576 



EVOLUTION OF ANIMALS 



Part V 



:.;^\^ 



Fig. 29.5. Examples of the variety of small marine crustaceans. A, B, C; minute 

 copepods of the surface waters of the open sea with appendages used in swimming 

 and floating. D, an equally small copepod of the tide-pools which lacks any elab- 

 orate equipment for floating. Copepods compose an important part of the basic 

 food supply of surface sea waters. (Courtesy, MacGinitie and MacGinitie: 

 Natural History of Marine Animals. New York, McGraw-Hill Book Co., 1949.) 



homes, lobsters in coastal sea waters and crayfishes in ponds and streams, 

 most often in limy regions. As a representative crustacean either animal is 

 attractive for study. Crayfishes offer the advantages of being widely dis- 

 tributed and in relatively small demand for food and in general structure they 

 are but smaller editions of lobsters. In North America crayfishes of the genus 

 Cambarus are common east of the Rocky Mountains and Potamobius 

 (Astacus) west of them. 



Ecology of the Crayfishes. Crayfishes hide in dark places and forage about 

 on pool bottoms walking on their claws as if on tiptoes, their great pincers 

 held out in front for instant attack, like hands in a reception line. Some species 

 do not burrow, such as Cambarus bartoni, one of the common dwellers in 

 small clear streams. Cambarus diogenes is a well-known burrower in swamps, 



