THE POLYZOA. 
63 
lively and varied : now it sails swiftly along, tumbling over and 
over occasionally in a rollicking fashion; now it rotates per- 
severingly on its axis ; now it moves restlessly over the surface 
of the vessel in which it is confined, as if searching for something. 
The decline of its activity is the sign that the first stage of its 
existence is drawing to a close. 
The cilia are borne on a delicate membranous envelope, 
within which the solid contents of the body are seen as an 
opaque white mass. At one pole of the body is placed a some- 
what horseshoe-shaped opening, at the extremity of which is a 
tuft of long bristles that lash the water vehemently and 
incessantly. This opening is without doubt a mouth ; I have 
seen rejectamenta passing from it. At the opposite pole is a 
circular depression, forming a kind of bowl, the margin of 
which is surrounded by very delicate cilia. This, as Nitsche 
conjectures, is very probably a sort of sucker by means of which 
the embryo attaches itself. I have little doubt that this is the 
true interpretation of it, as I have noticed that this portion of 
the body is always applied to the surface on which it rests. In 
some species minute red pigment spots are present at certain 
points on the surface of the body, in which a retractile corpuscle 
is embedded. Are these to be regarded as the equivalent of 
eyes ? 
When its term of free life is ended — that is, when it has at- 
tained the proper stage of development — the embryo fixes itself, 
loses its cilia and the whole of its characteristic structure, and 
now appears as a membranous chamber inclosing a mass of 
granular substance; and from this mass the polypide gradually 
buds. The foundation of the colony is thus laid. 
A word in conclusion respecting the curious appendages 
with which so many of the calcareous Polyzoa are furnished, 
and which bear the names of vibracula and avicularia* The 
former are long and slender bristles placed close to the cells, 
which move incessantly, with much energy and a certain rhyth- 
mical sweep, over the surface of the polyzoary. They are, as it 
were, jointed at the base and supplied with a special apparatus 
of muscles, and their function plainly is to prevent the accumu- 
lation of noxious matter in the neighbourhood of the polypides. 
The avicularia are developed in extraordinary profusion and 
variety throughout a large portion of the class. They are best 
described as miniature birds’-heads, and are furnished with a 
movable lower jaw w'hich they keep in constant play, now 
throwing it back with ludicrous vehemence, and after a w T hile 
dosing it with a malicious snap. Many of them are fixed, 
* Both these organs may he studied to great advantage in one of the 
commonest of our British species, the Scrupoccllaria scruposa. 
