All living things, whether plants or animals, are made up of tiny particles called cells. There is a separate article about the cell in this encyclopedia. Here we will pause only to explain that the cells that form animals are usually very small-it would take several thousand of them to measure an inch, so that they cannot be seen without a microscope- and that many millions or billions of them may be needed to make the body of a single animal. The simplest of all animals may have only a single cell.
Such animals are called protozoans. Most of them live in water, and some have shells, even though they are so small themselves. Many are parasites, which means they cannot live by themselves, but must live in the bodies of other animals, or in plants. The article on ameba, in another volume of this encyclopedia, tells how one of these one-celled animals lives. Some protozoans, small as they are, are very important as food for other animals; they are found in "plankton," which is a mass of tiny plants and animals drifting in the sea. Plankton is the chief food of the gigantic whale, and human beings could eat plankton without harm. Most animals, unlike the protozoans, are formed not only of many cells but of many different kinds of cell.
Cells of the same kind form a particular kind of solid matter, or tissue, in an animal's body. For instance, in the human body the nerves are made of nerve tissue, which is formed of nerve cells; and in the same way different cells form the tissue of our muscles, bones, skin, and so on. The cells in our body are constantly dying and being replaced by new cells that are made of the food we eat. For instance, your skin flakes off in tiny particles, often so small you do not see them, while new skin grows underneath. Losing old cells and growing new ones is part of the process we call "living."