THE GENESIS OF THE EYE 



115 



Specialization, however, occurred at a very early stage, for some 

 degree of a localization of the sensitivity to light is seen even among 

 Protozoa. The most elementary expression of this advance is the 

 accentuation of photosensitivity in one part of the cell, and since the 

 early response to the stimulation of light was motorial, this occurred 

 particularly in the anterior part of the organism or in close association 

 with the organs of locomotion, as is seen in the eye-spots of Flagellates 

 and Ciliates ; an appreciation of directional activity was thereby 

 gained. When unicellular organisms developed into multicellular, 

 however, the subdivision into cells gave the opportunity for more 

 intense specialization, and out of the generalized dermal sensitivity, 

 specific integumentary liglitsensitive cells were evolved ; these again 

 tended to accumulate in localities where the recejition of stimuli was 

 of most biological value — towards the head-end of the animal, as in 

 annelid worms, or in association with the motile organs such as the 

 tentacles of Coslenterates, or the siphon or mantle of Molluscs. 



Such a single cell, however, although able to appreciate the 

 presence of light, is unable to form images ; for this purpose a number 

 of photosensitive cells must be aggregated together to form an " organ". 

 The most primitive organ of this type is composed of a simple colony 

 of independent cells without functional relationship — the simple eye or 

 ocellus — and eventually such a grouj^ing of cells became structurally 

 and functionally related in the compound eye ; in either case the 

 receptor cells were usually provided with a focusing arrangement to 

 concentrate the light and a j^igment mantle to absorb any excess. In 

 this way eyes are found in some polychsete worms and higher Inverte- 

 brates which from the anatomical point of view can form the basis of 

 vision of varying degrees of sensitivity and resolution. 



Throughout Invertebrates there is therefore a wide range of 

 photoreceptor mechanisms ; they have, however, one thing in common 

 — that in contradistinction to the " cerebral eye " of Vertebrates, which 

 is essentially of one general type and is developed from the neural 

 ectoderm, with few exceptions (e.g., Rotifera) they are all derived from 

 the surface epithelium. It is to be remembered, however, that in some 

 Invertebrates, in addition to the integument and its derivatives, 

 portions of the central nervous system appear to be light-sensitive. 

 This api^lies, for examj)le, to the sixth abdominal ganglion of the 

 crayfish (Prosser, 1934) ; in the eyeless white cave crayfish, Cambarus 

 ayersii. Wells (1952) found that stimulation of the cerebral ganglion by 

 light results in an increased kinesis without the suggestion of a visual 

 sense. This is analogous to the light -sensitivity of portions of the 

 central nervous system, particularly of the ependymal cells, of some 

 Vertebrates,^ and the gonadotrophic action of light on the hypothalamus 

 of some birds such as the duck ^ (Benoit et al., 1952). 



Drosoph ila 



Rotifer 



p. 537. 



p. 559. 



