Flatworms are the simplest of the worm group. There are about 20,000 species in this group and they are found in many places. Flatworms are found in fresh water. A flatworm has a body shaped roughly like a bag, like the earlier hydras. There is no specific cardiovascular system or organs in order to transport nutrients and needed materials throughout the body. The flatworm body allows materials to diffuse straight through their skin and be absorbed. Flatworm 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.