Coelom is internal body cavity filled with a fluids. It is lined by-peritonium and probably is derived from mesoderm, It is either formed by splitting a mesoderm (know as shizocoelom as in annelids) or as outgrowth of embryonic gut (enterocoelom as in echinoderms and chordates) so coelom (body cavity) is found between mesoderm and endoderm (body wall).