Dallas and Ash remove Kane's helmet and we witness the facehugger in all its gruesome glory. When Ash messes around with it, the thing's prehensile tail tightens around Kane's throat in a clear sign of aggression.
Scanning it, Ash assumes the creature is feeding Kane oxygen to keep him alive (In a blink-and-you'll-miss-it cut, the monitor shows something entering Kane's blood stream; only Ash is watching the screen at this time).
Dallas orders the creature removed, but when Ash tries to cut off one of its fingers, it bleeds a molecular acid that almost eats through the ship's hull. Guess not, then.