Phylum

A phylum is one of the principal ranks in the classification of living organisms (taxonomy), used to organize life on the basis of body structure and evolutionary relationships. In the standard hierarchy, phylum is ranked below kingdom and above class.

Each phylum groups together organisms that share fundamental structural characteristics, such as the presence of a backbone, body cavity arrangement, or the organization of body parts. For example, vertebrate animals belong to the phylum Chordata, while insects, spiders, and crustaceans belong to the phylum Arthropoda.

Position in the classification hierarchy

The standard order of ranks proceeds as follows: kingdom, phylum, class, order, family, genus, and species. Intermediate ranks are sometimes inserted between these levels, such as subphylum or subclass. Phylum is therefore a broad rank, and each phylum may contain hundreds of thousands of species.

Terminology

Changes in understanding

Originally, phyla were defined solely on the basis of visible structural features. Since advances in DNA analysis, some groups have been split or merged, and their positions on the evolutionary tree have been revised. As a result, the number of accepted phyla varies depending on the classification system used and the time period.