All animals belong to the taxonomic Kingdom Animalia (also called Metazoa). All members of the Animalia are multicellular, and all are heterotrophs (i.e. they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
The bodies of animals (except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions. In most, tissues are organized into even more specialized organs.
Most animals are capable of complex and relatively rapid movement compared to plants and other organisms, and all animals are motile during some stage of life. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material. The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few division of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore. The cells of animals do not have cell walls.
Somewhere around 9 or 10 million species of animals inhabit the earth; the exact number is not known. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid. By far most species of animals are insects, with groups such as mollusks and nematodes also being especially diverse. By this measure our own group, the vertebrates, is relatively inconsequential.
The Animal Kingdom has the largest number of organisms yet recorded. It includes sponges, worms, insects, birds, mammals, crustaceans, as well as humans.