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indeterminate growth, Epithelial tissue, Arctic tundra, multicellular organism, peregrine falcon

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>  Animals in the Balance of Nature

Animal, multicellular organism that obtains energy by eating food. With over 2 million known species, and many more awaiting identification, animals are the most diverse forms of life on earth. They range in size from 30-m (100-ft) long whales to microscopic organisms only 0.05 mm (0.002 in) long. They live in a vast range of habitats, from deserts and Arctic tundra to the deep-sea floor. Animals are the only living things that have evolved nervous systems and sense organs that monitor their surroundings. They are also the only forms of life that show flexible patterns of behavior that can be shaped by past experience. The study of animals is known as zoology.

Animals are multicellular organisms, a characteristic they share with plants and many fungi. But they differ from plants and fungi in several important ways. Foremost among these is the way they obtain energy. Plants obtain energy directly from sunlight through the process of photosynthesis, and they use this energy to build up organic matter from simple raw materials. Animals, on the other hand, eat other living things or their dead remains. They then digest this food to release the energy that it contains. Fungi also take in food, but instead of digesting it internally as animals do, they digest it before they absorb it.

Most animals start life as a single fertilized cell, which divides many times to produce the thousands or millions of cells needed to form a functioning body. During this process, groups of cells develop different characteristics and arrange themselves in tissues that carry out specialized functions. Epithelial tissue covers the body’s inner and outer surfaces, while connective tissue binds it together and provides support. Nervous tissue conducts the signals that coordinate the body, and muscle tissue–which makes up over two-thirds of the body mass of some animals–contracts to make the body move. This mobility, coupled with rapid responses to opportunities and hazards, is one feature that distinguishes animals from other forms of life.

Some kinds of animal movement, such as the slow progress of a limpet as it creeps across rocks, are so slow that they are almost imperceptible. Others, such as the attacking dive of a peregrine falcon or the leap of a flea, are so fast that they are difficult or even impossible to follow. Many single-celled organisms can move, but in absolute terms, animals are by far the fastest-moving living things on earth.

Animal life spans vary from less than 3 weeks in some insects to over a century in giant tortoises. Some animals, such as sponges, mollusks, fish, and snakes, show indeterminate growth, which means that they continue to grow throughout life. Most, however, reach a pre-defined size at maturity, at which point their physical growth stops.


Burnie, David, B.S.

Professional writer of natural histiry and science books. Author of ''How Nature Works''.

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