Jellyfish are made of 95 percent water and 5 percent solid matter. The solid matter is composed of three layers: the epidermis is the outer layer, the mesoglea or the jelly is the middle layer and the gastrodermis is the inner layer.
Jellyfish are simple invertebrates. They do not have blood, hearts or brains. A jellyfish has a simple digestive system with one cavity for the intestine and stomach and one opening for the anus and mouth. The jellyfish's nervous system is a network of nerves that allow the jellyfish to detect light, smell and respond to stimuli. They have a radial symmetry; their body parts radiate from the center. This allows jellyfish to detect food and respond to danger from all directions. Jellyfish do not have a respiratory system. Their skin is thin and is oxygenated by diffusion. They have tentacles that can sting predators.