Sentence:  Those who have never seen a living <WSD>Martian</WSD> can scarcely imagine the strange horror of their appearance. The peculiar V-shaped mouth with its pointed upper lip, the absence of brow ridges, the absence of a chin beneath the wedge-like lower lip, the incessant quivering of this mouth, the Gorgon groups of tentacles, [...] 
Meaning of the word marked with <WSD> tags:  An inhabitant of the planet Mars.