EAR-SWIMBLADDER RELATION IN CLUPEOIDS 237 



body of the swimbladder either directly through the anal region 

 or even indirectly through the fluid-permeated tissues of the body 

 of the fish cannot be denied. But the requirements of the 

 theory as stated above are sufficient if it can be shown that 

 transmission of the pressure changes through the lateral recess 

 and the perilabyrinthine canals to the anterior membranous 

 vesicle is more direct, more immediate, and more sensitive to 

 slight variations than the transmission of pressure changes 

 through the body walls or through the postanal opening to the 

 body of the swimbladder. 



An auditory function may possibly be attributed to this 

 mechanism. This was the view expressed by most of the older 

 anatomical writers. Weber (1820), for example, considered that 

 the septum in the anterior osseous capsule functions as a tym- 

 panic membrane. The anatomical structure of this mechanism 

 hardly appears to support this view. To be effective as an 

 aid to hearing, it must be constructed so as to transmit molecu- 

 lar vibrations from the outside to the membranous labyrinth 

 more quickly or more efficiently than they can be transmitted 

 by other existing structures in the head of the fish. Whereas, 

 it would appear that the molecular vibrations of sound waves 

 would pass more easily to the membranous labyrinth directly 

 from the water through the bones and fluid-permeated tissues of 

 the head. This apparatus is so constructed that we have a 

 pathway adapted for a mass motion of fluid; it can hardly be 

 considered as a means of increasing efficiency of transmission 

 of the molecular motion of sound waves. These considerations 

 are similar to those which lead Bridge and Haddon to conclude 

 that the Weberian ossicles are for the transmission of hydro- 

 static pressures and not accessory to audition. 



If the functional significance of these structures can be shown 

 to be essentially as stated above, the anterior and posterior 

 divisions of the macula acustica utriculi are together to be con- 

 sidered as a 'depth sense organ' or, more accurately speaking, a 

 receptor which is stimulated by changes in hydrostatic pres- 

 sure which usually, or perhaps always in the normal life of the 

 fish, result from movements from one water level to another. 



