To obtain oxygen from their environments mollusks and crustaceans use which organs

The mantle creates a small empty space called a mantle cavity, which is modified for different functions in different groups of mollusks. Within the mantle cavity hang the gills, highly complex and greatly folded sheets of tissue. Gills are used to exchange oxygen and carbon dioxide in respiration

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Between the roach's body and the mantle is a space called the mantle cavity. It has gills for cockroaches to breathe