Even-toed ungulates are mammals which form the order Artiodactyla ("even-toed"). They bear their body weight about equally on the third and fourth toes, rather than mostly or entirely on the third as in perissodactyls. The artiodactyls are a large and remarkably diverse group of mammals, containing around 220 living species placed in 10 families. The majority live in relatively open habitats, such as plains and savannas, but others dwell in forests, and one group is semiaquatic. Within the order can be found some of the fastest-running mammals and most nimble animals such as deer, but the Artiodactyla also includes relatively slow and cumbersome species such as pigs and hippos.