Jellyfish, also known as sea jellies, are the medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria.
Jellyfish
A purple-striped jellyfish at the Monterey Bay Aquarium
Fossil jellyfish, Rhizostomites lithographicus, one of the Scypho-medusae, from the Kimmeridgian (late Jurassic, 157 to 152 mya) of Solnhofen, Germany
Stranded scyphozoans on a Cambrian tidal flat at Blackberry Hill, Wisconsin
Medusozoa is a clade in the phylum Cnidaria, and is often considered a subphylum. It includes the classes Hydrozoa, Scyphozoa, Staurozoa and Cubozoa, and possibly the parasitic Polypodiozoa. Medusozoans are distinguished by having a medusa stage in their often complex life cycle, a medusa typically being an umbrella-shaped body with stinging tentacles around the edge. With the exception of some Hydrozoa, all are called jellyfish in their free-swimming medusa phase.
Medusozoa
Polypodium hydriforme, parasites of obscure affinity
Pelagia noctiluca, a scyphozoan
An unidentified cubozoan