Various examples of Jellyfish, and the Portuguese Man o’ War, which is a colonial organism. It is also quite predatory, equipped with deadly stinging tentacles, which can deliver venom potent enough to kill a human being. Steering clear of the Man o’ War would seem to be a sound strategy. Certain Jellyfish happen to be quite capable of inflicting mortal wounds, as well, however.
Bideodromage II 