Animal

Animal, living organism distinguished from plants by locomotion, environmental reactivity, nutrient absorption, and cell structure. Animals move freely using a wide variety of mechanisms to do so, whereas plants are rooted to one place. Animals sense their environments and react to them. In the case of multicellular animals, they react by means of the nervous system; in more highly developed animals reactions to the environment are mediated by the nervous system combined with sense organs of touch, smell, taste, hearing, and sight. Plants may react to light, chemicals, and other stimuli, but such reactions are automatic and not nervous. Plants usually contain chlorophyll, which helps them build, from inorganic material, the organic substances of which they consist. Animals must consume organic food. And while both plants and animals are made up of cells, plants have a cell wall strengthened with cellulose (a woodlike substance), but animals contain no cellulose. Other minor distinctions include the ability of plants, lacking in animals, to periodically form new organs from undifferentiated cells.