Whorl

Definition

A single complete turn or volution in a spiral structure, such as the coiled whorls of a gastropod shell or the spiral arrangement of structures in development. In arthropod biology, the term describes concentric or helical patterns of segmentation, trichome arrangement, or coiled appendages.

Etymology

From Middle English whorl, wharle, possibly related to whirl; originally describing a flywheel or rotating object before application to spiral biological forms.

Example

The embryonic whorls of a developing gastropod shell expand logarithmically, with each whorl adding to the aperture diameter; in some terrestrial snails, the number of whorls serves as a diagnostic character for species identification.

Synonyms

• volution
• spire
• coil

Related Terms

• spiral
• coiling
• torsion
• apex
• aperture
• Suture
• protoconch
• teleoconch

Usage Notes

In malacology, whorls are counted from the apex downward, with the body whorl being the largest and most recent. The term is less common in insect morphology but applies to spiral tracheal coils, coiled proboscides in Lepidoptera, and helical patterns of scale arrangement. Distinguish from 'whorled' as an adjective describing phyllotaxy or radial arrangement (e.g., leaves in a circle around a stem), which derives from a different etymological root.