There is a snake on the turtle's back. What kind of story is it?

Hello, landlord! Four great beasts: Qinglong, White Tiger, Suzaku and Xuanwu, among which Xuanwu is the name of water god, Xuanwu is the god of the north, and it is a combination of tortoise and snake. Xuanwu has many different versions, and there are sculptures of faucets! Xuanwu originally meant Xuanming, and the ancient sounds of Wu and Ming are interlinked. Xuan means black; Ghost means yin. At first, Xuan Ming described the tortoise divination as follows: the tortoise's back is black, and the tortoise divination is to let the tortoise go to the underworld to ask his ancestors, bring back the answers and show them to the world in the form of divination. Therefore, the earliest Xuanwu is the tortoise. Since then, the meaning of Xuanming has been expanding. Turtles live in rivers, lakes and seas (including turtles), so Xuan Ming became a water god; The tortoise lives long, and Xuan Ming has become a symbol of immortality; It turned out that the underworld was in the north, and the divination of Oracle Bone Inscriptions in Shang Dynasty was' its divination will be in the north', so Xuan Ming became a northern god again. Hope to adopt, thank you.