Gas exchange
Gas exchange and excretion occur through cilia-lined sacs called bursae; each opens between the arm bases on the underside of the disk. Typically ten bursae are found, and each fits between two stomach digestive pouches. Water flows through the bursae by means of cilia or muscular contraction. Oxygen is transported through the body by the hemal system, a series of sinuses and vessels distinct from the water vascular system.