Pelecypoda. 



125 



and weak. The mussel is found attached to rocks by means of a num- 

 ber of yellowish threads, the byssus, which grow from the base of the 

 small foot. Mussels are found widely distributed along the coasts in 

 most seas. They are used to a considerable extent as food in some 

 countries. 



The Giant Clam. Probably the largest bivalve known is a marine 

 clam of the genus Tridacna, found in Eastern seas. The soft body 

 sometimes weighs twenty pounds, and the two valves of the shell 

 together may weigh five hundred pounds. 



The Scallop. The outline of the shell as seen from one side is 

 nearly circular. The two valves are not equal, one being less convex 



FIG. 78. SCALLOP. 



The crusaders' badge. 



than the other, sometimes perfectly flat. While at rest the scallop lies 

 on the bottom with its valves widely gaped open. The scallop has a 

 row of eyes along the margin of each mantle lobe. When an enemy 

 approaches, the shell is quickly and powerfully shut by the one strong 

 adductor muscle. This forcibly ejects the water, and by reaction the 

 scallop is driven through the water, hinge foremost. The foot is rudi- 

 mentary or lacking. The scallop is used for food ; the adductor muscle, 



