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 further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

or whorled growth of a

mollusc shell The mollusc (or mollusk) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled ...

. A spiral configuration of the shell is found in numerous

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

s, but it is also found in shelled

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

s including ''

Nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

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

ammonites Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

. A spiral shell can be visualized as consisting of a long conical tube, the growth of which is coiled into an overall helical or

planispiral Planispiral is a condition in which a tubicolous shell is coiled in a single horizontal plane and the diameter increases away from the axis of coiling. This term is usually used for describing gastropod shells. Many cephalopod A cephalopod is ...

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 Sir D'Arcy Wentworth Thompson CB FRS FRSE (2 May 1860 – 21 June 1948) was a Scottish biologist, mathematician and classics scholar. He was a pioneer of mathematical and theoretical biology, travelled on expeditions to the Bering Strait ...

's classic 1917 book ''

On Growth and Form ''On Growth and Form'' is a book by the Scottish mathematical biology, mathematical biologist D'Arcy Wentworth Thompson (1860–1948). The book is long – 793 pages in the first edition of 1917, 1116 pages in the second edition of 1942. The ...

'', and by

David Raup David M. Raup (April 24, 1933 – July 9, 2015) was a University of Chicago paleontologist. Raup studied the fossil record and the diversity of life on Earth. Raup contributed to the knowledge of extinction events along with his colleague Jack Se ...

. 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 In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

. The

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

marine gastropod families

Turritellidae Turritellidae, with the common name "tower shells" or "tower snails", is a Taxonomy (biology), taxonomic family (biology), family of small- to medium-sized sea snails, marine gastropod molluscs in the Sorbeoconcha clade. They are filter feeders; ...

and

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

, and the extinct

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

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

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

. 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 abalone, also known as ormers or pāua. Both the common name and the scientific name ar ...

'' 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 A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

(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 The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics) A-Bomb Abomination Absorbing Man Abraxas Abyss Abyss is the name of two characters appearing in Ameri ...

or tip of the shell of gastropods *

Body whorl The body whorl is part of the morphology (biology), morphology of the gastropod shell, shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk ...

—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 protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

—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