Phylum

In the Linnaean system of classification, organisms are sorted into nested ranks: domain, kingdom, phylum, class, order, family, genus and species. Phylum sits just below kingdom, so it is one of the broadest groupings within a kingdom. Animals in the same phylum share a fundamental body plan. Members of Chordata, which includes humans, all have a notochord, a dorsal nerve cord, pharyngeal slits and a tail at some stage of development. Other large animal phyla include Arthropoda (insects, spiders, crustaceans), Mollusca (snails, clams, squid), Annelida (segmented worms), Cnidaria (jellyfish, corals) and Echinodermata (sea stars, sea urchins).

In botany and mycology the equivalent rank has traditionally been called a division, although phylum is now also used. Because classification began with shared visible features, ranks do not always match evolutionary history, and phyla are revised as DNA evidence clarifies how groups are related. Biologists now aim for each phylum to be a clade, a group made up of one ancestor and all its descendants.

Students meet phyla when learning animal diversity, where each phylum is defined by a short list of key traits. The plural is phyla.