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In contrast, animals do not produce their own food but must eat other organisms to obtain it. Animals are generally more complex structurally. Unlike plants, they have nerves and muscles that aid in rapid, controlled movement around their environment. Animal cells usually do not have rigid walls like those of plants. This accounts for the fact that your skin and flesh are flexible and the trunk of a tree is not.
This simple dichotomy between plants and animals is not adequate to encompass all life forms. Some organisms have characteristics which do not qualify them to fit neatly into either kingdom. For instance, funguses and most bacteria do not photosynthesize and most of them lack a means of controlled locomotion. Some organisms have attributes of both plants and animals. For instance, there is a group of common single-cell species living in fresh water ponds called Euglena that photosynthesize and have their own means of locomotion (whip-like tail structures called flagella). Because of these and other exceptions, new kingdoms of living things had to be created.
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