162 



PRACTICAL ZOOLOGY 



Fresh-water Snails. The land snails and slugs belong to the 

 order Pulmonata, but in this order are also included the fresh- 

 water snails that are so common in ponds and sluggish streams. 

 Pond snails are very easily collected and kept in aquaria. 

 Their habits and structures differ but slightly from their ter- 

 restrial relatives. They are not truly aquatic, since they must 



FIG. 92. Shells of common snails. 



A, helicodiscus ; B, planorbis; C, polygyra ; D, physa ; E, pleurocera ; 

 F, goniobasis ; G, lymnaea. (From various authors.) 



come to the surface from time to time to breathe. Often threads 

 of mucus are formed which extend from the bottom to the sur- 

 face of the water, up which the snails travel when they wish a 

 fresh supply of air. The shells of pond snails are less liable to 

 injury than those that live on land and are correspondingly 

 thinner. 



Three common fresh-water snails are Physa, Lymncea, and 

 Planorbis. Physa (Fig. 92, 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 



