Animals are multicellular, eukaryotic organisms of the kingdom Animalia (also called Metazoa). All animals are motile, meaning they can move spontaneously and independently, at some point in their lives. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives. All animals are heterotrophs: they must ingest other organisms or their products for sustenance.
Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals are divided into various sub-groups, some of which are:vertebrates (birds, mammals, amphibians, reptiles, fish); molluscs (clams, oysters, octopuses, squid, snails); arthropods (millipedes, centipedes, insects, spiders, scorpions, crabs, lobsters, shrimp); annelids(earthworms, leeches); cnidarians (jellyfish, anemones, corals); and sponges.
It is estimated that around 9 or 10 million species of animals inhabit the earth; the exact number is not known and all estimates are rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as the giant squid. By far most species of animals are insects, with groups such as mollusks, crustaceans, and nematodes also being especially diverse. By this measure our own group, the vertebrates, is relatively inconsequential from a diversity perspective.
All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain prokaryotes (organisms made up of cells that lack a cell nucleus or any membrane-encased organelles, such as bacteria and blue-green algae) or protists (Kingdom Protista, which includes unicellular eukaryotic organisms). All members of Animalia are multicellular, and all are heterotrophs (i.e., they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions. In most, tissues are organized into even more specialized organs. Most animals are capable of complex and relatively rapid movement compared to plants and other organisms. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material.
The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few divisions of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore.
- animal noun [C] (LIVING THING)
› a living thing that can move and eat and react to the world through its senses, esp. of sight and hearing: Mammals, insects, reptiles, and birds are all animals.
‘What is an animal?’ outlines the criteria for defining an ‘animal’. There are characteristics by which we recognize animals — they are multicellular, get the energy necessary for life by eating other organisms, and can move and sense their environment — but these do not define an animal. In taxonomy, names are given to branches on evolutionary trees. Real or ‘natural’ groupings must encompass sets of organisms that have a shared evolutionary ancestor. This means that the term ‘animal’ must refer to a group of related species. The animals, therefore, are a natural group (or clade) descended from a shared common ancestor. This clade is called the Animal Kingdom, or Metazoa.
Simple Definition of Creature
- : an animal of any type
- : an imaginary or very strange kind of animal
- : a person usually of a specified type
a. A living being, especially an animal: land creatures; microscopic creatures in a drop of water.
b. A human.
c. An imaginary or fantastical being: mythological creatures; a creature from outer space.
2. One dependent on or subservient to another.
3. Something created.
› A creature can be a person when an opinion is beingexpressed about them:It seems clear to me that people are creatures of emotion.
an animal, especially a nonhuman:
the creatures of the woods and fields; a creature from outer space.
anything created, whether animate or inanimate.
person; human being:
She is a charming creature. The driver of a bus is sometimes an irritable creature.