366 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



-ph. 



One of the most interesting points about the Crossopterygians is the 

 clue they give to the evolutionary origin of the air-bladder so charac- 

 teristic of the modern teleosts. We find in fact in Polypterus in place 

 of a typical air-bladder a deeply bi-lobed lung corresponding with that of 

 the higher vertebrates (Fig. 153). This arises in the form of a pocket-like 

 downgrowth of the floor of the pharynx which grows backwards as a 

 pair of horn-hke projections — commonly termed the right and left lung. 

 The original communication with the cavity of the pharynx remains as a 

 longitudinal slit — the glottis (Fig. 153, gV) — 

 near the mid-ventral line. Each lung forms 

 a smooth thin-walled sac provided with a 

 network of capillary blood-vessels which 

 enable respiratory exchange to take place 

 between the blood circulating in the net- 

 work and the air in the lung. The living 

 Polypterus visits the surface of the water at 

 short intervals and swallows air which passes 

 through the glottis into the lung ; if it is 

 prevented from reaching the surface by wire 

 netting it drowns. 



There are certain details about the lung 

 apparatus of Polypterus which should be 

 particularly noted for their bearing upon the 

 evolutionary explanation of the teleostean 

 air-bladder. 



(i) The apparatus is certainly homologous 

 with the lungs of the higher animals. This is 

 demonstrated by its possessing the three 

 fundamental characteristics of the vertebrate 

 lung : (a) it is in its earliest stages in the 

 form of a mid-ventral outpushing of the floor 

 of the pharynxj (p) it receives its blood supply by a pulmonary artery 

 arising from the sixth aortic arch and (c) it is innervated by a special 

 pulmonary branch of the vagus. 



(2) While undoubtedly homologous it shows certain differences from 

 the typical lung, associated with the fact that in the adult Polypterus it 

 becomes strongly lop-sided — the right lobe or the right lung, to use the 

 ordinary nomenclature, increasing greatly in length (Fig. 154, B, r.l) 

 while the left (Z.Z) lags relatively behind in its development. In an 

 aquatic creature the lung, filled as it is with air, necessarily acts as a 

 float ; and in Polypterus, where the body is covered with a coating oi 



■nl. 



Fig. 153- 



Polypterus, dorsal view of lung. 

 gl, Glottis ; l.l, left lung ; oes, 

 oesophagus ; ph, pharynx ; r.l, right 

 lung. 



