Concept:Efficient gaseous exchange depends on a large, moist respiratory surface and an effective internal transport system.
Explanation:Flatworms are flat and simple. Gases diffuse directly through their moist outer surface because every cell lies close to the outside environment.
Annelids are elongated and cylindrical in shape. This gives them a relatively high surface area to volume ratio for gaseous exchange.
Their skin is moist and richly supplied with blood capillaries.
They also have a closed circulatory system. Blood transports oxygen to deeper body tissues and carries carbon dioxide back to the body surface.
Some aquatic annelids have gills, but the main advantage is the combination of a large respiratory surface and a transport system.
This makes gaseous exchange in annelids more advanced and efficient than diffusion alone in flatworms.
Answer:C. Their cylindrical shape gives high surface area to volume ratio.