CHAPTER XVII 



THE MUSSEL OR CLAM AND OTHER BIVALVES 



Habitat. — When we inquire into the details of everyday life 

 of animals, we soon learn that a struggle is all the time taking 

 place between each individual and others of its kind, between it 

 and individuals of other species, and between it and its physical 



Fig. 82. — Digging soft-shell clams on a mud-flat. (From Davenport.) 



and chemical surroundings. But there are some animals that 

 live in the same general habitat that seem to get along together 

 peacefully. Two examples are the crayfish and the fresh-water 

 mussel or clam. Both inhabit the same ponds or streams and 

 may live within a few inches of each other on the bottom; both 

 must live in water containing calcium carbonate from which part 

 of their shells is built up. Perhaps they live in peace because the 

 crayfish hides under a rock while the mussel plows through the 



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