POLYZOA : AVICULARIA AND V1BRACULA. 



583 



allow a number of these bodies to be in sight at once. It is still 

 very doubtful what is their precise function in the economy of the 

 animal ; whether it is to retain within the reach of the ciliary 

 current bodies that may serve as food ; or whether it is, like the 



Fig. 301. 



a, Portion of Cellularia ciliata, enlarged; B, one of the ' bird's- 

 head ' processes of Bugula avicularia, more highly magnified, 

 and seen in the act of grasping another. 



Pedicellariae of Echini (§ 432), to remove extraneous particles that 

 may be in contact with the surface of the Polyzoary. The latter 

 would seem to be the function of the ribracula, which are long 

 bristle-shaped organs (Fig. 300, a), each one springing at its base 

 out of a sort of cup that contains muscles by which it is kept in 

 almost constant motion, sweeping slowly and carefully over the 

 surface of the Polyzoary, and removing what might be injurious 

 to the delicate inhabitants of the cells when their tentacula are 

 protruded. Out of 191 species of Cheilostomatous Polyzoa de- 



