and maggie discovered a shell that sang so sweetly she couldn't remember her troubles,and (3-4)
In the manmade world, maybe maggie's troubles seem really important and significant. But out in the natural world, those troubles just seem to disappear, which suggests that maybe they aren't so important after all. Man has a way of magnifying trifling matters, while nature has a way of bringing those same troubles right down to earth where they belong.
milly befriended a stranded star whose rays five languid fingers were; (5-6)
The natural world and people don't seem so far apart in these lines. The speaker personifies that starfish and bridges the gap between man and nature in a way that's comforting to milly. Even loneliness doesn't seem to be exclusive to man alone. A stranded starfish can look just as lonely, which makes any lonely person feel like they're not alone in their experience.
may came home with a smooth round stone as small as a world and as large as alone. (9-10)
A small thing like a stone sure seems to carry an entire world of possibilities, in terms of how we can perceive it. In one light, maybe it looks small, but in another light, we might liken that stone to the world itself. The planet, after all, must look pretty small in an infinite universe. Nature lets us know that the world is never so black and white so as to permanently fix anything in a neat little definition.