OPISTHOBRANCHIATA, 351 
of its elongated trunk, twisting its body about, and swimming 
in every direction by the lateral movements of. its vertical, 
dilated foot. I have frequently seen them descend to the bottom 
_of the glass vessel in which they were kept, fix themselves there 
in the manner of 4 leech, by their sucking disk, and carefully 
examine the nature of their prison-house by protruding the front 
portion of the foot in every direction.” 
They swim shell downwards, with sudden jerks, by means of 
their compressed and fin-like foot. 
Oxyeyrus, Benson. 
Syn.—Ladas, Cantraine. Helicophlegma, d’Orb. 
Distr —4 sp. Atlantic, Mediterranean. 0. Keraudrenii, 
Rang (Ixxxvi, 6,7). 
Shell milky, narrowly umbilicated on both sides; nucleus not 
visible; back rounded, keeled only near the aperture ; body- 
whoil, near the aperture, and keel cartilaginous ; no apertural 
slit. Operculum trigonal, lamellar. 
SUBCLASS OPISTHOBRANCHIATA. 
Branchie exposed, or protected by a fold of the mantle and 
situated at the posterior centre of the back, and never in a 
cervical cavity. Sexes united. Some have an internal or external 
spiral or patelliform shell, testaceous or membranous, others are 
without shell. 
The mollusks of this subclass may be termed sea-slugs, since 
the shell, when it exists, is usually small and thin, and wholly 
or partially concealed by theanimal. When alarmed or removed 
from their native element, they retract their gills and tentacles, 
and present such a questionable Shape that the inexperienced 
naturalist will be likely enough to return them, with the refuse 
of the dredge, into the sea. Their internal structure presents 
many points of interest; in some the gizzard is armed with horny 
spines, or large shelly plates; in others the stomach is extremely 
complicated, its ramifications and those of the liver being pro- 
longed into the papilla, which are said to be branches of the 
respiratory organ. The tongue is armed, but the number and 
arrangement. of the lingual teeth is exceedingly variable, even 
in the same family ; usually the dental membrane is broad and 
_ Short, with many similar teeth in each row. 
__ The alimentary canal terminates more in the rear of the-body 
than.in the other univalve shellfish. The gills are behind the 
-heart, and the auricle behind the ventricle; conditions which 
characterize the embryonic state of the mollusca generally. 
Comparatively little is known of the geographical distribution 
of these animals ; they have been found wherever the requisite 
Search has been made, and are probably much more numerous 
