From ZooTerms (Dictionary of Invertebrate Zoology)
primary axis of symmetry.radius (noun; plural radii; Latin radius, ray): 1. In radially symmetrical animals, the
3. (Arthropoda) a. In Crustacea (sessile barnacles), the lateral part of a compartmental plate when marked off from the median triangular area by change in direction of growth lines. b. In Insecta, the third longitudinal wing vein.