Rhopalium
[1] In tandem with a “touch plate” which is situated near the spot-ocellus, tilt and gravity during the jellies' movements pulls downward on the dense lithocyst, which bends the entire rhopalial body such that the cells of the touch plate are either distant from or making contact with the lithocyst hood; this physical mechanism is how the rhopalia deduct sensory information into gravity-sensate behavioral responses[1] As cnidarians lack centralized ganglia and cephalization, the centralization of sensory mechanisms divided up among connections within the rhopalial centers is the nearest concept to a brain that we can place within the phylum.
[3] 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.
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 [1] In recent years, it has been found that rhopalia retain a sizable reservoir of undifferentiated, stem-like cells, which are thought to be poised for potential use in both cell turnover and regeneration in the rhopalia.
[4] 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.
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.
[4] During strobliation, a single polyp, which is now dubbed a Strobila, is set to undergo a process of transverse fission, which will result in the generation of one or multiple juvenile medusae, consequently referred to as ephyrae.
[1] When the process of strobilation is at its genesis, before rhopalia begin their formation, small swellings manifest at the bases of specific tentacles in the polyp; these will become the sites of rhopalial development.
[4] It has been observed that the removal of some of the rhopalia in the species Aurelia results in a period of two weeks to complete regrowth, with only subtle loss of size and pigmentation but virtually identical function.
