The order Cetacea comprises two extant sub-orders and one extinct sub-order. The extant sub-orders are Mysticeti (baleen whales) and Odontoceti (toothed whales). Both mysticetes and odontocetes are thought to be descendants of archaeocetes (Archaeoceti, ancient whales), an extinct sub-order. There are at least 83 living species of cetaceans, with 46 genera in 14 families. Of the two extant suborders, Odontoceti is larger and more diverse, with at least 70 species, 40 genera, and 10 families. Cetaceans, along with bats, are considered some of the most derived mammals on the planet. They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water.