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.
https://gizmodo.com/even-without-skin-the-chestburster-mechanism-test-is-n-1269321296
