16S THE MECHANICAL FUNCTIONS. 



§ 4. Structure and Formation of the Shells of Mollusca, 



The structure and formation of the shells of molluscous 

 animals is a subject of much interest in comparative physi- 

 ology, as presenting many beautiful illustrations of the laws 

 by which the inorganic parts of the living system are in- 

 creased in their dimensions. 



All shells are composed of two portions, the one consist- 

 ing of particles of carbonate of lime, the other having the 

 character of an animal substance, and corresponding in its 

 chemical properties either to albumen or to gelatine. The 

 mode in which these two constituent parts are united, as 

 well as the nature of the animal portion, differ much in dif- 

 ferent kinds of shell; and it is chiefly in reference to these 

 circumstances that shells have been divided into two classes, 

 namely, the memhranous and jiorcellaneQUS shells. 



In shells belonging to the first of these classes, the carbo- 

 nate of lime is united with a membranous substance deposit- 

 ed in layers, which may be separated from one another, 

 either by mechanical division with a sharp instrument, or 

 by the slow actions of air, water, or other decomposing che- 

 mical agents. The shells of the limpet, of the oyster, and 

 of almost all the larger bivalve mollusca which reside in the 

 oceaif are of this kind. They are usually covered with a 

 thick outer skin, or epidermis; and their texture is of a 

 coarser srain than that of other shells. 



If a shell of this description be immersed in an acid capable 

 of dissolving carbonate of lime, such as the muriatic or ni- 

 tric acids properly diluted, at first a brisk efiervescence is 

 produced, but this soon slackens, and the carbonate of lime 

 contained in the shell is slowly dissolved; the membranous 

 layers being left entire, and sufficiently coherent to retain 

 the figure of the shell, but, having lost the earthy material 

 which gave them hardness, they assume their natural form 

 of soft and flexible plates. 



Many membranous shells exhibit, on several parts of their 

 internal surface, a glistening, silvery, or iridescent appear- 



