What is a cnidarian such as a hydra or a fire coral?

Cnidarians are a diverse group of simple, aquatic animals that include species like hydra and fire coral. They are characterized by their radial symmetry, specialized stinging cells called cnidocytes, and a body plan that typically includes a central cavity. Hydra are freshwater organisms known for their regenerative abilities, while fire corals are marine species that form calcium carbonate structures and can deliver painful stings due to their nematocysts. Both play important roles in their ecosystems, contributing to biodiversity and habitat structure.