In the body of earthworm there are two median longitudinal vessels. The dorsal longitudinal vessel runs above the alimentary canal. The ven-tral longitudinal vessel runs below the alimentary canal. The dorsal vessel is contractile and blood flows forwards in it. There are paired valves inside this vessel which prevent the backward flow of the blood.
The ventral vessel is non contractile and blood flows backwards in it. The ventral vessel has no valves. In the anterior part of the body the dorsal vessel is connected with the ventral vessel by eight pairs of commissural vessels or the lateral hearts lying in the segments 6 to 13. These vessels run on either side of the alimen-tary canal and pump blood from the dorsal vessel to the ventral vessel.
Fig: Circulatory System
The dorsal vessel receives blood from various organs in the body. The ventral vessel supplies blood to the various organs.