ON THE ORIGIN OF VERTEBRATES. 265 



animal throughout would possess organs of vision as good as 

 before or after the period of transition. 



In confirmation of this suggestion, we find the very striking 

 statement that the lateral eyes of the Trilobites are not in all 

 cases facetted, but, as Korschelt and Heider point out, were 

 apparently of an Arachnid nature in Harpes vittatus and H. 

 uugula. They say ^ : " Paheoutologists have appropriately described 

 them as ocelli, although, from a zoological point of view, they 

 do not deserve this name, having most probably arisen in a way 

 similar to that conjectured in connection with the lateral eyes 

 of scorpions." This observation requires further investigation, 

 but distinctly points to the possibility that in the Trilobites the 

 Crustacean and Arachnid type of eye simultaneously existed. 



Further, we see that 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 difhcult 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, as is also the case in the 

 vertebrate lens, and not as bulgiugs 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 verte- 

 brate 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 



' Op. cif., part iii. p. 71. 



