Fascinating, elegant, and mysterious to watch in the water, take a jellyfish out of the water, and it becomes a much less fascinating blob. This is because jellyfish are about 95 percent water.
Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis. An elementary nervous system, or nerve net, allows jellyfish to smell, detect light, and respond to other stimuli. The simple digestive cavity of a jellyfish acts as both its stomach and intestine, with one opening for both the mouth and the anus.