{89}1. The optic ganglion of the Arthropoda consists of two parts, of which the one stands in direct inseparable connection with the facetted eye, and together with the layer of retinal rods forms the retina of the facetted eye, while the other part is connected rather with the brain, and is to be considered as an integral part of the brain in the narrower sense of the word. [Illustration: FIG. 38.--THE RETINA OF MUSCA. (After BERGER.) _Br._, brain; _O.n._, optic nerve; _n.l.o.g._, nuclear layer of ganglion of optic nerve; _m.l._, molecular layer (Punktsubstanz); _n.l.r.g.i._ and _n.l.r.g.o._, inner and outer nuclear layers of the ganglion of the retina; _f.br.r._, terminal fibre-layer of retina; _r._, layer of retinal end-cells (indicated only).] 2. In all arthropods examined by him, the retina consists of five layers, as follows:-- (1) The layer of rods and their nuclei. (2) The layer of nerve-bundles. (3) The nuclear layer. (4) The molecular layer. (5) The ganglion cell layer. Berger passes under review the structure and arrangement of the optic ganglion in a large number of different groups of arthropods, and concludes that in all cases one part of the optic ganglion is always closely attached to the visual end-cells, and this combination he calls the retina. On the other hand, the nerve-fibres which connect the peripheral part of the optic ganglion with the brain, the so-called optic nerve, are by no means homologous in the different groups; for in some cases, as in many of the stalk-eyed crustaceans, the whole optic ganglion is at the periphery, while in others, as in the Diptera, only the retinal ganglion is at the periphery, and the nerve-stalk connects this with the rest of the optic ganglion, the latter being fused with the main brain-mass. In the Diptera, in fact, according to Berger, the optic nerve {90}and retina are most nearly comparable to those of the vertebrate. For this reason I give Berger's picture of the retina of Musca (Fig. 38), in order to show the arrangement there of the retinal layers. [Illustration: FIG. 39.--THE BRAIN OF _Sphæroma serratum_. (After BELLONCI.) _Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _f.br.r._, terminal fibre-layer of retina; _Op. g. I._, first optic ganglion; _Op. g. II._, second optic ganglion; _O.n._, optic nerve-fibres forming an optic chiasma.]