Bearing in mind the fact that the retinulæ are the nerve-end cells of the fibres passing through the periopticon, it will be well to read the following account, by Hickson, of the terminal anastomosis of the optic fibrils in the periopticon of _Agrion bifurcatum_, and to examine his sketch (Fig. 266): “The terminal anastomosis of Agrion may be conveniently divided into four regions. First the region (1) lying nearest to the periopticon in which the nerve-cells are numerous, and the fibrils leaving the periopticon form a complicated plexus; the region (2) next to this, in which the fibrils have collected into bundles separated by spaces occupied by very thin-walled tracheæ in which there are no spiral markings, and lymph-spaces; next, the region (3) in which the fibrils form a final plexus, and in which there are again a considerable number of nerve-cells; and, lastly, the region (4) in which the fibrils are again collected into bundles, separated by spaces containing tracheæ, which perforate the basement membrane to supply the retinulæ.” It would seem as if the decussation of the optic nerve-fibrils were a matter of primary importance, as it so generally occurs, but in the young of that most generalized of all pterygote insects, the cockroach (Periplaneta), Hickson states that the optic nerve-fibrils which leave the periopticon pass without decussating to the ommateum, and in the adult there is only a partial decussation. In Nepa there is no decussation, but the anastomosis is complicated by the presence of looped and transverse anastomoses. Looking at the eye as a whole, Hickson regards all the nerve structure of the eye lying between the crystalline cone-layer and the true optic nerve to be analogous with the retina of other animals. With Ciaccio, Berger, and others, he does not regard the layer composed of the retinulæ and rhabdoms as the equivalent of the retina of vertebrates, etc. =Origin of the facetted eye.=—The two kinds of eye, the simple and the compound, are supposed to have been derived from a primitive type, resembling the single eye (ommatidium) of the acone eye of Tipula. As stated by Lang, “an increase of the elements of this primitive eye led to the formation of the ocellus; an increase in number of the primitive eyes, and their approximation, led to the formation of the compound facet eye.” This view is suggested, he says, by the groups of closely contiguous single eyes of the myriopods, considered in connection with the compound eye of Scutigera. Grenacher looks upon simple (ocelli) and compound eyes as “sisters,” not derived from one another, but from a common parentage. Immature insects rarely possess compound eyes; they are only known to occur in the nymphs of Odonata and Ephemeridæ, and in the larvæ and pupa of Corethra.