The phylum Coelenterata, also known as Cnidaria, includes the marine corals, sea fans, sea pens, sea anemones, hydras, and jellyfish. Hydras also have freshwater representatives and are the exception in the coelenterate taxon since they do not exhibit a medusa or "jellyfish" life stage. Specialized stinging cells known as nematocysts or cnidae (thus, the alternative phylum name) are used to capture, hold, and reel in prey, as well as to assist in locomotion and defense.

View a medium magnification image of an ancient Cnidaria fossil.

View a high magnification image of an ancient Cnidaria fossil.

Capable of either asexual or sexual reproduction, coelenterates are primitive invertebrates characterized by the radial symmetry typical of sessile, benthic filter feeders. In similarity to sponges, coelenterates are often colonial, but the tiny hydras are capable of living freely as individuals. Some coelenterates are free-swimming, such as the Portuguese man-of-war and the jellyfishes, while many species of hydroids are capable of some forms of mobility, including creeping slowing while sliding on their basal discs, or locomoting more rapidly in a series of somersaults or looping movements.

The poisonous tentacles of fire coral, jellyfishes, and the hydrozoan Portuguese man-of-war deliver painful stings upon contact with human skin, although they are intended for paralyzing fish and other small prey. If washed with fresh water, the stinging cells excrete even more irritants. Counterintuitively, a vinegar wash of the jellyfish stings is recommended as first aid before seeking medical advice. The venom associated with the skin-piercing nematocysts is highly toxic and in some cases, lethal to human victims.