Although they lack a stomach, mouth, and anus, giant tube worms have a complex closed cardiovascular system comparable to humans. Because the tube worm has a closed cardiovascular system, blood never leaves the network of blood vessels that continues to supply blood with oxygen from the lungs. Contrary to the Arthropoda's hemocyanin, the primary oxygen carrying protein that Giant Tube Worms use is hemoglobin. Oxygen binds to hemoglobin, allowing it to be delivered wherever it's needed within the body through the arteries.