The symbolism of the snake is a universal archtype over the ages in various cultures.
Imagine a snake crawling over your body and that you are a small child and that you are not aware that it is a snake. Or you have not learned to name it as a snake. What do you find? You find a supreme pleasure in its touch. It coils around your limbs and a beautiful massage is being given to you by the snake. In this situation you are not naming it and not identifying it with a situation that is potentially dangerous. You just play with that. This is the nature of Siva.

The moment you associate that situation with the notion of fear that it can kill you, then the fear is related to the muladhara chakra. On the one hand there is pleasure and on the other hand there is fear. This combination of the pleasure-fear complex is what is symbolized by the snake.

If you look at the philosophical structure behind this, you find that the snake is something that moves in a wavy curvy fashion, not straight. They say that when you are drunk you move in a wavy fashion, you are not clear in what direction you are moving.

But if a snake becomes drunk, what does it do? It moves straight. The mind and its thought patterns are like the snakes, going hither and dither in wavy fashions. But when the mind becomes steady and one-pointed, when it flows relatively straight then it is "drunken". This is the drink that they refer to in the tantra. The drink, the ambrosia which makes your mind one-pointed and straight.

The Kundalini shakti is flowing up the sushumna channels instead of going round the petals in whatever way it wants. This is the symbol of the snake.