4 MOLLTJSCA. 



of shells, is infinite ; most of them are calcareous ; some are simply 

 horny, but they always consist of matters deposited in layers, or 

 exuded from the skin under the epidermis, like the mucous covering, 

 nails, hairs, horns, scales, and even teeth. The tissue of shells differs 

 according as this transudation is deposited either in parallel laminae 

 or in crowded vertical filaments. 



All the modes of mastication and deglutition are illustrated in 

 the Mollusca; here the stomachs are simple, there they are com- 

 plicated, and frequently provided with a peculiar armature; their 

 intestines are variously prolonged. They most generally have 

 salivary glands, and always a large liver, but neither pancreas 

 nor mesentery : several have secretions which are peculiar to 

 them. 



They also present examples of all the varieties of the process of 

 generation. Several of them possess the faculty of self-impregna- 

 tion ; others, although hermaphrodites, require a reciprocal coitus, 

 while in many the sexes are separated. The first are viviparous, 

 and the others oviparous ; the eggs of the latter are sometimes en- 

 veloped with a harder or softer shell, and sometimes with a simple 

 viscosity. 



These varieties of the digestive and generative processes are found 

 in the same order, and sometimes in the same family. 



The Mollusca in general appear to be animals that are but slightly 

 developed, possessed of but little industry, and which are only pre- 

 served by their fecundity and their tenacity of life. 



Division of the Mollusca into Six Classes.* 



The general form of the body of the Mqllusca, being in propor- 

 tion to the complication of their internal organization, indicates their 

 natural division.! 



The body of some resembles a sac open in front, containing the 

 branchiae, whence issues a well developed head crowned with long 

 and strong fleshy productions, by means of which they crawl, and 

 seize various objects. These we term the Cephalopoda. 



That of others is closed ; the appendages of the head are either 

 wanting or are extremely reduced; the principal organs of locomotion 

 are two wings or membranous fins, situated on the sides of the neck, 



* M. de Blainville has substituted the name of Malacozoaires for that of Mol 

 lusca, separating from them the Chitons and CirrMpoda, which he calls Malenlo- 

 zoasrcs. 



f The whole of this arrangement of the Mollusca, and most of the secondary 

 subdivisions, belong exclusively to me. 



