Whales, porpoises, and dolphins are cetaceans. A cetacean is a member of the order of animals called Cetacea. Cetaceans are a member of the larger Mammalia class of animals; they are mammals.
Cetaceans are a group of mammals that live wholly in water. Cetaceans are just like other mammals; they breathe air with lungs, and they have mammary glands with which they suckle their young.
But cetaceans are specialized mammals: they have fish-shaped, hairless bodies, and their front limbs are shaped like flippers. They have vestigial hind limbs located within their body wall. The hind limbs have nearly disappeared over the course of evolution.
Cetaceans evolved from four-legged mammals that moved from the land into the sea millions of years ago. (There is some fossil evidence that cetaceans may have once looked something like the hippopotamus.) In the millions of years that cetaceans lived in water, they evolved into sleek and streamlined animals. The tail became a powerful paddle, the front limbs became flippers, and the nostrils moved to the top of the head to make breathing easier.
When cetaceans come to the surface to take in air, the nostril or blowhole opens and explosively blows out moist stale air in a spout or spray. A cetacean then breathes in rapidly, closes its blowhole, and dives beneath the surface.
To stay underwater for long periods, a cetacean’s heart rate slows by half and the pressure of the water squeezes blood into the animal’s vital organs. The lungs become compressed and oxygen is absorbed along the respiratory tract wall. Oxygen is also stored in muscle tissue. Some cetaceans can descend to 3,300 feet (1,000m) and can remain underwater for 45 minutes.