From ZooTerms (Dictionary of Invertebrate Zoology)
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callus (noun; plural calluses, calli; Latin callus, hard skin): 1. An unusually hardened or thickened area; a rounded swelling.

2. (Arthropoda: Insecta) In brachycerous Diptera, a knoblike swelling on the cuticle.

3. (Mollusca: Gastropoda) A shelly substance (inductura) on the parietal region or extending from the inner lip over base or into the umbilicus of the shell.