[Cubo-/Scyphomedusae] In both cubomedusae and scyphomedusae, a rhopalium or (marginal) sense organ is a small, roughly cylindrical, organ typically situated in a cleft in the margin, or in a niche or pit on the side of the bell. Most support light receptors and posture- and movement-detecting organs. They occur on or near the margin of the bell in scyphomedusae, and in pits in the exumbrellar surface in cubomedusae. The radii in which they occur are termed the rhopalar radii, those between being the inter-rhopalar radii. Rhopalar stomach-pouch: one in a rhopalar radius, as distinct from a tentacular stomach-pouch, in a tentacular radius.

Alternative forms for Rhopalium : inter-rhopalar, rhopalar, rhopalar niche.