Metazoa are multicellular, eukaryotic organisms in the biological kingdom Animalia

Characteristics

Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2).

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular.^[7][8] Unlike plants and algae, which produce their own nutrients^[9] animals are heterotrophic,^[8][10] feeding on organic material and digesting it internally.^[11] With very few exceptions, animals respire aerobically.^[12] All animals are motile^[13] (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryonic development that is unique to animals,^[14] (though it has been lost in some) allowing cells to be differentiated into specialised tissues and organs.

Structure

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.^[15] During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules.^[16] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.^[17] Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.^[18]

With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues.^[19] These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).^[20]