The Air-Bladder 



137 



Devonian fishes it opened into the lower side, for the air- 

 bladder or "lung" lay below the digestive organs. This 

 seems mechanically unsound, since the bladder, being light, 



Food 



Nostril - Pdssd^e for Air 



/{ir Bladder Stomach 



Food 



Nostril - No Air Pdssd^e. 



Stomach Air Bladder 



Figure 17. AIR AND FOOD SYSTEMS OF EARLY FISH (above) 

 AND MODERN FISH (below). 



should naturally be above the stomach, and it is possible that 

 the tendency of the bladder to rise is responsible for the 

 present position of the opening. Anyway, whatever the 

 reason, in the early fishes the opening was on the lower side 

 of the throat. The nostrils, however, were on the top of 

 the head, and when they succeeded in acquiring a passage 

 through the skull into the mouth cavity for the fish to 



