Whales belong to class Mammalia while fishes are included in class Pisces. Both are aquatic animals and chordates. However, both differ in several basic characters, such as: (l) Whales are warm-blooded animals while fishes are cold-blooded. (ii) Whales respire through lungs while fishes respire through gills. (iii) whales possess mammary glands which are absent in fishes. (iv) Whales have four chambered heart, while fishes have a two-chambered heart.