The Organs of Respiration 105 



their native habitat, the water. Nor did the change end here. 

 In time the waters were invaded by the reptiles, numerous swim- 

 ming forms appearing, which it is hkely were abundant in the 

 shallower shore-line of the ocean, while they sent many repre- 

 sentatives far out to sea. These were actively carnivorous, 

 making the fish their prey, the great mass of whom were doubt- 

 less driven into the deeper waters, beyond the reach of their air- 

 breathing foes. 



" In this change of conditions we seem to perceive an adequate 

 cause for the loss of air-breathing habits in those fishes in which 

 the lung development had not far progressed. It may indeed 

 have been a leading influence in the development of the Teleostean 

 or bony fishes, as it doubtless was in the loss of its primitive 

 function by, and the subsequent changes of, the air-bladder. 



"Such of the Crossopterygians and Dipnoans as survived in 

 their old condition had to contend with adverse circumstances. 

 Most of them in time vanished, while their descendants which 

 still exist have lost in great measure their air-breathing powers, 

 and the Dipnoans, in which the development of the lung had 

 gone too far for reversal, have degenerated into eel-like, mud- 

 haunting creatures, in which the organs of locomotion haA'e 

 become converted into the feeble paddle-like limbs of Neocera- 

 todus and the filamentary appendages of the other species. 



" As regards the presence of a large quantity of oxygen in the 

 bladders of deep-swimming marine fishes, it not unlikely has a 

 respiratory purpose, the bladder being, as suggested by Semper, 

 used as a reservoir for oxygen, to serve the fish when sleeping, 

 or when, from any cause, not actively breathing. The excess 

 of oxygen is not due to any like excess in the gaseous contents 

 of sea-water, for the percentage of oxygen decreases from the 

 surface downward, while that of nitrogen remains nearly un- 

 changed. In all cases, indeed, the bladder may preserve a share 

 of its old function, and act as an aid in respiration. Speaking 

 of this, Cuvier says: 'With regard to the presumed assistance 

 which the swim-bladder affords in respiration, it is a fact that 

 when a fish is deprived of that organ, the production of car- 

 bonic acid by the branchife is very trifling,' thus strongly indi- 

 cating that the bladder still plays a part in the oxygenation of 

 the blood. 



