Whales are mammals and have many of the features and systems of mammal anatomy. For example, whales breathe air with lungs, and the females nurse their young on milk. But whales differ significantly from almost all other mammals - a result of their gradual move from land to sea millions of years ago.
Whales evolved a streamlined shape, they lost their external hind limbs, and their forelimbs became flippers. Their nostrils became blowholes. Their lungs and circulatory system adapted to spending extended periods underwater.
From these changes came the remarkable, fully aquatic lives of whales.