The Organs of Respiration 101 



outgrowth from the fore part of the alimentary tract, the only 

 difference being that the air-bladder usually rises dorsally and 

 the lung ventrally. The fact already cited, that the pneumatic 

 duct is always present in the larval form in fishes that possess 

 a bladder, is equally significant. All the facts go to show that the 

 introduction of external air into the body was a former function 

 of the air-bladder, and that the atrophy of the duct in many 

 cases, and the disappearance of the bladder in others, are results 

 of the loss of this function. 



" Such an elaborate arrangement for the introduction of air 

 into the body could have, if we may judge from analogy, but 

 one purpose, that of breathing, to which purpose the muscular 

 and other apparatus for compressing and dilating the bladder, 

 now seemingly adapted to gravitative uses, may have been origi- 

 nally applied. The same may be said of the great development 

 of blood-capillaries in the inner tunic of the bladder. These 

 may now be used only for the secretion of gas into its interior, 

 but were perhaps originally employed in the respiratory secre- 

 tion of oxygen. In fact all the circumstances mentioned the 

 similarity in larval development between the bladder and lung, 

 the larval existence of the pneumatic duct, the arrangements for 

 compressing and dilating the bladder, and the capillary vessels 

 on its inner tunic point to the breathing of air as its original 

 purpose. 



" It is probable that the Ganoid, as well as the Dipnoan, air- 

 bladder is to some extent still used in breathing. The Dipnoans 

 have both lungs and gills, and probably breathe with the latter 

 in ordinary cases, but use their lungs when the inland waters in 

 which they live become thick and muddy, or are charged with 

 gases from decomposing organic matter. The Ganoid fishes to 

 some extent breathe the air. In Polypterus the air-bladder re- 

 sembles the Dipnoan lung in having lateral divisions and a ventral 

 connection with the oesophagus, while in Lepisosteus (the Amer- 

 ican garpike) it is cellular and lung-like. This fish keeps near 

 the surface, and may be seen to emit air-bubbles, probably 

 taking in a fresh supply of air. The American bowfin, or mud- 

 fish (A-mia), has a bladder of the same lung-like character, 

 and has been seen to come to the surface, open its jaws 

 widely, and apparently swallow a large quantity of air. He 



