Why madreporic canal of starfish is also called stone canal?

The madreporic canal of a starfish is often referred to as the stone canal because it is a rigid, calcified structure that connects the madreporite, a porous plate on the starfish's body, to the ring canal. This canal plays a crucial role in the water vascular system, allowing seawater to enter and circulate, which is essential for locomotion and feeding. The term "stone" reflects its hardened, mineralized composition, distinguishing it from softer tissues.