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. Most animals inhabit the seas, with fewer in fresh water and even fewer on land.

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.

Animal Phyla Mesozoa Phylum Mesozoa  Parazoa Phylum Porifera  Phylum Placozoa Eumetazoa Radiata Phylum Cnidaria  Phylum Ctenophora  Bilateria Protostomia Acoelomates Phylum Platyhelminthes  Phylum Nemertea  Pseudocoelomates Phylum Rotifera  Phylum Gastrotricha  Phylum Kinorhyncha  Phylum Gnathostomulida  Phylum Nematoda  Phylum Priapulida  Phylum Nematomorpha  Phylum Acanthocephala  Phylum Entoprocta  Phylum Loricifera  Eucoelomates Phylum Mollusca  Phylum Annelida  Phylum Arthropoda  Phylum Echiurida  Phylum Sipuncula  Phylum Tardigrada  Phylum Pentastomida  Phylum Onychophora  Phylum Pogonophora  Deuterostomia Phylum Phoronida  Phylum Ectoprocta  Phylum Brachiopoda  Phylum Echinodermata  Phylum Chaetognatha  Phylum Hemichordata  Phylum Chordata