ON THE ORIGIN OF VERTEBRATES. 227 



with the single exception of the eyes of vertebrates and their 

 degenerated allies ; a fact which points strongly to the proba- 

 bility that a reconsideration of the evidence upon which the 

 present teaching of the origin of the vertebrate eye is based will 

 show that here, too, a confusion has arisen between that part 

 formed from the epidermal surface and from the optic ganglion. 

 Such reconsideration I propose to give in this paper. 



In order to understand the retina of the vertebrate eyes it is 

 absolutely necessary to consider the nature of the retina of the 

 cephalic eyes of arthropods. These latter are in all cases 

 divisible into a median and a lateral group, of which the former 

 are always non-facetted eyes with a simple retina, and the latter 

 are either facetted with a compound retina, as in most crustaceans 

 and insects, or non-facetted with a simple retina, as in arachnids. 



The Arachnid Eyes. 



In the spiders we see that, although all the eyes possess a 

 simple retina, two very distinct types are found, as was first 

 pointed out by Grenacher.^ In the one the nuclei of the retinal 

 layer are post-bacillary in position, in the other they are pre- 

 bacillary; in other words, if we consider with Patten that the 

 retina is formed by a process of invagination of the hypodermis, 

 and that the rods always represent what was the original 

 cuticnlar surface, then we may describe in his phraseology the 

 eyes in Grenacher's first group as eyes with upright retina ; in 

 the second group as eyes with inverted retina, the rods being 

 on the side of the retinal cell furthest away from the source of 

 light, just as in the lateral eye of the vertebrate, which also 

 possesses an inverted retina. Moreover, according to Bertkau,- 

 the optic nerve fibres always enter the retinal cell at the nuclear 

 end, as must of necessity be the case if the rods are formed from 

 the cuticular surface of the hypodermal cells, so that in the 

 inverted eyes the optic nerve fibres lie between the retina and 

 the source of light, just as in the vertebrate lateral eye. Further, 

 it is found that in the eyes with inverted retina a marked post- 



^ Grenadier, Untcrsuchinujcn ilher das Schor<jun dcr AiihrojHxlen, Gottingeii, 

 1879. 



- Jrchivf. viikr. AiiaL, Bd. xxvii., 1886. 



