{82}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 Ammocoetes 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 Ammocoetes 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 Ammocoetes 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 {83}lamprey, and correspond, therefore, more nearly to the Petromyzon than to the Ammocoetes stage of the European lamprey. THE DIOPTRIC APPARATUS.