The Ouroboros symbol represents self-reflexivity or cyclicality, especially in the sense of something constantly re-creating itself, the eternal return, and other things perceived as cycles that begin anew as soon as they end (compare with phoenix).
Seal of Solomon. In Medieval Jewish, Christian and Islamic legends, the Seal of Solomon was a magical signet ring said to have been possessed by King Solomon, which variously gave him the power to command demons/genies, or to speak with animals.
Henry Cornelius Agrippa produced this pentagram in his century Three Books of Occult Philosophy. It displays humanity as microcosm, reflecting the influences of the wider macrocosm as indicated by the seven planetary symbols.