334 



THE EVOLUTION OF MAK. 



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. — Intestine of an embryonic Dog (which is representev^ in Fig. 

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

 pairs) ; 6, 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 ; /i, rectum. 



Fig. 285. — The same intestine, seen from the I'ight side : a, lungs ; t, 

 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 



