ARTHROPOD \ 



'''7 



cephalothorax, consisting of a fusion of all the thoracic segments 

 with the head, and the carapace forms a complete covering ex- 

 tending as far hack as the abdomen and covering the gills on 

 the sides of the body. The head hears the stalked, compound 

 eyes ami live pairs of appendages, — two pairs of antennae, and 

 three pairs of mouth parts. On the thorax are the usual eight 

 pairs of appendages, of which the first three are biramous mouth 

 parts and the succeeding five consist of endopodites only. The 

 abdomen possesses the usual seven segments with six pairs oi 

 appendages, mostly biramous. 



The gills are attached to the thoracic appendages, and lie in 

 the gill chamber. In some crabs which live the greater part of 



1 IG. 160. Astacus JluviatiliSy the fresh-water crayfish. Lateral aspect of male, aj, antennule; 

 0-2, antenna; ad, abdomen; cth, cephalothorax ; kd, gill-cover; r, rostrum ; 8, third maxil- 

 liped; 9-13, legs; 19, uropod; XIV, first abdominal segment; XIX, sixth abdominal 

 ment. (From Lang's Comparative Anatomy.) 



the time on land, this gill chamber is modified into a sort of 

 pulmonary or lung chamber, so that the animal may use the air 

 directly. The genital openings occur in the female at the base of 

 the fifth thoracic appendage, and in the male at the base of the 

 eighth ; the first pair of abdominal appendages in the male is 

 modified into acopulatory organ. The stomach is provided with 

 strong, chitinous teeth for grinding up the food; the anus is on 

 the ventral side of the telson. At the time of ecdysis, the animal 

 crawls away into some obscure corner, where it is likely to be 

 concealed from its enemies, and after cracking its shell, crawls 



