ON THE EYES OF SOME INVERTEBRATA. 677 



sade-shaped cells (10), the number of which corresponds with 

 that of the eye-units. Every one of these palisade cells 

 possesses an oblong nucleus at its foremost somewhat broader 

 end, and is broken into by one of the nerve-strings mentioned 

 above. 



With a Sarcophaga carnaria which I examined shortly 

 before the creeping out of the pupa, and the eye of which was 

 already quite developed and brown-pigmented, while the gan- 

 glion showed still somewhat the embryonal character, I could 

 determine by means of transverse and vertical sections that 

 the nerve-string in each palisade cell surrounds a refract- 

 ing chitinous or cuticular tube which lies in the midst of 

 the cell. 



In Musca vomitoria also, one sees that in every cell 

 lies a cylindrical axis, but on account of the small size of the 

 histological elements and for want of transverse sections, I 

 could not decide here if it be the nerve-string or such a chiti- 

 nous tube. Also, I did not succeed in obtaining sureuess as to 

 wliether the nerve-string perforates the palisade cell unaltered, 

 or if it only passes close to it and suffers here an interruption 

 by the substance contained in the tube or in the axial cylinder. 

 Inwardly the palisade layer is limited by a membrane (11) 

 containing nuclei; on the outside a string of several nerve- 

 fibres passes out of every cell, penetrates the layer of the small 

 ganglion cells (9) and reaches the central end of the retinula. 

 In vertical sections I could not of course see more than three 

 or four fibres in one string, but it seems certain to me, in 

 view of what occurs in other Arthropoda, that their number 

 answers to that of the retinula cells, and probably that already 

 the strings or bundles, formed after the crossing, consist each 

 of seven fibres. 



Though in most cases the peripheral ganglion-opticum is not 

 extended as a plane but more spherically, yet there is a specific 

 character which we always find in the higher and in many 

 lower Arthropoda with fan-eyes — namely, the existence of the 

 two ganglionic layers, and secondly, the crossing of the nerve- 

 fibres between them. In the Decapoda and Schizopoda there 



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