MOLLUSCA. 447 



of progressive motion. It is a sucker rather than a foot, 

 and enables the animal to adhere to objects when at rest, 

 and to crawl from one place to another by a succession of 

 adhesions, not unlike the leech. It is also used as a fin in 

 swimming. 



By the union of the cloak and foot laterally and poste- 

 riorly, a sac is formed, which is open in front for the pro- 

 trusion of the neck and head. The line of junction be- 

 tween the cloak and foot is marked, in general, by peculia- 

 rities in the condition of the margins of both. 



The neck is usually divided from the cloak by a cottar, 

 or thickened margin belonging to the cloak, or rather to 

 the shield, while in other cases it is continuous. Under- 

 neath, the neck is frequently attached to the foot. 



The head supports the tentacula and eyes, is free dor- 

 sally, but frequently intimately connected with the foot on 

 its ventral side. The portion between the tentacula and 

 the mouth is termed the snout, (le mufle of the French, 

 and its margin le chaperon). The mouth exhibits various 

 modifications of fleshy lips and corneous jaws. The inside 

 of the cheeks are covered in some species with reflected 

 teeth to aid deglutition. The tongue can scarcely be de- 

 tected in some of the genera ; while, in others, it is a simple 

 tubercle, or a strap-shaped, spiral organ, armed with trans- 

 verse rows of teeth. This spiral tongue, where it is fixed 

 to the base of the mouth, is broadest, and there also the 

 spinous processes are strongest. The spiral part is narrow- 

 est and softest, and folded up behind the pharynx. M, 

 CUVIEE conjectures, and apparently with plausibility, that 

 the spiral portion comes forward into the mouth to act as a 

 tongue, in proportion as the anterior part is worn by use 

 and absorbed. (See his Memoir e sur la Vivipare cTeau 

 douce, p. 12). 



