The nervous system of Platyhelminthes includes which features?

• A. Two nerve cords; Anterior centralized ganglia (brain), eyespots on some planarians
• B. Nerve net with no central brain
• C. Single nerve cord and ganglia
• D. No nervous system present

Answer: (A)

Platyhelminthes show cephalization with a more organized nervous system than a simple nerve net. They have two longitudinal nerve cords running along the ventral side, connected by transverse nerves, which gives a ladder-like arrangement. At the head end, these cords pool into a pair of anterior ganglia that function as a simple brain. From this brain, nerves extend to the rest of the body, and many flatworms, like planarians, have eyespots (ocelli) that detect light.

This combination—two ventral nerve cords plus an anterior centralized ganglion and sensory eyespots in some species—reflects a higher level of nervous system organization than a diffuse nerve net. The other descriptions either imply a diffuse, non-centralized system, a single nerve cord, or no nervous system at all, which don’t fit the typical planarian-grade organization.