Planarian
Planarians are non-parasitic worms of the turbellaria class. Their nervous system consists of a cerebral ganglia under the eyespots in the head. This ganglia is sometimes considered a brain, because it has been shown to electric activity similar to that of other animals. From the ganglion are two nerve cords that extend to the tail. Then there are traverse nerves connecting the two longitudinal nerve cords, making the system look like a ladder.