230 DE WALTER H. GASKELL. 



It is not necessary to suppose that they were ever functional 

 in the Vertebrate, for whether we look at the Cephalaspids, or 

 at the Eurypterids, or at the living Limulus, we see in all cases 

 that these median eyes are small, insignificant, and very 

 different to the large lateral eyes, and, judging from their 

 condition in Limulus,^ they had already lost the right to the 

 title of principal eyes, and were in full course of degeneration 

 in the merostomatous ancestors of the vertebrate. 



Comparison of the Comjjound Retina of the Lateral Eyes of 

 Arthroiiods and Vertebrates. 



Clearly the lateral eyes of vertebrates cannot be directly 

 compared with those of arachnids, for, as already stated, the 

 retina of the former is compound, not simple, like that of the 

 latter. In the crustaceans, however, the retina of the lateral 

 eye resembles that of the vertebrate in its possession of a 

 retinal ganglion. It is, therefore, in this direction that our 

 search for the origin of the lateral eye of the vertebrate is most 

 likely to be successful, and, indeed, there is hardly a single 

 writer who has described the structure of the facetted eye who 

 has not compared its layers with that of the vertebrate. 



The foundation of our knowledge of the compound retina is 

 Berger's - well-known paper, the results of which are summed 

 up by him in the following two main conclusions. 



1. The optic ganglion of the Arthropoda consists of two 

 parts, of which the one stands in direct inseparable connection 

 with the facetted eye, and together with the layer of retinal 

 rods forms the retina of the facetted eye, while the other part is 

 connected rather with the brain, and is to be considered as an 

 integral part of the brain in the narrower sense of the word. 



2. In all Arthropods examined by him, the retina consists of 

 five layers, as follows : — 



(1) The layer of rods and their nuclei. 



(2) The layer of nerve bundles. 



(3) The nuclear layer, 



(4) The molecular layer. 



(5) Tlie ganglion cell layer. 



^ Lankester and Bourne, Q. J. of Micr. ScL, vol. xxiii. p. 177. 

 - Arbeit, a. >/. Zo'd. IiistU. Wien, B^i. i., 1878. 



