An earthworm's ventral nerve cord carries signals from the brain to the body and allows for the coordination of the ganglia in each segment past the fourth. Ganglia are clusters of nerve cells, and an earthworm's brain is an organ made from the fused ganglia of the first three segments.
Each ganglion along the earthworm's ventral nerve cord is capable of doing its own processing and giving instructions to its particular segment, as well as coordinating with other ganglia along the cord. The brain is the origin of the cord, toward the front of the worm. It is shaped in a ring and occupies the third and fourth segments of the worm. The ring goes around the forward portion of the worm's gut.
Along the ventral nerve cord, each segment branches out with six short nerves to the gut that then branch further to the muscles along the body wall. These nerves do not connect or communicate with the nerves of any other segment, as this only occurs through the ventral nerve cord. These nerves both give instructions to the muscles and receive sensory data. Earthworms have no eyes, but they have light sensors along their back, sides and surrounding both ends of their bodies.