Incomplete vs complete digestive tracts.
Medium-sized and larger animals (as compared to fungi and things like that) have more appropriately sized digestive systems. Those with more primitive digestive systems are animals that have a gut with just one opening that serves as both mouth and anus. These are called incomplete digestive systems. An example of an animal with this type of digestive tract is the jellyfish.
Increasing in complexity are animals that have gut tubes, where foods are digested and the good stuff is absorbed, with a mouth at one end and an anus at the other. Although simple, this type of system is complete. An example of an animal with this type of system is a worm.
So, what’s the benefit of a complete digestive system? A “one way” system allows food that was just eaten to be digested before it gets pooped out along with materials taken in earlier. An organism with a complete system does not have to take in food constantly to replace food that was pooped out before nutrients could be acquired.