The most important appendix of the skin appears to be shell. This part is easily preserved, exhibits fine forms and beautiful colours, and has long occupied the attention of the conchologist. The matter of the shell is secreted by the corium, and the form which it assumes is regulated by the body of the animal. It is coeval with the existence of the animal, and appears previous to the exclusion from the egg ; nor can it be dispensed with during the continuance of existence. The solid matter of the shell consists of carbonate of lime, with a small por tion of animal matter, resembling coagulated albumen: The mouth of the shell is extended by the application of fresh layers of the shelly matter to the margin, and its thickness is increased by a coating on the inner surface. These assertions are abundantly confirmed by the obser vations of Rcaumur, (Alen:oh-es de rilcadimies des Scien ces, 1709.) whose accurate experiments have greatly con tributed to the elucidation of conchology. If a hole is made in the shell of a snail, and a piece of skin glued to the inner margin, so as to cover the opening, the shelly matter does not ooze out from the broken surface, so as to cover the external surface of the skin, but it forms a coating on its inner surface, thus proving it to have ex uded from the body of the animal. When a considerable part of the oral part of the spire of a snail is broken off, and a piece of skin glued to the inner margin, and re flected outwardly and fixed on the body of the shell, the spire is again renewed, and the matter added to the inner surface, thus leaving the skin interposed between the new formed portion and the fractured edge. Similar experiments, repeated on a variety of shells, both uni valve and bivalve, by different naturalists, leave no room to doubt that shells loci ease in size by the juxtaposition of shelly matter from the common integuments.
Each calcareous layer is more or less intimately mixed and enveloped in the animal matter, which we have al ready alluded to; so that the different layers of succes sive growth may, by various processes, be distinctly exhibited. If the shell is exposed for a short time in the fire, the animal matter becomes charred, and its black colour, contrasted with the white earthy matter, indicates the different strata : in the same manner as the ivory and enamel of a tooth can be distinguished, when subjected to similar treatment. The same satisfactory results may be obtained by a different process. If the shell be steeped in weak muriatic acid, the earthy matter will be dissolved, and the flakes of albumen will remain as the frame-work of the edifice.
The layers of growth may often be distinguished on the surface of the shell, in the form of stri?, or ridges more or less elevated, but parallel to the margin of the aperture. Other inequalities may likewise he observed on the surface, at right angles to the layers of growth, such as ridges, knobs, and spines. These last derive their origin from the inequalities of the skin on which they have been moulded.
In some univalve shells, the layers of growth parallel to the opening cannot be discerned ; when exposed in the fire, there is little darkening of colour; and when dissolved in acids, but a feeble trace of animal matter remains. In the fire, these shells crack in various directions, but exhibit no trace of a scaly structure. By a careful management with the file, the shell may be separated into a central layer contiguous to the skin, and a peripheral layer, both similar in structure, though frequently differing in colour. The shells exhihiting such characters have been termed Porcellaneous, from their dense structure, and the fine polish which their surface presents. The formation of shells of this kind must be executed in a different manner from those of the first kind which we have noticed.
If we attend to the form of a young shell of the genus Cyprza of Linnaeus, we may perceive that an addition of shclly matter to the margin of the aperture, in the manner in which it is applied in other shells, would not enlarge the cavity, but completely close the aperture. The increase of the shell, (accompanied with a corres ponding increase of its inhabitant,) must take place either by absorption of the accumulated shelly matter of the mouth, and an elongation m the direction of the greatest curvature of the shell ; or the old shell must be thrown off, and a new one produced, suited to the size of the animal. The former supposition has not been entertained, the latter is now generally received by naturalists. The inner coat of such shells appears to be a transudation from the body of the animal, the outer one applied to the surface by the loose reflected lobes of the cloak. In many other shells, portions of more compact matter than the other parts may be observed, spread on the pillar, and applied to the margin of the mouth by a similar pro cess. Mr. Platt, in support of Reaumur's opinion, that shells are formed by juxtaposition, against the objec tions of Mr. Poupart, (Phil. Trans. vol. liv. p. 43.) erro neously considers the different sizes of the Cyprex as depending on the thickness of the shell increasing according to age, without admitting a corresponding increase of the dimensions of the contained animal, or cavity for its reception.
The shells of the first kind which we have noticed, from the manner in which they are formed, of cones or layers applied to the inner edge of the margin, and ex tending beyond it, have an imbricated structure. Those of the second kind, consisting of layers regularly super imposed, have consequently a laminated structure ; but between the two kinds there are numerous intermediate links, formed by a combination of the two processes.
In some cases, the hard parts of the skin are not en titled to the appellation of shell, but may rather be con sidered as horn. Such are the coverings of the mandi bles of the Cuttic-fish, the branchial lid of the Aplysia, and the operculum of the Welk. The two last appen dices, however, though horny in some species, are shelly in others.