Cnidarians form a phylum of animals that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. Both cnidarians and

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Basic body forms
Most adult cnidarians appear as either free-swimming medusae or sessile polyps, and many

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Sexual
Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larvaswims until it finds a good site, and then becomes a

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Many cnidarians are limited to shallow waters because they depend on endosymbiotic algaefor much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless, major

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Fossil record
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from

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Jellyfish stings killed about 1,500 people in the 20th century, and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in East and Southeast Asia. Coral reefshave long been economically important as providers of

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Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Modern

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