Scyphozoa contains roughly 200 valid species, but as with other relatively poorly studied groups, the true diversity is likely to be significantly greater. Scyphozoa is divided into three subgroups, coronates, rhizostomes, and semaeostomes. Recent work has shown quite clearly that the latter two groups form a clade with semaeostomes being paraphyletic with respect to rhizostomes. Together, semaeostomes and rhizostomes are known as Discomedusae.
Nearly all scyphozoans have a life cycle that includes a sexually reproducing planktonic medusoid phase known as a jellyfish. This stage usually alternates with an asexual benthic polyp stage. Scyphozoans are unique in producing medusae through a process known as strobilation, a single polyp produces multiple, distinctively shaped juvenile medusae
(ephyrae) at its oral end.