Answer:
Oxygen molecules are small and nonpolar, which allows them to easily pass through the cell membrane.
The digestive tract overlaps in many layers (It can even reach microsocpic overlaping) allowing it to fit inside a person's body.
Jellyfish are forever but fish ain't.
Plus, jellyfish have those tentacle like things attached to thier body and you might confuse em for octopus for that very reason..
but you should really evade confusing em as an octopus cuz octopus might take it as an insult and the next thing you know will be a Kraken taking your house down.. which is pretty tragic :'(
