In nature, animals are multicellular, eukaryotic organisms of the kingdom Animalia (also called Metazoa). Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives.
Most animals are motile, meaning they can move spontaneously and independently. All animals must ingest other organisms or their products for sustenance (aka Heterotrophs).
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);
- mollusks (clams, oysters, snails, cephalopods (octopuses, squid);
- arthropods (millipedes, centipedes, insects, spiders, scorpions, crustaceans like lobsters, shrimp);
- annelids (earthworms, leeches);
- echinoderms (star fish)
- sponges (Porifera); and
Origin and usage of the word animal
The word "animal" comes from the Latin word animalis, meaning "having breath". In everyday colloquial usage the word incorrectly excludes humans—that is, "animal" is often used to refer only to non-human members of the kingdom Animalia. Sometimes, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.
The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects, and humans.
The scientific and overall study of animals is called zoology, although specific kinds of animals are referred to according the scientific name of the animal group.