SECTION III. 



THE CLASS OF CRUSTACEA OR CRUSTACEANS. 



SUB-SECTION I. 

 THE CRUSTACEA CONSIDERED AS A CLASS. 



THE Crustacea include all articulated animals which 

 are covered with a crust or shell, and which have the 

 head and thorax united into one piece called the cephalo- 

 thorax, and which respire by means of branchiae or 

 gills. The shell is leathery, horny, or calcareous, and 

 like that of Insects, contains chitine. 



Crustaceans are thus aquatic in their mode of respira- 

 tion, and most kinds live in the water a vast majority 

 in the ocean ; but some kinds live on the land, mainly in 

 damp places. 



As examples of this class, we may mention the Crabs 

 (Figs, vignette, 480, 481), Lobsters and Craw-fishes (Figs. 

 477,485), Shrimps, Sand-fleas (Fig. 490), Barnacles (Figs. 

 504, 506), etc. 



As already indicated, the head and thorax are so closely 

 united that they appear as one piece or segment, and so 

 the Crustaceans exhibit only two well-marked regions 



