Criticalmass is a space shootemup similar to Galaxian.

Your world has been infested by an aggressive army of space critters. Overrun and unprepared, your government was unable to defend its precious resources. As a last effort to recapture some of the 'goodies', you have been placed into a tiny spacecraft and sent after them. Critical Mass (aka Critter) is a shoot'em up (smup) similar to Galaxian. The basic idea is simple: you move your ship back and forth at the bottom of the screen, shooting up, while bad guys fly around and shoot at you.