 Mysticetes are also known as baleen whales . They have a pair of blowholes side @-@ by @-@ side and lack teeth , which renders them incapable of catching larger prey ; they instead have baleen plates which is a sieve @-@ like structure in the upper jaw made of keratin , which it uses to filter plankton and other food from the water ; this forces them to follow krill or plankton migrations . Some whales , such as the humpback , reside in the polar regions where they feed on a reliable source of schooling fish and krill . These animals rely on their well @-@ developed flippers and tail fin to propel themselves through the water ; they swim by moving their fore @-@ flippers and tail fin up and down . Whale ribs loosely articulate with their thoracic vertebrae at the proximal end , but do not form a rigid rib cage . This adaptation allows their chest to compress during deep dives as the pressure increases with depth . Mysticetes consist of four families : rorquals ( <unk> ) , cetotheriids , right whales ( <unk> ) , and gray whales ( <unk> ) . 
