Snail is a common name that is applied most often to land snails, terrestrial pulmonate gastropod molluscs. However, the common name "snail" is also applied to 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 thousands of species of sea snails and freshwater snails. Occasionally a few other molluscs that are not actually gastropods, such as the Monoplacophora, which superficially resemble small limpets, may also informally be referred to as "snails".
Snail-like animals that naturally lack a shell, or have only an internal shell, are usually called slugs, and land snails that have only a very small shell (that they cannot retract into) are often called semislugs.
Sea snail is a common name for snails that normally live in saltwater–marine gastropod molluscs. The taxonomic class Gastropoda also includes snails that live in other habitats such as land snails and freshwater snails. Many species are edible and exploited as food sources by humans.
Sea snails are marine gastropods with shells. Those marine gastropods with no shells, or only internal shells, are variously known by other common names, including sea slug, sea hare, nudibranch, etc. The diversity within sea snails is enormous. Many very different clades of gastropods are either dominated by, or consist exclusively of, sea snails. Because of this great variability, generalization about the feeding, reproduction, habitat, or other traits of sea snails is not possible. Instead, individual clades, families, genera, or species must be assessed.
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