100 PRIMITIVE ANIMALS 



living at present in widely separated and narrowly 

 circumscribed regions ; Ceratodus (Fig. 21) is known 

 only from the Burnett River in Queensland, Pro- 

 topterus inhabits the Gambia in North Africa, and 

 Lepidosiren the reedy swamps of the Gran Chaco 

 in tropical South America. The swamps of the Gran 

 Chaco, at the level of the southern tropic, support a 

 dense vegetation of papyrus and reeds ; the stagnant 

 water in the wet season varies from about four to 

 eight feet in depth, and an occasional sluggish stream 

 meanders round the bases of the papyri and swamp- 

 grasses. In the dry season the standing water may 

 almost completely disappear. Lepidosiren is a 

 sluggish fish, creeping about among the vegetation 

 with the help of its hind fins, which are employed 

 almost as legs. At regular intervals it visits the 

 surface of the water to expire the stale air from its 

 lungs and to inhale a fresh supply ; the frequency 

 with which these visits to the air are paid depends 

 on the condition of the water, and in very stagnant 

 and foul water the intervals between the breaths may 

 be only four or five minutes. It is clear therefore 

 that even in the wet season, when water is com- 

 paratively abundant, the lung acts as an important 

 accessory to the gills in the function of respiration. 

 Its most essential function, however, is exercised 

 in the dry season. Before the onset of this season 

 the Lepidosiren feeds actively on the molluscs and 



