In zoology, an annulus is an external circular ring. Annuli are commonly found in segmented animals such as

earthworms An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...

and

leeches Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodi ...

. The bodies of these

annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolo ...

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. For example, in fish, it is a series of concentric rings (or annuli) formed in the scales of bony fish. In bivalve mollusks, annuli are concentric growth rings in their shells.

References

Fish anatomy Incremental dating Animal anatomy {{bony-fish-stub