The Structure and Mode of Function of the Water Vascular System of a Brittlestar, Ophioderma appressum

Unlike the asteroids, which have large madreporite structures, the ophiuroid Ophioderma appressum possesses only two small hidden madreporite pores. Experiments with labeled amino acids, fluorescent microbeads, and surgical obstruction show that small amounts of seawater do routinely enter these pores and become distributed throughout the water vascular system; but this uptake does not seem essential. The flagellated stone canal draws its fluid from the axial sinus, to which the pores connect through a tortuous ampulla. Thus, the stone canal mainly recirculates fluid from hyponeural (perihemal) passages. That perihemal fluid is augmented by seawater from the pores. As perihemal fluid moves towards the stone canal, it passes by or through the axial organ, where nutritive materials may be removed and passed into the hemal channels. Pressure generated by the stone canal forces flow out to the oral tube feet, polian vesicles, and, through valves, eventually to the arm tube feet. Inflation of the tube feet also might occur through osmotic mechanisms, but their activity was not impeded by raising the external osmotic level with dextran. Observations indicate that negative coelomic pressures must be generated during respiratory movements, and these could lead to sufficient body fluid production (by filtration) that the need for substantive madreporitic inflows would be alleviated.