How does water enter an echinoderm water-vascular system through the madreporite?

The water-vascular system of an echinoderm is a hydraulic network of canals that facilitates movement and respiration. Water enters this system through the madreporite, a porous, button-shaped plate that acts as a sieve.12

While the madreporite is typically located externally to take in water from the surrounding environment, it is internally located in many holothurians (sea cucumbers), where it instead draws fluid from the body cavity. Once the fluid enters the madreporite, it passes through a duct known as the stone canal, which is often reinforced with calcareous deposits. This canal connects the madreporite to a circular ring vessel that surrounds the mouth, from which radial canals extend into the body to distribute fluid to the tube feet.213