232 THE WEALTH OF THE SEA 



If all of these eggs grew to be mature oysters, there 

 would be no water in our harbors, for they would 

 be entirely filled with oysters. About the same time 

 that the female spawns her eggs, each male oyster 

 discharges billions of sperms into the water, which 

 swim around until they come in contact with an egg. 

 A few hours after fertilization of an egg, a swimming 

 embryo is formed. The tiny oyster develops rapidly, 

 soon growing shell valves. Even when the shell has 

 grown so that it covers the little oyster, the larva 

 is very, very small, being only about one four-hun- 

 dredth of an inch long. For the first two weeks of the 

 oyster's existence, it swims rapidly around, pro- 

 pelled by a fine hair which resembles a tail. At the 

 end of this period it attaches itself to some clean 

 solid object on the bottom, and loses its power to 

 swim. This stage in the oyster's life is known as the 

 spat or set stage. When the oysters are large enough 

 to be handled individually, they are called seed- 

 oysters. In Northern waters, these little oysters grow 

 to be market oysters in about four years, but in the 

 warmer waters of the South they grow much faster, 

 attaining market size in two years. 



The shell of the oyster is lined with a thin mem- 

 brane called the mantle, which is fringed all around 

 the edge and unattached along the margin. The 

 oyster possesses four gills which are excellent filters, 

 removing from the water the exceedingly small living 

 animals, chiefly diatoms and dinoflagellates, which 

 the shell-fish eats. The gills are covered on both sides 

 with very fine hairs arranged in rows, which beat back 

 and forth, causing a current of sea-water to pass 



