Evolution of Animals 467 



pulmonary artery. Essentially the same arrangement persists 

 in the Urodela. 



But mention of the pulmonary artery leads to a study of 

 the lungs and lung circulation. Amongst the cyclostomes 

 no swim-bladder nor related structure apart from the gills 

 occurs. But in seeking for possible first beginnings of lungs 

 "that arise at the hinder border of the branchial region of 

 the pharynx" a structure — the oesophagoeo-cutaneous duct — 

 deserves mention. Regarding this Goodrich (14-6: 46) says as 

 to the myxinoids: "There is always on the left side a simple 

 tube leading from the pharynx to the exterior, and opening 

 in common with the last gill pouch." This he views as "prob- 

 ably a modified gill slit." With passage from an aquatic 

 to a semi-terrestrial and at length terrestrial existence and 

 coeval reduction of gill tissue, this might gradually have en- 

 larged into a simple sac below the pharynx at first, that later 

 might close off internally and dividing give rise to paired lungs. 

 Interesting in this connection also is Cope's observation (73: 

 363): "The habit of holding in the oesophagus large quantities 

 of air while engaged in seeking food in foul water, or on land, 

 on the part of vertebrates which normally oxygenated the 

 blood by means of gills, was probably the mechanical cause 

 of the development of a pouch, and afterwards of a divertic- 

 ulum of the oesophagus, which became ultimately a swim- 

 bladder or a lung. . . . The development of a lung at 

 once produced a change in the uses to which the various branch- 

 ial arches were put. The posterior, which supply the lung, 

 would be subjected to greater pressure owing to the increased 

 blood supply demanded by the lung, and a correspondingly 

 diminished pressure would be experienced by the now unused 

 branchial portions of the bows." 



In living Apoda, so far as observed, the lung is in the form 

 of a bilobed sac, of which the right is well developed, the left 

 is minute. This may be due to the habit that many, and 

 specially the females, show of curling circularly round the 

 eggs during the breeding season, but doubtless also is due as 

 in snakes to a certain unilateral tendency in which one side 

 comes more into play than another, as during rest and hiber- 

 nation. 



In the simpler Urodela like the Sirenidse and Proteidse, the 

 same inequality is shown, though the two lobes tend to become 

 increasingly near each other in size, till in Amhlij stoma they 

 may be said to be paired. In the Salamandridic they are 

 equal and of considerable size. Internally they show varying 

 transitions from thin-walled sacs (Proteus) to sacs with coarse 



