THE SIMPLEST LIVING UNIT A CELL 157 



called proteids, a familiar example of which is the white of egg 

 (albumen). Very little is known of the exact chemical structure 

 of the numerous proteids. Their molecules are very complex, 

 for they contain a large number of elements of remarkably 

 varied chemical properties, carbon, nitrogen, oxygen, hydro- 

 gen, sulphur, and in some cases phosphorus. But besides the 

 proteids and many other organic compounds (substances usually 

 formed only in association with life processes, as, for example, 

 the sugars, starch, and fats), protoplasm also contains certain 

 necessary inorganic substances, such as salts of sodium, potas- 

 sium, calcium, magnesium, and iron, and in addition to these a 

 very large amount of water. 



Although we know very little about the chemical nature 

 of protoplasm, certain remarkable structural peculiarities have 

 been recognized for a long time. Protoplasm always exists in 

 the form of units which are called cells. The simplest organisms 

 consist of solitary units, and are consequently termed one-celled 

 (unicellular). The higher organisms are made up of aggregates 

 of cells, and are termed many-celled (multicellular). 



The cells in many-celled organisms have each a separate indi- 

 viduality, but they are usually set apart for particular kinds of 

 work and depend upon one another for mutual assistance. The 

 many-celled organism has been termed a cell republic, because 

 all the cells, as individuals, work for the common good of the 

 community, and by a system of helpful division of labor benefit 

 one another. 



There is a large group of one-celled microscopic animals called 

 the Protozoa. This constitutes the lowest division of the animal 

 kingdom, and is quite distinct from the groups of many-celled 

 animals, although the latter are believed to have been derived 

 from it. There are likewise numerous one-celled plants, but 

 they are related to the higher many-celled forms by very com- 

 plete and interesting connecting links, so that botanists do not 

 make a group of one-celled plants, and can readily understand 

 the evolution of the many-celled forms from the single-celled. 



