LIFE PROCESSES IN ANIMALS 275 



of labor, each kind of work in the body of an animal 

 is performed better than it otherwise would be. 



Tissues, organs, and systems. As soon as there is a 

 difference in the work which collections of cells have 

 to do, there is also a difference in the structure of the 

 cells. A collection of similar cells performing the same 

 function is known as a tissue (see page 196). Several 

 kinds of tissues may have certain functions to perform, 

 together. Such a group of tissues forms an organ. 

 Groups of organs working together form a system. 

 For example, a certain part of our body is used to pre- 

 pare the food so that it can be used. This is known as 

 the digestive system. It consists of a number of 

 organs such as the stomach, the liver, and the pancreas. 

 Each of these organs in turn is made up of several 

 tissues, and each tissue is made up of a number of cells. 



In the higher animals certain tissues are always 

 present. The more common ones are muscle, connective, 

 epithelial, bone, and nerve tissues. Each of these is 

 composed of cells of a certain structure and having 

 certain definite properties. 



Animal functions. An ameba, we know, carries on 

 certain processes. These same general processes are 

 carried on in the higher animals. The difference 

 between an ameba and a dog, for example, is not so 

 much in what each does as in the way that it is done. 

 In other words, there are certain general processes 

 which must be performed by all animals if they continue 

 to live successfully. 



