Anatomy of Gorgonid.^. 137 



the other species studied, owing probably to the mobihty 

 of the caHcle Hps. 



The retractors are very large and long (PI. VI, fig. i, j, j) 

 and are by far the most powerful that I have found in the 

 GoRGONiD^E. In a transverse section of the gullet and mesen- 

 teries (Pl.V, fig. 2), these muscles are seen to consist of several 

 layers of fibers. In some sections that I have, the muscles 

 appear to be made up of a number of muscle plates, each 

 composed of many fibres which run parallel to each other and 

 are attached by the edges to the mesentery. 



There is another set of muscles in polyps of this species 

 which may serve both to aid in the protraction of the polyps 

 and to close the calicle lips. These muscles seem to be direct 

 continuations of a portion of each retractor which extends 

 from between the bases of the tentacles to the edge of the 

 calicular lip. 



There are then eight of these muscles which, for conveni- 

 ence, I shall call opercular muscles, the name being suggested 

 by the function which the calicular lips perform. 



These opercular muscles are seen in PI. VI, fig. i, i, i, and 

 also in PI. IV, fig. 5 where they are not lettered. It is prob- 

 able that the polyp in retracting closes the calicle by simply 

 pulling the calicular lips down after it by means of these mus- 

 cles. It can be easily seen by reference to PI. Ill, fig. 2, that 

 the contraction of these muscles when the polyp is fully re- 

 tracted would raise it toward the calicle mouth and give it a 

 start, as it were, in protraction. 



The opercular muscles must not be confounded with the 

 polyp wall which forms a sort of sack in which the tentacles 

 are bundled together when the polyp is retracted, as described 

 in Briaremn. This wall is formed by the endothelial calicular 

 lining and the ectodermal investment of the zoanthodeme, 

 with probably a mesodermal layer between, meeting at the 

 edge of the calicle mouth and together passing over the outside 

 of the retracted tentacles, and encircling their bases. When 

 the polyp is expanded this structure constitutes the polyp wall. 



