(Gk, koiloma. A hollow) The main body cavity of animals, formed between layers of mesoderm, in which internal organs are suspended. A secondary body cavity of vertebrates and most invertebrates, having a membranous lining composed of specialised cells containing the internal organs; developed between the mesoderm layers and forms, peritoneum and pericardial cavities. A similar secondary body cavity found in members of lower invertebrate phyla, such as roundworms and rotifers; it develops within the mesoderm and serves some of the functions of a true coelom