EAR-SWIMBLADDER RELATION IN CLUPEOIDS 235 



The mechanical relation of the swimbladder to other struc- 

 tures is shown diagrammatically in figure 8. The swimbladder 

 is a tube of very small diameter in comparison with the size of 

 the body of the fish and with the thickness of its muscular wall ; 

 it has no direct connection with the skin or other superficial 

 structures, as is the case in many other groups of fishes (with 

 the exception in some species of a postanal connection with 

 the exterior, which will be discussed below). It may there- 

 fore be assumed that the thick muscular wall of the body, sup- 

 ported by vertebrae, ribs, scales, spines, etc., interferes with the 

 direct and immediate transmission of changes in hydrostatic 

 pressure from the surrounding water to the swimbladder. In the 

 head, however, the anatomical structure indicates that there 

 exists a pathway for the transmission directly to the anterior 

 membranous vesicle of changes in the outside pressure resulting 

 from movement of the fish from one water level to another. 



The pressure transmission from the outside to the walls of the 

 vesicle may be assumed to take the following pathway (indi- 

 cated by the arrows A, Al, A2, fig. 8): from the lateral-line 

 canal in the lateral recess to the lateral perilabyrinthine spaces, 

 thence to the whole system of perilabyrinthine canals, particu- 

 larly to the utricular perilabyrinthine canal (with which the 

 lateral spaces are in direct connection), thence through the two 

 parts of that canal, around the lips of the fenestra, to the floor 

 of the recessus utriculi, then through the endolymph down 

 through the fenestra — and through the superior chamber of the 

 osseous capsule and the elastic septum to the wall of the anterior 

 membranous vesicle. Since the walls of the gas-filled vesicle 

 beneath the septum are thin and loosely attached to the sides 

 of the inferior chamber of the bony capsule, the vesicle easily 

 expands and contracts, and hence pressure transmission from 

 the outside to it will cause a mass movement of fluid along the 

 pathway indicated. The parts of the utricular wall immedi- 

 ately adjacent to and in between the anterior and posterior 

 divisions of the macula are thin with little or no condensed 

 tissue. The thin floor of the utriculus between the two divi- 

 sions of the macula overlies the fenestra, and hence is in the 



