Whale and walrus are aquatic and marine mammals. They do not belong to class Pisces. They do not have gills to breathe in dissolved oxygen in water. Neither they have scales on the body nor can they lay eggs. Whales and walrus have mammary glands like all other mammals. They give birth to live young one. They breathe only with the help of lungs by putting their nostrils out of the water at surface. Hence they are included in Mammalia.
