The shoal set off. There were millions now, darting here and there, their tiny round bodies flashing like crystal globules through the water, their belts of swimming hairs wafting the microscopic creatures of the sea into their ever-ready mouths. For days they travelled, growing every hour a little larger, but still defenceless in the savage sea. Sometimes lurking enemies dragged off stragglers from the edges of the shoal; sometimes a great fish drove through their millions with his mouth wide open, swallowing all that came within his path. Then the ranks closed up again and went onward as before; but the shoal was smaller than at first, and the babies grew more watchful. At last they were tired, and a little frightened too. "Let us find a settling-place and grow our shells," said one. They sank to the sea-floor. It was sand. That would not do. They drifted on. The sand gave place to mud. That would not do, either. They drifted on again. At last a stretch of gravel, clean and firm, lay beneath them. "A splendid place," said the babies, joyfully, remembering their mother's words. Down they dropped, each one settling on a stone and there fixing himself for life. Now came the marvellous making of those strong shells which were to be their safe retreat from every enemy. Furnished by the rich seafood, a limy fluid formed in each soft baby's body, to ooze through tiny pores in his outer skin, and there to harden into shell. Day by day, week by week, the beautiful growth went on, till a two-walled house was made, with lustrous pearly lining and a powerful hinge to pull the edges of the walls together. At first the shells were thin. Hungry whelks, finding them, could bore round holes in them with their sharp-pointed shells and so reach the juicy babies; wandering starfishes could clasp them in their long ray-arms and swallow shell and baby whole. But as the months and years passed by, and the surviving babies grew to greater size, layer after layer was added to the shells, until at last, rock-hard and strong, they kept out all intruders. Now the oysters were secure. From helpless, shell-less, reckless babies they had grown to cautious, well-defended dwellers in the sea, living quiet lives in peace within their firm shell walls. When no enemy was near their shells lay open; their fringed, delicate gills were hung out and waved to and fro to catch their food. But at the first alarm there was a quick withdrawing of the gills, an instantaneous closing of the shelly walls. To the enemy all was firm-locked, silent, hidden. The babies had grown into full knowledge; they had learned when to shut up. FANNY FLY