Editing Jellyfish (section)

=== Taxonomy ===
The subphylum Medusozoa includes all cnidarians with a medusa stage in their life cycle. The basic cycle is egg, [[planula]] larva, polyp, medusa, with the medusa being the sexual stage. The polyp stage is sometimes secondarily lost. The subphylum include the major taxa, [[Scyphozoa]] (large jellyfish), [[Cubozoa]] (box jellyfish) and [[Hydrozoa]] (small jellyfish), and excludes [[Anthozoa]] (corals and sea anemones).<ref name=Ruppert>{{cite book |title=Invertebrate Zoology, 7th edition |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard, S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=148–174 }}</ref> This suggests that the medusa form evolved after the polyps.<ref name="treeoflife">[http://tolweb.org/Cnidaria/ Cnidaria] {{webarchive|url=https://web.archive.org/web/20120921151117/http://tolweb.org/Cnidaria |date=2012-09-21 }}, Tree of Life.</ref> Medusozoans have tetramerous symmetry, with parts in fours or multiples of four.<ref name=Ruppert />

The four major classes of medusozoan Cnidaria are:

* [[Scyphozoa]] are sometimes called true jellyfish, though they are no more truly jellyfish than the others listed here. They have tetra-radial symmetry. Most have tentacles around the outer margin of the bowl-shaped bell, and long, oral arms around the mouth in the center of the subumbrella.<ref name=Ruppert />
* [[Cubozoa]] (box jellyfish) have a (rounded) box-shaped bell, and their velarium assists them to swim more quickly. Box jellyfish may be related more closely to scyphozoan jellyfish than either are to the Hydrozoa.<ref name="treeoflife" />
* [[Hydrozoa]] medusae also have tetra-radial symmetry, nearly always have a velum (diaphragm used in swimming) attached just inside the bell margin, do not have oral arms, but a much smaller central stalk-like structure, the manubrium, with terminal mouth opening, and are distinguished by the absence of cells in the mesoglea. Hydrozoa show great diversity of lifestyle; some species maintain the polyp form for their entire life and do not form medusae at all (such as [[Hydra (genus)|Hydra]], which is hence not considered a jellyfish), and a few are entirely medusal and have no polyp form.<ref name=Ruppert />
* [[Staurozoa]] (stalked jellyfish) are characterized by a medusa form that is generally sessile, oriented upside down and with a stalk emerging from the apex of the "calyx" (bell), which attaches to the substrate. At least some Staurozoa also have a polyp form that alternates with the medusoid portion of the life cycle. Until recently, Staurozoa were classified within the Scyphozoa.<ref name=Ruppert />

There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 50 species of Cubozoa, and the Hydrozoa includes about 1000–1500 species that produce medusae, but many more species that do not.<ref>{{Cite journal |doi=10.1111/j.1744-7410.2004.tb00139.x |last=Marques |first=A.C. |author2=A. G. Collins |s2cid=28342963 |year=2004 |title=Cladistic analysis of Medusozoa and cnidarian evolution |journal=Invertebrate Biology|volume=123|pages=23–42}}</ref><ref>{{Cite journal |last=Kramp |first=P.L. |year=1961 |title=Synopsis of the Medusae of the World|journal=Journal of the Marine Biological Association of the United Kingdom |volume=40 |pages=1–469 |doi=10.1017/s0025315400007347|doi-broken-date=1 November 2024 |bibcode=1961JMBUK..40....7K }}</ref>