How do crustaceans breathe?

Crustaceans (true and false crabs, lobsters and prawns etc) need oxygen, just like we do, but instead of using lungs inside the body they use gills outside the body to get it. Gills (and lungs) work because oxygen is a very small molecule. During respiration oxygen molecules first dissolve into a layer of moisture surrounding a thin membrane. Then the oxygen molecules, because they are so small, cross right through the membrane into the circulatory system (the blood) of the animal. The source of the oxygen can either be as gas in the air or already dissolved in another liquid - like the sea. It doesn't matter where the oxygen originally comes from, the most important factor in respiration is that the surface the oxygen molecules cross is wet. In decapods (10-legged crustaceans like crabs and lobsters) the gills are protected because they are enclosed in a chamber under the sides of the carapace (the hard shell that covers the head and thorax). Crustaceans that live in water have no trouble keeping their gills moist. Crustaceans that live on land or on rocky shores where the tide comes and goes keep their gills wet by using fluids from inside the body and by having the chamber well sealed so that very little moisture is lost. The gills themselves are feathery structures at the tops of the walking legs of decapod crustaceans. They are derived from part of the jointed walking leg.