lung bu' 



esophagus 



B C D 



Figure 9-31 . Five stages in the development of the lungs of the alligator. (After Broman) 



It is generally assumed that the swim bladder is the hom- 

 olog of the lung, although there are differences. The lung 

 arises primitively from paired anlagen from the ventral wall 

 of the pharynx, while the teleost swim bladder arises from 

 an unpaired dorsal or dorsolateral diverticulum of the esoph- 

 agus. The blood supply of the lung is from the sixth aortic 

 arch, while that of the swim bladder is usually from the 

 dorsal aorta. 



Actinislia The swim bladder of Latimeria is attached ante- 

 riorly to the ventral aspect of the esophagus. From here it 

 extends up and around the right side of the esophagus and 

 stomach to a dorsal position, where it extends back in the 

 dorsal mesentery the entire length of the body cavity. The 

 gonads are suspended from its sides anteriorly, and below 

 it hang the gut and its attendant organs. The walls of this 

 bladder are much thickened with oil-filled cells, and only 

 remnants of the lumen are observed. The vascular supply 

 has not been described. The walls of this structure were 

 calcified or ossified in the known Mesozoic fossil forms. 



Dipnoan In two of the lung fishes, Pwloplerus and Lepidosiren, 

 paired sac-like lungs extend the length of the body cavity. 

 These have been rotated around the right side of the gut 

 into a dorsal position, so that the original right lung is now 

 the left. The stem of these lungs attaches to the ventral as- 

 pect of the "esophagus" just behind the expanded branchial 

 chamber. There is a slit-like glottis opening into the phar- 

 ynx or esophagus, depending on the definition of these 

 terms. The lungs in these forms serve not only a respiratory 

 function but also a hydrostatic one— ventral lungs would 

 make it difficult to maintain an upright position, while an 

 anterior lung would make it hard to maintain a horizontal 

 position. 



Although paired diverticula probably arise in the larva, 

 only one retains its connection with the esophagus. Rotation 

 of these sacs brings them in contact, with the result that their 

 cavities become interconnected anteriorly. At the beginning 



of the development, the left lung is smaller than the right, 

 but in the adult this same lung, now the right one because 

 of rotation, is as large as the other. 



In Pwloplerus the lungs lie retroperitoneally above the level 

 of the kidney and the gonad (Figure 10-41), and they ex- 

 tend to the extreme posterior end of the body cavity. They 

 are broad anteriorly, narrower midway along the body, and 

 expanded in height posteriorly (Figure 9-12). 



The lungs are served ventrally by the left pulmonary 

 artery which passes below the esophagus and up the right 

 side to reach the lung. Here the artery bifurcates to pass 

 along the medial margin of both lungs. The right pulmo- 

 nary artery bifurcates and extends back on the mid-dorsal 

 surface of both lungs. The pulmonary veins lie along the 

 lateral margins of the lungs and unite below the fused an- 

 terior part. The single stem passes down to the right of the 

 esophagus, then along and to the right of the hepatic vein. 

 From here it extends forward above the sinus opening of the 

 right duct of Cuvier, then downward through a septum sep- 

 arating the recess of the sinus venosus to reach the auricular 

 wall. It opens through the auricular wall to the left of the 

 septum partially separating the cavity of the auricle into 

 halves. 



The left vagus nerve follows in part the same path as the left 

 pulmonary artery; a second part passes dorsally to the em- 

 bryonic left lung, now the right lung. The right vagus ap- 

 pears to lack a ventral division. 



The monopneumonous condition of Neoceralodus, in con- 

 trast to the dipneumonous or paired lung condition, has 

 been described as due to development of the right lung only, 

 the left being rudimentary and lost in the pneumatic duct. 

 Whether one or both lungs actually develop, the end result 

 is the same in terms of the vascular supply. Thus, it appears 

 that in Neoceralodus both lung buds fuse early and produce 

 the single sac of the adult with its bilateral sacculations and 

 blood supply. In Protopterus and Lepidosiren fusion occurs 

 only anteriorly, but the left pulmonary artery serves the 

 under side of both lun.gs just as in Neoceralodus. 



THE RESPIRATORY SYSTEM • 283 



