54 MOLLUSC A. 



etc. The eyes, when present, are in the borders of the 

 mantle, and often resemble gems. The foot is a muscular 

 organ that projects from the mantle nearly opposite the 

 siphon, by which the animal leaps, moves, or glues itself to 

 the rock, as the case may be. The ear is in the foot a 

 transparent sac containing a clear fluid in which floats a 

 glassy globule. 



BIVALVES WITHOUT SIPHONS. 



Oysters (Ostreida). The oyster is found in great beds 

 upon the coasts of many countries. About 425 different 

 living species are known, and over 1,400 fossil, some of 

 the latter being of gigantic size. A single oyster will de- 

 posit during July and September over 1,000,000 yellow 

 eggs. At first they remain in the gills, but finally leave 

 the parent, and, after swimming about for awhile by means 

 of cilia, during which their numbers are greatly dimin- 

 ished, they finally settle upon the bottom, and in five or 

 six weeks are as large as a grain of corn, three years, how- 

 ever, being required to attain full growth. They generally 

 lie upon their sides, and are often inhabited by one or 

 more small crabs {Pinnotheres). 



VALUE. Seven hundred and sixty-five million oysters are handled 

 yearly in New York alone, representing a capital of $1,577,000, the 

 industry giving employment to thousands. 



Comb Shells (Pectens). The Pectens are generally 



round, with radiating 

 ribs from the valves, and 

 are noted for their loco- 

 motive powers, and the 

 luster and brilliancy of 

 their gem-like eyes that 

 dot the mantle. They 



FIG. 57-Pecten swimming, by violently have also numbers of 

 opening and closing its valves. filaments that extend 



