Bivalve mollusks are a group of aquatic animals that have two hinged shells, or valves, which protect their soft bodies. Common examples include clams, oysters, and scallops. They primarily live in marine and freshwater environments and are known for their filter-feeding habits, where they draw in water to extract food particles. These organisms have a simple body structure, lacking a head and complex nervous system. Bivalves play a crucial role in their ecosystems by filtering water, which helps maintain water quality. They are also important to human economies, providing food and materials like pearls and shells.