THE BRYOZOA BASSLEE. 



369 



are indicated by the nature of the frontal and of the operculum. 

 The nature and shape of the operculum is thus very important and 

 whenever possible the student should make a special study of this 

 organ. 



Operculum. — This small chitinized organ closes at the same time 

 both the orifice of the polypide and of the compensation sac, its an- 

 terior part, or anter, closing the, former and the posterior part, or 



pharynx 

 esophagus 

 retractor muscles 



avicular ium 



anus 

 intestine 



ovary 



funiculus 



embryo: in ovicell 

 ooecium (ovicell) 



spermatidia 



Fig. 10. — General anatomy of the Cheilostomata. Two zociils of the 

 common corneous cheilostomatous bryozoan (Bugula aricularia 

 Linnaeus) from the Atlantic, showing the various parts cf the 

 polypide, highly magnified. (After Parker and Haswell.) 



poster, similarly closing the latter. Thus the shape of these two por- 

 tions is evidence of the nature of the tubes they close and the de- 

 termination of the operculum is an important feature. (Text fig. 12.) 

 In one large group of the Ascophora the orifice of the compensation 

 sac is very small and the operculum has a corresponding small nar- 

 row tongue ; in another group this orifice is quite large and the cor- 

 responding portion of the operculum is large ; again a special tube, 

 the spiramen (text fig. 13), may introduce the water into the com- 

 42803°— 22 24 



