 However, the common name "snail" is also used for most of the members of the molluscan class Gastropoda that have a coiled shell that is large enough for the animal to retract completely into.When the word "snail" is used in this most general sense, it includes not just land snails but also numerous species of sea snails and freshwater snails.Gastropods that naturally lack a shell, or have only an internal shell, are mostly called "slugs", and land snails that have only a very small shell (that they cannot retract into) are often called "semi-slugs".