70 patten: [Vol. I. 



highly specialized eyes of Cephalopods and of Pecien, the double 

 rods of the retinophorae have alone been retained. In Area the 

 optic cups possess both single and double rods ; in the convex 

 faceted eyes found side by side with the optic cups, there are, 

 however, only double rods, while the cells bearing single rods 

 have been modified to serve secondary purposes. 



The compound eyes of Arthropods consist of two parts: a 

 thin outer layer of cells, the eorneal hypodermis ( Fig. 14, c.^j/.), 

 which secretes the corneal facets ; the remaining portion of the 

 eye, or ommateal hypodermisy although it is often extremely 

 thick, represents but a single layer of cells. These facts are of 

 importance in determining the homologies of the compound 

 eye. While Grenacher and his followers have either overlooked 

 or misunderstood the corneal hypodermis, they have maintained 

 that the crystalline-cone cells and the surrounding pigmented 

 ones constitute a distinct outer layer, and the retinulae and so- 

 called rhabdoms an equally distinct inner layer of the ommateum. 

 That the ommateum proper, which does not include the corneal 

 hypodermis, is not a double layer is shown by the fact that the 

 retinulse and other pigmented cells extend through the whole 

 thickness of the ommateum ; and, above all, by the fact that the so- 

 called rhabdom is not produced by the retinulse, but by the in- 

 ward prolongation of the crystalline-cone cells. It follows, 

 therefore, that generalizations founded upon the supposition 

 that the ommateum is two-layered are no longer tenable. 



We find good reasons for believing that the ocelli are also 

 composed of ommatidia having essentially the same structure as 

 those of the compound eye. The ocelli of spiders consist of 

 groups of cells, each cluster containing a double colorless cell 

 with either double apical rods, as in Molluscs, or with double 

 axial rods and overlying nuclei. The rods in the latter case 

 coincide essentially with the crystalline cones of the compound 

 eye, and we therefore consider them as homologous structures.^ 

 The compound cells, or retinophorae, of the Arachnid ocellus, 

 like those in the compound eye, are surrounded by circles of 

 pigment-cells. Although the retinophorae of spiders, as far as 



^ My recent observations on the eyes of Phalangium show that its ommateum is com- 

 posed of ommatidia in all essential points like those of the compound eye. The 

 resemblance of the threefold conical rods of Phalangium to the fourfold crystalline cone 

 is especially evident. Vide my preliminary account in this journal. 



