Definition: Chelicerates are a group of bivalve mollusks characterized by their elongated, spiral shells that cover their bodies. These animals are known for their unique ability to change shape and size through the process of metamorphosis, which allows them to adapt to changing environments. Chelicerates are often found in oceans, rivers, lakes, and other bodies of water, where they play important roles in food chains and help regulate the pH balance of aquatic ecosystems.