CHAPTER VI 



THE LUNGFISH 



When the sharks sought sanctuary from the climatic vi- 

 cissitudes of the Devonian continents by turning to the 

 sea, they condemned themselves and their progeny to 

 perpetual existence as fishes. Other fresh-water fishes, 

 who took what was perhaps a more dangerous and cer- 

 tainly a more difficult course, are of more immediate 

 interest to those who can read the epic of life in books. 



Along with the elasmobranchs, the Devonian bony 

 fishes had inherited many features of the placoderms 

 and especially of the spiny sharks, and at least some of 

 them had deviated from the elasmobranchs in one im- 

 portant respect— they had begun swallowing air as an 

 accessory mode of respiration. Appearing alongside the 

 early Devonian ostracoderms and placoderms, and be- 

 fore the sharks, these air-breathing fishes had by the 

 Middle Devonian given rise to two groups, one of which 

 (the Actinopterygii) was to lead to the modem fishes; 

 the other (the Crossopterygii) to the ancestors of the 

 air-breathing, four-footed animals. 



It is presimied that the ancestors of the air-breathing 

 fishes at first simply swallowed air and either passed it 

 through the intestinal tract or regurgitated it, but shortly 

 they developed an air bladder or lung' which opened 

 as a blind sac oiBF the ventral side of the esophagus. In 

 the higher fishes this air bladder was ultimately to be 



