I think you should focus less on the history and more on the different types. You could go over the different animal phyla, describing who has an open circulatory system, who has closed ones, who doesn't have any and who has water vascular systems instead of a circulatory system. Then you can go to talk about the differences in the closed circulatory systems in chordates, explaining how some have two-chambered hearts and one circuit while others have 3 1/2 chambers and 2 1/2 circuits. You could also explain about xylem and phloem in plants and how this resemble circulatory systems in animals.