MUSEUM OF COMPARATIVE ZOOLOGY. 59 



and Bourne are to be accepted, it would appear that the lateral eyes pre- 

 sent a much simpler type than the median eyes, — so far, at least, as 

 regards the relation of the retinal layer to the hypodermis, the point upon 

 which the interpretation essentially turns. 



It is of importance in tlie consideration of this question that in neither of 

 their figures (Lankester and Bourne, Figs. 2, 3, 4) are the " interneural " 

 cells represented as reaching to the cuticular lens. They form a layer, 

 — uninterrupted except by the narrow nerve-fibre prolongations of the 

 retinal cells, — the individual elements of which are wedged in between 

 the posterior ends only of the cells composing the retina. Nothing in this 

 relation stands in the way of these interneural cells being directly com- 

 pared with the posterior layer of the retinal infolding in spiders' eyes. 

 The only serious obstacle to a direct comparison with triplostichous eyes 

 is the absence of a true " vitreous." 



The authors afiirm with great positiveness the entire absence of the 

 vitreous layer. There are two considerations which make it appear to rae 

 possible that Graber in figuring that layer may not have been so grossly 

 in error as they claim. There are great differences in the thickness of the 

 " vitreous " in the adult eyes of different Arthropods. (Compare Gren- 

 acher, '79, Figg. 28 and 31.) It is possible either that a very thin layer 

 of cells may have been overlooked by Lankester and Bourne, or that, 

 after secreting the substance of the cuticular lens, the " vitreous " cells 

 are in the adult crowded to the margin or completely obliterated. 



If, then, it should happen from any cause whatever (e. g. the extreme 

 thinness of the layer, or its prompt degeneration and disappearance after 

 secreting the lens) that the " vitreous body " had escaped the attention 

 of these authors, as suggested by Lowne ('84, p. 416), then one might 

 readily conceive that the lateral eyes of scorpions were formed on practi- 

 cally the same plan as the median eyes of the scorpion and the pre- 

 nuclear eyes of spiders. In that event the cells called by Lankester 

 and Bourne " interneural " Avould doubtless represent the posterior of 

 the infolded layers.* 



Although Graber ('79, Fig. 4) has given a figure of the lateral eye 

 (Scorpio europa3us) which in some respects is much less satisfactory than 



* If this were the case (comjiare Lankester and Bonrne, op. cit., "Explanation of 

 the small italics in all figures " and explanations of Figs. 7 and 8), the question 

 raised by the authors — ■ whether the " pignientiferous cells " (pp) witliin the retinal 

 capsule of the central eye were erjnivaleut to the " interneural epithelial cells" {cjq) 

 of the lateral eyes, or were "intracapsular (intrusive) connective tissue" — would 

 be answered in favor of the former of the two possibilities. 



