Crustaceans circulation

Barnacles are sessile crustaceans that live within protective plates. Positioned so that their heads are down and their legs are sticking out into the water, barnacles spend their lives attached to wood, rock, or some other solid surface. At low tide, when they are exposed to air, barnacles close their plates to prevent drying. These plates have a small opening that allows moist air to circulate over their gills, providing oxygen. 

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