Brachiopods are a class of marine invertebrates with a shell made up of two asymmetrical halves and two arms on the sides of the mouth. Brachiopods have a very long history of life on Earth, at least 550 million years. They first appear as fossils in rocks from the earliest Cambrian period, and their descendants survive, although relatively rarely, in the oceans and seas of today. They were particularly abundant during the Paleozoic (248–545 million years ago) and are often the most common fossils in rocks from that period.
Although relatively rare, modern brachiopods occupy a variety of seafloor habitats, from the tropics to the cold waters of the Arctic and, especially, Antarctica. Unlike bivalves, they are found only in marine environments and have shells made up of two asymmetrical halves.
Brachiopods are virtually defenseless, and their shell, which encloses the animal's organs, is the only protection against predators. Most are permanently attached by a fleshy stalk (pedicle) to a hard surface on the sea floor and are unable to actively pursue food. A few species can attach directly to soft sediment, while others remain unattached. The opening and closing of the shell allows food-carrying currents to pass through it, which the animal then filters for food.
