CHAPTER VI 



The Air-Bladder 



ONCE upon a time, all the bony fishes breathed air. This 

 was many, many million years ago, long before there were 

 any human beings. The bony fishes were then living in 

 streams, lakes, pools, and swamps, where they at first breathed 

 only water. When, in the Devonian era, the climate changed, 

 the inland waters began to dry up and stagnate. They no 

 longer contained enough oxygen to sustain animal life. By 

 processes which we still do not entirely understand but 

 which we name evolution, .the fish acquired a pouch, open- 

 ing out of the throat, into which air could be taken to pro- 

 vide the oxygen which the water no longer held. 



By the time the climate changed again and the waters 

 increased once more, some of the fishes, armed with the new 

 pouch which made them free of water, had gone out on 

 land and become amphibians and reptiles. A few remained 

 in the water but kept on breathing air, like the present Afri- 

 can lung-fish. The majority ceased to breathe air and went 

 back to complete water-dwelling, but they retained, and 

 still carry, the remains of that pouch without which they 

 would have perished. It is now called the air-bladder. And 

 so both the human lungs and the fish's air-bladder derive 

 from the same primitive respiratory sac. 



The sharks and rays lack all trace of an air-bladder. All 

 the bony fishes alive today either have an air-bladder, or 

 show that they once had it. The sharks and rays must have 

 left the fresh water and gone out to sea before the great 



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