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 largest, the blue whale, reaching 100 feet (30 meters) in length and weighing up to 300,000 pounds (136,000 kilograms). Animals are classified as vertebrates (having backbones) or invertebrates (without backbones).
Among the most primitive of the animals are the sponges, invertebrates that live in water. Sponges have no nerve cells or muscles. They are shaped like vases, with water flowing in through holes in their sides and leaving through an opening in their top. Sponges are remarkable for their ability to regenerate. Although many invertebrates are capable of growing new body parts, sponges are capable of growing into a new individual from even the tiniest fragment of the original body. It is believed that although sponges have lived successfully for about one billion years, yet it seems that they did not give rise to any other animal forms.
Some of the simplest animals with the oldest ancestral lines are invertebrates such as jellyfish, sea anemones (pronounced uh-NEH-muh-neez), and corals, which also live in water. They have radial body symmetry, which means that their bodies are arranged equally around a central point. This arrangement allows them to sense food and danger approaching from all directions. The simplest animals having bilateral symmetry (meaning that both the left and right sides of their bodies are mirror images of each other) include the invertebrate flatworms and roundworms. These animals live in water, on land, and as parasites in the body fluids of other organisms. Bilateral symmetry was an important evolutionary development because it allowed for forward movement of animals. It also is associated with the development of separate head and tail areas, as well as a distinction between the upper and under portions of an animal's body.
Another major step in the evolution of animals was the development of a body cavity called the coelom (pronounced SEE-luhm). The coelom is a cavity in the body between the gut and the body wall that houses the internal organs, such as the liver, stomach, and heart. Animal groups that have coeloms include Mollusca (snails, clams, octopus, and squid), Annelida (earthworms and leeches), Arthropoda (insects, spiders, and crabs), Echinodermata (sea urchins and starfish), and Chordata (fish, amphibians, reptiles, birds, and mammals). All of the vertebrates are included in the group Chordata.
About one million species of animals have been named. However, biologists estimate that a much larger number of animal species has yet to be discovered; the actual total could be as large as 30 to 50 million species.
Bilateral symmetry: Two-sided symmetry where the left and right sides of the body are identical, with each side containing similar structures.
Coelom: The cavity between the body wall and gut that is lined with specialized cells and that serves to protect the organs within.
Eukaryote: Multicellular organism whose cells contain true nuclei and membrane-bound structures called organelles.
Radial symmetry: Identical or similar body shape around one central point so that any line drawn through the center yields similar right and left halves.