BORING OF SHELLS. 135 



slight continuous movement of the tongue, to force the 

 food down into the gullet, whence it is carried to the 

 stomach. 



In the pectinibranchiate group, a still more com- 

 plex and surprising structure of the mouth exists, in 

 which the tongue takes the part of a boring instru- 

 ment, by which the animal is capable of piercing the 

 shells of other mollusks, so as to enable it to suck 

 out their contents. This structure is seen in the whelk, 

 (^Buccinum undatum?) ' The mouth may be described as 

 a flexible proboscis, moveable in various directions by 

 the action of muscles, and capable of being completely 

 retracted. This is effected, as we have seen in the horns 

 of the snail, by inverting the proboscis into itself: but 

 the action now referred to, is not so complete as in the 

 horn of the snail, half of the proboscis only being drawn 

 into the basal half, which thus forms a sheath ; this, how- 

 ever, can be retracted into the head of the animal bv 

 muscular fibres. To the end of this tubular proboscis, 

 enclosed within it, is carried the tongue, which is carti- 

 laginous, together with the gullet, or oesophagus, opening 

 near the tip of the former. The tongue is supported by 

 two cartilaginous slips, the extremities of which are 

 formed into a sort of lip ; these lips are capable of being 

 opened and closed, and the cartilages can be moved upon 



