SABELLIDS are marine polychaete worms, the blood system of which is peculiar, both anatomically and physiologically. Numerous capillaries are present, in the body wall, in the branchial crown, and projecting freely into the body cavity, all of them ending blindly. Most of the blood vessels in the body contract rhythmically. By peristalsis blood is forced along the continuous vessels, and at intervals of 10–20 seconds blood is expelled from the blind capillaries by regularly rhythmic centripetal contractions of their walls, to flow back again into these capillaries almost immediately. There may thus be said to be a true Galenic circulation in the capillaries. The blood contains the respiratory pigment chlorocruorin.