214 



GENERAL ZOOLOGY 



erable importance in locomotion; in fact, movement is not 

 possible without it. 



Physa is able to vary its density to some extent by pulling 

 the body forward or backward to increase or decrease the 

 air space at the apex of the shell and can therefore rise to the 

 surface of the water or fall to the bottom without being in 

 contact with any solid object. This permits it to spin 

 threads of slime in all directions between the aquatic plants 

 (Fig. 86). The slime threads are sticky and act as traps, 



A B C E F 



Fio. 86. Activities of the snail, Physa. A, eggs in jelly attached to a plant 

 stem; B, dropping from surface; C, taking a breath through the surface film; 

 D, resting; E, crawling on slime thread; F, crawling on bottom. 



catching minute plants and animals. After they have been 

 in the water for a time, Physa eats them to secure the ac- 

 cumulated food. They are also made use of as roadways in 

 passing from one part of a pond to another. 



Physa is largely vegetarian in its food habits. Aside 

 from the minute particles captured by the slime threads, it 

 gnaws away parts of plants with its rasp-like tongue, and 

 eats the scum off the surface of the water. The tongue, or 

 radula, of a snail is a very complicated mechanism, with a 



