panzerschiff:
Correct! There is no reason Anderssen(and not Andersen) to want to play "black moves" when he had white, since in that match he lost all his games as black against Morphy except one. So there was no reason to want to play black lines....
Also with 1.a3 he succeeded an equal result of 1 win, 1 lose and 1 draw that is 50% while against Morphy in the same match when he tried other opening he had 75%(1 win, 1 draw) so we can't consider 1.a3 a success for him.....
Also Anderssen may tried 1.a3 because he may liked to play a kind of reversed Sicilian Defense after 1...e5 Who knows what he had in mind?!?!
The best responses i know are 1...Nf6! 1...g6 and 1...d5 but 1...e5 is playable too.....