From ZooTerms (Dictionary of Invertebrate Zoology)
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keel (noun; Anglo-Saxon ceol, ship): 1. A prominent ridge or carina.

2. (Bryozoa) a. In Stenolaemata, a flat median portion of the zooidal wall between sinuses in recumbent part of endozone or as a synonym of carina. b. In Phylactolaemata, a median longitudinal ridge along recumbent tubular colony parts.

3. (Mollusca: Gastropoda) A spiral ridge usually marking a change of slope in the outline of the shell.