XI 



PHYLUM ARTHROPODA 



495 



nerve springing from the brain dilates into an optic ganglion, from 

 which fibres pass to the retinulae. 



The median eye is an ovoid body, and consists of four groups of 

 large sensory cells enclosing a mass of pigmented tissue : it is in 

 immediate contact with the brain, and receives a narrow canal 

 from the water-sac beneath the cuticle of the paired eyes. 



Reproductive Organs. The large majority of individuals 

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



hy 





FIG. 394. Diagram of two onimatidia from the paired eyes of Apus. re. vitreous cells ; cr, vit- 

 reous body ; cl, connective-tissue fibre ; hy, epidemi cells ; p. pigment cells ; re. retinuLe ; 

 rh. rhabdome. (From Bernard.) 



paratively rare occurrence. The ovary (Fig. 390, 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 on the eleventh or 

 last thoracic segment. As in Leeches (p. 474), 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. 



