Madreporite

It is visible as a small red or yellow button-like structure, looking like a small wart, on the aboral surface of the central disk of a sea star or sea urchin or the oral surface of Ophiuroidea.

[2] Close up, it is visibly structured, resembling a "madrepore" (stone coral, Scleractinia) colony.

The water vascular system of the sea star consists of a series of seawater-filled ducts that function in locomotion and feeding and respiration.

Each tube foot is a closed cylinder with muscular walls, having a sucker at the outer end and a bulb-like ampulla at its inner end within the body cavity.

The madreporite's function in maintaining fluid has been experimentally tested on Pisaster ochraceus, wherein sea stars with blocked madreporites were unable to readjust their body volume after exposure to hyperosmotic conditions.