Its not Mercury at all, in fact. Its the "Winged Liberty Head with Phrygian Cap." The wings at her temples (Yes, Liberty is always portrayed as a female) represents freedom of thought.
The Prhygians were Greek slaves, who rebelled and won their freedom from the Greeks. During the Roman Empire the Phrygian cap (Latin: pileus) was worn... by former slaves who had been emancipated by their master and whose descendants were therefore considered citizens of the Empire. This usage is often considered the root of its meaning as a symbol of liberty.
On the reverse is a fasces, a Roman symbol of strength through unity. A fasces is a spear, wrapped in a bundle of staves - stronger for the coming together of these separate parts than it would be alone. The coin was designed at a time of great upheaveal among the European empires of the day. These various elements were incorporated in the design to thumb our nose at Europe as a whole, with whom relations were quite strained.
Note, too, that Liberty is facing away from Europe, to the left. Another coin that used this theme to superb effect is the "Liberty, Walking" half dollar. On that beauty, Liberty is swaddled within the protecting folds of Old Glory and has turned her back on Europe. She is walking into the bright sun of a new day... away from Europe, the dark, tumultuous winds of Continental strife at her back.
Those designs were some awesome stuff, eh?
Mercury, by the way, was male and had winged FEET.
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