VOMPOUXD EYES OF ARTIIliOI'ODS. ;S3:{ 



the outside of the mediiin fold (wi./). Counting from the outside 

 we meet with tiiree consecutive stratii of basement membrane in tliis 

 region of tlie eye. 



Fig. 64. In this we come nearly to the posterior extremity of the 

 median fold («)./"). The median fold is further from the surface 

 basement menibriine than in Fig. 03. 



Plate XXXV. 



Figs. 65-09. Diagrams showing the probable evolution of the 

 three-layered ommatidium from the single-layered surface depres- 

 sion in the skin, by the gradual subsidence of the neuro-epithelial 

 elements, Rt and O, Fig. 65. In Fig. 60 the ommatidium of 

 Limulus is represented, which is considered a step furtlier advanced 

 from the condition shown in Fig. 65. The distal end of the 

 retinula (^0 instead of being pointed toward the exterior as in 

 Fig. 65, in Limulus it points towards the median axis of the 

 ommatidium. The chitinous substance being still secreted on the 

 outside, a distinct body of chitin beneath the lens-cone (C) is 

 formed, the rhabdoni {Rb). In Fig. 67 this deepening is supposed 

 to have gone still further, resulting in the formation of anotiier inde- 

 pendent chitinous body, the crystalline cone {G.c). In Fig. 68 this 

 deepening is considered to have advanced still further, the crystalline 

 cone (C. c) being entirely separated from the corneal lens (C) by a 

 distinct stratum of cell, the corneagen {c.y). In Fig. 69 an omma- 

 tidium with three strata of cells, each secreting chitinous substance 

 on the part of their surface, is formed. These three strata of cells 

 are known as the corneagen {eg), the vitrella (F), and the retinula 

 (_Rl). Three chitinous bodies secreted by each group of cells above 

 mentioned are the cornea (C), the crystalline cone (c. c), and the 

 rhabdomere ( Rb), respectively. 



Fig. 70. — Serolis. Diagram of the ommatidium of <S'eroZ<.s. Gen- 

 eral arrangement of cells in this is not very different from that shown 

 in Fig. 69. The place of ganglion cell in Fig. 69, G, is taken by a 

 pair (of whicii only one is shown in the diagram) of transparent 

 "hyaline cells" (H). 



FiG. 71. — Cambariis. This is introduced in comparison with the 

 hypothetical ommatidia. 



Fig. 72.— Callinectes. 



Fig. Ti.— Talorchestia. 



In the last three forms no element corresponding to the central 



