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- 

 plication ; 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. Mollusks feed upon these plants, in- 

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

 may eat living 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- 

 dent in shallow water in inland ponds and rivers, 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 

 Shell of fresh-water clam, the left half polished to with ft yiew to restocki the 



show the prismatic layer from which buttons . mi , , 



are made 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 gills of a fish and there lives 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 solid 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 



localities, for in such places they would be 



quickly smothered by the sediment in the 



water. They are found in nature clinging 



to stones or on shells or other objects which 



project a little above the bottom. Here 



Shell of oyster, showing asym- 

 metry. 



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- 



