486 



POLYZOA 



CHAP. 



movements of the avicularia and vibracula. It is obvious that 

 they may be defensive in character; and it cannot be doubted 



that the avicularia can prevent 

 inquisitive worms from stray- 

 ing at will over the surface of 

 the colony. There is no evi- 

 dence to show that animals are 

 discouraged from interfering 

 with a Bugula owing to the 

 presence of its defensive weapons. 

 It is not, indeed, certain 

 what are the enemies against 

 which the Polyzoa have specially 

 to guard. Sea-urchins and cer- 

 tain Molluscs are known to 

 browse on Polyzoa. Fresh- 

 water Polyzoa, in which avicu- 

 laria and vibracula are absent, 

 are attacked by the larvae of 

 Insects, and by Triclad Planar - 

 ians. I have found the latter 

 with their long pharynx everted 

 and completely buried in a 

 Cristatella colony. It is pos- 

 sible that some marine Cheilo- 

 stomes may be saved from 

 attacks of this kind owing to 

 the existence of their armoury 

 of avicularia and vibracula. 

 It is also possible that these 

 structures are of service by 

 removing foreign particles which 

 might otherwise settle on the 



Fig. 242. Caberea ellisii Flem., Norway, colony, and tend to block up 

 x 40. Back view of part of a branch. _ , f . 



The large vibracular zooecia (v.z) occupy its Orinces. It has IUrther 



nearly the whole of the surface. 

 Seta of vibraculum ; z, zooecia. 



been suggested that animals 

 seized by the avicularia may 

 be held until they die, and that their disintegrating particles 

 may then be carried to the mouths of the polypides by the ciliary 

 currents of the tentacles ; but proofs of this suggestion are 



