142 Profs. T. W. Bridge and A. 0. Haddon. [June 16, 



in the walls of the air-bladder, of extrinsic muscles in all but a few 

 genera (Pimelodinee), and of internal vibratory diaphragms, or other 

 obviously vocal structures, is sufficient to prove that the air-bladder 

 takes little or no part in this function, at all events by any of the 

 ordinary methods known in other Fishes. The feeble vascularity of 

 the air-bladder and the absence of any inspiratory or expiratory 

 mechanisms are serious objections to its use as an ordinary respiratory 

 organ. 



In many Fishes the bladder appears to have a secondary relation 

 to respiration by acting as a reservoir for the superabundance of 

 oxygen introduced into the blood by the gills, which can be re- 

 absorbed when required, but Moreau's experiments prove that those 

 Fishes provided with a Weberian mechanism have a far less capacity 

 for absorbing oxygen from the air-bladder than other Teleostei have 

 under precisely similar conditions, and, further, that the capacity for 

 oxygen absorption is always associated with the presence of retia 

 mirabilia or vaso-ganglia, which, as our investigations prove, are in- 

 variably absent in all Siluridae. 



Very little is known about the physiology of hearing in Fishes, but 

 we are unable to see that there is any need to assume that the con- 

 ditions of subaqueous audition are very different to those in air, ex- 

 cept in so far as the physical differences in the conductivity of the 

 respective media are concerned. Sound vibrations travel much more 

 rapidly in water than in air and to far greater distances, but they pass 

 with difficulty from water to air, and conversely. Those sound vibra- 

 tions which are too feeble to produce any appreciable effect on the 

 external surface of the skull when they pass through air can, never- 

 theless, strongly impress the ear when propagated in water and the 

 head of the observer is completely submerged (Colladon and Sturm). 

 Sound waves impinging on the surface of a Fish's skull would there- 

 fore be readily conveyed to the perilymph and endolymph of the ear, 

 and such sounds will, in all probability, be heard with greater rapidity 

 and from greater distances than could possibly be the case under 

 similar conditions in air. 



The strongest objections to the auditory function of the air-bladder 

 and Weberian mechanism (Weber's theory) are to be found in the 

 imperfections of the apparatus. If vibrations can pass at all from the 

 external medium to the gases contained within the air-bladder the 

 transmission mast be accompanied by a considerable loss of intensity, 

 and this must especially be the case in those Ostariophysese in which 

 the air-bladder is widely separated from the superficial skin by the 

 liver and other organs and tissues. In many Siluridee the walls of 

 the air-bladder are too thick to admit of their vibrating synchronously 

 with rapidly recurring sound waves. The inertia of the ossicles them- 

 selves, and the interposition between them of a compressible ligament, 



