292 
MR. J. D. MACDONALD ON THE ANATOMY OF 
I have not discovered any vestige of a float, although it is possible that such may 
exist normally, and be detached by the rush of water through the towing-net, or some 
other accident. 
The branchiae or gills are of two kinds, i. e. covered and naked. The covered gill, 
as far as I have been able to observe, is single, but of considerable length. It is 
beautifully pectinated and fringed with long vibratile cilia, representing doubtless 
the respiratory organ of the pectinibranchiate Gasteropoda. 
The basis of this structure is a long and narrow strip of a tough and fibrous mate- 
rial, folded upon itself so as to form a series of loops, invested with a coating of epi- 
thelium richly ciliated along the free border. 
The naked gills are four in number, similar both in situation and character to those 
of Macg 'ilLivrayia. 
Each gill is of an oval or elongated form, presenting a thin, frilled and corrugated 
border, beset with long whip-like cilia, which strike the water with a lashing move- 
ment. 
In the central parts muscular fibres are distinctly discernible, some disposed length- 
wise and others transversely, so that the whole structure and appearance of these 
organs, although very small, would recall to mind the sea-mouse and the numerous 
other marine annelidans of that character. 
The lingual strap is of considerable length, the anterior extremity lying between 
two club-shaped pieces of cartilage consisting of extremely minute cells. The rachis 
is formed of a single series of dental plates, which alternate with those of the pleurae. 
The latter consist of broad quadrilateral masses, each presenting one or two prin- 
cipal tubercles, the most internal of which is somewhat uncinated. The posterior 
extremity of the tubular sac, in the floor of which these peculiar teeth are arranged, 
is rounded and slightly enlarged. 
Besides the lingual teeth, the mouth is furnished with two file-like triturating 
plates, which are articulated with each other inferiorly (PI. XVI. fig. 7)- The two ten- 
tacula of each side appear as it were enclosed in one envelope, so as to form a single 
tactile instrument, bearing 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 enclosed in a little 
sac ; and the optic nerve, emerging from the supra-oesophageal ganglion, joins the 
miniature globe and expands into a retina. I have not been able to trace an opening 
through the pigment for the passage of luminous rays, but it is most probable that, 
as in the ocelli of insects, such exists at the central part, the pigment only encroach- 
ing on the sphere of the lens sufficiently to correct the aberration of light. 
At some distance behind the eyes, when the neighbouring parts are carefully 
removed with fine needles, the auditory capsules may be distinctly seen with the 
micioscope. They are of a rounded or oval form, containing each a beautifully 
transparent and highly refracting otolithe, much larger than the lens of the eye. 
