The word "animal" comes from the Latin word animal, of which animalia is the plural, and ultimately from anima, meaning vital breath or soul. The following general characteristics and structures pertain to animals:

  1. Animals are a major group of organisms, classified as the kingdom Animalia or Meta­zoa.
  2. In general they are multi­cellular, capable of locomotion, responsive to their environment, and feed by consuming other organisms.
  3. 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.
  4. Animals have several characteristics that set them apart from other living things.
  5. Animals are eukaryotic and usually multicellular (although see Myxozoa), which separates them from bacteria and most protists.
  6. They are heterotrophic, 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 cell walls.
  8. With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differen­tiated into separate tissues.
  9. These include muscles, which are able to contract and control locomotion, and a nervous system, which sends and processes signals.
  10. 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.
  11. 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.
  12. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth.