The circulatory system of Hutchinsoniella macracantha consists of a tubular heart in thoracic segments 1–6, a dorsoposterior channel, and body fluid (“haemocoelic”) sinuses. The heart, which is possibly closed anteriorly, has three paired ostia dorsolaterally in thoracic segments 2–4. The heart wall is formed by two cellular layers: an inner myocardium and an outer epicardium. The myofibrils are oriented in various directions within the cells. Hutchinsoniella differs from all other crustaceans by lacking a tubule system in the myocardium. The connections between the myocardial cells, called intercalated discs, also differ from those of other crustaceans. The construction of the dorsoposterior channel is unlike that of a typical blood vessel. The body space is organized into a dorsal and ventral haemocoel separated by a thick, cellular, multifunctional, horizontal septum. There is no dedicated pericardial septum. Additional septa and openings between the body cavities channel the circulation of haemocoelic fluid. Resting on the horizontal septum and filling the space between the intestine and the dorsal longitudinal muscles are large cells, here called balloon cells, with few organelles and an otherwise undetermined cytoplasmic content. The function of this new cell type remains obscure.