Central nervous system
The CNS consists of a bilobed brain (cerebral ganglia, or supra-pharyngeal ganglia), sub-pharyngeal ganglia, circum-pharyngeal connectives and a ventral nerve cord.
Earthworms' brains consist of a pair of pear-shaped cerebral ganglia. These are located in the dorsal side of the alimentary canal in the third segment, in a groove between the buccal cavity and pharynx.
A pair of circum-pharyngeal connectives from the brain encircle the pharynx and then connect with a pair of sub-pharyngeal ganglia located below the pharynx in the fourth segment. This arrangement means the brain, sub-pharyngeal ganglia and the circum-pharyngeal connectives form a nerve ring around the pharynx.
The ventral nerve cord (formed by nerve cells and nerve fibres) begins at the sub-pharyngeal ganglia and extends below the alimentary canal to the most posterior body segment. The ventral nerve cord has a swelling, or ganglion, in each segment, i.e. a segmental ganglion, which occurs from the fifth to the last segment of the body. There are also three giant axons, one medial giant axon (MGA) and two lateral giant axons (LGAs) on the mid-dorsal side of the ventral nerve cord. The MGA is 0.07 mm in diameter and transmits in an anterior-posterior direction at a rate of 32.2 m/s. The LGAs are slightly wider at 0.05 mm in diameter and transmit in a posterior-anterior direction at 12.6 m/s. The two LGAs are connected at regular intervals along the body and are therefore considered as one giant axon.
Peripheral nervous system
Eight to ten nerves arise from the cerebral ganglia to supply the prostomium, buccal chamber and pharynx.
Three pairs of nerves arise from the subpharyangeal ganglia to supply the 2nd, 3rd and 4th segment.
Three pairs of nerves extend from each segmental ganglia to supply various structures of the segment.
Sympathetic nervous system
The sympathetic nervous system consists of nerve plexuses in the epidermis and alimentary canal. (A plexus a web of nerve cells connected together in a two dimensional grid.) The nerves that run along the body wall pass between the outer circular and inner longitudinal muscle layers of the wall. They give off branches that form the intermuscular plexus and the subepidermal plexus. These nerves connect with the circumpharyngeal connective
