334 THE EVOLUTION OF MAX. 



the mouth ; in this way the circumference of the swim- 

 ming-bladder is diminished, and the fish becomes heavier 

 and sinks. When the animal is again about to ascend, 

 the swimming-bladder is distended by remitting the com- 





FIG. 284. Iiitestine of an embryonic Dog (which is represent^., in Fig. 

 137, vol. i. p. 382 ; after Bischoff), from the ventral side : a, gill-arches (four 

 pairs); b, rudimentary throat and larynx; c, lungs ; d, stomach ;/, liver ; g, 

 walls of the opened yelk-sac, into which the central intestine opens by a 

 wide aperture ; h, rectum. 



FIG. 285. The same intestine, seen from the right side : a, lungs ; b, 

 stomach ; c, liver ; d, yelk-sac ; e, rectum. 



pressing force. This hydrostatic apparatus begins to be 

 transformed into a respiratory organ in the Mud-fishes 

 (Dipneusta), the blood-vessels in the wall of the swim- 

 ming-bladder no longer merely separating air, but also 

 inhaling fresh air, which has come in through the air- 

 passage. This process is fully developed in all Amphibia. 

 The original swimming-bladder here generally becomes a 



