What does Xuanwu represent among the four beasts?

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.