WHALE, any of the marine mammals in the order Cetacea. They are unique among all mammals in that they carry out their complete life history, from birth to death, in water. The term cetacean is used to embrace all known species (about 79) of whales,dolphins, and porpoises. Whales are divided into two suborders: toothed whales and baleen whales. Most smaller whales, and all the dolphins and porpoises, belong to the toothed whale suborder. Those more than 4 to 5 m (13 to 16 ft) long are generally referred to as whales, whereas smaller species are known as dolphins or porpoises. Toothed whales have teeth that are uniform in size and shape although they vary considerably in number, and they feed on fish and invertebrates such as squid and crustaceans; one species, the killer whale, has a more varied diet that includes seabirds and marine mammals.