From ZooTerms (Dictionary of Invertebrate Zoology)
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lunule (noun; Latin diminutive luna, moon): 1. A crescent-shaped part or marking.

2. (Arthropoda: Crustacea) In Copepoda, small, sucker-like adhesion disc on the anterior margin.

3. (Echinodermata: Echinoidea) One of several perforations in the test of some sand dollars that may serve for passage of sand and water while burrowing.

4. (Mollusca: Bivalvia) Cordate shaped depression anterior to the beaks.