The Professor shook his head. "They're too clever to do that twice. The very fact that they kept half their number in reserve shows that they have some new trick to try. Otherwise they'd all have come at once in one supreme effort." He paced back and forth. "They're ingenious, intelligent. They're fighting for their very existence. They must have figured out some way of breathing in air, some way of attacking us on a more even basis in case that first rush went wrong. What can it be?" "I think you're borrowing trouble before it is necessary--" I began, smiling at his elaborate, scientific pessimism. But I was interrupted by a startled shout from Stanley. "Professor Martin," he cried, pointing to the tunnel mouth. "Look!" Like twin snakes crawling up to sun themselves, two tentacles had appeared over the rock rim. They hooked over the edge; and leisurely, with grim surety of invulnerability, the barrel-like head of a Quabo balanced itself on the ledge and glared at us. * * * * * For a moment we stared, paralyzed, at the Thing. And, during that moment it squatted there, as undistressed as though the air were its natural element, its gills flapping slowly up and down supplying it with oxygen. The thing that held us rooted to the spot with fearful amazement was the fantastic device that permitted it to be almost as much at home in air as in water. Over the great, globular head was set an oval glass shell. This was filled with water. A flexible metal tube hung down from the rear. Evidently it carried a constant stream of fresh water. As we gazed we saw intermittent trickles emerging from the bottom of the crystalline case. Point for point the creature's equipment was the same as diving equipment used by men, only it was exactly opposite in function. A helmet that enabled a fish to breathe in air, instead of a helmet to allow a man to breathe in water! Stanley was the first of us to recover from the shock of this spectacle. He faced about and raised his voice in shouts of warning to the resting Zyobites. For other glass encased monsters had appeared beside the first, now. One by one, in single file like a line of enormous marching insects, they crawled down the wall and humped along on their tentacles--around the ditch and toward us! * * * * * The deadly infallibility of that second attack! The Quabos advanced on us like armored tanks bearing down on defenceless savages. Their glass helmets, in addition to containing water for their breathing, protected them from our knives and axes. We were utterly helpless against them. They marched in ranks about twenty yards apart, each rank helping the one in front to carry the cumbersome water-hoses which trailed back to the central water supply in the tunnel. Their movements were slow, weighted down as they were by the great glass helmets, but they were appallingly sure.