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Awhorl is a single, complete 360° revolution or turn in thespiral orwhorled growth of amollusc shell. A spiral configuration is found in the shells of numerousgastropods, and in shelledcephalopods includingNautilus,Spirula and the large extinct subclass of cephalopods known as theammonites.
A spiral shell can be visualized as consisting of a longconical tube, the growth of which is coiled into an overallhelical orplanispiral shape, for reasons of strength and compactness.
The number of whorls in an adult shell of a particular species depends on mathematical factors in geometric growth, as described inD'Arcy Wentworth Thompson's classic 1917 bookOn Growth and Form, and byDavid Raup. The main factor is how rapidly the conical tube expands (or flares) 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 the conical diameter expands, then the adult shell has few whorls.
The number of whorls present in an adult shell differs bytaxa. Theextant marine gastropod familiesTurritellidae andTerebridae, and the extinctMesozoic familyNerineidae, have highspired shells with many whorls and a relatively smallaperture.
The shells of a few genera of gastropods, and of the cephalopod genusSpirula, have whorls that are disconnected.
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 methods have been proposed.[2][3] increasing whorl counts by one-fourth.[1]
This article incorporates CC-BY-3.0 text from the reference.[1]
Cephalopod anatomy | ||||||||||||||
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| Shell |
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| Mantle & funnel |
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| Head & limbs |
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| General | ||||||||||||||
Developmental stages:Spawn →Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult •Egg fossils •Protoconch (embryonic shell) | ||||||||||||||