The water vascular system of a sea star enables movement, digestion and respiration through the use of water to exert force on limbs known as tube feet. The vascular system is primarily water but also contains coelomocytes, protein and potassium salts.
The outer surface of a sea star is dotted with numerous small projections known as tube feet. According to the University of Alaska Southeast, the water vascular system manipulates the flow of water to contract muscles and push the tube feet against the ground.
The controlling mechanism is the maderoprite, a porous calcareous plate that replenishes the sea star's vascular system and filters out harmful particles. After water enters the maderoprite, it goes through a channel known as the radial canal. From there, lateral branches send the water to each individual arm. In every arm there is a valve called an ampulla. When the ampulla closes, it creates pressure that extends the tube feet. When the ampulla opens, it releases pressure and the tube feet withdraw.
Wikipedia explains that the tube feet are normally arranged in two rows of equal length but that some species have alternating rows of long and short lateral canals that give the appearance of four rows of tube feet.