HOW COME JELLY FISH DONT HAVE A BRAIN NOR BONES AND STILL KNOW WHEN & WHAT TO POISON WITH THEIR TENTACLES?

its just so weird how they produce poison and have something like a nervous system to react when something like a person or a fish touches its tentacles. so how do they do it without a brain or something? how do they eat? how do they produce poison or something through their tentacles?