Snails are members of a class of mollusks known as Gastropoda. The word is derived from the latin words for stomach and foot. So, these are "stomach-footed" animals, and I can't think of another species that typifies this description as much as the moon snail does.
Nearly all marine snails move using a muscular foot that oozes over the ocean bottom, lubricated by a continuously laid down layer of slime. In the moon snail's case, this foot can be expanded to a ridiculous size by engorging it with water. The foot expands to the point where it actually engulfs the entire shell.