Chapter XXXV: Part II: The Inhabitants of the Sea (8)

Besides the hooked acetabula, a cluster of small simple unarmed suckers may be observed at the base of the expanded part. These add greatly to the animal's prehensile powers, for when they are applied to one another (_e_), the tentacles are firmly locked together at that point, and the united strength of both the elongated peduncles can be applied to drag towards the mouth any resisting object which has been grappled by the terminal hooks. There is no mechanical contrivance which surpasses the admirable structure of this natural forceps.

The size of the arms and the arrangement of the suckers differ considerably in the various species. In the octopods or poulps, which generally lead a more sedentary creeping life, and, hidden in the crevices of rocks, await the passing prey, the arms, in accordance with their wants, are with rare exceptions longer, more muscular, and stronger, than in the actively swimming decapods, where the two elongated tentacles or peduncles are the chief organs of prehension. In some species we find the arms distinct--in others they are united by a membrane. Some have a double row of suckers on each arm, others four rows, others again but one. So wonderful are the variations which nature, that consummate artist, plays upon a single theme--so inexhaustible are the modifications she introduces into the formation of numerous species, all constructed upon the same fundamental plan, and all equally perfect in their kind.

_e._ Parts joined together by the mutual
apposition of the armed suckers.

_f._ Terminal expanded portions bearing
the hooks.
]

Thus well provided with the means for seizing and overcoming the struggles of a living prey, the Cephalopods likewise possess adequate weapons for completing its destruction; for their mouth is most formidably armed with two horny or calcareous jaws, shaped like the mandibles of a parrot, playing vertically on each other, and enclosing a large fleshy tongue bristling with recurved horny spines. Hard, indeed, must be the crab which can resist this terrible beak; and when the cuttle-fish has once fixed on the back of a fish, though much larger and stronger than himself, it is in vain for the tortured victim to fly through the water: he carries his enemy with him till he sinks exhausted under his murderous fangs.

Besides their arms, by help of which the Cephalopods either swim or creep, the forcible expulsion of the water through the respiratory tube or infundibulum serves them as a means of locomotion in a backward direction. By those which have an elongated body and comparatively strong muscles, this movement is performed with such violence that they shoot like arrows through the water, or even like the flying-fish perform a long curve through the air.

Thus Sir James Ross tells us, that once a number of cuttle-fish not only fell upon the deck of his ship, which rose fifteen or sixteen feet above the water, and where more than fifty were gathered, but even bolted right over the entire breadth of the vessel, like a sportsman over a five-barred gate. Finally, the fin-like expansion of their mantle renders the nimble decapods good service in swimming. In the Sepias this finny membrane runs along the sides of the body, while in the Calamary it forms a kind of terminal paddle.

_b._ Finny membrane running along the sides of the body.
_c._ Arms with four rows of suckers.
_d._ Elongated retractile tentacles.
_e._ Eyes.
]

It might be supposed that the dibranchiate cephalopods, by their swiftness, their arms, and their powerful jaws, were sufficiently provided with means of attack or defence; but it must be remembered that their body is soft and naked, and that, though well armed in front, they may readily be attacked in the rear. To afford them the additional protection they required, nature, ever ready to minister to the real wants of her children, has furnished them with an internal bag communicating with the respiratory tube, and secreting a large quantity of an inky fluid, which they can squirt out with force in the face of their foe, and which, mixing readily with the water, envelops them in an opaque cloud, and thus screens them from pursuit. But this inky fluid, thus useful to its owner, is often the cause of his destruction by man, who applies it to his own purpose, for the Italian pigment, called sepia, so invaluable to painters in water-colours, is prepared from the inspissated contents of the ink bag of a cuttle-fish. Such is the durability of this colour that even the inky fluid of fossil species has been found to retain its chromatic property. We are told that grains of wheat buried with Egyptian mummies three thousand years ago have germinated; but it is surely still more astonishing that an animal secretion, the origin of which is lost in the dark abyss of countless ages, should remain so long unaltered.

The cephalopods are scattered in vast numbers over the whole ocean, from the ice-bound shores of Boothia Felix to the open main; they seem, however, to be most abundant in temperate latitudes. Some, like the common poulp, constantly frequent the coasts, creeping among the rocks and stones at the bottom; others, like the Cirroteuthis and Ommastrephes, roam about the high seas at a vast distance from the land.

They are generally nocturnal or vespertine in their habits; they abound towards evening and at night on the surface of the seas, but sink to a greater depth, or retire into the crevices of the rocks, as soon as the sun rises above the horizon. Some are of a recluse disposition, and lead a solitary life in the anfractuosities of the littoral zone; others, of a more social temper, wander in large troops along the shores, or over the vast plains of ocean.

Possessing the organs of sense, and the means of locomotion in a high degree of development, the cephalopods may naturally be expected to be far more active and intelligent than the inferior orders of the molluscs. On moonlight nights, among the islands of the Indian Archipelago, Mr. Adams frequently observed the Sepiæ and Octopi in full predatory activity, and had considerable difficulty and trouble in securing them, so great was their restless vivacity, and so vigorous their endeavours to escape. "They dart from side to side of the pools," says the naturalist in his entertaining and instructive account of his journey to those distant gems of the tropical sea, "or fix themselves so tenaciously to the surface of the stones by means of their suckers that it requires great force and strength to detach them. Even when removed and thrown upon the sand, they progress rapidly, in a sidelong shuffling manner, throwing about their long arms, ejecting their ink-like fluid in sudden violent jets, and staring about with their big shining eyes (which at night appear luminous, like a cat's) in a very grotesque and hideous manner."

At the Cape de Verd islands, Mr. C. Darwin was also much amused by the various arts to escape detection used by a cuttle-fish, which seemed fully aware that he was watching it. Remaining for a time motionless, it would then stealthily advance an inch or two, like a cat after a mouse, and thus proceeded, till, having gained a deeper part, it darted away, leaving a dusky train of ink, to hide the hole into which it had crawled.

All the cephalopods are extremely voracious; they destroy on shallow banks the hopes of the fishermen, devour along the coasts and on the high seas countless myriads of young fish and naked molluscs, and kill, like the tiger, for the mere love of carnage. Thus they would become dangerous to the equilibrium of the seas if nature, to counterbalance their destructive habits, had not provided a great number of enemies for the thinning of their ranks.

They form the almost exclusive food of the sperm-whales, and the albatross and the petrels love to skim them from the surface of the ocean. Tunnies and bonitos devour them in vast numbers, the cod consumes whole shoals of squids, and man, as I have already mentioned, catches many millions to serve him as a bait for this valuable fish.

At Teneriffe, in the Brazils, in Peru and Chili, in India and China, various species of cephalopods are used as food. Along the eastern shores of the Mediterranean, the common sepia constitutes now, as in ancient times, a valuable part of the food of the poor. "One of the most striking spectacles," says Edward Forbes, "is to see at night on the shores of the Ægean the numerous torches glancing along the shores, and reflected by the still and clear sea, borne by poor fishermen, paddling as silently as possible over the rocky shallows in search of the cuttle-fish, which, when seen lying beneath the water in wait for his prey, they dexterously spear, ere the creature has time to dart with the rapidity of an arrow from the weapon about to transfix his soft but firm body."

Animals exposed to the attacks of so many enemies must necessarily multiply in an analogous ratio. Their numerous eggs are generally brought forth in the spring. In the species inhabiting the high seas, they float freely on the surface, carried along by the currents and winds, and form large gelatinous bunches or cylindrical rolls, sometimes as large as a man's leg.

The eggs of the littoral cephalopods appear in the form of dark-coloured, roundish or spindle-shaped bodies, of the size and colour of grapes, and hanging together in clusters. They are soft to the touch, with a tough skin, resembling india-rubber; one end is attenuated into a sort of point or nipple, and the other prolonged into a pedicle, which coils round sea-weed or other floating objects, and serves to fix the berry-like bag in its place. At an early stage these "sea-grapes," as they are called by the fishermen, contain a white yolk enclosed in a clear albumen, and nearer maturity the young cuttle-fish may be found within in various stages of formation, until finally, hatched by the heat of the sun, it emerges from the husk perfectly formed, and launches forth into the water.

Some species of cephalopods are only about the size of a finger, while others attain an astonishing size. Banks and Solander, in Cook's first voyage, found the dead carcass of a gigantic cuttle-fish floating between Cape Horn and the Polynesian islands. It was surrounded by aquatic birds, which were feeding on its remains. From the parts of this specimen, which are still preserved in the Hunterian collection, and which have always strongly excited the attention of naturalists, it must have measured at least six feet from the end of the tail to the end of the tentacles.

Near Van Diemen's Land, Péron saw a sepia about as big as a tun rolling about in the waters. Its enormous arms had the appearance of frightful snakes. Each of these organs was at least seven feet long, and measured seven or eight inches round the base. These well authenticated proportions are truly formidable, and fully justify the dread and abhorrence which the Polynesian divers entertain of those snake-armed monsters of the deep; but not satisfied with reality, some writers have magnified the size of the cephalopods to fabulous dimensions. Thus Pernetti mentions a colossal cuttle-fish, which, climbing up the rigging, overturned a three-masted ship; and Pliny notices a similar giant, with arms thirty feet long and a corresponding girth. But all this is nothing to the Norwegian kraken, a mass of a quarter of a mile in diameter, and a back covered with a thicket of sea-weeds. When it comes to the surface, which seems to be but rarely the case, it raises its arms mast-high into the air, and, having enjoyed for a time the lovely daylight, sinks slowly back again into abysmal darkness. Fishermen are said to have landed on a kraken, and to have kindled a fire upon the supposed island for the purpose of cooking their dinner. But even a kraken, thick-skinned as he may be, does not like his back to be converted into a hearth, and thus it happened that the treacherous ground gave way under the mistaken mariners, and overwhelmed them in the waters. Strange that the oriental tale of Sinbad the sailor should thus be re-echoed in the wild legends of the north.

All the dibranchiate cephalopods are destitute of an outward shell, with the sole exception of the Spirula, a small species chiefly found in the South Sea, and of the far more renowned Argonaut, which poets, ancient and modern, have celebrated as the model from which man took the first idea of navigation. Its two sail-like arms expanding in the air, and the six others rowing in the water, the keel of its elegant shell is pictured as dividing the surface of the tranquil sea. But as soon as the wind rises, or the least danger appears, the cautious argonaut takes in his sails, draws back his oars, creeps into his shell, and sinks instantly into a securer depth. Unfortunately there is not a word of truth in this pleasing tale. Like the common octopus, the argonaut generally creeps about at the bottom of the sea, or when he swims, he places his sails close to his shell, stretches his oars right out before him, and shoots backwards like most of his class by expelling the water from his respiratory tube.

As he sits loosely in his shell, he was supposed by some naturalists to be a parasite enjoying the house of the unknown murdered owner; but this is perfectly erroneous, as the young in the egg already show the rudiments of the future shell, and the full-grown animal repairs by reproduction any injury that may have happened to it.

The tetrabranchiate cephalopods, or Nautili, are very differently constructed from their dibranchiate relations. Here, instead of mighty muscular arms, furnished with suckers or raptorial claws, we find a number of small, sheathed, and retractile tentacles (_f_), surrounding the mouth in successive series, and amounting to little short of a hundred. The head is further provided with a large muscular disk (_g_), which, besides acting as a defence to the opening of the shell, serves also in all probability as an organ for creeping along the ground, like the foot in the Gasteropods. The mandibles are strengthened by a dense calcareous substance fit to break up the defensive armour of the crustacean or shell-fish on which the animal feeds. There is no ink-bag, no organ of hearing, and the eyes (_h_) are pedunculated, and of a more simple structure. The handsome pearl-mother and spirally wound shell is divided by transverse partitions (_a_), perforated in the centre, into numerous chambers (_b_). The animal takes up its abode in the foremost and largest (_b′_), but sends a communicating tube or siphon (_c_) through all the holes of the partitions to the very extremity of the spirally wound shell. Though the empty conch was frequently found swimming on the waters of the Indian Ocean, or cast ashore on the Moluccas or New Guinea, yet it was only in 1829 that the animal was known with any certainty, one having been caught alive by Mr. George Bennett, near the New Hebrides, which, preserved in spirits, is now in the museum of the College of Surgeons. Since then three different species have been found to abound in the waters of the above-named archipelago, of New Caledonia, and of the Feejee and Solomon Islands, where they principally sojourn among the coral reefs at depths of from three to six fathoms. They usually remain at the bottom of the water, where they creep along rather quickly, supporting themselves upon their tentacula, with their head downwards and the shell raised above. After stormy weather, as it becomes more calm, they may be seen in great numbers floating upon the surface of the sea with the head protruded, and the tentacula resting upon the water, the shell at the same time being undermost; they remain, however, but a short time sailing in this manner, as they can easily return to their situation at the bottom of the sea, by merely drawing in their tentacles and upsetting the shell. They are caught in baskets by the natives, who eat them roasted as a great delicacy.

What renders these animals peculiarly interesting is the circumstance that they are the only living representatives of a class which once filled in countless numbers the bosom of the primeval ocean, and whose fossil remains (Orthoceratites, Ammonites) furnish the naturalist with a series of historical documents, attesting the unmeasured age of our planet. What are the ruins, thirty or forty centuries old, that speak of the vanished glories of extinguished empires to these wonderful medals of creation that lead our thoughts through the dim vista of unnumbered centuries to the fathomless abyss of the past.

* * * * *

In point of development of organisation the Gasteropods or snails rank immediately after the Cephalopods. They also have a head plainly distinguishable from the rest of the body, and to which two brilliant black eyes give an animated expression. But their nervous system is far less developed, and while the lively cephalopod is able to swim about, and rapidly to seize a distant prey, almost all the gasteropods creep slowly along upon a flat disk or foot situated below the digestive organs, a formation to which they owe their name of gasteropods or stomach-footers.

The marine snails are divided into several groups according to the different position and arrangement of their gills. In some species these organs form naked or free-swimming tufts on the back (Nudibranchiata) but generally they are variously disposed either in special cavities or under the folds of the mantle. Thus in the Inferobranchiata they are arranged under its inferior border on both sides of the body, or upon one side only, while in the Tectibranchiata they are placed, as in the Nudibranchiata, upon the dorsal aspect of the body, but are protected by a fold of the skin. In the Cyclobranchiata they form a fringe round the margin of the body, between the edge of the mantle and the foot, and in the Scutibranchiata and Pectinibranchiata they are pectinated, or shaped like the teeth of a comb, and placed in a large hollow chamber, which opens externally at the side of the body or above the head.

Nothing can be more elegant or various than the form and arrangement of the gills in most of the nudibranchiate gasteropods. In the Glauci and Scyllææ, we see at each side of the elongated body long arms branching out into tufty filaments; in the Briarei a hundred furcated stems serve for the aëration of the blood. On the back of the Eolides the gills are arranged in rows; in the Dorides they form a wreath or garland round the posterior intestinal aperture.

The beauty of these animals corresponds with their charming mythological names, for every part of them which is not sparkling like the purest crystal shines with the liveliest colours, red, yellow, or azure. Some inhabit the coasts, where they creep along upon a well-developed foot, others live in the deep waters, where they cling to the stems of floating sea-weed with a narrow and furrowed foot, or swim upon their back, using the borders of the mantle and of the branchiæ as oars. Though chiefly living in the warmer latitudes, they are found in every sea, and many interesting species inhabit the British waters: such as the Sea-lemon (_Doris tuberculata_), which, when its horns and starry wreath of branchiæ are concealed, bears a curious resemblance in size, form, colour, and warty surface to the half of a citron divided longitudinally; the exquisite _Eolis coronata_, whose crowded clusters of branchial papillæ are radiant with crimson and cerulean tints; and the crested Antiopa, whose transparent breathing organs are tipped with silvery white.

Though they have no shell to cover them, the Nudibranchiata are not left defenceless to the mercy of their enemies. The transparency of their body is a cause of safety to many of them. Some conceal themselves under stones or among the branches of the madrepores, and some on contracting cast off a part of their mantle, which they leave in possession of their hungry foe, while they themselves make their escape.

Among the British Inferobranchiata we find the rare golden or orange-coloured _Pleurobranchus plumula_, thus named from its branchiæ projecting like a plume from between the mantle and foot in crawling; and among the Tectibranchiata the common sea-hare (_Aplysia punctata_), which resembles a great naked snail; its back opening with two wide lobes, which can be expanded or closed over the opening at the animal's will. When open, they expose to view on the right side the finely fringed and lobed branchiæ, seated in a deep hollow beneath a fold of the mantle. The uncomely creature glides along over the stones upon its flat fleshy foot and up the slender stems of sea-weeds by bringing the borders of the same locomotive apparatus to meet around the stem, thus tightly grasping it as if enclosed in a tube. While progressing, the fore part is poked forward as a narrow neck furnished with two pair of tentacles, one pair of which, standing erect and being formed of thin laminæ, bent round so as to bring the edges nearly into contact, look like the ears of the timid quadruped, from which the Aplysia has derived its common name. The colour is a dark-brownish purple studded with rings and spots of white. On being disturbed, the sea-hare pours out from beneath the mantle-lobes a copious fluid of the richest purple hue, which however quickly fades, and is of no value in the arts.

More than forty species of Aplysiæ are known, most of them inhabitants of the warmer seas. The acrid humour exuded by the depilatory aplysia, or _Aplysia depilans_, of the Mediterranean is still supposed by the Italian fishermen to occasion the loss of the hair, and was used by the ancient Romans in the composition of their venomous potions--though it is by no means poisonous. Such are the prejudices resulting from the propensity of man to associate evil qualities with an unprepossessing appearance.

To the Cyclobranchiate order belong the Limpets and the Chitons. The latter, which are the only multivalve shells among the Gasteropods, are spread in more than two hundred species over every shore from Iceland to the Indies, but they are particularly abundant on the coasts of Peru and Chili. Some of the smaller species inhabit our coasts, where they may be found adhering to stones near low water mark. They are coated with eight transverse shelly plates, folding over each other at their edges like the plates of ancient armour, and inserted into a tough marginal band, so as to form a complete shield to the animal. Thus encased in coat of mail, the chitons have the power of baffling the voracity of their enemies by rolling themselves up into a ball like the wood-louse or the armadillo: they are also able to cling with such tenacity to the rock that it is difficult to detach them without tearing them to pieces. The Limpets, or Patellæ, likewise attach their shield-like shell so firmly to a hard body that it requires the introduction of a knife between the shell and the stone to detach them. It has been calculated that the larger species are thus able to produce a resistance equivalent to a weight of 150 pounds, which, considering the sharp angle of the shell, is more than sufficient to defy the strength of a man to raise them. They often congregate in large numbers in one place, and an old writer compares them to nail-heads struck into the rock. More than a hundred species are known; one of which, the _Patella cochlear_ of the Cape, is almost invariably found squatting upon the shell of another species of limpet. The finest and largest varieties abound on the shores of the Oriental seas and the coasts of the Mediterranean, but several of the smaller species are very numerous in our littoral or sub-littoral zone, where they either feast on the green sea-weeds that we find covering at ebb-tide the stones with a thin emerald layer, or upon the coarser olive-coloured algæ. Thus _Patella pellucida_ and _Patella lævis_, both remarkable for longitudinal streaks of iridescent colours on an olive-shell, may generally be found feeding either on the broad fronds or on the roots and stems of the Laminariæ, or Oar-weeds. To their labours may indeed be partly attributed the annual destruction of these gigantic algæ, for, eating into the lower part of the stems, and destroying the branches of the roots, they so far weaken the base that it is unable to support the weight of the frond, and thus the plant is detached and driven on shore by the waves.

The beautiful Sea-ear, or Haliotis, is the chief representative of the scutibranchiate gasteropods. The flattened shell, perforated with small holes on one side, is characterised by a very wide mouth or aperture, the largest in any shell except the limpet. The outside is generally rough, or covered with marine substances; the inside presents the same enamelled appearance as mother-of-pearl, and exhibits the most beautiful colours. The holes with which the shell is perforated serve to admit water to the branchiæ, and are formed at regular intervals as it increases in size. The foot is very large, having the margin fringed all round, and is able, like that of the chiton or the limpet, to cling firmly to the rock. More than seventy species of Haliotis are known, the greater part occurring in the Pacific Ocean.

_c._ Series of perforations.
_d._ Eye peduncles.
_e._ Tentacles.
_g._ Foot.
]

To the scutibranchiate gasteropods also belong the strangely formed Carinariæ, which seem to be made up of disjointed parts. The gills (_g_) project from under a thin vitreous shell (_f_), which projects from the dorsal surface, and has a form not unlike that of the Argonaut or of a Phrygian cap. The foot (_b_) is not formed for creeping, but constitutes a muscular vertical paddle or fin, that serves them for swimming on the back, and is furnished with a sucking disk (_c_), with which they are enabled to attach themselves to floating objects.

The Pectinibranchiata comprise all the spiral univalve shells, and are by far the most numerous of all the gasteropods, as their species are not counted by hundreds, but by thousands. If their calcareous garment could be drawn out, it would be found to consist of a tube gradually widening from the apex to the base; but what an immense variety of form and ornaments, what a prodigality of splendid tints, has not Nature spread over this interminable host! The same fundamental idea appears to us in thousands of modifications, one yet more elegant and capricious than the other. Thus the passion of the shell collector is as conceivable as that of the lover of choice flowers, and when we read that rich tulip-amateurs have given thousands of florins for one single bulb, we cannot wonder that many of the Volutes, Cones, Mitres, and Harps, are worth several times their weight in gold; that more than a hundred pounds have been paid for a Chinese wentle-trap, and that the _Cypræa aurora_, which the Polynesian chiefs used to wear about the neck, is valued at thirty or forty guineas.

The mode in which these beautifully painted structures are formed is very similar to what takes place among bivalve shells. They are secreted by the glandular margin of the mantle or soft skin which clothes the upper part of the body of the snail, and their form depends on the shape of the body they are destined to cover, while the outline of the border is alike regulated by that of the mantle. In the border of the mantle are placed the glands through which colouring matter is added to the lime of which the shell consists, and here also the whole of the outer coat of the shell is formed by constant annual additions to the lip. The after-growth of the shell proceeds, layer over layer, from the general surface of the mantle, so that the calcareous robe constantly increases in thickness with the age of the animal.

However different the form of a shell may be, its use is invariably the same, affording the soft-bodied animal a shield or retreat against external injuries. In this respect it is not uninteresting to remark that those species which inhabit the littoral zone, and are most exposed to the violence of the waves, have a stronger shell than those which live in greater depths, and that the fresh-water molluscs have generally a much more delicate and fragile coat than those which live in the ocean. The greater the necessity of protection the better has Nature provided for the want. Thus most of the gasteropods, besides possessing a stone-hard dwelling, are also furnished at the extremity of the foot with an operculum, or calcareous lid, which fits exactly upon the opening of their house, and closes it like a fortress against the outer world. But no animal exists that is safe against every attack, for the large birds sometimes carry the ponderous sea-snails, whose entrance they cannot force with their beaks, high up into the air, and let them fall upon the rocks, where they are dashed to pieces.

The ordinary mode of locomotion of the testaceous sea-snails is by creeping along on their foot: those that have a very heavy house to carry, such as the Cassis or the Pteroceras, generally move along very slowly, while others, such as the Olivæ, that are possessed of a comparatively strong and broad foot, have rapid and lively movements, and quickly raise themselves again when they have been overturned. The Strombidæ and Rostellariæ place their powerful and elastic foot under the shell in a bent position, when suddenly by a muscular effort they straighten that organ and roll and leap over and over. The structure of the foot of the _Tornatella fasciata_, an inhabitant of our coast, is most remarkable: beaten incessantly by the waves, in the cavities of rocks which it frequents, nearly on a level with the surface of the sea, to the violence of which it is always exposed, it has need of additional powers for retaining its hold; its foot is therefore divided into two adhering portions, placed at each extremity, and separated by a wide interval; when it crawls, it fixes the posterior disc and advances the other, which it attaches firmly to the place of progression, and this being effected, the hinder sucker is detached and drawn forwards, locomotion being accomplished by the alternate adhesion of these two prehensile discs. In Cyclostoma the foot is likewise furnished with two longitudinal adhering lobes, which are advanced alternately. But the foot of the marine snails is not merely an instrument of progression on a solid surface, for in many species it is convertible at the will of the animal into a boat, by means of which the creature can suspend itself in an inverted position at the surface of the water, where by the aid of its mantle and tentacles it can row itself from place to place.

The Ianthinæ, or purple Sea-Snails, carry under their foot a vesicular organ like a congeries of foam-bubbles, that prevents creeping, but serves as a buoy to support them at the surface of the water.

When the sea is quiet, these little creatures,

Like little wanton boys that swim on bladders,

appear in vast shoals on the surface, but as soon as the wind ruffles the ocean, or an enemy approaches, they at once empty their air-cells, contract their float, and sink to the bottom, pouring out at the same time a darkened fluid like that of the Aplysia or the Murex, which no doubt serves them as a defence against their foes, and, according to Lesson, furnished the celebrated purple of the ancients. The Ianthinæ inhabit the Mediterranean and the warmer regions of the Atlantic, but especially towards the close of summer they are frequently drifted by the Gulf Stream to the west coast of Ireland.

While the vast majority of the gasteropods either creep or swim, some are doomed to the sedentary life of the oyster, and remain for ever fixed to the spot where they first attached themselves as small free-swimming larvæ. Thus the _Magilus antiquus_, which in its young state presents all the characters of a regular spiral univalve, establishes itself in the excavations of madrepores, and as the coral increases around it, the Magilus is obliged, in order to have its aperture on a level with the surrounding surface, to construct a tube, lengthening with the growth of the coral. As the tube goes on increasing, the animal abandons the spiral for the tubular part of the shell, and in the operation it leaves behind no partitions, but secretes a compact calcareous matter which reaches to the very summit of the spiral part, so that in an old specimen the posterior part of the shell presents a solid mass.

The Siliquariæ are generally found embedded in a similar manner in sponges or other soft bodies, while the Vermetus, or Worm-Shell, usually attaches itself, like the Serpulæ, to rocks, coral-reefs, or shells.

In these genera, which have been arranged by Cuvier in a separate order (Tubulibranchiata), the foot is naturally reduced to the state of an adhesive organ, its chief functions consisting in opening and closing the lid.

The sea-snails are either predaceous or herbivorous; among the pectinibranchiates, those with circular mouths to the shell are vegetable feeders, while such as have an aperture ending in a canal are animal feeders. Considerable modifications of internal structure indicate this difference of food; and the external organs, particularly about the mouth, exhibit a corresponding variety of form. In those which feed on vegetables the mouth is generally a slit furnished with more or less perfect lips, armed with a simple cutting apparatus, which is often powerful enough to divide or dismember comparatively hard substances.

In most animal feeders the mouth presents the appearance of a proboscis that can be protruded or shortened at the will of the animal, and which, grasping the food, conveys it to a spine-armed tongue, by the aid of which it is propelled into the gullet without mastication or any preparatory change.

In the Whelk and its shell-boring allies, the alternate protrusion and retraction of the proboscis, which is here of a much more complicated structure, causes the sharp tongue to act as a rasp or auger, capable of drilling holes into the hardest shells. It is this circumstance which renders the whelk so formidable an enemy to mussel and oyster banks. During the erection of Bell Rock lighthouse, an attempt was made to plant a colony of mussels on the wave-beaten cliff, as they were likely to be of great use to the workmen, and especially to the light keepers, the future inhabitants of the rock; but the mussels were soon observed to open and die in great numbers. "For some time," says Mr. Stevenson in his interesting narrative, "this was ascribed to the effects of the violent surge of the sea, but the Buccinum lapillus having greatly increased, it was ascertained that it had proved a successful enemy to the mussel. The buccinum was observed to perforate a small hole in the shell, and thus to suck out the finer parts of the body of the mussel; the valves of course opened, and the remainder of the shell-fish was washed away by the sea. The perforated hole is generally upon the thinnest part of the shell, and is perfectly circular, of a champhered form, being wider towards the outward side, and so perfectly smooth and regular as to have all the appearance of the most beautiful work of an expert artist. It became a matter extremely desirable to preserve the mussel, and it seemed practicable to extirpate the buccinum. But after we had picked up and destroyed many barrels of them, their extirpation was at length given up as a hopeless task. The mussels were consequently abandoned as their prey; and, in the course of the third year's operations, so successful had the ravages of the buccinum been that not a single member of the imported mussel colony was to be found upon the rock." Thus the engineer, whose skill and perseverance had gained so proud a triumph over the waves of the stormy ocean, was defeated by an ignoble whelk.

In the genera which have no proboscis, the tongue, acting as a prehensile and rasping or abrading organ, is frequently of considerable length; thus, in the Ear-shell, it is half as long as the body, and in the common Limpet even three times longer than the entire animal. From the two cartilaginous pieces (_b b_), placed on each side of its root, arise the short and powerful muscles which wield the organ. The surface of this curious piece of mechanism, a magnified view of which is given at B, is armed with minute, though strong, teeth, placed in transverse rows, and arranged in three series; each central group consists of four spines, while those on the sides contain but two a-piece. It is only at its anterior extremity (_d_), however, that the tongue, so armed, presents that horny hardness needful for the performance of its functions, the posterior part being comparatively soft; so that, probably in proportion as the anterior part is worn away, the parts behind it gradually assume the necessary firmness, and advance to supply its place. In the upper part of the circumference of the mouth, we find a semicircular horny plate, resembling an upper jaw, and the tongue, by triturating the food against this, gradually reduces substances however hard. On opening the limpet, the tongue is found doubled upon itself, and folded in a spiral manner beneath the viscera.

Many of the Gasteropods which live on coarse and refractory materials are provided with several digestive cavities, resembling in some degree the stomachs of the ruminating quadrupeds; and frequently the triturating power of these organs is still further increased by their being armed with teeth variously disposed.

In the Bulla, for instance, a genus belonging, like the sea-hares, to the tectibranchiate order, the gizzard, or second stomach, contains three plates of stony hardness attached to its walls, and so disposed that they perform the part of a most efficacious grinding mill.

On opening the gizzard of the Scyllæa, it is found to be still more formidably armed, for in its muscular walls there are embedded no less than twelve horny plates (_e_), which are extremely hard and as sharp as the blades of a knife.

The Sea-hare, however, furnishes us with the most curious form of these stomachal teeth, for here we see not only the gizzard (_b_) armed with horny pyramidal plates, whose tuberculated apices, meeting in the centre of the organ, must necessarily bruise by their action whatever passes through that cavity, but the third stomach (_d_) is also studded with sharp-pointed hooks (_c_), resembling canine teeth, and admirably adapted to pierce and subdivide the tough leathery fronds of the olive sea-weeds on which the animal feeds. Thus these deformed and disgusting molluscs afford us one of the most interesting examples of the adaptation of organs to their functions, which an enlightened research is continually finding in creation.

Though not so gifted as the cephalopods, many of the gasteropods possess all the organs of sense. Like them, they have an apparatus specially calculated to appreciate sonorous undulations, and consisting of a membranous vesicle attached to an auditive nerve, and containing either a single spherical otolithe or a larger number of similar smaller calcareous bodies, which by their vibrations communicate the impression of sound to the nerve. Their minute eyes are short-sighted, it is true, and frequently either entirely wanting or, as in the Nudibranchiates, scarcely able to distinguish light from darkness; but their inactive habits require no wide field of vision, and thus they see as much of the external world as is necessary for their humble sphere of existence. The organs of sight are generally situated either on a prominence at the base of the superior pair of tentacles or, as, for instance, in the Murex, at the extremity of these organs (_a_, _b_), a position which enables the animal to direct them readily to different objects.

_c._ Eye highly magnified.]

Many of the Gasteropods are evidently capable of perceiving odours; thus, animal substances let down in a net to the bottom will attract thousands of Nassæ in one night. We also may infer that they are not deficient in taste from the presence of papillæ at the bottom of their mouth, analogous to those found on the tongue of other animals; but, of all their senses, that of touch is undoubtedly the most perfect. The whole soft surface of the body is indeed of exquisite sensibility, but more especially the vascular foot, and the tentacles, or horns, which vary both in number and in shape in different genera. Yet, in spite of this delicacy in the organisation of the skin, which makes it so sensible of contact, it appears to have been beneficently ordered that animals so helpless and exposed to injury from every quarter are but little sensible to pain. Although they are deprived of all higher instincts, we find among the Gasteropods a few examples of concealment under extraneous objects, which remind us of the masks and artifices frequently employed by the insects and crustaceans.

The Agglutinating Top (_Trochus agglutinans_) covers itself with small stones and fragments of shells, and thus shielded from the view escapes the voracity of many an enemy but little suspecting the savoury morsel hidden under the mound of rubbish which he disdainfully passes by.

In animals which are only provided with passive means of defence, we may naturally expect a considerable degree of caution, and in this respect the gasteropods might give many useful lessons to man. How carefully they protrude their tentacles as far as possible to sound every obstacle in their way, before they creep onwards, and how rapidly they withdraw into their shell at the least symptom of danger! What an example to so many of us that leap before they look, and frequently break their necks in the fall!

Yet, in spite of all their prudence and of the protection of their stony dwellings, they serve as food to a host of powerful enemies. The sea-stars, their most dangerous foes, not only swallow the young fry but also seize with their long rays the full-grown gasteropods, and clasp them in a murderous embrace.

They are preyed upon by fishes, crustaceans, and sea-birds, who pick them up along the shores; but it will sometimes happen that a crow, while endeavouring to detach a limpet for its food, is caught by the tip of its bill, and held there until drowned by the advancing tide.

Man also consumes a vast number of sea-snails, for on every coast there are some edible species; and it may be said that, with the exception of very few that have a disagreeable taste, they are all of them used as food by the savage. The miserable inhabitants of Tierra del Fuego chiefly live upon a large limpet that abounds on the rocky shores of their inhospitable land, and but for this resource would most likely long since have been extirpated by hunger.

Many of the univalve shells are, moreover, highly prized as objects of ornament or use both by savage and civilised nations. The South Sea Islander makes use of a Triton as a war conch; the Patagonian drinks out of the Magellanic volute, the Arab of the Red Sea employs a large Buccinum as a water-jug, and the _Cypræa moneta_ is well known in commerce as the current coin of the natives of many parts of Africa. In Europe the iridescent Haliotis is frequently used for the inlaying of tables or boxes, and various species of Helmet-shells and Strombi (_Cassis rufa madagascariensis_, _Strombus gigas_), peculiar as being formed of several differently coloured layers, placed side by side, are in great request for the cutting of cameos, as they are soft enough to be worked with ease, and hard enough to resist wear. More than two hundred thousand of these shells are annually imported into France, and the value of cameos produced in Paris alone amounts to more than a hundred thousand pounds. A large number are also cut in the small town of Oberstein on the Nahe (a river flowing into the Rhine at Bingen), which has long been famous for the manufactory of agate ornaments and trinkets, and has now added this new branch of industry to the more ancient sources of its prosperity.

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The Pteropods, or Wing-footers, move about by means of two fin-like flaps, proceeding wing-like from the fore part of the body. They have no disk to walk upon, nor arms for the seizure of prey, like the cephalopods and gasteropods, but resemble them by the possession of a head distinct from the rest of the body, which some, like the Hyaleas and Cleodora, conceal in a thin transparent or translucent shell, in which they also hide their head and wings at the approach of danger, and immediately sink to the bottom; while others, like the blue and violet Clios, beautifully variegated with light red spots, are perfectly naked. They generally inhabit the high seas, and are but rarely drifted by storms or currents into the neighbourhood of the land. They mostly swim about freely, but sometimes also they are found clinging by their wings to floating sea-weeds. They are small creatures, but propagate so fast that the _Clio borealis_ and _Limacina arctica_ form the chief food of the colossal whale.

While these two little pteropods, in spite of their minute proportions, deserve to rank among the most important inhabitants of the northern seas, the Mediterranean species belong mainly to the genera Hyalea, Cleodora, and Criseis--forms wholly unknown to our own fauna except as waifs. Vast shoals of these animals frequent the deeper parts of that sea, leaving their remains strewed over its bed, between depths of from one hundred to two hundred fathoms; they are short-lived creatures, and have their seasons, being met with near the surface during spring and winter, sparkling in the water like needles of glass.

"The pteropods are the winged insects of the sea," says M. Godwin-Austen, "reminding us, in their free circling movements and crepuscular habits, of the gnats and moths of the atmosphere; they shun the light, and if the sun is bright, you may look in vain for them during the life-long day--as days sometimes are at sea; a passing cloud, however, suffices to bring some Cleodoræ to the surface. It is only as day declines that their true time begins, and thence onwards the watches of the night may be kept by observing the contents of the towing-net, as the hours of a summer day may be by the floral dial. The Cleodoræ are the earliest risers; as the sun sets, _Hyalæa gibbosa_ appears, darting about as if it had not a moment to spare, and, indeed, its period is brief, lasting only for the Mediterranean twilight. Then it is that _Hyalæa trispinosa_ and _Cleodora subula_ come up; _Hyalæa tridentata_, though it does not venture out till dusk, retires early, whilst some species, such as _Cleodora pyramidata_, are to be met with only during the midnight hours and the darkest nights. This tribe, like a higher one, has its few irregular spirits, who manage to keep it up the whole night through. All, however, are back to their homes below before dawn surprises them."

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The lamellibranchiate Acephala, or headless molluscs with comb-like gills, are distinguished from the preceding orders of molluscs by a more simple organisation and the peculiar formation of their external coverings. They are all contained within a bivalve shell, articulated after the manner of a hinge, and to which some of their families are attached by one strong muscle (Monomyaria), others by two (Dimyaria). In this shell, which is secreted by two large flaps or folds of their skin or mantle, they generally lie concealed like a book in its binding, and bid defiance to many of their enemies. When danger menaces, the sea-snail withdraws its head and closes the entrance of its hermitage with a lid, but the bivalve shuts its folding-doors when it wishes to avoid a disagreeable intruder. A strong elastic ligament connects the two valves, and opens them wide as soon as the muscular contraction which closed them ceases to act.

While the sea-snail creeps along upon a mighty foot, the bivalve is frequently doomed to a sedentary life, and the former protrudes from its shell a well-formed head, while the latter, like many a biped, has no head at all. The lamellibranchiate Acephala have, however, been treated by nature not quite so step-motherly as might be supposed from this deficiency, for many of them have eyes, or at least ocular spots, which enable them to distinguish light from darkness; and even auditory organs have been discovered in many of them. Their circulation is performed by a heart generally symmetrical, and their respiration by means of four branchial leaflets equal in size, and symmetrically arranged on either side of the body. The mouth is a simple orifice without any teeth, bordered by membranous lips, and placed at one end of the body between the two inner leaves of the branchiæ. The digestive apparatus consists of a stomach or intestine of different lengths, a liver, and several other accessory organs. A simple nervous system brings all the parts of the body into harmonious action.

In many lamellibranchiates the folds of the mantle are disjoined, as, for instance, in the oyster, which, on opening its shell, at once admits the water to its delicately fringed branchiæ; in others they are more or less united, so as to form a closed sack with several openings, an anterior one (_h_) for the passage of the foot, and two posterior ones (_g_, _f_) for the ingress and egress of the water which the animal requires for respiration. These posterior openings are often prolonged into shorter or longer tubes or siphons, sometimes separate, and sometimes grown together so as to form a single elongated fleshy mass. The use of these prolongations becomes at once apparent when we consider that they are chiefly developed in those species which burrow in sand, mud, wood, or stone, and which therefore require to be specially guarded against the danger of suffocation. The interior of these siphonal canals is lined with innumerable vibratory cilia, by the action of which the water is drawn towards the branchial orifice and conveyed in a current through the canal over the surface of the gills; then, having been deprived of its oxygen, it is expelled by a similar mechanism through the other tube; and it is by the force of this anal current that the passage is kept free from the deposit of mud or other substances, which would otherwise soon choke it up. The cleaning action of the anal current is assisted by the faculty the burrowing molluscs possess of elongating and contracting their siphons, and the degree to which this may be accomplished depends on the depth of the cavity which the species is accustomed to make. Yet since many particles of matter float even in clear water, which from their form or other qualities might be injurious to the delicate tissue of the viscera to be traversed, how is the entrance of these to be guarded against in an indiscriminating current? A beautiful contrivance is provided for this necessity. The margin of the branchial siphon, and sometimes, though more rarely, of the anal one, is set round with a number of short tentacular processes, endowed with an exquisite sensibility and expanding like feathery leaves. In _Pholas dactylus_ this apparatus, which is here confined to the oral tube, is of peculiar beauty, forming a network of exquisite tracery, through the interstices or meshes of which the water freely percolates, while they exclude all except the most minute floating atoms of extraneous matter. Thus admirably has the health and comfort of the lowly shell-fish been provided for that spend their whole life buried in sepulchres of stone or sand.

_a_, _b_. Siphons.]