Animal (Encyclopedia of Science)
Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms. They are multicelled, eukaryotic (pronounced yookar-ee-AH-tik) organisms, meaning their cells contain nuclei and other structures called organelles, all of which are enclosed by thin membranes. (Eukaryote means "true nucleus.") Unlike plants, their cells do not have cell walls. Animals are capable of moving their bodies, often in response to what they sense in their environment. For food, animals ingest plants and other organisms. The scientific study of animals is called zoology.
Animals have existed for millions of years, but it is not known when they first appeared on Earth. The earliest animals were soft-bodied, multicellular life-forms that did not preserve well as fossils. (A fossil is the remains or print of an organism from long ago that has been preserved in rock.) By the time animal parts became hardened in rock about 640 to 670 million years ago, numerous well-developed multicellular animals already existed. Therefore, the beginnings of the animal kingdom must have occurred earlier.
Most zoologists recognize the existence of 30 to 35 phyla (related groups) of animals, some of which are extinct (no longer exist) and are known only from their fossil record. Animals that live today come in many forms and sizes, the very smallest visible only under a microscope and the very...
(The entire section is 738 words.)
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