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The word cetacean is used to describe all whales, dolphins and porpoises in the order Cetacean. This word comes from the Latin Cetus meaning "a large sea animal," and the Greek word ketos, meaning "sea monster."
There are about 86 species of cetaceans. The term "about" is used because as scientists learn more about these fascinating animals, new species are discovered or populations are re-classified.

Cetaceans range in size from the tiniest dolphin, Hector's dolphin, which is just over 39 inches long, to the largest whale, the blue whale, which can be over 100 feet long. Cetaceans live in all of the oceans and many of the major rivers of the world.
Cetaceans are thought to have evolved from hooved land mammals that lived about 50 million years ago.

There are many types of cetaceans, which are divided largely according to how they feed.
The order Cetacean is divided into two sub-orders, the Mysticism (baleen whales) and the Odontocetes (toothed whales). The Odontocetes are more numerous, comprising 72 different species, compared to 14 baleen whale species.
The Mysticism include species such as the blue whale, fin whale, right whale and humpback whale.

Mysticism have hundreds of comb-like plates of baleen hanging from their upper jaw. Baleen whales feed by gulping large amounts of water containing hundreds or thousands of fish or plankton, then forcing the water out in between the baleen plates, leaving the prey inside to be swallowed whole.
Odontocetes include the sperm whale, orca (killer whale), beluga and all of the dolphins and porpoises. These animals have cone-shaped or spade-shaped teeth and usually capture one animal at a time and swallow it whole. Odontocetes feed mostly on fish and squid, although some orcas prey on other marine mammals.

Cetaceans are mammals, which means they are endothermic (commonly called warm-blooded) and their internal body temperature is about the same as a human's. They give birth to live young and breathe air through lungs just like we do. They even have hair.
Unlike fish, which swim by moving their heads from side-to-side to swing their tail, cetaceans propel themselves by moving their tail in a smooth, up-and-down motion. Some cetaceans, such as the Dall's porpoise and the orca (killer whale) can swim faster than 30 miles per hour.

Breathing

When a cetacean wants to breathe, it has to rise to the water surface and exhale and inhale out of the blowholes located on top of its head. When the cetacean comes to the surface and exhales, you can sometimes see the spout, or blow, which is the result of the warm air in the whale's lungs condensing upon reaching the cool air outside.

Insulation

Whales do not have a coat of fur to keep warm, so they have a thick layer of fat and connective tissue called blubber underneath their skin. This blubber layer can be as much as 24 inches thick in some whales.

Senses

Whales have a poor sense of smell, and depending on where they are, they may not be able to see well underwater. However, they have excellent hearing. They do not have external ears, but have tiny ear openings behind each eye. They can also tell the direction of sound underwater.

Diving

Whales have collapsible rib cages and flexible skeletons, which allows them to compensate for high water pressure when they dive. They can also tolerate higher levels of carbon dioxide in their blood, allowing them to stay underwater for up to 1 to 2 hours for large whales.