Animals with an incomplete digestive system are those in which the digestive tube has only one opening (cnidarians, platyhelminthes). Animals with a complete digestive system are those in which the digestive tube has two openings, mouth and anus (all other animal phyla, with the exception of poriferans, that do not have any digestive tube).

In animals with incomplete digestive tubes the digestion is mixed, it begins in the extracellular space and finishes in the intracellular space. In animals with complete digestive systems extracellular digestion within the digestive tube predominates.   _{Your Comment}