32 VERTEBRATE RESPIRATION 



(b) LUNGS AND THE SWIMBLADDER OF FISHES 



One part of the alimentary canal from which accessory 

 respiratory structures are frequently derived is of particular 

 interest because of its relationship to the breathing organs of 

 tetrapods. This is the pharynx from which diverticula form the 

 lungs of the Dipnoi (lungfishes), the primitive bony fish, Polyp- 

 terus, and the swimbladder of most teleost fishes. In some 

 instances, e.g., Erythrinus, the swimbladder has secondarily 

 acquired an accessory respiratory function. 



The distinction between lungs and swimbladders is usually 

 based upon their ventral or dorsal origin from the pharynx. As 

 in tetrapods, lungs have a ventral origin although they may come 

 to lie dorsal to the alimentary canal. Swimbladders always lie 

 dorsal to the alimentary canal and may or may not retain their 

 connection with the pharynx via the pneumatic duct. No definite 

 evidence is available for deciding which of these structures is the 

 more primitive, although some very early fish (e.g., the Silurian 

 Placoderm Bothriolepis) appear to have had ventral pharyngeal 

 diverticula. Comparative anatomy supports the view that a 

 bilobed lung was the most primitive condition. On the line of 

 evolution which gave rise to tetrapods this structure persisted 

 and is homologous with the tetrapod lung. The lungfishes 

 diverged from this line quite early, but the lung persisted and 

 enabled them to inhabit swamps liable to drying up. It is prob- 

 able that both of these lines of evolution took place in fresh 

 waters, but the other main line of bony fish evolution resulted in 

 the invasion of marine waters. Here the dangers of periodic 

 drying up were negligible and in association with this the air- 

 breathing function of the lung became lost. Correspondingly, 

 greater adaptation to a pelagic mode of life occurred and the 

 pharyngeal diverticula became hydrostatic in function. As this 

 is one of the most fascinating modifications of a respiratory 

 organ in vertebrates it will be considered in detail before dis- 

 cussing the breathing of lungfish and its relationship to the 

 evolution of the double circulation. 



