EXCRETION 
5 
nutrition, respiration, excretion, sensation (irritability), and 
reproduction. 
Motion. — All animals can move from place to place or 
move parts of their bodies. The higher animals move with 
ease as a result of highly developed muscular and nervous 
systems. The lower animals are more limited in their 
movements. The simpler plants move about in the water 
and at least the leaves of many of the higher plants 
move toward the sunlight, and the hop-vine and morning- 
glory not only grow but move in a certain direction around 
a pole. 
Nutrition. — This function includes the preparation of 
food so that the animal or plant may have it in the form of 
a solution. This is digestion. Next, the food must be taken 
into the veins of the animal or plant. This is absorption. 
Then the food must be moved to all parts of the animal or 
plant. This is circulation. And, lastly, each part of the 
animal or plant must take from the blood or sap the food 
that it needs. This is assimilation. Nutrition is the term 
under which are described the changes through which food 
passes from the time it enters into solution until it becomes 
a part of the living body of an animal or plant. 
Respiration. -— All animals and plants require energy in 
order to live. Part of this energy comes from the food and 
part from the oxygen used in the process known as respiration. 
As a result of respiration the needed energy is obtained and a 
by-product, carbon dioxide, is formed. Respiration, which 
takes place in every living cell, should not be confused with 
breathing, the process by which the higher animals get air 
in and out of their lungs. 
Excretion. — The formation of by-products in the bodies 
of animals and plants and their removal is called excretion. 
The skin, lungs, and kidneys in animals remove excretions. 
In plants they are not always removed but are isolated 
where they can do no harm. 
