The position of the shell with respect to the constituent layers of the integuments, exhibits very remarkable dif ferences. In some it appears instead of a cuticle, or at least external membrane investing it. In general, however, it occurs between the cuticle and the skin, a position which induces Curler, Let. &An. Comp. xiv. 11.) to consider it as analogous to the mucous web of the vertebral animals. Its intimate connection with the muscular system of the animal, and the protection which it yields, seem adverse to such a conclusion. In many species the testaceous substance occurs in folds of the corium, or inserted in its substance. In this position it never acquires the solid texture which shells exposed, or covered only by the cuticle, exhibit. Those which are thus concealed are in general white ; those which are more exposed are frequently coloured. The co louring, however, does not depend on the direct expo sure to the light, as some have imagined, for many shells which are destitute of a cuticle are white, while many of those covered with a dense cuticle are finely variegated beneath. The forms of the British species of shells have been described in detail under the article CONCHOLOGY, where an explanation of the terms by which their different parts are distinguished may like wise be found.
Between the skin and the shell neither vessels nor nerves have been traced ; and the manner in which the latter is formed forbids us to expect their existence. Yet the shell cannot be considered as dead matter, so long as it remains in connection with the living animal. In those animals in which the shell is external, there are muscles which connect the animal with its internal surface, and the bond of union being a substance soluble in water, the muscle can he detached by maceration. The ana logy between shell and bone is here obvious, although in the one case the connection between the muscle and the bone is permanent, in the other between the muscle and shell temporary, or frequently changed during the life of the animal. But the vitality of the shell, if I may use the expression, is demonstrated from the changes which it undergoes when detached from the animal : The plates of animal matter harden ; the epidermis dries, cracks, and falls off; and, in many cases, the co lours fade or disappear. We confess ourselves unable to point out the means employed by the animal to pre vent these changes from taking place, by any process similar to circulation. It is probably effected by the secretions of the skin, in the same manner as our cuticle and hair are lubricated. When the shelly covering con sists of two or more pieces, they are joined together as the articulated bones in the higher classes of animals, by ligaments. These, in some cases, are of great thickness and strength, and,in consequence of their in the motion of the different parts.
In the molluseous animals, the skin secretes a viscous, adhesive substance, differing according to the medium in which the animal resides, but in all cases calculated to resist its influence. It is probably owing to the lubri
cating agency of this secretion, that both the cuticle and shell are preserved from decomposition.
The skin likewise secretes the colouring matter by which the shells are variegated. The glands from which it proceeds vary much in different individuals, and even in the same individual in different periods of growth.
The characters furnished by the skin and its appen dices, are extensively employed in the systematical ar rangement of molluscous animals ; nearly all those cha racters which distinguish the species, and many of those on which genera are established, are derived from the form of the shell, the tentacula, or the colour. This last character, however, is one on which little dependence should he placed.
3. Muscular System.—Thcre is nothing peculiar in the muscular system of this class of animals. Where the muscles are inserted in the skin, as is usually the case, that organ is in some cases strengthened by con densed cellular substance ; where the muscles are in serted on the shell, although no intervening tendons can be perceived, the existence of a connecting link of a similar nature is rendered probable by the circumstance of boiling water not dissolving the muscle itself, yet de taching it from the surface of the shell. During the growth of the animal, such detachment is frequently performed voluntarily, as it increases in size, and alters its position with regard to the shell.
1\Iolluscous animals preserve themselves in a state of rest chiefly by suction and cementation. The organ which acts as a sucker, is in some cases simple, soft, and muscular, as the foot of the snail, while in others it is compound, and strengthened internally by hard parts, as in the arms of the cuttlefish. The force with which some animals adhere is very considerable, and is striking ly displayed, for example, when we attempt to detach a limpet from the rock.
The rest, which is maintained by cementation, in some cases depends on a glairy secretion, which glues the body of the animal to the substance to which it is dis posed to be attached. By such an expedient, the shells of snails adhere to rocks, stones, and plants. It is pro bable that the bivalve shells of the genus Cyclas, which readily adhere to the sides of a glass, obtain their tem porary attachment by means of their glutinous cuticle. In other animals threads are produced, (termed a Bys sus,) from particular glands, and while one extremity is glued to the rock, the other remains in connection with the animal. But there is an attachment more durable than any of these, which takes place in some shells, they being cemented to rocks or stones by calcareous matter, and retained in the same position during the whole term of their existence.