body. In Squalus, a second yolk sac is formed in tiie body 

 and this too shortens until it is lost. 



The development of the spiral valve involves torsion of 

 the gut lining within the mesodermal cover (Figure 9-24). 

 The first turns develop at the posterior end and extend for- 

 ward as counterclockwise rotation continues. This torsion 

 rotates the point of entrance of the bile duct to the dorsal 

 surface of the gut. Unlike the typical actinopterygian a sep- 

 arate midgut does not form; it is included in the spiral m- 

 testine. The liver arises as a ventral diverticulum as in the 

 higher forms, while the pancreas arises from dorsal and ven- 

 tral diverticula. 



Holocephalon Hydroiagus is without an identifiable stomach 

 or pyloric valve (Figure 9-25). The esophagus, with its ridged 

 epithelium, is constricted behind the pharynx and extends 

 from here back and downward through a short arc to open 

 directly into the intestine. The intestine receives the bile and 

 pancreatic ducts dorsally at its anterior expanded end There 

 is a spiral valve which begins at the entrance of the bile and 

 pancreatic ducts and extends, through four counterclockwise 

 turns, nearly to the anus. The dorsal mesentery is largely 

 missing, the ventral entirely so. A cloaca is lacking. There is 

 no rectal gland (digitiform gland), but in the submucosa of 

 the intestine are long branching glands which end blindly 

 anteriorly and open posteriorly through the mucous lining. 

 The two lobes of the liver are connected by a narrow band 

 ventral to the esophagus. The right lobe extends back to the 



-esophagus 



stomach 



pyloric caeca) evagination 



blood vessel/ Or 



B 



Figure 9-19. An early stage in the development of the gut of 

 tepisosteus, as seen in ventral view, A, and a later stage of pyloric 

 caeca development, B. (After Jocobshogen, 1937) 



posterior end of the body cavity and clasps the hindgut 

 from above in a deep groove. The gall bladder lies below 

 and to the right of the gut; its duct passes dorsally, then me- 

 dially, and back to enter the dorsal aspect of the anterior 



-pharynx 

 -esophagus 



bile duct- 



pylorus^ 



pancreasJ 



spiral valve-^ 

 in gut 



1 



-ovary 



■ liver 



folds of stomach 



lining 



-spleen 



, rectal gland 



.-Mijllerian duct 



-cloaca! aperture 



Figure 9-20. Ventral view of viscera of Squa/us with part of stomach 

 wall removed to show ridged lining. 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 273 



