30 GENERAL ZOOLOGY 



of the mouth is the tongue with its free, forked end pointed back 

 toward the esophagus. Between the spread tips of the tongue 

 is a slightly raised circular area with a longitudinal slit, the 

 glottis, which opens into the larynx. On either side of the floor 

 of the mouth in the male is a pit leading to a vocal sac, which is a 

 small pocket of the oral epithelium that can be expanded with 

 air. The lower jaw is without teeth. When the mouth closes 

 the lower jaw meets the upper jaw in a groove, the sulcus mar- 

 ginalis, which lies inside of the tooth-bearing margin of the 

 upper jaw. The tuberculum prelinguale is a rounded conical 

 prominence at the tip of the lower jaw. 



Live insects and worms form the principal diet of frogs. In 



capturing insects, the tongue, coated 

 with an adhesive secretion is extended 

 (Fig. 10), forked end foremost, and 

 then retracted with the prey sticking 

 to it. The nature of the teeth restricts 

 their function largely to grasping and 

 holding. The food moistened by the 

 saliva is swallowed, passing down the 

 Fig. 10.— Showing tongue of wide esophagus to the stomach and 



intestine where it is digested and ab- 

 sorbed. If a longitudinal slit is made in the abdominal region of 

 a frog, the cut passes through the skin and a thin layer of muscle. 

 The skin can be easily separated from the underlying muscle, 

 which is the real supporting element of the abdominal wall. 

 To expose the underlying parts between the forelegs it is necessary 

 to cut through the sternum which is part of the skeleton. Like- 

 wise the pubic portion of the pelvic girdle between the hind limbs 

 must be severed to follow the termination of the alimentary 

 canal. The cavity thus exposed is the pleuroperitoneal cavity 

 or coelom, in which lie the lungs, liver, pancreas, spleen, alimen- 

 tary tract, kidneys, and reproductive organs (Fig. 11). The 

 heart, it will be noted, is in a sac, the pericardium, which is really 

 a subdivision of the coelomic cavity. The wall of the coelom is 

 lined by a thin epithelium, the peritoneum or parietal peritoneum, 

 which is continued over the organs, in whole or in part, as an 

 investing membrane, the visceral peritoneum. The sac surround- 

 ing the heart, the pericardium, represents the parietal peritoneum 

 in that region. The small intestine is attached to the dorsal 



