The _retina_ (_r_) is the membrane into which the optic nerve penetrates, and in which its terminal filaments are distributed. It consists of several layers (Fig. 49). The outermost is an extremely delicate membrane (_a_), followed by a layer of nerve-cells (_b_), from which the terminal filaments issue, passing through several granular strata (_c_, _d_, _e_), on which the innermost stratum rests. This stratum is composed of cylindrical rods (_f_) vertically arranged, between which twin fusiform corpuscles (_g_) are intercalated. This last layer is thickly covered with a dark pigment. The retina extends over a portion of the iris, and a well-defined raised rim runs along its anterior margin. [Illustration: Fig. 49.--Vertical section of the Retina of the Perch, magn. X 350.] The _vitreous humour_ (Fig. 48, _h_) which fills the posterior cavity of the eyeball, is of a firmer consistency than in the higher Vertebrates. The _lens_ is spherical, or nearly so; firm, denser towards the centre, and lies in a hollow of the vitreous humour. When a falciform process is present, it is with one end attached to the lens, which is thus steadied in its position. It consists of concentric layers consisting of fibres, which in the nucleus of the body have marginal teeth, by which they are interlocked together. In _Petromyzon_ this serrature is absent, or but faintly indicated. [Illustration: Fig. 50.--Interlocking fibres of lens, highly magnified.] The anterior cavity of the eye is very small in Fishes, in consequence of the small degree of convexity of the cornea; the quantity of the aqueous humour, therefore, is very small, just sufficient to float the free border of the iris; and the lessened refractive power of the aqueous humour is compensated by the greater convexity of the lens. * * * * * _Organ of Hearing._--No trace of an organ of hearing has been found in _Branchiostoma_. In the _Cyclostomes_ the labyrinth is enclosed in externally visible cartilaginous capsules laterally attached to the skull; it consists of a single _semicircular canal_ in the Myxinoids, whilst the Petromyzontes possess two semicircular canals with a _vestibulum_. In all other fishes the labyrinth consists of a vestibule and three semicircular canals, the vestibule dilating into one or more sacs which contain the otoliths. A tympanum, tympanic cavity, and external parts, are entirely absent in the class of fishes. In the _Chondropterygians_ and _Dipnoi_, the labyrinth is enclosed in the cartilaginous substance of the skull. In the former the excavation in the cartilage is larger than the membranous labyrinth, but nearly corresponds to it in form; the part which receives the membranous vestibulum is called _Vestibulum cartilagineum_, from which a canal issues and penetrates to the surface of the skull, where it is closed by the skin in Sharks, but opens by a minute foramen in Rays. The otolithic contents are soft and chalklike.