What is the primary respiratory organ used by monotremes, marsupials, and placental mammals?

• A. Gills
• B. Lungs
• C. Skin diffusion
• D. Tracheal tubes

Answer: (B)

Breathing air through lungs is the main way these mammals take in oxygen. Monotremes, marsupials, and placental mammals all use lungs with branching airways and tiny air sacs called alveoli where oxygen moves into blood and carbon dioxide is released. Gills are used by fish and some amphibians, not mammals. Skin diffusion isn’t enough for a mammal’s metabolic needs, so it isn’t the primary route. Tracheal tubes are characteristic of insects, not mammals. So the lungs are the correct primary respiratory organ.