Seite - 249 - in The Origin of Species

THEORY OF NATURAL SELECTION 249 gradations the one could have been converted into the other : but it by no means follows from this that such gradations have not existed. As the chela: of Crustaceans resemble in some degree the avicularia of Polyzoa, both serving as pincers, it may be worth while to show that with the former a long series of serviceable gradations still exists. In the first and simplest stage, the terminal segment of a limb shuts down either on the square summit of the broad penultimate segment, or against one whole side ; and is thus enabled to catch hold of an object; but the limb still serves as an organ of locomotion. We next find one corner of the broad penultimate segment slightly prominent, sometimes furnished with irregular 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 modified and im- proved, the pincers are rendered more and more perfect, un- til we have at last an instrument as efificient as the chelse 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 ex- amined by me the vibracula were slightly curved and ser- rated 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, re- moving 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 ?lso 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