ix. ii THE AIR-BLADDER 263 



is also found in the modern lung- fishes and presumably in their 

 Devonian ancestors, from which we may suppose that the tetrapods 

 arose (p. 276). This ventral position of the air-bladder was one of the 

 features that for a long time led zoologists to suppose that Polypteriis 



A B 



Fig. 158. Diagrams illustrating the blood-supply of the air-bladder in A, Polypterus, 

 b, Ceratodus, c, Aviia, and d, a teleost. The blood-vessels are seen from behind, and 



cut short in transverse section. 

 a. dorsal aorta; aad. anterior dorsal artery from the coeliac; aav. ant. ventral artery; 

 ab. air-bladder; aid. anterior dorsal vein to the cardinal; ba r \ 4th aortic arch (6th of the 

 series); d. ductus Cuvieri; ev. coeliac artery; la. left pulmonary artery; oe. oesophagus; 

 pr. portal vein receiving posterior vein from air-bladder; ra. rii^ht 'pulmonary' artery ; rpv. 

 right (branch of) 'pulmonary'' vein; rv. right vein from air-bladder; v. left 'pulmonary' 

 vein. (From Goodrich, Vertebrata, A. & C. Black, Ltd.) 



was a member of the crossopterygian line of fishes. It is probable, 

 however, that the affinity is only that which persists between all 

 primitive members of both Actinopterygii and Crossopterygii and 

 is to be taken as an indication that the air-bladder was originally a 

 widely open respiratory sac, or perhaps pair of sacs. Once the power 

 to produce a pharyngeal diverticulum had been developed it is easy 

 to imagine that the actual position of the opening might shift either 

 dorsally, as in the later Actinopterygii, or ventrally, as in the tetrapods. 

 The blood-supply of the air-bladder should provide some indica- 

 tions both of its origin and function. In Polypterus and Dipnoi there 

 are pulmonary arteries springing from the last (sixth) branchial arch 



