The circulatory system of jawless fish features which heart structure?

The circulatory system of jawless fish features which heart structure?

• A. Three-chambered heart
• B. Four-chambered heart
• C. Two-chambered heart
• D. No heart

Answer: (C)

Jawless fish have a simple circulation with two main pumping chambers: an atrium that receives blood and a ventricle that pumps it out. Blood flows in a single loop—from the heart to the gills for oxygenation and then to the rest of the body before returning to the atrium. This two-chamber arrangement means there isn’t a complete separation of oxygen-rich and oxygen-poor blood, unlike later vertebrates that evolve extra chambers for more independent pathways. A three-chambered heart has two atria and one ventricle, a four-chambered heart has two atria and two ventricles, and having no heart isn’t accurate for these animals.

Jawless fish have a simple circulation with two main pumping chambers: an atrium that receives blood and a ventricle that pumps it out. Blood flows in a single loop—from the heart to the gills for oxygenation and then to the rest of the body before returning to the atrium. This two-chamber arrangement means there isn’t a complete separation of oxygen-rich and oxygen-poor blood, unlike later vertebrates that evolve extra chambers for more independent pathways. A three-chambered heart has two atria and one ventricle, a four-chambered heart has two atria and two ventricles, and having no heart isn’t accurate for these animals.