Cnidarians have very unusual digestion. It is only a one way system so any food, digested or not, goes through the same hole. After the mouth, the "stomach" of cnidarians is the gastrovacular cavity. This is where the actual digestion takes place.
Extracellular enzymes break down the food into small pieces inside the gastrovacular cavity. The rest of the digestion happens inter cellularly. (http://bio.sunyorange.edu)
Anything that is not digested then exits through the mouth of the cnidarian.