154. S. WATASE. 



Still wider field of vision. In this connection I may refer to a 

 series of diagrams (PI. XIX, figs. 10-17). The black heavy 

 layer represents the ectoderm, and the region in which the 

 ectoderm is thrown into folds the area of the compound eye. 

 The yellow-coloured layer outside represents the chitin, and 

 the dotted line beneath the ectoderm the basement mem- 

 brane. 



In Limulus (fig. 10) the ectoderm is thrown into a series 

 of shallow folds, which, when viewed from above, would be a 

 group of shallow pits in the skin. Each pit is an omraatidium. 

 In Serolis (fig. 11) the invagination of the skin is a little deeper 

 than that of Limulus, and the whole ocular area is more pro- 

 minent. Fig. 12 represents the condition of the ectodermal 

 folding inNotonecta, and fig. 13 that of A gr ion larva. Fig. 

 14 represents the eye of Branchipus, only a part of the stalk 

 being shown in the figure. Fig. 15 represents the eye of 

 Cambarus; fig. 16 that of Peneeus ; and fig. 17 that of 

 Lucifer. 



It must not be understood that the number of folds given 

 in the diagrams have anything to do with the actual number of 

 ommatidia that may exist in the actual specimens ; no more 

 than a morphological expression of the eye in a simplest 

 possible form was intended. If one suppose a single invagina- 

 tion of the skin, say of fig. 15, to be divided into three strata 

 and the cells in the bottom stratum to send out nerve-fibres, 

 those in the middle to form the crystalline cone and those in 

 the outermost to form the cornea (fig. 7), the interpretation of 

 the diagram will be complete. 



According to this view the compound eyes of Arthropods, 

 either in the sessile or in the stalked form, are nothing more 

 than a collection of ectoderm pits whose outer open ends face 

 towards the sources of light, and whose inner ends are con- 

 nected with the central nervous system by the optic nerve- 

 fibres. The cells forming the walls of the pit arrange them- 

 selves into three strata, in most cases accompanied by three 

 regional functional differentiations. Grenadier's classification 

 of the compound eyes of insects into " acone,'' " pseudocoue,'' 



