In Atari 2600 Centipede, the main objective is to shoot all the segments of a centipede as it winds its way down the playing field. The centipede moves horizontally across the screen, and when it hits a mushroom or the edge of the screen, it drops down one row and changes direction. The player controls a small bug blaster at the bottom of the screen, firing shots at the centipede and other enemies such as spiders, scorpions, and fleas. (Millipede, released in 1982, is the arcade sequel to Centipede. It features more gameplay variety and a wider array of insects than the original game.)
The player controls a small insect-like Bug Blaster. It is moved around the bottom area of the screen with a trackball and fires small darts at a segmented centipede advancing from the top of the screen through a field of mushrooms. Each segment of the centipede becomes a mushroom when shot; shooting one of the middle segments splits the centipede into two pieces at that point. Each piece then continues independently on its way down the screen, with the rear piece sprouting its own head. If the centipede head is destroyed, the segment behind it becomes the next head. Shooting the head is worth 100 points while the other segments are 10. The centipede starts at the top of the screen, traveling either left or right. When it touches a mushroom or reaches the edge of the screen, it descends one level and reverses direction. The player can destroy mushrooms (a point each) by shooting them, but each takes four shots to destroy. At higher levels, the screen can become increasingly crowded with mushrooms due to player/enemy actions, causing the centipede to descend more rapidly.
Once the centipede reaches the bottom of the screen, it stays within the player area and one-segment “head” centipedes will periodically appear from the side. This continues until the player has eliminated both the original centipede and all heads. When all the centipede’s segments are destroyed, another one enters from the top of the screen. The initial centipede is 10 or 12 segments long, including the head; each successive centipede is one segment shorter and accompanied by one detached, faster-moving head. This pattern continues until all segments are separate heads, after which it repeats with a single full-length centipede.
The player also encounters other creatures besides the centipedes. Fleas drop vertically and disappear upon touching the bottom of the screen, occasionally leaving a trail of mushrooms in their path when only a few mushrooms are in the player movement area; they are worth 200 points and takes two shots to destroy. Spiders move across the player area in a zig-zag pattern and eat some of the mushrooms; they are worth 300, 600, or 900 points depending on how close the player shoots it. Scorpions move horizontally across the screen, turning every mushroom they touch into poison mushrooms. Scorpions are also worth the most points of all enemies with 1,000 points each. A centipede touching a poison mushroom will attack straight down toward the bottom, then return to normal behavior upon reaching it. This “poisoned” centipede can be both beneficial and detrimental to the player; the player can destroy them rapidly as it descends down, while at the same time, they can be very challenging to avoid, especially if already split into multiple segments.
The Bug Blaster is destroyed when hit by any enemy, after which any poisonous or partially damaged mushrooms revert to normal. 5 points are awarded for each regenerated mushroom. An extra life is awarded every 12,000 points. (souce: https://en.wikipedia.org/wiki/Centipede_(video_game))