Jellyfish are 95% water and have no brain, heart, or bones.

Jellyfish have been around for more than 500 million years, making them older than dinosaurs. While humans are about 60% water, a jellyfish is almost entirely liquid. This high water content makes them about the same weight as the seawater around them, so they do not have to work hard to stay afloat.Instead of a brain, they use a simple web of nerves called a nerve net. This net helps them feel things like light, touch, and chemicals in the water. Even without a brain, scientists found in a 2017 study that some jellyfish, like the Cassiopea, actually sleep just like we do.They do not have lungs to breathe either. Instead, oxygen passes right through their thin skin. Their body is held together by a jelly-like material called mesoglea that sits between their inner and outer layers.If a jellyfish washes up on the sand, it will almost completely disappear in the sun. Since they are mostly water, the sun evaporates them until only a tiny, thin layer of protein is left on the beach.