The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish.
Scyphozoa
Fossilized stranded scyphozoans on a Cambrian tidal flat at Blackberry Hill, Wisconsin.
The developmental stages of scyphozoan jellyfish's life cycle: 1–3 Larva searches for site 4–8 Polyp grows 9–11 Polyp strobilates 12–14 Medusa grows
Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.
Pacific sea nettles, Chrysaora fuscescens
Oral end of actinodiscus polyp
Cerianthus filiformis (Ceriantharia)
Sea anemones (Actinaria, part of Hexacorallia)