150 



THE EYE IN EVOLUTION 



Sea-spider 



Web spider 



Scorpion 



different arrangement. In a further variation, seen in the median eyes 

 of scorpions (Scorpionidea) and in the median eyes of whip-tailed 

 scorpions, the visual cells are doubled upon themselves so that the 

 base of the cell is verted and the sensory end inverted. 



The simplest arrangement of an inverted retina in Arachnids 

 is seen in the lateral eyes of whip-tailed scorpions (Fig. 124) ; 

 the sensory ends of the inverted visual cells rest on the tapetum, 

 directed away from the incident light, and from the mid-point of the 

 cell-bodies the nerve fibres emerge to run to the periphery whence the 

 optic nerve emerges on the side of the eye (Versluys and Demoll, 

 1923). 



A different arrangement again is found in the sea-spiders 

 (Pycnogonids) (Fig. 125). In these, the hypodermal cells secrete a 

 cuticular lens in the anterior part of the eye and a reflecting tapetum 

 in the posterior part. The visual cells are unusually interesting. They 

 are large and triangular in shape, the apex of the triangle lying on the 

 tapetum ; the nuclei are placed distally at the base of the triangle and 

 the narrow proximal ends filled with granular material form the receptive 

 portion of the cell. The arrangement of the optic nerve fibres is unique 

 for they interA^ eave in the substance of the large retinal cells, reaching 

 distally towards the nuclei.^ 



An ingenious arrangement which probably has optical advantages is 

 seen in the lateral and posterior median eyes of web-spiders : the 

 (anterior) median eyes of these animals have direct, verted retinae (Wid- 

 mann, 1908). In the former the sensory portions of the elongated visual 

 cells point proximally to lie on the tapetum, while the cell-bodies are bent 

 on themselves at an angle of 90°, to run towards the periphery of the 

 retina where the nuclei lie (Fig. 126) ; this portion of the cell does not 

 therefore interpose itself in the path of incident light (Versluys and 

 Demoll, 1923). 



A semi-inimied retina is found in the median eyes both of 

 scorpions and of whip -scorpions. Here the visual cells, grouped 

 in retinules around rhabdomes, are bent upon themselves at 180°, their 

 nuclei lying proximally next to the tapetum and the receptor ends of the 

 cells being bent round so that their extremities lie alongside the nuclei : 

 here again there is the optical advantage that the incident light does 

 not travel through the bases of the visual cells (Fig. 127) (Scheuring, 

 1913 ; Versluys and Demoll, 1923). 



It will be remembered that the subepithelial eyes seen most typically in 

 platiarian and nemertine worms ^ wherein the visual cells dip downwards from 



' ; [organ (1891), Korsehelt and Heidei- (1893), Bouvier (1913) 

 Schlottke (1933). 

 " P 134. 



Wiren (1918), 



