us 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. 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 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
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 cell, with the
included zooid, could hardly have disappeared at once. In many cases the
vibracula have a grooved support at the base, which seems to represent
the fixed beak; though this support in some species is quite absent.
This view of the development of the vibracula, if trustworthy, is
interesting; for supposing that all the species provided with avicularia
had become extinct, no one with the most vivid imagination would ever
have thought that the vibracula had originally exi