Qí 18. refers to the appendages of fish and other aquatic animals similar to wings or paddles, which play the role of propulsion, balance and guidance. 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. Usually distributed in the body and tail, it is the main tool for exercising and balancing the body. Fins can generally be divided into two categories. One is unpaired odd fins, which are located behind the back, tail and anus, including dorsal fin, caudal fin and gluteal fin. The other is paired lateral fins, which are located at both sides of the body, including pectoral fins and ventral fins.

Under special environmental conditions, fins are transformed into various organs with special functions, such as feeding, adsorption, breathing, reproduction, gliding and so on. The first dorsal fin is specialized as a slender hook rod, and the end is expanded to attract fish and shrimp. ? The first dorsal fin of a fish becomes an oval sucker, and the sucker is attached to other fish or boats. The pectoral fin and ventral fin of climbing loach, which grows in ravine rapids, are connected together to form a big sucker, which can be adsorbed on rocks to avoid being washed away by rapids.