The advantages of the coelom in animals include the fact that the coelom, a cavity filled with fluid around the organs, provides a hydrostatic skeleton to aid movement, and allows for more efficient circulation of nutrients and removal of wastes. True coelems come in two varieties: schizocoelems and enterocoelems.
Prior to the development of a coelem, some animals relied on a solid layer of tissue between their exterior and gut as an anchor for movement. This meant that internal organs were embedded in solid material and could not move around to accommodate different positions. Many of these organisms also have incomplete digestive tracts, so that they must eat and release waste through the same opening. Many groups of animals developed what is known as a pseudocoelem, which includes a complete digestive tract and body cavity, granting many of the benefits of a true coelem.
A true coelem, unlike a pseudocoelem, has tissues that can form muscles next to the digestive tract, allowing them to push food along more efficiently. The two types of true coelem develop differently, but their end result is functionally identical. The biggest end difference, once the coelem has formed in an embryo, is that the openings that eventually become the mouth and the anus are opposite from the other type.