The coelom is defined as a body cavity or space than runs the length of a vertebrate. It separates the body into an inner tube and an outer tube. The coelom forms when the lateral plate mesoderm splits.

 I. Divisions

 In the early stages of embryological development, the coelom is a continuous cavity along the entire length of the trunk divided into right and left halves by the dorsal and ventral mesenteries. In the adult vertebrate, the coelom is divided into two or more cavities. How the coelom is further subdivided varies depending upon the taxa being considered. In an overly simplistic way (from a phylogenetic perspective), the coelom can be subdivided as follows:

 A) Fish:

  1) Pericardial Cavity -- surrounds the heart

  2) Pleuroperitoneal Cavity -- surrounds all other viscera


B) Mammals:

  1) Pericardial Cavity -- surrounds the heart

  2) Pleural Cavity -- surrounds the lungs

  3) Peritoneal (Abdominal) Cavity -- surrounds all other viscera


II) Linings

 The coelom is lined by tissue derived from mesoderm. This lining is generally called peritoneum. However, as development continues and the coelom is subdivided, different names are given to this lining based on its location and what it covers. This lining is subdivided as follows:

 A) Lining of the Pericardial Cavity

  1) Visceral Pericardium -- covers heart

  2) Parietal Pericardium -- outside visceral pericardium


B) Lining of the Pleural Cavity

  1) Visceral Pleura -- covers lung

  2) Parietal Pleura -- lines thoracic cavity (chest wall)


C) Lining of the Peritoneum

  1) Visceral Peritoneum -- covers abdominal viscera

  2) Parietal Peritoneum -- lines abdominal cavity (body wall)