Page "Cnidaria" Paragraph 1

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: the almost wholly 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 ( more complex animals ).