376 ZOOLOGY FOR MEDICAL STUDENTS chap. 



dwindles away and eventually all trace of it is lost, there being a single 

 air-bladder — the persistent right lung — precisely as- in a teleost except 

 that its cavity communicates with that of the alimentary canal, round 

 the right side of the latter, by means of the still ventral glottis (Fig. 

 ^54) C, p. 367). The blood supply of the Dipnoan lung is by a pair of 

 typical pulmonary arteries arising from the sixth aortic arch, and that 

 of the left side still curls round the ventral side of the alimentary canal 

 to the right side, thus marking out the track of the lung as it became 

 twisted from its original ventral to its definitive dorsal position. We 

 should expect the pulmonary branch of the left vagus nerve to pursue 

 the same course but as a matter of fact it crosses the right in an X-like 

 manner dorsal to the alimentary canal — a fact which seemed to con- 

 stitute an insoluble morphological puzzle until the investigation of 

 Polypterus, in which the lung retained a condition assumed to be ancestral 

 to that of the lung-fishes, showed that in that stage of evolution a direct 

 dorsal connexion had already been established between the right lung 

 and the left vagus. 



It is now possible to construct out of the facts that have been 

 mentioned in connexion with the several groups a fairly complete 

 history of the evolutionary changes undergone by the lung of fishes (see 

 Fig- 154, P- 367). 



It had at an early stage of its evolution, as shown by the early stages 

 of development in Crossopterygians and Lung-fish, the form of a bilobed 

 pocket of the ventral wall of the pharynx : i.e. it was identical with the 

 lung of terrestrial vertebrates (Fig. 154, A). As evolution proceeded the 

 left lung underwent reduction in size and this allowed the right lung to 

 twist round the alimentary canal into a dorsal position : this stage is 

 still represented in the adult Polypterus and in the young Lung-fish 

 (Fig. 154, B). With the complete disappearance of the left lung the 

 right assumed a completely dorsal and median position except that its 

 duct still passed down the right side of the alimentary canal to the 

 ventrally placed glottis : this stage is perpetuated in the adult Ceratodus 

 (Fig. 154, C). Economy of tissue now led to the shortening of the 

 duct so that the glottis came to be dorsal in position : this stage is 

 seen in physostomatous teleosts (Fig. 154, D). Finally the duct became 

 nipped across, giving the condition seen in physoclistic teleosts (Fig. 

 154, E). 



The lung or air-bladder of the Dipnoan necessarily fulfils a hydro- 

 static function, but it also functions as a breathing organ, so efficient 

 that during the dry season — when the water-pools are charged with 

 putrefying vegetation and the water consequently useless for gill breath- 



