SHELL

SHELL, in natural history, a hard, and, as it were, stony covering, with which certain animals are defended, and thence called shell-fish.

The singular regularity, beauty, and delicacy, in the structure of the shells of animals, and the variety and brilliancy in the colouring of many of them, at the same time that they strike the attention of the most incurious observers, have at all times excited philosophers to inquire into and detect, if possible, the causes and manner of their formation. But the attempts of naturalists, ancient and modern, to discover this process, have constantly proved unsuccessful. M. de Reaumur hitherto appears alone to have given a plausible account, at least, of the formation of the shell of the garden-snail in particular, founded on a course of very ingenious experiments, related in the Paris Memoirs *. He there endeavours to show, that this substance is produced merely by the perspirable matter of the animal, condensing and afterwards hardening on its surface, and accordingly taking the figure of its body, which has performed the office of a mould to it; in short, that the shell of a snail, and, as he supposed, of all other animals possessed of shells, was only the product of a viscous transudation from the body of the animal, containing earthy particles united by mere juxtaposition. This hypothesis, however, is liable to very great and insurmountable difficulties, if we apply it to the formation of some of the most common shells: for how, according to this system, it may be asked, can the oyster, for instance, considered simply as a mould, form to itself a covering so much exceeding its own body in dimensions?

M. Herissant, in the memoirs of the academy of sciences for 1766, has discovered the structure of shells to be organical. In the numerous experiments that he made on an immense number, and a very great variety, of animal-shells, he constantly found that they were composed of two distinct substances; one of which is a cretaceous or earthy matter; and the other appeared, from many experiments made upon it, by burning, distillation, and otherwise, to be evidently of an animal-nature. These two substances he dextrously separated from each other by a very easy chemical analysis; by the gentle operation of which they were exhibited distinctly to view, without any material alteration from the action of the solvent, or instrument, employed for that purpose. On an entire shell, or a fragment of one, contained in a glass vessel, he

poured a sufficient quantity of the nitrous acid, considerably diluted either with water or spirit of wine. After the liquor has dissolved all the earthy part of the shell, (which may be collected, after precipitation by a fixed or volatile alkali), there remains floating in it a soft substance, consisting of innumerable membranes of a retiform appearance, and disposed, in different shells, in a variety of positions, which constitutes the animal-part of it. This, as it has not been affected by the solvent, retains the exact figure of the shell; and, on being viewed through a microscope, exhibits satisfactory proofs of a vascular and organical structure. He shows that this membranous substance is an appendix to the body of the animal, or a continuation of the tendinous fibres that compose the ligaments by which it is fixed to its shell; and that this last owes its hardness to the earthy particles conveyed through the vessels of the animal, which fix themselves into, and incrust, as it were, the meshes formed by the reticular filaments of which this membranous substance is composed. In the shell called porcelaine, in particular, the delicacy of these membranes was so great, that he was obliged to put it into spirit of wine, to which he had the patience to add a single drop of spirit of nitre day by day, for the space of two months; lest the air generated, or let loose by the action of the acid on the earthy substance, should tear the compages of its fine membranous structure into shatters; as it certainly would have done, in a more hasty and less gentle dissolution. The delicate reticulated film, left after this operation, had all the tenuity of a spider's web; and accordingly he does not attempt to delineate its organisation. In other shells, he employed even five or six months in demonstrating the complicated membranous structure of this animal-substance by this kind of chemical anatomy. In general, however, the process does not require much time.

Of the many singular configurations and appearances of the membranous part of different shells, which are described in this memoir, and are delineated in several well executed plates, we shall mention only, as a specimen, the curious membranous structure observed in the laminae of mother-of-pearl, and other shells of the same kind, after having been exposed to the operation of the author's solvent. Beside the great variety of fixed or permanent colours with which he found the animal-filaments of these shells to be adorned, it is known that the shell itself presents to the view a succession of rich and changeable colours, the production of which he easily explains from the configurations of their membranes. Nature, he observes, always magnificent in her designs, but singularly frugal in the execution of them, produces these brilliant decorations at a very small expence. The membranous substance above-mentioned is plaited and rumpled, as it were, in such a manner, that its exterior laminae, incrust with their earthy and semi-transparent matter, form an infinite number of little prisms, placed in all kinds of directions, which refract the rays of light, and produce all the changes of colour observable in these shells.