Cetaceans are the mammals most fully adapted to aquatic life. Their body is fusiform (spindle-shaped). The forelimbs are modified into flippers. The hindlimbs are vestigial, they do not attach to the backbone and are not exteriorly visible. The tail has horizontal flukes.
Cetaceans are nearly hairless, and are insulated by a thick layer of blubber.
Cetacea contains 10 families, 78 species.