Final answer:
Retroperitoneal organs lose their mesenteries during development and become positioned behind the peritoneal cavity, against the dorsal body wall. These include the pancreas, duodenum, and portions of the large intestine.
Step-by-step explanation:
The organs mentioned in the question are referred to as retroperitoneal organs. These are structures in the abdominal cavity that originally have mesenteries; connective tissues which suspend the organs from the dorsal wall and contain blood vessels, lymphatics, and nerves. However, as development progresses, these organs lose their mesenteries, becoming pushed against the dorsal body wall, and exist behind the peritoneal cavity. The peritoneum wraps the abdominal organs and consists of both the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelops the abdominal organs. Notable retroperitoneal organs include the pancreas, duodenum, ascending and descending colons, and the rectum. These structures are characterized as such because they lie outside the peritoneal cavity, behind the visceral peritoneum.