288 S. WATASE. 



work. I am also indebted to Prof. Jolin A. Ilydcr uiid Dr. E. A. 

 Andrews for various courtesies. 



It soon bocaino evident that the eye of Limnlus is, as we 

 miglit expect from its great antiquity, a very primitive visual 

 organ, presenting in a simple form a most interesting type. 



For the further comprehension of its structure and significance 

 it became necessary to extend my researches to several other 

 Arthropods. Among the forms studied in this connection are 

 the following, besides five other pelagic Arthropods from the 

 Gulf Stream, whose generic n;iii;e> liave not yet been ascertained: 

 £ranchij)us,Esthena, T.ih;rh. ><int Jbjperia, Phnn,'n„<i, s, rnJlx^ 

 Ligia, Aselhis, Notonaiii, L'dnV nln, Ayrion, Gnjlhis^ Cunllira, 

 Camharits, Homarus, Ilippa, Alplieiis, Gehia, Penaeus, Squilla, 

 Callinecies, Gonodactylus and Lucifer. 



While only a few selected types are here described, all the 

 above named forms, representing three great groups of Arthro- 

 pods, Insects, Crustacea and Arachnids, were carefully studied, 

 and all the results have been embodied in the general observa- 

 tions which follow, although the principal subject of the paper is 

 the eye of Liinulus. 



The points ■which are at present receiving marked attention 

 among the workers on the morphology of tlie compound eyes 

 of Arthropods may be put in four principal categories, viz. 

 (1) the mode of termination of the optic nerve fibre in the 

 retinal cell; (2) the mode of formation of difierent "layers" in 

 the retina; (3) the phylogeny of the compound eye; and (4) the 

 homology of the optic ganglia. 



The problem therefore involves several inquiries. The funda- 

 mental step is to determine to what morphological category 

 the compound eye of the Arthropod belongs. To the elucidation 

 of this point our histological inquiries must be directed, while 

 our phylogenetie consideration of the eye may fitly be reserved 

 until we have learned something of its structure. 



The first thing which confronts us is this: "What is the 

 ommatidium or " eyelet," the repetition of which, often several 

 thousandfold, gives rise to the compound eye? What is the 

 significance of the characteristic arrangement of its component 

 cells ? Is it possible to reduce it to a simpler form, or to express 

 its structure and significance briefiy ? 



