Among flatworms with a distinguishable excretory system, the usual method for voiding waste is through flame cells that are arrayed along the sides of the worms' bodies and connected to main ducts that drain its tissues directly. Many flatworms lack a complete digestive system, as their primary method of extracting nutrients from the environment is simply to absorb them through their skin.
In vertebrates, the digestive system conveys nutrients to the circulatory system for distribution throughout the body. Waste excreted by the cells is picked up by the blood and carried back to the digestive system for elimination through the alimentary tract.
Some flatworms such as tapeworms absorb nutrients directly. This eliminates the need for a dedicated excretory apparatus. Other flatworms have a mouth and rudimentary digestive tract, but excrete through the same orifice that takes up food. Among the flatworms with more sophisticated digestive tracts, flame cells fulfill the role that an anus does in other groups.
Unlike vertebrates, flatworms with flame cells do not depend on a circulatory system to distribute nutrients and dispose of waste. Instead, the simple layout of their bodies makes it possible for the lateral ducts to collect cellular waste directly and convey it out of the body.