Animals are multicellular, heterotrophic eukaryotes. Because animals are unable to make their own food, they must have some means of ingesting food. They do this by consuming plants, other animals, or decomposing organic matter, or by absorbing nutrients directly from a host. Animals typically store food reserves in their body as glycogen. Animals have nerve tissues to gain information about the environment and muscle tissue to allow them to move. They have membrane-bound cells that lack rigid walls. Most animals reproduce sexually and spend most of their life cycle as diploid organisms. These are the characteristics that generally separate animals from other groups.

Comparison of basic characteristics of plants and animals.

By this definition the first animals appeared on Earth in the Precambrian oceans over 500 million years ago. Since that time animals have evolved into many diverse forms. Some of those forms have become extinct while others continue to thrive. At the start of the twenty-first century, more than one million species of animals are known on Earth, with more being discovered all the time. Animals are grouped into about thirty-five phyla. Over 95 percent of the animal species lack a vertebral column and are called invertebrates.

Animals are found in nearly all environments on Earth. The oceans are home to the largest number of animal phyla. Freshwater environments are home to a large number of phyla, but those environments are not as diverse as the oceans. Terrestrial environments have the smallest number of animal phyla.

Bibliography

Barnes-Svarney, Patricia, ed. The New York Public Library Desk Reference. New York: Macmillan USA, 1995.