Chapter Eleven 



MOLLUSCA 



Mollusks, Shellfish 



1. PELECYPODA Clams, oysters, etc. 



2. GASTROPODA Sea snails, etc. 



3. CEPHALOPODA Squid, Octopus, etc. 



PELECYPODA 



The name Pelecypoda means hatched-foot and 

 refers to the type of foot usually possessed by the 

 mollusks of this group. The more popular name, 

 bivalves, is often applied to the group and refers to 

 the fact that the animals have two shells. 



The muscles that the bivalve uses to open and 

 close its shell are known as the aductor muscles and 

 are often very strong; the scars where these muscles 

 are attached to the shells are sometimes conspicuous 

 even after the muscle itself has disintergrated. 



The two shells of the bivalve are held together 

 by the hinge ligament, a tough leathery substance 

 which often is lost when the shell is cast upon the 

 beach, and usually by one or more pairs of inter- 

 locking teeth. The arrangement of these teeth is 

 often of paramount importance in identifying the 

 families and genera of bivalves. 



The foot, as indicated by the name Pelecypoda, 

 is usually hatchet shaped. In some species the foot 

 is used for slow locomotion along the ocean bottom, 



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