970 FRESH-WATER BIOLOGY 



I 



Fig. 1404. The shell of a bivalve. 



1. Hinge. 6. Scar of protractor pedis. 



2. Beak. 7. Pallial line. 



3. Pseudo-cardinals. 8. Scar of posterior adductor. 



4. Scar of anterior retractor. 9. Scar of posterior retractor. 



5. Scar of anterior adductor. 10. Lateral teeth. 



Classification 



Of the several classes into which the subkingdom of the MoUusca 

 is divided, but two, the Gastropoda and Lamellibranchia, are 

 represented in the fresh-water fauna of North America. The for- 

 mer includes all the univalve species commonly called snails or 

 periwinkles, and the latter, all the bivalve forms usually known as 

 clams or mussels. 



The class Gastropoda, as the name implies, are moUusks in which 

 the ventral portion of the body is developed into a fleshy, more 

 or less expanded, creeping disk, called the foot, by the muscular 

 contractions of which the animal progresses. 



When fully expanded, the animal is seen to have a distinct head, 

 with a pair of tentacles, at the base of which are placed the eyes. 

 In the center of the head, below and between the tentacles, is the 

 mouth, in which, on the upper surface, are situated the jaws, from 

 one to three, and, on the lower side, the lingual ribbon or radula, 



