Rhopalia (singular: rhopalium) are small sensory structures of certain Scyphozoan (true jellyfish) and Cubozoan (box jellyfish) species. Description The structures typically occur in multiples of four, are bell shaped and face outward from invaginations around the bell of the jelly's mantle. They are each connected ectodermally to the periphery of other rhopalia by a stalk-like projections which join extremities in a skirt-like shape. These connections form the junctions of the cnidarian 'central nervous system', which synapse within the rhopalial centers. Rhopalia vary in form, size and number, but ubiquitously 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 the organ in Cubozoa, which exhibit the most complex rhopalial structure and visual sensory mechanisms, have been few, but are coming to reveal more detail about the mechanisms and origins of these structures. Sensory 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. In continuing to increase the breadth and depth of scientific knowledge about the origins of rhopalia and how they manifest during the process of metamorphosis is crucial to understanding sensory evolution in all metazoa, and could contribute to the progression of knowledge about the beginnings of nervous system evolution. Morphology and function Studies observing the genus Aurelia indicate that development of their rhopalial nervous system occurs in an organized, staged manner beginning in the strobila phase and results in bilaterally symmetrical organization of the organs and the stalk-like projections that connect them inwardly. Rhopalia uniformly manifest in multiples of four, regardless of their variations in morphology among the species that possess them. Each rhopalium is capped by a lithocyst, or statocyst, at the terminal end, which is known to sense gravitational change and orientation. On the oral side of the rhopalium near the lithocyst is a mass of sensory cells called the "pigment-cup ocellus", while on the aboral side, there is a “pigment-spot ocellus” just proximal to the lithocyst, formed by a patch of pigment cells. In tandem with a