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Cnidarians were the first animals to have muscles and nerves to produce behavior. They were also the first to have a mouth and stomach to digest food. We learn about nematocysts when we watch an anemone catching a goby and two anemones fighting. Cnidarians come in various body shapes and have different ways of living. Corals are cnidarians that build reefs. One anemone, Stomphia, can swim away from predators by contracting its entire body. The jellyfish body plan is like an anemone that has been turned upside down and a diverse group of cnidarians thrives at all depths of the ocean.