And as that dazzling light-band moved around the big moon-world's dark side, around the almost empty compartment-city that covered that dark side, we saw emerging into that compartment-city of the dark side, as though from its walls themselves, millions on millions of disk-bodied Neptunians that matched in number the vast swarms on the sunlit side! And as we gazed down in utter amazement we saw from whence they came. There were in the dark compartment-city's extent many compartments like those we had seen upon Neptune, with nothing in them save shelving, which formed in their walls myriads of shelfed openings a few feet in height and some four feet in width, one above the other. And in these narrow, flat shelf-openings countless Neptunians had been sleeping! Their disk-bodies, with the flexible legs drawn up, fitted snugly into those flat, strange openings in the walls, and vast hordes of them, countless millions of them, had been sleeping in the shelf-compartments on Triton's dark side! As that band of brilliant light swept swiftly across the dark side, though, they had awakened, were pouring forth in all their hordes into and through the compartments, all streaming toward the sunward side, while the more remote of them were heading toward the same side in flashing cylinders above. Then, as we gazed toward that sunward side, we saw the brilliant band of light reappearing there, moving swiftly still around Triton's surface, through the pale dim light of its sunward side, having in those moments moved completely around Triton! It moved on until in a moment more it had stopped where first it had formed, at the junction of the dark and sunward sides. There it hung for a moment, dazzling, and then had suddenly snapped out of being. And now we saw that all the crowding millions of Neptunians that had been busy upon the sunlit side were streaming through the compartments toward the dark side! "The Neptunians' day and night!" Marlin exclaimed, as we gazed downward. "Triton must keep one face always toward the sun and one dark, so these Neptunians spend their day on the sunlit side and sleep their night on the dark side!" "And that great band of light that traveled around Triton was their signal, then!" I added. * * * * *