Why are stinging cells effective for jellyfish and sea anemones to capture prey?

Stinging cells are called nematocysts. Anything that helps a predator to catch and kill prey is going to be effective. Nematocysts respond to touch, and act like tiny bow-and-arrow traps. Whenever the prey is touched by a tentacle, the nematocyst fires its stinging dart, and poison is shot into the prey. The prey gets more and more tangled up in tentacles, and eventually has so much poison that it dies. Then the jelly or anemone pulls it in and chows down!