THE CRAYFISH 131 



EXOSKELETON. The shell or exoskeleton is a sort of armor 

 encasing the body. As in the insect this consists of the sub- 

 stance called chitin, but is made stronger by the addition of cal- 

 careous salts. Crayfishes do not thrive well in water that does 

 not contain this mineral matter. From time to time the exo- 

 skeleton is shed to allow the growing body to expand. The new 

 shell is at first soft and the animal tries to hide until it becomes 

 hard. The body, like that of the insect, would be very unwieldy 

 if joints were not present (Fig. 74). In these joints the chitin 

 is thin and flexible. The two principal parts of the body differ 



Cefihalothorajc 



VOVG L. ! ^"" 



Antennute 



Ytalkinq leqs 



FIG. 74. External anatomy of a lobster. (After Caiman.) 



in their flexibility; the foremost or anterior portion corresponds 

 to the head and thorax of the insect combined, and is named the 

 cephalo thorax. A furrow, the cervical groove, indicates where 

 these two parts are united. 



COLOR. The color of the crayfish is likewise a means of 

 protection, since it closely matches the bottom of the body of 

 water in which the animal lives. It is in the shell and is formed 

 by green, brown, blue, and red pigments; the color of the body 

 depends upon which color is present in the greatest quantity. 

 When cooked, these pigments all turn red and the whole body 

 becomes " as red as a boiled lobster." 



