Biologists classify living things into various separate groups. This classification, known as taxonomy orsystematic biology, consists of hierarchical categories.
Living things are first divided into kingdoms, such as the plant and animal kingdoms, which are then subdivided into phyla.
In determining these phyla, each of all the different basic body types has been considered. For instance, arthropods (jointed legs) are one separate phylum, and all the species in it have a similar body plan. The phylum known as Chordata contains all those species with a central nervous system. All the animals familiar to us, such as fish, birds, reptiles and mammals represent a subdivision-vertebrates-of the phylum Chordata.
Among the different animal phyla there are very different categories, such as Mollusca, which include soft-bodied creatures such as octopus, and the phylum Nematode, which includes roundworms. The categories beneath phyla have basically similar body plans, but phyla are altogether different from one another.