This is a special cell found in some ocean critters — such as jellyfish, sea anemones and corals — that has a stinging barb coated in venom. The cell works a bit like a living harpoon. Before the nematocyst fires, its barb stays coiled inside the cell in a chamber where it is bathed in venom. When the nematocyst comes in contact with something else — such as a fish or your leg — the tiny harpoon fires. The barb sinks into the target, delivering a dose of venom.
Nematocysts are most associated with jellyfish. The cells’ venom can range from weak to strong. Australian box jellyfish, for instance, have venom that can kill a person. Corals also have nematocysts. They use them at night to catch tiny creatures floating by in the water.
In a sentence
Jellyfish have nematocysts, but their distant cousins the comb jellies are sting-free.