THE OYSTER. 73 



firm enough to keep it out of the soft mud, but near 

 enough to it to be within easy reach of the vast supply 

 of food which it affords. As a fixed animal does not 

 need to have the two sides of its body balanced, the 

 fixed oyster has become one-sided, and has thus been 

 still better fitted for its peculiar mode of life. 



These changes, while they are on the whole ad- 

 vantageous, since they enable the oysters to avail them- 

 selves of inexhaustible supplies of food, are not without 

 disadvantage. The oyster has become so perfectly 

 adapted for a life on those hard bodies which occur 

 in the soft mud of estuaries, that it cannot live any- 

 where else, and the young oysters that do not find a 

 proper home soon die. In shallow bays and sounds 

 hard substances are rare and far apart, and many 

 young oysters must perish from inability to find a 

 proper resting place. To meet this danger the oyster's 

 birth rate has been enormously increased, so that 

 among its innumerable descendants some few may be 

 able to find proper homes, and may grow up to maturity 

 in their turn. 



