268 



THE MOLLUSKS 



gives one basis for the support of these plants. The carbon dioxide from 

 the thousands of species of fish, mollusks, crustaceans, worms, and other 

 forms of animal life gives another source of raw food material for the 

 plant. The sunlight penetrating through the shallow waters supplies the 

 energy for making the food. Thus conditions are ideal for rapid multi- 

 pheation ; hence the water becomes alive with all kinds of plant life, 

 especially the lower forms. Among these plants are always found bac- 

 teria, both harmless and harmful. iMoUusks feed upon these plants, in- 

 cluding the bacteria ; man feeds on the mollusks, and, if he eats them raw, 

 may eat li\ing bacteria as well. Thus disease might result, and, as a mat- 

 ter of fact, epidemics of typhoid fever have been traced to such a source. 

 'Some Common Mollusks. — The fresh- water clam, a common resi- 

 lient in shallow water in inland ponds and ri^-ers, although not useful 



for food to man, has become 

 the source of a very impor- 

 tant industry. The making 

 of pearl buttons has so de- 

 pleted the number of adult 

 clams in our Middle West 

 that the state and United 

 States governments have un- 

 dertaken the study of the 

 life habits of these animals 

 with a view to restocking the 

 rivers. The development of 

 the fresh-water clam or 

 mussel is complicated. The egg develops into a free-swimming larval 

 form which fastens to the giUs of a fish and there hves as a parasite until 

 almost mature. Then it drops off and begins life in the sand of the river 

 or lake where it lives. 



"The Oyster. — The chief difference between the oyster and the clam 

 ''lies in the fact that the oyster is fastened by one valve to some soUd ob- 

 ject, while the clam or fresh-water mussel __ 

 moves about. This results in an asymmetry 

 in the shell of the oyster. 



Oysters are never found in muddy 

 looahties, for in such places they would be 

 quickly smothered by the sediment in the 

 water. They are found in nature cUnging 

 to stones or on shells or other objects which 

 project a little above the bottom. Here 

 food is abundant and oxygen is obtained from the water surrounding 

 them. Hence oyster raisers throw oyster shells into the water and the 

 young oysters attach themselves. 



In some parts of Europe and this country where oysters are raised ar- 



Shell of fresh-water clam, the left half polished to 

 show the prismatic layer from which buttons 

 are made. 



Shell of oyster, showing asym- 

 metry. 



