A major body division of the vertebrate trunk lying posterior to the thorax; and in mammals, bounded anteriorly by the diaphragm and extending to the pelvis. The diaphragm, found only in mammals, separates the abdominal or peritoneal cavity from the pleural and pericardial cavities of the thorax. In all pulmonate vertebrates (possessing lungs or lunglike organs) other than mammals, the lungs lie in the same cavity with the abdominal viscera, and this cavity is known as the pleuroperitoneal cavity.