A surface mesothelium and underlying connective tissue is universally apparent. It is lined by a layer of one cell thick mesothelium. Nerve supply is found underneath the mesothelium. The body is divided into coeloms, compartments lined with mesothelium. It is also present in the mesothelium of the thoracic cavity and the parietal pericardium. The enclosing mesothelium contains macrophages, lymphocytes and mast cells. In some animals, it is lined with mesothelium. All three of these phyla have a coelom, an internal cavity lined by mesothelium. The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes. Perimetrium consists of superficial mesothelium, and a thin layer of loose connective tissue beneath it. Mesothelium is the epithelial component of serosa. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.