No. I.] EYES OF ARTHROPODS. 215 



were not cut in that plane. I think, therefore, that the fact is 

 satisfactorily established that there are ocelli in the larvae of 

 Insects which are very similar to what, in my former paper, I 

 regarded as the ancestral eye of Arthropods, 



In that paper I also had occasion to refer to the lateral eyes 

 of Scorpions, which, for various reasons, I regarded, in opposi- 

 tion to Lankester's observations, as double-layered. Prof. 

 Lankester {26) mentions this point in his criticism of my paper 

 and maintains that there is not the ^^ slightest doubt" that the 

 lateral eyes are single-layered, " there being no folding in of the 

 edges of the depression so as to form a vesicle, and consequently 

 no duplication or triplication of the layers." This evidence 

 is far from being conclusive ; for it is well known that there are 

 few, if any, instances of admittedly two-layered ocelli in which 

 there is any evidence of infolding. It has been recently shown 

 that the compound eye is supplied with a layer of cells corre- 

 sponding to the vitreous layer of the ocelli, yet this layer has 

 been overlooked by the most careful students of this subject. 

 Claus could not find the corneagen in the compound eyes of 

 other Arthropods than the Edriopthalmidae, although, from 

 theoretical reasons, he confidently expected to find such a 

 layer. The lateral eyes of Scorpions are more difficult to study 

 than the compound eyes ; and yet, in spite of these facts. Prof. 

 Lankester asserts that there is not the "slightest doubt" that 

 the vitreous body is absent in these eyes, although nothing 

 whatever is known of the development, which alone can fur- 

 nish conclusive evidence in favor of his assertion. 



In the paper referred to I gave reasons for not accepting as 

 conclusive Lankester's observations on the lateral eyes of Scor- 

 pions. This proceeding of mine has been severely criticised by 

 Prof. Lankester. It seems my objections did not outweigh my 

 audacity in presuming to doubt the accuracy of his ob- 

 servations. It is reassuring, however, to think that the weight 

 of Prof. Lankester's indignation may be divided in the future, 

 since Prof. Mark,^ on essentially the same ground offered by 

 myself, has also presumed to question the decisiveness of the 

 evidence. 



' Simple Eyes in Arthropods. Bulletin of the Mus. Comp. Zool. Cambridge, 

 Vol. xiii, No. 3. Prof. Mark's paper was received as this article was going to the 

 printer. 



