The Air-B ladder 135 



goes to the other, have to work to keep below the surface 

 or to stay off bottom unless he can make an appropriate 

 change in his volume. Experiments with killifish, some of 

 which are able to stand abrupt change from fresh to salt 

 water or the reverse, show that when first placed in salt 

 water they have to swim constantly downward to overcome 

 their buoyancy, and when first placed in fresh water, con- 

 stantly upwards to overcome their weight, but that in fifteen 

 minutes they adjust themselves sufficiently to the changed 

 density to maneuver without any perceptible difficulty. 



In the third place we definitely know that the ajr-bladder ' 

 has been turned into an efficient hearing aid in one way or 

 another by many species of fish, including the predominating 

 group, both in number of species and number of individuals, 

 in all the fresh waters of the world. 



In the fourth place we definitely know that some fish use 

 the air-bladder to make noises. Best known is the weakfish 

 and its allies. It has a peculiar muscle by which it can set 

 the air-bladder into vibration and produce sounds. The 

 male only possesses this muscle, and the male only can 

 make the sound, from which we judge that it has some- 

 thing to do with mating; and so loud is this sound that it 

 has been heard six feet above water when the fish is fifty 

 feet under water. 



So much for the positive side — for the things which we 

 know that the air-bladder does do. As for the things which 

 we know that the air-bladder does not do, while at first 

 thought a summary of the negatives may seem of little value, 

 it is undertaken here to dispel the many misconceptions 

 which have been held in the past and which are to some 

 extent still held. 



We know that the air-bladder is not an active instrument 

 for changing levels — that fish do not raise or lower them- 

 selves by increasing or decreasing their size. For this reason 



