Jellyfish

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Jellyfish are marine invertebrates belonging to the Class Scyphozoa within the Phylum Cnidaria. They can be found in every ocean in the world. The use of the term "jellyfish" is actually a misnomer since scyphozoans are not fish, which are vertebrates. The term is also (incorrectly) commonly-applied to some close relatives of true scyphozoans, such as the Hydrozoa and the Cubozoa.

Most jellyfish have tentacles or oral arms coated with thousands of microscopic nematocysts; generally, each nematocyst has a "trigger" (cnidocil) paired with a capsule containing a coiled stinging filament, as well as barbs on the exterior. Upon contact, the filament rapidly unwinds, launches into the target, and injects toxins. It can then pull the victim into its mouth, if appropriate.

Although most jellyfish are not perniciously dangerous to humans, a few are highly toxic, such as Cyanea capillata. The recently discovered Carukia barnesi is also suspected of causing two deaths in Australia. Contrary to popular belief, the menacingly infamous Portuguese Man o' War (Physalia) is not a jellyfish, but a colony of hydrozoan polyps. Regardless of the actual toxicity of the stings, many victims find them very painful, and some individuals may have severe allergic reactions, anaphylactic shock, similar to bee sting allergic reactions
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Source: http://en.wikipedia.org/wiki/Jellyfish