1. The anatomy of the blood system of Sabella pavonina is described.
2. Differences from Sabella found in the structure of the blood system of the closely related Spirographis spallanzanii are pointed out.
3. Very young Spirographis show no difference in the structure of the blood system from Sabella. Certain of the differences are, however, found in slightly larger specimens of Spirographis which are smaller than fully grown Sabella. The differences are therefore regarded as specifically significant.
4. A preliminary account is given of the histology of the blood-vessels. The main longitudinal vessels show a distinct muscular layer, which is absent in circular and capillary vessels.
5. The course of the circulation is described. An explanation of the mechanism of the sorting of oxygenated and deoxygenated blood is put forward.
6. Peristalsis in the main longitudinal vessels occasionally reverses its direction; there is thus no physiological polarization of the direction of peristalsis.
7. The occurrence of blind-ending vascular structures in other polychaetes is reviewed and suggestions put forward concerning their function.