560 E. EAY LANKESTEB. 



Euarthropoda, and distinct from the Chi^topod eye, wliicli is 

 found only in tlie Onycliophora where the true Arthropod 

 eye is absent. The essential difference between these two 

 kinds of eye appears to be that the Cluetopod eye (in its 

 higher developments) is a vesicle enclosing the lens, whereas 

 the Arthropod eye is a pit or series of pits into which the 

 heavy chitinous cuticle dips and enlarges knobwise as a lens. 

 Two distinct forms of the Arthropod eye are observed — the 

 monomeniscous (simple) and the polymeniscous (compound). 

 The nerve-end cells, which lie below the lens, are part of the 

 general epidermis. They show in the monomeniscous eye 

 (see article Arachnida,^ fig. 26) a tendency to group them- 

 selves into " retinulas," consisting of five to twelve cells 

 united by vertical deposits of chitin (rhabdoms). In the 

 case of the polymeniscous eye (fig. 23, article Aeachnida) a 

 single retiuula or group of nerve-end cells is grouped beneath 

 each associated lens. A further complication occurs in each 

 of these two classes of eye. The monomeniscous eye is rarely 

 provided with a single layer of cells beneath its lens; when 

 it is so, it is called monostichous (simple lateral eye of 

 scorpion, fig. 22, article Aeachnida). More usually, by an 

 infolding of the layer of cells in development, we get three 

 layers under the lens; the front layer is the corueagen layer, 

 and is separated by a membrane from the other two, which 

 more or less fuse and contain the nerve-end cells (retinal 

 layer). These eyes are called diplostichous, and occur in 

 Arachnida and Hexapoda (fig. 24, in article Aeachnida.). 



On the other hand, the polymeniscous eye undergoes 

 special elaboration on its lines. The retinulaj become elon- 

 gated as deep and very narrow pits (fig. 11 and explanation), 

 and develop additional cells near the mouth of the narrow 

 pit. Those nearest to the lens are the corueagen cells of this 

 more elaborated eye, and those between the original retinula 

 cells and the corueagen cells become firm and transparent. 

 They are the crystalline cells or vitrella (see Watase, 7). 



' This article will be reproduced from the ' Eucjclopaedia* in ihe next 

 number of this Journal. — E. K. L. 



