Sea urchins have a closed circulatory system, in which blood remains inside the blood vessels. Sea urchins, like many echinoderms, do not have a brain and rely on their water-vascular system, which acts like a circulatory system and is made up of water-filled channels that run through the body. Also, the blood of the sea urchins contains coelmocytes, which is an essential part of blood clotting, but it also collects waste products and removes them through the gills and tube feet.