CHAP. VII.] THEORY OF NATURAL SELECTION. 18l 



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 

 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 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, without 

 having passed through the avicularian stage ; but it seems more 

 probable that they have passed through this stage, as during the 

 early stages of the transformation, the other parts of the cell with 

 the included zooid could hardly have disappeared at once. In 

 many cases the vibracula have a grooved support at the base, 

 which seems to represent the fixed beak ; though this support in 

 some species is quite absent. This view of the development of the 

 vibracula, if trustworthy, is interesting ; for supposing that all the 

 species provided with avicularia had become extinct, no one with 

 the most vivid imagination would ever have thought that the 

 vibracula had originally existed as part of an organ, resembling a 

 bird's head or an irregular box or hood. It is interesting to see 

 two such widely different organs developed from a common origin ; 

 and as the moveable lip of the cell serves as a protection to the 

 zooid, there is no difficulty in believing that all the gradations, by 



