THEORY OF NATURAL SELECTION. 215 



segment slightly prominent, sometimes furnished with irreg- 

 ular teeth, and against these the terminal segment shuts 

 down. By an increase in the size of this projection, with its 

 shape, as well as that of the terminal segment, slightly mod- 

 ified 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 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 afterward 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 avicula- 

 rium 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 under- 

 stand 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 movable 

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

 the presence of the upper or fixed beak alone serves to deter- 

 mine 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 

 eell, with the included zooid, could hardly have disappeared 



