"Don't be alarmed," Klalmar-lan reassured them. "I've got him under a neural anesthetic. He's suffered a bad radium burn, but I think he'll be all right. Should recover consciousness in a couple of hours." Klalmar-lan was at the controls, and they were rising rapidly. The little spot of green was visible through the rear port, falling away behind them. "I first met Denny on Venus, where I had been sent to watch for the coming of Ghlak-Ileth, or Hell-worms, as we call them; for they are no new experience to us Martians. Some three thousand Earth years ago, they turned our once beautiful planet into a red desert, almost exterminating our race. Three thousand years before that, our astronomers had watched as uninhabited Mercury gave up its treasure. According to all our calculations, Venus should have been next. When I talked to Denny in his jungle camp, he informed me that he had discovered remains of an ancient civilization on Venus. "I knew then that something was terribly wrong with our theory--for we had always considered Venus a very young planet, whose evolution of life had not even produced a mammalian form, and would not for millions of years. Now it seemed more plausible that at a remote age Venus was inhabited by intelligent beings, perhaps more highly developed than we on Earth or Mars, and that some great catastrophe wiped them out, leaving survivors, the ancestors of the present day fauna. "The answer," he went on, "was plain--the Ghlak-Ileth had already been to Venus! In all probability, Earth would suffer the effect of the next raid! Denny had started for Earth with his crew. I hurried to my ship and followed him. About two hours out, my mass detector indicated the presence of matter about ten thousand miles ahead, but moving _toward_ me. In a little while I saw it, approaching headon. A huge blob of a ship, gleaming like quicksilver, shaped like a great flat-bellied slug. The Ghosts of Outer Space had come again!" "Hold it!" cried Art. "This is getting beyond me. Who are these--" "We call them Ghosts, or Voornizar, because they bear little resemblance to anything mortal, although they are terribly real. They are the masters, the creators of these Hell-worms, whom they planted countless eons ago on the planets of our Solar System. The impelling energy of these Ghlak-Ileth, as with their masters, and in fact all the machinery they use, is the disintegration of radium, of which they are partially composed. They devour it for food.