PHYLUM MOLLUSCA 



263 



Economic Importance. Several of 

 considerable importance as food for 

 man. The most valuable are the 

 oyster and the long-neck or soft-shell 

 clam. Razor-shells, hen-clams, rnus- 

 sels, scallops, and a number of other 

 bivalves are also eaten. 



The oyster, Ostrea virginiana, in- 

 habits the shallow water along the 

 Atlantic coast from Massachusetts to 

 Florida. It is attached to rocks and 

 other objects by its left valve, and 

 does not move about in the adult 

 stage. The Chesapeake Bay oyster- 

 beds are large and important. The 

 value of the oyster industry along 

 the Atlantic seaboard is from twenty 

 to thirty million dollars annually. 

 Oysters lay an enormous number of 

 eggs. Professor Brooks placed the 

 number for a single female in one 

 season at nine million or more. 

 Those eggs which are fertilized and 

 not eaten by fishes and other animals 

 develop into free-swimming larvae 

 .which soon become fixed to some 

 object and grow into the adults. 

 The larvae are preyed upon by many 

 animals, especially crabs (Chap. XIII). 

 Those that reach the adult stage 

 may be attacked by starfishes 

 (p. 196), boring snails (p. 260), 

 sponges (p. 1 06), and parasites. 



The value of the pearl-button in- 

 dustry has already been mentioned 



the PELECYPODA are of 



FIG. 190. A ship " worm," 

 Teredo navalis, in a piece of 

 timber. P, pallets; SS, si- 

 phons ; T, tube ; U, valves 

 of shell. (From the Cam- 

 bridge Natural History, after 

 Mobius.) 



