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.
Seashells are made when marine mollusks such as snails, clams and oysters secrete minerals and proteins through their mantle, which is the outermost part of their body that comes in contact with the shells. Shells are made mostly of calcium carbonate; less than 2 percent of their mass is made of proteins. There are no living cells in shells. Shells are usually formed in three layers, which have distinct structures.