They looked at each other, and a general hush fell upon the group. A disciplined army—here? It made no sense at all. The upper caverns would not have organized any search party, and could not have kept one supplied this far. If the exit to the long trek were near, there might be human beings, and they might indeed march, but not from the direction the prison party had come.

The sound persisted, coming down the tunnel toward their dome, a measured beat increasing in volume. This, whatever it might be, was the thing that had driven the other animals before it.

Everyone in the dome had heard it now. Sleeping people were nudged awake, to listen apprehensively of a sound they could not understand.

“Great Chthon!” the guard exclaimed, drawing back in terror. The marching beat loudened. The source had rounded the last bend, though it was not yet in sight of the people in the dome.

Then it appeared. A gigantic, grotesque head poked through the entrance. It had enormous faceted eyes and antennae as thick as a finger and a foot long. It rotated slowly, mechanically, to gaze upon the assemblage; the tramping died away. Then it came forward—into their refuge.

The back of the head compressed into a neck scarcely two inches thick. The body emerged: a squat, irregular hump, supported by two thick legs that lifted and fell jerkily. The people nearest fell back, appalled, giving ground to the creature.

The body tapered down again to a two-inch-diameter tail. Then, astonishingly, a second body followed, similar to the first. A third, and a fourth. The thing was segmented!

Now the people before the head were crowding back in unabashed horror, desperate to get away from it, but finding no room to retreat. Some jumped into the water as the creature advanced relentlessly.

There was a general shuffle as those who could not swim, or who were afraid to, pushed roughly around the ledge to make way for the caterpillar. Aton and Bossman were now nearest to the great head. Bossman’s axe was ready, but he chose so far to retreat rather than to attack. Not enough was known about this thing, yet.

The ledge had become quite crowded. There had been little surplus room to begin with, and a considerable length of the monster had appeared. Ten, fifteen bodies; and more segments appearing endlessly, until it took up almost a quarter of the circumstance. When would it end?

Aton noticed that the latter parts were misshapen, grotesque even to one familiar with the standards of a creature such as this. They were no longer uniform, except in the synchronized motion of the legs. Some of the segments had extra limbs hanging uselessly by their sides, withered. Some segments appeared to have shrunken heads. It was as though they actually belonged to different species.

One segment even looked human.

Ridiculous! Aton backed away from the gross head.