536 PEOF. T. W. BRIDaE OK THE 



of gas through the ductus pneumaticus, so tLat oulj so much gas 

 will be eliminated as will suffice to maintain the Fish in a plaue 

 of least efFort, 



In attempting to extend this theory to such Teleosts as 

 Sargus, Syodon, Priacanthus, Notopterus, and Gliipea, it must 

 be at once admitted that there are grave difficulties in the way. 

 It is easy to conceive how distension or contraction of the air- 

 bladder, produced by variations in the superincumbent column of 

 water, would be competent to give rise to stimuli affecting the 

 auditory organ. A tendency to over-distension may be conceived 

 to produce such a bulgiiig of the anterior extremities of the 

 auditory caeca as would modify the condition of the perilymph 

 surrounding the auditory organs, and impart a stimulus to the 

 sensory epithelium of those organs. The objection to this view 

 is, however, that it is very difficult to see in vshat way the 

 contingent efferent impulses will find expression. In Fishes like 

 JSotopterus and Clupea, where a ductus pneumaticus is present, 

 the existence of some kind of regulatory control over the libera- 

 tion of gas from the air-bladder is possible ; but this suggestion 

 is obviously inapplicable to such Fishes as, for example, the 

 species of Sparus and Sargus in which the ductus atrophies in 

 the adult. In the latter genera, variations in the amount of gas 

 present in the air-bladder must depend upon the relatively slow 

 procc sses of gaseous secretion or absorption ; and it is at least 

 within the bounds of conjecture that the connection of the air- 

 bladder and auditory organ forms part of a reflex mechanism by 

 which the varying tensions of the gases of the air-bladder con- 

 stitute a stimulus to the auditory organ and central nervous 

 system, and, ultimately, by reflex action lead to such a modifica- 

 tion of the rate of secretion or absorption as will vary the amount 

 of gas in the bladder in accordance with the requirements oftiie 

 Fish. The special advantage to the Fish may be that secretion 

 and absorption will take place more rapidly than is the case 

 wliere pressure-stimuli have no special means of affecting a 

 sensory organ, and adjustment to varying hydrostatic pressures 

 effected with greater promptitude. Neveriheles?, it must be 

 acknowledged that there is at present but very little evidence, 

 either physiological or anatomical, which can be adduced in 

 support of these suggestions. 



Admitting, however, the want, or rather the paucity, of 

 evidence for such tentative suggestions, is it not possible that the 

 connection of the auditory organ and the air-bladder may have 



