MODIFICATIONS IN THE FROG 153 



food passes backward into a U-shaped portion or stomach. 

 From this stomach the remainder of the tube or intestine 

 passes at first forward and then turns and extends straight 

 backward to the anus. Opening into this forward tube 

 are a number of blind tubes called pyloric cieca. There 

 is also a liver with a large gall bladder connected to this 

 part of the intestine by a duct. The cteca and liver secrete 

 digestive fluids which they pour into the alimentary tube. 



The principal advance of this form of tract over that of 

 the worm is seen in the two devices employed to increase 

 the digestive surface, namely, by the folding and by the 

 extension of the walls at points to form external glands. 

 Other adaptations to the new needs are seen in the devel- 

 opment of a tongue and teeth in the mouth, and in the 

 increase in size of the stomach portion. 



Modifications in the frog. — The most striking adapta- 

 tions in the digestive tract of the frog are the mouth 

 structures, the increased coiling of the intestine, and 

 the division of the tube into the stomach, small and large 

 intestine. In addition to the liver the frog also possesses 

 a pancreas. 



The mouth of the frog contains no teeth with sockets, but 

 there is a single series of projections around the upper jaw 

 which serve, as those of the fish do, to prevent the escape 

 ■ of the food when swallowing. In the frog, the tongue, 

 unlike that of the fish, is movable, and plays a distinct 

 part in swallowing. Unlike ours it is fastened at the 

 front with the tip pointing down the throat. The end is 

 sticky and can bo extended to catch insects, which it snaps 

 back into the throat. 



The lai'ge intestine ends in an enlarged space called a 

 cloaca into which open also the egg or sperm ducts and the 



