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. The structure consists of two layers, the epidermis, or outer layer, and the gastrodermis, or inner layer. Between these two layers is a gel like substance called mesoglea.