What structures make up the nervous system of a free-living planarian?

The nervous system of a free-living planarian is a centralized system, marking a significant evolutionary development in invertebrates. It is composed of the following primary structures:1

• Brain: Located in the anterior portion of the animal, the brain consists of two cephalic ganglia joined by a broad connection known as a commissure.1
• Longitudinal Nerve Cords: Typically three to five pairs of these cords extend posteriorly from the brain.1
• Commissures and Lateral Nerves: The longitudinal nerve cords are connected by transverse commissures, and smaller lateral nerves extend from these cords.1
• Peripheral Nerve Plexuses: The lateral nerves give rise to interlacing networks of peripheral nerves known as nerve plexuses. These include a submuscular plexus situated in the mesenchyme and a subepidermal plexus located at the bases of the epithelial cells.1

Additionally, planarians are equipped with various sensory organs, including ocelli (eyes), statocysts for balance, and chemoreceptors found in ciliated pits and grooves.1