PHYLUM ARTHROPODA 



495 



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

 which fibres pass to the retinuhe. 



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 

 >th of Apus and Lepidurus are females; males are of corn- 



FIG. 394. 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 ; re. retinul* ; 

 rh. rhabdome. (From Bernard.) 



paratively rare occurrence. The owry (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. 



