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These mesenteries and ligaments are continuous with the visceral and parietal layers of the peritoneum. For example, the falciform ligament of the liver is formed as the visceral peritoneum that covers the liver and is reflected onto the internal surface of the anterior abdominal wall.
An organ, which is surrounded by the peritoneum and has a supporting mesentery, is defined as an intraperitoneal organ (e.g. spleen, liver, and transverse colon).An organ, which is only partially covered by the peritoneum and has no supporting mesentery, is defined as a retroperitoneal
or extraperitoneal organ (e.g. pancreas, kidney, ascending colon, and descending colon).
Links and References:
Grant's: 2.22, 2.24
Netter: 252
Rohen/Yokochi: 273, 286, 287