THE PHYLA ARTHROPODA AND ONYCHOPHORA 



Fig. 15.2. A fresh-water crayfish in its natural habitat, 

 partially concealed under a stone. Note the evident 

 utilitv to the animal of the elone;ate, sensitive antennae and 

 antcnnules and of the movable compound eyes. (Photos;raph 

 b\- Charles W. Schwartz.) 



islands, such as Tasmania, New Zealand, and Madagascar. Their distribu- 

 tion is limited by certain environmental factors, notably the availability of 

 calcium carbonate in the water. Different species are adapted to various 

 habitats: some frequent rapidly flowing streams; others are found only in stag- 

 nant ponds or sluggish streams; still others occur in wet meadows and 

 marshes. Species of the genera Cambarus and Orconectes are widely dis- 

 tributed in the more temperate regions of North America east of the Rocky 

 Mountains, and several species of the genus Astacus are found in the streams 

 of the Pacific Slope. The account of the crayfish that follows is applicable to 

 the common species of Cambarus and related forms. 



In nature, the crayfish is found crawling upon the bottom or concealed 

 under stones (Fig. 15.2), or in burrows which many species excavate into the 



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