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Rhopalia in the Phylum Cnidaria
Rhopalia (singular: rhopalium) are small sensory structures of Scyphozoa (typical jellyfish) and Cubozoa (box jellies). The structures typically occur in multiples of four, are bell shaped and face outward from the bell of the jelly's body, connected ectodermally to the periphery of other Rhopalia by a stalk-like projections that reach laterally about the circumference of the mantle. These connections form the junctions of the cnidarian central nervous system. Rhopalia consist of specialized structures to sense light (ocelli) which line the structure and regions to perceive gravity (statoliths) at their terminal tip. Rhopalia are unique to the medusoid forms of Cnidarians and are best studied in Scyphozoa, within the genus Aurelia, which exhibits the most typical arrangement and structure of rhopalia in marginal indentations around the skirt of the bell which are flanked by rhopalial lappets. However, studies about rhopalia in Cubozoa, which exhibit unique manifestations of rhopalial structure, and thus boast the most complex visual sensory mechanisms within the phylum, are coming to reveal more detail about the mechanisms and origins of these structures. Input from rhopalia are not only crucial for Cnidaria to sense light and spatial orientation, but help to gauge and control the pace of swimming and muscle contraction.

Phylogenetic and taxonomic origins
Rhopalia are

Rhopalial Ocelli and Regeneration
Cubozoa rhoplaial structures and ocelli are the most complex of Cnidarians, and resemble the image-forming eyes of squids, octopuses and vertebrates.