Coelom is the name of internal cavity which is filled with a fluid and is lined by peritoneum, derived from mesoderm. It is either formed by splitting a mesoderm (known as schizocoelom as in annelids) or as outgrowth of embryonic gut (enterocoelom as in echinoderms and chordates). So coelom (body cavity) is a cavity found between mesoderm and endoderm (body wall).