Original intention: Fins are flat limbs, tails or other structures used for swimming in water or other liquids. Many different creatures have evolved fins, especially most fish. Among mammals, whales and sea lions have fins. Others are a few reptiles, such as turtles; And birds, such as penguins. Sometimes species without fins will grow flippers because of abnormal development.

2. What is a fin? Fin is a motor organ for fish and other aquatic vertebrates to adapt to life in water.

As far as fins are concerned, they are generally composed of skin, soft fins and hard unsegmented fins and spines. There are dorsal fin, gluteal fin, caudal fin, pectoral fin and ventral fin in different parts, among which the first three are unpaired and called "odd fin", while the latter two are generally paired with forelimbs and hind limbs of other vertebrates and called "lateral fin". However, with the different living environment and habits, in the long-term evolution process, the functions and shapes of some fins have changed, and even some fins have degenerated or disappeared.