In vertebrates, extensions of the serosa layer of the GI tract help to support the digestive organs.  In humans, these consist of the greater omentum, lesser omentum, falciform ligament, mesentery, and mesocolon.  In some primitive coelomates (Phoronida), mesenteries support the intestine.  (Hickman, p. 288).  In hagfish extensions of the peritoneum exist which surround and support the organs of the abdominopelvic cavity (Romer, p. 318-9).  By the appearance of gnathostome fish, these extensions of the peritoneum are identifiable as the greater and lesser omenta (from the greater and lesser curvatures of the stomach), the falciform ligament (attaching the liver dorsally), the mesentery (to small intestine), and the mesocolon (to the large intestine).  As the small intestine became more coiled in tetrapods, so did the mesentery  (Romer, p. 318-9).  In higher tetrapods, the greater omentum may store adipose.

HAGFISH

LAMPREY

SHARK

GAR

PERCH

SALAMANDER

FROG

TURTLE

ALLIGATOR