Extracellular digestion takes place in a tubular cavity that extends throughout the length of the organism. All animals after flatworms have-a tubular alimentary organisation open at both ends. The development of extracellular digestion freed many animals from feeding continuously on small particles. They could now quickly ingest a few large chunks of food. The overall tubular organisation of the digestive tract also allows the food to travel in one direction passing through regions of digestive specialisation.
Figure: Generalised digestive tract. One way passage of food allows sequential stages in digestion. Dashed lines represent crop in some animals