What is the evolutionary advantage of having a true coleom (as seen in the annelids?)?

Question

And why?

Answer 1

According to this site:
http://www.britannica.com/eb/topic-124168/coelom

"The advantage of a true coelom is the ability of the inner mesenteric (mostly connective tissue) layer to suspend the central gut in the middle of the animal. Otherwise, in those animals with a body cavity used in locomotion, gravity would pull the gut down and severely curtail body size. Coelomates have attained vastly larger body sizes than has any other group of animals. "