Any of various wholly aquatic placental mammals belonging to the order Cetacea, which includes the dolphins, porpoises, and whales. Their bodies are fish-shaped and adapted for swimming, with forelimbs in the form of paddlelike flippers. Functional hindlimbs are not seen. The long tail ends in two tranverse flukes, horizontally flattened tail used for locomotion. Hair is found only on the muzzle. There is a thick layer of fat (blubber) beneath the skin.