MOLLUSCA. 145 



The cloak is either continuous, and usually more or less 

 arched, for the reception of the viscera underneath, or it is 

 interrupted by a projecting bag, in which are contained the 

 principal digestive and reproductive organs. This project- 

 ing bag is tapering and spiral, and always protected exter- 

 nally by a shell. When the cloak is continuous, the surface 

 is variously marked, and frequently exhibits a particular 

 portion more elevated than the rest, in some cases conceal- 

 ing a testaceous plate, which has been termed the shield. 



The foot situate on the ventral aspect, and in opposition 

 to the cloak, exhibits a flat, soft surface, consisting of inter- 

 laced muscular fibres. Its central surface serves as a sup- 

 port to the viscera, while externally it constitutes the organ 

 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 posteri- 

 orly, a sac is formed, which is open in front for the protrusion 

 of the neck and head. The line of junction between the 

 cloak and foot is marked, in general, by peculiarities in the 

 condition of the margins of both. 



The neck is usually divided from the cloak by a collar, or 

 thickened margin belonging to the cloak, or rather to the 

 shield;, while in other cases it is continuous. Underneath, 

 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 mo- 



