LATERAL AND CENTRAL EYES OP SCORPIO AND LIMULUS. 195 



adult Insects and of Crustacea ; also the " inulticorneal " lateral 

 eye of Limulus. In a more recent work Grenadier has described 

 (' Archiv fiir Mikr. Anat./ Bd. 18) the eyes of Myriapods. 



One of the chief conclusions which appears to us to follow 

 from Grenacher's work, when combined with our own observa- 

 tions on the central eyes of Scorpions, is that the primary dis- 

 tinction which has to be made amongst the various forms of 

 Arthropod eyes is not, as has hitherto been maintained, a dis- 

 tinction into (a) simple or unicorneal eyes, and (b) compound 

 or multicorneal eyes, but a distinction into (a) eyes with a one- 

 cell-layered ommateum (i. e. with no vitreous body separated 

 from the retinal body), and (b) eyes with a two-cell-layered om- 

 mateum (i. e. with a vitreous body, or layer of cells placed in 

 front of the retinal body, and usually separated from it by mem- 

 brane). These may be called respectively Monostichous 

 and Diplostichous eyes. In both these primary groups it 

 appears to be possible for the nerve-end cells (see woodcut, 

 fig. 1) to remain ungrouped — each equal and similar to its 

 neighbour, as is usual with cells building up cell layers — or, 

 on the other hand, the nerve-end cells of the ommateum may 

 segregate and group themselves as Retinulae (see fig. 14, PI. X). 

 These two conditions we propose to speak of as (1) eyes non- 

 retinulate, i.e. with the nerve-end cells autonomous, and (2) 

 eyes retinulate, i.e. with the nerve-end cells segregated. 



We have monostichous eyes which are non-retinulate in the 

 larvse of Insects, according to Greuacher's descriptions. We 

 have a monostichous eye which is feebly retinulate in the 

 lateral eye of the Scorpion, and a highly developed retinulate 

 monostichous eye in the lateral eye of Limulus. In the dorsal 

 eyes of Spiders and simple eyes of adult Insects we have ex- 

 amples of (according to Grenadier's description) a diplostichous 

 non-retinulate eye. In the central eyes of Scorpions, on the 

 other hand, we find for example of the strongly retinulate 

 diplostichous class. 



Further, the so-called compound eyes of Insects and Crus- 

 tacea are examples of diplostichous retinulate eyes, with cer- 

 tain additional peculiarities now to be noticed. 



