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Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Theirbody 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 alsoheterotrophs, meaning they must ingest other organisms forsustenance.
Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.
• 1 Etymology
• 2 Characteristics
o 2.1 Structure
o 2.2 Reproduction and development
o 2.3 Food and energy sourcing
• 3 Origin and fossil record
• 4 Groups of animals
o 4.1 Porifera, Radiata and basal Bilateria
o 4.2Deuterostomes
o 4.3 Ecdysozoa
o 4.4 Platyzoa
o 4.5 Lophotrochozoa
• 5 Model organisms
• 6 History of classification
• 7 See also
• 8 References
o 8.1 Notes
o 8.2 Bibliography
• 9 External links

The word "animal" comes from the Latin word animal (meaning with soul, from anima, soul). In everyday colloquial usage, the word usually refers to non-human animals.[1] Frequently only closerrelatives of humans such asvertebrates or mammals are meant in colloquial use.[citation needed] The biological definition of the word refers to all members of the Kingdom Animalia including humans.[2]
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and aremulticellular[3] (although see Myxozoa), which separates them frombacteria and most protists. They areheterotrophic,[4] generally digesting food in an internal chamber, which separates them from plants and algae.[5] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[6] All animals are motile,[7] if only at certain life stages. In most animals, embryos pass through a blastula stage, 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 tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization arecalled metazoans, or eumetazoans when the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and bereorganized, 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. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.
Reproduction and development

A newt lung cell stained with fluorescentdyes undergoingmitosis, specifically earlyanaphase.
Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to formzygotes, which develop into new individuals.
Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, wherefertile eggs are produced without mating, or in some cases through fragmentation.
A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It firstinvaginates to form a...
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