What does it mean that there is a snake on the tortoise shell of the dragon head?

Zhenwu Emperor, also known as Tian Xuan God, Xuanwu God and Tian Xuan God, is the famous Jade Emperor among Taoist immortals. At present, the main god of Wudang Mountain belief is Zhenwu Emperor, which is called "Zhenwu Emperor" in Taoist scriptures, or "Zhenwu Emperor" for short. Folks call it "demon Buddha, founder, founder". After the Ming dynasty, it had a great influence throughout the country, especially in modern folk beliefs. Zhenwu Emperor, also known as Xuanwu and Longxuan, was the son of Pangu. After the jade emperor abdicated, he became the third emperor of heaven and was born with Yan Huangdi. He was once a descendant of Fuxi, the body of a dragon and the ancestor of China. The origin of Zhenwu Emperor According to "Tai Shang Huang said that Tian Xuansheng's Zhenwu was a wonderful sutra of magical spells", Zhenwu Emperor was the eighty-second change of Taishang Laojun, born in Rhoda Palace, and Jingle King was better than the son of the queen. The queen dreamed of swallowing Japan, and was born in the palace in October for more than 400 years. When he grew up, he left his family, resigned from his parents and went to Wudang Mountain to practice. After 42 years of success, he ascended to heaven in the daytime. The jade emperor has an imperial edict, and his name is Taixuan. The town is located in the north. The word Xuanwu was originally a general term for seven of the twenty-eight inns in the north. There is a saying in Qu Yuan's "Farewell" in The Songs of Chu, "Call Xuanwu and run to the genus". Xuanwu Seven Nights is shaped like a tortoise snake, so it is noted that "Xuanwu is called tortoise snake, which is located in the north, so it is called Xuan, and it has scales, so it is called Wu". During the Kaibao period of the Northern Song Dynasty, Xuanwu God came to Zhong Nanshan. In the sixth year of Taiping and Xingguo (98 1), he was named the next general. In the seventh year of Song Zhenzong Dazhong Xiangfu (10 14), he was named the true king of St. Baode in the following year, and later changed to Xuanwu as Zhenwu to avoid the taboo of the holy ancestor Zhao.