Many animal kingdom classification charts include the categories of kingdom, phylum, class, order and family, and finish with genus and species. Some charts also show the division between unicellular and multicellular animals, as well as levels of organization within individual species in the animal kingdom.
Some factors that affect animal classification include the symmetry of the body, arrangement of cells, presence of the notochord and features of the coelom. The digestive, circulatory and reproductive systems also influence animal classification. Charts that show the levels of organization within species may categorize the levels as cells, tissues, organs and organ systems. Asymmetrical bodies, radial symmetry and bilateral symmetry also serve as a basis for classification.
Two major animal categories are unicellular and multicellular life. Protozoa falls in the unicellular category, and the multicellular category divides into invertebrates and vertebrates. Invertebrates include sponges, worms, arthropods, molluscs and echinoderms, whereas the vertebrate category includes fish, amphibians, reptiles, birds and mammals. The division between invertebrates and vertebrates lies mainly in the presence of a backbone or similar structure, which vertebrates possess and invertebrates lack.
The species is the fundamental unit in classification, being the description of one kind of animal. The closer a category is to the species (such as genus), the more animals within that group are related.