CLAMS, OYSTERS, AND MUSSELS 



lis 



no shell or onl}' a rui.limentary one. A head bearing eyes 

 and tentacles is present, and the foot forms a large flat 

 disk with which the 

 animal creeps over 

 the ground. 



Snails. — The pond 

 snails, Physa and 

 Lininfea, are common 

 examples of fresh- 

 water snails. Their 

 habits are much ahke. 

 Each has a spirally 

 coiled shell (Fig. 61) 



and each breathes air F^G- 61. — Pond snail (Limncea). 



directl^v, b}' coming to the surface of the water and taking 

 the air in through a tube, the ojiening of which is just 



within the edge of the 

 shell. These snails 

 are vegetable feeders, 

 living upon bits of 

 \\'ater plants rasped 

 off with a long, flat, 

 fleshy band, or rib- 

 bon, situated in the 

 mouth. This band is 

 beset with sharp teeth 

 and is kno\Mi as the 

 lingual rihhcm. The 

 flat, triangular por- 

 tion of the body, b}^ 

 T. , wT,, X f . means of which the 



tlG. (i2. — Pond snau (Physa) : a, toot.; 



m, mouth. snails move, is the 



