 Cetaceans are divided into two parvorders : the largest parvorder , Mysticeti ( baleen whales ) , is characterized by the presence of baleen , a sieve @-@ like structure in the upper jaw made of keratin , which it uses to filter plankton , among others , from the water ; Odontocetes ( toothed whales ) are characterized by bearing sharp teeth for hunting , as opposed to their counterparts ' baleen . 
