It can also be filled by consecutively shooting four aliens of the same type. When you do this, you get a power-up which can be activated by pressing a button on your gamepad or keyboard. The power-up varies between aliens. Blast four red aliens in a row and you'll receive the Vertical Blast, a shot that can take out an entire row at once. Take out four Anglers and you'll get to fire a Boomerang Bomb that circles the screen and then returns to the tank.
Different bosses arrive at the end of each wave.
They can be defeated with special moves unique to each. I had trouble with the Crush Boss on Pluto, but soon figured out what I was supposed to do and when. Later bosses gave me little trouble, including the disappointing final creature, the Golem. High scores are tracked. There are two difficulty levels, although neither poses much of a challenge to even rookie gamers.