OPERCULUM AND AVICULAPJUM. 99 



with a horny door or valve (operculum) for 

 closing the entrance, and somewhat resembling 

 a lip. Hence, the name of this group 

 Cheilostomata, signifying " lip-mouthed." 

 This door (or operculum) plays an important 

 part in the economy of this group, since it 

 gives rise to two very curious and interesting 

 modifications, viz., the Bird's-head organ and 

 the Vibraculum. 



Operculum in Ctenostomata. In 



another group (Ctenostomata), the margin of 

 the cell mouth is cut into numerous fine 

 segments, like the teeth of a comb. When 

 the tentacles of the animal are withdrawn, the 

 points of the segments meet overhead so as to 

 close the aperture. The word Ctenostomata 

 means " cut-mouthed " in allusion to this form 

 of operculum. An analogous contrivance 

 may be found in the operculate Hydroids. 



Avicularium. In certain of the Polyzoa 

 (Cheilostomata), there occurs an organ 

 called the Bird's-head organ (Avicularium), 

 since in its higher form it resembles very 

 strikingly the head of a bird. It is a peculiarly 

 interesting example of modification, and one 



