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The axial organ of two sea urchin genera, Echinometra and Eucidaris, has been analysed with the electron microscope. It is mainly built up by an extracoelomic tissue rich in various leucocytic cells and blood vessels lacking an endothelium. The axial sinus (axocoel) is located on the interior of the axial organ, and evaginations extend into this loosely constructed tissue. The epithelium of the axial sinus and its extensions are characterized by epitheliomuscular cells and abundant podocytes, which overlie the blood vessels. Since podocytes are the well known structural basis for ultrafiltration, it is postulated that this is a function of the axial organ at least in echinoids and asteroids.