The doliolid heart is briefly described: despite the absence of closed vessels, it produces an efficient circulation along defined paths within the haemocoel. When it reverses, the flow follows the same paths in reverse. There are very few cells within the haemocoel fluid, but the movements of the occasional cell permit entry velocity to the heart to be measured, and intracardiac pressures of some 2Pa to be calculated. Heart volume is around 0.15% of that of the haemocoel.