When a snake hisses, it is trying to say to animals that might threaten it: โleave me alone!โ At the same time, the snake needs to know what sort of animal it is dealing with, and a snake's tongue is an important way for it to find out what is going on around it.
The symbol of a serpent or dragon eating its tail, known as the Ouroboros, represents the unending cycle of birth, aging, and death.