Concept:Unicellular organisms exchange gases directly through their plasma membrane by simple diffusion.
Explanation:Paramecium is a single-celled organism, so it has no lungs, gills, or specialized respiratory organs.
Its thin plasma membrane allows oxygen to diffuse into the cell and carbon dioxide to diffuse out.
This diffusion occurs over the entire cell surface because the organism is microscopic and has a large surface-area-to-volume ratio.
Hydra is multicellular and exchanges gases through its general body surface.
Flatworms rely on their flattened body surface, but they are not single cells using a plasma membrane.
Earthworms carry out cutaneous respiration through their moist skin and capillaries.
Only in paramecium does gaseous exchange occur directly through the plasma membrane.
Answer:B. paramecium