OBJECTIONS TO THE THEORY 329 



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 simultaneously; 

 so that, acting like long oars, they swept a branch 

 rapidly across the object-glass of my microscope. When 

 a branch was placed on its face, the vibracula became 

 entangled, and they made violent efforts to free them- 

 selves. They are supposed to serve as a defence, and 

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

 carefully over the surface of the polyzoary, removing 

 what might be noxious to the delicate inhabitants of the 

 cells when their tentacula are protruded." The avicu- 

 laria, like the vibracula, probably serve for defence, but 

 they also catch and kill small living animals, which it 

 is believed are afterward swept by the currents within 

 reach of the tentacula of the zooids. Some species are 

 provided with avicularia and vibracula; some with avicu- 

 laria 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 

 cell. Hence we can understand how it is that these 

 organs graduate in some cases, as I am informed by 

 Mr. Busk, into each other. Thus with the avicularia of 

 everal species of Lepralia, the movable mandible is so 

 uch produced and is so like a bristle, that the presence 

 if the upper or fixed beak alone serves to determine its 

 vicularian nature. The vibracula may have been directly 

 eveloped from the lips of the cells, without having 



