A varix () is an anatomical feature of the

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

of certain

sea snail 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 ...

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

s. Gastropods whose shells have varices are primarily families and species within the

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

groups

Littorinimorpha Littorinimorpha is a large order of snails, gastropods, consisting primarily of sea snails ( marine species), but also including some freshwater snails ( aquatic species) and land snails (terrestrial species).Bouchet P. & Rocroi J.-P. (Ed.); Fr� ...

and

Neogastropoda Neogastropoda is an order of sea snails, both freshwater and marine gastropod molluscs. Description The available fossil record of Neogastropoda is relatively complete, and supports a widely accepted evolutionary scenario of an Early Creta ...

. The varix is a thickened axial ridge, a subcylindrical protrusion, in the

which exists in some families of marine gastropods. It is an important shell character in generic classification. A varix is located at intervals around the

whorl A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral ...

, and is formed by considerable thickening of the outer lip during a resting stage in the growth of the shell. In other words, in gastropods whose shells have varices, the shells are characterised by

episodic growth Episodic may refer to: * The nature of television series that are divided into short programs known as episodes * Episodic memory, types of memory that result from specific incidents in a lifetime * In Geology, episodic refers to events that occur ...

- the shell grows in spurts, and during the resting phase the varix forms. In many gastropod whose shells have varices, for example the

Cassinae The Cassidae are a taxonomic family of medium-sized, large, and sometimes very large sea snails commonly called helmet snails or bonnet snails. These are marine gastropod mollusks in the superfamily Tonnoidea and the clade Littorinimorpha.Go ...

, the varix is essentially merely a thickening and swelling of the shell at that point. But in some genera within the family

Muricidae Muricidae is a large and varied taxonomic family of small to large predatory sea snails, marine gastropod mollusks, commonly known as murex snails or rock snails. With about 1,600 living species, the Muricidae represent almost 10% of the Neogas ...

, such as ''

Chicoreus ''Chicoreus'' is a genus of medium- to large-sized predatory sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, the murexes or rock snails.Bouchet, P.; Houart, R. (2010). Chicoreus Montfort, 1810. In: Bouche ...

'', ''

Hexaplex ''Hexaplex'' is a genus of medium-sized to large sea snails, marine gastropod mollusks in the subfamily muracinae of the family Muricidae, the murex shells or rock snails.Houart, R.; Gofas, S. (2010). Hexaplex Perry, 1810. In: Bouchet, P.; Gofas ...

'', ''

Pteropurpura ''Pteropurpura'' is a genus of sea snails, marine gastropod mollusks in the subfamily Ocenebrinae of the family Muricidae, the murex snails or rock snails. Species Species within the genus ''Pteropurpura'' include: * '' Pteropurpura bendersky ...

'' and ''

Pterynotus ''Pterynotus'' is a genus of sea snails, marine gastropod mollusks in the subfamily Muricinae of the family Muricidae Muricidae is a large and varied taxonomic family of small to large predatory sea snails, marine gastropod mollusks, co ...

'', and also within the genus ''Biplex'', of the family

Ranellidae The Ranellidae, common name the triton shells or tritons, are a taxonomic family of small to very large predatory sea snails, marine gastropod mollusks in the order Littorinimorpha. Subfamilies According to the taxonomy of the Gastropoda by B ...

, the varices are characterised by elaborate ruffles, frills or lamellae. Some other genera, for example ''

Murex ''Murex'' is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly called "murexes" or "rock snails".Houart, R.; Gofas, S. (2010). Murex Linnaeus, 1758 ...

'', are armed with protective spines which may be straight or curved, and which are formed by the varices closing or curling around their axis.

External links

Rare Shells In Marco Island
{{Gastropod anatomy Mollusc shells Gastropod anatomy