What organ does the prawn use to 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.