SURVEY OF THE ARACHNIDAN EYES. 



75 



Lankester, derives them from a simple eye (stemma, ocellus) by the grouping 

 of the retinal elements into retinulae. The lateral eye of Limulus would, 

 according to this view, represent a later stage in which a separation of the 

 lenses has already taken place, which, progressing further, leads to the forma- 

 tion of facet-eyes. Although this view affords in some respects a satisfactory 

 explanation of the different forms of the eyes, we were not able to accept it, 

 because we could not find sufficient grounds for assuming that the continuous 

 retina breaks up into separate groups. 



The processes of development are more easily explicable by direct derivation 

 of the Arachnid eye from the ocellus. The invagination then corresponds to 

 the primary optic pit. If, however, we regard the eye as compound, as we did 

 before, it then consists of the sum of the single eyes, and the invagination is 



Fig. 30.— Diagrams illustrating the rise of the Arachnid eyes (chiefly after E. L. Mark), gl, 

 vitreous body ; h, hypodermis ; I, lens ; n, optic nerve ; pr, post-retinal layer ; r, retina ; st, 

 rods. 



not comparable with the primary optic pit, but must rather be considered as a 

 secondary structure, which displaced the whole of the visual surface inwards in 

 the form of an infolding, a process which, without further explanation, is incom- 

 prehensible. The process of inversion also then remains the same on the whole. 

 The simpler case of the derivation from the ocellus is explained by Mark 

 through degeneration of a part of the retina and greater development of the 

 other part, the lens shifting meantime towards the latter (Fig. 39 A-C). The 

 more highly developed portion of the retina conies to lie more and more beneath 

 the lens (Fig. 39 I) and E). While the nerves degenerate on the surfaces now 

 turned outward, others become connected with the inner ends of these cells (D 

 and E). The rods which lay originally behind the nuclei (B) are now found in 



