82 THE ORIGIN OF VERTEBRATES 



On the other hand, Dently describes in the New Zealand lamprey, 

 Gcotria 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 hahcnulcc. 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 Ammoccetes of Petromijzon 

 Planeri. Taking also into consideration the continuity of the mass 

 of small ganglion-cells which surround this atrium with the cells of 

 the ganglion habcnulce 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 habenulce 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 

 Ammoccetes 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 Gcotria 

 worked at by Dendy were in the ' Velasia ' stage of the New Zealand 



