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Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning 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.
Etymology
The word "animal" comes from the Latin word animalis, meaning "having breath".[1] In everyday colloquial usage, the word usually refers to non-human animals.[2] Sometimes, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.[3] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.[4]
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and mostly multicellular,[5] which separates them from bacteria and most protists. They are heterotrophic,[6] generally digesting food in an internal chamber, which separates them from plants and algae.[7] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[8] All animals are motile,[9] if only at certain life stages. In most animals, embryos pass through a blastula stage,[10] which is a characteristic exclusive to animals.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings.[11] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.[12]All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.[13] This may be calcified to form structures like shells, bones, and spicules.[14] During development, it forms a relatively flexible framework[15] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.[11] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.[16]
Food and energy sourcing
Main article: Animal nutrition
All animals are heterotrophs, meaning that they feed directly or indirectly on other living things.[28] They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.[29]Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).[30] Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.[31] The other main category of consumption is detritivory, the consumption of dead organic matter.[32] It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.[33]
Most animals feed indirectly from the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules carbon dioxide (CO2) and water (H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C6H12O6) and releases oxygen (O2). These sugars are then used as the building blocks which allow the plant to grow.[11] When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal.[34] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.[35] This process is known as glycolysis.[36]
Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight.[37] Instead chemosynthetic archaea and bacteria form the base of the food chain.[38]
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