MOLLUSKS 123 



constant current providing fresh water to breathe, and also drawing 

 into the shell the food of the oyster. The organisms serving as 

 food are caught in the slime covering the gills, and are swept into 

 the open mouth by the action of the cilia. Oysters are capable of 

 detecting sound, for they close immediately when a loud noise is 

 produced. 



The Surf Clam, (Mactra solidissima, Fig. SI). This is also 

 called the hen-clam. It extends from tlie Gulf of Mexico to Lab- 

 rador, and is 

 found on sandy 

 beaches from low 

 water to a depth 

 of about sixty 

 feet. The shell 

 is covered with a 

 horny, light- 

 brown skin, and 

 is not deeply fur- 

 rowed by lines of 

 growth. It grows 

 to be fully six 

 inches long and 



four wide The /="/;§-. ^// surf clam. From are. Voung and old. 



. . Annisquam, Mass. 



Siphon IS short 



and thick, while the foot is large, stout and muscular, and enables 

 the clam to dig rapidly into the sand. It does not construct a deep 

 burrow, however, but lives quite close to the surface, and is on this 

 account often cast ashore by storms. Its flesh is tough, and not 

 highly esteemed as food. 



The Soft-Shell Clam, (Mya arenaria, Fig. 82 J. This is often 

 called the "long clam"' or "nanninose." It is found in sandy or 

 muddy shores from the Carolinas to the Arctic, and also on the 

 Northern coasts of Europe. It is common in the sheltered banks 

 of bays and estuaries between tide limits, where it burrows in the 

 ground by means of its muscular foot, having its long, extensible 

 siphon pointing upward. When the tide is high the siphon is 

 extended so as to reach to the opening of the burrow, but it retracts 



