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 (Ama, 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. 
Necturus may well be expected to establish definitely. 
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