Cnidaria are early-evolving animals defined by cnidocytes; stinging cells unique to the group. These allow for effective prey capture and defense, setting cnidarians apart from other radially symmetrical animals like ctenophores.

They have radial symmetry, a diploblastic body plan (ectoderm and endoderm), and a gastrovascular cavity with a single opening. A simple nerve net and contractile tissues allow for coordinated movement. Most species show a life cycle alternating between polyp (sessile) and medusa (free-swimming) stages.

The clade includes Anthozoa (corals, anemones) and Medusozoa (jellyfish and allies). Though soft-bodied forms fossilize poorly, cnidarians have a long record through coral lineages and were among the first animals with true tissues, symmetry, and active predation.