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Animal

Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multicellular, capable of locomotion and responsive to their environment, and feed by consuming other organisms. 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. Biologically, human beings fall under the animal kingdom.

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus in the first hierarchial classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately. Colloquially humans have been considered to be separate from the animals, but biologically they are definitely included. The name animal comes from the Latin word animalis, of which animalia is the plural, and ultimately from anima, meaning vital breath or soul.

1. Structure

2. Reproduction and development

3. Origin and fossil record

4. Groups of animals

4.1 Deuterostomes
4.2 Ecdysozoa
4.3 Platyzoa
4.4 Lophotrochozoa

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 a nervous system, 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, so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

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