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A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. 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 are often called semi-slugs.
Gastropods, commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda.
Nudibranchs belong to the order Nudibranchia, a group of soft-bodied marine gastropod molluscs that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have been given colourful nicknames to match, such as "clown", "marigold", "splendid", "dancer", "dragon", and "sea rabbit". Currently, about 3,000 valid species of nudibranchs are known.
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 gastropods, i.e. they are sea snails that, over evolutionary time, have either entirely lost their shells or have seemingly lost their shells due to having a significantly reduced or internal shell. The name "sea slug" is often applied to nudibranchs and a paraphyletic set of other marine gastropods without apparent shells.
Opisthobranchs is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.
Sacoglossa are a superorder of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia known as sacoglossans. There are 284 valid species recognized within this superorder. Sacoglossans live by ingesting the cellular contents of algae, hence they are sometimes called "sap-sucking sea slugs". Some sacoglossans simply digest the fluid which they suck from the algae, but in some other species, the slugs sequester and use within their own tissues living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty, for the "stolen" plastids. This earns them the title of the "solar-powered sea slugs", and makes them unique among metazoan organisms, for otherwise kleptoplasty is known only among single-celled protists.
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.
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.
Doridoidea, commonly known as dorid nudibranchs, are a taxonomic superfamily of medium to large, shell-less sea slugs, marine gastropod mollusks in the clade Doridacea, included in the clade Nudibranchia.
Polyceroidea is a taxonomic superfamily of sea slugs, specifically dorid nudibranchs, marine gastropod mollusks in the clade Doridacea.
Lower Heterobranchia, also known as the Allogastropoda, is a group of rather specialized, highly evolved sea slugs and sea snails, within the subclass Heterobranchia.
Juliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group.
Nudipleura are a clade of sea snails and sea slugs, marine gastropod mollusks within the large clade Heterobranchia.
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.
The Dexiarchia are a suborder of sea slugs, shell-less marine gastropod molluscs in the order Nudibranchia. This classification is based on the study by Schrödl et al., published in 2001, who recognized within this clade two clades Pseudoeuctenidiacea and Cladobranchia.
The Cladobranchia are a taxonomic clade of nudibranchs, sea slugs, marine gastropod molluscs in the clade Dexiarchia.
Phyllidioidea is a taxonomic superfamily of colorful sea slugs, dorid nudibranchs, marine gastropod mollusks.
The Plakobranchacea are a clade of sea slugs, marine gastropod molluscs in the clade Sacoglossa.
Okadaiidae is a family of sea slugs, dorid nudibranchs, marine gastropod mollusks in the superfamily Polyceroidea. This family is within the clade Euctenidiacea.
Terrestrial molluscs or land molluscs (mollusks) are an ecological group that includes all molluscs that live on land in contrast to freshwater and marine molluscs. They probably first occurred in the Carboniferous, arising from freshwater ones.
References
- ↑ Bouchet, P. & Rocroi, J.-P. (2005). "Classification and Nomenclator of Gastropod Families". Malacologia. 47 (1–2).
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