302 MISCELLANEOUS OBJECTIONS TO THE [Chap. VIL 



increase in the size of this projection, with its shape, as 

 well as that of the terminal segment, slightly modified 

 and improved, the pincers are rendered more and more 

 perfect, until we have at last an instrument as efficient 

 as the chelae of a lobster; and all these gradations can 

 be actually traced. 



Besides the avicularia, the Polyzoa possess curious 

 organs called vibracula. These generally consist of long 

 bristles, capable of movement and easily excited. In 

 one species examined by me the vibracula were slightly 

 curved and serrated along the outer margin; and all 

 of them on the same polyzoary often moved simul- 

 taneously; so that, acting like long oars, they swept a 

 branch rapidly across the object-glass of my micro- 

 scope. When a branch was placed on its face, the vibrac- 

 ula became entangled, and they made violent efforts 

 to free themselves. They are supposed to serve as a 

 defence, and may be seen, as Mr. Busk remarks, "to 

 sweep slowly and cai;efully over the surface of the poly- 

 zoary, removing what might be noxious to the delicate 

 inhabitants of the cells when their tentacula are pro- 

 truded." The avicularia, like the vibracula, probably 

 serve for defence, but they also catch and kill small liv- 

 ing animals, which it is believed are afterwards swept 

 by the currents within reach of the tentacula of the 

 zooids. Some species are provided with avicularia and 

 vibracula; some with avicularia alone, and a few with 

 vibracula alone. 



It is not easy to imagine two objects more widely 

 different in appearance than a bristle or vibraculum, 

 and an avicularium like the head of a bird; yet they are 

 almost certainly homologous and have been developed 

 from the same common source, namely a zooid with its 



