XI 



PHYLUM ARTHROPODA 



527 



6. The Fresh-water Crayfish (Astacus fluviatilis). 



Astacus fluviatilis is common in streams and rivers in England 

 and the continent of Europe ; allied species occur in Asia and 



FM 



Fid. 437. Astacus fluviatilus, side view of male, a 1 , antennule ; a 2 , antenna ; ab, abdomen; 

 cth. eephalothorax ; kd, gill-cover ; r. rostrum ; 8, third maxillipede ; 9, first leg ; 10 13, 

 remaining legs ; 19, uropod ; XIV, first abdominal segment- XIX, sixth abdominal segment. 

 (From Lang's Comparative Anatomy.) 



North America ; and fresh-water Crayfishes belonging to other 

 genera, but agreeing with Astacus in all essential features, are 

 found in America, Australia, and New Zealand. 



External Char- 

 acters. The body 

 o f the C r a y fi s h 

 (Fig. 437) is divided 

 into two regions 

 a n anterior, the 

 cephalothorax (cth.), 

 which is unjointed, 

 and is covered by 

 a carapace resem- 

 bling that of Apus, 

 but of smaller pro- 

 portional size ; and 

 a posterior, the ab- 

 domen (ab), which 

 is divided into dis- 

 tinct segments, 



movable Upon One FIG . 438. Transverse section of abdomen of Crayfish. DA, 



annfVior in a irar+inal dorsal abdominal artery ; EM, dorsal muscles of the abdomen; 



n a Vei ACai Ep> gpace b e tween tne p] eur0 n and the appendage ; FM, 



Diane The Cet)ha- ventral muscles of the abdomen ; M, muscles of the appendage ; 



N, endopodite ; NG, nerve-ganglion ; P, protopodite ; PL, 



lOtnoraX IS again pleuron ; PR, hind-gut ; S, sternum ; T, tergum ; V, ventral 

 j;_ j^j :j. rt abdominal artery; X, exopodite. (From Parker's Practical 



divided into tWO zoology, after Marshall and Hurst.) 



