These are beautifuly dangerous animals. Almost every single one have one thing in common; they are all armed and potentialy dangerous. Coral, jellyfish, and anemones are just a few examlpes of Cnidaria. As diverse as they are they are all equipped with nematocysts.
Nematocysts are tiny cells that inject toxins into the victims. This is how the gather food and ward off any intruders that should want harm. The burning that divers feel when they have a brush in with jellyfishes is due mostly to the toxins. There is only one jellyfish in the known world that cannot sting and has gone against the main stream theorys of Cnidaria, and it resides in an extremely poisonous lake in Palau. This is not to say it lacks the nematocysts rather that they are no longer functional.
There are four main groups to note when studying Cnidara. They are Anthozoa: true corals, Anemones, and sea pens; Cubozoa: box jellies with complex eyes and potent toxins; Hydrozoa: siphonophores, hydroids, fire corals, and many medusae; and Scyphozoa: real jellyfish. Not all of these animals are salt water creatures. Cnidaria is from the Greek word "cnidos," which means stinging nettle.