Answer
The dorsal blood vessel runs along the length of the upper side of the earthworm. It contracts and pumps blood to the aortic arches. The ventral blood vessel runs along the lower length of the earthworm.
The top-side of an animal is called the dorsal surface. The dorsal surface of some species of earthworms is darker than its ventral surface.
The bottom-side of an animal is called the ventral surface, the ventral
surface is a lighter colour or shade than the dorsal surface.