6S THE OYSTER. 



microscopic life of the mud-flats. The gills then 

 gradually became modified and fitted for maintaining 

 the circulation of water, and for filtering out the 

 minute food particles it contains. 



Food is most abundant on the muddy bottom, but 

 in estuaries this is so deep and soft that a locomotor 

 animal would sink and smother in it, so the oyster 

 has gradually become converted into a fixture, and 

 has learned to fasten itself when young to something 

 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 

 themselves 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 

 anywhere else, and the young oysters which 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. 



