"Well, the job looked like a cinch, so I creased it across the spine with a heat-ray, just enough to double it up while we doped out a muzzle to cap that fiery mouth. It twisted into a knot, all right, but then the damnedest thing happened. The thing split down the middle like an over-ripe fruit and another Cacus popped out almost full-born. The new one spouted a terrific blast of fire at us, and while we ducked out of range, the new Cacus just sat down and made a meal off its mother's -- or is it its father's-insides. You could see the creature grow by inches till it got about the size of the original. Then it made for the ship.

  "Leeds and Machen were guarding the air-lock, and they gave the second Cacus full-power heat-ray. It never bothered the thing. It just burned the two of 'em to so much charcoal with a single breath and pushed on inside the ship." Strike's mouth twisted bitterly at the memory. "Most of the gang escaped, though a few are still in there, safe behind the emergency bulkheads and with some of the air still preserved. Don't think anyone else was hurt."

  The trio hurried toward The Ark.

  "So the Cacus is bisexuals" said Gerry wonderingly. "Self-fertilizing. That's amazing. And only one of them on the whole satellite! That's really amazing."

  Strike looked at her queerly.

  "You don't grasp the truly amazing part of it-the Cacus' imperviousness to Leeds' and Machen's heat guns. Don't you see, Gerry? When Cacus number one was attacked by the heatray, it promptly transferred all its life and intelligence to the youngster in its womb. But it also transferred the power of unbelievable adaptability, so when Cacus number two was born it was completely defended against that heat-ray forever henceforth.

  "It'd be the same for any other weapon we have for capturing an animal alive; it would simply let itself be born again fully adapted and protected. The only way we can stop this monstrosity is by suspending instantly all its vital functions, or by killing it outright."

  Gerry thought for a moment. "Well, why worry?" she said finally. "A cathode gun will always do the trick."

  "That's just it," said Strike with melancholy triumph. "The door to the arsenal was open when the Cacus entered the ship. Everyone ran out of there in a hurry, and there isn't a cathode gun in the crowd."

  Gerry snorted.

  "You certainly have a genius for getting into trouble. But it can't be as bad as you say. For one thing, this business about instant adaptability is so much moon-truffle. It's fantastic. Leeds' and Machen's guns simply failed. Or maybe they shot wildly."

  Strike expressed unutterable scorn. Gerry Carlyle's crew were all sharpshooters, and they simply never got rattled.

  "You'll soon see for yourself," was all he said.

  When the three of them approached The Ark, the crew gave a ragged cheer for their famous leader and rallied hopefully around, visibly heartened. Nothing in their experience had ever completely baffled Gerry Carlyle, except the strange case of the Venusian murri, and they had confidence she would get them out of this predicament.

  Gerry looked over the familiar faces with relief-Kranz, Barrows, Michaels-most of her veterans were all right.

  "Let's find out about this adaptability stuff first of all," she decided. "Anyone got a hypo rifle handy?"

  The original hunting party had carried several, and presently one of them cautiously approached the open port of The Ark to act as decoy while Gerry stood within easy range, rifle ready. The decoy peered gingerly inside the ship, passed the two grim chunks of seared flesh and fabric that marked the pyres of two brave men, then finally vanished inside. Minutes dragged by. Then a faint shout rang in the watchers' helmets, and suddenly the man tore out of The Ark as fast as be could run.

  Once outside, he gave a tremendous upward leap many feet high, and just cleared a sizzling tongue of hot flame that belched out of the door behind him.

  The Cacus, bulgy-eyed and hot-breathed, crouched angrily at the door.

  Quickly Gerry drove home three hypodermic bullets in the creature's soft flesh in the crevices between the armor-like coverings. They took quick effect. The Cacus' head drooped sleepily, and it moved uncertainly as if undecided whether to come out or stay in.

  Then suddenly a series of hideous abdominal convulsions wracked the thing.

  The monster rolled over, still inside the ship; as if an invisible surgeon slit the Cacus open for two-thirds its length, the abdomen parted. Like some strange phoenix of terror, a new Cacus struggled out of the dying body of the old, stood defiantly with the upper half of its body raised on the six legs.

  Unerringly and with no sign of nerves, Gerry deliberately emptied the hypodermic rifle into the new Cacus. The creature lowered itself to the metal floor, hunching along like a caterpillar. Then it turned and commenced ravenously to devour the soft inner parts of its host's anatomy.

  Jerkily it seemed to increase in size, like a speeded-up motion picture of subaqueous life.

  The hypo slugs had absolutely no effect upon it.

  Petulantly Gerry slammed the rifle to the ground, where it bounced lightly.

  "That's impossible!" she cried. "I've never heard of such a thing before in the entire Solar System!"

  "Maybe it got here from some other solar system," Tommy said. "Lord knows how, and isn't native here. But that won't help subduing it."

  "Rats! How about anesthetic gas? Any bombs available?"

  A dozen were turned up. The Cacus having disappeared from view, Kranz daringly ran up to The Ark, threw several of the bombs in, and shoved the port partly closed. In less than five minutes the port was nudged wide open again, and the Cacus, ugly and flame-wrapped, glared challengingly at the little group of scattered humans. Everyone saw instantly that the new Cacus was slightly smaller than the one before, and was still growing. The amazing re-birth had defeated the anesthetic gas as well.

  "Well," said Gerry cheerfully, "I guess we'll just have to quit playing games."

  Chapter XIII.

  Duval the Magnificent

  She quickly set up Lunde's model paralysis ray machine. It worked successfully on Kranz, to everyone's amusement, and Gerry advanced on The Ark. Instantly the Cacus, watchfully guarding the port, emitted a tremendous streamer of fire close to the ground, curling up at the end like an enormous prehensile tongue. Gerry marked the limit of that flame and stopped outside it. Aiming the paralysis ray at the Cacus, she flipped the activating switch.

  Nothing happened. Gerry fiddled with the lens to no avail. She moved closer, only to be forced to scamper out of range of the breath of fire. Then she remembered. Lunde had told her this was a small-scale model, with less than half the power of the working model. The Cacus out-ranged them; they couldn't get close enough to allow the smaller ray machine to take effect.

  The Cacus blew another fiery lance at the crew, as if in derision, then turned at some vibration within the ship and moved into its depths. Abandoning its sluggish mode of crawling, the Cacus coiled and raised its tail over its back much in the manner of the scorpion, and trotted off on its six curious legs in search of some incautious engineer who was seeking, perhaps, to sneak out to safety.

  Gerry wore a baffled expression.

  "That," she pronounced, "beats me. It looks like stalemate."

  "Pardon, mademoiselle. Not stalemate." Everyone turned to look at Duval, who had been completely forgotten in the excitement.

  "No?" said Strike. "Then it's a pretty good imitation of stalemate. He can't catch us in the open; we can't do anything to him."

  "But, monsieur, every second that passes works in favor of the enemy. Our oxygen supply grows short. It is a situation of the most desperate. I, Duval, say it."

  Immediately, though no one had noticed the mustiness of their air before, every person there gestured toward his throat and fumbled quickly with the oxygen valves. Breathing became consciously shallow, slow. There was no sign of panic among these veterans, but uneasiness was a definite presence among them.

  Gerry bit her lip. "Any suggestions, Duval? You've played aces every trick so far."

  "Merci bien. Yes, mademoiselle, I have the suggestion to offer. To combat our enemy, it is necessary that we study him, find his points vulnerable, if such he has."

  "And how'll you get that monstrosity under your microscope?"

  Duval's teeth flashed. "Ah. To study the present Monsieur Cacus, that is not possible. But his ancestors-eh?"

  Startled looks were exchanged.

  "Say, that's a thought!" Strike cried, and led a rapid trek across the plain to where the carcass of the first Cacus lay disemboweled. While not scientists in the strict sense, all the Carlyle crew had had scientific education and training. Almost at once a remarkable discovery was made by Kranz.

  "Captain, will you take a look at this?" He was holding up the dead creature's funnel-shaped mouth, spreading it wide apart with his hands. Instead of true teeth, the entire inner mouth was composed of a sort of flexible horny growth which probably served for mastication when and if necessary. But the extraordinary thing was that every available crevice was veined with a gray, spongy mass.

  "That," said Kranz, "is spongy platinum!"

  "And say!" someone chimed in impressively. "The whole Satellite must be rank with platinum if there's enough to impregnate the system of any animal life."

  Excitement over a possible bonanza discovery stirred them momentarily. Then Duval's ringing voice held them all again.

  "Ah! But more important, I believe, it is that we have here the explanation of the breath of fire! One may read in any textbook of chemistry elementary that when hydrogen or coal gas is made to pass over spongy platinum, it makes of fire, is it no? Well! One may also read that anerobic bacteria, acting upon matter of decomposition in swamps, generate methane, which is one of the constituents-as is hydrogen-of coal gas. Now! All the world knows we have in our digestive tracts many bacteria. Surely, Monsieur Cacus, within, contains anerobic bacteria which act on the decaying matter animal and vegetable, of which a decomposition product must be gas similar to coal gas. Thus the breath of fire!" Duval finished with a flourish.

  Everyone agreed: the Frenchman had something there. But how to turn it to advantage? Strike screwed his face up thoughtfully.

  "Spongy platinum, then," he groped hesitantly, "is a catalyst --"

  Instantly Gerry took him up.

  "Of course! A catalyst! And there are several things which, in combination with it, kill its action as a catalytic agent. The halogens, for instance-bromine, flourine. Or hydrogen cyanide -- '

  Everyone looked at everyone else, eager to advance Gerry's idea, uncertain just how to go about it.

  "That's smart brain-work, Gerry," said Strike, "but our supplies might as well be on Sirius for all the good they can do us. Where'll we get any of the things you mentioned?"

  "If it pleases you, mademoiselle -- " It was Duval again, and hopes soared at the confidence in his voice. "I, Duval, can perhaps solve this problem. You see these blossoms, so tiny, so unimportant?" He toed one of the little groups of close-clinging growths with the colorless, star-shaped blooms. "They are found, I believe, in one species or another, on all the satellites of Jupiter. We know them well. They are related, one might say, to the night-shade of Earth, because they have poison within them. It is, as you have said it, hydrogen cyanide."

  Without the necessity of a single command, the crew went to work. Three of them got furiously busy picking great handfuls of the plants which offered them salvation. Another ran back to the prow of The Ark, from which the man in the pilot house had dropped the important instruments, and had him toss out a space-suit helmet; it would make a perfect pot for boiling.

  The little remaining drinking water left in the pilot house was also lowered. A pair of low-power heat beams was arranged under a tripod made of three of the useless hypo rifles. In a very few minutes the mixture was bubbling merrily-it came to a boil quickly in the absence of much pressure-brewing a vengeful hell-broth for the Cacus.

  By the time it cooled to a scummy liquid with a brown substance deposited from the solution, the whole party was laboring for breath, with the exception of Gerry and Duval, who hadn't been in their space-suits as long as the others.

  Gerry peered around the row of blue-lipped faces; what she had to do now was hard. Someone had to be chosen to try conclusions with the Cacus; someone had to risk his life, perhaps lose it, in a desperate effort to introduce the HCN into the monster's mouth.

  True, it had to be done at close range; so why not try the paralysis ray? But Gerry had come to distrust the ray machine, which was the cause of all the trouble. Perhaps it didn't have the proper power even at close range. If a life had to be lost, it would simply be thrown away if the paralysis ray failed to work. But it might do some good if lost while putting into effect Duval's textbook chemistry.

  The crew would never under any circumstances allow Gerry to try it, so she was forced to call for volunteers. To the last member, they all stepped forward.

  But Tommy Strike stepped farthest, taking the bowl of deadly juice from Gerry's hands.

  "My job," be said briefly. "I'm sort of responsible for this mess. It's up to me to straighten things out."

  Gerry's eyes misted. She had no right to refuse him. Someone had to go and Strike, as co-captain, had authority to choose himself. And rigid discipline of the Carlyle expeditions insisted on no needless sacrifice of life or limb. Strike would go alone. Gerry needed all her iron control at that moment.

  Strike opened one of the meta-glass gas bombs to allow the gas to disperse, then filled it with most of the poison solution, saving a little for a second try in case he failed. With a crooked grin be waved salute and started toward The Ark. Deftly, and before anyone had the slightest inkling of what was happening, Duval slipped up behind Strike, tripped him, and threw him easily to the ground. He caught the meta-glass ball as it floated downward.

  Gerry yelled at him.

  "Duval! Stop it! You've done enough already, besides, you're not properly one of us at all. Put that down!"

  Duval's smile gleamed brightly. "But I have just made a flight impossible from Ganymede to Satellite Five in a scrap heap. Today is my day of luck! I cannot fail!"

  "Duval! Come back! We want no quixotic foolishness. If you understood our discipline you'd realize we just don't do things that way."

  And Duval of the empty life, whose passing none would mourn, who burned to do heroic things in the grand manner, said soberly:

  "And if you, mademoiselle, but understood the French, you would realize that we Gascons do things this way."

  And he was gone, running rapidly toward The Ark. Strike floundered finally to his feet, snarling. He seized the paralysis ray model and set out after Duval as fast as he could go. In a flash the entire crew made a concerted rush in the same direction. Only Gerry's savage commands halted them reluctantly.

  Duval reached the port, peered cautiously in, then vanish inside. Strike followed him less than a half minute later. Then nothing. The watchers outside listened intently at their helmet earphones, but no word came from either Duval or Strike They got in touch with those still trapped in the ship, but the latter reported nothing. That was natural, as the lethal game being played between Duval, Strike, and the Cacus was taking place along nearly airless passages where sound would not carry well.

  Presently the listeners were shocked to hear a high-pitched squeal like that of a wounded horse coming faintly through the earphones. It was nothing human: it must have been picked up by someone's helmet mike at a point very close to the screamer. At that, all restraint was flung aside and the crew, with Gerry in the lead, pounded pell-mell over the solid terrain and recklessly into The Ark.

  They burst in gasping on a climax of terrible ferocity. It was so swift, so savagely sudden, that it was all over before they could throw their feeble powers into the balance.

  The Cacus had evidently been prowling down a side passage, and Duval had attracted its attention, then ducked around a corner into the main corridor; when they met, it would be at close quarters where there was no chance for the Frenchman to miss. As the crew tumbled in, Duval was crouching by the passage corner and had just finished yammering at Tommy Strike to stay back and not be a fool. Strike had apparently started in the wrong direction and had just located the real theater of action; he was running purposely along the corridor to back up Duval's play.

  And then everything happened at once, like a badly-rehearsed bit of stage continuity in which the actors rush through their parts almost simultaneously.

  The Cacus, tail curled up and running on its six legs, skidded furiously into the main corridor of The Ark. At once it spied Duval and emitted another of those hideous shrilling sounds. Duval's arm went back, whipped forward. A glittering arc made a line straight for the ugly, horn-like snout of the beast. Strike, off to one side and several feet behind Duval, dropped to his knees and fumbled with the ray-box. A terrific blast of flame belched out from the Cacus to envelop head and shoulders of the doughty Frenchman.

  For a moment it appeared that the fiery stream had caught the container of HCN and demolished it. But no-the Frenchman had been the quicker; he had scored a bull's-eye. By the time the Cacus turned to annihilate Strike, the hydrogen cyanide had entered into combination with the spongy platinum, and nothing but a burst of gas came forth. From that moment the monster was through. Strike brought the miniature paralysis ray to bear, and instantly the Cacus collapsed in a twitching mound of nauseous flesh.

  Cathode guns were brought from the arsenal, and the Cacus was ruthlessly blasted out of existence. Then Gerry and Strike hurried to Duval's side. The Frenchman was terribly burned, his face a blackened, blinded travesty of a man. The spark of life was almost extinguished. But as the two knelt beside him, Duval's cracked lips managed a feeble grin.

  "Mademoiselle," he whispered, "will have to collect that wager I have won from the good Bullwer. We made the flight. He has lost a week's pay, that one." Something like a laugh bubbled up from his seared chest.

  Gerry groaned in anguish.

  "Duval! Oh, you magnificent fool, Duval! Why did you do it? Because of me, you must die. That's wrong --"