CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 143 



supply, for their numbers compensate for whatever may 

 be lacking in size. 



Chief characteristics of the Crustacea. — They breathe by 

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

 of legs; the head and thorax are combined into a cepnalo- 

 thorax, and the body is incased in a hard crust formed by 

 the deposition of lime carbonate in the skin. 



