Nervous System of Earthworm

Ganglion of ganglia

It is the fused form of nerves. It is found in all segment.

The nervous system of earthworm is well developed. It consists of 3 parts:

1. Central nervous System
2. Peripheral nervous system
3. Sympathetic nervous system

1. Central nervous system

• This part of nervous system lies along the mid-line of the body.
• It consists of 2 parts: Nerve ring and Ventral nerve cord.
  

  a. Nerve ring

  It is ring like spherical structure which lies around the pharynx in 3^rd and 4^th segment.

          It has 3 parts:

         i. Supra-pharyngeal ganglia

• Also called cerebral ganglia.
• They are bilobed in structure.
• A pair of whitish pear-shaped supra pharyngeal ganglia fused to form brain.
• Lies dorsally in the 3^rd segment in the depression between the buccal cavity and the pharynx.ii. Circum-pharyngeal connectives/ Peri-pharyngeal connectives
• From either side of supra-pharyngeal ganglia, a pair of thick short band circum-pharyngeal connectives arise which enclose the pharynx.
• It is outer buldging part which extends from 3^rd to 4^th segment.iii. Sub-pharyngeal ganglia
• Found in lower region of nerve ring which lies in 4^th segment.
• Ventrally Circum-pharyngeal ganglia meet with a pair of sub-pharyngeal ganglia. Thus a complete nerve ring is formed around the pharynx.
  

   b. Ventral nerve cord

• It is long thread like double layered structure, which arises from the posterior part of sub-pharyngeal ganglia and runs the posteriorly up to the last segment of the body.
• Extends from 5^th to last segment.

          NOTE: Nerve cord are two in number but they are fused. So they are called double ventral nerve cord.

          It has swelling bulb like structure in each segment called segmental ganglion.

2. Peripheral nervous system

Those nerve which arise from different parts of central nervous system are called peripheral nervous system.

Following pairs of nerve arise from brain:

1. 8 to 10 pairs of nerve arise from brain which communicates or innervate or supply to prostomium, buccal cavity and pharynx.
2. 2 pairs of nerve arise from cirum-pharyngeal connectives which supply to the wall of prostomium and peristomium segment.
3. 3 pairs of nerves arise from sub-pharyngeal ganglia which supply to the 2^nd, 3^rd and 4^th segment.
4. 3 pairs of nerves arise from each segmental ganglion which supply to the various parts of segment, gut wall, body wall and other internal organ.

3. Sympathetic nervous system

          Sensory and motor nerve fuse to form nerve plexuses which moves ventrally through alimentary canal.
These plexuses are connected with the peri-pharyngeal connectives.

Nervous System of Earthworm

Tags: Central Nervous System, Circum-pharyngeal connectives, Nerve ring, Nervous System of Earthworm, Sub-pharyngeal ganglia, Supra-pharyngeal ganglia, Ventral nerve cord