Mollusca respiration occurs through which structure?

• A. Lungs
• B. Gills
• C. Skin diffusion
• D. Tracheae

Answer: (B)

Most mollusks exchange gases with water through gills, the paired, thin, highly folded surfaces called ctenidia that sit in the mantle cavity. Water flows over these gill surfaces, allowing dissolved oxygen to diffuse into the blood and carbon dioxide to diffuse out. The gills’ large surface area, provided by the many lamellae, makes gas exchange efficient in an aquatic environment.

There are notable exceptions among mollusks: some land-dwelling groups have a lung-like mantle cavity instead of gills, which is why lungs can be used by certain terrestrial snails and slugs. However, the widespread and typical respiration across the mollusk group is via gills. Lungs are more characteristic of those terrestrial relatives, skin diffusion is not the main method for mollusks, and tracheae are features of insects and some other arthropods, not mollusks.