Cnidaria (pronounced Ne-dear-e-a with a silent c) is a phylum containing over 9000 species of animals found exclusively in aquatic, mostly marine, environments. The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps.

Cnidarians are classified into four main groups: sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War.