vertebra
From ZooTerms (Dictionary of Invertebrate Zoology)
vertebra (noun; plural vertebrae; Latin vertebra, turning joint): (Echinodermata) 1. In Asteroidea, the fused pair of opposite ambulacrals, articulating with adjacent vertebrae by ball-and-socket joints. 2. In Ophiuroidea, enclosed by a ventral arm plate and skin or a dorsal arm plate.
