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