SHELL. This term is employed to designate the hard outer coverings of a large num ber of invertebrate animals. Shells are met with in the echinodermata, in the great major ity of the mollusca (excluding the molluscoids), in a few of the annelida, as serpula, spiror bis, etc., in the cerropoda, and in the crustacea. The forms of the different varieties of shells are sufficiently noticed in the articles on the classes of animals to which they respectively belong; and we shall confine our remarks to the intimate structure of shell, which, until the publications of Carpenter, Rainey, and others, during the last quarter of a century, was altogether misunderstood. The doctrine formerly held, and still maintained in many popular hand-books of conchology, was, that shell is not only extra vascular (or devoid of vessels), but completely inorganic, being composed of an exuda tion of calcareous particles (chiefly carbonate of lime) cemented together by a kind of animal glue. It is now known that shell always possesses a more or less distinct organic structure, which in some cases resembles that of the epidermis of the higher anunals, while in others it approximates to that of the derma, or true skin. The nature of the organic structure is so different in the echinodermata, mollusca, and crustacea, that z..eparate description is required for each, and as Dr. Carpenter 'remarks: "Even in tae subordinate divisions of these groups, very characteristic diversities arc frequently observal6e, so that, as in the case of the teeth, it is often possible to determine the fam ily, somelmes the genus. and occasionally even the species, from the inspection of a minute fraonent of a shell, as well fossil as recent.
In the ,^c.'3inodertnata, the elementary structure 0. the skeleton exhibits the appear ance of a nct-work composed of calcareous and animal matter intimately united. The diameter of Vtose apertures or meshes of net-work varies to certain degree•u different parts of the same shell, the openings being larger in the inner than the outer layers. the extremes being and of an inch. The entire shell is made up of an immense number of such plates, which lie parallel to one another, separated by minute vertical pillars.
In the rtio/htsca, the shell is formed upon the surface of the mantle, which sponds to the true skin of other animals. Hence must be regarded as epidermic. It
consists of cells consolidated by a deposit of calcareous salts in their interior, but, as in the ease of many other tissues, the origina'l cellular organization often becomes so hidden by subsequent clmiges, as to cease to be recognizable. The typical condition of the in this sub-klogd,ot is best seen in certain bivalves—the genus pinna, for example.
On breaking off a small portion of the projecting margin of one of these shells, and examining it under the microscope, it is found to be made up of a vast number of prisms, hexagonal in form and nearly uniform in size, which are arranged perpen dicular to the surface of the. lamina of the shell, so that the thickness of the lamina is formed by their length, and its surfaces by their extremities. On submitting such a lamina to the action of a dilute acid, the calcareous salts are dissolved, and a membrane is left which shows the prismatic structure as perfectly as it was seen in the original shell, the hexagonal divisions being evidently the walls of cells resembling those occurring in the pith or bark of a plant. It sometimes happens iu recent, but more commonly in fossil shells, that the animal matter decays and leaves the prisms ununited, and easily separable from one another. It is only in a few families of bivalves that the cellular structure is seen in this very distinct form, or that it. makes up a large portion of the shell; and these families are closely allied to pinna. In many shells the external layer is formed.on the above plan, while the inter nal layer is nacreous; in many, again, the nacre, or "mother of pearl," and in others suttnacreous structure, constitutes nearly the whole thickness of the shell. The nacre, according to sir D. Brewster, consists of a multitude of layers of carbonate of lime, alternating with animal membrane; and the grooved lines on which iridescent luster depends are due to the wearing away of the edges of the animal lamina., while those composed of carbonate of lime stand out; it is, however, more probable, from Dr. Carpenter's researches, that the peculiar lineation of the surface of nacre is due to the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface.