Platyhelminthes are a category of organisms that are commonly known as flatworms, and are some of the most simple organisms around. These organisms have no circulatory or respiratory organs, forcing them to be flat. Flatworms also have only one entry and exit point for food and waste, meaning that they cannot eat food simultaneously. Many flatworms, nearly 50%, are parasites, and infect a certain host. Some, however, marine flatworms have outer pores and texture composed of bright and vibrant colors, and look quite pretty.