Well, I see you already got help, but since I went through making this, I'll post it here for the benefit of anyone else who needs it
If you had your ass layer as Layer B and your text as Layer A, and you wanted the ass to be behind the text, you'd go to the layers palette, click the Layer B layer and drag it below layer A on the pallette.
Or you could also use the (Layer > Arrange > Bring To Front) but that wouldn't work unless the two layers in question were indeed the top two layers. If there were other layers you would use (Layer > Arrange > Bring Forward) or Ctrl+J.