SECT. VI 



THE SENSORY ORGANS 



103 



paired " eye " seems to stand on the brain, as on four 

 stalks, between the stalks of the optic ganglia. The 

 stalk of the ventral retina is distinguished by the pre- 

 sence of several enormous ganglion cells, apparently 

 the largest in the whole nervous system of Apus. 



.dr 



FIG. 26. Lateral view of the unpaired "eye," diagrammatic, vr, ventral ; dr, dorsal ; 

 Ir, lateral retina ; b, brain ; S, water-space ; c , canal of the same ; /. tangle cf 

 pigment cells. Only the outer ends of the lateral retinal cells can be seen. 



A cross section of the posterior end of the organ 

 looks to the naked eye like an X. The retinae, bul- 

 ging in somewhat towards the interior of the cavity, 

 give its lumen this form. (See Fig. 27, 3.) The 

 pigment cells which fill the cavity form a tangle of 



