THE MORPHOLOGY OP THE CHEIT.OSTOMATA. 319 



condyles, as well us by the tension of the frontal membrane ^ 

 continuous with the base of the operculum. 



Although the distal occliisors are larger than the proximal 

 pair, their tendency to shift the operculum from its proper 

 position is probably adequately corrected by the proximal 

 occlusors, which, although weaker than the others, have a 

 more effective pull, in consequence of the greater obliquity 

 of their tendon. 



It appears to me that in certain positions assumed by the 

 operculum^ the distal occlusors would work to less advantage 

 than the others. When the operculum is opened so far that 

 its outer surface forms a rig'ht angle with the frontal mem- 

 brane, the projecting ends of the main sclerites extend far to 

 the distal side of the hinge. The operculum seen from its 

 edge would have the ap[)earauce of a right-angled triangle, 

 of which the hypotheuuse is formed by the main sclerite, and 

 a short side by the piece connecting the proximal end of the 

 same sclerite with the base of the operculum. Both pairs of 

 occlusors now probably pull, by means of their fascise inserted 

 into the projecting ends of the main sclerite, on the short side 

 of the triangle which is, in fact, an arm of a right-angled 

 lever hinged at its angle. In so acting, the proximal occhisor 

 probably pulls over the ligament {lig.) which connects the 

 operculum with the condyle, the ligament thus forming a 

 kind of pulley. The distal occlusors are probably specially 

 important in the final stages of the closure. 



Two muscles remain to be described. The depressor 

 muscles (hgs. (35, (jH, depr.) originate from the innermost part 

 of the lateral recess. Their tendon passes obliquely towards 

 the proximal end of the zooecium, ascending at the same time 

 to reach the frontal membrane, into which it is inserted (fig. 

 ^'6). Between the depressor and the proximal occlusor is a 

 group of muscles which constitute the divaricator [div.). 



' lu my I'oniier paper 1 described lliis as the epiUieca, a more general term 

 which il would be belter not to use ia eases where the homology of the mem- 

 braue with the frontal membrane of a Menibranipora seems to be demon- 

 strated. 



