phylum

A phylum is a principal rank in biological classification, below kingdom and above class. It groups together organisms that share a broad plan of body organization, developmental patterns, and evolutionary ancestry. Phyla are used to reflect major lineages within a kingdom, and the exact composition of phyla can vary with new data and different taxonomic systems.

In zoology, some well-known phyla include Chordata (animals with a notochord, dorsal nerve cord, and pharyngeal slits at some life stage), Arthropoda (jointed limbs and segmented bodies), Mollusca (soft-bodied with often a muscular foot and mantle), Annelida (segmented worms), Echinodermata (radially symmetric marine animals with a water vascular system), Cnidaria (jellyfish, corals), and Porifera (sponges). Vertebrates are a subphylum of Chordata. There are many additional phyla representing other lineages of animals.

In plants, the corresponding rank is division rather than phylum; some modern classifications coincide and use phylum for plant groups as well, especially in cladistic contexts. Across all life, the concept of a phylum is tied to a clade—group of organisms believed to include all descendants of a common ancestor.

Phyla are typically defined by distinctive features, derived traits, or, increasingly, by molecular phylogenetics. Because phylogenies are refined over time, the membership and number of phyla can change as new data are integrated.

Etymology: phylum derives from Greek phylon, meaning tribe or race, via Latin phylum. In use since the 19th century, the rank serves to organize the diversity of life into broad, related lineages.