It includes nerve ring and the nerve cord.
Nerve ring :
It is situated in the head around the oesophagus and consists of a pair of supra-oesophageal ganglia, a pair of circum-oesophageal connectives & a pair of sub- oesophageal ganglia. The supra-oesophageal ganglia are also called as brain and is formed by the fusion of three pairs of ganglia. The brain is a bilobed structure which has three parts-
- Protocerebrum
- Deuterocerebrum
- Tritocerebrum
The sub-oesophageal ganglia are also formed by the fusion of three pairs of ganglia. They remain situated below the oesophagus.
