PHYLUM MOLLUSCA 



259 



ncea, 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. PO, pul- 

 monary orifice. (From the Cambridge 

 Natural History.) 



B 



FIG. 185. The shells of certain GASTROPODA. A, Helicodiscus parallelus. 

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

 cera elcvatum, F, Goniobasis liviscens. G, Lymncea palustris, (From various 

 authors.) 



