A jellyfish has a nervous system consisting of a network of fibres which coordinate its movements so that the whole bell contracts at once, and is often connected with eyes which are sensitive to light though they cannot perceive form, and an organ to enable it to keep the right end upwards. The most familiar forms are sea anemones and jellyfish, but the corals are much more important, as they have been in the past, and still are, builders of masses of rock. A typical coelenterate has a number of tentacles with which it catches its prey, a mouth in the middle of them, through which it also rejects the undigested remains of its food, a body wall with muscles, and sexual organs whose products may burst through the skin or be shed through the mouth. It may be a swimmer like jellyfish, or a fixed polyp, like sea anemones and corals.