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XVIII. — On the Anatomy of Eurybia Gaudichaudi, m bearing %ipon Us Fositiou amongst 

 the Pteropoda. By John Denis Macdonald, Assistant-Surgeon of H. M.S. ' Herald,' 

 employed on Surceylng Sendee in the South-ioestern Pacific, tinder the command of 

 Cajjtain H. M. Denham, R.N., F.R.S. Co)nmunicated by G. Busk, F.E.S., F.L.S. 



Read February 18th, 1858. 



One of the most constant products of the towmg-uet, in the S.W. Pacific, is the little 

 Ptei'opod Eurybia 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-Mke pro- 

 cess at the inner side of the base. All these appendages are riclily 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 witliin 

 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 cutting 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-hke uncini in each pleura. The buccal mass is globose in 

 figure, and composed of fibro-cartilage with investing muscular fibres. 



A very distinct though rather small foot springs from the under sm-face of the body, 

 considerably behind the head, having a flat creeping-disc, with a subquadrate anterior 

 and a pointed posterior extremity, and in many particulars closely resembling the foot of 

 the true pelagic Gasteropods. 



The swimming-fins are unquestionably the epipodia of this foot, arising by a subcylin- 

 drical base, just above the lateral border, and near the middle of its root. These organs 

 at first diminish a little in size, and then gradually expand to form a broad and laterally 

 compressed paddle, widely emarginated at its extremity. 



The oesophagus holds a dorsal position, and is distinguished from the other parts of the 

 alimentary canal by the thickness of its walls and its richly ciliated lining. 



The stomach is lai*ge, consisting of several wide sacculated portions, the exterior of which 



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