Species of spaces

“We can look at ducks, and afterwards, we can look at the pictures and talk about ducks.”

—"Screen Time Higher Than Ever for Children," NYT, 10/25/11

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The creature periodically erects barriers inside its shell as it grows, leaving a series of unoccupied spaces behind. —"Loving the Chambered Nautilus to Death," NYT, 10/25/11

What is this a metaphor for?