322 JS. WATASE. 



sents the ectoderm, and the region in which the ectoderm is 

 thrown into folds the area of the compound eye. Tlie yclhnv 

 colored layer outside represents the chitin, and the dotted line 

 beneath the ectoderm, the basement membrane. 



In Zimv/us (Fig. 74) 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 ommatidium. In 

 Sei-olis {Fig. 75) the invagination of the skin is a little deeper than 

 that of ZimuluD, and the whole ocular area is more prominent. 

 Fig. 76 represents the condition of the ectodermal folding in 

 Notonecta, and Fig. 77 that oi' Agrioti larva. Fig. 78 represents 

 the eye of Branchipus, only a part of the stalk being shovvn in 

 the figure. Fig. 79 represents the eye of Cambarus ; Fig. 80 

 that oi Penaeus, and Fig. 81 that oi 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 morpliological expression of the eye in a simplest possible form 

 was intended. If one suppose a single invagination of the skin, 

 say of Fig. 79, 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. 71), the interpretation of the diagram will be com- 

 plete. 



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 ectodermic pits whose outer open ends face towards 

 the sources of light, and whose inner ends are connected with 

 the central nervous system by the optic nerve fibres. The cells 

 forming the walls of the pit arrange themselves into three strata, 

 in most cases accompanied by three regional functional difl'eren- 

 tiations. Grenadier's classification of the compound eyes of 

 insects into "acone," "pseudocone" and "eucone" types refers 

 to the condition of the cells and their products in the middle 

 stratum — the vitrellae. 



Morphologically, then, the compound eye of an Arthropod is 

 strictly single-layered, although, as is evident, the present concep- 

 tion is entirely diflerent from the monostichous theory main- 

 tained by some recent writers. From Limuliis to SquiUa we 



