So they lay down on the seaweed, heaped along one end of the Oysters’ armory, and were instantly asleep.

It may have been their natures, or it may have been the influence of the magic coats. But whatever the cause, it is certain that they lay down without fear, slept without dreams, and awoke without alarm when an Oyster corporal touched their arms and whispered, “Now!”

They were wide awake on the instant and started up, picking their oyster shields from the ground beside them.

“I feel just like a Roman soldier,” Cathay said. “Don’t you?”

And the others owned that so far as they knew the feelings of a Roman soldier, those feelings were their own.

The shadows of the guardroom were changed and shifted and flung here and there by the torches carried by the busy Oysters. Phosphorescent fish these torches were, and gave out a moony light like that of the pillars in the Cave of Learning. Outside the Lobster-guarded arch the water showed darkly clear. Large phosphorescent fish were twined round pillars of stone, rather like the fish you see on the lampposts on the Thames Embankment, only in this case the fish were the lamps. So strong was the illumination that you could see as clearly as you can on a moonlit night on the downs, where there are no trees to steal the light from the landscape and bury it in their thick branches.

All was hurry and bustle. The Salmoners had sent a detachment to harass the flank of the enemy, and the Sea Urchins, under the command of Reuben, were ready in their seaweed disguises.

There was a waiting time, and the children used it to practice with their shells, using the thick stems of seaweed—thick as a man’s arm—to represent the ankles of the invading force, and they were soon fairly expert at the trick which was their duty. Francis had just nipped an extra fat stalk and released it again by touching the secret spring when the word went around, “Every man to his post!”

The children proudly took up their post next to the Princess, and hardly had they done so when a faint yet growing sound knocked gently at their ears. It grew and grew and grew till it seemed to shake the ground on which they stood, and the Princess murmured, “It is the tramp of the army of the Under Folk. Now, be ready. We shall lurk among these rocks. Hold your good oyster shell in readiness, and when you see a foot near you clip it, and at the same time set down the base of the shell on the rock. The trusty shell will do the rest.”

“Yes, we know, thank you, dear Princess,” said Mavis. “Didn’t you see us practicing?”