Water Vascular System
One of the characteristics that define not only the sunflower sea star, but echinoderms as a whole, is the water vascular system. This feature is a hydraulic system that moves the tube feet of the sea star. It is controlled by the maderoprite, which is a porous calcareous plate on the aboral surface of the disc that periodically takes in water to replenish the water vascular system. The grooves in the maderoprite catch prevent large particles from entering, while cilia pick up smaller particles and flush them away back into the ocean. Once the water has entered the sea star, it flows through the radial canal into lateral branches that connect the tube feet within each arm. For movement of the tube feet to occur it has a valve, or ampulla, that creates pressure. When the ampulla is closed, the tube foot extends and when it is open, the pressure is released to withdraw the tube feet.
For support, sea stars have calcareous plates for an internal skeleton. These plates can be tightly linked or loosely connected in a mesh-like pattern. Pedicerllarie (spines) are attached to these plates and their names are derived from the type of plate they are attached to. For example, the sunflower sea star has crossed pedicerllarie which look very similar to a lobster or crab claw. The plates are held together by connective tissue in order to support the structure of the sea star.
Sea stars do not have a circulatory system, but they have something else that acts very similarly called the haemal system. The haemal system has basically the same function as blood in that a fluid surrounding the coelom of a sea star transports dissolved nutrients and respiratory gases.
Sea stars ingest food through their mouth which is on the aboral surface of the star. When food enters the mouth, it travels to the cardiac stomach and into a smaller pyloric stomach. Two pyloric ducts extend from the pyloric stomach and then branch into pyloric caeca. The job of the pyloric caeca is to secrete enzymes that break down food to be used as energy. The sea star must excrete waste, which travels down an intestine that leads to the anus, which protrudes from the center of the disc on the oral surface of the sea star.