MOLLUSKS 121 



inliabitants of tlie sea. They differ greatly in size and shape, vary- 

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

 as in the giant clam (Tridaena gigasj of the tropical Pacific and 

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

 with the sinuous gape of its shell upperaiost. It commonly remains 

 with its valves slightly open, showing the 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 twenty pounds and, although coarse, is sometimes eaten by 

 the natives. 



Excellent photographs showing these clams partially exj^osed 

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

 Reef of Australia," Plate XXIX. 



The American Oyster, (Ostrea virginiea, Fig. SoJ, is found 

 from the Crulf of St. Lawrence to Texas, but is most abundant in 

 Chesapeake and Delaware Bays and in Long Island and Pamlico 

 Sounds. It thrives best in shallow bays and estuaries where the 

 water is apt to be brackish. Our oyster fisheries are worth 

 $10,600,000 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 

 we will 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 miiscle that extends across from 

 both sides of the oyster's body and hinds the valves together. 

 This strong adductor muscle is commonly but erroneously called 

 the "heart." In the American oyster the place of attachment of 

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

 while in the common Ostrea edulisoi Europe this area is uncolored. 

 When undisturbed, and under normal conditions oysters commonly 

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

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

 grows otit 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 



