Which statement about circulatory systems is true for lancelets and tunicates?

Which statement about circulatory systems is true for lancelets and tunicates?

• A. Both have hearts
• B. Lancelets have a heart; tunicates do not
• C. Lancelets lack a heart but have contractile vessels; tunicates can have both closed and open circulatory systems
• D. Neither has a circulatory system

Answer: (C)

Circulatory organization varies a lot among basal chordates, and a true chambered heart isn’t universal outside vertebrates. In lancelets, there isn’t a true heart. Blood is moved by rhythmic contractions of contractile vessels along the body, with circulation driven by these vessels rather than by a single chambered pump. That fits “lancelets lack a heart but have contractile vessels.” Tunicates, on the other hand, show more variation. Their adults generally have an open circulatory system—blood moves into body sinuses rather than being confined to a network of vessels—yet some tunicate species or life stages can display a more closed pattern with a heart-like pumping mechanism. So it’s accurate to say tunicates can have either closed or open circulatory arrangements depending on the context. Putting that together, the statement that lancelets lack a heart but rely on contractile vessels, while tunicates can exhibit either closed or open circulation, best captures the natural diversity of circulatory organization in these groups.

Circulatory organization varies a lot among basal chordates, and a true chambered heart isn’t universal outside vertebrates. In lancelets, there isn’t a true heart. Blood is moved by rhythmic contractions of contractile vessels along the body, with circulation driven by these vessels rather than by a single chambered pump. That fits “lancelets lack a heart but have contractile vessels.”

Tunicates, on the other hand, show more variation. Their adults generally have an open circulatory system—blood moves into body sinuses rather than being confined to a network of vessels—yet some tunicate species or life stages can display a more closed pattern with a heart-like pumping mechanism. So it’s accurate to say tunicates can have either closed or open circulatory arrangements depending on the context.

Putting that together, the statement that lancelets lack a heart but rely on contractile vessels, while tunicates can exhibit either closed or open circulation, best captures the natural diversity of circulatory organization in these groups.