thumb|right|200px|Moon jellyfish ([[Aurelia aurita), with rhopalia visible in indentations of the bell's rim]]
Rhopalia (; ) are small sensory organs 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.]]
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. In its totality, the rhopalial nervous system is involved in deduction of the sensation of light (the complexity of which approaches visual processing in some species) and in spatial-behavioral control; because the sensory cells of both sight and physical touch are located within such proximity, studies have explored how related and potentially integral these two mechanisms are to one another in terms of cnidarian behavior. It has thus been observed that the rate of muscle contraction and swimming speed increases when adult Aurelia medusae are exposed to higher levels of light. Additionally that this positive phototactic behavior is absent when the same conditions are presented to jellies whose pigment-cup ocelli have removed (but not the spot-ocellus), insinuating that the pigment-cup ocellus on the oral side is primarily photosensory yet intrinsically related to normal functional behavior
Formation during metamorphosis
Rhopalia are exclusive to the medusoid form of the cnidarian life cycle; young jellies within the classes Schyphozoa and Cubozoa develop through their planula larval and polyp stages lacking these structures. This indicates not only that jellies must employ different sensory mechanisms than those offered by rhopalia before entering the medusa stage, but that the formation of these complex structures takes place completely within the stages of adult strobilation and not during gastrulation. In most schyphozoa and cubozoa, sessile polyps undergo a secondary round of metamorphosis in order to become free-swimming, sexually mature medusae, generally entailing a process known as strobilation. Cubozoan rhopalia also occur in multiples of four, where at least one rhopalia is embedded within the skirt of each side of the box jelly. Each rhopalial organ is lined with six total eyes of four varying morphological types. On each, there are two lens eyes, the upper and lower eye, accompanied by two different pairs of simpler pigment-pit eyes, summing to a total of 24 eyes per organism.