FIELD BOOK OF PONDS AND STREAMS 



important eating tool, a flexible file by which they rasp off 

 the cells of delicate plant stems and clean the alg^ from stones 

 and leaves. It is covered with small teeth and hooks, differ- 

 ently shaped and arranged in various species. The diversity 

 of the teeth in different kinds of snails forms the basis for a 

 large part of their classification. 



attoLcheJ 

 to mouth 



Fig. 241. — I, Diagram of snail; 2, jaws; 3, upper 

 surface of tongue or radula. 



Snails glide slowly about on their one large muscular foot 

 over the bottom of which wave-like contractions of muscles 

 constantly pass from front to rear. These quickly succeeding 

 waves can be clearly seen on the feel; of snails as they move 

 over the sides of an aquarium or beneath the surface film of 

 water (Fig. 242). 



Fig. 242. — Pond snail, showirig the under surface 

 of the foot against a glass and (dark bands) the suc- 

 ceeding waves of contraction in the muscles. 



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