Cool! The explosions are a nice touch, one I hadn't seen before in games like this, (well, I haven't played many of them) You know what else would be nice? If regular enemies could randomly explode like that when destroyed, that way the player will think twice before just shooting its way through waves of enemies... just an idea though.
Also, in case you're taking suggestion... now that you have the "outer space" and "alien base" levels, how about a "inside the beast" level to wrap the demo up, you now with fleshy walls and a brain or heart to shoot at the end of the stage. =)