AIK-B LADDER IN NOTOPTERUS BOR]S'EE]SrsiS. 535 



compartments bad become entirely separated from eacb. other, 

 and eacb bad remained continuous witb the auditory csecum of 

 its side, it is at least conjecturaMe tbat eacb cbamber would be 

 somewbat differently affected by sound-waves impinging on its 

 own side of tbe body and eventually propagated to tbe auditory 

 organ of tbe same side. In this way, it seems possible tbat 

 stimuli received by tbe auditory organ tbrougb tbe intervention 

 of tbe air-bladder might be competent to give rise to a sense 

 of direction. So far, bowever, from this being tbe case, tbe 

 longitudinal septum ceases at tbe very point wbere its forward 

 prolongation seems to be most desirable ; and, in consequence, the 

 auditory caeca arise from an anterior, median, tubular portion of 

 tbe bladder, which is only slightly divided longitudinally by the 

 carinate growth of the suprajacent axial skeleton. Hence, as in 

 non-septate air-bladders, sound-waves are propagated to the 

 auditory organs simultaneously and with equal intensity. 



There is also another point which deserves consideration in 

 discussing the possible auditory function of the air-bladder in 

 these Eishes. 



Well-marked cranial fontanelles are present in several of 

 them, and these structures are not only paired but each is in 

 immediate relation with the auditory organ of its side of the 

 bead. Do not these skin-closed fontanelles afford a better 

 channel for tKe transmission of sound-vibrations to the mem- 

 branous labyrinths than the air-bladder?, and is not one of these 

 structures somewhat superfluous if we regard both of them as 

 sound-transmitting organs ? Moreover, so far from interfering 

 with tbe sense of direction, cranial fontanelles would probably 

 facilitate the differential action of the two auditory organs. 



If the connection of tbe air-bladder witb tbe auditory organ is 

 not subservient to the sense of hearing, can any other function 

 be assigned to it ? 



In the case of the Ostariophysese, Eamsay "Wright (13) and 

 Bridge and Haddon (2) have supported the view that the 

 connection of the two organs was j)ossibly for the purpose of 

 enabling these Fishes to appreciate the varying degrees of tension 

 of the gases in the air-bladder, resulting from corresponding 

 variations of hydrostatic pressure produced by locomotor move- 

 ments involving differences of depth. Subsequent reflex or 

 voluntary impulses, it was suggested, might find expression in 

 the exercise of some form of regulatory control over the liberation 



