Brachiopoda, commonly known as lamp-shells, are solitary, sessile benthic marine invertebrates, mainly related to the other lophophorate phylum the Phoronida. They are two-shelled with a dorsal (formerly named brachial) valve and a ventral (formerly named pedicule) valve, and filter-feeding with a ciliated, tentaculated feeding organ named the lophophore, usually supported by the brachidium. Most brachiopods are attached to the substrate by means of a pedicle, while others are cemented or lie freely. Some species that have long pedicles do not attach to a hard substrate but anchor the pedicle deep in the sand. The two main types (with either articulated or inarticulated valves) are recorded since the Lower Cambrian (about 600 MY ago) but they probably date back to at least 800-1,000 MY ago. The brachiopods were particularly abundant in the Palaeozoic but have progressively decreased in diversity toward the Recent. From the about 5,000 described genera, only ~115 are extant and ~385 species from the ~30,000 described ones. Extant representatives are found from the littoral waters (generally subtidal) through to the abyssal zone, and are generally epifaunal on hard substrata; only the lingulides are exclusively infaunal in soft substrata. They range in size from one millimetre to over nine centimetres. Larvae are either planktotrophic or non-planktotrophic.