The blue whale's limbs degenerate into fins.

The fin shape of the limbs is the adaptation of whales to aquatic life. Whales are the largest mammals in the world, not fish; Whales live in the ocean. The ancestors of whales originally lived on land, but later, due to environmental changes, they lived in shallow seas near land. After a long evolution, the forelimbs and tails of whales gradually became fins, and the hind limbs completely degenerated. The whole body becomes like a fish (so people mistake it for a fish), adapts to the life of the ocean and becomes a mammal with the characteristics of viviparous lactation.