Cnidarians all have one unique characteristic; they have stingers made of cells called nematocysts. They are radially symmetric and have a basic nervous system. They are also invertebrates.

There are about 9,000 species of cnidarians.

One example is the pacific sea nettle, also known as Chrysaora fuscescens. This animal's sting is very poisonous, but predators still feed on it.

Another example is the Christmas anemone, also known as Urticina crassicornis. These animals have a mouth at the center of their tentacles that they use to eat prey such as sea urchins and barnacles.

http://www.oceanicresearch.org/education/wonders/cnidarian.html

http://animaldiversity.ummz.umich.edu/accounts/Cnidaria/