Whorl (mollusc)

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Counting gastropod whorls. In this example a shell with 3 3/4 whorls is shown, with the fourth whorl expanding more rapidly than whorls 1-3. Whorls.jpg
Counting gastropod whorls. In this example a shell with 3¾ whorls is shown, with the fourth whorl expanding more rapidly than whorls 1-3.
View of the spire side of the planispiral shell of the freshwater snail Anisus septemgyratus. This shell has seven and a half whorls Anisus septegyrus1pl.jpg
View of the spire side of the planispiral shell of the freshwater snail Anisus septemgyratus . This shell has seven and a half whorls
A fossil shell of the marine gastropod Turritella communis. This shell has nine whorls Turritella communis fossiel.jpg
A fossil shell of the marine gastropod Turritella communis . This shell has nine whorls

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in numerous gastropods, but it is also found in shelled cephalopods including Nautilus , Spirula and the large extinct subclass of cephalopods known as the ammonites.

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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 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 and Terebridae, and the extinct Mesozoic family Nerineidae, have very high spired shells with a large number of whorls, and a relatively small aperture.

The shells of a few genera of gastropods, and of the cephalopod genus Spirula , have whorls that are disconnected.

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. [1] 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. [1] From that point all whorls are counted towards the margin of the shell, estimating the ultimate whorl with an accuracy of a quarter whorl. [1]

Other authors (Ehrmann 1933; [2] Richter & Seapy 1999 [3] ) applied a slightly different measuring method, resulting in whorl numbers being a quarter higher. [1]

Terminology

Related Research Articles

In anatomy, an apex is part of the shell of a mollusk. The apex is the pointed tip of the shell of a gastropod, scaphopod, or cephalopod.

Spire (mollusc)

A spire is a part of the coiled shell of molluscs. The spire consists of all of the whorls except for the body whorl. Each spire whorl represents a rotation of 360°. A spire is part of the shell of a snail, a gastropod mollusc, a gastropod shell, and also the whorls of the shell in ammonites, which are fossil shelled cephalopods.

Protoconch

A protoconch 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 "nucleus". The protoconch may sometimes consist of several whorls, but when this is the case, the whorls show no growth lines.

Gastropod shell Part of the body of a gastropod or snail

The gastropod shell is part of the body of a gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group. An excellent source for terminology of the gastropod shell is "How to Know the Eastern Land Snails" by John B. Burch now freely available at the Hathi Trust Digital Library.

<i>Coelotrochus oppressus</i>

Coelotrochus oppressus, common name the shouldered top shell, is a species of very small sea snail, a marine gastropod mollusc in the family Trochidae, the top snails or top shells.

<i>Diloma coracinum</i>

Diloma coracinum is a species of small sea snail, a marine gastropod mollusc in the family Trochidae, the top snails. The Māori name is māihi.

<i>Trochus camelophorus</i> Species of sea snail

Trochus camelophorus is a species of small sea snail, a marine gastropod mollusc in the family Trochidae, the top snails.

<i>Cantharidus capillaceus</i>

Cantharidus capillaceus is a species of sea snail, a marine gastropod mollusc in the family Trochidae, the top snails.

<i>Cantharidus opalus</i>

Cantharidus opalus, common name the opal top shell or in the Maori language matangongore, is a species of large sea snail, a marine gastropod mollusc in the family Trochidae, the top snails.

<i>Pseudexomilus costicapitata</i>

Pseudexomilus costicapitata is a species of sea snail, a marine gastropod mollusk in the family Horaiclavidae, the turrids.

Fenimorea marmarina is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

Kermia bifasciata is a species of sea snail, a marine gastropod mollusk in the family Raphitomidae.

Pleurotomella pudens is a species of sea snail, a marine gastropod mollusk in the family Raphitomidae.

<i>Spirotropis lithocolleta</i>

Spirotropis lithocolleta is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

<i>Atlanta echinogyra</i>

Atlanta echinogyra is a species of sea snail, a holoplanktonic marine gastropod mollusk in the family Atlantidae.

<i>Oxygyrus</i>

Oxygyrus keraudrenii is a species of sea snail, a holoplanktonic marine gastropod mollusk in the family Atlantidae.

<i>Protatlanta souleyeti</i>

Protatlanta souleyeti is a species of sea snail, a holoplanktonic marine gastropod mollusk in the family Atlantidae.

<i>Protatlanta rotundata</i>

Protatlanta rotundata is an extinct species of sea snail, a holoplanktonic marine gastropod mollusk in the family Atlantidae.

<i>Borsonia mitromorphoides</i>

Borsonia mitromorphoides is an extinct species of sea snail, a marine gastropod mollusk in the family Borsoniidae.

<i>Raphitoma diozodes</i>

Raphitoma diozodes is an extinct species of sea snail, a marine gastropod mollusc in the family Raphitomidae.

References

This article incorporates CC-BY-3.0 text from the reference. [1]

  1. 1 2 3 4 5 6 Janssen, Ari (2007). "Holoplanktonic Mollusca (Gastropoda: Pterotracheoidea, Janthinoidea, Thecosomata and Gymnosomata) from the Pliocene of Pangasinan (Luzon, Philippines)". Scripta Geologica. 135: 29–178. ISSN   0375-7587.
  2. Ehrmann P. (1933). "Mollusca". In: Brohmer P., Ehrmann P. & Ulmer G. (eds.) Die Tierwelt mittel-europas2: 1-264. Quelle & Meyer, Leipzig.
  3. Richter G. & Seapy R. R. (1999). "Heteropoda". In: Boltovskoy D. (ed.). South Atlantic zooplankton, 1. Backhuys, Leiden: 621-647.

Further reading