374 



ESSENTIALS OF ZOOLOGY 



water. At the posterior end of the shell are two openings: the 

 ventral siphon which pulls in food and water, and the dorsal siphon 

 through which wastes and deoxygenated water are eliminated. 



Movement is varied among the pelecypods. Scallops may move 

 rapidly by suddenly contracting the valves, thus ejecting a jet of 

 water. Oysters are motile in their larval stages but in the adult 

 stage are attached to rocks and other objects. Many marine mus- 

 sels are attached to objects on the bottom or along the shore. At- 

 tachment is made possible by the dissolution of a part of the under 

 valve and adherence of a portion of the body thus exposed. 



Anodonta 

 Jtewort/ana 



Quadrula 

 forsheyi 



Proptera 

 purpurakQ 



Amblema 

 costata 



Leptod(?a 

 fracjilis- 



Canjnculina ^Musojlium 

 texasensis remssi 



Fig. 142. — Some common fresh -water bivalves. 



The life span of clams may be relatively long. It has been esti- 

 mated that Anodonta, one of our common genera of fresh-water 

 clams, attains its maximum growth in twelve to fourteen years. 



External Features 



Shell. — Unlike the snail whose shell is of one piece, the clam shell 

 is composed of two parts called valves (hence, bivalves) which are 

 attached together at the dorsal surface by a hingelike ligament. 

 The oldest part of the shell is the umbo which is usually a rounded 

 protuberance near the top of the valves and is frequently eroded 



