Page:Natural History, Mollusca.djvu/88

 solidity and hardness. The chemical composition of these shells, however, varies very little; they consist of carbonate of lime deposited in cells of animal albumen. In the porcelain shells, of which the Cowries (Cypræa) afford familiar and beautiful examples, the lime is compact, with so small a portion of animal matter, that when immersed in acid the shell is completely dissolved, no sensible trace remaining. In the pearly shells, such as the genus Trochus, the calcareous matter is deposited in layers; and these, when submitted to acids, leave behind an insoluble membrane of albumen which retains the form of the shell.

The shell is secreted by the mantle. In one family, that of the Chitons, it consists of several pieces, but in general it is simple, and takes the form of a hollow cone produced in various degrees. In the Limpets, which we see adhering so abundantly to our sea-side rocks, the cone is low and nearly symmetrical; but in the great majority of this class, the cone is greatly lengthened and twisted upon itself, so as to form a spire.

The mode in which the shell is formed has been well investigated by Mr. J. E. Gray, whose observations on the subject I shall here take occasion to cite:—

"The shell, which is peculiar to this division of the animal kingdom, may be seen covering the young animal in the egg before it has gained all its organs, as was observed by Swammerdam, and verified by the more extended observations of Pfeiffer, Turpin, and others. They are easily seen in the egg of the Limnæi, Physæ, Ancyli, and Bithiniæ, which have a transparent coat.