168 GENERAL ZOOLOGY 



the markets of the large cities for these " esculent bivalves." 

 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. 



Relation to Environment. According to the investigations 

 of Professor Brooks, Professor Ryder, and others, oysters are 

 to be found most abundantly in the quiet, semistagnant 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-organism 

 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 by the water-currents 

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



In times of storm the home of the oysters' food may become 

 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 un- 

 favorable to oysters, there are many animals which come into 

 definite and usually unfavorable relations to them. Only one 

 of these animals, so far as known, is anything but harmful 

 to the oyster. That one is a little crab, about 13 millimeters 

 (^ inch) wide, which spends its life in the mantle-cavity of 

 its messmate. Trie greatest enemy of the adult oyster is the 



