[0248 «Jo 
XVIII.—On the Anatomy of Eurybia Gaudichaudi, as bearing upon its Position amongst 
the Pteropoda. By Joux Denis MACDONALD, Assistant-Surgeon of H.M.S. * Herald, 
employed on Surveying Service in the South-western Pacific, under the command of 
Captain H. M. Denna, R.N., F.R.S. Communicated by G. Busk, F.R.S., F.L.S. 
Read February 18th, 1858. 
ONE of the most constant products of the towing-net, in the S.W. Pacific, is the little 
Pteropod Zurybia Gaudichaudi. It is often captured in the daytime, which is seldom 
the case with other members of its class. 
The enveloping mantle forms a sac of an oval figure and scarcely exceeding $th of an 
inch in length, with an anterior transverse subterminal slit on the ventral side, giving 
passage to the head with its tentacula, a rudimentary foot, and the swimming-fins. It is 
composed of large spheroidal cartilage-cells, in which the nuclei are distinctly visible, and 
a sparingly interspersed fibrous tissue, opposing an almost insuperable obstacle to the 
study of the internal anatomy of the animal. 
The Eurybia may be said to possess a distinct head and neck, the head bearing on 
either side a large tapering and gently curved tentaculum, with a small nipple-like pro- 
cess at the inner side of the base. All these appendages are richly ciliated, the cilia being 
generally disposed in parallel, longitudinal lines, and exhibiting a dextral, undulatory 
' motion. 
The mouth is a vertical opening, with moderately prominent lateral lips, just within 
the borders of which may be noticed, according to the age of the animal, one, two, or 
three longitudinal series of small square plates, with a delicate eutting edge near the 
outer part of each. 
The lingual ribbon is somewhat more lengthy than that of any other Pteropod with 
which I am acquainted. It presents a single row of simple, lancet-shaped teeth in the 
rachis, and a single row of fang-like uneini in each pleura. The buccal mass is globose in 
figure, and composed of fibro-cartilage with investing museular fibres. + 
Å very distinct though rather small foot springs from the under surface of the ly, 
considerably behind the head, having a flat creeping-dise, with a Ben on 
and a pointed posterior extremity, and in many particulars closely resembling the foot o: 
the true pelagie Gasteropods. i 
The swimming-fins are unquestionably the epipodia of this foot, arising by a subeylin- 
drieal base, just above the lateral border, and near the middle of its eei eg 
at first diminish a little in size, and then gradually expand to form a br y 
compressed paddle, widely emarginated at its extremity. Fe 
The æsophagus holds a dorsal position, and is distinguished en parts of the 
alimentary canal by the thickness of its walls and its richly eiliated lining. 
i i ior of which 
The stomach is large, consisting of several wide sacculated portions, the PE of whic 
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