THEORY OF NATURAL SELECTION. 229 



against one whole side, and is thus enabled to catch hold 

 of an object, but the limb still serves as an organ of loco- 

 motion. We next find one corner of the broad penulti- 

 mate segment slightly prominent, sometimes furnished 

 with irregular teeth, and against these the terminal seg- 

 ment shuts down. By an increase in the size of this pro- 

 jection, 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 chelge 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 vibracnla became entangled, and they made vio- 

 lent 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 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 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 dif- 

 ferent 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 movable 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 



