All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain the prokaryotes (Kingdom Monera, includes bacteria, blue-green algae) or the protists (Kingdom Protista, includes unicellular eukaryotic organisms). All members of the Animalia are multicellular, and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all 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. 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.
Somewhere around 9 or 10 million species of animals inhabit the earth; the exact number is not known and even our estimates are very rough. 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.
Research continues on the evolutionary relationships of the major groups of animals. For the sake of convenience, we shall follow the system outlined in Hickman and Roberts (1994), but for some groups we shall incorporate the results of current research in our classification and discussion.