534 PROF. T. "W. BRIDGE ON THE 



direct path through the bones and other structures of the head 

 to the membranous labyrinths. It may be also urged that this 

 objection loses none of its force when it is considered that the 

 cranial bones are far more pervious to sound-waves travelling in 

 water than, as is the case with terrestrial animals, in air. 



If this argument has any force, does it not suggest that the 

 "Weberian mechanism, considered as an accessory to audition, 

 may prove a positive disadvantage to the Fish in so far as the 

 sense of direction is concerned ? Finally, if the Weberian 

 mechanism is a means of increasing the acuteness of the sense of 

 hearing, is it not a little remarkable that sounds heard through 

 this agency convey no idea of direction, especially when the 

 latter is of so much importance to the animal, and that the 

 sense of direction should depend upon the perception of sounds 

 of an obviously more limited range of intensity which reach the 

 auditory organs directly through the head? 



Other objections which may be urged against Weber's theory 

 as applied to the Ostariophysese are scarcely pertinent to the 

 present discussion, which is' designedly restricted to those Fishes 

 in which there is no Weberian apparatus and the auditory organ 

 and air-bladder are connected by other methods. 



It may at once be affirmed that the same " real objection " is 

 equally applicable to such Fishes as Notopterus^ Sargus, Sparus, 

 Hyodon, Clupea. 



In some of these Fishes (e. g., Sparus, Sargus, and Clupea) 

 there is no longitudinal bipartition of the air-bladder, which, 

 therefore, for the greater part of its extent encloses a simple 

 undivided cavity. Sound-waves in the gases of the air-bladder 

 travelling along the auditory cseca will ultimately affect the 

 auditory organs to an equal extent and simultaneously. Hence, 

 these Fishes will derive no sense of direction from auditory 

 stimuli reaching the membranous labyrinths by such channels. 

 Whatever sense of direction they possess will be derived from 

 those stimuli which reach the auditory organs by the usual path 

 through the skull ; but here, as in the Ostariophysese, we are con- 

 fronted with the probability that such sense of direction will be 

 interfered with by the stimuli received throngh the air-bladder. 



From this point of view the condition of the air-bladder 

 in Notopterus is especially significant. For more than three- 

 fourths of its extent the organ consists of two lateral chambers ; 

 and if, by an anterior extension of the longitudinal septum, these 



