An open circulatory system, where the circulating fluid bathes organs directly, is characteristic of:

• A. Arthropoda
• B. Vertebrates only
• C. Cnidaria only
• D. Nematodes only

Answer: (A)

Open circulatory systems bathe organs directly in the body cavity with circulating fluid, rather than keeping all blood in a network of vessels. In arthropods like insects, spiders, and crustaceans, the heart pumps hemolymph into the hemocoel, a spacious body cavity, where the fluid bathes tissues and organs before returning to the heart through openings called ostia. This setup contrasts with vertebrates, which have a closed circulatory system where blood stays confined to vessels; cnidarians rely on diffusion within a gastrovascular cavity rather than a true circulatory system; and nematodes lack a true circulatory system altogether, functioning mainly through diffusion and a hydrostatic body plan. Because the described open, vessel-free flow of circulating fluid is a hallmark of arthropods, that is the best match.