Brachiopoda

Brachiopoda is a phylum of marine, sessile, filter-feeding invertebrates characterized by two calcareous shells, or valves, surrounding the body. The valves are dorsal and ventral, not left and right, and most species attach to the substrate by a fleshy stalk called a pedicle, though some are free-living or cemented.

The animal lies between the two valves, with a specialized feeding organ called the lophophore—a crown of ciliated tentacles that generates currents and captures suspended particles. The hinge line aligns with the dorsal-ventral axis, and some brachiopods are articulate, possessing teeth and sockets for a rigid hinge, while inarticulate forms lack such hinges, giving them more flexible articulation.

The pedicle typically passes through a pedicle opening in the ventral valve, and the lophophore is retracted into the shell when threatened. The body plan also includes a U-shaped gut and a simple, closed circulatory system, though soft tissues are reduced in many fossil forms.

Brachiopoda comprises three major clades: Linguliformea, Craniiformea, and Rhynchonelliformea, with the latter containing most articulated forms. The traditional split into articulate and inarticulate groups is still used informally, though modern classification emphasizes these deeper lineages.

Fossil evidence shows a long-ranging record from the early Cambrian, with a flourishing diversity in the Paleozoic. Today, about 300 living species remain, occupying shallow to deep marine environments worldwide, often attached to hard substrates or living in soft sediments.