A HISTORY Of VERTEBRATES: FISHES 



439 



in its wall. Under conditions of oxygen deficiency, the fish can utilize 

 the oxygen in the bladder, so the organ also functions as a temporary 

 storage site for this gas. 



In some bony fishes, the swim bladder is connected to the pharynx 

 by a pneumatic duct, and in a few, functional lungs are present instead. 

 This led many to postulate that the swim bladder was the precursor of 

 lungs. At present, however, the lungs are considered to be the precursor 

 of the swim bladder, for the organ is most lunglike in the most primi- 

 tive bony fishes. 



It is believed that the ancestral bony fishes had lungs similar to 

 those of the living African lungfish (Protopterus). In the lungfish (Fig. 

 22.11) a pair of saclike lungs develop as a ventral outgrowth from the 

 posterior part of the pharynx. The lungs enable the fish to survive 

 conditions of stagnant water and drought. The rivers in which the 

 African lungfish live may completely dry up, but the fish can survive 

 curled up within a mucous cocoon that it secretes around itself in the 

 dried mud. A small opening from the cocoon to the surface of the mud 

 enables the fish to breathe air during this period. The African lungfish 

 has become so dependent upon its lungs that it will die if it cannot 

 occasionally reach the surface to gulp air. 



Air breathing probably evolved in fishes as a supplement to gill 

 respiration. Presumably early bony fishes, or perhaps their placoderm 

 ancestors, evolved lungs as an adaptation to the unreliable fresh-water 

 conditions of the Devonian period. Geologic evidence indicates that the 

 Devonian was a period ol Irecjuent seasonal drought. Bodies of fresh 

 water undoubtedly either became stagnant swamps with a low oxygen 

 content, or dried up completely. Only fishes with such an adaptation 

 could survive these conditions. The others became extinct or migrated 

 to the sea, as did many later placoderms and the cartilaginous fishes. 



^:0 Lixng of 



land vertebrates 



Teleost 



Swrim blad-der 



Primitive 

 fish lang 



TrcLnsitional type. 



Figure 22.11. A diagram to illustrate the evolution of lungs and the swim blad- 

 der. (.After Dean.) 



