It's pretty much Xenophobe, an up to 4-player game for arcade, ported to other smaller systems.
Play continues until all aliens are eliminated from the base or ship, the base self destructs, players activate the evacuation sequence, or all player characters are killed. Should everyone survive, bonuses are totaled up and the intrepid crew heads to the next infestation site to beam down again.
Rocket Ship, Moon Base, Space City
- punch (unarmed)
- phazer (the starting weapon)
- laser pistol
- lightning rifle
- gas gun
- human skulls
- lab vials
- fire extinguishers
- teleportation device
- Pod (early stage): The first two stages are when the creatures are at their weakest. A few hits should kill them.
- Pod (late stage): Little difference between the two beginning egg stages and this, except if it changes again players will be facing a creature that can actually do damage.
- Critter: Resembling a cornucopia with legs sticking out, this little bugger crawls along the ground and attaches to an explorer if it gets close enough. They must throw it off as soon as possible, because it nibbles on them until it's thrown loose, causing damage over time.
- Rollerbaby: The next stage is one of the most troublesome. In its open form it merely crawls along the ground and does damage to the player character, but if it rolls up into a ball it is all but indestructible, going so far as to deflect weapons fire. Blasting it will also make it bounce around the room, causing characters damage if it makes contact with them.
- Snotterpillar (full-grown): This the deadliest of the normal aliens. It has a lot of hit points, and takes quick leaps in the rooms, bowling characters over and causing significant damage. It also flings acidic spit from a distance. These guys can be blasted apart in mid air as they're about to hit, which is super awesome.