114 STRUCTURE OF MOLLUSOA, 
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. 
™ vb ab b ov 
| 
Fig. 55.—Anatomy or Turxso 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 0, which serves 
to cloge the mouth of the shell when the body of the animal 
is drawn within it. At¢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 
: 
