136 The Life Story of the Fish 



it seems to me that the name "swim-bladder," which is 

 sometimes given it, is misleading. With locomotion in any 

 direction it has nothing to do. True, the mixture of gases 

 which it contains is not air, and it should really be called 

 a "gas-bladder," but at least the name "air-bladder" is more 

 indicative of its true nature than "swim-bladder." 



We know that it is not even a passive help to the fish in 

 making depth changes, as the first theory held. Fish with- 

 out the air-bladder are almost incompressible, and there- 

 fore have an almost constant volume at all depths. If they 

 are in equilibrium with the water at one level, they are in 

 equilibrium at all levels. They change from one depth to 

 another without disturbance. Fish with air-bladders are 

 compressible. In order to maintain their constancy of volume 

 they have therefore to increase or decrease the pressure in 

 their air-bladders. The air-bladder has to work to overcome 

 the weakness which it itself has introduced. It is in no way 

 a help; it merely succeeds in overcoming the obstacle of its 

 own presence. It is probably just as much of a nuisance to 

 the fish, in so far as depth changes are concerned, as our 

 lungs would be to us if we suddenly acquired gills and took 

 to living under the water. 



And we know that it is not an indispensable device to 

 keep the fish from too rapid or too great changes of level. If 

 such a pressure register were necessary to keep the fish within 

 safe limits, the halibut with its great changes of depth would 

 long since have perished. 



And it may be that some day exceptions will be found to 

 every one of the statements we have just madej but they 

 will still remain in general true. 



There is one more point about the air-bladder which is of 

 interest to human beings. In most present-day fish the tube 

 which connects the air-bladder to the throat opens into the 

 upper side of the gullet; but this was not always so. In the 



