Cetaceans, or whales (a group which includes the animals commonly known as dolphins and porpoises), are interesting from an evolutionary perspective because they are so different from their ancestors. The ancestors of whales were fur-covered land mammals that rarely entered the water, whereas whales today are fully aquatic and thrive in the oceans, seas, and—in some instances—great rivers of the world. The ancestors were four-footed and hoofed for running on land, whereas whales today have forelimbs modified as flippers, lack external hindlimbs, and move by pushing themselves through water using a powerfully muscled tail with a broad terminal water-foil called a fluke. The ancestors were omnivorous to herbivorous; in contrast, whales today are piscivorous (fish-eating) and planktivorous (plankton-eating). The ancestors perceived their surroundings using their eyes and noses; however, whales today “see” their surroundings with their ears.