The free extremity of the eyestalk presents a convex, soft, and transparent surface, limited by an oval contour. The cuticle in this region, which is termed the _cornea_, (fig. 28, _a_), is, in fact, somewhat thinner and less distinctly laminated than in the rest of the eyestalk, and it contains no calcareous matter. But it is directly continuous with the rest of the exoskeleton of the eyestalk, to which it stands in somewhat the same relation as the soft integument of an articulation does to the adjacent hard parts. [Illustration: FIG. 28.—_Astacus fluviatilis._—A, a vertical section of the eye-stalk (× 6); B, a small portion of the same, showing the visual apparatus more highly magnified; _a_, cornea; _b_, outer dark zone; _c_, outer white zone; _d_, middle dark zone; _e_, inner white zone; _f_, inner dark zone; _cr_, crystalline cones; _g_, optic ganglion; _op_, optic nerve; _sp_, striated spindles.] The _cornea_ is divided into a great number of minute, usually square facets, by faint lines, which cross it from side {119} to side nearly at right angles with one another. A longitudinal section shows that both the horizontal and the vertical contours of the cornea are very nearly semicircular, and that the lines which mark off the facets merely arise from a slight modification of its substance between the facets. The outer contour of each facet forms part of the general curvature of the outer face of the cornea; the inner contour sometimes exhibits a slight deviation {120} from the general curvature of the inner face, but usually nearly coincides with it. When a longitudinal or a transverse section is taken through the whole eyestalk, the optic nerve (fig. 28, A, _op_) is seen to traverse its centre. At first narrow and cylindrical, it expands towards its extremity into a sort of bulb (B, _g_), the outer surface of which is curved in correspondence with the inner surface of the cornea. The terminal half of the bulb contains a great quantity of dark colouring matter or pigment, and, in section, appears as what may be termed the _inner dark zone_ (_f_). Outside this, and in connection with it, follows a white line, the _inner white zone_ (_e_), then comes a _middle dark zone_ (_d_); outside this an outer pale band, which may be called the _outer white zone_ (_c_), and between this and the cornea (_a_) is another broad band of dark pigment, the _outer dark zone_ (_b_).