ARTHROPOD A — CRUSTACEA 275 



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Class A. Crustacea 275 



Sub-phylum 2, Tracheata (except many of Class D). — Air 

 breathers; breathe mostly by tracheae. 



Class B. Onychophora 308 



Class C. Myriopoda 309 



Class D. Arachnida 309 



Class E. Insecta , . . . . 317 



CLASS A, CRUSTACEA 



Type of the class, Cambarus (Fig. 123). 



The crayfish, Cambarus, lives in fresh water lakes, rivers 

 and pools and is found in North America east of the Rockies 

 and in eastern Asia. The only other genus of crayfish living 

 in the northern Hemisphere is Astacus ; it is . found in North 

 America west of the Rockies, in Europe and in western Asia. 

 There are also some blind crayfish of the genus Cambarus inhab- 

 iting the caves in Carniola, southern Austria, which point to the 

 former more extended occupation of Europe likewise by this 

 genus. 



The body is divided externally into the anterior unjointed 

 portion, — the cephalothorax (union of head and thorax) 

 covered with the carapace, and the posterior portion, the abdo- 

 men, made up of distinct segments and terminating in the large 

 horizontally flattened telson. The cephalothorax bears the eyes, 

 feelers, and the five large walking legs. This division into head, 

 thorax and abdomen is not equivalent to the like division of the 

 vertebrate body. 



The body consists of twenty segments bearing upon their under 

 side nineteen pairs of appendages. As can be seen by the ap- 

 pendages on the ventral side, the head consists of five segments 

 bearing pairs of antennae or feelers, the short and the long, 

 one of mandibles or jaws, and two of maxillae. The thorax is 

 made up of eight segments bearing five large posterior or walking 

 legs and three smaller anterior pairs or foot-jaws, with their 



