Fact about jellyfish.

Their bodies are so simple
Jellyfish are also famous for their very simple body structure. About 98 percent of their body consists of water, as reported by the How Stuff Works page. They don't have brains, but they do have a nervous system all over their epidermis. They don't even have a heart because they get oxygen from water by diffusion.

Even so simple, they do not have separate feeding and exhaust channels. That is, they eat and defecate through the same hole! But in matters of vision, some species are quite complex. The box jellyfish, for example, has 24 eyes around its body so it has 360-degree vision.