Damon Knight 
Double Meaning [EPUB ebook] 

The fate of the Earth Empire hung in the balance – and Security Commissioner Spangler knew it was up to him to find the monster, the Rithian Terror, as some called it. Seven Rithians had landed on Earth. Six had been disposed of. One was loose.Surely, Spangler reasoned, the stereoptic fluoroscope would flush it out. 'That’s one test the Rithian can’t meet, no matter how good his human disguise may be.’ Spangler explained to Pembun, the strange, little Colonial who had been sent to help find the monster. But Pembun didn’t agree. 'The trouble, ’ he said, 'is that the Rithi have no bones. Which would be indication enough under a fluoroscope, if it weren’t for the fact that it can easily swallow a skeleton.’Spangler shuddered.