542 ZOOLOGY SKIT. 



tunnel-like passage or sternal canal, containing the thoracic portion 

 of the nervous system. The entire cndopliragmal 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 cpisl-oma, 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 

 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 cara- 

 pace is produced in front into a large median spine, the mv/V//'/// 

 (Fig. 430, r) : immediately below it is a plate from which spring 

 two movably articulated cylindrical bodies, the eye-stalks, bearing 

 the eyes at their ends. 



The appendages are seen at a glance to differ from those of 

 Apus in their vastly greater degree of differentiation: obvious at 

 a glance are the long feelers (Fig. 430, a. 1, a. 2) attached to the 

 head, the five pairs of legs (9-lo) springing from the thorax, and 

 the little fin-like bodies arising from the sterna of the abdomen. 

 It will be convenient to begin with the last-named region. 



The third, fourth, and fifth segments of the abdomen bear 

 each a pair of small appendages, the swimming feet or yy/tv>/W.s 

 (Fig. 431, 10), the resemblance of which to the biramous limbs 

 of the larval Apus is obvious. There is an axis or protopodite 

 consisting of a very short proximal (pr. 1) and a long distal 

 (pr. 2) podomere, and bearing at its free end two jointed plates, 

 fringed with setae, the endopodite (en) and exopodite (c,c). These 

 appendages act as fins, moving backwards and forwards with a 

 regular swing, and probably aiding in the animal's forward 

 movements. 



In the female a similar appendage is borne on the second seg- 

 ment, while that of the first is more or less vestigial. In the 

 male the first and second pleopods (9) are modified into incom- 

 plete tubes which act as copulatory organs, serving to transfer 

 the spermatophores to the body of the female. The sixth pair of 

 abdominal limbs (11) are alike in the two sexes : they are very 

 large, both endopodite and exopodite having the form of broad flat 

 plates : in the natural position of the parts they lie one on each 

 side of the telson, forming with it a large five-lobed tail-fin 

 capable of being spread out after the manner of a fan ; they are 

 therefore conveniently called uropods or tail-feet. The telson itself 

 bears no appendages. 



The thoracic appendages are very different. The four posterior 

 segments bear long slender, join ted legs (8), upon which the animal 



