A phylum is a group in the classification of life (taxonomy) directly below kingdom and above class. It groups together organisms that share an evolutionary lineage and a fundamental body plan. For example, in animals this includes symmetry (radial versus bilateral) and the number of primary tissue layers formed during embryonic development, leading to familiar groups such as creatures with a backbone or those with soft bodies protected by a shell.
The animal kingdom alone contains roughly 30 to 40 recognized phyla (plural of phylum), although the vast majority of animal species belong to just a handful of them. The phylum with the largest number of species by far is arthropods (scientific name Arthropoda), which consists of animals with external joints, including insects, spiders and crustaceans (e.g., crabs, lobsters and shrimp). It contains more than 1.3 million described species, and some estimates suggest that the total could actually be 5 to 10 million.
Mollusks (Mollusca), which includes snails, clams and octopuses, follows with around 85,000 to 118,000 described species. Chordata, the phylum that includes all vertebrates, namely fish, amphibians, reptiles, birds and mammals, contains roughly 60,000 to 85,000 species.
Other significant phyla include segmented worms (e.g., earthworms and leeches) at roughly 17,000 to 22,000 species, flatworms with about 20,000 to 29,000 species, roundworms with around 25,000 species and echinoderms (such as starfish and sea urchins) with roughly 7,000 living species.
In the plant kingdom there are 10 to 14 recognized phyla (traditionally called divisions in botany), depending on the taxonomic classification system used. 10 of these contain living (extant) species, while the remaining four consist entirely of extinct fossil groups. The four largest and most important make up the vast majority of global plant diversity.
The largest, and overwhelmingly dominant, plant phylum is flowering plants (Magnoliophyta), with 250,000 to 400,000 species, accounting for 90 to 94 percent of all plant species. Currently, around 295,000 to 369,000 of these species have been formally described and accepted by scientists. They provide nearly all human food crops and structural materials.
The other most populous plant phyla include mosses, which are non-vascular land plants that lack true roots or seeds and absorb water directly through their leaves. The number of species is estimated at roughly 12,000 to 15,000.
This is followed closely by ferns and other vascular plants, such as club mosses and horsetails, that have true roots and leaves but reproduce via spores instead of seeds. The number of species is estimated at 10,000 to 12,000.
Additionally, seed-producing plants without flowers (gymnosperms) are split into several distinct phyla: conifers (such as pines, firs, and redwoods), which dominate vast northern boreal forests and high-altitude regions with about 600 to 650 extant species, alongside smaller, ancient groups such as cycads and ginkgos.
Regarding extinction risk, certain phyla face disproportionate threats due to habitat loss, climate change, pollution and overexploitation. Cnidaria, particularly reef-building corals, are among the most threatened, with indications that around 44 percent of reef-forming coral species are at risk of extinction.
Mollusks also face significant threats, especially freshwater species, with a large percentage classified as endangered due to habitat degradation and pollution. Chordata includes many high-profile endangered species, with amphibians in particular facing a 41 percent threat level and mammals at 27 percent. While Arthropoda contains the most species overall, certain groups (such as selected crustaceans) face elevated extinction risks, and many insect orders are in steep decline.