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 Studniçka. These terminations are free from pigment, and contain strikingly translucent 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. Studniçka has also seen these appearances, and figures them in his second paper on the nerve end-cells of the pineal eye of Ammocoetes. Up to this point the following conclusions may be drawn:-- 1. Ammocoetes possesses a pair of median eyes, just as was the case with the most ancient fishes, and with the members of the contemporary palæostracan 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 translucent 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 Studniçka'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 Studniçka 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.