- The advanced nervous system in annelids is generally compare with coelenterates. The well developed nervous system can be seen here in form of a pair of cerebral ganglia (brain) and a double ventral nerve cord. This ventral nerve cord bears ganglia and lateral nerve in each segment as annelids are true segmented animals.
Nervous system in Indian Earthworm
In Indian Earthworm (Pheretima posthuma), the nervous system is advanced and developed. There are three part of nervous system in Earthworm. 1. Central, 2.Peripferal and, 3.Sympathic nervous system.
1. Central Nervous System : This system consists a front nerve ring and posterior ventral nerve cord.
- Nerve ring: it is composed of two cerebral ganglia, circumpharyngeal connectives and sub pharyngeal ganglia. These two cerebral ganglia are pear shaped and form brain in earthworm. The brain is dorsally present in third segment between buccal cavity and pharynx. Again a pair of thread like connective emerge from brain in both sides around pharynx and fuse to sub-pharyngeal ganglia present in forth segment below pharynx. This connective is called peri- or circum-pharyngeal connective. Cerebral ganglia, circum-pharyngeal connective and sub-pharyngeal ganglion form the “nerve ring” around pharynx.
- Ventral nerve cord : It arises from sub-pharyngeal ganglia situated on mid and ventral position till end of the body. There is a ganglion in each segment on ventral nerve chord in form of enlargement. Ventral nerve cord is internally double in nature but it looks like single one. The two types of neuron, motor neurons and associated neurons are present in the ventral nerve cord, are responsible for contraction of body/ segments.
2. Peripherae nervous system : 8 to 10 lateral nerves are emerged from cerebral ganglion which pass through buccal cavity and prostomium. Also there are nerves from circum-pharyngeal connectives witch runs in buccual chamber and peristomium; whereas nerves from sub-pharyngeal ganglia are supplied to organs present from 2 to 4 segments. Three pairs of lateral nerves are also supplied by each segment ganglion
3. Sympathetic nerve system: an enlarged nerve plexus is present under epidermis, on elementary canal and within muscles of body wall. The plexuses are joined to peri-pharngeal connectives.