Starfish move using tube feet on their undersides. Their feet use seawater to form a hydraulic system. The starfish's vascular system is open and relies on the water around it rather than internal pressure. Starfish can only move outside of water for a short time. A starfish left out of water for too long loses its ability to move, and it dies if not quickly returned to the ocean.
A starfish's mouth and gills are also located on its underside. To feed, the starfish turns its stomach inside-out through its mouth. This unique ability allows the starfish to feed on bivalves, such as clams, without completely prying open the shell. Once the starfish uses its arms to open the shell slightly, it pushes its stomach through the opening and digests the contents of the shell.
Starfish have eye spots that allow them to sense light. One of these red spots is located at the end of each arm on the starfish's top side. Though they can differentiate between light and dark, these eyes cannot see color or make out details. The dark seafloor environment that starfish live in does not require great visual acuity, so basic light sensitivity is sufficient.
