236 HENRY C. TRACY 



direct pathway of the transmission of pressure. Since each of 

 the macular divisions is very loosely attached to the edge of 

 the fenestral lips, the fluid movement resulting from the trans- 

 mission of force through the fenestra may cause a slight motion 

 of the divisions of the macula which will result in a stimulation 

 of the macular cells by the cilia embedded in the otolithic 

 membrane. 



A possible objection to this theory of the mechanics of these 

 structures as stated above may be alleged, owing to the fact 

 that in some of the Clupeoid species the caudal end of the 

 swimbladder opens directly to the exterior just behind the anal 

 opening. In the majority of the Clupeoids and allied forms, 

 however, the postanal connection of the swimbladder with the 

 exterior is not present. De Beaufort ('09) names nine species 

 with an opening near the anus and fifteen without such an 

 opening. Of the American species (not included in the list of 

 de Beaufort), Alosa sapidissima, Brevoortia tyrannis, and 

 Stolephorus mitchilli have no anal communication. In species 

 in which the opening is present the communication takes place 

 through a slender thin-walled tube which curves around to the 

 left between the genital and kidney ducts to the left body 

 wall and opens to the exterior between the genital opening and 

 the anus (de Beaufort, '09, described in Clupea harengus). It 

 might be said that the expansion or compression of the gas in 

 the swimbladder may take place directly through the postanal 

 opening (in those species in which it exists) and thereby bal- 

 ances the pressure transmitted to the wall of the anterior mem- 

 branous vesicle along the pathway indicated above to such a 

 degree that pressure transmission through the head would not 

 be effective in originating a stimulus in the divisions of the 

 macula. Possibly this may be true to some degree for extreme 

 and very sudden changes in the hydrostatic pressure of the 

 surrounding medium. It seems hardly probable, however, that 

 the transmission of changes in hydrostatic pressure through a 

 slender, curved, thin-walled tube could be as direct or as imme- 

 diate as through the perilabyrinthine spaces. The possibility 

 of the transmission of changes in hydrostatic pressure to the 



