COMB-GILLED MOLLUSKS. 163 
ORDER PECTINIBRANCHIATA. 
(Comb-gilled Mollusks.) 
WE have now arrived at the most numerous 
division of the GASTEROPODA, which comprehends 
nearly the whole of the spiral univalves, and many 
with simply conical shells. Their distinctive cha- 
racter is the possession of gills composed of nu- 
merous leaflets, or fringes, ranged in parallel order, 
like the teeth of a comb, and attached, in one, two, 
or three lines, (according to the genus) to the ceiling 
of the breathing-chamber, a cavity opening by a 
wide orifice between the edge of the mantle and 
the body. All the members of the Order respire 
water, and nearly all are marine. 
A pair of tentacles are always present, accom- 
panied by a pair of eyes, often highly organized, 
carried, sometimes, on spinal footstalks, and some- 
times seated, as it were, on the side, or at the base 
of the tentacles. ‘The mouth takes the form of a 
proboscis more or less lengthened, and: conceals a 
tongue armed with small recurved hooks, which 
wear down the hardest bodies by slow and repeated 
friction. ‘The sexes are always separate. 
The shell is in general turbinated, or twisted 
spirally into a cone more or less regular; the aper- 
ture of which is sometimes entire, sometimes 
notched, sometimes drawn out into a canal. The 
orifice is in general capable of being closed by an 
operculum, a horny or shelly disk, attached to the 
