The ingested food is broken down to small food particles by this process. Since the process takes place in a cavity within the animal, rather than within the animal s cells, this is called extracellular digestion. The small food particles are then taken into the gastrodermis cells by endocytosis. Within food vacuoles formed by endocytosis, the food particles are further broken down to food molecules. These food molecules diffuse or are actively transported into the cytoplasm of the gastrodermis cell.
Food is distributed from the gastrodermis to other cells of the body by diffusion primarily.
Since flatworms are small distribution of food by diffusion is possible. This is released from the planarian through the mouth. The planarian has a system of tubules extending throughout its body from the anterior to posterior on both sided of the body. These tubules are called protonephridia and they collect both excess water and much of the nitrogen waste. The tubules have pores that lead out of the body.