The helmet jellyfish (Periphylla periphylla), sometimes called the merchant-cap, 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 and is one of the rare examples in Scyphozoa which life-cycle lacks a polyp stage.

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. They are the only known scyphozoan to undergo sexual propagation that lacks a planula stage.

Pe­ri­phylla pe­ri­phylla are deep-sea medusae with twelve stiff ten­ta­cles, which ex­tend from a thick, bell shaped body. These an­i­mals have dark red and/or brown stom­achs, and are bi­o­lu­mi­nes­cent. They can be up to 20 cm long, and their bod­ies, or cen­tral domes, have a di­am­e­ter of up to 35 cm.

The helmet jellyfish (Periphylla periphylla), is a species of crown jellyfish (Coronatae), and a vertical migrator, ascending from the depths at night to feed on copepods and other crustaceans. They are found in oceans around the world – with the exception of the Arctic Ocean – at depths ranging from thousands of metres to near the surface.