SWIM-BLADDER 



21 



To live a longer time out of water has been rendered 

 possible only by the appearance of a lung-like organ. Such 

 a structure, however, would have been of too great impor- 

 tance in the living economy of terrestrial vertebrates to 

 have had a sudden origin : it may most reasonably have 

 been derived from a similar structure occurring very gener- 

 ally among fishes. The lungs certainly resemble the swim- 

 bladder of fishes in so many important characters that it 

 seems difficult to regard these organs as morphologically 

 distinct. In itself the swim-bladder must be looked upon 

 as an ancient and essentially a generalized structure, for 

 within the groups of fishes it has already acquired a vari- 

 ety of modified characters : appearing in a lowly condition 

 in sharks, it acquires a balancing function in the majority 

 of bony fishes ; in some forms (carp, siluroids) its function 

 connects it with the auditory organ, often by a highly 

 elaborated apparatus : while in other forms (Amia, Gar- 

 pike, Dipnoans), it is unquestionably of respiratory value. 

 The wide range in the characters of the air-bladder (cf. 

 Figs. 13-19, and Table, p. 264), even among recent fishes, 

 would naturally favour its homology with the lungs : it may 

 thus be paired or unpaired, attached by its duct to either 

 the dorsal, lateral, or ventral wall of the gullet : it may 

 present the most varied characters in its lining membrane 

 or in its vascular supply. When, moreover, it becomes of 

 respiratory value (e.g. Dipnoans, Polypterus), the gills are 

 known to become in part degenerate. The larval history 

 of amphibians, presenting so perfect a transition between 

 gill-breathing and terrestrial vertebrates, should alone seem 

 to render more than probable the general homology of air- 

 bladder and lung an homology which a closer knowledge 

 of the conditions of the lungs of the lower urodeles (e.g. 

 Nccturns may well be expected to establish definitely. 



