4 SECOND GRAND DIVISION. 
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 modes of generation. 
Several of them possess the faculty of self-impregnation ; 
others, although hermaphrodites, have need of a reciprocal 
intercourse. Many have the sexes separated. Some are 
viviparous, others oviparous; the eggs of the latter are some- 
times enveloped with a shell more or less hard, sometimes with 
a simple viscosity. These varieties of the digestive and gene- 
rative 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 preserved by their fecundity and 
vital tenacity. 
DIVISION OF THE MOLLUSCA INTO SIX CLASSES. 
The general form of the body of the mollusca being in pro- 
portion to the complication of their internal organization, 
indicates their natural division. 
The body of some resembles a sac, open in front, containing 
the branchiz, 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, and which frequently support the 
branchial tissue. They constitute the PrEROPODA. 
Others, again, crawl by means of a fleshy disk on their 
belly, sometimes, though rarely, compressed into a fin, and 
i 
