Gaseous exchange in annelids is more advanced and efficient compared to flatworms because?

1. Analyze the question The question asks why annelids (e.g., earthworms) exchange gases more efficiently than flatworms (e.g., tapeworms, planarians).

2. Explain the concept

• Flatworms have no circulatory system and no respiratory organs. Gases move by simple diffusion across the body surface. This works only because their bodies are flat and thin, giving a large surface area to volume ratio, so every cell is close to the surface.
• Annelids are cylindrical and thicker, so diffusion alone through the body surface would not reach all cells. They have a closed circulatory system with blood containing a respiratory pigment (haemoglobin, dissolved in the plasma). The skin is moist and has a network of blood capillaries just beneath it. Some annelids, such as polychaetes (e.g., Nereis), also have specialised structures like parapodia and gills.

3. Go through the options

• A. Wrong. The epidermis of annelids is richly supplied with capillaries, which is the opposite of this statement.
• B. This is the best option. Annelids have more developed respiratory structures than flatworms: a vascularised moist skin, blood with a respiratory pigment, and gills or parapodia in some species.
• C. Wrong. A cylindrical shape gives a lower surface area to volume ratio than a flat shape. It is flatworms that benefit from their shape.
• D. This statement is true of annelids, but it would make gas exchange less efficient, not more. It cannot be the reason for greater efficiency.

4. Answer

5. Exam tip Remember this contrast: flatworms rely on a flat shape, while annelids rely on blood and capillaries. If a question asks why a thicker animal exchanges gas efficiently, look for the option mentioning blood, pigment, capillaries or specialised respiratory structures.