Coelenterates are soft-bodied organisms such as jellyfish, anemones, coral and hydras and are characterized by a gelatinous body, tentacles and the presence of nemadocysts or stinging cells. Their body structure is relatively simple, with two layers of cells and radial symmetry.
All coelenterates are aquatic and mostly live in marine environments, although there are some that can be found in fresh or brackish water. They have a single opening, and respiration and excretion are performed through diffusion. Coelenterates have a simple network of nerves across their body, and can be either free-swimming or immobile. Some coelenterates live in polymorphic colonies where individual members perform different functions.