It Has Tails and Lives in Water, Is It
In our daily life, people often call some aquatic creatures that have long tails＾fish￣, though they are not fish at all. Such as cuttlefish, starfish, baby fish (giant salamanders), E fish (crocodiles) etc. Though we call them all fish, they belong to different categories of animals actually. Cuttlefish is mollush; starfish is echinozoa; giant salamanders are amphibians; while turtles and crocodiles are reptiles; and the whale, is actually a kind of mammal. Then, what is fish? Fish is a kind of poikilothermal vertebrates, which lives in water, breathes with gills, and moves with fins. Their shapes are mostly like spindles and their bodies are covered with protective scales. Through more than 300 - 400 million years of evolution, fish has developed into a large family clan in the world of animals. In the vast sea waters and waters on the continent, there is a variety of fish. There are more than 20000 kinds of fish in the world today and they take up more than 42% of all vertebrates in the world. We can find them in high mountain streams as high as 6000 m above sea level and in deep seawaters as deep as 10000 m in the ocean.