148 S. WATASE. 



degrees of curvatures and by the varieties of its thickness. In 

 several of the Decapod Crustacea which I have examined, as 

 Peneeus, Cambarus, Homarus, Callinectes, Gebia, 

 &c., the curvatures of the individual cornea on both surfaces 

 are very slight ; it is biconvex in an extremely small degree. 

 In Talorchestia both surfaces of the cornea are parallel. In 

 Serolis, four species of which I have studied, all having well- 

 developed compound eyes, there exists a considerable diflFerence 

 in different species in the nature of the cornea. In some the 

 curyature on the proximal surface is very strong, and the whole 

 structure is quite thick; while in others the cornea is rather thin, 

 and a slight development of curvatures exists. This is interest- 

 in f, showing that even within the group of nearly allied species 

 there are considerable differences in this respect. 



This fact is easy to understand when we remember the func- 

 tional property of the cornea and the crystalline cone. As has 

 been noticed already, the crystalline cone is always dioptric in 

 function, while the cornea may be partly protective and partly 

 dioptric, or wholly protective. When the cornea becomes 

 partly dioptric, as in Scrolls and in several other Arthropods, 

 the dioptric function in an individual ommatidium 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 modi- 

 fication 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 different Arthropods, one 

 important point awaits our consideration, viz. the homology 

 and fate of the central ganglion-cell found in the ommatidium 

 of Limulus. 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 question that the central ganglion-cell is an 



