The most primitive animal with a central nervous system (CNS) located on the ventral side of body is-------.

Step-by-step solution

Sycon belongs to Porifera and lacks a nervous system. Hydra belongs to Cnidaria and has a primitive nervous system consisting of a diffuse nerve net, but lacks a central nervous system (CNS). Planaria belongs to Platyhelminthes, which are the most primitive animals to exhibit cephalization and a central nervous system. Their CNS consists of cerebral ganglia (brain) and two longitudinal nerve cords located on the ventral side of the body, connected by transverse commissures to form a ladder-like structure. Scoliodon