122 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
to that organ. The dorsal mesentery is usually more complete, but it is 
interrupted in various groups. Its regions are called mesogaster, mesentery 
proper, mesocolon, mesorectum, etc., accordingly as they support stomach, 
small intestine, colon and rectum. Except in the cyclostomes the alimentary 
canal is bent on itself and the bends are connected by similar membranes, here 
called omenta. These also have special names. Thus the gastrohepatic 
omentum (small omentum) connects stomach and liver; then there are gastro- 
splenic, doudeno-hepatic omenta, etc., while in mammals there is a great 
omentum, a double fold of mesogaster and mesocolon which connects the stomach 
YY 
AUT AAN \S 
Wy ee 
AAR , 
RR 
SSA = es y 
$o AVY _ Gs 
Fic. 131.—Diagrammatic section of a vertebrate to show the relation of the body walls, 
etc. av, aorta; c, coelom; e, ectoderm; ep, epaxial muscles; g, gonads; ha, hemal arch; hp, 
hypaxial muscles; 7, intestine; mes, mesentery; x, nephridium; 0, omentum; 7, rib; so, 
somatopleure; sp, splanchnopleure; v, vertebra. 
with the colon. This forms a large sac, the bursa omentalis, which opens into 
the rest of the body cavity by a small foramen of Winslow (foramen epiploicum) 
near the hinder end of the liver. 
Homologous structures are formed in connection with other organs. Thus 
in the formation of the heart there are formed temporary membranes, the meso- 
cardia, connecting it with the walls of the pericardium; while in the mammals a 
mediastinum, between the two pleural cavities binds the pericardium to the ventral 
body wall. Frequently the reproductive organs project so far into the body cavity 
that the serosa meets behind them, forming similar supports, mesovaria for the 
ovaries, mesorchia for the testes. 
The primitive body cavity extends from a point just behind the head 
back to the vent. It soon becomes divided into two cavities. Just 
in front of the liver a pair of blood-vessels, the Cuverian ducts, enter 
the heart from the sides. These arise in the ventral body wall but soon 
ascend, carrying the serosa before them. In this way they form a 
