CHAPTER XV 



THE CRAYFISH 



All night the crawfish deepens out her wells, 



As shows the clay that freshly curbs them round. 



J. P. Irvine, Summer Drought 



Habitat and Distribution. Crayfishes, also called craw- 

 fishes, are found in bodies of fresh water on every continent 

 except Africa, and on many of the large islands. All the 

 crayfishes of the United States east of the Rocky Moun- 

 tains belong to the single genus Cam'barus. On the Pacific 

 coast the crayfishes are slightly different. All of them be- 

 long to the genus As'tacus. An eastern American species 

 (Cam'barus limo'sus) is used as an example in this chapter. 

 In structure all members of the genus Cambarus are so 

 nearly identical that for a general laboratory study any 

 species available is entirely satisfactory. 



External Plan of Structure. The body, except for the ven- 

 tral surface of the abdomen, is covered with a thick wall, 

 formed, like the covering of insects, from the hardening of a 

 secretion of the outer layer of the skin. Unlike the insects, 

 this protecting sheath is filled with carbonate of lime. The 

 body is divided into a cephalothorax and abdomen (Fig. 83), 

 as in the arachnids. There are no indications on the dorsal 

 surface of separate somites in the cephalothorax, but on the 

 ventral surface transverse grooves and paired appendages 

 indicate a division into thirteen somites. The abdomen 

 (Fig. 83) plainly consists of seven somites, of which the 

 first six bear jointed appendages. The external plan upon 



which the crayfish is formed is similar to that of the insects 



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