AIR-BLADDER OF FISHES. 281 



however, do not affect its relation of homology with the divided air- 

 bladder of the allied ganoid genus Pohjpterus, any more than with 

 the divided air-bladders of the Cobitis barbatula or Arins gagora, in 

 which the divisions are confined to the anterior part of the abdomen, 

 and inclosed in osseous cups developed from vertebra; answering to 

 the second or third cervicals. 



Thus the series of transitions traceable in the organs universally 

 acknowledged as the air-bladders of fishes prove those of the Lepido- 

 siren to be the homologous organ ; whilst the development, relative 

 position, and connection of the lungs of the Batrachia equally prove 

 those lungs to be the homologues of the air-bladders of the Lepi- 

 dosiren. Therefore, it follows that the aii'-bladder of the fish is 

 homologous with the lungs of the Batrachian, and of all other air- 

 breathing Vertebrates ; although the air-bladder of the fish does not, 

 as a general rule, perform the functions of a lung. But the air- 

 bladder in most fishes is analogous to the ai^--chambers of the shell of 

 the polythalamous Cephalopods, and in some fishes it is analogous to 

 the tympanum of the higher Vertebrates.* 



In tracing the development of the windpipe and larynx, from the 

 more lung-like forms of the air-bladder in fishes, through the Peren- 

 nibranchiate Batrachia upwards, we obtain incontrovertible pi'oof 

 that the so-called ' ductus pneumaticus ' in fishes is the homologue 

 of the trachea. It follows, therefore, that the branchial cartilaginous 

 and osseous supports of the gills are not the homologues of the trachea 

 and bronchise, any more than the gills themselves are the homologues 

 of the lungs. We shall find the branchial arches and gills developed 

 in the larvte of Batrachia, and disappearing as the true trachea and 

 lungs are formed, without being converted into them. The only 

 parts of air-breathing Vertebrates with Avhich the branchiaj of fishes 

 are homologous are the jiersistent or deciduous branchiae of the 

 Batrachia. The relations of the branchial ai'clies and gills of fishes 

 with the trachea and lungs of higlier air-breathing Vertebrates arc 

 those of analogy merely. The branchial apparatus, in relation to the 

 entire vertebrate scheme or type of organization, is to be regarded 

 as a temporary graft on such type, introduced to serve the purposes 

 of the lowest embryo-like forms, and to give way to another and 

 higher and more persistent form of respiratory organ : in tliis respect 

 the branchial organization is to the vertebrate series what the pla- 

 centa is to the mammalian individual. 



* In tlic Loacli (Colitis) ilio wliok- alimentary canal is analogous to a lung, in- 

 asmuch as this fish swallows air and voids carbonic acid. 



