Whorl (mollusc)

A whorl is a single, complete 360° turn in the spiral growth of a mollusc shell. A mollusc shell is basically a long tube, usually coiled in a spiral for strength and compactness. The number of whorls in the shell depend on various factors in the geometric growth, as described in work of D’Arcy Wentworth Thompson ("On Growth and Form") and David Raup.

The number of whorls differs in various groups, with the extant family Turritellidae and the Mesozoic family Nerineidae having very high spired shells with a large number of whorls and hence a relatively small aperture.

*Apical whorls—those whorls near the apex or tip of the shell.
*Body whorl—The most recently formed whorl of a spiral shell.
*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.