late Ediacaran to present
The class does not have a medusa larval stage in its development, unlike the rest of the phylum. Like all Cnidaria, their food-catching and defense are done by nematocysts, which are extremely effective stinging cells. Over 6,100 species have been described.
There are three subclasses:
- Ceriantharia: the tube-dwelling anemones or cerianthids
- Hexacorallia: 4,300 species of colonial polyps, usually with 6-fold symmetry. This includes all of the stony corals, which are vital for coral reef formation, and all sea anemones, and zoanthids.
- Octocorallia: 3,000 species of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fans and sea whips).
- Daly M. et al 2007. The phylum Cnidaria: a review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa, 1668: 127–182, Wellington. Abstract - PDF
- Crowther A.L. (2011). Z.-Q. Zhang (ed.). "Animal biodiversity: an outline of higher-level classification and survey of taxonomic richness" (PDF). Zootaxa. 3148: 19–23.
- estimates of species numbers differ between authorities.