Jellyfish don't have a brain or a heart or blood and can't see or hear and yet they have managed to stick around in our oceans for 600 million years and have survived five mass extinctions. These drifting blobs are awesome and especially awesome to look at. Deep Look examines why.
Deep Look writes a bit on a Jellyfish's sting:
Jellyfish sting to paralyse their prey. They use special cells called nematocysts. Jellyfish don't have a brain or a central nervous system to control these stinging cells, so each one has it's own trip wire, called a cnidocil. When triggered, the nematocyst cells act like a combination of fishing hook and hypodermic needle. They fire a barb into the flesh of the jellyfish's prey at 10,000 times the force of gravity -- making it one of the fastest mechanisms in the animal kingdom. As the barb latches on, a thread-like filament bathed in toxin erupts from the barb and delivers the poison.