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Many cnidarian species exist in two different body forms during their life cycle: the free-swimming form, called the medusa (a jellyfish for instance), and the stationary form called the polyp (which resembles a tiny sea anemone). Both forms have a single opening that serves as the mouth and anus and is typically surrounded by a ring of tentacles packed with stinging cells called cnidocytes. Cnidarians have a defined top and bottom and are made up of two layers of tissue that include nerve and muscle cells. This body, formed from tissues with organized groupings of muscles and nerves, allowed ancestral cnidarians to be the first animals on the planet to show animated behavior.
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