PHYLUM MOLLUSCA 



249 



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. 



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. 



, . , Liqamentous hinqe Umbo 

 Ventral siphon -^ j^ ^^p„_^»-i^^ ^ 



Dorsal siphc 



, , Growth linns 



foot 



Anterior 

 protractor 

 &. retractor 

 muscle 



Pallial \inz 6 



Fig. 141. — External (A) and Internal (B) shell features of Lampsilis anodontoides. 



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

 protuberance near the top of the valves and is frequently eroded 

 due to carbonic acid in the water. Extending out from the umbo on 

 each valve in a concentric manner are the growth lines of the shell, 

 evidenced as slight, medium, or heavy ridges. 



