The brain is formed of the supra pharyngeal ganglia (segment earthworm). It is a bilobed mass of nervous tissue situated on the dorsal wall of the pharynx in 3rd segment. The ganglia found below the pharynx in the filth segments is called the subpharyngeal ganglia. The brain and the subpharyngeal ganglia are con-nected by a pair of circuin pharyngeal connectives.They run one on each side of the pharynx. Thus a nerve ring is formed around the anterior region of the alimentary canal. The double, solid ventral nerve cord runs backwards from the subpharyngeal ganglia, in the mid ventral line to the hind end of the body.
The ventral nerve cord has segmental ganglion one in each segment. From the brain nerves system are given off to the peristomium. From each ganglion of the ventral nerve cord, three pairs of nerves are given off to the body wall and other organs.
Receptors which are stimulated by the sense of touch(tactile re-ceptors), Chemical changes(chemoreceptors) and changes in temperature (thermoreceptors) are present in the body wall. These receptors are in the form of groups of slender columnar cells with short hairs projecting at the free end and connected with sensory fibres at the inner end. Receptors stimulated by changes in the intensity of light (Photoreceptors) are found on the dorsal surface of the body. Gustatory receptors (sense of taste) and olfactory receptors (sense of smell) are found in the buccal cavity.
