Final answer:
Sea squirts excrete waste through nephridia and exchange gases through gills or the mantle cavity. The excretory system helps maintain water balance.
Step-by-step explanation:
Sea squirts, which are mollusks, excrete waste through tubular organs called nephridia. These organs filter waste from the body fluids and release it into the coelom, the body cavity of mollusks. Sea squirts living in aquatic environments exchange gases with the surrounding water through gills, while terrestrial sea squirts exchange gases through the lining of the mantle cavity. The excretory system of sea squirts helps them to maintain water balance and remove metabolic wastes.