Eyes of MoUuscs aud Arthropods. 651 



tidium, to designate the individuai eyes of which the Arthropod omma- 

 teum is composed. Accordiui;- to my own reseaches, the strueture and 

 morphologieal signification of the latter term is entirely diffcrent from 

 that ascribcd to it by my predecessors. Two difliculties have presented 

 themselves, either uew terms must be invented to designate whatlcon- 

 sider to be the real unit of the Arthropod, or in fact of all eyes, or the 

 meaning of the old, must be exteuded to suit my conception of the 

 structure. I bave chosen the latter course, the less of the two evils. 



By the term ommatidium, I shall designate those constructive 

 Clements of ali eyes consistìng of single (?; or compound, colorless cells 

 the re tino p bora e, surrouuded by one or more cìrcles of pigmented 

 ones, or retinulae. 



A typical Molluscan ommatidium consists of a double, colorless 

 aud, in some cases, gland-like celi, the retinophora, contaiuing two 

 nuclei and an axial nerve fibre. The external surface of the retinophora 

 is provided ^ith nerve fibres, which at the outer ends of the cells break 

 up into numerous fibrillae directly continuous with simìlar ones from the 

 axial nerve. This network of cross fibrillae , the retinidium, is usually 

 supported bya cuticular secretion, or rod, of the retinophora. The latter, 

 except in the retinated types, always coutains a number of refractive 

 globules which serve as a kind of mirror, or argentinula, for each rod, so 

 that the rays of light are reflected back again , passing a second time 

 through the retinidium. The retinophorae are surrouuded by a circle of 

 pigmented cells, which mayalso be provided with nervous retiatermi- 

 nalia imbedded in a cuticular secretion; but these cells are in ali 

 cases single and coutaiu no axial fibres. In the simplest condì tion, the 

 terminal nerve fibrillae of the retinophorae, of the pìgmeut cells, or 

 retinulae, aud of the adjacent, uumoditìed epithelial cells, form a con- 

 tinuous nervous network. 



The retinophorae, iu ali the varied stages of their modification, 

 are distinguished by their pointed inner ends drawn out into the axial 

 nerve fibre, and by the presence of two nuclei ; one is faint and diftìcult 

 to observe, while the other is large and ovai, stains sharply but not 

 deeply, and alwa3'S coutains a nucleolus. 



The retinulae end iu several root-like fibres resting upon the 

 basai membrane. Their nuclei are comparatively small, staiu deeply 

 but not so clearly and sharply as those of the retinophorae, and, as far 

 as I have observed, never contain nucleoli. 



The retinulae may be transformed into gangliouic cells, which, 

 in the early stages, migrate inwards leaving the retineum, with 



