AIR-BLADDER OF FISHES. 499 



fishes. Indeed the essential distinction of those relations has 

 seldom been clearly kept in view. When we read in the 

 latest edition oi" the Comparative Anatomy of Cuvier : i the 

 gills are the lungs of animals absolutely aquatic ; ' x and, with 

 regard to the cartilaginous or osseous supports of the gills, 

 ( they are in our opinion, to the gills of fishes, what the carti- 

 laginous or osseous tracheal rings are to the lungs of the three 

 superior classes : ' 2 we are left in doubt whether it is meant 

 that the gills and their mechanical supports merely perform the 

 same function in Fishes which the lungs and windpipe do in 

 Mammals, or whether they are not also actually the same parts 

 differently modified in relation to the different respiratory media 

 of the two classes. Geoffroy St. Hilaire leaves no doubt as to 

 his meaning where he argues that the branchial arches of fishes 



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are the modified tracheal rings of the air-breathing classes ; we 

 perceive that he is enunciating his belief in a relation of homo- 

 logy. The truth of his proposition will be best tested by first 

 considering the homologies of the air-bladder of fishes. In the 



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Lepidosiren the notochord, the parapophyses, the attachment of 

 the scapula? to the occiput, the branchiostegal covering of the 

 permanent gills, the opercular bones, the presence of a spiral in- 

 testinal valve, the relative position of the anus, the extra-oral 

 nasal sacs, the scaly integuments, the mucous tubes on the head, 

 the •' lateral line,' in short, the totality of the organisation of 

 the Lepidosiren, exemplify its fundamental ichthyic nature. It 

 is extremely interesting to find the Ganoid Polypterus, which 

 of all osseous fishes most closely resembles the Lepidosiren in its 

 spiral intestinal valve, in the bipartition of the long air-bladder, 

 the origin of the arteries of that part, and the place and laryngeal 

 mode of communication of the short and wide air-duct or wind- 

 pipe, also presenting the closest agreement with the Lepidosiren 

 in the important character of the form of the brain. The common 

 objection to the view of the air-bladder of fishes being the rudi- 

 mental homologue of the lungs of air-breathing Vertebrates has 

 been, that the artery of the air-bladder carries arterial blood, that 

 of the lungs venous blood. But in the Polypterus and Lepido- 

 siren, in reference to this character, the arteries of air-bladders 

 are derived from the returning dorsal portions of the branchial 

 vascular arches before their union to form the aorta. In the 



1 'Les branchies sont les poumons des animaux absolument aquatiques.' (xm. 

 t. vii. p. 164.) 



2 ' Elles sont, a notre avis, aux branchies des poisson?, cc que les cerceaux carti- 

 lagineux ou osseux des voies aeriennes sont aux pouraons des trois classes supe- 

 rieures.' (lb. p. 177.) 



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