334 MARINE INVERTEBRATES 
specimen, break away the shell with a hammer, using care not to 
lacerate the soft, fleshy portion within; when this is accomplished, 
wash the animal carefully to remove the slimy exudation. Note 
the “visceral hump,” which is spiral, and which formerly ocecu- 
pied the upper-whorl portion of the shell. The thin skin covering 
it is the mantle, which below is greatly thickened and free, lying 
about the foot like a heavy fleshy flap. 
MANTLE 
The mantle-edge in both of these examples is simple; that is to 
say, it possesses no fringe of tentacles, nor is it supplemented by 
extra processes, characters which mark many genera of maine 
Gasteropoda. In Fulgur and Buccinum the mantle-edge does not 
protrude below the edge of the shell ; but in many genera, especially 
those which possess smooth, glossy shells, like the cowries (Cyprea) 
and the graceful Oliva, the mantle is proportionately very much 
larger. In these two genera, when the animal is extended, as in 
crawling about the sand, the mantle curves upward and incloses a 
large portion of the shell itself. Indeed, in some genera the shell 
is almost entirely concealed by this extension of the mantle (Siga- 
retus, Natica, ete.). 
When the shell is removed, the folding of the mantle which 
constitutes the siphon can be plainly seen. The office of the siphon 
has already been referred to, also the fact that the presence of a 
siphon in the gasteropod mollusk may always be determined by 
merely glancing at the shell alone, for a notch at the base of the 
aperture indicates the place through which the siphon passed. 
In Buccinwm this is merely a notch, but in Fulgur the siphonal 
canal of the shell is much longer. Just why the long siphons of 
some mollusks should be naked and exposed to danger, while 
others are so carefully protected by elongated portions of the 
shell, is a mystery, but nature is full of such contradictions. 
THE GASTEROPOD FOOT 
The foot is long, broad, and flat on the under side, like a 
disk. The variations in the gasteropod foot are almost infinite. 
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