XII 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,’ and besides oecurring in Auchenipterus is found also 
in the South American genera Doras, Oxydoras, Rhinodoras, and 
Huanemus ; 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 
im 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. 
* The mechanism is apparently absent in one species of Pangasius (P. micronema). 
Bridge and Haddon, op. cit. p. 220. 
