Toothed whales have only one opening. Dolphins, porpoises and whales have lost their hind limbs and have tail fins called flukes that are horizontally flattened. As their tail moves up and down, the animals are pushed through the water. Their flippers steer and stabilize. A streamlined shape helps them slip through the water.
Whales live in a dark and murky world.
Many toothed whales have systems of echolocation or sonar which can locate food or obstacles. Dolphins and porpoises are particularly adept at echolocation. They emit a series of clicks which are focused by the rounded melon on their head. The clicks travel out, hit and bounce back from animals and objects in their path such as other dolphins, potential prey, and irregularities. A good deal of dolphin and porpoise brains is devoted to processing information about sound.