In arthropods, the heart is positioned on the dorsal side (back) of the body — they have a dorsal, tubular heart. This contrasts with vertebrates, where the heart is ventral. The arthropod circulatory system is open: the dorsal heart pumps haemolymph into open sinuses (haemocoel) that bathe the tissues directly. The haemolymph re-enters the heart through lateral openings called ostia.