腕足动物, or brachiopods, are marine animals that have hard shells on the upper and lower surfaces. They are often mistaken for mollusks like clams, but they belong to a different phylum. Brachiopods have existed for over 500 million years and were once very abundant in the oceans. These creatures typically live on the seafloor and filter food from the water using a structure called a lophophore. Brachiopods come in various shapes and sizes, and their shells can be made of calcium carbonate or phosphate. Today, they are less common but still play a role in marine ecosystems.