54 MOLLUSCA. 
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 (Ostresde).—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 a while 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 
have also numbers of 
Fic. 57.—Pecten swimming, by violently 
opening and closing its valves. filaments that extend 
