ENVIRONMENTAL CONDITIONS 407 



The three existing genera of dipnoans or lung fishes show a wide dis- 

 tribution and marked adaptations to dry seasons and diminished waters. 



Ceratodus, the Australian lung fish, is known to have existed as far 

 back as the Triassic. 14 It lives readily in partially dried-up water-holes, 

 foul with dead fishes of various kinds. It rises to the surface at irregular 

 intervals to breathe air by means of its air-bladder, which functions as a 

 lung. It is also dependent, however, on its gills, and soon dies when 

 removed from the water unless kept moist. At present it is confined to 

 the Mary and Burnett rivers of the eastern coast. This extremely lim- 

 ited distribution in Australia, with threatened extinction after such a pro- 

 longed and world-wide geological history, is presumably due to unfavor- 

 able conditions of environment, beginning with the development of zonal 

 climates and accentuated now by its human enemies, owing to its large 

 size, the fondness of the Australian natives for its flesh, and the ease with 

 which it may be captured when confined to the shrunken water-holes. 



Protopterus, the African genus, has a wide distribution within that 

 continent, extending from the southern margin of the Sahara to the 

 northern limits of the Kalahari, living thus under equatorial latitudes, 

 in regions marked in large part by moderate precipitation, and practically 

 throughout by alternations of seasonal rainfall and drought. Its favorite 

 habitat is in the marshes in the vicinity of the rivers. In respiration it 

 uses its double swim-bladder as a lung, supplementing the use of its gills. 

 In the dry season the marshes dry up, the fish burrows into the ground 

 and passes into a summer sleep, during which it is entirely dependent on 

 its lungs for respiration and on its body fat and muscular tissues for food 



Lepidosiren, the South American genus, leads a somewhat similar life, 

 inhabiting the wide-spreading swamps and marshes of the Chaco country 

 and passing into a dormant state in the dry season. 



In limb development Ceratodus appears to be quite comparable to its 

 Devonian relatives, but those of the two other genera are long and slender. 

 Ceratodus is a bottom-loving fish and frequently supports the anterior 

 portion of the body on its pectoral fins. It is also able to push itself 

 backward by means of its fins, indicating that an elbow-like joint is func- 

 tional. It has also been observed to paddle forward, using alternate 

 movements of the pectoral fins. It is not, however, so amphibian-like in 

 its movements as Protopterus, which will "walk" forward, balancing itself 

 on its paired fins. 15 



"The living form is quite commonly distinguished from the Triassic genus as Neo- 

 ceratodus. 



15 Bashford Dean : Notes on the living specimens of the Australian lung-fish, Cerato- 

 dus forsteri, in the Zoological Society's collection. Proc. Zool. Soc, London, vol. i, 1906, 

 p. 176. 



