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Phylum - Cnidaria

(Jellyfish, Anemones and Hydras)


The phylum Cnidaria has animals like: jellyfish, anemones and hydras. All Cnidarians have stinging cells called nematocysts. Casually touching a cnidaria can trigger their nematocysts that will send out barbed threads tipped with poison.


This phylum containing some 11,000 species of relatively simple animals found in aquatic, mostly marine, environments. The basic body shapes of a cnidarians are the polyp of the hydra seen on the left, or the medusa seen on the right. The medusa is the classic jellyfish shape. The medusa has radial symmetry, while Hydras and sea anemones are polyps.


Asexual reproduction via budding is common among Cnidaria, particularly among the Hydrozoa class. Asexual larvae bud laterally from the adult polyps, which develop into polyps themselves. However, all cnidarians can also reproduce sexually.

Photo by Dennis Mojado, 29 July 2005, Sea Nettles

Picture from the University of California website: hydra viridissima

Photo by MadRabbit, 15 September 2006, Sea anemone at the Monterey Bay Aquarium Diagrams by Jeremy Scholz

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