224 FISHES 



to the length of four or five feet, and the body is covered 

 with scales (Fig. 160). It has gills like other fishes, but, 

 in addition, the air bladder is mocUfied into a respiratory 

 organ that opens into the pharynx, like the lungs of mam- 

 mals. The blood flows to this lung and is there purified 

 exactly as in the higher animals. The Ceratodus lives in 



Fig. 160. — Australian lung fish. 



still pools that become very stagnant during the dry sum- 

 mer season, and it survives only by rising to the surface 

 now and then to take fresh air directly into the lung. 



Of the other lung fishes, one species lives in South Africa 

 and one species is found in the rivers of South America. In 

 both of these species the modified air bladder, or lung, is 

 divided into two lobes, and in this respect they differ from 

 Ceratodus. The walls of each lobe are supphed with many 

 blood vessels filled with venous blood which is brought to 

 the lung by a special pulmonary artery to be purified. The 

 species in South Africa also lives in pools which dry up during 

 the hot season. Before the water wholly evaporates the 

 fish descends into the mud and hollows out a place in which 

 it lies until the rains come again. 



General character of fishes. — For the fir-st time we now 

 meet with animals that have a jaw bone and bony skeleton. 

 The skeletons of all fishes are not bony, for some have 

 cartilaginous skeletons. Moreover, the fishes have a true 

 skull, which, however, in some is rather rudimentary. The 



