Jellyfish are not actually fish, but a collection of plankton. Though relatively simple, they are able to survive without the presence of a complex circulatory, digestive or central nervous system.
Without the presence of a brain, Jellyfish interact with the outside world using a simple nervous system, known as a "nerve net," located in the epidermis that protects their inner organ tissue. The nerve net transmits neural impulses through a circular nerve ring and into the rhopalium, a simple sensory organ located in their bells. Using this system, jellyfish are able to perceive stimuli such as light, gravity, touch and react accordingly.