How do jellyfish defend themselves against predators?

The tentacles and the oral lobes both are covered with stinging cells. When the stinging cells are touched, they pop open and a tiny dart emerges from each stinging cell injecting poison into its victim.

To defend themselves, jellyfish use small structures in their tentacles which shoot out when touched by anything. Though they do shoot at other jellyfish they do not hurt them. Also in the movie Finding Nemo Marvin doesn't get stung because Clown fish and some other types have a protective coating which doesn't let the shooters shoot.