Chapter III (1)

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).

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]

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.

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).

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.

=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.

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.

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.

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.

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]

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=