Q. How is the nervous system characterized in beings of the phylum Annelida? How can one compare cephalization in annelids to cephalization in platyhelminthes and nematodes?

Annelids have a nervous system made of two ventral chords and one relatively big nervous cell concentration in its anterior portion resembling a primitive brain.

Nematodes have an anterior neural ring connected to two neural chords, a dorsal one and a ventral, while in planarias (platyhelminthes) there are only two small anterior "cerebral" ganglia from which neural chords split. Cephalization in annelids thus is more outstanding than in flatworms or in nematodes.