Three important parts of nervous system in annelidian phylum

It is of the typical annelidian type, consists of usual three parts:

(i) Central

(ii) Periphery

(iii) Sympathetic.

(i) Central nervous system:

Ventral haemocoeclomic channel encloses entire central nervous system.

It consists of (a) an anterior nerve ring (b) a ventral nerve cord and (c) a terminal ganglionic mass.

Image Source: 185.26.182.219

(a) Anterior nerve ring:

It consists of a pair of cerebral or supra-pharyngeal ganglia lying dorsally above the pharynx in 5^th segment.

It is connected with a ventral sub-pharyngeal ganglionic mass by a pair of lateral peripharyngeal connectives. Cerebral ganglia and sub-pharyngeal ganglionic mass represent the ganglia of first 5 segments.

(b) Ventral nerve cord:

It arises from the sub-pharyngeal ganglionic mass and extends mid-ventrally from 6^th to 26^th segment. It carries at wide intervals 21 well formed ganglia, each located in the first annulus of its own segment.

Nerve cord is made up of nerve cells and their process or nerve fibers and is covered by a sheath, neurilemma.

(c) Terminal ganglionic mass:

Posteriorly the ventral nerve cord ends in a large ovoid terminal ganglionic mass in the 26^th segment. It formed by the fusion of 7 pairs of ganglia of last 7 segments.

(ii) Peripheral nervous system:

It consists of paired nerves arising from ganglia of central nervous system.

A pair of stout optic nerves from cerebral ganglia and 4 pair of stout optic nerves from sub-pharyngeal ganglionic mass supply the 5 pair of eye orderly.

Other nerves arise from ganglionic mass supply rest of the organs of all 5 segments. In each segment two pairs of nerves, the anterior laterals and posterior-laterals arise from segmental gangia to supply entire viscera and body wall of its own segment.

Terminal ganglionic mass sends off several nerves supplying the receptor organs and other structures found in the posterior sucker.

(iii) Sympathetic or autonomous nervous system:

It consists of an extensive nerve plexus lying beneath epidermis within muscles and on gut wall. Nerve plexus have multipolar ganglionic cells that join to peripharyngeal connectives of both sides.