THE FROG AND ITS RELATIVES 263 



are probably good, and its varied life on land and water necessarily 

 presents a wider range of experiences and hence some advance 

 in intelligence. 



Excretory System. Excretion is provided for by a pair of well- 

 developed kidneys with a large bladder. Water, uric acid, and 

 other nitrogenous waste are removed by these organs, while the 

 lungs and skin also help dispose of waste matter, particularly 

 carbon dioxide and water. 



Reproduction. As in the fish, the sexes are separate, and the 

 reproductive organs are easily found upon dissection. The ovaries 

 appear as masses of eggs, the size depending on the season of 

 year. The sperm glands of the male are small oval organs near 

 the kidneys. Both sets of organs have coiled ducts which eventually 

 connect with the posterior part of the intestine (cloaca) into 

 which the bladder also empties. 



It may be well to remember that in the frog we find systems of 

 organs adapted to perform all the life functions, and that in the 

 higher animal forms, few new structures are developed, but rather, 

 these are carried to a greater complexity or perfection. 



The following list illustrates this and would apply in general to 

 most vertebrate animals. 



1. Digestive system 



Mouth, tongue, teeth, throat cavity, salivary glands 

 Gullet and stomach, gastric glands 

 Intestine, small and large, rectum, and cloaca 

 Liver and gall sac, pancreas 



2. Respiratory system 



Nostrils, mouth cavity, glottis, and trachea 



Lungs, air cells, and capillaries 



Skin 



3. Circulatory system 



Heart, auricles, and ventricle 

 Arteries, aorta, etc. 

 Capillaries, and veins 

 Lymph vessels, and spleen 



