404 



VERTEBRATE LIFE AND ORGANIZATION 



Subcutaneous 

 lymph Sa.C 



Dorsal a-orta 



Wolffian 

 duct 



Kidney 



StomacK 

 Pancreas 



S. common 

 bile duct 



Liver 

 Visceral peritone- 



Dorsolateral sktrLfold 

 Spinal cord 



3ubverfcebral 



lymph Sac 



Post. vena_cava. 

 Testis 



Mesenteries 



Small 

 inttsbine 



Duodenum 



Parietal 



peritoneum. 



Plearoperitoneal Cavity 



Ventral a.bdominaLvein. 



Figure 21.8. A diagrammatic cross section through the trunk of a frog viewed from 

 behind. At a more anterior level, a mesentery would pass to the stomach rather than 

 to the intestine. 



1 85. Digestive System 



Adult frogs are carnivorous and feed upon any animal small enough 

 for them to catch and swallow— worms, crustaceans, insects and the like. 

 Many of these are captured by a flick of the tongue, which is covered by 

 a sticky secretion. In this process the back of the tongue vaults over the 

 front, for the tongue is attached anteriorly (Fig. 21.9). The tongue is 

 protruded by muscular action and by a sudden filling of a lymph sac 

 at its base Food is held in the mouth by the teeth and then swallowed 

 whole. A lubricating mucous secretion, the tongue, the beating of micro- 

 scopic cilia on the cells lining the mouth cavity, and an inward move- 

 ment of the eyes all aid in swallowing. 



From the mouth cavity proper the food passes through the pharynx 

 (back of the apparent mouth cavity where food and air passages cross) 

 into a narrow esophagus. The esophagus is a short tube leading to the 

 stomach, where food is temporarily stored and its digestion initiated. 

 The stomach terminates in a muscular valve, the pyloric sphincter. 

 From the stomach a segment of the small intestine known as the duo- 

 denum passes anteriorly, receiving secretions from the liver and pancreas 

 by way of a common bile duct. The remainder of the small intestine 

 continues posteriorly in a number of convolutions, finally emptying into 

 the large intestine, or colon. Digestion is completed in the intestine 

 and the food is absorbed into the circulatory system. The large intestine 

 narrows posteriorly before entering the cloaca— a chamber receiving the 

 products of the digestive, excretory and genital systems. The cloaca 

 opens on the body surface through the cloacal aperture. 



The basic histology of the alimentary canal can be seen to advan- 

 tage in a cross section through the anterior part of the stomach (Fig. 

 21.10). Progressing from the coelom toward the lumen there is (1) the 

 visceral peritoneum, or serosa, consisting of a single layer of squamous 



