Roundworms have a ganglial nervous system with an anterior neural ring representing (evolutionarily) a primitive cephalization.
Nematodes have two main longitudinal ganglial chords that extend one dorsally and the other ventrally under the epidermis. There may also be nerves lateral to these main chords. The nervous system of a free living nematode, “Caenorhabditis elegans”, 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).