30 
Busk oil Avicularia. 
Caberea, it is toothed on one side ; and in others, as in the 
species forming the family Selenariadce, of which we have no 
British representative, the seta is very variously and curiously 
formed, in some being trifid or bifid at the extremity, and in 
one, Selenaria maculata, it is spirally contorted and minutely 
annulated, so as very closely to resemble the proboscis of a 
butterfly. 
As to the function of the vibracula, it would appear, in most 
cases, to be simply defensive. The seta may be observed in 
almost constant motion, sweeping slowly and carefully over 
the surface of the polyzoary, and removing what might be 
noxious to the delicate inhabitants of the cells when their 
tentacula are protruded. 
Another circumstance often to be observed with respect to 
these organs, is this, that each presents inferiorly a rounded 
perforation, as in Scrupocellaria and Cauda, sometimes chan- 
nelled as in Caberea, which indicates the point of attachment 
of a radical tube or fibre. That this connexion with a radical 
tube, however, is not an essential attribute of the vibracular 
organ is sufficiently obvious from the circumstance that those 
tubes are frequent where no such organs exist ; but where 
there are vibracula, the tubes invariably enter them, and not 
the cell itself. This is especially evident in the genus Cauda, 
of which the British species, Cauda (Cellularia) reptaus, affords 
an instance. 
In the case of the Selenariadce or Lunulites, I think it not 
improbable that the vibracula may be subservient to loco- 
motion. 
These organs, both avicularian and vibracular, appear to me 
to be of very considerable importance in a systematic point of 
view ; and, although from our imperfect knowledge of them, 
and in fact of the Polyzoa in general, the supposition can only 
be regarded as problematical, it seems not improbable that 
the presence or absence, especially of the avicularium, may be 
connected more directly with the intrinsic nature of the 
species upon which they are found, than has hitherto been 
supposed. It may, for instance, be the case, that those furnished 
with these offensive weapons live upon a kind of food different 
from that of the others who do not require such an aid in the 
capture or weakening of their prey. The Polyzoa may, per- 
haps, thus be divided into vegetable and animal feeders, or into 
feeders upon dead, and those which feed upon living organisms. 
One thing, however, is certain, that these organs afford, in many 
cases, excellent and available systematic characters ; and this 
part of the subject I will now proceed briefly to discuss. 
I have already stated that the accessory organs we are now 
