Busk on Avicularia. 
31 
considering are divided into two kinds, apparently with dis- 
tinct functions — avicularia and vibracula ; the one, probably 
prehensile, the other defensive. Of these, the avicularia are 
found by far the most extensively ; in fact, they are wanting 
in but few of the genera constituting the cheilostomatous 
class of Polyzoa. In employing them for the purpose of 
classification, it is necessary to subdivide them into three 
classes. 1, the pedunculate. 2, the sessile ; and 3, the 
immersed. The two latter classes, however, run insensibly 
into each other, whilst the pedunculate form is obviously 
quite distinct, inasmuch as it presents an additional member, 
in the form of a basal joint. It is this form of avicularium to 
which the term " birds," or " vultures' heads," is more pro- 
perly applied. It is well known in this form, as it occurs in 
Bugula avicularia^ B. plumosa^ and B. Jlahellata : it is also 
found in Bugula tridentata (Krauss), a South African species ; 
and in Bicellaria ciliata ; whilst it is wanting altogether in 
Bugula neritina, Bicellaria grandis, and Bicellaria gracilis. 
A second modification of pedunculate avicularium, where it 
assumes the form of a long trumpet-shaped or infundi- 
buliform tube, exists in Bicellaria tuba. So far as I know 
then, at present, the pedunculate form of avicularium is 
restricted to the genera Bugula and Bicellaria, though it does 
not exist in every species of either genus, and in one, assumes 
a form quite different from the ordinary. All that can be 
said about it, therefore, in those two genera, is that where the 
avicularia exist, they are of the pedunculate variety. The 
true " bird's-head " avicularia are always placed on the an- 
terior aspect of the cell, on one side below the level of the aper- 
ture, whilst the tubiform variety arises on the back of the cell. 
The sessile form of avicularium as I have observed, may 
be subdivided into the projecting and the immersed. Of 
these, the latter is much the more extensively distributed : it 
is placed either at the angles or margin of the cells, or on 
some other part, usually of their anterior aspect, but some- 
times on the posterior : instances of the latter are presented 
in Caherea nuda, where the vibracular organs, so characteristic 
of the other species in the same genus, are replaced by what 
may be termed avicularia, though, in fact, they should more 
properly be referred to the vibracular type, inasmuch as 
the radical tubes enter their bases. The genus Retepora, also 
offers an instance of the posterior position of true avicularia ; 
but, with these exceptions, I am not acquainted with any 
other in which the avicularia are not on the sides or front of 
the body, as indeed, if our surmises with respect to their func- 
tion be well founded, might be expected. 
