THE SIMPLE EYE 



149 



1915). It is interesting that in studying the electrical responses in the eye of 

 Pecten, Hartline (1938) found that the distal layer of the retina mediated a 

 strong off-response while the proximal layer discharged impulses whenever 

 illuminated. 



In AEACHNiDS, ail inverse retina is seen in the lateral and median 

 posterior eyes of spiders (Araneida), in all the ocelli of pseudo -scorpions 

 (Pseiidoscorpionidea). in the lateral eyes of whip-tailed scorpions 

 (Pedipalpi) and in sea-spiders (Pycnogonida). Each one of these has a 



Pseudo-scorpion 



Figs. 124 to 127. — Inverted and Semi-inverted Retin.e in Arachnids. 



Fig. 124. — The lateral eye of a whip- 

 tailed scorpion. 



C, cuticular lens ; X, optic nerve 

 fibres ; T, tapetum (after Versluys 

 and DenioU). 



Fig. 125. — -The eye of a sea-spider. 

 C, cuticle ; Ep, the hypodermal cells, the 

 central ones of which become extremely 

 elongated and surround the retinal cells, V. 

 In the distal part of the eye they give rise to 

 the cells of the lens, L, and in the proximal 

 part, to the tapetum, T. The retinal cells 

 themselves are elongated with a nucleus in 

 the distal part, while the proximal granular 

 part is the sensory receptor. Into these cells 

 the optic nerve fibrils, OX, ramify. The 

 whole eye is surrounded in a pigment cap- 

 sule, P (after Schlottke). 



Fig. 126. — The lateral eye of a spider. 

 C, cuticular lens ; X, optic nerve 

 fibres ; T, tapetum (after Versluys 

 and Demoll). 



Fig. 127. — The median eye of a whip- 

 tailed scorpion. 



C, cuticular lens ; X, optic nerve 

 fibres (after Versluys and Demoll). 



