It is well developed and consists of
central nervous system, peripheral nervous system and sympathetic nervous
system.
Central nervous system:-
It consists of nerve ring and ventral nerve cord.
Nerve ring consists of supra-pharyngeal ganglia, circum-pharyngeal connectives and sub-pharyngeal ganglia.
A pair of supra-pharyngeal ganglia
are present at the middle of 3rd
segment on dorsal surface of depression between buccal chamber and
pharynx. Each ganglion is small, round and white coloured structure and they remain fused with each
other at middle. They are also called as cerebral ganglia or brain.
From outer side of each
supra-pharyngeal ganglia a thick and stout circum-pharyngeal connectives arises
and pass posterior-ventrally encircling the pharynx and meet with
sub-pharyngeal ganglia.
A pair of sub-pharyngeal ganglia are
present ventrally below the pharynx in 4th segment. Each ganglion is
small, round and white coloured structure and they remain fused with each other
at middle.
Due to the presence of
supra-pharyngeal ganglia, circum-pharyngeal connectives and sub-pharyngeal
ganglia a complete ring like structure is formed around the pharynx and is
called as nerve ring or nerve collar.
Ventral nerve cord arises from sub-pharyngeal ganglia and runs mid-ventrally
upto posterior end of body. At the middle of each segment from 5th
segment , it remains enlarged to form segmental or ventral nerve cord ganglia.
Externally, it seems single but consists of two nerve cords each arising from
each sub-pharyngeal ganglia.
The ventral nerve cord consists of
nerve cells and nerve fibers. The fibers are present at center forming core of
fibers. Two cores of fibers are present in ventral nerve cord which proves
double nature of ventral nerve cord. These cores of fibers remain fused with
each other in segmental ganglia. The nerve cells are present on sides and under
surface of core of fibers. The number of cells is more in segmental ganglia.
Four giant nerve fibers are present dorsal to core of fibers. All these are
present in mass of connective tissue called as neuroglea and all these remain
inside a capsular structure called as epineurium outer to which a layer of
longitudinal muscles is present and externally it remains covered by
peritoneum.
Peripheral nervous system
It consists of
nerves arising from central nervous system. About 8-10 pairs of nerves arise
from supra-pharyngeal ganglia and pass into prostomium and buccal chamber. Two
pairs of nerves arise from circum-pharyngeal ganglia and they pass into
peristomium and buccal chamber. Three pairs of nerves arise from sub-pharyngeal
ganglia and each pair pass into 2nd, 3rd and 4th
segment respectively. Three pairs of nerves arise from each segmental ganglia
and pass in to different parts of that segment.
The nerves are of both types. Some are sensory or
afferent carrying information from
receptors up to central nervous system while some are motor or efferent
carrying information from central nervous system up to effectors.
Sympathetic nervous
system
It consists of network of nerves
present in muscles of body wall and muscles of alimentary canal. These network
of nerves i.e. nerve plexus remains connected with circum-pharyngeal connectives.