204 PROFESSOR E. RAY LANKESTER AND A. G. BOURNE. 



it has been suggested by Prof. Lankester that the comparison 

 to be made is not between a single lateral eye of the Scorpion 

 and a whole lateral eye of Limulus, but between the latter and 

 the complete group of lateral eyes occurring in Scorpions. 



When the comparison is thus made, we see that if we sup- 

 posed a common ancestor of the Scorpion and King Crab to 

 have exhibited a lateral " ocular area," which possessed a 

 single feebly developed cuticular lens, then by two slightly 

 divergent lines of differentiation we can obtain the grouped 

 eyes of Scorpio on the one hand, and the polymeniscous eye of 

 Limulus on the other hand. 



The ancestral eye was undoubtedly monostichous, an archaic 

 character which is retained by both descendants. The archaic 

 eye was at first non-retinulate, and commenced to exhibit a 

 tendency to retinulation before the actual divergence of the 

 Scorpionids and the Liinuloids. In the Limuloids the differen- 

 tiation ofretinulge and the corresponding differentiation of lens- 

 cones (facets) became definite and characteristic. It is pro- 

 bable, if we may judge from the condition of the extinct 

 Eurypterina, that the lens-cones were at first relatively larger 

 and shallower and the retinulse less concentrated (composed of 

 more numerous cells) in the Limuloids than they subsequently 

 became. 



In the Scorpionids the segregation of ommateum and lens 

 took different proportions. The original lens segregrated, not 

 into a number of contiguous lens-cones, but broke up into a 

 number of quite separate lenticules, to each of which a portion 

 of ommateum corresponded. This process was no doubt a very 

 gradual one, and was essentially determined by the fact that 

 well-marked retinulse did not develop themselves as optical units 

 in the ommateum of the lateral eye of these ancestral Scor- 

 pions. Probably the secondary eyes of these Scorpionids were 

 at first much more numerous and more closely set than in living 

 Scorpions. Gradually they have become reduced in number 

 and more widely separated from one another. At the present 

 day various genera of Scorpions differ in the number of eyelets 

 present on the lateral ocular area (from two to seven). It is also 



