A flatworm has a simple nervous system with two nerve cords running down both of its sides. It also has two simple brains, and two eyespots that help sense light. While a fish's nervous system is comprised of a central brain and a spinal cord along with many branching nerves, which allows them to sense things around them.
Step-by-step explanation:
A flatworm has a very mild nervous system with pair nerve cords moving down each side. It has two single brains called ganglia, which are pure packages of nerves. It has two eyespots that support it sense light. A flatworm has a gastrovascular hole with one opening.
Fish have extremely advanced nervous systems organized around a brain. Fish brains have rare clearly visible parts. Back to the brain is the spinal cord, which is the deep dorsal nerve cord that identifies chordates.