Busk on Avicularia. 
27 
Mr. Darwin, in the ' Voyage of the * Adventure ' and 
' Beagle,' ' adverts at some length to these organs, and de- 
scribes their actions in the living state very graphically. They 
have also been described with great care by Dr. Van Beneden 
and the late Professor John Reid, and also by Nordmann 
and Krohn, the latter (as I extract from Dr. Johnston's ' Hist. 
Brit. Zooph.') classifying them under three different forms : 
1, those which have the figure of the crab's arms ; 2, those 
which resemble pincers ; and 3, those which are formed like 
bristles or hairs ; the last corresponding to what are here 
termed the vibracula. 
In a paper read before this Society, October 27, 1847, and 
published in the ' Transactions,' I have described more particu- 
larly the structure of the curious and unique form presented 
by this organ in Notamia hursaria^ pointing out, I believe for 
the first time, that the muscles were divisible into two distinct 
sets, one for the closure and the other for the opening of 
the mandible, with other minute particulars of their mecha- 
nical arrangement previously unnoticed ; I also stated that the 
muscles not only in this organ but throughout, in this and 
other Polyzoa, are of the striped kind, or resembled those of 
the Brachiopoda more than of any other class in the Mollusca. 
I also indicated that the mandible and the beak of the cup 
were differently constituted to the rest of the organ, being com- 
posed of a horny instead of calcareous substance ; and that 
besides the two sets of muscles above noticed, the cup con- 
tained a " peculiar body of unknown nature.'^ 
I believe that up to the present time our knowledge of these 
organs is pretty nearly limited to the above particulars. With 
respect to their homologies and functions, nothing but con- 
jectures have been offered ; in fact, as regards the homo- 
logies of these organs with any existing in animals belonging 
to the same or any other class, no conjectures beyond the most 
vague have been offered. They may, perhaps, be regarded as 
analogous in function with the Pedicellarice of the Echino- 
derms, as well as with the little accessory cups filled with 
prehensile filaments, or thread cells, which are found in the 
Plumulariae and Campanulariadae ; but, I conceive that any 
homology with these organs is quite out of the question. 
They are as nearly related to the claws of a lobster or the feet 
of a Pygnogonum.* 
* Mr. Huxley has pointed out to me several points of resemblance be- 
tween the avicularia, especially as to their mechanical and muscular ar- 
rangements, and the shells of the Brachiopoda. An ingenious idea, and 
calculated to lead to the more serious consideration of the relationship 
between the Polyzoa and Brachiopoda, as suggested by Mr. Hancock 
