27 



ior the most part v-onliiu'd to llie coiiii)arativt'ly shallow, brackish 

 water ak)ng the shore. As previously mentioned, the diatoms 

 are most abundant in localities where tliere is a soft, muddy 

 bottom. 



^lost of the diatoms are motile, and during the day, under 

 the influence of the sunlight, they migrate from the bottom 

 toward the surface of the water, settling down to the bottom 

 again at night. During the periods of migration they are the 

 most abundant in the water above the oyster beds, and it has 

 been shown that the oysters procure the most of their food at 

 this time. During the night, when the diatoms are at the 

 bottom, they undergo repeated divisions, each into two new 

 individuals, so that the numbers are ver\' rapidly increased. 



The manner in Avhich the oyster obtains its food has been 

 previously mentioned in the chapter on anatomy. 



While there are several forms of diatoms always found in 

 the stomach contents of oysters from any locality, a few of 

 these are so much larger than the others that in order to get 

 an adequate idea of the richness of the food supply it is neces- 

 sary to determine the numbers of the larger forms only. 



The warm season in the summer and fall is the time when 

 the diatoms are the most abundant and when the greatest growth 

 of the oysters takes place. The fact that the oysters do not 

 usually get fat until rather late in the fall is because earlier 

 in the season their energies are expended in the production of 

 the sexual products, and the fattening takes place as soon as 

 this drain on their vitality is over. 



ENEMIES. 



In all stages of its development the oyster is preyed upon 

 by numerous, and more or less destructive, foes. In the adult 

 condition the presence of the rather thick, limy shells affords 

 protection against the weaker enemies; but in the earlier stages 

 of its growth it is entirely without any means of protection and 

 is preyed upon l\v many small forms. The miinite eggs and fry 

 are swallowed in immense numbers by the adult oysters, other 

 molluscs, worms, sponges, hydroids, etc. Fishes, like the men- 

 haden, which get their food l)y straining out the minute particles 

 from the water, doubth^ss cDiisnnie larL'o nuni])ei-s of the free- 

 swimming young. 



