Agnatha, which includes lampreys and hagfish, have a single-circuit circulatory system. This means that blood passes through the heart only once per complete circuit around the body.

Here's a breakdown:

* Heart: Agnathans have a two-chambered heart:

* Sinus venosus: Receives deoxygenated blood from the body.

* Atrium: Receives blood from the sinus venosus and pumps it to the ventricle.

* Ventricle: Pumps deoxygenated blood to the gills.

* Gills: Blood picks up oxygen and releases carbon dioxide in the gills.

* Body: Oxygenated blood circulates throughout the body and then returns to the heart, completing the circuit.

Key points:

* No separation of oxygenated and deoxygenated blood: Unlike more advanced vertebrates, Agnathans don't have a separate pulmonary circulation for oxygenating blood in the lungs.

* Lower pressure system: The single-circuit system operates at a lower pressure than the double-circuit system found in other vertebrates. This is reflected in the thinner-walled blood vessels and slower blood flow.

Let me know if you'd like more details on the circulatory system of any specific Agnathan species!