The madreporite is a porous, calcareous sieve plate on the aboral surface of sea stars (Asteroidea) and other echinoderms. It serves as the entry point for seawater into the water vascular system — the hydraulic system of canals and tube feet used for locomotion, feeding, and gas exchange. Ascaris is a nematode; Hydra is a cnidarian; the house fly is an insect — none have a water vascular system.