One of the freakier things about H.R. Giger's design of the Alien (from the movie of the same name) was that weird set of inner jaws that the alien had which would snap out at you when it got close.
The technical term for these sorts of second jaws are pharyngeal jaws. Such jaws actually do exist in nature, but, before now, no one had observed a set of pharyngeal jaws that could actually move independently of the outer jaw (they tend to be more like a second row of teeth). Recently, however, scientists have discovered that moray eels actually do have a pharyngeal set that do work something like the one's that the Alien has.
Let us hope that the rest of Geiger's more creative speculations remain in the realm of fiction.
Thanks to Arturo Magidin for pointing me to the NPR link