Phylum

Phylum (plural: phyla) is a taxon used in the scientific classification of life. "Phyla" is adopted from the Greek phylai the clan-based voting groups in Greek city-states. Phyla represent the largest generally accepted groupings of animals and other living things with certain evolutionary traits, although the phyla themselves may sometimes be grouped into superphyla (e.g. Ecdysozoa with eight phyla, including arthropods and roundworms; and Deuterostomia with the echinoderms, chordates, hemichordates and arrow worms). Informally, phyla can be thought of as grouping animals based on general body plan[1], e.g., having bones vs. having an exoskeleton. This is morphological grouping. But despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal organizations[2]. For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "Phylum" in reference to plants, the term "Division" is almost always used by botanists.

The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum humans belong to. Although there are approximately 35 phyla, these nine include the majority of the species. Many phyla are exclusively marine, and only one phylum is entirely absent from the world's oceans: the Onychophora or velvet worms. The most recently discovered phylum is Cycliophora[3] found in 1993; only three phyla were discovered in the last century.

The Cambrian explosion was a great flowering of life forms that occurred between roughly 542 and 530 million years ago; during this time all modern phyla (and many now-extinct ones) were established.[4] Over time the roles among different phyla have varied. For instance, during the Cambrian, the dominant megafauna, or large animals, were arthropods whereas now they are vertebrates (chordata).[5]

Scientific classification describes the following levels of organization (taxons) for classifying life forms: Kingdom, Phylum, Class, Order, Family, Genus, Species, Subspecies. Thus Phylum is a rather broad classification. Also, the Three Domain System is now widely used and adds the level of Domain above Kingdom as the broadest classification.