Yes, it would certainly have been nice if someone in the makeup department was thinking about such things and making the aliens plausible. It's strange that no-one would propose that having an alien unable to eat effectively isn't the soundest design. At least it leaves us opportunity to try and reason it out ourselves. I like the parasite idea (I suppose you're suggesting something akin to the pink puppet-things, the bluegill?). Perhaps this should have been another of TheGodBen's tally lists: "Aliens Who Give The Middle Finger To Darwin".