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CHAPTER Xll 

 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 crawfishes, 

 are found in bodies of fresh water on every continent except 

 Africa, and in many of the large islands. The eastern 

 American species, Cam'barus af 'finis (Fig. 67), which grows to 

 the length of live or six inches, may be taken as an example 

 of the several genera and the m'any species which are known 

 by the common name crayfish. 



External Plan of Structure. The body, except for the 

 ventral 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. 67, 1, 2), 

 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. 67, 2) 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, 

 arachnids, and myriapods ; that is, a series of somites placed 

 one after the other, with all appendages jointed or segmented. 



The Cephalothorax. The shell covering the dorsal and 

 lateral surfaces is distinct from the hard parts elsewhere on 

 the body, and is termed the carapace. At the anterior dorsal 



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