backbone would be as stiff as a—well as an iron poker. Then 111 make paddles on the sides, of the same horny substance that your shell is made of, but mixed with a softer substance so that I can furl and unfurl them like boat sails when I want to move through the water, and back up and turn and twist. The wings of those dragon-flies are made of just such stuff, but they aren't thick and tough enough for flying through the water—those beautiful gauzy wings.
So the oyster keeps his improved insides, shingles himself with scales made of the lobster's shell, changes the claw legs into fins, sets his tail on edge for a rudder, makes an inside backbone of his limy, outside house, but jointed like the crawfish's shell, and ringed like the muscles of the earthworm. He swallows some extra mouth-fuls of air in a little bag called his "swim bladder" to make himself lighter, and away he goes through the water!
When he gets to be a bird you will see him twist his tail back again, and carry it in the same position the lobster does. It will lie flat on the air pressing it down, just as the lobster's tail lies flat on the water!
What will he do that for?
That is the very first question we will take up when we come to birds—why does a bird have his tail set on like a crawfish?
But why not watch the next bird you see; notice how he uses his tail as a crawfish does. You can easily see this because he uses it in this way just when you can watch him best—when he drops to a perch.
