S WIM-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 {Ajuia, 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, Polypteriis), 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 uisodeles {e.g. 

 Nectunis may well be expected to establish definitely. 



