Nervous system of Earthworm (Pheretima posthuma)

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Nervous System of Earth WormZoology | Biology
Nervous system of Earthworm (Pheretima posthuma)
For: Science Class 11
Unit: Detailed Study of Earth Worm

The nervous system of earthworm consists of three parts
a) Central Nervous system
It includes following structures:

Cerebral ganglia
There are two pear shaped cerebral ganglia fused together at 3rd segment called brain.
circumpharyngeal connective
Two circumharyngeal connectives arise from each cerebral ganglion laterally. They encircle pharynx and fuse at 4th segment. The fused portion is called sub pharyngeal ganglia.

Ventral nerve cord
It is white rod like structure, which starts running from sub- pharyngeal ganglia towards posterior end. In each segment, ventral nerve cord swells which is called segmental ganglia. Actually, there are two cords fuse together to form single ventral nerve cord. Ventral nerve cord is composed of nerve cells and nerve fibers. There are 4 giant fibers o mid dorsal side of nerve cord which conducts impulses rapidly. The outer covering of ventral nerve cord is called peritoneum.

b) Peripheral nervous system
It includes nerve fibers or nerve, which arises from central nervous system.

  • From cerebral ganglia, 8-10 nerves arise and supply to prostomium, buccal chamber, and pharynx.
  • From circumpharyngeal connectives, two pairs of nerves arise and supply to 1st and 2nd segment.
  • From subpharyangeal ganglia, three pairs of nerve arise and supply to 2nd, 3rd and 4th segment.
  • From each segmental ganglion, three pairs of nerves arise and supply to respective segment.

c) Sympathetic nervous system
It consists of nerve plexuses extensively branched and distributed beneath epidermis, alimentary canal that is connected to circumpharyngeal connectives.

Working of nervous system
The sensory cells from different parts of the body receive impulse. Sensory fibers (afferent fibers) carry impulse to central nervous system. Motor fibers (efferent fibers) carry impulse back to effective organs or different parts of body from central nervous system. In earthworm, impulse is also travel from one ganglion to another ganglion through ventral nerve cord.

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