Cetacea

Cetacea, scientific order of exclusively aquatic mammals including whales, dolphins, and porpoises. Despite being aquatic, cetaceans share several characteristics with other mammals. They have lungs to breathe atmospheric air, and have warm blood and the remnants of body hair. They bear their young alive after placental, intrauterine development and suckle their young with milk. They have some unique characteristics as well. Although they have evolved from the basic anatomy of the mammal, their bodies are fusiform (tapered on both ends). The tail fins of cetaceans are horizontal instead of vertical as in fish, and the tail fins are not supported by a bony skeleton. Cetaceans have no external hind legs, and their fore flippers correspond with the pectoral fins of many fish. The nostrils of most cetaceans are situated on top of their heads. Their necks are extremely short: the seven neck, or cervical, vertebrae normally found in mammals are compressed into plates totaling 15 cm (6 in) in length.

Scientific classification: Cetaceans are classified in the family Cetacea, class Mammalia.