The ventral nerve cord is a chain of ganglia running posteriorly. The last ganglion is the preanal ganglion that give rise to two branches that pass dorsally into the pseudocoel to encircle the rectum forming the rectal commissure or posterior nerve ring (Figure 22.12). Peripheral nerves form a latticework of fibers that join with small commissures to supply nerves endings to sensory structures in the cuticle.
In parasitic species, the main sense organs are amphids, phasmids, cephalic papillae, and caudal papillae. Arrangement of cephalic nerves suggests that sixteen papillae existed in primitive nematodes and sixteen nerve endings can be found in modern nematodes even when far fewer papillae exist. The arrangement of papillae in nematodes is a strong taxonomic character. Sensory endings of papillae appear to be modified cilia and the papillae are probably sensory receptors.