THE AUDITORY ORGANS.—In its more typical condition each auditory organ consists of a membranous sac or vestibule, partially constricted into an upper portion or utriculus and a lower or sacculus (Fig. 221, A). Three semicircular canals are connected with the utriculus, of which two are vertical and at right angles to one another, and the third is horizontal. One end of each canal is dilated into an ampulla. A slender tube, the ductus endolymphaticus, leaves the sacculus, and ends in a sac-like swelling, the sinus endolymphaticus, which apparently represents a portion of the embryonic epidermic involution from which the auditory organ is formed. A smaller sac-like outgrowth from the sacculus, the _lagena_, corresponds to the cochlea of the higher Vertebrates. The epidermic lining of this system of cavities is differentiated into patches or ridges of sense-cells (maculae or cristae), separated by supporting cells and innervated by the terminal branches of the auditory nerve. There is a crista acustica in each ampulla; and maculae acusticae are present in the utriculus, sacculus, and lagena. A fluid, the _endolymph_, fills all the cavities, and a similar fluid or _perilymph_ occupies the spaces in the periotic capsule in which the various chambers are lodged. Among the more notable deviations from this type of auditory organ the Cyclostome _Myxine_, apparently, has but a single semicircular canal with an ampulla at each end, and the vestibule is a simple sac (Fig. 221, B). _Petromyzon_ has two canals, but lacks the horizontal canal. In Elasmobranchs, including _Chimaera_ (C), the ductus endolymphaticus retains its primitive connexion with the exterior by means of a pore on the dorsal surface of the head. In the Dipnoi (e.g. _Protopterus_) the paired endolymphatic sinuses divide into a number of caecal branches containing otoliths, which meet and interlace over the fourth ventricle (Fig. 217).[456] Otoliths, either in the form of fine, {388}mucus-connected, calcareous particles, as in Elasmobranchs, or as massive solid concretions in Teleosts, are present in relation with the sensory areas of the utriculus, sacculus, and lagena. [Illustration: FIG. 221.—Auditory organs of Fishes. A, of a typical Fish; B, of _Myxine_; C, of _Chimaera_; and D, of _Perca_. _a.c_, Anterior canal; _am_', _am_", _am_'", ampullae; _am.n_, nerves to ampullae; _c_, semicircular canal (in _Myxine_); _d.e_, ductus endo-lymphaticus; _h.c._ horizontal canal; _l_, lagena; _mc_, macula acustica; _m.s_, macula acustica of the sacculus; _n_, nerves to ampullae; _o_, external aperture of the ductus endo-lymphaticus; _p.c_, posterior canal; _s_, sacculus; _s.e_, sinus endo-lymphaticus; _sk_, superficial skin; _s.s_, sinus superior; _u_, utriculus; viii, auditory nerve. (From Wiedersheim, after Retzius.)]