Cnidarians are entirely aquatic animals.
They exist mainly in salt water, however there are a few of the over 9,000 species that live in fresh water as well. Cnidarians exhibit radial or biradial symmetry around a longitudinal axis. They have oral and aboral ends, but do not have a definite head. Cnidarians have two basic body forms. The polyp stage, which is a sessile form, and the medusa stage, which is free-swimming.