272 SFDNEY F. HAKMER. 



tioiij with I'egard to tlie existence of the '' parietal muscles " 

 which elsewhere dilate the compensatiou-sac ; but the analogy 

 of other Cheilostomes leaves little doubt as to their presence. 



The Operculum is a very remarkable piece of mechanism, 

 which forms a most efficient means of protecting the entrance 

 to the tentacle-sheath. Most of the existing descriptions of 

 Cheilostomatous opercula take account of the appearance 

 of the outer surface oidy of this structure — a very inadequate 

 way of arriving at its i-enl relations. Although E.clathrata 

 has an operculum which, judged by the descriptions available 

 for comparison, would appear to be unusually complex, it is 

 in tlie highest degree probable tli.it a renewed examination of 

 other Cheilostomes will show that it is by no means unique 

 in this particular. 



The first part of the introvert which leads to the mouth of 

 the polypide is constituted by the "diaphragm" or 

 " vestibule" (fig. 26, vest.), which is a musculnr invagination 

 projecting into the tentacle-sheath, and communicating with 

 it by a central aperture.^ The way in which the vestibule 

 opens and closes has been aptly compared with the action of 

 a clasp-purse. The structures which protect the vestibule 

 of E. clathrata have a superficial resemblance to the skull 

 of a turtle (Chelone), the skull with the upper jaw being- 

 represented by the operculum, and the lower jaw by a 

 chitinons lower lip (tigs. 23 — 27, Ih.), which I propose to term 

 the'' labium." The labium was described by Hincks^in a form 

 from the Queen Charlotte Islands, named by him Lepralia 

 bilabiata, in allusion to the existence of this structure. It 

 is probable that it will hereafter be found in numerous 

 Ciieilostomata. 



The labium can be clearly seen in those zooocia of the dry 

 colony in which the opei'culum is open (fig. 10). Between it 

 and the edge of the downwardly })rojecting flange {ji.) of the 



' See, for a desciipUon of (liis structure, Nifsclie (1871, p. 432), Jullien 

 (1888, 4, p. 38), and Calvet (1900, pp. 180, 201). 



■^ ' Aim. Ma!,'. Nat. Hist.' (5), xiii, 1S8I, p. 49. The labium is the " upper 

 lij) " of lliiicks's description. 



