|As food is digested, the blood in the circulatory system picks it up for distribution to all the cells of the body. In simpler we have studied so far, the digested food had only to diffuse a short distance in order to reach all cell of the body. But in higher forms, the distances are greater, and more food material is needed by the many specialized and active cells of the body.
The blood of the earthworm moves through a series of closed tubes, or vessels. It flows forward to the anterior end in a dorsal blood vessel and moves to the posterior end in a ventral blood vessel.
Small tubes connect the dorsal and ventral vessels throughout the animal, except in the segments 7 and 11. Here there are five pairs of connecting tubes which are large and muscular. By means of alternate contraction and relaxation,they keep the blood flowing. Not true hearts, they are called aortic arches.