200 THE SWIM-BLADDER OF FISHES 



held concerning- its nature and meaning. The famous Carl 

 Cegenbauv has referred to the existing uncertainty as to the 

 practical use of this structure (No. 8, p. 566), and one of tlie 

 most recent contributors to the subject says (No. 24, p. 125) 

 " even now there is much doubt as to the functions of the swim- 

 bladder." From the days of Aristotle its use has been involved 

 in obscurity ; but the ancient father of comparative anatomy 

 ventured on the theory that its purpose was to aid in the 

 production of sound, and his successors have again and again 

 revived the theory down to our own day. The Italian, Borelli, 

 regar-ded it as hydrostatic and an aid to fish in floating( No. 2). 

 A third interpretation is that the organ is respiratory, 

 and in the Ganoids and Dipnoans, it is a complex, vascular, lung- 

 like organ, with an undoubted pulmonary function ; but it is by 

 no means certain that the pharyngeal evagination or sac in 

 those highly specialised lung-fishes is homologous with the 

 swim-bladder, and I shall have occasion to point out that 

 exception may be justifiably taken to such a view as that of 

 Dr. A. S. Packard (No. 17, p. 444) who says, "the air-bladder 

 being homologous with tbe lungs of higher v^ertebrates, the 

 pneumatic duct is comparable with the trachea of birds and 

 mammals," a view similar to that recently expressed by 

 Professors Jordan and Evermann (No. 11, p. 11) that the swim- 

 bladder is " a sac filled with air lying beneath the backbone of 

 fishes and corresponding to the lungs of higher vertebrates." Pro- 

 fessor Arthur Thomson, on the other hand, has given his opinion 

 regarding the view just stated and says (No. 23, p. 397), " that 

 the lungs and air-bladder are homologous is by no means certain ; 

 but the comparison is plausible." A further view interprets the 

 swim-bladder as a barometer. Sagemahl (No. 21) regarded 

 it as such, so that like an aneroid instrument, it informs the 

 fish of changes in the atmospheric pressure aflPecting the sur- 

 rounding water. Minor modifications of these views liave been 

 broached by otlier authorities ; but a full examination of the 

 facts seems to lend little support to any of them. 



