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Animals
Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multicellular, capable of locomotion at some stage in their life cycle, responsive to their environment, and feed by consuming other organisms. more...
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Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on.
The word "animal" comes from the Latin word animal, of which animalia is the plural, and is derived from anima, meaning vital breath or soul. In everyday usage animal refers to any member of the animal kingdom that is not a human, and sometimes excludes insects (although including such arthropods as crabs). The use of the word animal in law typically reflects the common pre-scientific use of the word, roughly equivalent to what modern biology would classify as nonhuman mammal. For example, wildlife laws commonly use phrases such as "animals, birds and fish."
The biological definition of the word refers all members of the Kingdom Animalia. Therefore, when biologists use the word "animal" they include humans.
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera), 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 are called 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 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. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.
Reproduction and development
Nearly all animals undergo some form of sexual reproduction. Adults are diploid or polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.
Read more at Wikipedia.org
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