Insects have an open system with a tubular heart that pumps blood into and receives blood from the hemocoel .
Mollusks also have a partially open system with both arteries and veins.
Vertebrates have increasingly complex hearts in their closed systems . Hearts Fish have a two-chambered heart while mammals and birds have four chambered hearts . Figure 46.4 illustrates the changes that occur in heart structure from the more primitive to more advanced vertebrates. Note in which organisms blood is mixed and in which ones it is kept separated. Figure 46.5 illustrates the structures of the human heart and associated blood vessels.