Islets of Langerhans, the insulin-producing cells observed 

 in the pancreatic tissue of higher forms, have not been ob- 

 served in the lungfishes. 



Actinopterygians 



Teleost The teleost fishes usually have a hook-shaped 

 stomach extended posteriorly by a caecum. This is the 

 case in the perch but not the salmon (Figure 9-13). The py- 

 loric section of the stomach extends forward on the right 

 ventrolateral aspect. There is a pyloric valve from which 

 the duodenal loop extends forward and then posteriorly. 

 The bile and pancreatic ducts enter the intestine near the 

 pyloric valve. Pyloric caeca are present in most teleosts; in 

 the perch there are three, opening at the pyloris, and many 

 in the salmon. In the salmon these open into the ventro- 

 lateral aspect of either side of the duodenum throughout its 

 anteriorly directed limb. These caeca form an irregular mass 

 in which the separate tubes can be discerned. 



The intestine is divided in the perch into anterior, includ- 

 ing the duodenum, and posterior parts; these are separated 

 by a distinct valve. In the salmon the valve is only slightly 

 developed. The posterior part differs in having its walls 

 thrown into occasional transverse folds. The gut opens to 

 the outside separate from the urinary or genital systems. 

 Lacking a cloaca is typical of the teleosts although there are 

 some exceptions. 



The liver is a single mass, somewhat larger on the left. 

 The gall bladder lies dorsal to the liver and on the right. 

 The pancreas is generally diffuse although scraps of this tis- 

 sue can occasionally be observed in the general region of 

 the entrance of the bile duct into the intestine. In the perch 

 the pancreatic acini are mixed through a large ventral mass 

 of lymphoid tissue in which the stomach and intestine are 

 embedded. This mass extends back to the anus. In most 

 teleosts the lymphoid tissue is more limited and forms 

 a coating for restricted areas of the gut or strands between 

 loops of the gut. In the salmon the pancreatic tissue forms 

 a thin film over and around the pyloric caeca, and some 

 strips along the bile duct, stomach, and intestine. 



The spleen is a dark, regular-shaped mass between the 

 stomach and the first loop of the intestine. The spleen is very 

 small in some fishes and may be displaced posteriorly. One 

 or two other small spherical masses, the islets of Langerhans, 

 are usually seen above the liver and to the right of 

 the esophagus or in the first loop of the gut. In the perch 

 two small islets lie to the left of the spleen. 



Po/ypferus The esophagus is fairly long and leads into an 

 expanded stomach with a long posterior caecum (Figure 

 9-14). The pyloric section of the stomach extends forward 

 and curves around to the right to enter the intestine. The 

 intestine has a ventral pyloric caecum and a spiral valve 

 which starts above the opening into the caecum. The bile 

 duct enters the gut in a pocket between valve and caecum. 



The valve makes about five and a half turns in a counter- 

 clockwise direction as it progresses posteriorly. It tapers down 

 and disappears on the right lateral wall one-third of the 

 length of the intestine in front of the anus. 



The liver is a single mass lying largely below the stomach. 

 The pyloric part of the stomach loops forward below a me- 

 dian liver lobe, while a long, tapered intestinal lobe of the 

 liver extends back and above and attaches to the intestine. 

 The gall bladder lies above the right margin of the liver and 

 anterior to the pyloric loop of the stomach. Its bile duct, 

 accompanied by blood vessels and pancreatic ducts, enters 

 the anterior end of the intestine dorsally. The pancreas is 

 diffuse, but a small mass is exposed between intestine and 

 stomach. Strands extend from here along the venous drain- 

 age from the ventral intestinal fat body and the stomach 

 wall. The pancreatic mass passes around the left side of the 

 gut to the area of the entrance of the bile duct. Pancreatic 

 tissue is scattered in the tissue around the bile duct and ex- 

 tends into the medial lobe of the liver and back along the 

 undersurface of the anterior third of the intestinal lobe. 



The spleen is a long band, triangular in section, lying 

 above and to the right of the stomach and extending down 

 between the stomach and intestine. A fat mass hangs by a 

 mesentery below the gut, and bilateral fat bands hang 

 from the body wall to either side of the median (right) lung. 

 Fat also rims the liver. The lung, attached to the dorsal wall, 

 extends the length of the body. 



A ventral septum is lacking below the liver and digestive 

 tract. The gut is attached to the body wall for a short dis- 

 tance anterior to the anus. 



Acipenser and Po/yodon The stomach oiAcipenser is of a sim- 

 ple tubular form, without a posterior caecum. In its course it 

 makes a complete circle, curving to the left (and downward), 

 then forward, and across to the right below the esophagus ( Fig- 

 ure 9-15). The pyloric end has thick muscular walls form- 

 ing a massive pyloric sphincter. The intestine is divided into 

 two segments. The anterior segment has a tubular pyloric 

 mass opening into it at its anterior end. The anterior intes- 

 tine curves to the left, then extends back as a closed loop 

 which enters the posterior intestine on the right. There is a 

 valve-like constriction between these two parts of the intes- 

 tine. The posterior section contains a spiral valve which 

 makes about seven counterclockwise turns in its posterior 

 course. The valve ends dorsally near the anal opening. 



In Poh'odon the anterior division of the intestine is S-shaped 

 but much shorter than in Acipenser. Its constricted entrance 

 into the posterior intestine is valve-like. There are about 

 seven turns in the spiral valve. 



The liver is bilobed, and the left lobe extends further back 

 above the pyloric loop of the stomach. In Polyodon this lobe 

 of the liver is not developed; thus, the right lobe extends 

 somewhat further back than the left. Both genera agree in 

 having the ventromedial margin of the right lobe notched 

 to expose the gall bladder. 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 267 



