XVIII. CRUSTACEANS 



The Crayfish (Cambarus affinis). —Crayfish live in fresh- water 

 lakes and streams. There they may be caught under projecting 

 stones in clear streams by hand. From muddy streams they may 

 be taken by means of a weighted net, which is pulled along the 

 bottom. Although they prefer the water, they are sometimes 

 found at some distance from any large body of water; such 

 animals are supposed to be migrating. 



Crayfish; A., antennie; E., stalked eye; C.P., cephalothorax; .46., abdomen; C.F., caudal 

 fin; M., mouth; Ch., chelipeds. From photograph. 



The Structure and Activities of the Crayfish. — Living crayfish in dishes 

 of water should be provided for this exercise * Notice the color of the 

 living crayfish. In the natural habitat the colors blend with its surround- 

 ings, so that it is difficult to distinguish a crayfish from the bottom on which 

 it rests. The animal is thus said to be protectively colored. 



The body is composed of a series of rings or segments. Tliis fact is not 

 apparent at the anterior end of the body, where the head and mifldle region 

 (thoracic region) are covered by one piece of the skeleton. This is called 

 the cephalothorax. Count the number of segments in the abdomen (the 

 posterior region). This number is constant for every crayfish. 



The shell is a true exoskeleton, that is, it is formed by the skin. As in 



the exoskeleton of the insects, an animal material called chit in forms the 



basis, but in this case the skeleton is strengthened by the addition of lime. 



Test a piece of the shell with acid. What results? Is anything left 



* For full laboratory directions see Hunter and Valentine, Manual, page 121. 



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