1354 THE STORY OF THE UNIVERSE 



dinary fishes is to alter the specific gravity of the ani- 

 mals; and, by the compression or expansion of the 

 air or gases it contains, to enable them to sink or rise 

 in the water at will ; but it would also appear that in- 

 directly it may aid in the breathing of all fishes which 

 possess the organ. In the mud-fishes, however, the 

 air-bladder becomes divided externally into two 

 sacs, while internally each sac exhibits a cellular 

 structure resembling that seen in the lungs of higher 

 animals, with which structures, in fact, the swim- 

 ming-bladder of fishes actually corresponds. Then 

 also this elaborate air-bladder of the mud-fish com- 

 municates with the mouth and throat by a tube, 

 which corresponds to a windpipe. The nostrils of 

 the mud-fishes further open backward into the 

 mouth ; while, as already mentioned, in all other 

 fishes, save one genus, the nostrils are simple, closed, 

 pocket-like cavities. And it may lastly be noted that 

 the Lepidosirens are in addition provided with true 

 gills, like their ordinary and more commonplace 

 neighbors. 



These remarks serve to explain the "reason why" 

 these fishes can exist for months out of water. Thus, 

 on the approach of the hot season, the mud-fishes 

 leave their watery homes and wriggle into the soft 

 mud of their native rivers. Here they burrow out 

 a kind of nest, coiling head and tail together; and 

 as the mud dries and hardens, the fishes remain in 

 this temporary tomb ; breathing throughout the warm 

 season like true land-dwellers, by means of the lung- 

 like air-bladder. When the wet season once more re- 

 turns, the fishes are aroused from their semi-torpid 



