NERVE.
SHEL L.—This term is commonly employed to designate the hard envelopes in which the bodies and members of inany animals be longing to the Radiated, Molluscous, and Articulated sub-kingdoms are enclosed. Ge nerally speaking, it is applied to thbse only into whose composition mineral matter enters : thus, we speak of the shell of a Crustacean, whilst we do not give that appellation to the derrno-skeleton of an Insect or Myriapod. Still this rule is not strictly obser% ed ; for there are many Crustacea and Mollusca which are commonly spoken of as possessing shells although these bodies are entirely. destitute of calcareons matter, being as horny in their texture as the envelope of a beetle or a centi pede. Among radiated animals, the class of ECIIINODERMATA is the only one in which shells are met with ; and these are by no means universally present throughout the group. In the molluscous series, we meet with shells in every class save the TUNICATA ; all the animals of the class CONCIIIFERA, W he ther lamelli-branchiate or pallio-branchiate, bei n g furnished with them ; a considerable propor tion of GASTEROPODA oil of them, it would seem, in the embryonic state) possessing them ; whilst they' are occasionally found in the deli cate little PTEROPODA, and in the compara tively gigantic CEPHALOPODA. In this last class, however, the shells are not unfrequently internal ; an approach to this arrangement being seen in certain Gasteropoda and Ptero poda, in which the shells are covered-in by folds of the mantle, whilst really external to the body. In the articulated series, the pre sence of a shelly covering, according to the usual acceptation of the term, is more re stricted. It is possessed by a few ANNELIDA (e. g. Serpula, Spirorhis, Ste.), whose shelly tubes so much resemble those of certain Mol lusks as to be readily confounded with them. It is usually found, too, in the CIRRHOPODA, (a class whose articulated character is now quite settled) ; and it is generally present in the CRUSTACEA, although it is only in the larger and more highly developed forms of this class, that the shell is consolidated by mineral deposit, and really deserves the appel lation.
The external configuration of the prinCipal varieties of shelly covering having been suf ficiently described under the several heads above referred to, it is not requisite here to revert to that subject ; our present purpose being to give an account of the intimate struc ture of shell, on which an entirely new light has been thrown by microscopical en quiries. The prevalent doctrine respecting the nature of shell, as expressed even by the most recent conchological writers, has been that it is not only extravascular, but completely inorganic, being composed of an exudation of calcareous particles, cemented together by animal glue. It may now, however, be stated as an ascertained fact, that shell always pos sesses a more or less distinct organic struc ture* ; this being in some instances of the cha racter ofthat of the epidermis of higher animals, but ii) others having more resemblance to that of the dermis, or true skin. The nature of the organic structure is so entirely different in the Mollusca, Echinodermata, and Crustacea re spectively, that a separate description is re quired for each ; indeed, even in the subor dinate divisions of these groups very charac teristic diversities are frequently observable ; so that, as in the case of teeth, it is often possible to determine the family, sometimes the genus, and occasionally even the species, from the inspection of a minute fragment of a shell, as well fossil as recent ; whilst the degree of correspondence or difference in the intimate structure appears to be, in many groups, more valuable than any other single character as a basis for classification, because more indicative of the general organisation of the animal. Examples of both these applications will be presently given.