Whales and dolphins are animals which, along with porpoises, are part of the cetacean order. They are considered mammals due to the fact that they once had hair, contain mammary glands, skull and vertebrae, limbs, lungs, and fleshy ears among other features. However, because of the environment they inhabit, cetaceans have had to modify or adapt to the aquatic environment in order to be more hydrodynamic.
Cetacean species are divided into Baleen animals (mysticetes, large whales), and toothed animals (odontocete, dolphins, beaked whales and porpoises). The baleen whales do not have teeth, but thick, hair-like structures that allow them to filter feed animals like plankton and krill.