Animals

Animals are a major group of organisms, classified as the kingdom Animalia or Meta­zoa. In general they are multi­cellular, capable of locomotion, 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. More specifically, animals can be defined as heterotrophic eukaryotes without cell walls, which move through a blastula stage in early development.

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". A common usage of the term may be limited to so-called lower animals (as in "brutes" or "beasts") and refer to humans only in a contemptuous or humourous context .

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 distinguishes them from plants and algae. They are also distinguished from plants, algae, and fungi because their cells lack 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 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, 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