72 FROM FISH TO PHILOSOPHER 



converted into the swim bladder, and its use as a respira- 

 tory organ abandoned, but among certain of the De- 

 vonian fishes the air bladder proved to be a lifesav- 

 ing device. It persisted as a respiratory organ in two 

 lines: the lungfishes or Dipnoi {di = double; pnein = to 

 breathe), and the Crossopterygii (tassel-finned fishes), 

 which between them became the most common fresh- 

 water fishes in the Middle and Upper Devonian. No 

 crossopterygian survives today, but some insight can 

 be gained into the life of the Devonian air-breathing 

 fishes from studies of the surviving lungfishes, which are 

 closely related to them. 



Once numerous and widely distributed in the fresh 

 waters of the Paleozoic continents, the Ixmgfishes are to- 

 day reduced to three genera: Neoceratodus, with one 

 species in AustraUa, Lepidosiren, with one species in 

 South America, and Protopterus, with three species in 

 equatorial Africa (Figure 7). The last two are fairly 

 abundant, but Neoceratodus is so nearly extinct that 

 the government has placed it under its protection. 

 Lepidosiren and Protopterus undertake estivation (sum- 

 mer sleep) in the dry season, Neoceratodus does not. In 

 Lepidosiren and Protopterus the gills are vestigial and 

 the animals are so dependent on aerial respiration that 

 they are quickly asphyxiated if restrained under water, 

 while Neoceratodus retains functionally adequate gills 

 and can remain for indefinite periods under water, and 

 is reported to die if removed to air. 



During some years from 1928 onward the writer was 

 able to carry out extensive studies on the African lung- 

 fish, Protopterus aethiopicus (called Kamongo in Swa- 

 hili), both during estivation and in the active state. This 

 family inhabits the River Gambia, the Congo basin and 

 the rivers and lakes of equatorial East Africa, and is 

 among the common fishes eaten by the natives around 

 Lake Victoria. The natives catch it in nets in both deep 

 and shallow water, and also seek it out when it is estivat- 

 ing by probing every suspicious-looking hole in the mud 



