Cnidaria - Classification

Classification

Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and there is debate about whether Myxozoa and Polypodiozoa are cnidarians or closer to bilaterians.

Modern cnidarians are generally classified into four classes:

	Hydrozoa	Scyphozoa	Cubozoa	Anthozoa
Number of species	3,600	228	42	6,100
Examples	Hydra, siphonophores	Jellyfish	Box jellies	Sea anemones, corals, sea pens
Cells found in mesoglea	No	Yes	Yes	Yes
Nematocysts in exodermis	No	Yes	Yes	Yes
Medusa phase in life cycle	In some species	Yes, except for Stauromedusae if they are scyphozoans	Yes	No
Number of medusae produced per polyp	Many	Many	One	(not applicable)

Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa.

The Myxozoa, microscopic parasites, were first classified as protozoans, but recently as heavily modified cnidarians, and more closely related to Hydrozoa and Scyphozoa than to Anthozoa. However other recent research suggests that Polypodium hydriforme, a parasite within the egg cells of sturgeon, is closely related to the Myxozoa and that both Polypodium and the Myxozoa are intermediate between cnidarians and bilaterian animals.

Some researchers classify the extinct conulariids as cnidarians, while others propose that they form a completely separate phylum.