88 THE ORIGIN OF VERTEBRATES 



the dominant race was neither crustacean nor arachnid, but partook 

 of the characters of both ; also, as is characteristic of dominance, 

 there was very great variety of form, so that it seems more probable 

 than not that some of these forms may have combined the arachnid 

 and crustacean characteristics to the extent of possessing lateral eyes 

 with an inverted yet compound retina. A certain amount of 

 evidence points in this direction. As already stated, the compound 

 retina which characterizes the vertebrate lateral eyes is character- 

 istic of all facetted eyes, and in the trilobites facetted lateral eyes 

 are commonly found. From this it may be concluded that many of 

 the trilobites possessed eyes with a compound retina. There have, 

 however, been found in certain species, e.g. Harpcs vittatus and 

 Harpes ungula, lateral eyes which were not facetted, and are believed 

 by Korschelt and Heider to be of an arachnid nature. They say, 

 " Palaeontologists 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 con- 

 jectured in connection with the lateral eyes of scorpions." If this 

 conjecture is right, then in these forms the retina may have been 

 inverted, but because they belonged to the trilobite group, the retina 

 was most probably compound, so that here we may have had the 

 combination of the arachnid and crustacean characteristics. On the 

 other hand, in some forms of Branchipus, and many of the Gamma- 

 ridse, a single corneal lens is found in conjunction with an eye of the 

 crustacean type, so that a non-facetted lateral eye, found in a fossil 

 form, would not necessarily imply the arachnid type of eye with the 

 possibility of an inverted retina. Whatever may lie the ultimate 

 decision upon these particular forms, the striking fact remains, that 

 both in the vertebrate and in the arachnid the median eyes possess 

 a simple upright retina, while the lateral eyes possess an inverted 

 retina, and that both in the vertebrate and the crustacean the 

 median eyes possess a simple upright retina, while the lateral eyes 

 possess a compound retina. 



The resemblance of the retina of the lateral eyes of vertebrates 

 to that of the lateral eyes of many arthropods, especially crustaceans, 

 has been pointed out by nearly every one who has worked at these 

 invertebrate lateral eyes. The foundation of our knowledge of the 

 compound retina is Berger's well-known paper, the results of which 

 are summed up by him in the following two main conclusions. 



