524 



ZOOLOGY 



SECT. 



lens and vitreous humour. The inner portion is a group of 

 sensory cells, constituting a retinula (re.), and enclosing a re- 

 fractive rod, the rhabdome (rh.) : the retinula is the actual per- 

 cipient part of the ommatidium, its cells being comparable to our 

 own rods and cones. The retinula3 of adjacent ommatidia are 

 separated from one another by cells full of black pigment (p.), so 

 that each ommatidium is in a state of optical isolation from its 

 fellows, and the whole eye is what is called a compound eye. The 

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

 from which fibres pass to the retinulse. 



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

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



FIG. 435. Diagram of two ommatidia from tlie paired eyes of Apus. c.c vitreous cells ; cr. vit- 

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

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



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 compara- 

 tively rare occurrence. The ovary (Fig. 431, 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 



