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LETTER VIII.

ON THE LOCOMOTION OF THE MOLLUSCA.

THE great importance which, in the preceding arrangement, is attached to the locomotive organs, naturally draws our attention to these in the first place; and we shall find that their various modifications have induced a corresponding variety in the mode in which this class of beings move themselves. Hence, by throwing a cursory glance among them,

we might indulge ourselves with what some naturalists delight in a scene rising by a gradual concatenated series from the half zoophytical mollusk, whose fixidity is permanent as the rock on which it is placed, to the fish-like Cephalopod wandering at freedom through the ocean; passing from the oyster to the Ligula waving to and fro on a flexile stalk, thence to the burrowing bivalves, which gradually lessen in dependency on their furrows and mingle with their halting congeners; among whose tribes, again, there are some thatfloating on the surface-lead us to the Gasteropods, which, through many a linked transition, pass insensibly to the winged natatory Pteropods. There are, in this hasty sketch, many chasms which a little ingenuity might fill up with osculant groups, and connect the horrid gaps which the naturalists alluded to seem to abhor as much as Nature herself is said to abhor a vacuum; but the scene would then be

CONNECTION BETWEEN THE SHELL AND ANIMAL. 113

in truth after the fashion of the maker's mind, and little in accordance with what a sober survey of the reality discloses. This is not less beautiful nor less designed, but so far from rising on the spectator, like alpine scenery, the more perfect as he ascends, it lies wide before him with all its variety level to his gaze, and perplexing by its unpatterned intricacy. In every, or nearly in every, class or order, there are some families which swim whithersoever it listeth them, some which crawl or trail themselves along the solid ground, some which burrow and which confine themselves to their narrow cells; and others there are which have no power of changing their sites, but live and die on their natal spot. Instead therefore of making each class pass in separate review before you, exhibiting, in a coloured glass, their progressive rise in motive capabilities, I rather choose to leave system for the present; and, grouping, somewhat loosely, the molluscans into the swimmers, the crawlers, the burrowers, and the sedentaries, I will lay my illustrations before you under these several heads.

Before entering on the details, however, I must remind you of the close connection between the shell and its inmate, by which the former, instead of being a drag and hinderance, becomes essential to the movements of the latter. The connection between them is inseparable during life, and is effected by the medium of muscles which go from the animal to be inserted to the parietes of its dwelling. The mollusca with bivalved shells are in this manner attached by one or two large and powerful muscles, called sometimes transverse, because, passing through the body, they are inserted into both valves at opposite points; and sometimes adductors, because their office is to close the valves and keep them so in opposition to the elasticity of the ligament at the hinge which tends to separate them; and the astonishing force with which they act is well illustrated by the extreme difficulty of opening those of an oyster. In the mollusca which are covered with a shell in form of a case or sheath, as in some Pteropods and Gasteropods, the animal is connected to the base of the shell by a large dorsal retractor muscle. In simple conical univalves, as the Limpet (Patella, Linn.), the body "is fastened to the circumference of the shell by a ring of fibres, which are attached all round the shell, and which, after piercing the outward covering or cloak, are inserted in the edges of the foot, and interlaced with its circular fibres. Anteriorly they leave a free space for the passage of the head. This muscle, by its contractions, brings the foot and the shell closer together, and compresses the body; on relaxing,

I

it allows the shell to be raised up by the elasticity of the body." The snails of spiral shells are bound to them by two muscles, which arise from the pillar, and, having penetrated the body below its spiral part, run forward under the stomach, and spread their fibres in several slips, which interlace with those of the muscles proper to the foot, the substance of which they enter. It is obvious from this direction, that, on their contraction, the body of the snail must be drawn within the shell. When it wishes to reissue, the head and foot are forced out by circular fibres, which surround the body immediately above the foot.* The muscles, like the muscles of superior animals, are composed of parallel fibres, but of a bluish-white colour, † soft and jelly-like, and rather loosely connected; for the cellular substance, which binds together those of red-blooded animals, is here very generally wanting. They have, apparently, no tendons; but this is, according to Cuvier, owing to the colour being the same in the tendinous and the fleshy parts. The fibres are, in general, closely and inextricably interlaced, the insertions being lost in one another, or in the skin under which they lie, and from which, indeed, it seems impossible to separate them by any definite line. Chemically they consist of fibrine, but the medium which cements them to the shell appears to be gelatinous, for it is loosened and detached by maceration in spirits of wine and boiling, operations which have an opposite effect on fibrine.‡

I. SWIMMERS. The Pteropods are the most entirely natatory of all the mollusca. Created to occupy the high seas, they are organized in evident aptitude to the place assigned them, with a light shell, which affects not their buoyancy, and with fins for progressive motion. They are found far at sea, in large herds, swimming about in a lively manner, by undulatory or flapping movements of their membranous fins, which open and close like those of the butterfly while basking in the sun. They shun the light

*Cuvier, Comp. Anatomy.

+ Bohadtch says, that some of the muscular fibres of the Aplysia are red. -De Anim. Mar. 12.

"The muscles of mollusks either form a flat disk, or are distributed in the skin so as to dilate and contract it, or are arranged about the mouth and tentacles, which they put in motion. However varied the disposition of the muscles may be they always form very considerable masses, in proportion to the size of the animal, and have a soft and mucous appearance, such as is not seen in the contractile fibres of the other departments of the Animal Kingdom. This peculiar aspect no doubt arises from the numerous small cavities found in the muscles, and the mucous glands which are distributed through them."-AGASSIZ and GOULD's Princ. Zoology, i. 52.

Fig. 19.

some day, and sink, as the sun rises, into the bosom of the deep, to attain that shaded gloom which suits them; but on the evening's approach they gradually again ascend to the surface, and regain all their vivacity, so that if some will fancifully seek amongst them the molluscan analogues of insect-butterflies in their manner of natation, they will not fail to mark the correspondency between them and the moths in their crepuscular and nocturnal habits. The little Hyales (Fig. 19) first appear. About five in the afternoon, when the garish eye of day begins to grow dim, two or three species venture upwards to the field of their occupancy; as evening advances several small species of Cleodores rise in great number with other Hyales and Atlantes, but the larger kinds do not leave the abyss and mingle in the crowd until night lends them her friendly veil; and some species, as the Hyalæa balantium, are even so fearful of the light's malign influence that they do

not come to the surface excepting when the night is very dark. After a few hours' disport, the lesser species begin to descend and disappear; the larger follow at a little later hour, so that towards midnight only a few wandering individuals can be taken. These may possibly remain even to the dawn, but the sun's rise is the signal which recals them to their home. After this not a single Pteropod is to be seen either at the surface or at any depth to which the eye can penetrate. Each species has its own time at which it rises up and goeth down, determined not by the clock, as you will readily believe, but by the degree of obscurity in the heavens, so that in an overshadowed day they rise earlier than in a cloudless one, and sink earlier also to repose. From these habits M. d'Orbigny infers that each species dwells habitually in the water at a depth peculiar to itself, and where it can enjoy the shade of obscurity suited to its disposition, the light being of course tempered in exact proportion to the thickness of the layer or bed of water it has had to pass through. The species then comes to the surface only at that time of day when the dusk is nearly the same as that which reigns in the zone occupied by the creature in the bosom of the deep. As the sun rises, the Pteropod sinks lower until it has reached its maximum of descent; but when the sun has passed the meridian, the snail's upward course begins, and with a gradual ascent, regulated by the sun's decline, he passes up and up, until

the surface is reached. Light, therefore, and not the search after food, or the desire of breathing a freer atmosphere, as Rang believes, is, according to D'Orbigny, the true regulator of their diurnal movements. It is not a sufficient objection to this theory to remind me that the Pteropods are eyeless and blind, for numerous facts prove that many animals which have no organs of vision are still powerfully affected by the light, seeking or shunning it as their sensations teach them to find pleasure or pain under its influence.* But still we may ask, why do the Pteropods disappear, as the darkness thickens, to a retreat certainly still darker, and why are they not ready to greet the dawn in its approach as well as the evening fall, seeing that there must be, in both, periods at which the shade of light will be of equal intensity? Let it also be remarked, that their congregations on the surface are variable and inconstant. For some successive nights a species will throng the naturalist's fatal net, when, without any visible cause, it may be cast and drawn for two or three nights in succession and in vain-not a single individual has left his subaqueous haunt,—after which they will again suddenly rise as numerous as before. It is neither an instinctive prescience of a storm, nor a storm itself, which hinders them, for D'Orbigny often took them during stormy nights in abundance; and the belief that at such seasons they lay sunk in the abyss seems to have originated in its seeming reasonableness to the naturalist, who deemed it consequently fruitless to endeavour the capture of such fragile creatures among the billows of a troubled sea.

By what mechanism the Pteropods balance themselves in the deep and vary their position, is, perhaps, scarcely determined. Cuvier conjectures, with reference to the Clio, that there may be a collection of fluid or air in a space between the sac and the viscera, by compressing which the animal will sink; and it will rise when the air or fluid is allowed to distend the relaxed sac to its full capacity.† D'Orbigny seems to think that nothing is required beyond their muscular efforts and the movements of their fins, and though this explanation would seem to require a continuance of action, to which there is no relaxation or end, it is better to acquiesce in it than to call imaginary structures to our aid. The Pteropods, says D'Orbigny, have a peculiar mode of swimming in subservience to their form: the cephalic fins can propel the animal forwards, or sustain it afloat, only by *The Clio, it is now ascertained, has two eyes, apparently of a very complete character. See JONES's Animal Kingdom, p.428. + Mém. sur les Mollusq. ii. 6.