Planarians are pretty hype. They don't have a true "brain," just a "ganglia," which is a clump of nerves located at one end of their body, along with a primitive pair of light-sensitive "eyes." If you flash a light on them, and then pass a slight current through the water, they contract their bodies in response. After repeating this about 150 times, they begin to contract in response to just the flash of light.