Royal Society. 299 



The portion of the mantle which forms the respiratory siphon, is 

 short, and its opposite edges are merely in apposition, without 

 organic union. The branchiae are of two kinds, covered and naked. 

 The covered gill is single but of considerable length. It is beautifully 

 pectinated, and fringed with long cilia, and, doubtless, represents 

 the respiratory organ of the pectinibranchiate Gasteropoda. The 

 basis of this part is a long and narrow strip of a tough and fibrous 

 material, folded upon itself into a series of loops invested with a 

 coating of epithelium, and richly ciliated along the free border. 

 The naked gills are four in number, similar both in situation and 

 character to those of Macyillivrayia. Each gill is of an oval or 

 elongated form, presenting a thin, frilled and corrugated border, 

 beset with long whip-like cilia. In the central parts muscular fibres 

 are distinctly discernible, some disposed lengthwise and others 

 transversely. 



The lingual strap is next described, as well as two file-like tri- 

 turating plates with which the mouth is furnished. 



The two tentacula of each side appear as it were enclosed in one 

 envelope, so as to form a single tactile instrument, which bears a 

 large dark eye on its outer side near the base. To this latter organ 

 the tegumentary covering forms a kind of cornea, beneath which is 

 a spherical lens resting on a mass of black pigment, both being en- 

 closed in a little sac ; and the optic nerve, emerging from the sub- 

 cesophageal ganglion, joins the miniature globe and expands into a 

 retina. The author was unable to trace an opening through the 

 pigment for the passage of light, but thinks it probable that, as in 

 the ocelli of insects, such an aperture exists in the central part. The 

 auditory capsules are situated at some distance behind the eyes, and 

 may be distinctly seen with the microscope when the surrounding 

 parts are carefully removed with fine needles. They are of a rounded 

 or oval form, and each contains a beautifully transparent and highly 

 refracting otolithe, much larger than the lens of the eye. 



January 11, 1855.— Thomas Bell, Esq., V.P., in the Chair. 



The following communications were read : — 



" On the Development of Muscular Fibre in Mammalia." By 

 William S. Savory, M.D. 



The author's observations were made chiefly upon fcetal pigs, but 

 they have been confirmed by repeated examinations of the embryos 

 of many other animals, and of the human foetus. 



If a portion of tissue immediately beneath the surface from the 

 dorsal region of a fcetal pig, from one to two inches in length, be 

 examined microscopically, there will be seen, besides blood-corpuscles 

 in various stages of development, nucleated cells and free nuclei or 

 cytoblaste scattered through a clear and structureless blastema in 

 great abundance. These cytoblaste vary in shape and size ; the 

 mailer ones, which are by far the most numerous, being generally 

 round, and the larger ones more or less oval. Their outline is di- 

 stinct and well defined, and one or two nucleoli may be seen in their 

 interior as small, bright, highly refracting spots. The rest of their 

 substance is either uniformly nebulous or faintly granular. 



