THE CLAM AND OTHER BIVALVES 199 



lars are yielded annually by the oysters and their products. 

 On the coasts of Holland, Belgium, and France far greater 

 care is taken of their species (Ostrea ed'ulis) than we take 

 of ours ; but the natural conditions here are superior to the 

 natural conditions there. 



Oysters when subjected to sewage contamination may 

 become carriers of typhoid fever. When grown and mar- 

 keted under sanitary conditions they are not only harmless 

 but a very wholesome food. 



Relation to Environment. According to the pioneer in- 

 vestigations of Professor Brooks and others, oysters are to 

 be found most abundantly in the quiet, semi-stagnant water 

 of shallow inlets. Into such inlets slowly flowing creeks 

 enter, giving to the water of the inlet a brackish quality. 

 When food consisting of microscopic plants and animals is 

 carried to the oyster by the natural currents in the water, it 

 may enter at any point between the separate folds of the 

 mantle. Cilia on the inner surface of the mantle folds, and 

 on the four gills, sweep the minute organisms forward to the 

 mouth, which lies near the hinge. The four palps aid in the 

 process. In brackish water the most important food organ- 

 ism of the oyster multiplies in vast, invisible hordes. These 

 organisms are plants called diatoms. Diatoms live in the 

 soft mud at the bottom and are carried bj^ the water cur- 

 rents within range of the cilia in the oyster's mantle folds. 



In times of storm the home of the oysters' food may be- 

 come a source of great danger to them. Once covered with 

 mud or with shifting sand, the life of a bed of oysters is at an 

 end. At the mouths of rapidly flowing rivers no oysters 

 are to be found, chiefly because the silt (fine sediment) and 

 the debris of decaying plants are unfavorable to the growth 

 of the animal. 



Aside from the physical agencies which are favorable or 

 unfavorable to oysters, there are many animals which come 



