SYSTEMATIC DISTRIBUTION. 73 



II. Conchifers* — Having the mouth placed between 

 the gills, they and the body enclosed between the 

 two leaves of the mantle, which are covered with 

 two shelly valves united by a cartilage. 



B. Destitute of any foot. 



HI. BracMopodes. — Having the mouth placed at 

 the base of two spirally twisted ciliated arms, 

 between the two leaves of the mantle, which are 

 covered with two separate shelly valves : they 

 live attached to other marine bodies. 



IV. Pteropodes. — Having a prominent head, with 

 one or two pairs of fins on the sides of the neck, 

 by which they swim about in the ocean. The body 

 is often covered with a thin glassy conoidal shell. 



V. Cephalopodes, which have a large distinct head, 



furnished with eight or ten arms, by means of 

 which they walk head downwards. 



Linnaeus refers all the animals inhabiting shells 

 to five different genera ; viz., Limax, Ascidia, Alio- 

 mia, Clio, and Sepia. These genera may be re- 

 garded as the types of the classes proposed by Cuvier. 

 Poli had, before his time, considered three of them as 

 orders, under the names of Mollusca Reptantia, Sub- 

 silientia, and Brachiata. (i. 27.) 



The terrestrial or fluviatile Mollusca, of which alone 

 we have to treat in this little work, are confined to the 

 two first of these classes. 



The shell, which is peculiar to this division of the 

 animal kingdom, may be seen covering the young 

 animal in the egg, before it has gained all its organs, 



