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The Collaborative International Dictionary
Rhopalium

Rhopalium \Rho*pa"li*um\, n.; pl. Rhopalia. [NL.] (Zo["o]l.) One of the marginal sensory bodies of medus[ae] belonging to the Discophora.

Wiktionary
rhopalium

n. (context zoology English) One of the marginal sensory bodies of medusae belonging to the Discophora.

Wikipedia
Rhopalium

Rhopalia (singular: rhopalium) are small sensory structures of Scyphozoa (typical jellyfish) and Cubozoa (box jellies). In Aurelia they lie in marginal indententions around the bell and are flanked by rhopalial lappets. Specialized structures to sense light ( ocelli) and perceive gravity ( statoliths) are usually present. They also control the pace of swimming-muscle contraction.

Cubozoan ocelli are the most complex of Cnidarians, and resemble the image-forming eyes of squids, octopuses and vertebrates.