Cnidarians are invertebrates with stinging cells called nematocysts. Included in the cnidarian group are anemones, corals, and jellies.
A nematocyst is a cell that contains a coiled-up venomous barb. When touched, the barb is discharged into the skin of the prey and the venom is released.
Although a nematocyst is microscopic, when thousands of these are discharged into the skin, their effect can be dangerous or even deadly. Some of the most deadly animals in the world are the box jellies.