Busk on Avicularia, 29 
that species, also, that I first noticed (in 1852) the fact that 
when the mandible is thrown back, or, in other words, when 
the avicularium is open, a slight prominence comes into view, 
covered Avith delicate setose hairs, which do not seem to be of 
the nature of cilia, because they exhibit no motion. These 
minute set(je appear to be seated on the " peculiar body " above 
adverted to, and which again seems to be so connected with 
the muscles by which the mandible is closed, or rather, 
perhaps, to a membrane by which they are covered, or by 
which the opening of the cell is closed, when the mandible is 
thrown back, as to be protruded, simply by the throwing back 
of that process ; so that the setce are then made to project 
beyond the level of the cup, and are withdrawn as the man- 
dible closes. Fig. 2 represents this apparatus in Notamia. 
Fig. 7, the same thing in Bugula plumosa, and fig. 9, in 
Bugula avicularia. These are the only three species in 
which, up to the present time, I have been able to perceive 
this arrangement ; but not having had an opportunity of ex- 
amining the avicularia of any other, except Scrupocellaria 
scruposa, for the purpose, and in which I was unable to detect 
it (fig. IS), I am not prepared to say that it obtains univer- 
sally. It is probable, however, that in a modified form it may 
do so. I am inclined to the opinion that it is a tactile organ, 
the object of which is to apprize the occlusor muscles of the 
contact of any minute floating object, upon which the muscles 
immediately contract, and either close the avicularium against 
the invasion of a foe or capture the appropriate prey. 
A second point that I have also observed in these organs, 
and which, I believe, has not been before noticed, is that the 
portion of the cup in which the muscles and the greater part 
of the peculiar organ (which might probably be regarded as a 
nervous ganglion) are placed, is closed in by a delicate mem- 
branous tympanum, which has a central perforation, through 
which the conjoined tendon of the occlusor muscles passes, and 
also a second smaller opening (at all events in B. avicularia), 
the object of which I do not know. This tympanic mem- 
brane is shown in fig. 8. 
So much for the structure and conjectural function of the 
avicularia ; and to proceed to consider the vihracula in the 
same particulars. 
1. As to the structure of these organs, I have nothing new 
to offer. They consist, as I have said, of a cup containing the 
muscular apparatus, and of a movable seta, articulated to the 
cup, and which appears to be moved in the same way as the 
mandible of the avicularia. This seta is in most cases simple 
and terete ; in others, as, for instance, generally in the genus 
