![Cnidarian body forms. A cnidarian may display either the sessile polyp form or the free-swimming &elipsis; [Credit: © Merriam-Webster Inc.]](http://images.bookrags.com/images/eb/72139.gif)
Cnidarian body forms. A cnidarian may display either the sessile polyp form or the free-swimming &elipsis; [Credit: © Merriam-Webster Inc.]Any of about 9,000 species of mostly marine aquatic
invertebrates, constituting the phylum Cnidaria (or Coelenterata), that are unique in possessing specialized stinging cells (cnidocytes) borne on the tentacles. Cnidocytes contain fluid-filled capsules (nematocysts) with a harpoonlike coiled thread used for stinging, paralyzing, and capturing prey.
Cnidarians have no well-defined separate respiratory, circulatory, or excretory organs; their tissues, composed of two cell layers, surround a cavity known as a coelenteron (gastrovascular cavity), which is the basic internal organ. Tentacles surrounding the mouth are used to capture and ingest food. Cnidarians are carnivorous, feeding mostly on zooplankton but also on small crustaceans, fish eggs, worms, smaller cnidarians, and even small fish. Cnidarians range in size from nearly microscopic to more than 100 ft (30 m) long and more than a ton (910 kg) in weight. There are two basic body forms: the polyp (e.g., coral) and the medusa (e.g., jellyfish). &Seealso; hydra; Portuguese man-of-war; sea anemone.
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