82 THE ORIGIN OF VERTEBRATES 
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 habenulw. 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 Petromyzon 
Planert. Taking also into consideration the continuity of the mass 
of small ganglion-cells which surround this atrium with the cells of 
the ganglion habenule 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 Ammoccetes the cells 
of the ganglion habenule 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 
Ammoceetes 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 
