114 STEUCTURE 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. 



vb ab b ov 



Fig. 55. ANATOMY OF TURBO PICA. 



108. The accompanying figure of the interior of a Turbo 

 show the very large size of the digestive apparatus, and 

 of the other organs of nutrition. The muscular disk or foot 

 is seen at/>; and this carries the operculum o, which serves 

 to cloge 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 



