CHAPTER XXVII. 

 THE LUNG-FISH. 



EAELY in geological history the continents were much 

 smaller than new, but as soon as they began to attain their 

 present shape and size, when they were covered with for- 

 ests, and were diversified by mountain ranges, inland seas, 

 and great rivers, a new type of vertebrate life appeared 

 land animals with limbs and lungs. But their appearance 

 was not altogether sudden. 



They were preceded by a group of singular fishes. They 

 form perhaps an order of Ganoids, called Dipnoi or lung- 

 fishes. They are so called from the fact that, often living 

 in pools and streams liable to dry up, they breathe air di- 

 rectly, having true lungs, like those of frogs, as well as 

 gills. From the nature of their brain and their three-cham- 

 bered heart, that of other fishes being two-chambered, the 

 Dipnoans are quite different from all other fishes ; while, 

 on the other hand, the notocord is persistent, there being 

 no bony spinal column, and the skull is cartilaginous. 



The body of the Dipnoans is somewhat eel-shaped, 

 though not very long in proportion to its thickness, and is 

 covered with round scales. The pectoral and ventral fins 

 are long, narrow, and pointed, and the imperfect vertebral 

 column extends to the end of the caudal fin, which ends in 

 a point, not being two-lobed, as in other fishes. 



The Australian lung-fish (Fig. 171) has but a single 

 lung. It attains a length of six feet. It can breathe by 

 either gills or lungs alone. Ordinarily it uses its gills, but 

 when the fish is compelled to live during droughts in thick 

 muddy water charged with gases which are the product of 

 decomposing organic matter, it is obliged to use its lungs. 



