Cetaceans are the mammals most fully adapted to aquatic life. Their body 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 by a thick layer of blubber. Cetacea contains ten families, and about 80 species.
The classification here closely follows "Marine Mammals of the World: Systematics and Distribution" by Dale W. Rice (1998). The work has become the standard taxonomy reference in the field. Differences reflect usage of common names and further discoveries since the publication of that work.
Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp. See the Society's website (http://www.marinemammalogy.org/publications.htm) for further details.
Cetaceans are nearly hairless, and are insulated by a thick layer of blubber.
Cetaceans evolved from land mammals (most likely from certain hoofed carnivores distantly related to hippopotamuses) that returned to the sea about 50 million years ago.
From this, cetaceans can discern the size, shape, surface characteristics and movement of the object, as well as how far away it is. This is called echolocation, and with it cetaceans can search for, chase and catch fast-swimming prey in total darkness.
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