Nervous system
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Evolution of the eye
When a photon is absorbed by the chromophore, a chemical reaction causes the photon's energy to be transduced into electrical energy and relayed, in higher animals, to the nervous system. These photoreceptor cells form part of the retina, a thin layer of cells that relays visual information, including the light and day-length information needed by the circadian rhythm system, to the brain. However, some jellyfish, such as Cladonema, have elaborate eyes but no brain. Their eyes transmit a message directly to the muscles without the intermediate processing provided by a brain.