Whorl (mollusc)

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.

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

Counting gastropod whorls. In this example a shell with 3¾ whorls is shown, with the fourth whorl expanding more rapidly than whorls 1–3. [ 1 ]
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