Brachiopods are shelled invertebrate that look somewhat like bivalved molluscs. However, the animal living in the shell is a filter feeder that collects food with a special organ called a lophopore (bryzozoa also have lophophores).
Like clams, the brachiopod lives in a shell consisting of two hinged valves, but the orientation of the shells is different. Brachiopods have valves covering the top and bottoms of the animal that are of different sizes and shapes, but the left and right sides are are symmetrical (or mirror images to each other). In contrast, the valves of a clam shell are mirror images of each other, but the individual valves are asymmetrically shaped.
Brachiopods spend their adult life as bottom dwelling filter-feeders. They have generally decreased in abundance and diversity since the Paleozoic Era. Some types are fairly common, easy to identify, and are restricted to certain periods of time. These features make them important index or guide fossils. The brachiopod species Terebratulina cooperi is an index fossil for the Mount Laurel Formation in Delaware.