1008] Bohertson. — Incrusting Bryozoa. 299 



and guarded by two ehitinoiis lips. Ooecia immersed (fig. 64, 

 06.), formed by an extension of the zoa:'cial wall above the oper- 

 culum and marked by a large pore, p., covered with a chitinons 

 membrane. Avicidaria none. 



The orifice in Lcpralia hilabiata is characteristic and deserves 

 closer inspection. Unlike that of the usual Chilostomatous zooeci- 

 um the orifice here is not closed directly by the external opercul- 

 um. When dissected out (fig. 63), the orifice is seen to be closed 

 by two chitinons ribs, the one, rb'., forming the distal edge of a 

 thin, chitinous membrane lying against the inner side of the oral 

 extremity of the zocecium and forming: what may be regarded as 

 a dorsal or inner operculum, in. op. This is ordinarily hidden by 

 the external operculum when the polypide is retracted and the 

 orifice closed. The other, the ventral or outer operculum, ou. op., 

 consists of a relatively thick, chitinous membrane surrounded by 

 a strong chitinous framework. At its distal extremity there is a 

 folded membrane, fd. meyn., possessing on its inner side a chitin- 

 ous rib, rb., forming the outer or ventral lip of the orifice or 

 mouth, or. The inner rib, r6'./ and the outer rib, rb., are con- 

 tinuous at the corners forming a narrow elongated mouth through 

 which the polypide emerges. Extending from the corners of the 

 mouth between the lateral edges of the inner and outer opercula 

 is a lateral membrane, lat. mem., connecting them, the whole form- 

 ing the tube mentioned by Hincks ( '84), through which the poly- 

 pide passes as it emerges from the zocecium. When closed, the 

 outer operculum shuts down upon the two lips of the orifice, 

 making a sort of third or accessory lip, ac. 



If we regard the outer operculum as homologous with the 

 ordinary Chilostomatous operculum, then its function has changed 

 considerably. Instead of primarily effecting the closure of the 

 orifice, it does so only secondarily by its connection with the 

 folded membrane forming the ventral lip. In this case the inner 

 operculum and the oral lips are new structures peculiar to this 

 species. 



Hincks remarks that the entrance to the ooecium is closed by 

 the operculum. The ovicells are not abundant in this material 

 and this point could not be satisfactorily verified. There is a 

 possibility that the pore may be a point of egress for the embryo. 



