A Fascinating World of Animals

Animalia is the scientific term for the classification (at the level of kingdom) of organisms that are commonly called animals.  Animals are all multicellular organisms that obtain 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.

Like all living things, animals show similarities and differences that enable them to be classified into groups. Birds, for example, are the only animals that have feathers, while mammals are the only ones that have fur. The scientific classification of animals began in the late 18th century. At this time, animals were classified almost entirely by external features, mainly because these are easy to observe. But external features can sometimes be misleading. For example, in the past, comparison of physical features led to whales being classified as fish and some snakes being classified as worms.

Presently, animals are classified according to a broader range of characteristics, including their internal anatomy, patterns of development, and genetic makeup. These features provide a much more reliable guide to an animal's place in the living world. They also help to show how different species are linked through evolution. Scientists divide the animal kingdom into approximately 30 groups, each called a phylum (plural phyla).

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