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    Whales and dolphins (Cetacea) can be divided into two major groups or sub-orders. Baleen whales (Mysticetes) are distinctive for having two blowholes and whalebone (Baleen) plates hanging from the roof of the mouth to filter food. The rorqual whales (from the Norwegian rorhval meaning furrow) are also of this group and are distinct once again for having a series of longitudinal grooves or pleats, extending from under the throat to behind the pectoral fins. These groves allow the throat to expand - concertina style - and hugely increase capacity whilst feeding. Toothed whales (Odontocetes) have only one blowhole and by definition have teeth. Dolphins are in fact small, toothed whales of the family Delphinidae. All species of Cetacea have a horizontally flattened tail (fluke) and flattened forelimbs forming the pectoral fins whilst most species have a boneless (dorsal) fin on the back.
Cetaceans breathe using lungs and therefore rely on air from the surface - they are highly specialised, intelligent and otherwise totally aquatic mammals. They can produce a large and varying vocabulary of sounds as a means of communication and in many species, a form of echo-location.
The larger species of whale were almost hunted to extinction by the mid 1960's - see history of whaling. Recent conservation efforts have seen a steady and encouraging rise in some cetacean populations, in particular those of the southern right and humpback whales, however the conservation status of most species is at present unsure and must be regarded as still vulnerable. Since 1986 over 23 000 whales are known to have been hunted and killed. Another major concern is the current threat from commercial fishing practices, gill nets and pollution, to the many species of dolphin that inhabit coastal waters worldwide.
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