PARIETAL EYE OF APUS. 



139 



the exterior by a short duct or pore. The remnant of this duct is seen in the 

 adult in the deep recess on the posterior outer margin of the eye sac. (Fig. 97, 

 0. at., and 102, B.) 



The fold of skin that covers both lateral and median eyes was no doubt a 

 later formation, having nothing to do with the original parietal eye infolding. 



The parietal eye of Apus lies entirely below the surface. There are no over- 

 lying lenses, or thickenings of the adjacent ectoderm, to control the direction of 

 the light. The latter may enter the paired retinas from the sides, and the un- 

 paired ones from in front, or from behind. 



Fig. 98. — Parietal eye vesicle of Apus, in cross-section. 



Fig. 99. — Same as preceding figure, with 

 pigment removed, showing the coarsely 

 fibrillated visual rods, r.t.d., on the inner 

 ends of the retinal cells. 



There is no reason to doubt that the tri-oculate median eye of decapods, 

 copepods, trilobites, and merostomes, in structure and development is essentially 

 like that of Limulus, scorpions, spiders, Apus, and Branchipus. The evidence 

 presented clearly indicates that this group of ocelli is very constant throughout 

 the Crustacea and arachnids, and that it has certain remarkable features which 

 distinguish it from all other visual organs. There is no parallel to the way in 

 which these ocelli develop except in the parietal eye of vertebrates, and there is 

 no explanation available for the condition seen in vertebrates except the one offered 

 by the arachnids. 



The Parietal Eye of Vertebrates. 



The parietal eye of vertebrates was long ago demonstrated to be a vestigial 

 eye, although there are some authors who still refer to it as a mysterious organ of 



