FISHES 151 



anybody who has ever killed a fish will remember the 

 large structure filled with air. The bladder is sur- 

 rounded by muscles, and when it is compressed by 

 these the fish descends in the water. As the fish has 

 about the same specific gravity ^ as the water, it becomes 

 heavier when the air in the bladder is compressed, and 

 so descends ; when the pressure is relaxed, the air 

 expands in the bladder, the volume of the fish is 

 increased, it is specifically lighter, and it rises to the 

 surface. Pressure on the front part of the bladder 

 causes the head to sink; pressure at the back sends 

 down the tail. 



In about half the species of fishes the bladder is 

 connected with the gullet by a duct. This is especially 

 developed in the dipneust or mud-fishes, which live in 

 tropical waters that dry up in the rainless season. In 

 many species the bladder is double. When the water 

 disappears from the pond they live in, and they find 

 themselves on the mud, the bladder takes over the 

 function of breathing from the now useless gills. They 

 absorb the oxygen from the air that presses into the 

 bladder through the gullet, while the carbonic acid that 

 must be given off from the fish's body passes away by 

 the bladder and its duct. 



The mud-fishes are most interesting to us, because 

 they show that an organ which seems to make its 

 appearance suddenly in the land-vertebrates is really 

 found in a rudimentary form in the fishes. If we had 

 not the mud-fishes, we should scarcely be able to 



^ The reader will know that we mean by specific gravity the number 

 of times that a body is heavier than an equal volume of water. 



