the end of its thread. The last arm let go, and the zekolo landed safely on its other arms on the cavern floor.
Looking up, Don saw that the last arm still gripped the rocks above. It had remained behind. It had not broken, but the zekolo had snapped it off and thrown it away so that the recoil would not injure the two men who sat in its back. An arm more or less was nothing to the zekolo. It would soon grow another.
THE snake above was unable to follow them. But soon Wimpolo's guards, on their own snakes, reached it. Then Don saw another trick. Several snakes, twining their bodies together, formed a long living rope down which the rest of the snakes and guards traveled in a few seconds.
They were in a large natural cavern that seemed to be quite uninhabited. Not waiting for the snakes and guards, the zekolo raced spider-fashion over the rough floor. Don guessed that it was trailing its mistress by its sense of smell, or by whatever strange other sense took the place of smell with these creatures. Don let it go on its own way, trusting to the instincts of the beast, which knew their purpose quite well, even though it was unable to speak to them.
There were no beast-men to be seen, but it was possible that they might be lurking in the shadows around. A deathray might strike from some hidden point at any moment. The two men put out their lights and relied on the natural lights that glowed from between the stalked eyes of the zekolo, on the heads of snakes and on the life, plant and animal of the cavern.
Presently they passed a group of apemen gathered together on the cavern floor, but the zekolo took no notice, going straight on.
At last they emerged through a small opening into a much vaster, well lighted cavern. They saw, on a wide plain that stretched before them, a Martian city, beautifully designed and colored, an arm of a Martian ocean with ships lying at anchor, and cultivated fields and pastures. But signs of destruction and ruin were everywhere.
It was Selketh.
"Stop," Don whispered. "We must spy out the land before we go further."
BEHIND them they heard cries. Snakes hissed in wrath. Wimpolo's guards had come upon the group of apemen resting in the cavern. The zekolo tried to go back and join in the fight, but Don held it with a low word of command. Their business was to find the Princess, not to look for side-battles.
Two ape-men came running to get help for their companions. Don's ray knocked them both over. A score of death rays, blazing together, had wiped out the rest of the surprised group.
The guards had lost no time. Don saw that each man was being carried in a coil of one of the snakes, in which way they could travel almost as fast as the zekolo.
The scaly, sliding cavalry reached the opening where Don and Winterton waited. Wimpolo's snake, which knew Don, came up to him and rubbed its scaly snout against his leg.
"Where is the Princess?" the leader asked at once.
"This zekolo," Don said, "the Princess' personal pet and very attached to her, trailed her as far as this. That is all we know."
"Our snakes followed your zekolo," said the leader of the guards. "But if your animal is the Princess' pet he should be able to follow her without difficulty. The Princess is there."
He turned to his men.
"Soldiers of Usulor," he said, quietly,
