Flatworms digestive system is described as incomplete because, rather than them having a tube connecting to the mouth and anus they only have one hole in their body. A flatworms mouth is used to eat and to get rid of waste after it is digested and broken down so it may be descibed as a Sac-like system. Flatworms use little special cells to get rid of materials they do not need or want. These cells are called Flame Cells (also called solenocytes). Food goes through the mouth and is digested in a branched Gastrovascular Cavity. Anything that can't be digested is regurgitated back through the mouth. Flatworms also have developed a pharynx, which is a tube that is internal but can be strechted out externally to suck up prey. The pharynx pertrudes from the mouth (shown in the bottom right picture)