Whales are not included in the class pieces (fishes) and considered a part of class Mammalia because of many reasons.

• The main characteristic of the member of class Mammalia is the presence of the mammary glands. Whales have mammary glands and other fishes do not, hence, whales are kept in class Mammalia.

• Whales are viviparous (give birth to young ones) as other mammals whereas pieces are oviparous (lay eggs).

• Whales possess lungs for respiration as other mammals whereas fishes have gills.