252 PRACTICAL ZOOLOGY 



One result of combustion is the production of heat. In man 

 and other mammals and birds the body is kept at an even tem- 

 perature by the oxidation of protoplasm regardless of how cold 

 the surrounding air is. These animals are " warm-blooded." 

 The frogs and other vertebrates and the invertebrates are all 

 " cold-blooded," since the little heat that is produced is lost 

 rapidly. Their bodies are usually about the same temperature 

 as their surroundings. 



Excretion. — The waste products resulting from the oxida- 

 tion of protoplasm are carried by the blood to the excretory or- 

 gans. The cells of these organs take the waste matter from the 

 blood and excrete it. Some of it is excreted by the skin, liver, 

 and intestinal walls, but most of it is taken from the blood in the 

 two kidneys. 



The kidneys contain a great many coiled tubes (uriniferous 

 tubules) into which the excretions pass. From these tubes they 

 are carried into one large tube, the ureter, in each kidney, and 

 from these into a thin-walled sac, the bladder (Fig. 142, B). 

 From time to time the walls of the bladder contract and force 

 the excretory matter out of the body through the anal opening. 

 Excretions are poisonous to the animal and must be removed 

 from the body. If an animal's excretory organs do not perform 

 their functions properly, serious sickness results. 



Secretion. — We have found that excretions are of no use to 

 the body, but are really injurious and must be cast out to avoid 

 sickness. Secretions, on the other hand, are of use to the animal ; 

 in fact, life would be impossible without them, since, for example, 

 without saliva, gastric juice, pancreatic juice, and bile, digestion 

 would be impossible and starvation would result, even where 

 food is abundant. 



Secretions are produced within special secretory cells. When 

 a number of these cells are grouped together by connective 

 tissue, the resulting organ is designated a gland. Glands of vari- 

 ous complexity are present in the body of the frog. In the skin 

 are simple, saclike mucous glands and poison glands (Fig. 144). 



