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 zeck 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 muffle of the French, and 
its margin de chaperon). ‘The mouth exhibits various mo- 
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