MOLLUSCA. 
4 
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 laminse 
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 Avith 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, 
Avhile in many the sexes are separated. 4' he first are viviparous, 
and the others oviparous ; the eggs of the latter are sometimes en- 
veloped Avith a harder or softer shell, and sometimes Avith a simple 
Auscosity. 
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 
deA^eloped, possessed of but little industiy, and AA'hich 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 Mollusca, 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, Avhence issues a Avell developed head croAAUied Avith long' 
and strong fleshy productions, by means of Avhich they craAvl, and 
seize A^arious objects. These Ave term the Cephalopoda. 
That of others is closed; the appendages of the head are either 
Avanting or are extremely reduced; the principal organs of locomotion 
are tAvo Avings or membranous fins, situated on the sides of the neck. 
* M. cie Blainville has substituted the name of Malacozoaires for that of Mol 
luscn, separating from them the Chitons and Cirrhipoda, A\hich he calls Malento- 
zoasrcs. 
f The whole of this arrangement of the Mollusca, and most of the secondary 
subdivisions, belong e.xclusively to me. 
