1. The blood-system in sabellids of the following genera is described: Sabella, Potamilla, Branchiomma, Dasychone, Amphiglena, Fabricia, Jasmineira, Dialychone, and Myxicola.
2. The central blood-system of Sabella is typical of the family, but the peripheral blood-system is variable.
3. The dorsal vessel lacks the valve and muscular sphincter found in some serpulids.
4. Lateral vessels are present only in Sabella and Dasychone.
5. The differences and similarities between sabellid and serpulid blood-systems are discussed. Special attention is given to the functions of sub-epidermal and coelomic capillaries and the blood-supply of the body-wall musculature.