STRUCTURE OF THE VERTEBRATES 211 



slits they are a derivative of the digestive tract. In the fish, 

 however, the connection between the two systems is closer. Water, 

 the environmental medium, is drawn into the mouth with the 

 food, and the dissolved oxygen of the water is taken up as it 

 passes over the gills in its course to the outside. With the 

 development of lungs a certain degree of separation begins be- 

 tween the digestive and respiratory tracts, although the environ- 

 mental oxygen is drawn into the pharynx and passed into the 

 respiratory tissues. 



1. Evolution of the Lungs. The first homologue of the lungs 

 appears in the chondrostean fishes. In the sturgeon (page 41) 

 a dorsal outpocketing of the gut develops in the region between 

 the pharynx and the esophagus. The distal portion of the diver- 

 ticulum enlarges to form a swim-bladder, its duct maintaining a 

 free connection with the gut throughout life. The organ is slightly 

 vascular, the supply coming from the sixth branchial artery. In 

 Amia (a holostean fish) the swim-bladder is highly vascular, 

 and the connection with the gut remains in the adult. 



From the simple, slightly vascular condition, the swim-bladder 

 evolved in two directions: (1) toward the condition found in the 

 teleost fishes, where the organ is non-vascular, loses its connec- 

 tion with the gut, and remains as a dorsal sac solely hydrostatic 

 in function; and (2) toward a ventral position, the duct open, 

 and the function respiratory. The latter condition is found in the 

 Crossopterygii and Dipnoi which have a lung-like swim-bladder. 

 Polypterus (a recent crossopterygian fish) has a swim-bladder 

 which develops as a ventral outpocketing of the gut. The struc- 

 ture becomes bi-lobed and highly vascular, the blood supply 

 coming from the sixth branchial as in the other fish. During the 

 dry periods, or when the river waters become muddy, the fish 

 gulps air and carries on respiration with the bladder. The Dipnoi 

 (page 45) parallel the amphibia in development and have a 

 swim-bladder w^hich is functionally a lung. In the South Amer- 

 ican genus the structure is so specialized that the animal is 

 unable to survive if kept submerged in water. 



When the amphibia evolved the lung relationships of the 

 crossopterygian ancestors were retained. The lung develops as a 

 ventral outpocketing of the gut at the posterior end of the 

 pharynx, and soon divides into two distinct lobes. The opening 



