XI 



PHYLUM ARTHROPODA 



525 



segment. As in Leeches (p. 503), there is reason for thinking 

 that the cavity of the ovarian tube represents a shut-off portion of 

 the coelome, and the oviduct a nephridium. One species has 

 been shown to be hermaphrodite : in others males are occasionally 

 found, but reproduction appears to be, as a rule, parthenogenetic. 



Development. The eggs are centrolecithal, i.e., have an accumu- 

 lation of yolk in the centre surrounded by a superficial layer of 

 protoplasm. 



The embryo is hatched in the form shown in Fig. 436, A. The 



Fro 436. Three stages in the development of Apus. fs. frontal sensory organ ; L, digestive 

 gland ; s. carapace ; 14, cephalic appendages ; I XIII, body-segments and appendages. 

 (From Lang's Comparatire Anatomy.} 



body is oval, and is divisible into three regions a large anterior or 

 head-region ; an intermediate trunk-region, the hinder part of which 

 already shows signs of segmentation (I-V) ; and a posterior bilobed 

 anal region. The head-region bears a single median eye, and a 

 pair of small unjointed appendages (7), each with two large setae 

 at its extremity : these become the antennules of the adult. The 

 trunk-region bears two pairs of appendages, the first of which (2) 

 is very large and fringed with setae, but is chiefly remarkable for 

 being biramous or two-branchedbeing formed of a proximal 



