The original Chestburster from Alien lacked arms. They were added by propmasters in the sequel, which surprised special effects artist Stephen Norrington, who narrates this video about the creation of the little bastard. "I'm amazed at how much emotion comes out of those little arms," he says.
Some horror creatures become less frightening when you see the special effects that brought them to life. Not so with the Alien chestburster, which is still pretty damn creepy even during its mechanism test.
Stan Winston School posted this video of the mechanism test. Stephen Norrington's description of the chestburster mechanism as "serpentine" is perfect. Even when you can see all of its components, it looks read to chew its way through any obstacle.