The Origin of Vertebrates

by Walter Holbrook Gaskell

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.

On the other hand, Dendy describes in the New Zealand lamprey, Geotria australis, a cavity where the nerve enters into the eye, which he calls the atrium. This cavity is distinct from the main cavity of the eye, and is separated from it by a mass of cells similar in appearance to those of the cortex of the ganglion habenulæ. In these two eyes then, groups of cells, resembling in appearance those belonging to an optic ganglion, exist in the eyes themselves. This atrium is evidently that part of the central cavity which I have called the handle of the cornucopia in the European lamprey, and the very fact that it is separated from the rest of the central cavity is evidence that we are dealing here with a later stage in the history of the pineal eyes than in the case of the Ammocœtes of Petromyzon Planeri. Taking also into consideration the continuity of the mass of small ganglion-cells which surround this atrium with the cells of the ganglion habenulæ by means of the similar cells scattered along the course of the nerve, and also bearing in mind the fact already stated that in the more degenerate left eye of Ammocœtes the cells of the ganglion habenulæ extend right up to the eye itself, it seems more likely than not that these cells do not represent the original optic ganglion of a compound retina, but rather the subsequent invasion, by way of the pineal nerve, of ganglion-cells belonging to a portion of the brain. In the last chapter it has been suggested that the presence of the trochlear or fourth cranial nerve has given rise to the formation of the cerebellum by a similar spreading.

There is certainly no appearance in the least resembling a compound retina such as is seen in the vertebrate or crustacean lateral eye. In the median eyes of scorpions and of Limulus, cells are found within the capsule of the eye among the nerve-fibres and the nerve end-cells. These are especially numerous in the median eyes of Limulus, as described by Lankester and Bourne, and are called by them intrusive connective tissue cells. The meaning of these cells is not, to my mind, yet settled. It is sufficient for my purpose to point out that the presence of cells interneural in position among the nerve end-cells of the retina of the median eyes of Ammocœtes is more probable than not, on the assumption that the retina of these eyes is built up on the same plan as that of the median eyes in Limulus and the scorpions.

It is further to be borne in mind that these specimens of Geotria worked at by Dendy were in the 'Velasia' stage of the New Zealand lamprey, and correspond, therefore, more nearly to the Petromyzon than to the Ammocœtes stage of the European lamprey.

The Dioptric Apparatus.

Besides the retina, all eyes possess a dioptric apparatus. What is the evidence as to its nature in these vertebrate median eyes? Lankester and Bourne have divided the eyes of scorpions and Limulus into two kinds, monostichous and diplostichous. In the first the retinal cells are supposed to give rise to not only rhabdites but also the cuticular chitinous lens, so that the eye is one-layered; in the second the lens is formed by a well-marked hypodermal layer, in front of the retina, composed of elongated cells, so that these eyes are two-layered or diplostichous. The lateral eyes, according to them, are all monostichous, but the median eyes are diplostichous.

Fig. 35.—Eye of Acilius Larvæ. (After Patten.)