CRAYFISH, LDBSTIOKS, SI^IDKIJS, AND LXSECTS 143 



supply, foi' their numliers compensate for whatever may 

 be hickiug in size. 



Chief characteristics of the Crustacea. — Thej^ breathe by 

 gills, hence live in the water; they have at least five pairs 

 of legs; the head ami thoi-ax are combmed into a cephalo- 

 tliorax, and the body is incased in ;' hard crust formed by 

 the deposition of lime carbonate in the skin. 



