How is the nervous system of nematodes organized? Where are the neural chords located in their body?

Roundworms have a ganglial nervous system with an anterior neural ring showing (evolutionarily) a primitive cephalization.

Nematodes have two major longitudinal ganglial chords that extend one dorsally and the other ventrally under the epidermis. There might also be nerves lateral to these major chords. The nervous system of a Caenorhabditis elegans, free-living nematode, has been well-studied in neurophysiological research and presents 302 neurons.

The nematode C. Elegans was the organism used in the research on the genetic regulation of organogenesis and apoptosis whose researchers won the Nobel prize of Medicine in 2002 (Brenner, Horvitz and Sulston).