THE EVIDENCE OF THE ORGANS OF VISION 81 
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 Studnicgka. 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. 
Studnigka has also seen these appearances, and figures them in 
his second paper on the nerve end-cells of the pineal eye of 
Ammoceetes. 
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 palostracan 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 Studni¢ka’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 
Studnigka 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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