IO2 



ANIMAL BIOLOGY 



worm. Pearls may be formed by various kinds of mol- 

 luscs, and in some places pearl fisheries form an important 

 industry. 



Several species of clams found on the seashore are much 

 used as food. One of these, Mya arenaria, the common 

 long-neck clam, is obtained by digging in muddy beaches 

 at low tide. The two joined siphons in this species con- 

 stitute a long tube which projects upward as the clam 

 lies buried in the mud. When the clam is disturbed it 



frequently reveals its presence 



by squirting water out of its 

 siphon as it closes the valves of 

 its shell. 



Mussels are generally found 

 upon rocks to which they at- 

 tach themselves by a series of 

 threads called the byssus which 

 is secreted by a gland in the foot. 

 The common scallop, Pecten, 

 has the somewhat unusual habit 

 of swimming by alternately opening and closing the valves 

 of the shell. The most important bivalves are unquestion- 

 ably the oysters which are extensively cultivated in various 

 parts of the world. In its early stages the oyster is a free- 

 swimming larva; later it settles down and becomes at- 

 tached by the left valve of its shell. Oysters are planted 

 and cultivated in oyster beds. These beds are especially 

 numerous in Chesapeake Bay which has provided over 

 25,000,000 bushels of oysters a year. A very aberrant bi- 

 valve and one which looks more like a worm than a mollusc, 

 is the Teredo which has the habit of boring into the piles of 

 wharves and bottoms of wooden vessels where it does a 

 great deal of damage by riddling the wood with its holes. 

 One large division of the Mollusca, the Gastropoda, is 



FIG. 85. A scallop shell, 

 Pecten. 



