Most sea stars, sea urchins, and sea cucumbers have suction cups at the tips of their tube feet. Tube feet extend through the small holes in the skeleton to the outside. Figure 3.85 shows some examples of echinoderm tube feet. Tube feet are small, delicate projections attached along the side of a water-filled tube called a radial canal. An echinoderm moves by using many tube feet. Water enters and leaves the tubes through this sieve-like plate. The canals are water-filled tubes that open to the outside through a skeletal plate called the madreporite (from Latin root words madre meaning mother and pori meaning small hole) lying on the surface near the anal opening. It is a hydraulic pressure system that aids in movement. The water vascular system is a complex series of canals running through an echinoderm’s body (Fig.
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