A whorl is a single, complete 360° revolution or turn in the

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:mollusc shell. A spiral configuration of the shell is found in numerous

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 ...

s, but it is also found in shelled

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s including '' Nautilus'', '' Spirula'' and the large extinct subclass of cephalopods known as the

ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

. A spiral shell can be visualized as consisting of a long

conical A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines c ...

tube, the growth of which is coiled into an overall helical or planispiral shape, for reasons of both strength and compactness. The number of whorls which exist in an adult shell of a particular species depends on mathematical factors in the geometric growth, as described in D'Arcy Wentworth Thompson's classic 1917 book '' On Growth and Form'', and by David Raup. The main factor is how rapidly the conical tube expands (or flares-out) over time. When the rate of expansion is low, such that each subsequent whorl is not that much wider than the previous one, then the adult shell has numerous whorls. When the mathematical factors governing the pattern of growth are such that there is a very rapid expansion of the conical shape, of the shell tube, then the adult shell has very few whorls. The number of whorls present in an adult shell differs greatly in various taxa. The extant marine gastropod families

Turritellidae 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. These snails are filter feeders. This method of feeding is somewha ...

and

Terebridae The Terebridae, commonly referred to as auger shells or auger snails, is a group or family of small to large predatory marine gastropods in the superfamily Conoidea.Gofas, S. (2012). Terebridae. World Register of Marine Specie2012-10-12 These ...

, and the extinct

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

family Nerineidae, have very high spired shells with a large number of whorls, and a relatively small

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

. The shells of a few genera of gastropods, and of the cephalopod genus '' Spirula'', have whorls that are disconnected. File:Haliotis asinina.jpg, The shell of the marine gastropod ''

Haliotis asinina ''Haliotis asinina'', common name the ass's-ear abalone, is a fairly large species of sea snail, a tropical gastropod mollusk in the family Haliotidae, the abalones, also known as ormers or paua. Both the common name and the scientific name ...

'' has fewer than two whorls File:Spirula fg1.jpg, The shell of '' Spirula spirula'' has disconnected whorls

Counting the number of whorls

To count exactly the number of whorls in gastropods a straight line is drawn to separate the semi-circular nucleus ( protoconch (1 - see image) from the younger part of the shell. An arrow placed at a 90° angle on this line, following the course of the whorl, reaches the end of the first whorl where it is parallel to its starting position. From that point all whorls are counted towards the margin of the shell, estimating the ultimate whorl with an accuracy of a quarter whorl. Other authors (Ehrmann 1933; Richter & Seapy 1999Richter G. & Seapy R. R. (1999). "Heteropoda". In: Boltovskoy D. (ed.). ''South Atlantic zooplankton, 1''. Backhuys, Leiden: 621-647.) applied a slightly different measuring method, resulting in whorl numbers being a quarter higher.

Terminology

*Apical whorls—the whorls near the apex or tip of the shell of gastropods * Body whorl—The most recently formed whorl of a spiral shell, in which most of the body of the animal is found *Nuclear whorl(s)—small, generally smooth whorls formed within the egg, and constituting the apex of the shell * Protoconch—a larval shell of a mollusc; also refers to protoconch whorls of an adult shell *Teleoconch—all the whorls of a shell after the protoconch whorls *Nepionic whorls : the whorls immediately following the embryonic whorls.

References

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External links

{{Gastropod anatomy Mollusc shells

Counting the number of whorls

Terminology

References

Further reading

External links