146 



THE EYE IN EVOLUTION 



Eledone 



bipolar cells and ganglion cells in the nuclear layer proximal to the pigmented 

 layer. A considerable amount of pigment is found in association with the 

 visual cells which is most abundant near the narrow neck of the cell between the 

 rod and the cell-body, and in some species at any rate, it is claimed, migrates 

 towards the extremities of the rods in bright light {Eledone — Rawitz, 1891). 



Such an eye is thus a highly complex organ capable of image-formation and 

 structurally equipped to mediate pattern-vision, able to accommodate over a 

 considerable range and possessed of some power of adaptation. Indeed, in one 

 species, Bafkyteiifhis, the elements of a central retinal area become apparent since 



Figs. 



116 AND 117. — The Pupils of Cephai.opods in Various Stages 

 OF Contraction. 



Fig. 116. — The pupil of the octopus. 



Cj 



Fig. 117. — The pupil of the cuttlefish, Sepia. 



the rods are greatly elongated as if to form a primitive area centralis, a differen- 

 tiation suggesting the existence of a fixation mechanism endowed with con- 

 siderable visual sensitivity (Chun, 1903). 



The Inverted Retina 



A peculiar form of simple eye is associated with an inverted (or 

 inverse) retina, that is, a retina wherein the visual cells are orientated' 

 so that their sensory ends are directed away fro7n the incident light. As a 

 rule, inversion of the retina is associated with a secondary invagination 

 of the optic vesicle. In the usual form of verted (or converse) 

 retina, as we have seen, the cells lining the proximal (deep) portion of 

 the vesicle form the visual cells and their orientation is quite straight- 

 forward ; their receptive elements face the surface and the optic nerve 

 fibres lead directly away from their proximal ends (Figs. 118, 120). In 

 some cases, however, the cells lining the distal (superficial) portion of the 

 vesicle form the visual cells ; since the receptive elements face the 

 inter, of the vesicle, the light must traverse the cell-bodies before it 

 reac.'i the end-organ, and the nerve fibres, issuing superficially, must 



