From ZooTerms (Dictionary of Invertebrate Zoology)
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annulus (noun; plural annuli, annuluses; Latin annulus, ring): 1. Any ring-like circling of a joint, segment, spot or mark.

2. (Bryozoa: Phylactolaemata) The outer epidermal layer of a statoblast that encircles the protective capsule.

3. (NEMATA) The interstice area between the transverse striae of the body cuticle.