ON THE ORIGIN OF 7EKTEBKATES. 225 



certain of the lower crustaceans, such as some forms of Branchi- 

 pus, many of the Gammaridse, etc., possess undoubtedly a single 

 corneal lens, and at the same time an eye in other respects of 

 the crustacean type. The more natural division appears to me 

 to be based on the difference in the nature of the retina in the 

 two main groups, the one group possessing a simple, the other 

 a compound retina. 



Throughout the animal kingdom the word ' retina ' is used in 

 two senses, viz. : — 



1. In the strictly proper sense, as a layer of ceils bearing 

 visual rods — the retinal end cells. The retina in this sense 

 forms the special sense terminations of the fibres of the optic 

 nerve. Such a retina may be called a simple retina, and is 

 characteristic of all simple non-facetted eyes. 



2. In the sense in which it is used in vertebrate anatomy, 

 viz., a complicated structure composed of the retina in the 

 strict sense, i.e. the layer of cells bearing visual rods, and other 

 layers belonging to the central nervous system. Such a retina is 

 clearly compounded of the true or simple retina and a retinal 

 ganglion, and may be called a compound retina. It is character- 

 istic of all compound facetted eyes, the vertebrate lateral eyes, 

 and the lateral eyes of various lowly crustacean forms which 

 possess a compound eye as far as the retina is concerned, and 

 yet a simple cornea. 



With respect to the first type of eye, the evidence is Quite 

 conclusive that the retinal cells are derived from the peripheral 

 epidermal cells, the cells of the so-called hypodermis, the cuticle 

 of these cells forming the cuticular part of the visual rods. 

 Originally, at the first formation of the eye, the optic portion 

 of the brain, which is called the optic ganglion, is formed in close 

 contact with the cominencing retina or optic plate, and subse- 

 quently, as the brain sinks below the surface, the optic ganglion 

 travels with it, leaving as a connection between the ganglion 

 and the retina the strand of fibres known as the optic nerve. 



In the second type of eye the process starts in the same way, 

 but when the brain withdraws away from the surface a portion 

 of the optic ganglion is left behind in close contact with the 

 retina, forming the retinal ganglion, so that the stalk which 

 remains, in consequence of the withdrawal, as the connection 



