228 DR WALTER H. GASKELL. 



retinal layer exists characterised by pigment and coloration, 

 which is called the tapetum, while the existence of the pre- 

 retinal layer is very doubtful. On the contrary, in the eyes 

 with upright retina, the tapetum is absent and the pre-retinal 

 membrane is conspicuous ; the eyes then are frequently classed 

 as tapetal and non-tapetal eyes, and it is supposed that in the 

 tapetal eye — i.e., the eye with inverted retina — the image is 

 formed on the tapetum and reflected back on to the rods which 

 are in close contact with the tapetum. Similarly, in the 

 mammalian lateral eye, with its inverted retina, Johnson^ 

 describes the post -retinal pigmental layer as being frequently 

 coloured and shiny, and imagines that it reflects the image 

 back directly upon the rods. Finally , and this to my mind is 

 very significant, the non-tapetal eyes with upright retina are the 

 two anterior median eyes, while the rest of the eyes are 

 tapetal with inverted retina. These two median eyes are 

 called by Bertkau Hauptaugen (principal eyes) ; they are formed 

 distinctly sooner than the remaining eyes, called by him Neben- 

 augen (accessory eyes). 



We see, therefore, that the eyes of spiders, like the eyes of 

 vertebrates, fall naturally into two groups : — 



1. A. median group, consisting of two eyes, which, judging from 

 their development, are the most ancient ; these, like the two 

 median pineal eyes of vertebrates, possess an upright retina and 

 no tapetal layer, the pigment being around the retinal cells. 



2. A lateral group, consisting of many eyes, which possess a 

 well-marked tapetal layer, and, like the lateral eyes of verte- 

 brates, an inverted retina. 



Throughout this series of papers the consideration of each 



separate organ has led to the same conclusion — that the ancestor 



of the Vertebrates belonged to a proto-Arachnid type, so that 



it is natural enovigh to find that the vertebrate eyes present 



resemblances to those of spiders. The important question is, 



however, are these resemblances so close as to force us to the 



conclusion that a genetic relationship exists between the 



two ? 



' rroc. Roy. Sue, vol. Ixvi. p. 474, 19C0. 



