only tiny ripples running through the monster as it
fed.

Sue Guinness turned her head. But the two men for some reason could
not take their eyes away....

* * * * *

It was the girl's voice that jerked them back to reality. "The other!"
she gasped. "It's coming, behind!"

They had completely forgotten the mass in the tunnel. Turning, they
saw that it was only fifteen feet away and approaching fast, and
instinctively they ran out into the cavern, skirting the sphere
widely. When they came to Quade's wrecked borer Phil, who had snatched
a glance behind, dragged them down behind it. For he had seen their
pursuer abandon the chase and go to share in the meal of its fellow.

"We'd best not get too far away," he whispered. "When they leave the
front of the borer, maybe we can make a dash for it."

For minutes that went like hours the young man watched, waiting for
the creatures to be done, hoping that they would go away. Fortunately
the sphere lay between, and he was not forced to see too much. Only
one portion of one of the monsters was visible, lapping out from
behind the machine....

At last his body tensed, and he gripped Sue and her father's arm in
quick warning. The things were leaving the sphere. Or, rather, only
one was. For Phil saw that they had agglutenated--merged into
oneness--and now the monster that remained was the sum of the sizes of
the original two. And more....

They all watched. And they all saw the amoeba stop, hesitate for a
moment--and come straight for the wrecked borer behind which they were
hidden.

"Damn!" Phil whispered hoarsely. "It's still hungry--and it's after
us!"

David Guinness sighed wearily. "It's heavy and sluggish, now," he
said, "so maybe if we run again.... Though I don't know how I can last
any longer...."

Holmes did not answer. His eyes were narrowed; he was casting about
desperately for a plan. He hardly felt Sue's light touch on his arm as
she whispered:

"In case, Phil--in case.... This must be good-by...."

But the young man turned to her with gleaming eyes. "Good-by,
nothing!" he cried. "We've still got a card to play!"

* * * * *

She stared at him, wondering if he had cracked from the strain of what
he had passed through. But his next words assured her he had not. "Go
back, Sue," he said levelly. "Go far back. We'll win through this
yet."

She hesitated, then obeyed. She crept back from the