XI 



PHYLUM ARTHROPODA 



529 



of the nervous system. The entire endophragmal system, as it is 

 called, constitutes a kind of internal skeleton : its anterior end is 

 formed by a plate, the cephalic apodeme, having the same anatomical 

 relations as the similarly named structure in Apus. 



The head exhibits no segmentation : its sternal region is 

 formed largely by a shield-shaped plate, the epistoma, nearly vertical 

 in position. The ventral surface of the head is, in fact, bent so as 

 to face forwards instead of downwards. The epistoma is bounded 



ll.Uro|>< 



FIG. 439. Typical appendages of Astacus. en. 1 5, podomeres of tmdopodite ; ep. 

 epipodite ; ex. exopodite \fl. flagella ; g. gill ; pr. 1, pr. 2, podomeres of protopodite ; 1 3 

 podomeres of axis of antennule. (After Huxley.) 



laterally by the free edge of the carapace instead of passing 

 insensibly into it like the sub-frontal area of Apus, with which, 

 however, it agrees in having the labrum attached to the middle 

 of its posterior border. The cephalic region of the carapace is 

 produced in front into a large median spine, the rostrum (Fig. 437, r) : 

 immediately below it is a plate from which spring two movably 



VOL. I. MM 



