Box jellyfish

Box jellyfish are invertebrates belonging to the class Cubozoa, named for their cube-shaped medusae. The Cubozoans are categorized separately from other types of jellyfish, and are considered more evolved; Scyphozoans. Likewise, the best-known species of box jellyfish, Chironex fleckeri, while sometimes simply called "the Box Jellyfish", is only one of a category which actually contains about 19 different species. The name sea wasp is also applied to some species of cubozoans, including the afterforementioned Chironex fleckeri and Carybdea alata. Box jellies can be found in Australia, the Philippines, Hawaii[1], Vietnam, and many other tropical areas. Box jellyfish are extremely venomous and can kill humans.
Box jellyfish are best known for the extremely powerful venom possessed by some of their species. The Chironex fleckeri and the Carukia barnesi (Irukandji) species are the most venomous creatures in the world. Stings from such species are excruciatingly painful, either initially or as an after-effect, and are often fatal. However not all species of Box Jellyfish are this dangerous to humans.