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 



ha 



FIG. 131. Diagrammatic section of a vertebrate to show the relation of the body walls, 

 etc. av, aorta; c, ccelom; e, ectoderm; ep, epaxial muscles; g, gonads; ha, haemal arch; hp, 

 hypaxial muscles; i, intestine; mes, mesentery; n, nephridium; o, omentum; r, rib; so, 

 somatopleure; sp, splanchnopleure; v t 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 



