In zoology, an annulus is an external circular ring. Annuli are commonly found in segmented animals such as earthworms and leeches. The bodies of these annelids are externally marked by annuli that are arranged in series with each other.

An annulus may also be an indication of growth in certain species, similar to dendrochronology

Dendrochronology

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

. For example, in fish, it is a series of concentric rings (or annuli) formed in the scales of bony fish., whereas in bivalves, they are the concentric rings found on their shells.