xni SOUND-PRODUCING ORGANS 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, 1 and besides occurring in Auclienipterus is found also 

 in the South American genera Doras, Oxydoras, Bhinodoras, and 

 Euanemits ; in the African genera Synodontis and Malopterurus ; 

 and in at least four species of the Indian genus Pangasius? 

 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, without possessing special muscles of its own, 

 may, nevertheless, be partially invested by tendinous, or partly 

 muscular and partly tendinous, extensions from the muscles of 

 the body-wall (Fig. 208), or may be intimately related to certain 

 muscles connected with the pectoral girdle. Whatever the 

 precise relation of the air-bladder to its muscles it is probable 

 that the physiological effect is in most cases the same. By the 

 rapid alternating contraction and relaxation of the muscles, some 

 part of the wall of the bladder becomes alternately compressed 



1 The elastic -spring -mechanism has been described by several writers, who 

 had assigned to it various functions, but Sorensen (op. cit. pp. 85-91) was the 

 first to discover its vocal function by observations and experiments on Doras 

 maculatus. 



a The mechanism is apparently absent in one species ofPangasius (P. micronema). 

 Bridge and Haddon, op. cit. p. 220. 



