Sea stars have a circulatory system that is primarily made up of seawater rather than blood. Through the sieve plate, seawater is pumped into the animal's water vascular system. This is a trap door known as a madreporite, which is often visible as a light-colored spot on the starfish's top. Seawater moves into the sea star's tube feet from the madreporite, causing the arm to extend. To retract the limb, muscles within the tube feet are used.
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