[Illustration: FIG. 206.—The air-bladder and elastic-spring-mechanism in _Auchenipterus nodosus_. A, Cavity of the bladder exposed by the removal of its ventral wall: _a.c_, anterior chamber; _cl_, clavicle; _c.tr_, crescentic process of the tripus; _l.c_, left lateral chamber; _l.s_, longitudinal septum separating the two lateral chambers; _oes_, oesophagus; _p.s_, pectoral spine; _t.s_, the narrow transverse septum which partially separates the anterior from the two lateral chambers. B, Ventral view of the anterior vertebrae, to show the elastic springs: _es_, the oval bony plates in which the elastic springs terminate; _r^1_, first rib; _t.p^5_, transverse process of the fifth vertebra; _v^1_, first vertebral centrum; _cl_, _oes_, and _ps_, as in A. (From Bridge and Haddon.)] In the South American Siluroid, _Auchenipterus nodosus_, the transverse processes of the fourth vertebra are bent downwards and backwards, and at the same time become converted into flexible and highly elastic springs (Fig. 206, B). Their distal extremities expand into oval bony plates which are imbedded in the anterior wall of the air-bladder, and often cause the latter to bulge inwards (Fig. 206, A). From the occipital region of the skull arise two powerful muscles which pass backwards to {359}their insertion into the anterior faces of the two springs. By the contraction of these muscles the springs, and consequently also the front wall of the bladder, are drawn forwards; but directly the muscles relax, the elasticity of the springs causes them to move backwards to their former position, carrying with them the wall of the air-bladder. Hence it follows that the rapid alternating contraction and relaxation of the muscles will impart a vibratory movement to the anterior wall of the bladder and to the gaseous contents of that organ, with the result that a sound is produced. As a rule, those Fishes in which an elastic-spring-mechanism is present have the air-bladder subdivided by internal septa into a series of chambers freely communicating with one another; and no doubt the intensity of the sound is greatly increased by the vibratory movements of the gases across the free edges of the septa, and from one chamber to another. The elastic-spring type of vocal organ is apparently restricted to the Siluridae,[421] and besides occurring in _Auchenipterus_ is found also in the South American genera _Doras_, _Oxydoras_, _Rhinodoras_, and _Euanemus_; in the African genera _Synodontis_ and _Malopterurus_; and in at least four species of the Indian genus _Pangasius_.[422] There are also a few Teleosts in which the air-bladder is provided with special muscles, but, instead of being connected with elastic springs, the muscles extend from the skull, and are inserted directly into the wall of the bladder (Fig. 207); or, without being in any way attached to the skeleton, the muscles simply invest some portion of the surface of the air-bladder. In other Fishes the air-bladder,