During the whole of the Alabasta arc, the Straw Hat Pirates had wraps on their arms concealing that they all had a black "X" underneath for means for making sure that they could tell the real Straw Hat from the fake made by Mr. 2's devil fruit the Mane Mane no Mi. The "X"s were revealed right at the climax of the arc, when the crew was saying goodbye to Vivi, as a sign that they were still friends even though they couldn't say it out loud.
Well, as I was watching the anime, I noticed that it was not long after that the "X" disappeared off of the arms of the Straw Hat Crew. Is this just that time went on and they wore off or is there supposed to be some deeper meaning to it?