Jellies

The cnidarians exhibit phenomenal diversity within a simple body plan. This phylum includes the sea anemones, corals, sea fans, and hydroids. My definite favorite are the jellies. Claudia Mills, a researcher at Friday Harbor Labs, where I did my graduate work is one of the world's experts on gelatinous zooplankton, which includes familiar, large jellyfish (the scyphozoans), small hydromedusae, large pelagic hydrozoans like the Portuguese Man of War, and comb jellies (ctenophores). The Ctenophora are a separate phylum from the Cnidaria, which include the jellyfish, anemones and corals, as well as the Cubozoa, the scariest class of cnidarians. The sting of all cubozoan species is intensely painful and some are deadly.
Jellyfish blooms are currently in the news, although the causes of population fluctuations are not well understood.
There is a lot of information, some of it conflicting, about what to do if you are stung by a jellyfish. Here is a great place to get the lowdown on the most recent scientific research. They publish an online newsletter every six months that is an annotated bibliography of recent publications on jellyfish stings.