Cetacea-鲸鱼等分类.pdf

Cetacea From Wikipedia, the free encyclopedia The order Cetacea /s??te??(i)?/ includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean whale. Its original meaning, large sea animal, was more general. It comes from Ancient Greek κ?το? (kētos), used for whales and other huge fish or sea monsters. Cetology is the branch of marine science associated with the study of cetaceans. An ancient ancestor of the whale, Basilosaurus was thought to be a reptile until vestigial parts were recognized.[2] Fossil evidence suggests that the cetaceans share a common ancestor with land-dwelling mammals that began living in marine environments around 50 million years ago. Today, they are the mammals best adapted to aquatic life. The body of a cetacean is fusiform (spindle-shaped). The forelimbs are modified into flippers. The tiny hindlimbs are vestigial; they do not attach to the backbone and are hidden within the body. The tail has horizontal flukes. Cetaceans are nearly hairless, and are insulated from the cooler water they inhabit by a thick layer of blubber. Some species are attributed with high levels of intelligence. At the 2012 meeting of the American Association for the Advancement of Science, support was reiterated for a cetacean bill of rights, listing cetaceans as non-human persons.[3] The collective noun pod[4] is often used for a herd or school of marine mammals, especially a small herd or family group of whales or dolphins. Physical characteristics Anatomy Further information: Dolphin#Evolution and anatomy and Whale#Anatomy Respiration Cetaceans breathe air, and surface periodically to exhale carbon dioxide and inhale a fresh supply of oxygen. During diving, a muscular action closes the blowholes (nostrils), which remain closed until the cetacean returns to the surface; when it surfaces, the muscles open the blowholes and warm air is exhaled. A dolphin in the St