Scientists and biologists classify earth's organisms in one of three domains: archaebacteria (Archaea), eubacteria (Bacteria) and eukaryotes (Eukarya). Animals are eukaryotes. The Eukarya domain includes animals, plants, fungus and protists. Eukaryotes have complex body structures enclosed within membranes and cell walls, while prokaryotes lack nucleus or cell membranes.
Animals are a part of kingdom Animalia. While there are wide differences between animals, all species in the animal kingdom move with the assistance of muscular organs, flagella or cilia. Unlike other eukaryotic kingdoms -- plants, fungus and protists -- animal cells do not have cell walls, but assemble into tissues. Animals are also different from other eukaryotes since they have their own digestive systems. Humans are eukaryotes in the animal kingdom.
Kingdom Animalia has nine phylums. Humans belong in the Chordata phylum. Other animals classes in the Chordata phylum are fish, reptiles, mammals, birds and amphibians. Three of Kingdom Animalia phylums are made up of worms: Annelida (segmented worms), Nematoda (roundworms) and Platyhelminthes (flatworms). All insects, crustaceans and arachnids are in the Arthropoda phylum. The remainder of phylums represent marine animals. Sea stars and sea urchines are in the Echinodermata phylum, while the Mollusca phylum consist of gastropods (snails, slugs) and cephalopods (octopus and squid). The Porifera phylum covers sponges and Coelenterata phylum animals include jellyfish and hydras.
All animals belong to one of two subphlya, invertebrates (Invertebra) and vertebrates (Vertebra). The primary difference between these two subphyla of animals is the backbone; Vertebra have backbones and Invertebra do not. Approximately 95 percent of kingdom Animalia are in the Invertebra subphylum. This subphylum includes insects, cephalopods, gastropods, arachnids and worms. Mammals, birds, amphibians, reptiles and fish are members of the Vertebra subphylum. Humans are members of the Vertebra subphylum.
The classification system further categorizes animals into classes, orders, families, genera and species. These classification levels, also known as taxa, differentiate based on an animal's color, geographic range, eating habits, digestive systems, body heat regulation methods and reproduction systems. Humans are in the Mammalia class, Primate order, Hominidae family, Homo genus and Sapiens genus.