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sation for a purpose so definite and unequivocal * Cuvier 
says, the “ most obvious use of the swim-bladder is to keep the 
animal in equilibrium with the water, or to increase or reduce 
its relative weight, and thereby cause it to ascend or sink in 
proportion as that organ is dilated or compressed. For this 
purpose the fish contracts the ribs or allows them to expand. 
Certain we are, that when the air-bladder is burst, the animal 
remains at the bottom, turns up the belly, and becomes deficient 
in the powers of motion. A curious phenomenon is observed 
in fishes caught with hook and line at great depths and drawn 
up suddenly ; for the air contained in their swim -bladder ex- 
pands, as they are ascending, more rapidly than they can coun- 
teract by compression, and either it bursts and the abdomen 
becomes inflated, or by expanding forces the oesophagus and 
stomach out at the mouth.” Cuvier maintained that there is 
no sufficient foundation for assuming that the swim-bladder 
offers any analogy to lungs. Professor Eymer Jones, in speaking 
of this organ, remarks — ‘‘In connection with the locomotive 
organs we must here notice one of the most elegant con- 
trivances met with in the whole range of animated nature, by 
which the generality of fishes are enabled to ascend towards 
the surface or to sink to any required depth without exertion.” 
The general function of the swim-bladder, there can be little 
doubt, is a mechanical one ; nevertheless, it does not seem to 
me that this organ is of such importance, and such an elegant 
contrivance, as has been so repeatedly affirmed. For how is it, 
we may fairly ask, that in different genera of fishes of precisely 
similar habits, some have an air-bladder, others have not? We 
can understand why it should be absent in the Pleuronectidce, 
whose habits confine them to the bottom of the water, or in 
the angler with similar habits ; but when we find that one 
surface-swimming mackerel has a swim-bladder and another 
species of precisely similar habits is devoid of one, it is obvious, 
notwithstanding the general function of the organ when present, 
that it is by no means an essential adjunct to swimming. Of 
the two sun-fish {Ortliagoriscus) which occur in our own seas, 
one has a swim-bladder, the other none at all. Many of the 
Siluridce possess a large and sometimes complex swim-bladder, 
but genera occur in which there is no swim-bladder at all. So, 
again, fish that, for the most part, keep to the bottom are 
devoid of this organ ; but in the mud-loving eel we meet with 
swim-bladder and pneumatic duct. Experiments, too, if I am 
not mistaken, have been made according to which, after the 
puncture of the bladder and the severing of the muscles which 
act upon it, and even after its complete removal, the fishes have 
* ‘‘ Animal and Vegetable Physiology,” vol. i. page 429. 
