Answer:
[Figure: Nervous system of Planaria — a pair of eyespots at the head end; a bilobed cerebral ganglion (brain) below them; two ventral longitudinal nerve cords running along the body length, joined by many transverse nerves (commissures) giving a ladder-like pattern, with lateral nerves branching outwards.]
Planaria (a flatworm) has a simple centralised, ladder-like nervous system:
- Brain: at the anterior end there is a pair of cerebral ganglia forming a bilobed brain, which receives impulses from the eyespots (photoreceptors) and chemoreceptors/auricles.
- Nerve cords: two longitudinal ventral nerve cords arise from the brain and run along the length of the body.
- Transverse nerves: the nerve cords are connected by transverse commissures at regular intervals, giving a ladder-like appearance.
- Lateral nerves: branch from the nerve cords to all parts of the body.
This shows the beginning of cephalisation and centralisation; it helps planaria respond to light, touch and chemicals and to coordinate its movements.
