FIELD BOOK OF PONDS AND STREAMS 



clams" lying upon the bottoms of clear pools and brooks. All 

 of them have two shells made by the hardened secretion of the 



Fig. 259. — A. — A large mussel with its fleshy 

 foot pushed out into the mud. Arrows indicate 

 water passing in and out of the "neck" or siphon 

 (after Baker). B. — A "finger-nail clam" (Sphce- 

 ridcB), showing its delicate two-parted siphon 

 (modified after Baker). 



fleshy mantle (Fig. 261) beneath, and all have a soft more or 

 less hatchet-shaped foot which has given them the name 

 Pelecypoda or hatchet-footed. At the rear end of the body 

 is the tube-like siphon (Fig. 259) through which water passes 

 in and out of the cavity within the folds of the mantle. In 

 salt water clams the siphon is called the "neck," as in "little 

 neck clams". Neither clams nor mussels have heads but their 

 mouths are located at the end opposite the siphon, recogniz- 

 able from the outside by the greater width of the shell. 



Form and movements of the shell. — The two shells or valves 

 are joined by a spring-hinge at the back near the umbo (Fig. 

 260) and the closure is strengthened by hinge teeth. The 

 spring-hinge automatically opens the shells when they are 

 not pulled together by the animal. They are closed together 

 by means of two large adductor muscles whose opposite ends 

 are attached to opposing shells. Their attachment places 

 can be seen on the shell of any bivalve (Fig. 260). The 

 familiar "fried scallops" are the adductor muscles of the 

 scallop oysters. When a mussel relaxes its adductor muscles 

 they stretch and let the shells open automatically. This 

 occurs when a mussel extends its foot out between its shells 



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