Nervous System in Planaria

Planaria, a flatworm of the phylum Platyhelminthes, is the most primitive animal to possess a true central nervous system (CNS), positioned on the ventral side of the body. At its head end lies a mass of cerebral (cephalic) ganglion tissue shaped like an inverted U, from which nine branches radiate outward on each side. Running down from these ganglia are a pair of ventral nerve cords (VNC), connected to each other at intervals by transverse connecting fibres called commissures -- an arrangement that, viewed as a whole, looks like a ladder, so it is called the ladder-type nervous system. Lateral to the VNC, a peripheral nerve plexus carries sensory cells arranged in lateral cords, and a pair of photosensory eyes sit on the dorsal side of the brain, alongside scattered single sensory cells over the body.

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