dorsal aorta complex 



ventral aorta 



left anterior cardinal 



branchial sac 



phoryngocutaneous duct 



right duct of Cuvier 

 inferior jugular, 

 ventricle 



cystic vein 



coeliac artery v/ith hepatic branches 

 bile duct 



superior mesenteric vein 



nearly ripe egg in ovary 



efferent branchial ducts 



atrium 



anterior hepatic vein 



posterior hepatic portal 



mesovarium 



eft duct of Cuvier 



anterior hepatic portal 

 posterior hepatic vein 

 anterior bile duct 



gall bladder 



posterior bile duct 



liver 



gastrohepotic ligament (cut) 



nternal intestinal vein 



Figure 9-28. Anterior end of viscera of Myxine as seen in ventral view. The liver is displaced to 

 the reader's right. (After Marenelli and Strenger, 1956) 



condition. Diffusion of the pancreatic tissue in the actinop- 

 terygian appears to be related to the appearance and devel- 

 opment of pyloric caeca. The origin of the pancreas in the 

 several groups differs only in the number of diverticula; 

 usually there are dorsal and ventral anlagen. In the lamprey 

 a large number of tubules form, but these do not appear as 

 evaginations of the gut wall. The manner of origin in this 

 type is probably derived rather than primitive. 



The liver is remarkably constant in its relationships with 

 the gut and in having a gall gladder The association of bile 

 duct and pancreatic ducts is fairly constant. The segmen- 

 tation of the gut into esophagus, stomach, and intestme is 

 also consistent. The occurrence of two types of spiral valves 

 suggests that at first there was only a simple typhlosole. 

 Surface area was increased, for more efficient absorption, by 



growth of the free margin so as to form a rolled sheet or by 

 increase in length so as to cause spiraling. In more advanced 

 forms the spiral valve part of the intestine was reduced and 

 a "small intestine" of increasing length developed. In the 

 teleost the spiral valve is undeveloped and the intestine has 

 small and large divisions separated by a valve. This devel- 

 opment represents a parallelism to the situation in the tet- 

 rapod. 



In the fishes the spleen does not have the same relation- 

 ships that it has in the tetrapod, although that of the shark 

 joins these extremes. The shark is peculiar in having a rectal 

 gland, and comparisons of the shark and holocephalan are 

 no better in this area of anatomy than they are elsewhere. 

 Loss of the stomach in the holocephalan is a parallelism to 

 the situation in Prolopterus or the cyclostome. In the latter 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 279 



