The Flatworm's cardiovascular system
In flatworms, there is no specific cardiovascular system or organs in order to transport nutrients and needed materials throughout the body. Instead, the flatworm's thin and moist body allows for materials to diffuse straight through their skin and be absorbed. The phylum Platyhelminthes does not have an individual circulatory or respiratory system, but they do have their own digestive system. Because of the lack of either a circulatory or respiratory system, flatworms are limited to the size and shape of being flat and quite long in order to allow oxygen to enter the body through diffusion, and carbon dioxide to leave the body through diffusion. Flatworms are typically capable of living in both marine environments, as well as dark lands that are very moist.