CLAMS, UYSSTERS, AND MUSSELS 



115 



no shell or only a rutlimentary 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 pontl 



snails, Physa and 



Linmsea, are common 



examples of fresh- 

 water snails. Their 



habits are much alike. 



Each has a spirally 



coiled sheU (Fig. 61) 



and each breathes air Fig- 6i.— Pond snaii (Umnwa). 



directly, by coming to the surface of the water and taking 



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



within the edge of the 

 shell. These snails 

 are vegetable feeders, 

 living upon bits of 

 water plants rasped 

 off with a long, flat, 

 fleshy band, or rib- 

 l3on, situated in the 

 mouth. This band is 

 beset with sharp teeth 

 and is known as the 

 lingual ribbon. The 

 flat, triangular por- 

 tion of the body, by 

 „ , .,,„,, , ^ means of which the 



Fig. 62. — Pond snail (Physa) : a, foot ; 



m, mouth. snails move, is the 



