THE MORPHOLOGY OF THE CHEILOSTOMATA. 273 



operculum is seen the eutrauce to the vestibule, while the 

 aperture of the compensation-sac is situated between the base 

 of the operculum and the calcareous frontal wall of the 

 zooDcium. 



The relations of the same parts are explained by the thick 

 longitudinal section shown in fig. 26, in which the operculum 

 is very nearly closed. By deep focussing can be seen the 

 calcareous condyle {cond.) which constitutes the hinge. The 

 divaricator muscles {div.) of the operculum are paired ; 

 each originates from one of the lateral walls of the zooecium, 

 and passes obliquely towards its frontal surface, crossing the 

 condyle on its proximal side, to reach its insertion into the 

 basal sclerite of the operculum. The contraction of these 

 muscles will obviously have the double effect of opening the 

 vestibule and of opening the compensation-sac. The oper- 

 culum is prolonged laterally into a very large triangular 

 flange {fl.), whose plane is at right angles to that of its free 

 surface (see also figs. 24, 25, 27). The occlusor muscles 

 (occl.) are similarly paired, each originating from the lateral 

 wall of the zooecium at a deeper level than the divaricator, 

 and passing obliquely across the base of that muscle to 

 reach its insertion, on the distal side of the condyle, into 

 the tip of the triangular flange. Fig. 25 shows that the 

 insertion is by means of a broad tendon. Since the 

 labium articulates with the operculum in this region, the 

 effect of the contraction of the occlusor muscles will be, 

 not only to close the operculum itself, but also to retract 

 the labium from the position shown in fig. 24 to that shown 

 in fig. 25. In the closed condition, the labium lies just 

 inside the vertical flange of the operculum. 



The lateral flanges are not really independent structures, 

 but they pass into one another in a continuous curve round 

 the distal side of the operculum, their free border, continuing 

 the comparison with the turtle's skull, constituting the biting 

 edge of the upper jaw. In longitudinal section (tig. 20) the 

 upper jaw appears to be strengthened by a strong buttress 

 {huttr.) which passes from the free surface of the operculum. 



VOL. 46, PART 2. KKW SERIES. S 



