Integumentary Gas Exchange
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Integumentary gas exchange occurs directly across an animal's moist skin, working only for small or thin-bodied animals whose surface-to-volume ratio, per Bio 2727, keeps diffusion distances short.
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An earthworm exchanges gases across its moist skin alone, without lungs or gills.
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A dried-out skin surface halts gas exchange entirely, which is why this strategy is limited to consistently moist environments.