Helmet jellyfish

Helmet jellyfish
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Scyphozoa
Order:	Coronatae
Family:	Periphyllidae
Genus:	Periphylla F. Müller, 1861
Species:	P. periphylla
Binomial name
Periphylla periphylla (Péron & Lesueur, 1810)
Synonyms
Carybdea periphylla Peron & Lesueur, 1810 Medusa hyacinthina Faber, 1829 Periphylla humilis Fewkes, 1886 Periphylla hyacinthina Haeckel, 1880 Periphylla mirabilis Haeckel, 1880 Periphylla regina Haeckel, 1880

The helmet jellyfish (Periphylla periphylla), sometimes called the merchant-cap,^[1] 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. This species is photophobic and inhabits deeper parts of the oceans to avoid light. It may be found at the surface on dark nights.