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. 



^gment cefS 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 



