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Cnidarians include freshwater Hydrazoans, jellyfish or Scyphozoa, Anthozoa which are sea anenomes and coral animals and Cubozoa or box jellies. Cnidarians contain two layers of epithelial cells with a mesoglea or jelly-like layer in between. They have an oral cavity or mouth with tentacles around it. These have Cnidoblasts or stinging cells with tiny harpoon like structures. Most Cnidarians are predators of plankton, fish and other animals and their stinging cells use a toxin to paralyze the prey so that they can ingest them via their mouth. Most Cnidarians live a sessile lifestyle in the polyp phase. As long as nutrients are plentiful, they live and thrive attached to the substrate. Sometimes, they may even form a bud, which can break off and grow independently. This is a form of asexual reproduction. Another part of their life cycle includes the sexual or Medusa stage. In this stage, free-swimming male and female jellyfish, release sperm and eggs into the water, where external fertilization occurs. Eventually, the larvae that results will find a place to settle down and the sessile part of the life cycle will begin. They alternate between a sessile or polyp stage, and a free-swimming or Medusa stage.
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