26 Busk 07i Avicularia. 
all appearance considerably thinner than Micrastorias denti- 
culata. 
Remarks on the Structure and Function of the Avicularian 
and ViBRACULAR Organs of the Polyzoa; and on their 
value as diagnostic characters in the classification of those 
creatures, (Read Nov. 23, 1853.) 
The Polyzoa, or, more properly speaking, one class of the 
Polyzoa characterized by the possession of a movable semi-cres- 
centic lip, furnished with a corneous rim, at the mouth of the 
cell, — the cheilostomata as I have elsewhere termed them, or 
Celleporina of Ehrenberg, — are, many of them, distinguished 
by the presence of appendicular organs affixed to one part or 
another of the cells of which the polyzoarium is composed. 
These organs are of two kinds, the one a sort of pincers, and 
the other consisting of a long, slender, movable seta. To the 
former set of organs, of whatever form, the term avicularium is 
here applied, and the latter are designated as vihracula. 
With respect to the structure of these organs of either class it 
is sufficient to remark that, however diverse their appearance 
may be, they are all constructed upon the same general type, 
that is to say, the organ consists of a hollow cup or receptacle 
containing two sets of muscles for the movements of its motile 
portion, the mandible^ as I term it in the one case, and the 
seta in the other. Beyond this general conformity in type, 
however, my knowledge of the more intimate structure and 
contents of the cup in the vibracular organs, does not allow me 
to approximate them to the avicularia. 
The avicularia., besides the movable mandible, it may be 
observed, always have a corresponding fixed heah^ the opponent 
as it were of the mandible, and necessary to constitute the 
organ, what I presume it to be, an instrument of prehension. 
This beak is needless, and is therefore wanting in the vihra- 
cula^ and its absence in cases where the movable part is 
detached, would serve to distinguish one kind of organ from 
the other. 
I. The avicularia. — 
The first notice we have of the existence of these organs 
or rather of one form of them, is contained in Ellis's account 
of what he terms the " Bird's-head coralline," (Nat. Hist., 
Zooph., p. 36, pi. 20,) where he says, " On the outside of each 
cell we discover, by the microscope, the appearance of a bird's 
head, with a crooked beak opening very wide." 
