THE EVIDENCE OF THE ORGANS OF VISION 8 1 



median eyes of Limulus, Lankester and Bourne find it difficult to 

 determine how far the retinal end-cells contain pigment and how far 

 that pigment really is in the cells surrounding these nerve end-cells. 



The interior of the eye presents the appearance of a cavity in 

 shape like a cornucopia, the stalk of which terminates at the place 

 where the nerve enters. This cavity is not empty, but the posterior 

 part of it is filled with the termination of the nerve end-cells of the 

 retina, as pointed out by me and confirmed by Studnicka. These 

 terminations are free from pigment, and contain strikingly trans- 

 lucent bodies, which I have described in my paper in the Quarterly 

 Journal, and called rhabdites, for they present the same appearance 

 and are situated in the same position as are many of the rhabdites 

 on the terminations of the retinal end-cells of arthropod eyes. 

 Studnicka has also seen these appearances, and figures them in 

 his second paper on the nerve end-cells of the pineal eye of 

 Ammoccetes. 



Up to this point the following conclusions may be drawn : — 



1. Ammoccetes possesses a pair of median eyes, just as was the 



case with the most ancient fishes, and with the members of 

 the contemporary paheostracan group. 



2. The retina of one of these eyes is well-defined and upright, 



not inverted, and therefore in this respect agrees with that 

 of all median eyes. 



3. The presence of nerve end-cells, with pigment either in them or 



in cells around them, to the unpigmented ends of which trans- 

 lucent bodies resembling rhabdites are attached, is another 

 proof that this retina agrees with that of the median eyes of 

 arthropods. 



4. The simple nature of the nerve with its termination in an 



optic ganglion closely resembling in structure an arthropod 



optic ganglion, together with Studnicka' s statement that the 



nerve end-cells pass directly into the nerve, points directly 



to the conclusion that this retina is a simple, not a compound, 



retina, and that it therefore in this respect also agrees with 



the retina of all median eyes. 



With respect to this last conclusion, neither I myself nor 



Studnicka have been able to see any definite groups of cells 



between the nerve end-cells and the optic nerve such as a compound 



retina necessitates. 



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