Final answer:
Cnidarians and flatworms have an incomplete digestive system featuring a gastrovascular cavity with a single opening and utilize both extracellular and intracellular digestion.
Step-by-step explanation:
The characteristic of the digestive system in cnidarians and flatworms is that they both have an incomplete digestive system. This type of system, known as a gastrovascular cavity, consists of a single opening that serves both as the mouth for ingestion and the anus for egestion. The process within this cavity involves extracellular digestion, where enzymes are secreted into the cavity to break down food, followed by the nutrients being absorbed by the cells lining the cavity. Some degree of intracellular digestion also occurs as these cells take in and further digest the food particles.