The nervous system of earthworm is divided into 3 parts:
(i) Central nervous system,
(ii) Peripheral nervous system,
(iii) Sympathetic nervous system.
(i) Central Nervous System:
It consists of a bilobed brain or cerebral ganglia, sub-pharyngeal ganglia, circum- pharyngeal connectives and a ventral nerve cord (Fig. 1.1). The brain is present on the dorsal side of alimentary canal in the 3rd segment in a groove between the buccal cavity and pharynx. The two ganglia forming the brain are the cerebral ganglia or supra pharyngeal ganglia.
From the brain arises a pair of circum-pharyngeal connectives which encircle the pharynx and then unite with a pair of sub-pharyngeal ganglia lying below the pharynx in the 4th segment. In this manner the supra-pharyngeal ganglia (brain), sub-pharyngeal ganglia and the circum-pharyngeal connectives form a nerve ring around the pharynx.
A ventral nerve cord arises from the sub-pharyngeal ganglia and runs below the alimentary canal upto the posterior most body segment. The ventral nerve cord bears a swelling or ganglion in each segment from 5th to the posterior end of the body. The ventral nerve cord is actually double in nature being consists of two longitudinal nerves which are fused intimately.
The wall of the nerve cord is formed of three layers. The outer layer is peritoneum, the middle muscle layer and the inner is fibrous epineurium. Further, the epineurium divides the nerve cord into two equal parts. The dorsal region of nerve cord has four giant fibres, middle part has only fibres and neuroglia (supporting cells) and lateral part has polar and tri-polar nerve cells.
(ii) Peripheral Nervous System:
It consists of nerves arising from the central nervous system. From the cerebral ganglia 8 to 10 nerves arise and supply to prostomium and buccal chamber. The circumpharyngeal connectives gives off nerves which innervates the body wall of first and second body segments as well as the walls of pharynx and buccal chamber. The nerves from the subpharyngeal ganglion supplies the different parts of 2nd and 4th body segments. The segmental ganglia give off three pairs of nerves supplying various structures of the segment.
(iii) Sympathetic Nervous System:
It consists of nerve plexus which lie in the epidermis and alimentary canal. The sympathetic nerves are connected with the circumpharyngeal connective.