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. Sometimes the mesoglea contains cells and connective tissue. Only one oral opening is present in Cnidarians, and therefore serves as both a mouth and an anus.