What substance is being carried to the gills for disposal?

The substance being carried to the gills for disposal is carbon dioxide (CO2). As blood circulates through the body, it picks up CO2 produced by cellular metabolism. This CO2 is then transported to the gills, where it is exchanged for oxygen, allowing the organism to expel the waste gas into the surrounding water. This process is crucial for maintaining the acid-base balance and overall homeostasis in aquatic organisms.