MOLLUSKS 121 



inhabitants of tlie sea. Tliey differ greatly in size and shape, vary- 

 ing from microscopic dimensions to four and a half feet in width, 

 as in the giant clam fTridaana gigas) of the tropical Pacific and 

 Indian Ocean. This great clam Vixen half buried in the coral reefs 

 with the sinuous ga]ie of its shell uppermost. It commonly remains 

 ^vitli its valves slightly open, showing tlie rich, blue edges of 

 the mantle, but if the tide retreats or if it be suddenly disturbed 

 the valves close tightly. The animal, exclusive of its shell, often 

 weighs twent}' pounds and, although coarse, is sometimes eaten by 

 the natives. 



Excellent photographs showing these clams partially exposed 

 at low tide are given by W. Saville-Kent in " The Great Barrier 

 Reef of Australia," Plate XXIX. 



The American Oyster, ( Ostrea virginica, Fig. SO), is found 

 from the Gulf of St. Lawrence to Texas, Isut is most aljundant in 

 Chesapeake and Delaware Bays and in Long Island and Pamlico 

 Sounds. It thrives best in shallow baj's and estuaries where the 

 w^ater is apt to be brackish. Our oyster fisheries are worth 

 110,000,000 annually. 



Oysters usually lie upon their sides, and the lower valve fas- 

 tens itself to a rock or other firm anchorage by the shelly secretion 

 of the mantle. This lower shell is almost always upon the left side 

 of the body, and is quite deep and convex, while the upper shell 

 is much flatter or even concave. At the narrow end of the shell 

 Ave Avill see a dark brown, elastic hinge, which is so arranged 

 that it tends constantly to cause the two valves of the shell to open, 

 but this is prevented by a strong muscle that extends across from 

 laoth sides of the oyster's body and binds the valves together. 

 This strong adductor muscle is commonly but erroneously called 

 the "heart." In the American oyster tJie place of attadiment of 

 this muscle to the shell is marked by a dark brownish-purple area 

 while in the common Ostrea edulis oi Europe this area is uncolored. 

 When undistTirbed, and under normal conditions oysters commonh' 

 remain with their shells gaping slightly open. The inside of the 

 shell is lined with a delicate membrane called the mantle, which 

 grows out from both sides of the mid-dorsal line of the body and 

 secretes the shell itself. 



There are also a pair of gills which arise from the sides 



