THE EVIDENCE OF THE ORGANS OF VISION I 15 



retina took place before the complete amalgamation with the gut- 

 diverticulum, that, in fact, among the proto-crustacean, proto- 

 arachnid forms there were some sufficiently arachnid to have an 

 inverted retina, and at the same time sufficiently crustacean to 

 possess a compound retina, and therefore a compound inverted 

 retina after the vertebrate fashion existed in combination with the 

 anterior gut-diverticula. Thus, when the eye and optic nerve sank 

 into and amalgamated with the gut-diverticulum, neither the dioptric 

 apparatus nor the nervous arrangements would suffer any alteration, 

 and the animal throughout the whole process would possess organs 

 of vision as good as before or after the period of transition. 



Further, not only the retina but also the dioptric apparatus of 

 the vertebrate eye point to its origin from a type that combined 

 the peculiarities of the arachnids and the crustaceans. In the 

 former it is difficult to speak of a true lens, the function of a lens 

 being undertaken by the cuticular surface of the cells of the corneagen 

 (Mark's ' lentigtn '), while in the latter, in addition to the corneal 

 covering, a true lens exists in the shape of the crystalline cones. 

 Further, these crustacean lenses are true lenses in the vertebrate 

 sense, in that they are formed by modified hypodermal cells, and 

 not bulgings of the cuticle, as in the arachnid. We see, in fact, that 

 in the compound crustacean eye an extra layer of hypodermal cells has 

 become inserted between the cornea and the retina to form a lens. 

 So also in the vertebrate eye the lens is formed by an extra layer of 

 the epidermal cells between the cornea and the retina. The fact that 

 the vertebrate eye possesses a single lens, though its retina is composed 

 of a number of ommatidia, while the crustacean eye possesses a lens 

 to each ommatidium, may well be a consequence of the inversion of 

 the vertebrate retina. It is most probable, as Korschelt and Heider 

 have pointed out, that the retina of the arachnid eyes is composed 

 of a number of ommatidia, just as in the crustacean eyes and 

 in the inverted eyes it is probable that the image is focussed on 

 to the pigmented tapetal layer, and thence reflected on to the 

 percipient visual rods. In such a method of vision a single lens is a 

 necessity, and so it must also be if, as I suppose, eyes existed with 

 an inverted compound retina. Owing to the crustacean affinities of 

 such eyes, a lens would be formed and the retina would be compound : 

 owing to the arachnid affinities, the retina would be inverted and 

 the hypodermal cells which formed the lens would be massed 



