THE ECONOMIC IMPORTANCE OF ANIMALS 199 



Several other common crustaceans are near relatives of the crayfish. 

 Among them are the shrimp and prawn, thin-shelled, active crustaceans 

 common along our eastern coast. In spite of the fact that they form a 

 large part of the food supply of many marine animals, especially fish, 

 they do not appear to be decreasing in numbers. They are also used 



as food by man, the shrimp fish- 



eries in this country aggregating 

 over $1,000,000 yearly. 



Another edible crustacean of 

 considerable economic impor 

 tance is the blue crab. Crabs 

 are found inhabiting muddy bot 

 toms ; in such localities they are 

 caught in great numbers in nets 

 or traps baited with decaying 



The edible blue crab. (From a photograph 

 loaned by the American Museum of 

 Natural History.) 



meat. They are, indeed, among 

 our most valuable sea scavengers, 

 although they are carnivorous 

 hunters as well. The young crabs differ considerably in form from the 

 adult. They undergo a complete metamorphosis (change of form). 

 Immediately after molting or shedding of the outer shell in order to grow 

 larger, crabs are greatly desired by man as an article of food. They are 

 then known as " shedders," or soft-shelled crabs. 



Mollusks as Food. Oysters are never found in muddy localities, for in 

 such places they would be quickly smothered by the sediment in the 

 water. They are found in nature clinging to stones or on shells or other 

 objects which project a little above the bottom. Here food is abundant 



and oxygen is obtained from the water sur 

 rounding them. Hence oyster raisers throw 

 oyster shells into the water and the young 

 oysters attach themselves. 



In some parts of Europe and this country 

 where oysters are raised artificially, stakes 

 or brush are sunk in shallow water so that 

 the young oyster, which is at first free- 

 swimming, may escape the danger of smothering on the bottom. After 

 the oysters are a year or two old, they are taken up and put down in 

 deeper water as seed oysters. At the age of three and four years they 

 are ready for the market. 

 The oyster industry is one of the most profitable of our fisheries. Nearly 



The oyster. 



