SECT. VI 



THK 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. 



p< 



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

 Ir, lateral retina; />, 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 



