Cetaceans are a diverse order of mammals that includes whales, dolphins, and porpoises. They have adapted to life in the water, evolving over millions of years to have:
* Streamlined bodies: for efficient swimming.
* Flippers and flukes: for propulsion and steering.
* Blowholes: for breathing air.
* Blubber: for insulation and buoyancy.
* Echolocation: for navigating and finding prey in the water.
All cetaceans are placental mammals, meaning their young develop inside the mother's womb and are nourished by a placenta.
Other marine mammals, like seals, sea lions, and walruses, are also adapted to aquatic life, but they are not placental mammals. They are pinnipeds, belonging to the order Carnivora, and give birth to live young that develop inside their bodies, but they are not considered "placental" mammals.