Lancelets lack which heart-related structure?

Lancelets lack which heart-related structure?

• A. Heart
• B. Blood
• C. Gills
• D. Contractile blood vessels

Answer: (A)

Lancelets illustrate that a heart is not essential for all chordates. They have a simple circulatory system with blood transported through vessels, moving by the rhythmic contraction of those vessels rather than by a true, chambered heart. Gas exchange happens mainly through diffusion across the body surface and through pharyngeal slits used in feeding, which function similarly to gill structures in terms of respiration. They do have blood, and their vessels are contractile to push blood along, but there is no heart. So the heart-related structure they lack is a true heart. The other features—blood, gill-like pharyngeal slits, and contractile vessels—are present in lancelets, which is why those options aren’t the missing item.

Lancelets illustrate that a heart is not essential for all chordates. They have a simple circulatory system with blood transported through vessels, moving by the rhythmic contraction of those vessels rather than by a true, chambered heart. Gas exchange happens mainly through diffusion across the body surface and through pharyngeal slits used in feeding, which function similarly to gill structures in terms of respiration. They do have blood, and their vessels are contractile to push blood along, but there is no heart.

So the heart-related structure they lack is a true heart. The other features—blood, gill-like pharyngeal slits, and contractile vessels—are present in lancelets, which is why those options aren’t the missing item.