Respiration in Animals

Core concept

Gills work by allowing water to flow over thin membranes rich in blood vessels, where oxygen from the water diffuses into the blood while carbon dioxide diffuses out — this is why fish need constantly flowing water over their gills to survive. An earthworm's skin must stay moist for cutaneous respiration to work efficiently, which is why earthworms are usually found in damp soil and often come to the surface after rain when their burrows fill with water.