Cnidaria is a phylum with about 11,000 species of animals. All of them are simple and aquatic, and most of them live in the sea. Some are colonial, composed of zooids which may be clones. Cnidarian zooids may take the form of polyps or medusae at different phases of their life.
|East Coast sea nettle |
Chrysaora quinquecirrha (?)
Cnidaria take their name from special cells which have organelles that sting: the nematocysts. This device is largely responsible for their success: it is their main specialised and distinctive cell type.
The word Cnidaria is spoken without the initial "C", and with a long "i". So it sounds like "Naidaria". In a similar way, the term Ctenophore is pronounced as "Teenophore". Originally, a Greek kappa was in front of the words, and it was pronounced.
- Subphylum and class Anthozoa: the sea anemones and corals.
- Subphylum Medusozoa: the jellyfish 
Some Cnidarians, such as Polypodium hydriforme and the group Myxozoa are parasites. Some live in symbiosis with algae that do photosynthesis. Most of these algae are dinoflagellates, but sometimes they are green algae. The algae take the carbon dioxide and produce oxygen and carbohydrates, which the cnidarian uses as source of food.
- Daly M. et al 2007. The phylum Cnidaria: a review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa, 1668: 127–182, Wellington. 
- Classes in Medusozoa based on "The Taxonomicon - Taxon: Subphylum Medusozoa". Universal Taxonomic Services. Retrieved 2009-01-26.
- Allan G. Collins (2011). "Recent insights into Cnidarian phylogeny" (pdf). Proceedings of the Smithonian Marine Science Symposium. Smithonian Contributions to the Marine Sciences Number 38.
- Yelena Fishman; et al. "Expulsion of symbiotic algae during feeding by the green Hydra – a mechanism for regulating symbiont density?". PLoS ONE 3(7).
- Waggoner B. & Collins A.G. (2004). "Reductio ad absurdum: testing the evolutionary relationships of Ediacaran and Paleozoic problematic fossils using molecular divergence dates". Journal of Paleontology. 78: 51–61. doi:10.1666/0022-3360(2004)078<0051:RAATTE>2.0.CO;2.