Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are actually gastropods, i.e. they are sea snails that over evolutionary time have either completely lost their shells, or have seemingly lost their shells due to having a greatly reduced or internal shell. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells.
Heterobranchia, the heterobranchs, is a taxonomic clade of snails and slugs, which includes marine, aquatic and terrestrial gastropod mollusks.
Caenogastropoda is a taxonomic clade, a large diverse group which are mostly sea snails and other marine gastropod mollusks, but also includes some freshwater snails and some land snails. The clade is the most diverse and ecologically successful of the gastropods.
Sorbeoconcha is a taxonomic clade of snails, i.e. gastropods, mainly marine species with gills and opercula, within the clade Caenogastropoda.
Littorinimorpha is a large order of snails, gastropods, consisting primarily of sea snails, but also including some freshwater snails and land snails.
Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the absence of a visible shell.
The Systellommatophora is a clade of primitive, air-breathing slugs, according to the taxonomy of the Gastropoda.
The Vermetidae, the worm snails or worm shells, are a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Littorinimorpha. The shells of species in the family Vermetidae are extremely irregular, and do not resemble the average snail shell, hence the common name "worm shells" or "worm snails".
Turritellidae, common name the "tower shells" or "tower snails", is a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Sorbeoconcha.
Amathinidae, is a taxonomic family mostly consisting of small and minute sea snails, marine heterobranch gastropod molluscs or micromolluscs in the superfamily Pyramidelloidea.
Cylichnidae, common name the "chalice bubble snails" or "canoe bubble snails" is a family of sea snails or bubble snails, marine gastropod mollusks in the superfamily Cylichnoidea.
Neritimorpha is a taxonomic grouping, an unranked clade of snails, gastropod mollusks. This grouping includes land snails, sea snails, some deepwater limpets, and also freshwater snails. This clade used to be known as the superorder Neritopsina.
Sigmurethra is a taxonomic category of air-breathing land snails and slugs, terrestrial pulmonate gastropod molluscs. This is an informal group which includes most land snails and slugs.
The Ptenoglossa is an informal taxonomic group of sea snails. This group was considered paraphyletic or polyphyletic by Ponder and Lindberg (1997) in their classification of gastropod molluscs.
Runcinoidea is a taxonomic superfamily or a clade Runcinaecea of sea slugs, marine gastropod mollusks in the order Runcinida
Cylindrobulla is a genus of sea snails or bubble snails, marine gastropod mollusks in the clade Sacoglossa.
Hygrophila is a taxonomic superorder of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks within the clade Panpulmonata.
Rostellariidae is a family of sea snails, marine gastropod molluscs in the clade Littorinimorpha.
After excluding groups not related, the informal group Sigmurethra has become the suborder Helicina, with the following infraorders and a collection of families with no superfamily：
Trochida is an order of small to very large vetigastropod, Recent and extinct sea snails with gills and an operculum.
