _Mytilus edulis_ near Kiel is 8–9 cm. long, while near Gothland it only attains a length of 3–4 cm. Mollusca living at only a shallow depth (_e.g._ _Tellina balthica_, _Mya arenaria_, _Cardium edule_) do not differ much in size in different parts of the Baltic, but in the far eastern basin the calcareous layers of the shells of _Mya arenaria_ and _Tellina balthica_ are extraordinarily thin, and disappear very rapidly after death, leaving only the cuticular membrane, still united by the ligament, in a perfect state of preservation. These remarkable variations are no doubt to a large extent due to the violent changes of temperature which are experienced in the Baltic, and by which the steady development of the animals in question is interrupted and thrown out of gear. The same species occur on the coasts of Greenland and Iceland, where they attain a considerably larger size than in the Baltic, in spite of the lower mean temperature, probably because their development is not interrupted by any sudden change from cold to heat or _vice versâ_.

Karl Semper has shown that _Limnaea stagnalis_ is developed, lives and feeds best in a mean temperature of about 20° C. (= 68° F.). This mean, however, must not be the mean of two distant extremes, for the _Limnaea_ cannot digest its food and grow in a temperature which is less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C. (= 86° to 90° F.). In certain localities, therefore, the interruption to the growth of this species must be serious and prolonged, and may tend towards the production of more or less dwarfed varieties. Thus specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the sea, are permanently dwarfed, and have a very thin and fragile shell. _Limnaea peregra_ in the Pyrenees, Alps, and Himalayas is generally of a very delicate form and dwarfed habit, while the small variety known as _lacustris_ occurs, according to Jeffreys, only in mountain lakes in Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr. Bateson from lakes near the Sea of Aral, which are salt for some months and comparatively fresh for others, exhibit clearly the effect of changes in the environment (Figs. 33 and 34). Excess of heat produces similar results to excess of cold. _L. peregra_ var. _thermalis_, found in the warm springs of the Pyrenees and the Vosges, and the var. _geisericola_, from the hot water of the Iceland geysers, are alike thin and dwarfed forms.

Many instances may be given of ‘varieties due to locality.’ In some of these, the cause which predisposes towards variation can be inferred with some approach to certainty, in others we must be content to note the fact, without at present being able to perceive its explanation.

[Illustration: FIG. 33.--Four examples of _Limnaea peregra_ Müll., from salt marshes near the Sea of Aral, showing different effects produced by abnormal conditions of life.]

[Illustration: FIG. 34.--Four examples of _Limnaea stagnalis_ L., from marshes in the Aral district which are salt for several months in the year, illustrating variation produced by changes in the environment. × ½.]

Desert specimens of widely distributed species, _e.g._ _Helix pomatia_, _H. niciensis_, _H. pisana_, _Leucochroa candidissima_ are much thicker than the type, and tend to lose all trace of coloured bands. These modifications are clearly the means of preventing evaporation of moisture, the dull white or grayish brown colour being calculated to absorb the smallest possible amount of heat. Desert shells in all parts of the world (_e.g._ N. Africa, Arabia, Central Asia, S. Africa, W. America) have been noticed to exhibit these peculiarities.

A very singular case of the reverse process, _i.e._ the production of darkened forms of shell through cold, has been noticed by Fischer as characteristic of the marine shells of the west coast of South America.[186] This melanism is especially noticeable in _Trochus_, _Turbo_, _Chiton_, _Mitra_, and _Pleurotoma_, and is attested by the specific names, not merely expressive of actual blackness (_e.g._ _nigerrimus_, _ater_, _atramentarius_, _maurus_), but also of a generally lugubrious tone (_e.g._ _moestus_, _funebralis_, _tristis_, _lugubris_, _luctuosus_). It is highly probable that this concurrence of specific melanism (which stands quite alone in the world) is due to the cold polar current which impinges on the Chilian coasts, for the same genera occur on the opposite shores of the continent without exhibiting any trace whatever of this mournful characteristic.

It is a well-known fact, attested by many observers, that our common _Limax agrestis_ as well as the young of _Arion ater_ become decidedly darker in summer than in winter. If these slugs were accustomed to disport themselves in the sun, it might have been suggested that this increased darkness of colour tended to absorb more of the heat rays. But since this is not the case, the result is probably due to some unexplained effect of higher temperature. According to Lessona and Pollonera, the length of the keel in _Limax arborum_ varies greatly in different parts of Italy, being shorter in specimens from low ground, but much longer in those inhabiting more elevated regions. The longer the keel, the more obscure the colouring becomes, so that in the Upper Alps of Piedmont individuals are practically black. Roebuck has observed that Scottish specimens of this same slug are much darker and less translucent than English forms. According to Simroth, our common black slug, _Arion ater_, is a northern type, which in more southern latitudes assumes the form known as _A. rufus_. Similarly _Limax maximus_ “in its northern form _cinereo-niger_ is almost wholly black, but in the more genial climate of Italy develops a series of brilliantly coloured and strikingly marked variations which have received numerous distinctive names from Italian limacologists.”[187] According to Scharff, however[188] (who regards the colours of slugs as in the main protective), these dark forms are by no means exclusively northern, being found equally on the parched plains of Spain and Portugal, and in the bleak climate of Norway. The same authority observes that similar forms occur both in the dry regions of E. Germany, and in the very humid district of western Ireland.

It appears unquestionable that marine genera from high northern latitudes are provided with shells of uniform colour, or whitish with a pale brown epidermis; spots, bands, or stripes seldom occur. The arctic forms of _Buccinum_, _Trophon_, _Chrysodomus_, _Margarita_, _Crenella_, _Leda_, _Yoldia_, _Astarte_ illustrate this fact. In the more temperate seas of Europe, colours tend on the whole to increase, although there are certain genera (_e.g._ _Pecten_) which are not more brightly coloured in Mediterranean than in Icelandic waters.

Land Mollusca inhabiting the mainland of a continent not unfrequently become smaller when they have spread to adjacent islands where perhaps the rainfall is less abundant or the soil and food-supply less nicely adjusted to their wants. _Orthalicus undatus_ is decidedly larger on the mainland of S. America than on the adjacent islands of Trinidad and Grenada. Specimens of _Bulimulus exilis_ from Barbados are invariably broader and more obese than those from S. Thomas, while those from the volcanic island of S. Lucia, where lime is deficient, are small and very slender. _Streptaxis deformis_, as occurring at Trinidad, is only half the size of specimens from Georgetown, Demerara.[189]

Certain localities appear, for some unexplained reason, to be

## particularly favourable to the production of albino varieties. The

neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen of these forms of land Mollusca and five of fresh-water.[190]

Our common _Helix aspersa_, as found near Bristol, is said to be ‘dark coloured’; about Western-super-mare ‘brown, with black markings’; near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and large.’[191] Sometimes the same kind of variation is exhibited by different species in the same locality. Thus specimens of _H. aspersa_, _H. nemoralis_, and _H. hortensis_, taken from the same bank at Torquay, presented a straw-coloured tinge of ground colour, with red-brown bands or markings. Trochiform _H. nemoralis_ and _H. arbustorum_, sinistral _H. hortensis_ and _H. aspersa_, sinistral _H. aspersa_ and _H. virgata_, and similarly banded forms of _H. caperata_ and _H. virgata_, have been taken together.[192]

The immediate neighbourhood of the sea appears frequently to have the effect of dwarfing land Mollusca. Thus the var. _conoidea_ of _Helix aspersa_, which is small, conical, with a compressed mouth, occurs ‘on sandhills and cliffs at the seaside.’ The varieties _conica_ and _nana_ of _Helix hispida_ are found ‘near the sea.’ _Helix virgata_ is exceedingly small in similar localities, and tends to become unicoloured. _H. caperata_ var. _Gigaxii_, a small depressed form, occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact opposite is the case, for _H. nemoralis_ var. _major_, which is ‘much larger’ than the type, occurs on ‘sandhills and downs’ and is ‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of _Limnaea peregra_ known as _maritima_ appears to be confined to the neighbourhood of the sea.

Dwarfing of the shell seems frequently to be the result of an elevated locality, not perhaps so much as the direct consequence of purer air and less barometric pressure, as of changes in the character of the food supply and in the humidity of the air. Several species of _Helix_ have a variety _minor_ which is characteristic of an Alpine habitat. _Helix arbustorum_ var. _alpestris_, which is scarcely two-thirds the size of the type, occurs on the Swiss Alps in the region of perpetual snow. Sometimes a very slight elevation is sufficient to produce the dwarfed form. At Tenby the type form of _Helix pisana_ is scattered in countless numbers over the sandhills just above high-water mark. At the extreme western end of these sandhills rises abruptly to a height of over 100 feet the promontory known as Giltar Head, the vegetation of which is entirely distinct from that of the burrows below. There is a colony of _H. pisana_ at the end of Giltar, all of which are devoid of the characteristic markings of the typical form, and most are dwarfed and stunted in growth.

Occasionally the same variety will be found to be produced by surroundings of very different nature. Thus the var. _alpestris_, of _H. arbustorum_ mentioned above, besides being characteristic of high Alpine localities, also occurs abundantly in low marshes at Hoddesdon on the River Lea. _Helix pulchella_ var. _costata_, according to Jeffreys, is found in dry and sandy places, often under loose stones and bricks on walls, while other authorities have noticed it in wet and dry localities quite indifferently.

Sometimes the production of a variety may be traced to the intrusion of some other organism. According to Brot, nine-tenths of the _Limnaea peregra_ inhabiting a certain pond near Geneva, were, during one season, afflicted with a malformation of the base of the columella. This deformity coincided with the appearance, in the same waters, of extraordinary numbers of _Hydra viridis_. The next season, when the _Hydra_ disappeared, the next generation of _Limnaea_ was found to have resumed its normal form.

It has been noticed that a form of _Helix caperata_ with a flattened spire and wide umbilicus is restricted to tilled fields, especially the borders of clover fields, while a form with a more elevated spire and more compact whorls occurs exclusively in open downs and uncultivated places. The Rev. S. S. Pearce accounts[194] for this divergence by the explanation that the flatter spire enables the shell of the fields to creep about more easily under the leaves or matted weeds, seldom requiring to crawl up a stalk or stem, while on the short turf of the downs and pastures the smaller and more rounded shell enables the animal to manoeuvre in and out of the blades of grass, and even to crawl up them with considerable activity. The same writer endeavours to explain the causes which regulate the distribution of _H. caperata_ var. _ornata_. He found that this variety (dark bands on a white ground) occurred almost exclusively on downs which were fed upon by sheep, associated with the common or mottled form, while the latter form alone occurred in localities where sheep were not accustomed to feed. Assuming then, as is probably the case, that sheep, in the course of their close pasturing, devour many small snails, he believes that individuals of the more conspicuous form _ornata_ were more likely to be noticed, and therefore avoided, by the sheep, than the mottled form, which would more easily escape their observation. Hence the var. _ornata_ is due to the advantage which strikingly coloured individuals obtained owing to their conspicuous habit, as compared with the typical form, which would be less readily detected.

(_b_) _Changes in Soil, Station, Character of Water, etc._--A deficiency of lime in the composition of the soil of any particular locality produces very marked effects upon the shells of the Mollusca which inhabit it; they become small and very thin, occasionally almost transparent. The well-known var. _tenuis_ of _Helix aspersa_ occurs on downs in the Channel Islands where calcareous material is scarce. For similar reasons, _H. arbustorum_ develops a var. _fusca_, which is depressed, very thin, and transparent, at Scilly, and also at Lunna I., E. Zetland.

[Illustration: FIG. 35.--19 specimens of _Purpura lapillus_ L., Great Britain, illustrating variation.

(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined and coloured rock; (4) Herm, rather exposed; (5) Solent, very sheltered; (6) Land’s End, exposed rocks, small food supply; (7) Scilly, exposed rocks, fair food supply; (8) St. Leonards, flat mussel beds at extreme low water; (9) Robin Hood’s Bay, sheltered under boulders, good food supply; (10) Rhoscolyn, on oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed rocks; (12) Estuary of Conway, very sheltered, abundant food supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor food supply; (14) slightly monstrous; (15), (16), (17) Morthoe, rather exposed rocks, but abundant food supply; (18) St. Bride’s Bay; (19) L. Swilly, sheltered, but small food supply. All from the author’s collection, except (10).]

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

[Illustration: FIG. 36.--Valves of _Cardium edule_ from the four upper terraces of Shumish Kul, a dry salt lake adjacent to the Aral Sea. (After Bateson.)]

Mr. W. Bateson has made[195] some observations on the shells of _Cardium edule_ taken from a series of terraces on the border of certain salt lakes which once formed a portion of the Sea of Aral. As these lakes gradually became dry, the water they contained became salter, and thus the successive layers of dead shells deposited on their borders form an interesting record of the progressive variation of this species under conditions which, in one respect at least, can be clearly appreciated. At the same time the diminishing volume of water, and the increasing average temperature, would not be without their effect. It was found that the principal changes were as follows: the thickness, and consequently the weight, of the shells became diminished, the size of the beaks was reduced, the shell became highly coloured, and diminished considerably in size, and the breadth of the shells increased in proportion to their length (Fig. 36). Shells of the same species of _Cardium_, occurring in Lake Mareotis, were found to exhibit very similar variations as regards colour, size, shape, and thickness.

_Unio pictorum_ var. _compressa_ occurs near Norwich at two similar localities six or seven miles distant from one another, under circumstances which tend to show that similar conditions have produced similar results. The form occurs where the river, by bending sharply in horse-shoe shape, causes the current to rush across to the opposite side and form an eddy near the bank on the outside of the bend. Just at the edge of the sharp current next the eddy the shells are found, the peculiar form being probably due to the current continually washing away the soft particles of mud and compelling the shell to elongate itself in order to keep partly buried at the bottom.[196]

The rivers Ouse and Foss, which unite just below York, are rivers of strikingly different character, the Ouse being deep, rapid, with a bare, stony bottom, and little vegetable growth, and receiving a good deal of drainage, while the Foss is shallow, slow, muddy, full of weeds and with very little drainage. In the Foss, fine specimens of _Anodonta anatina_ occur, lustrous, with beautifully rayed shells. A few yards off, in the Ouse, the same species of _Anodonta_ is dull brown in colour, its interior clouded, the beaks and epidermis often deeply eroded. Precisely the same contrast is shown in specimens of _Unio tumidus_, taken from the same rivers, Ouse specimens being also slightly curved in form. Just above Yearsley Lock in the Foss, _Unio tumidus_ occurs, but always dwarfed and malformed, a result probably due to the effect of rapidly running water upon a species accustomed to live in still water.[197] Simroth records the occurrence of remarkably distorted varieties in two species of _Aetheria_ which lived in swift falls of the River Congo.[198]

A variety of _Limnaea peregra_ with a short spire and rather strong, stoutly built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-fach. It lives adhering to stones in places where there are very few weeds, its shape enabling it to withstand the surf of these large lakes, to which the ordinary form would probably succumb.[199]

Scalariform specimens of _Planorbis_ are said to occur most commonly in waters which are choked by vegetation, and it has been shown that this form of shell is able to make its way through masses of dense weed much more readily than specimens of normal shape.

Continental authorities have long considered _Limnaea peregra_ and _L. ovata_ as two distinct species. Hazay, however, has succeeded in rearing specimens of so-called _peregra_ from the ova of _ovata_, and so-called _ovata_ from the ova of _peregra_, simply by placing one species in running water, and the other in still water.

According to Mr. J. S. Gibbons[200] certain species of _Littorina_, in tropical and sub-tropical regions, are confined to water more or less brackish, being incapable of living in pure salt water. “I have met,” says Mr. Gibbons, “with three of these species, and in each case they have been distinguished from the truly marine species by the extreme (comparative) thinness of their shells, and by their colouring being richer and more varied; they are also usually more elaborately marked. They are to be met with under three different conditions--(1) in harbours and bays where the water is salt with but a slight admixture of fresh water; (2) in mangrove swamps where salt and fresh water mix in pretty equal volume; (3) on dry land, but near a marsh or the dry bed of one.

“_L. intermedia_ Reeve, a widely diffused E. African shell, attaches itself by a thin pellicle of dried mucus to grass growing by the margin of slightly brackish marshes near the coast, resembling in its mode of suspension the Old World _Cyclostoma_. I have found it in vast numbers in situations where, during the greater part of the year, it is exposed to the full glare of an almost vertical sun, its only source of moisture being a slight dew at night-time. The W. Indian _L. angulifera_ Lam., and a beautifully coloured E. African species (_? L. carinifera_), are found in mangrove swamps; they are, however, less independent of salt water than the last.”

Mr. Gibbons goes on to note that brackish water species (although not so solid as truly marine species) tend to become more solid as the water they inhabit becomes less salt. This is a curious fact, and the reverse of what one would expect. Specimens of _L. intermedia_ on stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are much smaller, darker, and more fragile, than those living on grass a few hundred yards away. _L. angulifera_ is unusually solid and heavy at Puerto Plata (S. Domingo) among mangroves, where the water is in a great measure fresh; at Havana and at Colon, where it lives on stakes in water but slightly brackish, it is thinner and smaller and also darker coloured.

(_c_) _Changes in the Volume of Water._--It has long been known that the largest specimens, _e.g._ of _Limnaea stagnalis_ and _Anodonta anatina_, only occurred in pieces of water of considerable size. Recent observation, however, has shown conclusively that the volume of water in which certain species live has a very close relation to the actual size of their shells, besides producing other effects. _Lymnaea megasoma_, when kept in an aquarium of limited size, deposited eggs which hatched out; this process was continued in the same aquarium for four generations in all, the form of the shell of the last generation having become such that an experienced conchologist gave it as his opinion that the first and last terms of the series could have no possible specific relation to one another. The size of the shell became greatly diminished, and in particular the spire became very slender.[201]

The same species being again kept in an aquarium under similar conditions, it was found that the third generation had a shell only four-sevenths the length of their great grandparents. It was noticed also that the sexual capacities of the animals changed as well. The liver was greatly reduced, and the male organs were entirely lost.[202]

K. Semper conducted some well-known experiments bearing on this point. He separated[203] specimens of _Limnaea stagnalis_ from the same mass of eggs as soon as they were hatched, and placed them simultaneously in bodies of water varying in volume from 100 to 2000 cubic centimetres. All the other conditions of life, and especially the food supply, were kept at the known optimum. He found, in the result, that the size of the shell varied directly in proportion to the volume of the water in which it lived, and that this was the case, whether an individual specimen was kept alone in a given quantity of water, or shared it with several others. At the close of 65 days the specimens raised in 100 cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9 mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in 2000 cubic cm. attained a length of 18 mm. (Fig. 37).

An interesting effect of a sudden fall of temperature was noticed by Semper in connection with the above experiments. Vessels of unequal size, containing specimens of the _Limnaea_, happened to stand before a window at a time when the temperature suddenly fell to about 55° F. The sun, which shone through the window, warmed the water in the smaller vessels, but had no effect upon the temperature of the larger. The result was, that the _Limnaea_ in 2000 cubic cm., which ought to have been 10 mm. long when 25 days old, were scarcely longer, at the end of that period, than those which had lived in the smaller vessels, but whose water had been sufficiently warm.

[Illustration: FIG. 37.--Four equally old shells of _Limnaea stagnalis_, hatched from the same mass of ova, but reared in different volumes of water: =A= in 100, =B= in 250, =C= in 600, and =D= in 2000 cubic centimetres. (After K. Semper.)]

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