Box jellyfish
The box jellies are found in tropical oceanic waters round the world. Their venom is delivered by stinging nematocysts, which cluster on tentacles, each with half a million stinging cells. The largest species, Chironex fleckeri, has caused many human deaths. The pain of an attack is agonising, and heart failure is a danger. The attack can be treated: first with vinegar, then remove stingers, then apply anti-venom.