THE EVIDENCE OF THE ORGANS OF VISION 115 
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 ‘lentigen ’), 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 
