Animal

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.

Words to Know

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.

[See also Arthropods; Mollusks ]

Animal

Animals are multicellular, eukaryotic organisms that are capable of voluntary, spontaneous movements, often in response to sensory input. Animal cells are differentiated from cells of other eukaryotes in that they do not have cellulose-containing walls. Animals require external sources of energy and nutrients, therefore they consume plants or other animals. Animals are classified in the Domain Eukarya and the Kingdom Animalia. Zoology is the scientific study of animals.

The first animals were likely soft-bodied metazoans. Because of the nature of their structure, this type of animal does not fossilize well, making the determination of the origin of animals from the fossil record difficult. Studies of the diversity of ancient stromatalites combined with molecular sequence data suggest that the origins of animals may date to about 1 billion years ago.

Most zoologists recognize the existence of 30-35 phyla of animals, some of which are extinct and only known from their fossil record. The simplest animals with the most ancient lineages are asymmetric or radially symmetric, for example, Porifera (sponges) and Cnidaria (jellyfish and sea anemones). The simplest of the bilaterally symmetric animals include Platyhelminthes (flatworms) and Nematoda (nematodes). Coelomates are a functional group of animals with an enclosed body cavity, the best known of which are Mollusca (snails, clams, octopus, and squid), Annelida (segmented worms and leeches), Arthropoda (insects, spiders, and crustacea), Echinodermata (sea urchins and starfish), and Chordata (fish, amphibians, reptiles, birds, and mammals).

About one million species of animals have been named. However, biologists estimate that a much larger number of animal species are yet to be discovered, and that the actual total could be as large as 30-50 million species. The undiscovered species mostly occur in Earth’s richest ecosystems, especially old-growth tropical rain forests, and perhaps the deep oceans. Most of the undiscovered species of terrestrial animals are believed to be insects, especially beetles.

See also Arrow worms; Arthropods; Brachiopods; Chordates; Horsehair worms; Mesozoa; Mollusks; Moss animals; Phoronids; Ribbon worms; Spiny-headed worms.

Bill Freedman

Animal

Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms (the others are: Monera or bacteria , Fungi , Protists or protozoans, and Plantae or plants). Animals are multicellular, eukaryotic organisms, with cells that do not have walls made of cellulose . Animals are capable of voluntary, spontaneous movements, often in response to sensory perceptions. For their nutrition , animals require sources of biologically fixed energy , that is, the biomass of either plants or other animals. Zoology is the scientific study of animals.

The time of the origin of animals during Earth's evolutionary history is not known, because the first animals were soft-bodied, multicellular life forms that did not preserve well as fossils. By the time multicellular animals became represented in the geological record about 640-670 million years ago, there were already numerous phyla of animals, so the actual origin of the kingdom must have occurred earlier.

Most zoologists recognize the existence of 30-35 phyla of animals, some of which are extinct and only known from their fossil record. The simplest animals with the most ancient lineages are asymmetric or radially symmetric, for example, Porifera (sponges ) and Cnidaria (jellyfish and sea anemones ). The simplest of the bilaterally symmetric animals include Platyhelminthes (flatworms ) and Nematoda (nematodes). Coelomates are a functional group of animals with an enclosed body cavity, the best known of which are Mollusca (snails , clams, octopus , and squid ), Annelida (segmented worms and leeches), Arthropoda (insects , spiders, and crustacea ), Echinodermata (sea urchins and starfish ), and Chordata (fish , amphibians , reptiles , birds , and mammals ).

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, and that the actual total could be as large as 30-50 million species. The undiscovered species mostly occur in Earth's richest ecosystems, especially old-growth tropical rain forests , and perhaps the deep oceans. Most of the undiscovered species of terrestrial animals are believed to be insects, especially beetles .

See also Arrow worms; Arthropods; Brachiopods; Chordates; Horsehair worms; Mesozoa; Mollusks; Moss animals; Phoronids; Ribbon worms; Spiny-headed worms.

Bill Freedman

an·i·mal / ˈanəməl/ • n. a living organism that feeds on organic matter, typically having specialized sense organs and nervous system and able to respond rapidly to stimuli: humans are the only animals who weep. ∎  any such living organism other than a human being: are humans superior to animals, or just different? ∎  a mammal, as opposed to a bird, reptile, fish, or insect: the snowfall seemed to have chased all birds, animals, and men indoors. ∎  a person whose behavior is regarded as devoid of human attributes or civilizing influences, esp. someone who is very cruel, violent, or repulsive. ∎  a particular type of person or thing: a regular party animal | the government that followed the election was a very different animal.• adj. of, relating to, or characteristic of animals: the evolution of animal life animal welfare. ∎  of animals as distinct from plants: tissues of animal and vegetable protein. ∎  characteristic of the physical and instinctive needs of animals; of the flesh rather than the spirit or intellect: a crude surrender to animal lust.

Animal

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.

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.

Allan B. Cobb

Bibliography

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