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.