INVERTEBRATE EYES 129 



peculiarly shaped ellipsoidal mass — the optic organelle (the " Binnen- 

 korper " of Hesse ; the " Glaskorper " of Apathy, 1897) — distinctly 

 marked off from the rest of the cytoj)lasm by its deeper staining, 

 occupying the centre of the cell and crowding the nucleus to one side ; 

 it is made up of a clear hyaline-like structure (a " lens ") surrounded 

 by a dense neurofibrillar network (the retinella). Hess (1925) 

 found experimentally that no matter from which direction light was 

 directed onto the cell, the " lens " brought it to a focus on the sur- 

 rounding network of the retinella, a circumstance which suggests that 

 the hyaline mass focuses the light which induces a direct stimulation 

 of the nerve-fibres, possibly by a photo-electrical rather than a 

 photochemical effect. 



Depending on the arrangement of these cells singly or in com- 

 munities to form an organ, invertebrate eyes may be classified morpho- 

 logically as follows. 



I. The SIMPLE EYE or ocellus. 



1. The unicellular eye. 



(a) epithelial, 

 (6) subepithelial. 



2. The multicellular eye. 



(a) the subepithelial eye, 

 (6) the epithelial invaginated eye. 

 (i) the flat eye, 



(ii) the cupulate eye, 



(iii) the vesicular eye. 



II. Intermediate forms. 



(a) The aggregate eye. 



(6) The composite ocellus. 



(c) The simple ommatidial eye. 



III. The compound eye. 



The Simple Eye 



A simple light-sensitive cell, an ectodermal cell differentiated from 

 its neighbours in order to receive incident light and transmit a physio- 

 logical imjjulse, ranks as a very primitive type of eye. With single 

 cells, each of which is responsive merely to the presence of light, a 

 light-sense may exist, but no definite image such as is necessary for the 

 development of the visual sense can be formed. If, however, these 

 cells multiply and group together in clumps l^o form an " organ ", some 

 conception of an object may be realized and a primitive directional 

 analysis may be made of visual space. Each single constituent cell, it 



S.O.— VOL. I. 



