26o 



AN INTRODUCTION TO ZOOLOGY. 



form a 



link between Ganoid Fish and Amphibia, being 

 the lowest kind of vertebrates 

 that have lungs. One, the 

 Barramunda or Ceratodus, is 

 found in Australia, one, the 

 Protopterus, in Africa, and 

 one, the Lejpidosiren, in Brazil. 

 While they resemble fishes in 

 having gills and a persistent 

 notochord supplemented by- 

 bony arches, four fins, and a 

 compressed tail, they resemble 

 Amphibians in having lungs. 

 These lungs are of a very 

 simple character, and arise 

 just in the same way as does 

 the swimming-bladder of or- 

 dinary fish, namely, as a sac, 

 paired or unpaired, which is 

 filled with air, and opens by 

 a duct into the pharynx. 

 Hence we see that the lungs 

 of vertebrates, although the 

 analogue of the fish's gills, 

 are the homologue of its swim- 

 ming-bladder; that is to say, 

 they fulfil the same use as the 

 former, but have the same 

 origin in development as the 

 latter. 



