OYSTERS 471 



young which hatch from fertilized eggs are totally unlike the 

 parent in appearance and habit; they have no shells and 

 swim about on the water. In time they secrete a thin 

 shell and sink. If they fall upon a muddy bottom, they are 

 smothered, but if they fall upon a gravelly, stony, or rocky 

 bottom, they live and grow and within a few years become full 

 grown. As the soft part of the young oyster grows the shell 

 also grows and is always large enough to cover the defenseless 

 creature within it. In spite of the fact that the female oyster 

 lays millions of eggs yearly, the supply of oysters is too small 

 to satisfy the demand for them. This is because many are 



FIG. 341. Oyster trays. (From Brooks's Oyster.) 



smothered in muddy bottoms, many are killed by overcrowd- 

 ing, and many more are attacked by enemies which destroy 

 them in spite of their hard shell. In order to increase the sup- 

 ply, oysters are raised artificially on some parts of the coast. 

 Large beds of stones, stakes', or trays (Fig. 341) are built at the 

 bottom of the water to prevent smothering, and care is taken 

 that the growing oysters are not overcrowded and are not dis- 

 turbed by enemies. 



Clams are soft-bodied animals protected by shells, and are 

 similar to oysters. They are eaten extensively in the United 

 States. Clams do not remain fixed in one place as do oysters, 



