Footnote

a The biological term phyla (singular, phylum) refers to a large group of animals that have the same distinctive body plan. One way that scientists classify all living things is by a seven-step system in which each step is more specific than the one before it. Step one is kingdom, the broadest category. Then come the categories phylum, class, order, family, genus, and species. For example, the horse is categorized in the following way: kingdom, Animalia; phylum, Chordata; class, Mammalia; order, Perissodactyla; family, Equidae; genus, Equus; species, Caballus.