Each of these joints is trihedral, the outer face being convex; the inner, applied to its fellow, flat; and the upper, on which the eyestalk rests, concave. On this upper face there is a narrow elongated oval aperture, the outer lip of which is beset with a flat brush of long close-set setæ, which lie horizontally over the aperture, and effectually close it. The aperture leads into a small sac (_au_) with delicate walls formed by a chitinous continuation of the general cuticula. The inferior and posterior wall of the sac is raised up along a curved line into a ridge which projects into its interior (fig. 27, A, _r_). Each side of this ridge is beset with a series of delicate setæ (_as_), the longest of which measures about 1‐50th of an inch; they thus form a longitudinal band bent upon itself. These _auditory setæ_ project into the fluid contents of the sac, and their apices are for the most part imbedded in a gelatinous mass, which contains irregular particles of sand {117} and sometimes of other foreign matter. A nerve (_n n′_,) is distributed to the sac, and its fibres enter the bases of the hairs, and may be traced to their apices, where they end in peculiar elongated rod-like bodies (fig. 27, C). Here is an auditory organ of the simplest description. It retains, in fact, throughout life, the condition of a simple sac or involution of the integument, such as is that of the vertebrate ear in its earliest stage. [Illustration: FIG. 27.—_Astacus fluviatilis._ A, the auditory sac detached and seen from the outside (× 15); B, auditory hair (× 100); C, the distal extremity of the same more highly magnified. _a_, aperture of sac; _as_, auditory setæ; _b_, its inner or posterior extremity; _n n′_, nerves; _r_, ridge.] {118} The sonorous vibrations transmitted through the water in which the crayfish lives to the fluid and solid contents of the auditory sac are taken up by the delicate hairs of the ridge, and give rise to molecular changes which traverse the auditory nerves and reach the cerebral ganglia. * * * * * The vibrations of the luminiferous ether are brought to bear upon the free ends of two large bundles of nerve fibres, termed the optic nerves (fig. 25, _on_), which proceed directly from the brain, by means of a highly complex _eye_. This is an apparatus, which, in part, sorts out the rays of light into as many very small pencils as there are separate endings of the fibres of the optic nerve, and, in part, serves as the medium by which the luminous vibrations are converted into molecular nerve changes.