AIR IN THE BODY OF FISHES. 321 



are a favourite article of food. They are caught in nets and 

 brought to the surface of the water ; they come up invariably 

 with the belly much distended ; the air in the swimming-blad- 

 der, being relieved from the pressure of the column of water, 

 has expanded greatly and occasioned this unnatural distension, 

 which renders the fish quite incapable of swimming. Under 

 these conditions the fish is naturally unable to live for any 

 length of time. But the fishermen of the lake have a very 

 simple remedy ; they prick into the air-bladder with a fine 

 needle ; the air escapes with some force, the distension subsides, 

 and the fishes are enabled to live under totally changed condi- 

 tions as to pressure, even in quite shallow water and at the 

 surface, swimming quite as freely as their companions, the 



FIj. 82. — The Kilch of the Late of Constance {Covpgonus U>maH\), showing the 

 distensiou caused by the expansiou of the air in the swimming-bladder. 



natives of the surface water. Hence the Kilch is confined to 

 a certain dejith, because it is not capable of accommodating the 

 tension of its swimming-bladder to the change of pressure in 

 the column of superincumbent water. Since, moreover, in the 

 Kilch the pressure from within outwards is the same as the ex- 

 ternal pressure, or must at any rate be very nearly the same, 

 the mechanical problem stated above has no existence for this 

 fish, nor for any other creatures living under the same condi- 

 tions. It can arise, in fact, only in those uncommon cases — such 

 as would seem to be oflfered, for instance, by the whale — where 

 an animal furnished with internal air-cavities plunges from the 

 surface of the sea down to considerable depths and remains 

 there for some length of time ; in these it is evident that some 



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