Why do all turtles always have 13 cracks on their backs?

Normal turtles are all 13 pieces, and chinemys reevesii's shell is also big, but its distribution will not change, with 5 pieces in the middle and 4 pieces on each side. There is indeed a case of wearing the wrong nail, which is not normal. The tortoise with the wrong armor has no appearance, which is very pitiful. But there is a kind of freshwater turtle called pig-nosed turtle, which has no hard armor and is similar to soft-shelled turtle. It lives in the water like a turtle. Other turtles are basically amphibians, just to climb ashore, such as basking in the sun and foraging. Pig-nosed turtles can't get out of the water either.

Why does the little turtle have 13 hexagonal patterns? There is an interesting legend!

A long time ago, turtles had no patterns. One day, the dragon king was ill, and the doctor said that eating rabbits would make him feel better. So the Dragon King sent a turtle to trick the rabbit into the palace. Later, the rabbit found that he was cheated by the tortoise, so he flew up and kicked the tortoise to a stone.

Since then, the tortoise shell has cracked into 13 hexagonal patterns.