An arcade game from Atari, Centipede was the predecessor of Millipede.
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The actual playfield is 30 cells by 30 cells (the diagram above is designed for proportion rather than cell-for-cell accuracy). Player can move in any direction within the bottom 6 rows and shoot (one shot on screen due to sprite limitations).

The spider

A spider wanders in from the side and moves randomly in eight directions. One shot will destroy it. You get more points for shooting it from close range; point blank nets you 900.

The centipede

A centipede of ten or so segments moves across the screen, moving one square toward the bottom when it hits a wall. One shot will take out a segment and turn it into a mushroom.

Mushrooms

It takes four shots to get rid of a mushroom, and you get only one measly point for doing so.

The flea

When you don't have a lot of mushrooms on the field, a flea will drop straight down the field, leaving mushrooms in its path. It takes two shots to kill a flea. Some people get rid of all their mushrooms to attract fleas, which are worth 100 points a piece.

The snake

Every once in a while, a snake will wander in from one side of the screen. Every mushroom it touches will turn poisonous; a centipede that touches a poisonous mushroom will head straight for the bottom of the screen. Shooting one will net you a metric buttload of points.

This game has touch damage; three touches from an enemy will end your game.