Primitive animals have just one body opening for their digestive tract. Food goes in through a mouth and is digested in the gut or digestive cavity. Wastes are expelled through the mouth.

A body plan with two openings for the digestive tract is more efficient. Most advanced animals have a mouth and an anus. The evolution of an anus was a major improvement over the single body opening. How would two body openings allow animals to evolve into bigger sizes?