170 ABSENCE OF RETINA. 



exist. Wagner,* indeed, observed that in some cases 

 the optic nerve embraces the end of the cone, and he 

 supposed that it thus forms a sort of retina, for which, 

 however, its form is little suited. 



I ought also to mention that Max Schultzef con- 

 sidered that he had, in some few cases — for instance, 

 in Syrphus — been able to observe that the termina- 

 tion of the nerve does divide into a number of fibres. 

 Patten, t more recently, has also maintained the 

 existence of numerous nerve-fibrils, which, however, 

 subsequent observers — for instance, Kingsley § and 

 Beddard || — have been unable to discover. Even, how- 

 ever, if we admit the perfect correctness of Schultze's 

 observation, these cases are exceptional, and the fibres 

 so few that they can hardly, I think, affect the general 

 conclusion. To give anything like a distinct vision, a 

 very large number would be required. 



A last objection is the extreme difficulty which 

 would exist of combining so many different images 

 into one idea, though it must be admitted that at first 

 sight this difficulty (though to a minor degree) exists 

 even in the case of simple eyes, the number of which 

 varies considerably. Spiders have six to eight ; some 

 aquatic larvae twelve ; while the OniscoidsB (wood-lice), 

 assuming that their eyes are aggregates of simple eyes, 

 as Miiller supposed, have as many as twenty to forty. 



* Einige Bemerk. iiber den Bau der zus. Augen," Arch, fur Nat., 

 1835. 



t " Unt. iiber die zus. Augen der Krebse und Insecten," 1868. 



X " Eyes of Molluscs and Arthropods," Mitth. Zool. St. Neapel, 1886. 



§ " On the Divisions of the Compound Eye," Journal of Morphology, 

 1887. 



II " On the Structure of the Eye in Cymothoidse," Trans. Boy. Soc. 

 Edtn., 1887. 



