Cnidarians

Cnidarians are a phylum of simple aquatic animals built around a single body opening and stinging cells called cnidocytes, used to capture prey and defend against predators. The group spans free swimming jellyfish, sessile anemones, and colonial reef building corals, unified by a body plan with only two true tissue layers surrounding a central gut cavity. Many cnidarians alternate between a polyp stage that anchors to a surface and a medusa stage that drifts with ocean currents, though some species keep only one form throughout their life. Reef building corals host symbiotic algae that supply much of their energy, making them central to the health of tropical reef ecosystems.