An Irukandji jellyfish. The bells of Irukandji jellyfish rarely grow to be larger than 10mm, but their tentacles can be up to a meter long. Unlike other jellyfish, they are able to fire their stingers at will. Their sting causes a condition called Irukandji Syndrome, which, if left untreated, can cause cardiac arrest within 20 minutes.
(via thejives)