Practice question

Most annelid taxa possess well-developed closed circulatory system representing advancement over open hemocoel of arthropods and molluscs except cephalopods. Basic plan includes two major trunks: dorsal vessel collecting blood from sinus and capillaries pumping anteriorly via peristaltic wave, and ventral vessel distributing oxygenated blood posteriorly interconnected by segmental commissural vessels aortic arches in parapodia and gills. Blood typically contains respiratory pigments hemoglobin dissolved or in corpuscles occasionally chlorocruorin. Auxiliary hearts from enlarged muscular commissurals in earthworms drive circulation. Closed system enables high pressure efficient transport supporting active burrowing lifestyles requiring sustained oxygen delivery and waste removal continuously.