302 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



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 

 vibracula 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 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 avicularia, like the vibracula, probably 

 serve for defence, but they also catch and kill small 

 living 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 

 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 several species of Lepralia, the moveable mandible 

 is so much produced and is so like a bristle, that 

 the presence of the upper or fixed beak alone serves to 

 determine its avicularian nature. The vibracula may 

 have been directly developed from the lips of the cells. 



