Ourobouros

The Ourobouros is a depiction of a snake curled in a circle and devouring its own tail.

The Ourobouros can be traced back to the Greek philosophers who used it as a symbol of their understanding of the nature of time as cyclic and, "plus la meme", could very well be used to symbolize the closed-system model of the universe of some physicists today.

Christians early adopted the Orobouros as a symbol of the limited confines of this world (that there is an "outside" being implied by the demarcation of an inside), and the self-consuming transitory nature of a mere this-worldly existence (following in the footsteps of the Preacher in Ecclesiastes).