234 MOLLUSCA. 



Mollusca; here the stomachs are simple, there multiple, and are 

 frequently provided with a peculiar armature. 



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 vital tenacity. 



Division of the Mollusca into Six Classes. 



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 loco- 

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

 neck, and which frequently support the branchial tissue. They con- 

 stitute the Pteropoda. 



Others again crawl by means of a fleshy disk on their belly, some- 

 times, though rarely, compressed into a fin, and have almost always a 

 distinct head before. We call these the Gasteropoda. 



A fourth class is composed of those where the mouth remains hidden 

 in the bottom of the mantle, which also incloses the branchiae and 

 viscera, and is open either throughout its length, at both ends, or 

 at one extremity only. Such are our Acephala. 



A fifth comprises those which, also inclosed in a mantle and without 

 an apparent head, have fleshy or membranous arms, furnished with 

 cilia of the same nature. We term these Brachiopoda. 



Finally, there are some which, although similar to the other 

 Mollusca in the mantle, branchiae, &c., differ from them in numerous 

 horny and articulated limbs, and in a nervous system more nearly 

 allied to that of the Articulata. They will constitute our last class, or 

 that of the Cirrhopoda. 



