GASTEROPODS 331 
the animal draws in water to supply the gills, lying just back 
of the siphon and concealed under the mantle in the cavity 
already spoken of as the mantle cavity. With the exception of 
that portion of the mantle which constitutes the siphon, no part 
of its margin, it will be observed, can be seen, the edge of the 
shell completely hiding it from view. 
In some genera the siphon is exceedingly long—in fact, longer 
than the body of the animal. In these cases the mollusk is 
generally fond of burying 
itself in the sand and main- 
taining communication with 
the world above only by 
means of this long, fleshy 
tube. In some genera the 
siphon is protected by an 
elongation of the shell, as in 
Fasciolaria (Plate LXXVI). ny Crete ee ae 
An extreme example of this is given in the figure on page 343, 
which represents a species of Murex, where the siphon is not only 
protected by the shell, but the shell itself is further protected by 
the double row of sharp spines with which it is beset. 
There are some genera which possess no siphons at all, in all 
of which cases there is no notch in the shell opening, the aperture 
being round, as in Natica and in Jnttorina (pages 368, 371). The 
mollusks of this type make a fold in the forward part of the 
mantle which serves as a siphon. Such a muscular folding in 
the mantle would be technically called a functional siphon. 
On the posterior dorsal side of the foot is attached a horny 
plate, of concentric structure about a central or subcentral 
point or nucleus. This is called the operculum, and serves 
to close the entrance of the shell when the animal is withdrawn, 
the object no doubt being protective. The opercula vary to a 
great extent in the different genera; in some cases they are cal- 
careous, in others cartilaginous. When the aperture of the shell 
is very large it often happens that the operculum does not entirely 
close the opening, as in Fulgur, but in Buccinum it is a close fit. 
The operculum is entirely wanting in the Pulmonata, one of the 
