OPISTHOBRANCHIATA. 351 



of its elongated trunk, twisting its body about, and swimming 

 in eveiy direction bj^ 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 a 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. 



OxYGYRUS, Benson. 



Syn. — Ladas, Cantraine. Helicophlegma, d'Orb. 



Diatr. — 4 sp. Atlantic, Mediterranean. 0. Keraudrenii^ 

 Rang (Ixxxvi, 6, 1 )• 



Shell milk}^, 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. 



Branchiae exposed, or protected by a fold of the mantle and 

 situated at the posterior centre of the back, and never in a 

 cervical cavit}'. 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 the animal. 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 papillas, 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 shell-fish. 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 



