Nervous System of Earthworm: Pheretima posthuma 

In Earthworm, the Nervous system is well developed and comprises three parts,

1. The central nervous system.

2. The peripheral nervous system.

3. The sympathetic nervous system.

 The Central Nervous System of Earthworm :

The central nervous system of earthworm is formed by,

a. The nerve ring on the anterior end,

b. The ventral nerve chord after that, continuing to the posterior end.

Nerve ring :

The Nerve ring is formed by,

• Cerebral ganglia,
• Circumpharyngeal connectives, 
• Subpharyngeal ganglia.

 Cerebral ganglia: 

The cerebral ganglia is also called " The brain". It is a pair of closely united, white-coloured and pear-shaped ganglia. It is also called supra-pharyngeal ganglia.

Location: It is located on the dorsal side in the depression formed in between the buccal cavity and pharynx.

Circumpharyngeal or peri-pharyngeal connectives:

 These pairs of thick, stout connectives of the ring, arise from the lateral side of the cerebral ganglia embracing the pharynx.

Sub-pharyngeal ganglia:

The connective cerebral ganglia meet ventrally as a pair of fused sub-pharyngeal ganglia.

Location: Beneath the pharynx in the fourth segment.

Thus a complete nerve ring is formed embracing the pharynx.

Nervous System of Earthworm

Ventral nerve cord :

Location: The ventral nerve chord runs beneath the gut close to the ventral wall of the coelom, from the 4th segment to the last segment of the body.

The sub-pharyngeal ganglion ( on the ventral side ), bifurcates into the circumpharyngeal ( circumoesophageal ) connectives, which pass anteriorly around either side of the oesophagus and meet as the bilobed cerebral ganglion on the dorsal surface of the pharynx in 3rd segment. 

This sub-pharyngeal ganglion on the ventral side posteriorly continues as the ventral nerve cord which passes through each segment, forms segmental ganglions as slightly enlarged areas.

The ventral nerve cord of Pheretima anatomically seems to be single but in the T.S. it shows two compactly united right and left cords. Also, each segmental ganglion represents the fusion of a pair of ganglia, one belonging to each cord of the double ventral nerve cord.

 Histology of the Nerve Cord of Pheretima posthuma :

     The nerve cord comprises nerve fibres and nerve cells. Externally, it is covered by a layer of visceral peritoneum, beneath which lies a thin layer of longitudinal muscle fibres. Surrounding this structure is a fibrous capsule known as the epineurium. 

T.S. Ventral Nerve Cord of Pheretima

Observing the Transverse - Section of the cord, two distinct cores can be observed within the core, highlighting the dual nature of the nerve cord.

At the segmental ganglia, these two cores of nerve fibres merge completely along the midline. 

Nerve cells, situated on the sides and beneath the cores, are categorized into two types: motor neurons and association neurons. 

The concentration of nerve cells is notably higher in the ganglia. Both nerve cells and fibres are embedded within a connective tissue matrix called neuroglia. 

     Dorsally, four large fibres one median, one submedian, and two lateral fibres extend through the connective tissue, running the length of the entire nerve cord. 

These functions are essential for facilitating rapid and forceful contractions of the body in response to stimuli. They are filled with a homogeneous, plasma-like fluid that aids in their contraction capabilities.

Peripheral Nervous system :

     Each cerebral ganglion laterally emits 8 to 10 nerves that innervate the prostomium and buccal chamber.

     In addition, nerves originating from the sub-pharyngeal ganglia supply structures located in the second, third, and fourth segments. 

     Every segmental ganglion of the ventral nerve cord generates three pairs of lateral nerves: one pair positioned anteriorly and two pairs situated posteriorly to the row of setae. 

These nerves innervate the gut wall, body wall, and various internal organs within their respective segments. 

The nerves are of a mixed type, comprising both afferent (sensory) fibres and efferent (motor) fibres.

Sympathetic Nervous System :

The sympathetic nervous system features an extensive network of nerve plexuses located beneath the epidermis, within the body wall muscles, and along the alimentary canal. These plexuses are interconnected with the peri-pharyngeal connectives.

How the Nervous System of Earthworm work?

The activities of earthworms are primarily regulated by their nervous system, although this does not exclusively involve the brain. 

The nervous system comprises both afferent and efferent fibers. 

Afferent fibers originate from sensory cells or receptor organs located in the epidermis and extend to form fine branches within the ventral nerve cord. Nearby these branches, efferent fibers emerge, creating synapses and running outward to connect with muscles. 

When stimuli or sensory impulses are detected, they are transmitted from the receptors via afferent fibers to the ventral nerve cord, which then relays motor impulses through efferent fibers to the muscles, resulting in contraction. 

Additionally, an adjuster neuron may be present between sensory and motor neurons, allowing impulses to be redirected to the opposite side of the same segment or to different segments, thereby facilitating coordination among various body parts.

 Furthermore, giant fibers within the nerve cord transmit impulses more swiftly than other fibers, enabling rapid movements of the entire body in response to strong stimuli at a specific point.