i8o 



Aquatic Organisms 



Molluscs — ^A large part of the population of lake and 

 river beds, shores, and [pools is made up of molluscs. 



They cling, they 

 climb, they bur- 

 row, they float — 

 they do every- 

 thing but swim in 

 the w^ater. They 

 are predominantly 

 herbivorous, and 

 constitute a large 

 proportion of the 

 producing class 

 among aquatic 

 animals. Two great 

 groups of molluscs 

 are common in 

 fresh water, the 

 familiar groups of 

 mussels and snails. 



Fresh-water mus- 

 sels (clams, or 

 bivalves) abound 

 in suitable places, 

 where they push 

 through the mud 

 or sand with their 

 muscular protrusi- 

 ble foot, and drag 

 the shell along in 

 a vertical position 

 leaving a channel- 

 like trail across the bottom. They feed on micro-organ- 

 isms. 



The two commonest sorts of fresh-water mussels are 

 roughly distinguished by size and reproductive habits 



Fig. 88. A living mussel, Atiodonta, with foot 

 retracted and shell tightly closed. A copious 

 growth of algae covers the portion of the 

 shell that is exposed above the mud in loco- 

 motion: the remainder is buried in oblique 

 position with the foot projecting still more 

 deeply into the mud. 



