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 existed as part of an organ,
resembling a bird's head, or an irregular box or hood.
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