Rhopalia (singular: rhopalium) are small sensory structures of scyphozoan and cubozoan jellyfish; 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 squid, octopuses, and vertebrates.… (More on Rhopalia)