Animals are multicellular, eukaryotic organisms of the kingdom Animalia.
Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms.
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes.
In biology, an organism is any individual entity that exhibits the properties of life.
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All animals are motile, meaning they can move spontaneously and independently at some point in their lives.
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Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives.
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.
A body plan, Bauplan, or ground plan is a set of morphological features common to many members of a phylum of animals.
All animals are heterotrophs: they must ingest other organisms or their products for sustenance.
A heterotroph is an organism that cannot produce its own food, relying instead on the intake of nutrition from other sources of organic carbon, mainly plant or animal matter.
Sustenance can refer to any means of subsistence or livelihood.
Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.
The Cambrian explosion or Cambrian radiation was the relatively short span event, occurring approximately 541 million years ago in the Cambrian period, during which most major animal phyla appeared, as indicated by the fossil record.
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon.
Animals can be divided broadly into vertebrates and invertebrates.
Invertebrates are animals that neither possess nor develop a vertebral column, derived from the notochord.
Vertebrates have a backbone or spine, and amount to less than five percent of all described animal species.
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity.
A bone is a rigid organ that constitutes part of the vertebrate skeleton.
They include fish, amphibians, reptiles, birds and mammals.
Birds, also known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.
Mammals are any vertebrates within the class Mammalia, a clade of endothermic amniotes distinguished from reptiles and birds by the possession of a neocortex, hair, three middle ear bones and mammary glands.
Reptiles are tetrapod animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives.
The remaining animals are the invertebrates, which lack a backbone.
These include molluscs ; arthropods ; annelids, cnidarians, and sponges.
The annelids, also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches.
Cnidaria is a phylum under Kingdom Animalia containing over 11,000 species of animals found exclusively in aquatic environments: they are predominantly marine.
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages.