From ZooTerms (Dictionary of Invertebrate Zoology)
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radius (noun; plural radii; Latin radius, ray): 1. In radially symmetrical animals, the primary axis of symmetry.

2. (Echinodermata: Crinoidea) One of 5 reference planes passing through the polar or central axis and median line.

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.