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 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 afterwards swept by the currents within reach of the
tentacula of the zooids.
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