"If these jellyfish are poisonous, how can they be eaten?"
It is fugu, a blow fish, that has the deadly nerve toxin in one of the
internal organs (either the liver of gall bladder, I forget which). There
are fugu restaurants in Japan, where the thrill of risking death by eating
even a slightly tainted sliver of meat is the whole point of the experience.
You must be a licensed fugu chef to work at such a restaurant, but there
are a number of "black market" fugu stands around, too.
However, fugu usually kills you within 15 minutes, not 24 hours. It starts
with a tingling in the fingers, and works its way up your arms and legs, until
you are completely paralyzed, and you die when it reaches your heart.