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Examination of dissections and reconstructions made from serial sections and observations of blood flow in small transparent animals have been used to describe the heart and associated blood vessels in the larva (ammocoete) of all three living families of lampreys. Particular emphasis has been placed on describing the condition in ammocoetes spanning the complete range in larval body size and on resolving the serious descriptive and nomenclatural anomalies in the literature. During early larval life (7-30 mm), the sinus venosus becomes almost entirely separated from the liver by a collagenous barrier which develops on the anterior face of the latter organ. However, at no stage in larval life is a transverse septum developed between the pericardial and perivisceral cavities. The four chambers of the heart (sinus venosus, atrium, ventricle and conus arteriosus) are separated by valves, each of which has a pair of leaflets which open and close. The junctions of the left anterior and posterior cardinal veins and of the right anterior and posterior cardinal veins are united transversely where they meet the right and left descending ducts of Cuvier. Since this allows the lateral transfer of blood between the right and left sides of the cardinal veins, it presumably facilitates the distribution of different volumes of blood to the different-sized ducts of Cuvier. The large right duct passes dorsoventrally straight into the sinus venosus, while the smaller and slightly more posterior left duct descends in a channel on the anterior face of the liver, from which it receives further blood before finally entering the sinus venosus below the level of entry of the right duct. In 47% of animals examined, the more dorsal region of the left duct also passed straight into the sinus venosus at a level comparable with that of the right duct. In addition to the ducts of Cuvier, the sinus venosus receives blood directly from a single ventral hepatic vein and a single inferior jugular vein. Except in very young ammocoetes (< 12 mm), the dorsal aorta, and the anterior mesenteric artery which arises from its dorsal surface, curve to the right in the cardiac region of the trunk. This shift in location, which parallels that described for fossil osteostracans, permits the oesophagus to be suspended above the heart by vertical strands of connective tissue hanging from the mid-ventral surface of the notochord.