THE FROG AND ITS RELATIVES 259 



A short gullet connects the large mouth cavity with the stomach 

 which is an oval enlargement of the digestive tube, set diagonally 

 in the body cavity and partly covered by the liver which is anterior 

 and ventral to it. Continuing from the stomach is the intestine, 

 of medium length, coiled, and enlarging near the vent into a short, 

 broad rectum and cloaca. The digestive tract is longer than that 

 of the fish, but the fingerlike projections (caeca) are lacking in 

 the stomach, the absorbing surface being increased by the coiled 

 intestine, instead. Connected with the food tube are the usual 

 digestive glands, the salivary and mucous glands in mouth and 

 gullet, gastric glands in the walls of the stomach, and the large 

 liver and smaller pancreas opening into the intestines. 



Here as usual we have the essential features of any vertebrate 

 digestive system: a tubular canal, provided with large extent of 

 surface for absorption by osmosis, and a series of glands which 

 secrete the fluids used to get the food into soluble form for this 

 absorption. 



Circulatory System. In so complicated an animal as the frog, 

 it would be expected that the circulatory system would need to be 

 better developed than in the fish, especially as the lungs are present 

 for the first time, to purify the blood. To provide for this added 

 burden, we find a three-chambered heart located well forward 

 in the body cavity, and consisting of two auricles and one muscular 

 ventricle. Extending from the ventricle is a large artery which 

 at once divides in two branches like a letter Y and each of the arms 

 again divides into three separate arteries on each side. The 

 anterior pair of these branches (the carotids) carries blood to the 

 head; the middle pair arch around to the back of the body cavity 

 and unite to form the dorsal aorta which supplies the muscles 

 and viscera; while the posterior (pulmonary) arteries carry the 

 blood to the lungs and skin for purification. 



The blood supplied to the muscles returns laden with carbon 

 dioxide and other oxidation products, while that going to the di- 

 gestive tract takes up the digested foods as well. It returns by 

 way of the veins, in part to the liver, and, finally, all to the right 

 auricle of the heart. Meanwhile the blood which went to the 



