PHYLUM MOLLUSCA 



2 59 



naa, and Planorbis. Physa (Fig. 185, D) lives in ponds and 

 brooks and feeds on vegetable matter. It is a sinistral snail, 

 since if the shell is held 

 so that the opening faces 

 the observer and the spire 

 points upward, the aper- 

 ture will be on the left. 

 Lymncea (Fig. 185, G) is 

 the common pond-snail. 

 Its shell is coiled in an 

 opposite direction from 

 that of Physa and is 

 called dextral. Both 

 Physa and Lymncea usu- 

 ally come to the surface 

 to breathe. In dry weather many snails have the power of se- 

 creting a mucous epiphragm over the mouth of the shell so as to 



Fig. 184. — Limax maximus. 

 monary orifice. (From the 

 Natural History.) 



PO, pul- 

 Cambridge 



Fig. 185. — The shells of certain Gastropoda. A, Helicodiscus parallelus. 

 B, Planorbis trivolvis. C, Polygyra albolabris. D, Physa gyrina. E, Pleuro- 

 cera elevalum. F, Goniobasis liviscens. G, Lymncea paluslris. (From varioug 

 authors.) 



