tends along the ventral aspect of the gut posterior to the bile 

 duct. The spleen is an elongate body bulging to the left of the 

 dorsal mesentery above the middle of the stomach. 



Cryptobranchus has a long tubular stomach which ends pos- 

 teriorly at the pyloris. The dorsal mesentery has a gap in 

 the pyloric region, but the gastrohepatic ligament is com- 

 plete as is the falciform ligament. There is no ventral mes- 

 entery for the intestine. From the stomach the duodenum 

 extends anteriorly next to the stomach. The duodenal loop 

 is open. Its first section extends forward, its second limb 

 back. The remainder of the small intestine is thrown into a 

 series of short open loops before entering the large intestine. 



The liver is basically a single ventral mass with right and 

 left lobes. The gall bladder lies just to the right of the mid- 

 line in the notch between the lobes. There is no postcaval 

 lobe in either Cryptobranchus or Neclurus. There is a falciform 

 ligament which extends posteriorly along the ventral body 

 wall as a strand enclosing the ventral abdominal vein. The 

 duodenum is broadly joined to the liver and stomach by the 

 gastrohepatic ligament (mesentery). The pancreas is long 

 and thin and lies along the first part of the duodenum, on 

 the side away from the stomach. Patches of similar tissue lie 

 between the stomach and duodenum. A part of the pan- 

 creas lies dorsal to the gut, just behind the entrance of the 

 main bile duct. The two duodenal parts are joined by a 

 narrow lymphoid stem. 



The frog is similar to the salamander, but differs in that 

 the stomach is hook-shaped. The spleen lies in the dorsal 

 mesentery in the area of suspension of the looped intestine. 

 In some anurans the large intestine has an anterior caecum 

 (Hyla arborea). 



Embryological development Several stages of development 

 can be observed in commercially prepared serial and sagit- 

 tal sections of the frog. The entoderm forms an ovoid mass 

 in which the lumen of the gut is best marked in the pharyn- 

 geal region (Figure 7-7). In the 4-mm frog, the gut wall is 

 several cells thick; behind the pharynx it is many cells thick 

 below the lumen. All of the entoderm cells contain yolk and 

 this is particularly abundant in the more ventral cells. 



A stomodeal invagination of ectoderm forms the mouth. 

 The pharyngeal pouches form and a liver diverticulum ex- 

 tends from the pharyngeal space down into the mass of yolk- 

 filled cells. The posterior end of the gut is connected at first 

 with the neural canal through the neurenteric canal (4-mm 

 stage). This connection is next to the closing blastopore. The 

 hindgut cavity extends ventrally below the neurenteric 

 canal and lies near an invagination of ectoderm, the procto- 

 deum. The cloaca is formed by union of these (4-mm larva) 

 following breakdown of the separating membrane. The 

 bladder is an anterior evagination from the hindgut portion 

 of the cloaca. 



With the establishment of a circulatory system, liver 

 strands are formed. The thickness of the yolk-filled walls of 

 the gut is continually reduced until the columnar epithelial 



of the older larva is achieved, but there is still much yolk 

 in the cells of the looped gut of the 10-mm frog. 



The pancreas first appears as a small, posteriorly directed 

 diverticulum from the bile duct near its point of entrance 

 into the gut. This is the ventral pancreas. Later this divertic- 

 ulum divides into two and a dorsal diverticulum appears. 

 These three outgrowths join to form a single mass which uti- 

 lizes the ventral duct now opening just behind the bile duct. 



The gut of the larval frog is interesting in that it is much 

 elongated and forms a coil visible through the abdominal 

 wall. At metamorphosis the gut shortens and attains the 

 adult form. The difference in length is related to an algal 

 versus an animal diet. Development of the salamander ap- 

 pears to be similar to that of the frog but is more direct. 



In summary, the amphibian agrees in general with the 

 least modified condition observed in the reptile, as for ex- 

 ample in the alligator. This agreement suggests that this pat- 

 tern is indeed similar to the ancestral vertebrate and that 

 the rotated gut of the mammal, turde, or bird is a speciali- 

 zation. 



Choanate fishes 



Actinistian Latimena has a wide esophagus opening into a 

 large stomach. This extends back to a tapered caecum 

 which reaches to the posterior end of the body cavity. The 

 pyloric region lies below the esophageal part and extends 

 anteriorly to the pyloric valve. 



The pyloric valve opens upward into a small chamber, 

 the bursa entiana (see Figure 9-21 C), which in turn opens 

 into the intestine extending posteriorly above the pyloric 

 section and to the right of the caecum of the stomach. The 

 intestine has a roll-type valve (see Figure 9-22) which makes 

 nearly 20 turns and is attached to the ventral wall of the in- 

 testine. Behind the valve is a short section of gut extending 

 back to the cloaca. The cloaca extends upward behind the 

 point of entrance of the rectum, where it receives the uro- 

 genital papilla. 



The liver has two large lobes: the right extends back about 

 two-thirds of the body cavity length; the left is smaller 

 There is a gall bladder on the right. The pancreas is a 

 band, triangular in section, lying along the right side of the 

 pyloric region of the stomach and below the intestine. There 

 is much hemopoietic tissue in the pancreas. The spleen is a 

 compact body lying in the mesentery above the intestine and 

 to the right of the stomach. 



Oipnoan Protoplerus lacks a distinct stomach (Figure 9-12). 

 There is a constricted esophageal region into which opens 

 the ventral glottis. Behind this is a slight enlargement dis- 

 placed to the left; this can be viewed as a part of the stom- 

 ach. From here the stomach leads back to the pyloric valve, 

 which is a flap separating the stomach cavity from the in- 

 testine. The intestine contains a typhlosole, or spiral valve, 



264 • THE VISCERA 



