178 



GENERAL ZOOLOGY 



rasping-tongue, running it in and out, to tear into bits the 

 soft plant tissues on which it feeds. A most interesting 

 experience is to watch a pond-snail as it crawls slowly up 





FIG. 91. Group of Living Pond-Snails. Natural size 



the side of an aquarium, feeding on the microscopic plants 

 as it goes. At such times one can see the mouth open and 

 the tongue sweep gracefully out and in, with tiny, rhythmic 

 movements. 



