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.