Jellyfish bodies are built from two epithelial layers separated by mesoglea. The mesoglea is not a tissue of packed cells but an extracellular matrix, mostly water bound in a network of collagen fibres and other proteins, with scattered cells running through it. In Aurelia aurita the water fraction is around 95 per cent.
That construction is what allows a large bell with almost no metabolic cost. There is no skeleton, no respiratory system and no blood; oxygen diffuses across surfaces because nothing in the body is far from seawater. The mesoglea also works as an elastic spring, storing energy from each contraction of the swimming muscles and returning it to reopen the bell.
The consequence on land is dramatic. Left on a beach, a stranded jelly loses its water and shrinks to a thin transparent smear. A related misunderstanding is that this proves the animal is simple or inert. Moon jellies have a nerve net with rhopalia carrying balance organs and light sensors, and they feed continuously by trapping plankton in mucus on the bell and moving it to the margin.