The arthropod circulatory system is:

• A. Open - hemolymph acts as blood
• B. Closed - blood in vessels
• C. Absent
• D. Lymphatic-based

Answer: (A)

Arthropods have an open circulatory system, where the circulating fluid called hemolymph is pumped by a dorsal heart into the body cavity and directly bathes the organs. It moves through spaces in the hemocoel rather than staying inside a continuous network of vessels. The hemolymph then returns to the heart through openings called ostia. Gas exchange in arthropods is handled by separate structures like the tracheal system in insects or gills in crustaceans, so the hemolymph isn’t primarily responsible for oxygen transport. This combination—fluid circulating through body cavities rather than being confined to vessels—best describes an open circulatory system. It isn’t absent, and it isn’t lymphatic-based, since arthropods do have a circulating fluid that functions similarly to blood but uses a different organization.