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XIV. Further Observations on the Anatomy of Macgillivrayia, Cheletropis, and allied 
genei'a of pelagic Gasteropoda. By iomi Denis Macdonald, R.N., Assistant- 
Surgeon H.M.S.V. ‘ Torch.' Communicated hy ^’/tM^illiam Burnett, K.C.B. 8 § c . 
Received February 22, — Read March 22, 1855. 
During a late voyage from Sydney to Moreton Bay, specimens of Macgillivrayia^ 
Cheletropis, and a few other genera of minute pelagic Gasteropoda, apparently unde- 
scribed, were daily taken in the towing-net, and having embraced this opportunity 
of determining the actual mode of attachment and connexions of the ciliated arms, 
at first presumed to be naked branchiae, I am anxious to append the following remarks 
to those which I have already made on this interesting subject. 
In a former paper I mentioned, more particularly with reference to Cheletropis 
Huxleyi, that the gills were of two kinds, i. e. “covered and naked;” the former, 
corresponding to that of the pectinibranchiate Gasteropoda generally, I have never- 
found absent in any of the genera ; but from careful observation of the so-called 
naked gills of these minute animals, while yet alive in their native element, I am dis- 
posed to believe that they are chiefly employed as organs of prehension, and may also 
assist in natation. When these ciliated appendages are fully extended, the line of 
cilia is perfectly straight and uninterrupted, so that the frilled border, noticed in the 
previous account, is a character simply depending on the partial contraction of the 
longitudinal muscular fibres, a preparatory step to complete retraction of the oi-gans. 
They have no connexion whatever with the mantle, but encircle the mouth, including 
the tentacula and eyes, communicating with each other at the base like the segments 
of a deeply-cleft calyx. In the accompanying figure of Macgillivrayia, which has 
been closely copied from nature, the vvhole scheme of arrangement is sufficiently well 
seen. In the specimens of this genus examined, the arms were quite transparent and 
marked at irregular intervals with transverse streaks of a brownish purple colour. 
In the extended form they were several times the length of the shell, and like the 
arms of a polype, when touched, they rolled themselves up, and started back into the 
shell with surprising rapidity. They were also exquisitely sensitive, exhibiting short 
twitching movements when minute atoms suspended in the water came in contact 
with them. 
It will be observed by reference to the figure, that the respiratory siphon is repre- 
sented as a simple process of the mantle converted into a tube by the apposition of 
its lateral borders without organic union, which I must confess is at variance with 
my former views ; moreover this process appeared to be much shorter than I had 
