Phylum

In biology, a phylum (/ˈfaɪləm/; plural: phyla) is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division was used instead of "phylum", although from 1993 the International Code of Nomenclature for algae, fungi, and plants accepted the designation "phylum". Depending on definitions, the kingdom Animalia contains approximately 35 phyla, Plantae contains about 12, and Fungi contains around 7. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four "embranchements" of Georges Cuvier.Haeckel introduced the term phylum, based on the Greek word phylon ('tribe' or 'stock'). In plant taxonomy, Eichler (1883) classified plants into five groups, named divisions.

Informally, phyla can be thought of as grouping organisms based on general specialization of body plan. At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.

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