Lancelets circulatory system is best described as?

Lancelets circulatory system is best described as?

• A. Open circulatory system with a heart
• B. Closed circulatory system, lacks heart, with contractile blood vessels
• C. Closed circulatory system with a prominent heart
• D. Diffusion only

Answer: (B)

Lancelets have a closed circulatory system, meaning blood stays inside vessels rather than bathing tissues directly. They lack a true heart; instead, contractile blood vessels generate the pumping action that moves blood through the dorsal and ventral vessels and into the capillary beds of the pharyngeal slits. This combination—blood confined to vessels with vessel-wall contractions but no central pumping chamber—best fits a closed system without a heart. A diffusion-only description would ignore the presence of vessels, and a closed system with a prominent heart would imply a true cardiac chamber, which lancelets do not have.

Lancelets have a closed circulatory system, meaning blood stays inside vessels rather than bathing tissues directly. They lack a true heart; instead, contractile blood vessels generate the pumping action that moves blood through the dorsal and ventral vessels and into the capillary beds of the pharyngeal slits. This combination—blood confined to vessels with vessel-wall contractions but no central pumping chamber—best fits a closed system without a heart. A diffusion-only description would ignore the presence of vessels, and a closed system with a prominent heart would imply a true cardiac chamber, which lancelets do not have.