Coelom - Metazoa
True coelom is a body cavity which arises within the embryonic mesoderm so that the cavity lies between the body wall (integument; ectoderm) and guts (endoderm) and is lined by mesodermal cells. This lining is called peritoneum in higher animals. Various internal organs are housed in the coelom and are lined by peritoneum along with many mesenteries thin membranes that keep the internal organs in place. The fluid inside the cavity acts as a shock absorber and gives rigidity to the body as well as functions in circulation, excretion and respiration in invertebrates.
Figure: Eucoelomate animals