THE MOLLUSCA 



973 



traction close the shell. They are destitute of jaws or radula, and 

 the cephalic region is furnished only with a pair of labial palps on 

 each side. They feed by ciliary action and breathe by gills sus- 

 pended on each side of the body. The digestive system consists 

 of a stomach, a liver, and a more or less convoluted intestinal canal 

 with its oral and anal extremities at the opposite ends of the body. 

 The edges of the mantle lobes in the fresh-water forms are usually 

 united between exhalent and inhalent orifices, and in some families 

 the posterior margins are extended in one or two siphons. The 

 foot is ventral, usually compressed, hatchet-shaped, and adapted 

 for burrowing. The nervous system consists of three principal 

 groups of ganglia (cerebral, pedal, and visceral), united by nerves. 

 They are monoecious or dioecious. 



The following diagram represents the classification of the fresh- 

 water Gastropoda as thus briefly outlined. 



Class 



Subclass 



GASTROPODA 



Streptoneura. 



r 71 



Order Pectinibranchia Aspidobranchia 



Suborder Taenioglossa Rhipidoglossa 



Superfamily Plat 



ypoda 



Euthyneura. 



Pulmonata. 

 Basommatophora. 



Viviparidae Neritidae Lymnaeidae Siphonariidae Auriculidae. 



Ampullariidae Planorbidae Gadiniidae 



Valvatidae Ancylidae 



Assimeniidae Physidae 



Amnicolidae 



Pleuroceratidae 



THE RADULA 



As the radula of the gastropod mollusca is very important as a 

 basis for classification, the following series of typical forms is given, 

 which should be used in connection with the key. 



