Cetaceans are usually referred to as marine mammals because most of them live in the sea (a few species live in rivers). But there are a number of other marine mammals also, including the Pinnipeds (seals, sea lions and walruses), the Sirenians (manatees and dugongs) and sea otters.
Whales, dolphins and porpoises have a spindle-like body shape (wider in the middle than at either end), just like seals or manatees. But unlike cetaceans, pinnipeds and sirenians still have their nostrils at the front of the head (rather than a blowhole). Seals, sea lions and walruses have not lost their hind limbs through evolution, but developed them into flexible paddles for steering and swimming. Their front limbs have also kept their flexibility; you can even count their fingers. Dugongs and manatees have lost their hind limbs, just like cetaceans. Their front limbs have also developed into paddles, and manatees (but not dugongs) still have finger nails. So, what exactly characterizes a cetacean? Check out our Anatomy section to find the answer!