Ventral nerve cord

The ventral nerve cords make up the nervous system of some phyla of the invertebrates particularly within the nematodes, annelids and the arthropods. It usually consists of cerebral ganglia  anteriorly with the nerve cords running down the ventral plane of the organism. This characteristic is important in qualifying the difference compared to the chordates which have a dorsal nerve cord.

Ventral nerve cords from anterior to posterior (the thoracic and abdominal tagma in the arthropods) are made up of segmented ganglia that are connected by a tract of nerve fibers passing from one side to the other of the nerve cord called commissures. The complete system bears some likeness to a rope ladder. In some animals the bilateral ganglia are fused into a single large ganglion per segment. This characteristic is found mostly in the insects.