Nervous system of Leech

The nervous system of Leech

The nervous system of Leech is basically annelidan but it shows advancement over the other annelids. It shows central, peripheral and autonomic nervous systems.

 

1. Central nervous system:

It is enclosed in the venteral haemocoebmic canal. It shows.

 

i) Cerebral ganglia and nerve ring

ii)Nerve cord

iii) Terminal ganglionic mass

The ganglia are made by nerve cells.

 

i) Cerebral ganglia and nerve ring: On the pharynx in the fifth segment a pair of cerebral of supra' pharyngeal ganglia are present. They are fused. It is called brain.

Below the pharynx in the fifth segment sub-pharyngeal ganglionic mass is present. It is formed by the union of the 4 pairs of ganglia.

The sub-pharyngeal ganglionic mass and cerebro (or) supra pharyngeal ganglionic mass will be connected by two circumpharyngeal connectives. Thus, a nerve ring is formed around the pharynx.

 

 

 

Hirudinaria-nervous system

 

ii) Nerve cord: From the subpharyngeal ganglionic mass the nerve cord

starts. It runs backwards. It is mid ventral in position. It is present below the

posterior sucker. It wjll join the terminal ganglionic, mass present near the

posterior sucker.

 

The nerve cord shows 21 ganglia, one ganglion in the first annulus of each segment from 6th to 26th segments of the body: Each ganglion is formed by the union of 2 ganglia. They give nerves or peripheral nerves. The nerve cord is made by nerve cells and nerve fibres.

 

iii) Terminal ganglionic Mass: In the posterior sucker of leech

terminal ganglionic mass is present. It is the fusion product of 7 pairs of ganglia.

 

 

2. Peripheral nervous system:

From the ganglia of the central nervous system many nerves will arise. These will constitute the peripheral nervous system.

i) One pair of nerves will arise from the front side of brain and go to first pair of eyes and prostomium.

 

ii) Four pairs of nerves arise from the sub-pharyngeal ganglionic mass. They go to 2, 3, 4, 5th pairs of eyes.

iii) Two pairs of nerves arise from each ganglion, and go to the body parts.

iv) Seven pairs of nerves will arise from terminal ganglionic mass and they go to posterior sucker.