228 



ZOOLOGY 



The fresh-water mussels (Anodonta and Unio) are extremely 

 common in the ponds, lakes, and rivers of North America. 

 Anodonta is more apt to be found in still waters and Unio in 

 running waters. They lie partly buried in the mud of the 

 bottom, with the valves of the sheU gaping open and the 

 partly united edges of the mantle protruding. They do not 

 lead wholly sedentary lives, but may burrow or plow along 

 the bottom. 



B. 



Fig. 217. — A, advanced embryo Anodonta; B, free Glochidium ; /, pro- 

 visional byssus ; s, shell ; s/i, hooks ; sm, adductor muscle ; so, sense organs ; 

 to, cilia. From Korschelt and Heider's " Embrj'ology." 



Their food is gained from organic particles borne along 

 in the current of the water. A part of this current is carried 

 through the mantle chamber and forced out again, deprived 

 of usable food and of the oxygen used in respiration. By 

 devouring organic matter these clams act as useful scavengers 

 of the water. On account of' the sessile habits of Unios they 

 can hardly go in search of their food, and so it is necessary 

 that the}' should live in a current of water which will bring 

 them food. 



The family of Unionidae, to which ITnio and Anodontia 

 belong, is of world-wide distribution, but nowhere else are 



