Periphylla periphylla

The helmet jellyfish (Periphylla periphylla) is a luminescent, red-colored jellyfish of the deep sea, belonging to the order Coronatae of the phylum Cnidaria. It is the only species in the monotypic genus Periphylla. P. periphylla represents an exception, very rarely found in the phylum Cnidaria: the medusae do not go through a polyp stage, thus presenting a 'holopelagic' life cycle. The medusae strew fertilized eggs in open water and these develop directly into medusae, whose development rests entirely upon the egg's high yolk supply. The ephyra stage common among other jellyfish is not observed in P. periphylla.