COMPOUND EVES OF A nTirHOPODS. 317 



fanotion, whilo tlie cornea may be partly protective and partly 

 dioptric, or wiiolly protective. Wlieii the cornea becomes partly 

 dioptric, as in Scrolis and in several other Arthropods, the diop- 

 tric function in an individual oiniiiatidinm comes to be performed 

 by two structures, the crystalline cone and the corneal lens. 

 When the two structures act together for the same end at the 

 same time, it is easy to see how a certain trivial peculiarity of 

 the one may induce a correlative modification of the other, and 

 how a slight specific peculiarity may appear exaggerated in the 

 thickness or in the degree of curvature of the corneal lenses in 

 different species. 



After so much has been said in regard to the unity of structure 

 of the ommatidium in diflerent Arthropods, one important point 

 awaits our consideration, viz. the homology and fate of the cen- 

 tral ganglion cell found in the oinmatidiuin of Limidus. Unless 

 a great many forms of ommatidia in different Arthropods be 

 compared, a discussion on this point appears to be unprofitable. 

 The consideration which follows is therefore a purely provisional 

 one. 



There can be no ijuostion that the central ganglion cell is an 

 important factor in the ommatidium of Lhaulu.s, nor can we 

 doubt the existence of a fundamental homology between the 

 retinulae o^ Limidus and those of all the other Arthropods which 

 I have examined. With the exception of a few problematical 

 bodies, such as the "hyaline cells" of Serolis, there are no struc- 

 tures in the ommatidia of most Arthropods which correspond to 

 the central ganglion cell of Zhmdus, in spite of the existence of 

 a fundamental homology in the other elements of the ommatidium. 



What has become of the central ganglionic element of the 

 ommatidium? Was it lost in the course of the phylogenetic 

 history of a more complex ommatidium ? Or is it reasonable to 

 suppose that some ommatidia came into existence without it 

 from the beginning? Or, if it were lost at all, is there any 

 evidence which makes this supposition probable ? 



The colorless ganglionic cell and the pigmented rod-bearing 

 cells which surround the former I consider as the two primitive 

 morphological factors in the unit of the sensory part of the 

 Arthropod retina, somewhat in the same way as the circle of 



