XI 



PHYLUM ARTHROPODA 



537 



canal from the water-sac bendath the cuticle of the paired 

 eyes. 



Reproductive Organs.- -The large majority of individuals 

 both of Apus and Lepidurus are females ; males are of com- 

 paratively rare occurrence. The ovary (Fig. 424, ovy.) is a branched 

 tube occupying a considerable portion of the body-cavity in 

 sexually mature individuals. The walls of the tube are lined 

 with epithelium, and give rise to ova, which pass into the lumen 

 of the tube and thence to a duct (ovd.) opening 011 the eleventh or 



FIG. 428. Diagram of two ommatidia from the paired eyes of Apus. re. vitreous cells ; cr. vit- 

 reous body ; cl. connective-tissue fibre; hy. epiderm cells; p. pigment cells; ;-, inner pirts 

 of ommatidia ; re. retinulns ; rh. rhabdome. (From Bernard.) 



last thoracic segment. As in Leeches (p. 515), there is reason 

 for thinking that the cavity of the ovarian tube represents a 

 shut-off portion of the ccelome, 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 

 accumulation of yolk in the centre surrounded by a superficial 

 layer of protoplasm. The process of segmentation and the forma- 

 tion of the germ-layers have not been observed. 



