114 
STRUCTURE OF MOLLUSCA. 
from its surface, in those species which form such a pro¬ 
tection. This envelope, which is called the mantle ,, is very 
loosely applied round the parts which it contains; and it 
frequently extends itself into folds or duplicatures, which 
wrap round the gills, and sometimes meet and adhere so as to 
inclose them within a cavity of their own. In the Cuttle¬ 
fish, the water within this cavity is renewed from time to time 
by the muscular movements of its walls; but usually a 
current of fluid is kept up over the surface of the gills, by the 
action of the cilia (§ 45) with which they are covered. 
d 
Fig. 55.— Anatomy of Turbo Pica. 
108. The accompanying figure of the interior of a Turbo 
will show the very large size of the digestive apparatus, and 
of the other organs of nutrition. The muscular disk or foot 
is seen at p; and this carries the operculum o, which serves 
to close the mouth of the shell when the body of the animal 
is drawn within it. At t is shown the proboscis, on either 
side of which are the tentacula or feelers, ta , bearing the eyes 
at y. Just behind the tentacula is seen the large cephalic 
ganglion, sending nerves to the eyes ; and behind this again 
are the salivary glands. The mantle, m, is opened and folded 
back to show the respiratory cavity, in which lie the gills 
