In a few marine and in a large number of freshwater Teleosts the auditory organ enters into a more or less intimate connexion with the air-bladder by one of three different methods. {389}[Illustration: FIG. 222.—Cavity of the air-bladder of a Siluroid (_Macrones nemurus_) exposed by the removal of its ventral wall. _a.c_, Anterior chamber; _b.o_, basioccipital; _b.w_, body wall, here reduced to the external skin; _cl_, clavicle; _l.c_, lateral chamber; _l.s_, longitudinal septum; _pt_, post-temporal; _tr.a_, anterior portion of the tripus; _tr.c_, crescentic portion of the tripus; _t.s_, transverse septum; _t.s_', shorter transverse septum. (From Bridge and Haddon.)] The first and simplest is by the apposition of the extremities of a pair of caecal tubular prolongations from the air-bladder to the outer surfaces of the fibrous membranes which close a pair of vacuities in the outer bony walls of the periotic capsules, the inner surfaces being bathed by the perilymph surrounding the auditory organs. This method is characteristic of certain Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae,[457] and probably in the Hyodontidae. In the second method, of which several Clupeidae (_e.g._ Herring, Pilchard, etc.) furnish examples, the periotic vacuities are open instead of closed, and the sac-like ends of the tubular extensions from the air-bladder are in actual contact with protruding outgrowths from the utriculus.[458] The third method, by far the most elaborate, is by the intervention of a series of movably connected "Weberian" ossicles, of which the most posterior on each side (the tripus) is inserted into the dorsal wall of the air-bladder (Fig. 223), while the anterior one (scaphium) forms the outer wall of a median backward prolongation (sinus impar) of the perilymph-containing spaces surrounding the two auditory organs. This in turn encloses a similar median prolongation (sinus endolymphaticus) from the two sub-cerebrally united endolymphatic ducts (Fig. 223).[459] This complex mechanism is present in the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence the term "Ostariophysi"[460] as a collective name for these families.[461] {390}The physiological _raison d'être_ of the connexion between the air-bladder and the auditory organ cannot yet be regarded as satisfactorily determined. It is possible, as Weber thought, that it may be an auxiliary to the function of hearing by transmitting to the ear sound-waves impinging on the surface of the body and affecting the gases in the air-bladder.[462] On the other hand, it may be urged with perhaps greater probability that the connexion exists for the purpose of conveying to the ear stimuli due to the varying degrees of distension of the air-bladder, such as, it may be presumed, are naturally brought about by the variations of hydrostatic pressure which a Fish encounters in the course of its ascent or descent in the water.[463]