There are approximately 9,000 species of Cnidarians. They are familiar to us as jellyfish and corals and one of the characteristics of the phylum is the nematocysts that can inject poison through human skin making the pelagic types the bane of swimmers worldwide. This toxin varies from the harmless moon jelly Aurelia aurita to the deadly box jellyfish (Chironex fleckeri, Carukia barnesi and Malo kingi).
An interesting Cindarian, recently filmed, is the Scyphomedusa Deepstaria enigmatica.
The description and classification of the species can be found here. It has so far been found in the mesopelagic zone; since this zone strateches from ~660 to 3300 feet down, you are unlikely to encounter it anytime soon. Because of the relatively few sightings, worldwide distribution is unknown.
Here is a remarkable video of the strange, and relatively unknown animal.