98 PRIMITIVE ANIMALS 



blood is returned to the general circulation by two 

 small veins passing to the posterior cardinal and the 

 portal veins. In these fishes the air-bladder has no 

 respiratory function. 



In the primitive Teleosts, such as Polypterm, 

 the blood supply is different, the arterial supply 

 coming from the sixth or posterior pair of branchial 

 arteries, while the veins open into the ductus 

 Cuvieri which passes directly to the heart. In these 

 fishes the air-bladder probably exercises some re- 

 spiratory function, but the complete transformation 

 of the air-bladder into the lungs is effected in the 

 curious fishes known as the Dipnoi. In the Dipnoi 

 (Fig. 20 B), or Lung-fishes, the air-bladder, which 

 lies dorsally to the oesophagus and opens into it 

 ventrally by a curved pneumatic duct, is a median 

 or bilobed sac with vascular and honeycombed walls 

 like a lung, and it is supplied by a pair of pulmonary 

 arteries which arise from the sixth branchial arteries, 

 and a pair of veins which empty directly into the 

 heart. The position of the air-bladder and its duct 

 in these fishes is exactly the same as that of the 

 lungs and trachea in a true land Vertebrate ; the 

 minute structure of its walls is the same, and its 

 blood supply is also essentially the same, since the 

 pulmonary arteries in the land Vertebrate always 

 arise in the embryo from the sixth branchial arches, 

 and the pulmonary veins pass back directly to the 



