A detailed description is given of the anatomy and histology of the blood systems of the maldanid polychaetes Clymenella torquata (Leidy) and Euclymene oerstedi (Claparède) (=Caesicirrus neglectus Arwidsson,1911–12). The systems are similar in many ways to that of the arenicolids. All the longitudinal vessels are contractile. Enlarged cells which contain chloragosomes, glycogen, and iron, are associated with the greater part of the ventral vessel and with the plexus of capillaries surrounding the oesophagus. A gut sinus is associated with the stomach and intestine. Intravasal tissue is present in the dorso-lateral regions of the gut sinus in the tail of Clymenella but not of Euclymene.