REPRODUCTION AND DEVELOPMENT OF ANIMALS 305 



cells, called cellulose, making a spongy rather than a pasty 

 mass. The digestive fluids can readily attack the starch, 

 while the cellulose becomes a part of the indigestible 

 residue. 



356. Blood. In all the higher animals there is a liquid, 

 the blood, that is driven through the body by a force pump, 

 the heart. The blood travels through a system of tubes 

 called blood vessels. This liquid is useful in distributing 

 food and oxygen among the cells, and in collecting and carry- 

 ing away from them the waste products from oxidation. 

 The blood carries the carbon dioxide and water vapor to the 

 breathing organs, and nitrogen compounds and water to the 

 kidneys. The kidneys of vertebrates are two organs 

 located inside the body near the anus, and opening by 

 tubes to the outside of the body. 



357. Body Temperatures. In the body of such animals 

 as worms, fishes, and frogs, oxidation is slower than it is in 

 many other animals, and so the heat generated in oxidation 

 does not raise the temperature of the blood perceptibly above 

 that of the water in which these animals live. For this rea- 

 son certain animals have commonly, though inaccurately, 

 been given the name " cold-blooded." The actual tempera- 

 ture of the body of a cold-blooded animal changes with the 

 temperature of its surroundings. 



In warm-blooded animals, the body^ maintains a certain 

 relatively high temperature all the time, regardless of the 

 temperature of the surroundings. The temperature of the 

 blood in the human body in health is slightly above 98 F. 

 In some birds it is higher than that. 



358. The Two Uses of Food. The oxygen and digested 

 foods that are distributed by the blood are taken up by the 

 different cells and are transformed into living protoplasm, 

 resulting in growth. Then as each cell does its work, part of 

 the substance is oxidized to release the energy for this work 

 and the temperature is kept at the degree required. 



