4 CLASS III. GASTEROPODA. ORDER I. CIRRHOBRANCHIATA. 
univalve, and mostly convoluted into a regular spire, the aperture being 
in most instances either partially or entirely closed by an operculum ; 
sometimes the shell is simple, entirely covering the animal in the shape 
of an inverted basin, and, though rarely, supported upon a testaceous 
basal cup or plate ; sometimes, however, it is small and simple, concealed 
within the animal merely for the protection of the branchiz. 
The Gasteropoda are very numerous, whether terrestrial, fluviatile, or 
marine, and are divided into seven orders, as follows : 
CIRRHOBRANCHIATA. NUCLEOBRANCHIATA. 
CycLOBRANCHIATA. PULMOBRANCHIATA. 
CERVICOBRANCHIATA. PECTINIBRANCHIATA. 
PLEUROBRANCHIATA. 
Order I. GASTEROPODA CIRRHOBRANCHIATA. 
Branchie cirrate, cristas duas equales, pediculo bilobato supra collum 
affixas, symmetricé formantes ; cristarum filamenta mollia, flexilia, 
claveeformia. 
In this order, recently established by De Blainville, the branchiz 
are cirrous, symmetrically divided into two equal tufts of soft flexible 
club-shaped filaments, attached to a bilobed pedicle on the neck. They 
are of very peculiar structure, and appear to serve a double purpose,— 
that of conveying the vital oxygen to the blood, and that of drawing a 
current of water by their tentacular activity. The following genus is the 
only one at present known : 
DENTALIUM. 
